In vivo effects of insulin-like growth factor-I (IGF-I) on prenatal and early postnatal development of the central nervous system.

The in vivo actions of insulin-like growth factor-I (IGF-I) on prenatal and early postnatal brain development were investigated in transgenic (Tg) mice that overexpress IGF-I prenatally under the control of regulatory sequences from the nestin gene. Tg mice demonstrated increases in brain weight of 6% by embryonic day (E) 18 and 27% by postnatal day (P) 12. In Tg embryos at E16, the volume of the cortical plate was significantly increased by 52% and total cell number was increased by 54%. S-phase labeling with 5-bromo-2'-deoxyuridine revealed a 13-15% increase in the proportion of labeled neuroepithelial cells in Tg embryos at E14. In Tg mice at P12, significant increases in regional tissue volumes were detected in the cerebral cortex (29%), subcortical white matter (52%), caudate-putamen (37%), hippocampus (49%), dentate gyrus (71%) and habenular complex (48%). Tg mice exhibited significant increases in the total number of neurons in the cerebral cortex (27%), caudate-putamen (27%), dentate gyrus (69%), medial habenular nucleus (61%) and lateral habenular nucleus (36%). In the cerebral cortex and subcortical white matter of Tg mice, the total numbers of glial cells were significantly increased by 37% and 42%, respectively. The numerical density of apoptotic cells in the cerebral cortex, labeled by antibodies against active caspase-3, was reduced by 26% in Tg mice at P7. Our results demonstrate that IGF-I can both promote proliferation of neural cells in the embryonic central nervous system in vivo and inhibit their apoptosis during postnatal life.

Down-regulation of 14-3-3 eta gene expression by IGF-I in mouse cerebellum during postnatal development.

Insulin-like growth factor I (IGF-I) overexpression in the postnatal cerebellum of transgenic (Tg) mice results in remarkable cerebellar overgrowth characterized by a near doubling of granule cell number that is predominantly due to inhibition of apoptosis. Using this Tg model we set out to investigate IGF-I anti-apoptotic mechanisms by defining the influence of IGF-I on gene expression. Using a cDNA array technique, we screened a total of 243 mouse apoptosis-related genes, and found that 14-3-3 eta gene expression was significantly reduced in the cerebella of Tg mice compared with their wild-type (Wt) littermates. Using Northern blot analysis to corroborate our microarray finding, we showed that 14-3-3 eta mRNA abundance was decreased from postnatal day P5 through P17. Nonetheless, the expression pattern of 14-3-3 eta in Tg mice followed the same pattern observed in Wt mice, and was indistinguishable from that in Wt mice at P20 and P23. 14-3-3 eta protein abundance, as determined by Western immunoblot analyses, showed similar decreases in the cerebella of Tg mice. In situ hybridization demonstrated that 14-3-3 eta was predominantly, if not exclusively, expressed and regulated in Purkinje cells. 14-3-3 proteins have multiple functions, including participation in pathways that favor cell survival. Our finding of IGF-I-induced down-regulation of 14-3-3 eta expression in Purkinje cell at a time when IGF-I promotes granule cell survival leads us to speculate that down-regulation of 14-3-3 eta may: (a) serve a negative feedback role to modulate Purkinje cell survival, i.e. limit Purkinje cell number, and/or (b) function as part of a distinct signaling mechanism, perhaps one that augments the capacity of Purkinje cells to promote granule cell survival.