REVIEW article

Front. Physiol., 26 October 2018

Sec. Cardiac Electrophysiology

Volume 9 - 2018 | https://doi.org/10.3389/fphys.2018.01518

The Mitochondrial Translocator Protein and the Emerging Link Between Oxidative Stress and Arrhythmias in the Diabetic Heart

  • Cardiovascular Research Center, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Abstract

The mitochondrial translocator protein (TSPO) is a key outer mitochondrial membrane protein that regulates the activity of energy-dissipating mitochondrial channels in response to oxidative stress. In this article, we provide an overview of the role of TSPO in the systematic amplification of reactive oxygen species (ROS) through an autocatalytic process known as ROS-induced ROS-release (RIRR). We describe how this TSPO-driven process destabilizes the mitochondrial membrane potential leading to electrical instability at the cellular and whole heart levels. Finally, we provide our perspective on the role of TSPO in the pathophysiology of diabetes, in general and diabetes-related arrhythmias, in particular.

Introduction

Diabetes mellitus is a global public health epidemic that continues to expand, in both its incidence and prevalence. Diabetic patients are predisposed to an increasing number of debilitating cardiovascular disorders such as stroke and myocardial infarction (; Yang et al., 2018). This metabolic disease is also an important risk factor in the development of cardiac rhythm disorders (; ; ). In addition to predisposing to atrial fibrillation (), diabetes along with its numerous complications, has been linked to increased prevalence of ventricular arrhythmias leading to sudden cardiac death (; ; Xie et al., 2015; ). Importantly, oxidative stress, a major factor in the pathophysiology of diabetes, has been linked to arrhythmias either directly or through exacerbation of atherogenic risk factors (; ). Oxidative stress arises from enhanced production of free radicals and defective antioxidant defense mechanisms in the diabetic heart (; ). This, in turn, contributes to the pathogenesis of numerous diabetes-related cardiovascular complications, including endothelial dysfunction (), atherosclerosis (), myocardial infarction (), and diabetic cardiomyopathy (), all of which as illustrated in Figure 1 can lead to sudden cardiac death (; ; ; Zhang et al., 2017). In this article, we focus on a key outer mitochondrial membrane protein known as the mitochondrial translocator protein (TSPO) as a source of oxidative stress-related cardiac dysfunction. We begin by highlighting its role in linking mitochondrial instability to arrhythmias in the heart through a regenerative process known as reactive oxygen species (ROS) -induced ROS-release (RIRR). We then provide a new perspective on its potential importance to the pathophysiology of diabetes, in general and diabetes-related arrhythmias, in particular.

FIGURE 1

The Mitochondrial Translocator Protein

TSPO, formerly known as the peripheral benzodiazepine receptor (PBR) (), is a structurally conserved molecule which is ubiquitously expressed in steroidogenic tissues, as well as brain, kidney, and heart cells (; ). It was discovered in 1977, and initially called PBR because of its ability to bind benzodiazepine drugs outside of the central nervous system (). The 18-kDa molecule carries out a variety of essential roles such as cholesterol transport (), steroidogenesis (; ), and programmed cell death (). In eukaryotes TSPO is mainly expressed on the outer mitochondrial membrane, in close physical association with other mitochondrial channels such as the voltage-dependent anion channel (VDAC) within the mitochondrial membrane transition pore (mPTP) complex, and the inner membrane anion channel (IMAC) (; ) (Figure 2). Cryo-electron microscopy and image analyses of the TSPO molecule from Rhodobacter sphaeroides revealed a dimeric quaternary structure, whereby each TSPO monomer consists of five transmembrane domains (; ). Although monomeric and oligomeric forms have been reported, the functional implications of TSPO polymerization have not been fully elucidated (; ; ). The revelation of the 3-dimensional high-resolution image of the mouse TSPO has provided the opportunity to study the molecular interactions between this mitochondrial protein and its ligands, such as the antagonist PK11195 (). The functional role of TSPO in various organs and cell types has been investigated primarily using TSPO agonists and antagonists (; ). In order to fully appreciate the role of TSPO as a mediator of cardiac pro-arrhythmic risk, we begin by reviewing the concept of RIRR which directly links mitochondrial instability to myocyte excitability.

