A Genetic modification that reduces ON-bipolar cells in hESC-derived retinas enhances functional integration after transplantation

hPSC-based treatment of retinal diseases - Current progress and challenges

Degenerative retinal diseases, such as age-related macular degeneration (AMD) and inherited retinal diseases (IRDs), cause visual impairment due to irreversible damage to the retinal pigment epithelium (RPE) and photoreceptor cells (PRCs). Currently, no definitive treatment exists. However, cell-based therapies using induced pluripotent stem cells (iPSCs) or embryonic stem cells (ESCs) offer potential solutions for restoring damaged retinal cells. This review summarizes recent advances in RPE and PRC transplantation, highlighting the benefits of each approach. For RPE transplantation, we focus on the outcomes of clinical studies involving three formulations: RPE sheets, RPE suspensions, and RPE strips. In the context of PRC transplantation, we trace the progress from fetal retinal transplantation to the latest studies. Additionally, we discuss our recent clinical work with retinal sheet transplantation and genome-edited retinal organoid sheets, which aim to improve functional integration by reducing bipolar cells in grafts. Finally, with the overall safety of the regenerative cell-based therapies demonstrated in past clinical applications, we explore future prospects for these therapies.

Evaluating the outcomes of pluripotent stem-cell-derived photoreceptor transplantation in retinal repair

In recent decades, numerous research groups have focused on restoring visual function through the transplantation of stem cells into animal models of retinal neurodegeneration. Significant advancements in surgical techniques, the maturation of donor cells, and the production of cell suspensions, along with ensuring proper synaptic connectivity with the host environment, are key considerations for the potential implementation of this strategy in clinical practice. In this review, we summarize the latest progress in the transplantation of stem cell-derived photoreceptors, emphasizing the outcomes related to visual function observed in the used animal models. Additionally, we analyze the various methods of stem cell differentiation and the surgical techniques selected for transplanting these photoreceptor precursors. Finally, we report on functional assessments from recent studies to highlight the considerable potential of stem cell-derived photoreceptor transplants as a therapeutic approach for retinal degenerative diseases.

Animal models for the evaluation of retinal stem cell therapies

Retinal degeneration (RD) diseases leading to severe vision loss can affect photoreceptors (PRs) that are responsible for phototransduction, or retinal pigmented epithelium (RPE) providing support for PRs. Human pluripotent stem cell (hPSC)-based therapies are a potential approach for restoration of retinal structure in patients with currently incurable RD diseases. Currently, there are two targeted hPSC therapeutics: PR rescue and PR replacement. PR rescue involves the transplantation of RPE or other neural progenitors into the subretinal space to slow down or prevent further RD. RPE transplantation plays a critical role in preserving photoreceptors by providing trophic support and maintaining retinal integrity, particularly in diseases like age-related macular degeneration (AMD). Advances in RPE transplantation methods, such as polarized monolayer cultures and scaffold-based approaches, have shown promise in enhancing graft survival and integration. However, limitations include inconsistent integration, variable neurotrophic factor secretion, and immune rejection risks in non-autologous transplants. In PR replacement, stem cell-derived photoreceptor-like cells or photoreceptor progenitors (PRP) obtained are transplanted into the eye. While PRPs are commonly obtained from retinal organoids (ROs), alternative sources, such as early differentiation stages or direct differentiation protocols, are also utilized to enhance the efficiency and scalability of PRP generation. Challenges include achieving proper integration, forming outer segments, rosette formation, and avoiding immune rejection or tumorigenicity. Various animal models that simulate human RD diseases are being used for establishing surgical feasibility, graft survival and visual functional recovery but fail to replicate clinical immune challenges. Rodent models lack macula-like structures and have limited reliability in detecting subtle functional changes, while larger animal models pose ethical, logistical, and financial challenges. Immunocompromised models have been developed for minimizing xenograft issues. Visual functional testing for efficacy includes optokinetic testing (OKN), electroretinography (ERG), and electrophysiological recordings from the retina and brain. These tests often fail to capture the complexity of human visual recovery, highlighting the need for advanced models and improved functional testing techniques. This review aims to aggregate current knowledge about approaches to stem cell transplantation, requirements of animal models chosen for validating vision benefits of transplantation studies, advantages of using specific disease models and their limitations. While promising strides have been made, addressing these limitations remains essential for translating stem cell-based therapies into clinical success.

