Lineage tracing of col10a1 cells identifies distinct progenitor populations for osteoblasts and joint cells in the regenerating fin of medaka (Oryzias latipes)

A candidate sex determination locus in amphibians which evolved by structural variation between X- and Y-chromosomes

Most vertebrates develop distinct females and males, where sex is determined by repeatedly evolved environmental or genetic triggers. Undifferentiated sex chromosomes and large genomes have caused major knowledge gaps in amphibians. Only a single master sex-determining gene, the dmrt1-paralogue (dm-w) of female-heterogametic clawed frogs (Xenopus; ZW♀/ZZ♂), is known across >8740 species of amphibians. In this study, by combining chromosome-scale female and male genomes of a non-model amphibian, the European green toad, Bufo(tes) viridis, with ddRAD- and whole genome pool-sequencing, we reveal a candidate master locus, governing a male-heterogametic system (XX♀/XY♂). Targeted sequencing across multiple taxa uncovered structural X/Y-variation in the 5'-regulatory region of the gene bod1l, where a Y-specific non-coding RNA (ncRNA-Y), only expressed in males, suggests that this locus initiates sex-specific differentiation. Developmental transcriptomes and RNA in-situ hybridization show timely and spatially relevant sex-specific ncRNA-Y and bod1l-gene expression in primordial gonads. This coincided with differential H3K4me-methylation in pre-granulosa/pre-Sertoli cells, pointing to a specific mechanism of amphibian sex determination.

A Collagen10a1 mutation disrupts cell polarity in a medaka model for metaphyseal chondrodysplasia type Schmid

Heterozygous mutations in COL10A1 lead to metaphyseal chondrodysplasia type Schmid (MCDS), a skeletal disorder characterized by epiphyseal abnormalities. Prior analysis revealed impaired trimerization and intracellular retention of mutant collagen type X alpha 1 chains as cause for elevated endoplasmic reticulum (ER) stress. However, how ER stress translates into structural defects remained unclear. We generated a medaka (Oryzias latipes) MCDS model harboring a 5 base pair deletion in col10a1, which led to a frameshift and disruption of 11 amino acids in the conserved trimerization domain. col10a1Δ633a heterozygotes recapitulated key features of MCDS and revealed early cell polarity defects as cause for dysregulated matrix secretion and deformed skeletal structures. Carbamazepine, an ER stress-reducing drug, rescued this polarity impairment and alleviated skeletal defects in col10a1Δ633a heterozygotes. Our data imply cell polarity dysregulation as a potential contributor to MCDS and suggest the col10a1Δ633a medaka mutant as an attractive MCDS animal model for drug screening.

Compartmentalization and synergy of osteoblasts drive bone formation in the regenerating fin

Zebrafish regenerate their fins which involves a component of cell plasticity. It is currently unclear how regenerate cells divide labor to allow for appropriate growth and patterning. Here, we studied lineage relationships of fluorescence-activated cell sorting-enriched epidermal, bone-forming (osteoblast), and (non-osteoblast) blastemal fin regenerate cells by single-cell RNA sequencing, lineage tracing, targeted osteoblast ablation, and electron microscopy. Most osteoblasts in the outgrowing regenerate derive from osterix+ osteoblasts, while mmp9+ cells reside at segment joints. Distal blastema cells contribute to distal osteoblast progenitors, suggesting compartmentalization of the regenerating appendage. Ablation of osterix+ osteoblasts impairs segment joint and bone matrix formation and decreases regenerate length which is partially compensated for by distal regenerate cells. Our study characterizes expression patterns and lineage relationships of rare fin regenerate cell populations, indicates inherent detection and compensation of impaired regeneration, suggests variable dependence on growth factor signaling, and demonstrates zonation of the elongating fin regenerate.

