Fasting-induced myocardial lipid accumulation in long-chain acyl-CoA dehydrogenase knockout mice is accompanied by impaired left ventricular function

A review of fatty acid oxidation disorder mouse models

Fatty acid oxidation disorders (FAODs) are a family of rare, genetic disorders that affect any part of the fatty acid oxidation pathway. Patients present with severe phenotypes, such as hypoketotic hypoglycemia, cardiomyopathy, and rhabdomyolysis, and currently manage these symptoms by the avoidance of fasting and maintaining a low-fat, high-carbohydrate diet. Because knowledge about FAODs is limited due to the small number of patients, rodent models have been crucial in learning more about these disorders, particularly in studying the molecular mechanisms involved in different phenotypes and in evaluating treatments for patients. The purpose of this review is to present the different FAOD mouse models and highlight the benefits and limitations of using these models. Specifically, we discuss the phenotypes of the available FAOD mouse models, the potential molecular causes of prominent FAOD phenotypes that have been studied using FAOD mouse models, and how FAOD mouse models have been used to evaluate treatments for patients.

Tracer-based lipidomics enables the discovery of disease-specific candidate biomarkers in mitochondrial β-oxidation disorders

Carnitine derivatives of disease-specific acyl-CoAs are the diagnostic hallmark for long-chain fatty acid β-oxidation disorders (lcFAOD), including carnitine shuttle deficiencies, very-long-chain acyl-CoA dehydrogenase deficiency (VLCADD), long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) and mitochondrial trifunctional protein deficiency (MPTD). The exact consequence of accumulating lcFAO-intermediates and their influence on cellular lipid homeostasis is, however, still unknown. To investigate the fate and cellular effects of the accumulating lcFAO-intermediates and to explore the presence of disease-specific markers, we used tracer-based lipidomics with deuterium-labeled oleic acid (D9-C18:1) in lcFAOD patient-derived fibroblasts. In line with previous studies, we observed a trend towards neutral lipid accumulation in lcFAOD. In addition, we detected a direct connection between the chain length and patterns of (un)saturation of accumulating acylcarnitines and the various enzyme deficiencies. Our results also identified two disease-specific candidate biomarkers. Lysophosphatidylcholine(14:1) (LPC(14:1)) was specifically increased in severe VLCADD compared to mild VLCADD and control samples. This was confirmed in plasma samples showing an inverse correlation with enzyme activity, which was better than the classic diagnostic marker C14:1-carnitine. The second candidate biomarker was an unknown lipid class, which we identified as S-(3-hydroxyacyl)cysteamines. We hypothesized that these were degradation products of the CoA moiety of accumulating 3-hydroxyacyl-CoAs. S-(3-hydroxyacyl)cysteamines were significantly increased in LCHADD compared to controls and other lcFAOD, including MTPD. Our findings suggest extensive alternative lipid metabolism in lcFAOD and confirm that lcFAOD accumulate neutral lipid species. In addition, we present two disease-specific candidate biomarkers for VLCADD and LCHADD, that may have significant relevance for disease diagnosis, prognosis, and monitoring.

Remodeling of lipid landscape in high fat fed very-long chain acyl-CoA dehydrogenase null mice favors pro-arrhythmic polyunsaturated fatty acids and their downstream metabolites

Very-long chain acyl-CoA dehydrogenase (VLCAD) catalyzes the initial step of mitochondrial long chain (LC) fatty acid β-oxidation (FAO). Inherited VLCAD deficiency (VLCADD) predisposes to neonatal arrhythmias whose pathophysiology is still not understood. We hypothesized that VLCADD results in global disruption of cardiac complex lipid homeostasis, which may set conditions predisposing to arrhythmia. To test this, we assessed the cardiac lipidome and related molecular markers in seven-month-old VLCAD-/- mice, which mimic to some extent the human cardiac phenotype. Mice were sacrificed in the fed or fasted state after receiving for two weeks a chow or a high-fat diet (HFD), the latter condition being known to worsen symptoms in human VLCADD. Compared to their littermate counterparts, HFD/fasted VLCAD-/- mouse hearts displayed the following lipid alterations: (1) Lower LC, but higher VLC-acylcarnitines accumulation, (2) higher levels of arachidonic acid (AA) and lower docosahexaenoic acid (DHA) contents in glycerophospholipids (GPLs), as well as (3) corresponding changes in pro-arrhythmogenic AA-derived isoprostanes and thromboxane B2 (higher), and anti-arrythmogenic DHA-derived neuroprostanes (lower). These changes were associated with remodeling in the expression of gene or protein markers of (1) GPLs remodeling: higher calcium-dependent phospholipase A2 and lysophosphatidylcholine-acyltransferase 2, (2) calcium handling perturbations, and (3) endoplasmic reticulum stress. Altogether, these results highlight global lipid dyshomeostasis beyond FAO in VLCAD-/- mouse hearts, which may set conditions predisposing the hearts to calcium mishandling and endoplasmic reticulum stress and thereby may contribute to the pathogenesis of arrhythmias in VLCADD in mice as well as in humans.

A G1528C Hadha knock-in mouse model recapitulates aspects of human clinical phenotypes for long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency

Long chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) is a fatty acid oxidation disorder (FAOD) caused by a pathogenic variant, c.1528 G > C, in HADHA encoding the alpha subunit of trifunctional protein (TFPα). Individuals with LCHADD develop chorioretinopathy and peripheral neuropathy not observed in other FAODs in addition to the more ubiquitous symptoms of hypoketotic hypoglycemia, rhabdomyolysis and cardiomyopathy. We report a CRISPR/Cas9 generated knock-in murine model of G1528C in Hadha that recapitulates aspects of the human LCHADD phenotype. Homozygous pups are less numerous than expected from Mendelian probability, but survivors exhibit similar viability with wildtype (WT) littermates. Tissues of LCHADD homozygotes express TFPα protein, but LCHADD mice oxidize less fat and accumulate plasma 3-hydroxyacylcarnitines compared to WT mice. LCHADD mice exhibit lower ketones with fasting, exhaust earlier during treadmill exercise and develop a dilated cardiomyopathy compared to WT mice. In addition, LCHADD mice exhibit decreased visual performance, decreased cone function, and disruption of retinal pigment epithelium. Neurological function is affected, with impaired motor function during wire hang test and reduced open field activity. The G1528C knock-in mouse exhibits a phenotype similar to that observed in human patients; this model will be useful to explore pathophysiology and treatments for LCHADD in the future.

Circulating sphingolipids in heart failure

Lack of significant advancements in early detection and treatment of heart failure have precipitated the need for discovery of novel biomarkers and therapeutic targets. Over the past decade, circulating sphingolipids have elicited promising results as biomarkers that premonish adverse cardiac events. Additionally, compelling evidence directly ties sphingolipids to these events in patients with incident heart failure. This review aims to summarize the current literature on circulating sphingolipids in both human cohorts and animal models of heart failure. The goal is to provide direction and focus for future mechanistic studies in heart failure, as well as pave the way for the development of new sphingolipid biomarkers.

Myocardial steatosis impairs left ventricular diastolic-systolic coupling in healthy humans

Mounting evidence suggests that myocardial steatosis contributes to left ventricular diastolic dysfunction, but definitive evidence in humans is lacking due to confounding comorbidities. As such, we utilized a 48-h food restriction model to acutely increase myocardial triglyceride (mTG) content - measured by ¹ H magnetic resonance spectroscopy - in 27 young healthy volunteers (13 men/14 women). Forty-eight hours of fasting caused a more than 3-fold increase in mTG content (P < 0.001). Diastolic function - defined as early diastolic circumferential strain rate (CSRd) - was unchanged following the 48-h fasting intervention, but systolic circumferential strain rate was elevated (P < 0.001), indicative of systolic-diastolic uncoupling. Indeed, in a separate control experiment in 10 individuals, administration of low-dose dobutamine (2 μg/kg/min) caused a similar change in systolic circumferential strain rate as was found during 48 h of food restriction, along with a proportionate increase in CSRd, such that the two metrics remained coupled. Taken together, these data indicate that myocardial steatosis contributes to diastolic dysfunction by impairing diastolic-systolic coupling in healthy adults, and suggest that steatosis may contribute to the progression of heart disease. KEY POINTS: Preclinical evidence strongly suggests that myocardial lipid accumulation (termed steatosis) is an important mechanism driving heart disease. Definitive evidence in humans is limited due to the confounding influence of multiple underlying comorbidities. Using a 48-h food restriction model to acutely increase myocardial triglyceride content in young healthy volunteers, we demonstrate an association between myocardial steatosis and left ventricular diastolic dysfunction. These data advance the hypothesis that myocardial steatosis may contribute to diastolic dysfunction and suggest myocardial steatosis as a putative therapeutic target.

