A Population Study of Common Ocular Abnormalities in C57BL/6N rd8 Mice

Identification of Keratoconus-Related Phenotypes in Three Ppip5k2 Mouse Models

Purpose

It is necessary to establish a mouse model of keratoconus (KC) for research and therapy. We aimed to determine corneal phenotypes in 3 Ppip5k2 mouse models.

Methods

Central corneal thickness (CCT) was determined using spectral domain optical coherence tomography (SD-OCT) in Ppip5k2+/K^ (n = 41 eyes), Ppip5k2K^/K^ (n = 17 eyes) and 2 knock-in mice, Ppip5k2S419A/+ (n = 54 eyes) and Ppip5k2S419A/S419A (n = 18 eyes), and Ppip5k2D843S/+ (n = 42 eyes) and Ppip5k2D843S/D843S (n = 44 eyes) at 3 and 6 months. Pachymetry maps were generated using the Mouse Corneal Analysis Program (MCAP) to process OCT images. Slit lamp biomicroscopy was used to determine any corneal abnormalities, and, last, hematoxylin and eosin (H&E) staining using corneal sections from these animals was used to examine morphological changes.

Results

CCT significantly decreased from 3 to 6 months in the Ppip5k2+/K^ and Ppip5k2K^/K^ mice compared to their littermate controls. OCT-based pachymetry maps revealed abnormally localized thinning in all three models compared to their wild-type (WT) controls. Slit lamp examinations revealed corneal abnormalities in the form of bullous keratopathy, stromal edema, stromal scarring, deep corneal neovascularization, and opacities in the heterozygous/homozygous mice of the three models in comparison with their controls. Corneal histological abnormalities, such as epithelial thickening and stromal layer damage, were observed in the heterozygous/homozygous mice of the three models in comparison with the WT controls.

Conclusions

We have identified phenotypic and histological changes in the corneas of three mouse lines that could be relevant in the development of animal models of KC.

Role of PKN1 in Retinal Cell Type Formation

We recently identified PKN1 as a developmentally active gatekeeper of the transcription factor neuronal differentiation-2 (NeuroD2) in several brain areas. Since NeuroD2 plays an important role in amacrine cell (AC) and retinal ganglion cell (RGC) type formation, we aimed to study the expression of NeuroD2 in the postnatal retina of WT and Pkn1-/- animals, with a particular focus on these two cell types. We show that PKN1 is broadly expressed in the retina and that the gross retinal structure is not different between both genotypes. Postnatal retinal NeuroD2 levels were elevated upon Pkn1 knockout, with Pkn1-/- retinae showing more NeuroD2+ cells in the lower portion of the inner nuclear layer. Accordingly, immunohistochemical analysis revealed an increased amount of AC in postnatal and adult Pkn1-/- retinae. There were no differences in horizontal cell, bipolar cell, glial cell and RGC numbers, nor defective axon guidance to the optic chiasm or tract upon Pkn1 knockout. Interestingly, we did, however, see a specific reduction in SMI-32+ α-RGC in Pkn1-/- retinae. These results suggest that PKN1 is important for retinal cell type formation and validate PKN1 for future studies focusing on AC and α-RGC specification and development.

Nfia Is Critical for AII Amacrine Cell Production: Selective Bipolar Cell Dependencies and Diminished ERG

The nuclear factor one (NFI) transcription factor genes Nfia, Nfib, and Nfix are all enriched in late-stage retinal progenitor cells, and their loss has been shown to retain these progenitors at the expense of later-generated retinal cell types. Whether they play any role in the specification of those later-generated fates is unknown, but the expression of one of these, Nfia, in a specific amacrine cell type may intimate such a role. Here, Nfia conditional knockout (Nfia-CKO) mice (both sexes) were assessed, finding a massive and largely selective absence of AII amacrine cells. There was, however, a partial reduction in type 2 cone bipolar cells (CBCs), being richly interconnected to AII cells. Counts of dying cells showed a significant increase in Nfia-CKO retinas at postnatal day (P)7, after AII cell numbers were already reduced but in advance of the loss of type 2 CBCs detected by P10. Those results suggest a role for Nfia in the specification of the AII amacrine cell fate and a dependency of the type 2 CBCs on them. Delaying the conditional loss of Nfia to the first postnatal week did not alter AII cell number nor differentiation, further suggesting that its role in AII cells is solely associated with their production. The physiological consequences of their loss were assessed using the ERG, finding the oscillatory potentials to be profoundly diminished. A slight reduction in the b-wave was also detected, attributed to an altered distribution of the terminals of rod bipolar cells, implicating a role of the AII amacrine cells in constraining their stratification.SIGNIFICANCE STATEMENT The transcription factor NFIA is shown to play a critical role in the specification of a single type of retinal amacrine cell, the AII cell. Using an Nfia-conditional knockout mouse to eliminate this population of retinal neurons, we demonstrate two selective bipolar cell dependencies on the AII cells; the terminals of rod bipolar cells become mis-stratified in the inner plexiform layer, and one type of cone bipolar cell undergoes enhanced cell death. The physiological consequence of this loss of the AII cells was also assessed, finding the cells to be a major contributor to the oscillatory potentials in the electroretinogram.