Lineage Tracing of Bone Cells in the Regenerating Fin and During Repair of Bone Lesions

Small teleost fishes such as zebrafish and medaka show remarkable regeneration capabilities upon tissue injury or amputation. To elucidate cellular mechanisms of teleost tissue repair and regeneration processes, the Cre/LoxP recombination system for cell lineage tracing is a widely used technique. In this chapter, we describe protocols used for inducible Cre/LoxP recombination-mediated lineage tracing of osteoblast progenitors during medaka fin regeneration as well as during the repair of osteoporosis-like bone lesions in the medaka vertebral column. Our approach can be adapted for lineage tracing of other cell populations in the regenerating teleost fin or in other tissues undergoing repair.

Zebrafish Fin: Complex Molecular Interactions and Cellular Mechanisms Guiding Regeneration

The zebrafish caudal fin has become a popular model to study cellular and molecular mechanisms of regeneration due to its high regenerative capacity, accessibility for experimental manipulations, and relatively simple anatomy. The formation of a regenerative epidermis and blastema are crucial initial events and tightly regulated. Both the regenerative epidermis and the blastema are highly organized structures containing distinct domains, and several signaling pathways regulate the formation and interaction of these domains. Bone is the major tissue regenerated from the progenitor cells of the blastema. Several cellular mechanisms can provide source cells for blastemal (pre-)osteoblasts, including dedifferentiation of differentiated osteoblasts and de novo formation from other cell types, providing intriguing examples of cellular plasticity. In recent years, omics analyses and single-cell approaches have elucidated genetic and epigenetic regulation, increasing our knowledge of the surprisingly complex coordination of various mechanisms to achieve successful restoration of a seemingly simple structure.

A non-disruptive and efficient knock-in approach allows fate tracing of resident osteoblast progenitors during repair of vertebral lesions in medaka

During bone development and repair, osteoblasts are recruited to bone deposition sites. To identify the origin of recruited osteoblasts, cell lineage tracing using Cre/loxP recombination is commonly used. However, a confounding factor is the use of transgenic Cre drivers that do not accurately recapitulate endogenous gene expression or the use of knock-in Cre drivers that alter endogenous protein activity or levels. Here, we describe a CRISPR/Cas9 homology-directed repair knock-in approach that allows efficient generation of Cre drivers controlled by the endogenous gene promoter. In addition, a self-cleaving peptide preserves the reading frame of the endogenous protein. Using this approach, we generated col10a1p2a-CreERT2 knock-in medaka and show that tamoxifen-inducible CreERT2 efficiently recombined loxP sites in col10a1 cells. Similar knock-in efficiencies were obtained when two unrelated loci (osr1 and col2a1a) were targeted. Using live imaging, we traced the fate of col10a1 osteoblast progenitors during bone lesion repair in the medaka vertebral column. We show that col10a1 cells at neural arches represent a mobilizable cellular source for bone repair. Together, our study describes a previously unreported strategy for precise cell lineage tracing via efficient and non-disruptive knock-in of Cre.

Transcriptome Profiling of Osteoblasts in a Medaka (Oryzias latipes) Osteoporosis Model Identifies Mmp13b as Crucial for Osteoclast Activation

Matrix metalloproteases (MMPs) play crucial roles in extracellular matrix (ECM) modulation during osteoclast-driven bone remodeling. In the present study, we used transcriptome profiling of bone cells in a medaka model for osteoporosis and bone regeneration to identify factors critical for bone remodeling and homeostasis. This identified mmp13b, which was strongly expressed in osteoblast progenitors and upregulated under osteoporotic conditions and during regeneration of bony fin rays. To characterize the role of mmp13b in bone remodeling, we generated medaka mmp13b mutants by CRISPR/Cas9. We found that mmp13b mutants form normal numbers of osteoblasts and osteoclasts. However, osteoclast activity was severely impaired under osteoporotic conditions. In mmp13b mutants and embryos treated with the MMP13 inhibitor CL-82198, unmineralized collagens and mineralized bone matrix failed to be degraded. In addition, the dynamic migratory behavior of activated osteoclasts was severely affected in mmp13b mutants. Expression analysis showed that maturation genes were downregulated in mmp13b deficient osteoclasts suggesting that they remain in an immature and non-activated state. We also found that fin regeneration was delayed in mmp13b mutants with a concomitant alteration of the ECM and reduced numbers of osteoblast progenitors in regenerating joint regions. Together, our findings suggest that osteoblast-derived Mmp13b alters the bone ECM to allow the maturation and activation of osteoclasts during bone remodeling in a paracrine manner. Mmp13b-induced ECM alterations are also required to facilitate osteoblast progenitor recruitment and full regeneration of bony fin rays.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Screening and identification of key biomarkers in alimentary tract cancers: A bioinformatic analysis

