Efficient generation of retinal progenitor cells from human embryonic stem cells

New frontiers in retinal transplantation

New frontiers about retinal cell transplantation for retinal degenerative diseases start from the idea that acting on stem cells can help regenerate retinal layers and establish new synapses among retinal cells. Deficiency or alterations of synaptic input and neurotrophic factors result in trans-neuronal degeneration of the inner retinal cells. Thus, the disruption of photoreceptors takes place. However, even in advanced forms of retinal degeneration, a good percentage of the ganglion cells and the inner nuclear layer neurons remain intact. This phenomenon provides evidence for obtaining retinal circuitry through the transplantation of photoreceptors into the subretinal region. The eye is regarded as an optimal organ for cell transplantation because of its immunological privilege and the relatively small number of cells collaborating to carry out visual activities. The eyeball's immunological privilege, characterized by the suppression of delayed-type hypersensitivity responses in ocular tissues, is responsible for the low rate of graft rejection in transplant patients. The main discoveries highlight the capacity of embryonic stem cells (ESCs) and induced pluripotent stem cells to regenerate damaged retinal regions. Recent progress has shown significant enhancements in transplant procedures and results. The research also explores the ethical ramifications linked to the utilization of stem cells, emphasizing the ongoing issue surrounding ESCs. The analysis centers on recent breakthroughs, including the fabrication of three-dimensional retinal organoids and the innovation of scaffolding for cell transportation. Moreover, researchers are currently assessing the possibility of CRISPR and other advanced gene editing technologies to enhance the outcomes of retinal transplantation. The widespread use of universally recognized safe surgical and imaging methods enables retinal transplantation and monitoring of transplanted cell growth toward the correct location. Currently, most therapy approaches are in the first phases of development and necessitate further research, including both pre-clinical and clinical trials, to attain favorable visual results for individuals suffering from retinal degenerative illnesses.

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.

Small Molecular Approaches for Cellular Reprogramming and Tissue Engineering: Functions as Mediators of the Cell Signaling Pathway

Utilizing induced pluripotent stem cells (iPSCs) in drug screening and cell replacement therapy has emerged as a method with revolutionary applications. With the advent of patient-specific iPSCs and the subsequent development of cells that exhibit disease phenotypes, the focus of medication research will now shift toward the pathology of human diseases. Regular iPSCs can also be utilized to generate cells that assess the negative impacts of medications. These cells provide a much more precise and cost-efficient approach compared to many animal models. In this review, we explore the utilization of small-molecule drugs to enhance the growth of iPSCs and gain insights into the process of reprogramming. We mainly focus on the functions of small molecules in modulating different signaling pathways, thereby modulating cell fate. Understanding the way small molecule drugs interact with iPSC technology has the potential to significantly enhance the understanding of physiological pathways in stem cells and practical applications of iPSC-based therapy and screening systems, revolutionizing the treatment of diseases.

Complexin vitromodels positioned for impact to drug testing in pharma: a review

Recent years have seen the creation and popularization of various complexin vitromodels (CIVMs), such as organoids and organs-on-chip, as a technology with the potential to reduce animal usage in pharma while also enhancing our ability to create safe and efficacious drugs for patients. Public awareness of CIVMs has increased, in part, due to the recent passage of the FDA Modernization Act 2.0. This visibility is expected to spur deeper investment in and adoption of such models. Thus, end-users and model developers alike require a framework to both understand the readiness of current models to enter the drug development process, and to assess upcoming models for the same. This review presents such a framework for model selection based on comparative -omics data (which we term model-omics), and metrics for qualification of specific test assays that a model may support that we term context-of-use (COU) assays. We surveyed existing healthy tissue models and assays for ten drug development-critical organs of the body, and provide evaluations of readiness and suggestions for improving model-omics and COU assays for each. In whole, this review comes from a pharma perspective, and seeks to provide an evaluation of where CIVMs are poised for maximum impact in the drug development process, and a roadmap for realizing that potential.

Retinitis pigmentosa and stem cell therapy

Retinitis pigmentosa (RP) is a group of genetic disorders characterized by progressive degeneration of photoreceptors and retinal pigment epithelium (RPE) cells. Its main clinical manifestations include night blindness and progressive loss of peripheral vision, making it a prevalent debilitating eye disease that significantly impacts patients' quality of life. RP exhibits significant phenotypic and genetic heterogeneity. For instance, numerous abnormal genes are implicated in RP, resulting in varying clinical presentations, disease progression rates, and pathological characteristics among different patients. Consequently, gene therapy for RP poses challenges due to these complexities. However, stem cells have garnered considerable attention in the field of RPE therapy since both RPE cells and photoreceptors can be derived from stem cells. In recent years, a large number of animal experiments and clinical trials based on stem cell transplantation attempts, especially cord blood mesenchymal stem cell (MSC) transplantation and bone marrow-derived MSC transplantation, have confirmed that stem cell therapy can effectively and safely improve the outer retinal function of the RP-affected eye. However, stem cell therapy also has certain limitations, such as the fact that RP patients may involve multiple types of retinal cytopathia, which brings great challenges to stem cell transplantation therapy, and further research is needed to solve various problems faced by this approach in the clinic. Through comprehensive analysis of the etiology and histopathological changes associated with RP, this study substantiates the efficacy and safety of stem cell therapy based on rigorous animal experimentation and clinical trials, while also highlighting the existing limitations that warrant further investigation.

Quantification of Proliferating and Mitotically Active Retinal Cells in Mice by Flow Cytometry

Adult mammals lack the ability to regenerate retinal neurons after injury. However, in previous studies from this lab, topical application of the selective alpha7 nicotinic acetylcholine receptor (nAChR) agonist, PNU-282987, has been associated with an increase in the number of retinal neurons in adult murine models both in the presence and absence of injury to the retina. Additionally, studies assaying mitotic markers have shown a substantial increase in the amount of mitotically active and proliferating cells with the topical application of the alpha7 nAChR agonist. However, these previous studies were performed using fluorescent immunolabeling and subsequent confocal microscopy, thus limiting the number of antibodies that can be multiplexed. As a result, we have developed a flow cytometry method that allows for the multiplexing and analysis of multiple external and internal markers in dissociated retinal cells. In this paper, a step-by-step protocol is described for the labeling of multiple retinal cell types such as retinal ganglion cells, rod photoreceptors, and Müller glia, concurrently with Müller glia-derived progenitor cells that arise after treatment with PNU-282987. Key features • Neurogenesis in the adult mammalian retina. • Flow cytometry of retinal cells. • PNU-282987-induced mitotic activity in the retina. • Dissociation of the retina for flow cytometry analysis. Graphical overview Schematic demonstrating the protocol for preparation of retinal cells for flow cytometry analysis. (A) Adult mice (3-6 months) are subjected to topical PBS eyedrop treatment containing DMSO (control groups) or PNU-282987 (experimental groups). Both eyedrop treatments contain 1 mg/mL of BrdU to label proliferating cells. After treatment, mice are euthanized, and retinae are harvested for dissociation using papain. (B) Dissociated retina cells are fixed and permeabilized before aliquots are taken for cell counts on a hemocytometer. After determining the number of cells present, conjugated antibodies and unconjugated primary antibodies are added at the appropriate dilutions. Fluorescent secondary antibodies are added for markers that are unconjugated. Cells are then subjected to flow cytometric analysis using a BD LSRFortessa.

