Structure and Function of Roundabout Receptors

Abstract

The creation of complex neuronal networks relies on ligand-receptor interactions that mediate attraction or repulsion towards specific targets. Roundabouts comprise a family of single-pass transmembrane receptors facilitating this process upon interaction with the soluble extracellular ligand Slit protein family emanating from the midline. Due to the complexity and flexible nature of Robo receptors, their overall structure has remained elusive until now. Recent structural studies of the Robo1 and Robo2 ectodomains have provided the basis for a better understanding of their signalling mechanism. These structures reveal how Robo receptors adopt an auto-inhibited conformation on the cell surface that can be further stabilised by cis and/or trans oligmerisation arrays. Upon Slit-N binding Robo receptors must undergo a conformational change for Ig4 mediated dimerisation and signaling, probably via endocytosis. Furthermore, it's become clear that Robo receptors do not only act alone, but as large and more complex cell surface receptor assemblies to manifest directional and growth effects in a concerted fashion. These context dependent assemblies provide a mechanism to fine tune attractive and repulsive signals in a combinatorial manner required during neuronal development. While a mechanistic understanding of Slit mediated Robo signaling has advanced significantly further structural studies on larger assemblies are required for the design of new experiments to elucidate their role in cell surface receptor complexes. These will be necessary to understand the role of Slit-Robo signaling in neurogenesis, angiogenesis, organ development and cancer progression. In this chapter, we provide a review of the current knowledge in the field with a particular focus on the Roundabout receptor family.

The CNS/PNS Extracellular Matrix Provides Instructive Guidance Cues to Neural Cells and Neuroregulatory Proteins in Neural Development and Repair

“…Slit is a large secreted 200 kDa LRR protein that provides important guidance cues in the developing nervous system [ 145 , 146 ]. The secreted Slit glycoproteins and their Roundabout (ROBO) receptors have major axon guidance roles in cell repulsion that prevent commissural axons from migrating inappropriately during the assembly of the nervous system but also have recently discovered roles in neurogenesis and angiogenesis [ 147 , 148 , 149 , 150 , 151 , 152 , 153 , 154 , 155 ]. Vertebrates contain four different Robos and three Slits: Robo1, Robo2, Robo3/Rig1 and Robo4; and Slit1, Slit2 and Slit3.…”

SLIT2/ROBO signaling in tumor-associated microglia and macrophages drives glioblastoma immunosuppression and vascular dysmorphia

“…SLIT ligands bind via the second leucine-rich repeat region (D2) to the Ig1 domain of ROBO1&2 (23), while mammalian ROBO3 and ROBO4 lack the SLIT binding residues and do not bind SLITs (24,25). SLIT binding triggers recruitment of adaptor proteins to the ROBO cytoplasmic domain that modulate the cytoskeleton, in turn regulating cell migration, adhesion and proliferation (22,26,27). SLIT/ROBO signaling was discovered in the developing nervous system as a guidance cue for axonal growth cones that regulates pathfinding of commissural axons and motor coordination between the left and right side of the body (20,21).…”

FLRTing Neurons in Cortical Migration During Cerebral Cortex Development

“…These homophilic bindings trigger cell adhesion and stimulate neurite outgrowth (Hivert et al, 2002). Robo receptors participate in widely diverse functions during development due to their ability to interact with different co-receptors through forming both cis and/or trans extracellular interactions, as well as intracellular cis-interactions, thus creating crosstalk between several distinct signaling pathways (Bisiak and McCarthy, 2019).…”

“…Robo (1-4) receptors are type-I single-pass transmembrane proteins. Their ectodomains are composed of five Ig and three FNIII domains (although Robo4 is smaller, containing only two of each domains), followed by a single transmembrane helix connected to a large ICD showing different conserved cytoplasmic (CC) motifs ( Bisiak and McCarthy, 2019 ). Their ectodomain can trigger both repulsive and adhesive signaling depending on its binding partner.…”