Conditional ablation of the choroideremia gene causes age-related changes in mouse retinal pigment epithelium

Gene therapy for choroideremia: progress, potential and pitfalls

INTRODUCTION

Choroideremia is a rare disease with a significant disease burden. Gene-supplementation methods for choroideremia gene therapy have been the most successful form of gene therapy thus far.

AREAS COVERED

The aim of the current review is to provide an overview of current progress of gene therapy trials to date, with a focus on potential and pitfalls of such trials. We propose a novel end point that may be clinically meaningful for obtaining regulatory approval in subsequent clinical trials. Additionally, we offer recommendations for further optimization of surgical techniques.

EXPERT OPINION

Lessons learnt from this phase 3 clinical trial, encompassing optimal vector design, delivery techniques, patient selection criteria, and long-term safety profiles can be used in the development of treatments for polygenic retinal disorders, which may necessitate a more nuanced approach due to genetic complexity.

A novel frameshift variant in LAMP2 gene mimicking choroideremia carrier retinopathy

BACKGROUND

Danon disease is a rare, multisystemic X-linked dominant disorder caused by variants in the LAMP2 gene. It can be associated with retinal degeneration, but this is not well characterized. Here we describe a late presentation of a mild retinal phenotype, initially diagnosed as choroideremia carrier, associated with a novel variant in the LAMP2 gene.

METHODS

Retrospective analysis of the case included medical history, ophthalmic examination, multimodal retinal imaging, and microperimetry. Genetic testing was conducted to establish the molecular diagnosis.

RESULTS

A 54-year-old female presented with worsening night vision, without any family history. BCVA was 6/6 bilaterally and fundus examination showed light peripheral pigmentary changes bilaterally. FAF demonstrated a widespread speckled pattern and OCT revealed hyper-reflective spots in the outer nuclear layer. Differentials included non-genetic and genetic causes, suspected of being a manifesting choroideremia carrier. However, initial genetic testing by targeted analysis of retinal disorders did not detect a pathogenic variant. Further systems review revealed that the patient had previously been diagnosed with dilated cardiomyopathy, mini-stroke and partial deafness. Subsequent whole mitochondrial genome sequencing analysis did not detect any pathogenic variants too. Finally, whole exome sequencing with targeted analysis of a panel of hypertrophic cardiomyopathy genes identified a novel pathogenic heterozygous variant (c.925del, p.(Ser309fs)) in the LAMP2 gene, confirming the diagnosis of X-linked Danon disease.

CONCLUSION

Recording previous medical history and extraocular symptoms is crucial. The similarity in choroideremia carrier and Danon disease retinal phenotypes suggests a possible common pathway in these two genes where pathogenic variants lead to retinal pigment epithelium degeneration.

Loss of REP1 impacts choroidal melanogenesis and vasculogenesis in choroideremia

Choroideremia (CHM) is a rare X-linked chorioretinal dystrophy affecting the photoreceptors, retinal pigment epithelium (RPE) and choroid, however, the involvement of the choroid in disease progression is not fully understood. CHM is caused by mutations in the CHM gene, encoding the ubiquitously expressed Rab escort protein 1 (REP1). REP1 plays an important role in intracellular trafficking of vesicles, including melanosomes. In this study, we examined the ultrastructure of the choroid in chmru848 fish and Chmnull/WT mouse models using transmission electron and confocal microscopy. Significant pigmentary disruptions were observed, with lack of melanosomes in the choroid of chmru848 fish from 4 days post fertilisation (4dpf), and a reduction in choroidal blood vessel diameter and interstitial pillars suggesting a defect in vasculogenesis. Total melanin and expression of melanogenesis genes tyr, tryp1a, mitf, dct and pmel were also reduced from 4dpf. In Chmnull/WT mice, choroidal melanosomes were significantly smaller at 1 month, with reduced eumelanin at 1 year. The choroid in CHM patients were also examined using spectral domain optical coherence tomography (SD-OCT) and OCT-angiography (OCT-A) and the area of preserved choriocapillaris (CC) was found to be smaller than that of overlying photoreceptors, suggesting that the choroid is degenerating at a faster rate. Histopathology of an enucleated eye from a 74-year-old CHM male patient revealed isolated areas of RPE but no associated underlying CC. Pigmentary disruptions in CHM animal models reveal an important role for REP1 in melanogenesis, and drugs that improve melanin production represent a potential novel therapeutic avenue.

Choroideremia: Toward Regulatory Approval of Retinal Gene Therapy

Choroideremia is an X-linked inherited retinal degeneration characterized by primary centripetal degeneration of the retinal pigment epithelium (RPE), with secondary degeneration of the choroid and retina. Affected individuals experience reduced night vision in early adulthood with blindness in late middle age. The underlying CHM gene encodes REP1, a protein involved in the prenylation of Rab GTPases essential for intracellular vesicle trafficking. Adeno-associated viral gene therapy has demonstrated some benefit in clinical trials for choroideremia. However, challenges remain in gaining regulatory approval. Choroideremia is slowly progressive, which presents difficulties in demonstrating benefit over short pivotal clinical trials that usually run for 1-2 years. Improvements in visual acuity are particularly challenging due to the initial negative effects of surgical detachment of the fovea. Despite these challenges, great progress toward a treatment has been made since choroideremia was first described in 1872.

Spatiotemporal Live-Cell Analysis of Photoreceptor Outer Segment Membrane Ingestion by the Retinal Pigment Epithelium Reveals Actin-Regulated Scission

The photoreceptor outer segment (OS) is the phototransductive organelle in the vertebrate retina. OS tips are regularly ingested and degraded by the adjacent retinal pigment epithelium (RPE), offsetting the addition of new disk membrane at the base of the OS. This catabolic role of the RPE is essential for photoreceptor health, with defects in ingestion or degradation underlying different forms of retinal degeneration and blindness. Although proteins required for OS tip ingestion have been identified, spatiotemporal analysis of the ingestion process in live RPE cells is lacking; hence, the literature reflects no common understanding of the cellular mechanisms that affect ingestion. We imaged live RPE cells from mice (both sexes) to elucidate the ingestion events in real time. Our imaging revealed roles for f-actin dynamics and specific dynamic localizations of two BAR (Bin-Amphiphysin-Rvs) proteins, FBP17 and AMPH1-BAR, in shaping the RPE apical membrane as it surrounds the OS tip. Completion of ingestion was observed to occur by scission of the OS tip from the remainder of the OS, with a transient concentration of f-actin forming around the site of imminent scission. Actin dynamics were also required for regulating the size of the ingested OS tip, and the time course of the overall ingestion process. The size of the ingested tip is consistent with the term "phagocytosis." However, phagocytosis usually refers to engulfment of an entire particle or cell, whereas our observations of OS tip scission indicate a process that is more specifically described as "trogocytosis," in which one cell "nibbles" another cell.SIGNIFICANCE STATEMENT The ingestion of the photoreceptor outer segment (OS) tips by the retinal pigment epithelium (RPE) is a dynamic cellular process that has fascinated scientists for 60 years. Yet its molecular mechanisms had not been addressed in living cells. We developed a live-cell imaging approach to investigate OS tip ingestion, and focused on the dynamic participation of actin filaments and membrane-shaping BAR proteins. We observed scission of OS tips for the first time, and were able to monitor local changes in protein concentration preceding, during, and following scission. Our approach revealed that actin filaments were concentrated at the site of OS scission and were required for regulating the size of the ingested OS tip and the time course of the ingestion process.