A mitochondrial long-chain fatty acid oxidation defect leads to transfer RNA uncharging and activation of the integrated stress response in the mouse heart

AIMS

Cardiomyopathy and arrhythmias can be severe presentations in patients with inherited defects of mitochondrial long-chain fatty acid β-oxidation (FAO). The pathophysiological mechanisms that underlie these cardiac abnormalities remain largely unknown. We investigated the molecular adaptations to a FAO deficiency in the heart using the long-chain acyl-CoA dehydrogenase (LCAD) knockout (KO) mouse model.

METHODS AND RESULTS

We observed enrichment of amino acid metabolic pathways and of ATF4 target genes among the upregulated genes in the LCAD KO heart transcriptome. We also found a prominent activation of the eIF2α/ATF4 axis at the protein level that was independent of the feeding status, in addition to a reduction of cardiac protein synthesis during a short period of food withdrawal. These findings are consistent with an activation of the integrated stress response (ISR) in the LCAD KO mouse heart. Notably, charging of several transfer RNAs (tRNAs), such as tRNAGln was decreased in LCAD KO hearts, reflecting a reduced availability of cardiac amino acids, in particular, glutamine. We replicated the activation of the ISR in the hearts of mice with muscle-specific deletion of carnitine palmitoyltransferase 2.

CONCLUSIONS

Our results show that perturbations in amino acid metabolism caused by long-chain FAO deficiency impact cardiac metabolic signalling, in particular the ISR. These results may serve as a foundation for investigating the role of the ISR in the cardiac pathology associated with long-chain FAO defects.Translational Perspective: The heart relies mainly on mitochondrial fatty acid β-oxidation (FAO) for its high energy requirements. The heart disease observed in patients with a genetic defect in this pathway highlights the importance of FAO for cardiac health. We show that the consequences of a FAO defect extend beyond cardiac energy homeostasis and include amino acid metabolism and associated signalling pathways such as the integrated stress response.

Insights Into the Metabolic Aspects of Aortic Stenosis With the Use of Magnetic Resonance Imaging

Pressure overload in aortic stenosis (AS) encompasses both structural and metabolic remodeling and increases the risk of decompensation into heart failure. A major component of metabolic derangement in AS is abnormal cardiac substrate use, with down-regulation of fatty acid oxidation, increased reliance on glucose metabolism, and subsequent myocardial lipid accumulation. These changes are associated with energetic and functional cardiac impairment in AS and can be assessed with the use of cardiac magnetic resonance spectroscopy (MRS). Proton MRS allows the assessment of myocardial triglyceride content and creatine concentration. Phosphorous MRS allows noninvasive in vivo quantification of the phosphocreatine-to-adenosine triphosphate ratio, a measure of cardiac energy status that is reduced in patients with severe AS. This review summarizes the changes to cardiac substrate and high-energy phosphorous metabolism and how they affect cardiac function in AS. The authors focus on the role of MRS to assess these metabolic changes, and potentially guide future (cellular) metabolic therapy in AS.

Mechanism of tonifying-kidney Chinese herbal medicine in the treatment of chronic heart failure

According to traditional Chinese medicine (TCM), chronic heart failure has the basic pathological characteristics of “heart-kidney yang deficiency.” Chronic heart failure with heart- and kidney-Yang deficiency has good overlap with New York Heart Association (NYHA) classes III and IV. Traditional Chinese medicine classical prescriptions for the treatment of chronic heart failure often take “warming and tonifying kidney-Yang” as the core, supplemented by herbal compositions with functions of “promoting blood circulation and dispersing blood stasis.” Nowadays, there are still many classical and folk prescriptions for chronic heart failure treatment, such as Zhenwu decoction, Bushen Huoxue decoction, Shenfu decoction, Sini decoction, as well as Qili Qiangxin capsule. This review focuses on classical formulations and their active constituents that play a key role in preventing chronic heart failure by suppressing inflammation and modulating immune and neurohumoral factors. In addition, given that mitochondrial metabolic reprogramming has intimate relation with inflammation, cardiac hypertrophy, and fibrosis, the regulatory role of classical prescriptions and their active components in metabolic reprogramming, including glycolysis and lipid β-oxidation, is also presented. Although the exact mechanism is unknown, the classical TCM prescriptions still have good clinical effects in treating chronic heart failure. This review will provide a modern pharmacological explanation for its mechanism and offer evidence for clinical medication by combining TCM syndrome differentiation with chronic heart failure clinical stages.

Animal Models of Dysregulated Cardiac Metabolism

As a muscular pump that contracts incessantly throughout life, the heart must constantly generate cellular energy to support contractile function and fuel ionic pumps to maintain electrical homeostasis. Thus, mitochondrial metabolism of multiple metabolic substrates such as fatty acids, glucose, ketones, and lactate is essential to ensuring an uninterrupted supply of ATP. Multiple metabolic pathways converge to maintain myocardial energy homeostasis. The regulation of these cardiac metabolic pathways has been intensely studied for many decades. Rapid adaptation of these pathways is essential for mediating the myocardial adaptation to stress, and dysregulation of these pathways contributes to myocardial pathophysiology as occurs in heart failure and in metabolic disorders such as diabetes. The regulation of these pathways reflects the complex interactions of cell-specific regulatory pathways, neurohumoral signals, and changes in substrate availability in the circulation. Significant advances have been made in the ability to study metabolic regulation in the heart, and animal models have played a central role in contributing to this knowledge. This review will summarize metabolic pathways in the heart and describe their contribution to maintaining myocardial contractile function in health and disease. The review will summarize lessons learned from animal models with altered systemic metabolism and those in which specific metabolic regulatory pathways have been genetically altered within the heart. The relationship between intrinsic and extrinsic regulators of cardiac metabolism and the pathophysiology of heart failure and how these have been informed by animal models will be discussed.

MicroRNA-126b-5p Exacerbates Development of Adipose Tissue and Diet-Induced Obesity

Obesity has become a worldwide epidemic, caused by many factors such as genetic regulatory elements, unhealthy diet, and lack of exercise. MicroRNAs (miRNAs) are non-coding single-stranded RNA classes, which are about 22 nucleotides in length and highly conserved among species. In the last decade, a series of miRNAs were identified as therapeutic targets for obesity. In the present study, we found that miR-126b-5p was associated with adipogenesis. miR-126b-5p overexpression promoted the proliferation of 3T3-L1 preadipocytes by upregulating the expression of proliferation-related genes and downregulating the expression of apoptosis-related genes; the inhibition of miR-126b-5p gave rise to opposite results. Similarly, miR-126b-5p overexpression could promote the differentiation of 3T3-L1 preadipocytes by increasing the expression of lipid deposition genes and triglyceride (TG) and total cholesterol (TC) levels. Moreover, luciferase reporter assay demonstrated that adiponectin receptor 2 (Adipor2) and acyl-CoA dehydrogenase, long chain (ACADL) were the direct target genes of miR-126b-5p. Moreover, overexpression of miR-126b-5p could exacerbate the clinical symptoms of obesity when mice were induced by a high-fat diet, including increased adipose tissue weight, adipocyte volume, and insulin resistance. Interestingly, overexpression of miR-126b-5p in preadipocytes and mice could significantly increase total fatty acid content and change the fatty acid composition of adipose tissue. Taken together, the present study showed that miR-126b-5p promotes lipid deposition in vivo and in vitro, indicating that miR-126b-5p is a potential target for treating obesity.

