Visual Cone Arrestin 4 Contributes to Visual Function and Cone Health.
ABSTRACT: Visual arrestins (ARR) play a critical role in shutoff of rod and cone phototransduction. When electrophysiological responses are measured for a single mouse cone photoreceptor, ARR1 expression can substitute for ARR4 in cone pigment desensitization; however, each arrestin may also contribute its own, unique role to modulate other cellular functions.A combination of ERG, optokinetic tracking, immunohistochemistry, and immunoblot analysis was used to investigate the retinal phenotypes of Arr4 null mice (Arr4-/-) compared with age-matched control, wild-type mice.When 2-month-old Arr4-/- mice were compared with wild-type mice, they had diminished visual acuity and contrast sensitivity, yet enhanced ERG flicker response and higher photopic ERG b-wave amplitudes. In contrast, in older Arr4-/- mice, all ERG amplitudes were significantly reduced in magnitude compared with age-matched controls. Furthermore, in older Arr4-/- mice, the total cone numbers decreased and cone opsin protein immunoreactive expression levels were significantly reduced, while overall photoreceptor outer nuclear layer thickness was unchanged.Our study demonstrates that Arr4-/- mice display distinct phenotypic differences when compared to controls, suggesting that ARR4 modulates essential functions in high acuity vision and downstream cellular signaling pathways that are not fulfilled or substituted by the coexpression of ARR1, despite its high expression levels in all mouse cones. Without normal ARR4 expression levels, cones slowly degenerate with increasing age, making this a new model to study age-related cone dystrophy.
Project description:Previous studies discovered cone phototransduction shutoff occurs normally for Arr1-/- and Arr4-/-; however, it is defective when both visual arrestins are simultaneously not expressed (Arr1-/-Arr4-/-). We investigated the roles of visual arrestins in an all-cone retina (Nrl-/-) since each arrestin has differential effects on visual function, including ARR1 for normal light adaptation, and ARR4 for normal contrast sensitivity and visual acuity.We examined Nrl-/-, Nrl-/-Arr1-/-, Nrl-/-Arr4-/-, and Nrl-/-Arr1-/-Arr4-/- mice with photopic electroretinography (ERG) to assess light adaptation and retinal responses, immunoblot and immunohistochemical localization analysis to measure retinal expression levels of M- and S-opsin, and optokinetic tracking (OKT) to measure the visual acuity and contrast sensitivity.Study results indicated that Nrl-/- and Nrl-/-Arr4-/- mice light adapted normally, while Nrl-/-Arr1-/- and Nrl-/-Arr1-/-Arr4-/- mice did not. Photopic ERG a-wave, b-wave, and flicker amplitudes followed a general pattern in which Nrl-/-Arr4-/- amplitudes were higher than the amplitudes of Nrl-/-, while the amplitudes of Nrl-/-Arr1-/- and Nrl-/-Arr1-/-Arr4-/- were lower. All three visual arrestin knockouts had faster implicit times than Nrl-/- mice. M-opsin expression is lower when ARR1 is not expressed, while S-opsin expression is lower when ARR4 is not expressed. Although M-opsin expression is mislocalized throughout the photoreceptor cells, S-opsin is confined to the outer segments in all genotypes. Contrast sensitivity is decreased when ARR4 is not expressed, while visual acuity was normal except in Nrl-/-Arr1-/-Arr4-/-.Based on the opposite visual phenotypes in an all-cone retina in the Nrl-/-Arr1-/- and Nrl-/-Arr4-/- mice, we conclude that ARR1 and ARR4 perform unique modulatory roles in cone photoreceptors.
