The cellular basis of cartilage growth and shape change in larval and metamorphosing Xenopus frogs

As the first and sometimes only skeletal tissue to appear, cartilage plays a fundamental role in the development and evolution of vertebrate body shapes. This is especially true for amphibians whose largely cartilaginous feeding skeleton exhibits unparalleled ontogenetic and phylogenetic diversification as a consequence of metamorphosis. Fully understanding the evolutionary history, evolvability and regenerative potential of cartilage requires in-depth analysis of how chondrocytes drive growth and shape change. This study is a cell-level description of the larval growth and postembryonic shape change of major cartilages of the feeding skeleton of a metamorphosing amphibian. Histology and immunohistochemistry are used to describe and quantify patterns and trends in chondrocyte size, shape, division, death, and arrangement, and in percent matrix from hatchling to froglet for the lower jaw, hyoid and branchial arch cartilages of Xenopus laevis. The results are interpreted and integrated into programs of cell behaviors that account for the larval growth and histology, and metamorphic remodeling of each element. These programs provide a baseline for investigating hormone-mediated remodeling, cartilage regeneration, and intrinsic shape regulating mechanisms. These programs also contain four features not previously described in vertebrates: hypertrophied chondrocytes being rejuvenated by rapid cell cycling to a prechondrogenic size and shape; chondrocytes dividing and rearranging to reshape a cartilage; cartilage that lacks a perichondrium and grows at single-cell dimensions; and an adult cartilage forming de novo in the center of a resorbing larval one. Also, the unexpected superimposition of cell behaviors for shape change onto ones for larval growth and the unprecedented exploitation of very large and small cell sizes provide new directions for investigating the development and evolution of skeletal shape and metamorphic ontogenies.

A novel cis-regulatory element drives early expression of Nkx3.2 in the gnathostome primary jaw joint

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.

The amazing and anomalous axolotls as scientific models

Ambystoma mexicanum (axolotl) embryos and juveniles have been used as model organisms for developmental and regenerative research for many years. This neotenic aquatic species maintains the unique capability to regenerate most, if not all, of its tissues well into adulthood. With large externally developing embryos, axolotls were one of the original model species for developmental biology. However, increased access to, and use of, organisms with sequenced and annotated genomes, such as Xenopus laevis and tropicalis and Danio rerio, reduced the prevalence of axolotls as models in embryogenesis studies. Recent sequencing of the large axolotl genome opens up new possibilities for defining the recipes that drive the formation and regeneration of tissues like the limbs and spinal cord. However, to decode the large Ambystoma mexicanum genome will take a herculean effort, community resources, and the development of novel techniques. Here, we provide an updated axolotl-staging chart ranging from 1-cell stage to immature adult paired with a perspective on both historical and current axolotl research that spans from their use in early studies of development to the recent cutting-edge research, employment of transgenesis, high resolution imaging, and study of mechanisms deployed in regeneration. This article is protected by copyright. All rights reserved.

The broad role of Nkx3.2 in the development of the zebrafish axial skeleton

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.

Conserved and unique transcriptional features of pharyngeal arches in the skate (Leucoraja erinacea) and evolution of the jaw

The origin of the jaw is a long-standing problem in vertebrate evolutionary biology. Classical hypotheses of serial homology propose that the upper and lower jaw evolved through modifications of dorsal and ventral gill arch skeletal elements, respectively. If the jaw and gill arches are derived members of a primitive branchial series, we predict that they would share common developmental patterning mechanisms. Using candidate and RNAseq/differential gene expression analyses, we find broad conservation of dorsoventral (DV) patterning mechanisms within the developing mandibular, hyoid, and gill arches of a cartilaginous fish, the skate (Leucoraja erinacea). Shared features include expression of genes encoding members of the ventralizing BMP and endothelin signaling pathways and their effectors, the joint markers nkx3.2 and gdf5 and prochondrogenic transcription factor barx1, and the dorsal territory marker pou3f3. Additionally, we find that mesenchymal expression of eya1/six1 is an ancestral feature of the mandibular arch of jawed vertebrates, whereas differences in notch signaling distinguish the mandibular and gill arches in skate. Comparative transcriptomic analyses of mandibular and gill arch tissues reveal additional genes differentially expressed along the DV axis of the pharyngeal arches, including scamp5 as a novel marker of the dorsal mandibular arch, as well as distinct transcriptional features of mandibular and gill arch muscle progenitors and developing gill buds. Taken together, our findings reveal conserved patterning mechanisms in the pharyngeal arches of jawed vertebrates, consistent with serial homology of their skeletal derivatives, as well as unique transcriptional features that may underpin distinct jaw and gill arch morphologies.

Knockdown of hspg2 is associated with abnormal mandibular joint formation and neural crest cell dysfunction in zebrafish

BACKGROUND

Heparan sulfate proteoglycan 2 (HSPG2) encodes for perlecan, a large proteoglycan that plays an important role in cartilage formation, cell adhesion, and basement membrane stability. Mutations in HSPG2 have been associated with Schwartz-Jampel Syndrome (SJS) and Dyssegmental Dysplasia Silverman-Handmaker Type (DDSH), two disorders characterized by skeletal abnormalities. These data indicate a function for HSPG2 in cartilage development/maintenance. However, the mechanisms in which HSPG2 regulates cartilage development are not completely understood. Here, we explored the relationship between this gene and craniofacial development through morpholino-mediated knockdown of hspg2 using zebrafish.

