Dramatic Accumulation of Triglycerides and Precipitation of Cardiac Hemodynamic Dysfunction during Brief Caloric Restriction in Transgenic Myocardium Expressing Human Calcium-independent Phospholipase A2γ

Steatosis in metabolic diseases: A focus on lipolysis and lipophagy

Fatty acids (FAs), as part of lipids, are involved in cell membrane composition, cellular energy storage, and cell signaling. FAs can also be toxic when their concentrations inside and/or outside the cell exceed physiological levels, which is called "lipotoxicity", and steatosis is a form of lipotoxity. To facilitate the storage of large quantities of FAs in cells, they undergo a process called lipolysis or lipophagy. This review focuses on the effects of lipolytic enzymes including cytoplasmic "neutral" lipolysis, lysosomal "acid" lipolysis, and lipophagy. Moreover, the impact of related lipolytic enzymes on lipid metabolism homeostasis and energy conservation, as well as their role in lipid-related metabolic diseases. In addition, we describe how they affect lipid metabolism homeostasis and energy conservation in lipid-related metabolic diseases with a focus on hepatic steatosis and cancer and the pathogenesis and therapeutic targets of AMPK/SIRTs/FOXOs, PI3K/Akt, PPARs/PGC-1α, MAPK/ERK1/2, TLR4/NF-κB, AMPK/mTOR/TFEB, Wnt/β-catenin through immune inflammation, oxidative stress and autophagy-related pathways. As well as the current application of lipolytic enzyme inhibitors (especially Monoacylglycerol lipase (MGL) inhibitors) to provide new strategies for future exploration of metabolic programming in metabolic diseases.

The phospholipase A2 superfamily as a central hub of bioactive lipids and beyond

In essence, "phospholipase A₂" (PLA₂) means a group of enzymes that release fatty acids and lysophospholipids by hydrolyzing the sn-2 position of glycerophospholipids. To date, more than 50 enzymes possessing PLA₂ or related lipid-metabolizing activities have been identified in mammals, and these are subdivided into several families in terms of their structures, catalytic mechanisms, tissue/cellular localizations, and evolutionary relationships. From a general viewpoint, the PLA₂ superfamily has mainly been implicated in signal transduction, driving the production of a wide variety of bioactive lipid mediators. However, a growing body of evidence indicates that PLA₂s also contribute to phospholipid remodeling or recycling for membrane homeostasis, fatty acid β-oxidation for energy production, and barrier lipid formation on the body surface. Accordingly, PLA₂ enzymes are considered one of the key regulators of a broad range of lipid metabolism, and perturbation of specific PLA₂-driven lipid pathways often disrupts tissue and cellular homeostasis and may be associated with a variety of diseases. This review covers current understanding of the physiological functions of the PLA₂ superfamily, focusing particularly on the two major intracellular PLA₂ families (Ca²⁺-dependent cytosolic PLA₂s and Ca²⁺-independent patatin-like PLA₂s) as well as other PLA₂ families, based on studies using gene-manipulated mice and human diseases in combination with comprehensive lipidomics.

Insights Into the Peroxisomal Protein Inventory of Zebrafish

Peroxisomes are ubiquitous, oxidative subcellular organelles with important functions in cellular lipid metabolism and redox homeostasis. Loss of peroxisomal functions causes severe disorders with developmental and neurological abnormalities. Zebrafish are emerging as an attractive vertebrate model to study peroxisomal disorders as well as cellular lipid metabolism. Here, we combined bioinformatics analyses with molecular cell biology and reveal the first comprehensive inventory of Danio rerio peroxisomal proteins, which we systematically compared with those of human peroxisomes. Through bioinformatics analysis of all PTS1-carrying proteins, we demonstrate that D. rerio lacks two well-known mammalian peroxisomal proteins (BAAT and ZADH2/PTGR3), but possesses a putative peroxisomal malate synthase (Mlsl) and verified differences in the presence of purine degrading enzymes. Furthermore, we revealed novel candidate peroxisomal proteins in D. rerio, whose function and localisation is discussed. Our findings confirm the suitability of zebrafish as a vertebrate model for peroxisome research and open possibilities for the study of novel peroxisomal candidate proteins in zebrafish and humans.

