Visual properties of human retinal ganglion cells

A synthetic opsin restores vision in patients with severe retinal degeneration

Inherited retinal degenerations are the leading cause of blindness worldwide, and, in advanced stages, cell loss makes gene replacement ineffective. Optogenetics offers a therapeutic opportunity to restore vision by photo-sensitizing remaining retinal neurons. However, current opsins are kinetically slow, partially activated in ambient light, unresponsive to different light colors, and target low-resolution retinal cell circuits. To overcome these limits, we engineered a synthopsin made of three selectively mutated non-mammalian proteins to achieve a broadband multi-characteristic opsin. The synthopsin was packaged into an optimized AAV2 gene-therapy vector that targets human retinal bipolar cells. In an investigator-initiated, open-label study, four blind retinitis pigmentosa patients with ABCA4 variants received a single intravitreal gene-therapy injection. Noninvasive imaging confirmed retinal gene expression via a fluorescent reporter protein. Patients showed improvement in vision, shape discrimination, and mobility through 52 weeks. There were no significant safety issues despite what is likely one of the most synthetic, non-mammalian proteins ever expressed in a human. This is the first report of a gene monotherapy that can restore vision in blind patients in a mutation-independent manner utilizing an optogenetics technology platform.

Full-Length Cryptochrome 1 in the Outer Segments of the Retinal Blue Cone Photoreceptors in Humans and Great Apes Suggests a Role Beyond Transcriptional Repression

Mammalian cryptochrome 1 (CRY1) is a central player in the circadian transcription-translation feedback loop, crucial for maintaining a roughly 24-h rhythm. CRY1 was suggested to also function as a blue-light photoreceptor in humans and has been found to be expressed at the mRNA level in various cell types of the inner retina. However, attempts to detect CRY1 at the protein level in the human retina have remained unsuccessful so far. Using various C-terminal specific antibodies recognizing full-length CRY1 protein, we consistently detected selective labeling in the outer segments of short wavelength-sensitive (SWS1, "blue") cone photoreceptor cells across human, bonobo, and gorilla retinae. No other retinal cell types were stained, which is in contrast to what would be expected of a ubiquitous clock protein. Subcellular fractionation experiments in transfected HEK cells using a C-terminal specific antibody located full-length CRY1 in the cytosol and membrane fractions. Our findings indicate that human CRY1 has several different functions including at least one nonclock function. Our results also raise the likely possibility that several different versions of CRY1 exist in humans. We suggest that truncation of the C-terminal tail, maybe to different degrees, may affect the localization and function of human CRY1.

Sustained Experimental Myopia Exacerbates the Effect of Eye Growth on Retinal Ganglion Cell Density and Function

The aim of this study is to describe the effect that sustained myopic eye growth has on the cellular distribution and function of retinal ganglion cells as myopia progresses over time. Ganglion cell density and the photopic negative response (PhNR) were assessed using immunochemistry and electroretinography (ERG), respectively, on twelve common marmoset eyes (Callithrix jacchus). Myopia was induced in six eyes using negative defocus (three eyes from 2 to 6 months of age, 6-month-old myopes; three eyes from 2 to 12 months of age, 12-month-old myopes). These six treated eyes were compared to six age-matched control eyes. Marmosets induced with myopia for four months showed a reduced pan-retinal ganglion cell density, which continued to decrease in the peripapillary area of marmosets induced with sustained myopia for ten months. Ganglion cell density decreased as a function of axial length. Full-field ERGs revealed a dampening of the PhNR in the 12-month-old, but not 6-month-old myopes. The myopic changes observed in ganglion cell density and retinal function suggest a reorganization of the ganglion cell template during myopia development and progression that increases over time with sustained myopic eye growth and translates into functional alterations at later stages of myopia development in the absence of degenerative changes. It remains unknown whether these changes positively or negatively impact retinal function and health.