Predicting late-stage age-related macular degeneration by integrating marginally weak SNPs in GWA studies

Introduction: Age-related macular degeneration (AMD) is a progressive neurodegenerative disease and the leading cause of blindness in developed countries. Current genome-wide association studies (GWAS) for late-stage age-related macular degeneration are mainly single-marker-based approaches, which investigate one Single-Nucleotide Polymorphism (SNP) at a time and postpone the integration of inter-marker Linkage-disequilibrium (LD) information in the downstream fine mappings. Recent studies showed that directly incorporating inter-marker connection/correlation into variants detection can help discover novel marginally weak single-nucleotide polymorphisms, which are often missed in conventional genome-wide association studies, and can also help improve disease prediction accuracy. Methods: Single-marker analysis is performed first to detect marginally strong single-nucleotide polymorphisms. Then the whole-genome linkage-disequilibrium spectrum is explored and used to search for high-linkage-disequilibrium connected single-nucleotide polymorphism clusters for each strong single-nucleotide polymorphism detected. Marginally weak single-nucleotide polymorphisms are selected via a joint linear discriminant model with the detected single-nucleotide polymorphism clusters. Prediction is made based on the selected strong and weak single-nucleotide polymorphisms. Results: Several previously identified late-stage age-related macular degeneration susceptibility genes, for example, BTBD16, C3, CFH, CFHR3, HTARA1, are confirmed. Novel genes DENND1B, PLK5, ARHGAP45, and BAG6 are discovered as marginally weak signals. Overall prediction accuracy of 76.8% and 73.2% was achieved with and without the inclusion of the identified marginally weak signals, respectively. Conclusion: Marginally weak single-nucleotide polymorphisms, detected from integrating inter-marker linkage-disequilibrium information, may have strong predictive effects on age-related macular degeneration. Detecting and integrating such marginally weak signals can help with a better understanding of the underlying disease-development mechanisms for age-related macular degeneration and more accurate prognostics.

Bioprinted 3D outer retina barrier uncovers RPE-dependent choroidal phenotype in advanced macular degeneration

Age-related macular degeneration (AMD), a leading cause of blindness, initiates in the outer-blood-retina-barrier (oBRB) formed by the retinal pigment epithelium (RPE), Bruch's membrane, and choriocapillaris. The mechanisms of AMD initiation and progression remain poorly understood owing to the lack of physiologically relevant human oBRB models. To this end, we engineered a native-like three-dimensional (3D) oBRB tissue (3D-oBRB) by bioprinting endothelial cells, pericytes, and fibroblasts on the basal side of a biodegradable scaffold and establishing an RPE monolayer on top. In this 3D-oBRB model, a fully-polarized RPE monolayer provides barrier resistance, induces choriocapillaris fenestration, and supports the formation of Bruch's-membrane-like structure by inducing changes in gene expression in cells of the choroid. Complement activation in the 3D-oBRB triggers dry AMD phenotypes (including subRPE lipid-rich deposits called drusen and choriocapillaris degeneration), and HIF-α stabilization or STAT3 overactivation induce choriocapillaris neovascularization and type-I wet AMD phenotype. The 3D-oBRB provides a physiologically relevant model to studying RPE-choriocapillaris interactions under healthy and diseased conditions.

Widespread subclinical cellular changes revealed across a neural-epithelial-vascular complex in choroideremia using adaptive optics

Choroideremia is an X-linked, blinding retinal degeneration with progressive loss of photoreceptors, retinal pigment epithelial (RPE) cells, and choriocapillaris. To study the extent to which these layers are disrupted in affected males and female carriers, we performed multimodal adaptive optics imaging to better visualize the in vivo pathogenesis of choroideremia in the living human eye. We demonstrate the presence of subclinical, widespread enlarged RPE cells present in all subjects imaged. In the fovea, the last area to be affected in choroideremia, we found greater disruption to the RPE than to either the photoreceptor or choriocapillaris layers. The unexpected finding of patches of photoreceptors that were fluorescently-labeled, but structurally and functionally normal, suggests that the RPE blood barrier function may be altered in choroideremia. Finally, we introduce a strategy for detecting enlarged cells using conventional ophthalmic imaging instrumentation. These findings establish that there is subclinical polymegathism of RPE cells in choroideremia.

AAV2-hCHM Subretinal Delivery to the Macula in Choroideremia: Two Year Interim Results of an Ongoing Phase I/II Gene Therapy Trial

PURPOSE

To assess the safety of the subretinal delivery of a recombinant adeno-associated virus serotype 2 (AAV2) vector carrying a human CHM-encoding cDNA in choroideremia (CHM).

DESIGN

Prospective, open-label, non-randomized, dose-escalation, phase 1/2 clinical trial.

SUBJECTS, PARTICIPANTS, AND/OR CONTROLS

Fifteen CHM patients (ages 20-57 years at dosing).

METHODS, INTERVENTION, OR TESTING

Patients received uniocular subfoveal injections of low dose (up to 5x10¹⁰ vector genome (vg) per eye, n=5) or high dose (up to 1x10¹¹ vg per eye, n=10) AAV2-hCHM. Patients were evaluated pre- and post-operatively for two years with ophthalmic examinations, multimodal retinal imaging and psychophysical testing.

MAIN OUTCOME

Measures: visual acuity (VA), perimetry (10-2 protocol), spectral-domain optical coherence tomography (SD-OCT) and short-wavelength fundus autofluorescence (SW-FAF).

RESULTS

We detected no vector-related or systemic toxicities. VA returned to within 15 letters of baseline in all but two patients (one developed acute foveal thinning, another patient, a macular hole); the rest showed no gross changes in foveal structure at two years. There were no significant differences between intervention and control eyes in mean light-adapted sensitivity by perimetry, or in the lateral extent of retinal pigment epithelium (RPE) relative preservation by SD-OCT and SW-FAF. Microperimetry showed non-significant (<3SD of the intervisit variability) gains in sensitivity in some locations and participants in the intervention eye. There were no obvious dose-dependent relationships.

CONCLUSIONS

VA was within 15 letters of baseline after the subfoveal AAV2-hCHM injections in 13/15 (87%) of the patients. Acute foveal thinning with unchanged perifoveal function in one patient and macular hole in a second suggests foveal vulnerability to the subretinal injections. Longer observation intervals will help establish the significance of the minor differences in sensitivities and rate of disease progression observed between intervention and control eyes.

Short-term Assessment of Subfoveal Injection of Adeno-Associated Virus-Mediated hCHM Gene Augmentation in Choroideremia Using Adaptive Optics Ophthalmoscopy

Importance

Subretinal injection for gene augmentation in retinal degenerations forcefully detaches the neural retina from the retinal pigment epithelium, potentially damaging photoreceptors and/or retinal pigment epithelium cells.

Objective

To use adaptive optics scanning light ophthalmoscopy (AOSLO) to assess the short-term integrity of the cone mosaic following subretinal injections of adeno-associated virus vector designed to deliver a functional version of the CHM gene (AAV2-hCHM) in patients with choroideremia.

Design, Setting, and Participants

This longitudinal case series study enrolled adult patients with choroideremia from February 2015 to January 2016 in the US. To be included in the study, study participants must have received uniocular subfoveal injections of low-dose (5 × 1010 vector genome per eye) or high-dose (1 × 1011 vector genome per eye) AAV2-hCHM. Analysis began February 2015.

