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Sarah E. Hadyniak, Kiara C. Eldred, Boris Brenerman, Kasia A. Huseey, Anna La Torre, Jay Neitz, Maureen Neitz, James Taylor, Robert Johnston; Human retinas and retinal organoids reveal temporal regulation of green and red cone photoreceptor specification. Invest. Ophthalmol. Vis. Sci. 2020;61(7):3799.
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© ARVO (1962-2015); The Authors (2016-present)
The mechanisms that diversify neuronal subtypes during human development are poorly understood. Trichromacy is unique to primates among mammals, enabled by specification of blue (S), green (M), and red (L) cones. During retinal development, cones choose between M and L cone subtype fates. Two non-exclusive models have been proposed: a stochastic model, in which a regulatory DNA element randomly loops to either the M- or L-opsin promoter to drive expression, and a temporal model, in which cone subtypes are generated in a developmental progression.
We sought to determine the developmental order of M and L cone generation. We quantified M- and L-opsin expression in human fetal and organoid RNA-seq datasets by analyzing absolute pileup counts for each SNP. We examined ratios of M and L cone subtypes in 750 adults and performed an association study on 20 relevant retinal patterning genes to find SNPs which correlated with changes in M:L ratios. We developed an in situ method to visually distinguish between the highly similar M- and L-opsin mRNA transcripts using two sets of duplex probes spanning 40-44 nucleotides, and examined expression in organoids and in human tissue. We altered retinoic acid (RA) signaling in human retinal organoids from day 120 of differentiation to day 200 to assay in changes of M and L cone generation.
Here we present data that support a temporal mechanism for M and L cone specification. We find that M-opsin is expressed prior to L-opsin in human fetal retinas and retinal organoids. By examining ratios of M to L cone subtypes in the human population, we found associations with polymorphisms in genes involved in retinoic acid (RA) signaling. Organoids differentiated in extended RA conditions displayed a dramatic increase in L-opsin-expressing cones and decrease in M-opsin-expressing cones.
Together, our data support a model in which temporal specification of M and L cones is controlled by RA signaling, where RA is sufficient to induce L cones and suppress M cones. These studies advance human retinal organoids as a model to study developmental processes unique to humans and primates.
This is a 2020 ARVO Annual Meeting abstract.
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