Samd7 preserves cell identity and enforces the 'one neuron-one receptor' rule in vertebrate photoreceptors [5dpf_td]

The exclusive expression of single sensory receptors in individual neurons (the 'one-neuron-one receptor' rule) is essential for vision and other sensory systems. Here, we show that the transcriptional corepressor Samd7 enforces this rule in vertebrate red cones and acts in other photoreceptor types to maintain cell identity. In the zebrafish samd7-/- retina, red cones are transformed to hybrid red/UV-sensitive cones, green cones are transfated to blue cones, and the number of rods is greatly reduced. In the mouse Samd7-/- retina, dorsal M-cones are transformed to hybrid M/S-cones—analogous to the transformation of red to red/UV cones that occurs in zebrafish—and rods aberrantly express cone genes including S-opsin. Altogether, Samd7 acts to repress short-wavelength cone gene expression in long-wavelength-sensitive cones, thereby sustaining the mutually exclusive patterns of opsin expression and cone identity required for color vision. Overall design: To perform RNA-seq on 5-dpf thrb:tdTomato+ cells, the following crosses were performed: WT × WT;thrb:tdTomato+/- or samd7stl888/stl888 × samd7stl888/stl888;thrb:tdTomato+/-. A subset of larvae were genotyped from each resultant clutch. TdTomato+ larvae were then separated for subsequent dissociation. 40-50 eyes from 20-25 larvae were dissected from each clutch as an individual replicate. The eyes were then dissociated into single cells and subjected to FACS as described in another section. 10,000-25,000 cells were collected and sorted directly to buffer RLT from the Rneasy minElute Cleanup Kit (Qiagen); RNA was then extracted using the Rneasy Micro Kit (Qiagen), and on-column DNAse treatment performed using the Rnase-Free Dnase Set (Qiagen). RNA concentrations ranged from 40-190 pg/µl (0.3-1.8 ng total), with RNA integrity (RIN) scores ranging from 7.4-8.4.