Genetic analysis of fin development in zebrafish identifies furin and hemicentin1 as potential novel fraser syndrome disease genes

Genetic Architecture of the Variation in Male-Specific Ossified Processes on the Anal Fins of Japanese Medaka

Traits involved in reproduction evolve rapidly and show great diversity among closely related species. However, the genetic mechanisms that underlie the diversification of courtship traits are mostly unknown. Japanese medaka fishes (Oryzias latipes) use anal fins to attract females and to grasp females during courtship; the males have longer anal fins with male-specific ossified papillary processes on the fin rays. However, anal fin morphology varies between populations: the southern populations tend to have longer anal fins and more processes than the northern populations. In the present study, we conducted quantitative trait locus (QTL) mapping to investigate the genetic architecture underlying the variation in the number of papillary processes of Japanese medaka fish and compared the QTL with previously identified QTL controlling anal fin length. First, we found that only a few QTL were shared between anal fin length and papillary process number. Second, we found that the numbers of papillary processes on different fin rays often were controlled by different QTL. Finally, we produced another independent cross and found that some QTL were repeatable between the two crosses, whereas others were specific to only one cross. These results suggest that variation in the number of papillary processes is polygenic and controlled by QTL that are distinct from those controlling anal fin length. Thus, different courtship traits in Japanese medaka share a small number of QTL and have the potential for independent evolution.

Basement Membranes in the Worm: A Dynamic Scaffolding that Instructs Cellular Behaviors and Shapes Tissues

The nematode worm Caenorhabditis elegans has all the major basement membrane proteins found in vertebrates, usually with a smaller gene family encoding each component. With its powerful forward genetics, optical clarity, simple tissue organization, and the capability to functionally tag most basement membrane components with fluorescent proteins, C. elegans has facilitated novel insights into the assembly and function of basement membranes. Although basement membranes are generally thought of as static structures, studies in C. elegans have revealed their active properties and essential functions in tissue formation and maintenance. Here, we review discoveries from C. elegans development that highlight dynamic aspects of basement membrane assembly, function, and regulation during organ growth, tissue polarity, cell migration, cell invasion, and tissue attachment. These studies have helped transform our view of basement membranes from static support structures to dynamic scaffoldings that play broad roles in regulating tissue organization and cellular behavior that are essential for development and have important implications in human diseases.

Transient laminin beta 1a Induction Defines the Wound Epidermis during Zebrafish Fin Regeneration

The first critical stage in salamander or teleost appendage regeneration is creation of a specialized epidermis that instructs growth from underlying stump tissue. Here, we performed a forward genetic screen for mutations that impair this process in amputated zebrafish fins. Positional cloning and complementation assays identified a temperature-sensitive allele of the ECM component laminin beta 1a (lamb1a) that blocks fin regeneration. lamb1a, but not its paralog lamb1b, is sharply induced in a subset of epithelial cells after fin amputation, where it is required to establish and maintain a polarized basal epithelial cell layer. These events facilitate expression of the morphogenetic factors shha and lef1, basolateral positioning of phosphorylated Igf1r, patterning of new osteoblasts, and regeneration of bone. By contrast, lamb1a function is dispensable for juvenile body growth, homeostatic adult tissue maintenance, repair of split fins, or renewal of genetically ablated osteoblasts. fgf20a mutations or transgenic Fgf receptor inhibition disrupt lamb1a expression, linking a central growth factor to epithelial maturation during regeneration. Our findings reveal transient induction of lamb1a in epithelial cells as a key, growth factor-guided step in formation of a signaling-competent regeneration epidermis.

Unlike mammals, adult teleost fish and urodele amphibians can fully regenerate lost appendages. Understanding what initiates regeneration in these vertebrates is of great interest to the scientific community. It has long been known that the epidermis that forms quickly over an amputated limb stump is critical for initiating regenerative programs. Yet, little of understood of the molecular and cellular mechanisms by which a simple adult epithelium transforms into this key signaling source. Here, we performed a large-scale, unbiased genetic screen for epithelial signaling deficiencies during the regeneration of amputated adult zebrafish fins, from which we identified several new mutants. One gene identified from this screen disrupts a specific component of the extracellular matrix material Laminin, Laminin beta 1a, a factor that we find to be dispensable in uninjured adult animals but required for all stages fin regeneration. Transient induction of this component by amputation polarizes the basal layer of the nascent epithelium, and, in turn, facilitates the synthesis of signaling factors, the positioning of ligand receptors, and the patterning of new bone cells. We also find that normal induction of Laminin beta 1a by injury relies on the function of Fibroblast growth factors, secreted polypeptide signals that are released early upon injury. Our results identify key early steps in the endogenous program for vertebrate appendage regeneration.

