Rhodopsin Mutations Research Articles

The heat-shock response co-inducer arimoclomol protects against retinal degeneration in rhodopsin retinitis pigmentosa.

Retinitis pigmentosa (RP) is a group of inherited diseases that cause blindness due to the progressive death of rod and cone photoreceptors in the retina. There are currently no effective treatments for RP. Inherited mutations in rhodopsin, the light-sensing protein of rod photoreceptor cells, are the most common cause of autosomal-dominant RP. The majority of mutations in rhodopsin, including the common P23H substitution, lead to protein misfolding, which is a feature in many neurodegenerative disorders. Previous studies have shown that upregulating molecular chaperone expression can delay disease progression in models of neurodegeneration. Here, we have explored the potential of the heat-shock protein co-inducer arimoclomol to ameliorate rhodopsin RP. In a cell model of P23H rod opsin RP, arimoclomol reduced P23H rod opsin aggregation and improved viability of mutant rhodopsin-expressing cells. In P23H rhodopsin transgenic rat models, pharmacological potentiation of the stress response with arimoclomol improved electroretinogram responses and prolonged photoreceptor survival, as assessed by measuring outer nuclear layer thickness in the retina. Furthermore, treated animal retinae showed improved photoreceptor outer segment structure and reduced rhodopsin aggregation compared with vehicle-treated controls. The heat-shock response (HSR) was activated in P23H retinae, and this was enhanced with arimoclomol treatment. Furthermore, the unfolded protein response (UPR), which is induced in P23H transgenic rats, was also enhanced in the retinae of arimoclomol-treated animals, suggesting that arimoclomol can potentiate the UPR as well as the HSR. These data suggest that pharmacological enhancement of cellular stress responses may be a potential treatment for rhodopsin RP and that arimoclomol could benefit diseases where ER stress is a factor.

A Pro23His Mutation Alters Prenatal Rod Photoreceptor Morphology in a Transgenic Swine Model of Retinitis Pigmentosa

Functional studies have detected deficits in retinal signaling in asymptomatic children from families with inherited autosomal dominant retinitis pigmentosa (RP). Whether retinal abnormalities are present earlier during gestation or shortly after birth in a subset of children with autosomal dominant RP is unknown and no appropriate animal RP model possessing visual function at birth has been available to examine this possibility. In a recently developed transgenic P23H (TgP23H) rhodopsin swine model of RP, we tracked changes in pre- and early postnatal retinal morphology, as well as early postnatal retinal function. Domestic swine inseminated with semen from a TgP23H miniswine founder produced TgP23H hybrid and wild type (Wt) littermates. Outer retinal morphology was assessed at light and electron microscopic levels between embryonic (E) and postnatal (P) day E85 to P3. Retinal function was evaluated using the full field electroretinogram at P3. Embryonic TgP23H rod photoreceptors are malformed and their rhodopsin expression pattern is abnormal. Consistent with morphological abnormalities, rod-driven function is absent at P3. In contrast, TgP23H and Wt cone photoreceptor morphology (E85-P3) and cone-driven retinal function (P3) are similar. Prenatal expression of mutant rhodopsin alters the normal morphological and functional development of rod photoreceptors in TgP23H swine embryos. Despite this significant change, cone photoreceptors are unaffected. Human infants with similarly aggressive RP might never have rod vision, although cone vision would be unaffected. Such aggressive forms of RP in preverbal children would require early intervention to delay or prevent functional blindness.

Unoprostone protects against cell death, reactive oxygen species, and apoptosis in a model retinitis pigmentosa system using expression of wild type and mutant rhodopsin (LB832)

