InteractiveFly: GeneBrief

Spindle-F : Biological Overview | References

BIOLOGICAL OVERVIEW

Stable localization of the signaling complex is essential for the robust morphogenesis of polarized cells. Cell elongation involves molecular signaling centers that coordinately regulate intracellular transport and cytoskeletal structures. In Drosophila bristle elongation, the protein kinase IKKε is activated at the distal tip of the growing bristle and regulates the shuttling movement of recycling endosomes and cytoskeletal organization. However, how the distal tip localization of IKKε is established and maintained during bristle elongation is unknown. This study demonstrates that IKKε distal tip localization is regulated by Spindle-F (Spn-F), which is stably retained at the distal tip and functions as an adaptor linking IKKε to cytoplasmic dynein. Javelin-like (Jvl) is a key regulator of Spn-F retention. In jvl mutant bristles, IKKε and Spn-F initially localize to the distal tip but fail to be retained there. In S2 cells, particles that stain positively for Jvl or Spn-F move in a microtubule-dependent manner, whereas Jvl and Spn-F double-positive particles are immobile, indicating that Jvl and Spn-F are transported separately and, upon forming a complex, immobilize each other. These results suggest that polarized transport and selective retention regulate the distal tip localization of the Spn-F-IKKε complex during bristle cell elongation (Otani, 2015).

Highly polarized cells, such as neurons and epithelial cells, rely heavily on intracellular transport mechanisms for their functional differentiation. Disrupted intracellular transport systems lead to a variety of diseases, including neurodegeneration and microvillus inclusion diseases. Accurate intracellular transport is ensured by the polarized cytoskeleton and by the adaptor protein-mediated recognition of specific cargoes by molecular motors. Molecular motors play central roles in intracellular transport, and have diversified through evolution. However, the diversity of molecular motors is not sufficient to explain transport specificity, as various cargoes often share the same motor while being transported to distinct locations. For example, cytoplasmic dynein is the major microtubule minus-end motor and transports a variety of cargoes including the Golgi apparatus, endosomes and RNAs. Evidence suggests that the fate of cargo is determined not only by cargo-motor recognition, which occurs upon cargo loading, but also at the cargo destination site. For instance, in axonal transport some cargoes, such as dense core vesicles and synaptic vesicles, are inefficiently captured at synaptic boutons and circulate within the axon, whereas others, such as mitochondria, are stably retained at synapses. Although the precise regulation of cargo transport is important for the functional differentiation of various polarized cells, the underlying molecular mechanisms remain poorly understood (Otani, 2015).

Cell elongation is a widely observed morphogenetic event that requires the coordinated input of intracellular transport, the cytoskeleton and cell polarity. Drosophila bristles, which are hair-like unicellular structures that function as external sensory organs, are formed by the elongation of trichogen cells, which can grow up to 350 microm in 1 day during the pupal stage. IκB kinase ε [IKKε; also known as IκB kinase-like 2 (Ik2)] acts at the distal tip of growing bristles and functions as a signaling center to regulate the bidirectional shuttling of Rab11-positive recycling endosomes during bristle elongation (Otani, 2011). Rab11-positive vesicles are transported to the distal tip by interacting with cytoplasmic dynein via an adaptor protein Nuf/Rab11FIP3. At the distal tip, IKKε phosphorylates Nuf to inactivate dynein-dependent trafficking, thereby promoting the directional switching of the recycling endosomes. In addition to its role in endosome trafficking, IKKε regulates the organization of both actin and microtubules. However, how IKKε is localized to the distal tip of growing bristles is unknown (Otani, 2015).

Spindle-F (Spn-F) is a coiled-coil protein that interacts with IKKε and has been implicated in regulating IKKε polarized activation (Abdu, 2006; Dubin-Bar, 2008). In oocytes, the intracellular localizations of Spn-F and IKKε depend on each other (Dubin-Bar, 2008), and spn-F and ikkε mutants show similar bristle morphology and oocyte polarization phenotypes, suggesting that they function together (Abdu, 2006; Koto, 2009; Oshima, 2006; Otani, 2011; Shapiro, 2006). Several proteins other than IKKε, including Cut up (Ctp)/dynein light chain (LC8) and Javelin-like (Jvl), are reported to interact with Spn-F (Abdu, 2006; Dubin-Bar, 2011). It was proposed that Spn-F interacts with cytoplasmic dynein via Ctp to localize the Spn-F-IKKε complex to microtubule minus ends. However, subsequent structural studies indicated that Ctp/LC8 cannot simultaneously bind dynein and cargo molecules, challenging this model. On the other hand, IKKε can phosphorylate Spn-F, suggesting that Spn-F might act downstream of IKKε (Dubin-Bar, 2008). Interestingly, another Spn-F-interacting protein, Jvl, was recently shown to regulate the polarized activation of IKKε in oocytes. Although Jvl can interact with microtubules (Dubin-Bar, 2011), how it regulates the polarized activation of IKKε is unknown (Otani, 2015).

This study demonstrated that the bristle tip is a sorting station for cytoplasmic dynein-dependent cargoes. The IKKε-Spn-F complex, which acts as the signaling center in bristle cell elongation, localizes to the distal tip by dynein-dependent polarized transport and Jvl-dependent selective retention. By contrast, Rab11-positive recycling endosomes undergo both dynein-dependent distal transport and proximal transport, which is probably mediated by kinesins (Otani, 2015 and references therein).

The distinct transport characteristics at the distal tip are specified by the nature of the adaptor proteins. IKKε is transported to the distal tip by dynein via the adaptor protein Spn-F, and the IKKε-Spn-F complex is stably retained at the distal tip by Jvl, a Spn-F-interacting protein. By contrast, Rab11-positive recycling endosomes are transported to the distal tip by dynein via the adaptor protein Nuf, where it is phosphorylated by IKKε (Otani, 2011). This phosphorylation inactivates the dynein-dependent transport of Rab11-positive recycling endosomes, thereby promoting their transport back to the cell body (Otani, 2011). Thus, the IKKε-Spn-F complex stably localizes to the distal tip by polarized transport followed by selective retention, whereas Rab11-positive recycling endosomes bidirectionally shuttle by polarized transport and motor switching. The pivotal step in this sorting decision is the specific recognition of the cargo adaptor proteins (Spn-F and Nuf) by their regulatory proteins (Jvl and IKKε) at the distal tip. These results support the emerging concept that cargo adaptor proteins are not merely physical linkers between cargoes and motors, but act as regulatory hubs where various signals converge (Otani, 2015).

