Functional Expression of Choline Transporters in Microglia and Their Regulation of Microglial M1/M2 Polarization

Abstract

Background: Microglia are key cells of the immune system in the central nervous system and are suggested to be deeply involved in the development of neurodegenerative diseases. It is well known that microglia have functional plasticity, with an inflammatory M1 phenotype and an anti-inflammatory M2 phenotype. Inhibition of choline transport in macrophages has been reported to suppress the secretion of inflammatory cytokines. However, the role of the choline transport system in regulating microglial M1/M2 polarization has not been fully elucidated to date. In this study, we investigated the mechanism of choline uptake in microglia, and its association with microglial M1/M2 polarization. Methods: The immortalized mouse microglial cell line SIM-A9 was used for [3H]choline uptake and expression analysis of choline transporters. The association between the choline uptake system and the M1/M2 polarization of microglia was also analyzed. Results: Choline transporter-like protein (CTL) 1 and CTL2 were highly expressed in SIM-A9 cells, and CTL1 and CTL2 were localized in the plasma membrane and mitochondria, respectively. Functional analysis of choline uptake demonstrated the existence of Na+-independent, pH-dependent, and intermediate-affinity choline transport systems. Choline uptake was concentration-dependently inhibited by hemicholinium-3 (HC-3), an inhibitor of choline uptake, and increased by lipopolysaccharide (LPS) and interleukin-4 (IL-4). Expression of the mRNA of M1 microglia markers IL-1β and IL-6 was increased by LPS, and their effects were suppressed by choline deprivation and HC-3. In contrast, mRNA expression of the M2 microglial marker arginase-1 (Arg-1) was increased by IL-4, and the effect was enhanced by choline deprivation and HC-3. Conclusions: Our results suggest that inhibition of CTL1-mediated choline uptake in microglia preferentially induces M2 microglia polarization, which is a potential therapeutic approach for inflammatory brain diseases.