Twist1 promotes heart valve cell proliferation and extracellular matrix gene expression during development in vivo and is expressed in human diseased aortic valves

Twist factor regulation of non-cardiomyocyte cell lineages in the developing heart

The heart is a complex organ that is composed of numerous cell types, which must integrate their programs for proper specification, differentiation and cardiac morphogenesis. During cardiogenesis members of the Twist-family of basic helix-loop-helix (bHLH) transcription factors play distinct roles within cardiac lineages such as the endocardium and extra-cardiac lineages such as the cardiac neural crest (cNCC) and epicardium. While the study of these cell populations is often eclipsed by that of cardiomyocytes, the contributions of non-cardiomyocytes to development and disease are increasingly being appreciated as both dynamic and essential. This review summarizes what is known regarding Twist-family bHLH function in extra-cardiac cell populations and the endocardium, with a focus on regulatory mechanisms, downstream targets, and expression profiles. Improving our understanding of the molecular pathways that Twist-family bHLH factors mediate in these lineages will be necessary to ascertain how their dysfunction leads to congenital disease and adult pathologies such as myocardial infarctions and cardiac fibroblast induced fibrosis. Indeed, this knowledge will prove to be critical to clinicians seeking to improve current treatments.

Animal models of organic heart valve disease

Heart valve disease is a frequently encountered pathology, related to high morbidity and mortality rates in industrialized and developing countries. Animal models are interesting to investigate the causality, but also underlying mechanisms and potential treatments of human valvular diseases. Recently, animal models of heart valve disease have been developed, which allow to investigate the pathophysiology, and to follow the progression and the potential regression of disease with therapeutics over time. The present review provides an overview of animal models of primary, organic heart valve disease: myxoid age-related, infectious, drug-induced, degenerative calcified, and mechanically induced valvular heart disease.

Differential activation of valvulogenic, chondrogenic, and osteogenic pathways in mouse models of myxomatous and calcific aortic valve disease

Studies of human diseased aortic valves have demonstrated increased expression of genetic markers of valve progenitors and osteogenic differentiation associated with pathogenesis. Three potential mouse models of valve disease were examined for cellular pathology, morphology, and induction of valvulogenic, chondrogenic, and osteogenic markers. Osteogenesis imperfecta murine (Oim) mice, with a mutation in Col1a2, have distal leaflet thickening and increased proteoglycan composition characteristic of myxomatous valve disease. Periostin null mice also exhibit dysregulation of the ECM with thickening in the aortic midvalve region, but do not have an overall increase in valve leaflet surface area. Klotho null mice are a model for premature aging and exhibit calcific nodules in the aortic valve hinge-region, but do not exhibit leaflet thickening, ECM disorganization, or inflammation. Oim/oim mice have increased expression of valve progenitor markers Twist1, Col2a1, Mmp13, Sox9 and Hapln1, in addition to increased Col10a1 and Asporin expression, consistent with increased proteoglycan composition. Periostin null aortic valves exhibit relatively normal gene expression with slightly increased expression of Mmp13 and Hapln1. In contrast, Klotho null aortic valves have increased expression of Runx2, consistent with the calcified phenotype, in addition to increased expression of Sox9, Col10a1, and osteopontin. Together these studies demonstrate that oim/oim mice exhibit histological and molecular characteristics of myxomatous valve disease and Klotho null mice are a new model for calcific aortic valve disease.

A tale with a Twist: a developmental gene with potential relevance for metabolic dysfunction and inflammation in adipose tissue

The Twist proteins (Twist-1 and -2) are highly conserved developmental proteins with key roles for the transcriptional regulation in mesenchymal cell lineages. They belong to the super-family of bHLH proteins and exhibit bi-functional roles as both activators and repressors of gene transcription. The Twist proteins are expressed at low levels in adult tissues but may become abundantly re-expressed in cells undergoing malignant transformation. This observation prompted extensive research on the roles of Twist proteins in cancer progression and metastasis. Very recent studies indicate a novel role for Twist-1 as a potential regulator of adipose tissue (AT) remodeling and inflammation. Several studies suggested that developmental genes are important determinants of obesity, fat distribution and remodeling capacity of different adipose depots. Twist-1 is abundantly and selectively expressed in the adult AT and its constitutive expression is significantly higher in subcutaneous (SAT) vs. visceral (VAT) fat in both mice and humans. Moreover, Twist1 expression is strongly correlated with BMI and insulin resistance in humans. However, the functional roles and transcriptional downstream targets of Twist1 in AT are largely unexplored. The purpose of this review is to highlight the major findings related to Twist1 expression in different fat depots and cellular components of AT and to discuss the potential mechanisms suggesting a role for Twist1 in AT metabolism, inflammation and remodeling.