Modeling inherited retinal diseases using human induced pluripotent stem cell derived photoreceptor cells and retinal pigment epithelial cells

Since the discovery of induced pluripotent stem cell (iPSC) technology, there have been many attempts to create cellular models of inherited retinal diseases (IRDs) for investigation of pathogenic processes to facilitate target discovery and validation activities. Consistency remains key in determining the utility of these findings. Despite the importance of consistency, quality control metrics are still not widely used. In this review, a toolkit for harnessing iPSC technology to generate photoreceptor, retinal pigment epithelial cell, and organoid disease models is provided. Considerations while developing iPSC-derived IRD models such as iPSC origin, reprogramming methods, quality control metrics, control strategies, and differentiation protocols are discussed. Various iPSC IRD models are dissected and the scientific hurdles of iPSC-based disease modeling are discussed to provide an overview of current methods and future directions in this field.

Exploring gellan gum-based hydrogels for regenerating human embryonic stem cells in age-related macular degeneration therapy: A literature review

Age-related macular degeneration (AMD) is a progressive ocular disease marked by the deterioration of retinal photoreceptor cells, leading to central vision decline, predominantly affecting the elderly population worldwide. Current treatment modalities, such as anti-VEGF agents, laser therapy, and photodynamic therapy, aim to manage the condition, with emerging strategies like stem cell replacement therapy showing promise. However, challenges like immune rejection and cell survival hinder the efficacy of stem cell interventions. Regenerative medicine faces obstacles in maximizing stem cell potential due to limitations in mimicking the dynamic cues of the extracellular matrix (ECM) crucial for guiding stem cell behaviour. Innovative biomaterials like gellan gum hydrogels offer tailored microenvironments conducive to enhancing stem cell culture efficacy and tissue regeneration. Gellan gum-based hydrogels, renowned for biocompatibility and customizable mechanical properties, provide crucial support for cell viability, differentiation, and controlled release of therapeutic factors, making them an ideal platform for culturing human embryonic stem cells (hESCs). These hydrogels mimic native tissue mechanics, promoting optimal hESC differentiation while minimizing immune responses and facilitating localized delivery. This review explores the potential of Gellan Gum-Based Hydrogels in regenerative AMD therapy, emphasizing their role in enhancing hESC regeneration and addressing current status, treatment limitations, and future directions.

Pluripotent stem cell-derived models of retinal disease: Elucidating pathogenesis, evaluating novel treatments, and estimating toxicity

Blindness poses a growing global challenge, with approximately 26% of cases attributed to degenerative retinal diseases. While gene therapy, optogenetic tools, photosensitive switches, and retinal prostheses offer hope for vision restoration, these high-cost therapies will benefit few patients. Understanding retinal diseases is therefore key to advance effective treatments, requiring in vitro models replicating pathology and allowing quantitative assessments for drug discovery. Pluripotent stem cells (PSCs) provide a unique solution given their limitless supply and ability to differentiate into light-responsive retinal tissues encompassing all cell types. This review focuses on the history and current state of photoreceptor and retinal pigment epithelium (RPE) cell generation from PSCs. We explore the applications of this technology in disease modelling, experimental therapy testing, biomarker identification, and toxicity studies. We consider challenges in scalability, standardisation, and reproducibility, and stress the importance of incorporating vasculature and immune cells into retinal organoids. We advocate for high-throughput automation in data acquisition and analyses and underscore the value of advanced micro-physiological systems that fully capture the interactions between the neural retina, RPE, and choriocapillaris.

New Perspectives in Stem Cell Transplantation and Associated Therapies to Treat Retinal Diseases: From Gene Editing to 3D Bioprinting

Inherited and non-inherited retinopathies can affect distinct cell types, leading to progressive cell death and visual loss. In the last years, new approaches have indicated exciting opportunities to treat retinopathies. Cell therapy in retinitis pigmentosa, age-related macular disease, and glaucoma have yielded encouraging results in rodents and humans. The first two diseases mainly impact the photoreceptors and the retinal pigmented epithelium, while glaucoma primarily affects the ganglion cell layer. Induced pluripotent stem cells and multipotent stem cells can be differentiated in vitro to obtain specific cell types for use in transplant as well as to assess the impact of candidate molecules aimed at treating retinal degeneration. Moreover, stem cell therapy is presented in combination with newly developed methods, such as gene editing, Müller cells dedifferentiation, sheet & drug delivery, virus-like particles, optogenetics, and 3D bioprinting. This review describes the recent advances in this field, by presenting an updated panel based on cell transplants and related therapies to treat retinopathies.

Transplanted human photoreceptors transfer cytoplasmic material but not to the recipient mouse retina

BACKGROUND

The discovery of material transfer between transplanted and host mouse photoreceptors has expanded the possibilities for utilizing transplanted photoreceptors as potential vehicles for delivering therapeutic cargo. However, previous research has not directly explored the capacity for human photoreceptors to engage in material transfer, as human photoreceptor transplantation has primarily been investigated in rodent models of late-stage retinal disease, which lack host photoreceptors.

METHODS

In this study, we transplanted human stem-cell derived photoreceptors purified from human retinal organoids at different ontological ages (weeks 10, 14, or 20) into mouse models with intact photoreceptors and assessed transfer of human proteins and organelles to mouse photoreceptors.

RESULTS

Unexpectedly, regardless of donor age or mouse recipient background, human photoreceptors did not transfer material in the mouse retina, though a rare subset of donor cells (< 5%) integrated into the mouse photoreceptor cell layer. To investigate the possibility that a species barrier impeded transfer, we used a flow cytometric assay to examine material transfer in vitro. Interestingly, dissociated human photoreceptors transferred fluorescent protein with each other in vitro, yet no transfer was detected in co-cultures of human and mouse photoreceptors, suggesting that material transfer is species specific.

