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

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.

Tackling visual impairment: emerging avenues in ophthalmology

Visual impairment, stemming from genetic, degenerative, and traumatic causes, affects millions globally. Recent advancements in ophthalmology present novel strategies for managing and potentially reversing these conditions. Here, we explore 10 emerging avenues-including gene therapy, stem cell therapy, advanced imaging, novel therapeutics, nanotechnology, artificial intelligence (AI) and machine learning, teleophthalmology, optogenetics, bionics, and neuro-ophthalmology-all making strides to improve diagnosis, treatment, and vision restoration. Among these, gene therapy and stem cell therapy are revolutionizing the treatment of retinal degenerative diseases, while advanced imaging technologies enable early detection and personalized care. Therapeutic advancements like anti-vascular endothelial growth factor therapies and neuroprotective agents, along with nanotechnology, have improved clinical outcomes for multiple ocular conditions. AI, especially machine learning, is enhancing diagnostic accuracy, facilitating early detection, and personalized treatment strategies, particularly when integrated with advanced imaging technologies. Teleophthalmology, further strengthened by AI, is expanding access to care, particularly in underserved regions, whereas emerging technologies like optogenetics, bionics, and neuro-ophthalmology offer new hope for patients with severe vision impairment. In light of ongoing research, we summarize the current clinical landscape and the potential advantages of these innovations to revolutionize the management of visual impairments. Additionally, we address the challenges and limitations associated with these emerging avenues in ophthalmology, providing insights into their future trajectories in clinical practice. Continued advancements in these fields promise to reshape the landscape of ophthalmic care, ultimately improving the quality of life for individuals with visual impairments.

Optogenetic therapy for retinal degenerative diseases: A review

Optogenetics, a cutting-edge tool in novel gene manipulation and drug discovery, holds significant therapeutic potential for a variety of neurological disorders, including retinal diseases. Retinal diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), significantly impair quality of life and cause severe visual impairment due to limited treatment options and a general lack of awareness. The increasing incidence of these degenerative conditions underscores the need for innovative solutions, such as optogenetics. Optogenetic therapy introduces genes coding for light-sensitive proteins, which are controlled by light signals to make neurons photosensitive. This precise targeting approach does not require specific gene intervention and can bypass dysfunctional photoreceptors, offering a treatment option for various degenerative and dystrophic eye diseases. Successful outcomes in patients with late-stage genetic retinal diseases and numerous clinical trials suggest that optogenetics could be an effective treatment for humans. This review provides an overview of the current landscape of optogenetic therapy, discusses its challenges, and summarizes the findings of ongoing clinical trials for neural and visual restoration.

The ERGtools2 package: a toolset for processing and analysing visual electrophysiology data

PURPOSE

To introduce ERGtools2, an open-source R package for processing, analysing and long-term storing visual electrophysiology data.

METHODS

A dataset comprising Electroretinogram (ERG) recordings of C57Bl/6J mice, subjected to standard ISCEV stimuli, was used to present the functionality of ERGtools2. ERGtools2 stores and organizes all recordings, metadata, and measurement information from an individual examination in a single object, maintaining raw data throughout the analysis process.

RESULTS

A standard workflow is presented exemplifying how ERGtools2 can be used to efficiently import, pre-process and analyse ERG data. Following this workflow, basic ERG measurements and visualisation of a single exam as well as group statistics are obtained. Moreover, special use cases are described, including for the handling of noisy data and the storage of data in the HDF5 format to ensure long-term preservation and accessibility.

CONCLUSIONS

ERGtools2 provides a comprehensive, flexible, and device-independent solution for visual electrophysiology data analysis. Its emphasis on maintaining raw data integrity, combined with advanced processing and analysis capabilities, makes it a useful tool for preclinical and clinical research applications. The open-source nature and the use of open data formats promote reproducibility and data sharing in visual neurosciences.