Protein-Trap Insertional Mutagenesis Uncovers New Genes Involved in Zebrafish Skin Development, Including a Neuregulin 2a-Based ErbB Signaling Pathway Required during Median Fin Fold Morphogenesis

Skin disorders are widespread, but available treatments are limited. A more comprehensive understanding of skin development mechanisms will drive identification of new treatment targets and modalities. Here we report the Zebrafish Integument Project (ZIP), an expression-driven platform for identifying new skin genes and phenotypes in the vertebrate model Danio rerio (zebrafish). In vivo selection for skin-specific expression of gene-break transposon (GBT) mutant lines identified eleven new, revertible GBT alleles of genes involved in skin development. Eight genes—fras1, grip1, hmcn1, msxc, col4a4, ahnak, capn12, and nrg2a—had been described in an integumentary context to varying degrees, while arhgef25b, fkbp10b, and megf6a emerged as novel skin genes. Embryos homozygous for a GBT insertion within neuregulin 2a (nrg2a) revealed a novel requirement for a Neuregulin 2a (Nrg2a) – ErbB2/3 – AKT signaling pathway governing the apicobasal organization of a subset of epidermal cells during median fin fold (MFF) morphogenesis. In nrg2a mutant larvae, the basal keratinocytes within the apical MFF, known as ridge cells, displayed reduced pAKT levels as well as reduced apical domains and exaggerated basolateral domains. Those defects compromised proper ridge cell elongation into a flattened epithelial morphology, resulting in thickened MFF edges. Pharmacological inhibition verified that Nrg2a signals through the ErbB receptor tyrosine kinase network. Moreover, knockdown of the epithelial polarity regulator and tumor suppressor lgl2 ameliorated the nrg2a mutant phenotype. Identifying Lgl2 as an antagonist of Nrg2a – ErbB signaling revealed a significantly earlier role for Lgl2 during epidermal morphogenesis than has been described to date. Furthermore, our findings demonstrated that successive, coordinated ridge cell shape changes drive apical MFF development, making MFF ridge cells a valuable model for investigating how the coordinated regulation of cell polarity and cell shape changes serves as a crucial mechanism of epithelial morphogenesis.

Congenital upper eyelid coloboma: embryologic, nomenclatorial, nosologic, etiologic, pathogenetic, epidemiologic, clinical, and management perspectives

Purpose:

To review the recent literature and describe the authors’ experience with congenital upper eyelid coloboma.

Methods:

In this review, we will summarize the embryologic and etiopathogenetic bases of congenital upper eyelid coloboma, and study the published clinical reports. We will also attempt to briefly shed some light on the rarer syndromic curiosities associated with upper eyelid coloboma.

Results:

Congenital upper eyelid colobomas are one of the few nontraumatic oculoplastic emergencies that may occasionally present in the first few days of life with a corneal ulcer and may even present with impending perforation. They can present with or without corneopalpebral adhesions, may be isolated findings or a part of a larger spectrum of congenital anomalies as in the case of Fraser syndrome or Goldenhar syndrome, or could be associated with other rare curiosities that could challenge the clinician with a huge diagnostic dilemma.

Conclusions:

Existing literature dealing with congenital colobomas of the upper eyelid is fraught with nosologic problems, confusing etiologies, and overlapping clinical features. We attempted to clarify the salient clinical features, outline the management principles, and until a time in the not-so-distant future where advances in molecular genetic testing would help redefine the etiology and the diverse clinical spectrum of genetic diseases associated with upper eyelid colobomas, we propose a simplified classification scheme based on the relation of the coloboma to the cornea, the presence or absence of systemic features, and all the syndromic and nonsyndromic associations of congenital coloboma of the upper eyelid known today.

In this review, the authors will describe the pathogenesis of upper eyelid coloboma, suggest a new simplified classification system, describe the clinical picture in detail, clarify the various syndromic associations of upper eyelid coloboma, and lay out the basic surgical principles of management.

An active role for basement membrane assembly and modification in tissue sculpting

Basement membranes are a dense, sheet-like form of extracellular matrix (ECM) that underlie epithelia and endothelia, and surround muscle, fat and Schwann cells. Basement membranes separate tissues and protect them from mechanical stress. Although traditionally thought of as a static support structure, a growing body of evidence suggests that dynamic basement membrane deposition and modification instructs coordinated cellular behaviors and acts mechanically to sculpt tissues. In this Commentary, we highlight recent studies that support the idea that far from being a passive matrix, basement membranes play formative roles in shaping tissues.

Canonical Wnt signalling regulates epithelial patterning by modulating levels of laminins in zebrafish appendages

The patterning and morphogenesis of body appendages – such as limbs and fins – is orchestrated by the activities of several developmental pathways. Wnt signalling is essential for the induction of limbs. However, it is unclear whether a canonical Wnt signalling gradient exists and regulates the patterning of epithelium in vertebrate appendages. Using an evolutionarily old appendage – the median fin in zebrafish – as a model, we show that the fin epithelium exhibits graded changes in cellular morphology along the proximo-distal axis. This epithelial pattern is strictly correlated with the gradient of canonical Wnt signalling activity. By combining genetic analyses with cellular imaging, we show that canonical Wnt signalling regulates epithelial cell morphology by modulating the levels of laminins, which are extracellular matrix components. We have unravelled a hitherto unknown mechanism involved in epithelial patterning, which is also conserved in the pectoral fins – evolutionarily recent appendages that are homologous to tetrapod limbs.

Highlighted article: In the zebrafish fin, a Wnt gradient dictates the expression of laminin α5, which signals via integrin α3 to control epithelial cell morphology.

Genome-wide linkage and exome analyses identify variants of HMCN1 for splenic epidermoid cyst

BACKGROUND

Splenic epidermoid cyst is a benign tumor-like lesion affecting the spleen and sometimes occurs in familial form. The causality of such rare diseases remain challenging, however recently, with the emergence of exome re-sequencing, the genetics of many diseases have been unveiled. In the present study, we performed a combinatorial approach of genome-wide parametric linkage and exome analyses for a moderate-sized Japanese family with frequent occurrence of splenic epidermoid cyst to identify the genetic causality of the disease.

