When a diffusible axon guidance cue stops diffusing: roles for netrins in adhesion and morphogenesis
Introduction
Netrins are an evolutionarily conserved family of ∼70 kilodalton proteins that have sequence homology to laminins (Figure 1a). Three secreted netrins, netrin-1, -3 and -4 are expressed in mammals, in addition to two related glycophosphatidyl inositol-linked membrane proteins, netrin-G1 and -G2. Netrins are bifunctional cues, functioning as either chemoattractants or chemorepellents for different cell types. Receptors for netrins in vertebrates include DCC (deleted in colorectal cancer), the DCC paralog neogenin, and four UNC5 homologs, UNC5A–D. DCC is a single pass transmembrane immunoglobulin (Ig) superfamily member that is required for attractant responses to netrin (Figure 1b). Chemorepellent responses to netrin require expression of an UNC5 protein, and in some cases are dependent on co-expression of DCC. UNC5 proteins are also transmembrane Ig superfamily members (Figure 1b). The predicted structure of neogenin is similar to that of DCC, but its function is relatively poorly understood [1]. Here, we review emerging evidence for short-range actions of netrins in regulating cell adhesion and tissue morphogenesis.
Section snippets
Netrin function at long-range
Studies of netrin function during development suggest roles as both a short-range and a long-range cue. Here, ‘short-range’ refers to factors that remain close to, or attached to, the surface of the cell secreting the cue, and, therefore, function in the immediate vicinity of the cellular source. Secreted long-range cues, by contrast, function many cells away from the source of the factor. Substantial evidence indicates that netrins, and in particular netrin-1, function as long-range
Netrin function at short-range: guidance, adhesion and tissue morphogenesis
In addition to this long-range function, evidence is accumulating that netrins also function as short-range cues. Initial studies indicated that local expression of netrin-1 at the optic disc of the mouse retina was not required for long-range axon guidance within the retina, but instead enables the axons of retinal ganglion cells to exit the eye and enter the optic nerve, a short-range function [7]. In addition, netrin functions as a target-derived short-range cue in regulating the development
Adhesive and instructive roles in tissue morphogenesis
Netrin expression is not limited to the nervous system. Recent studies have uncovered roles for netrin in the development of the mammary gland, lung, pancreas and vasculature. During development of mammary terminal end buds, the growing tips of the developing ductal network of the gland, netrin-1 is expressed by a layer of luminal epithelial cells, and the netrin receptor neogenin by an adjacent layer of cap cells (Figure 1g) [11]. The absence of either netrin-1 or neogenin function generates a
Netrin is associated with the cell surface and components of the extracellular matrix
Subcellular fractionation of embryonic or adult central nervous system (CNS) demonstrated that the vast majority of netrin-1 is bound to the extracellular matrix (ECM) or to cell membranes [1, 22]. Importantly, this is consistent with the initial purification of netrins, in which they were extracted with a concentrated salt solution from preparations of embryonic chick brain membranes, indicating that they are membrane-associated proteins [16]. Putative netrin binding partners on the cell
Netrin signaling and potential roles in adhesion
Netrin-1 binding to DCC exerts a powerful effect on the organization of F-actin, promoting filopodia formation and membrane extension [25••, 28]. Substrate-bound netrin-1 also regulates growth cone morphology and promotes adherence of DCC-expressing cells [25••], providing evidence that DCC and netrin-1 form an adhesive receptor–ligand pair. This finding suggests that netrin-1 promotes filopodia formation and membrane extension by two complementary mechanisms: DCC forms the core of a protein
Netrins, morphogenesis, tissue maintenance and cancer
The dcc gene was first identified as the putative tumor suppressor deleted in colorectal cancer [40]. Substantial evidence points to an anti-oncogenic role for DCC, but how disruption of netrin signaling might contribute to tumorigenesis is poorly understood. Both DCC and UNC5 proteins have been proposed to function as pro-apoptotic receptors, triggering apoptosis in the absence of netrin [41]. By contrast, disrupting either netrin or DCC function did not induce apoptosis in glial precursor
Conclusions
Netrins were first identified as axon guidance cues little more than a decade ago. The studies described above broaden our understanding of netrin function to include essential contributions to regulating cell–cell and cell–matrix adhesion, tissue morphogenesis, and the maintenance of appropriate cell–cell interactions. Although this research is in its initial stages, it is now clear that the netrin gene family exhibits a rich biology that goes beyond neural development and contributes to the
Update
A recent report indicates that removal of UNC5A by gene knock-out reduces levels of spinal motoneuron death, consistent with a pro-apoptotic role for UNC5A [46]. In contrast to the hypothesis that netrin-1 regulates apoptosis through a dependence mechanism that triggers cell death in the absence of ligand, loss of netrin-1 did not increase motoneuron apoptosis. This suggests that the role of UNC5A in motoneuron cell death involves either a non-netrin-1 ligand or is ligand independent.
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
Acknowledgements
We thank S Rodrigues and A Di Polo for comments on the manuscript. KA Baker was supported by a Jeanne Timmins Costello Fellowship, SW Moore by a Lloyd Carr-Harris studentship, AA Jarjour by a Multiple Sclerosis Society of Canada Studentship, and TE Kennedy by a Senior Bourses de Chercheurs-Boursiers Award from the Fonds de la Recherche en Santé du Québec.
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