Differentiation of Retinal Glial Cells From Human Embryonic Stem Cells by Promoting the Notch Signaling Pathway

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.

Transcriptomics of CD29+/CD44+ cells isolated from hPSC retinal organoids reveals a single cell population with retinal progenitor and Müller glia characteristics

Müller glia play very important and diverse roles in retinal homeostasis and disease. Although much is known of the physiological and morphological properties of mammalian Müller glia, there is still the need to further understand the profile of these cells during human retinal development. Using human embryonic stem cell-derived retinal organoids, we investigated the transcriptomic profiles of CD29+/CD44+ cells isolated from early and late stages of organoid development. Data showed that these cells express classic markers of retinal progenitors and Müller glia, including NFIX, RAX, PAX6, VSX2, HES1, WNT2B, SOX, NR2F1/2, ASCL1 and VIM, as early as days 10-20 after initiation of retinal differentiation. Expression of genes upregulated in CD29+/CD44+ cells isolated at later stages of organoid development (days 50-90), including NEUROG1, VSX2 and ASCL1 were gradually increased as retinal organoid maturation progressed. Based on the current observations that CD24+/CD44+ cells share the characteristics of early and late-stage retinal progenitors as well as of mature Müller glia, we propose that these cells constitute a single cell population that upon exposure to developmental cues regulates its gene expression to adapt to functions exerted by Müller glia in the postnatal and mature retina.

Spatial and Temporal Development of Müller Glial Cells in hiPSC-Derived Retinal Organoids Facilitates the Cell Enrichment and Transcriptome Analysis

Müller glial cells (MGCs) play important roles in human retina during physiological and pathological conditions. However, the development process of human MGCs in vivo remains unclear, and how to obtain large numbers of human MGCs with high quality faces technical challenges, which hinder the further study and application of MGCs. Human induced pluripotent stem cell (hiPSC)-derived retinal organoids (ROs) with all retinal cell subtypes provide an unlimited cell resource and a platform for the studies of retinal development and disorders. This study explored the development of human MGCs in hiPSC-derived ROs and developed an approach to select and expand the induced MGCs (iMGCs). In ROs, retinal progenitor cells progressively differentiated into SOX9+ Ki67– MGC precursors during differentiation day (D) 60 to D90, while mature MGCs expressing markers CRALBP and GS gradually appeared since D120, which spanned the entire thickness of the neural retina layer. Cells isolated from ROs aged older than 120 days was an optimal source for the enrichment of iMGCs with high purity and expansion ability. They had typical features of human MGCs in morphological, structural, molecular and functional aspects, and could be passaged serially at least 10 times, yielding large numbers of cells in a short period. The transcriptome pattern of the expanded iMGCs was also revealed. This study firstly clarified the timecourse of human MGC development in the RO model, where the iMGCs could be enriched and expanded, paving the way for downstream investigation and application in MGC-related retinal disorders.

Esrrb is a cell-cycle-dependent associated factor balancing pluripotency and XEN differentiation

Cell cycle and differentiation decisions are linked; however, the underlying principles that drive these decisions are unclear. Here, we combined cell-cycle reporter system and single-cell RNA sequencing (scRNA-seq) profiling to study the transcriptomes of embryonic stem cells (ESCs) in the context of cell-cycle states and differentiation. By applying retinoic acid, to G1 and G2/M ESCs, we show that, while both populations can differentiate toward epiblast stem cells (EpiSCs), only G2/M ESCs could differentiate into extraembryonic endoderm cells. We identified Esrrb, a pluripotency factor that is upregulated during G2/M, as a driver of extraembryonic endoderm stem cell (XEN) differentiation. Furthermore, enhancer chromatin states based on wild-type (WT) and ESRRB knockout (KO) ESCs show association of ESRRB with XEN poised enhancers. G1 cells overexpressing Esrrb allow ESCs to produce XENs, while ESRRB-KO ESCs lost their potential to differentiate into XEN. Overall, this study reveals a vital link between Esrrb and cell-cycle states during the exit from pluripotency.