This study identified Jvl as a key regulator of IKKε-Spn-F retention at the distal tip. Jvl interacts with microtubules (Dubin-Bar, 2011), and binding Spn-F promotes the microtubule binding activity of Jvl and induces microtubule bundling in S2 cells. Full-length Jvl localizes to punctate structures that were located along microtubules, whereas the C-terminal half of Jvl uniformly decorated microtubules. These results imply that Jvl microtubule binding activity is repressed by its N-terminal region, and that binding Spn-F could relieve this inhibition. Oligomerization of Spn-F could promote the formation of higher-order Spn-F-Jvl complexes to generate multivalent microtubule-binding sites, thereby increasing the microtubule binding activity of Jvl (Otani, 2015).

Spn-F and Jvl are independently transported to the distal tip in elongating bristles, indicating that their interaction occurs upon arrival at the tip. This interaction presumably activates Jvl microtubule binding activity, which then serves as a molecular brake to immobilize the complex on microtubules. Similar mechanisms have been proposed for the anchoring of mitochondria by Syntaphilin and Kinesin-1 in axonal mitochondrial transport, and for the immobilization of lysosomes in dendrites by the interaction of TMEM106B and MAP6. The coupling of cargo adaptor proteins with microtubule-binding proteins might be a general mechanism for regulating the transport of a particular cargo in a spatiotemporally controlled manner. As Spn-F and Jvl are also involved in the polarized activation of IKKε during oogenesis (Amsalem, 2013), similar mechanisms might help generate and maintain cell polarity in various cell types (Otani, 2015).

As an alternative to the molecular brake model, Jvl could act as a scaffolding protein to recruit enzymes that modify the IKKε-Spn-F complex to promote its retention, or as a regulator of microtubule organization at the distal tip to maintain the polarized organization of the cytoskeleton during bristle elongation. Further analysis of the molecular functions of Jvl will help in elucidating the mechanisms of IKKε-Spn-F retention (Otani, 2015).

The results suggest that Spn-F functions as a cargo adaptor for IKKε and cytoplasmic dynein. Structure-function analysis of Spn-F demonstrated that its dynein-binding region is required for localizing IKKε to the distal tip and for bristle morphogenesis. In contrast to the dynein-binding-deficient Spn-F mutant, which partially suppressed the spn-F mutant bristle morphology phenotype, a mutant lacking the IKKε-binding region completely failed to rescue, indicating that, in addition to its function as a cargo adaptor, Spn-F has a role in regulating IKKε activity. This role could involve regulating IKKε kinase activity or protein stability, or in scaffolding the components of the IKKε signaling pathway. IKKε overexpression could partially suppress the spn-F mutant bristle morphology phenotype despite IKKε delocalization from the distal tip, suggesting that increasing the dosage of IKKε can compensate for the loss of Spn-F to some extent. It is likely that the delocalized IKKε can phosphorylate some of its downstream target molecules (such as Nuf and Diap1) to partially support bristle morphogenesis (Kuranaga, 2006; Otani, 2011; Otani, 2015).

In summary, this study has demonstrated that the signaling center for bristle elongation is localized to the distal tip by polarized transport and selective retention mechanisms. The distal tip of bristles acts as a sorting center for cytoplasmic dynein cargoes, where regulatory proteins recognize cargo adaptor proteins and determine whether cargo is retained or sent back to the cell body. These findings support the idea that cargo adaptor proteins act as regulatory hubs where various signals converge. It would be interesting to test whether the differential regulation of cargo-motor interactions contributes to the formation of signaling centers during the morphogenesis of mammalian cells of complex shape, such as neurons and podocytes (Otani, 2015).

Spindle-F is the central mediator of Ik2 kinase-dependent dendrite pruning in Drosophila sensory neurons

During development, certain Drosophila sensory neurons undergo dendrite pruning that selectively eliminates their dendrites but leaves the axons intact. How these neurons regulate pruning activity in the dendrites remains unknown. This study identifies a coiled-coil protein Spindle-F (Spn-F) that is required for dendrite pruning in Drosophila sensory neurons. Spn-F acts downstream of IKK-related kinase Ik2 in the same pathway for dendrite pruning. Spn-F exhibits a punctate pattern in larval neurons, whereas these Spn-F puncta become redistributed in pupal neurons, a step that is essential for dendrite pruning. The redistribution of Spn-F from puncta in pupal neurons requires the phosphorylation of Spn-F by Ik2 kinase to decrease Spn-F self-association, and depends on the function of microtubule motor dynein complex. Spn-F is a key component to link Ik2 kinase to dynein motor complex, and the formation of Ik2/Spn-F/dynein complex is critical for Spn-F redistribution and for dendrite pruning. These findings reveal a novel regulatory mechanism for dendrite pruning achieved by temporal activation of Ik2 kinase and dynein-mediated redistribution of Ik2/Spn-F complex in neurons (Lin, 2015)

The precise assembly of neural circuits is crucial for the nervous system to function properly. The developing nervous systems often start with a primitive prototype, characterized by exuberant branches and excessive connections. Thus, further remodeling is required to refine the developing nervous systems to maturity. Neuronal pruning, one such remodeling mechanism, is a highly regulated self-destruct process that eliminates excessive neuronal branches in the absence of cell death. Pruning is widely observed in the nervous systems of both vertebrates and invertebrates, that not only ensures precise wiring during development, but also allows for adjustment of neuronal connections in response to injury and disease. Various studies have shown that defects in developmental pruning affect the function of the nervous systems in C. elegans and Drosophila. Moreover, a progressive loss of neurites far ahead of cell death is commonly observed in many neurodegenerative disorders. Thus, any dysregulation of pruning activity even at the level of individual neurons would bring catastrophic consequences to the nervous systems. Although the primary triggers for developmental pruning and pruning that ensues upon neuronal injury and disease are diverse, the downstream machinery that eliminates neuronal processes shared some common features. For example, microtubule disruption is the earliest cellular event observed in all types of pruning, and the ubiquitin-proteasome system is required in all circumstances (Lin, 2015)

Spindle-F is the central mediator of Ik2 kinase-dependent dendrite pruning in Drosophila sensory neurons

