Structure of cone photoreceptors

3D color-rendered MR neurography heatmaps in visualizing normal lumbosacral (LS) plexus and increasing conspicuity of LS plexopathy

PURPOSE

To determine whether color-rendered 3D MR neurography (MRN) images (heatmaps) improve diagnostic accuracy, reader confidence levels, and time savings to assess LS plexus lesions compared to the conventional grayscale images.

MATERIALS AND METHODS

A cross-sectional study included adults of all genders with randomly chosen MRNs of LS plexus and known reference standards of normal or neuropathy (plexopathy and radiculopathy). Heatmaps were constructed using 3D MRN STIR images and color rendered with higher intensity to yellow and lower intensity to darker-red colors in 1-2 min on average and were available on PACS for the readers. 2D plus 3D grayscale MIP images and 2D plus 3D MIP heatmaps were analyzed by four musculoskeletal radiologists (two faculty and two fellows) in two separate rounds blinded to the final diagnosis. Readers evaluated: neuropathy and number of nerves affected (neuropathy score: 0-normal; 1-one nerve affected; and 2-two or more nerves affected); final diagnosis; confidence levels; and time taken to evaluate the studies. Conger's kappa and paired t-test were used for analysis.

RESULTS

Among 70 MRNs from 70 patients, there were 32 males and 38 females with average age ± SD of 54.8 ± 20.1 years and 49.9 ± 16.6 years, respectively. There were 30 normal and 40 LS plexus lesion scans. Interreader agreements for neuropathy scores were substantial to moderate on conventional imaging and heat maps (Conger's kappa: 0.65; 95% CI: 0.55, 0.73, and 0.59; 95% CI: 0.47, 0.69), respectively. The mean neuropathy score and final diagnosis accuracies were similar in both rounds 85.7% ± 0.1% vs 83.2% ± 0.1% (p = 0.13), and 83.6% ± 0.1% vs 80.0% ± 0.1%; p = 0.16), respectively. Time savings were significant when using heatmaps for all readers (p < 0.001). Time savings using heatmaps ranged from 57.7% to 74.6% and 56.3% to 75% of the original time for the fellows and faculty, respectively. Average confidence levels for neuropathy score significantly increased using heatmaps for one fellow and one faculty (p < 0.05), while average confidence levels for final diagnosis improved for both fellows and one faculty (p < 0.05).

CONCLUSION

3D color-rendered MRN heatmaps show comparable diagnostic accuracy to conventional MRN imaging but with significant time savings to identify LS plexus lesions.

KEY POINTS

Question Do color-rendered 3D MRN images (heatmaps) improve accuracy, and confidence, and save time when assessing lumbosacral (LS) plexus lesions compared to conventional grayscale images? Findings 3D-rendered heatmaps showed comparable diagnostic accuracy with time savings ranging from 56.3% to 75%. Clinical relevance 3D color-rendered heatmaps increase time efficiency in evaluating MRNs of LS plexus, allowing for improved radiologist productivity and diagnostic confidence.

Engineered red Opto-mGluR6 Opsins, a red-shifted optogenetic excitation tool, an in vitro study

Degenerative eye diseases cause partial or complete blindness due to photoreceptor degeneration. Optogenetic gene therapy is a revolutionary technique combining genetics and optical methods to control the function of neurons. Due to the inherent risk of photochemical damage, the light intensity necessary to activate Opto-mGluR6 surpasses the safe threshold for retinal illumination. Conversely, red-shifted lights pose a significantly lower risk of inducing such damage compared to blue lights. We designed red-shifted Opto-mGluR6 photopigments with a wide, red-shifted working spectrum compared to Opto-mGluR6 and examined their excitation capability in vitro. ROM19, ROM18 and ROM17, red-shifted variants of Opto-mGluR6, were designed by careful bioinformatics/computational studies. The predicted molecules with the best scores were selected, synthesised and cloned into the pAAV-CMV-IRES-EGFP vector. Expression of constructs was confirmed by functional assessment in engineered HEK-GIRK cells. Spectrophotometry and patch clamp experiments demonstrated that the candidate molecules were sensitive to the desired wavelengths of the light and directly coupled light stimuli to G-protein signalling. Herein, we introduce ROM17, ROM18 and ROM19 as newly generated, red-shifted variants with maximum excitation red-shifted of ~ 40nm, 70 nm and 126 nm compared to Opto-mGluR6.

A Comparative Analysis of Models for AAV-Mediated Gene Therapy for Inherited Retinal Diseases

Inherited retinal diseases (IRDs) represent a diverse group of genetic disorders leading to progressive degeneration of the retina due to mutations in over 280 genes. This review focuses on the various methodologies for the preclinical characterization and evaluation of adeno-associated virus (AAV)-mediated gene therapy as a potential treatment option for IRDs, particularly focusing on gene therapies targeting mutations, such as those in the RPE65 and FAM161A genes. AAV vectors, such as AAV2 and AAV5, have been utilized to deliver therapeutic genes, showing promise in preserving vision and enhancing photoreceptor function in animal models. Despite their advantages-including high production efficiency, low pathogenicity, and minimal immunogenicity-AAV-mediated therapies face limitations such as immune responses beyond the retina, vector size constraints, and challenges in large-scale manufacturing. This review systematically compares different experimental models used to investigate AAV-mediated therapies, such as mouse models, human retinal explants (HREs), and induced pluripotent stem cell (iPSC)-derived retinal organoids. Mouse models are advantageous for genetic manipulation and detailed investigations of disease mechanisms; however, anatomical differences between mice and humans may limit the translational applicability of results. HREs offer valuable insights into human retinal pathophysiology but face challenges such as tissue degradation and lack of systemic physiological effects. Retinal organoids, on the other hand, provide a robust platform that closely mimics human retinal development, thereby enabling more comprehensive studies on disease mechanisms and therapeutic strategies, including AAV-based interventions. Specific outcomes targeted in these studies include vision preservation and functional improvements of retinas damaged by genetic mutations. This review highlights the strengths and weaknesses of each experimental model and advocates for their combined use in developing targeted gene therapies for IRDs. As research advances, optimizing AAV vector design and delivery methods will be critical for enhancing therapeutic efficacy and improving clinical outcomes for patients with IRDs.

Ultrafast transient absorption spectra and kinetics of human blue cone visual pigment at room temperature

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Review: The influence of light on pig welfare

While several countries impose minimum light requirements for pig housing, it remains unknown whether these requirements are beneficial for pig welfare. Therefore, we aim to review the current knowledge on the effects of light on pig welfare. In this paper, we explain concepts defining light, discuss the relevance of vision for pigs and systematically review the effects of light on pig welfare. Systematic literature searches were performed in two databases to find studies about light and welfare-related topics, including behaviour, health, hormonal secretions and productivity. After screening, 63 studies were reviewed. According to literature, light is relevant in pigs' lives as they are diurnal animals and use vision in combination with other senses to, for example, locate food and interact with conspecifics. Throughout this paper, the investigated light parameters are photoperiod, intensity and spectrum. Pigs seem to have p for a certain light intensity and spectrum, but these preferences vary over production phases. Photoperiod influences feed intake and growth, especially in piglets, but no conclusion can be drawn because of contradictory results. Furthermore, pigs' activity patterns adapt to the provided light schedule and show a diurnal rhythm with higher activity during lit hours. Photoperiod also plays a role in the diurnal secretion of hormones. Cortisol secretion increases shortly before the moment of light onset, and melatonin secretion is influenced by the light and dark contrast with a nocturnal rise after light offset. Some behaviours are impacted by light intensity; for instance, dim conditions are associated with resting and bright conditions with elimination behaviour. Moreover, a few studies showed that in dimmer conditions, more negative social interactions occur, while brighter conditions lead to more positive interactions. Lastly, even though light spectrum is the least explored light parameter, several studies showed that UV B light can activate the cutaneous synthesis of vitamin D3. A limitation in the current literature is that several studies tested light treatments differing in more than one light parameter, making the interpretation of each light parameter difficult. Moreover, most studies do not provide information on other light parameters not targeted by the study, particularly on light spectrum. Some clear knowledge gaps that emerged from this review are on light spectrum and on affective states of pigs in relation to light.

