Functional characterization and genetic mapping of alk8

Robust axis elongation by Nodal-dependent restriction of BMP signaling

Embryogenesis results from the coordinated activities of different signaling pathways controlling cell fate specification and morphogenesis. In vertebrate gastrulation, both Nodal and BMP signaling play key roles in germ layer specification and morphogenesis, yet their interplay to coordinate embryo patterning with morphogenesis is still insufficiently understood. Here, we took a reductionist approach using zebrafish embryonic explants to study the coordination of Nodal and BMP signaling for embryo patterning and morphogenesis. We show that Nodal signaling triggers explant elongation by inducing mesendodermal progenitors but also suppressing BMP signaling activity at the site of mesendoderm induction. Consistent with this, ectopic BMP signaling in the mesendoderm blocks cell alignment and oriented mesendoderm intercalations, key processes during explant elongation. Translating these ex vivo observations to the intact embryo showed that, similar to explants, Nodal signaling suppresses the effect of BMP signaling on cell intercalations in the dorsal domain, thus allowing robust embryonic axis elongation. These findings suggest a dual function of Nodal signaling in embryonic axis elongation by both inducing mesendoderm and suppressing BMP effects in the dorsal portion of the mesendoderm.

Navigating the Complex Landscape of Fibrodysplasia Ossificans Progressiva: From Current Paradigms to Therapeutic Frontiers

Fibrodysplasia ossificans progressiva (FOP) is an enigmatic, ultra-rare genetic disorder characterized by progressive heterotopic ossification, wherein soft connective tissues undergo pathological transformation into bone structures. This incapacitating process severely limits patient mobility and poses formidable challenges for therapeutic intervention. Predominantly caused by missense mutations in the ACVR1 gene, this disorder has hitherto defied comprehensive mechanistic understanding and effective treatment paradigms. This write-up offers a comprehensive overview of the contemporary understanding of FOP's complex pathobiology, underscored by advances in molecular genetics and proteomic studies. We delve into targeted therapy, spanning genetic therapeutics, enzymatic and transcriptional modulation, stem cell therapies, and innovative immunotherapies. We also highlight the intricate complexities surrounding clinical trial design for ultra-rare disorders like FOP, addressing fundamental statistical limitations, ethical conundrums, and methodological advancements essential for the success of interventional studies. We advocate for the adoption of a multi-disciplinary approach that converges bench-to-bedside research, clinical expertise, and ethical considerations to tackle the challenges of ultra-rare diseases like FOP and comparable ultra-rare diseases. In essence, this manuscript serves a dual purpose: as a definitive scientific resource for ongoing and future FOP research and a call to action for innovative solutions to address methodological and ethical challenges that impede progress in the broader field of medical research into ultra-rare conditions.

Significance of Premature Vertebral Mineralization in Zebrafish Models in Mechanistic and Pharmaceutical Research on Hereditary Multisystem Diseases

Zebrafish are increasingly becoming an important model organism for studying the pathophysiological mechanisms of human diseases and investigating how these mechanisms can be effectively targeted using compounds that may open avenues to novel treatments for patients. The zebrafish skeleton has been particularly instrumental in modeling bone diseases as-contrary to other model organisms-the lower load on the skeleton of an aquatic animal enables mutants to survive to early adulthood. In this respect, the axial skeletons of zebrafish have been a good read-out for congenital spinal deformities such as scoliosis and degenerative disorders such as osteoporosis and osteoarthritis, in which aberrant mineralization in humans is reflected in the respective zebrafish models. Interestingly, there have been several reports of hereditary multisystemic diseases that do not affect the vertebral column in human patients, while the corresponding zebrafish models systematically show anomalies in mineralization and morphology of the spine as their leading or, in some cases, only phenotype. In this review, we describe such examples, highlighting the underlying mechanisms, the already-used or potential power of these models to help us understand and amend the mineralization process, and the outstanding questions on how and why this specific axial type of aberrant mineralization occurs in these disease models.

A critical review on the role of abiotic factors on the transformation, environmental identity and toxicity of engineered nanomaterials in aquatic environment

Engineered nanomaterials (ENMs) are at the forefront of many technological breakthroughs in science and engineering. The extensive use of ENMs in several consumer products has resulted in their release to the aquatic environment. ENMs entering the aquatic ecosystem undergo a dynamic transformation as they interact with organic and inorganic constituents present in aquatic environment, specifically abiotic factors such as NOM and clay minerals, and attain an environmental identity. Thus, a greater understanding of ENM-abiotic factors interactions is required for an improved risk assessment and sustainable management of ENMs contamination in the aquatic environment. This review integrates fundamental aspects of ENMs transformation in aquatic environment as impacted by abiotic factors, and delineates the recent advances in bioavailability and ecotoxicity of ENMs in relation to risk assessment for ENMs-contaminated aquatic ecosystem. It specifically discusses the mechanism of transformation of different ENMs (metals, metal oxides and carbon based nanomaterials) following their interaction with the two most common abiotic factors NOM and clay minerals present within the aquatic ecosystem. The review critically discusses the impact of these mechanisms on the altered ecotoxicity of ENMs including the impact of such transformation at the genomic level. Finally, it identifies the gaps in our current understanding of the role of abiotic factors on the transformation of ENMs and paves the way for the future research areas.