In Vitro Models of Diabetes: Focus on Diabetic Retinopathy

Diabetic retinopathy is a major eye complication in patients with diabetes mellitus, and it is the leading cause of blindness and visual impairment in the world. Chronic hyperglycemia induces endothelial damage with consequent vascular lesions, resulting in global vasculitis, which affects the small vessels of the retina. These vascular lesions cause ischemic conditions in certain areas of the retina, with a consequent increase in the release of pro-angiogenic mediators. In addition to pharmacological interventions for controlling the blood glycaemic level, the main strategies for treating diabetic retinopathy are the intravitreal injections of drugs, surgical treatments, and vitrectomies. The complexity of diabetic retinopathy is due to its close interactions with different cell types (endothelial cells, astrocytes, and Müller cells). The evaluation of the efficacy of novel pharmacological strategies is mainly performed through in vivo models. However, the use of different animal species leads to heterogenic results and ethical concerns. For these reasons, the development of new and reliable in vitro models, such as cell co-cultures and eye organoids, represents an urgent need in this area of research. This review features an overview of the in vitro models used to date and highlights the advances in technology used to study this pathology.

Functional diversity in the output of the primate retina

The visual image transmitted by the retina to the brain has long been understood in terms of spatial filtering by the center-surround receptive fields of retinal ganglion cells (RGCs). Recently, this textbook view has been challenged by the stunning functional diversity and specificity observed in ∼40 distinct RGC types in the mouse retina. However, it is unclear whether the ∼20 morphologically and molecularly identified RGC types in primates exhibit similar functional diversity, or instead exhibit center-surround organization at different spatial scales. Here, we reveal striking and surprising functional diversity in macaque and human RGC types using large-scale multi-electrode recordings from isolated macaque and human retinas. In addition to the five well-known primate RGC types, 18-27 types were distinguished by their functional properties, likely revealing several previously unknown types. Surprisingly, many of these cell types exhibited striking non-classical receptive field structure, including irregular spatial and chromatic properties not previously reported in any species. Qualitatively similar results were observed in recordings from the human retina. The receptive fields of less-understood RGC types formed uniform mosaics covering visual space, confirming their classification, and the morphological counterparts of two types were established using single-cell recording. The striking receptive field diversity was paralleled by distinctive responses to natural movies and complexity of visual computation. These findings suggest that diverse RGC types, rather than merely filtering the scene at different spatial scales, instead play specialized roles in human vision.

Response of Human Retinal Networks to Electrical Stimulation using 3D Intra-retinal Microelectrodes

This research presents a computational model of the human retina's response to electrical stimulation using three-dimensional penetrating electrodes. Our model suggests that varying depths of insertion, coupled with specific stimulation parameters, can maximize the control over retinal ganglion cells. Notably, intra-retinal electrodes positioned in the inner plexiform layer can selectively modulate both ON and OFF functional pathways via direct activation of AII amacrine cells. In contrast, electrodes reaching the inner nuclear layer can indirectly inhibit the OFF pathway via gap junction coupling between bipolar cells and AII amacrine cells, and directly activate the ON pathway via ON bipolar cells. Furthermore, our simulations provide insights into the multi-faceted mechanisms underlying the differential retinal responses to intraretinal electrical stimulation. These mechanisms include differentially targeted neural subtypes and variations in the number of pre-synaptic neurons. New technologies capable of selective stimulation of different functional retinal pathways offer further hope to improve the level of artificial vision provided to blind patients. This powerful model allows us to assess how novel electrode and stimulation parameters influence retinal network electrical activity, guiding future development of more sophisticated retinal prosthetic devices and stimulation strategies to maximize therapeutic efficacy.

Temporal Sensitivity for Achromatic and Chromatic Flicker across the Visual Cortex

The retinal ganglion cells (RGCs) receive different combinations of L, M, and S cone inputs and give rise to one achromatic and two chromatic postreceptoral channels. The goal of the current study was to determine temporal sensitivity across the three postreceptoral channels in subcortical and cortical regions involved in human vision. We measured functional magnetic resonance imaging (fMRI) responses at 7 T from three participants (two males, one female) viewing a high-contrast, flickering, spatially uniform wide field (∼140°). Stimulus flicker frequency varied logarithmically between 2 and 64 Hz and targeted the L + M + S, L - M, and S - (L + M) cone combinations. These measurements were used to create temporal sensitivity functions of the primary visual cortex (V1) across eccentricity and spatially averaged responses from the lateral geniculate nucleus (LGN), and the V2/V3, hV4, and V3A/B regions. fMRI responses reflected the known properties of the visual system, including higher peak temporal sensitivity to achromatic versus chromatic stimuli and low-pass filtering between the LGN and V1. Peak temporal sensitivity increased across levels of the cortical visual hierarchy. Unexpectedly, peak temporal sensitivity varied little across eccentricity within area V1. Measures of adaptation and distributed pattern activity revealed a subtle influence of 64 Hz achromatic flicker in area V1, despite this stimulus evoking only a minimal overall response. The comparison of measured cortical responses to a model of the integrated retinal output to our stimuli demonstrates that extensive filtering and amplification are applied to postretinal signals.

