Remodeling of second-order neurons in the retina of rd/rd mutant mice

Molecular mechanisms underlying inherited photoreceptor degeneration as targets for therapeutic intervention

Retinitis pigmentosa (RP) is a form of retinal degeneration characterized by primary degeneration of rod photoreceptors followed by a secondary cone loss that leads to vision impairment and finally blindness. This is a rare disease with mutations in several genes and high genetic heterogeneity. A challenging effort has been the characterization of the molecular mechanisms underlying photoreceptor cell death during the progression of the disease. Some of the cell death pathways have been identified and comprise stress events found in several neurodegenerative diseases such as oxidative stress, inflammation, calcium imbalance and endoplasmic reticulum stress. Other cell death mechanisms appear more relevant to photoreceptor cells, such as high levels of cGMP and metabolic changes. Here we review some of the cell death pathways characterized in the RP mutant retina and discuss preclinical studies of therapeutic approaches targeting the molecular outcomes that lead to photoreceptor cell demise.

Sustained Extracellular Electrical Stimulation Modulates the Permeability of Gap Junctions in rd1 Mouse Retina with Photoreceptor Degeneration

Neurons build vast gap junction-coupled networks (GJ-nets) that are permeable to ions or small molecules, enabling lateral signaling. Herein, we investigate (1) the effect of blinding diseases on GJ-nets in mouse retinas and (2) the impact of electrical stimulation on GJ permeability. GJ permeability was traced in the acute retinal explants of blind retinal degeneration 1 (rd1) mice using the GJ tracer neurobiotin. The tracer was introduced via the edge cut method into the GJ-net, and its spread was visualized in histological preparations (fluorescent tagged) using microscopy. Sustained stimulation was applied to modulate GJ permeability using a single large electrode. Our findings are: (1) The blind rd1 retinas displayed extensive intercellular coupling via open GJs. Three GJ-nets were identified: horizontal, amacrine, and ganglion cell networks. (2) Sustained stimulation significantly diminished the tracer spread through the GJs in all the cell layers, as occurs with pharmaceutical inhibition with carbenoxolone. We concluded that the GJ-nets of rd1 retinas remain coupled and functional after blinding disease and that their permeability is regulatable by sustained stimulation. These findings are essential for understanding molecular signaling in diseases over coupled networks and therapeutic approaches using electrical implants, such as eliciting visual sensations or suppressing cortical seizures.

A New Preclinical Model of Retinitis Pigmentosa Due to Pde6g Deficiency

Purpose

Retinitis pigmentosa (RP) is the most common cause of inherited blindness, with onset occurring as early as 4 years of age in certain rare but severe forms caused by mutations in the gamma subunit of phosphodiesterase 6 (PDE6). Studies in humans and mice have shown that RP pathology begins with progressive photoreceptor death, which then drives changes in downstream neurons, neighboring retinal pigment epithelium (RPE), and vasculature. Here, we present the first detailed analysis of RP disease progression in Pde6g-deficient mice.

Design

Experimental study of an RP mouse model.

Subjects

We studied Pde6g-/- and Pde6g+/- mice at the age of 7, 16, 30, 44, and 56 days with n = 2 to 5 per group and time point.

Methods

Photoreceptor degeneration and retinal remodeling were analyzed in retinal sections by immunofluorescence. Retinal blood vessel degradation was analyzed in flat-mounted retinas immunolabeled for isolectin GS-IB4. Protein expression was measured by immunoblot. Acellular capillaries were assessed in trypsin-digested and hematoxylin-eosin-stained retinas at postnatal day (P) 44. Retinal pigment epithelium cells were delineated in flat-mounted RPE-choroid-sclera by immunolabeling for the cell-adhesion protein β-catenin.

Main Outcome Measures

Immunofluorescence and morphometry (quantitative analysis of outer nuclear layer, dendrite area, vessel area, acellular vessels, RPE cell size, number of nuclei per RPE cell, RPE cell eccentricity, and RPE cell solidity).

Results

This novel RP model exhibits early onset and rapid rod degeneration, with the vast majority gone by P16. This pathology leads to retinal remodeling, including changes of inner retinal neurons, early activation of glia cells, degradation of retinal vasculature, and structural abnormalities of the RPE.

Conclusions

The pathology in our Pde6g-/- mouse model precisely mirrors human RP progression. The results demonstrate the significant role of the gamma subunit in maintaining phosphodiesterase activity and provide new insights into the disease progression due to Pde6g deficiency.

Financial Disclosures

Proprietary or commercial disclosure may be found after the references.

Cellular migration into a subretinal honeycomb-shaped prosthesis for high-resolution prosthetic vision

In patients blinded by geographic atrophy, a subretinal photovoltaic implant with 100 µm pixels provided visual acuity closely matching the pixel pitch. However, such flat bipolar pixels cannot be scaled below 75 µm, limiting the attainable visual acuity. This limitation can be overcome by shaping the electric field with 3-dimensional (3-D) electrodes. In particular, elevating the return electrode on top of the honeycomb-shaped vertical walls surrounding each pixel extends the electric field vertically and decouples its penetration into tissue from the pixel width. This approach relies on migration of the retinal cells into the honeycomb wells. Here, we demonstrate that majority of the inner retinal neurons migrate into the 25 µm deep wells, leaving the third-order neurons, such as amacrine and ganglion cells, outside. This enables selective stimulation of the second-order neurons inside the wells, thus preserving the intraretinal signal processing in prosthetic vision. Comparable glial response to that with flat implants suggests that migration and separation of the retinal cells by the walls does not cause additional stress. Furthermore, retinal migration into the honeycombs does not negatively affect its electrical excitability, while grating acuity matches the pixel pitch down to 40 μm and reaches the 27 μm limit of natural resolution in rats with 20 μm pixels. These findings pave the way for 3-D subretinal prostheses with pixel sizes of cellular dimensions.

Utilization of the retinal organoid model to evaluate the feasibility of genetic strategies to ameliorate retinal disease(s)

Organoid models have quickly become a popular research tool to evaluate novel therapeutics on 3-D recapitulated tissue. This has enabled researchers to use physiologically relevant human tissue in vitro to augment the standard use of immortalized cells and animal models. Organoids can also provide a model when an engineered animal cannot recreate a specific disease phenotype. In particular, the retinal research field has taken advantage of this burgeoning technology to provide insight into inherited retinal disease(s) mechanisms and therapeutic intervention to ameliorate their effects. In this review we will discuss the use of both wild-type and patient-specific retinal organoids to further gene therapy research that could potentially prevent retinal disease(s) progression. Furthermore, we will discuss the pitfalls of current retinal organoid technology and present potential solutions that could overcome these hurdles in the near future.

The Phosphoproteome of the Rd1 Mouse Retina, a Model of Inherited Photoreceptor Degeneration, Changes after Protein Kinase G Inhibition

Retinitis pigmentosa (RP) is a frequent cause of blindness among the working population in industrial countries due to the inheritable death of photoreceptors. Though gene therapy was recently approved for mutations in the RPE65 gene, there is in general no effective treatment presently. Previously, abnormally high levels of cGMP and overactivation of its dependent protein kinase (PKG) have been suggested as causative for the fatal effects on photoreceptors, making it meaningful to explore the cGMP-PKG downstream signaling for more pathological insights and novel therapeutic target development purposes. Here, we manipulated the cGMP-PKG system in degenerating retinas from the rd1 mouse model pharmacologically via adding a PKG inhibitory cGMP-analogue to organotypic retinal explant cultures. A combination of phosphorylated peptide enrichment and mass spectrometry was then applied to study the cGMP-PKG-dependent phosphoproteome. We identified a host of novel potential cGMP-PKG downstream substrates and related kinases using this approach and selected the RAF1 protein, which may act as both a substrate and a kinase, for further validation. This showed that the RAS/RAF1/MAPK/ERK pathway may be involved in retinal degeneration in a yet unclarified mechanism, thus deserving further investigation in the future.

