Combining RNA-Seq and Somatic CRISPR Mutagenesis to Study Mouse Neural Development in vivo

We used RNA sequencing to identify candidate regulators of interactions between photoreceptor axons and bipolar cell (BCs) dendrites in developing mouse retina. We chose three time points: P7, just after the OPL forms and synaptogenesis with BCs begins; P13, as synaptogenesis nears completion and sublamination begins; and P30, when the OPL is mature. We purified cone and rod photoreceptors separately by fluorescence activated cell sorting (FACS) using transgene markers: Rhoicre;Ai9 for rods and HRGPcre;Ai9 for cones. We purified ON BCs, which include ON cone bipolars plus rod bipolars using Grm6:GFP. As appropriate transgenic lines to separate RBCs from CBCs were not available, we performed RNAseq on cells from Grm6:GFP mice that were fixed and immunostained prior to FACS, allowing us to purify RBCs (GFP+PKC+) and CBCs (GFP+PKC-) from the same retinas. As PKC is not highly expressed at P7, profiling of developing rod bipolars separate from developing ON cone bipolars was restricted to P13. RNA sequencing of purified rods, cones and ON-bipolar cells at three time points (P7, P13, P30).