Genome-wide screening reveals the genetic basis of mammalian embryonic eye development

BACKGROUND

Microphthalmia, anophthalmia, and coloboma (MAC) spectrum disease encompasses a group of eye malformations which play a role in childhood visual impairment. Although the predominant cause of eye malformations is known to be heritable in nature, with 80% of cases displaying loss-of-function mutations in the ocular developmental genes OTX2 or SOX2, the genetic abnormalities underlying the remaining cases of MAC are incompletely understood. This study intended to identify the novel genes and pathways required for early eye development. Additionally, pathways involved in eye formation during embryogenesis are also incompletely understood. This study aims to identify the novel genes and pathways required for early eye development through systematic forward screening of the mammalian genome.

RESULTS

Query of the International Mouse Phenotyping Consortium (IMPC) database (data release 17.0, August 01, 2022) identified 74 unique knockout lines (genes) with genetically associated eye defects in mouse embryos. The vast majority of eye abnormalities were small or absent eyes, findings most relevant to MAC spectrum disease in humans. A literature search showed that 27 of the 74 lines had previously published knockout mouse models, of which only 15 had ocular defects identified in the original publications. These 12 previously published gene knockouts with no reported ocular abnormalities and the 47 unpublished knockouts with ocular abnormalities identified by the IMPC represent 59 genes not previously associated with early eye development in mice. Of these 59, we identified 19 genes with a reported human eye phenotype. Overall, mining of the IMPC data yielded 40 previously unimplicated genes linked to mammalian eye development. Bioinformatic analysis showed that several of the IMPC genes colocalized to several protein anabolic and pluripotency pathways in early eye development. Of note, our analysis suggests that the serine-glycine pathway producing glycine, a mitochondrial one-carbon donator to folate one-carbon metabolism (FOCM), is essential for eye formation.

CONCLUSIONS

Using genome-wide phenotype screening of single-gene knockout mouse lines, STRING analysis, and bioinformatic methods, this study identified genes heretofore unassociated with MAC phenotypes providing models to research novel molecular and cellular mechanisms involved in eye development. These findings have the potential to hasten the diagnosis and treatment of this congenital blinding disease.

OCT and ERG Techniques in High-Throughput Phenotyping of Mouse Vision

The purpose of the study is to demonstrate coherent optical tomography and electroretinography techniques adopted from the human clinical practice to assess the morphology and function of the mouse retina in a high-throughput phenotyping environment. We present the normal range of wild-type C57Bl/6NCrl retinal parameters in six age groups between 10 and 100 weeks as well as examples of mild and severe pathologies resulting from knocking out a single protein-coding gene. We also show example data obtained by more detailed analysis or additional methods useful in eye research, for example, the angiography of a superficial and deep vascular complex. We discuss the feasibility of these techniques in conditions demanding a high-throughput approach such as the systemic phenotyping carried out by the International Mouse Phenotyping Consortium.

Atomic Force Microscopy-Based Measurements of Retinal Microvessel Stiffness in Mice with Endothelial-Specific Deletion of CCN1

Vascular stiffness is an independent predictor of human vascular diseases and is linked to ischemia, diabetes, high blood pressure, hyperlipidemia, and/or aging. Blood vessel stiffening increases owing to changes in the microscale architecture and/or content of extracellular, cytoskeletal, and nuclear matrix proteins. These alterations, while best appreciated in large blood vessels, also gradually occur in the microvasculature and play an important role in the initiation and progression of numerous microangiopathies including diabetic retinopathy. Although macroscopic measurements of arterial stiffness by pulse wave velocity are often used for clinical diagnosis, stiffness changes of intact microvessels and their causative factors have not been characterized. Herein, we describe the use of atomic force microscopy (AFM) to determine stiffness of mouse retinal capillaries and assess its regulation by the cellular communication network (CCN) 1, a stiffness-sensitive gene-encoded matricellular protein. AFM yields reproducible measurements of retinal capillary stiffness in lightly fixed freshly isolated retinal flat mounts. AFM measurements also show significant changes in compliance properties of the retinal microvasculature of mice with endothelial-specific deletion of CCN1, indicating that CCN1 expression, or lack thereof, affects the mechanical properties of microvascular cells in vivo. Thus, AFM has the force sensitivity and the spatial resolution necessary to measure the local modulus of retinal capillaries in situ and eventually to investigate microvascular compliance heterogeneities as key components of disease pathogenesis.

Cooperation of Angiopoietin-2 and Angiopoietin-4 in Schlemm's Canal Maintenance

Purpose

Defects in the iridocorneal angle tissues, including the trabecular meshwork (TM) and Schlemm's canal (SC), impair aqueous humor flow and increase the intraocular pressure (IOP), eventually resulting in glaucoma. Activation of endothelial tyrosine kinase receptor Tie2 by angiopoietin-1 (Angpt1) has been demonstrated to be essential for SC formation, but roles of the other two Tie2 ligands, Angpt2 and Angpt4, have been controversial or not yet characterized, respectively.

Methods

Angpt4 expression was investigated using genetic cell fate mapping and reporter mice. Congenital deletion of Angpt2 and Angpt4 and tamoxifen-inducible deletion of Angpt1 in mice were used to study the effects of Angpt4 deletion alone and in combination with the other angiopoietins. SC morphology was examined with immunofluorescent staining. IOP measurements, electron microscopy, and histologic evaluation were used to study glaucomatous changes.

