Impaired in vivo mitochondrial Krebs cycle activity after myocardial infarction assessed using hyperpolarized magnetic resonance spectroscopy

Hyperpolarized 13C and 31P MRS detects differences in cardiac energetics, metabolism, and function in obesity, and responses following treatment

Obesity is associated with important changes in cardiac energetics and function, and an increased risk of adverse cardiovascular outcomes. Multi-nuclear MRS and MRI techniques have the potential to provide a comprehensive non-invasive assessment of cardiac metabolic perturbation in obesity. A rat model of obesity was created by high-fat diet feeding. This model was characterized using in vivo hyperpolarized [1-13C]pyruvate and [2-13C]pyruvate MRS, echocardiography and perfused heart 31P MRS. Two groups of obese rats were subsequently treated with either caloric restriction or the glucagon-like peptide-1 analogue/agonist liraglutide, prior to reassessment. The model recapitulated cardiovascular consequences of human obesity, including mild left ventricular hypertrophy, and diastolic, but not systolic, dysfunction. Hyperpolarized 13C and 31P MRS demonstrated that obesity was associated with reduced myocardial pyruvate dehydrogenase flux, altered cardiac tricarboxylic acid (TCA) cycle metabolism, and impaired myocardial energetic status (lower phosphocreatine to adenosine triphosphate ratio and impaired cardiac ΔG~ATP). Both caloric restriction and liraglutide treatment were associated with normalization of metabolic changes, alongside improvement in cardiac diastolic function. In this model of obesity, hyperpolarized 13C and 31P MRS demonstrated abnormalities in cardiac metabolism at multiple levels, including myocardial substrate selection, TCA cycle, and high-energy phosphorus metabolism. Metabolic changes were linked with impairment of diastolic function and were reversed in concert following either caloric restriction or liraglutide treatment. With hyperpolarized 13C and 31P techniques now available for human use, the findings support a role for multi-nuclear MRS in the development of new therapies for obesity.

Multinomial machine learning identifies independent biomarkers by integrated metabolic analysis of acute coronary syndrome

A multi-class classification model for acute coronary syndrome (ACS) remains to be constructed based on multi-fluid metabolomics. Major confounders may exert spurious effects on the relationship between metabolism and ACS. The study aims to identify an independent biomarker panel for the multiclassification of HC, UA, and AMI by integrating serum and urinary metabolomics. We performed a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based metabolomics study on 300 serum and urine samples from 44 patients with unstable angina (UA), 77 with acute myocardial infarction (AMI), and 29 healthy controls (HC). Multinomial machine learning approaches, including multinomial adaptive least absolute shrinkage and selection operator (LASSO) regression and random forest (RF), and assessment of the confounders were applied to integrate a multi-class classification biomarker panel for HC, UA and AMI. Different metabolic landscapes were portrayed during the transition from HC to UA and then to AMI. Glycerophospholipid metabolism and arginine biosynthesis were predominant during the progression from HC to UA and then to AMI. The multiclass metabolic diagnostic model (MDM) dependent on ACS, including 2-ketobutyric acid, LysoPC(18:2(9Z,12Z)), argininosuccinic acid, and cyclic GMP, demarcated HC, UA, and AMI, providing a C-index of 0.84 (HC vs. UA), 0.98 (HC vs. AMI), and 0.89 (UA vs. AMI). The diagnostic value of MDM largely derives from the contribution of 2-ketobutyric acid, and LysoPC(18:2(9Z,12Z)) in serum. Higher 2-ketobutyric acid and cyclic GMP levels were positively correlated with ACS risk and atherosclerosis plaque burden, while LysoPC(18:2(9Z,12Z)) and argininosuccinic acid showed the reverse relationship. An independent multiclass biomarker panel for HC, UA, and AMI was constructed using the multinomial machine learning methods based on serum and urinary metabolite signatures.

Current Advances of Mitochondrial Dysfunction and Cardiovascular Disease and Promising Therapeutic Strategies

Mitochondria are cellular power stations and essential organelles for maintaining cellular homeostasis. Dysfunctional mitochondria have emerged as a key factor in the occurrence and development of cardiovascular disease. This review focuses on advances in the relationship between mitochondrial dysfunction and cardiovascular diseases such as atherosclerosis, heart failure, myocardial ischemia reperfusion injury, and pulmonary arterial hypertension. The clinical value and challenges of mitochondria-targeted strategies, including mitochondria-targeted antioxidants, mitochondrial quality control modulators, mitochondrial function protectors, mitochondrial biogenesis promoters, and recently developed mitochondrial transplants, are also discussed.

Cardiac Metabolism in Heart Failure and Implications for Uremic Cardiomyopathy

Chronic kidney disease is associated with an increased risk for the development and progression of cardiovascular disorders including hypertension, dyslipidemia, and coronary artery disease. Chronic kidney disease may also affect the myocardium through complex systemic changes, resulting in structural remodeling such as hypertrophy and fibrosis, as well as impairments in both diastolic and systolic function. These cardiac changes in the setting of chronic kidney disease define a specific cardiomyopathic phenotype known as uremic cardiomyopathy. Cardiac function is tightly linked to its metabolism, and research over the past 3 decades has revealed significant metabolic remodeling in the myocardium during the development of heart failure. Because the concept of uremic cardiomyopathy has only been recognized in recent years, there are limited data on metabolism in the uremic heart. Nonetheless, recent findings suggest overlapping mechanisms with heart failure. This work reviews key features of metabolic remodeling in the failing heart in the general population and extends this to patients with chronic kidney disease. The knowledge of similarities and differences in cardiac metabolism between heart failure and uremic cardiomyopathy may help identify new targets for mechanistic and therapeutic research on uremic cardiomyopathy.

Multi-nuclear magnetic resonance spectroscopy: state of the art and future directions

With the development of heteronuclear fluorine, sodium, phosphorus, and other probes and imaging technologies as well as the optimization of magnetic resonance imaging (MRI) equipment and sequences, multi-nuclear magnetic resonance (multi-NMR) has enabled localize molecular activities in vivo that are central to a variety of diseases, including cardiovascular disease, neurodegenerative pathologies, metabolic diseases, kidney, and tumor, to shift from the traditional morphological imaging to the molecular imaging, precision diagnosis, and treatment mode. However, due to the low natural abundance and low gyromagnetic ratios, the clinical application of multi-NMR has been hampered. Several techniques have been developed to amplify the NMR sensitivity such as the dynamic nuclear polarization, spin-exchange optical pumping, and brute-force polarization. Meanwhile, a wide range of nuclei can be hyperpolarized, such as ²H, ³He, ¹³C, ¹⁵ N, ³¹P, and ¹²⁹Xe. The signal can be increased and allows real-time observation of biological perfusion, metabolite transport, and metabolic reactions in vivo, overcoming the disadvantages of conventional magnetic resonance of low sensitivity. HP-NMR imaging of different nuclear substrates provides a unique opportunity and invention to map the metabolic changes in various organs without invasive procedures. This review aims to focus on the recent applications of multi-NMR technology not only in a range of preliminary animal experiments but also in various disease spectrum in human. Furthermore, we will discuss the future challenges and opportunities of this multi-NMR from a clinical perspective, in the hope of truly bridging the gap between cutting-edge molecular biology and clinical applications.

