Identification of the human mitochondrial linoleoyl-coenzyme A monolysocardiolipin acyltransferase (MLCL AT-1)

Cardiolipin and its different properties in mitophagy and apoptosis

Cardiolipin (CL) is a unique dimeric phospholipid that exists almost exclusively in the inner mitochondrial membrane (IMM) in eukaryotic cells. Two chiral carbons and four fatty acyl chains in CL result in a flexible body allowing interactions with respiratory chain complexes and mitochondrial substrate carriers. Due to its high content of unsaturated fatty acids, CL is particularly prone to reactive oxygen species (ROS)-induced oxidative attacks. Under mild mitochondrial damage, CL is redistributed to the outer mitochondrial membrane (OMM) and serves as a recognition signal for dysfunctional mitochondria, which are rapidly sequestered by autophagosomes. However, peroxidation of CL is far greater in response to severe stress than under normal or mild-damage conditions. The accumulation of oxidized CL on the OMM results in recruitment of Bax and formation of the mitochondrial permeability transition pore (MPTP), which releases Cytochrome c (Cyt c) from mitochondria. Over the past decade, the significance of CL in the function of mitochondrial bioenergy has been explored. Moreover, approaches to analyzing CL have become more effective and accurate. In this review, we discuss the unique structural features of CL as well as the current understanding of CL-based molecular mechanisms of mitophagy and apoptosis.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.

ALCAT1 controls mitochondrial etiology of fatty liver diseases, linking defective mitophagy to steatosis

Defective autophagy is implicated in the pathogenesis of nonalcoholic fatty liver diseases (NAFLD) through poorly defined mechanisms. Cardiolipin is a mitochondrial phospholipid required for bioenergetics and mitophagy from yeast to mammals. Here we investigated a role for ALCAT1 in the development of NAFLD. ALCAT1 is a lysocardiolipin acyltransferase that catalyzes pathological cardiolipin remodeling in several aging-related diseases. We show that the onset of diet-induced NAFLD caused autophagic arrest in hepatocytes, leading to oxidative stress, mitochondrial dysfunction, and insulin resistance. In contrast, targeted deletion of ALCAT1 in mice prevented the onset of NAFLD. ALCAT1 deficiency also restored mitophagy, mitochondrial architecture, mitochondrial DNA (mtDNA) fidelity, and oxidative phosphorylation. In support of a causative role of the enzyme in a mitochondrial etiology of the disease, hepatic ALCAT1 expression was significantly up-regulated in mouse models of NAFLD.

CONCLUSION

Forced expression of ALCAT1 in primary hepatocytes led to multiple defects that are highly reminiscent of NAFLD, including steatosis, defective autophagy, and mitochondrial dysfunction, linking pathological cardiolipin remodeling by ALCAT1 to the pathogenesis of NAFLD.

Cardiolipin remodeling: a regulatory hub for modulating cardiolipin metabolism and function

Cardiolipin (CL), the signature phospholipid of mitochondria, is involved in a plethora of cellular processes and is crucial for mitochondrial function and architecture. The de novo synthesis of CL in the mitochondria is followed by a unique remodeling process, in which CL undergoes cycles of deacylation and reacylation. Specific fatty acyl composition is acquired during this process, and remodeled CL contains predominantly unsaturated fatty acids. The importance of CL remodeling is underscored by the life-threatening genetic disorder Barth syndrome (BTHS), caused by mutations in tafazzin, which reacylates monolysocardiolipin (MLCL) generated from the deacylation of CL. Just as CL-deficient yeast mutants have been instrumental in elucidating functions of this lipid, the recently characterized CL-phospholipase mutant cld1Δ and the tafazzin mutant taz1Δ are powerful tools to understand the functions of CL remodeling. In this review, we discuss recent advances in understanding the role of CL in mitochondria with specific focus on the enigmatic functions of CL remodeling.

