This fundamental question in axon guidance led researchers to Robo, which was identified in a large-scale screening of Drosophila mutants in the early 1990s.
[11] Several years later, genetic evidence,[12] biochemical binding experiments, and explant assays[13] identified Slits as the repulsive ligands for Robo receptors in both Drosophila and vertebrates.
In Drosophila, the two proteins Abelson tyrosine kinase (Abl) and Enabled (Ena) mediate cytoskeletal remodeling downstream of Slit-Robo signaling.
[7] Abl is also thought to promote repulsive signaling by binding to adenylyl cyclase associated proteins (CAP), which regulate actin polymerization.
[32] Deletions of individual robos do not phenotypically match Slit mutants, indicating that Robos1-3 play distinct, complementary but not entirely overlapping roles in axon guidance.
In vertebrates, Robo1 and Robo2 work together to mediate repulsion from Slit ligands expressed at the floor plate, while Robo3/Rig-1 has the opposite activity, and functions to promote attraction to the midline (most likely by inhibiting the other two Robo receptors, via an unknown mechanism).
In fact Slit2-N, an N-terminal fragment of Slit2, has been shown to induce Dorsal Root Ganglion (DRG) elongation and branching, whereas full length Slit2 antagonizes this effect.
[41] In vertebrates, Slit1 plays an important role in vomeronasal organ (VNO) axonal targeting to the accessory olfactory bulb (AOB).
[42] In 2009, a combination of Slit-Robo and Netrin-Frazzled signaling in Drosophila was shown to govern the establishment of myotopic maps, which describe the innervation of motorneuron dendrites in the muscle field.
[50] Robo4, also known as magic roundabout,[51] is an endothelial specific Robo which, upon binding Slit2, blocks Src family kinase activation, thereby inhibiting VEGF-165-induced migration and permeability in vitro and vascular leak in vivo.
[52] This suggests that combined VEGF/Slit2 therapies could be useful in preventing tumor angiogenesis and vascular leak or edema after heart attack or stroke.
[53] The homozygous Robo3 mutations have been associated with typical ophthalmologic horizontal gaze palsy with progressive scoliosis, which is characterized by oculomotor problems and general disturbances in innervation.
[54] Robo1 has been implicated as one of 14 different candidate genes for dyslexia, and one of 10 that fit into a theoretical molecular network involved in neuronal migration and neurite outgrowth.