Ewsr1a Regulates the Development of the Axial Skeleton in Zebrafish

The EWSR1 (EWS RNA binding protein 1) was originally discovered in Ewing sarcoma, a pediatric bone cancer. In a previous study, we identified zebrafish ewsr1a, the homologue of human EWSR1. Using a zebrafish mutant for ewsr1a, we demonstrated that Ewsr1a promotes differentiation of chondrogenesis in Meckel's cartilage, a component of the facial skeletal elements. Furthermore, we demonstrated that Ewsr1a interacts with the transcription factor SRY-BOX transcription factor 9 (SOX9) and modulates the transcription of its target genes. In this study, the role of Ewsr1a was further investigated in axial skeletogenesis. The maternal zygotic (MZ) ewsr1a/ewsr1a zebrafish display curved spines derived from irregularly spaced intervertebral discs (IVDs) and aberrant differentiation of nucleus pulposus cells. Using an in vitro cell culture system, we further show that the N-terminus of Ewsr1a is required for the interaction with Sox9. In zebrafish, the Ewsr1a also interacts with a Sox9 target gene, specifically the first intron of col2a1a. Zebrafish with the MZ ewsr1a/ewsr1a genotype display an increased level of collagen type II protein in the notochord starting at 36 h post fertilization. We propose that Ewsr1a contributes to IVD formation by regulating the expression of col2a1a.