A look into retinal organoids: methods, analytical techniques, and applications

Graphene-polymer nanofibers enable optically induced electrical responses in stem cell-derived electrically excitable cells and brain organoids

Human pluripotent stem cell (hPSC)-derived electrically excitable cells provide a unique window into development, but they remain electrically immature partially due to the lack of chronic stimulation. Here, we fabricated electrospun polymer nanofibers containing light-reactive reduced graphene oxide (rGO) as part of a new classes of on-demand, electrically active biomaterials to enhance cell function. Fiber size, stiffness, and electrical conductivity varied with rGO concentration, which impacted hPSC-derived cardiomyocyte and neuron responses; with acute light stimulation, cardiomyocytes exhibited increased, synchronous calcium handling. Long-term, daily nanofiber light stimulation improves brain organoid electrical activity and activates photoreceptor pathways. This work outlines a tunable method where electrical cell functions can be titrated with rGO fibers and light stimulation, and it suggests that repetitive light stimulation may provide a novel method for retinal differentiation.

Single-cell RNA sequencing in the study of human retinal organoids

Single-cell RNA sequencing (scRNA-seq) has transformed the study of retinal development and diseases by enabling a detailed analysis of cellular diversity within retinal organoids (ROs). ROs generated from pluripotent stem cells mimic the essential characteristics of the human retina and provide a valuable in vitro model for investigating retinal development, cell interactions, and disease mechanisms. This review summarizes the application of scRNA-seq on RO research, emphasizing its capacity to identify distinct cell populations, uncover developmental trajectories, and reveal the molecular signatures of retinal diseases. scRNA-seq provides new insights into retinal neurogenesis, cellular diversity, and the pathophysiology of retinal degenerative diseases. This technology has enabled the identification of novel biomarkers and potential therapeutic targets. Integrating scRNA-seq with other technologies, such as spatial transcriptomics and CRISPR-based screening, can further deepen our understanding of retinal biology and improve treatment strategies.

Retinal Organoids: Innovative Tools for Understanding Retinal Degeneration

Retinal degenerative diseases (RDDs) comprise diverse genetic and phenotypic conditions that cause progressive retinal dysfunction and cell loss, leading to vision impairment or blindness. Most RDDs lack appropriate animal models for their study, which affects understanding their disease mechanisms and delays the progress of new treatment development. Recent advances in stem cell engineering, omics, and organoid technology are facilitating research into diseases for which there are no previously existing models. The development of retinal organoids produced from human stem cells has impacted the study of retinal development as well as the development of in vitro models of diseases, opening possibilities for applications in regenerative medicine, drug discovery, and precision medicine. In this review, we recapitulate research in the retinal organoid models for RDD, mentioning some of the main pathways underlying retinal neurodegeneration that can be studied in these new models, as well as their limitations and future challenges in this rapidly advancing field.

Retina-on-chip: engineering functional in vitro models of the human retina using organ-on-chip technology

The retina is a complex and highly metabolic tissue in the back of the eye essential for human vision. Retinal diseases can lead to loss of vision in early and late stages of life, significantly affecting patients' quality of life. Due to its accessibility for surgical interventions and its isolated nature, the retina is an attractive target for novel genetic therapies and stem cell-based regenerative medicine. Understanding disease mechanisms and evaluating new treatments require relevant and robust experimental models. Retina-on-chip models are microfluidic organ-on-chip systems based on human tissue that capture multi-cellular interactions and tissue-level functions in vitro. Various retina-on-chip models have been described in literature. Some of them capture basic retinal barrier functions while others replicate key events underlying vision. In addition, some of these cellular systems have also been used in studies to explore their added value in retinal disease modeling. Most existing retina-on-chip models capture limited aspects of the phenotypic complexity of human diseases. This limitation arises primarily from the challenges related to controlled recapitulation of retinal function, including the relevant multi-cellular interactions and functional read-outs. In this review, we provide an update on recent advancements in the field of retina-on-chip, and we discuss the biotechnical strategies to further enhance the physiological relevance of the models. We emphasize that developers and researchers should prioritize the incorporation of the full spectrum of retinal complexity to effectuate a direct impact of retina-on-chip models in disease modeling and development of therapeutic strategies.

Small molecule treatment alleviates photoreceptor cilia defects in LCA5-deficient human retinal organoids

Bialleleic pathogenic variants in LCA5 cause one of the most severe forms of Leber congenital amaurosis, an early-onset retinal disease that results in severe visual impairment. Here, we report the use of gene editing to generate isogenic LCA5 knock-out (LCA5 KO) induced pluripotent stem cells (iPSC) and their differentiation to retinal organoids. The molecular and cellular phenotype of the LCA5 KO retinal organoids was studied in detail and compared to isogenic controls as well as patient-derived retinal organoids. The absence of LCA5 was confirmed in retinal organoids by immunohistochemistry and western blotting. There were no major changes in retinal organoid differentiation or ciliation, however, the localisation of CEP290 and IFT88 was significantly altered in LCA5 KO and patient photoreceptor cilia with extension along the axoneme. The LCA5-deficient organoids also had shorter outer segments and rhodopsin was mislocalised to the outer nuclear layer. We also identified transcriptomic and proteomic changes associated with the loss of LCA5. Importantly, treatment with the small molecules eupatilin, fasudil or a combination of both drugs reduced CEP290 and IFT88 accumulation along the cilia. The treatments also improved rhodopsin traffic to the outer segment and reduced mislocalisation of rhodopsin in the outer nuclear layer (ONL). The improvements in cilia-associated protein localisation and traffic were accompanied by significant changes in the transcriptome towards control gene expression levels in many of the differentially expressed genes. In summary, iPSC-derived retinal organoids are a powerful model for investigating the molecular and cellular changes associated with loss of LCA5 function and highlight the therapeutic potential of small molecules to treat retinal ciliopathies.

Generation of a Double Reporter mES Cell Line to Simultaneously Trace the Generation of Retinal Progenitors and Photoreceptors

Three-dimensional retinal culture systems help to understand eye development and the pathology of disorders. There is a need for reporter stem cell lines to allow in vitro studies on retinal progenitors and photoreceptors and their developmental dynamics or properties and to test therapeutic approaches. The isolation of pure progenitor populations or photoreceptor precursors may serve for drug, gene, and cell therapy development. Here, we generated a dual-reporter mouse embryonic stem cell line Crx-GFP;Rax-mCherry enabling the visualization or isolation of photoreceptors and retinal progenitors from retinal organoid settings. From day 4 organoids, we isolated mCherry-positive cells to assess their early retinal progenitor identity with proliferation tests as well as transcriptomic and proteomic profiling. The timing of eye field transcription factor expression at the transcriptomic and protein levels is in accordance with mouse retinogenesis. This new line will be helpful for rapidly investigating biological questions or testing therapeutics before using human induced pluripotent stem cells (iPSCs), which require a much longer time for retinal organoid formation.