Theoretical prediction of broadband ambient light optogenetic vision restoration with ChRmine and its mutants

Vision restoration is one of the most promising applications of optogenetics. However, it is limited due to the poor-sensitivity, slow-kinetics and narrow band absorption spectra of opsins. Here, a detailed theoretical study of retinal ganglion neurons (RGNs) expressed with ChRmine, ReaChR, CoChR, CatCh and their mutants, with near monochromatic LEDs, and broadband sunlight, halogen lamp, RGB LED light, and pure white light sources has been presented. All the opsins exhibit improved light sensitivity and larger photocurrent on illuminating with broadband light sources compared to narrow band LEDs. ChRmine allows firing at ambient sunlight (1.5 nW/mm2) and pure white light (1.2 nW/mm2), which is lowest among the opsins considered. The broadband activation spectrum of ChRmine and its mutants is also useful to restore color sensitivity. Although ChRmine exhibits slower turn-off kinetics with broadband light, high-fidelity spikes can be evoked upto 50 Hz. This limit extends upto 80 Hz with the improved hsChRmine mutant although it requires double the irradiance compared to ChRmine. The present study shows that ChRmine and its mutants allow activation of RGNs with ambient light which is useful for goggle-free white light optogenetic retinal prostheses with improved quality of restored vision.

Temporal sensitivity for achromatic and chromatic flicker across the visual cortex

The retinal ganglion cells (RGCs) receive different combinations of L, M, and S cone inputs and give rise to one achromatic and two chromatic post-receptoral channels. Beyond the retina, RGC outputs are subject to filtering and normalization along the geniculo-striate pathway, ultimately producing the properties of human vision. The goal of the current study was to determine temporal sensitivity across the three post-receptoral channels in subcortical and cortical regions involved in vision. We measured functional magnetic resonance imaging (MRI) responses at 7 Tesla from three participants (two males, one female) viewing a high-contrast, flickering, spatially-uniform wide field (~140°). Stimulus flicker frequency varied logarithmically between 2 and 64 Hz and targeted the L+M+S, L-M, and S-[L+M] cone combinations. These measurements were used to create temporal sensitivity functions of primary visual cortex (V1) across eccentricity, and spatially averaged responses from lateral geniculate nucleus (LGN), V2/V3, hV4, and V3A/B. Functional MRI responses reflected known properties of the visual system, including higher peak temporal sensitivity to achromatic vs. chromatic stimuli, and low-pass filtering between the LGN and V1. Peak temporal sensitivity increased across levels of the cortical visual hierarchy. Unexpectedly, peak temporal sensitivity varied little across eccentricity within area V1. Measures of adaptation and distributed pattern activity revealed a subtle influence of 64 Hz achromatic flicker in area V1, despite this stimulus evoking only a minimal overall response. Comparison of measured cortical responses to a model of integrated retinal output to our stimuli demonstrates that extensive filtering and amplification is applied to post-retinal signals.

Oxidative stress in the eye and its role in the pathophysiology of ocular diseases

Oxidative stress occurs through an imbalance between the generation of reactive oxygen species (ROS) and the antioxidant defense mechanisms of cells. The eye is particularly exposed to oxidative stress because of its permanent exposure to light and due to several structures having high metabolic activities. The anterior part of the eye is highly exposed to ultraviolet (UV) radiation and possesses a complex antioxidant defense system to protect the retina from UV radiation. The posterior part of the eye exhibits high metabolic rates and oxygen consumption leading subsequently to a high production rate of ROS. Furthermore, inflammation, aging, genetic factors, and environmental pollution, are all elements promoting ROS generation and impairing antioxidant defense mechanisms and thereby representing risk factors leading to oxidative stress. An abnormal redox status was shown to be involved in the pathophysiology of various ocular diseases in the anterior and posterior segment of the eye. In this review, we aim to summarize the mechanisms of oxidative stress in ocular diseases to provide an updated understanding on the pathogenesis of common diseases affecting the ocular surface, the lens, the retina, and the optic nerve. Moreover, we discuss potential therapeutic approaches aimed at reducing oxidative stress in this context.

