PERIAXIN EXPRESSION IN MYELINATING SCHWANN-CELLS - MODULATION BY AXON-GLIAL INTERACTIONS AND POLARIZED LOCALIZATION DURING DEVELOPMENT

Periaxin is a newly described protein that is expressed exclusively by myelinating Schwann cells. In developing nerves, periaxin is first de tected as Schwann cells ensheathe axons, prior to the appearance of th e proteins that characterize the myelin sheath. Periaxin is initially concentrated in the adaxonal membrane (apposing the axon) but, during development, as myelin sheaths mature, periaxin becomes predominately localized at the abaxonal Schwann cell membrane (apposing the basal la mina). In permanently axotomized adult nerves, periaxin is lost from t he abaxonal and adaxonal membranes, becomes associated with degenerati ng myelin sheaths and is phagocytosed by macrophages. In crushed nerve s, in which axons regenerate and are remyelinated, periaxin is first d etected in the adaxonal membrane as Schwann cells ensheathe regenerati ng axons, but again prior to the appearance of other myelin proteins. Periaxin mRNA and protein levels change in parallel with those of othe r myelin-related genes after permanent axotomy and crush. These data d emonstrate that periaxin is expressed by myelinating Schwann cells in a dynamic, developmentally regulated manner. The shift in localization of periaxin in the Schwann cell after completion of the spiralization phase of myelination suggests that periaxin participates in membrane- protein interactions that are required to stabilize the mature myelin sheath.