Changes in the localization of NAP-22, a calmodulin binding membrane protein, during the development of neuronal polarity

NAP-22, a neuronal tissue-enriched acidic membrane protein, is a Ca2+-depen dent calmodulin binding protein and has similar biochemical characteristics to GAP-43 (neuromodulin). Recent biochemical studies have demonstrated tha t NAP-22 localizes in the membrane raft domain with a cholesterol-dependent manner. Since the raft domain is assumed to be important to establish and/ or to maintain the cell polarity, we have investigated the changes in the l ocalization of NAP-22 during the development of the neuronal polarity in vi tro and in vivo, using cultured hippocampal neurons and developing cerebell um neurons, respectively. Cultured hippocampal neurons initially extended s everal short processes, and at this stage NAP-22 was distributed more or le ss evenly among them. During the maturation of neuronal cells, NAP-22 was s orted preferentially into the axon. Throughout the developmental stages of hippocampal neurons, the localization change of NAP-22 was quite similar to that of tau, an axonal marker protein, but not to that of microtubule-asso ciated protein-2 (MAP-2), a dendritic marker protein. Further confocal micr oscopic observation demonstrated the colocalization of NAP-22 and either ta u or vesicle-associated protein-2 (VAMP-2). A comparison of the lime course of the axonal localization of NAP-22 and GAP-43 showed that NAP-22 localiz ation was much later than that of GAP-43. The correlation between the expre ssion of NAP-22 and synaptogenesis in the cerebellar granular layer, partic ularly in the synaptic glomeruli, was also investigated. There existed many VAMP-2 positive synapses but no NAP-22 positive ones in 1-week-old cerebel lum. On sections of 2-week-old cerebellum, accumulation of NAP-22 to the sy naptic glomeruli was clearly observed and this accumulation became clearer during the maturation of the synaptic structure. The present results sugges t the possibility that NAP-22 plays an important role in the maturation and /or the maintenance of synapses rather than in the process of the axonal ou tgrowth, by controlling cholesterol-dependent membrane dynamics. (C) 2000 E lsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved.