Expression of Mcl-1 in cerebellar granule neurons is regulated by IGF-I in a developmentally specific fashion.

Transgenic (Tg) mice that overexpress IGF-I during postnatal brain development exhibit remarkable cerebellar overgrowth characterized by significant increases in granule cell number that is predominantly due to IGF-I anti-apoptotic actions. Using these mice as a model to define the gene expression profile underlying the pro-survival actions of IGF-I, we screened 243 apoptosis-related genes by cDNA arrays and found that Mcl-1 was down-regulated in cerebella of IGF-I Tg mice. Contrary to the results obtained by cDNA array, Northern blot analyses showed that the Mcl-1 mRNA abundance in the cerebella of IGF-I Tg mice at postnatal day 14 (P14) was five times more than that of wild-type (Wt) controls. The increase in Mcl-1 mRNA expression in IGF-I Tg mice was detected as early as P8, peaked at P14, and remained detectable at P20. Both IGF-I Tg and Wt mice showed a similar expression pattern of Mcl-1 mRNA which coincided with the post-mitotic migration and the post-migratory maturation of granule cells. We measured the relative abundance of Mcl-1 protein in the cerebellum by immunoblots and found that anti-apoptotic Mcl-1L was the predominant form, while pro-apoptotic Mcl-1S was minimally detectable. Cerebellar Mcl-1L was 2.6-fold more abundant in IGF-I Tg mice compared with that in their Wt littermates. Using laser capture microdissection followed by RT-PCR, we determined that Mcl-1 mRNA was expressed in granule cells, but not in Purkinje cells. In summary, these findings show that the anti-apoptotic Mcl-1 isoform is expressed in cerebellar granule neurons, which undergo apoptosis during postnatal cerebellar cortical lamination, and Mcl-1 expression is up-regulated by IGF-I overexpression in a developmentally specific manner. These data suggest that anti-apoptotic Mcl-1 may mediate IGF-I pro-survival actions on granule neurons during the development of cerebellar cortex. They also point out pitfalls of cDNA array analyses.