Characterization of two new zebrafish members of the hedgehog family: atypical expression of a zebrafish indian hedgehog gene in skeletal elements of both endochondral and dermal origins

Cell expansion for notochord mechanics and endochondral bone lengthening in zebrafish depends on the 5'-inositol phosphatase Inppl1a

Cell size is a key contributor to tissue morphogenesis. As a notable example, growth plate hypertrophic chondrocytes use cellular biogenesis and disproportionate fluid uptake to expand 10 to 20 times in size to drive lengthening of endochondral bone. Similarly, notochord vacuolated cells expand to one of the largest cell types in the developing embryo to drive axial extension. In zebrafish, the notochord vacuolated cells undergo vacuole fusion to form a single large, fluid-filled vacuole that fills the cytoplasmic space and contributes to vacuolated cell expansion. When this process goes awry, the notochord lacks sufficient hydrostatic pressure to support vertebral bone deposition, resulting in adult spines with misshapen vertebral bones and scoliosis. However, it remains unclear whether endochondral bone and the notochord share common genetic and cellular mechanisms for regulating cell and tissue expansion. Here, we demonstrate that the 5'-inositol phosphatase gene, inppl1a, regulates notochord expansion independent of vacuole fusion, thereby genetically decoupling these processes. We demonstrate that inppl1a-dependent vacuolated cell expansion is essential to establish normal mechanical properties of the notochord and to facilitate the development of a straight spine. Finally, we find that inppl1a is also important for hypertrophic chondrocyte differentiation and endochondral bone lengthening in fish, as has been shown in the human INPPL1-related endochondral bone disorder, opsismodysplasia. Overall, this work reveals a shared mechanism of cell size regulation that influences disparate tissues critical for skeletal development and short-stature disorders.

Increased Netrin downstream of overactive Hedgehog signaling disrupts optic fissure formation

BACKGROUND

Uveal coloboma, a developmental eye defect, is caused by failed development of the optic fissure, a ventral structure in the optic stalk and cup where axons exit the eye and vasculature enters. The Hedgehog (Hh) signaling pathway regulates optic fissure development: loss-of-function mutations in the Hh receptor ptch2 produce overactive Hh signaling and can result in coloboma. We previously proposed a model where overactive Hh signaling disrupts optic fissure formation by upregulating transcriptional targets acting both cell- and non-cell-autonomously. Here, we examine the Netrin family of secreted ligands as candidate Hh target genes.

RESULTS

We find multiple Netrin ligands upregulated in the zebrafish ptch2 mutant during optic fissure development. Using a gain-of-function approach to overexpress Netrin in a spatiotemporally specific manner, we find that netrin1a or netrin1b overexpression is sufficient to cause coloboma and disrupt wild-type optic fissure formation. We used loss-of-function alleles, CRISPR/Cas9 mutagenesis, and morpholino knockdown to test if loss of Netrin can rescue coloboma in the ptch2 mutant: loss of netrin genes does not rescue the ptch2 mutant phenotype.

CONCLUSION

These results suggest that Netrin is sufficient but not required to disrupt optic fissure formation downstream of overactive Hh signaling in the ptch2 mutant.

Reduced ossification caused by 3D simulated microgravity exposure is short-term in larval zebrafish

Understanding how skeletal tissues respond to microgravity is ever more important with the increased interest in human space travel. Here, we exposed larval Danio rerio at 3.5 dpf to simulated microgravity (SMG) using a 3D mode of rotation in a ground-based experiment and then studied different cellular, molecular, and morphological bone responses both immediately after exposure and one week later. Our results indicate an overall decrease in ossification in several developing skeletal elements immediately after SMG exposure with the exception of the otoliths, however ossification returns to normal levels seven days after exposure. Coincident with the reduction in overall ossification tnfsf11 (RANKL) expression is highly elevated after 24 h of SMG exposure and also returns to normal levels seven days after exposure. We also show that genes associated with osteoblasts are unaffected immediately after SMG exposure. Thus, the observed reduction in ossification is primarily the result of a high level of bone resorption. This study sheds insight into the nuances of how osteoblasts and osteoclasts in the skeleton of a vertebrate organism respond to an external environmental disturbance, in this case simulated microgravity.

Early Mesozoic burst of morphological disparity in the slow-evolving coelacanth fish lineage

Since the split of the coelacanth lineage from other osteichthyans 420 million years ago, the morphological disparity of this clade has remained remarkably stable. Only few outliers with peculiar body shape stood out over the evolutionary history, but they were phylogenetically and stratigraphically independent of each other. Here, we report the discovery of a new clade of ancient latimeriid coelacanths representing a small flock of species present in the Western Tethys between 242 and 241 million years ago. Among the four species, two show highly derived anatomy. A new genus shows reversal to plesiomorphic conditions in its skull and caudal fin organisation. The new genus and its sister Foreyia have anatomical modules that moved from the general coelacanth Bauplau either in the same direction or in opposite direction that affect proportions of the body, opercle and fins. Comparisons with extant genetic models shows that changes of the regulatory network of the Hedgehog signal gene family may account for most of the altered anatomy. This unexpected, short and confined new clade represents the only known example of a burst of morphological disparity over the long history of coelacanths at a recovery period after the Permian-Triassic Mass Extinction.

Thermal imprinting during embryogenesis modifies skin repair in juvenile European sea bass (Dicentrarchus labrax)

Fish skin is a multifunctional tissue that develops during embryogenesis, a developmental stage highly susceptible to epigenetic marks. In this study, the impact of egg incubation temperature on the regeneration of a cutaneous wound caused by scale removal in juvenile European sea bass was evaluated. Sea bass eggs were incubated at 11, 13.5 and 16 °C until hatching and then were reared at a common temperature until 9 months when the skin was damaged and sampled at 0, 1 and 3 days after scale removal and compared to the intact skin from the other flank. Skin damage elicited an immediate significant (p < 0.001) up-regulation of pcna in fish from eggs incubated at higher temperatures. In fish from eggs incubated at 11 °C there was a significant (p < 0.001) up-regulation of krt2 compared to fish from higher thermal backgrounds 1 day after skin damage. Damaged epidermis was regenerated after 3 days in all fish irrespective of the thermal background, but in fish from eggs incubated at 11 °C the epidermis was significantly (p < 0.01) thinner compared to other groups, had less goblet cells and less melanomacrophages. The thickness of the dermis increased during regeneration of wounded skin irrespective of the thermal background and by 3 days was significantly (p < 0.01) thicker than the dermis from the intact flank. The expression of genes for ECM remodelling (mmp9, colXα, col1α1, sparc, and angptl2b) and innate immunity (lyg1, lalba, sod1, csf-1r and pparγ) changed during regeneration but were not affected by egg thermal regime. Overall, the results indicate that thermal imprinting of eggs modifies the damage-repair response in juvenile sea bass skin.