DNAJC19, a mitochondrial cochaperone associated with cardiomyopathy, forms a complex with prohibitins to regulate cardiolipin remodeling

Prohibitins form large protein and lipid scaffolds in the inner membrane of mitochondria that are required for mitochondrial morphogenesis, neuronal survival, and normal lifespan. Here, we have defined the interactome of PHB2 in mitochondria and identified DNAJC19, mutated in dilated cardiomyopathy with ataxia, as binding partner of PHB complexes. We observed impaired cell growth, defective cristae morphogenesis, and similar transcriptional responses in the absence of either DNAJC19 or PHB2. The loss of PHB/DNAJC19 complexes affects cardiolipin acylation and leads to the accumulation of cardiolipin species with altered acyl chains. Similar defects occur in cells lacking the transacylase tafazzin, which is mutated in Barth syndrome. Our experiments suggest that PHB/DNAJC19 membrane domains regulate cardiolipin remodeling by tafazzin and explain similar clinical symptoms in two inherited cardiomyopathies by an impaired cardiolipin metabolism in mitochondrial membranes.

Metabolism and function of mitochondrial cardiolipin

Since it has been recognized that mitochondria are crucial not only for energy metabolism but also for other cellular functions, there has been a growing interest in cardiolipin, the specific phospholipid of mitochondrial membranes. Indeed, cardiolipin is a universal component of mitochondria in all eukaryotes. It has a unique dimeric structure comprised of two phosphatidic acid residues linked by a glycerol bridge, which gives rise to unique physicochemical properties. Cardiolipin plays an important role in the structural organization and the function of mitochondrial membranes. In this article, we review the literature on cardiolipin biology, focusing on the most important discoveries of the past decade. Specifically, we describe the formation, the migration, and the degradation of cardiolipin and we discuss how cardiolipin affects mitochondrial function. We also give an overview of the various phenotypes of cardiolipin deficiency in different organisms.

Dietary macronutrients modulate the fatty acyl composition of rat liver mitochondrial cardiolipins

The interaction of dietary fats and carbohydrates on liver mitochondria were examined in male FBNF1 rats fed 20 different low-fat isocaloric diets. Animal growth rates and mitochondrial respiratory parameters were essentially unaffected, but mass spectrometry-based mitochondrial lipidomics profiling revealed increased levels of cardiolipins (CLs), a family of phospholipids essential for mitochondrial structure and function, in rats fed saturated or trans fat-based diets with a high glycemic index. These mitochondria showed elevated monolysocardiolipins (a CL precursor/product of CL degradation), elevated ratio of trans-phosphocholine (PC) (18:1/18:1) to cis-PC (18:1/18:1) (a marker of thiyl radical stress), and decreased ubiquinone Q9; the latter two of which imply a low-grade mitochondrial redox abnormality. Extended analysis demonstrated: i) dietary fats and, to a lesser extent, carbohydrates induce changes in the relative abundance of specific CL species; ii) fatty acid (FA) incorporation into mature CLs undergoes both positive (>400-fold) and negative (2.5-fold) regulation; and iii) dietary lipid abundance and incorporation of FAs into both the CL pool and specific mature tetra-acyl CLs are inversely related, suggesting previously unobserved compensatory regulation. This study reveals previously unobserved complexity/regulation of the central lipid in mitochondrial metabolism.

Deacylation on the matrix side of the mitochondrial inner membrane regulates cardiolipin remodeling

Our understanding of the clinically relevant tafazzin-mediated cardiolipin (CL) remodeling pathway is incomplete. In this study, a new trafficking step required for CL remodeling has been identified. Further, it is demonstrated that flux through this CL remodeling pathway is controlled by the strength of the mitochondrial electrochemical gradient.