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

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

Murine Retina Outer Plexiform Layer Development and Transcriptome Analysis of Pre-Synapses in Photoreceptors

Photoreceptors in the mammalian retina convert light signals into electrical and molecular signals through phototransduction and transfer the visual inputs to second-order neurons via specialized ribbon synapses. Two kinds of photoreceptors, rods and cones, possess distinct morphology and function. Currently, we have limited knowledge about rod versus (vs.) cone synapse development and the associated genes. The transcription factor neural retina leucine zipper (NRL) determines the rod vs. cone photoreceptor cell fate and is critical for rod differentiation. Nrl knockout mice fail to form rods, generating all cone or S-cone-like (SCL) photoreceptors in the retina, whereas ectopic expression of Nrl using a cone-rod homeobox (Crx) promoter (CrxpNrl) forms all rods. Here, we examined rod and cone pre-synapse development, including axonal elongation, terminal shaping, and synaptic lamination in the outer plexiform layer (OPL) in the presence or absence of Nrl. We show that NRL loss and knockdown result in delayed OPL maturation and plasticity with aberrant dendrites of bipolar neurons. The integrated analyses of the transcriptome in developing rods and SCLs with NRL CUT&RUN and synaptic gene ontology analyses identified G protein subunit beta (Gnb) 1 and p21 (RAC1) activated kinase 5 (Pak5 or Pak7) transcripts were upregulated in developing rods and down-regulated in developing SCLs. Notably, Gnb1 and Gnb5 are rod dominant, and Gnb3 is enriched in cones. NRL binds to the genes of Gnb1, Gnb3, and Gnb5. NRL also regulates pre-synapse ribbon genes, and their expression is altered in rods and SCLs. Our study of histological and gene analyses provides new insights into the morphogenesis of photoreceptor pre-synapse development and regulation of associated genes in the developing retina.

Cone photoreceptor differentiation regulated by thyroid hormone transporter MCT8 in the retinal pigment epithelium

The key role of a thyroid hormone receptor in determining the maturation and diversity of cone photoreceptors reflects a profound influence of endocrine signaling on the cells that mediate color vision. However, the route by which hormone reaches cones remains enigmatic as cones reside in the retinal photoreceptor layer, shielded by the blood-retina barrier. Using genetic approaches, we report that cone differentiation is regulated by a membrane transporter for thyroid hormone, MCT8 (SLC16A2), in the retinal pigment epithelium (RPE), which forms the outer blood-retina barrier. Mct8-deficient mice display hypothyroid-like cone gene expression and compromised electroretinogram responses. Mammalian color vision is typically facilitated by cone types that detect medium-long (M) and short (S) wavelengths of light but Mct8-deficient mice have a partial shift of M to S cone identity, resembling the phenotype of thyroid hormone receptor deficiency. RPE-specific ablation of Mct8 results in similar shifts in cone identity and hypothyroid-like gene expression whereas reexpression of MCT8 in the RPE in Mct8-deficient mice partly restores M cone identity, consistent with paracrine-like control of thyroid hormone signaling by the RPE. Our findings suggest that in addition to transport of essential solutes and homeostatic support for photoreceptors, the RPE regulates the thyroid hormone signal that promotes cone-mediated vision.

Visual pigment concentration and photoreceptor outer segment length in the human retina

PURPOSE

The Beer-Lambert law suggests that visual pigment optical density (OD) should be linearly related to the length of photoreceptor outer segments (POSs). Mammalian studies indicate that visual pigment concentration increases with POS length, but the nature of this relationship may vary due to factors such as visual pigment packing density or retinal eccentricity, and may not necessarily be linearly related. The purpose of this study was to establish the relationship between OD and POS length in humans.

METHODS

Spectral domain optical coherence tomography (OCT) was used to image POS, and imaging retinal densitometry (IRD) was used to measure OD at corresponding locations in 19 healthy participants (age range 25-82 years). POS length and OD measurements were extracted from OCT and IRD images at 23 discrete locations spanning the central 9° of the retina. The averaged data from all participants were fitted with models based on the Beer-Lambert law to establish the relationship between OD and POS length.

RESULTS

Visual pigment OD increased monotonically with POS length, but the relationship was non-linear, and a straight-line fit, based on a simple interpretation of the Beer-Lambert law, provided a poor description. A model allowing for different rod and cone visual pigment concentrations provided a superior fit. Specifically, the data were well described by a model where the molar concentration of visual pigment in cones and rods were 3.8 × 10-3 mol/L and 1.8 × 10-3mol/L, respectively.

CONCLUSIONS

In accordance with the Beer-Lambert law, the results indicate that OD increases monotonically with POS length in humans, but the precise relationship is dependent on photoreceptor type. These results suggest that visual pigment concentration in rods is only about 48% of that found in cones. This may be due to the ubiquitous nature of artificial light that works to reduce the concentration of rhodopsin in rod photoreceptors.

Light-evoked deformations in rod photoreceptors, pigment epithelium and subretinal space revealed by prolonged and multilayered optoretinography

Phototransduction involves changes in concentration of ions and other solutes within photoreceptors and in subretinal space, which affect osmotic pressure and the associated water flow. Corresponding expansion and contraction of cellular layers can be imaged using optoretinography (ORG), based on phase-resolved optical coherence tomography (OCT). Until now, ORG could reliably detect only photoisomerization and phototransduction in photoreceptors, primarily in cones under bright stimuli. Here, by employing a phase-restoring subpixel motion correction algorithm, which enables imaging of the nanometer-scale tissue dynamics during minute-long recordings, and unsupervised learning of spatiotemporal patterns, we discover optical signatures of the other retinal structures' response to visual stimuli. These include inner and outer segments of rod photoreceptors, retinal pigment epithelium, and subretinal space in general. The high sensitivity of our technique enables detection of the retinal responses to dim stimuli: down to 0.01% bleach level, corresponding to natural levels of scotopic illumination. We also demonstrate that with a single flash, the optoretinogram can map retinal responses across a 12° field of view, potentially replacing multifocal electroretinography. This technique expands the diagnostic capabilities and practical applicability of optoretinography, providing an alternative to electroretinography, while combining structural and functional retinal imaging in the same OCT machine.

DNA-Programmed Four-Bit Quaternary Fluorescence Encoding (FLUCO) Enables 51-Colored Bioimaging Analysis

Color encoding plays a crucial role in painting, digital photography, and spectral analysis. Achieving accurate, target-responsive color encoding at the molecular level has the potential to revolutionize scientific research and technological innovation, but significant challenges persist. Here, we propose a multibit DNA self-assembly system based on computer-aided design (CAD) technology, enabling accurate, target-responsive, amplified color encoding at the molecular level, termed fluorescence encoding (FLUCO). As a model, we establish a quaternary FLUCO system using four-bit DNA self-assembly, which can accurately encode 51 colors, presenting immense potential in applications such as spatial proteomic imaging and multitarget analysis. Notably, FLUCO enables the simultaneous imaging of multiple targets exceeding the limitations of channels using conventional imaging equipment, and marks the integration of computer science for molecular encoding and decoding. Overall, our work paves the way for target-responsive, controllable molecular encoding, facilitating spatial omics analysis, exfoliated cell analysis, and high-throughput liquid biopsy.

Molecular Mechanisms Governing Sight Loss in Inherited Cone Disorders

Inherited cone disorders (ICDs) are a heterogeneous sub-group of inherited retinal disorders (IRDs), the leading cause of sight loss in children and working-age adults. ICDs result from the dysfunction of the cone photoreceptors in the macula and manifest as the loss of colour vision and reduced visual acuity. Currently, 37 genes are associated with varying forms of ICD; however, almost half of all patients receive no molecular diagnosis. This review will discuss the known ICD genes, their molecular function, and the diseases they cause, with a focus on the most common forms of ICDs, including achromatopsia, progressive cone dystrophies (CODs), and cone-rod dystrophies (CORDs). It will discuss the gene-specific therapies that have emerged in recent years in order to treat patients with some of the more common ICDs.

