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

Abstract

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.

Insulin-Like Growth Factor-I Accelerates the Cell Cycle by Decreasing G1Phase Length and Increases Cell Cycle Reentry in the Embryonic Cerebral Cortex

“…Nestin/IGF-I Tg mice were generated at the University of North Carolina at Chapel Hill, according to a previously published protocol (Popken et al, 2004). Briefly, these mice carried a transgene composed of regulatory elements that included the second intron of the human nestin gene and the minimal promoter of herpes simplex virus immediate early gene ICP4 (infected cell protein 4), the human IGF-IA cDNA fused to a signal sequence from rat somatostatin, and a sequence containing polyadenylation signals and sites derived from the human growth hormone gene.…”

“…All subsequent breeding was with C57BL/6 mice, and all mice used in these studies were derived from 15 or more generations of matings of mice heterozygous for the transgene and C57BL/6 non-Tg normal mice. Expression of the nestin IGF-I transgene was detectable in the CNS as early as E13 and continued into postnatal life (Popken et al, 2004). On E13, transgene expression was evident in the VZ of the developing telencephalic wall, whereas at later ages, transgene expression was greatest in the cortical plate (CP) and cerebral cortex (Popken et al, 2004).…”

“…IGF-I and its cell surface receptor, IGF1R (the type 1 IGF receptor), are expressed throughout the brain during embryonic development (Bondy et al, 1990(Bondy et al, , 1992Ayer-Le Lievre et al, 1991;Bach et al, 1991;Brooker et al, 2000). The capacity of IGF-I to promote neuron progenitor proliferation has been documented in vitro (DiCicco-Bloom and Black, 1988;Torres-Aleman et al, 1990;Drago et al, 1991;Werther et al, 1993;Zackenfels et al, 1995;Arsenijevic et al, 2001) and in vivo Aberg et al, 2000;O'Kusky et al, 2000;Popken et al, 2004), but the influence of IGF-I on cell cycle kinetics in the embryonic brain has not been studied.…”

“…The present study was conducted to investigate the in vivo effects of IGF-I on the cell cycle kinetics of proliferating neuroepithelial cells in the ventricular zone (VZ) of the dorsal telencephalon during embryonic development, using a line of transgenic (Tg) mice (nestin/IGF-I transgenics) that overexpress IGF-I in the brain as early as embryonic day 13 (E13) (Popken et al, 2004). Cumulative S phase labeling with 5-bromo-2Ј-deoxyuridine (BrdU) was used to measure several cell cycle parameters, including the growth fraction [GF (proportion of proliferating cells)], total cell cycle duration [T C (mean length of the mitotic cycle)], and individual phase lengths [T G1 , T S , T G2 , and T M (lengths of the G 1 , S, G 2 , and M phases, respectively)].…”

“…For proliferating neuroepithelial cells in the VZ, the proliferative fraction gradually decreases, whereas the exiting fraction gradually increases, indicating that initially neurons are generated slowly, and the VZ expands rapidly (Takahashi et al, 1994(Takahashi et al, , 1996. Given that the transgene in nestin/IGF-I Tg embryos is expressed as early as E13 (Popken et al, 2004), these mice provide a unique opportunity to study the in vivo role of IGF-I in controlling cell cycle kinetics in the embryonic brain.…”