Enhanced restoration of visual code after targeting ON bipolar cells compared with retinal ganglion cells with optogenetic therapy

Optogenetic therapy is a promising vision restoration method where light-sensitive opsins are introduced to the surviving inner retina following photoreceptor degeneration. The cell type targeted for opsin expression will likely influence the quality of restored vision. However, a like-for-like preclinical comparison of visual responses evoked following equivalent opsin expression in the two major targets, ON bipolar (ON BCs) or retinal ganglion cells (RGCs), is absent. We address this deficit by comparing stimulus-response characteristics at single-unit resolution in the retina and dorsal lateral geniculate nucleus of retinally degenerate mice genetically engineered to express the opsin ReaChR in Grm6- or Brn3c-expressing cells (ON BC vs. RGCs, respectively). For both targeting strategies, we find ReaChR-evoked responses have equivalent sensitivity and can encode contrast across different background irradiances. Compared with ON BCs, targeting RGCs decreased response reproducibility and resulted in more stereotyped responses with reduced diversity in response polarity, contrast sensitivity, and temporal frequency tuning. Recording ReaChR-driven responses in visually intact retinas confirmed that RGC-targeted ReaChR expression disrupts visual feature selectivity of individual RGCs. Our data show that, while both approaches restore visual responses with impressive fidelity, ON BC targeting produces a richer visual code closer to that of wild-type mice.

Evolution of Light-Sensitive Proteins in Optogenetic Approaches for Vision Restoration: A Comprehensive Review

Retinal degenerations, such as age-related macular degeneration and retinitis pigmentosa, present significant challenges due to genetic heterogeneity, limited therapeutic options, and the progressive loss of photoreceptors in advanced stages. These challenges are compounded by difficulties in precisely targeting residual retinal neurons and ensuring the sustained efficacy of interventions. Optogenetics offers a novel approach to vision restoration by inducing light sensitivity in residual retinal neurons through gene delivery of light-sensitive opsins. This review traces the evolution of opsins in optogenetic therapies, highlighting advancements from early research on channelrhodopsin-2 (ChR2) to engineered variants addressing key limitations. Red-shifted opsins, including ReaChR and ChrimsonR, reduced phototoxicity by enabling activation under longer wavelengths, while Chronos introduced superior temporal kinetics for dynamic visual tracking. Further innovations, such as Multi-Characteristic Opsin 1 (MCO1), optimized opsin performance under ambient light, bridging the gap to real-world applications. Key milestones include the first partial vision restoration in a human patient using ChrimsonR with light-amplifying goggles and ongoing clinical trials exploring the efficacy of opsin-based therapies for advanced retinal degeneration. While significant progress has been made, challenges remain in achieving sufficient light sensitivity for functional vision under normal ambient lighting conditions in a manner that is both effective and safe, eliminating the need for external light-enhancing devices. As research progresses, optogenetic therapies are positioned to redefine the management of retinal degenerative diseases, offering new hope for millions affected by vision loss.

A membrane-targeted photoswitch restores physiological ON/OFF responses to light in the degenerate retina

The lack of effective therapies for visual restoration in Retinitis pigmentosa and macular degeneration has led to the development of new strategies, such as optogenetics and retinal prostheses. However, visual restoration is poor due to the massive light-evoked activation of retinal neurons, regardless of the segregation of visual information in ON and OFF channels, which is essential for contrast sensitivity and spatial resolution. Here, we show that Ziapin2, a membrane photoswitch that modulates neuronal capacitance and excitability in a light-dependent manner, is capable of reinstating, in mouse and rat genetic models of photoreceptor degeneration, brisk and sluggish ON, OFF, and ON-OFF responses in retinal ganglion cells evoked by full-field stimuli, with reactivation of their excitatory and inhibitory conductances. Intravitreally injected Ziapin2 in fully blind rd10 mice restores light-driven behavior and optomotor reflexes. The results indicate that Ziapin2 is a promising molecule for reinstating physiological visual responses in the late stages of retinal degeneration.

Syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) is a progressive inherited retinal dystrophy, characterized by the degeneration of photoreceptors, presenting as a rod-cone dystrophy. Approximately 20-30% of patients with RP also exhibit extra-ocular manifestations in the context of a syndrome. This manuscript discusses the broad spectrum of syndromes associated with RP, pathogenic mechanisms, clinical manifestations, differential diagnoses, clinical management approaches, and future perspectives. Given the diverse clinical and genetic landscape of syndromic RP, the diagnosis may be challenging. However, an accurate and timely diagnosis is essential for optimal clinical management, prognostication, and potential treatment. Broadly, the syndromes associated with RP can be categorized into ciliopathies, inherited metabolic disorders, mitochondrial disorders, and miscellaneous syndromes. Among the ciliopathies associated with RP, Usher syndrome and Bardet-Biedl syndrome are the most well-known. Less common ciliopathies include Cohen syndrome, Joubert syndrome, cranioectodermal dysplasia, asphyxiating thoracic dystrophy, Mainzer-Saldino syndrome, and RHYNS syndrome. Several inherited metabolic disorders can present with RP, including Zellweger spectrum disorders, adult Refsum disease, α-methylacyl-CoA racemase deficiency, certain mucopolysaccharidoses, ataxia with vitamin E deficiency, abetalipoproteinemia, several neuronal ceroid lipofuscinoses, mevalonic aciduria, PKAN/HARP syndrome, PHARC syndrome, and methylmalonic acidaemia with homocystinuria type cobalamin (cbl) C disease. Due to the mitochondria's essential role in supplying continuous energy to the retina, disruption of mitochondrial function can lead to RP, as seen in Kearns-Sayre syndrome, NARP syndrome, primary coenzyme Q10 deficiency, SSBP1-associated disease, and long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. Lastly, Cockayne syndrome and PERCHING syndrome can present with RP, but they do not fit the abovementioned hierarchy and are thus categorized as miscellaneous. Several first-in-human clinical trials are underway or in preparation for some of these syndromic forms of RP.

Bridging the gap of vision restoration

Retinitis pigmentosa (RP) and Age-Related Macular Degeneration (AMD) are similar in that both result in photoreceptor degeneration leading to permanent progressive vision loss. This affords the possibility of implementing vision restoration techniques, where light signaling is restored to spared retinal circuitry to recreate vision. There are far more AMD patients (Wong et al., 2014), yet more resources have been put towards researching and developing vision restoration strategies for RP despite it rarity, because of the tractability of RP disease models. The hope is that these therapies will extend to the AMD population, however, many questions remain about how the implementation of prosthetic or optogenetic vision restoration technologies will translate between RP and AMD patients. In this review, we discuss the difference and similarities of RP and AMD with a focus on aspects expected to impact vision restoration strategies, and we identify key gaps in knowledge needed to further improve vision restoration technologies for a broad patient population.

Deprivation of visual input alters specific subset of inhibitory neurons and affect thalamic afferent terminals in V1 of rd1 mouse

Retinitis Pigmentosa (RP) is a heterogenous group of inherited disorder, and its progression not only affects the retina but also the primary visual cortex. This manifests imbalances in the excitatory and inhibitory neurotransmission. Here, we investigated if changes in cortical functioning is linked to alterations in GABAergic population of neurons and its two important subsets, somatostatin (SST) and parvalbumin (PV) neuron in rd1 model of retinal degeneration (RD). We demonstrate marked decrease in the proportion of SST neurons in different layers of cortex whereas PV neurons were less affected. Moreover, we found reduced expression of glutamatergic thalamic afferents (VGLUT2) due to lack of visual activity. These results suggest PV neurons are likely recruited by the cortical circuitry to increase the inhibitory drive and compensate the disrupted inhibition-excitation balance. However, reduced SST expression perhaps results in weakening of stimulus selectivity. Delineating their functional role during RD will provide insights for acquisition of high-resolution vision thereby improving current state of vision restoration.

Current and Future Directions in Developing Effective Treatments for PRPH2-Associated Retinal Diseases: A Workshop Report

Purpose and Methods

A workshop of affected individuals and their families, clinicians, researchers, and industry representatives was convened in March 2023 to define the knowledge landscape of peripherin 2 (PRPH2) biology and identify challenges and opportunities towards developing PRPH2-associated inherited retinal disease (IRD) treatments.

Results

The results of an online survey and presentations from affected individuals and their family members revealed disease characteristics and impacts on daily living. Scientific sessions highlighted the significant heterogeneity in clinical presentation of PRPH2-related retinopathy; PRPH2's crucial function in rod and cone outer segment formation and maintenance; the usefulness of existing animal and cellular models for understanding disease pathophysiology; and possible therapeutic approaches for autosomal dominant PRPH2-associated IRDs, including gene-specific therapies and gene-agnostic approaches. Priority gaps identified by the workshop included having a more complete understanding of PRPH2's fundamental biology and factors contributing to PRPH2-related disease phenotypic diversity, establishing genotype-phenotype correlations, and creating additional models to probe the functional consequences of PRPH2 variants and to test therapies. Additionally, a natural history study involving a large number of participants is required to more fully characterize PRPH2-related disease progression, aiding in interventional clinical trial design.

