The proteasome subunit psmb1 is essential for craniofacial cartilage maturation and morphogenesis

Craniofacial dysmorphisms are among the most common birth defects. Proteasome mutations frequently result in craniofacial dysmorphisms including lower jaw malformations; however, the underlying mechanisms are unknown. Here we use a zebrafish proteasome subunit beta 1 (psmb1) mutant to define the cellular mechanisms underlying proteasome mutation-induced craniofacial dysmorphisms. psmb1 mutants exhibit a flattened ceratohyal and smaller Meckel's and palatoquadrate cartilages. Ceratohyal flattening is a result of failed chondrocyte convergent extension, accompanied by reduced numbers of chondrocytes in the lower jaw due to defects in chondrocyte differentiation. Morphogenesis of craniofacial muscles and tendons is similarly perturbed. psmb1 mutants lack the hyohyal muscles and craniofacial tendons are shortened and disorganized. We additionally identify a critical period for proteasome function in craniofacial development, specifically during chondrocyte and muscle differentiation. psmb1 overexpression in sox10+ cells of mutant embryos rescued both cartilage and tendon phenotypes but induced only a partial rescue of the muscle phenotype, indicating that psmb1 is required in both tissue-autonomous and non-autonomous fashions during craniofacial development. Overall, our work demonstrates that psmb1 is required for craniofacial cartilage, tendon, and muscle differentiation and morphogenesis. Overall design: To investigate the role of psmb1 in craniofacial chondrocyte and muscle development, we sorted sox10+ cells or mylz2+ cells from the heads of psmb1 mutant zebrafish vs. pooled wild-type and heterozygous larvae at 72hpf. We then performed bulk RNA-sequencing and differential gene expression analysis.