Main Outcomes and Measures

The macular regions of both eyes were imaged before and 1 month after injection using a custom-built multimodal AOSLO. Postinjection cone inner segment mosaics were compared with preinjection mosaics at multiple regions of interest. Colocalized spectral-domain optical coherence tomography and dark-adapted cone sensitivity was also acquired at each time point.

Results

Nine study participants ranged in age from 26 to 50 years at the time of enrollment, and all were White men. Postinjection AOSLO images showed preservation of the cone mosaic in all 9 AAV2-hCHM-injected eyes. Mosaics appeared intact and contiguous 1 month postinjection, with the exception of foveal disruption in 1 patient. Optical coherence tomography showed foveal cone outer segment shortening postinjection. Cone-mediated sensitivities were unchanged in 8 of 9 injected and 9 of 9 uninjected eyes. One participant showed acute loss of foveal optical coherence tomography cone outer segment-related signals along with cone sensitivity loss that colocalized with disruption of the mosaic on AOSLO.

Conclusions and Relevance

Integrity of the cone mosaic is maintained following subretinal delivery of AAV2-hCHM, providing strong evidence in support of the safety of the injections. Minor foveal thinning observed following surgery corresponds with short-term cone outer segment shortening rather than cone cell loss. Foveal cone loss in 1 participant raises the possibility of individual vulnerability to the subretinal injection.

Microtubule Motor Transport of Organelles in a Specialized Epithelium: The RPE

The retinal pigment epithelium (RPE) is a uniquely polarized epithelium that lies adjacent to the photoreceptor cells in the retina, and is essential for photoreceptor function and viability. Two major motile organelles present in the RPE are the melanosomes, which are important for absorbing stray light, and phagosomes that result from the phagocytosis of the distal tips of the photoreceptor cilium, known as the photoreceptor outer segment (POS). These organelles are transported along microtubules, aligned with the apical-basal axis of the RPE. Although they undergo a directional migration, the organelles exhibit bidirectional movements, indicating both kinesin and dynein motor function in their transport. Apical melanosome localization requires dynein; it has been suggested that kinesin contribution might be complex with the involvement of more than one type of kinesin. POS phagosomes undergo bidirectional movements; roles of both plus- and minus-end directed motors appear to be important in the efficient degradation of phagosomes. This function is directly related to retinal health, with defects in motor proteins, or in the association of the phagosomes with the motors, resulting in retinal degenerative pathologies.

Modeling Rare Human Disorders in Mice: The Finnish Disease Heritage

The modification of genes in animal models has evidently and comprehensively improved our knowledge on proteins and signaling pathways in human physiology and pathology. In this review, we discuss almost 40 monogenic rare diseases that are enriched in the Finnish population and defined as the Finnish disease heritage (FDH). We will highlight how gene-modified mouse models have greatly facilitated the understanding of the pathological manifestations of these diseases and how some of the diseases still lack proper models. We urge the establishment of subsequent international consortiums to cooperatively plan and carry out future human disease modeling strategies. Detailed information on disease mechanisms brings along broader understanding of the molecular pathways they act along both parallel and transverse to the proteins affected in rare diseases, therefore also aiding understanding of common disease pathologies.

Role of retinal pigment epithelium in age-related macular disease: a systematic review

Age-related macular disease (AMD) is a major cause of blindness and there is little treatment currently available by which the progress of the basic disorder can be modulated. Histological and clinical studies show that the major tissues involved are the outer retina, retinal pigment epithelium, Bruch's membrane and choroid. Because of a wide variation of phenotype from one case to another, it has been suggested that accurate phenotyping would be necessary for assessment of the effectiveness of treatment that is tissue-directed. However, based on findings from the study of human donor material and animal models of disease and of cell culture, it is concluded that retinal pigment epithelial dysfunction plays a central role in the disease process in most, if not all, cases of early AMD. The metabolism of phagosomal material, particularly lipids, and energy generation are interdependent, and dysfunction of both appears to be important in the genesis of disease. Evidence exists to suggest that both can be modulated therapeutically. These metabolic functions are amenable to further investigation in both the normal state and in disease. Once fully characterised, it is likely that treatment could be directed towards a limited number of functions in single tissue, thus simplifying treatment strategies.

Expression of Rab Prenylation Pathway Genes and Relation to Disease Progression in Choroideremia

Purpose

Choroideremia results from the deficiency of Rab Escort Protein 1 (REP1), encoded by CHM, involved in the prenylation of Rab GTPases. Here, we investigate whether the transcription and expression of other genes involved in the prenylation of Rab proteins correlates with disease progression in a cohort of patients with choroideremia.

Methods

Rates of retinal pigment epithelial area loss in 41 patients with choroideremia were measured using fundus autofluorescence imaging for up to 4 years. From lysates of cultured skin fibroblasts donated by patients (n = 15) and controls (n = 14), CHM, CHML, RABGGTB and RAB27A mRNA expression, and REP1 and REP2 protein expression were compared.

Results

The central autofluorescent island area loss in patients with choroideremia occurred with a mean half-life of 5.89 years (95% confidence interval [CI] = 5.09-6.70), with some patients demonstrating relatively fast or slow rates of progression (range = 3.3-14.1 years). Expression of CHM mRNA and REP1 protein were significantly decreased in all patients. No difference in expression of CHML, RABGGTB, RAB27A, or REP2 was seen between patients and controls. No correlation was seen between expression of the genes analyzed and rates of retinal degeneration. Non-sense induced transcriptional compensation of CHML, a CHM-like retrogene, was not observed in patients with CHM variants predicted to undergo non-sense mediated decay.

Conclusions

Patients with choroideremia, who are deficient for REP1, show normal levels of expression of other genes involved in Rab prenylation, which do not appear to play any modifying role in the rate of disease progression.

Translational Relevance

There remains little evidence for selection of patients for choroideremia gene therapy based on genotype.

Is subretinal AAV gene replacement still the only viable treatment option for choroideremia?

INTRODUCTION

Choroideremia is an X-linked inherited retinal degeneration resulting from mutations in the CHM gene, encoding Rab escort protein-1 (REP1), a protein regulating intracellular vesicular transport. Loss-of-function mutations in CHM lead to progressive loss of retinal pigment epithelium (RPE) with photoreceptor and choriocapillaris degeneration, leading to progressive visual field constriction and loss of visual acuity. Three hundred and fifty-four unique mutations have been reported in CHM. While gene augmentation remains an ideal therapeutic option for choroideremia, other potential future clinical strategies may exist.

AREAS COVERED

The authors examine the pathophysiology and genetic basis of choroideremia. They summarize the status of ongoing gene therapy trials and discuss CHM mutations amenable to other therapeutic approaches including CRISPR/Cas-based DNA and RNA editing, nonsense suppression of premature termination codons, and antisense oligonucleotides for splice modification. The authors undertook a literature search in PubMed and NIH Clinical Trials in October 2020.

EXPERT OPINION

The authors conclude that AAV-mediated gene augmentation remains the most effective approach for choroideremia. Given the heterogeneity of CHM mutations and potential risks and benefits, genome-editing approaches currently do not offer significant advantages. Nonsense suppression strategies and antisense oligonucleotides are exciting novel therapeutic options; however, their clinical viability remains to be determined.

