Insulin-like growth factor-I overexpression attenuates cerebellar apoptosis by altering the expression of Bcl family proteins in a developmentally specific manner

In studies of transgenic (Tg) mice that overexpress insulin-like growth fac tor-I (IGF-I) exclusively in the CNS, we demonstrated a dramatic increase i n cerebellar granule cell number that appeared to be attributable predomina ntly to enhanced survival. IGF-I anti-apoptotic actions are well establishe d in cultured neurons, but comparable studies in vivo are few. Using the sa me Tg mice, therefore, we set out to document IGF-I anti-apoptotic effects during cerebellar development and to probe IGF-I signaling mechanisms. Comp ared with cerebella (CBs) of non-Tg littermates, those of Tg mice had fewer apoptotic cells at postnatal day 7 (P7) and showed a similar tendency at P 14 and P21. At each age studied, procaspase-3 and caspase-3 were decreased in CBs of Tg mice. The caspase-3 decline was accompanied by decreases in th e 85 kDa fragment of Poly( ADP-ribose) polymerase, a known product of caspa se cleavage, suggesting decreased caspase activity. At P7 decreased apoptos is in Tg mice was associated with increased expression of the anti-apoptoti c Bcl genes, Bcl-x(L) and Bcl-2. The mRNA expression of the proapoptotic Bc l genes, Bax and Bad, also was increased, but no changes were observed in t he abundance of their proteins. At P14 Bcl-xL and Bcl-2 expression were sim ilar in normal and Tg mice; Bax mRNA was unchanged in Tg mice, but its prot ein abundance was decreased, and both Bad mRNA and protein abundance were d ecreased. At P21 Bcl-xL and Bcl-2 expression were unchanged, but Bax and Ba d expression were decreased. Our data show that IGF-I exerts anti-apoptotic actions during cerebellar development, and thereby alters the magnitude of naturally occurring apoptosis. IGF-I appears to affect multiple steps in t he apoptotic pathway in a developmentally specific manner. IGF-I decreases caspase-3 availability and activity, increases the expression of anti-apopt otic Bcl-xL and Bcl-2 during early postnatal development, and decreases pro apoptotic Bax and Bad expression at later developmental stages.