Single cell analysis of juvenile Nkx3.2 mutant zebrafish craniofacial skeleton

The specification and maintenance of distinct zones of chondrocytes within growth plates and joints ensures proper skeletal development through adulthood. Rare mutations in the transcription factor NKX3.2 underlie Spondylo-megaepiphyseal-metaphyseal dysplasia (SMMD), which is characterized by skeletal defects including scoliosis, large epiphyses, wide growth plates, and supernumerary joints in the distal limbs. Embryonic knockdown of nkx3.2 function in zebrafish had revealed a requirement in jaw joint specification, yet embryonic lethality of nkx3.2 knockdown zebrafish and mouse Nkx3.2 mutants had precluded an analysis of post-embryonic functions. Here we report adult viable nkx3.2 zebrafish mutants that display ectopic cartilage overgrowth in place of a missing jaw joint, as well as severe dysmorphologies of the facial skeleton, skullcap, and spine. We also isolate rare viable nkx3.2 knockdown animals that lack the jaw joint but fail to display ectopic cartilage growth and scoliosis, indicating post-embryonic roles for Nkx3.2 beyond jaw joint specification. Consistently, we observe nkx3.2 expression in the subarticular zone of the adult jaw joint and in pre-hypertrophic growth plate chondrocytes. Single-cell RNA sequencing reveals an upregulation of stress-induced pathways in mutants, including the prostaglandin D2 synthase ptgdsb.1 and the mTOR regulator sestrin1, which we confirm by in situ RNA analysis of the defective jaw joint region. Our data reveal a zebrafish model for the joint and spine defects of SMMD and point to post-embryonic roles for Nkx3.2 in buffering the stress response and dampening proliferation in joint-adjacent chondrocytes. Single cell RNA sequencing of FACS isolated cartilages (fli:GFP/sox10:DsRed+) from wildtype and nkx3.2 mutants