Transcription Enhancer Factor-1-dependent Expression of the α-Tropomyosin Gene in the Three Muscle Cell Types

Delineating the heterogeneity of matrix-directed differentiation toward soft and stiff tissue lineages via single-cell profiling

The clinical utility of mesenchymal stromal/stem cells (MSCs) in mediating immunosuppressive effects and supporting regenerative processes is broadly established. However, the inherent heterogeneity of MSCs compromises its biomedical efficacy and reproducibility. To study how cellular variation affects fate decision-making processes, we perform single-cell RNA sequencing at multiple time points during bipotential matrix-directed differentiation toward soft- and stiff tissue lineages. In this manner, we identify distinctive MSC subpopulations that are characterized by their multipotent differentiation capacity and mechanosensitivity. Also, whole-genome screening highlights TPM1 as a potent mechanotransducer of matrix signals and regulator of cell differentiation. Thus, by introducing single-cell methodologies into mechanobiology, we delineate the complexity of adult stem cell responses to extracellular cues in tissue regeneration and immunomodulation.

Mesenchymal stromal/stem cells (MSCs) form a heterogeneous population of multipotent progenitors that contribute to tissue regeneration and homeostasis. MSCs assess extracellular elasticity by probing resistance to applied forces via adhesion, cytoskeletal, and nuclear mechanotransducers that direct differentiation toward soft or stiff tissue lineages. Even under controlled culture conditions, MSC differentiation exhibits substantial cell-to-cell variation that remains poorly characterized. By single-cell transcriptional profiling of nonconditioned, matrix-conditioned, and early differentiating cells, we identified distinct MSC subpopulations with distinct mechanosensitivities, differentiation capacities, and cell cycling. We show that soft matrices support adipogenesis of multipotent cells and early endochondral ossification of nonadipogenic cells, whereas intramembranous ossification and preosteoblast proliferation are directed by stiff matrices. Using diffusion pseudotime mapping, we outline hierarchical matrix-directed differentiation and perform whole-genome screening of mechanoresponsive genes. Specifically, top-ranked tropomyosin-1 is highly sensitive to stiffness cues both at RNA and protein levels, and changes in TPM1 expression determine the differentiation toward soft versus stiff tissue lineage. Consistent with actin stress fiber stabilization, tropomyosin-1 overexpression maintains YAP1 nuclear localization, activates YAP1 target genes, and directs osteogenic differentiation. Knockdown of tropomyosin-1 reversed YAP1 nuclear localization consistent with relaxation of cellular contractility, suppressed osteogenesis, activated early endochondral ossification genes after 3 d of culture in induction medium, and facilitated adipogenic differentiation after 1 wk. Our results delineate cell-to-cell variation of matrix-directed MSC differentiation and highlight tropomyosin-mediated matrix sensing.

Sarcomeric TPM3α in developing chicken

Cloning and sequencing of various tropomyosin isoforms expressed in chickens have been described since the early 1980s. However, to the best of our knowledge, this is the first report on the molecular characterization and the expression of the sarcomeric isoform of the TPM3 gene in cardiac and skeletal muscles from developing as well as adult chickens. Expression of TPM3α was performed by conventional RT-PCR as well as qRT-PCR using relative expression (by ΔCT as well as ΔΔCT methods) and by determining absolute copy number. The results employing all these methods show that the expression level of TPM3α is maximum in embryonic (10-day/15-day old) skeletal muscle and can barely be detected in both cardiac and skeletal muscles from the adult chicken. Our various RT-PCR analyses suggest that the expression of high molecular weight TPM3 isoforms are regulated at the transcription level from the proximal promoter at the 5'-end of the TPM3 gene.

Vestigial-like 3 is a novel Ets1 interacting partner and regulates trigeminal nerve formation and cranial neural crest migration

Drosophila Vestigial is the founding member of a protein family containing a highly conserved domain, called Tondu, which mediates their interaction with members of the TEAD family of transcription factors (Scalloped in Drosophila). In Drosophila, the Vestigial/Scalloped complex controls wing development by regulating the expression of target genes through binding to MCAT sequences. In vertebrates, there are four Vestigial-like genes, the functions of which are still not well understood. Here, we describe the regulation and function of vestigial-like 3 (vgll3) during Xenopus early development. A combination of signals, including FGF8, Wnt8a, Hoxa2, Hoxb2 and retinoic acid, limits vgll3 expression to hindbrain rhombomere 2. We show that vgll3 regulates trigeminal placode and nerve formation and is required for normal neural crest development by affecting their migration and adhesion properties. At the molecular level, vgll3 is a potent activator of pax3, zic1, Wnt and FGF, which are important for brain patterning and neural crest cell formation. Vgll3 interacts in the embryo with Tead proteins but unexpectedly with Ets1, with which it is able to stimulate a MCAT driven luciferase reporter gene. Our findings highlight a critical function for vgll3 in vertebrate early development.

