Structural insights into the mechanism and dynamics of proteorhodopsin biogenesis and retinal scavenging.

Microbial ion-pumping rhodopsins (MRs) are extensively studied retinal-binding membrane proteins. However, their biogenesis, including oligomerisation and retinal incorporation, remains poorly understood. The bacterial green-light absorbing proton pump proteorhodopsin (GPR) has emerged as a model protein for MRs and is used here to address these open questions using cryo-electron microscopy (cryo-EM) and molecular dynamics (MD) simulations. Specifically, conflicting studies regarding GPR stoichiometry reported pentamer and hexamer mixtures without providing possible assembly mechanisms. We report the pentameric and hexameric cryo-EM structures of a GPR mutant, uncovering the role of the unprocessed N-terminal signal peptide in the assembly of hexameric GPR. Furthermore, certain proteorhodopsin-expressing bacteria lack retinal biosynthesis pathways, suggesting that they scavenge the cofactor from their environment. We shed light on this hypothesis by solving the cryo-EM structure of retinal-free proteoopsin, which together with mass spectrometry and MD simulations suggests that decanoate serves as a temporary placeholder for retinal in the chromophore binding pocket. Further MD simulations elucidate possible pathways for the exchange of decanoate and retinal, offering a mechanism for retinal scavenging. Collectively, our findings provide insights into the biogenesis of MRs, including their oligomeric assembly, variations in protomer stoichiometry and retinal incorporation through a potential cofactor scavenging mechanism.

Glutamine is the only amino acid that can adequately support the generation of NADPH in rod photoreceptors.

NADPH, the primary source of reducing equivalents in the cytosol, is used in vertebrate rod photoreceptor outer segments to reduce the all-trans retinal released from photoactivated visual pigment to all-trans retinol. Light activation of the visual pigment isomerizes the 11-cis retinal chromophore to all-trans, thereby destroying it and necessitating its regeneration. Release and reduction of all-trans retinal are the first steps in the series of reactions that regenerate the visual pigment. Glucose and glutamine can both support the reduction of all-trans retinal to retinol, indicating that the NADPH used in rod photoreceptor outer segments can be generated by the pentose phosphate pathway as well as by mitochondria-linked pathways. We have used the conversion of all-trans retinal to all-trans retinol to examine whether amino acids other than glutamine can also support the generation of NADPH in rod photoreceptors. We have measured this conversion in single isolated mouse rod photoreceptors by imaging the fluorescence of the all-trans retinal and retinol generated after exposure of the cells to light. In agreement with previous work, we find that 5 mM glucose or 0.5 mM glutamine support the conversion of ∼70-80% of all-trans retinal to retinol, corresponding to a reduced NADP fraction of ∼10%. All other amino acids at 0.5 mM concentration support the conversion to a much lesser extent, indicating reduced NADP fractions of 1-2% at most. Taurine was also ineffective at supporting NADPH generation, while formic acid, the toxic metabolite of methanol, suppressed the generation of NADPH by either glucose or glutamine.

Channelrhodopsins with distinct chromophores and binding patterns.

Channelrhodopsins are popular optogenetic tools in neuroscience, but remain poorly understood mechanistically. Here we report the cryo-EM structures of channelrhodopsin-2 (ChR2) from Chlamydomonas reinhardtii and H. catenoides kalium channelrhodopsin (KCR1). We show that ChR2 recruits an endogenous N-retinylidene-PE-like molecule to a previously unidentified lateral retinal binding pocket, exhibiting a reduced light response in HEK293 cells. In contrast, H. catenoides kalium channelrhodopsin (KCR1) binds an endogenous retinal in its canonical retinal binding pocket under identical condition. However, exogenous ATR reduces the photocurrent magnitude of wild type KCR1 and also inhibits its leaky mutant C110T. Our results uncover diverse retinal chromophores with distinct binding patterns for channelrhodopsins in mammalian cells, which may further inspire next generation optogenetics for complex tasks such as cell fate control.

Activating an invertebrate bistable opsin with the all-trans 6.11 retinal analog.

