Control of axon–axon attraction by Semaphorin reverse signaling

Abstract

Semaphorin family proteins are well-known axon guidance ligands. Recent studies indicate that certain transmembrane Semaphorins can also function as guidance receptors to mediate axon-axon attraction or repulsion. The mechanisms by which Semaphorin reverse signaling modulates axon-surface affinity, however, remain unknown. In this study, we reveal a novel mechanism underlying upregulation of axon-axon attraction by Semaphorin-1a (Sema1a) reverse signaling in the developing Drosophila visual system. Sema1a promotes the phosphorylation and activation of Moesin (Moe), a member of the ezrin/radixin/moesin family of proteins, and downregulates the level of active Rho1 in photoreceptor axons. We propose that Sema1a reverse signaling activates Moe, which in turn upregulates Fas2-mediated axon-axon attraction by inhibiting Rho1.T he Semaphorin family of proteins are well-known axon guidance cues or ligands, which activate their receptors on a variety of axons to control axonal pathfinding, fasciculation, branching, and target selection in vertebrates and invertebrates (1, 2). Recent studies demonstrate that certain transmembrane Semaphorins can also function as a receptor to mediate downstream signaling events in both vertebrates and invertebrates (3-7). 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). 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).To understand the mechanisms underlying upregulation of axon-axon attractions by Sema1a reverse signaling, we set out to examine potential genetic interactions between Sema1a and other genes in R-cell axon guidance. The establishment of R-cell-to-optic-lobe connections in the Drosophila adult visual system begins at the third-instar larval stage (9). At the thirdinstar larval stage, differentiating R cells in the eye-imaginal disk extend axons through the optic stalk into the developing optic lobe. R1-R6 axons terminate at the superficial lamina layer, where their growth cones closely associate with each other at the lamina termination site. R7 and R8 axons bypass the lamina and terminate in the deeper medulla layer.In this study, we present evidence that Sema1a reverse signaling promotes R-cell axon-axon attraction by upregulating the adhesive function of Fasciclin 2 (Fas2). Sema1a interacts genetically and physically with Moesin (Moe), a member of the ezrin/radixin/moesin (ERM) family proteins, and downregulates the level of active Rho1. Our results support that Sema1a-induced reduction in the level of active Rho1 in R-cell axons contributes to an increase in Fas2-mediated R-cell axon-axon attraction. Results Sema1aInteracts with Fas2 in Regulating R-Cell Axonal Projec...

Sema-1a Reverse Signaling Promotes Midline Crossing in Response to Secreted Semaphorins

“…Two classes of neurons in the Drosophila visual system, the laminar neurons and the photoreceptors were both found to employ Sema-1a reverse signaling and both bound the canonical binding partner PlexA; however, the laminar neurons exhibit a repulsive response to PlexA, while the photoreceptors show an adhesive response (Cafferty et al, 2006; Hsieh et al, 2014; Pecot et al, 2013). The discovery of competitive downstream effectors (Pbl and RhoGAPp190) with opposing effects on Rho1 begins to explain how Sema-1a reverse signaling could have multiple, and even opposite outputs.…”

“…Interestingly, the Sema-1a mediated adhesive response uncovered in the photoreceptors is also dependent on the down regulation of Rho1 (Hsieh et al, 2014). However, in the context of photoreceptor axon guidance, adhesion is mediated by FasII, which is not expressed in the commissural eagle neurons.…”

Glial and neuronal Semaphorin signaling instruct the development of a functional myotopic map for Drosophila walking

“…This suggests that PlexA/Sema1a signaling might be at the level of axon-axon interactions. A role of Plexin/Semaphorin signaling in axon-axon interactions has been documented in the developing visual and olfactory systems of Drosophila ( Sweeney et al, 2007 ; Hsieh et al, 2014 ). Alternatively, cis-interactions between PlexA and Sema1a within the affected motoneuron axons might also be involved.…”

Characterization of plexinA and two distinct semaphorin1a transcripts in the developing and adult cricket Gryllus bimaculatus

“…In invertebrates, however, Sema signaling may not always require Plex receptors; in D. melanogaster Sema1a can work in "reverse," by acting as a receptor for Plexin, Sema2a, or Sema2b (Godenschwege, Hu, Shan-Crofts, Goodman, & Murphey, 2002;Hernandez-Fleming et al, 2017;Yu, Zhou, Cheng, & Rao, 2010). When Sema1a is bound by a ligand, the cytoplasmic domain interacts with a number of different factors that influence GTPases, which in turn alter axon growth (Cho, Chak, Andreone, Wooley, & Kolodkin, 2012;Hsieh, Chang, Yu, & Rao, 2014;Jeong, Juhaszova, & Kolodkin, 2012). In cricket thoracic ganglia, both sema1a.2 and sema2a were expressed at reasonably high levels.…”