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

PURPOSE: Hyperpolarized [1-13 C]pyruvate MRS can measure cardiac metabolism in vivo. We investigated whether [1-13 C]pyruvate MRS could predict left ventricular remodeling following myocardial infarction (MI), long-term left ventricular effects of heart failure medication, and could identify responders to treatment. METHODS: Thirty-five rats were scanned with hyperpolarized [1-13 C]pyruvate MRS 3 days after MI or sham surgery. The animals were re-examined after 30 days of therapy with ß-blockers and ACE-inhibitors (active group, n = 12), placebo treatment (placebo group, n = 13) or no treatment (sham group, n = 10). Furthermore, heart tissue mitochondrial respiratory capacity was assessed by high-resolution respirometry. Metabolic results were compared between groups, over time and correlated to functional MR data at each time point. RESULTS: At 30 ± 0.5 days post MI, left ventricular ejection fraction (LVEF) differed between groups (sham, 77% ± 1%; placebo, 52% ± 3%; active, 63% ± 2%, P < .001). Cardiac metabolism, measured by both hyperpolarized [1-13 C]pyruvate MRS and respirometry, neither differed between groups nor between baseline and follow-up. Three days post MI, low bicarbonate + CO2 /pyruvate ratio was associated with low LVEF. At follow-up, in the active group, a poor recovery of LVEF was associated with high bicarbonate + CO2 /pyruvate ratio, as measured by hyperpolarized MRS. CONCLUSION: In a rat model of moderate heart failure, medical treatment improved function, but did not on average influence [1-13 C]pyruvate flux as measured by MRS; however, responders to heart failure medication had reduced capacity for carbohydrate metabolism.

Hyperpolarized [1,4-13C2]Fumarate Enables Magnetic Resonance-Based Imaging of Myocardial Necrosis.

OBJECTIVES: The aim of this study was to determine if hyperpolarized [1,4-13C2]malate imaging could measure cardiomyocyte necrosis after myocardial infarction (MI). BACKGROUND: MI is defined by an acute burst of cellular necrosis and the subsequent cascade of structural and functional adaptations. Quantifying necrosis in the clinic after MI remains challenging. Magnetic resonance-based detection of the conversion of hyperpolarized [1,4-13C2]fumarate to [1,4-13C2]malate, enabled by disrupted cell membrane integrity, has measured cellular necrosis in vivo in other tissue types. Our aim was to determine whether hyperpolarized [1,4-13C2]malate imaging could measure necrosis after MI. METHODS: Isolated perfused hearts were given hyperpolarized [1,4-13C2]fumarate at baseline, immediately after 20 min of ischemia, and after 45 min of reperfusion. Magnetic resonance spectroscopy measured conversion into [1,4-13C2]malate. Left ventricular function and energetics were monitored throughout the protocol, buffer samples were collected and hearts were preserved for further analyses. For in vivo studies, magnetic resonance spectroscopy and a novel spatial-spectral magnetic resonance imaging sequence were implemented to assess cardiomyocyte necrosis in rats, 1 day and 1 week after cryo-induced MI. RESULTS: In isolated hearts, [1,4-13C2]malate production became apparent after 45 min of reperfusion, and increased 2.7-fold compared with baseline. Expression of dicarboxylic acid transporter genes were negligible in healthy and reperfused hearts, and lactate dehydrogenase release and infarct size were significantly increased in reperfused hearts. Nonlinear regression revealed that [1,4-13C2]malate production was induced when adenosine triphosphate was depleted by >50%, below 5.3 mmol/l (R2 = 0.904). In vivo, the quantity of [1,4-13C2]malate visible increased 82-fold over controls 1 day after infarction, maintaining a 31-fold increase 7 days post-infarct. [1,4-13C2]Malate could be resolved using hyperpolarized magnetic resonance imaging in the infarct region one day after MI; [1,4-13C2]malate was not visible in control hearts. CONCLUSIONS: Malate production in the infarcted heart appears to provide a specific probe of necrosis acutely after MI, and for at least 1 week afterward. This technique could offer an alternative noninvasive method to measure cellular necrosis in heart disease, and warrants further investigation in patients.

Extramitochondrial domain rich in carbonic anhydrase activity improves myocardial energetics.

CO2 is produced abundantly by cardiac mitochondria. Thus an efficient means for its venting is required to support metabolism. Carbonic anhydrase (CA) enzymes, expressed at various sites in ventricular myocytes, may affect mitochondrial CO2 clearance by catalyzing CO2 hydration (to H(+) and HCO3(-)), thereby changing the gradient for CO2 venting. Using fluorescent dyes to measure changes in pH arising from the intracellular hydration of extracellularly supplied CO2, overall CA activity in the cytoplasm of isolated ventricular myocytes was found to be modest (2.7-fold above spontaneous kinetics). Experiments on ventricular mitochondria demonstrated negligible intramitochondrial CA activity. CA activity was also investigated in intact hearts by (13)C magnetic resonance spectroscopy from the rate of H(13)CO3(-) production from (13)CO2 released specifically from mitochondria by pyruvate dehydrogenase-mediated metabolism of hyperpolarized [1-(13)C]pyruvate. CA activity measured upon [1-(13)C]pyruvate infusion was fourfold higher than the cytoplasm-averaged value. A fluorescent CA ligand colocalized with a mitochondrial marker, indicating that mitochondria are near a CA-rich domain. Based on immunoreactivity, this domain comprises the nominally cytoplasmic CA isoform CAII and sarcoplasmic reticulum-associated CAXIV. Inhibition of extramitochondrial CA activity acidified the matrix (as determined by fluorescence measurements in permeabilized myocytes and isolated mitochondria), impaired cardiac energetics (indexed by the phosphocreatine-to-ATP ratio measured by (31)P magnetic resonance spectroscopy of perfused hearts), and reduced contractility (as measured from the pressure developed in perfused hearts). These data provide evidence for a functional domain of high CA activity around mitochondria to support CO2 venting, particularly during elevated and fluctuating respiratory activity. Aberrant distribution of CA activity therefore may reduce the heart's energetic efficiency.

