Smad proteins function as co-modulators for MEF2 transcriptional regulatory proteins

FoxP1 Represses MEF2A in Striated Muscle

Myocyte enhancer factor 2 (MEF2) proteins are involved in multiple developmental, physiological, and pathological processes in vertebrates. Protein-protein interactions underlie the plethora of biological processes impacted by MEF2A, necessitating a detailed characterization of the MEF2A interactome. A nanobody based affinity-purification/mass spectrometry strategy was employed to achieve this goal. Specifically, the MEF2A protein complexes were captured from myogenic lysates using a GFP-tagged MEF2A protein immobilized with a GBP-nanobody followed by LC-MS/MS proteomic analysis to identify MEF2A interactors. After bioinformatic analysis, we further characterized the interaction of MEF2A with a transcriptional repressor, FOXP1. FOXP1 coprecipitated with MEF2A in proliferating myogenic cells which diminished upon differentiation (myotube formation). Ectopic expression of FOXP1 inhibited MEF2A driven myogenic reporter genes (derived from the creatine kinase muscle and myogenin genes) and delayed induction of endogenous myogenin during differentiation. Conversely, FOXP1 depletion enhanced MEF2A transactivation properties and myogenin expression. The FoxP1:MEF2A interaction is also preserved in cardiomyocytes and FoxP1 depletion enhanced cardiomyocyte hypertrophy. FOXP1 prevented MEF2A phosphorylation and activation by the p38MAPK pathway. Overall, these data implicate FOXP1 in restricting MEF2A function in order to avoid premature differentiation in myogenic progenitors and also to possibly prevent re-activation of embryonic gene expression in cardiomyocyte hypertrophy.

Interaction of C/EBPβ with SMAD2 and SMAD4 genes induces the formation of lipid droplets in bovine myoblasts

As a key component of Transforming growth factor-β (TGF-β) pathway, Smad2 has many crucial roles in a variety of cellular processes, but it cannot bind DNA without complex formation with Smad4. In the present study, the molecular mechanism in the progress of myogenesis underlying transcriptional regulation of SMAD2 and SMAD4 had been clarified. The result showed the inhibition between SMAD2 and SMAD4, which promotes and inhibits bovine myoblast differentiation, respectively. Further, the characterization of promoter region of SMAD2 and SMAD4 was analyzed, and identified C/EBPβ directly bound to the core region of both SMAD2 and SMAD4 genes promoter and stimulated the transcriptional activity. However, C/EBPβ has lower expression in myoblasts which plays vital function in the transcriptional networks controlling adipogenesis, while the overexpression of C/EBPβ gene in myoblasts significantly increased SMAD2 and SMAD4 gene expression, induced the formation of lipid droplet in bovine myoblasts, and promoted the expression of adipogenesis-specific genes. Collectively, our results showed that C/EBPβ may play an important role in the trans-differentiation and dynamic equilibrium of myoblasts into adipocyte cells via promoting an increase in SMAD2 and SMAD4 gene levels. These results will provide an important basis for further understanding of the TGFβ pathway and C/EBPβ gene during myogenic differentiation.

AR cooperates with SMAD4 to maintain skeletal muscle homeostasis

Androgens and androgen-related molecules exert a plethora of functions across different tissues, mainly through binding to the transcription factor androgen receptor (AR). Despite widespread therapeutic use and misuse of androgens as potent anabolic agents, the molecular mechanisms of this effect on skeletal muscle are currently unknown. Muscle mass in adulthood is mainly regulated by the bone morphogenetic protein (BMP) axis of the transforming growth factor (TGF)-β pathway via recruitment of mothers against decapentaplegic homolog 4 (SMAD4) protein. Here we show that, upon activation, AR forms a transcriptional complex with SMAD4 to orchestrate a muscle hypertrophy programme by modulating SMAD4 chromatin binding dynamics and enhancing its transactivation activity. We challenged this mechanism of action using spinal and bulbar muscular atrophy (SBMA) as a model of study. This adult-onset neuromuscular disease is caused by a polyglutamine expansion (polyQ) in AR and is characterized by progressive muscle weakness and atrophy secondary to a combination of lower motor neuron degeneration and primary muscle atrophy. Here we found that the presence of an elongated polyQ tract impairs AR cooperativity with SMAD4, leading to an inability to mount an effective anti-atrophy gene expression programme in skeletal muscle in response to denervation. Furthermore, adeno-associated virus, serotype 9 (AAV9)-mediated muscle-restricted delivery of BMP7 is able to rescue the muscle atrophy in SBMA mice, supporting the development of treatments able to fine-tune AR-SMAD4 transcriptional cooperativity as a promising target for SBMA and other conditions associated with muscle loss.

TGF-β3/Smad3 Contributes to Isoflurane Postconditioning Against Cerebral Ischemia-Reperfusion Injury by Upregulating MEF2C

Isoflurane postconditioning alleviates cerebral ischemic-reperfusion injury (CIRI), but the underlying mechanism has not been fully clarified. We previously demonstrated that the transforming growth factor beta-1 (TGF-β1)/Smads signaling pathway is involved in the neuroprotective effect of isoflurane postconditioning. TGF-β3 has a highly homologous sequence relative to that of TGF-β1. In this study, we explored the roles of the TGF-β3/Smad3 signaling pathway and myocyte enhancer factor 2C (MEF2C) in neuroprotection induced by isoflurane postconditioning. A CIRI rat model was established by middle cerebral artery occlusion for 1.5 h, followed by 24 h of reperfusion. Isoflurane postconditioning led to lower infarct volumes and neurologic deficit scores, more surviving neurons, and less damaged and apoptotic neurons as compared with those of CIRI rats. Moreover, isoflurane postconditioning upregulated the expressions of TGF-β3, p-Smad3, and MEF2C. However, the neuroprotective effect was reversed by pirfenidone, a TGF-β3/Smad3 signaling pathway inhibitor. Also, pirfenidone treatment downregulated the expression of MEF2C. These results indicate that the TGF-β3/Smad3 signaling pathway contributes to the neuroprotection of isoflurane postconditioning after CIRI and is possibly related to MEF2C.

TGFβ/BMP Signaling Pathway in Cartilage Homeostasis

Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease.

Smad3 influences Smad2 expression via the transcription factor C/EBPα and C/EBPβ during bovine myoblast differentiation

Transforming growth factor β (TGFβ) has participated in a variety of cellular biological processes. Smad2 and Smad3 are equally important TGFβ downstream effectors in mediating TGFβ signals. However, genes involved in controlling the balance between these two signaling pathways are unknown. In this study, we showed that although Smad2 and Smad3 are structurally similar, with 89% amino acid sequence similarity in bovine, Smad3 significantly decreased Smad2 mRNA and protein expression during bovine myoblast differentiation, but not by binding on its promoter. Luciferase assays and electrophoretic mobility shift assays (EMSA) demonstrated that the transcription factors C/EBPα and C/EBPβ activate Smad2 promoter activity and expression under high serum medium (GM), whereas the opposite was observed under low serum medium (DM). Moreover, over-expression and interference assays revealed that Smad3 has a different effect on C/EBPα and C/EBPβ expression under GM versus DM conditions. After mutation of the C/EBPα and C/EBPβ binding sites, Smad3 had a reduced effect on Smad2 promoter activity. Therefore, these results demonstrated that Smad3 inhibits Smad2 expression via its transcription factors C/EBPα and C/EBPβ during bovine myoblast differentiation. This novel mechanism of the Smad2/3 genes may offer clues for further investigation of TGFβ signal function.

