Loss of MITF expression during human embryonic stem cell differentiation disrupts retinal pigment epithelium development and optic vesicle cell proliferation

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

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

Lactate-dependent transcriptional regulation controls mammalian eye morphogenesis

Mammalian retinal metabolism favors aerobic glycolysis. However, the role of glycolytic metabolism in retinal morphogenesis remains unknown. We report that aerobic glycolysis is necessary for the early stages of retinal development. Taking advantage of an unbiased approach that combines the use of eye organoids and single-cell RNA sequencing, we identify specific glucose transporters and glycolytic genes in retinal progenitors. Next, we determine that the optic vesicle territory of mouse embryos displays elevated levels of glycolytic activity. At the functional level, we show that removal of Glucose transporter 1 and Lactate dehydrogenase A gene activity from developing retinal progenitors arrests eye morphogenesis. Surprisingly, we uncover that lactate-mediated upregulation of key eye-field transcription factors is controlled by the epigenetic modification of histone H3 acetylation through histone deacetylase activity. Our results identify an unexpected bioenergetic independent role of lactate as a signaling molecule necessary for mammalian eye morphogenesis.

Application of Human Stem Cell Derived Retinal Organoids in the Exploration of the Mechanisms of Early Retinal Development

The intricate neural circuit of retina extracts salient features of the natural world and forms bioelectric impulse as the origin of vision. The early development of retina is a highly complex and coordinated process in morphogenesis and neurogenesis. Increasing evidence indicates that stem cells derived human retinal organoids (hROs) in vitro faithfully recapitulates the embryonic developmental process of human retina no matter in the transcriptome, cellular biology and histomorphology. The emergence of hROs greatly deepens on the understanding of early development of human retina. Here, we reviewed the events of early retinal development both in animal embryos and hROs studies, which mainly comprises the formation of optic vesicle and optic cup shape, differentiation of retinal ganglion cells (RGCs), photoreceptor cells (PRs) and its supportive retinal pigment epithelium cells (RPE). We also discussed the classic and frontier molecular pathways up to date to decipher the underlying mechanisms of early development of human retina and hROs. Finally, we summarized the application prospect, challenges and cutting-edge techniques of hROs for uncovering the principles and mechanisms of retinal development and related developmental disorder. hROs is a priori selection for studying human retinal development and function and may be a fundamental tool for unlocking the unknown insight into retinal development and disease.

Retinal organoid light responsivity: current status and future opportunities

The ability to generate human retinas in vitro from pluripotent stem cells opened unprecedented opportunities for basic science and for the development of therapeutic approaches for retinal degenerative diseases. Retinal organoid models not only mimic the histoarchitecture and cellular composition of the native retina, but they can achieve a remarkable level of maturation that allows them to respond to light stimulation. However, studies evaluating the nature, magnitude, and properties of light-evoked responsivity from each cell type, in each retinal organoid layer, have been sparse. In this review we discuss the current understanding of retinal organoid function, the technologies used for functional assessment in human retinal organoids, and the challenges and opportunities that lie ahead.

Development of Müller cell-based 3D biomimetic model using bioprinting technology

Müller cells are the principal glial cells for the maintenance of structural stability and metabolic homeostasis in the human retina. Although variousin vitroexperiments using two-dimensional (2D) monolayer cell cultures have been performed, the results provided only limited results because of the lack of 3D structural environment and different cellular morphology. We studied a Müller cell-based 3D biomimetic model for use in experiments on thein vivo-like functions of Müller cells within the sensory retina. Isolated primary Müller cells were bioprinted and a 3D-aligned architecture was induced, which aligned Müller cell structure in retinal tissue. The stereographic and functional characteristics of the biomimetic model were investigated and compared to those of the conventional 2D cultured group. The results showed the potential to generate Müller cell-based biomimetic models with characteristic morphological features such as endfeet, soma, and microvilli. Especially, the 3D Müller cell model under hyperglycemic conditions showed similar responses as observed in thein vivodiabetic model with retinal changes, whereas the conventional 2D cultured group showed different cytokine and growth factor secretions. These results show that our study is a first step toward providing advanced tools to investigate thein vivofunction of Müller cells and to develop complete 3D models of the vertebrate retina.

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

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

Deciphering the Retinal Epigenome during Development, Disease and Reprogramming: Advancements, Challenges and Perspectives

Retinal neurogenesis is driven by concerted actions of transcription factors, some of which are expressed in a continuum and across several cell subtypes throughout development. While seemingly redundant, many factors diversify their regulatory outcome on gene expression, by coordinating variations in chromatin landscapes to drive divergent retinal specification programs. Recent studies have furthered the understanding of the epigenetic contribution to the progression of age-related macular degeneration, a leading cause of blindness in the elderly. The knowledge of the epigenomic mechanisms that control the acquisition and stabilization of retinal cell fates and are evoked upon damage, holds the potential for the treatment of retinal degeneration. Herein, this review presents the state-of-the-art approaches to investigate the retinal epigenome during development, disease, and reprogramming. A pipeline is then reviewed to functionally interrogate the epigenetic and transcriptional networks underlying cell fate specification, relying on a truly unbiased screening of open chromatin states. The related work proposes an inferential model to identify gene regulatory networks, features the first footprinting analysis and the first tentative, systematic query of candidate pioneer factors in the retina ever conducted in any model organism, leading to the identification of previously uncharacterized master regulators of retinal cell identity, such as the nuclear factor I, NFI. This pipeline is virtually applicable to the study of genetic programs and candidate pioneer factors in any developmental context. Finally, challenges and limitations intrinsic to the current next-generation sequencing techniques are discussed, as well as recent advances in super-resolution imaging, enabling spatio-temporal resolution of the genome.

