Neuron-enriched acidic protein having a molecular mass of 22 kDa, NAP-22, is a Ca(2+)-dependent calmodulin-binding protein and is phosphorylated with protein kinase C (PKC). This protein is localized to the biological membrane via myristoylation and found in the membrane fraction of the brain and in the synaptic vesicle fraction. Recent studies showed that NAP-22 is localized in the membrane raft domain in a cholesterol-dependent manner and suggest a role for NAP-22 in maturation and/or maintenance of nerve terminals by controlling cholesterol-dependent membrane dynamics. The present study revealed the immunohistochemical distribution of NAP-22 in the peripheral nerves in rat muscles. In all examined muscles, nerve terminals in the motor endplates showed NAP-22 immunoreactivity associated with the membranes of synaptic vesicles and nerve terminals. In the muscle spindles, annulospiral endings, which made spirals around the intrafusal muscles, showed intense NAP-22 immunoreactivity. Autonomic nerve fibers around the intramuscular blood vessels also showed the immunoreactivity for NAP-22. NAP-22 immunoreactivity in these peripheral nerves was observed from birth to adulthood (100 days after birth). Though growth-associated protein-43 (GAP-43) immunoreactivity in these nerves was observed from birth, this immunoreactivity decreased from 20 days after birth. These findings suggest that NAP-22 is distributed and regulates functions in the motor, sensory and autonomic nerve terminals in the peripheral nervous system.
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