Production of Retinal Cells from Confluent Human iPS Cells

Patient stem cell-derived in vitro disease models for developing novel therapies of retinal ciliopathies

Primary cilia are specialized organelles on the surface of almost all cells in vertebrate tissues and are primarily involved in the detection of extracellular stimuli. In retinal photoreceptors, cilia are uniquely modified to form outer segments containing components required for the detection of light in stacks of membrane discs. Not surprisingly, vision impairment is a frequent phenotype associated with ciliopathies, a heterogeneous class of conditions caused by mutations in proteins required for formation, maintenance and/or function of primary cilia. Traditionally, immortalized cell lines and model organisms have been used to provide insights into the biology of ciliopathies. The advent of methods for reprogramming human somatic cells into pluripotent stem cells has enabled the generation of in vitro disease models directly from patients suffering from ciliopathies. Such models help us in investigating pathological mechanisms specific to human physiology and in developing novel therapeutic approaches. In this article, we review current protocols to differentiate human pluripotent stem cells into retinal cell types, and discuss how these cellular and/or organoid models can be utilized to interrogate pathobiology of ciliopathies affecting the retina and for testing prospective treatments.

Restoration of functional PAX6 in aniridia patient iPSC-derived ocular tissue models using repurposed nonsense suppression drugs

Congenital aniridia is a rare, pan-ocular disease causing severe sight loss, with only symptomatic intervention offered to patients. Approximately 40% of aniridia patients present with heterozygous nonsense variants in PAX6, resulting in haploinsufficiency. Translational readthrough-inducing drugs (TRIDs) have the ability to weaken the recognition of in-frame premature termination codons (PTCs), permitting full-length protein to be translated. We established induced pluripotent stem cell (iPSC)-derived 3D optic cups and 2D limbal epithelial stem cell (LESC) models from two aniridia patients with prevalent PAX6 nonsense mutations. Both in vitro models show reduced PAX6 protein levels, mimicking the disease. The repurposed TRIDs amlexanox and 2,6-diaminopurine (DAP) and the positive control compounds ataluren and G418 were tested for their efficiency. Amlexanox was identified as the most promising TRID, increasing full-length PAX6 levels in both models and rescuing the disease phenotype through normalization of VSX2 and cell proliferation in the optic cups and reduction of ABCG2 protein and SOX10 expression in LESCs. This study highlights the significance of patient iPSC-derived cells as a new model system for aniridia and proposes amlexanox as a new putative treatment for nonsense-mediated aniridia.

Inducible nonhuman primate models of retinal degeneration for testing end-stage therapies

The anatomical differences between the retinas of humans and most animal models pose a challenge for testing novel therapies. Nonhuman primate (NHP) retina is anatomically closest to the human retina. However, there is a lack of relevant NHP models of retinal degeneration (RD) suitable for preclinical studies. To address this unmet need, we generated three distinct inducible cynomolgus macaque models of RD. We developed two genetically targeted strategies using optogenetics and CRISPR-Cas9 to ablate rods and mimic rod-cone dystrophy. In addition, we created an acute model by physical separation of the photoreceptors and retinal pigment epithelium using a polymer patch. Among the three models, the CRISPR-Cas9-based approach was the most advantageous model in view of recapitulating disease-specific features and its ease of implementation. The acute model, however, resulted in the fastest degeneration, making it the most relevant model for testing end-stage vision restoration therapies such as stem cell transplantation.

Mesenchymal stem cell-derived extracellular vesicles for cell-free therapy of ocular diseases

Mesenchymal stem cells-derived extracellular vesicles (MSC-EVs) have noticeably attracted clinicians' attention in treating ocular diseases. As the paracrine factor of MSCs and an alternative for cell-free therapies, MSC-EVs can be conveniently dropped over the ocular surface or diffused through the retina upon intravitreal injection, without increasing the risks of cellular rejection and tumor formation. For clinical translation, a standardized and scalable production, as well as reprogramming the MSC-EVs, are highly encouraged. This review aims to assess the potential approaches for EV production and functional modification, in addition to summarizing the worldwide clinical trials initiated for various physiological systems and the specific biochemical effects of MSC-EVs on the therapy of eye diseases. Recent advances in the therapy of ocular diseases based on MSC-EVs are reviewed, and the associated challenges and prospects are discussed as well.