Dietary restriction in the long-chain acyl-CoA dehydrogenase knockout mouse

Patients with a disorder of mitochondrial long-chain fatty acid β-oxidation (FAO) have reduced fasting tolerance and may present with hypoketotic hypoglycemia, hepatomegaly, (cardio)myopathy and rhabdomyolysis. Patients should avoid a catabolic state because it increases reliance on FAO as energy source. It is currently unclear whether weight loss through a reduction of caloric intake is safe in patients with a FAO disorder. We used the long-chain acyl-CoA dehydrogenase knockout (LCAD KO) mouse model to study the impact of dietary restriction (DR) on the plasma metabolite profile and cardiac function. For this, LCAD KO and wild type (WT) mice were subjected to DR (70% of ad libitum chow intake) for 4 weeks and compared to ad libitum chow fed mice. We found that DR had a relatively small impact on the plasma metabolite profile of WT and LCAD KO mice. Echocardiography revealed a small decrease in left ventricular systolic function of LCAD KO mice, which was most noticeable after DR, but there was no evidence of DR-induced cardiac remodeling. Our results suggest that weight loss through DR does not have acute and detrimental consequences in a mouse model for FAO disorders.

Xbp1s-FoxO1 axis governs lipid accumulation and contractile performance in heart failure with preserved ejection fraction

Heart failure with preserved ejection fraction (HFpEF) is now the dominant form of heart failure and one for which no efficacious therapies exist. Obesity and lipid mishandling greatly contribute to HFpEF. However, molecular mechanism(s) governing metabolic alterations and perturbations in lipid homeostasis in HFpEF are largely unknown. Here, we report that cardiomyocyte steatosis in HFpEF is coupled with increases in the activity of the transcription factor FoxO1 (Forkhead box protein O1). FoxO1 depletion, as well as over-expression of the Xbp1s (spliced form of the X-box-binding protein 1) arm of the UPR (unfolded protein response) in cardiomyocytes each ameliorates the HFpEF phenotype in mice and reduces myocardial lipid accumulation. Mechanistically, forced expression of Xbp1s in cardiomyocytes triggers ubiquitination and proteasomal degradation of FoxO1 which occurs, in large part, through activation of the E3 ubiquitin ligase STUB1 (STIP1 homology and U-box-containing protein 1) a novel and direct transcriptional target of Xbp1s. Our findings uncover the Xbp1s-FoxO1 axis as a pivotal mechanism in the pathogenesis of cardiometabolic HFpEF and unveil previously unrecognized mechanisms whereby the UPR governs metabolic alterations in cardiomyocytes.

Quantification of Myocardial Creatine and Triglyceride Content in the Human Heart: Precision and Accuracy of in vivo Proton Magnetic Resonance Spectroscopy

Background

Proton magnetic resonance spectroscopy (¹H‐MRS) of the human heart is deemed to be a quantitative method to investigate myocardial metabolite content, but thorough validations of in vivo measurements against invasive techniques are lacking.

Purpose

To determine measurement precision and accuracy for quantifications of myocardial total creatine and triglyceride content with localized ¹H‐MRS.

Study type

Test–retest repeatability and measurement validation study.

Subjects

Sixteen volunteers and 22 patients scheduled for open‐heart aortic valve replacement or septal myectomy.

Field Strength/Sequence

Prospectively ECG‐triggered respiratory‐gated free‐breathing single‐voxel point‐resolved spectroscopy (PRESS) sequence at 3 T.

Assessment

Myocardial total creatine and triglyceride content were quantified relative to the total water content by fitting the ¹H‐MR spectra. Precision was assessed with measurement repeatability. Accuracy was assessed by validating in vivo ¹H‐MRS measurements against biochemical assays in myocardial tissue from the same subjects.

Statistical Tests

Intrasession and intersession repeatability was assessed using Bland–Altman analyses. Agreement between ¹H‐MRS measurements and biochemical assay was tested with regression analyses.

Results

The intersession repeatability coefficient for myocardial total creatine content was 41.8% with a mean value of 0.083% ± 0.020% of the total water signal, and 36.7% for myocardial triglyceride content with a mean value of 0.35% ± 0.13% of the total water signal. Ex vivo myocardial total creatine concentrations in tissue samples correlated with the in vivo myocardial total creatine content measured with ¹H‐MRS: n = 22, r = 0.44; P < 0.05. Likewise, ex vivo myocardial triglyceride concentrations correlated with the in vivo myocardial triglyceride content: n = 20, r = 0.50; P < 0.05.

Data Conclusion

We validated the use of localized ¹H‐MRS of the human heart at 3 T for quantitative assessments of in vivo myocardial tissue metabolite content by estimating the measurement precision and accuracy.

Level of Evidence

2

Technical Efficacy Stage

2

Cardiac 1H MR spectroscopy: development of the past five decades and future perspectives

Continued advances in laboratory medicine are required to realize the potential of individualized medicine to impact common cardiovascular diseases. Magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques have advanced over recent years and offer unique, powerful insights into cardiac anatomic and metabolic changes, respectively, occurring in both nascent and advanced heart disease. Although numerous MRI-based in vivo diagnostics are already used in routine clinical practice and more are anticipated, MRS has been less incorporated into routine clinical practice. Given the ability of ¹H MRS to identify and quantify specific molecules with high sensitivity and specificity, its potential utility should be successfully transition from "bench-to-bedside" is tantalizing. The present review will highlight the development of ¹H MRS techniques for cardiac applications, observations in seminal studies with ¹H MRS, and the prospects and challenges for widespread application in patients with cardiovascular disease.

Cardiac tissue citric acid cycle intermediates in exercised very long-chain acyl-CoA dehydrogenase-deficient mice fed triheptanoin or medium-chain triglyceride

Anaplerotic odd-chain fatty acid supplementation has been suggested as an approach to replenish citric acid cycle intermediate (CACi) pools and facilitate adenosine triphosphate (ATP) production in subjects with long-chain fatty acid oxidation disorders, but the evidence that cellular CACi depletion exists and that repletion occurs following anaplerotic substrate supplementation is limited. We exercised very long-chain acyl-CoA dehydrogenase-deficient (VLCAD-/-) and wild-type (WT) mice to exhaustion and collected cardiac tissue for measurement of CACi by targeted metabolomics. In a second experimental group, VLCAD-/- and WT mice that had been fed chow prepared with either medium-chain triglyceride (MCT) oil or triheptanoin for 4 weeks were exercised for 60 minutes. VLCAD-/- mice exhibited lower succinate in cardiac muscle at exhaustion than WT mice suggesting lower CACi in VLCAD-/- with prolonged exercise. In mice fed either MCT or triheptanoin, succinate and malate were greater in VLCAD-/- mice fed triheptanoin compared to VLCAD-/- animals fed MCT but lower than WT mice fed triheptanoin. Long-chain odd acylcarnitines such as C19 were elevated in VLCAD-/- and WT mice fed triheptanoin suggesting some elongation of the heptanoate, but it is unknown what proportion of heptanoate was oxidized vs elongated. Prolonged exercise was associated with decreased cardiac muscle succinate in VLCAD-/- mice in comparison to WT mice. VLCAD-/- fed triheptanoin had increased succinate compared to VLCAD-/- mice fed MCT but lower than WT mice fed triheptanoin. Cardiac CACi were higher following dietary ingestion of an anaplerotic substrate, triheptanoin, in comparison to MCT.