Animal vision depends on opsins, a category of G protein-coupled receptor (GPCR) that achieves light sensitivity by covalent attachment to retinal. Typically binding as an inverse agonist, 11-cis retinal photoisomerizes to the all-trans isomer and activates the receptor, initiating downstream signaling cascades. Retinal bound to bistable opsins isomerizes back to the 11-cis state after absorption of a second photon, inactivating the receptor. Bistable opsins are essential for invertebrate vision and nonvisual light perception across the animal kingdom. While crystal structures are available for bistable opsins in the inactive state, it has proven difficult to form homogeneous populations of activated bistable opsins either via illumination or reconstitution with all-trans retinal. Here, we show that a nonnatural retinal analog, all-trans retinal 6.11 (ATR6.11), can be reconstituted with the invertebrate bistable opsin, Jumping Spider Rhodopsin-1 (JSR1). Biochemical activity assays demonstrate that ATR6.11 functions as a JSR1 agonist. ATR6.11 binding also enables complex formation between JSR1 and signaling partners. Our findings demonstrate the utility of retinal analogs for biophysical characterization of bistable opsins, which will deepen our understanding of light perception in animals.

The First Steps of the Visual Cycle in Human Rod and Cone Photoreceptors.

Purpose: Light detection destroys the visual pigment. Its regeneration, necessary for the recovery of light sensitivity, is accomplished through the visual cycle. Release of all-trans retinal by the light-activated visual pigment and its reduction to all-trans retinol comprise the first steps of the visual cycle. In this study, we determined the kinetics of all-trans retinol formation in human rod and cone photoreceptors. Methods: Single living rod and cone photoreceptors were isolated from the retinas of human cadaver eyes (ages 21 to 90 years). Formation of all-trans retinol was measured by imaging its outer segment fluorescence (excitation, 360 nm; emission, >420 nm). The extent of conversion of released all-trans retinal to all-trans retinol was determined by measuring the fluorescence excited by 340 and 380 nm. Measurements were repeated with photoreceptors isolated from Macaca fascicularis retinas. Experiments were carried out at 37°C. Results: We found that ∼80% to 90% of all-trans retinal released by the light-activated pigment is converted to all-trans retinol, with a rate constant of 0.24 to 0.55 min-1 in human rods and ∼1.8 min-1 in human cones. In M. fascicularis rods and cones, the rate constants were 0.38 ± 0.08 min-1 and 4.0 ± 1.1 min-1, respectively. These kinetics are several times faster than those measured in other vertebrates. Interphotoreceptor retinoid-binding protein facilitated the removal of all-trans retinol from human rods. Conclusions: The first steps of the visual cycle in human photoreceptors are several times faster than in other vertebrates and in line with the rapid recovery of light sensitivity exhibited by the human visual system.

RDH12 allows cone photoreceptors to regenerate opsin visual pigments from a chromophore precursor to escape competition with rods.

Capture of a photon by an opsin visual pigment isomerizes its 11-cis-retinaldehyde (11cRAL) chromophore to all-trans-retinaldehyde (atRAL), which subsequently dissociates. To restore light sensitivity, the unliganded apo-opsin combines with another 11cRAL to make a new visual pigment. Two enzyme pathways supply chromophore to photoreceptors. The canonical visual cycle in retinal pigment epithelial cells supplies 11cRAL at low rates. The photic visual cycle in Müller cells supplies cones with 11-cis-retinol (11cROL) chromophore precursor at high rates. Although rods can only use 11cRAL to regenerate rhodopsin, cones can use 11cRAL or 11cROL to regenerate cone visual pigments. We performed a screen in zebrafish retinas and identified ZCRDH as a candidate for the enzyme that converts 11cROL to 11cRAL in cone inner segments. Retinoid analysis of eyes from Zcrdh-mutant zebrafish showed reduced 11cRAL and increased 11cROL levels, suggesting impaired conversion of 11cROL to 11cRAL. By microspectrophotometry, isolated Zcrdh-mutant cones lost the capacity to regenerate visual pigments from 11cROL. ZCRDH therefore possesses all predicted properties of the cone 11cROL dehydrogenase. The human protein most similar to ZCRDH is RDH12. By immunocytochemistry, ZCRDH was abundantly present in cone inner segments, similar to the reported distribution of RDH12. Finally, RDH12 was the only mammalian candidate protein to exhibit 11cROL-oxidase catalytic activity. These observations suggest that RDH12 in mammals is the functional ortholog of ZCRDH, which allows cones, but not rods, to regenerate visual pigments from 11cROL provided by Müller cells. This capacity permits cones to escape competition from rods for visual chromophore in daylight-exposed retinas.