Objective. This study was to determine if unoprostone protects against deleterious effects evident in retinitis pigmentosa (RP). With the P23H mutation in rhodopsin, it is a dominant negative disease, causing the unfolded protein response (UPR) and ER stress. This leads to reactive oxygen species (ROS) production, apoptosis and ultimately cell death.Methods. Flp‐In T‐Rex‐293 cells were transfected with wild type (WT) or P23H mutant (MUT) rhodopsin in pcDNA5/FRT/TO with pOG44 under control of a tetracycline (tet) promoter. Cell growth, ROS and caspase were measured in non, mock, WT and MUT transfected cells with 1 µg/ml tet ± 100 nM unoprostone. ROS was measured with (5&6)‐chloromethyl‐2’,7’‐dichlorodihydrofluorescein diacetate,acetyl ester and caspase was detected using FITC‐VAD‐FMK.Results. There were no significant differences between non‐, mock‐ and WT (WT) cells ± unoprostone. At 120 h (n=3) the number of WT cells increased from 54.6 ± 1.2 to 111.7 ± 3.1, while the number of MUT cells decreased from 56.7 ± 3.6 to 19.0 ± 1.3. Unoprostone increased significantly (P<0.005) the number of MUT cells to 32.0 ± 0.5. ROS (FU, n=6) increased from 332.9 ± 0.7 in WT cells to 2816.8 ± 6.4 in MUT cells and this was significantly decreased (P<0.0005) to 1331.0 ± 3.0 with unoprostone. Caspase (FU, n=6) at 120 h increased from 365.7 ± 5.3 in WT cells to 895.5 ± 10.2 in MUT cells and this was significantly reduced (P<0.0005) to 523.2 ± 11.3 with unoprostone.Conclusions. The data shows that unoprostone can slow the pathological events (ER stress, ROS production, and apoptosis) and cell death seen in RP with UPR. Increased ROS generation and apoptosis are also seen in age related macular degeneration (AMD), suggesting unoprostone should also be investigated in dry AMD.Grant Funding Source: Supported by Sucampo AG

Endoplasmic reticulum‐associated degradation of mutant rhodopsin disrupts protein homeostasis in photoreceptor cells during retinal degeneration (558.3)

Rhodopsin is a G‐protein coupled receptor that is expressed exclusively by rod photoreceptors of the retina and is essential for phototransduction and vision. Over 150 missense mutations in rhodopsin have been identified in the hereditary forms of retinal degeneration. Many of these missense mutations, such as P23H mutation, cause rhodopsin protein misfolding and ultimately lead to rod cell death and vision loss via poorly understood mechanisms. Here, we examined a P23H knock‐in mouse model that exactly recapitulates the gene dosage and retinal degeneration disease phenotype seen in human families carrying P23H rhodopsin alleles. In these retinas, we find that photoreceptors immediately target P23H rhodopsin for elimination as soon as it is produced. Through analysis of P23H knock‐in mice crossed with Xbp1‐Venus reporter mice, we find that photoreceptors expressing P23H rhodopsin strongly activate the IRE1 branch of the UPR. IRE1 activation leads to pronounced upregulation of ER‐associated protein degradation in photoreceptors, pronounced ubiquitination of P23H rhodopsin, and its rapid degradation. By contrast to heterologous cell culture studies of misfolded rhodopsin, virtually no misfolded rhodopsin protein accumulates within photoreceptors as ER‐retained aggregates. Consistent with the efficient elimination of P23H rhodopsin from the ER of photoreceptors, we find no increase in proapoptotic Chop, induced by persistent ER protein aggregation. Surprisingly, despite the induction of ERAD and rapid elimination of P23H rhodopsin, total cellular levels of ubiquitylated proteins markedly increase during retinal degeneration. Our findings show that the IRE1 branch of the UPR promotes robust ERAD of P23H rhodopsin in photoreceptors. We suggest that the increased buildup of damaged, ubiquitylated proteins arises as a consequence of ERAD of P23H rhodopsin, and disruption of cellular protein homeostasis leads to rod photoreceptor cell death and retinal degeneration.

