Retinoic acid and Cyp26b1 are critical regulators of osteogenesis in the axial skeleton

Zebrafish: An Emerging Model for Orthopedic Research

Advances in next-generation sequencing have transformed our ability to identify genetic variants associated with clinical disorders of the musculoskeletal system. However, the means to functionally validate and analyze the physiological repercussions of genetic variation have lagged behind the rate of genetic discovery. The zebrafish provides an efficient model to leverage genetic analysis in an in vivo context. Its utility for orthopedic research is becoming evident in regard to both candidate gene validation as well as therapeutic discovery in tissues such as bone, tendon, muscle, and cartilage. With the development of new genetic and analytical tools to better assay aspects of skeletal tissue morphology, mineralization, composition, and biomechanics, researchers are emboldened to systematically approach how the skeleton develops and to identify the root causes, and potential treatments, of skeletal disease. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:925-936, 2020.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

Cyp26b1 is an essential regulator of distal airway epithelial differentiation during lung development

Proper organ development depends on coordinated communication between multiple cell types. Retinoic acid (RA) is an autocrine and paracrine signaling molecule essential for the development of most organs, including the lung. Despite extensive work detailing effects of RA deficiency in early lung morphogenesis, little is known about how RA regulates late gestational lung maturation. Here, we investigate the role of the RA catabolizing protein Cyp26b1 in the lung. Cyp26b1 is highly enriched in lung endothelial cells (ECs) throughout development. We find that loss of Cyp26b1 leads to reduction of alveolar type 1 cells, failure of alveolar inflation and early postnatal lethality in mouse. Furthermore, we observe expansion of distal epithelial progenitors, but no appreciable changes in proximal airways, ECs or stromal populations. Exogenous administration of RA during late gestation partially mimics these defects; however, transcriptional analyses comparing Cyp26b1-/- with RA-treated lungs reveal overlapping, but distinct, responses. These data suggest that defects observed in Cyp26b1-/- lungs are caused by both RA-dependent and RA-independent mechanisms. This work reports crucial cellular crosstalk during lung development involving Cyp26b1-expressing endothelium and identifies a novel RA modulator in lung development.

Field cyanobacterial blooms producing retinoid compounds cause teratogenicity in zebrafish embryos

Cyanobacteria routinely release potentially harmful bioactive compounds into the aquatic environment. Several recent studies suggested a potential link between the teratogenicity of effects caused by cyanobacteria and production of retinoids. To investigate this relationship, we analysed the teratogenicity of field-collected cyanobacterial bloom samples by means of an in vivo zebrafish embryo test, an in vitro reporter gene bioassay and by the chemical analysis of retinoids. Extracts of biomass from cyanobacterial blooms with the dominance of Microcystis aeruginosa and Aphanizomenon klebahnii were collected from water bodies in the Czech Republic and showed significant retinoid-like activity in vitro, as well as high degrees of teratogenicity in vivo. Chemical analysis was then used to identify a set of retinoids in ng per gram of dry weight concentration range. Subsequent fractionation and bioassay-based characterization identified two fractions with significant in vitro retinoid-like activity. Moreover, in most of the retinoids eluted from these fractions, teratogenicity with malformations typical for retinoid signalling disruption was observed in zebrafish embryos after exposure to the total extracts and these in vitro effective fractions. The zebrafish embryo test proved to be a sensitive toxicity indicator of the biomass extracts, as the teratogenic effects occurred at even lower concentrations than those expected from the activity detected in vitro. In fact, teratogenicity with retinoid-like activity was detected at concentrations that are commonly found in biomasses and even in bulk water surrounding cyanobacterial blooms. Overall, these results provide evidence of a link between retinoid-like activity, teratogenicity and the retinoids produced by cyanobacterial water blooms in the surrounding environment.

Notochord vacuoles absorb compressive bone growth during zebrafish spine formation

The vertebral column or spine assembles around the notochord rod which contains a core made of large vacuolated cells. Each vacuolated cell possesses a single fluid-filled vacuole, and loss or fragmentation of these vacuoles in zebrafish leads to spine kinking. Here, we identified a mutation in the kinase gene dstyk that causes fragmentation of notochord vacuoles and a severe congenital scoliosis-like phenotype in zebrafish. Live imaging revealed that Dstyk regulates fusion of membranes with the vacuole. We find that localized disruption of notochord vacuoles causes vertebral malformation and curving of the spine axis at those sites. Accordingly, in dstyk mutants the spine curves increasingly over time as vertebral bone formation compresses the notochord asymmetrically, causing vertebral malformations and kinking of the axis. Together, our data show that notochord vacuoles function as a hydrostatic scaffold that guides symmetrical growth of vertebrae and spine formation.

A Rare Mutation in SMAD9 Associated With High Bone Mass Identifies the SMAD-Dependent BMP Signaling Pathway as a Potential Anabolic Target for Osteoporosis

Novel anabolic drug targets are needed to treat osteoporosis. Having established a large national cohort with unexplained high bone mass (HBM), we aimed to identify a novel monogenic cause of HBM and provide insight into a regulatory pathway potentially amenable to therapeutic intervention. We investigated a pedigree with unexplained HBM in whom previous sequencing had excluded known causes of monogenic HBM. Whole exome sequencing identified a rare (minor allele frequency 0.0023), highly evolutionarily conserved missense mutation in SMAD9 (c.65T>C, p.Leu22Pro) segregating with HBM in this autosomal dominant family. The same mutation was identified in another two unrelated individuals both with HBM. In silico protein modeling predicts the mutation severely disrupts the MH1 DNA-binding domain of SMAD9. Affected individuals have bone mineral density (BMD) Z-scores +3 to +5, mandible enlargement, a broad frame, torus palatinus/mandibularis, pes planus, increased shoe size, and a tendency to sink when swimming. Peripheral quantitative computed tomography (pQCT) measurement demonstrates increased trabecular volumetric BMD and increased cortical thickness conferring greater predicted bone strength; bone turnover markers are low/normal. Notably, fractures and nerve compression are not found. Both genome-wide and gene-based association testing involving estimated BMD measured at the heel in 362,924 white British subjects from the UK Biobank Study showed strong associations with SMAD9 (PGWAS = 6 × 10-16 ; PGENE = 8 × 10-17 ). Furthermore, we found Smad9 to be highly expressed in both murine cortical bone-derived osteocytes and skeletal elements of zebrafish larvae. Our findings support SMAD9 as a novel HBM gene and a potential novel osteoanabolic target for osteoporosis therapeutics. SMAD9 is thought to inhibit bone morphogenetic protein (BMP)-dependent target gene transcription to reduce osteoblast activity. Thus, we hypothesize SMAD9 c.65T>C is a loss-of-function mutation reducing BMP inhibition. Lowering SMAD9 as a potential novel anabolic mechanism for osteoporosis therapeutics warrants further investigation. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Zebrafish: A Resourceful Vertebrate Model to Investigate Skeletal Disorders

Animal models are essential tools for addressing fundamental scientific questions about skeletal diseases and for the development of new therapeutic approaches. Traditionally, mice have been the most common model organism in biomedical research, but their use is hampered by several limitations including complex generation, demanding investigation of early developmental stages, regulatory restrictions on breeding, and high maintenance cost. The zebrafish has been used as an efficient alternative vertebrate model for the study of human skeletal diseases, thanks to its easy genetic manipulation, high fecundity, external fertilization, transparency of rapidly developing embryos, and low maintenance cost. Furthermore, zebrafish share similar skeletal cells and ossification types with mammals. In the last decades, the use of both forward and new reverse genetics techniques has resulted in the generation of many mutant lines carrying skeletal phenotypes associated with human diseases. In addition, transgenic lines expressing fluorescent proteins under bone cell- or pathway- specific promoters enable in vivo imaging of differentiation and signaling at the cellular level. Despite the small size of the zebrafish, many traditional techniques for skeletal phenotyping, such as x-ray and microCT imaging and histological approaches, can be applied using the appropriate equipment and custom protocols. The ability of adult zebrafish to remodel skeletal tissues can be exploited as a unique tool to investigate bone formation and repair. Finally, the permeability of embryos to chemicals dissolved in water, together with the availability of large numbers of small-sized animals makes zebrafish a perfect model for high-throughput bone anabolic drug screening. This review aims to discuss the techniques that make zebrafish a powerful model to investigate the molecular and physiological basis of skeletal disorders.

