Three-dimensional neuroepithelial culture from human embryonic stem cells and its use for quantitative conversion to retinal pigment epithelium

Production of Neuroepithelial Organoids from Human-Induced Pluripotent Stem Cells for Mimicking Early Neural Tube Development

Organoids have emerged as robust tools for unravelling the mechanisms that underly tissue development. They also serve as important in vitro systems for studying fundamentals of stem cell behavior and for building advanced disease models. During early development, a crucial step in the formation of the central nervous system is patterning of the neural tube dorsal-ventral (DV) axis. Here we describe a simple and rapid culture protocol to produce human neuroepithelial (NE) cysts and DV-patterned organoids from single human-induced pluripotent stem cells (hiPSCs). Rather than being embedded within a matrix, hiPSCs undergo a 5-day differentiation process in medium containing soluble extracellular matrix and are allowed to self-organize into 3D cysts with defined central lumen structures that express early neuroepithelial markers. Moreover, upon stimulation with sonic hedgehog proteins and all-trans retinoic acid, NE cysts further develop into NE organoids with DV patterning. This rapid generation of patterned NE organoids using simple culture conditions enables mimicking, monitoring, and longitudinal manipulation of NE cell behavior. This straightforward culture system makes NE organoids a tractable model for studying neural stem cell self-organization and early neural tube developmental events.

Retinal Organoids: A Next-Generation Platform for High-Throughput Drug Discovery

Retinal diseases are leading causes of blindness globally. Developing new drugs is of great significance for preventing vision loss. Current drug discovery relies mainly on two-dimensional in vitro models and animal models, but translation to human efficacy and safety is biased. In recent years, the emergence of retinal organoid technology platforms, utilizing three-dimensional microenvironments to better mimic retinal structure and function, has provided new platforms for exploring pathogenic mechanisms and drug screening. This review summarizes the latest advances in retinal organoid technology, emphasizing its application advantages in high-throughput drug screening, efficacy and toxicity evaluation, and translational medicine research. The review also prospects the combination of emerging technologies such as organ-on-a-chip, 3D bioprinting, single cell sequencing, gene editing with retinal organoid technology, which is expected to further optimize retinal organoid models and advance the diagnosis and treatment of retinal diseases.

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

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

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ VSX2+ optic-disc cells. Single-cell RNA sequencing of these organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in these organoids differentially expressed FGF8 and FGF9; FGFR inhibitions drastically decreased early RGC differentiation and directional axon growth. Through the RGC-specific cell-surface marker CNTN2 identified here, electrophysiologically excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

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.

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ VSX2+ optic-disc cells. Single-cell RNA sequencing of these organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in these organoids differentially expressed FGF8 and FGF9; FGFR inhibitions drastically decreased early RGC differentiation and directional axon growth. Through the RGC-specific cell-surface marker CNTN2 identified here, electrophysiologically excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report the self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ optic-disc cells. Single-cell RNA sequencing of CONCEPT organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in CONCEPT organoids differentially expressed FGF8 and FGF9 ; FGFR inhibitions drastically decreased RGC differentiation and directional axon growth. Through the identified RGC-specific cell-surface marker CNTN2, electrophysiologically-excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

Impact statement

A human telencephalon-eye organoid model that exhibited axon growth and pathfinding from retinal ganglion cell (RGC) axons is reported; via cell surface marker CNTN2 identified using scRNA-seq, early RGCs were isolated under a native condition.

hPSC-derived RPE transplantation for the treatment of macular degeneration

Macular degeneration (MD) is a group of diseases characterized by irreversible and progressive vision loss. Patients with MD suffer from severely impaired central vision, especially elderly people. Currently, only one type of MD, wet age-related macular degeneration (AMD), can be treated with anti-vascular endothelium growth factor (VEGF) drugs. Other types of MD remain difficult to treat. With the advent of human pluripotent stem cells (hPSCs) and their differentiation into retinal pigmented epithelium (RPE), it is promising to treat patients with MD by transplantation of hPSC-derived RPE into the subretinal space. In this review, the current progress in hPSC-derived RPE transplantation for the treatment of patients with MD is described from bench to bedside, including hPSC differentiation into RPE and the characterization and usage of hPSC-derived RPE for transplantation into patients with MD.

High glucose causes developmental abnormalities in neuroepithelial cysts with actin and HK1 distribution changes

It has been reported that the offspring of diabetic pregnant women have an increased risk for neural tube defects. Previous studies in animal models suggested that high glucose induces cell apoptosis and epigenetic changes in the developing neural tube. However, effects on other cellular aspects such as the cell shape changes were not fully investigated. Actin dynamics plays essential roles in cell shape change. Disruption on actin dynamics is known to cause neural tube defects. In the present study, we used a 3D neuroepithelial cyst model and a rosette model, both cultured from human embryonic stem cells, to study the cellular effects caused by high glucose. By using these models, we observed couple of new changes besides increased apoptosis. First, we observed that high glucose disturbed the distribution of pH3 positive cells in the neuroepithelial cysts. Secondly, we found that high glucose exposure caused a relatively smaller actin inner boundary enclosed area, which was unlikely due to osmolarity changes. We further investigated key glucose metabolic enzymes in our models and the results showed that the distribution of hexokinase1 (HK1) was affected by high glucose. We observed that hexokinase1 has an apical-basal polarized distribution and is highest next to actin at the boundaries. hexokinase1 was more diffused and distributed less polarized under high glucose condition. Together, our observations broadened the cellular effects that may be caused by high glucose in the developing neural tube, especially in the secondary neurulation process.

Environmental concentrations of a delorazepam-based drug impact on embryonic development of non-target Xenopus laevis

Benzodiazepines, psychotropics drugs used for treating sleep disorders, anxiety and epilepsy, represent a major class of emerging water pollutants. As occurs for other pharmaceutical residues, they are not efficiently degraded during sewage treatment and persist in effluent waters. Bioaccumulation is already reported in fish and small crustaceans, but the impact and consequences on other "non-target" aquatic species are still unclear and nowadays of great interest. In this study, we investigated the effects of a pharmaceutical preparation containing the benzodiazepine delorazepam on the embryogenesis of Xenopus laevis, amphibian model species, taxa at high risk of exposure to water contaminants. Environmental (1 μg/L) and two higher (5 and 10 μg/L) concentrations were tested on tadpoles up to stage 45/46. Results demonstrate that delorazepam interferes with embryo development and that the effects are prevalently dose-dependent. Delorazepam reduces vitality by decreasing heart rate and motility, induces marked cephalic and abdominal edema, as well as intestinal and retinal defects. At the molecular level, delorazepam increases ROS production, modifies the expression of some master developmental genes and pro-inflammatory cytokines. The resulting stress condition significantly affects embryos' development and threatens their survival. Similar effects should be expected as well in embryos belonging to other aquatic species that have not been yet considered targets for these pharmaceutical residues.