Zebrafish Models of Photoreceptor Dysfunction and Degeneration

Zebrafish are an instrumental system for the generation of photoreceptor degeneration models, which can be utilized to determine underlying causes of photoreceptor dysfunction and death, and for the analysis of potential therapeutic compounds, as well as the characterization of regenerative responses. We review the wealth of information from existing zebrafish models of photoreceptor disease, specifically as they relate to currently accepted taxonomic classes of human rod and cone disease. We also highlight that rich, detailed information can be derived from studying photoreceptor development, structure, and function, including behavioural assessments and in vivo imaging of zebrafish. Zebrafish models are available for a diversity of photoreceptor diseases, including cone dystrophies, which are challenging to recapitulate in nocturnal mammalian systems. Newly discovered models of photoreceptor disease and drusenoid deposit formation may not only provide important insights into pathogenesis of disease, but also potential therapeutic approaches. Zebrafish have already shown their use in providing pre-clinical data prior to testing genetic therapies in clinical trials, such as antisense oligonucleotide therapy for Usher syndrome.

Chronically shortened rod outer segments accompany photoreceptor cell death in Choroideremia

X-linked choroideremia (CHM) is a disease characterized by gradual retinal degeneration caused by loss of the Rab Escort Protein, REP1. Despite partial compensation by REP2 the disease is characterized by prenylation defects in multiple members of the Rab protein family that are master regulators of membrane traffic. Remarkably, the eye is the only organ affected in CHM patients, possibly because of the huge membrane traffic burden of the post mitotic photoreceptors, which synthesise outer segments, and the adjacent retinal pigment epithelium that degrades the spent portions each day. In this study, we aimed to identify defects in membrane traffic that might lead to photoreceptor cell death in CHM. In a heterozygous null female mouse model of CHM (Chmnull/WT), degeneration of the photoreceptor layer was clearly evident from increased numbers of TUNEL positive cells compared to age matched controls, small numbers of cells exhibiting signs of mitochondrial stress and greatly increased microglial infiltration. However, most rod photoreceptors exhibited remarkably normal morphology with well-formed outer segments and no discernible accumulation of transport vesicles in the inner segment. The major evidence of membrane trafficking defects was a shortening of rod outer segments that was evident at 2 months of age but remained constant over the period during which the cells die. A decrease in rhodopsin density found in the outer segment may underlie the outer segment shortening but does not lead to rhodopsin accumulation in the inner segment. Our data argue against defects in rhodopsin transport or outer segment renewal as triggers of cell death in CHM.

Phagocytosis by the Retinal Pigment Epithelium: Recognition, Resolution, Recycling

Tissue-resident phagocytes are responsible for the routine binding, engulfment, and resolution of their meals. Such populations of cells express appropriate surface receptors that are tailored to recognize the phagocytic targets of their niche and initiate the actin polymerization that drives internalization. Tissue-resident phagocytes also harbor enzymes and transporters along the endocytic pathway that orchestrate the resolution of ingested macromolecules from the phagolysosome. Solutes fluxed from the endocytic pathway and into the cytosol can then be reutilized by the phagocyte or exported for their use by neighboring cells. Such a fundamental metabolic coupling between resident phagocytes and the tissue in which they reside is well-emphasized in the case of retinal pigment epithelial (RPE) cells; specialized phagocytes that are responsible for the turnover of photoreceptor outer segments (POS). Photoreceptors are prone to photo-oxidative damage and their long-term health depends enormously on the disposal of aged portions of the outer segment. The phagocytosis of the POS by the RPE is the sole means of this turnover and clearance. RPE are themselves mitotically quiescent and therefore must resolve the ingested material to prevent their toxic accumulation in the lysosome that otherwise leads to retinal disorders. Here we describe the sequence of events underlying the healthy turnover of photoreceptors by the RPE with an emphasis on the signaling that ensures the phagocytosis of the distal POS and on the transport of solutes from the phagosome that supersedes its resolution. While other systems may utilize different receptors and transporters, the biophysical and metabolic manifestations of such events are expected to apply to all tissue-resident phagocytes that perform regular phagocytic programs.

Transcriptome-Wide Analysis of CXCR5 Deficient Retinal Pigment Epithelial (RPE) Cells Reveals Molecular Signatures of RPE Homeostasis

Age-related macular degeneration (AMD) is the most common cause of irreversible blindness in the elderly population. In our previous studies, we found that deficiency of CXCR5 causes AMD-like pathological phenotypes in mice, characterized by abnormalities and dysfunction of the retinal pigment epithelium (RPE) cells. The abnormalities included abnormal cellular shape and impaired barrier function. In the present study, primary RPE cells were derived separately from CXCR5 knockout (KO) mice and from C57BL6 wild type (WT). The isolated primary cells were cultured for several days, and then total RNA was isolated and used for library preparation, sequencing, and the resultant raw data analyzed. Relative to the WT, a total of 1392 differentially expressed genes (DEG) were identified. Gene ontology analysis showed various biological processes, cellular components, and molecular functions were enriched. Pathway enrichment analysis revealed several pathways, including the PI3K-Akt signaling, mTOR signaling, FoxO, focal adhesion, endocytosis, ubiquitin-mediated proteolysis, TNFα-NF-kB Signaling, adipogenesis genes, p53 signaling, Ras, autophagy, epithelial–mesenchymal transition (EMT), and mitochondrial pathway. This study explores molecular signatures associated with deficiency of CXCR5 in RPE cells. Many of these signatures are important for homeostasis of this tissue. The identified pathways and genes require further evaluation to better understand the pathophysiology of AMD.

Remodeling of the Basal Labyrinth of Retinal Pigment Epithelial Cells With Osmotic Challenge, Age, and Disease

Purpose

The basal surface of the retinal pigment epithelium (RPE) is folded into a complex basal labyrinth thought to facilitate solute and water transport. We aimed to analyze and define the structural organization of the basal labyrinth of the RPE to enable quantitative analysis of structural changes in age and disease and to better understand the relationship between basal labyrinth structure and efficiency of transepithelial transport.

Methods

Conventional transmission and serial block-face scanning electron microscopy and electron tomography were used to examine the structure of the basal labyrinth in mouse eyes of different ages and genotypes and with and without osmotic shock before fixation.

Results

We identified structurally distinct zones (stacked and ribbon-like) within the RPE basal labyrinth that are largely organelle free and cisternal elements that make contact with the endoplasmic reticulum (ER) and mitochondria. These zones are lost in a hierarchic fashion with age and prematurely in a model of the progressive retinal degenerative disease, choroideremia. Junctional complexes crosslink closely opposed infoldings. Spacing between the basal infoldings was affected by subtle osmotic changes while osmotic shock induced dramatic remodeling of the infoldings.

Conclusions

The basal labyrinth has complex but ordered structural elements that break down with age and in choroideremia. The geometry of these elements and site of contact with ER and mitochondria likely facilitate the ion transport that drives water transport across the basal RPE surface. Changes in structure in response to local osmotic variation may allow transport to be modulated in order to maintain RPE volume.

Molecular Therapies for Choroideremia

Advances in molecular research have culminated in the development of novel gene-based therapies for inherited retinal diseases. We have recently witnessed several groundbreaking clinical studies that ultimately led to approval of Luxturna, the first gene therapy for an inherited retinal disease. In parallel, international research community has been engaged in conducting gene therapy trials for another more common inherited retinal disease known as choroideremia and with phase III clinical trials now underway, approval of this therapy is poised to follow suit. This chapter discusses new insights into clinical phenotyping and molecular genetic testing in choroideremia with review of molecular mechanisms implicated in its pathogenesis. We provide an update on current gene therapy trials and discuss potential inclusion of female carries in future clinical studies. Alternative molecular therapies are discussed including suitability of CRISPR gene editing, small molecule nonsense suppression therapy and vision restoration strategies in late stage choroideremia.

