Axonally Transported Peripheral Signals Regulate α-Internexin Expression in Regenerating Motoneurons

Abstract

The class IV neuronal intermediate filament (IF) family proteins includes the neurofilament (NF) triplet proteins NF-L, NF-M, and NF-H and also the more recently characterized α-internexin-NF66. It is well established that NF-L, -M, and -H protein and mRNA are downregulated after peripheral nerve injury. We examined α-internexin protein expression after three facial nerve lesion paradigms: crush, transection, and resection. α-Internexin immunoreactivity was absent in the perikarya of uninjured facial motoneurons but increased dramatically in all three injury paradigms, with maximum immunoreactivity observed at 7 d after injury. Twenty-eight days after nerve crush or transection, there was a dramatic decrease in the number of α-internexin-positive cells. In contrast, α-internexin remained elevated 28 d after nerve resection, an injury that hinders regeneration and target reinnervation. In situ hybridization studies showed an increase in α-internexin mRNA expression in the facial nucleus at 7 and 14 d after injury. Retrograde transport of fluorogold from the whisker pads to the facial nucleus was seen only in motoneurons that lacked α-internexin immunoreactivity, supporting the idea that target reinnervation and inhibitory signals from the periphery regulate the expression of α-internexin. Blockage of axonal transport through local colchicine application induced strong immunoreactivity in motoneurons. α-Internexin expression was also examined after central axotomy of rubrospinal neurons, which constitutively show α-internexin immunoreactivity. After rubrospinal tractotomy, α-internexin immunoreactivity transiently increased by 7 d after injury but returned to control levels by 14 d. We conclude that α-internexin upregulation in injured motoneurons suggests a role for this IF protein in neuronal regeneration.