Screening of a retinal-targeting Adeno-Associated Virus (AAV) via DNA shuffling

Due to its unique physiological structure and functions, the eye has received considerable attention in the field of adeno-associated virus (AAV) gene therapy. Inherited retinal degenerative diseases, which arise from pathogenic mutations in mRNA transcripts expressed in the eye's photoreceptor cells or retinal pigment epithelium (RPE), are the most common cause of vision loss. However, current retinal gene therapy mostly involves subretinal injection of therapeutic genes, which treats a limited area, entails retinal detachment, and requires sophisticated techniques. Intravitreal (IVT) injection provides an alternative method with less invasion and more convenience for retinal gene therapy. In the present study, we performed a directed evolution via DNA shuffling in RHO-GFP mice and identified a novel recombinant AAV vector (AAV-M04) suitable for IVT injection in the gene delivery of retinal tissue. Compared with AAV2, AAV9, and AAV2.7m8, AAV-M04 vector exhibited higher transduction efficiency in retinal ganglion cell line-5 (RGC-5) cells as well as in human embryonic stem cell derived retinal organoids. Importantly, when delivered via IVT injection in mice, the AAV-M04 vector also showed better delivery efficiency of transgene as indicated by the red fluorescence protein mScarlet. The red fluorescence was distributed in a wider retinal area of AAV-M04 injected mice, suggesting the potent retinal targeting of AAV-M04 vector. In addition, AAV-M04 qualities including the packaging efficiency, the thermal stability, and the capsid integrity were superior to controls, which were important in drug manufacture. In summary, we screened a novel AAV-M04 vector with great retinal-targeting via IVT injection, which provides the potential of AAV-M04 for effective gene therapy of retinal diseases.

Exonic splice variant discovery using in vitro models of inherited retinal disease

Correct identification of the molecular consequences of pathogenic genetic variants is essential to the development of allele-specific therapies. However, such molecular effects may remain ambiguous following genetic sequence analysis alone. Here, we identify exonic codon-altering variants that are also predicted to disrupt normal RNA splicing in the context of inherited retinal disease. NR2E3 c.932G>A (p.Arg311Gln) is a variant commonly associated with enhanced S cone syndrome. Previous studies using mutagenized cDNA constructs have shown that the arginine to glutamine substitution at position 311 of NR2E3 does not meaningfully diminish function of the rod-specific transcription factor. Using retinal organoids, we explored the molecular consequences of NR2E3 c.932G>A when expressed endogenously during human rod photoreceptor cell development. Retinal organoids carrying the NR2E3 c.932G>A allele expressed a transcript containing a 186-nucleotide deletion of exon 6 within the ligand binding domain. This short transcript was not detected in control organoids or control human donor retina samples. A minigene containing exons 5 and 6 of NR2E3 showed sufficiency of the c.932G>A variant to cause the observed splicing defect. These results support the hypothesis that the pathogenic NR2E3 c.932G>A variant leads to photoreceptor disease by causing a splice defect and not through an amino acid substitution as previously supposed. They also explain the relatively mild effect of Arg311Gln on NR2E3 function in vitro. We also used in silico prediction tools to show that similar changes are likely to affect other inherited retinal disease variants in genes such as CEP290, ABCA4, and BEST1.

PEGDA-based HistoBrick for increasing throughput of cryosectioning and immunohistochemistry in organoid and small tissue studies

Histology is the gold standard for analyzing tissue structure and cell morphology. Immunostaining on thin tissue sections enables precise visualization of antigens and proteins. However, for cryosectioning small tissues such as organoids, spheroids, and tumoroids there is a lack of standardized, time- and cost-effective methods, limiting the throughput of analysis. Here, we have adapted to cryosectioning our previously developed HistoBrick approach, in which small tissue arrangement is spatially controlled within arrayed mini-wells. By testing various embedding matrices, we show that an 8% PEGDA and 2.5% gelatine mixture is optimal, providing essential structural support to maintain sample integrity during cryosectioning. This embedding matrix preserves fragile substructures of human retinal organoids, which are particularly susceptible to damage during sample preparation. Using PEGDA-gelatine HistoBricks for the simultaneous embedding of 16 retinal organoids, we analyzed a time course of retinal organoid development. We observed the maintenance of photoreceptors cell bodies up to week 98 in culture, while photoreceptor outer segments were gradually lost. Further, we observed displaced photoreceptors in the region of outer segments. The PEGDA-gelatine HistoBrick is a cost-efficient tool that can be implemented for small tissue studies to increase throughput in experiments such as large-scale screenings or toxicology research.

Retinal Organoids from Induced Pluripotent Stem Cells of Patients with Inherited Retinal Diseases: A Systematic Review

BACKGROUND

Currently, most inherited retinal diseases lack curative interventions, and available treatment modalities are constrained to symptomatic approaches. Retinal organoid technology has emerged as a method for treating inherited retinal diseases, with growing academic interest in recent years. The purpose of this review was to systematically organize the current protocols for generating retinal organoids using induced pluripotent stem cells from patients with inherited retinal disease and to investigate the application of retinal organoids in inherited retinal disease research.

METHODS

Data were collected from the PubMed, Scopus, and Web of Science databases using a keyword search. The main search term used was "retinal organoid," accompanied by secondary keywords such as "optic cup," "three-dimensional," and "self-organizing." The final search was conducted on October 2, 2024.

RESULTS

Of the 2,129 studies retrieved, 130 were included in the qualitative synthesis. The protocols for the generation of retinal organoids in inherited retinal disease research use five major approaches, categorized into 3D and a combination of 2D/3D approaches, implemented with modifications. Disease phenotypes have been successfully reproduced via the generation of retinal organoids from the induced pluripotent stem cells of individuals with inherited retinal diseases, facilitating the progression of research into novel therapeutic developments. Cells have been obtained from retinal organoids for cell therapy, and progress toward their potential integration into clinical practice is underway. Considering their potential applications, retinal organoid technology has shown promise across various domains.

CONCLUSION

In this systematic review, we organized protocols for generating retinal organoids using induced pluripotent stem cells from patients with inherited retinal diseases. Retinal organoid technology has various applications including disease modeling, screening for novel therapies, and cell replacement therapy. Further advancements would make this technology a clinically significant tool for patients with inherited retinal diseases.

Inherited Retinal Degenerations and Non-Neovascular Age-Related Macular Degeneration: Progress and Unmet Needs

Inherited retinal degeneration (IRD) disease and age-related macular degeneration (AMD) are leading causes of irreversible vision loss and blindness. Although significant progress has advanced the field in the past 5 years, significant challenges remain. The current article reviews the accomplishments and research advances that have fueled the development of treatments for patients with IRD and AMD, including the first approved gene-augmentation treatment for RPE65-related retinal degeneration and complement inhibition therapies to slow progression of geographic atrophy (GA) in AMD. The article outlines opportunities to address gaps and unmet needs that should lead to additional progress toward the development of treatments for patients with IRDs and non-neovascular AMD in the future.