Retinoic Acid Signaling and the Zebrafish Dentition During Development and Evolution

Explaining how the extensive diversity in form of vertebrate teeth arose in evolution and the mechanisms by which teeth are made during embryogenesis are intertwined questions that can merit from a better understanding of the roles of retinoic acid (RA) in tooth development. Pioneering studies in rodents showed that dietary vitamin A (VA), and eventually RA (one of the major active metabolites of VA), are required for proper tooth formation and that dentin-forming odontoblast cells seem to be especially sensitive to changes in RA levels. Later, rodent studies further indicated that RA signaling interactions with other cell-signaling pathways are an important part of RA's actions in odontogenesis. Recent investigations employing zebrafish and other teleost fish continued this work in an evolutionary context, and specifically demonstrated that RA is required for the initiation of tooth development. RA is also sufficient in certain circumstances to induce de novo tooth formation. Both effects appear to involve cranial-neural crest cells, again suggesting that RA signaling has a particular influence on odontoblast development. These teleost studies have also highlighted both evolutionary conservation and change in how RA is employed during odontogenesis in different vertebrate lineages, and thus raises the possibility that developmental changes to RA signaling has led to some of the diversity of form seen across vertebrate dentitions. Future progress in this area will come at least in part from expanding the species examined to get a better picture of how often RA signaling has changed in evolution and how this relates to the evolution of dental form.

Hypomorphic zebrafish models mimic the musculoskeletal phenotype of β4GalT7-deficient Ehlers-Danlos syndrome

β4GalT7 is a transmembrane Golgi enzyme, encoded by B4GALT7, that plays a pivotal role in the proteoglycan linker region formation during proteoglycan biosynthesis. Defects in this enzyme give rise to a rare autosomal recessive form of Ehlers-Danlos syndrome (EDS), currently known as 'spondylodysplastic EDS (spEDS-B4GALT7)'. This EDS subtype is mainly characterized by short stature, hypotonia and skeletal abnormalities, thereby illustrating its pleiotropic importance during human development. Insights into the pathogenic mechanisms underlying this disabling disease are very limited, in part due to the lack of a relevant in vivo model. As the majority of mutations identified in patients with spEDS-B4GALT7 are hypomorphic, we generated zebrafish models with partial loss of B4galt7 function, including different knockdown (morphant) and mosaic knockout (crispant) b4galt7 zebrafish models and studied the morphologic, functional and molecular aspects in embryonic and larval stages. Morphant and crispant zebrafish show highly similar morphological abnormalities in early development including a small, round head, bowed pectoral fins, short body-axis and mild developmental delay. Several craniofacial cartilage and bone structures are absent or strongly misshapen. In addition, the total amount of sulfated glycosaminoglycans is significantly diminished and particularly heparan and chondroitin sulfate proteoglycan levels are greatly reduced. We also show impaired cartilage patterning and loss of chondrocyte organization in a cartilage-specific Tg(Col2a1aBAC:mcherry) zebrafish reporter line. The occurrence of the same abnormalities in the different models confirms these are specifically caused by B4galt7 deficiency. A disturbed actin pattern, along with a lack of muscle tone, was only noted in morphants in which translation of b4galt7 was blocked. In conclusion, we generated the first viable animal models for spEDS-B4GALT7, and show that in early development the human spEDS-B4GALT7 phenotype is faithfully mimicked in these zebrafish models. Our findings underscore a key role for β4GalT7 in early development of cartilage, bone and muscle. These models will lead to a better understanding of spEDS-B4GALT7 and can be used in future efforts focusing on therapeutic applications.

New Insights into the Control of Cell Fate Choices and Differentiation by Retinoic Acid in Cranial, Axial and Caudal Structures

Retinoic acid (RA) signaling is an important regulator of chordate development. RA binds to nuclear RA receptors that control the transcriptional activity of target genes. Controlled local degradation of RA by enzymes of the Cyp26a gene family contributes to the establishment of transient RA signaling gradients that control patterning, cell fate decisions and differentiation. Several steps in the lineage leading to the induction and differentiation of neuromesodermal progenitors and bone-producing osteogenic cells are controlled by RA. Changes to RA signaling activity have effects on the formation of the bones of the skull, the vertebrae and the development of teeth and regeneration of fin rays in fish. This review focuses on recent advances in these areas, with predominant emphasis on zebrafish, and highlights previously unknown roles for RA signaling in developmental processes.

Biochemical and physiological importance of the CYP26 retinoic acid hydroxylases

The Cytochrome P450 (CYP) family 26 enzymes contribute to retinoic acid (RA) metabolism and homeostasis in humans, mammals and other chordates. The three CYP26 family enzymes, CYP26A1, CYP26B1 and CYP26C1 have all been shown to metabolize all-trans-retinoic acid (atRA) it's 9-cisRA and 13-cisRA isomers and primary metabolites 4-OH-RA and 4-oxo-RA with high efficiency. While no crystal structures of CYP26 enzymes are available, the binding of various ligands has been extensively explored via homology modeling. All three CYP26 enzymes are inducible by treatment with atRA in various prenatal and postnatal tissues and cell types. However, current literature shows that in addition to regulation by atRA, CYP26 enzyme expression is also regulated by other endogenous processes and inflammatory cytokines. In humans and in animal models the expression patterns of CYP26 enzymes have been shown to be tissue and cell type specific, and the expression of the CYP26 enzymes is believed to regulate the formation of critical atRA concentration gradients in various tissue types. Yet, very little data exists on direct disease associations of altered CYP26 expression or activity. Nevertheless, data is emerging describing a variety of human genetic variations in the CYP26 enzymes that are associated with different pathologies. Interestingly, some of these genetic variants result in increased activity of the CYP26 enzymes potentially leading to complex gene-environment interactions due to variability in dietary intake of retinoids. This review highlights the current knowledge of structure-function of CYP26 enzymes and focuses on their role in human retinoid metabolism in different tissues.

