Transgenic mice carrying the H258N mutation in the gene encoding the beta-subunit of phosphodiesterase-6 (PDE6B) provide a model for human congenital stationary night blindness

The m6A reader YTHDC2 maintains visual function and retinal photoreceptor survival through modulating translation of PPEF2 and PDE6B

Inherited retinal dystrophies (IRDs) are major causes of visual impairment and irreversible blindness worldwide, while the precise molecular and genetic mechanisms are still elusive. N6-methyladenosine (m6A) modification is the most prevalent internal modification in eukaryotic mRNA. YTH domain containing 2 (YTHDC2), an m6A reader protein, has recently been identified as a key player in germline development and human cancer. However, its contribution to retinal function remains unknown. Here, we explore the role of YTHDC2 in the visual function of retinal rod photoreceptors by generating rod-specific Ythdc2 knockout mice. Results show that Ythdc2 deficiency in rods causes diminished scotopic ERG responses and progressive retinal degeneration. Multi-omics analysis further identifies Ppef2 and Pde6b as the potential targets of YTHDC2 in the retina. Specifically, via its YTH domain, YTHDC2 recognizes and binds m6A-modified Ppef2 mRNA at the coding sequence and Pde6b mRNA at the 5'-UTR, resulting in enhanced translation efficiency without affecting mRNA levels. Compromised translation efficiency of Ppef2 and Pde6b after YTHDC2 depletion ultimately leads to decreased protein levels in the retina, impaired retinal function, and progressive rod death. Collectively, our finding highlights the importance of YTHDC2 in visual function and photoreceptor survival, which provides an unreported elucidation of IRD pathogenesis via epitranscriptomics.

Probing the mechanism by which the retinal G protein transducin activates its biological effector PDE6

Phototransduction in retinal rods occurs when the G protein-coupled photoreceptor rhodopsin triggers the activation of phosphodiesterase 6 (PDE6) by GTP-bound alpha subunits of the G protein transducin (GαT). Recently, we presented a cryo-EM structure for a complex between two GTP-bound recombinant GαT subunits and native PDE6, that included a bivalent antibody bound to the C-terminal ends of GαT and the inhibitor vardenafil occupying the active sites on the PDEα and PDEβ subunits. We proposed GαT-activated PDE6 by inducing a striking reorientation of the PDEγ subunits away from the catalytic sites. However, questions remained including whether in the absence of the antibody GαT binds to PDE6 in a similar manner as observed when the antibody is present, does GαT activate PDE6 by enabling the substrate cGMP to access the catalytic sites, and how does the lipid membrane enhance PDE6 activation? Here, we demonstrate that 2:1 GαT-PDE6 complexes form with either recombinant or retinal GαT in the absence of the GαT antibody. We show that GαT binding is not necessary for cGMP nor competitive inhibitors to access the active sites; instead, occupancy of the substrate binding sites enables GαT to bind and reposition the PDE6γ subunits to promote catalytic activity. Moreover, we demonstrate by reconstituting GαT-stimulated PDE6 activity in lipid bilayer nanodiscs that the membrane-induced enhancement results from an increase in the apparent binding affinity of GαT for PDE6. These findings provide new insights into how the retinal G protein stimulates rapid catalytic turnover by PDE6 required for dim light vision.

A novel homozygous missense substitution p.Thr313Ile in the PDE6B gene underlies autosomal recessive retinitis pigmentosa in a consanguineous Pakistani family

BACKGROUND

Retinitis pigmentosa (RP) is one of the most frequent hereditary retinal diseases that often starts with night blindness and eventually leads to legal blindness. Our study aimed to identify the underlying genetic cause of autosomal recessive retinitis pigmentosa (arRP) in a consanguineous Pakistani family.

METHODS

Following a detailed ophthalmological examination of the patients by an ophthalmologist, whole-exome sequencing was performed on the proband's DNA to delineate the genetic cause of RP in the family. In-depth computational methods, in-silico analysis, and familial co-segregation study were performed for variant detection and validation.

RESULTS

We studied an inbred Pakistani family with two siblings affected by retinitis pigmentosa. The proband, a 32 years old female, was clinically diagnosed with RP at the age of 6 years. A classical night blindness symptom was reported in the proband since her early childhood. OCT report showed a major reduction in the outer nuclear layer and the ellipsoid zone width, leading to the progression of the disease. Exome sequencing revealed a novel homozygous missense mutation (c.938C > T;p.Thr313Ile) in exon 12 of the PDE6B gene. The mutation p.Thr313Ile co-segregated with RP phenotype in the family. The altered residue (p.Thr313) was super conserved evolutionarily across different vertebrate species, and all available in silico tools classified the mutation as highly pathogenic.

CONCLUSION

We present a novel homozygous pathogenic mutation in the PDE6B gene as the underlying cause of arRP in a consanguineous Pakistani family. Our findings highlight the importance of missense mutations in the PDE6B gene and expand the known mutational repertoire of PDE6B-related RP.

Mettl14-mediated m6A modification is essential for visual function and retinal photoreceptor survival

Background

As the most abundant epigenetic modification of eukaryotic mRNA, N6-methyladenosine (m6A) modification has been shown to play a role in mammalian nervous system development and function by regulating mRNA synthesis and degeneration. However, the role of m6A modification in retinal photoreceptors remains unknown.

