The Hox code responsible for the patterning of the anterior vertebrae in zebrafish.

The vertebral column is a characteristic structure of vertebrates. Genetic studies in mice have shown that Hox-mediated patterning plays a key role in specifying discrete anatomical regions of the vertebral column. Expression pattern analyses in several vertebrate embryos have provided correlative evidence that the anterior boundaries of Hox expression coincide with distinct anatomical vertebrae. However, because functional analyses have been limited to mice, it remains unclear which Hox genes actually function in vertebral patterning in other vertebrates. In this study, various zebrafish Hox mutants were generated for loss-of-function phenotypic analysis to functionally decipher the Hox code responsible for the zebrafish anterior vertebrae between the occipital and thoracic vertebrae. We found that Hox genes in HoxB- and HoxC-related clusters participate in regulating the morphology of the zebrafish anterior vertebrae. In addition, medaka hoxc6a was found to be responsible for anterior vertebral identity, as in zebrafish. Based on phenotypic similarities with Hoxc6 knockout mice, our results suggest that the Hox patterning system, including at least Hoxc6, may have been functionally established in the vertebral patterning of the common ancestor of ray-finned and lobe-finned fishes.

Teleost Hox code defines regional identities competent for the formation of dorsal and anal fins.

The dorsal and anal fins can vary widely in position and length along the anterior-posterior axis in teleost fishes. However, the molecular mechanisms underlying the diversification of these fins remain unknown. Here, we used genetic approaches in zebrafish and medaka, in which the relative positions of the dorsal and anal fins are opposite, to demonstrate the crucial role of hox genes in the patterning of the teleost posterior body, including the dorsal and anal fins. By the CRISPR-Cas9-induced frameshift mutations and positional cloning of spontaneous dorsalfinless medaka, we show that various hox mutants exhibit the absence of dorsal or anal fins, or a stepwise posterior extension of these fins, with vertebral abnormalities. Our results indicate that multiple hox genes, primarily from hoxc-related clusters, encompass the regions responsible for the dorsal and anal fin formation along the anterior-posterior axis. These results further suggest that shifts in the anterior boundaries of hox expression which vary among fish species, lead to diversification in the position and size of the dorsal and anal fins, similar to how modulations in Hox expression can alter the number of anatomically distinct vertebrae in tetrapods. Furthermore, we show that hox genes responsible for dorsal fin formation are different between zebrafish and medaka. Our results suggest that a novel mechanism has occurred during teleost evolution, in which the gene network responsible for fin formation might have switched to the regulation downstream of other hox genes, leading to the remarkable diversity in the dorsal fin position.

An atlas of seven zebrafish hox cluster mutants provides insights into sub/neofunctionalization of vertebrate Hox clusters.

Vertebrate Hox clusters are comprised of multiple Hox genes that control morphology and developmental timing along multiple body axes. Although results of genetic analyses using Hox-knockout mice have been accumulating, genetic studies in other vertebrates have not been sufficient for functional comparisons of vertebrate Hox genes. In this study, we isolated all of the seven hox cluster loss-of-function alleles in zebrafish using the CRISPR-Cas9 system. Comprehensive analysis of the embryonic phenotype and X-ray micro-computed tomography scan analysis of adult fish revealed several species-specific functional contributions of homologous Hox clusters along the appendicular axis, whereas important shared general principles were also confirmed, as exemplified by serial anterior vertebral transformations along the main body axis, observed in fish for the first time. Our results provide insights into discrete sub/neofunctionalization of vertebrate Hox clusters after quadruplication of the ancient Hox cluster. This set of seven complete hox cluster loss-of-function alleles provide a formidable resource for future developmental genetic analysis of the Hox patterning system in zebrafish.

A globin-family protein, Cytoglobin 1, is involved in the development of neural crest-derived tissues and organs in zebrafish.

