Caenorhabditis elegans TBX-2 Directly Regulates Its Own Expression in a Negative Autoregulatory Loop

Nfya-1 functions as a substrate of ERK-MAP kinase during Caenorhabditis elegans vulval development

A common bridge between a linear cytoplasmic signal and broad nuclear regulation is the family of MAP kinases which can translocate to the nucleus upon activation by the cytoplasmic signal. One pathway which functions to activate the ERK family of MAP kinases is the Ras signaling pathway which functions at multiple times and locations during the development of Caenorhabditis elegans including the development of the excretory cell, germ cells, male tail, and vulva. It has been most extensively characterized during the development of the vulva which is formed from the vulval precursor cells (VPCs), a set of six equivalent, epithelial cells designated P3.p - P8.p. Although LIN-1 appears to be a primary target of ERK MAP kinase during vulval development, it is likely that other developmentally important molecules are also regulated by ERK-mediated phosphorylation. The identification of physiological substrates of MAP kinases has been aided by the identification of docking site domains in substrate proteins that contribute to high-affinity interactions with kinases. Our laboratory has identified the C. elegans protein, T08D10.1/NFYA-1, as a potential ERK MAP kinase substrate in this manner, and we have initiated a characterization of its role during Ras-mediated development. T08D10.1 possesses significant homology to the CCAAT-box DNA-binding domain of the vertebrate nuclear transcription factor-Y, alpha (NF-YA) family of proteins. NF-Y proteins act as part of a complex to regulate the transcription of a large number of genes, in particular, genes that function in the G1/S cell cycle transition. T08D10.1/NFYA-1 is predicted to code for a protein containing multiple potential phosphorylation sites for ERK MAP kinase and a D-domain docking site. We demonstrate through biochemical analysis of purified NFYA-1 protein that it can act in vitro as a high affinity substrate for activated ERK MAP kinase. Growth factor activation of the Ras pathway in a tissue culture system has negligible effect on the protein's transactivation potential, however, the DNA-binding activity of the protein is reduced after treatment with activated ERK-MAP kinase. We demonstrate through mutant analysis that nfya-1 acts to inhibit vulval development and functions downstream or in parallel to let-60/ras. Both the NF-Y complex and the Ras signaling pathway play a fundamental role in cell proliferation and oncogenesis and the connection between the two is an important insight into the mechanisms of cell fate specification and cellular response.

TBX2, a Novel Regulator of Labour

Background and Objectives: Therapeutic interventions targeting molecular factors involved in the transition from uterine quiescence to overt labour are not substantially reducing the rate of spontaneous preterm labour. The identification of novel rational therapeutic targets are essential to prevent the most common cause of neonatal mortality. Based on our previous work showing that Tbx2 (T-Box transcription factor 2) is a putative upstream regulator preceding progesterone withdrawal in mouse myometrium, we now investigate the role of TBX2 in human myometrium. Materials and Methods: RNA microarray analysis of (A) preterm human myometrium samples and (B) myometrial cells overexpressing TBX2 in vitro, combined with subsequent analysis of the two publicly available datasets of (C) Chan et al. and (D) Sharp et al. The effect of TBX2 overexpression on cytokines/chemokines secreted to the myometrium cell culture medium were determined by Luminex assay. Results: Analysis shows that overexpression of TBX2 in myometrial cells results in downregulation of TNFα- and interferon signalling. This downregulation is consistent with the decreased expression of cytokines and chemokines of which a subset has been previously associated with the inflammatory pathways relevant for human labour. In contrast, CXCL5 (C-X-C motif chemokine ligand 5), CCL21 and IL-6 (Interleukin 6), previously reported in relation to parturition, do not seem to be under TBX2 control. The combined bioinformatical analysis of the four mRNA datasets identifies a subset of upstream regulators common to both preterm and term labour under control of TBX2. Surprisingly, TBX2 mRNA levels are increased in preterm contractile myometrium. Conclusions: We identified a subset of upstream regulators common to both preterm and term labour that are activated in labour and repressed by TBX2. The increased TBX2 mRNA expression in myometrium collected during a preterm caesarean section while in spontaneous preterm labour compared to tissue harvested during iatrogenic preterm delivery does not fit the bioinformatical model. We can only explain this by speculating that the in vivo activity of TBX2 in human myometrium depends not only on the TBX2 expression levels but also on levels of the accessory proteins necessary for TBX2 activity.

Endodermal Maternal Transcription Factors Establish Super-Enhancers during Zygotic Genome Activation

Elucidation of the sequence of events underlying the dynamic interaction between transcription factors and chromatin states is essential. Maternal transcription factors function at the top of the regulatory hierarchy to specify the primary germ layers at the onset of zygotic genome activation (ZGA). We focus on the formation of endoderm progenitor cells and examine the interactions between maternal transcription factors and chromatin state changes underlying the cell specification process. Endoderm-specific factors Otx1 and Vegt together with Foxh1 orchestrate endoderm formation by coordinated binding to select regulatory regions. These interactions occur before the deposition of enhancer histone marks around the regulatory regions, and these TFs recruit RNA polymerase II, regulate enhancer activity, and establish super-enhancers associated with important endodermal genes. Therefore, maternal transcription factors Otx1, Vegt, and Foxh1 combinatorially regulate the activity of super-enhancers, which in turn activate key lineage-specifying genes during ZGA.