Identification of the metabolic remodeling profile in the early-stage of myocardial ischemia and the contributory role of mitochondrion

Cardiac remodeling is the primary pathological feature of chronic heart failure. Prompt inhibition of remodeling in acute coronary syndrome has been a standard procedure, but the morbidity and mortality are still high. Exploring the characteristics of ischemia in much earlier stages and identifying its biomarkers are essential for introducing novel mechanisms and therapeutic strategies. Metabolic and structural remodeling of mitochondrion is identified to play key roles in ischemic heart disease. The mitochondrial metabolic features in early ischemia have not previously been described. In the present study, we established a mouse heart in early ischemia and explored the mitochondrial metabolic profile using metabolomics analysis. We also discussed the role of mitochondrion in the global cardiac metabolism. Transmission electron microscopy revealed that mitochondrial structural injury was invoked at 8 minutes post-coronary occlusion. In total, 75 metabolites in myocardium and 26 in mitochondria were screened out. About 23% of the differentiated metabolites in mitochondria overlapped with the differentiated metabolites in myocardium; Total 81% of the perturbed metabolic pathway in mitochondria overlapped with the perturbed pathway in myocardium, and these pathways accounted for 50% of the perturbed pathway in myocardium. Purine metabolism was striking and mechanically important. In conclusion, in the early ischemia, myocardium exacerbated metabolic remodeling. Mitochondrion was a contributor to the myocardial metabolic disorder. Purine metabolism may be a potential biomarker for early ischemia diagnosis. Our study introduced a perspective for prompt identification of ischemia.

Lactate and Myocadiac Energy Metabolism

The myocardium is capable of utilizing different energy substrates, which is referred to as “metabolic flexibility.” This process assures ATP production from fatty acids, glucose, lactate, amino acids, and ketones, in the face of varying metabolic contexts. In the normal physiological state, the oxidation of fatty acids contributes to approximately 60% of energy required, and the oxidation of other substrates provides the rest. The accumulation of lactate in ischemic and hypoxic tissues has traditionally be considered as a by-product, and of little utility. However, recent evidence suggests that lactate may represent an important fuel for the myocardium during exercise or myocadiac stress. This new paradigm drives increasing interest in understanding its role in cardiac metabolism under both physiological and pathological conditions. In recent years, blood lactate has been regarded as a signal of stress in cardiac disease, linking to prognosis in patients with myocardial ischemia or heart failure. In this review, we discuss the importance of lactate as an energy source and its relevance to the progression and management of heart diseases.

Remodeling after myocardial infarction and effects of heart failure treatment investigated by hyperpolarized [1-13 C]pyruvate magnetic resonance spectroscopy

PURPOSE

Hyperpolarized [1-¹³ C]pyruvate MRS can measure cardiac metabolism in vivo. We investigated whether [1-¹³ C]pyruvate MRS could predict left ventricular remodeling following myocardial infarction (MI), long-term left ventricular effects of heart failure medication, and could identify responders to treatment.

METHODS

Thirty-five rats were scanned with hyperpolarized [1-¹³ C]pyruvate MRS 3 days after MI or sham surgery. The animals were re-examined after 30 days of therapy with β-blockers and ACE-inhibitors (active group, n = 12), placebo treatment (placebo group, n = 13) or no treatment (sham group, n = 10). Furthermore, heart tissue mitochondrial respiratory capacity was assessed by high-resolution respirometry. Metabolic results were compared between groups, over time and correlated to functional MR data at each time point.

RESULTS

At 30 ± 0.5 days post MI, left ventricular ejection fraction (LVEF) differed between groups (sham, 77% ± 1%; placebo, 52% ± 3%; active, 63% ± 2%, P < .001). Cardiac metabolism, measured by both hyperpolarized [1-¹³ C]pyruvate MRS and respirometry, neither differed between groups nor between baseline and follow-up. Three days post MI, low bicarbonate + CO₂ /pyruvate ratio was associated with low LVEF. At follow-up, in the active group, a poor recovery of LVEF was associated with high bicarbonate + CO₂ /pyruvate ratio, as measured by hyperpolarized MRS.

CONCLUSION

In a rat model of moderate heart failure, medical treatment improved function, but did not on average influence [1-¹³ C]pyruvate flux as measured by MRS; however, responders to heart failure medication had reduced capacity for carbohydrate metabolism.

Emerging Techniques in Cardiac Magnetic Resonance Imaging

Cardiovascular disease is the leading cause of death and a significant contributor of health care costs. Noninvasive imaging plays an essential role in the management of patients with cardiovascular disease. Cardiac magnetic resonance (MR) can noninvasively assess heart and vascular abnormalities, including biventricular structure/function, blood hemodynamics, myocardial tissue composition, microstructure, perfusion, metabolism, coronary microvascular function, and aortic distensibility/stiffness. Its ability to characterize myocardial tissue composition is unique among alternative imaging modalities in cardiovascular disease. Significant growth in cardiac MR utilization, particularly in Europe in the last decade, has laid the necessary clinical groundwork to position cardiac MR as an important imaging modality in the workup of patients with cardiovascular disease. Although lack of availability, limited training, physician hesitation, and reimbursement issues have hampered widespread clinical adoption of cardiac MR in the United States, growing clinical evidence will ultimately overcome these challenges. Advances in cardiac MR techniques, particularly faster image acquisition, quantitative myocardial tissue characterization, and image analysis have been critical to its growth. In this review article, we discuss recent advances in established and emerging cardiac MR techniques that are expected to strengthen its capability in managing patients with cardiovascular disease. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 1.

Metabolomics and cardiovascular imaging: a combined approach for cardiovascular ageing

The purpose of this review is to explore how metabolomics can help uncover new biomarkers and mechanisms for cardiovascular ageing. Cardiovascular ageing refers to cardiovascular structural and functional alterations that occur with chronological ageing and that can lead to the development of cardiovascular disease. These alterations, which were previously only detectable on tissue histology or corroborated on blood samples, are now detectable with modern imaging techniques. Despite the emergence of powerful new imaging tools, clinical investigation into cardiovascular ageing is challenging because ageing is a life course phenomenon involving known and unknown risk factors that play out in a dynamic fashion. Metabolomic profiling measures large numbers of metabolites with diverse chemical properties. Metabolomics has the potential to capture changes in biochemistry brought about by pathophysiologic processes as well as by normal ageing. When combined with non-invasive cardiovascular imaging tools, metabolomics can be used to understand pathological consequences of cardiovascular ageing. This review will summarize previous metabolomics and imaging studies in cardiovascular ageing. These methods may be a clinically relevant and novel approach to identify mechanisms of cardiovascular ageing and formulate or personalize treatment strategies.

Multi-sample measurement of hyperpolarized pyruvate-to-lactate flux in melanoma cells

Hyperpolarized [1-13 C] pyruvate can be used to examine the metabolic state of cancer cells, highlighting a key metabolic characteristic of cancer: the upregulated metabolic flux to lactate, even in the presence of oxygen (Warburg effect). Thus, the rate constant of 13 C exchange of pyruvate to lactate, kPL , can serve as a metabolic biomarker of cancer presence, aggressiveness and therapy response. Established in vitro hyperpolarized experiments dissolve the probe for each cell sample independently, an inefficient process that consumes excessive time and resources. Expanding on our previous development of a microcoil with greatly increased detection sensitivity (103 -fold) compared with traditional in vitro methods, we present a novel microcoil equipped with a 10-μL vertical reservoir and an experimental protocol utilizing deuterated dissolution buffer to measure metabolic flux in multiple mass-limited cell suspension samples using a single dissolution. This method increases efficiency and potentially reduces the methodological variability associated with hyperpolarized experiments. This technique was used to measure pyruvate-to-lactate flux in melanoma cells to assess BRAF-inhibition treatment response. There was a significant reduction of kPL in BRAFV600E cells following 24 and 48 hours of treatment with 2 μM vemurafenib (P ≤ .05). This agrees with significant changes observed in the pool sizes of extracellular lactate (P ≤ .05) and glucose (P ≤ .001) following 6 and 48 hours of treatment, respectively, and a significant reduction in cell proliferation following 72 hours of treatment (P ≤ .01). BRAF inhibition had no significant effect on the metabolic flux of BRAFWT cells. These data demonstrate a 6-8-fold increase in efficiency for the measurement of kPL in cell suspension samples compared with traditional hyperpolarized in vitro methods.