Extrinsic electric field modulates neuronal development and increases photoreceptor population in retinal organoids

Introduction

Considering the significant role played by both intrinsic and extrinsic electric fields in the growth and maturation of the central nervous system, the impact of short exposure to external electric fields on the development and differentiation of retinal organoids was investigated.

Methods

Retinal organoids derived from human embryonic stem cells were used at day 80, a key stage in their differentiation. A single 60-minute exposure to a biphasic electrical field was administered to assess its influence on retinal cell populations and maturation markers. Immunohistochemistry, qPCR, and RNA sequencing were employed to evaluate cell type development and gene expression changes.

Results

Electrical stimulation significantly enhanced neuronal development and increased the population of photoreceptors within the organoids. RNA sequencing data showed upregulated expression of genes related to rod photoreceptors, Müller cells, horizontal cells, and amacrine cells, while genes associated with retinal pigment epithelium and retinal ganglion cells were downregulated. Variations in development and maturation were observed depending on the specific parameters of the applied electric field.

Discussion

These findings highlight the significant impact of extrinsic electrical fields on early retinal development and suggest that optimizing electrical field parameters could effectively address certain limitations in retinal organoid technology, potentially reducing the reliance on chemicals and small molecules.

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.

The mechanics of the retina: Müller glia role on retinal extracellular matrix and modelling

The retina is a highly heterogeneous tissue, both cell-wise but also regarding its extracellular matrix (ECM). The stiffness of the ECM is pivotal in retinal development and maturation and has also been associated with the onset and/or progression of numerous retinal pathologies, such as glaucoma, proliferative vitreoretinopathy (PVR), age-related macular degeneration (AMD), epiretinal membrane (ERM) formation or uveitis. Nonetheless, much remains unknown about the biomechanical milieu of the retina, and specifically the role that Müller glia play as principal mechanosensors and major producers of ECM constituents. So far, new approaches need to be developed to further the knowledge in the field of retinal mechanobiology for ECM-target applications to arise. In this review, we focus on the involvement of Müller glia in shaping and altering the retinal ECM under both physiological and pathological conditions and look into various biomaterial options to more accurately replicate the impact of matrix stiffness in vitro.

Rapid and efficient generation of mature retinal organoids derived from human pluripotent stem cells via optimized pharmacological modulation of Sonic hedgehog, activin A, and retinoic acid signal transduction

Human retinal organoids have become indispensable tools for retinal disease modeling and drug screening. Despite its versatile applications, the long timeframe for their differentiation and maturation limits the throughput of such research. Here, we successfully shortened this timeframe by accelerating human retinal organoid development using unique pharmacological approaches. Our method comprised three key steps: 1) a modified self-formed ectodermal autonomous multizone (SEAM) method, including dual SMAD inhibition and bone morphogenetic protein 4 treatment, for initial neural retinal induction; 2) the concurrent use of a Sonic hedgehog agonist SAG, activin A, and all-trans retinoic acid for rapid retinal cell specification; and 3) switching to SAG treatment alone for robust retinal maturation and lamination. The generated retinal organoids preserved typical morphological features of mature retinal organoids, including hair-like surface structures and well-organized outer layers. These features were substantiated by the spatial immunostaining patterns of several retinal cell markers, including rhodopsin and L/M opsin expression in the outermost layer, which was accompanied by reduced ectopic cone photoreceptor generation. Importantly, our method required only 90 days for retinal organoid maturation, which is approximately two-thirds the time necessary for other conventional methods. These results indicate that thoroughly optimized pharmacological interventions play a pivotal role in rapid and precise photoreceptor development during human retinal organoid differentiation and maturation. Thus, our present method may expedite human retinal organoid research, eventually contributing to the development of better treatment options for various degenerative retinal diseases.

Electrophysiological Analysis of Retinal Organoid Development Using 3D Microelectrodes of Liquid Metals

Despite of the substantial potential of human-derived retinal organoids, the degeneration of retinal ganglion cells (RGCs) during maturation limits their utility in assessing the functionality of later-born retinal cell subtypes. Additionally, conventional analyses primarily rely on fluorescent emissions, which limits the detection of actual cell functionality while risking damage to the 3D cytoarchitecture of organoids. Here, an electrophysiological analysis is presented to monitor RGC development in early to mid-stage retinal organoids, and compare distinct features with fully-mature mouse retina. This approach utilizes high-resolution 3D printing of liquid-metal microelectrodes, enabling precise targeting of specific inner retinal layers within organoids. The adaptable distribution and softness of these microelectrodes facilitate the spatiotemporal recording of inner retinal signals. This study not only demonstrates the functional properties of RGCs in retinal organoid development but also provides insights into their synaptic connectivity, reminiscent of fetal native retinas. Further comparison with fully-mature mouse retina in vivo verifies the organoid features, highlighting the potential of early-stage retinal organoids in biomedical research.

Non-canonical Wnt pathway expression in the developing mouse and human retina

The non-canonical Wnt pathway is an evolutionarily conserved pathway essential for tissue patterning and development across species and tissues. In mammals, this pathway plays a role in neuronal migration, dendritogenesis, axon growth, and synapse formation. However, its role in development and synaptogenesis of the human retina remains less established. In order to address this knowledge gap, we analyzed publicly available single-cell RNA sequencing (scRNAseq) datasets for mouse retina, human retina, and human retinal organoids over multiple developmental time points during outer retinal maturation. We identified ligands, receptors, and mediator genes with a putative role in retinal development, including those with novel or species-specific expression, and validated this expression using fluorescence in situ hybridization (FISH). By quantifying outer nuclear layer (ONL) versus inner nuclear layer (INL) expression, we provide evidence for the differential expression of certain non-canonical Wnt signaling components in the developing mouse and human retina during outer plexiform layer (OPL) development. Importantly, we identified distinct expression patterns of mouse and human FZD3 and WNT10A, as well as previously undescribed expression, such as for mouse Wnt2b in Chat+ starburst amacrine cells. Human retinal organoids largely recapitulated the human non-canonical Wnt pathway expression. Together, this work provides the basis for further study of non-canonical Wnt signaling in mouse and human retinal development and synaptogenesis.

Current trends and research topics regarding organoids: A bibliometric analysis of global research from 2000 to 2023

The use of animal models for biological experiments is no longer sufficient for research related to human life and disease. The development of organ tissues has replaced animal models by mimicking the structure, function, development and homeostasis of natural organs. This provides more opportunities to study human diseases such as cancer, infectious diseases and genetic disorders. In this study, bibliometric methods were used to analyze organoid-related articles published over the last 20+ years to identify emerging trends and frontiers in organoid research. A total of 13,143 articles from 4125 institutions in 86 countries or regions were included in the analysis. The number of papers increased steadily over the 20-year period. The United States was the leading country in terms of number of papers and citations. Harvard Medical School had the highest number of papers published. Keyword analysis revealed research trends and focus areas such as organ tissues, stem cells, 3D culture and tissue engineering. In conclusion, this study used bibliometric and visualization methods to explore the field of organoid research and found that organ tissues are receiving increasing attention in areas such as cancer, drug discovery, personalized medicine, genetic disease modelling and gene repair, making them a current research hotspot and a future research trend.

