Tetraspanin Cd9b and Cxcl12a/Cxcr4b have a synergistic effect on the control of collective cell migration

Collective cell migration is essential for embryonic development and homeostatic processes. During zebrafish development, the posterior lateral line primordium (pLLP) navigates along the embryo flank by collective cell migration. The chemokine receptors, Cxcr4b and Cxcr7b, as well as their cognate ligand, Cxcl12a, are essential for this process. We corroborate that knockdown of the zebrafish cd9 tetraspanin orthologue, cd9b, results in mild pLL abnormalities. Through generation of CRISPR and TALEN mutants, we show that cd9a and cd9b function partially redundantly in pLLP migration, which is delayed in the cd9b single and cd9a; cd9b double mutants. This delay led to a transient reduction in neuromast numbers. Loss of both Cd9a and Cd9b sensitized embryos to reduced Cxcr4b and Cxcl12a levels. Together these results provide evidence that Cd9 modulates collective cell migration of the pLLP during zebrafish development. One interpretation of these observations is that Cd9 contributes to more effective chemokine signalling.

A bi-modal function of Wnt signalling directs an FGF activity gradient to spatially regulate neuronal differentiation in the midbrain

FGFs and Wnts are important morphogens during midbrain development, but their importance and potential interactions during neurogenesis are poorly understood. We have employed a combination of genetic and pharmacological manipulations in zebrafish to show that during neurogenesis FGF activity occurs as a gradient along the anterior-posterior axis of the dorsal midbrain and directs spatially dynamic expression of the Hairy gene her5. As FGF activity diminishes during development, Her5 is lost and differentiation of neuronal progenitors occurs in an anterior-posterior manner. We generated mathematical models to explain how Wnt and FGFs direct the spatial differentiation of neurons in the midbrain through Wnt regulation of FGF signalling. These models suggested that a negative-feedback loop controlled by Wnt is crucial for regulating FGF activity. We tested Sprouty genes as mediators of this regulatory loop using conditional mouse knockouts and pharmacological manipulations in zebrafish. These reveal that Sprouty genes direct the positioning of early midbrain neurons and are Wnt responsive in the midbrain. We propose a model in which Wnt regulates FGF activity at the isthmus by driving both FGF and Sprouty gene expression. This controls a dynamic, posteriorly retracting expression of her5 that directs neuronal differentiation in a precise spatiotemporal manner in the midbrain.

Loss of function of parathyroid hormone receptor 1 induces Notch-dependent aortic defects during zebrafish vascular development

OBJECTIVE

Coarctation of the aorta is rarely associated with known gene defects. Blomstrand chondrodysplasia, caused by mutations in the parathyroid hormone receptor 1 (PTHR1) is associated with coarctation of the aorta in some cases, although it is unclear whether PTHR1 deficiency causes coarctation of the aorta directly. The zebrafish allows the study of vascular development using approaches not possible in other models. We therefore examined the effect of loss of function of PTHR1 or its ligand parathyroid hormone-related peptide (PTHrP) on aortic formation in zebrafish.

APPROACH AND RESULTS

Morpholino antisense oligonucleotide knockdown of either PTHR1 or PTHrP led to a localized occlusion of the mid-aorta in developing zebrafish. Confocal imaging of transgenic embryos showed that these defects were caused by loss of endothelium, rather than failure to lumenize. Using a Notch reporter transgenic ([CSL:Venus]qmc61), we found both PTHR1 and PTHrP knockdown-induced defective Notch signaling in the hypochord at the site of the aortic defect before onset of circulation, and the aortic occlusion was rescued by inducible Notch upregulation.

CONCLUSIONS

Loss of function of either PTHR1 or PTHrP leads to a localized aortic defect that is Notch dependent. These findings may underlie the aortic defect seen in Blomstrand chondrodysplasia, and reveal a link between parathyroid hormone and Notch signaling during aortic development.

