Canine retina has a primate fovea-like bouquet of cone photoreceptors which is affected by inherited macular degenerations

Canine Best disease as a translational model

In this review, we summarize the findings of several pre-clinical studies in the canine BEST1 disease model. To this end, client-owned and purpose bred dogs that were compound heterozygotes or homozygotes, respectively, for two or one of 3 different mutations in BEST1 were evaluated by ophthalmic examination, cSLO/sdOCT imaging, and retinal immunohistochemistry to characterize the clinical and microanatomic features of the disease. Subsequently AAV-mediated gene therapy was done to transfer the BEST1 transgene to the RPE under control of a hVMD2 promoter. We demonstrated that canine bestrophinopathies are an RPE-photoreceptor interface disease with underdeveloped RPE apical microvilli that invest rod and cone outer segments. This leads to microdetachments which later progress to clinically evident RPE-retinal separation and a spectrum of disease stages, ranging from vitelliform to vitelliruptive/atrophic lesions, similar to Best Vitelliform Macular Dystrophy (BVMD). Gene therapy corrects the microdetachments and reverses large lesions when delivered at the pseudohypopyon stage of disease. Because of the similar clinical and microstructural abnormalities between the canine model and BVMD, and positive response to gene therapy, the canine model is a valuable translational model for developing gene and other therapies for BVMD.

Canine models of inherited retinal diseases: from neglect to well-recognized translational value

Large animal models of inherited retinal diseases, particularly dogs, have been extensively used over the past decades to study disease natural history and evaluate therapeutic interventions. Our group of investigators at the University of Pennsylvania, School of Veterinary Medicine, has played a pivotal role in characterizing several of these animal models, documenting the natural history of their diseases, developing gene therapies, and conducting proof-of-concept studies. Additionally, we have assessed the potential toxicity of these therapies for human clinical trials, contributing to the regulatory approval of voretigene neparvovec-rzyl (Luxturna®) by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of patients with confirmed biallelic mutation-associated retinal dystrophy. In this review, we aim to summarize the clinical features of a subset of these diseases and reflect on the challenges encountered in integrating canine models into the translational pipeline.

Mapping protein distribution in the canine photoreceptor sensory cilium and calyceal processes by ultrastructure expansion microscopy

Photoreceptors are highly polarized sensory neurons, possessing a unique ciliary structure known as the photoreceptor sensory cilium (PSC). Vertebrates have two subtypes of photoreceptors: rods, which are responsible for night vision, and cones, which support daylight vision and color perception. Despite identifying functional and morphological differences between these subtypes, ultrastructural analyses of the PSC molecular architecture in rods and cones are still lacking. In this study, we employed ultrastructure expansion microscopy (U-ExM) to characterize the molecular architecture of the PSC in canine retina. We demonstrated that U-ExM is applicable to both non-frozen and cryopreserved retinal tissues with standard paraformaldehyde fixation. Using this validated U-ExM protocol, we revealed the molecular localization of numerous ciliopathy-related proteins in canine photoreceptors. Furthermore, we identified significant architectural differences in the PSC, ciliary rootlet, and calyceal processes between canine rods and cones. These findings pave the way for a better understanding of alterations in the molecular architecture of the PSC in canine models of retinal ciliopathies.

Differential gene expression between central and peripheral retinal regions in dogs and comparison with humans

The dog retina contains a central macula-like region, and there are reports of central retinal disorders in dogs with shared genetic etiologies with humans. Defining central/peripheral gene expression profiles may provide insight into the suitability of dogs as models for human disorders. We determined central/peripheral posterior eye gene expression profiles in dogs and interrogated inherited retinal and macular disease-associated genes for differential expression between central and peripheral regions. Bulk tissue RNA sequencing was performed on 8 mm samples of the dog central and superior peripheral regions, sampling retina and retinal pigmented epithelium/choroid separately. Reads were mapped to CanFam3.1, read counts were analyzed to determine significantly differentially expressed genes (DEGs). A similar analytic pipeline was used with a published bulk-tissue RNA sequencing human dataset. Pathways and processes involved in significantly DEGs were identified (Database for Annotation, Visualization and Integrated Discovery). Dogs and humans shared the extent and direction of central retinal differential gene expression, with multiple shared biological pathways implicated in differential expression. Many genes implicated in heritable retinal disorders in dogs and humans were differentially expressed between central and periphery. Approximately half of genes associated with human age-related macular degeneration were differentially expressed in human and dog tissues. We have identified similarities and differences in central/peripheral gene expression profiles between dogs and humans which can be applied to further define the relevance of dogs as models for human retinal disorders.

Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retinal Ganglion Cell Axon Bundles

We seek to develop techniques for high-resolution imaging of the tree shrew retina for visualizing and parameterizing retinal ganglion cell (RGC) axon bundles in vivo. We applied visible-light optical coherence tomography fibergraphy (vis-OCTF) and temporal speckle averaging (TSA) to visualize individual RGC axon bundles in the tree shrew retina. For the first time, we quantified individual RGC bundle width, height, and cross-sectional area and applied vis-OCT angiography (vis-OCTA) to visualize the retinal microvasculature in tree shrews. Throughout the retina, as the distance from the optic nerve head (ONH) increased from 0.5 mm to 2.5 mm, bundle width increased by 30%, height decreased by 67%, and cross-sectional area decreased by 36%. We also showed that axon bundles become vertically elongated as they converge toward the ONH. Ex vivo confocal microscopy of retinal flat-mounts immunostained with Tuj1 confirmed our in vivo vis-OCTF findings.

A combination treatment based on drug repurposing demonstrates mutation-agnostic efficacy in pre-clinical retinopathy models

Inherited retinopathies are devastating diseases that in most cases lack treatment options. Disease-modifying therapies that mitigate pathophysiology regardless of the underlying genetic lesion are desirable due to the diversity of mutations found in such diseases. We tested a systems pharmacology-based strategy that suppresses intracellular cAMP and Ca2+ activity via G protein-coupled receptor (GPCR) modulation using tamsulosin, metoprolol, and bromocriptine coadministration. The treatment improves cone photoreceptor function and slows degeneration in Pde6βrd10 and RhoP23H/WT retinitis pigmentosa mice. Cone degeneration is modestly mitigated after a 7-month-long drug infusion in PDE6A-/- dogs. The treatment also improves rod pathway function in an Rpe65-/- mouse model of Leber congenital amaurosis but does not protect from cone degeneration. RNA-sequencing analyses indicate improved metabolic function in drug-treated Rpe65-/- and rd10 mice. Our data show that catecholaminergic GPCR drug combinations that modify second messenger levels via multiple receptor actions provide a potential disease-modifying therapy against retinal degeneration.

Characterization of the development of the high-acuity area of the chick retina

The fovea is a small region within the central retina that is responsible for our high acuity daylight vision. Chickens also have a high acuity area (HAA), and are one of the few species that enables studies of the mechanisms of HAA development, due to accessible embryonic tissue and methods to readily perturb gene expression. To enable such studies, we characterized the development of the chick HAA using single molecule fluorescent in situ hybridization (smFISH), along with more classical methods. We found that Fgf8 provides a molecular marker for the HAA throughout development and into adult stages, allowing studies of the cellular composition of this area over time. The radial dimension of the ganglion cell layer (GCL) was seen to be the greatest at the HAA throughout development, beginning during the period of neurogenesis, suggesting that genesis, rather than cell death, creates a higher level of retinal ganglion cells (RGCs) in this area. In contrast, the HAA acquired its characteristic high density of cone photoreceptors post-hatching, which is well after the period of neurogenesis. We also confirmed that rod photoreceptors are not present in the HAA. Analyses of cell death in the developing photoreceptor layer, where rods would reside, did not show apoptotic cells, suggesting that lack of genesis, rather than death, created the "rod-free zone" (RFZ). Quantification of each cone photoreceptor subtype showed an ordered mosaic of most cone subtypes. The changes in cellular densities and cell subtypes between the developing and mature HAA provide some answers to the overarching strategy used by the retina to create this area and provide a framework for future studies of the mechanisms underlying its formation.

