Evaluation of Zebrafish Kidney Function Using a Fluorescent Clearance Assay

The zebrafish embryo offers a tractable model to study organogenesis and model human genetic disease. Despite its relative simplicity, the zebrafish ...

St. George's University of London

Pax3/7 paired homeodomain transcription factors are important markers of muscle stem cells. Pax3 is required upstream of myod for lateral dermomyotomal cells in the amniote somite to form particular muscle cells. Later Pax3/7-dependent cells generate satellite cells and most body muscle. Here we analyse early myogenesis from, and regulation of, a population of Pax3-expressing dermomyotome-like cells in the zebrafish. Zebrafish pax3 is widely expressed in the lateral somite and, along with pax7, becomes restricted anteriorly and then to the external cells on the lateral somite surface. Midline-derived Hedgehog signals appear to act directly on lateral somite cells to repress Pax3/7. Both Hedgehog and Fgf8, signals that induce muscle formation within the somite, suppress Pax3/7 and promote expression of myogenic regulatory factors (MRFs) myf5 and myod in specific muscle precursor cell populations. Loss of MRF function leads to loss of myogenesis by specific populations of muscle fibres, with parallel up-regulation of Pax3/7. Myod is required for lateral fast muscle differentiation from pax3-expressing cells. In contrast, either Myf5 or Myod is sufficient to promote slow muscle formation from adaxial cells. Thus, myogenic signals act to drive somite cells to a myogenic fate through up-regulation of distinct combinations of MRFs. Our data show that the relationship between Pax3/7 genes and myogenesis is evolutionarily ancient, but that changes in the MRF targets for particular signals contribute to myogenic differences between species.

Signals and myogenic regulatory factors restrict pax3 and pax7 expression to dermomyotome-like tissue in zebrafish.

Mrf4 (Myf6) is a member of the basic helix-loop-helix (bHLH) myogenic regulatory transcription factor (MRF) family, which also contains Myod, Myf5 and myogenin. Mrf4 is implicated in commitment of amniote cells to skeletal myogenesis and is also abundantly expressed in many adult muscle fibres. The specific role of Mrf4 is unclear both because mrf4 null mice are viable, suggesting redundancy with other MRFs, and because of genetic interactions at the complex mrf4/myf5 locus. We report the cloning and expression of an mrf4 gene from zebrafish, Danio rerio, which shows conservation of linkage to myf5. Mrf4 mRNA accumulates in a subset of terminally differentiated muscle fibres in parallel with myosin protein in the trunk and fin. Although most, possibly all, trunk muscle expresses mrf4, the level of mRNA is dynamically regulated. No expression is detected in muscle precursor cell populations prior to myosin accumulation. Moreover, mrf4 expression is not detected in head muscles, at least at early stages. As fish mature, mrf4 expression is pronounced in the region of slow muscle fibres.

Mrf4 (myf6) is dynamically expressed in differentiated zebrafish skeletal muscle.

The vestigial gene has been shown to control skeletal muscle formation in Drosophila and the related Vestigial-like 2 (Vgl-2) protein plays a similar role in mice. Vgl-family proteins are thought to regulate tissue-specific gene expression by binding to members of the broadly expressed Scalloped/Tef/TEAD transcription factor family. Zebrafish have at least four Vgl genes, including two Vgl-2s, and at least three TEAD genes, including two Tead3s. We describe the cloning and expression of one member from each family in the zebrafish. A novel gene, vgl-2b, with closest homology to mouse and human vgl-2, is expressed transiently in nascent notochord and in muscle fibres as they undergo terminal differentiation during somitogenesis. Muscle cells also express a TEAD-3 homologue, a possible partner of Vgl-2b, during myoblast differentiation and early fibre assembly. Tead-3a is also expressed in rhombomeres, eye and epiphysis regions.

Vestigial-like-2b (VITO-1b) and Tead-3a (Tef-5a) expression in zebrafish skeletal muscle, brain and notochord.

Myogenic regulatory factors of the Myod family (MRFs) are transcription factors essential for mammalian skeletal myogenesis. However, the roles of each gene in myogenesis remain unclear, owing partly to genetic linkage at the Myf5/Mrf4 locus and to rapid morphogenetic movements in the amniote somite. In mice, Myf5 is essential for the earliest epaxial myogenesis, whereas Myod is required for timely differentiation of hypaxially derived muscle. A second major subdivision of the somite is between primaxial muscle of the somite proper and abaxial somite-derived migratory muscle precursors. Here, we use a combination of mutant and morphant analysis to ablate the function of each of the four conserved MRF genes in zebrafish, an organism that has retained a more ancestral bodyplan. We show that a fundamental distinction in somite myogenesis is into medial versus lateral compartments, which correspond to neither epaxial/hypaxial nor primaxial/abaxial subdivisions. In the medial compartment, Myf5 and/or Myod drive adaxial slow fibre and medial fast fibre differentiation. Myod-driven Myogenin activity alone is sufficient for lateral fast somitic and pectoral fin fibre formation from the lateral compartment, as well as for cranial myogenesis. Myogenin activity is a significant contributor to fast fibre differentiation. Mrf4 does not contribute to early myogenesis in zebrafish. We suggest that the differential use of duplicated MRF paralogues in this novel two-component myogenic system facilitated the diversification of vertebrates.

