T-box Family Of Transcription Factors Research Articles

Functional promoter polymorphism in the TBX21 gene associated with aspirin-induced asthma

Asthma is a phenotypically heterogeneous disorder with many etiologic factors and clinical characteristics. T-bet, a Th1-specific transcription factor of T-box family, has been found to control interferon-gamma (IFN-gamma) expression in T cells. Mice lacking the T-bet gene (tbx21) demonstrate multiple physiological and inflammatory features reminiscent of human asthma. In order to examine whether polymorphisms in the candidate gene, TBX21, located on chromosome 17q21.32, are related to the risk of human asthma phenotypes, we have searched for genetic variations in the human TBX21 gene and identified 24 single nucleotide polymorphisms (SNPs), including five novel SNPs, by direct sequencing in Japanese subjects. Among asthma phenotypes, a promoter -1993T-->C SNP, which is in linkage disequilibrium with a synonymous coding 390A-->G SNP in exon 1, is significantly associated with a risk of aspirin-induced asthma (AIA; P = 0.004, P(c) = 0.016). This association has also been confirmed in additional independent samples of asthma with nasal polyposis (P = 0.008), regardless of aspirin hypersensitivity. Furthermore, our data indicate that the -1993T-->C substitution increases the affinity of a particular nuclear protein to the binding site of TBX21 covering the -1993 position, resulting in increased transcriptional activity of the TBX21 gene. Thus, in addition to the antigen-driven excess Th2 response, increased T-bet (and subsequent IFN-gamma) production in human airways of individuals with the -1993T-->C polymorphism could contribute to the development of certain asthma-related phenotypes, such as AIA.

Tbx2b is essential for neuronal differentiation along the dorsal/ventral axis of the zebrafish retina

The mechanisms by which retinal neurons are patterned along the dorsal/ventral axis remain largely unknown, yet this patterning is integral for the topographic mapping of visual space. With an interest in elucidating the mechanisms that regulate the development of this retinal axis, we have characterized a T-box family transcription factor, Tbx2b, during zebrafish retinogenesis. Tbx2b is expressed throughout all phases of retinal development with a striking asymmetry of distribution highest dorsally to lowest ventrally. To examine Tbx2b function during retinal development, two morpholino antisense oligonucleotides were created; one blocking the translational start site of Tbx2b and the other interfering with Tbx2b mRNA splicing. Injection of either of these morpholinos resulted in profound defects in the development of the dorsal retina. By using molecular markers for neuronal subtypes, the ventral retina contained all cell types, whereas in the dorsal retina, only retinal ganglion cells expressed markers of differentiation. The cells of the dorsal retina were postmitotic, however, as demonstrated by a lack of BrdUrd incorporation during the normal periods of retinal differentiation. Markers for dorsal and ventral retinal compartments were also expressed normally in Tbx2b morphants. Combined, these observations suggest that the cellular mechanisms regulating neuronal differentiation within the retina are asymmetric about the dorsal/ventral axis and that Tbx2b mediates this process within the dorsal retina.

Dll1 is a downstream target of Tbx6 in the paraxial mesoderm

Tbx6 is a member of the T-box family of transcription factors. In the mouse, Tbx6 is expressed in the primitive streak, tail bud, and presomitic mesoderm and is essential for the specification of posterior paraxial mesoderm; in its absence, posterior somites are replaced by ectopic neural tubes. Analysis of embryos expressing reduced levels of Tbx6 also revealed that it is required for the correct patterning of the somites as well as their initial specification. As a first step toward identifying downstream targets of Tbx6, we examined the DNA binding properties of Tbx6 and identified a Tbx6 consensus binding site. Previously, we have shown that expression of Dll1, which encodes a Notch ligand, is lost in the Tbx6 mutant and that Tbx6 and Dll1 genetically interact, indicating that Dll1 may be a direct target of Tbx6 in the paraxial mesoderm. We uncovered four putative Tbx6 binding sites within a Dll1 paraxial mesoderm enhancer and show that Tbx6 can bind two of these sites in vitro. Altogether, these results lend further support for Dll1 being a direct target of Tbx6 in the presomitic mesoderm.