Zebrafish embryonic explants undergo genetically encoded self-assembly

Embryonic stem cell cultures are thought to self-organize into embryoid bodies, able to undergo symmetry-breaking, germ layer specification and even morphogenesis. Yet, it is unclear how to reconcile this remarkable self-organization capacity with classical experiments demonstrating key roles for extrinsic biases by maternal factors and/or extraembryonic tissues in embryogenesis. Here, we show that zebrafish embryonic tissue explants, prepared prior to germ layer induction and lacking extraembryonic tissues, can specify all germ layers and form a seemingly complete mesendoderm anlage. Importantly, explant organization requires polarized inheritance of maternal factors from dorsal-marginal regions of the blastoderm. Moreover, induction of endoderm and head-mesoderm, which require peak Nodal-signaling levels, is highly variable in explants, reminiscent of embryos with reduced Nodal signals from the extraembryonic tissues. Together, these data suggest that zebrafish explants do not undergo bona fide self-organization, but rather display features of genetically encoded self-assembly, where intrinsic genetic programs control the emergence of order.

Combination of humic acid and clay reduce the ecotoxic effect of TiO2 NPs: A combined physico-chemical and genetic study using zebrafish embryo

The series of breakthroughs that have occurred within the realm of nanotechnology have been the source of several new products and technological interventions. One of the most salient examples in this regard is the widespread employment of titanium dioxide (TiO₂) nanoparticles across a range of consumer goods. Given that waste is generated at every stage of the consumer-product cycle (from production to disposal), many items with TiO₂ nanoparticles are likely to end up being discarded into water bodies. In order to understand the interaction of TiO₂ NPs with aquatic ecosystem, the ecological fate and toxicity of TiO₂ NPs was studied by exposing zebrafish embryos to a combination of abiotic factors (humic acid and clay) to assess its effect on the development of zebrafish embryos. The physiological changes were correlated with genetic marker analysis to holistically understand the effect on embryos development. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to analyze the interaction energy between TiO₂ NPs and natural organic matter (NOM) for understanding the aggregation behavior of engineered nanoparticles (ENPs) in media. The study revealed that combination of HA and clay stabilized TiO₂ NPs, compared to bare TiO₂ and HA or clay alone. TiO₂ NPs and TiO₂ NPs + Clay significantly altered the expression of genes involved in development of dorsoventral axis and neural network of zebrafish embryos. However, the presence of HA and HA + clay showed protective effect on zebrafish embryo development. The complete system analysis demonstrated the possible ameliorating effects of abiotic factors on the ecotoxicity of ENPs.

An Adult Zebrafish Model of Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare human skeletal disease caused by constitutively activating mutations in the gene ACVR1, which encodes a type I BMP/TGFβ family member receptor. FOP is characterized by progressive heterotopic ossification (HO) of fibrous tissues, including skeletal muscle, tendons, and ligaments, as well as malformation of the big toes, vertebral fusions, and osteochondromas. Surgical interventions in patients often result in enhanced HO, which can exacerbate rather than improve diagnostic outcomes. As a result of these difficulties, a variety of animal models are needed to study human FOP. Here we describe the methods for creating and characterizing zebrafish conditionally expressing Acvr1l^Q204D, the first adult zebrafish model for FOP.

Animal models of fibrodysplasia ossificans progressiva

Fibrodysplasia Ossificans Progressiva is a rare human disease of heterotopic ossification. FOP patients experience progressive development of ectopic bone within fibrous tissues that contributes to a gradual loss of mobility and can lead to early mortality. Due to lack of understanding of the etiology and progression of human FOP, and the fact that surgical interventions often exacerbate FOP disease progression, alternative therapeutic methods are needed, including modeling in animals, to study and improve understanding of human FOP. In this review we provide an overview of the existing animal models of FOP and the key mechanistic findings from each. In addition, we highlight the specific advantages of a new adult zebrafish model, generated by our lab, to study human FOP. Developmental Dynamics 247:279-288, 2018. © 2017 Wiley Periodicals, Inc.

