Development of chemical tools based on GSK-7975A to study store-operated calcium entry in cells

Many physiological functions, such as cell differentiation, proliferation, muscle contraction, neurotransmission and fertilisation, are regulated by changes of Ca²⁺ levels. The major Ca²⁺ store in cells is the endoplasmic reticulum (ER). Certain cellular processes induce ER store depletion, e.g. by activating IP₃ receptors, that in turn induces a store refilling process known as store-operated calcium entry (SOCE). This refilling process entails protein-protein interactions between Ca²⁺ sensing stromal interaction molecules (STIM) in the ER membrane and Orai proteins in the plasma membrane. Fully assembled STIM/Orai complexes then form highly selective Ca²⁺ channels called Ca²⁺ release-activated Ca²⁺ Channels (CRAC) through which Ca²⁺ ions flow into the cytosol and subsequently are pumped into the ER by the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). Abnormal SOCE has been associated with numerous human diseases and cancers, and therefore key players STIM and Orai have attracted significant therapeutic interest. Several potent experimental and clinical candidate compounds have been developed and have helped to study SOCE in various cell types. We have synthesized multiple novel small-molecule probes based on the known SOCE inhibitor GSK-7975A. Here we present GSK-7975A derivatives, which feature photo-caging, photo-crosslinking, biotin and clickable moieties, and also contain deuterium labels. Evaluation of these GSK-7975A probes using a fluorometric imaging plate reader (FLIPR)-Tetra-based Ca²⁺ imaging assay showed that most synthetic modifications did not have a detrimental impact on the SOCE inhibitory activity. The photo-caged GSK-7975A was also used in patch-clamp electrophysiology experiments. In summary, we have developed a number of active, GSK-7975A-based molecular probes that have interesting properties and therefore are useful experimental tools to study SOCE in various cells and settings.