Mesodermal Transcription Factors Research Articles

Non-Homologous Structured CRMs from theCionaGenome

The internal functional organization of cis-regulatory modules (CRMs) lies at the heart of our understanding the mode and tempo of gene regulatory evolution as well as practical efforts at deciphering and annotating genomic sequences. In an open-ended search for loose clusters of known mesodermal enhancer motifs in the Ciona intestinalis genome, I discovered the existence of a class of highly organized CRMs in otherwise unrelated genes expressed early in development. Each such CRM is composed of distinct motifs located at specific positions along approximately 160 bp of DNA sequence, and is able to drive expression in distinct mesodermal compartments descended from the B4.1 blastomere. The majority of the loci bearing these B4.1-specific modules encode important early mesodermal transcription factors at the snail, paraxis, and tbx6 orthologous loci of this invertebrate chordate system. These unrelated genes encode members of the C2H2 zinc-finger, bHLH, and T-box transcription factor families, and likely serve as a chordate-specific trans-code for paraxial mesoderm. One other similarly organized enhancer was discovered in the TNC3 muscle structural locus. These results suggest that organization of binding sites over the length of the enhancer sequence is a critical aspect of gene regulatory biology. The extent to which this is a general principle will facilitate our ability to identify, decipher, and categorize the regulatory functions contained in whole genome assemblies.

Wnt3a/β-catenin signaling controls posterior body development by coordinating mesoderm formation and segmentation

Somitogenesis is thought to be controlled by a segmentation clock, which consists of molecular oscillators in the Wnt3a, Fgf8 and Notch pathways. Using conditional alleles of Ctnnb1 (beta-catenin), we show that the canonical Wnt3a/beta-catenin pathway is necessary for molecular oscillations in all three signaling pathways but does not function as an integral component of the oscillator. Small, irregular somites persist in abnormally posterior locations in the absence of beta-catenin and cycling clock gene expression. Conversely, Notch pathway genes continue to oscillate in the presence of stabilized beta-catenin but boundary formation is delayed and anteriorized. Together, these results suggest that the Wnt3a/beta-catenin pathway is permissive but not instructive for oscillating clock genes and that it controls the anterior-posterior positioning of boundary formation in the presomitic mesoderm (PSM). The Wnt3a/beta-catenin pathway does so by regulating the activation of the segment boundary determination genes Mesp2 and Ripply2 in the PSM through the activation of the Notch ligand Dll1 and the mesodermal transcription factors T and Tbx6. Spatial restriction of Ripply2 to the anterior PSM is ensured by the Wnt3a/beta-catenin-mediated repression of Ripply2 in posterior PSM. Thus, Wnt3a regulates somitogenesis by activating a network of interacting target genes that promote mesodermal fates, activate the segmentation clock, and position boundary determination genes in the anterior PSM.

Colliertranscription in a singleDrosophilamuscle lineage: the combinatorial control of muscle identity

Specification of muscle identity in Drosophila is a multistep process: early positional information defines competence groups termed promuscular clusters, from which muscle progenitors are selected, followed by asymmetric division of progenitors into muscle founder cells (FCs). Each FC seeds the formation of an individual muscle with morphological and functional properties that have been proposed to reflect the combination of transcription factors expressed by its founder. However, it is still unclear how early patterning and muscle-specific differentiation are linked. We addressed this question, using Collier (Col; also known as Knot) expression as both a determinant and read-out of DA3 muscle identity. Characterization of the col upstream region driving DA3 muscle specific expression revealed the existence of three separate phases of cis-regulation, correlating with conserved binding sites for different mesodermal transcription factors. Examination of col transcription in col and nautilus (nau) loss-of-function and gain-of-function conditions showed that both factors are required for col activation in the ;naïve' myoblasts that fuse with the DA3 FC, thereby ensuring that all DA3 myofibre nuclei express the same identity programme. Together, these results indicate that separate sets of cis-regulatory elements control the expression of identity factors in muscle progenitors and myofibre nuclei and directly support the concept of combinatorial control of muscle identity.

The Human T-Box Mesodermal Transcription Factor Brachyury Is a Candidate Target for T-Cell–Mediated Cancer Immunotherapy

Identification of tumor antigens is essential in advancing immune-based therapeutic interventions in cancer. Particularly attractive targets are those molecules that are selectively expressed by malignant cells and that are also essential for tumor progression. We have used a computer-based differential display analysis tool for mining of expressed sequence tag clusters in the human Unigene database and identified Brachyury as a novel tumor antigen. Brachyury, a member of the T-box transcription factor family, is a key player in mesoderm specification during embryonic development. Moreover, transcription factors that control mesoderm have been implicated in the epithelial-mesenchymal transition (EMT), which has been postulated to be a key step during tumor progression to metastasis. Reverse transcription-PCR analysis validated the in silico predictions and showed Brachyury expression in tumors of the small intestine, stomach, kidney, bladder, uterus, ovary, and testis, as well as in cell lines derived from lung, colon, and prostate carcinomas, but not in the vast majority of the normal tissues tested. An HLA-A0201 epitope of human Brachyury was identified that was able to expand T lymphocytes from blood of cancer patients and normal donors with the ability to lyse Brachyury-expressing tumor cells. To our knowledge, this is the first demonstration that (a) a T-box transcription factor and (b) a molecule implicated in mesodermal development, i.e., EMT, can be a potential target for human T-cell-mediated cancer immunotherapy.

