SummaryThe role of long noncoding RNA (lncRNA) in adult hearts is unknown; also unclear is how lncRNA modulates nucleosome remodeling. An estimated 70% of mouse genes undergo antisense transcription1, including myosin heavy chain 7 (Myh7) that encodes molecular motor proteins for heart contraction2. Here, we identify a cluster of lncRNA transcripts from Myh7 loci and show a new lncRNA–chromatin mechanism for heart failure. In mice, these transcripts, which we named Myosin Heavy Chain Associated RNA Transcripts (MyHEART or Mhrt), are cardiac-specific and abundant in adult hearts. Pathological stress activates the Brg1-Hdac-Parp chromatin repressor complex3 to inhibit Mhrt transcription in the heart. Such stress-induced Mhrt repression is essential for cardiomyopathy to develop: restoring Mhrt to the pre-stress level protects the heart from hypertrophy and failure. Mhrt antagonizes the function of Brg1, a chromatin-remodeling factor that is activated by stress to trigger aberrant gene expression and cardiac myopathy3. Mhrt prevents Brg1 from recognizing its genomic DNA targets, thus inhibiting chromatin targeting and gene regulation by Brg1. Mhrt binds to the helicase domain of Brg1, and this domain is crucial for tethering Brg1 to chromatinized DNA targets. Brg1 helicase has dual nucleic acid-binding specificities: it is capable of binding lncRNA (Mhrt) and chromatinized—but not naked—DNA. This dual-binding feature of helicase enables a competitive inhibition mechanism by which Mhrt sequesters Brg1 from its genomic DNA targets to prevent chromatin remodeling. A Mhrt-Brg1 feedback circuit is thus crucial for heart function. Human MHRT also originates from MYH7 loci and is repressed in various types of myopathic hearts, suggesting a conserved lncRNA mechanism in human cardiomyopathy. Our studies identify the first cardioprotective lncRNA, define a new targeting mechanism for ATP-dependent chromatin-remodeling factors, and establish a new paradigm for lncRNA–chromatin interaction.

SUMMARYCardiac hypertrophy and failure are characterized by transcriptional reprogramming of gene expression. Adult cardiomyocytes in mice express primarily α-myosin heavy chain (α-MHC), whereas embryonic cardiomyocytes express β-MHC. Cardiac stress triggers adult hearts to undergo hypertrophy and a shift from α-MHC to fetal β-MHC expression. Here we show that Brg1, a chromatin-remodeling protein, plays critical roles in regulating cardiac growth, differentiation and gene expression. In embryos, Brg1 promotes myocyte proliferation by maintaining BMP10 and suppressing p57kip2 expression. It preserves fetal cardiac differentiation by interacting with HDAC and PARP to repress α-MHC and activate β-MHC. In adults, Brg1 is turned off in cardiomyocytes. It is reactivated by cardiac stresses and complexes with its embryonic partners, HDAC and PARP, to induce a pathological α- to β-MHC shift. Preventing Brg1 re-expression decreases hypertrophy and reverses such MHC switch. Brg1 is activated in certain patients with hypertrophic cardiomyopathy, its level correlating with disease severity and MHC changes. Our studies show that Brg1 maintains cardiomyocytes in an embryonic state, and demonstrate an epigenetic mechanism by which three classes of chromatin-modifying factors, Brg1, HDAC and PARP, cooperate to control developmental and pathological gene expression.

Developing myocardial cells respond to signals from the endocardial layer to form a network of trabeculae that characterize the ventricles of the vertebrate heart. Abnormal myocardial trabeculation results in specific cardiomyopathies in humans and yet trabecular development is poorly understood. We show that trabeculation requires Brg1, a chromatin remodeling protein, to repress ADAMTS1 expression in the endocardium that overlies the developing trabeculae. Repression of ADAMTS1, a secreted matrix metalloproteinase, allows the establishment of an extracellular environment in the cardiac jelly that supports trabecular growth. Later during embryogenesis, ADAMTS1 expression initiates in the endocardium to degrade the cardiac jelly and prevent excessive trabeculation. Thus, the composition of cardiac jelly essential for myocardial morphogenesis is dynamically controlled by ADAMTS1 and its chromatin-based transcriptional regulation. Modification of the intervening microenvironment provides a mechanism by which chromatin regulation within one tissue layer coordinates the morphogenesis of an adjacent layer.

Interstitial fibrosis plays a key role in the development and progression of heart failure. Here, we show that an enzyme that crosslinks collagen—Lysyl oxidase-like 2 (Loxl2)—is essential for interstitial fibrosis and mechanical dysfunction of pathologically stressed hearts. In mice, cardiac stress activates fibroblasts to express and secrete Loxl2 into the interstitium, triggering fibrosis, systolic and diastolic dysfunction of stressed hearts. Antibody-mediated inhibition or genetic disruption of Loxl2 greatly reduces stress-induced cardiac fibrosis and chamber dilatation, improving systolic and diastolic functions. Loxl2 stimulates cardiac fibroblasts through PI3K/AKT to produce TGF-β2, promoting fibroblast-to-myofibroblast transformation; Loxl2 also acts downstream of TGF-β2 to stimulate myofibroblast migration. In diseased human hearts, LOXL2 is upregulated in cardiac interstitium; its levels correlate with collagen crosslinking and cardiac dysfunction. LOXL2 is also elevated in the serum of heart failure (HF) patients, correlating with other HF biomarkers, suggesting a conserved LOXL2-mediated mechanism of human HF.