Zebrafish Trap230/Med12 is required as a coactivator for Sox9-dependent neural crest, cartilage and ear development

The Mediator complex protein Med31 is required for embryonic growth and cell proliferation during mammalian development

During development, the mammalian embryo must integrate signals to control growth and proliferation. A failure in the ability to respond to mitogenic stimuli can cause embryonic growth restriction. We have identified a mouse mutant, l11Jus15, from a mutagenesis screen that exhibits growth defects and late-gestation lethality. Here we demonstrate that this phenotype results from a mutation in the Mediator complex gene Med31, which causes degradation of Med31 protein. The Med31 mutant phenotype is not similar to other Mediator complex mouse mutants, and target genes of other Mediator proteins are expressed normally in Med31 mutants, suggesting that Med31 has distinct target genes required for mammalian development. Med31 mutant embryos have fewer proliferating cells than controls, especially in regions that expand rapidly during development such as the forelimb buds. Likewise, embryonic fibroblast cells cultured from mutant embryos have a severe proliferation defect, as well as reduced levels of the cell cycle protein Cdc2. Med31 mutants have normal limb bud patterning but defective or delayed chondrogenesis due to a lack of Sox9 and Col2a1 expression. As the Mediator complex is a transcriptional co-activator, our results suggest that Med31 functions to promote the transcription of genes required for embryonic growth and cell proliferation.

Disruption of the epigenetic code: an emerging mechanism in mental retardation

Mental retardation (MR) is a highly diverse group of cognitive disorders. Gene defects account for about half of all patients and mutations causative for impaired cognition have been identified in more than 400 genes. While there are numerous genetic defects underlying MR, a more limited number of pathways is emerging whose disruption appears to be shared by groups of MR genes. One of these common pathways is composed of MR genes that encode regulators of chromatin structure and of chromatin-mediated transcription regulation. Already more than 20 "epigenetic MR genes" have been identified and this number is likely to increase in the coming years when deep sequencing of exomes and genomes will become commonplace. Prominent examples of epigenetic MR genes include the methyl CpG-binding protein MECP2 and the CREB binding protein, CBP. Interestingly, several epigenetic MR proteins have been found to interact directly with one another or act together in complexes that regulate the local chromatin structure at target genes. Thus, it appears that the functions of individual epigenetic MR proteins converge onto similar biological processes that are crucial to neuronal processes. The next challenge will be to gain more insight into patterns of altered DNA methylation and histone modifications that are caused by epigenetic gene mutations and how these will disrupt the brain-specific expression of target genes. Such research may reveal that a wide variety of mutations in the genetic code result in a more limited number of disruptions to the epigenetic code. If so, this will provide a rationale for therapeutic strategies.

Characterization and classification of zebrafish brain morphology mutants

The mechanisms by which the vertebrate brain achieves its three-dimensional structure are clearly complex, requiring the functions of many genes. Using the zebrafish as a model, we have begun to define genes required for brain morphogenesis, including brain ventricle formation, by studying 16 mutants previously identified as having embryonic brain morphology defects. We report the phenotypic characterization of these mutants at several timepoints, using brain ventricle dye injection, imaging, and immunohistochemistry with neuronal markers. Most of these mutants display early phenotypes, affecting initial brain shaping, whereas others show later phenotypes, affecting brain ventricle expansion. In the early phenotype group, we further define four phenotypic classes and corresponding functions required for brain morphogenesis. Although we did not use known genotypes for this classification, basing it solely on phenotypes, many mutants with defects in functionally related genes clustered in a single class. In particular, Class 1 mutants show midline separation defects, corresponding to epithelial junction defects; Class 2 mutants show reduced brain ventricle size; Class 3 mutants show midbrain-hindbrain abnormalities, corresponding to basement membrane defects; and Class 4 mutants show absence of ventricle lumen inflation, corresponding to defective ion pumping. Later brain ventricle expansion requires the extracellular matrix, cardiovascular circulation, and transcription/splicing-dependent events. We suggest that these mutants define processes likely to be used during brain morphogenesis throughout the vertebrates. Anat Rec, 2009. (c) 2008 Wiley-Liss, Inc.