A Zebrafish Model of Human Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare, autosomal dominant genetic disorder in humans characterized by explosive inflammatory response to injury leading to gradual ossification within fibrous tissues, including skeletal muscle, tendons, and ligaments. A variety of animal models are needed to study and understand the etiology of human FOP. To address this need, here we present characterizations of the first adult zebrafish model for FOP. In humans, activating mutations in the Type I BMP/TGFβ family member receptor, ACVR1, are associated with FOP. Zebrafish acvr1l, previously known as alk8, is the functional ortholog of human ACVR1, and has been studied extensively in the developing zebrafish embryo, where it plays a role in early dorsoventral patterning. Constitutively active and dominant negative mutations in zebrafish acvr1l cause early lethal defects. Therefore, to study roles for activating acvr1l mutations in adult zebrafish, we created transgenic animals expressing mCherry-tagged, heat-shock-inducible constitutively active Acvr1l, Acvr1l^Q204D, to investigate phenotypes in juvenile and adult zebrafish. Our studies showed that adult zebrafish expressing heat-shock-induced Acvr1l^Q204D develop a number of human FOP-like phenotypes, including heterotopic ossification lesions, spinal lordosis, vertebral fusions, and malformed pelvic fins. Together, these results suggest that transgenic zebrafish expressing heat-shock-inducible Acvr1l^Q204D can serve as a model for human FOP.

Maternal control of axial-paraxial mesoderm patterning via direct transcriptional repression in zebrafish

Axial-paraxial mesoderm patterning is a special dorsal-ventral patterning event of establishing the vertebrate body plan. Though dorsal-ventral patterning has been extensively studied, the initiation of axial-paraxial mesoderm pattering remains largely unrevealed. In zebrafish, spt cell-autonomously regulates paraxial mesoderm specification and flh represses spt expression to promote axial mesoderm fate, but the expression domains of spt and flh initially overlap in the entire marginal zone of the embryo. Defining spt and flh territories is therefore a premise of axial-paraxial mesoderm patterning. In this study, we investigated why and how the initial expression of flh becomes repressed in the ventrolateral marginal cells during blastula stage. Loss- and gain-of-function experiments showed that a maternal transcription factor Vsx1 is essential for restricting flh expression within the dorsal margin and preserving spt expression and paraxial mesoderm specification in the ventrolateral margin of embryo. Chromatin immunoprecipitation and electrophoretic mobility shift assays in combination with core consensus sequence mutation analysis further revealed that Vsx1 can directly repress flh by binding to the proximal promoter at a specific site. Inhibiting maternal vsx1 translation resulted in confusion of axial and paraxial mesoderm markers expression and axial-paraxial mesoderm patterning. These results demonstrated that direct transcriptional repression of the decisive axial mesoderm gene by maternal ventralizing factor is a crucial regulatory mechanism of initiating axial-paraxial mesoderm patterning in vertebrates.

Kidney organogenesis in the zebrafish: insights into vertebrate nephrogenesis and regeneration

Vertebrates form a progressive series of up to three kidney organs during development-the pronephros, mesonephros, and metanephros. Each kidney derives from the intermediate mesoderm and is comprised of conserved excretory units called nephrons. The zebrafish is a powerful model for vertebrate developmental genetics, and recent studies have illustrated that zebrafish and mammals share numerous similarities in nephron composition and physiology. The zebrafish embryo forms an architecturally simple pronephros that has two nephrons, and these eventually become a scaffold onto which a mesonephros of several hundred nephrons is constructed during larval stages. In adult zebrafish, the mesonephros exhibits ongoing nephrogenesis, generating new nephrons from a local pool of renal progenitors during periods of growth or following kidney injury. The characteristics of the zebrafish pronephros and mesonephros make them genetically tractable kidney systems in which to study the functions of renal genes and address outstanding questions about the mechanisms of nephrogenesis. Here, we provide an overview of the formation and composition of these zebrafish kidney organs, and discuss how various zebrafish mutants, gene knockdowns, and transgenic models have created frameworks in which to further delineate nephrogenesis pathways.

Distinct and separable activities of the endocytic clathrin-coat components Fcho1/2 and AP-2 in developmental patterning

Clathrin-mediated endocytosis occurs at multiple independent import sites on the plasma membrane, but how these positions are selected and how different cargo is simultaneously recognized is obscure. FCHO1 and FCHO2 are early-arriving proteins at surface clathrin assemblies and are speculated to act as compulsory coat nucleators, preceding the core clathrin adaptor AP-2. Here, we show the μ-homology domain (μHD) of FCHO1/2 represents a novel endocytic interaction hub. Translational silencing of fcho1 in zebrafish embryos causes strong dorsoventral patterning defects analogous to Bmp signal failure. The Fcho1 μHD interacts with the Bmp receptor Alk8, uncovering a new endocytic component that positively modulates Bmp signal transmission. Still, the fcho1 morphant phenotype is distinct from severe embryonic defects apparent when AP-2 is depleted. Our data thus contradict the primacy of FCHO1/2 in coat initiation.

