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

In vitro elastic cartilage reconstruction using human auricular perichondrial chondroprogenitor cell-derived micro 3D spheroids

Morphologically stable scaffold-free elastic cartilage tissue is crucial for treating external ear abnormalities. However, establishing adequate mechanical strength is challenging, owing to the difficulty of achieving chondrogenic differentiation in vitro; thus, cartilage reconstruction is a complex task. Auricular perichondrial chondroprogenitor cells exhibit high proliferation potential and can be obtained with minimal invasion. Therefore, these cells are an ideal resource for elastic cartilage reconstruction. In this study, we aimed to develop a novel in vitro scaffold-free method for elastic cartilage reconstruction, using human auricular perichondrial chondroprogenitor cells. Inducing chondrogenesis by using microscopic spheroids similar to auricular hillocks significantly increased the chondrogenic potential. The size and elasticity of the tissue were maintained after craniofacial transplantation in immunodeficient mice, suggesting that the reconstructed tissue was morphologically stable. Our novel tissue reconstruction method may facilitate the development of future treatments for external ear abnormalities.

MED12-Related (Neuro)Developmental Disorders: A Question of Causality

MED12 is a member of the Mediator complex that is involved in the regulation of transcription. Missense variants in MED12 cause FG syndrome, Lujan-Fryns syndrome, and Ohdo syndrome, as well as non-syndromic intellectual disability (ID) in hemizygous males. Recently, female patients with de novo missense variants and de novo protein truncating variants in MED12 were described, resulting in a clinical spectrum centered around ID and Hardikar syndrome without ID. The missense variants are found throughout MED12, whether they are inherited in hemizygous males or de novo in females. They can result in syndromic or nonsyndromic ID. The de novo nonsense variants resulting in Hardikar syndrome that is characterized by facial clefting, pigmentary retinopathy, biliary anomalies, and intestinal malrotation, are found more N-terminally, whereas the more C-terminally positioned variants are de novo protein truncating variants that cause a severe, syndromic phenotype consisting of ID, facial dysmorphism, short stature, skeletal abnormalities, feeding difficulties, and variable other abnormalities. This broad range of distinct phenotypes calls for a method to distinguish between pathogenic and non-pathogenic variants in MED12. We propose an isogenic iNeuron model to establish the unique gene expression patterns that are associated with the specific MED12 variants. The discovery of these patterns would help in future diagnostics and determine the causality of the MED12 variants.

Somatic and de novo Germline Variants of MEDs in Human Neural Tube Defects

BACKGROUND

Neural tube defects (NTDs) are among the most common and severe congenital defects in humans. Their genetic etiology is complex and remains poorly understood. The Mediator complex (MED) plays a vital role in neural tube development in animal models. However, no studies have yet examined the role of its human homolog in the etiology of NTDs.

METHODS

In this study, 48 pairs of neural lesion site and umbilical cord tissues from NTD and 21 case-parent trios were involved in screening for NTD-related somatic and germline de novo variants. A series of functional cell assays were performed. We generated a Med12 p.Arg1784Cys knock-in mouse using CRISPR/Cas9 technology to validate the human findings.

RESULTS

One somatic variant, MED12 p.Arg1782Cys, was identified in the lesion site tissue from an NTD fetus. This variant was absent in any other normal tissue from different germ layers of the same case. In 21 case-parent trios, one de novo stop-gain variant, MED13L p.Arg1760∗, was identified. Cellular functional studies showed that MED12 p.Arg1782Cys decreased MED12 protein level and affected the regulation of MED12 on the canonical-WNT signaling pathway. The Med12 p.Arg1784Cys knock-in mouse exhibited exencephaly and spina bifida.

CONCLUSION

These findings provide strong evidence that functional variants of MED genes are associated with the etiology of some NTDs. We demonstrated a potentially important role for somatic variants in the occurrence of NTDs. Our study is the first study in which an NTD-related variant identified in humans was validated in mice using CRISPR/Cas9 technology.

Potential roles of mediator Complex Subunit 13 in Cardiac Diseases

Mediator complex subunit 13 (MED13, previously known as THRAP1 and TRAP240) is a subunit of the cyclin-dependent kinase 8 (CDK8) kinase module in the eukaryotic mediator complex. MED13 has been known to play critical roles in cell cycle, development, and growth. The purpose of this review is to comprehensively discuss its newly identified potential roles in myocardial energy metabolism and non-metabolic cardiovascular diseases. Evidence indicates that cardiac MED13 mainly participates in the regulation of nuclear receptor signaling, which drives the transcription of genes involved in modulating cardiac and systemic energy homeostasis. MED13 is also associated with several pathological conditions, such as metabolic syndrome and thyroid disease-associated heart failure. Therefore, MED13 constitutes a potential therapeutic target for the regulation of metabolic disorders and other cardiovascular diseases.

Med23 Regulates Sox9 Expression during Craniofacial Development

The etiology and pathogenesis of craniofacial birth defects are multifactorial and include both genetic and environmental factors. Despite the identification of numerous genes associated with congenital craniofacial anomalies, our understanding of their etiology remains incomplete, and many affected individuals have an unknown genetic diagnosis. Here, we show that conditional loss of a Mediator complex subunit protein, Med23 in mouse neural crest cells (Med23^fx/fx;Wnt1-Cre), results in micrognathia, glossoptosis, and cleft palate, mimicking the phenotype of Pierre Robin sequence. Sox9 messenger RNA and protein levels are both upregulated in neural crest cell-derived mesenchyme surrounding Meckel's cartilage and in the palatal shelves in Med23^fx/fx;Wnt1-Cre mutant embryos compared to controls. Consistent with these observations, we demonstrate that Med23 binds to the promoter region of Sox9 and represses Sox9 expression in vitro. Interestingly, Sox9 binding to β-catenin is enhanced in Med23^fx/fx;Wnt1-Cre mutant embryos, which, together with downregulation of Col2a1 and Wnt signaling target genes, results in decreased proliferation and altered jaw skeletal differentiation and cleft palate. Altogether, our data support a cell-autonomous requirement for Med23 in neural crest cells, potentially linking the global transcription machinery through Med23 to the etiology and pathogenesis of craniofacial anomalies such as micrognathia and cleft palate.

What is the impact of a novel MED12 variant on syndromic conotruncal heart defects? Analysis of case report on two male sibs

Background

Syndromic congenital heart disease accounts for 30% of cases and can be determined by genetic, environmental or multifactorial causes. In many cases the etiology remains uncertain. Many known genes are responsible for specific morphopathogenetic mechanisms during the development of the heart whose alteration can determine specific phenotypes of cardiac malformations.

Case presentation

We report on two cases of association of conotruncal heart defect with facial dysmorphisms in sibs. In both cases the malformations’ identification occurred by ultrasound in the prenatal period. It was followed by prenatal invasive diagnosis. The genetic analysis revealed no rearrangements in Array-CGH test, while gene panel sequencing identified a new hemizygous variant of uncertain significance (c.887G > A; p.Arg296Gln) in the MED12 gene, located on the X chromosome and inherited from the healthy mother.