Tbx3 expression is related to apoptosis and cell proliferation in rat bladder both hyperplastic epithelial cells and carcinoma cells

In the present study, Tbx3, a member of the T-box family of transcription factors, was identified as an up-regulated gene by mRNA differential display in the regression (apoptosis) stage after uracil-induction of papillomatosis in the rat urinary bladder. Immunohistochemical analysis revealed that apoptosis cells are negative and apoptosis resistant cells are positive for Tbx3 expression. That suggests that Tbx3 is an apoptosis resistant gene rather than an apoptosis induced gene. We have found the rat bladder carcinoma cell line, BC31, to feature Tbx3 expression detectable by RT-PCR. To investigate its role in such cancer cells, they were transfected with an anti-sense Tbx3 expression vector. The obtained stable transfectant clones showed reduced expression of Tbx3 and much slower cell growth, as compared with mock transfectants, and many apoptotic cells were observed under normal culture conditions. These results indicate that Tbx3 is a negative regulator of apoptosis in bladder normal epithelial cells and suppression of Tbx3 expression causes inhibition of cell proliferation and induction of apoptosis in BC31, a rat bladder carcinoma cells.

T-box genes in the ascidian Ciona intestinalis: characterization of cDNAs and spatial expression.

Members of the T-box family of transcription factors share an evolutionarily conserved DNA-binding domain and play significant roles in various processes of embryonic development. Vertebrate T-box genes are categorized into the following five major subfamilies (eight groups), depending on sequence similarities: Brachyury, Tbx1 (Tbx1/10, Tbx15/18/22, Tbx20), Tbx2/3/4/5 (Tbx2/3 and Tbx4/5), Tbx6, and Tbr/Eomes/TBX21. Ascidians are primitive chordates, and their tadpole larva are considered to represent the simplified and basic body plan of vertebrates. In addition, it has been revealed that the ascidian genome contains the basic ancestral complement of genes involved in development. The present characterization of cDNAs and survey of the Ciona intestinalis draft genome demonstrated that the Ciona genome contains a single copy gene for each of the Brachyury, Tbx1/10, Tbx15/18/22, Tbx20, Tbx2/3, and Tbr/Eomes/TBX21 groups, and at least three copies of the Tbx6 subfamily. Each of the Ciona T-box genes shows a characteristic expression pattern, although that of Tbx20 was not determined in the present study. These results provide basic information that will be useful for future studies of the function of each gene, genetic cascades of different T-box genes, and genome-wide surveys of evolutionary changes in the T-box gene structure and organization in this primitive chordate.

Development of the building plan of the heart.

In this communication we discuss the formation of the synchronously contracting chambered heart from a peristaltically contracting linear heart tube. It is proposed that members of the T-box family of transcription factors play a crucial role in the formation of the building plan of the formed heart. Tbx5 may confer venoarterial polarity to the heart tube, whereas Tbx2 initially and Tbx3 in later developmental stages prevent the cardiac inflow tract, atrioventricular region, outflow tract, as well as the cardiac inner curvatures from chamber differentiation. With the exception of the outflow tract that becomes incorporated into the ventricles, these regions contribute to the cardiac conduction system.

Absence of the T-bet gene coding for the Th1-related transcription factor does not affect diabetes-associated phenotypes in Balb/c mice.

The T-box expressed in T cells gene (T-bet) is a member of the T-box family of transcription factors. T-bet-deficient mice show normal lymphoid development, but exhibit profound defects in their Th1-mediated immune responses. As the balance between Th1- and Th2-mediated immune responses plays a role in autoimmune-prone diseases, we have investigated the diabetes-related insulin autoantibody (IAA) and cellular immune responses (insulitis), in the absence of Th1 lineage commitment, in T-bet KO Balb/c mice, after immunization with the B9-23 insulin peptide. We have therefore investigated whether absence of the T-bet gene influences diabetes-related phenotypes in Balb/c T-bet KO mice.

Cardiac T-box factor Tbx20 directly interacts with Nkx2-5, GATA4, and GATA5 in regulation of gene expression in the developing heart

Tbx20 is a member of the T-box transcription factor family expressed in the forming hearts of vertebrate and invertebrate embryos. We report here analysis of Tbx20 expression during murine cardiac development and assessment of DNA-binding and transcriptional properties of Tbx20 isoforms. Tbx20 was expressed in myocardium and endocardium, including high levels in endocardial cushions. cDNAs generated by alternative splicing encode at least four Tbx20 isoforms, and Tbx20a uniquely carried strong transactivation and transrepression domains in its C terminus. Isoforms with an intact T-box bound specifically to DNA sites resembling the consensus brachyury half site, although with less avidity compared with the related factor, Tbx5. Tbx20 physically interacted with cardiac transcription factors Nkx2-5, GATA4, and GATA5, collaborating to synergistically activate cardiac gene expression. Among cardiac GATA factors, there was preferential synergy with GATA5, implicated in endocardial differentiation. In Xenopus embryos, enforced expression of Tbx20a, but not Tbx20b, led to induction of mesodermal and endodermal lineage markers as well as cell migration, indicating that the long Tbx20a isoform uniquely bears functional domains that can alter gene expression and developmental behaviour in an in vivo context. We propose that Tbx20 plays an integrated role in the ancient myogenic program of the heart, and has been additionally coopted during evolution of vertebrates for endocardial cushion development.