Zebrafish Models of Human Skeletal Disorders: Embryo and Adult Swimming Together

Danio rerio (zebrafish) is an elective model organism for the study of vertebrate development because of its high degree of homology with human genes and organs, including bone. Zebrafish embryos, because of the optical clarity, small size, and fast development, can be easily used in large-scale mutagenesis experiments to isolate mutants with developmental skeletal defects and in high-throughput screenings to find new chemical compounds for the ability to revert the pathological phenotype. On the other hand, the adult zebrafish represents another powerful resource for pathogenic and therapeutic studies about adult human bone diseases. In fish, some characteristics such as bone turnover, reparation, and remodeling of the adult bone tissue cannot be found at the embryonic stage. Several pathological models have been established in adult zebrafish such as bone injury models, osteoporosis, and genetic diseases such as osteogenesis imperfecta. Given the growing interest for metabolic diseases and their complications, adult zebrafish models of type 2 diabetes and obesity have been recently generated and analyzed for bone complications using scales as model system. Interestingly, an osteoporosis-like phenotype has been found to be associated with metabolic alterations suggesting that bone complications share the same mechanisms in humans and fish. Embryo and adult represent powerful resources in rapid development to study bone physiology and pathology from different points of view.

Clinical pathologies of bone fracture modelled in zebrafish

Reduced bone quality or mineral density predict susceptibility to fracture and also attenuate subsequent repair. Bone regrowth is also compromised by bacterial infection, which exacerbates fracture site inflammation. Because of the cellular complexity of fracture repair, as well as genetic and environmental influences, there is a need for models that permit visualisation of the fracture repair process under clinically relevant conditions. To characterise the process of fracture repair in zebrafish, we employed a crush fracture of fin rays, coupled with histological and transgenic labelling of cellular responses; the results demonstrate a strong similarity to the phased response in humans. We applied our analysis to a zebrafish model of osteogenesis imperfecta (OI), which shows reduced bone quality, spontaneous fractures and propensity for non-unions. We found deficiencies in the formation of a bone callus during fracture repair in our OI model and showed that clinically employed antiresorptive bisphosphonates can reduce spontaneous fractures in OI fish and also measurably reduce fracture callus remodelling in wild-type fish. The csf1ra mutant, which has reduced osteoclast numbers, also showed reduced callus remodelling. Exposure to excessive bisphosphonate, however, disrupted callus repair. Intriguingly, neutrophils initially colonised the fracture site, but were later completely excluded. However, when fractures were infected with Staphylococcus aureus, neutrophils were retained and compromised repair. This work elevates the zebrafish bone fracture model and indicates its utility in assessing conditions of relevance to an orthopaedic setting with medium throughput.

This article has an associated First Person interview with the first author of the paper.

Summary: The effect of osteogenesis imperfecta, bisphosphonate treatment and bacterial infection on phases of bone fracture repair are determined using a zebrafish fracture model.

Retinoic acid disrupts osteogenesis in pre-osteoblasts by down-regulating WNT signaling

The skull bones are formed by osteoblasts by intramembranous ossification. WNT signaling is a regulator of bone formation. Retinoic Acid (RA) act as a teratogen affecting craniofacial development. We evaluated the effects of RA on the differentiation and mineralization of MC-3T3 cells, and on the expression of WNT components. MC-3T3 were cultured with or without 0.5 μM RA in osteogenic medium and mineralization was assessed by alizarin red staining. The expression of osteogenic marker genes and WNT genes was evaluated at several time points up to 28 days. RA significantly inhibited MC-3T3 mineralization (p < 0.01), without affecting ALP activity or Alp gene expression. Both parameters gradually increased in time. During culture, RA stimulated Runx2 expression at 14 and 28 days compared to the respective controls (p < 0.05). Also, RA significantly reduced Sp7 expression at days 14 and 21 (p < 0.05). Simultaneously, RA significantly reduced the expression of the WNT genes cMyc, Lef1, Lrp5, Lrp6 and Wnt11 compared to the controls (p < 0.05). In contrast, RA increased the expression of the WNT inhibitors Dkk1 at day 21 and Dkk2 at days 14 and 21 (p < 0.01). Our data indicate that RA disrupts osteogenic differentiation and mineralization by inhibiting WNT signaling.

Overexpression of Notch Signaling Induces Hyperosteogeny in Zebrafish

Notch signaling is one of the evolutionarily conserved signaling pathways in multicellular organisms. It plays an important role in embryonic development. During skeletal development of vertebrates, it regulates bone homeostasis by manipulating both osteoblastogenesis and osteoclastogenesis through different mechanisms. However, due to the different nature of Notch signaling in mesenchymal stem cell and osteoblast, regulation of Notch signaling in bone-related diseases remains unsettled. Previous studies by cell culture and mouse models showed contradictory results regarding the role of Notch signaling in bone homeostasis. To clarify the role of Notch signaling in osteogenesis, we established a zebrafish model, in which Notch1a intracellular domain (N1aICD) was specifically expressed in the osteoblasts. We found that overexpression of N1aICD in osteoblasts caused hyperosteogeny in the column region of zebrafish with the morphology of narrowed neural/hemal canals. Moreover, increased metabolic activity of osteoblasts instead of augmenting osteoblast number led to hyperosteogeny in N1aICD-overexpressed zebrafish. In summary, we successfully established a transgenic zebrafish line overexpressing N1aICD to clarify the in-vivo function of Notch signaling during osteoblastogenesis. In the future, this fish line can serve as a valuable tool to test the therapeutic drugs for hyperosteogeny.

Zebrafish and medaka as models for biomedical research of bone diseases

The identification of disease-causing mutations has in recent years progressed immensely due to whole genome sequencing approaches using patient material. The task accordingly is shifting from gene identification to functional analysis of putative disease-causing genes, preferably in an in vivo setting which also allows testing of drug candidates or biotherapeutics in whole animal disease models. In this review, we highlight the advances made in the field of bone diseases using small laboratory fish, focusing on zebrafish and medaka. We particularly highlight those human conditions where teleost models are available.

FGF signaling deregulation is associated with early developmental skeletal defects in animal models for mucopolysaccharidosis type II (MPSII)

Skeletal abnormalities represent a major clinical burden in patients affected by the lysosomal storage disorder mucopolysaccharidosis type II (MPSII, OMIM #309900). While extensive research has emphasized the detrimental role of stored glycosaminoglycans (GAGs) in the bone marrow (BM), a limited understanding of primary cellular mechanisms underlying bone defects in MPSII has hampered the development of bone-targeted therapeutic strategies beyond enzyme replacement therapy (ERT). We here investigated the involvement of key signaling pathways related to the loss of iduronate-2-sulfatase activity in two different MPSII animal models, D. rerio and M. musculus. We found that FGF pathway activity is impaired during early stages of bone development in IDS knockout mice and in a newly generated Ids mutant fish. In both models the FGF signaling deregulation anticipated a slow but progressive defect in bone differentiation, regardless of any extensive GAGs storage. We also show that MPSII patient fibroblasts harboring different mutations spanning the IDS gene exhibit perturbed FGF signaling-related markers expression. Our work opens a new venue to discover possible druggable novel key targets in MPSII.

Involvement of the retinoic acid signaling pathway in sex differentiation and pubertal development in the European sea bass Dicentrarchus labrax

Retinoic Acid (RA) is a vitamin A derivative present in many biological processes including embryogenesis, organ development and cell differentiation. The RA signaling pathway is essential for the onset of meiosis in tetrapods, although its role in fish reproduction needs further evidence. This study reports the expression profiles of several genes involved in this pathway during sex differentiation and the first reproductive season in European sea bass (Dicentrarchus labrax) gonads. The assessed genes are representative of several steps of the pathway including retinol transport, RA synthesis, nuclear receptors, RA transport and degradation. The study includes a synteny analysis of stra8, a tetrapod meiosis gatekeeper, in several taxa. The results show that, these genes were overexpressed during early gonad development and their expression decreased during meiosis progression in males and during vitellogenesis in females. Specifically, a decrease of cyp26a1, involved in RA degradation, together with an increase of aldh1a2 and aldh1a3, in charge of RA-synthesis, might ensure the availability of high RA levels at the time of meiosis in males and females. Moreover, the absence of stra8 in the European sea bass genome, as well as the conserved genomic neighbourhood found in other taxa, suggest a stra8 independent signaling for RA during meiosis. Taken together, our results might help to better understand the role of RA signaling in teleost gonad development.