The zebrafish is an excellent model animal that is amenable to forward genetics approaches. To uncover unknown developmental regulatory mechanisms in vertebrates, we conducted chemical mutagenesis screening and identified a novel mutation, kanazutsi (kzt). This mutation is recessive, and its homozygotes are embryonic lethal. Mutant embryos suffered from a variety of morphological defects, such as head flattening, pericardial edema, circulation defects, disrupted patterns of melanophore distribution, dwarf eyes, a defective jaw, and extensive apoptosis in the head, which indicates that the main affected tissues are derived from neural crest cells (NCCs). The expression of tissue-specific markers in kzt mutants showed that the early specification of NCCs was normal, but their later differentiation was severely affected. The mutation was mapped to chromosome 3 by linkage analyses, near cytoglobin 1 (cygb1), the product of which is a globin-family respiratory protein. cygb1 expression was activated during somitogenesis in somites and cranial NCCs in wild-type embryos but was significantly downregulated in mutant embryos, despite the normal primary structure of the gene product. The kzt mutation was phenocopied by cygb1 knockdown with low-dose morpholino oligos and was partially rescued by cygb1 overexpression. Both severe knockdown and null mutation of cygb1, established by the CRISPR/Cas9 technique, resulted in far more severe defects at early stages. Thus, it is highly likely that the downregulation of cygb1 is responsible for many, if not all, of the phenotypes of the kzt mutation. These results reveal a requirement for globin family proteins in vertebrate embryos, particularly in the differentiation and subsequent development of NCCs.

Transcriptional autoregulation of zebrafish tbx6 is required for somite segmentation.

The presumptive somite boundary in the presomitic mesoderm (PSM) is defined by the anterior border of the expression domain of Tbx6 protein. During somite segmentation, the expression domain of Tbx6 is regressed by Ripply-meditated degradation of Tbx6 protein. Although the expression of zebrafish tbx6 remains restricted to the PSM, the transcriptional regulation of tbx6 remains poorly understood. Here, we show that the expression of zebrafish tbx6 is maintained by transcriptional autoregulation. We find that a proximal-located cis-regulatory module, TR1, which contains two putative T-box sites, is required for somite segmentation in the intermediate body and for proper expression of segmentation genes. Embryos with deletion of TR1 exhibit significant reduction of tbx6 expression at the 12-somite stage, although its expression is initially observed. Additionally, Tbx6 is associated with TR1 and activates its own expression in the anterior PSM. Furthermore, the anterior expansion of tbx6 expression in ripply gene mutants is suppressed in a TR1-dependent manner. The results suggest that the autoregulatory loop of zebrafish tbx6 facilitates immediate removal of Tbx6 protein through termination of its own transcription at the anterior PSM.

Role of somite patterning in the formation of Weberian apparatus and pleural rib in zebrafish.

In the vertebrate body, a metameric structure is present along the anterior-posterior axis. Zebrafish tbx6-/- larvae, in which somite boundaries do not form during embryogenesis, were shown to exhibit abnormal skeletal morphology such as rib, neural arch and hemal arch. In this study, we investigated the role of somite patterning in the formation of anterior vertebrae and ribs in more detail. Using three-dimensional computed tomography scans, we found that anterior vertebrae including the Weberian apparatus were severely affected in tbx6-/- larvae. In addition, pleural ribs of tbx6 mutants exhibited severe defects in the initial ossification, extension of ossification, and formation of parapophyses. Two-colour staining revealed that bifurcation of ribs was caused by fusion or branching of ribs in tbx6-/- . The parapophyses in tbx6-/- juvenile fish showed irregular positioning to centra and abnormal attachment to ribs. Furthermore, we found that the ossification of the distal portion of ribs proceeded along myotome boundaries even in irregularly positioned myotome boundaries. These results provide evidence of the contribution of somite patterning to the formation of the Weberian apparatus and rib in zebrafish.

Posterior-anterior gradient of zebrafish hes6 expression in the presomitic mesoderm is established by the combinatorial functions of the downstream enhancer and 3'UTR.

