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Current Opinion in Structural Biology Aug 2021Rod photoreceptor phosphodiesterase (PDE6) is the key catalytic enzyme of visual phototransduction. PDE6 is the only member of the phosphodiesterase family that consists... (Review)
Review
Rod photoreceptor phosphodiesterase (PDE6) is the key catalytic enzyme of visual phototransduction. PDE6 is the only member of the phosphodiesterase family that consists of a heterodimeric catalytic core composed of PDE6α and PDE6β subunits and two inhibitory PDE6γ subunits. Both PDE6α and PDE6β contain two regulatory GAF domains and one catalytic domain. GAF domains and the tightly bound PDE6γ subunits allosterically regulate the activity of the catalytic domain in association with the GTP-bound transducin alpha subunit (G). Recent cryo-electron microscopy structures of the PDE6αγβγ and PDE6αγβγ-(G) complexes have provided valuable knowledge shedding additional light on the allosteric activation of PDE6 by G. Here we discuss recent developments in our understanding of the mechanism of PDE6 activation.
Topics: Catalytic Domain; Cryoelectron Microscopy; Cyclic Nucleotide Phosphodiesterases, Type 6; Phosphoric Diester Hydrolases; Retinal Rod Photoreceptor Cells
PubMed: 33945959
DOI: 10.1016/j.sbi.2021.03.016 -
Proceedings of the National Academy of... May 2022Myopia is the commonest visual impairment. Several genetic loci confer risk, but mechanisms by which they do this are unknown. Retinal signals drive eye growth, and...
Myopia is the commonest visual impairment. Several genetic loci confer risk, but mechanisms by which they do this are unknown. Retinal signals drive eye growth, and myopia usually results from an excessively long eye. The common variant most strongly associated with myopia is near the GJD2 gene, encoding connexin-36, which forms retinal gap junctions. Light-evoked responses of retinal neurons can be recorded noninvasively as the electroretinogram (ERG). We analyzed these responses from 186 adult twin volunteers who had been genotyped at this locus. Participants underwent detailed ERG recordings incorporating international standard stimuli as well as experimental protocols aiming to separate dark-adapted rod- and cone-driven responses. A mixed linear model was used to explore association between allelic dosage at the locus and international standard ERG parameters after adjustment for age, sex, and family structure. Significant associations were found for parameters of light-adapted, but not dark-adapted, responses. Further investigation of isolated rod- and cone-driven ERGs confirmed associations with cone-driven, but not rod-driven, a-wave amplitudes. Comparison with responses to similar experimental stimuli from a patient with a prior central retinal artery occlusion, and from two patients with selective loss of ON-bipolar cell signals, was consistent with the associated parameters being derived from signals from cone-driven OFF-bipolar cells. Analysis of single-cell transcriptome data revealed strongest GJD2 expression in cone photoreceptors; bipolar cell expression appeared strongest in OFF-bipolar cells and weakest in rod-driven ON-bipolar cells. Our findings support a potential role for altered signaling in cone-driven OFF pathways in myopia development.
Topics: Electroretinography; Genome-Wide Association Study; Humans; Myopia; Polymorphism, Genetic; Retinal Cone Photoreceptor Cells; Retinal Rod Photoreceptor Cells
PubMed: 35594404
DOI: 10.1073/pnas.2119675119 -
Proceedings of the National Academy of... Sep 2020Rod photoreceptors are composed of a soma and an inner segment (IS) connected to an outer segment (OS) by a thin cilium. OSs are composed of a stack of ∼800 lipid...
Rod photoreceptors are composed of a soma and an inner segment (IS) connected to an outer segment (OS) by a thin cilium. OSs are composed of a stack of ∼800 lipid discs surrounded by the plasma membrane where phototransduction takes place. Intracellular calcium plays a major role in phototransduction and is more concentrated in the discs, where it can be incorporated and released. To study calcium dynamics in rods, we used the fluorescent calcium dye CaSiR-1 AM working in the near-infrared (NIR) (excitation at 650 and emission at 664 nm), an advantage over previously used dyes. In this way, we investigated calcium dynamics with an unprecedented accuracy and most importantly in semidark-adapted conditions. We observed light-induced drops in [Ca] with kinetics similar to that of photoresponses recorded electrophysiologically. We show three properties of the rods. First, intracellular calcium and key proteins have concentrations that vary from the OS base to tip. At the OS base, [Ca] is ∼80 nM and increases up to ∼200 nM at the OS tip. Second, there are spontaneous calcium flares in healthy and functional rod OSs; these flares are highly localized and are more pronounced at the OS tip. Third, a bright flash of light at 488 nm induces a drop in [Ca] at the OS base but often a flare at the OS tip. Therefore, rod OSs are not homogenous structures but have a structural and functional gradient, which is a fundamental aspect of transduction in vertebrate photoreceptors.
