Ca2+ sensing of STIM1/2 shapes physiological Ca2+ signals

PD. Dr. Rainer Schindl, Medical University of Graz, Gottfried Schatz Research Center, 8010-Graz, Austria A. Univ.-Prof. Dr. Christoph Romanin, Institute of Biophysics, JKU Linz, 4020-Linz, Austria Calcium (Ca2+) plays an important role in many processes in a cells lifecycle. Therefore, a precise control of calcium levels within the cell is crucial. All cells have a specific calcium toolkit, which contains multiple calcium sensors, calcium buffers and calcium channels. Two essential proteins out of this toolkit are STIM and Orai. The stromal interaction molecule (STIM) is a calcium sensor within the endoplasmic reticulum that reaches towards the plasma membrane and activates the calcium channel Orai. STIM1 and its second isoform STIM2 precisely respond to the extent of Ca2+-depletion in the endoplasmic reticulum to trigger essential human immune responses including T-cell activation or mast cell degranulation in allergic diseases. In the present project we will determine the sequential activation mechanism as well as structural conformations of STIM1 and STIM2. Investigation of Ca2+-signaling pathways on a molecular level is especially important for many human diseases, including immunodeficiency. In a combination of computational simulations and live cell recordings, we aim to determine how STIM1 and STIM2 encode different cellular Ca2+-input signals. The research team will include profound experts in Ca2+-signaling (Prof. Christoph Romanin, University of Linz, Austria and Dr. Rainer Schindl, Medical University of Graz, Austria) and MD simulations (Prof. Rüdiger Ettrich, Larkin University, USA and Dr. Daniel Bonhenry, Academy of Sciences of the Czech Republic). Additionally, Dr. Peter Stathopulos from the University of Western Ontario will support our team with his expertise in biochemical studies of STIM1 and STIM2. The gained insights of the present project will provide a molecular understanding of Ca2+-signaling pathways to decode physiological and pathophysiological Ca2+-processes, possibly enabling a specific therapy for malfunction of Ca2+-dependent processes in human diseases.

Activation and Pathophysiological Role of the Calcium Sensor Protein STIM1 STIM1 (Stromal Interaction Molecule 1) is a key protein responsible for calcium regulation in various human cell types, such as immune cells. It is located in the membrane of the endoplasmic reticulum (ER) and becomes activated when the calcium concentration in the ER decreases, subsequently triggering the opening of so-called CRAC channels (Calcium Release-Activated Calcium). In its active form, STIM1 directly binds to the calcium-selective channel protein Orai1 in the plasma membrane. This activation is essential for numerous cellular processes such as gene expression and cell proliferation. A central element of this activation involves structural changes in STIM1, transitioning from an inactive, calcium-bound monomer into an active, multimerized form. Using molecular dynamics simulations and electrophysiological cell experiments, a project funded by the Austrian Science Fund (FWF) identified two critical regions within the so-called SAM domain (Sterile Alpha Motif) as essential for this multimerization. Point mutations in these regions impaired the formation of higher-order STIM1 complexes, leading to reduced calcium influx and dampened calcium oscillations in cells. These findings demonstrate that hydrophobic interactions within the ER-luminal domain of STIM1 are crucial for the protein's activation. This mechanism was published in 2024 in the prestigious journal Proceedings of the National Academy of Sciences of the USA (PNAS). Genetic mutations in STIM1 that disrupt its activation mechanism can have pathological consequences. One such example is Stormorken syndrome, a rare hereditary disorder caused by persistent overactivation of STIM1. A single specific mutation in STIM1 leads to a continuous opening of CRAC channels-regardless of ER calcium levels. Another key success of this FWF project was the discovery that deletion of a neighboring amino acid can correct this defect. In mouse models, this correction mutation completely restored normal STIM1 function. These results, published in the renowned journal Science Signaling in 2023, provide important insights into the molecular basis of STIM1-related diseases and offer potential therapeutic strategies for treating genetic disorders of calcium regulation. This research highlights the central role of STIM1 as a molecular switch for calcium signaling and demonstrates how precise molecular interventions can reverse disease-relevant dysfunctions.