During Drosophila metamorphosis, substantial neuronal remodeling takes place in both the central and peripheral nervous systems. Most of the larval peripheral neurons die during metamorphosis, whereas few, including some class IV dendritic arborization (C4da) neurons, survive and undergo large-scale dendrite pruning. Dendrite pruning of the dorsal C4da neuron ddaC starts with severing of the proximal dendrites at 4-6 h APF (after puparium formation). Subsequently these disconnected dendrites become fragmented and eventually eliminated by the surrounding epidermal cells by 16-18 h APF. In contrast to the central brain mushroom body (MB) gamma neurons where both larval dendrites and axons are pruned during development, the peripheral C4da neurons specifically prune their dendrites keeping the axons intact. The molecular basis for how the pruning activity is confined to the dendrites of C4da neurons remains unknown. It was reasoned that molecular differences between dendrites and axons should be considered for such differential pruning activity in C4da neurons. It is known that microtubule polarity is different in the dendrites and axons of neurons, including in the Drosophila sensory neurons. For example, C4da neurons have polarized microtubules in their proximal dendrites predominantly with microtubule minus end pointing away from the cell body, but have an opposite polarity in their axons. This difference in microtubule polarity is essential for maintaining the proper function and compartmental identities of dendrites and axons, and might be an important determinant for spatially restricting pruning activity in the dendritic compartments of C4da neurons. Based on this assumption, some molecules are required to connect the pruning activity with the distinctive microtubule polarity of the dendrites in C4da neurons during dendrite pruning (Lin, 2015)

Previous studies have shown that dendrite pruning in C4da neurons is initiated by the steroid hormone ecdysone and its heterodimeric receptors, ecdysone receptor B1 (EcR-B1) and Ultraspiracle (Usp). Through transcriptional regulation of sox14, ecdysone signaling activates the Sox14 target gene mical, which encodes a cytoskeletal regulator, to regulate dendrite pruning. A few other molecules mediating specific cellular activities have been shown to participate in dendrite pruning of C4da neurons, such as the ubiquitin-proteasome system, caspases, matrix metalloproteases, microtubule severing proteins and mediators of dendritic calcium transients. Previous studies identified Ik2 kinase, a homologue of vertebrate IKK-ε in Drosophila, that plays an essential role in dendrite pruning of pupal neurons, and further demonstrated that Ik2 is sufficient to induce precocious dendrite severing in larval neurons. Ik2 is the only known molecule sufficient to induce premature dendrite severing in larvae, reflecting a central role of Ik2 kinase in dendrite pruning. Therefore, this study aimed to elucidate the mechanism by which Ik2 kinase signaling is transduced and regulated in Drosophila sensory neurons during dendrite pruning (Lin, 2015)

To elucidate the mechanism of Ik2 kinase signaling, candidate molecules were sought that mediate Ik2 signals during dendrite pruning. Several lines of evidence suggested that Spn-F, a coil-coiled protein, is a good candidate. Firstly, spn-F mutant flies showed defects in developing oocytes and bristles (Abdu, 2006), similar to the phenotypes observed in ik2 mutants (Shapiro, 2006). Secondly, Spn-F physically interacts with Ik2 (Dubin-bar, 2008). It implied that ik2 and spn-F may act in the same pathway during oogenesis and bristle morphogenesis, and the possibility has been raised that a similar pathway might also be involved in dendrite pruning of C4da neurons. This study demonstrates Spn-F plays a key role in linking Ik2 kinase to microtubule motor dynein complex for dendrite pruning. Spn-F acts downstream of Ik2 kinase in the same pathway for dendrite pruning. Spn-F is shown to displays a punctate pattern in larval neurons and these Spn-F puncta become dispersed in pupal cells. The redistribution of Spn-F from puncta is essential for dendrite pruning, and depends on the activity of Ik2 kinase and the function of microtubule motor dynein complex. These data also demonstrate that Spn-F not only links Ik2 to dynein motor complex, but also mediates the formation of Ik2/Spn-F/dynein complex, that is critical for Spn-F punctum disassembly and dendrite pruning (Lin, 2015)

In addition to apoptosis, neurons have a second self-destruct program in their axons for axonal pruning during development and in response to neuronal injury and disorders. This study proposes a third self-destruct program, which is mediated by Ik2 kinase activity in Drosophila sensory neurons, specific for dendrite pruning. Ik2 is essential for dendrite severing in pupal C4da neurons (Lee, 2009), and currently is the only known molecule sufficient to cause precocious dendrite severing in larval cells (Lee, 2009), indicating that Ik2 activation must be regulated temporally. For temporal regulation, ecdysone signaling plays a key role in dendrite pruning (Williams, 2005; Lee, 2009). These studies show that no Ik2 activation is detected in pupal C4da neurons with impaired ecdysone signaling and thus places Ik2 kinase downstream of ecdysone signaling. Microarray studies have identified ik2 as one of the ecdysone/EcR up-regulated genes in brain MB γ neurons during axon pruning (Hoopfer, 2008). This suggests one possible mechanism where ecdysone/EcR regulates Ik2 activation through increasing ik2 expression in C4da neurons. Although Ik2 kinase activity is crucial for oogenesis and bristle morphogenesis (Shapiro, 2006; Otani, 2011), the activation mechanisms of Ik2 kinase in both processes remain unknown. Since pruning activity is considered as a self-destruct program, how to regulate this activity spatially in subcellular compartments within individual neurons is an intriguing issue to investigate. This study identifies Spn-F and cytoplasmic dynein complex as critical regulators of Ik2-mediated dendrite pruning activity in C4da neurons (Lin, 2015)

It is known that endogenous Spn-F exhibits a punctate pattern in nurse cells (Abdu, 2006), consistent with the observation of punctate Spn-F-GFP in larval C4da neurons. The formation of Spn-F puncta in cells is through self-association, and does not depend on the integrity of microtubule network or the function of cytoplasmic dynein. Since Ik2 could form oligomers in cells, the interaction between Ik2 and Spn-F might also play a role in Spn-F puncta formation. Indeed, it was observed that SpnF-ΔCC3-GFP has normal interaction with either SpnF-ΔCC3 or full-length Spn-F, but formed fewer puncta than the wild type Spn-F-GFP did in larval neurons. Therefore, the Spn-F puncta formation could be attributed not only to Spn-F self-association, but also to Ik2/Spn-F interaction and Ik2 oligomerization (Lin, 2015)

