Local control of mitochondrial membrane potential, permeability transition pore and reactive oxygen species by calcium and calmodulin in rat ventricular myocytes

CaMKII determines mitochondrial stress responses in heart

Myocardial cell death is initiated by excessive mitochondrial Ca²⁺ entry, causing Ca²⁺ overload, mitochondrial permeability transition pore (mPTP) opening and dissipation of the mitochondrial inner membrane potential (ΔΨm)^1,2. However, the signaling pathways that control mitochondrial Ca²⁺ entry through the inner membrane mitochondrial Ca²⁺ uniporter (MCU)^3–5 are not known. The multifunctional Ca²⁺ and calmodulin-dependent protein kinase II (CaMKII) is activated in ischemia reperfusion (I/R), myocardial infarction (MI) and neurohumoral injury, common causes of myocardial death and heart failure, suggesting CaMKII could couple disease stress to mitochondrial injury. Here we show that CaMKII promotes mPTP opening and myocardial death by increasing MCU current (I_MCU). Mitochondrial-targeted CaMKII inhibitory protein or cyclosporin A (CsA), an mPTP antagonist with clinical efficacy in I/R injury⁶, equivalently prevent mPTP opening, ΔΨm deterioration and diminish mitochondrial disruption and programmed cell death in response to I/R injury. Mice with myocardial and mitochondrial-targeted CaMKII inhibition are resistant to I/R injury, MI and neurohumoral injury, suggesting pathological actions of CaMKII are substantially mediated by increasing I_MCU. Our findings identify CaMKII activity as a central mechanism for mitochondrial Ca²⁺ entry and suggest mitochondrial-targeted CaMKII inhibition could prevent or reduce myocardial death and heart failure dysfunction in response to common experimental forms of pathophysiological stress.

Loss of DJ-1 does not affect mitochondrial respiration but increases ROS production and mitochondrial permeability transition pore opening

Background

Loss of function mutations in the DJ-1 gene have been linked to recessively inherited forms of Parkinsonism. Mitochondrial dysfunction and increased oxidative stress are thought to be key events in the pathogenesis of Parkinson’s disease. Although it has been reported that DJ-1 serves as scavenger for reactive oxidative species (ROS) by oxidation on its cysteine residues, how loss of DJ-1 affects mitochondrial function is less clear.

Methodology/Principal Findings

Using primary mouse embryonic fibroblasts (MEFs) or brains from DJ-1−/− mice, we found that loss of DJ-1 does not affect mitochondrial respiration. Specifically, endogenous respiratory activity as well as basal and maximal respiration are normal in intact DJ-1−/− MEFs, and substrate-specific state 3 and state 4 mitochondrial respiration are also unaffected in permeabilized DJ-1−/− MEFs and in isolated mitochondria from the cerebral cortex of DJ-1−/− mice at 3 months or 2 years of age. Expression levels and activities of all individual complexes composing the electron transport system are unchanged, but ATP production is reduced in DJ-1−/− MEFs. Mitochondrial transmembrane potential is decreased in the absence of DJ-1. Furthermore, mitochondrial permeability transition pore opening is increased, whereas mitochondrial calcium levels are unchanged in DJ-1−/− cells. Consistent with earlier reports, production of reactive oxygen species (ROS) is increased, though levels of antioxidative enzymes are unaltered. Interestingly, the decreased mitochondrial transmembrane potential and the increased mitochondrial permeability transition pore opening in DJ-1−/− MEFs can be restored by antioxidant treatment, whereas oxidative stress inducers have the opposite effects on mitochondrial transmembrane potential and mitochondrial permeability transition pore opening.

Conclusions/Significance

Our study shows that loss of DJ-1 does not affect mitochondrial respiration or mitochondrial calcium levels but increases ROS production, leading to elevated mitochondrial permeability transition pore opening and reduced mitochondrial transmembrane potential.

