We investigated the hypothesis that endothelial cells activated by erythropoietin (EPO) promote the migration of neuroblasts. This hypothesis is based on observations in vivo that treatment of focal cerebral ischemia with EPO enhances the migration of neuroblasts to the ischemic boundary, a site containing activated endothelial cells and angiogenic microvasculature. To model the microenvironment within the ischemic boundary zone, we used a coculture system of mouse brain endothelial cells (

Angiogenesis and neurogenesis are coupled processes. Using a coculture system, we tested the hypothesis that cerebral endothelial cells activated by ischemia enhance neural progenitor cell proliferation and differentiation, while neural progenitor cells isolated from the ischemic subventricular zone promote angiogenesis. Coculture of neural progenitor cells isolated from the subventricular zone of the adult normal rat with cerebral endothelial cells isolated from the stroke boundary substantially increased neural progenitor cell proliferation and neuronal differentiation and reduced astrocytic differentiation. Conditioned medium harvested from the stroke neural progenitor cells promoted capillary tube formation of normal cerebral endothelial cells. Blockage of vascular endothelial growth factor receptor 2 suppressed the effect of the endothelial cells activated by stroke on neurogenesis as well as the effect of the supernatant obtained from stroke neural progenitor cells on angiogenesis. These data suggest that angiogenesis couples to neurogenesis after stroke and vascular endothelial growth factor likely mediates this coupling.

The migratory behavior of neuroblasts after a stroke is poorly understood. Using time-lapse microscopy, we imaged migration of neuroblasts and cerebral vessels in living brain slices of adult doublecortin (DCX, a marker of neuroblasts) enhanced green fluorescent protein (eGFP) transgenic mice that were subjected to 7 days of stroke. Our results show that neuroblasts originating in the subventricular zone (SVZ) of adult mouse brain laterally migrated in chains or individually to reach the ischemic striatum. The chains were initially formed at the border between the SVZ and the striatum by neuroblasts in the SVZ and then extended to the striatum. The average speed of DCXeGFP-expressing cells within chains was 28.67±1.04 lm/h, which was significantly faster (P < 0.01) than the speed of the cells in the SVZ (17.98 ± 0.57 lm/h). Within the ischemic striatum, individual neuroblasts actively extended or retracted their processes, suggestive of probing the immediate microenvironment. The neuroblasts close to cerebral blood vessels exhibited multiple processes. Our data suggest that neuroblasts actively interact with the microenvironment to reach the ischemic striatum by multiple migratory routes.

Collectively, the present study suggests that stroke promotes cytokinesis of migrating neuroblasts, and these cells migrate toward the ischemic striatum with distinct migratory behaviors and retain the capacity for cell division during migration.

Neural progenitor cells in the subventricular zone (SVZ) of the lateral ventricular wall give rise to new neurons throughout rodent life. Ischemic stroke induces angiogenesis and neurogenesis. Using laser capture microdissection (LCM) in combination with microarrays containing approximately 400 known genes associated with stem cells and angiogenesis, we investigated gene profiles of SVZ cells in the adult mouse subjected to middle cerebral artery occlusion. Our data revealed that nonstroke SVZ cells expressed sets of genes that are important for neural progenitor cell proliferation, differentiation, and migration. In addition, stroke SVZ cells expressed many genes involved in neurogenesis during embryonic development but were not detected in nonstroke SVZ cells. Stroke upregulated genes were verified by real-time reverse transcriptase-polymerase chain reaction and immunostaining. These data indicate that adult SVZ cells recapture embryonic molecular signals after stroke and provide insight into the molecular mechanisms, which regulate the biological function of neural progenitor cells in the SVZ of adult rodent brain under physiological and stroke conditions.

