ALTERATIONS IN ACTIN-BINDING BETA-THYMOSIN EXPRESSION ACCOMPANY NEURONAL DIFFERENTIATION AND MIGRATION IN RAT CEREBELLUM

The beta4- and beta10-thymosins, recently identified as actin monomer- sequestering proteins, are developmentally regulated in brain. Using s pecific mRNA and protein probes, we have used in situ hybridization an d immunohistochemical techniques to investigate the distribution of th e beta-thymosin mRNAs and their proteins in developing rat cerebellum. Early in postnatal development, both beta-thymosin mRNAs were express ed at highest levels in the postmitotic, premigratory granule cells of the external granular layer; expression diminished as granule cells m igrated to and differentiated within the developing internal granular layer. In addition, both beta-thymosin proteins were present in bundle s of cerebellar afferent fibers in the white matter at this time. Thro ughout the maturation period, both proteins were present in elongating parallel fibers in the upper portion of the molecular layer. Later in cerebellar development, thymosin beta4, but not thymosin beta10, was expressed in Golgi epithelial cells and Bergmann processes. Thymosin b eta4 was expressed in a small population of cells with microglial morp hology scattered throughout the gray and white matter. Thymosin beta10 was detected in an even smaller population of glia. Expression of thy mosin beta4 and thymosin beta10 in premigratory granule cells and in g rowing neuronal processes is consistent with the possibility that both beta-thymosins are involved in the dynamics of actin polymerization d uring migration and process extension of neurons.