Involvement of N-type Ca²⁺ channel in microglial activation and its implications to aging-induced exaggerated...

Voltage-dependent calcium channel (VDCC) is generally believed to be active only in excitable cells. However, we have reported recently that N-type VDCC (Cav2.2) could become functional in non-excitable cells under pathological conditions. In the present study, we show that Cav2.2 channels are also functional in physiological microglial activation process. By using a mouse microglial cell line (MG6), we examined the effects of a Cav2.2 blocker on the activation of MG6 cells, when treated with lipopolysaccharide (LPS) / interferon γ (IFNγ) or with interleukin-4 (IL-4). As a result, blocking the activation of Cav2.2 enhanced so-called alternative activation process of microglia (transition to neuroprotective M2 microglia) without changing the efficacy of the transition to neuroinflammatory M1 microglia. This enhanced M2 transition involved the activation of a transcription factor hypoxia inducible factor 2 (HIF-2), since a specific blocker of HIF-2 completely abolished this enhancement. We then examined whether Cav2.2 activation was involved in aging-related neuroinflammation. Using primary culture of microglia, we found that the efficacy of microglial M1 transition was enhanced but that M2 transition was reduced by aging, in agreement with a general notion that aging induces enhanced neuroinflammation. Finally, we show here that the moderate blockade of Cav2.2 expression in microglia restores this age-dependent reduction of microglial M2 transition and reduces the aging-induced exaggerated cytokine response, as revealed by a fast recovery from depressive-like behaviors in microglia-specific Cav2.2 deficient mice. These results suggest a critical role for microglial Cav2.2 channel in the aging-related neuroinflammation.