Photic generation of 11-cis-retinal in bovine retinal pigment epithelium

The expression of opsins in the human skin and its implications for photobiomodulation: A Systematic Review

“…They reported that irradiation of cultured keratinocytes with violet light (380 nm) elicits the highest amplitude of Ca 2+ transients only in the presence of the all‐ trans ‐retinal ligand, suggesting that RRH may contribute to the phototransduction of violet light in keratinocytes. The spectral sensitivity of RRH is unknown, but neuropsin and RGR, two other phylogenetically related tetraopsins, have maximal absorption spectra at 380 and 470 nm, respectively 19,53 . However, the pathophysiological relevance of this RRH in human skin warrants further investigation.…”

“…Tetraopsins, also known as group 4 opsins, include retinal G protein–coupled receptor opsins (RGR), retinochrome, peropsin (RRH), neuropsin (OPN5), and Go‐opsins. In contrast to c‐opsins and r‐opsins, many tetraopsins are relatively poorly characterized although more recent evidence suggests their functional role in photoisomerization of trans‐form to cis‐ form of retinal 18‐20 …”

Pathways and disease-causing alterations in visual chromophore production for vertebrate vision

“…RGR, a 7-transmembrane spanning protein that appears to function as a retinal photoisomerase, not as the classical GPCR suggested by its name, is also expressed in both the RPE and Müller glia and may have distinct functional roles in the two cell types. In the RPE, RGR has been proposed to support visual chromophore production by a variety of mechanisms including acting directly as a photoisomerase (Chen et al, 2001a;Morshedian et al, 2019;Zhang et al, 2019), by stimulating the isomerase activity of the classical visual cycle (Radu et al, 2008;Wenzel et al, 2005), or by clearing potentially toxic isomers of retinal (Maeda et al, 2003). RGR may form 11-cis-retinal through its photoisomerase activity in conjunction with an 11-cis-RDH (Chen et al, 2001b), (e.g.…”

“…Regardless of the exact mechanisms of RGR, the impaired cone function observed in Rgr -/mice clearly shows that this protein plays an important role in visual chromophore production (Morshedian et al, 2019). However, RNAseq analysis has demonstrated that RGR expression is much lower in mouse Müller glia as compared to human and bovine Müller glia indicating that the phenotype in Rgr -/mice may not fully recapitulate the consequences of RGR loss of function in other species (Zhang et al, 2019). As shown in Figure 1A, the full complement of proteins that contribute to visual chromophore production in the Müller glia remain to be identified and some previously suspected players such as Des1 have been shown physiologically irrelevant to this process (Kiser et al, 2019;Ward et al, 2020).…”

“…In addition to all-trans-retinal, RGR was shown to bind13-cis-retinol, which is the undesirable byproduct of RPE65 catalysis and non-specific thermal isomerization. Upon absorption of visible light by the retinylidene adduct, RGR isomerizes both all-trans-retinal and 13-cisretinol to the 11-cis-configuration (Hao and Fong, 1999;Zhang et al, 2019). Unlike most other non-visual opsin proteins, which retain their retinal ligand in both the cis and trans forms and can be interconverted through stimulation by distinct electronic absorbance bands, the 11-cis-retinylidene complex with RGR is unstable and susceptible to hydrolysis, particularly in the presence of the 11-cis-retinal acceptor, CRALBP (Zhang et al, 2019) or potentially RDH enzymes (Morshedian et al, 2019).…”