Novel STIM1-dependent control of Ca²⁺ clearance regulates NFAT activity during T-cell activation

Antigen presentation to the T-cell receptor leads to sustained cytosolic Ca²⁺ elevation, which is critical for T-cell activation. We previously showed that in activated T cells, Ca²⁺ clearance is inhibited by the endoplasmic reticulum Ca²⁺ sensor stromal interacting molecule 1 (STIM1) via association with the plasma membrane Ca²⁺/ATPase 4 (PMCA4) Ca²⁺ pump. Having further observed that expression of both proteins is increased in activated T cells, the current study focused on mechanisms regulating both up-regulation of STIM1 and PMCA4 and assessing how this up-regulation contributes to control of Ca²⁺ clearance. Using a STIM1 promoter luciferase vector, we found that the zinc finger transcription factors early growth response (EGR) 1 and EGR4, but not EGR2 or EGR3, drive luciferase activity. We further found that neither STIM1 nor PMCA4 is up-regulatedwhen both EGR1 andEGR4 are knocked downusingRNAinterference. Further, under these conditions, activation-induced Ca²⁺ clearance inhibition was eliminated with little effect on Ca²⁺ entry. Finally, we found that nuclear factor of activated T-cell (NFAT) activity is profoundly attenuated if Ca²⁺ clearance is not inhibited by STIM1. These findings reveal a critical role for STIM1-mediated control of Ca²⁺ clearance in NFAT induction during T-cell activation.