Functional integrity of visual coding following advanced photoreceptor degeneration

Photoreceptor degeneration sufficient to produce severe visual loss often spares the inner retina. This raises hope for vision restoration treatments using optogenetics or electrical stimulation, which generate a replacement light input signal in surviving neurons. The success of these approaches is dependent on the capacity of surviving circuits of the visual system to generate and propagate an appropriate visual code in the face of neuroanatomical remodeling. To determine whether retinally degenerate animals possess this capacity, we generated a transgenic mouse model expressing the optogenetic actuator ReaChR in ON bipolar cells (second-order neurons in the visual projection). After crossing this with the rd1 model of photoreceptor degeneration, we compared ReaChR-derived responses with photoreceptor-driven responses in wild-type (WT) mice at the level of retinal ganglion cells and the visual thalamus. The ReaChR-driven responses in rd1 animals showed low photosensitivity, but in other respects generated a visual code that was very similar to the WT. ReaChR rd1 responses had high trial-to-trial reproducibility and showed sensitivity normalization to code contrast across background intensities. At the single unit level, ReaChR-derived responses exhibited broadly similar variations in response polarity, contrast sensitivity, and temporal frequency tuning as the WT. Units from the WT and ReaChR rd1 mice clustered together when subjected to unsupervised community detection based on stimulus-response properties. Our data reveal an impressive ability for surviving circuitry to recreate a rich visual code following advanced retinal degeneration and are promising for regenerative medicine in the central nervous system.

Retinal Pathoconnectomics: A Window into Neurodegeneration

Over the past decade, the field of retinal connectomics has made huge strides in describing the precise topologies underlying retinal visual processing. The same techniques that allowed these advancements are also applicable to understanding the progression of rewiring in retinal remodeling: retinal pathoconnectomics. Pathoconnectomics is unique in its unbiased approach to understanding the impacts of deafferentation on the remaining network components and identifying aberrant connectivities leading to visual processing defects. Pathoconnectomics also paves the way for identifying underlying rules of rewiring that may be recapitulated throughout the nervous system in other neurodegenerative diseases.

Loss of Müller cell glutamine synthetase immunoreactivity is associated with neuronal changes in late-stage retinal degeneration

Introduction

A hallmark of photoreceptor degenerations is progressive, aberrant remodeling of the surviving retinal neurons and glia following photoreceptor loss. The exact relationship between neurons and glia remodeling in this late stage of retinal degeneration, however, is unclear. This study assessed this by examining Müller cell dysfunction via glutamine synthetase immunoreactivity and its spatial association with retinal neuron subpopulations through various cell markers.

Methods

Aged Rd1 mice retinae (P150 - P536, n = minimum 5 per age) and control heterozygous rd1 mice retinae (P536, n = 5) were isolated, fixed and cryosectioned. Fluorescent immunolabeling of glutamine synthetase was performed and retinal areas quantified as having low glutamine synthetase immunoreactivity if proportion of labeled pixels in an area was less than two standard deviations of the mean of the total retina. Other Müller cell markers such as Sox9 and Glial fibrillary acidic protein along with neuronal cell markers Calbindin, Calretinin, recoverin, Protein kinase C-α, Glutamic acid decarboxylase 67, and Islet-1 were then quantified within areas of low and normal synthetase immunoreactivity.

Results

Glutamine synthetase immunoreactivity was lost as a function of age in the rd1 mouse retina (P150 - P536). Immunoreactivity of other Müller cell markers, however, were unaffected suggesting Müller cells were still present in these low glutamine synthetase immunoreactive regions. Glutamine synthetase immunoreactivity loss affected specific neuronal populations: Type 2, Type 8 cone, and rod bipolar cells, as well as AII amacrine cells based on reduced recoverin, protein kinase Ca and parvalbumin immunoreactivity, respectively. The number of cell nuclei within regions of low glutamine synthetase immunoreactivity was also reduced suggesting possible neuronal loss rather than reduced cell marker immunoreactivity.

Conclusion

These findings further support a strong interplay between glia-neuronal alterations in late-stage degeneration and highlight a need for future studies and consideration in intervention development.

Current perspective on retinal remodeling: Implications for therapeutics

The retinal degenerative diseases retinitis pigmentosa and age-related macular degeneration are a leading cause of irreversible vision loss. Both present with progressive photoreceptor degeneration that is further complicated by processes of retinal remodeling. In this perspective, we discuss the current state of the field of retinal remodeling and its implications for vision-restoring therapeutics currently in development. Here, we discuss the challenges and pitfalls retinal remodeling poses for each therapeutic strategy under the premise that understanding the features of retinal remodeling in totality will provide a basic framework with which therapeutics can interface. Additionally, we discuss the potential for approaching therapeutics using a combined strategy of using diffusible molecules in tandem with other vision-restoring therapeutics. We end by discussing the potential of the retina and retinal remodeling as a model system for more broadly understanding the progression of neurodegeneration across the central nervous system.

Short pulses of epiretinal prostheses evoke network-mediated responses in retinal ganglion cells by stimulating presynaptic neurons

OBJECTIVE

Microelectronic retinal implant aims to restore functional vision with electric stimulation. Short pulses are generally known to directly activate retinal ganglion cells (RGCs) with a notion of one or two spike(s) per pulse. In the present work, we systematically explore network-mediated responses that arise from various short pulses in both normal and degenerate retinas.

APPROACH

Cell-attached patch clamping was used to record spiking responses of RGCs in wild-type (C57BL/6J) and retinal degeneration (rd10) mice. Alpha RGCs of the mouse retinas were targeted by their large soma sizes and classified by their responses to spot flashes. Then, RGCs were electrically stimulated by various conditions such as duration (100-460 μs), count (1-10), amplitude (100-400 μA), and repeating frequency (10-40 Hz) of short pulses. Also, their responses were compared with each own response to a single 4-ms-long pulse which is known to evoke strong indirect responses.

MAIN RESULTS

Short pulses evoked strong network-mediated responses not only in both ON and OFF types of RGCs in the healthy retinas but also in RGCs of the severely degenerate retina. However, the spike timing consistency across repeats not decreased significantly in the rd10 RGCs compared to the healthy ON and OFF RGCs. Network-mediated responses of ON RGCs were highly dependent on the current amplitude of stimuli but much less on the pulse count and the repetition frequency. In contrast, responses of OFF RGCs were more influenced by the number of stimuli than the current amplitude.

SIGNIFICANCE

Our results demonstrate that short pulses also elicit indirect responses by activating presynaptic neurons. In the case of the commercial retinal prostheses using repeating short pulses, there is a possibility that the performance of clinical devices is highly related to the preserved retinal circuits. Therefore, examination of surviving retinal neurons in patients would be necessary to improve the efficacy of retinal prostheses.

Retinal ganglion cell desensitization is mitigated by varying parameter constant excitation pulse trains

Retinal prostheses partially restore vision in patients blinded by retinitis pigmentosa (RP) and age-related macular degeneration (AMD). One issue that limits the effectiveness of retinal stimulation is the desensitization of the retina response to repeated pulses. Rapid fading of percepts is reported in clinical studies. We studied the retinal output evoked by fixed pulse trains vs. pulse trains that have variable parameters pulse-to-pulse. We used the current clamp to record RGC spiking in the isolated mouse retina. Trains of biphasic current pulses at different frequencies and amplitudes were applied. The main results we report are: (1) RGC desensitization was induced by increasing stimulus frequency, but was unrelated to stimulus amplitude. Desensitization persisted when the 20 Hz stimulation pulses were applied to the retinal ganglion cells at 65 μA, 85 μA, and 105 μA. Subsequent pulses in the train evoked fewer spikes. There was no obvious desensitization when 2 Hz stimulation pulse trains were applied. (2) Blocking inhibitory GABA_A receptor increased spontaneous activity but did not reduce desensitization. (3) Pulse trains with constant charge or excitation (based on strength-duration curves) but varying pulse width, amplitude, and shape increased the number of evoked spikes/pulse throughout the pulse train. This suggests that retinal desensitization can be partially overcome by introducing variability into each pulse.