Cloning, tissue distribution of desert hedgehog (dhh) gene and expression profiling during different developmental stages of Pseudopleuronectes yokohamae

As a crucial member of the Hedgehog (Hh) protein family, desert hedgehog (dhh) plays a vital role in multiple developmental processes, cell differentiation and tissue homeostasis. However, it is unclear how it regulates development in fish. In this study, we cloned and characterized the dhh gene from Pseudopleuronectes yokohamae. The full-length cDNA of Pydhh comprises 3194 bp, with a 1386 bp open reading frame (ORF) that encodes a polypeptide of 461 amino acids with a typical HH-signal domain, Hint-N and Hint-C domains. Multiple sequence alignment revealed that the putative PyDHH protein sequence was highly conserved across species, especially in the typical domains. Phylogenetic analysis showed that the PyDHH clustered within the Pleuronectiformes. Real-time quantitative PCR showed that Pydhh was detected in fourteen different tissues in adult-female and adult-male marbled flounder, and nine different tissues in juvenile fish. During early embryonic development stages, the expression of Pydhh was revealed high levels at hatching stage of embryo development. Moreover, the relative expression of Pydhh was significantly higher in the juvenile liver than adults', and higher in the female skin than the male skin. To further investigate its location, the in situ hybridization (ISH) assay was performed, the results showed that the hybridization signal was obviously expressed in the immune organs of Pseudopleuronectes yokohamae, with weak signal expression in the other tissues. Our results suggested that Pydhh is highly conserved among species and plays a vital role in embryonic development and formation of immune related organs.

Vibration exposure uncovers a critical early developmental window for zebrafish caudal fin development

Mechanical influencers have long been shown to affect mature bone. Bone mechanosensation is a key feature that allows the skeleton to adapt to environmental constraints. In this study, we describe the response of immature, developing bones to a mechanical stimulus. To do so, zebrafish larvae at different stages of development were exposed to whole-body vibration (WBV) at a low frequency of 20 Hz, for up to 4 days. Whole mount Alizarin red and Alcian blue staining revealed age-related and bone type-specific defects. Specifically, the parhypural and hypural 1 caudal fin endoskeletal elements were affected when the exposure to WBV started early during their development. We show that these WBV-induced parhypural and hypural 1 patterning defects are triggered by a Sox9-independent pathway, potentially by reducing the distance separating adjacent chondrogenic condensations in the developing tail skeleton. The remaining hypurals were unaffected by the WBV treatment. Altogether, our results indicate that, upon exposure to vibration, chondrogenic cell progenitors can react to mechanical stimuli early during their development, which ultimately affects the skeletal patterning of the growing zebrafish larvae. These findings open a new research avenue to better understand the cellular processes involved in developing, patterning, and maintaining skeletal tissue.

Zebrafish caudal fin as a model to investigate the role of probiotics in bone regeneration

Probiotics are live microorganisms that confer several beneficial effects to the host, including enhancement of bone mineralization. However, probiotic action on bone regeneration is not well studied and therefore we analysed various effects of probiotic treatment on the caudal fin regeneration of zebrafish. Morphological analysis revealed an increased regenerated area with shorter and thicker lepidotrichia segments after probiotic treatment. Fourier transform infrared spectroscopy imaging analysis highlighted the distribution of phosphate groups in the regenerated fins and probiotic group showed higher amounts of well-crystallized hydroxyapatite. At the midpoint (5 days post amputation) of regeneration, probiotics were able to modulate various stages of osteoblast differentiation as confirmed by the upregulation of some key marker genes such as runx2b, sp7, col10a1a, spp1 and bglap, besides suppressing osteoclast activity as evidenced from the downregulation of ctsk. Probiotics also caused an enhanced cell cycle by regulating the expression of genes involved in Retinoic acid (rarga, cyp26b1) and Wnt/β-catenin (ctnnb1, ccnd1, axin2, sost) signaling pathways, and also modulated phosphate homeostasis by increasing the entpd5a levels. These findings provide new outlooks for the use of probiotics as a prophylactic treatment in accelerating bone regeneration and improving skeletal health in both aquaculture and biomedical fields.

Single-cell resolution of MET- and EMT-like programs in osteoblasts during zebrafish fin regeneration

Zebrafish regenerate fin rays following amputation through epimorphic regeneration, a process that has been proposed to involve the epithelial-to-mesenchymal transition (EMT). We performed single-cell RNA sequencing (scRNA-seq) to elucidate osteoblastic transcriptional programs during zebrafish caudal fin regeneration. We show that osteoprogenitors are enriched with components associated with EMT and its reverse, mesenchymal-to-epithelial transition (MET), and provide evidence that the EMT markers cdh11 and twist2 are co-expressed in dedifferentiating cells at the amputation stump at 1 dpa, and in differentiating osteoblastic cells in the regenerate, the latter of which are enriched in EMT signatures. We also show that esrp1, a regulator of alternative splicing in epithelial cells that is associated with MET, is expressed in a subset of osteoprogenitors during outgrowth. This study provides a single cell resource for the study of osteoblastic cells during zebrafish fin regeneration, and supports the contribution of MET- and EMT-associated components to this process.

An early Shh-H2O2 reciprocal regulatory interaction controls the regenerative program during zebrafish fin regeneration

Reactive oxygen species (ROS), originally classified as toxic molecules, have attracted increasing interest given their actions in cell signaling. Hydrogen peroxide (H2O2), the major ROS produced by cells, acts as a second messenger to modify redox-sensitive proteins or lipids. After caudal fin amputation, tight spatiotemporal regulation of ROS is required first for wound healing and later to initiate the regenerative program. However, the mechanisms carrying out this sustained ROS production and their integration with signaling pathways are still poorly understood. We focused on the early dialog between H2O2 and Sonic Hedgehog (Shh) during fin regeneration. We demonstrate that H2O2 controls Shh expression and that Shh in turn regulates the H2O2 level via a canonical pathway. Moreover, the means of this tight reciprocal control change during the successive phases of the regenerative program. Dysregulation of the Hedgehog pathway has been implicated in several developmental syndromes, diabetes and cancer. These data support the existence of an early positive crosstalk between Shh and H2O2 that might be more generally involved in various processes paving the way to improve regenerative processes, particularly in vertebrates.

Pthlha and mechanical force control early patterning of growth zones in the zebrafish craniofacial skeleton

Secreted signals in patterning systems often induce repressive signals that shape their distributions in space and time. In developing growth plates (GPs) of endochondral long bones, Parathyroid hormone-like hormone (Pthlh) inhibits Indian hedgehog (Ihh) to form a negative-feedback loop that controls GP progression and bone size. Whether similar systems operate in other bones and how they arise during embryogenesis remain unclear. We show that Pthlha expression in the zebrafish craniofacial skeleton precedes chondrocyte differentiation and restricts where cells undergo hypertrophy, thereby initiating a future GP. Loss of Pthlha leads to an expansion of cells expressing a novel early marker of the hypertrophic zone (HZ), entpd5a, and later HZ markers, such as ihha, whereas local Pthlha misexpression induces ectopic entpd5a expression. Formation of this early pre-HZ correlates with onset of muscle contraction and requires mechanical force; paralysis leads to loss of entpd5a and ihha expression in the pre-HZ, mislocalized pthlha expression and no subsequent ossification. These results suggest that local Pthlh sources combined with force determine HZ locations, establishing the negative-feedback loop that later maintains GPs.

