Phototransduction motifs and variations

Structure of the human cone photoreceptor cyclic nucleotide-gated channel

Cyclic nucleotide-gated (CNG) channels transduce light-induced chemical signals into electrical signals in retinal cone and rod photoreceptors. Structures of native CNG channels, which are heterotetramers formed by CNGA and CNGB subunits, have not been obtained. In the present study, we report a high-resolution cryo-electron microscopy structure of the human cone CNG channel in the apo closed state. The channel contains three CNGA3 and one CNGB3 subunits. Arg403 in the pore helix of CNGB3 projects into an asymmetric selectivity filter and forms hydrogen bonds with two pore-lining backbone carbonyl oxygens. Arg442 in S6 of CNGB3 protrudes into and occludes the pore below the hydrophobic cavity gate previously observed in homotetrameric CNGA channels. It is interesting that Arg403Gln is a disease mutation, and Arg442 is replaced by glutamine in some animal species with dichromatic or monochromatic vision. These and other unique structural features and the disease link conferred by CNGB3 indicate a critical role of CNGB3 in shaping cone photoresponses.

Carotenoid modifying enzymes in metazoans

Carotenoids constitute an essential dietary component of animals and other non-carotenogenic species which use these pigments in both their modified and unmodified forms. Animals utilize uncleaved carotenoids to mitigate light damage and oxidative stress and to signal fitness and health. Carotenoids also serve as precursors of apocarotenoids including retinol, and its retinoid metabolites, which carry out essential functions in animals by forming the visual chromophore 11-cis-retinaldehyde. Retinoids, such as all-trans-retinoic acid, can also act as ligands of nuclear hormone receptors. The fact that enzymes and biochemical pathways responsible for the metabolism of carotenoids in animals bear resemblance to the ones in plants and other carotenogenic species suggests an evolutionary relationship. We will explore some of the modes of transmission of carotenoid genes from carotenogenic species to metazoans. This apparent relationship has been successfully exploited in the past to identify and characterize new carotenoid and retinoid modifying enzymes. We will review approaches used to identify putative animal carotenoid enzymes, and we will describe methods used to functionally validate and analyze the biochemistry of carotenoid modifying enzymes encoded by animals.

Structural basis of the partially open central gate in the human CNGA1/CNGB1 channel explained by additional density for calmodulin in cryo-EM map

The recently reported structure of the human CNGA1/CNGB1 CNG channel in the open state (Xue et al., 2021a) shows that one CNGA1 and one CNGB1 subunit do not open the central hydrophobic gate completely upon cGMP binding. This is different from what has been reported for CNGA homomeric channels (Xue et al., 2021b; Zheng et al., 2020). In seeking to understand how this difference is due to the presence of the CNGB1 subunit, we find that the deposited density map (Xue et al., 2021a) (EMDB 24465) contains an additional density not reported in the images of the original publication. This additional density fits well the structure of calmodulin (CaM), and it unambiguously connects the newly identified D-helix of CNGB1 to one of the CNGA1 helices (A1_R) participating in the coiled-coil region. Interestingly, the CNGA1 subunit that engages in the interaction with this additional density is the one that, together with CNGB1, does not open completely the central gate. The sequence of the D-helix of CNGB1 contains a known CaM-binding site of exquisitely high affinity - named CaM2 (Weitz et al., 1998) -, and thus the presence of CaM in that region is not surprising. The mechanism through which CaM reduces currents across the membrane by acting on the native channel (Bauer, 1996; Hsu and Molday, 1993; Weitz et al., 1998) remains unclear. We suggest that the presence of CaM may explain the partially open central gate reported by Xue et al. (2021a). The structure of the open and closed states of the CNGA1/CNGB1 channel may be different with and without CaM present.

Molecular determinants of response kinetics of mouse M1 intrinsically-photosensitive retinal ganglion cells

Intrinsically-photosensitive retinal ganglion cells (ipRGCs) are non-rod/non-cone retinal photoreceptors expressing the visual pigment, melanopsin, to detect ambient irradiance for various non-image-forming visual functions. The M1-subtype, amongst the best studied, mediates primarily circadian photoentrainment and pupillary light reflex. Their intrinsic light responses are more prolonged than those of rods and cones even at the single-photon level, in accordance with the typically slower time course of non-image-forming vision. The short (OPN4S) and long (OPN4L) alternatively-spliced forms of melanopsin proteins are both present in M1-ipRGCs, but their functional difference is unclear. We have examined this point by genetically removing the Opn4 gene (Opn4-/-) in mouse and re-expressing either OPN4S or OPN4L singly in Opn4-/- mice by using adeno-associated virus, but found no obvious difference in their intrinsic dim-flash responses. Previous studies have indicated that two dominant slow steps in M1-ipRGC phototransduction dictate these cells' intrinsic dim-flash-response kinetics, with time constants (τ1 and τ2) at room temperature of ~ 2 s and ~ 20 s, respectively. Here we found that melanopsin inactivation by phosphorylation or by β-arrestins may not be one of these two steps, because their genetic disruptions did not prolong the two time constants or affect the response waveform. Disruption of GAP (GTPase-Activating-Protein) activity on the effector enzyme, PLCβ4, in M1-ipRGC phototransduction to slow down G-protein deactivation also did not prolong the response decay, but caused its rising phase to become slightly sigmoidal by giving rise to a third time constant, τ3, of ~ 2 s (room temperature). This last observation suggests that GAP-mediated G-protein deactivation does partake in the flash-response termination, although normally with a time constant too short to be visible in the response waveform.

Mitigating the pro-oxidant state and melanogenesis of Retinitis pigmentosa: by counteracting mitochondrial dysfunction

Retinitis pigmentosa (RP) is a group of mitochondrial diseases characterized by progressive degeneration of rods and cones leading to retinal loss of light sensitivity and, consequently, to blindness. To date, no cure is available according to the clinical literature. As a disease associated with pigmentation-related, pro-oxidant state, and mitochondrial dysfunction, RP may be viewed at the crossroads of different pathogenetic pathways involved in adverse health outcomes, where mitochondria play a preeminent role. RP has been investigated in a number of experimental and clinical studies aimed at delaying retinal hyperpigmentation by means of a number of natural and synthetic antioxidants, as well as mitochondrial cofactors, also termed mitochondrial nutrients (MNs), such as alpha-lipoic acid, coenzyme Q10 and carnitine. One should consider that each MN plays distinct-and indispensable-roles in mitochondrial function. Thus, a logical choice would imply the administration of MN combinations, instead of individual MNs, as performed in previous studies, and with limited, if any, positive outcomes. A rational study design aimed at comparing the protective effects of MNs, separately or in combinations, and in association with other antioxidants, might foresee the utilization of animal RP models. The results should verify a comparative optimization in preventing or effectively contrasting retinal oxidative stress in mouse RP models and, in prospect, in human RP cases.

Molecular Strategies for Intensity-Dependent Olfactory Processing in Caenorhabditis elegans

Various odorants trigger complex animal behaviors across species in both quality- and quantity-dependent manners. However, how the intensity of olfactory input is encoded remains largely unknown. Here we report that isoamyl alcohol (IAA) induces bi-directional currents through a Gα- guanylate cyclase (GC)- cGMP signaling pathway in Caenorhabditis elegans olfactory neuron amphid wing “C” cell (AWC), while two opposite cGMP signaling pathways are responsible for odor-sensing in olfactory neuron amphid wing “B” cell (AWB): (1) a depolarizing Gα (GPA-3)- phosphodiesterase (PDE) – cGMP pathway which can be activated by low concentrations of isoamyl alcohol (IAA), and (2) a hyperpolarizing Gα (ODR-3)- GC- cGMP pathway sensing high concentrations of IAA. Besides, IAA induces Gα (ODR-3)-TRPV(OSM-9)-dependent currents in amphid wing “A” cell (AWA) and amphid neuron “H” cell with single ciliated sensory ending (ASH) neurons with different thresholds. Our results demonstrate that an elaborate combination of multiple signaling machineries encode the intensity of olfactory input, shedding light on understanding the molecular strategies on sensory transduction.