Overview of Lipidomic Analysis of Triglyceride Molecular Species in Biological Lipid Extracts

Triglyceride (TG) is a class of neutral lipids, which functions as an energy storage depot and is important for cellular growth, metabolism, and function. The composition and content of TG molecular species are crucial factors for nutritional aspects in food chemistry and are directly associated with several diseases, including atherosclerosis, diabetes, obesity, stroke, etc. As a result of the complexities of aliphatic moieties and their different connections/locations to the glycerol backbone in TG molecules, accurate identification of individual TG molecular species and quantitative assessment of TG composition and content are particularly challenging, even at the current stage of lipidomics development. Herein, methods developed for analysis of TG species, such as liquid chromatography-mass spectrometry with a variety of columns and different mass spectrometric techniques, shotgun lipidomics approaches, and ion-mobility-based analysis, are reviewed. Moreover, the potential limitations of the methods are discussed. It is our sincere hope that the overviews and discussions can provide some insights for researchers to select an appropriate approach for TG analysis and can serve as the basis for those who would like to establish a methodology for TG analysis or develop a new method when novel tools become available. Biologically accurate analysis of TG species with an enabling method should lead us toward improving the nutritional quality, revealing the effects of TG on diseases, and uncovering the underlying biochemical mechanisms related to these diseases.

Antioxidant Synergy of Mitochondrial Phospholipase PNPLA8/iPLA2γ with Fatty Acid-Conducting SLC25 Gene Family Transporters

Patatin-like phospholipase domain-containing protein PNPLA8, also termed Ca²⁺-independent phospholipase A2γ (iPLA2γ), is addressed to the mitochondrial matrix (or peroxisomes), where it may manifest its unique activity to cleave phospholipid side-chains from both sn-1 and sn-2 positions, consequently releasing either saturated or unsaturated fatty acids (FAs), including oxidized FAs. Moreover, iPLA2γ is directly stimulated by H₂O₂ and, hence, is activated by redox signaling or oxidative stress. This redox activation permits the antioxidant synergy with mitochondrial uncoupling proteins (UCPs) or other SLC25 mitochondrial carrier family members by FA-mediated protonophoretic activity, termed mild uncoupling, that leads to diminishing of mitochondrial superoxide formation. This mechanism allows for the maintenance of the steady-state redox status of the cell. Besides the antioxidant role, we review the relations of iPLA2γ to lipid peroxidation since iPLA2γ is alternatively activated by cardiolipin hydroperoxides and hypothetically by structural alterations of lipid bilayer due to lipid peroxidation. Other iPLA2γ roles include the remodeling of mitochondrial (or peroxisomal) membranes and the generation of specific lipid second messengers. Thus, for example, during FA β-oxidation in pancreatic β-cells, H₂O₂-activated iPLA2γ supplies the GPR40 metabotropic FA receptor to amplify FA-stimulated insulin secretion. Cytoprotective roles of iPLA2γ in the heart and brain are also discussed.

Genome-wide association analysis identified both RNA-seq and DNA variants associated to paratuberculosis in Canadian Holstein cattle 'in vitro' experimentally infected macrophages

Background

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of paratuberculosis, or Johne’s disease (JD), an incurable bovine disease. The evidence for susceptibility to MAP disease points to multiple interacting factors, including the genetic predisposition to a dysregulation of the immune system. The endemic situation in cattle populations can be in part explained by a genetic susceptibility to MAP infection. In order to identify the best genetic improvement strategy that will lead to a significant reduction of JD in the population, we need to understand the link between genetic variability and the biological systems that MAP targets in its assault to dominate macrophages. MAP survives in macrophages where it disseminates. We used next-generation RNA (RNA-Seq) sequencing to study of the transcriptome in response to MAP infection of the macrophages from cows that have been naturally infected and identified as positive for JD (JD (+); n = 22) or negative for JD (healthy/resistant, JD (−); n = 28). In addition to identifying genetic variants from RNA-seq data, SNP variants were also identified using the Bovine SNP50 DNA chip.