Differentiated skeletal cells contribute to blastema formation during zebrafish fin regeneration

The origin of cells that generate the blastema following appendage amputation has been a long-standing question in epimorphic regeneration studies. The blastema is thought to originate from either stem (or progenitor) cells or differentiated cells of various tissues that undergo dedifferentiation. Here, we investigate the origin of cells that contribute to the regeneration of zebrafish caudal fin skeletal elements. We provide evidence that the process of lepidotrichia (bony rays) regeneration is initiated as early as 24 hours post-amputation and that differentiated scleroblasts acquire a proliferative state, detach from the lepidotrichia surface, migrate distally, integrate into the blastema and dedifferentiate. These findings provide novel insights into the origin of cells in epimorphic appendage regeneration in zebrafish and suggest conservation of regeneration mechanisms between fish and amphibians.

Regulation of zebrafish hatching by tetraspanin cd63

Tetraspanins cause the clustering of membrane proteins into a level of organisation essential for cellular function. Given the importance and complicated nature of this mechanism, we attempted a novel approach to identify the function of a single component in a biologically relevant context. A morpholino knockdown strategy was used to investigate the role of cd63, a membrane protein associated with intracellular transport and a melanoma marker, in embryonic zebrafish. By using three separate morpholinos targeting cd63, we were able to identify a specific phenotype. Strikingly, morphant fish failed to hatch due to the lack of secreted proteolytic enzymes required for chorion-softening. The morphology of the hatching gland at both the cellular and intracellular levels was disorganised, suggesting a role for cd63 in the functioning of this organ. This work identifies a specific role for cd63 in the zebrafish embryo and provides evidence for the suitability of zebrafish as a model system for the investigation of tetraspanin enriched microdomains.

Ret signalling integrates a craniofacial muscle module during development

An appropriate organisation of muscles is crucial for their function, yet it is not known how functionally related muscles are coordinated with each other during development. In this study, we show that the development of a subset of functionally related head muscles in the zebrafish is regulated by Ret tyrosine kinase signalling. Three genes in the Ret pathway (gfra3, artemin2 and ret) are required specifically for the development of muscles attaching to the opercular bone (gill cover), but not other adjacent muscles. In animals lacking Ret or Gfra3 function, myogenic gene expression is reduced in forming opercular muscles, but not in non-opercular muscles derived from the same muscle anlagen. These animals have a normal skeleton with small or missing opercular muscles and tightly closed mouths. Myogenic defects correlate with a highly restricted expression of artn2, gfra3 and ret in mesenchymal cells in and around the forming opercular muscles. ret(+) cells become restricted to the forming opercular muscles and a loss of Ret signalling results in reductions of only these, but not adjacent, muscles, revealing a specific role of Ret in a subset of head muscles. We propose that Ret signalling regulates myogenesis in head muscles in a modular manner and that this is achieved by restricting Ret function to a subset of muscle precursors.

A replication competent lentivirus (RCL) assay for equine infectious anaemia virus (EIAV)-based lentiviral vectors

Lentiviral vectors are being developed to satisfy a wide range of currently unmet medical needs. Vectors destined for clinical evaluation have been rendered multiply defective by deletion of all viral coding sequences and nonessential cis-acting sequences from the transfer genome. The viral envelope and accessory proteins are excluded from the production system. The vectors are produced from separate expression plasmids that are designed to minimize the potential for homologous recombination. These features ensure that the regeneration of the starting virus is impossible. It is a regulatory requirement to confirm the absence of any replication competent virus, so we describe here the development and validation of a replication competent lentivirus (RCL) assay for equine infectious anaemia virus (EIAV)-based vectors. The assay is based on the guidelines developed for testing retroviral vectors, and uses the F-PERT (fluorescent-product enhanced reverse transcriptase) assay to test for the presence of a transmissible reverse transcriptase. We have empirically modelled the replication kinetics of an EIAV-like entity in human cells and devised an amplification protocol by comparison with a replication competent MLV. The RCL assay has been validated at the 20 litre manufacturing scale, during which no RCL was detected. The assay is theoretically applicable to any lentiviral vector and pseudotype combination.

