Mice over-expressing the myocardial creatine transporter develop progressive heart failure and show decreased glycolytic capacity

Maintaining energy provision in the heart: the creatine kinase system in ischaemia-reperfusion injury and chronic heart failure

The non-stop provision of chemical energy is of critical importance to normal cardiac function, requiring the rapid turnover of ATP to power both relaxation and contraction. Central to this is the creatine kinase (CK) phosphagen system, which buffers local ATP levels to optimise the energy available from ATP hydrolysis, to stimulate energy production via the mitochondria and to smooth out mismatches between energy supply and demand. In this review, we discuss the changes that occur in high-energy phosphate metabolism (i.e., in ATP and phosphocreatine) during ischaemia and reperfusion, which represents an acute crisis of energy provision. Evidence is presented from preclinical models that augmentation of the CK system can reduce ischaemia-reperfusion injury and improve functional recovery. Energetic impairment is also a hallmark of chronic heart failure, in particular, down-regulation of the CK system and loss of adenine nucleotides, which may contribute to pathophysiology by limiting ATP supply. Herein, we discuss the evidence for this hypothesis based on preclinical studies and in patients using magnetic resonance spectroscopy. We conclude that the correlative evidence linking impaired energetics to cardiac dysfunction is compelling; however, causal evidence from loss-of-function models remains equivocal. Nevertheless, proof-of-principle studies suggest that augmentation of CK activity is a therapeutic target to improve cardiac function and remodelling in the failing heart. Further work is necessary to translate these findings to the clinic, in particular, a better understanding of the mechanisms by which the CK system is regulated in disease.

Cardiac 31P MR spectroscopy: development of the past five decades and future vision-will it be of diagnostic use in clinics?

In the past five decades, the use of the magnetic resonance (MR) technique for cardiovascular diseases has engendered much attention and raised the opportunity that the technique could be useful for clinical applications. MR has two arrows in its quiver: One is magnetic resonance imaging (MRI), and the other is magnetic resonance spectroscopy (MRS). Non-invasively, highly advanced MRI provides unique and profound information about the anatomical changes of the heart. Excellently developed MRS provides irreplaceable and insightful evidence of the real-time biochemistry of cardiac metabolism of underpinning diseases. Compared to MRI, which has already been successfully applied in routine clinical practice, MRS still has a long way to travel to be incorporated into routine diagnostics. Considering the exceptional potential of ³¹P MRS to measure the real-time metabolic changes of energetic molecules qualitatively and quantitatively, how far its powerful technique should be waited before a successful transition from "bench-to-bedside" is enticing. The present review highlights the seminal studies on the chronological development of cardiac ³¹P MRS in the past five decades and the future vision and challenges to incorporating it for routine diagnostics of cardiovascular disease.

Synergistic effect on cardiac energetics by targeting the creatine kinase system: in vivo application of high-resolution 31P-CMRS in the mouse

BACKGROUND

Phosphorus cardiovascular magnetic resonance spectroscopy (31P-CMRS) has emerged as an important tool for the preclinical assessment of myocardial energetics in vivo. However, the high rate and diminutive size of the mouse heart is a challenge, resulting in low resolution and poor signal-to-noise. Here we describe a refined high-resolution 31P-CMRS technique and apply it to a novel double transgenic mouse (dTg) with elevated myocardial creatine and creatine kinase (CK) activity. We hypothesised a synergistic effect to augment energetic status, evidenced by an increase in the ratio of phosphocreatine-to-adenosine-triphosphate (PCr/ATP).

METHODS AND RESULTS

Single transgenic Creatine Transporter overexpressing (CrT-OE, n = 7) and dTg mice (CrT-OE and CK, n = 6) mice were anaesthetised with isoflurane to acquire 31P-CMRS measurements of the left ventricle (LV) utilising a two-dimensional (2D), threefold under-sampled density-weighted chemical shift imaging (2D-CSI) sequence, which provided high-resolution data with nominal voxel size of 8.5 µl within 70 min. (1H-) cine-CMR data for cardiac function assessment were obtained in the same imaging session. Under a separate examination, mice received invasive haemodynamic assessment, after which tissue was collected for biochemical analysis. Myocardial creatine levels were elevated in all mouse hearts, but only dTg exhibited significantly elevated CK activity, resulting in a 51% higher PCr/ATP ratio in heart (3.01 ± 0.96 vs. 2.04 ± 0.57-mean ± SD; dTg vs. CrT-OE), that was absent from adjacent skeletal muscle. No significant differences were observed for any parameters of LV structure and function, confirming that augmentation of CK activity does not have unforeseen consequences for the heart.

CONCLUSIONS

We have developed an improved 31P-CMRS methodology for the in vivo assessment of energetics in the murine heart which enabled high-resolution imaging within acceptable scan times. Mice over-expressing both creatine and CK in the heart exhibited a synergistic elevation in PCr/ATP that can now be tested for therapeutic potential in models of chronic heart failure.

Clozapine-induced Myocarditis: Pathophysiologic Mechanisms and Implications for Therapeutic Approaches

Clozapine, a superior treatment for treatment-resistant schizophrenia can cause potentially life-threatening myocarditis and dilated cardiomyopathy. While the occurrence of this condition is well known, its molecular mechanisms are unclear and may be multifactorial. Putative mechanisms warrant an in-depth review not only from the perspective of toxicity but also for understanding the molecular mechanisms of the adverse cardiac effects of clozapine and the development of novel therapeutic approaches. Clozapine-induced cardiac toxicity encompasses a diverse set of pathways, including (i) immune modulation and proinflammatory processes encompassing an IgEmediated (type I hypersensitivity) response and perhaps a cytokine release syndrome (ii) catecholaminergic activation (iii) induction of free radicals and oxidative stress (iv) activation of cardiomyocyte cell death pathways, including apoptosis, ischemia through impairment in coronary blood flow via changes in endothelial production of NO and vasoconstriction induced by norepinephrine as well as other factors released from cardiac mast cells. (v) In addition, an extensive examination of the effects of clozapine on non-cardiac cellular proteins demonstrates that clozapine can impair enzymes involved in cellular metabolism, such as pyruvate kinase, mitochondrial malate dehydrogenase, and other proteins, including α-enolase, triosephosphate isomerase and cofilin, which might explain clozapine-induced reductions in myocardial energy generation for cell viability as well as contractile function. Pharmacologic antagonism of these cellular protein effects may lead to the development of strategies to antagonize the cardiac damage induced by clozapine.