Differential requirements for myogenic regulatory factors distinguish medial and lateral somitic, cranial and fin muscle fibre populations.

Centrioles are the foundation for centrosome and cilia formation. The biogenesis of centrioles is initiated by an assembly mechanism that first synthesizes the ninefold symmetrical cartwheel and subsequently leads to a stable cylindrical microtubule scaffold that is capable of withstanding microtubule-based forces generated by centrosomes and cilia. We report that the conserved WD40 repeat domain-containing cartwheel protein Poc1 is required for the structural maintenance of centrioles in Tetrahymena thermophila. Furthermore, human Poc1B is required for primary ciliogenesis, and in zebrafish, DrPoc1B knockdown causes ciliary defects and morphological phenotypes consistent with human ciliopathies. T. thermophila Poc1 exhibits a protein incorporation profile commonly associated with structural centriole components in which the majority of Poc1 is stably incorporated during new centriole assembly. A second dynamic population assembles throughout the cell cycle. Our experiments identify novel roles for Poc1 in centriole stability and ciliogenesis.

Basal body stability and ciliogenesis requires the conserved component Poc1.

Laterality defects such as situs inversus are not uncommonly encountered in humans, either in isolation or as part of another syndrome, but can have devastating developmental consequences. The events that break symmetry during early embryogenesis are highly conserved amongst vertebrates and involve the establishment of unidirectional flow by cilia within an organising centre such as the node in mammals or Kupffer's vesicle (KV) in teleosts. Disruption of this flow can lead to the failure to successfully establish left-right asymmetry. The correct apical-posterior cellular position of each node/KV cilium is critical for its optimal radial movement which serves to sweep fluid (and morphogens) in the same direction as its neighbours. Planar cell polarity (PCP) is an important conserved process that governs ciliary position and posterior tilt; however the underlying mechanism by which this occurs remains unclear. Here we show that Bbs8, a ciliary/basal body protein important for intraciliary/flagellar transport and the core PCP protein Vangl2 interact and are required for establishment and maintenance of left-right asymmetry during early embryogenesis in zebrafish. We discovered that loss of bbs8 and vangl2 results in laterality defects due to cilia disruption at the KV. We showed that perturbation of cell polarity following abrogation of vangl2 causes nuclear mislocalisation, implying defective centrosome/basal body migration and apical docking. Moreover, upon loss of bbs8 and vangl2, we observed defective actin organisation. These data suggest that bbs8 and vangl2 act synergistically on cell polarization to establish and maintain the appropriate length and number of cilia in the KV and thereby facilitate correct LR asymmetry.

Bbs8, together with the planar cell polarity protein Vangl2, is required to establish left-right asymmetry in zebrafish.

Differentiation often requires conversion of analogue signals to a stable binary output through positive feedback. Hedgehog (Hh) signalling promotes myogenesis in the vertebrate somite, in part by raising the activity of muscle regulatory factors (MRFs) of the Myod family above a threshold. Hh is known to enhance MRF expression. Here we show that Hh is also essential at a second step that increases Myod protein activity, permitting it to promote Myogenin expression. Hh acts by inducing expression of cdkn1c (p57(Kip2)) in slow muscle precursor cells, but neither Hh nor Cdkn1c is required for their cell cycle exit. Cdkn1c co-operates with Myod to drive differentiation of several early zebrafish muscle fibre types. Myod in turn up-regulates cdkn1c, thereby providing a positive feedback loop that switches myogenic cells to terminal differentiation.

Cdkn1c drives muscle differentiation through a positive feedback loop with Myod.

3MC syndrome has been proposed as a unifying term encompassing the overlapping Carnevale, Mingarelli, Malpuech and Michels syndromes. These rare autosomal recessive disorders exhibit a spectrum of developmental features, including characteristic facial dysmorphism, cleft lip and/or palate, craniosynostosis, learning disability and genital, limb and vesicorenal anomalies. Here we studied 11 families with 3MC syndrome and identified two mutated genes, COLEC11 and MASP1, both of which encode proteins in the lectin complement pathway (collectin kidney 1 (CL-K1) and MASP-1 and MASP-3, respectively). CL-K1 is highly expressed in embryonic murine craniofacial cartilage, heart, bronchi, kidney and vertebral bodies. Zebrafish morphants for either gene develop pigmentary defects and severe craniofacial abnormalities. Finally, we show that CL-K1 serves as a guidance cue for neural crest cell migration. Together, these findings demonstrate a role for complement pathway factors in fundamental developmental processes and in the etiology of 3MC syndrome.

Mutations in lectin complement pathway genes COLEC11 and MASP1 cause 3MC syndrome.

