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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.

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. The anti-apoptotic function of Bcl-xL in the heart is diminished by Mst1-mediated phosphorylation of Serine14. Here, the authors show that the Bcl-xL phosphorylation is also promoted by hemodynamic stress, which plays an essential role in mediating compensatory cardiac hypertrophy and contractility. [ABSTRACT FROM AUTHOR]

Aims Well-controlled mitochondrial homeostasis, including a mitochondria-specific form of autophagy (hereafter referred to as mitophagy), is essential for maintaining cardiac function. The molecular mechanism mediating mitophagy during pressure overload (PO) is poorly understood. We have shown previously that mitophagy in the heart is mediated primarily by Atg5/Atg7-independent mechanisms, including Unc-51-like kinase 1 (Ulk1)-dependent alternative mitophagy , during myocardial ischaemia. Here, we investigated the role of alternative mitophagy in the heart during PO-induced hypertrophy. Methods and results Mitophagy was observed in the heart in response to transverse aortic constriction (TAC), peaking at 3–5 days. Whereas mitophagy is transiently up-regulated by TAC through an Atg7-dependent mechanism in the heart, peaking at 1 day, it is also activated more strongly and with a delayed time course through an Ulk1-dependent mechanism. TAC induced more severe cardiac dysfunction, hypertrophy, and fibrosis in ulk1 cardiac-specific knock-out (cKO) mice than in wild-type mice. Delayed activation of mitophagy was characterized by the co-localization of Rab9 dots and mitochondria and phosphorylation of Rab9 at Ser179, major features of alternative mitophagy. Furthermore, TAC-induced decreases in the mitochondrial aspect ratio were abolished and the irregularity of mitochondrial cristae was exacerbated, suggesting that mitochondrial quality control mechanisms are impaired in ulk1 cKO mice in response to TAC. TAT-Beclin 1 activates mitophagy even in Ulk1-deficient conditions. TAT-Beclin 1 treatment rescued mitochondrial dysfunction and cardiac dysfunction in ulk1 cKO mice during PO. Conclusion Ulk1-mediated alternative mitophagy is a major mechanism mediating mitophagy in response to PO and plays an important role in mediating mitochondrial quality control mechanisms and protecting the heart against cardiac dysfunction. [ABSTRACT FROM AUTHOR]

The Hippo pathway plays a wide variety of roles in response to stress in the heart. Lats2, a component of the Hippo pathway, is phosphorylated by Mst1/2 and, in turn, phosphorylates YAP, causing inactivation of YAP. Lats2 stimulates apoptosis and negatively affects hypertrophy in cardiomyocytes. However, the role of Lats2 during cardiac stress is poorly understood in vivo. Lats2 is activated in the mouse heart in response to transverse aortic constriction (TAC). We used systemic Lats2 +/- mice to elucidate the role of endogenous Lats2. Cardiac hypertrophy and dysfunction induced by 4 weeks of TAC were attenuated in Lats2 +/- mice, and interstitial fibrosis and apoptosis were suppressed. Although TAC upregulated the Bcl-2 family proapoptotic (Bax and Bak) and anti-apoptotic (Bcl-2 and Bcl-xL) molecules in non-transgenic mice, TAC-induced upregulation of Bax and Bak was alleviated and that of Bcl-2 was enhanced in Lats2 +/- mice. TAC upregulated p53, but this upregulation was abolished in Lats2 +/- mice. Lats2-induced increases in apoptosis and decreases in survival in cardiomyocytes were inhibited by Pifithrin-α, a p53 inhibitor, suggesting that Lats2 stimulates apoptosis via a p53-dependent mechanism. In summary, Lats2 is activated by pressure overload, thereby promoting heart failure by stimulating p53-dependent mechanisms of cell death. [ABSTRACT FROM AUTHOR]