Results

The complementary strategy allowed the identification of 1,356,248 genetic variants, including 814,168 RNA-seq and 591,220 DNA chip variants. Annotation using SnpEff predicted that the 2435 RNA-seq genetic variants would produce high functional effect on known genes in comparison to the 33 DNA chip variants. Significant variants from JD(+/−) macrophages were identified by genome-wide association study and revealed two quantitative traits loci: BTA4 and 11 at (P < 5 × 10^− 7). Using BovineMine, gene expression levels together with significant genomic variants revealed pathways that potentially influence JD susceptibility, notably the energy-dependent regulation of mTOR by LKB1-AMPK and the metabolism of lipids.

Conclusion

In the present study, we succeeded in identifying genetic variants in regulatory pathways of the macrophages that may affect the susceptibility of cows that are healthy/resistant to MAP infection. RNA-seq provides an unprecedented opportunity to investigate gene expression and to link the genetic variations to biological pathways that MAP normally manipulate during the process of killing macrophages. A strategy incorporating functional markers into genetic selection may have a considerable impact in improving resistance to an incurable disease. Integrating the findings of this research into the conventional genetic selection program may allow faster and more lasting improvement in resistance to bovine paratuberculosis in dairy cattle.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07487-4.

TAZ encodes tafazzin, a transacylase essential for cardiolipin formation and central to the etiology of Barth syndrome

Tafazzin, which is encoded by the TAZ gene, catalyzes transacylation to form mature cardiolipin and shows preference for the transfer of a linoleic acid (LA) group from phosphatidylcholine (PC) to monolysocardiolipin (MLCL) with influence from mitochondrial membrane curvature. The protein contains domains and motifs involved in targeting, anchoring, and an active site for transacylase activity. Tafazzin activity affects many aspects of mitochondrial structure and function, including that of the electron transport chain, fission-fusion, as well as apoptotic signaling. TAZ mutations are implicated in Barth syndrome, an underdiagnosed and devastating disease that primarily affects male pediatric patients with a broad spectrum of disease pathologies that impact the cardiovascular, neuromuscular, metabolic, and hematologic systems.

Calcium-independent phospholipase A2γ (iPLA2γ) and its roles in cellular functions and diseases

Calcium-independent phospholipase A₂γ (iPLA₂γ)/patatin-like phospholipase domain-containing lipase 8 (PNPLA8) is one of the iPLA₂ enzymes, which do not require Ca²⁺ ion for their activity. iPLA₂γ is a membrane-bound enzyme with unique features, including the utilization of four distinct translation initiation sites and the presence of mitochondrial and peroxisomal localization signals. This enzyme is preferentially distributed in the mitochondria and peroxisomes and is thought to be responsible for the maintenance of lipid homeostasis in these organelles. Thus, both the overexpression and the deletion of iPLA₂γ in vivo caused mitochondrial abnormalities and dysfunction. Roles of iPLA₂γ in lipid mediator production and cytoprotection against oxidative stress have also been suggested by in vitro and in vivo studies. The dysregulation of iPLA₂γ can therefore be a critical factor in the development of many diseases, including metabolic diseases and cancer. In this review, we provide an overview of the biochemical properties of iPLA₂γ and then summarize the current understanding of the in vivo roles of iPLA₂γ revealed by knockout mouse studies.

Lipid Droplets in Cancer: Guardians of Fat in a Stressful World

Cancer cells possess remarkable abilities to adapt to adverse environmental conditions. Their survival during severe nutrient and oxidative stress depends on their capacity to acquire extracellular lipids and the plasticity of their mechanisms for intracellular lipid synthesis, mobilisation, and recycling. Lipid droplets, cytosolic fat storage organelles present in most cells from yeast to men, are emerging as major regulators of lipid metabolism, trafficking, and signalling in various cells and tissues exposed to stress. Their biogenesis is induced by nutrient and oxidative stress and they accumulate in various cancers. Lipid droplets act as switches that coordinate lipid trafficking and consumption for different purposes in the cell, such as energy production, protection against oxidative stress or membrane biogenesis during rapid cell growth. They sequester toxic lipids, such as fatty acids, cholesterol and ceramides, thereby preventing lipotoxic cell damage and engage in a complex relationship with autophagy. Here, we focus on the emerging mechanisms of stress-induced lipid droplet biogenesis; their roles during nutrient, lipotoxic, and oxidative stress; and the relationship between lipid droplets and autophagy. The recently discovered principles of lipid droplet biology can improve our understanding of the mechanisms that govern cancer cell adaptability and resilience to stress.

