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

Scale-type-specific requirement for the mosquito Aedes aegypti Spindle-F homologue by regulating microtubule organization

Insect epithelial cells contain unique cellular extensions such as bristles, hairs, and scales. In contrast to bristle and hair, which are not divergent in their shape, scale morphology shows high diversity. In our attempt to characterize the role of the insect-specific gene, Spindle-F (spn-F), in mosquito development, we revealed a scale-type specific requirement for the mosquito Aedes aegypti spn-F homologue. Using CRISPR-Cas9, we generated Ae-spn-F mutants and found that Ae-spn-F is an essential gene, but we were able to recover a few adult escapers. These escapers could not fly nor move, and died after 3 to 4 days. We found that in Ae-spn-F mutants, only the tip part of the bristle was affected with bulbous with misoriented ribs. We also show that in Ae-spn-F mutants, only in falcate scales, which are curved with a sharp or narrowly rounded apex, and not in other scale types, the tip region is strongly affected. Our analysis also revealed that in contrast to Drosophila spn-F, which show strong defects in both the actin and microtubule (MT) network in the bristle, the Ae-spn-F gene is required only for MT organization in scales and bristles. In summary, our results reveal that Ae-spn-F is required for shaping tapered epithelial cellular extension structures, namely, the bristle and falcate scales by affecting MT organization.

Ik2/TBK1 and Hook/Dynein, an adaptor complex for early endosome transport, are genetic modifiers of FTD-associated mutant CHMP2B toxicity in Drosophila

Mutations in CHMP2B, encoding a protein in the endosomal sorting complexes required for transport (ESCRT) machinery, causes frontotemporal dementia linked to chromosome 3 (FTD3). FTD, the second most common form of pre-senile dementia, can also be caused by genetic mutations in other genes, including TANK-binding kinase 1 (TBK1). How FTD-causing disease genes interact is largely unknown. We found that partial loss function of Ik2, the fly homologue of TBK1 also known as I-kappaB kinase ε (IKKε), enhanced the toxicity of mutant CHMP2B in the fly eye and that Ik2 overexpression suppressed the effect of mutant CHMP2B in neurons. Partial loss of function of Spn-F, a downstream phosphorylation target of Ik2, greatly enhanced the mutant CHMP2B phenotype. An interactome analysis to understand cellular processes regulated by Spn-F identified a network of interacting proteins including Spn-F, Ik2, dynein light chain, and Hook, an adaptor protein in early endosome transport. Partial loss of function of dynein light chain or Hook also enhanced mutant CHMP2B toxicity. These findings identify several evolutionarily conserved genes, including ik2/TBK1, cut up (encoding dynein light chain) and hook, as genetic modifiers of FTD3-associated mutant CHMP2B toxicity and implicate early endosome transport as a potential contributing pathway in FTD.

Rab11 activation by Ik2 kinase is required for dendrite pruning in Drosophila sensory neurons

Neuronal pruning is a commonly observed phenomenon for the developing nervous systems to ensure precise wiring of neural circuits. The function of Ik2 kinase and its downstream mediator, Spindle-F (Spn-F), are essential for dendrite pruning of Drosophila sensory neurons during development. However, little is known about how Ik2/Spn-F signaling is transduced in neurons and ultimately results in dendrite pruning. Our genetic analyses and rescue experiments demonstrated that the small GTPase Rab11, especially the active GTP-bound form, is required for dendrite pruning. We also found that Rab11 shows genetic interactions with spn-F and ik2 on pruning. Live imaging of single neurons and antibody staining reveal normal Ik2 kinase activation in Rab11 mutant neurons, suggesting that Rab11 could have a functional connection downstream of and/or parallel to the Ik2 kinase signaling. Moreover, we provide biochemical evidence that both the Ik2 kinase activity and the formation of Ik2/Spn-F/Rab11 complexes are central to promote Rab11 activation in cells. Together, our studies reveal that a critical role of Ik2/Spn-F signaling in neuronal pruning is to promote Rab11 activation, which is crucial for dendrite pruning in neurons.

