Role of Photoreceptor-specific Retinol Dehydrogenase in the Retinoid Cycle in Vivo

Abstract

The retinoid cycle is a recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. Photoreceptor-specific retinol dehydrogenase (prRDH) catalyzes reduction of all-trans-retinal to all-trans-retinol and is thought to be a key enzyme in the retinoid cycle. We disrupted mouse prRDH (human gene symbol RDH8) gene expression by targeted recombination and generated a homozygous prRDH knock-out (prRDH؊/؊) mouse. Histological analysis and electron microscopy of retinas from 6-to 8-week-old prRDH؊/؊ mice revealed no structural differences of the photoreceptors or inner retina. For brief light exposure, absence of prRDH did not affect the rate of 11-cis-retinal regeneration or the decay of Meta II, the activated form of rhodopsin. Absence of prRDH, however, caused significant accumulation of all-trans-retinal following exposure to bright lights and delayed recovery of rod function as measured by electroretinograms and single cell recordings. Retention of all-trans-retinal resulted in slight overproduction of A2E, a condensation product of all-trans-retinal and phosphatidylethanolamine. We conclude that prRDH is an enzyme that catalyzes reduction of all-trans-retinal in the rod outer segment, most noticeably at higher light intensities and prolonged illumination, but is not an essential enzyme of the retinoid cycle.Reduction and oxidation of retinoids are key reactions of the retinoid cycle (visual cycle), which is critical for the production of the chromophore of rhodopsin, 11-cis-retinal (1, 2). When light strikes the visual pigments (rhodopsin and cone opsins) in photoreceptors, it causes the 11-cis-retinylidene chromophore to isomerize to its all-trans configuration, before all-trans-retinal is released from the binding site of the pigments (3) (see Scheme 1). The NADPH-dependent reduction of all-trans-retinal in photoreceptor outer segments is the first step in the regeneration of bleached visual pigment. The reduction occurs directly on the cytoplasmic surface of outer segment disk membranes. Once all-trans-retinal escapes into the internal disk space, it is pumped out to the cytosol by a photoreceptorspecific ATP-binding transporter (4 -8). Several all-trans-retinol dehydrogenases (RDHs) 1 from the photoreceptor cells have been identified. First, Haeseleer et al. (9) cloned a cone-specific enzyme from the short-chain dehydrogenase/reductase (SDR) family with properties that suggest participation in the retinoid cycle. Next, Rattner and colleagues (10) reported the identification of a novel member of the SDR family, photoreceptor RDH (prRDH or RDH8), that localized to photoreceptors and possessed enzymatic properties closely matching those previously reported for RDH activity in ROS. The authors suggested that prRDH is the enzyme responsible for the reduction of all-trans-retinal to all-trans-retinol within the photoreceptor outer segment. The sequence homology among SDRs is typically low (20 -40%), but the structural homology is high and most protein folds are conserved (11). prR...

Steroid metabolism in cnidarians: Insights from Nematostella vectensis

“…HSD17B1 is primarily expressed in female reproductive tissues and catalyzes the reductive activation of estrogens (reviewed by Moeller and Adamski, 2006). As previously reported (Baker, 2001 and references therein) and as shown by our analysis, HSD17B1 is closely related to prRDH (photoreceptor-specific retinol dehydrogenase, also called RDH8), a reductive enzyme mediating the recycling of retinoids within the outer rods (Maeda et al, 2005). The steroidal activity of HSD17B1 may be derived from a retinoid-metabolizing ancestor, although additional phylogenetic analysis, structural modeling, and functional studies are needed to test this hypothesis (Baker, 2001;Mindnich and Adamski, 2007).…”

Retinoid processing in cone and Müller cell lines

“…RDH8 has been previously shown to be expressed in the outer segments of rod and cone photoreceptors in the rod dominated retina (Maeda, Maeda, Imanishi, Kuksa, Alekseev, Bronson, Zhang, Zhu, Sun, Saperstein, Rieke, Baehr and Palczewski, 2005). However, according to the q-RT-PCR results in the cone dominated Nrl −/− retina, it is barely expressed in cones compared to rods ( Figure 2B & C) and not expressed in 661W cells ( Figure 2A).…”

Aberrant Metabolites in Mouse Models of Congenital Blinding Diseases: Formation and Storage of Retinyl Esters

“…Rpe65-deficient mice were obtained from Dr. M. Redmond (NEI, National Institutes of Health) and genotyped as described previously (22). Prrdh-and Lrat-deficient mice were generated and characterized in our laboratory in collaboration with Dr. W. Baehr (University of Utah) as described previously (29,30). Double knockout mice were generated by cross-breeding single knockout mice to genetic homogeneity.…”