The fibrodysplasia ossificans progressiva R206H ACVR1 mutation activates BMP-independent chondrogenesis and zebrafish embryo ventralization

Patients with classic fibrodysplasia ossificans progressiva, a disorder characterized by extensive extraskeletal endochondral bone formation, share a recurrent mutation (R206H) within the glycine/serine-rich domain of ACVR1/ALK2, a bone morphogenetic protein type I receptor. Through a series of in vitro assays using several mammalian cell lines and chick limb bud micromass cultures, we determined that mutant R206H ACVR1 activated BMP signaling in the absence of BMP ligand and mediated BMP-independent chondrogenesis that was enhanced by BMP. We further investigated the interaction of mutant R206H ACVR1 with FKBP1A, a glycine/serine domain-binding protein that prevents leaky BMP type I receptor activation in the absence of ligand. The mutant protein exhibited reduced binding to FKBP1A in COS-7 simian kidney cell line assays, suggesting that increased BMP pathway activity in COS-7 cells with R206H ACVR1 is due, at least in part, to decreased binding of this inhibitory factor. Consistent with these findings, in vivo analyses of zebrafish embryos showed BMP-independent hyperactivation of BMP signaling in response to the R206H mutant, resulting in increased embryonic ventralization. These data support the conclusion that the mutant R206H ACVR1 receptor in FOP patients is an activating mutation that induces BMP signaling in a BMP-independent and BMP-responsive manner to promote chondrogenesis, consistent with the ectopic endochondral bone formation in these patients.

Insights from a rare genetic disorder of extra-skeletal bone formation, fibrodysplasia ossificans progressiva (FOP)

Fibrodysplasia ossificans progressiva (FOP) is a rare human genetic disorder of extensive and debilitating extra-skeletal bone formation. While the challenges of investigating a rare condition are many, the potential benefits are also great - not only for the specific disease under investigation, but also for the unique perspective on how cells normally function and the mechanisms that underlie more common disorders. This review will illustrate some of the many insights that we have gained by studying FOP.

HSPG modulation of BMP signaling in fibrodysplasia ossificans progressiva cells

Cell surface heparan sulfate proteoglycans (HSPGs) play important roles in morphogen gradient formation and cell signaling. Bone morphogenetic protein (BMP) signaling is dysregulated in fibrodysplasia ossificans progressiva (FOP), a disabling disorder of progressive heterotopic bone formation. Here, we investigated the role of HSPG glycosaminoglycan (GAG) side chains on BMP signaling and found increased total and HSPG-specific GAG chain levels and dysregulation in HSPG modulation of BMP signaling in FOP lymphoblastoid cells (LCLs). Specifically, HSPG profiling demonstrated abundant mRNA and protein levels of glypican 1 and syndecan 4 on control and FOP LCLs, with elevated core protein levels on FOP cells. Targeted downregulation of glypican 1 core protein synthesis by siRNA enhanced BMP signaling in control and FOP cells, while reduction of syndecan 4-core protein synthesis decreased BMP signaling in control, but not FOP cells. These results suggest that FOP cells are resistant to the stimulatory effects of cell surface HSPG GAG chains, but are susceptible to the inhibitory effects, as shown by downregulation of glypican 1. These data support that HSPG modulation of BMP signaling is altered in cells from patients with FOP and that altered HSPG-related BMP signaling may play a role in the pathogenesis of the disease.

Vhnf1 acts downstream of Bmp, Fgf, and RA signals to regulate endocrine beta cell development in zebrafish

Bmp, Fgf, and retinoic acid (RA) signals have been implicated as regulators of pancreas development. However, the integration of these signaling pathways in vivo is not fully understood. Variant hnf1 (Vhnf1) is a transcription factor involved in pancreas, liver, and kidney development and its mutation in zebrafish causes underdeveloped pancreas and liver. We investigated the signaling pathways that regulate vhnf1 expression during pancreas development. First, we showed that Bmp activity is required for vhnf1 expression in the endoderm. In chordin (a Bmp antagonist) morpholino (MO)-injected embryos, vhnf1 expression in endoderm and in endocrine beta cells is expanded. On the other hand, in alk8 (a type I TGFbeta receptor) MO-injected embryos, vhnf1 expression in the endoderm is significantly reduced. Second, we showed that Fgf signaling participates in regulation of pancreas development through the vhnf1 pathway. Third, we demonstrated that RA fails to rescue reduction of insulin expression in vhnf1 mutants, whereas overexpression of vhnf1 restores insulin expression that is repressed by treatment with a RA receptor inhibitor. And finally, we revealed that both Bmp and Fgf signals act genetically upstream of RA in directing pancreas development. Taken together, our data establish that vhnf1 acts downstream of the signaling pathways of RA, Bmp, and Fgf to regulate pancreas development in zebrafish.