Correlation between lactate dehydrogenase/pyruvate dehydrogenase activities ratio and tissue pH in the perfused mouse heart: A potential noninvasive indicator of cardiac pH provided by hyperpolarized magnetic resonance

Cardiovascular diseases account for more than 30% of all deaths worldwide and many could be ameliorated with early diagnosis. Current cardiac imaging modalities can assess blood flow, heart anatomy and mechanical function. However, for early diagnosis and improved treatment, further functional biomarkers are needed. One such functional biomarker could be the myocardium pH. Although tissue pH is already determinable via MR techniques, and has been since the early 1990s, it remains elusive to use practically. The objective of this study was to explore the possibility to evaluate cardiac pH noninvasively, using in-cell enzymatic rates of hyperpolarized [1-¹³ C]pyruvate metabolism (ie, moles of product produced per unit time) determined directly in real time using magnetic resonance spectroscopy in a perfused mouse heart model. As a gold standard for tissue pH we used ³¹ P spectroscopy and the chemical shift of the inorganic phosphate (Pi) signal. The nonhomogenous pH distribution of the perfused heart was analyzed using a multi-parametric analysis of this signal, thus taking into account the heterogeneous nature of this characteristic. As opposed to the signal ratio of hyperpolarized [¹³ C]bicarbonate to [¹³ CO₂ ], which has shown correlation to pH in other studies, we investigated here the ratio of two intracellular enzymatic rates: lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH), by way of determining the production rates of [1-¹³ C]lactate and [¹³ C]bicarbonate, respectively. The enzyme activities determined here are intracellular, while the pH determined using the Pi signal may contain an extracellular component, which could not be ruled out. Nevertheless, we report a strong correlation between the tissue pH and the LDH/PDH activities ratio. This work may pave the way for using the LDH/PDH activities ratio as an indicator of cardiac intracellular pH in vivo, in an MRI examination.

MRI of [2-13 C]Lactate without J-coupling artifacts

PURPOSE

Imaging of [2-¹³ C]lactate, a metabolic product of [2-¹³ C]pyruvate, is over considerable interest in hyperpolarized ¹³ C studies. However, artifact-free imaging of a J-coupled nuclear spin species can be challenging due to the peak-splitting induced by the spin-spin interactions. In this work, two new techniques resolving these J-modulated artifacts are presented.

THEORY AND METHODS

The Product Operator Formalism (POF) of density matrix theory is used to both numerically and analytically derive the coherences arising during radiofrequency excitation and readout of a J-coupled spin system. A combination of computer simulations and experiments with [2-¹³ C]lactate and ¹³ C-formate phantoms are then used to verify the performance of two imaging methods. In the first approach, a quadrature imaging technique is used to eliminate scalar coupling artifacts via the combination of in-phase and quadrature images acquired at echo times differing by 1/2J with an echoplanar readout. The second approach employs a highly narrowband RF excitation pulse to image a single peak from the J-coupled doublet.

RESULTS

Simulations using a numerical Shepp-Logan phantom, in vitro experiments using thermally polarized [2-¹³ C]lactate, thermally and hyperpolarized ¹³ C-formate phantoms, and in vivo imaging of [2-¹³ C]lactate produced in rat brain following injection of hyperpolarized [2-¹³ C]pyruvate show artifact-free images and demonstrate potential utility of these methods.

CONCLUSION

The quadrature imaging and the narrowband excitation techniques resolve the J-coupling induced ghosting and blurring artifacts present with conventional MRI of J-coupled signals such as [2-¹³ C]lactate.

Neutrophil-Mediated Cardiac Damage After Acute Myocardial Infarction: Significance of Defining a New Target Cell Type for Developing Cardioprotective Drugs

Significance: Acute myocardial infarction (AMI) is a leading cause of death worldwide. Post-AMI survival rates have increased with the introduction of angioplasty as a primary coronary intervention. However, reperfusion after angioplasty represents a clinical paradox, restoring blood flow to the ischemic myocardium while simultaneously inducing ion and metabolic imbalances that stimulate immune cell recruitment and activation, mitochondrial dysfunction and damaging oxidant production. Recent Advances: Preclinical data indicate that these metabolic imbalances contribute to subsequent heart failure through sustaining local recruitment of inflammatory leukocytes and oxidative stress, cardiomyocyte death, and coronary microvascular disturbances, which enhance adverse cardiac remodeling. Both left ventricular dysfunction and heart failure are strongly linked to inflammation and immune cell recruitment to the damaged myocardium. Critical Issues: Overall, therapeutic anti-inflammatory and antioxidant agents identified in preclinical trials have failed in clinical trials. Future Directions: The versatile neutrophil-derived heme enzyme, myeloperoxidase (MPO), is gaining attention as an important oxidative mediator of reperfusion injury, vascular dysfunction, adverse ventricular remodeling, and atrial fibrillation. Accordingly, there is interest in therapeutically targeting neutrophils and MPO activity in the setting of heart failure. Herein, we discuss the role of post-AMI inflammation linked to myocardial damage and heart failure, describe previous trials targeting inflammation and oxidative stress post-AMI, highlight the potential adverse impact of neutrophil and MPO, and detail therapeutic options available to target MPO clinically in AMI patients.

Clinical Cardiovascular Applications of Hyperpolarized Magnetic Resonance

Current cardiovascular magnetic resonance imaging techniques provide an exquisite assessment of the structure and function of the heart and great vessels, but their ability to assess the molecular processes that underpin changes in cardiac function in health and disease is limited by inherent insensitivity. Hyperpolarized magnetic resonance is a new technology which overcomes this limitation, generating molecular contrast agents with an improvement in magnetic resonance signal of up to five orders of magnitude. One key molecule, hyperpolarized [1-¹³C]pyruvate, shows particular promise for the assessment of cardiac energy metabolism and other fundamental biological processes in cardiovascular disease. This molecule has numerous potential applications of clinical relevance and has now been translated to human use in early clinical studies. This review outlines the principles of hyperpolarized magnetic resonance and key potential cardiovascular applications for this new technology. Finally, we provide an overview of the pipeline for forthcoming hyperpolarized agents and their potential applications in cardiovascular disease.

Incremental prognostic value of a novel metabolite-based biomarker score in congestive heart failure patients

Aims

The Cardiac Lipid Panel (CLP) is a newly discovered panel of metabolite‐based biomarkers that has shown to improve the diagnostic value of N terminal pro B type natriuretic peptide (NT‐proBNP). However, little is known about its usefulness in predicting outcomes. In this study, we developed a risk score for 4‐year cardiovascular death in elderly chronic heart failure (CHF) patients using the CLP.

