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Molecular and Cellular Biology, May 2008, p. 3236-3244, Vol. 28, No. 10
0270-7306/08/$08.00+0     doi:10.1128/MCB.01754-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Activator-to-Repressor Conversion of T-Box Transcription Factors by the Ripply Family of Groucho/TLE-Associated Mediators ^,

Akinori Kawamura,^1,2,3 Sumito Koshida,^1,4, and Shinji Takada^1,4*

Okazaki Institute for Integrative Biosciences, National Institutes of Natural Sciences, Okazaki, Aichi 444-8787, Japan,¹ Graduate School of Science and Engineering, Bioscience and Biomedical Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8050, Japan,² Department of Life Science, Graduate School of Science and Engineering, Saitama University, Sakura-ku, Saitama 338-8570, Japan,³ Department of Basic Biology, Graduate University for Advanced Studies (SOKENDAI), Okazaki, Aichi 444-8787, Japan⁴

Received 25 September 2007/ Returned for modification 26 November 2007/ Accepted 28 February 2008

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The T-box family of transcription factors, defined by a conserved DNA binding domain called the T-box, regulate various aspects of embryogenesis by activating and/or repressing downstream genes. In spite of the biological significance of the T-box proteins, how they regulate transcription remains to be elucidated. Here we show that the Groucho/TLE-associated protein Ripply converts T-box proteins from activators to repressors. In cultured cells, zebrafish Ripply1, an essential component in somite segmentation, and its structural relatives, Ripply2 and -3, suppress the transcriptional activation mediated by the T-box protein Tbx24, which is coexpressed with ripply1 during segmentation. Ripply1 associates with Tbx24 and converts it to a repressor. Ripply1 also antagonizes the transcriptional activation of another T-box protein, No tail (Ntl), the zebrafish ortholog of Brachyury. Furthermore, injection of a high dosage of ripply1 mRNA into zebrafish eggs causes defective development of the posterior trunk, similar to the phenotype observed in homozygous mutants of ntl. A mutant form of Ripply1 defective in association with Tbx24 also lacks activity in zebrafish embryos. These results indicate that the intrinsic transcriptional property of T-box proteins is controlled by Ripply family proteins, which act as specific adaptors that recruit the global corepressor Groucho/TLE to T-box proteins.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
T-box proteins (Tbx) play many crucial roles in development by activating and/or repressing the transcription of their target genes (23, 32). For instance, reduced function of T/Brachyury, the founder member of this family, causes a truncated tail in the mouse (10). The importance of T-box transcription factors is also shown by the fact that mutations in human T-box genes cause severe congenital disorders, such as DiGeorge, ulnar-mammary, and Holt-Oram syndromes (26). Furthermore, the Tbx family is evolutionally conserved from Caenorhabditis elegans to insects and vertebrates, showing remarkable functional conservation across species. However, in spite of the biological significance of the T-box proteins, how they regulate transcription remains to be elucidated.

One of the processes in which T-box proteins play crucial roles is somite segmentation in vertebrates (3, 4, 24). Somitogenesis is the sequential subdivision of segmented precursors of the vertebral column and musculature from the presomitic mesoderm (PSM) (27, 29). Prior to morphological subdivision, a segmental prepattern is established in the anterior PSM. Genetic analyses of the mouse and the zebrafish indicate that several Tbx genes are required for distinct processes of this prepattern formation (11, 24, 36), which also involves Notch signaling and Mesp2, a basic helix-loop-helix transcription factor (21, 33). For instance, Tbx6 induces the expression of Mesp2 and is required for proper patterning of somites of the mouse (36). Similarly, tbx24/fused somites, structurally related to mouse Tbx6, is expressed in the anterior PSM of the zebrafish embryo and required for the formation of somite boundaries and the expression of mesp-b, a zebrafish homolog of Mesp2 (24, 30), suggesting that Tbx24 is involved in the transcriptional regulation of the mesp-b locus in zebrafish. Because Mesp2, as well as mesp-b, is expressed in only a portion of Tbx-expressing cells (24, 28, 36), however, another molecule also seems to be involved in Tbx-mediated transcriptional regulation of mesp genes in the anterior PSM.

