Molecular mechanisms underlying sarcopenia in heart failure

The loss of skeletal muscle, also known as sarcopenia, is an aging-associated muscle disorder that is disproportionately present in heart failure (HF) patients. HF patients with sarcopenia have poor outcomes compared to the overall HF patient population. The prevalence of sarcopenia in HF is only expected to grow as the global population ages, and novel treatment strategies are needed to improve outcomes in this cohort. Multiple mechanistic pathways have emerged that may explain the increased prevalence of sarcopenia in the HF population, and a better understanding of these pathways may lead to the development of therapies to prevent muscle loss. This review article aims to explore the molecular mechanisms linking sarcopenia and HF, and to discuss treatment strategies aimed at addressing such molecular signals.

Metformin preconditioning protects against myocardial stunning and preserves protein translation in a mouse model of cardiac arrest

Cardiac arrest (CA) causes high mortality due to multi-system organ damage attributable to ischemia-reperfusion injury. Recent work in our group found that among diabetic patients who experienced cardiac arrest, those taking metformin had less evidence of cardiac and renal damage after cardiac arrest when compared to those not taking metformin. Based on these observations, we hypothesized that metformin's protective effects in the heart were mediated by AMPK signaling, and that AMPK signaling could be targeted as a therapeutic strategy following resuscitation from CA. The current study investigates metformin interventions on cardiac and renal outcomes in a non-diabetic CA mouse model. We found that two weeks of metformin pretreatment protects against reduced ejection fraction and reduces kidney ischemia-reperfusion injury at 24 h post-arrest. This cardiac and renal protection depends on AMPK signaling, as demonstrated by outcomes in mice pretreated with the AMPK activator AICAR or metformin plus the AMPK inhibitor compound C. At this 24-h time point, heart gene expression analysis showed that metformin pretreatment caused changes supporting autophagy, antioxidant response, and protein translation. Further investigation found associated improvements in mitochondrial structure and markers of autophagy. Notably, Western analysis indicated that protein synthesis was preserved in arrest hearts of animals pretreated with metformin. The AMPK activation-mediated preservation of protein synthesis was also observed in a hypoxia/reoxygenation cell culture model. Despite the positive impacts of pretreatment in vivo and in vitro, metformin did not preserve ejection fraction when deployed at resuscitation. Taken together, we propose that metformin's in vivo cardiac preservation occurs through AMPK activation, requires adaptation before arrest, and is associated with preserved protein translation.

A novel ultrasound-guided mouse model of sudden cardiac arrest

AIM

Mouse models of sudden cardiac arrest are limited by challenges with surgical technique and obtaining reliable venous access. To overcome this limitation, we sought to develop a simplified method in the mouse that uses ultrasound-guided injection of potassium chloride directly into the heart.

METHODS

Potassium chloride was delivered directly into the left ventricular cavity under ultrasound guidance in intubated mice, resulting in immediate asystole. Mice were resuscitated with injection of epinephrine and manual chest compressions and evaluated for survival, body temperature, cardiac function, kidney damage, and diffuse tissue injury.

RESULTS

The direct injection sudden cardiac arrest model causes rapid asystole with high surgical survival rates and short surgical duration. Sudden cardiac arrest mice with 8-min of asystole have significant cardiac dysfunction at 24 hours and high lethality within the first seven days, where after cardiac function begins to improve. Sudden cardiac arrest mice have secondary organ damage, including significant kidney injury but no significant change to neurologic function.

CONCLUSIONS

Ultrasound-guided direct injection of potassium chloride allows for rapid and reliable cardiac arrest in the mouse that mirrors human pathology without the need for intravenous access. This technique will improve investigators' ability to study the mechanisms underlying post-arrest changes in a mouse model.

Abstract 476: Mitochondrial DNA Preservation Preserves Cardiac Function Following Sudden Cardiac Arrest

Background: Sudden cardiac arrest (SCA) affects over 350,000 Americans yearly with greater than 70% mortality. Survivors frequently develop cardiomyopathy after SCA. Cardiac reperfusion causes mitochondrial ROS production, which is known to damage mitochondrial DNA (mtDNA), but the physiologic consequences of mtDNA damage are unclear. We investigated the role of mtDNA damage by altering the expression of TFAM, a nuclear-encoded transcription factor that protects mtDNA from ROS, in a mouse model of SCA.

Methods: WT and transgenic mice featuring cardiac-specific TFAM overexpression (TFAM-OE) and under-expression (TFAM Flox) underwent either 8 min of SCA or sham surgery followed by cardiopulmonary resuscitation. Survivors were assessed by echocardiography at 1-day, 1-week, and 4-weeks. Tissues were collected for assessment of mtDNA copy number and damage and assessment of mitochondrial morphology, protein expression, and function.