[Canine inherited retinal degeneration as model to study disease mechanisms and therapy for ciliopathies]

Humans have a highly developed retina and obtain approximately 80% of their external information from vision. Photoreceptor cells, which are located in the outermost layer of the neuroretina and recognize light signals, are highly specialized sensory cilia that share structural and functional features with primary cilia. Genetic disorders of the retina or photoreceptor cells are termed inherited retinal diseases (IRDs) and are caused by variants in one of more than 280 genes identified to date. Among the genes responsible for IRDs, many are shared with those responsible for ciliopathies. In studies of inherited diseases, mouse models are commonly used due to their advantages in breeding, handling, and relative feasibility in creating pathological models. On the other hand, structural, functional, and genetic differences in the retina between mice and humans can be a barrier in IRD research. To overcome the limitations of mouse models, larger vertebrate models of IRDs can be a useful research subject. In particular, canines have retinas that are structurally and functionally similar and eyes that are anatomically comparable to those of humans. In addition, due to their unique veterinary clinical surveillance and genetic background, naturally occurring canine IRDs are more likely to be identified than in other large animals. To date, pathogenic mutations related to canine IRDs have been identified in more than 30 genes, contributing to the understanding of pathogeneses and to the development of new therapies. This review provides an overview of the roles of the canine IRD models in ciliopathy research.

Canine and Feline Models of Inherited Retinal Diseases

Naturally occurring inherited retinal diseases (IRDs) in cats and dogs provide a rich source of potential models for human IRDs. In many cases, the phenotypes between the species with mutations of the homologous genes are very similar. Both cats and dogs have a high-acuity retinal region, the area centralis, an equivalent to the human macula, with tightly packed photoreceptors and higher cone density. This and the similarity in globe size to that of humans means these large animal models provide information not obtainable from rodent models. The established cat and dog models include those for Leber congenital amaurosis, retinitis pigmentosa (including recessive, dominant, and X-linked forms), achromatopsia, Best disease, congenital stationary night blindness and other synaptic dysfunctions, RDH5-associated retinopathy, and Stargardt disease. Several of these models have proven to be important in the development of translational therapies such as gene-augmentation therapies. Advances have been made in editing the canine genome, which necessitated overcoming challenges presented by the specifics of canine reproduction. Feline genome editing presents fewer challenges. We can anticipate the generation of specific cat and dog IRD models by genome editing in the future.

The Mongolian gerbil as an advanced model to study cone system physiology

In this work, we introduce a diurnal rodent, the Mongolian gerbil (Meriones unguiculatus) (MG) as an alternative to study retinal cone system physiology and pathophysiology in mice. The cone system is of particular importance, as it provides high-acuity and color vision and its impairment in retinal disorders is thus especially disabling. Despite their nocturnal lifestyle, mice are currently the most popular animals to study cone-related diseases due to the high availability of genetically modified models. However, the potential for successful translation of any cone-related results is limited due to the substantial differences in retinal organization between mice and humans. Alternatively, there are diurnal rodents such as the MG with a higher retinal proportion of cones and a macula-like specialized region for improved visual resolution, the visual streak. The focus of this work was the evaluation of the MG's cone system functionality using full-field electroretinography (ERG), together with a morphological assessment of its retinal/visual streak organization via angiography, optical coherence tomography (OCT), and photoreceptor immunohistochemistry. We found that rod system responses in MGs were comparable or slightly inferior to mice, while in contrast, cone system responses were much larger, more sensitive, and also faster than those in the murine counterparts, and in addition, it was possible to record sizeable ON and OFF ERG components. Morphologically, MG cone photoreceptor opsins were evenly distributed throughout the retina, while mice show a dorsoventral M- and S-opsin gradient. Additionally, each cone expressed a single opsin, in contrast to the typical co-expression of opsins in mice. Particular attention was given to the visual streak region, featuring a higher density of cones, elongated cone and rod outer segments (OSs), and an increased thickness of the inner and outer retinal layers in comparison to peripheral regions. In summary, our data render the MG a supreme model to investigate cone system physiology, pathophysiology, and to validate potential therapeutic strategies in that context.

Cone-Driven, Geniculocortical Responses in Canine Models of Outer Retinal Disease

Purpose

Canine models of inherited retinal degeneration are used for proof of concept of emerging gene and cell-based therapies that aim to produce functional restoration of cone-mediated vision. We examined functional magnetic resonance imaging (MRI) measures of the postretinal response to cone-directed stimulation in wild-type (WT) dogs, and in three different retinal disease models.

Methods

Temporal spectral modulation of a uniform field of light around a photopic background was used to target the canine L/M (hereafter "L") and S cones and rods. Stimuli were designed to separately target the postreceptoral luminance (L+S) and chrominance (L-S) pathways, the rods, and all photoreceptors jointly (light flux). These stimuli were presented to WT, and mutant PDE6B-RCD1, RPGR-XLPRA2, and NPHP5-CRD2 dogs during pupillometry and functional MRI (fMRI).

Results

Pupil responses in WT dogs to light flux, L+S, and rod-directed stimuli were consistent with responses being driven by cone signals alone. For WT animals, both luminance and chromatic (L-S) stimuli evoked fMRI responses in the lateral geniculate nucleus or visual cortex; RCD1 animals with predominant rod loss had similar responses. Responses to cone-directed stimulation were reduced in XLPRA2 and absent in CRD2. NPHP5 gene augmentation restored the cortical response to luminance stimulation in a CRD2 animal.

Conclusions

Cone-directed stimulation during fMRI can be used to measure the integrity of luminance and chrominance responses in the dog visual system. The NPHP5-CRD2 model is appealing for studies of recovered cone function.

Translational Relevance

fMRI assessment of cone-driven cortical response provides a tool to translate cell/gene therapies for vision restoration.

Screen interaction behavior in companion dogs: results from a dog owner survey

Despite availability of video content marketed for dog (Canis familiaris) entertainment, there is little information on dog behaviors when viewing content, nor describing which content is engaging. The aims of this study were to define demographics of dogs that engage with screens, owner observed behaviors, and perceived content interest. A digital survey was distributed to dog owners (03/2022-03/2023). We collected demographics, home environment, owner-rated behaviors, content interest, and interest in 4 presented videos. We compared the representation of dogs from different purebred dog groups (categorized by job/purpose by the American Kennel Club) with the estimated general purebred dog population. Most respondents (total n=1,246) lived in the USA (89%). Median age was 4 years, 54% were purebred, 51% were female. Most (86%, n=1,077) stated their dog watched screen content. Excitement behaviors were often described: 78% of dogs approached the screen, 76% vocalized. Many owners played videos for their dogs when left alone. Dogs most frequently engaged with animal content; dogs were the most popular animal. Age and visual status influenced the frequency of perceived interaction; age and breed influenced content interest. Within purebred dogs that were stated to watch content, there was a relative over-representation of "sporting" and "herding"-type breeds. A dog's age, visual status, and breed type may influence their interest in video content at home. Because many owners reported excitement in their dogs in reaction to screen content, owners may wish to determine whether video content would be suitable for use when their dogs are left alone.

Cone-driven, geniculo-cortical responses in canine models of outer retinal disease

Purpose

Canine models of inherited retinal degeneration are used for proof-of-concept of emerging gene and cell-based therapies that aim to produce functional restoration of cone-mediated vision. We examined functional MRI measures of the post-retinal response to cone-directed stimulation in wild type (WT) dogs, and in three different retinal disease models.

Methods

Temporal spectral modulation of a uniform field of light around a photopic background was used to target the canine L/M (hereafter "L") and S cones and rods. Stimuli were designed to separately target the post-receptoral luminance (L+S) and chrominance (L-S) pathways, the rods, and all photoreceptors jointly (light flux). These stimuli were presented to WT, and mutant PDE6B-RCD1, RPGR-XLPRA2, and NPHP5-CRD2 dogs during pupillometry and fMRI.