Rhodopsins at a glance

Rhodopsins are photoreceptive membrane proteins consisting of a common heptahelical transmembrane architecture that contains a retinal chromophore. Rhodopsin was first discovered in the animal retina in 1876, but a different type of rhodopsin, bacteriorhodopsin, was reported to be present in the cell membrane of an extreme halophilic archaeon, Halobacterium salinarum, 95 years later. Although these findings were made by physiological observation of pigmented tissue and cell bodies, recent progress in genomic and metagenomic analyses has revealed that there are more than 10,000 microbial rhodopsins and 9000 animal rhodopsins with large diversity and tremendous new functionality. In this Cell Science at a Glance article and accompanying poster, we provide an overview of the diversity of functions, structures, color discrimination mechanisms and optogenetic applications of these two rhodopsin families, and will also highlight the third distinctive rhodopsin family, heliorhodopsin.

Adhesion GPCR Latrophilin 3 regulates synaptic function of cone photoreceptors in a trans-synaptic manner

Cone photoreceptors mediate daylight vision in vertebrates. Changes in neurotransmitter release at cone synapses encode visual information and is subject to precise control by negative feedback from enigmatic horizontal cells. However, the mechanisms that orchestrate this modulation are poorly understood due to a virtually unknown landscape of molecular players. Here, we report a molecular player operating selectively at cone synapses that modulates effects of horizontal cells on synaptic release. Using an unbiased proteomic screen, we identified an adhesion GPCR Latrophilin3 (LPHN3) in horizontal cell dendrites that engages in transsynaptic control of cones. We detected and characterized a prominent splice isoform of LPHN3 that excludes a element with inhibitory influence on transsynaptic interactions. A gain-of-function mouse model specifically routing LPHN3 splicing to this isoform but not knockout of LPHN3 diminished Ca_V1.4 calcium channel activity profoundly disrupted synaptic release by cones and resulted in synaptic transmission deficits. These findings offer molecular insight into horizontal cell modulation on cone synaptic function and more broadly demonstrate the importance of alternative splicing in adhesion GPCRs for their physiological function.

[Biophysical and Biochemical Research of Animal Rhodopsins]

Opsins (also called animal rhodopsins) are universal photoreceptive proteins that provide the molecular basis of visual and nonvisual photoreception in animals, including humans. Opsins consist of seven helical α-transmembrane domains and use retinal, a derivative of vitamin A, as a chromophore. In many opsins, light absorption triggers photo-isomerization from 11-cis retinal to all-trans retinal, resulting in activation via dynamic structural changes in the protein moiety. Activated opsins stimulate cognate trimeric G proteins to induce signal transduction cascades in cells. Recently, molecular and physiological analyses of diverse opsins have progressively advanced. This review introduces the molecular basis and physiological functions of opsins. Based on the functions of opsins, I will discuss the potential of opsins as target molecules to treat and prevent visual and nonvisual diseases such as sleep disorder and depression.

Drosophila fabp is required for light-dependent Rhodopsin-1 clearance and photoreceptor survival

Rhodopsins are light-detecting proteins coupled with retinal chromophores essential for visual function. Coincidentally, dysfunctional Rhodopsin homeostasis underlies retinal degeneration in humans and model organisms. Drosophila ninaEG69D mutant is one such example, where the encoded Rh1 protein imposes endoplasmic reticulum (ER) stress and causes light-dependent retinal degeneration. The underlying reason for such light-dependency remains unknown. Here, we report that Drosophila fatty acid binding protein (fabp) is a gene induced in ninaEG69D/+ photoreceptors, and regulates light-dependent Rhodopsin-1 (Rh1) protein clearance and photoreceptor survival. Specifically, our photoreceptor-specific gene expression profiling study in ninaEG69D/+ flies revealed increased expression of fabp together with other genes that control light-dependent Rh1 protein degradation. fabp induction in ninaEG69D photoreceptors required vitamin A and its transporter genes. In flies reared under light, loss of fabp caused an accumulation of Rh1 proteins in cytoplasmic vesicles. The increase in Rh1 levels under these conditions was dependent on Arrestin2 that mediates feedback inhibition of light-activated Rh1. fabp mutants exhibited light-dependent retinal degeneration, a phenotype also found in other mutants that block light-induced Rh1 degradation. These observations reveal a previously unrecognized link between light-dependent Rh1 proteostasis and the ER-stress imposing ninaEG69D mutant that cause retinal degeneration.

Bidirectional communication between neural and immune systems

The immune and nervous systems share many features, including receptor and ligand expression, enabling efficient communication between the two. Accumulating evidence suggests that the communication is bidirectional, with the neural system regulating immune cell functions and vice versa. Steroid hormones from the hypothalamus-pituitary-adrenal gland axis are examples of systemic regulators for this communication. Neural reflexes describe regional regulation mechanisms that are a historically new concept that helps to explain how the neural and body systems including immune system communicate. Several recently identified neural reflexes, including the inflammatory reflex and gateway reflex, significantly impact the activation status of the immune system and are associated with inflammatory diseases and disorders. Either pro-inflammatory or anti-inflammatory effects can be elicited by these neural reflexes. On the other hand, the activities of immune cells during inflammation, for example the secretion of inflammatory mediators, can affect the functions of neuronal systems via neural reflexes and modulate biological outputs via specific neural pathways. In this review article, we discuss recent advances in the understanding of bidirectional neuro-immune interactions, with a particular focus on neural reflexes.

Ultrasensitivity and bistability in covalent-modification cycles with positive autoregulation

Switch-like behaviours in biochemical networks are of fundamental significance in biological signal processing, and exist as two distinct types: ultra-sensitivity and bistability. Here we propose two new models of a reversible covalent-modification cycle with positive autoregulation (PAR), a motif structure that is thought to be capable of both ultrasensitivity and bistability in different parameter regimes. These new models appeal to a modelling framework that we call complex-complete, which accounts fully for the molecular complexities of the underlying signalling mechanisms. Each of the two new models encodes a specific molecular mechanism for PAR. We demonstrate that the modelling simplifications for PAR models that have been used in previous work, which rely on Michaelian approximations, are unable to accurately recapitulate the qualitative signalling responses supported by our detailed models. Strikingly, we show that complex-complete PAR models are capable of new qualitative responses such as one-way switches and a 'prozone' effect, depending on the specific PAR-encoding mechanism, which are not supported by Michaelian simplifications. Our results highlight the critical importance of accurately representing the molecular details of biochemical signalling mechanisms, and strongly suggest that the Michaelian approximation is inadequate for predictive models of enzyme-mediated chemical reactions with added regulations such as PAR.

Opsins outside the eye and the skin: a more complex scenario than originally thought for a classical light sensor

Since the discovery of melanopsin as a retinal non-visual photopigment, opsins have been described in several organs and cells. This distribution is strikingly different from the classical localization of photopigments in light-exposed tissues such as the eyes and the skin. More than 10 years ago, a new paradigm in the field was created as opsins were shown, to detect not only light, but also thermal energy in Drosophila. In agreement with these findings, thermal detection by opsins was also reported in mammalian cells. Considering the presence of opsins in tissues not reached by light, an intriguing question has emerged: What is the role of a classical light-sensor, and more recently appreciated thermo-sensor, in these tissues? To tackle this question, we address in this review the most recent studies in the field, with emphasis in mammals. We provide the present view about the role of opsins in peripheral tissues, aiming to integrate the current knowledge of the presence and function of opsins in organs that are not directly affected by light.