Oxidative Stress and Dysfunctional Intracellular Traffic Linked to an Unhealthy Diet Results in Impaired Cargo Transport in the Retinal Pigment Epithelium (RPE)

SCOPE

Oxidative stress and dysregulated intracellular trafficking are associated with an unhealthy diet which underlies pathology. Here, these effects on photoreceptor outer segment (POS) trafficking in the retinal pigment epithelium (RPE), a major pathway of disease underlying irreversible sight-loss, are studied.

METHODS AND RESULTS

POS trafficking is studied in ARPE-19 cells using an algorithm-based quantification of confocal-immunofluorescence data supported by ultrastructural studies. It is shown that although POS are tightly regulated and trafficked via Rab5, Rab7 vesicles, LAMP1/2 lysosomes and LC3b-autophagosomes, there is also a considerable degree of variation and flexibility in this process. Treatment with H2 O2 and bafilomycin A1 reveals that oxidative stress and dysregulated autophagy target intracellular compartments and trafficking in strikingly different ways. These effects appear limited to POS-containing vesicles, suggesting a cargo-specific effect.

CONCLUSION

The findings offer insights into how RPE cells cope with stress, and how mechanisms influencing POS transport/degradation can have different outcomes in the senescent retina. These shed new light on cellular processes underlying retinopathies such as age-related macular degeneration. The discoveries reveal how diet and nutrition can cause fundamental alterations at a cellular level, thus contributing to a better understanding of the diet-disease axis.

Phagosomal and mitochondrial alterations in RPE may contribute to KCNJ13 retinopathy

Mutations in KCNJ13 are associated with two retinal disorders; Leber congenital amaurosis (LCA) and snowflake vitreoretinal degeneration (SVD). We examined the retina of kcnj13 mutant zebrafish (obelix^td15, c.502T > C p.[Phe168Leu]) to provide new insights into the pathophysiology underlying these conditions. Detailed phenotyping of obelix^td15 fish revealed a late onset retinal degeneration at 12 months. Electron microscopy of the obelix^td15 retinal pigment epithelium (RPE) uncovered reduced phagosome clearance and increased mitochondrial number and size prior any signs of retinal degeneration. Melanosome distribution was also affected in dark-adapted 12-month obelix^td15 fish. At 6 and 12 months, ATP levels were found to be reduced along with increased expression of glial fibrillary acidic protein and heat shock protein 60. Quantitative RT-PCR of polg2, fis1, opa1, sod1/2 and bcl2a from isolated retina showed expression changes consistent with altered mitochondrial activity and retinal stress. We propose that the retinal disease in this model is primarily a failure of phagosome physiology with a secondary mitochondrial dysfunction. Our findings suggest that alterations in the RPE and photoreceptor cellular organelles may contribute to KCNJ13-related retinal degeneration and provide a therapeutic target.

Cell Culture Analysis of the Phagocytosis of Photoreceptor Outer Segments by Primary Mouse RPE Cells

The phagocytosis of photoreceptor outer segments (POSs) by the retinal pigment epithelium (RPE) is essential for retinal homeostasis. Defects in this process can be caused by mutations in the photoreceptor cells, the RPE cells, or both cell types. This function can be experimentally investigated by performing an in vitro phagocytosis assay, in which cultured RPE cells are challenged with isolated POSs, and subsequently tested for their ability to degrade the POSs. A significant advantage of this approach is that mutant phenotypes can be attributed either to the photoreceptor or the RPE cells, by experimenting with different permutations of mutant and control photoreceptor and RPE cells. In this chapter, we detail the method for a double-immunofluorescence assay for analysis of the binding, ingestion, and subsequent degradation of isolated mouse POSs by cultured mouse primary RPE cells.

Choroideremia: from genetic and clinical phenotyping to gene therapy and future treatments

Choroideremia is an X-linked inherited chorioretinal dystrophy leading to blindness by late adulthood. Choroideremia is caused by mutations in the CHM gene which encodes Rab escort protein 1 (REP1), an ubiquitously expressed protein involved in intracellular trafficking and prenylation activity. The exact site of pathogenesis remains unclear but results in degeneration of the photoreceptors, retinal pigment epithelium and choroid. Animal and stem cell models have been used to study the molecular defects in choroideremia and test effectiveness of treatment interventions. Natural history studies of choroideremia have provided additional insight into the clinical phenotype of the condition and prepared the way for clinical trials aiming to investigate the safety and efficacy of suitable therapies. In this review, we provide a summary of the current knowledge on the genetics, pathophysiology, clinical features and therapeutic strategies that might become available for choroideremia in the future, including gene therapy, stem cell treatment and small-molecule drugs with nonsense suppression action.

Soft Drusen in Age-Related Macular Degeneration: Biology and Targeting Via the Oil Spill Strategies

AMD is a major cause of legal blindness in older adults approachable through multidisciplinary research involving human tissues and patients. AMD is a vascular-metabolic-inflammatory disease, in which two sets of extracellular deposits, soft drusen/basal linear deposit (BLinD) and subretinal drusenoid deposit (SDD), confer risk for end-stages of atrophy and neovascularization. Understanding how deposits form can lead to insights for new preventions and therapy. The topographic correspondence of BLinD and SDD with cones and rods, respectively, suggest newly realized exchange pathways among outer retinal cells and across Bruch's membrane and the subretinal space, in service of highly evolved, eye-specific physiology. This review focuses on soft drusen/BLinD, summarizing evidence that a major ultrastructural component is large apolipoprotein B,E-containing, cholesterol-rich lipoproteins secreted by the retinal pigment epithelium (RPE) that offload unneeded lipids of dietary and outer segment origin to create an atherosclerosis-like progression in the subRPE-basal lamina space. Clinical observations and an RPE cell culture system combine to suggest that soft drusen/BLinD form when secretions of functional RPE back up in the subRPE-basal lamina space by impaired egress across aged Bruch's membrane-choriocapillary endothelium. The soft drusen lifecycle includes growth, anterior migration of RPE atop drusen, then collapse, and atrophy. Proof-of-concept studies in humans and animal models suggest that targeting the “Oil Spill in Bruch's membrane” offers promise of treating a process in early AMD that underlies progression to both end-stages. A companion article addresses the antecedents of soft drusen within the biology of the macula.

Impaired Cargo Clearance in the Retinal Pigment Epithelium (RPE) Underlies Irreversible Blinding Diseases

Chronic degeneration of the Retinal Pigment Epithelium (RPE) is a precursor to pathological changes in the outer retina. The RPE monolayer, which lies beneath the neuroretina, daily internalises and digests large volumes of spent photoreceptor outer segments. Impaired cargo handling and processing in the endocytic/phagosome and autophagy pathways lead to the accumulation of lipofuscin and pyridinium bis-retinoid A2E aggregates and chemically modified compounds such as malondialdehyde and 4-hydroxynonenal within RPE. These contribute to increased proteolytic and oxidative stress, resulting in irreversible damage to post-mitotic RPE cells and development of blinding conditions such as age-related macular degeneration, Stargardt disease and choroideremia. Here, we review how impaired cargo handling in the RPE results in their dysfunction, discuss new findings from our laboratory and consider how newly discovered roles for lysosomes and the autophagy pathway could provide insights into retinopathies. Studies of these dynamic, molecular events have also been spurred on by recent advances in optics and imaging technology. Mechanisms underpinning lysosomal impairment in other degenerative conditions including storage disorders, α-synuclein pathologies and Alzheimer's disease are also discussed. Collectively, these findings help transcend conventional understanding of these intracellular compartments as simple waste disposal bags to bring about a paradigm shift in the way lysosomes are perceived.