Hyperpolarized (13)C magnetic resonance reveals early- and late-onset changes to in vivo pyruvate metabolism in the failing heart.

AIMS: Impaired energy metabolism has been implicated in the pathogenesis of heart failure. Hyperpolarized (13)C magnetic resonance (MR), in which (13)C-labelled metabolites are followed using MR imaging (MRI) or spectroscopy (MRS), has enabled non-invasive assessment of pyruvate metabolism. We investigated the hypothesis that if we serially examined a model of heart failure using non-invasive hyperpolarized [(13)C]pyruvate with MR, the profile of in vivo pyruvate oxidation would change throughout the course of the disease. METHODS AND RESULTS: Dilated cardiomyopathy (DCM) was induced in pigs (n = 5) by rapid pacing. Pigs were examined using MR at weekly time points: cine-MRI assessed cardiac structure and function; hyperpolarized [2-(13)C]pyruvate was administered intravenously, and (13)C MRS monitored [(13)C]glutamate production; (31)P MRS assessed cardiac energetics [phosphocreatine (PCr)/ATP]; and hyperpolarized [1-(13)C]pyruvate was administered for MRI of pyruvate dehydrogenase complex (PDC)-mediated pyruvate oxidation via [(13)C]bicarbonate production. Early in pacing, the cardiac index decreased by 25%, PCr/ATP decreased by 26%, and [(13)C]glutamate production decreased by 51%. After clinical features of DCM appeared, end-diastolic volume increased by 40% and [(13)C]bicarbonate production decreased by 67%. Pyruvate dehydrogenase kinase 4 protein increased by two-fold, and phosphorylated Akt decreased by half. Peroxisome proliferator-activated receptor-α and carnitine palmitoyltransferase-1 gene expression decreased by a half and a third, respectively. CONCLUSION: Despite early changes associated with cardiac energetics and (13)C incorporation into the Krebs cycle, pyruvate oxidation was maintained until DCM developed, when the heart's capacity to oxidize both pyruvate and fats was reduced. Hyperpolarized (13)C MR may be important to characterize metabolic changes that occur during heart failure progression.

In vivo alterations in cardiac metabolism and function in the spontaneously hypertensive rat heart.

AIMS: The aim of this work was to use hyperpolarized carbon-13 ((13)C) magnetic resonance (MR) spectroscopy and cine MR imaging (MRI) to assess in vivo cardiac metabolism and function in the 15-week-old spontaneously hypertensive rat (SHR) heart. At this time point, the SHR displays hypertension and concentric hypertrophy. One of the cellular adaptations to hypertrophy is a reduction in ß-oxidation, and it has previously been shown that in response to hypertrophy the SHR heart switches to a glycolytic/glucose-oxidative phenotype. METHODS AND RESULTS: Cine-MRI (magnetic resonance imaging) was used to assess cardiac function and degree of cardiac hypertrophy. Wistar rats were used as controls. SHRs displayed functional changes in stroke volume, heart rate, and late peak-diastolic filling alongside significant hypertrophy (a 56% increase in left ventricular mass). Using hyperpolarized [1-(13)C] and [2-(13)C]pyruvate, an 85% increase in (13)C label flux through pyruvate dehydrogenase (PDH) was seen in the SHR heart and (13)C label incorporation into citrate, acetylcarnitine, and glutamate pools was elevated in proportion to the increase in PDH flux. These findings were confirmed using biochemical analysis of PDH activity and protein expression of PDH regulatory enzymes. CONCLUSIONS: Functional and structural alterations in the SHR heart are consistent with the hypertrophied phenotype. Our in vivo work indicates a preference for glucose metabolism in the SHR heart, a move away from predominantly fatty acid oxidative metabolism. Interestingly, (13)C label flux into lactate was unchanged, indicating no switch to an anaerobic glycolytic phenotype, but rather an increased reliance on glucose oxidation in the SHR heart.

Multicentric carpotarsal osteolysis is caused by mutations clustering in the amino-terminal transcriptional activation domain of MAFB.

Multicentric carpotarsal osteolysis (MCTO) is a rare skeletal dysplasia characterized by aggressive osteolysis, particularly affecting the carpal and tarsal bones, and is frequently associated with progressive renal failure. Using exome capture and next-generation sequencing in five unrelated simplex cases of MCTO, we identified previously unreported missense mutations clustering within a 51 base pair region of the single exon of MAFB, validated by Sanger sequencing. A further six unrelated simplex cases with MCTO were also heterozygous for previously unreported mutations within this same region, as were affected members of two families with autosomal-dominant MCTO. MAFB encodes a transcription factor that negatively regulates RANKL-induced osteoclastogenesis and is essential for normal renal development. Identification of this gene paves the way for development of novel therapeutic approaches for this crippling disease and provides insight into normal bone and kidney development.