ESC-derived retinal pigmented epithelial cell transplants in patients: so far, so good

Differentiation of Human Embryonic/Induced-Pluripotent Stem Cells to Retinal Ganglion Cells

The generation of retinal ganglion cells (RGCs) differentiated from human embryonic stem cell (hESC) or induced-pluripotent stem cells (iPSC) could aid with understanding of human RGC development, neuronal biology, drug discovery, potential cell-based therapies, and gene regulation. Here, we present a protocol for differentiation of hESC to RGCs using a 40-day protocol, significantly shorter than typical retinal organoids while still yielding cells with RGC-enriched markers and show physiological and morphological properties typical of RGCs.

Cell Sources for Retinal Regeneration: Implication for Data Translation in Biomedicine of the Eye

The main degenerative diseases of the retina include macular degeneration, proliferative vitreoretinopathy, retinitis pigmentosa, and glaucoma. Novel approaches for treating retinal diseases are based on cell replacement therapy using a variety of exogenous stem cells. An alternative and complementary approach is the potential use of retinal regeneration cell sources (RRCSs) containing retinal pigment epithelium, ciliary body, Müller glia, and retinal ciliary region. RRCSs in lower vertebrates in vivo and in mammals mostly in vitro are able to proliferate and exhibit gene expression and epigenetic characteristics typical for neural/retinal cell progenitors. Here, we review research on the factors controlling the RRCSs' properties, such as the cell microenvironment, growth factors, cytokines, hormones, etc., that determine the regenerative responses and alterations underlying the RRCS-associated pathologies. We also discuss how the current data on molecular features and regulatory mechanisms of RRCSs could be translated in retinal biomedicine with a special focus on (1) attempts to obtain retinal neurons de novo both in vivo and in vitro to replace damaged retinal cells; and (2) investigations of the key molecular networks stimulating regenerative responses and preventing RRCS-related pathologies.

Retina stem cells, hopes and obstacles

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

Development-related mitochondrial properties of retinal pigment epithelium cells derived from hEROs

AIM

To explore the temporal mitochondrial characteristics of retinal pigment epithelium (RPE) cells obtained from human embryonic stem cells (hESC)-derived retinal organoids (hEROs-RPE), to verify the optimal period for using hEROs-RPE as donor cells from the aspect of mitochondria and to optimize RPE cell-based therapeutic strategies for age-related macular degeneration (AMD).

METHODS

RPE cells were obtained from hEROs and from spontaneous differentiation (SD-RPE). The mitochondrial characteristics were analyzed every 20d from day 60 to 160. Mitochondrial quantity was measured by MitoTracker Green staining. Transmission electron microscopy (TEM) was adopted to assess the morphological features of the mitochondria, including their distribution, length, and cristae. Mitochondrial membrane potentials (MMPs) were determined by JC-1 staining and evaluated by flow cytometry, reactive oxygen species (ROS) levels were evaluated by flow cytometry, and adenosine triphosphate (ATP) levels were measured by a luminometer. Differences between two groups were analyzed by the independent-samples t-test, and comparisons among multiple groups were made using one-way ANOVA or Kruskal-Wallis H test when equal variance was not assumed.

RESULTS

hEROs-RPE and SD-RPE cells from day 60 to 160 were successfully differentiated from hESCs and expressed RPE markers (Pax6, MITF, Bestrophin-1, RPE65, Cralbp). RPE features, including a cobblestone-like morphology with tight junctions (ZO-1), pigments and microvilli, were also observed in both hEROs-RPE and SD-RPE cells. The mitochondrial quantities of hEROs-RPE and SD-RPE cells both peaked at day 80. However, the cristae of hEROs-RPE mitochondria were less mature and abundant than those of SD-RPE mitochondria at day 80, with hEROs-RPE mitochondria becoming mature at day 100. Both hEROs-RPE and SD-RPE cells showed low ROS levels from day 100 to 140 and maintained a normal MMP during this period. However, hEROs-RPE mitochondria maintained a longer time to produce high levels of ATP (from day 120 to 140) than SD-RPE cells (only day 120).