CONCLUSIONS

While xenograft models are not a tractable system to study material transfer of human photoreceptors, these findings demonstrate that human retinal organoid-derived photoreceptors are competent donors for material transfer and thus may be useful to treat retinal degenerative disease.

Rapid and efficient generation of a transplantable population of functional retinal ganglion cells from fibroblasts

Glaucoma and other optic neuropathies lead to progressive and irreversible vision loss by damaging retinal ganglion cells (RGCs) and their axons. Cell replacement therapy is a potential promising treatment. However, current methods to obtain RGCs have inherent limitations, including time-consuming procedures, inefficient yields and complex protocols, which hinder their practical application. Here, we have developed a straightforward, rapid and efficient approach for directly inducing RGCs from mouse embryonic fibroblasts (MEFs) using a combination of triple transcription factors (TFs): ASCL1, BRN3B and PAX6 (ABP). We showed that on the 6th day following ABP induction, neurons with molecular characteristics of RGCs were observed, and more than 60% of induced neurons became iRGCs (induced retinal ganglion cells) in the end. Transplanted iRGCs had the ability to survive and appropriately integrate into the RGC layer of mouse retinal explants and N-methyl-D-aspartic acid (NMDA)-damaged retinas. Moreover, they exhibited electrophysiological properties typical of RGCs, and were able to regrow dendrites and axons and form synaptic connections with host retinal cells. Together, we have established a rapid and efficient approach to acquire functional RGCs for potential cell replacement therapy to treat glaucoma and other optic neuropathies.

Generating Retinas through Guided Pluripotent Stem Cell Differentiation and Direct Somatic Cell Reprogramming

Retinal degeneration diseases affect millions of people worldwide but are among the most difficult eye diseases to cure. Studying the mechanisms and developing new therapies for these blinding diseases requires researchers to have access to many retinal cells. In recent years there has been substantial advances in the field of biotechnology in generating retinal cells and even tissues in vitro, either through programmed sequential stem cell differentiation or direct somatic cell lineage reprogramming. The resemblance of these in vitro-generated retinal cells to native cells has been increasingly utilized by researchers. With the help of these in vitro retinal models, we now have a better understanding of human retinas and retinal diseases. Furthermore, these in vitro-generated retinal cells can be used as donor cells which solves a major hurdle in the development of cell replacement therapy for retinal degeneration diseases, while providing a promising option for patients suffering from these diseases. In this review, we summarize the development of pluripotent stem cell-to-retinal cell differentiation methods, the recent advances in generating retinal cells through direct somatic cell reprogramming, and the translational applications of retinal cells generated in vitro. Finally, we discuss the limitations of the current protocols and possible future directions for improvement.

Development of a simplified human embryonic stem cell-based retinal pre-organoid model for toxicity evaluations of common pollutants

OBJECTIVE

To explore the retinal toxicity of pharmaceuticals and personal care products (PPCPs), flame retardants, bisphenols, phthalates, and polycyclic aromatic hydrocarbons (PAHs) on human retinal progenitor cells (RPCs) and retinal pigment epithelial (RPE) cells, which are the primary cell types at the early stages of retinal development, vital for subsequent functional cell type differentiation, and closely related to retinal diseases.

MATERIALS AND METHODS

After 23 days of differentiation, human embryonic stem cell (hESC)-based retinal pre-organoids, containing RPCs and RPE cells, were exposed to 10, 100, and 1000 nM pesticides (butachlor, terbutryn, imidacloprid, deltamethrin, pendimethalin, and carbaryl), flame retardants (PFOS, TBBPA, DBDPE, and TDCIPP), PPCPs (climbazole and BHT), and other typical pollutants (phenanthrene, DCHP, and BPA) for seven days. Then, mRNA expression changes were monitored and compared.

RESULTS

(1) The selected pollutants did not show strong effects at environmental and human-relevant concentrations, although the effects of flame retardants were more potent than those of other categories of chemicals. Surprisingly, some pollutants with distinct structures showed similar adverse effects. (2) Exposure to pollutants induced different degrees of cell detachment, probably due to alterations in extracellular matrix and/or cell adhesion.

CONCLUSIONS

In this study, we established a retinal pre-organoid model suitable for evaluating multiple pollutants' effects, and pointed out the potential retinal toxicity of flame retardants, among other pollutants. Nevertheless, the potential mechanisms of toxicity and the effects on cell detachment are still unclear and deserve further exploration. Additionally, this model holds promise for screening interventions aimed at mitigating the detrimental effects of these pollutants.

Therapeutic strategies for glaucoma and optic neuropathies

Glaucoma is a neurodegenerative eye disease that causes permanent vision impairment. The main pathological characteristics of glaucoma are retinal ganglion cell (RGC) loss and optic nerve degeneration. Glaucoma can be caused by elevated intraocular pressure (IOP), although some cases are congenital or occur in patients with normal IOP. Current glaucoma treatments rely on medicine and surgery to lower IOP, which only delays disease progression. First-line glaucoma medicines are supported by pharmacotherapy advancements such as Rho kinase inhibitors and innovative drug delivery systems. Glaucoma surgery has shifted to safer minimally invasive (or microinvasive) glaucoma surgery, but further trials are needed to validate long-term efficacy. Further, growing evidence shows that adeno-associated virus gene transduction and stem cell-based RGC replacement therapy hold potential to treat optic nerve fiber degeneration and glaucoma. However, better understanding of the regulatory mechanisms of RGC development is needed to provide insight into RGC differentiation from stem cells and help choose target genes for viral therapy. In this review, we overview current progress in RGC development research, optic nerve fiber regeneration, and human stem cell-derived RGC differentiation and transplantation. We also provide an outlook on perspectives and challenges in the field.

Continuous non-adherent culture promotes transdifferentiation of human adipose-derived stem cells into retinal lineage

Non-adherent culture is critical for the transdifferentiation of stem cells from mesoderm to neuroectoderm. Sphere culture has been reported to directly induce the adipose tissue cells to neural stem cells. Here we aimed to evaluate continuous non-adherent culture on the transdifferentiation potential of human adipose-derived stem cells (ASCs) into retinal lineage. Human ASCs were induced into retinal lineage by the treatment of noggin, dickkopf-related protein 1, and IGF-1 (NDI) under adherent and non-adherent culture. The NDI induction treatment with the adherent culture for 21 days promoted robust expression of retinal markers in the induced ASCs as compared to those without NDI induction on the adherent culture. With continuous non-adherent culture for 21 days, human ASCs could highly express retinal marker genes even without NDI induction treatment as compared to those on the adherent culture. The combination of continuous non-adherent culture with the NDI induction did not show a significant upregulation of the retinal marker expression as compared to either NDI induction with the adherent culture or continuous non-adherent culture without NDI induction treatment. In summary, both non-adherent culture and NDI induction medium could independently promote the transdifferentiation of human ASCs into retinal lineage. Yet, their combination did not produce an enhancement effect.

