Hyperpolarized butyrate: a metabolic probe of short chain fatty acid metabolism in the heart

Hyperpolarized magnetic resonance as a sensitive detector of metabolic function

Hyperpolarized magnetic resonance allows for noninvasive measurements of biochemical reactions in vivo. Although this technique provides a unique tool for assaying enzymatic activities in intact organs, the scope of its application is still elusive for the wider scientific community. The purpose of this review is to provide key principles and parameters to guide the researcher interested in adopting this technology to address a biochemical, biomedical, or medical issue. It is presented in the form of a compendium containing the underlying essential physical concepts as well as suggestions to help assess the potential of the technique within the framework of specific research environments. Explicit examples are used to illustrate the power as well as the limitations of hyperpolarized magnetic resonance.

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

BACKGROUND

Myocardial infarction (MI) is one of the leading causes of heart failure. An increasing body of evidence links alterations in cardiac metabolism and mitochondrial function with the progression of heart disease. The aim of this work was to, therefore, follow the in vivo mitochondrial metabolic alterations caused by MI, thereby allowing a greater understanding of the interplay between metabolic and functional abnormalities.

METHODS AND RESULTS

Using hyperpolarized carbon-13 ((13)C)-magnetic resonance spectroscopy, in vivo alterations in mitochondrial metabolism were assessed for 22 weeks after surgically induced MI with reperfusion in female Wister rats. One week after MI, there were no detectable alterations in in vivo cardiac mitochondrial metabolism over the range of ejection fractions observed (from 28% to 84%). At 6 weeks after MI, in vivo mitochondrial Krebs cycle activity was impaired, with decreased (13)C-label flux into citrate, glutamate, and acetylcarnitine, which correlated with the degree of cardiac dysfunction. These changes were independent of alterations in pyruvate dehydrogenase flux. By 22 weeks, alterations were also seen in pyruvate dehydrogenase flux, which decreased at lower ejection fractions. These results were confirmed using in vitro analysis of enzyme activities and metabolomic profiles of key intermediates.

CONCLUSIONS

The in vivo decrease in Krebs cycle activity in the 6-week post-MI heart may represent an early maladaptive phase in the metabolic alterations after MI in which reductions in Krebs cycle activity precede a reduction in pyruvate dehydrogenase flux. Changes in mitochondrial metabolism in heart disease are progressive and proportional to the degree of cardiac impairment.

Resolving confounding enrichment kinetics due to overlapping resonance signals from 13C-enriched long chain fatty acid oxidation and uptake within intact hearts

PURPOSE

Long chain fatty acid (LCFA) oxidation measurements in the intact heart from 13C-NMR rely on detection of 13C-enriched glutamate. However, progressive increases in overlapping resonance signal from LCFA can confound detection of the glutamate 4-carbon (GLU-C4) signal. We evaluated alternative 13C labeling for exogenous LCFA and developed a simple scheme to distinguish kinetics of LCFA uptake and storage from oxidation.

METHODS

Sequential 13C-NMR spectra were acquired from isolated rat hearts perfused with 13C LCFA and glucose. Spectra were evaluated from hearts supplied: U 13C LCFA, [2,4,6,8,10,12,14,16-(13) C8 ] palmitate, [2,4,6,8,10,12,14,16,18-(13) C9 ] oleate, [4,6,8,10,12,14,16-(13) C7 ] palmitate, or [4,6,8,10,12,14,16,18-(13) C8 ] oleate.

RESULTS

13C signal reflected the progressive enrichment at 34.6 ppm from GLU-C4, confounded by additional signal with distinct kinetics attributed to 13C-enriched LCFA 2-carbon (34.0 ppm). Excluding 13C at the 2-carbon of both palmitate and oleate eliminated signal overlap and enabled detection of the exponential enrichment of GLU-C4 for assessing LCFA oxidation.

CONCLUSION

Eliminating enrichment at the 2-carbon of 13C LCFA resolved confounding kinetics between GLU-C4 and LCFA 2-carbon signals. With this enrichment scheme, oxidation of LCFA, the primary fuel for cardiac ATP synthesis, can now be more consistently examined in whole organs with dynamic mode, proton-decoupled (13C-NMR

Hyperpolarized NMR probes for biological assays

During the last decade, the development of nuclear spin polarization enhanced (hyperpolarized) molecular probes has opened up new opportunities for studying the inner workings of living cells in real time. The hyperpolarized probes are produced ex situ, introduced into biological systems and detected with high sensitivity and contrast against background signals using high resolution NMR spectroscopy. A variety of natural, derivatized and designed hyperpolarized probes has emerged for diverse biological studies including assays of intracellular reaction progression, pathway kinetics, probe uptake and export, pH, redox state, reactive oxygen species, ion concentrations, drug efficacy or oncogenic signaling. These probes are readily used directly under natural conditions in biofluids and are often directly developed and optimized for cellular assays, thus leaving little doubt about their specificity and utility under biologically relevant conditions. Hyperpolarized molecular probes for biological NMR spectroscopy enable the unbiased detection of complex processes by virtue of the high spectral resolution, structural specificity and quantifiability of NMR signals. Here, we provide a survey of strategies used for the selection, design and use of hyperpolarized NMR probes in biological assays, and describe current limitations and developments.

Clinical implications of cardiac hyperpolarized magnetic resonance imaging

Alterations in cardiac metabolism are now considered a cause, rather than a result, of cardiac disease. Although magnetic resonance spectroscopy has allowed investigation of myocardial energetics, the inherently low sensitivity of the technique has limited its clinical application in the study of cardiac metabolism. The development of a novel hyperpolarization technique, based on the process of dynamic nuclear polarization, when combined with the metabolic tracers [1-(13)C] and [2-(13)C] pyruvate, has resulted in significant advances in the understanding of real time myocardial metabolism in the normal and diseased heart in vivo. This review focuses on the changes in myocardial substrate selection and downstream metabolism of hyperpolarized 13C labelled pyruvate that have been shown in diabetes, ischaemic heart disease, cardiac hypertrophy and heart failure in animal models of disease and how these could translate into clinical practice with the advent of clinical grade hyperpolarizer systems.