Zebrafish as an Emerging Model for Osteoporosis: A Primary Testing Platform for Screening New Osteo-Active Compounds

Osteoporosis is metabolic bone disease caused by an altered balance between bone anabolism and catabolism. This dysregulated balance is responsible for fragile bones that fracture easily after minor falls. With an aging population, the incidence is rising and as yet pharmaceutical options to restore this imbalance is limited, especially stimulating osteoblast bone-building activity. Excitingly, output from large genetic studies on people with high bone mass (HBM) cases and genome wide association studies (GWAS) on the population, yielded new insights into pathways containing osteo-anabolic players that have potential for drug target development. However, a bottleneck in development of new treatments targeting these putative osteo-anabolic genes is the lack of animal models for rapid and affordable testing to generate functional data and that simultaneously can be used as a compound testing platform. Zebrafish, a small teleost fish, are increasingly used in functional genomics and drug screening assays which resulted in new treatments in the clinic for other diseases. In this review we outline the zebrafish as a powerful model for osteoporosis research to validate potential therapeutic candidates, describe the tools and assays that can be used to study bone homeostasis, and affordable (semi-)high-throughput compound testing.

Retinoic Acid Maintains Function of Neural Crest-Derived Ocular and Craniofacial Structures in Adult Zebrafish

Purpose

Retinoic acid (RA) is required for embryonic formation of the anterior segment of the eye and craniofacial structures. The present study further investigated the role of RA in maintaining the function of these neural crest–derived structures in adult zebrafish.

Methods

Morphology and histology were analyzed by using live imaging, methylacrylate sections, and TUNEL assay. Functional analysis of vision and aqueous humor outflow were assayed with real-time imaging.

Results

Both decreased and increased RA signaling altered craniofacial and ocular structures in adult zebrafish. Exogenous treatment with all-trans RA for 5 days resulted in a prognathic jaw, while inhibition of endogenous RA synthesis through treatment with 4-diethylaminobenzaldehyde (DEAB) decreased head height. In adult eyes, RA activity was localized to the retinal pigment epithelium, photoreceptors, outer plexiform layer, inner plexiform layer, iris stroma, and ventral canalicular network. Exogenous RA increased apoptosis in the iris stroma and canalicular network in the ventral iridocorneal angle, resulting in the loss of these structures and decreased aqueous outflow. DEAB, which decreased RA activity throughout the eye, induced widespread apoptosis, resulting in corneal edema, cataracts, retinal atrophy, and loss of iridocorneal angle structures. DEAB-treated fish were blind with no optokinetic response and no aqueous outflow from the anterior chamber.

Conclusions

Tight control of RA levels is required for normal structure and function of the adult anterior segment. These studies demonstrated that RA plays an important role in maintaining ocular and craniofacial structures in adult zebrafish.

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.

Pituicyte Cues Regulate the Development of Permeable Neuro-Vascular Interfaces

The hypothalamo-neurohypophyseal system (HNS) regulates homeostasis through the passage of neurohormones and blood-borne proteins via permeable blood capillaries that lack the blood-brain barrier (BBB). Why neurohypophyseal capillaries become permeable while the neighboring vasculature of the brain forms BBB remains unclear. We show that pituicytes, the resident astroglial cells of the neurohypophysis, express genes that are associated with BBB breakdown during neuroinflammation. Pituicyte-enriched factors provide a local microenvironment that instructs a permeable neurovascular conduit. Thus, genetic and pharmacological perturbations of Vegfa and Tgfβ3 affected HNS vascular morphogenesis and permeability and impaired the expression of the fenestral marker plvap. The anti-inflammatory agent dexamethasone decreased HNS permeability and downregulated the pituicyte-specific cyp26b gene, encoding a retinoic acid catabolic enzyme. Inhibition of Cyp26b activity led to upregulation of tight junction protein Claudin-5 and decreased permeability. We conclude that pituicyte-derived factors regulate the "decision" of endothelial cells to adopt a permeable endothelial fate instead of forming a BBB.

Adult Zebrafish Injury Models to Study the Effects of Prednisolone in Regenerating Bone Tissue

Zebrafish are able to regenerate various organs, including appendages (fins) after amputation. This involves the regeneration of bone, which regrows within roughly two weeks after injury. Furthermore, zebrafish are able to heal bone rapidly after trepanation of the skull, and repair fractures that can be easily introduced into zebrafish bony fin rays. These injury assays represent feasible experimental paradigms to test the effect of administered drugs on rapidly forming bone. Here, we describe the use of these 3 injury models and their combined use with systemic glucocorticoid treatment, which exerts bone inhibitory and immunosuppressive effects. We provide a workflow on how to prepare for immunosuppressive treatment in adult zebrafish, illustrate how to perform fin amputation, trepanation of calvarial bones, and fin fractures, and describe how the use of glucocorticoids affects both bone forming osteoblasts and cells of the monocyte/macrophage lineage as part of innate immunity in bone tissue.

Direct activation of chordoblasts by retinoic acid is required for segmented centra mineralization during zebrafish spine development

Zebrafish mutants with increased retinoic acid (RA) signaling due to the loss of the RA-inactivating enzyme Cyp26b1 develop a hyper-mineralized spine with gradually fusing vertebral body precursors (centra). However, the underlying cellular mechanisms remain incompletely understood. Here, we show that cells of the notochord epithelium named chordoblasts are sensitive to RA signaling. Chordoblasts are uniformly distributed along the anteroposterior axis and initially generate the continuous collagenous notochord sheath. However, subsequently and iteratively, subsets of these cells undergo further RA-dependent differentiation steps, acquire a stellate-like shape, downregulate expression of the collagen gene col2a1a, switch on cyp26b1 expression and trigger metameric sheath mineralization. This mineralization fails to appear upon chordoblast-specific cell ablation or RA signal transduction blockade. Together, our data reveal that, despite their different developmental origins, the activities and regulation of chordoblasts are very similar to those of osteoblasts, including their RA-induced transition from osteoid-producing cells to osteoid-mineralizing ones. Furthermore, our data point to a requirement for locally controlled RA activity within the chordoblast layer in order to generate the segmented vertebral column.

Segmentation of the zebrafish axial skeleton relies on notochord sheath cells and not on the segmentation clock

Segmentation of the axial skeleton in amniotes depends on the segmentation clock, which patterns the paraxial mesoderm and the sclerotome. While the segmentation clock clearly operates in teleosts, the role of the sclerotome in establishing the axial skeleton is unclear. We severely disrupt zebrafish paraxial segmentation, yet observe a largely normal segmentation process of the chordacentra. We demonstrate that axial entpd5+ notochord sheath cells are responsible for chordacentrum mineralization, and serve as a marker for axial segmentation. While autonomous within the notochord sheath, entpd5 expression and centrum formation show some plasticity and can respond to myotome pattern. These observations reveal for the first time the dynamics of notochord segmentation in a teleost, and are consistent with an autonomous patterning mechanism that is influenced, but not determined by adjacent paraxial mesoderm. This behavior is not consistent with a clock-type mechanism in the notochord.