In a previous study, we showed that Ripply1, which is associated with the global transcriptional corepressor Groucho/TLE, terminates the expression of segmentation genes in the anterior PSM of zebrafish embryos (16). In ripply1-deficient embryos, the expression of mesp-b is up-regulated in a cell-autonomous manner, whereas in embryos injected with ripply1 mRNA, the expression of mesp-b is highly suppressed in the anterior PSM (16). These results suggest that Ripply1 regulates the proper expression of mesp-b in the anterior PSM. Taking into account that the expression of mesp-b could be induced by T-box proteins, we can speculate that Ripply1 may antagonize the function of T-box proteins in the transcription of mesp genes. Therefore, we examined precisely the relationship between Tbx24 and Ripply1 in the transcription of mesp-b. We showed that Ripply1 is coprecipitated with Tbx24 and converts it from an activator to a repressor in cultured cells. The other members of the Ripply family, Ripply2 and Ripply3, can also antagonize the transcriptional activation mediated by Tbx24. On the other hand, Ripply1 also antagonizes the transcriptional activation of another T-box protein, No tail (Ntl), both in vitro and in vivo. These results reveal a molecular mechanism underlying dynamic conversion of the regulatory property of T-box proteins in the transcription of their targets.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Plasmid construction. The mesp-b 2.5-kb luciferase reporter plasmid was constructed by inserting a 2.5-kb genomic fragment upstream of the translational initiation site of zebrafish mesp-b fused to the firefly luciferase gene in the pGL3 Basic vector (Promega). For pBP-TbxX2 and pBP-mutTbxX2, a T-box site (5'-AATTCACACCT-3') and a mutT-box sequence (5'-AATTCAgACCT-3'), respectively, were inserted into the beta interferon basal promoter fused to the luciferase gene.

Luciferase assay and TSA treatment. Human embryonic kidney 293T cells were cultured at 37°C in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal calf serum and antibiotics. For luciferase assay, 293T cells (1.0 x 10⁴/well) were seeded into 24-well collagen-coated plates (Iwaki). The cells were transiently transfected with the reporter gene plasmids along with the indicated expression vector, using Fugene 6 reagent (Roche). The total amount of DNA in each transfection mix was kept the same by supplementation with the empty vector pCS2+. After incubation for 24 h, the cells were washed with phosphate-buffered saline (Ca²⁺ and Mg²⁺ free) and lysed, and luciferase activity was assayed by using a dual-luciferase reporter assay system (Promega). For normalization of the transfection efficiencies, pRL-tk-Luc (Promega), carrying a Renilla luciferase reporter gene under the control of the thymidine kinase promoter, was cotransfected. The average normalized firefly luciferase level with mock transfection was set at 100%, and error bars represent standard deviations. All transfections were performed in triplicate, and similar results were obtained in at least duplicate experiments. For trichostatin A (TSA) treatment, the cells were first incubated with DMEM for 12 h posttransfection and with DMEM containing 300 nM TSA (Sigma). After incubation for another 24 h, the cells were lysed for analysis of luciferase activity.

Coimmunoprecipitation. Whole-cell extracts were prepared from 293T cells transfected with the various expression vectors and incubated with either anti-Myc (4A6; Upstate Biotechnology) or anti-FLAG (D-8; Santa Cruz Biotechnology) antibody for 1 h at 4°C. The antigen-antibody complexes were collected using protein G-Sepharose (GE Healthcare) and washed several times. Precipitated proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretically transferred to polyvinylidene difluoride membranes. Western blotting was carried out according to a standard procedure by using rabbit polyclonal anti-FLAG or mouse monoclonal anti-Myc antibodies.

