ORAI1, stromal interaction molecules 1/2, and ryanodine receptor type 1 shape sub-second Ca2+ microdomains upon T cell activation
Diercks, B. P., Werner, R., Weidemüller, P., Czarniak, F., Hernandez, L., Lehmann, C., ... & Guse, A. H.
The earliest intracellular signals determined in T cell activation are local, sub-second Ca2+ microdomains. Here we identify a Ca2+ entry component involved in Ca2+ microdomain formation in both non-stimulated and stimulated cells. In non-stimulated cells, spontaneous small Ca2+ microdomains depend on expression of ORAI1, STIM1, and STIM2. Using T cells stably transfected with ORAI1 fused to a genetically encoded Ca2+ indicator for optical imaging spontaneous Ca2+ microdomains depending on ORAI1 were also detected. Super resolution microscopy of non-stimulated T cells resulted in identification of a circular subplasmalemmal region with a diameter of approx. 300 nm with preformed patches of co-localized ORAI1, ryanodine receptors (RYR), and STIM1. Preformed complexes of STIM1 and ORAI1 in non-stimulated cells were confirmed by co-immunoprecipitation and Förster resonance energy transfer studies. Furthermore, within the first second of T cell receptor (TCR) stimulation, Ca2+ microdomain numbers increase in the subplasmalemmal space, an effect not observed upon genetic deletion of Orai1, Stim2 or Ryr1 or upon antagonism of the Ca2+ mobilizing second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). Taken together, while preformed clusters of STIM and ORAI1 allow for local Ca2+ entry events in non-stimulated cells, upon TCR activation, NAADP-evoked Ca2+ release via RYR1, in tight interplay with Ca2+ entry via ORAI1 and STIM, rapidly increases the number of Ca2+ microdomains, thereby initiating spread of Ca2+ signals deeper into the cytoplasm to promote full T cell activation.