Results

We generated the first retina-specific Mettl14-knockout mouse models using the Rho-Cre and HRGP-Cre lines and investigated the functions of Mettl14 in retinal rod and cone photoreceptors. Our data showed that loss of Mettl14 in rod cells causes a weakened scotopic photoresponse and rod degeneration. Further study revealed the ectopic accumulation of multiple outer segment (OS) proteins in the inner segment (IS). Deficiency of Mettl14 in cone cells led to the mislocalization of cone opsin proteins and the progressive death of cone cells. Moreover, Mettl14 depletion resulted in drastic decreases in METTL3/WTAP levels and reduced m6A methylation levels. Mechanistically, transcriptomic analyses in combination with MeRIP-seq illustrated that m6A depletion via inactivation of Mettl14 resulted in reduced expression levels of multiple phototransduction- and cilium-associated genes, which subsequently led to compromised ciliogenesis and impaired synthesis and transport of OS-residing proteins in rod cells.

Conclusions

Our data demonstrate that Mettl14 plays an important role in regulating phototransduction and ciliogenesis events and is essential for photoreceptor function and survival, highlighting the importance of m6A modification in visual function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01335-x.

New focus on regulation of the rod photoreceptor phosphodiesterase

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_tα-GTP). Recent cryo-electron microscopy structures of the PDE6αγβγ and PDE6αγβγ-(G_tα-GTP)₂ complexes have provided valuable knowledge shedding additional light on the allosteric activation of PDE6 by G_tα-GTP. Here we discuss recent developments in our understanding of the mechanism of PDE6 activation.

Clinical Phenotype of PDE6B-Associated Retinitis Pigmentosa

In this retrospective, longitudinal, observational cohort study, we investigated the phenotypic and genotypic features of retinitis pigmentosa associated with variants in the PDE6B gene. Patients underwent clinical examination and genetic testing at a single tertiary referral center, including best-corrected visual acuity (BCVA), kinetic visual field (VF), full-field electroretinography, full-field stimulus threshold, spectral domain optical coherence tomography, and fundus autofluorescence imaging. The genetic testing comprised candidate gene sequencing, inherited retinal disease gene panel sequencing, whole-genome sequencing, and testing for familial variants by Sanger sequencing. Twenty-four patients with mutations in PDE6B from 21 families were included in the study (mean age at the first visit: 32.1 ± 13.5 years). The majority of variants were putative splicing defects (8/23) and missense (7/23) mutations. Seventy-nine percent (38/48) of eyes had no visual acuity impairment at the first visit. Visual acuity impairment was mild in 4% (2/48), moderate in 13% (6/48), and severe in 4% (2/48). BCVA was symmetrical in the right and left eyes. The kinetic VF measurements were highly symmetrical in the right and left eyes, as was the horizontal ellipsoid zone (EZ) width. Regarding the genetic findings, 43% of the PDE6B variants found in our patients were novel. Thus, this study contributed substantially to the PDE6B mutation spectrum. The visual acuity impairment was mild in 83% of eyes, providing a window of opportunity for investigational new drugs. The EZ width was reduced in all patients and was highly symmetric between the eyes, making it a promising outcome measure. We expect these findings to have implications on the design of future PDE6B-related retinitis pigmentosa (RP) clinical trials.

Clinical characteristics and disease progression of retinitis pigmentosa associated with PDE6B mutations in Korean patients

Due to the genotype-phenotype heterogeneity in retinitis pigmentosa (RP), molecular diagnoses and prediction of disease progression is difficult. This study aimed to report ocular and genetic data from Korean patients with PDE6B-associated RP (PDE6B-RP), and establish genotype-phenotype correlations to predict the clinical course. We retrospectively reviewed targeted next-generation sequencing or whole exome sequencing data for 305 patients with RP, and identified PDE6B-RP in 15 patients (median age, 40.0 years). Amongst these patients, ten previously reported PDE6B variants (c.1280G > A, c.1488del, c.1547T > C, c.1604T > A, c.1669C > T, c.1712C > T, c.2395C > T, c.2492C > T, c.592G > A, and c.815G > A) and one novel variant (c.712del) were identified. Thirteen patients (86.7%) experienced night blindness as the first symptom at a median age of 10.0 years. Median age at diagnosis was 21.0 years and median visual acuity (VA) was 0.20 LogMAR at the time of genetic analysis. Nonlinear mixed models were developed and analysis revealed that VA exponentially decreased over time, while optical coherence tomography parameters linearly decreased, and this was related with visual field constriction. A high proportion of patients with the c.1669C > T variant (7/9, 77.8%) had cystoid macular edema; despite this, patients with this variant did not show a higher rate of functional or structural progression. This study will help clinicians predict functional and structural progression in patients with PDE6B-RP.

A Practical Approach to Retinal Dystrophies

Genomic approaches to developing new diagnostic and therapeutic strategies in retinal dystrophies are among the most advanced applications of genetics (Tsang SH, Gouras P (1996) Molecular physiology and pathology of the retina. In: Duane TD, Tasman W, Jaeger AE (eds) Duane's clinical opthalmology. Lippincott-Raven, Philadelphia). The notion that "nothing can be done" for patients with retinal dystrophies is no longer true. Electrophysiological testing and autofluorescence imaging help to diagnose and predict the patient's course of disease. Better phenotyping can contribute to better-directed, cost-efficient genotyping. Combining fundoscopy, autofluorescent imaging, and electrophysiological testing is essential in approaching patients with retinal dystrophies. Emerging are new gene-based treatments for these devastating conditions.

Insight into the molecular genetics of myopia

Myopia is the most common cause of visual impairment worldwide. Genetic and environmental factors contribute to the development of myopia. Studies on the molecular genetics of myopia are well established and have implicated the important role of genetic factors. With linkage analysis, association studies, sequencing analysis, and experimental myopia studies, many of the loci and genes associated with myopia have been identified. Thus far, there has been no systemic review of the loci and genes related to non-syndromic and syndromic myopia based on the different approaches. Such a systemic review of the molecular genetics of myopia will provide clues to identify additional plausible genes for myopia and help us to understand the molecular mechanisms underlying myopia. This paper reviews recent genetic studies on myopia, summarizes all possible reported genes and loci related to myopia, and suggests implications for future studies on the molecular genetics of myopia.