3D Interfacial and Spatiotemporal Regulation of Human Neuroepithelial Organoids

Neuroepithelial (NE) organoids with dorsal-ventral patterning provide a useful three-dimensional (3D) in vitro model to interrogate neural tube formation during early development of the central nervous system. Understanding the fundamental processes behind the cellular self-organization in NE organoids holds the key to the engineering of organoids with higher, more in vivo-like complexity. However, little is known about the cellular regulation driving the NE development, especially in the presence of interfacial cues from the microenvironment. Here a simple 3D culture system that allows generation and manipulation of NE organoids from human-induced pluripotent stem cells (hiPSCs), displaying developmental phases of hiPSC differentiation and self-aggregation, first into NE cysts with lumen structure and then toward NE organoids with floor-plate patterning, is established. Longitudinal inhibition reveals distinct and dynamic roles of actomyosin contractility and yes-associated protein (YAP) signaling in governing these phases. By growing NE organoids on culture chips containing anisotropic surfaces or confining microniches, it is further demonstrated that interfacial cues can sensitively exert dimension-dependent influence on luminal cyst and organoid morphology, successful floor-plate patterning, as well as cytoskeletal regulation and YAP activity. This study therefore sheds new light on how organoid and tissue architecture can be steered through intracellular and extracellular means.

Human-induced pluripotent stem cells-derived retinal pigmented epithelium, a new horizon for cells-based therapies for age-related macular degeneration

Retinal pigment epithelium (RPE) degeneration is the hallmark of age-related macular degeneration (AMD). AMD, as one of the most common causes of irreversible visual impairment worldwide, remains in need of an appropriate approach to restore retinal function. Wet AMD, which is characterized by neovascular formation, can be stabilized by currently available therapies, including laser photocoagulation, photodynamic therapy, and intraocular injections of anti-VEFG (anti-vascular endothelial growth factor) therapy or a combination of these modalities. Unlike wet AMD, there is no effective therapy for progressive dry (non-neovascular) AMD. However, stem cell-based therapies, a part of regenerative medicine, have shown promising results for retinal degenerative diseases such as AMD. The goal of RPE cell therapy is to return the normal structure and function of the retina by re-establishing its interaction with photoreceptors, which is essential to vision. Considering the limited source of naturally occurring RPE cells, recent progress in stem cell research has allowed the generation of RPE cells from human pluripotent cells, both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSC). Since iPSCs face neither ethical arguments nor significant immunological considerations when compared to ESCs, they open a new horizon for cell therapy of AMD. The current study aims to discuss AMD, review the protocols for making human iPSCs-derived RPEs, and summarize recent developments in the field of iPSC-derived RPEs cell therapy.

The Role of Small Molecules and Their Effect on the Molecular Mechanisms of Early Retinal Organoid Development

Early in vivo embryonic retinal development is a well-documented and evolutionary conserved process. The specification towards eye development is temporally controlled by consecutive activation or inhibition of multiple key signaling pathways, such as the Wnt and hedgehog signaling pathways. Recently, with the use of retinal organoids, researchers aim to manipulate these pathways to achieve better human representative models for retinal development and disease. To achieve this, a plethora of different small molecules and signaling factors have been used at various time points and concentrations in retinal organoid differentiations, with varying success. Additions differ from protocol to protocol, but their usefulness or efficiency has not yet been systematically reviewed. Interestingly, many of these small molecules affect the same and/or multiple pathways, leading to reduced reproducibility and high variability between studies. In this review, we make an inventory of the key signaling pathways involved in early retinogenesis and their effect on the development of the early retina in vitro. Further, we provide a comprehensive overview of the small molecules and signaling factors that are added to retinal organoid differentiation protocols, documenting the molecular and functional effects of these additions. Lastly, we comparatively evaluate several of these factors using our established retinal organoid methodology.

Optimized Adeno-Associated Virus Vectors for Efficient Transduction of Human Retinal Organoids

The most widely used vectors for gene delivery in the retina are recombinant adeno-associated virus (rAAV) vectors. They have proven to be safe and effective in retinal gene therapy studies aimed to treat inherited retinal dystrophies, although with various limitations in transduction efficiency. Novel variants with modified capsid sequences have been engineered to improve transduction and overcome limitations of naturally occurring variants. Although preclinical evaluation of rAAV vectors based on such novel capsids is mostly done in animal models, the use of human induced pluripotent stem cell (hiPSC)-derived organoids offers an accessible and abundant human testing platform for rAAV evaluation. In this study, we tested the novel capsids, AAV9.GL and AAV9.NN, for their tropism and transduction efficiency in hiPSC-derived human retinal organoids (HROs) with all major neuronal and glial cell types in a laminated structure. These variants are based on the AAV9 capsid and were engineered to display specific surface-exposed peptide sequences, previously shown to improve the retinal transduction properties in the context of AAV2. To this end, HROs were transduced with increasing concentrations of rAAV9, rAAV9.GL, or rAAV9.NN carrying a self-complementary genome with a cytomegalovirus-enhanced green fluorescent protein (eGFP) cassette and were monitored for eGFP expression. The rAAV vectors transduced HROs in a dose-dependent manner, with rAAV9.NN achieving the highest efficiency and fastest onset kinetics, leading to detectable eGFP signals in photoreceptors, some interneurons, and Müller glia already at 2 days post-transduction. The potency-enhancing effect of the NN peptide insert was replicated when using the corresponding AAV2-based version (rAAV2.NN). Taken together, we report the application of an HRO system for screening novel AAV vectors and introduce novel vector candidates with enhanced transduction efficiency for human retinal cells.

