Proteomic analysis of optic nerve lipid rafts reveals new paranodal proteins

Neuron-glia interactions at paranodal junctions play important roles in action potential propagation. Among their many functions, they contribute to the passive electrical properties of myelinated nerve fibers and actively regulate the polarized distribution of ion channels along axons. Despite their importance, relatively little is known about the molecules responsible for paranode formation and function. Paranodal junction formation apparently depends on interactions among three cell adhesion molecules: caspr and contactin on the axon and neurofascin 155 (NF-155) on the glial membrane. Using Caspr-null paranodal mutant mice, we demonstrate that loss of paranodal junctions causes failure of NF-155 to partition into lipid rafts, indicating that proteins located at paranodal junctions have biochemical characteristics of lipid raft-associated proteins. Based on this property of paranodal junctions, mass spectrometry of lipid rafts isolated from a pure white matter tract (optic nerve) was used to search for new paranodal proteins. Because we used a relatively crude biochemical preparation, we identified several hundred different proteins. Among these, we found all previously described paranodal proteins. Further analysis based on antibody staining of central and peripheral nerves revealed beta-adducin, septin 2, and sh3p8 as putative paranodal proteins. We describe the localization of these proteins in relation to other markers of nodes, paranodes, and juxtaparanodes in adult and developing nerve fibers. Finally, we describe their distribution in dysmyelinating TremblerJ mice, a model for the peripheral neuropathy Charcot-Marie-Tooth disease.