Creation of an Isogenic Human iPSC-Based RGC Model of Dominant Optic Atrophy Harboring the Pathogenic Variant c.1861C>T (p.Gln621Ter) in the OPA1 Gene

Autosomal dominant optic atrophy (ADOA) is a rare progressive disease mainly caused by mutations in OPA1, a nuclear gene encoding for a mitochondrial protein that plays an essential role in mitochondrial dynamics, cell survival, oxidative phosphorylation, and mtDNA maintenance. ADOA is characterized by the degeneration of retinal ganglion cells (RGCs). This causes visual loss, which can lead to legal blindness in many cases. Nowadays, there is no effective treatment for ADOA. In this article, we have established an isogenic human RGC model for ADOA using iPSC technology and the genome editing tool CRISPR/Cas9 from a previously generated iPSC line of an ADOA plus patient harboring the pathogenic variant NM_015560.3: c.1861C>T (p.Gln621Ter) in heterozygosis in OPA1. To this end, a protocol based on supplementing the iPSC culture media with several small molecules and defined factors trying to mimic embryonic development has been employed. Subsequently, the created model was validated, confirming the presence of a defect of intergenomic communication, impaired mitochondrial respiration, and an increase in apoptosis and ROS generation. Finally, we propose the analysis of OPA1 expression by qPCR as an easy read-out method to carry out future drug screening studies using the created RGC model. In summary, this model provides a useful platform for further investigation of the underlying pathophysiological mechanisms of ADOA plus and for testing compounds with potential pharmacological action.

Disruption of common ocular developmental pathways in patient-derived optic vesicle models of microphthalmia

Genetic perturbations influencing early eye development can result in microphthalmia, anophthalmia, and coloboma (MAC). Over 100 genes are associated with MAC, but little is known about common disease mechanisms. In this study, we generated induced pluripotent stem cell (iPSC)-derived optic vesicles (OVs) from two unrelated microphthalmia patients and healthy controls. At day 20, 35, and 50, microphthalmia patient OV diameters were significantly smaller, recapitulating the "small eye" phenotype. RNA sequencing (RNA-seq) analysis revealed upregulation of apoptosis-initiating and extracellular matrix (ECM) genes at day 20 and 35. Western blot and immunohistochemistry revealed increased expression of lumican, nidogen, and collagen type IV, suggesting ECM overproduction. Increased apoptosis was observed in microphthalmia OVs with reduced phospho-histone 3 (pH3+) cells confirming decreased cell proliferation at day 35. Pharmacological inhibition of caspase-8 activity with Z-IETD-FMK decreased apoptosis in one patient model, highlighting a potential therapeutic approach. These data reveal shared pathophysiological mechanisms contributing to a microphthalmia phenotype.

Model-free robust motion control for biological optical microscopy using time-delay estimation with an adaptive RBFNN compensator

The field of large numerical aperture microscopy has witnessed significant advancements in spatial and temporal resolution, as well as improvements in optical microscope imaging quality. However, these advancements have concurrently raised the demand for enhanced precision, extended range, and increased load-bearing capacity in objective motion carrier (OMC). To address this challenge, this study introduces an innovative OMC that employs a ball screw mechanism as its primary driving component. Furthermore, a robust nonlinear motion control strategy has been developed, which integrates fast nonsingular terminal sliding mode, experimental estimation techniques, and adaptive radial basis neural network, to mitigate the impact of nonlinear friction within the ball screw mechanism on motion precision. The stability of the closed-loop control system has been rigorously demonstrated through Lyapunov theory. Compared with other enhanced sliding mode control strategies, the maximum error and root mean square error of this controller are improved by 33% and 34% respectively. The implementation of the novel OMC has enabled the establishment of a high-resolution bio-optical microscope, which has proven its effectiveness in the microscopic imaging of retinal organoids.

The application of retinal organoids in ophthalmic regenerative medicine: A mini-review

Retinal organoids are three-dimensional (3D) microscopic tissues that are induced and differentiated from stem cells or progenitor cells in vitro and have a highly similar structure to the retina. With the optimization and development of 3D retinal culture system and the improvement of induced differentiation technology, retinal organoids have broad application prospects in retinal development, regenerative medicine, biomaterial evaluation, disease mechanism investigation, and drug screening. In this review we summarize recent development of retinal organoids and their applications in ophthalmic regenerative medicine. In particular, we highlight the promise and challenges in the use of retinal organoids in disease modeling and drug discovery.

Hyaluronan improves photoreceptor differentiation and maturation in human retinal organoids

Human stem cell-derived organoids enable both disease modeling and serve as a source of cells for transplantation. Human retinal organoids are particularly important as a source of human photoreceptors; however, the long differentiation period required and lack of vascularization in the organoid often results in a necrotic core and death of inner retinal cells before photoreceptors are fully mature. Manipulating the in vitro environment of differentiating retinal organoids through the incorporation of extracellular matrix components could influence retinal development. We investigated the addition of hyaluronan (HA), a component of the interphotoreceptor matrix, as an additive to promote long-term organoid survival and enhance retinal maturation. HA treatment had a significant reduction in the proportion of proliferating (Ki67+) cells and increase in the proportion of photoreceptors (CRX+), suggesting that HA accelerated photoreceptor commitment in vitro. HA significantly upregulated genes specific to photoreceptor maturation and outer segment development. Interestingly, prolonged HA-treatment significantly decreased the length of the brush border layer compared to those in control retinal organoids, where the photoreceptor outer segments reside; however, HA-treated organoids also had more mature outer segments with organized discs structures, as revealed by transmission electron microscopy. The brush border layer length was inversely proportional to the molar mass and viscosity of the hyaluronan added. This is the first study to investigate the role of exogenous HA, viscosity, and polymer molar mass on photoreceptor maturation, emphasizing the importance of material properties on organoid culture. STATEMENT OF SIGNIFICANCE: Retinal organoids are a powerful tool to study retinal development in vitro, though like many other organoid systems, can be highly variable. In this work, Shoichet and colleagues investigated the use of hyaluronan (HA), a native component of the interphotoreceptor matrix, to improve photoreceptor maturation in developing human retinal organoids. HA promoted human photoreceptor differentiation leading to mature outer segments with disc formation and more uniform and healthy retinal organoids. These findings highlight the importance of adding components native to the developing retina to generate more physiologically relevant photoreceptors for cell therapy and in vitro models to drive drug discovery and uncover novel disease mechanisms.

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.