RESULTS

Knockdown of hspg2 resulted in abnormal development of the mandibular jaw joint at 5 days post fertilization (DPF). We surmised that defects in mandible development were a consequence of neural crest cell (NCC) dysfunction, as these multipotent progenitors produce the cartilage of the head. Early NCC development was normal in morphant animals as measured by distal-less homeobox 2a (dlx2a) and SRY-box transcription factor 10 (sox10) expression at 1 DPF. However, subsequent analysis at later stages of development (4 DPF) revealed a decrease in the number of Sox10 + and Collagen, type II, alpha 1a (Col2a1a)+ cells within the mandibular jaw joint region of morphants relative to random control injected embryos. Concurrently, morphants showed a decreased expression of nkx3.2, a marker of jaw joint formation, at 4 DPF.

CONCLUSIONS

Collectively, these data suggest a complex role for hspg2 in jaw joint formation and late stage NCC differentiation.

Zebrafish model for spondylo-megaepiphyseal-metaphyseal dysplasia reveals post-embryonic roles of Nkx3.2 in the skeleton

The regulated expansion of chondrocytes within growth plates and joints ensures proper skeletal development through adulthood. 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 distal limb joints. Whereas nkx3.2 knockdown zebrafish and mouse Nkx3.2 mutants display embryonic lethal jaw joint fusions and skeletal reductions, respectively, they lack the skeletal overgrowth seen in SMMD patients. Here, we report adult viable nkx3.2 mutant zebrafish displaying cartilage overgrowth in place of a missing jaw joint, as well as severe dysmorphologies of the facial skeleton, skullcap and spine. In contrast, cartilage overgrowth and scoliosis are absent in rare viable nkx3.2 knockdown animals that lack jaw joints, supporting post-embryonic roles for Nkx3.2. Single-cell RNA-sequencing and in vivo validation reveal increased proliferation and upregulation of stress-induced pathways, including prostaglandin synthases, in mutant chondrocytes. By generating a zebrafish model for the skeletal overgrowth defects of SMMD, we reveal post-embryonic roles for Nkx3.2 in dampening proliferation and buffering the stress response in joint-associated chondrocytes.

nkx3.2 mutant zebrafish accommodate jaw joint loss through a phenocopy of the head shapes of Paleozoic jawless fish

The vertebrate jaw is a versatile feeding apparatus. To function, it requires a joint between the upper and lower jaws, so jaw joint defects are often highly disruptive and difficult to study. To describe the consequences of jaw joint dysfunction, we engineered two independent null alleles of a single jaw joint marker gene, nkx3.2, in zebrafish. These mutations caused zebrafish to become functionally jawless via fusion of the upper and lower jaw cartilages (ankylosis). Despite lacking jaw joints, nkx3.2 mutants survived to adulthood and accommodated this defect by: (a) having a remodeled skull with a fixed open gape, reduced snout and enlarged branchial region; and (b) performing ram feeding in the absence of jaw-generated suction. The late onset and broad extent of phenotypic changes in the mutants suggest that modifications to the skull are induced by functional agnathia, secondarily to nkx3.2 loss of function. Interestingly, nkx3.2 mutants superficially resemble ancient jawless vertebrates (anaspids and furcacaudiid thelodonts) in overall head shape. Because no homology exists in individual skull elements between these taxa, the adult nkx3.2 phenotype is not a reversal but rather a convergence due to similar functional requirements of feeding without moveable jaws. This remarkable analogy strongly suggests that jaw movements themselves dramatically influence the development of jawed vertebrate skulls. Thus, these mutants provide a unique model with which to: (a) investigate adaptive responses to perturbation in skeletal development; (b) re-evaluate evolutionarily inspired interpretations of phenocopies generated by gene knockdowns and knockouts; and (c) gain insight into feeding mechanics of the extinct agnathans.

Sequence of chondrocranial development in the oriental fire bellied toad Bombina orientalis

The vertebrate head as a major novelty is directly linked to the evolutionary success of the vertebrates. Sequential information on the embryonic pattern of cartilaginous head development are scarce, but important for the understanding of its evolution. In this study, we use the oriental fire bellied toad, Bombina orientalis, a basal anuran to investigate the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen in post-neurulation stages until the premetamorphic larvae. We use different methodological approaches like classic histology, clearing and staining, and antibody staining to examine the larval skeletal morphology. Our results show that in contrast to other vertebrates, the ceratohyals are the first centers of chondrification. They are followed by the palatoquadrate and the basihyal. The latter later fuses to the ceratohyal and the branchial basket. Anterior elements like Meckel's cartilage and the rostralia are delayed in development and alter the ancestral anterior posterior pattern observed in other vertebrates. The ceratobranchials I-IV, components of the branchial basket, follow this strict anterior-posterior pattern of chondrification as reported in other amphibians. Chondrification of different skeletal elements follows a distinct pattern and the larval skeleton is nearly fully developed at Gosner Stage 28. We provide baseline data on the pattern and timing of early cartilage development in a basal anuran species, which may serve as guidance for further experimental studies in this species as well as an important basis for the understanding of the evolutionary changes in head development among amphibians and vertebrates.