In vertebrates, the periodic formation of somites from the presomitic mesoderm (PSM) is driven by the molecular oscillator known as the segmentation clock. The hairy-related gene, hes6/her13.2, functions as a hub by dimerizing with other oscillators of the segmentation clock in zebrafish embryos. Although hes6 exhibits a posterior-anterior expression gradient in the posterior PSM with a peak at the tailbud, the detailed mechanisms underlying this unique expression pattern have not yet been clarified. By establishing several transgenic lines, we found that the transcriptional regulatory region downstream of hes6 in combination with the hes6 3'UTR recapitulates the endogenous gradient of hes6 mRNA expression. This downstream region, which we termed the PT enhancer, possessed several putative binding sites for the T-box and Ets transcription factors that were required for the regulatory activity. Indeed, the T-box transcription factor (Tbx16) and Ets transcription factor (Pea3) bound specifically to the putative binding sites and regulated the enhancer activity in zebrafish embryos. In addition, the 3'UTR of hes6 is required for recapitulation of the endogenous mRNA expression. Furthermore, the PT enhancer with the 3'UTR of hes6 responded to the inhibition of retinoic acid synthesis and fibroblast growth factor signaling in a manner similar to endogenous hes6. The results showed that transcriptional regulation by the T-box and Ets transcription factors, combined with the mRNA stability given by the 3'UTR, is responsible for the unique expression gradient of hes6 mRNA in the posterior PSM of zebrafish embryos.

Enhancer activity-based identification of functional enhancers using zebrafish embryos.

Chromatin immunoprecipitation (ChIP) against enhancer-associated marks with massive sequencing is a powerful approach to identify genome-wide distributions of putative enhancers. However, functional in vivo analysis is required to elucidate the activities of predicted enhancers. Using zebrafish embryos, we established a ChIP-Injection method that enables identification of functional enhancers based on their enhancer activities in embryos. Each reporter gene possessing the enhancer-associated genomic region enriched by ChIP was injected into zebrafish embryos to analyze the activity of putative enhancers. By using the ChIP-Injection, we identified 32 distinct putative enhancers that drove specific expression. Additionally, we generated transgenic lines that exhibit distributions of the EGFP signal as was observed in the screening. Furthermore, the expression pattern driven by the identified somite-specific enhancer resembled that of the gene acta2. The results indicate that ChIP-Injection provides an efficient approach for identification of active enhancers in a potentially wide variety of developmental tissues and stages.

Ripply3, a Tbx1 repressor, is required for development of the pharyngeal apparatus and its derivatives in mice.

The pharyngeal apparatus is a transient structure that gives rise to the thymus and the parathyroid glands and also contributes to the development of arteries and the cardiac outflow tract. A typical developmental disorder of the pharyngeal apparatus is the 22q11 deletion syndrome (22q11DS), for which Tbx1 is responsible. Here, we show that Ripply3 can modulate Tbx1 activity and plays a role in the development of the pharyngeal apparatus. Ripply3 expression is observed in the pharyngeal ectoderm and endoderm and overlaps with strong expression of Tbx1 in the caudal pharyngeal endoderm. Ripply3 suppresses transcriptional activation by Tbx1 in luciferase assays in vitro. Ripply3-deficient mice exhibit abnormal development of pharyngeal derivatives, including ectopic formation of the thymus and the parathyroid gland, as well as cardiovascular malformation. Corresponding with these defects, Ripply3-deficient embryos show hypotrophy of the caudal pharyngeal apparatus. Ripply3 represses Tbx1-induced expression of Pax9 in luciferase assays in vitro, and Ripply3-deficient embryos exhibit upregulated Pax9 expression. Together, our results show that Ripply3 plays a role in pharyngeal development, probably by regulating Tbx1 activity.

The effects of interleukin-6 signal blockade on immune system, reproductive and skeletal development in juvenile mice.

Interleukin-6 (IL-6) is involved in the pathogenesis of multiple disorders, including juvenile autoimmune diseases. IL-6 participates in a broad spectrum of physiological events, and the IL-6 receptor (IL-6R) is widely distributed across multiple organs. The interrelationship of development phases in juveniles together with organs involved in IL-6 signaling called for evaluations of anti-IL-6R antibody induced effects in a juvenile mouse model to assess the safety of such an approach in human juvenile arthritis. Here we show that naive mice in which IL-6 signals have been transiently blocked during the juvenile period develop normally. The fatal immunogenic reactions recorded earlier by repeated administration of the chosen rat anti-mouse IL-6R antibody, MR16-1, to mice were avoided successfully by application of a high loading dose followed by lower maintenance doses, with the support of modeling data. The high loading-dose regimen enabled us to conduct assessments without any major interference due to immunogenicity. Transient and complete inhibition of IL-6 signals from postnatal days 22 to 79 in mice exhibited no biologically important changes in sexual maturation or development of immune and skeletal systems. Although tendencies toward reductions of peripheral blood T-cell counts were observed, normal levels of antigen-specific IgG/IgM antibody productions indicating sufficient immunological functions were confirmed. Our results demonstrate that blockage of IL-6R by the neutralizing antibody does not affect juvenile development. This may be in part due to the generation or existence of compensatory pathways in the whole body system.

