AFos inhibits phenylephrine-mediated contractile dysfunction by altering phospholamban phosphorylation

Metabolic Interventions to Prevent Hypertrophy-Induced Alterations in Contractile Properties In Vitro

(1) Background: The exact mechanism(s) underlying pathological changes in a heart in transition to hypertrophy and failure are not yet fully understood. However, alterations in cardiac energy metabolism seem to be an important contributor. We characterized an in vitro model of adrenergic stimulation-induced cardiac hypertrophy for studying metabolic, structural, and functional changes over time. Accordingly, we investigated whether metabolic interventions prevent cardiac structural and functional changes; (2) Methods: Primary rat cardiomyocytes were treated with phenylephrine (PE) for 16 h, 24 h, or 48 h, whereafter hypertrophic marker expression, protein synthesis rate, glucose uptake, and contractile function were assessed; (3) Results: 24 h PE treatment increased expression of hypertrophic markers, phosphorylation of hypertrophy-related signaling kinases, protein synthesis, and glucose uptake. Importantly, the increased glucose uptake preceded structural and functional changes, suggesting a causal role for metabolism in the onset of PE-induced hypertrophy. Indeed, PE treatment in the presence of a PAN-Akt inhibitor or of a GLUT4 inhibitor dipyridamole prevented PE-induced increases in cellular glucose uptake and ameliorated PE-induced contractile alterations; (4) Conclusions: Pharmacological interventions, forcing substrate metabolism away from glucose utilization, improved contractile properties in PE-treated cardiomyocytes, suggesting that targeting glucose uptake, independent from protein synthesis, forms a promising strategy to prevent hypertrophy and hypertrophy-induced cardiac dysfunction.

Ex vivo Methods for Measuring Cardiac Muscle Mechanical Properties

Cardiovascular disease continues to be the leading cause of morbidity and mortality in the United States and thousands of manuscripts each year are aimed at elucidating mechanisms underlying cardiac disease. The methods for quantifying cardiac performance are quite varied, with each technique assessing unique features of cardiac muscle mechanical properties. Accordingly, in this review, we discuss current ex vivo methods for quantifying cardiac muscle performance, highlighting what can be learned from each method, and how each technique can be used in conjunction to complement others for a more comprehensive understanding of cardiac function. Importantly, cardiac function can be assessed at several different levels, from the whole organ down to individual protein-protein interactions. Here, we take a reductionist view of methods that are commonly used to measure the distinct aspects of cardiac mechanical function, beginning with whole heart preparations and finishing with the in vitro motility assay. While each of the techniques are individually well-documented in the literature, there is a significant need for a comparison of the techniques, delineating the mechanical parameters that can are best measured with each technique, as well as the strengths and weaknesses inherent to each method. Additionally, we will consider complementary techniques and how these methods can be used in combination to improve our understanding of cardiac mechanical function. By presenting each of these methods, with their strengths and limitations, in a single manuscript, this review will assist cardiovascular biologists in understanding the existing literature on cardiac mechanical function, as well as designing future experiments.

Site-specific acetyl-mimetic modification of cardiac troponin I modulates myofilament relaxation and calcium sensitivity

OBJECTIVE

Cardiac troponin I (cTnI) is an essential physiological and pathological regulator of cardiac relaxation. Significant to this regulation, the post-translational modification of cTnI through phosphorylation functions as a key mechanism to accelerate myofibril relaxation. Similar to phosphorylation, post-translational modification by acetylation alters amino acid charge and protein function. Recent studies have demonstrated that the acetylation of cardiac myofibril proteins accelerates relaxation and that cTnI is acetylated in the heart. These findings highlight the potential significance of myofilament acetylation; however, it is not known if site-specific acetylation of cTnI can lead to changes in myofilament, myofibril, and/or cellular mechanics. The objective of this study was to determine the effects of mimicking acetylation at a single site of cTnI (lysine-132; K132) on myofilament, myofibril, and cellular mechanics and elucidate its influence on molecular function.