Magnetic resonance imaging of cardiac metabolism in heart failure: how far have we come?

As one of the highest energy consumer organs in the body, the heart requires tremendous amount of adenosine triphosphate (ATP) to maintain its continuous mechanical work. Fatty acids, glucose, and ketone bodies are the primary fuel source of the heart to generate ATP with perturbations in ATP generation possibly leading to contractile dysfunction. Cardiac metabolic imaging with magnetic resonance imaging (MRI) plays a crucial role in understanding the dynamic metabolic changes occurring in the failing heart, where the cardiac metabolism is deranged. Also, targeting and quantifying metabolic changes in vivo noninvasively is a promising approach to facilitate diagnosis, determine prognosis, and evaluate therapeutic response. Here, we summarize novel MRI techniques used for detailed investigation of cardiac metabolism in heart failure including magnetic resonance spectroscopy (MRS), hyperpolarized MRS, and chemical exchange saturation transfer based on evidence from preclinical and clinical studies and to discuss the potential clinical application in heart failure.

The Pitfalls of in vivo Cardiac Physiology in Genetically Modified Mice - Lessons Learnt the Hard Way in the Creatine Kinase System

In order to fully understand gene function, at some point, it is necessary to study the effects in an intact organism. The creation of the first knockout mouse in the late 1980's gave rise to a revolution in the field of integrative physiology that continues to this day. There are many complex choices when selecting a strategy for genetic modification, some of which will be touched on in this review, but the principal focus is to highlight the potential problems and pitfalls arising from the interpretation of in vivo cardiac phenotypes. As an exemplar, we will scrutinize the field of cardiac energetics and the attempts to understand the role of the creatine kinase (CK) energy buffering and transport system in the intact organism. This story highlights the confounding effects of genetic background, sex, and age, as well as the difficulties in interpreting knockout models in light of promiscuous proteins and metabolic redundancy. It will consider the dose-dependent effects and unintended consequences of transgene overexpression, and the need for experimental rigour in the context of in vivo phenotyping techniques. It is intended that this review will not only bring clarity to the field of cardiac energetics, but also aid the non-expert in evaluating and critically assessing data arising from in vivo genetic modification.

Role of Creatine in the Heart: Health and Disease

Creatine is a key player in heart contraction and energy metabolism. Creatine supplementation (throughout the paper, only supplementation with creatine monohydrate will be reviewed, as this is by far the most used and best-known way of supplementing creatine) increases creatine content even in the normal heart, and it is generally safe. In heart failure, creatine and phosphocreatine decrease because of decreased expression of the creatine transporter, and because phosphocreatine degrades to prevent adenosine triphosphate (ATP) exhaustion. This causes decreased contractility reserve of the myocardium and correlates with left ventricular ejection fraction, and it is a predictor of mortality. Thus, there is a strong rationale to supplement with creatine the failing heart. Pending additional trials, creatine supplementation in heart failure may be useful given data showing its effectiveness (1) against specific parameters of heart failure, and (2) against the decrease in muscle strength and endurance of heart failure patients. In heart ischemia, the majority of trials used phosphocreatine, whose mechanism of action is mostly unrelated to changes in the ergogenic creatine-phosphocreatine system. Nevertheless, preliminary data with creatine supplementation are encouraging, and warrant additional studies. Prevention of cardiac toxicity of the chemotherapy compounds anthracyclines is a novel field where creatine supplementation may also be useful. Creatine effectiveness in this case may be because anthracyclines reduce expression of the creatine transporter, and because of the pleiotropic antioxidant properties of creatine. Moreover, creatine may also reduce concomitant muscle damage by anthracyclines.

Water-soluble alkaloids extracted from Aconiti Radix lateralis praeparata protect against chronic heart failure in rats via a calcium signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Many studies have shown the beneficial effects of aconite water-soluble alkaloid extract (AWA) in experimental models of heart disease, which have been ascribed to the presence of aconine, hypaconine, talatisamine, fuziline, neoline, and songorine. This study evaluated the effects of a chemically characterized AWA by chemical content, evaluated its effects in suprarenal abdominal aortic coarctation surgery (AAC)-induced chronic heart failure (CHF) in rats, and revealed the underlying mechanisms of action by proteomics.

METHODS

Rats were distributed into different groups: sham, model, and AWA-treated groups (10, 20, and 40 mg/kg/day). Sham rats received surgery without AAC, whereas model rats an AWA-treated groups underwent AAC surgery. after 8 weeks, the treatment group was fed AWA for 4 weeks, and body weight was assessed weekly. At the end of the treatment, heart function was tested by echocardiography. AAC-induced chronic heart failure, including myocardial fibrosis, cardiomyocyte hypertrophy, and apoptosis, was evaluated in heart tissue and plasma by RT-qPCR, ELISA, hematoxylin and eosin (H&E) staining, Masson's trichrome staining, TUNEL staining, and immunofluorescence staining of α-SMA, Col Ⅰ, and Col Ⅲ. Then, a proteomics approach was used to explore the underlying mechanisms of action of AWA in chronic heart failure.

RESULTS

AWA administration reduced body weight gain, myocardial fibrosis, cardiomyocyte hypertrophy, and apoptosis, and rats showed improvement in cardiac function compared to model group. The extract significantly ameliorated the AAC-induced altered expression of heart failure markers such as ANP, NT-proBNP, and β-MHC, as well as fibrosis, hypertrophy markers MMP-2 and MMP-9, and other heart failure-related factors including plasma levels of TNF-α and IL-6. Furthermore, the extract reduced the protein expression of α-SMA, Col Ⅰ, and Col Ⅲ in the left ventricular (LV), thus inhibiting the LV remodeling associated with CHF. In addition, proteomics characterization of differentially expressed proteins showed that AWA administration inhibited left ventricular remodeling in CHF rats via a calcium signaling pathway, and reversed the expression of RyR2 and SERCA2a.