Retinitis pigmentosa-associated rhodopsin mutant T17M induces endoplasmic reticulum (ER) stress and sensitizes cells to ER stress-induced cell death

Retinitis pigmentosa (RP) is a group of inherited diseases that primarily affect light‑sensitive rods and cones in the retina. Rhodopsin mutations, including the T17M mutation, are associated with the autosomal dominant form of retinitis pigmentosa (ADRP) and have been linked to abnormal protein folding. However, the molecular mechanisms underlying T17M rhodopsin‑induced retinal degeneration are yet to be elucidated. In the present study, Human embryonic kidney (HEK) 293 and ARPE‑19 cells were transfected with myc‑tagged wild‑type (WT) and T17M rhodopsin constructs. Cells were fixed and stained with anti‑myc antibodies and the localization of WT and T17M rhodopsin was visualized using immunofluorescence microscopy. Turnover rates of WT and T17M rhodopsin were measured using western blot analysis. In addition, endoplasmic reticulum (ER) stress‑induced cell death was analyzed in WT and T17M rhodopsin‑transfected cells using nuclear staining. Misfolded T17M rhodopsin was observed to be abnormally localized in the ER, while WT rhodopsin was predominantly found at the plasma membrane. Protein turnover analysis revealed that T17M rhodopsin was more rapidly degraded by proteasomes than WT rhodopsin. Furthermore, overexpression of T17M rhodopsin was observed to induce cell death and increase cytotoxicity; predisposing cells to ER stress‑induced cell death. These findings show novel insight into the properties of T17M rhodopsin and highlight the role of ER stress in T17M‑associated RP.

Ablation of the proapoptotic genes CHOP or Ask1 does not prevent or delay loss of visual function in a P23H transgenic mouse model of retinitis pigmentosa.

The P23H mutation in rhodopsin (RhoP23H) is a prevalent cause of autosomal dominant retinitis pigmentosa. We examined the role of the ER stress proteins, Chop and Ask1, in regulating the death of rod photoreceptors in a mouse line harboring the RhoP23H rhodopsin transgene (GHL+). We used knockout mice models to determine whether Chop and Ask1 regulate rod survival or retinal degeneration. Electrophysiological recordings showed similar retinal responses and sensitivities for GHL+, GHL+/Chop−/− and GHL+/Ask1−/− animals between 4–28 weeks, by which time all three mouse lines exhibited severe loss of retinal function. Histologically, ablation of Chop and Ask1 did not rescue photoreceptor loss in young animals. However, in older mice, a regional protective effect was observed in the central retina of GHL+/Chop−/− and GHL+/Ask1−/−, a region that was severely degenerated in GHL+ mice. Our results show that in the presence of the RhoP23H transgene, the rate of decline in retinal sensitivity is similar in Chop or Ask1 ablated and wild-type retinas, suggesting that these proteins do not play a major role during the acute phase of photoreceptor loss in GHL+ mice. Instead they may be involved in regulating secondary pathological responses such as inflammation that are upregulated during later stages of disease progression.

RHO Mutations (p.W126L and p.A346P) in Two Japanese Families with Autosomal Dominant Retinitis Pigmentosa.

Purpose. To investigate genetic and clinical features of patients with rhodopsin (RHO) mutations in two Japanese families with autosomal dominant retinitis pigmentosa (adRP). Methods. Whole-exome sequence analysis was performed in ten adRP families. Identified RHO mutations for the cosegregation analysis were confirmed by Sanger sequencing. Ophthalmic examinations were performed to evaluate the RP phenotypes. The impact of the RHO mutation on the rhodopsin conformation was examined by molecular modeling analysis. Results. In two adRP families, we identified two RHO mutations (c.377G>T (p.W126L) and c.1036G>C (p.A346P)), one of which was novel. Complete cosegregation was confirmed for each mutation exhibiting the RP phenotype in both families. Molecular modeling predicted that the novel mutation (p.W126L) might impair rhodopsin function by affecting its conformational transition in the light-adapted form. Clinical phenotypes showed that patients with p.W126L exhibited sector RP, whereas patients with p.A346P exhibited classic RP. Conclusions. Our findings demonstrated that the novel mutation (p.W126L) may be associated with the phenotype of sector RP. Identification of RHO mutations is a very useful tool for predicting disease severity and providing precise genetic counseling.

Low-Temperature Trapping of Photointermediates of the Rhodopsin E181Q Mutant.

