The Deisseroth and Hegemann groups have just published a newly engineered channelrhodopsin, ChETA, in the Nature Neuroscience paper, Ultrafast optogenetic control. Gunaydin et al rationally targeted mutations to the opsin pocket of channelrhodopsin-2 to increase the speed of channel deactivation/closing. ChETA provides higher fidelity optical control of spiking at high expression levels or firing frequencies (up to 200Hz!) and eliminates plateau potentials during sustained spike trains.
ChETA clearly provides higher precision in optical control of spiking, particularly at high spike rates. However, a big problem limiting some in vivo channelrhodopsin use has been insufficient conductance. Some groups have sought to increase single channel conductance, but this approach can lead to increased ChR toxicity and/or spurious spikes. At first glance, increasing deactivation rates, and thus decreasing single channel current from a brief light pulse, seems to make life MORE difficult for situations where light and conductance levels are limiting. ChETA produces a very low number of successful spikes at 1ms illumination (Fig 3f), as compared to ChR2. ChETA response peaks within 1ms but requires 2ms illumination and >10Hz trains to induces spikes more reliably than ChR2. Is this due to a decreased peak single channel conductance in ChETA or just the activation/deactivation rate differences? I couldn’t find a direct single-channel conductance comparison in the paper.
A reduced conductance might not be a bad thing though. ChETA’s increased deactivation rate might make less toxic to cells, allowing a higher expression level, which would compensate for a reduced single channel current flow. It all depends on what causes ChR2 toxicity. Is toxicity caused by a non-illuminated leak current or something else? Is the deactivation rate correlated with a leak current and/or toxicity? I would love to see a quantitative comparison of expression level and toxicity between wt ChR2 and ChETA. Maybe our readers can post their experiences with it in the coming weeks.