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Assessment of cardiac function in mice lacking the mitochondrial calcium uniporter

https://doi.org/10.1016/j.yjmcc.2015.05.022Get rights and content

Highlights

  • MCU expression is required for rapid calcium uptake in cardiac mitochondria.

  • Hearts lacking MCU have normal basal cardiac function.

  • Deletion of MCU does not alter the response to transverse aortic constriction.

Abstract

Mitochondrial calcium is thought to play an important role in the regulation of cardiac bioenergetics and function. The entry of calcium into the mitochondrial matrix requires that the divalent cation pass through the inner mitochondrial membrane via a specialized pore known as the mitochondrial calcium uniporter (MCU). Here, we use mice deficient of MCU expression to rigorously assess the role of mitochondrial calcium in cardiac function. Mitochondria isolated from MCU−/− mice have reduced matrix calcium levels, impaired calcium uptake and a defect in calcium-stimulated respiration. Nonetheless, we find that the absence of MCU expression does not affect basal cardiac function at either 12 or 20 months of age. Moreover, the physiological response of MCU−/− mice to isoproterenol challenge or transverse aortic constriction appears similar to control mice. Thus, while mitochondria derived from MCU−/− mice have markedly impaired mitochondrial calcium handling, the hearts of these animals surprisingly appear to function relatively normally under basal conditions and during stress.

Introduction

To achieve its ongoing energy demand, the heart relies almost exclusively on ATP production from the mitochondria. Cardiac myocytes are particularly well constructed for this energetic task with approximately ten to twenty thousand mitochondria per myocyte and with the mitochondria comprising roughly a third of the cardiac myocyte cellular volume [1]. In certain situations, for instance with exercise or after adrenergic stimulation, the heart sees a dramatic increase in its energetic demand. Experimental evidence has demonstrated that during such episodes, the ratio of ADP/ATP remains relatively constant [2], [3]. This suggests that during these periods of suddenly increased demand, the heart has the capacity to rapidly augment ATP production.

One mechanism for how ATP production could be rapidly augmented came from observations in the 1970s that various mitochondrial matrix enzymes were regulated by calcium. In particular, the key metabolic enzymes pyruvate dehydrogenase, oxoglutarate dehydrogenase and isocitrate dehydrogenase all demonstrated calcium-induced activation [4]. These observations led to the hypothesis that entry of mitochondrial calcium across the inner membrane results in a rise in matrix calcium, which activates matrix dehydrogenases, leading to an increase in ATP production necessary to meet the increase in demand. Recent studies have also demonstrated that calcium also activates specific components of the electron transport chain including Complex V [5].

For many years, the molecular details of how calcium enters the mitochondria remained elusive. Pharmacological, biochemical and electrophysiological evidence all pointed to an inner mitochondrial membrane protein that acted as a calcium selective pore, relying on the large mitochondrial membrane potential as a driving force for calcium entry. Nonetheless, the molecular identity of the mitochondrial calcium uniporter (MCU) was unknown until several years ago, when two groups independently identified a 40 kD protein as the long sought uniporter [6], [7]. Both groups were able to demonstrate that knockdown of MCU attenuated mitochondrial calcium uptake.

In order to characterize the physiological role of mitochondrial calcium, we recently described a MCU mouse knockout model [8]. Remarkably, in a mixed genetic background, MCU−/− mice were viable and except for a smaller size, displayed remarkably few outwardly discernable phenotypes. We did observe that when challenged with an increase in workload, the skeletal muscle of MCU−/− mice appeared impaired and were unable to perform as well as their wild type littermates. Given the significant experimental data concerning the role of mitochondrial calcium in the heart [9], [10], we sought to further characterize the cardiac function of our MCU−/− mice under basal and stress conditions. Here, we report that the absence of MCU expression surprisingly does not alter basal cardiac function, nor does it seemingly markedly impair the ability of the heart to respond to pharmacological or physiological stress.

Section snippets

Methods

Detailed methods are available in the online supplement.

Results and discussion

MCU protein expression could readily be detected in mitochondria isolated from the hearts of wild type mice, although this expression, as expected, was seemingly absent in mitochondria obtained from the hearts of MCU−/− mice (Fig. 1A). The absence of MCU also resulted in the corresponding reduction of EMRE (essential MCU regulator), a known component of the uniporter complex [11]. Previous in vitro observations have also noted that reducing MCU expression results in the destabilization of EMRE

Disclosures

None declared.

Acknowledgments

We thank Hong San for assistance with the TAC surgery, Zuxi Yu for help with pathology and Ilsa I Rovira for help for the manuscript. This work was supported by a grant from the Leducq Foundation and with NIH Intramural Funds.

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