METHODS

Twelve individuals from the family were subject to SNP typing and exome re-sequencing was done for 8 family members and 4 unrelated patients from Kosovo. Linkage was estimated using multi-point parametric linkage analysis assuming a dominant mode of inheritance. All of the candidate variants from exome analysis were confirmed by direct sequencing.

RESULTS

The parametric linkage analysis suggested two loci on 1q and 14q with a maximal LOD score of 2.5 . Exome generated variants were prioritized based on; impact on the protein coding sequence, novelty or rareness in public databases, and position within the linkage loci. This approach identified three variants; variants of HMCN1 and CNTN2 on 1q and a variant of DDHD1 on 14q. The variant of HMCN1 (p.R5205H) showed the best co-segregation in the family after validation with Sanger sequencing. Additionally, rare missense variants (p.A4704V, p.T5004I, and p.H5244Q) were detected in three unrelated Kosovo patients. The identified variants of HMCN1 are on conserved domains, particularly the two variants on calcium-binding epidermal growth factor domain.

CONCLUSIONS

The present study, by combining linkage and exome analyses, identified HMCN1 as a genetic causality of splenic epidermoid cyst. Understanding the biology of the disease is a key step toward developing innovative approaches of intervention.

B-LINK: a hemicentin, plakin, and integrin-dependent adhesion system that links tissues by connecting adjacent basement membranes

Basement membrane (BM), a sheet-like form of extracellular matrix, surrounds most tissues. During organogenesis, specific adhesions between adjoining tissues frequently occur; however, their molecular basis is unclear. Using live-cell imaging and electron microscopy, we identify an adhesion system that connects the uterine and gonadal tissues through their juxtaposed BMs at the site of anchor cell (AC) invasion in C. elegans. We find that the extracellular matrix component hemicentin (HIM-4), found between BMs, forms punctate accumulations under the AC and controls BM linkage to promote rapid invasion. Through targeted screening, we identify the integrin-binding cytolinker plakin (VAB-10A) and integrin (INA-1/PAT-3) as key BM-BM linkage regulators: VAB-10A localizes to the AC-BM interface and tethers hemicentin to the AC while integrin promotes hemicentin punctae formation. Together, plakin, integrin, and hemicentin are founding components of a cell-directed adhesion system, which we name a BM-LINKage (B-LINK), that connects adjacent tissues through adjoining BMs.

Hecate/Grip2a acts to reorganize the cytoskeleton in the symmetry-breaking event of embryonic axis induction

Maternal homozygosity for three independent mutant hecate alleles results in embryos with reduced expression of dorsal organizer genes and defects in the formation of dorsoanterior structures. A positional cloning approach identified all hecate mutations as stop codons affecting the same gene, revealing that hecate encodes the Glutamate receptor interacting protein 2a (Grip2a), a protein containing multiple PDZ domains known to interact with membrane-associated factors including components of the Wnt signaling pathway. We find that grip2a mRNA is localized to the vegetal pole of the oocyte and early embryo, and that during egg activation this mRNA shifts to an off-center vegetal position corresponding to the previously proposed teleost cortical rotation. hecate mutants show defects in the alignment and bundling of microtubules at the vegetal cortex, which result in defects in the asymmetric movement of wnt8a mRNA as well as anchoring of the kinesin-associated cargo adaptor Syntabulin. We also find that, although short-range shifts in vegetal signals are affected in hecate mutant embryos, these mutants exhibit normal long-range, animally directed translocation of cortically injected dorsal beads that occurs in lateral regions of the yolk cortex. Furthermore, we show that such animally-directed movement along the lateral cortex is not restricted to a single arc corresponding to the prospective dorsal region, but occur in multiple meridional arcs even in opposite regions of the embryo. Together, our results reveal a role for Grip2a function in the reorganization and bundling of microtubules at the vegetal cortex to mediate a symmetry-breaking short-range shift corresponding to the teleost cortical rotation. The slight asymmetry achieved by this directed process is subsequently amplified by a general cortical animally-directed transport mechanism that is neither dependent on hecate function nor restricted to the prospective dorsal axis.

One of the earliest and most crucial events in animal development is the establishment of the embryonic dorsal axis. In amphibians and fish, this event depends on the transport of so-called “dorsal determinants” from one region of the egg, at the pole opposite from the site where the oocyte nucleus lies, towards the site of axis induction. There, the dorsal determinant activates the Wnt signaling pathway, which in turn triggers dorsal gene expression. Dorsal determinant transport is mediated by the reorganization of a cellular network composed of microtubules. We determine that hecate, a zebrafish gene active during egg formation that is essential for embryonic axis induction, is required for an early step in this microtubule reorganization. We find that hecate corresponds to glutamate receptor interacting protein 2a, which participates in other animal systems in Wnt-based pathways. We also show that the microtubule reorganization dependent on hecate results in a subtle symmetry-breaking event that subsequently becomes amplified by a more general transport process independent of hecate function. Our data reveal new links between glutamate receptor interacting protein 2a, Wnt signaling and axis induction, and highlights basic mechanisms by which small changes early in development translate into global changes in the embryo.

