Transcriptional repression by Msx-1 does not require homeodomain DNA-binding sites

The mechanism of biogenesis and potential function of the two alternatively spliced mRNAs encoded by the murine Msx3 gene

The homeodomain-containing Msx3 gene, the newest member of the Msx family, encodes two mRNAs, with an unknown relationship to each other. To elucidate how Msx3 gene generates the two transcripts, we cloned their corresponding cDNAs from an E10.5 mouse embryo cDNA library. The alignment of sequences of the two Msx3-specific cDNAs with the corresponding regions of the genomic DNA revealed that read-through of the sequences preceding a cryptic splice donor site in the first intron of Max3 gene generated the longer transcript. The longer Msx3 mRNA (Msx3-l) contains 66 nucleotides spliced in frame that would encode a protein with 22 additional amino acids. These extra 22 amino acids are inserted between the residues 72 and 73, exactly 14 amino acids upstream of the homeodomain of the smaller Msx3 protein. In situ hybridization and competitive RT-PCR experiments revealed that both Msx3-s and Msx3-l mRNAs elicited similar spatio-temporal patterns of expression in the developing embryo, with maximal expression of both mRNAs occurring in the embryos between 8.5 and 12.5 days post-coitus. We found that while Msx3-s down-regulated Msx1 promoter in transfected C2C12 cells, co-expression of Msx3-l alone did not affect the activity of the Msx1 promoter. In contrast, a concomitant expression of Msx3-s and Msx3-l in the transfected C2C12 cells neutralized the repressive effect of Msx3-s on the Msx1 promoter. In transient expression assays, the repressive action of Msx3-s on Msx1 promoter could also be reversed by co-expression of exogenous Sp1. Our data indicate that the potential interactions among the protein products encoded by the alternately spliced Msx3 mRNAs and the putative constituents of transcriptional co-activators and co-repressors may have functional consequences in vivo.

Developmental regulation of gonadotropin-releasing hormone gene expression by the MSX and DLX homeodomain protein families

Gonadotropin-releasing hormone (GnRH) is the central regulator of the hypothalamic-pituitary-gonadal axis, controlling sexual maturation and fertility in diverse species from fish to humans. GnRH gene expression is limited to a discrete population of neurons that migrate through the nasal region into the hypothalamus during embryonic development. The GnRH regulatory region contains four conserved homeodomain binding sites (ATTA) that are essential for basal promoter activity and cell-specific expression of the GnRH gene. MSX and DLX are members of the Antennapedia class of non-Hox homeodomain transcription factors that regulate gene expression and influence development of the craniofacial structures and anterior forebrain. Here, we report that expression patterns of the Msx and Dlx families of homeodomain transcription factors largely coincide with the migratory route of GnRH neurons and co-express with GnRH in neurons during embryonic development. In addition, MSX and DLX family members bind directly to the ATTA consensus sequences and regulate transcriptional activity of the GnRH promoter. Finally, mice lacking MSX1 or DLX1 and 2 show altered numbers of GnRH-expressing cells in regions where these factors likely function. These findings strongly support a role for MSX and DLX in contributing to spatiotemporal regulation of GnRH transcription during development.

Transcriptional activity of the paired-like homeodomain proteins CHX10 and VSX1

CHX10 and VSX1 are homeodomain (HD) proteins essential for normal retinal development. CHX10 is required first for retinal progenitor cell proliferation and later for bipolar cell differentiation, whereas VSX1 is important in the terminal differentiation of a subset of bipolar cells. Elucidating the transcriptional activity of CHX10 and VSX1 is required to understand how these factors control retinal development. We show that CHX10 and Vsx1 can function as transcriptional repressors. When tethered to a promoter by a heterologous LexA DNA-binding domain or its HD, CHX10 repressed multiple classes of activators in different immortalized cell lines. CHX10 blocked TATA-containing and TATA-less promoters, repressed at a distance, and inhibited a complex enhancer positioned upstream or downstream of the reporter gene, whereas retinoblastoma protein (RB) inhibited the downstream enhancer only. Interestingly, CHX10 mildly potentiated a subset of activators in chick neuronal cultures. Thus, CHX10 is both a versatile repressor and a context-specific weak activator. The CHX10 HD and CVC domains were sufficient for DNA binding and repression. VSX1 contains closely related homeo and CVC domains and, like CHX10, also repressed transcription. A VSX1 HD mutation, R166W, that impairs DNA binding and causes keratoconus in humans, hindered repressor function. Therefore, CHX10 and VSX1 may control retinal bipolar cell specification or differentiation by repressing genes required for the development of other cell types.

The p75NTR-interacting protein SC1 inhibits cell cycle progression by transcriptional repression of cyclin E

Schwann cell factor 1 (SC1), a p75 neurotrophin receptor–interacting protein, is a member of the positive regulatory/suppressor of variegation, enhancer of zeste, trithorax (PR/SET) domain-containing zinc finger protein family, and it has been shown to be regulated by serum and neurotrophins. SC1 shows a differential cytoplasmic and nuclear distribution, and its presence in the nucleus correlates strongly with the absence of bromodeoxyuridine (BrdU) in these nuclei. Here, we investigated potential transcriptional activities of SC1 and analyzed the function of its various domains. We show that SC1 acts as a transcriptional repressor when it is tethered to Gal4 DNA-binding domain. The repressive activity requires a trichostatin A–sensitive histone deacetylase (HDAC) activity, and SC1 is found in a complex with HDACs 1, 2, and 3. Transcriptional repression exerted by SC1 requires the presence of its zinc finger domains and the PR domain. Additionally, these two domains are involved in the efficient block of BrdU incorporation by SC1. The zinc finger domains are also necessary to direct SC1's nuclear localization. Lastly, SC1 represses the promoter of a promitotic gene, cyclin E, suggesting a mechanism for how growth arrest is regulated by SC1.

MSX1 cooperates with histone H1b for inhibition of transcription and myogenesis

During embryogenesis, differentiation of skeletal muscle is regulated by transcription factors that include members of the Msx homeoprotein family. By investigating Msx1 function in repression of myogenic gene expression, we identified a physical interaction between Msx1 and H1b, a specific isoform of mouse histone H1. We found that Msx1 and H1b bind to a key regulatory element of MyoD, a central regulator of skeletal muscle differentiation, where they induce repressed chromatin. Moreover, Msx1 and H1b cooperate to inhibit muscle differentiation in cell culture and in Xenopus animal caps. Our findings define a previously unknown function for "linker" histones in gene-specific transcriptional regulation.

Does Vitamin D play a role on Msx1 homeoprotein expression involving an endogenous antisense mRNA?

Msx1 homeobox gene, a member of Msx family, has been implicated in numerous organs. Its participation was established in different events, such as morphogenetic field determinism and epithelio-mesenchymal interactions. Most of Msx1 target organs are also known for their sensitivity to Vitamin D: such as bone, tooth germ, and hair follicle. Whereas, the expression of Msx2, another member of Msx family, has been shown to be controlled by Vitamin D, no information is available for Msx1. This study aims to analyze the potential relationships between Vitamin D and Msx1 through: (1) comparative analysis of Vitamin D receptor (VDR) and Msx1 protein expression, (2) investigation of Msx1 expression in VDR null mutant mice, and (3) study of Msx1 overexpression impact on osteocalcin VDR expression in immortalized MO6-G3 odontoblasts. Results show the existence of cross-talks between Vitamin D and Msx1 regulation pathways. In odontoblastic cells, Msx1 overexpression decrease VDR expression, whereas in rickets Msx1 sense transcript expression is decreased. These cross-talks may open a new window in the analysis of rickets mineralized tissues physiopathology. In Vitamin D null mutants, the study of the natural Msx1 antisense transcript which has been recently described should be informative.