A zebrafish model for Waardenburg syndrome type IV reveals diverse roles for Sox10 in the otic vesicle

In humans, mutations in the SOX10 gene are a cause of the auditory-pigmentary disorder Waardenburg syndrome type IV (WS4) and related variants. SOX10 encodes an Sry-related HMG box protein essential for the development of the neural crest; deafness in WS4 and other Waardenburg syndromes is usually attributed to loss of neural-crest-derived melanocytes in the stria vascularis of the cochlea. However, SOX10 is strongly expressed in the developing otic vesicle and so direct roles for SOX10 in the otic epithelium might also be important. Here, we examine the otic phenotype of zebrafish sox10 mutants, a model for WS4. As a cochlea is not present in the fish ear, the severe otic phenotype in these mutants cannot be attributed to effects on this tissue. In zebrafish sox10 mutants, we see abnormalities in all otic placodal derivatives. Gene expression studies indicate deregulated expression of several otic genes, including fgf8, in sox10 mutants. Using a combination of mutant and morphant data, we show that the three sox genes belonging to group E (sox9a, sox9b and sox10) provide a link between otic induction pathways and subsequent otic patterning: they act redundantly to maintain sox10 expression throughout otic tissue and to restrict fgf8 expression to anterior macula regions. Single-cell labelling experiments indicate a small and transient neural crest contribution to the zebrafish ear during normal development, but this is unlikely to account for the strong defects seen in the sox10 mutant. We discuss the implication that the deafness in WS4 patients with SOX10 mutations might reflect a haploinsufficiency for SOX10 in the otic epithelium, resulting in patterning and functional abnormalities in the inner ear.

The human CDK8 subcomplex is a histone kinase that requires Med12 for activity and can function independently of mediator

The four proteins CDK8, cyclin C, Med12, and Med13 can associate with Mediator and are presumed to form a stable "CDK8 subcomplex" in cells. We describe here the isolation and enzymatic activity of the 600-kDa CDK8 subcomplex purified directly from human cells and also via recombinant expression in insect cells. Biochemical analysis of the recombinant CDK8 subcomplex identifies predicted (TFIIH and RNA polymerase II C-terminal domain [Pol II CTD]) and novel (histone H3, Med13, and CDK8 itself) substrates for the CDK8 kinase. Notably, these novel substrates appear to be metazoan-specific. Such diverse targets imply strict regulation of CDK8 kinase activity. Along these lines, we observe that Mediator itself enables CDK8 kinase activity on chromatin, and we identify Med12--but not Med13--to be essential for activating the CDK8 kinase. Moreover, mass spectrometry analysis of the endogenous CDK8 subcomplex reveals several associated factors, including GCN1L1 and the TRiC chaperonin, that may help control its biological function. In support of this, electron microscopy analysis suggests TRiC sequesters the CDK8 subcomplex and kinase assays reveal the endogenous CDK8 subcomplex--unlike the recombinant submodule--is unable to phosphorylate the Pol II CTD.

Sox9a regulation of ff1a in zebrafish (Danio rerio) suggests an involvement of ff1a in cartilage development

The NR5A family of orphan nuclear receptors has been implicated in development of the vertebrate embryo, but their exact role remains largely unknown. To evaluate the regulation and developmental role for ff1a (NR5A2) in zebrafish (Danio rerio), we performed morpholino knockdown to block translation of the ff1a gene and the upstream located sox9a gene during embryogenesis. Using a newly developed antibody against Ff1a we could show that the ff1a morpholinos were functional and that a reduction in the expression of Ff1a correlated to altered phenotypes. The role of Sox9a in ff1a gene regulation and function was studied using sox9a morpholinos. Knock-down of sox9a resulted in abolished ff1a signals in the somites, mandibular arches and pharyngeal arches, while the pectoral fin signal remained. The reduction in Ff1a levels correlated to truncated tails and cranio-facial malformation. As Sox9a is involved in chondrocyte development we analysed for cartilage formation and found that blocking translation of either sox9a or ff1a also blocked cartilage formation. In light of the results, the present study suggests a novel function of ff1a in chondrocyte development.

