NRL- and CRX-guided gene network modulates photoreceptor presynapse size and positioning during retinal development

Unique morphologies of rod and cone photoreceptor presynaptic terminals permit the formation of synapses onto interneurons during retina development. We integrated multiple “omics” datasets of developing rod and S-cone-like photoreceptors and identified 719 genes that are regulated by NRL and CRX, critical transcriptional regulators of rod differentiation, as candidates for controlling presynapse morphology. In vivo knockdown of 72 candidate genes in the developing retina uncovered 26 genes that alter size and/or positioning of rod spherules in the outer plexiform layer. Co-expression of seven cDNAs with their cognate shRNAs rescued the rod presynapse phenotype. Loss of function of four genes in germline or by an AAV-mediated CRISPR/Cas9 strategy validated RNAi screen findings. A protein interaction network analysis of the 26 positive effectors revealed additional candidates in the NRL/CRX-regulated presynapse morphology-associated gene network. Follow-up knockdowns of two novel candidates support the proposed network. Our studies demonstrate a requirement of multiple components in a modular network for rod presynapse morphogenesis and provide a functional genomic framework for deciphering genetic determinants of morphological specification during development.