The genome of Nautilus pompilius illuminates eye evolution and biomineralization

Nautilus is the sole surviving externally shelled cephalopod from the Palaeozoic. It is unique within cephalopod genealogy and critical to understanding the evolutionary novelties of cephalopods. Here, we present a complete Nautilus pompilius genome as a fundamental genomic reference on cephalopod innovations, such as the pinhole eye and biomineralization. Nautilus shows a compact, minimalist genome with few encoding genes and slow evolutionary rates in both non-coding and coding regions among known cephalopods. Importantly, multiple genomic innovations including gene losses, independent contraction and expansion of specific gene families and their associated regulatory networks likely moulded the evolution of the nautilus pinhole eye. The conserved molluscan biomineralization toolkit and lineage-specific repetitive low-complexity domains are essential to the construction of the nautilus shell. The nautilus genome constitutes a valuable resource for reconstructing the evolutionary scenarios and genomic innovations that shape the extant cephalopods.

CRISPR/Cas9-mediated knockout of LW-opsin reduces the efficiency of phototaxis in the diamondback moth Plutella xylostella

BACKGROUND

Opsins are crucial for animal vision. The identity and function of opsins in Plutella xylostella remain unknown. The aim of the research is to confirm which opsin gene(s) contribute to phototaxis of P. xylostella.

RESULTS

LW-opsin, BL-opsin and UV-opsin, were identified in the P. xylostella genome. LW-opsin was more highly expressed than the other two opsin genes, and all three genes were specifically expressed in the head. Three P. xylostella strains, LW-13 with a 13-bp deletion in LW-opsin, BL + 2 with a 2-bp insertion in BL-opsin, and UV-29 with a 5-bp insertion and a 34-bp deletion in UV-opsin, were established from the strain G88 using the CRISPR/Cas9 system. Among the three opsin-knockout strains, only male and female LW-13 exhibited weaker phototaxis to lights of different wavelengths and white light than G88 at 2.5 lx due to defective locomotion, and LW-13 was defective to sense white, green and infrared lights. The locomotion of LW-13 was reduced compared with G88 at 2.5, 10, 20, 60, 80, 100, and 200 lx under the green light, but the locomotion of LW-13 female was recovered at 80, 100 and 200 lx. The defective phototaxis to the green light of male LW-13 was not affected by light intensity, while the defective phototaxis to the green light of female LW-13 was recovered at 10, 20, 60, 80, 100, and 200 lx.

CONCLUSION

LW-opsin is involved in light sensing and locomotion of P. xylostella, providing a potential target gene for controlling the pest. © 2021 Society of Chemical Industry.

Sildenafil in ophthalmology: An update

Sildenafil citrate, a selective oral phosphodiesterase 5 inhibitor, is a widely used drug for erectile dysfunction that acts by elevating cGMP levels and causing smooth muscle relaxation. It also has 10% activity against PDE6, a key enzyme in phototransduction cascade in the retina. Recent ocular imaging developments have further revealed the influence of sildenafil on ocular hemodynamics, particularly choroidal perfusion. Choroidal thickness is increased, and choroidal perfusion is also enhanced by autoregulatory mechanisms that are further dependent on age and microvascular abnormalities. Studies demonstrating high intraocular pressure via a "parallel pathway" from increased choroidal volume and blood flow to the ciliary body have challenged previous concepts. Another new observation is the effect of sildenafil on bipolar cells and cyclic-nucleotide gated channels. We discuss potential deleterious effects (central serous chorioretinopathy, glaucoma, ischemic optic neuropathy, and risks to recessive carriers of retinitis pigmentosa), potential beneficial effects (ameliorate choroidal ischemia, prevent thickening of Bruch membrane, and promote recovery of the ellipsoid zone) in macular degeneration, as well as potential drug interactions of sildenafil.

Genetic network regulating visual acuity makes limited contribution to visually guided eye emmetropization

During postnatal development, the eye undergoes a refinement process whereby optical defocus guides eye growth towards sharp vision in a process of emmetropization. Optical defocus activates a signaling cascade originating in the retina and propagating across the back of the eye to the sclera. Several observations suggest that visual acuity might be important for optical defocus detection and processing in the retina; however, direct experimental evidence supporting or refuting the role of visual acuity in refractive eye development is lacking. Here, we used genome-wide transcriptomics to determine the relative contribution of the retinal genetic network regulating visual acuity to the signaling cascade underlying visually guided eye emmetropization. Our results provide evidence that visual acuity is regulated at the level of molecular signaling in the retina by an extensive genetic network. The genetic network regulating visual acuity makes relatively small contribution to the signaling cascade underlying refractive eye development. This genetic network primarily affects baseline refractive eye development and this influence is primarily facilitated by the biological processes related to melatonin signaling, nitric oxide signaling, phototransduction, synaptic transmission, and dopamine signaling. We also observed that the visual-acuity-related genes associated with the development of human myopia are chiefly involved in light perception and phototransduction. Our results suggest that the visual-acuity-related genetic network primarily contributes to the signaling underlying baseline refractive eye development, whereas its impact on visually guided eye emmetropization is modest.

Color discrimination properties arising from optimal decoding in the early stages of visual systems

In order to interpret animal behavior we need to understand how they see the world. As directly testing color discrimination in animals is difficult and time consuming, it is important to develop theoretical models based in the properties of visual systems. One of the most successful for the prediction of color discrimination behavior is the receptor noise-limited (RNL) model, which depends only on the level of noise in photoreceptors and opponent mechanisms. Here a complementary approach to model construction is used, and optimal color discrimination properties are obtained using information theoretical tools, for the early stages of visual systems. It is shown here that, for most biologically relevant conditions the optimal discrimination function of an ideal observer coincides with the one obtained with the RNL model. Furthermore, within this framework the influence of opponency can be studied by considering models with and without that mechanism but with exactly the same parameters at the level of photoreceptors. As an example, it is shown here that opponency is necessary to explain the discrimination of monochromatic stimuli in honeybees, but not in budgerigars. Since this is a consequence of the narrowing of absorption spectra of photoreceptors, produced by the presence of oil droplets, this could also be true for most other species of birds. This suggests that in order to study opponency in birds, stimuli should have a relatively wide spectrum.

Mutant Nmnat1 leads to a retina-specific decrease of NAD+ accompanied by increased poly(ADP-ribose) in a mouse model of NMNAT1-associated retinal degeneration

Nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1) is required for nuclear nicotinamide adenine mononucleotide (NAD+) biosynthesis in all nucleated cells, and despite its functional ubiquity, mutations in this gene lead to an isolated retinal degeneration. The mechanisms underlying how mutant NMNAT1 causes disease are not well understood, nor is the reason why the pathology is confined to the retina. Using a mouse model of NMNAT1-associated retinal degeneration that harbors the p.Val9Met mutation, we tested the hypothesis that decreased function of mutant NMNAT1 has a greater effect on the levels of NAD+ in the retina than elsewhere in the body. Measurements by liquid chromatography with tandem mass spectrometry showed an early and sustained decrease of NAD+ in mutant retinas that was not observed in other tissues. To understand how consumers of nuclear NAD+ are affected by the reduced availability of NAD+ in mutant retinas, poly(ADP-ribose) polymerase (PARP) and nuclear sirtuin activity were evaluated. PARP activity was elevated during disease progression, as evidenced by overproduction of poly(ADP-ribose) (PAR) in photoreceptors, whereas histone deacetylation activity of nuclear sirtuins was not altered. We hypothesized that PARP could be activated because of elevated levels of oxidative stress; however, we did not observe oxidative DNA damage, lipid peroxidation, or a low glutathione to oxidized glutathione ratio. Terminal deoxynucleotidyl transferase dUTP nick end labeling staining revealed that photoreceptors appear to ultimately die by apoptosis, although the low NAD+ levels and overproduction of PAR suggest that cell death may include aspects of the parthanatos cell death pathway.

