Step-Function Chronos

Technology

This technology builds upon four previously established channelrhodopsins, Chronos, Chrimson, CoChR, and CsChR, which span the blue-to-red range of the visible spectrum and are each distinguished respectively by fast kinetics and high light sensitivity, red-shifted activation enabling deeper tissue penetration, large photocurrents, and green-shifted activation. Each parent protein is modified by substituting a conserved Cysteine in transmembrane helix 3, the structural region that interfaces directly with the all-trans retinal Schiff base chromophore, with Serine, weakening the molecular interactions that normally drive rapid channel closure and extending the conducting open state from milliseconds to seconds following a single brief light pulse. In some embodiments, a second modification substitutes a corresponding residue with Alanine, further altering the retinal Schiff base environment and creating a more stable step-function opsin in which both channel opening and closing are slowed. When contacted with an appropriate wavelength of light, the channels conduct an inward flux of cations, leading to membrane depolarization. Because the slow mutant versions remain in their conducting state significantly longer than their parents, more channels can accumulate simultaneously in the open state, achieving sustained membrane depolarization at lower light intensities than their unmodified parents.  

Problem Addressed

Because most light-activated ion channels close within milliseconds, maintaining cell activation requires continuous high-intensity illumination (>1 mW/mm²). Sustained illumination, however, can cause phototoxic damage to cells and tissues, thus limiting the utility of light-activated ion channels in long time-scale experiments and clinical applications. The slow mutant channelrhodopsins presented here remain open for longer periods after a single brief light pulse, enabling sustained depolarization and cell activation without continuous illumination.  

Advantages

• Sustained cell activation from a single brief light pulse: Step-function kinetics extend the channel open state enabling effective membrane depolarization at light intensities significantly below those required by unmodified parent channelrhodopsins
• Full blue-to-red spectral coverage enabling multi-color independent cell control: Slow mutant variants of four spectrally distinct channelrhodopsins allow simultaneous, independent activation of distinct cell populations using different wavelengths of light within a single experimental or therapeutic system.
• Light-activated platform for candidate compound screening: The patent claims use of these channels as a standardized optogenetic tool for assessing the effect of candidate compounds on membrane ion conductivity and cell polarization, enabling all-optical drug screening assays.  
• Vector-compatible platform for targeted expression and in vivo applications: Encoding sequences for these slow-mutant opsins are compatible with existing viral vector delivery platforms, including AAV, enabling targeted expression in specific cell types through cell-type-specific promoters and subcellular targeting sequences, and can be incorporated into fusion proteins with fluorescent markers, supporting applications spanning drug screening, therapeutic delivery, and in vivo circuit control.