Crosstalk between mitochondrial and sarcoplasmic reticulum Ca2+ cycling modulates cardiac pacemaker cell automaticity

Background

Mitochondria dynamically buffer cytosolic Ca²⁺ in cardiac ventricular cells and this affects the Ca²⁺ load of the sarcoplasmic reticulum (SR). In sinoatrial-node cells (SANC) the SR generates periodic local, subsarcolemmal Ca²⁺ releases (LCRs) that depend upon the SR load and are involved in SANC automaticity: LCRs activate an inward Na⁺-Ca²⁺ exchange current to accelerate the diastolic depolarization, prompting the ensemble of surface membrane ion channels to generate the next action potential (AP).

Objective

To determine if mitochondrial Ca²⁺ (Ca²⁺_m), cytosolic Ca²⁺ (Ca²⁺_c)-SR-Ca²⁺ crosstalk occurs in single rabbit SANC, and how this may relate to SANC normal automaticity.

Results

Inhibition of mitochondrial Ca²⁺ influx into (Ru360) or Ca²⁺ efflux from (CGP-37157) decreased [Ca²⁺]_m to 80±8% control or increased [Ca²⁺]_m to 119±7% control, respectively. Concurrent with inhibition of mitochondrial Ca²⁺ influx or efflux, the SR Ca²⁺ load, and LCR size, duration, amplitude and period (imaged via confocal linescan) significantly increased or decreased, respectively. Changes in total ensemble LCR Ca²⁺ signal were highly correlated with the change in the SR Ca²⁺ load (r² = 0.97). Changes in the spontaneous AP cycle length (Ru360, 111±1% control; CGP-37157, 89±2% control) in response to changes in [Ca²⁺]_m were predicted by concurrent changes in LCR period (r² = 0.84).

Conclusion

A change in SANC Ca²⁺_m flux translates into a change in the AP firing rate by effecting changes in Ca²⁺_c and SR Ca²⁺ loading, which affects the characteristics of spontaneous SR Ca²⁺ release.

Regulation of mitochondrial permeability transition pore by PINK1

Background

Loss-of-function mutations in PTEN-induced kinase 1 (PINK1) have been linked to familial Parkinson’s disease, but the underlying pathogenic mechanism remains unclear. We previously reported that loss of PINK1 impairs mitochondrial respiratory activity in mouse brains.

Results

In this study, we investigate how loss of PINK1 impairs mitochondrial respiration using cultured primary fibroblasts and neurons. We found that intact mitochondria in PINK1−/− cells recapitulate the respiratory defect in isolated mitochondria from PINK1−/− mouse brains, suggesting that these PINK1−/− cells are a valid experimental system to study the underlying mechanisms. Enzymatic activities of the electron transport system complexes are normal in PINK1−/− cells, but mitochondrial transmembrane potential is reduced. Interestingly, the opening of the mitochondrial permeability transition pore (mPTP) is increased in PINK1−/− cells, and this genotypic difference between PINK1−/− and control cells is eliminated by agonists or inhibitors of the mPTP. Furthermore, inhibition of mPTP opening rescues the defects in transmembrane potential and respiration in PINK1−/− cells. Consistent with our earlier findings in mouse brains, mitochondrial morphology is similar between PINK1−/− and wild-type cells, indicating that the observed mitochondrial functional defects are not due to morphological changes. Following FCCP treatment, calcium increases in the cytosol are higher in PINK1−/− compared to wild-type cells, suggesting that intra-mitochondrial calcium concentration is higher in the absence of PINK1.

Conclusions

Our findings show that loss of PINK1 causes selective increases in mPTP opening and mitochondrial calcium, and that the excessive mPTP opening may underlie the mitochondrial functional defects observed in PINK1−/− cells.