Ultrafast terahertz Stark spectroscopy reveals the excited-state dipole moments of retinal in bacteriorhodopsin.

The photoinduced all-trans to 13-cis isomerization of the retinal Schiff base represents the ultrafast first step in the reaction cycle of bacteriorhodopsin (BR). Extensive experimental and theoretical work has addressed excited-state dynamics and isomerization via a conical intersection with the ground state. In conflicting molecular pictures, the excited state potential energy surface has been modeled as a pure S[Formula: see text] state that intersects with the ground state, or in a 3-state picture involving the S[Formula: see text] and S[Formula: see text] states. Here, the photoexcited system passes two crossing regions to return to the ground state. The electric dipole moment of the Schiff base in the S[Formula: see text] and S[Formula: see text] state differs strongly and, thus, its measurement allows for assessing the character of the excited-state potential. We apply the method of ultrafast terahertz (THz) Stark spectroscopy to measure electric dipole changes of wild-type BR and a BR D85T mutant upon electronic excitation. A fully reversible transient broadening and spectral shift of electronic absorption is induced by a picosecond THz field of several megavolts/cm and mapped by a 120-fs optical probe pulse. For both BR variants, we derive a moderate electric dipole change of 5 [Formula: see text] 1 Debye, which is markedly smaller than predicted for a neat S[Formula: see text]-character of the excited state. In contrast, S[Formula: see text]-admixture and temporal averaging of excited-state dynamics over the probe pulse duration gives a dipole change in line with experiment. Our results support a picture of electronic and nuclear dynamics governed by the interaction of S[Formula: see text] and S[Formula: see text] states in a 3-state model.

Photoisomerization pathway of the microbial rhodopsin chromophore in solution.

Photoisomerization is a key photochemical reaction in microbial and animal rhodopsins. It is well established that such photoisomerization is highly selective; all-trans to 13-cis, and 11-cis to all-trans forms in microbial and animal rhodopsins, respectively. Nevertheless, unusual photoisomerization pathways have been discovered recently in microbial rhodopsins. In an enzymerhodopsin NeoR, the all-trans chromophore is isomerized into the 7-cis form exclusively, which is stable at room temperature. Although, the 7-cis form is produced by illumination of retinal, formation of the 7-cis form was never reported for a protonated Schiff base of all-trans retinal in solution. Present HPLC analysis of retinal oximes prepared by hydroxylamine reaction revealed that all-trans and 7-cis forms cannot be separated from the syn peaks under the standard HPLC conditions, while it is possible by the analysis of the anti-peaks. Consequently, we found formation of the 7-cis form by the photoreaction of all-trans chromophore in solution, regardless of the protonation state of the Schiff base. Upon light absorption of all-trans protonated retinal Schiff base in solution, excited-state relaxation accompanies double-bond isomerization, producing 7-cis, 9-cis, 11-cis, or 13-cis form. In contrast, specific chromophore-protein interaction enforces selective isomerization into the 13-cis form in many microbial rhodopsins, but into 7-cis in NeoR.

Spectral and mass characterization of kinetic conversion from retinoids to retinoic acid in an in vitro 3-D human skin equivalent model.

