New striatal neurons form projections to substantia nigra in adult rat brain after stroke.

Previous studies have demonstrated that newborn striatal neurons can functionally integrate with local neural networks in adult rat brain after injury. In the present study, we determined whether these newly generated striatal neurons can develop projections to the substantia nigra, a target of striatal projection neurons. We used 5'-bromodeoxyuridine (BrdU) and a retroviral vector expressing green fluorescent protein (GFP) combined with multiple immunostaining labels of newborn striatal neurons, and nigral microinjection of fluorogold (FG) to trace the striatonigral projection in adult rat brain at different weeks following a transient middle cerebral artery occlusion (MCAO). We found that FG positive (FG(+)) cells could be detected in newly generated neurons (BrdU(+)-NeuN(+) and GFP(+)-NeuN(+)) in ipsilateral striatum clearly at 12, but not 2 weeks after MCAO. The data suggest that ischemia-induced newborn striatal projection neurons could form long axons that targeted the substantia nigra (striatonigral projection pathway) and that have intact axonal transport from the nerve terminal to cell body. These new striatal neurons express glutamate NR2 and dopamine D2L receptors, which form the molecular basis for responding to the inputs from cortical glutamatergic and nigral dopaminergic projection neurons. Our data provide the first morphological evidence that newborn neurons in the striatum, a non-neurogenic region, can establish new striatonigral neural circuits, important pathways for the maintenance of motor function. These results help us to understand endogenous cellular mechanisms of brain repair, and suggest that increasing adult neurogenesis could be a practical strategy for enhancing the efficacy of rehabilitative therapy in stroke patients.

Neurogenic effect of VEGF is related to increase of astrocytes transdifferentiation into new mature neurons in rat brains after stroke.

To study the cellular mechanism of vascular endothelial growth factor (VEGF)-enhanced neurogenesis in ischemic brain injury, we used middle cerebral artery occlusion (MCAO) model to induce transient focal ischemic brain injury. The results showed that ischemic injury significantly increased glial fibrillary acidic protein immunopositive (GFAP(+)) and nestin(+) cells in ipsilateral striatum 3 days following MCAO. Most GFAP(+) cells colocalized with nestin (GFAP(+)-nestin(+)), Pax6 (GFAP(+)-Pax6(+)), or Olig2 (GFAP(+)-Olig2(+)). VEGF further increased GFAP(+)-nestin(+) and GFAP(+)-Pax6(+) cells, and decreased GFAP(+)-Olig2(+) cells. We used striatal injection of GFAP targeted enhanced green fluorescence protein (pGfa2-EGFP) vectors combined with multiple immunofluorescent staining to trace the neural fates of EGFP-expressing (GFP(+)) reactive astrocytes. The results showed that MCAO-induced striatal reactive astrocytes differentiated into neural stem cells (GFP(+)-nestin(+) cells) at 3 days after MCAO, immature (GFP(+)-Tuj-1(+) cells) at 1 week and mature neurons (GFP(+)-MAP-2(+) or GFP(+)-NeuN(+) cells) at 2 weeks. VEGF increased GFP(+)-NeuN(+) and BrdU(+)-MAP-2(+) newborn neurons after MCAO. Fluorocitrate, an astrocytic inhibitor, significantly decreased GFAP and nestin expression in ischemic brains, and also reduced VEGF-enhanced neurogenic effects. This study is the first time to report that VEGF-mediated increase of newly generated neurons is dependent on the presence of reactive astrocytes. The results also illustrate cellular mechanism of VEGF-enhanced neural repair and functional plasticity in the brains after ischemic injury. We concluded that neurogenic effect of VEGF is related to increase of striatal astrocytes transdifferentiation into new mature neurons, which should be very important for the reconstruction of neurovascular units/networks in non-neurogenic regions of the mammalian brain.

Exercise promotes axon regeneration of newborn striatonigral and corticonigral projection neurons in rats after ischemic stroke.

Newborn striatal neurons induced by middle cerebral artery occlusion (MCAO) can form functional projections targeting into the substantia nigra, which should be very important for the recovery of motor function. Exercise training post-stroke improves motor recovery in clinic patients and increases striatal neurogenesis in experimental animals. This study aimed to investigate the effects of exercise on axon regeneration of newborn projection neurons in adult rat brains following ischemic stroke. Rats were subjected to a transient MCAO to induce focal cerebral ischemic injury, followed by 30 minutes of exercise training daily from 5 to 28 days after MCAO. Motor function was tested using the rotarod test. We used fluorogold (FG) nigral injection to trace striatonigral and corticonigral projection neurons, and green fluorescent protein (GFP)-targeting retroviral vectors combined with FG double labeling (GFP(+) -FG(+)) to detect newborn projection neurons. The results showed that exercise improved the recovery of motor function of rats after MCAO. Meanwhile, exercise also increased the levels of BDNF and VEGF, and reduced Nogo-A in ischemic brain. On this condition, we further found that exercise significantly increased the number of GFP(+) -FG(+) neurons in the striatum and frontal and parietal cortex ipsilateral to MCAO, suggesting an increase of newborn striatonigral and corticonigral projection neurons by exercise post-stroke. In addition, we found that exercise also increased NeuN(+) and FG(+) cells in the striatum and frontal and parietal cortex, the ischemic territory, and tyrosine hydroxylase (TH) immunopositive staining cells in the substantia nigra, a region remote from the ischemic territory. Our results provide the first evidence that exercise can effectively enhance the capacity for regeneration of newborn projection neurons in ischemic injured mammalian brains while improving motor function. Our results provide a very important cellular mechanism to illustrate the effectiveness of rehabilitative treatment post-stroke in the clinic.