FIGURE 2

Ros-Induced Ros-Release and Mitochondrial Instability as a Mediator of Cardiac Arrhythmias

Mitochondria have long been recognized as indispensable sources of adenosine triphosphate (ATP) in energy-reliant organs such as the heart. Almost counterintuitively, it later became apparent that these specialized organelles can also control cell death in response to injury. In healthy mammalian cells, the preservation of ATP synthesis by complex V is achieved by maintaining a proton gradient across the inner mitochondrial membrane (,), which in turn, generates an electrochemical gradient that is responsible for maintaining a polarized mitochondrial membrane potential (). Mitochondrial respiration is always accompanied with ROS production through leakage of electrons that subsequently react with oxygen to form superoxide anions (O-2) (). Under certain pathological conditions such as diabetes, the production of ROS can exceed the capacity with which protective antioxidant defense systems eliminate these toxic agents. Oxidative stress, as well as secondary factors such as mitochondrial Ca2+ overload can prime the formation of mPTP on the inner mitochondrial membrane (Zorov et al., 2000; , , ; ). This is responsible, at least in part, for mitochondrial membrane permeabilization (), which can be underpinned by the process of RIRR (Zorov et al., 2000; , ; ; Yang et al., 2010). Traditionally, mPTP has been thought to exist as a complex of proteins comprising of VDAC, adenine nucleotide translocator (ANT), and cyclophilin D (CypD) (Figure 2). Nevertheless, genetic studies in recent years have challenged this traditionally accepted model of mPTP structure. For more details on this subject matter, we refer the reader to another review ().

Sollot and colleagues (Zorov et al., 2000) pioneered the concept of RIRR to describe how ROS injuries confined to distinct areas of a cardiomyocyte are able to quickly spread through a wider network of mitochondria, culminating in oxidative stress at a cellular level (Zorov et al., 2006). RIRR is responsible for the autocatalytic amplification of ROS levels, eventually leading to cell death. Two modes of RIRR have been proposed based on the identity of the mitochondrial pathway that mediates the process, namely the mPTP or the IMAC (; Yang et al., 2010). Initially, the connection between mPTP opening and oxidative stress-dependent destabilization of ΔΨm, leading to cell death was demonstrated by Zorov et al. (2000). This was followed by studies by who provided strong evidence for the involvement of IMAC as a mediator of RIRR in metabolic oscillations. Pharmacological studies confirmed that IMAC facilitates superoxide release (; ), providing the mechanism by which this anion channel contributes to RIRR. TSPO antagonists such as 4′-chlorodiazepam and PK11195 inhibit anion transport by IMAC, consistent with a strong modulatory role of TSPO on this ROS-sensitive channel (; ). In response to stress, IMAC activation occurs first, ultimately followed by mPTP activation at higher stress levels (; ). Collectively, these are the key elements in the series of events leading up to ROS-induced cell death. Although brief perturbations in ΔΨm may not influence cell survival to a large extent, prolonged periods of ΔΨm instability are known to mediate mitochondrial dysfunction and cell death (; Zamzami et al., 2005).

The relevance of metabolic oscillations to electrophysiological behavior was examined using photo-induced oxidation of cardiomyocytes. These seminal studies demonstrated that cyclical oscillations of the action potentials (AP) were generated in phase with ΔΨm oscillations. AP recovery was found to depend upon ΔΨm recovery, and this suggested a profound mitochondrial control of myocyte excitability, at least in vitro (). More recently, we and others (Zhou et al., 2014; ) examined the relationship between ΔΨm stability and arrhythmogenesis in response to oxidative stress. AP oscillations were generated by “out-of-phase” oscillations of sarcolemmal KATP channels during RIRR (). Furthermore, the opening of sarcolemmal KATP channels may give rise to the phenomenon of “metabolic sink”, whereby conduction wavefronts are hindered when they encounter heterogeneous current sinks in the tissue. These current sinks are formed in regions having high open probability of sarcolemmal KATP channels (; ; Zhou et al., 2014). The testing of the anti-arrhythmic effects of KATP channel inhibition using glibenclamide resulted in conflicting results, including reports of adverse effects (), whilst the pro-arrhythmic effects of channel activation have been demonstrated by multiple groups (; Xie et al., 2015). In our studies sarcolemmal KATP channel inhibition using glibenclamide did not prevent the initiation of reperfusion arrhythmias in the ex vivo perfused guinea pig heart (). This highlighted the necessity for a better understanding of the upstream elements such as TSPO which could potentially modulate the deleterious opening of sarcolemmal KATP channels during RIRR.

Consistent with cellular studies of RIRR, we demonstrated that exposure of intact hearts to high doses of exogenous pro-oxidants such as H2O2 provoked two distinct ROS peaks. While the initial low amplitude peak coincided with the exogenous stressor, the second (large amplitude) peak (which we termed P2) occurred following not during the exogenous stress, consistent with a RIRR response (). Functionally, hearts that exhibited P2 were prone to ventricular fibrillation, whereas those that did not were relatively more protected (). In a subsequent study, we investigated the relationship between the stability of the mitochondrial membrane in response to oxidative stress and the pro-arrhythmic potential of guinea pig hearts (Xie et al., 2014). Specifically, we modulated the threshold and rate of decline of the mitochondrial membrane potential in response to exogenous pro-oxidant challenge using a variety of agents that affected the activity of key mitochondrial ion channels. Once again, hearts that exhibited rapid ΔΨm decline were associated with low thresholds for sustained arrhythmias (Xie et al., 2014). More recently, elegantly demonstrated that the stabilization of the mitochondrial membrane potential may underpin exercise-mediated protection against reperfusion arrhythmias.