Differential effects of sPLA2-GV and GX on cellular proliferation and lipid accumulation in HT29 colon cancer cells

Secretory phospholipase A₂ (sPLA₂) group of enzymes have been shown to hydrolyze phospholipids, among which sPLA₂ Group V (GV) and Group X (GX) exhibit high selectivity towards phosphatidylcholine-rich cellular plasma membranes. The enzymes have recently emerged as key regulators in lipid droplets formation and it is hypothesized that sPLA₂-GV and GX enhanced cell proliferation and lipid droplet accumulation in colon cancer cells (HT29). In this study, cell viability and lipid droplet accumulation were assessed by Resazurin assay and Oil-Red-O staining. Interestingly, both sPLA₂-GV and GX enzymes reduced intracellular lipid droplet accumulation and did not significantly affect cell proliferation in HT29 cells. Incubation with varespladib, a pan-inhibitor of sPLA₂-Group IIA/V/X, further suppressed lipid droplets accumulation in sPLA₂-GV but have no effects in sPLA₂-GX-treated cells. Further studies using catalytically inactive sPLA₂ enzymes showed that the enzymes intrinsic catalytic activity is required for the net reduction of lipid accumulation. Meanwhile, inhibition of intracellular phospholipases (iPLA₂-γ and cPLA₂-α) unexpectedly enhanced lipid droplet accumulation in both sPLA₂-GV and GX-treated cells. The findings suggested an interconnected relationship between extracellular and intracellular phospholipases in lipid cycling. Previous studies indicated that sPLA₂ enzymes are linked to cancer development due to their ability to induce release of arachidonic acid and eicosanoids as well as the stimulation of lipid droplet formation. This study showed that the two enzymes work in a distinct manner and they neither confer proliferative advantage nor enhanced the net lipid droplet accumulation in HT29 cells.

Rate of acyl migration in lysophosphatidylcholine (LPC) is dependent upon the nature of the acyl group. Greater stability of sn-2 docosahexaenoyl LPC compared to the more saturated LPC species

Several previous studies reported that sn-2 acyl lysophosphatidylcholines (LPCs) undergo rapid isomerization due to acyl migration, especially at physiological pH and temperature. However, these studies have been carried out using mostly sn-2 palmitoyl LPC, whereas the naturally occurring sn-2 LPCs are predominantly unsaturated. In this study, we investigated the acyl migration in four naturally occurring sn-2 acyl LPCs (sn-2 16:0, sn-2 18:1, sn-2 20:4, and sn-2 22:6) stored at various temperatures in aqueous or organic solvents, employing LC/MS to analyze the isomer composition. At 37°C and pH 7.4, the order of acyl migration rates (from sn-2 to sn-1) in aqueous buffer was 16:0 LPC> 18:1 LPC> 20:4 LPC> 22:6 LPC. The rate of isomerization of sn-2 16:0 LPC was 2–5 times greater than that of sn-2 22:6 under these conditions. Complexing the LPCs to serum albumin accelerated the acyl migration of all species, but sn-2 22:6 LPC was least affected by the presence of albumin. The migration rates were lower at lower temperatures (22°C, 4°C, and -20°C), but the differences between the LPC species persisted. All the sn-2 acyl LPCs were more stable in organic solvent (chloroform: methanol, 2:1 v/v), but the effect of the acyl groups on acyl migration was evident in the solvent also, at all temperatures. Storage of sn-2 22:6 LPC at -20°C for 4 weeks in the organic solvent resulted in about 10% isomerization, compared to 55% isomerization for sn-2 16:0. These results show that the sn-2 polyunsaturated LPCs can be stored at -20°C or below for several days without appreciable isomerization. Furthermore, they demonstrate that the sn-2 polyunsaturated LPCs generated in vivo are much more stable under physiological conditions than previously assumed.