Insights on the Regeneration Potential of Müller Glia in the Mammalian Retina

Müller glia, the major glial cell types in the retina, maintain retinal homeostasis and provide structural support to retinal photoreceptors. They also possess regenerative potential that might be used for retinal repair in response to injury or disease. In teleost fish (such as zebrafish), the Müller glia response to injury involves reprogramming events that result in a population of proliferative neural progenitors that can regenerate the injured retina. Recent studies have revealed several important mechanisms for the regenerative capacity of Müller glia in fish, which may shed more light on the mechanisms of Müller glia reprogramming and regeneration in mammals. Mammalian Müller glia can adopt stem cell characteristics, and in response to special conditions, be persuaded to proliferate and regenerate, although their native regeneration potential is limited. In this review, we consider the work to date revealing the regenerative potential of the mammalian Müller glia and discuss whether they are a potential source for cell regeneration therapy in humans.

Repopulation of decellularized retinas with hiPSC-derived retinal pigment epithelial and ocular progenitor cells shows cell engraftment, organization and differentiation

The retinal extracellular matrix (ECM) provides architectural support, adhesion and signal guidance that controls retinal development. Decellularization of the ECM affords great potential to tissue engineering; however, how structural retinal ECM affects in vitro development, differentiation and maturation of ocular cells remains to be elucidated. Here, mouse and porcine retinas were decellularized and the protein profile analyzed. Acellular retinal ECM (arECM) scaffolds were then repopulated with human iPSC-derived retinal pigment epithelial (RPE) cells or ocular progenitor cells (OPC) to assess their integration, proliferation and organization. 3837 and 2612 unique proteins were identified in mouse and porcine arECM, respectively, of which 93 and 116 proteins belong to the matrisome. GO analysis shows that matrisome-related proteins were associated with the extracellular region and cell junction and KEGG pathways related to signalling transduction, nervous and endocrine systems and cell junctions were enriched. Interestingly, mouse and porcine arECMs were successfully repopulated with both RPE and OPC, the latter exhibiting cell lineage-specific clusters. Retinal cells organized into different layers containing well-defined areas with pigmented cells, photoreceptors, Müller glia, astrocytes, and ganglion cells, whereas in other areas, conjunctival/limbal, corneal and lens cells re-arranged in cell-specific self-organized areas. In conclusion, our results demonstrated that decellularization of both mouse and porcine retinas retains common native ECM components that upon cell repopulation could guide similar ocular cell adhesion, migration and organization.

Urine-derived induced pluripotent/neural stem cells for modeling neurological diseases

Neurological diseases are mainly modeled using rodents through gene editing, surgery or injury approaches. However, differences between humans and rodents in terms of genetics, neural development, and physiology pose limitations on studying disease pathogenesis in rodent models for neuroscience research. In the past decade, the generation of induced pluripotent stem cells (iPSCs) and induced neural stem cells (iNSCs) by reprogramming somatic cells offers a powerful alternative for modeling neurological diseases and for testing regenerative medicines. Among the different somatic cell types, urine-derived stem cells (USCs) are an ideal cell source for iPSC and iNSC reprogramming, as USCs are highly proliferative, multipotent, epithelial in nature, and easier to reprogram than skin fibroblasts. In addition, the use of USCs represents a simple, low-cost and non-invasive procedure for generating iPSCs/iNSCs. This review describes the cellular and molecular properties of USCs, their differentiation potency, different reprogramming methods for the generation of iPSCs/iNSCs, and their potential applications in modeling neurological diseases.

Reproducing diabetic retinopathy features using newly developed human induced-pluripotent stem cell-derived retinal Müller glial cells

Muller glial cells (MGCs) are responsible for the homeostatic and metabolic support of the retina. Despite the importance of MGCs in retinal disorders, reliable and accessible human cell sources to be used to model MGC-associated diseases are lacking. Although primary human MGCs (pMGCs) can be purified from post-mortem retinal tissues, the donor scarcity limits their use. To overcome this problem, we developed a protocol to generate and bank human induced pluripotent stem cell-derived MGCs (hiMGCs). Using a transcriptome analysis, we showed that the three genetically independent hiMGCs generated were homogeneous and showed phenotypic characteristics and transcriptomic profile of pMGCs. These cells expressed key MGC markers, including Vimentin, CLU, DKK3, SOX9, SOX2, S100A16, ITGB1, and CD44 and could be cultured up to passage 8. Under our culture conditions, hiMGCs and pMGCs expressed low transcript levels of RLPB1, AQP4, KCNJ1, KCJN10, and SLC1A3. Using a disease modeling approach, we showed that hiMGCs could be used to model the features of diabetic retinopathy (DR)-associated dyslipidemia. Indeed, palmitate, a major free fatty acid with elevated plasma levels in diabetic patients, induced the expression of inflammatory cytokines found in the ocular fluid of DR patients such as CXCL8 (IL-8) and ANGPTL4. Moreover, the analysis of palmitate-treated hiMGC secretome showed an upregulation of proangiogenic factors strongly related to DR, including ANG2, Endoglin, IL-1β, CXCL8, MMP-9, PDGF-AA, and VEGF. Thus, hiMGCs could be an alternative to pMGCs and an extremely valuable tool to help to understand and model glial cell involvement in retinal disorders, including DR.