Primary cilia are involved in important developmental and disease pathways, such as the regulation of neurogenesis and tumorigenesis. They function as sensory antennae and are essential in the regulation of key extracellular signalling systems. We have investigated the effects of cell stress on primary cilia. Exposure of mammalian cells in vitro, and zebrafish cells in vivo, to elevated temperature resulted in the rapid loss of cilia by resorption. In mammalian cells loss of cilia correlated with a reduction in hedgehog signalling. Heat-shock-dependent loss of cilia was decreased in cells where histone deacetylases (HDACs) were inhibited, suggesting resorption is mediated by the axoneme-localised tubulin deacetylase HDAC6. In thermotolerant cells the rate of ciliary resorption was reduced. This implies a role for molecular chaperones in the maintenance of primary cilia. The cytosolic chaperone Hsp90 localises to the ciliary axoneme and its inhibition resulted in cilia loss. In the cytoplasm of unstressed cells, Hsp90 is known to exist in a complex with HDAC6. Moreover, immediately after heat shock Hsp90 levels were reduced in the remaining cilia. We hypothesise that ciliary resorption serves to attenuate cilia-mediated signalling pathways in response to extracellular stress, and that this mechanism is regulated in part by HDAC6 and Hsp90.

Heat shock induces rapid resorption of primary cilia.

Bardet-Biedl syndrome (BBS) is a genetically heterogeneous disorder that is generally inherited in an autosomal recessive fashion. However, in some families, trans mutant alleles interact with the primary causal locus to modulate the penetrance and/or the expressivity of the phenotype. CCDC28B (MGC1203) was identified as a second site modifier of BBS encoding a protein of unknown function. Here we report the first functional characterization of this protein and show it affects ciliogenesis both in cultured cells and in vivo in zebrafish. Consistent with this biological role, our in silico analysis shows that the presence of CCDC28B homologous sequences is restricted to ciliated metazoa. Depletion of Ccdc28b in zebrafish results in defective ciliogenesis and consequently causes a number of phenotypes that are characteristic of BBS and other ciliopathy mutants including hydrocephalus, left-right axis determination defects and renal function impairment. Thus, this work reports CCDC28B as a novel protein involved in the process of ciliogenesis whilst providing functional insight into the cellular basis of its modifier effect in BBS patients.

Characterization of CCDC28B reveals its role in ciliogenesis and provides insight to understand its modifier effect on Bardet-Biedl syndrome.

CCDC28B encodes a coiled coil domain-containing protein involved in ciliogenesis that was originally identified as a second site modifier of the ciliopathy Bardet-Biedl syndrome. We have previously shown that the depletion of CCDC28B leads to shortened cilia; however, the mechanism underlying how this protein controls ciliary length is unknown. Here, we show that CCDC28B interacts with SIN1, a component of the mTOR complex 2 (mTORC2), and that this interaction is important both in the context of mTOR signaling and in a hitherto unknown, mTORC-independent role of SIN1 in cilia biology. We show that CCDC28B is a positive regulator of mTORC2, participating in its assembly/stability and modulating its activity, while not affecting mTORC1 function. Further, we show that Ccdc28b regulates cilia length in vivo, at least in part, through its interaction with Sin1. Importantly, depletion of Rictor, another core component of mTORC2, does not result in shortened cilia. Taken together, our findings implicate CCDC28B in the regulation of mTORC2, and uncover a novel function of SIN1 regulating cilia length that is likely independent of mTOR signaling.

The Bardet-Biedl syndrome-related protein CCDC28B modulates mTORC2 function and interacts with SIN1 to control cilia length independently of the mTOR complex.

Common intronic variants in the Human fat mass and obesity-associated gene (FTO) are found to be associated with an increased risk of obesity. Overexpression of FTO correlates with increased food intake and obesity, whilst loss-of-function results in lethality and severe developmental defects. Despite intense scientific discussions around the role of FTO in energy metabolism, the function of FTO during development remains undefined. Here, we show that loss of Fto leads to developmental defects such as growth retardation, craniofacial dysmorphism and aberrant neural crest cells migration in Zebrafish. We find that the important developmental pathway, Wnt, is compromised in the absence of FTO, both in vivo (zebrafish) and in vitro (Fto(-/-) MEFs and HEK293T). Canonical Wnt signalling is down regulated by abrogated β-Catenin translocation to the nucleus whilst non-canonical Wnt/Ca(2+) pathway is activated via its key signal mediators CaMKII and PKCδ. Moreover, we demonstrate that loss of Fto results in short, absent or disorganised cilia leading to situs inversus, renal cystogenesis, neural crest cell defects and microcephaly in Zebrafish. Congruently, Fto knockout mice display aberrant tissue specific cilia. These data identify FTO as a protein-regulator of the balanced activation between canonical and non-canonical branches of the Wnt pathway. Furthermore, we present the first evidence that FTO plays a role in development and cilia formation/function.

Daniel P. S. Osborn

Molecular Cell Science Research Centre

St. George's University of London

Evaluation of Zebrafish Kidney Function Using a Fluorescent Clearance Assay

Daniel P. S. Osborn

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Molecular Cell Science Research Centre

St. George's University of London

Evaluation of Zebrafish Kidney Function Using a Fluorescent Clearance Assay

Molecular Cell Science Research Centre