Results

Angpt4 was postnatally expressed in the TM. While Angpt4 deletion alone did not affect SC and Angpt4 deletion did not aggravate Angpt1 deletion phenotype, absence of Angpt4 combined with Angpt2 deletion had detrimental effects on SC morphology in adult mice. Consequently, Angpt2^−/−;Angpt4^−/− mice displayed glaucomatous changes in the eye. Mice with Angpt2 deletion alone showed only moderate SC defects, but Angpt2 was necessary for proper limbal vasculature development. Mechanistically, analysis of Tie2 phosphorylation suggested that Angpt2 and Angpt4 cooperate as agonistic Tie2 ligands in maintaining SC integrity.

Conclusions

Our results indicated an additive effect of Angpt4 in SC maintenance and Tie2 activation and a spatiotemporally regulated interplay between the angiopoietins in the mouse iridocorneal angle.

Retinal Ganglion Cells: Global Number, Density and Vulnerability to Glaucomatous Injury in Common Laboratory Mice

How many RBPMS+ retinal ganglion cells (RGCs) does a standard C57BL/6 laboratory mouse have on average and is this number substrain- or sex-dependent? Do RGCs of (European) C57BL/6J and -N mice show a different intrinsic vulnerability upon glaucomatous injury? Global RGC numbers and densities of common laboratory mice were previously determined via axon counts, retrograde tracing or BRN3A immunohistochemistry. Here, we report the global RGC number and density by exploiting the freely available tool RGCode to automatically count RGC numbers and densities on entire retinal wholemounts immunostained for the pan-RGC marker RBPMS. The intrinsic vulnerability of RGCs from different substrains to glaucomatous injury was evaluated upon introduction of the microbead occlusion model, followed by RBPMS counts, retrograde tracing and electroretinography five weeks post-injury. We demonstrate that the global RGC number and density varies between substrains, yet is not sex-dependent. C57BL/6J mice have on average 46K ± 2K RBPMS+ RGCs per retina, representing a global RGC density of 3268 ± 177 RGCs/mm2. C57BL/6N mice, on the other hand, have on average less RBPMS+ RGCs (41K ± 3K RGCs) and a lower density (3018 ± 189 RGCs/mm2). The vulnerability of the RGC population of the two C57BL/6 substrains to glaucomatous injury did, however, not differ in any of the interrogated parameters.

CD73 controls ocular adenosine levels and protects retina from light-induced phototoxicity

ATP and adenosine have emerged as important signaling molecules involved in vascular remodeling, retinal functioning and neurovascular coupling in the mammalian eye. However, little is known about the regulatory mechanisms of purinergic signaling in the eye. Here, we used three-dimensional multiplexed imaging, in situ enzyme histochemistry, flow cytometric analysis, and single cell transcriptomics to characterize the whole pattern of purine metabolism in mouse and human eyes. This study identified ecto-nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39), NTPDase2, and ecto-5′-nucleotidase/CD73 as major ocular ecto-nucleotidases, which are selectively expressed in the photoreceptor layer (CD73), optic nerve head, retinal vasculature and microglia (CD39), as well as in neuronal processes and cornea (CD39, NTPDase2). Specifically, microglial cells can create a spatially arranged network in the retinal parenchyma by extending and retracting their branched CD39^high/CD73^low processes and forming local “purinergic junctions” with CD39^low/CD73⁻ neuronal cell bodies and CD39^high/CD73⁻ retinal blood vessels. The relevance of the CD73–adenosine pathway was confirmed by flash electroretinography showing that pharmacological inhibition of adenosine production by injection of highly selective CD73 inhibitor PSB-12489 in the vitreous cavity of dark-adapted mouse eyes rendered the animals hypersensitive to prolonged bright light, manifested as decreased a-wave and b-wave amplitudes. The impaired electrical responses of retinal cells in PSB-12489-treated mice were not accompanied by decrease in total thickness of the retina or death of photoreceptors and retinal ganglion cells. Our study thus defines ocular adenosine metabolism as a complex and spatially integrated network and further characterizes the critical role of CD73 in maintaining the functional activity of retinal cells.

Retinal degeneration in mice and humans with neuronal ceroid lipofuscinosis type 8

Background

Ceroid lipofuscinosis type 8 belongs to a heterogenous group of vision and life-threatening neurodegenerative diseases, neuronal ceroid lipofuscinosis (NCL). Effective therapy is limited to a single drug for treatment of ceroid lipofuscinosis type 2, necessitating animal disease models to facilitate further therapeutic development. Murine models are advantageous for therapeutic development due to easy genetic manipulation and rapid breeding, however appropriate genetic models need to be identified and characterized before being used for therapy testing. To date, murine models of ocular disease associated with ceroid lipofuscinosis type 8 have only been characterized in motor neuron degeneration mice.

Methods

Cln8^−/− mice were produced by CRISPR/Cas9 genome editing through the International Mouse Phenotyping Consortium. Ophthalmic examination, optical coherence tomography, electroretinography, and ocular histology was performed on Cln8^−/− mice and controls at 16 weeks of age. Quantification of all retinal layers, retinal pigmented epithelium, and the choriocapillaris was performed using images acquired with ocular coherence tomography and planimetry of histologic sections. Necropsy was performed to investigate concurrent systemic abnormalities. Clinical correlation with human patients with CLN8-associated retinopathy is provided.