Extracellular Matrix in Neural Plasticity and Regeneration

The extracellular matrix (ECM) is a fundamental component of biological tissues. The ECM in the central nervous system (CNS) is unique in both composition and function. Functions such as learning, memory, synaptogenesis, and plasticity are regulated by numerous ECM molecules. The neural ECM acts as a non-specific physical barrier that modulates neuronal plasticity and axon regeneration. There are two specialized types of ECM in the CNS, diffuse perisynaptic ECM and condensed ECM, which selectively surround the perikaryon and initial part of dendritic trees in subtypes of neurons, forming perineuronal nets. This review presents the current knowledge about the role of important neuronal ECM molecules in maintaining the basic functions of a neuron, including electrogenesis and the ability to form neural circuits. The review mainly focuses on the role of ECM components that participate in the control of key events such as cell survival, axonal growth, and synaptic remodeling. Particular attention is drawn to the numerous molecular partners of the main ECM components. These regulatory molecules are integrated into the cell membrane or disposed into the matrix itself in solid or soluble form. The interaction of the main matrix components with molecular partners seems essential in molecular mechanisms controlling neuronal functions. Special attention is paid to the chondroitin sulfate proteoglycan 4, type 1 transmembrane protein, neural-glial antigen 2 (NG2/CSPG4), whose cleaved extracellular domain is such a molecular partner that it not only acts directly on neural and vascular cells, but also exerts its influence indirectly by binding to resident ECM molecules.

Targeting the Notch and TGF-β signaling pathways to prevent retinal fibrosis in vitro and in vivo

Rationale: The Notch and transforming growth factor-β (TGFβ) signaling pathways are two intracellular mechanisms that control fibrosis in general but whether they play a major role in retinal fibrosis is less clear. Here we study how these two signaling pathways regulate Müller cell-dominated retinal fibrosis in vitro and in vivo.

Methods: Human MIO-M1 Müller cells were treated with Notch ligands and TGFβ1, either alone or in combination. Western blots were performed to study changes in γ-secretase proteases, Notch downstream effectors, endogenous TGFβ1, phosphorylated Smad3 (p-Smad3) and extracellular matrix (ECM) proteins. We also studied the effects of RO4929097, a selective γ-secretase inhibitor, on expression of ECM proteins after ligand stimulation. Müller cell viability was studied by AlamarBlue and cytotoxicity by lactate cytotoxicity assays. Finally, we studied changes in Notch and TGFβ signaling and tested the effect of intravitreal injections of the Notch pathway inhibitor RO4929097 on retinal fibrosis resulted from Sodium iodate (NaIO₃)-induced retinal injury in mice. We also studied the safety of intravitreal injections of RO4929097 in normal mice.

Results: Treatment of Müller cells with Notch ligands upregulated γ-secretase proteases and Notch downstream effectors, with increased expression of endogenous TGFβ1, TGFβ receptors and p-Smad3. TGFβ1 upregulated the expression of proteins associated with both signaling pathways in a similar manner. Notch ligands and TGFβ1 had additive effects on overexpression of ECM proteins in Müller cells which were inhibited by RO4929097. Notch and TGFβ ligands stimulated Müller cell proliferation which was inhibited by RO4929097 without damaging the cells. NaIO₃-induced retinal injury activated both Notch and TGFβ signaling pathways in vivo. Intravitreal injection of RO4929097 prevented Müller cell gliosis and inhibited overexpression of ECM proteins in this murine model. We found no safety concerns for up to 17 days after an intravitreal injection of RO4929097.

Conclusions: Inhibiting Notch signaling might be an effective way to prevent retinal fibrosis. This study is of clinical significance in developing a treatment for preventing fibrosis in proliferative vitreoretinopathy, proliferative diabetic retinopathy and wet age-related macular degeneration.