The mitochondrial-specific lipid cardiolipin (CL) is required for numerous processes therein. After its synthesis on the matrix-facing leaflet of the inner membrane (IM), CL undergoes acyl chain remodeling to achieve its final form. In yeast, this process is completed by the transacylase tafazzin, which associates with intermembrane space (IMS)-facing membrane leaflets. Mutations in TAZ1 result in the X-linked cardiomyopathy Barth syndrome. Amazingly, despite this clear pathophysiological association, the physiological importance of CL remodeling is unresolved. In this paper, we show that the lipase initiating CL remodeling, Cld1p, is associated with the matrix-facing leaflet of the mitochondrial IM. Thus monolysocardiolipin generated by Cld1p must be transported to IMS-facing membrane leaflets to gain access to tafazzin, identifying a previously unknown step required for CL remodeling. Additionally, we show that Cld1p is the major site of regulation in CL remodeling; and that, like CL biosynthesis, CL remodeling is augmented in growth conditions requiring mitochondrially produced energy. However, unlike CL biosynthesis, dissipation of the mitochondrial membrane potential stimulates CL remodeling, identifying a novel feedback mechanism linking CL remodeling to oxidative phosphorylation capacity.

Emerging roles of cardiolipin remodeling in mitochondrial dysfunction associated with diabetes, obesity, and cardiovascular diseases

Cardiolipin (CL) is a phospholipid exclusively localized in inner mitochondrial membrane where it is required for oxidative phosphorylation, ATP synthesis, and mitochondrial bioenergetics. The biological functions of CL are thought to depend on its acyl chain composition which is dominated by linoleic acids in metabolically active tissues. This unique feature is not derived from the de novo biosynthesis of CL, rather from a remodeling process that involves in phospholipases and transacylase/acyltransferase. The remodeling process is also believed to be responsible for generation of CL species that causes oxidative stress and mitochondrial dysfunction. CL is highly sensitive to oxidative damages by reactive oxygen species (ROS) due to its high content in polyunsaturated fatty acids and location near the site of ROS production. Consequently, pathological remodeling of CL has been implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, heart failure, neurodegeneration, and aging that are characterized by oxidative stress, CL deficiency, and abnormal CL species. This review summarizes recent progresses in molecular, enzymatic, lipidomic, and metabolic studies that support a critical regulatory role of pathological CL remodeling as a missing link between oxidative stress and mitochondrial dysfunction in metabolic diseases and aging.

Identification of small subunit of serine palmitoyltransferase a as a lysophosphatidylinositol acyltransferase 1-interacting protein

Lysophosphatidylinositol acyltransferase 1 (LPIAT1), also known as MBOAT7, is a phospholipid acyltransferase that selectively incorporates arachidonic acid (AA) into the sn-2 position of phosphatidylinositol (PI). We previously demonstrated that LPIAT1 regulates AA content in PI and plays a crucial role in brain development in mice. However, how LPIAT1 is regulated and which proteins function cooperatively with LPIAT1 are unknown. In this study, using a split-ubiquitin membrane yeast two-hybrid system, we identified the small subunit of serine palmitoyltransferase a (ssSPTa) as an LPIAT1-interacting protein. ssSPTa co-immunoprecipitated and colocalized with LPIAT1 in cultured mammalian cells. Knockdown of ssSPTa decreased the LPIAT1-dependent incorporation of exogenous AA into PI but did not affect the in vitro enzyme activity of LPIAT1 in the microsomal fraction. Interestingly, knockdown of ssSPTa decreased the protein level of LPIAT1 in the crude mitochondrial fraction but not in total homogenate or the microsomal fraction. LPIAT1 was localized to the mitochondria-associated membrane (MAM), where AA-selective acyl-CoA synthetase is enriched. These results suggest that ssSPTa plays a role in fatty acid remodeling of PI, probably by facilitating the MAM localization of LPIAT1.

Cardiolipin-mediated cellular signaling

This review focuses on recent studies showing that cardiolipin (CL), a unique mitochondrial phospholipid, regulates many cellular functions and signaling pathways, both inside and outside the mitochondria. Inside the mitochondria, CL is a critical target of mitochondrial generated reactive oxygen species (ROS) and regulates signaling events related to apoptosis and aging. CL deficiency causes perturbation of signaling pathways outside the mitochondria, including the PKC-Slt2 cell integrity pathway and the high osmolarity glycerol (HOG) pathway, and is a key player in the cross-talk between the mitochondria and the vacuole. Understanding these connections may shed light on the pathology of Barth syndrome, a disorder of CL remodeling.

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.