Mesendoderm specification depends on the function of Pou2, the class V POU-type transcription factor, during zebrafish embryogenesis.

Zebrafish pou2, encoding the class V POU transcription factor orthologous to mouse Oct-3/4, has been implicated in different aspects of development, such as dorsoventral patterning, endoderm formation, and brain regionalization, by analyzing pou2 mutant embryos. In the present study, we first conducted overexpression of pou2-modified genes by mRNA injection, and found that pou2 and its active form (vp-pou2) augmented mesoderm formation and suppressed endoderm specification, whereas repressive pou2 (en-pou2) affected the formation of the mesoderm and endoderm in a different manner. To avoid complications that might arise from different pou2 functions during the course of development, we used a transgenic line harboring inducible en-pou2 (HEP), which could function in a dominant-negative manner. We found that suppressing endogenous pou2 by HEP induction at the mid-blastula stage enhanced endoderm development at the expense of mesoderm, whereas the same treatment in the late blastulae promoted mesoderm formation and suppressed the endoderm specification. Further analyses using HEP induction revealed that, from late epiboly to early somitogenesis, pou2 regulated additional developmental aspects, such as brain regionalization, heart development, and tail formation. Our findings suggest that Pou2 functions in multiple aspects of vertebrate development, especially in the binary decision of the mesendoderm to mesoderm and endoderm in different ways depending on the developmental stage.

The effects of interleukin-6 signal blockade on fertility, embryo-fetal development, and immunization in vivo.

Possible effects of interleukin-6 (IL-6) on reproductive performance, embryonal development, parturition, and postnatal development have been suggested based on protein/mRNA expression level of IL-6 in related organs, but less is known about functions of IL-6 signals in these areas. Following two different approaches have been employed to investigate the role of IL-6 signals in fertility and pre-/postnatal development: administration of a rat anti-mouse IL-6 receptor antibody, MR16-1, to mice as a neutralizing antibody system, and B6.129S2-Il6(tm1Kopf)/J (IL-6 knockout [KO]) mice as a KO system. By intravenously dosing 50 mg/kg of MR16-1 every 3 days, animals in male and female fertility studies and dams in a pre-/postnatal development study exhibited plasma MR16-1 concentrations much higher than the effective plasma concentration, indicating that MR16-1 exposure was sufficient to completely block IL-6 signals. The concentration of MR16-1 in the plasma of fetuses exceeded that in the plasma of pregnant animals, and MR16-1 concentration in milk was about one-fourth of that in plasma. Both the transient IL-6 signal blockade by MR16-1, and the constitutive IL-6 signal inhibition using IL-6 KO mice in a combined fertility and pre-/postnatal development study, revealed no biologically important effects on fertility, early embryonic development to implantation, or pre-/postnatal development, including IgG/IgM production by keyhole limpet hemocyanin sensitization. These results indicate that IL-6 signals have no unique, noncompensable roles in reproduction and development in the whole body system, although contributions of IL-6 in the signaling network appear to exist, as suggested by previously published investigations.

FGF receptor gene expression and its regulation by FGF signaling during early zebrafish development.

The expression of all four fgfr genes was extensively examined throughout early embryogenesis of the zebrafish (Danio rerio). fgfr1 alone was expressed maternally throughout the blastoderm, and then zygotically in the anterior neural plate and presomitic mesoderm. fgfr4 expression was first detected in late blastulae and was gradually restricted to the brain. fgfr2 and fgfr3 expression were initiated in early and late gastrulae, respectively; fgfr2 was expressed in the anterior neural plate and somitic mesoderm, whereas fgfr3 was activated in the axial mesoderm and then in the midbrain and somitic mesoderm. During somitogenesis, each of these fgfr genes was expressed in a characteristic manner in the brain. Using an FGF signal inhibitor, dominant-negative FGF receptors and fgf8.1/fgf8a mutants, we found that fgfr expression is directly or indirectly regulated by FGF signaling during epiboly and at the end of somitogenesis, revealing the presence of an autoregulatory mechanism.