Cloning, expression and functional analysis of the desert hedgehog (dhh) gene in Chinese tongue sole (Cynoglossus semilaevis)

Desert hedgehog (dhh) is a gene that is crucial for spermatogenesis and Leydig cell differentiation, but little is known regarding its influence on gonadal differentiation and development in fish. To understand its function, we cloned and characterized the dhh gene from Cynoglossus semilaevis (csdhh). The full length csdhh cDNA was 2473 bp, including a 1386 bp open reading frame (ORF), a 475 bp 5'-UTR, and a 612 bp 3'-UTR, encoding a predicted protein of 461 amino acid residues. Phylogenetic analysis showed that the putative protein belongs to the hedgehog (HH) family, and contains typical HH-N and HH-C domains. Amino acid sequence analysis revealed that CsDhh shares many features with Dhh analogues in other teleost species. Real-time quantitative PCR showed that csdhh was detected in eight different tissues in male and female tongue sole. During early embryonic development, the relative expression of the csdhh was significantly higher in the neural stage than in other embryonic developmental stages (P < 0.05). csdhh was detected at 20 days after hatching (dah) and at the critical period of male gonadal differentiation (80-95 dah), the relative expression of the csdhh was significantly higher in the male gonads than the female gonads. In 5, 8, and 12 month old gonads, the relative expression of the csdhh was significantly higher in male and pseudo-male than in female fish. The in situ hybridization (ISH) results showed that the hybridization signal was strongly expressed in primary and secondary spermatocytes, spermatids, and sertoli cells of the 1-year-old fish testis, with only weak signal expression in the corresponding ovarian tissue. These results suggest that csdhh is highly conserved in evolution and plays an important role in spermatogenesis in males and pseudo-males.

Endochondral growth zone pattern and activity in the zebrafish pharyngeal skeleton

BACKGROUND

Endochondral ossification is a major bone forming mechanism in vertebrates, defects in which can result in skeletal dysplasia or craniofacial anomalies in humans. The zebrafish holds great potential to advance our understanding of endochondral growth zone development and genetics, yet several important aspects of its biology remain unexplored. Here we provide a comprehensive description of endochondral growth zones in the pharyngeal skeleton, including their developmental progression, cellular activity, and adult fates.

RESULTS

Postembryonic growth of the pharyngeal skeleton is supported by endochondral growth zones located either at skeletal epiphyses or synchondroses. Col2a1a and col10a1a in situ hybridization and anti-PCNA immunostaining identify resting-, hypertrophic- and proliferative zones, respectively, in pharyngeal synchondroses. Cellular hypertrophy and matrix deposition contribute little, if at all, to axial growth in most skeletal elements. Zebrafish endochondral growth zones develop during metamorphosis and arrest in adults.

CONCLUSIONS

Two endochondral growth zone configurations in the zebrafish pharyngeal skeleton produce either unidirectional (epiphyses) or bidirectional (synchondroses) growth. Cell proliferation drives endochondral growth and its modulation, in contrast to mammalian long bones in which bone length depends more on cell enlargement during hypertrophy and intramembranous ossification is the default mechanism of bone growth in zebrafish adults.

Zebrafish models of skeletal dysplasia induced by cholesterol biosynthesis deficiency

Human disorders of the post-squalene cholesterol biosynthesis pathway frequently result in skeletal abnormalities, yet our understanding of the mechanisms involved is limited. In a forward-genetic approach, we have found that a late-onset skeletal mutant, named kolibernu7 , is the result of a cis-acting regulatory mutation leading to loss of methylsterol monooxygenase 1 (msmo1) expression within pre-hypertrophic chondrocytes. Generated msmo1nu81 knockdown mutation resulted in lethality at larval stage. We demonstrated that this is a result of both cholesterol deprivation and sterol intermediate accumulation by creating a mutation eliminating activity of Lanosterol synthase (Lss). Our results indicate that double lssnu60;msmo1nu81 and single lssnu60 mutants survive significantly longer than msmo1nu81 homozygotes. Liver-specific restoration of either Msmo1 or Lss in corresponding mutant backgrounds suppresses larval lethality. Rescued mutants develop dramatic skeletal abnormalities, with a loss of Msmo1 activity resulting in a more-severe patterning defect of a near-complete loss of hypertrophic chondrocytes marked by col10a1a expression. Our analysis suggests that hypertrophic chondrocytes depend on endogenous cholesterol synthesis, and blocking C4 demethylation exacerbates the cholesterol deficiency phenotype. Our findings offer new insight into the genetic control of bone development and provide new zebrafish models for human disorders of the cholesterol biosynthesis pathway.

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.

Exposure to Metalaxyl Disturbs the Skeletal Development of Zebrafish Embryos

Metalaxyl is broadly applied in agriculture to control peronosporales-caused diseases in plant. To investigate the toxic effects, zebrafish embryos were exposed to metalaxyl at 5, 50 and 500 ng/L for 72 h, the development of larvae were assessed. A significant decreased survival rate, body length, hatching rate (48 h post-fertilization), and a significant increased spinal curvature rate were observed in the 500 ng/L treatment. The lengths of lower jaw, upper jaw and hyomandibular were significantly decreased in the 5, 50 and 500 ng/L groups; while the lower jaw width was significantly increased in the 500 ng/L group. The lengths of palatoquadrate, ceratohyal and ethmoid plate were reduced. Though cyp26a1 mRNA levels showed no significant change, the transcription of bmp2b (in the 500 ng/L group), ihh (in the 50 and 500 ng/L groups), shh (in the 5, 50 and 500 ng/L groups) were significantly up-regulated, which may be related to the abnormal development of the skeleton.

Comparative and evolutionary analysis of the reptilian hedgehog gene family (Shh, Dhh, and Ihh)

The hedgehog signaling pathway plays a vital role in human and animal patterning and cell proliferation during the developmental process. The hedgehog gene family of vertebrate species includes three genes, Shh, Dhh, and Ihh, which possess different functions and expression patterns. Despite the importance of hedgehog genes, genomic evidence of this gene family in reptiles is lacking. In this study, the available genomes of a number of representative reptile species were explored by utilizing adaptive evolutionary analysis methods to characterize the evolutionary patterns of the hedgehog gene family. Altogether, 33 sonic hedgehog (Shh), 25 desert hedgehog (Dhh), and 20 Indian hedgehog (Ihh) genes were obtained from reptiles, and six avian and five mammalian sequences were added to the analysis. The phylogenetic maximum likelihood (ML) tree of the Shh, Dhh, and Ihh genes revealed a similar topology, which is approximately consistent with the traditional taxonomic group. No shared positive selection site was identified by the PAML site model or the three methods in the Data Monkey Server. Branch model and Clade model C analyses revealed that the Dhh and Ihh genes experienced different evolutionary forces in reptiles and other vertebrates, while the Shh gene was not significantly different in terms of selection pressure. The different evolutionary rates of the Dhh and Ihh genes suggest that these genes may be potential contributors to the discrepant sperm and body development of different clades. The different adaptive evolutionary history of the Shh, Dhh, and Ihh genes among reptiles may be due to their different functions in regulating cellular events of development from the embryonic stages to adulthood. Overall, this study has provided meaningful information regarding the evolution of the hedgehog gene family in reptiles and a theoretical foundation for further analyses on the functional and molecular mechanisms that have shaped the reptilian hedgehog genes.

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.

