InteractiveFly: GeneBrief

Insulin-like peptide 7: Biological Overview | References

Cellular Insulin signaling (IS) shows a remarkable high molecular and functional conservation. Insulin-producing cells respond directly to nutritional cues in circulation and receive modulatory input from connected neuronal networks. Neuronal control integrates a wide range of variables including dietary change or environmental temperature. Although it is shown that neuronal input is sufficient to regulate Insulin-producing cells, the physiological relevance of this network remains elusive. In Drosophila melanogaster, Insulin-like peptide7-producing neurons are wired with Insulin-producing cells. The former cells regulate the latter to facilitate larval development at high temperatures, and to regulate systemic Insulin signaling in adults feeding on calorie-rich food lacking dietary yeast. These results demonstrate a role for neuronal innervation of Insulin-producing cells important for fruit flies to survive unfavorable environmental conditions (Prince, 2021).

This study has analyzed the role of dIlp7-producing neurons in different thermal treatments. D7Ns are active on yeast diets, but show no activity in animals kept on yeast-free corn food (CF). Activated D7Ns are required to respond to heat stress. In addition, dIlp7 produced by D7Ns regulates dIlp2/dIlp3-induced Insulin signaling (IS) on CF, and yeast products are able to supplement efficiently for the loss of this neuropeptide (Prince, 2021).

The generative cycle of Drosophila is divided into feeding and non-feeding stages. Due to the absence of food intake during embryonic and pupal development these stages highly rely on internal energy stores. In contrast, larvae and adults need to absorb food to survive and develop. The insulin signaling cascade is one metabolic circuit to regulate the absorption and internal turnover of macronutrients. In addition, the cascade is essential to provide thermal resistance for ectothermic insects. All feeding stages of Drosophila express four neuronal Insulin-like peptides, namely dIlp 2, 3, 5, and 7 . Larvae with functionally compromised Insulin-producing cells (IPCs) kept on yeast diets are heat sensitive, slow in development and small in size (Prince, 2021).

Dietary yeast increase intracellular Ca2+ levels of IPCs, elevate systemic IS and support survival at high temperatures. This study found that IPCs with high Ca2+ are not sufficient to rescue larval survival at high temperatures on yeast-free CF. Therefore, it was speculated that yeast products likely activate additional neurons involved in heat stress responses. It was shown that animals kept on yeast increase Ca2+ in D7Ns (Linneweber, 2014). D7Ns connect to IPCs and are able to stimulate the latter. This study shows that, on CF, D7Ns are low on Ca2+ with respect to yeast-fed animals and that induced Ca2+ levels in D7Ns improve larval heat resistance on CF. In addition, larvae with inactivated D7Ns kept on yeast show poor survival at high temperatures. Thus, D7Ns are one integral part of the heat response and it is speculated that these neurons directly communicate with IPCs. D7Ns secrete a multitude of neuropeptides including dIlp7. DIlp7 mutants kept on yeast food (YF) are slightly heat sensitive, and due to such relative high survival rates, is is deemed unlikely that dIlp7 is one main cue crucial to withstand thermal treatments (Prince, 2021).

D7Ns are inactive on CF and attempts were made to identify dIlp candidates responsible for IS on yeast-free diets. Interestingly, dIlp2, dIlp3, and dIlp7 were identified as essential for larval development. Moreover, genetic interactions revealed that δdIlp2,3 double mutants are unable to survive on CF. In stark contrast, δdilp2,7 and δdilp2-3,7 animals rescued the lethality shown by single mutants. These findings indicate a new metabolic link between dIlp7 and dIlp2 essential for larval development in yeast-free environments. However, wild larvae grow in microbe-rich environments, such as rotting fruits, and have likely access to dietary yeast. Adult flies sometimes feed on yeast-poor diets or avoid yeast in response to cold. Therefore, adults kept on CF were sampled. Adult δdilp7 flies show reduced IS levels and higher lethality rates with respect to genetic controls. Moreover, the combined absence of dIlp2 and dIlp7 pronounced the observed adult lethality on CF. Thus, larval and adult dIlp7 signaling is likely very different (Prince, 2021).