Mechanisms of mutant PDE6 proteins underlying retinal diseases

Mutations in PDE6 genes encoding the effector enzymes in rods and cones underlie severe retinal diseases including retinitis pigmentosa (RP), autosomal dominant congenital stationary night blindness (adCSNB), and achromatopsia (ACHM). Here we examined a spectrum of pathogenic missense mutations in PDE6 using the system based on co-expression of cone PDE6C with its specialized chaperone AIPL1 and the regulatory Pγ subunit as a potent co-chaperone. We uncovered two mechanisms of PDE6C mutations underlying ACHM: (a) folding defects leading to expression of catalytically inactive proteins and (b) markedly diminished ability of Pγ to co-chaperone mutant PDE6C proteins thereby dramatically reducing the levels of functional enzyme. The mechanism of the Rambusch adCSNB associated with the H258N substitution in PDE6B was probed through the analysis of the model mutant PDE6C-H262N. We identified two interrelated deficits of PDE6C-H262N: disruption of the inhibitory interaction of Pγ with mutant PDE6C that markedly reduced the ability of Pγ to augment the enzyme folding. Thus, we conclude that the Rambusch adCSNB is triggered by low levels of the constitutively active PDE6. Finally, we examined PDE6C-L858V, which models PDE6B-L854V, an RP-linked mutation that alters the protein isoprenyl modification. This analysis suggests that the type of prenyl modifications does not impact the folding of PDE6, but it modulates the enzyme affinity for its trafficking partner PDE6D. Hence, the pathogenicity of PDE6B-L854V likely arises from its trafficking deficiency. Taken together, our results demonstrate the effectiveness of the PDE6C expression system to evaluate pathogenicity and elucidate the mechanisms of PDE6 mutations in retinal diseases.

The mutation p.E113K in the Schiff base counterion of rhodopsin is associated with two distinct retinal phenotypes within the same family

The diagnoses of retinitis pigmentosa (RP) and stationary night blindness (CSNB) are two distinct clinical entities belonging to a group of clinically and genetically heterogeneous retinal diseases. The current study focused on the identification of causative mutations in the RP-affected index patient and in several members of the same family that reported a phenotype resembling CSNB. Ophthalmological examinations of the index patient confirmed a typical form of RP. In contrast, clinical characterizations and ERGs of another affected family member showed the Riggs-type CSNB lacking signs of RP. Applying whole exome sequencing we detected the non-synonymous substitution c.337G > A, p.E113 K in the rhodopsin (RHO) gene. The mutation co-segregated with the diseases. The identification of the pathogenic variant p.E113 K is the first description of a naturally-occurring mutation in the Schiff base counterion of RHO in human patients. The heterozygous mutation c.337G > A in exon 1 was confirmed in the index patient as well as in five CSNB-affected relatives. This pathogenic sequence change was excluded in a healthy family member and in 199 ethnically matched controls. Our findings suggest that a mutation in the biochemically well-characterized counterion p.E113 in RHO can be associated with RP or Riggs-type CSNB, even within the same family.

Biology and therapy of inherited retinal degenerative disease: insights from mouse models

Retinal neurodegeneration associated with the dysfunction or death of photoreceptors is a major cause of incurable vision loss. Tremendous progress has been made over the last two decades in discovering genes and genetic defects that lead to retinal diseases. The primary focus has now shifted to uncovering disease mechanisms and designing treatment strategies, especially inspired by the successful application of gene therapy in some forms of congenital blindness in humans. Both spontaneous and laboratory-generated mouse mutants have been valuable for providing fundamental insights into normal retinal development and for deciphering disease pathology. Here, we provide a review of mouse models of human retinal degeneration, with a primary focus on diseases affecting photoreceptor function. We also describe models associated with retinal pigment epithelium dysfunction or synaptic abnormalities. Furthermore, we highlight the crucial role of mouse models in elucidating retinal and photoreceptor biology in health and disease, and in the assessment of novel therapeutic modalities, including gene- and stem-cell-based therapies, for retinal degenerative diseases.

Rod metabolic demand drives progression in retinopathies

Various factors are thought to cause the development and progression of disease in macular degeneration, diabetic retinopathy, and retinitis pigmentosa. Some of the deleterious processes include oxidative stress, hypoxia, metabolic derangement, genetics, and vasculopathy. In this review, we present a unified theory for the pathophysiology of several retinopathies based on the unique and intense metabolism of rod photoreceptors.

Variable phenotypic expressivity in inbred retinal degeneration mouse lines: A comparative study of C3H/HeOu and FVB/N rd1 mice

PURPOSE

Recent advances in optogenetics and gene therapy have led to promising new treatment strategies for blindness caused by retinal photoreceptor loss. Preclinical studies often rely on the retinal degeneration 1 (rd1 or Pde6b(rd1)) retinitis pigmentosa (RP) mouse model. The rd1 founder mutation is present in more than 100 actively used mouse lines. Since secondary genetic traits are well-known to modify the phenotypic progression of photoreceptor degeneration in animal models and human patients with RP, negligence of the genetic background in the rd1 mouse model is unwarranted. Moreover, the success of various potential therapies, including optogenetic gene therapy and prosthetic implants, depends on the progress of retinal degeneration, which might differ between rd1 mice. To examine the prospect of phenotypic expressivity in the rd1 mouse model, we compared the progress of retinal degeneration in two common rd1 lines, C3H/HeOu and FVB/N.

METHODS

We followed retinal degeneration over 24 weeks in FVB/N, C3H/HeOu, and congenic Pde6b(+) seeing mouse lines, using a range of experimental techniques including extracellular recordings from retinal ganglion cells, PCR quantification of cone opsin and Pde6b transcripts, in vivo flash electroretinogram (ERG), and behavioral optokinetic reflex (OKR) recordings.

