Using Zinc Finger Nuclease Technology to Generate CRX-Reporter Human Embryonic Stem Cells as a Tool to Identify and Study the Emergence of Photoreceptors Precursors During Pluripotent Stem Cell Differentiation

Transplanted human photoreceptors transfer cytoplasmic material but not to the recipient mouse retina

BACKGROUND

The discovery of material transfer between transplanted and host mouse photoreceptors has expanded the possibilities for utilizing transplanted photoreceptors as potential vehicles for delivering therapeutic cargo. However, previous research has not directly explored the capacity for human photoreceptors to engage in material transfer, as human photoreceptor transplantation has primarily been investigated in rodent models of late-stage retinal disease, which lack host photoreceptors.

METHODS

In this study, we transplanted human stem-cell derived photoreceptors purified from human retinal organoids at different ontological ages (weeks 10, 14, or 20) into mouse models with intact photoreceptors and assessed transfer of human proteins and organelles to mouse photoreceptors.

RESULTS

Unexpectedly, regardless of donor age or mouse recipient background, human photoreceptors did not transfer material in the mouse retina, though a rare subset of donor cells (< 5%) integrated into the mouse photoreceptor cell layer. To investigate the possibility that a species barrier impeded transfer, we used a flow cytometric assay to examine material transfer in vitro. Interestingly, dissociated human photoreceptors transferred fluorescent protein with each other in vitro, yet no transfer was detected in co-cultures of human and mouse photoreceptors, suggesting that material transfer is species specific.

CONCLUSIONS

While xenograft models are not a tractable system to study material transfer of human photoreceptors, these findings demonstrate that human retinal organoid-derived photoreceptors are competent donors for material transfer and thus may be useful to treat retinal degenerative disease.

The Prospects for Retinal Organoids in Treatment of Retinal Diseases

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

Generation of an RCVRN-eGFP Reporter hiPSC Line by CRISPR/Cas9 to Monitor Photoreceptor Cell Development and Facilitate the Cell Enrichment for Transplantation

Stem cell-based cell therapies are considered to be promising treatments for retinal disorders with dysfunction or death of photoreceptors. However, the enrichment of human photoreceptors suitable for transplantation has been highly challenging so far. This study aimed to generate a photoreceptor-specific reporter human induced pluripotent stem cell (hiPSC) line using CRISPR/Cas9 genome editing, which harbored an enhanced green fluorescent protein (eGFP) sequence at the endogenous locus of the pan photoreceptor marker recoverin (RCVRN). After confirmation of successful targeting and gene stability, three-dimensional retinal organoids were induced from this reporter line. The RCVRN-eGFP reporter faithfully replicated endogenous protein expression of recoverin and revealed the developmental characteristics of photoreceptors during retinal differentiation. The RCVRN-eGFP specifically and steadily labeled photoreceptor cells from photoreceptor precursors to mature rods and cones. Additionally, abundant eGFP-positive photoreceptors were enriched by fluorescence-activated cell sorting, and their transcriptome signatures were revealed by RNA sequencing and data analysis. Moreover, potential clusters of differentiation (CD) biomarkers were extracted for the enrichment of photoreceptors for clinical applications, such as CD133 for the positive selection of photoreceptors. Altogether, the RCVRN-eGFP reporter hiPSC line was successfully established and the first global expression database of recoverin-positive photoreceptors was constructed. These achievements will provide a powerful tool for dynamically monitoring photoreceptor cell development and purification of human photoreceptors, thus facilitating photoreceptor cell therapy for advanced retinal disorders.