It was reported that dIlp7 is expressed in the subesophageal ganglion region of the brain and suggested that D7Ns regulate the feeding behavior (Cognigni, 2011). Therefore, reduced feeding of dIlp7 mutants could explain the lower IS levels on CF. This study has shown that, on CF, δdilp2, and δdilp7 mutants ingest food faster, have a longer retention time of the ingested material and are able to absorb macronutrients. Therefore, the idea that these flies are starving on CF is not favored. It is more likely that dIlp7 is required to stimulate IPCs to maintain basic dIlp levels in circulation. To test for this possibility, wthe predicted target receptor of dIlp7, the G-protein-coupled rector Lgr3 was knocked down. The loss of Lgr3 results in low IS levels on CF. In contrast, on YF, all tested genotypes show IS comparable to controls. Taken together, it is concluded that neuronal dIlp7/Lgr3 signaling controls IPCs in adults kept on yeast-free diets. As such dIlp7 secures a basic amount of systemic IS and therefore, likely contributes to thermal resistance of adult flies. However, required adult tracking on CF at low temperatures appeared impractical to confirm this idea (Prince, 2021).

Neuronal innervation of IPCs is established in many animals and modulates metabolic signals. The current findings indicate that food products can overwrite such neuronal stimulation. In Drosophila, a dual role for D7Ns was found: (1) these neurons facilitate the heat response of larvae feeding on yeast and (2) they form a metabolic circuit that enables adult flies to thrive on yeast-free diets if required. In mice and humans, pancreatic islets are directly innervated; however, the role of this neuronal stimulation in response to dietary cues is not well understood. This study has identified the importance of D7Ns and their product, dIlp7, in regulating IS in response to dietary quality. These findings provide new insights into the neuronal stimulation of IPCs within a given ecological context and provide a model to study neuronal innervation of insulin producing cells (Prince, 2021).

A neuropeptidergic circuit gates selective escape behavior of Drosophila larvae

Animals display selective escape behaviors when faced with environmental threats. Selection of the appropriate response by the underlying neuronal network is key to maximizing chances of survival, yet the underlying network mechanisms are so far not fully understood. Using synapse-level reconstruction of the Drosophila larval network paired with physiological and behavioral readouts, this study uncovered a circuit that gates selective escape behavior for noxious light through acute and input-specific neuropeptide action. Sensory neurons required for avoidance of noxious light and escape in response to harsh touch, each converge on discrete domains of neuromodulatory hub neurons. Acute release of hub neuron-derived insulin-like peptide 7 (Ilp7) and cognate relaxin family receptor (Lgr4) signaling in downstream neurons are required for noxious light avoidance, but not harsh touch responses. This work highlights a role for compartmentalized circuit organization and neuropeptide release from regulatory hubs, acting as central circuit elements gating escape responses (Imambocus, 2021).

Insulin-like peptides regulate feeding preference and metabolism in Drosophila

Fruit flies have eight identified Drosophila insulin-like peptides (DILPs) that are involved in the regulation of carbohydrate concentrations in hemolymph as well as in accumulation of storage metabolites. This study investigated diet-dependent roles of DILPs encoded by the genes dilp1-5, and dilp7 in the regulation of insect appetite, food choice, accumulation of triglycerides, glycogen, glucose, and trehalose in fruit fly bodies and carbohydrates in hemolymph. The wild type and the mutant lines demonstrate compensatory feeding for carbohydrates. However, mutants on dilp2, dilp3, dilp5, and dilp7 showed higher consumption of proteins on high yeast diets. High nutrient diets led to a moderate increase in concentration of glucose in hemolymph of the wild type flies. Mutations on dilp genes changed this pattern. The dilp2 mutation led to a drop in glycogen levels independently of diet. Lack of dilp3 led to dramatic increase in circulating trehalose and glycogen levels, especially at low protein consumption. Lack of dilp5 led to decreased levels of glycogen and triglycerides for all diets, whereas knockout on dilp7 caused increase in glycogen levels and simultaneous decrease in triglyceride levels at low protein consumption. Fruit fly appetite was influenced by dilp3 and dilp7 genes. These data contribute to the understanding of Drosophila as a model for further studies of metabolic diseases (Semaniuk, 2018).