Melanopsin Signaling Pathway in HEK293 Cell Line with Stable Expression of Human Melanopsin: Possible Participation of Phospholipase C beta 4 and Diacylglycerol

Melanopsin, a member of the G protein-coupled receptors family, is involved in non-image-forming functions including circadian rhythm, sleep regulation and pupil response. In spite of significant research efforts, the signaling cascade involving melanopsin photoactivation remains poorly characterized. Here, we analyzed the effects of photoactivation of melanopsin on phospholipase C (PLC) and diacylglycerol. As an in vitro model, HEK293 cells with stable expression of human melanopsin were used. Although both the PLC_β1 and PLC_β4 subtypes were activated by the cell exposure to blue light, only PLC_β4 appeared to play a significant role in the studied melanopsin signaling pathway. We have demonstrated, for the first time, that cells expressing human melanopsin and enriched with 11-cis-retinal exhibited significantly increased diacylglycerol level. To determine the role of phospholipase C and involvement of diacylglycerols, two approaches were employed: inhibition of the G protein and phospholipase C (using the BIM-46187 and U73122 inhibitors, respectively), and gene silencing using siRNA of PLC_β1 and PLC_β4 . While silencing the PLC_β4 gene and using U73122 inhibited the diacylglycerol and calcium ion responses, the FOS gene expression level was only partially reduced. These results may facilitate a better understanding of the role of phospholipase C and diacylglycerols in the melanopsin signaling pathway.

The PDE-Opathies: Diverse Phenotypes Produced by a Functionally Related Multigene Family

The phosphodiesterase (PDE)-opathies, an expanding set of disorders caused by germline mutations in cyclic nucleotide PDEs, present an intriguing paradox. The enzymes encoded by the PDE family all hydrolyze cAMP and/or cGMP, but mutations in different family members produce very divergent phenotypes. Three interacting factors have been shown recently to contribute to this phenotypic diversity: (i) the 21 genes encode over 80 different isoforms, using alternative mRNA splicing and related mechanisms; (ii) the various isoforms have different regulatory mechanisms, mediated by their unique amino-terminal regulatory domains; (iii) the isoforms differ widely in their pattern of tissue expression. These mechanisms explain why many PDE-opathies are gain-of-function mutations and how they exemplify uniqueness and redundancy within a multigene family.

Transduction and Adaptation Mechanisms in the Cilium or Microvilli of Photoreceptors and Olfactory Receptors From Insects to Humans

Sensing changes in the environment is crucial for survival. Animals from invertebrates to vertebrates use both visual and olfactory stimuli to direct survival behaviors including identification of food sources, finding mates, and predator avoidance. In primary sensory neurons there are signal transduction mechanisms that convert chemical or light signals into an electrical response through ligand binding or photoactivation of a receptor, that can be propagated to the olfactory and visual centers of the brain to create a perception of the odor and visual landscapes surrounding us. The fundamental principles of olfactory and phototransduction pathways within vertebrates are somewhat analogous. Signal transduction in both systems takes place in the ciliary sub-compartments of the sensory cells and relies upon the activation of G protein-coupled receptors (GPCRs) to close cyclic nucleotide-gated (CNG) cation channels in photoreceptors to produce a hyperpolarization of the cell, or in olfactory sensory neurons open CNG channels to produce a depolarization. However, while invertebrate phototransduction also involves GPCRs, invertebrate photoreceptors can be either ciliary and/or microvillar with hyperpolarizing and depolarizing responses to light, respectively. Moreover, olfactory transduction in invertebrates may be a mixture of metabotropic G protein and ionotropic signaling pathways. This review will highlight differences of the visual and olfactory transduction mechanisms between vertebrates and invertebrates, focusing on the implications to the gain of the transduction processes, and how they are modulated to allow detection of small changes in odor concentration and light intensity over a wide range of background stimulus levels.

Mechanisms of vitamin A metabolism and deficiency in the mammalian and fly visual system

Vitamin A deficiency can cause human pathologies that range from blindness to embryonic malformations. This diversity is due to the lack of two major vitamin A metabolites with very different functions: the chromophore 11-cis-retinal (vitamin A aldehyde) is a critical component of the visual pigment that mediates phototransduction, while the signaling molecule all-trans-retinoic acid regulates the development of various tissues and is required for the function of the immune system. Since animals cannot synthesize vitamin A de novo, they must obtain it either as preformed vitamin A from animal products or as carotenoid precursors from plant sources. Due to its essential role in the visual system, acute vitamin A deprivation impairs photoreceptor function and causes night blindness (poor vision under dim light conditions), while chronic deprivation results in retinal dystrophies and photoreceptor cell death. Chronic vitamin A deficiency is the leading cause of preventable childhood blindness according to the World Health Organization. Due to the requirement of vitamin A for retinoic acid signaling in development and in the immune system, vitamin A deficiency also causes increased mortality in children and pregnant women in developing countries. Drosophila melanogaster is an excellent model to study the effects of vitamin A deprivation on the eye because vitamin A is not essential for Drosophila development and chronic deficiency does not cause lethality. Moreover, genetic screens in Drosophila have identified evolutionarily conserved factors that mediate the production of vitamin A and its cellular uptake. Here, we review our current knowledge about the role of vitamin A in the visual system of mammals and Drosophila melanogaster. We compare the molecular mechanisms that mediate the uptake of dietary vitamin A precursors and the metabolism of vitamin A, as well as the consequences of vitamin A deficiency for the structure and function of the eye.

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

Retinal degeneration-3 protein promotes photoreceptor survival by suppressing activation of guanylyl cyclase rather than accelerating GMP recycling

Retinal degeneration-3 protein (RD3) deficiency causes photoreceptor dysfunction and rapid degeneration in the rd3 mouse strain and in human Leber's congenital amaurosis, a congenital retinal dystrophy that results in early vision loss. However, the mechanisms responsible for photoreceptor death remain unclear. Here, we tested two hypothesized biochemical events that may underlie photoreceptor death: (i) the failure to prevent aberrant activation of retinal guanylyl cyclase (RetGC) by calcium-sensor proteins (GCAPs) versus (ii) the reduction of GMP phosphorylation rate, preventing its recycling to GDP/GTP. We found that GMP converts to GDP/GTP in the photoreceptor fraction of the retina ∼24-fold faster in WT mice and ∼400-fold faster in rd3 mice than GTP conversion to cGMP by RetGC. Adding purified RD3 to the retinal extracts inhibited RetGC 4-fold but did not affect GMP phosphorylation in wildtype or rd3 retinas. RD3-deficient photoreceptors rapidly degenerated in rd3 mice that were reared in constant darkness to prevent light-activated GTP consumption via RetGC and phosphodiesterase 6. In contrast, rd3 degeneration was alleviated by deletion of GCAPs. After 2.5 months, only ∼40% of photoreceptors remained in rd3/rd3 retinas. Deletion of GCAP1 or GCAP2 alone preserved 68% and 57% of photoreceptors, respectively, whereas deletion of GCAP1 and GCAP2 together preserved 86%. Taken together, our in vitro and in vivo results support the hypothesis that RD3 prevents photoreceptor death primarily by suppressing activation of RetGC by both GCAP1 and GCAP2 but do not support the hypothesis that RD3 plays a significant role in GMP recycling.