In larval C4da neurons, Ik2 kinase is inactive and associates with Spn-F as puncta in the cytosol. After puparium formation, Ik2 kinase becomes activated promptly and phosphorylates Spn-F in C4da neurons. This Ik2-dependent phosphorylation on Spn-F decreases Spn-F self-association, and subsequently the numbers and sizes of Spn-F puncta were reduced. One may question that protein degradation might contribute to decrease the numbers and sizes of Spn-F puncta in C4da neurons during dendrite pruning. It was known that Ik2 promotes caspase inhibitor DIAP1 degradation via proteasomes during the development of sensory organ precursors; therefore, Ik2 might promote Spn-F degradation in C4da neurons during dendrite pruning. However, Ik2 overexpression does not alter the protein level of Spn-F in either S2 or germline cells (Dubin-Bar, 2008). Thus, protein degradation by proteasomes is unlikely the mechanism leading to decreased Spn-F puncta after Ik2 activation. Since P-Ik2 signals were indistinguishable between wild-type and Dhc64C RNAi neurons, it is reasonable to presume that both Ik2 activation and Spn-F phosphorylation occur normally in dynein mutant neurons. No significant differences were found between the pruning defects of C4da neurons in spn-F mutants and that in spn-F mutants with Dhc-RNAi, and between the pruning phenotypes observed in Dhc mutants and that in Dhc mutants with ik2-RNAi. These findings further support that Ik2, Spn-F and dynein complex function together in the same pathway in dendrite pruning of C4da neurons. However, the finding of Spn-F puncta in mutant pupal neurons with impaired dynein function indicated that dynein is required for Spn-F redistribution after Ik2 activation. Furthermore, Spn-F remains punctate in S2 cells with Ik2 overexpression even after microtubule depolymerization and inhibition of dynein function, suggesting that dynein might redistribute Ik2/Spn-F complexes via transporting complexes toward the minus ends of microtubules in C4da neurons during dendrite pruning. The results in this study and studies in germline cells, (the fact that more Spn-F puncta accumulated in nurse cells with colchicine treatment and with Dhc mutation) (Abdu, 2006), favor the mechanism of protein redistribution for Spn-F punctum reduction in dendrite pruning of C4da neurons (Lin, 2015)

It has been shown that during Drosophila bristle elongation, directional transport of activated Ik2 and of Spn-F to the bristle tips, where the microtubule minus ends are concentrated, requires the function of cytoplasmic dynein, and Spn-F acts as an adaptor to link Ik2 to dynein complexes. These are similar to the current findings that both Ik2 activation and dynein complex are essential for Spn-F redistribution, and Spn-F plays a central role in the formation of Ik2/Spn-F/dynein complex, which is crucial for Spn-F redistribution and for dendrite pruning in C4da neurons. However, the studies in bristle elongation indicating that spn-F acts upstream of ik2 (Otani, 2015) disagree with the current finding that ik2 acts upstream of spn-F in dendrite pruning. The discrepancy between the epistasis analyses of ik2 and spn-F in these two different processes might be due to different cell-type specific factors in these two types of cells that affect the morphological readouts in genetic studies. Moreover, this study demonstrated that Ik2-dependent phosphorylation of Spn-F decreases Spn-F self-association, promotes Spn-F redistribution, and finally leads to dendrite pruning in C4da neurons (Lin, 2015)

The activated Ik2 signals accumulate at the microtubule minus ends in cells with polarized microtubule distribution, such as oocytes, follicle cells and bristles. This is consistent with the conclusion that dynein transports activated Ik2 toward microtubule minus ends in C4da neurons. Since Drosophila sensory neurons have polarized microtubules in their proximal dendrites predominantly with microtubule minus end pointing away from the cell body, these studies revealed a possible mechanism that Spn-F and minus-end directed motor dynein complex confine Ik2-dependent pruning activity to the somatodendritic compartments of C4da neurons. During Drosophila bristle elongation, the accumulation of endogenous Spn-F observed at the bristle tip, where the microtubule minus ends are enriched, led to an examination of Spn-F-GFP signals along the dendrites of C4da neurons during dendrite pruning. However, no enriched of Spn-F-GFP signals in the proximal dendrites, where dendrite severing is expected to occur, was observed, by live imaging during pruning. Previous studies (Lee, 2009) showed that microtubules are first disassembled in the proximal dendrites of C4da neurons during dendrite severing. This local disassembly of microtubules is suppressed in ik2 mutant neurons. Since the current genetic studies indicate that both ik2 and spn-F act in the same pathway of dendrite pruning, tests were performed to see whether local microtubule disassembly happens normally in spn-F mutants. Local breakage of microtubules found in the proximal dendrites of C4da neurons was also suppressed in spn-F RNAi mutants, suggesting that Spn-F, like Ik2, plays a role in dendrite severing that involves local microtubule disassembly. However, the molecular mechanisms by which activated Ik2 and Spn-F lead to dendrite severing in the proximal dendrites of C4da neurons will be an important question for future studies (Lin, 2015)

It is known that there is no decrease in cell death in wing discs with ik2 knockdown and in ik2 mutant embryos (Kuranaga, 2006), indicating that the primary function of Ik2 is not involved in the apoptotic pathway during development. However, ectopic Ik2 activation by overexpression leads to cell death in fly compound eyes (Kuranaga, 2006) and in C4da neurons (Lee, 2009), suggesting that excessive Ik2 kinase signaling could trigger a crosstalk with signaling molecules in apoptotic pathways and result in apoptosis. It is known that Ik2 kinase regulates the nonapoptotic function of caspase through promoting DIAP1 degradation (Kuranaga, 2006). In a similar manner, the confinement of activated Ik2 kinase in the dendritic compartments might restrict the detected caspase activity in the degenerating dendrites after separating from the soma of C4da neurons during dendrite pruning. Therefore, this raises a possibility that de-regulation of pruning activity in neurons may trigger a crosstalk with molecules in apoptotic pathways and lead to undesired cell death during neuronal injury and disorders. Recently, a caspase cascade, including caspase 3 and 6, was identified in mice to play a role in developmental axon pruning and in sensory axon pruning after trophic factor withdrawal. Moreover, activated caspase 6 was detected in human patient brains of Alzheimer and Huntington diseases long before cell death, highlighting a critical role in regulating caspase activity in both diseases. Understanding the regulatory mechanisms that confine pruning activity into proper subcellular compartments of the neuron might provide molecular insights into the pathogenesis of neural disorders (Lin, 2015)

