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 specific mRNA and protein probes, we have used in situ hybridization and immunohistochemical techniques to investigate the distribution of the beta-thymosin mRNAs and their proteins in developing rat cerebellum. Early in postnatal development, both beta-thymosin mRNAs were expressed at highest levels in the postmitotic, premigratory granule cells of the external granular layer; expression diminished as granule cells migrated to and differentiated within the developing internal granular layer. In addition, both beta-thymosin proteins were present in bundles of cerebellar afferent fibers in the white matter at this time. Throughout 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 beta4 was expressed in a small population of cells with microglial morphology scattered throughout the gray and white matter. Thymosin beta10 was detected in an even smaller population of glia. Expression of thymosin beta4 and thymosin beta10 in premigratory granule cells and in growing neuronal processes is consistent with the possibility that both beta-thymosins are involved in the dynamics of actin polymerization during migration and process extension of neurons.