Retinoids and Retinal Diseases

“…protein investigation: RPE65 gene therapy to restore a functional RPE-based retinoid cycle (Acland et al. 2001, Bainbridge et al. 2008, Maguire et al. 2008) and treatment with an artificial chromophore (9-cis-RAL) to bypass the metabolic blockade caused by the lack of RPE65 activity…”

“…cellular retinaldehyde-binding protein investigation: RPE65 gene therapy to restore a functional RPE-based retinoid cycle (Acland et al. 2001, Bainbridge et al. 2008, Maguire et al. 2008) and treatment with an artificial chromophore (9-cis-RAL) to bypass the metabolic blockade caused by the…”

“…result in a severe, juvenile-onset autosomal recessive retinopathy called Stargardt disease (SGD), characterized by the loss of central vision, disruption of retinal structure, and accumulation of retinoid-containing lipofuscin-like material (Figure 5d ) (Allikmets et al. 1997b, Sun et al. 2000).”

“Shortly after elucidation of the rhodopsin primary structure, the gene encoding the β1-adrenergic receptor (β1-AR) was identified, which revealed that these two proteins belong to the G protein–coupled receptor superfamily (Dixon et al. 1986).”

“…protein CRALBP: cellular retinaldehyde-binding protein investigation: RPE65 gene therapy to restore a functional RPE-based retinoid cycle (Acland et al. 2001, Bainbridge et al. 2008, Maguire et al. 2008) and treatment with an artificial chromophore (9-cis-RAL) to bypass the metabolic…”

“These light receptors belong to the broad type 2 opsin family and typically signal through G proteins (Ernst et al. 2014).”

“A RHO mutation resulting in a Pro → His substitution at position 23 in the amino acid sequence (P23H) was the first reported pathological mutation in this gene, and it is a particularly common cause of inherited autosomal dominant RP (Dryja et al. 1990).”

“This consisted of nearly flat ERG responses, a lack of ocular 11-cis-retinoids, and an overaccumulation of all-trans-retinyl esters in the RPE (Redmond et al. 1998).”

“The figure was generated with PyMol (Schrödinger, https://www.pymol.org/). its loss by glomerular filtration and renal excretion (Kanai et al. 1968).”

“…result in a severe, juvenile-onset autosomal recessive retinopathy called Stargardt disease (SGD), characterized by the loss of central vision, disruption of retinal structure, and accumulation of retinoid-containing lipofuscin-like material (Figure 5d ) (Allikmets et al. 1997b, Sun et al. 2000).”

“Additionally, the RPE takes up vitamin A from holo-RBP4 circulating in the choriocapillaris through the STRA6 transporter expressed on its basolateral surface (Amengual et al. 2012, Isken et al. 2008, Kawaguchi et al. 2007).”

“…retinyl esters within the RPE (Radu et al. 2008, Wenzel et al. 2005), melanopsin, expressed in photosensitive ganglion cells that trigger the pupillary light response (Lucas et al. 2003, Panda et al. 2003), and related proteins, which are involved in circadian rhythmicity (Van Gelder 2008).”

“Upon photoisomerization, the Schiff base loses its proton to form the signaling-competent Meta II rhodopsin state, with a blue-shifted absorbance spectrum (Wald 1968).”

“This topology was confirmed by the projection structure of rhodopsin (Schertler et al. 1993), which provided the first detailed information concerning the configuration of the α-helical bundle of this protein.”

“Conformational changes in the visual pigment elicited by this isomerization allow the receptor to couple to its cognate G protein and initiate the phototransduction cascade (Palczewski 2006).”

“…retinyl esters within the RPE (Radu et al. 2008, Wenzel et al. 2005), melanopsin, expressed in photosensitive ganglion cells that trigger the pupillary light response (Lucas et al. 2003, Panda et al. 2003), and related proteins, which are involved in circadian rhythmicity (Van Gelder 2008).”

“In rhodopsin’s ground state, the Schiff base is protonated, with the glutamate113 side chain carboxylate serving as the counterion (Nathans 1990, Sakmar et al. 1989).”

“…of retinylidene proteins expressed in lower organisms that exhibit significant structural similarity to type 2 opsins, including a seven-transmembrane-spanning α-helical fold and a retinal-binding lysine (Lys) residue located at a position equivalent to Lys296 in rhodopsin (Spudich et al. 2000).”