Salvianolic Acid B Inhibits Activation of Human Primary Hepatic Stellate Cells Through Downregulation of the Myocyte Enhancer Factor 2 Signaling Pathway

Various isoforms of myocyte enhancer factor 2 (MEF2) have been shown to play a role in the activation of rat hepatic stellate cells (HSCs) in culture. The signals that regulate MEF2 in HSCs are unknown. In addition, whether MEF2s regulate the activation of human HSCs (H-HSCs) is unclear. Here, we studied the expression and function of MEF2s in H-HSCs. Our data showed that the levels of MEF2A, C, and D proteins were high in liver tissues from patients with cirrhosis and increased during culture-induced activation of primary H-HSCs. Exposure of H-HSCs to transforming growth factor beta 1 (TGF-β1) led to a significant increase in MEF2A and C protein levels and enhanced MEF2 activity. Interestingly, TGF-β1 did not further enhance MEF2D levels. Furthermore, TGF-β1 activated p38 mitogen-activated protein kinase (MAPK) and led to increased phosphorylation of MEF2C at its p38 recognition site. Inhibition of p38 MAPK inhibited both TGF-β1- and culture-induced activation of MEF2. The activity of collagen I reporter in H-HSCs was significantly reduced when MEF2A and MEF2C were blocked with overexpression of dominant negative MEF2 mutants. Salvianolic-acid B (SA-B), a water-soluble element of Salvia miltiorrhiza known to have anti-fibrosis effects, attenuated both basal and TGF-β1-induced increased levels of MEF2A and C mRNA and protein. In addition, SA-B inhibited MEF2 activity, which correlated with reduced expression of the HSC activation markers, α-smooth muscle actin (α-SMA), and collagen I. Administration of SA-B reduced MEF2A in vivo, which was accompanied by reduced levels of α-SMA in a model of dimethylnitrosamine-induced rat liver fibrosis. We concluded that the MEF2 transcription factor was stimulated by TGF-β1 in H-HSCs. Antagonizing TGF-β1-induced activation of the MEF2 signaling pathway may account in part for the anti-fibrosis effects of SA-B.

Systemic Activation of Activin A Signaling Causes Chronic Kidney Disease-Mineral Bone Disorder

The high cardiovascular mortality associated with chronic kidney disease (CKD) is caused in part by the CKD-mineral bone disorder (CKD-MBD) syndrome. The CKD-MBD consists of skeletal, vascular and cardiac pathology caused by metabolic derangements produced by kidney disease. The prevalence of osteopenia/osteoporosis resulting from the skeletal component of the CKD-MBD, renal osteodystrophy (ROD), in patients with CKD exceeds that of the general population and is a major public health concern. That CKD is associated with compromised bone health is widely accepted, yet the mechanisms underlying impaired bone metabolism in CKD are not fully understood. Therefore, clarification of the molecular mechanisms by which CKD produces ROD is of crucial significance. We have shown that activin A, a member of the transforming growth factor (TGF)-β super family, is an important positive regulator of receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis with Smad-mediated signaling being crucial for inducing osteoclast development and function. Recently, we have demonstrated systemic activation of activin receptors and activin A levels in CKD mouse models, such as diabetic CKD and Alport (AL) syndrome. In these CKD mouse models, bone remodeling caused by increased osteoclast numbers and activated osteoclastic bone resorption was observed and treatment with an activin receptor ligand trap repaired CKD-induced-osteoclastic bone resorption and stimulated individual osteoblastic bone formation, irrespective of parathyroid hormone (PTH) elevation. These findings have opened a new field for exploring mechanisms of activin A-enhanced osteoclast formation and function in CKD. Activin A appears to be a strong candidate for CKD-induced high-turnover ROD. Therefore, the treatment with the decoy receptor for activin A might be a good candidate for treatment for CKD-induced osteopenia or osteoporosis, indicating that the new findings from in these studies will lead to the identification of novel therapeutic targets for CKD-related and osteopenia and osteoporosis in general. In this review, we describe the impact of CKD-induced Smad signaling in osteoclasts, osteoblasts and vascular cells in CKD.

Inhibition of myocyte-specific enhancer factor 2A improved diabetic cardiac fibrosis partially by regulating endothelial-to-mesenchymal transition

Cardiac fibrosis is an important pathological process of diabetic cardiomyopathy, the underlying mechanism remains elusive. This study sought to identify whether inhibition of Myocyte enhancer factor 2A (MEF2A) alleviates cardiac fibrosis by partially regulating Endothelial-to-mesenchymal transition (EndMT). We induced type 1 diabetes mellitus using the toxin streptozotocin (STZ) in mice and injected with lentivirus-mediated short-hairpin RNA (shRNA) in myocardium to inhibit MEF2A expression. Protein expression, histological and functional parameters were examined twenty-one weeks post-STZ injection. We found that Diabetes mellitus increased cardiac MEF2A expression, aggravated cardiac dysfunction and myocardial fibrosis through the accumulation of fibroblasts via EndMT. All of these features were abolished by MEF2A inhibition. MEF2A gene silencing by shRNA in cultured human umbilical vein endothelial cells (HUVECs) ameliorated high glucose-induced phenotypic transition and acquisition of mesenchymal markers through interaction with p38MAPK and Smad2. We conclude that inhibition of endothelial cell-derived MEF2A might be beneficial in the prevention of diabetes mellitus-induced cardiac fibrosis by partially inhibiting EndMT through interaction with p38MAPK and Smad2.

Mef2 and the skeletal muscle differentiation program

Mef2 is a conserved and significant transcription factor in the control of muscle gene expression. In cell culture Mef2 synergises with MyoD-family members in the activation of gene expression and in the conversion of fibroblasts into myoblasts. Amongst its in vivo roles, Mef2 is required for both Drosophila muscle development and mammalian muscle regeneration. Mef2 has functions in other cell-types too, but this review focuses on skeletal muscle and surveys key findings on Mef2 from its discovery, shortly after that of MyoD, up to the present day. In particular, in vivo functions, underpinning mechanisms and areas of uncertainty are highlighted. We describe how Mef2 sits at a nexus in the gene expression network that controls the muscle differentiation program, and how Mef2 activity must be regulated in time and space to orchestrate specific outputs within the different aspects of muscle development. A theme that emerges is that there is much to be learnt about the different Mef2 proteins (from different paralogous genes, spliced transcripts and species) and how the activity of these proteins is controlled.

Cardiac fibrosis in the ageing heart: Contributors and mechanisms

Cardiac fibrosis refers to an excessive deposition of extracellular matrix (ECM) in cardiac tissue. Fibrotic tissue is stiffer and less compliant, resulting in subsequent cardiac dysfunction and heart failure. Cardiac fibrosis in the ageing heart may involve activation of fibrogenic signalling and inhibition of anti-fibrotic signalling, leading to an imbalance of ECM turnover. Excessive accumulation of ECM such as collagen in older patients contributes to progressive ventricular dysfunction. Overexpression of collagen is derived from various sources, including higher levels of fibrogenic growth factors, proliferation of fibroblasts and cellular transdifferentiation. These may be triggered by factors, such as oxidative stress, inflammation, hypertension, cellular senescence and cell death, contributing to age-related fibrotic cardiac remodelling. In this review, we will discuss the fibrogenic contributors in age-related cardiac fibrosis, and the potential mechanisms by which fibrogenic processes can be interrupted for therapeutic intent.

