Monitoring β-arrestin recruitment via β-lactamase enzyme fragment complementation: purification of peptide E as a low-affinity ligand for mammalian bombesin receptors

Identification of cognate ligands for G protein-coupled receptors (GPCRs) provides a starting point for understanding novel regulatory mechanisms. Although GPCR ligands have typically been evaluated through the activation of heterotrimeric G proteins, recent studies have shown that GPCRs signal not only through G proteins but also through β-arrestins. As such, monitoring β-arrestin signaling instead of G protein signaling will increase the likelihood of identifying currently unknown ligands, including β-arrestin-biased agonists. Here, we developed a cell-based assay for monitoring ligand-dependent GPCR-β-arrestin interaction via β-lactamase enzyme fragment complementation. Inter alia, β-lactamase is a superior reporter enzyme because of its cell-permeable fluorescent substrate. This substrate makes the assay non-destructive and compatible with fluorescence-activated cell sorting (FACS). In a reporter cell, complementary fragments of β-lactamase (α and ω) were fused to β-arrestin 2 and GPCR, respectively. Ligand stimulation initiated the interaction of these chimeric proteins (β-arrestin-α and GPCR-ω), and this inducible interaction was measured through reconstituted β-lactamase activity. Utilizing this system, we screened various mammalian tissue extracts for agonistic activities on human bombesin receptor subtype 3 (hBRS3). We purified peptide E as a low-affinity ligand for hBRS3, which was also found to be an agonist for the other two mammalian bombesin receptors such as gastrin-releasing peptide receptor (GRPR) and neuromedin B receptor (NMBR). Successful purification of peptide E has validated the robustness of this assay. We conclude that our newly developed system will facilitate the discovery of GPCR ligands.

Quantitative Measurement of GPCR Endocytosis via Pulse-Chase Covalent Labeling

G protein-coupled receptors (GPCRs) play a critical role in many physiological systems and represent one of the largest families of signal-transducing receptors. The number of GPCRs at the cell surface regulates cellular responsiveness to their cognate ligands, and the number of GPCRs, in turn, is dynamically controlled by receptor endocytosis. Recent studies have demonstrated that GPCR endocytosis, in addition to affecting receptor desensitization and resensitization, contributes to acute G protein-mediated signaling. Thus, endocytic GPCR behavior has a significant impact on various aspects of physiology. In this study, we developed a novel GPCR internalization assay to facilitate characterization of endocytic GPCR behavior. We genetically engineered chimeric GPCRs by fusing HaloTag (a catalytically inactive derivative of a bacterial hydrolase) to the N-terminal end of the receptor (HT-GPCR). HaloTag has the ability to form a stable covalent bond with synthetic HaloTag ligands that contain fluorophores or a high-affinity handle (such as biotin) and the HaloTag reactive linker. We selectively labeled HT-GPCRs at the cell surface with a HaloTag PEG ligand, and this pulse-chase covalent labeling allowed us to directly monitor the relative number of internalized GPCRs after agonist stimulation. Because the endocytic activities of GPCR ligands are not necessarily correlated with their agonistic activities, applying this novel methodology to orphan GPCRs, or even to already characterized GPCRs, will increase the likelihood of identifying currently unknown ligands that have been missed by conventional pharmacological assays.

Modulation of cardiac fibrosis by Krüppel-like factor 6 through transcriptional control of thrombospondin 4 in cardiomyocytes

AIMS

Krüppel-like factors (KLFs) are a family of transcription factors which play important roles in the heart under pathological and developmental conditions. We previously identified and cloned Klf6 whose homozygous mutation in mice results in embryonic lethality suggesting a role in cardiovascular development. Effects of KLF6 on pathological regulation of the heart were investigated in the present study.

METHODS AND RESULTS

Mice heterozygous for Klf6 resulted in significantly diminished levels of cardiac fibrosis in response to angiotensin II infusion. Intriguingly, a similar phenotype was seen in cardiomyocyte-specific Klf6 knockout mice, but not in cardiac fibroblast-specific knockout mice. Microarray analysis revealed increased levels of the extracellular matrix factor, thrombospondin 4 (TSP4), in the Klf6-ablated heart. Mechanistically, KLF6 directly suppressed Tsp4 expression levels, and cardiac TSP4 regulated the activation of cardiac fibroblasts to regulate cardiac fibrosis.

CONCLUSION

Our present studies on the cardiac function of KLF6 show a new mechanism whereby cardiomyocytes regulate cardiac fibrosis through transcriptional control of the extracellular matrix factor, TSP4, which, in turn, modulates activation of cardiac fibroblasts.

Phosphodiesterase 9A controls nitric-oxide-independent cGMP and hypertrophic heart disease

Cyclic guanosine monophosphate (cGMP) is a second messenger molecule that transduces nitric oxide (NO) and natriuretic peptide (NP) coupled signaling, stimulating phosphorylation changes by protein kinase G (PKG). Enhancing cGMP synthesis or blocking its degradation by phosphodiesterase type 5A (PDE5A) protects against cardiovascular disease^1,2. However, cGMP stimulation alone is limited by counter-adaptions including PDE upregulation³. Furthermore, though PDE5A regulates NO-generated cGMP^4,5, NO-signaling is often depressed by heart disease⁶. PDEs controlling NP-coupled cGMP remain uncertain. Here we show that cGMP-selective PDE9A^7,8 is expressed in mammalian heart including humans, and is upregulated by hypertrophy and cardiac failure. PDE9A regulates NP rather than NO-stimulated cGMP in heart myocytes and muscle, and its genetic or selective pharmacological inhibition protects against pathological responses to neuro-hormones, and sustained pressure-overload stress. PDE9A inhibition reverses pre-established heart disease independent of NO-synthase (NOS) activity, whereas PDE5A inhibition requires active NOS. Transcription factor activation and phospho-proteome analyses of myocytes with each PDE selectively inhibited reveals substantial differential targeting, with phosphorylation changes from PDE5A inhibition being more sensitive to NOS activation. Thus, unlike PDE5A, PDE9A can regulate cGMP signaling independent of the NO-pathway, and its role in stress-induced heart disease suggests potential as a therapeutic target.

Regulation of Mitochondrial Dynamics by Dynamin-Related Protein-1 in Acute Cardiorenal Syndrome

Experimental evidence has clarified distant organ dysfunctions induced by AKI. Crosstalk between the kidney and heart, which has been recognized recently as cardiorenal syndrome, appears to have an important role in clinical settings, but the mechanisms by which AKI causes cardiac injury remain poorly understood. Both the kidney and heart are highly energy-demanding organs that are rich in mitochondria. Therefore, we investigated the role of mitochondrial dynamics in kidney-heart organ crosstalk. Renal ischemia reperfusion (IR) injury was induced by bilateral renal artery clamping for 30 min in 8-week-old male C57BL/6 mice. Electron microscopy showed a significant increase of mitochondrial fragmentation in the heart at 24 h. Cardiomyocyte apoptosis and cardiac dysfunction, evaluated by echocardiography, were observed at 72 h. Among the mitochondrial dynamics regulating molecules, dynamin-related protein 1 (Drp1), which regulates fission, and mitofusin 1, mitofusin 2, and optic atrophy 1, which regulate fusion, only Drp1 was increased in the mitochondrial fraction of the heart. A Drp1 inhibitor, mdivi-1, administered before IR decreased mitochondrial fragmentation and cardiomyocyte apoptosis significantly and improved cardiac dysfunction induced by renal IR. This study showed that renal IR injury induced fragmentation of mitochondria in a fission-dominant manner with Drp1 activation and subsequent cardiomyocyte apoptosis in the heart. Furthermore, cardiac dysfunction induced by renal IR was improved by Drp1 inhibition. These data suggest that mitochondrial fragmentation by fission machinery may be a new therapeutic target in cardiac dysfunction induced by AKI.

Soluble guanylate cyclase is required for systemic vasodilation but not positive inotropy induced by nitroxyl in the mouse

Nitroxyl (HNO), the reduced and protonated form of nitric oxide (NO·), confers unique physiological effects including vasorelaxation and enhanced cardiac contractility. These features have spawned current pharmaceutical development of HNO donors as heart failure therapeutics. HNO interacts with selective redox sensitive cysteines to effect signaling but is also proposed to activate soluble guanylate cyclase (sGC) in vitro to induce vasodilation and potentially enhance contractility. Here, we tested whether sGC stimulation is required for these HNO effects in vivo and if HNO also modifies a redox-sensitive cysteine (C42) in protein kinase G-1α to control vasorelaxation. Intact mice and isolated arteries lacking the sGC-β subunit (sGCKO, results in full sGC deficiency) or expressing solely a redox-dead C42S mutant protein kinase G-1α were exposed to the pure HNO donor, CXL-1020. CXL-1020 induced dose-dependent systemic vasodilation while increasing contractility in controls; however, vasodilator effects were absent in sGCKO mice whereas contractility response remained. The CXL-1020 dose reversing 50% of preconstricted force in aortic rings was ≈400-fold greater in sGCKO than controls. Cyclic-GMP and cAMP levels were unaltered in myocardium exposed to CXL-1020, despite its inotropic-vasodilator activity. In protein kinase G-1α(C42S) mice, CXL-1020 induced identical vasorelaxation in vivo and in isolated aortic and mesenteric vessels as in littermate controls. In both groups, dilation was near fully blocked by pharmacologically inhibiting sGC. Thus, sGC and cGMP-dependent signaling are necessary and sufficient for HNO-induced vasodilation in vivo but are not required for positive inotropic action. Redox modulation of protein kinase G-1α is not a mechanism for HNO-mediated vasodilation.