Topics: Animals; Calcium; Cell Membrane; Cytoplasm; Female; Kinetics; Light Signal Transduction; Male; Retinal Rod Photoreceptor Cells; Rod Cell Outer Segment; Xenopus laevis
PubMed: 32817426
DOI: 10.1073/pnas.2004909117 -
Progress in Retinal and Eye Research May 2022The main aim of the paper is to discuss current knowledge on how Age Related Macular Degeneration (AMD) affects Dark Adaptation (DA). The paper is divided into three... (Review)
Review
The main aim of the paper is to discuss current knowledge on how Age Related Macular Degeneration (AMD) affects Dark Adaptation (DA). The paper is divided into three parts. Firstly, we outline some of the molecular mechanisms that control DA. Secondly, we review the psychophysical issues and the corresponding analytical techniques. Finally, we characterise the link between slowed DA and the morphological abnormalities in early AMD. Historically, DA has been regarded as too cumbersome for widespread clinical application. Yet the technique is extremely useful; it is widely accepted that the psychophysically obtained slope of the second rod-mediated phase of the dark adaptation function is an accurate assay of photoreceptor pigment regeneration kinetics. Technological developments have prompted new ways of generating the DA curve, but analytical problems remain. A simple potential solution to these, based on the application of a novel fast mathematical algorithm, is presented. This allows the calculation of the parameters of the DA curve in real time. Improving current management of AMD will depend on identifying a satisfactory endpoint for evaluating future therapeutic strategies. This must be implemented before the onset of severe disease. Morphological changes progress too slowly to act as a satisfactory endpoint for new therapies whereas functional changes, such as those seen in DA, may have more potential in this regard. It is important to recognise, however, that the functional changes are not confined to rods and that building a mathematical model of the DA curve enables the separation of rod and cone dysfunction and allows more versatility in terms of the range of disease severity that can be monitored. Examples are presented that show how analysing the DA curve into its constituent components can improve our understanding of the morphological changes in early AMD.
Topics: Dark Adaptation; Humans; Macular Degeneration; Retinal Rod Photoreceptor Cells; Visual Acuity
PubMed: 34626782
DOI: 10.1016/j.preteyeres.2021.101015 -
Experimental Eye Research Apr 2021We assess the effect of autophagy inhibition on photoreceptor (PR) survival during experimental retinal detachment (RD) and examine the and examine the relationship...
We assess the effect of autophagy inhibition on photoreceptor (PR) survival during experimental retinal detachment (RD) and examine the and examine the relationship between autophagy and the expression of glycolytic enzymes HK2 and PKM2 in the retina. We find that inhibiting autophagy by genetic knock out of the autophagy activator Atg5 in rod PRs resulted in increased apoptotic and necroptotic cell death during RD, demonstrated by elevated terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells, caspase 8 activity, transcript levels of Fas receptor and RIPK3 as compared to controls. The absence of autophagy in rods resulted in downregulation of hexokinase 2 and pyruvate kinase muscle isozyme 2 levels. More than 460 proteins were identified by mass spectroscopy in autophagosomes isolated from detached retinas compared with less than 150 proteins identified in autophagosomes from attached retinas. Among various cellular compartments, proteins from cytoskeleton, cytoplasm and intracellular organelles constituted a large portion of increased autophagosome contents. These proteins represent numerous biological processes, including phototransduction, cell-cell signaling, metabolism and inflammation. Our findings suggest that competent autophagy machinery is necessary for PR homeostasis and improving PR survival during periods of nutrient deprivation.
Topics: Animals; Autophagosomes; Autophagy; Autophagy-Related Protein 5; Blotting, Western; Caspase 8; Cell Survival; Gene Deletion; Green Fluorescent Proteins; Hexokinase; Immunohistochemistry; In Situ Nick-End Labeling; Mass Spectrometry; Mice; Mice, Inbred C57BL; Mice, Knockout; Mice, Transgenic; Pyruvate Kinase; Real-Time Polymerase Chain Reaction; Retinal Detachment; Retinal Rod Photoreceptor Cells
PubMed: 33609513
DOI: 10.1016/j.exer.2021.108492 -
Advances in Experimental Medicine and... 2019PRCD (progressive rod-cone degeneration) is a small ~6 kDa protein with unknown function that specifically resides in photoreceptor discs and interacts with rhodopsin.... (Review)
Review
PRCD (progressive rod-cone degeneration) is a small ~6 kDa protein with unknown function that specifically resides in photoreceptor discs and interacts with rhodopsin. PRCD's discovery resulted from decades-long study of a canine retinal disease called progressive rod-cone degeneration which is one of the most frequent causes of blindness in dogs characterized by the slow, progressive death of rod photoreceptors followed by cones. A series of genetic studies eventually mapped the disease to a single point mutation in a novel gene which was then named Prcd. Highlighting the importance of this gene, this and several other mutations have been identified in human patients suffering from retinitis pigmentosa. In this review, we highlight what is currently known about PRCD protein, including the etiology and pathology of the retinal disease caused by its mutation, the protein's trafficking, localization, and biochemical characterization.