RESULTS

We demonstrated a substantial difference in the speed of retinal degeneration and accompanying loss of visual function between the two rd1 lines. Photoreceptor degeneration and loss of vision were faster with an earlier onset in the FVB/N mice compared to C3H/HeOu mice, whereas the performance of the Pde6b(+) mice did not differ significantly in any of the tests. By postnatal week 4, the FVB/N mice expressed significantly less cone opsin and Pde6b mRNA and had neither ERG nor OKR responses. At 12 weeks of age, the retinal ganglion cells of the FVB/N mice had lost all light responses. In contrast, 4-week-old C3H/HeOu mice still had ERG and OKR responses, and we still recorded light responses from C3H/HeOu retinal ganglion cells until the age of 24 weeks. These results show that genetic background plays an important role in the rd1 mouse pathology.

CONCLUSIONS

Analogous to human RP, the mouse genetic background strongly influences the rd1 phenotype. Thus, different rd1 mouse lines may follow different timelines of retinal degeneration, making exact knowledge of genetic background imperative in all studies that use rd1 models.

Reduced rod electroretinograms in carrier parents of two Japanese siblings with autosomal recessive retinitis pigmentosa associated with PDE6B gene mutations

PURPOSE

To present the clinical and genetic findings in two siblings with autosomal recessive retinitis pigmentosa (RP) and their non-symptomatic parents.

METHODS

We studied two siblings, a 48-year-old woman and her 44-year-old brother, and their parents. They had general ophthalmic examinations including ophthalmoscopy, perimetry, and electroretinography (ERG). Their whole exomes were analyzed by the next-generation sequence technique.

RESULTS

The two siblings had night blindness for a long time, and clinical examinations revealed diffuse retinal degeneration with bone spicule pigmentation, constriction of the visual field, and non-recordable ERGs. Their parents were non-symptomatic and had normal fundi; however, their rod ERGs were reduced. Genetic examination revealed compound heterozygous mutations of I535N and H557Y in the PDE6B gene in the siblings, and the parents were heterozygous carriers of the mutations.

CONCLUSIONS

Heterozygous mutation in the PDE6B gene can cause a reduction in the rod function to different degrees. The retinal function of non-symptomatic carriers of autosomal recessive RP should be evaluated with care.

Silencing of tuberin enhances photoreceptor survival and function in a preclinical model of retinitis pigmentosa (an american ophthalmological society thesis)

PURPOSE

To assess the functional consequences of silencing of tuberin, an inhibitor of the mTOR signaling pathway, in a preclinical model of retinitis pigmentosa (RP) in order to test the hypothesis that insufficient induction of the protein kinase B (PKB)-regulated tuberin/mTOR self-survival pathway initiates apoptosis.

METHODS

In an unbiased genome-scale approach, kinase peptide substrate arrays were used to analyze self-survival pathways at the onset of photoreceptor degeneration. The mutant Pde6b(H620Q)/Pde6b(H620Q) at P14 and P18 photoreceptor outer segment (OS) lysates were labeled with P-ATP and hybridized to an array of 1,164 different synthetic peptide substrates. At this stage, OS of Pde6b(H620Q)/Pde6b(H620Q) rods are morphologically normal. In vitro kinase assays and immunohistochemistry were used to validate phosphorylation. Short hairpin RNA (shRNA) gene silencing was used to validate tuberin's role in regulating survival.

RESULTS

At the onset of degeneration, 162 peptides were differentially phosphorylated. Protein kinases A, G, C (AGC kinases), and B exhibited increased activity in both peptide array and in vitro kinase assays. Immunohistochemical data confirmed altered phosphorylation patterns for phosphoinositide-dependent kinase-1 (PDK1), ribosomal protein S6 (RPS6), and tuberin. Tuberin gene silencing rescued photoreceptors from degeneration.

CONCLUSIONS

Phosphorylation of tuberin and RPS6 is due to the upregulated activity of PKB. PKB/tuberin cell growth/survival signaling is activated before the onset of degeneration. Substrates of the AGC kinases in the PKB/tuberin pathway are phosphorylated to promote cell survival. Knockdown of tuberin, the inhibitor of the mTOR pathway, increased photoreceptor survival and function in a preclinical model of RP.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Next-generation sequencing revealed a novel mutation in the gene encoding the beta subunit of rod phosphodiesterase

PURPOSE

To report the phenotypes caused by a novel mutation in the PDE6B gene in a family with two affected siblings and one affected cousin with a 2-year follow-up.

DESIGN

Three patients from a family with a history of retinitis pigmentosa underwent clinical evaluations. The affected patients' DNA was analyzed using next-generation sequencing and segregation analyses were performed for the family.

SETTING

Edward S. Harkness Eye Institute, New York Presbyterian Hospital.

PARTICIPANTS

Two siblings, one cousin, and five unaffected family members.

MAIN OUTCOME MEASURES

Macular appearance assessed by funduscopy, autofluorescence imaging, spectral-domain optical coherence tomography and visual function assessed by electroretinography.

RESULTS

The proband, brother, and cousin had rod-cone degeneration with cystoid macular edema. Fundus autofluorescence showed hyperautofluorescent ring constriction over time. Spectral-domain optical coherence tomography revealed retinal pigment epithelium atrophy, loss of external limiting membrane, retinal layer thinning, and reduction in ellipsoid zone length over time. Next-generation whole exome sequencing revealed a homozygous c.1923_1969ins6del47 nonsense PDE6B mutation, which has not been previously described, that segregated with the disease in the family.