Myocyte enhancer factor (MEF)-2 plays essential roles in T-cell transformation associated with HTLV-1 infection by stabilizing complex between Tax and CREB

Background

The exact molecular mechanisms regarding HTLV-1 Tax-mediated viral gene expression and CD4 T-cell transformation have yet to be fully delineated. Herein, utilizing virus-infected primary CD4⁺ T cells and the virus-producing cell line, MT-2, we describe the involvement and regulation of Myocyte enhancer factor-2 (specifically MEF-2A) during the course of HTLV-1 infection and associated disease syndrome.

Results

Inhibition of MEF-2 expression by shRNA and its activity by HDAC9 led to reduced viral replication and T-cell transformation in correlation with a heightened expression of MEF-2 in ATL patients. Mechanistically, MEF-2 was recruited to the viral promoter (LTR, long terminal repeat) in the context of chromatin, and constituted Tax/CREB transcriptional complex via direct binding to the HTLV-1 LTR. Furthermore, an increase in MEF-2 expression was observed upon infection in an extent similar to CREB (known Tax-interacting transcription factor), and HATs (p300, CBP, and p/CAF). Confocal imaging confirmed MEF-2 co-localization with Tax and these proteins were also shown to interact by co-immunoprecipitation. MEF-2 stabilization of Tax/CREB complex was confirmed by a novel promoter-binding assay that highlighted the involvement of NFAT (nuclear factor of activated T cells) in this process via Tax-mediated activation of calcineurin (a calcium-dependent serine-threonine phosphatase). MEF-2-integrated signaling pathways (PI3K/Akt, NF-κB, MAPK, JAK/STAT, and TGF-β) were also activated during HTLV-1 infection of primary CD4⁺ T cells, possibly regulating MEF-2 activity.

Conclusions

We demonstrate the involvement of MEF-2 in Tax-mediated LTR activation, viral replication, and T-cell transformation in correlation with its heightened expression in ATL patients through direct binding to DNA within the HTLV-1 LTR.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0140-1) contains supplementary material, which is available to authorized users.

Epstein-Barr Virus Lytic Cycle Reactivation

Epstein-Barr virus, which mainly infects B cells and epithelial cells, has two modes of infection: latent and lytic. Epstein-Barr virus infection is predominantly latent; however, lytic infection is detected in healthy seropositive individuals and becomes more prominent in certain pathological conditions. Lytic infection is divided into several stages: early gene expression, DNA replication, late gene expression, assembly, and egress. This chapter summarizes the most recent progress made toward understanding the molecular mechanisms that regulate the different lytic stages leading to production of viral progeny. In addition, the chapter highlights the potential role of lytic infection in disease development and current attempts to purposely induce lytic infection as a therapeutic approach.

Developments in sclerostin biology: regulation of gene expression, mechanisms of action, and physiological functions

The SOST gene, which encodes the protein sclerostin, was identified through genetic linkage analysis of sclerosteosis and van Buchem's disease patients. Sclerostin is a secreted glycoprotein that binds to the low-density lipoprotein receptor-related proteins 4, 5, and 6 to inhibit Wnt signaling. Since the initial discovery of sclerostin, much understanding has been gained into the role of this protein in the regulation of skeletal biology. In this article, we discuss the latest findings in the regulation of SOST expression, sclerostin mechanisms of action, and the potential utility of targeting sclerostin in conditions of low bone mass.

Transcriptional regulation of mesoderm genes by MEF2D during early Xenopus development

In Xenopus, specification of the three germ layers is one of the earliest developmental decisions occurring prior to gastrulation. The maternally-expressed vegetally-localized transcription factor VegT has a central role in cell autonomous specification of endoderm and in the generation of mesoderm-inducing signals. Yet, marginally-expressed transcription factors that cooperate with mesoderm-inducing signals are less investigated. Here we report that the transcription factors MEF2A and MEF2D are expressed in the animal hemisphere before mid-blastula transition. At the initiation of zygotic transcription, expression of MEF2D expands into the marginal region that gives rise to mesoderm. Knockdown of MEF2D delayed gastrulation movements, prevented embryo elongation at the subsequent tailbud stage and caused severe defects in axial tissues. At the molecular level, MEF2D knockdown reduced the expression of genes involved in mesoderm formation and patterning. We also report that MEF2D functions with FGF signaling in a positive feedback loop; each augments the expression of the other in the marginal region and both are necessary for mesodermal gene expression. One target of MEF2D is the Nodal-related 1 gene (Xnr1) that mediates some of MEF2D mesodermal activities. Chromatin immunoprecipitation analysis revealed that MEF2D associates with transcriptional regulatory sequences of the Xnr1 gene. Several MEF2 binding sites within the proximal promoter region of Xnr1 were identified by their in vitro association with MEF2D protein. The same promoter region was necessary but not sufficient to mediate MEF2D activity in a reporter gene assay. In sum, our results indicate that the MEF2D protein is a key transcription factor in the marginal zone acting in a positive feedback loop with FGF signaling that promotes mesoderm specification at late blastula stages.

Transcription factors in heart: promising therapeutic targets in cardiac hypertrophy

Regulation of gene expression is central to cell growth, differentiation and diseases. Context specific and signal dependent regulation of gene expression is achieved to a large part by transcription factors. Cardiac transcription factors regulate heart development and are also involved in stress regulation of the adult heart, which may lead to cardiac hypertrophy. Hypertrophy of cardiac myocytes is an outcome of the imbalance between prohypertrophic factors and anti-hypertrophic factors. This is initially a compensatory mechanism but sustained hypertrophy may lead to heart failure. The growing knowledge of transcriptional control mechanisms is helpful in the development of novel therapies. This review summarizes the role of cardiac transcription factors in cardiac hypertrophy, emphasizing their potential as attractive therapeutic targets to prevent the onset of heart failure and sudden death as they can be converging targets for current therapy.

Krüppel-like factor 6 (KLF6) promotes cell proliferation in skeletal myoblasts in response to TGFβ/Smad3 signaling

Background

Krüppel-like factor 6 (KLF6) has been recently identified as a MEF2D target gene involved in neuronal cell survival. In addition, KLF6 and TGFβ have been shown to regulate each other’s expression in non-myogenic cell types. Since MEF2D and TGFβ also fulfill crucial roles in skeletal myogenesis, we wanted to identify whether KLF6 functions in a myogenic context.

Methods

KLF6 protein expression levels and promoter activity were analyzed using standard cellular and molecular techniques in cell culture.