Opsin 5 is a key regulator of ultraviolet radiation-induced melanogenesis in human epidermal melanocytes

Background

Human skin, which is constantly exposed to solar ultraviolet radiation (UVR), has a unique ability to respond by increasing its pigmentation in a protective process driven by melanogenesis in human epidermal melanocytes (HEMs). However, the molecular mechanisms used by HEMs to detect and respond to UVR remain unclear.

Objectives

To investigate the function and potential mechanism of opsin 5 (OPN5), a photoreceptor responsive to UVR wavelengths, in melanogenesis in HEMs.

Methods

Melanin content in HEMs was determined using the NaOH method, and activity of tyrosinase (TYR) (a key enzyme in melanin synthesis) was determined by the l‐DOPA method. OPN5 expression in UVR‐treated vs. untreated HEMs and explant tissues was detected by reverse‐transcription quantitative polymerase chain reaction (RT‐qPCR), Western blotting and immunofluorescence. Short interfering RNA‐mediated OPN5 knockdown and a lentivirus OPN5 overexpression model were used to examine their respective effects on TYR, tyrosinase‐related protein 1 (TRP1), TRP2 and microphthalmia‐associated transcription factor (MITF) expression, under UVR. Changes in expression of TYR, TRP1 and TRP2 caused by changes in OPN5 expression level were detected by RT‐qPCR and Western blot. Furthermore, changes in signalling pathway proteins were assayed.

Results

We found that OPN5 is the key sensor in HEMs responsible for UVR‐induced melanogenesis. OPN5‐induced melanogenesis required Ca²⁺‐dependent G protein‐coupled receptor‐ and protein kinase C signal transduction, thus contributing to the UVR‐induced MITF response to mediate downstream cellular effects, and providing evidence of OPN5 function in mammalian phototransduction. Remarkably, OPN5 activation was necessary for UVR‐induced increase in cellular melanin and has an inherent function in melanocyte melanogenesis.

Conclusions

Our results provide insight into the molecular mechanisms of UVR sensing and phototransduction in melanocytes, and may reveal molecular targets for preventing pigmentation or pigment diseases.

What is already known about this topic?

Ultraviolet radiation (UVR) induces a protective response to DNA damage mediated by melanin synthesis in human epidermal melanocytes (HEMs).

Tyrosinase (TYR), with tyrosinase‐related proteins (TRP1, TRP2), are the key enzymes for melanin synthesis.

Microphthalmia‐associated transcription factor regulates key genes for melanocyte development and differentiation, and can stimulate melanogenesis by activating transcription of TYR and other pigmentation genes, including TRP1.

Opsin 5 (OPN5) is known to function as a photoreceptor responsive to wavelengths in the near UV spectrum.

What does this study add?

UVR induces melanogenesis in HEMs via OPN5.

OPN5 regulates expression of TYR, TRP1 and TRP2 through the calcium‐dependent G protein‐coupled and protein kinase C signalling pathways.

OPN5 has an inherent role in HEMs in mediating melanogenesis.

What is the translational message?

OPN5 was discovered as a key sensor for UVR‐induced melanogenesis in human skin melanocytes.

It could be a target for early treatment of pigmentation or pigment diseases, to provide a more personalized and economically feasible method.

Linked Comment: L.V.M. de Assis and A.M. de Lauro Castrucci. Br J Dermatol 2021; 185:249–250.

Plain language summary available online

Optogenetic Modulation of Ion Channels by Photoreceptive Proteins

In these 15 years, researches to control cellular responses by light have flourished dramatically to establish "optogenetics" as a research field. In particular, light-dependent excitation/inhibition of neural cells using channelrhodopsins or other microbial rhodopsins is the most powerful and the most widely used optogenetic technique. New channelrhodopsin-based optogenetic tools having favorable characteristics have been identified from a wide variety of organisms or created through mutagenesis. Despite the great efforts, some neuronal activities are still hard to be manipulated by the channelrhodopsin-based tools, indicating that complementary approaches are needed to make optogenetics more comprehensive. One of the feasible and complementary approaches is optical control of ion channels using photoreceptive proteins other than channelrhodopsins. In particular, animal opsins can modulate various ion channels via light-dependent G protein activation. In this chapter, we summarize how such alternative optogenetic tools work and they will be improved.

Energy sources that fuel metabolic processes in protruding finger-like organelles

Cells possess a variety of organelles with characteristic structure and subcellular localization intimately linked to their specific function. While most are intracellular and found in virtually all eukaryotic cells, there is a small group of organelles of elongated cylindrical shapes in highly specialized cells that protrude into the extracellular space, such as cilia, flagella, and microvilli. The ATP required by intracellular organelles is amply available in the cytosol, largely generated by mitochondria. However, such is not the case for cilia and flagella, whose slender structures cannot accommodate mitochondria. These organelles consume massive amounts of ATP to carry out high energy-demanding functions, such as sensory transduction or motility. ATP from the nearest mitochondria or other reactions within the cell body is severely limited by diffusion and generally insufficient to fuel the entire length of cilia and flagella. These organelles overcome this fuel restriction by local generation of ATP, using mechanisms that vary depending on the nutrients that are available in their particular external environment. Here, we review, with emphasis in mammals, the remarkable adaptations that cilia and flagella use to fuel their metabolic needs. Additionally, we discuss how a decrease in nutrients surrounding olfactory cilia might impair olfaction in COVID-19 patients.

GUCY2D mutations in retinal guanylyl cyclase 1 provide biochemical reasons for dominant cone-rod dystrophy but not for stationary night blindness

Mutations in the GUCY2D gene coding for the dimeric human retinal membrane guanylyl cyclase (RetGC) isozyme RetGC1 cause various forms of blindness, ranging from rod dysfunction to rod and cone degeneration. We tested how the mutations causing recessive congenital stationary night blindness (CSNB), recessive Leber's congenital amaurosis (LCA1), and dominant cone-rod dystrophy-6 (CORD6) affected RetGC1 activity and regulation by RetGC-activating proteins (GCAPs) and retinal degeneration-3 protein (RD3). CSNB mutations R666W, R761W, and L911F, as well as LCA1 mutations R768W and G982VfsX39, disabled RetGC1 activation by human GCAP1, -2, and -3. The R666W and R761W substitutions compromised binding of GCAP1 with RetGC1 in HEK293 cells. In contrast, G982VfsX39 and L911F RetGC1 retained the ability to bind GCAP1 in cyto but failed to effectively bind RD3. R768W RetGC1 did not bind either GCAP1 or RD3. The co-expression of GUCY2D allelic combinations linked to CSNB did not restore RetGC1 activity in vitro The CORD6 mutation R838S in the RetGC1 dimerization domain strongly dominated the Ca2+ sensitivity of cyclase regulation by GCAP1 in RetGC1 heterodimer produced by co-expression of WT and the R838S subunits. It required higher Ca2+ concentrations to decelerate GCAP-activated RetGC1 heterodimer-6-fold higher than WT and 2-fold higher than the Ser838-harboring homodimer. The heterodimer was also more resistant than homodimers to inhibition by RD3. The observed biochemical changes can explain the dominant CORD6 blindness and recessive LCA1 blindness, both of which affect rods and cones, but they cannot explain the selective loss of rod function in recessive CSNB.

