ReviewSlit proteins: key regulators of axon guidance, axonal branching, and cell migration
Introduction
Dramatic progress has been made over the past years in identifying molecules that function as attractive or repulsive cues to guide developing axons during the assembly of the nervous system (for reviews, see 1, 2). Some of these molecules, such as the netrins, are bifunctional, capable of attracting some axons while repelling others. In addition, many of these molecules, including the netrins, semaphorins, and ephrins, as well as their receptors, are also phylogenetically conserved, playing similar or related roles in axon guidance and/or cell migration or morphogenesis in both vertebrates and invertebrates [3].
Two lines of investigation have led, this year, to the identification of another important family of ligands that functions in axon guidance and cell migration: the Slit proteins. The first line of inquiry stemmed from previous studies of a phylogenetically conserved receptor mechanism involving proteins of the Robo family. Members of this family were first identified and implicated in axon guidance through genetic studies in Drosophila (Roundabout, or Robo) and C. elegans (Sax-3), and by 1998 the evidence, summarized briefly below, indicated that Robo was probably a receptor that mediates repulsive responses to a ligand that, at the time was still unidentified 4, 5••, 6••, 7. This year the repulsive Robo ligand has been identified as the Drosophila Slit protein [5••], and additional studies have shown that the repulsive action of Slit proteins is conserved between flies and vertebrates and can function in repelling both migrating cells and axons 8••, 9•, 10••, 11••, 12••, 13••, 14••, 15•.
The second line of investigation that led to the identification of Slit proteins focused on a completely different problem: the search for factors that can stimulate axonal branching. No link was suspected at the time between repulsion and branching. It was therefore quite remarkable that the biochemical purification of a branching activity in brain extracts identified a cleavage fragment of the mammalian Slit2 protein as a stimulator of elongation and branching of rat spinal sensory axons [16••]. Together, these studies have identified Slit proteins as key regulators of cell and axon migration, and have suggested important links between axonal repulsion and axonal branching. We begin by reviewing studies on Drosophila Slit, before turning to the repulsive functions of mammalian Slit proteins, and then to their roles in axonal branching.
Section snippets
Slit as the midline repellent in Drosophila
In organisms as diverse as flies, worms and vertebrates, the midline acts as an important choice point for navigating axons (for a review, see [1]). The midline is known to be a source of both attractive cues for drawing axons in and repulsive cues for pushing axons away. Robo was identified in a mutant screen in Drosophila for genes involved in regulating crossing at the midline [4]. Robo was found to encode a protein which is a transmembrane receptor and a member of the immunoglobulin
Slit proteins mediate axon repulsion in vertebrates
Given the conservation of guidance mechanisms across species that was alluded to earlier, it may not be too surprising that Robo and Slit proteins are evolutionarily conserved and have also been implicated in axonal repulsion in vertebrates. To date, three mammalian homologues of Drosophila Robo, Robo1, Robo2, and Rig1 5••, 10••, 23•, and three mammalian Slit homologues, Slit1, Slit2, and Slit3, have been identified 5••, 10••, 11••, 24•, 25•, 26, 27•, 28•. In addition, partial sequences of a
Slit proteins also direct cell migrations
In addition to guiding axons, Slit and Robo proteins also play roles in directing embryonic cell migrations. The first evidence for this again came from Drosophila, where it was found that Slit is required for the migration of muscle precursor cells away from the midline, consistent with a repulsive action of Slit in directing those migrations [21••]. The fact that precursor cells expressing the Robo–Fra chimera (the attractive Slit receptor) were attracted to the midline provides further
Slit proteins and the regulation of axon branching
At the same time that Slit proteins were implicated in axon and cell repulsion by the experiments described above, an independent line of investigation identified a Slit protein as a positive regulator of sensory axon elongation and branching in vertebrates. Most neurons, at least in vertebrates, make connections with multiple target cells, and the manner in which many or most of these connections are made is through a process of collateral branching, in which a neuron first sends out a primary
Other slit binding interactions
Slit proteins have been found to interact with a number of proteins other than Robo receptors. Based on its homology to biglycan and decorin (which are both proteoglycans and laminin-binding proteins), Slit2 was found to bind with high affinity to both laminin-1 and netrin-1 [10••], both of which can also affect axon outgrowth and guidance. Although it remains unclear what the functional significance of these interactions might be, one possibility is that Slit proteins act as part of a larger
Conclusions and future directions
In the last year, there has been an explosion of research findings related to Robo and Slit proteins. As well as functioning in growth cone repulsion, Slit proteins have been shown to be capable of repelling migrating mesodermal and neuronal cell bodies. Furthermore, work implicating Slit proteins in collateral branch formation indicates that these proteins are bifunctional, with positive as well as negative effects on extending axons. It will of course be necessary to extend all of the in vitro
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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