Methods and results

From the Cardiac Insufficiency Bisoprolol Study in Elderly trial, we included 280 patients with CHF aged >65 years. A targeted metabolomic analysis of the CLP biomarkers was performed on baseline serum samples. Cox regression was used to determine the association of the biomarkers with the outcome after accounting for established risk factors. A risk score ranging from 0 to 4 was calculated by counting the number of biomarkers above the cut‐offs, using Youden index. During the mean (standard deviation) follow‐up period of 50 (8) months, 35 (18%) subjects met the primary endpoint of cardiovascular death. The area under the receiver operating curve for the model based on clinical variables was 0.84, the second model with NT‐proBNP was 0.86, and the final model with the CLP was 0.90. The categorical net reclassification index was 0.25 using three risk categories: 0–60% (low), 60–85% (intermediate), and >85% (high). The continuous net reclassification index was 0.772, and the integrated discrimination index was 0.104.

Conclusions

In patients with CHF, incorporating a panel of three metabolite‐based biomarkers into a risk score improved the prognostic utility of NT‐proBNP by predicting long‐term cardiovascular death more precisely. This novel approach holds promise to improve clinical risk assessment in CHF patients.

Metabolomics and the pig model reveal aberrant cardiac energy metabolism in metabolic syndrome

Although metabolic syndrome (MS) is a significant risk of cardiovascular disease (CVD), the cardiac response (MR) to MS remains unclear due to traditional MS models' narrow scope around a limited number of cell-cycle regulation biomarkers and drawbacks of limited human tissue samples. To date, we developed the most comprehensive platform studying MR to MS in a pig model tightly related to human MS criteria. By incorporating comparative metabolomic, transcriptomic, functional analyses, and unsupervised machine learning (UML), we can discover unknown metabolic pathways connections and links on numerous biomarkers across the MS-associated issues in the heart. For the first time, we show severely diminished availability of glycolytic and citric acid cycle (CAC) pathways metabolites, altered expression, GlcNAcylation, and activity of involved enzymes. A notable exception, however, is the excessive succinate accumulation despite reduced succinate dehydrogenase complex iron-sulfur subunit b (SDHB) expression and decreased content of precursor metabolites. Finally, the expression of metabolites and enzymes from the GABA-glutamate, GABA-putrescine, and the glyoxylate pathways significantly increase, suggesting an alternative cardiac means to replenish succinate and malate in MS. Our platform discovers potential therapeutic targets for MS-associated CVD within pathways that were previously unknown to corelate with the disease.

Targeting mitochondria for cardiovascular disorders: therapeutic potential and obstacles

A large body of evidence indicates that mitochondrial dysfunction has a major role in the pathogenesis of multiple cardiovascular disorders. Over the past 2 decades, extraordinary efforts have been focused on the development of agents that specifically target mitochondria for the treatment of cardiovascular disease. Despite such an intensive wave of investigation, no drugs specifically conceived to modulate mitochondrial functions are currently available for the clinical management of cardiovascular disease. In this Review, we discuss the therapeutic potential of targeting mitochondria in patients with cardiovascular disease, examine the obstacles that have restrained the development of mitochondria-targeting agents thus far, and identify strategies that might empower the full clinical potential of this approach.

UPLC-QTOF/MS-based metabolomics analysis of plasma reveals an effect of Xue-Fu-Zhu-Yu capsules on blood-stasis syndrome in CHD rats

ETHNOPHARMACOLOGICAL RELEVANCE

Blood-stasis syndrome (BSS) is a specific ZHENG type of coronary heart disease (CHD) in traditional Chinese medicine (TCM). The Xue-Fu-Zhu-Yu (XFZY) decoction is a common herbal formula that has been used for several centuries to treat BSS, but its mechanism has not been thoroughly elucidated to date.

AIM OF THE STUDY

In this study, serum lipid, blood haemorheology and metabolomics analyses were performed to depict a complete profile of XFZY capsules for the treatment of CHD with BSS and to reveal the potential mechanism of the XFZY capsules.

MATERIALS AND METHODS

A rat model of CHD with BSS was generated by combining a high-fat diet (HFD) with a left anterior descending coronary artery (LAD) ligation. After four weeks of treatment with XFZY capsules or simvastatin pills, an echocardiography was performed for a therapeutic evaluation. Blood samples and heart tissues were then collected for further analyses. A UPLC-QTOF/MS-based metabolomics analysis of the plasma was performed, and all metabolic features were fit by PCA and OPLS-DA pattern for the biomarker screen. The identified biomarkers were later implemented into a metabolic pathway analysis. Furthermore, we used qRT-PCR and Western blot analyses to verify the treatment effects of the XFZY capsules.

RESULTS

A total of 49 metabolites (VIP>1.0, p < 0.05, RSD%<20%) were identified in the Model rats, and 27 metabolites (VIP>1.0, p < 0.05, RSD%<20%) were identified in the XFZY-H rats. The results of the pathway analysis indicated that the XFZY capsules treated CHD primarily by regulating cardiac energy, phospholipid, polyunsaturated fatty acid (PUFA) and amino acid metabolism. In addition, blood viscosity and serum lipid assays suggested that XFZY capsules could decrease serum triglycerides, total cholesterol, low-density lipoprotein cholesterol and whole blood viscosity at a low shear rate.

CONCLUSION

This study demonstrated that the XFZY capsule effectively decreases serum lipids and whole blood viscosity in CHD with BSS. The underlying metabolic mechanism mainly included improving cardiac energy supply, reducing phospholipid peroxide, maintaining the PUFA metabolic balance and regulating amino acid metabolism.

Hyperpolarized 13C MRI: State of the Art and Future Directions

Hyperpolarized (HP) carbon 13 (¹³C) MRI is an emerging molecular imaging method that allows rapid, noninvasive, and pathway-specific investigation of dynamic metabolic and physiologic processes that were previously inaccessible to imaging. This technique has enabled real-time in vivo investigations of metabolism that are central to a variety of diseases, including cancer, cardiovascular disease, and metabolic diseases of the liver and kidney. This review provides an overview of the methods of hyperpolarization and ¹³C probes investigated to date in preclinical models of disease. The article then discusses the progress that has been made in translating this technology for clinical investigation. In particular, the potential roles and emerging clinical applications of HP [1-¹³C]pyruvate MRI will be highlighted. The future directions to enable the adoption of this technology to advance the basic understanding of metabolism, to improve disease diagnosis, and to accelerate treatment assessment are also detailed.

Mitochondria as a therapeutic target for common pathologies

Although the development of mitochondrial therapies has largely focused on diseases caused by mutations in mitochondrial DNA or in nuclear genes encoding mitochondrial proteins, it has been found that mitochondrial dysfunction also contributes to the pathology of many common disorders, including neurodegeneration, metabolic disease, heart failure, ischaemia-reperfusion injury and protozoal infections. Mitochondria therefore represent an important drug target for these highly prevalent diseases. Several strategies aimed at therapeutically restoring mitochondrial function are emerging, and a small number of agents have entered clinical trials. This Review discusses the opportunities and challenges faced for the further development of mitochondrial pharmacology for common pathologies.

Changing Metabolism in Differentiating Cardiac Progenitor Cells-Can Stem Cells Become Metabolically Flexible Cardiomyocytes?

The heart is a metabolic omnivore and the adult heart selects the substrate best suited for each circumstance, with fatty acid oxidation preferred in order to fulfill the high energy demand of the contracting myocardium. The fetal heart exists in an hypoxic environment and obtains the bulk of its energy via glycolysis. After birth, the "fetal switch" to oxidative metabolism of glucose and fatty acids has been linked to the loss of the regenerative phenotype. Various stem cell types have been used in differentiation studies, but most are cultured in high glucose media. This does not change in the majority of cardiac differentiation protocols. Despite the fact that metabolic state affects marker expression and cellular function and activity, the substrate composition is currently being overlooked. In this review we discuss changes in cardiac metabolism during development, the various protocols used to differentiate progenitor cells to cardiomyocytes, what is known about stem cell metabolism and how consideration of metabolism can contribute toward maturation of stem cell-derived cardiomyocytes.