Simvastatin impairs ADP-stimulated respiration and increases mitochondrial oxidative stress in primary human skeletal myotubes

Statins, the widely prescribed cholesterol-lowering drugs for the treatment of cardiovascular disease, cause adverse skeletal muscle side effects ranging from fatigue to fatal rhabdomyolysis. The purpose of this study was to determine the effects of simvastatin on mitochondrial respiration, oxidative stress, and cell death in differentiated primary human skeletal muscle cells (i.e., myotubes). Simvastatin induced a dose-dependent decrease in viability of proliferating and differentiating primary human muscle precursor cells, and a similar dose-dependent effect was noted in differentiated myoblasts and myotubes. Additionally, there were decreases in myotube number and size following 48 h of simvastatin treatment (5 μM). In permeabilized myotubes, maximal ADP-stimulated oxygen consumption, supported by palmitoylcarnitine+malate (PCM, complex I and II substrates) and glutamate+malate (GM, complex I substrates), was 32-37% lower (P<0.05) in simvastatin-treated (5 μM) vs control myotubes, providing evidence of impaired respiration at complex I. Mitochondrial superoxide and hydrogen peroxide generation were significantly greater in the simvastatin-treated human skeletal myotube cultures compared to control. In addition, simvastatin markedly increased protein levels of Bax (proapoptotic, +53%) and Bcl-2 (antiapoptotic, +100%, P<0.05), mitochondrial PTP opening (+44%, P<0.05), and TUNEL-positive nuclei in human skeletal myotubes, demonstrating up-regulation of mitochondrial-mediated myonuclear apoptotic mechanisms. These data demonstrate that simvastatin induces myotube atrophy and cell loss associated with impaired ADP-stimulated maximal mitochondrial respiratory capacity, mitochondrial oxidative stress, and apoptosis in primary human skeletal myotubes, suggesting that mitochondrial dysfunction may underlie human statin-induced myopathy.

Ca2+ dynamics in the mitochondria - state of the art

The importance of [Ca2+] in the mitochondrial matrix, [Ca2+]mito, had been proposed by early work of Carafoli and others [1 ], [2 ] and [3 ]. The key suggestion in the 1970s [4 ] was that regulatory [Ca2+]mito played a role in controlling the rate of activation of tricarboxylic acid cycle dehydrogenases, important in the regulation of ATP production by the electron transport chain (ETC) during oxidative phosphorylation. This view is now established [5 ] and [6 ] and the key questions currently debated are to what extent do the mitochondria acquire and release Ca2+, and what impact do mitochondria have on the dynamic Ca2+ signal in the cardiac ventricular myocyte [7 ]. Although investigations of Ca2+ dynamics in mitochondria have been problematic, disparate and inconclusive, they have also been both provocative and exciting. A recent special issue of this journal presented contrasting perspectives on the speed, extent and mechanisms of changes in [Ca2+]mito, and how these changes may influence cellular spatio-temporal [Ca2+]i dynamics [8 ]. An audio discussion is also available online [9 ]. The uncertain nature of the signaling pathways is noted in Table 1 (see below) which shows mitochondrial proteins and processes that are of current focus and which remain contentious. Each of the “items” listed is largely unsettled, or is a “work in progress”. There may be advocates for opposing positions noted or recent discoveries that must still be tested at multiple levels by diverse laboratories. Currently, the first item, the mitochondrial sodium/calcium exchanger (NCLX) [10 ], appears the most solid with respect to the molecular identification and physiological function, whereas, the recently described candidates of the mitochondrial Ca2+ uniporter (MCU) [11 ] and [12 ] still need to be verified and broadly examined by the scientific community.

Mitochondrial dysfunction in long-term neuronal cultures mimics changes with aging

Background

Aging is a highly complex process that affects various tissues and systems in the body. Senescent changes are relatively more prevalent and severe in the postmitotic cells. Mitochondria play an important role in the aging process. Recently, cell cultures have been widely used as an in vitro model to study aging. The present study was designed to investigate mitochondrial dysfunction associated with aging in a long-term cell culture system.

Material/Methods

Rat hippocampal neurons were maintained in culture in serum-free medium for 30 days in vitro (DIV). The morphology and development of hippocampal neurons was observed by phase contrast microscope. The levels of cellular senescence were evaluated by cytochemical staining of senescence-associated β-galactosidase (SA-β-Gal) at DIV 5, 10, 15, 20, 25 and 30. In addition, we investigated the changes in mitochondrial membrane potential (Δψm) and intracellular reactive oxygen species (ROS) generation of hippocampal neurons by flow cytometry at different ages.