Inherited Retinal Diseases and Retinal Organoids as Preclinical Cell Models for Inherited Retinal Disease Research

Inherited retinal diseases (IRDs) are a large group of genetically and clinically diverse blinding eye conditions that result in progressive and irreversible photoreceptor degeneration and vision loss. To date, no cures have been found, although strides toward treatments for specific IRDs have been made in recent years. To accelerate treatment discovery, retinal organoids provide an ideal human IRD model. This review aims to give background on the development and importance of retinal organoids for the human-based in vitro study of the retina and human retinogenesis and retinal pathologies. From there, we explore retinal pathologies in the context of IRDs and the current landscape of IRD treatment discovery. We discuss the usefulness of retinal organoids in this context (as a patient-derived cell model for IRDs) to precisely understand the pathogenesis and potential mechanisms behind a specific IRD-causing variant of interest. Finally, we discuss the importance and promise of retinal organoids in treatment discovery for IRDs, now and in the future.

Deep learning based characterization of human organoids using optical coherence tomography

Organoids, derived from human induced pluripotent stem cells (hiPSCs), are intricate three-dimensional in vitro structures that mimic many key aspects of the complex morphology and functions of in vivo organs such as the retina and heart. Traditional histological methods, while crucial, often fall short in analyzing these dynamic structures due to their inherently static and destructive nature. In this study, we leveraged the capabilities of optical coherence tomography (OCT) for rapid, non-invasive imaging of both retinal, cerebral, and cardiac organoids. Complementing this, we developed a sophisticated deep learning approach to automatically segment the organoid tissues and their internal structures, such as hollows and chambers. Utilizing this advanced imaging and analysis platform, we quantitatively assessed critical parameters, including size, area, volume, and cardiac beating, offering a comprehensive live characterization and classification of the organoids. These findings provide profound insights into the differentiation and developmental processes of organoids, positioning quantitative OCT imaging as a potentially transformative tool for future organoid research.

Understanding and Rescuing the Splicing Defect Caused by the Frequent ABCA4 Variant c.4253+43G>A Underlying Stargardt Disease

Pathogenic variants in ABCA4 are the underlying molecular cause of Stargardt disease (STGD1), an autosomal recessive macular dystrophy characterized by a progressive loss of central vision. Among intronic ABCA4 variants, c.4253+43G>A is frequently detected in STGD1 cases and is classified as a hypomorphic allele, generally associated with late-onset cases. This variant was previously reported to alter splicing regulatory sequences, but the splicing outcome is not fully understood yet. In this study, we attempted to better understand its effect on splicing and to rescue the aberrant splicing via antisense oligonucleotides (AONs). Wild-type and c.4253+43G>A variant-harboring maxigene vectors revealed additional skipping events, which were not previously detected upon transfection in HEK293T cells. To restore exon inclusion, we designed a set of 27 AONs targeting either splicing silencer motifs or the variant region and screened these in maxigene-transfected HEK293T cells. Candidate AONs able to promote exon inclusion were selected for further testing in patient-derived photoreceptor precursor cells. Surprisingly, no robust splicing modulation was observed in this model system. Overall, this research helped to adequately characterize the splicing alteration caused by the c.4253+43G>A variant, although future development of AON-mediated exon inclusion therapy for ABCA4 is needed.

Proof-of-concept for multiple AON delivery by a single U7snRNA vector to restore splicing defects in ABCA4

The high allelic heterogeneity in Stargardt disease (STGD1) complicates the design of intervention strategies. A significant proportion of pathogenic intronic ABCA4 variants alters the pre-mRNA splicing process. Antisense oligonucleotides (AONs) are an attractive yet mutation-specific therapeutic strategy to restore these splicing defects. In this study, we experimentally assessed the potential of a splicing modulation therapy to target multiple intronic ABCA4 variants. AONs were inserted into U7snRNA gene cassettes and tested in midigene-based splice assays. Five potent antisense sequences were selected to generate a multiple U7snRNA cassette construct, and this combination vector showed substantial rescue of all of the splicing defects. Therefore, the combination cassette was used for viral synthesis and assessment in patient-derived photoreceptor precursor cells (PPCs). Simultaneous delivery of several modified U7snRNAs through a single AAV, however, did not show substantial splicing correction, probably due to suboptimal transduction efficiency in PPCs and/or a heterogeneous viral population containing incomplete AAV genomes. Overall, these data demonstrate the potential of the U7snRNA system to rescue multiple splicing defects, but also suggest that AAV-associated challenges are still a limiting step, underscoring the need for further optimization before implementing this strategy as a potential treatment for STGD1.

Toward Retinal Organoids in High-Throughput

Human retinal organoids recapitulate the cellular diversity, arrangement, gene expression, and functional aspects of the human retina. Protocols to generate human retinal organoids from pluripotent stem cells are typically labor intensive, include many manual handling steps, and the organoids need to be maintained for several months until they mature. To generate large numbers of human retinal organoids for therapy development and screening purposes, scaling up retinal organoid production, maintenance, and analysis is of utmost importance. In this review, we discuss strategies to increase the number of high-quality retinal organoids while reducing manual handling steps. We further review different approaches to analyze thousands of retinal organoids with currently available technologies and point to challenges that still await to be overcome both in culture and analysis of retinal organoids.

Retinal Ciliopathies and Potential Gene Therapies: A Focus on Human iPSC-Derived Organoid Models

The human photoreceptor function is dependent on a highly specialised cilium. Perturbation of cilial function can often lead to death of the photoreceptor and loss of vision. Retinal ciliopathies are a genetically diverse range of inherited retinal disorders affecting aspects of the photoreceptor cilium. Despite advances in the understanding of retinal ciliopathies utilising animal disease models, they can often lack the ability to accurately mimic the observed patient phenotype, possibly due to structural and functional deviations from the human retina. Human-induced pluripotent stem cells (hiPSCs) can be utilised to generate an alternative disease model, the 3D retinal organoid, which contains all major retinal cell types including photoreceptors complete with cilial structures. These retinal organoids facilitate the study of disease mechanisms and potential therapies in a human-derived system. Three-dimensional retinal organoids are still a developing technology, and despite impressive progress, several limitations remain. This review will discuss the state of hiPSC-derived retinal organoid technology for accurately modelling prominent retinal ciliopathies related to genes, including RPGR, CEP290, MYO7A, and USH2A. Additionally, we will discuss the development of novel gene therapy approaches targeting retinal ciliopathies, including the delivery of large genes and gene-editing techniques.