BACKGROUND

Alimentary tract cancers (ATCs) are the most malignant cancers in the world. Numerous studies have revealed the tumorigenesis, diagnosis and treatment of ATCs, but many mechanisms remain to be explored.

METHODS

To identify the key genes of ATCs, microarray datasets of oesophageal cancer, gastric cancer and colorectal cancer were obtained from the Gene Expression Omnibus (GEO) database. In total, 207 differentially expressed genes (DEGs) were screened. KEGG and GO function enrichment analyses were conducted, and a protein-protein interaction (PPI) network was generated and gene modules analysis was performed using STRING and Cytoscape.

RESULTS

Five hub genes were screened, and the associated biological processes indicated that these genes were mainly enriched in cellular processes, protein binding and metabolic processes. Clinical survival analysis showed that COL10A1 and KIF14 may be significantly associated with the tumorigenesis or pathology grade of ATCs. In addition, relative human ATC cell lines along with blood samples and tumour tissues of ATC patients were obtained. The data proved that high expression of COL10A1 and KIF14 was associated with tumorigenesis and could be detected in blood.

CONCLUSION

In conclusion, the identification of hub genes in the present study helped us to elucidate the molecular mechanisms of tumorigenesis and identify potential diagnostic indicators and targeted treatment for ATCs.

Skeletal Biology and Disease Modeling in Zebrafish

Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Cxcl9l and Cxcr3.2 regulate recruitment of osteoclast progenitors to bone matrix in a medaka osteoporosis model

Bone remodeling requires a balanced interplay of osteoblasts and osteoclasts. While the intercellular signaling that triggers bone cell differentiation is well understood, it remains unclear how bone progenitor cells are recruited to remodeling sites. Various chemokines are upregulated under osteoporotic conditions. However, whether they are involved in progenitor recruitment or instead have inflammatory roles is unknown. Here we used a medaka fish osteoporosis model to identify the chemokine ligand Cxcl9l and receptor Cxcr3.2 as essential to control osteoclast progenitor recruitment and differentiation at bone resorption sites. Cxcr3.2 activity can be blocked by small-molecule inhibitors that protect bone from osteoporotic insult. Our study demonstrates the potential of fish for osteoporosis drug discovery and opens avenues for future osteoporosis therapy.

Bone homeostasis requires continuous remodeling of bone matrix to maintain structural integrity. This involves extensive communication between bone-forming osteoblasts and bone-resorbing osteoclasts to orchestrate balanced progenitor cell recruitment and activation. Only a few mediators controlling progenitor activation are known to date and have been targeted for intervention of bone disorders such as osteoporosis. To identify druggable pathways, we generated a medaka (Oryzias latipes) osteoporosis model, where inducible expression of receptor-activator of nuclear factor kappa-Β ligand (Rankl) leads to ectopic formation of osteoclasts and excessive bone resorption, which can be assessed by live imaging. Here we show that upon Rankl induction, osteoblast progenitors up-regulate expression of the chemokine ligand Cxcl9l. Ectopic expression of Cxcl9l recruits mpeg1-positive macrophages to bone matrix and triggers their differentiation into osteoclasts. We also demonstrate that the chemokine receptor Cxcr3.2 is expressed in a distinct subset of macrophages in the aorta-gonad-mesonephros (AGM). Live imaging revealed that upon Rankl induction, Cxcr3.2-positive macrophages get activated, migrate to bone matrix, and differentiate into osteoclasts. Importantly, mutations in cxcr3.2 prevent macrophage recruitment and osteoclast differentiation. Furthermore, Cxcr3.2 inhibition by the chemical antagonists AMG487 and NBI-74330 also reduced osteoclast recruitment and protected bone integrity against osteoporotic insult. Our data identify a mechanism for progenitor recruitment to bone resorption sites and Cxcl9l and Cxcr3.2 as potential druggable regulators of bone homeostasis and osteoporosis.

