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Vision is initiated by the rhodopsin family of light-sensitive G protein-coupled receptors (GPCRs) 1 . A photon is absorbed by the 11- cis retinal chromophore of rhodopsin, which isomerizes within 200 femtoseconds to the all- trans conformation 2 , thereby initiating the cellular signal transduction processes that ultimately lead to vision. However, the intramolecular mechanism by which the photoactivated retinal induces the activation events inside rhodopsin remains experimentally unclear. Here we use ultrafast time-resolved crystallography at room temperature 3 to determine how an isomerized twisted all -trans retinal stores the photon energy that is required to initiate the protein conformational changes associated with the formation of the G protein-binding signalling state. The distorted retinal at a 1-ps time delay after photoactivation has pulled away from half of its numerous interactions with its binding pocket, and the excess of the photon energy is released through an anisotropic protein breathing motion in the direction of the extracellular space. Notably, the very early structural motions in the protein side chains of rhodopsin appear in regions that are involved in later stages of the conserved class A GPCR activation mechanism. Our study sheds light on the earliest stages of vision in vertebrates and points to fundamental aspects of the molecular mechanisms of agonist-mediated GPCR activation. One picosecond after photoactivation, isomerized retinal pulls away from half of its numerous interactions with its binding pocket, and the excess of the photon energy is released through an anisotropic protein breathing motion in the direction of the extracellular space.

The visual phototransduction cascade begins with a cis–trans photoisomerization of a retinylidene chromophore associated with the visual pigments of rod and cone photoreceptors. Visual opsins release their all-trans-retinal chromophore following photoactivation, which necessitates the existence of pathways that produce 11-cis-retinal for continued formation of visual pigments and sustained vision. Proteins in the retinal pigment epithelium (RPE), a cell layer adjacent to the photoreceptor outer segments, form the well-established “dark” regeneration pathway known as the classical visual cycle. This pathway is sufficient to maintain continuous rod function and support cone photoreceptors as well although its throughput has to be augmented by additional mechanism(s) to maintain pigment levels in the face of high rates of photon capture. Recent studies indicate that the classical visual cycle works together with light-dependent processes in both the RPE and neural retina to ensure adequate 11-cis-retinal production under natural illuminances that can span ten orders of magnitude. Further elucidation of the interplay between these complementary systems is fundamental to understanding how cone-mediated vision is sustained in vivo. Here, we describe recent advances in understanding how 11-cis-retinal is synthesized via light-dependent mechanisms.

Krokinobacter eikastus rhodopsin 2 (KR2) is the first light-driven Na + pump discovered, and is viewed as a potential next-generation optogenetics tool. Since the positively charged Schiff base proton, located within the ion-conducting pathway of all light-driven ion pumps, was thought to prohibit the transport of a non-proton cation, the discovery of KR2 raised the question of how it achieves Na + transport. Here we present crystal structures of KR2 under neutral and acidic conditions, which represent the resting and M-like intermediate states, respectively. Structural and spectroscopic analyses revealed the gating mechanism, whereby the flipping of Asp116 sequesters the Schiff base proton from the conducting pathway to facilitate Na + transport. Together with the structure-based engineering of the first light-driven K + pumps, electrophysiological assays in mammalian neurons and behavioural assays in a nematode, our studies reveal the molecular basis for light-driven non-proton cation pumps and thus provide a framework that may advance the development of next-generation optogenetics. KR2 light-driven Na + pump structure Known microbial rhodopsins were classified into two groups, either outward proton pumps or inward chloride pumps, until the recent discovery of a light-driven Na + -pumping rhodopsin from the marine bacterium Krokinobacter eikastus . This novel protein, termed KR2, is attracting attention as a potential tool for use in optogenetics: its activation would change the sodium concentration of a targeted cell, not just the pH or chloride concentration. Now Osamu Nureki and colleagues have solved two X-ray crystal structures of KR2 and they use them to propose a working model for Na + transport. Based on these structures the authors have designed several mutants of KR2 and successfully engineered a K + -transporting pump.