Cardiac troponin I Pro82Ser variant induces diastolic dysfunction, blunts β-adrenergic response, and impairs myofilament cooperativity

Troponin I (TnI) variant Pro82Ser (cTnIP82S) was initially considered a disease-causing mutation; however, later studies suggested the contrary. We tested the hypothesis of whether a causal link exists between cTnIP82S and cardiac structural and functional remodeling, such as during aging or chronic pressure overload. A cardiac-specific transgenic (Tg) mouse model of cTnIP82S was created to test this hypothesis. During aging, Tg cTnIP82S displayed diastolic dysfunction, characterized by longer isovolumetric relaxation time, and impaired ejection and relaxation time. In young, Tg mice in vivo pressure-volume loops and intact trabecular preparations revealed normal cardiac contractility at baseline. However, upon β-adrenergic stimulation, a blunted contractile reserve and no hastening in left ventricle relaxation were evident in vivo, whereas, in isolated muscles, Ca(2+) transient amplitude isoproterenol dose-response was blunted. In addition, when exposed to chronic pressure overload, Tg mice show exacerbated hypertrophy and decreased contractility compared with age-matched non-Tg littermates. At the molecular level, this mutation significantly impairs myofilament cooperative activation. Importantly, this occurs in the absence of alterations in TnI or myosin-binding protein C phosphorylation. The cTnIP82S variant occurs near a region of interactions with troponin T; therefore, structural changes in this region could explain its meaningful effects on myofilament cooperativity. Our data indicate that cTnIP82S mutation modifies age-dependent diastolic dysfunction and impairs overall contractility after β-adrenergic stimulation or chronic pressure overload. Thus cTnIP82S variant should be regarded as a disease-modifying factor for dysfunction and adverse remodeling with aging and chronic pressure overload.

Abstract 292: Brain Derived Neurotrophic Factor Induced Upregulation Of Peroxisome Proliferator-activated Receptor Gamma Coactivator 1-alpha Signaling Prevents Hearts From Heart Failure Progression Against Pressure Overload

Background: The heart is under the influence of neurotrophins (NTs) secreted from peripheral sympathetic nerves, including brain derived neurotrophic factor (BDNF). BDNF is indispensible for cardiac development and vascular wall integrity. Yet, whether BDNF signaling plays a role in governing cardiac function in response to stress is unclear.

Hypothesis: BDNF signaling contributes to maintain proper cardiac structure/function in pressure overloaded mice.

Results: BDNF expression is markedly down-regulated in hearts subjected to transverse aortic constriction (TAC). Cardiac specific over-expression of BDNF (BDNF-TG) or administration of a BDNF-mimetic agonist (LM22A-4) preserved cardiac function against pressure overload. In contrast, cardiac-selective deletion of the BDNF receptor, Tropomyosin related kinase receptor B (TrkB), further exacerbated heart failure. In neurons, BDNF up-regulates Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) that regulates energy metabolism and mitochondrial function/biogenesis. Oxidative stress is a major negative modulator of PGC-1a expression/activity. Exposing neonatal rat ventricular myocytes (NRVMs) to hydrogen peroxide downregulated PGC-1α, and BDNF restored it to normal levels, with a concomitant up-regulation of downstream genes involved in both mitochondrial biogenesis and oxidative stress, resulting in attenuated ROS production and increased mitochondrial biogenesis. Consistent with the cultured myocyte findings, PGC-1α and downstream genes were up-regulated in BDNF-TG mice subjected to TAC, associated with attenuated oxidative stress and improved mitochondrial biogenesis; whereas TrkB-/- mice subjected to TAC displayed further decreased PGC-1α expression with worsened oxidative stress and impaired mitochondrial biogenesis.

Conclusion: Our data show that BDNF confers protection against pressure overload via enhanced PGC-1α signaling that in turn prevents oxidative stress and improves mitochondrial biogenesis. Our data suggest BDNF/trkB is a promising new therapeutic avenue to prevent or retard heart failure.

PDE5 inhibitor efficacy is estrogen dependent in female heart disease

Inhibition of cGMP-specific phosphodiesterase 5 (PDE5) ameliorates pathological cardiac remodeling and has been gaining attention as a potential therapy for heart failure. Despite promising results in males, the efficacy of the PDE5 inhibitor sildenafil in female cardiac pathologies has not been determined and might be affected by estrogen levels, given the hormone's involvement in cGMP synthesis. Here, we determined that the heart-protective effect of sildenafil in female mice depends on the presence of estrogen via a mechanism that involves myocyte eNOS-dependent cGMP synthesis and the cGMP-dependent protein kinase Iα (PKGIα). Sildenafil treatment failed to exert antiremodeling properties in female pathological hearts from Gαq-overexpressing or pressure-overloaded mice after ovary removal; however, estrogen replacement restored the effectiveness of sildenafil in these animals. In females, sildenafil-elicited myocardial PKG activity required estrogen, which stimulated tonic cardiomyocyte cGMP synthesis via an eNOS/soluble guanylate cyclase pathway. In contrast, eNOS activation, cGMP synthesis, and sildenafil efficacy were not estrogen dependent in male hearts. Estrogen and sildenafil had no impact on pressure-overloaded hearts from animals expressing dysfunctional PKGIα, indicating that PKGIα mediates antiremodeling effects. These results support the importance of sex differences in the use of PDE5 inhibitors for treating heart disease and the critical role of estrogen status when these agents are used in females.

Hypoxia-induced mitogenic factor (FIZZ1/RELMα) induces endothelial cell apoptosis and subsequent interleukin-4-dependent pulmonary hypertension

Pulmonary hypertension (PH) is characterized by elevated pulmonary artery pressure that leads to progressive right heart failure and ultimately death. Injury to endothelium and consequent wound repair cascades have been suggested to trigger pulmonary vascular remodeling, such as that observed during PH. The relationship between injury to endothelium and disease pathogenesis in this disorder remains poorly understood. We and others have shown that, in mice, hypoxia-induced mitogenic factor (HIMF, also known as FIZZ1 or RELMα) plays a critical role in the pathogenesis of lung inflammation and the development of PH. In this study, we dissected the mechanism by which HIMF and its human homolog resistin (hRETN) induce pulmonary endothelial cell (EC) apoptosis and subsequent lung inflammation-mediated PH, which exhibits many of the hallmarks of the human disease. Systemic administration of HIMF caused increases in EC apoptosis and interleukin (IL)-4-dependent vascular inflammatory marker expression in mouse lung during the early inflammation phase. In vitro, HIMF, hRETN, and IL-4 activated pulmonary microvascular ECs (PMVECs) by increasing angiopoietin-2 expression and induced PMVEC apoptosis. In addition, the conditioned medium from hRETN-treated ECs had elevated levels of endothelin-1 and caused significant increases in pulmonary vascular smooth muscle cell proliferation. Last, HIMF treatment caused development of PH that was characterized by pulmonary vascular remodeling and right heart failure in wild-type mice but not in IL-4 knockout mice. These data suggest that HIMF contributes to activation of vascular inflammation at least in part by inducing EC apoptosis in the lung. These events lead to subsequent PH.

Inhibiting mitochondrial Na+/Ca2+ exchange prevents sudden death in a Guinea pig model of heart failure

RATIONALE

In cardiomyocytes from failing hearts, insufficient mitochondrial Ca(2+) accumulation secondary to cytoplasmic Na(+) overload decreases NAD(P)H/NAD(P)(+) redox potential and increases oxidative stress when workload increases. These effects are abolished by enhancing mitochondrial Ca(2+) with acute treatment with CGP-37157 (CGP), an inhibitor of the mitochondrial Na(+)/Ca(2+) exchanger.

OBJECTIVE

Our aim was to determine whether chronic CGP treatment mitigates contractile dysfunction and arrhythmias in an animal model of heart failure (HF) and sudden cardiac death (SCD).

METHODS AND RESULTS

Here, we describe a novel guinea pig HF/SCD model using aortic constriction combined with daily β-adrenergic receptor stimulation (ACi) and show that chronic CGP treatment (ACi plus CGP) attenuates cardiac hypertrophic remodeling, pulmonary edema, and interstitial fibrosis and prevents cardiac dysfunction and SCD. In the ACi group 4 weeks after pressure overload, fractional shortening and the rate of left ventricular pressure development decreased by 36% and 32%, respectively, compared with sham-operated controls; in contrast, cardiac function was completely preserved in the ACi plus CGP group. CGP treatment also significantly reduced the incidence of premature ventricular beats and prevented fatal episodes of ventricular fibrillation, but did not prevent QT prolongation. Without CGP treatment, mortality was 61% in the ACi group <4 weeks of aortic constriction, whereas the death rate in the ACi plus CGP group was not different from sham-operated animals.

