Osteogenic role of endosomal chloride channels in MC3T3-E1 cells

ClC-3, ClC-4, and ClC-5 belong to the voltage gated chloride channels (ClCs) and facilitate the endosomal acidification. The mutations of these endosomal chloride channel genes cause different genetic diseases with various bone disorders. We hypothesized that these endosomal ClCs might be involved in the bone development or osteoblast differentiation. Here we used MC3T3-E1 osteoprogenitor cell line and primarily cultured mouse osteoblasts and detected the expression of Clcn3, Clcn4, and Clcn5 in these cells. We analyzed the relationships between three endosomal ClCs and the osteogenic phenotype using osteoinductive treatment, overexpressing of ClCs and RNAi of ClCs. We found the increased mRNA levels of osteogenic markers [alkaline phosphatase (Alp), osteocalcin (Oc), bone sialoprotein (Bsp), and runt-related transcription factor 2 (Runx2)] were in parallel to that of Clcn3, Clcn4 and Clcn5 with osteoinductive treatment and overexpressed ClCs. Overexpressed ClCs were localized in intracellular periphery and also promoted the mineralization of cells in vitro. While RNAi mediated gene silencing of ClC-3, ClC-4, and ClC-5 down regulated the expression of the four osteogenic markers. The positive relationship between endosomal ClCs and the osteogenic markers suggested a new function of endosomal ClCs in osteogenic differentiation.