Results

We found that KLF6 and MEF2D are co-localized in the nuclei of mononucleated but not multinucleated myogenic cells and, that the MEF2 cis element is a key component of the KLF6 promoter region. In addition, TGFβ potently enhanced KLF6 protein levels and this effect was repressed by pharmacological inhibition of Smad3. Interestingly, pharmacological inhibition of MEK/ERK (1/2) signaling resulted in re-activation of the differentiation program in myoblasts treated with TGFβ, which is ordinarily repressed by TGFβ treatment. Conversely, MEK/ERK (1/2) inhibition had no effect on TGFβ-induced KLF6 expression whereas Smad3 inhibition negated this effect, together supporting the existence of two separable arms of TGFβ signaling in myogenic cells. Loss of function analysis using siRNA-mediated KLF6 depletion resulted in enhanced myogenic differentiation whereas TGFβ stimulation of myoblast proliferation was reduced in KLF6 depleted cells.

Conclusions

Collectively these data implicate KLF6 in myoblast proliferation and survival in response to TGFβ with consequences for our understanding of muscle development and a variety of muscle pathologies.

Class IIa histone deacetylases and myocyte enhancer factor 2 proteins regulate the mesenchymal-to-epithelial transition of somatic cell reprogramming

Class IIa histone deacetylases (HDACs) and myocyte enhancer factor 2 (MEF2) proteins compose a signaling module that orchestrates lineage specification during embryogenesis. We show here that this module also regulates the generation of mouse induced pluripotent stem cells by defined transcription factors. Class IIa HDACs and MEF2 proteins rise steadily during fibroblast reprogramming to induced pluripotent stem cells. MEF2 proteins tend to block the process by inducing the expression of Tgfβ cytokines, which impairs the necessary phase of mesenchymal-to-epithelial transition (MET). Conversely, class IIa HDACs endeavor to suppress the activity of MEF2 proteins, thus enhancing the MET and colony formation efficiency. Our work highlights an unexpected role for a developmental axis in somatic cell reprogramming and provides new insight into how the MET is regulated in this context.

TGF-β family signaling in stem cells

BACKGROUND

The diversity of cell types and tissue types that originate throughout development derives from the differentiation potential of embryonic stem cells and somatic stem cells. While the former are pluripotent, and thus can give rise to a full differentiation spectrum, the latter have limited differentiation potential but drive tissue remodeling. Additionally cancer tissues also have a small population of self-renewing cells with stem cell properties. These cancer stem cells may arise through dedifferentiation from non-stem cells in cancer tissues, illustrating their plasticity, and may greatly contribute to the resistance of cancers to chemotherapies.

SCOPE OF REVIEW

The capacity of the different types of stem cells for self-renewal, the establishment and maintenance of their differentiation potential, and the selection of differentiation programs are greatly defined by the interplay of signaling molecules provided by both the stem cells themselves, and their microenvironment, the niche. Here we discuss common and divergent roles of TGF-β family signaling in the regulation of embryonic, reprogrammed pluripotent, somatic, and cancer stem cells.

MAJOR CONCLUSIONS

Increasing evidence highlights the similarities between responses of normal and cancer stem cells to signaling molecules, provided or activated by their microenvironment. While TGF-β family signaling regulates stemness of normal and cancer stem cells, its effects are diverse and depend on the cell types and physiological state of the cells.

GENERAL SIGNIFICANCE

Further mechanistic studies will provide a better understanding of the roles of TGF-β family signaling in the regulation of stem cells. These basic studies may lead to the development of a new therapeutic or prognostic strategies for the treatment of cancers. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

TGF-β regulates sclerostin expression via the ECR5 enhancer

Wnt signaling is critical for skeletal development and homeostasis. Sclerostin (Sost) has emerged as a potent inhibitor of Wnt signaling and, thereby, bone formation. Thus, strategies to reduce sclerostin expression may be used to treat osteoporosis or non-union fractures. Transforming growth factor-beta (TGF-β) elicits various effects upon the skeleton both in vitro and in vivo depending on the duration and timing of administration. In vitro and in vivo studies demonstrate that TGF-β increases osteoprogenitor differentiation but decreases matrix mineralization of committed osteoblasts. Because sclerostin decreases matrix mineralization, this study aimed to examine whether TGF-β achieves such inhibitory effects via transcriptional modulation of Sost. Using the UMR106.01 mature osteoblast cell line, we demonstrated that TGF-βTGF-β(1)-β(2)-β(3) and Activin A increase Sost transcript expression. Pharmacologic inhibition of Alk4/5/7 in vitro and in vivo decreased endogenous Sost expression, and siRNA against Alk4 and Alk5 demonstrated their requirement for endogenous Sost expression. TGF-β(1) targeted the Sost bone enhancer ECR5 and did not affect the transcriptional activity of the endogenous Sost promoter. These results indicate that TGF-β(1) controls Sost transcription in mature osteoblasts, suggesting that sclerostin may mediate the inhibitory effect of TGF-β upon osteoblast differentiation.

Transforming growth factor beta-induced reactivation of Epstein-Barr virus involves multiple Smad-binding elements cooperatively activating expression of the latent-lytic switch BZLF1 gene

Transforming growth factor β (TGF-β) physiologically induces Epstein-Barr virus (EBV) lytic infection by activating the expression of EBV's latent-lytic switch BZLF1 gene. Liang et al. (J. Biol. Chem. 277:23345-23357, 2002) previously identified a Smad-binding element (SBE) within the BZLF1 promoter, Zp; however, it accounts for only 20 to 30% of TGF-β-mediated activation of transcription from Zp. Here, we identified additional factors responsible for the rest of this activation. The incubation of EBV-positive MutuI cells with a TGF-β neutralizing antibody or inhibitors of the TGF-β type I receptor (TβRI) or Smad3 eliminated the TGF-β-induced reactivation of EBV. The coexpression of Smad2, Smad3, and Smad4 together with a constitutively active form of TβRI induced 15- to 25-fold transcription from Zp in gastric carcinoma AGS cells. By electrophoretic mobility shift assays, we identified four additional Smad-binding elements, named SBE2 to SBE5. Substitution mutations in individual SBEs reduced Smad-mediated activation of Zp by 20 to 60%; together, these mutations essentially eliminated it. Chromatin immunoprecipitation assays confirmed that Smad4 newly bound the Zp region of the EBV genome following the incubation of MutuI cells with TGF-β. SBE2 overlaps the ZEB-binding ZV silencing element of Zp. Depending upon posttranslational modifications, Smad4 either competed with ZEB1 for binding or formed a complex with ZEB1 on the Zp ZV element in a cell-free assay system. In transiently transfected cells, exogenously expressed ZEB1 inhibited Smad-mediated transcriptional activation from Zp. We conclude that TGF-β induces EBV lytic reactivation via the canonical Smad pathway by activating BZLF1 gene expression through multiple SBEs acting in concert.

