Expression and regulation of the LIM-class homeobox gene rlim-1 in neuronal progenitors of the rat cerebellum

To investigate LIM gene function in the rat cerebellar system, we analyzed expression and regulation of the rat homologue of frog Xlim-1 (rlim-1) in v ivo and in cultured cells. In developing cerebellum, peak levels of rlim-1 mRNA at postnatal day 8 (p8) are coincident with the peak period of granule cell proliferation. Analysis of rlim-1 protein with a specific antibody sh owed that expression was also maximal at p8, In situ hybridization showed t hat at p8 rlim-1 mRNA was expressed in Purkinje and granule cells. Both the proliferative and the premigratory granule cells in the external germinal zone displayed high levels of rlim-1 mRNA expression. Immunocytochemical st aining demonstrated that at p8 rlim-1 protein was also present in prolifera tive and premigratory granule cells, In adult cerebellum (p30), rlim-1 mRNA and protein expression in granule cells was strongly attenuated. The down- regulation of rlim-1 mRNA occurred in granule cells just after the time of final division, coinciding with the onset of their migration. rlim-1 protei n was detected in migratory granule neurons. The developmental decrease in rlim-1 mRNA and protein found in vivo was reproduced in pure cerebellar gra nule cell cultures. In these cultures, granule neurons were postmitotic I d ay after plating but still displayed high levels of rlim-1 protein expressi on up to 3 days in vitro. Our findings indicate that 1) rlim-1 is likely to act in concert with other genes to specify granule cell fate, 2) rlim-1 ex pression in granule neurons is regulated autonomously, and 3) rlim-1 protei n may also play an important role in granule neuron differentiation and sur vival. Published 2001 Wiley-Liss, Inc.dagger