Repulsive guidance molecule (RGM) is a protein implicated in both axonal guidance and neural tube closure. We report RGMa as a potent inhibitor of axon regeneration in the adult central nervous system (CNS). RGMa inhibits mammalian CNS neurite outgrowth by a mechanism dependent on the activation of the RhoA–Rho kinase pathway. RGMa expression is observed in oligodendrocytes, myelinated fibers, and neurons of the adult rat spinal cord and is induced around the injury site after spinal cord injury. We developed an antibody to RGMa that efficiently blocks the effect of RGMa in vitro. Intrathecal administration of the antibody to rats with thoracic spinal cord hemisection results in significant axonal growth of the corticospinal tract and improves functional recovery. Thus, RGMa plays an important role in limiting axonal regeneration after CNS injury and the RGMa antibody offers a possible therapeutic agent in clinical conditions characterized by a failure of CNS regeneration.

Netrin-1 induces repulsive axon guidance by binding to the mammalian Unc5 receptor family (Unc5A-Unc5D). Mouse genetic analysis of selected members of the Unc5 family, however, revealed essential functions independent of Netrin-1, suggesting the presence of other ligands. Unc5B was recently shown to bind fibronectin and leucine-rich transmembrane protein-3 (FLRT3), although the relevance of this interaction for nervous system development remained unclear. Here, we show that the related Unc5D receptor binds specifically to another FLRT protein, FLRT2. During development, FLRT2/3 ectodomains (ECDs) are shed from neurons and act as repulsive guidance molecules for axons and somata of Unc5-positive neurons. In the developing mammalian neocortex, Unc5D is expressed by neurons in the subventricular zone (SVZ), which display delayed migration to the FLRT2-expressing cortical plate (CP). Deletion of either FLRT2 or Unc5D causes a subset of SVZ-derived neurons to prematurely migrate towards the CP, whereas overexpression of Unc5D has opposite effects. Hence, the shed FLRT2 and FLRT3 ECDs represent a novel family of chemorepellents for Unc5-positive neurons and FLRT2/ Unc5D signalling modulates cortical neuron migration.

Bone morphogenetic proteins (BMPs) regulate many mammalian physiologic and pathophysiologic processes. These proteins bind with the kinase receptors BMPR-I and BMPR-II, thereby activating Smad transcription factor. In this study, we demonstrate that neogenin, a receptor for netrins and proteins of the repulsive guidance molecule family, is a receptor for BMPs and modulates Smad signal transduction. Neogenin was found to bind directly with BMP-2, BMP-4, BMP-6, and BMP-7. Knockdown of neogenin in C2C12 cells resulted in the enhancement of the BMP-2-induced processes of osteoblastic differentiation and phosphorylation of Smad1, Smad5, and Smad8. Conversely, overexpression of neogenin in C2C12 cells suppressed these processes. Our results also indicated that BMP-induced activation of RhoA was mediated by neogenin. Inhibition of RhoA promoted BMP-2-induced processes of osteoblastic differentiation and phosphorylation of Smad1/ 5/8. However, treatment with Y-27632, an inhibitor of Rhoassociated protein kinase, did not modulate BMP-induced phosphorylation of Smad1/5/8. Taken together, our findings suggest that neogenin negatively regulates the functions of BMP and that this effect of neogenin is mediated by the activation of RhoA.Bone morphogenetic proteins (BMPs) 2 are a unique family belonging to the transforming growth factor-␤ superfamily. BMPs regulate various processes associated with the differentiation, growth, and death of cells (1). BMPs bind to two different serine/threonine kinase receptors and mediate their signals through Smad-dependent and Smad-independent pathways (1). The biological activities of these proteins are regulated by intracellular signaling cascades triggered by various growth factors and cytokines. In particular, the functions of BMPs are known to be modulated by small GTPase Rho. Rho and one of its effectors, ROCK, participate in a variety of biological processes such as vascular contraction, tumor invasion, and bone formation. We have previously reported that in mice, continuous infusion of the ROCK-specific inhibitor Y-27632 enhanced ectopic bone formation induced by recombinant BMP-2 (rhBMP-2) impregnated into an atelocollagen carrier, without affecting the systemic bone metabolism (2). The expression of a dominant negative mutant of ROCK in mesenchymal ST2 cells promoted osteoblastic differentiation of these cells, whereas that of a constitutively active mutant of ROCK attenuated osteoblastic differentiation; moreover, the administration of a ROCK inhibitor reversed this phenotype. These findings suggest that ROCK negatively regulates BMPinduced osteogenesis. However, the precise molecular mechanism by which Rho is involved in the BMP signals remains to be elucidated.The results of this study show that neogenin is a receptor for BMPs and uncover the missing link between the BMPs and Rho. Neogenin, a receptor for the repulsive guidance molecule (RGM) and netrins, is a single-membrane-spanning protein and a member of the Ig superfamily. Neogenin consists of four immunoglobulin-like dom...

Bone morphogenetic proteins (BMPs) are multifunctional growth factors that belong to the transforming growth factor‐β superfamily. BMPs regulate several crucial aspects of embryonic development and organogenesis. The reemergence of BMPs in the injured adult CNS suggests their involvement in the pathogenesis of the lesion. Here, we demonstrate that BMPs are potent inhibitors of axonal regeneration in the adult spinal cord. The expression of BMP‐2/4 is elevated in oligodendrocytes and astrocytes around the injury site following spinal cord contusion. Intrathecal administration of noggin – a soluble BMP antagonist—leads to enhanced locomotor activity and reveals significant regrowth of the corticospinal tract after spinal cord contusion. Thus, BMPs play a role in inhibiting axonal regeneration and limiting functional recovery following injury to the CNS.