The transcription factor TEAD1 represses smooth muscle-specific gene expression by abolishing myocardin function

The TEAD (transcriptional enhancer activator domain) proteins share an evolutionarily conserved DNA-binding TEA domain, which binds to the MCAT cis-acting regulatory element. Previous studies have shown that TEAD proteins are involved in regulating the expression of smooth muscle α-actin. However, it remains undetermined whether TEAD proteins play a broader role in regulating expression of other genes in vascular smooth muscle cells. In this study, we show that the expression of TEAD1 is significantly induced during smooth muscle cell phenotypic modulation and negatively correlates with smooth muscle-specific gene expression. We further demonstrate that TEAD1 plays a novel role in suppressing expression of smooth muscle-specific genes, including smooth muscle α-actin, by abolishing the promyogenic function of myocardin, a key mediator of smooth muscle differentiation. Mechanistically, we found that TEAD1 competes with myocardin for binding to serum response factor (SRF), resulting in disruption of myocardin and SRF interactions and thereby attenuating expression of smooth muscle-specific genes. This study provides the first evidence demonstrating that TEAD1 is a novel general repressor of smooth muscle-specific gene expression through interfering with myocardin binding to SRF.

Functional structure of the promoter regions for the predominant low molecular weight isoforms of tropomyosin in human kidney cells

High and low molecular weight (LMW) tropomyosin isoforms, by regulation of actin filaments, have a major role in the regulation of cell behaviour. They affect malignant transformation, motility, differentiation, metastasis and cell membrane protein presentation. Expression of LMW isoforms from the TPM1 and TPM3 genes have an important role in these effects but the regulation of their expression is unknown. Luciferase assays on a progressively truncated 1.7 kb fragment upstream of the exon 1b translation start site in the TPM1 and TPM3 genes in HEK-293 cells showed upstream activation sequences in TPM1 between -152 and -139 bp and in TPM3 between -154 and -102 bp. The effect of mutating candidate transcription factor binding sites identified an AML1-like transcription factor binding site in TPM1 and a cAMP response element in TPM3. Downstream from the primary activation sequence in TPM1 was a repressor region corresponding to two Sp/KLF family binding GC boxes. Band shift assays confirmed that deletion of these sites altered transcription factor binding and ChIP assays confirmed the presence of AML1 and CREB at the TPM1 and TPM3 activation sequences in the respective promoters. Expression of LMW isoforms from TPM1 and TPM3 genes is regulated very differently. This facilitates regulation of the many cell processes involving these proteins. In situations where these proteins have a critical role, such as cancer metastasis, it also facilitates specific intervention.

Absence of mutation at the 5'-upstream promoter region of the TPM4 gene from cardiac mutant axolotl (Ambystoma mexicanum)

Tropomyosins are a family of actin-binding proteins that show cell-specific diversity by a combination of multiple genes and alternative RNA splicing. Of the 4 different tropomyosin genes, TPM4 plays a pivotal role in myofibrillogenesis as well as cardiac contractility in amphibians. In this study, we amplified and sequenced the upstream regulatory region of the TPM4 gene from both normal and mutant axolotl hearts. To identify the cis-elements that are essential for the expression of the TPM4, we created various deletion mutants of the TPM4 promoter DNA, inserted the deleted segments into PGL3 vector, and performed promoter-reporter assay using luciferase as the reporter gene. Comparison of sequences of the promoter region of the TPM4 gene from normal and mutant axolotl revealed no mutations in the promoter sequence of the mutant TPM4 gene. CArG box elements that are generally involved in controlling the expression of several other muscle-specific gene promoters were not found in the upstream regulatory region of the TPM4 gene. In deletion experiments, loss of activity of the reporter gene was noted upon deletion which was then restored upon further deletion suggesting the presence of both positive and negative cis-elements in the upstream regulatory region of the TPM4 gene. We believe that this is the first axolotl promoter that has ever been cloned and studied with clear evidence that it functions in mammalian cell lines. Although striated muscle-specific cis-acting elements are absent from the promoter region of TPM4 gene, our results suggest the presence of positive and negative cis-elements in the promoter region, which in conjunction with positive and negative trans-elements may be involved in regulating the expression of TPM4 gene in a tissue-specific manner.

