Overexpressed TMEM203 depletes ER calcium stores
TMEM203 might increase cytoplasmic calcium levels via either directly activating SOCE or via lowering ER calcium stores. To distinguish between these models, we first examined ER calcium stores indirectly by measuring the increase in cytoplasmic calcium levels after release of ER stores. HEK293 cells stably containing an IPTG-inducible TMEM203 construct were treated with various levels of IPTG and the ER flux into the cytosol was measured in the absence of extracellular calcium after exposure to thapsigargin (TG) or ionomycin. IPTG induced TMEM203 expression (Fig 2C) resulted in significantly lowered calcium release from the ER in response to both TG and ionomycin (Fig 2D–2F). This demonstrates that TMEM203 overexpression causes a significant depletion of ER calcium stores. Influx of extracellular calcium after TG induced store depletion was unaffected by TMEM203 over-expression (Fig 2F) suggesting that TMEM203 does not affect SOCE directly.