Organoid cultures represent a unique tool to investigate the developmental complexity of tissues like the human retina. NRL is a transcription factor required for the specification and homeostasis of mammalian rod photoreceptors. In Nrl-deficient mice, photoreceptor precursor cells do not differentiate into rods, and instead follow a default photoreceptor specification pathway to generate S-cone-like cells. To investigate whether this genetic switch mechanism is conserved in humans, we used CRISPR/Cas9 gene editing to engineer an NRL-deficient embryonic stem cell (ESC) line (NRL ), and differentiated it into retinal organoids. Retinal organoids self-organize and resemble embryonic optic vesicles (OVs) that recapitulate the natural histogenesis of rods and cone photoreceptors. NRL OVs develop comparably to controls, and exhibit a laminated, organized retinal structure with markers of photoreceptor synaptogenesis. Using immunohistochemistry and quantitative PCR, we observed that NRL OVs do not express NRL, or other rod photoreceptor markers directly or indirectly regulated by NRL. On the contrary, they show an abnormal number of photoreceptors positive for S-OPSIN, which define a primordial subtype of cone, and overexpress other cone genes indicating a conserved molecular switch in mammals. This study represents the first evidence in a human in vitro ESC-derived organoid system that NRL is required to define rod identity, and that in its absence S-cone-like cells develop as the default photoreceptor cell type. It shows how gene edited retinal organoids provide a useful system to investigate human photoreceptor specification, relevant for efforts to generate cells for transplantation in retinal degenerative diseases. © AlphaMed Press 2021 SIGNIFICANCE STATEMENT: Photoreceptor cells located in the retina are essential for vision and their degeneration leads to a large proportion of global blindness. Rod photoreceptors needed for night vision are prevalent, whereas cone photoreceptors needed for high acuity day light vision are rare. This study has engineered a human pluripotent embryonic stem cell line that lacks NRL gene function. When differentiated in vitro into 3D retinal organoids, the engineered organoids contain cone cells, but no rods. This study shows that NRL is required to differentiate rod photoreceptors, and represents a powerful tool to generate enriched populations of cone photoreceptor cells in the laboratory.© 2021 AlphaMed Press.
About The Expert
Jane C Sowden