This study has found that fruit flies consume more food on diets with low rather than higher percentages of carbohydrates, indicative of compensatory feeding for carbohydrates. DILPs are known to be connected with signaling pathways that control appetite, and diminution of insulin signaling leads to increased consumption of unpalatable food by fruit flies or their larvae. In this study, the lack of a functional DILP4, DILP5, or DILP7 led to significantly increased food consumption on almost all diets. However, the lack of either DILP2 or DILP3 led to increased food consumption only on high protein diets. Notably, dilp7 mutants had the most pronounced increase in food intake on low sucrose concentrations. These mutants also showed compensatory responses to changes in concentration of the yeast solution, for example consuming a substantially greater volume of food than wild type flies on the 3S-3Y (3% sucrose and 3% yeast or yeast autolysate) and the 3S-6Y diets. DILP7 is believed to be a relaxin-like peptide in Drosophila, regulating egg laying decisions and thus it would have a major role to play for the female flies used in the present study. It is notable that protein and carbohydrate intake are essential for reproduction, with protein appetite being tightly coupled to mating and egg production. Interestingly, human relaxins have been shown to be orexigenic. Partial co-expression of dilp7 with short neuropeptide F is involved in feeding behavior and, additionally, DILP7 has been reported to be involved in the regulation of feeding behavior by other authors. Interestingly, the effect was obtained by inactivation of Dilp7 producing neurons. Increased appetite is also characteristic of other DILP mutants, though to a lesser degree. This may suggest that other DILPs also function at least partially as relaxin-like peptides. However, this poses the question as to whether specific DILPs, especially DILP7, may bind to receptors other than the Drosophila insulin receptor (dInR). Human relaxins and insulin-like peptides are known to bind to specific G-protein coupled receptors distinct from the tyrosine kinase type insulin receptor (Semaniuk, 2018).

It was shown recently that responses to a continuous lack of protein and carbohydrates are mediated by distinct dopaminergic neurons. Moreover, activation of protein feeding simultaneously inhibited carbohydrate feeding. These results add complexity to these data, suggesting that the response is additionally mediated by DILPs (Semaniuk, 2018).

The only DILP mutation which led to substantial increase in the level of hemolymph glucose was DILP2. Mutation of all the other DILPs resulted in a slight decrease in hemolymph glucose on some diet treatments and a slight increase on others (e.g., dilp5 mutant). The regulation of hemolymph glucose levels by DILP2 has also been confirmed by other authors, and its physiological proximity to insulin is supported by the similarity of amino acid sequences of the two peptides. The dependence of hemolymph glucose on protein intake by dilp3, dilp5, and dilp7 mutants suggests that DILPs may also be involved in regulation of distinct branches of glucose metabolism such as glycolysis, gluconeogenesis, glycogenesis, and the pentose phosphate pathway. Particularly, the level of circulating glucose may depend on the balance between cellular influx and efflux of glucose, glycogen synthesis and breakdown, synthesis of glucose from organic or amino acids via gluconeogenesis, as well as on glucose catabolism via glycolysis and/or pentose phosphate pathways. In fruit flies, glucose can also be used for trehalose synthesis. However, circulating trehalose levels showed an inverse relationship to circulating glucose in the dilp3 mutant. This suggests that a lack of DILP3 may result in conversion of glycogen stores into trehalose instead of glucose. This possible conversion was inhibited by an increased intake of protein. Therefore, the current data imply a role of DILP3 in regulating trehalose concentration in hemolymph. This finding is not consistent with previous data, which showed that a knockdown of the dilp2 gene but not dilp3 gene led to an increase in trehalose content in whole flies. However, the diet used in that study was more concentrated than most of the diets in the current study. Notably, it has been shown that trehalose promotes selective secretion of DILP3. Interestingly, dilp3 mutants also had high levels of glucose in their bodies. A similar pattern of diet-dependent accumulation of trehalose in the body was observed for dilp7 mutants although this accumulation was not reflected in an increase in hemolymph trehalose levels, suggesting that dilp7 is not involved in regulation of this branch of metabolism (Semaniuk, 2018).