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

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

Generation of a Cone Photoreceptor-specific GNGT2 Reporter Line in Human Pluripotent Stem Cells

Fluorescent reporter lines generated in human pluripotent stem cells are a highly useful tool to track, isolate, and analyze cell types and lineages in live cultures. Here, we generate the first human cone photoreceptor reporter cell line by CRISPR/Cas9 genome editing of a human embryonic stem cell (hESC) line to tag both alleles of the Guanine nucleotide-binding protein subunit gamma-T2 (GNGT2) gene with a mCherry reporter cassette. Three-dimensional optic vesicle-like structures were produced to verify reporter fidelity and track cones throughout their development in culture. The GNGT2-T2A-mCherry hESC line faithfully and robustly labels GNGT2-expressing cones throughout the entirety of their differentiation in vitro, recapitulating normal fetal expression of this gene. Our observations indicate that human cones undergo significant migratory activity during the course of differentiation in vitro. Consistent with this, our analysis of human fetal retinae from different stages of development finds positional differences of the cone population depending on their state of maturation. This novel reporter line will provide a useful tool for investigating human cone development and disease.

Retinal Organoids Long-Term Functional Characterization Using Two-Photon Fluorescence Lifetime and Hyperspectral Microscopy

Pluripotent stem cell-derived organoid technologies have opened avenues to preclinical basic science research, drug discovery, and transplantation therapy in organ systems. Stem cell-derived organoids follow a time course similar to species-specific organ gestation in vivo. However, heterogeneous tissue yields, and subjective tissue selection reduce the repeatability of organoid-based scientific experiments and clinical studies. To improve the quality control of organoids, we introduced a live imaging technique based on two-photon microscopy to non-invasively monitor and characterize retinal organoids’ (RtOgs’) long-term development. Fluorescence lifetime imaging microscopy (FLIM) was used to monitor the metabolic trajectory, and hyperspectral imaging was applied to characterize structural and molecular changes. We further validated the live imaging experimental results with endpoint biological tests, including quantitative polymerase chain reaction (qPCR), single-cell RNA sequencing, and immunohistochemistry. With FLIM results, we analyzed the free/bound nicotinamide adenine dinucleotide (f/b NADH) ratio of the imaged regions and found that there was a metabolic shift from glycolysis to oxidative phosphorylation. This shift occurred between the second and third months of differentiation. The total metabolic activity shifted slightly back toward glycolysis between the third and fourth months and stayed relatively stable between the fourth and sixth months. Consistency in organoid development among cell lines and production lots was examined. Molecular analysis showed that retinal progenitor genes were expressed in all groups between days 51 and 159. Photoreceptor gene expression emerged around the second month of differentiation, which corresponded to the shift in the f/b NADH ratio. RtOgs between 3 and 6 months of differentiation exhibited photoreceptor gene expression levels that were between the native human fetal and adult retina gene expression levels. The occurrence of cone opsin expression (OPN1 SW and OPN1 LW) indicated the maturation of photoreceptors in the fourth month of differentiation, which was consistent with the stabilized level of f/b NADH ratio starting from 4 months. Endpoint single-cell RNA and immunohistology data showed that the cellular compositions and lamination of RtOgs at different developmental stages followed those in vivo.

Co-grafts of Human Embryonic Stem Cell Derived Retina Organoids and Retinal Pigment Epithelium for Retinal Reconstruction in Immunodeficient Retinal Degenerate Royal College of Surgeons Rats

End-stage age-related macular degeneration (AMD) and retinitis pigmentosa (RP) are two major retinal degenerative (RD) conditions that result in irreversible vision loss. Permanent eye damage can also occur in battlefields or due to accidents. This suggests there is an unmet need for developing effective strategies for treating permanent retinal damages. In previous studies, co-grafted sheets of fetal retina with its retinal pigment epithelium (RPE) have demonstrated vision improvement in rat retinal disease models and in patients, but this has not yet been attempted with stem-cell derived tissue. Here we demonstrate a cellular therapy for irreversible retinal eye injuries using a "total retina patch" consisting of retinal photoreceptor progenitor sheets and healthy RPE cells on an artificial Bruch's membrane (BM). For this, retina organoids (ROs) (cultured in suspension) and polarized RPE sheets (cultured on an ultrathin parylene substrate) were made into a co-graft using bio-adhesives [gelatin, growth factor-reduced matrigel, and medium viscosity (MVG) alginate]. In vivo transplantation experiments were conducted in immunodeficient Royal College of Surgeons (RCS) rats at advanced stages of retinal degeneration. Structural reconstruction of the severely damaged retina was observed based on histological assessments and optical coherence tomography (OCT) imaging. Visual functional assessments were conducted by optokinetic behavioral testing and superior colliculus electrophysiology. Long-term survival of the co-graft in the rat subretinal space and improvement in visual function were observed. Immunohistochemistry showed that co-grafts grew, generated new photoreceptors and developed neuronal processes that were integrated into the host retina. This novel approach can be considered as a new therapy for complete replacement of a degenerated retina.