Three active-site components in rhodopsin play a key role in the stability and function of the protein: 1) the counter-ion residues which stabilize the protonated Schiff base, 2) water molecules, and 3) the hydrogen-bonding network. The ionizable residue Glu-181, which is involved in an extended hydrogen-bonding network with Ser-186, Tyr-268, Tyr-192, and key water molecules within the active site of rhodopsin, has been shown to be involved in a complex counter-ion switch mechanism with Glu-113 during the photobleaching sequence of the protein. Herein, we examine the photobleaching sequence of the E181Q rhodopsin mutant by using cryogenic UV-visible spectroscopy to further elucidate the role of Glu-181 during photoactivation of the protein. We find that lower temperatures are required to trap the early photostationary states of the E181Q mutant compared to native rhodopsin. Additionally, a Blue Shifted Intermediate (BSI, λmax = 498 nm, 100 K) is observed after the formation of E181Q Bathorhodopsin (Batho, λmax = 556 nm, 10 K) but prior to formation of E181Q Lumirhodopsin (Lumi, λmax = 506 nm, 220 K). A potential energy diagram of the observed photointermediates suggests the E181Q Batho intermediate has an enthalpy value 7.99 KJ/mol higher than E181Q BSI, whereas in rhodopsin, the BSI is 10.02 KJ/mol higher in enthalpy than Batho. Thus, the Batho to BSI transition is enthalpically driven in E181Q and entropically driven in native rhodopsin. We conclude that the substitution of Glu-181 with Gln-181 results in a significant perturbation of the hydrogen-bonding network within the active site of rhodopsin. In addition, the removal of a key electrostatic interaction between the chromophore and the protein destabilizes the protein in both the dark state and Batho intermediate conformations while having a stabilizing effect on the BSI conformation. The observed destabilization upon this substitution further supports that Glu-181 is negatively charged in the early intermediates of the photobleaching sequence of rhodopsin.

Mutagenesis Study of Retinal Entry Pathway of Rhodopsin

The visual photoreceptor rhodopsin binds a photosensitive ligand 11-cis-retinal to respond to light stimulation. The regeneration of rhodopsin with 11-cis-retinal completes the photocycle of visual signaling. The molecular determinants of retinal entry can be assessed by measuring the retinal uptake kinetics of site-directed rhodopsin mutants. Here we report an assay for this purpose based on the energy transfer between 11-cis-retinal and a fluorescent probe covalently attached to the receptor. Previously, we developed an approach for site-specific labeling of G protein-coupled receptors by combining amber codon suppression technology and strain-promoted alkyne-azide cycloaddition reaction[1]. We have demonstrated robust fluorescent labeling of receptors without generating a cysteine-free background. We utilized this approach to label rhodopsin with Alexa488 as the FRET donor. Compared to tryptophan fluorescence spectroscopy, our assay exhibits a greater sensitivity and enables the characterization of mutant with slow retinal uptake kinetics. We showed that the heterologously expressed rhodopsin labeled at the second intracellular loop retains the wild-type retinal uptake kinetics. We further introduced a series of substitutions for the residues located in proximity of the retinal β-ionone binding pocket. We observed three orders of magnitude changes in retinal uptake kinetics. By varying the residue substitution we further studied the effect of hydrogen bonding and sterical hindrance. The insights into retinal entry not only shed light on a physiologically relevant process, but also facilitate understanding of retinal diseases.[1] H Tian, TP Sakmar, T Huber. Site-specific labeling of genetically encoded azido groups for multicolor, single-molecule fluorescence imaging of GPCRs. Methods in Cell Biology. In press.

S-opsin knockout mice with the endogenous M-opsin gene replaced by an L-opsin variant.