Expression of fibulin-6 in failing hearts and its role for cardiac fibroblast migration

AIMS

The cardiac extracellular matrix (ECM) undergoes a dynamic transition following myocardial infarction. Fibulin-6 is expressed in cell junctions particularly in tissues subjected to significant mechanical stress. Fibulin-6 deficiency results in defective cell migration in nematodes and early embryonic lethality in mice. The role of fibulin-6 in healthy and failing myocardium is unknown. We have examined the expression and distribution pattern of fibulin-6 during myocardial remodelling (MR) and detailed its effect on the migratory function of cardiac fibroblasts (CFs) in response to TGF-β1.

METHODS AND RESULTS

In healthy murine myocardium, fibulin-6 expression is largely confined to larger coronary arteries. It is induced during the early and the late phase of remodelling after infarction in murine hearts predominantly in the scar-muscle junction. Similar results are obtained in human ischaemic cardiomyopathy. Fibulin-6 is mostly expressed in close vicinity to vimentin-positive cells and is also abundantly expressed in vitro in cultured neonatal CF. TGF-β1 does not induce smooth muscle actin in fibroblasts deficient of fibulin-6, which also compromised their migration. Cells that had migrated expressed more fibulin-6 compared with stationary cells. Plated on fibulin-6-depleted matrix, stress fibre induction in fibroblast in response to TGF-β1 was impaired. In ex vivo explant cultures from post-infarct myocardium, the number of emigrating fibroblasts was also significantly reduced by fibulin-6 siRNA knockdown.

CONCLUSION

Fibulin-6, a fibroblast-released ECM protein, may play an important role during MR by imparting an effect on CF migration in close and complementary interplay with TGF-β1 signalling.

A new component of the Fraser complex

In embryos, the Fraser Complex (FC) mediates epithelial-connective tissue interactions. Loss of expression of FC components leads to Fraser Syndrome (FS) in which cohesion of epithelial tissues and stroma is perturbed. Using zebrafish, Richardson et al (in this issue) identified the protein AMACO in the FC. We discuss the utility of zebrafish in determining FC functions and identifying FS targets.

Basal keratinocytes contribute to all strata of the adult zebrafish epidermis

The epidermis of terrestrial vertebrates is a stratified epithelium and forms an essential protective barrier. It is continually renewed, with dead corneocytes shed from the surface and replaced from a basal keratinocyte stem cell population. Whilst mouse is the prime model system used for epidermal studies, there is increasing employment of the zebrafish to analyse epidermis development and homeostasis, however the architecture and ontogeny of the epidermis in this system are incompletely described. In particular, it is unclear if adult zebrafish epidermis is derived entirely from the basal epidermal stem cell layer, as in the mouse, or if the most superficial keratinocyte layer is a remnant of the embryonic periderm. Furthermore, a relative paucity of cellular markers and genetic reagents to label and manipulate the basal epidermal stem cell compartment has hampered research. Here we show that the type I keratin, krtt1c19e, is a suitable marker of the basal epidermal layer and identify a krtt1c19e promoter fragment able to drive strong and specific expression in this cell type. Use of this promoter to express an inducible Cre recombinase allowed permanent labelling of basal cells during embryogenesis, and demonstrated that these cells do indeed generate keratinocytes of all strata in the adult epidermis. Further deployment of the Cre-Lox system highlighted the transient nature of the embryonic periderm. We thus show that the epidermis of adult zebrafish, as in the mouse, derives from basal stem cells, further expanding the similarities of epidermal ontogeny across vertebrates. Future use of this promoter will assist genetic analysis of basal keratinocyte biology in zebrafish.

AMACO is a component of the basement membrane-associated Fraser complex

Fraser syndrome (FS) is a phenotypically variable, autosomal recessive disorder characterized by cryptophthalmus, cutaneous syndactyly, and other malformations resulting from mutations in FRAS1, FREM2, and GRIP1. Transient embryonic epidermal blistering causes the characteristic defects of the disorder. Fras1, Frem1, and Frem2 form the extracellular Fraser complex, which is believed to stabilize the basement membrane. However, several cases of FS could not be attributed to mutations in FRAS1, FREM2, or GRIP1, and FS displays high clinical variability, suggesting that there is an additional genetic, possibly modifying contribution to this disorder. An extracellular matrix protein containing VWA-like domains related to those in matrilins and collagens (AMACO), encoded by the VWA2 gene, has a very similar tissue distribution to the Fraser complex proteins in both mouse and zebrafish. Here, we show that AMACO deposition is lost in Fras1-deficient zebrafish and mice and that Fras1 and AMACO interact directly via their chondroitin sulfate proteoglycan (CSPG) and P2 domains. Knockdown of vwa2, which alone causes no phenotype, enhances the phenotype of hypomorphic Fras1 mutant zebrafish. Together, our data suggest that AMACO represents a member of the Fraser complex.

