Neurogenesis and transplantation in stroke-damaged brain

Abstract: Stroke in young adult rodents triggers neurogenesis in the damaged striatum and intact hippocampus. as stroke happens frequently in aged individuals it is of great importance to know whether aging affects brain recovery mechanisms. Aged and young adult rats were subjected to stroke by transient occlusion of the middle cerebral artery and proliferating cells were labeled by intraperitoneal administration of the mitotic marker BrdU. Animals were sacrificed at 7 weeks after infarct and new cells were examined for expression of BrdU and neuronal and glial markers with epifluorescent and confocal microscopy. Young and aged rats showed similarly increased number of new neurons in the striatum, although basal proliferation was reduced in the aged subventricular zone. In contrast, both basal proliferation and the generation of new neurons was significantly reduced in aged subgranular zone and granule cell layer of the hippocampus. This study shows that basal neurogenesis is impaired in the aged rat brain compared to young, but the brain responds to damage with increased neurogenesis. This increase was similar in the striatum of both young and old animals, indicating the existence of potential self-repair mechanisms in the aged brain. Stem cell transplantation is considered one of the future therapeutic methods for treatment of stroke. Therefore identification and characterization of viable neural stem cell lines is essential for the development of successful therapies. Neural stem cells (NSCs) isolated from fetal human striatum and cortex were studied and compared for their neurogenic capacity in vitro and after transplantation in neonatal intact or adult, stroke-damaged brain. Cortex- and striatum-derived NSCs expanded as neurospheres did not differ in proliferative capacity, growth rate, secondary sphere formation, and expression of general neural markers. However, whereas cortical NSCs produced higher number of glutamatergic and tyrosine hydroxylase- and calretinin-positive neurons, several-fold more neurons expressing the striatal projection neuron marker, DARPP-32, were observed in cultures of striatal NSCs. Human cortical and striatal NSCs survived and migrated equally well after transplantation in neonatal rats. The two NSC types also generated similar numbers of mature NeuN+ neurons, which were several-fold higher at 4 months as compared to at 1 month after grafting. At 4 months, the grafts contained cells with morphological characteristics of neurons, astrocytes, and oligodendrocytes, the majority of neurons expressing parvalbumin. Striatal and cortical NSCs exhibited similar robust survival (30%) at 1 month after transplantation in stroke-subjected animals and migrated throughout the damaged striatum. Striatal NSCs migrated longer distance and occupied a bigger volume of striatum. In the transplantation core, cells were undifferentiated, virtually all expressing cellular markers of immature neural lineage such as nestin, and to lesser extent also GFAP, ?III-tubulin, DCX and calretinin. Immunocytochemistry with proliferation markers (p-H3 and Ki67) revealed that grafted striatal and cortical NSCs cease to proliferate. Human cells outside the transplantation core differentiated, exhibited mature neuronal morphology and expressed adult neuronal markers such as HuD, calbindin and parvalbumin. Interestingly, striatal NSCs generate greater number of parvalbumin+ and calbindin+ neurons and virtually none of the grafted cells differentiated into astrocytes or oligodendrocytes. Based on these data, human fetal striatum- and cortex-derived NSCs could be considered safe and viable sources with strong neurogenic potential for further exploration in animal models of stroke.