Msx homeobox gene family and craniofacial development

Vertebrate Msx genes are unlinked, homeobox-containing genes that bear homology to the Drosophila muscle segment homeobox gene. These genes are expressed at multiple sites of tissue-tissue interactions during vertebrate embryonic development. Inductive interactions mediated by the Msx genes are essential for normal craniofacial, limb and ectodermal organ morphogenesis, and are also essential to survival in mice, as manifested by the phenotypic abnormalities shown in knockout mice and in humans. This review summarizes studies on the expression, regulation, and functional analysis of Msx genes that bear relevance to craniofacial development in humans and mice. Key words: Msx genes, craniofacial, tooth, cleft palate, suture, development, transcription factor, signaling molecule.

Distinct activities of Msx1 and Msx3 in dorsal neural tube development

Patterning of the dorsal neural tube involves Bmp signaling, which results in activation of multiple pathways leading to the formation of neural crest,roof plate and dorsal interneuron cell types. We show that constitutive activation of Bmp signaling at early stages (HH10-12) of chick neural tube development induces roof-plate cell fate, accompanied by an increase of programmed cell death and a repression of neuronal differentiation. These activities are mimicked by the overexpression of the homeodomain transcription factor Msx1, a factor known to be induced by Bmp signaling. By contrast, the closely related factor, Msx3, does not have these activities. At later stages of neural tube development (HH14-16), dorsal progenitor cells lose their competence to generate roof-plate cells in response to Bmp signaling and instead generate dorsal interneurons. This aspect of Bmp signaling is phenocopied by the overexpression of Msx3 but not Msx1. Taken together, these results suggest that these two different Msx family members can mediate distinct aspects of Bmp signaling during neural tube development.

Molecules mimicking Smad1 interacting with Hox stimulate bone formation

Bone morphogenetic proteins (BMPs) induce osteoblast differentiation and bone formation. Smads, a group of functionally and structurally related intracellular effectors, mediate signaling initiated by BMPs and regulate cell definite commitment. Previously, we showed that Smad1 activates osteopontin and osteoprotegerin gene expression by dislodging Hoxc-8 from its DNA binding sites. A domain of Smad1, termed Smad1C, was characterized as interacting with Hoxc-8 and then crippling its DNA-binding ability. Ectopic expression of Smad1C is able to bypass BMP signaling in the induction of osteoblast differentiation and bone formation in vitro. To test the function of Smad1C on osteogenesis in vivo, we generated transgenic mice in which Smad1C expression was induced with doxycycline and localized in bone by using a tetracycline-inducible expression system (Tet-on) modified with a bone-specific gene promoter, type I collagen alpha1. The mice expressing Smad1C showed increased skeletal bone mineral density compared with their littermates. Bone histomorphometric analysis of mouse tibiae showed that Smad1C significantly increases trabecular bone area and length of trabecular surface covered with osteoid and up-regulates bone marker gene (OPN, Cbfa1, Col I alpha1, BSP, ALP) expression in vivo. Moreover, stromal cells isolated from mice expressing Smad1C displayed a higher potential for differentiating into osteoblasts than the other mice. These results indicate that Smad1C mimics BMPs in the induction of osteogenesis in vivo. Most important, using a high throughput screening assay based on mimicking Smad1C's displacement of Hoxc-8 binding to DNA, we identified chemical entities that exhibit bone anabolic activity in cell and bone organ cultures, suggesting the possibility that the compounds may be used as bone anabolic agents to treat bone pathologies.

JUMONJI, a critical factor for cardiac development, functions as a transcriptional repressor

JUMONJI (JMJ) is a nuclear factor that is critical for normal cardiovascular development, evidenced by the analysis of jmj homozygous mutant mice. However, the molecular function of JMJ remains to be elucidated. In the present study, we investigated whether JMJ is a transcriptional modulator. Reporter gene assays using the GAL4-DNA binding domain fused to JMJ and a reporter gene consisting of the GAL4 binding sites upstream of a luciferase reporter gene indicated that JMJ functions as a powerful transcriptional repressor. The DNA binding motif of JMJ was determined using CASTing experiments by incubating a random oligonucleotide library with the GST-JMJ fusion protein coupled to agarose beads. Among the selected binding oligonucleotides, the high affinity DNA binding sequences were identified by gel retardation assays. JMJ repressed expression of the reporter genes containing the high affinity JMJ binding sequences, indicating that JMJ is a DNA-binding transcriptional repressor. The domains for transcriptional repression, DNA binding, and nuclear localization signal were mapped by mutational analyses using reporter gene assays, gel retardation assays, and immunostaining experiments, respectively. The present data demonstrate for the first time that JMJ functions as a DNA-binding transcriptional repressor. Therefore, JMJ may play a critical role in transcription factor cascade to regulate expression of heart-specific genes and normal cardiac development.

Molecular basis of non-syndromic tooth agenesis: mutations of MSX1 and PAX9 reflect their role in patterning human dentition

Tooth agenesis constitutes the most common anomalies of dental development in man. Despite this, little is known about the genetic defects responsible for this complex condition. To date, the only genes associated with the non-syndromic form of tooth agenesis are MSX1 and PAX9, which encode transcription factors that play a critical role during tooth development. This paper aims to review current literature about the molecular mechanisms responsible for selective tooth agenesis in humans.

HOX and non-HOX homeobox genes in leukemic hematopoiesis

Dysregulation of homeobox (HB)-containing genes is becoming increasingly recognized as the underlying basis of many hematologic malignancies. Expression of clustered HB (HOX) genes within the hematopoietic system, and enforced overexpression and knockout studies have provided support for the concept that these homeodomain-containing transcription factors play a significant role in the developmental biology of hematopoietic cells. Diverged HB (non-HOX) genes have recently been identified as either cofactors and/or accelerators of leukemic disease mediated by HOX genes or as bona fide oncogenes. In this review, we examine the evidence that supports a central role for HB genes in normal and malignant hematopoiesis, paying particular attention to the non-HOX class and the possible mechanisms through which they contribute to leukemic transformation.

Conserved homeodomain proteins interact with MADS box protein Mcm1 to restrict ECB-dependent transcription to the M/G1 phase of the cell cycle

Two homeodomain proteins, Yox1 and Yhp1, act as repressors at early cell cycle boxes (ECBs) to restrict their activity to the M/G1 phase of the cell cycle in budding yeast. These proteins bind to Mcm1 and to a typical homeodomain binding site. The expression of Yox1 is periodic and directly correlated with its binding to, and repression of, ECB activity. The absence of Yox1 and Yhp1 or the constitutive expression of Yox1 leads to the loss of cell-cycle regulation of ECB activity. Therefore, the cell-cycle-regulated expression of these repressors defines the interval of ECB-dependent transcription. Twenty-eight genes, including MCM2-7, CDC6, SWI4, CLN3, and a number of genes required during late M phase have been identified that are coordinately regulated by this pathway.