ChIP experiments. Chromatin immunoprecipitation (ChIP) experiments were performed by using some modifications of a previously described method (20). Briefly, 293T cells (2.0 x 10⁶/10-cm dish) were transfected with a linearized mesp-b 2.5-kb luciferase gene and each expression vector. After a 40-h incubation, the cells were cross-linked with formaldehyde, and the genomic DNA was sheared to an average size of 400 bp by sonication. Sonicated samples were incubated with anti-Myc antibody (9E10; Santa Cruz Biotechnology) or with anti-FLAG antibody (D9; SantaCruz), collected with protein G-Sepharose (GE Healthcare), and washed several times. After reversal of the cross-link, the DNA isolated by ChIP was amplified by PCR using the following primers: T-site containing region-s, 5'-CAACAAACACAAAAAGCACACGTT-3'; T-site containing region-as, 5'-GGTGAAAGGAGGATGGAGGTTTAT-3'; Proximal-region-s, 5'-GCATCTCTATTGACGATATC-3'; Proximal-region-as, 5'-CATTTCATTGCTCTAGCTCA-3'; Distal-region-s, 5'-TTTCAGAGCACAACAAAAGCAGTG-3'; and Distal-region-as, 5'-TTATGTATGCATGAACGCAGATGG-3'.

Fish and microinjection. The zebrafish TL2 inbred line was used for fish experiments (18). Capped mRNA encoding the entire open reading frame of ripply1 was synthesized from linearized pCS2+ripply1 (8) by using an mMessage mMachine Sp6 kit (Ambion). The sequences of morpholino antisense oligonucleotides (MOs) used in the study were as follows: ripply1 MO, 5'-CATCGTCACTGTGTTTTTCGTTTTG-3'; tbx24 MO, 5'-CATTTCCACACCCAGCATGTCTCGG-3'; and deltaD MO, 5'-AACAGCTATCATTAGTCGTCCCATG-3'.

Synthesized mRNA or MO was injected into the yolks of fertilized embryos at the one-cell stage.

Whole-mount in situ hybridization. Whole-mount in situ hybridization was performed as described previously (17). An antisense RNA probe was transcribed according to a standard procedure.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Up-regulation of mesp-b mRNA in ripply1-deficient embryos is dependent on Tbx24. In ripply1-deficient embryos, the expression of mesp-b was earlier shown to be up-regulated in the anterior PSM and extended anteriorly (16). The expression pattern of mesp-b in the anterior PSM is also known to be perturbed in zebrafish embryos defective in either Notch signaling or the transcription factor Tbx24 (12, 24, 30). Thus, as a first approach to address how Ripply1 regulates mesp-b expression, we investigated the epistatic relationship between ripply1 and tbx24 or deltaD, encoding a Notch ligand. As previously described, the expression of mesp-b was significantly reduced in tbx24 MO-injected embryos (Fig. 1A) (n = 12; 100% were affected), suggesting the activator function of Tbx24 (24, 30), but was up-regulated in ripply1 MO-injected embryos (n = 15; 100% were affected) (16). In embryos injected with both tbx24 MO and ripply1 MO, the mesp-b mRNA was almost completely suppressed, as in the tbx24 MO-injected embryos (Fig. 1A) (n = 7; 100% were affected). In contrast, the anteriorly expanded expression of mesp-b caused by the ripply1 MO injection was not reduced in the embryos injected with deltaD MO and ripply1 MO, although its expression in the PSM was perturbed, showing a scattered salt-and-pepper pattern (Fig. 1B) (n = 9; 100% were affected). These results suggest that the transcriptional suppression of mesp-b mRNA by Ripply1 depends on the function of Tbx24 in zebrafish embryos.

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FIG. 1. Tbx24-dependent transcriptional suppression of mesp-b mRNA by Ripply1 in zebrafish embryos, shown by whole-mount in situ hybridization. The stained embryos were flat mounted by removing the yolk (dorsal view and anterior to the top). The MOs were injected into the fertilized embryos at the appropriate concentration as previous described. (A) Ectopic expression of mesp-b mRNA is not detectable in tbx24 and ripply1 MO-injected embryos. The embryos were simultaneously stained with the probes for no tail (notochord and tailbud) and mesp-b (segmental expression in the anterior PSM). Expression of mesp-b, which is normally restricted to the anterior PSM, was significantly decreased in tbx24 MO-injected embryos. In ripply1 MO-injected embryos, mesp-b mRNA was ectopically expressed in the presumptive somitic regions. In both the tbx24 and ripply1 MO-injected embryos, mesp-b mRNA was not ectopically expressed in the somitic regions, which is totally different from what was observed in ripply1 MO-injected embryos. Staining of no tail appeared to be comparable in either case. (B) Ectopic expression of mesp-b mRNA was detected in the deltaD and ripply1 MO-injected embryos. The embryos were stained with the probe for mesp-b. Expression of mesp-b mRNA was scattered in the anterior PSM of deltaD MO-injected embryos. In deltaD and ripply1 MO-injected embryos, expression of mesp-b mRNA was not properly terminated in the anterior PSM, in a similar manner to that in ripply1 MO-injected embryos.