Specific variants of human long-wavelength (L) and middle-wavelength (M) cone opsin genes have recently been associated with a variety of vision disorders caused by cone malfunction, including red-green color vision deficiency, blue cone monochromacy, myopia, and cone dystrophy. Strikingly, unlike disease-causing mutations in rhodopsin, most of the cone opsin alleles that are associated with vision disorders do not have deleterious point mutations. Instead, specific combinations of normal polymorphisms that arose by genetic recombination between the genes encoding L and M opsins appear to cause disease. Knockout/knock-in mice promise to make it possible to study how these deleterious cone opsin variants affect the structure, function, and viability of the cone photoreceptors. Ideally, we would like to evaluate different variants that cause vision disorders in humans against a control pigment that is not associated with vision disorders, and each variant should be expressed as the sole photopigment in each mouse cone, as is the case in humans. To evaluate the feasibility of this approach, we created a line of mice to serve as the control in the analysis of disease-causing mutations by replacing exon 2 through 6 of the mouse M-opsin gene with the corresponding cDNA for a human L-opsin variant that is associated with normal vision. Experiments reported here establish that the resulting pigment, which differs from the endogenous mouse M opsin at 35 amino acid positions, functions normally in mouse cones. This pigment was evaluated in mice with and without coexpression of the mouse short wavelength (S) opsin. Here, the creation and validation of two lines of genetically engineered mice that can be used to study disease-causing variants of human L/M-opsins, in vivo, are described.

Hsp90 inhibition protects against inherited retinal degeneration

The molecular chaperone Hsp90 is important for the functional maturation of many client proteins, and inhibitors are in clinical trials for multiple indications in cancer. Hsp90 inhibition activates the heat shock response and can improve viability in a cell model of the P23H misfolding mutation in rhodopsin that causes autosomal dominant retinitis pigmentosa (adRP). Here, we show that a single low dose of the Hsp90 inhibitor HSP990 enhanced visual function and delayed photoreceptor degeneration in a P23H transgenic rat model. This was associated with the induction of heat shock protein expression and reduced rhodopsin aggregation. We then investigated the effect of Hsp90 inhibition on a different type of rod opsin mutant, R135L, which is hyperphosphorylated, binds arrestin and disrupts vesicular traffic. Hsp90 inhibition with 17-AAG reduced the intracellular accumulation of R135L and abolished arrestin binding in cells. Hsf-1−/− cells revealed that the effect of 17-AAG on P23H aggregation was dependent on HSF-1, whereas the effect on R135L was HSF-1 independent. Instead, the effect on R135L was mediated by a requirement of Hsp90 for rhodopsin kinase (GRK1) maturation and function. Importantly, Hsp90 inhibition restored R135L rod opsin localization to wild-type (WT) phenotype in vivo in rat retina. Prolonged Hsp90 inhibition with HSP990 in vivo led to a posttranslational reduction in GRK1 and phosphodiesterase (PDE6) protein levels, identifying them as Hsp90 clients. These data suggest that Hsp90 represents a potential therapeutic target for different types of rhodopsin adRP through distinct mechanisms, but also indicate that sustained Hsp90 inhibition might adversely affect visual function.

The Severe Autosomal Dominant Retinitis Pigmentosa Rhodopsin Mutant Ter349Glu Mislocalizes and Induces Rapid Rod Cell Death

Mutations in the rhodopsin gene cause approximately one-tenth of retinitis pigmentosa cases worldwide, and most result in endoplasmic reticulum retention and apoptosis. Other rhodopsin mutations cause receptor mislocalization, diminished/constitutive activity, or faulty protein-protein interactions. The purpose of this study was to test for mechanisms by which the autosomal dominant rhodopsin mutation Ter349Glu causes an early, rapid retinal degeneration in patients. The mutation adds an additional 51 amino acids to the C terminus of the protein. Folding and ligand interaction of Ter349Glu rhodopsin were tested by ultraviolet-visible (UV-visible) spectrophotometry. The ability of the mutant to initiate phototransduction was tested using a radioactive filter binding assay. Photoreceptor localization was assessed both in vitro and in vivo utilizing fluorescent immunochemistry on transfected cells, transgenic Xenopus laevis, and knock-in mice. Photoreceptor ultrastructure was observed by transmission electron microscopy. Spectrally, Ter349Glu rhodopsin behaves similarly to wild-type rhodopsin, absorbing maximally at 500 nm. The mutant protein also displays in vitro G protein activation similar to that of WT. In cultured cells, mislocalization was observed at high expression levels whereas ciliary localization occurred at low expression levels. Similarly, transgenic X. laevis expressing Ter349Glu rhodopsin exhibited partial mislocalization. Analysis of the Ter349Glu rhodopsin knock-in mouse showed a rapid, early onset degeneration in homozygotes with a loss of proper rod outer segment development and improper disc formation. Together, the data show that both mislocalization and rod outer segment morphogenesis are likely associated with the human phenotype.

