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...

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Bone morphogenetic proteins (BMPs) are multifunctional proteins that comprise the largest subfamily of the transforming growth factors‐beta. BMPs transduce the signal through BMP type I and type II receptors. Here we identify a new protein that directly binds to BMPs. This protein has a transmembrane domain that localizes at the plasma membrane. Although BMP‐2 as well as BMP‐4 induces osteoblast differentiation of ST2 cells, knockdown of this protein results in suppression of the osteoblast differentiation and phosphorylation of smad1/5/8 induced by BMPs, suggesting that the protein enhances the BMPs signals. The osteoblast differentiation maintained by BMPs binding to the protein appears to be dependent on the activity of RhoA. Thus, we uncover a new mechanism that regulates the signaling pathway from BMPs to smads and osteoblast differentiation.