Results

The proportion of the senescent cells steadily increased with age in neuron cultures. Δψm decreased gradually with age in long-term culture, while ROS generation increased.

Conclusions

This study indicates an age-related decrease in mitochondrial function in long-term hippocampal neuronal culture and suggests that DIV 25 neurons could possibly serve as a platform for the future study of anti-aging from the perspective of mitochondrial function.

Mitochondria-targeted cardioprotection in aldosteronism

Chronic aldosterone/salt treatment (ALDOST) is accompanied by an adverse structural remodeling of myocardium that includes multiple foci of microscopic scarring representing morphologic footprints of cardiomyocyte necrosis. Our previous studies suggested that signal-transducer-effector pathway leading to necrotic cell death during ALDOST includes intramitochondrial Ca overloading, together with an induction of oxidative stress and opening of the mitochondrial permeability transition pore (mPTP). To further validate this concept, we hypothesized that mitochondria-targeted interventions will prove to be cardioprotective. Accordingly, 8-week-old male Sprague-Dawley rats receiving 4 weeks ALDOST were cotreated with either quercetin, a flavonoid with mitochondrial antioxidant properties, or cyclosporine A (CsA), an mPTP inhibitor, and compared with ALDOST alone or untreated, age/sex-matched controls. We monitored mitochondrial free Ca and biomarkers of oxidative stress, including 8-isoprostane and H2O2 production; mPTP opening; total Ca in cardiac tissue; and collagen volume fraction to quantify replacement fibrosis, a biomarker of cardiomyocyte necrosis, and employed terminal deoxynucleotidyl transferase dUTP nick end labeling assay to address apoptosis in coronal sections of ventricular myocardium. Compared with controls, at 4 weeks ALDOST we found a marked increase in mitochondrial H2O2 production and 8-isoprostane levels, an increased propensity for mPTP opening, and greater concentrations of mitochondrial free [Ca]m and total tissue Ca, coupled with a 5-fold rise in collagen volume fraction without any terminal deoxynucleotidyl transferase dUTP nick end labeling-based evidence of cardiomyocyte apoptosis. Each of these pathophysiologic responses to ALDOST was prevented by quercetin or cyclosporine A cotreatment. Thus, mitochondria play a central role in initiating the cellular-subcellular mechanisms that lead to necrotic cell death and myocardial scarring. This destructive cycle can be interrupted and myocardium salvaged with its structure preserved by mitochondria-targeted cardioprotective strategies.

Two close, too close: sarcoplasmic reticulum-mitochondrial crosstalk and cardiomyocyte fate

Mitochondria are key organelles in cell life whose dysfunction is associated with a variety of diseases. Their crucial role in intermediary metabolism and energy conversion makes them a preferred target in tissues, such as the heart, where the energetic demands are very high. In the cardiomyocyte, the spatial organization of mitochondria favors their interaction with the sarcoplasmic reticulum, thereby offering a mechanism for Ca(2+)-mediated crosstalk between these 2 organelles. Recently, the molecular basis for this interaction has begun to be unraveled, and we are learning how endoplasmic reticulum-mitochondrial interactions are often exploited by death signals, such as proapoptotic Bcl-2 family members, to amplify the cell death cascade. Here, we review our present understanding of the structural basis and the functional consequences of the close interaction between sarcoplasmic reticulum and mitochondria on cardiomyocyte function and death.

Mechanisms of non-apoptotic programmed cell death in diabetes and heart failure

Programmed cell elimination is an important pathological mediator of disease. Multiple pathways to programmed cell death have been delineated, including apoptosis, autophagy and programmed necrosis. Cross-talk between the signaling pathways mediating each process has made it difficult to define specific mechanisms of in vivo programmed cell death. For this reason, many "apoptotic" diseases may involve other death signaling pathways. Recent advances in genetic complementation using mouse knock-out models are helping to dissect apoptotic and necrotic cell death in different pathological states. The current state of research in this area is reviewed, focusing upon new findings describing the role of programmed necrosis induced by the mitochondrial permeability transition in mouse models of heart failure and diabetes.