Knockdown of col22a1 gene in zebrafish induces a muscular dystrophy by disruption of the myotendinous junction

The myotendinous junction (MTJ) is the major site of force transfer in skeletal muscle, and defects in its structure correlate with a subset of muscular dystrophies. Col22a1 encodes the MTJ component collagen XXII, the function of which remains unknown. Here, we have cloned and characterized the zebrafish col22a1 gene and conducted morpholino-based loss-of-function studies in developing embryos. We showed that col22a1 transcripts localize at muscle ends when the MTJ forms and that COLXXII protein integrates the junctional extracellular matrix. Knockdown of COLXXII expression resulted in muscular dystrophy-like phenotype, including swimming impairment, curvature of embryo trunk/tail, strong reduction of twitch-contraction amplitude and contraction-induced muscle fiber detachment, and provoked significant activation of the survival factor Akt. Electron microscopy and immunofluorescence studies revealed that absence of COLXXII caused a strong reduction of MTJ folds and defects in myoseptal structure. These defects resulted in reduced contractile force and susceptibility of junctional extracellular matrix to rupture when subjected to repeated mechanical stress. Co-injection of sub-phenotypic doses of morpholinos against col22a1 and genes of the major muscle linkage systems showed a synergistic gene interaction between col22a1 and itga7 (α7β1 integrin) that was not observed with dag1 (dystroglycan). Finally, pertinent to a conserved role in humans, the dystrophic phenotype was rescued by microinjection of recombinant human COLXXII. Our findings indicate that COLXXII contributes to the stabilization of myotendinous junctions and strengthens skeletal muscle attachments during contractile activity.

Exxon Valdez to Deepwater Horizon: comparable toxicity of both crude oils to fish early life stages

The 2010 Deepwater Horizon disaster in the Gulf of Mexico was the largest oil spill in United States history. Crude oils are highly toxic to developing fish embryos, and many pelagic fish species were spawning in the northern Gulf in the months before containment of the damaged Mississippi Canyon 252 (MC252) wellhead (April-July). The largest prior U.S. spill was the 1989 grounding of the Exxon Valdez that released 11 million gallons of Alaska North Slope crude oil (ANSCO) into Prince William Sound. Numerous studies in the aftermath of the Exxon Valdez spill defined a conventional crude oil injury phenotype in fish early life stages, mediated primarily by toxicity to the developing heart. To determine whether this type of injury extends to fishes exposed to crude oil from the Deepwater Horizon - MC252 incident, we used zebrafish to compare the embryotoxicity of ANSCO alongside unweathered and weathered MC252 oil. We also developed a standardized protocol for generating dispersed oil water-accommodated fractions containing microdroplets of crude oil in the size range of those detected in subsurface plumes in the Gulf. We show here that MC252 oil and ANSCO cause similar cardiotoxicity and photo-induced toxicity in zebrafish embryos. Morphological defects and patterns of cytochrome P450 induction were largely indistinguishable and generally correlated with polycyclic aromatic compound (PAC) composition of each oil type. Analyses of embryos exposed during different developmental windows provided additional insight into mechanisms of crude oil cardiotoxicity. These findings indicate that the impacts of MC252 crude oil on fish embryos and larvae are consistent with the canonical ANSCO cardiac injury phenotype. For those marine fish species that spawned in the northern Gulf of Mexico during and after the Deepwater Horizon incident, the established literature can therefore inform the assessment of natural resource injury in the form of potential year-class losses.

Semicircular canal morphogenesis in the zebrafish inner ear requires the function of gpr126 (lauscher), an adhesion class G protein-coupled receptor gene

Morphogenesis of the semicircular canal ducts in the vertebrate inner ear is a dramatic example of epithelial remodelling in the embryo, and failure of normal canal development results in vestibular dysfunction. In zebrafish and Xenopus, semicircular canal ducts develop when projections of epithelium, driven by extracellular matrix production, push into the otic vesicle and fuse to form pillars. We show that in the zebrafish, extracellular matrix gene expression is high during projection outgrowth and then rapidly downregulated after fusion. Enzymatic disruption of hyaluronan in the projections leads to their collapse and a failure to form pillars: as a result, the ears swell. We have cloned a zebrafish mutant, lauscher (lau), identified by its swollen ear phenotype. The primary defect in the ear is abnormal projection outgrowth and a failure of fusion to form the semicircular canal pillars. Otic expression of extracellular matrix components is highly disrupted: several genes fail to become downregulated and remain expressed at abnormally high levels into late larval stages. The lau mutations disrupt gpr126, an adhesion class G protein-coupled receptor gene. Expression of gpr126 is similar to that of sox10, an ear and neural crest marker, and is partially dependent on sox10 activity. Fusion of canal projections and downregulation of otic versican expression in a hypomorphic lau allele can be restored by cAMP agonists. We propose that Gpr126 acts through a cAMP-mediated pathway to control the outgrowth and adhesion of canal projections in the zebrafish ear via the regulation of extracellular matrix gene expression.

A stem cell proliferation burst forms new layers of P63 expressing suprabasal cells during zebrafish postembryonic epidermal development

Organ growth during development is a highly regulated process with both temporal and spatial constraints. Epidermal stratification is essential for skin growth and development. Although the zebrafish has been well studied, it is not known when and how epidermal stratification occurs. This is because beyond the first five days of development our knowledge is currently limited. We found that epidermal stratification in zebrafish begins when the larvae reach a standard length (SL) of 6 mm at approximately 25 days of age. Over the next four days (from a SL of 6 to 9 mm), epidermis thickness increases almost four-fold. This represents a sudden increase in organ size, since for the previous 20 days of development, the epidermis has been only two layers thick. This pattern is different from that observed in mammals that undergo continuous stratification from E14.5–E18.5. To study how stem cell proliferation gives rise to the new epidermal layers, we used a combination of markers: one for cell proliferation (proliferating cell nuclear-antigen PCNA) and one for epidermal stem cells (P63 transcription factor). We identified, throughout the stratification process, two different waves of cell division. Initially, the most basal epidermal cells divided and generated a subset of suprabasal cells (possibly transient-amplifying cells); within the next several days, the basal cells stopped dividing, and the suprabasal cells began proliferation, giving rise to most of the cell types in the new layers. This part of the process is similar to what has been recently found during epidermal stratification in mammals.