To investigate the effect of retinoids, such as retinol (ROL), retinal (RAL), and retinyl palmitate (RP), on epidermal integrity, skin deposition, and bioconversion to retinoic acid (RA). 3-D human skin equivalent model (EpiDermFT™) was used. Epidermal cellular integrity measured by TEER values was significantly higher for a topical treatment of ROL and RAL than RP (p < 0.05). The skin deposition (µM) of ROL and RAL was approximately 269.54 ± 73.94 and 211.35 ± 20.96, respectively, greater than that of RP (63.70 ± 37.97) over 2 h incubation. Spectral changes were revealed that the CO maximum absorbance occurred between 1600∼1800 cm-1 and was greater from ROL than that from RAL and RP, indicating conjugation of R-OH to R-CHO or R-COOH could strongly occur after ROL treatment. Subsequently, a metabolite from the bioconversion of ROL and RAL was identified as RA, which has a product ion of m/z 283.06, by using liquid a chromatography-mass spectrometry (LC-MS) - total ion chromatogram (TIC). The amount of bioconversion from ROL and RAL to RA in artificial skin was 0.68 ± 0.13 and 0.70 ± 0.10 µM at 2 h and 0.60 ± 0.04 and 0.57 ± 0.06 µM at 24 h, respectively. RA was not detected in the skin and the receiver compartment after RP treatment. ROL could be a useful dermatological ingredient to maintain epidermal integrity more effectively, more stably deposit on the skin, and more steadily metabolize to RA than other retinoids such as RAL and RP.

Retinoid Synthesis Regulation by Retinal Cells in Health and Disease.

Vision starts in retinal photoreceptors when specialized proteins (opsins) sense photons via their covalently bonded vitamin A derivative 11cis retinaldehyde (11cis-RAL). The reaction of non-enzymatic aldehydes with amino groups lacks specificity, and the reaction products may trigger cell damage. However, the reduced synthesis of 11cis-RAL results in photoreceptor demise and suggests the need for careful control over 11cis-RAL handling by retinal cells. This perspective focuses on retinoid(s) synthesis, their control in the adult retina, and their role during retina development. It also explores the potential importance of 9cis vitamin A derivatives in regulating retinoid synthesis and their impact on photoreceptor development and survival. Additionally, recent advancements suggesting the pivotal nature of retinoid synthesis regulation for cone cell viability are discussed.

Uncommon opsin's retinal isomer is involved in mammalian sperm thermotaxis.

In recent years it became apparent that, in mammals, rhodopsin and other opsins, known to act as photosensors in the visual system, are also present in spermatozoa, where they function as highly sensitive thermosensors for thermotaxis. The intriguing question how a well-conserved protein functions as a photosensor in one type of cells and as a thermosensor in another type of cells is unresolved. Since the moiety that confers photosensitivity on opsins is the chromophore retinal, we examined whether retinal is substituted in spermatozoa with a thermosensitive molecule. We found by both functional assays and mass spectrometry that retinal is present in spermatozoa and required for thermotaxis. Thus, starvation of mice for vitamin A (a precursor of retinal) resulted in loss of sperm thermotaxis, without affecting motility and the physiological state of the spermatozoa. Thermotaxis was restored after replenishment of vitamin A. Using reversed-phase ultra-performance liquid chromatography mass spectrometry, we detected the presence of retinal in extracts of mouse and human spermatozoa. By employing UltraPerformance convergence chromatography, we identified a unique retinal isomer in the sperm extracts-tri-cis retinal, different from the photosensitive 11-cis isomer in the visual system. The facts (a) that opsins are thermosensors for sperm thermotaxis, (b) that retinal is essential for thermotaxis, and (c) that tri-cis retinal isomer uniquely resides in spermatozoa and is relatively thermally unstable, suggest that tri-cis retinal is involved in the thermosensing activity of spermatozoa.

Unraveling the mystery of ocular retinoid turnover: Insights from albino mice and the role of STRA6.