Single-cell-derived mesenchymal stem cells overexpressing Csx/Nkx2.5 and GATA4 undergo the stochastic cardiomyogenic fate and behave like transient amplifying cells

Bone marrow-derived stromal cells can give rise to cardiomyocytes as well as adipocytes, osteocytes, and chondrocytes in vitro. The existence of mesenchymal stem cells has been proposed, but it remains unclear if a single-cell-derived stem cell stochastically commits toward a cardiac lineage. By single-cell marking, we performed a follow-up study of individual cells during the differentiation of 9-15c mesenchymal stromal cells derived from bone marrow cells. Three types of cells, i.e., cardiac myoblasts, cardiac progenitors and multipotent stem cells were differentiated from a single cell, implying that cardiomyocytes are generated stochastically from a single-cell-derived stem cell. We also demonstrated that overexpression of Csx/Nkx2.5 and GATA4, precardiac mesodermal transcription factors, enhanced cardiomyogenic differentiation of 9-15c cells, and the frequency of cardiomyogenic differentiation was increased by co-culturing with fetal cardiomyocytes. Single-cell-derived mesenchymal stem cells overexpressing Csx/Nkx2.5 and GATA4 behaved like cardiac transient amplifying cells, and still retained their plasticity in vivo.

Maintenance of embryonic stem cell pluripotency by Nanog-mediated reversal of mesoderm specification

Embryonic stem cells (ESCs) can be propagated indefinitely in culture, while retaining the ability to differentiate into any cell type in the organism. The molecular and cellular mechanisms underlying ESC pluripotency are, however, poorly understood. We characterize a population of early mesoderm-specified (EM) progenitors that is generated from mouse ESCs by bone morphogenetic protein stimulation. We further show that pluripotent ESCs are actively regenerated from EM progenitors by the action of the divergent homeodomain-containing protein Nanog, which, in turn, is upregulated in EM progenitors by the combined action of leukemia inhibitory factor and the early mesoderm transcription factor T/Brachyury. These findings uncover specific roles of leukemia inhibitory factor, Nanog, and bone morphogenetic protein in the self-renewal of ESCs and provide novel insights into the cellular bases of ESC pluripotency.

The initiation of Hox gene expression in Xenopus laevis is controlled by Brachyury and BMP-4

Hox genes encode a family of transcription factors that specify positional identities along the anterior–posterior (AP) axis during the development of vertebrate embryos. The earliest Hox expression in vertebrates is during gastrulation, at a position distant from the organiser or its equivalent. However, the mechanism that initiates this early expression is still not clear. Guided by the expression pattern, we identified upstream regulators in Xenopus laevis. The mesodermal transcription factor brachyury ( Xbra) controls the early Hox expression domain in the animal–vegetal direction and the secreted growth factor BMP-4 limits it in the organiser/non-organiser direction. The overlap of these two signals, indicated by a Cartesian coordinate system, defines the initial Hox expression domain. We postulate that this system is a general mechanism for the activation of all Hox genes expressed during gastrulation.

Intersecting signalling and transcriptional pathways inDrosophilaheart specification

TheDrosophilaheart, also called the dorsal vessel, is a linear tube consisting of two major cell types, namely cardioblasts which serve as cardiomyocytes and pericardial cells which surround its outer surface. This organ is derived from segmental clusters of cells in the dorsal mesoderm during early embryonic development. During the past few years, genetic and molecular studies have led to significant advances in our understanding of the regulatory mechanisms that pattern the early mesoderm by defining these cell clusters and ultimately specifying individual cells within them as heart progenitors. These studies established that the patterning events involve specific combinations of localized inductive signals that act in concert with mesoderm-autonomous transcription factors to achieve a progressive subdivision of the mesoderm. Some of the synergistic interactions between mesodermal transcription factors and external signalling molecules have been defined at the molecular level. With respect to the pericardial cells, the final specification steps were found to employ cell-intrinsic lineage mechanisms. Because of the many striking similarities betweenDrosophilaand early vertebrate heart development, which appear to extend to the molecular level, these insights will be of significant help in defining related events during vertebrate cardiogenesis