Assessing the optimal preparation strategy to minimize the variability of cardiac pyruvate dehydrogenase flux measurements with hyperpolarized MRS

Hyperpolarized [1-13 C] pyruvate MRS can measure cardiac pyruvate dehydrogenase (PDH) flux in vivo through 13 C-label incorporation into bicarbonate. Using this technology, substrate availability as well as pathology have been shown to modulate PDH flux. Clinical protocols attempt to standardize PDH flux with oral glucose loading prior to scanning, while rodents in preclinical studies are usually scanned in the fed state. We aimed to establish which strategy was optimal to maximize PDH flux and minimize its variability in both control and Type II diabetic rats, without affecting the pathological variation being assessed. We found similar variances in the bicarbonate to pyruvate ratio, reflecting PDH flux, in fed and fasted/glucose-loaded animals, which showed no statistically significant differences. Furthermore, fasting/glucose loading did not alter the low PDH flux seen in Type II diabetic rats. Overall this suggests that preclinical cardiac hyperpolarized magnetic resonance studies could be performed either in the fed or in the fasted/glucose-loaded state. Centres planning to start new clinical studies with cardiac hyperpolarized magnetic resonance in man may find it beneficial to run small proof-of-concept trials to determine whether metabolic standardizations by oral or intravenous glucose load are beneficial compared with scanning patients in the fed state.

Increasing cardiac pyruvate dehydrogenase flux during chronic hypoxia improves acute hypoxic tolerance

KEY POINTS

The cardiac metabolic reprogramming seen in heart diseases such as myocardial infarction and hypertrophy shares similarities with that seen in chronic hypoxia, but understanding of how the hypoxic heart responds to further hypoxic challenge - hypoxic tolerance - is limited. The pyruvate dehydrogenase complex serves to control irreversible decarboxylation of pyruvate within mitochondria, and is a key regulator of substrate metabolism, potentially regulating hypoxic tolerance. Acute activation of the pyruvate dehydrogenase complex did not improve cardiac function during acute hypoxia; however, simultaneous activation of the pyruvate dehydrogenase complex during chronic hypoxic exposure improved tolerance to subsequent acute hypoxia. Activation of the pyruvate dehydrogenase complex during chronic hypoxia stockpiled cardiac acetylcarnitine, and this was used during acute hypoxia. This maintained cardiac ATP and glycogen, and improved hypoxic tolerance as a result. These findings demonstrate that pyruvate dehydrogenase complex activation can improve cardiac function under hypoxia.

ABSTRACT

The pattern of metabolic reprogramming in chronic hypoxia shares similarities with that following myocardial infarction or hypertrophy; however, the response of the chronically hypoxic heart to subsequent acute injury, and the role of metabolism is not well understood. Here, we determined the myocardial tolerance of the chronically hypoxic heart to subsequent acute injury, and hypothesised that activation of a key regulator of myocardial metabolism, the pyruvate dehydrogenase complex (PDC), could improve hypoxic tolerance. Mouse hearts, perfused in Langendorff mode, were exposed to 30 min of hypoxia, and lost 80% of pre-hypoxic function (P = 0.001), with only 51% recovery of pre-hypoxic function with 30 min of reoxygenation (P = 0.046). Activation of the PDC with infusion of 1 mm dichloroacetate (DCA) during hypoxia and reoxygenation did not alter function. Acute hypoxic tolerance was assessed in hearts of mice housed in hypoxia for 3 weeks. Chronic hypoxia reduced cardiac tolerance to subsequent acute hypoxia, with recovery of function 22% of pre-acute hypoxic levels vs. 39% in normoxic control hearts (P = 0.012). DCA feeding in chronic hypoxia (per os, 70 mg kg-1  day-1 ) doubled cardiac acetylcarnitine content, and this fell following acute hypoxia. This acetylcarnitine use maintained cardiac ATP and glycogen content during acute hypoxia, with hypoxic tolerance normalised. In summary, chronic hypoxia renders the heart more susceptible to acute hypoxic injury, which can be improved by activation of the PDC and pooling of acetylcarnitine. This is the first study showing functional improvement of the chronically hypoxic heart with activation of the PDC, and offers therapeutic potential in cardiac disease with a hypoxic component.

Loss of Metabolic Flexibility in the Failing Heart

To maintain its high energy demand the heart is equipped with a highly complex and efficient enzymatic machinery that orchestrates ATP production using multiple energy substrates, namely fatty acids, carbohydrates (glucose and lactate), ketones and amino acids. The contribution of these individual substrates to ATP production can dramatically change, depending on such variables as substrate availability, hormonal status and energy demand. This "metabolic flexibility" is a remarkable virtue of the heart, which allows utilization of different energy substrates at different rates to maintain contractile function. In heart failure, cardiac function is reduced, which is accompanied by discernible energy metabolism perturbations and impaired metabolic flexibility. While it is generally agreed that overall mitochondrial ATP production is impaired in the failing heart, there is less consensus as to what actual switches in energy substrate preference occur. The failing heart shift toward a greater reliance on glycolysis and ketone body oxidation as a source of energy, with a decrease in the contribution of glucose oxidation to mitochondrial oxidative metabolism. The heart also becomes insulin resistant. However, there is less consensus as to what happens to fatty acid oxidation in heart failure. While it is generally believed that fatty acid oxidation decreases, a number of clinical and experimental studies suggest that fatty acid oxidation is either not changed or is increased in heart failure. Of importance, is that any metabolic shift that does occur has the potential to aggravate cardiac dysfunction and the progression of the heart failure. An increasing body of evidence shows that increasing cardiac ATP production and/or modulating cardiac energy substrate preference positively correlates with heart function and can lead to better outcomes. This includes increasing glucose and ketone oxidation and decreasing fatty acid oxidation. In this review we present the physiology of the energy metabolism pathways in the heart and the changes that occur in these pathways in heart failure. We also look at the interventions which are aimed at manipulating the myocardial metabolic pathways toward more efficient substrate utilization which will eventually improve cardiac performance.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

Hyperpolarised magnetic resonance for in vivo real-time metabolic imaging

Although non-invasive perfusion and viability imaging often provide the gateway to coronary revascularisation, current non-invasive imaging methods only report the surrogate markers of inducible hypoperfusion and presence or absence of myocardial scar, rather than actually visualising areas of ischaemia and/or viable myocardium. This may lead to suboptimal revascularisation decisions. Normally respiring (viable) cardiomyocytes convert pyruvate to acetyl-CoA and CO₂/bicarbonate (via pyruvate dehydrogenase), but under ischaemic conditions characteristically shift this conversion to lactate (by lactate dehydrogenase). Imaging pyruvate metabolism thus has the potential to improve upon current imaging techniques. Using the novel hyperpolarisation technique of dynamic nuclear polarisation (DNP), the magnetic resonance signal of injected [1-¹³C]pyruvate can be transiently magnified >10 000 times over that seen in conventional MR spectroscopy, allowing the characteristic metabolic signatures of ischaemia (lactate production) and viability (CO₂/bicarbonate production) to be directly imaged. As such DNP imaging of the downstream metabolism of [1-¹³C]pyruvate could surpass the diagnostic capabilities of contemporary ischaemia and viability testing. Here we review the technique, and with brief reference to the salient biochemistry, discuss its potential applications within cardiology. These include ischaemia and viability testing, and further characterisation of the altered metabolism seen at different stages during the natural history of heart failure.