Twist1 directly regulates genes that promote cell proliferation and migration in developing heart valves

Twist1, a basic helix-loop-helix transcription factor, is expressed in mesenchymal precursor populations during embryogenesis and in metastatic cancer cells. In the developing heart, Twist1 is highly expressed in endocardial cushion (ECC) valve mesenchymal cells and is down regulated during valve differentiation and remodeling. Previous studies demonstrated that Twist1 promotes cell proliferation, migration, and expression of primitive extracellular matrix (ECM) molecules in ECC mesenchymal cells. Furthermore, Twist1 expression is induced in human pediatric and adult diseased heart valves. However, the Twist1 downstream target genes that mediate increased cell proliferation and migration during early heart valve development remain largely unknown. Candidate gene and global gene profiling approaches were used to identify transcriptional targets of Twist1 during heart valve development. Candidate target genes were analyzed for evolutionarily conserved regions (ECRs) containing E-box consensus sequences that are potential Twist1 binding sites. ECRs containing conserved E-box sequences were identified for Twist1 responsive genes Tbx20, Cdh11, Sema3C, Rab39b, and Gadd45a. Twist1 binding to these sequences in vivo was determined by chromatin immunoprecipitation (ChIP) assays, and binding was detected in ECCs but not late stage remodeling valves. In addition identified Twist1 target genes are highly expressed in ECCs and have reduced expression during heart valve remodeling in vivo, which is consistent with the expression pattern of Twist1. Together these analyses identify multiple new genes involved in cell proliferation and migration that are differentially expressed in the developing heart valves, are responsive to Twist1 transcriptional function, and contain Twist1-responsive regulatory sequences.

Tbx20 regulation of cardiac cell proliferation and lineage specialization during embryonic and fetal development in vivo

TBX20 gain-of-function mutations in humans are associated with congenital heart malformations and myocardial defects. However the effects of increased Tbx20 function during cardiac chamber development and maturation have not been reported previously. CAG-CAT-Tbx20 transgenic mice were generated for Cre-dependent induction of Tbx20 in myocardial lineages in the developing heart. βMHCCre-mediated overexpression of Tbx20 in fetal ventricular cardiomyocytes results in increased thickness of compact myocardium, induction of cardiomyocyte proliferation, and increased expression of Bmp10 and pSmad1/5/8 at embryonic day (E) 14.5. βMHCCre-mediated Tbx20 overexpression also leads to increased expression of cardiac conduction system (CCS) genes Tbx5, Cx40, and Cx43 throughout the ventricular myocardium. In contrast, Nkx2.5Cre mediated overexpression of Tbx20 in the embryonic heart results in reduced cardiomyocyte proliferation, increased expression of a cell cycle inhibitor, p21(CIP1), and decreased expression of Tbx2, Tbx5, and N-myc1 at E9.5, concomitant with decreased phospho-ERK1/2 expression. Together, these analyses demonstrate that Tbx20 differentially regulates cell proliferation and cardiac lineage specification in embryonic versus fetal cardiomyocytes. Induction of pSmad1/5/8 at E14.5 and inhibition of dpERK expression at E9.5 are consistent with selective Tbx20 regulation of these pathways in association with stage-specific effects on cardiomyocyte proliferation. Together, these in vivo data support distinct functions for Tbx20 in regulation of cardiomyocyte lineage maturation and cell proliferation at embryonic and fetal stages of heart development.

Transforming growth factor beta signaling in adult cardiovascular diseases and repair

The majority of children with congenital heart disease now live into adulthood due to the remarkable surgical and medical advances that have taken place over the past half century. Because of this, adults now represent the largest age group with adult cardiovascular diseases. It includes patients with heart diseases that were not detected or not treated during childhood, those whose defects were surgically corrected but now need revision due to maladaptive responses to the procedure, those with exercise problems and those with age-related degenerative diseases. Because adult cardiovascular diseases in this population are relatively new, they are not well understood. It is therefore necessary to understand the molecular and physiological pathways involved if we are to improve treatments. Since there is a developmental basis to adult cardiovascular disease, transforming growth factor beta (TGFβ) signaling pathways that are essential for proper cardiovascular development may also play critical roles in the homeostatic, repair and stress response processes involved in adult cardiovascular diseases. Consequently, we have chosen to summarize the current information on a subset of TGFβ ligand and receptor genes and related effector genes that, when dysregulated, are known to lead to cardiovascular diseases and adult cardiovascular deficiencies and/or pathologies. A better understanding of the TGFβ signaling network in cardiovascular disease and repair will impact genetic and physiologic investigations of cardiovascular diseases in elderly patients and lead to an improvement in clinical interventions.

Calcific aortic valve stenosis: methods, models, and mechanisms

Calcific aortic valve stenosis (CAVS) is a major health problem facing aging societies. The identification of osteoblast-like and osteoclast-like cells in human tissue has led to a major paradigm shift in the field. CAVS was thought to be a passive, degenerative process, whereas now the progression of calcification in CAVS is considered to be actively regulated. Mechanistic studies examining the contributions of true ectopic osteogenesis, nonosseous calcification, and ectopic osteoblast-like cells (that appear to function differently from skeletal osteoblasts) to valvular dysfunction have been facilitated by the development of mouse models of CAVS. Recent studies also suggest that valvular fibrosis, as well as calcification, may play an important role in restricting cusp movement, and CAVS may be more appropriately viewed as a fibrocalcific disease. High-resolution echocardiography and magnetic resonance imaging have emerged as useful tools for testing the efficacy of pharmacological and genetic interventions in vivo. Key studies in humans and animals are reviewed that have shaped current paradigms in the field of CAVS, and suggest promising future areas for research.