Conclusion

No other reports about the involvement of MED12 gene in syndromic conotruncal heart defects are actually available from the literature and the international genomic databases. This novel variant is a likely pathogenic variant of uncertain significance and it could broaden the spectrum of genes involved in the development of congenital heart diseases and the phenotypic range of MED12-related disorders.

The Effects of Age and Cell Isolation on Collagen II Synthesis by Articular Chondrocytes: Evidence for Transcriptional and Posttranscriptional Regulation

Adult articular cartilage synthesises very little type II collagen in comparison to young cartilage. The age-related difference in collagen II synthesis is poorly understood. This is the first systematic investigation of age-related differences in extracellular matrix synthesis in fresh articular cartilage and following isolation of chondrocytes. A histological comparison of 3-year-old skeletally mature and 6-month-old juvenile porcine cartilage was made. Differences in collagen II, aggrecan, and Sox5, 6, and 9 mRNA and protein expression and mRNA stability were measured. Adult cartilage was found to be thinner than juvenile cartilage but with similar chondrocyte density. Procollagen α1(II) and Sox9 mRNA levels were 10-fold and 3-fold reduced in adult cartilage. Sox9 protein was halved and collagen II protein synthesis was almost undetectable and calculated to be at least 30-fold reduced. Aggrecan expression did not differ. Isolation of chondrocytes caused a drop in procollagen α1(II) and Sox9 mRNA in both adult and juvenile cells along with a marked reduction in Sox9 mRNA stability. Interestingly, juvenile chondrocytes continued to synthesise collagen II protein with mRNA levels similar to those seen in adult articular cartilage. Age-related differences in collagen II protein synthesis are due to both transcriptional and posttranscription regulation. A better understanding of these regulatory mechanisms would be an important step in improving current cartilage regeneration techniques.

The SOXE transcription factors-SOX8, SOX9 and SOX10-share a bi-partite transactivation mechanism

SOX8, SOX9 and SOX10 compose the SOXE transcription factor group. They govern cell fate and differentiation in many lineages, and mutations impairing their activity cause severe diseases, including campomelic dysplasia (SOX9), sex determination disorders (SOX8 and SOX9) and Waardenburg-Shah syndrome (SOX10). However, incomplete knowledge of their modes of action limits disease understanding. We here uncover that the proteins share a bipartite transactivation mechanism, whereby a transactivation domain in the middle of the proteins (TAM) synergizes with a C-terminal one (TAC). TAM comprises amphipathic α-helices predicted to form a protein-binding pocket and overlapping with minimal transactivation motifs (9-aa-TAD) described in many transcription factors. One 9-aa-TAD sequence includes an evolutionarily conserved and functionally required EΦ[D/E]QYΦ motif. SOXF proteins (SOX7, SOX17 and SOX18) contain an identical motif, suggesting evolution from a common ancestor already harboring this motif, whereas TAC and other transactivating SOX proteins feature only remotely related motifs. Missense variants in this SOXE/SOXF-specific motif are rare in control individuals, but have been detected in cancers, supporting its importance in development and physiology. By deepening understanding of mechanisms underlying the central transactivation function of SOXE proteins, these findings should help further decipher molecular networks essential for development and health and dysregulated in diseases.

Molecular and Phylogenetic Analyses of the Mediator Subunit Genes in Solanum lycopersicum

The Mediator complex is a multi-subunit protein assembly that serves as a central scaffold to help regulate DNA-binding transcription factors (TFs) and RNA polymerase II (Pol II) activity controlled gene expression programmed in response to developmental or environmental factors. However, litter information about Mediator complex subunit (MED) genes in tomato is available, although it is an essential model plant. In this study, we retrieved 46 candidate SlMED genes from the genome of tomato, and a comprehensive analysis was conducted, including their phylogenetic relationship, chromosomal locations, gene structure, cis-regulatory elements prediction, as well as gene expression. The expression profiling of 46 SlMED genes was analyzed using publicly available RNA-seq data. Furthermore, we selected some SlMED genes to evaluate their expression patterns in various tissues and under different abiotic stress treatments by quantitative reverse transcription PCR experiments. This is the first detailed report to elucidate the molecular and phylogenetic features of the MED genes in tomato, and it provides valuable clues for further functional analysis in order to clarify the role of the SlMED genes in diverse plant growth, development and abiotic stress response.

Mediator complex subunit Med12 regulates cardiac jelly development and AV valve formation in zebrafish

The molecular mechanism essential for the formation of heart valves involves complex interactions of signaling molecules and transcription factors. The Mediator Complex (MC) functions as multi-subunit machinery to orchestrate gene transcription, especially for tissue-specific fine-tuning of transcriptional processes during development, also in the heart. Here, we analyzed the role of the MC subunit Med12 during atrioventricular canal (AVC) development and endocardial cushion formation, using the Med12-deficient zebrafish mutant trapped (tpd). Whereas primary heart formation was only slightly affected in tpd, we identified defects in AVC development and cardiac jelly formation. We found that although misexpression of bmp4 and versican in tpd hearts can be restored by overexpression of a modified version of the Sox9b transcription factor (harboring VP16 transactivation domain) that functions independent of its co-activator Med12, endocardial cushion development in tpd was not reconstituted. Interestingly, expression of tbx2b and its target hyaluronan synthase 2 (has2) - the synthase of hyaluronan (HA) in the heart - was absent in both uninjected and Sox9b-VP16 overexpressing tpd hearts. HA is a major ECM component of the cardiac jelly and required for endocardial cushion formation. Furthermore, we found secreted phosphoprotein 1 (spp1), an endocardial marker of activated AV endocardial cells, completely absent in tpd hearts, suggesting that crucial steps of the transformation of AV endocardial cells into endocardial cushions is blocked. We demonstrate that Med12 controls cardiac jelly formation Sox9-independently by regulating tbx2b and has2 expression and therefore the production of the glycosaminoglycan HA at the AVC to guarantee proper endocardial cushion development.

MED12-related XLID disorders are dose-dependent of immediate early genes (IEGs) expression

Mediator occupies a key role in protein coding genes expression in mediating the contacts between gene specific factors and the basal transcription machinery but little is known regarding the role of each Mediator subunits. Mutations in MED12 are linked with a broad spectrum of genetic disorders with X-linked intellectual disability that are difficult to range as Lujan, Opitz-Kaveggia or Ohdo syndromes. Here, we investigated several MED12 patients mutations (p.R206Q, p.N898D, p.R961W, p.N1007S, p.R1148H, p.S1165P and p.R1295H) and show that each MED12 mutations cause specific expression patterns of JUN, FOS and EGR1 immediate early genes (IEGs), reflected by the presence or absence of MED12 containing complex at their respective promoters. Moreover, the effect of MED12 mutations has cell-type specificity on IEG expression. As a consequence, the expression of late responsive genes such as the matrix metalloproteinase-3 and the RE1 silencing transcription factor implicated respectively in neural plasticity and the specific expression of neuronal genes is disturbed as documented for MED12/p.R1295H mutation. In such case, JUN and FOS failed to be properly recruited at their AP1-binding site. Our results suggest that the differences between MED12-related phenotypes are essentially the result of distinct IEGs expression patterns, the later ones depending on the accurate formation of the transcription initiation complex. This might challenge clinicians to rethink the traditional syndromes boundaries and to include genetic criterion in patients' diagnostic.

