Human cone elongation responses can be explained by photoactivated cone opsin and membrane swelling and osmotic response to phosphate produced by RGS9-catalyzed GTPase

Human cone outer segment (COS) length changes in response to stimuli bleaching up to 99% of L- and M-cone opsins were measured with high resolution, phase-resolved optical coherence tomography (OCT). Responses comprised a fast phase (similar to 5 ms), during which COSs shrink, and two slower phases (1.5 s), during which COSs elongate. The slower components saturated in amplitude (similar to 425 nm) and initial rate (similar to 3 nm ms(-1)) and are well described over the 200-fold bleaching range as the sum of two exponentially rising functions with time constants of 80 to 90 ms (component 1) and 1,000 to 1,250 ms (component 2). Measurements with adaptive optics reflection densitometry revealed component 2 to be linearly related to cone pigment bleaching, and the hypothesis is proposed that it arises from cone opsin and disk membrane swelling triggered by isomerization and rate-limited by chromophore hydrolysis and its reduction to membrane-localized all-trans retinol. The light sensitivity and kinetics of component 1 suggested that the underlying mechanism is an osmotic response to an amplified soluble by-product of phototransduction. The hypotheses that component 1 corresponds to G-protein subunits dissociating from the membrane, metabolites of cyclic guanosine monophosphate (cGMP) hydrolysis, or by-products of activated guanylate cyclase are rejected, while the hypothesis that it corresponds to phosphate produced by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in G protein-phosphodiesterase complexes was found to be consistent with the results. These results provide a basis for the assessment with optoretinography of phototransduction in individual cone photoreceptors in health and during disease progression and therapeutic interventions.

Automated summary

Using phase-resolved optical coherence tomography, this study measured the changes in length of human cone outer segments in response to bleaching stimuli. The results showed that these changes consist of a fast phase and two slower phases, with the slower phases being linearly related to cone pigment bleaching. The study also proposed hypotheses about the underlying mechanism of these changes.