METHODS

To determine if pseudo-acetylation of cTnI at 132 modulates thin filament regulation of the acto-myosin interaction, we reconstituted thin filaments containing WT or K132Q (to mimic acetylation) cTnI and assessed in vitro motility. To test if mimicking acetylation at K132 alters cellular relaxation, adult rat ventricular cardiomyocytes were infected with adenoviral constructs expressing either cTnI K132Q or K132 replaced with arginine (K132R; to prevent acetylation) and cell shortening and isolated myofibril mechanics were measured. Finally, to confirm that changes in cell shortening and myofibril mechanics were directly due to pseudo-acetylation of cTnI at K132, we exchanged troponin containing WT or K132Q cTnI into isolated myofibrils and measured myofibril mechanical properties.

RESULTS

Reconstituted thin filaments containing K132Q cTnI exhibited decreased calcium sensitivity compared to thin filaments reconstituted with WT cTnI. Cardiomyocytes expressing K132Q cTnI had faster relengthening and myofibrils isolated from these cells had faster relaxation along with decreased calcium sensitivity compared to cardiomyocytes expressing WT or K132R cTnI. Myofibrils exchanged with K132Q cTnI ex vivo demonstrated faster relaxation and decreased calcium sensitivity.

CONCLUSIONS

Our results indicate for the first time that mimicking acetylation of a specific cTnI lysine accelerates myofilament, myofibril, and myocyte relaxation. This work underscores the importance of understanding how acetylation of specific sarcomeric proteins affects cardiac homeostasis and disease and suggests that modulation of myofilament lysine acetylation may represent a novel therapeutic target to alter cardiac relaxation.

A Novel Method of Isolating Myofibrils From Primary Cardiomyocyte Culture Suitable for Myofibril Mechanical Study

Myofibril based mechanical studies allow evaluation of sarcomeric protein function. We describe a novel method of obtaining myofibrils from primary cardiomyocyte culture. Adult rat ventricular myocytes (ARVMs) were obtained by enzymatic digestion and maintained in serum free condition. ARVMs were homogenized in relaxing solution (pCa 9.0) with 20% sucrose, and myofibril suspension was made. Myofibrils were Ca²⁺-activated and relaxed at 15°C. Results from ARVM myofibrils were compared to myofibrils obtained from ventricular tissue skinned with Triton X-100. At maximal Ca²⁺-activation (pCa 4.5) myofibril mechanical parameters from ARVMs were 6.8 ± 0.9 mN/mm² (resting tension), 146.8 ± 13.8 mN/mm² (maximal active tension, P₀), 5.4 ± 0.22 s⁻¹ (rate of force activation), 53.4 ± 4.4 ms (linear relaxation duration), 0.69 ± 0.36 s⁻¹ (linear relaxation rate), and 10.8 ± 1.3 s⁻¹ (exponential relaxation rate). Force-pCa curves were constructed from Triton skinned tissue, ARVM culture day 1, and ARVM culture day 3 myofibrils, and pCa₅₀ were 5.79 ± 0.01, 5.69 ± 0.01, and 5.71 ± 0.01, respectively. Mechanical parameters from myofibrils isolated from ARVMs treated with phenylephrine were compared to myofibrils isolated from time-matched non-treated ARVMs. Phenylephrine treatment did not change the kinetics of activation or relaxation but decreased the pCa₅₀ to 5.56 ± 0.03 (vehicle treated control: 5.67 ± 0.03). For determination of protein expression and post-translational modifications, myofibril slurry was re-suspended and resolved for immunoblotting and protein staining. Troponin I phosphorylation was significantly increased at serine 23/24 in phenylephrine treated group. Myofibrils obtained from ARVMs are a viable method to study myofibril mechanics. Phenylephrine treatment led to significant decrease in Ca²⁺-sensitivity that is due to increased phosphorylation of TnI at serine 23/24. This culture based approach to obtaining myofibrils will allow pharmacological and genetic manipulation of the cardiomyocytes to correlate biochemical and biophysical properties.