Topics: Animals; Carrier Proteins; Dogs; Eye Proteins; Humans; Mutation; Retinal Cone Photoreceptor Cells; Retinal Rod Photoreceptor Cells; Retinitis Pigmentosa; Rhodopsin
PubMed: 31884666
DOI: 10.1007/978-3-030-27378-1_87 -
Cells Jun 2020Phosphoinositides are known to play multiple roles in eukaryotic cells. Although dysregulation of phosphoinositide metabolism in the retina has been reported to cause...
Phosphoinositides are known to play multiple roles in eukaryotic cells. Although dysregulation of phosphoinositide metabolism in the retina has been reported to cause visual dysfunction in animal models and human patients, our understanding of the phosphoinositide composition of the retina is limited. Here, we report a characterization of the phosphoinositide profile of the mouse retina and an analysis of the subcellular localization of major phosphorylated phosphoinositide forms in light-sensitive photoreceptor neurons. Using chromatography of deacylated phosphatidylinositol headgroups, we established PI(4,5)P and PI(4)P as two major phosphorylated phosphoinositides in the retina. Using high-resolution mass spectrometry, we revealed 18:0/20:4 and 16:0/20:4 as major fatty-acyl chains of retinal phosphoinositides. Finally, analysis of fluorescent phosphoinositide sensors in rod photoreceptors demonstrated distinct subcellular distribution patterns of major phosphoinositides. The PI(4,5)P reporter was enriched in the inner segments and synapses, but was barely detected in the light-sensitive outer segments. The PI(4)P reporter was mostly found in the outer and inner segments and the areas around nuclei, but to a lesser degree in the synaptic region. These findings provide support for future mechanistic studies defining the biological significance of major mono- (PI(4)P) and bisphosphate (PI(4,5)P) phosphatidylinositols in photoreceptor biology and retinal health.
Topics: Animals; Fatty Acids; Lipidomics; Mice, Inbred C57BL; Phosphatidylinositols; Phosphorylation; Retina; Retinal Rod Photoreceptor Cells; Subcellular Fractions
PubMed: 32517352
DOI: 10.3390/cells9061417 -
The Journal of Neuroscience : the... Nov 2022At intermediate (mesopic) light levels, rods and cones are both active and can contribute to vision. This presents a challenge to the retina because the visual responses...
At intermediate (mesopic) light levels, rods and cones are both active and can contribute to vision. This presents a challenge to the retina because the visual responses originating with rods and cones are distinct, yet their visual responses must be seamlessly combined. The current study aimed to establish how the circadian clock regulates rod and/or cone vision in these conditions, given the strong time-of-day change in the reliance on each photoreceptor. Visual responses were recorded in the retina and visual thalamus of anaesthetized male mice at distinct circadian time points, and the method of receptor silent substitution was used to selectively stimulate different photoreceptor types. With stimuli designed to only activate rods, responses in the mesopic range were highly rhythmic and peaked in amplitude in the subjective night. This rhythm was abolished following intravitreal injection of the gap junction blocker meclofenamic acid, consistent with a circadian variation in the strength of electrical coupling of photoreceptors. In contrast, responses to stimuli designed to only activate cones were arrhythmic within the mesopic to photopic range when adapted to the background irradiance. The outcome was that combined rod-plus-cone responses showed a stable contrast-response relationship across mesopic-photopic backgrounds in the circadian day, whereas at night, responses were significantly amplified at lower light levels. These data support the idea that the circadian clock is a key regulator of vision, in this case defining the relative amplitude of rod/cone vision across the mesopic transition according to time of day. Although the importance of circadian clocks in regulating vision has been long recognized, less is known about how the clock shapes vision in conditions where both rods and cones are active (mesopic conditions). Here, the novel approach of receptor silent substitution has been applied to trace rod and cone visual responses in mice across the circadian cycle and has identified pronounced rhythms in rod, but not cone, vision. This has the effect of boosting responses in dimmer backgrounds at night at the cost of impaired contrast-response stability across the mesopic to photopic range. Thus, the circadian clock drives anticipatory changes in the relative contribution of rods versus cones to vision, which match the prevailing visual environment.