Salamanders: The molecular basis of tissue regeneration and its relevance to human disease

Salamanders are recognized for their ability to regenerate a broad range of tissues. They have also have been used for hundreds of years for classical developmental biology studies because of their large accessible embryos. The range of tissues these animals can regenerate is fascinating, from full limbs to parts of the brain or heart, a potential that is missing in humans. Many promising research efforts are working to decipher the molecular blueprints shared across the organisms that naturally have the capacity to regenerate different tissues and organs. Salamanders are an excellent example of a vertebrate that can functionally regenerate a wide range of tissue types. In this review, we outline some of the significant insights that have been made that are aiding in understanding the cellular and molecular mechanisms of tissue regeneration in salamanders and discuss why salamanders are a worthy model in which to study regenerative biology and how this may benefit research fields like regenerative medicine to develop therapies for humans in the future.

A Micropatterned Human-Specific Neuroepithelial Tissue for Modeling Gene and Drug-Induced Neurodevelopmental Defects

The generation of structurally standardized human pluripotent stem cell (hPSC)-derived neural embryonic tissues has the potential to model genetic and environmental mediators of early neurodevelopmental defects. Current neural patterning systems have so far focused on directing cell fate specification spatio-temporally but not morphogenetic processes. Here, the formation of a structurally reproducible and highly-organized neuroepithelium (NE) tissue is directed from hPSCs, which recapitulates morphogenetic cellular processes relevant to early neurulation. These include having a continuous, polarized epithelium and a distinct invagination-like folding, where primitive ectodermal cells undergo E-to-N-cadherin switching and apical constriction as they acquire a NE fate. This is accomplished by spatio-temporal patterning of the mesoendoderm, which guides the development and self-organization of the adjacent primitive ectoderm into the NE. It is uncovered that TGFβ signaling emanating from endodermal cells support tissue folding of the prospective NE. Evaluation of NE tissue structural dysmorphia, which is uniquely achievable in the model, enables the detection of apical constriction and cell adhesion dysfunctions in patient-derived hPSCs as well as differentiating between different classes of neural tube defect-inducing drugs.

MERTK-Dependent Ensheathment of Photoreceptor Outer Segments by Human Pluripotent Stem Cell-Derived Retinal Pigment Epithelium

Maintenance of a healthy photoreceptor-retinal pigment epithelium (RPE) interface is essential for vision. At the center of this interface, apical membrane protrusions stemming from the RPE ensheath photoreceptor outer segments (POS), and are possibly involved in the recycling of POS through phagocytosis. The molecules that regulate POS ensheathment and its relationship to phagocytosis remain to be deciphered. By means of ultrastructural analysis, we revealed that Mer receptor tyrosine kinase (MERTK) ligands, GAS6 and PROS1, rather than αVβ5 integrin receptor ligands, triggered POS ensheathment by human embryonic stem cell (hESC)-derived RPE. Furthermore, we found that ensheathment is required for POS fragmentation before internalization. Consistently, POS ensheathment, fragmentation, and internalization were abolished in MERTK mutant RPE, and rescue of MERTK expression in retinitis pigmentosa (RP38) patient RPE counteracted these defects. Our results suggest that loss of ensheathment due to MERTK dysfunction might contribute to vision impairment in RP38 patients.

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POS are ensheathed in vitro by human embryonic stem cell-derived RPE

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POS ensheathment is upregulated by MERTK ligands: GAS6 and PROS1

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αVβ5 integrin receptor ligands do not stimulate POS ensheathment

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MERTK is essential for POS ensheathment and fragmentation before internalization

An alternative approach to produce versatile retinal organoids with accelerated ganglion cell development

Genetically complex ocular neuropathies, such as glaucoma, are a major cause of visual impairment worldwide. There is a growing need to generate suitable human representative in vitro and in vivo models, as there is no effective treatment available once damage has occured. Retinal organoids are increasingly being used for experimental gene therapy, stem cell replacement therapy and small molecule therapy. There are multiple protocols for the development of retinal organoids available, however, one potential drawback of the current methods is that the organoids can take between 6 weeks and 12 months on average to develop and mature, depending on the specific cell type wanted. Here, we describe and characterise a protocol focused on the generation of retinal ganglion cells within an accelerated four week timeframe without any external small molecules or growth factors. Subsequent long term cultures yield fully differentiated organoids displaying all major retinal cell types. RPE, Horizontal, Amacrine and Photoreceptors cells were generated using external factors to maintain lamination.

A Human Retinal Pigment Epithelium-Based Screening Platform Reveals Inducers of Photoreceptor Outer Segments Phagocytosis

Phagocytosis is a key function in various cells throughout the body. A deficiency in photoreceptor outer segment (POS) phagocytosis by the retinal pigment epithelium (RPE) causes vision loss in inherited retinal diseases and possibly age-related macular degeneration. To date, there are no effective therapies available aiming at recovering the lost phagocytosis function. Here, we developed a high-throughput screening assay based on RPE derived from human embryonic stem cells (hRPE) to reveal enhancers of POS phagocytosis. One of the hits, ramoplanin (RM), reproducibly enhanced POS phagocytosis and ensheathment in hRPE, and enhanced the expression of proteins known to regulate membrane dynamics and ensheathment in other cell systems. Additionally, RM rescued POS internalization defect in Mer receptor tyrosine kinase (MERTK) mutant hRPE, derived from retinitis pigmentosa patient induced pluripotent stem cells. Our platform, including a primary phenotypic screening phagocytosis assay together with orthogonal assays, establishes a basis for RPE-based therapy discovery aiming at a broad patient spectrum.

Studying Human Neurodevelopment and Diseases Using 3D Brain Organoids

In vitro differentiation of pluripotent stem cells provides a systematic platform to study development and disease. Recent advances in brain organoid technology have created new opportunities to investigate the formation and function of the human brain, under physiological and pathological conditions. Brain organoids can be generated to model the cellular and structural development of the human brain, and allow the investigation of the intricate interactions between resident neural and glial cell types. Combined with new advances in gene editing, imaging, and genomic analysis, brain organoid technology can be applied to address questions pertinent to human brain development, disease, and evolution. However, the current iterations of brain organoids also have limitations in faithfully recapitulating the in vivo processes. In this perspective, we evaluate the recent progress in brain organoid technology, and discuss the experimental considerations for its utilization.Dual Perspectives Companion Paper: Integrating CRISPR Engineering and hiPSC-Derived 2D Disease Modeling Systems, by Kristina Rehbach, Michael B. Fernando, and Kristen J. Brennand.

