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.

The health and function of our visual system relies on accurate gene expression. While many genetic mutations are associated with visual impairment and blindness, we are just beginning to understand the complex interplay between gene regulation and retinal pathologies. MicroRNAs (miRNAs), a class of non-coding RNAs, are important regulators of gene expression that exert their function through post-transcriptional silencing of complementary mRNA targets. According to recent transcriptomic analyses, certain miRNA species are expressed in all retinal cell types, while others are cell type-specific. As miRNAs play important roles in homeostasis, cellular function, and survival of differentiated retinal cell types, their dysregulation is associated with retinal degenerative diseases. Thus, advancing our understanding of the genetic networks modulated by miRNAs is central to harnessing their potential as therapeutic agents to overcome visual impairment. In this review, we summarize the role of distinct miRNAs in specific retinal cell types, the current knowledge on their implication in inherited retinal disorders, and their potential as therapeutic agents.

MicroRNAs (miRNAs) are important regulators of gene expression. These small, non-coding RNAs post-transcriptionally silence messenger RNAs (mRNAs) in a sequence-specific manner. In this way, miRNAs control important regulatory functions, also in the retina. If dysregulated, these molecules are involved in several retinal pathologies. For example, several miRNAs have been linked to essential photoreceptor functions, including light sensitivity, synaptic transmission, and modulation of inflammatory responses. Mechanistic miRNA knockout and knockdown studies further linked their functions to degenerative retinal diseases. Of note, the type and timing of genetic manipulation before, during, or after retinal development, is important when studying specific miRNA knockout effects. Within this review, we focus on miR-124 and the miR-183/96/182 cluster, which have assigned functions in photoreceptors in health and disease. As a single miRNA can regulate hundreds of mRNAs, we will also discuss the experimental validation and manipulation approaches to study complex miRNA/mRNA regulatory networks. Revealing these networks is essential to understand retinal pathologies and to harness miRNAs as precise therapeutic and diagnostic tools to stabilize the photoreceptors’ transcriptomes and, thereby, function.

Cell transplantation is a promising therapeutic approach to recover loss of neurons and vision in patient retinas. So far, human photoreceptor transplants restored some visual function in degenerating mouse retina. Whether retinal cell transplants also integrate into human retina, and how to optimize this for different pathologies are still unknown. Here, we sought to determine if human retina organoids generated from pluripotent stem cells might assist cell replacement therapy development in a human-to-human setting. Models for intra- and subretinal cell transplantation strategies were explored: Photoreceptor donor cells carrying a transgenic fluorescent reporter were enriched from acutely dissociated human retinal organoids. Donor cells were precisely transplanted by microinjection into the retina of host organoids, but high cell numbers might require multiple injections posing potential damage. Alternatively, donor cells were transplanted in large numbers by placing them in subretinal-like contact to the apical organoid surface. Using postmitotic retinal organoids (age >170-days) as a source for donor cells and as hosts, we show that six weeks after subretinal-like transplantation, large clusters of photoreceptors reproducibly incorporate into the host retina. Transplanted clusters frequently are located within or across the host photoreceptor layer, include cone and rod photoreceptors, and become infiltrated by cell processes of host Müller glia, indicative of structural integration. Histological and ultrastructural data of virally-labeled photoreceptor transplants show characteristic morphological and structural features of polarized photoreceptors: inner segments and ribbon synapses, and donor-host cell contacts develop contributing to the retinal outer limiting membrane. These results demonstrate that human retinal organoids provide a preclinical research system for cell replacement therapies.Graphical abstract

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