Members of the TEAD family of transcription factors regulate the expression of Myf5 in ventral somitic compartments

The transcriptional regulation of the Mrf4/Myf5 locus depends on a multitude of enhancers that, in equilibria with transcription balancing sequences and the promoters, regulate the expression of the two genes throughout embryonic development and in the adult. Transcription in a particular set of muscle progenitors can be driven by the combined outputs of several enhancers that are not able to recapitulate the entire expression pattern in isolation, or by the action of a single enhancer the activity of which in isolation is equivalent to that within the context of the locus. We identified a new enhancer element of this second class, ECR111, which is highly conserved in all vertebrate species and is necessary and sufficient to drive Myf5 expression in ventro-caudal and ventro-rostral somitic compartments in the mouse embryo. EMSA analyses and data obtained from binding-site mutations in transgenic embryos show that a binding site for a TEA Domain (TEAD) transcription factor is essential for the function of this new enhancer, while ChIP assays show that at least two members of the family of transcription factors bind to it in vivo.

Myocardin is required for cardiomyocyte survival and maintenance of heart function

Despite intense investigation over the past century, the molecular mechanisms that regulate maintenance and adaptation of the heart during postnatal development are poorly understood. Myocardin is a remarkably potent transcriptional coactivator expressed exclusively in cardiac myocytes and smooth muscle cells during postnatal development. Here we show that myocardin is required for maintenance of cardiomyocyte structure and sarcomeric organization and that cell-autonomous loss of myocardin in cardiac myocytes triggers programmed cell death. Mice harboring a cardiomyocyte-restricted null mutation in the myocardin gene (Myocd) develop dilated cardiomyopathy and succumb from heart failure within a year. Remarkably, ablation of the Myocd gene in the adult heart leads to the rapid-onset of heart failure, dilated cardiomyopathy, and death within a week. Myocd gene ablation is accompanied by dissolution of sarcomeric organization, disruption of the intercalated disc, and cell-autonomous loss of cardiomyocytes via apoptosis. Expression of myocardin/serum response factor-regulated myofibrillar genes is extinguished, or profoundly attenuated, in myocardin-deficient hearts. Conversely, proapoptotic factors are induced and activated in myocardin-deficient hearts. We conclude that the transcriptional coactivator myocardin is required for maintenance of heart function and ultimately cardiomyocyte survival.

Induction and modulation of smooth muscle differentiation in Xenopus embryonic cells

By comparison with skeletal or cardiac developmental programs, little is known regarding the specific factors that promote specification and differentiation of smooth muscle cells from pluripotent cells. We have analyzed the developmental expression of a subset of smooth muscle genes during Xenopus early development and showed that similar to mammals and avians, Xenopus smooth muscle myosin heavy chain (SM-MHC) is a highly specific marker of smooth muscle differentiation. Embryonic cells from animal pole explants of Xenopus blastula can be induced by basic fibroblast growth factor, Wnt, and bone morphogenetic protein signals to adopt the smooth muscle pathway. Explants from early embryos that contain neural crest cells can also differentiate into cells expressing smooth muscle genes. We examined the interplay of several transcription factors, that is SRF, myocardin, and GATA6, that induce the expression of SM-MHC in animal cap cells and found that myocardin-dependent expression of smooth muscle genes in animal cap cells is synergized by SRF but is strongly antagonized by GATA6.