Subclinical effects of long-chain fatty acid β-oxidation deficiency on the adult heart: A case-control magnetic resonance study

Cardiomyopathy can be a severe complication in patients with long-chain fatty acid β-oxidation disorders (LCFAOD), particularly during episodes of metabolic derangement. It is unknown whether latent cardiac abnormalities exist in adult patients. To investigate cardiac involvement in LCFAOD, we used proton magnetic resonance imaging (MRI) and spectroscopy (¹ H-MRS) to quantify heart function, myocardial tissue characteristics, and myocardial lipid content in 14 adult patients (two with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD); four with carnitine palmitoyltransferase II deficiency (CPT2D); and eight with very long-chain acyl-CoA dehydrogenase deficiency (VLCADD)) and 14 gender-, age-, and BMI-matched control subjects. Examinations included cine MRI, MR tagging, native myocardial T₁ and T₂ mapping, and localized ¹ H-MRS at 3 Tesla. Left ventricular (LV) myocardial mass (P = .011) and the LV myocardial mass-to-volume ratio (P = .008) were higher in patients, while ejection fraction (EF) was normal (P = .397). LV torsion was higher in patients (P = .026), whereas circumferential shortening was similar compared with controls (P = .875). LV hypertrophy was accompanied by high myocardial T₁ values (indicative of diffuse fibrosis) in two patients, and additionally a low EF in one case. Myocardial lipid content was similar in patients and controls. We identified subclinical morphological and functional differences between the hearts of LCFAOD patients and matched control subjects using state-of-the-art MR methods. Our results suggest a chronic cardiac disease phenotype and hypertrophic LV remodeling of the heart in LCFAOD, potentially triggered by a mild, but chronic, energy deficiency, rather than by lipotoxic effects of accumulating lipid metabolites.

Slc22a5 haploinsufficiency does not aggravate the phenotype of the long-chain acyl-CoA dehydrogenase KO mouse

Secondary carnitine deficiency is commonly observed in inherited metabolic diseases characterised by the accumulation of acylcarnitines such as mitochondrial fatty acid oxidation (FAO) disorders. It is currently unclear if carnitine deficiency and/or acylcarnitine accumulation play a role in the pathophysiology of FAO disorders. The long-chain acyl-CoA dehydrogenase (LCAD) KO mouse is a model for long-chain FAO disorders and is characterised by decreased levels of tissue and plasma free carnitine. Tissue levels of carnitine are controlled by SLC22A5, the plasmalemmal carnitine transporter. Here, we have further decreased carnitine availability in the LCAD KO mouse through a genetic intervention by introducing one defective Slc22a5 allele (jvs). Slc22a5 haploinsufficiency decreased free carnitine levels in liver, kidney, and heart of LCAD KO animals. The resulting decrease in the tissue long-chain acylcarnitines levels had a similar magnitude as the decrease in free carnitine. Levels of cardiac deoxycarnitine, a carnitine biosynthesis intermediate, were elevated due to Slc22a5 haploinsufficiency in LCAD KO mice. A similar increase in heart and muscle deoxycarnitine was observed in an independent experiment using Slc22a5^jvs/jvs mice. Cardiac hypertrophy, fasting-induced hypoglycemia and increased liver weight, the major phenotypes of the LCAD KO mouse, were not affected by Slc22a5 haploinsufficiency. This may suggest that secondary carnitine deficiency does not play a major role in the pathophysiology of these phenotypes. Similarly, our data do not support a major role for toxicity of long-chain acylcarnitines in the phenotype of the LCAD KO mouse.

Myocardial adipose deposition and the development of heart failure with preserved ejection fraction

AIMS

It has been proposed that an increase of myocardial adiposity is related to left ventricular (LV) diastolic dysfunction. The specific roles of myocardial steatosis including epicardial fat and intramyocardial fat for diastolic function are unknown in those patients suffering heart failure (HF) with reduced (HFrEF) or preserved ejection fraction (HFpEF). This study aims to determine the complex relationship between myocardial adiposity in patients with HFrEF or HFpEF.

METHODS AND RESULTS

Using cardiac magnetic resonance imaging (CMRI), myocardial steatosis was measured in 305 subjects (34 patients with HFrEF, 163 with HFpEF, and 108 non-HF controls). We also evaluated cardiac structure and diastolic and systolic function by echocardiography and CMRI. Patients with HFpEF had significantly more intramyocardial fat than HFrEF patients or non-HF controls [intramyocardial fat content (%), 1.56 (1.26, 1.89) vs. 0.75 (0.50, 0.87) and 1.0 (0.79, 1.15), P < 0.05]. Intramyocardial fat amount (%) was higher in HFpEF women than in men [1.91% (1.17%, 2.32%) vs. 1.22 (0.87%, 2.02%), P = 0.01]. When estimated by CMRI (left ventricular peak filling rate), echocardiographic E/e' level, or left atrial volume index, intramyocardial fat correlated with LV diastolic dysfunction parameters in HFpEF patients, and this was independent of age, co-morbidities, body mass index, gender, and myocardial fibrosis (standardized coefficient: β = -0.34, P = 0.03; β = 0.29, P = 0.025; and β = 0.25, P = 0.02, respectively).

CONCLUSIONS

Patients with HFpEF had significantly more intramyocardial fat than HFrEF patients or non-HF controls. Independent of risk factors or gender, intramyocardial fat correlated with LV diastolic dysfunction parameters in HFpEF patients.

Increased cardiac fatty acid oxidation in a mouse model with decreased malonyl-CoA sensitivity of CPT1B

Aims

Mitochondrial fatty acid oxidation (FAO) is an important energy provider for cardiac work and changes in cardiac substrate preference are associated with different heart diseases. Carnitine palmitoyltransferase 1B (CPT1B) is thought to perform the rate limiting enzyme step in FAO and is inhibited by malonyl-CoA. The role of CPT1B in cardiac metabolism has been addressed by inhibiting or decreasing CPT1B protein or after modulation of tissue malonyl-CoA metabolism. We assessed the role of CPT1B malonyl-CoA sensitivity in cardiac metabolism.

Methods and results

We generated and characterized a knock in mouse model expressing the CPT1BE3A mutant enzyme, which has reduced sensitivity to malonyl-CoA. In isolated perfused hearts, FAO was 1.9-fold higher in Cpt1bE3A/E3A hearts compared with Cpt1bWT/WT hearts. Metabolomic, proteomic and transcriptomic analysis showed increased levels of malonylcarnitine, decreased concentration of CPT1B protein and a small but coordinated downregulation of the mRNA expression of genes involved in FAO in Cpt1bE3A/E3A hearts, all of which aim to limit FAO. In vivo assessment of cardiac function revealed only minor changes, cardiac hypertrophy was absent and histological analysis did not reveal fibrosis.

Conclusions

Malonyl-CoA-dependent inhibition of CPT1B plays a crucial role in regulating FAO rate in the heart. Chronic elevation of FAO has a relatively subtle impact on cardiac function at least under baseline conditions.

Obesity and its cardiovascular effects

Obesity is described in terms of body fat percentage or body mass index (BMI), despite the fact that these measures do not give full insight about the body fat distribution. It is presently a consistently growing universal challenge since it has tripled in the last 10 years, killing approximately 28 million people each year. In this review, we aim to clarify the different results of obesity on the working and physiology of the cardiovascular system and to reveal changes in the obesity "paradox"-a variety of cardiovascular outcomes in typical/overweight people. Central fat build-up in ordinary/overweight populaces has been related to expanded occurrences of myocardial infarction, heart failure, or all-cause mortality when contrasted with the obese populace. These discoveries are additionally clarified as the abundance and prolonged vulnerability to free fatty acids (FFAs) in obesity. This has been believed to cause the myocardial substrate to move from glucose to FFAs digestion, which causes lipid gathering in cardiomyocytes, spilling over to other lean tissues, and prompting a general atherogenic impact. This cardiomyocyte lipid aggregation has been demonstrated to cause insulin resistance and cardiovascular hypertrophy, and to lessen the heart functions in general. There is a proof backing the fact that fat tissue is not only an energy reservoir, it also coordinates hormones and proinflammatory cytokines and deals with the energy transition of the body by putting away abundant lipids in diverse tissues.