Results: WT, TFAM-OE, and TFAM Flox mice had no significant changes to baseline body weight or ejection fraction (EF). There were no changes in time to return of spontaneous circulation or body temperature between groups. 1 day after SCA, WT mice have reduced EF (38.49±3.76%) compared to sham WT mice (59.73±1.42). EF is protected in TFAM-OE mice (51.11±2.95%) and exacerbated in TFAM-UE mice (29.36±5.40%). TFAM-OE have significantly higher survival at 4 weeks (80%, 8 of 10) when compared to WT mice (38%, 5 of 13), but there is no change in TFAM-Flox mice (43%, 3 of 7). TFAM OE mice have higher mtDNA copy number and lower mtDNA damage when compared to WT mice.

Conclusions: TFAM OE protects cardiac function 1-day after SCA and improves 4-week survival. This is likely driven by TFAM-mediated protection of mtDNA. TFAM-Flox mice have lower EF at one day but no change to survival. This work suggests a role for mtDNA damage as a mechanism and potential therapeutic target of cardiomyopathy after SCA.

Abstract 105: Downregulation Of Cardiac Na+ Channel In Myocardial Infarction Is Prevented By A Mitochondria-targeted Antioxidant

Objectives: The aim of this study was to investigate the effect of ischemia on cardiac Na+ channel (Nav1.5) and possible treatments in a mouse model of myocardial infarction (MI).

Methods: MI was induced in 12-week old C57BL/6 mice by coronary artery occlusion. Sham-operated mice were used as controls. Two weeks following surgery, MI mice were either given a mitochondria-targeted antioxidant, mitoTEMPO (0.7 mg/kg/day, intraperitoneally), or left untreated for two weeks. Cardiomyocytes isolated from the scar border of MI mice or from the left ventricular (LV) anterior wall of sham-operated mice were utilized for whole-cell patch clamp recording of Na+ currents (INa) and for measurements of mitochondrial reactive oxygen species (mitoROS) using flow cytometry. Nav1.5 protein expression levels were determined in the LV from MI and sham-operated mice. Echocardiography was performed 2- and 4-weeks following MI.

Results: The peak INa densities of the isolated LV cardiomyocytes were significantly lower (P<0.05) in MI (-14.3±1.4 pA/pF), compared to sham (-24.0±1.8 pA/pF). The mitoROS levels were elevated to 1.5±0.2 fold in MI mice (P<0.05). INa was increased (-19.4±0.8 pA/pF, P<0.05) and mitoROS was decreased to 1.2±0.2 fold (P<0.05) with mitoTEMPO treatment. The Nav1.5 channel protein level was not altered in the heart tissue of MI mice. There were no significant differences in echocardiography parameters between untreated and mitoTEMPO groups to explain the increase in INa.

Conclusions: Ischemic cardiomyopathy leads to downregulation of cardiac Nav1.5 currents and overproduction of mitochondrial ROS. The mitochondria-targeted antioxidant can mitigate these changes and may help reduce arrhythmic risk after myocardial infarction.

Caveolin-1 modulates cardiac gap junction homeostasis and arrhythmogenecity by regulating cSrc tyrosine kinase

BACKGROUND

Genome-wide association studies have revealed significant association of caveolin-1 (Cav1) gene variants with increased risk of cardiac arrhythmias. Nevertheless, the mechanism for this linkage is unclear.

METHODS AND RESULTS

Using adult Cav1(-/-) mice, we revealed a marked reduction in the left ventricular conduction velocity in the absence of myocardial Cav1, which is accompanied with increased inducibility of ventricular arrhythmias. Further studies demonstrated that loss of Cav1 leads to the activation of cSrc tyrosine kinase, resulting in the downregulation of connexin 43 and subsequent electric abnormalities. Pharmacological inhibition of cSrc mitigates connexin 43 downregulation, slowed conduction, and arrhythmia inducibility in Cav1(-/-) animals. Using a transgenic mouse model with cardiac-specific overexpression of angiotensin-converting enzyme (ACE8/8), we demonstrated that, on enhanced cardiac renin-angiotensin system activity, Cav1 dissociated from cSrc because of increased Cav1 S-nitrosation at Cys(156), leading to cSrc activation, connexin 43 reduction, impaired gap junction function, and subsequent increase in the propensity for ventricular arrhythmias and sudden cardiac death. Renin-angiotensin system-induced Cav1 S-nitrosation was associated with increased Cav1-endothelial nitric oxide synthase binding in response to increased mitochondrial reactive oxidative species generation.