Retinal organoids with X-linked retinoschisis RS1 (E72K) mutation exhibit a photoreceptor developmental delay and are rescued by gene augmentation therapy

BACKGROUND

X-linked juvenile retinoschisis (XLRS) is an inherited disease caused by RS1 gene mutation, which leads to retinal splitting and visual impairment. The mechanism of RS1-associated retinal degeneration is not fully understood. Besides, animal models of XLRS have limitations in the study of XLRS. Here, we used human induced pluripotent stem cell (hiPSC)-derived retinal organoids (ROs) to investigate the disease mechanisms and potential treatments for XLRS.

METHODS

hiPSCs reprogrammed from peripheral blood mononuclear cells of two RS1 mutant (E72K) XLRS patients were differentiated into ROs. Subsequently, we explored whether RS1 mutation could affect RO development and explore the effectiveness of RS1 gene augmentation therapy.

RESULTS

ROs derived from RS1 (E72K) mutation hiPSCs exhibited a developmental delay in the photoreceptor, retinoschisin (RS1) deficiency, and altered spontaneous activity compared with control ROs. Furthermore, the delays in development were associated with decreased expression of rod-specific precursor markers (NRL) and photoreceptor-specific markers (RCVRN). Adeno-associated virus (AAV)-mediated gene augmentation with RS1 at the photoreceptor immature stage rescued the rod photoreceptor developmental delay in ROs with the RS1 (E72K) mutation.

CONCLUSIONS

The RS1 (E72K) mutation results in the photoreceptor development delay in ROs and can be partially rescued by the RS1 gene augmentation therapy.

Automated quantification of photoreceptor outer segments in developing and degenerating retinas on microscopy images across scales

The functionality of photoreceptors, rods, and cones is highly dependent on their outer segments (POS), a cellular compartment containing highly organized membranous structures that generate biochemical signals from incident light. While POS formation and degeneration are qualitatively assessed on microscopy images, reliable methodology for quantitative analyses is still limited. Here, we developed methods to quantify POS (QuaPOS) maturation and quality on retinal sections using automated image analyses. POS formation was examined during the development and in adulthood of wild-type mice via light microscopy (LM) and transmission electron microscopy (TEM). To quantify the number, size, shape, and fluorescence intensity of POS, retinal cryosections were immunostained for the cone POS marker S-opsin. Fluorescence images were used to train the robust classifier QuaPOS-LM based on supervised machine learning for automated image segmentation. Characteristic features of segmentation results were extracted to quantify the maturation of cone POS. Subsequently, this quantification method was applied to characterize POS degeneration in "cone photoreceptor function loss 1" mice. TEM images were used to establish the ultrastructural quantification method QuaPOS-TEM for the alignment of POS membranes. Images were analyzed using a custom-written MATLAB code to extract the orientation of membranes from the image gradient and their alignment (coherency). This analysis was used to quantify the POS morphology of wild-type and two inherited retinal degeneration ("retinal degeneration 19" and "rhodopsin knock-out") mouse lines. Both automated analysis technologies provided robust characterization and quantification of POS based on LM or TEM images. Automated image segmentation by the classifier QuaPOS-LM and analysis of the orientation of membrane stacks by QuaPOS-TEM using fluorescent or TEM images allowed quantitative evaluation of POS formation and quality. The assessments showed an increase in POS number, volume, and membrane coherency during wild-type postnatal development, while a decrease in all three observables was detected in different retinal degeneration mouse models. All the code used for the presented analysis is open source, including example datasets to reproduce the findings. Hence, the QuaPOS quantification methods are useful for in-depth characterization of POS on retinal sections in developmental studies, for disease modeling, or after therapeutic interventions affecting photoreceptors.

Artificial Retina Based on Organic Heterojunction Transistors for Mobile Recognition

The flicker frequency of incident light constitutes a critical determinant in biology. Nevertheless, the exploration of methods to simulate external light stimuli with varying frequencies and develop artificial retinal neurons capable of responsive behavior remains an open question. This study presents an artificial neuron comprising organic phototransistors. The triggering properties of neurons are modulated by optical input, enabling them to execute rudimentary synaptic functions, emulating the biological characteristics of retinal neurons. The artificial retinal neuron exhibits varying responses to incoming light frequencies, allowing it to replicate the persistent visual behavior of the human eye and facilitating image discrimination. Additionally, through seamless integration with circuitry, it can execute motion recognition on a machine cart, preventing collisions with high-speed obstacles. The artificial retinal neuron offers a cost-effective and energy-efficient route for future mobile robot processors.

Next generation therapeutics for retinal neurodegenerative diseases

Neurodegenerative diseases affecting the visual system encompass glaucoma, macular degeneration, retinopathies, and inherited genetic disorders such as retinitis pigmentosa. These ocular pathologies pose a serious burden of visual impairment and blindness worldwide. Current treatment modalities include small molecule drugs, biologics, or gene therapies, most of which are administered topically as eye drops or as injectables. However, the topical route of administration faces challenges in effectively reaching the posterior segment and achieving desired concentrations at the target site, while injections and implants risk severe complications, such as retinal detachment and endophthalmitis. This necessitates the development of innovative therapeutic strategies that can prolong drug release, deliver effective concentrations to the back of the eye with minimal systemic exposure, and improve patient compliance and safety. In this review, we introduce retinal degenerative diseases, followed by a discussion of the existing clinical standard of care. We then delve into detail about drug and gene delivery systems currently in preclinical and clinical development, including formulation and delivery advantages/drawbacks, with a special emphasis on potential for clinical translation.

Shotgun lipidomics of human subretinal fluids under rod-dominant retina reveals cone-dominated lipids

Subretinal fluid (SRF) accumulates between photoreceptor outer segments and retinal pigment epithelium during rhegmatogenous retinal detachment. Biomolecular components such as lipids originate from cells surrounding the SRF. Knowledge of the composition of these molecules in SRF potentially provides mechanistic insight into the physiologic transfer of lipids between retinal tissue compartments. Using mass spectrometry and tandem mass spectrometry analysis on an electrospray ionization quadrupole-time-of-flight mass spectrometer, we identified a total of 115 lipid molecular species of 11 subclasses and 9 classes in two samples from two patients with rhegmatogenous retinal detachment. These included 47 glycerophosphocholines, 6 glycerophosphoethanolamines, 1 glycerophosphoinositol, 18 sphingomyelins, 9 cholesteryl esters, free cholesterol, 3 ceramides, 22 triacylglycerols and 8 free fatty acids. Glycerophosphocholines were of the highest intensity. By minimizing the formation of different adduct forms or clustering ions of different adducts, we determined the relative intensity of lipid molecular species within the same subclasses. The profiles were compared with those of retinal cells available in the published literature. The glycerophosphocholine profile of SRF was similar to that of cone outer segments, suggesting that outer segment degradation products are constitutively released into the interphotoreceptor matrix, appearing in SRF during detachment. This hypothesis was supported by the retinal distributions of corresponding lipid synthases' mRNA expression obtained from an online resource based on publicly available single-cell sequencing data. In contrast, based on lipid profiles and relevant gene expression in this study, the sources of free cholesterol and cholesteryl esters in SRF appeared more ambiguous, possibly reflecting that outer retina takes up plasma lipoproteins. Further studies to identify and quantify lipids in SRF will help better understand etiology of diseases relevant to outer retina.

Hierarchies in Visual Pathway: Functions and Inspired Artificial Vision

The development of artificial intelligence has posed a challenge to machine vision based on conventional complementary metal-oxide semiconductor (CMOS) circuits owing to its high latency and inefficient power consumption originating from the data shuffling between memory and computation units. Gaining more insights into the function of every part of the visual pathway for visual perception can bring the capabilities of machine vision in terms of robustness and generality. Hardware acceleration of more energy-efficient and biorealistic artificial vision highly necessitates neuromorphic devices and circuits that are able to mimic the function of each part of the visual pathway. In this paper, we review the structure and function of the entire class of visual neurons from the retina to the primate visual cortex within reach (Chapter 2) are reviewed. Based on the extraction of biological principles, the recent hardware-implemented visual neurons located in different parts of the visual pathway are discussed in detail in Chapters 3 and 4. Furthermore, valuable applications of inspired artificial vision in different scenarios (Chapter 5) are provided. The functional description of the visual pathway and its inspired neuromorphic devices/circuits are expected to provide valuable insights for the design of next-generation artificial visual perception systems.