Transplantation of genome-edited retinal organoids restores some fundamental physiological functions coordinated with severely degenerated host retinas

We have previously shown that the transplantation of stem cell-derived retinal organoid (RO) sheets into animal models of end-stage retinal degeneration can lead to host-graft synaptic connectivity and restoration of vision, which was further improved using genome-edited Islet1-/- ROs (gROs) with a reduced number of ON-bipolar cells. However, the details of visual function restoration using this regenerative therapeutic approach have not yet been characterized. Here, we evaluated the electrophysiological properties of end-stage rd1 retinas after transplantation (TP-rd1) and compared them with those of wild-type (WT) retinas using multi-electrode arrays. Notably, retinal ganglion cells (RGCs) in TP-rd1 retinas acquired light sensitivity comparable to that of WT retinas. Furthermore, RGCs in TP-rd1 retinas showed light adaptation to a photopic background and responded to flickering stimuli. These results demonstrate that transplantation of gRO sheets may restore some fundamental physiological functions, possibly coordinating with the remaining functions in retinas with end-stage degeneration.

Progress in photoreceptor replacement therapy for retinal degenerative diseases

Retinal degenerative diseases encompass a diverse range of eye conditions that result in blindness, many due to photoreceptor dysfunction and loss. Regrettably, current clinical treatments are frequently not overly effective. However, photoreceptor transplantation shows promise as a potential therapy for late-stage retinal degenerative diseases. This article will review the various donor cell sources for this transplantation, as well as the mechanisms and factors that impact donor cell integration and material transfer, donor cell maturation, and other auxiliary methods that can be combined with photoreceptor transplantation to treat these degenerative retinal diseases.

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.

Cell-cell interactions between transplanted retinal organoid cells and recipient tissues

The transplantation of human organoid-derived retinal cells is being studied as a potentially viable strategy to treat vision loss due to retinal degeneration. Experiments in animal models have demonstrated the feasibility of organoid-derived photoreceptor transplantation in various recipient contexts. In some cases, vision repair has been shown. However, recipient-donor cell-cell interactions are incompletely understood. This review briefly summarizes these interactions, categorizing them as synaptic structure formation, cellular component transfer, glial activation, immune cell infiltration, and cellular migration. Each of these interactions may affect the survival and functionality of the donor cells and, ultimately, their efficacy as a treatment substrate. Additionally, recipient characteristics, such as the cytoarchitecture of the retina and immune status, may also impact the type and frequency of cell-cell interactions. Despite the procedural challenges associated with culturing human retinal organoids and the technical difficulties in transplanting donor cells into the delicate recipient retina, transplantation of retinal organoid-derived cells is a promising tool for degenerative retinal disease treatment.

Transplantation of human pluripotent stem cell-derived retinal sheet in a primate model of macular hole

Macular hole (MH) is a retinal break involving the fovea that causes impaired vision. Although advances in vitreoretinal surgical techniques achieve >90% MH closure rate, refractory cases still exist. For such cases, autologous retinal transplantation is an optional therapy showing good anatomic success, but visual improvement is limited and peripheral visual field defects are inevitable after graft harvesting. Here, using a non-human primate model, we evaluated whether human embryonic stem cell-derived retinal organoid (RO) sheet transplantation can be an effective option for treating MH. After transplantation, MH was successfully closed by continuous filling of the MH space with the RO sheet, resulting in improved visual function, although no host-graft synaptic connections were confirmed. Mild xeno-transplantation rejection was controlled by additional focal steroid injections and rod/cone photoreceptors developed in the graft. Overall, our findings suggest pluripotent stem cell-derived RO sheet transplantation as a practical option for refractory MH treatment.