Deep Learning-Assisted Multiphoton Microscopy to Reduce Light Exposure and Expedite Imaging in Tissues With High and Low Light Sensitivity

Purpose

Two-photon excitation fluorescence (2PEF) reveals information about tissue function. Concerns for phototoxicity demand lower light exposure during imaging. Reducing excitation light reduces the quality of the image by limiting fluorescence emission. We applied deep learning (DL) super-resolution techniques to images acquired from low light exposure to yield high-resolution images of retinal and skin tissues.

Methods

We analyzed two methods: a method based on U-Net and a patch-based regression method using paired images of skin (550) and retina (1200), each with low- and high-resolution paired images. The retina dataset was acquired at low and high laser powers from retinal organoids, and the skin dataset was obtained from averaging 7 to 15 frames or 70 frames. Mean squared error (MSE) and the structural similarity index measure (SSIM) were outcome measures for DL algorithm performance.

Results

For the skin dataset, the patches method achieved a lower MSE (3.768) compared with U-Net (4.032) and a high SSIM (0.824) compared with U-Net (0.783). For the retinal dataset, the patches method achieved an average MSE of 27,611 compared with 146,855 for the U-Net method and an average SSIM of 0.636 compared with 0.607 for the U-Net method. The patches method was slower (303 seconds) than the U-Net method (<1 second).

Conclusions

DL can reduce excitation light exposure in 2PEF imaging while preserving image quality metrics.

Translational Relevance

DL methods will aid in translating 2PEF imaging from benchtop systems to in vivo imaging of light-sensitive tissues such as the retina.

Retinal organoids on-a-chip: a micro-millifluidic bioreactor for long-term organoid maintenance

Retinal degeneration is a leading cause of vision impairment and blindness worldwide and medical care for advanced disease does not exist. Stem cell-derived retinal organoids (RtOgs) became an emerging tool for tissue replacement therapy. However, existing RtOg production methods are highly heterogeneous. Controlled and predictable methodology and tools are needed to standardize RtOg production and maintenance. In this study, we designed a shear stress-free micro-millifluidic bioreactor for nearly labor-free retinal organoid maintenance. We used a stereolithography (SLA) 3D printer to fabricate a mold from which Polydimethylsiloxane (PDMS) was cast. We optimized the chip design using in silico simulations and in vitro evaluation to optimize mass transfer efficiency and concentration uniformity in each culture chamber. We successfully cultured RtOgs at three different differentiation stages (day 41, 88, and 128) on an optimized bioreactor chip for more than 1 month. We used different quantitative and qualitative techniques to fully characterize the RtOgs produced by static dish culture and bioreactor culture methods. By analyzing the results from phase contrast microscopy, single-cell RNA sequencing (scRNA seq), quantitative polymerase chain reaction (qPCR), immunohistology, and electron microscopy, we found that bioreactor-cultured RtOgs developed cell types and morphology comparable to static cultured ones and exhibited similar retinal genes expression levels. We also evaluated the metabolic activity of RtOgs in both groups using fluorescence lifetime imaging (FLIM), and found that the outer surface region of bioreactor cultured RtOgs had a comparable free/bound NADH ratio and overall lower long lifetime species (LLS) ratio than static cultured RtOgs during imaging. To summarize, we validated an automated micro-millifluidic device with significantly reduced shear stress to produce RtOgs of comparable quality to those maintained in conventional static culture.