CONCLUSIONS

AWA extract exerts beneficial effects in an AAC-induced CHF model in rats, which was associated with an improvement in LV function, hypertrophy, fibrosis, and apoptotic status. These effects may be related to the regulation of calcium signaling by the altered expression of RyR2 and SERCA2a.

XPO1 Gene Therapy Attenuates Cardiac Dysfunction in Rats with Chronic Induced Myocardial Infarction

Transcriptomic signature of XPO1 was highly expressed and inversely related to left ventricular function in ischemic cardiomyopathy patients. We hypothesized that treatment with AAV9-shXPO1 attenuates left ventricular dysfunction and remodeling in a myocardial infarction rat model. We induced myocardial infarction by coronary ligation in Sprague-Dawley rats (n = 10), which received AAV9-shXPO1 (n = 5) or placebo AAV9-scramble (n = 5) treatment. Serial echocardiographic assessment was performed throughout the study. After myocardial infarction, AAV9-shXPO1-treated rats showed partial recovery of left ventricular fractional shortening (16.8 ± 2.8 vs 24.6 ± 4.1%, P < 0.05) and a maintained left ventricular dimension (6.17 ± 0.95 vs 4.70 ± 0.93 mm, P < 0.05), which was not observed in non-treated rats. Furthermore, lower levels of EXP-1 (P < 0.05) and lower collagen fibers and fibrosis in cardiac tissue were observed. However, no differences were found in the IL-6 or TNFR1 plasma levels of the myocardium of AAV9-shXPO1 rats. AAV9-shXPO1 administration attenuates cardiac dysfunction and remodeling in rats after myocardial infarction, producing the gene silencing of XPO1.

The creatine kinase system as a therapeutic target for myocardial ischaemia-reperfusion injury

Restoring blood flow following an acute myocardial infarction saves lives, but results in tissue damage due to ischaemia–reperfusion injury (I/R). Ameliorating this damage is a major research goal to improve recovery and reduce subsequent morbidity due to heart failure. Both the ischaemic and reperfusion phases represent crises of cellular energy provision in which the mitochondria play a central role. This mini-review will explore the rationale and therapeutic potential of augmenting the creatine kinase (CK) energy shuttle, which constitutes the primary short-term energy buffer and transport system in the cardiomyocyte. Proof-of-principle data from several transgenic mouse models have demonstrated robust cardioprotection by either raising myocardial creatine levels or by overexpressing specific CK isoforms. The effect on cardiac function, high-energy phosphates and myocardial injury will be discussed and possible directions for future research highlighted. We conclude that the CK system represents a viable target for therapeutic intervention in I/R injury; however, much needed translational studies will require the development of new pharmacological tools.

Histone methyltransferase Smyd1 regulates mitochondrial energetics in the heart

Smyd1, a muscle-specific histone methyltransferase, has established roles in skeletal and cardiac muscle development, but its role in the adult heart remains poorly understood. Our prior work demonstrated that cardiac-specific deletion of Smyd1 in adult mice (Smyd1-KO) leads to hypertrophy and heart failure. Here we show that down-regulation of mitochondrial energetics is an early event in these Smyd1-KO mice preceding the onset of structural abnormalities. This early impairment of mitochondrial energetics in Smyd1-KO mice is associated with a significant reduction in gene and protein expression of PGC-1α, PPARα, and RXRα, the master regulators of cardiac energetics. The effect of Smyd1 on PGC-1α was recapitulated in primary cultured rat ventricular myocytes, in which acute siRNA-mediated silencing of Smyd1 resulted in a greater than twofold decrease in PGC-1α expression without affecting that of PPARα or RXRα. In addition, enrichment of histone H3 lysine 4 trimethylation (a mark of gene activation) at the PGC-1α locus was markedly reduced in Smyd1-KO mice, and Smyd1-induced transcriptional activation of PGC-1α was confirmed by luciferase reporter assays. Functional confirmation of Smyd1's involvement showed an increase in mitochondrial respiration capacity induced by overexpression of Smyd1, which was abolished by siRNA-mediated PGC-1α knockdown. Conversely, overexpression of PGC-1α rescued transcript expression and mitochondrial respiration caused by silencing Smyd1 in cardiomyocytes. These findings provide functional evidence for a role of Smyd1, or any member of the Smyd family, in regulating cardiac energetics in the adult heart, which is mediated, at least in part, via modulating PGC-1α.

Atorvastatin Attenuates Metabolic Remodeling in Ischemic Myocardium through the Downregulation of UCP2 Expression

Uncoupling protein 2 (UCP2) is primarily expressed in the myocardium and is closely related to myocardial ischemia/reperfusion injury and myocardial metabolism. To explore the effects and the mechanisms of UCP2 on atorvastatin-mediated myocardium protection, the rat model of myocardial ischemia was established by ligation of the left anterior descending coronary arteries (LADs). The rats were divided into the sham operation (SO) group, myocardial infarction (MI) group and MI-atorvastatin group. The study that atorvastatin reduced myocardial remodeling and improved the disturbed myocardial energy metabolism after MI. Furthermore, the mechanisms of myocardial metabolic remodeling affected by atorvastatin were explored. The atorvastatin group showed a significantly decreased expression of UCP2 mRNA and protein. Furthermore, the primary rat cardiomyocytes were cultured and treated with angiotensin II (Ang II) to induce cardiomyocyte hypertrophy. The results showed that in the atorvastatin group, the surface area of the cardiomyocytes, the total protein content per unit of cells, and the expression of the UCP2 protein were significantly decreased. These data suggested that atorvastatin significantly attenuated the myocardial remodeling by downregulating the expression of UCP2 that was found to improve the myocardial energy metabolism, inhibit myocardial hypertrophy, and eventually reduce myocardial remodeling.

