Signaling by bone morphogenetic proteins and Smad1 modulates the postnatal differentiation of cerebellar cells

Previous studies have demonstrated that bone morphogenetic proteins (BMPs) activate the Smad1 signaling pathway to regulate cell determination and differentiation in the embryonic nervous system. Studies examining gene and protein expression in the rat cerebellum suggest that this pathway also regulates postnatal differentiation. Using microarrays, we found that Smad1 mRNA expression in the cerebellum increases transiently at postnatal day 6 (P6). Immunohistochemistry and Western blots showed that Smad1 and BMP4 proteins are present in the cerebellum, and that their expression also changes postnatally. The proteins are detectable at P4-P6, a stage at which most cerebellar cells reside in the external germinal layer (EGL), where they extensively differentiate. The levels become maximal at P8-P10, when neurons begin to migrate from the EGL into their mature positions in the internal granule layer. In cerebellar cultures prepared at P6 or P10, BMP4 activates Smad1 signaling to modulate cell differentiation. Brief BMP4 application caused Smad1 translocation from the neuronal cytoplasm into the nucleus, where it is known to regulate transcription in association with Smad4. Longer BMP4 treatment promoted the differentiation of both neuronal and non-neuronal cells. By 3 d, neuronal processes appeared more fasciculated, and the level of synaptotagmin, a protein found in synaptic vesicles, increased. In addition, many astroglial cells became more branched and stellate in morphology. The BMP-induced changes were reduced by treatment with antisense oligonucleotides to Smad1 or Smad4. These findings in vivo and in culture suggest that BMP4 and Smad1 signaling participate in regulating postnatal cerebellar differentiation.