Mixed lineage kinase-3 prevents cardiac dysfunction and structural remodeling with pressure overload

Myocardial hypertrophy is an independent risk factor for heart failure (HF), yet the mechanisms underlying pathological cardiomyocyte growth are incompletely understood. The c-Jun NH2-terminal kinase (JNK) signaling cascade modulates cardiac hypertrophic remodeling, but the upstream factors regulating myocardial JNK activity remain unclear. In this study, we sought to identify JNK-activating molecules as novel regulators of cardiac remodeling in HF. We investigated mixed lineage kinase-3 (MLK3), a master regulator of upstream JNK-activating kinases, whose role in the remodeling process had not previously been studied. We observed increased MLK3 protein expression in myocardium from patients with nonischemic and hypertrophic cardiomyopathy and in hearts of mice subjected to transverse aortic constriction (TAC). Mice with genetic deletion of MLK3 (MLK3-/-) exhibited baseline cardiac hypertrophy with preserved cardiac function. MLK3-/- mice subjected to chronic left ventricular (LV) pressure overload (TAC, 4 wk) developed worsened cardiac dysfunction and increased LV chamber size compared with MLK3+/+ littermates ( n = 8). LV mass, pathological markers of hypertrophy ( Nppa, Nppb), and cardiomyocyte size were elevated in MLK3-/- TAC hearts. Phosphorylation of JNK, but not other MAPK pathways, was selectively impaired in MLK3-/- TAC hearts. In adult rat cardiomyocytes, pharmacological MLK3 kinase inhibition using URMC-099 blocked JNK phosphorylation induced by neurohormonal agents and oxidants. Sustained URMC-099 exposure induced cardiomyocyte hypertrophy. These data demonstrate that MLK3 prevents adverse cardiac remodeling in the setting of pressure overload. Mechanistically, MLK3 activates JNK, which in turn opposes cardiomyocyte hypertrophy. These results support modulation of MLK3 as a potential therapeutic approach in HF. NEW & NOTEWORTHY Here, we identified a role for mixed lineage kinase-3 (MLK3) as a novel antihypertrophic and antiremodeling molecule in response to cardiac pressure overload. MLK3 regulates phosphorylation of the stress-responsive JNK kinase in response to pressure overload and in cultured cardiomyocytes stimulated with hypertrophic agonists and oxidants. This study reveals MLK3-JNK signaling as a novel cardioprotective signaling axis in the setting of pressure overload.

Lung Vascular Remodeling, Cardiac Hypertrophy, and Inflammatory Cytokines in SHIVnef-Infected Macaques

Fatal pulmonary arterial hypertension (PAH) affects HIV-infected individuals at significantly higher frequencies. We previously showed plexiform-like lesions characterized by recanalized lumenal obliteration, intimal disruption, medial hypertrophy, and thrombosis consistent with PAH in rhesus macaques infected with chimeric SHIVnef but not with the parental SIVmac239, suggesting that Nef is implicated in the pathophysiology of HIV-PAH. However, the current literature on non-human primates as animal models for SIV(HIV)-associated pulmonary disease reports the ultimate pathogenic pulmonary outcomes of the research efforts; however, the variability and features in the actual disease progression remain poorly described, particularly when using different viral sources for infection. We analyzed lung histopathology, performed immunophenotyping of cells in plexogenic lesions pathognomonic of PAH, and measured cardiac hypertrophy biomarkers and cytokine expression in plasma and lung of juvenile SHIVnef-infected macaques. Here, we report significant hematopathologies, changes in cardiac biomarkers consistent with ventricular hypertrophy, significantly increased levels of interleukin-12 and GM-CSF and significantly decreased sCD40 L, CCL-2, and CXCL-1 in plasma of the SHIVnef group. Pathway analysis of inflammatory gene expression predicted activation of NF-κB transcription factor RelB and inhibition of bone morphogenetic protein type-2 in the setting of SHIVnef infection. Our findings highlight the utility of SHIVnef-infected macaques as suitable models of HIV-associated pulmonary vascular remodeling as pathogenetic changes are concordant with features of idiopathic, familial, scleroderma, and HIV-PAH.