Structure of the MADS-box/MEF2 domain of MEF2A bound to DNA and its implication for myocardin recruitment

Myocyte enhancer factor 2 (MEF2) regulates specific gene expression in diverse developmental programs and adaptive responses. MEF2 recognizes DNA and interacts with transcription cofactors through a highly conserved N-terminal domain referred to as the MADS-box/MEF2 domain. Here we present the crystal structure of the MADS-box/MEF2 domain of MEF2A bound to DNA. In contrast to previous structural studies showing that the MEF2 domain of MEF2A is partially unstructured, the present study reveals that the MEF2 domain participates with the MADS-box in both dimerization and DNA binding as a single domain. The sequence divergence at and immediately following the C-terminal end of the MEF2 domain may allow different MEF2 dimers to recognize different DNA sequences in the flanking regions. The current structure also suggests that the ligand-binding pocket previously observed in the Cabin1-MEF2B-DNA complex and the HDAC9 (histone deacetylase 9)-MEF2B-DNA complex is not induced by cofactor binding but rather preformed by intrinsic folding. However, the structure of the ligand-binding pocket does undergo subtle but significant conformational changes upon cofactor binding. On the basis of these observations, we generated a homology model of MEF2 bound to a myocardin family protein, MASTR, that acts as a potent coactivator of MEF2-dependent gene expression. The model shows excellent shape and chemical complementarity at the binding interface and is consistent with existing mutagenesis data. The apo structure presented here can also serve as a target for virtual screening and soaking studies of small molecules that can modulate the function of MEF2 as research tools and therapeutic leads.

Transcriptional induction of MMP-10 by TGF-beta, mediated by activation of MEF2A and downregulation of class IIa HDACs

Transforming growth factor (TGF)-beta regulates the expression of matrix metalloproteinases (MMPs) and components of the extracellular matrix, thereby profoundly affecting the microenvironment of cells including cancerous ones. We studied MMP-10 induction by TGF-beta in mammary epithelial cells and found that the induction was dependent on the myocyte enhancer factor (MEF)-2 transcription factor. TGF-beta upregulated the gene promoter through the MEF2 site, and knockdown of the MEF2A transcription factor negatively affected MMP-10 induction, whereas its overexpression had a positive effect on the induction. In response to TGF-beta, acetylation and concomitant binding of MEF2A to the promoter region increased, thus suggesting a critical role of MEF2A in transactivation of MMP-10 by TGF-beta. Consistent with the fact that class IIa histone deacetylases (HDACs) interact with MEF2 and suppress transcription, knockdown of HDACs increased and their overexpression inhibited MMP-10 expression. Intriguingly, TGF-beta promoted proteasome-dependent degradation of HDACs. Consistent with this, acetylation of core histones was increased around the MEF2 site of the MMP-10 promoter by TGF-beta and alleviated by overexpression of HDACs. Collectively, it is possible that TGF-beta transcriptionally upregulated MMP-10 through activation of MEF2A, concomitant with acetylation of core histones increasing around the promoter, as a consequence of degradation of the class IIa HDACs.

Transcriptional regulation of hepatic stellate cells

Hepatic stellate cell (HSC) activation is a process of cellular transdifferentiation in which, upon liver injury, the quiescent vitamin A storing perisinusoidal HSC is converted into a wound-healing myofibroblast and acquires potent pro-inflammatory and pro-fibrogenic activities. This remarkable phenotypic transformation is underpinned by changes in the expression of a vast number of genes. In this review we survey current knowledge of the transcription factors that either control HSC activation or which regulate specific fibrogenic functions of the activated HSC such as collagen expression, proliferation and resistance to apoptosis.

Menin expression modulates mesenchymal cell commitment to the myogenic and osteogenic lineages

Menin plays an established role in the differentiation of mesenchymal cells to the osteogenic lineage. Conversely, whether Menin influences the commitment of mesenschymal cells to the myogenic lineage, despite expression in the developing somite was previously unclear. We observed that Menin is down-regulated in C2C12 and C3H10T1/2 mesenchymal cells when muscle differentiation is induced. Moreover, maintenance of Menin expression by constitutive ectopic expression inhibited muscle cell differentiation. Reduction of Menin expression by siRNA technology results in precocious muscle differentiation and concomitantly attenuates BMP-2 induced osteogenesis. Reduced Menin expression antagonizes BMP-2 and TGF-beta1 mediated inhibition of myogenesis. Furthermore, Menin was found to directly interact with and potentiate the transactivation properties of Smad3 in response to TGF-beta1. Finally in concert with these observations, tissue-specific inactivation of Men1 in Pax3-expressing somite precursor cells leads to a patterning defect of rib formation and increased muscle mass in the intercostal region. These data invoke a pivotal role for Menin in the competence of mesenchymal cells to respond to TGF-beta1 and BMP-2 signals. Thus, by modulating cytokine responsiveness Menin functions to alter the balance of multipotent mesenchymal cell commitment to the osteogenic or myogenic lineages.

The molecular regulation of Chinook salmon gonadotropin beta-subunit gene transcription

Expression of the gonadotropin beta-subunit genes is tightly regulated both cell-specifically and by the regulatory hormones to achieve the appropriate gonadotropic hormone levels required for reproductive development and function. Although the cDNA sequences of these genes are highly conserved across species, their promoter sequences are not and few functional studies have been carried out to understand the molecular mechanisms through which their expression is regulated. We and others have carried out several studies on the LHbeta gene promoter of Chinook salmon (Oncorhynchus tschawytscha), and also isolated the FSHbeta gene from the same species. We present here a review of these studies and also novel data pertaining to both genes, in an attempt to collate the current understanding of the molecular regulation of the gonadotropin beta-subunit genes in these fish.

TGF-beta activates ERK5 in human renal epithelial cells

The role of the MAP kinase, extracellular signal-regulated kinase 5 (ERK5) remains unknown, however it is involved in cell differentiation and survival as highlighted by the embryonic lethality of the ERK5 knockout. ERK5 can be activated by growth factors and other extracellular signals. TGF-beta, a powerful controller of epithelial cell phenotype, is known to activate the MAP kinase, ERK1/2 however its effect on ERK5 remains unknown. This study demonstrates, fort the first time, ERK5 activation by TGF-beta, observed in both transformed and primary adult human PTEC; activation required ALK-5 receptor activity. In addition this work demonstrates expression of myocyte enhancer factor-2 (MEF2C) by PTEC and that TGF-beta increased the association of MEK5 with phospho-ERK5 and MEF2C. ERK5 activation by either TGF-beta or epidermal growth factor (EGF) was also inhibited by the p38 MAP kinase inhibitor, SB-202190.

Protein kinase A represses skeletal myogenesis by targeting myocyte enhancer factor 2D

Activation of protein kinase A (PKA) by elevation of the intracellular cyclic AMP (cAMP) level inhibits skeletal myogenesis. Previously, an indirect modulation of the myogenic regulatory factors (MRFs) was implicated as the mechanism. Because myocyte enhancer factor 2 (MEF2) proteins are key regulators of myogenesis and obligatory partners for the MRFs, here we assessed whether these proteins could be involved in PKA-mediated myogenic repression. Initially, in silico analysis revealed several consensus PKA phosphoacceptor sites on MEF2, and subsequent analysis by in vitro kinase assays indicated that PKA directly and efficiently phosphorylates MEF2D. Using mass spectrometric determination of phosphorylated residues, we document that MEF2D serine 121 and serine 190 are targeted by PKA. Transcriptional reporter gene assays to assess MEF2D function revealed that PKA potently represses the transactivation properties of MEF2D. Furthermore, engineered mutation of MEF2D PKA phosphoacceptor sites (serines 121 and 190 to alanine) rendered a PKA-resistant MEF2D protein, which efficiently rescues myogenesis from PKA-mediated repression. Concomitantly, increased intracellular cAMP-mediated PKA activation also resulted in an enhanced nuclear accumulation of histone deacetylase 4 (HDAC4) and a subsequent increase in the MEF2D-HDAC4 repressor complex. Collectively, these data identify MEF2D as a primary target of PKA signaling in myoblasts that leads to inhibition of the skeletal muscle differentiation program.

