Notwithstanding whether MAP2 binds to CRMP5 through direct interaction or indirectly via tubulin, its presence within the complex implies that it is a determinant factor in the inhibitory role of CRMP5 notably at the dendritic level. At later stages, when dendrites begin to extend, CRMP5 expression is absent. However, CRMP2 is constantly expressed. Overexpression of CRMP5 with CRMP2 inhibits CRMP2-induced outgrowth both on the axonal and dendritic levels. Deficiency of CRMP5 expression enhanced the CRMP2 effect. This antagonizing effect of CRMP5 is exerted through a tubulin-based mechanism. Thus, the CRMP5 binding to tubulin modulates CRMP2 regulation of neurite outgrowth and neuronal polarity during brain development. Introduction Collapsin response mediator proteins (CRMPs) are cytosolic phosphoproteins, highly expressed in the developing nervous system. They act as signaling molecules involved in the regulation of microtubule polymerization, actin bundling, and endocytosis leading to neuronal differentiation and axonal growth (Arimura and Kaibuchi, 2007). Among CRMPs, CRMP2 was originally identified as the signaling molecule of the repulsive guidance cue, semaphorin 3A, inducing growth cone collapse (Goshima et al., 1995). CRMP2 has been reported to regulate neuronal polarity and axon elongation (Yoshimura et al., 2005); its overexpression induces multiple axon formation and primary axon elongation in hippocampal neurons (Inagaki et al., 2001). CRMP2 modulates axon growth through direct binding to tubulin and promoting microtubule dynamics (Fukata et al., 2002). By binding to the kinesin-1 light chain, CRMP2 regulates the transport of soluble tubulin to the distal parts of growing axons, thereby modulating axon growth (Kimura et al., 2005). Microtubule assembly can be disrupted by CRMP2 phosphorylation (Yoshimura et al., 2005). Some studies have suggested Peramivir trihydrate a role for CRMP2 in JWS the etiology of Alzheimer’s disease since hyperphosphorylation of CRMP2 may interfere with tubulin assembly in neurites (Cole et al., 2004; Uchida et al., 2005). In addition to its role in the transport of soluble molecules, CRMP2 can act Peramivir trihydrate as a cargo receptor in the transport of vesicles (Arimura et al., 2009). Another member of the CRMP family, CRMP5, was independently identified by different approaches (Fukada et al., 2000; Inatome et al., 2000; Ricard et al., 2001). CRMP5 expression is high in developing brain, but decreases in adult brain. Our group has studied the spatiotemporal expression of CRMP5 in the cortex, hippocampus, and cerebellum, and in postmitotic neuronal precursors, suggesting a role in process extension (Ricard et al., 2001). Other studies have reported the localization of CRMP5 at the filopodia of growth cones, suggesting that it plays a role in regulating filopodial Peramivir trihydrate dynamics and growth cone development (Hotta et al., 2005). The association of CRMP5 with tyrosine kinase Fes/Fps or mitochondrial septin Peramivir trihydrate has been previously reported (Mitsui et al., 2002; Takahashi et al., 2003), although the functional significance of these interactions has not been elucidated. In addition to its expression during development, CRMP5 plays a role in neurological disorders. In adults, the reexpression of CRMP5 causes paraneoplastic neurological syndromes, as a result of cancer-induced autoimmune process. CRMP5 autoantibodies were reported as markers of small-cell lung cancer and thymoma (Ricard et al., 2001; Honnorat et al., 2009). Considering the importance of CRMP5 in development and disease, it is possible that it helps the remodeling of neuronal networks, although, to date, its function is still poorly understood. In the present study, we show that CRMP5 forms a ternary complex with microtubule-associated protein 2 (MAP2) and tubulin, resulting in the inhibition of neurite elongation. In hippocampal neurons, CRMP5 is.