Exogenous human beta amyloid peptide interferes osteogenesis through Sox9a in embryonic zebrafish

The two major hallmarks of Alzheimer's disease (AD) are beta-amyloid plaques and neurofibrillary tangles. Amyloid peptide aggregations in the brain cause loss of synaptic connections and subsequent neurotoxicity leading to neurodegeneration and memory deficits. However, the physiological effects of beta-amyloid on early embryonic development still remain unclear. Administration of human beta-amyloid peptide (1-42) through cerebrospinal ventricular injection was carried out at 24 hpf (hours post fertilization) and it was uptaken into the cellular layers of the early ventricular development without any plaque aggregation. Whole-mount Immunostaining of zebrafish embryos injected with the beta-amyloid at 60 hpf revealed the delay in Sox9a expression. Decreased level of cartilage to bone transformation rate in 15 dpf (days post fertilization) zebrafish was observed by differential staining. These results suggest the possible existence of a genetic relationship between extrinsic amyloid peptide and Sox9a expression. Thus, our results demonstrated that the human beta-amyloid influences bone development through Sox9a expression during osteogenesis in zebrafish.

Hedgehog signaling regulates osteoblast differentiation in zebrafish larvae through modulation of autophagy

Impaired osteoblast differentiation may result in bone metabolic diseases such as osteoporosis. It was reported recently that hedgehog (Hh) signaling and autophagy are two important regulators of bone differentiation. In order to further dissect their relationship in bone development, we used a zebrafish larvae model to investigate how disruption of one of these signals affects the function of the other and impacts osteoblast differentiation. Our results showed that activation of Hh signaling negatively regulated autophagy. However, suppression of autophagy by knocking down atg5 expression did not alter Hh signaling, but dramatically upregulated the expression of osteoblast-related genes and increased bone mineralization, especially in the den region. On the contrary, inhibition of the Hh signaling pathway by cyclopamine treatment suppressed the expression of osteoblast-related genes and decreased bone mineralization. In agreement with these findings, blocking Hh signaling through knockdown SHH and Gli2 genes led to defective osteoblast differentiation, while promoting Hh signaling by knockdown Ptch1 was beneficial to osteoblast differentiation. Our results thus support that activation of the Hh signaling pathway negatively regulates autophagy and consequentially promotes osteoblast differentiation. On the contrary, induction of autophagy inhibits osteoblast differentiation. Our work reveals the mechanism underlying Hh signaling pathway regulation of bone development.

Summary: Our report of an essential regulation role of hedgehog signaling and autophagy on osteoblast differentiation may contribute to research on bone development biology, hedgehog signaling and the autophagy pathway.

Adult islet1 Expression Outlines Ventralized Derivatives Along Zebrafish Neuraxis

Signals issued by dorsal roof and ventral floor plates, respectively, underlie the major patterning process of dorsalization and ventralization during vertebrate neural tube development. The ventrally produced morphogen Sonic hedgehog (SHH) is crucial for vertebrate hindbrain and spinal motor neuron development. One diagnostic gene for motor neurons is the LIM/homeodomain gene islet1, which has additional ventral expression domains extending into mid- and forebrain. In order to corroborate motor neuron development and, in particular, to improve on the identification of poorly documented zebrafish forebrain islet1 populations, we studied adult brains of transgenic islet1-GFP zebrafish (3 and 6 months). This molecular neuroanatomical analysis was supported by immunostaining these brains for tyrosine hydroxylase (TH) or choline acetyltransferase (ChAT), respectively, revealing zebrafish catecholaminergic and cholinergic neurons. The present analysis of ChAT and islet1-GFP label confirms ongoing adult expression of islet1 in zebrafish (basal plate) midbrain, hindbrain, and spinal motor neurons. In contrast, non-motor cholinergic systems lack islet1 expression. Additional presumed basal plate islet1 positive systems are described in detail, aided by TH staining which is particularly informative in the diencephalon. Finally, alar plate zebrafish forebrain systems with islet1 expression are described (i.e., thalamus, preoptic region, and subpallium). We conclude that adult zebrafish continue to express islet1 in the same brain systems as in the larva. Further, pending functional confirmation we hypothesize that the larval expression of sonic hedgehog (shh) might causally underlie much of adult islet1 expression because it explains findings beyond ventrally located systems, for example regarding shh expression in the zona limitans intrathalamica and correlated islet1-GFP expression in the thalamus.

hdac4 mediates perichondral ossification and pharyngeal skeleton development in the zebrafish

Background

Histone deacetylases (HDACs) are epigenetic factors that function to repress gene transcription by removing acetyl groups from the N-terminal of histone lysines. Histone deacetylase 4 (HDAC4), a class IIa HDAC, has previously been shown to regulate the process of endochondral ossification in mice via repression of Myocyte enhancer factor 2c (MEF2C), a transcriptional activator of Runx2, which in turn promotes chondrocyte maturation and production of bone by osteoblasts.

Methods & Materials

In this study, we generated two zebrafish lines with mutations in hdac4 using CRISPR/Cas9 and analyzed mutants for skeletal phenotypes and expression of genes known to be affected by Hdac4 expression.

Results

Lines have insertions causing a frameshift in a proximal exon of hdac4 and a premature stop codon. Mutations are predicted to result in aberrant protein sequence and a truncated protein, eliminating the Mef2c binding domain and Hdac domain. Zygotic mutants from two separate lines show a significant increase in ossification of pharyngeal ceratohyal cartilages at 7 days post fertilization (dpf) (p < 0.01, p < 0.001). At 4 dpf, mutant larvae have a significant increase of expression of runx2a and runx2b in the ceratohyal cartilage (p < 0.05 and p < 0.01, respectively). A subset of maternal-zygotic (mz) mutant and heterozygote larvae (40%) have dramatically increased ossification at 7 dpf compared to zygotic mutants, including formation of a premature anguloarticular bone and mineralization of the first and second ceratobranchial cartilages and symplectic cartilages, which normally does not occur until fish are approximately 10 or 12 dpf. Some maternal-zygotic mutants and heterozygotes show loss of pharyngeal first arch elements (25.9% and 10.2%, respectively) and neurocranium defects (30.8% and 15.2%, respectively). Analysis of RNA-seq mRNA transcript levels and in situ hybridizations from zygotic stages to 75–90% epiboly indicates that hdac4 is highly expressed in early embryos, but diminishes by late epiboly, becoming expressed again in larval stages.

Discussion

Loss of function of hdac4 in zebrafish is associated with increased expression of runx2a and runx2b targets indicating that a role for hdac4 in zebrafish is to repress activation of ossification of cartilage. These findings are consistent with observations of precocious cartilage ossification in Hdac4 mutant mice, demonstrating that the function of Hdac4 in skeletal development is conserved among vertebrates. Expression of hdac4 mRNA in embryos younger than 256–512 cells indicates that there is a maternal contribution of hdac4 to the early embryo. The increase in ossification and profound loss of first pharyngeal arch elements and anterior neurocranium in a subset of maternal-zygotic mutant and heterozygote larvae suggests that maternal hdac4 functions in cartilage ossification and development of cranial neural crest-derived structures.