Transforming growth factor-beta and myostatin signaling in skeletal muscle

The superfamily of transforming growth factor-beta (TGF-beta) cytokines has been shown to have profound effects on cellular proliferation, differentiation, and growth. Recently, there have been major advances in our understanding of the signaling pathway(s) conveying TGF-beta signals to the nucleus to ultimately control gene expression. One tissue that is potently influenced by TGF-beta superfamily signaling is skeletal muscle. Skeletal muscle ontogeny and postnatal physiology have proven to be exquisitely sensitive to the TGF-beta superfamily cytokine milieu in various animal systems from mice to humans. Recently, major strides have been made in understanding the role of TGF-beta and its closely related family member, myostatin, in these processes. In this overview, we will review recent advances in our understanding of the TGF-beta and myostatin signaling pathways and, in particular, focus on the implications of this signaling pathway for skeletal muscle development, physiology, and pathology.

A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

Potentiation of Smad-mediated transcriptional activation by the RNA-binding protein RBPMS

Smad2, Smad3 and Smad4 proteins are considered to be key mediators of transforming growth factor-beta (TGF-beta) signaling. However, the identities of the Smad partners mediating TGF-beta signaling are not fully understood. Here, we show that RNA-binding protein with multiple splicing (RBPMS), a member of the RNA-binding protein family, physically interacts with Smad2, Smad3 and Smad4 both in vitro and in vivo. The presence of TGF-beta increases the binding of RBPMS with these Smad proteins. Consistent with the binding results, overexpression of RBPMS enhances Smad-dependent transcriptional activity in a TGF-beta-dependent manner, whereas knockdown of RBPMS decreases this activity. RBPMS interacts with TGF-beta receptor type I (TbetaR-I), increases phosphorylation of C-terminal SSXS regions in Smad2 and Smad3, and promotes the nuclear accumulation of the Smad proteins. Moreover, RBPMS fails to enhance the transcriptional activity of Smad2 and Smad3 that lack the C-terminal phosphorylation sites. Our data provide the first evidence for an RNA-binding protein playing a role in regulation of Smad-mediated transcriptional activity and suggest that RBPMS stimulates Smad-mediated transactivation possibly through enhanced phosphorylation of Smad2 and Smad3 at the C-terminus and promotion of the nuclear accumulation of the Smad proteins.

Smad7 promotes and enhances skeletal muscle differentiation

Transforming growth factor beta1 (TGF-beta1) and myostatin signaling, mediated by the same Smad downstream effectors, potently repress skeletal muscle cell differentiation. Smad7 inhibits these cytokine signaling pathways. The role of Smad7 during skeletal muscle cell differentiation was assessed. In these studies, we document that increased expression of Smad7 abrogates myostatin- but not TGF-beta1-mediated repression of myogenesis. Further, constitutive expression of exogenous Smad7 potently enhanced skeletal muscle differentiation and cellular hypertrophy. Conversely, targeting of endogenous Smad7 by small interfering RNA inhibited C2C12 muscle cell differentiation, indicating an essential role for Smad7 during myogenesis. Congruent with a role for Smad7 in myogenesis, we observed that the muscle regulatory factor (MyoD) binds to and transactivates the Smad7 proximal promoter region. Finally, we document that Smad7 directly interacts with MyoD and enhances MyoD transcriptional activity. Thus, Smad7 cooperates with MyoD, creating a positive loop to induce Smad7 expression and to promote MyoD driven myogenesis. Taken together, these data implicate Smad7 as a fundamental regulator of differentiation in skeletal muscle cells.

SUMO-1 modification of MEF2A regulates its transcriptional activity

Myocyte enhancer factor 2 (MEF2) transcription factors are crucial regulators controlling muscle-specific and growth factor-inducible genes. Numerous studies have reported that the activity of these transcription factors is tightly modulated by posttranslational modifications such as activation by specific phosphorylation as well as repression by class II histone deacetylases (HDACs). We hypothesized that MEF2 could also be regulated by covalent modification by SUMO-1, a reversible posttranslational modification which has been shown to regulate key proteins involved in cell proliferation, differentiation and tumor suppression. In this study, we demonstrate that MEF2A undergoes sumoylation primarily at a single lysine residue (K395) both in vitro and in vivo. We also show that the nuclear E3 ligase, PIAS1, promotes sumoylation of MEF2A. Mutation of lysine 395 to arginine abolishes MEF2A sumoylation and the sumoylation incompetent mutant protein has enhanced transcriptional activity compared to the wild type protein. Our results suggest that protein sumoylation could play a pivotal role in controlling MEF2 transcriptional activity.

A novel strategy to identify the regulatory DNA-organized cooperations among transcription factors

To identify the functional contributions of cooperations among transcription factors on regulatory DNA is critical for understanding transcription activation. But so far there is a great lack of effective identifying methods. Here we describe a novel strategy, based on comprehensively perturbed experiments and a computational model, to identify the cooperations among NF-kappaB (p65), CREB, and AP-1 in transcription activation of human cytomegalovirus major IE1 promoter/enhancer (MIEP). In this strategy, functional profiles of protein-MIEP association and RNA synthesis are achieved through comprehensively perturbing the association of p65, CREB or AP-1 with MIEP and then subjected to the computational model. Consequently, the 'real' cooperations contributing to MIEP activation are found to comprise five but not seven types of potential cooperations. Thus, our research provides a facile systematic approach to identifying the DNA-organized cooperations among transcription factors and understanding transcription activation.

Alternative Pre-mRNA Splicing Governs Expression of a Conserved Acidic Transactivation Domain in Myocyte Enhancer Factor 2 Factors of Striated Muscle and Brain

Myocyte enhancer factor 2 (MEF2) transcription factors play pivotal roles in striated muscle, neuron, and lymphocyte gene expression and are targets of stress- and calcium-mediated signaling. All MEF2 gene products have a common DNA binding and dimerization domain, but MEF2 transcripts are alternatively spliced among coding exons to produce splicing isoforms. In vertebrate MEF2A, -C, and -D, a splice versus no-splice option gives forms that include or exclude a short domain that we designate beta. We show that mRNAs containing beta are expressed predominantly in striated muscle and brain and that splicing to include beta is induced during myocyte differentiation. MEF2 beta+ isoforms are more robust than beta- forms in activating MEF2-responsive reporters despite similar expression levels. One-hybrid transcription assays using Gal4-MEF2 fusions show similar distinctions in the transactivation produced by beta+ versus beta- isoforms in all cell types tested, including myocytes. beta function is position-independent and exists in all MEF2 splicing variant contexts. The activity is not due to cis effects on MEF2 DNA binding or dimerization nor are established transcription factor or coactivator interactions involved. Each MEF2 beta domain contains multiple acidic residues, mutation of which abolishes function. Despite a location between the p38 MAPK docking domain and Thr phosphoacceptors of MEF2A and MEF2C, inclusion of beta does not influence responses of these factors to this signaling pathway. Thus, a conserved pattern of alternative splicing in vertebrate MEF2 genes generates an acidic activation domain in MEF2 proteins selectively in tissues where MEF2 target genes are highly expressed.