Negative transcriptional regulation of the human periostin gene by YingYang-1 transcription factor

Periostin (POSTN), an osteoblast-specific secreted protein known to be associated with cell adhesion activity for bone formation and development by the epithelial cell-derived tumors, leads to a significant enhancement in angiogenesis and tumorigenesis. At present, little is known about the mechanisms underlying its transcriptional control either in physiological or neoplastic conditions. In this study we demonstrate that the ability of the human POSTN promoter to drive transcription mostly depends on the activity of YingYang-1 (YY1) zinc finger transcription factor. YY1, whose regulatory role in biology includes, besides transcriptional control, also chromatin remodeling, DNA damage repair and tumorigenesis, acts as a strong negative modulator of the POSTN expression. We retain that the identification of the functional role of YY1 in the transcriptional control of the human POSTN gene adds new insights in the studies focused on gene expression in normal and transformed cells.

Animal models of calcific aortic valve disease

Calcific aortic valve disease (CAVD), once thought to be a degenerative disease, is now recognized to be an active pathobiological process, with chronic inflammation emerging as a predominant, and possibly driving, factor. However, many details of the pathobiological mechanisms of CAVD remain to be described, and new approaches to treat CAVD need to be identified. Animal models are emerging as vital tools to this end, facilitated by the advent of new models and improved understanding of the utility of existing models. In this paper, we summarize and critically appraise current small and large animal models of CAVD, discuss the utility of animal models for priority CAVD research areas, and provide recommendations for future animal model studies of CAVD.

Heart valve structure and function in development and disease

The mature heart valves are made up of highly organized extracellular matrix (ECM) and valve interstitial cells (VICs) surrounded by an endothelial cell layer. The ECM of the valves is stratified into elastin-, proteoglycan-, and collagen-rich layers that confer distinct biomechanical properties to the leaflets and supporting structures. Signaling pathways have critical functions in primary valvulogenesis as well as the maintenance of valve structure and function over time. Animal models provide powerful tools to study valve development and disease processes. Valve disease is a significant public health problem, and increasing evidence implicates aberrant developmental mechanisms underlying pathogenesis. Further studies are necessary to determine regulatory pathway interactions underlying valve pathogenesis in order to generate new avenues for novel therapeutics.

Molecular and developmental mechanisms of congenital heart valve disease

Congenital heart disease occurs in approximately 1% of all live births and includes structural abnormalities of the heart valves. However, this statistic underestimates congenital valve lesions, such as bicuspic aortic valve (BAV) and mitral valve prolapse (MVP), that typically become apparent later in life as progressive valve dysfunction and disease. At present, the standard treatment for valve disease is replacement, and approximately 95,000 surgical procedures are performed each year in the United States. The most common forms of congenital valve disease include abnormal valve cusp morphogenesis, as in the case of BAV, or defects in extracellular matrix (ECM) organization and homeostasis, as occurs in MVP. The etiology of these common valve diseases is largely unknown. However, the study of murine and avian model systems, along with human genetic linkage studies, have led to the identification of genes and regulatory processes that contribute to valve structural malformations and disease. This review focuses on the current understanding and therapeutic implications of molecular regulatory pathways that control valve development and contribute to valve disease.

Differential expression of cartilage and bone-related proteins in pediatric and adult diseased aortic valves

Approximately 5 million people are affected with aortic valve disease (AoVD) in the United States. The most common treatment is aortic valve (AoV) replacement surgery, however, replacement valves are susceptible to failure, necessitating additional surgeries. The molecular mechanisms underlying disease progression and late AoV calcification are not well understood. Recent studies suggest that genes involved in bone and cartilage development play an active role in osteogenic-like calcification in human calcific AoVD (CAVD). In an effort to define the molecular pathways involved in AoVD progression and calcification, expression of markers of valve mesenchymal progenitors, chondrogenic precursors, and osteogenic differentiation was compared in pediatric non-calcified and adult calcified AoV specimens. Valvular interstitial cell (VIC) activation, extracellular matrix (ECM) disorganization, and markers of valve mesenchymal and skeletal chondrogenic progenitor cells were observed in both pediatric and adult AoVD. However, activated BMP signaling, increased expression of cartilage and bone-type collagens, and increased expression of the osteogenic marker Runx2 are observed in adult diseased AoVs. They are not observed in the majority of pediatric diseased valves, representing a marked distinction in the molecular profile between pediatric and adult diseased AoVs. The combined evidence suggests that an actively regulated osteochondrogenic disease process underlies the pathological changes affecting AoVD progression, ultimately resulting in stenotic AoVD. Both pediatric and adult diseased AoVs express protein markers of valve mesenchymal and chondrogenic progenitor cells while adult diseased AoVs also express proteins involved in osteogenic calcification. These findings provide specific molecular indicators of AoVD progression, which may lead to identification of early disease markers and the development of potential therapeutics.