CONCLUSION

hEROs-RPE mitochondria develop more slowly and maintain a longer time to supply high-level energy than SD-RPE mitochondria. From the mitochondrial perspective, hEROs-RPE cells from day 100 to 140 are an optimal cell source for treating AMD.

The road to restore vision with photoreceptor regeneration

Neuroretinal diseases are the predominant cause of irreversible blindness worldwide, mainly due to photoreceptor loss. Currently, there are no radical treatments to fully reverse the degeneration or even stop the disease progression. Thus, it is urgent to develop new biological therapeutics for these diseases on the clinical side. Stem cell-based treatments have become a promising therapeutic for neuroretinal diseases through the replacement of damaged cells with photoreceptors and some allied cells. To date, considerable efforts have been made to regenerate the diseased retina based on stem cell technology. In this review, we overview the current status of stem cell-based treatments for photoreceptor regeneration, including the major cell sources derived from different stem cells in pre-clinical or clinical trial stages. Additionally, we discuss herein the major challenges ahead for and potential new strategy toward photoreceptor regeneration.

Generation of a Retina Reporter hiPSC Line to Label Progenitor, Ganglion, and Photoreceptor Cell Types

Purpose

Early in mammalian eye development, VSX2, BRN3b, and RCVRN expression marks neural retinal progenitors (NRPs), retinal ganglion cells (RGCs), and photoreceptors (PRs), respectively. The ability to create retinal organoids from human induced pluripotent stem cells (hiPSC) holds great potential for modeling both human retinal development and retinal disease. However, no methods allowing the simultaneous, real-time monitoring of multiple specific retinal cell types during development currently exist.

Methods

CRISPR/Cas9-mediated homology-directed repair (HDR) in hiPSCs facilitated the replacement of the VSX2 (Progenitor), BRN3b (Ganglion), and RCVRN (Photoreceptor) stop codons with sequences encoding a viral P2A peptide fused to Cerulean, green fluorescent protein, and mCherry reporter genes, respectively, to generate a triple transgenic reporter hiPSC line called PGP1. This was accomplished by co-electroporating HDR templates and sgRNA/Cas9 vectors into hiPSCs followed by antibiotic selection. Functional validation of the PGP1 hiPSC line included the ability to generate retinal organoids, with all major retinal cell types, displaying the expression of the three fluorescent reporters consistent with the onset of target gene expression. Disaggregated organoids were also analyzed by fluorescence-activated cell sorting and fluorescent populations were tested for the expression of the targeted gene.

Results

Retinal organoids formed from the PGP1 line expressed appropriate fluorescent proteins consistent with the differentiation of NRPs, RGCs, and PRs. Organoids produced from the PGP1 line expressed transcripts consistent with the development of all major retinal cell types.

Conclusions and Translational Relevance

The PGP1 line offers a powerful new tool to study retinal development, retinal reprogramming, and therapeutic drug screening.

The Universal 3D3 Antibody of Human PODXL Is Pluripotent Cytotoxic, and Identifies a Residual Population After Extended Differentiation of Pluripotent Stem Cells

Podocalyxin-like protein (PODXL) is a member of CD34 family proteins. It is the protein that carries many post-translational epitopes responsible for various pluripotent surface markers including TRA-1-60, TRA-1-81, GCTM2, GP200, and mAb84. However, PODXL has not attracted the attention of stem cell biologists. Here, we report several features of PODXL mRNA and protein in pluripotent stem cells. Similar to the modification-dependent pluripotent epitopes, PODXL transcripts and carrier protein are also features of pluripotency. PODXL is highly expressed in early human embryos from oocytes up to four-cell stages. During reprogramming of human cells to pluripotency, in contrast to TRA-1-60 and TRA-1-81, PODXL is activated by KLF4 at a very early time of reprogramming. Although TRA-1-60 and TRA-1-81 are completely lost upon differentiation, a residual PODXL(+) population exists even after extended differentiation and they were identified by the universal human PODXL epitope 3D3. Unlike TRA-1-60 and TRA-1-81 epitopes that are unique to primate pluripotent stem cells (PSCs), PODXL carrier protein can be used as a murine surface marker. Most importantly, antibody to 3D3 epitope causes massive necrosis and apoptosis of human PSCs (hPSCs). We suggest that 3D3 antibody could be employed to eliminate the tumorigenic pluripotent cells in hPSC-derived cells for cell transplantation.