Aims A diet with modified components, such as a ketogenic low-carbohydrate (LC) diet, potentially extends longevity and healthspan. However, how an LC diet impacts on cardiac pathology during haemodynamic stress remains elusive. This study evaluated the effects of an LC diet high in either fat (Fat-LC) or protein (Pro-LC) in a mouse model of chronic hypertensive cardiac remodelling. Methods and results Wild-type mice were subjected to transverse aortic constriction, followed by feeding with the Fat-LC, the Pro-LC, or a high-carbohydrate control diet. After 4 weeks, echocardiographic, haemodynamic, histological, and biochemical analyses were performed. LC diet consumption after pressure overload inhibited the development of pathological hypertrophy and systolic dysfunction compared to the control diet. An anti-hypertrophic serine/threonine kinase, GSK-3β, was re-activated by both LC diets; however, the Fat-LC, but not the Pro-LC, diet exerted cardioprotection in GSK-3β cardiac-specific knockout mice. β-hydroxybutyrate, a major ketone body in mammals, was increased in the hearts of mice fed the Fat-LC, but not the Pro-LC, diet. In cardiomyocytes, ketone body supplementation inhibited phenylephrine-induced hypertrophy, in part by suppressing mTOR signalling. Conclusion Strict carbohydrate restriction suppresses pathological cardiac growth and heart failure after pressure overload through distinct anti-hypertrophic mechanisms elicited by supplemented macronutrients. [ABSTRACT FROM AUTHOR]

Summary: High energy production in mitochondria is essential for maintaining cardiac contraction in the heart. Genes regulating mitochondrial function are commonly downregulated during heart failure. Here we show that both PPARα and Sirt1 are upregulated by pressure overload in the heart. Haploinsufficiency of either PPARα or Sirt1 attenuated pressure overload-induced cardiac hypertrophy and failure, whereas simultaneous upregulation of PPARα and Sirt1 exacerbated the cardiac dysfunction. PPARα and Sirt1 coordinately suppressed genes involved in mitochondrial function that are regulated by estrogen-related receptors (ERRs). PPARα bound and recruited Sirt1 to the ERR response element (ERRE), thereby suppressing ERR target genes in an RXR-independent manner. Downregulation of ERR target genes was also observed during fasting, and this appeared to be an adaptive response of the heart. These results suggest that suppression of the ERR transcriptional pathway by PPARα/Sirt1, a physiological fasting response, is involved in the progression of heart failure by promoting mitochondrial dysfunction. [ABSTRACT FROM AUTHOR]

Cardiac hypertrophy is enlargement of the heart in response to physiological or pathological stimuli, chiefly involving growth of myocytes in size rather than in number. Previous studies have shown that the expression pattern of a group of genes in hypertrophied heart induced by pressure overload resembles that at the embryonic stage of heart development, a phenomenon known as activation of the ''fetal gene program''. Here, using a genome-wide approach we systematically defined genes and pathways regulated in short- and long-term cardiac hypertrophy conditions using mice with transverse aortic constriction (TAC), and compared them with those regulated at different stages of embryonic and postnatal development. In addition, exon-level analysis revealed widespread mRNA isoform changes during cardiac hypertrophy resulting from alternative usage of terminal or internal exons, some of which are also developmentally regulated and may be attributable to decreased expression of Fox-1 protein in cardiac hypertrophy. Genes with functions in certain pathways, such as cell adhesion and cell morphology, are more likely to be regulated by alternative splicing. Moreover, we found 39UTRs of mRNAs were generally shortened through alternative cleavage and polyadenylation in hypertrophy, and microRNA target genes were generally de-repressed, suggesting coordinated mechanisms to increase mRNA stability and protein production during hypertrophy. Taken together, our results comprehensively delineated gene and mRNA isoform regulation events in cardiac hypertrophy and revealed their relations to those in development, and suggested that modulation of mRNA isoform expression plays an importance role in heart remodeling under pressure overload. [ABSTRACT FROM AUTHOR]

The article presents a study on the cardiac function of Lats 2, a mammalian homologs of Warts. It evaluates whether Lats 2 mediated the proapoptotic and antihypertrophic actions of mammalian sterile 20-like kinase (Mst) 1 in the heart and increases the apoptosis in a cultured cardiac myocytes. It depicts that Lats 2 affects both the growth and death of cardiac myocytes, but it regulates the size of the heart and serves as an endogenous negative regulator of cardiac hypertrophy.