Acute intramyocardial lipid accumulation in rats does not slow cardiac conduction per se

Diabetic patients suffer from both cardiac lipid accumulation and an increased risk of arrhythmias and sudden cardiac death. This correlation suggests a link between diabetes induced cardiac steatosis and electrical abnormalities, however, the underlying mechanism remains unknown. We previously showed that cardiac conduction velocity slows in Zucker diabetic fatty rats and in fructose-fat fed rats, models that both exhibit prominent cardiac steatosis. The aim of this study was to investigate whether acute cardiac lipid accumulation reduces conduction velocity per se. Cardiac lipid accumulation was induced acutely by perfusing isolated rat hearts with palmitate-glucose buffer, or subacutely by fasting rats overnight. Subsequently, longitudinal cardiac conduction velocity was measured in right ventricular tissue strips, and intramyocardial triglyceride and lipid droplet content was determined by thin layer chromatography and BODIPY staining, respectively. Perfusion with palmitate-glucose buffer significantly increased intramyocardial triglyceride levels compared to perfusion with glucose (2.16 ± 0.17 (n = 10) vs. 0.92 ± 0.33 nmol/mg WW (n = 9), P < 0.01), but the number of lipid droplets was very low in both groups. Fasting of rats, however, resulted in both significantly elevated intramyocardial triglyceride levels compared to fed rats (3.27 ± 0.43 (n = 10) vs. 1.45 ± 0.24 nmol/mg WW (n = 10)), as well as a larger volume of lipid droplets (0.60 ± 0.13 (n = 10) vs. 0.21 ± 0.06% (n = 10), P < 0.05). There was no significant difference in longitudinal conduction velocity between palmitate-glucose perfused and control hearts (0.77 ± 0.025 (n = 10) vs. 0.75 m/sec ± 0.029 (n = 9)), or between fed and fasted rats (0.75 ± 0.042 m/sec (n = 10) vs. 0.79 ± 0.047 (n = 10)). In conclusion, intramyocardial lipid accumulation does not slow cardiac longitudinal conduction velocity per se. This is true for both increased intramyocardial triglyceride content, induced by palmitate-glucose perfusion, and increased intramyocardial triglyceride and lipid droplet content, generated by fasting.

Follow-up of fatty acid β-oxidation disorders in expanded newborn screening era

Fatty acid β-oxidation (FAO) disorders have a wide variety of symptoms, not usually evident between episodes of acute decompensations. Cardiac involvement is frequent, and severe ventricular arrhythmias are suspected of causing sudden death. Expanded newborn screening (ENS) for these disorders, hopefully, contribute to prevent potentially acute life-threatening events. In order to characterize acute decompensations observed in FAO-deficient cases identified by ENS, a retrospective analysis was performed, covering a period of 9 years. Demographic data, number/type of acute decompensations, treatment, and follow-up were considered. Eighty-three clinical charts, including 66 medium-chain acyl-CoA dehydrogenase deficiency (MCADD), 5 carnitine-uptake deficiency (CUD), 3 carnitine palmitoyltransferase I and II (CPT I/II) deficiency, 5 very long-chain acyl-CoA dehydrogenase deficiency (VLCADD), and 4 multiple acyl-CoA dehydrogenase deficiency (MADD) cases were reviewed. Nineteen patients had acute decompensations (1 CPT I, 1 CPT II, 3 MADD, 14 MCADD). Six patients developed symptoms previously to ENS diagnosis. Severe clinical manifestations included multiple organ failure, liver failure, heart failure, and sudden death. Long-chain FAO disorders had the highest number of decompensations per patient.Conclusion: Despite earlier diagnosis by ENS, sudden deaths were not avoided and acute decompensations with severe clinical manifestations still occur as well. What is Known: • Severe ventricular arrhythmias are suspected to cause unexpected death in FAO disorders. • Neonatal screening intends to reduce the incidence of severe metabolic crisis and death. What is New: • Acute severe decompensations occurred in FAO disorders diagnosed through neonatal screening. • Sudden deaths were not avoided by starting treatment precociously.

Guidelines for measuring cardiac physiology in mice

Cardiovascular disease is a leading cause of death, and translational research is needed to understand better mechanisms whereby the left ventricle responds to injury. Mouse models of heart disease have provided valuable insights into mechanisms that occur during cardiac aging and in response to a variety of pathologies. The assessment of cardiovascular physiological responses to injury or insult is an important and necessary component of this research. With increasing consideration for rigor and reproducibility, the goal of this guidelines review is to provide best-practice information regarding how to measure accurately cardiac physiology in animal models. In this article, we define guidelines for the measurement of cardiac physiology in mice, as the most commonly used animal model in cardiovascular research.

Listen to this article’s corresponding podcast at http://ajpheart.podbean.com/e/guidelines-for-measuring-cardiac-physiology-in-mice/.

The impact of obesity in the cardiac lipidome and its consequences in the cardiac damage observed in obese rats

AIMS

To explore the impact of obesity on the cardiac lipid profile in rats with diet-induced obesity, as well as to evaluate whether or not the specific changes in lipid species are associated with cardiac fibrosis.

METHODS

Male Wistar rats were fed either a high-fat diet (HFD, 35% fat) or standard diet (3.5% fat) for 6 weeks. Cardiac lipids were analyzed using by liquid chromatography-tandem mass spectrometry.

RESULTS

HFD rats showed cardiac fibrosis and enhanced levels of cardiac superoxide anion (O₂), HOMA index, adiposity, and plasma leptin, as well as a reduction in those of cardiac glucose transporter (GLUT 4), compared with control animals. Cardiac lipid profile analysis showed a significant increase in triglycerides, especially those enriched with palmitic, stearic, and arachidonic acid. An increase in levels of diacylglycerol (DAG) was also observed. No changes in cardiac levels of diacyl phosphatidylcholine, or even a reduction in total levels of diacyl phosphatidylethanolamine, diacyl phosphatidylinositol, and sphingomyelins (SM) was observed in HFD, as compared with control animals. After adjustment for other variables (oxidative stress, HOMA, cardiac hypertrophy), total levels of DAG were independent predictors of cardiac fibrosis while the levels of total SM were independent predictors of the cardiac levels of GLUT 4.

CONCLUSIONS

These data suggest that obesity has a significant impact on cardiac lipid composition, although it does not modulate the different species in a similar manner. Nonetheless, these changes are likely to participate in the cardiac damage in the context of obesity, since total DAG levels can facilitate the development of cardiac fibrosis, and SM levels predict GLUT4 levels.

Very long-chain acyl-CoA dehydrogenase (VLCAD-) deficiency-studies on treatment effects and long-term outcomes in mouse models

Very-long-chain-acyl-CoA-dehydrogenase deficiency is the most common disorder of mitochondrial long-chain fatty acid (LCFA) oxidation, with an incidence of 1:50,000-1:100,000 in newborns. Catabolic situations contribute to the aggravation of symptoms and induce severe metabolic derangement. Treatment for VLCAD-deficiency includes avoidance of fasting and a long-chain fat-restricted and fat-modified diet in which LCFAs are fully or partially replaced by medium-chain triglycerides (MCT). The aim of this work was to investigate the outcome and the effects of long-term treatment in a mouse model of VLCAD-deficiency. The application of a single MCT bolus in a mouse model of VLCAD-deficiency (VLCAD^-/- mice) immediately prior to exercise protected the muscles from the accumulation of acylcarnitines providing the required energy and it did not affect hepatic lipid metabolism. However, when MCT was applied over the course of a year as a regular part of the diet, female VLCAD^-/- mice developed a severe clinical phenotype comparable to the human metabolic syndrome. Indeed, they were characterized by massive visceral fat infiltration, hepatosteatosis, disturbed fatty acid composition, hyperlipidemia, and systemic oxidative stress. In contrast, male VLCAD^-/- mice seemed to be protected and displayed only signs of insulin resistance. Besides the sex-specific response to MCT supplementation with regard to the lipid metabolism, all VLCAD^-/- mice developed progressive cardiac dysfunction over time which worsened when they were treated with regular MCT resulting in severe dilated cardiomyopathy. While long term use of MCT oil in mice has adverse effects, no such effects have been demonstrated in humans, likely reflecting the differences in long chain fatty acid oxidation between the two species.

High field magnetic resonance imaging of rodents in cardiovascular research

Transgenic and gene knockout rodent models are primordial to study pathophysiological processes in cardiovascular research. Over time, cardiac MRI has become a gold standard for in vivo evaluation of such models. Technical advances have led to the development of magnets with increasingly high field strength, allowing specific investigation of cardiac anatomy, global and regional function, viability, perfusion or vascular parameters. The aim of this report is to provide a review of the various sequences and techniques available to image mice on 7-11.7 T magnets and relevant to the clinical setting in humans. Specific technical aspects due to the rise of the magnetic field are also discussed.