Maternal control of vertebrate dorsoventral axis formation and epiboly by the POU domain protein Spg/Pou2/Oct4

Dorsoventral (DV) axis formation of the vertebrate embryo is controlled by the maternal genome and is subsequently refined zygotically. In the zygote, repression of ventralizing Bmp activity on the dorsal side through chordin and noggin is crucial for establishment of a dorsally located organizer. This interplay generates a zygotic Bmp activity gradient that defines distinct positional values along the DV axis. The maternal processes that control expression of the zygotic genes implicated in DV patterning are largely unknown. spiel-ohne-grenzen (spg/pou2) is a maternally and zygotically expressed zebrafish gene that encodes the POU domain transcription factor Pou2, an ortholog of mammalian Oct4/Pou5f1. We show that embryos that are genetically depleted of both maternal and zygotic pou2 function (MZspg) exhibit extreme DV patterning defects and, independently, a blastoderm-specific arrest of epiboly. Dorsal tissues expand to the ventral side at the expense of ventrolateral tissue in MZspg embryos. Dorsally expressed Bmp-antagonists, such as Chd and Nog1, and Gsc are ectopically activated at ventral levels in MZspg. Lack of ventral specification is apparent very early, suggesting that maternal processes are affected in MZspg. Indeed, maternal pou2 function is necessary to initiate zygotic expression of ventrally expressed genes such as bmp2b and bmp4, and for proper activation of bmp7, vox, vent and eve1. A constitutively active Alk8-TGFbeta-receptor can ectopically induce bmp2b and bmp4 and rescues the dorsalization of MZspg. This indicates that pou2 acts upstream of Alk8, a maternally provided receptor implicated in the activation of zygotic bmp2b and bmp4 transcription. Consistent with this possibility, Bmp gene misexpression can rescue MZspg embryos, indicating that TGFbeta-mediated signal transduction itself is intact in absence of Pou2. Inhibition of Fgf signaling, another pathway with early dorsalizing activity, can also restore and even ventralize MZspg embryos. The requirement for pou2 to initiate bmp2b expression can therefore be bypassed by releasing the repressive function of Fgf signaling upon bmp2b transcription. In transplantation experiments, we find that dorsalized cells from prospective ventrolateral regions of MZspg embryos are non cell-autonomously respecified to a ventral fate within wild-type host embryos. Analysis of pou2 mRNA injected MZspg embryos shows that pou2 is required on the ventral side of cleavage stage embryos. Based on the maternal requirement for pou2 in ventral specification, we propose that ventral specification employs an active, pou2-dependent maternal induction step, rather than a default ventralizing program.

A recurrent mutation in the BMP type I receptor ACVR1 causes inherited and sporadic fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disorder of skeletal malformations and progressive extraskeletal ossification. We mapped FOP to chromosome 2q23-24 by linkage analysis and identified an identical heterozygous mutation (617G --> A; R206H) in the glycine-serine (GS) activation domain of ACVR1, a BMP type I receptor, in all affected individuals examined. Protein modeling predicts destabilization of the GS domain, consistent with constitutive activation of ACVR1 as the underlying cause of the ectopic chondrogenesis, osteogenesis and joint fusions seen in FOP.

Mutations of TGFbeta signaling molecules in human disease

The transforming growth factor beta (TGFbeta) signaling pathway regulates several biological processes including cellular proliferation, differentiation, apoptosis, migration, and extracellular matrix deposition. Ligand and receptor family members signal through two main Smad signaling branches, TGFbeta/activin to Smad2/3 (Sma and MAD-related proteins) and bone morphogenetic protein (BMP) to Smad1/5. At the molecular level, TGFbeta acts by modifying cytoskeletal organization and ultimately regulating expression of specific target genes. Germline disruption of TGFbeta signaling leads to several types of hereditary congenital malformation or dysfunction of the skeletal, muscular and/or cardiovascular systems, and to cancer predisposition syndromes. In this review, the molecular etiology of TGFbeta-associated disorders is examined, together with a discussion of clinical overlap between syndromes and possible biological explanations underlying the variable penetrance and expressivity of clinical characteristics. Increasing our understanding of the molecular etiology underlying genotype-phenotype correlations will ultimately provide a molecular-based approach that should result in better prognostic tools, smart therapeutics and individualized disease management, not only for these rare syndromes, but for more generalized disorders of the cardiovascular and musculoskeletal systems and cancer. The clinical consequence of TGFbeta signaling mutations appears to depend on environmental factors and on the basal levels of ongoing signaling transduction networks specific to each individual. In this respect, genetic background might be a central factor in determining disease outcome and treatment strategy for TGFbeta-associated diseases.