Results

Pupil responses in WT dogs to light flux, L+S, and rod-directed stimuli were consistent with responses being driven by cone signals alone. For WT animals, both luminance and chromatic (L-S) stimuli evoked fMRI responses in the lateral geniculate nucleus (LGN) or visual cortex; RCD1 animals with predominant rod loss had similar responses. Responses to cone-directed stimulation were reduced in XLPRA2 and absent in CRD2. NPHP5 gene augmentation restored the cortical response to luminance stimulation in a CRD2 animal.

Conclusions

Cone-directed stimulation during fMRI can be used to measure the integrity of luminance and chrominance responses in the dog visual system. The NPHP5-CRD2 model is appealing for studies of recovered cone function.

Translational Relevance

fMRI assessment of cone driven cortical response provides a tool to translate cell/gene therapies for vision restoration.

Subjective vision assessment in companion dogs using dogVLQ demonstrates age-associated visual dysfunction

Introduction

Dim light vision as assessed by proxy and clinical tools is commonly impaired in older humans and impacts quality of life. Although proxy visual assessment tools have been developed for dogs, it is unclear if they are sensitive enough to detect subtle visual dysfunction in older dogs. We sought to determine if a newly designed proxy visual function questionnaire could detect age-associated differences in visual behaviors in varying lighting conditions in dogs.

Methods

A 27-item questionnaire (the dog variable lighting questionnaire, dogVLQ) was designed to assess visual behavior in dogs in different lighting settings. We conducted the dogVLQ, a previously validated visual function questionnaire the dog vision impairment score and performed light- and dark-adapted electroretinography (ERG) on a subset of dogs. Questionnaire scores were analyzed for dog age associations using correlation analysis.

Results

Questionnaire responses from 235 dog owners were obtained (122 female, 112 male dogs), 79 of which underwent ERG (43 female, 36 male dogs). Bright light visual behavior was significantly associated with light-adapted bright flash ERG amplitudes, visual behavior in near darkness was associated with dark-adapted ERG amplitudes. The dogVLQ identified worse vision in older dogs in bright light, dim light, and darkness; predicted onset was younger for vision in near darkness. Older dogs had more difficulty navigating transitions between lighting conditions.

Discussion

Subjective dog owner assessment of visual function associates with objective measurement of retinal function in dogs and supports reduced vision-mediated behaviors in older dogs.

Age-associated changes in electroretinography measures in companion dogs

PURPOSE

To determine the association between age and retinal full-field electroretinographic (ERG) measures in companion (pet) dogs, an important translational model species for human neurologic aging.

METHODS

Healthy adult dogs with no significant ophthalmic abnormalities were included. Unilateral full-field light- and dark-adapted electroretinography was performed using a handheld device, with mydriasis and topical anaesthesia. Partial least squares effect screening analysis was performed to determine the effect of age, sex, body weight and use of anxiolytic medication on log-transformed ERG peak times and amplitudes; age and anxiolytic usage had significant effects on multiple ERG outcomes. Mixed model analysis was performed on data from dogs not receiving anxiolytic medications.

RESULTS

In dogs not receiving anxiolytics, median age was 118 months (interquartile range 72-140 months, n = 77, 44 purebred, 33 mixed breed dogs). Age was significantly associated with prolonged peak times of a-waves (dark-adapted 3 and 10 cds/m2 flash p < 0.0001) and b-waves (cone flicker p = 0.03, dark-adapted 0.01 cds/m2 flash p = 0.001). Age was also significantly associated with reduced amplitudes of a-waves (dark-adapted 3 cds/m2 flash p < 0.0001, 10 cds/m2 flash p = 0.005) and b-waves (light-adapted 3 cds/m2 flash p < 0.0001, dark-adapted 0.01 cds/m2 flash p = 0.0004, 3 cds/m2 flash p < 0.0001, 10 cds/m2 flash p = 0.007) and flicker (light-adapted 30 Hz 3 cds/m2 p = 0.0004). Within the Golden Retriever breed, these trends were matched in a cross-sectional analysis of 6 individuals that received no anxiolytic medication.

CONCLUSIONS

Aged companion dogs have slower and reduced amplitude responses in both rod- and cone-mediated ERG. Consideration of anxiolytic medication use should be made when conducting ERG studies in dogs.

Visual perception of emotion cues in dogs: a critical review of methodologies

Comparative studies of human-dog cognition have grown exponentially since the 2000's, but the focus on how dogs look at us (as well as other dogs) as social partners is a more recent phenomenon despite its importance to human-dog interactions. Here, we briefly summarise the current state of research in visual perception of emotion cues in dogs and why this area is important; we then critically review its most commonly used methods, by discussing conceptual and methodological challenges and associated limitations in depth; finally, we suggest some possible solutions and recommend best practice for future research. Typically, most studies in this field have concentrated on facial emotional cues, with full body information rarely considered. There are many challenges in the way studies are conceptually designed (e.g., use of non-naturalistic stimuli) and the way researchers incorporate biases (e.g., anthropomorphism) into experimental designs, which may lead to problematic conclusions. However, technological and scientific advances offer the opportunity to gather much more valid, objective, and systematic data in this rapidly expanding field of study. Solving conceptual and methodological challenges in the field of emotion perception research in dogs will not only be beneficial in improving research in dog-human interactions, but also within the comparative psychology area, in which dogs are an important model species to study evolutionary processes.

Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retinal Ganglion Cell Axon Bundles

We seek to develop techniques for high-resolution imaging of the tree shrew retina for visualizing and parameterizing retinal ganglion cell (RGC) axon bundles in vivo. We applied visible-light optical coherence tomography fibergraphy (vis-OCTF) and temporal speckle averaging (TSA) to visualize individual RGC axon bundles in the tree shrew retina. For the first time, we quantified individual RGC bundle width, height, and cross-sectional area and applied vis-OCT angiography (vis-OCTA) to visualize the retinal microvasculature in tree shrews. Throughout the retina, as the distance from the optic nerve head (ONH) increased from 0.5 mm to 2.5 mm, bundle width increased by 30%, height decreased by 67%, and cross-sectional area decreased by 36%. We also showed that axon bundles become vertically elongated as they converge toward the ONH. Ex vivo confocal microscopy of retinal flat-mounts immunostained with Tuj1 confirmed our in vivo vis-OCTF findings.

Photochemical Restoration of Light Sensitivity in the Degenerated Canine Retina

Photopharmacological compounds such as azobenzene-based photoswitches have been shown to control the conductivity of ionic channels in a light-dependent manner and are considered a potential strategy to restore vision in patients with end-stage photoreceptor degeneration. Here, we report the effects of DENAQ, a second-generation azobenzene-based photoswitch on retinal ganglion cells (RGC) in canine retinas using multi-electrode array (MEA) recordings (from nine degenerated and six WT retinas). DENAQ treatment conferred increased light sensitivity to RGCs in degenerated canine retinas. RGC light responses were observed in degenerated retinas following ex vivo application of 1 mM DENAQ (n = 6) or after in vivo DENAQ injection (n = 3, 150 μL, 3-10 mM) using 455 nm light at intensities as low as 0.2 mW/cm². The number of light-sensitive cells and the per cell response amplitude increased with light intensity up to the maximum tested intensity of 85 mW/cm². Application of DENAQ to degenerated retinas with partially preserved cone function caused appearance of DENAQ-driven responses both in cone-driven and previously non-responsive RGCs, and disappearance of cone-driven responses. Repeated stimulation slowed activation and accelerated recovery of the DENAQ-driven responses. The latter is likely responsible for the delayed appearance of a response to 4 Hz flicker stimulation. Limited aqueous solubility of DENAQ results in focal drug aggregates associated with ocular toxicity. While this limits the therapeutic potential of DENAQ, more potent third-generation photoswitches may be more promising, especially when delivered in a slow-release formulation that prevents drug aggregation.

Retinal Vascular Plexuses Are Unequally Affected in Canine Inherited Retinal Degenerations

Purpose

To characterize the progression of vascular changes that occur in each retinal plexus, in three canine models of inherited retinal degeneration.