Lentiviral mediated RPE65 gene transfer in healthy hiPSCs-derived retinal pigment epithelial cells markedly increased RPE65 mRNA, but modestly protein level

The retinal pigment epithelium (RPE) is a monolayer of cobblestone-like epithelial cells that accomplishes critical functions for the retina. Several protocols have been published to differentiate pluripotent stem cells into RPE cells suitable for disease modelling and therapy development. In our study, the RPE identity of human induced pluripotent stem cell (hiPSC)-derived RPE (iRPE) was extensively characterized, and then used to test a lentiviral-mediated RPE65 gene augmentation therapy. A dose study of the lentiviral vector revealed a dose-dependent effect of the vector on RPE65 mRNA levels. A marked increase of the RPE65 mRNA was also observed in the iRPE (100-fold) as well as in an experimental set with RPE derived from another hiPSC source and from foetal human RPE. Although iRPE displayed features close to bona fide RPE, no or a modest increase of the RPE65 protein level was observed depending on the protein detection method. Similar results were observed with the two other cell lines. The mechanism of RPE65 protein regulation remains to be elucidated, but the current work suggests that high vector expression will not produce an excess of the normal RPE65 protein level.

Rapid generation of purified human RPE from pluripotent stem cells using 2D cultures and lipoprotein uptake-based sorting

Background

Despite increasing demand, current protocols for human pluripotent stem cell (hPSC)-derived retinal pigment epithelium (RPE) remain time, labor, and cost intensive. Additionally, absence of robust methods for selective RPE purification and removal of non-RPE cell impurities prevents upscaling of clinical quality RPE production. We aimed to address these challenges by developing a simplified hPSC-derived RPE production and purification system that yields high-quality RPE monolayers within 90 days.

Methods

Human pluripotent stem cells were differentiated into RPE using an innovative time and cost-effective protocol relying entirely on 2D cultures and minimal use of cytokines. Once RPE identity was obtained, cells were transferred onto permeable membranes to acquire mature RPE morphology. RPE differentiation was verified by electron microscopy, polarized VEGF expression, establishment of high transepithelial electrical resistance and photoreceptor phagocytosis assay. After 4 weeks on permeable membranes, RPE cell cultures were incubated with Dil-AcLDL (DiI-conjugated acetylated low-density lipoproteins) and subjected to fluorescence-activated cell sorting (FACS) for purification and subculture.

Results

Using our 2D cytokine scarce protocol, hPSC-derived functional RPE cells can be obtained within 2 months. Nevertheless, at this stage, most samples contain a percentage of non-RPE/early RPE progenitor cells that make them unsuitable for clinical application. We demonstrate that functional RPE cells express high levels of lipoprotein receptors and that this correlates with their ability to uptake lipoproteins. Combining photoreceptor uptake assay with lipoprotein uptake assay further confirms that only functional RPE cells uptake AcLDL. Incubation of mixed RPE/non-RPE cell cultures with fluorophore conjugated AcLDL and subsequent FACS-based isolation of labeled cells allows selective purification of mature functional RPE. When subcultured, DiI-AcLDL-labeled cells rapidly form pure homogenous high-quality RPE monolayers.

Conclusions

Pure functional RPE monolayers can be derived from hPSC within 90 days using simplified 2D cultures in conjunction with our RPE PLUS protocol (RPE Purification by Lipoprotein Uptake-based Sorting). The simplicity of this protocol makes it scalable, and the rapidity of production and purification allows for high-quality RPE to be produced in a short span of time making them ideally suited for downstream clinical and in vitro applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-020-1568-3) contains supplementary material, which is available to authorized users.

Inhibition of Ca2+ channel surface expression by mutant bestrophin-1 in RPE cells

The BEST1 gene product bestrophin-1, a Ca²⁺ -dependent anion channel, interacts with Ca_V 1.3 Ca²⁺ channels in the retinal pigment epithelium (RPE). BEST1 mutations lead to Best vitelliform macular dystrophy. A common functional defect of these mutations is reduced trafficking of bestrophin-1 into the plasma membrane. We hypothesized that this defect affects the interaction partner Ca_V 1.3 channel affecting Ca²⁺ signaling and altered RPE function. Thus, we investigated the protein interaction between Ca_V 1.3 channels and bestrophin-1 by immunoprecipitation, Ca_V 1.3 activity in the presence of mutant bestrophin-1 and intracellular trafficking of the interaction partners in confluent RPE monolayers. We selected four BEST1 mutations, each representing one mutational hotspot of the disease: T6P, F80L, R218C, and F305S. Heterologously expressed L-type channels and mutant bestrophin-1 showed reduced interaction, reduced Ca_V 1.3 channel activity, and changes in surface expression. Transfection of polarized RPE (porcine primary cells, iPSC-RPE) that endogenously express Ca_V 1.3 and wild-type bestrophin-1, with mutant bestrophin-1 confirmed reduction of Ca_V 1.3 surface expression. For the four selected BEST1 mutations, presence of mutant bestrophin-1 led to reduced Ca_V 1.3 activity by modulating pore-function or decreasing surface expression. Reduced Ca_V 1.3 activity might open new ways to understand symptoms of Best vitelliform macular dystrophy such as reduced electro-oculogram, lipofuscin accumulation, and vision impairment.

Stem Cell Transplantation Therapy for Retinal Degenerative Diseases

In the past decade, progress in the research on human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) has provided the solid basis to derive retinal pigment epithelium, photoreceptors, and ganglion cells from hESCs/iPSCs for transplantation therapy of retinal degenerative diseases (RDD). Recently, the iPSC-derived retinal pigment epithelium cells have achieved efficacy in treating patients with age-related macular degeneration (AMD). However, there is still much work to be done about the differentiation of hESCs/iPSCs into clinically required retinal cells and improvement in the methods to deliver the cells into the retina of patients. Here we will review the research advances in stem cell transplantation in animal studies and clinical trials as well as propose the challenges for improving the clinical efficacy and safety of hESCs/iPSCs-derived retinal neural cells in treating retinal degenerative diseases.

