Fundamental Mechanisms of Regulated Cell Death and Implications for Heart Disease.

Twelve regulated cell death programs have been described. We review in detail the basic biology of nine including death receptor-mediated apoptosis, death receptor-mediated necrosis (necroptosis), mitochondrial-mediated apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, ferroptosis, pyroptosis, parthanatos, and immunogenic cell death. This is followed by a dissection of the roles of these cell death programs in the major cardiac syndromes: myocardial infarction and heart failure. The most important conclusion relevant to heart disease is that regulated forms of cardiomyocyte death play important roles in both myocardial infarction with reperfusion (ischemia/reperfusion) and heart failure. While a role for apoptosis in ischemia/reperfusion cannot be excluded, regulated forms of necrosis, through both death receptor and mitochondrial pathways, are critical. Ferroptosis and parthanatos are also likely important in ischemia/reperfusion, although it is unclear if these entities are functioning as independent death programs or as amplification mechanisms for necrotic cell death. Pyroptosis may also contribute to ischemia/reperfusion injury, but potentially through effects in non-cardiomyocytes. Cardiomyocyte loss through apoptosis and necrosis is also an important component in the pathogenesis of heart failure and is mediated by both death receptor and mitochondrial signaling. Roles for immunogenic cell death in cardiac disease remain to be defined but merit study in this era of immune checkpoint cancer therapy. Biology-based approaches to inhibit cell death in the various cardiac syndromes are also discussed.

Decoding the Impact of the Hippo Pathway on Different Cell Types in Heart Failure.

The evolutionarily conserved Hippo pathway plays a pivotal role in governing a variety of biological processes. Heart failure (HF) is a major global health problem with a significant risk of mortality. This review provides a contemporary understanding of the Hippo pathway in regulating different cell types during HF. Through a systematic analysis of each component's regulatory mechanisms within the Hippo pathway, we elucidate their specific effects on cardiomyocytes, fibroblasts, endothelial cells, and macrophages in response to various cardiac injuries. Insights gleaned from both in vitro and in vivo studies highlight the therapeutic promise of targeting the Hippo pathway to address cardiovascular diseases, particularly HF.

Suppression of myeloid YAP antagonizes adverse cardiac remodeling during pressure overload stress.

Inflammation is an integral component of cardiovascular disease and is thought to contribute to cardiac dysfunction and heart failure. While ischemia-induced inflammation has been extensively studied in the heart, relatively less is known regarding cardiac inflammation during non-ischemic stress. Recent work has implicated a role for Yes-associated protein (YAP) in modulating inflammation in response to ischemic injury; however, whether YAP influences inflammation in the heart during non-ischemic stress is not described. We hypothesized that YAP mediates a pro-inflammatory response during pressure overload (PO)-induced non-ischemic injury, and that targeted YAP inhibition in the myeloid compartment is cardioprotective. In mice, PO elicited myeloid YAP activation, and myeloid-specific YAP knockout mice (YAPF/F;LysMCre) subjected to PO stress had better systolic function, and attenuated pathological remodeling compared to control mice. Inflammatory indicators were also significantly attenuated, while pro-resolving genes including Vegfa were enhanced, in the myocardium, and in isolated macrophages, of myeloid YAP KO mice after PO. Experiments using bone marrow-derived macrophages (BMDMs) from YAP KO and control mice demonstrated that YAP suppression shifted polarization toward a resolving phenotype. We also observed attenuated NLRP3 inflammasome priming and function in YAP deficient BMDMs, as well as in myeloid YAP KO hearts following PO, indicating disruption of inflammasome induction. Finally, we leveraged nanoparticle-mediated delivery of the YAP inhibitor verteporfin and observed attenuated PO-induced pathological remodeling compared to DMSO nanoparticle control treatment. These data implicate myeloid YAP as an important molecular nodal point that facilitates cardiac inflammation and fibrosis during PO stress and suggest that selective inhibition of YAP may prove a novel therapeutic target in non-ischemic heart disease.