A delicate balance between photon absorption for vision and the protection of photoreceptors from light damage is pivotal for ocular health. This equilibrium is governed by the light-absorbing 11-cis-retinylidene chromophore of visual pigments, which, upon bleaching, transforms into all-trans-retinal and undergoes regeneration through an enzymatic pathway, named the visual cycle. Chemical side reactions of retinaldehyde during the recycling process can generate by-products that may result in a depletion of retinoids. In our study, we have clarified the crucial roles played by melanin pigmentation and the retinoid transporter STRA6 in preventing this loss and preserving the integrity of the visual cycle. Our experiments initially confirmed that consecutive green and blue light bleaching of isolated bovine rhodopsin produced 9-cis and 13-cis retinal. The same unusual retinoids were found in the retinas of mice exposed to intense light, with elevated concentrations observed in albino mice. Examining the metabolic fate of these visual cycle byproducts revealed that 9-cis-retinal, but not 13-cis-retinal, was recycled back to all-trans-retinal through an intermediate called isorhodopsin. However, investigations in Stra6 knockout mice unveiled that the generation of these visual cycle byproducts correlated with a light-induced loss of ocular retinoids and visual impairment. Collectively, our findings uncover important novel aspects of visual cycle dynamics, with implications for ocular health and photoreceptor integrity.

Carotenoid cleavage enzymes evolved convergently to generate the visual chromophore.

The retinal light response in animals originates from the photoisomerization of an opsin-coupled 11-cis-retinaldehyde chromophore. This visual chromophore is enzymatically produced through the action of carotenoid cleavage dioxygenases. Vertebrates require two carotenoid cleavage dioxygenases, ß-carotene oxygenase 1 and retinal pigment epithelium 65 (RPE65), to form 11-cis-retinaldehyde from carotenoid substrates, whereas invertebrates such as insects use a single enzyme known as Neither Inactivation Nor Afterpotential B (NinaB). RPE65 and NinaB couple trans-cis isomerization with hydrolysis and oxygenation, respectively, but the mechanistic relationship of their isomerase activities remains unknown. Here we report the structure of NinaB, revealing details of its active site architecture and mode of membrane binding. Structure-guided mutagenesis studies identify a residue cluster deep within the NinaB substrate-binding cleft that controls its isomerization activity. Our data demonstrate that isomerization activity is mediated by distinct active site regions in NinaB and RPE65-an evolutionary convergence that deepens our understanding of visual system diversity.

Distinct mouse models of Stargardt disease display differences in pharmacological targeting of ceramides and inflammatory responses.

Mutations in many visual cycle enzymes in photoreceptors and retinal pigment epithelium (RPE) cells can lead to the chronic accumulation of toxic retinoid byproducts, which poison photoreceptors and the underlying RPE if left unchecked. Without a functional ATP-binding cassette, sub-family A, member 4 (ABCA4), there is an elevation of all-trans-retinal and prolonged buildup of all-trans-retinal adducts, resulting in a retinal degenerative disease known as Stargardt-1 disease. Even in this monogenic disorder, there is significant heterogeneity in the time to onset of symptoms among patients. Using a combination of molecular techniques, we studied Abca4 knockout (simulating human noncoding disease variants) and Abca4 knock-in mice (simulating human misfolded, catalytically inactive protein variants), which serve as models for Stargardt-1 disease. We compared the two strains to ascertain whether they exhibit differential responses to agents that affect cytokine signaling and/or ceramide metabolism, as alterations in either of these pathways can exacerbate retinal degenerative phenotypes. We found different degrees of responsiveness to maraviroc, a known immunomodulatory CCR5 antagonist, and to the ceramide-lowering agent AdipoRon, an agonist of the ADIPOR1 and ADIPOR2 receptors. The two strains also display different degrees of transcriptional deviation from matched WT controls. Our phenotypic comparison of the two distinct Abca4 mutant-mouse models sheds light on potential therapeutic avenues previously unexplored in the treatment of Stargardt disease and provides a surrogate assay for assessing the effectiveness for genome editing.

Convergent evolution of dim light vision in owls and deep-diving whales.