Ischemic stroke stimulates neurogenesis in the adult rodent brain. The molecules that mediate stroke-induced neurogenesis have not been fully investigated. Using a microarray containing 113 known genes associated with angiogenesis, we analyzed transcriptional profiles in subventricular zone (SVZ) tissue and in cultured neural progenitor cells isolated from the SVZ of adult mice subjected to middle cerebral artery occlusion (MCAo). Among the genes most robustly up-regulated by MCAo were chemokine ligand 2 (CCL2) and chemokine ligand 10 (CXCL10). Consistent with the mRNA data, immunofluorescent staining revealed that MCAo substantially increased the number of CCL2-positive cells in the ipsilateral SVZ and that CCL2-positive cells were positive for both glial fibrillary acidic protein (GFAP) and nestin. In vitro studies showed that incubation of neural progenitor cells with recombinant human CCL2 substantially increased the number of Tuj1-positive cells dose dependently compared with the number in the control group, indicating that CCL2 promotes neuronal differentiation. Blockage of CCL2 with a neutralized antibody against CCL2 abolished the effects of CCL2 on neural progenitor cell migration and differentiation. Treatment of neural progenitor cells with CCL2 did not alter the number of BrdU cells and the number of apoptotic cells compared with those in the control group, suggesting that CCL2 does not affect neural progenitor cell proliferation and cell survival. These data demonstrate that in addition to its role in cell motility, CCL2 plays an important role in neuronal differentiation.

Carbamylated erythropoietin (CEPO), a well characterized erythropoietin (EPO) derivative, does not bind to the classical EPO receptor and does not stimulate erythropoiesis. Using neural progenitor cells derived from the subventricular zone of the adult mouse, we investigated the effect of CEPO on neurogenesis and the associated signaling pathways in vitro. We found that CEPO significantly increased neural progenitor cell proliferation and promoted neural progenitor cell differentiation into neurons, which was associated with up-regulation of Sonic hedgehog (Shh), its receptor ptc, and mammalian achaete-scute homolog 1 (Mash1), a pro-neuron basic helix-loop-helix protein transcription factor. Blockage of the Shh signaling pathway with a pharmacological inhibitor, cyclopamine, abolished the CEPOinduced neurogenesis. Attenuation of endogenous Mash1 expression by short-interfering RNA blocked CEPO-promoted neuronal differentiation. In addition, recombinant mouse Shh up-regulated Mash1 expression in neural progenitor cells. These results demonstrate that the Shh signaling pathway mediates CEPO-enhanced neurogenesis and Mash1 is a downstream target of the Shh signaling pathway that regulates CEPO-enhanced neuronal differentiation.

Recombinant human erythropoietin (rhEPO) induces neurogenesis and angiogenesis. Using a coculture system of mouse brain endothelial cells (MBECs) and neural progenitor cells derived from the subventricular zone of adult mouse, we investigated the hypothesis that neural progenitor cells treated with rhEPO promote angiogenesis. Treatment of neural progenitor cells with rhEPO significantly increased their expression and secretion of vascular endothelial growth factor (VEGF) and activated phosphatidylinositol 3-kinase/Akt (PI3K/Akt) and extracellular signal-regulated kinase (ERK1/2). Selective inhibition of the Akt and ERK1/2 signaling pathways significantly attenuated the rhEPO-induced VEGF expression in neural progenitor cells. The supernatant harvested from neural progenitor cells treated with rhEPO significantly increased the capillary-like tube formation of MBECs. SU1498, a specific VEGF type-2 receptor (VEGFR2) antagonist, abolished the supernatant-enhanced angiogenesis. In addition, coculture of MBECs with neural progenitor cells treated with rhEPO substantially increased VEGFR2 mRNA and protein levels in MBECs. These in vitro results suggest that EPO enhances VEGF secretion in neural progenitor cells through activation of the PI3K/Akt and ERK1/2 signaling pathways and that neural progenitor cells treated with rhEPO upregulate VEGFR2 expression in cerebral endothelial cells, which along with VEGF secreted by neural progenitor cells promotes angiogenesis.