The expression and phosphorylation of SMAD3 protein in microglia and astrocytes of the rat hippocampus after transient global cerebral ischemia.

SMAD3 protein transduces signals from TGF-β and activins. In vitro studies have shown that SMAD3 plays an important role in regulating of micoglia and astrocytic function. However, there is little information on the association between SMAD3 signaling and the pathophysiology of the glial cells in the post-ischemic hippocampus. In this study, we examined the time-course changes in the expression and phosphorylation of SMAD3 in the rat hippocampus using a rat model of global cerebral ischemia. Most pyramidal neuronal cells in the CA1 region died within 7 days after ischemia. The number of SMAD3- or phosphorylated SMAD3 (p-SMAD3)-immunopositive microglia or astrocytes increased in the CA1 region 7 days after ischemia. Real-time PCR analysis showed an increase in the level of TGF-β1 mRNA in the hippocampus after ischemia. Intracerebroventricular injection of SB525334, a selective inhibitor of TGF-β receptor I kinase (ALK5), reduced the ischemia-induced p-SMAD3 immunoreactivity in the microglia and astrocytes. By contrast, intracerebroventricular injection of SB525334 did not affect the ischemia-induced neuronal cell death. These results suggest that ischemia-induced SMAD3 phosphorylation in the microglia and astrocytes of post-ischemic hippocampi is associated with tissue repair and not neuroprotection.