Post-stroke neurogenesis and the neurovascular niche: Newly born neuroblasts localize to peri-infarct cortex in close association with the vascular endothelium

Abstract

Stroke induces a process of neurogenesis, whereby newly born neuroblasts migrate to areas of injury. This process may provide for endogenous neural replacement after stroke if properly harnessed. Previous studies have defined post-stroke neurogenesis using rat models of large hemispheric stroke, in which the infarct borders the subventricular zone or adjacent medial striatum. We used a model of small cortical stroke in the mouse to study the molecular mechanisms of long distance post-stroke neuroblast migration and localization to peri-infarct cortex. Systemic BrdU injections (i.p.) label doublecortin-and PSA-NCAM-positive neuroblasts predominantly in white matter at day 3 and predominantly in cortex at 7 day post-stroke. BrdU injections into the subventricular zone (SVZ) label doublecortin-positive neuroblasts migrating through white matter to peri-infarct cortex. Stereological quantification of doublecortin-positive neuroblasts in peri-infarct cortex shows that there are no doublecortin positive cells in control cortex, 468+/-231 neuroblasts at day 3 and 4304+/-1545 doublecortin positive neuroblasts by day 7 post-stroke. By three weeks post-stoke there are many cells that express NeuN that are BrdU positive. These are being quantified. These data show that stroke induces a robust process of migration of newly born neuroblasts from SVZ to peri-infarct cortex over the first week post-stroke, and that a portion of these cells survive and express more mature neuronal markers. In the SVZ neuroblasts develop within close relationship to vascular endothelial cells in an environment termed the neurovascular niche. Because stroke alters the molecular makeup of peri-infarct microvasculature, we hypothesized that stroke induces neuronal migration and localization of neuroblasts through the induction of a novel neurovascular niche in peri-infarct cortex. Mice were given a stroke and allowed to survive 5 days, an intermediate time point for neuroblast migration in this model. Brain sections were stained for two markers of migratory neuroblasts, doublecortin and PSA-NCAM; and three markers of the vascular endothelium: laminin, PE-CAM and intravascularly-perfused lycopersicon esculentum (tomato lectin). Laminin staining visualizes the basal lamina surrounding blood vessels. PE-CAM is expressed by endothelial cells. Tomato lectin, when administered intravascularly prior to sacrifice, labels all endothelial cells that receive blood flow. Neuroblasts closely associate with vascular endothelial cells. Confocal microscopy with three dimensional reconstruction of image stacks shows that neuroblasts wrap around vascular endothelial cells in peri-infarct cortex. In sections stained for PE-CAM, and visualized with intravascular tomato lection, doublecortin-positive neuroblasts associate with blood vessels at the border of normally perfused and hypoperfused cortical tissue near the infarct. At this site, neuroblasts express alpha v integrin, a receptor for laminin, and Tie-2, an angiopoietin receptor. These data indicate that stroke induces neuroblast migration to peri-infarct cortex in close association with vascular endothelial cells in a microenvironment that involves specific vascular growth factors and adhesion molecules.