UP-REGULATION OF B-50 GAP-43 IN SCHWANN-CELLS AT DENERVATED MOTOR END-PLATES AND IN MOTONEURONS AFTER RAT FACIAL-NERVE CRUSH/

Crush or transection of peripheral nerves of the adult rat is accompan ied by changes in protein expression, including the growth associated protein (GAP-43) B-50. Following peripheral nerve crush in rat enhance d B-50 immunoreactivity was observed in regenerating nerve fibres and in newly formed axon terminals. However, before reinnervation was appa rent, an unexpected transient increase in B-50 immunoreactivity was ob served at denervated motor endplates [J. Neurosci. 8 (1988) 1759]. Thi s study was performed to clarify this observation. Four days following facial nerve crush B-50 immunoreactivity was detected by double immun ofluorescence microscopy in S100-positive Schwann cells covering the d enervated endplates. Using diluted polyclonal and monoclonal B-50 anti bodies we found that B-50 immunoreactivity at the denervated motor end plates was strongly increased in comparison to innervated motor endpla tes in which B-50 immunoreactivity was hardly detectable. However, whe n a high concentration of B-50 antibodies was applied the normal inner vated motor endplates were also B-50 immunoreactive. Muscle fibres did not display B-50 immunoreactivity. Northern blot analysis revealed el evated B-50 mRNA in denervated muscle and in degenerating nerve with r espect to the controls. The B-50 mRNA levels in these non-neuronal tis sues were very low compared to the intact and injured facial nucleus c ontaining the neuronal cell bodies. Electron microscopy demonstrated t hat the B-50 protein was localized in the processes of Schwann cells c overing axon terminals of intact and vacant motor endplates and in axo n varicosities of sympathetic nerves. This study has confirmed that pr ior to reinnervation B-50 immunoreactivity is increased at denervated motor endplates and shows that B-50 is co-localized with S100 in Schwa nn cells. Therefore, upregulation of B-50 expression in Schwann cells may explain the early occurrence of B-50 immunoreactivity at the motor endplate.