A regulatory pathway involving retinoic acid and calcineurin demarcates and maintains joint cells and osteoblasts in regenerating fin

During zebrafish fin regeneration, blastema cells lining the epidermis differentiate into osteoblasts and joint cells to reconstruct the segmented bony rays. We show that osteoblasts and joint cells originate from a common cell lineage, but are committed to different cell fates. Pre-osteoblasts expressing runx2a/b commit to the osteoblast lineage upon expressing sp7, whereas the strong upregulation of hoxa13a correlates with a commitment to a joint cell type. In the distal regenerate, hoxa13a, evx1 and pthlha are sequentially upregulated at regular intervals to define the newly identified presumptive joint cells. Presumptive joint cells mature into joint-forming cells, a distinct cell cluster that maintains the expression of these factors. Analysis of evx1 null mutants reveals that evx1 is acting upstream of pthlha and downstream of or in parallel with hoxa13a Calcineurin activity, potentially through the inhibition of retinoic acid signaling, regulates evx1, pthlha and hoxa13a expression during joint formation. Furthermore, retinoic acid treatment induces osteoblast differentiation in mature joint cells, leading to ectopic bone deposition in joint regions. Overall, our data reveal a novel regulatory pathway essential for joint formation in the regenerating fin.

Crosstalk between Hedgehog pathway and energy pathways in human adipose-derived stem cells: A deep sequencing analysis of polysome-associated RNA

Adult stem cells are considered promising candidates for cellular therapies due to their capacity to differentiate and self-renew. Differentiation leads to changes in the metabolism, structure, and gene expression patterns of cells. Hedgehog is one of the pathways that is involved in the enhancement of osteogenesis and chondrogenesis in adult stem cells, but its mechanisms are poorly understood. In this study, we treated adipose tissue-derived stem cells (ADSC) with two well-characterized drugs, purmorphamine (Hedgehog pathway activator) and cyclopamine (Hedgehog pathway inhibitor), and identified mRNAs associated with polysomes in each treatment group to determine the post transcriptional genetic networks governed by the Hedgehog pathway. Activation of the Hedgehog pathway by purmorphamine results in significant upregulation of mRNAs associated with cellular communication and signal transduction. Furthermore, our experiments show that cyclopamine acts late downregulating GLI1 expression in ADSCs but promotes the upregulation of mRNAs associated with energy pathways and metabolism at early times. Through in silico analysis, we identified some miRNAs, such as miR-355, that could regulate these mRNAs association with polysomes and thereby modulate the Hedgehog pathway. Our results suggest that activation of the Hedgehog pathway by purmorphamine also results in a negative regulation of mRNAs in the protein translation machinery.

Widening control of fin inter-rays in zebrafish and inferences about actinopterygian fins

The amputation of a teleost fin rapidly triggers an intricate maze of hierarchically regulated signalling processes which ultimately reconstruct the diverse tissues of the appendage. Whereas the generation of the fin pattern along the proximodistal axis brings with it several well-known developmental regulators, the mechanisms by which the fin widens along its dorsoventral axis remain poorly understood. Utilizing the zebrafish as an experimental model of fin regeneration and studying more than 1000 actinopterygian species, we hypothesized a connection between specific inter-ray regulatory mechanisms and the morphological variability of inter-ray membranes found in nature. To tackle these issues, both cellular and molecular approaches have been adopted and our results suggest the existence of two distinguishable inter-ray areas in the zebrafish caudal fin, a marginal and a central region. The present work associates the activity of the cell membrane potassium channel kcnk5b, the fibroblast growth factor receptor 1 and the sonic hedgehog pathway to the control of several cell functions involved in inter-ray wound healing or dorsoventral regeneration of the zebrafish caudal fin. This ray-dependent regulation controls cell migration, cell-type patterning and gene expression. The possibility that modifications of these mechanisms are responsible for phenotypic variations found in euteleostean species, is discussed.

Shh promotes direct interactions between epidermal cells and osteoblast progenitors to shape regenerated zebrafish bone

Zebrafish innately regenerate amputated fins by mechanisms that expand and precisely position injury-induced progenitor cells to re-form tissue of the original size and pattern. For example, cell signaling networks direct osteoblast progenitors (pObs) to rebuild thin cylindrical bony rays with a stereotypical branched morphology. Hedgehog/Smoothened (Hh/Smo) signaling has been variably proposed to stimulate overall fin regenerative outgrowth or promote ray branching. Using a photoconvertible patched2 reporter, we resolve active Hh/Smo output to a narrow distal regenerate zone comprising pObs and adjacent motile basal epidermal cells. This Hh/Smo activity is driven by epidermal Sonic hedgehog a (Shha) rather than Ob-derived Indian hedgehog a (Ihha), which nevertheless functions atypically to support bone maturation. Using BMS-833923, a uniquely effective Smo inhibitor, and high-resolution imaging, we show that Shha/Smo is functionally dedicated to ray branching during fin regeneration. Hh/Smo activation enables transiently divided clusters of Shha-expressing epidermis to escort pObs into similarly split groups. This co-movement likely depends on epidermal cellular protrusions that directly contact pObs only where an otherwise occluding basement membrane remains incompletely assembled. Progressively separated pObs pools then continue regenerating independently to collectively re-form a now branched skeletal structure.

Nerves, H2O2 and Shh: Three players in the game of regeneration

The tight control of reactive oxygen species (ROS) levels is required during regeneration. H₂O₂ in particular assumes clear signalling functions at different steps in this process. Injured nerves induce high levels of H₂O₂ through the activation of the Hedgehog (Shh) pathway, providing an environment that promotes cell plasticity, progenitor recruitment and blastema formation. In turn, high H₂O₂ levels contribute to growing axon attraction. Once re-innervation is completed, nerves subsequently downregulate H₂O₂ levels to their original state. A similar regulatory loop between H₂O₂ levels and nerves also exists during development. This suggests that redox signalling is a major actor in cell plasticity.

Functional bone histology of zebrafish reveals two types of endochondral ossification, different types of osteoblast clusters and a new bone type

The zebrafish is as an important vertebrate animal model system for studying developmental processes, gene functions and signalling pathways. It is also used as a model system for the understanding of human developmental diseases including those related to the skeleton. However, surprisingly little is known about normal zebrafish skeletogenesis and osteogenesis. As in most vertebrates, it is commonly known that the bones of adult zebrafish are cellular unlike that of some other teleosts. After careful histological analyses of each zebrafish adult bone, we identified several acellular bones, with no entrapped osteocytes in addition to several cellular bones. We show that both cellular and acellular bones can even occur within the same skeletal element and transitions between these two cell types can be found. Furthermore, we describe two types of osteoblast clusters during skeletogenesis and two different types of endochondral ossification. The epiphyseal plate, for example, lacks a zone of calcification and a degradation zone with osteoblasts. A new bone type that we term tubular bone was also identified. This bone is completely filled with adipose tissue, unlike spongy bones. This study provides important insight on how osteogenesis takes place in zebrafish, and especially on the transition from cellular to acellular bones. Overall, this study leads to a deeper understanding of the functional histological composition of adult zebrafish bones.

Distinct requirements of wls, wnt9a, wnt5b and gpc4 in regulating chondrocyte maturation and timing of endochondral ossification

Formation of the mandible requires progressive morphologic change, proliferation, differentiation and organization of chondrocytes preceding osteogenesis. The Wnt signaling pathway is involved in regulating bone development and maintenance. Chondrocytes that are fated to become bone require Wnt to polarize and orientate appropriately to initiate the endochondral ossification program. Although the canonical Wnt signaling has been well studied in the context of bone development, the effects of non-canonical Wnt signaling in regulating the timing of cartilage maturation and subsequent bone formation in shaping ventral craniofacial structure is not fully understood.. Here we examined the role of the non-canonical Wnt signaling pathway (wls, gpc4, wnt5b and wnt9a) in regulating zebrafish Meckel's cartilage maturation to the onset of osteogenic differentiation. We found that disruption of wls resulted in a significant loss of craniofacial bone, whereas lack of gpc4, wnt5b and wnt9a resulted in severely delayed endochondral ossification. This study demonstrates the importance of the non-canonical Wnt pathway in regulating coordinated ventral cartilage morphogenesis and ossification.