LvTbx2/3: a T-box family transcription factor involved in formation of the oral/aboral axis of the sea urchin embryo.

T-box family transcription factors have been identified in many organisms and are frequently associated with patterning events during embryonic development. With an interest in the molecular basis of patterning in the sea urchin embryo, we identified several members of the T-box family in Lytechinus variegatus. Here, we report the cloning and characterization of an ortholog of the Tbx2/3 subfamily, LvTbx2/3. To characterize the spatial distribution of LvTbx2/3 protein throughout sea urchin embryogenesis, a polyclonal antiserum was generated. Nuclear localization of LvTbx2/3 initiated at the mesenchyme blastula stage and protein was present into the pluteus stage. Localization was asymmetric throughout this period and costaining with marker genes indicated that asymmetry was about the oral/aboral (O/A) axis. Asymmetric distribution of LvTbx2/3 was observed in the aboral territories of all three germ layers. In the skeletogenic mesoderm lineage, LvTbx2/3 expression was dynamic because expression appeared initially in all skeletogenic mesenchyme cells (PMCs) but, subsequently, became refined solely to the aboral ones during skeletogenesis. To determine if the aboral expression of LvTbx2/3 is linked between germ layers, and to place LvTbx2/3 in the sequence of events that specifies the O/A axis, the effects of a series of perturbations to O/A polarity on LvTbx2/3 expression in each germ layer were examined. Preventing the nuclear localization of beta-catenin, pharmacological disruption of the O/A axis with NiCl(2), overexpression of BMP2/4 and disruption of the extracellular matrix all blocked LvTbx2/3 expression in all germ layers. This indicates that expression of LvTbx2/3 in the aboral territories of each germ layer is a common aspect of O/A specification, downstream of the molecular events that specify the axis. Furthermore, blocking the nuclear localization of beta-catenin, overexpression of BMP2/4 and disruption of the extracellular matrix also prevented the oral (stomodael) expression of LvBrachyury (LvBrac) protein, indicating that the O/A axis is established by a complex series of events. Last, the function of LvTbx2/3 in the formation of the O/A axis was characterized by examining the phenotypic consequences of ectopic expression of LvTbx2/3 mRNA on embryonic development and the expression of marker genes that identify specific germ layers and tissues. Ectopic expression of LvTbx2/3 produced profound morphogenetic defects in derivatives of each germ layer with no apparent loss in specification events in those tissues. This indicates that LvTbx2/3 functions as a regulator of morphogenetic movements in the aboral compartments of the ectoderm, endoderm and mesoderm.

Expression of mouse Tbx22 supports its role in palatogenesis and glossogenesis.

TBX22 belongs to the T-box family of transcription factors and was originally found in an in silico approach designed to identify new genes on the human Xq12-q21 region. Mutations in TBX22 have been reported in families with X-linked cleft palate and ankyloglossia (CPX), but the underlying pathogenetic mechanism remained unknown. We have identified mouse Tbx22 and analyzed its expression during embryogenesis by reverse transcriptase-polymerase chain reaction and in situ hybridization. In mouse embryos, it is expressed in distinct areas of the head, namely the mesenchyme of the inferior nasal septum, the posterior palatal shelf before fusion, the attachment of the tongue, and mesenchymal cells surrounding the eye anlage. The localization in the tongue frenulum perfectly correlates with the ankyloglossia phenotype in CPX. Furthermore, we identified positionally conserved binding sites for transcription factors, two of which have been implicated previously in palatogenesis (MSX1, PRX2).

The heartstrings mutation in zebrafish causes heart/fin Tbx5 deficiency syndrome.