CONCLUSIONS

The findings demonstrate the critical role played by altered mitochondrial Ca(2+) dynamics in the development of HF and HF-associated SCD; moreover, they reveal a novel strategy for treating SCD and cardiac decompensation in HF.

Monoamine oxidase B prompts mitochondrial and cardiac dysfunction in pressure overloaded hearts

AIMS

Monoamine oxidases (MAOs) are mitochondrial flavoenzymes responsible for neurotransmitter and biogenic amines catabolism. MAO-A contributes to heart failure progression via enhanced norepinephrine catabolism and oxidative stress. The potential pathogenetic role of the isoenzyme MAO-B in cardiac diseases is currently unknown. Moreover, it is has not been determined yet whether MAO activation can directly affect mitochondrial function.

RESULTS

In wild type mice, pressure overload induced by transverse aortic constriction (TAC) resulted in enhanced dopamine catabolism, left ventricular (LV) remodeling, and dysfunction. Conversely, mice lacking MAO-B (MAO-B(-/-)) subjected to TAC maintained concentric hypertrophy accompanied by extracellular signal regulated kinase (ERK)1/2 activation, and preserved LV function, both at early (3 weeks) and late stages (9 weeks). Enhanced MAO activation triggered oxidative stress, and dropped mitochondrial membrane potential in the presence of ATP synthase inhibitor oligomycin both in neonatal and adult cardiomyocytes. The MAO-B inhibitor pargyline completely offset this change, suggesting that MAO activation induces a latent mitochondrial dysfunction, causing these organelles to hydrolyze ATP. Moreover, MAO-dependent aldehyde formation due to inhibition of aldehyde dehydrogenase 2 activity also contributed to alter mitochondrial bioenergetics.

INNOVATION

Our study unravels a novel role for MAO-B in the pathogenesis of heart failure, showing that both MAO-driven reactive oxygen species production and impaired aldehyde metabolism affect mitochondrial function.

CONCLUSION

Under conditions of chronic hemodynamic stress, enhanced MAO-B activity is a major determinant of cardiac structural and functional disarrangement. Both increased oxidative stress and the accumulation of aldehyde intermediates are likely liable for these adverse morphological and mechanical changes by directly targeting mitochondria.

Nitroxyl (HNO): A novel approach for the acute treatment of heart failure

BACKGROUND

The nitroxyl (HNO) donor, Angeli's salt, exerts positive inotropic, lusitropic, and vasodilator effects in vivo that are cAMP independent. Its clinical usefulness is limited by chemical instability and cogeneration of nitrite which itself has vascular effects. Here, we report on effects of a novel, stable, pure HNO donor (CXL-1020) in isolated myoctyes and intact hearts in experimental models and in patients with heart failure (HF).

METHODS AND RESULTS

CXL-1020 converts solely to HNO and inactive CXL-1051 with a t1/2 of 2 minutes. In adult mouse ventricular myocytes, it dose dependently increased sarcomere shortening by 75% to 210% (50-500 μmol/L), with a ≈30% rise in the peak Ca(2+) transient only at higher doses. Neither inhibition of protein kinase A nor soluble guanylate cyclase altered this contractile response. Unlike isoproterenol, CXL-1020 was equally effective in myocytes from normal or failing hearts. In anesthetized dogs with coronary microembolization-induced HF, CXL-1020 reduced left ventricular end-diastolic pressure and myocardial oxygen consumption while increasing ejection fraction from 27% to 40% and maximal ventricular power index by 42% (both P<0.05). In conscious dogs with tachypacing-induced HF, CXL-1020 increased contractility assessed by end-systolic elastance and provided venoarterial dilation. Heart rate was minimally altered. In patients with systolic HF, CXL-1020 reduced both left and right heart filling pressures and systemic vascular resistance, while increasing cardiac and stroke volume index. Heart rate was unchanged, and arterial pressure declined modestly.

CONCLUSIONS

These data show the functional efficacy of a novel pure HNO donor to enhance myocardial function and present first-in-man evidence for its potential usefulness in HF.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifiers: NCT01096043, NCT01092325.

Abstract 191: Phosphodiesterase 9a Modulates Cardiomyocyte Natriuretic Peptide-Stimulated cGMP Pool and Cardiac Hypertrophy

Cardiac hypertrophy in response to pathologic stress is a major contributor to heart disease. Many studies have revealed that compartmentalized cyclic guanosine monophosphate (cGMP) regulation by phosphodiesterases (PDEs) can potently modify acute and chronic cardiac stress responses. PDE5a normally regulates cGMP generated from nitric oxide (NO)-stimulated soluble guanylate cyclase (sGC) but not natriuretic peptide (NP)-stimulated cGMP. Its inhibition blunts maladaptive hypertrophy and remodeling. Regulators of NP-cGMP remain uncertain. Here we reveal that PDE9a, a highly cGMP-specific PDE (×100 affinity vs PDE5a) that is expressed predominantly in the brain modulates myocyte cGMP as well, targeting NP-stimulated pools. PDE9a gene expression is observed in rat neonatal cardiac myocytes (RNCM) and adult mouse myocytes. Protein expression was identified in myocytes by immunohistochemistry, using gene silencing models as a negative control. PDE9a expression is upregulated by various hypertrophic stimuli (phenylephrine (PE) or endothelin-1 (ET-1), and observed in myocytes from pressure-overloaded whole myocardium. PDE9a inhibition with a selective antagonist (PF-9613) or gene silencing significantly abrogated PE- or ET-1-dependent upregulation of pathological-hypertrophy fetal genes and NFAT activity in RNCM, and PDE9a inhibition attenuated ET-1-induced hypertrophy in adult mouse myocytes. PF-9613 had no impact in cells lacking PDE9a (from knockout mice) or after gene silencing. Biochemical and cGMP-sensitive fluorescent probe studies shows PDE9a inhibition augments ANP- but not NO-dependent cGMP stimuli in RNCM and adult myocytes. PDE9a knockout (KO) mice were protected against hypertrophy/remodeling after pressure-overload, accompanied by a rise in myocardial cGMP. In PDE9a KO mice also exhibited less fibrosis and reduced expression of fibrotsis-related genes over wild-type mice. Together, these data identify PDE9a as a novel regulator of myocyte cGMP and hypertrophy that impacts a different compartmentalized cGMP pool to that by PDE5a. Our observations may provide a novel therapeutic approach in the treatment of heart failure.

Mutation of the protein kinase I alpha leucine zipper domain produces hypertension and progressive left ventricular hypertrophy: a novel mouse model of age-dependent hypertensive heart disease

Hypertensive heart disease causes significant mortality in older patients, yet there is an incomplete understanding of molecular mechanisms that regulate age-dependent hypertensive left ventricular hypertrophy (LVH). Therefore, we tested the hypothesis that the cGMP-dependent protein kinase G I alpha (PKGIα) attenuates hypertensive LVH by evaluating the cardiac phenotype in mice with selective mutations of the PKGIα leucine zipper domain. These leucine zipper mutant (LZM) mice develop basal hypertension. Compared with wild-type controls, 8-month-old adult LZM mice developed increased left ventricular end-diastolic pressure but without frank LVH. In advanced age (15 months), the LZM mice developed overt pathological LVH. These findings reveal a role of PKGIα in normally attenuating hypertensive LVH. Therefore, mutation of the PKGIα LZ domain produces a clinically relevant model for hypertensive heart disease of aging.

Protein kinase g iα inhibits pressure overload-induced cardiac remodeling and is required for the cardioprotective effect of sildenafil in vivo

BACKGROUND

Cyclic GMP (cGMP) signaling attenuates cardiac remodeling, but it is unclear which cGMP effectors mediate these effects and thus might serve as novel therapeutic targets. Therefore, we tested whether the cGMP downstream effector, cGMP-dependent protein kinase G Iα (PKGIα), attenuates pressure overload-induced remodeling in vivo.

METHODS AND RESULTS

The effect of transaortic constriction (TAC)-induced left ventricular (LV) pressure overload was examined in mice with selective mutations in the PKGIα leucine zipper interaction domain. Compared with wild-type littermate controls, in response to TAC, these Leucine Zipper Mutant (LZM) mice developed significant LV systolic and diastolic dysfunction by 48 hours (n=6 WT sham, 6 WT TAC, 5 LZM sham, 9 LZM TAC). In response to 7-day TAC, the LZM mice developed increased pathologic hypertrophy compared with controls (n=5 WT sham, 4 LZM sham, 8 WT TAC, 11 LZM TAC). In WT mice, but not in LZM mice, phosphodiesterase 5 (PDE5) inhibition with sildenafil (Sil) significantly inhibited TAC-induced cardiac hypertrophy and LV systolic dysfunction in WT mice, but this was abolished in the LZM mice (n=3 WT sham, 4 LZM sham, 3 WT TAC vehicle, 6 LZM TAC vehicle, 4 WT TAC Sil, 6 LZM TAC Sil). And in response to prolonged, 21-day TAC (n=8 WT sham, 7 LZM sham, 21 WT TAC, 15 LZM TAC), the LZM mice developed markedly accelerated mortality and congestive heart failure. TAC induced activation of JNK, which inhibits cardiac remodeling in vivo, in WT, but not in LZM, hearts, identifying a novel signaling pathway activated by PKGIα in the heart in response to LV pressure overload.