Drosophila oocyte polarity and cytoskeleton organization require regulation of Ik2 activity by Spn-F and Javelin-like

The Drosophila melanogaster Spn-F, Ik2, and Javelin-like (Jvl) proteins interact to regulate oocyte mRNA localization and cytoskeleton organization. However, the mechanism by which these proteins interact remains unclear. Using antibodies to activated Ik2, this protein was found at the region of oocyte and follicle cell where microtubule minus ends are enriched. Germ line Ik2 activation is diminished both in jvl and in spn-F mutant ovaries. Structure-function analysis of Spn-F revealed that the C-terminal end is critical for protein function, since it alone was able to rescue spn-F sterility. On the other hand, germ line expression of Spn-F lacking its conserved C-terminal region (Spn-FDeltaC) phenocopied ik2, leading to production of ventralized eggshell and bicaudal embryos. In Spn-FDeltaC-expressing oocytes, Gurken protein is mislocalized and oskar mRNA and protein localization is disrupted. Expression of Ik2 rescued Spn-FDeltaC ovarian phenotypes. Whereas Spn-F physically interacts with Ik2 and Jvl, Spn-FDeltaC physically interacts with Ik2 but not with Jvl. Thus, expression of Spn-FDeltaC, which lacks the Jvl-interacting domain, probably interferes with interaction of Ik2 and Jvl. In summary, these results demonstrate that Spn-F mediates the interaction between Ik2 and Jvl to control Ik2 activity (Amsalem, 2013).

Previously work showed that Ik2 is activated locally at the tips of bristles. This study has demonstrated that during oogenesis, Ik2 is also locally activated at MT minus-end regions in the oocyte and follicle cells, as well as in the nurse cells, where Ik2 presents a punctate pattern. In the germ line, jvl and spn-F have been shown to be required for activation of Ik2. To better understand the mechanism by which Spn-F affects Ik2 activation, structure-function analysis was performed of the Spn-F protein. Using this approach, it was demonstrated that the C-terminal end of Spn-F is sufficient for protein function. Following the expression of several truncated Spn-F-encoding constructs in the germ line, expression of Spn-F lacking 84 amino acids from the C-terminal end produced defects in both the dorsal-ventral and anterior-posterior axes that were similar to those found in ik2 loss-of-function ovaries. A high percentage of ventralized eggs and bicaudal embryos are produced by both the ik2 mutant and Spn-FΔC-expressing females. Most importantly, the fact that expression of Ik2 was able to significantly rescue defects in eggshell and embryo development, as detected by Spn-FΔC expression, suggests that the C-terminal end of Spn-F regulates Ik2 protein function (Amsalem, 2013).

The results of the current study demonstrated that in terms of mRNA localization, the expression of Spn-FΔC protein produced, similarly to the case for ik2 mutants, high percentages of bicaudal embryos due to defects in osk mRNA localization. Previous work has shown that females mutant for spn-F produce a low percentage of bicaudal embryos, ranging from embryos with the typical reduced head skeleton to rare symmetrical bicaudals. Thus, it is believed that Ik2 function in oocyte anterior-posterior patterning has two components, one that depends on Spn-F and one that does not (Amsalem, 2013).

One can thus ask how Ik2 activity affects osk mRNA localization. The role of the cytoskeleton in transporting osk mRNA to its final destination required cooperation between MTs and between MTs and actin motor proteins. Initially, osk mRNAs are transcribed in nurse cells and transported into the MT minus end at the anterior of the oocyte by dynein, along with the accessory factors BicD and Egalitarian. Within the oocyte, it was shown that the localization of osk to the posterior end requires MTs, Khc, and myosin V. Several models explaining how osk transcripts are transported toward the posterior of the oocyte have been proposed, including active transport to the posterior, diffusion and trapping, or exclusion from the anterior and lateral cortex. One recent model suggested that osk mRNA is actively transported along microtubules in all directions, with a slight bias toward the posterior. As to the role of Khc in osk mRNA transport, it was suggested that Khc is required either directly or indirectly. The present study demonstrated that overexpression of Spn-FΔC, which eliminates Ik2 activity, affects Khc-LacZ function and the posterior localization of osk mRNA. Based on these results, it is possible that regulation of Ik2 by the Spn-F C-terminal region affects MT-biased polarity toward the oocyte posterior, thus indirectly affecting Khc and osk mRNA localization. Alternatively, the effect on osk mRNA localization could be due to a direct effect on the regulation of Khc motor protein activity (Amsalem, 2013).

This study has demonstrated that the defects in the dorsoventral axis in flies expressing Spn-FΔC are due to the Grk protein but not mRNA mislocalization. Moreover, Grk protein, a secreted protein, as well as spectrin, a microfilament-related protein, is localized to ectopic actin clumps in the oocyte. The localization of Grk to ectopic actin clumps was reported for several mutants, including Bic-C, trailer hitch (tral), spn-F, and ik2 mutants. In all of these mutants, grk mRNA was mislocalized but in a different pattern than was Grk protein, suggesting that defects in grk mRNA localization cannot account for defects in Grk protein localization. It was suggested that Bic-C and tral are part of the same pathway that regulates efficient Grk secretion. Accumulation of ectopic Grk protein in the oocyte was also found in Khc and Dhc mutants, and it was suggested that both genes are also required for Grk protein exocytosis. Thus, it is suggested that the aberrant localization of Grk protein in ik2 mutants and in flies expressing Spn-FΔC reveals a role of these proteins in regulating Grk protein secretion (Amsalem, 2013).

To better understand the mechanism by which overexpression of Spn-FΔC affects Ik2 activity, this study has examined interactions between Spn-FΔC and the Ik2 and Jvl proteins. Previously, it was shown that Spn-F is able to directly bind Ik2 and Jvl. This study demonstrated that whereas Ik2 physically interacts with Spn-FΔC, Jvl was not able to bind Spn-FΔC. It was also found that Ik2 binds to the second coiled-coil domain of Spn-F, while Jvl interacts with the conserved C-terminal region of this protein. Moreover, Spn-F but not Spn-FΔC protein was shown to form a complex with Ik2 and Jvl. It is believed that expression of Spn-FΔC, which is able to bind Ik2 but not Jvl, interfered with the interaction between Ik2 and Jvl. These results suggest that specific interference with the interaction between Ik2 and Jvl, as revealed upon Spn-FΔC expression, is critical for Ik2 core functions during oogenesis. The fact that Spn-F mediates Ik2 interaction with Jvl, an MT-associated protein, and the finding that Ik2 is activated at the MT minus end, together with the specific effects of ik2 on oocyte MT organization, suggest that Ik2 plays a crucial role in MT organization and/or function during oogenesis (Amsalem, 2013).