Methods

In this retrospective cohort study, we examined the retinal imaging records of 44 dogs from a research colony that had undergone optical coherence tomography angiography (OCTA) imaging. Animals enrolled included crd2/NPHP5 and xlpra2/RPGR mutant dogs imaged at different stages of photoreceptor loss, as well as RHO^T4R/+ dogs after acute light-induced rod degeneration. Also included were normal controls imaged at similar ages. OCT angiograms of the superficial vascular plexus combined with the intermediate capillary plexus (SVP + ICP), and the deep capillary plexus (DCP) were analyzed using the AngioTool software to calculate vessel density and other vascular parameters.

Results

A reduction in vessel density was seen over time in both the SVP + ICP and DCP in all mutant dogs but was more pronounced in the DCP. Scans were subclassified based on outer nuclear layer (ONL) thinning compared to age-matched normal controls. When ONL loss was 0% to 50%, vessel density in the DCP was significantly lower than in age-matched controls. In all cases, when ONL loss exceeded 87.5%, vessel density in the SVP + ICP was significantly reduced as well. In the acute light-induced rod degeneration model, the vascular regression changes were observed mainly in the DCP.

Conclusions

Vessel density reduction in dogs undergoing retinal degeneration is first detected by OCTA in the DCP, and only at later stages in the SVP + ICP.

Quantitative and qualitative characterization of retinal dystrophies in canine models of inherited retinal diseases using spectral domain optical coherence tomography (SD-OCT)

The purpose of this study was to establish spectral domain optical coherence tomography (SD-OCT) assessment data in well-established canine models of inherited retinal dystrophies: PDE6B-rod-cone dysplasia 1 (RCD1: early onset retinitis pigmentosa), PRCD-progressive rod-cone degeneration (PRCD: late onset retinitis pigmentosa), CNGB3-achromatopsia, and RPE65-Leber congenital amaurosis (LCA). High resolution SD-OCT images of the retina were acquired from both eyes in 5 planes: temporal; superotemporal; superior; nasal; and inferior in adult dogs with: RCD1 (n = 4 dogs, median age: 1.5 yrs); PRCD (n = 2, 4.3 yrs); LCA (n = 3, 5.2 yrs); achromatopsia (n = 3, 4.2 yrs); and wild types (wt, n = 6, 5.5 yrs). Total, inner and outer retinal thicknesses and ellipsoid zone were analyzed. In selected animals, histomorphometric evaluations were performed. In dogs with RCD1, PRCD, and LCA, the thickness of the outer retina was, compared to wt, significantly decreased (p ≤ 0.02) in all OCT imaging planes, and in superotemporal and inferior imaging planes in dogs with achromatopsia. No significant thinning was observed in inner retina thickness in any disease model except in the inferior imaging plane in dogs with RCD1. Dogs with RCD1, PRCD, and LCA had significantly more areas with disrupted ellipsoid zone in the presumed area centralis than wt (p ≤ 0.001). OCT findings provide baseline information for research of retinal dystrophies using these canine models.

Outer retinal thickness and visibility of the choriocapillaris in four distinct retinal regions imaged with spectral domain optical coherence tomography in dogs and cats

PURPOSE

To evaluate the outer retinal band thickness and choriocapillaris (CC) visibility in four distinct retinal regions in dogs and cats imaged with spectral domain optical coherence tomography (SD-OCT). To attempt delineation of a fovea-like region in canine and feline SD-OCT scans, aided by the identification of outer retinal thickness differences between retinal regions.

METHODS

Spectralis® HRA + OCT SD-OCT scans from healthy, anesthetized dogs (n = 10) and cats (n = 12) were analyzed. Scanlines on which the CC was identifiable were counted and CC visibility was scored. Outer nuclear layer (ONL) thickness and the distances from external limiting membrane (ELM) to retinal pigment epithelium/Bruch's membrane complex (RPE/BM) and ELM to CC were measured in the area centralis (AC), a visually identified fovea-like region, and in regions superior and inferior to the optic nerve head (ONH). Measurements were analyzed using a multilevel regression.

RESULTS

The CC was visible in over 90% of scanlines from dogs and cats. The ONL was consistently thinnest in the fovea-like region. The outer retina (ELM-RPE and ELM-CC) was thickest within the AC compared with superior and inferior to the ONH in dogs and cats (p < .001 for all comparisons).

CONCLUSIONS

The CC appears a valid, albeit less than ideal outer retinal boundary marker in tapetal species. The AC can be objectively differentiated from the surrounding retina on SD-OCT images of dogs and cats; a fovea-like region was identified in dogs and its presence was suggested in cats. These findings allow targeted imaging and image evaluation of these regions of retinal specialization.

Ten Questions for a Theory of Vision

By and large, the remarkable progress in visual object recognition in the last few years has been fueled by the availability of huge amounts of labelled data paired with powerful, bespoke computational resources. This has opened the doors to the massive use of deep learning, which has led to remarkable improvements on new challenging benchmarks. While acknowledging this point of view, in this paper I claim that the time has come to begin working towards a deeper understanding of visual computational processes that, instead of being regarded as applications of general purpose machine learning algorithms, are likely to require tailored learning schemes. A major claim of in this paper is that current approaches to object recognition lead to facing a problem that is significantly more difficult than the one offered by nature. This is because of learning algorithms that work on images in isolation, while neglecting the crucial role of temporal coherence. Starting from this remark, this paper raises ten questions concerning visual computational processes that might contribute to better solutions to a number of challenging computer vision tasks. While this paper is far from being able to provide answers to those questions, it contains some insights that might stimulate an in-depth re-thinking in object perception, while suggesting research directions in the control of object-directed action.

Developing Non-Human Primate Models of Inherited Retinal Diseases

Inherited retinal diseases (IRDs) represent a genetically and clinically heterogenous group of diseases that can eventually lead to blindness. Advances in sequencing technologies have resulted in better molecular characterization and genotype-phenotype correlation of IRDs. This has fueled research into therapeutic development over the recent years. Animal models are required for pre-clinical efficacy assessment. Non-human primates (NHP) are ideal due to the anatomical and genetic similarities shared with humans. However, developing NHP disease to recapitulate the disease phenotype for specific IRDs may be challenging from both technical and cost perspectives. This review discusses the currently available NHP IRD models and the methods used for development, with a particular focus on gene-editing technologies.

Abnormal Appearance of the Area Centralis in Labrador Retrievers With an ABCA4 Loss-of-function Mutation

PURPOSE

To study retinal appearance and morphology in Labrador retrievers (LRs) heterozygous and homozygous for an ABCA4 loss-of-function mutation.

METHODS

Ophthalmic examination, including ophthalmoscopy and simple testing of vision, was performed in five ABCA4wt/wt, four ABCA4wt/InsC, and six ABCA4InsC/InsC LRs. Retinas were also examined with confocal scanning laser ophthalmoscopy (cSLO) and optical coherence tomography (OCT). Infrared and fundus autofluorescence (FAF) images were studied, and outer nuclear layer (ONL) and neuroretinal thickness were measured in the central and peripheral area centralis.

RESULTS

Clinical signs in young ABCA4InsC/InsC LRs were subtle, whereas ophthalmoscopic findings and signs of visual impairment were obvious in old ABCA4InsC/InsC LRs. Retinal appearance and vision testing was unremarkable in heterozygous LRs regardless of age. The cSLO/OCT showed abnormal morphology including ONL thinning, abnormal outer retinal layer segmentation, and focal loss of retinal pigment epithelium in the fovea equivalent in juvenile ABCA4InsC/InsC LRs. The abnormal appearance extended into the area centralis and visual streak in middle-aged ABCA4InsC/InsC and then spread more peripherally. A mild phenotype was seen on cSLO/OCT and FAF in middle-aged to old ABCA4wt/InsC LRs.

CONCLUSIONS

Abnormal appearance and morphology in the fovea equivalent are present in juvenile ABCA4InsC/InsC. In the older affected LRs, the visual streak and then the peripheral retina also develop an abnormal appearance. Vision deteriorates slowly, but some vision is retained throughout life. Older heterozygotes may show a mild retinal phenotype but no obvious visual impairment. The ABCA4InsC/InsC LR is a potential model for ABCA4-mediated retinopathies/juvenile-onset Stargardt disease in a species with human-sized eyes.