During metamorphosis in Drosophila, both the central and peripheral nervous systems undergo substantial neuronal remodeling, such as the cell death of most larval neurons and regeneration of adult neurons, while few larval neurons remain alive and prune their branches. Pruning is a self-destruction program, and thus requires to be tightly controlled within single neurons spatially and temporally during development. Recent studies have shown a strong correlation between pruning and human psychiatric disorders, such as schizophrenia and autism. Drosophila sensory neurons that undergo dendrite pruning provide us an opportunity to study the regulatory mechanism of neuronal pruning. Previously, we identified an IKK-related kinase Ik2 that is essential and sufficient for dendrite pruning, and a coiled-coil protein Spindle-F that mediates Ik2-dependent pruning activity in neurons. However, what are the downstream targets of Ik2/Spindle-F signaling in dendrite pruning remains unclear. In this study, we found that the small GTPase Rab11, especially the active GTP-bound form, is required for dendrite pruning in neurons. We further demonstrated that both the Ik2 kinase activity and Ik2/Spindle-F complexes are essential to enhance Rab11 activation in neurons during dendrite pruning.

The plus-tip tracking and microtubule stabilizing activities of Javelin-like regulate microtubule organization and cell polarity

Cell polarity is essential for building cell asymmetry in all eukaryotic cells. Drosophila oocyte and bristle development require the newly characterized Spn-F protein complex, which includes Spn-F, IKKε, and Javelin-like (Jvl), to establish polarity. Jvl is a novel microtubule (MT)-associated protein; however, the mechanism by which it regulates MT organization is still unknown. We found that overexpression of Jvl stabilizes MTs and that jvl is needed for stable MT arrangement at the bristle tip and organization of the dynamic MT throughout the bristle shaft. At low levels of expression in cultured cells, Jvl behaved as a microtubule plus-end tracking protein. We demonstrated that Jvl physically interacts with the highly conserved MT end-binding protein 1 (EB1) using yeast two-hybrid and GST pull-down assays. This interaction is, however, dispensable for Jvl function in oocyte and bristle development. In addition, using a MT-binding assay, we saw that Jvl-C terminus directly binds to MTs. We also revealed that oocyte developmental arrest caused by Jvl overexpression in the germline can be rescued by mutations in its partners, spn-F and ikkε, suggesting that complex formation with Spn-F and IKKε is required for Jvl function in vivo. In summary, our results show that the microtubule plus-end tracking and stabilizing activities of Jvl are central for controlling cell polarity of oocytes and bristles.

GFP-Forked, a genetic reporter for studying Drosophila oocyte polarity

The polarized organization of the Drosophila oocyte can be visualized by examining the asymmetric localization of mRNAs, which is supported by networks of polarized microtubules (MTs). In this study, we used the gene forked, the putative Drosophila homologue of espin, to develop a unique genetic reporter for asymmetric oocyte organization. We generated a null allele of the forked gene using the CRISPR-Cas9 system and found that forked is not required for determining the axes of the Drosophila embryo. However, ectopic expression of a truncated form of GFP-Forked generated a distinct network of asymmetric Forked, which first accumulated at the oocyte posterior and was then restricted to the anterolateral region of the oocyte cortex in mid-oogenesis. This localization pattern resembled that reported for the polarized MTs network. Indeed, pharmacological and genetic manipulation of the polarized organization of the oocyte showed that the filamentous Forked network diffused throughout the entire cortical surface of the oocyte, as would be expected upon perturbation of oocyte polarization. Finally, we demonstrated that Forked associated with Short-stop and Patronin foci, which assemble non-centrosomal MT-organizing centers. Our results thus show that clear visualization of asymmetric GFP-Forked network localization can be used as a novel tool for studying oocyte polarity.

Summary: The novel asymmetric Forked network could be used as a genetic reporter for visualizing and studying oocyte polarity.

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. Here, we identify 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. Our 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.

A transport and retention mechanism for the sustained distal localization of Spn-F-IKKε during Drosophila bristle elongation

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. Here, we demonstrate 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. We found that 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.

Summary: In the Drosophila bristle, the microtubule binding protein Jvl, the adaptor Spn-F and cytoplasmic dynein are required for localised transport and retention of polarised signalling factors.