Project description:The cone cyclic nucleotide-gated (CNG) channel is essential for central and color vision and visual acuity. This channel is composed of two structurally related subunits, CNGA3 and CNGB3; CNGA3 is the ion-conducting subunit, whereas CNGB3 is a modulatory subunit. Mutations in both subunits are associated with achromatopsia and progressive cone dystrophy, with mutations in CNGB3 alone accounting for 50% of all known cases of achromatopsia. However, the molecular mechanisms underlying cone diseases that result from CNGB3 deficiency are unknown. This study investigated the role of CNGB3 in cones, using CNGB3(-/-) mice. Cone dysfunction was apparent at the earliest time point examined (post-natal day 30) in CNGB3(-/-) mice. When compared with wild-type (WT) controls: photopic electroretingraphic (ERG) responses were decreased by approximately 75%, whereas scotopic ERG responses were unchanged; visual acuity was decreased by approximately 20%, whereas contrast sensitivity was unchanged; cone density was reduced by approximately 40%; photoreceptor apoptosis was detected; and outer segment disorganization was observed in some cones. Notably, CNGA3 protein and mRNA levels were significantly decreased in CNGB3(-/-) mice; in contrast, mRNA levels of S-opsin, Gnat2 and Pde6c were unchanged, relative to WT mice. Hence, we show that loss of CNGB3 reduces biosynthesis of CNGA3 and impairs cone CNG channel function. We suggest that down-regulation of CNGA3 contributes to the pathogenic mechanism by which CNGB3 mutations lead to human cone disease.
Project description:Purpose:The present study tested the hypothesis that connexin-36 (Cx36) and gap junctions between photoreceptor cells contribute to the circadian rhythm of the photopic electroretinogram (ERG) b-wave amplitude. Methods:Cone-specific disruption of Cx36 was obtained in mice with a floxed Gjd2 gene and human red/green pigment promoter (HRGP)-driven Cre recombinase. Standard ERG, spectral-domain optical coherence tomography (SD-OCT) and histochemical methods were used. Results:HRGPcreGjd2fl/fl mice had a selective reduction in Cx36 protein in the outer plexiform layer; no reduction in Cx36 was observed in the inner plexiform layer. Cx36 disruption had no effect on the number of cones, the thickness of the photoreceptor layer, or the scotopic ERG responses. However, there was a reduction of the photopic ERG circadian rhythm, with b-wave amplitudes in the day and the night locked in the daytime, light-adapted state. In HRGPcreGjd2+/+and Gjd2fl/fl controls, the circadian rhythm of light-adapted ERG persisted, similar to that in wild type mice. Conclusions:Cx36 regulation contributes to the circadian rhythm of light-adapted ERG; in the absence of photoreceptor gap junctions, mice appear to be in a fully light-adapted state regardless of the time of day. The higher amplitudes and reduced circadian regulation of the b-wave of HRGPcreGjd2fl/fl mice may be due to increased synaptic strength at the cone to ON bipolar cell synapse due to electrotonic isolation of the terminals lacking gap junctions.
Project description:Arrestins are proteins that arrest the activity of G protein-coupled receptors (GPCRs). While it is well established that normal inactivation of photoexcited rhodopsin, the GPCR of rod phototransduction, requires arrestin (Arr1), it has been controversial whether the same requirement holds for cone opsin inactivation. Mouse cone photoreceptors express two distinct visual arrestins: Arr1 and Arr4. By means of recordings from cones of mice with one or both arrestins knocked out, this investigation establishes that a visual arrestin is required for normal cone inactivation. Arrestin-independent inactivation is 70-fold more rapid in cones than in rods, however. Dual arrestin expression in cones could be a holdover from ancient genome duplication events that led to multiple isoforms of arrestin, allowing evolutionary specialization of one form while the other maintains the basic function.
Project description:Irreversible photoreceptor cell death is a major cause of blindness in many retinal dystrophies. A better understanding of the molecular mechanisms underlying the progressive loss of photoreceptor cells remains therefore crucial. Abnormal expression of microRNAs (miRNAs) has been linked with the aetiology of a number of retinal dystrophies. However, their role during the degenerative process remains poorly understood. Loss of cone photoreceptors in the human macula has the greatest impact on sight as these cells provide high acuity vision. Using a Chrnb4-cre; Dicerflox/flox conditional knockout mouse (Dicer CKO) to delete Dicer1 from cone cells, we show that cone photoreceptor cells degenerate and die in the Dicer-deleted retina. Embryonic eye morphogenesis appeared normal in Dicer CKO mice. Cone photoreceptor abnormalities were apparent by 3 weeks of age, displaying either very short or absent outer segments. By 4 months 50% of cones were lost and cone function was impaired as assessed by electroretinography (ERG). RNAseq analysis of the Dicer CKO retina revealed altered expression of genes involved in the visual perception pathway. These data show that loss of Dicer1 leads to early-onset cone cell degeneration and suggest that Dicer1 is essential for cone photoreceptor survival and homeostasis.
