Phospholamban (PLN) is a key regulator of sarcolemma calcium uptake in cardiomyocyte, its inhibitory activity to SERCA is regulated by phosphorylation. PLN hypophosphorylation is a common molecular feature in failing heart. The current study provided evidence at molecular, cellular and whole heart levels to implicate a sarcolemma membrane targeted protein phosphatase, PP2Ce, as a specific and potent PLN phosphatase. PP2Ce expression was elevated in failing human heart and induced acutely at protein level by β -adrenergic stimulation or oxidative stress in cardiomyocytes. PP2Ce expression in mouse heart blunted β-adrenergic response and exacerbated ischemia/reperfusion injury. Therefore, PP2Ce is a new regulator for cardiac function and pathogenesis.

Heart failure is associated with induction of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). The serine/threonine protein kinase/endoribonuclease IRE1α is a key protein in ER stress signal transduction. IRE1α activity can induce both protective UPR and apoptotic downstream signaling events, but the specific role for IRE1α activity in the heart is unknown. A major aim of this study was to characterize the specific contribution of IRE1α in cardiac physiology and pathogenesis. We used both cultured myocytes and a transgenic mouse line with inducible and cardiomyocyte-specific IRE1α overexpression as experimental models to achieve targeted IRE1α activation. IRE1α expression induced a potent but transient ER stress response in cardiomyocytes and did not cause significant effects in the intact heart under normal physiological conditions. Furthermore, the IRE1α-activated transgenic heart responding to pressure overload exhibited preserved function and reduced fibrotic area, associated with increased adaptive UPR signaling and with blunted inflammatory and pathological gene expression. Therefore, we conclude that IRE1α induces transient ER stress signaling and confers a protective effect against pressure overload-induced pathological remodeling in the heart. To our knowledge, this report provides first direct evidence of a specific and protective role for IRE1α in the heart and reveals an interaction between ER stress signaling and inflammatory regulation in the pathologically stressed heart.

Stress-induced p38 mitogen-activated protein kinase (MAPK) activity is implicated in pathological remodeling in the heart. For example, constitutive p38 MAPK activation in cardiomyocytes induces pathological features, including myocyte hypertrophy, apoptosis, contractile dysfunction, and fetal gene expression. However, the physiological function of cardiomyocyte p38 MAPK activity in beneficial compensatory vascular remodeling is unclear. This report investigated the functional role and the underlying mechanisms of cardiomyocyte p38 MAPK activity in cardiac remodeling induced by chronic stress. Using both in vitro and in vivo model systems, we found that p38 MAPK activity is required for hypoxia-induced pro-angiogenic activity from cardiomyocytes and that p38 MAPK activation in cardiomyocyte is sufficient to promote paracrine signaling-mediated, pro-angiogenic activity. We further demonstrate that VEGF is a paracrine factor responsible for the p38 MAPK-mediated pro-angiogenic activity from cardiomyocytes and that p38 MAPK pathway activation is sufficient for inducing VEGF secretion from cardiomyocytes in an Sp1-dependent manner. More significantly, cardiomyocyte-specific inactivation of p38α in mouse heart impaired compensatory angiogenesis after pressure overload and promoted early onset of heart failure. In summary, p38αMAPK has a critical role in the cross-talk between cardiomyocytes and vasculature by regulating stress-induced VEGF expression and secretion in cardiomyocytes. We conclude that as part of a stress-induced signaling pathway, p38 MAPK activity significantly contributes to both pathological and compensatory remodeling in the heart.

Hematopoietic progenitors are enriched in the endocardial cushion and contribute, in a Nkx2-5-dependent manner, to tissue macrophages required for the remodeling of cardiac valves and septa. However, little is known about the molecular mechanism of endocardial-hematopoietic transition. In the current study, we identified the regulatory network of endocardial hematopoiesis. Signal network analysis from scRNA-seq datasets revealed that genes in Notch and retinoic acid (RA) signaling are significantly downregulated in Nkx2-5-null endocardial cells. In vivo and ex vivo analyses validate that the Nkx2-5-Notch axis is essential for the generation of both hemogenic and cushion endocardial cells, and the suppression of RA signaling via Dhrs3 expression plays important roles in further differentiation into macrophages. Genetic ablation study revealed that these macrophages are essential in cardiac valve remodeling. In summary, the study demonstrates that the Nkx2-5/Notch/RA signaling plays a pivotal role in macrophage differentiation from hematopoietic progenitors.

In the mammalian heart, the left ventricle (LV) rapidly becomes more dominant in size and function over the right ventricle (RV) after birth. The molecular regulators responsible for this chamber-specific differential growth are largely unknown. We found that cardiomyocytes in the neonatal mouse RV had lower proliferation, more apoptosis, and a smaller average size compared with the LV. This chamber-specific growth pattern was associated with a selective activation of p38 mitogen-activated protein kinase (MAPK) activity in the RV and simultaneous inactivation in the LV. Cardiomyocyte-specific deletion of both the Mapk14 and Mapk11 genes in mice resulted in loss of p38 MAPK expression and activity in the neonatal heart. Inactivation of p38 activity led to a marked increase in cardiomyocyte proliferation and hypertrophy but diminished cardiomyocyte apoptosis, specifically in the RV. Consequently, the p38-inactivated hearts showed RV-specific enlargement postnatally, progressing to pulmonary hypertension and right heart failure at the adult stage. Chamber-specific p38 activity was associated with differential expression of dual-specific phosphatases (DUSPs) in neonatal hearts, including DUSP26. Unbiased transcriptome analysis revealed that IRE1α/XBP1-mediated gene regulation contributed to p38 MAPK-dependent regulation of neonatal cardiomyocyte proliferation and binucleation. These findings establish an obligatory role of DUSP/p38/IRE1α signaling in cardiomyocytes for chamber-specific growth in the postnatal heart.