A novel form of transducin-dependent retinal degeneration: accelerated retinal degeneration in the absence of rod transducin

RNA-Seq reveals differential expression profiles and functional annotation of genes involved in retinal degeneration in Pde6c mutant Danio rerio

BACKGROUND

Retinal degenerative diseases affect millions of people and represent the leading cause of vision loss around the world. Retinal degeneration has been attributed to a wide variety of causes, such as disruption of genes involved in phototransduction, biosynthesis, folding of the rhodopsin molecule, and the structural support of the retina. The molecular pathogenesis of the biological events in retinal degeneration is unclear; however, the molecular basis of the retinal pathological defect can be potentially determined by gene-expression profiling of the whole retina. In the present study, we analyzed the differential gene expression profile of the retina from a wild-type zebrafish and phosphodiesterase 6c (pde6c) mutant.

RESULTS

The datasets were downloaded from the Sequence Read Archive (SRA), and adaptors and unbiased bases were removed, and sequences were checked to ensure the quality. The reads were further aligned to the reference genome of zebrafish, and the gene expression was calculated. The differentially expressed genes (DEGs) were filtered based on the log fold change (logFC) (±4) and p-values (p < 0.001). We performed gene annotation (molecular function [MF], biological process [BP], cellular component [CC]), and determined the functional pathways Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway for the DEGs. Our result showed 216 upregulated and 3527 downregulated genes between normal and pde6c mutant zebrafish. These DEGs are involved in various KEGG pathways, such as the phototransduction (12 genes), mRNA surveillance (17 genes), phagosome (25 genes), glycolysis/gluconeogenesis (15 genes), adrenergic signaling in cardiomyocytes (29 genes), ribosome (20 genes), the citrate cycle (TCA cycle; 8 genes), insulin signaling (24 genes), oxidative phosphorylation (20 genes), and RNA transport (22 genes) pathways. Many more of all the pathway genes were down-regulated, while fewer were up-regulated in the retina of pde6c mutant zebrafish.

CONCLUSIONS

Our data strongly indicate that, among these genes, the above-mentioned pathways' genes as well as calcium-binding, neural damage, peptidase, immunological, and apoptosis proteins are mostly involved in the retinal and neural degeneration that cause abnormalities in photoreceptors or retinal pigment epithelium (RPE) cells.

Ultrahigh Resolution Mouse Optical Coherence Tomography to Aid Intraocular Injection in Retinal Gene Therapy Research

HR-SD-OCT is utilized to monitor the progression of photoreceptor degeneration in live mouse models, assess the delivery of therapeutic agents into the subretinal space, and to evaluate toxicity and efficacy in vivo. HR-SD-OCT uses near infrared light (800-880 nm) and has optics specifically designed for the unique optics of the mouse eye with sub-2-micron axial resolution. Transgenic mouse models of outer retinal (photoreceptor) degeneration and controls were imaged to assess the disease progression. Pulled glass microneedles were used to deliver sub retinal injections of adeno-associated virus (AAV) or nanoparticles (NP) via a trans-scleral and trans-choroidal approach. Careful positioning of the needle into the subretinal space was required prior to a calibrated pressure injection, which delivers fluid into the sub retinal space. Real time subretinal surgery was conducted on our retinal imaging system (RIS). HR-SD-OCT demonstrated progressive uniform retinal degeneration due to expression of a toxic mutant human mutant rhodopsin (P347S) (RHO^P347S) transgene in mice. HR-SD-OCT allows rigorous quantification of all the retinal layers. Outer nuclear layer (ONL) thickness and photoreceptor outer segment length (OSL) measurements correlate with photoreceptor vitality, degeneration, or rescue. The RIS delivery system allows real-time visualization of subretinal injections in neonatal (~P10-14) or adult mice, and HR-SD-OCT immediately determines success of delivery and maps areal extent. HR-SD-OCT is a powerful tool that can evaluate the success of subretinal surgery in mice, in addition to measuring vitality of photoreceptors in vivo. HR-SD-OCT can also be used to identify uniform animal cohorts to evaluate the extent of retinal degeneration, toxicity, and therapeutic rescue in preclinical gene therapy research studies.

MRI of Retinal Free Radical Production With Laminar Resolution In Vivo

Purpose

Recent studies have suggested the hypothesis that quench-assisted 1/T1 magnetic resonance imaging (MRI) measures free radical production with laminar resolution in vivo without the need of a contrast agent. Here, we test this hypothesis further by examining the spatial and detection sensitivity of quench-assisted 1/T1 MRI to strain, age, or retinal cell layer-specific genetic manipulations.