“Crystal structures of rhodopsin, including Meta II–like states, indicate that the isomerization of retinylidene is communicated to the G protein–interacting surface via alterations in a watermediated hydrogen bonding network (Angel et al. 2009, Choe et al. 2011).”

“In the most severe cases, RPE65 mutations cause the childhood retinopathy Leber congenital amaurosis (LCA), which results in childhood blindness (Gu et al. 1997, Marlhens et al. 1997, Morimura et al. 1998).”

“In the most severe cases, RPE65 mutations cause the childhood retinopathy Leber congenital amaurosis (LCA), which results in childhood blindness (Gu et al. 1997, Marlhens et al. 1997, Morimura et al. 1998).”

“…as the visual cycle isomerase was not generally accepted until 2005, when three independent groups used reconstituted visual cycle systems in cultured cells to show that RPE65 is the key component conferring retinoid isomerase activity ( Jin et al. 2005, Moiseyev et al. 2005, Redmond et al. 2005).”

“Rhodopsin constitutes an integral component of the rod outer segment structure as demonstrated by the absence of photoreceptor outer segments in Rho−/− mice (Humphries et al. 1997).”

“Determination of the bovine rhodopsin amino acid sequence (Hargrave et al. 1983, Nathans & Hogness 1983) has revealed seven distinct stretches of hydrophobic amino acids of appropriate length to be embedded as α-helices in a phospholipid membrane.”

“Rbp4−/− mice are viable and fertile, but exhibit a non-developmental retinal dysfunction that is correctable by consumption of a vitamin A–enriched diet (Quadro et al. 1999).”

“In the most severe cases, RPE65 mutations cause the childhood retinopathy Leber congenital amaurosis (LCA), which results in childhood blindness (Gu et al. 1997, Marlhens et al. 1997, Morimura et al. 1998).”

“The molecular steps in recycling chromophores phagocytized by the RPE (Kevany & Palczewski 2010) must be defined as well as their utilization in regenerating visual pigments.”

“…as the visual cycle isomerase was not generally accepted until 2005, when three independent groups used reconstituted visual cycle systems in cultured cells to show that RPE65 is the key component conferring retinoid isomerase activity ( Jin et al. 2005, Moiseyev et al. 2005, Redmond et al. 2005).”

“Rhodopsin, like all known vertebrate visual pigments, senses light through a Schiff base–linked 11-cis-retinylidene chromophore (Hubbard & Wald 1952).”

“Mutations in human RLBP1 are associated with several different retinopathies, including RP (Maw et al. 1997), Bothnia dystrophy, retinitis punctata albescens, and Newfoundland rod–cone dystrophy (Travis et al. 2007).”

“In the circulating RBP–TTR complex, the ROL hydroxyl group is covered by the tip of an extended loop region of TTR composed of residues 76–87 (Monaco et al. 1995).”

“…as the visual cycle isomerase was not generally accepted until 2005, when three independent groups used reconstituted visual cycle systems in cultured cells to show that RPE65 is the key component conferring retinoid isomerase activity ( Jin et al. 2005, Moiseyev et al. 2005, Redmond et al. 2005).”

“A single ROL-binding site is formed by the core, eight-stranded, β-barrel fold of RBP4 (Figure 9a) (Newcomer et al. 1984).”

“…downstream second messenger–generating enzymes (PDE6 and adenylyl cyclase for, respectively, rhodopsin and β1-AR) by catalyzing the exchange of guanosine triphosphate for guanosine diphosphate in an intermediary signaling, heterotrimeric G protein called transducin (Kwok-Keung Fung & Stryer 1980).”

“The resulting condensation products and their oxidized derivatives reportedly are toxic to the RPE (reviewed in Sparrow et al. 2012).”

“Defined enzymatic activities thought important for cone-specific regeneration have been identified in the retinas of cone-dominant species (Figure 3) (Mata et al. 2002, Muniz et al. 2009).”

“9-cis-Retinyl acetate and LRAT gene replacement therapy are under investigation as treatments for RP associated with loss-of-function LRAT mutations (Batten et al. 2005, Koenekoop et al. 2014).”

“Sustained vision thus relies on a mechanism for regenerating ground-state visual pigments (Kiser et al. 2014, McBee et al. 2001).”