Overestimation of cardiac lactate production caused by liver metabolism of hyperpolarized [1-13 C]pyruvate

PURPOSE

The purpose of this work was to study the contribution of liver [1-¹³ C]lactate to the lactate signal detected in the heart following injection of hyperpolarized [1-¹³ C]pyruvate.

METHODS

A slice-selective saturation scheme was incorporated into a hybrid metabolic imaging and spectroscopy approach to selectively presaturate lactate in the liver. Imaging and slice-selective spectroscopy of [1-¹³ C]pyruvate and its downstream metabolites were sequentially interleaved in the same experiment with optional presaturation of liver [1-¹³ C]lactate. Six healthy rats were measured, and metabolic data in the heart acquired with and without presaturation of liver lactate were compared.

RESULTS

When using liver lactate presaturation, a statistically significant reduction of the lactate/pyruvate ratio was observed in the spectroscopic data of the left ventricle (0.18 ± 0.03 versus 0.24 ± 0.04; p < .05) as well as in the imaging data of the blood pool (0.05 ± 0.01 versus 0.11 ± 0.01; p < .05). No significant difference in myocardial lactate was observed when using myocardium only as the region of interest in the imaging data (0.08 ± 0.01 versus 0.11 ± 0.02; p = .2).

CONCLUSION

Liver metabolism leads to statistically significant overestimation of cardiac lactate production in slice-selective or nonselective spectroscopic experiments. Therefore, metabolic imaging is preferred over spectroscopy to separate left-ventricular compartments within the slice and hence avoid contamination of cardiac lactate signals. Alternatively, presaturation pulses should be used in combination with spectroscopy approaches.

Cardiac and placental mitochondrial characterization in a rabbit model of intrauterine growth restriction

BACKGROUND

Intrauterine growth restriction (IUGR) is associated with cardiovascular remodeling persisting into adulthood. Mitochondrial bioenergetics, essential for embryonic development and cardiovascular function, are regulated by nuclear effectors as sirtuins. A rabbit model of IUGR and cardiovascular remodeling was generated, in which heart mitochondrial alterations were observed by microscopic and transcriptomic analysis. We aimed to evaluate if such alterations are translated at a functional mitochondrial level to establish the etiopathology and potential therapeutic targets for this obstetric complication.

METHODS

Hearts and placentas from 16 IUGR-offspring and 14 controls were included to characterize mitochondrial function.

RESULTS

Enzymatic activities of complexes II, IV and II + III in IUGR-hearts (-11.96 ± 3.16%; -15.58 ± 5.32%; -14.73 ± 4.37%; p < 0.05) and II and II + III in IUGR-placentas (-17.22 ± 3.46%; p < 0.005 and -29.64 ± 4.43%; p < 0.001) significantly decreased. This was accompanied by a not significant reduction in CI-stimulated oxygen consumption and significantly decreased complex II SDHB subunit expression in placenta (-44.12 ± 5.88%; p < 0.001). Levels of mitochondrial content, Coenzyme Q and cellular ATP were conserved. Lipid peroxidation significantly decreased in IUGR-hearts (-39.02 ± 4.35%; p < 0.001), but not significantly increased in IUGR-placentas. Sirtuin3 protein expression significantly increased in IUGR-hearts (84.21 ± 31.58%; p < 0.05) despite conserved anti-oxidant SOD2 protein expression and activity in both tissues.

CONCLUSIONS

IUGR is associated with cardiac and placental mitochondrial CII dysfunction. Up-regulated expression of Sirtuin3 may explain attenuation of cardiac oxidative damage and preserved ATP levels under CII deficiency.

GENERAL SIGNIFICANCE

These findings may allow the design of dietary interventions to modulate Sirtuin3 expression and consequent regulation of mitochondrial imbalance associated with IUGR and derived cardiovascular remodeling.

Probing the cardiac malate-aspartate shuttle non-invasively using hyperpolarized [1,2-13 C2 ]pyruvate

Previous studies have demonstrated that using hyperpolarized [2-¹³ C]pyruvate as a contrast agent can reveal ¹³ C signals from metabolites associated with the tricarboxylic acid (TCA) cycle. However, the metabolites detectable from TCA cycle-mediated oxidation of [2-¹³ C]pyruvate are the result of several metabolic steps. In the instance of the [5-¹³ C]glutamate signal, the amplitude can be modulated by changes to the rates of pyruvate dehydrogenase (PDH) flux, TCA cycle flux and metabolite pool size. Also key is the malate-aspartate shuttle, which facilitates the transport of cytosolic reducing equivalents into the mitochondria for oxidation via the malate-α-ketoglutarate transporter, a process coupled to the exchange of cytosolic malate for mitochondrial α-ketoglutarate. In this study, we investigated the mechanism driving the observed changes to hyperpolarized [2-¹³ C]pyruvate metabolism. Using hyperpolarized [1,2-¹³ C]pyruvate with magnetic resonance spectroscopy (MRS) in the porcine heart with different workloads, it was possible to probe ¹³ C-glutamate labeling relative to rates of cytosolic metabolism, PDH flux and TCA cycle turnover in a single experiment non-invasively. Via the [1-¹³ C]pyruvate label, we observed more than a five-fold increase in the cytosolic conversion of pyruvate to [1-¹³ C]lactate and [1-¹³ C]alanine with higher workload. ¹³ C-Bicarbonate production by PDH was increased by a factor of 2.2. Cardiac cine imaging measured a two-fold increase in cardiac output, which is known to couple to TCA cycle turnover. Via the [2-¹³ C]pyruvate label, we observed that ¹³ C-acetylcarnitine production increased 2.5-fold in proportion to the ¹³ C-bicarbonate signal, whereas the ¹³ C-glutamate metabolic flux remained constant on adrenergic activation. Thus, the ¹³ C-glutamate signal relative to the amount of ¹³ C-labeled acetyl-coenzyme A (acetyl-CoA) entering the TCA cycle was decreased by 40%. The data strongly suggest that NADH (reduced form of nicotinamide adenine dinucleotide) shuttling from the cytosol to the mitochondria via the malate-aspartate shuttle is limited on adrenergic activation. Changes in [5-¹³ C]glutamate production from [2-¹³ C]pyruvate may play an important future role in non-invasive myocardial assessment in patients with cardiovascular diseases, but careful interpretation of the results is required.

Mitochondrial Bioenergetics and Dysfunction in Failing Heart

Energy insufficiency has been recognized as a key feature of systolic heart failure. Although mitochondria have long been known to sustain myocardial work energy supply, the capacity to therapeutically target mitochondrial bioenergetics dysfunction is hampered by a complex interplay of multiple perturbations that progressively compound causing myocardial failure and collapse. Compared to non-failing human donor hearts, activity rates of complexes I and IV, nicotinamide nucleotide transhydrogenase (NADPH-transhydrogenase, Nnt) and the Krebs cycle enzymes isocitrate dehydrogenase, malate dehydrogenase and aconitase are markedly decreased in end-stage heart failure. Diminished REDOX capacity with lower total glutathione and coenzyme Q₁₀ levels are also a feature of chronic left ventricular failure. Decreased enzyme activities in part relate to abundant and highly specific oxidative, nitrosylative, and hyperacetylation modifications. In this brief review we highlight that energy deficiency in end-stage failing human left ventricle predominantly involves concomitantly impaired activities of key electron transport chain and Krebs cycle enzymes rather than altered expression of respective genes or proteins. Augmented oxidative modification of these enzyme subunit structures, and the formation of highly reactive secondary metabolites, implicates dysfunction due to diminished capacity for management of mitochondrial reactive oxygen species, which contribute further to progressive decreases in bioenergetic capacity and contractile function in human heart failure.

