Combining different ion-selective channelrhodopsins to control water flux by light

Water transport through water channels, aquaporins (AQPs), is vital for many physiological processes including epithelial fluid secretion, cell migration and adipocyte metabolism. Water flux through AQPs is driven by the osmotic gradient that results from concentration differences of solutes including ions. Here, we developed a novel optogenetic toolkit that combines the light-gated anion channel GtACR1 either with the light-gated K⁺ channel HcKCR1 or the new Na⁺ channelrhodopsin HcNCR1 with high Na⁺ permeability, to manipulate water transport in Xenopus oocytes non-invasively. Water efflux through AQP was achieved by light-activating K⁺ and Cl^- efflux through HcKCR1 and GtACR1. Contrarily, when GtACR1 was co-expressed with HcNCR1, inward movement of Na⁺ and Cl^- was light-triggered, and the resulting osmotic gradient led to water influx through AQP1. In sum, we demonstrate a novel optogenetic strategy to manipulate water movement into or out of Xenopus oocytes non-invasively. This approach provides a new avenue to interfere with water homeostasis as a means to study related biological phenomena across cell types and organisms.