CONCLUSIONS

The present studies reveal the critical role of Cav1 in modulating cSrc activation, gap junction remodeling, and ventricular arrhythmias. These data provide a mechanistic explanation for the observed genetic link between Cav1 and cardiac arrhythmias in humans and suggest that targeted regulation of Cav1 may reduce arrhythmic risk in cardiac diseases associated with renin-angiotensin system activation.

c-Src kinase inhibition reduces arrhythmia inducibility and connexin43 dysregulation after myocardial infarction

OBJECTIVES

The aim of this study was to evaluate the role of tyrosine kinase cellular-Src (c-Src) inhibition on connexin43 (Cx43) regulation in a mouse model of myocardial infarction (MI).

BACKGROUND

MI is associated with decreased expression of Cx43, the principal gap junction protein responsible for propagating current in ventricles. Activated c-Src has been linked to Cx43 dysregulation.

METHODS

MI was induced in 12-week-old mice by coronary artery occlusion. MI mice were treated with c-Src inhibitors (PP1 or AZD0530), PP3 (an inactive analogue of PP1), or saline. Treated hearts were compared to sham mice by echocardiography, optical mapping, telemetry electrocardiographic monitoring, and inducibility studies. Tissues were collected for immunoblotting, quantitative polymerase chain reaction, and immunohistochemistry.

RESULTS

Active c-Src was elevated in PP3-treated MI mice compared to sham at the scar border (280%, p = 0.003) and distal ventricle (346%, p = 0.013). PP1 treatment restored active c-Src to sham levels at the scar border (86%, p = 0.95) and distal ventricle (94%, p = 1.0). PP1 raised Cx43 expression by 69% in the scar border (p = 0.048) and by 73% in the distal ventricle (p = 0.043) compared with PP3 mice. PP1-treated mice had restored conduction velocity at the scar border (PP3: 32 cm/s, PP1: 41 cm/s, p < 0.05) and lower arrhythmic inducibility (PP3: 71%, PP1: 35%, p < 0.05) than PP3 mice. PP1 did not change infarct size, electrocardiographic pattern, or cardiac function. AZD0530 treatment demonstrated restoration of Cx43 comparable to PP1.

CONCLUSIONS

c-Src inhibition improved Cx43 levels and conduction velocity and lowered arrhythmia inducibility after MI, suggesting a new approach for arrhythmia reduction following MI.

Mitochondria oxidative stress, connexin43 remodeling, and sudden arrhythmic death

BACKGROUND

Previously, we showed that a mouse model (ACE8/8) of cardiac renin-angiotensin system activation has a high rate of spontaneous ventricular tachycardia and sudden cardiac death secondary to a reduction in connexin43 level. Angiotensin-II activation increases reactive oxygen species (ROS) production, and ACE8/8 mice show increased cardiac ROS. We sought to determine the source of ROS and whether ROS played a role in the arrhythmogenesis.

METHODS AND RESULTS

Wild-type and ACE8/8 mice with and without 2 weeks of treatment with L-NIO (NO synthase inhibitor), sepiapterin (precursor of tetrahydrobiopterin), MitoTEMPO (mitochondria-targeted antioxidant), TEMPOL (a general antioxidant), apocynin (nicotinamide adenine dinucleotide phosphate oxidase inhibitor), allopurinol (xanthine oxidase inhibitor), and ACE8/8 crossed with P67 dominant negative mice to inhibit the nicotinamide adenine dinucleotide phosphate oxidase were studied. Western blotting, detection of mitochondrial ROS by MitoSOX Red, electron microscopy, immunohistochemistry, fluorescent dye diffusion technique for functional assessment of connexin43, telemetry monitoring, and in vivo electrophysiology studies were performed. Treatment with MitoTEMPO reduced sudden cardiac death in ACE8/8 mice (from 74% to 18%; P<0.005), decreased spontaneous ventricular premature beats, decreased ventricular tachycardia inducibility (from 90% to 17%; P<0.05), diminished elevated mitochondrial ROS to the control level, prevented structural damage to mitochondria, resulted in 2.6-fold increase in connexin43 level at the gap junctions, and corrected gap junction conduction. None of the other antioxidant therapies prevented ventricular tachycardia and sudden cardiac death in ACE8/8 mice.

CONCLUSIONS

Mitochondrial oxidative stress plays a central role in angiotensin II-induced gap junction remodeling and arrhythmia. Mitochondria-targeted antioxidants may be effective antiarrhythmic drugs in cases of renin-angiotensin system activation.