The Analysis of Embryoid Body Formation and Its Role in Retinal Organoid Development

Within the last decade, a wide variety of protocols have emerged for the generation of retinal organoids. A subset of studies have compared protocols based on stem cell source, the physical features of the microenvironment, and both internal and external signals, all features that influence embryoid body and retinal organoid formation. Most of these comparisons have focused on the effect of signaling pathways on retinal organoid development. In this study, our aim is to understand whether starting cell conditions, specifically those involved in embryoid body formation, affect the development of retinal organoids in terms of differentiation capacity and reproducibility. To investigate this, we used the popular 3D floating culture method to generate retinal organoids from stem cells. This method starts with either small clumps of stem cells generated from larger clones (clumps protocol, CP) or with an aggregation of single cells (single cells protocol, SCP). Using histological analysis and gene-expression comparison, we found a retention of the pluripotency capacity on embryoid bodies generated through the SCP compared to the CP. Nonetheless, these early developmental differences seem not to impact the final retinal organoid formation, suggesting a potential compensatory mechanism during the neurosphere stage. This study not only facilitates an in-depth exploration of embryoid body development but also provides valuable insights for the selection of the most suitable protocol in order to study retinal development and to model inherited retinal disorders in vitro.

Multimodal spatiotemporal phenotyping of human retinal organoid development

Organoids generated from human pluripotent stem cells provide experimental systems to study development and disease, but quantitative measurements across different spatial scales and molecular modalities are lacking. In this study, we generated multiplexed protein maps over a retinal organoid time course and primary adult human retinal tissue. We developed a toolkit to visualize progenitor and neuron location, the spatial arrangements of extracellular and subcellular components and global patterning in each organoid and primary tissue. In addition, we generated a single-cell transcriptome and chromatin accessibility timecourse dataset and inferred a gene regulatory network underlying organoid development. We integrated genomic data with spatially segmented nuclei into a multimodal atlas to explore organoid patterning and retinal ganglion cell (RGC) spatial neighborhoods, highlighting pathways involved in RGC cell death and showing that mosaic genetic perturbations in retinal organoids provide insight into cell fate regulation.

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.

Retinal organoids in disease modeling and drug discovery: Opportunities and challenges

Diseases leading to retinal cell loss can cause severe visual impairment and blindness. The lack of effective therapies to address retinal cell loss and the absence of intrinsic regeneration in the human retina leads to an irreversible pathological condition. Progress in recent years in the generation of human three-dimensional retinal organoids from pluripotent stem cells makes it possible to recreate the cytoarchitecture and associated cell-cell interactions of the human retina in remarkable detail. These human three-dimensional retinal organoid systems made of distinct retinal cell types and possessing contextual physiological responses allow the study of human retina development and retinal disease pathology in a way animal model and two-dimensional cell cultures were unable to achieve. We describe the derivation of retinal organoids from human pluripotent stem cells and their application for modeling retinal disease pathologies, while outlining the opportunities and challenges for its application in academia and industry.

Retinal organoid and gene editing for basic and translational research

The rapid evolution of two technologies has greatly transformed the basic, translational, and clinical research in the mammalian retina. One is the retinal organoid (RO) technology. Various induction methods have been created or adapted to generate species-specific, disease-specific, and experimental-targeted retinal organoids (ROs). The process of generating ROs can highly mimic the in vivo retinal development, and consequently, the ROs resemble the retina in many aspects including the molecular and cellular profiles. The other technology is the gene editing, represented by the classical CRISPR-Cas9 editing and its derivatives such as prime editing, homology independent targeted integration (HITI), base editing and others. The combination of ROs and gene editing has opened up countless possibilities in the study of retinal development, pathogenesis, and therapeutics. We review recent advances in the ROs, gene editing methodologies, delivery vectors, and related topics that are particularly relevant to retinal studies.

Cell Reprogramming Techniques: Contributions to Cancer Therapy

The reprogramming of terminally differentiated cells over the past few years has become important for induced pluripotent stem cells (iPSCs) in the field of regenerative medicine and disease drug modeling. At the same time, iPSCs have also played an important role in human cancer research. iPSCs derived from cancer patients can be used to simulate the early progression of cancer, for drug testing, and to study the molecular mechanism of cancer occurrence. In recent years, with the application of cellular immunotherapy in cancer therapy, patient-derived iPSC-induced immune cells (T, natural killer, and macrophage cells) solve the problem of immune rejection and have higher immunogenicity, which greatly improves the therapeutic efficiency of immune cell therapy. With the continuous progress of cancer differentiation therapy, iPSC technology can reprogram cancer cells to a more primitive pluripotent undifferentiated state, and successfully reverse cancer cells to a benign phenotype by changing the epigenetic inheritance of cancer cells. This article reviews the recent progress of cell reprogramming technology in human cancer research, focuses on the application of reprogramming technology in cancer immunotherapy and the problems solved, and summarizes the malignant phenotype changes of cancer cells in the process of reprogramming and subsequent differentiation.

Experimental Model Systems Used in the Preclinical Development of Nucleic Acid Therapeutics

Preclinical evaluation of nucleic acid therapeutics (NATs) in relevant experimental model systems is essential for NAT drug development. As part of COST Action "DARTER" (Delivery of Antisense RNA ThERapeutics), a network of researchers in the field of RNA therapeutics, we have conducted a survey on the experimental model systems routinely used by our members in preclinical NAT development. The questionnaire focused on both cellular and animal models. Our survey results suggest that skin fibroblast cultures derived from patients is the most commonly used cellular model, while induced pluripotent stem cell-derived models are also highly reported, highlighting the increasing potential of this technology. Splice-switching antisense oligonucleotide is the most frequently investigated RNA molecule, followed by small interfering RNA. Animal models are less prevalent but also widely used among groups in the network, with transgenic mouse models ranking the top. Concerning the research fields represented in our survey, the mostly studied disease area is neuromuscular disorders, followed by neurometabolic diseases and cancers. Brain, skeletal muscle, heart, and liver are the top four tissues of interest reported. We expect that this snapshot of the current preclinical models will facilitate decision making and the share of resources between academics and industry worldwide to facilitate the development of NATs.

Retinal organoids from human-induced pluripotent stem cells: From studying retinal dystrophies to early diagnosis of Alzheimer's and Parkinson's disease

Human-induced pluripotent stem cells (hiPSCs) have provided new methods to study neurodegenerative diseases. In addition to their wide application in neuronal disorders, hiPSCs technology can also encompass specific conditions, such as inherited retinal dystrophies. The possibility of evaluating alterations related to retinal disorders in 3D organoids increases the truthfulness of in vitro models. Moreover, both Alzheimer's (AD) and Parkinson's disease (PD) have been described as causing early retinal alterations, generating beta-amyloid protein accumulation, or affecting dopaminergic amacrine cells. This review addresses recent advances and future perspectives obtained from in vitro modeling of retinal diseases, focusing on retinitis pigmentosa (RP). Additionally, we depicted the possibility of evaluating changes related to AD and PD in retinal organoids obtained from potential patients long before the onset of the disease, constituting a valuable tool in early diagnosis. With this, we pointed out prospects in the study of retinal dystrophies and early diagnosis of AD and PD.