Background Minimally‐to‐moderately invasive facial rejuvenation procedures, such as chemical peels, hyaluronic acid (HA) injections and fractional lasers, yield visible improvements in skin texture, tone and wrinkles; their long‐term benefits can be optimized through a targeted anti‐aging skincare regimen. Aims This controlled study aimed to evaluate the tolerability and efficacy of a cream containing three evidence‐based active ingredients (retinaldehyde, niacinamide and haritaki fruit extract) to support and maintain the benefits of minimally to moderately invasive facial rejuvenation procedures. Methods A monocentric, controlled, randomized, split‐face study was conducted over a three‐month period in subjects who had undergone one type of rejuvenation procedure, with assessments beginning after re‐epidermization. Subjects were instructed to apply the test product on the randomized hemiface once daily. Results This comparative controlled study, conducted under close dermatological supervision in 66 subjects who had undergone either chemical peels, HA injections, or fractional laser (n = 22 per group), demonstrated that the cream significantly improved multiple signs of aging. Throughout the entire study, compliance was very good. At 1 month (M1), wrinkles were significantly reduced, and the skin was significantly firmer and plumper, with improved texture. Skin tone homogeneity and radiance were significantly enhanced as of M1 (and M2 for skin smoothness) versus the control hemiface. Conclusion Regular application of the study product, which contains retinaldehyde, niacinamide, and haritaki fruit extract, yielded significantly visible anti‐aging results already at 1 month compared with the control, with very good skin tolerance. It can be used to maximize and maintain the benefits of rejuvenation procedures and promote long‐term skin health. Trial Registration ClinicalTrials.gov identifier: NCT06942403

Visual pigment consists of opsin covalently linked to the vitamin A-derived chromophore, 11-cis-retinaldehyde. Photon absorption causes the chromophore to isomerize from the 11-cis- to all-trans-retinal configuration. Continued light sensitivity necessitates the regeneration of 11-cis-retinal via a series of enzyme-catalyzed steps within the visual cycle. During this process, vitamin A aldehyde is shepherded within photoreceptors and retinal pigment epithelial cells to facilitate retinoid trafficking, to prevent nonspecific reactivity, and to conserve the 11-cis configuration. Here we show that redundancy in this system is provided by a protonated Schiff base adduct of retinaldehyde and taurine (A1-taurine, A1T) that forms reversibly by nonenzymatic reaction. A1T was present as 9-cis, 11-cis, 13-cis, and all-trans isomers, and the total levels were higher in neural retina than in retinal pigment epithelium (RPE). A1T was also more abundant under conditions in which 11-cis-retinaldehyde was higher; this included black versus albino mice, dark-adapted versus light-adapted mice, and mice carrying the Rpe65-Leu450 versus Rpe65-450Met variant. Taurine levels paralleled these differences in A1T. Moreover, A1T was substantially reduced in mice deficient in the Rpe65 isomerase and in mice deficient in cellular retinaldehyde-binding protein; in these models the production of 11-cis-retinal is compromised. A1T is an amphiphilic small molecule that may represent a mechanism for escorting retinaldehyde. The transient Schiff base conjugate that the primary amine of taurine forms with retinaldehyde would readily hydrolyze to release the retinoid and thus may embody a pool of 11-cis-retinal that can be marshalled in photoreceptor cells.

Rhodopsin activation Structural studies of active states of the visual pigment rhodopsin, a G protein-coupled receptor, have previously been limited to apoprotein or opsin forms that do not contain the agonist all- trans -retinal. Two groups now report structures that reveal more details of the transformations involved in rhodopsin activation. Choe et al . solve the X-ray crystal structure of the metarhodopsin II intermediate of the photoreceptor rhodopsin, and Standfuss et al . determine the structure of a constitutively active mutant of rhodopsin bound to a peptide derived from the C-terminus of the G protein transducin. This study solves the X-ray crystal structure of a constitutively active mutant of rhodopsin, a G-protein-coupled receptor, bound to a peptide derived from the C-terminus of the G protein transducin. Comparison of this structure with the structure of ground-state rhodopsin suggests how translocation of the retinal β-ionone ring leads to a rotational tilt of transmembrane helix 6, the critical conformational change that occurs upon activation. G-protein-coupled receptors (GPCRs) comprise the largest family of membrane proteins in the human genome and mediate cellular responses to an extensive array of hormones, neurotransmitters and sensory stimuli. Although some crystal structures have been determined for GPCRs, most are for modified forms, showing little basal activity, and are bound to inverse agonists or antagonists. Consequently, these structures correspond to receptors in their inactive states. The visual pigment rhodopsin is the only GPCR for which structures exist that are thought to be in the active state 1 , 2 . However, these structures are for the apoprotein, or opsin, form that does not contain the agonist all- trans retinal. Here we present a crystal structure at a resolution of 3 Å for the constitutively active rhodopsin mutant Glu 113 Gln 3 , 4 , 5 in complex with a peptide derived from the carboxy terminus of the α-subunit of the G protein transducin. The protein is in an active conformation that retains retinal in the binding pocket after photoactivation. Comparison with the structure of ground-state rhodopsin 6 suggests how translocation of the retinal β-ionone ring leads to a rotation of transmembrane helix 6, which is the critical conformational change on activation 7 . A key feature of this conformational change is a reorganization of water-mediated hydrogen-bond networks between the retinal-binding pocket and three of the most conserved GPCR sequence motifs. We thus show how an agonist ligand can activate its GPCR.