Dioxygen and Metabolism; Dangerous Liaisons in Cardiac Function and Disease

The heart must consume a significant amount of energy to sustain its contractile activity. Although the fuel demands are huge, the stock remains very low. Thus, in order to supply its daily needs, the heart must have amazing adaptive abilities, which are dependent on dioxygen availability. However, in myriad cardiovascular diseases, “fuel” depletion and hypoxia are common features, leading cardiomyocytes to favor low-dioxygen-consuming glycolysis rather than oxidation of fatty acids. This metabolic switch makes it challenging to distinguish causes from consequences in cardiac pathologies. Finally, despite the progress achieved in the past few decades, medical treatments have not improved substantially, either. In such a situation, it seems clear that much remains to be learned about cardiac diseases. Therefore, in this review, we will discuss how reconciling dioxygen availability and cardiac metabolic adaptations may contribute to develop full and innovative strategies from bench to bedside.

Evaluation of cardiac energetics by non-invasive 31P magnetic resonance spectroscopy

Alterations in myocardial energy metabolism have been implicated in the pathophysiology of cardiac diseases such as heart failure and diabetic cardiomyopathy. ³¹P magnetic resonance spectroscopy (MRS) is a powerful tool to investigate cardiac energetics non-invasively in vivo, by detecting phosphorus (³¹P)-containing metabolites involved in energy supply and buffering. In this article, we review the historical development of cardiac ³¹P MRS, the readouts used to assess cardiac energetics from ³¹P MRS, and how ³¹P MRS studies have contributed to the understanding of cardiac energy metabolism in heart failure and diabetes. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Metabolic reprogramming via PPARα signaling in cardiac hypertrophy and failure: From metabolomics to epigenetics

Studies using omics-based approaches have advanced our knowledge of metabolic remodeling in cardiac hypertrophy and failure. Metabolomic analysis of the failing heart has revealed global changes in mitochondrial substrate metabolism. Peroxisome proliferator-activated receptor-α (PPARα) plays a critical role in synergistic regulation of cardiac metabolism through transcriptional control. Metabolic reprogramming via PPARα signaling in heart failure ultimately propagates into myocardial energetics. However, emerging evidence suggests that the expression level of PPARα per se does not always explain the energetic state in the heart. The transcriptional activities of PPARα are dynamic, yet highly coordinated. An additional level of complexity in the PPARα regulatory mechanism arises from its ability to interact with various partners, which ultimately determines the metabolic phenotype of the diseased heart. This review summarizes our current knowledge of the PPARα regulatory mechanisms in cardiac metabolism and the possible role of PPARα in epigenetic modifications in the diseased heart. In addition, we discuss how metabolomics can contribute to a better understanding of the role of PPARα in the progression of cardiac hypertrophy and failure.

Augmentation of Creatine in the Heart

Creatine is a principle component of the creatine kinase (CK) phosphagen system common to all vertebrates. It is found in excitable cells, such as cardiomyocytes, where it plays an important role in the buffering and transport of chemical energy to ensure that supply meets the dynamic demands of the heart. Multiple components of the CK system, including intracellular creatine levels, are reduced in heart failure, while ischaemia and hypoxia represent acute crises of energy provision. Elevation of myocardial creatine levels has therefore been suggested as potentially beneficial, however, achieving this goal is not trivial. This mini-review outlines the evidence in support of creatine elevation and critically examines the pharmacological approaches that are currently available. In particular, dietary creatine-supplementation does not sufficiently elevate creatine levels in the heart due to subsequent down-regulation of the plasma membrane creatine transporter (CrT). Attempts to increase passive diffusion and bypass the CrT, e.g. via creatine esters, have yet to be tested in the heart. However, studies in mice with genetic overexpression of the CrT demonstrate proof-of-principle that elevated creatine protects the heart from ischaemia-reperfusion injury. This suggests activation of the CrT as a major unmet pharmacological target. However, translation of this finding to the clinic will require a greater understanding of CrT regulation in health and disease and the development of small molecule activators.

Proteomic and metabolomic changes driven by elevating myocardial creatine suggest novel metabolic feedback mechanisms

Mice over-expressing the creatine transporter have elevated myocardial creatine levels [Cr] and are protected against ischaemia/reperfusion injury via improved energy reserve. However, mice with very high [Cr] develop cardiac hypertrophy and dysfunction. To investigate these contrasting effects, we applied a non-biased hypothesis-generating approach to quantify global protein and metabolite changes in the LV of mice stratified for [Cr] levels: wildtype, moderately elevated, and high [Cr] (65-85; 100-135; 160-250 nmol/mg protein, respectively). Male mice received an echocardiogram at 7 weeks of age with tissue harvested at 8 weeks. RV was used for [Cr] quantification by HPLC to select LV tissue for subsequent analysis. Two-dimensional difference in-gel electrophoresis identified differentially expressed proteins, which were manually picked and trypsin digested for nano-LC-MS/MS. Principal component analysis (PCA) showed efficient group separation (ANOVA P ≤ 0.05) and peptide sequences were identified by mouse database (UniProt 201203) using Mascot. A total of 27 unique proteins were found to be differentially expressed between normal and high [Cr], with proteins showing [Cr]-dependent differential expression, chosen for confirmation, e.g. α-crystallin B, a heat shock protein implicated in cardio-protection and myozenin-2, which could contribute to the hypertrophic phenotype. Nuclear magnetic resonance (¹H-NMR at 700 MHz) identified multiple strong correlations between [Cr] and key cardiac metabolites. For example, positive correlations with α-glucose (r² = 0.45; P = 0.002), acetyl-carnitine (r² = 0.50; P = 0.001), glutamine (r² = 0.59; P = 0.0002); and negative correlations with taurine (r² = 0.74; P < 0.0001), fumarate (r² = 0.45; P = 0.003), aspartate (r² = 0.59; P = 0.0002), alanine (r² = 0.66; P < 0.0001) and phosphocholine (r² = 0.60; P = 0.0002). These findings suggest wide-ranging and hitherto unexpected adaptations in substrate utilisation and energy metabolism with a general pattern of impaired energy generating pathways in mice with very high creatine levels.