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.

Specific structure and unique function define the hemicentin

Hemicentin has come a long way from when it was first identified in C. elegans as him-4 (High incidence of males). The protein is now a recognized player in maintaining the architectural integrity of vertebrate tissues and organs. Highly conserved hemicentin sequences across species indicate this gene's ancient evolutionary roots and functional importance. In mouse, hemicentin is liberally distributed on the cell surface of many cell types, including epithelial cells, endothelial cells of the eye, lung, and uterus, and trophectodermal cells of blastocyst. Recent discoveries have uncovered yet another vital purpose of hemicentin 1. The protein also serves a unique function in mitotic cytokinesis, during which this extracellular matrix protein plays a key role in cleavage furrow maturation. Though understanding of hemicentin function has improved through new discoveries, much about this protein remains mysterious.

An exclusively mesodermal origin of fin mesenchyme demonstrates that zebrafish trunk neural crest does not generate ectomesenchyme

The neural crest is a multipotent stem cell population that arises from the dorsal aspect of the neural tube and generates both non-ectomesenchymal (melanocytes, peripheral neurons and glia) and ectomesenchymal (skeletogenic, odontogenic, cartilaginous and connective tissue) derivatives. In amniotes, only cranial neural crest generates both classes, with trunk neural crest restricted to non-ectomesenchyme. By contrast, it has been suggested that anamniotes might generate derivatives of both classes at all axial levels, with trunk neural crest generating fin osteoblasts, scale mineral-forming cells and connective tissue cells; however, this has not been fully tested. The cause and evolutionary significance of this cranial/trunk dichotomy, and its absence in anamniotes, are debated. Recent experiments have disputed the contribution of fish trunk neural crest to fin osteoblasts and scale mineral-forming cells. This prompted us to test the contribution of anamniote trunk neural crest to fin connective tissue cells. Using genetics-based lineage tracing in zebrafish, we find that these fin mesenchyme cells derive entirely from the mesoderm and that neural crest makes no contribution. Furthermore, contrary to previous suggestions, larval fin mesenchyme cells do not generate the skeletogenic cells of the adult fin, but persist to form fibroblasts associated with adult fin rays. Our data demonstrate that zebrafish trunk neural crest does not generate ectomesenchymal derivatives and challenge long-held ideas about trunk neural crest fate. These findings have important implications for the ontogeny and evolution of the neural crest.

Fraser syndrome: epidemiological study in a European population

Fraser syndrome is a rare autosomal recessive disorder characterized by cryptophthalmos, cutaneous syndactyly, laryngeal, and urogenital malformations. We present a population-based epidemiological study using data provided by the European Surveillance of Congenital Anomalies (EUROCAT) network of birth defect registries. Between January 1990 and December 2008, we identified 26 cases of Fraser syndrome in the monitored population of 12,886,464 births (minimal estimated prevalence of 0.20 per 100,000 or 1:495,633 births). Most cases (18/26; 69%) were registered in the western part of Europe, where the mean prevalence is 1 in 230,695 births, compared to the prevalence 1 in 1,091,175 for the rest of Europe (P = 0.0003). Consanguinity was present in 7/26 (27%) families. Ten (38%) cases were liveborn, 14 (54%) pregnancies were terminated following prenatal detection of a serious anomaly, and 2 (8%) were stillborn. Eye anomalies were found in 20/24 (83%), syndactyly in 14/24 (58%), and laryngeal anomalies in 5/24 (21%) patients. Ambiguous genitalia were observed in 3/24 (13%) cases. Bilateral renal agenesis was present in 12/24 (50%) and unilateral in 4/24 (17%) cases. The frequency of anorectal anomalies was particularly high (42%). Most cases of Fraser syndrome (85%) are suspected prenatally, often due to the presence of the association of renal agenesis and cryptophthalmos. In the European population, a high proportion (82%) of pregnancies is terminated, thus reducing the live birth prevalence to a third of the total prevalence rate.

fras1 shapes endodermal pouch 1 and stabilizes zebrafish pharyngeal skeletal development

Lesions in the epithelially expressed human gene FRAS1 cause Fraser syndrome, a complex disease with variable symptoms, including facial deformities and conductive hearing loss. The developmental basis of facial defects in Fraser syndrome has not been elucidated. Here we show that zebrafish fras1 mutants exhibit defects in facial epithelia and facial skeleton. Specifically, fras1 mutants fail to generate a late-forming portion of pharyngeal pouch 1 (termed late-p1) and skeletal elements adjacent to late-p1 are disrupted. Transplantation studies indicate that fras1 acts in endoderm to ensure normal morphology of both skeleton and endoderm, consistent with well-established epithelial expression of fras1. Late-p1 formation is concurrent with facial skeletal morphogenesis, and some skeletal defects in fras1 mutants arise during late-p1 morphogenesis, indicating a temporal connection between late-p1 and skeletal morphogenesis. Furthermore, fras1 mutants often show prominent second arch skeletal fusions through space occupied by late-p1 in wild type. Whereas every fras1 mutant shows defects in late-p1 formation, skeletal defects are less penetrant and often vary in severity, even between the left and right sides of the same individual. We interpret the fluctuating asymmetry in fras1 mutant skeleton and the changes in fras1 mutant skeletal defects through time as indicators that skeletal formation is destabilized. We propose a model wherein fras1 prompts late-p1 formation and thereby stabilizes skeletal formation during zebrafish facial development. Similar mechanisms of stochastic developmental instability might also account for the high phenotypic variation observed in human FRAS1 patients.