“Compared with classical NlpC/P60 enzymes in which the nucleophilic cysteine residue resides on the N terminus, LRATlike members are circularly permuted, with the catalytic cysteine relocated to the C-terminal end (Anantharaman & Aravind 2003).”

“Mutations in STRA6 are associated with a severe developmental disorder known as the Matthew-Wood syndrome (Golzio et al. 2007, Pasutto et al. 2007).”

“Sequestration of All-trans-Retinal to Prevent Toxicity A series of studies have demonstrated that one candidate toxin in the retinoid cycle is all-transRAL released from photoactivated visual pigments (reviewed in Kiser et al. 2014).”

“This intraretinal visual cycle specifically supplies cone photoreceptors with quantities of visual chromophore sufficient for operation under photopic conditions (Wang & Kefalov 2011).”

“This developmentally regulated microsomal protein was cloned in the early 1990s (Hamel et al. 1993), but its involvement in retinal health and function remained unclear until later in the decade when it was discovered that RPE65 mutations cause severe retinopathies in humans.”

“Mutations in human RLBP1 are associated with several different retinopathies, including RP (Maw et al. 1997), Bothnia dystrophy, retinitis punctata albescens, and Newfoundland rod–cone dystrophy (Travis et al. 2007).”

“In rhodopsin’s ground state, the Schiff base is protonated, with the glutamate113 side chain carboxylate serving as the counterion (Nathans 1990, Sakmar et al. 1989).”

“…products, vertebrate photoreceptors express an approximately 250 kDa adenosine triphosphate (ATP)-driven transporter called ABCA4 (for ATP-binding cassette transporter 4, also known as ABCR) that belongs to the large ABC transporter family (Figure 5c) (Illing et al. 1997, Tsybovsky et al. 2010).”

“In humans, loss-of-function RDH5 mutations cause a mild retinal disease known as fundus albipunctatus, characterized by night blindness, delayed dark adaptation, and the presence of small, pale yellow deposits in the subretinal space (Yamamoto et al. 1999).”

“This P23H rhodopsin is highly susceptible to degradation within the endoplasmic reticulum (ER), impeding rhodopsin trafficking to the outer segments, disrupting disc morphogenesis, and, ultimately, causing photoreceptor cell death (Sakami et al. 2011).”

“In vivo, effective RDH8 function depends on the ABCA4 transporter, which flips N-retinylidenephosphatidylethanolamine conjugates to the cytosolic leaflet of disks where they can dissociate, and the released all-trans-RAL becomes accessible to the RDH8 active site (Quazi et al. 2012).”

“ABCA4 is also expressed in cone photoreceptor outer segments and thus could participate in retinoid cycling within cones (Molday et al. 2000).”

“This limitation has been circumvented by using dual AAV vectors containing portions of the ABCA4 coding sequence that reconstitute the complete coding sequence in situ via concatemerization (Trapani et al. 2014).”

“The latter compound is then transferred to the RPE where it undergoes esterification by a microsomal enzyme (Andrews & Futterman 1964), lecithin:retinol acyltransferase (LRAT) (Ruiz et al. 1999), to form all-trans-retinyl esters.”

“F or p er so na l u se o nl y. 986 445 1, 00 0 555 1,000 814 0.6 RDH14 cRDH13L RDH12 RDH11 RDH13 RDH5 RDH8 RDH10 DHRS3 11-cis/NAD(H) All-trans/NADP(H) Dual specificity/NADP(H) Figure 6 An unrooted molecular phylogenetic tree illustrating relationships among groups of RDH homologs.”

“Rlbp1−/− mice exhibit delayed dark adaption consistent with a key role of CRALBP in efficient visual cycle function (Saari et al. 2001).”

“Additionally, all-trans-RAL released from photoactivated visual pigments is cytotoxic at high concentrations and must be rapidly converted to nonelectrophilic all-trans-ROL to maintain the health of the retina (Chen et al. 2012, Maeda et al. 2009a).”

“Consistent with this strong murine phenotype, mutations in RDH12 cause RP in humans ( Janecke et al. 2004).”

