Scalloped homologs in yeast

Diverse eukaryotic organisms share developmental transcription factors with homologous DNA-binding domains. The developmental regulator AbaA, a member of the ATTS/TEA (AbaA, TEF-1, TEC1, Scalloped/TEF-1, TEC1, AbaA) class of transcription factors of the filamentous fungus Aspergillus nidulans, induces pseudohyphal development in the yeast Saccharomyces cerevisiae. The S. cerevisiae homolog of AbaA, TEC1p, is required for this morphological transition. Evidence is provided that TEC1p functions in co-operation with STE12p to induce pseudohyphal development (Gavrias, 1996).

In yeast, an overlapping set of mitogen-activated protein kinase (MAPK) signaling components controls mating, haploid invasion, and pseudohyphal development. Paradoxically, a single downstream transcription factor, Ste12, is necessary for the execution of these distinct programs. Developmental specificity was found to require a transcription factor of the TEA/ATTS family, Tec1, which cooperates with Ste12 during filamentous and invasive growth. Purified derivatives of Ste12 and Tec1 bind cooperatively to enhancer elements called filamentation and invasion response elements (FREs), which program transcription that is specifically responsive to the MAPK signaling components required for filamentous growth. An FRE in the TEC1 promoter functions in a positive feedback loop required for pseudohyphal development (Madhani, 1997).

The transcription factor Tec1 is involved in pseudohyphal differentiation and agar-invasive growth of Saccharomyces cerevisiae cells. The sole element in the TEC1 promoter that has thus far been shown to control Tec1 function is the filament response element. The TEC1 promoter also contains several pheromone response element sequences that are likely to be functional: TEC1 transcription is induced by mating factor; it is cell cycle regulated and dependent on the Ste4, Ste18 and Ste5, all components of the mating factor signal transduction pathway. Using alleles of the transcription factor Ste12 that are defective in DNA binding, transcriptional induction or cooperativity with other transcription factors, little correlation is found between TEC1 transcript levels and agar-invasive growth (Oehlen, 1998).

Diploid yeast develop pseudohyphae in response to nitrogen starvation, while haploid yeast produce invasive filaments that penetrate the agar in rich medium. This study has identified the gene FLO11, which encodes a cell wall protein that is critically required for both invasion and pseudohyphae formation in response to nitrogen starvation. FLO11 encodes a cell surface flocculin with a structure similar to the class of yeast serine/threonine-rich GPI-anchored cell wall proteins. Cells of the Saccharomyces cerevisiae strain Sigma1278b with deletions of FLO11 do not form pseudohyphae as diploids nor invade agar as haploids. In rich media, FLO11 is regulated by mating type; it is expressed in haploid cells but not in diploids. However, upon transfer to nitrogen starvation media, FLO11 transcripts accumulate in diploid cells, but not in haploids. Overexpression of FLO11 in diploid cells, which are otherwise not invasive, enables them to invade agar. Thus, the mating type repression of FLO11 in diploids grown in rich media suffices to explain the inability of these cells to invade. The promoter of FLO11 contains a consensus binding sequence for Ste12p and Tec1p, proteins known to cooperatively activate transcription of Ty1 elements and the TEC1 gene during development of pseudohyphae. Yeast with a deletion of STE12 does not express FLO11 transcripts, indicating that STE12 is required for FLO11 expression. These ste12-deletion strains also do not invade agar. However, the ability to invade can be restored by overexpressing FLO11. Activation of FLO11 may thus be the primary means by which Ste12p and Tec1p cause invasive growth (Lo, 1998).

Specialized gene expression programs are induced by signaling pathways that act on transcription factors. Whether these transcription factors can function in multiple developmental programs through a global switch in promoter selection is not known. Genome-wide location analysis has been used to show that the yeast Ste12 transcription factor, which regulates mating and filamentous growth, is bound to distinct program-specific target genes dependent on the developmental condition. This condition-dependent distribution of Ste12 requires concurrent binding of the transcription factor Tec1 during filamentation and is differentially regulated by the MAP kinases Fus3 and Kss1. Program-specific distribution across the genome may be a general mechanism by which transcription factors regulate distinct gene expression programs in response to signaling (Zeitlinger, 2003).

In response to pheromone from a mating partner, Ste12 induces the expression of mating gene, whereas under certain starvation conditions, Ste12 induces genes involved in filamentous growth, which facilitates foraging for nutrients. Ste12 can activate transcription alone or in partnership with other transcription factors. Some mating genes are induced in response to pheromone by Ste12 alone, whereas other mating genes are regulated by Ste12 together with Mcm1. For its role in filamentous growth, Ste12 requires the function of Tec1, a well-conserved transcription factor of the ATTS/TEA (AbaA, TEF-1, TEC1, Scalloped/TEF-1, TEC1, AbaA) family. Ste12 binds cooperatively with Tec1 at filamentation-responsive elements (FREs) in vitro. These studies suggest that differential gene expression by Ste12 might be achieved through selective partnership with other transcription factors. It is not known, however, whether the two MAPK pathways that induce mating and filamentation differentially regulate Ste12 by activating distinct complexes already bound to DNA or by regulating the partnership and DNA binding behaviors of these complexes (Zeitlinger, 2003 and references therein).

Activation of Ste12 during mating and filamentation involves two different MAPK signal transduction pathways that share components. Since they share the MAPK kinase and the MAPK kinase, two MAPKs, Fus3 and Kss1, may be activated by the mating and filamentation pathways. Genetic studies indicate that Fus3 and Kss1 act in a partially redundant fashion during mating. Deletion of both MAPKs, but not either one alone, abolishes Ste12-dependent induction of mating genes in response to pheromone. The two MAPKs, however, have different effects on Ste12-dependent induction of filamentous growth. The kinase activity of Kss1 increases filamentation, whereas the kinase activity of Fus3 appears to suppress filamentation. The mechanism by which Fus3 and Kss1 might differentially regulate Ste12 is not clear. Genetic assays indicate that both MAPKs activate Ste12 through regulation of two inhibitors, Dig1 and Dig2. Ste12 can form a complex with these two proteins and is released from the complex upon activation by pheromone. Both MAPKs phosphorylate Dig1 and Dig2, as well as Ste12 (Zeitlinger, 2003 and references therein).