In light of studies showing that TSPO blockade was highly effective in abolishing ΔΨm instability, we and others examined the impact of TSPO inhibition on arrhythmia propensity. Indeed, TSPO inhibition protected against ischemia-induced AP duration (APD) shortening and inexcitability (). In contrast, IMAC activation using the TSPO agonist FGIN-1-27 enhanced APD shortening and promoted conduction failure under ischemic conditions (). In these hearts, high-resolution optical AP mapping revealed areas of conduction block, which gave rise to sustained re-entrant arrhythmias upon reperfusion. In contrast, TSPO inhibition protected against ischemia-induced conduction block and reperfusion-related arrhythmias. Highlighting the role of TSPO as a chief mediator of post-ischemic arrhythmias, Brown and colleagues observed similar anti-arrhythmic effects of TSPO blockade in a rabbit model of ischemia-reperfusion injury, which were not apparent in those hearts treated with the mPTP blocker, CsA (; ; ). In addition to pharmacological inhibition of TSPO, cardiac-specific knockdown of this gene also proved to be protective against reperfusion arrhythmias in spontaneously hypertensive rats (). Ongoing studies will help determine if this novel cardiotropic TSPO gene silencing approach may have a role in combatting oxidative stress-related arrhythmias in the heart.

Tspo in Diabetic Pathophysiology

The use of TSPO ligands in a variety of experimental settings has led to their translation to clinical trials for treatment of neurological and psychiatric diseases (). The utility of these ligands in metabolic diseases, however, has been the subject of very few investigations. A notable exception is an elegant study by in which treatment of zebrafish larvae with 4′-chlorodiazepam or PK11195 caused a marked decrease in systemic glucose levels, suggesting a potential role for treatment of diabetic complications. The compounds were also found to be activators of a fasting-like energy state, protecting obese mice from the undesirable effects of metabolic dysregulation (; ). Other groups studied the effects of pharmacological manipulation of TSPO on adipocyte functions. Since adipose tissue is a vital integrator of glucose homeostasis, it plays a major role in the pathophysiology of metabolic diseases including diabetes (). The Papadopoulos laboratory postulated that TSPO in adipose tissues could serve as a pharmacological target in the treatment of type-2 diabetes mellitus (). To that end, they demonstrated the efficacy of two separate TSPO ligands in improving glucose uptake and adipogenesis through TSPO activation (). These authors argued that the anti-diabetic effects of these ligands are mediated via modulation of mitochondrial function, and in particular, cholesterol transport thereby improving biogenesis of the lipid bilayer (). Of note, TSPO expression is reduced in adipocytes from obese and diabetic mice and humans compared to those from their healthy non-diabetic counterparts (; ; ). The significance of these observations was underscored by genetic knockdown studies. In particular, TSPO depletion in adipocytes led to impaired glucose uptake and adipogenesis. These findings are consistent with the notion that TSPO plays a critical role in the maintenance of normal adipocyte homeostasis ().

Recent evidence also indicates that mitochondrial cholesterol buildup may be a key step in disease progression (; ). reported that reperfusion of ischemic rat myocardium is linked with an accumulation of mitochondrial cholesterol, which in turn, causes the generation of oxysterols via oxidation of cholesterol by ROS. Interestingly, 4′-chlorodiazepam inhibited cholesterol accumulation and mitochondrial injury through oxysterol formation (). These findings revealed a novel mechanism of TSPO-related mitochondrial dysfunction that is distinct from RIRR. This alternative mechanism has been hypothesized to be of particular relevance to hypercholesterolemia, a hallmark of type-2 diabetes mellitus (). Indeed, elevated cholesterol levels are well-known risk factors for various cardiovascular diseases including thrombosis and cardiac ischemia-reperfusion injury. Moreover, there is substantial evidence for exacerbation of cardiac injury (; ; ), and defective cardioprotective pathways in hypercholesterolemic and diabetic conditions (; ; ; Wu et al., 2014). In a follow-up study, demonstrated enhanced oxysterol formation in a standard rat model of type-2 diabetes mellitus. Remarkably, 4′-chlorodiazepam inhibited cholesterol transfer into mitochondria and reduced oxysterol buildup, reinstating oxidative phosphorylation and preventing mPTP opening (). Therefore, the inhibition of cholesterol uptake by 4′-chlorodiazepam may represent a potential therapeutic strategy against ischemia-reperfusion injury in diabetes mellitus and other metabolic diseases. Preliminary work by our lab examined the role of TSPO ligands in post-ischemic arrhythmogenesis of the diabetic heart (). In a rat model of obesity and type-2 diabetes mellitus, in which we and others found that classically cardioprotective pathways targeting mitochondria are generally impaired, we verified the effectiveness of TSPO inhibition by 4′-chlorodiazepam in protection against these arrhythmias. Future studies employing genetic knockdown and over-expression strategies are needed to better understand the role of TSPO in the electrophysiology of the diabetic heart both at baseline and in response to oxidative stress.