An ON-type direction-selective ganglion cell in primate retina

To maintain a stable and clear image of the world, our eyes reflexively follow the direction in which a visual scene is moving. Such gaze-stabilization mechanisms reduce image blur as we move in the environment. In non-primate mammals, this behaviour is initiated by retinal output neurons called ON-type direction-selective ganglion cells (ON-DSGCs), which detect the direction of image motion and transmit signals to brainstem nuclei that drive compensatory eye movements1. However, ON-DSGCs have not yet been identified in the retina of primates, raising the possibility that this reflex is mediated by cortical visual areas. Here we mined single-cell RNA transcriptomic data from primate retina to identify a candidate ON-DSGC. We then combined two-photon calcium imaging, molecular identification and morphological analysis to reveal a population of ON-DSGCs in the macaque retina. The morphology, molecular signature and GABA (γ-aminobutyric acid)-dependent mechanisms that underlie direction selectivity in primate ON-DSGCs are highly conserved with those in other mammals. We further identify a candidate ON-DSGC in human retina. The presence of ON-DSGCs in primates highlights the need to examine the contribution of subcortical retinal mechanisms to normal and aberrant gaze stabilization in the developing and mature visual system.

Cone-Opponent Ganglion Cells in the Primate Fovea Tuned to Non-Cardinal Color Directions

A long-standing question in vision science is how the three cone photoreceptor types - long (L), medium (M) and short (S) wavelength sensitive - combine to generate our perception of color. Hue perception can be described along two opponent axes: red-green and blue-yellow. Psychophysical measurements of color appearance indicate that the cone inputs to the red-green and blue-yellow opponent axes are M vs. L+S and L vs. M+S, respectively. However, the "cardinal directions of color space" revealed by psychophysical measurements of color detection thresholds are L vs. M and S vs. L+M. The cardinal directions match the most common cone-opponent retinal ganglion cells (RGCs) in the primate retina. Accordingly, the cone opponency necessary for color appearance is thought to be established in cortex. However, small populations with the appropriate M vs. L+S and L vs. M+S cone-opponency have been reported in large surveys of cone inputs to primate RGCs and their projections to the lateral geniculate nucleus (LGN) yet their existence continues to be debated. Resolving this long-standing open question is needed as a complete account of the cone-opponency in the retinal output is critical for efforts to understand how downstream neural circuits process color. Here, we performed adaptive optics calcium imaging to longitudinally and noninvasively measurements of the foveal RGC light responses in the living macaque eye. We confirm the presence of L vs. M+S and M vs. L+S neurons with non-cardinal cone-opponency and demonstrate that cone-opponent signals in the retinal output are substantially more diverse than classically thought.

The Bovine Ex Vivo Retina: A Versatile Model for Retinal Neuroscience

Purpose

The isolated ex vivo retina is the standard model in retinal physiology and neuroscience. During isolation, the retina is peeled from the retinal pigment epithelium (RPE), which plays a key role in the visual cycle. Here we introduce the choroid-attached bovine retina as an in vivo-like model for retinal physiology. We find that-in the bovine eye-the choroid and retina can be peeled from the sclera as a single thin sheet. Importantly, the retina remains tightly associated with the RPE, which is sandwiched between the retina and the choroid. Furthermore, bovine tissue is readily available and cheap, and there are no ethical concerns related to the use of animals solely for research purposes.

Methods

We combine multi-electrode array and single-cell patch-clamp recordings to characterize light responses in the choroid-attached bovine ex vivo retina.

