Cytosol alkalization induces cytosolic and mitochondrial calcium elevation in human cells

Calcium ions (Ca²⁺) orchestrate cellular physiology as a versatile and ubiquitous second messenger, regulating a wide range of intracellular processes, including muscle contraction, secretion, metabolism, cell proliferation, and cell death. Here, we show that optogenetic cytosolic alkalization induced by the outward proton-pumping microbial rhodopsin Arch3 from Halorubrum sodomense triggers extracellular calcium influx into the cytosol of human cells. This elevation in cytosolic Ca²⁺ is accompanied by a subsequent increase in mitochondrial matrix Ca²⁺ concentration and depletion of cellular ATP. Together, these findings demonstrate how changes in cytosolic pH can modulate calcium signaling, mitochondrial physiology, and the induction of cell death. These insights are important for fundamental studies of cell physiology and may contribute to the development of improved anticancer therapeutic strategies.