C33(S), a novel PDE9A inhibitor, protects against rat cardiac hypertrophy through upregulating cGMP signaling

Phosphodiesterase-9A (PDE9A) expression is upregulated during cardiac hypertrophy and heart failure. Accumulating evidence suggests that PDE9A might be a promising therapeutic target for heart diseases. The present study sought to investigate the effects and underlying mechanisms of C33(S), a novel selective PDE9A inhibitor, on cardiac hypertrophy in vitro and in vivo. Treatment of neonatal rat cardiomyocytes (NRCMs) with PE (100 μmol/L) or ISO (1 μmol/L) induced cardiac hypertrophy characterized by significantly increased cell surface areas and increased expression of fetal genes (ANF and BNP). Furthermore, PE or ISO significantly increased the expression of PDE9A in the cells; whereas knockdown of PDE9A significantly alleviated PE-induced hypertrophic responses. Moreover, pretreatment with PDE9A inhibitor C33(S) (50 and 500 nmol/L) or PF-7943 (2 μmol/L) also alleviated the cardiac hypertrophic responses in PE-treated NRCMs. Abdominal aortic constriction (AAC)-induced cardiac hypertrophy and ISO-induced heart failure were established in SD rats. In ISO-treated rats, oral administration of C33(S) (9, 3, and 1 mg·kg^-1·d^-1, for 3 consecutive weeks) significantly increased fractional shortening (43.55%±3.98%, 54.79%±1.95%, 43.98%±7.96% vs 32.18%±6.28%), ejection fraction (72.97%±4.64%, 84.29%±1.56%, 73.41%±9.37% vs 49.17%±4.20%) and cardiac output (60.01±9.11, 69.40±11.63, 58.08±8.47 mL/min vs 48.97±2.11 mL/min) but decreased the left ventricular internal diameter, suggesting that the transition to heart failure was postponed by C33(S). We further revealed that C33(S) significantly elevated intracellular cGMP levels, phosphorylation of phospholamban (PLB) and expression of SERCA2a in PE-treated NRCMs in vitro and in ISO-induced heart failure model in vivo. Our results demonstrate that C33(S) effectively protects against cardiac hypertrophy and postpones the transition to heart failure, suggesting that it is a promising agent in the treatment of cardiac diseases.

Biomimetic Polymers for Cardiac Tissue Engineering

Heart failure is a morbid disorder characterized by progressive cardiomyocyte (CM) dysfunction and death. Interest in cell-based therapies is growing, but sustainability of injected CMs remains a challenge. To mitigate this, we developed an injectable biomimetic Reverse Thermal Gel (RTG) specifically engineered to support long-term CM survival. This RTG biopolymer provided a solution-based delivery vehicle of CMs, which transitioned to a gel-based matrix shortly after reaching body temperature. In this study we tested the suitability of this biopolymer to sustain CM viability. The RTG was biomolecule-functionalized with poly-l-lysine or laminin. Neonatal rat ventricular myocytes (NRVM) and adult rat ventricular myocytes (ARVM) were cultured in plain-RTG and biomolecule-functionalized-RTG both under 3-dimensional (3D) conditions. Traditional 2D biomolecule-coated dishes were used as controls. We found that the RTG-lysine stimulated NRVM to spread and form heart-like functional syncytia. Regarding cell contraction, in both RTG and RTG-lysine, beating cells were recorded after 21 days. Additionally, more than 50% (p value < 0.05; n = 5) viable ARVMs, characterized by a well-defined cardiac phenotype represented by sarcomeric cross-striations, were found in the RTG-laminin after 8 days. These results exhibit the tremendous potential of a minimally invasive CM transplantation through our designed RTG-cell therapy platform.