Stem Cell Therapy for Treatment of Ocular Disorders

Sustenance of visual function is the ultimate focus of ophthalmologists. Failure of complete recovery of visual function and complications that follow conventional treatments have shifted search to a new form of therapy using stem cells. Stem cell progenitors play a major role in replenishing degenerated cells despite being present in low quantity and quiescence in our body. Unlike other tissues and cells, regeneration of new optic cells responsible for visual function is rarely observed. Understanding the transcription factors and genes responsible for optic cells development will assist scientists in formulating a strategy to activate and direct stem cells renewal and differentiation. We review the processes of human eye development and address the strategies that have been exploited in an effort to regain visual function in the preclinical and clinical state. The update of clinical findings of patients receiving stem cell treatment is also presented.

Induced Pluripotent Stem Cell Therapies for Degenerative Disease of the Outer Retina: Disease Modeling and Cell Replacement

Stem cell therapies are being explored as potential treatments for retinal disease. How to replace neurons in a degenerated retina presents a continued challenge for the regenerative medicine field that, if achieved, could restore sight. The major issues are: (i) the source and availability of donor cells for transplantation; (ii) the differentiation of stem cells into the required retinal cells; and (iii) the delivery, integration, functionality, and survival of new cells in the host neural network. This review considers the use of induced pluripotent stem cells (iPSC), currently under intense investigation, as a platform for cell transplantation therapy. Moreover, patient-specific iPSC are being developed for autologous cell transplantation and as a tool for modeling specific retinal diseases, testing gene therapies, and drug screening.

Differentiation of human ESCs to retinal ganglion cells using a CRISPR engineered reporter cell line

Retinal ganglion cell (RGC) injury and cell death from glaucoma and other forms of optic nerve disease is a major cause of irreversible vision loss and blindness. Human pluripotent stem cell (hPSC)-derived RGCs could provide a source of cells for the development of novel therapeutic molecules as well as for potential cell-based therapies. In addition, such cells could provide insights into human RGC development, gene regulation, and neuronal biology. Here, we report a simple, adherent cell culture protocol for differentiation of hPSCs to RGCs using a CRISPR-engineered RGC fluorescent reporter stem cell line. Fluorescence-activated cell sorting of the differentiated cultures yields a highly purified population of cells that express a range of RGC-enriched markers and exhibit morphological and physiological properties typical of RGCs. Additionally, we demonstrate that aligned nanofiber matrices can be used to guide the axonal outgrowth of hPSC-derived RGCs for in vitro optic nerve-like modeling. Lastly, using this protocol we identified forskolin as a potent promoter of RGC differentiation.

c-Kit+ cells isolated from human fetal retinas represent a new population of retinal progenitor cells

Definitive surface markers for retinal progenitor cells (RPCs) are still lacking. Therefore, we sorted c-Kit+ and stage-specific embryonic antigen-4− (SSEA4−) retinal cells for further biological characterization. RPCs were isolated from human fetal retinas (gestational age of 12–14 weeks). c-Kit+/SSEA4− RPCs were sorted by fluorescence-activated cell sorting, and their proliferation and differentiation capabilities were evaluated by using immunocytochemistry and flow cytometry. The effectiveness and safety were assessed following injection of c-Kit+/SSEA4− cells into the subretina of Royal College of Surgeons (RCS) rats. c-Kit+ cells were found in the inner part of the fetal retina. Sorted c-Kit+/SSEA4− cells expressed retinal stem cell markers. Our results clearly demonstrate the proliferative potential of these cells. Moreover, c-Kit+/SSEA4− cells differentiated into retinal cells that expressed markers of photoreceptor cells, ganglion cells and glial cells. These cells survived for at least 3 months after transplantation into the host subretinal space. Teratomas were not observed in the c-Kit+/SSEA4−-cell group. Thus, c-Kit can be used as a surface marker for RPCs, and c-Kit+/SSEA4− RPCs exhibit the ability to self-renew and differentiate into retinal cells.