InteractiveFly: GeneBrief

Vesicular glutamate transporter: Biological Overview | References

BIOLOGICAL OVERVIEW

Neuronal activity can result in transient acidification of presynaptic terminals and such shifts in cytosolic pH (pH_cyto) likely influence mechanisms underlying forms of synaptic plasticity with a presynaptic locus. As neuronal activity drives acid loading in presynaptic terminals it was hypothesized that the same activity might drive acid efflux mechanisms to maintain pH_cyto homeostasis. To better understand the integration of neuronal activity and pH_cyto regulation this study investigated the acid extrusion mechanisms at Drosophila glutamatergic motorneuron terminals. Expression of a fluorescent genetically-encoded pH-indicator (GEpHI), named 'pHerry', in the presynaptic cytosol revealed acid efflux following nerve activity to be greater than that predicted from measurements of the intrinsic rate of acid efflux. Analysis of activity-induced acid transients in terminals deficient in either endocytosis or exocytosis revealed an acid efflux mechanism reliant upon synaptic vesicle exocytosis. Pharmacological and genetic dissection in situ and in a heterologous expression system indicate that this acid efflux is mediated by conventional plasma membrane acid transporters, and also by previously unrecognized intrinsic H+ /Na+ exchange via the Drosophila vesicular glutamate transporter (DVGLUT). DVGLUT functions not only as a vesicular glutamate transporter but also serves as an acid extruding protein when deposited on the plasma membrane (Rossano, 2016).

Neuronal activity generates large cytosolic pH (pH_cyto) transients in presynaptic terminals and such transients are likely to influence mechanisms underlying neurotransmitter release. Minute changes in pH_cyto can influence presynaptic processes such as voltage-gated Ca²⁺ channel gating, endocytosis, synaptic vesicle (SV) filling, cytosolic Ca²⁺ buffering, and both Ca²⁺/calmodulin-dependent kinase II and cyclic-AMP-based forms of synaptic plasticity. pH_cyto in quiescent neurons is set by the equilibrium between standing acid influx and efflux. Neuronal activity, however, results in enhanced acid influx, primarily due to electroneutral H⁺/Ca²⁺ exchange across the plasma membrane Ca²⁺-ATPase (PMCA) as it clears cytosolic Ca²⁺. pH homeostasis would be well served if nerve activity triggered acid efflux to offset acid influx (Rossano, 2016).

Nerve activity may enhance acid efflux in many ways. Standing efflux mechanisms might be directly enhanced by decreasing pH_cyto or through intracellular signalling linked to Ca²⁺ entry. For example, anion exchangers (AEs) are known to be modulated by Ca²⁺/calmodulin. Also, various mammalian Na⁺/H⁺ exchangers (NHEs) are modified by Ca²⁺-dependent kinases and protein kinase A and C pathways. Finally, but perhaps most significantly, activity-induced acid efflux could be mediated by translocation of acid extruders, such as the vesicular H⁺ ATPase (vATPase), from intracellular compartments to the plasmamembrane (PM), as described for cholinergic mouse motorneuron (MN) terminals. While mammalian NHE5, NHE6 and NHE9 are present on intracellular membranes in neurons, there are no reports of NHE translocation to the PM during nerve activity (Rossano, 2016).

Whether activity-induced acid efflux occurs at glutamatergic nerve terminals is unknown, but Drosophila glutamatergic MN terminals represent a tractable system for investigation. vATPases are present in these terminals and may extrude acid when deposited on the PM. Drosophila NHE and AE expression patterns have only been grossly characterized. Vesicular glutamate transporters (VGLUTs) have not been previously implicated in pH_cyto homeostasis but a recent report of VGLUT1 cation/H⁺ exchange activity in mammalian SVs (Preobraschenski, 2014) led to the speculation that VGLUTs contribute to acid efflux (Rossano, 2016).

VGLUTs transport glutamate into vesicles using the electrical portion (ΔΨ_H+) rather than the chemical portion (ΔpH) of the electrochemical proton gradient (Δμ_H+), and most models of VGLUT bioenergetics suggest VGLUTs are coupled to cation/H⁺ exchangers (Goh, 2011; Preobraschenski, 2014) or a Cl⁻ shunt. While these mechanisms have never been validated beyond in vitro preparations, H⁺/cation exchange by VGLUT may provide a novel acid efflux mechanism when VGLUTs are deposited on the PM during SV exocytosis and exposed to the strong PM electrochemical Na⁺ gradient (Δμ_Na+) (Rossano, 2016).

In this study, enhanced activity-induced acid transients under exocytotic blockade suggest that Drosophila MN terminals usually extrude acid through the translocation of vesicular acid extruders to the PM. While translocation of the vATPase accounts for some acid efflux, pharmacological and genetic tools revealed a portion of the efflux is mediated directly by the Drosophila vesicular glutamate transporter (DVGLUT). This is the first report of pH_cyto modulation in situ by a VGLUT in any cell. Expression of DVGLUT in Xenopus oocytes revealed intrinsic Na⁺/H⁺ exchange, which marks the first description of ion transport by DVGLUT. Modulation of activity-induced pH_cyto transients by DVGLUT though Na⁺/H⁺ exchange at the PM demonstrates novel integration of pH_cyto, vesicular recycling, glutamate loading and [Ca²⁺]_cyto in presynaptic terminals (Rossano, 2016).

This study examined the extent to which SV exocytosis shapes activity-induced pH_cyto transients in glutamatergic MN terminals through translocation of acid transporters to the PM. Cytosolic expression of the pseudo-ratiometric fluorescent GEpHI pHerry permitted measurement of acid dynamics within individual MN terminals. While intrinsic acid extrusion from MN terminals accelerates when pH_cyto falls, acid extrusion following action potentials trains is much faster than predicted by activation of acid extruders by low pH_cyto. Activity-induced acid extrusion partially offsets Ca²⁺-dependent acidification during nerve activity and continues for seconds after nerve activity. Complementary genetic and pharmacological approaches revealed this activity-induced acid efflux to be mediated by NHEs, the vATPase and Na⁺/H⁺ exchange through DVGLUT (Rossano, 2016).

Previous work has shown depolarization produces significant acid loading in soma, axons and presynaptic terminals of many neuronal preparations. While less common, there are several reports of depolarization-induced acid efflux from neurons. This report expands upon previous studies by characterizing mechanisms which shape rapid pH_cyto transients in the context of established models of vesicular trafficking and [Ca²⁺]_cyto dynamics. The latter is particularly important as [Ca²⁺]_cyto is tightly regulated in presynaptic terminals and [Ca²⁺]_cyto levels drive rapid acid influx and efflux mechanisms through direct Ca²⁺/H⁺ exchange via the PMCA and Ca²⁺-dependent trafficking of vesicular H⁺ transporters, respectively (Rossano, 2016).

Careful analysis of the relationship between pH_cyto and [Ca²⁺]_cyto was necessary to determine if alterations in activity-induced pH_cyto transients were directly due to changes in the location and activity of acid transporters or secondary to changes in [Ca²⁺]_cyto levels. To this end the relationships between [Ca²⁺]_cyto, J_H+ and acid extrusion (τ_rec) as well as resting pH_cyto and [Ca²⁺]_e were quantified. Many manipulations produced changes in resting pH_cyto which cannot be explained by alterations in Ca²⁺ handling as resting pH_cyto is independent of Ca²⁺ in Drosophila MN terminals. Similarly, manipulations which altered τ_rec probably altered acid efflux directly as τ_rec is independent of Ca²⁺ loading during stimulation. Interpreting manipulations which only altered J_H+ during stimulation proved the most challenging as J_H+ is proportional to Ca²⁺ loading during stimulation and thus changes in J_H+ may represent changes in acid influx due to changes in Ca²⁺ handling or direct changes in acid efflux. MN terminals with altered expression of DVGLUT only differed from their controls with respect to J_H+. As there were no differences in bulk [Ca²⁺]_cyto at rest or during stimulation between genotypes with different DVGLUT expression levels, differences in J_H+ between genotypes are probably not due to changes in Ca²⁺ handling, with the caveat that potential alterations in the Ca²⁺ and pH near-membrane micro-environments are not addressed in this analysis (Rossano, 2016).

The data in this study indicate that activity-induced acid efflux is mediated by multiple PM and vesicular acid extruders, namely NHEs, the vATPase and DVGLUT. The contribution of NHEs is unsurprising as NHE gene products are highly expressed in Drosophila larvae (Giannakou, 2001) and spontaneous vesicular fusion at the larval NMJ is sensitive to the NHE inhibitor amiloride (Caldwell, 2013). These results are corroborated in this study as application of the amiloride derivative EIPA decreased resting pH_cyto and delayed acid extrusion in MN terminals. The contributions of the vATPase and DVGLUT to pH_cyto transients are probably attributable to selective trafficking of these transporters to the PM during exocytosis as they are primarily vesicular proteins, although BafA1-sensitive vATPases are constitutively present on the PM of many cells as well. Furthermore, genetic manipulations to impair vesicular endocytosis and exocytosis revealed that stopping vesicular fusion impaired acid extrusion while locking vesicular membrane at the PM enhanced acid clearance following both NH₄⁺ withdrawal and activity-induced acid loading. The conclusion that both the vATPase and DVGLUT are functional components of the recycling pool of vesicular proteins which shape activity-induced pH_cyto transients is further supported by the observation that application of both the glutamate transporter inhibitor EB and BafA1, an established vATPase inhibitor, decrease acid clearance following activity-induced acid loading. These results agree with a previous report that trafficking of the vATPase to the PM can alkalinize the cytosol of mouse cholinergic MN terminals (Zhang, 2010; Rossano, 2016 and references therein).

The notion that the DVGLUT can function at the PM as an acid extruder requires careful consideration as exact transport mechanisms of VGLUTs are unclear and no previous studies have been conducted to elucidate the transport mechanisms of DVGLUT, although it is reasonable to assume its transport modalities are generally similar to those of mammalian VGLUT1 (Preobraschenski, 2014). Here, a combined pharmacological and genetic approach provided the most compelling evidence for acid efflux mediated by DVGLUT. This conclusion is further supported by the observation that EB can inhibit DVGLUT-mediated Na⁺/H⁺ exchange in oocytes. Experiments in which EB has been used to inhibit VGLUTs have been historically interpreted by assuming a primary effect on glutamate transport, not H⁺ dynamics. If intrinsic Na⁺/H⁺ exchange is a shared property of mammalian VGLUTs it is possible that prior work with EB has erroneously ascribed changes in neurotransmitter loading to direct inhibition of glutamate transport rather than secondary effects of inhibited cation/H⁺ exchange (Rossano, 2016).

The bioenergetics of vesicular glutamate transporters are undoubtedly vital to modulation of neurotransmitter release. Studies of mammalian VGLUTs in heterologous expression systems suggest that VGLUTs are primarily driven by ΔΨ_H+ of Δμ_H+ across the vesicular membrane, which is established by the vATPase. Maintenance of ΔΨ_H+ requires dissipation of ΔpH via H⁺ exchange with another cation to enable continuous proton pumping and glutamate transport (Goh, 2011). It is unclear if H⁺/cation exchange is due to co-transport with another ion transporter, as has been described in insect midgut where co-expression of vATPase and Na⁺-coupled nutrient amino acid transporters form a functional NHE (Harvey, 2009), or is an intrinsic property VGLUTs, as has been described in mammalian VGLUT1 (Preobraschenski, 2014). The data presented in this study provide the first direct evidence that intrinsic EB-sensitive Na⁺/H⁺ exchange is a property of DVGLUT. Taken with the observation that the in situ effects of EB are only additive with those of BafA1 in the presence of significant DVGLUT expression it is very likely that Na⁺/H⁺ exchange via DVGLUT contributes to activity-enhanced acid efflux across the PM of MN terminals. The electroneutrality of ion exchange by DVGLUT requires further investigation as changes in V_m upon removal of Na⁺ from oocytes expressing DVGLUT is probably attributable to decrease in the PM Na⁺ gradient rather than electrogenic Na⁺/H⁺ exchange by DVGLUT (Rossano, 2016).

The following model of pH_cyto regulation by DVGLUT reconciles the observation that DVGLUT is a mediator of activity-induced acid extrusion with the available data from mammalian VGLUT ion transport mechanisms. In quiescent nerve terminals NHEs mediate a standing acid efflux and DVGLUT is primarily on the vesicular membrane where it loads glutamate into the vesicular lumen via ΔΨ_H+ generated by the vATPase. The growing ΔpH across the vesicular membrane is dissipated by DVGLUT through cation/H⁺ exchange. Under these conditions K⁺/H⁺ exchange is most likely as K⁺ is much more abundant than Na⁺ in the cytosol and previous work has demonstrated functional K⁺/H⁺ exchange across organellar membranes in rat synaptosomes (Goh, 2011) and amphibian vestibular hair cells. Upon exocytosis to the PM the directionality of DVGLUT reverses and acid efflux is mediated by Na⁺/H⁺ exchange driven by the strong Δμ_Na+ at the PM. Upon cessation of neuronal activity, DVGLUT contributes to an early phase of accelerated acid efflux until it is retrieved from the PM by endocytosis. In recently endocytosed vesicles the elevated Cl⁻ concentration of the vesicular lumen drives glutamate into the vesicle by a Cl⁻ shunt mechanism (Schenck, 2009; Preobraschenski, 2014). As the vesicular ΔΨ_H+ generated by the vATPase is re-established the cycle completes. The model presented above describes a mechanism by which acid efflux can scale to effectively clear the overall net acid load through enhanced trafficking of acid-extruding proteins, including DVGLUT, to the PM, thus maintaining tight control of pH_cyto in the face of large PMCA-medicated acid loads during nerve activity (Rossano, 2016).

