Regulation of the expression and phosphorylation of microtubule-associatedprotein 1B during regeneration of adult dorsal root ganglion neurons

Microtubule-associated protein 1B is a major constituent of the neuronal cy toskeleton during the early stages of development. This protein and its pho sphorylated isoform, microtubule-associated protein 1B-P, defined by the mo noclonal antibody IB-P [Boyne L. J. et al. (1995) J. Neurosci. Res. 40,439- 450], are present in growing axons and concentrated in the distal end near the growth cone. In most regions of the central nervous system, microtubule -associated protein 1B and microtubule-associated protein IB-P are developm entally down-regulated. They remain, however, at relatively high levels in the adult peripheral nervous system, where microtubule-associated protein 1 B-P is localized exclusively in axons. The aim of this study was to examine the levels of microtubule-associated protein 1B and its phosphorylated iso form during regenerative growth of peripheral axons, Following transection and re-apposition of the sciatic nerve at midthigh, the levels of total mic rotubule-associated protein 1B, microtubule-associated protein 1B-P and mic rotubule-associated protein 1B messenger RNA were analysed in dorsal root g anglion neurons and sciatic nerve axons using western blots and RNase prote ction assays. After the lesion, there was a small decrease in the levels of microtubule-associated protein 1B and its messenger RNA in dorsal root gan glion neurons. The proximal axonal stump showed a similar decrease in the l evels of microtubule-associated protein 1B 30 days after lesion and returne d to normal 60-90 days post-lesion. In the distal stump of the sciatic nerv e, the levels of microtubule-associated protein 1B increased dramatically a nd rapidly between three and 14 days, but the protein was localized mainly in activated Schwann cells and myelin-like structures, and not in axons [Ma D. et at. (1999) Brain Res. 823, 141-153]. With the regeneration of axons into the distal stump, an intense expression of microtubule-associated prot ein 1B was observed in these axons. Microtubule-associated protein IB-P, ho wever, disappeared from the degenerated distal axonal stump as early as thr ee days post-operation, and was absent in the regenerating axons and in Sch wann cells between three and 14 days. The levels of microtubule-associated protein 1B-P recovered slowly and did not reach the normal levels even afte r 90 days post-operation. In contrast to the response following transection , the levels of microtubule-associated protein 1B and microtubule-associate d protein IB-P were much less affected after nerve crush. We propose that the relatively high levels of microtubule-associated protei n 1B and its messenger RNA in adult dorsal root ganglions support periphera l neuron regeneration. The presence of microtubule-associated protein 1B in the regenerating axons suggests that microtubule-associated protein 1B is involved in axonal growth during peripheral nerve regeneration. However, th e phosphorylated microtubule-associated protein 1B-P isoform, associated wi th growing axons during development, is not present in the regenerating axo ns after transection, presumably because of changes in the activities of ki nases and phosphatases associated with the injury. These observations under score the difference between axonal development and regeneration and the im portance of injury-related effects that occur locally. (C) 2000 IBRO. Publi shed by Elsevier Science Ltd. All rights reserved.