Functional role of cardiolipin in mitochondrial bioenergetics

Cardiolipin is a unique phospholipid which is almost exclusively located in the inner mitochondrial membrane where it is biosynthesized. Considerable progress has recently been made in understanding the role of cardiolipin in mitochondrial function and bioenergetics. This phospholipid is associated with membranes designed to generate an electrochemical gradient that is used to produce ATP, such as bacterial plasma membranes and inner mitochondrial membrane. This ubiquitous and intimate association between cardiolipin and energy transducing membranes indicates an important role for cardiolipin in mitochondrial bioenergetic processes. Cardiolipin has been shown to interact with a number of proteins, including the respiratory chain complexes and substrate carrier proteins. Over the past decade, the significance of cardiolipin in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Moreover, cardiolipin is involved in different stages of the mitochondrial apoptotic process, as well as in mitochondrial membrane stability and dynamics. This review discusses the current understanding of the functional role that cardiolipin plays in several reactions and processes involved in mitochondrial bioenergetics. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.     

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.     

Role of cardiolipin peroxidation and Ca in mitochondrial dysfunction and disease

Cardiolipin is a unique phospholipid which is almost exclusively located at the level of the inner mitochondrial membrane where it is biosynthesized. This phospholipid is known to be intimately involved in several mitochondrial bioenergetic processes. In addition, cardiolipin also has active roles in several of the mitochondrial-dependent steps of apoptosis and in mitochondrial membrane dynamics. Alterations in cardiolipin structure, content and acyl chains composition have been associated with mitochondrial dysfunction in multiple tissues in several physiopathological conditions, including ischemia/reperfusion, different thyroid states, diabetes, aging and heart failure. Cardiolipin is particularly susceptible to ROS attack due to its high content of unsaturated fatty acids. Oxidative damage to cardiolipin would negatively impact the biochemical function of the mitochondrial membranes altering membrane fluidity, ion permeability, structure and function of components of the mitochondrial electron transport chain, resulting in reduced mitochondrial oxidative phosphorylation efficiency and apoptosis. Diseases in which mitochondrial dysfunction has been linked to cardiolipin peroxidation are described. Ca²⁺, particularly at high concentrations, appears to have several negative effects on mitochondrial function, some of these effects being linked to CL peroxidation. Cardiolipin peroxidation has been shown to participate, together with Ca²⁺, in mitochondrial permeability transition. In this review, we provide an overview of the role of CL peroxidation and Ca²⁺ in mitochondrial dysfunction and disease.     

Cardiolipin remodeling in Barth syndrome and other hereditary cardiomyopathies

Mitochondria play a prominent role in cardiac energy metabolism, and their function is critically dependent on the integrity of mitochondrial membranes. Disorders characterized by mitochondrial dysfunction are commonly associated with cardiac disease. The mitochondrial phospholipid cardiolipin directly interacts with a number of essential protein complexes in the mitochondrial membranes including the respiratory chain, mitochondrial metabolite carriers, and proteins critical for mitochondrial morphology. Barth syndrome is an X-linked disorder caused by an inherited defect in the biogenesis of the mitochondrial phospholipid cardiolipin. How cardiolipin deficiency impacts on mitochondrial function and how mitochondrial dysfunction causes cardiomyopathy has been intensively studied in cellular and animal models of Barth syndrome. These findings may also have implications for the molecular mechanisms underlying other inherited disorders associated with defects in cardiolipin, such as Sengers syndrome and dilated cardiomyopathy with ataxia (DCMA).     