Using nutritional geometry, this study has shown that both dilp2 and dilp5 are involved in regulation of glycogen synthesis in fruit flies, a function that has not been reported previously for these DILPs. Glycogen, along with proteins, contributes to fly weight, what is reflected in this study where dilp2 and dilp5 mutants were significantly lighter than controls on a high carbohydrate diet. Earlier, it was shown that dilp5 expression was dependent on dietary carbohydrate. On the other hand, in a previous study no significant change was found in dilp5 expression with an increase in sugar concentration in the larval diet. However, dilp5 expression was dependent on the type of carbohydrate: increased glucose concentration did not change dilp5 expression, but fructose suppressed steady-state levels of dilp5 transcripts (Semaniuk, 2018).

The current data show that TAG and glycogen accumulation were most pronounced on high sucrose diets in wild type flies. Lack of DILP5 led to a decrease in TAG content for all treatments. Hence, DILP5 seems to be especially necessary for nutrient sensing on high carbohydrate and high yeast diets, and also for converting ingested food into carbohydrate and lipid storage on these diets. Recent data on dilp5 expression confirm these suggestions. In particular, expression of dilp5 is dependent on both consumed carbohydrate and proteinin, and was triggered by yeast in larvae in another study (Semaniuk, 2018).

The role of Dilp5 in regulation of glycogen and TAG synthesis/breakdown has not been reported previously. This study design was notable in two respects. First, flies were allowed to choose between protein and carbohydrate sources. This choice takes into account the impact of a dilp mutation on nutrient sensing. Indeed, dilp expression and release of the peptides from neurosecretory cells is dependent on availability of particular nutrients. The sensory responses to particular nutrients which are transduced to insulin-producing cells was shown to be mediated by biogenic amines and food odors. Secondly, conditions were created where flies were relatively free to fly and otherwise move around, thereby expending energy and creating a demand for glycogen and TAG catabolism (Semaniuk, 2018).

As compared with control flies, lower levels of TAG were also found in the dilp7 mutants. The effects of dilp7 knockout on lipid metabolism were also indirectly confirmed by recent data showing that down-regulation of insulin signaling led to reduced lipid accumulation. In previous studies, ablation of insulin-producing cells either resulted in no change in lipid content or slightly increased it (Semaniuk, 2018).

The present study explored the influence of dilp knockouts on (1) food intake and (2) levels of stored and circulating metabolites. The functions of DILPs in Drosophila resemble those of insulin in mammals, namely the lowering of glucose levels in blood (hemolymph in insects) by increasing glucose uptake by cells and directing it into production of storage metabolites, including glycogen and lipids. The current data showed that the balance of dietary macronutrients (proteins and carbohydrates) influences the outcomes of an insulin-like peptide deficiency. Moreover, this study has shown that knockouts of specific insulin-like peptides of Drosophila affect feeding behavior. It was clearly shown that different DILPs mediate regulation of particular aspects of diet-dependent metabolism. From the current data, this study showed that DILP2 and DILP5 can be involved in diet-independent accumulation of glycogen. DILP3 was shown to influence trehalose synthesis and/or release into the hemolymph, whereas DILP5 and DILP7 mainly affected TAG synthesis on high carbohydrate diets. These specific roles for DILPs are likely conferred by nutrient-dependent DILP release by neurosecretory cells and specific target cells for DILPs. The specific connection between feeding behavior and DILPs, found in the current study, may reflect an impact of nutrient cues on signaling effects of DILPs. In other words, specific ratios of macronutrients can evoke DILP release and subsequent regulation of metabolism in particular tissues (Semaniuk, 2018).