The Function of Rho-Associated Kinases ROCK1 and ROCK2 in the Pathogenesis of Cardiovascular Disease

Rho-associated kinases ROCK1 and ROCK2 are serine/threonine kinases that are downstream targets of the small GTPases RhoA, RhoB, and RhoC. ROCKs are involved in diverse cellular activities including actin cytoskeleton organization, cell adhesion and motility, proliferation and apoptosis, remodeling of the extracellular matrix and smooth muscle cell contraction. The role of ROCK1 and ROCK2 has long been considered to be similar; however, it is now clear that they do not always have the same functions. Moreover, depending on their subcellular localization, activation, and other environmental factors, ROCK signaling can have different effects on cellular function. With respect to the heart, findings in isoform-specific knockout mice argue for a role of ROCK1 and ROCK2 in the pathogenesis of cardiac fibrosis and cardiac hypertrophy, respectively. Increased ROCK activity could play a pivotal role in processes leading to cardiovascular diseases such as hypertension, pulmonary hypertension, angina pectoris, vasospastic angina, heart failure, and stroke, and thus ROCK activity is a potential new biomarker for heart disease. Pharmacological ROCK inhibition reduces the enhanced ROCK activity in patients, accompanied with a measurable improvement in medical condition. In this review, we focus on recent findings regarding ROCK signaling in the pathogenesis of cardiovascular disease, with a special focus on differences between ROCK1 and ROCK2 function.

MAP kinase kinase kinase-2 (MEKK2) regulates hypertrophic remodeling of the right ventricle in hypoxia-induced pulmonary hypertension

Pulmonary hypertension (PH) results in pressure overload of the right ventricle (RV) of the heart, initiating pathological RV remodeling and ultimately leading to right heart failure. Substantial research indicates that signaling through the MAPK superfamily mediates pathological cardiac remodeling. These considerations led us to test the hypothesis that the regulatory protein MAPKKK-2 (MEKK2) contributes to RV hypertrophy in hypoxia-induced PH. Transgenic mice with global knockout of MEKK2 (MEKK2(-/-) mice) and age-matched wild-type (WT) mice were exposed to chronic hypobaric hypoxia (10% O(2), 6 wk) and compared with animals under normoxia. Exposure to chronic hypoxia induced PH in WT and MEKK2(-/-) mice. In response to PH, WT mice showed RV hypertrophy, demonstrated as increased ratio of RV weight to body weight, increased RV wall thickness at diastole, and increased cardiac myocyte size compared with normoxic control animals. In contrast, each of these measures of RV hypertrophy seen in WT mice after chronic hypoxia was attenuated in MEKK2(-/-) mice. Furthermore, chronic hypoxia elicited altered programs of hypertrophic and inflammatory gene expression consistent with pathological RV remodeling in WT mice; MEKK2 deletion selectively inhibited inflammatory gene expression compared with WT mice. The actions of MEKK2 were mediated in part through regulation of the abundance and phosphorylation of its effector, ERK5. In conclusion, signaling by MEKK2 contributes to RV hypertrophy and altered myocardial inflammatory gene expression in response to hypoxia-induced PH. Therapies targeting MEKK2 may protect the myocardium from hypertrophy and pathological remodeling in human PH.

CD36 protein influences myocardial Ca2+ homeostasis and phospholipid metabolism: conduction anomalies in CD36-deficient mice during fasting

Sarcolemmal CD36 facilitates myocardial fatty acid (FA) uptake, which is markedly reduced in CD36-deficient rodents and humans. CD36 also mediates signal transduction events involving a number of cellular pathways. In taste cells and macrophages, CD36 signaling was recently shown to regulate store-responsive Ca(2+) flux and activation of Ca(2+)-dependent phospholipases A(2) that cycle polyunsaturated FA into phospholipids. It is unknown whether CD36 deficiency influences myocardial Ca(2+) handling and phospholipid metabolism, which could compromise the heart, typically during stresses. Myocardial function was examined in fed or fasted (18-22 h) CD36(-/-) and WT mice. Echocardiography and telemetry identified conduction anomalies that were associated with the incidence of sudden death in fasted CD36(-/-) mice. No anomalies or death occurred in WT mice during fasting. Optical imaging of perfused hearts from fasted CD36(-/-) mice documented prolongation of Ca(2+) transients. Consistent with this, knockdown of CD36 in cardiomyocytes delayed clearance of cytosolic Ca(2+). Hearts of CD36(-/-) mice (fed or fasted) had 3-fold higher SERCA2a and 40% lower phospholamban levels. Phospholamban phosphorylation by protein kinase A (PKA) was enhanced after fasting reflecting increased PKA activity and cAMP levels in CD36(-/-) hearts. Abnormal Ca(2+) homeostasis in the CD36(-/-) myocardium associated with increased lysophospholipid content and a higher proportion of 22:6 FA in phospholipids suggests altered phospholipase A(2) activity and changes in membrane dynamics. The data support the role of CD36 in coordinating Ca(2+) homeostasis and lipid metabolism and the importance of this role during myocardial adaptation to fasting. Potential relevance of the findings to CD36-deficient humans would need to be determined.