Human trifunctional protein alpha links cardiolipin remodeling to beta-oxidation

Cardiolipin (CL) is a mitochondrial membrane phospholipid which plays a key role in apoptosis and supports mitochondrial respiratory chain complexes involved in the generation of ATP. In order to facilitate its role CL must be remodeled with appropriate fatty acids. We previously identified a human monolysocardiolipin acyltransferase activity which remodels CL via acylation of monolysocardiolipin (MLCL) to CL and was identical to the alpha subunit of trifunctional protein (αTFP) lacking the first 227 amino acids. Full length αTFP is an enzyme that plays a prominent role in mitochondrial β-oxidation, and in this study we assessed the role, if any, which this metabolic enzyme plays in the remodeling of CL. Purified human recombinant αTFP exhibited acyl-CoA acyltransferase activity in the acylation of MLCL to CL with linoleoyl-CoA, oleoyl-CoA and palmitoyl-CoA as substrates. Expression of αTFP increased radioactive linoleate or oleate or palmitate incorporation into CL in HeLa cells. Expression of αTFP in Barth Syndrome lymphoblasts, which exhibit reduced tetralinoleoyl-CL, elevated linoleoyl-CoA acylation of MLCL to CL in vitro, increased mitochondrial respiratory Complex proteins and increased linoleate-containing species of CL. Knock down of αTFP in Barth Syndrome lymphoblasts resulted in greater accumulation of MLCL than those with normal αTFP levels. The results clearly indicate that the human αTFP exhibits MLCL acyltransferase activity for the resynthesis of CL from MLCL and directly links an enzyme of mitochondrial β-oxidation to CL remodeling.

Implication of liver cardiolipins in mitochondrial energy metabolism disorder in cancer cachexia

Mitochondrial membranes are essential for the good functioning of the organelle. For instance, the inner mitochondrial membrane contains the oxidative phosphorylation system that permits ATP synthesis. Phospholipids environment and especially cardiolipin are crucial for the mitochondrial energy metabolism. Indeed, cardiolipin is known to provide essential structural and functional support to several proteins involved in oxidative phosphorylation. Alterations in cardiolipin structure, content and fatty acids composition have been associated with mitochondrial dysfunction in several physiopathological conditions and diseases. Cancer cachexia is a complex and dynamic process characterized by a negative energy balance induced by anorexia and hypermetabolism which leads to a drastic loss in body weight that aggravate prognosis of cancer patients. The underlying mechanisms of hypermetabolism are not fully understood. Whether the mitochondrial energy metabolism is altered during this disease and may participate to hypermetabolism is not clear. This mini-review focuses on cardiolipin especially its biosynthesis and remodeling pathways, its relation with mitochondrial energy metabolism and its possible implication in the cancer cachexia syndrome.

Myocardial regulation of lipidomic flux by cardiolipin synthase: setting the beat for bioenergetic efficiency

Lipidomic regulation of mitochondrial cardiolipin content and molecular species composition is a prominent regulator of bioenergetic efficiency. However, the mechanisms controlling cardiolipin metabolism during health or disease progression have remained elusive. Herein, we demonstrate that cardiac myocyte-specific transgenic expression of cardiolipin synthase results in accelerated cardiolipin lipidomic flux that impacts multiple aspects of mitochondrial bioenergetics and signaling. During the postnatal period, cardiolipin synthase transgene expression results in marked changes in the temporal maturation of cardiolipin molecular species during development. In adult myocardium, cardiolipin synthase transgene expression leads to a marked increase in symmetric tetra-18:2 molecular species without a change in total cardiolipin content. Mechanistic analysis demonstrated that these alterations result from increased cardiolipin remodeling by sequential phospholipase and transacylase/acyltransferase activities in conjunction with a decrease in phosphatidylglycerol content. Moreover, cardiolipin synthase transgene expression results in alterations in signaling metabolites, including a marked increase in the cardioprotective eicosanoid 14,15-epoxyeicosatrienoic acid. Examination of mitochondrial bioenergetic function by high resolution respirometry demonstrated that cardiolipin synthase transgene expression resulted in improved mitochondrial bioenergetic efficiency as evidenced by enhanced electron transport chain coupling using multiple substrates as well as by salutary changes in Complex III and IV activities. Furthermore, transgenic expression of cardiolipin synthase attenuated maladaptive cardiolipin remodeling and bioenergetic inefficiency in myocardium rendered diabetic by streptozotocin treatment. Collectively, these results demonstrate the unanticipated role of cardiolipin synthase in maintaining physiologic membrane structure and function even under metabolic stress, thereby identifying cardiolipin synthase as a novel therapeutic target to attenuate mitochondrial dysfunction in diabetic myocardium.