Cellular and circuit remodeling of the primate foveal midget pathway after acute photoreceptor loss

The retinal fovea in human and nonhuman primates is essential for high acuity and color vision. Within the fovea lies specialized circuitry in which signals from a single cone photoreceptor are largely conveyed to one ON and one OFF type midget bipolar cell (MBC), which in turn connect to a single ON or OFF midget ganglion cell (MGC), respectively. Restoring foveal vision requires not only photoreceptor replacement but also appropriate reconnection with surviving ON and OFF MBCs and MGCs. However, our current understanding of the effects of cone loss on the remaining foveal midget pathway is limited. We thus used serial block-face electron microscopy to determine the degree of plasticity and potential remodeling of this pathway in adult Macaca fascicularis several months after acute photoreceptor loss upon photocoagulation. We reconstructed MBC structure and connectivity within and adjacent to the region of cone loss. We found that MBC dendrites within the scotoma retracted and failed to reach surviving cones to form new connections. However, both surviving cones and ON and OFF MBC dendrites at the scotoma border exhibited remodeling, suggesting that these neurons can demonstrate plasticity and rewiring at maturity. At six months postlesion, disconnected OFF MBCs clearly lost output ribbon synapses with their postsynaptic partners, whereas the majority of ON MBCs maintained their axonal ribbon numbers, suggesting differential timing or extent in ON and OFF midget circuit remodeling after cone loss. Our findings raise rewiring considerations for cell replacement approaches in the restoration of foveal vision.

Auditory hair cells and spiral ganglion neurons regenerate synapses with refined release properties in vitro

Ribbon synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs) in the inner ear are damaged by noise trauma and with aging, causing "synaptopathy" and hearing loss. Cocultures of neonatal denervated organs of Corti and newly introduced SGNs have been developed to find strategies for improving IHC synapse regeneration, but evidence of the physiological normality of regenerated synapses is missing. This study utilizes IHC optogenetic stimulation and SGN recordings, showing that, when P3-5 denervated organs of Corti are cocultured with SGNs, newly formed IHC/SGN synapses are indeed functional, exhibiting glutamatergic excitatory postsynaptic currents. When using older organs of Corti at P10-11, synaptic activity probed by deconvolution showed more mature release properties, closer to the specialized mode of IHC synaptic transmission crucial for coding the sound signal. This functional assessment of newly formed IHC synapses developed here, provides a powerful tool for testing approaches to improve synapse regeneration.

Acoustic vibration promotes in vitro expansion of human embryonic stem cells

OBJECTIVES

This study aimed to investigate the effect of acoustic vibration on the pluripotency of human embryonic stem cells (hESCs) and evaluate cell proliferation and self-renewal ability post-treatment.

METHODS

The human ES cell line H1 was used for the experiments. hESCs were treated with an acoustic vibration device. Their proliferative ability was subsequently detected using a colony formation assay, while the expression of pluripotency-related markers was detected via immunofluorescence staining. Finally, changes in gene expression levels were examined using quantitative polymerase chain reaction (qPCR) in the presence of appropriate primers.

RESULTS

Compared with normal cells in the control group, the morphology of experimental cells subjected to acoustic vibration did not significantly change. Contrastingly, the colony-forming efficiency of the experimental cells significantly increased. Immunofluorescence staining results showed the cells in experimental group were positive for the pluripotency markers NANOG, octamer-binding transcription factor 4 gene (OCT4), and SRY (sex determining region Y)-box 2 (SOX2). In addition, the expression levels of pluripotency genes NANOG, OCT4, SOX2, and Yes-associated protein (YAP)-related genes were up-regulated following acoustic vibration.

CONCLUSIONS

Our results revealed that acoustic vibration enhanced the proliferative ability of hESCs and increased the expression levels of NANOG, OCT4, SOX2, and YAP-related genes, indicating that acoustic vibration can optimize the self-renewal ability of hESCs and that the YAP signaling pathway may play a critical role in the functional process of acoustic vibration.

Status and prospects for the development of regenerative therapies for corneal and ocular diseases

Among the regenerative therapies being put into clinical use, the field of corneal regenerative therapy is one of the most advanced, with several regulatory approved products. This article describes the progress from initial development through to clinical application in the eye field, with a particular focus on therapies for corneal epithelial and endothelial diseases that have already been regulatory approved as regenerative therapy products. The applications of regenerative therapy to the corneal epithelium were attempted and confirmed earlier than other parts of the cornea, following advancements in basic research on corneal epithelial stem cells. Based on these advances, four regenerative therapy products for corneal epithelial disease, each employing distinct cell sources and culture techniques, have been commercialized since the regulatory approval of Holoclar® in Italy as a regenerative therapy product for corneal epithelial disease in 2015. Corneal endothelial regenerative therapy was started by the development of an in vitro method to expand corneal endothelial cells which do not proliferate in adults. The product was approved in Japan as Vyznova® in 2023. The development of regenerative therapies for retinal and ocular surface diseases is actively being pursued, and these therapies use somatic stem cells and pluripotent stem cells (PSCs), especially induced pluripotent stem cells (iPSCs). Accordingly, the eye field is anticipated to play a pioneering role in regenerative therapy development going forward.