Epigenetic histone modification and cardiovascular lineage programming in mouse embryonic stem cells exposed to laminar shear stress

Experimental evidence indicates that shear stress (SS) exerts a morphogenetic function during cardiac development of mouse and zebrafish embryos. However, the molecular basis for this effect is still elusive. Our previous work described that in adult endothelial cells, SS regulates gene expression by inducing epigenetic modification of histones and activation of transcription complexes bearing acetyltransferase activity. In this study, we evaluated whether SS treatment could epigenetically modify histones and influence cell differentiation in mouse embryonic stem (ES) cells. Cells were exposed to a laminar SS of 10 dyne per cm2/s(-1), or kept in static conditions in the presence or absence of the histone deacetylase inhibitor trichostatin A (TSA). These experiments revealed that SS enhanced lysine acetylation of histone H3 at position 14 (K14), as well as serine phosphorylation at position 10 (S10) and lysine methylation at position 79 (K79), and cooperated with TSA, inducing acetylation of histone H4 and phosphoacetylation of S10 and K14 of histone H3. In addition, ES cells exposed to SS strongly activated transcription from the vascular endothelial growth factor (VEGF) receptor 2 promoter. This effect was paralleled by an early induction of cardiovascular markers, including smooth muscle actin, smooth muscle protein 22-alpha, platelet-endothelial cell adhesion molecule-1, VEGF receptor 2, myocyte enhancer factor-2C (MEF2C), and alpha-sarcomeric actin. In this condition, transcription factors MEF2C and Sma/MAD homolog protein 4 could be isolated from SS-treated ES cells complexed with the cAMP response element-binding protein acetyltransferase. These results provide molecular basis for the SS-dependent cardiovascular commitment of mouse ES cells and suggest that laminar flow may be successfully applied for the in vitro production of cardiovascular precursors.

Regulation of hepatic stellate cell activation and growth by transcription factor myocyte enhancer factor 2

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) undergo activation during the development of liver fibrosis. Transcriptional regulation plays a key role in this process. We studied the role of transcription factor myocyte enhancer factor 2 (MEF2) during HSC activation.

METHODS

Culture of HSCs isolated from rat liver on plastic dishes and HSC-T6 on a basement membrane-like matrix were used as models of HSC activation and deactivation, respectively. The expression and activity of MEF2 were correlated with HSC activation. The roles of MEF2 during HSC activation were assessed in vitro and in vivo by animal models of fibrosis.

RESULTS

Early induction of MEF2 messenger RNA and protein accompanied culture-induced HSC activation. This was associated with enhanced MEF2 DNA binding and transactivation activity. p38 mitogen-activated protein kinase but not extracellular signal-regulated kinase pathway was required for increased MEF2 activity during HSC activation. Increased MEF2 protein also correlated with fibrosis in vivo. Reversal of HSC activation was paralleled by a marked decrease in MEF2 protein and activity. Functionally, enhancing MEF2 significantly increased the expression of alpha-smooth muscle actin (alpha-SMA), activated collagen I promoter activity, and stimulated HSC proliferation. MEF2 interference RNA significantly inhibited expression of alpha-SMA, collagen alpha1(I), and proliferating cell nuclear antigen.

CONCLUSIONS

The studies provide the first evidence for the presence of MEF2 in the liver and show that MEF2 regulates multiple aspects of HSC activation. These studies show a novel role of MEF2 as a key nuclear mediator that may participate in the pathologic process of liver fibrogenesis in vivo.

Immunity in Caenorhabditis elegans

Until very recently it was not known whether the invertebrate Caenorhabditis elegans was capable of mounting a specific immune response to protect itself from pathogens. It has only just become clear that this simple nematode in fact possesses a complex innate immune system, involving multiple signalling pathways and an armoury of antimicrobial proteins and peptides. Genetic and microarray approaches are now revealing the molecular cross-talk that exists between the different signalling cascades.

Immunity and aging: the enemy within?

Functional analyses of changes in the immune response indicate that aging is associated with a decline of adaptive immunity whereas innate immunity is ramped up. Gene expression studies also support age-dependent changes in immunity. Studies using a large panel of methodologies and multiple species show that some of the most dramatic transcriptional changes that occur during aging are associated with immunity. This observation leads to two fundamental questions: (1) Why is the immune response altered with age? (2) Is this a consequence of aging or does it contribute to it? The origin of these changes and the mechanistic relationship among them as well as with aging must be identified. In mammals, this task is complicated by the interdependence of the innate and adaptive immune systems. The value of invertebrates as model organisms to help answer these questions is presented. This includes a description of the immune response in invertebrate models and how it compares with vertebrates, focusing on conserved pathways. Finally, these questions are explored in light of recent reports and data from our laboratory. Experimental alterations of longevity indicate that the differential expression of immunity-related genes during aging is linked to the rate of aging. Long-lived nematodes are more resistant to pathogens and blocking the expression of immune-related genes can prevent lifespan extension. These observations suggest that the immune response has a positive effect on longevity, possibly by increasing fitness. By contrast, it has been reported that activation of the immune system can reduce longevity upon starvation. We also observed that deregulation of the immune response has drastic effects on viability and longevity in Drosophila. These data suggest that the immune response results in a trade-off between beneficial and detrimental effects that might profoundly affect the aging process. Given this, immunity may be an ally early in life, but turns out to be an enemy as we age.

Responses to infection and possible recognition strategies in the innate immune system of Caenorhabditis elegans

In recent years, researchers investigating innate immunity have begun to use C. elegans as a new model system. The worm has been found to mount protective responses to a variety of fungal and bacterial pathogens. Four signalling pathways involved in such responses have been identified so far: the p38 MAP kinase pathway, the programmed cell death pathway, the TGF-beta pathway and the DAF-2 insulin/IGF-I like signalling pathway. Activation of these pathways can lead to the production of immune effector molecules such as lysozymes, lipases and saposin-like proteins, which can act directly against the invading microorganisms. The signalling pathways used and the effectors produced depend on the nature of the infection, indicating that the worm can detect and discriminate between infecting microorganisms. However, the molecules involved in recognition of pathogens have yet to be identified. The worm genome encodes various proteins which might have this recognition function, such as numerous proteins containing C-type lectin domains. These and other candidates are discussed.

Regulation of large-scale chromatin unfolding by Smad4

The tumor suppressor Smad4 plays a critical role in the transforming growth factor-beta (TGF-beta signaling pathway. Smad4 is an essential component of transcriptional complexes mediating the activation of Smad-dependent target genes. However, how Smad4 regulates various chromosomal events remains unknown. Here, we report that Smad4 induces large-scale chromatin unfolding by targeting Smad4 to an amplified, lac operator-containing chromosome region in mammalian cells. This unfolding activity maps to the subdomains within the transactivation domain of Smad4. Both the N-terminal missense and the C-terminal truncated tumor-derived mutations increased the chromatin unfolding activity of Smad4. These data identify a novel function of Smad4 and suggest that regulation of large-scale chromatin unfolding by Smad4 may be responsible for cancer development.