Glia-specific enhancers and chromatin structure regulate NFIA expression and glioma tumorigenesis

Long-range enhancer interactions critically regulate gene expression, yet little is known about how their coordinated activities contribute to CNS development or how this may, in turn, relate to disease states. By examining the regulation of the transcription factor NFIA in the developing spinal cord, we identified long-range enhancers that recapitulate NFIA expression across glial and neuronal lineages in vivo. Complementary genetic studies found that Sox9-Brn2 and Isl1-Lhx3 regulate enhancer activity and NFIA expression in glial and neuronal populations. Chromatin conformation analysis revealed that these enhancers and transcription factors form distinct architectures within these lineages in the spinal cord. In glioma models, the glia-specific architecture is present in tumors, and these enhancers are required for NFIA expression and contribute to glioma formation. By delineating three-dimensional mechanisms of gene expression regulation, our studies identify lineage-specific chromatin architectures and associated enhancers that regulate cell fate and tumorigenesis in the CNS.

The MEDIATOR genes MED12 and MED13 control Arabidopsis root system configuration influencing sugar and auxin responses

Arabidopsis med12 and med13 mutants exhibit shoot and root phenotypes related to an altered auxin homeostasis. Sucrose supplementation reactivates both cell division and elongation in primary roots as well as auxin-responsive and stem cell niche gene expression in these mutants. An analysis of primary root growth of WT, med12, aux1-7 and med12 aux1 single and double mutants in response to sucrose and/or N-1-naphthylphthalamic acid (NPA) placed MED12 upstream of auxin transport for the sugar modulation of root growth. The MEDIATOR (MED) complex plays diverse functions in plant development, hormone signaling and biotic and abiotic stress tolerance through coordination of transcription. Here, we performed genetic, developmental, molecular and pharmacological analyses to characterize the role of MED12 and MED13 on the configuration of root architecture and its relationship with auxin and sugar responses. Arabidopsis med12 and med13 single mutants exhibit shoot and root phenotypes consistent with altered auxin homeostasis including altered primary root growth, lateral root development, and root hair elongation. MED12 and MED13 were required for activation of cell division and elongation in primary roots, as well as auxin-responsive and stem cell niche gene expression. Remarkably, most of these mutant phenotypes were rescued by supplying sucrose to the growth medium. The growth response of primary roots of WT, med12, aux1-7 and med12 aux1 single and double mutants to sucrose and application of auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) revealed the correlation of med12 phenotype with the activity of the auxin intake permease and suggests that MED12 acts upstream of AUX1 in the root growth response to sugar. These data provide compelling evidence that MEDIATOR links sugar sensing to auxin transport and distribution during root morphogenesis.

MED12 regulates a transcriptional network of calcium-handling genes in the heart

The Mediator complex regulates gene transcription by linking basal transcriptional machinery with DNA-bound transcription factors. The activity of the Mediator complex is mainly controlled by a kinase submodule that is composed of 4 proteins, including MED12. Although ubiquitously expressed, Mediator subunits can differentially regulate gene expression in a tissue-specific manner. Here, we report that MED12 is required for normal cardiac function, such that mice with conditional cardiac-specific deletion of MED12 display progressive dilated cardiomyopathy. Loss of MED12 perturbs expression of calcium-handling genes in the heart, consequently altering calcium cycling in cardiomyocytes and disrupting cardiac electrical activity. We identified transcription factors that regulate expression of calcium-handling genes that are downregulated in the heart in the absence of MED12, and we found that MED12 localizes to transcription factor consensus sequences within calcium-handling genes. We showed that MED12 interacts with one such transcription factor, MEF2, in cardiomyocytes and that MED12 and MEF2 co-occupy promoters of calcium-handling genes. Furthermore, we demonstrated that MED12 enhances MEF2 transcriptional activity and that overexpression of both increases expression of calcium-handling genes in cardiomyocytes. Our data support a role for MED12 as a coordinator of transcription through MEF2 and other transcription factors. We conclude that MED12 is a regulator of a network of calcium-handling genes, consequently mediating contractility in the mammalian heart.

Mediator: A key regulator of plant development

Mediator is a multiprotein complex that regulates transcription at the level of RNA pol II assembly, as well as through regulation of chromatin architecture, RNA processing and recruitment of epigenetic marks. Though its modular structure is conserved in eukaryotes, its subunit composition has diverged during evolution and varies in response to environmental and tissue-specific inputs, suggesting different functions for each subunit and/or Mediator conformation. In animals, Mediator has been implicated in the control of differentiation and morphogenesis through modulation of numerous signaling pathways. In plants, studies have revealed roles for Mediator in regulation of cell division, cell fate and organogenesis, as well as developmental timing and hormone responses. We begin this review with an overview of biochemical mechanisms of yeast and animal Mediator that are likely to be conserved in all eukaryotes, as well as a brief discussion of the role of Mediator in animal development. We then present a comprehensive review of studies of the role of Mediator in plant development. Finally, we point to important questions for future research on the role of Mediator as a master coordinator of development.

Mediator subunit Med12 contributes to the maintenance of neural stem cell identity

BACKGROUND

The RNA polymerase II transcriptional Mediator subunit Med12 is broadly implicated in vertebrate brain development, and genetic variation in human MED12 is associated with X-linked intellectual disability and neuropsychiatric disorders. Although prior studies have begun to elaborate the functional contribution of Med12 within key neurodevelopmental pathways, a more complete description of Med12 function in the developing nervous system, including the specific biological networks and cellular processes under its regulatory influence, remains to be established. Herein, we sought to clarify the global contribution of Med12 to neural stem cell (NSC) biology through unbiased transcriptome profiling of mouse embryonic stem (ES) cell-derived NSCs following RNAi-mediated Med12 depletion.

RESULTS

A total of 240 genes (177 up, 73 down) were differentially expressed in Med12-knockdown versus control mouse NS-5 (mNS-5) NSCs. Gene set enrichment analysis revealed Med12 to be prominently linked with "cell-to-cell interaction" and "cell cycle" networks, and subsequent functional studies confirmed these associations. Targeted depletion of Med12 led to enhanced NSC adhesion and upregulation of cell adhesion genes, including Syndecan 2 (Sdc2). Concomitant depletion of both Sdc2 and Med12 reversed enhanced cell adhesion triggered by Med12 knockdown alone, confirming that Med12 negatively regulates NSC cell adhesion by suppressing the expression of cell adhesion molecules. Med12-mediated suppression of NSC adhesion is a dynamically regulated process in vitro, enforced in self-renewing NSCs and alleviated during the course of neuronal differentiation. Accordingly, Med12 depletion enhanced adhesion and prolonged survival of mNS-5 NSCs induced to differentiate on gelatin, effects that were bypassed completely by growth on laminin. On the other hand, Med12 depletion in mNS-5 NSCs led to reduced expression of G1/S phase cell cycle regulators and a concordant G1/S phase cell cycle block without evidence of apoptosis, resulting in a severe proliferation defect.

