Abstract
Screens for genes that orchestrate neural circuit formation in mammals have been hindered by practical constraints of germline mutagenesis. To overcome these limitations, we combined RNA-seq with somatic CRISPR mutagenesis to study synapse development in the mouse retina. Here synapses occur between cellular layers, forming two multilayered neuropils. The outer neuropil, the outer plexiform layer (OPL), contains synapses made by rod and cone photoreceptor axons on rod and cone bipolar dendrites, respectively. We used RNA-seq to identify selectively expressed genes encoding cell surface and secreted proteins and CRISPR-Cas9 electroporation with cell-specific promoters to assess their roles in OPL development. Among the genes identified in this way are Wnt5a and Wnt5b. They are produced by rod bipolars and activate a non-canonical signaling pathway in rods to regulate early OPL patterning. The approach we use here can be applied to other parts of the brain.
•Lamination in synaptic neuropil of the outer retina arises in a series of steps•RNA-seq reveals distinct cell surface genes expressed by four outer retinal cell types•CRISPR-based electroporation inactivates genes in photoreceptor and bipolar neurons•Wnt5 acts through Ryk to regulate neuropil formation in the outer retina
Photoreceptors form synapses on interneurons in the retina. Sarin et al. used RNA-seq and somatic CRISPR/Cas9 mutagenesis to seek genes required for this process. They show that Wnt5 produced by bipolar interneurons acts on rod photoreceptors to regulate synapse location.