Cardiac mitochondrial energy metabolism in heart failure: Role of cardiolipin and sirtuins

Mitochondrial oxidation of fatty acids accounts for the majority of cardiac ATP production in the heart. Fatty acid utilization by cardiac mitochondria is controlled at the level of fatty acid uptake, lipid synthesis, mobilization and mitochondrial import and oxidation. Consequently defective mitochondrial function appears to be central to the development of heart failure. Cardiolipin is a key mitochondrial phospholipid required for the activity of the electron transport chain. In heart failure, loss of cardiolipin and tetralinoleoylcardiolipin helps to fuel the generation of excessive reactive oxygen species that are a by-product of inefficient mitochondrial electron transport chain complexes I and III. In this vicious cycle, reactive oxygen species generate lipid peroxides and may, in turn, cause oxidation of cardiolipin catalyzed by cytochrome c leading to cardiomyocyte apoptosis. Hence, preservation of cardiolipin and mitochondrial function may be keys to the prevention of heart failure development. In this review, we summarize cardiac energy metabolism and the important role that fatty acid uptake and metabolism play in this process and how defects in these result in heart failure. We highlight the key role that cardiolipin and sirtuins play in cardiac mitochondrial β-oxidation. In addition, we review the potential of pharmacological modulation of cardiolipin through the polyphenolic molecule resveratrol as a sirtuin-activator in attenuating mitochondrial dysfunction. Finally, we provide novel experimental evidence that resveratrol treatment increases cardiolipin in isolated H9c2 cardiac myocytes and tetralinoleoylcardiolipin in the heart of the spontaneously hypertensive rat and hypothesize that this leads to improvement in mitochondrial function. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.     

Cardiolipin remodeling and the function of tafazzin

Cardiolipin, the specific phospholipid of mitochondria, is involved in the biogenesis, the dynamics, and the supramolecular organization of mitochondrial membranes. Cardiolipin acquires a characteristic composition of fatty acids by post-synthetic remodeling, a process that is crucial for cardiolipin homeostasis and function. The remodeling of cardiolipin depends on the activity of tafazzin, a non-specific phospholipid–lysophospholipid transacylase. This review article discusses recent findings that suggest a novel function of tafazzin in mitochondrial membranes. By shuffling fatty acids between molecular species, tafazzin transforms the lipid composition and by doing so supports changes in the membrane conformation, specifically the generation of membrane curvature. Tafazzin activity is critical for the differentiation of cardiomyocytes, in which the characteristic cristae-rich morphology of cardiac mitochondria evolves. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.     

CARDIOLIPIN CONTENT OF WILD TYPE AND MUTANT YEASTS IN RELATION TO MITOCHONDRIAL FUNCTION AND DEVELOPMENT

The phospholipid composition of various strains of the yeast, Saccharomyces cerevisiae, and several of their derived mitochondrial mutants grown under conditions designed to induce variations in the complement of mitochondrial membranes has been examined. Wild type and petite (cytoplasmic respiratory deficient) yeasts were fractionated into various subcellular fractions, which were monitored by electron microscopy and analyzed for cytochrome oxidase (in wild type) and phospholipid composition. 90% or more of the phospholipid, cardiolipin was found in the mitochondrial membranes of wild type and petite yeast. Cardiolipin content differed markedly under various growth conditions. Stationary yeast grown in glucose had better developed mitochondria and more cardiolipin than repressed log phase yeast. Aerobic yeast contained more cardiolipin than anaerobic yeast. Respiration-deficient cytoplasmic mitochondrial mutants, both suppressive and neutral, contained less cardiolipin than corresponding wild types. A chromosomal mutant lacking respiratory function had normal cardiolipin content. Log phase cells grown in galactose and lactate, which do not readily repress the development of mitochondrial membranes, contained as much cardiolipin as stationary phase cells grown in glucose. Cytoplasmic mitochondrial mutants respond to changes in the glucose concentration of the growth medium by variations in their cardiolipin content in the same way as wild type yeast does under similar growth conditions. It is concluded that cardiolipin content of yeast is correlated with, and is a good indicator of, the state of development of mitochondrial membrane.     