A treatment within sight: challenges in the development of stem cell-derived photoreceptor therapies for retinal degenerative diseases

Stem cell therapies can potentially treat various retinal degenerative diseases, including age-related macular degeneration (AMD) and inherited retinal diseases like retinitis pigmentosa. For these diseases, transplanted cells may include stem cell-derived retinal pigmented epithelial (RPE) cells, photoreceptors, or a combination of both. Although stem cell-derived RPE cells have progressed to human clinical trials, therapies using photoreceptors and other retinal cell types are lagging. In this review, we discuss the potential use of human pluripotent stem cell (hPSC)-derived photoreceptors for the treatment of retinal degeneration and highlight the progress and challenges for their efficient production and clinical application in regenerative medicine.

Factors Affecting Stem Cell-Based Regenerative Approaches in Retinal Degeneration

Inherited and age-associated vision loss is often associated with degeneration of the cells of the retina, the light-sensitive layer at the back of the eye. The mammalian retina, being a postmitotic neural tissue, does not have the capacity to repair itself through endogenous regeneration. There has been considerable excitement for the development of cell replacement approaches since the isolation and development of culture methods for human pluripotent stem cells, as well as the generation of induced pluripotent stem cells. This has now been combined with novel three-dimensional organoid culture systems that closely mimic human retinal development in vitro. In this review, we cover the current state of the field, with emphasis on the cell delivery challenges, role of the recipient immunological microenvironment, and challenges related to connectivity between transplanted cells and host circuitry both locally and centrally to the different areas of the brain.

Establishing Functional Retina in a Dish: Progress and Promises of Induced Pluripotent Stem Cell-Based Retinal Neuron Differentiation

The human eye plays a critical role in vision perception, but various retinal degenerative diseases such as retinitis pigmentosa (RP), glaucoma, and age-related macular degeneration (AMD) can lead to vision loss or blindness. Although progress has been made in understanding retinal development and in clinical research, current treatments remain inadequate for curing or reversing these degenerative conditions. Animal models have limited relevance to humans, and obtaining human eye tissue samples is challenging due to ethical and legal considerations. Consequently, researchers have turned to stem cell-based approaches, specifically induced pluripotent stem cells (iPSCs), to generate distinct retinal cell populations and develop cell replacement therapies. iPSCs offer a novel platform for studying the key stages of human retinogenesis and disease-specific mechanisms. Stem cell technology has facilitated the production of diverse retinal cell types, including retinal ganglion cells (RGCs) and photoreceptors, and the development of retinal organoids has emerged as a valuable in vitro tool for investigating retinal neuron differentiation and modeling retinal diseases. This review focuses on the protocols, culture conditions, and techniques employed in differentiating retinal neurons from iPSCs. Furthermore, it emphasizes the significance of molecular and functional validation of the differentiated cells.

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.

Reproducible generation of human retinal ganglion cells from banked retinal progenitor cells: analysis of target recognition and IGF-1-mediated axon regeneration

The selective degeneration of retinal ganglion cells (RGCs) is a common feature in glaucoma, a complex group of diseases, leading to irreversible vision loss. Stem cell-based glaucoma disease modeling, cell replacement, and axon regeneration are viable approaches to understand mechanisms underlying glaucomatous degeneration for neuroprotection, ex vivo stem cell therapy, and therapeutic regeneration. These approaches require direct and facile generation of human RGCs (hRGCs) from pluripotent stem cells. Here, we demonstrate a method for rapid generation of hRGCs from banked human pluripotent stem cell-derived retinal progenitor cells (hRPCs) by recapitulating the developmental mechanism. The resulting hRGCs are stable, functional, and transplantable and have the potential for target recognition, demonstrating their suitability for both ex vivo stem cell approaches to glaucomatous degeneration and disease modeling. Additionally, we demonstrate that hRGCs derived from banked hRPCs are capable of regenerating their axons through an evolutionarily conserved mechanism involving insulin-like growth factor 1 and the mTOR axis, demonstrating their potential to identify and characterize the underlying mechanism(s) that can be targeted for therapeutic regeneration.

Non-canonical Wnt signaling in the eye

Wnt signaling comprises a group of complex signal transduction pathways that play critical roles in cell proliferation, differentiation, and apoptosis during development, as well as in stem cell maintenance and adult tissue homeostasis. Wnt pathways are classified into two major groups, canonical (β-catenin-dependent) or non-canonical (β-catenin-independent). Most previous studies in the eye have focused on canonical Wnt signaling, and the role of non-canonical signaling remains poorly understood. Additionally, the crosstalk between canonical and non-canonical Wnt signaling in the eye has hardly been explored. In this review, we present an overview of available data on ocular non-canonical Wnt signaling, including developmental and functional aspects in different eye compartments. We also discuss important changes of this signaling in various ocular conditions, such as keratoconus, aniridia-related keratopathy, diabetes, age-related macular degeneration, optic nerve damage, pathological angiogenesis, and abnormalities in the trabecular meshwork and conjunctival cells, and limbal stem cell deficiency.

Survival of Neural Progenitors Derived from Human Embryonic Stem Cells Following Subretinal Transplantation in Rodents

Purpose: To examine the survival of neural progenitors (NPs) cells derived from human embryonic stem cells (hESCs) following subretinal (SR) transplantation in rodents. Methods: hESCs engineered to express enhanced green fluorescent protein (eGFP) were differentiated in vitro toward an NP fate using a 4-week protocol. State of differentiation was characterized by quantitative-PCR. NPs in suspension (75,000/μl) were transplanted to the SR-space of Royal College of Surgeons (RCS) rats (n = 66), nude-RCS rats (n = 18), and NOD scid gamma (NSG) mice (n = 53). Success of engraftment was determined at 4 weeks post-transplant by in vivo visualization of GFP-expression using a properly filtered rodent fundus camera. Transplanted eyes were examined in vivo at set time points using the fundus camera, and in select cases, by optical coherence tomography imaging, and after enucleation, by retinal histology and immunohistochemistry. Results: In RCS rats, cell rejection was observed in 29% of eyes at 6 weeks, rising to 92% at 8 weeks. In the more immunodeficient nude-RCS rats, the rejection rate was still high reaching 62% of eyes at 6 weeks post-transplant. Following transplantation in highly immunodeficient NSG mice, survival of the hESC-derived NPs was much improved, with 100% survival at 9 weeks and 72% at 20 weeks. A small number of eyes that were followed past 20 weeks showed survival also at 22 weeks. Conclusions: Immune status of recipient animals influences transplant survival. Highly immunodeficient NSG mice provide a better model for studying long-term survival, differentiation, and possible integration of hESC-derived NPs. Clinical Trial Registration numbers: NCT02286089, NCT05626114.