The phospholipase iPLA2γ is a major mediator releasing oxidized aliphatic chains from cardiolipin, integrating mitochondrial bioenergetics and signaling

Cardiolipin (CL) is a dimeric phospholipid with critical roles in mitochondrial bioenergetics and signaling. Recently, inhibition of the release of oxidized fatty acyl chains from CL by the calcium-independent phospholipase A₂γ (iPLA₂γ)-selective inhibitor (R)-BEL suggested that iPLA₂γ is responsible for the hydrolysis of oxidized CL and subsequent signaling mediated by the released oxidized fatty acids. However, chemical inhibition by BEL is subject to off-target pharmacologic effects. Accordingly, to unambiguously determine the role of iPLA₂γ in the hydrolysis of oxidized CL, we compared alterations in oxidized CLs and the release of oxidized aliphatic chains from CL in experiments with purified recombinant iPLA₂γ, germ-line iPLA₂γ^-/- mice, cardiac myocyte-specific iPLA₂γ transgenic mice, and wild-type mice. Using charge-switch high mass accuracy LC-MS/MS with selected reaction monitoring and product ion accurate masses, we demonstrated that iPLA₂γ is the major enzyme responsible for the release of oxidized aliphatic chains from CL. Our results also indicated that iPLA₂γ selectively hydrolyzes 9-hydroxy-octadecenoic acid in comparison to 13-hydroxy-octadecenoic acid from oxidized CLs. Moreover, oxidative stress (ADP, NADPH, and Fe³⁺) resulted in the robust production of oxidized CLs in intact mitochondria from iPLA₂γ^-/- mice. In sharp contrast, oxidized CLs were readily hydrolyzed in mitochondria from wild-type mice during oxidative stress. Finally, we demonstrated that CL activates the iPLA₂γ-mediated hydrolysis of arachidonic acid from phosphatidylcholine, thereby integrating the production of lipid messengers from different lipid classes in mitochondria. Collectively, these results demonstrate the integrated roles of CL and iPLA₂γ in lipid second-messenger production and mitochondrial bioenergetics during oxidative stress.

Cardiac Myocyte-specific Knock-out of Calcium-independent Phospholipase A2γ (iPLA2γ) Decreases Oxidized Fatty Acids during Ischemia/Reperfusion and Reduces Infarct Size

Calcium-independent phospholipase A2γ (iPLA2γ) is a mitochondrial enzyme that produces lipid second messengers that facilitate opening of the mitochondrial permeability transition pore (mPTP) and contribute to the production of oxidized fatty acids in myocardium. To specifically identify the roles of iPLA2γ in cardiac myocytes, we generated cardiac myocyte-specific iPLA2γ knock-out (CMiPLA2γKO) mice by removing the exon encoding the active site serine (Ser-477). Hearts of CMiPLA2γKO mice exhibited normal hemodynamic function, glycerophospholipid molecular species composition, and normal rates of mitochondrial respiration and ATP production. In contrast, CMiPLA2γKO mice demonstrated attenuated Ca(2+)-induced mPTP opening that could be rapidly restored by the addition of palmitate and substantially reduced production of oxidized polyunsaturated fatty acids (PUFAs). Furthermore, myocardial ischemia/reperfusion (I/R) in CMiPLA2γKO mice (30 min of ischemia followed by 30 min of reperfusion in vivo) dramatically decreased oxidized fatty acid production in the ischemic border zones. Moreover, CMiPLA2γKO mice subjected to 30 min of ischemia followed by 24 h of reperfusion in vivo developed substantially less cardiac necrosis in the area-at-risk in comparison with their WT littermates. Furthermore, we found that membrane depolarization in murine heart mitochondria was sensitized to Ca(2+) by the presence of oxidized PUFAs. Because mitochondrial membrane depolarization and calcium are known to activate iPLA2γ, these results are consistent with salvage of myocardium after I/R by iPLA2γ loss of function through decreasing mPTP opening, diminishing production of proinflammatory oxidized fatty acids, and attenuating the deleterious effects of abrupt increases in calcium ion on membrane potential during reperfusion.