Project description:Inactivating mutations in the retinoid isomerase (RPE65) or lecithin:retinol acyltransferase (LRAT) genes cause Leber congenital amaurosis (LCA), a severe visual impairment in humans. Both enzymes participate in the retinoid (visual) cycle, the enzymatic pathway that continuously generates 11-cis-retinal, the chromophore of visual pigments in rod and cone photoreceptor cells needed for vision. We investigated human RPE65-LCA patients and mice with visual cycle abnormalities to determine the impact of chronic chromophore deprivation on cones. Young patients with RPE65 mutations showed foveal cone loss along with shortened inner and outer segments of remaining cones; cone cell loss also was dramatic in young mice lacking Rpe65 or Lrat gene function. To selectively evaluate cone pathophysiology, we eliminated the rod contribution to electroretinographic (ERG) responses by generating double knockout mice lacking Lrat or Rpe65 together with an inactivated rod-specific G protein transducin gene (Gnat1-/-). Cone ERG responses were absent in Gnat1-/-Lrat-/- mice which also showed progressive degeneration of cones. Cone ERG responses in Gnat1-/-Rpe65-/- mice were markedly reduced and declined over weeks. Treatment of these mice with the artificial chromophore pro-drug, 9-cis-retinyl acetate, partially protected inferior retinal cones as evidenced by improved ERGs and retinal histochemistry. Gnat1-/- mice chronically treated with retinylamine, a selective inhibitor of RPE65, also showed a decline in the number of cones that was ameliorated by 9-cis-retinyl acetate. These results suggest that chronic lack of chromophore leads to progressive loss of cones in mice and humans. Therapy for LCA patients should be geared toward early adequate delivery of chromophore to cone photoreceptors.
Project description:Expression of T17M rhodopsin (T17M) in rods activates the Unfolded Protein Response (UPR) and leads to the development of autosomal dominant retinitis pigmentosa (adRP). The rod death occurs in adRP retinas prior to cone photoreceptor death, so the mechanism by which cone photoreceptors die remains unclear. Therefore, the goal of the study was to verify whether UPR in rods induces TNFa-mediated signaling to the cones and to determine whether the TNFa deficit could prevent adRP cone cell death. Primary rod photoreceptors and cone-derived 661W cells transfected with siRNA against TNFa were treated with tunicamycin to mimic activation of UPR in T17M retinas expressing normal and reduced TNFa levels. The 661W cells were then exposed to recombinant TNFa to evaluate cell viability. In vivo, the role of TNFa was assessed in T17M TNFa+/- mice by electroretinography, optical coherence tomography, histology, immunohistochemistry, and a cytokine enzyme-linked immunosorbent assay. Rods overexpressed and secreted TNFa in response to UPR activation. The recombinant TNFa treatment lowered the number of viable cones, inducing cell death through elevation of pro-inflammatory cytokines and caspase-3/7 activity. The TNFa deficiency significantly protected adRP retinas. The photopic ERG amplitudes and the number of surviving cones dramatically increased in T17M TNFa+/- mice. This neuroprotection was associated with a reduced level of pro-inflammatory cytokines. Our results indicate that rod photoreceptors, following UPR activation during adRP progression, secrete TNFa and signal a self-destructive program to the cones, resulting in their cell death. TNFa therefore holds promise as a therapeutic target for treatment of adRP.