Light-induced charge generation in polymeric nanoparticles restores vision in advanced-stage retinitis pigmentosa rats

Retinal dystrophies such as Retinitis pigmentosa are among the most prevalent causes of inherited legal blindness, for which treatments are in demand. Retinal prostheses have been developed to stimulate the inner retinal network that, initially spared by degeneration, deteriorates in the late stages of the disease. We recently reported that conjugated polymer nanoparticles persistently rescue visual activities after a single subretinal injection in the Royal College of Surgeons rat model of Retinitis pigmentosa. Here we demonstrate that conjugated polymer nanoparticles can reinstate physiological signals at the cortical level and visually driven activities when microinjected in 10-months-old Royal College of Surgeons rats bearing fully light-insensitive retinas. The extent of visual restoration positively correlates with the nanoparticle density and hybrid contacts with second-order retinal neurons. The results establish the functional role of organic photovoltaic nanoparticles in restoring visual activities in fully degenerate retinas with intense inner retina rewiring, a stage of the disease in which patients are subjected to prosthetic interventions.

Retinal dystrophies such as Retinitis pigmentosa are among the most prevalent causes of inherited incurable legal blindness. Here the authors demonstrate that conjugated polymer nanoparticles reinstate visual functions in aged rats with fully degenerated and rewired retinas.

Riboflavin deficiency leads to irreversible cellular changes in the RPE and disrupts retinal function through alterations in cellular metabolic homeostasis

Ariboflavinosis is a pathological condition occurring as a result of riboflavin deficiency. This condition is treatable if detected early enough, but it lacks timely diagnosis. Critical symptoms of ariboflavinosis include neurological and visual manifestations, yet the effects of flavin deficiency on the retina are not well investigated. Here, using a diet induced mouse model of riboflavin deficiency, we provide the first evidence of how retinal function and metabolism are closely intertwined with riboflavin homeostasis. We find that diet induced riboflavin deficiency causes severe decreases in retinal function accompanied by structural changes in the neural retina and retinal pigment epithelium (RPE). This is preceded by increased signs of cellular oxidative stress and metabolic disorder, in particular dysregulation in lipid metabolism, which is essential for both photoreceptors and the RPE. Though many of these deleterious phenotypes can be ameliorated by riboflavin supplementation, our data suggests that some patients may continue to suffer from multiple pathologies at later ages. These studies provide an essential cellular and mechanistic foundation linking defects in cellular flavin levels with the manifestation of functional deficiencies in the visual system and paves the way for a more in-depth understanding of the cellular consequences of ariboflavinosis.

Progress on Designing a Chemical Retinal Prosthesis

The last major review of progress toward a chemical retinal prosthesis was a decade ago. Many important advancements have been made since then with the aim of producing an implantable device for animal testing. We review that work here discussing the potential advantages a chemical retinal prosthesis may possess, the spatial and temporal resolutions it might provide, the materials from which an implant might be constructed and its likely effectiveness in stimulating the retina in a natural fashion. Consideration is also given to implant biocompatibility, excitotoxicity of dispensed glutamate and known changes to photoreceptor degenerate retinas.

Optogenetics for visual restoration: From proof of principle to translational challenges

Degenerative retinal disorders are a diverse family of diseases commonly leading to irreversible photoreceptor death, while leaving the inner retina relatively intact. Over recent years, innovative gene replacement therapies aiming to halt the progression of certain inherited retinal disorders have made their way into clinics. By rendering surviving retinal neurons light sensitive optogenetic gene therapy now offers a feasible treatment option that can restore lost vision, even in late disease stages and widely independent of the underlying cause of degeneration. Since proof-of-concept almost fifteen years ago, this field has rapidly evolved and a detailed first report on a treated patient has recently been published. In this article, we provide a review of optogenetic approaches for vision restoration. We discuss the currently available optogenetic tools and their relative advantages and disadvantages. Possible cellular targets will be discussed and we will address the question how retinal remodelling may affect the choice of the target and to what extent it may limit the outcomes of optogenetic vision restoration. Finally, we will analyse the evidence for and against optogenetic tool mediated toxicity and will discuss the challenges associated with clinical translation of this promising therapeutic concept.

Correlated Activity in the Degenerate Retina Inhibits Focal Response to Electrical Stimulation

Retinal prostheses have shown some clinical success in patients with retinitis pigmentosa and age-related macular degeneration. However, even after the implantation of a retinal prosthesis, the patient’s visual acuity is at best less than 20/420. Reduced visual acuity may be explained by a decrease in the signal-to-noise ratio due to the spontaneous hyperactivity of retinal ganglion cells (RGCs) found in degenerate retinas. Unfortunately, abnormal retinal rewiring, commonly observed in degenerate retinas, has rarely been considered for the development of retinal prostheses. The purpose of this study was to investigate the aberrant retinal network response to electrical stimulation in terms of the spatial distribution of the electrically evoked RGC population. An 8 × 8 multielectrode array was used to measure the spiking activity of the RGC population. RGC spikes were recorded in wild-type [C57BL/6J; P56 (postnatal day 56)], rd1 (P56), rd10 (P14 and P56) mice, and macaque [wild-type and drug-induced retinal degeneration (RD) model] retinas. First, we performed a spike correlation analysis between RGCs to determine RGC connectivity. No correlation was observed between RGCs in the control group, including wild-type mice, rd10 P14 mice, and wild-type macaque retinas. In contrast, for the RD group, including rd1, rd10 P56, and RD macaque retinas, RGCs, up to approximately 400–600 μm apart, were significantly correlated. Moreover, to investigate the RGC population response to electrical stimulation, the number of electrically evoked RGC spikes was measured as a function of the distance between the stimulation and recording electrodes. With an increase in the interelectrode distance, the number of electrically evoked RGC spikes decreased exponentially in the control group. In contrast, electrically evoked RGC spikes were observed throughout the retina in the RD group, regardless of the inter-electrode distance. Taken together, in the degenerate retina, a more strongly coupled retinal network resulted in the widespread distribution of electrically evoked RGC spikes. This finding could explain the low-resolution vision in prosthesis-implanted patients.

Insulin receptor activation by proinsulin preserves synapses and vision in retinitis pigmentosa

Synaptic loss, neuronal death, and circuit remodeling are common features of central nervous system neurodegenerative disorders. Retinitis pigmentosa (RP), the leading cause of inherited blindness, is a group of retinal dystrophies characterized by photoreceptor dysfunction and death. The insulin receptor, a key controller of metabolism, also regulates neuronal survival and synaptic formation, maintenance, and activity. Indeed, deficient insulin receptor signaling has been implicated in several brain neurodegenerative pathologies. We present evidence linking impaired insulin receptor signaling with RP. We describe a selective decrease in the levels of the insulin receptor and its downstream effector phospho-S6 in retinal horizontal cell terminals in the rd10 mouse model of RP, as well as aberrant synapses between rod photoreceptors and the postsynaptic terminals of horizontal and bipolar cells. A gene therapy strategy to induce sustained proinsulin, the insulin precursor, production restored retinal insulin receptor signaling, by increasing S6 phosphorylation, without peripheral metabolic consequences. Moreover, proinsulin preserved photoreceptor synaptic connectivity and prolonged visual function in electroretinogram and optomotor tests. These findings point to a disease-modifying role of insulin receptor and support the therapeutic potential of proinsulin in retinitis pigmentosa.

Single-cell RNA sequencing of the retina in a model of retinitis pigmentosa reveals early responses to degeneration in rods and cones

Background

In inherited retinal disorders such as retinitis pigmentosa (RP), rod photoreceptor-specific mutations cause primary rod degeneration that is followed by secondary cone death and loss of high-acuity vision. Mechanistic studies of retinal degeneration are challenging because of retinal heterogeneity. Moreover, the detection of early cone responses to rod death is especially difficult due to the paucity of cones in the retina. To resolve heterogeneity in the degenerating retina and investigate events in both types of photoreceptors during primary rod degeneration, we utilized droplet-based single-cell RNA sequencing in an RP mouse model, rd10.

Results

Using trajectory analysis, we defined two consecutive phases of rod degeneration at P21, characterized by the early transient upregulation of Egr1 and the later induction of Cebpd. EGR1 was the transcription factor most significantly associated with the promoters of differentially regulated genes in Egr1-positive rods in silico. Silencing Egr1 affected the expression levels of two of these genes in vitro. Degenerating rods exhibited changes associated with metabolism, neuroprotection, and modifications to synapses and microtubules. Egr1 was also the most strongly upregulated transcript in cones. Its upregulation in cones accompanied potential early respiratory dysfunction and changes in signaling pathways. The expression pattern of EGR1 in the retina was dynamic during degeneration, with a transient increase of EGR1 immunoreactivity in both rods and cones during the early stages of their degenerative processes.