The ‘Goldilocks zone’ of fatty acid metabolism; to ensure that the relationship with cardiac function is just right

Fatty acids (FA) are the main fuel used by the healthy heart to power contraction, supplying 60–70% of the ATP required. FA generate more ATP per carbon molecule than glucose, but require more oxygen to produce the ATP, making them a more energy dense but less oxygen efficient fuel compared with glucose. The pathways involved in myocardial FA metabolism are regulated at various subcellular levels, and can be divided into sarcolemmal FA uptake, cytosolic activation and storage, mitochondrial uptake and β-oxidation. An understanding of the critical involvement of each of these steps has been amassed from genetic mouse models, where forcing the heart to metabolize too much or too little fat was accompanied by cardiac contractile dysfunction and hypertrophy. In cardiac pathologies, such as heart disease and diabetes, aberrations in FA metabolism occur concomitantly with changes in cardiac function. In heart failure, FA oxidation is decreased, correlating with systolic dysfunction and hypertrophy. In contrast, in type 2 diabetes, FA oxidation and triglyceride storage are increased, and correlate with diastolic dysfunction and insulin resistance. Therefore, too much FA metabolism is as detrimental as too little FA metabolism in these settings. Therapeutic compounds that rebalance FA metabolism may provide a mechanism to improve cardiac function in disease. Just like Goldilocks and her porridge, the heart needs to maintain FA metabolism in a zone that is ‘just right’ to support contractile function.

Dl-3-n-butylphthalide protects the heart against ischemic injury and H9c2 cardiomyoblasts against oxidative stress: involvement of mitochondrial function and biogenesis

Background

Myocardial infarction (MI) is an acute and fatal condition that threatens human health. Dl-3-n-butylphthalide (NBP) has been used for the treatment of acute ischemic stroke. Mitochondria may play a protective role in MI injury. However, there are few reports on the cardioprotective effect of NBP or the potential mitochondrial mechanism for the NBP-induced protection against cardiac ischemia injury. We investigated the therapeutic effects of NBP in an in vivo MI model and an in vitro oxidative stress model, as well as the potential mitochondrial mechanism.

Methods

This study comprised two different experiments. The aim of experiment 1 was to determine the protective effects of NBP on MI and the underlying mechanisms in vivo. In part 1, myocardial infarct size was measured by staining with 2,3,5-triphenyltetrazoliumchloride (TTC). Myocardial enzymes and mitochondrial enzymes were assayed. The aim of experiment 2 was to investigate the role of NBP in H₂O₂-induced myocardial ischemic injury in H9c2 cells and to determine the potential mechanism. In part 2, H9c2 cell viability was evaluated. ROS levels, mitochondrial morphology, and mitochondrial membrane potential of H9c2 cells were measured. ATP levels were evaluated using an assay kit; mitochondrial DNA (mtDNA), the expressions of NRF-1 and TFAM, and mitochondrial biogenesis factors were determined.

Results

NBP treatment significantly reduced the infarct ratio, as observed by TTC staining, decreased serum myocardial enzymes in MI, and restored heart mitochondrial enzymes (isocitrate dehydrogenase (ICDH), succinate dehydrogenase (SDH), malate dehydrogenase (MDH), and a-ketoglutarate dehydrogenase (a-KGDH) activities after MI. Moreover, in in vitro studies, NBP significantly increased the viability of H9c2 cells in a dose-dependent manner, reduced cell apoptosis, protected mitochondrial functions, elevated the cellular ATP levels, and promoted H₂O₂-induced mitochondrial biogenesis in H9c2 cardiomyoblasts.

Conclusion

Collectively, the results from both the in vivo and in vitro experiments suggested that NBP exerted a cardioprotective effect on cardiac ischemic injury via the regulation of mitochondrial function and biogenesis.

Imaging oxygen metabolism with hyperpolarized magnetic resonance: a novel approach for the examination of cardiac and renal function

Every tissue in the body critically depends on meeting its energetic demands with sufficient oxygen supply. Oxygen supply/demand imbalances underlie the diseases that inflict the greatest socio-economic burden globally. The purpose of this review is to examine how hyperpolarized contrast media, used in combination with MR data acquisition methods, may advance our ability to assess oxygen metabolism non-invasively and thus improve management of clinical disease. We first introduce the concept of hyperpolarization and how hyperpolarized contrast media have been practically implemented to achieve translational and clinical research. We will then analyse how incorporating hyperpolarized contrast media could enable realization of unmet technical needs in clinical practice. We will focus on imaging cardiac and renal oxygen metabolism, as both organs have unique physiological demands to satisfy their requirements for tissue oxygenation, their dysfunction plays a fundamental role in society’s most prevalent diseases, and each organ presents unique imaging challenges. It is our aim that this review attracts a multi-disciplinary audience and sparks collaborations that utilize an exciting, emergent technology to advance our ability to treat patients adversely affected by an oxygen supply/demand mismatch.

Combined Inhibition of the Renin-Angiotensin System and Neprilysin Positively Influences Complex Mitochondrial Adaptations in Progressive Experimental Heart Failure

BACKGROUND

Inhibitors of the renin angiotensin system and neprilysin (RAS-/NEP-inhibitors) proved to be extraordinarily beneficial in systolic heart failure. Furthermore, compelling evidence exists that impaired mitochondrial pathways are causatively involved in progressive left ventricular (LV) dysfunction. Consequently, we aimed to assess whether RAS-/NEP-inhibition can attenuate mitochondrial adaptations in experimental heart failure (HF).

METHODS AND RESULTS

By progressive right ventricular pacing, distinct HF stages were induced in 15 rabbits, and 6 animals served as controls (CTRL). Six animals with manifest HF (CHF) were treated with the RAS-/NEP-inhibitor omapatrilat. Echocardiographic studies and invasive blood pressure measurements were undertaken during HF progression. Mitochondria were isolated from LV tissue, respectively, and further worked up for proteomic analysis using the SWATH technique. Enzymatic activities of citrate synthase and the electron transfer chain (ETC) complexes I, II, and IV were assessed. Ultrastructural analyses were performed by transmission electron microscopy. During progression to overt HF, intricate expression changes were mainly detected for proteins belonging to the tricarboxylic acid cycle, glucose and fat metabolism, and the ETC complexes, even though ETC complex I, II, or IV enzymatic activities were not significantly influenced. Treatment with a RAS-/NEP-inhibitor then reversed some maladaptive metabolic adaptations, positively influenced the decline of citrate synthase activity, and altered the composition of each respiratory chain complex, even though this was again not accompanied by altered ETC complex enzymatic activities. Finally, ultrastructural evidence pointed to a reduction of autophagolytic and degenerative processes with omapatrilat-treatment.

CONCLUSIONS

This study describes complex adaptations of the mitochondrial proteome in experimental tachycardia-induced heart failure and shows that a combined RAS-/NEP-inhibition can beneficially influence mitochondrial key pathways.

Science to Practice: Hyperpolarized Metabolic MR Imaging--The Light at the End of the Tunnel for Clinical (13)C MR Spectroscopy?

