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.