Neuronal depolarization drives increased dopamine synaptic vesicle loading via VGLUT

The ability of presynaptic dopamine terminals to tune neurotransmitter release to meet the demands of neuronal activity is critical to neurotransmission. Although vesicle content has been assumed to be static, in vitro data increasingly suggest that cell activity modulates vesicle content. This study used a coordinated genetic, pharmacological, and imaging approach in Drosophila to study the presynaptic machinery responsible for these vesicular processes in vivo. Cell depolarization was shown to increase synaptic vesicle dopamine content prior to release via vesicular hyperacidification. This depolarization-induced hyperacidification is mediated by the vesicular glutamate transporter (VGLUT). Remarkably, both depolarization-induced dopamine vesicle hyperacidification and its dependence on VGLUT2 are seen in ventral midbrain dopamine neurons in the mouse. Together, these data suggest that in response to depolarization, dopamine vesicles utilize a cascade of vesicular transporters to dynamically increase the vesicular pH gradient, thereby increasing dopamine vesicle content (Aguilar, 2017).

Depolarization-induced synaptic vesicle (SV) release is dynamic, requiring feedback between the cellular machinery regulating vesicle recruitment, fusion, and recycling. Although initial work on presynaptic neurotransmitter release assumed that SV content was static with neuronal activity, more recent evidence suggests that vesicles modulate their content to meet the demands of repeated neuronal firing. The ability to tune the amount of neurotransmitter released per vesicle in response to firing adds another level of regulation to synaptic neurotransmission. However, the mechanisms underlying this regulation remain poorly understood, particularly in vivo (Aguilar, 2017).

SVs utilize the H⁺ electrochemical gradient (Δμ_H⁺) across the vesicle membrane to accumulate and retain neurotransmitter cargo. The vacuolar-type H⁺-ATPase (V-ATPase) generates this Δμ_H⁺ gradient, which is comprised of both a chemical H⁺ gradient (ΔpH) and an electrical potential (ΔΨ), where Δμ_H⁺ = ΔpH + ΔΨ (Blakely, 2012). Vesicular neurotransmitter transporters, depending on their substrates, rely on ΔpH or ΔΨ to differing extents to fill SVs. Manipulating either SV ΔpH, ΔΨ, or both modifies vesicle content, including glutamate, GABA, and dopamine (DA). While SV loading of glutamate across the vesicular glutamate transporter (VGLUT) is largely ΔΨ driven, both the loading and retention of monoamines, including DA, are principally ΔpH driven (Aguilar, 2017).

Cytoplasmic DA is transported across the SV membrane through the vesicular monoamine transporter (VMAT), which exchanges two luminal H⁺ out of the vesicle for every DA molecule that enters. This exchange leads to a transient net intraluminal H⁺ loss and establishes dependence on ΔpH for VMAT-mediated DA vesicle loading. Evidence suggests that the interplay between H⁺-exchanging vesicle transporters and the V-ATPase dynamically modulates both vesicular ΔpH and quantal size (Hnasko, 2012; Aguilar, 2017 and references therein).

Consistent with the idea that vesicular ΔpH is dynamic, in vitro stimulation increases secretory vesicle acidification in several secretory cell types, including adrenal chromaffin and thyroid parafollicular cells. These increases in vesicle ΔpH increase catecholamine content in vitro. Although such results suggest that depolarization-induced increases in content and intraluminal acidification are part of a unified process, their precise relationship to one another is not known. Moreover, it is unclear whether these phenomena occur in neurons in vivo (Aguilar, 2017).

This study combined genetic, optical, and pharmacological approaches in Drosophila melanogaster to study the mechanisms underlying in vivo regulation of DA SV pH and content during neuronal stimulation within a viable whole brain. A recently developed second-generation fluorescent false neurotransmitter (FFN), FFN206, was used as a fluorescent DA surrogate (Hu, 2013) to visualize the dynamics of SV DA loading and release in presynaptic DA terminals. dVMAT-pHluorin, a genetically encoded SV pH biosensor (Grygoruk, 2014), was also employed to observe intraluminal pH changes in response to stimulation. By co-expressing dVMAT-pHluorin with tetanus toxin light chain (TeTxLC), an inhibitor of SV fusion, depolarization-induced pH changes within an intact SV pool were studied, independently of exocytosis. The findings demonstrate that neuronal depolarization increases DA SV loading in response to increased vesicular acidification prior to exocytic release. Moreover, it was shown that VGLUT is a critical mediator of this depolarization-induced SV hyperacidification (Aguilar, 2017).

The depolarization-induced effects in Drosophila are conserved in mammals. A pH-responsive dopaminergic FFN, FFN102 (Rodriguez, 2013) was used to monitor effects of depolarization on SV pH in acute mouse striatal slices. DA vesicles were found to hyperacidify in response to neuronal depolarization in a VGLUT2-dependent manner in mice. Therefore, these findings reveal that these mechanisms are conserved across species. These data introduce a new paradigm for DA/glutamate co-transmission, arguing that vesicular glutamate transport mediates dynamic changes in the SV pH gradient and content during neuronal activity (Aguilar, 2017).

This study shows regulation of vesicular DA content in response to depolarization in vivo. Increases in DA SV content during periods of neuronal activity were shown to require the interplay of several transporters, including VGLUT, to enhance ΔpH and thus drive DA vesicle filling through VMAT. Specifically, depolarization produces a concomitant increase in DA SV content and acidification prior to exocytic release. Importantly, the results suggest that DA vesicle ΔpH is dynamic, changing in response to cellular states. Cell stimulation has been shown to modulate the concentrations of intracellular ions, including H⁺, in the cytosol. Likewise, the intraluminal pH of vesicular compartments, including secretory granules, changes in response to stimulation. Nevertheless, the mechanisms underlying these depolarization-induced changes in SV ΔpH have been unclear (Aguilar, 2017).

Cl⁻ is an abundant cellular anion and acts as a modulator of ΔpH in several cell organelles, including SVs. In mammalian cells, the vesicular Cl⁻ channel ClC3 has also been implicated in mediating Cl⁻-induced SV acidification. Acute inhibition of ClCs does not attenuate the magnitude of DA SV hyperacidification following stimulation, suggesting that ClCs are not the principal determinant of this change in ΔpH. Most studies examining ClC3’s role in SV acidification have been conducted in glutamatergic systems. However, far less is known about the contribution of ClCs to DA SV acidification and vesicle content. The current data are consistent with in vitro measurements in purified vesicle preparations, demonstrating no change in DA uptake in the absence of ClC3 expression. These results do not rule out the possibility that Cl⁻ influx drives hyperacidification since there are alternative routes of Cl⁻ entry into the vesicle lumen. To address further the role of Cl⁻ influx in mediating DA SV ΔpH, extracellular Cl⁻ was decreased by isosmotic gluconate substitution. This effectively decreased the concentration gradient driving Cl⁻ entry into the vesicle lumen through ClCs and/or additional routes. Diminishing the driving force for Cl⁻ entry using this approach did not attenuate the magnitude of depolarization-induced hyperacidification. However, Cl⁻ substitution significantly delayed SV hyperacidification in response to stimulation. These data suggest that although Cl⁻ does not itself drive hyperacidification in DA SVs, it may modulate the timing of this phenomenon (Aguilar, 2017).

In the absence of experimental data demonstrating that Cl⁻ mediates the magnitude of depolarization-induced DA SV hyperacidification, it was hypothesized that another counterion may be driving this process. It was speculated that glutamate may be this alternate counterion given: (1) increasing evidence demonstrating that specific neuronal populations release both monoamines and glutamate in mammals and (2) findings in reconstituted vesicle systems showing that VGLUT is sufficient to increase ΔpH (Schenck, 2009). Specifically, it was hypothesized that glutamate transport through VGLUT is necessary to decrease ΔΨ and subsequently drive depolarization-induced hyperacidification. Though several studies have shown co-localization between VMAT2 and VGLUT2 in mammalian DA neurons, it was unknown whether this was also the case in Drosophila. This study found that dVGLUT and dVMAT co-localized in limited subpopulations of presynaptic DA terminals in Drosophila adult central brain, including MB-MV1. This suggests region-specific enrichment of dVGLUT co-localization in fly DA terminals, analogous to increased VGLUT2 co-localization in DA terminals of the nucleus accumbens in mammals. The percentages of co-localization fit well within the values reported in rodent nucleus accumbens ranging from 2% to 25%. This suggests that co-localization between these two transporters is conserved across species (Aguilar, 2017).

To date, the anatomical and functional relationships between VGLUT and VMAT remain controversial. Neuroanatomical evidence from rodent VTA DA neurons shows that subpopulations of their projections can release both DA and glutamate. Moreover, VGLUT2 and VMAT2 are co-expressed in these same neurons. Currently, however, there is no ultrastructural evidence demonstrating co-localization of these transporters to the same vesicles. VGLUT2 and VMAT2 have been shown to segregate in discrete subpopulations of vesicles within the same axons of mesoaccumbens fibers. Though these ultrastructural immunolabeling studies examined fields in great detail, the areas sampled to determine transporter localization in these earlier studies are limited. Therefore, the possibility that earlier ultrastructural studies may have missed populations of SVs with lower levels of VGLUT2/VMAT2 co-expression and co-localization cannot be excluded. Indeed, as little as a single vesicular glutamate transporter is sufficient to fill a SV. The data are consistent with earlier biochemical evidence showing that VGLUT2(+) SVs isolated from rat ventral striatum also expressed VMAT2 and vice versa by reciprocal immunoprecipitation. Moreover, previous electrophysiological evidence showed that: (1) VGLUT2 knockdown in DA neurons decreases evoked DA release as measured by ex vivo fast-scan cyclic voltammetry and (2) glutamate stimulates monoamine uptake in a subpopulation of DA SVs. Thus, although it remains possible that VGLUT2 may regulate DA vesicle content indirectly while localized to distinct non-VMAT(+) SV population(s), a parsimonious explanation is that VMAT2 and VGLUT2 are co-expressed in vesicles in varying amounts within DA terminals (Aguilar, 2017).

To study further VGLUT's role in DA neurotransmission in vivo, AMPH was used to stimulate vesicular DA release independent of depolarization. AMPH-stimulated hyperlocomotion increased with dVGLUT overexpression in presynaptic DA neurons, consistent with the idea that glutamate enhances vesicular monoamine uptake. Moreover, overexpression of dVGLUT in DA neurons increased sensitivity to AMPH action in vivo. Conversely, DA neuron-selective RNAi-mediated knockdown of dVGLUT decreased both basal locomotion and AMPH-induced hyperlocomotion, suggesting that DA vesicle content is diminished in these neurons. Similarly, earlier work in mice showed that VGLUT2 knockdown in DA neurons reduced loco-motor response to AMPH or cocaine. While cocaine and AMPH work through distinct mechanisms, both induce hyperlocomotion by causing increases in extracellular DA. Consequently, decreased locomotion in response to either drug suggests reduced vesicular DA stores. Taken together, these behavioral data in Drosophila validate the significance of VGLUT in mediating vesicular DA content (Aguilar, 2017).

Consistent with the importance of dVGLUT in DA-mediated behavior, this study found that decreasing the expression of dVGLUT in presynaptic DA terminals significantly decreased depolarization-induced hyperacidification in whole, living brain, while over-expression of dVGLUT enhanced this effect. These data suggest that glutamate, as an anion, dissipates ΔΨ and thus promotes an increase in ΔpH across the DA SV membrane. The results therefore demonstrate that DA neurons across species take advantage of this VGLUT-dependent mechanism to augment ΔpH and increase DA vesicle content during depolarization. Whether VGLUT-mediated Cl⁻ transport contributed to depolarization-induced hyperacidification was also investigated. Although dVGLUT knockdown diminished hyperacidification, limiting Cl⁻ within this genetic background did not modify the SV pH gradient further. Interestingly, Cl⁻ substitution with gluconate, a VGLUT-impermeant anion, significantly delayed the time course of hyperacidification. This delay may be explained by previous studies showing that Cl⁻ modulates VGLUT activity, where decreasing external Cl⁻ concentrations significantly reduces VGLUT2-associated currents (Eriksen, 2016). Thus, it is possible that under low Cl⁻ conditions, VGLUT-dependent hyperacidification is delayed because glutamate transport by VGLUT is slowed (Aguilar, 2017).

