Small Molecule Cyclin-Dependent Kinase Inhibitors Protect Against Neuronal Cell Death in the Ischemic-Reperfused Rat Retina

Abstract

Recently, cell cycle reentry has been reported in the experimental brain ischemia model and Alzheimer's disease brain. Cyclin-dependent kinases (CDKs) are known to be key molecules that regulate the cell cycle. The aims of the present study were to determine the change in expression of the proteins involved in regulation of G1 phase, and to test the effect of small molecule CDK inhibitors on neuronal injury in the ischemic-reperfused rat retina. Male Sprague-Dawley rats were anesthetized and subjected to 60 min of retinal ischemia by raising intraocular pressure to 130 mmHg. One day after this procedure, the retinal tissues were homogenized and immunoblot analyses were done to check the expression levels of cell cycle regulatory proteins. Additional groups of rats received 3 mg/kg roscovitine, a CDK inhibitor, and 1 mg/kg CDK4 inhibitor intravenously (i.v.) 15 min before ischemia and underwent 60 min of ischemia. On the 7th day of the retinal ischemia-reperfusion, eyes were subjected to morphometry. Immunoblot analysis revealed that ischemia-reperfusion significantly increased expression levels of cyclin D1 and CDK4, and decreased those of p16(INK4) and p27(KIP1). Treatment with a CDK4 inhibitor (1 mg/kg, i.v.) 15 min before ischemia significantly reduced death of the retinal ganglion cells. Upregulation of cell cycle regulatory proteins and activation of nonmitotic CDK5 are shown to be associated with neuronal cell death. Although the CDK4 inhibitor used in the present study is known to reduce the activity of mitotic CDK4, there is no information about its blocking effect on other CDKs, such as nonmitotic CDK5. The results in the present study suggest that abnormal progression of the cell cycle and/or activation of nonmitotic CDK are involved in the neuronal cell death induced by retinal ischemia-reperfusion. Furthermore, as shown with a CDK4 inhibitor, agents that alter the activity of CDK may be good candidates for inhibitors of neuronal cell death induced by ischemia-reperfusion injury.