Lipid metabolism and signaling in cardiac lipotoxicity

The heart balances uptake, metabolism and oxidation of fatty acids (FAs) to maintain ATP production, membrane biosynthesis and lipid signaling. Under conditions where FA uptake outpaces FA oxidation and FA sequestration as triacylglycerols in lipid droplets, toxic FA metabolites such as ceramides, diacylglycerols, long-chain acyl-CoAs, and acylcarnitines can accumulate in cardiomyocytes and cause cardiomyopathy. Moreover, studies using mutant mice have shown that dysregulation of enzymes involved in triacylglycerol, phospholipid, and sphingolipid metabolism in the heart can lead to the excess deposition of toxic lipid species that adversely affect cardiomyocyte function. This review summarizes our current understanding of lipid uptake, metabolism and signaling pathways that have been implicated in the development of lipotoxic cardiomyopathy under conditions including obesity, diabetes, aging, and myocardial ischemia-reperfusion. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Loss of function variants in human PNPLA8 encoding calcium-independent phospholipase A2 γ recapitulate the mitochondriopathy of the homologous null mouse

Mitochondriopathies are a group of clinically heterogeneous genetic diseases caused by defects in mitochondrial metabolism, bioenergetic efficiency, and/or signaling functions. The large majority of proteins involved in mitochondrial function are encoded by nuclear genes, with many yet to be associated with human disease. We performed exome sequencing on a young girl with a suspected mitochondrial myopathy that manifested as progressive muscle weakness, hypotonia, seizures, poor weight gain, and lactic acidosis. She was compound heterozygous for two frameshift mutations, p.Asn112HisfsX29 and p.Leu659AlafsX4, in the PNPLA8 gene, which encodes mitochondrial calcium-independent phospholipase A2 γ (iPLA2 γ). Western blot analysis of affected muscle displayed the absence of PNPLA8 protein. iPLA2 s are critical mediators of a variety of cellular processes including growth, metabolism, and lipid second messenger generation, exerting their functions through catalyzing the cleavage of the acyl groups in glycerophospholipids. The clinical presentation, muscle histology and the mitochondrial ultrastructural abnormalities of this proband are highly reminiscent of Pnpla8 null mice. Although other iPLA2 -related diseases have been identified, namely, infantile neuroaxonal dystrophy and neutral lipid storage disease with myopathy, this is the first report of PNPLA8-related disease in a human. We suggest PNPLA8 join the increasing list of human genes involved in lipid metabolism associated with neuromuscular diseases due to mitochondrial dysfunction.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Phospholipase A2 regulation of lipid droplet formation

The classical regard of lipid droplets as mere static energy-storage organelles has evolved dramatically. Nowadays these organelles are known to participate in key processes of cell homeostasis, and their abnormal regulation is linked to several disorders including metabolic diseases (diabetes, obesity, atherosclerosis or hepatic steatosis), inflammatory responses in leukocytes, cancer development and neurodegenerative diseases. Hence, the importance of unraveling the cell mechanisms controlling lipid droplet biosynthesis, homeostasis and degradation seems evident Phospholipase A2s, a family of enzymes whose common feature is to hydrolyze the fatty acid present at the sn-2 position of phospholipids, play pivotal roles in cell signaling and inflammation. These enzymes have recently emerged as key regulators of lipid droplet homeostasis, regulating their formation at different levels. This review summarizes recent results on the roles that various phospholipase A2 forms play in the regulation of lipid droplet biogenesis under different conditions. These roles expand the already wide range of functions that these enzymes play in cell physiology and pathophysiology.

Dysregulation of cardiolipin biosynthesis in pediatric heart failure

Cardiolipin, a unique phospholipid in the inner mitochondrial membrane, is critical for optimal mitochondrial function. CL abnormalities have been demonstrated in the failing rodent and adult human heart. The aim of this study was to determine whether abnormalities in CL content and the CL biosynthesis and remodeling pathways are present in pediatric idiopathic dilated cardiomyopathy (IDC). A cross-sectional analysis of myocardial tissue from 119 IDC and non-failing (NF) control samples was performed. Electrospray ionizing mass spectrometry was used to measure total CL and CL species content in LV tissue. RT-PCR was employed to measure gene expression of the enzymes in the CL biosynthesis and remodeling pathways in both the adult and pediatric heart. Significantly lower total and (18:2)4CL (the beneficial species) content was demonstrated in myocardium from pediatric patients with IDC compared to NF controls. Analysis of mitochondrial gene transcripts was used to demonstrate that there is no decrease in mitochondrial content. Expression of two biosynthesis enzymes and one remodeling enzyme was significantly lower in pediatric IDC compared to NF controls. Expression of two phospholipases involved in CL degradation were also altered, one up- and one down-regulated. Except for one remodeling enzyme, these changes are unique from those in the failing adult heart. Similar to what has been seen in adults and in a rat model of IDC, total and (18:2)4CL are lower in pediatric IDC. Unique CL species profiles are seen in heart tissue from children with IDC compared to adults. Differences in CL biosynthesis and remodeling enzyme expression likely explain the differences in CL profiles observed in IDC and implicate unique age-related mechanisms of disease.