The identification of small molecules that stimulate retinal pigment epithelial cells: potential novel therapeutic options for treating retinopathies

INTRODUCTION

Combinatory strategies using pharmacology and stem cell therapy have emerged due to their potential in the treatment of retinal pigment epithelium (RPE) cell related diseases, and a variety of different stem cell sources have been evaluated both in animal models and in humans. RPE cells derived from human embryonic stem cells (hESCs) and human induced pluripotent cells (hiPSCs) are already in clinical trials, holding great promise for the treatment of age-related macular disease (AMD) and hereditary RPE-related retinal dystrophies. Highly efficient protocol for RPE generations have been developed, but they are still time-consuming and laborious. Areas covered: The authors review RPE related diseases, as well as the known functions of RPE cells in retinal homeostasis. The authors also discuss small molecules that target RPE in vivo as well as in vitro to aid RPE differentiation from pluripotent stem cells clinically. The authors base this review on literature searches performed through PubMed. Expert opinion: Using high-throughput systems, technology will provide the possibility of identifying and optimizing molecules/drugs that could lead to faster and simpler protocols for RPE differentiation. This could be crucial in moving forward to create safer and more efficient RPE-based personalized therapies.

Effects of Ranibizumab, Bevacizumab, and Aflibercept on Senescent Retinal Pigment Epithelial Cells

PURPOSE

Anti-vascular endothelial growth factor (VEGF) agents have been used for the last 10 years, but their safety profile, including cytotoxicity against various ocular cells such as retinal pigment epithelial (RPE) cells, remains a serious concern. Safety studies of VEGF agents conducted to date have primarily relied on healthy RPE cells. In this study, we assessed the safety of three anti-VEGF agents, namely, ranibizumab, bevacizumab, and aflibercept, on senescent RPE cells.

METHODS

Senescent human induced pluripotent stem cell-derived RPE cells were generated by continuous replication and confirmed with senescence biomarkers. The viability, proliferation, protein expression, and phagocytosis of the senescent RPE cells were characterized 3 days after anti-VEGF treatment with clinical doses of ranibizumab, bevacizumab, or aflibercept.

RESULTS

Clinical doses of ranibizumab, bevacizumab, or aflibercept did not decrease the viability or alter proliferation of senescent RPE cells. In addition, the anti-VEGF agents did not induce additional senescence, impair the protein expression of zonula occludens-1 and RPE65, or reduce the phagocytosis capacity of senescent RPE cells.

CONCLUSIONS

Clinical dosages of ranibizumab, bevacizumab, or aflibercept do not induce significant cytotoxicity in senescent RPE cells.

Human ESC-derived retinal epithelial cell sheets potentiate rescue of photoreceptor cell loss in rats with retinal degeneration

Replacing defective retinal pigment epithelial (RPE) cells with those derived from human embryonic stem cells (hESCs) or human-induced pluripotent stem cells (hiPSCs) is a potential strategy for treating retinal degenerative diseases. Early clinical trials have demonstrated that hESC-derived or hiPSC-derived RPE cells can be delivered safely as a suspension to the human eye. The next step is transplantation of hESC/hiPSC-derived RPE cells as cell sheets that are more physiological. We have developed a tissue-engineered product consisting of hESC-derived RPE cells grown as sheets on human amniotic membrane as a biocompatible substrate. We established a surgical approach to engraft this tissue-engineered product into the subretinal space of the eyes of rats with photoreceptor cell loss. We show that transplantation of the hESC-RPE cell sheets grown on a human amniotic membrane scaffold resulted in rescue of photoreceptor cell death and improved visual acuity in rats with retinal degeneration compared to hESC-RPE cells injected as a cell suspension. These results suggest that tissue-engineered hESC-RPE cell sheets produced under good manufacturing practice conditions may be a useful approach for treating diseases of retinal degeneration.

Possibilities for Using Pluripotent Stem Cells for Restoring Damaged Eye Retinal Pigment Epithelium

The retinal pigment epithelium is a monolayer of pigmented, hexagonal cells connected by tight junctions. These cells compose part of the outer blood-retina barrier, protect the eye from excessive light, have important secretory functions, and support the function of photoreceptors, ensuring the coordination of a variety of regulatory mechanisms. It is the degeneration of the pigment epithelium that is the root cause of many retinal degenerative diseases. The search for reliable cell sources for the transplantation of retinal pigment epithelium is of extreme urgency. Pluripotent stem cells (embryonic stem or induced pluripotent) can be differentiated with high efficiency into the pigment epithelium of the retina, which opens up possibilities for cellular therapy in macular degeneration and can slow down the development of pathology and, perhaps, restore a patient's vision. Pioneering clinical trials on transplantation of retinal pigment epithelial cells differentiated from pluripotent stem cells in the United States and Japan confirmed the need for developing and optimizing such approaches to cell therapy. For effective use, pigment epithelial cells differentiated from pluripotent stem cells should have a set of functional properties characteristic of such cells in vivo. This review summarizes the current state of preclinical and clinical studies in the field of retinal pigment epithelial transplantation therapy. We also discuss different differentiation protocols based on data in the literature and our own data, and the problems holding back the widespread therapeutic application of retinal pigment epithelium differentiated from pluripotent stem cells.

Stem Cell Derived Retinal Pigment Epithelium: The Role of Pigmentation as Maturation Marker and Gene Expression Profile Comparison with Human Endogenous Retinal Pigment Epithelium

In age-related macular degeneration (AMD) the retinal pigment epithelium (RPE) deteriorates, leading to photoreceptor decay and severe vision loss. New therapeutic strategies aim at RPE replacement by transplantation of pluripotent stem cell (PSC)-derived RPE. Several protocols to generate RPE have been developed where appearance of pigmentation is commonly used as indicator of RPE differentiation and maturation. It is, however, unclear how different pigmentation stages reflect developmental stages and functionality of PSC-derived RPE cells. We generated human embryonic stem cell-derived RPE (hESC-RPE) cells and investigated their gene expression profiles at early pigmentation (EP) and late pigmentation (LP) stages. In addition, we compared the hESC-RPE samples with human endogenous RPE. We used a common reference design microarray (44 K). Our analysis showed that maturing hESC-RPE, upon acquiring pigmentation, expresses markers specific for human RPE. Interestingly, our analysis revealed that EP and LP hESC-RPE do not differ much in gene expression. Our data further showed that pigmented hESC-RPE has a significant lower expression than human endogenous RPE in the visual cycle and oxidative stress pathways. In contrast, we observed a significantly higher expression of pathways related to the process adhesion-to-polarity model that is typical of developing epithelial cells. We conclude that, in vitro, the first appearance of pigmentation hallmarks differentiated RPE. However, further increase in pigmentation does not result in much significant gene expression changes and does not add important RPE functionalities. Consequently, our results suggest that the time span for obtaining differentiated hESC-RPE cells, that are suitable for transplantation, may be greatly reduced.