Fuel availability and fate in cardiac metabolism: A tale of two substrates

The heart's extraordinary metabolic flexibility allows it to adapt to normal changes in physiology in order to preserve its function. Alterations in the metabolic profile of the heart have also been attributed to pathological conditions such as ischemia and hypertrophy; however, research during the past decade has established that cardiac metabolic adaptations can precede the onset of pathologies. It is therefore critical to understand how changes in cardiac substrate availability and use trigger events that ultimately result in heart dysfunction. This review examines the mechanisms by which the heart obtains fuels from the circulation or from mobilization of intracellular stores. We next describe experimental models that exhibit either an increase in glucose use or a decrease in FA oxidation, and how these aberrant conditions affect cardiac metabolism and function. Finally, we highlight the importance of alternative, relatively under-investigated strategies for the treatment of heart failure. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Lipid metabolism and signaling in cardiac lipotoxicity

The heart balances uptake, metabolism and oxidation of fatty acids (FAs) to maintain ATP production, membrane biosynthesis and lipid signaling. Under conditions where FA uptake outpaces FA oxidation and FA sequestration as triacylglycerols in lipid droplets, toxic FA metabolites such as ceramides, diacylglycerols, long-chain acyl-CoAs, and acylcarnitines can accumulate in cardiomyocytes and cause cardiomyopathy. Moreover, studies using mutant mice have shown that dysregulation of enzymes involved in triacylglycerol, phospholipid, and sphingolipid metabolism in the heart can lead to the excess deposition of toxic lipid species that adversely affect cardiomyocyte function. This review summarizes our current understanding of lipid uptake, metabolism and signaling pathways that have been implicated in the development of lipotoxic cardiomyopathy under conditions including obesity, diabetes, aging, and myocardial ischemia-reperfusion. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Myocardial steatosis as a possible mechanistic link between diastolic dysfunction and coronary microvascular dysfunction in women

Women with coronary microvascular dysfunction (CMD) and no obstructive coronary artery disease (CAD) have increased rates of heart failure with preserved ejection fraction (HFpEF). The mechanisms of HFpEF are not well understood. Ectopic fat deposition in the myocardium, termed myocardial steatosis, is frequently associated with diastolic dysfunction in other metabolic diseases. We investigated the prevalence of myocardial steatosis and diastolic dysfunction in women with CMD and subclinical HFpEF. In 13 women, including eight reference controls and five women with CMD and evidence of subclinical HFpEF (left ventricular end-diastolic pressure >12 mmHg), we measured myocardial triglyceride content (TG) and diastolic function, by proton magnetic resonance spectroscopy and magnetic resonance tissue tagging, respectively. When compared with reference controls, women with CMD had higher myocardial TG content (0.83 ± 0.12% vs. 0.43 ± 0.06%; P = 0.025) and lower diastolic circumferential strain rate (168 ± 12 vs. 217 ± 15%/s; P = 0.012), with myocardial TG content correlating inversely with diastolic circumferential strain rate (r = -0.779; P = 0.002). This study provides proof-of-concept that myocardial steatosis may play an important mechanistic role in the development of diastolic dysfunction in women with CMD and no obstructive CAD. Detailed longitudinal studies are warranted to explore specific treatment strategies targeting myocardial steatosis and its effect on diastolic function.

Neuropsychological Development in Patients with Long-Chain 3-Hydroxyacyl-CoA Dehydrogenase (LCHAD) Deficiency

BACKGROUND

Reports on cognitive outcomes in long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHADD) are scarce. We present results from neuropsychological assessments of eight patients diagnosed with LCHADD prior to newborn screening with regard to clinical disease severity.

METHODS

Intellectual ability and adaptive and executive functions were assessed using age-appropriate Wechsler Scales, Adaptive Behavior Assessment Scales (ABAS), and Behavior Rating Inventory of Executive Function (BRIEF).

RESULTS

Five patients performed in the normal range on IQ tests but with lower scores on verbal working memory. In addition, they had lower parent-rated adaptive and executive functions.Three patients had intellectual disabilities with IQs below normal and/or autism spectrum disorders. In addition, they had low results on parent-rated adaptive functions. (Two of these patients had epilepsy.) Conclusions: Patients with LCHADD seem to have a specific cognitive pattern, with presentation as intellectual disability and specific autistic deficiencies or a normal IQ with weaknesses in auditive verbal memory and adaptive and executive functions. Future studies are warranted to investigate whether newborn screening programs and early treatment may promote improved neuropsychological development and outcomes.

Time course of cardiometabolic alterations in a high fat high sucrose diet mice model and improvement after GLP-1 analog treatment using multimodal cardiovascular magnetic resonance

BACKGROUND

Cardiovascular complications of obesity and diabetes are major health problems. Assessing their development, their link with ectopic fat deposition and their flexibility with therapeutic intervention is essential. The aim of this study was to longitudinally investigate cardiac alterations and ectopic fat accumulation associated with diet-induced obesity using multimodal cardiovascular magnetic resonance (CMR) in mice. The second objective was to monitor cardiac response to exendin-4 (GLP-1 receptor agonist).

METHODS

Male C57BL6R mice subjected to a high fat (35 %) high sucrose (34 %) (HFHSD) or a standard diet (SD) during 4 months were explored every month with multimodal CMR to determine hepatic and myocardial triglyceride content (HTGC, MTGC) using proton MR spectroscopy, cardiac function with cine cardiac MR (CMR) and myocardial perfusion with arterial spin labeling CMR. Furthermore, mice treated with exendin-4 (30 μg/kg SC BID) after 4 months of diet were explored before and 14 days post-treatment with multimodal CMR.

RESULTS

HFHSD mice became significantly heavier (+33 %) and displayed glucose homeostasis impairment (1-month) as compared to SD mice, and developed early increase in HTGC (1 month, +59 %) and MTGC (2-month, +63 %). After 3 months, HFHSD mice developed cardiac dysfunction with significantly higher diastolic septum wall thickness (sWtnD) (1.28 ± 0.03 mm vs. 1.12 ± 0.03 mm) and lower cardiac index (0.45 ± 0.06 mL/min/g vs. 0.68 ± 0.07 mL/min/g, p = 0.02) compared to SD mice. A significantly lower cardiac perfusion was also observed (4 months:7.5 ± 0.8 mL/g/min vs. 10.0 ± 0.7 mL/g/min, p = 0.03). Cardiac function at 4 months was negatively correlated to both HTGC and MTGC (p < 0.05). 14-day treatment with Exendin-4 (Ex-4) dramatically reversed all these alterations in comparison with placebo-treated HFHSD. Ex-4 diminished myocardial triglyceride content (-57.8 ± 4.1 %), improved cardiac index (+38.9 ± 10.9 %) and restored myocardial perfusion (+52.8 ± 16.4 %) under isoflurane anesthesia. Interestingly, increased wall thickness and hepatic steatosis reductions were independent of weight loss and glycemia decrease in multivariate analysis (p < 0.05).

CONCLUSION

CMR longitudinal follow-up of cardiac consequences of obesity and diabetes showed early accumulation of ectopic fat in mice before the occurrence of microvascular and contractile dysfunction. This study also supports a cardioprotective effect of glucagon-like peptide-1 receptor agonist.

Increased pericardial fat accumulation is associated with increased intramyocardial lipid content and duration of highly active antiretroviral therapy exposure in patients infected with human immunodeficiency virus: a 3T cardiovascular magnetic resonance feasibility study

BACKGROUND

The aim of the current study was to examine whether the use of highly active antiretroviral therapy (HAART) in patients with HIV is associated with changes in pericardial fat and myocardial lipid content measured by cardiovascular magnetic resonance (CMR).

METHODS

In this prospective case-control study, we compared 27 HIV seropositive (+) male subjects receiving HAART to 22 control male subjects without HIV matched for age, ethnicity and body mass index. All participants underwent CMR imaging for determination of pericardial fat [as volume at the level of the origin of the left main coronary artery (LM) and at the right ventricular free wall] and magnetic resonance spectroscopy (MRS) for evaluation of intramyocardial lipid content (% of fat to water in a single voxel at the interventricular septum). All measurements were made by two experienced readers blinded to the clinical history of the study participants. Two-sample t-test, Spearman's correlation coefficient or Pearson's correlation coefficient and multivariable logistic regression were used for statistical analysis.