Application of bone transgenic zebrafish in anti-osteoporosis chemical screening

BACKGROUND

The zebrafish (Danio rerio) has recently been shown to be an ideal model to study bone disease including osteoporosis. The zebrafish osteoporosis model could be induced by glucocorticoid treatment with chemical staining for reflecting the level of bone mineralization. However, this methodology was unstable. Here, we developed a novel methodology to directly evaluate the bone mass and density.

METHODS

We generated and used the bone of transgenic zebrafish Tg (ola.sp7:nlsGFP) to evaluate the bone mass and density by measuring the areal extent and the integrated optical density (IOD) of enhanced green fluorescent protein (eGFP). This methodology was further compared with the traditional chemically stained method showing the bone mineralization. Furthermore, genes related to zebrafish osteoporosis were analyzed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR).

RESULTS

Our results of new methods were consistent with those from chemically stained fish, following glucocorticoid-induction or epimedium flavonoid (FE)-rescue treatments. qRT-PCR analyses on mRNA levels revealed that glucocorticoid induces osteoporosis by downregulating the expression of osteoblast-related factors osterix, osteocalcin, and osteopontin, and upregulating the expression of osteoclast-related factor tartrate-resistant acid phosphatase. In FE-rescued fish, the expression of osteogenic factors osterix, osteocalcin, and osteopontin were increased.

CONCLUSION

Compared to the traditional chemical staining methods, the new osteoporosis model using Tg(ola.sp7:nlsGFP) is more convenient and efficient for studying osteoporosis in vivo, and especially for high-throughput anti-osteoporosis drug screening.

Dysregulatory effects of retinoic acid isomers in late zebrafish embryos

All-trans retinoic acid (atRA) and 9-cis retinoic acid (9cRA) are two natural derivatives of vitamin A that contribute to the normal vertebrate development by affecting gene expression through the retinoic acid signalling pathway. We show transcriptomic effects of the ectopic addition of atRA or 9cRA to zebrafish embryos at the posthatching embryonic stage. Exposure for 24 or 72 h to sublethal concentrations of both isomers resulted in characteristic transcriptome changes, in which many proliferation and development-related genes became underexpressed, whereas genes related to retinoid metabolism and some metabolic functions became overrepresented. While short and long exposures elicit essentially the same set of genes, atRA specifically induced expression of a specific subset of proteases, likely acting at the extracellular level, and of elements of the response to xenobiotics. These results reflect the well-known antiproliferative activity of retinoids, and they suggest a dysregulation of the developmental process at final stages of embryogenesis. They also indicate a potential role of endopeptidases as markers of developmental alterations, as well as their possible control by the retinoic signalling pathway. We propose to monitor mRNA levels of cyp16a, cyp16b, and cyp16c in zebrafish embryos as a bioassay for retinoid disruption.

In vivo zebrafish morphogenesis shows Cyp26b1 promotes tendon condensation and musculoskeletal patterning in the embryonic jaw

Integrated development of diverse tissues gives rise to a functional, mobile vertebrate musculoskeletal system. However, the genetics and cellular interactions that drive the integration of muscle, tendon, and skeleton are poorly understood. In the vertebrate head, neural crest cells, from which cranial tendons derive, pattern developing muscles just as tendons have been shown to in limb and trunk tissue, yet the mechanisms of this patterning are unknown. From a forward genetic screen, we determined that cyp26b1 is critical for musculoskeletal integration in the ventral pharyngeal arches, particularly in the mandibulohyoid junction where first and second arch muscles interconnect. Using time-lapse confocal analyses, we detail musculoskeletal integration in wild-type and cyp26b1 mutant zebrafish. In wild-type fish, tenoblasts are present in apposition to elongating muscles and condense in discrete muscle attachment sites. In the absence of cyp26b1, tenoblasts are generated in normal numbers but fail to condense into nascent tendons within the ventral arches and, subsequently, muscles project into ectopic locales. These ectopic muscle fibers eventually associate with ectopic tendon marker expression. Genetic mosaic analysis demonstrates that neural crest cells require Cyp26b1 function for proper musculoskeletal development. Using an inhibitor, we find that Cyp26 function is required in a short time window that overlaps the dynamic window of tenoblast condensation. However, cyp26b1 expression is largely restricted to regions between tenoblast condensations during this time. Our results suggest that degradation of RA by this previously undescribed population of neural crest cells is critical to promote condensation of adjacent scxa-expressing tenoblasts and that these condensations are subsequently required for proper musculoskeletal integration.

Immune Suppressive and Bone Inhibitory Effects of Prednisolone in Growing and Regenerating Zebrafish Tissues

Glucocorticoids are widely used as therapeutic agents to treat immune-mediated diseases in humans because of their anti-inflammatory and immunosuppressive effects. However, glucocorticoids have various adverse effects, in particular rapid and pronounced bone loss associated with fractures in glucocorticoid-induced osteoporosis, a common form of secondary osteoporosis. In zebrafish, which are increasingly used to study processes of bone regeneration and disease, glucocorticoids show detrimental effects on bone tissue; however, the underlying cellular mechanisms are incompletely understood. Here, we show that treatment with the glucocorticoid prednisolone impacts on the number, activity and differentiation of osteoblasts, osteoclasts, and immune cells during ontogenetic growth, homeostasis, and regeneration of zebrafish bone. Macrophage numbers are reduced in both larval and adult tissues, correlating with decreased generation of myelomonocytes and enhanced apoptosis of these cells. In contrast, osteoblasts fail to proliferate, show decreased activity, and undergo incomplete differentiation. In addition, prednisolone treatment mitigates the number and recruitment of osteoclasts to sites of bone regeneration in adult fish. In combination, these effects delay bone growth and impair bone regeneration. Our study demonstrates the many-faceted effects of glucocorticoids in non-mammalian vertebrates and helps to further establish the zebrafish as a model to study glucocorticoid-induced osteoporosis. © 2017 American Society for Bone and Mineral Research.

MicroCT-based phenomics in the zebrafish skeleton reveals virtues of deep phenotyping in a distributed organ system

Phenomics, which ideally involves in-depth phenotyping at the whole-organism scale, may enhance our functional understanding of genetic variation. Here, we demonstrate methods to profile hundreds of phenotypic measures comprised of morphological and densitometric traits at a large number of sites within the axial skeleton of adult zebrafish. We show the potential for vertebral patterns to confer heightened sensitivity, with similar specificity, in discriminating mutant populations compared to analyzing individual vertebrae in isolation. We identify phenotypes associated with human brittle bone disease and thyroid stimulating hormone receptor hyperactivity. Finally, we develop allometric models and show their potential to aid in the discrimination of mutant phenotypes masked by alterations in growth. Our studies demonstrate virtues of deep phenotyping in a spatially distributed organ system. Analyzing phenotypic patterns may increase productivity in genetic screens, and facilitate the study of genetic variants associated with smaller effect sizes, such as those that underlie complex diseases.

