Directional axon extension and cell migration require both cytoskeletal reorganization and membrane insertion to extend the leading edge of a cell. In this issue, Cotrufo et al. (2012) provide evidence that the chemoattractant netrin-1 directs membrane extension during neuronal cell migration by promoting local exocytosis.
Netrins are chemotropic guidance cues that direct axon extension, cell migration and neurite branching during neural development (Lai Wing Sun et al., 2011). Signal transduction mechanisms activated by the netrin-1 receptor DCC (Deleted in Colorectal Cancer) regulate cytoskeletal organization, and understanding how chemotropic guidance cues regulate the cytoskeleton has been a major focus of the field. The findings presented by Cotrufo et al. (2012) now fundamentally expand the role of DCC, providing evidence that netrin-1 binding may cause DCC to direct a localized increase in membrane insertion, facilitating the formation of cellular protrusions.
Cotrufo et al. (2011) had previously used hippocampal neurons to determine that netrin-1 increases the association between DCC and syntaxin-1, which is a plasma membrane target membrane-Soluble NSF Attachment Protein Receptor (SNARE) with a well-established function in synaptic vesicle exocytosis. In the current study, the authors extend these findings to cell migration using a well-characterized assay of chemotropic neuronal cell migration in the embryonic mammalian brain. During early development, post-mitotic neurons are directed by netrin-1 to migrate from the lower rhombic lip to form the pontine nuclei (Alcantara et al., 2000). The authors provide evidence for a direct interaction between DCC and syntaxin-1 in migrating neurons, and demonstrate that disrupting syntaxin-1 function blocks the chemoattractant migration of these cells to netrin-1. During synaptic vesicle release, syntaxin-1 forms a canonical SNARE complex with SNAP25 (Synaptosomal-associated protein 25), a plasma membrane SNARE, and synaptobrevin, a vesicle-SNARE on synaptic vesicles. Interestingly, disrupting the function of either SNAP25 or synaptobrevin-2 [also called vesicle-associated membrane protein (VAMP)2] had no effect on netrin-mediated axon guidance (Cotrufo et al., 2011) or cell migration (Cotrufo et al., 2012), suggesting that, if DCC functions with syntaxin-1 to promote exocytosis, this may occur via a novel, and perhaps atypical, complex of SNARE proteins. Along these lines, the authors detect a complex of DCC and the vesicle-SNARE tetanus neurotoxin-insensitive VAMP (also called VAMP7) in migrating cells (Cotrufo et al., 2012) and have shown that disrupting tetanus neurotoxin-insensitive VAMP function blocks the chemoattractant axon extension to netrin-1 (Cotrufo et al., 2011). The authors propose a model where netrin-1 binding recruits syntaxin-1 to DCC, which then promotes the fusion of vesicles required for local membrane extension.
Of greatest significance to the field, the interaction between DCC and syntaxin-1 provides a possible link between polarized membrane insertion and the established cytoskeletal rearrangements mediated by DCC. Netrins are multifunctional proteins. It is tempting to speculate that the findings presented in this article, along with past studies (Bouchard et al., 2004, 2008), indicate that different pools of vesicles regulate different aspects of the various responses made to netrin-1, in some cases influencing axon outgrowth and cell migration, and in others controlling axon arborization and the polarity of chemotropic turning.