Abstract
Purpose :
In the disc membranes of photoreceptors, the photoisomerization of the retinylidene chromophore of rhodopsin initiates phototransduction. Its bleaching products, metarhodopsin and all-trans-retinal, can also absorb light and undergo photochemical reactions. We carried out experiments with isolated bovine and with various mouse lines to scrutinize the physiological relevance of these processes.
Methods :
The spectral characteristics and retinal composition of isolated bovine rhodopsin upon green (510nm) and blue light (405nm) bleaching were determined by UV-vis spectroscopy and HPLC, respectively. Pigmented and albino wild-type mice were employed to study the effects of different light qualities on the retinoid composition of photoreceptors of the retina. We used pigmented and albino Stra6-/- mice to study the light-dependent catabolic turnover of ocular retinoids. The retinoid composition was determined by HPLC analysis, and the ratio between rhodopsin and opsin was determined spectroscopically.
Results :
Greenlight bleaching converted bovine rhodopsin into metarhodopsin. Bleaching metarhodospin with blue light led to a bathochromic shift of the absorption maximum from 380 nm (metarhodopsin) to 480 nm. HPLC analyses revealed that blue light converted the all-trans-retinylidene chromophore of metarhodopsin in 9, 11, and 13-cis-retinal diastereomers. Upon bleaching with white light, we measured significant concentrations of 9- and 13-cis-retinal in the albino mouse retina, indicating similar reactions in the eyes. An additional green light bleach converted the 9-cis-retinal into all-trans-retinal, meaning that 9-cis-retinal existed in opsin-bound form (isorhodopsin). Analysis of dark and dark/light-raised albino and pigmented Stra6-/- revealed that light affected the retinoid concentration of the mouse eyes. The retinoid-deficient phenotype of albino Stra6-/- mice was not associated with the accumulation of retinaldehyde adducts(bisretinoids), suggesting that light-dependent catabolic turnover triggered the loss of retinoids.
Conclusions :
Together, our studies showed that blue light triggers photochemical reactions of rhodopsin bleaching products and that melanin pigmentation and STRA6 are critical components to maintaining retinoid homeostasis in the retina upon illumination.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.