Hemicentin 2 and Fibulin 1 are required for epidermal-dermal junction formation and fin mesenchymal cell migration during zebrafish development

Hemicentin 1 (Hmcn1) and Hemicentin 2 (Hmcn2) belong to the fibulin family of extracellular matrix (ECM) proteins that play pivotal roles during development and homeostasis of a variety of vertebrate tissues. Recently, we have shown that mutations in zebrafish Hmcn1, also called Fibulin 6, lead to massive fin blistering, similar to the defects caused by the Fraser syndrome gene Fras1. In contrast, the role of Hmcn2 during vertebrate development has thus far been uncharacterized. In zebrafish, hmcn2, like fibulin 1 (fbln1), another member of the fibulin family, is predominantly expressed in fin mesenchymal cells and developing somites, contrasting the strict epithelial expression of hmcn1. While antisense morpholino oligonucleotide (MO)-based knockdown of hmcn2 did not yield any discernable defects, hmcn2/fbln1 double knockdown fish displayed blistering in the trunk, pointing to an essential contribution of these proteins from mesodermal sources for proper epidermal-dermal junction formation. In contrast, and unlike hmcn1 mutants, epidermal-dermal junctions in the fin folds of hmcn2/fbln1 double knockdown fish were only moderately affected. Instead, they displayed impaired migration of fin mesenchymal cells into the fin folds, pointing to a crucial role of Hmcn2 and Fbln1 to remodel the ECM of the fin fold interepidermal space, which is a prerequisite for fibroblast ingrowth. TEM analyses suggest that this ECM remodeling occurs at the level of actinotrichia, the collageneous migration substrate of mesenchymal cells, and at the level of cross fibers, which resemble mammalian microfibers. This work provides first insights into the role of Hmcn2 during vertebrate development, identifying it as an evolutionary conserved protein that acts in functional redundancy with Fbln1C and/or Fbln1D isoforms to regulate tissue adhesion and cell migration, while extending the current knowledge of the functions of vertebrate Fbln1.

Expression of Fraser syndrome genes in normal and polycystic murine kidneys

BACKGROUND

Fraser syndrome (FS) features renal agenesis and cystic kidneys. Mutations of FRAS1 (Fraser syndrome 1)and FREM2 (FRAS1-related extracellular matrix protein 2)cause FS. They code for basement membrane proteins expressed in metanephric epithelia where they mediate epithelial/mesenchymal signalling. Little is known about whether and where these molecules are expressed in more mature kidneys.

METHODS

In healthy and congenital polycystic kidney (cpk)mouse kidneys we sought Frem2 expression using a LacZ reporter gene and quantified Fras family transcripts. Fras1 immunohistochemistry was undertaken in cystic kidneys from cpk mice and PCK (Pkhd1 mutant) rats (models of autosomal recessive polycystic kidney disease) and in wildtype metanephroi rendered cystic by dexamethasone.

RESULTS

Nascent nephrons transiently expressed Frem2 in both tubule and podocyte epithelia. Maturing and adult collecting ducts also expressed Frem2. Frem2 was expressed in cpk cystic epithelia although Frem2 haploinsufficiency did not significantly modify cystogenesis in vivo. Fras1 transcripts were significantly upregulated, and Frem3 downregulated, in polycystic kidneys versus the non-cystic kidneys of littermates. Fras1 was immunodetected in cpk, PCK and dexamethasone-induced cystepithelia.

CONCLUSIONS

These descriptive results are consistent with the hypothesis that Fras family molecules play diverse roles in kidney epithelia. In future, this should be tested by conditional deletion of FS genes in nephron segments and collecting ducts.

Basement membrane assembly of the integrin α8β1 ligand nephronectin requires Fraser syndrome-associated proteins

QBRICK facilitates the integrin α8β1–dependent interactions of cells with basement membranes by regulating the basement membrane assembly of nephronectin.

Dysfunction of the basement membrane protein QBRICK provokes Fraser syndrome, which results in renal dysmorphogenesis, cryptophthalmos, syndactyly, and dystrophic epidermolysis bullosa through unknown mechanisms. Here, we show that integrin α8β1 binding to basement membranes was significantly impaired in Qbrick-null mice. This impaired integrin α8β1 binding was not a direct consequence of the loss of QBRICK, which itself is a ligand of integrin α8β1, because knock-in mice with a mutation in the integrin-binding site of QBRICK developed normally and do not exhibit any defects in integrin α8β1 binding. Instead, the loss of QBRICK significantly diminished the expression of nephronectin, an integrin α8β1 ligand necessary for renal development. In vivo, nephronectin associated with QBRICK and localized at the sublamina densa region, where QBRICK was also located. Collectively, these findings indicate that QBRICK facilitates the integrin α8β1–dependent interactions of cells with basement membranes by regulating the basement membrane assembly of nephronectin and explain why renal defects occur in Fraser syndrome.