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.

Photoreceptor laminin drives differentiation of human pluripotent stem cells to photoreceptor progenitors that partially restore retina function

Blindness caused by advanced stages of inherited retinal diseases and age-related macular degeneration are characterized by photoreceptor loss. Cell therapy involving replacement with functional photoreceptor-like cells generated from human pluripotent stem cells holds great promise. Here, we generated a human recombinant retina-specific laminin isoform, LN523, and demonstrated the role in promoting the differentiation of human embryonic stem cells into photoreceptor progenitors. This chemically defined and xenogen-free method enables reproducible production of photoreceptor progenitors within 32 days. We observed that the transplantation into rd10 mice were able to protect the host photoreceptor outer nuclear layer (ONL) up to 2 weeks post transplantation as measured by full-field electroretinogram. At 4 weeks post transplantation, the engrafted cells were found to survive, mature, and associate with the host's rod bipolar cells. Visual behavioral assessment using the water maze swimming test demonstrated visual improvement in the cell-transplanted rodents. At 20 weeks post transplantation, the maturing engrafted cells were able to replace the loss of host ONL by extensive association with host bipolar cells and synapses. Post-transplanted rabbit model also provided congruent evidence for synaptic connectivity with the degenerated host retina. The results may pave the way for the development of stem cell-based therapeutics for retina degeneration.

Self-organization, quality control, and preclinical studies of human iPSC-derived retinal sheets for tissue-transplantation therapy

Three-dimensional retinal organoids (3D-retinas) are a promising graft source for transplantation therapy. We previously developed self-organizing culture for 3D-retina generation from human pluripotent stem cells (hPSCs). Here we present a quality control method and preclinical studies for tissue-sheet transplantation. Self-organizing hPSCs differentiated into both retinal and off-target tissues. Gene expression analyses identified the major off-target tissues as eye-related, cortex-like, and spinal cord-like tissues. For quality control, we developed a qPCR-based test in which each hPSC-derived neuroepithelium was dissected into two tissue-sheets: inner-central sheet for transplantation and outer-peripheral sheet for qPCR to ensure retinal tissue selection. During qPCR, tissue-sheets were stored for 3-4 days using a newly developed preservation method. In a rat tumorigenicity study, no transplant-related adverse events were observed. In retinal degeneration model rats, retinal transplants differentiated into mature photoreceptors and exhibited light responses in electrophysiology assays. These results demonstrate our rationale toward self-organizing retinal sheet transplantation therapy.

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.

Human Retinal Organoids in Therapeutic Discovery: A Review of Applications

Human embryonic stem cells (hESCs)- and induced pluripotent stem cells (hiPSCs)-derived retinal organoids (ROs) are three-dimensional laminar structures that recapitulate the developmental trajectory of the human retina. The ROs provide a fascinating tool for basic science research, eye disease modeling, treatment development, and biobanking for tissue/cell replacement. Here we review the previous studies that paved the way for RO technology, the two most widely accepted, standardized protocols to generate ROs, and the utilization of ROs in medical discovery. This review is conducted from the perspective of basic science research, transplantation for regenerative medicine, disease modeling, and therapeutic development for drug screening and gene therapy. ROs have opened avenues for new technologies such as assembloids, coculture with other organoids, vasculature or immune cells, microfluidic devices (organ-on-chip), extracellular vesicles for drug delivery, biomaterial engineering, advanced imaging techniques, and artificial intelligence (AI). Nevertheless, some shortcomings of ROs currently limit their translation for medical applications and pose a challenge for future research. Despite these limitations, ROs are a powerful tool for functional studies and therapeutic strategies for retinal diseases.

Toward the next generation of vascularized human neural organoids

Thanks to progress in the development of three-dimensional (3D) culture technologies, human central nervous system (CNS) development and diseases have been gradually deciphered by using organoids derived from human embryonic stem cells (hESCs) or human induced pluripotent stem cells (hiPSCs). Selforganized neural organoids (NOs) have been used to mimic morphogenesis and functions of specific organs in vitro. Many NOs have been reproduced in vitro, such as those mimicking the human brain, retina, and spinal cord. However, NOs fail to capitulate to the maturation and complexity of in vivo neural tissues. The persistent issues with current NO cultivation protocols are inadequate oxygen supply and nutrient diffusion due to the absence of vascular networks. In vivo, the developing CNS is interpenetrated by vasculature that not only supplies oxygen and nutrients but also provides a structural template for neuronal growth. To address these deficiencies, recent studies have begun to couple NO culture with bioengineering techniques and methodologies, including genetic engineering, coculture, multidifferentiation, microfluidics and 3D bioprinting, and transplantation, which might promote NO maturation and create more functional NOs. These cutting-edge methods could generate an ever more reliable NO model in vitro for deciphering the codes of human CNS development, disease progression, and translational application. In this review, we will summarize recent technological advances in culture strategies to generate vascularized NOs (vNOs), with a special focus on cerebral- and retinal-organoid models.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

An Altered Neurovascular System in Aging-Related Eye Diseases

The eye has a complex and metabolically active neurovascular system. Repeated light injuries induce aging and trigger age-dependent eye diseases. Damage to blood vessels is related to the disruption of the blood-retinal barrier (BRB), altered cellular communication, disrupted mitochondrial functions, and exacerbated aggregated protein accumulation. Vascular complications, such as insufficient blood supply and BRB disruption, have been suggested to play a role in glaucoma, age-related macular degeneration (AMD), and Alzheimer's disease (AD), resulting in neuronal cell death. Neuronal loss can induce vision loss. In this review, we discuss the importance of the neurovascular system in the eye, especially in aging-related diseases such as glaucoma, AMD, and AD. Beneficial molecular pathways to prevent or slow down retinal pathologic processes will also be discussed.