CONCLUSIONS

The homozygous PDE6B mutation causes retinitis pigmentosa. Acetazolamide treatment improved visual acuity but rod degeneration continued. Despite having the same mutation and living in the same environment, the proband's brother progressed at a faster rate starting at a younger age, suggesting that gene modifiers may influence the expressivity of the phenotype. Next-generation sequencing, used to discover this mutation, is a practical new technology that can detect novel disease-causing alleles, where previous arrayed primer extension (APEX) technology could not.

Strains and stressors: an analysis of touchscreen learning in genetically diverse mouse strains

Touchscreen-based systems are growing in popularity as a tractable, translational approach for studying learning and cognition in rodents. However, while mouse strains are well known to differ in learning across various settings, performance variation between strains in touchscreen learning has not been well described. The selection of appropriate genetic strains and backgrounds is critical to the design of touchscreen-based studies and provides a basis for elucidating genetic factors moderating behavior. Here we provide a quantitative foundation for visual discrimination and reversal learning using touchscreen assays across a total of 35 genotypes. We found significant differences in operant performance and learning, including faster reversal learning in DBA/2J compared to C57BL/6J mice. We then assessed DBA/2J and C57BL/6J for differential sensitivity to an environmental insult by testing for alterations in reversal learning following exposure to repeated swim stress. Stress facilitated reversal learning (selectively during the late stage of reversal) in C57BL/6J, but did not affect learning in DBA/2J. To dissect genetic factors underlying these differences, we phenotyped a family of 27 BXD strains generated by crossing C57BL/6J and DBA/2J. There was marked variation in discrimination, reversal and extinction learning across the BXD strains, suggesting this task may be useful for identifying underlying genetic differences. Moreover, different measures of touchscreen learning were only modestly correlated in the BXD strains, indicating that these processes are comparatively independent at both genetic and phenotypic levels. Finally, we examined the behavioral structure of learning via principal component analysis of the current data, plus an archival dataset, totaling 765 mice. This revealed 5 independent factors suggestive of "reversal learning," "motivation-related late reversal learning," "discrimination learning," "speed to respond," and "motivation during discrimination." Together, these findings provide a valuable reference to inform the choice of strains and genetic backgrounds in future studies using touchscreen-based tasks.

EnRICH: Extraction and Ranking using Integration and Criteria Heuristics

Background

High throughput screening technologies enable biologists to generate candidate genes at a rate that, due to time and cost constraints, cannot be studied by experimental approaches in the laboratory. Thus, it has become increasingly important to prioritize candidate genes for experiments. To accomplish this, researchers need to apply selection requirements based on their knowledge, which necessitates qualitative integration of heterogeneous data sources and filtration using multiple criteria. A similar approach can also be applied to putative candidate gene relationships. While automation can assist in this routine and imperative procedure, flexibility of data sources and criteria must not be sacrificed. A tool that can optimize the trade-off between automation and flexibility to simultaneously filter and qualitatively integrate data is needed to prioritize candidate genes and generate composite networks from heterogeneous data sources.

Results

We developed the java application, EnRICH (Extraction and Ranking using Integration and Criteria Heuristics), in order to alleviate this need. Here we present a case study in which we used EnRICH to integrate and filter multiple candidate gene lists in order to identify potential retinal disease genes. As a result of this procedure, a candidate pool of several hundred genes was narrowed down to five candidate genes, of which four are confirmed retinal disease genes and one is associated with a retinal disease state.

Conclusions

We developed a platform-independent tool that is able to qualitatively integrate multiple heterogeneous datasets and use different selection criteria to filter each of them, provided the datasets are tables that have distinct identifiers (required) and attributes (optional). With the flexibility to specify data sources and filtering criteria, EnRICH automatically prioritizes candidate genes or gene relationships for biologists based on their specific requirements. Here, we also demonstrate that this tool can be effectively and easily used to apply highly specific user-defined criteria and can efficiently identify high quality candidate genes from relatively sparse datasets.

shRNA knockdown of guanylate cyclase 2e or cyclic nucleotide gated channel alpha 1 increases photoreceptor survival in a cGMP phosphodiesterase mouse model of retinitis pigmentosa

In vertebrate rods, dark and light conditions produce changes in guanosine 3′,5′-cyclic monophosphate (cGMP) and calcium (Ca²⁺) levels, which are regulated by the opposing function of several proteins. During the recovery of a bright flash, guanylate cyclase (GUCY) helps raise cGMP to levels that open cGMP-gated calcium sodium channels (CNG) to increase Na⁺ and Ca²⁺ influx in the outer segment. In contrast, light activates cGMP phosphodiesterase 6 (PDE6) causing rapid hydrolysis of cGMP, CNG closure, and reduced Na⁺ and Ca²⁺ levels. In Pde6b mouse models of retinitis pigmentosa (RP), photoreceptor death is preceded by abnormally high cGMP and Ca²⁺ levels, likely because of continued synthesis of cGMP by guanylate cyclases and unregulated influx of Ca²⁺ to toxic levels through CNG channels. To reverse the effects of Pde6b loss of function, we employed an shRNA knockdown approach to reduce the expression of Gucy2e or Cnga1 in Pde6b^H620Q photoreceptors prior to degeneration. Gucy2e- or Cnga1-shRNA lentiviral-mediated knockdown GUCY2E and CNGA1 expression increase visual function and photoreceptor survival in Pde6b^H620Q mice. We demonstrated that effective knockdown of GUCY2E and CNGA1 expression to counteract loss of PDE6 function may develop into a valuable approach for treating some patients with RP.