Ripply1, as well as Ripply2 and -3, antagonizes the transcriptional activation of Tbx24 through the T-box binding site. To investigate more precisely the relationship between Tbx24 and Ripply1 in the transcription of mesp-b, we next carried out in vitro luciferase reporter assays in which the reporter gene was under the control of the 2.5-kb genomic fragment upstream of the mesp-b initiation codon (Fig. 2A). We found that Tbx24 significantly activated the luciferase activity in a dose-dependent manner (Fig. 2B). This Tbx24-mediated activation of the reporter gene, however, was completely prevented by cotransfection with Ripply1 (Fig. 2C). In contrast, Ripply1 did not significantly affect the enhancement of reporter activity by the Notch intracellular domain of notch1a (Fig. 2D), which is a constitutively active form of the Notch receptor (34). These results indicate that Ripply1 specifically suppresses transcriptional activation by Tbx24.

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FIG. 2. Ripply1 antagonizes luciferase reporter gene activation mediated by Tbx24. Luciferase assays with 293T cells were conducted to examine the transcriptional relationship between Ripply1 and Tbx24. The amounts of plasmids used for transfection are shown under each bar. The average normalized firefly luciferase activity with pCS2+ alone was set at 100%, and error bars represent the standard deviations. (A) Schematic representation of the mesp-b 2.5-kb luciferase reporter construct. Two T sites and RBP-J-binding sequences are emphasized with boxes, and numbers show their locations relative to the mesp-b initiation codon. (B) Tbx24 significantly activates reporter gene activity in 293T cells. (C) Ripply1 completely suppresses the enhancement of Tbx24 reporter activity. (D) Ripply1 does not suppress the transcriptional up-regulation of the reporter gene by the Notch intracellular domain (NICD).

To identify the regulatory element targeted by Ripply1, we next constructed a series of reporter plasmids containing progressive 5' deletions of the 2.5-kb mesp-b fragment fused to the luciferase gene. As summarized in Fig. 3A, reporter genes containing at least 82 bp upstream of mesp-b could be activated by Tbx24 and suppressed by Ripply1, but those with 56 bp upstream of mesp-b were not affected by either Tbx24 or Ripply1. Because a putative T-box binding site (T site) exists between 82 and 56 bp upstream of mesp-b, we next determined whether the T site (5'-AATTCACACCT-3') is sufficient for the suppression of Tbx24 activity by Ripply1. The suppression by Ripply1 was also recapitulated by pBP-TbxX2, which contains two tandem T sites upstream of the beta interferon basal promoter and the firefly luciferase gene (Fig. 3B). This suppression by Ripply1 was abolished by substitution of a single nucleotide within the T site that is crucial for T-box binding (mutT) (8, 22), suggesting that the regulatory element targeted by Ripply1 cannot be separated from the T site targeted by Tbx24.

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FIG. 3. Ripply1 antagonizes the transcriptional activation of Tbx24 through the Tbx binding site. (A) Deletion constructs of the mesp-b luciferase reporter gene. The effects of Tbx24 and Ripply1 on each reporter gene are summarized on the right side of the panel. (B) T site is sufficient for the transcriptional suppression of Tbx24 by Ripply1. The sequence of each T site or mutated T site inserted upstream of the beta interferon basal promoter of the luciferase reporter gene is shown. (C) Using the pBP-TbxX2 reporter gene, zebrafish Ripply1, as well as Ripply2 and -3, antagonizes the transcriptional activation of Tbx24 through the T site in a dose-dependent manner.

In addition to Ripply1, zebrafish Ripply2 and Ripply3 also antagonized Tbx24 activation in a dose-dependent manner (Fig. 3C). Thus, the Ripply family proteins can commonly antagonize transcriptional activation by Tbx24.