Regulation of photosensation by hydrogen peroxide and antioxidants in C. elegans

The eyeless C. elegans exhibits robust phototaxis behavior in response to short-wavelength light, particularly UV light. C. elegans senses light through LITE-1, a unique photoreceptor protein that belongs to the invertebrate taste receptor family. However, it remains unclear how LITE-1 is regulated. Here, we performed a forward genetic screen for genes that when mutated suppress LITE-1 function. One group of lite-1 suppressors are the genes required for producing the two primary antioxidants thioredoxin and glutathione, suggesting that oxidization of LITE-1 inhibits its function. Indeed, the oxidant hydrogen peroxide (H₂O₂) suppresses phototaxis behavior and inhibits the photoresponse in photoreceptor neurons, whereas other sensory behaviors are relatively less vulnerable to H₂O₂. Conversely, antioxidants can rescue the phenotype of lite-1 suppressor mutants and promote the photoresponse. As UV light illumination generates H₂O₂, we propose that upon light activation of LITE-1, light-produced H₂O₂ then deactivates LITE-1 to terminate the photoresponse, while antioxidants may promote LITE-1’s recovery from its inactive state. Our studies provide a potential mechanism by which H₂O₂ and antioxidants act synergistically to regulate photosensation in C. elegans.

The nematode C. elegans possesses a unique photoreceptor protein, LITE-1, which mediates a light-avoidance behavior upon light exposure. C. elegans avoids short-wavelength light, particularly UV light, providing a potential mechanism by which worms escape from the dangerous UV rays in the sunlight. However, it is not clear how LITE-1 is regulated. Here, we performed a genetic screen to identify genes regulating LITE-1. We uncovered six genes that when mutated suppress LITE-1 function. All these genes are involved in regenerating cellular antioxidants that function to clear reactive oxygen species, particularly hydrogen peroxide (H₂O₂), suggesting that the function of LITE-1 is vulnerable to H₂O₂. Indeed, we show that H₂O₂ exposure inhibits LITE-1 function, while antioxidants promote it. Notably, other sensory functions are relatively less sensitive to H₂O₂. As UV light illumination is known to generate H₂O₂ within the cell, this provides a potential mechanism to turn off LITE-1. Our results uncover a potential mechanism of LITE-1 regulation, where antioxidants and oxidants act to promote and suppress LITE-1 function, respectively.

Non-visual Opsins and Novel Photo-Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spectrum Region

In recent decades, a number of novel non-visual opsin photopigments belonging to the family of G protein- coupled receptors, likely involved in a number of non-image-forming processes, have been identified and characterized in cells of the inner retina of vertebrates. It is now known that the vertebrate retina is composed of visual photoreceptor cones and rods responsible for diurnal/color and nocturnal/black and white vision, and cells like the intrinsically photosensitive retinal ganglion cells (ipRGCs) and photosensitive horizontal cells in the inner retina, both detecting blue light and expressing the photopigment melanopsin (Opn4). Remarkably, these non-visual photopigments can continue to operate even in the absence of vision under retinal degeneration. Moreover, inner retinal neurons and Müller glial cells have been shown to express other photopigments such as the photoisomerase retinal G protein-coupled receptor (RGR), encephalopsin (Opn3), and neuropsin (Opn5), all able to detect blue/violet light and implicated in chromophore recycling, retinal clock synchronization, neuron-to-glia communication, and other activities. The discovery of these new photopigments in the inner retina of vertebrates is strong evidence of novel light-regulated activities. This review focuses on the features, localization, photocascade, and putative functions of these novel non-visual opsins in an attempt to shed light on their role in the inner retina of vertebrates and in the physiology of the whole organism.

Visual metamorphoses in insects and malacostracans: Transitions between an aquatic and terrestrial life

Arthropods operate in an outrageous diversity of environments. From the deep sea to dense tropical forests, to wide open arctic tundra, they have colonized almost every possible habitat. Within these environments, the presence of light is nearly ubiquitous, varying in intensity, wavelength, and polarization. Light provides critical information about the environment, such as time of day or where food sources may be located. Animals take advantage of this prevalent and informative cue to make behavioral choices. However, the types of choices animals face depend greatly on their environments and needs at any given time. In particular, animals that undergo metamorphosis, with arthropods being the prime example, experience dramatic changes in both behavior and ecology, which in turn may require altering the structure and function of sensory systems such as vision. Amphibiotic organisms maintain aquatic lifestyles as juveniles before transitioning to terrestrial lifestyles as adults. However, light behaves differently in water than in air, resulting in distinct aquatic and terrestrial optical environments. Visual changes in response to these optical differences can occur on multiple levels, from corneal structure down to neural organization. In this review, we summarize examples of alterations in the visual systems of amphibiotic larval and adult insects and malacostracan crustaceans, specifically those attributed to environmental differences between metamorphic phases.

Rhodopsin-mediated light-off-induced protein kinase A activation in mouse rod photoreceptor cells

Light-induced extrasynaptic dopamine release in the retina reduces adenosine 3',5'-cyclic monophosphate (cAMP) in rod photoreceptor cells, which is thought to mediate light-dependent desensitization. However, the fine time course of the cAMP dynamics in rods remains elusive due to technical difficulty. Here, we visualized the spatiotemporal regulation of cAMP-dependent protein kinase (PKA) in mouse rods by two-photon live imaging of retinal explants of PKAchu mice, which express a fluorescent biosensor for PKA. Unexpectedly, in addition to the light-on-induced suppression, we observed prominent light-off-induced PKA activation. This activation required photopic light intensity and was confined to the illuminated rods. The estimated maximum spectral sensitivity of 489 nm and loss of the light-off-induced PKA activation in rod-transducin-knockout retinas strongly suggest the involvement of rhodopsin. In support of this notion, rhodopsin-deficient retinal explants showed only the light-on-induced PKA suppression. Taken together, these results suggest that, upon photopic light stimulation, rhodopsin and dopamine signals are integrated to shape the light-off-induced cAMP production and following PKA activation. This may support the dark adaptation of rods.

Diversity of Light Sensing Molecules and Their Expression During the Embryogenesis of the Cuttlefish (Sepia officinalis)

Eyes morphologies may differ but those differences are not reflected at the molecular level. Indeed, the ability to perceive light is thought to come from the same conserved gene families: opsins and cryptochromes. Even though cuttlefish (Cephalopoda) are known for their visually guided behaviors, there is a lack of data about the different opsins and cryptochromes orthologs represented in the genome and their expressions. Here we studied the evolutionary history of opsins, cryptochromes but also visual arrestins in molluscs with an emphasis on cephalopods. We identified 6 opsins, 2 cryptochromes and 1 visual arrestin in Sepia officinalis and we showed these families undergo several duplication events in Mollusca: one duplication in the arrestin family and two in the opsin family. In cuttlefish, we studied the temporal expression of these genes in the eyes of embryos from stage 23 to hatching and their expression in two extraocular tissues, skin and central nervous system (CNS = brain + optic lobes). We showed in embryos that some of these genes (Sof_CRY₆, Sof_reti-1, Sof_reti-2, Sof_r-opsin1 and Sof_v-arr) are expressed in the eyes and not in the skin or CNS. By looking at a juvenile and an adult S. officinalis, it seems that some of these genes (Sof_r-opsin1 and Sof_reti1) are used for light detection in these extraocular tissues but that they set-up later in development than in the eyes. We also showed that their expression (except for Sof_CRY₆) undergoes an increase in the eyes from stage 25 to 28 thus confirming their role in the ability of the cuttlefish embryos to perceive light through the egg capsule. This study raises the question of the role of Sof_CRY₆ in the developing eyes in cuttlefish embryos and the role and localization of xenopsins and r-opsin2. Consequently, the diversity of molecular actors involved in light detection both in the eyes and extraocular tissues is higher than previously known. These results open the way for studying new molecules such as those of the signal transduction cascade.