Acute inhibition of NHE activity attenuated SV hyperacidification and eliminated the increase in DA loading in SVs in response to depolarization. Although the experimental data in both flies and mice suggest that NHEs mediate hyperacidification (increase in ΔpH), these transporters also diminish ΔpH through their function as H⁺ antiporters. To explain this potential discrepancy, it is proposed that NHE-mediated cation/H⁺ exchange promotes the conversion of vesicular ΔpH to ΔΨ, as previously shown in glutamate-containing SVs (Goh, 2011). One possibility is that SVs maintain a constant electrochemical gradient (Δμ_H⁺) across the vesicle membrane in response to NHE activity. Thus, NHE-mediated decreases in ΔpH cause a corresponding increase in vesicular membrane potential (ΔΨ), according to the equation Δμ_H⁺ = ΔpH + ΔΨ. This rise in ΔΨ increases the driving force for VGLUT-mediated glutamate uptake. VGLUT functions as a substrate:H⁺ antiporter, exchanging one H⁺ for every glutamate/Cl⁻ transported into the vesicle lumen (Blakely, 2012). Consequently, the rise in intraluminal negative charge (which diminishes ΔΨ), and loss of H⁺ (which diminishes ΔpH), leads to an overall decrease in Δμ_H⁺. It is suggested that, to overcome this drop in Δμ_H⁺, the V-ATPase pumps additional H⁺ into the vesicle lumen. This leads to an overshoot in intraluminal H⁺, which produces hyperacidification during depolarization, consistent with glutamate’s ability to increase DA vesicle acidification more generally. Finally, the rise in ΔpH increases the driving force for VMAT-dependent loading of DA into SVs (Aguilar, 2017).

Alternatively, it is possible that Δμ_H⁺ does not remain constant in response to NHE activity during depolarization. Rather, NHEs may contribute to a decrease in Δμ_H⁺ by diminishing ΔpH without significantly changing ΔΨ since these transporters carry out an electroneutral (Na⁺ or K⁺)/H⁺ exchange (Goh, 2011). In this scenario, VGLUT further decreases Δμ_H⁺ as above, ultimately leading to the observed increases in intraluminal ΔpH and SV content through the combined actions of the V-ATPase and VMAT. Ultimately, both scenarios lead to a speculation that NHE-mediated cation exchange couples depolarization-induced cation influx through the plasma and SV membranes with DA SV hyperacidification. Nevertheless, the precise mechanisms by which NHEs mediate these changes in intraluminal pH remain a subject of open discussion, requiring further clarification (Aguilar, 2017).

Significantly, this study found that depolarization-induced hyperacidification in DA SVs is phylogenetically conserved in mammals. As in Drosophila, the ex vivo data in ventral striatal slices show that the underlying mechanisms driving this increase in ΔpH require the activities of both VGLUT2 and NHEs. cKO of VGLUT2 expression and pharmacological inhibition of NHE activity both attenuated depolarization-induced changes in ΔpH. The findings provide a mechanism for earlier observations made by Hnasko and colleagues, who showed that VGLUT2 KO mice have decreased evoked DA release in the ventral striatum (Hnasko, 2010; Aguilar, 2017 and references therein).

In addition to use of elevated extracellular K⁺, this study employed electrical stimulation to achieve neuronal depolarization under more physiologic circumstances. Although the physiological firing rates of DA neurons in Drosophila central brain are poorly characterized, the activity of DA neurons in rodents has been studied extensively. VTA neurons in mice tonically fire at 1–9 Hz and may exhibit periods of burst firing at ~20 Hz. This study paired electrical stimulation with FSCV in mouse NAc to detect effects of acute NHE inhibition on evoked DA release. NHE inhibition decreased evoked DA release following repeated trains of stimulation but had no significant effect in response to SP stimulation. Conversely, sustained stimulation enhanced evoked DA release in the control condition. These data suggest that DA neurons meet the increased demands of sustained firing with increased DA levels in an NHE-dependent manner through the mechanism outlined below (Aguilar, 2017).

The following model is proposed for increases in DA SV acidification and content in response to neuronal stimulation: (1) DA neuron depolarization causes an influx of cations, including Na⁺, into the cytoplasm. (2) Monovalent cation:H⁺ exchange via vesicular NHEs produces a transient drop in intraluminal H⁺ (ΔpH). To maintain a constant Δμ_H⁺, ΔΨ rises. (3) This increase in ΔΨ provides the driving force for increased glutamate transport across VGLUT into the vesicle lumen. (4) The resulting buildup of intraluminal negative charge increases the vesicular proton-motive force, causing the V-ATPase to pump more H⁺ into the vesicle lumen. (5) The rise in ΔpH increases the driving force for VMAT-dependent loading of DA into SVs. This model suggests that several transporters, including NHEs, VGLUT, VMAT, as well as the V-ATPase, work in concert to facilitate dynamic changes in DA vesicle ΔpH and content in response to neuronal stimulation (Aguilar, 2017).

Consistent with this model, earlier work showed that DAT and VMAT function in tandem to concentrate DA first in the cytoplasm and ultimately into SVs, respectively (Freyberg, 2016). The experimental system used here preserves the cellular function and localization of the presynaptic machinery, which allowed resolution of the individual contributions of NHE, VGLUT, and VMAT to these changes. Taken together, these findings suggest that DA SVs dynamically modulate their content through changes in ΔpH. During periods of heightened neuronal activity, increases in ΔpH drive further DA loading into SVs prior to fusion, thereby maximizing DA release. Such a mechanism may ultimately shed light on how subsets of actively firing DA neurons rapidly tune DA vesicle content to meet increased synaptic demand (Aguilar, 2017).

Activity-dependent global downscaling of evoked neurotransmitter release across glutamatergic inputs in Drosophila

Within mammalian brain circuits, activity-dependent synaptic adaptations such as synaptic scaling stabilise neuronal activity in the face of perturbations. Stability afforded through synaptic scaling involves uniform scaling of quantal amplitudes across all synaptic inputs formed on neurons, as well as on the postsynaptic side. It remains unclear whether activity-dependent uniform scaling also operates within peripheral circuits. This study tested for such scaling in a Drosophila larval neuromuscular circuit, where the muscle receives synaptic inputs from different motoneurons. Motoneuron-specific genetic manipulations were employed to increase the activity of only one motoneuron and recordings of postsynaptic currents from inputs formed by the different motoneurons. An adaptation was detected which caused uniform downscaling of evoked neurotransmitter release across all inputs through decreases in release probabilities. This 'presynaptic downscaling' maintained the relative differences in neurotransmitter release across all inputs around a homeostatic set point, caused a compensatory decrease in synaptic drive to the muscle affording robust and stable muscle activity, and was induced within hours. Presynaptic downscaling was associated with an activity-dependent increase in Drosophila vesicular glutamate transporter (DVGLUT) expression. Activity-dependent uniform scaling can therefore manifest also on the presynaptic side to produce robust and stable circuit outputs. Within brain circuits, uniform downscaling on the postsynaptic side is implicated in sleep- and memory-related processes. These results suggest that evaluation of such processes might be broadened to include uniform downscaling on the presynaptic side (Karunanithi, 2020).

Changes in neuronal activity can alter synaptic strength and refine synaptic connections as part of normal developmental and behavioral plasticity. Abnormal levels of activity could disrupt normal activity-dependent synaptic modifications, leading to pathophysiological imbalances in activity within neural circuits. Neurons have however evolved the capacity to functionally adapt and maintain stable activities in the face of perturbations. The most intensely studied adaptation, which stabilizes neuronal activity within mammalian brain circuits, is synaptic scaling. It operates globally by uniformly scaling quantal amplitudes across all synaptic inputs formed on a neuron, through changes in postsynaptic receptor numbers. Synaptic scaling causes compensatory changes in excitatory synaptic drive to the neuron in a direction that enables its firing rate, which has strayed out of set point range, to return back within range. Uniform scaling is also proposed to maintain the relative differences in synaptic weights among inputs, thereby maintaining the ability of neural circuits to appropriately process and interpret divergent inputs with physiological relevance. Considering that maintenance of stable activity is imperative for the robust function of the nervous system as a whole, this study tested whether activity-dependent uniform scaling could also operate within peripheral circuits (Karunanithi, 2020).

Peripheral circuits also exhibit compensatory adaptations and activity-dependent synaptic modifications. One such circuit is the Drosophila larval neuromuscular circuit which drives crawling. This is a converging circuit, where different glutamatergic motoneurons form discrete neuromuscular junctions (NMJs) on each muscle. The highly active 1b and the less active 1s motoneurons form NMJs on virtually all larval muscles. Compensatory adaptations observed at those NMJs are mainly those which preserve strong muscular excitation by homeostatically maintaining synaptic drive to the muscle. They include presynaptic homeostatic potentiation, presynaptic homeostatic depression (PHD), and synaptic homeostasis through structural compensations. Those adaptations are recruited through changes at synapses themselves, rather than through changes in neuronal and muscle activities (firing rates), and are therefore not regarded as activity-dependent. It is unclear whether there are activity-dependent adaptations that manifest at NMJs, involving uniform scaling, to afford robust muscle activity (Karunanithi, 2020).

Activity-dependent synaptic modifications, which appear within the larval neuromuscular circuit following long-term increases in neuronal activity, have been associated with synapse formation and maturation, and with synaptic modifications correlated to information storage. Such work used fly strains where activity was chronically increased in all neurons or motoneurons. With current knowledge regarding activity-dependent compensatory adaptations, it was asked whether such synaptic modifications were fully or partly compensatory to afford robust muscle activity. It was also asked whether compensatory adaptations could be recruited by increasing the activity of only one of the two motoneuron types (Karunanithi, 2020).

Using genetic manipulations, the activity of the 1s motoneuron, innervating muscle 2, was transformed from being lowly active to being highly active. Then whether that transformation recruited compensatory adaptations was assessed by testing for alterations in muscle activity and NMJ properties formed by the genetically manipulated 1s and the unmanipulated 1b motoneurons. A compensatory adaptation was uncovered that was termed 'presynaptic downscaling,' which uniformly downscaled 1b and 1s evoked neurotransmitter release to afford robust and stable muscle activity. Activity-dependent uniform scaling can therefore manifest also within the periphery and on the presynaptic side (Karunanithi, 2020).

Activity-dependent uniform scaling of a determinant of synaptic strength has previously been reported to occur only on the postsynaptic side within central circuits. This work showed that such scaling of a determinant of synaptic strength can also occur on the presynaptic side within peripheral circuits. Presynaptic downscaling describes the uniform decreases in QCs across all inputs, maintaining their relative neurotransmitter release differences around a homeostatic set point. Presynaptic downscaling was induced by increasing the activity of only the 1s motoneuron but was manifest by decreasing release probabilities at both the manipulated 1s and the unmanipulated 1b boutons. Presynaptic downscaling decreased synaptic drive to the muscle: (1) stabilizing muscle firing rates around the downshifted set point and homeostatically maintaining the wide variation in firing rates around that set point; and (2) maintaining robust muscle activity by preventing muscle firing rates from being displaced outside of the physiological range. An activity-dependent increase in 1s DVGLUT expression resulted in presynaptic downscaling, adding support to the idea that regulation of VGLUT expression plays a role in the recruitment of presynaptic compensatory adaptations (Karunanithi, 2020).

The findings regarding muscle activity appear to fit within the framework of biological robustness, a concept in systems biology which accounts for variations in set points to maintain robust function against perturbations. Within that framework, a robust system/component can operate stably at different set points. That framework also accounts for set point shifts, as in this study, where excessive 1s neural drive to the muscle causes the muscle firing rate set point to downshift, without firing rates overshooting the physiological range. Through such shifts, robust muscle activity is afforded to potentially generate a wide range of contractions to drive varied movements under adverse conditions (Karunanithi, 2020).

In synaptic downscaling, which operates centrally on the postsynaptic side, the quantal amplitudes are uniformly downscaled across all inputs, decreasing synaptic drive to the postsynaptic neuron. That decrease in synaptic drive affords stability by lowering firing rates back within the narrow set point range. In presynaptic downscaling, QCs are uniformly downscaled across all glutamatergic inputs through decreases in release probabilities (although uniform downscaling of QCs could also occur through decreases in active release site numbers). The downscaling of QCs decreases synaptic drive, affording robust and stable muscle activity. Activity-dependent uniform scaling appears to therefore operate on both the presynaptic and postsynaptic sides by scaling different determinants of synaptic strength to maintain target cell firing rates within operational range. When the operational range is narrow, as in neurons, uniform downscaling stabilizes firing rates around a set point. But when the range is wide, as in larval muscle, uniform downscaling stabilizes firing rates around a different set point without displacing firing rates outside of the physiological range, maintaining robust outputs. Activity-dependent uniform downscaling appears to therefore operate in different capacities in mammalian neurons and larval muscle (Karunanithi, 2020).