Fluorosis: a new model and new insights

Fluoride is an effective agent for the prevention of dental caries. However, the mechanism of how excessive fluoride exposure causes fluorosis remains uncertain. Zebrafish (Danio rerio) exhibit periodic tooth replacement throughout their lives, thereby providing continuous access to teeth at developmental stages susceptible to fluoride exposure. Zebrafish teeth do not contain true enamel, but consist of a hard enameloid surface. Therefore, we asked whether zebrafish could be used as a model organism for the study of dental fluorosis. Scanning electron microscopy of fluoride-treated teeth demonstrated that the enameloid was pitted and rough, and FTIR analysis demonstrated that the teeth also contained a significantly higher organic content when compared with untreated controls. Furthermore, we demonstrate for the first time that decreased expression of an important signaling molecule (Alk8) in tooth development may contribute to the observed fluorotic phenotype, and that increased cell apoptosis may also play a role in the mechanism of fluorosis.

D-glucuronyl C5-epimerase acts in dorso-ventral axis formation in zebrafish

BACKGROUND

Heparan sulfate (HS) is an ubiquitous component of the extracellular matrix that binds and modulates the activity of growth factors, cytokines and proteases. Animals with defective HS biosynthesis display major developmental abnormalities however the processes that are affected remain to be defined. D-glucuronyl-C5-epimerase (Glce) is a key HS chain modifying enzyme that catalyses the conversion of glucuronic acid into iduronic acid, a biosynthetic step that enhances HS biological activity. In this study the role of Glce during early zebrafish development has been investigated.

RESULTS

Two Glce-like proteins (Glce-A and -B) are expressed in zebrafish at all times. They are the products of two distinct genes that, based on chromosomal mapping, are both orthologues of the same single human gene. Transcripts for both proteins were detected in fertilized zebrafish embryos prior to the onset of zygotic transcription indicating their maternal origin. At later developmental stages the epimerases are expressed widely throughout gastrulation and then become restricted to the hindbrain at 24 h post-fertilization. By monitoring the expression of well characterized marker genes during gastrulation, we have found that misexpression of Glce causes a dose-dependent expansion of the ventral structures, whereas protein knockdown using targeted antisense morpholino oligonucleotides promotes axis dorsalization. The ventralizing activity of Bmp2b is enhanced by Glce overexpression whereas Glce knockdown impairs Bmp2b activity.

CONCLUSION

Glce activity is an important determinant of of dorso-ventral axis formation and patterning in zebrafish. In particular Glce acts during gastrulation by affecting Bmp-mediated cell specification. The results obtained further corroborate the concept that HS encodes information that affect morphogenesis during early vertebrate development.

Zebrafishacvr2a andacvr2b exhibit distinct roles in craniofacial development

To examine the roles of activin type II receptor signaling in craniofacial development, full-length zebrafish acvr2a and acvr2b clones were isolated. Although ubiquitously expressed as maternal mRNAs and in early embryogenesis, by 24 hr postfertilization (hpf), acvr2a and acvr2b exhibit restricted expression in neural, hindbrain, and neural crest cells (NCCs). A morpholino-based targeted protein depletion approach was used to reveal discrete functions for each acvr2 gene product. The acvr2a morphants exhibited defects in the development of most cranial NCC-derived cartilage, bone, and pharyngeal tooth structures, whereas acvr2b morphant defects were largely restricted to posterior arch structures and included the absence and/or aberrant migration of posterior NCC streams, defects in NCC-derived posterior arch cartilages, and dysmorphic pharyngeal tooth development. These studies revealed previously uncharacterized roles for acvr2a and acvr2b in hindbrain and NCC patterning, in NCC derived pharyngeal arch cartilage and joint formation, and in tooth development.

Immunolocalization of Alk8 during replacement tooth development in zebrafish

The novel type I transforming growth factor-beta (TGF-beta) family member receptor Alk8 was previously identified in a degenerate RT-PCR screen for zebrafish type I and II TGF-beta family member receptors. Functional analyses revealed that Alk8 acts through Bmp signaling pathways in early embryonic dorsoventral patterning, in neural crest cell specification, and in patterning and differentiation of neural crest cell-derived pharyngeal arch cartilages. In addition, Alk8 forms active signaling complexes with TGF-beta1 and the TGF-beta RII receptor, suggesting that Alk8 mediates cross talk between Bmp and TGF-beta subfamily members. In this study, immunohistochemical analysis was performed on zebrafish aged 2 days postfertilization to 1 year, revealing immunolocalization of Alk8 to tissues of the tooth-bearing ceratobranchial 5 (cb5) arch including dental epithelial and mesenchymal tooth tissues of developing primary and replacement teeth, mucous-producing crypt epithelium, keratinized bite plate, and developing taste buds. These results suggest roles for Alk8 in patterning tooth-bearing pharyngeal epithelium, in the initiation of tooth development, in odontoblast and ameloblast differentiation, and in osteoblast maturation. The ability for zebrafish to continuously form teeth throughout their lives allows for the comparison of Alk8 expression in both primary and replacement tooth development, revealing identical Alk8 expression profiles. This study advances our current understanding of the functions of Alk8, particularly with respect to primary and replacement tooth formation, reveals additional roles for Alk8 in dental epithelial patterning and in odontoblast, ameloblast and osteoblast differentiation, and demonstrates the utility of the zebrafish as a model for primary and replacement tooth development.