Group VIB Phospholipase A(2) promotes proliferation of INS-1 insulinoma cells and attenuates lipid peroxidation and apoptosis induced by inflammatory cytokines and oxidant agents

Group VIB Phospholipase A(2) (iPLA(2)γ) is distributed in membranous organelles in which β-oxidation occurs, that is, mitochondria and peroxisomes, and is expressed by insulin-secreting pancreatic islet β-cells and INS-1 insulinoma cells, which can be injured by inflammatory cytokines, for example, IL-1β and IFN-γ, and by oxidants, for example, streptozotocin (STZ) or t-butyl-hydroperoxide (TBHP), via processes pertinent to mechanisms of β-cell loss in types 1 and 2 diabetes mellitus. We find that incubating INS-1 cells with IL-1β and IFN-γ, with STZ, or with TBHP causes increased expression of iPLA(2)γ mRNA and protein. We prepared INS-1 knockdown (KD) cell lines with reduced iPLA(2)γ expression, and they proliferate more slowly than control INS-1 cells and undergo increased membrane peroxidation in response to cytokines or oxidants. Accumulation of oxidized phospholipid molecular species in STZ-treated INS-1 cells was demonstrated by LC/MS/MS scanning, and the levels in iPLA(2)γ-KD cells exceeded those in control cells. iPLA(2)γ-KD INS-1 cells also exhibited higher levels of apoptosis than control cells when incubated with STZ or with IL-1β and IFN-γ. These findings suggest that iPLA(2)γ promotes β-cell proliferation and that its expression is increased during inflammation or oxidative stress as a mechanism to mitigate membrane injury that may enhance β-cell survival.

CD36 protein influences myocardial Ca2+ homeostasis and phospholipid metabolism: conduction anomalies in CD36-deficient mice during fasting

Sarcolemmal CD36 facilitates myocardial fatty acid (FA) uptake, which is markedly reduced in CD36-deficient rodents and humans. CD36 also mediates signal transduction events involving a number of cellular pathways. In taste cells and macrophages, CD36 signaling was recently shown to regulate store-responsive Ca(2+) flux and activation of Ca(2+)-dependent phospholipases A(2) that cycle polyunsaturated FA into phospholipids. It is unknown whether CD36 deficiency influences myocardial Ca(2+) handling and phospholipid metabolism, which could compromise the heart, typically during stresses. Myocardial function was examined in fed or fasted (18-22 h) CD36(-/-) and WT mice. Echocardiography and telemetry identified conduction anomalies that were associated with the incidence of sudden death in fasted CD36(-/-) mice. No anomalies or death occurred in WT mice during fasting. Optical imaging of perfused hearts from fasted CD36(-/-) mice documented prolongation of Ca(2+) transients. Consistent with this, knockdown of CD36 in cardiomyocytes delayed clearance of cytosolic Ca(2+). Hearts of CD36(-/-) mice (fed or fasted) had 3-fold higher SERCA2a and 40% lower phospholamban levels. Phospholamban phosphorylation by protein kinase A (PKA) was enhanced after fasting reflecting increased PKA activity and cAMP levels in CD36(-/-) hearts. Abnormal Ca(2+) homeostasis in the CD36(-/-) myocardium associated with increased lysophospholipid content and a higher proportion of 22:6 FA in phospholipids suggests altered phospholipase A(2) activity and changes in membrane dynamics. The data support the role of CD36 in coordinating Ca(2+) homeostasis and lipid metabolism and the importance of this role during myocardial adaptation to fasting. Potential relevance of the findings to CD36-deficient humans would need to be determined.