Bankable human iPSC-derived retinal progenitors represent a valuable source of multipotent cells

Retinal progenitor cells (RPCs) are the source of all retinal cell types during retinogenesis. Until now, the isolation and expansion of RPCs has been at the expense of their multipotency. Here, we report simple methods and media for the generation, expansion, and cryopreservation of human induced pluripotent stem-cell derived-RPCs (hiRPCs). Thawed and passed hiRPCs maintained biochemical and transcriptional RPC phenotypes and their ability to differentiate into all retinal cell types. Specific conditions allowed the generation of large cultures of photoreceptor precursors enriched up to 90% within a few weeks and without a purification step. Combined RNA-seq analysis between hiRPCs and retinal organoids identified genes involved in developmental or degenerative retinal diseases. Thus, hiRPC lines could provide a valuable source of retinal cells for cell-based therapies or drug discovery and could be an advanced cellular tool to better understand retinal dystrophies.

CRISPR-Cas9 correction of a nonsense mutation in LCA5 rescues lebercilin expression and localization in human retinal organoids

Mutations in the lebercilin-encoding gene LCA5 cause one of the most severe forms of Leber congenital amaurosis, an early-onset retinal disease that results in severe visual impairment. Here, we report on the generation of a patient-specific cellular model to study LCA5-associated retinal disease. CRISPR-Cas9 technology was used to correct a homozygous nonsense variant in LCA5 (c.835C>T; p.Q279∗) in patient-derived induced pluripotent stem cells (iPSCs). The absence of off-target editing in gene-corrected (isogenic) control iPSCs was demonstrated by whole-genome sequencing. We differentiated the patient, gene-corrected, and unrelated control iPSCs into three-dimensional retina-like cells, so-called retinal organoids. We observed opsin and rhodopsin mislocalization to the outer nuclear layer in patient-derived but not in the gene-corrected or unrelated control organoids. We also confirmed the rescue of lebercilin expression and localization along the ciliary axoneme within the gene-corrected organoids. Here, we show the potential of combining precise single-nucleotide gene editing with the iPSC-derived retinal organoid system for the generation of a cellular model of early-onset retinal disease.

Pollution effects on retinal health: A review on current methodologies and findings

In our daily life, we are exposed to numerous industrial chemicals that may be harmful to the retina, which is a delicate and sensitive part of our eyes. This could lead to irreversible changes and cause retinal diseases or blindness. Current retinal environmental health studies primarily utilize animal models, isolated mammalian retinas, animal- or human-derived retinal cells, and retinal organoids, to address both pre- and postnatal exposure. However, as there is limited toxicological information available for specific populations, human induced pluripotent stem cell (hiPSC)-induced models could be effective tools to supplement such data. In order to obtain more comprehensive and reliable toxicological information, we need more appropriate models, novel evaluation methods, and computational technologies to develop portable equipment. This review mainly focused on current toxicology models with particular emphasis on retinal organoids, and it looks forward to future models, analytical methods, and equipment that can efficiently and accurately evaluate retinal toxicity.

Emerging strategies of engineering retinal organoids and organoid-on-a-chip in modeling intraocular drug delivery: current progress and future perspectives

Retinal diseases are a rising concern as major causes of blindness in an aging society; therapeutic options are limited, and the precise pathogenesis of these diseases remains largely unknown. Intraocular drug delivery and nanomedicines offering targeted, sustained, and controllable delivery are the most challenging and popular topics in ocular drug development and toxicological evaluation. Retinal organoids (ROs) and organoid-on-a-chip (ROoC) are both emerging as promising in-vitro models to faithfully recapitulate human eyes for retinal research in the replacement of experimental animals and primary cells. In this study, we review the generation and application of ROs resembling the human retina in cell subtypes and laminated structures and introduce the emerging engineered ROoC as a technological opportunity to address critical issues. On-chip vascularization, perfusion, and close inter-tissue interactions recreate physiological environments in vitro, whilst integrating with biosensors facilitates real-time analysis and monitoring during organogenesis of the retina representing engineering efforts in ROoC models. We also emphasize that ROs and ROoCs hold the potential for applications in modeling intraocular drug delivery in vitro and developing next-generation retinal drug delivery strategies.

Antisense oligonucleotide therapy corrects splicing in the common Stargardt disease type 1-causing variant ABCA4 c.5461-10T>C

Stargardt disease type 1 (STGD1) is the most common hereditary form of maculopathy and remains untreatable. STGD1 is caused by biallelic variants in the ABCA4 gene, which encodes the ATP-binding cassette (type 4) protein (ABCA4) that clears toxic byproducts of the visual cycle. The c.5461-10T>C p.[Thr1821Aspfs∗6,Thr1821Valfs∗13] variant is the most common severe disease-associated variant, and leads to exon skipping and out-of-frame ABCA4 transcripts that prevent translation of functional ABCA4 protein. Homozygous individuals typically display early onset STGD1 and are legally blind by early adulthood. Here, we applied antisense oligonucleotides (AONs) to promote exon inclusion and restore wild-type RNA splicing of ABCA4 c.5461-10T>C. The effect of AONs was first investigated in vitro using an ABCA4 midigene model. Subsequently, the best performing AONs were administered to homozygous c.5461-10T>C 3D human retinal organoids. Isoform-specific digital polymerase chain reaction revealed a significant increase in correctly spliced transcripts after treatment with the lead AON, QR-1011, up to 53% correct transcripts at a 3 μM dose. Furthermore, western blot and immunohistochemistry analyses identified restoration of ABCA4 protein after treatment. Collectively, we identified QR-1011 as a potent splice-correcting AON and a possible therapeutic intervention for patients harboring the severe ABCA4 c.5461-10T>C variant.

Characterization of Primary Cilia Formation in Human ESC-Derived Retinal Organoids

Objectives

Primary cilia are conserved organelles found in polarized mammalian cells that regulate neuronal growth, migration, and differentiation. Proper cilia formation is essential during eye development. Our previous reports found that both amacrine and retinal ganglion cells (RGCs) contain primary cilia in primate and rodent retinas. However, whether primary cilia are present in the inner retina of human retinal organoids remains unknown. The purpose of this study is to characterize the primary cilia distribution in human embryonic stem cell (hESC-derived retinal organoid development.

Materials and Methods

Retinal organoids were differentiated from a hESC line, harvested at various developmental timepoints (day 44-day 266), and immunostained with antibodies for primary cilia, including Arl13b (for the axoneme), AC3, and Centrin3 (for the basal body). AP2α, Prox1, GAD67, Calretinin, GFAP, PKCα, and Chx10 antibodies as well as Brn3b-promoted tdTomato expression were used to visualize retinal cell types.

Results

A group of ciliated cells were present in the inner aspects of retinal organoids from day 44 to day 266 in culture. Ciliated Chx10-positive retinal progenitor cells, GFAP-positive astrocytes, and PKCα-positive rod-bipolar cells were detected later during development (day 176 to day 266). Ciliation persisted during all stages of retinal developmental in AP2α-positive amacrine cells, but it was decreased in Brn3b-positive retinal ganglion cells (RGCs) at later time points. Additionally, AC3-positive astrocytes significantly decreased during the later stages of organoid formation.