Signaling-Dependent Control of Apical Membrane Size and Self-Renewal in Rosette-Stage Human Neuroepithelial Stem Cells

In the developing nervous system, neural stem cells are polarized and maintain an apical domain facing a central lumen. The presence of apical membrane is thought to have a profound influence on maintaining the stem cell state. With the onset of neurogenesis, cells lose their polarization, and the concomitant loss of the apical domain coincides with a loss of the stem cell identity. Little is known about the molecular signals controlling apical membrane size. Here, we use two neuroepithelial cell systems, one derived from regenerating axolotl spinal cord and the other from human embryonic stem cells, to identify a molecular signaling pathway initiated by lysophosphatidic acid that controls apical membrane size and consequently controls and maintains epithelial organization and lumen size in neuroepithelial rosettes. This apical domain size increase occurs independently of effects on proliferation and involves a serum response factor-dependent transcriptional induction of junctional and apical membrane components.

Cellular regeneration strategies for macular degeneration: past, present and future

Despite considerable effort and significant therapeutic advances, age-related macular degeneration (AMD) remains the commonest cause of blindness in the developed world. Progressive late-stage AMD with outer retinal degeneration currently has no proven treatment. There has been significant interest in the possibility that cellular treatments may slow or reverse visual loss in AMD. A number of modes of action have been suggested, including cell replacement and rescue, as well as immune modulation to delay the neurodegenerative process. Their appeal in this enigmatic disease relate to their generic, non-pathway-specific effects. The outer retina in particular has been at the forefront of developments in cellular regenerative therapies being surgically accessible, easily observable, as well as having a relatively simple architecture. Both the retinal pigment epithelium (RPE) and photoreceptors have been considered for replacement therapies as both sheets and cell suspensions. Studies using autologous RPE, and to a lesser extent, foetal retina, have shown proof of principle. A wide variety of cell sources have been proposed with pluripotent stem cell-derived cells currently holding the centre stage. Recent early-phase trials using these cells for RPE replacement have met safety endpoints and hinted at possible efficacy. Animal studies have confirmed the promise that photoreceptor replacement, even in a completely degenerated outer retina may restore some vision. Many challenges, however, remain, not least of which include avoiding immune rejection, ensuring long-term cellular survival and maximising effect. This review provides an overview of progress made, ongoing studies and challenges ahead.

Application of stem cell-derived retinal pigmented epithelium in retinal degenerative diseases: present and future

As a constituent of blood-retinal barrier and retinal outer segment (ROS) scavenger, retinal pigmented epithelium (RPE) is fundamental to normal function of retina. Malfunctioning of RPE contributes to the onset and advance of retinal degenerative diseases. Up to date, RPE replacement therapy is the only possible method to completely reverse retinal degeneration. Transplantation of human RPE stem cell-derived RPE (hRPESC-RPE) has shown some good results in animal models. With promising results in terms of safety and visual improvement, human embryonic stem cell-derived RPE (hESC-RPE) can be expected in clinical settings in the near future. Despite twists and turns, induced pluripotent stem cell-derived RPE (iPSC-RPE) is now being intensely investigated to overcome genetic and epigenetic instability. By far, only one patient has received iPSC-RPE transplant, which is a hallmark of iPSC technology development. During follow-up, no major complications such as immunogenicity or tumorigenesis have been observed. Future trials should keep focusing on the safety of stem cell-derived RPE (SC-RPE) especially in long period, and better understanding of the nature of stem cell and the molecular events in the process to generate SC-RPE is necessary to the prosperity of SC-RPE clinical application.

Organoid technology for retinal repair

A major cause for vision impairment and blindness in industrialized countries is the loss of the light-sensing retinal tissue in the eye. Photoreceptor damage is one of the main characteristics found in retinal degeneration diseases, such as Retinitis Pigmentosa or age-related macular degeneration. The lack of effective therapies to stop photoreceptor loss together with the absence of significant intrinsic regeneration in the human retina converts such degenerative diseases into permanent conditions that are currently irreversible. Cell replacement by means of photoreceptor transplantation has been proposed as a potential approach to tackle cell loss in the retina. Since the first attempt of photoreceptor transplantation in humans, about twenty years ago, several research groups have focused in the development and improvement of technologies necessary to bring cell transplantation for retinal degeneration diseases to reality. Progress in recent years in the generation of human tissue derived from pluripotent stem cells (PSCs) has significantly improved our tools to study human development and disease in the dish. Particularly the availability of 3D culture systems for the generation of PSC-derived organoids, including the human retina, has dramatically increased access to human material for basic and medical research. In this review, we focus on important milestones towards the generation of transplantable photoreceptor precursors from PSC-derived retinal organoids and discuss recent pre-clinical transplantation studies using organoid-derived photoreceptors in context to related in vivo work using primary photoreceptors as donor material. Additionally, we summarize remaining challenges for developing photoreceptor transplantation towards clinical application.

Defined Medium Conditions for the Induction and Expansion of Human Pluripotent Stem Cell-Derived Retinal Pigment Epithelium

We demonstrate that a combination of Noggin, Dickkopf-1, Insulin Growth Factor 1 and basic Fibroblast Growth Factor, promotes the differentiation of human pluripotent stem cells into retinal pigment epithelium (RPE) cells. We describe an efficient one-step approach that allows the generation of RPE cells from both human embryonic stem cells and human induced pluripotent stem cells within 40-60 days without the need for manual excision, floating aggregates or imbedded cysts. Compared to methods that rely on spontaneous differentiation, our protocol results in faster differentiation into RPE cells. This pro-retinal culture medium promotes the growth of functional RPE cells that exhibit key characteristics of the RPE including pigmentation, polygonal morphology, expression of mature RPE markers, electrophysiological membrane potential and the ability to phagocytose photoreceptor outer segments. This protocol can be adapted for feeder, feeder-free and serum-free conditions. This method thereby provides a rapid and simplified production of RPE cells for downstream applications such as disease modelling and drug screening.