Mst1 promotes cardiac myocyte apoptosis through phosphorylation and inhibition of Bcl-xL.

The Hippo pathway, evolutionarily conserved from flies to mammals, promotes cell death and inhibits cell proliferation to regulate organ size. The core component of this cascade, Mst1 in mammalian cells, is sufficient to promote apoptosis. However, the mechanisms underlying both its activation and its ability to elicit cell death remain largely undefined. We here identify a signaling cassette in cardiac myocytes consisting of K-Ras, the scaffold RASSF1A, and Mst1 that is localized to mitochondria and promotes Mst1 activation in response to oxidative stress. Activated Mst1 phosphorylates Bcl-xL at Ser14, which resides in the BH4 domain, thereby antagonizing Bcl-xL-Bax binding. This, in turn, causes activation of Bax and subsequent mitochondria-mediated apoptotic death. Our findings demonstrate mitochondrial localization of Hippo signaling and identify Bcl-xL as a target that is directly modified to promote apoptosis.

Guidelines for in vivo mouse models of myocardial infarction.

Despite significant improvements in reperfusion strategies, acute coronary syndromes all too often culminate in a myocardial infarction (MI). The consequent MI can, in turn, lead to remodeling of the left ventricle (LV), the development of LV dysfunction, and ultimately progression to heart failure (HF). Accordingly, an improved understanding of the underlying mechanisms of MI remodeling and progression to HF is necessary. One common approach to examine MI pathology is with murine models that recapitulate components of the clinical context of acute coronary syndrome and subsequent MI. We evaluated the different approaches used to produce MI in mouse models and identified opportunities to consolidate methods, recognizing that reperfused and nonreperfused MI yield different responses. The overall goal in compiling this consensus statement is to unify best practices regarding mouse MI models to improve interpretation and allow comparative examination across studies and laboratories. These guidelines will help to establish rigor and reproducibility and provide increased potential for clinical translation.

Hippo Deficiency Leads to Cardiac Dysfunction Accompanied by Cardiomyocyte Dedifferentiation During Pressure Overload.

RATIONALE: The Hippo pathway plays an important role in determining organ size through regulation of cell proliferation and apoptosis. Hippo inactivation and consequent activation of YAP (Yes-associated protein), a transcription cofactor, have been proposed as a strategy to promote myocardial regeneration after myocardial infarction. However, the long-term effects of Hippo deficiency on cardiac function under stress remain unknown. OBJECTIVE: We investigated the long-term effect of Hippo deficiency on cardiac function in the presence of pressure overload (PO). METHODS AND RESULTS: We used mice with cardiac-specific homozygous knockout of WW45 (WW45cKO), in which activation of Mst1 (Mammalian sterile 20-like 1) and Lats2 (large tumor suppressor kinase 2), the upstream kinases of the Hippo pathway, is effectively suppressed because of the absence of the scaffolding protein. We used male mice at 3 to 4 month of age in all animal experiments. We subjected WW45cKO mice to transverse aortic constriction for up to 12 weeks. WW45cKO mice exhibited higher levels of nuclear YAP in cardiomyocytes during PO. Unexpectedly, the progression of cardiac dysfunction induced by PO was exacerbated in WW45cKO mice, despite decreased apoptosis and activated cardiomyocyte cell cycle reentry. WW45cKO mice exhibited cardiomyocyte sarcomere disarray and upregulation of TEAD1 (transcriptional enhancer factor) target genes involved in cardiomyocyte dedifferentiation during PO. Genetic and pharmacological inactivation of the YAP-TEAD1 pathway reduced the PO-induced cardiac dysfunction in WW45cKO mice and attenuated cardiomyocyte dedifferentiation. Furthermore, the YAP-TEAD1 pathway upregulated OSM (oncostatin M) and OSM receptors, which played an essential role in mediating cardiomyocyte dedifferentiation. OSM also upregulated YAP and TEAD1 and promoted cardiomyocyte dedifferentiation, indicating the existence of a positive feedback mechanism consisting of YAP, TEAD1, and OSM. CONCLUSIONS: Although activation of YAP promotes cardiomyocyte regeneration after cardiac injury, it induces cardiomyocyte dedifferentiation and heart failure in the long-term in the presence of PO through activation of the YAP-TEAD1-OSM positive feedback mechanism.