Results

Retinal degeneration characterized by retinal pigment epithelium mottling, scattered drusen, and retinal vascular attenuation was noted in all Cln8^−/− mice. Loss of inner and outer photoreceptor segment demarcation was noted on optical coherence tomography, with significant thinning of the whole retina (P=1e-9), outer nuclear layer (P=1e-9), and combined photoreceptor segments (P=1e-9). A global reduction in scotopic and photopic electroretinographic waveforms was noted in all Cln8^−/− mice. Slight thickening of the inner plexiform layer (P=0.02) and inner nuclear layer (P=0.004), with significant thinning of the whole retina (P=0.03), outer nuclear layer (P=0.01), and outer photoreceptor segments (P=0.001) was appreciated on histologic sections. Scattered lipid vacuoles were noted in splenic red pulp of all Cln8^−/− mice, though no gross systemic abnormalities were detected on necropsy. Retinal findings are consistent with those seen in patients with ceroid lipofuscinosis type 8.

Conclusions

This study provides detailed clinical characterization of retinopathy in adult Cln8^−/− mice. Findings suggest that Cln8^−/− mice may provide a useful murine model for development of novel therapeutics needed for treating ocular disease in patients with ceroid lipofuscinosis type 8.

Animals Models of Inherited Retinal Disease

Inherited retinal diseases (IRDs) are an important cause of blindness worldwide. Over 270 genes have been associated with IRD. Genetic testing can determine the cause of the clinical disease in the majority of patients. However, at least 25-50% of patients with clinical diagnosis of IRD remain unsolved even after whole genome sequencing. Animal models of IRD can be useful for expanding the set of established IRD genes, to gain biological understanding of the function of these genes in the retina, and to test advanced therapeutics prior to human clinical trials. In this chapter some small and large animal models of IRD are discussed including some of the advantages and limitations of each for various forms of retinopathy.

Early Onset of Age-Related Cataracts in Cystine/Glutamate Antiporter Knockout Mice

Purpose

The purpose of this study was to determine the importance of the xCT is a subunit. The cystine/glutamate antiporter is actually system xc-xCT subunit of the cystine/glutamate antiporter in maintaining redox balance by investigating the effects of the loss of xCT on lens transparency and cystine/cysteine balance in the aqueous humour.

Methods

C57Bl/6 wild-type and xCT knockout mice at five age groups (6 weeks to 12 months) were used. Lens transparency was examined using a slit-lamp and morphological changes visualized by immunolabelling and confocal microscopy. Quantification of glutathione in lenses and cysteine and cystine levels in the aqueous was conducted by liquid chromatography tandem mass spectrometry (LC-MS/MS).

Results

Slit-lamp examinations revealed that 3-month-old wild-type mice and xCT knockout mice lenses exhibited an anterior localized cataract. The frequency of this cataract significantly increased in the knockout mice compared to the wild-type mice. Morphological studies revealed a localized swelling of the lens fiber cells at the anterior pole. Glutathione levels in whole lenses were similar between wild-type and knockout mice. However, glutathione levels were significantly decreased at 3 months in the knockout mice in the lens epithelium compared to the wild-type mice. Aqueous cysteine levels remained similar between wild-type and knockout mice at all age groups, whereas cystine levels were significantly increased in 3-, 9-, and 12-month-old knockout mice compared to wild-type mice.

Conclusions

Loss of xCT resulted in the depletion of glutathione in the epithelium and an oxidative shift in the cysteine/cystine ratio of the aqueous. Together, these oxidative changes may contribute to the accelerated development of an anterior cataract in knockout mice, which appears to be a normal feature of aging in wild-type mice.

Cohen Syndrome-Associated Cataract Is Explained by VPS13B Functions in Lens Homeostasis and Is Modified by Additional Genetic Factors

Purpose

Cohen syndrome (CS) is a rare genetic disorder caused by variants of the VPS13B gene. CS patients are affected with a severe form of retinal dystrophy, and in several cases cataracts also develop. The purpose of this study was to investigate the mechanisms and risk factors for cataract in CS, as well as to report on cataract surgeries in CS patients.

Methods

To understand how VPS13B is associated with visual impairments in CS, we generated the Vps13b^{∆Ex3/∆Ex3} mouse model. Mice from 1 to 3 months of age were followed by ophthalmoscopy and slit-lamp examinations. Phenotypes were investigated by histology, immunohistochemistry, and western blot. Literature analysis was performed to determine specific characteristic features of cataract in CS and to identify potential genotype–phenotype correlations.

Results

Cataracts rapidly developed in 2-month-old knockout mice and were present in almost all lenses at 3 months. Eye fundi appeared normal until cataract development. Lens immunostaining revealed that cataract formation was associated with the appearance of large vacuoles in the cortical area, epithelial–mesenchymal transition, and fibrosis. In later stages, cataracts became hypermature, leading to profound retinal remodeling due to inflammatory events. Literature analysis showed that CS-related cataracts display specific features compared to other forms of retinitis pigmentosa-related cataracts, and their onset is modified by additional genetic factors. Corroboratively, we were able to isolate a subline of the Vps13b^{∆Ex3/∆Ex3} model with delayed cataract onset.

Conclusions

VPS13B participates in lens homeostasis, and the CS-related cataract development dynamic is linked to additional genetic factors.