IGFBPs mediate IGF-1's functions in retinal lamination and photoreceptor development during pluripotent stem cell differentiation to retinal organoids

Development of the retina is regulated by growth factors, such as insulin-like growth factors 1 and 2 (IGF-1/2), which coordinate proliferation, differentiation, and maturation of the neuroepithelial precursors cells. In the circulation, IGF-1/2 are transported by the insulin growth factor binding proteins (IGFBPs) family members. IGFBPs can impact positively and negatively on IGF-1, by making it available or sequestering IGF-1 to or from its receptor. In this study, we investigated the expression of IGFBPs and their role in the generation of human retinal organoids from human pluripotent stem cells, showing a dynamic expression pattern suggestive of different IGFBPs being used in a stage-specific manner to mediate IGF-1 functions. Our data show that IGF-1 addition to culture media facilitated the generation of retinal organoids displaying the typical laminated structure and photoreceptor maturation. The organoids cultured in the absence of IGF-1, lacked the typical laminated structure at the early stages of differentiation and contained significantly less photoreceptors and more retinal ganglion cells at the later stages of differentiation, confirming the positive effects of IGF-1 on retinal lamination and photoreceptor development. The organoids cultured with the IGFBP inhibitor (NBI-31772) and IGF-1 showed lack of retinal lamination at the early stages of differentiation, an increased propensity to generate horizontal cells at mid-stages of differentiation and reduced photoreceptor development at the later stages of differentiation. Together these data suggest that IGFBPs enable IGF-1's role in retinal lamination and photoreceptor development in a stage-specific manner.

Electrophysiologic Characterization of Developing Human Embryonic Stem Cell-Derived Photoreceptor Precursors

Purpose

Photoreceptor precursor cells (PRPs) differentiated from human embryonic stem cells can serve as a source for cell replacement therapy aimed at vision restoration in patients suffering from degenerative diseases of the outer retina, such as retinitis pigmentosa and AMD. In this work, we studied the electrophysiologic maturation of PRPs throughout the differentiation process.

Methods

Human embryonic stem cells were differentiated into PRPs and whole-cell recordings were performed for electrophysiologic characterization at days 0, 30, 60, and 90 along with quantitative PCR analysis to characterize the expression level of various ion channels, which shape the electrophysiologic response. Finally, to characterize the electrically induced calcium currents, we employed calcium imaging (rhod4) to visualize intracellular calcium dynamics in response to electrical activation.

Results

Our results revealed an early and steady presence (approximately 100% of responsive cells) of the delayed potassium rectifier current. In contrast, the percentage of cells exhibiting voltage-gated sodium currents increased with maturation (from 0% to almost 90% of responsive cells at 90 days). Moreover, calcium imaging revealed the presence of voltage-gated calcium currents, which play a major role in vision formation. These results were further supported by quantitative PCR analysis, which revealed a significant and continuous (3- to 50-fold) increase in the expression of various voltage-gated channels concomitantly with the increase in the expression of the photoreceptor marker CRX.

Conclusions

These results can shed light on the electrophysiologic maturation of neurons in general and PRP in particular and can form the basis for devising and optimizing cell replacement-based vision restoration strategies.

Transplanted pluripotent stem cell-derived photoreceptor precursors elicit conventional and unusual light responses in mice with advanced retinal degeneration

Retinal dystrophies often lead to blindness. Developing therapeutic interventions to restore vision is therefore of paramount importance. Here we demonstrate the ability of pluripotent stem cell-derived cone precursors to engraft and restore light responses in the Pde6brd1 mouse, an end-stage photoreceptor degeneration model. Our data show that up to 1.5% of precursors integrate into the host retina, differentiate into cones, and engraft in close apposition to the host bipolar cells. Half of the transplanted mice exhibited visual behavior and of these 33% showed binocular light sensitivity. The majority of retinal ganglion cells exhibited contrast-sensitive ON, OFF or ON-OFF light responses and even motion sensitivity; however, quite a few exhibited unusual responses (eg, light-induced suppression), presumably reflecting remodeling of the neural retina. Our data indicate that despite relatively low engraftment yield, pluripotent stem cell-derived cone precursors can elicit light responsiveness even at advanced degeneration stages. Further work is needed to improve engraftment yield and counteract retinal remodeling to achieve useful clinical applications.