Holt-Oram syndrome is one of the autosomal dominant human "heart-hand" disorders, with a combination of upper limb malformations and cardiac defects. Holt-Oram syndrome is caused by mutations in the TBX5 gene, a member of a large family of T-box transcription factors that play important roles in cell-type specification and morphogenesis. In a screen for mutations affecting zebrafish cardiac function, we isolated the recessive lethal mutant heartstrings, which lacks pectoral fins and exhibits severe cardiac dysfunction, beginning with a slow heart rate and progressing to a stretched, non-functional heart. We mapped and cloned the heartstrings mutation and find it to encode the zebrafish ortholog of the TBX5 gene. The heartstrings mutation causes premature termination at amino acid 316. Homozygous mutant embryos never develop pectoral fin buds and do not express several markers of early fin differentiation. The total absence of any fin bud differentiation distinguishes heartstrings from most other mutations that affect zebrafish fin development, suggesting that Tbx5 functions very early in the pectoral fin induction pathway. Moderate reduction of Tbx5 by morpholino causes fin malformations, revealing an additional early requirement for Tbx5 in coordinating the axes of fin outgrowth. The heart of heartstrings mutant embryos appears to form and function normally through the early heart tube stage, manifesting only a slight bradycardia compared with wild-type siblings. However, the heart fails to loop and then progressively deteriorates, a process affecting the ventricle as well as the atrium. Relative to mammals, fish require lower levels of Tbx5 to produce malformed appendages and display whole-heart rather than atrial-predominant cardiac defects. However, the syndromic deficiencies of tbx5 mutation are remarkably well retained between fish and mammals.

Tbx3 impinges on the p53 pathway to suppress apoptosis, facilitate cell transformation and block myogenic differentiation.

Tbx3 is a member of the T-box family of transcription factors. Mutations in Tbx3 cause ulnar-mammary syndrome, an autosomal dominant disorder characterized by upper limb defects, apocrine-gland defects including mammary hypoplasia, and tooth, hair and genital defects. In cell culture, Tbx3 and its close relative Tbx2 are capable of immortalizing mouse embryo fibroblasts. We show that expression of Tbx3 together with Myc or oncogenic Ras (H-Ras(Val17)) leads to efficient transformation of mouse embryo fibroblasts. Oncogene cooperation by Tbx3 correlates with an ability of Tbx3 to suppress the induction of p19ARF and p53 that is typically caused by overexpression Myc and Ras, and to protect against Myc-induced apoptosis. Whereas Tbx3 is capable of interfering with apoptosis caused by excessive Myc levels, a Tbx3 mutant lacking its C-terminal repression domain shows no anti-apoptotic activity and fails to repress levels of p19ARF or p53. Consistent with an ability to suppress p53 pathway function, we find that Tbx3, but not a Tbx3 C-terminal mutant, efficiently blocks myogenic differentiation of C2C12 myoblasts. Our results support the idea that deregulation and/or excessive levels of Tbx3 may have oncogenic potential in vivo.

Ascent on T Cell Control

The T-box family transcription factor, T-bet, has recently been identified as a possible master regulator in T helper 1 (TH1) immune responses through the transactivation of the gene for interferon-γ (IFN-γ). Two studies now look at the effect of T-bet deficiencies. Szabo et al. found that the absence of T-bet expression in mice profoundly attenuated production of IFN-γ by natural killer and CD4+ T cells and also shifted the cytokine profile of CD4+ T cells toward that of a TH2 phenotype. Surprisingly, IFN-γ expression by CD8+ T cells was not affected. Finotto et al. observed that T-bet-deficient mice develop spontaneous airway inflammation resembling acute and chronic forms of human asthma. T-bet expression is also reduced in airway tissue from asthmatic patients, which suggests that dysregulation of this transcription factor might contribute to airway hyper-responsiveness in humans.S. J. Szabo, B. M. Sullivan, C. Stemmann, A. R. Satoskar, B. P. Sleckman, L. H. Glimcher, Distinct effects of T-bet in TH1 lineage commitment and IFN-γ production in CD4 and CD8 T cells. Science 295, 338-342 (2002).[Abstract][Full Text]S. Finotto, M. F. Neurath, J. N. Glickman, S. Qin, H. A. Lehr, F. H. Y. Green, K. Ackerman, K. Haley, P. R. Galle, S. J. Szabo, J. M. Drazen, G. T. De Sanctis, Laurie H. Glimcher, Development of spontaneous airway changes consistent with human asthma in mice lacking T-bet. Science 295, 336-338 (2002).[Abstract][Full Text]