Introduction: Reactive oxygen species (ROS), including superoxide and hydrogen peroxide, play an important role in the stress response in the heart. It is well known that p22phox is associated with NADPH oxidase (NOX) isoforms, thereby generating ROS in various tissues and organs, including the heart. However, the exact role of p22phox in cardiomyocytes remains to be elucidated. Purpose: To investigate the role of endogenous p22phox during pressure overload (PO). Methods and Results: Expression of p22phox in adult cardiomyocytes (ACM) isolated from mouse hearts subjected to transverse aortic constriction (TAC) for 1 one week was significantly higher than in those from sham- operated mouse hearts, both at the mRNA and protein level. We generated cardiac-specific p22phox knockout (p22phox cKO) mice. These mice did not have any significant cardiac phenotype at baseline. However, one week after TAC, p22phox cKO mice (n=6) exhibited a lower left ventricular (LV) ejection fraction (LVEF) and higher lung weight (LW) and LV weight to tibia length (TL) ratios than control mice (n=4) (LVEF: 64.3±4.3 vs. 46.4±4.8 %;, LW/TL: 8.2±0.3 vs. 13.4±4.8;, LV/TL: 6.0±0.5 vs. 7.5±0.4, p<0.05), indicating that endogenous p22phox plays a protective role against heart failure during the early phase of pressure overload (PO). 4 Four weeks after TAC, p22phox cKO mice (n=6) exhibited further deceases in LVEF and increases in LW/TL, and increased cardiac fibrosis, compared to control mice (n=6) (LVEF: 60.2±18.1 vs. 33.7±10.7 %;, LW/TL: 13.1±7.2 vs. 26.2±4.3;, relative fibrosis: 1±0.75 vs. 4.4±1.0, p<0.02). However, there was no significant difference in LV/TL or cardiomyocyte cell size between p22phox cKO mice and control mice, suggesting that cardiac hypertrophy is suppressed at the later stage of PO. Conclusion: These results indicate that endogenous p22phox is protective against PO and mediates compensatory cardiac hypertrophy during the late phase of PO. [ABSTRACT FROM AUTHOR]

The heart utilizes multiple adaptive mechanisms to maintain pump function. Compensatory cardiac hypertrophy reduces wall stress and oxygen consumption, thereby protecting the heart against acute blood pressure elevation. The nuclear effector of the Hippo pathway, Yes-associated protein 1 (YAP), is activated and mediates compensatory cardiac hypertrophy in response to acute pressure overload (PO). In this study, YAP promoted glycolysis by upregulating glucose transporter 1 (GLUT1), which in turn caused accumulation of intermediates and metabolites of the glycolytic, auxiliary, and anaplerotic pathways during acute PO. Cardiac hypertrophy was inhibited and heart failure was exacerbated in mice with YAP haploinsufficiency in the presence of acute PO. However, normalization of GLUT1 rescued the detrimental phenotype. PO induced the accumulation of glycolytic metabolites, including l-serine, l-aspartate, and malate, in a YAP-dependent manner, thereby promoting cardiac hypertrophy. YAP upregulated the GLUT1 gene through interaction with TEA domain family member 1 (TEAD1) and HIF-1α in cardiomyocytes. Thus, YAP induces compensatory cardiac hypertrophy through activation of the Warburg effect. [ABSTRACT FROM AUTHOR]

Summary: Thioredoxin 1 (Trx1) facilitates the reduction of signaling molecules and transcription factors by cysteine thiol-disulfide exchange, thereby regulating cell growth and death. Here we studied the molecular mechanism by which Trx1 attenuates cardiac hypertrophy. Trx1 upregulates DnaJb5, a heat shock protein 40, and forms a multiple-protein complex with DnaJb5 and class II histone deacetylases (HDACs), master negative regulators of cardiac hypertrophy. Both Cys-274/Cys-276 in DnaJb5 and Cys-667/Cys-669 in HDAC4 are oxidized and form intramolecular disulfide bonds in response to reactive oxygen species (ROS)-generating hypertrophic stimuli, such as phenylephrine, whereas they are reduced by Trx1. Whereas reduction of Cys-274/Cys-276 in DnaJb5 is essential for interaction between DnaJb5 and HDAC4, reduction of Cys-667/Cys-669 in HDAC4 inhibits its nuclear export, independently of its phosphorylation status. Our study reveals a novel regulatory mechanism of cardiac hypertrophy through which the nucleocytoplasmic shuttling of class II HDACs is modulated by their redox modification in a Trx1-sensitive manner. [Copyright &y& Elsevier]

