Electroretinogram of the Cone-Dominant Thirteen-Lined Ground Squirrel during Euthermia and Hibernation in Comparison with the Rod-Dominant Brown Norway Rat

Retinal neuroanatomy of two emerging model organisms, the spiny mouse (Acomys dimidiatus) and the Mongolian gerbil (Meriones unguiculatus)

Current research using animal models to investigate retinal cell biology and model retinal degenerative diseases largely utilize small mammals that are nocturnal and lack the ability to restore lost vision. In contrast, the Mongolian gerbil (Meriones) is a diurnal rodent with good photopic vision, and the spiny mouse (Acomys) is a small desert-dwelling rodent with remarkable regenerative capabilities. The goal of this study was to identify antibodies that detect retinal cell classes in Meriones and Acomys, and to describe the retinal anatomy of these two species in comparison to outbred laboratory mice (Mus musculus). Immunohistochemistry was performed on retinal sections with antibodies for various retinal cell types. Sections were imaged by light, fluorescence, and confocal microscopy. Cell density, morphology, and placement were compared between species qualitatively and quantitatively. Our analyses revealed a classic assembly of retinal cells in Meriones and Acomys, with a few deviations compared to Mus. Meriones displayed the highest density of cones and Acomys the lowest. A higher density of bipolar cell bodies in the proximal portion of the inner nuclear layer was observed in both Acomys and Meriones compared to Mus, and both species exhibited an increase in amacrine cell density compared to Mus. Our results provide a foundation for future research into the visual system adaptations of these interesting species.

Chemically induced cone degeneration in the 13-lined ground squirrel

Animal models of retinal degeneration are critical for understanding disease and testing potential therapies. Inducing degeneration commonly involves the administration of chemicals that kill photoreceptors by disrupting metabolic pathways, signaling pathways, or protein synthesis. While chemically induced degeneration has been demonstrated in a variety of animals (mice, rats, rabbits, felines, 13-lined ground squirrels (13-LGS), pigs, chicks), few studies have used noninvasive high-resolution retinal imaging to monitor the in vivo cellular effects. Here, we used longitudinal scanning light ophthalmoscopy (SLO), optical coherence tomography, and adaptive optics SLO imaging in the euthermic, cone-dominant 13-LGS (46 animals, 52 eyes) to examine retinal structure following intravitreal injections of chemicals, which were previously shown to induce photoreceptor degeneration, throughout the active season of 2019 and 2020. We found that iodoacetic acid induced severe pan-retinal damage in all but one eye, which received the lowest concentration. While sodium nitroprusside successfully induced degeneration of the outer retinal layers, the results were variable, and damage was also observed in 50% of contralateral control eyes. Adenosine triphosphate and tunicamycin induced outer retinal specific damage with varying results, while eyes injected with thapsigargin did not show signs of degeneration. Given the variability of damage we observed, follow-up studies examining the possible physiological origins of this variability are critical. These additional studies should further advance the utility of chemically induced photoreceptor degeneration models in the cone-dominant 13-LGS.

Seasonal changes in membrane structure and excitability in retinal neurons of goldfish (Carassius auratus) under constant environmental conditions

Seasonal modifications in the structure of cellular membranes occur as an adaptive measure to withstand exposure to prolonged environmental change. Little is known about whether such changes may occur independently of external cues, such as photoperiod or temperature, or how they may impact the central nervous system. We compared membrane properties of neurons isolated from the retina of goldfish (Carassius auratus), an organism well-adapted to extreme environmental change, during the summer and winter months. Goldfish were maintained in a facility under constant environmental conditions throughout the year. Analysis of whole-retina phospholipid composition using mass spectrometry-based lipidomics revealed a two-fold increase in phosphatidylethanolamine species during the winter, suggesting an increase in cell membrane fluidity. Atomic force microscopy was used to produce localized, nanoscale-force deformation of neuronal membranes. Measurement of Young's modulus indicated increased membrane-cortical stiffness (or decreased elasticity) in neurons isolated during the winter. Voltage-clamp electrophysiology was used to assess physiological changes in neurons between seasons. Winter neurons displayed a hyperpolarized reversal potential (Vrev) and a significantly lower input resistance (Rin) compared to summer neurons. This was indicative of a decrease in membrane excitability during the winter. Subsequent measurement of intracellular Ca2+ activity using Fura-2 microspectrofluorometry confirmed a reduction in action potential activity, including duration and action potential profile, in neurons isolated during the winter. These studies demonstrate chemical and biophysical changes that occur in retinal neurons of goldfish throughout the year without exposure to seasonal cues, and suggest a novel mechanism of seasonal regulation of retinal activity.