Inducing cellular dedifferentiation: a potential method for enhancing endogenous regeneration in mammals

Salamanders have the remarkable ability to regenerate lost body parts and injured organs. This regenerative ability requires fully-differentiated cells in the vicinity of the injury to dedifferentiate, proliferate, and then redifferentiate to form the specialized cells that comprise the regenerated structure or organ. The dedifferentiation stage plays a crucial role in the regenerative response and distinguishes the salamander from other vertebrates with more limited regenerative abilities. Recently, several investigators have shown that certain mammalian cell types can be induced to dedifferentiate to progenitor cells when stimulated with the appropriate signals. This discovery opens the possibility that researchers might one day enhance the endogenous regenerative capacity of mammals by inducing cellular dedifferentiation in vivo.

Stimulation of Cdx1 by oncogenic beta-catenin/Tcf4 in colon cancer cells; opposite effect of the CDX2 homeoprotein

The homeobox gene Cdx1 is a regulator of intestinal epithelial cell proliferation and differentiation. Using a transfection approach, we showed here that the oncogenic activation of the beta-catenin pathway stimulates the endogenous expression of the Cdx1 mRNA as well as the activity of the Cdx1 promoter in cancer cells of the human colon. Reciprocally, the paralogue homeobox gene Cdx2 exerts an inhibitory effect on the basal and on the beta-catenin-stimulated activity of the Cdx1 promoter. The inhibitory effect of CDX2 requires the intact homeodomain. It is not dependent on canonical CDX binding sites in the Cdx1 promoter nor on the cis-elements specifically targeted by the beta-catenin/Tcf complex. We conclude that the oncogenically activated beta-catenin and CDX2 have opposite and independent effects on the Cdx1 homeobox gene.

The role of MSX1 in human tooth agenesis

MSX1 has a critical role in craniofacial development, as indicated by expression assays and transgenic mouse phenotypes. Previously, MSX1 mutations have been identified in three families with autosomal-dominant tooth agenesis. To test the hypothesis that MSX1 mutations are a common cause of congenital tooth agenesis, we screened 92 affected individuals, representing 82 nuclear families, for mutations, using single-strand conformation analysis. A Met61Lys substitution was found in two siblings from a large family with autosomal-dominant tooth agenesis. Complete concordance of the mutation with tooth agenesis was observed in the extended family. The siblings have a pattern of severe tooth agenesis similar that in to previous reports, suggesting that mutations in MSX1 are responsible for a specific pattern of inherited tooth agenesis. Supporting this theory, no mutations were found in more common cases of incisor or premolar agenesis, indicating that these have a different etiology.

Evidence that GRIP, a PDZ-domain protein which is expressed in the embryonic forebrain, co-activates transcription with DLX homeodomain proteins

The DLX homeodomain proteins control development of the basal ganglia and branchial arches. To identify co-factors that regulate DLX function we utilized the yeast two-hybrid assay, and found a DLX interacting protein (DIP2) which binds to the N-terminal region of DLX2 via a PDZ domain. DIP2 appears to be an alternatively spliced form of GRIP1, a protein known to bind AMPA glutamate receptors via PDZ domains. Thus, we named it GRIP1b. We provide evidence that GRIP1b can function as a transcriptional co-activator of DLX2 and DLX5. Glutamate receptors inhibit this co-activation. These results suggest that some PDZ proteins may regulate transcription via their interactions with homeodomain proteins. Furthermore, these results suggest a link between glutamate receptors, PDZ proteins and the DLX transcription factors, all of which are co-expressed in the developing basal ganglia.

Msx homeobox genes inhibit differentiation through upregulation ofcyclin D1

During development, patterning and morphogenesis of tissues are intimately coordinated through control of cellular proliferation and differentiation. We describe a mechanism by which vertebrate Msx homeobox genes inhibit cellular differentiation by regulation of the cell cycle. We show that misexpression of Msx1 via retroviral gene transfer inhibits differentiation of multiple mesenchymal and epithelial progenitor cell types in culture. This activity of Msx1 is associated with its ability to upregulate cyclin D1 expression and Cdk4 activity, while Msx1 has minimal effects on cellular proliferation. Transgenic mice that express Msx1 under the control of the mouse mammary tumor virus long terminal repeat (MMTV LTR) display impaired differentiation of the mammary epithelium during pregnancy, which is accompanied by elevated levels of cyclin D1 expression. We propose that Msx1 gene expression maintains cyclin D1 expression and prevents exit from the cell cycle, thereby inhibiting terminal differentiation of progenitor cells. Our model provides a framework for reconciling the mutant phenotypes of Msx and other homeobox genes with their functions as regulators of cellular proliferation and differentiation during embryogenesis.

The identification of Prx1 transcription regulatory domains provides a mechanism for unequal compensation by the Prx1 and Prx2 loci

Transcription regulatory domains of the Prx1a and Prx1b homeoproteins were analyzed in transient transfection assays using artificial promoters as well as an established downstream target promoter (tenascin-c). Activation and repression domains were detected in their common amino end. In the carboxyl end of Prx1a an activation domain and an inhibition/masking region (OAR domain) were detected. The Prx1b isoform, generated by alternative splicing, does not contain these carboxyl activation or inhibition domains. Instead, the data demonstrate that the carboxyl tail of Prx1b contains a potent repressor region. This difference in the carboxyl tail accounts for a 45-fold difference observed in transcription regulatory activity between Prx1a and Prx1b. The data also support the likelihood that this difference between Prx1a and Prx1b is higher in the presence of still undetermined cofactors. DNA binding affinities of Prx1a, Prx1b, and a series of truncation mutants were also examined. The carboxyl tail of Prx1a, which inhibited transcription activation in the transfection assays, also inhibited DNA binding. These differences in biochemical function between Prx1a and Prx1b, as well as the recently described activities of Prx2, provide a mechanism for the unequal compensation between the Prx1 and Prx2 loci.

Identification of domains mediating transcription activation, repression, and inhibition in the paired-related homeobox protein, Prx2 (S8)

Despite the growing information concerning the developmental importance of the Prx2 protein, the structural determinants of Prx2 function are poorly understood. To gain insight into the transcription regulatory regions of the Prx2 protein, we generated a series of truncation mutants. Both the Prx2 response element (PRE) and a portion of the tenascin promoter, a downstream target of Prx2, were used as reporters in transient transfection assays. This analysis showed that a conserved domain (PRX), found in both Prx1 and Prx2, activated transcription in NIH 3T3 cells. This PRX domain, as well as other functional regions of Prx2, demonstrated both cell-specific and promoter-dependent transcriptional regulation. A second important region, the OAR (aristaless) domain, which is conserved among 35 Paired-type homeodomain proteins, was observed to inhibit transcription. Deletion of this element resulted in a 20-fold increase of transcription from the tenascin reporter in NIH 3T3 cells but not in C2C12 cells. The OAR domain did not function as a repressor in chimeric fusions with the Gal4 DNA binding domain in either cell type, characterizing it as an inhibitor instead of a repressor. These results give insight into the function of the Prx2 transcription factor while establishing the framework for comparison with the two isoforms of Prx1.