The tumor suppressor RASSF1A modulates inflammation and injury in the reperfused murine myocardium.

Inflammation is a central feature of cardiovascular disease, including myocardial infarction and heart failure. Reperfusion of the ischemic myocardium triggers a complex inflammatory response that can exacerbate injury and worsen heart function, as well as prevent myocardial rupture and mediate wound healing. Therefore, a more complete understanding of this process could contribute to interventions that properly balance inflammatory responses for improved outcomes. In this study, we leveraged several approaches, including global and regional ischemia/reperfusion (I/R), genetically modified mice, and primary cell culture, to investigate the cell type-specific function of the tumor suppressor Ras association domain family member 1 isoform A (RASSF1A) in cardiac inflammation. Our results revealed that genetic inhibition of RASSF1A in cardiomyocytes affords cardioprotection, whereas myeloid-specific deletion of RASSF1A exacerbates inflammation and injury caused by I/R in mice. Cell-based studies revealed that RASSF1A negatively regulates NF-κB and thereby attenuates inflammatory cytokine expression. These findings indicate that myeloid RASSF1A antagonizes I/R-induced myocardial inflammation and suggest that RASSF1A may be a promising target in immunomodulatory therapy for the management of acute heart injury.

Yes-associated protein (YAP) mediates adaptive cardiac hypertrophy in response to pressure overload.

Cardiovascular disease (CVD) remains the leading cause of death globally, and heart failure is a major component of CVD-related morbidity and mortality. The development of cardiac hypertrophy in response to hemodynamic overload is initially considered to be beneficial; however, this adaptive response is limited and, in the presence of prolonged stress, will transition to heart failure. Yes-associated protein (YAP), the central downstream effector of the Hippo signaling pathway, regulates proliferation and survival in mammalian cells. Our previous work demonstrated that cardiac-specific loss of YAP leads to increased cardiomyocyte (CM) apoptosis and impaired CM hypertrophy during chronic myocardial infarction (MI) in the mouse heart. Because of its documented cardioprotective effects, we sought to determine the importance of YAP in response to acute pressure overload (PO). Our results indicate that endogenous YAP is activated in the heart during acute PO. YAP activation that depended upon RhoA was also observed in CMs subjected to cyclic stretch. To examine the function of endogenous YAP during acute PO, Yap+/flox;Creα-MHC (YAP-CHKO) and Yap+/flox mice were subjected to transverse aortic constriction (TAC). We found that YAP-CHKO mice had attenuated cardiac hypertrophy and significant increases in CM apoptosis and fibrosis that correlated with worsened cardiac function after 1 week of TAC. Loss of CM YAP also impaired activation of the cardioprotective kinase Akt, which may underlie the YAP-CHKO phenotype. Together, these data indicate a prohypertrophic, prosurvival function of endogenous YAP and suggest a critical role for CM YAP in the adaptive response to acute PO.

NF2 Activates Hippo Signaling and Promotes Ischemia/Reperfusion Injury in the Heart.