“ABCA4 localizes specifically to the rim regions of rod photoreceptor disks, hence its prior designation as a Rim protein (Papermaster et al. 1978), possibly because of the large size of its extracellular domains, which could exclude the protein from the narrow interior of the disks’ central regions (Figure 5c) (Tsybov…”

“Additionally, all-trans-RAL released from photoactivated visual pigments is cytotoxic at high concentrations and must be rapidly converted to nonelectrophilic all-trans-ROL to maintain the health of the retina (Chen et al. 2012, Maeda et al. 2009a).”

“Inhibitors of Retinal Pigment Epithelium–Specific 65 kDa Protein Another idea is that slowing the regeneration of visual pigments by inhibiting the retinoid cycle will decrease the amount of free all-trans-RAL and its toxic condensation products (Radu et al. 2003).”

“A dose of 50,000 IU/day of vitamin A administered orally diminishes night blindness in patients with early stages of this disease ( Jacobson et al. 1995).”

“inner segments, accepts trans- and cis-retinoid isomers as well as some other lipophilic aldehydes and alcohols as substrates (Haeseleer et al. 2002).”

“2007), so named because of its retinoic acid (RA)-inducible nature (Bouillet et al. 1997).”

“This protein displays selectivity for all-trans-retinoid substrates, and utilizes NADP(H) as a cofactor, signifying that it catalyzes reduction in cells (Rattner et al. 2000).”

“…cellular retinaldehyde-binding protein investigation: RPE65 gene therapy to restore a functional RPE-based retinoid cycle (Acland et al. 2001, Bainbridge et al. 2008, Maguire et al. 2008) and treatment with an artificial chromophore (9-cis-RAL) to bypass the metabolic blockade caused by the…”

“…products, vertebrate photoreceptors express an approximately 250 kDa adenosine triphosphate (ATP)-driven transporter called ABCA4 (for ATP-binding cassette transporter 4, also known as ABCR) that belongs to the large ABC transporter family (Figure 5c) (Illing et al. 1997, Tsybovsky et al. 2010).”

“9-cis-Retinoids are the only other isomers capable of binding CRALBP with high affinity.”

“Mutations in STRA6 are associated with a severe developmental disorder known as the Matthew-Wood syndrome (Golzio et al. 2007, Pasutto et al. 2007).”

“Similar malformations have been observed in a STRA6 knockdown zebrafish model of Matthew-Wood syndrome (Isken et al. 2008), whereas Stra6−/− mice exhibit a more mild and restricted phenotype, primarily affecting the eye (Amengual et al.”

“The resulting all-trans-retinyl esters are efficiently stored in lipid droplet–like structures within the RPE referred to as retinosomes (Imanishi et al. 2004).”

“ABCA4 gene therapy for the treatment of SGD is an active and promising area of research (Cideciyan et al. 2009).”

“Consistent with the knockout mouse phenotype, loss-of-function mutations in LRAT cause RP in humans (Thompson et al. 2001).”

“In the 1980s, a phospholipid membrane-dependent retinoid isomerase activity was discovered in RPE cellular fractions (Bernstein et al. 1987, Deigner et al. 1989), some 50 years after Wald’s first description of the visual cycle pathway.”

“Retinal SDR1 (or RetSDR1), also known as DHRS3, is an all-trans-specific, NADP(H)-dependent RDH with retinal expression restricted to cone photoreceptor outer segments (Haeseleer et al. 1998).”

“[Importantly, profound suppression of visual pigment regeneration is detrimental to retinal health because free opsin has constitutive signaling activity that eventually causes the death of photoreceptor cells, as mentioned in Section 5 (Woodruff et al. 2003).]”

“Like RBP4, CRBP1 is a small, water soluble protein that contains a single ROL-binding site formed by its β-barrel fold (Figure 9b) (Cowan et al. 1993).”

“Crystallographic data have indicated that RPE65 forms a homodimeric structure mediated by a sequence insertion found in vertebrate carotenoid cleavage oxygenases (Kiser et al. 2012a).”

“In the 1980s, a phospholipid membrane-dependent retinoid isomerase activity was discovered in RPE cellular fractions (Bernstein et al. 1987, Deigner et al. 1989), some 50 years after Wald’s first description of the visual cycle pathway.”

“But when reared under nonextreme lighting conditions, these mice have structurally and functionally intact retinas, revealing that other enzymes can compensate for the loss of RDH8 enzymatic activity at this step of the retinoid cycle (Maeda et al. 2005).”