Drosophila female-specific Ilp7 motoneurons are generated by Fruitless-dependent cell death in males and by a double-assurance survival role for Transformer in females

Female-specific Ilp7 neuropeptide-expressing motoneurons (FS-Ilp7 motoneurons) are required in Drosophila for oviduct function in egg laying. This study uncovered cellular and genetic mechanisms underlying their female-specific generation. Programmed cell death (PCD) eliminates FS-Ilp7 motoneurons in males, and that this requires male-specific splicing of the sex-determination gene fruitless (fru) into the Fru(MC) isoform. However, in females, fru alleles that only generate Fru(M) isoforms failed to kill FS-Ilp7 motoneurons. This blockade of Fru(M)-dependent PCD was not attributable to doublesex gene function but to a non-canonical role for transformer (tra), a gene encoding the RNA splicing activator that regulates female-specific splicing of fru and dsx transcripts. In both sexes, Tra was shown to prevent PCD even when the Fru(M) isoform is expressed. In addition, it was found that Fru(MC) eliminated FS-Ilp7 motoneurons in both sexes, but only when Tra was absent. Thus, Fru(MC)-dependent PCD eliminates female-specific neurons in males, and Tra plays a double-assurance function in females to establish and reinforce the decision to generate female-specific neurons (Garner, 2018).

Molecular evolution of the ligands of the insulin-signaling pathway: dilp genes in the genus Drosophila

Drosophila melanogaster, unlike mammals, has seven insulin-like peptides (DILPS). In Drosophila, all seven genes (dilp1-7) are single copy in the 12 species studied, except for D. grimshawi with two tandem copies of dilp2. This comparative analysis revealed that genes dilp1-dilp7 exhibit differential functional constraint, which is indicative of some functional divergence. Species of the subgenera Sophophora and Drosophila differ in some traits likely affected by the insulin-signaling pathway, such as adult body size. It is in the branch connecting the two subgenera that the footprint left by positive selection driving nonsynonymous changes at some dilp1 codons to fixation was found. Finally, the similar rate at which the two dilp2 copies of D. grimshawi have evolved since their duplication and the presence of a putative regulatory region highly conserved between the two paralogs would suggest that both copies were preserved either because of subfunctionalization or dose dependency rather than by the neofunctionalization of one of the two copies (Guirao-Rico, 2011).

Postmitotic specification of Drosophila insulinergic neurons from pioneer neurons

Insulin and related peptides play important and conserved functions in growth and metabolism. Although Drosophila has proved useful for the genetic analysis of insulin functions, little is known about the transcription factors and cell lineages involved in insulin production. Within the embryonic central nervous system, the MP2 neuroblast divides once to generate a dMP2 neuron that initially functions as a pioneer, guiding the axons of other later-born embryonic neurons. Later during development, dMP2 neurons in anterior segments undergo apoptosis but their posterior counterparts persist. Surviving posterior dMP2 neurons no longer function in axonal scaffolding but differentiate into neuroendocrine cells that express insulin-like peptide 7 (Ilp7) and innervate the hindgut. The find that the postmitotic transition from pioneer to insulin-producing neuron is a multistep process requiring retrograde bone morphogenetic protein (BMP) signalling and four transcription factors: Abdominal-B, Hb9, Forkhead, and Dimmed. These five inputs contribute in a partially overlapping manner to combinatorial codes for dMP2 apoptosis, survival, and insulinergic differentiation. Ectopic reconstitution of this code is sufficient to activate Ilp7 expression in other postmitotic neurons. These studies reveal striking similarities between the transcription factors regulating insulin expression in insect neurons and mammalian pancreatic beta-cells (Miguel-Aliaga, 2008).

