The aim of the present study was to characterize the post-stroke neurogenesis after long-term survival in a mouse model of neonatal ischemic strokes. The areas of interest were the newborn cells in the ipsilateral injured and contralateral uninjured DG originating from the SGZ niche of neural progenitors and newborn cells in the ipsi- and contralateral striatum and neocortex most likely originating from the SVZ. Although we cannot exclude the possibility that some of the newly generated cells in the striatum and neocortex are derived from local precursors, previous work using retroviruses in rats suggested that the SVZ is the main source of new cells in the neocortex and striatum (Yang, 2007
). There are three important findings in this study. First, neonatal ischemia significantly reduced new cell counts as seen after long survival delays in the injured brains in the ipsilaterally atrophied hippocampi. Importantly new cell counts were also significantly reduced in the uninjured contralateral hippocampi. Second, overall percent neuronal commitment of SGZ derived new cells did not change significantly in spite of the lower counts. Third, SVZ derived neurogenesis in the injured brains, as assessed by new cell densities in the striatum and neocortex, were significantly amplified in injured brains both ipsi- and contralaterally as quantified by counts of new cells in the striatum and neocortex compared to control. However, cell lineage commitment after injury was predominantly non-neuronal and therefore remained similar to control. The comparatively lower percent neuronal commitment observed in the ipsilateral striatum of ligation injured mice (Results 2.3
) may among other possibilities indicate a trend towards increased non-neuronal commitment in SVZ derived cells that migrate towards the site of injury in the striatum. Although we do not have direct evidence that new cells quantified in the neocortex and striatum solely migrated from the SVZ, previous research indicates that local progenitor populations are minimal at best and SVZ proliferation is responsible for most of the cells that migrate into the striatum and neocortex after ischemic injuries (Parent et al., 2002b
, Gould et al., 1999
). Our main observation for the study is that postnatally, striatal and neocortical neurogenesis is non-neuronal in sham controls and remains non-neuronal after ischemia (i.e., new cell counts go up significantly but cell commitment fates do not change).
These results indicate that neonatal stroke clearly has a significant effect on post stroke neurogenesis. If pools of restricted and unrestricted precursors exist within the neurogenic niches of the SGZ and SVZ then neonatal stroke in this mouse model did not alter overall cell-lineage commitment profiles. However the two neurogenic zones underwent differential post-stroke plasticity. The cell proliferation in the striatum and neocortex showed amplification both ipsilaterally as was expected but also contralaterally in the week following the injury. The SGZ on the other hand showed marked suppression of neurogenesis derived new granule cell populations during the same time period. This result presents two alternatives for the SGZ derived new cells; 1) the ischemic injury directly affects the rate of neural precursor cell proliferation, or 2) the ischemic injury and the resulting lesion allows fewer newborn cells to survive the process of maturation and circuit integration. Further studies to look at rate of proliferation of new cells following the neonatal stroke at temporally spaced out time points after the stroke are underway. This additional data will help understand whether the rate of proliferation is amplified or suppressed within the SGZ after the neonatal stroke. If the rates of proliferation are indeed amplified like seen in the SVZ, it would mean from the findings of the current study that most of them degenerate over time. Since counts of newborn granule cells were also significantly lower in the contralateral SGZ derived cells, the unilateral ischemia negatively modulated contralateral endogenous neurogenesis that was independent of the ischemia related massive cell loss and atrophy noted in the ipsilateral hippocampi.
Very few of the SVZ derived cells that migrated to the striatum and neocortex became neurons. Specifically new BrdU labeled cells that had migrated to infarct borders of the neocortical and striatal lesions were not found to co-label with NeuN. This finding is interesting in lieu of studies that have shown SVZ related amplification of neurogenesis associated with a substantial increase in doublecortin (DcX) and neuron specific β-tubulin (TUJ1) positive new cells within the SVZ and in migratory mode moving towards infarct borders when immunohistochemistry was done at more acute time points after ischemic insults in neonatal rats (Yang and Levison, 2006
). Paucity of SVZ derived neurons after long survival delays may indicate that DcX and TuJ1, that are considered to be markers of committed immature neurons, either have a transient phase of upregulation in the post-stroke phase or that the neuronal progenitors begin to migrate towards the striatum and neocortex but do not survive the process of maturation and circuit integration over time. Studies quantifying temporal patterns of expression of immature and mature neuron markers with new cells identified by BrdU incorporation after stroke in neonatal rats have shown a similar trend (Yang et al., 2007
). With only ~5% of the new cells co-labeling with NeuN both in the ligation injured and control mice our findings are consistent with data from other studies that suggest that most newly generated neuronal precursors derived from SVZ neurogenesis do not survive maturation (Arvidsson, 2002
;Thored, 2006). Interestingly similar trends noted in a MCAO model of unilateral ischemia revealed that erythropoietin (EPO) treatment following the insult significantly changed new cell commitment profiles in the striatum towards neuronal commitment without changing the overall counts of new cells (Gonzalez et al., 2007
). This data indicate that exogenous factors can potentially alter cell commitment fates after ischemic injuries which may also have a possible therapeutic value.
In conclusion we have demonstrated that neonatal stroke due to an ischemic insult in CD1 mice results in gross perturbation of SGZ and SVZ derived neurogenesis that are paradoxical to each other. The results of this study will help determine long-term effects of commonly used pharmacological agents on post-stroke neurogenesis in future studies.