Ripply1-Tbx24 complex associates with T-box binding site. The inability to separate the regulatory element for Ripply from the Tbx-binding element indicated that Ripply1 might inhibit Tbx24 transcriptional activation via a physical interaction. To examine this possibility, we generated Myc-tagged Ripply1 and FLAG-tagged Tbx24 proteins. We confirmed that both proteins were functional in the luciferase assay (see Fig. S1 in the supplemental material). The two proteins were predominantly colocalized in the nucleus (Fig. 4A), suggesting that the suppression by Ripply1 is not due to removal of Tbx24 from the nucleus. Rather, coimmunoprecipitation assays showed preferential association of Tbx24-FLAG with Myc-Ripply1, and vice versa, in 293T cell lysates (Fig. 4B). To examine whether this Tbx24-Ripply1 complex is associated with DNA at the T site, a ChIP assay was performed with 293T cells. Myc-Ripply1 was specifically associated with the DNA region containing T sites (T region), but not with another two regions in the 5' region upstream of mesp-b, only in the presence of Tbx24-FLAG (Fig. 4C). On the other hand, Tbx24-FLAG was consistently associated with the T region independently of Ripply1 (Fig. 4C). These results strongly suggest that Ripply1 represses the function of Tbx24 by forming a complex with Tbx24 that binds to the T site.

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FIG. 4. The Tbx24-Ripply1 protein complex associates with the T site. (A) Colocalization of Myc-Ripply1 and Tbx24-FLAG proteins in the nuclei of 293T cells. (B) Myc-Ripply1 and Tbx24-FLAG proteins preferentially coimmunoprecipitate from lysates of 293T cells. (C) ChIP analysis reveals that Myc-Ripply1 and Tbx24-FLAG complexes, but not Myc-Ripply1 alone, preferentially associate with the DNA sequence containing the T sites. The DNA region containing the T sites (T region) was coprecipitated with Myc-Ripply1 in the presence of Tbx24, while a DNA region close to the T sites (proximal region), which includes potential binding sequences for the Pax, Hox, TCF/Lef1, and E47 transcription factors, as well as another region 2.0 kb upstream from the T sites (distal region), was not immunoprecipitated. The length of the amplified band was 250 bp in each case. On the other hand, Tbx24-FLAG was consistently associated with the T region in the presence or absence of Ripply1.

T domain and Ripply homology domain are indispensable for interactions between Tbx24 and Ripply1. To gain further mechanistic insights into the interactions between Tbx24 and Ripply1, we next prepared several deletion or amino acid substitution constructs of Tbx24 and Ripply1 (Fig. 5A). By conducting immunoprecipitation assays, we found that the T domain alone, which is highly conserved among the Tbx family proteins, is sufficient for the association with Ripply1 but that other regions of Tbx24 are dispensable for the interactions (Fig. 5D).

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FIG. 5. Ripply homology domain is required for interactions with T domain of Tbx24 and suppression of mesp-b mRNA in zebrafish embryos. (A) Schematic representation of modified FLAG-tagged Tbx24 and Myc-tagged Ripply1 variants used in this study. Conserved domains of Tbx24 and Ripply1 are emphasized with different colors. (B) Comparisons of Ripply homology domains in zebrafish Ripply proteins. Identical amino acid residues are shown on a black background, and similar residues are shown on a gray background. A four-amino-acid stretch (the 97th to 100th amino acids) was replaced with an alanine stretch (emphasized with asterisks). This variant was designated ripply1-mutFPVQ. (C) Luciferase reporter assay. The assay employed a luciferase reporter in which two T sites were placed upstream of the basal promoter used for the experiment shown in Fig. 3B. In contrast to wild-type Ripply1, replacement of the highly conserved sequences in the Ripply homology domain with a stretch of four alanines resulted in a significant reduction of the transcriptional repression of the pBP-TbxX2 reporter construct. (D and E) Immunoprecipitation assays. The T domain alone (173 amino acids) preferentially coimmunoprecipitated with Myc-Ripply1 (D). Substitution of four amino acid residues highly conserved in the Ripply homology domain results in a significant reduction of affinity for Tbx24 (E). (F) Whole-mount in situ staining with mesp-b and mesp-a probes. Injection of ripply1 mRNA resulted in a significant reduction of mesp-b, but not mesp-a, mRNA in the anterior PSM (arrowheads) at the three-somite stage. However, the same amount of substituted ripply1-mutFPVQ mRNA did not cause such suppression of mesp-b mRNA. Stained embryos are viewed from the dorsal side and anterior to the top.