A genome-wide binding assay that combines chromatin immunoprecipitation (ChIP) with DNA microarray technology was used to study the binding behavior of Ste12 during mating and filamentation. Ste12 binds to distinct promoters in vivo under different conditions and the different sets of genes specify distinct developmental programs. The global switch in target gene specificity of Ste12 depends on the transcription factor Tec1 during filamentation and is differentially regulated by the two MAPKs. Both MAPKs have the ability to induce mating genes in response to pheromone, but Fus3 has an additional activity that inhibits Ste12 binding to filamentation genes under the same condition. Thus, regulation of Ste12 binding, and not selective activation of transcription factor complexes already bound to DNA, is the mechanism by which the two MAPKs regulate distinct gene expression programs and direct cells toward specific developmental fates (Zeitlinger, 2003 and references therein).

Scalloped homologs in C. elegans

In wild-type Caenorhabditis elegans, six cells develop as receptors for gentle touch. In egl-44 (Drosophila homolog: Scalloped) and egl-46 (Drosophila homologs: Nerfin I and II) mutants, two other neurons, the FLP cells, express touch receptor-like features. The FLP cells normally express lin-14 as well as unc-86 (Drosophila homolog ACJ6/IPOU and mec-3 that code for a LIM homeodomain transcription factor). These cells do not express touch cell characteristics because of the action of two genes, egl-44 and egl-46. Mutation of either egl gene results in a transformation of the FLP cells into cells that resemble the touch cells. Instead of differentiating as FLP neurons, the cells in the mutants express the mec-4 and mec-7 touch function genes and have processes that lie adjacent to the normal touch cell processes and that also have the large-diameter microtubules and extracellular matrix characteristic of the touch cells. egl-44 and egl-46 also affect the differentiation of other neurons, including the HSN neurons, two cells needed for egg laying (Wu, 2001).

The egl-44 gene encodes a putative transcription regulatory protein of 471 amino acids similar to transcription enhancer factor (TEF) proteins. TEF-1-like proteins, which have been found from yeast to humans, are involved in a variety of developmental processes. For example, mutations in the Drosophila TEF gene scalloped affect the development of sensory bristles and central neurons needed for taste, and human TEF-5 is expressed in the placenta and activates the chorionic somatomammotropin gene. The most conserved region among family members is the 70-amino-acid TEA/ATTS DNA-binding domain at the N terminus. EGL-44, the Drosophila TEF Scalloped (Sd), and the human TEF-5 protein (the human TEF most similar to EGL-44) are 82% identical in the TEA/ATTS DNA-binding domain. The egl-44 mutations are in this domain. The C-terminal half of EGL-44, Sd, and TEF-5 are also 47% identical, and this region in TEF-1 contains Pro-rich, STY-rich, and other sequences that are needed together for transcriptional activation. Although EGL-44 does not contain sequences that match the activation domains in other TEF proteins, its C-terminal half is rich in Pro, Ser, Thr, and Tyr (Wu, 2001).

egl-46 encodes a predicted protein of 286 amino acids. The most notable features of EGL-46 are three closely spaced zinc finger motifs in the C-terminal region of EGL-46. The first two zinc finger motifs are separated by 9 amino acids and may form a pair, and the third motif is 19 amino acids C-terminal to the first two. The second finger motif of EGL-46 conforms to the TFIIIA (C2H2) consensus, whereas the other two fingers differ slightly. In the first and third fingers, the last His is replaced by Cys; in the first finger, the spacing between the His and the last Cys residue differs from the consensus. Nonetheless, the overall spacing and the conservation of other residues indicate that these are variants of the TFIIIA type. These three zinc fingers may mediate DNA binding by EGL-46. Consistent with a role in the nucleus, EGL-46 contains a potential nuclear localization signal (amino acids 110-126). Also consistent with a role as a transcription factor, EGL-46 contains a glutamine-rich region (amino acids 61-75), which may act as a transcriptional activation domain. EGL-46 and several proteins with which it shares similarity appear to form a new family of zinc finger proteins. These proteins include the human and mouse IA-1 proteins, the mouse MLT-1 protein, a human protein tentatively named R-355C3p, and two proteins from Drosophila melanogaster, Nerfin-1 and Nerfin-2 (Stivers, 2000). No other closely similar sequences are found in the C. elegans genome. All seven proteins have three zinc finger regions; the mammalian proteins have two additional zinc finger sequences C-terminal to these three. The first two zinc fingers show considerable similarity and equal spacing in all seven proteins. The second zinc finger of the pair is 90% identical for all the proteins and has a conserved potential PKC phosphorylation site. The high degree of similarity in this zinc finger pair region indicates that the region is functionally important. The egl-46(n1127) mutation produces a Cys to Phe substitution in the first zinc finger of this pair. All seven proteins share an additional region of similarity N-terminal to the zinc fingers (26 amino acids in EGL-46) that contains a potential nuclear localization signal. N-terminal to this region, all seven proteins have regions that are proline-rich (although EGL-46 is less so than the others). The mammalian proteins also contain a short transcriptional repression domain, but this sequence is not conserved in EGL-46 or the Drosophila proteins (Wu, 2001).

