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.

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Development of the cerebral vessels, pharyngeal arch arteries (PAAs). and cardiac outflow tract (OFT) requires multipotent neural crest cells (NCCs) that migrate from the neural tube to target tissue destinations. Little is known about how mammalian NCC development is orchestrated by gene programming at the chromatin level, however. Here we show that Brahma-related gene 1 (Brg1), an ATPase subunit of the Brg1/Brahma-associated factor (BAF) chromatin-remodeling complex, is required in NCCs to direct cardiovascular development. Mouse embryos lacking Brg1 in NCCs display immature cerebral vessels, aberrant PAA patterning, and shortened OFT. Brg1 suppresses an apoptosis factor, Apoptosis signal-regulating kinase 1 (Ask1), and a cell cycle inhibitor, p21 cip1 , to inhibit apoptosis and promote proliferation of NCCs, thereby maintaining a multipotent cell reservoir at the neural crest. Brg1 also supports Myosin heavy chain 11 (Myh11) expression to allow NCCs to develop into mature vascular smooth muscle cells of cerebral vessels. Within NCCs, Brg1 partners with chromatin remodeler Chromodomain-helicase-DNAbinding protein 7 (Chd7) on the PlexinA2 promoter to activate PlexinA2, which encodes a receptor for semaphorin to guide NCCs into the OFT. Our findings reveal an important role for Brg1 and its downstream pathways in the survival, differentiation, and migration of the multipotent NCCs critical for mammalian cardiovascular development.N eural crest cells (NCCs) originate from the neural crest of the dorsal neural tube and migrate to many regions of the embryo, where they differentiate into a variety of local cells, including cardiovascular tissues (1). NCCs that emigrate from the neural crest of rhombomere 6-8 to pharyngeal arches and the heart are essential for the patterning of pharyngeal arch arteries (PAAs) and the cardiac outflow tract (OFT) (2, 3). These NCCs also differentiate into vascular smooth muscle cells (SMCs) of PAAs and the muscular septum of the aorta and pulmonary trunk (4, 5). In contrast, NCCs from the cephalic neural tube migrate to the face and forebrain to form craniofacial bones, as well as SMCs of facial and forebrain vessels (6). Thus, NCCs are critical for the formation of cardiac OFT and vascular supplies of large areas of the body.Disruption of NCC development, either directly or indirectly, results in many forms of human birth defects with cardiovascular malformations, including Alagille, Carpenter, Ivemark, Leopard, Williams, DiGeorge, and CHARGE syndromes (7). These syndromes involve defects in PAAs or cardiac OFT, such as coarctation of the aorta, interrupted aortic arch, pulmonary artery stenosis, double-outlet right ventricle, tetralogy of Fallot, or persistent truncus arteriosus. During PAA and OFT development, NCCs are regulated by numerous transcription factors, including Pax3, Pbx1/2/3, Tbx1/2/3/20, Msx1/2, Hand2, AP2a, Cited2, Pitx2, Sox4, Foxc1/c2/d3/h1, Fog2, Gata3/4/6, and Notch/NICD (8). Such extensive involvement of transcription factors indicates the importance of gene pr...