The distribution of tubulin, microtubule-associated protein 2 and Tau in the spinal cords of bullfrog tadpoles during development and after transection was studied. α-Tubulin or β-tubulin immunoreactivity was present in the axons, neuronal perikarya and dendrites, as revealed by immunocytochemistry. The axonal staining intensity of the tubulins in the tadpoles was significantly stronger than that in the adult bullfrog. Microtubule-associated protein 2 immunoreactivity was localized largely to dendrites and expanded from distal to proximal dendrites with time; a high-molecular-weight microtubule-associated protein 2 was seen on the immunoblots of cord homogenates throughout development. Tau1 stained mainly the axons. Two-dimensional gel immunoblotting disclosed that the tadpole contained a greater number of isoforms of Tau than the frog. Complete transection of the spinal cords of stage IV tadpoles was followed by regeneration of the damaged cord region. The levels of tubulin and Tau immunoreactivity in the regenerating axons of the ventral fasciculi were generally increased. Strikingly, microtubule-associated protein 2 immunoreactivity appeared in the regenerating axons and the chromatolytic cell bodies of axotomized motor neurons, paralleling the induction of microtubule-associated protein 2c in the regenerating cord segment shown by immunoblotting. The chromatolytic cell bodies were also markedly labeled by Tau1, whereas the high-molecular-weight microtubule-associated protein 2 diminished on the immunoblots, in accordance with the reduced level of staining for the dendrites. It is apparent that the changes in the cytoskeletal proteins in the regenerating axons mostly recapitulated their developmental patterns. Moreover, the data indicate a close relationship between tubulin and microtubule-associated proteins in axonal growth as well as providing evidence for similar molecular mechanisms underlying successful regeneration for central and peripheral axons.
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