Directly induced human retinal ganglion cells mimic fetal RGCs and are neuroprotective after transplantation in vivo

Retinal ganglion cell (RGC) replacement therapy could restore vision in glaucoma and other optic neuropathies. We developed a rapid protocol for directly induced RGC (iRGC) differentiation from human stem cells, leveraging overexpression of NGN2. Neuronal morphology and neurite growth were observed within 1 week of induction; characteristic RGC-specific gene expression confirmed identity. Calcium imaging demonstrated γ-aminobutyric acid (GABA)-induced excitation characteristic of immature RGCs. Single-cell RNA sequencing showed more similarities between iRGCs and early-stage fetal human RGCs than retinal organoid-derived RGCs. Intravitreally transplanted iRGCs survived and migrated into host retinas independent of prior optic nerve trauma, but iRGCs protected host RGCs from neurodegeneration. These data demonstrate rapid iRGC generation in vitro into an immature cell with high similarity to human fetal RGCs and capacity for retinal integration after transplantation and neuroprotective function after optic nerve injury. The simplicity of this system may benefit translational studies on human RGCs.

Retinoic acid delays initial photoreceptor differentiation and results in a highly structured mature retinal organoid

BACKGROUND

Human-induced pluripotent stem cell-derived retinal organoids are a valuable tool for disease modelling and therapeutic development. Many efforts have been made over the last decade to optimise protocols for the generation of organoids that correctly mimic the human retina. Most protocols use common media supplements; however, protocol-dependent variability impacts data interpretation. To date, the lack of a systematic comparison of a given protocol with or without supplements makes it difficult to determine how they influence the differentiation process and morphology of the retinal organoids.

METHODS

A 2D-3D differentiation method was used to generate retinal organoids, which were cultured with or without the most commonly used media supplements, notably retinoic acid. Gene expression was assayed using qPCR analysis, protein expression using immunofluorescence studies, ultrastructure using electron microscopy and 3D morphology using confocal and biphoton microscopy of whole organoids.

RESULTS

Retinoic acid delayed the initial stages of differentiation by modulating photoreceptor gene expression. At later stages, the presence of retinoic acid led to the generation of mature retinal organoids with a well-structured stratified photoreceptor layer containing a predominant rod population. By contrast, the absence of retinoic acid led to cone-rich organoids with a less organised and non-stratified photoreceptor layer.

CONCLUSIONS

This study proves the importance of supplemented media for culturing retinal organoids. More importantly, we demonstrate for the first time that the role of retinoic acid goes beyond inducing a rod cell fate to enhancing the organisation of the photoreceptor layer of the mature organoid.

Timed Notch Inhibition Drives Photoreceptor Fate Specification in Human Retinal Organoids

Purpose

Transplanting photoreceptors from human pluripotent stem cell-derived retinal organoids have the potential to reverse vision loss in affected individuals. However, transplantable photoreceptors are only a subset of all cells in the organoids. Hence, the goal of our current study was to accelerate and synchronize photoreceptor differentiation in retinal organoids by inhibiting the Notch signaling pathway at different developmental time-points using a small molecule, PF-03084014 (PF).

Methods

Human induced pluripotent stem cell- and human embryonic stem cells-derived retinal organoids were treated with 10 µM PF for 3 days starting at day 45 (D45), D60, D90, and D120 of differentiation. Organoids were collected at post-treatment days 14, 28, and 42 and analyzed for progenitor and photoreceptor markers and Notch pathway inhibition by immunohistochemistry (IHC), quantitative PCR, and bulk RNA sequencing (n = 3-5 organoids from three independent experiments).

Results

Retinal organoids collected after treatment showed a decrease in progenitor markers (KI67, VSX2, PAX6, and LHX2) and an increase in differentiated pan-photoreceptor markers (OTX2, CRX, and RCVRN) at all organoid stages except D120. PF-treated organoids at D45 and D60 exhibited an increase in cone photoreceptor markers (RXRG and ARR3). PF treatment at D90 revealed an increase in cone and rod photoreceptors markers (ARR3, NRL, and NR2E3). Bulk RNA sequencing analysis mirrored the immunohistochemistry data and quantitative PCR confirmed Notch effector inhibition.

Conclusions

Timing the Notch pathway inhibition in human retinal organoids to align with progenitor competency stages can yield an enriched population of early cone or rod photoreceptors.

Advances in cell therapies using stem cells/progenitors as a novel approach for neurovascular repair of the diabetic retina

BACKGROUND

Diabetic retinopathy, a major complication of diabetes mellitus, is a leading cause of sigh-loss in working age adults. Progressive loss of integrity of the retinal neurovascular unit is a central element in the disease pathogenesis. Retinal ischemia and inflammatory processes drive interrelated pathologies such as blood retinal barrier disruption, fluid accumulation, gliosis, neuronal loss and/or aberrant neovascularisation. Current treatment options are somewhat limited to late-stages of the disease where there is already significant damage to the retinal architecture arising from degenerative, edematous and proliferative pathology. New preventive and interventional treatments to target early vasodegenerative and neurodegenerative stages of the disease are needed to ensure avoidance of sight-loss.

MAIN BODY

Historically, diabetic retinopathy has been considered a primarily microvascular disease of the retina and clinically it is classified based on the presence and severity of vascular lesions. It is now known that neurodegeneration plays a significant role during the pathogenesis. Loss of neurons has been documented at early stages in pre-clinical models as well as in individuals with diabetes and, in some, even prior to the onset of clinically overt diabetic retinopathy. Recent studies suggest that some patients have a primarily neurodegenerative phenotype. Retinal pigment epithelial cells and the choroid are also affected during the disease pathogenesis and these tissues may also need to be addressed by new regenerative treatments. Most stem cell research for diabetic retinopathy to date has focused on addressing vasculopathy. Pre-clinical and clinical studies aiming to restore damaged vasculature using vasoactive progenitors including mesenchymal stromal/stem cells, adipose stem cells, CD34+ cells, endothelial colony forming cells and induced pluripotent stem cell derived endothelial cells are discussed in this review. Stem cells that could replace dying neurons such as retinal progenitor cells, pluripotent stem cell derived photoreceptors and ganglion cells as well as Müller stem cells are also discussed. Finally, challenges of stem cell therapies relevant to diabetic retinopathy are considered.

CONCLUSION

Stem cell therapies hold great potential to replace dying cells during early and even late stages of diabetic retinopathy. However, due to the presence of different phenotypes, selecting the most suitable stem cell product for individual patients will be crucial for successful treatment.

New tricks and emerging applications from contemporary azobenzene research

Azobenzenes have many faces. They are well-known as dyes, but most of all, azobenzenes are versatile photoswitchable molecules with powerful photochemical properties. Azobenzene photochemistry has been extensively studied for decades, but only relatively recently research has taken a steer towards applications, ranging from photonics and robotics to photobiology. In this perspective, after an overview of the recent trends in the molecular design of azobenzenes, we highlight three research areas where the azobenzene photoswitches may bring about promising technological innovations: chemical sensing, organic transistors, and cell signaling. Ingenious molecular designs have enabled versatile control of azobenzene photochemical properties, which has in turn facilitated the development of chemical sensors and photoswitchable organic transistors. Finally, the power of azobenzenes in biology is exemplified by vision restoration and photactivation of neural signaling. Although the selected examples reveal only some of the faces of azobenzenes, we expect the fields presented to develop rapidly in the near future, and that azobenzenes will play a central role in this development.