RESULTS

Pericardial fat volume at the level of LM origin was higher in HIV (+) subjects (33.4 cm(3) vs. 27.4 cm(3), p = 0.03). On multivariable analysis adjusted for age, Framingham risk score (FRS) and waist/hip ratio, pericardial fat remained significantly associated to HIV-status (OR 1.09, p = 0.047). For both HIV (+) and HIV (-) subjects, pericardial fat volume showed strong correlation with intramyocardial lipid content (r = 0.58, p < 0.0001) and FRS (r = 0.53, p = 0.0002). Among HIV (+) subjects, pericardial fat was significantly higher in patients with lipo-accumulation (37 cm(3) vs. 27.1 cm(3), p = 0.03) and showed significant correlation with duration of both HIV infection (r = 0.5, p = 0.01) and HAART (r = 0.46, p = 0.02).

CONCLUSIONS

Pericardial fat content is increased in HIV (+) subjects on chronic HAART (>5 years), who demonstrate HAART-related lipo-accumulation and prolonged HIV duration of infection. Further investigation is warranted to determine whether increased pericardial fat is associated with higher cardiovascular risk leading to premature cardiovascular events in this patient population.

The Biochemistry and Physiology of Mitochondrial Fatty Acid β-Oxidation and Its Genetic Disorders

Mitochondrial fatty acid β-oxidation (FAO) is the major pathway for the degradation of fatty acids and is essential for maintaining energy homeostasis in the human body. Fatty acids are a crucial energy source in the postabsorptive and fasted states when glucose supply is limiting. But even when glucose is abundantly available, FAO is a main energy source for the heart, skeletal muscle, and kidney. A series of enzymes, transporters, and other facilitating proteins are involved in FAO. Recessively inherited defects are known for most of the genes encoding these proteins. The clinical presentation of these disorders may include hypoketotic hypoglycemia, (cardio)myopathy, arrhythmia, and rhabdomyolysis and illustrates the importance of FAO during fasting and in hepatic and (cardio)muscular function. In this review, we present the current state of knowledge on the biochemistry and physiological functions of FAO and discuss the pathophysiological processes associated with FAO disorders.

In vivo mouse myocardial (31)P MRS using three-dimensional image-selected in vivo spectroscopy (3D ISIS): technical considerations and biochemical validations

(31)P MRS provides a unique non-invasive window into myocardial energy homeostasis. Mouse models of cardiac disease are widely used in preclinical studies, but the application of (31)P MRS in the in vivo mouse heart has been limited. The small-sized, fast-beating mouse heart imposes challenges regarding localized signal acquisition devoid of contamination with signal originating from surrounding tissues. Here, we report the implementation and validation of three-dimensional image-selected in vivo spectroscopy (3D ISIS) for localized (31)P MRS of the in vivo mouse heart at 9.4 T. Cardiac (31)P MR spectra were acquired in vivo in healthy mice (n = 9) and in transverse aortic constricted (TAC) mice (n = 8) using respiratory-gated, cardiac-triggered 3D ISIS. Localization and potential signal contamination were assessed with (31)P MRS experiments in the anterior myocardial wall, liver, skeletal muscle and blood. For healthy hearts, results were validated against ex vivo biochemical assays. Effects of isoflurane anesthesia were assessed by measuring in vivo hemodynamics and blood gases. The myocardial energy status, assessed via the phosphocreatine (PCr) to adenosine 5'-triphosphate (ATP) ratio, was approximately 25% lower in TAC mice compared with controls (0.76 ± 0.13 versus 1.00 ± 0.15; P < 0.01). Localization with one-dimensional (1D) ISIS resulted in two-fold higher PCr/ATP ratios than measured with 3D ISIS, because of the high PCr levels of chest skeletal muscle that contaminate the 1D ISIS measurements. Ex vivo determinations of the myocardial PCr/ATP ratio (0.94 ± 0.24; n = 8) confirmed the in vivo observations in control mice. Heart rate (497 ± 76 beats/min), mean arterial pressure (90 ± 3.3 mmHg) and blood oxygen saturation (96.2 ± 0.6%) during the experimental conditions of in vivo (31)P MRS were within the normal physiological range. Our results show that respiratory-gated, cardiac-triggered 3D ISIS allows for non-invasive assessments of in vivo mouse myocardial energy homeostasis with (31)P MRS under physiological conditions.

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.

Good and bad consequences of altered fatty acid metabolism in heart failure: evidence from mouse models

The shift in substrate preference away from fatty acid oxidation (FAO) towards increased glucose utilization in heart failure has long been interpreted as an oxygen-sparing mechanism. Inhibition of FAO has therefore evolved as an accepted approach to treat heart failure. However, recent data indicate that increased reliance on glucose might be detrimental rather than beneficial for the failing heart. This review discusses new insights into metabolic adaptations in heart failure. A particular focus lies on data obtained from mouse models with modulations of cardiac FA metabolism at different levels of the FA metabolic pathway and how these differently affect cardiac function. Based on studies in which these mouse models were exposed to ischaemic and non-ischaemic heart failure, we discuss whether and when modulations in FA metabolism are protective against heart failure.

Increased and early lipolysis in children with long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency during fast

Children with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency (LCHAD) have a defect in the degradation of long-chain fatty acids and are at risk of hypoketotic hypoglycemia and insufficient energy production as well as accumulation of toxic fatty acid intermediates. Knowledge on substrate metabolism in children with LCHAD deficiency during fasting is limited. Treatment guidelines differ between centers, both as far as length of fasting periods and need for night feeds are concerned. To increase the understanding of fasting intolerance and improve treatment recommendations, children with LCHAD deficiency were investigated with stable isotope technique, microdialysis, and indirect calometry, in order to assess lipolysis and glucose production during 6 h of fasting. We found an early and increased lipolysis and accumulation of long chain acylcarnitines after 4 h of fasting, albeit no patients developed hypoglycemia. The rate of glycerol production, reflecting lipolysis, averaged 7.7 ± 1.6 µmol/kg/min, which is higher compared to that of peers. The rate of glucose production was normal for age; 19.6 ± 3.4 µmol/kg/min (3.5 ± 0.6 mg/kg/min). Resting energy expenditure was also normal, even though the respiratory quotient was increased indicating mainly glucose oxidation. The results show that lipolysis and accumulation of long chain acylcarnitines occurs before hypoglycemia in fasting children with LCHAD, which may indicate more limited fasting tolerance than previously suggested.

In vivo proton T1 relaxation times of mouse myocardial metabolites at 9.4 T

PURPOSE

Proton magnetic resonance spectroscopy ((1) H-MRS) for quantitative in vivo assessment of mouse myocardial metabolism requires accurate acquisition timing to minimize motion artifacts and corrections for T1 -dependent partial saturation effects. In this study, mouse myocardial water and metabolite T1 relaxation time constants were quantified.

METHODS

Cardiac-triggered and respiratory-gated PRESS-localized (1) H-MRS was employed at 9.4 T to acquire signal from a 4-µL voxel in the septum of healthy mice (n = 10) while maintaining a steady state of magnetization using dummy scans during respiratory gates. Signal stability was assessed via standard deviations (SD) of zero-order phases and amplitudes of water spectra. Saturation-recovery experiments were performed to determine T1 values.

RESULTS

Phase SD did not vary for different repetition times (TR), and was 13.1° ± 4.5°. Maximal amplitude SD was 14.2% ± 5.1% at TR = 500 ms. Myocardial T1 values (mean ± SD) were quantified for water (1.71 ± 0.25 s), taurine (2.18 ± 0.62 s), trimethylamine from choline-containing compounds and carnitine (1.67 ± 0.25 s), creatine-methyl (1.34 ± 0.19 s), triglyceride-methylene (0.60 ± 0.15 s), and triglyceride-methyl (0.90 ± 0.17 s) protons.

CONCLUSION

This work provides in vivo quantifications of proton T1 values for mouse myocardial water and metabolites at 9.4 T.

