Frazzled cytoplasmic P-motifs are differentially required for axon pathway formation in the Drosophila embryonic CNS

De novo assembly of a transcriptome for the cricket Gryllus bimaculatus prothoracic ganglion: An invertebrate model for investigating adult central nervous system compensatory plasticity

The auditory system of the cricket, Gryllus bimaculatus, demonstrates an unusual amount of anatomical plasticity in response to injury, even in adults. Unilateral removal of the ear causes deafferented auditory neurons in the prothoracic ganglion to sprout dendrites across the midline, a boundary they typically respect, and become synaptically connected to the auditory afferents of the contralateral ear. The molecular basis of this sprouting and novel synaptogenesis in the adult is not understood. We hypothesize that well-conserved developmental guidance cues may recapitulate their guidance functions in the adult in order to facilitate this compensatory growth. As a first step in testing this hypothesis, we have generated a de novo assembly of a prothoracic ganglion transcriptome derived from control and deafferented adult individuals. We have mined this transcriptome for orthologues of guidance molecules from four well-conserved signaling families: Slit, Netrin, Ephrin, and Semaphorin. Here we report that transcripts encoding putative orthologues of most of the candidate developmental ligands and receptors from these signaling families were present in the assembly, indicating expression in the adult G. bimaculatus prothoracic ganglion.

Frazzled can act through distinct molecular pathways in epithelial cells to regulate motility, apical constriction, and localisation of E-Cadherin

Netrin receptors of the DCC/NEO/UNC-40/Frazzled family have well established roles in cell migration and axon guidance but can also regulate epithelial features such as adhesion, polarity and adherens junction (AJ) stability. Previously, we have shown that overexpression of Drosophila Frazzled (Fra) in the peripodial epithelium (PE) inhibits wing disc eversion and also generates cellular protrusions typical of motile cells. Here, we tested whether the molecular pathways by which Fra inhibits eversion are distinct from those driving motility. We show that in disc proper (DP) epithelial cells Fra, in addition to inducing F-Actin rich protrusions, can affect localization of AJ components and columnar cell shape. We then show that these phenotypes have different requirements for the three conserved Fra cytoplasmic P-motifs and for downstream genes. The formation of protrusions required the P3 motif of Fra, as well as integrins (mys and mew), the Rac pathway (Rac1, wave and, arpc3) and myosin regulatory light chain (Sqh). In contrast, apico-basal cell shape change, which was accompanied by increased myosin phosphorylation, was critically dependent upon the P1 motif and was promoted by RhoGef2 but inhibited by Rac1. Fra also caused a loss of AJ proteins (DE-Cad and Arm) from basolateral regions of epithelial cells. This phenotype required all 3 P-motifs, and was dependent upon the polarity factor par6. par6 was not required for protrusions or cell shape change, but was required to block eversion suggesting that control of AJ components may underlie the ability of Fra to promote epithelial stability. The results imply that multiple molecular pathways act downstream of Fra in epithelial cells.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

Control of male and female fertility by the netrin axon guidance genes

The netrin axon guidance genes have previously been implicated in fertility in C. elegans and in vertebrates. Here we show that adult Drosophila lacking both netrin genes, NetA and NetB, have fertility defects in both sexes together with an inability to fly and reduced viability. NetAB females produce fertilized eggs at a much lower rate than wild type. Oocyte development and ovarian innervation are unaffected in NetAB females, and the reproductive tract appears normal. A small gene, hog, that resides in an intron of NetB does not contribute to the NetAB phenotype. Restoring endogenous NetB expression rescues egg-laying, but additional genetic manipulations, such as restoration of netrin midline expression and inhibition of cell death have no effect on fertility. NetAB males induce reduced egg-laying in wild type females and display mirror movements of their wings during courtship. Measurement of courtship parameters revealed no difference compared to wild type males. Transgenic manipulations failed to rescue male fertility and mirror movements. Additional genetic manipulations, such as removal of the enabled gene, a known suppressor of the NetAB embryonic CNS phenotype, did not improve the behavioral defects. The ability to fly was rescued by inhibition of neuronal cell death and pan-neural NetA expression. Based on our results we hypothesize that the adult fertility defects of NetAB mutants are due to ovulation defects in females and a failure to properly transfer sperm proteins in males, and are likely to involve multiple neural circuits.

Dscam1-mediated self-avoidance counters netrin-dependent targeting of dendrites in Drosophila

Dendrites and axons show precise targeting and spacing patterns for proper reception and transmission of information in the nervous system. Self-avoidance promotes complete territory coverage and nonoverlapping spacing between processes from the same cell [1, 2]. Neurons that lack Drosophila Down syndrome cell adhesion molecule 1 (Dscam1) show aberrant overlap, fasciculation, and accumulation of dendrites and axons, demonstrating a role in self-recognition and repulsion leading to self-avoidance [3-11]. Fasciculation and accumulation of processes suggested that Dscam1 might promote process spacing by counterbalancing developmental signals that otherwise promote self-association [9, 12]. Here we show that Dscam1 functions to counter Drosophila sensory neuron dendritic targeting signals provided by secreted Netrin-B and Frazzled, a netrin receptor. Loss of Dscam1 function resulted in aberrant dendrite accumulation at a Netrin-B-expressing target, whereas concomitant loss of Frazzled prevented accumulation and caused severe deficits in dendritic territory coverage. Netrin misexpression was sufficient to induce ectopic dendritic targeting in a Frazzled-dependent manner, whereas Dscam1 was required to prevent ectopic accumulation, consistent with separable roles for these receptors. Our results suggest that Dscam1-mediated self-avoidance counters extrinsic signals that are required for normal dendritic patterning, but whose action would otherwise favor neurite accumulation. Counterbalancing roles for Dscam1 may be deployed in diverse contexts during neural circuit formation.

Roles for netrin signaling outside of axon guidance: a view from the worm

The Netrin family of extracellular ligands and their receptors were the first identified signaling pathway regulating axon guidance. Subsequent work across model systems has begun to reveal the interactions that take place downstream of Netrin reception to facilitate growth cone migration. Though intensely studied, many aspects of this signaling system remain unclear. Even less understood are the growing number of contexts in which Netrin signaling influences cells beyond axon guidance and even outside the nervous system. Genetic and cell-biological studies in C. elegans have played an instrumental role in identifying critical functions for Netrin ligands in setting up specialized and potentially adhesive membrane-associated domains within a broad range of cell types. Here we review recent literature implicating Netrin or its receptors in morphogenetic processes outside of growth cone regulation with a special focus on studies in C. elegans that suggest cell biological mechanisms for Netrin signaling.