Frequency of incidental ocular findings during pre-employment screening at a tertiary care Eye hospital

Objective

To highlight the prevalence of incidentally discovered ocular findings harvested amongst candidates of different age groups presented for pre-employment screening in a tertiary care eye hospital.

Methods

This Cross sectional prospective study was conducted in Armed Forces Institute of Ophthalmology, Rawalpindi, from Jun 2018 to Dec 2019. Data was collected using non-probability consecutive sampling technique. All candidates who appeared for medical fitness examination were included. Candidates belonged to various regions of Pakistan. Complete ophthalmic checkup including visual acuity, best corrected visual acuity, anterior and posterior segment examination was performed. The data analysis was done by IBM SPSS 2.0 software.

Results

One thousand and five hundred (1500) candidates underwent ophthalmic medical fitness examination during Jun 2018 to Dec 2019, out of these 86% (1290) were males and 14% (210) were females. Mean age of the candidates was 23.14 ± 5.66 years. The most common incidental ocular findings were amblyopia 24.6% (369), strabismus 10% (150), cataract 7.3% (110), macular scar 6.5% (100) and colour vision deficiencies 5.5% (82).

Conclusion

The study demonstrates that out of total patients, 77% (1095) were found to be asymptomatic and 23% (405) were symptomatic. The study provides frequency for prevailing diseases and can help in improvement of eye care screening.

Hearing impairment due to Mir183/96/182 mutations suggests both loss and gain of function effects

The microRNA miR-96 is important for hearing, as point mutations in humans and mice result in dominant progressive hearing loss. Mir96 is expressed in sensory cells along with Mir182 and Mir183, but the roles of these closely-linked microRNAs are as yet unknown. Here we analyse mice carrying null alleles of Mir182, and of Mir183 and Mir96 together to investigate their roles in hearing. We found that Mir183/96 heterozygous mice had normal hearing and homozygotes were completely deaf with abnormal hair cell stereocilia bundles and reduced numbers of inner hair cell synapses at four weeks old. Mir182 knockout mice developed normal hearing then exhibited progressive hearing loss. Our transcriptional analyses revealed significant changes in a range of other genes, but surprisingly there were fewer genes with altered expression in the organ of Corti of Mir183/96 null mice compared with our previous findings in Mir96 Dmdo mutants, which have a point mutation in the miR-96 seed region. This suggests the more severe phenotype of Mir96 Dmdo mutants compared with Mir183/96 mutants, including progressive hearing loss in Mir96 Dmdo heterozygotes, is likely to be mediated by the gain of novel target genes in addition to the loss of its normal targets. We propose three mechanisms of action of mutant miRNAs; loss of targets that are normally completely repressed, loss of targets whose transcription is normally buffered by the miRNA, and gain of novel targets. Any of these mechanisms could lead to a partial loss of a robust cellular identity and consequent dysfunction.

Characterization of indoleamine-2,3-dioxygenase 1, tryptophan-2,3-dioxygenase, and Ido1/Tdo2 knockout mice

Indoleamine-2,3-dioxygenase 1 (IDO1) and tryptophan-2,3-dioxygenase 2 (TDO2) degrade tryptophan (Trp) to kynurenine (Kyn), and these enzymes have promise as therapeutic targets. A comprehensive characterization of potential safety liabilities of IDO1 and TDO2 inhibitors using knockout (KO) mice has not been assessed, nor has the dual Ido1/Tdo2 KO been reported. Here we characterized male and female mice with KOs for Ido1, Tdo2, and Ido1/Tdo2 and compared findings to the wild type (WT) mouse strain, evaluated for 14 days, using metabolomics, transcriptional profiling, behavioral analysis, spleen immunophenotyping, comprehensive histopathological analysis, and serum clinical chemistry. Multiple metabolomic changes were seen in KO mice. For catabolism of Trp to Kyn and anthranilic acid, both substrates were decreased in liver of Tdo2 and dual KO mice. Metabolism of Trp to serotonin and its metabolites resulted in an increase in 5-Hydroxyindole-3-acetic acid in the Tdo2 and dual KO mice. Ido1 and dual KO mice displayed a Kyn reduction in plasma but not in liver. Nicotinamide synthesis and conversion of glucose to lactic acid were not impacted. A slight decrease in serum alkaline phosphatase was seen in all KOs, and small changes in liver gene expression of genes unrelated to tryptophan metabolism were observed. Regarding other parameters, no genotype-specific changes were observed. In summary, this work shows metabolomic pathway changes for metabolites downstream of tryptophan in these KO mice, and suggests that inhibition of the IDO1 and TDO2 enzymes would be well tolerated whether inhibited individually or in combination since no safety liabilities were uncovered.

