Blockage of Store-Operated Ca(2+) Influx by Synta66 is Mediated by Direct Inhibition of the Ca(2+) Selective Orai1 Pore

SIMPLE SUMMARY: Store-operated calcium channels constituted from the proteins Orai and STIM are important targets for development of new drugs, especially for the treatment of auto-immune diseases. Also, interference with channel function is linked to reduced cancer cell progression, making these channels potential targets for anti-cancer drug development. Therefore, inhibitors need to be evaluated for both their binding selectivity and their potential to interfere with cancer progression. Here, we investigated the inhibitor Synta66 and determined its site of binding via both patch clamp recordings and computational approaches and evaluated its potency as anti-cancer agent in glioblastoma multiforme cells. Our findings show that Synta66 is a highly selective ligand to the Orai1 pore and efficiently blocks store operated calcium entry in glioblastoma cells. Still, in the tested cell lines, Synta66 did not reduce cell viability. We therefore suggest Synta66 as a precise tool to observe interference of store-operated Orai1 channel function in vitro and of resulting downstream effects. ABSTRACT: The Ca(2+) sensor STIM1 and the Ca(2+) channel Orai1 that form the store-operated Ca(2+) (SOC) channel complex are key targets for drug development. Selective SOC inhibitors are currently undergoing clinical evaluation for the treatment of auto-immune and inflammatory responses and are also deemed promising anti-neoplastic agents since SOC channels are linked with enhanced cancer cell progression. Here, we describe an investigation of the site of binding of the selective inhibitor Synta66 to the SOC channel Orai1 using docking and molecular dynamics simulations, and live cell recordings. Synta66 binding was localized to the extracellular site close to the transmembrane (TM)1 and TM3 helices and the extracellular loop segments, which, importantly, are adjacent to the Orai1-selectivity filter. Synta66-sensitivity of the Orai1 pore was, in fact, diminished by both Orai1 mutations affecting Ca(2+) selectivity and permeation of Na(+) in the absence of Ca(2+). Synta66 also efficiently blocked SOC in three glioblastoma cell lines but failed to interfere with cell viability, division and migration. These experiments provide new structural and functional insights into selective drug inhibition of the Orai1 Ca(2+) channel by a high-affinity pore blocker.