Neuropilin is a neuronal cell surface protein and has been shown to function as a receptor for a secreted protein, semaphorin III/D, that can induce neuronal growth cone collapse and repulsion of neurites in vitro. The roles of neuropilin in vivo, however, are unknown. Here, we report that neuropilin-deficient mutant mice produced by targeted disruption of the neuropilin gene show severe abnormalities in the trajectory of efferent fibers of the PNS. We also describe that neuropilin-deprived dorsal root ganglion neurons are perfectly protected from growth cone collapse elicited by semaphorin III/D. Our results indicate that neuropilin-semaphorin III/D-mediated chemorepulsive signals play a major role in guidance of PNS efferents.

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Summary We have investigated the source(s) and targeting of components to PNS nodes of Ranvier. We show adhesion molecules are diffusible within the axon membrane and accumulate at forming nodes from local axonal sources. In contrast, ion channels and cytoskeletal components exhibit limited planar mobility and require transport to the node. We have characterized further targeting of NF186, an axonal adhesion molecule, which pioneers node formation. NF186 redistributes to nascent nodes from a mobile, surface pool. This initial accumulation, and clearance from the internode require extracellular interactions whereas subsequent targeting to mature nodes, i.e. those flanked by paranodal junctions, requires intracellular interactions. After incorporation into the node, NF186 is immobile, stable, and promotes node integrity. Thus nodes assemble from two sources: adhesion molecules, which initiate assembly, accumulate by diffusion trapping via interactions with Schwann cells whereas ion channels and cytoskeletal components accumulate via subsequent transport. In mature nodes, components turnover slowly and are replenished via transport.

At the nodes of Ranvier, excitable axon membranes are exposed directly to the extracellular fluid. Cations are accumulated and depleted in the local extracellular nodal region during action potential propagation, but the impact of the extranodal micromilieu on signal propagation still remains unclear. Brain-specific hyaluronan-binding link protein, Bral1, colocalizes and forms complexes with negatively charged extracellular matrix (ECM) proteins, such as versican V2 and brevican, at the nodes of Ranvier in the myelinated white matter. The link protein family, including Bral1, appears to be the linchpin of these hyaluronan-bound ECM complexes. Here we report that the hyaluronan-associated ECM no longer shows a nodal pattern and that CNS nerve conduction is markedly decreased in Bral1-deficient mice even though there were no differences between wild-type and mutant mice in the clustering or transition of ion channels at the nodes or in the tissue morphology around the nodes of Ranvier. However, changes in the extracellular space diffusion parameters, measured by the real-time iontophoretic method and diffusion-weighted magnetic resonance imaging (MRI), suggest a reduction in the diffusion hindrances in the white matter of mutant mice. These findings provide a better understanding of the mechanisms underlying the accumulation of cations due to diffusion barriers around the nodes during saltatory conduction, which further implies the importance of the Bral1-based extramilieu for neuronal conductivity.

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Brevican is known to be an abundant extracellular matrix component in the adult brain and a structural constituent of perineuronal nets. We herein show that brevican, tenascin‐R (TN‐R) and phosphacan are present at the nodes of Ranvier on myelinated axons with a particularly large diameter in the central nervous system. A brevican deficiency resulted in a reorganization of the nodal matrices, which was characterized by the shift of TN‐R, and concomitantly phosphacan, from an axonal diameter‐dependent association with nodes to an axonal diameter independent association. Supported by the co‐immunoprecipitation results, these observations indicate that the presence of TN‐R and phosphacan at nodes is normally brevican‐dependent, while in the absence of brevican these molecules can also be recruited by versican V2. The versican V2 and Bral1 distribution was not affected, thus indicating a brevican‐independent role of these two molecules for establishing hyaluronan‐binding matrices at the nodes. Our results revealed that brevican plays a crucial role in determining the specialization of the hyaluronan‐binding nodal matrix assemblies in large diameter nodes.

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Perineuronal nets (PNNs) are pericellular coats of condensed matrix that enwrap the cell bodies and dendrites of many adult central nervous system (CNS) neurons. These extracellular matrices (ECMs) play a structural role as well as instructive roles in the control of CNS plasticity and the termination of critical periods. The cartilage link protein Crtl1/Hapln1 was reported to be a trigger for the formation of PNNs in the visual cortex. Bral2/Hapln4 is another link protein that is expressed in PNNs, mainly in the brainstem and cerebellum. To assess the role of Bral2 in PNN formation, we examined the expression of PNN components in targeted mouse mutants lacking Bral2. We show here that Bral2-deficient mice have attenuated PNNs, but the overall levels of chondroitin sulfate proteoglycans, lecticans, are unchanged with the exception of neurocan. Bral2 deficiency markedly affected the localization of brevican in all of the nuclei tested, and neurocan concomitant with Crtl1 in some of the nuclei, whereas no effect was seen on aggrecan even with the attenuation of Crtl1. Bral2 may have a role in the organization of the PNN, in association with brevican, that is independent of aggrecan binding. There was a heterogenous attenuation of PNN components, including glycosaminoglycans, indicating the elaborate molecular organization of the PNN components. Strikingly, a slight decrease in the number of synapses in deep cerebellar nuclei neurons was found. Taken together, these results imply that Bral2-brevican interaction may play a key role in synaptic stabilization and the structural integrity of the PNN.