A difference between the induction of synaptic scaling centrally and of presynaptic downscaling peripherally is that the former can be produced cell-autonomously, but not the latter. Synaptic scaling can be produced by directly changing the activity of a postsynaptic neuron. Such scaling can be swift, produced in an hour, as the signals activated in response to changes in activity directly affect the synapses formed on that postsynaptic neuron. Presynaptic downscaling, on the other hand, is produced by increasing the activity of a single input neuron. Induction signals, once initiated in that input neuron, would need to travel anterograde to the target cell, and then retrograde from the target cell to the different synaptic inputs to produce uniform downscaling. Induction signals could also involve signaling between 1b and 1s inputs, possibly like those reported among mammalian glutamatergic inputs. Those signaling pathways suggest that presynaptic downscaling induction is a relatively slow (requiring more than an hour) and nonautonomous process. Some candidate molecules carrying anterograde signals being vesicular glutamate and postsynaptic receptors, and retrograde signals could include Glass bottom boat, Dystrophin, and Semaphorin. Molecules evidenced to mediate signaling among mammalian glutamatergic inputs include glutamate, calcium, protein kinase A, and neurotrophic factors (Karunanithi, 2020).

The experiments argue for presynaptic downscaling resulting from increased 1s DVGLUT expression following increased 1s motoneuron activity. That order of process leading to presynaptic downscaling does not however imply direct causality among those steps. A number of intermediate steps may likely be involved, including retrograde signaling from the muscle (discussed above) to calibrate neurotransmitter release in order to prevent muscle activity from overshooting its physiological range. These possibilities need to be investigated in future work (Karunanithi, 2020).

Uniform scaling is proposed to maintain the relative differences among synapses around a set point to prevent disruption of mechanisms which rely on such differences being preserved. For example, in postsynaptic neurons within brain circuits, synaptic downscaling would enable synaptic inputs which underwent Hebbian potentiation to retain their stronger synaptic weighting relative to other inputs, while concurrently preventing such strengthening from causing runaway excitation. In the periphery, recent work at the larval NMJ shows that 1b synapses primarily drive larval muscle contractions during crawling, whereas 1s synapses drive intersegmental coordination of muscle contractions (Newman, 2017). Neurotransmitter release differences between 1b and 1s synapses underlie the differential execution of these two functions. The current findings therefore suggest that uniform downscaling could preserve the relative neurotransmitter release differences to enable 1b and 1s synapses to continue driving these two functions against perturbations. Uniform downscaling on the presynaptic or postsynaptic sides could therefore afford stability within circuits by maintaining the relative differences among synaptic inputs around a homeostatic set point (Karunanithi, 2020).

Larval NMJs display forms of compensatory adaptations, which preserve strong muscular excitation in the face of perturbations. In those forms, EJP amplitude is homeostatically maintained through compensatory changes in both QC and quantal size. PHD is one such adaptation, where the increases in quantal sizes are offset by compensatory decreases in QCs at both 1b and 1s boutons. PHD is induced by directly overexpressing DVGLUT into both 1b and 1s motoneurons, and as such, has not been shown to be induced by altered activity. By contrast, presynaptic downscaling causes a compensatory decrease in EJP amplitude to afford robust and stable muscle activity. It results following an activity-dependent increase in only 1s DVGLUT expression, causing the uniform downscaling of both 1b and 1s QCs. PHD is therefore a compensatory adaptation that preserves muscular excitation by homeostatically maintaining EJP amplitude, whereas presynaptic downscaling is a compensatory adaptation which stabilizes muscle activity by decreasing EJP amplitude. The two different adaptations are produced by two different patterns of DVGLUT expression, but both involve uniform decreases in QCs across inputs. However, in the case of presynaptic downscaling, the uniform decreases are activity-dependent. In this preparation, compensatory adaptations therefore not only regulate muscle excitation, but also muscle activity (Karunanithi, 2020).

Uniform downscaling has been implicated in processes related to sleep and memory. Work in mammalian brain circuits proposes that uniform downscaling of quantal amplitudes on the postsynaptic side renormalizes synapses during sleep following Hebbian potentiation throughout the day, and prevents runaway excitation following Hebbian potentiation during memory formation. This finding suggests that the scope for evaluating such a process should be broadened to include any uniform downscaling on the presynaptic side, and questions whether such downscaling would involve changes in vesicular neurotransmitter transporter expression (Karunanithi, 2020).

In future work, the molecular mechanisms underlying presynaptic downscaling will need to be elucidated, including identification of retrograde and anterograde signals, and the role of DVGLUT in such signaling. Assessments are also needed of how regulatory molecules, and changes in the activity profiles of the 1b motoneuron and both 1b and 1s motoneurons, contribute to robust and stable circuit function. Finally, detailed analysis of the frequency components within muscle bursts could provide better understanding of the mechanisms shaping muscle activity (Karunanithi, 2020).

Octopamine neuron dependent aggression requires dVGLUT from dual-transmitting neurons

Neuromodulators such as monoamines are often expressed in neurons that also release at least one fast-acting neurotransmitter. The release of a combination of transmitters provides both 'classical' and 'modulatory' signals that could produce diverse and/or complementary effects in associated circuits. This study establishes that the majority of Drosophila octopamine (OA) neurons are also glutamatergic and identifed the individual contributions of each neurotransmitter on sex-specific behaviors. Males without OA display low levels of aggression and high levels of inter-male courtship. Males deficient for dVGLUT solely in OA-glutamate neurons (OGNs) also exhibit a reduction in aggression, but without a concurrent increase in inter-male courtship. Within OGNs, a portion of VMAT and dVGLUT puncta differ in localization suggesting spatial differences in OA signaling. These findings establish a previously undetermined role for dVGLUT in OA neurons and suggests that glutamate uncouples aggression from OA-dependent courtship-related behavior. These results indicate that dual neurotransmission can increase the efficacy of individual neurotransmitters while maintaining unique functions within a multi-functional social behavior neuronal network (Sherer, 2020).

Addressing the functional complexities of 'one neuron, multiple transmitters' is critical to understanding how neuron communication, circuit computation, and behavior can be regulated by a single neuron. Over many decades, significant progress has been made elucidating the functional properties of neurons co-expressing neuropeptides and small molecule neurotransmitters, where the neuropeptide acts as a co-transmitter and modulates the action of the neurotransmitter. Only recently have studies begun to examine the functional significance of co-transmission by a fast-acting neurotransmitter and a slow-acting monoamine (Sherer, 2020).

This study has demonstrated that OA neurons express dVGLUT and has utilized a new genetic tool to remove dVGLUT in OA-glutamate neurons. Quantifying changes in the complex social behaviors of aggression and courtship revealed that dVGLUT in brain OGNs is required to promote aggressive behavior and a specific behavioral pattern, the lunge. In contrast, males deficient for dVGLUT function do not exhibit an increase in inter-male courtship. These results establish a previously undetermined role for dVGLUT in OA neurons located in the adult brain and reveal glutamate uncouples aggression from inter-male courtship. It has been suggested that classical neurotransmitters and monoamines present in the same neuron modulate each other's packaging into synaptic vesicles or after release via autoreceptors. For example, a reduction of dVGLUT in DA-glutamate neurons resulted in decreased AMPH-stimulated hyperlocomotion in Drosophila and mice suggesting a key function of dVGLUT is the mediation of vesicular DA content. In this study, the independent behavioral changes suggests enhancing the packaging of OA into vesicles is not the sole function of dVGLUT co-expression and suggests differences in signaling by OA from OGNs on courtship-related circuitry (Sherer, 2020).

Co-transmission can generate distinct circuit-level effects via multiple mechanisms. One mechanism includes spatial segregation; the release of two neurotransmitters or a neurotransmitter and monoamine from a single neuron occurring at different axon terminals or presynaptic zones. Recent studies examining this possible mechanism have described (1) the release of GLU and DA from different synaptic vesicles in midbrain dopamine neurons and (2) the presence of VMAT and VGLUT microdomains in a subset of rodent mesoaccumbens DA neurons. This study expressed a new conditionally expressed epitope-tagged version of VMAT in OGNs and visualized endogenous dVGLUT via antibody labeling. Within OGNs, the colocalization of VMAT and dVGLUT puncta was not complete suggesting the observed behavioral phenotype differences may be due to spatial differences in OA signaling (Sherer, 2020).

A second mechanism by which co-transmission may generate unique functional properties relies on activating distinct postsynaptic receptors. In Drosophila, recent work has identified a small population of male-specific neurons that express the alpha-like adrenergic receptor, OAMB, as aggression-promoting circuit-level neuronal targets of OA modulation independent of any effect on arousal and separately knockdown of the Rdl GABAa receptor in a specific doublesex+ population stimulated male aggression (Watanabe, 2017). Future experiments identifying downstream targets that express both glutamate and octopamine receptors would be informative, as well as using additional split-Gal4 lines to determine if segregation of transporters is a hallmark of the majority of OGNs. Finally, a third possible mechanism is Glu may be co-released from OGNs and act on autoreceptors to regulate presynaptic OA release (Sherer, 2020).

Deciphering the signaling complexity that allows neural networks to integrate external stimuli with internal states to generate context-appropriate social behavior is a challenging endeavor. Neuromodulators including monoamines are released to signal changes in an animal's environment and positively or negatively reinforce network output. In invertebrates, a role for OA in responding to external chemosensory cues as well as promoting aggression has been well-established. In terms of identifying specific aggression circuit-components that utilize OA, previous results determined OA neurons directly receive male-specific pheromone information and the aSP2 neurons serve as a hub through which OA can bias output from a multi-functional social behavior network towards aggression. The ability of OA to bias behavioral decisions based on positive and negative reinforcement was also recently described for food odors. In vertebrates, it has been proposed that DA-GLU cotransmission in the NAc medial shell might facilitate behavioral switching. The finding that the majority of OA neurons are glutamatergic, suggests that the complex social behavior of aggression may rely on small subsets of neurons that both signal the rapid temporal coding of critical external stimuli as well as the frequency coding of such stimuli resulting in the enhancement of this behavioral network. One implication of the finding regarding the separable OA-dependent inhibition of inter-male courtship is the possibility of identifying specific synapses or axon terminals that when activated gate two different behavioral outcomes. A second implication is that aggressive behavior in other systems may be modified by targeting GLU function in monoamine neurons (Sherer, 2020).

Finally, monoamine-expressing neurons play key roles in human behavior including aggression and illnesses that have an aggressive component such as depression, addiction, anxiety, and Alzheimer's. While progress is being made in addressing the functional complexities of dual transmission, the possible pathological implications of glutamate co-release by monoamine neurons remains virtually unknown. Analyzing the synaptic vesicle and release properties of monoamine-glutamate neurons could offer new possibilities for therapeutic interventions aimed at controlling out-of-context aggression (Sherer, 2020).

A pre-synaptic regulatory system acts trans-synaptically via Mon1 to regulate Glutamate receptor levels in Drosophila

Mon1 is an evolutionarily conserved protein involved in the conversion of Rab5 positive early endosomes to late endosomes through the recruitment of Rab7. This study has identified a role for Drosophila Mon1 in regulating glutamate receptor levels at the larval neuromuscular junction. Mutants were generated in Dmon1 through P-element excision. These mutants are short-lived with strong motor defects. At the synapse, the mutants show altered bouton morphology with several small supernumerary or satellite boutons surrounding a mature bouton; a significant increase in expression of GluRIIA and reduced expression of Bruchpilot. Neuronal knockdown of Dmon1 is sufficient to increase GluRIIA levels suggesting its involvement in a pre-synaptic mechanism that regulates post-synaptic receptor levels. Ultrastructural analysis of mutant synapses reveals significantly smaller synaptic vesicles. Overexpression of vglut suppresses the defects in synaptic morphology and also downregulates GluRIIA levels in Dmon1 mutants suggesting that homeostatic mechanisms are not affected in these mutants. It is proposed that DMon1 is part of a pre-synaptically regulated trans-synaptic mechanism that regulates GluRIIA levels at the larval neuromuscular junction (Deivasigamani, 2015).

