The pattern of E2F1 and c-myb immunoreactivities in the CA1 region is different from those in the CA2/3 region of the gerbil hippocampus induced by transient ischemia

Cdc25A Is a Critical Mediator of Ischemic Neuronal Death In Vitro and In Vivo

Dysregulation of cell cycle machinery is implicated in a number of neuronal death contexts, including stroke. Increasing evidence suggests that cyclin-dependent kinases (Cdks) are inappropriately activated in mature neurons under ischemic stress conditions. We previously demonstrated a functional role for the cyclin D1/Cdk4/pRb (retinoblastoma tumor suppressor protein) pathway in delayed neuronal death induced by ischemia. However, the molecular signals leading to cyclin D/Cdk4/pRb activation following ischemic insult are presently not clear. Here, we investigate the cell division cycle 25 (Cdc25) dual-specificity phosphatases as potential upstream regulators of ischemic neuronal death and Cdk4 activation. We show that a pharmacologic inhibitor of Cdc25 family members (A, B, and C) protects mouse primary neurons from hypoxia-induced delayed death. The major contributor to the death process appears to be Cdc25A. shRNA-mediated knockdown of Cdc25A protects neurons in a delayed model of hypoxia-induced death in vitro Similar results were observed in vivo following global ischemia in the rat. In contrast, neurons singly or doubly deficient for Cdc25B/C were not significantly protective. We show that Cdc25A activity, but not level, is upregulated in vitro following hypoxia and global ischemic insult in vivo Finally, we show that shRNA targeting Cdc25A blocks Ser795 pRb phosphorylation. Overall, our results indicate a role for Cdc25A in delayed neuronal death mediated by ischemia.SIGNIFICANCE STATEMENT A major challenge in stroke is finding an effective neuroprotective strategy to treat cerebral ischemic injury. Cdc25 family member A (Cdc25A) is a phosphatase normally activated during cell division in proliferating cells. We found that Cdc25A is activated in neurons undergoing ischemic stress mediated by hypoxia in vitro and global cerebral ischemia in rats in vivo We show that pharmacologic or genetic inhibition of Cdc25A activity protects neurons from delayed death in vitro and in vivo Downregulation of Cdc25A led to reduction in retinoblastoma tumor suppressor protein (pRb) phosphorylation. An increase in pRb phosphorylation has been previously linked to ischemic neuronal death. Our results identify Cdc25A as a potential target for neuroprotectant strategy for the treatment of delayed ischemic neuronal death.

Cell cycle machinery and stroke

Stroke results from a transient or permanent reduction in blood flow to the brain. The mechanisms involving neuronal death following ischemic insult are complex and not fully understood. One signal which may control ischemic neuronal death is the inappropriate activation of cell cycle regulators including cyclins, cyclin dependent kinases (CDKs) and endogenous cyclin dependent kinase inhibitors (CDKIs). In dividing cells, activation of cell cycle machinery induces cell proliferation. In the context of terminally differentiated-neurons, however, aberrant activation of these elements triggers neuronal death. Indeed, there are several lines of correlative and functional evidence supporting this "cell cycle/neuronal death hypothesis". The objective of this review is to summarize the findings implicating cell cycle machinery in ischemic neuronal death from in vitro and in vivo studies. Importantly, determining and blocking the signaling pathway(s) by which these molecules act to mediate ischemic neuronal death, in conjunction with other targets may provide a viable therapeutic strategy for stroke damage.

c-Myb immunoreactivity, protein and mRNA levels significantly increase in the aged hippocampus proper in gerbils

Myb genes are a family of transcription factors and have been implicated in the control of the proliferation and differentiation of normal and transformed cells. c-Myb is the best characterized member of the myb family. In the present study, we investigated age-dependent changes of c-myb immunoreactivity, its protein and mRNA level in the hippocampus proper (CA1-3 regions) at various age stages in gerbils. In the postnatal month 1 (PM 1) group, c-myb immunoreactivity was detected in non-pyramidal neurons of the strata oriens and radiatum as well as in pyramidal neurons of the stratum pyramidale. At PM 3, c-myb immunoreactivity and its protein level were similar to those at PM 1. Thereafter, c-myb immunoreactivity and its protein level were increased with time. In the PM 24 group, c-myb immunoreactivity, its protein and mRNA levels were highest. These results suggest that the significant increase of c-myb immunoreactivity, protein and mRNA levels in the aged hippocampus may be associated with neuronal aging.