Conclusion

Our results identify early and late changes in degenerating rd10 rod photoreceptors and reveal early responses to rod degeneration in cones not expressing the disease-causing mutation, pointing to mechanisms relevant for secondary cone degeneration. In addition, our data implicate EGR1 as a potential key regulator of early degenerative events in rods and cones, providing a potential broad target for modulating photoreceptor degeneration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01280-9.

Optogenetic therapy restores retinal activity in primate for at least a year following photoreceptor ablation

All retina-based vision restoration approaches rely on the assumption that photoreceptor loss does not preclude reactivation of the remaining retinal architecture. Whether extended periods of vision loss limit the efficacy of restorative therapies at the retinal level is unknown. We examined long-term changes in optogenetic responsivity of foveal retinal ganglion cells (RGCs) in non-human primates following localized photoreceptor ablation by high-intensity laser exposure. By performing fluorescence adaptive optics scanning light ophthalmoscopy (AOSLO) of RGCs expressing both the calcium indicator GCaMP6s and the optogenetic actuator ChrimsonR, it was possible to track optogenetic-mediated calcium responses in deafferented RGCs over time. Fluorescence fundus photography revealed a 40% reduction in ChrimsonR fluorescence from RGCs lacking photoreceptor input over the 3 weeks following photoreceptor ablation. Despite this, in vivo imaging revealed good cellular preservation of RGCs 3 months after the loss of photoreceptor input, and histology confirmed good structural preservation at 2 years. Optogenetic responses of RGCs in primate persisted for at least 1 year after the loss of photoreceptor input, with a sensitivity index similar to optogenetic responses recorded in intact retina. These results are promising for all potential therapeutic approaches to vision restoration that rely on preservation and reactivation of RGCs.

Functional Correlates of Outer Retina Remodeling in Intermediate Age-Related Macular Degeneration Using Microperimetry

Purpose

To assess the effect of drusen morphometric changes and choroidal vascular modifications on retinal sensitivity (RS) evaluated through microperimetry in intermediate age-related macular degeneration (iAMD).

Methods

A retrospective review of 18 iAMD patients (18 eyes) with a 12-month follow-up was performed. Eye-tracked spectral-domain optical coherence tomography was obtained, with automatic segmentation of the outer retinal layer (ORL) delineating the drusen area from the external limiting membrane to Bruch's membrane and outer nuclear layer (ONL) thickness maps adjusted manually, as needed. Advanced retinal pigment epithelium analysis was also performed with a ZEISS PLEX Elite 900. Microperimetry obtained under mesopic conditions was overlaid with the corresponding thickness maps with Fiji software. The choroidal vascularity index (CVI) was calculated in the subfoveal b-scan and volumetric in the central 1-mm subfield.

Results

A reduced central ONL thickness was strongly associated with RS decline at the same region (r = 0.69, P = 0.002) and globally (r = 0.80, P < 0.001) at baseline, but also at 1 year in the central subfield (central: r = 0.70, P = 0.001). One-year subfoveal CVI variation, differently from volumetric CVI, directly influenced the central (r = 0.64, P = 0.004) and global RS (r = 0.59, P = 0.009), indicating that a CVI reduction negatively affected RS. A greater volumetric CVI within central 1-mm was associated with ORL thickening at 1 year (r = 0.61, P = 0.008).

Conclusions

Progressive degeneration of the ONL is related to irreversible photoreceptor dysfunction in iAMD. Likewise, choroidal vascular modifications are associated with a significant functional decline in the central region and diffusely.

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.

Visual Cortex Engagement in Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a family of inherited disorders caused by the progressive degeneration of retinal photoreceptors. There is no cure for RP, but recent research advances have provided promising results from many clinical trials. All these therapeutic strategies are focused on preserving existing photoreceptors or substituting light-responsive elements. Vision recovery, however, strongly relies on the anatomical and functional integrity of the visual system beyond photoreceptors. Although the retinal structure and optic pathway are substantially preserved at least in early stages of RP, studies describing the visual cortex status are missing. Using a well-established mouse model of RP, we analyzed the response of visual cortical circuits to the progressive degeneration of photoreceptors. We demonstrated that the visual cortex goes through a transient and previously undescribed alteration in the local excitation/inhibition balance, with a net shift towards increased intracortical inhibition leading to improved filtering and decoding of corrupted visual inputs. These results suggest a compensatory action of the visual cortex that increases the range of residual visual sensitivity in RP.

Neuroprotection of the Inner Retina Also Prevents Secondary Outer Retinal Pathology in a Mouse Model of Glaucoma

Purpose

We examined structural and functional changes in the outer retina of a mouse model of glaucoma. We examined whether these changes are a secondary consequence of damage in the inner retina and whether neuroprotection of the inner retina also prevents outer retinal changes.

Methods

We used an established microbead occlusion model of glaucoma whereby intraocular pressure (IOP) was elevated. Specific antibodies were used to label rod and cone bipolar cells (BCs), horizontal cells (HCs), and retinal ganglion cells (RGCs), as well as synaptic components in control and glaucomatous eyes, to assess structural damage and cell loss. ERG recordings were made to assess outer retina function.

Results

We found structural and functional damage of BCs, including significant cell loss and dendritic/axonal remodeling of HCs, following IOP elevation. The first significant loss of both BCs occurred at 4 to 5 weeks after microbead injection. However, early changes in the dendritic structure of RGCs were observed at 3 weeks, but significant changes in the rod BC axon terminal structure were not seen until 4 weeks. We found that protection of inner retinal neurons in glaucomatous eyes by pharmacological blockade of gap junctions or genetic ablation of connexin 36 largely prevented outer retinal damage.

Conclusions

Together, our results indicate that outer retinal impairments in glaucoma are a secondary sequalae of primary damage in the inner retina. The finding that neuroprotection of the inner retina can also prevent outer retinal damage has important implications with regard to the targets for effective neuroprotective therapy.

Impaired Ca2+ Sensitivity of a Novel GCAP1 Variant Causes Cone Dystrophy and Leads to Abnormal Synaptic Transmission Between Photoreceptors and Bipolar Cells

Guanylate cyclase-activating protein 1 (GCAP1) is involved in the shutdown of the phototransduction cascade by regulating the enzymatic activity of retinal guanylate cyclase via a Ca²⁺/cGMP negative feedback. While the phototransduction-associated role of GCAP1 in the photoreceptor outer segment is widely established, its implication in synaptic transmission to downstream neurons remains to be clarified. Here, we present clinical and biochemical data on a novel isolate GCAP1 variant leading to a double amino acid substitution (p.N104K and p.G105R) and associated with cone dystrophy (COD) with an unusual phenotype. Severe alterations of the electroretinogram were observed under both scotopic and photopic conditions, with a negative pattern and abnormally attenuated b-wave component. The biochemical and biophysical analysis of the heterologously expressed N104K-G105R variant corroborated by molecular dynamics simulations highlighted a severely compromised Ca²⁺-sensitivity, accompanied by minor structural and stability alterations. Such differences reflected on the dysregulation of both guanylate cyclase isoforms (RetGC1 and RetGC2), resulting in the constitutive activation of both enzymes at physiological levels of Ca²⁺. As observed with other GCAP1-associated COD, perturbation of the homeostasis of Ca²⁺ and cGMP may lead to the toxic accumulation of second messengers, ultimately triggering cell death. However, the abnormal electroretinogram recorded in this patient also suggested that the dysregulation of the GCAP1–cyclase complex further propagates to the synaptic terminal, thereby altering the ON-pathway related to the b-wave generation. In conclusion, the pathological phenotype may rise from a combination of second messengers’ accumulation and dysfunctional synaptic communication with bipolar cells, whose molecular mechanisms remain to be clarified.