Structural and functional rescue of cones carrying the most common cone opsin C203R missense mutation

An arginine to cysteine substitution at amino acid position 203 (C203R) is the most common missense mutation in human cone opsin. Linked to color blindness and blue cone monochromacy (BCM), C203 is involved in a crucial disulfide bond required for proper folding. It has previously been postulated that expression of mutant C203R cone opsin exerts a toxic effect on cone photoreceptors, similar to some well-characterized missense mutations in rhodopsin that lead to protein misfolding. In this study, we generated and characterized a BCM mouse model carrying the equivalent C203R mutation (Opn1mwC198R Opn1sw-/-) to investigate the disease mechanism and develop a gene therapy approach for this disorder. Untreated Opn1mwC198R Opn1sw-/- cones phenocopied affected cones in human patients with the equivalent mutation, exhibiting shortened or absent cone outer segments and loss of function. We determined that gene augmentation targeting cones specifically yielded robust rescue of cone function and structure when Opn1mwC198R Opn1sw-/- mice were treated at early ages. Importantly, treated cones displayed elaborated outer segments and replenished expression of crucial cone phototransduction proteins. Interestingly, we were unable to detect OPN1MWC198R mutant opsin at any age. We believe this is the first proof-of-concept study exploring the efficacy of gene therapy in BCM associated with a C203R mutation.

Dyschromatopsia: a comprehensive analysis of mechanisms and cutting-edge treatments for color vision deficiency

Color blindness is a retinal disease that mainly manifests as a color vision disorder, characterized by achromatopsia, red-green color blindness, and blue-yellow color blindness. With the development of technology and progress in theory, extensive research has been conducted on the genetic basis of color blindness, and various approaches have been explored for its treatment. This article aims to provide a comprehensive review of recent advances in understanding the pathological mechanism, clinical symptoms, and treatment options for color blindness. Additionally, we discuss the various treatment approaches that have been developed to address color blindness, including gene therapy, pharmacological interventions, and visual aids. Furthermore, we highlight the promising results from clinical trials of these treatments, as well as the ongoing challenges that must be addressed to achieve effective and long-lasting therapeutic outcomes. Overall, this review provides valuable insights into the current state of research on color blindness, with the intention of informing further investigation and development of effective treatments for this disease.

Orchestrating Blood Flow in the Retina: Interpericyte Tunnelling Nanotube Communication

The retina transforms light into electrical signals, which are sent to the brain via the optic nerve to form our visual perception. This complex signal processing is performed by the retinal neuron and requires a significant amount of energy. Since neurons are unable to store energy, they must obtain glucose and oxygen from the bloodstream to produce energy to match metabolic needs. This process is called neurovascular coupling (NVC), and it is based on a precise mechanism that is not totally understood. The discovery of fine tubular processes termed tunnelling nanotubes (TNTs) set a new type of cell-to-cell communication. TNTs are extensions of the cellular membrane that allow the transfer of material between connected cells. Recently, they have been reported in the brain and retina of living mice, where they connect pericytes, which are vascular mural cells that regulate vessel diameter. Accordingly, these TNTs were termed interpericyte tunnelling nanotubes (IPTNTs), which showed a vital role in blood delivery and NVC. In this chapter, we review the involvement of TNTs in NVC and discuss their implications in retinal neurodegeneration.

Lipofuscin, Its Origin, Properties, and Contribution to Retinal Fluorescence as a Potential Biomarker of Oxidative Damage to the Retina

Lipofuscin accumulates with age as intracellular fluorescent granules originating from incomplete lysosomal digestion of phagocytosed and autophagocytosed material. The purpose of this review is to provide an update on the current understanding of the role of oxidative stress and/or lysosomal dysfunction in lipofuscin accumulation and its consequences, particularly for retinal pigment epithelium (RPE). Next, the fluorescence of lipofuscin, spectral changes induced by oxidation, and its contribution to retinal fluorescence are discussed. This is followed by reviewing recent developments in fluorescence imaging of the retina and the current evidence on the prognostic value of retinal fluorescence for the progression of age-related macular degeneration (AMD), the major blinding disease affecting elderly people in developed countries. The evidence of lipofuscin oxidation in vivo and the evidence of increased oxidative damage in AMD retina ex vivo lead to the conclusion that imaging of spectral characteristics of lipofuscin fluorescence may serve as a useful biomarker of oxidative damage, which can be helpful in assessing the efficacy of potential antioxidant therapies in retinal degenerations associated with accumulation of lipofuscin and increased oxidative stress. Finally, amendments to currently used fluorescence imaging instruments are suggested to be more sensitive and specific for imaging spectral characteristics of lipofuscin fluorescence.

Going beyond the 20th century color space to evaluate LED color consistency

The color matching possibilities between (reference) phosphor-converted LEDs (pc-LEDs) and replacement metameric LEDs made by color mixing technology (cm-LEDs) were evaluated in the classical 20th century CIE 1976 color space developed for perpendicular viewing (based on a 2° colorimetric observer) and in the latest CIE 2015 cone fundamental color space developed for wide field of view observers (10° colorimetric observer). For each given pc-LED 10 different sets of cm-LEDs were designed and evaluated for color consistency in 2°and 10° color spaces. There were 10 different B-color LEDs considered along with constant RGA LEDs. There are thousands of possible distributions that are metameric in the CIE 1976 color space and thousands of possible distributions that are metameric in the CIE 2015 color space for each set of LEDs. From the population of SPDs, we selected 10 metameric SPDs characterized by maximum differences between chromaticities. The results provide evidence that evaluating LED color consistency based only on the CIE 1976 color space is not fully informative because it may provide inaccurate information about light color consistency when the observer has a wide field of view. There are cases showed in this paper where cm-LEDs are color consistent in the CIE 2015 color space but are not color consistent in the CIE 1976 color space and vice versa. Including color consistency in the new CIE 2015 color space should be treated as an additional evaluation tool proving the user additional information relevant to the intended use of the LED. The results illustrate differences in LED color consistency evaluated in different color spaces and provides incentive go beyond the 20th century color space in the evaluation of cm-LED color consistency.

Episodic live imaging of cone photoreceptor maturation in GNAT2-EGFP retinal organoids

Fluorescent reporter pluripotent stem cell-derived retinal organoids are powerful tools to investigate cell type-specific development and disease phenotypes. When combined with live imaging, they enable direct and repeated observation of cell behaviors within a developing retinal tissue. Here, we generated a human cone photoreceptor reporter line by CRISPR/Cas9 genome editing of WTC11-mTagRFPT-LMNB1 human induced pluripotent stem cells (iPSCs) by inserting enhanced green fluorescent protein (EGFP) coding sequences and a 2A self-cleaving peptide at the N-terminus of guanine nucleotide-binding protein subunit alpha transducin 2 (GNAT2). In retinal organoids generated from these iPSCs, the GNAT2-EGFP alleles robustly and exclusively labeled immature and mature cones. Episodic confocal live imaging of hydrogel immobilized retinal organoids allowed tracking of the morphological maturation of individual cones for >18 weeks and revealed inner segment accumulation of mitochondria and growth at 12.2 μm3 per day from day 126 to day 153. Immobilized GNAT2-EGFP cone reporter organoids provide a valuable tool for investigating human cone development and disease.

USH2A variants causing retinitis pigmentosa or Usher syndrome provoke differential retinal phenotypes in disease-specific organoids

There is an emblematic clinical and genetic heterogeneity associated with inherited retinal diseases (IRDs). The most common form is retinitis pigmentosa (RP), a rod-cone dystrophy caused by pathogenic variants in over 80 different genes. Further complexifying diagnosis, different variants in individual RP genes can also alter the clinical phenotype. USH2A is the most prevalent gene for autosomal-recessive RP and one of the most challenging because of its large size and, hence, large number of variants. Moreover, USH2A variants give rise to non-syndromic and syndromic RP, known as Usher syndrome (USH) type 2, which is associated with vision and hearing loss. The lack of a clear genotype-phenotype correlation or prognostic models renders diagnosis highly challenging. We report here a long-awaited differential non-syndromic RP and USH phenotype in three human disease-specific models: fibroblasts, induced pluripotent stem cells (iPSCs), and mature iPSC-derived retinal organoids. Moreover, we identified distinct retinal phenotypes in organoids from multiple RP and USH individuals, which were validated by isogenic-corrected controls. Non-syndromic RP organoids showed compromised photoreceptor differentiation, whereas USH organoids showed a striking and unexpected cone phenotype. Furthermore, complementary clinical investigations identified macular atrophy in a high proportion of USH compared with RP individuals, further validating our observations that USH2A variants differentially affect cones. Overall, identification of distinct non-syndromic RP and USH phenotypes in multiple models provides valuable and robust readouts for testing the pathogenicity of USH2A variants as well as the efficacy of therapeutic approaches in complementary cell types.

