Calcium-permeable channelrhodopsins for the photocontrol of calcium signalling.

Channelrhodopsins are light-gated ion channels used to control excitability of designated cells in large networks with high spatiotemporal resolution. While ChRs selective for H, Na, K and anions have been discovered or engineered, Ca-selective ChRs have not been reported to date. Here, we analyse ChRs and mutant derivatives with regard to their Ca permeability and improve their Ca affinity by targeted mutagenesis at the central selectivity filter. The engineered channels, termed CapChR1 and CapChR2 for calcium-permeable channelrhodopsins, exhibit reduced sodium and proton conductance in connection with strongly improved Ca permeation at negative voltage and low extracellular Ca concentrations. In cultured cells and neurons, CapChR2 reliably increases intracellular Ca concentrations. Moreover, CapChR2 can robustly trigger Ca signalling in hippocampal neurons. When expressed together with genetically encoded Ca indicators in Drosophila melanogaster mushroom body output neurons, CapChRs mediate light-evoked Ca entry in brain explants.