Alk8 is required for neural crest cell formation and development of pharyngeal arch cartilages

The type I TGFbeta family member receptor alk8 acts in bone morphogenetic protein (BMP) signaling pathways to establish dorsoventral patterning in the early zebrafish embryo. Here, we present evidence that alk8 is required for neural crest cell (NCC) formation and that alk8 signaling gradients direct the proper patterning of premigratory NCCs. We extend our previous functional studies of alk8 to demonstrate that ectopic expression of constitutively active and dominant negative Alk8, consistently results in more medially or laterally positioned premigratory NCCs, respectively. We also demonstrate that patterning defects in premigratory NCCs, induced by alk8 misexpression, correlate with subsequent defects in NCC-derived pharyngeal arch cartilages. Furthermore, an anteroposterior effect is revealed, where overexpression of Alk8 more severely affects anterior arch cartilages and decreased Alk8 activity more severely affects posterior arch cartilage formation. Ectopic expression studies of alk8 are supported by analyses of zygotic and maternal-zygotic laf/alk8 mutants and of several BMP pathway mutants. Pharyngeal mesodermal and endodermal defects in laf/alk8 mutants suggest additional roles for alk8 in patterning of these tissues. Our results provide insight into alk8-mediated BMP signaling gradients and the establishment of premigratory NCC mediolateral positioning, and extend the model for BMP patterning of the neural crest to include that of NCC-derived pharyngeal arch cartilages.

Making a zebrafish kidney: a tale of two tubes

The kidney can be thought of as the pairing of two tubes: an epithelial tube (the nephron), carrying filtered blood and engaged in ion and water transport; and endothelial tubes (the blood vessels), delivering blood and carrying away recovered solute. The development of the nephron presents several interesting questions. How does an epithelial tube form and how is it patterned into functionally distinct components and segments? What guides the interaction between the vasculature and kidney epithelia? How are epithelial cell shape and lumen diameter maintained, and what goes wrong when kidney tubules balloon into cysts? Here, I outline the progress that has been made in answering these questions using the zebrafish pronephros as a simple, accessible model of nephron development.

From endoderm formation to liver and pancreas development in zebrafish

Recent studies in zebrafish have contributed to our understanding of early endoderm formation in vertebrates. Specifically, they have illustrated the importance of Nodal signaling as well as three transcription factors, Faust/Gata5, Bonnie and Clyde, and Casanova, in this process. Ongoing genetic and embryological studies in zebrafish are also contributing to our understanding of later aspects of endoderm development, including the formation of the gut and its associated organs, the liver and pancreas. The generation of transgenic lines expressing GFP in these organs promises to be particularly helpful in such studies.

Genetic analysis of the mammalian transforming growth factor-beta superfamily

Members of the TGF-beta superfamily, which includes TGF-betas, growth differentiation factors, bone morphogenetic proteins, activins, inhibins, and glial cell line-derived neurotrophic factor, are synthesized as prepropeptide precursors and then processed and secreted as homodimers or heterodimers. Most ligands of the family signal through transmembrane serine/threonine kinase receptors and SMAD proteins to regulate cellular functions. Many studies have reported the characterization of knockout and knock-in transgenic mice as well as humans or other mammals with naturally occurring genetic mutations in superfamily members or their regulatory proteins. These investigations have revealed that TGF-beta superfamily ligands, receptors, SMADs, and upstream and downstream regulators function in diverse developmental and physiological pathways. This review attempts to collate and integrate the extensive body of in vivo mammalian studies produced over the last decade.

Molecular dissection of craniofacial development using zebrafish

The zebrafish, Danio rerio, is a small, freshwater teleost that only began to be used as a vertebrate genetic model by the late George Streisinger in the early 1980s. The strengths of the zebrafish complement genetic studies in mice and embryological studies in avians. Its advantages include high fecundity, externally fertilized eggs and transparent embryos that can be easily manipulated, inexpensive maintenance, and the fact that large-scale mutagenesis screens can be performed. Here we review studies that have used the zebrafish as a model for craniofacial development. Lineage studies in zebrafish have defined the origins of the cranial skeleton at the single-cell level and followed the morphogenetic behaviors of these cells in skeletal condensations. Furthermore, genes identified by random mutational screening have now revealed genetic pathways controlling patterning of the jaw and other pharyngeal arches, as well as the midline of the skull, that are conserved between fish and humans. We discuss the potential impact of specialized mutagenesis screens and the future applications of this versatile, vertebrate developmental model system in the molecular dissection of craniofacial development.