Alternative requirements for Vestigial, Scalloped, and Dmef2 during muscle differentiation in Drosophila melanogaster

Vertebrate development requires the activity of the myocyte enhancer factor 2 (mef2) gene family for muscle cell specification and subsequent differentiation. Additionally, several muscle-specific functions of MEF2 family proteins require binding additional cofactors including members of the Transcription Enhancing Factor-1 (TEF-1) and Vestigial-like protein families. In Drosophila there is a single mef2 (Dmef2) gene as well single homologues of TEF-1 and vestigial-like, scalloped (sd), and vestigial (vg), respectively. To clarify the role(s) of these factors, we examined the requirements for Vg and Sd during Drosophila muscle specification. We found that both are required for muscle differentiation as loss of sd or vg leads to a reproducible loss of a subset of either cardiac or somatic muscle cells in developing embryos. This muscle requirement for Sd or Vg is cell specific, as ubiquitous overexpression of either or both of these proteins in muscle cells has a deleterious effect on muscle differentiation. Finally, using both in vitro and in vivo binding assays, we determined that Sd, Vg, and Dmef2 can interact directly. Thus, the muscle-specific phenotypes we have associated with Vg or Sd may be a consequence of alternative binding of Vg and/or Sd to Dmef2 forming alternative protein complexes that modify Dmef2 activity.

Porcine TEF1 and RTEF1: molecular characterization and association analyses with growth traits

TEA domain transcription factors play vital roles in myogenesis by binding the M-CAT motif in the promoter of the muscle-specific genes. In the present study, we cloned two porcine TEA domain family genes, TEF1 and RTEF1, and identified two different variants respectively. RT-PCR revealed that the TEF1-a variant was highly expressed and up-regulated with the development of the porcine skeletal muscle, indicating its potential regulatory function for muscle development. Promoter analysis revealed porcine TEF1 was regulated, in a TATA-independent manner, by a specific intact initiator element, and numerous binding motifs of multiple transcription factors, including SP1, CREB/ATF and AREB6. A substitution G93A was identified in the 5'-flanking sequence and used for the linkage mapping of TEF1. Association analyses in a BerkshirexYorkshire F(2) population revealed that the substitution of G93A has a significant effect on average daily gain from birth to weaning (p<0.05) and 16-day weight (p<0.05), and a suggestive effect on loin eye area (p<0.06), average back fat (p<0.07) and lumbar back fat (p<0.08). The association analyses results are in agreement with the gene's localization demonstrated by linkage analysis, SCHP and RH mapping to the QTL region of growth and carcass traits on chromosome 2p14-17.

Transcriptional enhancer factor 1 (TEF-1/TEAD1) mediates activation of IFITM3 gene by BRGl

The interferon inducible transmembrane (IFITM) proteins mediate several cellular processes such as homotypic cell adhesion functions of interferons (IFNs) and cellular anti-proliferative activities. We show that the BAF complex-mediated induction of IFITM3 is dependent on binding of the transcriptional enhancer factor 1 (TEF-1/TEAD1) to the M-CAT like elements of its promoter. TEF-1 knock-down reduced the BAF complex-mediated activation of IFITM3 promoter. In the absence of the BAF complex, TEF-1 is repressive to IFITM3 expression. The regulation of IFITM3 by TEF-1 demonstrates that TEF-1 dependent regulation is more widespread than its previously established role in the expression of muscle specific genes.

MCAT elements and the TEF-1 family of transcription factors in muscle development and disease

MCAT elements are located in the promoter-enhancer regions of cardiac, smooth, and skeletal muscle-specific genes including cardiac troponin T, beta-myosin heavy chain, smooth muscle alpha-actin, and skeletal alpha-actin, and play a key role in the regulation of these genes during muscle development and disease. The binding factors of MCAT elements are members of the transcriptional enhancer factor-1 (TEF-1) family. However, it has not been fully understood how these transcription factors confer cell-specific expression in muscle, because their expression patterns are relatively broad. Results of recent studies revealed multiple mechanisms whereby TEF-1 family members control MCAT element-dependent muscle-specific gene expression, including posttranslational modifications of TEF-1 family members, the presence of muscle-selective TEF-1 cofactors, and cell-selective control of TEF-1 accessibility to MCAT elements. In addition, of particular interest, recent studies regarding MCAT element-dependent transcription of the myocardin gene and the smooth muscle alpha-actin gene in muscle provide evidence for the transcriptional diversity among distinct cell types and subtypes. This article summarizes the role of MCAT elements and the TEF-1 family of transcription factors in muscle development and disease, and reviews recent progress in our understanding of the transcriptional regulatory mechanisms involved in MCAT element-dependent muscle-specific gene expression.