Three-Dimensional Model of Sub-Plasmalemmal Ca2+ Microdomains Evoked by T Cell Receptor/CD3 Complex Stimulation

Abstract

Ca2+ signalling 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 of reduced spatial and temporal extents develop in the junctions between the plasma membrane and the endoplasmic reticulum (ER). These microdomains rely on Ca2+ entry from the extracellular medium, via the ORAI1/STIM1/STIM2 system that mediates store operated Ca2+ entry Store operated calcium entry. The mechanism leading to local store depletion and subsequent Ca2+ entry depends on the activation state of the cells. The initial, smaller microdomains are triggered by D-myo-inositol 1,4,5-trisphosphate (IP3) signalling in response to T cell adhesion. T cell receptor (TCR)/CD3 stimulation then initiates nicotinic acid adenine dinucleotide phosphate signalling, which activates ryanodine receptors (RYR). We have recently developed a mathematical model to elucidate the spatiotemporal Ca2+ dynamics of the microdomains triggered by IP3 signalling in response to T cell adhesion (Gil et al., 2021). This reaction-diffusion model describes the evolution of the cytosolic and endoplasmic reticulum Ca2+ concentrations in a three-dimensional ER-PM junction and was solved using COMSOL Multiphysics. Modelling predicted that adhesion-dependent microdomains result from the concerted activity of IP3 receptors and pre-formed ORAI1-STIM2 complexes. In the present study, we extend this model to include the role of RYRs rapidly after TCR/CD3 stimulation. The involvement of STIM1, which has a lower KD for Ca2+ than STIM2, is also considered. Detailed 3D spatio-temporal simulations show that these Ca2+ microdomains rely on the concerted opening of ∼7 RYRs that are simultaneously active in response to the increase in NAADP induced by T cell stimulation. Opening of these RYRs provoke a local depletion of ER Ca2+ that triggers Ca2+ flux through the ORAI1 channels. Simulations predict that RYRs are most probably located around the junction and that the increase in junctional Ca2+ concentration results from the combination between diffusion of Ca2+ released through the RYRs and Ca2+ entry through ORAI1 in the junction. The computational model moreover provides a tool allowing to investigate how Ca2+ microdomains occur, extend and interact in various states of T cell activation.

Emerging optogenetics technologies in biomedical applications

“…158,159 Research has shown that the production of the inositol 1,4,5-trisphosphate (IP 3 ) is associated with the inhibition of cytotoxic T cells (CTL) killing function by Treg factors, indicating that the reduction of the IP 3 concentration leads to a decrease in T cell receptor (TCR)-dependent intracellular Ca 2þ concentration. [160][161][162] However, the process of regulating the calcium ion concentration through the IP3 protein pathway is complex and slow. Therefore, researchers have proposed the use of optogenetics to increase Ca 2þ concentration, which can rapidly reduce the local Treg factor influence on CTL and relieve the immunosuppression of the microenvironment.…”

“…Research has shown that the production of the inositol 1,4,5‐trisphosphate (IP 3 ) is associated with the inhibition of cytotoxic T cells (CTL) killing function by Treg factors, indicating that the reduction of the IP 3 concentration leads to a decrease in T cell receptor (TCR)‐dependent intracellular Ca 2+ concentration 160–162 . However, the process of regulating the calcium ion concentration through the IP3 protein pathway is complex and slow.…”

T cell Ca2+ microdomains through the lens of computational modeling

“…In this study, the channel locations in the PM, ER or mitochondrial membranes are not specified and Ca 2+ fluxes are homogeneously distributed across these membranes. The detailed impact of Ca 2+ channel locations on the spatio-temporal profiles of Ca 2+ changes inside the ER-PM junctions were analyzed in 1D by ( 18 ) and modelled in 3D by ( 61 , 62 ) and ( 63 , 64 ). In this section, we describe these models and their main relying assumptions, while their physiological outcomes are presented in the next section.…”

“…Although NAADP mobilizes lysosomal Ca 2+ ( 67 ), the involvement of this organelle in the formation of T cells Ca 2+ microdomains remains to be established ( 68 ). Instead of considering [IP 3 ] and [NAADP] explicitly, their effects can be modelled by changing the opening states of their receptors directly ( 63 , 64 ). This simplification is however not valid when considering the transition from local to global Ca 2+ signaling which involves considering diffusion and metabolism of the messengers.…”

“…To predict the exact spatio-temporal dynamics underlying Ca 2+ increases in microdomains, Ca 2+ fluxes and diffusion must be simulated in a realistic computational description of the ER-PM junction configuration. A typical configuration, used with slight variations in ( 61 , 62 ) and 63 ); 64 ), is schematized in Figure 2A . ER-PM junctions are regions of the cell where both membranes are closely apposed and correspond to the locations where STIM and Orai co-localize upon store depletion.…”