BMP signalling regulates anteroposterior endoderm patterning in zebrafish

In vertebrates, the embryonic dorsoventral asymmetry is regulated by the bone morphogenetic proteins (Bmp) activity gradient. In the present study, we have used dorsalized swirl (bmp2b) and ventralized chordino (chordin) zebrafish mutants to investigate the effects of dorsoventral signalling on endoderm patterning and on the differentiation and positioning of its derivatives. Alterations of dorsoventral Bmp signalling do not perturb the induction of endodermal precursors, as shown by normal amounts of cells expressing cas and sox17 in swirl and chordino gastrulae, but affect dramatically the expression pattern of her5, a regulator of endoderm anteroposterior patterning in zebrafish. In particular, increased levels of Bmp signalling in chordino gastrulae are associated with a markedly reduced her5 expression domain, that may be abolished by injecting bmp2b mRNA. Conversely, in swirl mutants, lacking Bmp2b, the her5 expression domain is expanded. Thus, a gradient of Bmp2b signalling defines the extension of the her5 expression domain at gastrulation and the allocation of anterior endodermal precursors. A balanced Bmp2b signalling is also required for the normal development of the pancreas, as shown by the sharp reduction of the pancreatic primordium in swirl embryos and its expansion in chordino mutants. In the latter, at 3 days post-fertilization, the increased Bmp signalling does not compromise the endocrine/exocrine pancreas compartmentalization, but the right/left positioning of the pancreas and liver is randomized. Our results suggest that by regulating the expression of her5, the Bmp2b/Chordin gradient directs the anteroposterior patterning of endoderm in zebrafish embryos.

Disruption ofacvrl1increases endothelial cell number in zebrafish cranial vessels

The zebrafish mutant violet beauregarde (vbg) can be identified at two days post-fertilization by an abnormal circulation pattern in which most blood cells flow through a limited number of dilated cranial vessels and fail to perfuse the trunk and tail. This phenotype cannot be explained by caudal vessel abnormalities or by a defect in cranial vessel patterning, but instead stems from an increase in endothelial cell number in specific cranial vessels. We show that vbg encodes activin receptor-like kinase 1 (Acvrl1; also known as Alk1), a TGFβ type I receptor that is expressed predominantly in the endothelium of the vessels that become dilated in vbg mutants. Thus, vbg provides a model for the human autosomal dominant disorder, hereditary hemorrhagic telangiectasia type 2, in which disruption of ACVRL1 causes vessel malformations that may result in hemorrhage or stroke.

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The novel type I serine-threonine kinase receptor Alk8 binds TGF-beta in the presence of TGF-betaRII

The TGF-beta superfamily consists of an array of ligands including BMP, TGF-beta, activin, and nodal subfamilies. The extensive range of biological effects elicited by TGF-beta family signaling is due in part to the large numbers and promiscuity of types I and II TGF-beta family member receptors. Alk8 is a novel type I TGF-beta family member receptor first identified in zebrafish [Dev. Dyn. 211 (4) (1998) 352], which participates in BMP signaling pathways [Development 128 (6) (2001) 849; Development 128 (6) (2001) 859; Mech. Dev. 100 (2) (2001) 275; J. Dent. Res. 80 (11) (2001) 1968]. Here we report that Alk8 also forms active signaling complexes with TGF-beta in the presence of TGF-betaRII. These results expand the signaling repertoire of zAlk8 by demonstrating an ability to participate in two distinct TGF-beta subfamily signaling pathways.

casanova encodes a novel Sox-related protein necessary and sufficient for early endoderm formation in zebrafish

Early endoderm formation in zebrafish requires at least three loci that function downstream of Nodal signaling but upstream of the early endodermal marker sox17: bonnie and clyde (bon), faust (fau), and casanova (cas). cas mutants show the most severe phenotype as they do not form any gut tissue and lack all sox17 expression. Activation of the Nodal signaling pathway or overexpression of Bon or Fau/Gata5 fails to restore any sox17 expression in cas mutants, demonstrating that cas plays a central role in endoderm formation. Here we show that cas encodes a novel member of the Sox family of transcription factors. Initial cas expression appears in the dorsal yolk syncytial layer (YSL) in the early blastula, and is independent of Nodal signaling. In contrast, endodermal expression of cas, which begins in the late blastula, is regulated by Nodal signaling. Cas is a potent inducer of sox17 expression in wild-type embryos as well as in bon and fau/gata5 mutants. Cas is also a potent inducer of sox17 expression in MZoep mutants, which cannot respond to Nodal signaling. In addition, ectopic expression of cas in presumptive mesodermal cells leads to their transfating into endoderm. Altogether, these data indicate that Cas is the principal transcriptional effector of Nodal signaling during zebrafish endoderm formation.