The transcription factor Nkx3.2 (Bapx1) is an important chondrocyte maturation inhibitor. Previous Nkx3.2 knockdown and overexpression studies in non-mammalian gnathostomes have focused on its role in primary jaw joint development, while the function of this gene in broader skeletal development is not fully described. We generated a mutant allele of nkx3.2 in zebrafish with CRISPR/Cas9 and applied a range of techniques to characterize skeletal phenotypes at developmental stages from larva to adult, revealing loss of the jaw joint, fusions in bones of the occiput, morphological changes in the Weberian apparatus, and the loss or deformation of bony elements derived from basiventral cartilages of the vertebrae. Axial phenotypes are reminiscent of Nkx3.2 knockout in mammals, suggesting that the function of this gene in axial skeletal development is ancestral to osteichthyans. Our results highlight the broad role of nkx3.2 in zebrafish skeletal development and its context-specific functions in different skeletal elements.

Optical projection tomography (OPT) is a 3D imaging alternative to conventional microscopy which allows imaging of millimeter-sized object with isotropic micrometer resolution. The zebrafish is an established model organism and an important tool used in genetic and chemical screening. The size and optical transparency of the embryo and larva makes them well suited for imaging using OPT. Here, we present an open-source implementation of an OPT platform, built around a customized sample stage, 3D-printed parts and open source algorithms optimized for the system. We developed a versatile automated workflow including a two-step image processing approach for correcting the center of rotation and generating accurate 3D reconstructions. Our results demonstrate high-quality 3D reconstruction using synthetic data as well as real data of live and fixed zebrafish. The presented 3D-printable OPT platform represents a fully open design, low-cost and rapid loading and unloading of samples. Our system offers the opportunity for researchers with different backgrounds to setup and run OPT for large scale experiments, particularly in studies using zebrafish larvae as their key model organism.

The transcription factor Nkx3.2 (Bapx1) is an important chondrocyte maturation inhibitor. Previous Nkx3.2 knock-down and overexpression studies in non-mammalian gnathostomes have focused on its role in primary jaw joint development, while little is known about the function of this gene in broader skeletal development. We generated CRISPR/Cas9 knockout of nkx3.2 in zebrafish and applied a range of techniques to characterize skeletal phenotypes at developmental stages from larva to adult, revealing fusions in bones of the occiput, the loss or deformation of bony elements derived from basiventral cartilages of the vertebrae, and an increased length of the proximal radials of the dorsal and anal fins. These phenotypes are reminiscent of Nkx3.2 knockout phenotypes in mammals, suggesting that the function of this gene in axial skeletal development is ancestral to osteichthyans. Our results highlight the broad role of nkx3.2 in zebrafish skeletal development and its context-specific functions in different skeletal elements.

Background: The development of the vertebrate limb skeleton requires a complex interaction of multiple factors to facilitate the correct shaping and positioning of bones and joints. Growth and differentiation factor 5 (Gdf5) is involved in patterning appendicular skeletal elements including joints. Expression of gdf5 in zebrafish has been detected in fin mesenchyme condensations and segmentation zones as well as the jaw joint, however, little is known about the functional role of Gdf5 outside of Amniota.Results: We generated CRISPR/Cas9 knockout of gdf5 in zebrafish and analyzed the resulting phenotype at different developmental stages. Homozygous gdf5 mutant zebrafish displayed changes in segmentation of the endoskeletal disc and, as a consequence, loss of posterior radials in the pectoral fins. Mutant fish also displayed disorganization and reduced length of endoskeletal elements in the median fins, while joints and mineralization seemed unaffected. Conclusions: Our study demonstrates the importance of Gdf5 in the development of the zebrafish pectoral and median fin endoskeleton and reveals that the severity of the effect increases from anterior to posterior elements. Our findings are consistent with phenotypes observed in the human and mouse appendicular skeleton in response to Gdf5 knockout, suggesting a broadly conserved role for Gdf5 in Osteichthyes.endoskeleton, fin radials, Gdf5 mutant, pectoral fin, zebrafish appendages 1 | INTRODUCTION Skeletal development requires a complex interaction of multiple factors to ensure correct positioning and shaping of cartilage elements and bones including the proper formation of joints articulating the skeleton. Skeletogenesis via endochondral ossification involves three major processes starting with mesenchyme condensation followed by chondrogenesis and finally the invasion of blood vessels and chondrocyte replacement by and transformation Laura Waldmann and Jake Leyhr contributed equally to this study.

The acquisition of movable jaws was a major event during vertebrate evolution. The role of NK3 homeobox 2 (Nkx3.2) transcription factor in patterning the primary jaw joint of gnathostomes (jawed vertebrates) is well known, however knowledge about its regulatory mechanism is lacking. In this study, we report a proximal enhancer element of Nkx3.2 that is deeply conserved in most gnathostomes but undetectable in the jawless hagfish and lamprey. This enhancer is active in the developing jaw joint region of the zebrafish Danio rerio, and was thus designated as jaw joint regulatory sequence 1 (JRS1). We further show that JRS1 enhancer sequences from a range of gnathostome species, including a chondrichthyan and mammals, have the same activity in the jaw joint as the native zebrafish enhancer, indicating a high degree of functional conservation despite the divergence of cartilaginous and bony fish lineages or the transition of the primary jaw joint into the middle ear of mammals. Finally, we show that deletion of JRS1 from the zebrafish genome using CRISPR/Cas9 results in a significant reduction of early gene expression of nkx3.2 and leads to a transient jaw joint deformation and partial fusion. Emergence of this Nkx3.2 enhancer in early gnathostomes may have contributed to the origin and shaping of the articulating surfaces of vertebrate jaws.