Mouse eye enucleation for remote high-throughput phenotyping

The mouse eye is an important genetic model for the translational study of human ophthalmic disease. Blinding diseases in humans, such as macular degeneration, photoreceptor degeneration, cataract, glaucoma, retinoblastoma, and diabetic retinopathy have been recapitulated in transgenic mice.(1-5) Most transgenic and knockout mice have been generated by laboratories to study non-ophthalmic diseases, but genetic conservation between organ systems suggests that many of the same genes may also play a role in ocular development and disease. Hence, these mice represent an important resource for discovering new genotype-phenotype correlations in the eye. Because these mice are scattered across the globe, it is difficult to acquire, maintain, and phenotype them in an efficient, cost-effective manner. Thus, most high-throughput ophthalmic phenotyping screens are restricted to a few locations that require on-site, ophthalmic expertise to examine eyes in live mice. (6-9) An alternative approach developed by our laboratory is a method for remote tissue-acquisition that can be used in large or small-scale surveys of transgenic mouse eyes. Standardized procedures for video-based surgical skill transfer, tissue fixation, and shipping allow any lab to collect whole eyes from mutant animals and send them for molecular and morphological phenotyping. In this video article, we present techniques to enucleate and transfer both unfixed and perfusion fixed mouse eyes for remote phenotyping analyses.

Lentivirus-mediated expression of cDNA and shRNA slows degeneration in retinitis pigmentosa

Mutations in Pde6b lead to high levels of signaling molecules cyclic guanosine monophosphate (cGMP) and Ca(2+), which ultimately result in photoreceptor cell death in certain forms of retinitis pigmentosa (RP). The level of cGMP, which is controlled by opposing activities of guanylate cyclase (GUCY) and photoreceptor phosphodiesterase-6 (PDE6), regulates the opening of cyclic nucleotide-gated ion channels [CNG] and thereby controls Ca(2+) influx into the outer segments. Using a lentiviral gene therapy approach, we have previously shown that degeneration can be temporarily slowed either by introducing wild-type PDE6β or knocking down expression of GUCY2E and CNGA1 in photoreceptors of Pde6b(H620Q), a mouse model for RP. Rescue was transient with either approach. Therefore, we tested a novel combination therapy using bipartite lentiviral vectors designed to both introduce wild-type PDE6β expression and knockdown GUCY2E or CNGA1. Immunoblot analysis shows simultaneous increases in PDE6β and decreases in GUCY2E or CNGA1 in retinas transduced by the vectors, indicating successful transduction. In Pde6b(H620Q) mutants, we observe rescue of photoreceptor function and an increase in photoreceptor rows as compared with untreated controls. However, no evidence of prolonged rescue beyond the limit of the previously tested single therapy was observed.

Effect of the ILE86TER mutation in the γ subunit of cGMP phosphodiesterase (PDE6) on rod photoreceptor signaling

The light-dependent decrease in cyclic guanosine monophosphate (cGMP) in the rod outer segment is produced by a phosphodiesterase (PDE6), consisting of catalytic α and β subunits and two inhibitory γ subunits. The molecular mechanism of PDE6γ regulation of the catalytic subunits is uncertain. To study this mechanism in vivo, we introduced a modified Pde6g gene for PDE6γ into a line of Pde6g(tm1)/Pde6g(tm1) mice that do not express PDE6γ. The resulting ILE86TER mice have a PDE6γ that lacks the two final carboxyl-terminal Ile(86) and Ile(87) residues, a mutation previously shown in vitro to reduce inhibition by PDE6γ. ILE86TER rods showed a decreased sensitivity and rate of activation, probably the result of a decreased level of expression of PDE6 in ILE86TER rods. More importantly, they showed a decreased rate of decay of the photoresponse, consistent with decreased inhibition of PDE6 α and β by PDE6γ. Furthermore, ILE86TER rods had a higher rate of spontaneous activation of PDE6 than WT rods. Circulating current in ILE86TER rods that also lacked both guanylyl cyclase activating proteins (GCAPs) could be increased several fold by perfusion with 100μM of the PDE6 inhibitor 3-isobutyl-1-methylxanthine (IBMX), consistent with a higher rate of dark PDE6 activity in the mutant photoreceptors. In contrast, IBMX had little effect on the circulating current of WT rods, unlike previous results from amphibians. Our results show for the first time that the Ile(86) and Ile(87) residues are necessary for normal inhibition of PDE6 catalytic activity in vivo, and that increased basal activity of PDE can be partially compensated by GCAP-dependent regulation of guanylyl cyclase.

Transplantation of reprogrammed embryonic stem cells improves visual function in a mouse model for retinitis pigmentosa

BACKGROUND

To study whether C57BL/6J-Tyr/J (C2J) mouse embryonic stem (ES) cells can differentiate into retinal pigment epithelial (RPE) cells in vitro and then restore retinal function in a model for retinitis pigmentosa: Rpe65/Rpe65 C57BL6 mice.

METHODS

Yellow fluorescent protein (YFP)-labeled C2J ES cells were induced to differentiate into RPE-like structures on PA6 feeders. RPE-specific markers are expressed from differentiated cells in vitro. After differentiation, ES cell-derived RPE-like cells were transplanted into the subretinal space of postnatal day 5 Rpe65/Rpe65 mice. Live imaging of YFP-labeled C2J ES cells demonstrated survival of the graft. Electroretinograms (ERGs) were performed on transplanted mice to evaluate the functional outcome of transplantation.

RESULTS

RPE-like cells derived from ES cells sequentially express multiple RPE-specific markers. After transplantation, YFP-labeled cells can be tracked with live imaging for as long as 7 months. Although more than half of the mice were complicated with retinal detachments or tumor development, one fourth of the mice showed increased electroretinogram responses in the transplanted eyes. Rpe65/Rpe65 mice transplanted with RPE-like cells showed significant visual recovery during a 7-month period, whereas those injected with saline, PA6 feeders, or undifferentiated ES cells showed no rescue.