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

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

Safety and stable survival of stem-cell-derived retinal organoid for 2 years in patients with retinitis pigmentosa

Transplantation of induced pluripotent stem cell (iPSC)-derived retinal organoids into retinal disease animal models has yielded promising results, and several clinical trials on iPSC-derived retinal pigment epithelial cell transplantation have confirmed its safety. In this study, we performed allogeneic iPSC-derived retinal organoid sheet transplantation in two subjects with advanced retinitis pigmentosa (jRCTa050200027). The primary endpoint was the survival and safety of the transplanted retinal organoid sheets in the first year post-transplantation. The secondary endpoints were the safety of the transplantation procedure and visual function evaluation. The grafts survived in a stable condition for 2 years, and the retinal thickness increased at the transplant site without serious adverse events in both subjects. Changes in visual function were less progressive than those of the untreated eye during the follow-up. Allogeneic iPSC-derived retinal organoid sheet transplantation is a potential therapeutic approach, and the treatment's safety and efficacy for visual function should be investigated further.

Auditory Hair Cells and Spiral Ganglion Neurons Regenerate Synapses with Refined Release Properties In Vitro

Ribbon synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs) in the inner ear are damaged by noise trauma and with aging, causing 'synaptopathy 'and hearing loss. Co-cultures of neonatal denervated organs of Corti and newly introduced SGNs have been developed to find strategies for improving IHC synapse regeneration, but evidence of the physiological normality of regenerated synapses is missing. This study utilizes IHC optogenetic stimulation and SGN recordings, showing that newly formed IHC synapses are indeed functional, exhibiting glutamatergic excitatory postsynaptic currents. When older organs of Corti were plated, synaptic activity probed by deconvolution, showed more mature release properties, closer to the highly specialized mode of IHC synaptic transmission that is crucial for coding the sound signal. This newly developed functional assessment of regenerated IHC synapses provides a powerful tool for testing approaches to improve synapse regeneration.

Application of patient-derived induced pluripotent stem cells and organoids in inherited retinal diseases

Inherited retinal diseases (IRDs) can induce severe sight-threatening retinal degeneration and impose a considerable economic burden on patients and society, making efforts to cure blindness imperative. Transgenic animals mimicking human genetic diseases have long been used as a primary research tool to decipher the underlying pathogenesis, but there are still some obvious limitations. As an alternative strategy, patient-derived induced pluripotent stem cells (iPSCs), particularly three-dimensional (3D) organoid technology, are considered a promising platform for modeling different forms of IRDs, including retinitis pigmentosa, Leber congenital amaurosis, X-linked recessive retinoschisis, Batten disease, achromatopsia, and best vitelliform macular dystrophy. Here, this paper focuses on the status of patient-derived iPSCs and organoids in IRDs in recent years concerning disease modeling and therapeutic exploration, along with potential challenges for translating laboratory research to clinical application. Finally, the importance of human iPSCs and organoids in combination with emerging technologies such as multi-omics integration analysis, 3D bioprinting, or microfluidic chip platform are highlighted. Patient-derived retinal organoids may be a preferred choice for more accurately uncovering the mechanisms of human retinal diseases and will contribute to clinical practice.

Single-cell transcriptome analysis of xenotransplanted human retinal organoids defines two migratory cell populations of nonretinal origin

Human retinal organoid transplantation could potentially be a treatment for degenerative retinal diseases. How the recipient retina regulates the survival, maturation, and proliferation of transplanted organoid cells is unknown. We transplanted human retinal organoid-derived cells into photoreceptor-deficient mice and conducted histology and single-cell RNA sequencing alongside time-matched cultured retinal organoids. Unexpectedly, we observed human cells that migrated into all recipient retinal layers and traveled long distances. Using an unbiased approach, we identified these cells as astrocytes and brain/spinal cord-like neural precursors that were absent or rare in stage-matched cultured organoids. In contrast, retinal progenitor-derived rods and cones remained in the subretinal space, maturing more rapidly than those in the cultured controls. These data suggest that recipient microenvironment promotes the maturation of transplanted photoreceptors while inducing or facilitating the survival of migratory cell populations that are not normally derived from retinal progenitors. These findings have important implications for potential cell-based treatments of retinal diseases.