The epidemiology, genetics and future management of syndactyly

Syndactyly is a condition well documented in current literature due to it being the most common congenital hand defect, with a large aesthetic and functional significance.There are currently nine types of phenotypically diverse non-syndromic syndactyly, an increase since the original classification by Temtamy and McKusick(1978). Non-syndromic syndactyly is inherited as an autosomal dominant trait, although the more severe presenting types and sub types appear to have autosomal recessive and in some cases X-linked hereditary.Gene research has found that these phenotypes appear to not only be one gene specific, although having individual localised loci, but dependant on a wide range of genes and subsequent signalling pathways involved in limb formation. The principal genes so far defined to be involved in congenital syndactyly concern mainly the Zone of Polarizing Activity and Shh pathway.Research into the individual phenotypes appears to complicate classification as new genes are found both linked, and not linked, to each malformation. Consequently anatomical, phenotypical and genotypical classifications can be used, but are variable in significance, depending on the audience.Currently, management is surgical, with a technique unchanged for several decades, although future development will hopefully bring alternatives in both earlier diagnosis and gene manipulation for therapy.

Zebrafish kidney development: basic science to translational research

The zebrafish has become a significant model system for studying renal organogenesis and disease, as well as for the quest for new therapeutics, because of the structural and functional simplicity of the embryonic kidney. Inroads to the nature and disease states of kidney-related ciliopathies and acute kidney injury (AKI) have been advanced by zebrafish studies. This model organism has been instrumental in the analysis of mutant gene function for human disease with respect to ciliopathies. Additionally, in the AKI field, recent work in the zebrafish has identified a bona fide adult zebrafish renal progenitor (stem) cell that is required for neo-nephrogenesis, both during the normal lifespan and in response to renal injury. Taken together, these studies solidify the zebrafish as a successful model system for studying the broad spectrum of ciliopathies and AKI that affect millions of humans worldwide, and point to a very promising future of zebrafish drug discovery. The emphasis of this review will be on the role of the zebrafish as a model for human kidney-related ciliopathies and AKI, and how our understanding of these complex pathologies is being furthered by this tiny teleost.

A new job for ancient extracellular matrix proteins: Hemicentins stabilize cleavage furrows

Interactions between extracellular matrix (ECM) proteins and transmembrane receptors mediate changes in cell shape during cell migration, adhesion, differentiation and polarization. Cytokinesis is the final step in cell division as cells employ a contractile ring composed of actin and myosin to partition one cell into two. During the partition process, an invagination in nascent membrane forms a new extracellular space called the cleavage furrow. Despite the dramatic changes in cell shape during cytokinesis, existing models include no role for the ECM. In a recent paper, we show that hemicentins assemble in the cleavage furrow of C. elegans germ cells and mouse embryo blastomeres. Hemicentin depletion results in membrane destabilization, cleavage furrow retraction and cytokinesis failure. The data suggest that hemicentins and other ECM proteins stabilize the cleavage furrow during cytokinesis of multiple cell types.

In vivo protein trapping produces a functional expression codex of the vertebrate proteome

We describe a conditional in vivo protein-trap mutagenesis system that reveals spatiotemporal protein expression dynamics and can be used to assess gene function in the vertebrate Danio rerio. Integration of pGBT-RP2.1 (RP2), a gene-breaking transposon containing a protein trap, efficiently disrupts gene expression with >97% knockdown of normal transcript amounts and simultaneously reports protein expression for each locus. The mutant alleles are revertible in somatic tissues via Cre recombinase or splice-site-blocking morpholinos and are thus to our knowledge the first systematic conditional mutant alleles outside the mouse model. We report a collection of 350 zebrafish lines that include diverse molecular loci. RP2 integrations reveal the complexity of genomic architecture and gene function in a living organism and can provide information on protein subcellular localization. The RP2 mutagenesis system is a step toward a unified 'codex' of protein expression and direct functional annotation of the vertebrate genome.

Basement membrane diseases in zebrafish

Basement membranes (BMs) are a complex, sheet-like network of specialized extracellular matrix that underlies epithelial cells and surrounds muscle cells. They provide adherence between neighboring tissues, permit some flexibility of these adherent structures, and can act as a store for growth factors and as a guide for cell migration. The BM is not just a static structure; its deposition and remodeling are important for many processes including embryonic development, immune response, and wound healing. To date, dysfunction in BM deposition or remodeling has been linked to many human congenital disorders and diseases, affecting many different tissues in the body, including malformations, dystrophies, and cancer. However, many questions remain to be answered on the role BM proteins, and their mutations, play in the pathogenesis of human disease. In recent years, the zebrafish (Danio rerio) has emerged as a powerful animal model for human development and disease. In the first part of this chapter, we provide an overview of described defects caused by BM dysfunction in zebrafish, including development and function of notochord, muscle, central nervous system, skin, cardiovascular system, and kidney. In the second part, we will describe details of methods used to visualize and assess the structure of the BM in zebrafish, and to functionally analyze its different components.