A hand for the epicardium.

In mammals, formation of the properly septated 4-chambered heart and coronary vasculature is essential for unidirectional blood flow and survival of the organism. This process involves complex cell-cell interactions, the dysregulation of which can profoundly alter organogenesis, leading to a plethora of structural or functional cardiac defects. No wonder that congenital heart disease (CHD) represents the largest class of birth defects in humans. CHD, even the more subtle, subclinical form, is a major risk factor for early onset of serious cardiovascular complications and premature death. Despite important advances in diagnosis and surgical repair, preventive or therapeutic approaches have been hampered by our incomplete understanding of the molecular basis of most CHDs. Over the past 15 years, work in several model systems has identified genes required for normal heart development and started unraveling intricate regulatory relationships among many of them. Importantly, mutations in several of these cardiac regulators were subsequently linked to human CHD.1 Of those, the basic helix-loop-helix transcription factors Hand1 (eHAND) and Hand2 (dHAND) have emerged as important regulators of heart, limb, and neural crest cell development. Within the developing heart, Hand1 and Hand2 have overlapping expression in the cardiac crescent, but Hand1 expression becomes more restricted to the left ventricle after cardiac looping, whereas Hand2 becomes more localized to the right ventricle and derivatives of the secondary heart field (SHF).2 Gene targeting in transgenic mice indicated critical function for both factors in heart and vascular development.3–7a Moreover, cell-specific inactivation revealed a role for the Hand proteins in several cardiac and extracardiac cell types involved in heart formation. In the case of Hand2, a role in secondary heart field cell proliferation as well as in myocardial and cardiac neural crest cell differentiation and survival was identified. Indeed, deletion of Hand2 from these cells using a variety of cell-specific Cre recombinase–driven promoters led to CHD (Table). Interestingly, outflow tract (OFT) defects and hypoplastic ventricles were common findings. For example, conditional deletion of Hand2 in neural crest cells caused double-outlet right ventricle, membranous ventricular septal defects, and pulmonary stenosis and interrupted aortic artery.8,9 Myocardial-specific loss of Hand2 caused hypoplastic right ventricle and OFT defects leading to death by E12.5.8 When Hand2 was specifically ablated in domains of the SHF, mutant embryos displayed hypoplastic right ventricles, thin myocardium, tricuspid atresia, and shortened OFT.10 Barnes et al11 report that in addition to its role in SHF, myocardial, and cardiac neural crest–derived cells, Hand2 is a major regulator of epicardial development. To dissect the role of Hand2 within the epicardium, the authors deleted Hand2in the Hand1-expressing lineage using a new Cre line (Hand1), which marks the left ventricular myocardium, a subpopulation of cardiac neural crest cells and the epicardium.11 The resulting line (H2CKO) displayed OFT defects, including persistent truncus arteriosus and double-outlet right ventricle as well as ventricular septal defects, a finding reminiscent of the defects obtained when Hand2 was deleted from the cardiac neural crest cells. H2CKO embryos also displayed hypertrabeculation, noncompaction of the ventricle, and absence of epicardium and coronary lumens, which are thought to be related to an epicardial phenotype. The same cardiac phenotype and embryonic lethality were observed when Hand2 was deleted from the epicardium using the WT1 line. Using epicardial primary cultures obtained from control and H2CKO hearts, Barnes et al provide evidence for major alterations in gene expression in cells lacking Hand2. Among others, expression of Pdgfr is significantly decreased in cultured cells and embryos, whereas -integrin 4 (Itg 4), the fibronectin1 receptor, was significantly upregulated. Furthermore, fibronectin1 fibrils were disorganized around the coronary vessels of E12.5 mutants, similar to what was previously reported in zebrafish Hand2 mutants.12 The potential role for Hand2 in extracellular matrix assembly was further supported by the increased alcian blue staining in H2CKO epicardium. Thus, dysregulated fibronectin/integrin signaling may be the mechanism by which altered extracellular matrix deposition occurs. Together, the data point to an important role for Hand2 in epicardial cell differentiation. The epicardium, a flat layer of mesothelium tissue that covers the outer surface of the myocardium, has generated a lot of interest lately; in addition to its recognized contribution to coronary vasculature, studies have suggested that it might be home to the cardiac stem cell niche.13 Epicardial progenitors are derived from the proepicardial organ, which forms finger-like protrusions. A subset of these cells will undergo an epithelial-to-mesenchymal transformation and migrate into the subepicardial space or invade the underlying myocardium, where they will differentiate into subepicardial mesenchyme, interstitial fibroblasts, coronary endothelium, and coronary smooth muscle cells.14 The report of Barnes et al identifies Hand2 as an important regulator of epicardial cell differentiation. The decrease in interstitial fibroblasts in The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association. From the Department of Biochemistry, Microbiology, and Immunology, University of Ottawa, Ottawa, Ontario, Canada. Correspondence to Dr Mona Nemer, Department of Biochemistry, Microbiology and Immunology, University of Ottawa, 550 Cumberland (246), Ottawa, ON K1N 6N5 Canada. E-mail [email protected] (Circ Res. 2011;108:900-902.) © 2011 American Heart Association, Inc.