CONCLUSIONS

Med12 contributes to the maintenance of NSC identity through a functionally bipartite role in suppression and activation of gene expression programs dedicated to cell adhesion and G1/S phase cell cycle progression, respectively. Med12 may thus contribute to the regulatory apparatus that controls the balance between NSC self-renewal and differentiation, with important implications for MED12-linked neurodevelopmental disorders.

Using mouse and zebrafish models to understand the etiology of developmental defects in Cornelia de Lange Syndrome

Cornelia de Lange Syndrome (CdLS) is a multisystem birth defects disorder that affects every tissue and organ system in the body. Understanding the factors that contribute to the origins, prevalence, and severity of these developmental defects provides the most direct approach for developing screens and potential treatments for individuals with CdLS. Since the majority of cases of CdLS are caused by haploinsufficiency for NIPBL (Nipped-B-like, which encodes a cohesin-associated protein), we have developed mouse and zebrafish models of CdLS by using molecular genetic tools to create Nipbl-deficient mice and zebrafish (Nipbl(+/-) mice, zebrafish nipbl morphants). Studies of these vertebrate animal models have yielded novel insights into the developmental etiology and genes/gene pathways that contribute to CdLS-associated birth defects, particularly defects of the gut, heart, craniofacial structures, nervous system, and limbs. Studies of these mouse and zebrafish CdLS models have helped clarify how deficiency for NIPBL, a protein that associates with cohesin and other transcriptional regulators in the nucleus, affects processes important to the emergence of the structural and physiological birth defects observed in CdLS: NIPBL exerts chromosome position-specific effects on gene expression; it influences long-range interactions between different regulatory elements of genes; and it regulates combinatorial and synergistic actions of genes in developing tissues. Our current understanding is that CdLS should be considered as not only a cohesinopathy, but also a "transcriptomopathy," that is, a disease whose underlying etiology is the global dysregulation of gene expression throughout the organism. © 2016 Wiley Periodicals, Inc.

Mediator MED23 cooperates with RUNX2 to drive osteoblast differentiation and bone development

How lineage specifiers are regulated during development is an outstanding question, and the molecular regulation of osteogenic factor RUNX2 remains to be fully understood. Here we report that the Mediator subunit MED23 cooperates with RUNX2 to regulate osteoblast differentiation and bone development. Med23 deletion in mesenchymal stem cells or osteoblast precursors results in multiple bone defects similar to those observed in Runx2(+/-) mice. In vitro, Med23-deficient progenitor cells are refractory to osteoblast differentiation, and Med23 deficiency reduces Runx2-target gene activity without changing Runx2 expression. Mechanistically, MED23 binds to RUNX2 and modulates its transcriptional activity. Moreover, Med23 deficiency in osteoprogenitor cells exacerbates the skeletal abnormalities observed in Runx2(+/-) mice. Collectively, our results establish a genetic and physical interaction between RUNX2 and MED23, suggesting that MED23 constitutes a molecular node in the regulatory network of anabolic bone formation and related diseases.

MED12 methylation by CARM1 sensitizes human breast cancer cells to chemotherapy drugs

The RNA polymerase II mediator complex subunit 12 (MED12) is frequently mutated in human cancers, and loss of MED12 has been shown to induce drug resistance through activation of transforming growth factor-β receptor (TGF-βR) signaling. We identified MED12 as a substrate for coactivator-associated arginine methyltransferase 1 (CARM1). Not only are the expression levels of CARM1 and MED12 positively correlated, but their high expression also predicts better prognosis in human breast cancers after chemotherapy. MED12 was methylated at R1862 and R1912 by CARM1, and mutation of these sites in cell lines resulted in resistance to chemotherapy drugs. Furthermore, we showed that the methylation-dependent drug response mechanism is distinct from activation of TGF-βR signaling, because methylated MED12 potently suppresses p21/WAF1 transcription. Cells defective in MED12 methylation have up-regulated p21 protein, which correlates with poor prognosis in breast cancer patients treated with chemotherapy. Collectively, this study identifies MED12 methylation as a sensor for predicting response to commonly used chemotherapy drugs in human cancers.

Differential expression of the aryl hydrocarbon receptor pathway associates with craniofacial polymorphism in sympatric Arctic charr

Background

The developmental basis of craniofacial morphology hinges on interactions of numerous signalling systems. Extensive craniofacial variation in the polymorphic Arctic charr, a member of the salmonid family, from Lake Thingvallavatn (Iceland), offers opportunities to find and study such signalling pathways and their key regulators, thereby shedding light on the developmental pathways, and the genetics of trophic divergence.

Results

To identify genes involved in the craniofacial differences between benthic and limnetic Arctic charr, we used transcriptome data from different morphs, spanning early development, together with data on craniofacial expression patterns and skeletogenesis in model vertebrate species. Out of 20 genes identified, 7 showed lower gene expression in benthic than in limnetic charr morphs. We had previously identified a conserved gene network involved in extracellular matrix (ECM) organization and skeletogenesis, showing higher expression in developing craniofacial elements of benthic than in limnetic Arctic charr morphs. The present study adds a second set of genes constituting an expanded gene network with strong, benthic–limnetic differential expression. To identify putative upstream regulators, we performed knowledge-based motif enrichment analyses on the regulatory sequences of the identified genes which yielded potential binding sites for a set of known transcription factors (TFs). Of the 8 TFs that we examined using qPCR, two (Ahr2b and Ap2) were found to be differentially expressed between benthic and limnetic charr. Expression analysis of several known AhR targets indicated higher activity of the AhR pathway during craniofacial development in benthic charr morphotypes.

Conclusion

These results suggest a key role of the aryl hydrocarbon receptor (AhR) pathway in the observed craniofacial differences between distinct charr morphotypes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0022-6) contains supplementary material, which is available to authorized users.