The Morphogenesis of Cranial Sutures in Zebrafish

Using morphological, histological, and TEM analyses of the cranium, we provide a detailed description of bone and suture growth in zebrafish. Based on expression patterns and localization, we identified osteoblasts at different degrees of maturation. Our data confirm that, unlike in humans, zebrafish cranial sutures maintain lifelong patency to sustain skull growth. The cranial vault develops in a coordinated manner resulting in a structure that protects the brain. The zebrafish cranial roof parallels that of higher vertebrates and contains five major bones: one pair of frontal bones, one pair of parietal bones, and the supraoccipital bone. Parietal and frontal bones are formed by intramembranous ossification within a layer of mesenchyme positioned between the dermal mesenchyme and meninges surrounding the brain. The supraoccipital bone has an endochondral origin. Cranial bones are separated by connective tissue with a distinctive architecture of osteogenic cells and collagen fibrils. Here we show RNA in situ hybridization for col1a1a, col2a1a, col10a1, bglap/osteocalcin, fgfr1a, fgfr1b, fgfr2, fgfr3, foxq1, twist2, twist3, runx2a, runx2b, sp7/osterix, and spp1/ osteopontin, indicating that the expression of genes involved in suture development in mammals is preserved in zebrafish. We also present methods for examining the cranium and its sutures, which permit the study of the mechanisms involved in suture patency as well as their pathological obliteration. The model we develop has implications for the study of human disorders, including craniosynostosis, which affects 1 in 2,500 live births.

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.

Transcriptome analysis of vertebral bone in the flounder, Paralichthys olivaceus (Teleostei, Pleuronectiformes), using Illumina sequencing

The processes underlying vertebral development in teleosts and tetrapods differ markedly in a variety of ways. At present, the molecular basis of teleost vertebral development and growth is poorly understood. Understanding vertebral development at the molecular level is important for aquaculture to prevent vertebral anomalies that can arise from a variety of factors, including excess vitamin A (all-trans retinol, VA) in the diet. To facilitate studies on teloest vertebral development, we performed transcriptome analysis of four month old flounder, Paralichthys olivaceus, vertebrae using next-generation sequencing. Expression profile obtained demonstrates that some members of the hh, bmp, fgf, wnt gene families, and their receptors, hox, pax, sox, dlx and tbx gene families and ntl, which are known to function in notochord and somite development in embryos, are expressed in the vertebrae. It was also showed that in addition to the retinoic acid receptor (Rar), the vertebrae express alcohol dehydrogenase 1 and retinal dehydrogenase 2 which convert VA to all-trans-retinoic acid (RA). The assembled contigs also included cytochrome p450 family members, which inactivate RA, as well as phosphatidylcholine-retinol O-acetyltransferase, which converts VA to all-trans-retinyl ester, a stock form of VA. These data suggest that in teleost vertebrae, expression of various signals and transcription factors which function in the notochord and somite development is maintained until adult stage, and RA metabolism and signaling are active to regulate transcription of RA-responsible genes, such as hedgehog and hox genes. This is the first transcriptome analysis of teleost fish vertebrae.

Building the backbone: the development and evolution of vertebral patterning

The segmented vertebral column comprises a repeat series of vertebrae, each consisting of two key components: the vertebral body (or centrum) and the vertebral arches. Despite being a defining feature of the vertebrates, much remains to be understood about vertebral development and evolution. Particular controversy surrounds whether vertebral component structures are homologous across vertebrates, how somite and vertebral patterning are connected, and the developmental origin of vertebral bone-mineralizing cells. Here, we assemble evidence from ichthyologists, palaeontologists and developmental biologists to consider these issues. Vertebral arch elements were present in early stem vertebrates, whereas centra arose later. We argue that centra are homologous among jawed vertebrates, and review evidence in teleosts that the notochord plays an instructive role in segmental patterning, alongside the somites, and contributes to mineralization. By clarifying the evolutionary relationship between centra and arches, and their varying modes of skeletal mineralization, we can better appreciate the detailed mechanisms that regulate and diversify vertebral patterning.

Hedgehog Signaling during Appendage Development and Regeneration

Regulatory networks that govern embryonic development have been well defined. While a common hypothesis supports the notion that the embryonic regulatory cascades are reexpressed following injury and tissue regeneration, the mechanistic regulatory pathways that mediate the regenerative response in higher organisms remain undefined. Relative to mammals, lower vertebrates, including zebrafish and newts, have a tremendous regenerative capacity to repair and regenerate a number of organs including: appendages, retina, heart, jaw and nervous system. Elucidation of the pathways that govern regeneration in these lower organisms may provide cues that will enhance the capacity for the regeneration of mammalian organs. Signaling pathways, such as the hedgehog pathway, have been shown to play critical functions during development and during regeneration in lower organisms. These signaling pathways have been shown to modulate multiple processes including cellular origin, positional identity and cellular maturation. The present review will focus on the cellular and molecular regulation of the hedgehog (HH) signaling pathway and its interaction with other signaling factors during appendage development and regeneration.

Denervation impairs regeneration of amputated zebrafish fins

BACKGROUND

Zebrafish are able to regenerate many of its tissues and organs after damage. In amphibians this process is regulated by nerve fibres present at the site of injury, which have been proposed to release factors into the amputated limbs/fins, promoting and sustaining the proliferation of blastemal cells. Although some candidate factors have been proposed to mediate the nerve dependency of regeneration, the molecular mechanisms involved in this process remain unclear.

RESULTS

We have used zebrafish as a model system to address the role of nerve fibres in fin regeneration. We have developed a protocol for pectoral fin denervation followed by amputation and analysed the regenerative process under this experimental conditions. Upon denervation fins were able to close the wound and form a wound epidermis, but could not establish a functional apical epithelial cap, with a posterior failure of blastema formation and outgrowth, and the accumulation of several defects. The expression patterns of genes known to be key players during fin regeneration were altered upon denervation, suggesting that nerves can contribute to the regulation of the Fgf, Wnt and Shh pathways during zebrafish fin regeneration.

CONCLUSIONS

Our results demonstrate that proper innervation of the zebrafish pectoral fin is essential for a successful regenerative process, and establish this organism as a useful model to understand the molecular and cellular mechanisms of nerve dependence, during vertebrate regeneration.

Evolutionary genomics and adaptive evolution of the Hedgehog gene family (Shh, Ihh and Dhh) in vertebrates

The Hedgehog (Hh) gene family codes for a class of secreted proteins composed of two active domains that act as signalling molecules during embryo development, namely for the development of the nervous and skeletal systems and the formation of the testis cord. While only one Hh gene is found typically in invertebrate genomes, most vertebrates species have three (Sonic hedgehog – Shh; Indian hedgehog – Ihh; and Desert hedgehog – Dhh), each with different expression patterns and functions, which likely helped promote the increasing complexity of vertebrates and their successful diversification. In this study, we used comparative genomic and adaptive evolutionary analyses to characterize the evolution of the Hh genes in vertebrates following the two major whole genome duplication (WGD) events. To overcome the lack of Hh-coding sequences on avian publicly available databases, we used an extensive dataset of 45 avian and three non-avian reptilian genomes to show that birds have all three Hh paralogs. We find suggestions that following the WGD events, vertebrate Hh paralogous genes evolved independently within similar linkage groups and under different evolutionary rates, especially within the catalytic domain. The structural regions around the ion-binding site were identified to be under positive selection in the signaling domain. These findings contrast with those observed in invertebrates, where different lineages that experienced gene duplication retained similar selective constraints in the Hh orthologs. Our results provide new insights on the evolutionary history of the Hh gene family, the functional roles of these paralogs in vertebrate species, and on the location of mutational hotspots.