Neurotransmitter release at the synapse is modulated by factors that control synaptic growth, synaptic vesicle recycling, and receptor turnover at postsynaptic sites. Endolysosomal trafficking modulates the function of these factors and therefore plays an important role in regulating synaptic development and function. Intracellular trafficking is regulated by Rabs, which are small GTPases. These proteins control specific steps in the trafficking process. A clear understanding of the role of Rabs at the synapse is still nascent. Drosophila has 31 Rabs, and most of these are expressed in the nervous system. Rab5 and Rab7, present on early and late endosomes, respectively, are critical regulators of endolysosomal trafficking and loss of this regulation affects neuronal viability underscored by the fact that mutations in Rab7 are associated with neurodegeneration. Rab5 along with Rab3 is present on synaptic vesicles, and both play a role in regulating neurotransmitter release. In Drosophila, Rab3 is involved in the assembly of active zones by controlling the level of both Bruchpilot-a core active zone protein-and the calcium channels surrounding the active zone. In hippocampal and cortex neurons, Rab5 facilitates LTD through removal of AMPA receptors from the synapse. In Drosophila, Rab5 regulates neurotransmission; it also functions to maintain synaptic vesicle size by preventing homotypic fusion. Compared to Rab5 or Rab3, less is known about the roles of Rab7 at the synapse. In spinal cord motor neurons, Rab7 mediates sorting and retrograde transport of neurotrophin-carrying vesicles. In Drosophila, tbc1D17-a known GAP for Rab7-affects GluRIIA levels; the effect of this on neurotransmission has not been evaluated. Excessive trafficking via the endolysosomal pathway also affects neurotransmission. This has been observed in mutants for tbc1D24-a GAP for Rab35. A high rate of turnover of synaptic vesicle proteins in these mutants is seen to increase neurotransmitter release (Deivasigamani, 2015 and references therein).

This study has examined the synaptic role of DMon1-a key regulator of endosomal maturation. Multiple synaptic phenotypes are found associated with Dmon1 loss of function, and one of these is altered synaptic morphology. Boutons in Dmon1 mutants are larger with more satellite or supernumerary boutons-a phenotype strongly associated with endocytic mutants. Formation of satellite boutons is thought to occur due to loss of bouton maturation, with the initial step of bouton budding being controlled postsynaptically and the maturation step being regulated presynaptically. Supporting this, a recent study shows that miniature neurotransmission is required for bouton maturation. The presence of excess satellite boutons in Dmon1 mutants suggests that the number of 'miniature' events is likely to be affected in these mutants. The fact that this phenotype can be rescued upon expression of vGlut supports this possibility. However, this does not fit with the observed decrease in size and intensity of Brp positive puncta in these mutants. Active zones with low or nonfunctional Brp are known to be more strongly associated with increased spontaneous neurotransmission. Considering the involvement of postsynaptic signaling in initiating satellite bouton formation, it is thought that altered neurotransmission possibly together with impaired postsynaptic or retrograde signaling, contributes to the altered synaptic morphology in Dmon1 mutants. This may also explain why no satellite boutons are observed in neuronal RNAi animals (Deivasigamani, 2015).

A striking phenotype associated with loss of Dmon1 is the increase in GluRIIA levels. This phenotype seems presynaptic in origin since neuronal loss of Dmon1 is sufficient to increase GluRIIA levels. Is the increase in GluRIIA due to trafficking defects in the neuron? This seems unlikely for the following reasons: First, it has been shown that although neuronal overexpression of wild-type and dominant negative Rab5 alters evoked response in a reciprocal manner, there is no change in synaptic morphology, glutamate receptor localization and density, or change in synaptic vesicle size. The role of Rab7 at the synapse is less clear. In a recent study, loss of tbc1D15-17, which functions as a GAP for Rab7, was shown to increase GluRIIA levels at the synapse. Selective knockdown of the gene in muscles, and not neurons, was seen to increase GluRIIA levels, indicating that the function of the gene is primarily postsynaptic. These data are not consistent with the current results from neuronal knockdown of Dmon1, suggesting that the presynaptic role of Dmon1 in regulating GluRIIA levels is likely to be independent of Rab5 and Rab7 and therefore novel (Deivasigamani, 2015).

The current experiments to evaluate the postsynaptic role of Dmon1 have been less clear. Although a modest increase in GluRIIA levels are seen upon knockdown in muscles, the increase is not always significant when compared to controls. However, the fact that muscle expression of Dmon1 can rescue the GluRIIA phenotype in the mutant suggests that it is likely to be one of the players in regulating GluRIIA postsynaptically. Further, it is to be noted, that while overexpression of vGlut leads to down-regulation of the receptor at the synapse, the receptors do not seem to get trapped in the muscle, suggesting that multiple pathways are likely to be involved in regulating receptor turnover in the muscle, and the DMon1-Rab7-mediated pathway may be just one of them (Deivasigamani, 2015).

How might neuronal Dmon1 regulate receptor expression? One possibility is that the increase in receptor levels is a postsynaptic homeostatic response to defects in neurotransmission, given that Dmon1^Δ181 mutants have smaller synaptic vesicles. However, in dvglut mutants, presence of smaller synaptic vesicles does not lead to any change in GluRIIA levels, given that receptors at the synapse are generally expressed at saturating levels. Therefore, it seems unlikely that the increase in GluRIIA is part of a homeostatic response, although one cannot rule this out completely. The other possibility is that DMon1 is part of a transsynaptic signaling mechanism that regulates GluRIIA levels in a post-transcriptional manner. The observation that presynaptically expressed DMon1 localizes to postsynaptic regions and the results from neuronal RNAi and rescue experiments support this possibility. The involvement of transsynaptic signaling in regulating synaptic growth and function has been demonstrated in the case of signaling molecules such as Ephrins, Wingless, and Syt4. In Drosophila, both Wingless and Syt4 are released by the presynaptic terminal via exosomes to mediate their effects in the postsynaptic compartment. It was hypothesized that DMon1 released from the boutons either directly regulates GluRIIA levels or facilitates the release of an unknown factor required to maintain receptor levels. The function of DMon1 in the muscle is likely to be more consistent with its role in cellular trafficking and may mediate one of the pathways regulating GluRIIA turnover. These possibilities will need to be tested to gain a mechanistic understanding of receptor regulation by Dmon1 (Deivasigamani, 2015).

Inhibiting glutamate activity during consolidation suppresses age-related long-term memory impairment in Drosophila

In Drosophila, long-term memory (LTM) formation requires increases in glial gene expression. Klingon (Klg), a cell adhesion molecule expressed in both neurons and glia, induces expression of the glial transcription factor, Repo. However, glial signaling downstream of Repo has been unclear. This study demonstrates that Repo increases expression of the glutamate transporter, EAAT1, and EAAT1 is required during consolidation of LTM. The expressions of Klg, Repo, and EAAT1 decrease upon aging, suggesting that age-related impairments in LTM are caused by dysfunction of the Klg-Repo-EAAT1 pathway. Supporting this idea, overexpression of Repo or EAAT1 rescues age-associated impairments in LTM. Pharmacological inhibition of glutamate activity during consolidation improves LTM in klg mutants and aged flies. Altogether, the results indicate that LTM formation requires glial-dependent inhibition of glutamate signaling during memory consolidation, and aging disrupts this process by inhibiting the Klg-Repo-EAAT1 pathway (Matsuno, 2019).

Changes in glial transcription due to neuronal activity have been studied previously, but a specific role of glial transcription in LTM has been less characterized. Expression of the glial transcription factor, Repo, increases shortly after spaced training, and this increase is required for LTM formation. This report has identified Eaat1 as a Repo-regulated glial gene required for LTM consolidation. Eaat1 encodes a glial glutamate transporter that removes glutamate from synaptic sites and transports it into astrocytes. Thus, the data indicate that glutamate signaling needs to be inhibited during LTM consolidation (Matsuno, 2019).

To identify Eaat1, a screen was performed for various genes regulating glial physiology for altered expression during LTM formation. Expression of Eaat1 and crammer was found to increase after spaced training. As Eaat1, but not crammer, is expressed exclusively in glia, focus was placed on Eaat1 as a likely Repo-regulated gene. Indeed, spaced-training-induced increases in EAAT1 depend on Repo and Klg activity. Interestingly, expression of the glial gene, genderblind, which encodes another glial glutamate transporter, required Repo activity for expression, but was not affected by spaced training, suggesting that other transcriptional regulatory factors besides Repo are likely necessary to differentially regulate genes required for memory consolidation from those required for other glial functions (Matsuno, 2019).

Because only screened selected genes were screened, it is possible that Repo induces the expression of other unidentified genes after spaced training. However, somewhat unexpectedly, it was found that overexpression of Eaat1 alone in glial cells is sufficient to rescue the LTM defects of klg and repo mutants. This indicates that the major function of the Klg/Repo signaling pathway is to induce glial expression of Eaat1. It further suggests that one function of astrocytes is to decrease glutamate signaling during LTM consolidation (Matsuno, 2019).

Combined with results from previous studies, this work identifies a putative pathway linking neuronal activity to glial inhibition of glutamate signaling. In flies, the homophilic cell adhesion molecule, Klingon, is expressed in both neurons and glia, and needs to be expressed in both cell types for normal LTM (Matsuno, 2015). Repo expression normally increases after spaced training, whereas it fails to do so in klg mutants, indicating that Klg-mediated neuron-glia communication is necessary for this increase (Matsuno, 2015). Thus, it is proposed that spaced training increases neuronal activity, which induces signaling to glia via the cell adhesion molecule Klg. This results in increased Repo activity in glia, which increases Eaat1 expression, and subsequently decreases glutamate signaling (Matsuno, 2019).

Previous work from various groups including has shown that glutamate signaling through NMDA-type receptors (NRs) is necessary for learning and memory. Overexpression of NRs in mice enhances learning and memory formation, and it has been shown that glial production of D-serine, a neuromodulator that functions as a coactivator of NRs, is necessary for short-lasting memory. In the current study, focus was placed on glutamate activity specifically during memory consolidation, instead of during initial learning and memory formation. Considering the current findings with those of previous studies, it is proposed that NR-dependent glutamate signaling needs to be initially high, during formation of short-lasting memories, but low during a later phase where short-lasting memories are consolidated into LTM. This suggests that glia play at least two roles in memory. They produce D-serine that contributes to high NR activity during memory formation and also produce EAAT1 after learning, which functions to reduce glutamate signaling during memory consolidation (Matsuno, 2019).

Age-related impairments in Drosophila memory do not consist of a general decrease in all forms of learning and memory, but instead consist of decreases in two specific phases of memory, MTM and LTM. The current results suggest that both these memory effects are caused by age-related glial dysfunction. Glia in young flies are able to produce sufficient amounts of D-serine for normal MTM, whereas D-serine amounts decrease 2-fold in aged flies. This decrease is responsible for age-related impairments in 1-h memory, because increasing glial production of D-serine, or directly feeding of D-serine to aged flies, rescues this impairment. Likewise, glial dysfunction is also responsible for age-related impairments in LTM because aged glia are unable to inhibit glutamate signaling during consolidation. Thus, in contrast to young flies, aged flies are unable to modulate glutamate activity during learning and consolidation, leading to defects in the two memory phases (Matsuno, 2019).

The model that EAAT1 inhibits glutamate activity during consolidation stems from EAAT1's role in clearing glutamate from synaptic sites and transporting it into astrocytes. This model is consistent with several mammalian studies that demonstrated decreased expression of astrocytic glutamate transporters upon aging, with a consequent reduction of glutamate uptake. Further supporting this model, it was found that feeding flies memantine or MK801, NMDA receptor antagonists, after spaced training, restores normal LTM in klg mutants and restores LTM in aged flies to youthful levels. This effect requires feeding after training during the consolidation phase. Similar results were obtained by feeding riluzole, a glutamate modulator, which decreases glutamate release and increases astrocytic glutamate uptake. Riluzole has also been reported to ameliorate age-related cognitive decline in mammals, suggesting that the mechanisms of AMI may be conserved between species. In contrast, this study found that D-serine feeding, which rescues age-related declines in short-lasting (1-h) memory, does not improve declines in LTM, but rather attenuates it. This is consistent with the model wherein declines in short-lasting memory and LTM are caused by distinct or opposing mechanisms and glutamate signaling needs to be suppressed during consolidation. Somewhat unexpectedly, it was also found that (s)-4C3HPG, the mGluR1 antagonist/mGluR2 agonist, also ameliorated age-related impairments in LTM. This result indicates that glutamate activity through both ionotropic and metabotropic glutamate receptors antagonizes memory consolidation (Matsuno, 2019).

Currently, it is unclear why glutamate signaling needs to be inhibited during consolidation, but a previous study has shown that Mg2+ block mutations in NMDA-type glutamate receptors (NRs) cause specific defects in LTM in Drosophila. Although Mg2+ block mutations have various effects, one effect is to increase NR activity. Increased NR activity results in increased activity of dCREB2b, an inhibitory isoform of CREB. CREB-dependent gene expression is required during consolidation of LTM, suggesting that consolidation may be preferentially sensitive to NR activity (Matsuno, 2019).

Alternatively, it is possible that neuronal activity needs to be inhibited globally during memory consolidation. Sleep is known to be important for LTM. Sleep deprivation during consolidation prevents LTM formation, whereas artificially inducing sleep after training has been reported to improve LTM. Thus a second possibility is suggested that inhibition of glutamate signaling after spaced training may be a brain-wide phenomenon that promotes consolidation by inducing the organism to sleep. Thus far, gross alterations in sleep duration in klg and repo mutants have not been detected, although this does not preclude minor disruptions in sleep quality that may not be detectable by motion-based sleep assays. Finally, a third possibility is envisioned wherein neuronal inhibition may be required as a neuroprotective mechanism that may be necessary to prevent cell death in neurons that were extensively stimulated during spaced training (Matsuno, 2019).