A global Slc7a7 knockout mouse model demonstrates characteristic phenotypes of human lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is an inborn error of cationic amino acid (arginine, lysine, ornithine) transport caused by biallelic pathogenic variants in SLC7A7, which encodes the light subunit of the y+LAT1 transporter. Treatments for the complications of LPI, including growth failure, renal disease, pulmonary alveolar proteinosis, autoimmune disorders and osteoporosis, are limited. Given the early lethality of the only published global Slc7a7 knockout mouse model, a viable animal model to investigate global SLC7A7 deficiency is needed. Hence, we generated two mouse models with global Slc7a7 deficiency (Slc7a7em1Lbu/em1Lbu; Slc7a7Lbu/Lbu and Slc7a7em1(IMPC)Bay/em1(IMPC)Bay; Slc7a7Bay/Bay) using CRISPR/Cas9 technology by introducing a deletion of exons 3 and 4. Perinatal lethality was observed in Slc7a7Lbu/Lbu and Slc7a7Bay/Bay mice on the C57BL/6 and C57BL/6NJ inbred genetic backgrounds, respectively. We noted improved survival of Slc7a7Lbu/Lbu mice on the 129 Sv/Ev × C57BL/6 F2 background, but postnatal growth failure occurred. Consistent with human LPI, these Slc7a7Lbu/Lbu mice exhibited reduced plasma and increased urinary concentrations of the cationic amino acids. Histopathological assessment revealed loss of brush border and lipid vacuolation in the renal cortex of Slc7a7Lbu/Lbu mice, which combined with aminoaciduria suggests proximal tubular dysfunction. Micro-computed tomography of L4 vertebrae and skeletal radiographs showed delayed skeletal development and suggested decreased mineralization in Slc7a7Lbu/Lbu mice, respectively. In addition to delayed skeletal development and delayed development in the kidneys, the lungs and liver were observed based on histopathological assessment. Overall, our Slc7a7Lbu/Lbu mouse model on the F2 mixed background recapitulates multiple human LPI phenotypes and may be useful for future studies of LPI pathology.

An eye on the dog as the scientist's best friend for translational research in ophthalmology: Focus on the ocular surface

Preclinical animal studies provide valuable opportunities to better understand human diseases and contribute to major advances in medicine. This review provides a comprehensive overview of ocular parameters in humans and selected animals, with a focus on the ocular surface, detailing species differences in ocular surface anatomy, physiology, tear film dynamics and tear film composition. We describe major pitfalls that tremendously limit the translational potential of traditional laboratory animals (i.e., rabbits, mice, and rats) in ophthalmic research, and highlight the benefits of integrating companion dogs with clinical analogues to human diseases into preclinical pharmacology studies. This One Health approach can help accelerate and improve the framework in which ophthalmic research is translated to the human clinic. Studies can be conducted in canine subjects with naturally occurring or noninvasively induced ocular surface disorders (e.g., dry eye disease, conjunctivitis), reviewed herein, and tear fluid can be easily retrieved from canine eyes for various bioanalytical purposes. In this review, we discuss common tear collection methods, including capillary tubes and Schirmer tear strips, and provide guidelines for tear sampling and extraction to improve the reliability of analyte quantification (drugs, proteins, others).

Comprehensive identification of mRNA isoforms reveals the diversity of neural cell-surface molecules with roles in retinal development and disease

Genes encoding cell-surface proteins control nervous system development and are implicated in neurological disorders. These genes produce alternative mRNA isoforms which remain poorly characterized, impeding understanding of how disease-associated mutations cause pathology. Here we introduce a strategy to define complete portfolios of full-length isoforms encoded by individual genes. Applying this approach to neural cell-surface molecules, we identify thousands of unannotated isoforms expressed in retina and brain. By mass spectrometry we confirm expression of newly-discovered proteins on the cell surface in vivo. Remarkably, we discover that the major isoform of a retinal degeneration gene, CRB1, was previously overlooked. This CRB1 isoform is the only one expressed by photoreceptors, the affected cells in CRB1 disease. Using mouse mutants, we identify a function for this isoform at photoreceptor-glial junctions and demonstrate that loss of this isoform accelerates photoreceptor death. Therefore, our isoform identification strategy enables discovery of new gene functions relevant to disease.

Endogenous Apelin Is Protective Against Age-Associated Loss of Retinal Ganglion Cells in Mice

Age-associated loss of retinal ganglion cells (RGCs) causes visual deficits, but there is not yet any therapeutic agent to prevent the loss of these cells. Herein, we report that apelin, an endogenous peptide ligand of APJ receptor, is protective against the age-related loss of RGCs in mice. The mRNA expression of apelin was reduced in the retina of old mice compared with that in young mice, whereas retinal APJ expression increased with age. Immunofluorescence staining showed that APJ was present in RGCs and their surrounding cells expressed apelin. In addition, both functional and histological analyses demonstrated that apelin deficiency accelerated the loss of RGCs associated with age in mice. These results suggest that endogenous apelin plays a protective role against the degeneration of RGCs and that the apelinergic axis may be a new target for preventing age-related visual impairment.

ERG Alteration Due to the rd8 Mutation of the Crb1 Gene in Cln3 +/+ rd8-/rd8- Mice

Mattapallil et al. described that vendor lines for C57BL/6 N mice may carry the rd8 mutation that leads to an ocular phenotype, which could be mistaken for an induced retinal degeneration. This mouse strain is widely used in ophthalmic research as a background for modeling retinal degeneration. In the process of studying Cln3^Δex7/8 knock-in mice on a C57BL/6 N background, we became aware of this issue. The aim of this study thus was to use electroretinography to investigate the age-dependent functional loss in Cln3^{+/+ rd8-/rd8-} mice and compare it to C57BL/6 J mice.The scotopic and photopic amplitudes of the a-wave and b-wave decrease significantly in mutant mice with increasing age, and the implicit time is prolonged. Especially the oscillatory potentials arising from inner retinal interaction seem to be notably affected by the rd8 mutation. Surprisingly, the amplitudes in young C57BL/6 J mice were lower than those measured in C57BL/6 N at any time point.Our results indicate that the rd8 mutation present in C57BL/6 N mice affects the function of the inner and outer retina. This is surprising given that the major retinal morphological alterations due to the rd8 mutation are found in the outer retina.We conclude that the rd8 mutation does affect the retinal function in Cln3^{+/+ rd8-/rd8-} mice in a variable manner. Epigenetic factors and modifying genes lead to a phenotype shift in these mice. Interpreting the results of previous studies in mutant mice on C57BL/6 N background is challenging as comparing results obtained in independent studies or on other mouse backgrounds may be misleading. Using littermates as controls remains the only valid option.