“Crystallographic data have indicated that RPE65 forms a homodimeric structure mediated by a sequence insertion found in vertebrate carotenoid cleavage oxygenases (Kiser et al. 2012a).”

“Thus, fenretinide, an amide of RA, had been hypothesized to have an antagonistic effect on the RBP4–TTR complex (Mata et al. 2013).”

“The reactions described above comprise the classical visual cycle essential for both rod and cone cell function in humans (Figure 3) ( Jacobson et al. 2007, Saari 2012).”

“Photoreceptors secrete an approximately 136 kDa lipoglycoprotein called IRBP that mediates intercellular retinoid transfer (Bunt-Milam & Saari 1983, Lai et al. 1982).”

“Moreover, there is experimental support for the existence of a second pathway for 11-cis-RAL biosynthesis in the retina (Fleisch et al. 2008, Wang et al. 2009).”

“Nonviral DNA nanoparticles, lacking the size limitations associated with AAV vectors, have also been successfully employed for delivering ABCA4 into the retinas of Abca4−/− mice (Han et al. 2012).”

“…is reversible, a small portion of the conjugates undergo secondary reactions with an additional alltrans-RAL molecule that result in the irreversible formation of a number of bis-retinoid adducts, the most thoroughly characterized of which are A2E and all-trans-RAL-dimer (Kim et al. 2007).”

“All-trans-RAL is prone to undergoing reactions with primary amine-containing compounds, such as proteins (which can directly cause cellular toxicity), small molecular drugs (Maeda et al. 2012), and phosphatidylethanolamine (PE).”

“9-cis-Retinyl acetate and LRAT gene replacement therapy are under investigation as treatments for RP associated with loss-of-function LRAT mutations (Batten et al. 2005, Koenekoop et al. 2014).”

“Similarly, treatment with pharmacological doses of vitamin A restored vision in Stra6−/- mice lacking adequate vitamin A supplementation to the eye (Amengual et al. 2014).”

“Rdh5−/- mice exhibit a mild phenotype consisting of delayed dark adaptation at high bleaching light levels and the accumulation of ocular 11/13-cis-ROL/retinyl esters (Driessen et al. 2000).”

“These mice exhibit delayed dark adaptation and slowly progressive rod–cone dystrophy, but can still regenerate visual pigments, indicating the presence of additional RAL reductases in mouse retina (Maeda et al. 2007).”

“Notably, LRAT is fairly promiscuous in its choice of acyl acceptors and can readily esterify or amidate a number of primary alcohol and amine compounds, including the retinoid cycle inhibitor retinylamine (Golczak et al. 2005a).”

“The systematic comparison of different inhibitors has been reported (Maeda et al. 2006a) and, based on those results, emixustat was developed by Acucela Inc. (Bavik et al. 2015), but human clinical trials have revealed significant side effects (Kubota et al. 2012).”

“…are based on the retinoid structure; thus their specific delivery to the eye can employ the same retinoid mechanism, for example, transport by RBP4, delivery via STRA6, or enhancement of retention through esterification or amidation in the retina with the help of LRAT (reviewed in Palczewski 2010).”

“Consistent with an important role for IRBP in retinoid trafficking within the retina, mutations in the RBP3 gene are associated with autosomal recessive RP (den Hollander et al. 2009).”

“RDH11, a dual-substrate specificity RDH, is one such enzyme that contributes to 11-cis-RAL reduction in the RPE (Haeseleer et al. 2002, Kim et al. 2005).”

“A current model of the Müller cell–based cone visual cycle posits key roles for RDHs in the selective delivery of visual chromophore to cone visual pigments (Kaylor et al. 2013).”

“Similar malformations have been observed in a STRA6 knockdown zebrafish model of Matthew-Wood syndrome (Isken et al. 2008), whereas Stra6−/− mice exhibit a more mild and restricted phenotype, primarily affecting the eye (Amengual et al. 2014, Ruiz et al. 2012).”

“9-cis-Retinyl acetate and LRAT gene replacement therapy are under investigation as treatments for RP associated with loss-of-function LRAT mutations (Batten et al. 2005, Koenekoop et al. 2014).”

“2015), but human clinical trials have revealed significant side effects (Kubota et al. 2012).”

“Interestingly, retSDR1 and a related RDH also involved in RA biosynthesis, RDH10, modulate each other’s activity to precisely control RA levels (Figure 6) (Adams et al. 2014). www.annualreviews.org • The Visual Cycle and Its Modulators 215 A nn u.”