(1)H-MR spectroscopy for analysis of cardiac lipid and creatine metabolism

Magnetic resonance spectroscopy (MRS) is the only non-invasive, non-radiation-based technique for investigating the metabolism of living tissue. MRS of protons (¹H-MRS), which provides the highest sensitivity of all MR-visible nuclei, is a method capable of detecting and quantifying specific cardiac biomolecules, such as lipids and creatine in normal and diseased hearts in both animal models and humans. This can be used to study mechanisms of heart failure development in a longitudinal manner, for example, the potential contribution of myocardial lipid accumulation in the context of ageing and obesity. Similarly, quantifying creatine levels provides insight into the energy storage and buffering capacity in the heart. Creatine depletion is consistently observed in heart failure independent of aetiology, but its contribution to pathophysiology remains a matter of debate. These and other questions can in theory be answered with cardiac MRS, but fundamental technical challenges have limited its use. The metabolites studied with MRS are much lower concentration than water protons, requiring methods to suppress the dominant water signal and resulting in larger voxel sizes and longer scan times compared to MRI. However, recent technical advances in MR hardware and software have facilitated the application of ¹H-MRS in humans and animal models of heart disease as detailed in this review.

Muscle MRI in patients with long-chain fatty acid oxidation disorders

INTRODUCTION

Muscle magnetic resonance imaging (MRI) is a useful tool for visualizing abnormalities in neuromuscular disorders. The value of muscle MRI has not been studied in long-chain fatty acid oxidation (lcFAO) disorders. LcFAO disorders may present with metabolic myopathy including episodic rhabdomyolysis.

OBJECTIVE

To investigate whether lcFAO disorders are associated with muscle MRI abnormalities.

METHODS

Lower body MRI was performed in 20 patients with lcFAO disorders, i.e. three carnitine palmitoyltransferase 2 deficiency (CPT2D), 12 very long-chain acyl-CoA dehydrogenase deficiency (VLCADD), three mitochondrial trifunctional protein deficiency (MTPD) and two isolated long-chain hydroxyacyl-CoA dehydrogenase deficiency (LCHADD).

RESULTS

At the time of MRI, four patients had muscle weakness, 14 had muscle pain and 13 were exercise intolerant. Median creatine kinase (CK) level of patients at the day of MRI was 398 U/L (range 35-12,483). T1W and STIR signal intensity (SI) were markedly increased in MTPD patients from girdle to lower leg. VLCADD patients showed predominantly proximal T1W SI changes, whereas LCHADD patients mostly showed distal T1W SI changes. Prominent STIR weighted signal intensity increases of almost all muscle groups were observed in patients with VLCADD and LCHADD with very high CK (>11.000) levels.

CONCLUSIONS AND RELEVANCE

lcFAO disorders are associated with specific patterns of increased T1W and STIR signal intensity. These patterns may reflect lipid accumulation and inflammation secondary to lcFAO defects and progressive muscle damage. Future studies are needed to investigate whether muscle MRI might be a useful tool to monitor disease course and to study pathogenesis of lcFAO related myopathy.

Food withdrawal lowers energy expenditure and induces inactivity in long-chain fatty acid oxidation-deficient mouse models

Very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is an inherited disorder of mitochondrial long-chain fatty acid β-oxidation (FAO). Patients with VLCAD deficiency may present with hypoglycemia, hepatomegaly, cardiomyopathy, and myopathy. Although several mouse models have been developed to aid in the study of the pathogenesis of long-chain FAO defects, the muscular phenotype is underexposed. To address the muscular phenotype, we used a newly developed mouse model on a mixed genetic background with a more severe defect in FAO (LCAD(-/-); VLCAD(+/-)) in addition to a validated mouse model (LCAD(-/-); VLCAD(+/+)) and compared them with wild-type (WT) mice. We found that both mouse models show a 20% reduction in energy expenditure (EE) and a 3-fold decrease in locomotor activity in the unfed state. In addition, we found a 1.7°C drop in body temperature in unfed LCAD(-/-); VLCAD(+/+) mice compared with WT body temperature. We conclude that food withdrawal-induced inactivity, hypothermia, and reduction in EE are novel phenotypes associated with FAO deficiency in mice. Unexpectedly, inactivity was not explained by rhabdomyolysis, but rather reflected the overall reduced capacity of these mice to generate heat. We suggest that mice are partly protected against the negative consequence of an FAO defect.-Diekman, E. F., van Weeghel, M., Wanders, R. J. A., Visser, G., Houten, S. M. Food withdrawal lowers energy expenditure and induces inactivity in long-chain fatty acid oxidation-deficient mouse models.

Development and pathomechanisms of cardiomyopathy in very long-chain acyl-CoA dehydrogenase deficient (VLCAD(-/-)) mice

Hypertrophic cardiomyopathy is a typical manifestation of very long-chain acyl-CoA dehydrogenase deficiency (VLCADD), the most common long-chain β-oxidation defects in humans; however in some patients cardiac function is fully compensated. Cardiomyopathy may also be reversed by supplementation of medium-chain triglycerides (MCT). We here characterize cardiac function of VLCAD-deficient (VLCAD(-/-)) mice over one year. Furthermore, we investigate the long-term effect of a continuous MCT diet on the cardiac phenotype. We assessed cardiac morphology and function in VLCAD(-/-) mice by in vivo MRI. Cardiac energetics were measured by (31)P-MRS and myocardial glucose uptake was quantified by positron-emission-tomography (PET). Metabolic adaptations were identified by the expression of genes regulating glucose and lipid metabolism using real-time-PCR. VLCAD(-/-) mice showed a progressive decrease in heart function over 12 months accompanied by a reduced phosphocreatine-to-ATP-ratio indicative of chronic energy deficiency. Long-term MCT supplementation aggravated the cardiac phenotype into dilated cardiomyopathy with features similar to diabetic heart disease. Cardiac energy production and function in mice with a β-oxidation defect cannot be maintained with age. Compensatory mechanisms are insufficient to preserve the cardiac energy state over time. However, energy deficiency by impaired β-oxidation and long-term MCT induce cardiomyopathy by different mechanisms. Cardiac MRI and MRS may be excellent tools to assess minor changes in cardiac function and energetics in patients with β-oxidation defects for preventive therapy.

Carnitine supplementation attenuates myocardial lipid accumulation in long-chain acyl-CoA dehydrogenase knockout mice

PURPOSE

Elevation of long-chain acylcarnitine levels is a hallmark of long-chain mitochondrial β-oxidation (FAO) disorders, and can be accompanied by secondary carnitine deficiency. To restore free carnitine levels, and to increase myocardial export of long-chain fatty acyl-CoA esters, supplementation of L-carnitine in patients has been proposed. However, carnitine supplementation is controversial, because it may enhance the potentially lipotoxic buildup of long-chain acylcarnitines in the FAO-deficient heart. In this longitudinal study, we investigated the effects of carnitine supplementation in an animal model of long-chain FAO deficiency, the long-chain acyl-CoA dehydrogenase (LCAD) knockout (KO) mouse.

METHODS

Cardiac size and function, and triglyceride (TG) levels were quantified using proton magnetic resonance imaging (MRI) and spectroscopy ((1)H-MRS) in LCAD KO and wild-type (WT) mice. Carnitine was supplemented orally for 4 weeks starting at 5 weeks of age. Non-supplemented animals served as controls. In vivo data were complemented with ex vivo biochemical assays.

RESULTS

LCAD KO mice displayed cardiac hypertrophy and elevated levels of myocardial TG compared to WT mice. Carnitine supplementation lowered myocardial TG, normalizing myocardial TG levels in LCAD KO mice. Furthermore, carnitine supplementation did not affect cardiac performance and hypertrophy, or induce an accumulation of potentially toxic long-chain acylcarnitines in the LCAD KO heart.

CONCLUSION

This study lends support to the proposed beneficial effect of carnitine supplementation alleviating toxicity by exporting acylcarnitines out of the FAO-deficient myocardium, rather than to the concern about a potentially detrimental effect of supplementation-induced production of lipotoxic long-chain acylcarnitines.