CONCLUSIONS

These findings reveal direct roles for PKGIα in attenuating pressure overload-induced remodeling in vivo and as a required effector for the cardioprotective effects of sildenafil.

Abstract 242: Cardiac Remodeling Induced by Chronic β-Adrenergic Simulation Is Blocked by Myocyte-TGF-β Signaling

[Background] Emerging evidence from cell-specific conditional gene manipulation models has revealed complex interactions with differential roles of cardiomyocyte (CM) and non-CM signaling in the evolution of pathological cardiac remodeling. A major factor involved in such cross talk is transforming growth factor-beta (TGFβ), which exists in CM,fibroblasts, and vascular cells. Both mechanical overload and prolonged catecholamine stimulation are critical determinants of hypertensive heart disease. Here, we tested the role of cell-specific TGFβ signaling in chronic pressure-overload or isoproterenol (Iso)-induced cardiac remodeling using CM-specific gene suppression of TGFβ type2 receptor in mice.

[Methods and Results] αMHC-driven tamoxifen-inducible Cre (MCM) x Tgfbr2 floxed mice (MCMR2), which achieved CM-specific knockdown of TGFβ signaling, showed a striking suppression of cardiac dilatation and dysfunction induced by chronic pressure-overload. Chronic Iso infusion induced modest cardiac hypertrophy with moderate myocardial fibrosis. Interestingly, in contrast to TAC, myocardial fibrosis induced by the chronic Iso exposure was not inhibited, rather worsened in MCMR2 mice. Systemic treatment with TGFβ neutralizing antibody (NAb) for Chronic Iso significantly inhibited myocardial fibrosis, yet cardiac function and hypertrophy were not improved. In Iso-treated cultured cardiomyocytes, profibrotic genes were up-regulated by TGFβ-receptor inhibition, while they were inhibited in cultured cardiac fibroblasts.

[Conclusion] CM-specific TGFβ signaling inhibition has marked protective effect for pressure-overload induced cardiac remodeling but not for Iso-induced cardiac remodeling. These results suggest that the role of TGFβ signaling may be determined by targeted cell types and pathological stresses in a development of heart failure.

Abstract 97: Ablation of Calpain-Targeted Site in Cardiac Myosin Binding Protein C Is Cardioprotective

Rationale: Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is essential for normal heart function. We recently demonstrated a direct correlation between cMyBP-C dephosphorylation and its degradation during ischemia-reperfusion (I-R) injury. Strikingly, cMyBP-C phosphorylation protects the heart from I-R injury. However, the mechanism of cMyBP-C-mediated cardioprotection is unknown.

Objective: To determine if preventing cMyBP-C proteolysis and cleavage confers cardioprotection during I-R injury.

Methods and Results: cMyBP-C is a substrate for calpains and dephosphorylation makes cMyBP-C more susceptible to proteolysis. In patients with heart failure, such proteolysis leads to the cleavage and release of the predominant N'-fragment (40 kDa) in association with increased calpain activity. MS/MS analysis identified a cardiac-specific phosphorylation motif responsible for the release of the 40 kDa fragment, 272-TSLAGAGRR-280, which we term the calpain-targeted site (CTS). Next, we utilized cardiac-specific transgenic mice expressing cMyBP-C[[Unable to Display Character: △]]CTS in which CTS motif was ablated and bred into cMyBP-C null (t/t) mice. We hypothesized that ablation of the CTS motif would confer resistance to calpain-mediated proteolysis and thus protect the heart from I-R injury. An animal that transgenically expressed the normal cardiac isoform, cMyBP-CWT, served as a control. Histological, electron microscopic and immunofluorescence analyses confirm that cMyBP-C[[Unable to Display Character: &#8710;]]CTS incorporates normally into the cardiac sarcomere and results in complete rescue of the cMyBP-Ct/t phenotype, compared control groups. Moreover, echocardiography and in vivo hemodynamic studies revealed that cMyBP-C[[Unable to Display Character: &#8710;]]CTS:t/t mice have normal cardiac function, similar to control mice. Altogether, these data suggest that cMyBP-C[[Unable to Display Character: &#8710;]]CTS is benign. Compared to cMyBP-CWT, cMyBP-C[[Unable to Display Character: &#8710;]]CTS protein is protected from calpain-mediated degradation. Finally, we determined that ablation of the CTS reduced infarct size and preserved cMyBP-C stability and cardiac function during I-R injury.

Conclusion: Our data suggest that preserving cMyBP-C stability by thwarting calpain-mediated proteolysis protects cardiac structure and function from I-R injury, as a supportive therapy.

Cyclic GMP-dependent signaling in cardiac myocytes

Cyclic GMP (cGMP) and its effector kinase PKG regulate diverse cellular functions. In cardiac myocytes, cGMP is produced by soluble and particulate guanylyl cyclases (GCs), the former stimulated by nitric oxide and the latter by natriuretic peptides, and is hydrolyzed to inactive 5'-GMP by cGMP-phosphodiesterases (PDEs). cGMP-PKG modulates cardiac contractility, hypertrophy and remodeling, and exerts cardioprotection. Although early research efforts have mostly focused on cGMP synthetic pathways, recent studies have revealed that cGMP degradation controlled by PDEs plays a critical role in the physiological action of cGMP. Among several cGMP-PDEs, cGMP-specific PDE5 has been intensively investigated. Studies in experimental animal models and humans consistently demonstrate benefits from PDE5 inhibitors in various cardiac pathologies. Several clinical trials are ongoing or planned to test the efficacy of PDE5 inhibitors in human heart disease, including a large multicenter clinical trial (RELAX) led by the NIH evaluating sildenafil efficacy in heart failure with preserved ejection fraction. This review underscores the current understanding of cGMP-PKG signal regulation and its pathophysiological role in the heart, focusing on cardiac myocytes.

Pathological cardiac hypertrophy alters intracellular targeting of phosphodiesterase type 5 from nitric oxide synthase-3 to natriuretic peptide signaling

BACKGROUND

In the normal heart, phosphodiesterase type 5 (PDE5) hydrolyzes cGMP coupled to nitric oxide- (specifically from nitric oxide synthase 3) but not natriuretic peptide (NP)-stimulated guanylyl cyclase. PDE5 is upregulated in hypertrophied and failing hearts and is thought to contribute to their pathophysiology. Because nitric oxide signaling declines whereas NP-derived cGMP rises in such diseases, we hypothesized that PDE5 substrate selectivity is retargeted to blunt NP-derived signaling.

METHODS AND RESULTS

Mice with cardiac myocyte inducible PDE5 overexpression (P5(+)) were crossed to those lacking nitric oxide synthase 3 (N3(-)), and each model, the double cross, and controls were subjected to transaortic constriction. P5(+) mice developed worse dysfunction and hypertrophy and enhanced NP stimulation, whereas N3(-) mice were protected. However, P5(+)/N3(-) mice behaved similarly to P5(+) mice despite the lack of nitric oxide synthase 3-coupled cGMP generation, with protein kinase G activity suppressed in both models. PDE5 inhibition did not alter atrial natriuretic peptide-stimulated cGMP in the resting heart but augmented it in the transaortic constriction heart. This functional retargeting was associated with PDE5 translocation from sarcomeres to a dispersed distribution. P5(+) hearts exhibited higher oxidative stress, whereas P5(+)/N3(-) hearts had low levels (likely owing to the absence of nitric oxide synthase 3 uncoupling). This highlights the importance of myocyte protein kinase G activity as a protection for pathological remodeling.

CONCLUSIONS

These data provide the first evidence for functional retargeting of PDE5 from one compartment to another, revealing a role for natriuretic peptide-derived cGMP hydrolysis by this esterase in diseased heart myocardium. Retargeting likely affects the pathophysiological consequence and the therapeutic impact of PDE5 modulation in heart disease.