“The retinoid-binding site is located within the membrane-proximal region of the cavity (Kiser et al. 2015), which exhibits a negative electrostatic potential likely needed to stabilize a carbocation intermediate of the isomerization reaction (Figure 8a) (McBee et al. 2000).”

“Of the retinoid isomers, it binds all-trans-ROL with the highest affinity, consistent with vitamin A being the physiological ligand (MacDonald & Ong 1987, Ong & Chytil 1975).”

“Additionally, the RPE takes up vitamin A from holo-RBP4 circulating in the choriocapillaris through the STRA6 transporter expressed on its basolateral surface (Amengual et al. 2012, Isken et al. 2008, Kawaguchi et al. 2007).”

“…mediate color vision, the retinal G protein–coupled receptor (RGR), which is involved in mobilizing retinyl esters within the RPE (Radu et al. 2008, Wenzel et al. 2005), melanopsin, expressed in photosensitive ganglion cells that trigger the pupillary light response (Lucas et al. 2003, Panda et…”

“Nonretinoid antagonists of the RBP4 complex are being developed (Dobri et al. 2013).”

“STRA6 contains an extracellular loop that mediates its interaction with holo-RBP4 to allow extraction of all-trans-ROL from the RBP4-binding pocket (Figure 5b) (Kawaguchi et al. 2008).”

“The retinoid-binding site is located within the membrane-proximal region of the cavity (Kiser et al. 2015), which exhibits a negative electrostatic potential likely needed to stabilize a carbocation intermediate of the isomerization reaction (Figure 8a) (McBee et al. 2000).”

“Additionally, RDH10 plays a decisive part in regulating RA levels and, hence, developmental processes, as evidenced by the embryonic lethality in Rdh10−/mice (Farjo et al. 2011).”

“…pigments, which mediate color vision, the retinal G protein–coupled receptor (RGR), which is involved in mobilizing retinyl esters within the RPE (Radu et al. 2008, Wenzel et al. 2005), melanopsin, expressed in photosensitive ganglion cells that trigger the pupillary light response (Lucas et…”

“Additionally, all-trans-RAL released from photoactivated visual pigments is cytotoxic at high concentrations and must be rapidly converted to nonelectrophilic all-trans-ROL to maintain the health of the retina (Chen et al. 2012, Maeda et al. 2009a).”

“Long-term administration of deuterium-enriched vitamin A, namely C20-D3–vitamin A, slowed the formation of all-trans-RAL condensation products (Ma et al. 2011).”

“The systematic comparison of different inhibitors has been reported (Maeda et al. 2006a) and, based on those results, emixustat was developed by Acucela Inc. (Bavik et al. 2015), but human clinical trials have revealed significant side effects (Kubota et al. 2012).”

“F or p er so na l u se o nl y. function upon vitamin A supplementation has also been observed in a mouse model with a specific rhodopsin mutation related to RP (Sibulesky et al. 1999).”

“The systematic comparison of different inhibitors has been reported (Maeda et al. 2006a) and, based on those results, emixustat was developed by Acucela Inc. (Bavik et al. 2015), but human clinical trials have revealed significant side effects (Kubota et al. 2012).”

“Abca4−/− mice display some features associated with SGD, although the phenotypes are mild relative to the human disease.”

“F or p er so na l u se o nl y. all-trans-ROL yet binds to STRA6 with a higher affinity than wild-type RBP4, as discussed in the section on STRA6 (Chou et al. 2015).”

“Finally, recent studies have questioned the mechanism of action of emixustat and suggested that sequestration of all-trans-RAL should be considered as discussed in Section 8.5 (Figure 10b) (Zhang et al. 2015).”

“The strong binding selectivity for these two isomers has been explained by the presence of a curved binding pocket that has high complementarity to the 11-cis- and 9-cis-configurations (Figure 9c) (He et al. 2009).”

“RDH10 is expressed in Müller glial cells, although at much lower levels than in the RPE (Wu et al. 2004).”

“STRA6 contains an extracellular loop that mediates its interaction with holo-RBP4 to allow extraction of all-trans-ROL from the RBP4-binding pocket (Figure 5b) (Kawaguchi et al. 2008).”