TRANSLATIONAL RELEVANCE

The ABCA4InsC mutation causes juvenile-onset abnormal appearance of the fovea equivalent in affected dogs that slowly spreads in the retina, while only a mild phenotype is seen in older carriers. This is the first non-primate, large-animal model for ABCA4-related/STGD1 retinopathies in a species with a fovea equivalent.

Bio-Inspired Multimodal Imaging in Reduced Visibility

The visual systems found in nature rely on capturing light under different modalities, in terms of spectral sensitivities and polarization sensitivities. Numerous imaging techniques are inspired by this variety, among which, the most famous is color imaging inspired by the trichromacy theory of the human visual system. We investigate the spectral and polarimetric properties of biological imaging systems that will lead to the best performance on scene imaging through haze, i.e., dehazing. We design a benchmark experiment based on modalities inspired by several visual systems, and adapt state-of-the-art image reconstruction algorithms to those modalities. We show the difference in performance of each studied systems and discuss it in front of our methodology and the statistical relevance of our data.

Characterization of the Canine Retinal Vasculature With Optical Coherence Tomography Angiography: Comparisons With Histology and Fluorescein Angiography

Purpose: To present a methodology for quantification of the canine retinal vasculature imaged by optical coherence tomography angiography (OCTA) and validate this approach by comparison with fluorescein angiography (FA) and confocal imaging of retinal wholemounts labelled by immunohistochemistry (IHC). Methods: Six normal adult dogs underwent retinal OCTA imaging in both eyes. The images extracted from the different microvascular plexuses at eight retinal locations spanning the central and mid-peripheral fundus were analyzed using the AngioTool software. FA was performed in one eye and was compared to the OCTA images. Six eyes from three dogs were processed by IHC to examine the retinal vasculature. Results: A total of four retinal plexuses were identified by OCTA in the canine retina, and their density and topographical pattern varied with eccentricity. OCTA offered improved resolution over FA with the advantage of allowing imaging of the individual plexuses. Detection by OCTA of small vessels within the deep capillary plexus was possible and approached the level of resolution achieved with ex vivo imaging of the retinal vasculature by confocal microscopy/IHC. The plexuses herein described are analogous to human retinal vasculature. Conclusion: OCTA can be used to image and quantify non-invasively the vascular retinal networks of the canine retina. We provide normative data in eight different retinal locations that can be imaged non-invasively with this technology. This could support analysis of retinal vascular changes associated with disease and following therapeutic intervention.

Scientific and Regulatory Policy Committee Points to Consider: Fixation, Trimming, and Sectioning of Nonrodent Eyes and Ocular Tissues for Examination in Ocular and General Toxicity Studies

A Working Group of the Society of Toxicologic Pathology's Scientific and Regulatory Policy Committee conducted a technical and scientific review of current practices relating to the fixation, trimming, and sectioning of the nonrodent eye to identify key points and species-specific anatomical landmarks to consider when preparing and evaluating eyes of rabbits, dogs, minipigs, and nonhuman primates from ocular and general toxicity studies. The topics addressed in this Points to Consider article include determination of situations when more comprehensive evaluation of the globe and/or associated extraocular tissues should be implemented (expanded ocular sampling), and what constitutes expanded ocular sampling. In addition, this manuscript highlights the practical aspects of fixing, trimming, and sectioning the eye to ensure adequate histopathological evaluation of all major ocular structures, including the cone-dense areas (visual streak/macula/fovea) of the retina for rabbits, dogs, minipigs, and nonhuman primates, which is a current regulatory expectation for ocular toxicity studies.[Box: see text].

Subretinal Transplantation of Human Embryonic Stem Cell-Derived Retinal Tissue in a Feline Large Animal Model

Retinal degenerative (RD) conditions associated with photoreceptor loss such as age-related macular degeneration (AMD), retinitis pigmentosa (RP) and Leber Congenital Amaurosis (LCA) cause progressive and debilitating vision loss. There is an unmet need for therapies that can restore vision once photoreceptors have been lost. Transplantation of human pluripotent stem cell (hPSC)-derived retinal tissue (organoids) into the subretinal space of an eye with advanced RD brings retinal tissue sheets with thousands of healthy mutation-free photoreceptors and has a potential to treat most/all blinding diseases associated with photoreceptor degeneration with one approved protocol. Transplantation of fetal retinal tissue into the subretinal space of animal models and people with advanced RD has been developed successfully but cannot be used as a routine therapy due to ethical concerns and limited tissue supply. Large eye inherited retinal degeneration (IRD) animal models are valuable for developing vision restoration therapies utilizing advanced surgical approaches to transplant retinal cells/tissue into the subretinal space. The similarities in globe size, and photoreceptor distribution (e.g., presence of macula-like region area centralis) and availability of IRD models closely recapitulating human IRD would facilitate rapid translation of a promising therapy to the clinic. Presented here is a surgical technique of transplanting hPSC-derived retinal tissue into the subretinal space of a large animal model allowing assessment of this promising approach in animal models.

ERG assessment of altered retinal function in canine models of retinitis pigmentosa and monitoring of response to translatable gene augmentation therapy

PURPOSE

To analyze ERG responses from two dog models of retinitis pigmentosa, one due to a PDE6A mutation and the other a CNGB1 mutation, both to assess the effect of these mutations on retinal function and the ability of gene augmentation therapy to restore normal function.

METHODS

Scotopic and photopic ERGs from young affected and normal control dogs and affected dogs following AAV-mediated gene augmentation therapy were analyzed. Parameters reflecting rod and cone function were collected by modeling the descending slope of the a-wave to measure receptor response and sensitivity. Rod-driven responses were further assessed by Naka-Rushton fitting of the first limb of the scotopic b-wave luminance-response plot.

RESULTS

PDE6A^-/- dogs showed a dramatic decrease in rod-driven responses with very reduced rod maximal responses and sensitivity. There was a minor reduction in the amplitude of maximal cone responses. In contrast, CNGB1^-/- dogs had some residual rod responses with reduced amplitude and sensitivity and normal cone responses. Following gene augmentation therapy, rod parameters were substantially improved in both models with restoration of sensitivity parameters log S and log K and a large increase in log R_max in keeping with rescue of normal rod phototransduction in the treated retinal regions.

CONCLUSIONS

Modeling of rod and cone a-waves and the luminance-response function of the scotopic b-wave characterized the loss of rod photoreceptor function in two dog models of retinitis pigmentosa and showed the effectiveness of gene augmentation therapy in restoring normal functional parameters.

Gene therapy reforms photoreceptor structure and restores vision in NPHP5-associated Leber congenital amaurosis

The inherited childhood blindness caused by mutations in NPHP5, a form of Leber congenital amaurosis, results in abnormal development, dysfunction, and degeneration of photoreceptors. A naturally occurring NPHP5 mutation in dogs leads to a phenotype that very nearly duplicates the human retinopathy in terms of the photoreceptors involved, spatial distribution of degeneration, and the natural history of vision loss. We show that adeno-associated virus (AAV)-mediated NPHP5 gene augmentation of mutant canine retinas at the time of active degeneration and peak cell death stably restores photoreceptor structure, function, and vision with either the canine or human NPHP5 transgenes. Mutant cone photoreceptors, which failed to form outer segments during development, reform this structure after treatment. Degenerating rod photoreceptor outer segments are stabilized and develop normal structure. This process begins within 8 weeks after treatment and remains stable throughout the 6-month posttreatment period. In both photoreceptor cell classes mislocalization of rod and cone opsins is minimized or reversed. Retinal function and functional vision are restored. Efficacy of gene therapy in this large animal ciliopathy model of Leber congenital amaurosis provides a path for translation to human treatment.

Bestrophinopathies: perspectives on clinical disease, Bestrophin-1 function and developing therapies

Bestrophinopathies are a group of clinically distinct inherited retinal dystrophies that typically affect the macular region, an area synonymous with central high acuity vision. This spectrum of disorders is caused by mutations in bestrophin1 (BEST1), a protein thought to act as a Ca²⁺-activated Cl^- channel in the retinal pigment epithelium (RPE) of the eye. Although bestrophinopathies are rare, over 250 individual pathological mutations have been identified in the BEST1 gene, with many reported to have various clinical expressivity and incomplete penetrance. With no current clinical treatments available for patients with bestrophinopathies, understanding the role of BEST1 in cells and the pathological pathways underlying disease has become a priority. Induced pluripotent stem cell (iPSC) technology is helping to uncover disease mechanisms and develop treatments for RPE diseases, like bestrophinopathies. Here, we provide a comprehensive review of the pathophysiology of bestrophinopathies and highlight how patient-derived iPSC-RPE are being used to test new genomic therapies in vitro.