Determining Maximum Glycolytic Capacity Using Extracellular Flux Measurements

Measurements of glycolytic rate and maximum glycolytic capacity using extracellular flux analysis can give crucial information about cell status and phenotype during normal operation, development of pathology, differentiation, and malignant transformation. They are also of great use when assessing the effects of chemical or drug treatments. Here, we experimentally define maximum glycolytic capacity, demonstrate how it differs from glycolytic rate, and provide a protocol for determining the basal glycolytic rate and maximum glycolytic capacity in cells using extracellular flux measurements. The results illustrate the power of extracellular flux analysis to describe the energetics of adherent cells in culture in a fully quantitative way.

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.

L-homoarginine and cardiovascular disease

PURPOSE OF REVIEW

An increasing number of reports indicate that low levels of the endogenous amino acid L-homoarginine are linked to cardiovascular disease. In this article, we review the current findings regarding L-homoarginine metabolism and (patho-)physiology with a focus on its clinical impact.

RECENT FINDINGS

Recent clinical and epidemiological studies revealed a strong association of low circulating L-homoarginine with cardiovascular outcomes and mortality. Human and murine studies identified L-arginine:glycine amidinotransferase (AGAT) as the responsible enzyme for endogenous L-homoarginine formation, suggesting a further important function of AGAT apart from its involvement in creatine and energy metabolism. Further studies related L-homoarginine to smoking and hypertension, and metabolic phenotypes.

SUMMARY

AGAT deficiency results in diminished intracellular energy stores (i.e., ATP and phosphocreatine), as well as a lack of L-homoarginine, and has been linked to an improved metabolic risk profile, but also to impaired cardiac and cerebrovascular function. L-homoarginine's structural similarity to L-arginine suggested physiological interference with L-arginine pathways (e.g., nitric oxide). Animal experiments and clinical trials are needed to improve knowledge on the physiology of L-homoarginine and differentiate its role as marker and mediator in cardiovascular disease.

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.

Normal cardiac function in mice with supraphysiological cardiac creatine levels

Creatine and phosphocreatine levels are decreased in heart failure, and reductions in myocellular phosphocreatine levels predict the severity of the disease and portend adverse outcomes. Previous studies of transgenic mouse models with increased creatine content higher than two times baseline showed the development of heart failure and shortened lifespan. Given phosphocreatine's role in buffering ATP content, we tested the hypothesis whether elevated cardiac creatine content would alter cardiac function under normal physiological conditions. Here, we report the creation of transgenic mice that overexpress the human creatine transporter (CrT) in cardiac muscle under the control of the α-myosin heavy chain promoter. Cardiac transgene expression was quantified by qRT-PCR, and human CrT protein expression was documented on Western blots and immunohistochemistry using a specific anti-CrT antibody. High-energy phosphate metabolites and cardiac function were measured in transgenic animals and compared with age-matched, wild-type controls. Adult transgenic animals showed increases of 5.7- and 4.7-fold in the content of creatine and free ADP, respectively. Phosphocreatine and ATP levels were two times as high in young transgenic animals but declined to control levels by the time the animals reached 8 wk of age. Transgenic mice appeared to be healthy and had normal life spans. Cardiac morphometry, conscious echocardiography, and pressure-volume loop studies demonstrated mild hypertrophy but normal function. Based on our characterization of the human CrT protein expression, creatine and phosphocreatine content, and cardiac morphometry and function, these transgenic mice provide an in vivo model for examining the therapeutic value of elevated creatine content for cardiac pathologies.

A role for thioredoxin-interacting protein (Txnip) in cellular creatine homeostasis

Creatine is important for energy metabolism, yet excitable cells such as cardiomyocytes do not synthesize creatine and rely on uptake via a specific membrane creatine transporter (CrT; SLC6A8). This process is tightly controlled with downregulation of CrT upon continued exposure to high creatine via mechanisms that are poorly understood. Our aim was to identify candidate endogenous CrT inhibitors. In 3T3 cells overexpressing the CrT, creatine uptake plateaued at 3 h in response to 5 mM creatine but peaked 33% higher (P < 0.01) in the presence of cycloheximide, suggesting CrT regulation depends on new protein synthesis. Global gene expression analysis identified thioredoxin-interacting protein (Txnip) as the only significantly upregulated gene (by 46%) under these conditions (P = 0.036), subsequently verified independently at mRNA and protein levels. There was no change in Txnip expression with exposure to 5 mM taurine, confirming a specific response to creatine rather than osmotic stress. Small-interfering RNA against Txnip prevented Txnip upregulation in response to high creatine, maintained normal levels of creatine uptake, and prevented downregulation of CrT mRNA. These findings were relevant to the in vivo heart since creatine-deficient mice showed 39.71% lower levels of Txnip mRNA, whereas mice overexpressing the CrT had 57.6% higher Txnip mRNA levels and 28.7% higher protein expression compared with wild types (mean myocardial creatine concentration 124 and 74 nmol/mg protein, respectively). In conclusion, we have identified Txnip as a novel negative regulator of creatine levels in vitro and in vivo, responsible for mediating substrate feedback inhibition and a potential target for modulating creatine homeostasis.

Heart failure entails significant changes in human nucleocytoplasmic transport gene expression

BACKGROUND

Heart failure (HF) induces alterations in nucleocytoplasmic transport, which is essential to the cardiomyocyte biology. The objective of this study was to analyze the changes in gene expression in human HF, particularly focusing on nucleocytoplasmic transport-related genes.

METHODS AND RESULTS

29 RNA heart samples from dilated cardiomyopathy (DCM, n = 12) and ischemic cardiomyopathy (ICM, n = 12) patients undergoing heart transplantation and control donors (CNT, n = 5) were extracted to perform a microarray profiling using Affymetrix Human Gene® 1.0 ST arrays. We focused on the study of 5 nucleocytoplasmic transport-related genes, since this functional category has not previously been studied in HF. XPO1, GABPB2, and RANBP17 were upregulated, while KALRN was downregulated in both DCM and ICM, and XPO5 only in DCM. Validation of the results by RT-qPCR increasing the total heart samples up to 41 showed a high degree of consistency with microarray results. Moreover, we observed a strong relationship between the XPO1 mRNA and robust left ventricular function parameters in ICM: left ventricular end-systolic (r = 0.81, p<0.0001) and end-diastolic diameters (r = 0.80, p<0.0001), and ejection fraction (r = -0.57, p<0.05).