PRH represses transcription in hematopoietic cells by at least two independent mechanisms

PRH (proline-rich homeodomain protein) is strongly expressed in the hematopoietic compartment. Here we show that PRH is a repressor of transcription in hematopoietic cells. A fragment of PRH that includes the homeodomain can bind to TATA box sequences in vitro and can also bind to the TATA box-binding protein. PRH represses transcription from TATA box-containing promoters in intact cells but does not repress transcription from a promoter lacking a TATA box. A mutation in the PRH homeodomain that blocks binding to DNA but that has little or no effect on binding to the TATA box-binding protein significantly reduces the ability of the protein to repress transcription and provides the first clear demonstration that a homeodomain can bring about transcriptional repression in vivo by binding to a TATA box. However, we also show that mutation of the PRH homeodomain does not block the ability of PRH to repress transcription when this protein is tethered upstream of the TATA box via a heterologous DNA-binding domain. PRH also contains an N-terminal proline-rich repression domain that is separate from the homeodomain. Deletion mapping suggests that this repression domain contains at least two regions that both independently contribute to transcriptional repression.

Molecular cloning and characterization of DEC2, a new member of basic helix-loop-helix proteins

DEC1 is a basic helix-loop-helix (bHLH) protein related to Drosophila Hairy, Enhancer of split and HES, and involved in the control of proliferation and/or differentiation of chondrocytes, neurons, etc. We report here the identification and characterization of human, mouse and rat DEC2, a novel member of the DEC subfamily. DEC2 had high (97%) and moderate (52%) similarities in the bHLH region and the Orange domain with DEC1, respectively. However, DEC2, but not DEC1, had alanine and glycine-rich regions in the C-terminal half. Unlike Hairy, Enhancer of split and HES, DEC2 lacked the WRPW motif for interaction with the corepressor Groucho. The DEC2 gene was mapped to human chromosome 12p11.23-p12.1, mouse chromosome 6 G2-G3 and rat chromosome 4q43 distal-q4, where the conserved linkage homology has been identified among these species. Unlike DEC1, which was broadly expressed in many tissues, DEC2 showed a more restricted pattern of mRNA expression. The DEC subfamily proteins may play an important role in tissue development.

Msx3 protein recruits histone deacetylase to down-regulate the Msx1 promoter

Msx1 promoter is known to be repressed by Msx1 protein [Shetty, Takahashi, Matsui, Iyengar and Raghow (1999) Biochem. J. 339, 751-758]. We show that in the transiently transfected C(2)C(12) myoblasts, co-expression of Msx3 also causes potent repression of Msx1 promoter that can be relieved by exogenous expression of cAMP-response-element-binding protein-binding protein (CBP) and p300 in a dose-dependent manner. Co-immunoprecipitation and Western blot analyses revealed that Msx3 interacts with CBP and p300 and this interaction significantly decreases the histone acetyltransferase (HAT) activity of both proteins. We also discovered that Msx3-mediated repression of Msx1 promoter is synergized by the exogenous co-expression of histone deacetylase 1 (HDAC1). Furthermore, the repression of Msx1 promoter by Msx3 could be relieved by treating transfected cells with trichostatin A, an inhibitor of HDAC(s). Finally, we show that Msx3 and HDAC1 can be co-immunoprecipitated in a complex that does not contain CBP and that Msx3 and HDAC1 proteins are co-localized in the nucleus. Taken together, our results strongly suggest that two distinct multiprotein complexes are present within the nuclei of C(2)C(12) cells: one containing Msx3 and HDAC(s) and another containing Msx3 and CBP and/or p300. On the basis of these results, we propose a dual mechanism of repression by Msx3 protein that involves the squelching of the HAT activity of co-activators, CBP and p300, and recruitment of HDAC(s).

Dedifferentiation of mammalian myotubes induced by msx1

The process of cellular differentiation culminating in terminally differentiated mammalian cells is thought to be irreversible. Here, we present evidence that terminally differentiated murine myotubes can be induced to dedifferentiate. Ectopic expression of msx1 in C2C12 myotubes reduced the nuclear muscle proteins MyoD, myogenin, MRF4, and p21 to undetectable levels in 20%-50% of the myotubes. Approximately 9% of the myotubes cleave to produce either smaller multinucleated myotubes or proliferating, mononucleated cells. Finally, clonal populations of the myotube-derived mononucleated cells can be induced to redifferentiate into cells expressing chondrogenic, adipogenic, myogenic, and osteogenic markers. These results suggest that terminally differentiated mammalian myotubes can dedifferentiate when stimulated with the appropriate signals and that msx1 can contribute to the dedifferentiation process.

The Dlx3 protein harbors basic residues required for nuclear localization, transcriptional activity and binding to Msx1

The murine Dlx3 protein is a putative transcriptional activator that has been implicated during development and differentiation of epithelial tissue. Dlx3 contains a homeodomain and mutational analysis has revealed two regions, one N-terminal and one C-terminal to the homeodomain, that act as transcriptional activators in a yeast one-hybrid assay. In addition to transactivation, data are presented to demonstrate specific DNA binding and an association between Dlx3 and the Msx1 protein in vitro. Immunohistochemical analysis confirmed coexpression of Dlx3 and Msx1 proteins in the differentiated layers of murine epidermal tissues. Transcription factor function requires nuclear localization. In this study, the intracellular localization of the green fluorescent protein fused to Dlx3 was examined in keratinocytes induced to differentiate by calcium and is shown to localize to the nucleus. A bipartite nuclear localization signal (NLS) was identified by mutational analysis and shown to be sufficient for nuclear localization. This was demonstrated by insertion of the Dlx3 bipartite NLS sequence into a cytoplasmic fusion protein, GFP-keratin 14, which functionally redirected GFP-keratin 14 expression to the nucleus. Further analysis of Dlx3 NLS mutants revealed that the Dlx3 NLS sequences are required for specific DNA binding, transactivation potential and interactions with the Msx1 protein.

Evolution of Hoxa-11 in lineages phylogenetically positioned along the fin-limb transition

HOXA11 is a transcription factor implicated in paired appendage development. To identify signatures of evolutionary change in the structural, and putative functional, domains of HOXA11, we studied its evolution in tetrapod and nontetrapod lineages that represent approximately 1.5 billion years of evolutionary time. Here, Hoxa-11 gene proper sequences were determined for frog (Xenopus tropicalis), coelacanth (Latimeria chalumnae), common zebrafish (Danio rerio; Hoxa-11a and Hoxa-11b paralogs), and giant zebrafish (D. aequipinnatus; Hoxa-11b) and aligned against previously published Hoxa-11 sequences of human, mouse, chick, and newt. Based on aligned Hoxa-11 amino acid sequences, the protein was demarcated into three segments: Domains I (N-terminal) and III (homeobox + C-terminal), which varied slightly in rates and patterns of evolution, and a variable, overall hydrophilic region (HyD), which partially overlaps with Domain I. As judged by character reconstructions of HOXA11 Domains I and III, no significant changes in rates of coding sequence evolution occurred in tetrapods (frog and chick), relative to coelacanth (a lobe-finned fish), i.e., across the fin-limb transition. Accelerated rates of Hoxa-11 coding sequence evolution were observed for the mammalian and newt lineages. This was shown to be a gene-specific phenomenon. The duplicated Hoxa-11a and Hoxa-11b genes of zebrafish exhibited accelerated rates of evolution and accumulated substitutions at sites that are conserved among coelacanth and all tetrapods examined. Amino acid sequence comparisons of the HyD of HOXA11 suggested that a putative repressor subdomain, containing stretches of consecutive alanine residues, emerged within the tetrapods. A high degree of nucleotide conservation in the 5' half of the Hoxa-11 intron was observed for tetrapod and nontetrapod lineages. Using electrophoretic mobility shift assays, a 35-bp intron sequence, which is 100% conserved in all Hoxa-11 loci except for the zebrafish Hoxa-11a paralog, was found to bind protein(s) in HeLa and chick whole-cell extracts.