Formation of molecular species of mitochondrial cardiolipin: 2. A mathematical model of pattern formation by phospholipid transacylation

Formation of the unique molecular species of mitochondrial cardiolipin requires tafazzin, a transacylase that exchanges acyl groups between phospholipid molecular species without strict specificity for acyl groups, head groups, or carbon positions. However, it is not known whether phospholipid transacylations can cause the accumulation of specific fatty acids in cardiolipin. Here, a model is shown in linear algebra representation, in which acyl specificity emerges from the transacylation equilibrium of multiple molecular species, provided that different species have different free energies. The model defines the conditions and energy terms, under which transacylations may generate the characteristic composition of mitochondrial cardiolipin. It is concluded that acyl-specific cardiolipin patterns could arise from phospholipid transacylations in the tafazzin domain, even if tafazzin itself does not have substrate specificity.     

Cardiolipin in Central Nervous System Physiology and Pathology

Cardiolipin, an anionic phospholipid found primarily in the inner mitochondrial membrane, has many well-defined roles within the peripheral tissues, including the maintenance of mitochondrial membrane fluidity and the regulation of mitochondrial functions. Within the central nervous system (CNS), cardiolipin is found within both neuronal and non-neuronal glial cells, where it regulates metabolic processes, supports mitochondrial functions, and promotes brain cell viability. Furthermore, cardiolipin has been shown to act as an elimination signal and participate in programmed cell death by apoptosis of both neurons and glia. Since cardiolipin is associated with regulating brain homeostasis, the modification of its structure, or even a decrease in the overall levels of cardiolipin, can result in mitochondrial dysfunction, which is a characteristic feature of many diseases. In this review, we outline the various functions of cardiolipin within the cells of the CNS, including neurons, astrocytes, microglia, and oligodendrocytes. In addition, we discuss the role cardiolipin may play in the pathogenesis of the neurodegenerative disorders Alzheimer’s disease and Parkinson’s disease, as well as traumatic brain injury.     

Cell biology of cardiac mitochondrial phospholipids.

Phospholipids are important structural and functional components of all biological membranes and define the compartmentation of organelles. Mitochondrial phospholipids comprise a significant proportion of the entire phospholipid content of most eukaroytic cells. In the heart, a tissue rich in mitochondria, the mitochondrial phospholipids provide for diverse roles in the regulation of various mitochondrial processes including apoptosis, electron transport, and mitochondrial lipid and protein import. It is well documented that alteration in the content and fatty acid composition of phospholipids within the heart is linked to alterations in myocardial electrical activity. In addition, reduction in the specific mitochondrial phospholipid cardiolipin is an underlying biochemical cause of Barth Syndrome, a rare and often fatal X-linked genetic disease that is associated with cardiomyopathy. Thus, maintenance of both the content and molecular composition of phospholipids synthesized within the mitochondria is essential for normal cardiac function. This review will focus on the function and regulation of the biosynthesis and resynthesis of mitochondrial phospholipids in the mammalian heart.     

Barth syndrome, a human disorder of cardiolipin metabolism

Barth syndrome is an X-linked recessive disease caused by mutations in the tafazzin gene. Patients have reduced concentration and altered composition of cardiolipin, the specific mitochondrial phospholipid, and they have variable clinical findings, often including heart failure, myopathy, neutropenia, and growth retardation. This article provides an overview of the molecular basis of Barth syndrome. It is argued that tafazzin, a phospholipid acyltransferase, is involved in acyl-specific remodeling of cardiolipin, which promotes structural uniformity and molecular symmetry among the cardiolipin molecular species. Inhibition of this pathway leads to changes in mitochondrial architecture and function.     

The effect of aging and acetyl-L-carnitine on the activity of the phosphate carrier and on the phospholipid composition in rat heart mitochondria.

The effect of aging and treatment with acetyl-L-carnitine on the activity of the phosphate carrier and on the phospholipid composition in rat heart mitochondria was studied. It was found that the activity of the phosphate carrier was reduced by aging. Treatment of aged rats with acetyl-L-carnitine reversed this effect. The mitochondrial level of cardiolipin was decreased with aging. Treatment of aged rats with acetyl-L-carnitine restored the level of cardiolipin to that of young rats. It is proposed that acetyl-L-carnitine may restore the correct phospholipid composition (cardiolipin level) of the mitochondrial membrane, altered by aging, thereby restoring the activity of the phosphate carrier.     