Results

We demonstrate robust and consistent light responses in choroid-attached preparations. Importantly, light responses adapt to different levels of background illumination and rapidly recover from photobleaching. The choroid-attached retina is also thin enough to permit targeted electrophysiological recording from individual retinal neurons using standard differential interference contrast microscopy. We also characterize light responses and membrane properties of bovine retinal ganglion cells and compare data obtained from bovine and murine retinas.

Conclusions

The choroid-attached retinal model retains the advantages of the isolated retina but with an intact visual cycle and represents a useful tool to elucidate retinal physiology.

The Role of Nrf2/sMAF Signalling in Retina Ageing and Retinal Diseases

Age-related diseases, such as Parkinson's disease, Alzheimer's disease, cardiovascular diseases, cancers, and age-related macular disease, have become increasingly prominent as the population ages. Oxygen is essential for living organisms, but it may also cause disease when it is transformed into reactive oxygen species via biological processes in cells. Most of the production of ROS occurs in mitochondrial complexes I and III. The accumulation of ROS in cells causes oxidative stress, which plays a crucial role in human ageing and many diseases. Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a key antioxidant transcription factor that plays a central role in many diseases and ageing in general. It regulates many downstream antioxidative enzymes when cells are exposed to oxidative stress. A basic-region leucine zipper (bZIP) transcription factor, MAF, specifically the small MAF subfamily (sMAFs), forms heterodimers with Nrf2, which bind with Maf-recognition elements (MAREs) in response to oxidative stress. The role of this complex in the human retina remains unclear. This review summarises the current knowledge about Nrf2 and its downstream signalling, especially its cofactor-MAF, in ageing and diseases, with a focus on the retina. Since Nrf2 is the master regulator of redox homeostasis in cells, we hypothesise that targeting Nrf2 is a promising therapeutic approach for many age-related diseases.

Differences in visual stimulation between reading and walking and implications for myopia development

Visual input plays an important role in the development of myopia (nearsightedness), a visual disorder that blurs vision at far distances. The risk of myopia progression increases with the time spent reading and decreases with outdoor activity for reasons that remain poorly understood. To investigate the stimulus parameters driving this disorder, we compared the visual input to the retina of humans performing two tasks associated with different risks of myopia progression, reading and walking. Human subjects performed the two tasks while wearing glasses with cameras and sensors that recorded visual scenes and visuomotor activity. When compared with walking, reading black text in white background reduced spatiotemporal contrast in central vision and increased it in peripheral vision, leading to a pronounced reduction in the ratio of central/peripheral strength of visual stimulation. It also made the luminance distribution heavily skewed toward negative dark contrast in central vision and positive light contrast in peripheral vision, decreasing the central/peripheral stimulation ratio of ON visual pathways. It also decreased fixation distance, blink rate, pupil size, and head-eye coordination reflexes dominated by ON pathways. Taken together with previous work, these results support the hypothesis that reading drives myopia progression by understimulating ON visual pathways.

No evidence for age-related alterations in the marmoset retina

The physiological aging process of the retina is accompanied by various and sometimes extensive changes: Macular degeneration, retinopathies and glaucoma are the most common findings in the elderly and can potentially lead to irreversible visual disablements up to blindness. To study the aging process and to identify possible therapeutic targets to counteract these diseases, the use of appropriate animal models is mandatory. Besides the most commonly used rodent species, a non-human primate, the common marmoset (Callithrix jacchus) emerged as a promising animal model of human aging over the last years. However, the visual aging process in this species is only partially characterized, especially with regard to retinal aberrations. Therefore, we assessed here for the first time potential changes in retinal morphology of the common marmoset of different age groups. By cell type specific immunolabeling, we analyzed different cell types and distributions, potential photoreceptor and ganglion cell loss, and structural reorganization. We detected no signs of age-related differences in staining patterns or densities of various cell populations. For example, there were no signs of photoreceptor degeneration, and there was only minimal sprouting of rod bipolar cells in aged retinas. Altogether, we describe here the maintenance of a stable neuronal architecture, distribution and number of different cell populations with only mild aberrations during the aging process in the common marmoset retina. These findings are in stark contrast to previously reported findings in rodent species and humans and deserve further investigations to identify the underlying mechanisms and possible therapeutic targets.