Mediator kinase module and human tumorigenesis

Mediator is a conserved multi-subunit signal processor through which regulatory informatiosn conveyed by gene-specific transcription factors is transduced to RNA Polymerase II (Pol II). In humans, MED13, MED12, CDK8 and Cyclin C (CycC) comprise a four-subunit "kinase" module that exists in variable association with a 26-subunit Mediator core. Genetic and biochemical studies have established the Mediator kinase module as a major ingress of developmental and oncogenic signaling through Mediator, and much of its function in signal-dependent gene regulation derives from its resident CDK8 kinase activity. For example, CDK8-targeted substrate phosphorylation impacts transcription factor half-life, Pol II activity and chromatin chemistry and functional status. Recent structural and biochemical studies have revealed a precise network of physical and functional subunit interactions required for proper kinase module activity. Accordingly, pathologic change in this activity through altered expression or mutation of constituent kinase module subunits can have profound consequences for altered signaling and tumor formation. Herein, we review the structural organization, biological function and oncogenic potential of the Mediator kinase module. We focus principally on tumor-associated alterations in kinase module subunits for which mechanistic relationships as opposed to strictly correlative associations are established. These considerations point to an emerging picture of the Mediator kinase module as an oncogenic unit, one in which pathogenic activation/deactivation through component change drives tumor formation through perturbation of signal-dependent gene regulation. It follows that therapeutic strategies to combat CDK8-driven tumors will involve targeted modulation of CDK8 activity or pharmacologic manipulation of dysregulated CDK8-dependent signaling pathways.

Transcription factors Sox10 and Sox2 functionally interact with positive transcription elongation factor b in Schwann cells

Sox proteins are mechanistically versatile regulators with established relevance to different developmental processes and crucial impact on chromatin structure, DNA conformation, and transcriptional initiation. Here, we show that Sox2 and Sox10, two Sox proteins important for Schwann cell development, also have the capability to activate transcriptional elongation in a Schwann cell line by recruiting the positive transcription elongation factor b. Recruitment is mediated by physical interaction between the carboxyterminal transactivation domains of the two Sox proteins and the Cyclin T1 subunit of positive transcription elongation factor b, with interaction interfaces for the two Sox proteins being mapped to adjacent regions of the central part of Cyclin T1. Supporting the relevance of this interaction to Schwann cell development, transcription of myelin genes appears regulated at the level of elongation. Our results thus add a new facet to the activity of Sox proteins and expand the functional repertoire of this important group of developmental regulators. Sox transcription factors are important regulators of nervous system development. While they are known to regulate transcription by recruiting and stabilizing the RNA polymerase II preinitiation complex directly or with help of the Mediator complex, this study provides evidence that Sox10 and Sox2 additionally influence transcription in glial cells at the elongation stage by recruiting P-TEFb. Cdk9, cyclin-dependent kinase 9; P-TEFb, positive transcription elongation factor b; Pol II, RNA polymerase II; Sox, Sox2 or Sox10 protein.

Nipbl and mediator cooperatively regulate gene expression to control limb development

Haploinsufficiency for Nipbl, a cohesin loading protein, causes Cornelia de Lange Syndrome (CdLS), the most common “cohesinopathy”. It has been proposed that the effects of Nipbl-haploinsufficiency result from disruption of long-range communication between DNA elements. Here we use zebrafish and mouse models of CdLS to examine how transcriptional changes caused by Nipbl deficiency give rise to limb defects, a common condition in individuals with CdLS. In the zebrafish pectoral fin (forelimb), knockdown of Nipbl expression led to size reductions and patterning defects that were preceded by dysregulated expression of key early limb development genes, including fgfs, shha, hand2 and multiple hox genes. In limb buds of Nipbl-haploinsufficient mice, transcriptome analysis revealed many similar gene expression changes, as well as altered expression of additional classes of genes that play roles in limb development. In both species, the pattern of dysregulation of hox-gene expression depended on genomic location within the Hox clusters. In view of studies suggesting that Nipbl colocalizes with the mediator complex, which facilitates enhancer-promoter communication, we also examined zebrafish deficient for the Med12 Mediator subunit, and found they resembled Nipbl-deficient fish in both morphology and gene expression. Moreover, combined partial reduction of both Nipbl and Med12 had a strongly synergistic effect, consistent with both molecules acting in a common pathway. In addition, three-dimensional fluorescent in situ hybridization revealed that Nipbl and Med12 are required to bring regions containing long-range enhancers into close proximity with the zebrafish hoxda cluster. These data demonstrate a crucial role for Nipbl in limb development, and support the view that its actions on multiple gene pathways result from its influence, together with Mediator, on regulation of long-range chromosomal interactions.

Limb malformations are a striking feature of Cornelia de Lange Syndrome (CdLS), a multi-system birth defects disorder most commonly caused by haploinsufficiency for NIPBL. In addition to its role as a cohesin-loading factor, Nipbl also regulates gene expression, but how partial Nipbl deficiency causes limb defects is unknown. Using zebrafish and mouse models, we show that expression of multiple key regulators of early limb development, including shha, hand2 and hox genes, are sensitive to Nipbl deficiency. Furthermore, we find morphological and gene expression abnormalities similar to those of Nipbl-deficient zebrafish in the limb buds of zebrafish deficient for the Med12 subunit of Mediator—a protein complex that mediates physical interactions between enhancers and promoters—and genetic interaction studies support the view that Mediator and Nipbl act together. Strikingly, depletion of either Nipbl or Med12 leads to characteristic changes in hox gene expression that reflect the locations of genes within their chromosomal clusters, as well as to disruption of large-scale chromosome organization around the hoxda cluster, consistent with impairment of long-range enhancer-promoter interaction. Together, these findings provide insights into both the etiology of limb defects in CdLS, and the mechanisms by which Nipbl and Mediator influence gene expression.

The Mediator complex: a master coordinator of transcription and cell lineage development

Mediator is a multiprotein complex that is required for gene transcription by RNA polymerase II. Multiple subunits of the complex show specificity in relaying information from signals and transcription factors to the RNA polymerase II machinery, thus enabling control of the expression of specific genes. Recent studies have also provided novel mechanistic insights into the roles of Mediator in epigenetic regulation, transcriptional elongation, termination, mRNA processing, noncoding RNA activation and super enhancer formation. Based on these specific roles in gene regulation, Mediator has emerged as a master coordinator of development and cell lineage determination. Here, we describe the most recent advances in understanding the mechanisms of Mediator function, with an emphasis on its role during development and disease.