The observed death of some Drosophila pioneer neurons has been used to argue that their function is transient, but persistence in other cases suggested that, either they continue to play an axonal-scaffolding role, or that they adopt some other identity. The current findings resolve this long-standing issue by clearly demonstrating that, for dMP2 neurons, the axonal scaffolding function is only transient. After this role is no longer required, surviving dMP2 neurons become insulinergic and innervate the hindgut. The other known innervation of the Drosophila gut occurs much more anteriorly, in the foregut and anterior midgut, from neuronal cell bodies located in the peripheral ganglia of the stomatogastric nervous system. Unlike dMP2 neurons, however, the individual identities of the stomatogastric neurons and their cell lineages remain to be clearly defined. Thus, dMP2 neurons may provide a simple and well-characterised system for studies of the guidance cues involved in enteric innervation. Future studies, however, will be needed to determine the functions of Ilp7 in dMP2 neurons. It will be important to distinguish if this posterior neural source of insulin acts humorally to promote growth, like the more anterior brain mNSCs, or if it has more local effects in abdominal tissues. In this regard, the presence of Ilp7-expressing neurites in close proximity to the Ilp2-producing mNSCs is intriguing (Miguel-Aliaga, 2008).

The transition from pioneer to neuroendocrine neuron is not unique to dMP2 neurons, as Drosophila MP1 pioneer neurons also become neuropeptidergic at larval stages (Wheeler, 2006). In the grasshopper, segment-specific survival of pioneer neurons has also been reported, raising the possibility that they too may become neuroendocrine. Studies in other species, including vertebrates, will be needed to reveal the extent to which the linkage between pioneer and neuroendocrine functions is conserved. Identifying pioneer neurons with an 'ancestral' neuroendocrine identity in other phyla would lend further support to the proposal that pioneer neurons are highly conserved in evolution (Miguel-Aliaga, 2008).

Apoptosis of postmitotic neurons is a widespread feature of normal VNC development, but few developmental regulators of core pro-apoptotic genes such as grim, hid, and rpr have been identified. This study uncovers roles for Fkh and Hb9. Hb9, at least, appears linked to cell death in neurons other than dMP2: in Df(3L)H99 mutant embryos, where apoptosis is blocked, ectopic Hb9-positive RP motor neurons are observed in segments A7-A8. Hb9 is an important regulator of motor neuron identity in both Drosophila and vertebrates. Finding of a pro-apoptotic function for Hb9 in Drosophila, together with the neurotrophic requirement for motor neuron survival in vertebrates, raises the possibility that the same genetic programs specifying the identities of motor neurons also sensitize them for postmitotic editing via apoptosis (Miguel-Aliaga, 2008).

Fkh function in CNS development has not been characterized. Fkh is expressed in segmentally repeated clusters of midline neurons, including dMP2, vMP2, MP1 neurons, and the VUM interneurons. Within the MP2 lineage, Fkh is first expressed in the MP2 neuroblast at stage 9-10 and continues to be expressed in both the dMP2 and vMP2 daughters throughout embryonic and larval stages. In fkh mutants, 95% of anterior dMP2 neurons fail to undergo apoptosis, and 95.3% of posterior dMP2 neurons (and 100% of ectopic anterior counterparts) fail to express Ilp7. Both of these dramatic phenotypes could be rescued to near wild-type levels by reintroducing Fkh under odd-GAL4 regulation, indicating a cell-autonomous requirement for promoting dMP2 apoptosis and Ilp7 expression (Miguel-Aliaga, 2008).

Hb9 and Fkh expression in many neurons that do not die suggests a combinatorial mechanism for the control of developmental apoptosis. One possibility is that several transcription factors function in combination to activate the core pro-apoptotic genes. Given the proposed role for Foxa proteins in chromatin accessibility, Fkh expression in dMP2 neurons may render the promoters of core pro-apoptotic genes responsive to activation by Hb9. An alternative but not mutually exclusive mechanism involves individual transcription factors activating different pro-apoptotic genes such that a combination of these would then be required to trigger neuronal death. For example, Hb9 could be required for rpr/skl but not grim expression. Some support for this idea comes from the observation that loss of hb9 activity blocks rpr/skl-mediated death of dMP2 neurons but not the largely grim-dependent apoptosis of anterior MP1 neurons (Miguel-Aliaga, 2008).

