Role of cell cycle-associated proteins in microglial proliferation in the axotomized rat facial nucleus

We analyzed cell cycle-associated proteins, including cyclins, cyclin-dependent protein kinases (Cdks), and Cdk inhibitors (CdkIs) in the axotomized rat facial nucleus. Immunoblotting revealed that cyclin A and cyclin D are induced 3-5 days after transection. The induced cyclin A was immunohistochemically recognized in microglia. Cdk2 and Cdk4 were also detected in the facial nucleus. The CdkI p21 was elevated 5 days after axotomy. Inhibition experiments in vitro using a cFms (receptor for macrophage-colony stimulating factor, M-CSF) inhibitor indicated that M-CSF-cFms signaling leads to upregulation of the levels of cyclin A, cyclin D, proliferating cell nuclear antigen (PCNA), and cFms in microglia. The role of cyclin A/Cdk2 activity in M-CSF-dependent microglial proliferation was ascertained using the specific inhibitor purvalanol A. Experiments using specific mitogen-activated protein kinase inhibitors suggested that c-Jun N-terminal kinase (JNK) is associated with M-CSF-dependent induction of cyclins and PCNA, whereas p38 is associated with cFms induction. Both JNK and p38 were proved to be phosphorylated by stimulation with M-CSF. Our results indicated that cyclin A, cyclin D, Cdk2, Cdk4, and p21 are involved in microglial proliferation in the transected facial nucleus, and that the M-CSF-dependent upregulations of cyclins/PCNA and cFms in microglia are differentially regulated by JNK and p38.