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.

Understanding the complexity of retina and pluripotent stem cell derived retinal organoids with single cell RNA sequencing: current progress, remaining challenges and future prospective

Single-cell sequencing technologies have emerged as a revolutionary tool with transformative new methods to profile genetic, epigenetic, spatial, and lineage information in individual cells. Single-cell RNA sequencing (scRNA-Seq) allows researchers to collect large datasets detailing the transcriptomes of individual cells in space and time and is increasingly being applied to reveal cellular heterogeneity in retinal development, normal physiology, and disease, and provide new insights into cell-type specific markers and signaling pathways. In recent years, scRNA-Seq datasets have been generated from retinal tissue and pluripotent stem cell-derived retinal organoids. Their cross-comparison enables staging of retinal organoids, identification of specific cells in developing and adult human neural retina and provides deeper insights into cell-type sub-specification and geographical differences. In this article, we review the recent rapid progress in scRNA-Seq analyses of retina and retinal organoids, the questions that remain unanswered and the technical challenges that need to be overcome to achieve consistent results that reflect the complexity, functionality, and interactions of all retinal cell types.

Stem Cell Transplantation Therapy for Retinal Degenerative Diseases

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

Systematic Comparison of Retinal Organoid Differentiation from Human Pluripotent Stem Cells Reveals Stage Specific, Cell Line, and Methodological Differences

A major goal in the stem cell field is to generate tissues that can be utilized as a universal tool for in vitro models of development and disease, drug development, or as a resource for patients suffering from disease or injury. Great efforts are being made to differentiate human pluripotent stem cells in vitro toward retinal tissue, which is akin to native human retina in its cytoarchitecture and function, yet the numerous existing retinal induction protocols remain variable in their efficiency and do not routinely produce morphologically or functionally mature photoreceptors. Herein, we determine the impact that the method of embryoid body (EB) formation and maintenance as well as cell line background has on retinal organoid differentiation from human embryonic stem cells and human induced pluripotent stem cells. Our data indicate that cell line-specific differences dominate the variables that underline the differentiation efficiency in the early stages of differentiation. In contrast, the EB generation method and maintenance conditions determine the later differentiation and maturation of retinal organoids. Of the latter, the mechanical method of EB generation under static conditions, accompanied by media supplementation with Y27632 for the first 48 hours of differentiation, results in the most consistent formation of laminated retinal neuroepithelium containing mature and electrophysiologically responsive photoreceptors. Collectively, our data provide substantive evidence for stage-specific differences in the ability to give rise to laminated retinae, which is determined by cell line-specific differences in the early stages of differentiation and EB generation/organoid maintenance methods at later stages.

The histone variant H2A.Z in gene regulation

The histone variant H2A.Z is involved in several processes such as transcriptional control, DNA repair, regulation of centromeric heterochromatin and, not surprisingly, is implicated in diseases such as cancer. Here, we review the recent developments on H2A.Z focusing on its role in transcriptional activation and repression. H2A.Z, as a replication-independent histone, has been studied in several model organisms and inducible mammalian model systems. Its loading machinery and several modifying enzymes have been recently identified, and some of the long-standing discrepancies in transcriptional activation and/or repression are about to be resolved. The buffering functions of H2A.Z, as supported by genome-wide localization and analyzed in several dynamic systems, are an excellent example of transcriptional control. Posttranslational modifications such as acetylation and ubiquitination of H2A.Z, as well as its specific binding partners, are in our view central players in the control of gene expression. Understanding the key-mechanisms in either turnover or stabilization of H2A.Z-containing nucleosomes as well as defining the H2A.Z interactome will pave the way for therapeutic applications in the future.