Galphas-biased beta2-adrenergic receptor signaling from restoring synchronous contraction in the failing heart

Cardiac resynchronization therapy (CRT), in which both ventricles are paced to recoordinate contraction in hearts that are dyssynchronous from conduction delay, is the only heart failure (HF) therapy to date to clinically improve acute and chronic function while also lowering mortality. CRT acutely enhances chamber mechanical efficiency but chronically alters myocyte signaling, including improving β-adrenergic receptor reserve. We speculated that the latter would identify unique CRT effects that might themselves be effective for HF more generally. HF was induced in dogs by 6 weeks of atrial rapid pacing with (HFdys, left bundle ablated) or without (HFsyn) dyssynchrony. We used dyssynchronous followed by resynchronized tachypacing (each 3 weeks) for CRT. Both HFdys and HFsyn myocytes had similarly depressed rest and β-adrenergic receptor sarcomere and calcium responses, particularly the β2-adrenergic response, whereas cells subjected to CRT behaved similarly to those from healthy controls. CRT myocytes exhibited suppressed Gαi signaling linked to increased regulator of G protein (heterotrimeric guanine nucleotide-binding protein) signaling (RGS2, RGS3), yielding Gαs-biased β2-adrenergic responses. This included increased adenosine cyclic AMP responsiveness and activation of sarcoplasmic reticulum-localized protein kinase A. Human CRT responders also showed up-regulated myocardial RGS2 and RGS3. Inhibition of Gαi (with pertussis toxin, RGS3, or RGS2 transfection), stimulation with a Gαs-biased β2 agonist (fenoterol), or transient (2-week) exposure to dyssynchrony restored β-adrenergic receptor responses in HFsyn to the values obtained after CRT. These results identify a key pathway that is triggered by restoring contractile synchrony and that may represent a new therapeutic approach for a broad population of HF patients.

Pressure-overload-induced subcellular relocalization/oxidation of soluble guanylyl cyclase in the heart modulates enzyme stimulation

RATIONALE

Soluble guanylyl cyclase (sGC) generates cyclic guanosine monophophate (cGMP) upon activation by nitric oxide (NO). Cardiac NO-sGC-cGMP signaling blunts cardiac stress responses, including pressure-overload-induced hypertrophy. The latter itself depresses signaling through this pathway by reducing NO generation and enhancing cGMP hydrolysis.

OBJECTIVE

We tested the hypothesis that the sGC response to NO also declines with pressure-overload stress and assessed the role of heme-oxidation and altered intracellular compartmentation of sGC as potential mechanisms.

METHODS AND RESULTS

C57BL/6 mice subjected to transverse aortic constriction (TAC) developed cardiac hypertrophy and dysfunction. NO-stimulated sGC activity was markedly depressed, whereas NO- and heme-independent sGC activation by BAY 60-2770 was preserved. Total sGCα(1) and β(1) expression were unchanged by TAC; however, sGCβ(1) subunits shifted out of caveolin-enriched microdomains. NO-stimulated sGC activity was 2- to 3-fold greater in Cav3-containing lipid raft versus nonlipid raft domains in control and 6-fold greater after TAC. In contrast, BAY 60-2770 responses were >10 fold higher in non-Cav3 domains with and without TAC, declining about 60% after TAC within each compartment. Mice genetically lacking Cav3 had reduced NO- and BAY-stimulated sGC activity in microdomains containing Cav3 for controls but no change within non-Cav3-enriched domains.

CONCLUSIONS

Pressure overload depresses NO/heme-dependent sGC activation in the heart, consistent with enhanced oxidation. The data reveal a novel additional mechanism for reduced NO-coupled sGC activity related to dynamic shifts in membrane microdomain localization, with Cav3-microdomains protecting sGC from heme-oxidation and facilitating NO responsiveness. Translocation of sGC out of this domain favors sGC oxidation and contributes to depressed NO-stimulated sGC activity.

Abstract P249: Cardiac Troponin I Pro82Ser Variant Impairs Myofilament Cooperativity, Induces Diastolic Dysfunction, and Blunts β-Adrenergic Response

Cardiac troponin I is a key regulatory protein for muscle contraction and necessary for an adequate β-adrenergic response. The Proline 82 Serine (P82S) sequence variant of troponin I gene ( TNNI3 ) is associated with late onset hypertrophic cardiomyopathy (HCM). Our preliminary data has shown that cTnIP82S is heterozygous in 3% of African Americans and is associated with increased LV mass in hypertensive black men. We hypothesize that cTnIP82S variant could influence the hypertrophic response to hypertension and/or the malignancy of the phenotypic expression when combined with known HCM disease causing myofilament mutations. We created a transgenic cTnIP82S mouse model. Echocardiography at baseline in older Tg and NTg mice (64-65 wks) showed impaired ejection time and relaxation, Tg display longer isovolumetric relaxation time (IVRT) (Tg 29.7±0.26 vs NTg 23± 0.23 msec, n=8 vs n=7 p<0.05) and a significantly higher TEI index. In the young NTg and Tg (17 wks) there were no echocardiographic differences. However, preliminary data suggest that 1 wk after transverse aortic coarctation (TAC), diastolic dysfunction was evident in Tg, and 10 weeks after TAC LV mass was also increased compared to NTg. Skinned fibers studies showed in the Tg mice borderline lower maximal Force (F max ) and myofilament calcium sensitivity, whereas n Hill showed a marked depression (Tg 1.72±0.17 vs NTg 3.09±0.44, n=7 vs n=5, p<0.05), this effect was not due to the global myofilament phosphorylation pattern. In addition, intact twitching cardiac muscle studies in Tg mice revealed an impaired dose-dependent β-adrenergic acceleration of relaxation, evidenced by a failure to increase relaxation ([-dF/dt min]/[+dF/dt max]) and accelerate calcium transients decay after isoproterenol. Pressure-volume (PV) loop studies confirm that Tg mice fail to increase: Δ change in dP/dt max (NTg 4,928.3±7.7 vs Tg 843.3±10.7, n=4 vs n=4 p<0.05) in response to isoproterenol. TnIP82S variant is near a region of TnI-TnT interaction; this could explain its dramatic effects on myofilament cooperativity. Overall these studies suggest that the expression of cTnIP82S variant in the mice heart induces diastolic dysfunction, impairs relaxation at baseline and after β-adrenergic stimulation.

Bi-modal dose-dependent cardiac response to tetrahydrobiopterin in pressure-overload induced hypertrophy and heart failure

The exogenous administration of tetrahydrobiopterin (BH4), an essential cofactor of nitric oxide synthase (NOS), has been shown to reduce left ventricular hypertrophy, fibrosis, and cardiac dysfunction in mice with pre-established heart disease induced by pressure-overload. In this setting, BH4 re-coupled endothelial NOS (eNOS), with subsequent reduction of NOS-dependent oxidative stress and reversal of maladaptive remodeling. However, recent studies suggest the effective BH4 dosing may be narrower than previously thought, potentially due to its oxidation upon oral consumption. Accordingly, we assessed the dose response of daily oral synthetic sapropterin dihydrochloride (6-R-l-erythro-5,6,7,8-tetrahydrobiopterin, 6R-BH4) on pre-established pressure-overload cardiac disease. Mice (n=64) were administered 0-400mg/kg/d BH4 by ingesting small pre-made pellets (consumed over 15-30 min). In a dose range of 36-200mg/kg/d, 6R-BH4 suppressed cardiac chamber remodeling, hypertrophy, fibrosis, and oxidative stress with pressure-overload. However, at both lower and higher doses, BH4 had less or no ameliorative effects. The effective doses correlated with a higher myocardial BH4/BH2 ratio. However, BH2 rose linearly with dose, and at the 400mg/kg/d, this lowered the BH4/BH2 ratio back toward control. These results expose a potential limitation for the clinical use of BH4, as variability of cellular redox and perhaps heart disease could produce a variable therapeutic window among individuals. This article is part of a special issue entitled ''Key Signaling Molecules in Hypertrophy and Heart Failure.''

Pivotal role of cardiomyocyte TGF-β signaling in the murine pathological response to sustained pressure overload

The cardiac pathological response to sustained pressure overload involves myocyte hypertrophy and dysfunction along with interstitial changes such as fibrosis and reduced capillary density. These changes are orchestrated by mechanical forces and factors secreted between cells. One such secreted factor is TGF-β, which is generated by and interacts with multiple cell types. Here we have shown that TGF-β suppression in cardiomyocytes was required to protect against maladaptive remodeling and involved noncanonical (non-Smad-related) signaling. Mouse hearts subjected to pressure overload and treated with a TGF-β-neutralizing Ab had suppressed Smad activation in the interstitium but not in myocytes, and noncanonical (TGF-β-activated kinase 1 [TAK1]) activation remained. Although fibrosis was greatly reduced, chamber dysfunction and dilation persisted. Induced myocyte knockdown of TGF-β type 2 receptor (TβR2) blocked all maladaptive responses, inhibiting myocyte and interstitial Smad and TAK1. Myocyte knockdown of TβR1 suppressed myocyte but not interstitial Smad, nor TAK1, modestly reducing fibrosis without improving chamber function or hypertrophy. Only TβR2 knockdown preserved capillary density after pressure overload, enhancing BMP7, a regulator of the endothelial-mesenchymal transition. BMP7 enhancement also was coupled to TAK1 suppression. Thus, myocyte targeting is required to modulate TGF-β in hearts subjected to pressure overload, with noncanonical pathways predominantly affecting the maladaptive hypertrophy/dysfunction.

