Alterations in Neural Stem Cell Fate Following Focal Ischemia
INTRODUCTION: The purpose of this experiment is to determine the differentiation identity of the neural stem cells (NSC) in the subventricular zone (SVZ) of adult mouse brain after a middle cerebral artery occlusion (MCAO). Injury to the brain causes a large number of changes including inflammation and apoptosis, but the reaction of NSC's has been more difficult to characterize because of the transient nature of their response. Previously, adult neural stem cells (NSC) in the SVZ have been observed to differentiate predominantly into cells with neuronal characteristics. This theory is questioned via a tamoxifen-inducible cre-recombinase (Cre-ERT2) expression mouse model system. METHOD: The Cre-ERT2 expression mouse model system is driven by the Cystatin-C promoter to label NSC's in a time specific manner and track their cell fate after MCAO. After the ischemia, these brain sections were stained with different immunohistochemicals at three separate time points. One set was co-labeled with GFAP, an astrocyte marker, and BrdU, a proliferation marker. Another set was co-labeled with DCX, a neuronal marker, and BrdU. This was used to differentiate between latent NSCs and proliferating NSCs by comparing the ipsilateral side (ischemic) with the contralateral side (control) of the brain. RESULTS: Compared to the contralateral, the ipsilateral side had a significant increase in GFAP/BrdU positive cells between day 3 and day 7 time points. The cell quantity dropped between day 7 to day 14 time points. Compared to the contralateral, the ipsilateral side had a decrease in DCX/BrdU positive cells between day 3 and day 7 time points. The cell quantity significantly increased between day 7 to day 14 time points, and the quantity at day 14 was about twice to that of the day 3 time point. DISCUSSION: This data demonstrated that after the MCAO, the stem cells are not just undergoing neurogenesis, but are for certain period of time, also differentiating into astrocytes that are migrating towards the site of injury. This phenomenon is only witnessed in the NSCs towards the day 7 time point. Afterwards and leading up to day 14, the NSCs seem to be changing their cell fate programming from the astrocyte pathway back to the intended neuronal pathway. Thus, the staining results verify that after an ischemia, NSCs within the SVZ regions of the brain undergo a constant change of programmed cell fate, alternating between immature neurons and astrocytes implicating future aims for "programmed" neurogenesis in the development of therapeutic strategies for the treatment of brain damage and disease.