Cell replacement therapy after stroke by recruitment of endogenous neural progenitors

Central nervous system injuries, such as stroke, lead to significant neuronal death in vulnerable regions of the brain, accompanied by corresponding loss of neuronal function. Recent evidence that neural stem cells or progenitors still persist in the adult mammalian brain has provided new possibility to repair such neuronal damage by recruitment of these progenitors. Here, we sought whether such neuronal regeneration could be achieved from endogenous neural progenitors in the hippocampal CA1 region following global ischemia. Rats were subjected to 6 minute global ischemia using 4—vessel occlusion and hypotension, and the growth factors (EGF and FGF-2) were infused into the ventricle from Day 2 to Day5 to augment the endogenous capacity. At Day 7, severe neuronal loss was observed even after the treatment, where only 2% of cells remained. Though these growth factors may have neuroprotective action, the treatment initiated from Day2 did not show neuroprotection. At Day 28, however, a significant increase in number of neurons, stained with cresyl-violet and neuronal marker NeuN, were observed in the treated animals, which amounted to 40 % of normal controls. These neurons were positively stained by BrdU after labeling during the treatment, indicating that they are newly produced neurons. Immunostaining for BrdU, progenitor markers (Pax 6, Mash 1), and DiI/retrovirus labeling, indicated that these newly produced neurons are derived from dormant neural progenitors residing in the paraventricular region above the CA1 field, a region that has not been recognized to harbor such progenitors. Morphologically, the regenerated neurons made synaptic connections with the existing neuronal circuit, as evidenced from electron microscopic studies. Electrophysiologically, fEPSP, which were lost after ischemia, was partially recovered, as well as long-term potentiation. In behavioral study using the water maze task, the treated animals exhibited higher performance compared with the untreated animal. Collectively, these data showed that dormant neural stem cells or progenitors exist in the periventricular region, where neurogenesis does not normally occur physiologically in the mature adult brain. Once neuronal death after ischemia takes place, these progenitors respond to the insult. However, endogenous capacity of this response is limited in the adult brain. Augmentation of the proliferation by growth factors after ischemia is capable of potentiating this endogenous capacity to replace the lost neurons, which also led to improved neuronal function. The data presented here showed a remarkable capacity of adult neural progenitors to repair ischemic injury in the hippocampal CA1. Accumulating evidence from other studies revealed that, in addition to classical neurogenic regions (subgranular layer of the hyppocampal dentate gyrus and anterior subventricular zone), dormant or slowly proliferating neural stem or progenitors exist widely, such as around the third ventricle and the midbrain aqueduct. Recruitment of these progenitors to repair various ischemic injury may provide a new therapeutic approach for stroke.