Interplay between cell-adhesion molecules governs synaptic wiring of cone photoreceptors

Establishment of functional synaptic connections in a selective manner is essential for nervous system operation. In mammalian retinas, rod and cone photoreceptors form selective synaptic connections with different classes of bipolar cells (BCs) to propagate light signals. While there has been progress in elucidating rod wiring, molecular mechanisms used by cones to establish functional synapses with BCs have remained unknown. Using an unbiased proteomic strategy in cone-dominant species, we identified the cell-adhesion molecule ELFN2 to be pivotal for the functional wiring of cones with the ON type of BC. It is selectively expressed in cones and transsynaptically recruits the key neurotransmitter receptor mGluR6 in ON-BCs to enable synaptic transmission. Remarkably, ELFN2 in cone terminals functions in synergy with a related adhesion molecule, ELFN1, and their concerted interplay during development specifies selective wiring and transmission of cone signals. These findings identify a synaptic connectivity mechanism of cones and illustrate how interplay between adhesion molecules and postsynaptic transmitter receptors orchestrates functional synaptic specification in a neural circuit.

Identification of Key Genes and Pathways Associated with Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of severe, permanent vision loss among the elderly in the developed world. The cellular and molecular pathogenesis of initiation and development of AMD remain poorly delineated. The limited resources of the human AMD RPE/choroid tissues impeded the extensive study of the disease. To better understand the molecular and pathway changes in human AMD RPE/choroid tissues, we searched the literature and found three independent studies using high-throughput technology to analyze gene expression in 54 human AMD RPE/choroid tissues and 46 age-matched healthy controls. We downloaded these data, pooled them together, and reanalyzed the difference between molecular and pathways by the Ingenuity Pathway Analysis (IPA) database. Totally, 353 differentially expressed genes (DEGs) were identified, among which 181 genes were downregulated and 172 genes were upregulated in RPE/choroid of AMD patients. Furthermore, several significantly enriched biological processes, including cancer, organismal injury and abnormalities, and ophthalmic disease, were identified associated with these DEGs. By analysis of canonical pathway, the phototransduction pathway and atherosclerosis signaling were the top two significant canonical pathways altered in RPE/choroid tissues in human AMD. As expected, several ophthalmic disease-related molecules, including RHO, PDE6A, 3',5'-cyclic-GMP phosphodiesterase, and G protein alpha, were in the central nodes of disease network. The bioinformatics technology combined with the existing high-throughput data was applied to evaluate the underlying key genes and pathways in human AMD tissues, which may predict downstream and upstream biological processes and identify potential therapeutic intervention targets in human AMD.

Fovea-like Photoreceptor Specializations Underlie Single UV Cone Driven Prey-Capture Behavior in Zebrafish

In the eye, the function of same-type photoreceptors must be regionally adjusted to process a highly asymmetrical natural visual world. Here, we show that UV cones in the larval zebrafish area temporalis are specifically tuned for UV-bright prey capture in their upper frontal visual field, which may use the signal from a single cone at a time. For this, UV-photon detection probability is regionally boosted more than 10-fold. Next, in vivo two-photon imaging, transcriptomics, and computational modeling reveal that these cones use an elevated baseline of synaptic calcium to facilitate the encoding of bright objects, which in turn results from expressional tuning of phototransduction genes. Moreover, the light-driven synaptic calcium signal is regionally slowed by interactions with horizontal cells and later accentuated at the level of glutamate release driving retinal networks. These regional differences tally with variations between peripheral and foveal cones in primates and hint at a common mechanistic origin.

Mislocalization of cone nuclei impairs cone function in mice

The nuclei of cone photoreceptors are located on the apical side of the outer nuclear layer (ONL) in vertebrate retinas. However, the functional role of this evolutionarily conserved localization of cone nuclei is unknown. We previously showed that Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) are essential for the apical migration of cone nuclei during development. Here, we developed an efficient genetic strategy to disrupt cone LINC complexes in mice. Experiments with animals from both sexes revealed that disrupting cone LINC complexes resulted in mislocalization of cone nuclei to the basal side of ONL in mouse retina. This, in turn, disrupted cone pedicle morphology, and appeared to reduce the efficiency of synaptic transmission from cones to bipolar cells. Although we did not observe other developmental or phototransduction defects in cones with mislocalized nuclei, their dark adaptation was impaired, consistent with a deficiency in chromophore recycling. These findings demonstrate that the apical localization of cone nuclei in the ONL is required for the timely dark adaptation and efficient synaptic transmission in cone photoreceptors.

Solar light induces the release of acetylcholine from skin keratinocytes affecting melanogenesis

Cholinergic system conducts signal transmission in brain and muscle. Besides nervous system, the nonneuronal functions of cholinergic system have been proposed in various tissues. The expression of cholinergic proteins and release of acetylcholine in human skin have been reported, but its mechanism and influence on dermatological functions is not elucidated. Here, the expression profile of cholinergic markers was further investigated in skin and keratinocyte. The expression levels of choline acetyltransferase (ChAT), acetylcholinesterase (AChE), vesicular acetylcholine transporter (VAChT), and synaptophysin, were upregulated during differentiation of keratinocytes. In cultured keratinocytes, a transient exposure of solar light induced the release of acetylcholine, which was mediated by intracellular Ca²⁺ mobilization. The light-induced acetylcholine release was mediated by the present of opsin. The light-induced melanogenesis was inhibited by acetylcholine or AChE inhibitor in melanocyte in vitro and mouse skin ex vivo. These results indicated that the potential role of cholinergic system could be a negative regulator in skin pigmentation.

Advances in exogenous docosahexaenoic acid-containing phospholipids: Sources, positional isomerism, biological activities, and advantages

In recent years, docosahexaenoic acid-containing phospholipids (DHA-PLs) have attracted much attention because of theirs unique health benefits. Compared with other forms of docosahexaenoic acid (DHA), DHA-PLs possess superior biological effects (e.g., anticancer, lipid metabolism regulation, visual development, and brain and nervous system biochemical reactions), more intricate metabolism mechanisms, and a stronger attraction to consumer. The production of DHA-PLs is hampered by several challenges associated with the limited content of DHA-PLs in natural sources, incomplete utilization of by-products, few microorganisms for DHA-PLs production, high cost, and complex process of artificial preparation of DHA-PLs. In this article, the sources, biological activities, and commercial applications of DHA-PLs were summarized, with intensive discussions on advantages of DHA-PLs over DHA, isomerism of DHA in phospholipids (PLs), and brain health. The excellent biological characteristics of DHA-PLs are primarily concerned with DHA and PLs. The metabolic fate of different DHA-PLs varies from the position of DHA in PLs to polar groups in DHA-PLs. Overall, well understanding of DHA-PLs about their sources and characteristics is critical to accelerate the production of DHA-PLs, economically enhance the value of DHA-PLs, and improve the applicability of DHA-PLs and the acceptance of consumers.

Aerobic Glycolysis in the Retina: Functional Roles of Pyruvate Kinase Isoforms

One hundred years ago, Otto Heinrich Warburg observed that postmitotic retinal cells are the highest oxygen-consuming cells in the body. He compared these cells to actively growing mitotic tumor cells since both cells reprogram glucose for anabolic processes, which include lipid, protein, and RNA/DNA synthesis, and for antioxidant metabolism. To achieve this metabolic reprogramming, cancer cells preferentially express a less active dimeric form, the M2 isoform of pyruvate kinase (PKM2), which shuttles glucose toward the accumulation of glycolytic intermediates that redirect cell activities into anabolic processes. Similar to cancer cells, retinal photoreceptors predominantly express the M2 isoform of PKM2. This isoform performs both metabolic and non-metabolic functions in photoreceptor cells. This review focuses on the metabolic and non-metabolic roles of pyruvate kinases in photoreceptor cell functions.

