Retinal degenerative diseases resulting in the loss of photoreceptors are one of the major causes of blindness. Photoreceptor replacement therapy is a promising treatment because the transplantation of retina-derived photoreceptors can be applied now to different murine retinopathies to restore visual function. To have an unlimited source of photoreceptors, we derived a transgenic embryonic stem cell (ESC) line in which the Crx-GFP transgene is expressed in photoreceptors and assessed the capacity of a 3D culture protocol to produce integration-competent photoreceptors. This culture system allows the production of a large number of photoreceptors recapitulating the in vivo development. After transplantation, integrated cells showed the typical morphology of mature rods bearing external segments and ribbon synapses. We conclude that a 3D protocol coupled with ESCs provides a safe and renewable source of photoreceptors displaying a development and transplantation competence comparable to photoreceptors from age-matched retinas.

The development of improved methods to culture retinal organoids is relevant for the investigation of mechanisms of retinal development under pathophysiological conditions, for screening of neuroprotective compounds, and for providing a cellular source for clinical transplantation. We report a tissue-engineering approach to accelerate and standardize the production of retinal organoids by culturing mouse embryonic stem cells (mESC) in optimal physico-chemical microenvironments. Arrayed round-bottom milliwells composed of biomimetic hydrogels, combined with an optimized medium formulation, promoted the rapid generation of retina-like tissue from mESC aggregates in a highly efficient and stereotypical manner: ∼93% of the aggregates contained retinal organoid structures. 26 day-old retinal organoids were composed of ∼80% of photoreceptors, of which ∼22% are GNAT2-positive cones, an important and rare sensory cell type that is difficult to study in rodent models. The compartmentalization of retinal organoids into predefined locations on a two-dimensional array not only allowed us to derive almost all aggregates into retinal organoids, but also to reliably capture the dynamics of individual organoids, an advantageous requirement for high-throughput experimentation. Our improved retinal organoid culture system should be useful for applications that require scalability and single-organoid traceability.

Light toxicity is suspected to enhance certain retinal degenerative processes such as age-related macular degeneration. Death of photoreceptors can be induced by their exposure to the visible light, and although cellular processes within photoreceptors have been characterized extensively, the role of the retinal pigment epithelium (RPE) in this model is less well understood. We demonstrate that exposition to intense light causes the immediate breakdown of the outer blood–retinal barrier (BRB). In a molecular level, we observed the slackening of adherens junctions tying up the RPE and massive leakage of albumin into the neural retina. Retinal pigment epithelial cells normally secrete vascular endothelial growth factor (VEGF) at their basolateral side; light damage in contrast leads to VEGF increase on the apical side – that is, in the neuroretina. Blocking VEGF, by means of lentiviral gene transfer to express an anti-VEGF antibody in RPE cells, inhibits outer BRB breakdown and retinal degeneration, as illustrated by functional, behavioral and morphometric analysis. Our data show that exposure to high levels of visible light induces hyperpermeability of the RPE, likely involving VEGF signaling. The resulting retinal edema contributes to irreversible damage to photoreceptors. These data suggest that anti-VEGF compounds are of therapeutic interest when the outer BRB is altered by retinal stresses.

The cone function is essential to mediate high visual acuity, color vision, and daylight vision. Inherited cone dystrophies and age-related macular degeneration affect a substantial percentage of the world population. To identify and isolate the most competent cells for transplantation and integration into the retina, cone tracing during development would be an important added value. To that aim, the Chrnb4-EGFP mouse line was characterized throughout retinogenesis. It revealed a sub-population of early retinal progenitors expressing the reporter gene that is progressively restricted to mature cones during retina development. The presence of the native CHRNB4 protein was confirmed in EGFP-positive cells, and it presents a similar pattern in the human retina. Sub-retinal transplantations of distinct subpopulations of Chrnb4-EGFP-expressing cells revealed the embryonic day 15.5 high-EGFP population the most efficient cells to interact with host retinas to provoke the appearance of EGFP-positive cones in the photoreceptor layer. Importantly, transplantations into the DsRed retinas revealed material exchanges between donor and host retinas, as >80% of transplanted EGFP-positive cones also were DsRed positive. Whether this cell material fusion is of significant therapeutic advantage requires further thorough investigations. The Chrnb4-EGFP mouse line definitely opens new research perspectives in cone genesis and retina repair.

Disease‐causing variants of a large number of genes trigger inherited retinal degeneration leading to photoreceptor loss. Because cones are essential for daylight and central vision such as reading, mobility, and face recognition, this review focuses on a variety of animal models for cone diseases. The pertinence of using these models to reveal genotype/phenotype correlations and to evaluate new therapeutic strategies is discussed. Interestingly, several large animal models recapitulate human diseases and can serve as a strong base from which to study the biology of disease and to assess the scale‐up of new therapies. Examples of innovative approaches will be presented such as lentiviral‐based transgenesis in pigs and adeno‐associated virus (AAV)‐gene transfer into the monkey eye to investigate the neural circuitry plasticity of the visual system. The models reported herein permit the exploration of common mechanisms that exist between different species and the identification and highlighting of pathways that may be specific to primates, including humans. © 2015 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.