Axon Sorting in the Optic Tract Requires HSPG Synthesis by ext2 (dackel) and extl3 (boxer)

Retinal ganglion cell (RGC) axons are topographically ordered in the optic tract according to their retinal origin. In zebrafish dackel (dak) and boxer (box) mutants, some dorsal RGC axons missort in the optic tract but innervate the tectum topographically. Molecular cloning reveals that dak and box encode ext2 and extl3, glycosyltransferases implicated in heparan sulfate (HS) biosynthesis. Both genes are required for HS synthesis, as shown by biochemical and immunohistochemical analysis, and are expressed maternally and then ubiquitously, likely playing permissive roles. Missorting in box can be rescued by overexpression of extl3. dak;box double mutants show synthetic pathfinding phenotypes that phenocopy robo2 mutants, suggesting that Robo2 function requires HS in vivo; however, tract sorting does not require Robo function, since it is normal in robo2 null mutants. This genetic evidence that heparan sulfate proteoglycan function is required for optic tract sorting provides clues to begin understanding the underlying molecular mechanisms.

The deafness genedfna5is crucial forugdhexpression and HA production in the developing ear in zebrafish

Over 30 genes responsible for human hereditary hearing loss have been identified during the last 10 years. The proteins encoded by these genes play roles in a diverse set of cellular functions ranging from transcriptional regulation to K+ recycling. In a few cases, the genes are novel and do not give much insight into the cellular or molecular cause for the hearing loss. Among these poorly understood deafness genes is DFNA5. How the truncation of the encoded protein DFNA5 leads to an autosomal dominant form of hearing loss is not clear. In order to understand the biological role of Dfna5, we took a reversegenetic approach in zebrafish. Here we show that morpholino antisense nucleotide knock-down of dfna5 function in zebrafish leads to disorganization of the developing semicircular canals and reduction of pharyngeal cartilage. This phenotype closely resembles previously isolated zebrafish craniofacial mutants including the mutant jekyll. jekyll encodes Ugdh [uridine 5′-diphosphate (UDP)-glucose dehydrogenase], an enzyme that is crucial for production of the extracellular matrix component hyaluronic acid (HA). In dfna5 morphants, expression of ugdh is absent in the developing ear and pharyngeal arches, and HA levels are strongly reduced in the outgrowing protrusions of the developing semicircular canals. Previous studies suggest that HA is essential for differentiating cartilage and directed outgrowth of the epithelial protrusions in the developing ear. We hypothesize that the reduction of HA production leads to uncoordinated outgrowth of the canal columns and impaired facial cartilage differentiation.

Slb/Wnt11 controls hypoblast cell migration and morphogenesis at the onset of zebrafish gastrulation

During vertebrate gastrulation, highly coordinated cellular rearrangements lead to the formation of the three germ layers, ectoderm, mesoderm and endoderm. In zebrafish, silberblick (slb)/wnt11 regulates normal gastrulation movements by activating a signalling pathway similar to the Frizzled-signalling pathway, which establishes epithelial planar cell polarity (PCP) in Drosophila. However, the cellular mechanisms by which slb/wnt11 functions during zebrafish gastrulation are still unclear. Using high-resolution two-photon confocal imaging followed by computer-assisted reconstruction and motion analysis, we have analysed the movement and morphology of individual cells in three dimensions during the course of gastrulation. We show that in slb-mutant embryos, hypoblast cells within the forming germ ring have slower, less directed migratory movements at the onset of gastrulation. These aberrant cell movements are accompanied by defects in the orientation of cellular processes along the individual movement directions of these cells. We conclude that slb/wnt11-mediated orientation of cellular processes plays a role in facilitating and stabilising movements of hypoblast cells in the germ ring, thereby pointing at a novel function of the slb/wnt11 signalling pathway for the regulation of migratory cell movements at early stages of gastrulation.