Mammalian Retinal Cell Quantification

Normal retina and its cell layers are essential for processing visual stimuli, and loss of its integrity has been documented in many disease processes. The numbers and the axonal processes of retinal ganglion cells are reduced substantially in glaucoma, leading to vision loss and blindness. Similarly, selective loss of photoreceptors in age-related macular degeneration and hereditary retinal dystrophies also results in the compromise of visual acuity. Development of genetically modified mice has led to increased understanding of the pathogenesis of many retinal diseases. Similarly, in this digital era, usage of modalities to quantify the retinal cell loss has grown exponentially leading to a better understanding of the suitability of animal models to study human retinal diseases. These quantification modalities provide valuable quantifiable data in studying pathogenesis and disease progression. This review will discuss the immunohistochemical markers for various retinal cells, available automated tools to quantify retinal cells, and present an example of retinal ganglion cell quantification using HALO image analysis platform. Additionally, we briefly review retinal cell types and subtypes, salient features of retina in various laboratory animal species, and a few of the main disease processes that affect retinal cell numbers in humans.

Dogs accurately track a moving object on a screen and anticipate its destination

The prediction of upcoming events is of importance not only to humans and non-human primates but also to other animals that live in complex environments with lurking threats or moving prey. In this study, we examined motion tracking and anticipatory looking in dogs in two eye-tracking experiments. In Experiment 1, we presented pet dogs (N = 14) with a video depicting how two players threw a Frisbee back and forth multiple times. The horizontal movement of the Frisbee explained a substantial amount of variance of the dogs' horizontal eye movements. With increasing duration of the video, the dogs looked at the catcher before the Frisbee arrived. In Experiment 2, we showed the dogs (N = 12) the same video recording. This time, however, we froze and rewound parts of the video to examine how the dogs would react to surprising events (i.e., the Frisbee hovering in midair and reversing its direction). The Frisbee again captured the dogs' attention, particularly when the video was frozen and rewound for the first time. Additionally, the dogs looked faster at the catcher when the video moved forward compared to when it was rewound. We conclude that motion tracking and anticipatory looking paradigms provide promising tools for future cognitive research with canids.

Diffusion tensor imaging of the visual pathway in dogs with primary angle-closure glaucoma

OBJECTIVE

To describe measurements of in vivo structures of the visual pathway beyond the retina and optic nerve head associated with canine primary angle-closure glaucoma (PACG).

METHODS

A prospective pilot study was conducted using magnetic resonance diffusion tensor imaging (DTI) to obtain quantitative measures of the optic nerve, chiasm, tract, and lateral geniculate nucleus (LGN) in dogs with and without PACG. 3-Tesla DTI was performed on six affected dogs and five breed, age- and sex-matched controls. DTI indices of the optic nerve, optic chiasm, optic tracts, and LGN were compared between normal, unilateral PACG, and bilateral PACG groups. Intra-class correlation coefficient (ICC) was calculated to assess intra-observer reliability.

RESULTS

Quantitative measurements of the optic nerve, optic tract, optic chiasm, and LGN were obtained in all dogs. There was a trend for reduced fractional anisotropy (FA) associated with disease for all structures assessed. Compared to the same structure in normal dogs, FA, and radial diffusivity (RD) of the optic nerve was consistently higher in the unaffected eye in dogs with unilateral PACG. Intra-observer reliability was excellent for measurements of the optic nerve (ICC: 0.92), good for measurements of the optic tract (ICC: 0.89) and acceptable for measures of the optic chiasm (ICC: 0.71) and lateral geniculate nuclei (ICC: 0.76).

CONCLUSION

Diffusivity and anisotropy measures provide a quantifiable means to evaluate the visual pathway in dogs. DTI has potential to provide in vivo measures of axonal and myelin injury and transsynaptic degeneration in canine PACG.

The brain of the African wild dog. IV. The visual system

The variegated pelage and social complexity of the African wild dog (Lycaon pictus) hint at the possibility of specializations of the visual system. Here, using a range of architectural and immunohistochemical stains, we describe the systems-level organization of the image-forming, nonimage forming, oculomotor, and accessory optic, vision-associated systems in the brain of one representative individual of the African wild dog. For all of these systems, the organization, in terms of location, parcellation and topology (internal and external), is very similar to that reported in other carnivores. The image-forming visual system consists of the superior colliculus, visual dorsal thalamus (dorsal lateral geniculate nucleus, pulvinar and lateral posterior nucleus) and visual cortex (occipital, parietal, suprasylvian, temporal and splenial visual regions). The nonimage forming visual system comprises the suprachiasmatic nucleus, ventral lateral geniculate nucleus, pretectal nuclear complex and the Edinger-Westphal nucleus. The oculomotor system incorporates the oculomotor, trochlear and abducens cranial nerve nuclei as well as the parabigeminal nucleus, while the accessory optic system includes the dorsal, lateral and medial terminal nuclei. The extent of similarity to other carnivores in the systems-level organization of these systems indicates that the manner in which these systems process visual information is likely to be consistent with that found, for example, in the well-studied domestic cat. It would appear that the sociality of the African wild dog is dependent upon the processing of information extracted from the visual system in the higher-order cognitive and affective neural systems.

Changes in retinal layer thickness with maturation in the dog: an in vivo spectral domain - optical coherence tomography imaging study

BACKGROUND

Retinal diseases are common in dogs. Some hereditary retinal dystrophies in dogs are important not only because they lead to vision loss but also because they show strong similarities to the orthologous human conditions. Advances in in vivo non-invasive retinal imaging allow the capture of retinal cross-section images that parallel low power microscopic examination of histological sections. Spectral domain - optical coherence tomography (SD-OCT) allows the measurement of retinal layer thicknesses and gives the opportunity for repeat examination to investigate changes in thicknesses in health (such as changes with maturation and age) and disease (following the course of retinal degenerative conditions). The purpose of this study was to use SD-OCT to measure retinal layer thicknesses in the dog during retinal maturation and over the first year of life. SD-OCT was performed on normal beagle cross dogs from 4 weeks of age to 52 weeks of age. To assess changes in layer thickness with age, measurements were taken from fixed regions in each of the 4 quadrants and the area centralis (the region important for most detailed vision). Additionally, changes in retinal layer thickness along vertical and horizontal planes passing through the optic nerve head were assessed.

RESULTS

In the four quadrants an initial thinning of retinal layers occurred over the first 12 to 15 weeks of life after which there was little change in thickness. However, in the area centralis there was a thickening of the photoreceptor layer over this time period which was mostly due to a lengthening of the photoreceptor inner/outer segment layer. The retina thinned with greater distances from the optic nerve head in both vertical and horizontal planes with the dorsal retina being thicker than the ventral retina. Most of the change in thickness with distance from the optic nerve head was due to difference in thickness of the inner retinal layers. The outer retinal layers remained more constant in thickness, particularly in the horizontal plane and dorsal to the optic nerve head.

CONCLUSIONS

These measurements will provide normative data for future studies.