An important conclusion from this study is that the combinatorial transcription factor code controlling apoptosis partially overlaps with that regulating insulinergic identity. Thus, Fkh and Hb9 are both essential components of the codes for anterior apoptosis and also Ilp7 expression, illustrating that these transcription factors play surprising dual roles as pro-apoptotic and pro-differentiation factors within the same neuronal subtype. Importantly, the results also show that the segment-specific Hox protein Abd-B acts as a postmitotic switch, converting the pro-apoptotic Fkh⁺ Hb9⁺ code into an insulinergic Fkh⁺ Hb9⁺ Abd-B⁺ code (Miguel-Aliaga, 2008).

Three Ilp7 regulators (Hb9, Abd-B, and Fkh) are expressed at least 12 h before Ilp7 is first activated: from the time when the MP2 neuroblast exits the cell cycle. In the case of Hb9, it was not possible to uncouple two temporally separable functions. Early postmitotic expression of Hb9 is important for its death-activating function, whereas later expression suffices for activating Ilp7. Similarly, the Hox protein Abd-B generates a segment-specific neuropeptide pattern via postmitotic regulation of posterior dMP2 survival and also Ilp7 activation. As vertebrate neuropeptides are also expressed in restricted neuronal populations within specific rostrocaudal domains, they may be similarly regulated by Hox survival/neuroendocrine inputs. In the case of Fkh, it is required for many different aspects of the progression from the early to the late postmitotic dMP2 fate. Fkh expression is restricted to VNC midline neurons and its vertebrate orthologue Foxa2 functions in the differentiation of the floor plate and ventral dopaminergic and serotonergic neurons. Thus, in both the Drosophila midline and its vertebrate counterpart, the floor plate, Fkh proteins play a conserved role in the differentiation of ventral neuronal subtypes (Miguel-Aliaga, 2008).

The other two dMP2 regulators identified in this study, Dimm and the BMP pathway, are switched on shortly before the onset of Ilp7 expression. The timing of onset of these two broad neuroendocrine regulators is likely to specify when Ilp7 is first activated, whereas the earlier factors Fkh, Hb9, and Abd-B may contribute more specifically to insulinergic identity. Together, the genetic and expression analyses in this study demonstrate that the combinatorial code of genetic inputs required for Ilp7 expression is assembled in a step-wise manner during postmitotic maturation. Importantly, this allows a subset of the components to be shared (such as Fkh and Hb9) between sequential neuronal programmes (survival and Ilp7 expression) without losing output specificity (Miguel-Aliaga, 2008).

Two observations from this study indicate that insulinergic combinatorial codes can vary from cell-to-cell and also from one Ilp to another. (1) None of the regulators of Ilp7 in dMP2 neurons appear to regulate it in DP neurons. (2) The dMP2 insulinergic code is sufficient to trigger ectopic expression of Ilp7 but not Ilp2 or other neuropeptides such as FMRFa. These findings suggest the existence of additional, as yet unidentified, insulinergic factors in DP neurons and also in the brain mNSCs where Ilp2 is expressed. Identification of the neural progenitor for these mNSCs (Wang, 2007) should facilitate characterization of the Ilp1/Ilp2/Ilp3/Ilp5 combinatorial codes and thus clarify the extent to which different insulinergic transcriptional programmes overlap (Miguel-Aliaga, 2008).

The finding that an Ilp7-expressing neuron derives from the MP2 lineage reveals that at least some insulinergic regulators are similar in insects and mammals. Three apparent similarities may not be very insulin-specific but reflect more general processes shared by neural and endocrine programmes in many species. (1) Notch signalling singles out the MP2 neuroblast and distinguishes its two progeny neurons, while in mammals, it limits pancreatic expression of the 'proneural' gene Ngn3 to prospective endocrine cells. (2) The survival and pro-Ilp7 functions mediated by Abd-B in the dMP2 neuron could also have their postmitotic counterparts in ß-cells, either mediated by related Hox genes or via another homeobox gene, Pdx-1, following its early input into pancreatic induction. (3) Nerfin-1 is required for dMP2 pioneer function (Kuzin, 2005), while its mammalian orthologue Insm1/IA1 is important for pancreatic ß-cell specification (Miguel-Aliaga, 2008).