Correlative light and immuno-electron microscopy of retinal tissue cryostat sections

Correlative light-electron microscopy (CLEM) is a powerful technique allowing localisation of specific macromolecules within fluorescence microscopy (FM) images to be mapped onto corresponding high-resolution electron microscopy (EM) images. Existing methods are applicable to limited sample types and are technically challenging. Here we describe novel methods to perform CLEM and immuno-electron microscopy (iEM) on cryostat sections utilising the popular FM embedding solution, optimal cutting temperature (OCT) compound. Utilising these approaches, we have (i) identified the same phagosomes by FM and EM in the retinal pigment epithelium (RPE) of retinal tissue (ii) shown the correct localisation of rhodopsin on photoreceptor outer segment disc like-structures in iPSC derived optic cups and (iii) identified a novel interaction between peroxisomes and melanosomes as well as phagosomes in the RPE. These data show that cryostat sections allow easy characterisation of target macromolecule localisation within tissue samples, thus providing a substantial improvement over many conventional methods that are limited to cultured cells. As OCT embedding is routinely used for FM this provides an easily accessible and robust method for further analysis of existing samples by high resolution EM.

Genetic analysis and clinical phenotype of two Indian families with X-linked choroideremia

Purpose:

This study aims to describe the phenotype and genotype of two Indian families affected with X-linked choroideremia (CHM).

Materials and Methods:

In these two families, the affected individuals and unaffected family members underwent a comprehensive ophthalmic examination including an optical coherence tomography (OCT) and electroretinogram. Blood samples were collected from the families for genetic analysis. Next generation sequencing (NGS) was done using a panel of 184 genes, which covered previously associated genes with retinal dystrophies. Sequencing data were analyzed for the CHM, RPGR, and RP2 genes that have been implicated in CHM and X-linked retinitis pigmentosa (XLRP), respectively. The identified variants were confirmed by Sanger sequencing in available individuals and unrelated controls.

Results:

In two unrelated male patients, NGS analysis revealed a previously reported 3’-splice site change c.820-1G>C in the CHM gene in the first family and hemizygous mutation c.653G>C (p.Ser218X) in the second family. The asymptomatic family members were carriers for these mutations. Spectral domain-OCT showed loss of outer retina, preservation of the inner retina, and choroidal thinning in the affected males and retinal pigment epithelial changes in the asymptomatic carriers. The identified mutations were not present in 100 controls of Indian origin. There were no potential mutations found in XLRP-associated (RPGR and RP2) genes.

Conclusion:

This report describes the genotype and phenotype findings in patients with CHM from India. The identified genetic mutation leads to lack of Rab escort protein-1 (REP-1) or affects the production of a REP-1 protein that is likely to cause retinal abnormalities in patients.

Oncogenic role of rab escort protein 1 through EGFR and STAT3 pathway

Rab escort protein-1 (REP1) is linked to choroideremia (CHM), an X-linked degenerative disorder caused by mutations of the gene encoding REP1 (CHM). REP1 mutant zebrafish showed excessive cell death throughout the body, including the eyes, indicating that REP1 is critical for cell survival, a hallmark of cancer. In the present study, we found that REP1 is overexpressed in human tumor tissues from cervical, lung, and colorectal cancer patients, whereas it is expressed at relatively low levels in the normal tissue counterparts. REP1 expression was also elevated in A549 lung cancer cells and HT-29 colon cancer cells compared with BEAS-2B normal lung and CCD-18Co normal colon epithelial cells, respectively. Interestingly, short interfering RNA (siRNA)-mediated REP1 knockdown-induced growth inhibition of cancer cell lines via downregulation of EGFR and inactivation of STAT3, but had a negligible effect on normal cell lines. Moreover, overexpression of REP1 in BEAS-2B cells enhanced cell growth and anchorage-independent colony formation with little increase in EGFR level and STAT3 activation. Furthermore, REP1 knockdown effectively reduced tumor growth in a mouse xenograft model via EGFR downregulation and STAT3 inactivation in vivo. These data suggest that REP1 plays an oncogenic role, driving tumorigenicity via EGFR and STAT3 signaling, and is a potential therapeutic target to control cancers.

REP1 inhibits FOXO3-mediated apoptosis to promote cancer cell survival

Rab escort protein 1 (REP1) is a component of Rab geranyl-geranyl transferase 2 complex. Mutations in REP1 cause a disease called choroideremia (CHM), which is an X-linked eye disease. Although it is postulated that REP1 has functions in cell survival or death of various tissues in addition to the eye, how REP1 functions in normal and cancer cells remains to be elucidated. Here, we demonstrated that REP1 is required for the survival of intestinal cells in addition to eyes or a variety of cells in zebrafish, and also has important roles in tumorigenesis. Notably, REP1 is highly expressed in colon cancer tissues and cell lines, and silencing of REP1 sensitizes colon cancer cells to serum starvation- and 5-FU-induced apoptosis. In an effort to elucidate the molecular mechanisms underlying REP1-mediated cell survival under those stress conditions, we identified FOXO3 as a binding partner of REP1 using a yeast two-hybrid (Y2H) assay system, and we demonstrated that REP1 blocked the nuclear trans-localization of FOXO3 through physically interacting with FOXO3, thereby suppressing FOXO3-mediated apoptosis. Importantly, the inhibition of REP1 combined with 5-FU treatment could lead to significant retarded tumor growth in a xenograft tumor model of human cancer cells. Thus, our results suggest that REP1 could be a new therapeutic target in combination treatment for colon cancer patients.

Tyrosinase-Cre-Mediated Deletion of the Autophagy Gene Atg7 Leads to Accumulation of the RPE65 Variant M450 in the Retinal Pigment Epithelium of C57BL/6 Mice

Targeted gene knockout mouse models have helped to identify roles of autophagy in many tissues. Here, we investigated the retinal pigment epithelium (RPE) of Atg7f/f Tyr-Cre mice (on a C57BL/6 background), in which Cre recombinase is expressed under the control of the tyrosinase promoter to delete the autophagy gene Atg7. In line with pigment cell-directed blockade of autophagy, the RPE and the melanocytes of the choroid showed strong accumulation of the autophagy adaptor and substrate, sequestosome 1 (Sqstm1)/p62, relative to the levels in control mice. Immunofluorescence and Western blot analysis demonstrated that the RPE, but not the choroid melanocytes, of Atg7f/f Tyr-Cre mice also had strongly increased levels of retinoid isomerohydrolase RPE65, a pivotal enzyme for the maintenance of visual perception. In contrast to Sqstm1, genes involved in retinal regeneration, i.e. Lrat, Rdh5, Rgr, and Rpe65, were expressed at higher mRNA levels. Sequencing of the Rpe65 gene showed that Atg7f/f and Atg7f/f Tyr-Cre mice carry a point mutation (L450M) that is characteristic for the C57BL/6 mouse strain and reportedly causes enhanced degradation of the RPE65 protein by an as-yet unknown mechanism. These results suggest that the increased abundance of RPE65 M450 in the RPE of Atg7f/f Tyr-Cre mice is, at least partly, mediated by upregulation of Rpe65 transcription; however, our data are also compatible with the hypothesis that the RPE65 M450 protein is degraded by Atg7-dependent autophagy in Atg7f/f mice. Further studies in mice of different genetic backgrounds are necessary to determine the relative contributions of these mechanisms.