Hyperpolarized metabolic magnetic resonance (MR) imaging offers greatly enhanced sensitivity to multinuclear MR spectroscopy, opening up a new tool with which to noninvasively assess metabolic changes in the diseased heart. In this issue of Radiology, O h-Ici et al ( 1 ) have demonstrated this ability by exploring the metabolic changes that occur in the setting of ischemia-reperfusion. They have shown the acute metabolic alterations that occur immediately after reperfusion (increased anaerobic and decreased oxidative metabolism) that then normalize over the following 60 minutes. This demonstration paves the way for the use of hyperpolarized metabolic MR imaging in the assessment of coronary artery disease in humans and shows the potential for this new tool to aid in the assessment of the diseased heart.

Measuring changes in substrate utilization in the myocardium in response to fasting using hyperpolarized [1-(13)C]butyrate and [1-(13)C]pyruvate

Cardiac dysfunction is often associated with a shift in substrate preference for ATP production. Hyperpolarized (HP) ¹³C magnetic resonance spectroscopy (MRS) has the unique ability to detect real-time metabolic changes in vivo due to its high sensitivity and specificity. Here a protocol using HP [1-¹³C]pyruvate and [1-¹³C]butyrate is used to measure carbohydrate versus fatty acid metabolism in vivo. Metabolic changes in fed and fasted Sprague Dawley rats (n = 36) were studied at 9.4 T after tail vein injections. Pyruvate and butyrate competed for acetyl-CoA production, as evidenced by significant changes in [¹³C]bicarbonate (−48%), [1-¹³C]acetylcarnitine (+113%), and [5-¹³C]glutamate (−63%), following fasting. Butyrate uptake was unaffected by fasting, as indicated by [1-¹³C]butyrylcarnitine. Mitochondrial pseudoketogenesis facilitated the labeling of the ketone bodies [1-¹³C]acetoacetate and [1-¹³C]β-hydroxybutyryate, without evidence of true ketogenesis. HP [1-¹³C]acetoacetate was increased in fasting (250%) but decreased during pyruvate co-injection (−82%). Combining HP ¹³C technology and co-administration of separate imaging agents enables noninvasive and simultaneous monitoring of both fatty acid and carbohydrate oxidation. This protocol illustrates a novel method for assessing metabolic flux through different enzymatic pathways simultaneously and enables mechanistic studies of the changing myocardial energetics often associated with disease.

Evidence for Intramyocardial Disruption of Lipid Metabolism and Increased Myocardial Ketone Utilization in Advanced Human Heart Failure

BACKGROUND

The failing human heart is characterized by metabolic abnormalities, but these defects remains incompletely understood. In animal models of heart failure there is a switch from a predominance of fatty acid utilization to the more oxygen-sparing carbohydrate metabolism. Recent studies have reported decreases in myocardial lipid content, but the inclusion of diabetic and nondiabetic patients obscures the distinction of adaptations to metabolic derangements from adaptations to heart failure per se.

METHODS AND RESULTS

We performed both unbiased and targeted myocardial lipid surveys using liquid chromatography-mass spectroscopy in nondiabetic, lean, predominantly nonischemic, advanced heart failure patients at the time of heart transplantation or left ventricular assist device implantation. We identified significantly decreased concentrations of the majority of myocardial lipid intermediates, including long-chain acylcarnitines, the primary subset of energetic lipid substrate for mitochondrial fatty acid oxidation. We report for the first time significantly reduced levels of intermediate and anaplerotic acyl-coenzyme A (CoA) species incorporated into the Krebs cycle, whereas the myocardial concentration of acetyl-CoA was significantly increased in end-stage heart failure. In contrast, we observed an increased abundance of ketogenic β-hydroxybutyryl-CoA, in association with increased myocardial utilization of β-hydroxybutyrate. We observed a significant increase in the expression of the gene encoding succinyl-CoA:3-oxoacid-CoA transferase, the rate-limiting enzyme for myocardial oxidation of β-hydroxybutyrate and acetoacetate.

CONCLUSIONS

These findings indicate increased ketone utilization in the severely failing human heart independent of diabetes mellitus, and they support the role of ketone bodies as an alternative fuel and myocardial ketone oxidation as a key metabolic adaptation in the failing human heart.

An intact small animal model of myocardial ischemia-reperfusion: Characterization of metabolic changes by hyperpolarized 13C MR spectroscopy

Hyperpolarized carbon-13 magnetic resonance spectroscopy ((13)C MRS) enables the sensitive and noninvasive assessment of the metabolic changes occurring during myocardial ischemia-reperfusion. Ischemia-reperfusion models using hyperpolarized (13)C MRS are established in heart preparations ex vivo and in large animals in vivo, but an in vivo model in small animals would be advantageous to allow the study of reperfusion metabolism with neuroendocrine and inflammatory responses intact with the option to perform a greater number of experiments. A novel intact rat model of ischemia-reperfusion is presented that incorporates hyperpolarized (13)C MRS to characterize reperfusion metabolism. Typically, in an in vivo model, a tissue input function (TIF) is required to account for apparent changes in the metabolism of injected hyperpolarized [1-(13)C]pyruvate resulting from changes in perfusion. Whereas the measurement of a TIF by metabolic imaging is particularly challenging in small animals, the ratios of downstream metabolites can be used as an alternative. The ratio of [(13)C]bicarbonate:[1-(13)C]lactate (RatioBic/Lac) measured within 1-2 min after coronary release decreased vs. baseline in ischemic rats (n = 10, 15-min occlusion, controls: n = 10; P = 0.017 for interaction, 2-way ANOVA). The decrease in oxidative pyruvate metabolism [RatioBic/Lac(Ischemia)/RatioBic/Lac(Baseline)] modestly correlated with area at risk (r = 0.66; P = 0.002). Hyperpolarized (13)C MRS was also used to examine alanine production during ischemia, which is observed in ex vivo models, but no significant change was noted; metrics incorporating [1-(13)C]alanine did not substantially improve the discrimination of ischemic-reperfused myocardium from nonischemic myocardium. This intact rat model, which mimics the human situation of reperfused myocardial infarction, could be highly valuable for the testing of new drugs to treat reperfusion injury, thereby facilitating translational research.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

MRS: a noninvasive window into cardiac metabolism

A well-functioning heart requires a constant supply of a balanced mixture of nutrients to be used for the production of adequate amounts of adenosine triphosphate, which is the main energy source for most cellular functions. Defects in cardiac energy metabolism are linked to several myocardial disorders. MRS can be used to study in vivo changes in cardiac metabolism noninvasively. MR techniques allow repeated measurements, so that disease progression and the response to treatment or to a lifestyle intervention can be monitored. It has also been shown that MRS can predict clinical heart failure and death. This article focuses on in vivo MRS to assess cardiac metabolism in humans and experimental animals, as experimental animals are often used to investigate the mechanisms underlying the development of metabolic diseases. Various MR techniques, such as cardiac (31) P-MRS, (1) H-MRS, hyperpolarized (13) C-MRS and Dixon MRI, are described. A short overview of current and emerging applications is given. Cardiac MRS is a promising technique for the investigation of the relationship between cardiac metabolism and cardiac disease. However, further optimization of scan time and signal-to-noise ratio is required before broad clinical application. In this respect, the ongoing development of advanced shimming algorithms, radiofrequency pulses, pulse sequences, (multichannel) detection coils, the use of hyperpolarized nuclei and scanning at higher magnetic field strengths offer future perspective for clinical applications of MRS.