Mediator links epigenetic silencing of neuronal gene expression with x-linked mental retardation

Mediator occupies a central role in RNA polymerase II transcription as a sensor, integrator, and processor of regulatory signals that converge on protein-coding gene promoters. Compared to its role in gene activation, little is known regarding the molecular mechanisms and biological implications of Mediator as a transducer of repressive signals. Here we describe a protein interaction network required for extraneuronal gene silencing comprising Mediator, G9a histone methyltransferase, and the RE1 silencing transcription factor (REST; also known as neuron restrictive silencer factor, NRSF). We show that the MED12 interface in Mediator links REST with G9a-dependent histone H3K9 dimethylation to suppress neuronal genes in nonneuronal cells. Notably, missense mutations in MED12 causing the X-linked mental retardation (XLMR) disorders FG syndrome and Lujan syndrome disrupt its REST corepressor function. These findings implicate Mediator in epigenetic restriction of neuronal gene expression to the nervous system and suggest a pathologic basis for MED12-associated XLMR involving impaired REST-dependent neuronal gene regulation.

Pygopus activates Wingless target gene transcription through the mediator complex subunits Med12 and Med13

Wnt target gene transcription is mediated by nuclear translocation of stabilized beta-catenin, which binds to TCF and recruits Pygopus, a cofactor with an unknown mechanism of action. The mediator complex is essential for the transcription of RNA polymerase II-dependent genes; it associates with an accessory subcomplex consisting of the Med12, Med13, Cdk8, and Cyclin C subunits. We show here that the Med12 and Med13 subunits of the Drosophila mediator complex, encoded by kohtalo and skuld, are essential for the transcription of Wingless target genes. kohtalo and skuld act downstream of beta-catenin stabilization both in vivo and in cell culture. They are required for transcriptional activation by the N-terminal domain of Pygopus, and their physical interaction with Pygopus depends on this domain. We propose that Pygopus promotes Wnt target gene transcription by recruiting the mediator complex through interactions with Med12 and Med13.

Control of chondrogenesis by the transcription factor Sox9

Cell-fate determination of pluripotent cells, cell proliferation, differentiation, and maturation, as well as the maintenance of stem cells, are essential cellular events during organogenesis. Previous reports show that some distinct cell-specific transcription factors are the master genes that control cell lineage commitment and the subsequent cell proliferation and differentiation. Some of these transcription factors generate hierarchical regulation of expression and act in concert to fulfill their roles. This review discusses the molecular properties and mechanisms of Sry-related high-mobility-group box (Sox) transcription factor, Sox9, in chondrogenesis.

Multiple roles for Med12 in vertebrate endoderm development

In zebrafish, the endoderm originates at the blastula stage from the most marginal blastomeres. Through a series of complex morphogenetic movements and differentiation events, the endodermal germ layer gives rise to the epithelial lining of the digestive tract as well as its associated organs such as the liver, pancreas, and swim bladder. How endodermal cells differentiate into distinct cell types such as hepatocytes or endocrine and exocrine pancreatic cells remains a major question. In a forward genetic screen for genes regulating endodermal organ development, we identified mutations at the shiri locus that cause defects in the development of a number of endodermal organs including the liver and pancreas. Detailed phenotypic analyses indicate that these defects are partially due to a reduction in endodermal expression of the hairy/enhancer of split-related gene, her5, at mid to late gastrulation stages. Using the Tg(0.7her5:EGFP)(ne2067) line, we show that her5 is expressed in the endodermal precursors that populate the pharyngeal region as well as the organ-forming region. We also find that knocking down her5 recapitulates some of the endodermal phenotypes of shiri mutants, further revealing the role of her5 in endoderm development. Positional cloning reveals that shiri encodes Med12, a regulatory subunit of the transcriptional Mediator complex recently associated with two human syndromes. Additional studies indicate that Med12 modulates the ability of Casanova/Sox32 to induce sox17 expression. Thus, detailed phenotypic analyses of embryos defective in a component of the Mediator complex have revealed new insights into discrete aspects of vertebrate endoderm development, and provide possible explanations for the craniofacial and digestive system defects observed in humans with mutations in MED12.