“Moreover, there is experimental support for the existence of a second pathway for 11-cis-RAL biosynthesis in the retina (Fleisch et al. 2008, Wang et al. 2009).”

“RDH10 also is expressed in the RPE (Wu et al. 2002), where it contributes to 11- cis-ROL oxidation (Farjo et al. 2009).”

“RPE65 adopts a seven-bladed β-propeller structure containing long insertions on its top face that form a mostly α-helical cap constituting the enzymatic active site (Figure 8) (Kiser et al. 2009, 2012a).”

“Notably, LRAT is fairly promiscuous in its choice of acyl acceptors and can readily esterify or amidate a number of primary alcohol and amine compounds, including the retinoid cycle inhibitor retinylamine (Golczak et al. 2005a).”

“Intravitreal treatment with β-cyclodextrins reduced bis-retinoids in the RPE of mice by 48% (Nociari et al. 2014).”

“Reduction of All-trans-Retinal to All-trans-Retinol in Photoreceptor Cells The first enzymatic step of the visual cycle is the conversion of all-trans-RAL into all-trans-ROL, which occurs mostly in photoreceptor outer segments (Zimmerman et al. 1975).”

“An amphipathic N-terminal region can also mediate monotopic membrane interactions (Moise et al. 2007).”

“This approach has been enabled by the advent of in vivo two-photon microscopy imaging of the retina, used in conjunction with traditional imaging and histological modalities, as a means to track disease pathogenesis and treatment efficacy (Maeda et al. 2014).”

“Grafting this LRAT-specific region into the HRASLS3 sequence has resulted in a chimeric protein with sn1-specific LRAT activity akin to that of native LRAT (Golczak et al. 2015).”

“LRAT-catalyzed acyl transfer occurs via a ping-pong mechanism featuring a covalent acyl-enzyme intermediate involving a conserved cysteine residue (Cys161 in the human sequence) (Mondal et al. 2000).”

“That the retinoid cycle is slowed only in the setting of an RDH5 loss-of-function mutation indicates that additional enzymes are capable of carrying out oxidation of 11-cis-ROL in the retina ( Jang et al. 2000).”

“…regions of rod photoreceptor disks, hence its prior designation as a Rim protein (Papermaster et al. 1978), possibly because of the large size of its extracellular domains, which could exclude the protein from the narrow interior of the disks’ central regions (Figure 5c) (Tsybovsky et al. 2013).”

“The structure of an IRBP module from Xenopus has revealed that the modules adopt a two-domain architecture (Loew & Gonzalez-Fernandez 2002).”

“RP: retinitis pigmentosa has been relocated within the helical bundle suggests that type 1 and 2 opsins descended from a common ancestral sequence (Devine et al. 2013).”

“RPE65 activity appears to have been first acquired by a BCO II–like sequence in the last common ancestor of jawless and jawed vertebrates (Poliakov et al. 2012).”

“The systematic comparison of different inhibitors has been reported (Maeda et al. 2006a) and, based on those results, emixustat was developed by Acucela Inc. (Bavik et al. 2015), but human clinical trials have revealed significant side effects (Kubota et al. 2012).”

“Defined enzymatic activities thought important for cone-specific regeneration have been identified in the retinas of cone-dominant species (Figure 3) (Mata et al. 2002, Muniz et al. 2009).”

“The lipophilic nature of rhodopsin was first noted by Kühne (1977), who observed that it could be solubilized in a spectrally intact form by treating rod outer segments with bile detergents.”

“This integral membrane enzyme uses NAD(H) as a cofactor (Simon et al. 1995, Zimmerman 1976).”

“Retinas from Nrl−/−mice, which have only S-cone photoreceptors, express RDH13 and RDH14 at high levels compared with rod-dominant, wild-type retinas, indicating that these candidate enzymes may be responsible for the cone RDH activity (Kanan et al. 2008).”

“Kühne made the critical discovery that the color of the retina could be restored if it was positioned back in contact with the retinal pigment epithelium (RPE) and placed in the dark, thus establishing the existence of a two-cell type system required for visual pigment regeneration.”

“The site of action is throughout the tissues, where both retinol and β-carotene are oxidized to retinal and retinoic acid, which are essential for the variety of biological roles of vitamin A [26].”

“18 65 kDa protein (RPE65) a retinoid isomerohydrolase required for the vision cycle [123].”