Drosophila javelin-like encodes a novel microtubule-associated protein and is required for mRNA localization during oogenesis

Asymmetrical localization of mRNA transcripts during Drosophila oogenesis determines the anteroposterior and dorsoventral axes of the Drosophila embryo. Correct localization of these mRNAs requires both microtubule (MT) and actin networks. This study identified a novel gene, CG43162, that regulates mRNA localization during oogenesis and also affects bristle development. The Drosophila gene javelin-like, which was identified based on its bristle phenotype, is an allele of the CG43162 gene. Female mutants for jvl produce ventralized eggs owing to the defects in the localization and translation of gurken mRNA during mid-oogenesis. Mutations in jvl also affect oskar and bicoid mRNA localization. Analysis of cytoskeleton organization in the mutants reveal defects in both MT and actin networks. Jvl protein colocalizes with MT network in Schneider cells, in mammalian cells and in the Drosophila oocyte. Both in the oocyte and in the bristle cells, the protein localizes to a region where MT minus-ends are enriched. Jvl physically interacts with SpnF and is required for its localization. Overexpression of Jvl in the germline affects MT-dependent processes: oocyte growth and oocyte nucleus anchoring. Thus, these results show that a novel MT-associated protein affects mRNA localization in the oocyte by regulating MT organization (Dubin-Bar, 2011).

In order to investigate further the role of Spn-F in MT organization, new proteins that interact with Spn-F or Ik2 were sought. This study led to identification of the gene CG43162 as a novel MT-associated protein, which is part of this complex. Moreover, the study showed that CG43162 encodes the javelin-like (jvl) gene. Several lines of evidence suggest that that jvl encodes the CG43162 gene: (1) Using fine deficiency mapping of jvl mutants showed that jvl is found in CG43162 region; (2) it was shown that downregulation of CG43162 specifically in the bristles led to defects in bristle morphology, similar to the defects found in jvl mutants; 3) furthermore, a mutation in CG43162 (CG43162^D590) failed to complement jvl in both ovarian and bristle phenotypes, suggesting that CG43162^D590 and jvl are two different alleles of the same gene; and (4) expression of CG43162 protein in oocytes was found to rescue jvl female sterility. Considering all of these results, it is concluded that the CG43162 gene encodes jvl (Dubin-Bar, 2011).

Moreover, it is suggested that Jvl is part of the Spn-F and Ik2 complex, based on the following evidence: (1) Spn-F physically interacts with Jvl (yeast two hybrid and GST pull-down assays); (2) Spn-F physically interacts with Ik2 (Dubin-Bar, 2008); (3) jvl shares similar mRNA localization and bristle defects to spn-F and ik2; (4) Spn-F and Ik2 colocalize with Jvl to MT, where Jvl determines this localization pattern (Dubin-Bar, 2011).

For further analysis of the jvl gene, Jvl protein localization was characterized. For this purpose, the localization of Jvl protein in S2R⁺ cells and human cells was analyzed. GFP-Jvl fusion protein was localized to the MT network. Next, the localization pattern of Jvl during oogenesis was analyzed. Using an antibody raised against the Jvl protein, it was found that Jvl is localized to the region where the MT minus-ends reside. At early stages of oogenesis, Jvl protein localizes as a tight crescent in the posterior pole of the oocytes. During mid-oogenesis, Jvl protein is localized all around the cortex, with enrichment at the anterior pole. It was also demonstrated that GFP-Jvl colocalizes with MTs in the nurse cells. Moreover, in the bristles, GFP-Jvl is localized asymmetrically, accumulating at the bristle tip, where other MT minus-end markers are found. Considering these results, indicating that Jvl localizes with the MT network in S2R⁺ and human cells along with its localization in the egg chamber and developing bristle, it is concluded that Jvl protein is associated with the MT network, specifically with the MT minus-ends (Dubin-Bar, 2011).

jvl¹ mutants are female fertile. However, flies hemizygous for jvl¹ and flies transheterozygous for jvl (jvl¹/jvl²) are female sterile. Beside sterility, it was noticed that the jvl mutant females laid eggs with dorsal-ventral defects. Determination of dorsal-ventral polarity of the eggshell depends on Grk protein signaling. In the hemizygous mutants, grk mRNA localizes in the anterior margins of the oocyte and in ectopic sites inside the oocyte. It has been suggested that grk mRNA moves in two distinct steps, both of which require MT and the motor protein Dynein. Each step depends on a different MT network. First grk mRNA moves towards the anterior of the oocyte, where it localizes transiently, and then to its final localization in the dorsal anterior corner of the oocyte. In jvl mutants, grk mRNA does not reach its final localization in the dorsal anterior corner of the oocyte, suggesting that the MT network upon which this step depends might be impaired in jvl mutants. This MT network is specifically associated with the oocyte nucleus and the minus-end in the dorsal-anterior corner of the oocyte. Next, it was found that Grk protein in jvl mutants is also mislocalized. Grk protein is colocalized with ectopic actin puncta close to the anterior of the oocyte. This localization pattern is also observed in Bicaudal-C and trailer-hitch mutants. It has been suggested that the sequestration of Grk in the actin cages interfered with the signaling to the follicle cells; therefore, it is suggested that sequestration of Grk in the actin cages in jvl mutant females similarly led to the dorsal-ventral polarity defects of the eggshell. In addition to the effect on grk mRNA and protein localization, jvl also affects bcd and osk mRNA localization. In wild-type, bcd mRNA is localized to the anterior pole of the oocyte facing the nurse cells, whereas osk mRNA is localized to the opposite posterior pole. The polar localization of these two mRNAs is maintained throughout the rest of oogenesis and well into early embryogenesis. The anterior localization of bcd requires both intact MTs and dynein motor protein function. osk localization to the posterior pole is achieved by two phases of transport: long-range MT-dependent transport by kinesin to the posterior, followed by actomyosin V-dependent positioning at the oocyte cortex (Dubin-Bar, 2011).