Mapping the glutamatergic neurons whose activity is inhibited during consolidation will be of great interest in the future. As aversive olfactory memories are formed and stored in the Drosophila MBs, it is possible that specific glutamatergic MB output neurons (MBONs) are inhibited during consolidation. Several glutamatergic MBONs are involved in feedback networks with the lobes of the MBs, suggesting that altering the activity of these neurons may modulate memory consolidation and memory-associated behavioral responses (Matsuno, 2019).

This study has demonstrate that increased expression of Eaat1 is required for LTM consolidation. Based on numerous results from other groups, it is hypothesized that Eaat1 functions to reduce glutamate signaling, and support for this model is provided by demonstrating that pharmacological inhibition of glutamate signaling during consolidation improves LTM under various conditions. However, due to technical limitations, it was not possible to actually measure glutamate concentrations at synapses during memory consolidation and it is not known where and how much glutamate signaling has to be inhibited for optimal LTM consolidation (Matsuno, 2019).

A neuroprotective role for microRNA miR-1000 mediated by limiting glutamate excitotoxicity

Evidence has begun to emerge for microRNAs as regulators of synaptic signaling, specifically acting to control postsynaptic responsiveness during synaptic transmission. This report provides evidence that Drosophila melanogaster miR-1000 acts presynaptically to regulate glutamate release at the synapse by controlling expression of the vesicular glutamate transporter (VGlut). Genetic deletion of miR-1000 led to elevated apoptosis in the brain as a result of glutamatergic excitotoxicity. The seed-similar miR-137 regulates VGluT2 expression in mouse neurons. These conserved miRNAs share a neuroprotective function in the brains of flies and mice. Drosophila miR-1000 showed activity-dependent expression, which might serve as a mechanism to allow neuronal activity to fine-tune the strength of excitatory synaptic transmission (Verma, 2015).

miRNAs have emerged in recent years as important regulators of homeostatic mechanisms. Changes in miRNA expression and activity have been linked to neurodegenerative disorders. A growing body of evidence suggests that miRNAs are neuroprotective in the aging brain, as well as in the control of synaptic function and plasticity. Mouse miR-134 acts postsynaptically to regulate synapse strength, and miR-181 and miR-223 regulate glutamate receptors, thereby affecting postsynaptic responsiveness to glutamate. miR-1, a muscle-specific miRNA, acts in a retrograde fashion at the neuromuscular junction to regulate the kinetics of synaptic vesicle exocytosis. However, there are few examples of miRNAs acting directly in the presynaptic terminal to control synaptic strength. miR-485, which is found presynaptically, has been shown to control the expression of synaptic vesicle protein SV2A, thereby affecting synapse density and GluR2 receptor levels (Verma, 2015).

This paper reports that Drosophila miR-1000 regulates neurotransmitter release from presynaptic terminals. miR-1000 regulates expression of the VGlut, which loads glutamate into synaptic vesicles. Genetic ablation of miR-1000 leads to glutamate excitotoxicity, resulting in early-onset neuronal death. Presynaptic regulation of miR-1000 is activity dependent and may serve as a mechanism for tuning synaptic transmission. Evidence is presented that this regulatory relationship is conserved in the mammalian CNS, with a seed-similar miRNA, miR-137, conferring neuroprotection through regulation of VGluT2. The consequences of misregulation of glutamatergic signaling can be severe: excitotoxicity due to excessive glutamate release has been implicated in ischemia and traumatic brain injury, as well as in neurodegenerative conditions such as Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis (Verma, 2015).

Although postsynaptic regulation of glutamate receptor activity has been well studied, much less is known about presynaptic regulation of glutamatergic signaling. These findings suggest that miR-1000 acts presynaptically to regulate VGlut expression and thereby control synaptic glutamate release. It is tempting to speculate that this could provide a mechanism for tuning synaptic output and locally modulating synaptic strength. Such a mechanism would be most useful if the miRNA itself could be regulated in an activity-dependent manner. Evidence is provided that miR-1000 expression is regulated by light in vivo, presumably reflecting photoreceptor activity in the eye. This in turn leads to light-regulated regulation of VGlut reporter levels. These findings lend support to the notion of activity-dependent regulation of miR-1000 activity. An in depth exploration of these issues awaits the development of methods to monitor changes in presynaptic miRNA levels in real time (Verma, 2015).

Failure of miR-1000-mediated regulation of VGlut led to excess glutamate release and resulted in excitotoxicity. Consistent with these findings, Gal4-directed overexpression of VGlut has been reported to cause neurodegeneration. Notably, elevated levels of vertebrate VGluTs have been associated with excitotoxicity in animal models of epilepsy and traumatic brain injury. The GAERS rat epilepsy model shows elevated levels of VGluT2 but not of VGluT1. Similarly, in a model of stroke, ischemic injury was found to result in elevated expression of VGluT1 but not of VGluT2. VGluT1 levels are regulated by methamphetamine treatment, likely contributing to excitotoxic consequences of methamphetamine abuse. VGluT1 levels have also been reported to increase in rat brains following antidepressant treatment (Verma, 2015).

In the mouse, miR-223 acts on postsynaptic glutamate receptors and has a neuroprotective role in vivo. These findings provide evidence that miR-1000 has a neuroprotective role mediated through regulation of presynaptic glutamate release and that this regulatory mechanism is conserved for miR-137 and VGluT2 in the mouse. Together, these studies show that miRNA-mediated regulation of glutamatergic activity acts pre- and post-synaptically to modulate synaptic transmission and to protect against excitotoxicity. In this context, it is noteworthy that miR-137 is reported to be enriched at synapses. miR-137 levels were found to be low in a subset of Alzheimer's patients with elevated serine palmitoyltransferase 1 expression leading to increased ceramide production. Single nucleotide polymorphisms affecting miR-137 target sites could lead to low-level constitutive overexpression of its targets, even when the SNP is present in a single copy. A single nucleotide polymorphism affecting miR-137 has also been identified as a risk factor for schizophrenia. It will be of interest to learn whether misregulation of VGluT2 expression contributes to these neurological conditions. The current findings raise the possibility that miRNA mediated regulation makes VGluT and other miRNA targets possible risk factors in neurodegenerative disease (Verma, 2015).

Identification of inhibitory premotor interneurons activated at a late phase in a motor cycle during Drosophila larval locomotion

Rhythmic motor patterns underlying many types of locomotion are thought to be produced by central pattern generators (CPGs). This study used the motor circuitry underlying crawling in larval Drosophila as a model to try to understand how segmentally coordinated rhythmic motor patterns are generated. Whereas muscles, motoneurons and sensory neurons have been well investigated in this system, far less is known about the identities and function of interneurons. A recent study identified a class of glutamatergic premotor interneurons, PMSIs (period-positive median segmental interneurons), that regulate the speed of locomotion. This study reports on the identification of a distinct class of glutamatergic premotor interneurons called Glutamatergic Ventro-Lateral Interneurons (GVLIs). Calcium imaging was used to search for interneurons that show rhythmic activity, and GVLIs were identified as interneurons showing wave-like activity during peristalsis. Paired GVLIs were present in each abdominal segment A1-A7 and locally extended an axon towards a dorsal neuropile region, where they formed GRASP-positive putative synaptic contacts with motoneurons. The interneurons expressed vesicular glutamate transporter (vGluT) and thus likely secrete glutamate, a neurotransmitter known to inhibit motoneurons. These anatomical results suggest that GVLIs are premotor interneurons that locally inhibit motoneurons in the same segment. Consistent with this, optogenetic activation of GVLIs with the red-shifted channelrhodopsin, CsChrimson ceased ongoing peristalsis in crawling larvae. Simultaneous calcium imaging of the activity of GVLIs and motoneurons showed that GVLIs' wave-like activity lagged behind that of motoneurons by several segments. Thus, GVLIs are activated when the front of a forward motor wave reaches the second or third anterior segment. It is proposed that GVLIs are part of the feedback inhibition system that terminates motor activity once the front of the motor wave proceeds to anterior segments (Itakura, 2015).

Increased vesicular glutamate transporter expression causes excitotoxic neurodegeneration

Increases in vesicular glutamate transporter (VGLUT) levels are observed after a variety of insults including hypoxic injury, stress, methamphetamine treatment, and in genetic seizure models. Such overexpression can cause an increase in the amount of glutamate released from each vesicle, but it is unknown whether this is sufficient to induce excitotoxic neurodegeneration. This study shows that overexpression of the Drosophila vesicular glutamate transporter (DVGLUT) leads to excess glutamate release, with some vesicles releasing several times the normal amount of glutamate. Increased DVGLUT expression also leads to an age-dependent loss of motor function and shortened lifespan, accompanied by a progressive neurodegeneration in the postsynaptic targets of the DVGLUT-overexpressing neurons. The early onset lethality, behavioral deficits, and neuronal pathology require overexpression of a functional DVGLUT transgene. Thus overexpression of DVGLUT is sufficient to generate excitotoxic neuropathological phenotypes and therefore reducing VGLUT levels after nervous system injury or stress may mitigate further damage (Daniels, 2011).

Membrane topology of the Drosophila vesicular glutamate transporter

The vesicular glutamate transporters (VGLUTs) are responsible for packaging glutamate into synaptic vesicles, and are part of a family of structurally related proteins that mediate organic anion transport. Standard computer-based predictions of transmembrane domains have led to divergent topological models, indicating the need for experimentally derived predictions. This study presents data on the topology of the VGLUT ortholog from Drosophila melanogaster (DVGLUT). Using immunofluorescence assays of DVGLUT transiently localized to the plasma membrane of heterologously transfected cells, the accessibility of epitope tags inserted into the lumenal/extracellular face of the protein was determined. Using immunoisolation, this study identified complementary tagged sites that face the cytoplasm. The data show that DVGLUT contains 10 hydrophobic regions that completely span the membrane (TMs 1-10) and that the amino and carboxyl termini are cytosolic. Importantly, between TMs 4 and 5 is an unforeseen cytosolic loop of some 50 residues. Other domains exposed to the cytosol include loops between TMs 6-7 and 8-9, and regions C-terminal to TM2 and N-terminal to TM3. Between TM2 and 3 is a potentially hydrophobic, but topologically ambiguous region. Lumenal domains include sequences between TMs 1-2, 3-4, 5-6, 7-8 and 9-10. These data provide a basis for determining structure-function relationships for DVGLUT and other related proteins (Fei, 2007).

A single vesicular glutamate transporter is sufficient to fill a synaptic vesicle

Quantal size is the postsynaptic response to the release of a single synaptic vesicle and is determined in part by the amount of transmitter within that vesicle. At glutamatergic synapses, the vesicular glutamate transporter (VGLUT) fills vesicles with glutamate. While elevated VGLUT expression increases quantal size, the minimum number of transporters required to fill a vesicle is unknown. In Drosophila DVGLUT mutants, reduced transporter levels lead to a dose-dependent reduction in the frequency of spontaneous quantal release with no change in quantal size. Quantal frequency is not limited by vesicle number or impaired exocytosis. This suggests that a single functional unit of transporter is both necessary and sufficient to fill a vesicle to completion and that vesicles without DVGLUT are empty. Consistent with the presence of empty vesicles, at dvglut mutant synapses synaptic vesicles are smaller, suggesting that vesicle filling and/or transporter level is an important determinant of vesicle size (Daniels, 2006).

Increased expression of the Drosophila vesicular glutamate transporter leads to excess glutamate release and a compensatory decrease in quantal content

Quantal size is a fundamental parameter controlling the strength of synaptic transmission. The transmitter content of synaptic vesicles is one mechanism that can affect the physiological response to the release of a single vesicle. At glutamatergic synapses, vesicular glutamate transporters (VGLUTs) are responsible for filling synaptic vesicles with glutamate. To investigate how VGLUT expression can regulate synaptic strength in vivo, this study has identified the Drosophila vesicular glutamate transporter, which was named DVGLUT. DVGLUT mRNA is expressed in glutamatergic motoneurons and a large number of interneurons in the Drosophila CNS. DVGLUT protein resides on synaptic vesicles and localizes to the presynaptic terminals of all known glutamatergic neuromuscular junctions as well as to synapses throughout the CNS neuropil. Increasing the expression of DVGLUT in motoneurons leads to an increase in quantal size that is accompanied by an increase in synaptic vesicle volume. At synapses confronted with increased glutamate release from each vesicle, there is a compensatory decrease in the number of synaptic vesicles released that maintains normal levels of synaptic excitation. These results demonstrate that (1) expression of DVGLUT determines the size and glutamate content of synaptic vesicles and (2) homeostatic mechanisms exist to attenuate the excitatory effects of excess glutamate release (Daniels, 2004).