Conclusions

Because PRPH2-associated IRDs are rare, maximizing opportunities for communication and collaboration among stakeholders, such as that provided by the workshop, is crucial to overcome the challenges to developing effective treatments and improve the lives of affected individuals.

Translational Relevance

Fostering communication among stakeholders to identify knowledge gaps, therapeutic challenges, and potential opportunities toward developing effective treatments for PRPH2-related IRDs.

Advances and Challenges in Gene Therapy for Inherited Retinal Dystrophies: A Comprehensive Review

Inherited retinal dystrophies (IRDs) are a diverse group of genetic disorders leading to progressive vision loss due to the degeneration of retinal photoreceptors. Gene therapy has emerged as a promising approach to address the underlying genetic causes of IRDs, offering the potential for restoring vision and halting disease progression. This review provides a comprehensive overview of gene therapy innovations for IRDs, focusing on the mechanisms, recent advancements, and ongoing challenges. We discuss the fundamental principles of gene therapy, including the use of viral and non-viral vectors, and highlight key developments such as the approval of Luxturna for RPE65-mediated retinal dystrophy and the application of gene editing technologies like CRISPR/Cas9. Despite these advancements, significant challenges remain, including vector delivery, long-term safety, and variable patient responses. This review also explores the future directions of gene therapy, emphasizing the need for further research to address these challenges and enhance therapeutic efficacy. By examining the current state of gene therapy for IRDs, this review aims to provide valuable insights into the potential for these treatments to transform the management of retinal diseases and improve the quality of life for affected individuals.

Correlations between the Michigan Retinal Degeneration Questionnaire and visual function parameters in patients with retinitis pigmentosa

PURPOSE

To validate the use of best-corrected visual acuity (BCVA), low-luminance visual acuity (LLVA), low-luminance deficit (LLD; the difference between BCVA and LLVA), mean macular sensitivity and fixation stability as parameters of vision-related quality of life based on the novel Michigan Retinal Degeneration Questionnaire (MRDQ) in retinitis pigmentosa (RP) patients.

METHODS

In this prospective cross sectional study, 30 patients with RP (47% female) were included with a median age of 41.0 years (interquartile range: 24.1-58.3 years). BCVA, LLVA and LLD were measured with Early Treatment Diabetic Retinopathy Study (ETDRS) charts. Mesopic microperimetry was performed to measure mean macular sensitivity and fixation stability. Patients completed a Dutch translation of the MRDQ which results in an experienced disability (Θ-)score of seven domains. Spearman's rank correlation was used.

RESULTS

BCVA correlated significantly to the MRDQ domain of central vision (r = 0.657; p < 0.001) and colour vision (r = 0.524; p = 0.003). Lower LLVA significantly correlated to higher experienced disability in the MRDQ domains for central vision (=0.550; p = 0.002) and contrast sensitivity (r = 0.502; p = 0.005). LLD was significantly correlated to the MRDQ domains of scotopic function (r = -0.484; p = 0.007) and mesopic peripheral function (r = -0.533; p = 0.002). Lower mean macular sensitivity was significantly associated with high experienced disability in all domains except for photosensitivity.

CONCLUSIONS

The majority of the MRDQ domains is strongly associated with visual function parameters. These findings show that visual function measurements, especially LLVA, LLD and mean macular sensitivity on microperimetry, reflect vision-related quality of life and can be used as relevant outcome measures in clinical trials for RP.

Information Transmission through Biotic-Abiotic Interfaces to Restore or Enhance Human Function

Advancements in reliable information transfer across biotic-abiotic interfaces have enabled the restoration of lost human function. For example, communication between neuronal cells and electrical devices restores the ability to walk to a tetraplegic patient and vision to patients blinded by retinal disease. These impactful medical achievements are aided by tailored biotic-abiotic interfaces that maximize information transfer fidelity by considering the physical properties of the underlying biological and synthetic components. This Review develops a modular framework to define and describe the engineering of biotic and abiotic components as well as the design of interfaces to facilitate biotic-abiotic information transfer using light or electricity. Delineating the properties of the biotic, interface, and abiotic components that enable communication can serve as a guide for future research in this highly interdisciplinary field. Application of synthetic biology to engineer light-sensitive proteins has facilitated the control of neural signaling and the restoration of rudimentary vision after retinal blindness. Electrophysiological methodologies that use brain-computer interfaces and stimulating implants to bypass spinal column injuries have led to the rehabilitation of limb movement and walking ability. Cellular interfacing methodologies and on-chip learning capability have been made possible by organic transistors that mimic the information processing capacity of neurons. The collaboration of molecular biologists, material scientists, and electrical engineers in the emerging field of biotic-abiotic interfacing will lead to the development of prosthetics capable of responding to thought and experiencing touch sensation via direct integration into the human nervous system. Further interdisciplinary research will improve electrical and optical interfacing technologies for the restoration of vision, offering greater visual acuity and potentially color vision in the near future.

