Rhodopsin is a tunable capacitor buffering the toxic, desensitizing retinoids of the vertebrate eye

Abstract

The eye is a marvel of evolution, but visual sensitivity introduces the risk of blinding photodamage. Here we reveal that the locus of sensitivity--the visual pigment rhodopsin --moonlights as a tunable mechanism of retinal photoprotection. Independent of signaling, light activated rhodopsin (R*) serves as an overflow capacitor buffering all-trans retinal (atRAL), a toxic and desensitizing retinoid agonist that accumulates as lipofuscin--a clinical marker of macular degeneration. Across mammals, R* stability reflects binding affinity and sequestration of atRAL (capacitance), thereby mitigating phototoxicity to a degree proportional to species photodamage risk, with human R* uniquely non-protective. A mouse model of defective atRAL clearance treated with a synthetic R* of unnaturally high atRAL capacitance preserved retinal function following light damage. This gene therapy also provided supra-physiological scotopic sensitivity despite being a signal-silent receptor, shielding neighboring dark-state receptors from agonist interference during regeneration, while also enhancing the signaling of endogenous R*. While the consensus human rhodopsin is not photoprotective, during recent evolution, mutations that protect against light damage emerged in high irradiance human environments and are now significantly associated with a 38% reduced risk of blindness and low vision. Together, our findings redefine rhodopsin as a tunable light buffer that can be leveraged to enhance photoreceptor function beyond natural limits.