Toxicity of triphenyltin on the development of retinal axons in zebrafish at low dose

The impacts of triphenyltin (TPT) on ecological health are of particular concern due to the unexpectedly high levels found in wild fish around the world. Here, zebrafish embryos were exposed to TPT via in ovo nano-injection to study its toxicity on the development of retinal axons in fish. Lipophilic dye labeling revealed obvious defects in retinal axon development in larvae with normally shaped eyes, with incidences of 0, 1.08%, 2.66%, 4.26%, and 6.85% observed in the control, 0.8, 4.0, 20.0, and 100ng TPT-Cl/g wet weight (ww) exposure groups, respectively, showing a dose-dependent increase. Since the lowest observable effective concentration of TPT to induce retinal axon development defects was 0.8ng TPT-Cl/g ww, which is lower than the concentrations in wild fish eggs, this defect would occur in wild fish larvae. Alterations in the expressions of pax6 and ephrinBs, which regulate the establishment of retinal polarity, were correlated with defect incidence. Expression levels of the CYP26A1 gene and protein were significantly up-regulated in all exposure groups compared with the control, which may lead to significant decreases in concentrations of all-trans retinoic acid (atRA). Such a disruption of RA metabolism would, at least partly, contribute to the incidence of developmental defects in retinal axons. This study is the first to report that TPT can interfere with development of retinal axons in fish at low dose.

Dynamics of actinotrichia regeneration in the adult zebrafish fin

The skeleton of adult zebrafish fins comprises lepidotrichia, which are dermal bones of the rays, and actinotrichia, which are non-mineralized spicules at the distal margin of the appendage. Little is known about the regenerative dynamics of the actinotrichia-specific structural proteins called Actinodins. Here, we used immunofluorescence analysis to determine the contribution of two paralogous Actinodin proteins, And1/2, in regenerating fins. Both proteins were detected in the secretory organelles in the mesenchymal cells of the blastema, but only And1 was detected in the epithelial cells of the wound epithelium. The analysis of whole mount fins throughout the entire regenerative process and longitudinal sections revealed that And1-positive fibers are complementary to the lepidotrichia. The analysis of another longfin fish, a gain-of-function mutation in the potassium channel kcnk5b, revealed that the long-fin phenotype is associated with an extended size of actinotrichia during homeostasis and regeneration. Finally, we investigated the role of several signaling pathways in actinotrichia formation and maintenance. This revealed that the pulse-inhibition of either TGFβ/Activin-βA or FGF are sufficient to impair deposition of Actinodin during regeneration. Thus, the dynamic turnover of Actinodin during fin regeneration is regulated by multiple factors, including the osteoblasts, growth rate in a potassium channel mutant, and instructive signaling networks between the epithelium and the blastema of the regenerating fin.

Nuclear receptor research in zebrafish

Nuclear receptors (NRs) form a superfamily of transcription factors that can be activated by ligands and are involved in a wide range of physiological processes. NRs are well conserved between vertebrate species. The zebrafish, an increasingly popular animal model system, contains a total of 73 NR genes, and orthologues of almost all human NRs are present. In this review article, an overview is presented of NR research in which the zebrafish has been used as a model. Research is described on the three most studied zebrafish NRs: the estrogen receptors (ERs), retinoic acid receptors (RARs) and peroxisome proliferator-activated receptors (PPARs). The studies on these receptors illustrate the versatility of the zebrafish as a model for ecotoxicological, developmental and biomedical research. Although the use of the zebrafish in NR research is still relatively limited, it is expected that in the next decade the full potential of this animal model will be exploited.

Vitamin A Affects Flatfish Development in a Thyroid Hormone Signaling and Metamorphic Stage Dependent Manner

Vitamin A (VA) and retinoid derivatives are known morphogens controlling vertebrate development. Despite the research effort conducted during the last decade, the precise mechanism of how VA induces post-natal bone changes, and particularly those operating through crosstalk with the thyroid hormones (THs) remain to be fully understood. Since effects and mechanisms seem to be dose and time-dependent, flatfish are an interesting study model as they undergo a characteristic process of metamorphosis driven by THs that can be followed by external appearance. Here, we studied the effects of VA imbalance that might determine Senegalese sole (Solea senegalensis) skeletogenetic phenotype through development of thyroid follicles, THs homeostasis and signaling when a dietary VA excess was specifically provided during pre-, pro- or post-metamorphic stages using enriched rotifers and Artemia as carriers. The increased VA content in enriched live prey was associated to a higher VA content in fish at all developmental stages. Dietary VA content clearly affected thyroid follicle development, T3 and T4 immunoreactive staining, skeletogenesis and mineralization in a dose and time-dependent fashion. Gene expression analysis showed that VA levels modified the mRNA abundance of VA- and TH-specific nuclear receptors at specific developmental stages. Present results provide new and key knowledge to better understand how VA and TH pathways interact at tissue, cellular and nuclear level at different developmental periods in Senegalese sole, unveiling how dietary modulation might determine juvenile phenotype and physiology.

Developmental toxicity and molecular responses of marine medaka (Oryzias melastigma) embryos to ciguatoxin P-CTX-1 exposure

Ciguatoxins are produced by toxic benthic dinoflagellates and cause ciguatera fish poisoning worldwide, but the toxic effects on developing marine fish have not been well investigated. The Pacific ciguatoxin (P-CTX-1), is a potent sodium channel agonist, which is one of the most toxic members among all CTXs. This study evaluated the toxic effects of microinjecting purified Pacific ciguatoxin-1 (P-CTX-1) on embryonic development of marine medaka Oryzias melastigma. A lower 96h-LD₅₀ value was estimated for eleuthero-embryos (1.32ngg^-1) than that for embryos (1.71ngg^-1), indicating that P-CTX-1 is more lethal to newly hatched medaka larvae. P-CTX-1 induced detrimental effects during embryonic development, including hatching failure, abnormalities in physical development (caudal fin malformation and spinal deformities), internal damage (green coloration of the gall bladder and hemorrhaging), immune dysfunction, and altered muscle physiology (bradycardia and hyperkinetic twitching). The results of a transcriptional expression analysis of genes related to the stress/immune responses, cardiac and bone development, and apoptosis supported the observed developmental abnormalities. This study advanced the understanding of P-CTX-1 mediated toxic mechanisms in the development of early life stages of a fish, and thus contributed to the toxicity assessment of CTXs in marine ecosystems.

Drug Treatment and In Vivo Imaging of Osteoblast-Osteoclast Interactions in a Medaka Fish Osteoporosis Model

Bone-forming osteoblasts interact with bone-resorbing osteoclasts to coordinate the turnover of bone matrix and to control skeletal homeostasis. Medaka and zebrafish larvae are widely used to analyze the behavior of bone cells during bone formation, degeneration, and repair. Their optical clarity allows the visualization of fluorescently labeled bone cells and fluorescent dyes bound to the mineralized skeletal matrix. Our lab has generated transgenic medaka fish that express the osteoclast-inducing factor Receptor Activator of Nuclear-factor κB Ligand (RANKL) under the control of a heat shock-inducible promoter. Ectopic expression of RANKL results in the excess formation of activated osteoclasts, which can be visualized in reporter lines with nlGFP expression under the control of the cathepsin K (ctsk) promoter. RANKL induction and ectopic osteoclast formation leads to severe osteoporosis-like phenotypes. Compound transgenic medaka lines that express ctsk:nlGFP in osteoclasts, as well as mCherry under the control of the osterix (osx) promoter in premature osteoblasts, can be used to study the interaction of both cell types. This facilitates the in vivo observation of cellular behavior under conditions of bone degeneration and repair. Here, we describe the use of this system to test a drug commonly used in human osteoporosis therapy and describe a protocol for live imaging. The medaka model complements studies in cell culture and mice, and offers a novel system for the in vivo analysis of drug action in the skeletal system.