Inhibitory Peptide of Mitochondrial μ-Calpain Protects against Photoreceptor Degeneration in Rhodopsin Transgenic S334ter and P23H Rats

Mitochondrial μ-calpain and apoptosis-inducing factor (AIF)-dependent photoreceptor cell death has been seen in several rat and mouse models of retinitis pigmentosa (RP). Previously, we demonstrated that the specific peptide inhibitor of mitochondrial μ-calpain, Tat-µCL, protected against retinal degeneration following intravitreal injection or topical eye-drop application in Mertk gene-mutated Royal College of Surgeons rats, one of the animal models of RP. Because of the high rate of rhodopsin mutations in RP patients, the present study was performed to confirm the protective effects of Tat-µCL against retinal degeneration in rhodopsin transgenic S334ter and P23H rats. We examined the effects of intravitreal injection or topical application of the peptide on retinal degeneration in S334ter and P23H rats by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay, electroretinogram (ERG), immunohistochemistry for AIF, and histological staining. In S334ter rats, we found that intravitreal injection or topical application of the peptide prevented photoreceptor cell death from postnatal (PN) 15 to 18 days, the time of early-stage retinal degeneration. Topical application of the peptide also delayed attenuation of ERG responses from PN 28 to 56 days. In P23H rats, topical application of the peptide protected against photoreceptor cell death and nuclear translocation of AIF on PN 30, 40, and 50 days, as the primary stages of degeneration. We observed that topical application of the peptide inhibited the thinning of the outer nuclear layer and delayed ERG attenuations from PN 30 to 90 days. Our results demonstrate that the mitochondrial μ-calpain and AIF pathway is involved in early-stage retinal degeneration in rhodopsin transgenic S334ter and P23H rats, and inhibition of this pathway shows curative potential for rhodopsin mutation-caused RP.

Correlation between SD‐OCT, immunocytochemistry and functional findings in an animal model of retinal degeneration

AbstractPurpose The P23H rhodopsin mutation is an extensively studied model of ADRP. We evaluated the anatomical changes using SD‐OCT and correlate the findings and retinal thickness values with immunocytochemistry. Functional changes were analyzedMethods Heterozygous P23H pigmented transgenic rats aged from P18 to P180 were studied. LE rats bred with Sprague Dawley (SD) 1 month old served as wild type controls. Visual acuity and contrast sensitivity evaluation was performed every month. Corneal ERGs were recorded under scotopic and photopic conditions. Retinal thicknesses at different levels (total thickness, ONL + RPE, ONL and IPL), fundus autofluorescence (FAF) and fluorescein angiography was performed in 3 animals at P150 using Spectralis OCT and HRA (Heidelberg Engineering, Germany). Retinas were immunostained for ICC.Results Retinal thicknesses diminution was seen in OCT sections, with a clear loss of ONL and morphological modifications. Statically differences were found between groups in all evaluated thicknesses. In the P23H rats, change in FAF was noted comparing to control group, as sparse autofluorescent dots. No relevant changes were observed in the angiography pattern. ICC showed a progressive decrease in ONL thickness. Functional changes were progressive with time.Conclusion Anatomical changes in pigmented P23H can be observed using SD‐OCT and immunocytochemistry, with a good correlation between their values. SD‐OCT and FAF are important tools for research in retinal degenerations.

