Combination of CRISPR-Cas9-RNP and Single-Cell RNAseq to Identify Cell State-Specific FOXJ1 Functions in the Human Airway Epithelium.

The study of the airway epithelium in vitro is routinely performed using air-liquid culture (ALI) models from nasal or bronchial basal cells. These 3D experimental models allow to follow the regeneration steps of fully differentiated mucociliary epithelium and to study gene function by performing gene invalidation. Recent progress made with CRISPR-based techniques has overcome the experimental difficulty of this approach, by a direct transfection of ribonucleoprotein complexes combining a mix of synthetic small guide RNAs (sgRNAs) and recombinant Cas9. The approach shows more than 95% efficiency and does not require any selection step. A limitation of this approach is that it generates cell populations that contain heterogeneous deletions, which makes the evaluation of invalidation efficiency difficult. We have successfully used Flongle sequencing (Nanopore) to quantify the number of distinct deletions. We describe the use of CRISPR-Cas9 RNP in combination with single-cell RNA sequencing to functionally characterize the impact of gene invalidation in ALI cultures. The complex ecosystem of the airway epithelium, composed of many cell types, makes single-cell approaches particularly relevant to study cell type, or cell state-specific events. This protocol describes the invalidation of FOXJ1 in ALI cultures through the following steps: (1) Establishment of basal cell cultures from nasal turbinates, (2) CRISPR-Cas9 RNP invalidation of FOXJ1, (3) Quantification of FOXJ1 invalidation efficiency by Nanopore sequencing, (4) Dissociation of ALI cultures and single-cell RNAseq, (5) Analysis of single-cell RNAseq data from FOXJ1-invalidated cells.We confirm here that FOXJ1 invalidation impairs the final differentiation step of multiciliated cells and provides a framework to explore other gene functions.