What could be the function of Jvl protein during oogenesis? The effects of jvl on grk and bcd mRNA localization, along with the particular changes affecting cytoskeletal organization close to the oocyte nuclear membrane as evident for Nod:KHC:β-gal localization and Tau mislocalization, suggest that jvl might be involved in either transport to the minus-end of MTs or in the organization of the minus-ends of the microtubule around the oocyte nucleus, as been suggested for its interactor, Spn-F (Abdu, 2006). However, it was also noticed that in jvl mutants, osk mRNA and protein are mislocalized. These phenotypes are probably not due to defects in either transport or organization of the MT plus-end, as the plus-end motor protein Kinesin I was properly localized as in the wild type. Examination of the cytoskeleton components of the oocyte shows that both actin and MTs are misorganized in jvl mutants. The MT levels along the anterior cortex of the oocyte were reduced with specific effects on the MT that surrounds the oocyte nucleus. However, ectopic aggregations of the actin cages were found in the middle of the oocyte. The defects in the organization of both actin and MT network, together with the defects in osk mRNA and protein localization, suggest that jvl could provide a connection between the actin and MT network. In summary, these results suggest that jvl plays a role in organization of the MT in the oocyte or in the stabilization of the connection between MT and actin cytoskeleton in the oocyte (Dubin-Bar, 2011).

This study also examined the effects of overexpression of Jvl in the germline. Overexpression of Jvl with different germline-specific Gal 4 affects oocyte growth, oocyte localization and, in later stages, oocyte nucleus localization. Interestingly enough, all of these phenotypes could arise from effects on MT network function (Dubin-Bar, 2011).

Oocyte growth depends on several processes: early in oogenesis, until stage 7, the oocyte grows at approximately the same rate as a single nurse cell. At these stages, oocyte growth is due to the transport of mRNAs and proteins, including products of early pattern-formation genes from the nurse cells to the oocyte. This transport is a microtubule-dependent process. Later in oogenesis, after stage 7, oocyte growth depends on the transport of components such as lipid droplets, mitochondria and other single particles from the nurse cells into the oocyte. This transport is an actin-dependent process. Beginning in stage 8, the oocyte expands through the uptake of yolk from the surrounding follicle cells and hemolymph. Consequently, oocyte growth is more rapid than nurse cell growth. During stage 11, the remaining nurse cell cytoplasm is rapidly transferred to the oocyte, resulting in doubling the oocyte volume. Overexpression of Jvl affects oocyte growth during stage 6 to stage 8, although the egg chamber size seems to be similar to that of wild-type stage 6 to 8 egg chambers. In these stages, oocyte growth depends on the transport of nutrients from the nurse cells to the oocyte, suggesting that overexpression of Jvl disrupted this transport. The fact that Orb protein is not detected in Jvl-overexpressing small oocytes strengthens this possibility (Dubin-Bar, 2011).

Another phenotype that was obtained in moderate overexpression of Jvl is mislocalization of the oocyte nucleus in 15% of stage 9 egg chambers. During early stages of oogenesis, the oocyte nucleus localizes to the posterior pole of the oocyte. After stage 7, following Grk signal and reorganization of the MT network, the nucleus migrates towards the anterodorsal corner of the oocyte. Positioning of the oocyte nucleus involves two anchoring steps: first anchoring to the lateral membrane, which requires dynein but not kinesin motor protein; and, second, after it localizes to the anterodorsal corner, anchoring to the anterior cortex of the oocyte, which requires both dynein and kinesin motor proteins. Moreover, nucleus anchoring also requires correct organization of the MT scaffold that surrounds the oocyte nucleus. Moderate expression of Jvl did not affect nucleus position in stage 8 egg chambers. At this stage, the nucleus was always at the dorsal anterior corner, as in the wild type. This finding implies that anchoring to the lateral cortex and migration of the oocyte nucleus is not affected in Jvl-overexpressing ovaries. However, the anchoring of the nucleus to the anterior membrane was affected. This could be due to misorganization of the MT scaffold that surrounds the nucleus. Thus, these results demonstrate that overexpression of Jvl protein affects MT-dependent processes such as transport of determinants from the nurse cells to the oocyte, and anchoring of oocyte nucleus to the anterior cortex of the oocyte. Taking into account the phenotypes detected in jvl mutants, the finding that Jvl is an MT-associated protein, together with the effects of Jvl overexpression on MT-dependent processes during oogenesis, it seems likely that jvl has a role in MT organization during oogenesis (Dubin-Bar, 2011).

Most importantly, although jvl encodes for a protein with no homology beside insects, its association with MT network in mammalian cells, along with its effect on MT network in Drosophila, may suggest the existence of mammalian protein(s) with a function analogous to Jvl (Dubin-Bar, 2011).

The Drosophila IKK-related kinase (Ik2) and Spindle-F proteins are part of a complex that regulates cytoskeleton organization during oogenesis

IkappaB kinases (IKKs) regulate the activity of Rel/NF-kappaB transcription factors by targeting their inhibitory partner proteins, IkappaBs, for degradation. The Drosophila genome encodes two members of the IKK family. Whereas the first is a kinase essential for activation of the NF-kappaB pathway, the latter does not act as IkappaB kinase. Instead, recent findings indicate that Ik2 regulates F-actin assembly by mediating the function of nonapoptotic caspases via degradation of DIAP1. Also, it has been suggested that ik2 regulates interactions between the minus ends of the microtubules and the actin-rich cortex in the oocyte. Since spn-F mutants display oocyte defects similar to those of ik2 mutants, whether Spn-F could be a direct regulatory target of Ik2 was investigated. It was found that Ik2 binds physically to Spn-F, biomolecular interaction analysis of Spn-F and Ik2 demonstrating that both proteins bind directly and form a complex. Ik2 phosphorylates Spn-F, and it was demonstrated that this phosphorylation does not lead to Spn-F degradation. Ik2 is localized to the anterior ring of the oocyte and to punctate structures in the nurse cells together with Spn-F protein, and both proteins are mutually required for their localization. It is concluded that Ik2 and Spn-F form a complex, which regulates cytoskeleton organization during Drosophila oogenesis and in which Spn-F is the direct regulatory target for Ik2. Interestingly, Ik2 in this complex does not function as a typical IKK in that it does not direct SpnF for degradation following phosphorylation (Dubin-Bar, 2008).