RATIONALE: NF2 (neurofibromin 2) is an established tumor suppressor that promotes apoptosis and inhibits growth in a variety of cell types, yet its function in cardiomyocytes remains largely unknown. OBJECTIVE: We sought to determine the role of NF2 in cardiomyocyte apoptosis and ischemia/reperfusion (I/R) injury in the heart. METHODS AND RESULTS: We investigated the function of NF2 in isolated cardiomyocytes and mouse myocardium at baseline and in response to oxidative stress. NF2 was activated in cardiomyocytes subjected to H2O2 and in murine hearts subjected to I/R. Increased NF2 expression promoted the activation of Mst1 (mammalian sterile 20-like kinase 1) and the inhibition of Yap (Yes-associated protein), whereas knockdown of NF2 attenuated these responses after oxidative stress. NF2 increased the apoptosis of cardiomyocytes that appeared dependent on Mst1 activity. Mice deficient for NF2 in cardiomyocytes, NF2 cardiomyocyte-specific knockout (CKO), were protected against global I/R ex vivo and showed improved cardiac functional recovery. Moreover, NF2 cardiomyocyte-specific knockout mice were protected against I/R injury in vivo and showed the upregulation of Yap target gene expression. Mechanistically, we observed nuclear association between NF2 and its activator MYPT-1 (myosin phosphatase target subunit 1) in cardiomyocytes, and a subpopulation of stress-induced nuclear Mst1 was diminished in NF2 CKO hearts. Finally, mice deficient for both NF2 and Yap failed to show protection against I/R indicating that Yap is an important target of NF2 in the adult heart. CONCLUSIONS: NF2 is activated by oxidative stress in cardiomyocytes and mouse myocardium and facilitates apoptosis. NF2 promotes I/R injury through the activation of Mst1 and inhibition of Yap, thereby regulating Hippo signaling in the adult heart.

miR-206 Mediates YAP-Induced Cardiac Hypertrophy and Survival.

RATIONALE: In Drosophila, the Hippo signaling pathway negatively regulates organ size by suppressing cell proliferation and survival through the inhibition of Yorkie, a transcriptional cofactor. Yes-associated protein (YAP), the mammalian homolog of Yorkie, promotes cardiomyocyte growth and survival in postnatal hearts. However, the underlying mechanism responsible for the beneficial effect of YAP in cardiomyocytes remains unclear. OBJECTIVES: We investigated whether miR-206, a microRNA known to promote hypertrophy in skeletal muscle, mediates the effect of YAP on promotion of survival and hypertrophy in cardiomyocytes. METHODS AND RESULTS: Microarray analysis indicated that YAP increased miR-206 expression in cardiomyocytes. Increased miR-206 expression induced cardiac hypertrophy and inhibited cell death in cultured cardiomyocytes, similar to that of YAP. Downregulation of endogenous miR-206 in cardiomyocytes attenuated YAP-induced cardiac hypertrophy and survival, suggesting that miR-206 plays a critical role in mediating YAP function. Cardiac-specific overexpression of miR-206 in mice induced hypertrophy and protected the heart from ischemia/reperfusion injury, whereas suppression of miR-206 exacerbated ischemia/reperfusion injury and prevented pressure overload-induced cardiac hypertrophy. miR-206 negatively regulates Forkhead box protein P1 expression in cardiomyocytes and overexpression of Forkhead box protein P1 attenuated miR-206-induced cardiac hypertrophy and survival, suggesting that Forkhead box protein P1 is a functional target of miR-206. CONCLUSIONS: YAP increases the abundance of miR-206, which in turn plays an essential role in mediating hypertrophy and survival by silencing Forkhead box protein P1 in cardiomyocytes.

Hippo Signaling in the Heart　- Non-Canonical Pathways Impact Growth, Survival and Function.

Initially identified inDrosophila melanogaster, the Hippo signaling pathway regulates organ size through modulation of cell proliferation, survival and differentiation. This pathway is evolutionarily conserved and canonical signaling involves a kinase cascade that phosphorylates and inhibits the downstream effector Yes-associated protein (YAP). Recent research has demonstrated a fundamental role of Hippo signaling in cardiac development, homeostasis, injury and regeneration, and remains the subject of intense investigation. However, 2 prominent members of this pathway, RASSF1A and Mst1, have been shown to influence heart function and stress responses through YAP-independent mechanisms. This review summarizes non-canonical targets of RASSF1A and Mst1 and discusses their role in the context of cardiac hypertrophy, autophagy, apoptosis and function. (Circ J 2016; 80: 1504-1510).