On the other hand, an 50-amino-acid sequence at the carboxyl terminus, called the Ripply homology domain, is conserved among the Ripply family proteins. Especially highly conserved sequences exist in this domain (Fig. 5B). Replacement of some of these sequences with an alanine stretch (Ripply1-mutFPVQ) resulted in a significant reduction in the transcriptional repression of the pBP-TbxX2 reporter construct (Fig. 5C), indicating that this conserved sequence is required for Ripply-mediated transcriptional repression. This is at least caused by a significantly decreased affinity of Ripply1-mutFPVQ for Tbx24, because the immunoprecipitation assay revealed that Ripply1-mutFPVQ was not efficiently precipitated with Tbx24 compared with the coprecipitation with wild-type Ripply1 (Fig. 5E).

Ripply1 converts the transcriptional property of Tbx24 from activator to repressor by recruiting the transcriptional corepressor Groucho/TLE. Ripply1 interacts with the transcriptional corepressor Groucho/TLE through the WRPW motif (7, 16). To determine whether Groucho/TLE is involved in the suppression of Tbx24 by Ripply, we next examined the effect of removing the WRPW motif. Compared with the wild-type Ripply1 protein, Ripply1 lacking the WRPW tetrapeptide did not efficiently suppress Tbx24-mediated enhancement of luciferase activity (Fig. 6A). Because multimeric Groucho/TLE complexes contain histone deacetylases (HDACs) in Drosophila and mammals (5, 7), we next examined the effect of TSA, a chemical inhibitor of HDACs (37). We found that TSA partially rescued the suppression of luciferase activity by Ripply1 (Fig. 6B). These results suggest that Ripply1 counteracts the transcriptional activity of Tbx24 by recruiting the Groucho/TLE complex via the WRPW motif. Specifically, Ripply acts as an adaptor that converts Tbx24 from an activator to a repressor. In fact, whereas Tbx24 activates the reporter genes in the absence of Ripply1 in 293T cells, it acts as a transcriptional repressor when there is an excess of Ripply1 (Fig. 6C). Thus, Ripply1 controls the characteristics of transcriptional regulation by Tbx24 from activator to repressor.

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FIG. 6. Ripply1 exerts Tbx24-dependent transcriptional repression by recruiting the Groucho/TLE-HDAC complex. (A) Suppression of Tbx24 transcriptional activity depends on the Groucho/TLE-binding motif WRPW of Ripply1. (B) Transcriptional suppression of Tbx24 by Ripply1 involves HDAC activity. (C) Transcriptional property of Tbx24 proteins is converted from activation to repression by Ripply1. The assay employed a luciferase reporter in which two T sites were placed upstream of the thymidine kinase promoter and the luciferase gene of pST0/TLN (25). Although Tbx24 normally acts as a transcriptional activator, Tbx24 acts as a repressor in the presence of Ripply1 (100 ng).

Ripply1 can antagonize the transcriptional activation of another T-box gene, no tail, in vivo and in vitro. The result showing that the T domain, which is conserved among all of the Tbx family proteins, is necessary and sufficient for the association between Ripply1 and Tbx24 suggests that Ripply acts on other T-box transcription factors (Fig. 3C). Indeed, Ripply1 suppressed transcriptional activation by Ntl, a zebrafish ortholog of the mouse T-box protein T/Brachyury (9, 31) (Fig. 7A). Thus, Ripply proteins appear to suppress the function of various T-box transactivators in vitro.