Zebrafish pea3 and erm are general targets of FGF8 signaling

Phenotypic analysis of both zebrafish and mouse has shown that fibroblast growth factor 8 (FGF8) is required for many developmental decisions. To further our understanding of the FGF8 signaling process, we sought to identify new transcriptional targets of the pathway. Here, we propose that two zebrafish ETS genes, pea3 and erm, are general targets of FGF8 signaling, based upon the following observations: both genes are expressed around all early FGF8 signaling sources, both genes are downregulated in fgf8 mutant embryos in all tissues known to require fgf8 function, a pharmacological inhibitor of the FGF pathway completely abolishes expression of both genes, and ectopic expression of fgf8 is sufficient to induce both genes. The finding that pea3 and erm are common transcriptional targets of FGF8 signaling suggests that they are general mediators of FGF8 signaling during development. In addition, we observed that pea3 is often expressed close to an FGF8 source, and erm is expressed in a broader domain. To test whether this differential expression is established by FGF8, we have induced FGF8 ectopically and show that it is sufficient to recapitulate the endogenous nested expression pattern of pea3 and erm.

A radiation hybrid map of the zebrafish genome

Recent large-scale mutagenesis screens have made the zebrafish the first vertebrate organism to allow a forward genetic approach to the discovery of developmental control genes. Mutations can be cloned positionally, or placed on a simple sequence length polymorphism (SSLP) map to match them with mapped candidate genes and expressed sequence tags (ESTs). To facilitate the mapping of candidate genes and to increase the density of markers available for positional cloning, we have created a radiation hybrid (RH) map of the zebrafish genome. This technique is based on somatic cell hybrid lines produced by fusion of lethally irradiated cells of the species of interest with a rodent cell line. Random fragments of the donor chromosomes are integrated into recipient chromosomes or retained as separate minichromosomes. The radiation-induced breakpoints can be used for mapping in a manner analogous to genetic mapping, but at higher resolution and without a need for polymorphism. Genome-wide maps exist for the human, based on three RH panels of different resolutions, as well as for the dog, rat and mouse. For our map of the zebrafish genome, we used an existing RH panel and 1,451 sequence tagged site (STS) markers, including SSLPs, cloned candidate genes and ESTs. Of these, 1,275 (87.9%) have significant linkage to at least one other marker. The fraction of ESTs with significant linkage, which can be used as an estimate of map coverage, is 81.9%. We found the average marker retention frequency to be 18.4%. One cR3000 is equivalent to 61 kb, resulting in a potential resolution of approximately 350 kb.

The origin and evolution of animal appendages

Animals have evolved diverse appendages adapted for locomotion, feeding and other functions. The genetics underlying appendage formation are best understood in insects and vertebrates. The expression of the Distal-less (Dll) homeoprotein during arthropod limb outgrowth and of Dll orthologs (Dlx) in fish fin and tetrapod limb buds led us to examine whether expression of this regulatory gene may be a general feature of appendage formation in protostomes and deuterostomes. We find that Dll is expressed along the proximodistal axis of developing polychaete annelid parapodia, onychophoran lobopodia, ascidian ampullae, and even echinoderm tube feet. Dll/Dlx expression in such diverse appendages in these six coelomate phyla could be convergent, but this would have required the independent co-option of Dll/Dlx several times in evolution. It appears more likely that ectodermal Dll/Dlx expression along proximodistal axes originated once in a common ancestor and has been used subsequently to pattern body wall outgrowths in a variety of organisms. We suggest that this pre-Cambrian ancestor of most protostomes and the deuterostomes possessed elements of the genetic machinery for and may have even borne appendages.

