STIM is a Ca2+ sensor essential for Ca2+-store-depletion-triggered Ca2+ influx

Abstract

Ca(2+) signaling in nonexcitable cells is typically initiated by receptor-triggered production of inositol-1,4,5-trisphosphate and the release of Ca(2+) from intracellular stores. An elusive signaling process senses the Ca(2+) store depletion and triggers the opening of plasma membrane Ca(2+) channels. The resulting sustained Ca(2+) signals are required for many physiological responses, such as T cell activation and differentiation. Here, we monitored receptor-triggered Ca(2+) signals in cells transfected with siRNAs against 2,304 human signaling proteins, and we identified two proteins required for Ca(2+)-store-depletion-mediated Ca(2+) influx, STIM1 and STIM2. These proteins have a single transmembrane region with a putative Ca(2+) binding domain in the lumen of the endoplasmic reticulum. Ca(2+) store depletion led to a rapid translocation of STIM1 into puncta that accumulated near the plasma membrane. Introducing a point mutation in the STIM1 Ca(2+) binding domain resulted in prelocalization of the protein in puncta, and this mutant failed to respond to store depletion. Our study suggests that STIM proteins function as Ca(2+) store sensors in the signaling pathway connecting Ca(2+) store depletion to Ca(2+) influx.

Figure 1. STIM1 and STIM2 Are Identified in a Kinetic Ca²⁺ Screen for Suppression of SOC Influx with 2,304 Human Signaling siRNAs

(A) Comparison of the Ca²⁺ time course in STIM1 knockdown cells to the averaged reference time course. HeLa cells were transfected with the siRNA signaling set at an average concentration of 10 nM for 2 days. Fura-2 Ca²⁺ time courses were measured in a microplate reader with automated stimulus addition. (B) Positional heat-map analysis of relative Ca²⁺ plateau values in the two microplates containing STIM1 and STIM2 siRNA-transfected cells (24 duplicate siRNAs per plate). Relative Ca²⁺ plateau values were calculated by dividing the plateau (the average of the last 5 data points) by the peak (the average of 3 peak points) fluorescence. (C) Domain structure of STIM1 and STIM2. Domains include an EF-hand motif (EF), a SAM domain, a single transmembrane domain (TM), and an ERM domain arranged from N to C terminus of both proteins with the C terminus in the cytosol. (D–F) Suppression of Ca²⁺ influx by STIM siRNAs measured by “Ca²⁺ add-back” in HeLa cells transfected with 10 nM STIM1 and/or 10 nM STIM2 siRNA for 2 days. One hundred micromolars histamine plus two micromolars thapsigargin (D), histamine alone (E), or thapsigargin alone (F) was used to deplete Ca²⁺ stores. (G) Titration of STIM1 siRNA. Total siRNA concentration was kept at 20 nM for all samples with control siRNA. (H) Near-complete inhibition of Ca²⁺ influx in cells transfected for 3 days with 20 nM STIM1 plus 20 nM STIM2 siRNA. Data shown in (D)–(H) are the average of 3 bulk-cell Ca²⁺ measurements obtained with a microplate reader. (I) Suppression of T-cell-receptor-triggered Ca²⁺ influx by STIM1 siRNA. Ca²⁺ add-back experiments were done in Jurkat T cells transfected with pYFP-Nuc (transfection marker) plus 72 nM STIM1 or control siRNA for 2 days. Ca²⁺ stores were depleted with 20 μg/ml anti-human CD3 antibody. Shown are the average Ca²⁺ responses of 48 control and 84 STIM1 siRNA-transfected (YFP-positive) single cells.

Figure 2. STIM Regulates SOC Influx

(A) Direct measurement of Ca²⁺ influx with an Mn²⁺ quench assay. HeLa cells were transfected for 2 days with 20 nM control or a mix of 10 nM STIM1 and 10 nM STIM2 siRNAs. Two millimolars Mn²⁺ was added before image acquisition, and 100 μM histamine plus 2 μM thapsigargin was added 50 s after. The left panel shows the relative Fura-2 fluorescence intensity measured with 360 nm excitation as a function of time in control and STIM knockdown cells. Each trace represents the average quench response of three separate experiments, with 660–1300 individual cells analyzed per experiment. A Matlab program was used to calculate the ΔF/F quench rate in each cell before (4–44 s) and after (100–140 s) stimulus addition. The average ΔF/F is shown in a bar graph on the right. (B) Overexpression of YFP-STIM1 enhances SOC influx. HeLa cells transfected with 40 ng YFP-STIM1 or control vector for 1 day were subjected to Mn²⁺ quench assays as described in (A). A SOC influx inhibitor, SKF 96365, was used at 20 μM. Over 150 individual cells were analyzed for each data set. Error bars are 95% confidence bounds.

Figure 3. STIM1 Senses ER Ca²⁺ Depletion with Its Luminal EF-Hand

(A) YFP-STIM1 redistributes into punctate structures after Ca²⁺ store depletion. HeLa cells were cotransfected with YFP-STIM1 and a CFP-tagged ER marker. CFP/YFP confocal images of the same cell were taken before (top two panels) and 8 min after (bottom two panels) histamine plus thapsigargin stimulation. The arrows point to peripheral sites rich in puncta. The magnified panels on the right show the colocalization of STIM1 with the ER marker before stimulation. (B) The EF-hand mutant of STIM1 is already localized to puncta and does not respond to store depletion. HeLa cells were cotransfected with YFP-STIM1(D76A) and a CFP-ER marker. CFP/YFP confocal images of the same cell were taken before (top two panels) and 8 min after (bottom two panels) histamine plus thapsigargin stimulation. The scale bar represents 10 μm. (C) STIM1 knockdown cells expressing the EF-hand mutant show elevated influx prior to stimulation and are unresponsive to Ca²⁺ store depletion. Mn²⁺ quench assays were performed in HeLa cells transfected with 40 nM STIM1 UTR or control siRNA plus YFP-STIM1(D76A) or the YFP vector as described in Figure 2A. Error bars are 95% confidence bounds.

Figure 4. STIM1 Exists in Two Localization States and Is Rapidly Redistributed into Puncta Near the Plasma Membrane after Ca²⁺ Store Depletion

(A) Colocalization of wild-type and the EF-hand mutant of STIM1 in puncta after Ca²⁺ store depletion. HeLa cells were cotransfected with CFP-STIM1 and YFP-STIM1(D76A). CFP/YFP confocal images were taken near the adhesion surface of the same cell before (top two panels) and 2.5 min after (bottom two panels) histamine plus thapsigargin stimulation. The magnified panels on the right show STIM1 puncta formation and the colocalization of wild-type and the EF-hand mutant of STIM1 after Ca²⁺ store depletion. (B) TIRF microcopy shows that many YFP-STIM1 puncta are rapidly formed within 100 nm of the plasma membrane. HeLa cells were cotransfected with YFP-STIM1 and CFP-CAAX. CFP/YFP TIRF images were taken in the same cells at different time points after histamine plus thapsigargin stimulation. The scale bar represents 10 μm. (C) Kinetic analysis of the average relative fluorescence intensity in near-plasma-membrane YFP-STIM1 puncta (n = 212; an exponential fit is shown).