Noninvasive imaging-guided ultrasonic neurostimulation with arbitrary 2D patterns and its application for high-quality vision restoration

Retinal degeneration, a leading cause of irreversible low vision and blindness globally, can be partially addressed by retina prostheses which stimulate remaining neurons in the retina. However, existing electrode-based treatments are invasive, posing substantial risks to patients and healthcare providers. Here, we introduce a completely noninvasive ultrasonic retina prosthesis, featuring a customized ultrasound two-dimensional array which allows for simultaneous imaging and stimulation. With synchronous three-dimensional imaging guidance and auto-alignment technology, ultrasonic retina prosthesis can generate programmed ultrasound waves to dynamically and precisely form arbitrary wave patterns on the retina. Neuron responses in the brain's visual center mirrored these patterns, evidencing successful artificial vision creation, which was further corroborated in behavior experiments. Quantitative analysis of the spatial-temporal resolution and field of view demonstrated advanced performance of ultrasonic retina prosthesis and elucidated the biophysical mechanism of retinal stimulation. As a noninvasive blindness prosthesis, ultrasonic retina prosthesis could lead to a more effective, widely acceptable treatment for blind patients. Its real-time imaging-guided stimulation strategy with a single ultrasound array, could also benefit ultrasound neurostimulation in other diseases.

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.

Nanoparticle-based optical interfaces for retinal neuromodulation: a review

Degeneration of photoreceptors in the retina is a leading cause of blindness, but commonly leaves the retinal ganglion cells (RGCs) and/or bipolar cells extant. Consequently, these cells are an attractive target for the invasive electrical implants colloquially known as "bionic eyes." However, after more than two decades of concerted effort, interfaces based on conventional electrical stimulation approaches have delivered limited efficacy, primarily due to the current spread in retinal tissue, which precludes high-acuity vision. The ideal prosthetic solution would be less invasive, provide single-cell resolution and an ability to differentiate between different cell types. Nanoparticle-mediated approaches can address some of these requirements, with particular attention being directed at light-sensitive nanoparticles that can be accessed via the intrinsic optics of the eye. Here we survey the available known nanoparticle-based optical transduction mechanisms that can be exploited for neuromodulation. We review the rapid progress in the field, together with outstanding challenges that must be addressed to translate these techniques to clinical practice. In particular, successful translation will likely require efficient delivery of nanoparticles to stable and precisely defined locations in the retinal tissues. Therefore, we also emphasize the current literature relating to the pharmacokinetics of nanoparticles in the eye. While considerable challenges remain to be overcome, progress to date shows great potential for nanoparticle-based interfaces to revolutionize the field of visual prostheses.

Micromolded honeycomb scaffold design to support the generation of a bilayered RPE and photoreceptor cell construct

Age-related macular degeneration (AMD) causes blindness due to loss of retinal pigment epithelium (RPE) and photoreceptors (PRs), which comprise the two outermost layers of the retina. Given the small size of the macula and the importance of direct contact between RPE and PRs, the use of scaffolds for targeted reconstruction of the outer retina in later stage AMD and other macular dystrophies is particularly attractive. We developed microfabricated, honeycomb-patterned, biodegradable poly(glycerol sebacate) (PGS) scaffolds to deliver organized, adjacent layers of RPE and PRs to the subretinal space. Furthermore, an optimized process was developed to photocure PGS, shortening scaffold production time from days to minutes. The resulting scaffolds robustly supported the seeding of human pluripotent stem cell-derived RPE and PRs, either separately or as a dual cell-layered construct. These advanced, economical, and versatile scaffolds can accelerate retinal cell transplantation efforts and benefit patients with AMD and other retinal degenerative diseases.