BF-1 interferes with transforming growth factor beta signaling by associating with Smad partners

The winged-helix (WH) BF-1 gene, which encodes brain factor 1 (BF-1) (also known as foxg1), is essential for the proliferation of the progenitor cells of the cerebral cortex. Here we show that BF-1-deficient telencephalic progenitor cells are more apt to leave the cell cycle in response to transforming growth factor beta (TGF-beta) and activin. We found that ectopic expression of BF-1 in vitro inhibits TGF-beta mediated growth inhibition and transcriptional activation. Surprisingly, we found that the ability of BF-1 to function as a TGF-beta antagonist does not require its DNA binding activity. Therefore, we investigated whether BF-1 can inhibit Smad-dependent transcriptional responses by interacting with Smads or Smad binding partners. We found that BF-1 does not interact with Smads. Because the identities of the Smad partners mediating growth inhibition by TGF-beta are not clearly established, we examined a model reporter system which is known to be activated by activin and TGF-beta through Smads and the WH factor FAST-2. We demonstrate that BF-1 associates with FAST-2. This interaction is dependent on the same region of protein which mediates its ability to interfere with the antiproliferative activity of TGF-beta and with TGF-beta-dependent transcriptional activation. Furthermore, the interaction of FAST-2 with BF-1 is mediated by the same domain which is required for FAST-2 to interact with Smad2. We propose a model in which BF-1 interferes with transcriptional responses to TGF-beta by interacting with FAST-2 or with other DNA binding proteins which function as Smad2 partners and which have a common mode of interaction with Smad2.

Gtx, an oligodendrocyte-specific homeodomain protein, has repressor activity

Myelin, a multilamellar membrane structure that facilitates nerve conduction, is synthesized in the central nervous system (CNS) by oligodendrocytes. Gtx, a member of the homeodomain family of transcriptional factors, is a candidate regulator of myelin gene expression, because it is uniquely expressed in myelinating oligodendrocytes in postnatal rodent brain. To analyze the regulatory activity of Gtx, we first identified the optimal Gtx-binding sequence using an in vitro DNA-binding assay. This sequence, (A/T)TTAATGA, contains a TAAT core and is similar, but not identical, to that of other homeodomain protein binding sites. When coexpressed in cultured cells along with a minimal promoter containing five tandem repeats of this optimal Gtx-binding sequence, Gtx demonstrated repressor activity, which was also present when Gtx was tethered to DNA by way of the strong GAL4 DNA-binding domain. Truncations of the GAL4-Gtx fusion identified a portable repressor domain within a relatively proline/alanine-rich region N-terminal to the Gtx homeodomain. Cotransfection of a Gtx expression vector into a variety of cell lines, including oligodendrocytes, along with constructs containing portions of the PLP, MBP, or Gtx promoters fused to a reporter gene, however, did not modulate transcription from any of these promoter constructs. These data support the notion that the oligodendrocyte-specific homeodomain protein Gtx can act as a transcriptional repressor. In addition, they suggest that interaction of Gtx with other, as yet undefined, transcriptional regulators modifies Gtx activity in oligodendrocytes.

Repression of transcription by HoxC11 upon phorbol ester stimulation

Hox genes encode transcription factors with a conserved DNA-binding domain and exhibit similar DNA-binding preferences. The in vivo specificity required for their biological function is brought about by combinatorial interactions with other factors. Such interactions also modulate their activation state. Here we show that HoxC11 can either activate or repress transcription in a signal-specific manner. We report the isolation of HoxC11 in a yeast one-hybrid screen for factors binding to a phorbol-ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) response element (VLTRE), which is also a target for TPA-induced binding of Rel factors in gel-shift experiments. Although we detect no binding of in vitro translated HoxC11 to the TPA response element in EMSA, overexpression of HoxC11 in the HepG2 cell line leads to a complete block of TPA-induced transcription from a VLTRE-luciferase reporter. There is, however, no repression of the basal levels. The repression is furthermore not dependent on homeo-domain DNA binding. Our data suggest an interaction of HoxC11 with the basal-transcription machinery. We propose that HoxC11 is capable of mediating transcriptional activation or repression in a signal-specific manner and that its activation of the DNA target sequence in yeast might reflect in vivo recruitment to the promoter complex.

Xenopus msx-1 regulates dorso-ventral axis formation by suppressing the expression of organizer genes

We demonstrated previously that Xmsx-1 is involved in mesoderm patterning along the dorso-ventral axis, under the regulation of BMP-4 signaling. When Xmsx-1 RNA was injected into the dorsal blastomeres, a mass of muscle tissue formed instead of notochord. This activity was similar to that of Xwnt-8 reported previously. In this study, we investigated whether the activity of Xmsx-1 is related to the ventralizing signal and myogenesis promoting factor, Xwnt-8. Whole-mount in situ hybridization showed that Xmsx-1, Xwnt-8, and XmyoD were expressed in overlapping areas, including the ventro-lateral marginal zone at mid-gastrula stage. The expression of XmyoD was induced by the ectopic expression of either Xmsx-1 or Xwnt-8 in dorsal blastomeres, and Xwnt-8 was induced by the ectopic expression of Xmsx-1. On the other hand, the expression of Xmsx-1 was not affected by the loading of pCSKA-Xwnt-8 or dominant-negative Xwnt-8 (DN-Xwnt-8) RNA. In addition, Xmsx-1 RNA did not abrogate the formation of notochord if coinjected with DN-Xwnt-8 RNA. These results suggest that Xmsx-1 functions upstream of the Xwnt-8 signal. Furthermore, the antagonistic function of Xmsx-1 to the expression of organizer genes, such as Xlim-1 and goosecoid, was shown by in situ hybridization analysis and luciferase reporter assay using the goosecoid promoter construct. Finally if Xmsx-1/VP-16 fusion RNA, which was expected to function as a dominant-negative Xmsx-1, was injected into ventral blastomeres, a partial secondary axis formed in a significant number of embryos. In such embryos, the activity of luciferase, under the control of goosecoid promoter sequence, was significantly elevated at gastrula stage. These results led us to conclude that Xmsx-1 plays a central role in establishing dorso-ventral axis in gastrulating embryo, by suppressing the expression of organizer genes.

Complex interactions between epidermal POU domain and activator protein 1 transcription factors regulate the expression of the profilaggrin gene in normal human epidermal keratinocytes

The human profilaggrin gene is expressed in the granular layer during the late stages of the epidermal differentiation. The proximal promoter region of the gene confers high levels of keratinocyte-specific transcription via interactions with c-Jun/c-Fos heterodimers. Here we provide evidence for another level of complexity in the regulation of the profilaggrin promoter activity. The POU domain proteins Oct1, Skn1a/i, and Oct6, which are abundantly expressed in the epidermal cells, act to both stimulate and repress transcription in a general and a cell type-specific mode. While binding to specific recognition elements within the promoter region, they exert their effects by either stimulating or antagonizing the c-Jun-dependent activity of the promoter. The response of the promoter to forced expression of the POU domain proteins reflects the effect of these transcription factors on the endogenous profilaggrin mRNA synthesis and suggests that the latter requires a fine balance in the amounts and the activities of the individual activator protein 1 and POU domain proteins.