Dose Range Finding Studies with Two RPGR Transgenes in a Canine Model of X-Linked Retinitis Pigmentosa Treated with Subretinal Gene Therapy

Recombinant adeno-associated viral (rAAV) vector-mediated gene therapy is being developed to treat X-linked retinitis pigmentosa (XLRP) in patients with mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene. In preparation for a clinical gene therapy trial, we conducted dose range finding (DRF) studies with an AAV2 capsid with three surface tyrosine residues changed to phenylalanine (AAV2tYF) vector administered by subretinal injection in a naturally occurring RPGR-mutant canine model (XLPRA2) to compare two different human RPGR (hRPGR) transgenes and to establish a reasonable starting dose for a clinical trial. Different dose levels of two candidate vectors (0.15 mL at 1.2 × 10¹⁰-3.0 × 10¹² vg/mL of rAAV2tYF-GRK1-hRPGRco or 4 × 10¹⁰-3.0 × 10¹² vg/mL of rAAV2tYF-GRK1-hRPGRstb), 6.0 × 10¹¹ vg/mL rAAV5-GRK1-hRPGRco reference vector or Vehicle were subretinally administered, and the dogs were followed for 8 weeks postdose. Ophthalmic examinations, analyses of retinal structure by in vivo imaging using confocal scanning laser ophthalmoscopy (cSLO)/optical coherence tomography (OCT) in the Lower (4.0 × 10¹⁰ vg/mL) and Lowest (1.2 × 10¹⁰ vg/mL) Doses, immunological responses by cell based assays or enzyme-linked immunosorbent assay, RPGR transgene expression, and reversal of opsin mislocalization by immunohistochemistry were performed. No sustained signs of ocular discomfort or ophthalmic complications were noted in any of the injected eyes except some in the High Dose group (3.0 × 10¹² vg/mL), which showed signs of retinal detachment and inflammation. A change in fundus reflectivity suggestive of a rescue effect was seen in the High, Mid (6.0 × 10¹¹ vg/mL), and Low (1.2 × 10¹¹ vg/mL) Dose groups. cSLO/OCT demonstrated qualitative and quantitative evidence of rescue effect in eyes treated with the Lower Dose. Anti-hRPGR antibodies were absent, but neutralizing antibody titers against AAV2 were detected in all animals dosed with rAAV2tYF in an apparent dose-related pattern. RPGR expression was stronger for rAAV2tYF-GRK1-hRPGRco compared to rAAV2tYF-GRK1-hRPGRstb at all dose levels. Subretinal administration of rAAV2tYF-GRK1-hRPGRco and rAAV2tYF-GRK1-hRPGRstb both corrected rod and cone opsin mislocalization, two early markers of disease in the XLPRA2 canine model of RPGR-XLRP. These results support the selection and use of rAAV2tYF-GRK1-hRPGRco (AGTC-501) and guided the initial doses in clinical studies in patients with XLRP caused by RPGR mutations.

Advancing Clinical Trials for Inherited Retinal Diseases: Recommendations from the Second Monaciano Symposium

Major advances in the study of inherited retinal diseases (IRDs) have placed efforts to develop treatments for these blinding conditions at the forefront of the emerging field of precision medicine. As a result, the growth of clinical trials for IRDs has increased rapidly over the past decade and is expected to further accelerate as more therapeutic possibilities emerge and qualified participants are identified. Although guided by established principles, these specialized trials, requiring analysis of novel outcome measures and endpoints in small patient populations, present multiple challenges relative to study design and ethical considerations. This position paper reviews recent accomplishments and existing challenges in clinical trials for IRDs and presents a set of recommendations aimed at rapidly advancing future progress. The goal is to stimulate discussions among researchers, funding agencies, industry, and policy makers that will further the design, conduct, and analysis of clinical trials needed to accelerate the approval of effective treatments for IRDs, while promoting advocacy and ensuring patient safety.

In vivo-directed evolution of adeno-associated virus in the primate retina

Efficient adeno-associated virus-mediated (AAV-mediated) gene delivery remains a significant obstacle to effective retinal gene therapies. Here, we apply directed evolution - guided by deep sequencing and followed by direct in vivo secondary selection of high-performing vectors with a GFP-barcoded library - to create AAV viral capsids with the capability to deliver genes to the outer retina in primates. A replication-incompetent library, produced via providing rep in trans, was created to mitigate risk of AAV propagation. Six rounds of in vivo selection with this library in primates - involving intravitreal library administration, recovery of genomes from outer retina, and extensive next-generation sequencing of each round - resulted in vectors with redirected tropism to the outer retina and increased gene delivery efficiency to retinal cells. These viral vectors expand the toolbox of vectors available for primate retina, and they may enable less invasive delivery of therapeutic genes to patients, potentially offering retina-wide infection at a similar dosage to vectors currently in clinical use.

Large Animal Models of Inherited Retinal Degenerations: A Review

Studies utilizing large animal models of inherited retinal degeneration (IRD) have proven important in not only the development of translational therapeutic approaches, but also in improving our understanding of disease mechanisms. The dog is the predominant species utilized because spontaneous IRD is common in the canine pet population. Cats are also a source of spontaneous IRDs. Other large animal models with spontaneous IRDs include sheep, horses and non-human primates (NHP). The pig has also proven valuable due to the ease in which transgenic animals can be generated and work is ongoing to produce engineered models of other large animal species including NHP. These large animal models offer important advantages over the widely used laboratory rodent models. The globe size and dimensions more closely parallel those of humans and, most importantly, they have a retinal region of high cone density and denser photoreceptor packing for high acuity vision. Laboratory rodents lack such a retinal region and, as macular disease is a critical cause for vision loss in humans, having a comparable retinal region in model species is particularly important. This review will discuss several large animal models which have been used to study disease mechanisms relevant for the equivalent human IRD.

Dog eye movements are slower than human eye movements

Eye movement of a species reflects the visual behavior strategy that it has adapted to during its evolution. What are eye movements of domestic dogs (Canis lupus familiaris) like? Investigations of dog eye movements per se have not been done, despite the increasing number of visuo-cognitive studies in dogs using eye-tracking systems. To fill this gap, we have recorded dog eye movements using a video-based eye-tracking system, and compared the dog data to that of humans. We found dog saccades follow the systematic relationships between saccade metrics previously shown in humans and other animal species. Yet, the details of the relationships, and the quantities of each metric of dog saccades and fixations differed from those of humans. Overall, dog saccades were slower and fixations were longer than those of humans. We hope our findings contribute to existing comparative analyses of eye movement across animal species, and also to improvement of algorithms used for classifying eye movement data of dogs.

Restoring vision at the fovea

In humans high quality, high acuity visual experience is mediated by the fovea, a tiny, specialized patch of retina containing the locus of fixation. Despite this, vision restoration strategies are typically developed in animal models without a fovea. While electrical prostheses have been approved by regulators, as yet they have failed to restore high quality, high acuity vision in patients. Approaches under pre-clinical development include regenerative cell therapies, optogenetics and chemical photosensitizers. All retinal vision restoration therapies require reactivation of inner retina that has lost photoreceptor input and that the restored signals can be interpreted at a behavioural level. A greater emphasis on tackling these challenges at the fovea may accelerate progress toward high quality vision restoration.

Focal/multifocal and geographic retinal dysplasia in the dog-In vivo retinal microanatomy analyses

PURPOSE

To examine the in vivo microanatomy of retinal folds and geographic lesions in dogs with acquired or inherited retinal dysplasia.

MATERIAL AND METHODS

Thirteen dogs had retinal microanatomy evaluation under general anesthesia using cSLO/sdOCT; two eyes had noninherited multifocal retinal folds, five had inherited multifocal retinal folds (drd1 or drd2), and 10 geographic retinal dysplasia. Retinas from two drd2 carrier dogs were examined by histology and immunohistochemistry (IHC) after in vivo imaging.

RESULTS

Retinal folds are the common feature of acquired focal/multifocal or geographic retinal dysplasia, are indistinguishable structurally from those associated with syndromic oculoskeletal dysplasia, and represent outer nuclear layer invaginations and rosettes visible by sdOCT. In dogs heterozygous for oculoskeletal dysplasia, the folds form clusters in a perivascular distribution along superior central vessels. IHC confirmed photoreceptor identity in the retinal folds. The geographic dysplasia plaques are not focally detached, but have inner retinal disorganization and intense autofluorescence in cSLO autofluorescence mode that is mainly limited to the geographic lesion, but is not uniform and in some extends beyond the plaques.

CONCLUSION

We propose that the autofluorescent characteristic of the geographic lesions is associated with an inner retinal disruption associated with perivascular or infiltrating macrophages and phagocytosis of cellular debris. As well, we suggest restructuring the examination forms to distinguish the folds that are sporadically distributed from those that have a perivascular distribution as the latter likely represent carriers for drd. In this latter group, DNA testing would be a helpful tool to provide specific breeding advice.