Groucho-associated transcriptional repressor ripply1 is required for proper transition from the presomitic mesoderm to somites.

Concomitant with the transition from the presomitic mesoderm (PSM) to somites, the periodical gene expression characteristic of the PSM is drastically changed and translated into the segmental structure. However, the molecular mechanism underlying this transition has remained obscure. Here, we show that ripply1, encoding a nuclear protein associated with the transcriptional corepressor Groucho, is required for this transition. Zebrafish ripply1 is expressed in the anterior PSM and in several newly formed somites. Ripply1 represses mesp-b expression in the PSM through a Groucho-interacting motif. In ripply1-deficient embryos, somite boundaries do not form, the characteristic gene expression in the PSM is not properly terminated, and the initially established rostrocaudal polarity in the segmental unit is not maintained, whereas paraxial mesoderm cells become differentiated. Thus, ripply1 plays dual roles in the transition from the PSM to somites: termination of the segmentation program in the PSM and maintenance of the rostrocaudal polarity.

Deadenylation by the CCR4-NOT complex contributes to the turnover of hairy-related mRNAs in the zebrafish segmentation clock.

In the zebrafish segmentation clock, hairy/enhancer of split-related genes her1, her7, and hes6 encodes components of core oscillators. Since the expression of cyclic genes proceeds rapidly in the presomitic mesoderm (PSM), these hairy-related mRNAs are subject to strict post-transcriptional regulation. In this study, we demonstrate that inhibition of the CCR4-NOT deadenylase complex lengthens poly(A) tails of hairy-related mRNAs and increases the amount of these mRNAs, which is accompanied by defective somite segmentation. In transgenic embryos, we show that EGFP mRNAs with 3'UTRs of hairy-related genes exhibit turnover similar to endogenous mRNAs. Our results suggest that turnover rates of her1, her7, and hes6 mRNAs are differently regulated by the CCR4-NOT deadenylase complex possibly through their 3'UTRs in the zebrafish PSM.

Functional roles of the Ripply-mediated suppression of segmentation gene expression at the anterior presomitic mesoderm in zebrafish.

Somites sequentially form with a regular interval by the segmentation from the anterior region of the presomitic mesoderm (PSM). The expression of several genes involved in the somite segmentation is switched off at the transition from the anterior PSM to somites. Zebrafish Ripply1, which down-regulates a T-box transcription factor Tbx6, is required for the suppression of segmentation gene expression. However, the functional roles of the Ripply-mediated suppression of segmentation gene expression at the anterior PSM remain elusive. In this study, we generated ripply1 mutants and examined genetic interaction between ripply1/2 and tbx6. Zebrafish ripply1-/- embryos failed to form the somite boundaries as was observed in knockdown embryos. We found that somite segmentation defects in ripply1 mutants were suppressed by heterozygous mutation of tbx6 or partial translational inhibition of tbx6 by antisense morpholino. We further showed that somite boundaries that were recovered in tbx6+/-; ripply1-/- embryos were dependent on the function of ripply2, indicating that relative gene dosage between ripply1/2 and tbx6 plays a critical role in the somite formation. Interestingly, the expression of segmentation genes such mesp as was still not fully suppressed at the anterior PSM of tbx6+/-; ripply1-/- embryos although the somite formation and rostral-caudal polarity of somites were properly established. Furthermore, impaired myogenesis was observed in the segmented somites in tbx6+/-; ripply1-/- embryos. These results revealed that partial suppression of the segmentation gene expression by Ripply is sufficient to establish the rostral-caudal polarity of somites, and that stronger suppression of the segmentation gene expression by Ripply is required for proper myogenesis in zebrafish embryos.

Polyglutamation of a novel antifolate, MX-68, is not necessary for its anti-arthritic effect.