In yeast,cardiolipin unsaturation level plays a key role in mitochondrial function and inner membrane integrity

Cardiolipin is the signature phospholipid of the mitochondrial inner membrane. It participates in shaping the inner membrane as well as in modulating the activity of many membrane-bound proteins. The acyl chain composition of cardiolipin is finely tuned post-biosynthesis depending on the surrounding phospholipids to produce mature or unsaturated cardiolipin. However, experimental evidence showing that immature and mature cardiolipin are functionally equivalents for mitochondria poses doubts on the relevance of cardiolipin remodeling. In this work, we studied the role of cardiolipin acyl chain composition in mitochondrial bioenergetics, including a detailed bioenergetic profile of yeast mitochondria. Cardiolipin acyl chains were modified by genetic and nutritional manipulation. We found that both the bioenergetic efficiency and osmotic stability of mitochondria are dependent on the unsaturation level of cardiolipin acyl chains. It is proposed that cardiolipin remodeling and, consequently, mature cardiolipins play an important role in mitochondrial inner membrane integrity and functionality.     

Dynamic simulation of cardiolipin remodeling: greasing the wheels for an interpretative approach to lipidomics

Cardiolipin is a class of mitochondrial specific phospholipid, which is intricately involved in mitochondrial functionality. Differences in cardiolipin species exist in a variety of tissues and diseases. It has been demonstrated that the cardiolipin profile is a key modulator of the functions of many mitochondrial proteins. However, the chemical mechanism(s) leading to normal and/or pathological distribution of cardiolipin species remain elusive. Herein, we describe a novel approach for investigating the molecular mechanism of cardiolipin remodeling through a dynamic simulation. This approach applied data from shotgun lipidomic analyses of the heart, liver, brain, and lung mitochondrial lipidomes to model cardiolipin remodeling, including relative content, regiospecificity, and isomeric composition of cardiolipin species. Generated cardiolipin profiles were nearly identical to those determined by shotgun lipidomics. Importantly, the simulated isomeric compositions of cardiolipin species were further substantiated through product ion analysis. Finally, unique enzymatic activities involved in cardiolipin remodeling were assessed from the parameters used in the dynamic simulation of cardiolipin profiles. Collectively, we described, verified, and demonstrated a novel approach by integrating both lipidomic analysis and dynamic simulation to study cardiolipin biology. We believe this study provides a foundation to investigate cardiolipin metabolism and bioenergetic homeostasis in normal and disease states.     

Binding of cardiolipin/lecithin and monoamine oxidase to lipid-depleted mitochondrial membrane structure.

Lipid-depleted pig liver mitochondrial residues were incubated with different proportions of the acidic phospholipid cardiolipin and the zwitterionic phospholipid lecithin in either separate or mixed liposomes. When cardiolipin and lecithin were present in separate liposomes all of the cardiolipin but no lecithin bound to the residues. When present in the same liposomes, cardiolipin also caused binding of lecithin to the mitochondrial residues. When monoamine oxidase solubilized from pig liver mitochondria by extraction of the phospholipids was included in the incubation, binding of the enzyme to the residues occurred in the presence of cardiolipin. The percentage of enzyme bound followed the same trend as the binding of phospholipids to the mitochondrial residues.     

Mitochondrial dysfunction in metabolic and cardiovascular diseases associated with cardiolipin remodeling

Cardiolipin (CL) is an acidic phospholipid almost exclusively found in the inner mitochondrial membrane, that not only stabilizes the structure and function of individual components of the mitochondrial electron transport chain, but regulates relevant mitochondrial processes, like mitochondrial dynamics and cristae structure maintenance among others. Alterations in CL due to peroxidation, correlates with loss of such mitochondrial activities and disease progression. In this review it is recapitulated the current state of knowledge of the role of cardiolipin remodeling associated with mitochondrial dysfunction in metabolic and cardiovascular diseases.     

