Neuregulin 1 Sustains the Gene Regulatory Network in Both Trabecular and Nontrabecular Myocardium

Abstract

Rationale:The cardiac gene regulatory network (GRN) is controlled by transcription factors and signaling inputs, but network logic in development and it unraveling in disease is poorly understood. In development, the membrane-tethered signaling ligand Neuregulin (Nrg)1, expressed in endocardium, is essential for ventricular morphogenesis. In adults, Nrg1 protects against heart failure and can induce cardiomyocytes to divide.Objective: To understand the role of Nrg1 in heart development through analysis of null and hypomorphic Nrg1 mutant mice. Methods and Results:Chamber domains were correctly specified in Nrg1 mutants, although chamber-restricted genes Hand1 and Cited1 failed to be activated. The chamber GRN subsequently decayed with individual genes exhibiting decay patterns unrelated to known patterning boundaries. Both trabecular and nontrabecular myocardium were affected. Network demise was spatiotemporally dynamic, the most sensitive region being the central part of the left ventricle, in which the GRN underwent complete collapse. Other regions were partially affected with graded sensitivity. In vitro, Nrg1 promoted phospho-Erk1/2-dependent transcription factor expression, cardiomyocyte maturation and cell cycle inhibition. We monitored cardiac pErk1/2 in embryos and found that expression was Nrg1-dependent and levels correlated with cardiac GRN sensitivity in mutants. Conclusions:The chamber GRN is fundamentally labile and dependent on signaling from extracardiac sources.Nrg1-ErbB1/4 -Erk1/2 signaling critically sustains elements of the GRN in trabecular and nontrabecular myocardium, challenging our understanding of Nrg1 function. Transcriptional decay patterns induced by reduced Nrg1 suggest a novel mechanism for cardiac transcriptional regulation and dysfunction in disease, potentially linking biomechanical feedback to molecular pathways for growth and differentiation. (Circ Res. 2010;107:715-727.) Key Words: neuregulin 1 Ⅲ cardiac gene regulation Ⅲ heart development Ⅲ cardiac gene regulatory network A n early patterning event in vertebrate heart development is the specification of myocardium of the atrial and ventricular chambers, a specialized muscle adapted to pumping blood through a closed circulatory system at high pressure. 1 Luminal myocytes of the cardiac chambers develop sponge-like convolutions termed trabeculae, which in development serve as a morphological marker for chamber specification. The trabecular zone is also marked by a unique set of genes. Chamber muscle is an electric syncytium through which action potentials spread via gap junctions, guided by caudal pacemaker myocytes and their conduction and Purkinje fiber tracts.Nontrabecular myocardium of the atrium, inner curvature, atrioventricular (AV) canal and outflow tract (OFT) is less specialized for contraction and gives rise to the myogenic layers of the outflow and inflow vessels, and cells of the proximal conduction system including the sinoatrial (SA) and AV nodes. 1 Nontrabecular myocardium also plays a critical role in induct...

Dependence of cardiac trabeculation on neuregulin signaling and blood flow in zebrafish

“…Due to the important role of Neuregulin signaling in promoting trabeculation in mouse (Gassmann et al, 1995;Lee et al, 1995;Meyer and Birchmeier, 1995;Lai et al, 2010), we hypothesized that Neuregulin signals generated by the zebrafish endocardium could have a potent impact on the morphogenesis of the myocardium. In zebrafish, neuregulin 1 is expressed in the endocardium (Milan et al, 2006), and multiple ErbB receptor genes, such as erbb2 and erbb1a, are expressed broadly, including in the myocardium (Goishi et al, 2003;Thisse and Thisse, 2004).…”

“…The shear forces generated by circulating blood are known to have a significant impact on heart development (Hove et al, 2003;Vermot et al, 2009;Culver and Dickinson, 2010), and the exposure of the endocardium to shear forces could alter its signaling to the myocardium. Notably, it is not yet known whether alteration of hemodynamics can affect Neuregulin signal transduction between the endocardium and myocardium (Lai et al, 2010). In addition to the impact of shear forces on the endocardium, the stretch forces experienced during normal ventricular loading could provoke intracellular signaling within the myocardium to drive trabeculation (Culver and Dickinson, 2010).…”

“…In mouse, Neuregulin 1 (Nrg1) is expressed by the endocardium and ErbB2 and ErbB4 are expressed by the adjacent myocardium (Meyer and Birchmeier, 1995). Loss-of-function of Nrg1, ErbB2, or ErbB4 in mouse results in reduced or absent trabeculation (Gassmann et al, 1995;Lee et al, 1995;Meyer and Birchmeier, 1995;Lai et al, 2010). These data strongly suggest that Neuregulin signals, delivered from the endocardium to the myocardium, are necessary for the successful completion of trabeculation.…”

“…However, several compelling studies in mouse highlight Neuregulin signaling as an important influence on trabeculation (Gassmann et al, 1995;Lee et al, 1995;Meyer and Birchmeier, 1995;Lai et al, 2010). Neuregulins are growth factors that serve as ligands of ErbB receptor tyrosine kinases and induce cell survival, proliferation, and differentiation in several different settings (Mei and Xiong, 2008;Pentassuglia and Sawyer, 2009).…”

Cardiac contraction activates endocardial Notch signaling to modulate chamber maturation in zebrafish

“…Trabeculae increase cardiac output and allow for nutrition and oxygenation of the myocardium before coronary vascularization and are required for establishment of the mature conduction system of the developing ventricle (Lai et al, 2010;. Thus, failure to form cardiac trabeculae causes embryonic lethality, and subtle perturbations of this process could lead to congenital cardiomyopathy (Jenni et al, 1999).…”