Photopharmacology for vision restoration

Blinding diseases that are caused by degeneration of rod and cone photoreceptor cells often spare the rest of the retinal circuit, from bipolar cells, which are directly innervated by photoreceptor cells, to the output ganglion cells that project axons to the brain. A strategy for restoring vision is to introduce light sensitivity to the surviving cells of the retina. One approach is optogenetics, in which surviving cells are virally transfected with a gene encoding a signaling protein that becomes sensitive to light by binding to the biologically available chromophore retinal, the same chromophore that is used by the opsin photo-detectors of rods and cones. A second approach uses photopharmacology, in which a synthetic photoswitch associates with a native or engineered ion channel or receptor. We review these approaches and look ahead to the next generation of advances that could reconstitute core aspects of natural vision.

Revival of light signalling in the postmortem mouse and human retina

Death is defined as the irreversible cessation of circulatory, respiratory or brain activity. Many peripheral human organs can be transplanted from deceased donors using protocols to optimize viability. However, tissues from the central nervous system rapidly lose viability after circulation ceases1,2, impeding their potential for transplantation. The time course and mechanisms causing neuronal death and the potential for revival remain poorly defined. Here, using the retina as a model of the central nervous system, we systemically examine the kinetics of death and neuronal revival. We demonstrate the swift decline of neuronal signalling and identify conditions for reviving synchronous in vivo-like trans-synaptic transmission in postmortem mouse and human retina. We measure light-evoked responses in human macular photoreceptors in eyes removed up to 5 h after death and identify modifiable factors that drive reversible and irreversible loss of light signalling after death. Finally, we quantify the rate-limiting deactivation reaction of phototransduction, a model G protein signalling cascade, in peripheral and macular human and macaque retina. Our approach will have broad applications and impact by enabling transformative studies in the human central nervous system, raising questions about the irreversibility of neuronal cell death, and providing new avenues for visual rehabilitation.

Improving the spatial resolution of artificial vision using midget retinal ganglion cell populations modelled at the human fovea

OBJECTIVE

Retinal prostheses seek to create artificial vision by stimulating surviving retinal neurons of patients with profound vision impairment. Notwithstanding tremendous research efforts, the performance of all implants tested to date has remained rudimentary, incapable of overcoming the threshold for legal blindness. To maximize the perceptual efficacy of retinal prostheses, a device must be capable of controlling retinal neurons with greater spatiotemporal precision. Most studies of retinal stimulation were derived from either non-primate species or the peripheral primate retina. We investigated if artificial stimulation could leverage the high spatial resolution afforded by the neural substrates at the primate fovea and surrounding regions to achieve improved percept qualities.

APPROACH

We began by developing a new computational model capable of generating anatomically accurate retinal ganglion cell (RGC) populations within the human central retina. Next, multiple RGC populations across the central retina were stimulated in-silico to compare clinical and recently proposed neurostimulation configurations based on their ability to improve perceptual efficacy and reduce activation thresholds.

MAIN RESULTS

Our model uniquely upholds eccentricity-dependent characteristics such as RGC density and dendritic field diameter, whilst incorporating anatomically accurate features such as axon projection and three-dimensional RGC layering, features often forgone in favor of reduced computational complexity. Following epiretinal stimulation, the RGCs in our model produced response patterns in shapes akin to the complex percepts reported in clinical trials. Our results also demonstrated that even within the neuron-dense central retina, epiretinal stimulation using a multi-return hexapolar electrode arrangement could reliably achieve spatially focused RGC activation and could achieve single-cell excitation in 74% of all tested locations.

SIGNIFICANCE

This study establishes an anatomically accurate three-dimensional model of the human central retina and demonstrates the potential for an epiretinal hexapolar configuration to achieve consistent, spatially confined retinal responses, even within the neuron-dense foveal region. Our results promote the prospect and optimization of higher spatial resolution in future epiretinal implants.