Diurnal rodents as pertinent animal models of human retinal physiology and pathology

This presentation will survey the retinal architecture, advantages, and limitations of several lesser-known rodent species that provide a useful diurnal complement to rats and mice. These diurnal rodents also possess unusually cone-rich photoreceptor mosaics that facilitate the study of cone cells and pathways. Species to be presented include principally the Sudanian Unstriped Grass Rat and Nile Rat (Arvicanthis spp.), the Fat Sand Rat (Psammomys obesus), the degu (Octodon degus) and the 13-lined ground squirrel (Ictidomys tridecemlineatus). The retina and optic nerve in several of these species demonstrate unusual resilience in the face of neuronal injury, itself an interesting phenomenon with potential translational value.

Long-Term Structural Outcomes of Late-Stage RPE65 Gene Therapy

The form of hereditary childhood blindness Leber congenital amaurosis (LCA) caused by biallelic RPE65 mutations is considered treatable with a gene therapy product approved in the US and Europe. The resulting vision improvement is well accepted, but long-term outcomes on the natural history of retinal degeneration are controversial. We treated four RPE65-mutant dogs in mid-life (age = 5-6 years) and followed them long-term (4-5 years). At the time of the intervention at mid-life, there were intra-ocular and inter-animal differences in local photoreceptor layer health ranging from near normal to complete degeneration. Treated locations having more than 63% of normal photoreceptors showed robust treatment-related retention of photoreceptors in the long term. Treated regions with less retained photoreceptors at the time of the intervention showed progressive degeneration similar to untreated regions with matched initial stage of disease. Unexpectedly, both treated and untreated regions in study eyes tended to show less degeneration compared to matched locations in untreated control eyes. These results support the hypothesis that successful long-term arrest of progression with RPE65 gene therapy may only occur in retinal regions with relatively retained photoreceptors at the time of the intervention, and there may be heretofore unknown mechanisms causing long-distance partial treatment effects beyond the region of subretinal injection.

Noninvasive imaging of the tree shrew eye: Wavefront analysis and retinal imaging with correlative histology

Tree shrews are small mammals with excellent vision and are closely related to primates. They have been used extensively as a model for studying refractive development, myopia, and central visual processing and are becoming an important model for vision research. Their cone dominant retina (∼95% cones) provides a potential avenue to create new damage/disease models of human macular pathology and to monitor progression or treatment response. To continue the development of the tree shrew as an animal model, we provide here the first measurements of higher order aberrations along with adaptive optics scanning light ophthalmoscopy (AOSLO) images of the photoreceptor mosaic in the tree shrew retina. To compare intra-animal in vivo and ex vivo cone density measurements, the AOSLO images were matched to whole-mount immunofluorescence microscopy. Analysis of the tree shrew wavefront indicated that the optics are well-matched to the sampling of the cone mosaic and is consistent with the suggestion that juvenile tree shrews are nearly emmetropic (slightly hyperopic). Compared with in vivo measurements, consistently higher cone density was measured ex vivo, likely due to tissue shrinkage during histological processing. Tree shrews also possess massive mitochondria ("megamitochondria") in their cone inner segments, providing a natural model to assess how mitochondrial size affects in vivo retinal imagery. Intra-animal in vivo and ex vivo axial distance measurements were made in the outer retina with optical coherence tomography (OCT) and transmission electron microscopy (TEM), respectively, to determine the origin of sub-cellular cone reflectivity seen on OCT. These results demonstrate that these megamitochondria create an additional hyper-reflective outer retinal reflective band in OCT images. The ability to use noninvasive retinal imaging in tree shrews supports development of this species as a model of cone disorders.

Optical coherence tomography of the retina, nerve fiber layer, and optic nerve head in dogs with glaucoma

OBJECTIVE

To evaluate the retina and optic nerve head (ONH) in canine eyes predisposed to glaucoma using optical coherence tomography (OCT).

ANIMALS

Twenty-five eyes (24 dogs).

METHODS

Measures of peripapillary retinal, retinal nerve fiber layer (RNFL), and ganglion cell complex (GCC) thickness and ONH parameters were obtained in vivo by OCT of the unaffected eye in dogs diagnosed with unilateral primary glaucoma (predisposed; n = 12) and compared with measures of healthy control eyes (normal; n = 13). Repeatability and intrarater reliability were explored using intraclass correlation coefficients (ICC).

RESULTS

Compared to normal eyes, predisposed eyes had a thinner retina in the temporal (P = 0.005), inferior quadrants (P = 0.003), and decreased inner retinal thickness (superior: P = 0.003, temporal: P = 0.001, inferior: P < 0.001, nasal: P = 0.001). Predisposed eyes had a thinner RNFL compared to normal eyes (P = 0.005), and when analyzed in quadrants, it was thinner in the superior (P < 0.001), temporal (P = 0.034), and nasal quadrants (P = 0.001). Repeatability (ICC 0.763-0.835) and intrarater reliability (ICC 0.824-0.942) were good to excellent for measures of retinal thickness and adequate for RNFL measurements (ICC 0.701-0.798). Reliable measurements of optic disk area were obtained and were similar between groups (P = 0.597). Measurements of parameters relying on automated software detection (GCC, optic cup, optic rim) had inadequate repeatability and reliability.

CONCLUSION

Statistically significant differences in retinal and RNFL thicknesses were identified in normal and predisposed eyes. Reliable and consistent measurements of variables with manual adjustment of software detected parameters were obtained. Validation of OCT as a diagnostic tool for clinical assessment in canine glaucoma is warranted.

Retinal Organ Cultures as Alternative Research Models

Ex vivo organ cultures represent unique research models, as they combine the advantages of cell cultures with those of animal models. Being able to mimic in vivo situations through the use of organ cultures provides an excellent opportunity to investigate cellular processes, molecular pathways and cell–cell interactions, as well as structural and synaptic organisation. Human and animal organ cultures are now well established and comprise sensitive, easy-to-manipulate experimental systems that raise minimal ethical concerns. The eye, in particular, is a very complex organ that is not easy to reproduce in vitro. However, a lot of research has been dedicated to the development of suitable ocular organ cultures. This review covers the various ex vivo retinal organ culture systems available for use in ophthalmology research and compares them with commonly used animal models. In particular, bovine and porcine retinal organ culture systems are described, because the size, anatomy, physiology and vessel morphology of bovine and porcine eyes are similar to the human eye in an undisputed way, thus making them good models. In addition, these animals are widely used by the food industry and the eyes are considered surplus material. A short overview of murine, rat, rabbit, cat, canine and simian retinal organ cultures is also provided.

Baseline retinal OCT measurements in normal female beagles: The effects of eccentricity, meridian, and age on retinal layer thickness

OBJECTIVE

Our aim was to generate baseline optical coherence tomography (OCT) measurements of retinal thickness in female Beagles and to determine how these are affected by meridian, eccentricity, and age.

METHODS

Twenty-three female Beagles, including six puppies (<6 months old), six mature (1.8-8.2 years old), and 11 elderly dogs (>11 years old) were studied. Both retinas of each dog were scanned (in 4 principal meridians) using the Heidelberg Spectralis following ophthalmic examination, refraction, and sedation. In each eye and each meridian, total retinal, outer retinal, and nerve fiber layer (NFL) thickness were measured from the disc rim up to 6 mm peripherally.

RESULTS

The canine retina is thickest dorsally and thinnest ventrally. Total retinal, outer retinal, and NFL thickness decrease progressively and significantly as a function of eccentricity. The greatest eccentricity-dependent thinning occurs dorsally. This thinning is due mostly to NFL tapering, while the eccentricity-dependent change in outer retinal thickness is more moderate, especially in the lateral meridian, possibly due to the presence of the visual streak. The retina is thickest in puppies, but there were no significant differences between mature and elderly dogs.

CONCLUSIONS

Our results provide normative values for total, outer, and inner retinal thickness in female dogs and may facilitate OCT use in the diagnosis of canine glaucoma and inherited retinopathies.