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To determine the function of VEGF-A in nervous system development, we have utilized the Nestin promoter-driven Cre recombinase transgene, in conjunction with a conditional and hypomorphic VEGF-A allele, to lower VEGF-A activity in neural progenitor cells. Mice with intermediate levels of VEGF-A activity showed decreased blood vessel branching and density in the cortex and retina, resulting in a thinner retina and aberrant structural organization of the cortex. Severe reductions in VEGF-A led to decreases in vascularity and subsequent hypoxia, resulting in the specific degeneration of the cerebral cortex and neonatal lethality. Decreased neuronal proliferation and hypoxia was evident at E11.5, leading to increased neuronal apoptosis in the cortex by E15.5. In order to address whether the observed changes in the structural organization of the nervous system were due to a direct and autocrine role of VEGF-A on the neural population, we conditionally inactivated the main VEGF-A receptor, Flk1, specifically in neuronal lineages, by using the Nestin Cre transgene. The normality of these mice ruled out the possibility that VEGF-A/Flk1 signaling has a significant autocrine role in CNS development. VEGF-A dosage is therefore a critical parameter regulating the density of the vascular plexus in the developing CNS that is in turn a key determinant in the development and architectural organization of the nervous system.

A better understanding of the molecular effects of aging in the brain may help to reveal important aspects of organismal aging, as well as processes that lead to age-related brain dysfunction. In this study, we have examined differences in gene expression in the hypothalamus and cortex of young and aged mice by using high-density oligonucleotide arrays. A number of key genes involved in neuronal structure and signaling are differentially expressed in both the aged hypothalamus and cortex, including synaptotagmin I, cAMP-dependent protein kinase C beta, apolipoprotein E, protein phosphatase 2A, and prostaglandin D. Misregulation of these proteins may contribute to age-related memory deficits and neurodegenerative diseases. In addition, many proteases that play essential roles in regulating neuropeptide metabolism, amyloid precursor protein processing, and neuronal apoptosis are up-regulated in the aged brain and likely contribute significantly to brain aging. Finally, a subset of these genes whose expression is affected by aging are oppositely affected by exposure of mice to an enriched environment, suggesting that these genes may play important roles in learning and memory.