Phototriggered Apoptotic Cell Death (PTA) Using the Light-Driven Outward Proton Pump Rhodopsin Archaerhodopsin-3

Apoptosis is a type of programmed cell death that commonly occurs in multicellular organisms including humans and that is essential to eliminate unnecessary cells to keep organisms healthy. Indeed, inappropriate apoptosis leads to various diseases such as cancer and autoimmune disease. Here, we developed an optical method to regulate apoptotic cell death by controlling the intracellular pH with outward or inward proton pump rhodopsins, Archaerhodopsin-3 (AR3) or Rubricoccus marinas xenorhodopsin (RmXeR), respectively. The alkalization-induced shrinking of human HeLa cells cultured at pH 9.0 was significantly accelerated or decelerated by light-activated AR3 or RmXeR, respectively, implying the contribution of intracellular alkalization to the cell death. The light-activated AR3 induced cell shrinking at a physiologically neutral pH 7.4 and biochemical analysis revealed that the intracellular alkalization caused by AR3 triggered the mitochondrial apoptotic signaling pathway, which resulted in cell death accompanied by morphological changes. Phototriggered apoptosis (PTA) was also observed for other human cell lines, SH-SY5Y and A549 cells, implying its general applicability. We then used the PTA method with the nematode Caenorhabditis elegans as a model for living animals. Irradiation of transgenic worms expressing AR3 in chemosensing amphid sensory neurons significantly decreased their chemotaxis responses, which suggests that AR3 induced the cell death of amphid sensory neurons and the depression of chemotaxis responses. Thus, the PTA method has a high applicability both in vivo and in vitro, which suggests its potential as an optogenetic tool to selectively eliminate target cells with a high spatiotemporal resolution.