Reliability of human retina organoid generation from hiPSC-derived neuroepithelial cysts

The possible applications for human retinal organoids (HROs) derived from human induced pluripotent stem cells (hiPSC) rely on the robustness and transferability of the methodology for their generation. Standardized strategies and parameters to effectively assess, compare, and optimize organoid protocols are starting to be established, but are not yet complete. To advance this, we explored the efficiency and reliability of a differentiation method, called CYST protocol, that facilitates retina generation by forming neuroepithelial cysts from hiPSC clusters. Here, we tested seven different hiPSC lines which reproducibly generated HROs. Histological and ultrastructural analyses indicate that HRO differentiation and maturation are regulated. The different hiPSC lines appeared to be a larger source of variance than experimental rounds. Although previous reports have shown that HROs in several other protocols contain a rather low number of cones, HROs from the CYST protocol are consistently richer in cones and with a comparable ratio of cones, rods, and Müller glia. To provide further insight into HRO cell composition, we studied single cell RNA sequencing data and applied CaSTLe, a transfer learning approach. Additionally, we devised a potential strategy to systematically evaluate different organoid protocols side-by-side through parallel differentiation from the same hiPSC batches: In an explorative study, the CYST protocol was compared to a conceptually different protocol based on the formation of cell aggregates from single hiPSCs. Comparing four hiPSC lines showed that both protocols reproduced key characteristics of retinal epithelial structure and cell composition, but the CYST protocol provided a higher HRO yield. So far, our data suggest that CYST-derived HROs remained stable up to at least day 200, while single hiPSC-derived HROs showed spontaneous pathologic changes by day 200. Overall, our data provide insights into the efficiency, reproducibility, and stability of the CYST protocol for generating HROs, which will be useful for further optimizing organoid systems, as well as for basic and translational research applications.

Ultrastructural Characteristics and Synaptic Connectivity of Photoreceptors in the Simplex Retina of Little Skate (Leucoraja erinacea)

The retinas of the vast majority of vertebrate species are termed "duplex," that is, they contain both rod and cone photoreceptor neurons in different ratios. The retina of little skate (Leucoraja erinacea) is a rarity among vertebrates because it contains only a single photoreceptor cell type and is thus "simplex." This unique retina provides us with an important comparative model and an exciting opportunity to study retinal circuitry within the context of a visual system with a single photoreceptor cell type. What is perhaps even more intriguing is the fact that the Leucoraja retina is able use that single photoreceptor cell type to function under both scotopic and photopic ranges of illumination. Although some ultrastructural characteristics of skate photoreceptors have been examined previously, leading to a general description of them as "rods" largely based on outer segment (OS) morphology and rhodopsin expression, a detailed study of the fine anatomy of the entire cell and its synaptic connectivity is still lacking. To address this gap in knowledge, we performed serial block-face electron microscopy imaging and examined the structure of skate photoreceptors and their postsynaptic partners. We find that skate photoreceptors exhibit unusual ultrastructural characteristics that are either common to rods or cones in other vertebrates (e.g., outer segment architecture, synaptic ribbon number, terminal extensions), or are somewhere in between those of a typical vertebrate rod or cone (e.g., number of invaginating contacts, clustering of multiple ribbons over a single synaptic invagination). We suggest that some of the ultrastructural characteristics we observe may play a role in the ability of the skate retina to function across scotopic and photopic ranges of illumination. Our findings have the potential to reveal as yet undescribed principles of vertebrate retinal design.

New Insights into RPE-Photoreceptor Complex Ultrastructure using Focused Ion Beam-Scanning Election Microscopy (FIB-SEM)

Photoreceptors in the retina are specialized neuronal cells that perceive light and play a central role in the visual system. Damage to photoreceptors is a clinical feature often associated with various retinal degenerative disorders. The photoreceptor bed comprises a unique extracellular matrix (ECM) scaffold often described as the interphotoreceptor matrix (IPM) in the subretinal space, vital during retinal development and homeostasis. In this study, we used focused ion beam scanning electron microscopy (FIB-SEM) and transmission electron microscopy (TEM) to analyze the ultrastructural architecture of the retinal pigmented epithelium (RPE)-photoreceptor complex in mice. Additionally, we describe methods for retinal preparation in EM imaging. TEM images display ultrastructural retina layers, including Bruch's membrane and the interdigitation zone (IZ). The 3-dimensional reconstruction of the outer retina revealed individual photoreceptors, the connection between their inner and outer segment via the photoreceptor cilia, and photoreceptor interaction with the RPE ciliary processes. Our findings highlight the importance of FIB-SEM in deciphering the ultrastructural details of RPE-photoreceptor interactions in the IPM complex which are essential for the maintenance of retinal architecture.

Ultrafast Transient Absorption Spectra and Kinetics of Rod and Cone Visual Pigments

Rods and cones are the photoreceptor cells containing the visual pigment proteins that initiate visual phototransduction following the absorption of a photon. Photon absorption induces the photochemical transformation of a visual pigment, which results in the sequential formation of distinct photo-intermediate species on the femtosecond to millisecond timescales, whereupon a visual electrical signal is generated and transmitted to the brain. Time-resolved spectroscopic studies of the rod and cone photo-intermediaries enable the detailed understanding of initial events in vision, namely the key differences that underlie the functionally distinct scotopic (rod) and photopic (cone) visual systems. In this paper, we review our recent ultrafast (picoseconds to milliseconds) transient absorption studies of rod and cone visual pigments with a detailed comparison of the transient molecular spectra and kinetics of their respective photo-intermediaries. Key results include the characterization of the porphyropsin (carp fish rhodopsin) and human green-cone opsin photobleaching sequences, which show significant spectral and kinetic differences when compared against that of bovine rhodopsin. These results altogether reveal a rather strong interplay between the visual pigment structure and its corresponding photobleaching sequence, and relevant outstanding questions that will be further investigated through a forthcoming study of the human blue-cone visual pigment are discussed.

Effect of Trace Metal Ions on the Conformational Stability of the Visual Photoreceptor Rhodopsin

Trace metals are essential elements that play key roles in a number of biochemical processes governing human visual physiology in health and disease. Several trace metals, such as zinc, have been shown to play important roles in the visual phototransduction process. In spite of this, there has been little research conducted on the direct effect of trace metal elements on the visual photoreceptor rhodopsin. In the current study, we have determined the effect of several metal ions, such as iron, copper, chromium, manganese, and nickel, on the conformational stability of rhodopsin. To this aim, we analyzed, by means of UV-visible and fluorescence spectroscopic methods, the effects of these trace elements on the thermal stability of dark rhodopsin, the stability of its active Metarhodopsin II conformation, and its chromophore regeneration. Our results show that copper prevented rhodopsin regeneration and slowed down the retinal release process after illumination. In turn, Fe³⁺, but not Fe²⁺, increased the thermal stability of the dark inactive conformation of rhodopsin, whereas copper ions markedly decreased it. These findings stress the important role of trace metals in retinal physiology at the photoreceptor level and may be useful for the development of novel therapeutic strategies to treat retinal disease.