Preservation of Visual Cortex Plasticity in Retinitis Pigmentosa

Retinitis Pigmentosa (RP) is a class of inherited disorders caused by the progressive death of photoreceptors in the retina. RP is still orphan of an effective treatment, with increasing optimism deriving from research aimed at arresting neurodegeneration or replacing light-responsive elements. All these therapeutic strategies rely on the functional integrity of the visual system downstream of photoreceptors. Whereas the inner retinal structure and optic radiation are known to be considerably preserved at least in early stages of RP, very little is known about the visual cortex. Remarkably, it remains completely unclear whether visual cortex plasticity is still present in RP. Using a well-established murine model of RP, the rd10 mouse, we report that visual cortical circuits retain high levels of plasticity, preserving their capability of input-dependent remodelling even at a late stage of retinal degeneration.

Blockade of Retinal Oscillations by Benzodiazepines Improves Efficiency of Electrical Stimulation in the Mouse Model of RP, rd10

Purpose

In RP, photoreceptors degenerate. Retinal prostheses are considered a suitable strategy to restore vision. In animal models of RP, a pathologic rhythmic activity seems to compromise the efficiency of retinal ganglion cell stimulation by an electrical prosthesis. We, therefore, strove to eliminate this pathologic activity.

Methods

Electrophysiologic recordings of local field potentials and spike activity of retinal ganglion cells were obtained in vitro from retinae of wild-type and rd10 mice using multielectrode arrays. Retinae were stimulated electrically.

Results

The efficiency of electrical stimulation was lower in rd10 retina than in wild-type retina and this was highly correlated with the presence of oscillations in retinal activity. Glycine and GABA, as well as the benzodiazepines diazepam, lorazepam, and flunitrazepam, abolished retinal oscillations and, most important, increased the efficiency of electrical stimulation to values similar to those in wild-type retina.

Conclusions

Treatment of patients with these benzodiazepines may offer a way to improve the performance of retinal implants in cases with poor implant proficiency. This study may open the way to a therapy that supports electrical stimulation by prostheses with pharmacologic treatment.

Neuroplasticity of the visual cortex: in sickness and in health

Brain plasticity refers to the ability of synaptic connections to adapt their function and structure in response to experience, including environmental changes, sensory deprivation and injuries. Plasticity is a distinctive, but not exclusive, property of the developing nervous system. This review introduces the concept of neuroplasticity and describes classic paradigms to illustrate cellular and molecular mechanisms underlying synapse modifiability. Then, we summarize a growing number of studies showing that the adult cerebral cortex retains a significant degree of plasticity highlighting how the identification of strategies to enhance the plastic potential of the adult brain could pave the way for the development of novel therapeutic approaches aimed at treating amblyopia and other neurodevelopmental disorders. Finally, we analyze how the visual system adjusts to neurodegenerative conditions leading to blindness and we discuss the crucial role of spared plasticity in the visual system for sight recovery.

Retinal Degeneration Reduces Consistency of Network-Mediated Responses Arising in Ganglion Cells to Electric Stimulation

Retinal prostheses use periodic repetition of electrical stimuli to form artificial vision. To enhance the reliability of evoked visual percepts, repeating stimuli need to evoke consistent spiking activity in individual retinal ganglion cells (RGCs). However, it is not well known whether outer retinal degeneration alters the consistency of RGC responses. Hence, here we systematically investigated the trial-to-trial variability in network-mediated responses as a function of the degeneration level. We patch-clamp recorded spikes in ON and OFF types of alpha RGCs from r d10 mice at four different postnatal days (P15, P19, P31, and P60), representing distinct stages of degeneration. To assess the consistency of responses, we analyzed variances in spike count and timing across repeats of the same stimulus delivered multiple times. We found the trial-to-trial variability of network-mediated responses increased considerably as the disease progressed. Compared to responses taken before degeneration onset, those of degenerate retinas showed up to ~70% higher variability (Fano Factor) in spike counts (p < 0.001) and ~95% lower correlation level in spike timing (p < 0.001). These results indicate consistency weakens significantly in electrically-evoked network-mediated responses and therefore raise concerns about the ability of microelectronic retinal implants to elicit consistent visual percepts at advanced stages of retinal degeneration.

Development of Stem Cell Therapies for Retinal Degeneration

Degenerative retinal disease is the major cause of sight loss in the developed world and currently there is a lack of effective treatments. As the loss of vision is directly the result of the loss of retinal cells, effective cell replacement through stem-cell-based therapies may have the potential to treat a great number of retinal diseases whatever their underlying etiology. The eye is an ideal organ to develop cell therapies as it is immune privileged, and modern surgical techniques enable precise delivery of cells to the retina. Furthermore, a range of noninvasive diagnostic tests and high-resolution imaging techniques facilitate the evaluation of any therapeutic intervention. In this review, we evaluate the progress to date of current cell therapy strategies for retinal repair, focusing on transplantation of pluripotent stem-cell-derived retinal pigment epithelium (RPE) and photoreceptor cells.

Partial Cone Loss Triggers Synapse-Specific Remodeling and Spatial Receptive Field Rearrangements in a Mature Retinal Circuit

Resilience of neural circuits has been observed in the persistence of function despite neuronal loss. In vision, acuity and sensitivity can be retained after 50% loss of cones. While neurons in the cortex can remodel after input loss, the contributions of cell-type-specific circuits to resilience are unknown. Here, we study the effects of partial cone loss in mature mouse retina where cell types and connections are known. At first-order synapses, bipolar cell dendrites remodel and synaptic proteins diminish at sites of input loss. Sites of remaining inputs preserve synaptic proteins. Second-order synapses between bipolar and ganglion cells remain stable. Functionally, ganglion cell spatio-temporal receptive fields retain center-surround structure following partial cone loss. We find evidence for slower temporal filters and expanded receptive field surrounds, derived mainly from inhibitory inputs. Surround expansion is absent in partially stimulated control retina. Results demonstrate functional resilience to input loss beyond pre-existing mechanisms in control retina.

Care et al. find that photoreceptor ablation causes structural rearrangement of bipolar cell input synapses while output synapses endure. Functionally, recipient ganglion cells show altered receptive field sizes, an effect not seen after partial stimulation of control retina, demonstrating de novo changes that occur in inhibitory circuitry after photoreceptor loss.

Non-photoreceptor Expression of Tulp1 May Contribute to Extensive Retinal Degeneration in Tulp1-/- Mice

Mutations in tubby like protein 1 gene (TULP1) are causative of early-onset recessive inherited retinal degenerations (IRDs); similarly, the Tulp1-/- mouse is also characterized by a rapid IRD. Tulp1 mRNA and protein expression was analyzed in wild type mouse retinas and expression data sets (NCBI) during early postnatal development. Comparative histology was undertaken in Tulp1-/-, rhodopsin-/- (Rho-/-) and retinal degeneration slow-/- (Rds-/-) mouse retinas. Bioinformatic analysis of predicted TULP1 interactors and IRD genes was performed. Peak expression of Tulp1 in healthy mouse retinas was detected at p8; of note, TULP1 was detected in both the outer and inner retina. Bioinformatic analysis indicated Tulp1 expression in retinal progenitor, photoreceptor and non-photoreceptor cells. While common features of photoreceptor degeneration were detected in Tulp1-/-, Rho-/-, and Rds-/- retinas, other alterations in bipolar, amacrine and ganglion cells were specific to Tulp1-/- mice. Additionally, predicted TULP1 interactors differed in various retinal cell types and new functions for TULP1 were suggested. A pilot bioinformatic analysis indicated that in a similar fashion to Tulp1, many other IRD genes were expressed in both inner and outer retinal cells at p4-p7. Our data indicate that expression of Tulp1 extends to multiple retinal cell types; lack of TULP1 may lead to primary degeneration not only of photoreceptor but also non-photoreceptor cells. Predicted interactors suggest widespread retinal functions for TULP1. Early and widespread expression of TULP1 and some other IRD genes in both the inner and outer retina highlights potential hurdles in the development of treatments for these IRDs.