Choroideremia Is a Systemic Disease With Lymphocyte Crystals and Plasma Lipid and RBC Membrane Abnormalities

PURPOSE

Photoreceptor neuronal degenerations are common, incurable causes of human blindness affecting 1 in 2000 patients worldwide. Only half of all patients are associated with known mutations in over 250 disease genes, prompting our research program to identify the remaining new genes. Most retinal degenerations are restricted to the retina, but photoreceptor degenerations can also be found in a wide variety of systemic diseases. We identified an X-linked family from Sri Lanka with a severe choroidal degeneration and postulated a new disease entity. Because of phenotypic overlaps with Bietti's crystalline dystrophy, which was recently found to have systemic features, we hypothesized that a systemic disease may be present in this new disease as well.

METHODS

For phenotyping, we performed detailed eye exams with in vivo retinal imaging by optical coherence tomography. For genotyping, we performed whole exome sequencing, followed by Sanger sequencing confirmations and cosegregation. Systemic investigations included electron microscopy studies of peripheral blood cells in patients and in normal controls and detailed fatty acid profiles (both plasma and red blood cell [RBC] membranes). Fatty acid levels were compared to normal controls, and only values two standard deviations above or below normal controls were further evaluated.

RESULTS

The family segregated a REP1 mutation, suggesting choroideremia (CHM). We then found crystals in peripheral blood lymphocytes and discovered significant plasma fatty acid abnormalities and RBC membrane abnormalities (i.e., elevated plasmalogens). To replicate our discoveries, we expanded the cohort to nine CHM patients, genotyped them for REP1 mutations, and found the same abnormalities (crystals and fatty acid abnormalities) in all patients.

CONCLUSIONS

Previously, CHM was thought to be restricted to the retina. We show, to our knowledge for the first time, that CHM is a systemic condition with prominent crystals in lymphocytes and significant fatty acid abnormalities.

Pharmacological Modulation of Photoreceptor Outer Segment Degradation in a Human iPS Cell Model of Inherited Macular Degeneration

Degradation of photoreceptor outer segments (POS) by retinal pigment epithelium (RPE) is essential for vision, and studies have implicated altered POS processing in the pathogenesis of some retinal degenerative diseases. Consistent with this concept, a recently established hiPSC-RPE model of inherited macular degeneration, Best disease (BD), displayed reduced rates of POS breakdown. Herein we utilized this model to determine (i) if disturbances in protein degradation pathways are associated with delayed POS digestion and (ii) whether such defect(s) can be pharmacologically targeted. We found that BD hiPSC-RPE cultures possessed increased protein oxidation, decreased free-ubiquitin levels, and altered rates of exosome secretion, consistent with altered POS processing. Application of valproic acid (VPA) with or without rapamycin increased rates of POS degradation in our model, whereas application of bafilomycin-A1 decreased such rates. Importantly, the negative effect of bafilomycin-A1 could be fully reversed by VPA. The utility of hiPSC-RPE for VPA testing was further evident following examination of its efficacy and metabolism in a complementary canine disease model. Our findings suggest that disturbances in protein degradation pathways contribute to the POS processing defect observed in BD hiPSC-RPE, which can be manipulated pharmacologically. These results have therapeutic implications for BD and perhaps other maculopathies.

Microtubule motors transport phagosomes in the RPE, and lack of KLC1 leads to AMD-like pathogenesis

The degradation of phagosomes, derived from the ingestion of photoreceptor outer segment (POS) disk membranes, is a major role of the retinal pigment epithelium (RPE). Here, POS phagosomes were observed to associate with myosin-7a, and then kinesin-1, as they moved from the apical region of the RPE. Live-cell imaging showed that the phagosomes moved bidirectionally along microtubules in RPE cells, with kinesin-1 light chain 1 (KLC1) remaining associated in both directions and during pauses. Lack of KLC1 did not inhibit phagosome speed, but run length was decreased, and phagosome localization and degradation were impaired. In old mice, lack of KLC1 resulted in RPE pathogenesis that was strikingly comparable to aspects of age-related macular degeneration (AMD), with an excessive accumulation of RPE and sub-RPE deposits, as well as oxidative and inflammatory stress responses. These results elucidate mechanisms of POS phagosome transport in relation to degradation, and demonstrate that defective microtubule motor transport in the RPE leads to phenotypes associated with AMD.

A comparison of some organizational characteristics of the mouse central retina and the human macula

Mouse models have greatly assisted our understanding of retinal degenerations. However, the mouse retina does not have a macula, leading to the question of whether the mouse is a relevant model for macular degeneration. In the present study, a quantitative comparison between the organization of the central mouse retina and the human macula was made, focusing on some structural characteristics that have been suggested to be important in predisposing the macula to stresses leading to degeneration: photoreceptor density, phagocytic load on the RPE, and the relative thinness of Bruch’s membrane. Light and electron microscopy measurements from retinas of two strains of mice, together with published data on human retinas, were used for calculations and subsequent comparisons. As in the human retina, the central region of the mouse retina possesses a higher photoreceptor cell density and a thinner Bruch’s membrane than in the periphery; however, the magnitudes of these periphery to center gradients are larger in the human. Of potentially greater relevance is the actual photoreceptor cell density, which is much greater in the mouse central retina than in the human macula, underlying a higher phagocytic load for the mouse RPE. Moreover, at eccentricities that correspond to the peripheral half of the human macula, the rod to cone ratio is similar between mouse and human. Hence, with respect to photoreceptor density and phagocytic load of the RPE, the central mouse retina models at least the more peripheral part of the macula, where macular degeneration is often first evident.

Multimodal assessment of choroideremia patients defines pre-treatment characteristics

PURPOSE

Choroideremia (CHM) is a X-chromosomal disorder leading to blindness by progressive degeneration of choroid, retinal pigment epithelium (RPE), and retinal neurons. A current clinical gene therapy trial (NCT01461213) showed promising safety and efficacy data in a carefully selected patient population. The present study was performed to shed light on pre-treatment characteristics of a larger cohort of CHM patients using a high resolution multi-modal approach.

METHODS

In a retrospective cross-sectional study, data from 58 eyes of 29 patients with clinically confirmed CHM were analysed including best-corrected visual acuity (BCVA), refractive error, spectral-domain optical coherence tomography (SD-OCT), fundus autofluorescence (FAF), perimetry, and tonometry. Residual retinal volume, area of residual RPE, and foveal thickness were quantified to further define natural disease progression and assess symmetry.