CONCLUSIONS

We show that the expression of nucleocytoplasmic transport-related genes is altered in HF. Furthermore, XPO1 mRNA level is closely related with robust left ventricular function parameters in ICM patients. These changes may help to distinguish DCM and ICM in HF at the level of the transcriptome and provide a base for novel therapeutic approaches.

Magnetic resonance imaging and spectroscopy of the murine cardiovascular system

Magnetic resonance imaging (MRI) has emerged as a powerful and reliable tool to noninvasively study the cardiovascular system in clinical practice. Because transgenic mouse models have assumed a critical role in cardiovascular research, technological advances in MRI have been extended to mice over the last decade. These have provided critical insights into cardiac and vascular morphology, function, and physiology/pathophysiology in many murine models of heart disease. Furthermore, magnetic resonance spectroscopy (MRS) has allowed the nondestructive study of myocardial metabolism in both isolated hearts and in intact mice. This article reviews the current techniques and important pathophysiological insights from the application of MRI/MRS technology to murine models of cardiovascular disease.

Heart failure induces significant changes in nuclear pore complex of human cardiomyocytes

Aims

The objectives of this study were to analyse the effect of heart failure (HF) on several proteins of nuclear pore complex (NPC) and their relationship with the human ventricular function.

Methods and Results

A total of 88 human heart samples from ischemic (ICM, n = 52) and dilated (DCM, n = 36) patients undergoing heart transplant and control donors (CNT, n = 9) were analyzed by Western blot. Subcellular distribution of nucleoporins was analysed by fluorescence and immunocytochemistry. When we compared protein levels according to etiology, ICM showed significant higher levels of NDC1 (65%, p<0.0001), Nup160 (88%, p<0.0001) and Nup153 (137%, p = 0.004) than those of the CNT levels. Furthermore, DCM group showed significant differences for NDC1 (41%, p<0.0001), Nup160 (65%, p<0.0001), Nup153 (155%, p = 0.006) and Nup93 (88%, p<0.0001) compared with CNT. However, Nup155 and translocated promoter region (TPR) did not show significant differences in their levels in any etiology. Regarding the distribution of these proteins in cell nucleus, only NDC1 showed differences in HF. In addition, in the pathological group we obtained good relationship between the ventricular function parameters (LVEDD and LVESD) and Nup160 (r = −0382, p = 0.004; r = −0.290, p = 0.033; respectively).

Conclusions

This study shows alterations in specific proteins (NDC1, Nup160, Nup153 and Nup93) that compose NPC in ischaemic and dilated human heart. These changes, related to ventricular function, could be accompanied by alterations in the nucleocytoplasmic transport. Therefore, our findings may be the basis for a new approach to HF management.

Investigating cardiac energetics in heart failure

The energetic requirements of the heart are, weight for weight, higher than for any other organ. The heart provides non-stop function for a lifetime, while maintaining energy in reserve in order to respond to increased demand. This demand is met by continuously recycling a relatively small pool of ATP, with the creatine kinase (CK) system acting as a spatial and temporal buffer. In the failing heart, key components of this system are downregulated, but whether these energetic changes are biomarkers or drivers of dysfunction and whether they represent therapeutic targets are the subjects of ongoing research. Key methodologies are now becoming available in vivo to help address these questions in mouse models, such as (31)P magnetic resonance spectroscopy to detect high-energy phosphates and (1)H magnetic resonance spectroscopy to detect total creatine. This report briefly discusses the challenges involved in using these technologies, the application and pitfalls of murine surgical models of heart failure, and how this has contributed to our understanding of pathophysiology in recent years.

Moderate elevation of intracellular creatine by targeting the creatine transporter protects mice from acute myocardial infarction

Aims

Increasing energy storage capacity by elevating creatine and phosphocreatine (PCr) levels to increase ATP availability is an attractive concept for protecting against ischaemia and heart failure. However, testing this hypothesis has not been possible since oral creatine supplementation is ineffectual at elevating myocardial creatine levels. We therefore used mice overexpressing creatine transporter in the heart (CrT-OE) to test for the first time whether elevated creatine is beneficial in clinically relevant disease models of heart failure and ischaemia/reperfusion (I/R) injury.

Methods and results

CrT-OE mice were selected for left ventricular (LV) creatine 20–100% above wild-type values and subjected to acute and chronic coronary artery ligation. Increasing myocardial creatine up to 100% was not detrimental even in ageing CrT-OE. In chronic heart failure, creatine elevation was neither beneficial nor detrimental, with no effect on survival, LV remodelling or dysfunction. However, CrT-OE hearts were protected against I/R injury in vivo in a dose-dependent manner (average 27% less myocardial necrosis) and exhibited greatly improved functional recovery following ex vivo I/R (59% of baseline vs. 29%). Mechanisms contributing to ischaemic protection in CrT-OE hearts include elevated PCr and glycogen levels and improved energy reserve. Furthermore, creatine loading in HL-1 cells did not alter antioxidant defences, but delayed mitochondrial permeability transition pore opening in response to oxidative stress, suggesting an additional mechanism to prevent reperfusion injury.

Conclusion

Elevation of myocardial creatine by 20–100% reduced myocardial stunning and I/R injury via pleiotropic mechanisms, suggesting CrT activation as a novel, potentially translatable target for cardiac protection from ischaemia.