CRX Expression in Pluripotent Stem Cell-Derived Photoreceptors Marks a Transplantable Subpopulation of Early Cones

Death of photoreceptors is a common cause of age-related and inherited retinal dystrophies, and thus their replenishment from renewable stem cell sources is a highly desirable therapeutic goal. Human pluripotent stem cells provide a useful cell source in view of their limitless self-renewal capacity and potential to not only differentiate into cells of the retina but also self-organize into tissue with structure akin to the human retina as part of three-dimensional retinal organoids. Photoreceptor precursors have been isolated from differentiating human pluripotent stem cells through application of cell surface markers or fluorescent reporter approaches and shown to have a similar transcriptome to fetal photoreceptors. In this study, we investigated the transcriptional profile of CRX-expressing photoreceptor precursors derived from human pluripotent stem cells and their engraftment capacity in an animal model of retinitis pigmentosa (Pde6brd1), which is characterized by rapid photoreceptor degeneration. Single cell RNA-Seq analysis revealed the presence of a dominant cell cluster comprising 72% of the cells, which displayed the hallmarks of early cone photoreceptor expression. When transplanted subretinally into the Pde6brd1 mice, the CRX⁺ cells settled next to the inner nuclear layer and made connections with the inner neurons of the host retina, and approximately one-third of them expressed the pan cone marker, Arrestin 3, indicating further maturation upon integration into the host retina. Together, our data provide valuable molecular insights into the transcriptional profile of human pluripotent stem cells-derived CRX⁺ photoreceptor precursors and indicate their usefulness as a source of transplantable cone photoreceptors. Stem Cells 2019;37:609-622.

Gene Editing in Human Pluripotent Stem Cells: Recent Advances for Clinical Therapies

The identification of human embryonic stem cells and reprogramming technology to obtain induced pluripotent stem cells from adult somatic cells have provided unique opportunity to create human disease models, gene editing strategies and cell therapy options.Development of pluripotent stem cells from somatic cells and genomic manipulation tools enabled to use site specific nucleases in the cell therapy research. Identification of efficient gene manipulation, safe differentiation and use will provide a novel strategy to treat many diseases in the near future. Current available registered clinical trials clearly indicate the need for pluripotent stem cell and gene therapy treatment options. Although gene editing based pluripotent stem cell research is a popular field for research worldwide, improvement of clinical approaches for treatment still remains to be investigated. In this review, we summarized the current situation of gene editing based pluripotent cell therapy developments and applications in clinics.

Transplantation of photoreceptors into the degenerative retina: Current state and future perspectives

The mammalian retina displays no intrinsic regenerative capacities, therefore retinal degenerative diseases such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP) result in a permanent loss of the light-sensing photoreceptor cells. The degeneration of photoreceptors leads to vision impairment and, in later stages, complete blindness. Several therapeutic strategies have been developed to slow down or prevent further retinal degeneration, however a definitive cure i.e. replacement of the lost photoreceptors, has not yet been established. Cell-based treatment approaches, by means of photoreceptor transplantation, have been studied in pre-clinical animal models over the last three decades. The introduction of pluripotent stem cell-derived retinal organoids represents, in principle, an unlimited source for the generation of transplantable human photoreceptors. However, safety, immunological and reproducibility-related issues regarding the use of such cells still need to be solved. Moreover, the recent finding of cytoplasmic material transfer between donor and host photoreceptors demands reinterpretation of several former transplantation studies. At the same time, material transfer between healthy donor and dysfunctional patient photoreceptors also offers a potential alternative strategy for therapeutic intervention. In this review we discuss the history and current state of photoreceptor transplantation, the techniques used to assess rescue of visual function, the prerequisites for effective transplantation as well as the main roadblocks, including safety and immune response to the graft, that need to be overcome for successful clinical translation of photoreceptor transplantation approaches.

