Conserved and divergent aspects of Robo receptor signaling and regulation betweenDrosophilaRobo1 andC. elegansSAX-3

Abstract

The evolutionarily conserved Roundabout (Robo) family of axon guidance receptors control midline crossing of axons in response to the midline repellant ligand Slit in bilaterian animals including insects, nematodes, and vertebrates. Despite this strong evolutionary conservation, it is unclear whether the signaling mechanism(s) downstream of Robo receptors are similarly conserved. To directly compare midline repulsive signaling in Robo family members from different species, here we use a transgenic approach to express the Robo family receptor SAX-3 from the nematode Caenorhabditis elegans in neurons of the fruit fly, Drosophila melanogaster. We examine SAX-3’s ability to repel Drosophila axons from the Slit-expressing midline in gain of function assays, and test SAX-3’s ability to substitute for Drosophila Robo1 during fly embryonic development in genetic rescue experiments. We show that C. elegans SAX-3 is properly translated and localized to neuronal axons when expressed in the Drosophila embryonic CNS, and that SAX-3 can signal midline repulsion in Drosophila embryonic neurons, although not as efficiently as Drosophila Robo1. Using a series of Robo1/SAX-3 chimeras, we show that the SAX-3 cytoplasmic domain can signal midline repulsion to the same extent as Robo1 when combined with the Robo1 ectodomain. We show that SAX-3 is not subject to endosomal sorting by the negative regulator Commissureless (Comm) in Drosophila neurons in vivo, and that peri-membrane and ectodomain sequences are both required for Comm sorting of Drosophila Robo1.

Single-cell RNA sequencing of mid-to-late stage spider embryos: new insights into spider development

“…These genes are all involved in neuron development, neuron differentiation, and neuronal pathfinding, and thus also in differentiated neurons (e.g. [ 36 , 43 , 66 , 75 , 92 , 140 , 151 , 191 , 203 ]. Beyond that, many of the lesser markers of this cluster indeed also represent genes typically expressed in neurons, such as the Dscams (e.g.…”

Structure and evolution of neuronal wiring receptors and ligands

“… 63 , 75 The first IG (IG1) domain (except in mammalian Robo3 and vertebrate Robo4) binds to the repulsive cue Slit, 76 and shows higher sequence identity among bilaterian Robo IG1 domains compared to other domains, suggesting that selective pressures act differently on each individual domain. 77 Slits and Robos have been shown to interact with heparin, creating a composite HSPG‐binding site, which may direct the axon guidance activity of Robo‐Slit complexes. 78 , 79 , 80 , 81 , 82 …”

“… 88 Robo paralogs in Drosophila play similar and overlapping roles in mediating Slit‐dependent repulsion but are not completely redundant. 89 , 90 Axon repulsive functions are mostly conserved, as C. elegans SAX‐3 is capable of rescuing robo1 knockout function in Drosophila but cannot be trafficked by Drosophila commissureless, 77 a protein required for sorting Robo1 to endosomes to prevent repulsion by Slit before crossing the midline. 91 Robo internalization is preserved in vertebrates, but it is mediated by the unrelated Ndfip proteins rather than commissureless, which does not exist in vertebrates.…”