The Control of Semaphorin-1a-Mediated Reverse Signaling by Opposing Pebble and RhoGAPp190 Functions in Drosophila

Abstract

SUMMARY Transmembrane semaphorins (Semas) serve evolutionarily conserved guidance roles, and some function as both ligands and receptors. However, the molecular mechanisms underlying the transduction of these signals to the cytoskeleton remain largely unknown. We have identified two direct regulators of Rho family small GTPases, pebble (a Rho guanine nucleotide exchange factor (GEF)) and RhoGAPp190 (a GTPase activating protein (GAP)), that show robust interactions with the cytoplasmic domain of the Drosophila Sema-1a protein. Neuronal pebble and RhoGAPp190 are required to control motor axon defasciculation at specific pathway choice points and also for target recognition during Drosophila neuromuscular development. Sema-1a–mediated motor axon defasciculation is promoted by pebble and inhibited by RhoGAPp190. Genetic analyses show that opposing pebble and RhoGAPp190 functions mediate Sema-1a reverse signaling through the regulation of Rho1 activity. Therefore, pebble and RhoGAPp190 transduce transmembrane semaphorin–mediated guidance cue information that regulates the establishment of neuronal connectivity during Drosophila development.

Control of axon–axon attraction by Semaphorin reverse signaling

“…Recent studies demonstrate that certain transmembrane Semaphorins can also function as a receptor to mediate downstream signaling events in both vertebrates and invertebrates (3)(4)(5)(6)(7). For example, we show that the transmembrane Semaphorin1a (Sema1a) functions as an axon guidance receptor for PlexinA (PlexA) in mediating reverse signaling in the developing Drosophila visual system (3,8).…”

“…In addition to the role of Rho1 in the Sema1a reverse-signaling pathway in the visual system, a recent study by Kolodkin and colleagues shows that Sema1a reverse signaling also regulates Rho1 in Drosophila motor axon guidance (6). The difference between photoreceptor and motor axons in the modulation of Rho1 by Sema1a reverse signaling may account for distinct effects on axonaxon interactions.…”

“…One likely explanation is that Sema1a uses different downstream effectors to modulate Rho1 in different cell types. Whereas Moe is activated by Sema1a in photoreceptor axons, Pebble and RhoGAPp190 appear to link Sema1a reverse signaling with the modulation of Rho1 activity in motor axons (6). It remains unknown how Sema1a-induced changes in Rho1 activity modulate cell-surface adhesiveness leading to axon-axon repulsion in motor axon guidance.…”

“…Sema1a reverse signaling promotes photoreceptor (R cell) axon-axon attractions during the establishment of R-cell-to-optic-lobe connections (8). A recent study by Kolodkin and colleagues also demonstrates that Sema1a reverse signaling mediates axon-axon repulsion in Drosophila motor axon guidance (6).…”

Semaphorin signalling during development

“…Sema-1a, the invertebrate semaphorin most closely related to Sema6 proteins, can serve as a receptor in synaptogenesis and during the targeting of axons and dendrites in Drosophila (Cafferty et al, 2006;Godenschwege et al, 2002;Jeong et al, 2012;Komiyama et al, 2007;Yu et al, 2010). Downstream of Sema-1a, RhoGAPp190 and Pebble, a Rho-GEF, function to regulate Rho activity and control motor axon fasciculation and target recognition (Jeong et al, 2012). The ability of such membrane-associated semaphorins to interact with signalling proteins through their cytoplasmic domains suggests that many other semaphorins might be capable of mediating reverse signalling events.…”