Oxidative stress caused by mitochondrial calcium overload

Mitochondrial oxidative stress has been reported as the result of respiratory complex anomalies, genetic defects, or insufficient oxygen or glucose supply. Although Ca(2+) has no direct effect on respiratory chain function or oxidation/reduction process, mitochondrial Ca(2+) overload can lead to reactive oxygen species (ROS) increase. Even though Ca(2+) is well known for its role as crucial second messenger in modulating many cellular physiological functions, Ca(2+) overload is detrimental to mitochondrial function and may present as an important cause of mitochondrial ROS generation. Possible mechanisms include Ca(2+) stimulated increase of metabolic rate, Ca(2+) stimulated nitric oxide production, Ca(2+) induced cytochrome c dissociation, Ca(2+) induced cardiolipin peroxidation, Ca(2+) induced mitochondrial permeability transition pore opening with release of cytochrome c and GSH-antioxidative enzymes, and Ca(2+)-calmodulin dependent protein kinases activation. Different mechanisms may exist under different mitochondrial preparations (isolated mitochondria vs. mitochondria in intact cells), tissue sources, animal species, or inhibitors used. Furthermore, mitochondrial ROS rise can modulate Ca(2+) dynamics and augment Ca(2+) surge. The reciprocal interactions between Ca(2+) induced ROS increase and ROS modulated Ca(2+) upsurge may cause a feedforward, self-amplified loop createing cellular damage far beyond direct Ca(2+) induced damage.

Melatonin signaling and cell protection function

Besides its well-known regulatory role on circadian rhythm, the pineal gland hormone melatonin has other biological functions and a distinct metabolism in various cell types and peripheral tissues. In different tissues and organs, melatonin has been described to act as a paracrine and also as an intracrine and autocrine agent with overall homeostatic functions and pleiotropic effects that include cell protection and prosurvival factor. These latter effects, documented in a number of in vitro and in vivo studies, are sustained through both receptor-dependent and -independent mechanisms that control detoxification and stress response genes, thus conferring protection against a number of xenobiotics and endobiotics produced by acute and chronic noxious stimuli. Redox-sensitive components are included in the cell protection signaling of melatonin and in the resulting transcriptional response that involves the control of NF-κB, AP-1, and Nrf2. By these pathways, melatonin stimulates the expression of antioxidant and detoxification genes, acting in turn as a glutathione system enhancer. A further and converging mechanism of cell protection by this indoleamine described in different models seems to lie in the control of damage and signaling function of mitochondria that involves decreased production of reactive oxygen species and activation of the antiapoptotic and redox-sensitive element Bcl2. Recent evidence suggests that upstream components in this mitochondrial route include the calmodulin pathway with its central role in melatonin signaling and the survival-promoting component of MAPKs, ERK1/2. In this review article, we will discuss these and other molecular aspects of melatonin signaling relevant to cell protection and survival mechanisms.

Tumor necrosis factor-mediated cell death: to break or to burst, that's the question

In this review, we discuss the signal-transduction pathways of three major cellular responses induced by tumor necrosis factor (TNF): cell survival through NF-kappaB activation, apoptosis, and necrosis. Recruitment and activation of caspases plays a crucial role in the initiation and execution of TNF-induced apoptosis. However, experimental inhibition of caspases reveals an alternative cell death pathway, namely necrosis, also called necroptosis, suggesting that caspases actively suppress the latter outcome. TNF-induced necrotic cell death crucially depends on the kinase activity of receptor interacting protein serine-threonine kinase 1 (RIP1) and RIP3. It was recently demonstrated that ubiquitination of RIP1 determines whether it will function as a pro-survival or pro-cell death molecule. Deeper insight into the mechanisms that control the molecular switches between cell survival and cell death will help us to understand why TNF can exert so many different biological functions in the etiology and pathogenesis of human diseases.