Self-Organization of the Retina during Eye Development, Retinal Regeneration In Vivo, and in Retinal 3D Organoids In Vitro

Self-organization is a process that ensures histogenesis of the eye retina. This highly intricate phenomenon is not sufficiently studied due to its biological complexity and genetic heterogeneity. The review aims to summarize the existing central theories and ideas for a better understanding of retinal self-organization, as well as to address various practical problems of retinal biomedicine. The phenomenon of self-organization is discussed in the spatiotemporal context and illustrated by key findings during vertebrate retina development in vivo and retinal regeneration in amphibians in situ. Described also are histotypic 3D structures obtained from the disaggregated retinal progenitor cells of birds and retinal 3D organoids derived from the mouse and human pluripotent stem cells. The review highlights integral parts of retinal development in these conditions. On the cellular level, these include competence, differentiation, proliferation, apoptosis, cooperative movements, and migration. On the physical level, the focus is on the mechanical properties of cell- and cell layer-derived forces and on the molecular level on factors responsible for gene regulation, such as transcription factors, signaling molecules, and epigenetic changes. Finally, the self-organization phenomenon is discussed as a basis for the production of retinal organoids, a promising model for a wide range of basic scientific and medical applications.

TCF7L2 promotes ER stress signaling in diabetic retinopathy

Diabetic retinopathy (DR) is one of the most common blindness in working-age adults. Transcription factor 7 like 2 (TCF7L2) is a susceptibility gene of DR, however, its roles in the pathogenesis of DR are still largely unknown. In this study, we found that TCF7L2 was mainly located in the cell nucleus of retinal ganglion cell layer (GCL) and inner nuclear layer (INL), while it was not expressed in the cell nucleus of retinal outer nuclear layer (ONL). Expression of TCF7L2 was significantly elevated in the retinas of db/db diabetic mice and oxygen-induced retinopathy (OIR) mice. Also, in Ad-hTCF7L2 treated hiPSCs-derived retinal progenitor cells (RPCs), activating transcription factor 6 (ATF6)-related endoplasmic reticulum (ER) stress signaling was remarkably activated. Moreover, knockdown of TCF7L2 significantly inhibited ATF6-related ER stress signaling. Furthermore, the data of endothelial permeability assay showed that RPCs pretreated with Ad-hTCF7L2 lead to enhanced monolayer permeability of human umbilical vein endothelial cells (HUVECs), and knockdown of TCF7L2 or ATF6 in RPCs could alleviate the monolayer permeability of HUVECs. Thus, our results showed that TCF7L2 could trigger ATF6-related ER stress signaling and promote vein endothelial cell permeability, which will provide important insight into the role of TCF7L2 in the pathogenesis of DR and contribute to designing potential therapies.

Bioelectric Potential in Next-Generation Organoids: Electrical Stimulation to Enhance 3D Structures of the Central Nervous System

Pluripotent stem cell-derived organoid models of the central nervous system represent one of the most exciting areas in in vitro tissue engineering. Classically, organoids of the brain, retina and spinal cord have been generated via recapitulation of in vivo developmental cues, including biochemical and biomechanical. However, a lesser studied cue, bioelectricity, has been shown to regulate central nervous system development and function. In particular, electrical stimulation of neural cells has generated some important phenotypes relating to development and differentiation. Emerging techniques in bioengineering and biomaterials utilise electrical stimulation using conductive polymers. However, state-of-the-art pluripotent stem cell technology has not yet merged with this exciting area of bioelectricity. Here, we discuss recent findings in the field of bioelectricity relating to the central nervous system, possible mechanisms, and how electrical stimulation may be utilised as a novel technique to engineer “next-generation” organoids.

Gene-independent therapeutic interventions to maintain and restore light sensitivity in degenerating photoreceptors

Neurodegenerative retinal diseases are a prime cause of blindness in industrialized countries. In many cases, there are no therapeutic treatments, although they are essential to improve patients' quality of life. A set of disease-causing genes, which primarily affect photoreceptors, has already been identified and is of major interest for developing gene therapies. Nevertheless, depending on the nature and the state of the disease, gene-independent strategies are needed. Various strategies to halt disease progression or maintain function of the retina are under research. These therapeutic interventions include neuroprotection, direct reprogramming of affected photoreceptors, the application of non-coding RNAs, the generation of artificial photoreceptors by optogenetics and cell replacement strategies. During recent years, major breakthroughs have been made such as the first optogenetic application to a blind patient whose visual function partially recovered by targeting retinal ganglion cells. Also, RPE cell transplantation therapies are under clinical investigation and show great promise to improve visual function in blind patients. These cells are generated from human stem cells. Similar therapies for replacing photoreceptors are extensively tested in pre-clinical models. This marks just the start of promising new cures taking advantage of developments in the areas of genetic engineering, optogenetics, and stem-cell research. In this review, we present the recent therapeutic advances of gene-independent approaches that are currently under clinical evaluation. Our main focus is on photoreceptors as these sensory cells are highly vulnerable to degenerative diseases, and are crucial for light detection.

In vivo study to assess dosage of allogeneic pig retinal progenitor cells: Long-term survival, engraftment, differentiation and safety

Despite notable efforts and significant therapeutical advances, age‐related macular degeneration remains the single most common reason for vision loss. Retinal progenitor cells (RPCs) are considered promising candidates for cellular treatments that repair and restore vision. In this allogenic study, the phenotypic profile of pig and human RPCs derived using similar manufacturing processes is compared. The long‐term (12‐week) survival of green fluorescent protein‐pig retinal progenitor cells GFP‐pRPC after subretinal transplantation into normal miniature pig (mini‐pig) retina is investigated. Human eyes are both anatomically and physiologically mimicked by pig eyes, so the pig is an ideal model to show an equivalent way of delivering cells, immunological response and dosage. The phenotypic equivalency of porcine and clinically intended human RPCs was established. Thirty‐nine mini‐pigs are used in this study, and vehicle‐injected eyes and non‐injected eyes serve as controls. Six groups are given different dosages of pRPCs, and the cells are found to survive well in all groups. At 12 weeks, strong evidence of integration is indicated by the location of the grafted cells within the neuro‐retina, extension of processes to the plexiform layers and expression of key retinal markers such as recoverin, rhodopsin and synaptophysin. No immunosuppression is used, and no immune response is found in any of the groups. No pRPC‐related histopathology findings are reported in the major organs investigated. An initial dose of 250 k cells in 100 µl of buffer is established as an appropriate initial dose for future human clinical trials.

