Measurement of intracellular Ca2+ release in permeabilized cells using 45Ca2+

Synthesis and Characterization of Store-Operated Calcium Entry Inhibitors Active in the Submicromolar Range

The store-operated calcium entry, better known as SOCE, forms the main Ca²⁺ influx pathway in non-excitable cells, especially in leukocytes, where it is required for cell activation and the immune response. During the past decades, several inhibitors were developed, but they lack specificity or efficacy. From the non-specific SOCE inhibitor 2-aminoethyl diphenylborinate (2-APB), we synthetized 16 new analogues by replacing/modifying the phenyl groups. Among them, our compound P11 showed the best inhibitory capacity with a K_i ≈ 75 nM. Furthermore, below 1 µM, P11 was devoid of any inhibitory activity on the two other main cellular targets of 2-APB, the IP₃ receptors, and the SERCA pumps. Interestingly, Jurkat T cells secrete interleukin-2 under phytohemagglutinin stimulation but undergo cell death and stop IL-2 synthesis when stimulated in the presence of increasing P11 concentrations. Thus, P11 could represent the first member of a new and potent family of immunosuppressors.

Ion and pH Sensitivity of a TMBIM Ca2+ Channel

The anti-apoptotic transmembrane Bax inhibitor motif (TMBIM) containing protein family regulates Ca2+ homeostasis, cell death, and the progression of diseases including cancers. The recent crystal structures of the TMBIM homolog BsYetJ reveal a conserved Asp171-Asp195 dyad that is proposed in regulating a pH-dependent Ca2+ translocation. Here we show that BsYetJ mediates Ca2+ fluxes in permeabilized mammalian cells, and its interaction with Ca2+ is sensitive to protons and other cations. We report crystal structures of BsYetJ in additional states, revealing the flexibility of the dyad in a closed state and a pore-opening mechanism. Functional studies show that the dyad is responsible for both Ca2+ affinity and pH dependence. Computational simulations suggest that protonation of Asp171 weakens its interaction with Arg60, leading to an open state. Our integrated analysis provides insights into the regulation of the BsYetJ Ca2+ channel that may inform understanding of human TMBIM proteins regarding their roles in cell death and diseases.

Non-canonical function of IRE1α determines mitochondria-associated endoplasmic reticulum composition to control calcium transfer and bioenergetics

Mitochondria-associated membranes (MAMs) are central microdomains that fine-tune bioenergetics by the local transfer of calcium from the endoplasmic reticulum to the mitochondrial matrix. Here, we report an unexpected function of the endoplasmic reticulum stress transducer IRE1α as a structural determinant of MAMs that controls mitochondrial calcium uptake. IRE1α deficiency resulted in marked alterations in mitochondrial physiology and energy metabolism under resting conditions. IRE1α determined the distribution of inositol-1,4,5-trisphosphate receptors at MAMs by operating as a scaffold. Using mutagenesis analysis, we separated the housekeeping activity of IRE1α at MAMs from its canonical role in the unfolded protein response. These observations were validated in vivo in the liver of IRE1α conditional knockout mice, revealing broad implications for cellular metabolism. Our results support an alternative function of IRE1α in orchestrating the communication between the endoplasmic reticulum and mitochondria to sustain bioenergetics.

Measurement of intracellular Ca2+ release in intact and permeabilized cells using 45Ca2+

Ca(2+) is an important ion that controls almost every function in a cell. Activator Ca(2+) can be released from intracellular Ca(2+) stores, and there are various ways to study this release. Here, we introduce a technique that uses radioactive (45)Ca(2+) to quantitatively measure the unidirectional release of Ca(2+) from the nonmitochondrial Ca(2+) stores in monolayers of cultured cells. This technique can be used in cells with an intact plasma membrane as well as in cells in which the plasma membrane has been permeabilized.

Measurement of intracellular Ca2+ release in intact cells using 45Ca2+

This protocol describes a technique using (45)Ca(2+) to measure the release of Ca(2+) from the intracellular stores in monolayers of intact cells cultured in 12-well 4-cm(2) clusters. The (45)Ca(2+)-flux technique described here can only be applied to cell types that adhere to plastic. We describe the loading of the stores with (45)Ca(2+), and the subsequent (45)Ca(2+) efflux.