How do maternal transcription factors interact with chromatin regions to coordinate the endodermal gene regulatory program? Paraiso et al. demonstrate that combinatorial binding of maternal Otx1, Vegt, and Foxh1 to select enhancers and super-enhancers in the genome controls endodermal cell fate specification during zygotic gene activation.

NF-Y in invertebrates

Both Drosophila melanogaster and Caenorhabditis elegans (C. elegans) are useful model organisms to study in vivo roles of NF-Y during development. Drosophila NF-Y (dNF-Y) consists of three subunits dNF-YA, dNF-YB and dNF-YC. In some tissues, dNF-YC-related protein Mes4 may replace dNF-YC in dNF-Y complex. Studies with eye imaginal disc-specific dNF-Y-knockdown flies revealed that dNF-Y positively regulates the sevenless gene encoding a receptor tyrosine kinase, a component of the ERK pathway and negatively regulates the Sensless gene encoding a transcription factor to ensure proper development of R7 photoreceptor cells together with proper R7 axon targeting. dNF-Y also controls the Drosophila Bcl-2 (debcl) to regulate apoptosis. In thorax development, dNF-Y is necessary for both proper Drosophila JNK (basket) expression and JNK signaling activity that is responsible for thorax development. Drosophila p53 gene was also identified as one of the dNF-Y target genes in this system. C. elegans contains two forms of NF-YA subunit, CeNF-YA1 and CeNF-YA2. C. elegans NF-Y (CeNF-Y) therefore consists of CeNF-YB, CeNF-YC and either CeNF-YA1 or CeNF-YA2. CeNF-Y negatively regulates expression of the Hox gene egl-5 (ortholog of Drosophila Abdominal-B) that is involved in tail patterning. CeNF-Y also negatively regulates expression of the tbx-2 gene that is essential for development of the pharyngeal muscles, specification of neural cell fate and adaptation in olfactory neurons. Negative regulation of the expression of egl-5 and tbx-2 by CeNF-Y provides new insight into the physiological meaning of negative regulation of gene expression by NF-Y during development. In addition, studies on NF-Y in platyhelminths are also summarized. This article is part of a Special Issue entitled: Nuclear Factor Y in Development and Disease, edited by Prof. Roberto Mantovani.

The Remarkably Diverse Family of T-Box Factors in Caenorhabditis elegans

The nematode Caenorhabditis elegans is a simple metazoan animal that is widely used as a model to understand the genetic control of development. The completely sequenced C. elegans genome contains 22 T-box genes, and they encode factors that show remarkable diversity in sequence, DNA-binding specificity, and function. Only three of the C. elegans T-box factors can be grouped into the conserved subfamilies found in other organisms, while the remaining factors are significantly diverged and unlike those in most other animals. While some of the C. elegans factors can bind canonical T-box binding elements, others bind and regulate target gene expression through distinct sequences. The nine genetically characterized T-box factors have varied functions in development and morphogenesis of muscle, hypodermal tissues, and neurons, as well as in early blastomere fate specification, cell migration, apoptosis, and sex determination, but the functions of most of the C. elegans T-box factors have not yet been extensively characterized. Like T-box factors in other animals, interaction with a Groucho-family corepressor and posttranslational SUMOylation have been shown to affect C. elegans T-box factor activity, and it is likely that additional mechanisms affecting T-box factor activity will be discovered using the effective genetic approaches in this organism.

Function of the C. elegans T-box factor TBX-2 depends on interaction with the UNC-37/Groucho corepressor

T-box transcription factors are important regulators of development in all animals, and altered expression of T-box factors has been identified in an increasing number of diseases and cancers. Despite these important roles, the mechanism of T-box factor activity is not well understood. We have previously shown that the Caenorhabditis elegans Tbx2 subfamily member TBX-2 functions as a transcriptional repressor to specify ABa-derived pharyngeal muscle, and that this function depends on SUMOylation. Here we show that TBX-2 function also depends on interaction with the Groucho-family corepressor UNC-37. TBX-2 interacts with UNC-37 in yeast two-hybrid assays via a highly conserved engrailed homology 1 (eh1) motif located near the TBX-2 C-terminus. Reducing unc-37 phenocopies tbx-2 mutants, resulting in a specific loss of anterior ABa-derived pharyngeal muscles and derepression of the tbx-2 promoter. Moreover, double mutants containing hypomorphic alleles of unc-37 and tbx-2 exhibit enhanced phenotypes, providing strong genetic evidence that unc-37 and tbx-2 share common functions in vivo. To test whether interaction with UNC-37 is necessary for TBX-2 activity, we developed a transgene rescue assay using a tbx-2 containing fosmid and found that mutating the tbx-2 eh1 motif reduced rescue of a tbx-2 null mutant. These results indicate that TBX-2 function in vivo depends on interaction with UNC-37. As many T-box factors contain eh1 motifs, we suggest that interaction with Groucho-family corepressors is a common mechanism contributing to their activity.