Retina organoids: Window into the biophysics of neuronal systems

With a kind of magnetism, the human retina draws the eye of neuroscientist and physicist alike. It is attractive as a self-organizing system, which forms as a part of the central nervous system via biochemical and mechanical cues. The retina is also intriguing as an electro-optical device, converting photons into voltages to perform on-the-fly filtering before the signals are sent to our brain. Here, we consider how the advent of stem cell derived in vitro analogs of the retina, termed retina organoids, opens up an exploration of the interplay between optics, electrics, and mechanics in a complex neuronal network, all in a Petri dish. This review presents state-of-the-art retina organoid protocols by emphasizing links to the biochemical and mechanical signals of in vivo retinogenesis. Electrophysiological recording of active signal processing becomes possible as retina organoids generate light sensitive and synaptically connected photoreceptors. Experimental biophysical tools provide data to steer the development of mathematical models operating at different levels of coarse-graining. In concert, they provide a means to study how mechanical factors guide retina self-assembly. In turn, this understanding informs the engineering of mechanical signals required to tailor the growth of neuronal network morphology. Tackling the complex developmental and computational processes in the retina requires an interdisciplinary endeavor combining experiment and theory, physics, and biology. The reward is enticing: in the next few years, retina organoids could offer a glimpse inside the machinery of simultaneous cellular self-assembly and signal processing, all in an in vitro setting.

Otx2-Genetically Modified Retinal Pigment Epithelial Cells Rescue Photoreceptors after Transplantation

Inherited retinal degenerations are blinding diseases characterized by the loss of photoreceptors. Their extreme genetic heterogeneity complicates treatment by gene therapy. This has motivated broader strategies for transplantation of healthy retinal pigmented epithelium to protect photoreceptors independently of the gene causing the disease. The limited clinical benefit for visual function reported up to now is mainly due to dedifferentiation of the transplanted cells that undergo an epithelial-mesenchymal transition. We have studied this mechanism in vitro and revealed the role of the homeogene OTX2 in preventing dedifferentiation through the regulation of target genes. We have overexpressed OTX2 in retinal pigmented epithelial cells before their transplantation in the eye of a model of retinitis pigmentosa carrying a mutation in Mertk, a gene specifically expressed by retinal pigmented epithelial cells. OTX2 increases significantly the protection of photoreceptors as seen by histological and functional analyses. We observed that the beneficial effect of OTX2 is non-cell autonomous, and it is at least partly mediated by unidentified trophic factors. Transplantation of OTX2-genetically modified cells may be medically effective for other retinal diseases involving the retinal pigmented epithelium as age-related macular degeneration.

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

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

The immune response of stem cells in subretinal transplantation

Stem cell transplantation is a potential curative treatment for degenerative diseases of the retina. Among cell injection sites, the subretinal space (SRS) is particularly advantageous as it is maintained as an immune privileged site by the retinal pigment epithelium (RPE) layer. Thus, the success of subretinal transplantation depends on maintenance of RPE integrity. Moreover, both embryonic stem cells (ESCs) and mesenchymal stem cells (MSCs) have negligible immunogenicity and in fact are immunosuppressive. Indeed, many studies have demonstrated that immunosuppressive drugs are not necessary for subretinal transplantation of stem cells if the blood-retinal barrier is not breached during surgery. The immunogenicity of induced pluripotent stem cells (iPSCs) appears more complex, and requires careful study before clinical application. Despite low rates of graft rejection in animal models, survival rates for ESCs, MSCs, and iPSCs in retina are generally poor, possibly due to resident microglia activated by cell transplantation. To improve graft survival in SRS transplantation, damage to the blood-retinal barrier must be minimized using appropriate surgical techniques. In addition, agents that inhibit microglial activation may be required. Finally, immunosuppressants may be required, at least temporarily, until the blood-retinal barrier heals. We review surgical methods and drug regimens to enhance the likelihood of graft survival after SRS transplantation.