Inducible Pluripotent Stem Cells to Model and Treat Inherited Degenerative Diseases of the Outer Retina: 3D-Organoids Limitations and Bioengineering Solutions

Inherited retinal degenerations (IRD) affecting either photoreceptors or pigment epithelial cells cause progressive visual loss and severe disability, up to complete blindness. Retinal organoids (ROs) technologies opened up the development of human inducible pluripotent stem cells (hiPSC) for disease modeling and replacement therapies. However, hiPSC-derived ROs applications to IRD presently display limited maturation and functionality, with most photoreceptors lacking well-developed outer segments (OS) and light responsiveness comparable to their adult retinal counterparts. In this review, we address for the first time the microenvironment where OS mature, i.e., the subretinal space (SRS), and discuss SRS role in photoreceptors metabolic reprogramming required for OS generation. We also address bioengineering issues to improve culture systems proficiency to promote OS maturation in hiPSC-derived ROs. This issue is crucial, as satisfying the demanding metabolic needs of photoreceptors may unleash hiPSC-derived ROs full potential for disease modeling, drug development, and replacement therapies.

Efficient differentiation of human embryonic stem cells to retinal pigment epithelium under defined conditions

Abstract

Age-related macular degeneration (AMD) is a highly prevalent form of blindness caused by loss death of cells of the retinal pigment epithelium (RPE). Transplantation of pluripotent stem cell (PSC)-derived RPE cells is considered a promising therapy to regenerate cell function and vision.

Objective

The objective of this study is to develop a rapid directed differentiation method for production of RPE cells from PSC which is rapid, efficient, and fully defined and produces cells suitable for clinical use.

Design

A protocol for cell growth and differentiation from hESCs was developed to induce differentiation through screening small molecules which regulated a primary stage of differentiation to the eyefield progenitor, and then, a subsequent set of molecules to drive differentiation to RPE cells. Methods for cell plating and maintenance have been optimized to give a homogeneous population of cells in a short 14-day period, followed by a procedure to support maturation of cell function.

Results

We show here the efficient production of RPE cells from human embryonic stem cells (hESCs) using small molecules in a feeder-free system using xeno-free/defined medium. Flow cytometry at day 14 showed ~ 90% of cells expressed the RPE markers MITF and PMEL17. Temporal gene analysis confirmed differentiation through defined cell intermediates. Mature hESC-RPE cell monolayers exhibited key morphological, molecular, and functional characteristics of the endogenous RPE.

Conclusion

This study identifies a novel cell differentiation process for rapid and efficient production of retinal RPE cells directly from hESCs. The described protocol has utility for clinical-grade cell production for human therapy to treat AMD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02316-7.

Single-Cell Transcriptomic Comparison of Human Fetal Retina, hPSC-Derived Retinal Organoids, and Long-Term Retinal Cultures

To study the development of the human retina, we use single-cell RNA sequencing (RNA-seq) at key fetal stages and follow the development of the major cell types as well as populations of transitional cells. We also analyze stem cell (hPSC)-derived retinal organoids; although organoids have a very similar cellular composition at equivalent ages as the fetal retina, there are some differences in gene expression of particular cell types. Moreover, the inner retinal lamination is disrupted at more advanced stages of organoids compared with fetal retina. To determine whether the disorganization in the inner retina is due to the culture conditions, we analyze retinal development in fetal retina maintained under similar conditions. These retinospheres develop for at least 6 months, displaying better inner retinal lamination than retinal organoids. Our single-cell RNA sequencing (scRNA-seq) comparisons of fetal retina, retinal organoids, and retinospheres provide a resource for developing better in vitro models for retinal disease.

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.

Retinal organoids: a window into human retinal development

Retinal development and maturation are orchestrated by a series of interacting signalling networks that drive the morphogenetic transformation of the anterior developing brain. Studies in model organisms continue to elucidate these complex series of events. However, the human retina shows many differences from that of other organisms and the investigation of human eye development now benefits from stem cell-derived organoids. Retinal differentiation methods have progressed from simple 2D adherent cultures to self-organising micro-physiological systems. As models of development, these have collectively offered new insights into the previously unexplored early development of the human retina and informed our knowledge of the key cell fate decisions that govern the specification of light-sensitive photoreceptors. Although the developmental trajectories of other retinal cell types remain more elusive, the collation of omics datasets, combined with advanced culture methodology, will enable modelling of the intricate process of human retinogenesis and retinal disease in vitro.

Summary: Retinal organoid systems derived from human pluripotent stem cells are micro-physiological systems that offer new insights into previously unexplored human retina development.

Transcriptomic Profiling of Human Pluripotent Stem Cell-derived Retinal Pigment Epithelium over Time

Human pluripotent stem cell (hPSC)-derived progenies are immature versions of cells, presenting a potential limitation to the accurate modelling of diseases associated with maturity or age. Hence, it is important to characterise how closely cells used in culture resemble their native counterparts. In order to select appropriate time points of retinal pigment epithelium (RPE) cultures that reflect native counterparts, we characterised the transcriptomic profiles of the hPSC-derived RPE cells from 1- and 12-month cultures. We differentiated the human embryonic stem cell line H9 into RPE cells, performed single-cell RNA-sequencing of a total of 16,576 cells to assess the molecular changes of the RPE cells across these two culture time points. Our results indicate the stability of the RPE transcriptomic signature, with no evidence of an epithelial–mesenchymal transition, and with the maturing populations of the RPE observed with time in culture. Assessment of Gene Ontology pathways revealed that as the cultures age, RPE cells upregulate expression of genes involved in metal binding and antioxidant functions. This might reflect an increased ability to handle oxidative stress as cells mature. Comparison with native human RPE data confirms a maturing transcriptional profile of RPE cells in culture. These results suggest that long-term in vitro culture of RPE cells allows the modelling of specific phenotypes observed in native mature tissues. Our work highlights the transcriptional landscape of hPSC-derived RPE cells as they age in culture, which provides a reference for native and patient samples to be benchmarked against.