This study investigated how the expression of a vesicular glutamate transporter regulates the strength of a glutamatergic synapse in vivo. The goal was to perform these studies at the Drosophila neuromuscular junction, a well characterized glutamatergic synapse, and thereby avoid the use of cultured neurons.DVGLUT is a synaptic vesicle protein that is present in the synaptic terminals of all glutamatergic motoneurons as well as many synaptic terminals in the CNS. Increased expression of DVGLUT at the NMJ leads to an increase in quantal size. Hence, DVGLUT expression can regulate the glutamate content of a synaptic vesicle. A corresponding increase in vesicle volume suggesting that the vesicular glutamate concentration is likely to remain roughly constant despite excess glutamate content. Release of this excess glutamate triggers a homeostatic mechanism that downregulates the number of vesicles released by the motoneuron, thereby maintaining normal levels of synaptic excitation (Daniels, 2004).

The predicted Drosophila protein CG9887 was identified as a candidate DVGLUT because its amino acid sequence and hydrophobicity profile are similar to those of known VGLUTs and its mRNA is strongly expressed in a subset of neurons. CG9887 was shown to be a synaptic vesicle protein, is expressed at synaptic terminals of all known glutamatergic motoneurons, and can regulate the glutamate content of synaptic vesicles. Based on these findings, it is concluded that CG9887 is a Drosophila VGLUT. Because CG9887 is the only Drosophila gene similar to the vertebrate VGLUTs that is expressed in neurons, CG9887 may be the only Drosophila VGLUT (Daniels, 2004).

Although the Drosophila NMJ is a well characterized glutamatergic synapse, the role of glutamate as a transmitter elsewhere in the Drosophila nervous system has received little attention. Widespread expression of DVGLUT was detected in the synaptic neuropil of the brain and nerve cord, suggesting that glutamate is an important transmitter for interneurons. The ability to detect glutamatergic terminals using the antibody to DVGLUT will aid in the identification of the transmitter phenotype of identified central neurons in Drosophila (Daniels, 2004).

Overexpression of transporters for acetylcholine, monoamines, and glutamate in cultured cells can increase quantal size, suggesting that changing the expression or activity of vesicular transporters may be a general mechanism for controlling the transmitter content of vesicles. This study tested this hypothesis for a vesicular glutamate transporter at an intact synapse in vivo (Daniels, 2004).

Transgenic flies were generated that overexpress DVGLUT at the glutamatergic NMJ. Increased expression of DVGLUT led to a large increase in the postsynaptic response to the fusion of single vesicles. This increase in the mean response was attributable to two factors. First, the entire population of spontaneous events shifted to larger amplitude. The median amplitude increased by 35%, and the mEJP amplitude distribution was recapitulated by scaling the wild-type distribution by a factor of 1.35. Second, a small population of very large spontaneous events appeared with DVGLUT overexpression. These events persisted in the presence of TTX and absence of external calcium, so they do not represent spontaneous evoked release and could not be evoked by an action potential. Regardless of the nature of these rare events, the key finding is that increasing the levels of synaptic DVGLUT increases the postsynaptic response to the entire pool of synaptic vesicles. Hence, the number of DVGLUT molecules in a vesicle regulates the glutamate content of that vesicle (Daniels, 2004).

The concentration and volume of glutamate in the vesicle will affect the concentration and time course of glutamate in the synaptic cleft, which are important determinants of the postsynaptic response. The overexpression of DVGLUT loads more glutamate into vesicles, so either the concentration of glutamate or the size of the vesicle must have increased. To determine which mechanism is underlying the observed increase in quantal size, the diameter of synaptic vesicles from wild-type and DVGLUT-overexpressing synapses was measured. It was observed that the entire population of synaptic vesicles is larger when DVGLUT is overexpressed. The calculated increases in the median (61%) and mean (95%) vesicle volume are more than sufficient to explain the observed increase in the median (35%) and mean (60%) quantal size. Therefore, it is likely that the glutamate concentration in the vesicle does not increase with DVGLUT overexpression; however, because quantal size does not change as much as vesicular volume, the glutamate concentration may actually be somewhat lower than in wild type. Alternatively, postsynaptic glutamate receptors may be approaching saturation in the mutant and unable to faithfully record the full extent of the increase in released glutamate, although the increase in quantal size that was observe demonstrates that in wild type the glutamate receptors are not saturated (Daniels, 2004).

In previous studies of VGLUT and VAChT overexpression, the increases in quantal size were interpreted as increases in the concentration of vesicular transmitter. This led to a steady-state model of vesicle filling, in which inflow through the transporters is balanced by a leak from the vesicle: more inflow caused by more transporter therefore leads to a higher steady-state equilibrium concentration of transmitter. This was contrasted with a set-point model, in which the concentration of transmitter is held constant, and more transporters would only be expected to increase the rate, but not the extent, of vesicle filling; however, these models do not take into account the possibility that the volume of the vesicle could change. The current data argue for a modified set-point model: the concentration of transmitter is held roughly constant, but the vesicular volume is changed by the number of transporter molecules in the vesicle. A similar model has been described for the loading of monoamines into secretory vesicles. The current findings suggest that despite differences between secretory vesicles and synaptic vesicles, a similar mechanism can increase transmitter content (Daniels, 2004).

Two models could explain the relationship between transporter expression and vesicle volume. First, the volume of the vesicle may be affected by its transmitter content; more functional transporter would add more glutamate, which by an unknown mechanism would lead to a larger vesicle. Alternatively, the vesicle volume may be sensitive to the physical addition of the transporter with its many membrane-spanning domains. These models could be distinguished by the expression of a nonfunctional transporter (Daniels, 2004).

Having used transgenic techniques to manipulate DVGLUT expression in vivo, it was possible to ask about the physiological consequences of this excess glutamate at an intact synapse. Total synaptic excitation was found to be unchanged despite the increase in glutamate per vesicle because the presynaptic terminal releases fewer vesicles with each evoked event. It is possible that overexpression of DVGLUT directly decreases release probability; however, it would be an unlikely coincidence that this would exactly offset the increase in quantal size and produce a normal-sized evoked event. Instead, the model is favored that a modest and persistent increase in glutamate release from each vesicle leads to a compensatory decrease in the number of released vesicles that attenuates the excitatory effect of glutamate. Other homeostatic mechanisms have been described at the fly NMJ. Various studies have demonstrated that decreasing postsynaptic activity, either by manipulating glutamate receptors or postsynaptic potassium channels, triggers a compensatory increase in presynaptic transmitter release. In addition, nonvesicular leak of glutamate can homeostatically regulate postsynaptic glutamate receptor levels. The current findings are the first to demonstrate that a change in the release of vesicular glutamate can trigger a homeostatic change in presynaptic release properties (Daniels, 2004).

What could trigger this compensatory downregulation of presynaptic release? Is it caused by increased postsynaptic excitation, or is it a direct effect of the increased glutamate in the cleft? Previously work manipulated postsynaptic expression of glutamate receptors leading to an increase in quantal size that is commensurate with the increase seen with DVGLUT overexpression. With increased postsynaptic excitation but no change in extracellular glutamate, there is no compensation; quantal content is unchanged and evoked synaptic events are larger (Petersen, 1997). So increasing glutamate release by overexpressing DVGLUT induces compensation, whereas directly increasing postsynaptic excitation by manipulating glutamate receptors does not affect cleft glutamate and does not trigger a compensatory decrease in quantal content. In summary, postsynaptic reductions in quantal size do initiate homeostatic compensation and postsynaptic increases in quantal size do not cause such compensation, but presynaptic increases in quantal size do trigger homeostatic decreases in presynaptic release. These data are consistent with the model that increased glutamate in the cleft directly triggers a compensatory downregulation of release. Similar findings in culture demonstrate that the persistent excitation of vertebrate neurons leads to a compensatory decrease in glutamate release (Moulder, 2004). Such homeostatic mechanisms could serve to limit the spread of excitotoxicity (Daniels, 2004).

Modular organization of Cis-regulatory control information of neurotransmitter pathway genes in Caenorhabditis elegans

This study explored the cis-regulatory logic that dictates gene expression in specific cell types in the nervous system. We focus on a set of eight genes involved in the synthesis, transport, and breakdown of three neurotransmitter systems: acetylcholine (unc-17 /VAChT, cha-1 /ChAT, cho-1/ChT, and ace-2 /AChE), glutamate (eat-4 /VGluT), and gamma-aminobutyric acid (unc-25 /GAD, unc-46 /LAMP, and unc-47 /VGAT). These genes are specifically expressed in defined subsets of cells in the nervous system. Through transgenic reporter gene assays, the cellular specificity of expression of all of these genes was found to be controlled in a modular manner through distinct cis-regulatory elements, corroborating the previously inferred piecemeal nature of specification of neurotransmitter identity. This modularity provides the mechanistic basis for the phenomenon of 'phenotypic convergence,' in which distinct regulatory pathways can generate similar phenotypic outcomes (i.e., the acquisition of a specific neurotransmitter identity) in different neuron classes. Cases of enhancer pleiotropy were identified, in which the same cis-regulatory element is utilized to control gene expression in distinct neuron types. A cis-regulatory allele of the vesicular acetylcholine transporter, unc-17 /VAChT, was engineered to assess the functional contribution of a 'shadowed' enhancer. A selective loss was observed of unc-17 /VAChT expression in one cholinergic pharyngeal pacemaker motor neuron class and a behavioral phenotype that matches microsurgical removal of this neuron. This analysis illustrates the value of understanding cis-regulatory information to manipulate gene expression and control animal behavior (Serrano-Saiz, 2020).

Past work has defined the modular nature of neurotransmitter identity specification in C. elegans from two different perspectives. First, distinct terminal selector-type transcription factors were found to be required to specify the same neurotransmitter identity in different neuron types. In several cases, it was explicitly shown that this specificity is achieved through the binding of the respective transcription factor to defined cis-regulatory elements within neurotransmitter pathway genes. Second, reporter constructs that capture distinct regions of the loci that encode for neurotransmitter pathway genes were shown to be selectively expressed in distinct subsets of ACh, glutamate, GABA, or monoaminergic neurons, respectively. This study has expanded the analysis of cis-regulatory control regions of neurotransmitter pathway genes, thereby corroborating the largely modular control of neurotransmitter identity. However, not every neuron type is controlled by an independent, dedicated module, since the same cis-regulatory element can be employed by the same transcription factor in different cell types, likely by acting in the context of a distinct combinatorial code of transcription factors. Hence, cell type-specific gene expression profiles are controlled by combinations of different types of regulatory constellations (Serrano-Saiz, 2020).

This and past analysis clearly shows that distinct, modular regulatory inputs control the expression of neurotransmitter pathway systems in distinct neuron types (see Illustrations of the organization of cis-regulatory elements for neuron type-specific gene expression). Apart from using transgenic reporter gene assays, this study provides a striking example of this modularity by engineering a cis-regulatory allele of the unc-17/VAChT locus in which a small, phylogenetically conserved motif was mutated. This manipulation not only leads to a selective loss of unc-17/VAChT expression in a single neuron class, but also causes the phenotype that one would expect from dysfunctionality of this neuron (Serrano-Saiz, 2020).

Nevertheless, past work, as well as the work presented in this study, also demonstrates that regulatory elements that drive expression in distinct neuron types can be difficult to disentangle, i.e., even small cis-regulatory elements still drive expression in multiple neuron classes. Does this observation contrast with the modular control model and support a singular control model? Theoretically, a more finely grained mutational analysis may still separate distinct elements for distinct cell types. However, there are already clear cases where it is known that the same transcription factor acts in distinct cells to specify, for example, their cholinergic identity, seemingly in support of the singular model. However, in many if not all cases, this singular transcription factor may act in combination with different sets of cell type-specific partner proteins. For example, the COE-type transcription factor unc-3 controls unc-17/VAChT and cho-1/ChT expression in VNC motor neurons and their presynaptic command interneurons, which are all cholinergic and specified by unc-3. Mutation of the unc-3-binding site leads to loss of expression in all these neuron classes. Similarly, a regulatory element from the unc-17/VAChT locus shows expression in the IL2, URA, and URB neuron classes, all of which require unc-86 for their proper neurotransmitter identity acquisition. Likewise, a regulatory element from the eat-4 locus is expressed in the AIM and PHC neuron classes, both of which require unc-86 for their glutamatergic identity specification. Cis-regulatory elements that control expression of GABA pathway genes in the RIS and AVL neuron classes (both controlled by the nhr-67 and lim-6 transcription factors) can also not be easily separated, while cis-regulatory elements for GABAergic VNC motor neurons (which are dependent on the unc-30 transcription factor) can be separated from RIS and AVL. These examples demonstrate that the modularity of cis-regulatory control does not need to manifest itself by the existence of distinct, completely separable cis-regulatory elements that drive expression in distinct neuron types. Rather, the same cis-regulatory element and its cognate transcription factor may be deployed in different cell types because transcription factors operate in distinct, cell type-specific combinations (Serrano-Saiz, 2020).