Lysosomal dysfunction caused by mutations in lysosomal genes results in lysosomal storage disorder (LSD), characterized by accumulation of damaged proteins and organelles in cells and functional abnormalities in major organs, including the heart, skeletal muscle, and liver. In LSD, autophagy is inhibited at the lysosomal degradation step and accumulation of autophagosomes is observed. Enlargement of the left ventricle (LV) and contractile dysfunction were observed in RagA/B cardiac-specific KO (cKO) mice, a mouse model of LSD in which lysosomal acidification is impaired irreversibly. YAP, a downstream effector of the Hippo pathway, was accumulated in RagA/B cKO mouse hearts. Inhibition of YAP ameliorated cardiac hypertrophy and contractile dysfunction and attenuated accumulation of autophagosomes without affecting lysosomal function, suggesting that YAP plays an important role in mediating cardiomyopathy in RagA/B cKO mice. Cardiomyopathy was also alleviated by downregulation of Atg7, an intervention to inhibit autophagy, whereas it was exacerbated by stimulation of autophagy. YAP physically interacted with transcription factor EB (TFEB), a master transcription factor that controls autophagic and lysosomal gene expression, thereby facilitating accumulation of autophagosomes without degradation. These results indicate that accumulation of YAP in the presence of LSD promotes cardiomyopathy by stimulating accumulation of autophagosomes through activation of TFEB. [ABSTRACT FROM AUTHOR]

Introduction: Neurofibromin 2 (NF2) is a tumor suppressor that modulates cell proliferation and survival. We previously demonstrated that NF2 mediates cardiomyocyte apoptosis and injury caused by acute myocardial infarction (MI). However, the function of NF2 in the heart remains largely uncharacterized. Our current study sought to determine whether NF2 contributes to cardiac dysfunction and failure due to chronic pressure overload (PO) stress. Methods and Results: To generate PO, we used transverse aortic constriction (TAC) in 8 week old control and cardiomyocyte-specific NF2 knockout (CKO) mice. We found that NF2 is upregulated in mouse myocardium in response to pressure overload. Importantly, we also observed increased NF2 expression in human failing hearts, indicating potential relevance to human disease. At baseline, NF2 CKO mice had normal cardiac morphology and function. However, following TAC, NF2 CKO hearts unexpectedly showed decreased systolic function (LVEF) and increased chamber dilation compared to littermate controls. Hemodynamic analysis revealed decreased dP/dtmax and dP/dtmin in knockout hearts indicating worsened cardiac function in NF2 CKO mice. Interestingly, no difference in cardiac hypertrophy was observed between CKO and control mice. To investigate the mechanism underlying NF2 function, we performed RNAseq. Our analysis indicated downregulation of several metabolic pathways including oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO) in unstressed NF2 CKO hearts, which was confirmed by qPCR. We also determined ATP content and found significantly reduced ATP levels in NF2 CKO hearts at baseline. Lin28, an RNA binding protein and promoter of OXPHOS gene expression, is increased by stress in the heart, but was significantly downregulated in NF2 CKO hearts. Conclusion and Future Studies: Myocardial NF2 is upregulated and may be compensatory during chronic stress through preservation of metabolic gene expression and ATP content in the heart. Ongoing studies will examine mitochondrial function in control and NF2 CKO hearts, and determine whether Lin28 mediates the NF2 CKO phenotype. [ABSTRACT FROM AUTHOR]

Introduction: Well-controlled mitochondrial homeostasis, including mitochondrial autophagy, is essential for maintaining cardiac function. Mitochondrial autophagy (hereafter referred to as mitophagy) is partly mediated by Atg5/Atg7-independent mechanisms, including Unc-51-like kinase1 (Ulk1)-dependent autophagy. Autophagy in the heart is activated transiently in response to pressure overload (PO), and mitophagy is observed even after conventional autophagy is downregulated. Inactivation of mitophagy triggers the development of mitochondrial dysfunction and heart failure during PO. Hypothesis: We investigated whether Ulk1-dependent autophagy mediates mitophagy, thereby protecting the heart during PO, using cardiac-specific Ulk1KO (ulk1 cKO) mice. Methods and Results: ulk1 cKO mice exhibited normal cardiac phenotype and function under basal conditions. We evaluated autophagy by conversion of LC3 and mitophagy by electron microscopy (EM) and using mice with cardiac-specific expression of mito-Keima, a mitochondrially-targeted pH-sensitive fluorescent protein. Conventional autophagy was activated only transiently in both wild-type (WT) and ulk1 cKO mice within 24 hours of transverse aortic constriction (TAC). However, mitophagy was activated 5 days post-TAC in WT mice but not in ulk1 cKO mice (relative mitochondrial Drp1 (fold): 3.05±0.68 vs 0.98±0.10, p <0.05; acidic mitochondria area (%): 8.12±0.72 vs 2.11±0.26, p <0.01). EM analyses confirmed that the number of autophagosomes containing mitochondria 5 days after TAC was significantly lower in ulk1 cKO mice than in WT mice. ulk1 cKO mice displayed aggravated cardiac dysfunction with early development of cardiac hypertrophy and fibrosis compared to WT mice 5 days after TAC (Left ventricular ejection fraction (%): 70.42±0.55 vs. 38.65±2.35, p <0.01; Heart weight/Tibia length (mg/mm): 9.57±0.43 vs 11.47±0.14, p <0.05). In addition, mortality within 1 week after TAC was drastically greater in ulk1 cKO than in WT. Conclusions: These results suggest that TAC-induced mitophagy is mediated through Ulk1-dependent mechanisms and that Ulk1-dependent mechanisms, including mitophagy, play an important role in mediating protection against PO. [ABSTRACT FROM AUTHOR]