Holmgren's principle of delamination during fin skeletogenesis

During fin morphogenesis, several mesenchyme condensations occur to give rise to the dermal skeleton. Although each of them seems to create distinctive and unique structures, they all follow the premises of the same morphogenetic principle. Holmgren's principle of delamination was first proposed to describe the morphogenesis of skeletal elements of the cranium, but Jarvik extended it to the development of the fin exoskeleton. Since then, some cellular or molecular explanations, such as the "flypaper" model (Thorogood et al.), or the evolutionary description by Moss, have tried to clarify this topic. In this article, we review new data from zebrafish studies to meet these criteria described by Holmgren and other authors. The variety of cell lineages involved in these skeletogenic condensations sheds light on an open discussion of the contributions of mesoderm- versus neural crest-derived cell lineages to the development of the head and trunk skeleton. Moreover, we discuss emerging molecular studies that are disclosing conserved regulatory mechanisms for dermal skeletogenesis and similarities during fin development and regeneration, which may have important implications in the potential use of the zebrafish fin as a model for regenerative medicine.

The dlx5a/dlx6a genes play essential roles in the early development of zebrafish median fin and pectoral structures

The Dlx5 and Dlx6 genes encode homeodomain transcription factors essential for the proper development of limbs in mammalian species. However, the role of their teleost counterparts in fin development has received little attention. Here, we show that dlx5a is an early marker of apical ectodermal cells of the pectoral fin buds and of the median fin fold, but also of cleithrum precursor cells during pectoral girdle development. We propose that early median fin fold establishment results from the medial convergence of dlx5a-expressing cells at the lateral edges of the neural keel. Expression analysis also shows involvement of dlx5a during appendage skeletogenesis. Using morpholino-mediated knock down, we demonstrate that disrupted dlx5a/6a function results in pectoral fin agenesis associated with misexpression of bmp4, fgf8a, and1 and msx genes. In contrast, the median fin fold presents defects in mesenchymal cell migration and actinotrichia formation, whereas the initial specification seems to occur normally. Our results demonstrate that the dlx5a/6a genes are essential for the induction of pectoral fin outgrowth, but are not required during median fin fold specification. The dlx5a/6a knock down also causes a failure of cleithrum formation associated with a drastic loss of runx2b and col10a1 expression. The data indicate distinct requirements for dlx5a/6a during median and pectoral fin development suggesting that initiation of unpaired and paired fin formation are not directed through the same molecular mechanisms. Our results refocus arguments on the mechanistic basis of paired appendage genesis during vertebrate evolution.

SNP discovery from transcriptome of the swimbladder of Takifugu rubripes

Single nucleotide polymorphisms (SNPs) have become the marker of choice for genome-wide association studies in many species. High-throughput sequencing of RNA was developed primarily to analyze global gene expression, while it is an efficient way to discover SNPs from the expressed genes. In this study, we conducted transcriptome sequencing of the swimbladder of Takifugu rubripes using Illumina HiSeq2000 platform to identify gene-associated SNPs in the swimbladder. A total of 30,312,181 unique-mapped-reads were obtained from 44,736,850 raw reads. A total of 62,270 putative SNPs were discovered, which were located in 11,306 expressed genes and 2,246 scaffolds. The average minor allele frequency (MAF) of the SNPs was 0.26. GO and KEGG pathway analysis were conducted to analyze the genes containing SNPs. Validation of selected SNPs revealed that 54% of SNPs (26/48) were true SNPs. The results suggest that RNA-Seq is an efficient and cost-effective approach to discover gene-associated SNPs. In this study, a large number of SNPs were identified and these data will be useful resources for population genetic study, evolution analysis, resource assessment, genetic linkage analysis and genome-wide association studies.

Transcriptome sequencing of Atlantic salmon (Salmo salar L.) notochord prior to development of the vertebrae provides clues to regulation of positional fate, chordoblast lineage and mineralisation

Background

In teleosts such as Atlantic salmon (Salmo salar L.), segmentation and subsequent mineralisation of the notochord during embryonic stages are essential for normal vertebrae formation. However, the molecular mechanisms leading to segmentation and mineralisation of the notochord are poorly understood. The aim of this study was to identify genes/pathways acting in gradients over time and along the anterior-posterior axis during notochord segmentation and immediately prior to mineralisation of the vertebral bodies in Atlantic salmon.

Results

Notochord samples were collected from unsegmented, pre-segmented and segmented developmental stages. In each stage, the cellular core of the notochord was cut into three pieces along the longitudinal axis (anterior, mid, posterior). RNA was sequenced (22 million pair-end 100 bp/ library) and mapped to the salmon genome. 66569 transcripts were predicted and 55775 were annotated. In order to identify possible gradients leading to segmentation of the notochord, all 71 notochord-expressed hox genes were investigated, most of them displaying a typical anterior-posterior expression pattern along the notochord axis. The clustering of hox genes revealed a pattern that could be related to notochord segmentation. We further investigated how mineralisation is initiated in the notochord, and several factors related to chondrogenic lineage were identified (sox9, sox5, sox6, tgfb3, ihhb and col2a1), suggesting a cartilage-like character of the notochord. KEGG analysis of differentially expressed genes between stages revealed down-regulation of pathways associated with ECM, cell division, metabolism and development at onset of notochord segmentation. This implies that inhibitory signals produce segmentation of the notochord. One such potential inhibitory signal was identified, col11a2, which was detected in segments of non-mineralising notochord.

Conclusions

An incomplete salmon genome was successfully used to analyse RNA-seq data from the cellular core of the Atlantic salmon notochord. In transcriptome we found; hox gene patterns possibly linked to segmentation; down-regulation of pathways in the notochord at onset of segmentation; segmented expression of col11a2 in non-mineralised segments of the notochord; and a chondroblast-like footprint in the notochord.

Role of mef2ca in developmental buffering of the zebrafish larval hyoid dermal skeleton

Phenotypic robustness requires a process of developmental buffering that is largely not understood, but which can be disrupted by mutations. Here we show that in mef2ca(b1086) loss of function mutant embryos and early larvae, development of craniofacial hyoid bones, the opercle (Op) and branchiostegal ray (BR), becomes remarkably unstable; the large magnitude of the instability serves as a positive attribute to learn about features of this developmental buffering. The OpBR mutant phenotype variably includes bone expansion and fusion, Op duplication, and BR homeosis. Formation of a novel bone strut, or a bone bridge connecting the Op and BR together occurs frequently. We find no evidence that the phenotypic stability in the wild type is provided by redundancy between mef2ca and its co-ortholog mef2cb, or that it is related to the selector (homeotic) gene function of mef2ca. Changes in dorsal-ventral patterning of the hyoid arch also might not contribute to phenotypic instability in mutants. However, subsequent development of the bone lineage itself, including osteoblast differentiation and morphogenetic outgrowth, shows marked variation. Hence, steps along the developmental trajectory appear differentially sensitive to the loss of buffering, providing focus for the future study.