Blue cone monochromacy and gene therapy

Blue cone monochromacy (BCM) is a congenital vision disorder characterized by complete loss or severely reduced long- and middle-wavelength cone function, caused by mutations in the OPN1LW/OPN1MW gene cluster on the X-chromosome. BCM patients typically suffer from poor visual acuity, severely impaired color discrimination, myopia, and nystagmus. In this review, we cover the genetic causes of BCM, clinical features of BCM patients, genetic testing, and clinical outcome measurements for future BCM clinical trials. However, our emphasis is on detailing the animal models for BCM and gene therapy using adeno-associated vectors (AAV). We describe two mouse models resembling the two most common causes of BCM, current progress in proof-of-concept studies to treat BCM with deletion mutations, the challenges we face, and future directions.

Comparison of Diabetic Retinopathy Lesions Identified Using Ultrawide Field Imaging and Optical Coherence Tomography Angiography

INTRODUCTION

Optical coherence tomography (OCT) angiography (OCTA) has the potential to influence the diagnosis and management of diabetic eye disease. This study aims to determine the correlation between diabetic retinopathy (DR) findings on ultrawide field (UWF) color photography (UWF-CP), UWF fluorescein angiography (UWF-FA), and OCTA.

METHODS

This is a cross-sectional, prospective study. One hundred and fourteen eyes from 57 patients with diabetes underwent mydriatic UWF-CP, UWF-FA, and OCTA. DR severity was assessed. Ischemic areas were identified on UWF-FA using ImageJ and the nonperfusion index (NPI) was calculated. Diabetic macular edema (DME) was assessed using OCT. Superficial capillary plexus vessel density (VD), vessel perfusion (VP), and foveal avascular zone (FAZ) area were automatically measured on OCTA. Pearson correlation coefficient between the imaging modalities was determined.

RESULTS

Forty-five eyes were excluded due to non-DR findings or prior laser photocoagulation; 69 eyes were analyzed. DR severity was associated with larger NPI (r = 0.55944, p < 0.0001) even after distinguishing between cones (Cone Nonperfusion Index [CPI]: r = 0.55617, p < 0.0001) and rods (Rod Nonperfusion Index [RPI]: r = 0.55285, p < 0.0001). In eyes with nonproliferative DR (NPDR), NPI is correlated with DME (r = 0.51156, p = 0.0017) and central subfield thickness (CST) (r = 0.67496, p < 0.0001). UWF-FA macular nonperfusion correlated with NPI (r = 0.42899, p = 0.0101), CPI (r = 0.50028, p = 0.0022), and RPI (r = 0.49027, p = 0.0028). Central VD and VP correlated with the DME presence (r = 0.52456, p < 0.0001; r = 0.51952, p < 0.0001) and CST (r = 0.50133, p < 0.0001; r = 0.48731, p < 0.0001). Central VD and VP were correlated with macular nonperfusion (r = 0.44503, p = 0.0065; r = 0.44239, p = 0.0069) in eyes with NPDR. Larger FAZ was correlated with decreased central VD (r = -0.60089, p = 0.0001) and decreased central VP (r = -0.59224, p = 0.0001).

CONCLUSION

UWF-CP, UWF-FA, and OCTA findings provide relevant clinical information on diabetic eyes. Nonperfusion on UWF-FA is correlated with DR severity and DME. OCTA metrics of the superficial capillary plexus correlate with the incidence of DME and macular ischemia.

Nuclear Receptor Subfamily 2 Group E Member 3 (NR2E3): Role in Retinal Development and Disease

NR2E3 is a nuclear hormone receptor gene required for the correct development of the retinal rod photoreceptors. Expression of NR2E3 protein in rod cell precursors suppresses cone-specific gene expression and, in concert with other transcription factors including NRL, activates the expression of rod-specific genes. Pathogenic variants involving NR2E3 cause a spectrum of retinopathies, including enhanced S-cone syndrome, Goldmann-Favre syndrome, retinitis pigmentosa, and clumped pigmentary retinal degeneration, with limited evidence of genotype-phenotype correlations. A common feature of NR2E3-related disease is an abnormally high number of cone photoreceptors that are sensitive to short wavelength light, the S-cones. This characteristic has been supported by mouse studies, which have also revealed that loss of Nr2e3 function causes photoreceptors to develop as cells that are intermediate between rods and cones. While there is currently no available cure for NR2E3-related retinopathies, there are a number of emerging therapeutic strategies under investigation, including the use of viral gene therapy and gene editing, that have shown promise for the future treatment of patients with NR2E3 variants and other inherited retinal diseases. This review provides a detailed overview of the current understanding of the role of NR2E3 in normal development and disease, and the associated clinical phenotypes, animal models, and therapeutic studies.

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.

New insights into the retinal microstructure-diurnal activity relationship in the African five-lined skink (Trachylepis quinquetaeniata) (Lichtenstein, 1823)

BACKGROUND

The retinae of diurnal vertebrates have characteristics. Most lizards are strictly diurnal, and their retinal morphology is still unknown.

MATERIALS AND METHODS

The retina of the African five-lined skink (Trachylepis quinquetaeniata) was studied using light and transmission electron microscopy.

RESULTS

The retina's ten layers were all detected. The inner nuclear layer was the thickest by an average of 67.66 μm, and the inner plexiform layer was 57.564 μm. There were elliptical, long cylindrical, and spherical melanosomes (small and large) in the pigment epithelial layer of the retina. The cylindrical melanosomes had a large area on the lateral surfaces of cones to increase light scatter absorption. The photoreceptor layer of the retina had cones only. There were single and double cones, with the double cones consisting of principal and accessory cones. The cones had inner and outer segments separated by oil droplets. A spherical paraboloid body existed between the limiting membrane and the ellipsoid. All single cones had a paraboloid, and double cones had a large paraboloid in the accessory cone. The presence of paraboloids and large ellipsoids with mitochondria of varying sizes may have helped focus the light on cone segments.

CONCLUSION

The African five-lined skink's eye was light-adapted due to a variety of retinal specializations related to the demands of its diurnal lifestyle in its environment.

Affinity-controlled release of rod-derived cone viability factor enhances cone photoreceptor survival

Retinitis pigmentosa (RP) is a group of genetic diseases that results in rod photoreceptor cell degeneration, which subsequently leads to cone photoreceptor cell death, impaired vision and eventual blindness. Rod-derived cone viability factor (RdCVF) is a protein which has two isoforms: a short form (RdCVF) and a long form (RdCVFL) which act on cone photoreceptors in the retina. RdCVFL protects photoreceptors by reducing hyperoxia in the retina; however, sustained delivery of RdCVFL remains challenging. We developed an affinity-controlled release strategy for RdCVFL. An injectable physical blend of hyaluronan and methylcellulose (HAMC) was covalently modified with a peptide binding partner of the Src homology 3 (SH3) domain. This domain was expressed as a fusion protein with RdCVFL, thereby enabling its controlled release from HAMC-binding peptide. Sustained release of RdCVFL was demonstrated for the first time as RdCVFL-SH3 from HAMC-binding peptide for 7 d in vitro. To assess bioactivity, chick retinal dissociates were harvested and treated with the affinity-released recombinant protein from the HAMC-binding peptide vehicle. After 6 d in culture, cone cell viability was greater when cultured with released RdCVFL-SH3 relative to controls. We utilized computational fluid dynamics to model release of RdCVFL-SH3 from our delivery vehicle in the vitreous of the human eye. We demonstrate that our delivery vehicle can prolong the bioavailability of RdCVFL-SH3 in the retina, potentially enhancing its therapeutic effects. Our affinity-based system constitutes a versatile delivery platform for ultimate intraocular injection in the treatment of retinal degenerative diseases. STATEMENT OF SIGNIFICANCE: Retinitis pigmentosa (RP) is the leading cause of inherited blindness in the world. Rod-derived cone viability factor (RdCVF), a novel protein paracrine factor, is effective in preclinical models of RP. To extend its therapeutic effects, we developed an affinity-controlled release strategy for the long form of RdCVF, RdCVFL. We expressed RdCVFL as a fusion protein with an Src homology 3 domain (SH3). We then utilized a hydrogel composed of hyaluronan and methylcellulose (HAMC) and modified it with SH3 binding peptides to investigate its release in vitro. Furthermore, we designed a mathematical model of the human eye to investigate delivery of the protein from the delivery vehicle. This work paves the way for future investigation of controlled release RdCVF.

