A novel role for Rabconnectin-3a in endosome maturation regulates Wnt signaling, migration, and fate specification in zebrafish neural crest cells.

The trafficking mechanisms and transcriptional targets downstream of long-range neurotrophic factor ligand/receptor signaling that promote axon growth are incompletely understood. Zebrafish carrying a null mutation in a neurotrophic factor receptor, Ret, displayed defects in peripheral sensory axon growth cone morphology and dynamics. Ret receptor was highly enriched in sensory pioneer neurons and Ret51 isoform was required for pioneer axon outgrowth. Loss-of-function of a cargo adaptor, Jip3, partially phenocopied Ret axonal defects, led to accumulation of activated Ret in pioneer growth cones, and reduced retrograde Ret51 transport. Jip3 and Ret51 were also retrogradely co-transported, ultimately suggesting Jip3 is a retrograde adapter of active Ret51. Finally, loss of Ret reduced transcription and growth cone localization of Myosin-X, an initiator of filopodial formation. These results show a specific role for Ret51 in pioneer axon growth, and suggest a critical role for long-range retrograde Ret signaling in regulating growth cone dynamics through downstream transcriptional changes.

Peripheral somatosensory neurons innervate the skin and sense the environment. Whereas many studies focus on initial axon outgrowth and pathfinding, how signaling pathways contribute to maintenance of the established axon arbors and terminals within the skin is largely unknown. This question is particularly relevant to the many types of neuropathies that affect mature neuronal arbors. We show that a receptor tyrosine kinase (RTK), c-Kit, contributes to maintenance, but not initial development, of cutaneous axons in the larval zebrafish before sex determination. Downregulation of Kit signaling rapidly induced retraction of established axon terminals in the skin and a reduction in axonal density. Conversely, misexpression of c-Kit ligand in the skin in larval zebrafish induced increases in local sensory axon density, suggesting an important role for Kit signaling in cutaneous axon maintenance. We found Src family kinases (SFKs) act directly downstream to mediate Kit's role in regulating cutaneous axon density. Our data demonstrate a requirement for skin-to-axon signaling to maintain axonal networks and elucidate novel roles for Kit and SFK signaling in this context. This Kit-SFK signaling axis offers a potential pathway to therapeutically target in sensory neuropathies and to further explore in other neurobiological processes.

22Cell migration requires dynamic regulation of cell-cell signaling and cell adhesion. Neural crest (NC) cells 23 are highly migratory cells, which undergo an epithelial-mesenchymal transition (EMT) to leave the neural 24 epithelium and migrate throughout the body to give rise to many different derivatives. We have identified 25 a Lim-domain only (Lmo) protein, Lmo7a, expressed in early NC cells that controls both actin cytoskeletal 26 dynamics and Wnt signaling during NC migration. In embryos deficient in Lmo7a, many NC cells fail to 27 migrate away from the dorsal midline, and form aggregates. Unlike the majority of NC cells that appear to 28 migrate normally, cells that aggregate in Lmo7a-deficient embryos mislocalize paxillin (Pxn) and have 29 reduced levels of phosphorylated focal adhesion kinase (pFAK). Lmo7a loss-of-function also disrupts 30canonical Wnt signaling such that after the onset of NC cell migration, Wnt responses and nuclear β-31 catenin levels increase in the cells that aggregate. However, this increase in Wnt signaling appears 32 secondary to the defect in migration. Similar to mutants for other Wnt regulators in NC cells, the NC cells 33in Lmo7a-deficient aggregates exhibit gene expression signatures of pigment cell progenitors, but also 34 express markers of Schwann cell progenitors, suggesting a role for Lmo7a in pigment-glial specification. 35We propose that Lmo7a modulates cell adhesion to facilitate both robust NC cell migration and a subset 36 of lineage decisions.

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