Methods

We studied: adult wild-type mice; mice at postnatal day 7 (P7); cre dependent retinal pigment epithelium (RPE)-specific MnSOD knockout mice; doxycycline-treated Sod2^flox/flox mice lacking the cre transgene; and α-transducin knockout (Gnat1^−/−) mice on a C57Bl/6 background. Transretinal 1/T1 profiles were mapped in vivo in the dark without or with antioxidant treatment, or followed by light exposure. We calibrated profiles spatially using optical coherence tomography.

Results

Dark-adapted RPE-specific MnSOD knockout mice had greater than normal 1/T1 in the RPE and outer nuclear layers that was corrected to wild-type levels by antioxidant treatment. Dark and light Gnat1^−/− mice also had greater than normal outer retinal 1/T1 values. In adult wild-type mice, dark values of 1/T1 in the ellipsoid region and in the outer segment were suppressed by 13 minutes of light. By 29 minutes of light, 1/T1 reduction extended to the outer nuclear layer. Gnat1^−/− mice demonstrated a faster light-evoked suppression of 1/T1 values in the outer retina. In P7 mice, transretinal 1/T1 profiles were the same in dark and light.

Conclusions

Quench-assisted MRI has the laminar resolution and detection sensitivity to evaluate normal and pathologic production of free radicals in vivo.

A Novel, Real-Time, In Vivo Mouse Retinal Imaging System

PURPOSE

To develop an efficient, low-cost instrument for robust real-time imaging of the mouse retina in vivo, and assess system capabilities by evaluating various animal models.

METHODS

Following multiple disappointing attempts to visualize the mouse retina during a subretinal injection using commercially available systems, we identified the key limitation to be inadequate illumination due to off axis illumination and poor optical train optimization. Therefore, we designed a paraxial illumination system for Greenough-type stereo dissecting microscope incorporating an optimized optical launch and an efficiently coupled fiber optic delivery system. Excitation and emission filters control spectral bandwidth. A color coupled-charged device (CCD) camera is coupled to the microscope for image capture. Although, field of view (FOV) is constrained by the small pupil aperture, the high optical power of the mouse eye, and the long working distance (needed for surgical manipulations), these limitations can be compensated by eye positioning in order to observe the entire retina.

RESULTS

The retinal imaging system delivers an adjustable narrow beam to the dilated pupil with minimal vignetting. The optic nerve, vasculature, and posterior pole are crisply visualized and the entire retina can be observed through eye positioning. Normal and degenerative retinal phenotypes can be followed over time. Subretinal or intraocular injection procedures are followed in real time. Real-time, intravenous fluorescein angiography for the live mouse has been achieved.

CONCLUSIONS

A novel device is established for real-time viewing and image capture of the small animal retina during subretinal injections for preclinical gene therapy studies.

Photoreceptors in whirler mice show defective transducin translocation and are susceptible to short-term light/dark changes-induced degeneration

Usher syndrome combines congenital hearing loss and retinitis pigmentosa (RP). Mutations in the whirlin gene (DFNB31/WHRN) cause a subtype of Usher syndrome (USH2D). Whirler mice have a defective whirlin gene. They have inner ear defects but usually do not develop retinal degeneration. Here we report that, in whirler mouse photoreceptors, the light-activated rod transducin translocation is delayed and its activation threshold is shifted to a higher level. Rhodopsin mis-localization is observed in rod inner segments. Continuous moderate light exposure can induce significant rod photoreceptor degeneration. Whirler mice reared under a 1500 lux light/dark cycle also develop severe photoreceptor degeneration. Previously, we have reported that shaker1 mice, a USH1B model, show moderate light-induced photoreceptor degeneration with delayed transducin translocation. Here, we further show that, in both whirler and shaker1 mice, short-term moderate light/dark changes can induce rod degeneration as severe as that induced by continuous light exposure. The results from shaker1 and whirler mice suggest that defective transducin translocation may be functionally related to light-induced degeneration, and these two symptoms may be caused by defects in Usher protein function in rods. Furthermore, these results indicate that both Usher syndrome mouse models possess a light-induced retinal phenotype and may share a closely related pathobiological mechanism.

Moderate light-induced degeneration of rod photoreceptors with delayed transducin translocation in shaker1 mice

PURPOSE. Usher syndrome is characterized by congenital deafness associated with retinitis pigmentosa (RP). Mutations in the myosin VIIa gene (MYO7A) cause a common and severe subtype of Usher syndrome (USH1B). Shaker1 mice have mutant MYO7A. They are deaf and have vestibular dysfunction but do not develop photoreceptor degeneration. The goal of this study was to investigate abnormalities of photoreceptors in shaker1 mice. METHODS. Immunocytochemistry and hydroethidine-based detection of intracellular superoxide production were used. Photoreceptor cell densities under various conditions of light/dark exposures were evaluated. RESULTS. In shaker1 mice, the rod transducin translocation is delayed because of a shift of its light activation threshold to a higher level. Even moderate light exposure can induce oxidative damage and significant rod degeneration in shaker1 mice. Shaker1 mice reared under a moderate light/dark cycle develop severe retinal degeneration in less than 6 months. CONCLUSIONS. These findings show that, contrary to earlier studies, shaker1 mice possess a robust retinal phenotype that may link to defective rod protein translocation. Importantly, USH1B animal models are likely vulnerable to light-induced photoreceptor damage, even under moderate light.