spn-F encodes a novel protein that affects oocyte patterning and bristle morphology in Drosophila

The anteroposterior and dorsoventral axes of the Drosophila embryo are established during oogenesis through the activities of Gurken (Grk), a Tgfα-like protein, and the Epidermal growth factor receptor (Egfr). spn-F mutant females produce ventralized eggs similar to the phenotype produced by mutations in the grk-Egfr pathway. The ventralization of the eggshell in spn-F mutants is due to defects in the localization and translation of grk mRNA during mid-oogenesis. Analysis of the microtubule network revealed defects in the organization of the microtubules around the oocyte nucleus. In addition, spn-F mutants have defective bristles. spn-F was clond and found to encodes a novel coiled-coil protein that localizes to the minus end of microtubules in the oocyte, and this localization requires the microtubule network and a Dynein heavy chain gene. Spn-F interacts directly with the Dynein light chain Ddlc-1 (Cut up). These results show that this novel protein affects oocyte axis determination and the organization of microtubules during Drosophila oogenesis (Abdu, 2006; full text of article).

In a global two-hybrid screen, Spn-F (CG12114) was found to interact with the Ik2 (CG2615) protein. Mutations in Ik2 have been isolated and characterized, ik2 mutants share many phenotypes with spn-F, including a very similar bristle phenotype and specific effects on MT organization in oogenesis. However, ik2 mutants are lethal, whereas spn-F homozygotes survive. In addition, whereas spnF mutations have only mild effects on Oskar protein localization and a low frequency of bicaudal phenotypes, such effects are more pronounced in the ik2 mutants. Nevertheless, the striking similarities strongly suggest that the two genes function in a common pathway that affects certain types of MT more strongly than others. In normal mitotic cells, the minus ends of MTs are usually focused by the centrosomes in the interior of the cell, and plus ends contact the cortex. However, in specialized cells, such as the Drosophila oocyte, there are minus ends that make contact with the cortex. It is therefore possible that Spn-F and Ik2 are required for providing a stable connection between such cortical MT minus ends and cortical actin for subsets of MTs involved in specialized transport processes. Future experiments will address the interactions of Spn-F and Ik2 directly, and will determine whether, for instance, Spn-F might be a target of Ik2 (Abdu, 2006).

Search PubMed for articles about Drosophila Spindle-F

Abdu, U., Bar, D. and Schupbach, T. (2006). spn-F encodes a novel protein that affects oocyte patterning and bristle morphology in Drosophila. Development 133: 1477-1484. PubMed ID: 16540510

Amsalem, S., Bakrhat, A., Otani, T., Hayashi, S., Goldstein, B. and Abdu, U. (2013). Drosophila oocyte polarity and cytoskeleton organization require regulation of Ik2 activity by Spn-F and Javelin-like. Mol Cell Biol 33: 4371-4380. PubMed ID: 24019068

Dubin-Bar, D., Bitan, A., Bakhrat, A., Kaiden-Hasson, R., Etzion, S., Shaanan, B. and Abdu, U. (2008). The Drosophila IKK-related kinase (Ik2) and Spindle-F proteins are part of a complex that regulates cytoskeleton organization during oogenesis. BMC Cell Biol 9: 51. PubMed ID: 18796167

Dubin-Bar, D., Bitan, A., Bakhrat, A., Amsalem, S. and Abdu, U. (2011). Drosophila javelin-like encodes a novel microtubule-associated protein and is required for mRNA localization during oogenesis. Development 138(21): 4661-71. PubMed Citation: 21989913

Hoopfer, E. D., Penton, A., Watts, R. J. and Luo, L. (2008). Genomic analysis of Drosophila neuronal remodeling: a role for the RNA-binding protein Boule as a negative regulator of axon pruning. J Neurosci 28: 6092-6103. PubMed ID: 18550751

Koto, A., Kuranaga, E. and Miura, M. (2009). Temporal regulation of Drosophila IAP1 determines caspase functions in sensory organ development. J Cell Biol 187: 219-231. PubMed ID: 19822670

Kuranaga, E., Kanuka, H., Tonoki, A., Takemoto, K., Tomioka, T., Kobayashi, M., Hayashi, S. and Miura, M. (2006). Drosophila IKK-related kinase regulates nonapoptotic function of caspases via degradation of IAPs. Cell 126: 583-596. PubMed ID: 16887178

Lee, H. H., Jan, L. Y. and Jan, Y. N. (2009). Drosophila IKK-related kinase Ik2 and Katanin p60-like 1 regulate dendrite pruning of sensory neuron during metamorphosis. Proc Natl Acad Sci U S A 106: 6363-6368. PubMed ID: 19329489

Lin, T., (2015). Spindle-F is the central mediator of Ik2 kinase-dependent dendrite pruning in Drosophila sensory neurons. PLoS Genet 11: e1005642. PubMed ID: 26540204

Oshima, K., Takeda, M., Kuranaga, E., Ueda, R., Aigaki, T., Miura, M. and Hayashi, S. (2006). IKK epsilon regulates F actin assembly and interacts with Drosophila IAP1 in cellular morphogenesis. Curr Biol 16: 1531-1537. PubMed ID: 16887350

Otani, T., Oshima, K., Onishi, S., Takeda, M., Shinmyozu, K., Yonemura, S. and Hayashi, S. (2011). IKKε regulates cell elongation through recycling endosome shuttling. Dev Cell 20: 219-232. PubMed ID: 21316589

Otani, T., Oshima, K., Kimpara, A., Takeda, M., Abdu, U. and Hayashi, S. (2015). A transport and retention mechanism for the sustained distal localization of Spn-F-IKKε during Drosophila bristle elongation. Development 142: 2338-2351. PubMed ID: 26092846

Shapiro, R. S. and Anderson, K. V. (2006). Drosophila Ik2, a member of the I kappa B kinase family, is required for mRNA localization during oogenesis. Development 133: 1467-1475. PubMed ID: 16540511

Williams, D. W. and Truman, J. W. (2005). Cellular mechanisms of dendrite pruning in Drosophila: insights from in vivo time-lapse of remodeling dendritic arborizing sensory neurons. Development 132: 3631-3642. PubMed ID: 16033801

date revised: 10 January, 2016

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