Retinal Plasticity

Brain plasticity is a well-established concept designating the ability of central nervous system (CNS) neurons to rearrange as a result of learning, when adapting to changeable environmental conditions or else while reacting to injurious factors. As a part of the CNS, the retina has been repeatedly probed for its possible ability to respond plastically to a variably altered environment or to pathological insults. However, numerous studies support the conclusion that the retina, outside the developmental stage, is endowed with only limited plasticity, exhibiting, instead, a remarkable ability to maintain a stable architectural and functional organization. Reviewed here are representative examples of hippocampal and cortical paradigms of plasticity and of retinal structural rearrangements found in organization and circuitry following altered developmental conditions or occurrence of genetic diseases leading to neuronal degeneration. The variable rate of plastic changes found in mammalian retinal neurons in different circumstances is discussed, focusing on structural plasticity. The likely adaptive value of maintaining a low level of plasticity in an organ subserving a sensory modality that is dominant for the human species and that requires elevated fidelity is discussed.

Clinical findings in eyes with BEST1-related retinopathy complicated by choroidal neovascularization

PURPOSE

To determine the characteristics of eyes diagnosed with Best vitelliform macular dystrophy (BVMD) and autosomal recessive bestrophinopathy (ARB) complicated by choroidal neovascularization (CNV).

METHODS

This was a retrospective, multicenter observational case series. Fourteen genetically confirmed BVMD patients and 9 ARB patients who had been examined in 2 ophthalmological institutions in Japan were studied. The findings in a series of ophthalmic examinations including B-scan optical coherence tomography (OCT) and OCT angiography (OCTA) were reviewed.

RESULTS

CNV was identified in 5 eyes (17.9%) of BVMD patients and in 2 eyes (11.1%) of ARB patients. Three of 5 eyes with BVMD were classified as being at the vitelliruptive stage and 2 eyes at the atrophic stage. The CNV in 2 BVMD eyes were diagnosed as exudative because of acute visual acuity reduction, retinal hemorrhage, and intraretinal fluid, while the CNV in 3 BVMD eyes and 2 ARB eyes were diagnosed as non-exudative. The visual acuity of the two eyes with exudative CNV did not improve despite anti-VEGF treatments. None of the eyes with non-exudative CNV had a reduction of their visual acuity for at least 4 years. All of the CNV were located within hyperreflective materials which were detected in 16 eyes (57.1%) of the BVMD eyes and in 7 eyes (38.9%) of the ARB eyes.

CONCLUSIONS

CNV is a relatively common complication in BEST1-related retinopathy in Asian population as well. The prognosis of eyes with exudative CNV is not always good, and OCTA can detect CNV in eyes possessing hyperreflective materials.

Pre-retinal delivery of recombinant adeno-associated virus vector significantly improves retinal transduction efficiency

Intravitreal injection is the most widely used injection technique for ocular gene delivery. However, vector diffusion is attenuated by physical barriers and neutralizing antibodies in the vitreous. The 13-lined ground squirrel (13-LGS), as in humans, has a larger relative vitreous body volume than the more common rodent models such as rats and mice, which would further reduce transduction efficiency with the intravitreal injection route. We report here a “pre-retinal” injection approach that leads to detachment of the posterior hyaloid membrane and delivers vector into the space between vitreous and inner retina. Vectors carrying a ubiquitously expressing mCherry reporter were injected into the deep vitreous or pre-retinal space in adult wild-type 13-LGSs. Then, adeno-associated virus (AAV)-mediated mCherry expression was evaluated with non-invasive imaging, immunofluorescence, and flow cytometry. Compared to deep vitreous delivery, pre-retinal administration achieved pan-retinal gene expression with a lower vector dose volume and significantly increased the number of transduced cone photoreceptors. These results suggest that pre-retinal injection is a promising tool in the development of gene therapy strategies in animal models and is a potential approach for use in human research, particularly in younger individuals with an intact posterior hyaloid membrane and stable vitreous.

Measuring the spatial distribution of multiply scattered light using a de-scanned image sensor for examining retinal structure contrast

An optical platform is presented for examining intrinsic contrast detection strategies when imaging retinal structure using ex vivo tissue. A custom microscope was developed that scans intact tissue and collects scattered light distribution at every image pixel, allowing digital masks to be applied after image collection. With this novel approach at measuring the spatial distribution of multiply scattered light, known and novel methods of detecting intrinsic cellular contrast can be explored, compared, and optimized for retinal structures of interest.