Research ArticleCalcium signaling

ORAI1, STIM1/2, and RYR1 shape subsecond Ca2+ microdomains upon T cell activation

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Science Signaling  18 Dec 2018:
Vol. 11, Issue 561, eaat0358
DOI: 10.1126/scisignal.aat0358

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A snapshot after T cell activation

Stimulation of the T cell receptor (TCR) initially elicits local Ca2+ signals at the plasma membrane that propagate to trigger global Ca2+ signals that fully activate T cells. Using high-resolution imaging and biochemical techniques to analyze mouse T cells and a human T cell line, Diercks et al. identified the proteins that generate Ca2+ signals within the first second after T cell activation. In unstimulated cells, the plasma membrane–localized Ca2+ channel ORAI1 was constitutively associated with the ER-localized Ca2+ sensors STIM1 and STIM2 and generated brief, spontaneous, and small-amplitude Ca2+ signals. Stimulation of the TCR induced production of the second messenger NAADP, which promoted Ca2+ release through the ryanodine receptor RYR1, ensuring the Ca2+ influx required for the sustained T cell activation. These results identify the Ca2+-handling proteins and show how they are poised to respond to TCR stimulation.

Abstract

The earliest intracellular signals that occur after T cell activation are local, subsecond Ca2+ microdomains. Here, we identified a Ca2+ entry component involved in Ca2+ microdomain formation in both unstimulated and stimulated T cells. In unstimulated T cells, spontaneously generated small Ca2+ microdomains required ORAI1, STIM1, and STIM2. Super-resolution microscopy of unstimulated T cells identified a circular subplasmalemmal region with a diameter of about 300 nm with preformed patches of colocalized ORAI1, ryanodine receptors (RYRs), and STIM1. Preformed complexes of STIM1 and ORAI1 in unstimulated cells were confirmed by coimmunoprecipitation and Förster resonance energy transfer studies. Furthermore, within the first second after T cell receptor (TCR) stimulation, the number of Ca2+ microdomains increased in the subplasmalemmal space, an effect that required ORAI1, STIM2, RYR1, and the Ca2+ mobilizing second messenger NAADP (nicotinic acid adenine dinucleotide phosphate). These results indicate that preformed clusters of STIM and ORAI1 enable local Ca2+ entry events in unstimulated cells. Upon TCR activation, NAADP-evoked Ca2+ release through RYR1, in coordination with Ca2+ entry through 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.

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