Organoids and organ chips in ophthalmology

Recent advances have driven the development of stem cell-derived, self-organizing, three-dimensional miniature organs, termed organoids, which mimic different eye tissues including the retina, cornea, and lens. Organoids and engineered microfluidic organ-on-chips (organ chips) are transformative technologies that show promise in simulating the architectural and functional complexity of native organs. Accordingly, they enable exploration of facets of human disease and development not accurately recapitulated by animal models. Together, these technologies will increase our understanding of the basic physiology of different eye structures, enable us to interrogate unknown aspects of ophthalmic disease pathogenesis, and serve as clinically-relevant surrogates for the evaluation of ocular therapeutics. Both the burden and prevalence of monogenic and multifactorial ophthalmic diseases, which can cause visual impairment or blindness, in the human population warrants a paradigm shift towards organoids and organ chips that can provide sensitive, quantitative, and scalable phenotypic assays. In this article, we review the current situation of organoids and organ chips in ophthalmology and discuss how they can be leveraged for translational applications.

The Use of Induced Pluripotent Stem Cells as a Model for Developmental Eye Disorders

Approximately one-third of childhood blindness is attributed to developmental eye disorders, of which 80% have a genetic cause. Eye morphogenesis is tightly regulated by a highly conserved network of transcription factors when disrupted by genetic mutations can result in severe ocular malformation. Human-induced pluripotent stem cells (hiPSCs) are an attractive tool to study early eye development as they are more physiologically relevant than animal models, can be patient-specific and their use does not elicit the ethical concerns associated with human embryonic stem cells. The generation of self-organizing hiPSC-derived optic cups is a major advancement to understanding mechanisms of ocular development and disease. Their development in vitro has been found to mirror that of the human eye and these early organoids have been used to effectively model microphthalmia caused by a VSX2 variant. hiPSC-derived optic cups, retina, and cornea organoids are powerful tools for future modeling of disease phenotypes and will enable a greater understanding of the pathophysiology of many other developmental eye disorders. These models will also provide an effective platform for identifying molecular therapeutic targets and for future clinical applications.

The X-Linked DDX3X RNA Helicase Dictates Translation Reprogramming and Metastasis in Melanoma

The X-linked DDX3X gene encodes an ATP-dependent DEAD-box RNA helicase frequently altered in various human cancers, including melanomas. Despite its important roles in translation and splicing, how DDX3X dysfunction specifically rewires gene expression in melanoma remains completely unknown. Here, we uncover a DDX3X-driven post-transcriptional program that dictates melanoma phenotype and poor disease prognosis. Through an unbiased analysis of translating ribosomes, we identified the microphthalmia-associated transcription factor, MITF, as a key DDX3X translational target that directs a proliferative-to-metastatic phenotypic switch in melanoma cells. Mechanistically, DDX3X controls MITF mRNA translation via an internal ribosome entry site (IRES) embedded within the 5' UTR. Through this exquisite translation-based regulatory mechanism, DDX3X steers MITF protein levels dictating melanoma metastatic potential in vivo and response to targeted therapy. Together, these findings unravel a post-transcriptional layer of gene regulation that may provide a unique therapeutic vulnerability in aggressive male melanomas.

PAX6D instructs neural retinal specification from human embryonic stem cell-derived neuroectoderm

PAX6 is essential for neural retina (NR) and forebrain development but how PAX6 instructs NR versus forebrain specification remains unknown. We found that the paired-less PAX6, PAX6D, is expressed in NR cells during human eye development and along human embryonic stem cell (hESC) specification to retinal cells. hESCs deficient for PAX6D failed to enter NR specification. Induced expression of PAX6D but not PAX6A in a PAX6-null background restored the NR specification capacity. ChIP-Seq, confirmed by functional assays, revealed a set of retinal genes and non-retinal neural genes that are potential targets of PAX6D, including WNT8B. Inhibition of WNTs or knocking down of WNT8B restored the NR specification capacity of neuroepithelia with PAX6D knockout, whereas activation of WNTs blocked NR specification even when PAX6D was induced. Thus, PAX6D specifies neuroepithelia to NR cells via the regulation of WNT8B.

Mitf-family transcription factor function is required within cranial neural crest cells to promote choroid fissure closure

A crucial step in eye development is the closure of the choroid fissure (CF), a transient structure in the ventral optic cup through which vasculature enters the eye and ganglion cell axons exit. Although many factors have been identified that function during CF closure, the molecular and cellular mechanisms mediating this process remain poorly understood. Failure of CF closure results in colobomas. Recently, MITF was shown to be mutated in a subset of individuals with colobomas, but how MITF functions during CF closure is unknown. To address this issue, zebrafish with mutations in mitfa and tfec, two members of the Mitf family of transcription factors, were analyzed and their functions during CF closure determined. mitfa;tfec mutants possess severe colobomas and our data demonstrate that Mitf activity is required within cranial neural crest cells (cNCCs) during CF closure. In the absence of Mitf function, cNCC migration and localization in the optic cup are perturbed. These data shed light on the cellular mechanisms underlying colobomas in individuals with MITF mutations and identify a novel role for Mitf function in cNCCs during CF closure.