The evo-devo of molluscs: Insights from a genomic perspective

Molluscs represent one of ancient and evolutionarily most successful groups of marine invertebrates, with a tremendous diversity of morphology, behavior, and lifestyle. Molluscs are excellent subjects for evo-devo studies; however, understanding of the evo-devo of molluscs has been largely hampered by incomplete fossil records and limited molecular data. Recent advancement of genomics and other technologies has greatly fueled the molluscan "evo-devo" field, and decoding of several molluscan genomes provides unprecedented insights into molluscan biology and evolution. Here, we review the recent progress of molluscan genome sequencing as well as novel insights gained from their genomes, by emphasizing how molluscan genomics enhances our understanding of the evo-devo of molluscs.

Functions of Opsins in Drosophila Taste

Rhodopsin is a light receptor comprised of an opsin protein and a light-sensitive retinal chromophore. Despite more than a century of scrutiny, there is no evidence that opsins function in chemosensation. Here, we demonstrate that three Drosophila opsins, Rh1, Rh4, and Rh7, are needed in gustatory receptor neurons to sense a plant-derived bitter compound, aristolochic acid (ARI). The gustatory requirements for these opsins are light-independent and do not require retinal. The opsins enabled flies to detect lower concentrations of aristolochic acid by initiating an amplification cascade that includes a G-protein, phospholipase Cβ, and the TRP channel, TRPA1. In contrast, responses to higher levels of the bitter compound were mediated through direct activation of TRPA1. Our study reveals roles for opsins in chemosensation and raise questions concerning the original roles for these classical G-protein-coupled receptors.

A review of electroretinography waveforms and models and their application in the dog

Electroretinography (ERG) is a commonly used technique to study retinal function in both clinical and research ophthalmology. ERG responses can be divided into component waveforms, analysis of which can provide insight into the health and function of different types and populations of retinal cells. In dogs, ERG has been used in the characterization of normal retinal function, as well as the diagnosis of retinal diseases and measuring effects of treatment. While many components of the recorded waveform are similar across species, dogs have several notable features that should be differentiated from the responses in humans and other animals. Additionally, modifications of standard protocols, such as changing flash frequency and stimulus color, and mathematical models of ERG waveforms have been used in studies of human retinal function but have been infrequently applied to visual electrophysiology in dogs. This review provides an overview of the origins and applications of ERG in addition to potential avenues for further characterization of responses in the dog.

Activation of an Endogenous Opsin 3 Light Receptor Mediates Photo-Relaxation of Pre-Contracting Late Gestation Human Uterine Smooth Muscle Ex Vivo

Spontaneous preterm birth (sPTB) remains a worldwide healthcare challenge. Preterm labor (PTL) is thought to be the largest reversible cause of sPTB, but current tocolytic therapies are ineffective and associated with systemic side effects from chronic use. Therefore, identifying novel mechanisms that promote human uterine smooth muscle (hUSM) relaxation is essential to improving clinical management of PTL. Here, we aimed to determine if an extraocular opsin receptor (OPN 3,4,5) system is expressed in pregnant hUSM and to characterize how photo-mediated relaxation of pre-contracting hUSM may be facilitated by external application of light. Translational studies were performed with hUSM from healthy late gestation patients (n = 8) and non-pregnant, similarly aged patients undergoing hysterectomy (n = 4). First, RT-PCR screened for mRNA coding for components of the classical extraocular light receptors (OPN 3,4,5). We found a restricted repertoire of opsin receptors (OPN3) expressed in pregnant hUSM tissue. Immunohistochemistry was performed to confirm protein expression. Pre-contracting late gestation hUSM strips were studied in functional organ bath studies to determine if photo-mediated relaxation is intensity or wavelength dependent. Functional organ bath studies revealed acute photo-mediated relaxation occurring in an intensity- and wavelength-dependent manner. Finally, coimmunoprecipitation of OPN3 with Gs following light activation suggests that a component of photo-relaxation occurs via G protein-coupled receptor machinery. This is the first report of light-mediated relaxation of pre-contracted human myometrium. Activation of endogenous light receptors on human myometrium may become a novel, non-invasive tocolytic strategy.

Gene expression patterns of novel visual and non-visual opsin families in immature and mature Japanese eel males

This study was carried out to identify and estimate physiological function of a new type of opsin subfamily present in the retina and whole brain tissues of Japanese eel using RNA–Seq transcriptome method. A total of 18 opsin subfamilies were identified through RNA–seq. The visual opsin family included Rh2, SWS2, FWO, DSO, and Exo-Rhod. The non-visual opsin family included four types of melanopsin subfamily (Opn4x1, Opn4x2, Opn4m1, and Opn4m2), peropsin, two types of neuropsin subfamily (Opn5-like, Opn5), Opn3, three types of TMT opsin subfamily (TMT1, 2, 3), VA-opsin, and parapinopsin. In terms of changes in photoreceptor gene expression in the retina of sexually mature and immature male eels, DSO mRNA increased in the maturation group. Analysis of expression of opsin family gene in male eel brain before and after maturation revealed that DSO and SWS2 expression in terms of visual opsin mRNA increased in the sexually mature group. In terms of non-visual opsin mRNA, parapinopsin mRNA increased whereas that of TMT2 decreased in the fore-brain of the sexually mature group. The mRNA for parapinopsin increased in the mid-brain of the sexually mature group, whereas those of TMT1 and TMT3 increased in the hind-brain of the sexually mature group. DSO mRNA also increased in the retina after sexual maturation, and DSO and SWS2 mRNA increased in whole brain part, suggesting that DSO and SWS2 are closely related to sexual maturation.

Membrane conductances of mouse cone photoreceptors

Vertebrate photoreceptor cells respond to light through a closure of CNG channels located in the outer segment. Multiple voltage-sensitive channels in the photoreceptor inner segment serve to transform and transmit the light-induced outer-segment current response. Despite extensive studies in lower vertebrates, we do not know how these channels produce the photoresponse of mammalian photoreceptors. Here we examined these ionic conductances recorded from single mouse cones in unlabeled, dark-adapted retinal slices. First, we show measurements of the voltage dependence of the light response. After block of voltage-gated Ca2+ channels, the light-dependent current was nearly linear within the physiological range of voltages with constant chord conductance and a reversal potential similar to that previously determined in lower vertebrate photoreceptors. At a dark resting membrane potential of -45 mV, cones maintain a standing Ca2+ current (iCa) between 15 and 20 pA. We characterized the time and voltage dependence of iCa and a calcium-activated anion channel. After constitutive closure of the CNG channels by the nonhydrolysable analogue GTP-γ-S, we observed a light-dependent increase in iCa followed by a Ca2+-activated K+ current, both probably the result of feedback from horizontal cells. We also recorded the hyperpolarization-activated cyclic nucleotide-gated (HCN) conductance (ih) and measured its current-voltage relationship and reversal potential. With small hyperpolarizations, ih activated with a time constant of 25 ms; activation was speeded with larger hyperpolarizations. Finally, we characterized two voltage-gated K+-conductances (iK). Depolarizing steps beginning at -10 mV activated a transient, outwardly rectifying iK blocked by 4-AP and insensitive to TEA. A sustained iK isolated through subtraction was blocked by TEA but was insensitive to 4-AP. The sustained iK had a nearly linear voltage dependence throughout the physiological voltage range of the cone. Together these data constitute the first comprehensive study of the channel conductances of mouse photoreceptors.