Role of Slit proteins in the vertebrate brain
Introduction
Mounting evidence indicates that in the developing central nervous system, growth cones can be guided at a distance by diffusible molecules secreted by non-target cells [42]. Many of these factors function as chemorepellents: they induce growth cone collapse and oriented axonal outgrowth away from the source of the factor. Chemorepulsive molecules are produced in a variety of central nervous system (CNS) regions, such as the ventral spinal cord, the floor plate or the thalamus [5]. Most chemorepulsive factors are members of the semaphorin, netrin and slit families. We have been studying the function of these molecules in the developing telencephalon, and particularly in the developing olfactory system.
Section snippets
Chemotropism in the developing olfactory system
The organization of axonal projections in the rodent olfactory system has been extensively characterized. Axons from olfactory receptor neurons in the olfactory epithelium project ipsilaterally to glomeruli in the main olfactory bulb, where they synapse on the dendrites of the mitral and tufted cells. These neurons project ipsilaterally to the anterior olfactory nucleus and to higher olfactory centers including the piriform and entorhinal cortex, and some amygdaloid nuclei, collectively
Structure and function of Slit proteins
The Slits is the most recently discovered family of chemotropic factors [2]. Slit (d-Slit) was first identified in Drosophila embryo as a gene involved in the patterning of larval cuticle. Subsequently, it was shown that d-Slit is synthesized in the central nervous system by midline glia cells and that in the absence of slit, longitudinal and commissural axons all converge and coalesce at the midline [2], [17], [27], [37]. More recent works have demonstrated that d-Slit is a chemorepulsive
Robos are receptors for Slits
One major breakthrough toward the understanding of Slit function has been the recent discovery that the Roundabout (robo) proteins are Slit receptors [1], [17], [19]. The first robo gene, robo1, was identified in Drosophila during a comprehensive screen for genes controlling CNS midline crossing [35]. In robo1 mutants, ipsilateral axons that normally avoid the midline cross it, and commissural axons cross and recross it repeatedly [35]. Robo is an evolutionary conserved family of transmembrane
Slit2 proteolytic fragments have distinct axon guidance properties
As mentioned above, Slit2 is cleaved in vivo and in vitro in two fragments. These have different cell association characteristics in cell culture suggesting that they may also have different extents of diffusion, different binding properties, and, hence, different functional activities in vivo. This possibility was supported by several studies. First, the purification of Slit as a DRG elongation- and branch-promoting activity revealed that only the N-terminal fragment of Slit2 is capable of
Functions and pharmacology of Slit2-N and Slit2-U in repulsion and branching
We focused on OB and DRG neurons because they both express Robo2 but not Robo1 mRNAs [22], [45] and because they showed dramatically different responses to Slit2, with olfactory axons being repelled [22] and DRG axons stimulated to elongate and branch [45]. Slit2-N and Slit2-U were found to have similar activities in repelling OB axons in the collagen gel repulsion assay (Fig. 2B) [23]. In contrast, the C-terminal portion of Slit2 (Slit2-C) had no repulsive activity. Other studies have shown
Substrate-bound Slit2 can guide sensory axons
Because of the strong binding of Slit2 fragments to cell membranes, growth cones are very likely to be confronted with immobilized Slit2. We examined whether substrate-bound Slit2 is able to guide developing sensory axons in the so-called “stripe” assay [43], [44] in which the axons grow parallel to alternating stripes of two different proteins or protein combinations, making it possible to test the axons' preference for one over the other [24]. We first examined the responses of E15 rat DRG
ECM molecules influence sensory axon response to substrate-bound Slit2 fragments
Given these results, we were curious to examine the response of DRG axons to purified Slit2 proteins in the stripe assay (Fig. 2C,D,E) [24]. Because ECM molecules, specially laminin-1 have been shown to influence the response of retinal axons to netrin-1, another chemotropic molecules [11], we compared the activity of Slit2 fragments in the presence of two different ECM molecules laminin or fibronectin which are both excellent substrates for DRG axons in culture. As expected, when Slit2-U or
Conclusion
Although extensive progress has been made toward the understanding of Slit function in the developing CNS there are still many unanswered questions. For instance, could Slit2-C have a role in axon guidance? Although the sole purpose of the cleavage could be to generate bioactive Slit2-N, there are nonetheless reasons, based on its structure, for thinking that Slit2-C is also bioactive. However, it had been shown that in the brain, Slit-2 is a ligand for the glycosylphosphatidylinositol-anchored
Acknowledgements
This work is supported by the Institut de la Santé et de le Recherche Médicale, the Ministère de la Recherche et de la Technologie (ACI) and the Association pour la Recherche sur le Cancer (No.5249). K N-B-C is supported by the Fondation de France.
References (51)
- et al.
Slit proteins bind Robo receptors and have an evolutionarily conserved role in repulsive axon guidance
Cell
(1999) - et al.
Slit proteinskey regulators of axon guidance, axonal branching, and cell migration
Curr. Opin. Neurobiol.
(2000) - et al.
Identification and characterization of roundabout orthologs in zebrafish
Mech. Dev
(2001) - et al.
The crystal structure of a laminin G-like module reveals the molecular basis of alpha-dystroglycan binding to laminins, perlecan, and agrin
Mol. Cell
(1999) - et al.