The impact of miR-183/182/96 gene regulation on the maturation, survival, and function of photoreceptor cells in the retina

MicroRNAs (MiRNAs) play important roles in posttranscriptional processes to regulate gene expression. MiRNAs control various biological processes, such as growth, development, and differentiation. The continuous physiological function of photoreceptors and retinal pigment epithelium requires precise regulation to maintain their homeostasis and function; hence, these cells are highly susceptible to premature death in retinal degenerative disorders. MiRNAs are essential for the retinal cell maturation and function; the miR-183 cluster represents one of the most important regulatory factors for the photoreceptor cells. Various studies together with bioinformatics analyses have shown that many genes contributing to the differentiation pathway of photoreceptors are targets of the miR-183 cluster, and the miR-183 cluster dysregulation causes certain defects in the differentiation of the photoreceptors and other retinal neurons by influencing the expression of target genes. Misexpression of miR-183 cluster in the human retinal epithelial cells leads to the reprogramming and transformation of these cells to neuron- and photoreceptor-like cells, which are associated with the expression of neuron- and photoreceptor-specific markers in human retinal pigment epitheliums cells. The knockout of this cluster causes the destruction of the outer segment of the photoreceptors, which subsequently causes the cells to exhibit severe susceptibility to light and eventually degenerate. Hundreds of target genes in this family are likely to affect the development and maintenance of the retina. Identifying the genes that are regulated by the miRNA-183 cluster provides researchers with important insights into the complex development and regeneration mechanism of the retina and may offer a new way for maintaining and regenerating photoreceptor cells in neurodegenerative diseases.

The Role of FGF9 in the Production of Neural Retina and RPE in a Pluripotent Stem Cell Model of Early Human Retinal Development

PURPOSE

To investigate the role of fibroblast growth factors (FGFs) in the production of neural retina (NR) and retinal pigmented epithelium (RPE) in a human pluripotent stem cell model of early retinal development.

METHODS

Human induced pluripotent stem cell (hiPSC) lines from an individual with microphthalmia caused by a functional null mutation (R200Q) in visual system homeobox 2 (VSX2), a transcription factor involved in early NR progenitor cell (NRPC) production, and a normal sibling were differentiated along the retinal and forebrain lineages using an established protocol. Quantitative and global gene expression analyses (microarray and RNAseq) were used to investigate endogenous FGF expression profiles in these cultures over time. Based on these results, mutant and control hiPSC cultures were treated exogenously with selected FGFs and subjected to gene and protein expression analyses to determine their effects on RPE and NR production.

RESULTS

We found that FGF9 and FGF19 were selectively increased in early hiPSC-derived optic vesicles (OVs) when compared to isogenic cultures of hiPSC-derived forebrain neurospheres. Furthermore, these same FGFs were downregulated over time in (R200Q)VSX2 hiPSC-OVs relative to sibling control hiPSC-OVs. Interestingly, long-term supplementation with FGF9, but not FGF19, partially rescued the mutant retinal phenotype of the (R200Q)VSX2 hiPSC-OV model. However, antagonizing FGF9 in wild-type control hiPSCs did not alter OV development.

CONCLUSIONS

Our results show that FGF9 acts in concert with VSX2 to promote NR differentiation in hiPSC-OVs and has potential to be used to manipulate early retinogenesis and mitigate ocular defects caused by functional loss of VSX2 activity. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

The Molecular Basis of Human Anophthalmia and Microphthalmia

Human eye development is coordinated through an extensive network of genetic signalling pathways. Disruption of key regulatory genes in the early stages of eye development can result in aborted eye formation, resulting in an absent eye (anophthalmia) or a small underdeveloped eye (microphthalmia) phenotype. Anophthalmia and microphthalmia (AM) are part of the same clinical spectrum and have high genetic heterogeneity, with >90 identified associated genes. By understanding the roles of these genes in development, including their temporal expression, the phenotypic variation associated with AM can be better understood, improving diagnosis and management. This review describes the genetic and structural basis of eye development, focusing on the function of key genes known to be associated with AM. In addition, we highlight some promising avenues of research involving multiomic approaches and disease modelling with induced pluripotent stem cell (iPSC) technology, which will aid in developing novel therapies.

Improved Ocular Tissue Models and Eye-On-A-Chip Technologies Will Facilitate Ophthalmic Drug Development

In this study, we describe efforts by the National Eye Institute (NEI) and National Center for Advancing Translational Science (NCATS) to catalyze advances in 3-dimensional (3-D) ocular organoid and microphysiological systems (MPS). We reviewed the recent literature regarding ocular organoids and tissue chips. Animal models, 2-dimensional cell culture models, and postmortem human tissue samples provide the vision research community with insights critical to understanding pathophysiology and therapeutic development. The advent of induced pluripotent stem cell technologies provide researchers with enticing new approaches and tools that augment study in more traditional models to provide the scientific community with insights that have previously been impossible to obtain. Efforts by the National Institutes of Health (NIH) have already accelerated the pace of scientific discovery, and recent advances in ocular organoid and MPS modeling approaches have opened new avenues of investigation. In addition to more closely recapitulating the morphologies and physiological responses of in vivo human tissue, key breakthroughs have been made in the past year to resolve long-standing scientific questions regarding tissue development, molecular signaling, and pathophysiological mechanisms that promise to provide advances critical to therapeutic development and patient care. 3-D tissue culture modeling and MPS offer platforms for future high-throughput testing of therapeutic candidates and studies of gene interactions to improve models of complex genetic diseases with no well-defined etiology, such as age-related macular degeneration and Fuchs' dystrophy.

Self-Formation of RPE Spheroids Facilitates Enrichment and Expansion of hiPSC-Derived RPE Generated on Retinal Organoid Induction Platform

Purpose

Retinal pigment epithelium (RPE) and neural retina could be generated concurrently through retinal organoid induction approaches using human induced pluripotent stem cells (hiPSCs), providing valuable sources for cell therapy of retinal degenerations. This study aims to enrich and expand hiPSC-RPE acquired with this platform and explore characteristics of serially passaged RPE cells.

Methods

RPE has been differentiated from hiPSCs with a published retinal organoid induction method. After detachment of neural retina on the 4th week, the remaining mixture was scraped from the dish and subjected to suspension culture for the formation of RPE spheroids. RPE sheets were isolated and digested for expansion. The cellular, molecular, and functional features of expanded RPE cells were evaluated by different assays.