Homeostatic Plasticity Shapes the Retinal Response to Photoreceptor Degeneration

Homeostatic plasticity stabilizes input and activity levels during neural development, but whether it can restore connectivity and preserve circuit function during neurodegeneration is unknown. Photoreceptor degeneration is the most common cause of blindness in the industrialized world. Visual deficits are dominated by cone loss, which progresses slowly, leaving a window during which rewiring of second-order neurons (i.e., bipolar cells) could preserve function. Here we establish a transgenic model to induce cone degeneration with precise control and analyze bipolar cell responses and their effects on vision through anatomical reconstructions, in vivo electrophysiology, and behavioral assays. In young retinas, we find that three bipolar cell types precisely restore input synapse numbers when 50% of cones degenerate but one does not. Of the three bipolar cell types that rewire, two contact new cones within stable dendritic territories, whereas one expands its dendrite arbors to reach new partners. In mature retinas, only one of four bipolar cell types rewires homeostatically. This steep decline in homeostatic plasticity is accompanied by reduced light responses of bipolar cells and deficits in visual behaviors. By contrast, light responses and behavioral performance are preserved when cones degenerate in young mice. Our results reveal unexpected cell type specificity and a steep maturational decline of homeostatic plasticity. The effect of homeostatic plasticity on functional outcomes identify it as a promising therapeutic target for retinal and other neurodegenerative diseases.

Visual Contrast Sensitivity Correlates to the Retinal Degeneration in Rhodopsin Knockout Mice

Purpose

Clinical manifestations of photoreceptor degeneration include gradual thinning of the outer nuclear layer (ONL) and progressive reduction of electroretinogram (ERG) amplitudes and vision loss. Although preclinical evaluations of treatment strategies greatly depend on rodent models, the courses of these changes in mice remain unclear. We thus sought to investigate the temporal correlations in changes of spatial vision, ERG response, and ONL thickness in mice with progressive photoreceptor degeneration.

Methods

Adult wild-type (WT) mice and mice carrying rhodopsin deficiency (Rho^−/−), a frequently used mouse model of human retinitis pigmentosa, were selected for investigation. Mouse spatial vision, including visual acuity (VA) and contrast sensitivity (CS), was determined using optomotor response (OMR) assays; ONL thickness was quantified by spectral-domain optical coherence tomography (SD-OCT), and ERG was performed to evaluate retinal functions. The mice were killed when they were 14 weeks old, and the cone photoreceptors in retinal sections were counted.

Results

Spatial vision, ONL thickness, and ERG amplitudes remained stable in WT mice at all examined time points. While 6-week-old Rho^−/− mice had VA, CS, as well as ERG responses similar to those of WT mice, progressive reductions in the spatial vision and retinal functions were recorded thereafter. Most tested 12-week-old Rho^−/− mice had no visual-evoked OMR and ERG responses. Moreover, CS, but not VA, displayed a linear decline that was closely associated with ONL thinning, reduction of ERG amplitudes, and loss of cones.

Conclusions

We presented a comprehensive study of the relation between the changes of spatial vision, retinal function, and ONL thickness in postnatal week (PW)6 to PW12 Rho^−/− mice. CS is a more sensitive indicator of spatial vision compared to VA, although both are required as separate parameters for monitoring the visual changes in retina undergoing photoreceptor degeneration.

Persistent remodeling and neurodegeneration in late-stage retinal degeneration

Retinal remodeling is a progressive series of negative plasticity revisions that arise from retinal degeneration, and are seen in retinitis pigmentosa, age-related macular degeneration and other forms of retinal disease. These processes occur regardless of the precipitating event leading to degeneration. Retinal remodeling then culminates in a late-stage neurodegeneration that is indistinguishable from progressive central nervous system (CNS) proteinopathies. Following long-term deafferentation from photoreceptor cell death in humans, and long-lived animal models of retinal degeneration, most retinal neurons reprogram, then die. Glial cells reprogram into multiple anomalous metabolic phenotypes. At the same time, survivor neurons display degenerative inclusions that appear identical to progressive CNS neurodegenerative disease, and contain aberrant α-synuclein (α-syn) and phosphorylated α-syn. In addition, ultrastructural analysis indicates a novel potential mechanism for misfolded protein transfer that may explain how proteinopathies spread. While neurodegeneration poses a barrier to prospective retinal interventions that target primary photoreceptor loss, understanding the progression and time-course of retinal remodeling will be essential for the establishment of windows of therapeutic intervention and appropriate tuning and design of interventions. Finally, the development of protein aggregates and widespread neurodegeneration in numerous retinal degenerative diseases positions the retina as a ideal platform for the study of proteinopathies, and mechanisms of neurodegeneration that drive devastating CNS diseases.

Inner retinal preservation in the photoinducible I307N rhodopsin mutant mouse, a model of autosomal dominant retinitis pigmentosa

Rod‐cone degenerations, for example, retinitis pigmentosa are leading causes of blindness worldwide. Despite slow disease progression in humans, vision loss is inevitable; therefore, development of vision restoration strategies is crucial. Among others, promising approaches include optogenetics and prosthetic implants, which aim to bypass lost photoreceptors (PRs). Naturally, the efficacy of these therapeutic strategies will depend on inner retinal structural and functional preservation. The present study shows that in photoinducible I307N rhodopsin mice (Translational Vision Research Model 4 [Tvrm4]), a 12k lux light exposure eliminates PRs in the central retina in 1 week, but interneurons and their synapses are maintained for as long as 9 weeks postinduction. Despite bipolar cell dendritic retraction and moderate loss of horizontal cells, the survival rate of various cell types is very high. Significant preservation of conventional synapses and gap junctions in the inner plexiform layer is also observed. We found the number of synaptic ribbons to gradually decline and their ultrastructure to become transiently abnormal, although based on our findings intrinsic retinal architecture is maintained despite complete loss of PRs. Unlike common rodent models of PR degeneration, where the disease phenotype often interferes with retinal development, in Tvrm4 mice, the degenerative process can be induced after retinal development is complete. This time course more closely mimics the timing of disease onset in affected patients. Stability of the inner retina found in these mutants 2 months after PR degeneration suggests moderate, stereotyped remodeling in the early stages of the human disease and represents a promising finding for prompt approaches of vision restoration.

Development of a Pde6b Gene Knockout Rat Model for Studies of Degenerative Retinal Diseases

Purpose

To describe the phenotypes of a newly developed Pde6b-deficient rat model of retinal degeneration.

Methods

Pde6b knockout rats were produced by CRISPR-Cpf1 technology. Pde6b knockout rats were evaluated for ocular abnormalities by comparison with wild-type eyes. Eyes were imaged using fundus photography and optical coherence tomography (OCT), stained by hematoxylin and eosin (H&E), and examined by TUNEL assay. Finally, eyes were functionally assessed by electroretinograms (ERGs).

Results

Pde6b knockout rats exhibited visible photoreceptor degeneration at 3 weeks of postnatal age. The fundus appearance of mutants was notable for pigmentary changes, vascular attenuation with an irregular vascular pattern, and outer retinal thinning, which resembled retinitis pigmentosa (RP) in humans. OCT showed profound retinal thinning in Pde6b knockout rats; the outer nuclear layer (ONL) was significantly thinner in Pde6b knockout rats, with relative preservation of the inner retina at 3 weeks of postnatal age. H&E staining confirmed extensive degeneration of the ONL, beginning at 3 weeks of postnatal age; no ONL remained in the retina by 16 weeks of postnatal age. Retinal sections of Pde6b knockout rats were highly positive for TUNEL, specifically in the ONL. In ERGs, Pde6b knockout rats showed no detectable a- or b-waves at 8 weeks of postnatal age.

Conclusions

The Pde6b knockout rat exhibits photoreceptor degeneration. It may provide a better model for experimental therapy for RP because of its slower progression and larger anatomic architecture than the corresponding mouse model. Further studies in this rat model may yield insights into effective therapies for human RP.