Cytoglobin deficiency potentiates Crb1-mediated retinal degeneration in rd8 mice

PURPOSE

The purpose of this study is to determine the effect of Cytoglobin (Cygb) deficiency on Crb1-related retinopathy. The Crb1 cell polarity complex is required for photoreceptor function and survival. Crb1-related retinopathies encompass a broad range of phenotypes which are not completely explained by the variability of Crb1 mutations. Genes thought to modify Crb1 function are therefore important targets of research. The biological function of Cygb involves oxygen delivery, scavenging of reactive oxygen species, and nitric oxide metabolism. However, the relationship of Cygb to diseases involving the Crb1 cell polarity complex is unknown.

METHODS

Cygb knockout mice homozygous for the rd8 mutation (Cygb^-/-rd8/rd8) were screened for ocular abnormalities and imaged using optical coherence tomography and fundus photography. Electroretinography was performed, as was histology and immunohistochemistry. Quantitative PCR was used to determine the effect of Cygb deficiency on transcription of Crb1 related cell polarity genes.

RESULTS

Cygb^-/-rd8/rd8 mice develop an abnormal retina with severe lamination abnormalities. The retina undergoes progressive degeneration with the ventral retina more severely affected than the dorsal retina. Cygb expression is in neurons of the retinal ganglion cell layer and inner nuclear layer. Immunohistochemical studies suggest that cell death predominates in the photoreceptors. Electroretinography amplitudes show reduced a- and b-waves, consistent with photoreceptor disease. Cygb deficient retinas had only modest transcriptional perturbations of Crb1-related cell polarity genes. Cygb^-/- mice without the rd8 mutation did not exhibit obvious retinal abnormalities.

CONCLUSIONS

Cygb is necessary for retinal lamination, maintenance of cell polarity, and photoreceptor survival in rd8 mice. These results are consistent with Cygb as a disease modifying gene in Crb1-related retinopathy. Further studies are necessary to investigate the role of Cygb in the human retina.

Delineating the role of MITF isoforms in pigmentation and tissue homeostasis

MITF, a gene that is mutated in familial melanoma and Waardenburg syndrome, encodes multiple isoforms expressed from alternative promoters that share common coding exons but have unique amino termini. It is not completely understood how these isoforms influence pigmentation in different tissues and how the expression of these independent isoforms of MITF is regulated. Here, we show that melanocytes express two isoforms of MITF, MITF-A and MITF-M. The expression of MITF-A is partially regulated by a newly identified retinoid enhancer element located upstream of the MITF-A promoter. Mitf-A knockout mice have only subtle changes in melanin accumulation in the hair and reduced Tyr expression in the eye. In contrast, Mitf-M-null mice have enlarged kidneys, lack neural crest-derived melanocytes in the skin, choroid, and iris stroma, yet maintain pigmentation within the retinal pigment epithelium and iris pigment epithelium of the eye. Taken together, these studies identify a critical role for MITF-M in melanocytes, a minor role for MITF-A in regulating pigmentation in the hair and Tyr expression in the eye, and a novel role for MITF-M in size control of the kidney.

Genome-wide screening of mouse knockouts reveals novel genes required for normal integumentary and oculocutaneous structure and function

Oculocutaneous syndromes are often due to mutations in single genes. In some cases, mouse models for these diseases exist in spontaneously occurring mutations, or in mice resulting from forward mutatagenesis screens. Here we present novel genes that may be causative for oculocutaneous disease in humans, discovered as part of a genome-wide screen of knockout-mice in a targeted single-gene deletion project. The International Mouse Phenotyping Consortium (IMPC) database (data release 10.0) was interrogated for all mouse strains with integument abnormalities, which were then cross-referenced individually to identify knockouts with concomitant ocular abnormalities attributed to the same targeted gene deletion. The search yielded 307 knockout strains from unique genes with integument abnormalities, 226 of which have not been previously associated with oculocutaneous conditions. Of the 307 knockout strains with integument abnormalities, 52 were determined to have ocular changes attributed to the targeted deletion, 35 of which represent novel oculocutaneous genes. Some examples of various integument abnormalities are shown, as well as two examples of knockout strains with oculocutaneous phenotypes. Each of the novel genes provided here are potentially relevant to the pathophysiology of human integumentary, or oculocutaneous conditions, such as albinism, phakomatoses, or other multi-system syndromes. The novel genes reported here may implicate molecular pathways relevant to these human diseases and may contribute to the discovery of novel therapeutic targets.