Losing, preserving, and restoring vision from neurodegeneration in the eye

The retina is a part of the brain that sits at the back of the eye, looking out onto the world. The first neurons of the retina are the rod and cone photoreceptors, which convert changes in photon flux into electrical signals that are the basis of vision. Rods and cones are frequent targets of heritable neurodegenerative diseases that cause visual impairment, including blindness, in millions of people worldwide. This review summarizes the diverse genetic causes of inherited retinal degenerations (IRDs) and their convergence onto common pathogenic mechanisms of vision loss. Currently, there are few effective treatments for IRDs, but recent advances in disparate areas of biology and technology (e.g., genome editing, viral engineering, 3D organoids, optogenetics, semiconductor arrays) discussed here enable promising efforts to preserve and restore vision in IRD patients with implications for neurodegeneration in less approachable brain areas.

Suppressing Retinal Remodeling to Mitigate Vision Loss in Photoreceptor Degenerative Disorders

Rod and cone photoreceptors degenerate in retinitis pigmentosa and age-related macular degeneration, robbing the visual system of light-triggered signals necessary for sight. However, changes in the retina do not stop with the photoreceptors. A stereotypical set of morphological and physiological changes, known as remodeling, occur in downstream retinal neurons. Some aspects of remodeling are homeostatic, with structural or functional changes compensating for partial loss of visual inputs. However, other aspects are nonhomeostatic, corrupting retinal information processing to obscure vision mediated naturally by surviving photoreceptors or artificially by vision-restoration technologies. In this review, I consider the mechanism of remodeling and its consequences for residual and restored visual function; discuss the role of retinoic acid, a critical molecular trigger of detrimental remodeling; and discuss strategies for suppressing retinoic acid biosynthesis or signaling as therapeutic possibilities for mitigating vision loss.

Current approaches to vision restoration using optogenetic therapy

Inherited progressive degeneration of photoreceptors such as retinitis pigmentosa (RP) is the most common cause of blindness leading to severe vision impairment affecting ~1 in 5,000 people worldwide. Although the function and morphology of the photoreceptors get disrupted, there is evidence that the inner retinal neurons such as bipolar cells and the retinal ganglion cells are left intact until later stages. Among several innovative therapeutic options aiming to restore vision, optogenetic therapy can bestow light sensitivity to remaining retinal neurons by ectopic expression of light-sensitive proteins. Since the advent of this technique, a diverse class of opsins (microbial and mammalian opsins), chimeric proteins, ligand-gated ion channels, and switchable opsins have been used to study their potential in vision restoration. These proteins differ in their excitation spectra, response kinetics, and signal amplification cascade. Although most of the studies have reported high fidelity of responses in the retina, only a handful of them have achieved functional vision in the visual cortex. This review is a summary of the visuocortical and behavioral responses after optogenetic treatment of the degenerated retina. This clarifies to what extent improved and meaningful vision can be obtained for therapeutic efficacy and continued clinical progress.

HYPERREFLECTIVE BAND IN THE GANGLION CELL LAYER IN RETINITIS PIGMENTOSA

PURPOSE

To describe a sign that takes the form of a continuous hyperreflective band within the thickness of the ganglion cell layer (GCL), thus dubbed the "hyperreflective ganglion cell layer band" (HGB), which the authors detected in a fraction of patients affected by retinitis pigmentosa (RP).

METHODS

Retrospective, cross-sectional, observational study. Optical coherence tomography (OCT) images of patients with RP examined between May 2015 and June 2021 were retrospectively reviewed for the presence of HGB, epiretinal membrane (ERM), macular hole, and cystoid macular edema (CME). The ellipsoid zone (EZ) width was also measured. A subgroup of patients underwent microperimetry in the central 2°, 4°, and 10°.

RESULTS

One hundred and fifty-four eyes from 77 subjects were included in the study. The HGB was present in 39 (25.3%) eyes with RP. Mean best-corrected visual acuity (BCVA) was 0.39 ± 0.05 logMAR (approximately 20/50 Snellen equivalent) and 0.18 ± 0.03 logMAR (approximately 20/32 Snellen equivalent) in eyes with and without HGB, respectively ( P < 0.001). The two groups did not differ regarding EZ width; mean 2°, 4°, and 10° retinal sensitivity; and prevalence of CME, ERM, and macular hole. The multivariable analysis showed the presence of HGB to be a predictor of poorer BCVA ( P < 0.001).