Results

Under suspension culture, hiPSC-RPE spheroids with pigmentation self-formed were readily enriched by removing the non-retinal tissues. RPE sheets were further dissected and purified from the spheroids. The individualized RPE cells could be passaged every week for at least 5 times in serum medium, yielding large numbers of cells with high quality in a short period. In addition, when switched to a serum-free medium, the passaged RPE cells could mature in cellular, molecular, and physiological levels, including repigmentation, markers expression, and phagocytosis.

Conclusions

We developed a simple and novel RPE spheroids formation approach to enrich and expand hiPSC-RPE cells generated along with retinal neurons on a universal retinal organoid induction platform. This achievement will reduce the cost and time in producing retinal cells for basic and translational researches, in particular for retinal cell therapy.

Defined Conditions for Differentiation of Functional Retinal Ganglion Cells From Human Pluripotent Stem Cells

Purpose

We aimed to establish an efficient method for retinal ganglion cell (RGC) differentiation from human pluripotent stem cells (hPSCs) using defined factors.

Methods

To define the contribution of specific signal pathways to RGC development and optimize the differentiation of hPSCs toward RGCs, we examined RGC differentiation in three stages: (1) eye field progenitors expressing the eye field transcription factors (EFTFs), (2) RGC progenitors expressing MATH5, and (3) RGCs expressing BRN3B and ISLET1. By monitoring the condition that elicited the highest yield of cells expressing stage-specific markers, we determined the optimal concentrations and combinations of signaling pathways required for efficient generation of RGCs from hPSCs.

Results

Precise modulation of signaling pathways, including Wnt, insulin growth factor-1, and fibroblast growth factor, in combination with mechanical isolation of neural rosette cell clusters significantly enriched RX and PAX6 double-positive eye field progenitors from hPSCs by day 12. Furthermore, Notch signal inhibition facilitated differentiation into MATH5-positive progenitors at 90% efficiency by day 20, and these cells further differentiated to BRN3B and ISLET1 double-positive RGCs at 45% efficiency by day 40. RGCs differentiated via this method were functional as exemplified by their ability to generate action potentials, express microfilament components on neuronal processes, and exhibit axonal transportation of mitochondria.

Conclusions

This protocol offers highly defined culture conditions for RGC differentiation from hPSCs and in vitro disease model and cell source for transplantation for diseases related to RGCs.

Gene Augmentation and Readthrough Rescue Channelopathy in an iPSC-RPE Model of Congenital Blindness

Pathogenic variants of the KCNJ13 gene are known to cause Leber congenital amaurosis (LCA16), an inherited pediatric blindness. KCNJ13 encodes the Kir7.1 subunit that acts as a tetrameric, inwardly rectifying potassium ion channel in the retinal pigment epithelium (RPE) to maintain ionic homeostasis and allow photoreceptors to encode visual information. We sought to determine whether genetic approaches might be effective in treating blindness arising from pathogenic variants in KCNJ13. We derived human induced pluripotent stem cell (hiPSC)-RPE cells from an individual carrying a homozygous c.158G>A (p.Trp53^∗) pathogenic variant of KCNJ13. We performed biochemical and electrophysiology assays to confirm Kir7.1 function. We tested both small-molecule readthrough drug and gene-therapy approaches for this "disease-in-a-dish" approach. We found that the LCA16 hiPSC-RPE cells had normal morphology but did not express a functional Kir7.1 channel and were unable to demonstrate normal physiology. After readthrough drug treatment, the LCA16 hiPSC cells were hyperpolarized by 30 mV, and the Kir7.1 current was restored. Similarly, we rescued Kir7.1 channel function after lentiviral gene delivery to the hiPSC-RPE cells. In both approaches, Kir7.1 was expressed normally, and there was restoration of membrane potential and the Kir7.1 current. Loss-of-function variants of Kir7.1 are one cause of LCA. Using either readthrough therapy or gene augmentation, we rescued Kir7.1 channel function in iPSC-RPE cells derived from an affected individual. This supports the development of precision-medicine approaches for the treatment of clinical LCA16.

Adult Stem Cells, Tools for Repairing the Retina

Purpose

Retinal degenerative diseases lead to the death of retinal neurons causing visual impairment and blindness. In lower order vertebrates, the retina and its surrounding tissue contain stem cell niches capable of regenerating damaged tissue. Here we examine these niches and review their capacity to be used as retinal stem/progenitor cells (RSC/RPCs) for retinal repair.

Recent Findings

Exogenous factors can control the in vitro activation of RSCs/PCs found in several niches within the adult eye including cells in the ciliary margin, the retinal pigment epithelium, iris pigment epithelium as well as the inducement of Müller and amacrine cells within the neural retina itself. Recently, factors have been identified for the activation of adult mammalian Müller cells to a RPC state in vivo.

Summary

Whereas cell transplantation still holds potential for retinal repair, activation of the dormant native regeneration process may lead to a more successful process including greater integration efficiency and proper synaptic targeting.

Retinal Ganglion Cell Replacement: Current Status and Challenges Ahead

The neurons of the retina can be affected by a wide variety of inherited or environmental degenerations that can lead to vision loss and even blindness. Retinal ganglion cell (RGC) degeneration is the hallmark of glaucoma and other optic neuropathies that affect millions of people worldwide. Numerous strategies are being trialed to replace lost neurons in different degeneration models, and in recent years, stem cell technologies have opened promising avenues to obtain donor cells for retinal repair. Stem cell-based transplantation has been most frequently used for the replacement of rod photoreceptors, but the same tools could potentially be used for other retinal cell types, including RGCs. However, RGCs are not abundant in stem cell-derived cultures, and in contrast to the short-distance wiring of photoreceptors, RGC axons take a long and intricate journey to connect with numerous brain nuclei. Hence, a number of challenges still remain, such as the ability to scale up the production of RGCs and a reliable and functional integration into the adult diseased retina upon transplantation. In this review, we discuss the recent advancements in the development of replacement therapies for RGC degenerations and the challenges that we need to overcome before these technologies can be applied to the clinic. Developmental Dynamics 248:118-128, 2019. © 2018 Wiley Periodicals, Inc.

