Mechanisms of Microglia Proliferation in a Rat Model of Facial Nerve Anatomy

SIMPLE SUMMARY: It has remained obscure whether microglia proliferate in the diseased or injured brain. This study analyzed the mitotic ability of microglia in vivo in a rat facial nerve axotomy model in which the blood cells do not enter the parenchyma. Microglia were found to proliferate in the axotomized facial nucleus at 5–7 days post-insult. They enhanced the levels of macrophage colony-stimulating factor (M-CSF) as a mitotic factor, its receptor cFms, and cell-cycle-related proteins, such as proliferating cell nuclear antigen (PCNA) and cyclins in vivo. In the M-CSF-dependent proliferation system in vitro, c-Jun N-terminal kinase (JNK) and p38 were shown to function in the induction of PCNA/cyclins and cFms in microglia, respectively. These analyses revealed that some specific signaling cascades are linked to the proliferative reaction of microglia. Microglia in the axotomized facial nucleus were considered to exhibit a mitotic ability to rescue injured motoneurons as neurotrophic cells. ABSTRACT: Although microglia exist as a minor glial cell type in the normal state of the brain, they increase in number in response to various disorders and insults. However, it remains unclear whether microglia proliferate in the affected area, and the mechanism of the proliferation has long attracted the attention of researchers. We analyzed microglial mitosis using a facial nerve transection model in which the blood–brain barrier is left unimpaired when the nerves are axotomized. Our results showed that the levels of macrophage colony-stimulating factor (M-CSF), cFms (the receptor for M-CSF), cyclin A/D, and proliferating cell nuclear antigen (PCNA) were increased in microglia in the axotomized facial nucleus (axotFN). In vitro experiments revealed that M-CSF induced cFms, cyclin A/D, and PCNA in microglia, suggesting that microglia proliferate in response to M-CSF in vivo. In addition, M-CSF caused the activation of c-Jun N-terminal kinase (JNK) and p38, and the specific inhibitors of JNK and p38 arrested the microglial mitosis. JNK and p38 were shown to play roles in the induction of cyclins/PCNA and cFms, respectively. cFms was suggested to be induced through a signaling cascade of p38-mitogen- and stress-activated kinase-1 (MSK1)-cAMP-responsive element binding protein (CREB) and/or p38-activating transcription factor 2 (ATF2). Microglia proliferating in the axotFN are anticipated to serve as neuroprotective cells by supplying neurotrophic factors and/or scavenging excite toxins and reactive oxygen radicals.