The Mediator complex and transcription regulation

The Mediator complex is a multi-subunit assembly that appears to be required for regulating expression of most RNA polymerase II (pol II) transcripts, which include protein-coding and most non-coding RNA genes. Mediator and pol II function within the pre-initiation complex (PIC), which consists of Mediator, pol II, TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH and is approximately 4.0 MDa in size. Mediator serves as a central scaffold within the PIC and helps regulate pol II activity in ways that remain poorly understood. Mediator is also generally targeted by sequence-specific, DNA-binding transcription factors (TFs) that work to control gene expression programs in response to developmental or environmental cues. At a basic level, Mediator functions by relaying signals from TFs directly to the pol II enzyme, thereby facilitating TF-dependent regulation of gene expression. Thus, Mediator is essential for converting biological inputs (communicated by TFs) to physiological responses (via changes in gene expression). In this review, we summarize an expansive body of research on the Mediator complex, with an emphasis on yeast and mammalian complexes. We focus on the basics that underlie Mediator function, such as its structure and subunit composition, and describe its broad regulatory influence on gene expression, ranging from chromatin architecture to transcription initiation and elongation, to mRNA processing. We also describe factors that influence Mediator structure and activity, including TFs, non-coding RNAs and the CDK8 module.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

The polycomb group protein ring1b/rnf2 is specifically required for craniofacial development

Polycomb group (PcG) genes are chromatin modifiers that mediate epigenetic silencing of target genes. PcG-mediated epigenetic silencing is implicated in embryonic development, stem cell plasticity, cell fate maintenance, cellular differentiation and cancer. However, analysis of the roles of PcG proteins in maintaining differentiation programs during vertebrate embryogenesis has been hampered due to the early embryonic lethality of several PcG knock-outs in the mouse. Here, we show that zebrafish Ring1b/Rnf2, the single E3 ubiquitin ligase in the Polycomb Repressive Complex 1, critically regulates the developmental program of craniofacial cell lineages. Zebrafish ring1b mutants display a severe craniofacial phenotype, which includes an almost complete absence of all cranial cartilage, bone and musculature. We show that Cranial Neural Crest (CNC)-derived cartilage precursors migrate correctly into the pharyngeal arches, but fail to differentiate into chondrocytes. This phenotype is specific for cartilage precursors, since other neural crest-derived cell lineages, including glia, neurons and chromatophores, are formed normally in ring1b mutants. Our results therefore reveal a critical and specific role for Ring1b in promoting the differentiation of cranial neural crest cells into chondrocytes. The molecular mechanisms underlying the pathogenesis of craniofacial abnormalities, which are among the most common genetic birth defects in humans, remain poorly understood. The zebrafish ring1b mutant provides a molecular model for investigating these mechanisms and may lead to the discovery of new treatments or preventions of craniofacial abnormalities.

Sox10 cooperates with the mediator subunit 12 during terminal differentiation of myelinating glia

Several transcription factors are essential for terminal differentiation of myelinating glia, among them the high-mobility-group-domain-containing protein Sox10. To better understand how these factors exert their effects and shape glial expression programs, we identified and characterized a physical and functional link between Sox10 and the Med12 subunit of the Mediator complex that serves as a conserved multiprotein interphase between transcription factors and the general transcription machinery. We found that Sox10 bound with two of its conserved domains to the C-terminal region of Med12 and its close relative, Med12-like. In contrast to Med12-like, substantial amounts of Med12 were detected in both Schwann cells and oligodendrocytes. Its conditional glia-specific deletion in mice led to terminal differentiation defects that were highly reminiscent of those obtained after Sox10 deletion. In support of a functional cooperation, both proteins were jointly required for Krox20 induction and were physically associated with the critical regulatory region of the Krox20 gene in myelinating Schwann cells. We conclude that Sox10 functions during terminal differentiation of myelinating glia, at least in part by Med12-dependent recruitment of the Mediator complex.

The C. elegans CDK8 Mediator module regulates axon guidance decisions in the ventral nerve cord and during dorsal axon navigation

Receptors expressed on the growth cone of outgrowing axons detect cues required for proper navigation. The pathway choices available to an axon are in part defined by the set of guidance receptors present on the growth cone. Regulated expression of receptors and genes controlling the localization and activity of receptors ensures that axons respond only to guidance cues relevant for reaching their targets. In genetic screens for axon guidance mutants, we isolated an allele of let-19/mdt-13, a component of the Mediator, a large ~30 subunit protein complex essential for gene transcription by RNA polymerase II. LET-19/MDT-13 is part of the CDK8 module of the Mediator. By testing other Mediator components, we found that all subunits of the CDK8 module as well as some other Mediator components are required for specific axon navigation decisions in a subset of neurons. Expression profiling demonstrated that let-19/mdt-13 regulates the expression of a large number of genes in interneurons. A mutation in the sax-3 gene, encoding a receptor for the repulsive guidance cue SLT-1, suppresses the commissure navigation defects found in cdk-8 mutants. This suggests that the CDK8 module specifically represses the SAX-3/ROBO pathway to ensure proper commissure navigation.

Increased dietary intake of vitamin A promotes aortic valve calcification in vivo

OBJECTIVE

Calcific aortic valve disease (CAVD) is a major public health problem with no effective treatment available other than surgery. We previously showed that mature heart valves calcify in response to retinoic acid (RA) treatment through downregulation of the SRY transcription factor Sox9. In this study, we investigated the effects of excess vitamin A and its metabolite RA on heart valve structure and function in vivo and examined the molecular mechanisms of RA signaling during the calcification process in vitro.

METHODS AND RESULTS

Using a combination of approaches, we defined calcific aortic valve disease pathogenesis in mice fed 200 IU/g and 20 IU/g of retinyl palmitate for 12 months at molecular, cellular, and functional levels. We show that mice fed excess vitamin A develop aortic valve stenosis and leaflet calcification associated with increased expression of osteogenic genes and decreased expression of cartilaginous markers. Using a pharmacological approach, we show that RA-mediated Sox9 repression and calcification is regulated by classical RA signaling and requires both RA and retinoid X receptors.

CONCLUSIONS

Our studies demonstrate that excess vitamin A dietary intake promotes heart valve calcification in vivo. Therefore suggesting that hypervitaminosis A could serve as a new risk factor of calcific aortic valve disease in the human population.

Transcriptional mediators Kto and Skd are involved in the regulation of the IMD pathway and anti-Plasmodium defense in Anopheles gambiae

The malarial parasite Plasmodium must complete a complex lifecycle in its Anopheles mosquito host, the main vector for Plasmodium. The mosquito resists infection with the human malarial parasite P. falciparum by engaging the NF-κB immune signaling pathway, IMD. Here we show that the conserved transcriptional mediators Kto and Skd are involved in the regulation of the mosquito IMD pathway. RNAi-mediated depletion of Kto and Skd in the Anopheles gambiae cell line L5-3 resulted in a decrease in the transcript abundance of Cec1, which is controlled by the IMD pathway. Silencing the two genes also resulted in an increased susceptibility of the mosquito to bacterial and Plasmodium falciparum infection, but not to infection with the rodent malaria parasite P. berghei. We also showed that Kto and Skd are not transcriptional co-activators of Rel2 or other key factors of the IMD pathway; however, they participate in the regulation of the IMD pathway, which is crucial for the mosquito’s defense against P. falciparum.