Role of MED12 in transcription and human behavior

The Mediator complex is a fluid assemblage of approximately 25 proteins that is essential for eukaryotic transcriptional regulation. Mediator of RNA polymerase II transcription (MED)12 (HOPA) is a 25-kb Xq13 member of the Mediator complex that plays a key role in the complex and directly moderates receptor tyrosine kinase, nuclear receptor and Wnt pathway signaling. Sequence variation in two MED12 protein domains has been linked to neuropsychiatric illness. First, variants in the Leu-Ser domain have been linked to Opitz-Kaveggia and Lujan syndromes, which are forms of X-linked mental retardation. Second, a balanced polymorphism in the C terminus opposite-paired domain, a key motif in the MED12-mediated transcriptional repression of Wnt signaling, has been associated with increased risk for psychosis. We conclude that variation of MED12 is associated with a wide variety of clinical presentations whose severity is dependent on the location and nature of the variation, and that a thorough understanding of MED12's role in transcriptional regulation could have significant benefits for human healthcare.

Identification of novel ciliogenesis factors using a new in vivo model for mucociliary epithelial development

Mucociliary epithelia are essential for homeostasis of many organs and consist of mucus-secreting goblet cells and ciliated cells. Here, we present the ciliated epidermis of Xenopus embryos as a facile model system for in vivo molecular studies of mucociliary epithelial development. Using an in situ hybridization-based approach, we identified numerous genes expressed differentially in mucus-secreting cells or in ciliated cells. Focusing on genes expressed in ciliated cells, we have identified new candidate ciliogenesis factors, including several not present in the current ciliome. We find that TTC25-GFP is localized to the base of cilia and to ciliary axonemes, and disruption of TTC25 function disrupts ciliogenesis. Mig12-GFP localizes very strongly to the base of cilia and confocal imaging of this construct allows for simple visualization of the planar polarity of basal bodies that underlies polarized ciliary beating. Knockdown of Mig12 disrupts ciliogenesis. Finally, we show that ciliogenesis factors identified in the Xenopus epidermis are required in the midline to facilitate neural tube closure. These results provide further evidence of a requirement for cilia in neural tube morphogenesis and suggest that genes identified in the Xenopus epidermis play broad roles in ciliogenesis. The suites of genes identified here will provide a foundation for future studies, and may also contribute to our understanding of pathological changes in mucociliary epithelia that accompany diseases such as asthma.

The association of the HOPA(12bp) polymorphism with schizophrenia in the NIMH Genetics Initiative for Schizophrenia sample

HOPA (MED12) is an X-chromosome gene that codes for a critical member of the Mediator Complex, a group of proteins that regulates transcription via the nuclear receptor, Wnt and Receptor Tyrosine Kinase pathways. In prior association and meta-analyses, we have shown that the presence of an evolutionarily conserved, 12 bp (4 amino acid) insertional polymorphism in exon 43 of this gene is associated with increased risk for an endophenotype of schizophrenia. In this communication, we describe the results of our work with subjects and data from the National Institutes of Mental Health (NIMH) Genetics Initiative for Schizophrenia. We report that the presence of the HOPA(12bp) polymorphism is associated with increased risk for schizophrenia in subjects of European ancestry. In the light of this new study and the prior wealth of clinical and basic science data, we conclude that the HOPA(12bp) allele is a risk factor for schizophrenia in subjects of European ancestry and suggest that further studies to define the endophenotype and mechanisms of illness associated with this polymorphism are indicated.

Cdk8 is essential for preimplantation mouse development

The Cdk8 kinase and associated proteins form a nonessential transcriptional repressor module of the Mediator in the budding yeast Saccharomyces cerevisiae. Genetic analyses of this module have demonstrated functions ranging from environmental responses in budding yeast to organogenesis and development in worms, flies, and zebrafish. Here we have investigated the function of mammalian Cdk8 using mice harboring a gene trap insertion at the Cdk8 locus inactivating this kinase. No phenotypes were noted in heterozygote Cdk8+/- mice, but intercrossing these did not produce homozygous Cdk8-/- offspring. Developmental analysis demonstrated a requirement for Cdk8 prior to implantation at embryonic days 2.5 to 3.0. Cdk8-/- preimplantation embryos had fragmented blastomeres and did not proceed to compaction. As Cdk8 deficiency in cultured metazoan cells did not affect cell viability, the results suggest that transcriptional repression of genes critical for early-cell-fate determination underlies the requirement of Cdk8 in embryogenesis.