3-D retina organoids: Building platforms for therapies of the future

Across scientific disciplines, 3-D organoid culture systems offer platforms to integrate basic research findings with clinical care. The National Eye Institute mounted a $1.1 million 3-D Retina Organoid Challenge. Organoids developed through the Challenge will be valuable resources for drug screening, disease modeling, and precision and regenerative medicine.

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.

Regulation of cell proliferation in the retinal pigment epithelium: Differential regulation of the death-associated protein like-1 DAPL1 by alternative MITF splice forms

Vertebrate eye development and homoeostasis critically depend on the regulation of proliferation of cells forming the retinal pigment epithelium (RPE). Previous results indicated that the death-associated protein like-1 DAPL1 cell autonomously suppresses RPE proliferation in vivo and in vitro. Here, we show in human RPE cell lines that the pigment cell transcription factor MITF regulates RPE cell proliferation by upregulating DAPL1 expression. DAPL1 regulation by MITF is, however, mediated predominantly by (-) MITF, one of two alternative splice isoforms of MITF that lacks six residues located upstream of the DNA-binding basic domain. Furthermore, we find that the regulation of DAPL1 by MITF is indirect in that (-) MITF stimulates the transcription of Musashi homolog-2 (MSI2), which negatively regulates the processing of the anti-DAPL1 microRNA miR-7. Our results provide molecular insights into the regulation of RPE cell proliferation and quiescence and may help us understand the mechanisms of normal RPE maintenance and of eye diseases associated with either RPE hyperproliferation or the lack of regenerative proliferation.

Creation of miniature pig model of human Waardenburg syndrome type 2A by ENU mutagenesis

Human Waardenburg syndrome 2A (WS2A) is a dominant hearing loss (HL) syndrome caused by mutations in the microphthalmia-associated transcription factor (MITF) gene. In mouse models with MITF mutations, WS2A is transmitted in a recessive pattern, which limits the study of hearing loss (HL) pathology. In the current study, we performed ENU (ethylnitrosourea) mutagenesis that resulted in substituting a conserved lysine with a serine (p. L247S) in the DNA-binding domain of the MITF gene to generate a novel miniature pig model of WS2A. The heterozygous mutant pig (MITF ^+/L247S) exhibits a dominant form of profound HL and hypopigmentation in skin, hair, and iris, accompanied by degeneration of stria vascularis (SV), fused hair cells, and the absence of endocochlear potential, which indicate the pathology of human WS2A. Besides hypopigmentation and bilateral HL, the homozygous mutant pig (MITF ^L247S/L247S) and CRISPR/Cas9-mediated MITF bi-allelic knockout pigs both exhibited anophthalmia. Three WS2 patients carrying MITF mutations adjacent to the corresponding region were also identified. The pig models resemble the clinical symptom and molecular pathology of human WS2A patients perfectly, which will provide new clues for better understanding the etiology and development of novel treatment strategies for human HL.

BMP-induced reprogramming of the neural retina into retinal pigment epithelium requires Wnt signalling

In vertebrates, the retinal pigment epithelium (RPE) and photoreceptors of the neural retina (NR) comprise a functional unit required for vision. During vertebrate eye development, a conversion of the RPE into NR can be induced by growth factors in vivo at optic cup stages, but the reverse process, the conversion of NR tissue into RPE, has not been reported. Here, we show that bone morphogenetic protein (BMP) signalling can reprogram the NR into RPE at optic cup stages in chick. Shortly after BMP application, expression of Microphthalmia-associated transcription factor (Mitf) is induced in the NR and selective cell death on the basal side of the NR induces an RPE-like morphology. The newly induced RPE differentiates and expresses Melanosomalmatrix protein 115 (Mmp115) and RPE65. BMP-induced Wnt2b expression is observed in regions of the NR that become pigmented. Loss of function studies show that conversion of the NR into RPE requires both BMP and Wnt signalling. Simultaneous to the appearance of ectopic RPE tissue, BMP application reprogrammed the proximal RPE into multi-layered retinal tissue. The newly induced NR expresses visual segment homeobox-containing gene (Vsx2), and the ganglion and photoreceptor cell markers Brn3α and Visinin are detected. Our results show that high BMP concentrations are required to induce the conversion of NR into RPE, while low BMP concentrations can still induce transdifferentiation of the RPE into NR. This knowledge may contribute to the development of efficient standardized protocols for RPE and NR generation for cell replacement therapies.

Stem Cell Sources and Their Potential for the Treatment of Retinal Degenerations

Stem cells offer unprecedented opportunities for the development of strategies geared toward the treatment of retinal degenerative diseases. A variety of cellular sources have been investigated for various potential clinical applications, including tissue regeneration, disease modeling, and screening for non–cell-based therapeutic agents. As the field transitions from more than a decade of preclinical research to the first phase I/II clinical trials, we provide a concise overview of the stem cell sources most commonly used, weighing their therapeutic potential on the basis of their technical strengths/limitations, their ethical implications, and the extent of the progress achieved to date. This article serves as a framework for further in-depth analyses presented in the following chapters of this Special Issue.