Retinitis Pigmentosa: over‐expression of anti‐ageing protein Klotho in degenerating photoreceptors

Retinitis Pigmentosa involves a hereditary degeneration of photoreceptors by as yet unresolved mechanisms. The secretable protein α-Klotho has a function related to ageing processes, and α-Klotho-deficient mice have reduced lifespan and declining functions in several tissues. Here, we studied Klotho in connection with inherited photoreceptor degeneration. Increased nuclear immunostaining for α-Klotho protein was seen in degenerating photoreceptors in four different Retinitis Pigmentosa models (rd1, rd2 mice; P23H, S334ter rhodopsin mutant rats). Correspondingly, in rd1 retina α-Klotho mRNA expression was significantly up-regulated. Moreover, immunostaining for another Klotho family protein, β-Klotho, also co-localized with degenerating rd1 photoreceptors. The rd1 retina displayed reduced levels of fibroblast growth factor 15, a member of the fibroblast growth factor subfamily for which Klotho acts as a co-receptor. Exogenous α-Klotho protein added to retinal explant cultures did not affect cell death in rd1 retinae, but caused a severe layer disordering in wild-type retinae. Our study suggests Klotho as a novel player in the retina, with a clear connection to photoreceptor cell death as well as with an influence on retinal organization.

Constitutively active rhodopsin mutants causing night blindness are effectively phosphorylated by GRKs but differ in arrestin-1 binding

The effects of activating mutations associated with night blindness on the stoichiometry of rhodopsin interactions with G protein-coupled receptor kinase 1 (GRK1) and arrestin-1 have not been reported. Here we show that the monomeric form of WT rhodopsin and its constitutively active mutants M257Y, G90D, and T94I, reconstituted into HDL particles are effectively phosphorylated by GRK1, as well as two more ubiquitously expressed subtypes, GRK2 and GRK5. All versions of arrestin-1 tested (WT, pre-activated, and constitutively monomeric mutants) bind to monomeric rhodopsin and show the same selectivity for different functional forms of rhodopsin as in native disc membranes. Rhodopsin phosphorylation by GRK1 and GRK2 promotes arrestin-1 binding to a comparable extent, whereas similar phosphorylation by GRK5 is less effective, suggesting that not all phosphorylation sites on rhodopsin are equivalent in promoting arrestin-1 binding. The binding of WT arrestin-1 to phospho-opsin is comparable to the binding to its preferred target, P-Rh*, suggesting that in photoreceptors arrestin-1 only dissociates after opsin regeneration with 11-cis-retinal, which converts phospho-opsin into inactive phospho-rhodopsin that has lower affinity for arrestin-1. Reduced binding of arrestin-1 to the phospho-opsin form of G90D mutant likely contributes to night blindness caused by this mutation in humans.

Detection and analysis of the Rhodopsin gene in a consanguineous Chinese Han autosomal dominant retinitis pigmentosa family

Background Rhodopsin (RHO) gene is the most common disease gene for autosomal dominant retinitis pigmentosa (adRP),one of the main pathogenesis is that misfolded mutant RHO proteins accumulate in the endoplasmic reticulum and cause endoplasmic reticulum stress (ERS).Objective This study aimed to determine the genetic basis for a consanguineous Chinese Han adRP family.Methods This study procedure complied with Helsinki Declaration.All participants in the family were investigated under the informed consent.Regular ocular examination was performed on the patients in this family.Next-generation sequencing (NGS) was carried out to screen the mutations in 189 genes associated with hereditary retinal diseases (HRDs).After being analyzed and filtered,variations detected by NGS were validated by Sanger sequencing and evaluating of pathogenicity.The wild-type RHOWT and mutant RHOP53Rwere cloned into the vector pEGFP-N1.Then the two plasmids were transfected into adult retinal pigmentosa epithelium cell line(ARPE19) and human embryo kidney 293 line (HEK293) to observe the location of rhodopsin-GFP fusion protein in cells,and the expression of ERS related protein XBP1 in the cells was detected by quantitative-PCR and Western blot.Results This family included 5 generations with the typical adRP characteristics.Genetic analysis identified a heterozygous variation,p.P53R in RHO gene,which was fully cosegregated in the family.Wild-type RHOWT-GFP fusion proteins showed the green fluorescence on the endoplasmic reticulum and cytomembrane,but the misfolded mutant RHO-GFP fusion protein gathered only in endoplasmic reticulum.Compared to wild-type RHOWT,the XBP1 was activated and increased by (1.28 ±0.09) fold.The introns of 26 bases in XBP1 mRNA were removed in the HEK293 cells with mutant RHO-GFP fusion protein,and the expression of XBP1 was stronger in the HEK293 cells with mutant RHO-GFP than that in HEK293 cells with wild type RHO-GFP and cells with blank pEGFP-N1 plasmid.Conclusions Heterozygous variant RHO p.P53R is very likely the pathogenical mutation in the adRP family.The RHOP53R mutant rhodopsin protein can not be delivery effectively from the endoplasmic reticulum to the cell membrane,and these proteins accumulate in the endoplasmic reticulum,which causes ERS. Key words: Autosomal dominant retinitis pigmentosa; Rhodopsin; Next-generation sequencing; Endoplasmic reticulum stress