Predicting human disease mutations and identifying drug targets from mouse gene knockout phenotyping campaigns

Two large-scale mouse gene knockout phenotyping campaigns have provided extensive data on the functions of thousands of mammalian genes. The ongoing International Mouse Phenotyping Consortium (IMPC), with the goal of examining all ∼20,000 mouse genes, has examined 5115 genes since 2011, and phenotypic data from several analyses are available on the IMPC website (www.mousephenotype.org). Mutant mice having at least one human genetic disease-associated phenotype are available for 185 IMPC genes. Lexicon Pharmaceuticals' Genome5000™ campaign performed similar analyses between 2000 and the end of 2008 focusing on the druggable genome, including enzymes, receptors, transporters, channels and secreted proteins. Mutants (4654 genes, with 3762 viable adult homozygous lines) with therapeutically interesting phenotypes were studied extensively. Importantly, phenotypes for 29 Lexicon mouse gene knockouts were published prior to observations of similar phenotypes resulting from homologous mutations in human genetic disorders. Knockout mouse phenotypes for an additional 30 genes mimicked previously published human genetic disorders. Several of these models have helped develop effective treatments for human diseases. For example, studying Tph1 knockout mice (lacking peripheral serotonin) aided the development of telotristat ethyl, an approved treatment for carcinoid syndrome. Sglt1 (also known as Slc5a1) and Sglt2 (also known as Slc5a2) knockout mice were employed to develop sotagliflozin, a dual SGLT1/SGLT2 inhibitor having success in clinical trials for diabetes. Clinical trials evaluating inhibitors of AAK1 (neuropathic pain) and SGLT1 (diabetes) are underway. The research community can take advantage of these unbiased analyses of gene function in mice, including the minimally studied 'ignorome' genes.

Ocular phenotypic consequences of a single copy deletion of the Yap1 gene (Yap1 +/-) in mice

PURPOSE

To identify the effects of a single copy deletion of Yap1 (Yap1 ^+/-) in the mouse eye, the ocular phenotypic consequences of Yap1 ^+/- were determined in detail.

METHODS

Complete ophthalmic examinations, as well as corneal esthesiometry, the phenol red thread test, intraocular pressure, and Fourier-domain optical coherence tomography were performed on Yap1 ^+/- and age-matched wild-type (WT) mice between eyelid opening (2 weeks after birth) and adulthood (2 months and 1 year after birth). Following euthanasia, enucleated eyes were characterized histologically.

RESULTS

Microphthalmia with small palpebral fissures, corneal fibrosis, and reduced corneal sensation were common findings in the Yap1 ^+/- mice. Generalized corneal fibrosis precluded clinical examination of the posterior structures. Histologically, thinning and keratinization of the corneal epithelium were observed in the Yap1 ^+/- mice in comparison with the WT mice. Distorted collagen fiber arrangement and hypercellularity of keratocytes were observed in the stroma. Descemet's membrane was extremely thin and lacked an endothelial layer in the Yap1 ^+/- mice. The iris was adherent to the posterior cornea along most of its surface creating a distorted contour. Most of the Yap1 ^+/- eyes were microphakic with swollen fibers and bladder cells. The retinas of the Yap1 ^+/- mice were normal at 2 weeks and 2 months of age, but the presence of retinal abnormalities, including retinoschisis and detachment, was markedly increased in the Yap1 ^+/- mice at 1 year of age.

CONCLUSIONS

The results show that the heterozygous deletion of the Yap1 gene in mice leads to complex ocular abnormalities, including microphthalmia, corneal fibrosis, anterior segment dysgenesis, and cataract.

Naturally Occurring Inherited Forms of Retinal Degeneration in Vertebrate Animal Species: A Comparative and Evolutionary Perspective

The ability to noninvasively monitor retinal abnormalities using imaging and cognitive and electrophysiological assessment has made it possible to carefully characterize genetic influences on retinal health. Because genetic retinal traits in animal species are not commonly detrimental to survival beyond birth, it is possible to document the natural history of retinal disease. Human quality of life is greatly impacted by retinal disease, and blindness carries a significant financial burden to society. Because of these compelling reasons, there is an ongoing medical need to study the effect of genetic mutations on retinal health and to develop therapies to address them. Transgenic animal models have aided in these missions, but there are opportunities for novel gene discovery and a development of greater understanding of retinal physiology using animal models that develop naturally occurring heritable retinal disorders. In this chapter, the advantages and disadvantages of transgenic and spontaneous vertebrate animal models of human inherited retinal disease are debated, in particular those of carnivore species, and the potential resource of spontaneous heritable retinal disorders in inbred nondomestic carnivore species is discussed.

Identification of genes required for eye development by high-throughput screening of mouse knockouts

Despite advances in next generation sequencing technologies, determining the genetic basis of ocular disease remains a major challenge due to the limited access and prohibitive cost of human forward genetics. Thus, less than 4,000 genes currently have available phenotype information for any organ system. Here we report the ophthalmic findings from the International Mouse Phenotyping Consortium, a large-scale functional genetic screen with the goal of generating and phenotyping a null mutant for every mouse gene. Of 4364 genes evaluated, 347 were identified to influence ocular phenotypes, 75% of which are entirely novel in ocular pathology. This discovery greatly increases the current number of genes known to contribute to ophthalmic disease, and it is likely that many of the genes will subsequently prove to be important in human ocular development and disease.

Bret Moore et al. from the International Mouse Phenotyping Consortium report the identification of 347 mouse genes that influence ocular phenotypes when knocked out. 75% of the identified genes have not previously been associated with any ocular pathology.