Two features of egl-46 indicate that its product may be regulated posttranscriptionally: (1) the N-terminal region of the protein contains a 25-amino-acid putative PEST sequence that could target the protein for rapid degradation; (2) the egl-46 mRNA 3' UTR contains sequences that may target the mRNA for degradation. The 3' UTR contains three AUUUA motifs, which have been associated with RNA instability. More recent work, however, has found that a single AUUUA motif is not sufficient to cause mRNA instability and that a UUAUUUA(U/A)(U/A) sequence may be required. egl-46 has the core of this sequence, UAUUUAU, which when tested in three copies promoted mRNA degradation. Several other members of this gene family share these features. The Nerfin-1 (but not Nerfin-2), hIA-1 (but not mIA-1), MLT 1, and R-355C3p proteins have predicted PEST sequences. Nerfin-1, hIA-1, and mlt 1 mRNAs contain AUUUA repeats in their 3' UTRs. Little is known about the EGL-46-related proteins, although they are often found in neuronal tissues, and at least Nerfin-1 and IA-1 are found in dividing neural precursors. Of the Drosophila proteins, Nerfin-1 is distributed widely throughout the nervous system and is observed in many neuronal precursors; Nerfin-2 is found in only a few neurons (Stivers, 2000). The human IA-1 protein is found in many neuroendocrine tumor cell lines and virtually all small cell lung cancer cell lines. The mouse protein MLT 1 has been identified in brain and kidney (Wu, 2001).

Both egl-44 and egl-46 are expressed in FLP and HSN neurons (and other cells); expression of egl-46 is dependent on egl-44 in the FLP cells but not in the HSN cells. Wild-type touch cells express egl-46 but not egl-44. Moreover, ectopic expression of egl-44 in the touch cells prevents touch cell differentiation in an egl-46-dependent manner. The sequences of these genes and their nuclear location as seen with GFP fusions indicate that they repress transcription of touch cell characteristics in the FLP cells (Wu, 2001).

egl-44 appears to repress the expression of touch cell features in the FLP cells in two ways: (1) egl-44 is required for the wild-type levels of egl-46 expression in these neurons; (2) egl-44 must be expressed with egl-46 to repress touch cell differentiation. This conclusion is supported by the ectopic expression of these genes in the touch cells and by the finding that these cells normally express egl-46. (The expression of egl-46 in the touch cells may account for the minor touch cell process and morphological defects in egl-46 mutants. EGL-44 may interact directly with EGL-46, because TEF proteins in other organisms act as transcription cofactors (Wu, 2001).

These considerations extend the model of combinatorial control of touch cell development. In the six touch cells, unc-86 promotes mec-3 expression, and the UNC-86/MEC-3 heterodimer activates the expression of touch genes. In the FLP neurons, egl-44 promotes egl-46 expression, presumably with some other factor(s), and EGL-44 and EGL-46 together, presumably also with some other factor(s), inhibit touch gene expression to secure the normal differentiation of FLP neurons. Because mec-3 and unc-86 are expressed in FLP cells normally, it is possible that EGL-44 and EGL-46 repress touch cell fate by directly antagonizing activation by MEC-3 or/and UNC-86 (Wu, 2001).

Although both egl-44 and egl-46 are expressed in the HSN neurons, and expression of the wild-type genes in the HSN cells complements egl-44 and egl-46 mutations, the timing of their expression is unexpected given the phenotypes of the mutant cells. egl-44 and egl-46 mutations affect HSN differentiation in three ways: (1) the cells migrate further than wild-type cells; (2) their axons are misdirected, and (3) they have reduced production of the neurotransmitter serotonin. Of these three processes, only cell migration occurs in the embryos; the others arise as the animals become adults. In contrast, the HSN cells express egl-44 in the embryo and express egl-46 in the embryo and transiently and weakly in L2 larvae. The embryonic expression could underlie a role for these genes in the regulation of HSN migration. The expression pattern of these genes is less easily reconciled with the late larval outgrowth defects and adult serotonin defect (both of which are incompletely penetrant). One explanation is that these genes act early to establish the ability of the cells to generate serotonin or grow appropriately. If so, the genes could act indirectly within the HSN to influence these later traits. It has been suggested, for example, that interactions of the HSN cells with their muscle targets result in the lowered levels of serotonin in the mutant HSN cells. An early defect in the HSN cells could lead to these abnormal interactions. Alternatively, because the genes are expressed in many other cells, their influence on axonal outgrowth and/or serotonin production could be caused by the loss of gene activity in other cells; for example, some of the HSN phenotypes are not the result of the cell-autonomous action of the genes. The AVM and PVM touch cells also have a low penetrant outgrowth defect in egl-44 and egl-46 animals, but the cells do not detectably express egl-44. Perhaps the loss of egl-44 expression in the hypodermis underlies the touch cell and HSN outgrowth defects (Wu, 2001).

Scalloped homologs in vertebrates

scalloped is homologous to human transcription enhancer factor TEF-1, with 68% homology (Campbell, 1992). The TEA domain of scalloped is 98% homologous to that of TEF-1. Vertebrate TEF-1 binding sites activate transcription form the SV40 early promoter and mediate large T antigen activation of the SV40 late promoter. The avian homolog has been shown to be involved in muscle-specific gene expression (Hwang, 1993).

A retroviral gene trap was used in embryonic stem (ES) cells to derive a recessive embryonic lethal mouse strain: ROSA beta-geo5. Mutant embryos display an enlarged pericardial cavity, bradycardia, a dilated fourth ventricle in the brain; they die between embryonic days 11 and 12. Whereas heart development in the mutant embryos is extensive, the ventricular wall is abnormally thin with a reduced number of trabeculae. Cloning of the trapped gene indicates that proviral insertion creates a null mutation in the transcriptional enhancer factor 1 (TEF-1) gene, a scalloped homolog. Although transcription of a number of muscle-specific genes believed to be TEF-1 targets appears normal, the defect in cardiogenesis is likely attributable to diminished transcription of one or several cardiac-specific genes (Chen, 1994).