Stem Cell Models of Human Brain Development

Recent breakthroughs in pluripotent stem cell technologies have enabled a new class of in vitro systems for functional modeling of human brain development. These advances, in combination with improvements in neural differentiation methods, allow the generation of in vitro systems that reproduce many in vivo features of the brain with remarkable similarity. Here, we describe advances in the development of these methods, focusing on neural rosette and organoid approaches, and compare their relative capabilities and limitations. We also discuss current technical hurdles for recreating the cell-type complexity and spatial architecture of the brain in culture and offer potential solutions.

Zelltherapie am Augenhintergrund – gestern, heute, morgen

Der gemeinsame Endpunkt vieler Netzhautdegenerationen ist ein Zelluntergang im retinalen Pigmentepithel und/oder der neurosensorischen Retina und ein damit verbundener irreversibler Visusverlust. Therapieansätze in fortgeschrittenen Erkrankungsstadien müssen folglich ebenfalls den Ersatz dieser verloren gegangenen Zellen und Gewebe adressieren. Hier zeichnen sich in den letzten Jahren vor allem auf dem Gebiet der stammzellbasierten zellulären Transplantationstherapie rasante Fortschritte in Grundlagenforschung und klinischer Anwendung ab. Besonders die induzierten pluripotenten Stammzellen scheinen die personalisierte Medizin signifikant voranbringen zu können, falls es gelingt wesentliche Bedenken und Limitationen zu überwinden. Diese Übersicht benennt retinale Krankheitsbilder, bei denen Zelltherapie eine potenzielle Therapieoption darstellt, und gibt einen kurzen Einblick in bisherige Therapiemöglichkeiten. Darüber hinaus werden insbesondere die potenziellen Anwendungsbereiche induzierter pluripotenter Stammzellen mit ihren Vorteilen, aber auch Problemen beleuchtet. Der Hauptfokus liegt auf dem stammzellbasierten Ersatz des retinalen Pigmentepithels, da dieser im Hinblick auf eine therapeutische Anwendung am Menschen, im Vergleich zu anderen Zellen der neurosensorischen Netzhaut, die größten Fortschritte verzeichnet. Abschließend wird ein Überblick über bereits laufende klinische Studien zur Therapie von Netzhautdegenerationen mittels stammzellbasierter zellulärer Transplantationstherapie gegeben.

Stem cell therapies for retinal diseases: recapitulating development to replace degenerated cells

Retinal degenerative diseases are the leading causes of blindness worldwide. Replacing lost retinal cells via stem cell-based therapies is an exciting, rapidly advancing area of translational research that has already entered the clinic. Here, we review the status of these clinical efforts for several significant retinal diseases, describe the challenges involved and discuss how basic developmental studies have contributed to and are needed to advance clinical goals.

Polarized, Cobblestone, Human Retinal Pigment Epithelial Cell Maturation on a Synthetic PEG Matrix

Cell attachment is essential for the growth and polarization of retinal pigment epithelial (RPE) cells. Currently, surface coatings derived from biological proteins are used as the gold standard for cell culture. However, downstream processing and purification of these biological products can be cumbersome and expensive. In this study, we constructed a library of chemically modified nanofibers to mimic the Bruch's membrane of the retinal pigment epithelium. Using atmospheric-pressure plasma-induced graft polymerization with a high-throughput screening platform to modify the nanofibers, we identified three polyethylene glycol (PEG)-grafted nanofiber surfaces (PEG methyl ether methacrylate, n = 4, 8, and 45) from a library of 62 different surfaces as favorable for RPE cell attachment, proliferation, and maturation in vitro with cobblestone morphology. Compared with the biologically derived culture matrices such as vitronectin-based peptide Synthemax, our newly discovered synthetic PEG surfaces exhibit similar growth and polarization of retinal pigment epithelial (RPE) cells. However, they are chemically defined, are easy to synthesize on a large scale, are cost-effective, are stable with long-term storage capability, and provide a more physiologically accurate environment for RPE cell culture. To our knowledge, no one has reported that PEG derivatives directly support attachment and growth of RPE cells with cobblestone morphology. This study offers a unique PEG-modified 3D cell culture system that supports RPE proliferation, differentiation, and maturation with cobblestone morphology, providing a new avenue for RPE cell culture, disease modeling, and cell replacement therapy.

Generation of Storable Retinal Organoids and Retinal Pigmented Epithelium from Adherent Human iPS Cells in Xeno-Free and Feeder-Free Conditions

Human induced pluripotent stem cells (hiPSCs) are potentially useful in regenerative therapies for retinal disease. For medical applications, therapeutic retinal cells, such as retinal pigmented epithelial (RPE) cells or photoreceptor precursors, must be generated under completely defined conditions. To this purpose, we have developed a two-step xeno-free/feeder-free (XF/FF) culture system to efficiently differentiate hiPSCs into retinal cells. This simple method, relies only on adherent hiPSCs cultured in chemically defined media, bypassing embryoid body formation. In less than 1 month, adherent hiPSCs are able to generate self-forming neuroretinal-like structures containing retinal progenitor cells (RPCs). Floating cultures of isolated structures enabled the differentiation of RPCs into all types of retinal cells in a sequential overlapping order, with the generation of transplantation-compatible CD73⁺ photoreceptor precursors in less than 100 days. Our XF/FF culture conditions allow the maintenance of both mature cones and rods in retinal organoids until 280 days with specific photoreceptor ultrastructures. Moreover, both hiPSC-derived retinal organoids and dissociated retinal cells can be easily cryopreserved while retaining their phenotypic characteristics and the preservation of CD73⁺ photoreceptor precursors. Concomitantly to neural retina, this process allows the generation of RPE cells that can be effortlessly amplified, passaged, and frozen while retaining a proper RPE phenotype. These results demonstrate that simple and efficient retinal differentiation of adherent hiPSCs can be accomplished in XF/FF conditions. This new method is amenable to the development of an in vitro GMP-compliant retinal cell manufacturing protocol allowing large-scale production and banking of hiPSC-derived retinal cells and tissues. Stem Cells 2017;35:1176-1188.

