Constitutive Apo-Opsin Activity Across Retinal Cone Subtypes

Abstract

Retinal rod and cone photoreceptors mediate vision in dim and bright light, respectively, by transducing absorbed photons into electrical signals. Their phototransduction mechanisms are essentially identical. However, one distinct difference between them is that, whereas a rod visual pigment remains stable in darkness, a cone pigment is liable to spontaneously dissociate into apo-opsin and retinal (the chromophore) without isomerization. This cone-pigment property has long been reported, but it has often been overlooked in the field. Importantly, because apo-opsin has constitutive activity, it triggers transduction signaling to produce electrical noise even in darkness. Currently, the precise apo-opsin content in dark-adapted cones is unknown, as is its associated dark activity. We report here a systematic study of red (L), green (M) and blue (S) cones, finding different percentages of apo-opsin across them in darkness. Surprisingly, we also found that apo-opsin signaling in L and M cones in darkness is actually much higher than the well-known holo-pigment thermal isomerization activity. As such, given the very low signal amplification at the pigment-to-transducin/phosphodiesterase step in cone transduction, apo-opsin noise may not be easily distinguishable from light-triggered responses and may affect cone vision near threshold.