Several more specific regulatory similarities exist between the insulinergic differentiation factors active in postmitotic dMP2 neurons. For example, the role of fkh in dMP2 neurosecretory differentiation described in this study is similar to the functions of HNF3b/Foxa2 in islet maturation and insulin secretion. In addition, mammalian Nkx2.2 is important for pancreatic ß-cell specification and is known to activate transcription of the insulin regulator Nkx6.1: an important late event in ß-cell differentiation. Intriguingly, the Drosophila orthologue of Nkx2.2, Vnd, is required for dMP2 formation. Drosophila Nkx6.1, the orthologue of mammalian Nkx6 (FlyBase name HGTX), is expressed by postmitotic dMP2 neurons, and it will be interesting to determine whether it too functions downstream of Vnd during Ilp7 regulation. Most strikingly, mammalian equivalents of two of the insulinergic inputs identified in this study, Hb9 and BMP signalling, are also required for several aspects of late ß-cell differentiation including the expression of Nkx6.1 and insulin. Together, these insect-mammalian comparisons provide evidence that, although the cell types involved look very different, some of the genetic circuitry regulating insulin is conserved between arthropods and chordates. This suggests that the power of fly genetics can now be harnessed to identify additional mammalian regulators of neuroendocrine cell fates and insulin expression (Miguel-Aliaga, 2008).

Search PubMed for articles about Drosophila Insulin-like peptide 7

Cognigni, P., Bailey, A. P. and Miguel-Aliaga, I. (2011). Enteric neurons and systemic signals couple nutritional and reproductive status with intestinal homeostasis. Cell Metab 13(1): 92-104. PubMed ID: 21195352

Garner, S. R. C., Castellanos, M. C., Baillie, K. E., Lian, T. and Allan, D. W. (2018). Drosophila female-specific Ilp7 motoneurons are generated by Fruitless-dependent cell death in males and by a double-assurance survival role for Transformer in females. Development 145(1). PubMed ID: 29229771

Guirao-Rico, S. and Aguade, M. (2011). Molecular evolution of the ligands of the insulin-signaling pathway: dilp genes in the genus Drosophila. Mol Biol Evol 28(5): 1557-1560. PubMed ID: 21196470

Imambocus, B. N., Zhou, F., Formozov, A., Wittich, A., Tenedini, F. M., Hu, C., Sauter, K., Macarenhas Varela, E., Heredia, F., Casimiro, A. P., Macedo, A., Schlegel, P., Yang, C. H., Miguel-Aliaga, I., Wiegert, J. S., Pankratz, M. J., Gontijo, A. M., Cardona, A. and Soba, P. (2021). A neuropeptidergic circuit gates selective escape behavior of Drosophila larvae. Curr Biol. PubMed ID: 34798050

Linneweber, G. A., Jacobson, J., Busch, K. E., Hudry, B., Christov, C. P., Dormann, D., Yuan, M., Otani, T., Knust, E., de Bono, M. and Miguel-Aliaga, I. (2014). Neuronal control of metabolism through nutrient-dependent modulation of tracheal branching. Cell 156(1-2): 69-83. PubMed ID: 24439370

Miguel-Aliaga, I., Thor, S. and Gould, A. P. (2008). Postmitotic specification of Drosophila insulinergic neurons from pioneer neurons. PLoS Biol. 6(3): e58. PubMed Citation: 18336071

Prince, E., Kretzschmar, J., Trautenberg, L. C., Broschk, S. and Brankatschk, M. (2021). DIlp7-Producing Neurons Regulate Insulin-Producing Cells in Drosophila. Front Physiol 12: 630390. PubMed ID: 34385929

Semaniuk, U. V., Gospodaryov, D. V., Feden'ko, K. M., Yurkevych, I. S., Vaiserman, A. M., Storey, K. B., Simpson, S. J. and Lushchak, O. (2018). Insulin-like peptides regulate feeding preference and metabolism in Drosophila. Front Physiol 9: 1083. PubMed ID: 30197596

date revised: 15 August 2022

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