Molecular cloning of a cell cycle regulation gene cyclin H from ischemic rat brain: Expression in neurons after global cerebral ischemia

Gene expression plays an important role in determining the fate of neurons after ischemia. To identify additional genes that promote survival or execu te programmed cell death in ischemic neurons, a subtractive cDNA library wa s constructed from hippocampus of rats subjected to global ischemia. With u se of a differential screening technique, a cDNA was identified that was up regulated after ischemia, The cDNA was found to have high homology with hum an cyclin H at both the nucleotide level (89%) and the amino acid level (93 %). Northern blotting detected cyclin H mRNA in nonischemic and ischemic br ains. In situ hybridization studies revealed that cyclin H message was foun d in hippocampal neurons in nonischemic brain. After ischemia, expression w as increased primarily in the dentate gyrus and CA3 regions of hippocampus. Expression of cyclin H protein, detected by western blotting of hippocampa l tissue, was increased after global ischemia, but expression of cyclins B1 and D1 and other related cell cycle genes (Cdk7 and Cdc2) was not increase d. Cyclin H immunoreactivity was found exclusively within neurons. After is chemia, there was increased immunoreactivity within neurons in dentate gyru s, CA3, and cortex. Thus, cyclin H is expressed in normal postmitotic neuro ns and expression is increased in neurons that are ischemic yet survive. Th ese results suggest that cyclin H may have functions in neurons other than cell cycle regulation, including other known functions such as DNA repair.