The Polycomb group protein Ring1b is essential for pectoral fin development

Polycomb group (PcG) proteins are transcriptional repressors that mediate epigenetic gene silencing by chromatin modification. PcG-mediated gene repression is implicated in development, cell differentiation, stem-cell fate maintenance and cancer. However, analysis of the roles of PcG proteins in orchestrating vertebrate developmental programs in vivo has been hampered by the early embryonic lethality of several PcG gene knockouts in mice. Here, we demonstrate that zebrafish Ring1b, the E3 ligase in Polycomb Repressive Complex 1 (PRC1), is essential for pectoral fin development. We show that differentiation of lateral plate mesoderm (LPM) cells into presumptive pectoral fin precursors is initiated normally in ring1b mutants, but fin bud outgrowth is impaired. Fgf signaling, which is essential for migration, proliferation and cell-fate maintenance during fin development, is not sufficiently activated in ring1b mutants. Exogenous application of FGF4, as well as enhanced stimulation of Fgf signaling by overactivated Wnt signaling in apc mutants, partially restores the fin developmental program. These results reveal that, in the absence of functional Ring1b, fin bud cells fail to execute the pectoral fin developmental program. Together, our results demonstrate that PcG-mediated gene regulation is essential for sustained Fgf signaling in vertebrate limb development.

CRYPTIC PRECOCIOUS/MED12 is a novel flowering regulator with multiple target steps in Arabidopsis

The proper timing of flowering is of crucial importance for reproductive success of plants. Regulation of flowering is orchestrated by inputs from both environmental and endogenous signals such as daylength, light quality, temperature and hormones, and key flowering regulators construct several parallel and interactive genetic pathways. This integrative regulatory network has been proposed to create robustness as well as plasticity of the regulation. Although knowledge of key genes and their regulation has been accumulated, there still remains much to learn about how they are organized into an integrative regulatory network. Here, we have analyzed the CRYPTIC PRECOCIOUS (CRP) gene for the Arabidopsis counterpart of the MED12 subunit of the Mediator. A novel dominant mutant, crp-1D, which causes up-regulation of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), FRUITFULL (FUL) and APETALA1 (AP1) expression in a FLOWERING LOCUS T (FT)-dependent manner, was identified in an enhancer screen of the early-flowering phenotype of 35S::FT. Genetic and molecular analysis of both crp-1D and crp loss-of-function alleles showed that MED12/CRP is required not only for proper regulation of SOC1, FUL and AP1, but also for up-regulation of FT, TWIN SISTER OF FT (TSF) and FD, and down-regulation of FLOWERING LOCUS C (FLC). These observations suggest that MED12/CRP is a novel flowering regulator with multiple regulatory target steps both upstream and downstream of the key flowering regulators including FT florigen. Our work, taken together with recent studies of other Mediator subunit genes, supports an emerging view that the Mediator plays multiple roles in the regulation of flowering.

Gene-specific transcription activation via long-range allosteric shape-shifting

How is specificity transmitted over long distances at the molecular level? REs (regulatory elements) are often far from transcription start sites. In the present review we discuss possible mechanisms to explain how information from specific REs is conveyed to the basal transcription machinery through TFs (transcription factors) and the Mediator complex. We hypothesize that this occurs through allosteric pathways: binding of a TF to a RE results in changes in the AD (activation domain) of the TF, which binds to Mediator and alters the distribution of the Mediator conformations, thereby affecting transcription initiation/activation. We argue that Mediator is formed by highly disordered proteins with large densely packed interfaces that make efficient long-range signal propagation possible. We suggest two possible general mechanisms for Mediator action: one in which Mediator influences PIC (pre-initiation complex) assembly and transcription initiation, and another in which Mediator exerts its effect on the already assembled but stalled transcription complex. We summarize (i) relevant information from the literature about Mediator composition, organization and structure; (ii) Mediator interaction partners and their effect on Mediator conformation, function and correlation to the RNA Pol II (polymerase II) CTD (C-terminal domain) phosphorylation; and (iii) propose that different allosteric signal propagation pathways in Mediator relate to PIC assembly and polymerase activation of the stalled transcription complex. The emerging picture provides for the first time a mechanistic view of allosteric signalling from the RE sequence to transcription activation, and an insight into how gene specificity and signal transmission can take place in transcription initiation.

Mediator subunit 12 is required for neutrophil development in zebrafish

Hematopoiesis requires the spatiotemporal organization of regulatory factors to successfully orchestrate diverse lineage specificity from stem and progenitor cells. Med12 is a regulatory component of the large Mediator complex that enables contact between the general RNA polymerase II transcriptional machinery and enhancer bound regulatory factors. We have identified a new zebrafish med12 allele, syr, with a single missense mutation causing a valine to aspartic acid change at position 1046. Syr shows defects in hematopoiesis, which predominantly affect the myeloid lineage. Syr has identified a hematopoietic cell-specific requirement for Med12, suggesting a new role for this transcriptional regulator.

Mediator complex proteins are required for diverse developmental processes

The Mediator complex serves a crucial function in gene regulation, forming a link between gene-specific transcription factors and RNA polymerase II. Most protein-coding genes therefore require Mediator complex activity for transcriptional regulation. Given the essential functions performed by Mediator complex proteins in gene regulation, it is not surprising that mutations in Mediator complex genes disrupt animal and plant development. What is more intriguing is that the phenotypes of individual Mediator complex mutants are distinct from each other, demonstrating that certain developmental processes have a greater requirement for specific Mediator complex genes. Additionally, the range of developmental processes that are altered in Mediator complex mutants is broad, affecting a variety of cell types and physiological systems. Gene expression defects in Mediator complex mutants reveal distinct roles for individual Mediator proteins in transcriptional regulation, suggesting that the deletion of one Mediator complex protein does not interfere with transcription in general, but instead alters the expression of specific target genes. Mediator complex proteins may have diverse roles in different organisms as well, as mutants in the same Mediator gene in different species can display dissimilar phenotypes.

Mediator and human disease

Since the identification of a metazoan counterpart to yeast Mediator nearly 15 years ago, a convergent body of biochemical and molecular genetic studies have confirmed their structural and functional relationship as an integrative hub through which regulatory information conveyed by signal activated transcription factors is transduced to RNA polymerase II. Nonetheless, metazoan Mediator complexes have been shaped during evolution by substantive diversification and expansion in both the number and sequence of their constituent subunits, with important implications for the development of multicellular organisms. The appearance of unique interaction surfaces within metazoan Mediator complexes for transcription factors of diverse species-specific origins extended the role of Mediator to include an essential function in coupling developmentally coded signals with precise gene expression output sufficient to specify cell fate and function. The biological significance of Mediator in human development, suggested by genetic studies in lower metazoans, is emphatically illustrated by an expanding list of human pathologies linked to genetic variation or aberrant expression of its individual subunits. Here, we review our current body of knowledge concerning associations between individual Mediator subunits and specific pathological disorders. When established, molecular etiologies underlying genotype-phenotype correlations are addressed, and we anticipate that future progress in this critical area will help identify therapeutic targets across a range of human pathologies.