The human transcription enhancer factor-1 (TEF-1) belongs to a family of evolutionarily conserved proteins that have a DNA binding TEA domain. TEF-1 shares a 98% homology with Drosophila Scalloped in the DNA binding domain and a 50% similarity in the activation domain. Human TEF-1 was expressed in Drosophila under the hsp-70 promoter and it was found that TEF-1 can substitute for SD function. The transformants rescue the wingblade defects as well as the lethality of loss-of-function alleles. Observation of reporter activity in the imaginal wing discs of the enhancer-trap alleles suggests that TEF-1 is capable of promoting sd gene regulation. The functional capability of the TEF-1 product was assessed by comparing the extent of rescue by heat shock (hs)-TEF-1 with that of hs-sd. The finding that TEF-1 can function in vivo during wingblade development offers a potent genetic system for the analysis of its function and in the identification of the molecular partners of TEF-1 (Deshpande, 1997).

mTEF-1 is the prototype of a family of mouse transcription factors that share the same TEA DNA binding domain (mTEAD genes) and are widely expressed in adult tissues. At least one member of this family is expressed at the beginning of mouse development, because mTEAD transcription factor activity is not detected in oocytes, but first appears at the 2-cell stage in development, concomitant with the onset of zygotic gene expression. Since embryos survive until day 11 in the absence of mTEAD-1 (TEF-1), another family member likely accounts for this activity. Screening an EC cell cDNA library yielded mTEAD-1, 2 and 3 genes. RT-PCR detects RNA from all three of these genes in oocytes, but upon fertilization, mTEAD-1 and 3 mRNAs disappear. mTEAD-2 mRNA, initially present at approx. 5,000 copies per egg, decreases to approx. 2,000 copies in 2-cell embryos before accumulating to approx. 100,000 copies in blastocysts, consistent with degradation of maternal mTEAD mRNAs, followed by selective transcription of mTEAD-2 from the zygotic genome. In situ hybridization does not detect mTEAD RNA in oocytes, and only mTEAD-2 is detected in day-7 embryos. Northern analysis detects all three RNAs at varying levels in day-9 embryos and in various adult tissues. A fourth mTEAD gene, recently cloned from a myotube cDNA library, is not detected by RT-PCR in either oocytes or preimplantation embryos. Together, these results reveal that mTEAD-2 is selectively expressed for the first 7 days of embryonic development, and is therefore most likely responsible for the mTEAD transcription factor activity that appears upon zygotic gene activation (Kaneko, 1997).

The M-CAT binding factor transcription enhancer factor 1 known as TEF-1 (Drosophila homolog: Scalloped) has been implicated in the regulation of several cardiac and skeletal muscle genes. An E-box-M-CAT hybrid (EM) motif has been identified that is responsible for the basal and cyclic AMP-inducible expression of the rat cardiac alpha-myosin heavy chain (alpha-MHC) gene in cardiac myocytes. Two factors, TEF-1 and a basic helix-loop-helix leucine zipper protein, Max, bind to the alpha-MHC EM motif. Max is a part of the cardiac troponin T M-CAT-TEF-1 complex even when the DNA template does not contain an apparent E-box binding site. In the protein-protein interaction assay, a stable association of Max with TEF-1 is observed when glutathione S-transferase (GST)-TEF-1 or GST-Max is used to pull down in vitro-translated Max or TEF-1, respectively. In addition, Max is coimmunoprecipitated with TEF-1, thus documenting an in vivo TEF-1-Max interaction. In the transient transcription assay, overexpression of either Max or TEF-1 results a mild activation of the alpha-MHC-chloramphenicol acetyltransferase (CAT) reporter gene at lower concentrations and repression of this gene at higher concentrations. However, when Max and TEF-1 expression plasmids are transfected together, the repression mediated by a single expression plasmid is alleviated and a three- to fourfold transactivation of the alpha-MHC-CAT reporter gene is observed. This effect is abolished once the EM motif in the promoter-reporter construct is mutated, suggesting that the synergistic transactivation function of the TEF-1/Max heterotypic complex is mediated through binding of the complex to the EM motif. These results demonstrate a novel association between Max and TEF-1 and indicate a positive cooperation between these two factors in alpha-MHC gene regulation (Gupta, 1997).

Expression of many skeletal muscle-specific genes depends on TEF-1 (transcription enhancer factor-1) and MEF2 transcription factors. In Drosophila, the TEF-1 homolog Scalloped interacts with the cofactor Vestigial to drive differentiation of the wing and indirect flight muscles. Three mammalian vestigial-like genes, Vgl-1, Vgl-2, and Vgl-3, have been identified that share homology in a TEF-1 interaction domain. Vgl-1 and Vgl-3 transcripts are enriched in the placenta, whereas Vgl-2 is expressed in the differentiating somites and branchial arches during embryogenesis and is skeletal muscle-specific in the adult. During muscle differentiation, Vgl-2 mRNA levels increase and Vgl-2 protein translocates from the cytoplasm to the nucleus. In situ hybridization revealed co-expression of Vgl-2 with myogenin in the differentiating muscle of embryonic myotomes but not in newly formed somites prior to muscle differentiation. Like Vgl-1, Vgl-2 interacts with TEF-1. In addition, Vgl-2 interacts with MEF2 in a mammalian two-hybrid assay and Vgl-2 selectively binds to MEF2 in vitro. Co-expression of Vgl-2 with MEF2 markedly co-activates an MEF2-dependent promoter through its MEF2 element. Overexpression of Vgl-2 in MyoD-transfected 10T(1/2) cells markedly increases myosin heavy chain expression, a marker of terminal muscle differentiation. These results identify Vgl-2 as an important new component of the myogenic program (Maeda, 2002).