Yes-associated protein isoform 1 (Yap1) promotes cardiomyocyte survival and growth to protect against myocardial ischemic injury.

Yap1 is an important regulator of cardiomyocyte proliferation and embryonic heart development, yet the function of endogenous Yap1 in the adult heart remains unknown. We studied the role of Yap1 in maintaining basal cardiac function and in modulating injury after chronic myocardial infarction (MI). Cardiomyocyte-specific homozygous inactivation of Yap1 in the postnatal heart (Yap(F/F)Cre) elicited increased myocyte apoptosis and fibrosis, dilated cardiomyopathy, and premature death. Heterozygous deletion (Yap(+/F)Cre) did not cause an overt cardiac phenotype compared with Yap(F/F) control mice at base line. In response to stress (MI), nuclear Yap1 was found selectively in the border zone and not in the remote area of the heart. After chronic MI (28 days), Yap(+/F)Cre mice had significantly increased myocyte apoptosis and fibrosis, with attenuated compensatory cardiomyocyte hypertrophy, and further impaired function versus Yap(+/F) control mice. Studies in isolated cardiomyocytes demonstrated that Yap1 expression is sufficient to promote increased cell size and hypertrophic gene expression and protected cardiomyocytes against H(2)O(2)-induced cell death, whereas Yap1 depletion attenuated phenylephrine-induced hypertrophy and augmented apoptosis. Finally, we observed a significant decrease in cardiomyocyte proliferation in Yap(+/F)Cre hearts compared with Yap(+/F) controls after MI and demonstrated that Yap1 is sufficient to promote cardiomyocyte proliferation in isolated cardiomyocytes. Our findings suggest that Yap1 is critical for basal heart homeostasis and that Yap1 deficiency exacerbates injury in response to chronic MI.

Inflammation in Myocardial Ischemia/Reperfusion Injury: Underlying Mechanisms and Therapeutic Potential.

Acute myocardial infarction (MI) occurs when blood flow to the myocardium is restricted, leading to cardiac damage and massive loss of viable cardiomyocytes. Timely restoration of coronary flow is considered the gold standard treatment for MI patients and limits infarct size; however, this intervention, known as reperfusion, initiates a complex pathological process that somewhat paradoxically also contributes to cardiac injury. Despite being a sterile environment, ischemia/reperfusion (I/R) injury triggers inflammation, which contributes to infarct expansion and subsequent cardiac remodeling and wound healing. The immune response is comprised of subsets of both myeloid and lymphoid-derived cells that act in concert to modulate the pathogenesis and resolution of I/R injury. Multiple mechanisms, including altered metabolic status, regulate immune cell activation and function in the setting of acute MI, yet our understanding remains incomplete. While numerous studies demonstrated cardiac benefit following strategies that target inflammation in preclinical models, therapeutic attempts to mitigate I/R injury in patients were less successful. Therefore, further investigation leveraging emerging technologies is needed to better characterize this intricate inflammatory response and elucidate its influence on cardiac injury and the progression to heart failure.

Ser14 phosphorylation of Bcl-xL mediates compensatory cardiac hypertrophy in male mice.