Retinal Phenotype in the rd9 Mutant Mouse, a Model of X-Linked RP

Retinal degeneration 9 (rd9) mice carry a mutation in the retina specific “Retinitis Pigmentosa GTPase Regulator (RPGR)” Open Reading Frame (ORF) 15 gene, located on the X chromosome and represent a rare model of X-linked Retinitis Pigmentosa (XLRP), a common and severe form of retinal degeneration (Wright et al., 2010; Tsang and Sharma, 2018). The rd9 RPGR-ORF15 mutation in mice causes lack of the protein in photoreceptors and a slow degeneration of these cells with consequent decrease in Outer Nuclear Layer (ONL) thickness and amplitude of ERG responses, as previously described (Thompson et al., 2012). However, relative rates of rod and cone photoreceptor loss, as well as secondary alterations occurring in neuronal and non-neuronal retinal cell types of rd9 mutants remain to be assessed. Aim of this study is to extend phenotype analysis of the rd9 mouse retina focusing on changes occurring in cells directly interacting with photoreceptors. To this purpose, first we estimated rod and cone survival and its degree of intraretinal variation over time; then, we studied the morphology of horizontal and bipolar cells and of the retinal pigment epithelium (RPE), extending our observations to glial cell reactivity. We found that in rd9 retinas rod (but not cone) death is the main cause of decrease in ONL thickness and that degeneration shows a high degree of intraretinal variation. Rod loss drives remodeling in the outer retina, with sprouting of second-order neurons of the rod-pathway and relative sparing of cone pathway elements. Remarkably, despite cone survival, functional defects can be clearly detected in ERG recordings in both scotopic and photopic conditions. Moderate levels of Muller cells and microglial reactivity are sided by striking attenuation of staining for RPE tight junctions, suggesting altered integrity of the outer Blood Retina Barrier (BRB). Because of many features resembling slowly progressing photoreceptor degeneration paradigms or early stages of more aggressive forms of RP, the rd9 mouse model can be considered a rare and useful tool to investigate retinal changes associated to a process of photoreceptor death sustained throughout life and to reveal disease biomarkers (e.g., BRB alterations) of human XLRP.

Visual responses in the dorsal lateral geniculate nucleus at early stages of retinal degeneration in rd1 PDE6β mice

Inherited retinal degenerations encompass a wide range of diseases that result in the death of rod and cone photoreceptors, eventually leading to irreversible blindness. Low vision survives at early stages of degeneration, at which point it could rely on residual populations of rod/cone photoreceptors as well as the inner retinal photoreceptor, melanopsin. To date, the impact of partial retinal degeneration on visual responses in the primary visual thalamus (dorsal lateral geniculate nucleus, dLGN) remains unknown, as does their relative reliance on surviving rod and cone photoreceptors vs. melanopsin. To answer these questions, we recorded visually evoked responses in the dLGN of anesthetized rd¹ mice using in vivo electrophysiology at an age (3-5 wk) at which cones are partially degenerate and rods are absent. We found that excitatory (ON) responses to light had lower amplitude and longer latency in rd¹ mice compared with age-matched visually intact controls; however, contrast sensitivity and spatial receptive field size were largely unaffected at this early stage of degeneration. Responses were retained when those wavelengths to which melanopsin is most sensitive were depleted, indicating that they were driven primarily by surviving cones. Inhibitory responses appeared absent in the rd¹ thalamus, as did light-evoked gamma oscillations in firing. This description of fundamental features of the dLGN visual response at this intermediate stage of retinal degeneration provides a context for emerging attempts to restore vision by introducing ectopic photoreception to the degenerate retina.NEW & NOTEWORTHY This study provides new therapeutically relevant insights to visual responses in the dorsal lateral geniculate nucleus during progressive retinal degeneration. Using in vivo electrophysiology, we demonstrate that visual responses have lower amplitude and longer latency during degeneration, but contrast sensitivity and spatial receptive fields remain unaffected. Such visual responses are driven predominantly by surviving cones rather than melanopsin photoreceptors. The functional integrity of this visual pathway is encouraging for emerging attempts at visual restoration.

Retinal degeneration rat model: A study on the structural and functional changes in the retina following injection of sodium iodate

Retinal disorders account for a large proportion of ocular disorders that can lead to visual impairment or blindness, and yet our limited knowledge in the pathogenesis and choice of appropriate animal models for new treatment modalities may contribute to ineffective therapies. Although genetic in vivo models are favored, the variable expressivity and penetrance of these heterogeneous disorders can cause difficulties in assessing potential treatments against retinal degeneration. Hence, an attractive alternative is to develop a chemically-induced model that is both cost-friendly and standardizable. Sodium iodate is an oxidative chemical that is used to simulate late stage retinitis pigmentosa and age-related macular degeneration. In this study, retinal degeneration was induced through systemic administration of sodium iodate (NaIO₃) at varying doses up to 80 mg/kg in Sprague-Dawley rats. An analysis on the visual response of the rats by electroretinography (ERG) showed a decrease in photoreceptor function with NaIO₃ administration at a dose of 40 mg/kg or greater. The results correlated with the TUNEL assay, which revealed signs of DNA damage throughout the retina. Histomorphological analysis also revealed extensive structural lesions throughout the outer retina and parts of the inner retina. Our results provided a detailed view of NaIO₃-induced retinal degeneration, and showed that the administration of 40 mg/kg NaIO₃ was sufficient to generate disturbances in retinal function. The pathological findings in this model reveal a degenerating retina, and can be further utilized to develop effective therapies for RPE, photoreceptor, and bipolar cell regeneration.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Comparative Optical Coherence Tomography Angiography of Wild-Type and rd10 Mouse Retinas

Purpose

To conduct longitudinal optical coherence tomography angiography (OCTA) to characterize dynamic changes of trilaminar vascular plexuses in wild-type (WT) and retinal degeneration 10 (rd10) mouse retinas.

Methods

Longitudinal in vivo OCT/OCTA measurements of WT and rd10 mouse retinas were conducted at postnatal day 14 (P14), P17, P21, P24, and P28. OCT images were used to quantify retinal thickness changes, while OCTA images were used to investigate vascular dynamics within the trilaminar vascular plexuses, that is, superficial vascular plexus (SVP), intermediate capillary plexus (ICP), and deep capillary plexus (DCP). Blood vessel densities of all three plexus layers were quantitatively evaluated separately. The caliber of first-order blood vessel branches in the SVP layer was also measured.

Results

Vascular densities in all three plexuses continuously decreased with aging in both WT and rd10. However, abnormal density reduction in rd10 occurred at P17 in both ICP (P < 0.001) and DCP (P < 0.001). While the ICP of rd10 showed density recovery at P24, the DCP of rd10 showed significantly low density. Remarkable vascular narrowing in rd10 was also observed in the SVP, especially at P28.

Conclusions

The most severe vascular impairment happened in the DCP, while the ICP showed the transient recovery of vascular density after the onset of retinal degeneration. The SVP was most resistant to the retinal degeneration, but the first-order blood vessel branches within the SVP showed progressive narrowing.

Translational Relevance

Better understanding of the vascular changes correlated with retinal development, and retinal degeneration can provide insights in advanced development of treatment protocols of retinal degenerative diseases.

Electrical activation of degenerated photoreceptors in blind mouse retina elicited network-mediated responses in different types of ganglion cells

Electrical (e-) stimulation is explored in schemes to rescue the vision of blind people, e.g. those affected by Retinitis Pigmentosa (RP). We e-activated subretinally the surviving degenerated photoreceptors (d-Phrs) of the rd1 mouse (RP model) and evoked visual responses in the blind retina. The e-stimulation was applied with a single platinum/iridium electrode. The d-Phrs (calcium-imaging) and ganglion cells (GC) activity (MEA-recording) were recorded in simultaneous multilayer recordings. The findings of this study confirm that the d-Phrs responded to e-stimulation and modulated the retinal network-activity. The application of blockers revealed that the synaptic interactions were dependent on voltage-gated calcium channels and mediated by the transmitters glutamate and GABA. Moreover, the gap junctions coupled networks promoted the lateral-spread of the e-evoked activity in the outer (~60 µm) and inner (~120 µm) retina. The activated GCs were identified as subtypes of the ON, OFF and ON-OFF classes. In conclusion, d-Phrs are the ideal interface partners for implants to elicit enhanced visual responses at higher temporal and spatial resolution. Furthermore, the retina's intact circuity at the onset of complete blindness makes it a tempting target when considering the implantation of implants into young patients to provide a seamless transition from blinding to chip-aided vision.

Network-mediated responses of ON ganglion cells to electric stimulation become less consistent across trials during retinal degeneration

Microelectronic retinal prostheses are being developed to restore sight in individuals blinded by outer retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Unfortunately, the quality of vision restored by these devices is still limited. To improve the quality of elicited vision, our group studies the responses of retinal neurons to electric stimulation. Our previous work showed that responses mediated through the retinal network are reproducible with high temporal precision, even for spikes that occur >100 ms after stimulus onset. Because they arise through the network, it is important to understand whether such reliability changes in the degenerate retina. Here, we examined response variability at several different stages of degeneration: postnatal day 14 (P14), P18, P31 and P60 in a well-established mouse model of degeneration (rd10). Spiking responses of ON alpha RGCs were recorded multiple times to an identical electric stimulus. We found that the trial-to-trial variability increased over the course of retinal degeneration. This finding may help to explain the reported variability in the quality of elicited vision across subjects using these devices.