Diabetic patients develop cardiomyopathy characterized by hypertrophy and diastolic dysfunction, and intracellular lipid accumulation, termed lipotoxicity. Diabetic hearts utilize fatty acids as a major energy source, which produces high levels of oxidative stress, thereby inducing mitochondrial dysfunction. How mitochondrial function is regulated in diabetic cardiomyopathy is poorly understood. Mice were fed either a normal diet (ND) or high fat diet (HFD, 60 kcal % fat). Although autophagic flux was activated by HFD consumption, peaking at 6 weeks (p<0.05), it was attenuated thereafter. Mitophagy, evaluated with Mito-Keima, was increased after 3 weeks of HFD feeding (mitophagy area: 8.3% per cell with ND and 12.4% with HFD) and continued to increase even after 2 months (p<0.05). By isolating adult cardiomyocytes from GFP-LC3 transgenic mice fed HFD, we confirmed that mitochondria were sequestrated by LC3 positive autophagosomes during mitophagy. In wild type (WT) mice, cardiac hypertrophy, diastolic dysfunction (EDPVR = 0.051±0.009 in ND and 0.11±0.004 in HFD) and lipid accumulation occurred within 2 months of HFD feeding (p<0.05). Deletion of atg7 impaired mitophagy, increased lipid accumulation, exacerbated diastolic dysfunction (EDPVR=0.11±0.004 in WT and 0.152±0.019 in atg7 cKO, p<0.05) and induced systolic dysfunction (ESPVR=24.86±2.46 in WT and 15.93±1.76 in atg7 cKO, p<0.05) during HFD feeding. Deletion of Parkin partially inhibited mitophagy, increased lipid accumulation and exacerbated diastolic dysfunction (EDPVR=0.124±0.005 in WT and 0.176±0.018 in Parkin KO, p<0.05) in response to HFD feeding. Injection of Tat-Beclin1 (TB1), a peptide that activates autophagy, activated mitophagy, attenuated mitochondrial dysfunction, decreased lipid accumulation, and protected against cardiac diastolic dysfunction (EDPVR=0.110±0.009 in Control peptide and 0.078±0.015 in TB1, p<0.05) during HFD feeding. In summary, mitophagy serves as an essential quality control mechanism for mitochondria in the heart during HFD feeding. Impairment of mitophagy induces mitochondrial dysfunction and lipid accumulation, thereby exacerbating the development of diabetic cardiomyopathy. Conversely, activation of mitophagy protects against HFD-induced diabetic cardiomyopathy. [ABSTRACT FROM AUTHOR]

Background: Mitochondrial autophagy is an important mediator of mitochondrial quality control in cardiomyocytes. The occurrence of mitochondrial autophagy and its significance during cardiac hypertrophy are not well understood.Methods and Results: Mice were subjected to transverse aortic constriction (TAC) and observed at multiple time points up to 30 days. Cardiac hypertrophy developed after 5 days, the ejection fraction was reduced after 14 days, and heart failure was observed 30 days after TAC. General autophagy was upregulated between 1 and 12 hours after TAC but was downregulated below physiological levels 5 days after TAC. Mitochondrial autophagy, evaluated by electron microscopy, mitochondrial content, and Keima with mitochondrial localization signal, was transiently activated at ≈3 to 7 days post-TAC, coinciding with mitochondrial translocation of Drp1. However, it was downregulated thereafter, followed by mitochondrial dysfunction. Haploinsufficiency of Drp1 abolished mitochondrial autophagy and exacerbated the development of both mitochondrial dysfunction and heart failure after TAC. Injection of Tat-Beclin 1, a potent inducer of autophagy, but not control peptide, on day 7 after TAC, partially rescued mitochondrial autophagy and attenuated mitochondrial dysfunction and heart failure induced by overload. Haploinsufficiency of either drp1 or beclin 1 prevented the rescue by Tat-Beclin 1, suggesting that its effect is mediated in part through autophagy, including mitochondrial autophagy.Conclusions: Mitochondrial autophagy is transiently activated and then downregulated in the mouse heart in response to pressure overload. Downregulation of mitochondrial autophagy plays an important role in mediating the development of mitochondrial dysfunction and heart failure, whereas restoration of mitochondrial autophagy attenuates dysfunction in the heart during pressure overload. [ABSTRACT FROM AUTHOR]

