Changes in cytoskeletal proteins in the rat facial nucleus following axotomy

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Abstract
Changes in L-35S-methionine incorporation into cytoskeletal proteins of the facial nucleus of the rat were studied at various times after unilateral crush or resection of the facial nerve by using 2- dimensional gel electrophoresis and fluorography. We found an increase in labeling of actin and tubulin and a decrease in the 68 kDa and 150 kDa neurofilament polypeptides (200 kDa was not studied). The increase in actin and decrease in neurofilament polypeptide labeling was already significant by 24 hr after nerve resection. These changes were more pronounced after nerve resection than after nerve crush on day 7; actin labeling increased to 270%, and tubulin to 205% of contralateral normal nuclei after resection, whereas both proteins increased to only 165% after crush. Neurofilament labeling decreased to 28% of the contralateral side after resection and to 50% after crush. Immunocytochemistry with a monoclonal antibody to the 150 kDa neurofilament component revealed decreased immunoreactivity in the axotomized facial axons at the inner facial genu, 1 cm proximal to the crush. In contrast, neurofilament immunoreactivity was not decreased in the axotomized perikarya of the facial motoneurons. All changes returned to normal 3 weeks after crush. When axonal regeneration was impeded by nerve resection, incorporation into tubulin remained elevated and into neurofilament proteins remained depressed. Actin returned to normal after either nerve resection or crush. We conclude that the synthesis of tubulin and neurofilament proteins following axotomy is regulated by successful axonal regeneration and/or target contact. Actin synthesis seems to be regulated independently of target- derived factors.