DataSheet_1_Three-Dimensional Model of Sub-Plasmalemmal Ca2+ Microdomains Evoked by the Interplay Between ORAI1 and InsP3 Receptors.pdf

Ca2+ signaling plays an essential role in T cell activation, which is a key step to start an adaptive immune response. During the transition from a quiescent to a fully activated state, Ca2+ microdomains characterized by reduced spatial and temporal extents are observed in the junctions between the plasma membrane (PM) and the endoplasmic reticulum (ER). Such Ca2+ responses can also occur in response to T cell adhesion to other cells or extracellular matrix proteins in otherwise unstimulated T cells. These non-TCR/CD3-dependent Ca2+ microdomains rely on d-myo-inositol 1,4,5-trisphosphate (IP3) signaling and subsequent store operated Ca2+ entry (SOCE) via the ORAI/STIM system. The detailed molecular mechanism of adhesion-dependent Ca2+ microdomain formation remains to be fully elucidated. We used mathematical modeling to investigate the spatiotemporal characteristics of T cell Ca2+ microdomains and their molecular regulators. We developed a reaction-diffusion model using COMSOL Multiphysics to describe the evolution of cytosolic and ER Ca2+ concentrations in a three-dimensional ER-PM junction. Equations are based on a previously proposed realistic description of the junction, which is extended to take into account IP3 receptors (IP3R) that are located next to the junction. The first model only considered the ORAI channels and the SERCA pumps. Taking into account the existence of preformed clusters of ORAI1 and STIM2, ORAI1 slightly opens in conditions of a full ER. These simulated Ca2+ microdomains are too small as compared to those observed in unstimulated T cells. When considering the opening of the IP3Rs located near the junction, the local depletion of ER Ca2+ allows for larger Ca2+ fluxes through the ORAI1 channels and hence larger local Ca2+ concentrations. Computational results moreover show that Ca2+ diffusion in the ER has a major impact on the Ca2+ changes in the junction, by affecting the local Ca2+ gradients in the sub-PM ER. Besides pointing out the likely involvement of the spontaneous openings of IP3Rs in the activation of SOCE in conditions of T cell adhesion prior to full activation, the model provides a tool to investigate how Ca2+ microdomains extent and interact in response to T cell receptor activation.

钙信号转导（Ca2+ signaling）在T细胞活化过程中发挥关键作用，而T细胞活化是启动适应性免疫应答的核心环节。在从静息状态向完全活化状态的转变过程中，研究人员在质膜（plasma membrane, PM）与内质网（endoplasmic reticulum, ER）之间的连接区域，观察到了空间和时间范围均受限的钙信号微区（Ca2+ microdomains）。在未受特异性刺激的T细胞中，当T细胞黏附于其他细胞或细胞外基质蛋白时，同样可观察到此类钙信号响应。此类不依赖T细胞受体（T cell receptor, TCR）与CD3的钙信号微区（Ca2+ microdomains），依赖于肌醇1,4,5-三磷酸（d-myo-inositol 1,4,5-trisphosphate, IP3）信号转导，以及后续通过ORAI/STIM系统介导的钙池操控性钙内流（store operated Ca2+ entry, SOCE）。黏附依赖型钙信号微区形成的具体分子机制，仍有待完全阐明。本研究通过数学建模，探究了T细胞钙信号微区（Ca2+ microdomains）及其分子调控因子的时空特征。我们利用COMSOL Multiphysics构建了反应扩散模型，用于描述三维内质网-质膜连接区域内胞质与内质网（ER）钙离子浓度的动态变化。模型方程基于此前提出的该连接区域的写实性描述，并进行了拓展，纳入了定位于连接区域附近的肌醇1,4,5-三磷酸受体（IP3 receptors, IP3R）。初始模型仅考虑了ORAI通道与肌浆网/内质网钙ATP酶（SERCA pumps）。考虑到ORAI1与STIM2预先形成簇状结构的特性，当内质网钙库处于充盈状态时，ORAI1会出现轻微开放。但该模拟得到的钙信号微区尺寸，与未受刺激T细胞中观察到的钙信号微区相比明显过小。当考虑定位于连接区域附近的IP3R开放时，内质网钙离子的局部耗竭可促使更多钙离子通过ORAI1通道流动，进而提升局部钙离子浓度。此外，计算结果表明，内质网内的钙离子扩散通过影响质膜下内质网区域的局部钙离子梯度，对连接区域的钙离子变化产生显著影响。本模型不仅揭示了在T细胞完全活化前的黏附阶段，IP3R的自发性开放可能参与了SOCE的激活过程，同时也为探究T细胞受体活化时钙信号微区的扩展与相互作用机制提供了有效的研究工具。