RESULTS

We evaluated 98 data points of BCVA [0.34 ± 0.06 (logMAR); mean ± 95 % confidence interval], 80 of IOP (14.6 ± 0.6 mmHg), and 98 of refraction (-2.16 ± 1.08 spherical equivalent). Visual fields (n  = 76) demonstrated variable degrees of concentric constriction (54 % <10°, 25 % 10-30°, 21 % >30°). Mean residual RPE area on FAF (n  = 64) measured 8.47 ± 1.91 mm(2) (range 0.30-38.5 mm(2)), while mean neuroretinal volume (n  = 42) was found to be 1.76 ± 0.12 mm(3). Age at examination was exponentially associated with BCVA, while logarithmic functions best described progressive loss of retinal area and volume. A high degree of left to right symmetry was found in all modalities with structural markers showing the best correlation (r (2) area = 0.83; r (2) volume = 0.75).

CONCLUSION

Analysis of these widely available clinical data defines the natural disease characteristics of a relevant patient population eligible for gene therapeutic intervention. In the wake of preliminary reports on safety and efficacy of CHM gene therapy (NCT01461213), this multi-modal assessment of a cohort of CHM patients provides important evidence of the natural rate of disease progression and degree of symmetry between eyes.

Cleavage of Mer tyrosine kinase (MerTK) from the cell surface contributes to the regulation of retinal phagocytosis

Phagocytosis of apoptotic cells by macrophages and spent photoreceptor outer segments (POS) by retinal pigment epithelial (RPE) cells requires several proteins, including MerTK receptors and associated Gas6 and protein S ligands. In the retina, POS phagocytosis is rhythmic, and MerTK is activated promptly after light onset via the αvβ5 integrin receptor and its ligand MFG-E8, thus generating a phagocytic peak. The phagocytic burst is limited in time, suggesting a down-regulation mechanism that limits its duration. Our previous data showed that MerTK helps control POS binding of integrin receptors at the RPE cell surface as a negative feedback loop. Our present results show that a soluble form of MerTK (sMerTK) is released in the conditioned media of RPE-J cells during phagocytosis and in the interphotoreceptor matrix of the mouse retina during the morning phagocytic peak. In contrast to macrophages, the two cognate MerTK ligands have an opposite effect on phagocytosis and sMerTK release, whereas the integrin ligand MFG-E8 markedly increases both phagocytosis and sMerTK levels. sMerTK acts as a decoy receptor blocking the effect of both MerTK ligands. Interestingly, stimulation of sMerTK release decreases POS binding. Conversely, blocking MerTK cleavage increased mostly POS binding by RPE cells. Therefore, our data suggest that MerTK cleavage contributes to the acute regulation of RPE phagocytosis by limiting POS binding to the cell surface.

The role of rab proteins in neuronal cells and in the trafficking of neurotrophin receptors

Neurotrophins are a family of proteins that are important for neuronal development, neuronal survival and neuronal functions. Neurotrophins exert their role by binding to their receptors, the Trk family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to receptors triggers a complex series of signal transduction events, which are able to induce neuronal differentiation but are also responsible for neuronal maintenance and neuronal functions. Rab proteins are small GTPases localized to the cytosolic surface of specific intracellular compartments and are involved in controlling vesicular transport. Rab proteins, acting as master regulators of the membrane trafficking network, play a central role in both trafficking and signaling pathways of neurotrophin receptors. Axonal transport represents the Achilles' heel of neurons, due to the long-range distance that molecules, organelles and, in particular, neurotrophin-receptor complexes have to cover. Indeed, alterations of axonal transport and, specifically, of axonal trafficking of neurotrophin receptors are responsible for several human neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis and some forms of Charcot-Marie-Tooth disease. In this review, we will discuss the link between Rab proteins and neurotrophin receptor trafficking and their influence on downstream signaling pathways.

Phagosome maturation during endosome interaction revealed by partial rhodopsin processing in retinal pigment epithelium

Defects in phagocytosis and degradation of photoreceptor outer segments (POS) by the retinal pigment epithelium (RPE) are associated with aging and retinal disease. The daily burst of rod outer segment (ROS) phagocytosis by the RPE provides a unique opportunity to analyse phagosome processing in vivo. In mouse retinae, phagosomes containing stacked rhodopsin-rich discs were identified by immuno-electron microscopy. Early apical phagosomes stained with antibodies against both cytoplasmic and intradiscal domains of rhodopsin. During phagosome maturation, a remarkably synchronised loss of the cytoplasmic epitope coincided with movement to the cell body and preceded phagosome-lysosome fusion and disc degradation. Loss of the intradiscal rhodopsin epitope and disc digestion occurred upon fusion with cathepsin-D-positive lysosomes. The same sequential stages of phagosome maturation were identified in cultured RPE and macrophages challenged with isolated POS. Loss of the cytoplasmic rhodopsin epitope was insensitive to pH but sensitive to protease inhibition and coincided with the interaction of phagosomes with endosomes. Thus, during pre-lysosomal maturation of ROS-containing phagosomes, limited rhodopsin processing occurs upon interaction with endosomes. This potentially provides a sensitive readout of phagosome-endosome interactions that is applicable to multiple phagocytes.

Cholesterol in the retina: the best is yet to come

Historically understudied, cholesterol in the retina is receiving more attention now because of genetic studies showing that several cholesterol-related genes are risk factors for age-related macular degeneration (AMD) and because of eye pathology studies showing high cholesterol content of drusen, aging Bruch's membrane, and newly found subretinal lesions. The challenge before us is determining how the cholesterol-AMD link is realized. Meeting this challenge will require an excellent understanding these genes' roles in retinal physiology and how chorioretinal cholesterol is maintained. In the first half of this review, we will succinctly summarize physico-chemical properties of cholesterol, its distribution in the human body, general principles of maintenance and metabolism, and differences in cholesterol handling in human and mouse that impact on experimental approaches. This information will provide a backdrop to the second part of the review focusing on unique aspects of chorioretinal cholesterol homeostasis, aging in Bruch's membrane, cholesterol in AMD lesions, a model for lesion biogenesis, a model for macular vulnerability based on vascular biology, and alignment of AMD-related genes and pathobiology using cholesterol and an atherosclerosis-like progression as unifying features. We conclude with recommendations for the most important research steps we can take towards delineating the cholesterol-AMD link.

R9AP overexpression alters phototransduction kinetics in iCre75 mice

PURPOSE

Determine the impact of rod photoreceptor-specific expression of Cre recombinase on the kinetics of phototransduction in the mouse eye and identify changes in gene expression that underlie any observed phenotypic differences.

METHODS

Transretinal ERG and single-cell suction electrode recordings were used to measure the kinetics of phototransduction in a mouse line exhibiting rod photoreceptor-specific Cre recombinase expression, and the results were compared with those from control non-Cre-expressing littermates. Gene expression changes were evaluated using RNA sequencing transcriptome analysis. The pattern of expression of Rgs9bp was determined by mapping sequencing reads to the mouse genome and performing 3'-rapid amplification of cDNA ends (3'-RACE).

RESULTS

Expression of the rod-specific iCre75 transgene was accompanied by accelerated phototransduction inactivation, likely due to overexpression of the Rgs9bp gene, which encodes the Rgs9 anchor protein (R9AP). R9AP upregulation stabilized the RGS9 GAP complex, altering phototransduction kinetics. 3'-Race identified an abundant, unexpected Rgs9bp-Prm1 fusion mRNA in Cre-expressing mouse retinas, which was determined to be derived from a second transgene present in the iCre75 line.

CONCLUSIONS

Here we report the presence of a second, R9AP-expressing transgene in the iCre75 mouse line, leading to altered kinetics of phototransduction. These results highlight an important caveat that must be considered when utilizing this mouse line for rod photoreceptor-specific gene loss of function studies.