Thermal Stability of Rhodopsin and Progression of Retinitis Pigmentosa

Over 100 point mutations in the rhodopsin gene have been associated with retinitis pigmentosa (RP), a family of inherited visual disorders. Among these, we focused on characterizing the S186W mutation. We compared the thermal properties of the S186W mutant with another RP-causing mutant, D190N, and with WT rhodopsin. To assess thermal stability, we measured the rate of two thermal reactions contributing to the thermal decay of rhodopsin as follows: thermal isomerization of 11-cis-retinal and hydrolysis of the protonated Schiff base linkage between the 11-cis-retinal chromophore and opsin protein. We used UV-visible spectroscopy and HPLC to examine the kinetics of these reactions at 37 and 55 °C for WT and mutant rhodopsin purified from HEK293 cells. Compared with WT rhodopsin and the D190N mutant, the S186W mutation dramatically increases the rates of both thermal isomerization and dark state hydrolysis of the Schiff base by 1-2 orders of magnitude. The results suggest that the S186W mutant thermally destabilizes rhodopsin by disrupting a hydrogen bond network at the receptor's active site. The decrease in the thermal stability of dark state rhodopsin is likely to be associated with higher levels of dark noise that undermine the sensitivity of rhodopsin, potentially accounting for night blindness in the early stages of RP. Further studies of the thermal stability of additional pathogenic rhodopsin mutations in conjunction with clinical studies are expected to provide insight into the molecular mechanism of RP and test the correlation between rhodopsin's thermal stability and RP progression in patients.

Visual arrestin interaction with clathrin adaptor AP-2 regulates photoreceptor survival in the vertebrate retina

Arrestins bind ligand-activated, phosphorylated G protein-coupled receptors (GPCRs) and terminate the activation of G proteins. Additionally, nonvisual arrestin/GPCR complex can initiate G protein-independent intracellular signals through their ability to act as scaffolds that bring other signaling molecules to the internalized GPCR. Like nonvisual arrestins, vertebrate visual arrestin (ARR1) terminates G protein signaling from light-activated, phosphorylated GPCR, rhodopsin. Unlike nonvisual arrestins, its role as a transducer of signaling from internalized rhodopsin has not been reported in the vertebrate retina. Formation of signaling complexes with arrestins often requires recruitment of the endocytic adaptor protein, AP-2. We have previously shown that Lys296 → Glu (K296E), which is a naturally occurring rhodopsin mutation in certain humans diagnosed with autosomal dominant retinitis pigmentosa, causes toxicity through forming a stable complex with ARR1. Here we investigated whether recruitment of AP-2 by the K296E/ARR1 complex plays a role in generating the cell death signal in a transgenic mouse model of retinal degeneration. We measured the binding affinity of ARR1 for AP-2 and found that, although the affinity is much lower than that of the other arrestins, the unusually high concentration of ARR1 in rods would favor this interaction. We further demonstrate that p44, a splice variant of ARR1 that binds light-activated, phosphorylated rhodopsin but lacks the AP-2 binding motif, prevents retinal degeneration and rescues visual function in K296E mice. These results reveal a unique role of ARR1 in a G protein-independent signaling cascade in the vertebrate retina.

225. In Vitro Repair of Rhodopsin Mutation by Spliceosome-Mediated RNA Trans-Splicing

225. In Vitro Repair of Rhodopsin Mutation by Spliceosome-Mediated RNA Trans-Splicing