Generation of 3D retinal tissue from human pluripotent stem cells using a directed small molecule-based serum-free microwell platform

Retinal degenerative diseases are a leading cause of blindness worldwide with debilitating life-long consequences for the affected individuals. Cell therapy is considered a potential future clinical intervention to restore and preserve sight by replacing lost photoreceptors and/or retinal pigment epithelium. Development of protocols to generate retinal tissue from human pluripotent stem cells (hPSC), reliably and at scale, can provide a platform to generate photoreceptors for cell therapy and to model retinal disease in vitro. Here, we describe an improved differentiation platform to generate retinal organoids from hPSC at scale and free from time-consuming manual microdissection steps. The scale up was achieved using an agarose mould platform enabling generation of uniform self-assembled 3D spheres from dissociated hPSC in microwells. Subsequent retinal differentiation was efficiently achieved via a stepwise differentiation protocol using a number of small molecules. To facilitate clinical translation, xeno-free approaches were developed by substituting Matrigel™ and foetal bovine serum with recombinant laminin and human platelet lysate, respectively. Generated retinal organoids exhibited important features reminiscent of retinal tissue including correct site-specific localisation of proteins involved in phototransduction.

Retinal Organoids and Retinal Prostheses: An Overview

Despite the progress of modern medicine in the last decades, millions of people diagnosed with retinal dystrophies (RDs), such as retinitis pigmentosa, or age-related diseases, such as age-related macular degeneration, are suffering from severe visual impairment or even legal blindness. On the one hand, the reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) and the progress of three-dimensional (3D) retinal organoids (ROs) technology provide a great opportunity to study, understand, and even treat retinal diseases. On the other hand, research advances in the field of electronic retinal prosthesis using inorganic photovoltaic polymers and the emergence of organic semiconductors represent an encouraging therapeutical strategy to restore vision to patients at the late onset of the disease. This review will provide an overview of the latest advancement in both fields. We first describe the retina and the photoreceptors, briefly mention the most used RD animal models, then focus on the latest RO differentiation protocols, carry out an overview of the current technology on inorganic and organic retinal prostheses to restore vision, and finally summarize the potential utility and applications of ROs.

Role of mTORC1 activity during early retinal development and lamination in human-induced pluripotent stem cell-derived retinal organoids

Retinal organoids derived from human-induced pluripotent stem cells (hiPSC) are powerful tools for studying retinal development as they model spatial and temporal differentiation of retinal cell types. Vertebrate retinal development involves a delicate and coordinated process of retinal progenitor cell (RPC) differentiation, and the mammalian target of rapamycin complex 1 (mTORC1) has been reported to play a significant role in this complex process. Herein, using hiPSC-derived retinal organoids, we identify the time-dependent role of mTORC1 in retinal development, specifically in retinal ganglion cell (RGC) differentiation and the retinal lamination process, during the early stages of retinal organoid (RO) development. mTORC1 activity in ROs was the highest at 40 days of differentiation. MHY1485-induced hyperactivation of mTORC1 during this period resulted in a significant increase in the overall size of ROs compared to the untreated controls and rapamycin-treated Ros; there was also a marked increase in proliferative activity within the inner and outer layers of ROs. Moreover, the MHY1485-treated ROs showed a significant increase in the number of ectopic RGCs in the outer layers (indicating disruption of retinal laminar structure), with robust expression of HuC/D-binding proteins in the inner layers. These results demonstrate that mTORC1 plays a critical role in the development of hiPSC-derived ROs, especially during the early stages of differentiation.

Cadaveric Stem Cells: Their Research Potential and Limitations

In the era of growing interest in stem cells, the availability of donors for transplantation has become a problem. The isolation of embryonic and fetal cells raises ethical controversies, and the number of adult donors is deficient. Stem cells isolated from deceased donors, known as cadaveric stem cells (CaSCs), may alleviate this problem. So far, it was possible to isolate from deceased donors mesenchymal stem cells (MSCs), adipose delivered stem cells (ADSCs), neural stem cells (NSCs), retinal progenitor cells (RPCs), induced pluripotent stem cells (iPSCs), and hematopoietic stem cells (HSCs). Recent studies have shown that it is possible to collect and use CaSCs from cadavers, even these with an extended postmortem interval (PMI) provided proper storage conditions (like cadaver heparinization or liquid nitrogen storage) are maintained. The presented review summarizes the latest research on CaSCs and their current therapeutic applications. It describes the developments in thanatotranscriptome and scaffolding for cadaver cells, summarizes their potential applications in regenerative medicine, and lists their limitations, such as donor’s unknown medical condition in criminal cases, limited differentiation potential, higher risk of carcinogenesis, or changing DNA quality. Finally, the review underlines the need to develop procedures determining the safe CaSCs harvesting and use.

Retina stem cells, hopes and obstacles

Retinal degeneration is a major contributor to visual dysfunction worldwide. Although it comprises several eye diseases, loss of retinal pigment epithelial (RPE) and photoreceptor cells are the major contributors to their pathogenesis. Early therapies included diverse treatments, such as provision of anti-vascular endothelial growth factor and many survival and trophic factors that, in some cases, slow down the progression of the degeneration, but do not effectively prevent it. The finding of stem cells (SC) in the eye has led to the proposal of cell replacement strategies for retina degeneration. Therapies using different types of SC, such as retinal progenitor cells (RPCs), embryonic SC, pluripotent SCs (PSCs), induced PSCs (iPSCs), and mesenchymal stromal cells, capable of self-renewal and of differentiating into multiple cell types, have gained ample support. Numerous preclinical studies have assessed transplantation of SC in animal models, with encouraging results. The aim of this work is to revise the different preclinical and clinical approaches, analyzing the SC type used, their efficacy, safety, cell attachment and integration, absence of tumor formation and immunorejection, in order to establish which were the most relevant and successful. In addition, we examine the questions and concerns still open in the field. The data demonstrate the existence of two main approaches, aimed at replacing either RPE cells or photoreceptors. Emerging evidence suggests that RPCs and iPSC are the best candidates, presenting no ethical concerns and a low risk of immunorejection. Clinical trials have already supported the safety and efficacy of SC treatments. Serious concerns are pending, such as the risk of tumor formation, lack of attachment or integration of transplanted cells into host retinas, immunorejection, cell death, and also ethical. However, the amazing progress in the field in the last few years makes it possible to envisage safe and effective treatments to restore vision loss in a near future.