Development of an in vitro cell system from zebrafish suitable to study bone cell differentiation and extracellular matrix mineralization

Mechanisms of bone formation and skeletal development have been successfully investigated in zebrafish using a variety of in vivo approaches, but in vitro studies have been hindered due to a lack of homologous cell lines capable of producing an extracellular matrix (ECM) suitable for mineral deposition. Here we describe the development and characterization of a new cell line termed ZFB1, derived from zebrafish calcified tissues. ZFB1 cells have an epithelium-like phenotype, grow at 28°C in a regular L-15 medium supplemented with 15% of fetal bovine serum, and are maintained and manipulated using standard methods (e.g., trypsinization, cryopreservation, and transfection). They can therefore be propagated and maintained easily in most cell culture facilities. ZFB1 cells show aneuploidy with 2n=78 chromosomes, indicative of cell transformation. Furthermore, because DNA can be efficiently delivered into their intracellular space by nucleofection, ZFB1 cells are suitable for gene targeting approaches and for assessing gene promoter activity. ZFB1 cells can also differentiate toward osteoblast or chondroblast lineages, as demonstrated by expression of osteoblast- and chondrocyte-specific markers, they exhibit an alkaline phosphatase activity, a marker of bone formation in vivo, and they can mineralize their ECM. Therefore, they represent a valuable zebrafish-derived in vitro system for investigating bone cell differentiation and extracellular matrix mineralization.

Comparative analysis of a teleost skeleton transcriptome provides insight into its regulation

An articulated endoskeleton that is calcified is a unifying innovation of the vertebrates, however the molecular basis of the structural divergence between terrestrial and aquatic vertebrates, such as teleost fish, has not been determined. In the present study long-read next generation sequencing (NGS, Roche 454 platform) was used to characterize acellular perichondral bone (vertebrae) and chondroid bone (gill arch) in the gilthead sea bream (Sparus auratus). A total of 15.97 and 14.53Mb were produced, respectively from vertebrae and gill arch cDNA libraries and yielded 32,374 and 28,371 contigs (consensus sequences) respectively. 10,455 contigs from vertebrae and 10,625 contigs from gill arches were annotated with gene ontology terms. Comparative analysis of the global transcriptome revealed 4249 unique transcripts in vertebrae, 4201 unique transcripts in the gill arches and 3700 common transcripts. Several core gene networks were conserved between the gilthead sea bream and mammalian skeleton. Transcripts for putative endocrine factors were identified in acellular gilthead sea bream bone suggesting that in common with mammalian bone it can act as an endocrine tissue. The acellular bone of the vertebra, in contrast to current opinion based on histological analysis, was responsive to a short fast and significant (p<0.05) down-regulation of several transcripts identified by NGS, osteonectin, osteocalcin, cathepsin K and IGFI occurred. In gill arches fasting caused a significant (p<0.05) down-regulation of osteocalcin and up-regulation of MMP9.

A col10a1:nlGFP transgenic line displays putative osteoblast precursors at the medaka notochordal sheath prior to mineralization

In teleosts, such as medaka, ossification of the vertebral column starts with the mineralization of the notochordal sheath in a segmental pattern. This establishes the chordal centrum, which serves as the basis for further ossifications by sclerotome derived osteoblasts generating the vertebral body. So far, it is unclear which cells produce the notochordal sheath and how a segmental pattern of mineralization is established in teleosts. Here, we use a transgenic medaka line that expresses nlGFP under the control of the col10a1 promoter for in vivo analysis of vertebral body formation. We show that col10a1:nlGFP expression recapitulates endogenous col10a1 expression. In the axial skeleton, col10a1:nlGFP cells appear prior to the mineralization of the notochordal sheath in a segmental pattern. These cells remain on the outer surface of the chordal centra during mineralization as well as subsequent perichordal ossification of the vertebral bodies. Using twist1a1:dsRed and osx:mCherry transgenic lines we show that a subset of col10a1:nlGFP cells is derived from sclerotomal precursors and differentiates into future osteoblasts. For the first time, this shows a segmental occurrence of putative osteoblast precursors in the vertebral centra prior to ossification of the notochordal sheath. This opens the possibility that sclerotome derived cells in teleosts are implicated in the establishment of the mineralized vertebral column in a similar manner as previously described for tetrapods.

Evolution of the vertebrate bone matrix: an expression analysis of the network forming collagen paralogues in amphibian osteoblasts

The emergence of vertebrates is closely associated to the evolution of mineralized bone tissue. However, the molecular basis underlying the origin and subsequent diversification of the skeletal mineralized matrix is still poorly understood. One efficient way to tackle this issue is to compare the expression, between vertebrate species, of osteoblastic genes coding for bone matrix proteins. In this work, we have focused on the evolution of the network forming collagen family which contains the Col8a1, Col8a2, and Col10a1 genes. Both phylogeny and synteny reveal that these three paralogues are vertebrate-specific and derive from two independent duplications in the vertebrate lineage. To shed light on the evolution of this family, we have analyzed the osteoblastic expression of the network forming collagens in endochondral and intramembraneous skeletal elements of the amphibian Xenopus tropicalis. Remarkably, we find that amphibian osteoblasts express Col10a1, a gene strongly expressed in osteoblasts in actinopterygians but not in amniotes. In addition, while Col8a1 is known to be robustly expressed in mammalian osteoblasts, the expression levels of its amphibian orthologue are dramatically reduced. Our work reveals that while a skeletal expression of network forming collagen members is widespread throughout vertebrates, osteoblasts from divergent vertebrate lineages express different combinations of network forming collagen paralogues.

New tools for studying osteoarthritis genetics in zebrafish

OBJECTIVE

Increasing evidence points to a strong genetic component to osteoarthritis (OA) and that certain changes that occur in osteoarthritic cartilage recapitulate the developmental process of endochondral ossification. As zebrafish are a well validated model for genetic studies and developmental biology, our objective was to establish the spatiotemporal expression pattern of a number of OA susceptibility genes in the larval zebrafish providing a platform for functional studies into the role of these genes in OA.

DESIGN

We identified the zebrafish homologues for Mcf2l, Gdf5, PthrP/Pthlh, Col9a2, and Col10a1 from the Ensembl genome browser. Labelled probes were generated for these genes and in situ hybridisations were performed on wild type zebrafish larvae. In addition, we generated transgenic reporter lines by modification of bacterial artificial chromosomes (BACs) containing full length promoters for col2a1 and col10a1.

RESULTS

For the first time, we show the spatiotemporal expression pattern of Mcf2l. Furthermore, we show that all six putative OA genes are dynamically expressed during zebrafish larval development, and that all are expressed in the developing skeletal system. Furthermore, we demonstrate that the transgenic reporters we have generated for col2a1 and col10a1 can be used to visualise chondrocyte hypertrophy in vivo.

CONCLUSION

In this study we describe the expression pattern of six OA susceptibility genes in zebrafish larvae and the generation of two new transgenic lines marking chondrocytes at different stages of maturation. Moreover, the tools used demonstrate the utility of the zebrafish model for functional studies on genes identified as playing a role in OA.