Episodic live imaging of cone photoreceptor maturation in GNAT2-EGFP retinal organoids

Fluorescent reporter pluripotent stem cell (PSC) derived retinal organoids are powerful tools to investigate cell type-specific development and disease phenotypes. When combined with live imaging, they enable direct and repeated observation of cell behaviors within a developing retinal tissue. Here, we generated a human cone photoreceptor reporter line by CRISPR/Cas9 genome editing of WTC11-mTagRFPT-LMNB1 human induced pluripotent stem cells (iPSCs) by inserting enhanced green fluorescent protein (EGFP) coding sequences and a 2A self-cleaving peptide at the N-terminus of Guanine Nucleotide-Binding Protein Subunit Alpha Transducin 2 (GNAT2). In retinal organoids generated from these iPSCs, the GNAT2-EGFP allele robustly and exclusively labeled both immature and mature cones starting at culture day 34. Episodic confocal live imaging of hydrogel immobilized retinal organoids allowed tracking of morphological maturation of individual cones for >18 weeks and revealed inner segment accumulation of mitochondria and growth at 12.2 cubic microns per day from day 126 to day 153. Immobilized GNAT2-EGFP cone reporter organoids provide a valuable tool for investigating human cone development and disease.

Kinetics of cone specific G-protein signaling in avian photoreceptor cells

Cone photoreceptor cells of night-migratory songbirds seem to process the primary steps of two different senses, vision and magnetoreception. The molecular basis of phototransduction is a prototypical G protein-coupled receptor pathway starting with the photoexcitation of rhodopsin or cone opsin thereby activating a heterotrimeric G protein named transducin. This interaction is well understood in vertebrate rod cells, but parameter describing protein-protein interactions of cone specific proteins are rare and not available for migratory birds. European robin is a model organism for studying the orientation of birds in the earth magnetic field. Recent findings showed a link between the putative magnetoreceptor cryptochrome 4a and the cone specific G-protein of European robin. In the present work, we investigated the interaction of European robin cone specific G protein and cytoplasmic regions of long wavelength opsin. We identified the second loop in opsin connecting transmembrane regions three and four as a critical binding interface. Surface plasmon resonance studies using a synthetic peptide representing the second cytoplasmic loop and purified G protein α-subunit showed a high affinity interaction with a K _D value of 21 nM. Truncation of the G protein α-subunit at the C-terminus by six amino acids slightly decreased the affinity. Our results suggest that binding of the G protein to cryptochrome can compete with the interaction of G protein and non-photoexcited long wavelength opsin. Thus, the parallel presence of two different sensory pathways in bird cone photoreceptors is reasonable under dark-adapted conditions or during illumination with short wavelengths.

Ultrafast spectra and kinetics of human green-cone visual pigment at room temperature

Rhodopsin is the pigment that enables night vision, whereas cone opsins are the pigments responsible for color vision in bright-light conditions. Despite their importance for vision, cone opsins are poorly characterized at the molecular level compared to rhodopsin. Spectra and kinetics of the intermediate states of human green-cone visual pigment (mid-wavelength sensitive, or MWS opsin) were measured and compared with the intermediates and kinetics of bovine rhodopsin. All the major intermediates of the MWS opsin were recorded in the picosecond to millisecond time range. Several intermediates in MWS opsin appear to have characteristics similar to the intermediates of bovine rhodopsin; however, there are some marked differences. One of the most striking differences is in their kinetics, where the kinetics of the MWS opsin intermediates are slower compared to those of the bovine rhodopsin intermediates.

Nanoparticles-mediated CRISPR-Cas9 gene therapy in inherited retinal diseases: applications, challenges, and emerging opportunities

Inherited Retinal Diseases (IRDs) are considered one of the leading causes of blindness worldwide. However, the majority of them still lack a safe and effective treatment due to their complexity and genetic heterogeneity. Recently, gene therapy is gaining importance as an efficient strategy to address IRDs which were previously considered incurable. The development of the clustered regularly-interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) system has strongly empowered the field of gene therapy. However, successful gene modifications rely on the efficient delivery of CRISPR-Cas9 components into the complex three-dimensional (3D) architecture of the human retinal tissue. Intriguing findings in the field of nanoparticles (NPs) meet all the criteria required for CRISPR-Cas9 delivery and have made a great contribution toward its therapeutic applications. In addition, exploiting induced pluripotent stem cell (iPSC) technology and in vitro 3D retinal organoids paved the way for prospective clinical trials of the CRISPR-Cas9 system in treating IRDs. This review highlights important advances in NP-based gene therapy, the CRISPR-Cas9 system, and iPSC-derived retinal organoids with a focus on IRDs. Collectively, these studies establish a multidisciplinary approach by integrating nanomedicine and stem cell technologies and demonstrate the utility of retina organoids in developing effective therapies for IRDs.

Case Report: Green Light Exposure Relieves Chronic Headache Pain in a Colorblind Patient

Patients with chronic headaches sometimes prefer non-pharmacological methods for pain management. We have shown previously that green light exposure (GLED, Green Light Emitting Diode) reversed thermal hyperalgesia and mechanical allodynia in a rat model of neuropathic pain. This effect is mediated through the visual system. Moreover, we recently showed that GLED was effective in decreasing the severity of headache pain and the number of headache-days per month in migraine patients. The visual system is comprised of image-forming and non-image-forming pathways; however, the contribution of different photosensitive cells to the effect of GLED is not yet known. Here, we report a 66-year-old man with headaches attributed to other disorders of homeostasis and color blindness who was recruited in the GLED study. The subject, diagnosed with protanomaly, cannot differentiate green, yellow, orange, and red colors. After completing the GLED exposure protocol, the subject noted significant decreases in headache pain intensity without reduction in the number of headache-days per month. The subject also reported improvement in the quality of his sleep. These findings suggest that green light therapy mediates the decrease of the headache pain intensity through non-image-forming intrinsically photosensitive retinal ganglion cells. However, the subject did not report a change in the frequency of his headaches, suggesting the involvement of cones in reduction of headache frequency by GLED. This is the first case reported of a colorblind man with chronic headache using GLED to manage his headache pain and may increase our understanding of the contribution of different photosensitive cells in mediating the pain-relieving effects of GLED.

Zebrafish and inherited photoreceptor disease: Models and insights

Photoreceptor dysfunctions and degenerative diseases are significant causes of vision loss in patients, with few effective treatments available. Targeted interventions to prevent or reverse photoreceptor-related vision loss are not possible without a thorough understanding of the underlying mechanism leading to disease, which is exceedingly difficult to accomplish in the human system. Cone diseases are particularly challenging to model, as some popular genetically modifiable model animals are nocturnal with a rod-dominant visual system and cones that have dissimilarities to human cones. As a result, cone diseases, which affect visual acuity, colour perception, and central vision in patients, are generally poorly understood in terms of pathology and mechanism. Zebrafish (Danio rerio) provide the opportunity to model photoreceptor diseases in a diurnal vertebrate with a cone-rich retina which develops many macular degeneration-like pathologies. Zebrafish undergo external development, allowing early-onset retinal diseases to be detected and studied, and many ophthalmic tools are available for zebrafish visual assessment during development and adulthood. There are numerous zebrafish models of photoreceptor disease, spanning the various types of photoreceptor disease (developmental, rod, cone, and mixed photoreceptor diseases) and genetic/molecular cause. In this review, we explore the features of zebrafish that make them uniquely poised to model cone diseases, summarize the established zebrafish models of inherited photoreceptor disease, and discuss how disease in these models compares to the human presentation, where applicable. Further, we highlight the contributions of these zebrafish models to our understanding of photoreceptor biology and disease, and discuss future directions for utilising and investigating these diverse models.