While the focus of this article is on the role of myocyte TSPO expression in arrhythmogenesis through the regenerative process of RIRR, mechanisms by which TSPO can contribute to sudden death is likely to be multi-factorial and not merely restricted to this phenomenon. Indeed, TSPO is expressed in numerous cell types and not just myocytes. In fact there is substantial evidence of robust TSPO expression in the endothelium, vascular smooth muscle cells, adipose tissue, platelets, and macrophages (; ; ; ). Of note, because TSPO expression in non-myocyte populations (namely macrophages) increases markedly during inflammation, TSPO serves as powerful biomarker in diabetes mellitus, atherosclerosis and other inflammatory diseases in an ever-growing number of PET studies (; ; ). In addition to serving as a biomarker of inflammatory disease, TSPO actively participates in the regulation of non-myocyte cellular functions that likely influence arrhythmia vulnerability. For example, in macrophages, genetic and pharmacologic TSPO inhibition reduces cellular lipid content and prevents foam cell formation during atherogenesis (). Use of specific ligands also suggested an interesting role for TSPO in mediating white and brown adipose tissue homeostasis, pointing to its potential as a therapeutic target in the metabolic syndrome (). Interestingly, both epicardial adipose tissue and macrophages secrete inflammatory adipokines and cytokines which can induce structural and electrical remodeling of the myocardium (; ; ; ; ; ). This provides a plausible link between non-myocyte TSPO activity and arrhythmogenesis, especially in the setting of diabetes mellitus.

Conclusion and Future Directions

A growing body of evidence highlights the role of oxidative stress as a major mediator of arrhythmias in the setting of metabolic diseases such as diabetes (; ; ; Zhang et al., 2017). Glucose fluctuations in diabetic patients promote excessive production of ROS (; Wu et al., 2016), which can supersede the protective antioxidant defense systems that normally operate in healthy myocardium. This leads to oxidative stress and mitochondrial dysfunction, and is often regarded as a hallmark feature of the diabetic heart. Mitochondrial dysfunction gives rise to and exacerbates numerous cardiovascular complications including endothelial dysfunction (), atherosclerosis (), myocardial infarction (), and diabetic cardiomyopathy (), all of which can lead to sudden cardiac death (Zipes and Wellens, 1998). In addition, the pathological phenomenon of RIRR is a major mediator of oxidative stress-driven cellular electrical dysfunction and death (Zorov et al., 2000; , , ). This process generates ROS endogenously as a response to elevated ROS levels. Given its well-characterized links to ROS-releasing mitochondrial channels, TSPO has emerged as a key hub in the regulation of mitochondrial function and the cardiac response to oxidative stress. Our understanding of the role of TSPO in diabetes will expand by combining insights gained from pharmacological and genetic studies targeting this critical outer mitochondrial membrane protein in the diabetic heart.

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Author contributions

Both authors have contributed to the drafting, writing, and final editing of this article.

Funding

This work was supported by NIH grants to FGA: R01 HL137259, R21 AG054211, R21 HL114378, R01 HL113497.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

Summary

Keywords

arrhythmias, reactive oxygen species, oxidative stress, mitochondria, diabetes

Citation

Ilkan Z and Akar FG (2018) The Mitochondrial Translocator Protein and the Emerging Link Between Oxidative Stress and Arrhythmias in the Diabetic Heart. Front. Physiol. 9:1518. doi: 10.3389/fphys.2018.01518

Received

27 July 2018

Accepted

09 October 2018

Published

26 October 2018

Volume

9 - 2018

Edited by

Gaetano Santulli, Columbia University, United States

Reviewed by

Jin O-Uchi, University of Minnesota Twin Cities, United States; Crystal M. Ripplinger, University of California, Davis, United States

Updates

Copyright

*Correspondence: Fadi G. Akar,

This article was submitted to Cardiac Electrophysiology, a section of the journal Frontiers in Physiology

Disclaimer

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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