Quantitative assessment of the regenerative and mineralogenic performances of the zebrafish caudal fin

The ability of zebrafish to fully regenerate its caudal fin has been explored to better understand the mechanisms underlying de novo bone formation and to develop screening methods towards the discovery of compounds with therapeutic potential. Quantifying caudal fin regeneration largely depends on successfully measuring new tissue formation through methods that require optimization and standardization. Here, we present an improved methodology to characterize and analyse overall caudal fin and bone regeneration in adult zebrafish. First, regenerated and mineralized areas are evaluated through broad, rapid and specific chronological and morphometric analysis in alizarin red stained fins. Then, following a more refined strategy, the intensity of the staining within a 2D longitudinal plane is determined through pixel intensity analysis, as an indicator of density or thickness/volume. The applicability of this methodology on live specimens, to reduce animal experimentation and provide a tool for in vivo tracking of the regenerative process, was successfully demonstrated. Finally, the methodology was validated on retinoic acid- and warfarin-treated specimens, and further confirmed by micro-computed tomography. Because it is easily implementable, accurate and does not require sophisticated equipment, the present methodology will certainly provide valuable technical standardization for research in tissue engineering, regenerative medicine and skeletal biology.

A vertebrate-specific and essential role for osterix in osteogenesis revealed by gene knockout in the teleost medaka

osterix (osx; sp7) encodes a zinc-finger transcription factor that controls osteoblast differentiation in mammals. Although identified in all vertebrate lineages, its role in non-mammalian bone formation remains elusive. Here, we show that an osx mutation in medaka results in severe bone defects and larval lethality. Pre-osteoblasts fail to differentiate leading to severe intramembranous and perichondral ossification defects. The notochord sheath mineralizes normally, supporting the idea of an osteoblast-independent mechanism for teleost vertebral centra formation. This study establishes a key role for Osx for bone formation in a non-mammalian species, and reveals conserved and non-conserved features in vertebrate bone formation.

Osteocrin, a peptide secreted from the heart and other tissues, contributes to cranial osteogenesis and chondrogenesis in zebrafish

The heart is an endocrine organ, as cardiomyocytes (CMs) secrete natriuretic peptide (NP) hormones. Since the discovery of NPs, no other peptide hormones that affect remote organs have been identified from the heart. We identified osteocrin (Ostn) as an osteogenesis/chondrogenesis regulatory hormone secreted from CMs in zebrafish. ostn mutant larvae exhibit impaired membranous and chondral bone formation. The impaired bones were recovered by CM-specific overexpression of OSTN. We analyzed the parasphenoid (ps) as a representative of membranous bones. In the shortened ps of ostn morphants, nuclear Yap1/Wwtr1-dependent transcription was increased, suggesting that Ostn might induce the nuclear export of Yap1/Wwtr1 in osteoblasts. Although OSTN is proposed to bind to NPR3 (clearance receptor for NPs) to enhance the binding of NPs to NPR1 or NPR2, OSTN enhanced C-type NP (CNP)-dependent nuclear export of YAP1/WWTR1 of cultured mouse osteoblasts stimulated with saturable CNP. OSTN might therefore activate unidentified receptors that augment protein kinase G signaling mediated by a CNP-NPR2 signaling axis. These data demonstrate that Ostn secreted from the heart contributes to bone formation as an endocrine hormone.

New Insights Into the Roles of Retinoic Acid Signaling in Nervous System Development and the Establishment of Neurotransmitter Systems

Secreted chiefly from the underlying mesoderm, the morphogen retinoic acid (RA) is well known to contribute to the specification, patterning, and differentiation of neural progenitors in the developing vertebrate nervous system. Furthermore, RA influences the subtype identity and neurotransmitter phenotype of subsets of maturing neurons, although relatively little is known about how these functions are mediated. This review provides a comprehensive overview of the roles played by RA signaling during the formation of the central and peripheral nervous systems of vertebrates and highlights its effects on the differentiation of several neurotransmitter systems. In addition, the evolutionary history of the RA signaling system is discussed, revealing both conserved properties and alternate modes of RA action. It is proposed that comparative approaches should be employed systematically to expand our knowledge of the context-dependent cellular mechanisms controlled by the multifunctional signaling molecule RA.

Loss of Type I Collagen Telopeptide Lysyl Hydroxylation Causes Musculoskeletal Abnormalities in a Zebrafish Model of Bruck Syndrome

Bruck syndrome (BS) is a disorder characterized by joint flexion contractures and skeletal dysplasia that shows strong clinical overlap with the brittle bone disease osteogenesis imperfecta (OI). BS is caused by biallelic mutations in either the FKBP10 or the PLOD2 gene. PLOD2 encodes the lysyl hydroxylase 2 (LH2) enzyme, which is responsible for the hydroxylation of lysine residues in fibrillar collagen telopeptides. This hydroxylation directs crosslinking of collagen fibrils in the extracellular matrix, which is necessary to provide stability and tensile integrity to the collagen fibrils. To further elucidate the function of LH2 in vertebrate skeletal development, we created a zebrafish model harboring a homozygous plod2 nonsense mutation resulting in reduced telopeptide hydroxylation and crosslinking of bone type I collagen. Adult plod2 mutants present with a shortened body axis and severe skeletal abnormalities with evidence of bone fragility and fractures. The vertebral column of plod2 mutants is short and scoliotic with compressed vertebrae that show excessive bone formation at the vertebral end plates, and increased tissue mineral density in the vertebral centra. The muscle fibers of mutant zebrafish have a reduced diameter near the horizontal myoseptum. The endomysium, a layer of connective tissue ensheathing the individual muscle fibers, is enlarged. Transmission electron microscopy of mutant vertebral bone shows type I collagen fibrils that are less organized with loss of the typical plywood-like structure. In conclusion, plod2 mutant zebrafish show molecular and tissue abnormalities in the musculoskeletal system that are concordant with clinical findings in BS patients. Therefore, the plod2 zebrafish mutant is a promising model for the elucidation of the underlying pathogenetic mechanisms leading to BS and the development of novel therapeutic avenues in this syndrome. © 2016 American Society for Bone and Mineral Research.

Small teleost fish provide new insights into human skeletal diseases

Small teleost fish such as zebrafish and medaka are increasingly studied as models for human skeletal diseases. Efficient new genome editing tools combined with advances in the analysis of skeletal phenotypes provide new insights into fundamental processes of skeletal development. The skeleton among vertebrates is a highly conserved organ system, but teleost fish and mammals have evolved unique traits or have lost particular skeletal elements in each lineage. Several unique features of the skeleton relate to the extremely small size of early fish embryos and the small size of adult fish used as models. A detailed analysis of the plethora of interesting skeletal phenotypes in zebrafish and medaka pushes available skeletal imaging techniques to their respective limits and promotes the development of new imaging techniques. Impressive numbers of zebrafish and medaka mutants with interesting skeletal phenotypes have been characterized, complemented by transgenic zebrafish and medaka lines. The advent of efficient genome editing tools, such as TALEN and CRISPR/Cas9, allows to introduce targeted deficiencies in genes of model teleosts to generate skeletal phenotypes that resemble human skeletal diseases. This review will also discuss other attractive aspects of the teleost skeleton. This includes the capacity for lifelong tooth replacement and for the regeneration of dermal skeletal elements, such as scales and fin rays, which further increases the value of zebrafish and medaka models for skeletal research.