Distinct but overlapping expression patterns of two vertebrate slit homologs implies functional roles in CNS development and organogenesis
Mech. Dev.
(1998) Chemorepulsion of neuronal migration by slit2 in the developing mammalian forebrain
Neuron
(1999)- et al.
A septum-derived chemorepulsive factor for migrating olfactory interneuron precursors
Neuron
(1996) - et al.
Cloning and expressions of three mammalian homologues of Drosophila slit suggest possible roles for slit in the formation and maintenance of the nervous system
Mol. Brain Res.
(1998) - et al.
Slit is the midline repellent for the robo receptor in Drosophila
Cell
(1999) - et al.
Roundabout controls axon crossing of the CNS midline and defines a novel subfamily of evolutionarily conserved guidance receptors
Cell
(1998)
Vertebrate Slit, a secreted ligand for the transmembrane protein roundabout, is a repellent for olfactory bulb axons
Cell
Mammalian homologues of the Drosophila slit protein are ligands of the heparan sulfate proteoglycan glypican-1 in brain
J. Biol. Chem.
Early onset of the rat olfactory bulb projections
Neuroscience
Crossing the midlineroles and regulation of Robo receptors
Neuron
Selecting a longitudinal pathwayRobo receptors specify the lateral position of axons in the Drosophila CNS
Cell
Characterization of Slit protein interactions with glypican-1
J. Biol. Chem.
Modularity of the Slit protein characterization of a conserved carboxy-terminal sequence in secreted proteins and a motif implicated in extarcellular protein interactions
J. Mol. Biol.
The structure of the ligand-binding domain of neurexin Ibetaregulation of LNS domain function by alternative splicing
Cell
Mutations affecting growth cone guidance in Drosophilagenes necessary for guidance toward or away from the midline
Neuron
Short-range and long-range guidance by slit and its Robo receptors. Robo and Robo2 play distinct roles in midline guidance
Neuron
Signal transduction underlying growth cone guidance by diffusible factors
Curr. Opin. Neurobiol.
Biochemical purification of a mammalian Slit protein as a positive regulator of sensory axon elongation and branching
Cell
Cloning and functional studies of a novel gene aberrantly expressed in RB-deficient embryos
Dev. Biol.
The conserved immunoglobulin superfamily member SAX-3/Robo directs multiple aspects of axon guidance in C. elegans
Cell
Cellular and molecular guidance of GABAergic neuronal migration from an extracortical origin to the neocortex
Neuron
Cited by (63)
Common cues wire the spinal cord: Axon guidance molecules in spinal neuron migration
2019, Seminars in Cell and Developmental BiologyCitation Excerpt :The latter function often involves UNC5 receptors. The secreted SLIT ligands and their Roundabout (ROBO) receptors are highly conserved, and have important roles both inside and outside the nervous system [55–58]. The SLIT/ROBO pathway signals chemorepulsion within commissural axons, with floorplate–expressed SLITs blocking commissural axons from crossing the midline [22,24].
A decade from discovery to therapy: Lingo-1, the dark horse in neurological and psychiatric disorders
2015, Neuroscience and Biobehavioral ReviewsCitation Excerpt :Proteins containing leucine-rich repeats have been shown to play an important role in protein–protein interactions (Kajava et al., 1995; Kobe and Deisenhofer, 1994; Kobe and Kajava, 2001) in a wide variety of cellular processes (Carim-Todd et al., 2003) including ligand recognition (Brose et al., 1999; Chen et al., 2001; Li et al., 1999; Vourc’h et al., 2003; Wang et al., 2002b). Their implication in important neurodevelopmental functions such as neuronal differentiation and growth (Halegoua et al., 1991) and the regulation of axon guidance, axon branching, cell-migration and regeneration processes (Bormann et al., 1999; Brose and Tessier-Lavigne, 2000; Ishii et al., 1996; Nguyen-Ba-Charvet and Chédotal, 2002), results consequently in their involvement in neurological diseases such as hereditary epilepsy and X-linked stationary night blindness (Bech-Hansen et al., 2000; Kalachikov et al., 2002; Morante-Redolat et al., 2002; Pusch et al., 2000). Since Lingo-1 is a leucine rich repeat protein, and considering its genetic locus is in a chromosomic region associated with a high risk for a number of psychiatric disorders, Lingo-1 makes for an ideal candidate for study in a vast array of neurological disorders.
Slit/Robo pathway: A promising therapeutic target for cancer
2015, Drug Discovery TodayCitation Excerpt :Slit and Robo proteins were first discovered as secreted proteins in Drosophila [5–7]. Thereafter, homologs of Slit and Robo proteins have been discovered in rat, mice and humans [8]. Many reports have suggested that, in addition to axon guidance, the Slit/Robo pathway is also involved in the developmental processes and in the regulation of several physiological processes.
SrGAP3 promotes neurite outgrowth of dorsal root ganglion neurons by inactivating RAC1
2014, Asian Pacific Journal of Tropical MedicineThe SLIT/ROBO Pathway in Liver Fibrosis and Cancer
2023, BiomoleculesSecreted Glycoproteins That Regulate Synaptic Function: the Dispatchers in the Central Nervous System
2023, Molecular Neurobiology