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, β-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. NinaB is an isomerooxygenase that generates visual chromophore (11- cis -retinal) from carotenoid substrates. Here Solano et al. reveal the structural basis for NinaB isomerase activity, providing new insights into the evolution of visual chromophore synthesis by carotenoid cleavage enzymes.

Organisms from bacteria to humans sense and react to light. Proteins that contain the light-sensitive molecule retinal couple absorption of light to conformational changes that produce a signal or move ions across a membrane. Nogly et al. used an x-ray laser to probe the earliest structural changes to the retinal chromophore within microcrystals of the ion pump bacteriorhodopsin (see the Perspective by Moffat). The excited-state retinal wiggles but is held in place so that only one double bond of retinal is capable of isomerizing. A water molecule adjacent to the proton-pumping Schiff base responds to changes in charge distribution in the chromophore even before the movement of atoms begins. Science , this issue p. eaat0094 ; see also p. 127 Ultrafast crystallography captures the response of the pigment of bacteriorhodopsin to absorption of light. Ultrafast isomerization of retinal is the primary step in photoresponsive biological functions including vision in humans and ion transport across bacterial membranes. We used an x-ray laser to study the subpicosecond structural dynamics of retinal isomerization in the light-driven proton pump bacteriorhodopsin. A series of structural snapshots with near-atomic spatial resolution and temporal resolution in the femtosecond regime show how the excited all-trans retinal samples conformational states within the protein binding pocket before passing through a twisted geometry and emerging in the 13-cis conformation. Our findings suggest ultrafast collective motions of aspartic acid residues and functional water molecules in the proximity of the retinal Schiff base as a key facet of this stereoselective and efficient photochemical reaction.

Intracellular Fe plays a key role in redox active energy and electron transfer. We sought to understand how Fe levels impact the retina, given that retinal pigment epithelial (RPE) cells are also challenged by accumulations of vitamin A aldehyde adducts (bisretinoid lipofuscin) that photogenerate reactive oxygen species and photodecompose into damaging aldehyde- and dicarbonyl-bearing species. In mice treated with the Fe chelator deferiprone (DFP), intracellular Fe levels, as reflected in transferrin receptor mRNA expression, were reduced. DFP-treated albino Abca4 −/− and agouti wild-type mice exhibited elevated bisretinoid levels as measured by high-performance liquid chromatography or noninvasively by quantitative fundus autofluorescence. Thinning of the outer nuclear layer, a parameter indicative of the loss of photoreceptor cell viability, was also reduced in DFP-treated albino Abca4 −/−. In contrast to the effects of the Fe chelator, mice burdened with increased intracellular Fe in RPE due to deficiency in the Fe export proteins hephaestin and ceruloplasmin, presented with reduced bisretinoid levels. These findings indicate that intracellular Fe promotes bisretinoid oxidation and degradation. This interpretation was supported by experiments showing that DFP decreased the oxidative/degradation of the bisretinoid A2E in the presence of light and reduced cell death in cell-based experiments. Moreover, light-independent oxidation and degradation of A2E by Fenton chemistry products were evidenced by the consumption of A2E, release of dicarbonyls, and generation of oxidized A2E species in cell-free assays.

Bacteriorhodopsin (bR) is a light-driven proton pump and a model membrane transport protein. We used time-resolved serial femtosecond crystallography at an x-ray free electron laser to visualize conformational changes in bR from nanoseconds to milliseconds following photoactivation. An initially twisted retinal chromophore displaces a conserved tryptophan residue of transmembrane helix F on the cytoplasmic side of the protein while dislodging a key water molecule on the extracellular side. The resulting cascade of structural changes throughout the protein shows how motions are choreographed as bR transports protons uphill against a transmembrane concentration gradient.