Exogenous expression of Msx1 renders myoblasts refractory to differentiation into myotubes and elicits enhanced biosynthesis of four unique mRNAs

Murine myoblast cell lines stably transfected with expression vectors containing homeobox Msx1 cDNA in sense (F31-c) or antisense (F3R1) orientation have contrasting phenotypes. F3R1 cells readily differentiate in medium containing low serum whereas F31-c cells fail to differentiate under these conditions. The mechanism by which exogenous overexpression of Msx1 leads to the altered phenotype of F31-c cells and the downstream targets of Msx1 are unknown. Using the method of differential display, we have identified four cDNAs that represent transcripts up-regulated in F31-c. Two of these cDNAs are homologous to ribosomal proteins S23 and S24 while the third has homology to sequences in the murine Tcp-1 gene. A fourth cDNA does not have appreciable homology to cDNA sequences deposited in the NIH GenBank. Since withdrawal from the cell cycle and enhanced expression of MyoD commonly precede differentiation of myoblasts into myotubes, we also examined regulation of the major cell cycle proteins as well as MyoD by Western blot analysis. We show that the levels of Cdks 2, 4 and 6, cyclins A and D, and the Cdk inhibitor p27 in both proliferating and serum-starved F31-c cells were similar to those in F3R1. Finally, although MyoD protein levels increased in both cell types after 72 h incubation in serum depleted medium, the levels of MyoD in serum-starved F31-c cells were 2-4 fold lower. We postulate that the reduced amount of MyoD is sufficient to permit reversible withdrawal of F31-c cells from the cell cycle, but is inadequate to permit myogenesis.

Roles for Msx and Dlx homeoproteins in vertebrate development

This review provides a comparative analysis of the expression patterns, functions, and biochemical properties of Msx and Dlx homeobox genes. These comprise multi-gene families that are closely related with respect to sequence features as well as expression patterns during vertebrate development. Thus, members of the Msx and Dlx families are expressed in overlapping, but distinct, patterns and display complementary or antagonistic functions, depending upon the context. A common theme shared among Msx and Dlx genes is that they are required during early, middle, and late phases of development where their differential expression mediates patterning, morphogenesis, and histogenesis of tissues in which they are expressed. With respect to their biochemical properties, Msx proteins function as transcriptional repressors, while Dlx proteins are transcriptional activators. Moreover, their ability to oppose each other's transcriptional actions implies a mechanism underlying their complementary or antagonistic functions during development.

Positionally-dependent chondrogenesis induced by BMP4 is co-regulated by Sox9 and Msx2

Cranial neural crest cells emigrate from the posterior midbrain and anterior hindbrain to populate the first branchial arch and eventually differentiate into multiple cell lineages in the maxilla and mandible during craniofacial morphogenesis. In the developing mouse mandibular process, the expression profiles of BMP4, Msx2, Sox9, and type II collagen demonstrate temporally and spatially restrictive localization patterns suggestive of their functions in the patterning and differentiation of cartilage. Under serumless culture conditions, beads soaked in BMP4 and implanted into embryonic day 10 (E10) mouse mandibular explants induced ectopic cartilage formation in the proximal position of the explant. However, BMP4-soaked beads implanted at the rostral position did not have an inductive effect. Ectopic chondrogenesis was associated with the up-regulation of Sox9 and Msx2 expression in the immediate vicinity of the BMP4 beads 24 hours after implantation. Control beads had no effect on cartilage induction or Msx2 and Sox9 expression. Sox9 was induced at all sites of BMP4 bead implantation. In contrast, Msx2 expression was induced more intensely at the rostral position when compared with the proximal position, and suggested that Msx2 expression was inhibitory to chondrogenesis. To test the hypothesis that over-expression of Msx2 inhibits chondrogenesis, we ectopically expressed Msx2 in the mandibular process organ culture system using adenovirus gene delivery strategy. Microinjection of the Msx2-adenovirus to the proximal position inhibited BMP4-induced chondrogenesis. Over-expression of Msx2 also resulted in the abrogation of endogenous cartilage and the down-regulation of type II collagen expression. Taken together, these results suggest that BMP4 induces chondrogenesis, the pattern of which is positively regulated by Sox9 and negatively by Msx2. Chondrogenesis only occurs at sites where Sox9 expression is high relative to that of Msx2. The combinatorial action of these transcription factors appear to establish a threshold for Sox9 function and thereby restricts the position of chondrogenesis.

Biomineralization, life-time of odontogenic cells and differential expression of the two homeobox genes MSX-1 and DLX-2 in transgenic mice

Msx and Dlx homeobox genes encode for transcription factors that control early morphogenesis. More specifically, Msx-1, Msx-2, and Dlx-2 homeobox genes contribute to the initial patterning of the dentition. The present study is devoted to the potential role of those homeobox genes during the late formation of mineralized tissues, using the rodent incisor as an experimental system. The continuously erupting mandibular incisor allows (1) the coinvestigation of the whole sequences of amelogenesis and dentinogenesis, aligned along the main dental axis in a single sample in situ and (2) the differential characterization of transcripts generated by epithelial and ectomesenchymal odontogenic cells. Northern blot experiments on microdissected cells showed the continuing expression of Msx-2 and Dlx-2 in the later stages of dental biomineralization, differentially in epithelial and ectomesenchymal compartments. Transgenic mice produced with LacZ reporter constructs for Dlx-2 and Msx-1 were used to detect different components of the gene expression patterns with the sensitive beta-galactosidase histoenzymology. The results show a prominent epithelial involvement of Dlx-2, with stage-specific variations in the cells involved in enamel formation. Quantitative analyses identified specific modulations of Dlx-2 expression in ameloblasts depending on the anatomical sites of the incisor, showing more specifically an inverse linear relationship between the Dlx-2 promoter activity level and enamel thickness. This investigation extends the role of homeoproteins to postmitotic stages, which would control secretory cell activity, in a site-specific manner as shown here for Dlx-2.

Smad1 domains interacting with Hoxc-8 induce osteoblast differentiation

Bone morphogenetic proteins are potent osteotropic agents that induce osteoblast differentiation and bone formation. The signal transduction of bone morphogenetic proteins has recently been discovered to involve Smad proteins. Smad1 is an essential intracellular component that is specifically phosphorylated by bone morphogenetic protein receptors and translocated into the nucleus upon ligand stimulation. Previously, we have reported that Smad1 activates osteopontin gene expression in response to bone morphogenetic protein simulation through an interaction with a homeodomain transcription factor, Hoxc-8. In the present study, the interaction domains between the two proteins were characterized by deletional analysis in both yeast two-hybrid and gel shift assays. Two regions within the amino-terminal 87 amino acid residues of Smad1 were mapped to interact with Hoxc-8, one of which binds to the homeodomain. Overexpression of recombinant cDNAs encoding the Hoxc-8 interaction domains of Smad1 effectively activated osteopontin gene transcription in transient transfection assays. Furthermore, stable expression of these Smad1 fragments in 2T3 osteoblast precursor cells stimulated osteoblast differentiation-related gene expression and led to mineralized bone matrix formation. Our data suggest that the interaction of amino-terminal Smad1 with Hoxc-8 mimics bone morphogenetic protein signaling and is sufficient to induce osteoblast differentiation and bone cell formation.