Animals with enhanced dim-light sensitivity are at higher risk of light-induced retinal degeneration when exposed to bright light conditions.1,2,3,4 This trade-off is mediated by the rod photoreceptor sensory protein, rhodopsin (RHO), and its toxic vitamin A chromophore by-product, all-trans retinal.5,6,7,8 Rod arrestin (Arr-1) binds to RHO and promotes sequestration of excess all-trans retinal,9,10 which has recently been suggested as a protective mechanism against photoreceptor cell death.2,11 We investigated Arr-1 evolution in animals at high risk of retinal damage due to periodic bright-light exposure of rod-dominated retinas. Here, we find the convergent evolution of enhanced Arr-1/RHO all-trans-retinal sequestration in owls and deep-diving whales. Statistical analyses reveal a parallel acceleration of Arr-1 evolutionary rates in these lineages, which is associated with the introduction of a rare Arr-1 mutation (Q69R) into the RHO-Arr-1 binding interface. Using in vitro assays, we find that this single mutation significantly enhances RHO-all-trans-retinal sequestration by ∼30%. This functional convergence across 300 million years of evolutionary divergence suggests that Arr-1 and RHO may play an underappreciated role in the photoprotection of the eye, with potentially vast clinical significance.

Rapid RGR-dependent visual pigment recycling is mediated by the RPE and specialized Müller glia.

In daylight, demand for visual chromophore (11-cis-retinal) exceeds supply by the classical visual cycle. This shortfall is compensated, in part, by the retinal G-protein-coupled receptor (RGR) photoisomerase, which is expressed in both the retinal pigment epithelium (RPE) and in Müller cells. The relative contributions of these two cellular pools of RGR to the maintenance of photoreceptor light responses are not known. Here, we use a cell-specific gene reactivation approach to elucidate the kinetics of RGR-mediated recovery of photoreceptor responses following light exposure. Electroretinographic measurements in mice with RGR expression limited to either cell type reveal that the RPE and a specialized subset of Müller glia contribute both to scotopic and photopic function. We demonstrate that 11-cis-retinal formed through photoisomerization is rapidly hydrolyzed, consistent with its role in a rapid visual pigment regeneration process. Our study shows that RGR provides a pan-retinal sink for all-trans-retinal released under sustained light conditions and supports rapid chromophore regeneration through the photic visual cycle.

A short story on how chromophore is hydrolyzed from rhodopsin for recycling.

The photocycle of visual opsins is essential to maintain the light sensitivity of the retina. The early physical observations of the rhodopsin photocycle by Böll and Kühne in the 1870s inspired over a century's worth of investigations on rhodopsin biochemistry. A single photon isomerizes the Schiff-base linked 11-cis-retinylidene chromophore of rhodopsin, converting it to the all-trans agonist to elicit phototransduction through photoactivated rhodopsin (Rho*). Schiff base hydrolysis of the agonist is a key step in the photocycle, not only diminishing ongoing phototransduction but also allowing for entry and binding of fresh 11-cis chromophore to regenerate the rhodopsin pigment and maintain light sensitivity. Many challenges have been encountered in measuring the rate of this hydrolysis, but recent advancements have facilitated studies of the hydrolysis within the native membrane environment of rhodopsin. These techniques can now be applied to study hydrolysis of agonist in other opsin proteins that mediate phototransduction or chromophore turnover. In this review, we discuss the progress that has been made in characterizing the rhodopsin photocycle and the journey to characterize the hydrolysis of its all-trans-retinylidene agonist.

Therapeutic Extracellular Vesicles from Tonsil-Derived Mesenchymal Stem Cells for the Treatment of Retinal Degenerative Disease.

