Microglial α7-Nicotinic Acetylcholine Receptors Are Expressed in Mitochondria Rather Than on the Plasma Membrane: Roles in Mitochondrial Function.

The subcellular localization and cellular functions of the microglial α7-nicotinic acetylcholine receptor (α7-nAChR) were investigated in detail. Although α7-nAChR mRNA was present in microglia isolated from mouse brains by fluorescence-activated cell sorting, we observed low levels of mRNA for NACHO (novel acetylcholine receptor chaperone) and RIC3 (resistance to inhibitors of cholinesterase 3), which are crucial chaperones for the functional expression of α7-nAChR as an ionotropic receptor. Limited localization of α7-nAChR on the cell membrane of isolated microglia suggested an intracellular distribution of this receptor preferentially. To examine the function of α7-nAChR as a ligand-gated ion channel receptor, we treated primary cultured microglia with the α7-nAChR agonist choline; however, no increase in the intracellular calcium ion concentration was observed. Cell staining with α-bungarotoxin and an α7-nAChR antibody suggested that the α7-nAChR expressed in microglia is localized intracellularly, particularly in mitochondria, rather than at the cell membrane. Treatment of primary cultured microglia with choline significantly increased intracellular ATP levels, an indicator of mitochondrial function. This increase in ATP production was significantly suppressed by pretreatment with the α7-nAChR antagonist methyllycaconitine. In microglia with relatively low expression levels of NACHO and RIC3, the population of α7-nAChRs functioning as ion channel receptors at the plasma membrane is expected to be limited. This study reveals a newly described cellular function of microglial α7-nAChR in mitochondria. This finding improves our understanding of the multifaceted roles of α7-nAChRs in the central nervous system and opens new avenues for exploring their potential as a therapeutic target in microglia-related central nervous system disorders.