RGS2 is a primary terminator of β₂-adrenergic receptor-mediated G(i) signaling

Two major β-adrenergic receptor (βAR) subtypes, β(1)AR and β(2)AR, are expressed in mammalian heart with β(1)AR coupling to G(s) and β(2)AR dually coupling to G(s) and G(i) proteins. In many types of chronic heart failure, myocardial contractile response to both β(1)AR and β(2)AR stimulation is severely impaired. The dysfunction of βAR signaling in failing hearts is largely attributable to an increase in G(i) signaling, because disruption of the G(i) signaling restores myocardial contractile response to β(1)AR as well as β(2)AR stimulation. However, the mechanism terminating the β(2)AR-G(i) signaling remains elusive, while it has been shown activation of the G(i) signaling is dependent on agonist stimulation and subsequent PKA-mediated phosphorylation of the receptor. Here we demonstrate that regulator of G protein signaling 2 (RGS2) is a primary terminator of the β(2)AR-G(i) signaling. Specifically, prolonged absence of agonist stimulation for 24h impairs the β(2)AR-G(i) signaling, resulting in enhanced β(2)AR- but not β(1)AR-mediated contractile response in cultured adult mouse cardiomyocytes. Increased β(2)AR contractile response is accompanied by a selective upregulation of RGS2 in the absence of alterations in other major cardiac RGS proteins (RGS3-5) or G(s), G(i) or βAR subtypes. Administration of a βAR agonist, isoproterenol (ISO, 1.0 nM), prevents RGS2 upregulation and restores the β(2)AR-G(i) signaling in cultured cells. Furthermore, RGS2 ablation, similar to βAR agonist stimulation, sustains the β(2)AR-G(i) signaling in cultured cells, whereas adenoviral overexpression of RGS2 suppresses agonist-activated β(2)AR-G(i) signaling in cardiomyocytes and HEK293 cells. These findings not only define RGS2 as a novel negative regulator of the β(2)AR-G(i) signaling but also provide a potential novel target for the treatment of chronic heart failure.

Myocardial remodeling is controlled by myocyte-targeted gene regulation of phosphodiesterase type 5

OBJECTIVES

we tested the hypothesis that bi-directional, gene-targeted regulation of cardiomyocyte cyclic guanosine monophosphate-selective phosphodiesterase type 5 (PDE5) influences maladaptive remodeling in hearts subjected to sustained pressure overload.

BACKGROUND

PDE5 expression is up-regulated in human hypertrophied and failing hearts, and its inhibition (e.g., by sildenafil) stimulates protein kinase G activity, suppressing and reversing maladaptive hypertrophy, fibrosis, and contractile dysfunction. Sildenafil is currently being clinically tested for the treatment of heart failure. However, researchers of new studies have questioned the role of myocyte PDE5 and protein kinase G (PKG) to this process, proposing alternative targets and mechanisms.

METHODS

mice with doxycycline-controllable myocyte-specific PDE5 gene expression were generated (medium transgenic [TG] and high TG expression lines) and subjected to sustained pressure overload.

RESULTS

Rest myocyte and heart function, histology, and molecular profiling were normal in both TG lines versus controls at 2 months of age. However, upon exposure to pressure overload (aortic banding), TG hearts developed more eccentric remodeling, maladaptive molecular signaling, depressed function, and amplified fibrosis with up-regulation of tissue growth factor signaling pathways. PKG activation was inhibited in TG myocytes versus controls. After establishing a severe cardiomyopathic state, high-TG mice received doxycycline to suppress PDE5 expression/activity only in myocytes. This in turn enhanced PKG activity and reversed all previously amplified maladaptive responses, despite sustained pressure overload. Sildenafil was also effective in this regard.

CONCLUSIONS

these data strongly support a primary role of myocyte PDE5 regulation to myocardial pathobiology and PDE5 targeting therapy in vivo and reveal a novel mechanism of myocyte-orchestrated extracellular matrix remodeling via PDE5/cyclic guanosine monophosphate-PKG regulatory pathways.

Cyclic GMP/PKG-dependent inhibition of TRPC6 channel activity and expression negatively regulates cardiomyocyte NFAT activation Novel mechanism of cardiac stress modulation by PDE5 inhibition

Increased cyclic GMP from enhanced synthesis or suppressed catabolism (e.g. PDE5 inhibition by sildenafil, SIL) activates protein kinase G (PKG) and blunts cardiac pathological hypertrophy. Suppressed calcineurin (Cn)-NFAT (nuclear factor of activated T-cells) signaling appears to be involved, though it remains unclear how this is achieved. One potential mechanism involves activation of Cn/NFAT by calcium entering via transient receptor potential canonical (TRPC) channels (notably TRPC6). Here, we tested the hypothesis that PKG blocks Cn/NFAT activation by modifying and thus inhibiting TRPC6 current to break the positive feedback loop involving NFAT and NFAT-dependent TRPC6 upregulation. TRPC6 expression rose with pressure-overload in vivo, and angiotensin (ATII) or endothelin (ET1) stimulation in neonatal and adult cardiomyocytes in vitro. 8Br-cGMP and SIL reduced ET1-stimulated TRPC6 expression and NFAT dephosphorylation (activity). TRPC6 upregulation was absent if its promoter was mutated with non-functional NFAT binding sites, whereas constitutively active NFAT triggered TRPC6 expression that was not inhibited by SIL. PKG phosphorylated TRPC6, and both T70 and S322 were targeted. Both sites were functionally relevant, as 8Br-cGMP strongly suppressed current in wild-type TRPC6 channels, but not in those with phospho-silencing mutations (T70A, S322A or S322Q). NFAT activation and increased protein synthesis stimulated by ATII or ET1 was blocked by 8Br-cGMP or SIL. However, transfection with T70A or S322Q TRPC6 mutants blocked this inhibitory effect, whereas phospho-mimetic mutants (T70E, S322E, and both combined) suppressed NFAT activation. Thus PDE5-inhibition blocks TRPC6 channel activation and associated Cn/NFAT activation signaling by PKG-dependent channel phosphorylation.

Monoamine oxidase A-mediated enhanced catabolism of norepinephrine contributes to adverse remodeling and pump failure in hearts with pressure overload

RATIONALE

Monoamine oxidases (MAOs) are mitochondrial enzymes that catabolize prohypertrophic neurotransmitters, such as norepinephrine and serotonin, generating hydrogen peroxide. Because excess reactive oxygen species and catecholamines are major contributors to the pathophysiology of congestive heart failure, MAOs could play an important role in this process.

OBJECTIVE

Here, we investigated the role of MAO-A in maladaptive hypertrophy and heart failure.

METHODS AND RESULTS

We report that MAO-A activity is triggered in isolated neonatal and adult myocytes on stimulation with norepinephrine, followed by increase in cell size, reactive oxygen species production, and signs of maladaptive hypertrophy. All of these in vitro changes occur, in part, independently from alpha- and beta-adrenergic receptor-operated signaling and are inhibited by the specific MAO-A inhibitor clorgyline. In mice with left ventricular dilation and pump failure attributable to pressure overload, norepinephrine catabolism by MAO-A is increased accompanied by exacerbated oxidative stress. MAO-A inhibition prevents these changes, and also reverses fetal gene reprogramming, metalloproteinase and caspase-3 activation, as well as myocardial apoptosis. The specific role of MAO-A was further tested in mice expressing a dominant-negative MAO-A (MAO-A(neo)), which were more protected against pressure overload than their wild-type littermates.

CONCLUSIONS

In addition to adrenergic receptor-dependent mechanisms, enhanced MAO-A activity coupled with increased intramyocardial norepinephrine availability results in augmented reactive oxygen species generation, contributing to maladaptive remodeling and left ventricular dysfunction in hearts subjected to chronic stress.

Avoidance of transient cardiomyopathy in cardiomyocyte-targeted tamoxifen-induced MerCreMer gene deletion models

Cardiac myocyte targeted MerCreMer transgenic mice expressing tamoxifen-inducible Cre driven by the alpha-myosin heavy chain promoter are increasingly used to control gene expression in the adult heart. Here, we show tamoxifen-mediated MerCreMer (MCM) nuclear translocation can induce severe transient dilated cardiomyopathy in mice with or without loxP transgenes. The cardiomyopathy is accompanied by marked reduction of energy/metabolism and calcium-handling gene expression (eg, PGC1-alpha, peroxisome proliferator-activated alpha, SERCA2A), all fully normalized with recovery. MCM-negative/flox-positive controls display no dysfunction with tamoxifen. Nuclear Cre translocation and equally effective gene knockdown without cardiomyopathy is achievable with raloxifene, suggesting toxicity is not simply from Cre. Careful attention to controls, reduced tamoxifen dosing and/or use of raloxifene is advised with this model.

Sildenafil stops progressive chamber, cellular, and molecular remodeling and improves calcium handling and function in hearts with pre-existing advanced hypertrophy caused by pressure overload

OBJECTIVE

This study sought to test the efficacy of phosphodiesterase type 5A (PDE5A) inhibition for treating advanced hypertrophy/remodeling caused by pressure overload, and to elucidate cellular and molecular mechanisms for this response.