Ascent on T Cell Control

The T-box family transcription factor, T-bet, has recently been identified as a possible master regulator in T helper 1 (TH1) immune responses through the transactivation of the gene for interferon-γ (IFN-γ). Two studies now look at the effect of T-bet deficiencies. Szabo et al. found that the absence of T-bet expression in mice profoundly attenuated production of IFN-γ by natural killer and CD4 + T cells and also shifted the cytokine profile of CD4 + T cells toward that of a TH2 phenotype. Surprisingly, IFN-γ expression by CD8 + T cells was not affected. Finotto et al. observed that T-bet-deficient mice develop spontaneous airway inflammation resembling acute and chronic forms of human asthma. T-bet expression is also reduced in airway tissue from asthmatic patients, which suggests that dysregulation of this transcription factor might contribute to airway hyper-responsiveness in humans. S. J. Szabo, B. M. Sullivan, C. Stemmann, A. R. Satoskar, B. P. Sleckman, L. H. Glimcher, Distinct effects of T-bet in TH1 lineage commitment and IFN-γ production in CD4 and CD8 T cells. Science 295 , 338-342 (2002). [Abstract] [Full Text] S. Finotto, M. F. Neurath, J. N. Glickman, S. Qin, H. A. Lehr, F. H. Y. Green, K. Ackerman, K. Haley, P. R. Galle, S. J. Szabo, J. M. Drazen, G. T. De Sanctis, Laurie H. Glimcher, Development of spontaneous airway changes consistent with human asthma in mice lacking T-bet. Science 295 , 336-338 (2002). [Abstract] [Full Text]

Distinct effects of T-bet in TH1 lineage commitment and IFN-gamma production in CD4 and CD8 T cells.

T-bet is a member of the T-box family of transcription factors that appears to regulate lineage commitment in CD4 T helper (TH) lymphocytes in part by activating the hallmark TH1 cytokine, interferon-gamma (IFN-gamma). IFN-gamma is also produced by natural killer (NK) cells and most prominently by CD8 cytotoxic T cells, and is vital for the control of microbial pathogens. Although T-bet is expressed in all these cell types, it is required for control of IFN-gamma production in CD4 and NK cells, but not in CD8 cells. This difference is also apparent in the function of these cell subsets. Thus, the regulation of a single cytokine, IFN-gamma, is controlled by distinct transcriptional mechanisms within the T cell lineage.

T-bet is rapidly induced by interferon-gamma in lymphoid and myeloid cells.

Differentiation of naive CD4(+) T cells into IFN-gamma-producing T helper 1 (T(H)1) cells is pivotal for protective immune responses against intracellular pathogens. T-bet, a recently discovered member of the T-box transcription factor family, has been reported to play a critical role in this process, promoting IFN-gamma production. Although terminal T(H)1 differentiation occurs over days, we now show that challenge of mice with a prototypical T(H)1-inducing stimulus, Toxoplasma gondii soluble extract, rapidly induced IFN-gamma and T-bet; T-bet induction was substantially lower in IFN-gamma-deficient mice. Naive T cells expressed little T-bet, but this transcription factor was induced markedly by the combination of IFN-gamma and cognate antigen. Human myeloid antigen-presenting cells showed T-bet induction after IFN-gamma stimulation alone, and this induction was antagonized by IL-4 and granulocyte/macrophage colony-stimulating factor. Although T-bet was induced rapidly and directly by IFN-gamma, it was not induced by IFN-alpha, lipopolysaccharide, or IL-1, indicating that this action of IFN-gamma was specific. Moreover, T-bet induction was dependent on Stat1 but not Stat4. These data argue for a model in which IFN-gamma gene regulation involves an autocrine loop, whereby the cytokine regulates a transcription factor that promotes its own production. These findings substantially alter the current view of T-bet in IFN-gamma regulation and promotion of cell-mediated immune responses.

The Role of Brachyury (T) during Gastrulation Movements in the Sea Urchin Lytechinus variegatus