Project description:Cone phototransduction and survival of cones in the human macula is essential for color vision and for visual acuity. Progressive cone degeneration in age-related macular degeneration, Stargardt disease, and recessive cone dystrophies is a major cause of blindness. Thyroid hormone (TH) signaling, which regulates cell proliferation, differentiation, and apoptosis, plays a central role in cone opsin expression and patterning in the retina. Here, we investigated whether TH signaling affects cone viability in inherited retinal degeneration mouse models. Retinol isomerase RPE65-deficient mice [a model of Leber congenital amaurosis (LCA) with rapid cone loss] and cone photoreceptor function loss type 1 mice (severe recessive achromatopsia) were used to determine whether suppressing TH signaling with antithyroid treatment reduces cone death. Further, cone cyclic nucleotide-gated channel B subunit-deficient mice (moderate achromatopsia) and guanylate cyclase 2e-deficient mice (LCA with slower cone loss) were used to determine whether triiodothyronine (T3) treatment (stimulating TH signaling) causes deterioration of cones. We found that cone density in retinol isomerase RPE65-deficient and cone photoreceptor function loss type 1 mice increased about sixfold following antithyroid treatment. Cone density in cone cyclic nucleotide-gated channel B subunit-deficient and guanylate cyclase 2e-deficient mice decreased about 40% following T3 treatment. The effect of TH signaling on cone viability appears to be independent of its regulation on cone opsin expression. This work demonstrates that suppressing TH signaling in retina dystrophy mouse models is protective of cones, providing insights into cone preservation and therapeutic interventions.
Project description:BACKGROUND: Transplantation as a therapeutic strategy for inherited retinal degeneration has been historically viewed to restore vision as a method by replacing the lost retinal cells and attempting to reconstruct the neural circuitry with stem cells, progenitor cells and mature neural retinal cells. METHODS AND FINDINGS: We present evidence for an alternative strategy aimed at preventing the secondary loss of cones, the most crucial photoreceptors for vision, by transplanting normal photoreceptors cells into the eye of the P23H rat, a model of dominant retinitis pigmentosa. We carried out transplantation of photoreceptors or total neural retina in 3-month-old P23H rats and evaluated the function and cell counts 6 months after surgery. In both groups, cone loss was significantly reduced (10%) in the transplanted eyes where the cone outer segments were found to be considerably longer. This morphological effect correlated with maintenance of the visual function of cones as scored by photopic ERG recording, but more precisely with an increase in the photopic b-wave amplitudes by 100% and 78% for photoreceptor transplantation and whole retinal transplantation respectively. CONCLUSIONS: We demonstrate here that the transplanted tissue prevents the loss of cone function, which is further translated into cone survival.
Project description:Retinitis pigmentosa (RP), a common group of human retinopathic diseases, is characterized by late-onset night blindness, loss of peripheral vision, and diminished or absent electroretinogram (ERG) responses. Mutations in the photoreceptor-specific gene RP1 account for 5-10% of cases of autosomal dominant RP. We generated a mouse model of the RP1 form of RP by targeted disruption of the mouse ortholog (Rp1) of human RP1. In Rp1(-/-) mice, the number of rod photoreceptors decreased progressively over a period of 1 year, whereas that of cone photoreceptors did not change for at least 10 months. Light and electron microscopic analysis revealed that outer segments of Rp1(-/-) rods and cones were morphologically abnormal and became progressively shorter in length. Before photoreceptor cell death, rhodopsin was mislocalized in inner segments and cell bodies of Rp1(-/-) rods. Rod ERG amplitudes of Rp1(-/-) mice were significantly smaller than those of Rp1(+/+) mice over a period of 12 months, whereas those of Rp1(+/-) mice were intermediate. The decreases in cone ERG amplitudes were slower and less severe than those in rods. These findings demonstrate that Rp1 is required for normal morphogenesis of photoreceptor outer segments and also may play a role in rhodopsin transport to the outer segments. The phenotype of Rp1 mutant mice resembles the human RP1 disease. Thus, these mice provide a useful model for studies of RP1 function, disease pathology, and therapeutic interventions.