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

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

A Novel Approach to Single Cell RNA-Sequence Analysis Facilitates In Silico Gene Reporting of Human Pluripotent Stem Cell-Derived Retinal Cell Types

Cell type-specific investigations commonly use gene reporters or single-cell analytical techniques. However, reporter line development is arduous and generally limited to a single gene of interest, while single-cell RNA (scRNA)-sequencing (seq) frequently yields equivocal results that preclude definitive cell identification. To examine gene expression profiles of multiple retinal cell types derived from human pluripotent stem cells (hPSCs), we performed scRNA-seq on optic vesicle (OV)-like structures cultured under cGMP-compatible conditions. However, efforts to apply traditional scRNA-seq analytical methods based on unbiased algorithms were unrevealing. Therefore, we developed a simple, versatile, and universally applicable approach that generates gene expression data akin to those obtained from reporter lines. This method ranks single cells by expression level of a bait gene and searches the transcriptome for genes whose cell-to-cell rank order expression most closely matches that of the bait. Moreover, multiple bait genes can be combined to refine datasets. Using this approach, we provide further evidence for the authenticity of hPSC-derived retinal cell types. Stem Cells 2018;36:313-324.

Generation of a rod-specific NRL reporter line in human pluripotent stem cells

Reporter lines generated in human pluripotent stem cells can be highly useful for the analysis of specific cell types and lineages in live cultures. We created the first human rod reporter line using CRISPR/Cas9 genome editing to replace one allele of the Neural Retina Leucine zipper (NRL) gene with an eGFP transgene in the WA09 human embryonic stem cell (hESC) line. After confirming successful targeting, three-dimensional optic vesicle structures were produced to examine reporter specificity and to track rod differentiation in culture. The NRL^+/eGFP hESC line robustly and exclusively labeled the entirety of rods throughout differentiation, eventually revealing highly mature structural features. This line provides a valuable tool for studying human rod development and disease and testing therapeutic strategies for retinitis pigmentosa.

Concise Review: Getting to the Core of Inherited Bone Marrow Failures

Bone marrow failure syndromes (BMFS) are a group of disorders with complex pathophysiology characterized by a common phenotype of peripheral cytopenia and/or hypoplastic bone marrow. Understanding genetic factors contributing to the pathophysiology of BMFS has enabled the identification of causative genes and development of diagnostic tests. To date more than 40 mutations in genes involved in maintenance of genomic stability, DNA repair, ribosome and telomere biology have been identified. In addition, pathophysiological studies have provided insights into several biological pathways leading to the characterization of genotype/phenotype correlations as well as the development of diagnostic approaches and management strategies. Recent developments in bone marrow transplant techniques and the choice of conditioning regimens have helped improve transplant outcomes. However, current morbidity and mortality remain unacceptable underlining the need for further research in this area. Studies in mice have largely been unable to mimic disease phenotype in humans due to difficulties in fully replicating the human mutations and the differences between mouse and human cells with regard to telomere length regulation, processing of reactive oxygen species and lifespan. Recent advances in induced pluripotency have provided novel insights into disease pathogenesis and have generated excellent platforms for identifying signaling pathways and functional mapping of haplo‐insufficient genes involved in large‐scale chromosomal deletions–associated disorders. In this review, we have summarized the current state of knowledge in the field of BMFS with specific focus on modeling the inherited forms and how to best utilize these models for the development of targeted therapies. Stem Cells2017;35:284–298

The Stagnant Adaptation of Defined and Xeno-Free Culture of iPSCs in Academia

Pluripotent Stem Cells were originally derived and cultured using a feeder layer of cells. Movements have been undertaken to transition from this method to one more defined, high-throughput, and without xenogenic factors. Tremendous research has been done in this area and many products have been developed, however, based on our analysis of recent publications in stem cell related journals many in academia are still using older methods like a feeder layer. In this short communication, we discuss the feasibility of transitioning to defined, xeno-free methods, how a standardized method could improve the field and industry, and that a study bringing together multiple institutions comparing culture methods could be done to evaluate the efficacy of these new methods.