Roles of the RAM and ANK domains in signaling by the C. elegans GLP-1 receptor

In Caenorhabditis elegans, the GLP-1 receptor acts with a downstream transcriptional regulator, LAG-1, to mediate intercellular signaling. GLP-1 and LAG-1 are homologs of Drosophila Notch and Su(H) respectively. Here, we investigate the functions of two regions of the GLP-1 intracellular domain: the ANK repeat domain, which includes six cdc10/ankyrin repeats plus flanking amino acids, and the RAM domain, which spans approximately 60 amino acids just inside the transmembrane domain. First, we demonstrate that both ANK and RAM domains interact with the LAG-1 transcription factor. The interaction between the ANK domain and LAG-1 is only observed in nematodes by a co-localization assay and, therefore, may be either direct or indirect. By contrast, the interaction between the RAM domain and LAG-1 is likely to be direct, since it is observed by co-precipitation of the proteins in vitro as well as by yeast two-hybrid experiments. Second, we demonstrate that the RAM domain, when expressed in nematodes without a functional ANK repeat domain, does not mimic the unregulated receptor in directing cell fates or interfere with signaling by endogenous components. Finally, we show in yeast that the ANK repeats are strong transcriptional activators. Furthermore, missense mutations that eliminate receptor activity also abolish transcriptional activation by the GLP-1 ANK repeats in yeast. We speculate that one possible function for the ANK repeat domain is to act as a transcriptional co-activator with LAG-1.

Control of cell fate in C. elegans by a GLP-1 peptide consisting primarily of ankyrin repeats

The homologous proteins GLP-1 and LIN-12 are required for cell interactions during nematode development. glp-1 and lin-12 are members of a gene family that includes Drosophila Notch and several vertebrate homologues. The members of this family have a single transmembrane domain and a similar arrangement of repeated amino-acid motifs (see Fig. 1). The mechanism by which proteins in this family function is not understood. One hypothesis is that these proteins are receptors, each with an extracellular domain that binds a ligand and an intracellular domain that influences the activity of downstream cell fate regulators. Here we report that a region of the GLP-1 intracellular domain, consisting primarily of six ankyrin repeats, is sufficient to direct cell fate. The cell fate transformations seen are similar to transformations caused by gain-of-function mutations in either glp-1 or lin-12 and do not rely on endogenous lin-12 or glp-1 activity. We propose that the ankyrin repeat region of GLP-1 is responsible for controlling downstream regulators of cell fate.

Characterization of the gene encoding herpesvirus of turkeys gp57-65: comparison to Marek's disease virus gp57-65 and herpes simplex virus glycoprotein C

A gene encoding herpesvirus of turkeys (HVT) strain FC 126 gp57-65 has been mapped to the viral genome and sequenced. The HVT (FC 126) gp57-65 gene maps to BamHI fragments K1 and M, colinear with the gene from Marek's disease virus (MDV) strain GA. HVT gp57-65 gene sequences were compared to the MDV strain GA gp57-65 gene that we sequenced previously. Overall, the two sequences are 66% identical, with greater similarity in the 3' proximal two thirds of the genes. HVT gp57-65 gene sequences have a slightly higher overall guanosine plus cytosine (G + C) content than MDV gp57-65 gene sequences (46% vs. 41%, respectively). A single, long open reading frame capable of encoding 523 amino acids was identified within the HVT gp57-65 gene region. The predicted precursor polypeptide derived from this open reading frame would have a calculated molecular weight of 58,587. The predicted HVT gp57-65 amino acid sequences contain six potential N-linked glycosylation sites (asn-x-ser/thr). Five of these six potential N-linked glycosylation sites are conserved between the HVT and MDV predicted amino acid sequences. Hydropathic analysis of the predicted HVT gp57-65 amino acid sequences indicate the presence of an amino-terminal hydrophobic sequence, which may function as a signal peptide, and a hydrophobic carboxyl terminal sequence, which may function as a membrane anchor sequence. Overall, MDV gp57-65 and HVT gp57-65 precursor polypeptide sequences are 73% homologous and share many potential antigenic epitopes. Predicted MDV and HVT gp57-65 protein sequences are similar to those of herpes simplex virus glycoprotein C (gC) and gC-like proteins from other herpes-viruses. Similarities are scattered throughout the molecule, with a primary concentration near the carboxyl half of the molecule. One stretch of 60 amino acids (HVT amino acids 378-437 and MDV amino acids 350-410) are relatively well conserved among gC-like proteins from six herpesviruses. The possible implications of these homologies and the potential roles of gC-like proteins in virus infection, growth, and replication are discussed.