BACKGROUND: Retinal degenerative disease (RDD), one of the most common causes of blindness, is predominantly caused by the gradual death of retinal pigment epithelial cells (RPEs) and photoreceptors due to various causes. Cell-based therapies, such as stem cell implantation, have been developed for the treatment of RDD, but potential risks, including teratogenicity and immune reactions, have hampered their clinical application. Stem cell-derived extracellular vesicles (EVs) have recently emerged as a cell-free alternative therapeutic strategy; however, additional invasiveness and low yield of the stem cell extraction process is problematic. METHODS: To overcome these limitations, we developed therapeutic EVs for the treatment of RDD which were extracted from tonsil-derived mesenchymal stem cells obtained from human tonsil tissue discarded as medical waste following tonsillectomy (T-MSC EVs). To verify the biocompatibility and cytoprotective effect of T-MSC EVs, we measured cell viability by co-culture with human RPE without or with toxic all-trans-retinal. To elucidate the cytoprotective mechanism of T-MSC EVs, we performed transcriptome sequencing using RNA extracted from RPEs. The in vivo protective effect of T-MSC EVs was evaluated using Pde6b gene knockout rats as an animal model of retinitis pigmentosa. RESULTS: T-MSC EVs showed high biocompatibility and the human pigment epithelial cells were significantly protected in the presence of T-MSC EVs from the toxic effect of all-trans-retinal. In addition, T-MSC EVs showed a dose-dependent cell death-delaying effect in real-time quantification of cell death. Transcriptome sequencing analysis revealed that the efficient ability of T-MSC EVs to regulate intracellular oxidative stress may be one of the reasons explaining their excellent cytoprotective effect. Additionally, intravitreally injected T-MSC EVs had an inhibitory effect on the destruction of the outer nuclear layer in the Pde6b gene knockout rat. CONCLUSIONS: Together, the results of this study indicate the preventive and therapeutic effects of T-MSC EVs during the initiation and development of retinal degeneration, which may be a beneficial alternative for the treatment of RDD.

Tuning the Metabolic Stability of Visual Cycle Modulators through Modification of an RPE65 Recognition Motif.

In the eye, the isomerization of all-trans-retinal to 11-cis-retinal is accomplished by a metabolic pathway termed the visual cycle that is critical for vision. RPE65 is the essential trans-cis isomerase of this pathway. Emixustat, a retinoid-mimetic RPE65 inhibitor, was developed as a therapeutic visual cycle modulator and used for the treatment of retinopathies. However, pharmacokinetic liabilities limit its further development including: (1) metabolic deamination of the γ-amino-α-aryl alcohol, which mediates targeted RPE65 inhibition, and (2) unwanted long-lasting RPE65 inhibition. We sought to address these issues by more broadly defining the structure-activity relationships of the RPE65 recognition motif via the synthesis of a family of novel derivatives, which were tested in vitro and in vivo for RPE65 inhibition. We identified a potent secondary amine derivative with resistance to deamination and preserved RPE65 inhibitory activity. Our data provide insights into activity-preserving modifications of the emixustat molecule that can be employed to tune its pharmacological properties.

eIF2α incites photoreceptor cell and retina damage by all-trans-retinal.

Dry age-related macular degeneration (AMD) and recessive Stargardt's disease (STGD1) lead to irreversible blindness in humans. The accumulation of all-trans-retinal (atRAL) induced by chaos in visual cycle is closely associated with retinal atrophy in dry AMD and STGD1 but its critical downstream signaling molecules remain ambiguous. Here, we reported that activation of eukaryotic translation initiation factor 2α (eIF2α) by atRAL promoted retinal degeneration and photoreceptor loss through activating c-Jun N-terminal kinase (JNK) signaling-dependent apoptosis and gasdermin E (GSDME)-mediated pyroptosis. We determined that eIF2α activation by atRAL in photoreceptor cells resulted from endoplasmic reticulum homeostasis disruption caused at least in part by reactive oxygen species production, and it activated JNK signaling independent of and dependent on activating transcription factor 4 and the activating transcription factor 4/transcription factor C/EBP homologous protein (CHOP) axis. CHOP overexpression induced apoptosis of atRAL-loaded photoreceptor cells through activating JNK signaling rather than inhibiting the expression of antiapoptotic gene Bcl2. JNK activation by eIF2α facilitated photoreceptor cell apoptosis caused by atRAL via caspase-3 activation and DNA damage. Additionally, we demonstrated that eIF2α was activated in neural retina of light-exposed Abca4-/-Rdh8-/- mice, a model that shows severe defects in atRAL clearance and displays primary features of human dry AMD and STGD1. Of note, inhibition of eIF2α activation by salubrinal effectively ameliorated retinal degeneration and photoreceptor apoptosis in Abca4-/-Rdh8-/- mice upon light exposure. The results of this study suggest that eIF2α is an important target to develop drug therapies for the treatment of dry AMD and STGD1.