Topics: Male; Mice; Animals; Retinal Rod Photoreceptor Cells; Mesopic Vision; Retinal Cone Photoreceptor Cells; Retina; Color Vision
PubMed: 36216501
DOI: 10.1523/JNEUROSCI.0486-22.2022 -
Experimental Eye Research Jan 2019Teleosts are unique in their ability to undergo persistent neurogenesis and to regenerate damaged and lost retinal neurons in adults. This contrasts with the human...
Teleosts are unique in their ability to undergo persistent neurogenesis and to regenerate damaged and lost retinal neurons in adults. This contrasts with the human retina, which is incapable of replacing lost retinal neurons causing vision loss/blindness in the affected individuals. Two cell populations within the adult teleost retina generate new retinal neurons throughout life. Stem cells within the ciliary marginal zone give rise to all retinal cell types except for rod photoreceptors, which are produced by the resident Müller glia that are located within the inner nuclear layer of the entire retina. Understanding the mechanisms that regulate the generation of photoreceptors in the adult teleost retina may ultimately aid developing strategies to overcome vision loss in diseases such as retinitis pigmentosa. Here, we investigated whether photic deprivation alters the proliferative capacity of rod precursor cells, which are generated from Müller glia. In dark-adapted retinas, rod precursor cell proliferation increased, while the number of proliferating Müller glia and their derived olig2:EGFP-positive neuronal progenitor cells was not significantly changed. Cell death of rod photoreceptors was excluded as the inducer of rod precursor cell proliferation, as the number of TUNEL-positive cells and l-plastin-positive microglia in both the outer (ONL) and inner nuclear layer (INL) remained at a similar level throughout the dark-adaptation timecourse. Rod precursor cell proliferation in response to dark-adaptation was characterized by an increased number of EdU-positive cells, i.e. cells that were undergoing DNA replication. These proliferating rod precursor cells in dark-adapted zebrafish differentiated into rod photoreceptors at a comparable percentage and in a similar time frame as those maintained under standard light conditions suggesting that the cell cycle did not stall in dark-adapted retinas. Inhibition of IGF1-receptor signaling reduced the dark-adaptation-mediated proliferation response; however, caloric restriction which has been suggested to be integrated by the IGF1/growth hormone signaling axis did not influence rod precursor cell proliferation in dark-adapted retinas, as similar numbers were observed in starved and normal fed zebrafish. In summary, photic deprivation induces cell cycle entry of rod precursor cells via IGF1-receptor signaling independent of Müller glia proliferation.
Topics: Animals; Animals, Genetically Modified; Cell Differentiation; Cell Proliferation; Dark Adaptation; Ependymoglial Cells; In Situ Nick-End Labeling; Injections, Intraperitoneal; Light; Neurogenesis; Photic Stimulation; Pyrimidines; Pyrroles; Receptor, IGF Type 1; Retinal Rod Photoreceptor Cells; Stem Cells; Taurine; Zebrafish
PubMed: 30267656
DOI: 10.1016/j.exer.2018.09.015 -
Advances in Protein Chemistry and... 2020Most vertebrates express four arrestin subtypes: two visual ones in photoreceptor cells and two non-visuals expressed ubiquitously. The latter two interact with hundreds... (Review)
Review
Most vertebrates express four arrestin subtypes: two visual ones in photoreceptor cells and two non-visuals expressed ubiquitously. The latter two interact with hundreds of G protein-coupled receptors, certain receptors of other types, and numerous non-receptor partners. Arrestins have no enzymatic activity and work by interacting with other proteins, often assembling multi-protein signaling complexes. Arrestin binding to every partner affects cell signaling, including pathways regulating cell survival, proliferation, and death. Thus, targeting individual arrestin interactions has therapeutic potential. This requires precise identification of protein-protein interaction sites of both participants and the choice of the side of each interaction which would be most advantageous to target. The interfaces involved in each interaction can be disrupted by small molecule therapeutics, as well as by carefully selected peptides of the other partner that do not participate in the interactions that should not be targeted.
Topics: Animals; Arrestins; Binding Sites; Gene Expression Regulation; Genetic Therapy; Humans; Leber Congenital Amaurosis; Molecular Targeted Therapy; Mutation; Protein Binding; Receptors, G-Protein-Coupled; Retinal Rod Photoreceptor Cells; Signal Transduction; Small Molecule Libraries
PubMed: 32312421
DOI: 10.1016/bs.apcsb.2019.11.011