CONCLUSION

HGB is an OCT finding detectable in approximately a quarter of eyes with RP and is associated with a poorer visual function. In the discussion, the authors speculate about possible morphogenetic scenarios to explain this observation.

Chromatic pupillometry isolation and evaluation of intrinsically photosensitive retinal ganglion cell-driven pupillary light response in patients with retinitis pigmentosa

Purpose

The pupil light response (PLR) is driven by rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs). We aimed to isolate ipRGC-driven pupil responses using chromatic pupillometry and to determine the effect of advanced retinitis pigmentosa (RP) on ipRGC function.

Methods

A total of 100 eyes from 67 patients with advanced RP and 18 healthy controls (HCs) were included. Patients were divided into groups according to severity of visual impairment: no light perception (NLP, 9 eyes), light perception (LP, 19 eyes), faint form perception (FFP, 34 eyes), or form perception (FP, 38 eyes). Pupil responses to rod-weighted (487 nm, -1 log cd/m2, 1 s), cone-weighted (630 nm, 2 log cd/m2, 1 s), and ipRGC-weighted (487 nm, 2 log cd/m2, 1 s) stimuli were recorded. ipRGC function was evaluated by the postillumination pupil response (PIPR) and three metrics of pupil kinetics: maximal contraction velocity (MCV), contraction duration, and maximum dilation velocity (MDV).

Results

We found a slow, sustained PLR response to the ipRGC-weighted stimulus in most patients with NLP (8/9), but these patients had no detectable rod- or cone-driven PLR. The ipRGC-driven PLR had an MCV of 0.269 ± 0.150%/s and contraction duration of 2.562 ± 0.902 s, both of which were significantly lower than those of the rod and cone responses. The PIPRs of the RP groups did not decrease compared with those of the HCs group and were even enhanced in the LP group. At advanced stages, ipRGC responses gradually became the main component of the PLR.

Conclusion

Chromatic pupillometry successfully isolated an ipRGC-driven PLR in patients with advanced RP. This PLR remained stable and gradually became the main driver of pupil contraction in more advanced cases of RP. Here, we present baseline data on ipRGC function; we expect these findings to contribute to evaluating and screening candidates for novel therapies.

Functional integrity of visual coding following advanced photoreceptor degeneration

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

Every nano-step counts: a critical reflection on do's and don'ts in researching nanomedicines for retinal gene therapy

INTRODUCTION

Retinal disease affects millions of people worldwide, generating a massive social and economic burden. Current clinical trials for retinal diseases are dominated by gene augmentation therapies delivered with recombinant viruses as key players. As an alternative, nanoparticles hold great promise for the delivery of nucleic acid therapeutics as well. Nevertheless, despite numerous attempts, 'nano' is in practice not as successful as aspired and major breakthroughs in retinal gene therapy applying nanomaterials are yet to be seen.

AREAS COVERED

In this review, we summarize the advantages of nanomaterials and give an overview of nanoparticles designed for retinal nucleic acid delivery up to now. We furthermore critically reflect on the predominant issues that currently limit nano to progress to the clinic, where faulty study design and the absence of representative models play key roles.

EXPERT OPINION

Since the current approach of in vitro - in vivo experimentation is highly inefficient and creates misinformation, we advocate for a more prominent role for ex vivo testing early on in nanoparticle research. In addition, we elaborate on several concepts, including systematic studies and open science, which could aid in pushing the field of nanomedicine beyond the preclinical stage.

Gene-agnostic therapeutic approaches for inherited retinal degenerations

Inherited retinal diseases (IRDs) are associated with mutations in over 250 genes and represent a major cause of irreversible blindness worldwide. While gene augmentation or gene editing therapies could address the underlying genetic mutations in a small subset of patients, their utility remains limited by the great genetic heterogeneity of IRDs and the costs of developing individualised therapies. Gene-agnostic therapeutic approaches target common pathogenic pathways that drive retinal degeneration or provide functional rescue of vision independent of the genetic cause, thus offering potential clinical benefits to all IRD patients. Here, we review the key gene-agnostic approaches, including retinal cell reprogramming and replacement, neurotrophic support, immune modulation and optogenetics. The relative benefits and limitations of these strategies and the timing of clinical interventions are discussed.