Comparison of cardiolipins from Drosophila strains with mutations in putative remodeling enzymes

Cardiolipin is a dimeric phospholipid with a characteristic acyl composition that is generated by fatty acid remodeling after de novo synthesis. Several enzymes have been proposed to participate in acyl remodeling of cardiolipin. In order to compare the effect of these enzymes, we determined the pattern of cardiolipin molecular species in Drosophila strains with specific enzyme deletions, using MALDI-TOF mass spectrometry with internal standards. We established the linear range of the method for cardiolipin quantification, determined the relative signal intensities of several cardiolipin standards, and demonstrated satisfying signal-to-noise ratios in cardiolipin spectra from a single fly. Our data demonstrate changes in the cardiolipin composition during the Drosophila life cycle. Comparison of cardiolipin spectra, using vector algebra, showed that inactivation of tafazzin had a large effect on the molecular composition of cardiolipin, inactivation of calcium-independent phospholipase A(2) had a small effect, whereas inactivation of acyl-CoA:lysocardiolipin-acyltransferase and of the trifunctional enzyme did not affect the cardiolipin composition.

Mitochondria targeting of non-peroxidizable triphenylphosphonium conjugated oleic acid protects mouse embryonic cells against apoptosis: role of cardiolipin remodeling

Peroxidation of cardiolipin in mitochondria is essential for the execution of apoptosis. We suggested that integration of oleic acid into cardiolipin generates non-oxidizable cardiolipin species hence protects cells against apoptosis. We synthesized mitochondria-targeted triphenylphosphonium oleic acid ester. Using lipidomics analysis we found that pretreatment of mouse embryonic cells with triphenylphosphonium oleic acid ester resulted in decreased contents of polyunsaturated cardiolipins and elevation of its species containing oleic acid residues. This caused suppression of apoptosis induced by actinomycin D. Triacsin C, an inhibitor of acyl-CoA synthase, blocked integration of oleic acid into cardiolipin and restored cell sensitivity to apoptosis.

Delineating the role of alterations in lipid metabolism to the pathogenesis of inherited skeletal and cardiac muscle disorders: Thematic Review Series: Genetics of Human Lipid Diseases

As the specific composition of lipids is essential for the maintenance of membrane integrity, enzyme function, ion channels, and membrane receptors, an alteration in lipid composition or metabolism may be one of the crucial changes occurring during skeletal and cardiac myopathies. Although the inheritance (autosomal dominant, autosomal recessive, and X-linked traits) and underlying/defining mutations causing these myopathies are known, the contribution of lipid homeostasis in the progression of these diseases needs to be established. The purpose of this review is to present the current knowledge relating to lipid changes in inherited skeletal muscle disorders, such as Duchenne/Becker muscular dystrophy, myotonic muscular dystrophy, limb-girdle myopathic dystrophies, desminopathies, rostrocaudal muscular dystrophy, and Dunnigan-type familial lipodystrophy. The lipid modifications in familial hypertrophic and dilated cardiomyopathies, as well as Barth syndrome and several other cardiac disorders associated with abnormal lipid storage, are discussed. Information on lipid alterations occurring in these myopathies will aid in the design of improved methods of screening and therapy in children and young adults with or without a family history of genetic diseases.

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.

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.