ON and OFF Signaling Pathways in the Retina and the Visual System

Visual processing starts at the retina of the eye, and signals are then transferred primarily to the visual cortex and the tectum. In the retina, multiple neural networks encode different aspects of visual input, such as color and motion. Subsequently, multiple neural streams in parallel convey unique aspects of visual information to cortical and subcortical regions. Bipolar cells, which are the second order neurons of the retina, separate visual signals evoked by light and dark contrasts and encode them to ON and OFF pathways, respectively. The interplay between ON and OFF neural signals is the foundation for visual processing for object contrast which underlies higher order stimulus processing. ON and OFF pathways have been classically thought to signal in a mirror-symmetric manner. However, while these two pathways contribute synergistically to visual perception in some instances, they have pronounced asymmetries suggesting independent operation in other cases. In this review, we summarize the role of the ON-OFF dichotomy in visual signaling, aiming to contribute to the understanding of visual recognition.

Relationship Between Flicker Modulation Sensitivity and Retinal Ganglion Cell Related Layer Thicknesses

Purpose

Early detection of structural changes in retinal ganglion cells (RGCs) and corresponding changes in visual function is important in early degenerative diseases of the retina, but the sensitivity of both measurements is limited by the inherent variability in healthy subjects. This study investigates the relationships between RGC-related layer thicknesses and foveal and parafoveal flicker modulation sensitivity (FMS) across photopic and mesopic light levels in healthy subjects.

Methods

Photopic and mesopic FMS was measured in 56 young adults, at the point of fixation and at an eccentricity of 5 degrees, in each of the four quadrants. Spectral-domain optical coherence tomography (SD-OCT) was used to measure retinal thicknesses. Relationships between foveal and parafoveal FMS and the retinal thickness in the corresponding region were examined after adjusting for confounding variables.

Results

Total macular and inner retinal layer (IRL) thicknesses in the parafoveal ring were significant predictors of photopic (P = 0.034) and mesopic (P = 0.034) parafoveal FMS, respectively. The superior peripapillary retinal nerve fiber layer (pRNFL) thickness was a contributing factor to the inferior parafoveal FMS (photopic: P = 0.006 and mesopic: P = 0.021) and the inferior pRNFL thickness was also a contributing factor to the superior parafoveal FMS (photopic: P < 0.001 and mesopic: P = 0.015).

Conclusions

The pRNFL thicknesses predict parafoveal FMS for both mesopic and photopic conditions in healthy eyes.

Translational Relevance

The measurement of rapid flicker sensitivity in the parafoveal retina together with the pRNFL thickness profiles measured before the onset of disease, may provide a more sensitive biomarker for detecting loss of sensitivity caused by the earliest neurodegenerative changes in the eyes.

Morphology, Molecular Characterization, and Connections of Ganglion Cells in Primate Retina

The eye sends information about the visual world to the brain on over 20 parallel signal pathways, each specialized to signal features such as spectral reflection (color), edges, and motion of objects in the environment. Each pathway is formed by the axons of a separate type of retinal output neuron (retinal ganglion cell). In this review, we summarize what is known about the excitatory retinal inputs, brain targets, and gene expression patterns of ganglion cells in humans and nonhuman primates. We describe how most ganglion cell types receive their input from only one or two of the 11 types of cone bipolar cell and project selectively to only one or two target regions in the brain. We also highlight how genetic methods are providing tools to characterize ganglion cells and establish cross-species homologies.

Intrinsic Morphologic and Physiologic Development of Human Derived Retinal Ganglion Cells In Vitro

Purpose

Human retinal ganglion cells (hRGC) derived from human pluripotent stem cells are promising candidates to model, protect, and replace degenerating RGCs. Here, we examined intrinsic morphologic and physiologic development of hRGCs.

Methods

We used CRISPR-Cas9 to selectively express tdTomato under the RGC-specific promoter, BRN3B. Human pluripotent stem cells were chemically differentiated into hRGCs and cultured up to 7 weeks. We measured soma area, neurite complexity, synaptic protein, axon-related messenger RNA and protein, and voltage-dependent responses.

Results

Soma area, neurite complexity, and postsynaptic density protein 95 increased over time. Soma area and neurite complexity increased proportionally week to week, and this relationship was dynamic, strengthening between 2 and 3 weeks and diminishing by 4 weeks. Postsynaptic density 95 localization was dependent on culture duration. After 1 to 2 weeks, postsynaptic density 95 localized within somas but redistributed along neurites after 3 to 4 weeks. Axon initial segment scaffolding protein, Ankyrin G, expression also increased over time, and by 7 weeks, Ankyrin G often localized within putative axons. Voltage-gated inward currents progressively developed, but outward currents matured by 4 weeks. Current-induced spike generation increased over time but limited by depolarization block.