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

Animal models of retinal disease

Diseases of the retina are the leading causes of blindness in the industrialized world. The recognition that animals develop retinal diseases with similar traits to humans has led to not only a dramatic improvement in our understanding of the pathogenesis of retinal disease but also provided a means for testing possible treatment regimes and successful gene therapy trials. With the advent of genetic and molecular biological tools, the association between specific gene mutations and retinal signs has been made. Animals carrying natural mutations usually in one gene now provide well-established models for a host of inherited retinal diseases, including retinitis pigmentosa, Leber congenital amaurosis, inherited macular degeneration, and optic nerve diseases. In addition, the development of transgenic technologies has provided a means by which to study the effects of these and novel induced mutations on retinal structure and function. Despite these advances, there is a paucity of suitable animal models for complex diseases, including age-related macular degeneration (AMD) and diabetic retinopathy, largely because these diseases are not caused by single gene defects, but involve complex genetics and/or exacerbation through environmental factors, epigenetic, or other modes of genetic influence. In this review, we outline in detail the available animal models for inherited retinal diseases and how this information has furthered our understanding of retinal diseases. We also examine how transgenic technologies have helped to develop our understanding of the role of isolated genes or pathways in complex diseases like AMD, diabetes, and glaucoma.

High levels of retinal docosahexaenoic acid do not protect photoreceptor degeneration in VPP transgenic mice

PURPOSE

To determine whether docosahexaenoic acid can protect against hereditary retinal degenerations in transgenic mice expressing the V20G, P23H, and P27L (VPP) rhodopsin mutations.

METHODS

Female transgenic mice expressing the VPP rhodopsin mutation, known to cause a retinal degeneration, were bred to male transgenic mice expressing the fat-1 gene, which can convert n6 to n3 polyunsaturated fatty acids (PUFA). Several weeks before breeding, the female mice were fed a standard diet containing 10% safflower oil (SFO), which is high in n6 and low in n3 PUFA (n6/n3=273). Offspring were genotyped and four groups identified: Fat1(+)/VPP(+), Fat1(-)/VPP(+), Fat1(+)/VPP(-), and Fat1(-)/VPP(-). Dams were maintained on the SFO diet during the nursing period and offspring were kept on the SFO diet after weaning. At 4, 16, and 28 weeks of age, retinal function was evaluated by electroretinography (ERG), photoreceptor cell loss was quantified by measuring outer nuclear layer thickness, and rhodopsin levels were determined. Fatty acid profiles were analyzed in whole retina, plasma, and liver at 4 and 28 weeks of age.

RESULTS

Expression of fat-1 in the absence of dietary n3 PUFA led to the generation of two groups of mice with distinctly different levels of n3 and n6 PUFA in the three tissues that were analyzed. Already at four weeks of age, the retinas of fat-1 positive animals had higher levels of n3 PUFA than their wild-type counterparts (23%-29% versus 6.4%-6.5%). In addition, by four weeks of age, there was a significant loss of rod photoreceptor cells in the VPP mice. Progression of retinal degeneration occurred with increasing age in VPP positive mice, with photoreceptor cell death occurring in both inferior and superior regions. Amplitudes of the a- and b-waves of the ERG were significantly reduced with age, with VPP positive mice showing the greatest change. Rhodopsin content was lower in the VPP transgenic mice. There were no significant differences in outer nuclear layer thickness or ERG amplitudes between Fat1(+)/VPP(+) and Fat1(-)/VPP(+), or between Fat1(+)/VPP(-)and Fat1(-)/VPP(-) mice at any of the three ages.

CONCLUSIONS

High levels of retinal docosahexaenoic acid do not protect mice expressing the VPP rhodopsin mutation from retinal degeneration.