Gene Therapy in Opn1mw-/-/Opn1sw-/- Mice and Implications for Blue Cone Monochromacy Patients with Deletion Mutations

Blue cone monochromacy (BCM) is a congenital vision disorder affecting both middle-wavelength (M) and long-wavelength (L) cone photoreceptors of the human retina. BCM results from abolished expression of green and red light-sensitive visual pigments expressed in M- and L-cones, respectively. Previously, we showed that gene augmentation therapy to deliver either human L- or M-opsin rescues dorsal M-opsin dominant cone photoreceptors structurally and functionally in treated M-opsin knockout (Opn1mw-/-) mice. Although Opn1mw-/- mice represent a disease model for BCM patients with deletion mutations, at the cellular level, dorsal cones of Opn1mw-/- mice still express low levels of S-opsin, which are different from L- and M-cones of BCM patients carrying a congenital opsin deletion. To determine whether BCM cones lacking complete opsin expression from birth would benefit from AAV-mediated gene therapy, we evaluated the outcome of gene therapy, and determined the therapeutic window and longevity of rescue in a mouse model lacking both M- and S-opsin (Opn1mw-/-/Opn1sw-/-). Our data show that cones of Opn1mw-/-/Opn1sw-/- mice are viable at younger ages but undergo rapid degeneration. AAV-mediated expression of human L-opsin promoted cone outer segment regeneration and rescued cone-mediated function when mice were injected subretinally at 2 months of age or younger. Cone-mediated function and visually guided behavior were maintained for at least 8 months post-treatment. However, when mice were treated at 5 and 7 months of age, the chance and effectiveness of rescue was significantly reduced, although cones were still present in the retina. Crossing Opn1mw-/-/Opn1sw-/- mice with proteasomal activity reporter mice (UbG76V-GFP) did not reveal GFP accumulation in Opn1mw-/-/Opn1sw-/- cones eliminating impaired degradation of ubiquitinated proteins as stress factor contributing to cone loss. Our results demonstrate that AAV-mediated gene augmentation therapy can rescue cone structure and function in a mouse model with a congenital opsin deletion, but also emphasize the importance that early intervention is crucial for successful therapy.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

The role of motor proteins in photoreceptor protein transport and visual function

BACKGROUND

Rods and cones are photoreceptor neurons in the retina that are required for visual sensation in vertebrates, wherein the perception of vision is initiated when these neurons respond to photons in the light stimuli. The photoreceptor cell is structurally studied as outer segments (OS) and inner segments (IS) where proper protein sorting, localization, and compartmentalization are critical for phototransduction, visual function, and survival. In human retinal diseases, improper protein transport to the OS or mislocalization of proteins to the IS and other cellular compartments could lead to impaired visual responses and photoreceptor cell degeneration that ultimately cause loss of visual function.

RESULTS

Therefore, studying and identifying mechanisms involved in facilitating and maintaining proper protein transport in photoreceptor cells would help our understanding of pathologies involving retinal cell degeneration in inherited retinal dystrophies, age-related macular degeneration, and Usher Syndrome.

CONCLUSIONS

Our mini-review will discuss mechanisms of protein transport within photoreceptors and introduce a novel role for an unconventional motor protein, MYO1C, in actin-based motor transport of the visual chromophore Rhodopsin to the OS, in support of phototransduction and visual function.

Proteins Associated with Phagocytosis Alteration in Retinal Pigment Epithelial Cells Derived from Age-Related Macular Degeneration Patients

Age-related macular degeneration (AMD) is partially characterized by retinal pigment epithelial (RPE) cell dysfunction. This study focused on phagocytosis activity and its involvement in AMD. Phagocytic activity was analyzed by flow cytometry using porcine photoreceptor outer segment (POS) and fluorescent beads in basal and under oxidative stress condition induced by Fe-NTA in fifteen hiPSC-RPE cell lines (six controls, six atrophic AMD and three exudative AMD). Oxidative stress exposure inhibited phagocytosis in the same manner for control, atrophic AMD (AMDa) and exudative AMD (AMDe) cell lines. However, altered phagocytosis in basal condition in hiPSC-RPE AMDa/e was observed compared to control cell lines. Gene expression after 3 or 24 h of POS incubation was analyzed by RNA-Seq based transcriptomic profiling. Differential gene expression was observed by RNA seq after 3 and 24 h POS exposure. We have focused on the genes involved in mTOR/PI3K-AKT/MEK-ERK pathway. We investigated differences in gene expression by analyzing the expression levels and activity of the corresponding proteins by Western blot. We showed the involvement of three proteins essential for phagocytosis activity: fak, tuberin and rictor. These findings demonstrate that hiPSC-RPE AMDa/e cells have a typical disease phenotype characterized by alteration of the main function of RPE cells, phagocytosis activity.

Emerging Lysosomal Functions for Photoreceptor Cell Homeostasis and Survival

Lysosomes are membrane-bound cell organelles that respond to nutrient changes and are implicated in cell homeostasis and clearance mechanisms, allowing effective adaptation to specific cellular needs. The relevance of the lysosome has been elucidated in a number of different contexts. Of these, the retina represents an interesting scenario to appreciate the various functions of this organelle in both physiological and pathological conditions. Growing evidence suggests a role for lysosome-related mechanisms in retinal degeneration. Abnormal lysosomal activation or inhibition has dramatic consequences on photoreceptor cell homeostasis and impacts extensive cellular function, which in turn affects vision. Based on these findings, a series of therapeutic methods targeting lysosomal processes could offer treatment for blindness conditions. Here, we review the recent findings on membrane trafficking, subcellular organization, mechanisms by which lysosome/autophagy pathway impairment affects photoreceptor cell homeostasis and the recent advances on developing efficient lysosomal-based therapies for retinal disorders.

Identification of small molecule allosteric modulators that act as enhancers/disrupters of rhodopsin oligomerization

The elongated cilia of the outer segment of rod and cone photoreceptor cells can contain concentrations of visual pigments of up to 5 mM. The rod visual pigments, G protein–coupled receptors called rhodopsins, have a propensity to self-aggregate, a property conserved among many G protein–coupled receptors. However, the effect of rhodopsin oligomerization on G protein signaling in native cells is less clear. Here, we address this gap in knowledge by studying rod phototransduction. As the rod outer segment is known to adjust its size proportionally to overexpression or reduction of rhodopsin expression, genetic perturbation of rhodopsin cannot be used to resolve this question. Therefore, we turned to high-throughput screening of a diverse library of 50,000 small molecules and used a novel assay for the detection of rhodopsin dimerization. This screen identified nine small molecules that either disrupted or enhanced rhodopsin dimer contacts in vitro. In a subsequent cell-free binding study, we found that all nine compounds decreased intrinsic fluorescence without affecting the overall UV-visible spectrum of rhodopsin, supporting their actions as allosteric modulators. Furthermore, ex vivo electrophysiological recordings revealed that a disruptive, hit compound #7 significantly slowed down the light response kinetics of intact rods, whereas compound #1, an enhancing hit candidate, did not substantially affect the photoresponse kinetics but did cause a significant reduction in light sensitivity. This study provides a monitoring tool for future investigation of the rhodopsin signaling cascade and reports the discovery of new allosteric modulators of rhodopsin dimerization that can also alter rod photoreceptor physiology.

The role of dark adaptation in understanding early AMD

The main aim of the paper is to discuss current knowledge on how Age Related Macular Degeneration (AMD) affects Dark Adaptation (DA). The paper is divided into three parts. Firstly, we outline some of the molecular mechanisms that control DA. Secondly, we review the psychophysical issues and the corresponding analytical techniques. Finally, we characterise the link between slowed DA and the morphological abnormalities in early AMD. Historically, DA has been regarded as too cumbersome for widespread clinical application. Yet the technique is extremely useful; it is widely accepted that the psychophysically obtained slope of the second rod-mediated phase of the dark adaptation function is an accurate assay of photoreceptor pigment regeneration kinetics. Technological developments have prompted new ways of generating the DA curve, but analytical problems remain. A simple potential solution to these, based on the application of a novel fast mathematical algorithm, is presented. This allows the calculation of the parameters of the DA curve in real time. Improving current management of AMD will depend on identifying a satisfactory endpoint for evaluating future therapeutic strategies. This must be implemented before the onset of severe disease. Morphological changes progress too slowly to act as a satisfactory endpoint for new therapies whereas functional changes, such as those seen in DA, may have more potential in this regard. It is important to recognise, however, that the functional changes are not confined to rods and that building a mathematical model of the DA curve enables the separation of rod and cone dysfunction and allows more versatility in terms of the range of disease severity that can be monitored. Examples are presented that show how analysing the DA curve into its constituent components can improve our understanding of the morphological changes in early AMD.