Muscle energy stores and stroke rates of emperor penguins: implications for muscle metabolism and dive performance

In diving birds and mammals, bradycardia and peripheral vasoconstriction potentially isolate muscle from the circulation. During complete ischemia, ATP production is dependent on the size of the myoglobin oxygen (O(2)) store and the concentrations of phosphocreatine (PCr) and glycogen (Gly). Therefore, we measured PCr and Gly concentrations in the primary underwater locomotory muscle of emperor penguin and modeled the depletion of muscle O(2) and those energy stores under conditions of complete ischemia and a previously determined muscle metabolic rate. We also analyzed stroke rate to assess muscle workload variation during dives and evaluate potential limitations on the model. Measured PCr and Gly concentrations, 20.8 and 54.6 mmol kg(-1), respectively, were similar to published values for nondiving animals. The model demonstrated that PCr and Gly provide a large anaerobic energy store, even for dives longer than 20 min. Stroke rate varied throughout the dive profile, indicating muscle workload was not constant during dives as was assumed in the model. The stroke rate during the first 30 s of dives increased with increased dive depth. In extremely long dives, lower overall stroke rates were observed. Although O(2) consumption and energy store depletion may vary during dives, the model demonstrated that PCr and Gly, even at concentrations typical of terrestrial birds and mammals, are a significant anaerobic energy store and can play an important role in the emperor penguin's ability to perform long dives.

Rearrangement of energetic and substrate utilization networks compensate for chronic myocardial creatine kinase deficiency

Plasticity of the cellular bioenergetic system is fundamental to every organ function, stress adaptation and disease tolerance. Here, remodelling of phosphotransfer and substrate utilization networks in response to chronic creatine kinase (CK) deficiency, a hallmark of cardiovascular disease, has been revealed in transgenic mouse models lacking either cytosolic M-CK (M-CK(-/-)) or both M-CK and sarcomeric mitochondrial CK (M-CK/ScCKmit(-/-)) isoforms. The dynamic metabolomic signatures of these adaptations have also been defined. Tracking perturbations in metabolic dynamics with (18)O and (13)C isotopes and (31)P NMR and mass spectrometry demonstrate that hearts lacking M-CK have lower phosphocreatine (PCr) turnover but increased glucose-6-phosphate (G-6-P) turnover, glucose utilization and inorganic phosphate compartmentation with normal ATP γ-phosphoryl dynamics. Hearts lacking both M-CK and sarcomeric mitochondrial CK have diminished PCr turnover, total phosphotransfer capacity and intracellular energetic communication but increased dynamics of β-phosphoryls of ADP/ATP, G-6-P and γ-/β-phosphoryls of GTP, indicating redistribution of flux through adenylate kinase (AK), glycolytic and guanine nucleotide phosphotransfer circuits. Higher glycolytic and mitochondrial capacities and increased glucose tolerance contributed to metabolic resilience of M-CK/ScCKmit(-/-) mice. Multivariate analysis revealed unique metabolomic signatures for M-CK(-/-) and M-CK/ScCKmit(-/-) hearts suggesting that rearrangements in phosphotransfer and substrate utilization networks provide compensation for genetic CK deficiency. This new information highlights the significance of integrated CK-, AK-, guanine nucleotide- and glycolytic enzyme-catalysed phosphotransfer networks in supporting the adaptivity and robustness of the cellular energetic system.

Homogenous protein programming in the mammalian left and right ventricle free walls

Despite identical cardiac outputs, the right (RV) and left ventricle (LV) have very different embryological origins and resting workload. These differences suggest that the ventricles have different protein programming with regard to energy metabolism and contractile elements. The objective of this study was to determine the relative RV and LV protein expression levels, with an emphasis on energy metabolism. The RV and LV protein contents of the rabbit and porcine heart were determined with quantitative gel electrophoresis (2D-DIGE), mass spectrometry, and optical spectroscopy techniques. Surprisingly, the expression levels for more than 600 RV and LV proteins detected were similar. This included proteins many different compartments and metabolic pathways. In addition, no isoelectric shifts were detected in 2D-DIGE consistent with no differential posttranslational modifications in these proteins. Analysis of the RV and LV metabolic response to work revealed that the metabolic rate increases much faster with workload in the RV compared with LV. This implies that the generally lower metabolic stress of the RV actually approaches LV metabolic stress at maximum workloads. Thus, identical levels of energy conversion and mechanical elements in the RV and LV may be driven by the performance requirements at maximum workloads. In summary, the ventricles of the heart manage the differences in overall workload by modifying the amounts of cytosol, not its composition. The constant myocyte composition in the LV and RV implies that the ratio of energy metabolism and contractile elements may be optimal for the sustained cardiac contractile activity in the mammalian heart.

Intersectional innovation in biomarker development for patient-centric medicine

The pharmaceutical and healthcare industries are being revolutionized by the use of genomics, proteomics, metabolomics, bioinformatics and molecular imaging. Patient friendly diagnosis, treatment and disease management options that utilize the combination of these technologies are currently in development. New innovations in pharmaceutical advancement are taking place at the intersection of these technologies, and will be coupled with societal changes as we move to a fully networked and individual-centric consumer base. Numerous examples of the combinations of molecular characterization technologies aimed at better preclinical and clinical disease understanding, diagnosis and treatment are highlighted that are ideally situated to generate the intersectional innovation that drives healthcare advancement. The true value in patient-centric medicine will only be realized as the improved molecular characterization of disease provided by these technologies is integrated across platforms that operate directly in the patient and care provider space to provide a comprehensive view of health. Molecular profiling and imaging technologies must become fully integrated and amenable for patient and physician use in a networked environment that can provide a personal health avatar approach to medicine.

