Timing of Expression of r and Its Encoding mRNAs in the Developing Cerebral Neocortex and Cerebellum of the Mouse

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Abstract
The expression of .tau. mRNA and of the corresponding encoded protein variants was studied during postnatal development in two brain regions differing in their timing of differentiation; the cerebral neocortex and the cerebellum. (a) The expression of .tau. mRNA was different in the two regions. Maximal contents were found at early stages in the cerebral neocortex, with a 10-fold decrease at later stages. In the cerebellum, two peaks of .tau. mRNA were observed soon after birth and in adulthood, with minimal values at postnatal day 6. (b) The expression of total .tau. proteins was similar to that of their encoding mRNAs in the cerebral neocortex, i.e., high concentrations after birth and low contents at later stages. In contrast, two peaks of .tau. proteins were observed in the cerebellum: the first perinatally and the second with a maximum at postnatal day 15. (c) Both in the cerebral neocortex and especially in the cerebellum, increasing concentrations of mature .tau. variants were expressed at late developmental stages, i.e., when total .tau. protein contents were decreased. In conclusion, the fluctuations in expression of .tau. and of its encoding mRNA seen in the cerebellum seem to reflect differences in the timing of differentiation of the various cell types, i.e., the macroneurons and the interneurons, present in this brain region. The adult .tau. variants appear in both the neocortex and the cerebellum only at late developmental stages, i.e., when most of the circuitry has been established, although these two regions markedly differ in their timing of differentiation.