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FIG. 7. Transcriptional activation of Ntl, a zebrafish ortholog of T/Brachyury, is antagonized by Ripply1. (A) Luciferase assays with 293T cells. Zebrafish Ripply1 suppresses the transcriptional activation of Ntl proteins. (B) Injection of ripply1 mRNA causes truncation of the posterior body, similar to the phenotype of ntl homozygous mutants (9). Synthetic ripply1 mRNA was injected into fertilized embryos at a concentration of 10 ng/µl. The image shown is a lateral view at 36 h postfertilization (hpf). (C) The notochord (N) and horizontal myoseptum (arrowhead) are absent in ripply1 mRNA-injected embryos. Transverse sections (7 µm) at the trunk region above the yolk tube of embryos at 36 h postfertilization are shown. The sections were briefly stained with hematoxylin. DA, dorsal aorta. (D) Expression of myod in adaxial cells, induced by the presence of the notochord, was absent from ripply1 mRNA-injected embryos at the one-somite stage. In contrast, expression of mesp-a in the anterior PSM appears indistinguishable between the control and ripply1 mRNA-injected embryos. (E) Bilateral expression of myod is fused in the posterior trunk (*), as observed in ntl homozygous mutants (1, 35). (F) Expression of ntl appears indistinguishable between control and ripply1 mRNA-injected embryos. The images show the animal pole view at the shield stage.

In addition, we investigated the ability of Ripply1 to antagonize T-box proteins in vivo by injecting ripply1 mRNA into eggs at the one-cell stage. As previously reported, injection of wild-type ripply1 mRNA at a low dosage caused segmental disruption of somites and suppression of mesp-b expression in the PSM. On the other hand, misexpression of ripply1 mRNA at a higher dosage resulted in truncation of the posterior body and the absence of a notochord and horizontal myoseptum (Fig. 7B). Although the altered somite patterning is similar to the phenotype observed for fused somites/tbx24 mutants (24), the posterior truncation and the absence of a differentiated notochord and horizontal myoseptum, regardless of the presence of a neural tube and somites (Fig. 7C) (n = 3; 100% were affected), are strikingly similar to the phenotype of ntl homozygous mutants (9). Furthermore, as observed for ntl homozygous mutants (1, 9, 35), in the ripply1 mRNA-injected embryos, the expression of myod in the adaxial cells, which is induced by notochord, was missing at the one-somite stage (Fig. 7D). In a later stage, bilateral expression of myod was fused in the posterior trunk, as observed in ntl homozygous mutants (Fig. 7E). In contrast, the expression of mesp-a was normal in the ripply1 mRNA-injected embryos (Fig. 7D). Of note, the expression of ntl itself was also unchanged in the ripply1 mRNA-injected embryos (Fig. 7F). Interestingly, conversion of the Xenopus homolog of T/Brachyury into a transcriptional repressor by fusion with the En1 repressor domain also produces a similar phenotype to that of ntl homozygous mutants (6). Therefore, the morphological defect observed in ripply-injected embryos could be consistent with the expected defect when Ntl is converted into a repressor. Collectively, our results suggest that Ripply proteins are regulators of a wide range of T-box proteins.

Association with T-box protein is required for the function of Ripply in embryos. Finally, to see whether interaction with T-box proteins is required for Ripply activities in vivo, we injected mRNA encoding either ripply1 or ripply1-mutFPVQ into zebrafish embryos. As previously described, injection of the wild-type ripply1 mRNA resulted in reduced expression of mesp-b but not mesp-a (Fig. 5F). In contrast, the injection of the mutated ripply1 mRNA did not cause an apparent decrease in mesp-b expression in the anterior PSM (Fig. 5F). Thus, the interaction between Ripply and T-box protein is required for the activity of Ripply1, even in embryos.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Previously, we showed that zebrafish ripply1 plays roles in the repression of gene expression in the anterior PSM. Although we suggested that the Groucho/TLE transcriptional corepressor is involved in this suppression, the molecular mechanism underlying this suppression has mostly remained. Here we have provided evidence to support a model by which Ripply proteins convert the transcriptional property of T-box proteins from activators to repressors. During the preparation of this manuscript, Kondow et al. reported that Bowline, a Xenopus counterpart of Ripply, suppresses the activity of an activated form of Tbx6 fused with VP16 in cultured cells (19). Although this report implies potential interaction between Ripply/Bowline and a T-box protein, it still remains unclear whether Ripply/Bowline can actually suppress the activity of intact T-box proteins. In contrast, our results clearly indicate that the activity of the intact Tbx24 molecule is modulated by zebrafish Ripply1 and that Tbx24 can actually be converted from an activator to a repressor by Ripply1. Furthermore, treatment with an inhibitor specific for HDAC activity strongly suggests that HDAC, which is known to be associated with Groucho/TLE, is actually involved in the Ripply-mediated suppression. We also show that Ripply1 is actually recruited to the T-box protein-binding DNA sequence through Tbx24. In addition, the T domain and a highly conserved amino acid stretch in the Ripply homology domain are indispensable for the interactions between Tbx24 and Ripply1, and as anticipated from this result, other members of either the Ripply or Tbx family are also interactive with Tbx24 or Ripply1, respectively. Finally, we show that this interaction is required for the activity of Ripply in embryos. Based on these results, we conclude that T-box proteins are converted from activators to repressors through specific interactions with Ripply proteins, which recruit the global corepressor Groucho/TLE.