Enhancers that are employed to drive expression in different cell types have been termed 'pleiotropic enhancers'. Such pleiotropy has been mostly inferred from genome-wide binding profiles of histone modification marks that indicate (but do not prove) enhancer activity at a relatively coarse level. Specifically, marks of enhancer activity can be found in the same location in distinct tissue types. The underlying mechanistic basis for enhancer pleiotropy is often not clear. The cis-regulatory analysis conducted in this and previous papers not only supports the concept of enhancer pleiotropy in an orthogonal and more granular manner, but also strongly hints at its mechanistic basis. Different neuron types utilize the same cis-regulatory element because this regulatory element contains binding sites for the same trans-acting factor (Serrano-Saiz, 2020).

While the unc-3 case provides the clearest evidence for the same cis-regulatory motif to be employed in different neuron types, this analysis also reveals that a transcription factor required for acquisition of the same neurotransmitter identity in distinct neuron types does not necessarily always act through the same cis-regulatory element. For example, UNC-86 operates in more than a dozen different neuron types to specify neurotransmitter identity. It appears that UNC-86 functions through separable cis-regulatory elements in the unc-17 locus to enable cholinergic gene expression in different cell types. These distinctive types of engagement of UNC-86 with its target promoters are a likely reflection of distinct molecular partnerships that UNC-86 may engage in (Serrano-Saiz, 2020).

In several neurotransmitter pathway genes, the existence is noted of separable elements that drive expression in the same neuron types. For example, RME motor neuron expression is controlled by separable cis-regulatory elements in the unc-25/GAD locus. This is akin to the 'shadow enhancers' that have been observed to drive the expression of developmental control genes in Drosophila. One immediate question that arises from such a constellation is whether this is an indication of true enhancer redundancies. Complete redundancy would predict that deletion of one of these elements would result in no expression defect because of the existence of the other element. Alternatively, the sufficiency of isolated elements to drive expression could be a result of the multicopy nature of concatamerized C. elegans transgenes. In the natural genomic context, several separable cis-regulatory elements may need to cooperate to turn on expression in a specific cell type. This notion was tested explicitly through mutating a small conserved motif that is located in one of two separate cis-regulatory elements of the unc-17 locus, which each drive expression in the same set of pharyngeal neurons. Completely penetrant defects in unc-17 expression were found in a single neuron class, thereby indicating that, in their endogenous context, a cis-regulatory element may be required even if another element produces, in the context of a multicopy array, the same pattern of expression. This finding is not without precedent. Two separate cis-regulatory elements were defined in the cog-1 homeobox gene locus that are alone sufficient to drive expression in the ASE neuron. Nevertheless, a cis-regulatory allele of cog-1 was isolated from a mutant screen in which only one of those two elements is mutated. These examples illustrate an important limitation for the interpretation of reporter gene analysis: the expression of isolated elements may demonstrate that an element is sufficient to produce an expression pattern, but they do not prove that such an element is required in its normal genomic context (Serrano-Saiz, 2020).

Pioneering work by Eric Davidson and colleagues in the sea urchin embryo revealed that 'nuts and bolts' differentiation genes, i.e., genes that do not encode regulatory factors, but rather 'function' proteins that endow a cell with specific anatomical or functional properties, are largely controlled by positive regulatory inputs, i.e., by transcriptional activators. This cis-regulatory analysis of neurotransmitter pathway genes provides further support for this notion. If the expression patterns of neurotransmitter pathway genes were frequently shaped by repressive mechanisms, successive deletion of regulatory elements should have revealed ectopic expression of reporter genes. While such derepression was observed in some rare cases, repression does not appear to be a major mechanism in shaping the expression of neurotransmitter pathway genes. The cis-regulatory analysis of other neuronal differentiation genes, undertaken by this laboratory over the years , also provides only limited evidence in favor of repression being an important mechanism to shape neuron type-specific gene expression profiles. This is not to say that transcriptional repression plays no role in shaping the terminal differentiation program. Several studies have revealed that transcriptional repression is used in a number of distinct cellular contexts (sensory neurons and motor neurons) to establish subtle, but important molecular differences between members of the same neuron class [e.g., differences between left vs. right ASE neurons or differences between different VNC motor neuron classes (Serrano-Saiz, 2020).

Lastly, the dissection of cis-regulatory elements of genes that define specific phenotypic traits of distinct neuron types (i.e., neurotransmitter identities) highlights the underlying mechanistic basis of phenotypic convergence. In evolutionary theory, phenotypic convergence refers to independent evolution of similar traits in different organisms. This terminology has recently been applied to the acquisition of the phenotypic trait of neurotransmitter identity. In both C. elegans and Drosophila, distinct transcription factors control the same phenotypic trait in distinct neuron types (Serrano-Saiz, 2013; Konstantinides, 2018; Kratsios and Hobert 2018). Cis-regulatory analyses like the ones described here demonstrate that it is the cis-regulatory control regions of neurotransmitter pathway genes that integrate distinct regulatory inputs to turn on a common neurotransmitter trait in distinct neuron types.

Merlin modulates process outgrowth and synaptogenesis in the cerebellum

Neurofibromatosis type 2 (NF2) patients are prone to develop glial-derived tumors in the peripheral and central nervous system (CNS). The Nf2 gene product Merlin (see Drosophila Merlin) is not only expressed in glia, but also in neurons of the CNS, where its function still remains elusive. This study shows that cerebellar Purkinje cells (PCs) of isoform-specific Merlin-deficient mice were innervated by smaller vGluT2-positive clusters (see Drosophila Vesicular glutamate transporter) at presynaptic terminals than those of wild-type mice. This was paralleled by a reduction in frequency and amplitude of miniature excitatory postsynaptic currents (mEPSC). On the contrary, in conditional transgenic mice in which Merlin expression was specifically ablated in PCs (L7Cre;Nf2(fl/fl)), enlarged vGluT2-positive clusters were found in their presynaptic buttons together with increased amplitudes of miniature postsynaptic currents. The presynaptic terminals of these PCs innervating neurons of the deep cerebellar nuclei were also enlarged. When exploring mice with Merlin-deficient granule cells (GCs) (Math1Cre;Nf2(fl/fl)), this study found cerebellar extracts to contain higher amounts of vGluT1 present in parallel fiber terminals. In parallel, mEPSC frequency was increased in Math1Cre;Nf2(fl/fl) mice. On the contrary, VGluT2 clusters in cerebellar glomeruli composed of NF2-deficient presynaptic Mossy fiber terminals and NF2-deficient postsynaptic GC were reduced in size as shown for isoform-specific knockout mice. These changes in Math1Cre;Nf2(fl/fl)-deficient mice were paralleled by an increased activation of Rac1-Cofilin signaling which is known to impact on cytoskeletal reorganization and synapse formation. Consistent with the observed synaptic alterations in these transgenic mice, this study observed altered ultrasonic vocalization, which is known to rely on proper cerebellar function. No gross morphological changes or motor coordination deficits were observed in any of these transgenic mice. It is therefore concluded that Merlin does not regulate overall cerebellar development, but impacts on pre- and post-synaptic terminal organization (Toledo, 2019).

VGLUT1 functions as a glutamate/proton exchanger with chloride channel activity in hippocampal glutamatergic synapses

Glutamate is the major excitatory transmitter in the vertebrate nervous system. To maintain synaptic efficacy, recycling synaptic vesicles (SV) are refilled with glutamate by vesicular glutamate transporters (VGLUTs). The dynamics and mechanism of glutamate uptake in intact neurons are still largely unknown. This study show by live-cell imaging with pH- and chloride-sensitive fluorescent probes in cultured hippocampal neurons of wild-type and VGLUT1-deficient mice that in SVs VGLUT functions as a glutamate/proton exchanger associated with a channel-like chloride conductance. After endocytosis most internalized Cl- is substituted by glutamate in an electrically, and presumably osmotically, neutral manner, and this process is driven by both the Cl- gradient itself and the proton motive force provided by the vacuolar H+-ATPase. These results shed light on the transport mechanism of VGLUT under physiological conditions and provide a framework for how modulation of glutamate transport via Cl- and pH can change synaptic strength (Martineau, 2017).

The CaM Kinase CMK-1 mediates a negative feedback mechanism coupling the C. elegans Glutamate Receptor GLR-1 with its own transcription

Regulation of synaptic AMPA receptor levels is a major mechanism underlying homeostatic synaptic scaling. While in vitro studies have implicated several molecules in synaptic scaling, the in vivo mechanisms linking chronic changes in synaptic activity to alterations in AMPA receptor expression are not well understood. This study used a genetic approach in C. elegans to dissect a negative feedback pathway coupling levels of the AMPA receptor GLR-1 with its own transcription. GLR-1 trafficking mutants with decreased synaptic receptors in the ventral nerve cord (VNC) exhibit compensatory increases in glr-1 mRNA, which can be attributed to increased glr-1 transcription. Glutamatergic transmission mutants lacking presynaptic eat-4/VGLUT or postsynaptic glr-1, exhibit compensatory increases in glr-1 transcription, suggesting that loss of GLR-1 activity is sufficient to trigger the feedback pathway. Direct and specific inhibition of GLR-1-expressing neurons using a chemical genetic silencing approach also results in increased glr-1 transcription. Conversely, expression of a constitutively active version of GLR-1 results in decreased glr-1 transcription, suggesting that bidirectional changes in GLR-1 signaling results in reciprocal alterations in glr-1 transcription. CMK-1/CaMK signaling axis was identified as a mediator of the glr-1 transcriptional feedback mechanism. Loss-of-function mutations in the upstream kinase ckk-1/CaMKK, the CaM kinase cmk-1/CaMK, or a downstream transcription factor crh-1/CREB, result in increased glr-1 transcription, suggesting that the CMK-1 signaling pathway functions to repress glr-1 transcription. Genetic double mutant analyses suggest that CMK-1 signaling is required for the glr-1 transcriptional feedback pathway. Furthermore, alterations in GLR-1 signaling that trigger the feedback mechanism also regulate the nucleocytoplasmic distribution of CMK-1, and activated, nuclear-localized CMK-1 blocks the feedback pathway. A model is proposed in which synaptic activity regulates the nuclear localization of CMK-1 to mediate a negative feedback mechanism coupling GLR-1 activity with its own transcription (Moss, 2016).

Gata2 and Gata3 regulate the differentiation of serotonergic and glutamatergic neuron subtypes of the dorsal raphe

Serotonergic and glutamatergic neurons of the dorsal raphe regulate many brain functions and are important for mental health. Their functional diversity is based on molecularly distinct subtypes; however, the development of this heterogeneity is poorly understood. This study shows that the ventral neuroepithelium of mouse anterior hindbrain is divided into specific subdomains giving rise to serotonergic neurons as well as other types of neurons and glia. The newly born serotonergic precursors are segregated into distinct subpopulations expressing vesicular glutamate transporter 3 (Vglut3) or serotonin transporter (Sert). These populations differ in their requirements for transcription factors Gata2 and Gata3 (see Drosophila Serpent), activated in the post-mitotic precursors. Gata2 operates upstream of Gata3 as a cell fate selector in both populations, whereas Gata3 is important for the differentiation of the Sert+ precursors and for the serotonergic identity of the Vglut3+ precursors. Similar to the serotonergic neurons, the Vglut3 expressing glutamatergic neurons, located in the central dorsal raphe, are derived from neural progenitors in the ventral hindbrain and express Pet1. Furthermore, both Gata2 and Gata3 are redundantly required for their differentiation. This study demonstrates lineage relationships of the dorsal raphe neurons and suggests that functionally significant heterogeneity of these neurons is established early during their differentiation (Haugas, 2016).

Vesicular glutamate transporters use flexible anion and cation binding sites for efficient accumulation of neurotransmitter

Vesicular glutamate transporters (VGLUTs) accumulate the neurotransmitter glutamate in synaptic vesicles. Transport depends on a V-ATPase-dependent electrochemical proton gradient (DeltamuH+) and requires chloride ions, but how chloride acts and how ionic and charge balance is maintained during transport is controversial. Using a reconstitution approach, an exogenous proton pump was used to drive VGLUT-mediated transport either in liposomes containing purified VGLUT1 or in synaptic vesicles fused with proton-pump-containing liposomes. The data show that chloride stimulation can be induced at both sides of the membrane. Moreover, chloride competes with glutamate at high concentrations. In addition, VGLUT1 possesses a cation binding site capable of binding H+ or K+ ions, allowing for proton antiport or K+ / H+ exchange. It is concluded that VGLUTs contain two anion binding sites and one cation binding site, allowing the transporter to adjust to the changing ionic conditions during vesicle filling without being dependent on other transporters or channels (Preobraschenski, 2014).

REFERENCES

Search PubMed for articles about Drosophila VGlut

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date revised: 1 March 2024

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