Non-rectangular waveforms are more charge-efficient than rectangular one in eliciting network-mediated responses of ON type retinal ganglion cells

OBJECTIVE

For individuals blinded by outer retinal degenerative diseases, retinal prostheses would be a promising option to restore sight. Unfortunately, however, the best performance of existing devices is still far removed from normal vision. One possible reason for the shortcoming is thought to be suboptimal stimulation conditions such as the waveform shape of electric stimulus. In this study, we explored the effects of varying waveforms on network-mediated responses arising in retinal ganglion cells (RGCs).

APPROACH

We used a cell-attached patch clamp technique to record RGC spiking activities in the isolated mouse retina. ON alpha RGCs were targeted by soma size and their light responses to stationary spot flashes. Spiking in targeted RGCs was measured in response to an epiretinally-delivered cathodal current pulse in four waveforms: rectangular, center triangular, increasing and decreasing ramp shapes. Each waveform was tested at three durations (20, 10, and 5 ms) with adjusted amplitude for a range of total charges (50-400 nC).

MAIN RESULTS

ON alpha RGCs always generated two bursts of spikes in responses to all stimuli conditions we tested. However, at a given charge, effects of differing waveforms were distinct in the two bursts. For the first burst, the increasing ramp was most effective among the four waveforms (p  <  0.05 for all pairwise comparisons with other waveforms). For example, in responses arising from 20 ms-long stimuli, the increasing ramp evoked ~44% more spikes on average than the rectangular shape which is the typical choice of neural stimulation. Also, the rectangular stimulus evoked the weakest response in the delayed burst arising from pulses of every duration. For instance, 20 ms-long stimuli in the three non-rectangular waveforms showed ~23% or more increment in spike counts compared to response arising from the rectangular one; but there was no statistical difference in response magnitudes across the non-rectangular waveforms.

SIGNIFICANCE

Although the rectangular waveform has been primarily used in retinal prostheses our results indicate that rectangular stimulus is not optimal for network-mediated responses of ON alpha RGCs. Instead, non-rectangular waveforms evoke stronger responses at a given charge, indicating higher charge-efficiency. Therefore, non-rectangular waveforms are expected to enhance clinical efficacy of retinal prostheses.

Responsive Neural Activities in the Primary Visual Cortex of Retina-Degenerated Rats

To study the responsive neural activities in the primary visual cortex (V1) of retinal degeneration (RD) models, experiments involving the wild-type (WT) and RD rats were conducted. The neural responses in the V1 were recorded extracellularly, while a visual stimulus with varied light intensity was given to the subjects. First, the firing rate and its relationship with light intensity were compared between the WT and RD groups. Second, the mutual information (MI) between the visual stimulus and neural response was determined for every isolated unit to quantify the amount and efficiency of information transmission in the V1 for both the control and experimental groups. Third, the local field potential (LFP) signal was characterized and its power used to compute the MI and further evaluate the function change in the RD model regarding information transmission. Analysis of spiking activity showed that the RD group exhibited a relatively decreased firing rate, information amount and efficiency compared with the control group. However, the information transmission performance of the RD model was similar to that of the WT group in the context of LFP activity. Therefore, for the RD rats, the early stage of the visual system was impaired, while the later stage of the visual system, V1, was able to capture the information about the visual stimulus, especially at the population level. Thus, this pathway could be used to restore visual ability, such as by visual prostheses.

Tissue inhibitor of metalloproteinases 1 enhances rod survival in the rd1 mouse retina

Retinitis pigmentosa (RP), an inherited retinal degenerative disease, is characterized by a progressive loss of rod photoreceptors followed by loss of cone photoreceptors. Previously, when tissue inhibitor of metalloproteinase 1 (TIMP1), a key extracellular matrix (ECM) regulator that binds to and inhibits activation of Matrix metallopeptidase 9 (MMP9) was intravitreal injected into eyes of a transgenic rhodopsin rat model of RP, S334ter-line3, we discovered cone outer segments are partially protected. In parallel, we reported that a specific MMP9 and MMP2 inhibitor, SB-3CT, interferes with mechanisms leading to rod photoreceptor cell death in an MMP9 dependent manner. Here, we extend our initial rat studies to examine the potential of TIMP1 as a treatment in retinal degeneration by investigating neuroprotective effects in a classic mouse retinal degeneration model, rdPde6b-/- (rd1). The results clearly demonstrate that intravitreal injections of TIMP1 produce extended protection to delay rod photoreceptor cell death. The mean total number of rods in whole-mount retinas was significantly greater in TIMP-treated rd1 retinas (postnatal (P) 30, P35 (P<0.0001) and P45 (P<0.05) than in saline-treated rd1 retinas. In contrast, SB-3CT did not delay rod cell death, leading us to further investigate alternative pathways that do not involve MMPs. In addition to inducing phosphorylated ERK1/2, TIMP1 significantly reduces BAX activity and delays attenuation of the outer nuclear layer (ONL). Physiological responses using scotopic electroretinograms (ERG) reveal b-wave amplitudes from TIMP1-treated retinas are significantly greater than from saline-treated rd1 retinas (P<0.05). In later degenerative stages of rd1 retinas, photopic b-wave amplitudes from TIMP1-treated rd1 retinas are significantly larger than from saline-treated rd1 retinas (P<0.05). Our findings demonstrate that TIMP1 delays photoreceptor cell death. Furthermore, this study provides new insights into how TIMP1 works in the mouse animal model of RP.

Ganglion Cell Assessment in Rodents with Retinal Degeneration

Analysis of how retinal ganglion cells change in retinal degeneration is critical for evaluating the potential of photoreceptor restorative therapies. Immunocytochemistry in combination with image analysis provides a way for quantifying not only the density of ganglion cells during disease, but also information about their morphology and an evaluation of excitatory and inhibitory synaptic inputs. Here, we describe how indirect immunofluorescence can be used in retinal whole mounts to obtain information about ganglion cells in retinal degeneration.

Non-Visual Photopigments Effects of Constant Light-Emitting Diode Light Exposure on the Inner Retina of Wistar Rats

The retina is part of the central nervous system specially adapted to capture light photons and transmit this information to the brain through photosensitive retinal cells involved in visual and non-visual activities. However, excessive light exposure may accelerate genetic retinal diseases or induce photoreceptor cell (PRC) death, finally leading to retinal degeneration (RD). Light pollution (LP) caused by the characteristic use of artificial light in modern day life may accelerate degenerative diseases or promote RD and circadian desynchrony. We have developed a working model to study RD mechanisms in a low light environment using light-emitting diode (LED) sources, at constant or long exposure times under LP conditions. The mechanism of PRC death is still not fully understood. Our main goal is to study the biochemical mechanisms of RD. We have previously demonstrated that constant light (LL) exposure to white LED produces a significant reduction in the outer nuclear layer (ONL) by classical PRC death after 7 days of LL exposure. The PRCs showed TUNEL-positive labeling and a caspase-3-independent mechanism of cell death. Here, we investigate whether constant LED exposure affects the inner-retinal organization and structure, cell survival and the expression of photopigments; in particular we look into whether constant LED exposure causes the death of retinal ganglion cells (RGCs), of intrinsically photosensitive RGCs (ipRGCs), or of other inner-retinal cells. Wistar rats exposed to 200 lx of LED for 2 to 8 days (LL 2 and LL 8) were processed for histological and protein. The results show no differences in the number of nucleus or TUNEL positive RGCs nor inner structural damage in any of LL groups studied, indicating that LL exposure affects ONL but does not produce RGC death. However, the photopigments melanopsin (OPN4) and neuropsin (OPN5) expressed in the inner retina were seen to modify their localization and expression during LL exposure. Our findings suggest that constant light during several days produces retinal remodeling and ONL cell death as well as significant changes in opsin expression in the inner nuclear layer.