The spatiotemporal regulation of gene expression, which is critical for proper development, is managed by a number of molecular processes, including the replacement of coactivator and corepressor complexes. For instance, external signals, such as Wnt and Notch, displace corepressor complexes, which maintain "default repression" of these signals' targets, from DNA-binding transcriptional regulators (2, 13-15). Here we show that spatiotemporal gene regulation is also accomplished by another fashion of replacement of corepressor complexes. Our study provides evidence that T-box proteins are converted from activators to repressors through specific interactions with Ripply proteins, which recruit the global corepressor Groucho/TLE-HDAC complex. During somite segmentation in zebrafish, the expression of mesp-b, a target of Tbx24, is dynamically regulated to yield the characteristic stripe pattern in the anterior PSM (30), even though Tbx24 is constantly expressed in this region (24). Considering that ripply1 is expressed in a stripe fashion in the anterior PSM (16), Ripply1 appears to provide conversion of the transcriptional property of Tbx24 in a stripe fashion in this region, and this conversion regulates the expression of mesp-b. Because defective function of Ripply1 or Mesp-b results in abnormal anteroposterior compartmentation within a single somite unit in zebrafish embryos, the Ripply1-mediated transcriptional control of mesp-b appears to be crucial for proper segmentation of somites. Thus, the regulatory recruitment of corepressor complexes to T-box proteins, established by the dynamic expression of ripply1, is likely to be a key process to establish segmental gene expression during somite development.

The identification of Ripply1 as a switching molecule for T-box genes suggests a novel transcriptional mechanism that could participate in other aspects of development, although the relationship between the Ripply family and T-box factors besides Tbx24 and Ntl remains to be elucidated. The overlapping expression of ripply and T-box genes observed in various developing tissues and organs (A. Kawamura and S. Takada, unpublished data) supports the idea that the two proteins cooperate in development. Further understanding of the cooperative transcriptional regulation by Ripply and T-box proteins could help to elucidate the mechanisms underlying the disproportionate activation or repression found in human genetic disorders associated with mutations in T-box genes.

 
We thank T. Ohyama, J. Campos-Ortega, K. Hoshijima, and M. Nikaido for reagents and technical advice. We are also grateful to T. Higashinakagawa, H. Kondoh, K. Inoue, and K. Yamasu for support and to the members of the Takada and Higashinakagawa laboratories for critical discussions.

This work was supported by a grant-in-aid for scientific research from the Ministry of Education, Science, Culture, and Sports of Japan to S.T. and S.K. A.K. was supported by a research fellowship from the Japan Society for the Promotion of Science.

 
* Corresponding author. Mailing address: Okazaki Institute for Integrative Biosciences, National Institutes of Natural Sciences, Okazaki, Aichi 444-8787, Japan. Phone: 81-564-59-5241. Fax: 81-564-59-5240. E-mail: stakada{at}nibb.ac.jp

Published ahead of print on 10 March 2008.

Supplemental material for this article may be found at http://mcb.asm.org/.

Present address: Department of Biological Sciences, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Molecular and Cellular Biology, May 2008, p. 3236-3244, Vol. 28, No. 10
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