CONCLUSIONS

ES cells can differentiate, morphologically, and functionally, into RPE-like cells. Based on these findings, differentiated ES cells have the potential for the development of new therapeutic approaches for RPE-specific diseases such as certain forms of retinitis pigmentosa and macular degeneration. Nevertheless, stringent control of retinal detachment and teratoma development will be necessary before initiation of treatment trials.

Light-dependent phosphorylation of the gamma subunit of cGMP-phophodiesterase (PDE6gamma) at residue threonine 22 in intact photoreceptor neurons

The gamma subunit of rod-specific cGMP phosphodiesterase 6 (PDE6gamma), an effector of the G-protein GNAT1, is a key regulator of phototransduction. The results of several in vitro biochemical reconstitution experiments conducted to examine the effects of phosphorylation of PDE6gamma on its ability to regulate the PDE6 catalytic core have been inconsistent, showing that phosphorylation of PDE6gamma may increase or decrease the ability of PDE6gamma to deactivate phototransduction. To resolve role of phosphorylation of PDE6gamma in living photoreceptors, we generated transgenic mice in which either one or both Threonine (T) sites in PDE6gamma (T22 and T35), which are candidates for putative regulatory phosphorylation, were substituted with alanine (A). Phosphorylation of these sites was examined as a function of light exposure. We found that phosphorylation of T22 increases with light exposure in intact mouse rods while constitutive phosphorylation of T35 is unaffected by light in intact mouse rods and cones. Phosphorylation of the cone isoform of PDE6gamma, PDE6H, is constitutively phosphorylated at the T20 residue. Light-induced T22 phosphorylation was lost in T35A transgenic rods, and T35 phosphorylation was extinguished in T22A transgenic rods. The interdependency of phosphorylation of T22 and T35 suggests that light-induced, post-translational modification of PDE6gamma is essential for the regulation of G-protein signaling.

The retinal cGMP phosphodiesterase gamma-subunit - a chameleon

Intrinsically disordered proteins (IDPs) represent an emerging class of proteins (or domains) that are characterized by a lack of ordered secondary and tertiary structure. This group of proteins has recently attracted tremendous interest primarily because of a unique feature: they can bind to different targets due to their structural plasticity, and thus fulfill diverse functions. The inhibitory gamma-subunit (PDEgamma) of retinal PDE6 is an intriguing IDP, of which unique protein properties are being uncovered. PDEgamma critically regulates the turn on as well as the turn off of visual signaling through alternate interactions with the PDE6 catalytic core, transducin, and the regulator of G protein signaling RGS9-1. The intrinsic disorder of PDEgamma does not compromise, but rather, optimizes its functionality. PDEgamma "curls up" when free in solution but "stretches out" when binding with the PDE6 catalytic core. Conformational changes of PDEgamma also likely occur in its C-terminal PDE6-binding region upon interacting with transducin during PDE6 activation. Growing evidence shows that PDEgamma is also a player in non-phototransduction pathways, suggesting additional protein targets. Thus, PDEgamma is highly likely to be adaptive in its structure and function, hence a "chameleon".

Cellular and molecular origin of circumpapillary dysgenesis of the pigment epithelium

PURPOSE

We studied clinical phenotyping and TEAD1 expression in mice and humans to gain a better understanding of the primary origin in the pathogenesis of circumpapillary dysgenesis of the pigment epithelium.

DESIGN

Observational case series and experimental study.

PARTICIPANTS

Three female patients from an affected family were included for phenotypic study. Mice and human tissues were used for biochemistry and immunohistochemistry studies.

METHODS

We performed genetic analyses and longitudinal clinical, imaging, and electrophysiologic studies in a 3-generation family. Western blotting and immunohistochemistry were used to detect TEAD1 expression in mice and human retinal tissues.

MAIN OUTCOME MEASURES

Autofluorescence and optical coherence tomography (OCT) imaging were compared and reviewed from 3 patients. TEAD1 expression was compared in different tissues from mice and human samples.

RESULTS

A point mutation at T1261 in TEAD1 was detected in the mother. Autofluorescence and OCT imaging studies revealed choroid is involved earlier than retinal pigment epithelium (RPE). From immunoblot analysis, we discovered that TEAD1 and its cofactors YAP65 and FOXA2 are expressed in the choroid. Immunohistochemical analysis on frozen sections of mouse retina supports immunoblot results.

CONCLUSIONS

The primary cellular origin of circumpapillary dysgenesis of the pigment epithelium is within the choroid instead of the pigment epithelium. The loss of the RPE and photoreceptors in later stages of the disease is a secondary consequence of choroidal degeneration. Studies of the downstream targets of TEAD1 in choroidal cells will provide promising new research opportunities for the development of treatments for choroidal diseases.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Phenotype-genotype correlations in autosomal dominant retinitis pigmentosa caused by RHO, D190N

PURPOSE

To phenotype a family with RHO (Asp190Asn or D190N) dominantly inherited retinitis pigmentosa (RP) and to describe an approach to surveying affected families.

METHODS

Four patients from a family with a history of autosomal dominant RP had complete clinical examinations and underwent full-field electroretinography (ERG), fundus autofluorescence (AF) imaging, and genetic testing. One patient had microperimetry (MP) mapping.

RESULTS

The patients' ages ranged from 6 years to 47 years. The proband, the father, had fundoscopic findings typical of RP. A small hyperfluorescent ring centered at the fovea was apparent on AF. MP showed preservation of central 7 degrees of visual field within this ring. The three children were all asymptomatic with visual acuity of 20/15 in each eye. One child had mild retinal pigment epithelium migration on fundoscopy; the other two children had normal fundoscopic examinations. Two children showed increased parafoveal AF. In the two affected children, average ERG b-wave implicit times were delayed in scotopic conditions, and maximal ERG tracings had abnormal waveforms. Genetic analysis confirmed that two of three asymptomatic children carried the D190N allele.