The cell population and the activity of the organizer change during the course of development. The mechanism of mouse node development has been addressed via an analysis of the node/notochord enhancer (NE) of Foxa2. The core element (CE) of the enhancer, which in multimeric form drives gene expression in the node, was identified. The CE is activated in Wnt/ß-catenin-treated P19 cells with a time lag, and this activation is dependent on two separate sequence motifs within the CE. These same motifs are also required for enhancer activity in transgenic embryos. The Tead family of transcription factors was identified as binding proteins for the 3' motif. Teads and their co-factor YAP65 (Drosophila homolog: Yorkie) activate the CE in P19 cells, and binding of Tead to CE is essential for enhancer activity. Inhibition of Tead activity by repressor-modified Tead compromises NE enhancer activation and notochord development in transgenic mouse embryos. Furthermore, manipulation of Tead activity in zebrafish embryos leads to altered expression of foxa2 in the embryonic shield. These results suggest that Tead activates the Foxa2 enhancer core element in the mouse node in cooperation with a second factor that binds to the 5' element, and that a similar mechanism also operates in the zebrafish shield (Sawada, 2005).

Redundant roles of Tead1 and Tead2 in notochord development and the regulation of cell proliferation and survival

Four members of the TEAD/TEF family of transcription factors are expressed widely in mouse embryos and adult tissues. Although in vitro studies have suggested various roles for TEAD proteins, their in vivo functions remain poorly understood. This study examined the role of Tead genes by generating mouse mutants for Tead1 and Tead2. Tead2-/- mice appeared normal, but Tead1-/-; Tead2-/- embryos die at embryonic day 9.5 (E9.5) with severe growth defects and morphological abnormalities. At E8.5, Tead1-/-; Tead2-/- embryos are already small and lack characteristic structures such as a closed neural tube, a notochord, and somites. Despite these overt abnormalities, differentiation and patterning of the neural plate and endoderm are relatively normal. In contrast, the paraxial mesoderm and lateral plate mesoderm are displaced laterally, and a differentiated notochord is not maintained. These abnormalities and defects in yolk sac vasculature organization resemble those of mutants for Yap, which encodes a coactivator of TEAD proteins. Moreover, genetic interactions were demonstrated between Tead1 and Tead2 and Yap. Finally, Tead1-/-; Tead2-/- embryos showed reduced cell proliferation and increased apoptosis. These results suggest that Tead1 and Tead2 are functionally redundant, use YAP as a major coactivator, and support notochord maintenance as well as cell proliferation and survival in mouse development (Sawada, 2008).

TEAD/TEF transcription factors utilize the activation domain of YAP65, a Src/Yes-associated protein localized in the cytoplasm

Mammals express four highly conserved TEAD/TEF transcription factors that bind the same DNA sequence, but serve different functions during development. TEAD-2/TEF-4 protein purified from mouse cells is associated predominantly with a novel TEAD-binding domain at the amino terminus of YAP65 (Drosophila homolog: Yorkie), a powerful transcriptional coactivator. YAP65 interacts specifically with the carboxyl terminus of all four TEAD proteins. Both this interaction and sequence-specific DNA binding by TEAD are required for transcriptional activation in mouse cells. Expression of YAP in lymphocytic cells that normally do not support TEAD-dependent transcription (e.g., MPC11) result in up to 300-fold induction of TEAD activity. Conversely, TEAD overexpression squelches YAP activity. Therefore, the carboxy-terminal acidic activation domain in YAP is the transcriptional activation domain for TEAD transcription factors. However, whereas TEAD was concentrated in the nucleus, excess YAP65 accumulates in the cytoplasm as a complex with the cytoplasmic localization protein, 14-3-3. Because TEAD-dependent transcription is limited by YAP65, and YAP65 also binds Src/Yes protein tyrosine kinases, it is proposed that YAP65 regulates TEAD-dependent transcription in response to mitogenic signals (Vassilev, 2001).

The transcriptional co-activator TAZ interacts TEF-1 family members

Members of the highly related TEF-1 (transcriptional enhancer factor-1) family (also known as TEAD, for TEF-1, TEC1, ABAA domain) bind to MCAT (muscle C, A and T sites) and A/T-rich sites in promoters active in cardiac, skeletal and smooth muscle, placenta, and neural crest. TEF-1 activity is regulated by interactions with transcriptional co-factors [p160, TONDU (Vgl-1, Vestigial-like protein-1), Vgl-2 and YAP65 (Yes-associated protein 65 kDa)]. The strong transcriptional co-activator YAP65 interacts with all TEF-1 family members, and, since YAP65 is related to TAZ (transcriptional co-activator with PDZ-binding motif), it was asked whether TAZ also interacts with members of the TEF-1 family. In the present study, it was shown by GST (glutathione S-transferase) pull-down assays, by co-immunoprecipitation and by modified mammalian two-hybrid assays that TEF-1 interacts with TAZ in vitro and in vivo. Electrophoretic mobility-shift assays with purified TEF-1 and GST-TAZ fusion protein showed that TAZ interacts with TEF-1 bound to MCAT DNA. TAZ can interact with endogenous TEF-1 proteins, since exogenous TAZ activates MCAT-dependent reporter promoters. Like YAP65, TAZ interacts with all four TEF-1 family members. GST pull-down assays with increasing amounts of [35S]TEF-1 and [35S]RTEF-1 (related TEF-1) showed that TAZ interacts more efficiently with TEF-1 than with RTEF-1. This differential interaction also extended to the interaction of TEF-1 and RTEF-1 with TAZ in vivo, as assayed by a modified mammalian two-hybrid experiment. These data show that differential association of TEF-1 proteins with transcriptional co-activators may regulate the activity of TEF-1 family members (Mahoney, 2005).