Conclusions

Amacrine cells in retinal organoids retain cilia throughout development, whereas RGC ciliation gradually and progressively decreases with organoid maturation.

Comprehensive characterization of fetal and mature retinal cell identity to assess the fidelity of retinal organoids

Characterizing cell identity in complex tissues such as the human retina is essential for studying its development and disease. While retinal organoids derived from pluripotent stem cells have been widely used to model development and disease of the human retina, there is a lack of studies that have systematically evaluated the molecular and cellular fidelity of the organoids derived from various culture protocols in recapitulating their in vivo counterpart. To this end, we performed an extensive meta-atlas characterization of cellular identities of the human eye, covering a wide range of developmental stages. The resulting map uncovered previously unknown biomarkers of major retinal cell types and those associated with cell-type-specific maturation. Using our retinal-cell-identity map from the fetal and adult tissues, we systematically assessed the fidelity of the retinal organoids in mimicking the human eye, enabling us to comprehensively benchmark the current protocols for retinal organoid generation.

Age-related macular degeneration: A disease of extracellular complement amplification

Age-related macular degeneration (AMD) is a major cause of vision impairment in the Western World, and with the aging world population, its incidence is increasing. As of today, for the majority of patients, no treatment exists. Multiple genetic and biochemical studies have shown a strong association with components in the complement system and AMD, and evidence suggests a major role of remodeling of the extracellular matrix underlying the outer blood/retinal barrier. As part of the innate immune system, the complement cascade acts as a first-line defense against pathogens, and upon activation, its amplification loop ensures a strong, rapid, and sustained response. Excessive activation, however, can lead to host tissue damage and cause complement-associated diseases like AMD. AMD patients present with aberrant activation of the alternative pathway, especially in ocular tissues but also on a systemic level. Here, we review the latest findings of complement activation in AMD, and we will discuss in vivo observations made in human tissue, cellular models, the potential synergy of different AMD-associated pathways, and conclude on current clinical trials and the future outlook.

Induced Pluripotent Stem Cells and Genome-Editing Tools in Determining Gene Function and Therapy for Inherited Retinal Disorders

Inherited retinal disorders (IRDs) affect millions of people worldwide and are a major cause of irreversible blindness. Therapies based on drugs, gene augmentation or transplantation approaches have been widely investigated and proposed. Among gene therapies for retinal degenerative diseases, the fast-evolving genome-editing CRISPR/Cas technology has emerged as a new potential treatment. The CRISPR/Cas system has been developed as a powerful genome-editing tool in ophthalmic studies and has been applied not only to gain proof of principle for gene therapies in vivo, but has also been extensively used in basic research to model diseases-in-a-dish. Indeed, the CRISPR/Cas technology has been exploited to genetically modify human induced pluripotent stem cells (iPSCs) to model retinal disorders in vitro, to test in vitro drugs and therapies and to provide a cell source for autologous transplantation. In this review, we will focus on the technological advances in iPSC-based cellular reprogramming and gene editing technologies to create human in vitro models that accurately recapitulate IRD mechanisms towards the development of treatments for retinal degenerative diseases.

PAX6 disease models for aniridia

Aniridia is a pan-ocular genetic developmental eye disorder characterized by complete or partial iris and foveal hypoplasia, for which there is no treatment currently. Progressive sight loss can arise from cataracts, glaucoma, and aniridia-related keratopathy, which can be managed conservatively or through surgical intervention. The vast majority of patients harbor heterozygous mutations involving the PAX6 gene, which is considered the master transcription factor of early eye development. Over the past decades, several disease models have been investigated to gain a better understanding of the molecular pathophysiology, including several mouse and zebrafish strains and, more recently, human-induced pluripotent stem cells (hiPSCs) derived from aniridia patients. The latter provides a more faithful cellular system to study early human eye development. This review outlines the main aniridia-related animal and cellular models used to study aniridia and highlights the key discoveries that are bringing us closer to a therapy for patients.

Bilirubin-Induced Neurological Damage: Current and Emerging iPSC-Derived Brain Organoid Models

Bilirubin-induced neurological damage (BIND) has been a subject of studies for decades, yet the molecular mechanisms at the core of this damage remain largely unknown. Throughout the years, many in vivo chronic bilirubin encephalopathy models, such as the Gunn rat and transgenic mice, have further elucidated the molecular basis of bilirubin neurotoxicity as well as the correlations between high levels of unconjugated bilirubin (UCB) and brain damage. Regardless of being invaluable, these models cannot accurately recapitulate the human brain and liver system; therefore, establishing a physiologically recapitulating in vitro model has become a prerequisite to unveil the breadth of complexities that accompany the detrimental effects of UCB on the liver and developing human brain. Stem-cell-derived 3D brain organoid models offer a promising platform as they bear more resemblance to the human brain system compared to existing models. This review provides an explicit picture of the current state of the art, advancements, and challenges faced by the various models as well as the possibilities of using stem-cell-derived 3D organoids as an efficient tool to be included in research, drug screening, and therapeutic strategies for future clinical applications.

A Novel Hypothesis on Choroideremia-Manifesting Female Carriers: Could CHM In-Frame Variants Exert a Dominant Negative Effect? A Case Report

Choroideremia is an X-linked recessive condition presenting in males, with progressive degeneration of retinal and choroidal tissues leading to progressive visual loss. Its pathological mechanism is due to alterations in the CHM gene that encodes for REP1, a protein required for prenylation of Rab by the Rab geranylgeranyl transferase (RGGT). Even though female carriers are predicted to be not affected by the disease, a wide phenotypic spectrum ranging from mild to severe cases has been reported in women. The reason why Choroideremia manifests in female carriers remains elusive. While X chromosome inactivation (XCI) skewing has been proposed as a leading putative mechanism, emerging evidence has shown that CHM could variably escape from XCI. We described a family with an initial clinical suspicion of Retinitis Pigmentosa in which a novel CHM pathogenic splicing variant was found by exome sequencing. The variant, initially found in the 63-year-old female presenting with impaired visual acuity and severe retinal degeneration, segregated in the 31-year-old daughter and the 37-year-old son, both presenting with fundus anomalies. mRNA studies revealed a shorter in-frame CHM isoform lacking exon 10. Molecular modeling of the ternary REP1/Rab/RGGT protein complex predicted significant impairing of REP1/Rab binding without alteration of REP1/RGGT interaction. We suggest that, in our female cases, the biallelic expression of CHM may have led to the production of both the mutant and wild type REP1. The mutant isoform, sequestrating RGGT, could reduce its available amount for Rab prenylation, thus exerting a dominant-negative effect. If confirmed with further studies and in large cohorts of female carriers, the here proposed molecular mechanism could help to explain the complexity of manifestation of Choroideremia in females.