Neural induction

The formation of the vertebrate nervous system is initiated at gastrula stages of development, when signals from a specialized cluster of cells (the organizer) trigger neural development in the ectoderm. This process, termed neural induction, was first described in 1924 and stemmed from experiments on amphibia (Spemann & Mangold 1924). In recent years, the molecular mechanisms underlying neural induction in the amphibian have been elucidated. Surprisingly, neuralizing agents secreted by the organizer do not act via receptor-mediated signaling events; rather, these factors antagonize local epidermal inducers within the cells of the dorsal ectoderm and function to uncover the latent neural fate of these cells. Many of the recent advances in our understanding of vertebrate neural induction come from studies on the frog, Xenopus laevis. It is now clear that a blockade of signaling of the bone morphogenetic proteins (BMPs) during gastrula stages is sufficient to initiate neuralization of the ectoderm in this species. Thus this review first details our current understanding of neural induction, using the amphibian as a model. We then use data emerging from other systems to examine the extent to which the Xenopus studies can be applied to other vertebrate species. The initiation of the neurectoderm-specific gene expression program and subsequent steps in patterning and neuronal development are only touched on here. We focus primarily on the initial establishment of the neural fate in the vertebrate gastrula ectoderm.

Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors

The migration of myogenic precursors to the vertebrate limb exemplifies a common problem in development - namely, how migratory cells that are committed to a specific lineage postpone terminal differentiation until they reach their destination. Here we show that in chicken embryos, expression of the Msx1 homeobox gene overlaps with Pax3 in migrating limb muscle precursors, which are committed myoblasts that do not express myogenic differentiation genes such as MyoD. We find that ectopic expression of Msx1 in the forelimb and somites of chicken embryos inhibits MyoD expression as well as muscle differentiation. Conversely, ectopic expression of Pax3 activates MyoD expression, while co-ectopic expression of Msx1 and Pax3 neutralizes their effects on MyoD. Moreover, we find that Msx1 represses and Pax3 activates MyoD regulatory elements in cell culture, while in combination, Msx1 and Pax3 oppose each other's trancriptional actions on MyoD. Finally, we show that the Msx1 protein interacts with Pax3 in vitro, thereby inhibiting DNA binding by Pax3. Thus, we propose that Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors via direct protein-protein interaction. Our results implicate functional antagonism through competitive protein-protein interactions as a mechanism for regulating the differentiation state of migrating cells.

Identification of core sequences involved in metabolism-dependent nuclear protein binding to the rat insulin-like growth factor I gene

In the liver, most insulin-like growth factor I (IGF-I) transcripts originate in exon 1, where important cis-regulatory regions are located downstream from the major transcription initiation sites. Within these regions, we have attempted to identify sequences which are involved in the decrease in IGF-I gene transcription associated with diabetes mellitus. The function of different genomic templates was assessed by in vitro transcription, which revealed a consistent 50-80% decrease in the activity of nuclear extracts from streptozotocin-diabetic as compared with normal rats. The disparity in transcriptional activity between normal and diabetic nuclear extracts was reduced with templates containing 11-bp mutations within DNase I protected regions III or V (+42 and +129 bp, respectively, from the major transcription initiation site), but a mutation between regions IV and V had little effect. Within region III, gel mobility shift analysis and methylation interference studies indicated that DNA-protein interactions involve a GCGC core sequence. In region V, gel mobility shift studies and uracil interference analysis revealed interactions involving a TTAT core. While gel mobility shift analysis and transient transfection studies indicate that the GCGC core sequence in region III recognizes C/EBP, the AT-rich sequence in region V is likely to recognize a protein with homeodomain characteristics. Identification of the nuclear factor(s) interacting with regions III and V, downstream from exon 1 initiation sites, will be important for understanding the mechanism of reduced IGF-I gene transcription due to diabetes mellitus.

Epidermal dysplasia and abnormal hair follicles in transgenic mice overexpressing homeobox gene MSX-2

The homeobox gene Msx-2 is expressed specifically in sites of skin appendage formation. To explore its part in skin morphogenesis, we produced transgenic mice expressing Msx-2 under the control of the cytomegalovirus promoter. The skin of these transgenic mice was flaky, exhibiting desquamation and shorter hairs. Histologic analysis showed thickened epidermis with hyperproliferation, which was restricted to the basal layer. Hyperkeratosis was also evident. A wide zone of suprabasal cells were misaligned and coexpressed keratins 14 and 10. There was reduced expression of integrin beta 1 and DCC in the basal layer. Hair follicles were misaligned with a shrunken matrix region. The dermis showed increased cellularity and empty vacuoles. We suggest that Msx-2 is involved in the growth control of skin and skin appendages.

NUP98 Is Fused to PMX1 Homeobox Gene in Human Acute Myelogenous Leukemia With Chromosome Translocation t(1;11)(q23;p15)

The nucleoporin gene NUP98 was found fused to theHOXA9, HOXD13, or DDX10 genes in human acute myelogenous leukemia (AML) with chromosome translocations t(7;11)(p15;p15), t(2;11)(q35;p15), or inv(11)(p15;q22), respectively. We report here the fusion between the NUP98 gene and another homeobox gene PMX1 in a case of human AML with a t(1;11)(q23;p15) translocation. The chimeric NUP98-PMX1transcript was detected; however, there was no reciprocalPMX1-NUP98 fusion transcript. Like the NUP98-HOXA9fusion, NUP98 and PMX1 were fused in frame and the N-terminal GLFG-rich docking region of the NUP98 and the PMX1 homeodomain were conserved in the NUP98-PMX1 fusion, suggesting that PMX1 homeodomain expression is upregulated and that the fusion protein may act as an oncogenic transcription factor. The fusion to NUP98 results in the addition of the strong transcriptional activation domain located in the N-terminal region of NUP98 to PMX1. These findings suggest that constitutive expression and alteration of the transcriptional activity of the PMX1 homeodomain protein may be critical for myeloid leukemogenesis.

NUP98 Is Fused to PMX1 Homeobox Gene in Human Acute Myelogenous Leukemia With Chromosome Translocation t(1;11)(q23;p15)

The nucleoporin gene NUP98 was found fused to theHOXA9, HOXD13, or DDX10 genes in human acute myelogenous leukemia (AML) with chromosome translocations t(7;11)(p15;p15), t(2;11)(q35;p15), or inv(11)(p15;q22), respectively. We report here the fusion between the NUP98 gene and another homeobox gene PMX1 in a case of human AML with a t(1;11)(q23;p15) translocation. The chimeric NUP98-PMX1transcript was detected; however, there was no reciprocalPMX1-NUP98 fusion transcript. Like the NUP98-HOXA9fusion, NUP98 and PMX1 were fused in frame and the N-terminal GLFG-rich docking region of the NUP98 and the PMX1 homeodomain were conserved in the NUP98-PMX1 fusion, suggesting that PMX1 homeodomain expression is upregulated and that the fusion protein may act as an oncogenic transcription factor. The fusion to NUP98 results in the addition of the strong transcriptional activation domain located in the N-terminal region of NUP98 to PMX1. These findings suggest that constitutive expression and alteration of the transcriptional activity of the PMX1 homeodomain protein may be critical for myeloid leukemogenesis.