Progress of iPS cell-based transplantation therapy for retinal diseases

The discovery of induced Pluripotent Stem) (iPS) cells has instigated innovation in various fields, including ophthalmology. Cell therapy has shown tremendous progress in translational research on retinal diseases, including the first-in-human transplantation of autologous iPS cell-derived retinal pigment epithelium (RPE) cells for patients with age-related macular degeneration (AMD). Cell therapy for retinitis pigmentosa (RP) has also been developed. Retinal organoid and photoreceptor cell transplantation has been shown to incorporate into the degenerated host retina, forming synapses with host neurons and resulting in functional recovery. Based on preclinical data, first-in-human transplantation of iPS cell-derived retinal sheets has been conducted. In this review, we summarize the current progress in iPS cell-based retinal cell transplantation research for retinal diseases, addressing some remaining challenges and future prospects.

Pluripotent stem cell-derived retinal organoid/cells for retinal regeneration therapies: A review

In recent decades, many researchers have attempted to restore vision via transplantation of retina/retinal cells in eyes with retinal degeneration. The advent of induced pluripotent stem cells (iPSC) and retinal organoid induction technologies has boosted research on retinal regeneration therapy. Although the recognition of functional integration of graft photoreceptor cells in the host retina from 2006 has been disputed a decade later by the newly evidenced phenomenon denoted as "material transfer," several reports support possible reconstruction of the host-graft network in the retinas of both end-stage degeneration and in progressing degeneration cases. Based on proof of concept (POC) studies in animal models, a clinical study was conducted in Kobe, Japan in 2020 and showed the feasibility of cell-based therapy using iPSC retinal organoid technology. Although the graft potency of human embryonic stem (ES)/iPS cell-derived retinal organoid/retinal cells has been suggested by previous studies, much is still unknown regarding host capability, that is, how long-standing human degenerating retinas are capable of rewiring with transplanted cells. This review summarizes past POC studies on photoreceptor replacement therapy and introduces some new challenges to maximize the possible efficacy in future human clinical studies of regenerative therapy.

Mesenchymal Stem Cell Derived Extracellular Vesicles: Promising Nanomedicine for Cutaneous Wound Treatment

A skin wound represents a rupture caused by external damage or the existence of underlying pathological conditions. Sometimes, skin wound healing processes may place a heavy burden on patients, families, and society. Wound healing processes mainly consist of several continuous, dynamic, but overlapping stages, namely, the coagulation stage, inflammation stage, proliferation stage, and remodeling stage. Bacterial infection, excessive inflammation, impaired angiogenesis, and scar formation constitute the four significant factors impeding the recovery efficacy of skin wounds. This encourages scientists to develop multifunctional nanomedicines to meet challenging needs. As we know, mesenchymal stem cells (MSCs) have been widely explored for wound repair owing to their unique capability for self-renewal and multipotency. However, problems including immune concerns and legal restrictions should be properly resolved before MSC-based therapeutics are safely and widely used in clinics. Besides, maintaining the high viability/proliferation capability of MSCs during administration processes and therapy procedures is also one of the biggest technical bottlenecks. Extracellular vesicles (EVs) are cell-derived nanovesicles, that not only possess the basic characteristics and functions of their corresponding maternal cells but also contain several outstanding advantages including abundant sources, excellent biocompatibility, and convenient administration routes. Furthermore, the membrane surface and cavity are easy to flexibly modify to meet versatile application needs. Recently, MSC-derived EVs have emerged as promising therapeutics for skin wound repair. However, current reviews are too broad and rarely focused on the specific roles of EVs in the different stages of wound recovery. Therefore, it is quite necessary to demonstrate the significance of stem cell-derived EVs in promoting wound healing from several specific aspects. Here, this review primarily tries to provide critical comments on current advances in EVs derived from MSCs for wound repair, particularly elaborating on their impressive roles in effectively eliminating infections, inhibiting inflammation, promoting angiogenesis, and reducing scar formation. Last but not least, current limitations and future prospects of EVs derived from MSCs in the areas of wound repair are also objectively analyzed.