N-[[4-[(2,4-diaminopteridin-6-yl)methyl]-3,4-dihydro-2H-1,4-benzothiazin-7-yl]-carbonyl]-L-homoglutamic acid (MX-68), a derivative of methotrexate, was chemically designed to resist polyglutamation and to have a high affinity for dihydrofolate reductase, in an attempt to reduce the side effects of methotrexate. We confirmed that MX-68 did not undergo polyglutamation and investigated the pharmacological activities of MX-68 compared with methotrexate. (1) In vitro: MX-68 inhibited the activity of dihydrofolate reductase to the same degree as methotrexate-tetraglutamate. MX-68 treatment produced a similar anti-proliferative effect to that of methotrexate. However, the intracellular concentration of MX-68 was much lower than the sum of the levels of methotrexate and its polyglutamate, and the effects of MX-68 disappeared when it was removed from the culture medium. (2) In vivo: Oral administration of MX-68 suppressed the development of collagen-induced arthritis in mice and adjuvant-induced arthritis in rats, in a similar fashion to that of methotrexate. These results indicate that polyglutamation is not essential for the anti-arthritic effect of antifolates.

Gbx2 functions as a transcriptional repressor to regulate the specification and morphogenesis of the mid-hindbrain junction in a dosage- and stage-dependent manner.

The Gbx subfamily of homeodomain transcription factors is involved in the positioning of the isthmus, which patterns the midbrain and cerebellum in vertebrates. To uncover the details of Gbx functions, we first examined the dose dependency of its effects on brain formation in zebrafish and found that high-dose gbx2 mRNA injection affected the entire forebrain and midbrain, whereas low-dose mRNA specifically disrupted the isthmic folding at the midbrain-hindbrain boundary (MHB) but only weakly affected the expression of genes involved in MHB specification. Thus, isthmus morphogenesis, and not its early specification, is highly sensitive to gbx2. Transient induction of heat-inducible gbx2 using transgenic fish showed that MHB specification is most sensitive to gbx2 at the end of epiboly and further suggested that otx2 is the direct target gene. These together demonstrate that gbx2 regulates both specification and morphogenesis of the MHB/isthmus region. Deletion analyses showed that both the N- and C-terminal regions contribute to the suppressive activity of Gbx2 against the anterior brain and that the N-terminal core region, including the Eh1 and proline-rich sequences, is required for this Gbx2 activity. Comparison of the effects of activated and repressive forms with wild-type Gbx2 suggested that Gbx2 functions as a transcriptional repressor, which was further evidenced by a luciferase assay in which gbx2 repressed the MHB enhancer of fgf8a in mouse P19 cells.

Pou2, a class V POU-type transcription factor in zebrafish, regulates dorsoventral patterning and convergent extension movement at different blastula stages.

Zebrafish pou2, which encodes a class V POU transcription factor considered to be an orthologue of mouse Oct-3/4, has been implicated by mutant analysis in dorsoventral (DV) patterning, gastrulation, and endoderm formation in early embryos and later in the regionalization of the neural plate. A series of gain-of-function experiments were conducted in the present study to directly reveal the roles pou2 plays in embryogenesis. We first revealed that injecting low-dose wild-type pou2 mRNA ventralizes embryos. Similar overexpression of activated (vp-) pou2 resulted in the same effects, whereas repressive (en-) pou2 caused dorsalization, supporting the previously proposed idea that pou2 is involved in DV patterning and that pou2 is a transcriptional activator. In contrast, high-dose mRNA for pou2 and its modified genes affected convergent extension (CE) movement. We observed similar activities for mouse Oct-3/4, suggesting conservation of the roles of this POU family in vertebrate development. To determine the critical stage for the functions of pou2 in embryos, we established a transgenic (Tg) fish line harboring en-pou2 under regulation of a heat-shock promoter (HEP) and found that the exposure of HEP Tg embryos to heat shock at the midblastula (sphere) stage dorsalized embryos, whereas induction of HEP at the late blastula stage (30-50% epiboly) affected CE movement. The defects due to HEP induction were rescued by introducing wild-type pou2 mRNA before the heat treatments. Collectively, these data demonstrated that pou2 regulates DV patterning and CE movement in zebrafish embryos at the midblastula and late blastula stages, respectively.

Activator-to-repressor conversion of T-box transcription factors by the Ripply family of Groucho/TLE-associated mediators.

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.