Mammalian sterile 20-like kinase 1 (Mst1) is a mammalian homolog of Drosophila Hippo, the master regulator of cell death, proliferation, and organ size in flies. It is the chief component of the mammalian Hippo pathway and promotes apoptosis and inhibits compensatory cardiac hypertrophy, playing a critical role in mediating heart failure. How Mst1 is regulated, however, remains unclear. Using genetically altered mice in which expression of the tumor suppressor Ras-association domain family 1 isoform A (Rassf1A) was modulated in a cell type-specific manner, we demonstrate here that Rassf1A is an endogenous activator of Mst1 in the heart. Although the Rassf1A/Mst1 pathway promoted apoptosis in cardiomyocytes, thereby playing a detrimental role, the same pathway surprisingly inhibited fibroblast proliferation and cardiac hypertrophy through both cell-autonomous and autocrine/paracrine mechanisms, playing a protective role during pressure overload. In cardiac fibroblasts, the Rassf1A/Mst1 pathway negatively regulated TNF-α, a key mediator of hypertrophy, fibrosis, and resulting cardiac dysfunction. These results suggest that the functional consequence of activating the proapoptotic Rassf1A/Mst1 pathway during pressure overload is cell type dependent in the heart and that suppressing this mechanism in cardiac fibroblasts could be detrimental. [ABSTRACT FROM AUTHOR]

Lysosomal storage disorders (LSDs) cause severe cardiomyopathy with impairment of autophagy. There is currently no effective treatment for LSDs. Rag proteins play a critical role in regulating lysosomal functions. Muscle-specific deletion of RagA and RagB (RagA/B) suppresses autophagy and induces cardiomyopathy reminiscent of LSDs. Here, we investigated the underlying molecular mechanism of cardiomyopathy in RagA/B knockout (KO) mice. We generated cardiac-specific RagA/BKO (RagA/BcKO) mice, using αMHC-Cre. RagA/BcKO mice exhibited prominent cardiac hypertrophy (CH) (heart weight (HW)/ tibial length (TL): RagA/BcKO 12.35 ± 0.41 vs. control 7.12 ± 0.15, p< 0.01) at 3 months of age. RagA/BcKO mice exhibited significantly more phospho-histone H3-positive cardiomyocytes (CMs) than control mice, suggesting that CM proliferation is stimulated. However, RagA/BcKO mice exhibited a significantly smaller fractional shortening (%FS: 23.0 ± 0.82 vs. 45.8 ± 1.06, p< 0.01) and significantly higher mortality than control mice (50% vs. 0%, p<0.01). Furthermore, RNA sequencing analysis revealed significant upregulation of genes associated with Yes-associated protein (YAP), a major transcriptional target of the Hippo pathway that controls cell growth and survival. Indeed, YAP accumulated significantly more (5.9-fold, p<0.05) in RagA/BcKO hearts than in control hearts. To elucidate the role of YAP in mediating cardiomyopathy, we crossed RagA/BcKO mice with cardiac-specific heterozygous YAPKO mice. Heterozygous deletion of YAP in RagA/BcKO mice attenuated CH and improved cardiac function. Furthermore, pharmacological inhibition of YAP with verteporfin treatment also ameliorated cardiomyopathy in RagA/BcKO mice. YAP interacts with transcriptional factor EB (TFEB), a master transcription factor that controls autophagic and lysosomal gene expression, in the presence of lysosomal dysfunction. Inhibition of YAP suppresses death of cardiomyocytes by reducing accumulation of autophagosomes without normalizing lysosomal dysfunction. These results indicate that YAP promotes cardiac dysfunction in LSDs, possibly through stimulation of TFEB and accumulation of autophagosomes. YAP may be targeted for treatment of cardiomyopathy in LSDs. [ABSTRACT FROM AUTHOR]