Photoreceptor Cell Replacement Using Pluripotent Stem Cells: Current Knowledge and Remaining Questions

Retinal degeneration is an increasing global burden without cure for the majority of patients. Once retinal cells have degenerated, vision is permanently lost. Different strategies have been developed in recent years to prevent retinal degeneration or to restore sight (e.g., gene therapy, cell therapy, and electronic implants). Herein, we present current treatment strategies with a focus on cell therapy for photoreceptor replacement using human pluripotent stem cells. We will describe the state of the art and discuss obstacles and limitations observed in preclinical animal models as well as future directions to improve graft integration and functionality.

Competency of iPSC-derived retinas in MHC-mismatched transplantation in non-human primates

Transplantation of embryonic/induced pluripotent stem cell-derived retina (ESC/iPSC-retina) restores host retinal ganglion cell light responses in end-stage retinal degeneration models with host-graft synapse formation. We studied the immunological features of iPSC-retina transplantation using major histocompatibility complex (MHC)-homozygote monkey iPSC-retinas in monkeys with laser-induced retinal degeneration in MHC-matched and -mismatched transplantation. MHC-mismatched transplantation without immune suppression showed no evident clinical signs of rejection and histologically showed graft maturation without lymphocytic infiltration, although immunological tests using peripheral blood monocytes suggested subclinical rejection in three of four MHC-mismatched monkeys. Although extensive photoreceptor rosette formation was observed on histology, evaluation of functional integration using mouse models such as mouse ESC-retina (C57BL/6) transplanted into rd1(C3H/HeJ, MHC-mismatched model) elicited light responses in the host retinal ganglion cells after transplantation but with less responsiveness than that in rd1-2J mice (C57BL/6, MHC-matched model). These results suggest the reasonable use of ESC/iPSC-retina in MHC-mismatched transplantation, albeit with caution.

The Prospects for Retinal Organoids in Treatment of Retinal Diseases

Retinal degeneration (RD) is a significant cause of incurable blindness worldwide. Photoreceptors and retinal pigmented epithelium are irreversibly damaged in advanced RD. Functional replacement of photoreceptors and/or retinal pigmented epithelium cells is a promising approach to restoring vision. This paper reviews the current status and explores future prospects of the transplantation therapy provided by pluripotent stem cell-derived retinal organoids (ROs). This review summarizes the status of rodent RD disease models and discusses RO culture and analytical tools to evaluate RO quality and function. Finally, we review and discuss the studies in which RO-derived cells or sheets were transplanted. In conclusion, methods to derive ROs from pluripotent stem cells have significantly improved and become more efficient in recent years. Meanwhile, more novel technologies are applied to characterize and validate RO quality. However, opportunity remains to optimize tissue differentiation protocols and achieve better RO reproducibility. In order to screen high-quality ROs for downstream applications, approaches such as noninvasive and label-free imaging and electrophysiological functional testing are promising and worth further investigation. Lastly, transplanted RO-derived tissues have allowed improvements in visual function in several RD models, showing promises for clinical applications in the future.

Transplanted human cones incorporate and function in a murine cone degeneration model

Once human photoreceptors die, they do not regenerate, thus, photoreceptor transplantation has emerged as a potential treatment approach for blinding diseases. Improvements in transplant organization, donor cell maturation, and synaptic connectivity to the host will be critical in advancing this technology for use in clinical practice. Unlike the unstructured grafts of prior cell-suspension transplantations into end-stage degeneration models, we describe the extensive incorporation of induced pluripotent stem cell (iPSC) retinal organoid–derived human photoreceptors into mice with cone dysfunction. This incorporative phenotype was validated in both cone-only as well as pan-photoreceptor transplantations. Rather than forming a glial barrier, Müller cells extended throughout the graft, even forming a series of adherens junctions between mouse and human cells, reminiscent of an outer limiting membrane. Donor-host interaction appeared to promote polarization as well as the development of morphological features critical for light detection, namely the formation of inner and well-stacked outer segments oriented toward the retinal pigment epithelium. Putative synapse formation and graft function were evident at both structural and electrophysiological levels. Overall, these results show that human photoreceptors interacted readily with a partially degenerated retina. Moreover, incorporation into the host retina appeared to be beneficial to graft maturation, polarization, and function.