Metabolic homeostasis is maintained in myocardial hibernation by adaptive changes in the transcriptome and proteome

A transgenic mouse model for conditional induction of long-term hibernation via myocardium-specific expression of a VEGF-sequestering soluble receptor allowed the dissection of the hibernation process into an initiation and a maintenance phase. The hypoxic initiation phase was characterized by peak levels of K(ATP) channel and glucose transporter 1 (GLUT1) expression. Glibenclamide, an inhibitor of K(ATP) channels, blocked GLUT1 induction. In the maintenance phase, tissue hypoxia and GLUT1 expression were reduced. Thus, we employed a combined "-omics" approach to resolve this cardioprotective adaptation process. Unguided bioinformatics analysis on the transcriptomic, proteomic and metabolomic datasets confirmed that anaerobic glycolysis was affected and that the observed enzymatic changes in cardiac metabolism were directly linked to hypoxia-inducible factor (HIF)-1 activation. Although metabolite concentrations were kept relatively constant, the combination of the proteomic and transcriptomic dataset improved the statistical confidence of the pathway analysis by 2 orders of magnitude. Importantly, proteomics revealed a reduced phosphorylation state of myosin light chain 2 and cardiac troponin I within the contractile apparatus of hibernating hearts in the absence of changes in protein abundance. Our study demonstrates how combining different "-omics" datasets aids in the identification of key biological pathways: chronic hypoxia resulted in a pronounced adaptive response at the transcript and the protein level to keep metabolite levels steady. This preservation of metabolic homeostasis is likely to contribute to the long-term survival of the hibernating myocardium.

Systems bioenergetics of creatine kinase networks: physiological roles of creatine and phosphocreatine in regulation of cardiac cell function

Physiological role of creatine (Cr) became first evident in the experiments of Belitzer and Tsybakova in 1939, who showed that oxygen consumption in a well-washed skeletal muscle homogenate increases strongly in the presence of creatine and with this results in phosphocreatine (PCr) production with PCr/O(2) ratio of about 5-6. This was the beginning of quantitative analysis in bioenergetics. It was also observed in many physiological experiments that the contractile force changes in parallel with the alteration in the PCr content. On the other hand, it was shown that when heart function is governed by Frank-Starling law, work performance and oxygen consumption rate increase in parallel without any changes in PCr and ATP tissue contents (metabolic homeostasis). Studies of cellular mechanisms of all these important phenomena helped in shaping new approach to bioenergetics, Molecular System Bioenergetics, a part of Systems Biology. This approach takes into consideration intracellular interactions that lead to novel mechanisms of regulation of energy fluxes. In particular, interactions between mitochondria and cytoskeleton resulting in selective restriction of permeability of outer mitochondrial membrane anion channel (VDAC) for adenine nucleotides and thus their recycling in mitochondria coupled to effective synthesis of PCr by mitochondrial creatine kinase, MtCK. Therefore, Cr concentration and the PCr/Cr ratio became important kinetic parameters in the regulation of respiration and energy fluxes in muscle cells. Decrease in the intracellular contents of Cr and PCr results in a hypodynamic state of muscle and muscle pathology. Many experimental studies have revealed that PCr may play two important roles in the regulation of muscle energetics: first by maintaining local ATP pools via compartmentalized creatine kinase reactions, and secondly by stabilizing cellular membranes due to electrostatic interactions with phospholipids. The second mechanism decreases the production of lysophosphoglycerides in hypoxic heart, protects the cardiac cells sarcolemma against ischemic damage, decreases the frequency of arrhythmias and increases the post-ischemic recovery of contractile function. PCr is used as a pharmacological product Neoton in cardiac surgery as one of the components of cardioplegic solutions for protection of the heart against intraoperational injury and injected intravenously in acute myocardial ischemic conditions for improving the hemodynamic response and clinical conditions of patients with heart failure.

Stoichiometry of STAT3 and mitochondrial proteins: Implications for the regulation of oxidative phosphorylation by protein-protein interactions

The signal transducer and activator of transcription 3 (STAT3) is a transcription factor and downstream product of cytokine and growth factor pathways. Among members of the STAT family, STAT3 has garnered particular interest due to its role in cancer and development. Recently, it was proposed that STAT3 regulates cardiac ATP generation in vivo through protein interaction with the mitochondrial complexes of oxidative phosphorylation, specifically Complexes I/II. For this mechanism to work effectively, the cellular ratio of Complexes I/II and STAT3 must approach one. However, using three different proteomic approaches in cardiac tissue, we determined the ratio of Complexes I/II and STAT3 to be ∼10⁵. This finding suggests that direct protein interaction between Complexes I/II and STAT3 cannot be required for optimal ATP production, nor can it dramatically modulate oxidative phosphorylation in vivo. Thus, STAT3 is likely altering mitochondrial function via transcriptional regulation or indirect signaling pathways that warrant further investigation.

Ins(1,4,5)P(3) facilitates ATP accumulation via phosphocreatine/creatine kinase in the endoplasmic reticulum extracted from MDCK cells

So far, the content and accumulation of ATP in isolated endoplasmic reticulum (ER) are little understood. First, we confirmed using electron microscopic and Western blotting techniques that the samples extracted from MDCK cells are endoplasmic reticulum (ER). The amounts of ATP in the extracted ER were measured from the filtrate after a spinning down of ultrafiltration spin column packed with ER. When the ER sample (5mug) after 3days freezing was suspended in intracellular medium (ICM), 0.1% Triton X and ultrapure water (UPW), ATP amounts from the ER with UPW were the highest and over 10 times compared with that from the control with ICM, indicating that UPW is the most effective tool in destroying the ER membrane. After a 10-min-incubation with ICM containing phosphocreatine (PCr)/creatine kinase (CK) of the fresh ER. ATP amounts in the filtrate obtained by spinning down were not changed from that in the control (no PCr/CK). However, ATP amounts in the filtrate from the second spinning down of the ER (treated with PCr/CK) suspended in UPW became over 10-fold compared with the control. When 1muM inositol(1,4,5)trisphosphate (Ins(1,4,5)P(3)) was added in the incubation medium (ICM with PCr/CK), ATP amounts from the filtrate after the second spinning down were further enhanced around three times. This enhancement was almost canceled by Ca(2+)-removal from ICM and by adding thapsigargin, a Ca(2+)-ATPase inhibitor, but not by 2-APB and heparin, Ins(1,4,5)P(3) receptor antagonists. Administration of 500muM adenosine to the incubation medium (with PCr/CK) failed to enhance the accumulation of ATP in the ER. These findings suggest that the ER originally contains ATP and ATP accumulation in the ER is promoted by PCr/CK and Ins(1,4,5)P(3).