Changes in Smad1/5/9 expression and phosphorylation in astrocytes of the rat hippocampus after transient global cerebral ischemia

Smad proteins are known to transduce the actions of the transforming growth factor-? (TGF-?) family including TGF-?s, activins, and bone morphogenetic proteins (BMPs). We previously reported that Smad1/5/9 immunoreactivity was observed in astrocytes of various rat brain regions including the hippocampus, suggesting that Smad1/5/9 may be associated with the physiology of astrocytes. However, the Smad1/5/9 expression and activation in the hippocampal astrocytes after global cerebral ischemia has not been yet elucidated. In this study, we examined temporal changes in the expression and phosphorylation of Smad1/5/9 in the hippocampus using a rat model of global cerebral ischemia. Furthermore, we examined the candidate ligand involved in the phosphorylation of Smad1/5/9 in the hippocampus after ischemia. Pyramidal neuronal cell death in the CA1 regions was visible at 3 days, and maximum death occurred within 7 days after ischemia. At 7 days after ischemia, astrocytes that showed strong immunoreactivity for Smad1/5/9 were frequently observed in the CA1 region. Additionally, there was an increase in phosphorylated Smad1/5/9 (phospho-Smad1/5/9) -immunopositive astrocytes in the CA1 region 7 days after ischemia. Real-time PCR analysis showed an increase in the expression level of TGF-?1 mRNA in the hippocampus after ischemia. Intracerebroventricular injection of SB525334, an inhibitor of TGF-?/Smad signaling, reduced immunoreactivity for phospho-Smad1/5/9 in astrocytes. These results suggest that TGF-?1 may be a key molecule for ischemia-induced Smad1/5/9 phosphorylation in astrocytes, and TGF-?1-Smad1/5/9 signaling may play a role in post-ischemic events, including brain inflammation or tissue repair rather than neuroprotection of the hippocampus.

Automated summary

This study investigated the expression and phosphorylation of Smad1/5/9 in the hippocampus after global cerebral ischemia in rats, finding that activins and bone morphogenetic proteins may be involved in the phosphorylation of Smad1/5/9 after ischemia. Additionally, transforming growth factor-β1 was identified as a key molecule for ischemia-induced Smad1/5/9 phosphorylation in astrocytes.