Conclusions

Human RGCs develop up to 7 weeks after culture. Thus, the state of hRGC maturation should be accounted for in designing models and treatments for optic neuropathies.

Translational Relevance

We characterized hRGC morphologic and physiologic development towards identifying key time points when hRGCs express mechanisms that may be harnessed to enhance the efficacy of neuroprotective and cell replacement therapies.

Theoretical analysis of optogenetic spiking with ChRmine, bReaChES and CsChrimson-expressing neurons for retinal prostheses

Objective.Optogenetics has emerged as a promising technique for neural prosthetics, especially retinal prostheses, with unprecedented spatiotemporal resolution. Newly discovered opsins with high light sensitivity and fast temporal kinetics can provide sufficient temporal resolution at safe light powers and overcome the limitations of presently used opsins. It is also important to formulate accurate mathematical models for optogenetic retinal prostheses, which can facilitate optimization of photostimulation factors to improve the performance.Approach.A detailed theoretical analysis of optogenetic excitation of model retinal ganglion neurons (RGNs) and hippocampal neurons expressed with already tested opsins for retinal prostheses, namely, ChR2, ReaChR and ChrimsonR, and also with recently discovered potent opsins CsChrimson, bReaChES and ChRmine, was carried out.Main results.Under continuous illumination, ChRmine-expressing RGNs begin to respond at very low irradiances ∼10^-4mW mm^-2, and evoke firing upto ∼280 Hz, highest among other opsin-expressing RGNs, at 10^-2mW mm^-2. Under pulsed illumination at randomized photon fluxes, ChRmine-expressing RGNs respond to changes in pulse to pulse irradiances upto four logs, although very bright pulses >10¹⁴photons mm^-2s^-1block firing in these neurons. The minimum irradiance threshold for ChRmine-expressing RGNs is lower by two orders of magnitude, whereas, the first spike latency in ChRmine-expressing RGNs is shorter by an order of magnitude, alongwith stable latency of subsequest spikes compared to others. Further, a good set of photostimulation parameters were determined to achieve high-frequency control with single spike resolution at minimal power. Although ChrimsonR enables spiking upto 100 Hz in RGNs, it requires very high irradiances. ChRmine provides control at light powers that are two orders of magnitude smaller than that required with experimentally studied opsins, while maintaining single spike temporal resolution upto 40 Hz.Significance.The present study highlights the importance of ChRmine as a potential opsin for optogenetic retinal prostheses.

Intrinsically Photosensitive Retinal Ganglion Cells of the Human Retina

Light profoundly affects our mental and physical health. In particular, light, when not delivered at the appropriate time, may have detrimental effects. In mammals, light is perceived not only by rods and cones but also by a subset of retinal ganglion cells that express the photopigment melanopsin that renders them intrinsically photosensitive (ipRGCs). ipRGCs participate in contrast detection and play critical roles in non-image-forming vision, a set of light responses that include circadian entrainment, pupillary light reflex (PLR), and the modulation of sleep/alertness, and mood. ipRGCs are also found in the human retina, and their response to light has been characterized indirectly through the suppression of nocturnal melatonin and PLR. However, until recently, human ipRGCs had rarely been investigated directly. This gap is progressively being filled as, over the last years, an increasing number of studies provided descriptions of their morphology, responses to light, and gene expression. Here, I review the progress in our knowledge of human ipRGCs, in particular, the different morphological and functional subtypes described so far and how they match the murine subtypes. I also highlight questions that remain to be addressed. Investigating ipRGCs is critical as these few cells play a major role in our well-being. Additionally, as ipRGCs display increased vulnerability or resilience to certain disorders compared to conventional RGCs, a deeper knowledge of their function could help identify therapeutic approaches or develop diagnostic tools. Overall, a better understanding of how light is perceived by the human eye will help deliver precise light usage recommendations and implement light-based therapeutic interventions to improve cognitive performance, mood, and life quality.