Early steps of paired fin development in zebrafish compared with tetrapod limb development

The development of zebrafish paired fins and tetrapod forelimbs and hindlimbs show striking similarities at the molecular level. In recent years, the zebrafish, Danio rerio has become a valuable model for the study of the development of vertebrate paired appendages and several large-scale mutagenesis screens have identified novel fin mutants. This review summarizes recent advances in research into zebrafish paired fin development and highlights features that are shared with and distinct from limb development in other main animal models.

A recurrent mutation in MED12 leading to R961W causes Opitz-Kaveggia syndrome

Opitz-Kaveggia syndrome (also known as FG syndrome) is an X-linked disorder characterized by mental retardation, relative macrocephaly, hypotonia and constipation. We report here that the original family for whom the condition is named and five other families have a recurrent mutation (2881C>T, leading to R961W) in MED12 (also called TRAP230 or HOPA), a gene located at Xq13 that functions as a thyroid receptor-associated protein in the Mediator complex.

Depletion of Med10 enhances Wnt and suppresses Nodal signaling during zebrafish embryogenesis

The transcriptional Mediator (MED) is a multiprotein complex that transmits information from transcription factors to RNA polymerase II (PolII) to regulate transcription. At present, the role of distinct MED subunits in general transcription versus transcription stimulated by specific signaling pathways is unclear. By means of positional cloning, we reveal that the zebrafish mutant tennismatch is a hypomorphic allele of Med10, a conserved MED middle domain subunit. Using morpholino antisense oligonucleotides, we further demonstrate that reduction of Med10 levels led to an enhancement of the Wnt signaling pathway, while also suggesting a role for Med10 in mediating the Nodal signaling pathway. In contrast to the dual roles of Med10, reduction of Med12 and Med13 levels, two MED subunits in the regulatory domain, led to an enhancement of the Wnt signaling pathway but not the Nodal pathway, while reduction of Med15 levels, a MED subunit in the tail domain, suppressed the Nodal signaling pathway but not the Wnt signaling pathway. Thus, Med10 appears to be a unique MED subunit that differentially transduces information from distinct signaling pathways during zebrafish embryogenesis.

A subunit of the mediator complex regulates vertebrate neuronal development

The unique profiles of gene expression dictate distinct cellular identity. How these profiles are established during development is not clear. Here we report that the mutant motionless (mot), identified in a genetic screen for mutations that affect neuronal development in zebrafish, displays deficits of monoaminergic neurons and cranial sensory ganglia, whereas expression of the pan-neuronal marker Hu is largely unperturbed; GABAergic and subsets of cranial motor neurons do not appear to be deficient. Positional cloning reveals that mot encodes Med12, a component of the evolutionarily conserved Mediator complex, whose in vivo function is not well understood in vertebrates. mot/med12 transcripts are enriched in the embryonic brain and appear distinct from two other Mediator components Med17 and Med21. Delivery of human med12 RNA into zebrafish restores normality to the mot mutant and, strikingly, leads to premature neuronal differentiation and an increased production of monoaminergic neuronal subtypes in WT. Further investigation reveals that mot/med12 is necessary to regulate, and when overexpressed is capable of increasing, the expression of distinct neuronal determination genes, including zash1a and lim1, and serves as an in vivo cofactor for Sox9 in this process. Together, our analyses reveal a regulatory role of Mot/Med12 in vertebrate neuronal development.

The role of TGFbetas and Sox9 during limb chondrogenesis

The majority of the skeletal elements, except the flat bones of the skull, are formed by endochondral ossification, in which cartilage is replaced by bone. The formation of cartilage is a multi-step process termed chondrogenesis, during which undifferentiated mesenchymal cells condense and undergo differentiation towards chondrocytes. Notwithstanding recent advances, our knowledge of the detailed mechanisms implicated in cartilage and bone formation is still scarce. Recent genetic, cellular and biochemical studies have highlighted the importance of TGFbeta signaling and the activity of the transcription factor Sox9 during the early stages of vertebrate limb chondrogenesis.