Cells Isolated from Regenerating Caudal Fin of Sparus aurata Can Differentiate into Distinct Bone Cell Lineages

Teleosts have the ability to regenerate their caudal fin upon amputation. A highly proliferative mass of undifferentiated cells called blastema forms beneath wound epidermis and differentiates to regenerate all missing parts of the fin. To date, the origin and fate of the blastema is not completely understood. However, current hypotheses suggest that the blastema is comprised of lineage-restricted dedifferentiated cells. To investigate the differentiation capacity of regenerating fin-derived cells, primary cultures were initiated from the explants of 2-days post-amputation (dpa) regenerates of juvenile gilthead seabream (Sparus aurata). These cells were subcultured for over 30 passages and were named as BSa2. After 10 passages they were characterized for their ability to differentiate towards different bone cell lineages and mineralize their extracellular matrix, through immunocytochemistry, histology, and RT-PCR. Exogenous DNA was efficiently delivered into these cells by nucleofection. Assessment of lineage-specific markers revealed that BSa2 cells were capable of osteo/chondroblastic differentiation. BSa2 cells were also found to be capable of osteoclastic differentiation, as demonstrated through TRAP-specific staining and pit resorption assay. Here, we describe the development of the first successful cell line viz., BSa2, from S. aurata 2-dpa regenerating caudal fins, which has the ability of multilineage differentiation and is capable of in vitro mineralization. The availability of such in vitro cell systems has the potential to stimulate research on the mechanisms of cell differentiation during fin regeneration and provide new insights into the mechanisms of bone formation.

Glucocorticoid Treatment Leads to Aberrant Ion and Macromolecular Transport in Regenerating Zebrafish Fins

Long-term glucocorticoid administration in patients undergoing immunosuppressive and anti-inflammatory treatment is accompanied by impaired bone formation and increased fracture risk. Furthermore, glucocorticoid treatment can lead to impaired wound healing and altered cell metabolism. Recently, we showed that exposure of zebrafish to the glucocorticoid prednisolone during fin regeneration impacts negatively on the length, bone formation, and osteoblast function of the regenerate. The underlying cellular and molecular mechanisms of impairment, however, remain incompletely understood. In order to further elucidate the anti-regenerative effects of continued glucocorticoid exposure on fin tissues, we performed proteome profiling of fin regenerates undergoing prednisolone treatment, in addition to profiling of homeostatic fin tissue and fins undergoing undisturbed regeneration. By using LC-MS (liquid chromatography-mass spectrometry) we identified more than 6,000 proteins across all tissue samples. In agreement with previous reports, fin amputation induces changes in chromatin structure and extracellular matrix (ECM) composition within the tissue. Notably, prednisolone treatment leads to impaired expression of selected ECM components in the fin regenerate. Moreover, the function of ion transporting ATPases and other proteins involved in macromolecule and vesicular transport mechanisms of the cell appears to be altered by prednisolone treatment. In particular, acidification of membrane-enclosed organelles such as lysosomes is inhibited. Taken together, our data indicate that continued synthetic glucocorticoid exposure in zebrafish deteriorates cellular trafficking processes in the regenerating fin, which interferes with appropriate tissue restoration upon injury.