Q344ter mutation causes mislocalization of rhodopsin molecules that are catalytically active: a mouse model of Q344ter-induced retinal degeneration

Q344ter is a naturally occurring rhodopsin mutation in humans that causes autosomal dominant retinal degeneration through mechanisms that are not fully understood, but are thought to involve an early termination that removed the trafficking signal, QVAPA, leading to its mislocalization in the rod photoreceptor cell. To better understand the disease mechanism(s), transgenic mice that express Q344ter were generated and crossed with rhodopsin knockout mice. Dark-reared Q344ter(rho+/-) mice exhibited retinal degeneration, demonstrating that rhodopsin mislocalization caused photoreceptor cell death. This degeneration is exacerbated by light-exposure and is correlated with the activation of transducin as well as other G-protein signaling pathways. We observed numerous sub-micrometer sized vesicles in the inter-photoreceptor space of Q344ter(rho+/-) and Q344ter(rho-/-) retinas, similar to that seen in another rhodopsin mutant, P347S. Whereas light microscopy failed to reveal outer segment structures in Q344ter(rho-/-) rods, shortened and disorganized rod outer segment structures were visible using electron microscopy. Thus, some Q344ter molecules trafficked to the outer segment and formed disc structures, albeit inefficiently, in the absence of full length wildtype rhodopsin. These findings helped to establish the in vivo role of the QVAPA domain as well as the pathways leading to Q344ter-induced retinal degeneration.

Effect of g protein-coupled receptor kinase 1 (Grk1) overexpression on rod photoreceptor cell viability

PURPOSE

Photoreceptor rhodopsin kinase (Rk, G protein-dependent receptor kinase 1 [Grk1]) phosphorylates light-activated opsins and channels them into an inactive complex with visual arrestins. Grk1 deficiency leads to human retinopathy and heightened susceptibility to light-induced photoreceptor cell death in the mouse. The goal of this study was to determine whether excess Grk1 activity is protective against photoreceptor cell death.

METHODS

Grk1-overexpressing transgenic mice (Grk1(+)) were generated by using a bacterial artificial chromosome (BAC) construct containing mouse Grk1, along with its flanking sequences. Quantitative reverse transcription-PCR, immunoblot analysis, immunostaining, and activity assays were combined with electrophysiology and morphometric analysis, to evaluate Grk1 overexpression and its effect on physiologic and morphologic retinal integrity. Morphometry and nucleosome release assays measured differences in resistance to photoreceptor cell loss between control and transgenic mice exposed to intense light.

RESULTS

Compared with control animals, the Grk1(+) transgenic line had approximately a threefold increase in Grk1 transcript and immunoreactive protein. Phosphorylated opsin immunochemical staining and in vitro phosphorylation assays confirmed proportionately higher Grk1 enzyme activity. Grk1(+) mice retained normal rod function, normal retinal appearance, and lacked evidence of spontaneous apoptosis when reared in cyclic light. In intense light, Grk1(+) mice showed photoreceptor damage, and their susceptibility was more pronounced than that of control mice with prolonged exposure times.

CONCLUSIONS

Enhancing visual pigment deactivation does not appear to protect against apoptosis; however, excess flow of opsin into the deactivation pathway may actually increase susceptibility to stress-induced cell death similar to some forms of retinal degeneration.

Rhodopsin-mediated retinitis pigmentosa

Retinitis pigmentosa (RP) is a genetically and phenotypically heterogeneous group of diseases that cause blindness. Mutations within the rhodopsin gene account for approximately 25% of autosomal dominantly inherited RP cases. Therefore, understanding the mechanisms causing rhodopsin-mediated RP has a significant health impact. To date, results from multiple labs indicate that rhodopsin-mediated RP pathogenesis does not share a common mechanism of degeneration. There is strong evidence that multiple mechanisms are involved, including protein misfolding, mislocalization, release of toxic products, and aberrant signaling. Development of effective treatments requires investigation of the mechanism involved in the different rhodopsin mutations. This chapter focuses on the mechanisms by which rhodopsin mutations cause retinal degeneration, as well as potential therapeutic strategies to treat the disease.

A nonsense mutation in Gnat1, encoding the alpha subunit of rod transducin, in spontaneous mouse models of retinal dysfunction

ICR-derived retinal dysfunction (IRD) 1 and IRD2 mice are new spontaneous mouse models of rod-cone and rod dysfunctions, respectively. In this study, we investigated the cause of rod dysfunction in IRD1 and IRD2 mice. Gene expression of rod phototransduction proteins was analyzed by quantitative real-time RT-PCR. mRNA levels of Gnat1, which encodes the alpha subunit of rod transducin (Tralpha), were severely reduced. Tralpha protein was immunohistochemically undetectable in both IRD1 and IRD2 mice. Sequencing of Tralpha cDNA revealed a 48-base pair (bp) insertion between exons 4 and 5 in both mutant strains. The insertion changed codon 150 (TAC) to a stop codon (TAG) (Tyr150Ter). The truncated Tralpha protein was undetectable in the retinas of both mutants by western blot analysis using a primary antibody against the N-terminal region. A 57-bp deletion was identified in intron 4 of the Gnat1 gene, which encodes the Tralpha protein, and included the last two bases of the splice donor site of intron 4. Thus our results showed that IRD1 and IRD2 mice harbor a nonsense mutation in the Gnat1 gene, resulting in the absence or suppressed expression of the Tralpha protein, which is the likely cause of rod dysfunction in both mutants.