Turnover of nonessential fatty acids in cardiolipin from the rat heart

Cardiolipin (CL) is a unique phospholipid (PL) found in the mitochondria of mammalian cells. CL remodeling is accompanied by turnover of its fatty acid acyl groups. Abnormalities in CL remodeling have been found in Barth's syndrome, diabetes, and obesity. The objective of this study was to determine nonessential fatty acid turnover in CL and phosphatidylethanolamine (PE) in the rat heart in vivo. Sprague-Dawley rats were fed either a regular chow or a high-fat diet for 15 weeks, and consumed 6% deuterium-enriched drinking water as a tracer for 14 days. CL and PE were extracted from cardiac tissue and isolated by TLC. Fatty acids from CL, PE, and plasma were analyzed by GC/MS for deuterium incorporation. Results showed oleate and vaccenate turnover were the highest in CL whereas palmitate and stearate turnover were low. Among the nonessential fatty acids in PE, turnover of stearate and vaccenate were the highest. The high turnover rate in vaccenate was unexpected, because vaccenate previously had no known metabolic or physiologic function. In conclusion, the similarly high turnover rates of both oleate and vaccenate readily suggest that remodeling is an important functional aspect of PL metabolism in CL.

Persistent pulmonary hypertension results in reduced tetralinoleoyl-cardiolipin and mitochondrial complex II + III during the development of right ventricular hypertrophy in the neonatal pig heart

Persistent pulmonary hypertension of the newborn (PPHN) results in right ventricular (RV) hypertrophy followed by right heart failure and an associated mitochondrial dysfunction. The phospholipid cardiolipin plays a key role in maintaining mitochondrial respiratory and cardiac function via modulation of the activities of enzymes involved in oxidative phosphorylation. In this study, changes in cardiolipin and cardiolipin metabolism were investigated during the development of right heart failure. Newborn piglets (<24 h old) were exposed to a hypoxic (10% O(2)) environment for 3 days, resulting in the induction of PPHN. Two sets of control piglets were used: 1) newborn or 2) exposed to a normoxic (21% O(2)) environment for 3 days. Cardiolipin biosynthetic and remodeling enzymes, mitochondrial complex II + III activity, incorporation of [1-(14)C]linoleoyl-CoA into cardiolipin precursors, and the tetralinoleoyl-cardiolipin pool size were determined in both the RV and left ventricle (LV). PPHN resulted in an increased heart-to-body weight ratio, RV-to-LV plus septum weight ratio, and expression of brain naturetic peptide in RV. In addition, PPHN reduced cardiolipin biosynthesis and remodeling in the RV and LV, which resulted in decreased tetralinoleoyl-cardiolipin levels and reduced complex II + III activity and protein levels of mitochondrial complexes II, III, and IV in the RV. This is the first study to examine the pattern of cardiolipin metabolism during the early development of both the RV and LV of the newborn piglet and to demonstrate that PPHN-induced alterations in cardiolipin biosynthetic and remodeling enzymes contribute to reduced tetralinoleoyl-cardiolipin and mitochondrial respiratory chain function during the development of RV hypertrophy. These defects in cardiolipin may play an important role in the rapid development of RV dysfunction and right heart failure in PPHN.

A mathematical model for the determination of steady-state cardiolipin remodeling mechanisms using lipidomic data

Technical advances in lipidomic analysis have generated tremendous amounts of quantitative lipid molecular species data, whose value has not been fully explored. We describe a novel computational method to infer mechanisms of de novo lipid synthesis and remodeling from lipidomic data. We focus on the mitochondrial-specific lipid cardiolipin (CL), a polyglycerol phospholipid with four acyl chains. The lengths and degree of unsaturation of these acyl chains vary across CL molecules, and regulation of these differences is important for mitochondrial energy metabolism. We developed a novel mathematical approach to determine mechanisms controlling the steady-state distribution of acyl chain combinations in CL . We analyzed mitochondrial lipids from 18 types of steady-state samples, each with at least 3 replicates, from mouse brain, heart, lung, liver, tumor cells, and tumors grown in vitro. Using a mathematical model for the CL remodeling mechanisms and a maximum likelihood approach to infer parameters, we found that for most samples the four chain positions have an independent and identical distribution, indicating they are remodeled by the same processes. Furthermore, for most brain samples and liver, the distribution of acyl chains is well-fit by a simple linear combination of the pools of acyl chains in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG). This suggests that headgroup chemistry is the key determinant of acyl donation into CL, with chain length/saturation less important. This canonical remodeling behavior appears damaged in some tumor samples, which display a consistent excess of CL molecules having particular masses. For heart and lung, the “proportional incorporation” assumption is not adequate to explain the CL distribution, suggesting additional acyl CoA-dependent remodeling that is chain-type specific. Our findings indicate that CL remodeling processes can be described by a small set of quantitative relationships, and that bioinformatic approaches can help determine these processes from high-throughput lipidomic data.