CONCLUSIONS

Patients with RHO (D190N) autosomal dominant retinitis pigmentosa (adRP) can show classic signs of RP on fundus examination and may be able to maintain good central visual acuity into adulthood. By combining clinical examination with AF imaging and electrophysiology, it is possible to offer presymptomatic clinical evaluation to families with this RP.

A novel mutation and phenotypes in phosphodiesterase 6 deficiency

PURPOSE

To develop a systematic approach for the molecular diagnosis of retinitis pigmentosa (RP) and to report new genotype-phenotype correlations for phosphodiesterase 6 (PDE6)-based RP mutations.

DESIGN

Clinical and molecular studies on a retrospective case series.

METHODS

We screened 40 unrelated RP patients with an autosomal recessive RP microarray. Individuals with RP caused by PDE6 deficiency underwent genetic segregation and phenotype analysis.

RESULTS

A disease-associated allele was identified in 32% of patients. Two probands (5%) had PDE6 mutations. The first proband was a compound heterozygote for known R102C and N216S alleles in PDE6A (MIM#180071). Pedigree analysis determined that the N216S variant was benign and direct sequencing discovered a novel, S303C allele. The second proband had a homozygous D600N mutation in the PDE6B gene (MIM#180072). Visual acuities of PDE6-deficient patients ranged from 20/40 to 20/200. Clinical studies showed unusual vitreomacular traction, cystoid macular edema, macular atrophy, and ring hyperfluorescence in PDE6-deficient patients. Such extensive vitreoretinal degeneration is not characteristic of photoreceptor-specific enzyme deficiencies.

CONCLUSION

High-throughput deoxyribonucleic acid microarray chips can be used in combination with clinical imaging to precisely characterize patients with RP. Identifying the precise mutation in RP may become the standard of care as gene therapy emerges.

Functional rescue of degenerating photoreceptors in mice homozygous for a hypomorphic cGMP phosphodiesterase 6 b allele (Pde6bH620Q)

PURPOSE

Approximately 8% of autosomal recessive retinitis pigmentosa (RP) cases worldwide are due to defects in rod-specific phosphodiesterase PDE6, a tetramer consisting of catalytic (PDE6alpha and PDE6beta) and two regulatory (PDE6gamma) subunits. In mice homozygous for a nonsense Pde6b(rd1) allele, absence of PDE6 activity is associated with retinal disease similar to humans. Although studied for 80 years, the rapid degeneration Pde6b(rd1) phenotype has limited analyses and therapeutic modeling. Moreover, this model does not represent human RP involving PDE6B missense mutations. In the current study the mouse missense allele, Pde6b(H620Q) was characterized further.

METHODS

Photoreceptor degeneration in Pde6b(H620Q) homozygotes was documented by histochemistry, whereas PDE6beta expression and activity were monitored by immunoblotting and cGMP assays. To measure changes in rod physiology, electroretinograms and intracellular Ca(2+) recording were performed. To test the effectiveness of gene therapy, Opsin::Pde6b lentivirus was subretinally injected into Pde6b(H620Q) homozygotes.

RESULTS

Within 3 weeks of birth, the Pde6b(H620Q) homozygotes displayed relatively normal photoreceptors, but by 7 weeks degeneration was largely complete. Before degeneration, PDE6beta expression and PDE6 activity were reduced. Although light-/dark-adapted total cGMP levels appeared normal, Pde6b(H620Q) homozygotes exhibited depressed rod function and elevated outer segment Ca(2+). Transduction with Opsin::Pde6b lentivirus resulted in histologic and functional rescue of photoreceptors.

CONCLUSIONS

Pde6b(H620Q) homozygous mice exhibit a hypomorphic phenotype with partial PDE6 activity that may result in an increased Ca(2+) to promote photoreceptor death. As degeneration in Pde6b(H620Q) mutants is slower than in Pde6b(rd1) mice and can be suppressed by Pde6b transduction, this Pde6b(H620Q) model may provide an alternate means to explore new treatments of RP.

Direct allosteric regulation between the GAF domain and catalytic domain of photoreceptor phosphodiesterase PDE6

Photoreceptor cGMP phosphodiesterase (PDE6) is the central enzyme in the visual transduction cascade. The PDE6 catalytic subunit contains a catalytic domain and regulatory GAF domains. Unlike most GAF domain-containing cyclic nucleotide phosphodiesterases, little is known about direct allosteric communication of PDE6. In this study, we demonstrate for the first time direct, inter-domain allosteric communication between the GAF and catalytic domains in PDE6. The binding affinity of PDE6 for pharmacological inhibitors or for the C-terminal region of the inhibitory gamma subunit (Pgamma), known to directly inhibit PDE6 catalysis, was increased approximately 2-fold by ligands binding to the GAF domain. Binding of the N-terminal half of Pgamma to the GAF domains suffices to induce this allosteric effect. Allosteric communication between GAF and catalytic domains is reciprocal, in that drug binding to the catalytic domain slowed cGMP dissociation from the GAF domain. Although cGMP hydrolysis was not affected by binding of Pgamma1-60, Pgamma lacking its last seven amino acids decreased the Michaelis constant of PDE6 by 2.5-fold. Pgamma1-60 binding to the GAF domain increased vardenafil but not cGMP affinity, indicating that substrate- and inhibitor-binding sites do not totally overlap. In addition, prolonged incubation of PDE6 with vardenafil or sildenafil (but not 3-isobutyl-1-methylxanthine and zaprinast) induced a distinct conformational change in the catalytic domain without affecting the binding properties of the GAF domains. We conclude that although Pgamma-mediated regulation plays the dominant role in visual excitation, the direct, inter-domain allosteric regulation described in this study may play a feedback role in light adaptational processes during phototransduction.