Targeting orphan G protein-coupled receptors for the treatment of diabetes and its complications: C-peptide and GPR146

G protein-coupled receptors (GPCRs) are the most abundant receptor family encoded by the human genome and are the targets of a high percentage of drugs currently in use or in clinical trials for the treatment of diseases such as diabetes and its associated complications. Thus, orphan GPCRs, for which the ligand is unknown, represent an important untapped source of therapeutic potential for the treatment of many diseases. We have identified the previously orphan GPCR, GPR146, as the putative receptor of proinsulin C-peptide, which may prove to be an effective treatment for diabetes-associated complications. For example, we have found a potential role of C-peptide and GPR146 in regulating the function of the retinal pigment epithelium, a monolayer of cells in the retina that serves as part of the blood-retinal barrier and is disrupted in diabetic macular oedema. However, C-peptide signalling in this cell type appears to depend at least in part on extracellular glucose concentration and its interaction with insulin. In this review, we discuss the therapeutic potential of orphan GPCRs with a special focus on C-peptide and GPR146, including past and current strategies used to 'deorphanize' this diverse family of receptors, past successes and the inherent difficulties of this process.

cGMP production of patient-specific iPSCs and photoreceptor precursor cells to treat retinal degenerative blindness

Immunologically-matched, induced pluripotent stem cell (iPSC)-derived photoreceptor precursor cells have the potential to restore vision to patients with retinal degenerative diseases like retinitis pigmentosa. The purpose of this study was to develop clinically-compatible methods for manufacturing photoreceptor precursor cells from adult skin in a non-profit cGMP environment. Biopsies were obtained from 35 adult patients with inherited retinal degeneration and fibroblast lines were established under ISO class 5 cGMP conditions. Patient-specific iPSCs were then generated, clonally expanded and validated. Post-mitotic photoreceptor precursor cells were generated using a stepwise cGMP-compliant 3D differentiation protocol. The recapitulation of the enhanced S-cone phenotype in retinal organoids generated from a patient with NR2E3 mutations demonstrated the fidelity of these protocols. Transplantation into immune compromised animals revealed no evidence of abnormal proliferation or tumor formation. These studies will enable clinical trials to test the safety and efficiency of patient-specific photoreceptor cell replacement in humans.

Regulation of WNT Signaling by VSX2 During Optic Vesicle Patterning in Human Induced Pluripotent Stem Cells

Few gene targets of Visual System Homeobox 2 (VSX2) have been identified despite its broad and critical role in the maintenance of neural retina (NR) fate during early retinogenesis. We performed VSX2 ChIP-seq and ChIP-PCR assays on early stage optic vesicle-like structures (OVs) derived from human iPS cells (hiPSCs), which highlighted WNT pathway genes as direct regulatory targets of VSX2. Examination of early NR patterning in hiPSC-OVs from a patient with a functional null mutation in VSX2 revealed mis-expression and upregulation of WNT pathway components and retinal pigmented epithelium (RPE) markers in comparison to control hiPSC-OVs. Furthermore, pharmacological inhibition of WNT signaling rescued the early mutant phenotype, whereas augmentation of WNT signaling in control hiPSC-OVs phenocopied the mutant. These findings reveal an important role for VSX2 as a regulator of WNT signaling and suggest that VSX2 may act to maintain NR identity at the expense of RPE in part by direct repression of WNT pathway constituents. Stem Cells 2016;34:2625-2634.

Nanofiber Scaffold-Based Tissue-Engineered Retinal Pigment Epithelium to Treat Degenerative Eye Diseases

Clinical-grade manufacturing of a functional retinal pigment epithelium (RPE) monolayer requires reproducing, as closely as possible, the natural environment in which RPE grows. In vitro, this can be achieved by a tissue engineering approach, in which the RPE is grown on a nanofibrous biological or synthetic scaffold. Recent research has shown that nanofiber scaffolds perform better for cell growth and transplantability compared with their membrane counterparts and that the success of the scaffold in promoting cell growth/function is not heavily material dependent. With these strides, the field has advanced enough to begin to consider implementation of one, or a combination, of the tissue engineering strategies discussed herein. In this study, we review the current state of tissue engineering research for in vitro culture of RPE/scaffolds and the parameters for optimal scaffold design that have been uncovered during this research. Next, we discuss production methods and manufacturers that are capable of producing the nanofiber scaffolds in such a way that would be biologically, regulatory, clinically, and commercially viable. Then, a discussion of how the scaffolds could be characterized, both morphologically and mechanically, to develop a testing process that is viable for regulatory screening is performed. Finally, an example of a tissue-engineered RPE/scaffold construct is given to provide the reader a framework for understanding how these pieces could fit together to develop a tissue-engineered RPE/scaffold construct that could pass regulatory scrutiny and can be commercially successful.

Robust Differentiation of mRNA-Reprogrammed Human Induced Pluripotent Stem Cells Toward a Retinal Lineage

The ability and efficiency of mRNA-reprogrammed human induced pluripotent stem cells (hiPSCs) to yield retinal cell types in a directed, stepwise manner was tested. hiPSCs derived through mRNA-based reprogramming strategies offer numerous advantages owing to the lack of genomic integration or constitutive expression of pluripotency genes. Such methods represent a promising new approach for retinal stem cell research, especially translational applications.

The derivation of human induced pluripotent stem cells (hiPSCs) from patient-specific sources has allowed for the development of novel approaches to studies of human development and disease. However, traditional methods of generating hiPSCs involve the risks of genomic integration and potential constitutive expression of pluripotency factors and often exhibit low reprogramming efficiencies. The recent description of cellular reprogramming using synthetic mRNA molecules might eliminate these shortcomings; however, the ability of mRNA-reprogrammed hiPSCs to effectively give rise to retinal cell lineages has yet to be demonstrated. Thus, efforts were undertaken to test the ability and efficiency of mRNA-reprogrammed hiPSCs to yield retinal cell types in a directed, stepwise manner. hiPSCs were generated from human fibroblasts via mRNA reprogramming, with parallel cultures of isogenic human fibroblasts reprogrammed via retroviral delivery of reprogramming factors. New lines of mRNA-reprogrammed hiPSCs were established and were subsequently differentiated into a retinal fate using established protocols in a directed, stepwise fashion. The efficiency of retinal differentiation from these lines was compared with retroviral-derived cell lines at various stages of development. On differentiation, mRNA-reprogrammed hiPSCs were capable of robust differentiation to a retinal fate, including the derivation of photoreceptors and retinal ganglion cells, at efficiencies often equal to or greater than their retroviral-derived hiPSC counterparts. Thus, given that hiPSCs derived through mRNA-based reprogramming strategies offer numerous advantages owing to the lack of genomic integration or constitutive expression of pluripotency genes, such methods likely represent a promising new approach for retinal stem cell research, in particular, those for translational applications.