Wwp2 is essential for palatogenesis mediated by the interaction between Sox9 and mediator subunit 25

Sox9 is a direct transcriptional activator of cartilage-specific extracellular matrix genes and has essential roles in chondrogenesis. Mutations in or around the SOX9 gene cause campomelic dysplasia or Pierre Robin Sequence. However, Sox9-dependent transcriptional control in chondrogenesis remains largely unknown. Here we identify Wwp2 as a direct target of Sox9. Wwp2 interacts physically with Sox9 and is associated with Sox9 transcriptional activity via its nuclear translocation. A yeast two-hybrid screen using a cDNA library reveals that Wwp2 interacts with Med25, a component of the Mediator complex. The positive regulation of Sox9 transcriptional activity by Wwp2 is mediated by the binding between Sox9 and Med25. In zebrafish, morpholino-mediated knockdown of either wwp2 or med25 induces palatal malformation, which is comparable to that in sox9 mutants. These results provide evidence that the regulatory interaction between Sox9, Wwp2 and Med25 defines the Sox9 transcriptional mechanisms of chondrogenesis in the forming palate.

Unraveling the transcriptional regulatory machinery in chondrogenesis

Since the discovery of SOX9 mutations in the severe human skeletal malformation syndrome campomelic dysplasia in 1994, Sox9 was shown to be both required and sufficient for chondrocyte specification and differentiation. At the same time, its distant relatives Sox5 and Sox6 were shown to act in redundancy with each other to robustly enhance its functions. The Sox trio is currently best known for its ability to activate the genes for cartilage-specific extracellular matrix components. Sox9 and Sox5/6 homodimerize through domains adjacent to their Sry-related high-mobility-group DNA-binding domain to increase the efficiency of their cooperative binding to chondrocyte-specific enhancers. Sox9 possesses a potent transactivation domain and thereby recruits diverse transcriptional co-activators, histone-modifying enzymes, subunits of the mediator complex, and components of the general transcriptional machinery, such as CBP/p300, Med12, Med25, and Wwp2. This information helps us begin to unravel the mechanisms responsible for Sox9-mediated transcription. We review here the discovery of this master chondrogenic trio and its roles in chondrogenesis in vivo and at the molecular level, and we discuss how these pioneering studies open the way for many additional studies that are needed to further increase our understanding of the transcriptional regulatory machinery operating in chondrogenesis.

The transcriptional mediator component Med12 is required for hindbrain boundary formation

Background

Rhombomere boundaries form during hindbrain segmentation and are critical for maintaining segmental integrity and regulating migration in the hindbrain. Some genetic models affecting hindbrain boundary formation have been described, but involvement of components of the transcriptional mediator complex in boundary formation has not reported so far.

Principal Findings

The kto/med12 mutant zebrafish, which affects the Mediator component Med12, causes specific loss of rhombomere boundary cells even though segmentation of the hindbrain takes place at least in part. In kto mutant embryos, cells forming rhombomere boundaries were largely absent as indicated by the use of several marker genes. While no obvious increase in cell death was observed, we found a notable reduction of cell proliferation in the hindbrain of kto mutant zebrafish.

Conclusions

The kto/med12 mutation results in specific defects of boundary cell formation in the zebrafish hindbrain.

MACCHI-BOU 2 is required for early embryo patterning and cotyledon organogenesis in Arabidopsis

The phytohormone auxin is a key regulator of organogenesis in plants and is distributed asymmetrically via polar transport. However, the precise mechanisms underlying auxin-mediated organogenesis remain elusive. Here, we have analyzed the macchi-bou 2 (mab2) mutant identified in a pinoid (pid) enhancer mutant screen. Seedlings homozygous for either mab2 or pid showed only mild phenotypic effects on cotyledon positions and/or numbers. In contrast, mab2 pid double mutant seedlings completely lacked cotyledons, indicating a synergistic interaction. We found that mab2 homozygous embryos had defective patterns of cell division and showed aberrant cotyledon organogenesis. Further analysis revealed that the mab2 mutation affected auxin response but not auxin transport in the embryos, suggesting the involvement of MAB2 in auxin response during embryogenesis. MAB2 encodes an Arabidopsis ortholog of MED13, a putative regulatory module component of the Mediator complex. Mediator is a multicomponent complex that is evolutionarily conserved in eukaryotes and its regulatory module associates with Mediator to control the interaction of Mediator and RNA polymerase II. MAB2 interacts with a regulatory module component in yeast cells. Taken together, our data suggest that MAB2 plays a crucial role in embryo patterning and cotyledon organogenesis, possibly through modulating expression of specific genes such as auxin-responsive genes.

The novel BTB/POZ and zinc finger factor Zbtb45 is essential for proper glial differentiation of neural and oligodendrocyte progenitor cells

Understanding the regulatory mechanisms controlling the fate decisions of neural stem cells (NSCs) is a crucial issue to shed new light on mammalian central nervous system (CNS) development in health and disease. We have investigated a possible role for the previously uncharacterized BTB/POZ-domain containing zinc finger factor Zbtb45 in the differentiation of NSCs and postnatal oligodendrocyte precursors. In situ hybridization histochemistry and RT-qPCR analysis revealed that Zbtb45 mRNA was ubiquitously expressed in the developing CNS in mouse embryos at embryonic day (E) 12.5 and 14.5. Zbtb45 mRNA knockdown in embryonic forebrain NSCs by siRNA resulted in a rapid decrease in the expression of oligodendrocyte-characteristic genes after mitogen (FGF2) withdrawal, whereas the expression of astrocyte-associated genes such as CD44 and GFAP increased compared to control. Accordingly, the number of astrocytes was significantly increased seven days after Zbtb45 siRNA delivery to NSCs, in contrast to the numbers of neuronal and oligodendrocyte-like cells. Surprisingly, mRNA knockdown of the Zbtb45-associated factor Med31, a subunit of the Mediator complex, did not result in any detectable effect on NSC differentiation. Similar to NSCs, Zbtb45 mRNA knockdown in oligodendrocyte precursors (CG-4) reduced oligodendrocyte maturation upon mitogen withdrawal associated with down-regulation of the mRNA expression and protein levels of markers for oligodendrocytic differentiation. Zbtb45 mRNA knockdown did not significantly affect proliferation or cell death in any of the cell types. Based on these observations, we propose that Zbtb45 is a novel regulator of glial differentiation.

Med12 is essential for early mouse development and for canonical Wnt and Wnt/PCP signaling

The Mediator complex is commonly seen as a molecular bridge that connects DNA-bound transcription factors to the RNA polymerase II (Pol II) machinery. It is a large complex of 30 subunits that is present in all eukaryotes. The Med12 subunit has been implicated not only in the regulation of Pol II activity, but also in the binding of transcription factors to the bulk of the Mediator complex. We targeted Med12 in mouse embryonic stem cells to investigate the in vivo function of this subunit. We report here the developmental defects of Med12 hypomorphic mutants that have a drastic reduction in Med12 protein levels. These mutants fail to develop beyond embryonic day 10 and have severe defects in neural tube closure, axis elongation, somitogenesis and heart formation. We show that in Med12 hypomorphic embryos, the Wnt/planar cell polarity pathway is disrupted and that canonical Wnt/beta-catenin signaling is impaired. In agreement with this, embryos that are incapable of Med12 expression failed to establish the anterior visceral endoderm or activate brachyury expression, and did not complete gastrulation.