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Shimada, Issei

Anatomy and Neurobiology

2010

PhD

Stroke is a leading cause of disability in the United States. Despite numerous studies, there is not a single drug that can prevent infarct damage after the onset of stroke. The lack of knowledge regarding critical cellular targets after stroke contributes to this treatment gap. The emergence of reactive astrocytes occurs following diverse CNS injuries. Importantly, there is compelling evidence that reactive astrocytes protect against brain damage. Therefore, understanding the mechanisms of reactive astrocytesmay help to identify potential pharmacological targets to modify or enhance astrocyte function. To examine cellular responses during cerebral ischemia, a reproducible permanent distal middle cerebral artery occlusion (dMCAO) model was performed in mice. In this model, regions of infarction were observed mainly in the cortex, but not in the striatum. Therefore, the permanent dMCAO stroke is a reproducible model that can be used to determine the mechanisms of mild cortical stroke in mice.
Reactive astrocytes residing in the most proximal layer to the infarct core expressed the radial glial cell marker RC2 following dMCAO. The RC2-positive reactive astrocytes derived from mature astrocytes and expressed several gammasecretase cleavage products such as Notch intracellular domain (NICD) and Amyloid Precursor Protein intracellular domain (AICD). Gamma-secretase inhibitor treatment after stroke decreased the number of RC2-positive reactive astrocytes, increased the number of infiItrating monocytes in the peri-infarct area and increased cerebral infarct volumes compared with vehicle treatment. Gamma-secretase inhibitor treatment after stab injUry directly decreased the number of RC2-positive reactive astrocytes and changed their morphology. Cultured RC2-positive astrocytes reduced T cell invasion in vitro. This study indicates that RC2-positive reactive astrocytes are regulated by gamma-secretase-mediated pathways and are protective against stroke injUry, in part, through immune suppression.
Multipotent stem/progenitor cells from the peri-infarct area grew clonally as neural spheres 3 days after dMCAO. Focal cortical stroke did not induce neuroblast mjgration from the SVZ to the infarct area following dMCAO, indicating a local origin for the NSC clones. Reactive astrocytes that co-expressed radial glial cell markers, and proliferating neural progenitor cells (immature neurons) were observed in the peri-infarct area. However, any newborn mature neurons were not observed 28 days after stroke. Therefore, the local stem/progenitor cells were neurogenic in culture but not in vivo after stroke, and the immature neurons did not survive in the injury environment. Finally, using a Crelloxp system, GFAP-positive reactive astrocytes in the peri-infarct area after stroke were identified as a source of multipotent neural spheres in vitro. This stUdy indicates that GFAP-positive reactive astrocytes derived from the peri-infarct area have the capacity of multipotent neural stem/progenitor cells after stroke.