Mammalian Tead proteins regulate cell proliferation and contact inhibition as transcriptional mediators of Hippo signaling

Regulation of organ size is important for development and tissue homeostasis. In Drosophila, Hippo signaling controls organ size by regulating the activity of a TEAD transcription factor, Scalloped, through modulation of its co-activator protein Yki. This study shows that mouse Tead proteins regulate cell proliferation by mediating Hippo signaling. In NIH3T3 cells, cell density and Hippo signaling regulated the activity of endogenous Tead proteins by modulating nuclear localization of a Yki homolog, Yap1, and the resulting change in Tead activity altered cell proliferation. Tead2-VP16 mimicked Yap1 overexpression, including increased cell proliferation, reduced cell death, promotion of EMT, lack of cell contact inhibition and promotion of tumor formation. Growth-promoting activities of various Yap1 mutants correlated with their Tead-co-activator activities. Tead2-VP16 and Yap1 regulated largely overlapping sets of genes. However, only a few of the Tead/Yap1-regulated genes in NIH3T3 cells were affected in Tead1-/-;Tead2-/- or Yap1-/- embryos. Most of the previously identified Yap1-regulated genes were not affected in NIH3T3 cells or mutant mice. In embryos, levels of nuclear Yap1 and Tead1 varied depending on cell type. Strong nuclear accumulation of Yap1 and Tead1 were seen in myocardium, correlating with requirements of Tead1 for proliferation. However, their distribution did not always correlate with proliferation. Taken together, mammalian Tead proteins regulate cell proliferation and contact inhibition as a transcriptional mediator of Hippo signaling, but the mechanisms by which Tead/Yap1 regulate cell proliferation differ depending on the cell type, and Tead, Yap1 and Hippo signaling may play multiple roles in mouse embryos (Ota, 2009).

Structural insights into the YAP and TEAD complex

The Yes-associated protein (YAP) transcriptional coactivator is a key regulator of organ size and a candidate human oncogene inhibited by the Hippo tumor suppressor pathway. The TEAD family of transcription factors binds directly to and mediates YAP-induced gene expression. This study reports the three-dimensional structure of the YAP (residues 50-171)-TEAD1 (residues 194-411) complex, in which YAP wraps around the globular structure of TEAD1 and forms extensive interactions via three highly conserved interfaces. Interface 3, including YAP residues 86-100, is most critical for complex formation. This study reveals the biochemical nature of the YAP-TEAD interaction, and provides a basis for pharmacological intervention of YAP-TEAD hyperactivation in human diseases (Li, 2010).

Structural basis of YAP recognition by TEAD4 in the hippo pathway

The Hippo signaling pathway controls cell growth, proliferation, and apoptosis by regulating the expression of target genes that execute these processes. Acting downstream from this pathway is the YAP transcriptional coactivator, whose biological function is mediated by the conserved TEAD family transcription factors. The interaction of YAP with TEADs is critical to regulate Hippo pathway-responsive genes. This study describe the crystal structure of the YAP-interacting C-terminal domain of TEAD4 in complex with the TEAD-interacting N-terminal domain of YAP. The structure reveals that the N-terminal region of YAP is folded into two short helices with an extended loop containing the PXXPhiP motif in between, while the C-terminal domain of TEAD4 has an immunoglobulin-like fold. YAP interacts with TEAD4 mainly through the two short helices. Point mutations of TEAD4 indicate that the residues important for YAP interaction are required for its transforming activity. Mutagenesis reveals that the PXXPhiP motif of YAP, although making few contacts with TEAD4, is important for TEAD4 interaction as well as for the transforming activity (Chen, 2010).

Transcription factor TEAD4 specifies the trophectoderm lineage at the beginning of mammalian development

Specification of cell lineages in mammals begins shortly after fertilization with formation of a blastocyst consisting of trophectoderm, which contributes exclusively to the placenta, and inner cell mass (ICM), from which the embryo develops. This study reports that ablation of the mouse Tead4 gene results in a preimplantation lethal phenotype, and TEAD4 is one of two highly homologous TEAD transcription factors that are expressed during zygotic gene activation in mouse 2-cell embryos. Tead4-/- embryos do not express trophectoderm-specific genes, such as Cdx2, but do express ICM-specific genes, such as Oct4 (also known as Pou5f1). Consequently, Tead4-/- morulae do not produce trophoblast stem cells, trophectoderm or blastocoel cavities, and therefore do not implant into the uterine endometrium. However, Tead4-/- embryos can produce embryonic stem cells, a derivative of ICM, and if the Tead4 allele is not disrupted until after implantation, then Tead4-/- embryos complete development. Thus, Tead4 is the earliest gene shown to be uniquely required for specification of the trophectoderm lineage (Yagi, 2007).

Tead4 is required for specification of trophectoderm in pre-implantation mouse embryos

During pre-implantation mouse development, embryos form blastocysts with establishment of the first two cell lineages: the trophectoderm (TE) which gives rise to the placenta, and the inner cell mass (ICM) which will form the embryo proper. Differentiation of TE is regulated by the transcription factor Caudal-related homeobox 2 (Cdx2), but the mechanisms which act upstream of Cdx2 expression remain unknown. This study shows that the TEA domain family transcription factor, Tead4, is required for TE development. Tead1, Tead2 and Tead4 were expressed in pre-implantation embryos, and at least Tead1 and Tead4 were expressed widely in both TE and ICM lineages. Tead4-/- embryos died at pre-implantation stages without forming the blastocoel. The mutant embryos continued cell proliferation, and adherens junction and cell polarity were not significantly affected. In Tead4-/- embryos, Cdx2 was weakly expressed at the morula stage but was not expressed in later stages. None of the TE specific genes, including Eomes and a Cdx2 independent gene, Fgfr2, was detected in Tead4-/- embryos. Instead, the ICM specific transcription factors, Oct3/4 and Nanog, were expressed in all the blastomeres. Tead4-/- embryos also failed to differentiate trophoblast giant cells when they were cultured in vitro. ES cells with normal in vitro differentiation abilities were established from Tead4-/- embryos. These results suggest that Tead4 has a distinct role from Tead1 and Tead2 in trophectoderm specification of pre-implantation embryos, and that Tead4 is an early transcription factor required for specification and development of the trophectoderm lineage, which includes expression of Cdx2 (Nishioka, 2008).