Cardiolipin remodeling by ALCAT1 links oxidative stress and mitochondrial dysfunction to obesity

Oxidative stress causes mitochondrial dysfunction and metabolic complications through unknown mechanisms. Cardiolipin (CL) is a key mitochondrial phospholipid required for oxidative phosphorylation. Oxidative damage to CL from pathological remodeling is implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, and other metabolic diseases. Here, we show that ALCAT1, a lyso-CL acyltransferase upregulated by oxidative stress and diet-induced obesity (DIO), catalyzes the synthesis of CL species that are highly sensitive to oxidative damage, leading to mitochondrial dysfunction, ROS production, and insulin resistance. These metabolic disorders were reminiscent of those observed in type 2 diabetes and were reversed by rosiglitazone treatment. Consequently, ALCAT1 deficiency prevented the onset of DIO and significantly improved mitochondrial complex I activity, lipid oxidation, and insulin signaling in ALCAT1(-/-) mice. Collectively, these findings identify a key role of ALCAT1 in regulating CL remodeling, mitochondrial dysfunction, and susceptibility to DIO.

The dynamics of cardiolipin synthesis post-mitochondrial fusion

Alteration in mitochondrial fusion may regulate mitochondrial metabolism. Since the phospholipid cardiolipin (CL) is required for function of the mitochondrial respiratory chain, we examined the dynamics of CL synthesis in growing Hela cells immediately after and 12h post-fusion. Cells were transiently transfected with Mfn-2, to promote fusion, or Mfn-2 expressing an inactive GTPase for 24h and de novo CL biosynthesis was examined immediately after or 12h post-fusion. Western blot analysis confirmed elevated Mfn-2 expression and electron microscopic analysis revealed that Hela cell mitochondrial structure was normal immediately after and 12h post-fusion. Cells expressing Mfn-2 exhibited reduced CL de novo biosynthesis from [1,3-(3)H]glycerol immediately after fusion and this was due to a decrease in phosphatidylglycerol phosphate synthase (PGPS) activity and its mRNA expression. In contrast, 12h post-mitochondrial fusion cells expressing Mfn-2 exhibited increased CL de novo biosynthesis from [1,3-(3)H]glycerol and this was due to an increase in PGPS activity and its mRNA expression. Cells expressing Mfn-2 with an inactive GTPase activity did not exhibit alterations in CL de novo biosynthesis immediately after or 12h post-fusion. The Mfn-2 mediated alterations in CL de novo biosynthesis were not accompanied by alterations in CL or monolysoCL mass. [1-(14)C]Oleate incorporation into CL was elevated at 12h post-fusion indicating increased CL resynthesis. The reason for the increased CL resynthesis was an increased mRNA expression of tafazzin, a mitochondrial CL resynthesis enzyme. Ceramide-induced expression of PGPS in Hela cells or in CHO cells did not alter expression of Mfn-2 indicating that Mfn-2 expression is independent of altered CL synthesis mediated by elevated PGPS. In addition, Mfn-2 expression was not altered in Hela cells expressing phospholipid scramblase-3 or a disrupted scramblase indicating that proper CL localization within mitochondria is not essential for Mfn-2 expression. The results suggest that immediately post-mitochondrial fusion CL de novo biosynthesis is "slowed down" and then 12h post-fusion it is "upregulated". The implications of this are discussed.

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.