Ubiquitin tagged dominant negative induces degradation of B-ZIP proteins

B-ZIP transcription factors heterodimerize with dominant negative designs, termed A-ZIPs, in a dimerization specific manner and inhibit its ability to bind DNA. Different A-ZIPs produce unique phenotypes in vivo suggesting that they have distinct B-ZIP heterodimerization partners. However, the identification of the in vivo heterodimerization partners of different A-ZIPs remains problematic. To identify the in vivo heterodimerization partners, a chimeric protein containing two ubiquitin motifs at the N-terminal of the A-ZIP domain was designed. The presence of ubiquitin reduced the concentration of specific co-transfected B-ZIP proteins. The ubiquitin enhanced degradation of the B-ZIP heterodimeric partner is inhibited by the proteasome inhibitor MG-132. These ubiquitin tagged A-ZIP dominant negatives may be more active in vivo because their endogenous heterodimerization partners are degraded more efficiently. This may be a general strategy to identify protein interaction partners.

Adiponectin mediates cardioprotection in oxidative stress-induced cardiac myocyte remodeling

Reactive oxygen species (ROS) induce matrix metalloproteinase (MMP) activity that mediates hypertrophy and cardiac remodeling. Adiponectin (APN), an adipokine, modulates cardiac hypertrophy, but it is unknown if APN inhibits ROS-induced cardiomyocyte remodeling. We tested the hypothesis that APN ameliorates ROS-induced cardiomyocyte remodeling and investigated the mechanisms involved. Cultured adult rat ventricular myocytes (ARVM) were pretreated with recombinant APN (30 μg/ml, 18 h) followed by exposure to physiologic concentrations of H(2)O(2) (1-200 μM). ARVM hypertrophy was measured by [(3)H]leucine incorporation and atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) gene expression by RT-PCR. MMP activity was assessed by in-gel zymography. ROS was induced with angiotensin (ANG)-II (3.2 mg·kg(-1)·day(-1) for 14 days) in wild-type (WT) and APN-deficient (APN-KO) mice. Myocardial MMPs, tissue inhibitors of MMPs (TIMPs), p-AMPK, and p-ERK protein expression were determined. APN significantly decreased H(2)O(2)-induced cardiomyocyte hypertrophy by decreasing total protein, protein synthesis, ANF, and BNP expression. H(2)O(2)-induced MMP-9 and MMP-2 activities were also significantly diminished by APN. APN significantly increased p-AMPK in both nonstimulated and H(2)O(2)-treated ARVM. H(2)O(2)-induced p-ERK activity and NF-κB activity were both abrogated by APN pretreatment. ANG II significantly decreased myocardial p-AMPK and increased p-ERK expression in vivo in APN-KO vs. WT mice. ANG II infusion enhanced cardiac fibrosis and MMP-2-to-TIMP-2 and MMP-9-to-TIMP-1 ratios in APN-KO vs. WT mice. Thus APN inhibits ROS-induced cardiomyocyte remodeling by activating AMPK and inhibiting ERK signaling and NF-κB activity. Its effects on ROS and ultimately on MMP expression define the protective role of APN against ROS-induced cardiac remodeling.