Microglial Proliferation and Cell Cycle Protein Upregulation in the Rat Hippocampus Following Forebrain Ischemia

Abstract

We investigated the expression of cell cycle proteins in proliferating microglial cells in the hippocampus in comparison with CA1 neurons that are destined to die after 10 min of forebrain ischemia in the rat. The animals were sacrificed 1 d, 2 d, and 7d after ischemia. Immunohistochemistry was performed using antibodies raised against microglial response factor-1 (MRF-1, a microglia/macrophage marker), glial fibrillary acidic protein (GFAP, an astrocyte marker), and cell cycle proteins, i.e., proliferating cell nuclear antigen (PCNA), cyclin D1, and cydin-dependent kinase-4 (cdk4). Microglial cells with immunoreactivities to PCNA, cyclin D1, and cdk4 in their nuclei started to increase in number 1 d after ischemia, especially in CA1 and dentate hilus, reached a peak after 2 d, and declined after 7 d, when microglial proliferation was striking Limited expression of these proteins in astrocytes was observed after 7 d, when limited astroglial proliferation was seen. No remarkable expressions of these proteins were observed in CA1 neurons during the observation period. Thus, the upregulation of cell cycle proteins preceded the microglial proliferation, and may be involved in the onset of micro-glial activation and proliferation. However, the role of cell cycle protein expression in CA1 neuronal death was not suggested in this study.