Single-cell endogenous protein labeling via CRISPR-Cas9-mediated genome editing in the mouse brain.

High-precision mapping of endogenous proteins is essential for understanding the molecular mechanism underlying neuronal functions in the brain. The SLENDR (single-cell labeling of endogenous proteins by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9-mediated homology-directed repair) technique provides single-cell endogenous protein labeling with genetically encoded tags within the mammalian brain through precise genome editing via homology-directed repair (HDR). This technique is based on the introduction of HDR-mediated genome editing into neuronal progenitors in embryonic brains by in utero electroporation. Subsequent histological analyses enable high-resolution interrogation of the subcellular distribution of endogenous proteins within a single neuron using conventional fluorescent microscopy. Here, we describe a step-by-step protocol for the SLENDR technique to label endogenous proteins with genetically encoded tags in single pyramidal cells of the mouse primary somatosensory cortex. This protocol would be helpful to visualize the molecular organization underlying biological processes at single-neuron levels in the brain, such as signal processing from synaptic inputs to neuronal outputs across different scales.