The Hippo signaling pathway components Lats and Yap pattern Tead4 activity to distinguish mouse trophectoderm from inner cell mass

Outside cells of the preimplantation mouse embryo form the trophectoderm (TE), a process requiring the transcription factor Tead4. This study shows that transcriptionally active Tead4 can induce Cdx2 and other trophoblast genes in parallel in embryonic stem cells. In embryos, the Tead4 coactivator protein Yap localizes to nuclei of outside cells, and modulation of Tead4 or Yap activity leads to changes in Cdx2 expression. In inside cells, Yap is phosphorylated and cytoplasmic, and this involves the Hippo signaling pathway component Lats. It is proposed that active Tead4 promotes TE development in outside cells, whereas Tead4 activity is suppressed in inside cells by cell contact- and Lats-mediated inhibition of nuclear Yap localization. Thus, differential signaling between inside and outside cell populations leads to changes in cell fate specification during TE formation (Nishioka, 2009).

During mouse development, the first lineage specified is the trophoblast/placenta lineage, set aside during blastocyst formation. In the blastocyst, the trophoblast, or trophectoderm (TE), surrounds the inner cell mass (ICM), which will give rise to the fetus and other extraembryonic tissues. The homeodomain transcription factor Cdx2 is expressed in the TE at the blastocyst stage. Cdx2 is required for TE development and is sufficient to promote trophoblast fate in ICM-derived embryonic stem (ES) cells, including suppression of ICM and induction of TE genes. Conversely, ICM fates are regulated by a distinct set of transcription factors, including the POU family transcription factor Oct3/4 (encoded by Pou5f1). Prior to blastocyst formation, Cdx2 and Oct3/4 are initially coexpressed throughout the embryo. Mutual antagonism between these two factors may contribute to the eventual segregation of their expression domains, with Cdx2 in outside cells of the TE and Oct3/4 in inside cells of the ICM. However, molecular mechanisms that initially interpret inside/outside positional information within the embryo to establish this pattern are not known (Nishioka, 2009 and references therein).

The TEAD/TEF family transcription factor Tead4 is essential for TE development and Cdx2 expression prior to the blastocyst stage. This observation provided the first clue about molecular mechanisms acting upstream of the TE/ICM lineage distinction. However, whether Tead4 acts permissively or instructively in this process has been unclear, since Tead4 itself is not restricted to outside cells (Nishioka, 2009).

This study sought to identify cofactors and signaling components that could impart positional information to spatially regulate Tead4 activity in the embryo. Many lines of evidence have suggested that TEAD-mediated transcription is regulated by the Ser/Thr kinase Hippo in Drosophila, or Stk3 (Mst) in mice. In Drosophila, Hippo inhibits the Yorkie (Yki) coactivator and suppresses cell proliferation. These activities are mediated by a Tead protein, Scalloped. In mammals, Hippo signaling comprises a growth-regulating pathway, which controls cell contact-mediated inhibition of proliferation. In this context, cell-cell contact regulates nuclear accumulation of a Yki homolog, Yes-associated protein 1 (Yap1, Yap hereafter), through Hippo signaling and controls cell proliferation by regulating transcriptional activity of Tead proteins. In the mouse embryo, Tead1-/-; Tead2-/- mutants die soon after implantation due to reduced cell proliferation and increased apoptosis. Tead1/2 interact genetically with Yap, suggesting that the roles of these genes in Hippo signaling are conserved in mice (Nishioka, 2009).

This study examined the role of Tead4 in TE development using both ES cells and preimplantation embryos. Active Tead4 can promote multiple trophoblast genes in parallel, including Cdx2. In the embryo, the Tead coactivator Yap localizes to nuclei only in outside cells and is excluded from inside cell nuclei by the Hippo signaling pathway component Lats. These observations suggest that Tead4/Yap interpret positional information along the inside/outside axis of the embryo to restrict expression of Cdx2 and TE fates to outside cells (Nishioka, 2009).

Acetylation of VGLL4 regulates Hippo-YAP signaling and postnatal cardiac growth

Binding of the transcriptional co-activator YAP (see Drosophila yki) with the transcription factor TEAD (see Drosophila sd) stimulates growth of the heart (see dorsal vessel in Drosophila) and other organs. YAP overexpression potently stimulates fetal cardiomyocyte (CM) proliferation, but YAP's mitogenic potency declines postnatally. While investigating factors that limit YAP's postnatal mitogenic activity, this study found that the CM-enriched TEAD1 binding protein VGLL4 (see Drosophila CG1737) inhibits CM proliferation by inhibiting TEAD1-YAP interaction and by targeting TEAD1 for degradation. Importantly, VGLL4 acetylation at lysine 225 negatively regulated its binding to TEAD1. This developmentally regulated acetylation event critically governs postnatal heart growth, since overexpression of an acetylation-refractory VGLL4 mutant enhanced TEAD1 degradation, limits neonatal CM proliferation, and causes CM necrosis. These data define an acetylation-mediated, VGLL4-dependent switch that regulates TEAD stability and YAP-TEAD activity. These insights may improve targeted modulation of TEAD-YAP activity in applications from cardiac regeneration to cancer (Lin, 2016).

