Considerations for using isolated cell systems to understand cardiac metabolism and biology

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Abstract

Changes in myocardial metabolic activity are fundamentally linked to cardiac health and remodeling. Primary cardiomyocytes, induced pluripotent stem cell-derived cardiomyocytes, and transformed cardiomyocyte cell lines are common models used to understand how (patho)physiological conditions or stimuli contribute to changes in cardiac metabolism. These cell models are helpful also for defining metabolic mechanisms of cardiac dysfunction and remodeling. Although technical advances have improved our capacity to measure cardiomyocyte metabolism, there is often heterogeneity in metabolic assay protocols and cell models, which could hinder data interpretation and discernment of the mechanisms of cardiac (patho)physiology. In this review, we discuss considerations for integrating cardiomyocyte cell models with techniques that have become relatively common in the field, such as respirometry and extracellular flux analysis. Furthermore, we provide overviews of metabolic assays that complement XF analyses and that provide information on not only catabolic pathway activity, but biosynthetic pathway activity and redox status as well. Cultivating a more widespread understanding of the advantages and limitations of metabolic measurements in cardiomyocyte cell models will continue to be essential for the development of coherent metabolic mechanisms of cardiac health and pathophysiology.

Introduction

Metabolism is an area of traditional biochemistry that, until recently, was considered a relatively mature field. The backbone of metabolic knowledge reflects the success of our scientific approaches and technological advances. New methods for understanding complex system behavior have further propelled the scientific community’s quest to understand metabolism, not only by increasing the breadth of exploration, but also by increasing the throughput and analysis of metabolic data. In addition, the development of cell models of cardiovascular disease, such as induced pluripotent stem cells (iPSCs), have expanded interest in understanding the metabolic properties of cultured cells, which could be used to predict disease or to devise actionable interventions. Experimental systems that integrate cell models with techniques for measuring metabolic activity have been used to generate volumes of data that must be carefully weighed for elements of rigor and for their meaningfulness for understanding in vivo biology. Ultimately, the value of data from isolated cell systems depends upon their usefulness to contribute to coherent explanations of biological phenomena. In this review, we discuss methodological considerations for measuring cardiomyocyte cell metabolism.

Section snippets

How does the heart’s metabolic machinery process substrates?

A general goal of many metabolic studies is to understand how living systems regulate the production of useable energy. Because the heart has an extremely high energetic demand, it is a model organ for understanding how cells control ATP levels (reviewed in [1]). Large quantities of ATP are required to maintain cardiac ion homeostasis and contractile function, and this ATP demand is met via catabolism of a variety of circulating hydrocarbon substrates [[2], [3], [4], [5], [6], [7], [8]]. For

Respirometry using isolated mitochondria

Purified mitochondrial fractions can be used to understand specific aspects of mitochondrial biology, including respiratory efficiency and capacity [[16], [17], [18]]. Typically, respirometry uses oxygen electrode- or fluorophore-based systems to measure the consumption of O2 as an index of catabolic activity in isolated mitochondria, cells, tissues, or whole organisms. Because the vast majority of O2 in most cells is consumed at cytochrome c oxidase, the rate of O2 consumption is used to

Considerations for other metabolic assays in cultured cardiomyocytes

Other techniques in the investigator’s metabolic toolbox, discussed briefly below, complement XF data and provide additional information on catabolic pathway flux, redox state, and biosynthetic pathway activity (e.g., see: [88,89]).

Summary

Sensible application of metabolic assays can facilitate understanding of how metabolic changes influence cardiomyocyte biology. Initial experiments to optimize cell number, substrate media composition, and pharmacological compound concentrations are critical for deriving reproducible and interpretable results. Moreover, complementary metabolic assays can provide additional rigor and depth to experimental results and interpretation. Considerations for the advantages and limitations of each

Acknowledgements

This work was supported in part by grants from the NIH (HL130174, HL147844, ES028268, HL078825, GM127607, HL154663).

Disclosures

None.

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