Significance

In the current report, the ability to derive mRNA-reprogrammed human induced pluripotent stem cells (hiPSCs), followed by the differentiation of these cells toward a retinal lineage, including photoreceptors, retinal ganglion cells, and retinal pigment epithelium, has been demonstrated. The use of mRNA reprogramming to yield pluripotency represents a unique ability to derive pluripotent stem cells without the use of DNA vectors, ensuring the lack of genomic integration and constitutive expression. The studies reported in the present article serve to establish a more reproducible system with which to derive retinal cell types from hiPSCs through the prevention of genomic integration of delivered genes and should also eliminate the risk of constitutive expression of these genes. Such ability has important implications for the study of, and development of potential treatments for, retinal degenerative disorders and the development of novel therapeutic approaches to the treatment of these diseases.

Quinazoline-based tricyclic compounds that regulate programmed cell death, induce neuronal differentiation, and are curative in animal models for excitotoxicity and hereditary brain disease

Expanding on a quinazoline scaffold, we developed tricyclic compounds with biological activity. These compounds bind to the 18 kDa translocator protein (TSPO) and protect U118MG (glioblastoma cell line of glial origin) cells from glutamate-induced cell death. Fascinating, they can induce neuronal differentiation of PC12 cells (cell line of pheochromocytoma origin with neuronal characteristics) known to display neuronal characteristics, including outgrowth of neurites, tubulin expression, and NeuN (antigen known as 'neuronal nuclei', also known as Rbfox3) expression. As part of the neurodifferentiation process, they can amplify cell death induced by glutamate. Interestingly, the compound 2-phenylquinazolin-4-yl dimethylcarbamate (MGV-1) can induce expansive neurite sprouting on its own and also in synergy with nerve growth factor and with glutamate. Glycine is not required, indicating that N-methyl-D-aspartate receptors are not involved in this activity. These diverse effects on cells of glial origin and on cells with neuronal characteristics induced in culture by this one compound, MGV-1, as reported in this article, mimic the diverse events that take place during embryonic development of the brain (maintenance of glial integrity, differentiation of progenitor cells to mature neurons, and weeding out of non-differentiating progenitor cells). Such mechanisms are also important for protective, curative, and restorative processes that occur during and after brain injury and brain disease. Indeed, we found in a rat model of systemic kainic acid injection that MGV-1 can prevent seizures, counteract the process of ongoing brain damage, including edema, and restore behavior defects to normal patterns. Furthermore, in the R6-2 (transgenic mouse model for Huntington disease; Strain name: B6CBA-Tg(HDexon1)62Gpb/3J) transgenic mouse model for Huntington disease, derivatives of MGV-1 can increase lifespan by >20% and reduce incidence of abnormal movements. Also in vitro, these derivatives were more effective than MGV-1.

Generation of highly enriched populations of optic vesicle-like retinal cells from human pluripotent stem cells

The protocol outlined below is used to differentiate human pluripotent stem cells (hPSCs) into retinal cell types through a process that faithfully recapitulates the stepwise progression observed in vivo. From pluripotency, cells are differentiated to a primitive anterior neural fate, followed by progression into two distinct populations of retinal progenitors and forebrain progenitors, each of which can be manually separated and purified. The hPSC-derived retinal progenitors are found to self-organize into three-dimensional optic vesicle-like structures, with each aggregate possessing the ability to differentiate into all major retinal cell types. The ability to faithfully recapitulate the stepwise in vivo development in a three-dimensional cell culture system allows for the study of mechanisms underlying human retinogenesis. Furthermore, this methodology allows for the study of retinal dysfunction and disease modeling using patient-derived cells, as well as high-throughput pharmacological screening and eventually patient-specific therapies.

Efficient delivery and functional expression of transfected modified mRNA in human embryonic stem cell-derived retinal pigmented epithelial cells

Gene- and cell-based therapies are promising strategies for the treatment of degenerative retinal diseases such as age-related macular degeneration, Stargardt disease, and retinitis pigmentosa. Cellular engineering before transplantation may allow the delivery of cellular factors that can promote functional improvements, such as increased engraftment or survival of transplanted cells. A current challenge in traditional DNA-based vector transfection is to find a delivery system that is both safe and efficient, but using mRNA as an alternative to DNA can circumvent these major roadblocks. In this study, we show that both unmodified and modified mRNA can be delivered to retinal pigmented epithelial (RPE) cells with a high efficiency compared with conventional plasmid delivery systems. On the other hand, administration of unmodified mRNA induced a strong innate immune response that was almost absent when using modified mRNA. Importantly, transfection of mRNA encoding a key regulator of RPE gene expression, microphthalmia-associated transcription factor (MITF), confirmed the functionality of the delivered mRNA. Immunostaining showed that transfection with either type of mRNA led to the expression of roughly equal levels of MITF, primarily localized in the nucleus. Despite these findings, quantitative RT-PCR analyses showed that the activation of the expression of MITF target genes was higher following transfection with modified mRNA compared with unmodified mRNA. Our findings, therefore, show that modified mRNA transfection can be applied to human embryonic stem cell-derived RPE cells and that the method is safe, efficient, and functional.