Lats2 Modulates Adipocyte Proliferation and Differentiation via Hippo Signaling

First identified in Drosophila and highly conserved in mammals, the Hippo pathway controls organ size. Lats2 is one of the core kinases of the Hippo pathway and plays major roles in cell proliferation by interacting with the downstream transcriptional cofactors YAP and TAZ. Although the function of the Hippo pathway and Lats2 is relatively well understood in several tissues and organs, less is known about the function of Lats2 and Hippo signaling in adipose development. This study shows that Lats2 is an important modulator of adipocyte proliferation and differentiation via Hippo signaling. Upon activation, Lats2 phosphorylates YAP and TAZ, leading to their retention in the cytoplasm, preventing them from activating the transcription factor TEAD in the nucleus. Because TAZ remains in the cytoplasm, PPARgamma regains its transcriptional activity. Furthermore, cytoplasmic TAZ acts as an inhibitor of Wnt signaling by suppressing DVL2, thereby preventing beta-catenin from entering the nucleus to stimulate TCF/LEF transcriptional activity. The above effects contribute to the phenotype of repressed proliferation and accelerated differentiation in adipocytes. Thus, Lats2 regulates the balance between proliferation and differentiation during adipose development. Interestingly, this study provides evidence that Lats2 not only negatively modulates cell proliferation but also positively regulates cell differentiation (An, 2013).

YAP1 is amplified and up-regulated in hedgehog-associated medulloblastomas and mediates Sonic hedgehog-driven neural precursor proliferation

Medulloblastoma is the most common solid malignancy of childhood, with treatment side effects reducing survivors' quality of life and lethality being associated with tumor recurrence. Activation of the Sonic hedgehog (Shh) signaling pathway is implicated in human medulloblastomas. Cerebellar granule neuron precursors (CGNPs) depend on signaling by the morphogen Shh for expansion during development, and have been suggested as a cell of origin for certain medulloblastomas. Mechanisms contributing to Shh pathway-mediated proliferation and transformation remain poorly understood. This study investigated interactions between Shh signaling and the recently described tumor-suppressive Hippo pathway in the developing brain and medulloblastomas. Up-regulation is reported of the oncogenic transcriptional coactivator yes-associated protein 1 (YAP1; homolog of Drosophila Yorkie), which is negatively regulated by the Hippo pathway, in human medulloblastomas with aberrant Shh signaling. Consistent with conserved mechanisms between brain tumorigenesis and development, Shh induces YAP1 expression in CGNPs. Shh also promotes YAP1 nuclear localization in CGNPs, and YAP1 can drive CGNP proliferation. Furthermore, YAP1 is found in cells of the perivascular niche, where proposed tumor-repopulating cells reside. Post-irradiation, YAP1 was found in newly growing tumor cells. These findings implicate YAP1 as a new Shh effector that may be targeted by medulloblastoma therapies aimed at eliminating medulloblastoma recurrence (Fernandez-L, 2009).

This study demonstrates that YAP1 and its transcriptional partner, TEAD1, are highly expressed in Shh-driven medulloblastomas in both humans and mice. YAP1 is amplified in a subset of human medulloblastomas -- specifically, SHH-associated medulloblastomas. Moreover, YAP1 expression is up-regulated by the Shh pathway in proliferating CGNPs. Shh signaling regulates YAP1 nuclear localization through its binding to IRS1, and YAP1 activity promotes CGNP proliferation, at least in part through interactions with TEAD1. In mouse medulloblastomas, YAP1 protein localized to the cells occupying the perivascular niche (PVN) that have been proposed to have cancer stem cell properties. Indeed, YAP1-positive cells remain alive and disseminated through the tumor after the tumor bulk cells have been eradicated by radiation. These findings mark YAP1 as a mediator of normal proliferation in the developing cerebellum, and as a potential target for medulloblastoma therapies aimed at eliminating tumor-reinitiating cells (Fernandez-L, 2009).

Genetic and pharmacological disruption of the TEAD-YAP complex suppresses the oncogenic activity of YAP

The Drosophila TEAD ortholog Scalloped is required for Yki-mediated overgrowth but is largely dispensable for normal tissue growth, suggesting that its mammalian counterpart may be exploited for selective inhibition of oncogenic growth driven by YAP hyperactivation. This hypothesis was tested genetically and pharmacologically. A dominant-negative TEAD molecule was shown not to perturb normal liver growth but potently suppresses hepatomegaly/tumorigenesis resulting from YAP overexpression or Neurofibromin 2 (NF2)/Merlin inactivation. Verteporfin was identified as a small molecule that inhibits TEAD-YAP association and YAP-induced liver overgrowth. These findings provide proof of principle that inhibiting TEAD-YAP interactions is a pharmacologically viable strategy against the YAP oncoprotein (Liu-Chittenden, 2012).

Scalloped homologs and viral gene expression

The large tumor antigen (TAg) of simian virus 40 regulates transcription of the viral genes. The early promoter is repressed when TAg binds to the origin and DNA replication begins, whereas the late promoter is activated by TAg through both replication-dependent and -independent mechanisms. Activation is diminished when a site in the viral enhancer to which the factor TEF-1 binds is disrupted. The NH2-terminal region of TAg binds to the TEA domain of TEF-1, a DNA binding domain also found in Drosophila Scalloped and the S. cerevisiae TEC1 proteins. The interaction inhibits DNA binding by TEF-1 and activates transcription in vitro from a subset of naturally occurring late start sites. These sites are also activated by mutations in the DNA motifs to which TEF-1 binds. Therefore, TEF-1 appears to function as a repressor of late transcription, and is involved in the early-to-late shift in viral transcription. The mutation of Ser-189 in TAg, which reduces transformation efficiency in certain assays, disrupts the interaction with TEF-1. Thus, TEF-1 might also regulate genes involved in growth control (Berger, 1996).

scalloped: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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