心磷脂重构与心血管疾病

心磷脂（cardiolipin, CL）是线粒体内膜的特征性磷脂,参与线粒体嵴的形成。心磷脂在线粒体内的合成伴随着特殊的分子重构过程,从而使其自身的4条酰基链形成特定的组成,以发挥其特殊的生理功能。研究发现,心磷脂重构对维持线粒体的形态及功能至关重要,其重构异常是大多数心血管疾病（cardiovascular disease, CVD）共有的病理现象,相应的分子机制研究得到了广泛关注。本文主要对心磷脂的理化特性及其生物合成途径,以及心磷脂重构在巴氏综合征（Barth syndrome, BTHS）、糖尿病心肌病（diabetic cardiomyopathy, DCM）以及心力衰竭（heart failure, HF）等心血管疾病的病理生理过程研究中的进展进行综述,以期为与心磷脂重构相关的心血管疾病的病理生理基础研究和药物干预的分子机制研究提供参考。     

Mitochondrial dysfunction in rat brain with aging Involvement of complex I, reactive oxygen species and cardiolipin

Reactive oxygen species (ROS) are considered a key factor in brain aging process. Mitochondrial respiration is an important site of ROS production and hence a potential contributor to brain functional changes with aging. In this study we examined the effect of aging on complex I activity, oxygen consumption, ROS production and phospholipid composition in rat brain mitochondria. The activity of complex I was reduced by 30% in brain mitochondria from 24 months aged rats relative to young animals. These changes in complex I activity were associated with parallel changes in state 3 respiration. H(2)O(2) generation was significantly increased in mitochondria isolated from aged rats. The mitochondrial content of cardiolipin, a phospholipid required for optimal activity of complex I, decreased by 31% as function of aging, while there was a significant increase in the level of peroxidized cardiolipin. The age-related decrease in complex I activity in brain mitochondria could be reversed by exogenously added cardiolipin. This effect of cardiolipin could not be replaced by other phospholipids. It is proposed that aging causes brain mitochondrial complex I dysfunction which can be attributed to ROS-induced cardiolipin oxidation. These findings may prove useful in elucidating the mechanism underlying mitochondrial dysfunction associated with brain aging.     

Known unknowns of cardiolipin signaling: The best is yet to come

Since its discovery 75 years ago, a wealth of knowledge has accumulated on the role of cardiolipin, the hallmark phospholipid of mitochondria, in bioenergetics and particularly on the structural organization of the inner mitochondrial membrane. A surge of interest in this anionic doubly-charged tetra-acylated lipid found in both prokaryotes and mitochondria has emerged based on its newly discovered signaling functions. Cardiolipin displays organ, tissue, cellular and transmembrane distribution asymmetries. A collapse of the membrane asymmetry represents a pro-mitophageal mechanism whereby externalized cardiolipin acts as an “eat-me” signal. Oxidation of cardiolipin's polyunsaturated acyl chains - catalyzed by cardiolipin complexes with cytochrome c. - is a pro-apoptotic signal. The messaging functions of myriads of cardiolipin species and their oxidation products are now being recognized as important intracellular and extracellular signals for innate and adaptive immune systems. This newly developing field of research exploring cardiolipin signaling is the main subject of this review. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.     

PTPMT1: connecting cardiolipin biosynthesis to mitochondrial function

Cardiolipin, a phospholipid component of the inner mitochondrial membrane, is required for mitochondrial metabolism. In this issue, Zhang et?al. (2011) highlight a critical role for PTPMT1, a mitochondrial phosphatase, in cardiolipin biogenesis and possibly in cardiolipin deficiency diseases. Their findings also unveil a yet uncharacterized pathway affecting cell growth.