Vascular endothelial growth factor (VEGF) increases production, migration and survival of neural stem cells during hypoxia and ischemia

Background and aims: Neurons are susceptible to hypoxia and ischemia. However, endogenous adaptive responses aim at protecting the tissue from hypoxic-ischemic injury. Angiogenesis and neuroprotection are two such adaptive responses, and both processes seem to be governed by the same hypoxia-induced growth factors, of which vascular endothelial growth factor (VEGF) is a prominent example. VEGF is upregulated during hypoxia and ischemia and improves outcome after stroke. New neurons are generated continuously in the subventricular zone (SVZ) and dentate gyrus of the adult brain. Neuropathological processes, including cerebral ischemia, can enhance neurogenesis, as can growth factors and other physiological stimuli. VEGF has been implicated in the hypoxia-induced regulation of neural stem cells (NSC), but it is unknown whether VEGF can enhance migration of newborn neurons toward sites of ischemic injury, where they might be able to replace neurons that undergo ischemic death. Methods: We previously generated transgenic mice, overexpressing human VEGF165 under the neuron-specific enolase promoter (V1 mice) [1-3]. V1 mice were subjected to middle cerebral artery occlusion (MCAO). Cell proliferation and neurogenesis were assessed with bromodeoxyuridine (Brdu) labeling and immunostaining for cell type-specific markers. Effects of VEGF on differentiation, proliferation and apoptosis of isolated NSC were analyzed in vitro. Results: In V1 mice, brains examined 7-28 days after MCAO showed markedly increased SVZ neurogenesis, chains of neuroblasts extending from the SVZ to the peri-infarct cortex, and an increase in the number of newly generated cortical neurons at 14-28 days after ischemia. In concert with these effects, VEGF overexpression reduced infarct volume and improved postischemic motor function. NSC isolated from the SVZ zone of V1 mice expressed more VEGF and differentiated faster into more mature neuronal cells. Furthermore, VEGF overexpression promoted NSC survival and proliferation under hypoxic conditions and protected NSC from apoptosis, induced by growth factor depletion. Finally, V1 NSC showed an increased phosphorylation of p44/42 MAP kinase and Akt, accompanied by reduced apoptosis under hypoxic conditions which was reversed when the PI3-kinase/Akt pathway was blocked by Wortmannin. Conclusions: Our results indicate VEGF as a trophic factor for neurogenesis and neuromigration in the adult brain after cerebral ischemia in vivo and for NSC in vitro involving an Akt/PI3-kinase dependent mechanism. Our data suggest that in addition to its neuroprotective effects, which are associated with improved outcome in the acute phase after cerebral ischemia, VEGF enhances postischemic neurogenesis, which could provide a therapeutic target for more chronic brain repair.