Both adult and juvenile tau microtubule-associated proteins are axon specific in the developing and adult rat cerebellum.

Abstract Several antibodies directed against the heterogenous microtubule-associated protein group tau have been used to determine the immunocytochemical localization of these proteins in the developing rat cerebellum. Immunoblot analysis of brain extracts showed that both monoclonal and polyclonal anti-tau antibodies revealed not only the adult tau proteins (50,000–70,000 mol. wt) but also the immature (48,000 mol. wt) tau form. Immunocytochemical studies showed that, whatever the stage of development, anti-tau antibodies stained several types of axonal fibres. The Purkinje cell bodies and their dendrites were never significantly labelled. This means that immature tau is, as adult tau, localized essentially in axons. Axonal labelling seems to follow the cerebellar developmental pattern. For instance, the climbing fibres which reach the cerebellum during the embryonic life were stained soon after birth by the anti-tau antibodies. In contrast, the parallel fibres, that begin to develop perinatally, do not express tau at early (5 days) postnatal stages; a clear labelling of the deeper parallel fibres (which are more mature than the superficial ones) was seen at day 10 after birth in the vicinity of the developing dendrites of the Purkinje cells. This suggests that (1) the appearance of tau immunoreactivity reflects a certain stage of maturity of the parallel fibre; (2) both immature and mature tau microtubule-associated proteins seem to be axon specific in the developing rat cerebellum. Juvenile tau, which is expressed only (Francon et al., 1982, Eur. J. Biochem. 129, 465–471; Drubin et al., 1984, J. Cell Biol. 98, 1090–1097) during the period of intensive neurite outgrowth, might be required to build up the growing axons, whereas microtubules formed with adult tau may contribute to the definition of the cytostructure of stable adult axons.