BACKGROUND

Sildenafil (SIL) inhibits cyclic guanosine monophosphate-specific PDE5A and can blunt the evolution of cardiac hypertrophy and dysfunction in mice subjected to pressure overload. Whether and how it ameliorates more established advanced disease and dysfunction is unknown.

METHODS

Mice were subjected to transverse aortic constriction (TAC) for 3 weeks to establish hypertrophy/dilation, and subsequently treated with SIL (100 mg/kg/day) or placebo for 6 weeks of additional TAC.

RESULTS

The SIL arrested further progressive chamber dilation, dysfunction, fibrosis, and molecular remodeling, increasing myocardial protein kinase G activity. Isolated myocytes from TAC-SIL hearts showed greater sarcomere shortening and relaxation, and enhanced Ca(2+) transients and decay compared with nontreated TAC hearts. The SIL treatment restored gene and protein expression of sarcoplasmic reticulum Ca(2+) uptake adenosine triphosphatase (SERCA2a), phospholamban (PLB), and increased PLB phosphorylation (S16), consistent with improved calcium handling. The phosphatase calcineurin (Cn) and/or protein kinase C-alpha (PKCalpha) can both lower phosphorylated phospholamban and depress myocyte calcium cycling. The Cn expression and PKCalpha activation (outer membrane translocation) were enhanced by chronic TAC and reduced by SIL treatment. Expression of PKCdelta and PKCepsilon also increased with TAC but were unaltered by SIL treatment.

CONCLUSIONS

SIL treatment applied to well-established hypertrophic cardiac disease can prevent further cardiac and myocyte dysfunction and progressive remodeling. This is associated with improved calcium cycling, and reduction of Cn and PKCalpha activation may be important to this improvement.

Regulator of G protein signaling 2 mediates cardiac compensation to pressure overload and antihypertrophic effects of PDE5 inhibition in mice

The heart initially compensates for hypertension-mediated pressure overload by enhancing its contractile force and developing hypertrophy without dilation. Gq protein-coupled receptor pathways become activated and can depress function, leading to cardiac failure. Initial adaptation mechanisms to reduce cardiac damage during such stimulation remain largely unknown. Here we have shown that this initial adaptation requires regulator of G protein signaling 2 (RGS2). Mice lacking RGS2 had a normal basal cardiac phenotype, yet responded rapidly to pressure overload, with increased myocardial Gq signaling, marked cardiac hypertrophy and failure, and early mortality. Swimming exercise, which is not accompanied by Gq activation, induced a normal cardiac response, while Rgs2 deletion in Galphaq-overexpressing hearts exacerbated hypertrophy and dilation. In vascular smooth muscle, RGS2 is activated by cGMP-dependent protein kinase (PKG), suppressing Gq-stimulated vascular contraction. In normal mice, but not Rgs2-/- mice, PKG activation by the chronic inhibition of cGMP-selective phosphodiesterase 5 (PDE5) suppressed maladaptive cardiac hypertrophy, inhibiting Gq-coupled stimuli. Importantly, PKG was similarly activated by PDE5 inhibition in myocardium from both genotypes, but PKG plasma membrane translocation was more transient in Rgs2-/- myocytes than in controls and was unaffected by PDE5 inhibition. Thus, RGS2 is required for early myocardial compensation to pressure overload and mediates the initial antihypertrophic and cardioprotective effects of PDE5 inhibitors.

Phosphodiesterase 5 inhibition blocks pressure overload-induced cardiac hypertrophy independent of the calcineurin pathway

AIMS

Cyclic GMP (cGMP)-specific phosphodiesterase 5 (PDE5) inhibition by sildenafil (SIL) activates myocardial cGMP-dependent protein kinase G (PKG) and blunts cardiac hypertrophy. To date, the only documented target of PKG in myocardium is the serine-threonine phosphatase calcineurin (Cn), which is central to pathological cardiac hypertrophy. We tested whether Cn suppression is necessary in order to observe anti-hypertrophic effects of SIL.

METHODS AND RESULTS

Mice lacking the Cn-Abeta subunit (CnAbeta(-/-)) and wild-type (WT) controls were subjected to transverse aorta constriction (TAC) with or without SIL (200 mg/kg/day, p.o.) for 3 weeks. TAC-induced elevation of Cn expression and activity in WT was absent in CnAbeta(-/-) hearts, and the latter accordingly developed less cardiac hypertrophy (50 vs. 100% increase in heart weight/tibia length, P < 0.03) and chamber dilation. SIL remained effective in CnAbeta(-/-) mice, increasing PKG activity similarly as in WT, suppressing hypertrophy and fetal gene expression, and enhancing heart function without altering afterload. TAC-stimulated calcium-calmodulin kinase II, Akt, and glycogen synthase kinase 3beta in both groups (the first rising more in CnAbeta(-/-) hearts), and SIL also suppressed these similarly. Activation of extracellular signal-regulated kinase observed in WT-TAC but not CnAbeta(-/-) hearts was also suppressed by SIL.

CONCLUSION

PDE5A inhibition and its accompanying PKG activation blunt hypertrophy and improve heart function even without Cn activation. This occurs by its modulation of several alternative pathways which may result from concomitant distal targeting, or activity against a common proximal node.

Expression, activity, and pro-hypertrophic effects of PDE5A in cardiac myocytes

Cyclic GMP-selective phosphodiesterase type 5 (PDE5) has been traditionally thought to play a little role in cardiac myocytes, yet recent studies using selective inhibitors such as sildenafil suggest it can potently modulate acute and chronic cardiac stress responses. To date, evidence for myocyte PDE5 expression and regulation has relied on small-molecule inhibitors and anti-sera, leaving open concerns regarding non-specific immune-reactivity, and off-target drug effects. To directly address both issues, we engineered a robust PDE5-gene silencing shRNA (inserted into miRNA-155 cassette) and DsRed-PDE5 fusion protein, both coupled to a CMV promoter and incorporated into adenoviral vectors. PDE5 mRNA and protein knock-down eliminated anti-sera positivity on immunoblots and fluorescent immuno-histochemistry in neonatal and adult cardiomyocytes, and suppressed PDE5 enzyme activity. Stimulation of myocyte hypertrophy by phenylephrine was blunted by PDE5 gene silencing in a protein kinase G dependent manner, and this effect was similar to that from sildenafil with no additive response by both combined. DsRed-PDE5 fusion protein expression showed normal z-band localization in adult myocytes but was diffused in eNOS(-/-) myocytes; echoing reported findings with anti-sera. PDE5 overexpression increased enzyme activity and amplified natriuretic peptide gene expression from phenylephrine stimulation. These data confirm PDE5 expression, activity, and targeted inhibition by sildenafil in cardiomyocytes, as well as the role of this PDE in cardiomyocyte hypertrophy modulation.

Exposure to inhaled particulate matter impairs cardiac function in senescent mice

Daily exposure to particulate matter (PM) is known to adversely affect cardiac function and is also known to be exaggerated with senescence. This study tests the hypothesis that cardiac function is uniquely altered by PM exposure in senescent mice. A mechanism for PM-induced cardiac effects is also postulated by examining the activity of nitric oxide synthase (NOS) and the generation of reactive oxygen species (ROS) in heart tissue. Echocardiography is performed in awake 18- and 28-mo-old mice at baseline and immediately following 3-h exposures to either filtered air or carbon black (CB; approximately 400 microg/m3) on 4 days. At 28 mo, left ventricular diameter at end-systole and end-diastole is significantly (P < 0.05) elevated, and fractional shortening is significantly reduced (49 +/- 3% vs. 56 +/- 3%) with CB exposure. In vivo hemodynamic measurements at 28 mo also demonstrate significant (P < 0.05) reductions in ejection fraction and increases in right ventricular and pulmonary vascular pressures following CB exposure. Functional changes at 28 mo are associated with increased ROS production as suggested by enhanced luminol activity. This elevated ROS production with aging and CB exposure is attributable to NOS uncoupling. Measurements of natriuretic peptide (atrial and brain) transcription and matrix metalloproteinase (MMP2 and MMP9) activity in heart tissue are significantly (P < 0.05) amplified with senescence and exposure to CB, pointing to increased cardiac stress and remodeling. These results demonstrate that acute PM exposure reduces cardiac contractility in senescent mice, and this decline in function is associated with increased ROS production linked to NOS uncoupling.

Reversal of cardiac hypertrophy and fibrosis from pressure overload by tetrahydrobiopterin: efficacy of recoupling nitric oxide synthase as a therapeutic strategy

BACKGROUND

Sustained pressure overload induces pathological cardiac hypertrophy and dysfunction. Oxidative stress linked to nitric oxide synthase (NOS) uncoupling may play an important role. We tested whether tetrahydrobiopterin (BH4) can recouple NOS and reverse preestablished advanced hypertrophy, fibrosis, and dysfunction.

