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Endocardial Brg1 disruption illustrates the EMT origins of semilunar valve disease

ABSTRACT: Discreet defects during prenatal semilunar valve (SLV) development frequently progress to pathological states later in life and often require valve replacement surgery. As such, it is challenging to distinguish between the disrupted developmental processes, and their genetic and environmental influences, that trigger valve defects from mechanisms that drive the progression of an anatomically abnormal valve into a disease state. This distinction, which is essential to inform the rationale design of diagnostics and therapeutics, requires carefully characterizing when and where an implicated gene or pathway functions during valve development and/or homeostasis. Disrupted growth, differentiation, and patterning events that trigger SLV disease are coordinated by gene expression changes in endocardial, myocardial, and cushion mesenchymal cells. We explored the roles of chromatin regulation in valve gene regulatory networks via conditional inactivation of the mouse Brg1 associated factor (BAF) chromatin-remodeling complex in the endocardial lineage. Endocardial Brg1-deficient embryos develop thickened and mal-patterned SLV cusps that frequently become bicuspid and myxomatous, including in surviving adults. These SLV disease-like phenotypes originate from deficient endocardial-mesenchymal transformation (EMT) in the proximal outflow tract (pOFT) cushions. Mesenchymal cells of neural crest or other cardiac origins subsequently replace the missing EMT-derived cells but are incompetent to pattern the valve interstitium into regions with distinct extracellular matrix composition. Transcriptomics reveal genes that may promote growth and patterning of SLVs and/or serve as biomarkers of their diseased state. Mechanistic studies of SLV disease genes will distinguish between disease origins and progression; the latter may largely reflect secondary responses to a disrupted developmental system.

ORGANISM(S): Mus musculus

PROVIDER: GSE67786 | GEO | 2015/07/16

SECONDARY ACCESSION(S): PRJNA280967

REPOSITORIES: GEO

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Project description:Malformations of the cardiovascular system are the most common type of birth defect in humans, affecting predominantly the formation of valves and septa. During heart valve and septa formation, cells from the atrio-ventricular canal (AVC) and outflow tract (OFT) regions of the heart undergo an epithelial-to-mesenchymal transformation (EMT) and invade the underlying extracellular matrix to give rise to endocardial cushions. Subsequent maturation of newly formed mesenchyme cells leads to thin stress-resistant leaflets. TWIST1 is a basic helix-loop-helix transcription factor expressed in newly formed mesenchyme cells of the AVC and OFT that has been shown to play roles in cell survival, cell proliferation and differentiation. However, the role and downstream targets of TWIST1 during heart valve formation remain unclear. To identify genes important for heart valve development downstream of Twist1 we performed global gene expression profiling of AVC, OFT, atria and ventricles of the embryonic day 10.5 mouse heart by tag-sequencing (Tag-seq). Using this resource we identified a novel set of 1246 genes, including 201 regulators of transcription, enriched in the valve forming regions of the heart. We compared these genes to a Tag-seq library from the Twist1 null developing valves revealing significant gene expression changes. These changes were consistent with a role of TWIST1 in controlling differentiation of mesenchymal cells following their transformation from endothelium in the mouse. To study the role of TWIST1 at the DNA level we performed chromatin immunoprecipitation and identified novel direct targets of TWIST1 in the developing heart valves. Our findings are consistent with a role for TWIST1 in the differentiation of AVC mesenchyme post-EMT in the mouse, and suggest that TWIST1 exerts its function by direct DNA binding to activate valve specific gene expression. Profiled the AVC, OFT, atria and ventricles of the embryonic day 10.5 mouse heart by tag-sequencing (Tag-seq) (no replicates). We also produced a Tag-seq library from Twist1 null developing valves to reveal the gene expression changes associated with loss of this gene.

Project description:E12.5 AV cushion and E17.5 AV valve from wild-type FVB/N mice and in vitro cultured MC3T3 cells; In the study we demonstrated shared gene expression in embryonic heart valve development and Osteoblast progenitor cells. The atrioventricular (AV) valves of the heart develop from undifferentiated mesenchymal endocardial cushions, that later remodel into stratified valves with diversified extracellular matrix (ECM). Because the mature valves express genes associated with osteogenesis and exhibit disease-associated calcification, we hypothesized the existence of shared regulatory pathways active in the remodeling AV valves and in bone progenitor cells. In order to define gene regulatory programs of valvulogenesis relative to osteoblast progenitors, we undertook Affymetrix gene expression profiling analysis of murine embryonic day (E)12.5 AV cushions compared to E17.5 remodeled AV valves (mitral and tri-cuspid) and to pre-osteoblast MC3T3-E1 (subclone4) cells. Overall MC3T3 cells were significantly more similar to E17.5 valves than to E12.5 cushions, supporting the hypothesis that valve remodeling involves the expression of many genes also expressed in osteoblasts. Several transcription factors characteristic of mesenchymal and osteoblast precursor cells, including Twist1 are predominant in E12.5 cushion. Valve remodeling also includes differential regulation of matrix metalloproteinases and their inhibitors as well as characteristic collagen isoform switching. Among the most highly enriched genes during valvulogenesis were members of the small leucine-rich proteoglycan (SLRP) family including Asporin, a known negative regulator of osteoblast differentiation and mineralization. Together, these data support shared gene expression profiles of the remodeling valves and osteoblast bone precursor cells in normal valve development and homeostasis with potential functions in calcific valve disease. Experiment Overall Design: In the study, we hybridized RNA from E12.5 AV cushion and E17.5 AV valve from wild-type FVB/N mice and in vitro cultured MC3T3 cells to Affymetrix MOE430 2 GeneChipÂ® arrays.

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Endocardial Brg1 disruption illustrates the EMT origins of semilunar valve disease

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