Species-specific developmental timing is maintained by pluripotent stem cells ex utero

How species-specific developmental timing is controlled is largely unknown. By following human embryonic stem (ES) cell and mouse epiblast stem (EpiS) cell differentiation through detailed RNA-sequencing time courses, here we show that pluripotent stem cells closely retain in vivo species-specific developmental timing in vitro. In identical neural differentiation conditions in vitro, gene expression profiles are accelerated in mouse EpiS cells compared to human ES cells with relative rates of differentiation closely reflecting the rates of progression through the Carnegie stages in utero. Dynamic Time Warping analysis identified 3389 genes that were regulated more quickly in mouse EpiS cells and identified none that were regulated more quickly in human ES cells. Interestingly, we also find that human ES cells differentiated in teratomas maintain the same rate of differentiation observed in vitro in spite of being grown in a mouse host. These results suggest the existence of a cell autonomous, species-specific developmental clock that pluripotent stem cells maintain even out of context of an intact embryo.

The promise of stem cell-based therapeutics in ophthalmology

The promising role of cellular therapies in the preservation and restoration of visual function has prompted intensive efforts to characterize embryonic, adult, and induced pluripotent stem cells for regenerative purposes. Three main approaches to the use of stem cells have been described: sustained drug delivery, immunomodulation, and differentiation into various ocular structures. Studies of the differentiation capacity of all three types of stem cells into epithelial, neural, glial and vascular phenotypes have reached proof-of-concept in culture, but the correction of vision is still in the early developmental stages, and the requirements for effective in vivo implementation are still unclear. We present an overview of some of the preclinical findings on stem-cell rescue and regeneration of the cornea and retina in acute injury and degenerative disorders.

Directing Differentiation of Pluripotent Stem Cells Toward Retinal Pigment Epithelium Lineage

Development of efficient and reproducible conditions for directed differentiation of pluripotent stem cells into specific cell types is important not only to understand early human development but also to enable more practical applications, such as in vitro disease modeling, drug discovery, and cell therapies. The differentiation of stem cells to retinal pigment epithelium (RPE) in particular holds promise as a source of cells for therapeutic replacement in age‐related macular degeneration. Here we show development of an efficient method for deriving homogeneous RPE populations in a period of 45 days using an adherent, monolayer system and defined xeno‐free media and matrices. The method utilizes sequential inhibition and activation of the Activin and bone morphogenetic protein signaling pathways and can be applied to both human embryonic stem cells and induced pluripotent stem cells as the starting population. In addition, we use whole genome transcript analysis to characterize cells at different stages of differentiation that provides further understanding of the developmental dynamics and fate specification of RPE. We show that with the described method, RPE develop through stages consistent with their formation during embryonic development. This characterization— together with the absence of steps involving embryoid bodies, three‐dimensional culture, or manual dissections, which are common features of other protocols—makes this process very attractive for use in research as well as for clinical applications. Stem Cells Translational Medicine2017;6:490–501

Induced Pluripotent Stem Cell-Derived Retinal Pigmented Epithelium: A Comparative Study Between Cell Lines and Differentiation Methods

PURPOSE

The application of induced pluripotent stem cell-derived retinal pigmented epithelium (iPSC-RPE) in patients with retinal degenerative disease is making headway toward the clinic, with clinical trials already underway. Multiple groups have developed methods for RPE differentiation from pluripotent cells, but previous studies have shown variability in iPSC propensity to differentiate into RPE.

METHODS

This study provides a comparison between 2 different methods for RPE differentiation: (1) a commonly used spontaneous continuously adherent culture (SCAC) protocol and (2) a more rapid, directed differentiation using growth factors. Integration-free iPSC lines were differentiated to RPE, which were characterized with respect to global gene expression, expression of RPE markers, and cellular function.

RESULTS

We found that all 5 iPSC lines (iPSC-1, iPSC-2, iPSC-3, iPSC-4, and iPSC-12) generated RPE using the directed differentiation protocol; however, 2 of the 5 iPSC lines (iPSC-4 and iPSC-12) did not yield RPE using the SCAC method. Both methods can yield bona fide RPE that expresses signature RPE genes and carry out RPE functions, and are similar, but not identical to fetal RPE. No differences between methods were detected in transcript levels, protein localization, or functional analyses between iPSC-1-RPE, iPSC-2-RPE, and iPSC-3-RPE. Directed iPSC-3-RPE showed enhanced transcript levels of RPE65 compared to directed iPSC-2-RPE and increased BEST1 expression and pigment epithelium-derived factor (PEDF) secretion compared to directed iPSC-1-RPE. In addition, SCAC iPSC-3-RPE secreted more PEDF than SCAC iPSC-1-RPE.

CONCLUSIONS

The directed protocol is a more reliable method for differentiating RPE from various pluripotent sources and some iPSC lines are more amenable to RPE differentiation.

Intercellular Adhesion-Dependent Cell Survival and ROCK-Regulated Actomyosin-Driven Forces Mediate Self-Formation of a Retinal Organoid

In this study we dissected retinal organoid morphogenesis in human embryonic stem cell (hESC)-derived cultures and established a convenient method for isolating large quantities of retinal organoids for modeling human retinal development and disease. Epithelialized cysts were generated via floating culture of clumps of Matrigel/hESCs. Upon spontaneous attachment and spreading of the cysts, patterned retinal monolayers with tight junctions formed. Dispase-mediated detachment of the monolayers and subsequent floating culture led to self-formation of retinal organoids comprising patterned neuroretina, ciliary margin, and retinal pigment epithelium. Intercellular adhesion-dependent cell survival and ROCK-regulated actomyosin-driven forces are required for the self-organization. Our data supports a hypothesis that newly specified neuroretina progenitors form characteristic structures in equilibrium through minimization of cell surface tension. In long-term culture, the retinal organoids autonomously generated stratified retinal tissues, including photoreceptors with ultrastructure of outer segments. Our system requires minimal manual manipulation, has been validated in two lines of human pluripotent stem cells, and provides insight into optic cup invagination in vivo.

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Established a method for isolating large amounts of retinal organoids from hESCs

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Dispase-mediated cell detachment led to self-formation of the retinal organoids

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Intercellular adhesions in the floating cultures are required for cell survival

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ROCK-regulated actomyosin-driven forces are required for the self-organization