METHODS AND RESULTS

C57/Bl6 mice underwent transverse aortic constriction for 4 weeks, increasing cardiac mass (190%) and diastolic dimension (144%), lowering ejection fraction (-46%), and triggering NOS uncoupling and oxidative stress. Oral BH4 was then administered for 5 more weeks of pressure overload. Without reducing loading, BH4 reversed hypertrophy and fibrosis, recoupled endothelial NOS, lowered oxidant stress, and improved chamber and myocyte function, whereas untreated hearts worsened. If BH4 was started at the onset of pressure overload, it did not suppress hypertrophy over the first week when NOS activity remained preserved even in untreated transverse aortic constriction hearts. However, BH4 stopped subsequent remodeling when NOS activity was otherwise declining. A broad antioxidant, Tempol, also reduced oxidant stress yet did not recouple NOS or reverse worsened hypertrophy/fibrosis from sustained transverse aortic constriction. Microarray analysis revealed very different gene expression profiles for both treatments. BH4 did not enhance net protein kinase G activity. Finally, transgenic mice with enhanced BH4 synthesis confined to endothelial cells were unprotected against pressure overload, indicating that exogenous BH4 targeted myocytes and fibroblasts.

CONCLUSIONS

NOS recoupling by exogenous BH4 ameliorates preexisting advanced cardiac hypertrophy/fibrosis and is more effective than a less targeted antioxidant approach (Tempol). These data highlight the importance of myocyte NOS uncoupling in hypertrophic heart disease and support BH4 as a potential new approach to treat this disorder.

Sustained soluble guanylate cyclase stimulation offsets nitric-oxide synthase inhibition to restore acute cardiac modulation by sildenafil

Phosphodiesterase type 5 (PDE5) inhibitors are used to treat erectile dysfunction, and growing evidence supports potential cardiovascular utility. Their efficacy declines with reduced nitric-oxide synthase (NOS) activity common to various diseases. We tested whether direct soluble guanylate cyclase (sGC) stimulation restores in vivo cardiovascular modulation by PDE5 inhibition despite acute or chronically suppressed NOS activity. Mice (C57/Bl6; n = 62) were studied by in vivo pressure-volume analysis to assess acute modulation by the PDE5 inhibitor sildenafil (SIL; 100 microg/kg/min) of the cardiac response to isoproterenol (ISO) with or without NOS inhibition [N(omega)-nitro-L-arginine methyl ester (L-NAME)] and cotreatment by the sGC stimulator 2-[1-(2-fluorobenzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-5-(4-morpholinyl)pyrimidine-4,6-diamine (BAY 41-8543). SIL induced mild vasodilation but no basal cardiac effects and markedly blunted ISO-stimulated contractility. Acute BAY 41-8543 at a dose lacking cardiovascular effects did not alter ISO responses. However, after acute L-NAME, SIL ceased to influence cardiovascular function, but adding BAY 41-8543 fully restored SIL effects. After 1 week of L-NAME, neither SIL nor SIL + BAY 41-8543 acutely induced vasodilation or blunted ISO responses. However, sustained BAY 41-8543 despite concurrent NOS inhibition restored the cardiovascular efficacy of SIL. The disparity between acute and chronic NOS inhibition related to diffusion of PDE5 away from myocyte z-bands coupled with reduced protein kinase G activation. Both were restored by sustained sGC costimulation. Thus, PDE5 regulation of adrenergic reserve and systemic vasodilation depends upon NOS-induced cGMP/protein kinase G and can be enhanced by sustained low-level stimulation of sGC. This may prove beneficial for enhancing the efficacy of PDE5 inhibitors in conditions with chronically reduced NOS activity.

Control of in vivo left ventricular [correction] contraction/relaxation kinetics by myosin binding protein C: protein kinase A phosphorylation dependent and independent regulation

BACKGROUND

Cardiac myosin binding protein-C (cMyBP-C) is a thick-filament protein whose presence and phosphorylation by protein kinase A (PKA) regulates cross-bridge formation and kinetics in isolated myocardium. We tested the influence of cMyBP-C and its PKA-phosphorylation on contraction/relaxation kinetics in intact hearts and revealed its essential role in several classic properties of cardiac function.

METHODS AND RESULTS

Comprehensive in situ cardiac pressure-volume analysis was performed in mice harboring a truncation mutation of cMyBP-C (cMyBP-C(t/t)) that resulted in nondetectable protein versus hearts re-expressing solely wild-type (cMyBP-C(WT:(t/t))) or mutated protein in which known PKA-phosphorylation sites were constitutively suppressed (cMyBP-C(AllP-:(t/t))). Hearts lacking cMyBP-C had faster early systolic activation, which then terminated prematurely, limiting ejection. Systole remained short at faster heart rates; thus, cMyBP-C(t/t) hearts displayed minimal rate-dependent decline in diastolic time and cardiac preload. Furthermore, prolongation of pressure relaxation by afterload was markedly blunted in cMyBP-C(t/t) hearts. All 3 properties were similarly restored to normal in cMyBP-C(WT:(t/t)) and cMyBP-C(AllP-:(t/t)) hearts, which supports independence of PKA-phosphorylation. However, the dependence of peak rate of pressure rise on preload was specifically depressed in cMyBP-C(AllP-:(t/t)) hearts, whereas cMyBP-C(t/t) and cMyBP-C(AllP-:(t/t)) hearts had similar blunted adrenergic and rate-dependent contractile reserve, which supports linkage of these behaviors to PKA-cMyBP-C modification.

CONCLUSIONS

cMyBP-C is essential for major properties of cardiac function, including sustaining systole during ejection, the heart-rate dependence of the diastolic time period, and relaxation delay from increased arterial afterload. These are independent of its phosphorylation by PKA, which more specifically modulates early pressure rise rate and adrenergic/heart rate reserve.

Compartmentalization of cardiac beta-adrenergic inotropy modulation by phosphodiesterase type 5

BACKGROUND

Recent cell-based studies have found that cGMP synthesis and hydrolysis by phosphodiesterase (PDE) appear compartmentalized, with nitric oxide synthase-derived and/or PDE type 5 (PDE-5)-hydrolyzable cGMP undetected at the sarcolemmal membrane in contrast to cGMP stimulated by natriuretic peptide. In the present study, we determine the functional significance of such compartments with a comparison of beta-adrenergic modulation by PDE-5 inhibition to that of natriuretic peptide stimulation in both cardiomyocytes and intact hearts. The potential role of differential cGMP and protein kinase G stimulation by these 2 modulators was also studied.

METHODS AND RESULTS

Intact C57/BL6 mouse hearts were studied with pressure-volume analysis, and adult isolated myocytes were studied with fluorescence microscopy. PDE-5 inhibition with 0.1 to 1 micromol/L sildenafil (SIL) suppressed isoproterenol (ISO)-stimulated contractility, whereas 10 micromol/L atrial natriuretic peptide (ANP) had no effect. ISO suppression by SIL was prevented in cells pretreated with a protein kinase G inhibitor. Surprisingly, myocardial cGMP changed little with SIL+ISO yet rose nearly 5-fold with ANP, whereas protein kinase G activation (vasodilator-stimulated protein phosphorylation; ELISA assay) displayed the opposite: increased with SIL+ISO but unaltered by ANP+ISO. PDE-5 and ANP compartments were functionally separated, as inhibition of nitric oxide synthase by N(w)-nitro-L-arginine methyl ester eliminated antiadrenergic effects of SIL, yet this was not restorable by co-stimulation with ANP.

CONCLUSIONS

Regulation of cardiac beta-adrenergic response by cGMP is specifically linked to a nitric oxide-synthesis/PDE-5-hydrolyzed pool signaling via protein kinase G. Natriuretic peptide stimulation achieves greater detectable increases in cGMP but not protein kinase G activity and does not modulate beta-adrenergic response. Such disparities likely contribute to differential cardiac regulation by drugs that modulate cGMP synthesis and hydrolysis.

Phosphodiesterase regulation of nitric oxide signaling

Nitric oxide regulation of the cardiovascular system involves both cGMP-dependent and independent mechanisms. The former directly interacts with the family of catabolic phosphodiesterases (PDEs) that control cGMP levels and thus distal effects such as protein kinase G stimulation. Growing evidence supports an important role of several PDEs, including PDE1, PDE2, and PDE5, in the regulation of cGMP in both vascular smooth muscle and cardiac myocytes. These PDEs have relatively little impact on resting function, but they can potently modulate acute contractile tone in cells stimulated by external agonists such as angiotensin or catecholamines. Regulation by PDEs is compartmentalized, with selective interactions occurring between a given source of cGMP and PDE hydrolysis. PDE1 and/or PDE5 are also reportedly up-regulated in chronic disease conditions such as atherosclerosis or cardiac pressure-load stress and heart failure as well as in response to long-term exposure to nitrates. Such up-regulation is thought to contribute to vascular and cardiac pathophysiology and to drug tolerance. Recent studies utilizing selective PDE5 inhibitors support significant cross-signaling with NO-cGMP synthetic pathways that may be particularly helpful in treating certain disease states.