Biomedical subjects

Simon W Moore

Publications and source records attributed to Simon W Moore.

5 recordsLinked to original sources

PubMed2006

When a diffusible axon guidance cue stops diffusing: roles for netrins in adhesion and morphogenesis.

Netrins are a small family of secreted proteins that are best known for their role as secreted long-range chemotropic guidance cues. Extracellular gradients of netrin protein, established by diffusion, are thought to direct cell and axon migration during neural development. In addition to this long-range role, recent findings provide increasing support for short-range functions, in which secreted netrin protein remains closely associated with its cellular source. Emerging evidence for short-range actions of netrins suggests that they contribute to tissue morphogenesis by regulating cell-cell and cell-matrix adhesion.

Animals↗

PubMed2006

Protein kinase A regulates the sensitivity of spinal commissural axon turning to netrin-1 but does not switch between chemoattraction and chemorepulsion.

Bifunctional axon guidance cues have been grouped into two classes depending on whether changes in intracellular cAMP or cGMP switch the response of the growth cone between attraction and repulsion. According to this model, axons respond to netrin-1, a group I guidance cue, as a chemoattractant when cAMP levels are high in the growth cone but switch and are repelled when the intraneuronal concentration of cAMP is low. The model is complicated by the proposal that cAMP-dependent kinase, protein kinase A (PKA), functions as a downstream effector for several guidance cues, including netrin-1, suggesting a close inter-relationship between guidance cue signal transduction and mechanisms regulating the switch between attraction and repulsion. Here, we examine possible interactions between netrin-1-mediated axon guidance and cAMP signaling in embryonic rat spinal commissural neurons. We report that netrin-1 does not alter the concentration of cAMP or PKA activity in these neurons across a wide range of netrin-1 concentrations and time points after application, leading us to conclude that netrin-1 does not regulate PKA in these cells. In contrast to the cyclic nucleotide switch model, we report that, despite inhibiting PKA, embryonic spinal commissural axons were always attracted to netrin-1 and never repelled. Instead, manipulating PKA regulated the sensitivity of chemoattraction to netrin-1: PKA inhibition reduced, and PKA activation increased, the distance over which axons turn toward a source of netrin-1. These findings indicate that the mechanisms underlying cyclic nucleotide-regulated switching are separable from the signal transduction mechanisms required for chemoattraction to netrin-1.

Animals↗

PubMed2005

Deleted in colorectal cancer binding netrin-1 mediates cell substrate adhesion and recruits Cdc42, Rac1, Pak1, and N-WASP into an intracellular signaling complex that promotes growth cone expansion.

Extracellular cues direct axon extension by regulating growth cone morphology. The netrin-1 receptor deleted in colorectal cancer (DCC) is required for commissural axon extension to the floor plate in the embryonic spinal cord. Here we demonstrate that challenging embryonic rat spinal commissural neurons with netrin-1, either in solution or as a substrate, causes DCC-dependent increases in growth cone surface area and filopodia number, which we term growth cone expansion. We provide evidence that DCC influences growth cone morphology by at least two mechanisms. First, DCC mediates an adhesive interaction with substrate-bound netrin-1. Second, netrin-1 binding to DCC recruits an intracellular signaling complex that directs the organization of actin. We show that netrin-1-induced growth cone expansion requires Cdc42 (cell division cycle 42), Rac1 (Ras-related C3 botulinum toxin substrate 1), Pak1 (p21-activated kinase), and N-WASP (neuronal Wiskott-Aldrich syndrome protein) and that the application of netrin-1 rapidly activates Cdc42, Rac1, and Pak1. Furthermore, netrin-1 recruits Cdc42, Rac1, Pak1, and N-WASP into a complex with the intracellular domain of DCC and Nck1. These findings suggest that DCC influences growth cone morphology by acting both as a transmembrane bridge that links extracellular netrin-1 to the actin cytoskeleton and as the core of a protein complex that directs the organization of actin.

Analysis of Variance↗

PubMed2004

Protein kinase A activation promotes plasma membrane insertion of DCC from an intracellular pool: A novel mechanism regulating commissural axon extension.

Protein kinase A (PKA) exerts a profound influence on axon extension during development and regeneration; however, the molecular mechanisms underlying these effects of PKA are not understood. Here, we show that DCC (deleted in colorectal cancer), a receptor for the axon guidance cue netrin-1, is distributed both at the plasma membrane and in a pre-existing intracellular vesicular pool in embryonic rat spinal commissural neurons. We hypothesized that the intracellular pool of DCC could be mobilized to the plasma membrane and enhance the response to netrin-1. Consistent with this, we show that application of netrin-1 causes a modest increase in cell surface DCC, without increasing the intracellular concentration of cAMP or activating PKA. Intriguingly, activation of PKA enhances the effect of netrin-1 on DCC mobilization and increases axon extension in response to netrin-1. PKA-dependent mobilization of DCC to the plasma membrane is selective, because the distributions of transient axonal glycoprotein-1, neural cell adhesion molecule, and trkB are not altered by PKA in these cells. Inhibiting adenylate cyclase, PKA, or exocytosis blocks DCC translocation on PKA activation. These findings indicate that netrin-1 increases the amount of cell surface DCC, that PKA potentiates the mobilization of DCC to the neuronal plasma membrane from an intracellular vesicular store, and that translocation of DCC to the cell surface increases axon outgrowth in response to netrin-1.

Adenylyl Cyclase Inhibitors↗

PubMed2003

Netrin-1 is a chemorepellent for oligodendrocyte precursor cells in the embryonic spinal cord.

Netrin-1, secreted by floor plate cells, orients axon extension in relation to the ventral midline of the embryonic spinal cord. Oligodendrocyte precursor (OP) cells are born close to the ventral midline and migrate away from the floor plate. Here we show that OP cells, identified by expression of the platelet-derived growth factor alpha receptor, express the netrin receptors dcc and unc5h1 but do not express netrin-1. Using a microchemotaxis assay, we demonstrate that migrating OPs are repelled by a gradient of netrin-1 in vitro. Furthermore, application of netrin-1 to OPs in vitro triggers retraction of OP processes. In the absence of netrin-1 or Deleted in Colorectal Cancer (DCC) function in vivo, fewer OP cells migrate from the ventral to the dorsal embryonic spinal cord, consistent with netrin-1 acting as a repellent. In addition to their role regulating cell movement, DCC and UNC-5 homologs have been suggested to function as proapoptotic dependence receptors, triggering cell death in the absence of netrin-1. In contrast, we report no evidence of increased OP cell death in vivo or in vitro in the absence of either netrin-1 or DCC. These findings indicate that netrin-1 is a repellent cue for migrating OPs in the embryonic spinal cord.

Animals↗