The studies described here sought to identify and characterize genes involved in the gastrulation and morphogenetic movements that occur during sea urchin embryogenesis. An orthologue of the T-box family transcription factor, Brachyury, was cloned through a candidate gene approach. Brachyury (T) is the founding member of this T-box transcription factor family and has been implicated in gastrulation movements in Xenopus, zebrafish, and mouse embryogenesis. Polyclonal serum was generated to LvBrac in order to characterize protein expression. LvBrac initially appears at mesenchyme blastula stage in two distinct regions with embryonic expression perduring until pluteus stage. Vegetally, LvBrac expression is in endoderm and lies circumferentially around the blastopore. This torus-shaped area of LvBrac expression remains constant in size as endoderm cells express LvBrac upon moving into that circumference and cease LvBrac expression as they leave the circumference. Vegetal expression remains around the anus through pluteus stage. The second domain of LvBrac expression first appears broadly in the oral ectoderm at mesenchyme blastula stage and at later embryonic stages is refined to just the stomodael opening. Vegetal LvBrac expression depends on autonomous β-catenin signaling in macromeres and does not require micromere or veg2-inductive signals. It was then determined that LvBrac is necessary for the morphogenetic movements occurring in both expression regions. A dominant-interfering construct was generated by fusing the DNA binding domain of LvBrac to the transcriptional repression module of the Drosophila Engrailed gene in order to perturb gene function. Microinjection of mRNA encoding this LvBrac-EN construct resulted in a block in gastrulation movements but not expression of endoderm and mesoderm marker genes. Furthermore, injection of LvBrac-EN into one of two blastomeres resulted in normal gastrulation movements of tissues derived from the injected blastomere, indicating that LvBrac downstream function may be nonautonomous during sea urchin gastrulation.

χ/Autosomal Translocations in the χq Critical Region Associated with Premature Ovarian Failure Fall within and outside Genes

Premature ovarian failure curtails female reproductive life and is often linked to balanced χq/autosomal translocations in a critical region. We mapped regions around translocations at the edges of this zone (one in χq13.3, two in χq26) in large-insert clones and analyzed their sequence. One χq26 region is extensively transcribed and, in agreement with a recent independent analysis, the breakpoint interrupts a gene that encodes a widely expressed peptidase. In contrast 430 kb around the second χq26 breakpoint has no putative or detected gene content. In 260 kb around the χq13 translocation, the breakpoint falls among a cluster of repetitive elements at least 59 kb from the only detected gene (a rarely expressed T-box family transcription factor). We discuss our results in relation to models that ascribe premature ovarian failure to interruption of ovarian genes or to a failure of interactions involving DNA of the critical region during follicle development.

Tbx1 haploinsufficieny in the DiGeorge syndrome region causes aortic arch defects in mice.

DiGeorge syndrome is characterized by cardiovascular, thymus and parathyroid defects and craniofacial anomalies, and is usually caused by a heterozygous deletion of chromosomal region 22q11.2 (del22q11) (ref. 1). A targeted, heterozygous deletion, named Df(16)1, encompassing around 1 megabase of the homologous region in mouse causes cardiovascular abnormalities characteristic of the human disease. Here we have used a combination of chromosome engineering and P1 artificial chromosome transgenesis to localize the haploinsufficient gene in the region, Tbx1. We show that Tbx1, a member of the T-box transcription factor family, is required for normal development of the pharyngeal arch arteries in a gene dosage-dependent manner. Deletion of one copy of Tbx1 affects the development of the fourth pharyngeal arch arteries, whereas homozygous mutation severely disrupts the pharyngeal arch artery system. Our data show that haploinsufficiency of Tbx1 is sufficient to generate at least one important component of the DiGeorge syndrome phenotype in mice, and demonstrate the suitability of the mouse for the genetic dissection of microdeletion syndromes.

A member of the T-box family of transcription factors mediates cartilage formation in mesenchymal progenitors C3H10T1/2

The bone morphogenetic protein 2 (BMP2)-dependent onset of osteo-/chondrogenic differentiation in the acknowledged pluripotent murine mesenchymal progenitor cell line C3H10T1/2 is accompanied by the immediate upregulation of fibroblast growth factor receptor3 (FGFR3) and by a delayed upregulation of FGFR2. This direct FGFR3-mediated expression seems to be essential for the onset of chondrogenesis since the forced expression of activated FGFR3 is sufficient for differentiation into the chondrogenic lineage. The screening for FGFR3-regulated transcription factors exhibiting a chondrogenic capacity in C3H10T1/2 indentified a T-box containing transcription factor. Forced expression of this factor is sufficient for the initiation of chondrogenic differentiationin mesenchymal progenitors C3H10T1/2 in vitro. Implantation of these recombinant progenitors at ectopic intramuscular sites of the mouse was followed by massive cartilage formation substantiating the chondrogenic potential of this transcription factor. A potential role for this T-box factor in chondrogenesis is also suggested by its expression in various skeletal elements at late stages of murine embryonic development as demonstrated by in situ hybridization. These studies indicate that BMP2 triggers onset of FGFR3-dependent signaling in mesenchymal progenitors C3H10T1/2 to induce a novel type of transcription factor for the initiation of chondrogenic differentiation.