Btf, a novel death-promoting transcriptional repressor that interacts with Bcl-2-related proteins

The adenovirus E1B 19,000-molecular-weight (19K) protein is a potent inhibitor of apoptosis and cooperates with E1A to transform primary rodent cells. E1B 19K shows sequence and functional homology to the mammalian antiapoptotic gene product, Bcl-2. Like Bcl-2, the biochemical mechanism of E1B 19K function includes binding to and antagonization of cellular proapoptotic proteins such as Bax, Bak, and Nbk/Bik. In addition, there is evidence that E1B 19K can affect gene expression, but whether this contributes to its antiapoptotic function has not been determined. In an effort to further understand the functions of E1B 19K, we screened for 19K-associated proteins by the yeast two-hybrid system. A novel protein, Btf (Bcl-2-associated transcription factor), that interacts with E1B 19K as well as with the antiapoptotic family members Bcl-2 and Bcl-xL but not with the proapoptotic protein Bax was identified. btf is a widely expressed gene that encodes a protein with homology to the basic zipper (bZip) and Myb DNA binding domains. Btf binds DNA in vitro and represses transcription in reporter assays. E1B 19K, Bcl-2, and Bcl-xL sequester Btf in the cytoplasm and block its transcriptional repression activity. Expression of Btf also inhibited transformation by E1A with either E1B 19K or mutant p53, suggesting a role in either promotion of apoptosis or cell cycle arrest. Indeed, the sustained overexpression of Btf in HeLa cells induced apoptosis, which was inhibited by E1B 19K. Furthermore, the chromosomal localization of btf (6q22-23) maps to a region that is deleted in some cancers, consistent with a role for Btf in tumor suppression. Thus, btf may represent a novel tumor suppressor gene residing in a unique pathway by which the Bcl-2 family can regulate apoptosis.

Smad1 interacts with homeobox DNA-binding proteins in bone morphogenetic protein signaling

Bone morphogenetic proteins (BMP) transduce their signals into the cell through a family of mediator proteins known as Smads. Upon phosphorylation by the BMP receptors, Smad1 interacts with Smad4 and translocates into the nucleus where the complex recruits DNA-binding protein(s) to activate specific gene transcription. However, the DNA-binding protein(s) involved in BMP signaling has not been identified. Using a yeast two-hybrid approach, we found that Smad1 interacts with Hoxc-8, a homeodomain transcription factor. The interaction between Smad1 and Hoxc-8 was confirmed by a "pull-down" assay and a co-immunoprecipitation experiment in COS-1 cells. Interestingly, purified Smad1 inhibited Hoxc-8 binding to the osteopontin Hoxc-8 site in a concentration-dependent manner. Transient transfection studies showed that native osteopontin promoter activity was elevated upon BMP stimulation. Consistent with the gel shift assay, overexpression of Hoxc-8 abolished the BMP stimulation. When a wild type or mutant Hoxc-8 binding element was linked to an SV40 promoter-driven reporter gene, the wild type but not the mutant Hoxc-8 binding site responded to BMP stimulation. Again, overexpression of Hoxc-8 suppressed the BMP-induced activity of the wild type reporter construct. Our findings suggest that Smad1 interaction with Hoxc-8 dislodges Hoxc-8 from its DNA binding element, resulting in the induction of gene expression.

Transcriptional autorepression of Msx1 gene is mediated by interactions of Msx1 protein with a multi-protein transcriptional complex containing TATA-binding protein, Sp1 and cAMP-response-element-binding protein-binding protein (CBP/p300)

The TATA-less murine Msx1 promoter contains two Msx1-binding motifs, located at -568 to -573 and +25 to +30, and is subject to potent autorepression [Takahashi, Guron, Shetty, Matsui and Raghow (1997) J. Biol. Chem. 272, 22667-22678]. To investigate the molecular mechanism by which Msx1 represses the activity of its own promoter, we transfected C2C12 myoblasts with Msx1-promoter-luciferase constructs and assessed reporter gene activity, with and without the exogenous expression of Msx1. We demonstrate that Msx1-mediated autorepression remained unaffected, regardless of the presence or absence of the Msx1 recognition motifs on the promoter. Furthermore, graded exogenous expression of TATA-binding protein (TBP), Sp1 or cAMP-response-element-binding protein-binding protein (CBP/p300) could counteract the autoinhibitory activity of Msx1. Finally, we demonstrate that Msx1 protein can be immunoprecipitated in a multiprotein complex containing TBP, Sp1 and CBP/p300. We hypothesize that the interaction of Msx1 protein with one or more ubiquitous or tissue-restricted transcription factors mediates transcriptional autorepression of the Msx1 gene.

Activation of the glycoprotein hormone alpha-subunit gene promoter in thyrotropes

Pituitary expression of the glycoprotein hormone alpha-subunit gene localizes to thyrotropes and gonadotropes. Factors that activate the alpha-subunit promoter exclusively in thyrotropes have not been described. In the current studies, alpha-subunit promoter activity was compared in alphaTSH thyrotropic and alphaT3 gonadotropic cells. Deletional analysis revealed a more important contribution of the -453/-381 region in thyrotropic cells and the -341/-297 region in gonadotropic cells, while other deletions had similar effects. Mutational analysis revealed some regions that were functionally similar, while others were active only in thyrotropic cells, such as the regions - 434/ - 421, - 398/ - 385, - 376/- 364 and - 363/- 351, that decreased activity by approximately 2-fold each. Combined mutation of the regions - 434/ - 421, - 398/ - 385 and - 376/ - 364 decreased activity by 5-fold in thyrotropic cells and not at all in gonadotropic cells. Cotransfection with Ptx1 in CV-1 cells resulted in a strong stimulation of alpha-subunit promoter activity, that was significantly diminished with mutation of the - 398/ - 385 region that disrupts the Ptx1 binding site, suggesting that this factor may play a role in thyrotrope-specific activation. This analysis provides basic information important to identify common and unique factors contributing to the cell-specific expression of the alpha-subunit gene.

Haploinsufficiency of MSX1: a mechanism for selective tooth agenesis

Previously, we found that the cause of autosomal dominant selective tooth agenesis in one family is a missense mutation resulting in an arginine-to-proline substitution in the homeodomain of MSX1. To determine whether the tooth agenesis phenotype may result from haploinsufficiency or a dominant-negative mechanism, we have performed biochemical and functional analyses of the mutant protein Msx1(R31P). We show that Msx1(R31P) has perturbed structure and reduced thermostability compared with wild-type Msx1. As a consequence, the biochemical activities of Msx1(R31P) are severely impaired, since it exhibits little or no ability to interact with DNA or other protein factors or to function in transcriptional repression. We also show that Msx1(R31P) is inactive in vivo, since it does not display the activities of wild-type Msx1 in assays of ectopic expression in the limb. Furthermore, Msx1(R31P) does not antagonize the activity of wild-type Msx1 in any of these assays. Because Msx1(R31P) appears to be inactive and does not affect the action of wild-type Msx1, we propose that the phenotype of affected individuals with selective tooth agenesis is due to haploinsufficiency.