TGF-beta-activated Smad3 represses MEF2-dependent transcription in myogenic differentiation

Transforming growth factor beta (TGF-beta) inhibits myogenesis and associated gene expression. We previously reported that the TGF-beta signaling effector Smad3 mediates this inhibition, by interfering with the assembly of myogenic bHLH transcription factor heterodimers on E-box sequences in the regulatory regions of muscle-specific genes. We now show that TGF-beta-activated Smad3 suppresses the function of MEF2, a second class of essential myogenic factors. TGF-beta signaling through Smad3 represses myogenin expression independently of E-boxes, and prevents a tethered MyoD-E47 dimer to activate transcription indirectly through MEF2-binding sites. In addition, Smad3 interacts with MEF2C, which requires its MADS domain, and disrupts its association with the SRC-family coactivator GRIP-1, thus diminishing the transcription activity of MEF2C. Consistent with this physical displacement, TGF-beta signaling blocks the GRIP-1-induced redistribution of MEF2C to discrete nuclear subdomains in 10T1/2 cells, and the recruitment of GRIP-1 to the myogenin promoter in differentiating myoblasts. These findings indicate that the TGF-beta/Smad3 pathway targets two critical components of the myogenic transcription machinery to inhibit terminal differentiation.

Evolution of the innate immune system: the worm perspective

Simple model organisms that are amenable to comprehensive experimental analysis can be used to elucidate the molecular genetic architecture of complex traits. They can thereby enhance our understanding of these traits in other organisms, including humans. Here, we describe the use of the nematode Caenorhabditis elegans as a tractable model system to study innate immunity. We detail our current understanding of the worm's immune system, which seems to be characterized by four main signaling cascades: a p38 mitogen-activated protein kinase, a transforming growth factor-beta-like, a programed cell death, and an insulin-like receptor pathway. Many details, especially regarding pathogen recognition and immune effectors, are only poorly characterized and clearly warrant further investigation. We additionally speculate on the evolution of the C. elegans immune system, taking into special consideration the relationship between immunity, stress responses and digestion, the diversification of the different parts of the immune system in response to multiple and/or coevolving pathogens, and the trade-off between immunity and host life history traits. Using C. elegans to address these different facets of host-pathogen interactions provides a fresh perspective on our understanding of the structure and complexity of innate immune systems in animals and plants.

Activation of p38 and Smads mediates BMP-2 effects on human trabecular bone-derived osteoblasts

The bone morphogenetic proteins (BMPs) are potent osteoinductive factors that accelerate osteoblast maturation, accompanied by increased cell-substrate adhesion. BMP-2 treatment of osteoblastic cells increases phosphorylation of the cytoplasmic BMP-2 signaling molecules, Smad1 and Smad5. We have previously reported that BMP-2 treatment increase cytoskeletal organization of human trabecular bone-derived osteoblast-like cells (osteoblasts), which is also accompanied by an activation of the focal adhesion kinase p125(FAK). We report here that activation of p125(FAK) occurs with the same kinetics as the phosphorylation of Smad1, suggesting that BMP-2 initiates cross-talk between Smad signaling and the adhesion-mediated signaling pathway. As an adjunct to these effects, we examined activation of mitogen-activated protein (MAP) kinase family members in response to focal adhesion contact formation. Although phosphorylated forms of all three kinases were apparent, only SAPK2alpha/p38 (p38) was activated in response to BMP-2 treatment. Inhibition of p38 kinase activity suppressed BMP-2 induced Smad1 phosphorylation, as well as its translocation to the nucleus, suggesting the integration of p38 activation with Smad1 signaling. Finally, inhibition of p38 in osteoblasts also led to the complete abrogation of BMP-2 induced osteocalcin gene expression and matrix mineralization. These findings suggest that BMP-2 must activate p38 in order to mediate osteogenic differentiation and maturation.

Phosphatidylinositol 3-kinase regulates bone morphogenetic protein-2 (BMP-2)-induced myocyte enhancer factor 2A-dependent transcription of BMP-2 gene in cardiomyocyte precursor cells

The growth and differentiation factor bone morphogenetic protein-2 (BMP-2) regulates cardiac development during vertebrate embryogenesis. In cardiac precursor cells, BMP-2 has recently been shown to induce expression of cardiac transcription factors, including myocyte enhancer factor 2A (MEF-2A). The specific signal transduction mechanism by which BMP-2 regulates these actions is not known. We investigated the role of phosphatidylinositol (PI) 3-kinase in regulating these processes in cardiomyocyte precursor CL6 cells. BMP-2 increased PI 3-kinase activity in these cells in a time-dependent manner, resulting in increased expression of sarcomeric myosin heavy chain (MHC) and MEF-2A. Inhibition of PI 3-kinase abolished these actions of BMP-2, indicating the involvement of PI 3-kinase in these processes. Furthermore, BMP-2 stimulated specific protein.DNA complex formation when an MEF-2 DNA recognition element was used as probe. Antibody supershift assay confirmed the presence of MEF-2A in this protein.DNA complex. Inhibition of PI 3-kinase activity completely prevented the MEF-2A.DNA complex formation. BMP-2 also increased transcription of a reporter gene driven by an MEF-2-specific DNA element in a PI 3-kinase-dependent manner. Ectopic expression of MEF-2A increased BMP-2 transcription to the same extent induced by BMP-2, indicating that MEF-2A may participate in BMP-2 autoregulation in CL6 cells. Expression of dominant negative PI 3-kinase completely abolished BMP-2-induced as well as MEF-2A-mediated BMP-2 transcription. Furthermore expression of MEF-2A increased MHC expression in a PI 3-kinase-dependent manner. Together these data provide the first evidence that BMP-2-induced PI 3-kinase signaling regulates MEF-2A expression and define a mechanism of MEF-2A-dependent BMP-2 transcription.

Roles of cardiac transcription factors in cardiac hypertrophy

Different cell types, equipped with unique structure and function, synthesize different sets of proteins on the basis of different patterns of gene expression, even though their genomes are identical. Cardiac transcription factors have been reported to control a cardiac gene program and thus to play a crucial role in transcriptional regulation during embryogenesis. Recently, postnatal roles of cardiac transcription factors have been extensively investigated. Consistent with the direct transactivation of numerous cardiac genes reactivated in response to hypertrophic stimulation, cardiac transcription factors are profoundly involved in the generation of cardiac hypertrophy or in cardioprotection from cytotoxic stress in the adult heart. In this review, the regulation of a cardiac gene program by cardiac transcription factors is summarized, with an emphasis on their potential role in the generation of cardiac hypertrophy.

Phosphorylation motifs regulating the stability and function of myocyte enhancer factor 2A

The phosphorylation status of the myocyte enhancer factor 2 (MEF2) transcriptional regulator is a critical determinant of its tissue-specific functions. However, due to the complexity of its phosphorylation pattern in vivo, a systematic inventory of MEF2A phosphorylation sites in mammalian cells has been difficult to obtain. We employed modern affinity purification techniques, combined with mass spectrometry, to identify several novel MEF2 phosphoacceptor sites. These include an evolutionarily conserved KSP motif, which we show is important in regulating the stability and function of MEF2A. Also, an indirect pathway in which a protein kinase casein kinase 2 phosphoacceptor site is phosphorylated by activation of p38 MAPK signaling was documented. Together, these findings identify several novel aspects of MEF2 regulation that may prove important in the control of gene expression in neuronal and muscle cells.