The calcium feedback loop and T cell activation: How cytoskeleton networks control intracellular calcium flux

During T cell activation, the engagement of a T cell with an antigen-presenting cell (APC) results in rapid cytoskeletal rearrangements and a dramatic increase of intracellular calcium (Ca^2 +) concentration, downstream to T cell antigen receptor (TCR) ligation. These events facilitate the organization of an immunological synapse (IS), which supports the redistribution of receptors, signaling molecules and organelles towards the T cell–APC interface to induce downstream signaling events, ultimately supporting T cell effector functions. Thus, Ca^2 + signaling and cytoskeleton rearrangements are essential for T cell activation and T cell-dependent immune response. Rapid release of Ca^2 + from intracellular stores, e.g. the endoplasmic reticulum (ER), triggers the opening of Ca^2 + release-activated Ca^2 + (CRAC) channels, residing in the plasma membrane. These channels facilitate a sustained influx of extracellular Ca^2 + across the plasma membrane in a process termed store-operated Ca^2 + entry (SOCE). Because CRAC channels are themselves inhibited by Ca^2 + ions, additional factors are suggested to enable the sustained Ca^2 + influx required for T cell function. Among these factors, we focus here on the contribution of the actin and microtubule cytoskeleton. The TCR-mediated increase in intracellular Ca^2 + evokes a rapid cytoskeleton-dependent polarization, which involves actin cytoskeleton rearrangements and microtubule-organizing center (MTOC) reorientation. Here, we review the molecular mechanisms of Ca^2 + flux and cytoskeletal rearrangements, and further describe the way by which the cytoskeletal networks feedback to Ca^2 + signaling by controlling the spatial and temporal distribution of Ca^2 + sources and sinks, modulating TCR-dependent Ca^2 + signals, which are required for an appropriate T cell response. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.