Retinoic acid (RA) is a member of the retinoid family of lipids and the active metabolite of vitamin A. Within the brain, its role as a neural differentiation factor and a regulator of synaptic plasticity is very well established. Excess and lack of Vitamin A or RA is associated with malformations in the developing brain as well as defects in higher cognitive functions in the adult brain. These abnormalities likely not only result from functional defects in neurons, but may be a consequence of an effect of RA on adjacent cell types other than neurons. How RA influences cell types other than neurons is not fully understood. In this dissertation, I have identified novel roles of RA in 1) regulating blood vessel development in the embryonic brain and 2) neural stem and progenitor proliferation in the adult brain.

I began my thesis work aiming to elucidate the role of RA in regulating angiogenesis in the developing forebrain, specifically the cerebral cortex. During cortical development, RA comes from a tissue covering the brain, the meninges. Utilizing Foxc1 mouse mutants, which lack the meninges, I demonstrate that lack of RA is a major contributing factor to the cerebrovascular growth defects seen in these mutants. Furthermore, I went on to demonstrate that meningeal-derived RA ensures adequate growth of the neocortical vasculature by regulating expression of WNT pathway proteins and neural progenitor derived VEGF-A. These findings offer the first evidence for a role of meningeal-derived RA in brain vascular development. Importantly, patients with Foxc1 mutations display meningeal defects similar to the mouse model system. My research may provide new insight into potential causes of the cerebrovascular defects in this patient population.

The second part of my thesis work involved understanding the function of RA in the adult brain. RA signaling is widespread in neural stem cells during embryonic brain development, becoming progressively more restricted to specialized neurogenic regions in the adult brain. One of these adult neurogenic regions is the hippocampus, where new neurons are generated throughout adulthood in both mice and humans. Although modulation of this process by RA has been studied extensively, the results are conflicting. The signaling pathways downstream of RA, as well as the question whether RA acts cell autonomously in neural stem and progenitor cells (NSPCs) to regulate neurogenesis, have not been resolved. My research aimed to clarify the role of RA in vivo, using genetic disruption of RA signaling in NSPCs or inhibition of synthesis of RA in the adult hippocampus. Using transgenic RA- signaling reporter mice, compounds to manipulate RA in vitro and in vivo, and transgenic mice conditionally expressing a dominant-negative RA receptor, I showed that RA signaling modulates hippocampal neurogenesis by regulating NSPC proliferation. Additionally, I delineated a novel pathway linking RA/HIF1alpha/VEGF signaling as a mechanism for the regulation of NSPC proliferation in the adult hippocampus. These studies demonstrate, for the first time, a cell autonomous role for RA signaling in hippocampal NSPCs that substantially broadens RA's function beyond its well described role in neuronal differentiation in the adult brain. All in all, my thesis work revealed previously unexplored roles of RA in regulating processes important to maintain a healthy brain both embryonically and in the adult. This knowledge of RA's widespread functions will aid in the development of RA as a therapeutic target for the treatment of embryonic and adult brain disorders which show defects in either blood vessel formation or adult neurogenesis.