Zebrafish fin regeneration after cryoinjury-induced tissue damage

Although fin regeneration following an amputation procedure has been well characterized, little is known about the impact of prolonged tissue damage on the execution of the regenerative programme in the zebrafish appendages. To induce histolytic processes in the caudal fin, we developed a new cryolesion model that combines the detrimental effects of freezing/thawing and ischemia. In contrast to the common transection model, the damaged part of the fin was spontaneously shed within two days after cryoinjury. The remaining stump contained a distorted margin with a mixture of dead material and healthy cells that concomitantly induced two opposing processes of tissue debris degradation and cellular proliferation, respectively. Between two and seven days after cryoinjury, this reparative/proliferative phase was morphologically featured by displaced fragments of broken bones. A blastemal marker msxB was induced in the intact mesenchyme below the damaged stump margin. Live imaging of epithelial and osteoblastic transgenic reporter lines revealed that the tissue-specific regenerative programmes were initiated after the clearance of damaged material. Despite histolytic perturbation during the first week after cryoinjury, the fin regeneration resumed and was completed without further alteration in comparison to the simple amputation model. This model reveals the powerful ability of the zebrafish to restore the original appendage architecture after the extended histolysis of the stump.

Summary: Fin cryolesion resulted in histolysis and a delayed tissue loss. Despite prolonged destruction of the stump architecture, fin regeneration resumed and was normally completed, revealing robustness of the regenerative capacity.

Citalopram and sertraline exposure compromises embryonic bone development

Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed treatments for depression and, as a class of drugs, are among the most used medications in the world. Concern regarding possible effects of SSRI treatment on fetal development has arisen recently as studies have suggested a link between maternal SSRI use and an increase in birth defects such as persistent pulmonary hypertension, seizures and craniosynostosis. Furthermore, SSRI exposure in adults is associated with decreased bone mineral density and increased fracture risk, and serotonin receptors are expressed in human osteoblasts and osteoclasts. To determine possible effects of SSRI exposure on developing bone, we treated both zebrafish, during embryonic development, and human mesenchymal stem cells (MSCs), during differentiation into osteoblasts, with the two most prescribed SSRIs, citalopram and sertraline. SSRI treatment in zebrafish decreased bone mineralization, visualized by alizarin red staining and decreased the expression of mature osteoblast-specific markers during embryogenesis. Furthermore, we showed that this inhibition was not associated with increased apoptosis. In differentiating human MSCs, we observed a decrease in osteoblast activity that was associated with a decrease in expression of the osteoblast-specific genes Runx2, Sparc and Spp1, measured with quantitative real-time PCR (qRT-PCR). Similar to the developing zebrafish, no increase in expression of the apoptotic marker Caspase 3 was observed. Therefore, we propose that SSRIs inhibit bone development by affecting osteoblast maturation during embryonic development and MSC differentiation. These results highlight the need to further investigate the risks of SSRI use during pregnancy in exposing unborn babies to potential skeletal abnormalities.

Targeted disruption of sp7 and myostatin with CRISPR-Cas9 results in severe bone defects and more muscular cells in common carp

The common carp (Cyprinus carpio) as one of the most important aquaculture fishes produces over 3 million metric tones annually, approximately 10% the annual production of the all farmed freshwater fish worldwide. However, the tetraploidy genome and long generation-time of the common carp have made its breeding and genetic studies extremely difficult. Here, TALEN and CRISPR-Cas9, two versatile genome-editing tools, are employed to target common carp bone-related genes sp7, runx2, bmp2a, spp1, opg, and muscle suppressor gene mstn. TALEN were shown to induce mutations in the target coding sites of sp7, runx2, spp1 and mstn. With CRISPR-Cas9, the two common carp sp7 genes, sp7a and sp7b, were mutated individually, all resulting in severe bone defects; while mstnba mutated fish have grown significantly more muscle cells. We also employed CRISPR-Cas9 to generate double mutant fish of sp7a;mstnba with high efficiencies in a single step. These results demonstrate that both TALEN and CRISPR-Cas9 are highly efficient tools for modifying the common carp genome, and open avenues for facilitating common carp genetic studies and breeding.

Retinoic acid-induced premature osteoblast-to-preosteocyte transitioning has multiple effects on calvarial development

We have previously shown that, in human and zebrafish, hypomorphic mutations of the gene encoding the retinoic acid (RA)-metabolizing enzyme Cyp26b1 result in coronal craniosynostosis, caused by an RA-induced premature transitioning of suture osteoblasts to preosteocytes, inducing ectopic mineralization of the suture's osteoid matrix. In addition, we showed that human CYP26B1 null patients have more severe and seemingly opposite skull defects, characterized by smaller and fragmented calvaria, but the cellular basis of these defects remained largely unclear. Here, by treating juvenile zebrafish with exogenous RA or a chemical Cyp26 inhibitor in the presence or absence of osteogenic cells or bone-resorbing osteoclasts, we demonstrate that both reduced calvarial size and calvarial fragmentation are also caused by RA-induced premature osteoblast-to-preosteocyte transitioning. During calvarial growth, the resulting osteoblast deprival leads to decreased osteoid production and thereby smaller and thinner calvaria, whereas calvarial fragmentation is caused by increased osteoclast stimulation through the gained preosteocytes. Together, our data demonstrate that RA-induced osteoblast-to-preosteocyte transitioning has multiple effects on developing bone in Cyp26b1 mutants, ranging from gain to loss of bone, depending on the allelic strength, the developmental stage and the cellular context.

Altered retinoic acid signalling underpins dentition evolution

Small variations in signalling pathways have been linked to phenotypic diversity and speciation. In vertebrates, teeth represent a reservoir of adaptive morphological structures that are prone to evolutionary change. Cyprinid fish display an impressive diversity in tooth number, but the signals that generate such diversity are unknown. Here, we show that retinoic acid (RA) availability influences tooth number size in Cyprinids. Heterozygous adult zebrafish heterozygous for the cyp26b1 mutant that encodes an enzyme able to degrade RA possess an extra tooth in the ventral row. Expression analysis of pharyngeal mesenchyme markers such as dlx2a and lhx6 shows lateral, anterior and dorsal expansion of these markers in RA-treated embryos, whereas the expression of the dental epithelium markers dlx2b and dlx3b is unchanged. Our analysis suggests that changes in RA signalling play an important role in the diversification of teeth in Cyprinids. Our work illustrates that through subtle changes in the expression of rate-limiting enzymes, the RA pathway is an active player of tooth evolution in fish.

Expression of osterix Is Regulated by FGF and Wnt/β-Catenin Signalling during Osteoblast Differentiation

Osteoblast differentiation from mesenchymal cells is regulated by multiple signalling pathways. Here we have analysed the roles of Fibroblast Growth Factor (FGF) and canonical Wingless-type MMTV integration site (Wnt/β-Catenin) signalling pathways on zebrafish osteogenesis. We have used transgenic and chemical interference approaches to manipulate these pathways and have found that both pathways are required for osteoblast differentiation in vivo. Our analysis of bone markers suggests that these pathways act at the same stage of differentiation to initiate expression of the osteoblast master regulatory gene osterix (osx). We use two independent approaches that suggest that osx is a direct target of these pathways. Firstly, we manipulate signalling and show that osx gene expression responds with similar kinetics to that of known transcriptional targets of the FGF and Wnt pathways. Secondly, we have performed ChIP with transcription factors for both pathways and our data suggest that a genomic region in the first intron of osx mediates transcriptional activation. Based upon these data, we propose that FGF and Wnt/β-Catenin pathways act in part by directing transcription of osx to promote osteoblast differentiation at sites of bone formation.