The anti-apoptotic function of Bcl-xL in the heart during ischemia/reperfusion is diminished by K-Ras-Mst1-mediated phosphorylation of Ser14, which allows dissociation of Bcl-xL from Bax and promotes cardiomyocyte death. Here we show that Ser14 phosphorylation of Bcl-xL is also promoted by hemodynamic stress in the heart, through the H-Ras-ERK pathway. Bcl-xL Ser14 phosphorylation-resistant knock-in male mice develop less cardiac hypertrophy and exhibit contractile dysfunction and increased mortality during acute pressure overload. Bcl-xL Ser14 phosphorylation enhances the Ca2+ transient by blocking the inhibitory interaction between Bcl-xL and IP3Rs, thereby promoting Ca2+ release and activation of the calcineurin-NFAT pathway, a Ca2+-dependent mechanism that promotes cardiac hypertrophy. These results suggest that phosphorylation of Bcl-xL at Ser14 in response to acute pressure overload plays an essential role in mediating compensatory hypertrophy by inducing the release of Bcl-xL from IP3Rs, alleviating the negative constraint of Bcl-xL upon the IP3R-NFAT pathway.

Hippo-Yap signaling in cardiac and fibrotic remodeling.

Cardiac injury initiates a tissue remodeling process in which aberrant fibrosis plays a significant part, contributing to impaired contractility of the myocardium and the progression to heart failure. Fibrotic remodeling is characterized by the activation, proliferation, and differentiation of quiescent fibroblasts to myofibroblasts, and the resulting effects on the extracellular matrix and inflammatory milieu. Molecular mechanisms underlying fibroblast fate decisions and subsequent cardiac fibrosis are complex and remain incompletely understood. Emerging evidence has implicated the Hippo-Yap signaling pathway, originally discovered as a fundamental regulator of organ size, as an important mechanism that modulates fibroblast activity and adverse remodeling in the heart, while also exerting distinct cell type-specific functions that dictate opposing outcomes on heart failure. This brief review will focus on Hippo-Yap signaling in cardiomyocytes, cardiac fibroblasts, and other non-myocytes, and present mechanisms by which it may influence the course of cardiac fibrosis and dysfunction.

Mechanisms of Ischemic Heart Injury.

Ischemic heart disease is a leading cause of morbidity and mortality worldwide [...].

AAV-mediated YAP expression in cardiac fibroblasts promotes inflammation and increases fibrosis.

Fibrosis is a hallmark of heart disease independent of etiology and is thought to contribute to impaired cardiac dysfunction and development of heart failure. However, the underlying mechanisms that regulate the differentiation of fibroblasts to myofibroblasts and fibrotic responses remain incompletely defined. As a result, effective treatments to mitigate excessive fibrosis are lacking. We recently demonstrated that the Hippo pathway effector Yes-associated protein (YAP) is an important mediator of myofibroblast differentiation and fibrosis in the infarcted heart. Yet, whether YAP activation in cardiac fibroblasts is sufficient to drive fibrosis, and how fibroblast YAP affects myocardial inflammation, a significant component of adverse cardiac remodeling, are largely unknown. In this study, we leveraged adeno-associated virus (AAV) to target cardiac fibroblasts and demonstrate that chronic YAP expression upregulated indices of fibrosis and inflammation in the absence of additional stress. YAP occupied the Ccl2 gene and promoted Ccl2 expression, which was associated with increased macrophage infiltration, pro-inflammatory cytokine expression, collagen deposition, and cardiac dysfunction in mice with cardiac fibroblast-targeted YAP overexpression. These results are consistent with other recent reports and extend our understanding of YAP function in modulating fibrotic and inflammatory responses in the heart.

Blockade of Fibroblast YAP Attenuates Cardiac Fibrosis and Dysfunction Through MRTF-A Inhibition.

Fibrotic remodeling of the heart in response to injury contributes to heart failure, yet therapies to treat fibrosis remain elusive. Yes-associated protein (YAP) is activated in cardiac fibroblasts by myocardial infarction, and genetic inhibition of fibroblast YAP attenuates myocardial infarction-induced cardiac dysfunction and fibrosis. YAP promotes myofibroblast differentiation and associated extracellular matrix gene expression through engagement of TEA domain transcription factor 1 and subsequent de novo expression of myocardin-related transcription factor A. Thus, fibroblast YAP is a promising therapeutic target to prevent fibrotic remodeling and heart failure.