Activation of mitochondrial calcium-independent phospholipase A2γ (iPLA2γ) by divalent cations mediating arachidonate release and production of downstream eicosanoids

Calcium-independent phospholipase A(2)γ (iPLA(2)γ) (PNPLA8) is the predominant phospholipase activity in mammalian mitochondria. However, the chemical mechanisms that regulate its activity are unknown. Here, we utilize iPLA(2)γ gain of function and loss of function genetic models to demonstrate the robust activation of iPLA(2)γ in murine myocardial mitochondria by Ca(2+) or Mg(2+) ions. Calcium ion stimulated the production of 2-arachidonoyl-lysophosphatidylcholine (2-AA-LPC) from 1-palmitoyl-2-[(14)C]arachidonoyl-sn-glycero-3-phosphocholine during incubations with wild-type heart mitochondrial homogenates. Furthermore, incubation of mitochondrial homogenates from transgenic myocardium expressing iPLA(2)γ resulted in 13- and 25-fold increases in the initial rate of radiolabeled 2-AA-LPC and arachidonic acid (AA) production, respectively, in the presence of calcium ion. Mass spectrometric analysis of the products of calcium-activated hydrolysis of endogenous mitochondrial phospholipids in transgenic iPLA(2)γ mitochondria revealed the robust production of AA, 2-AA-LPC, and 2-docosahexaenoyl-LPC that was over 10-fold greater than wild-type mitochondria. The mechanism-based inhibitor (R)-(E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one (BEL) (iPLA(2)γ selective), but not its enantiomer, (S)-BEL (iPLA(2)β selective) or pyrrolidine (cytosolic PLA(2)α selective), markedly attenuated Ca(2+)-dependent fatty acid release and polyunsaturated LPC production. Moreover, Ca(2+)-induced iPLA(2)γ activation was accompanied by the production of downstream eicosanoid metabolites that were nearly completely ablated by (R)-BEL or by genetic ablation of iPLA(2)γ. Intriguingly, Ca(2+)-induced iPLA(2)γ activation was completely inhibited by long-chain acyl-CoA (IC(50) ∼20 μm) as well as by a nonhydrolyzable acyl-CoA thioether analog. Collectively, these results demonstrate that mitochondrial iPLA(2)γ is activated by divalent cations and inhibited by acyl-CoA modulating the generation of biologically active metabolites that regulate mitochondrial bioenergetic and signaling functions.

Multi-dimensional mass spectrometry-based shotgun lipidomics and novel strategies for lipidomic analyses

Since our last comprehensive review on multi-dimensional mass spectrometry-based shotgun lipidomics (Mass Spectrom. Rev. 24 (2005), 367), many new developments in the field of lipidomics have occurred. These developments include new strategies and refinements for shotgun lipidomic approaches that use direct infusion, including novel fragmentation strategies, identification of multiple new informative dimensions for mass spectrometric interrogation, and the development of new bioinformatic approaches for enhanced identification and quantitation of the individual molecular constituents that comprise each cell's lipidome. Concurrently, advances in liquid chromatography-based platforms and novel strategies for quantitative matrix-assisted laser desorption/ionization mass spectrometry for lipidomic analyses have been developed. Through the synergistic use of this repertoire of new mass spectrometric approaches, the power and scope of lipidomics has been greatly expanded to accelerate progress toward the comprehensive understanding of the pleiotropic roles of lipids in biological systems.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

[Proteins sharing PNPLA domain, a new family of enzymes regulating lipid metabolism]

Genome sequencing technologies led to tremendous breakthrough in biology uncovering numerous genes unknown so far and thus opening the field of deep investigations to understand their associated biological functions. As a matter of fact, functional genomics have been progressively replacing sequence genomics with as a main objective to yield insight into cellular physiology. Recently, an emerging group of genes coding for proteins bearing a common domain termed patatin (PNPLA domain) have been discovered. Members of this new enzymatic family displaying lipase and transacylase properties appeared to have major roles in the regulation of lipid metabolism. The aim of this review is to make an overview on the latest discoveries concerning this new family of proteins and their relationship with lipid metabolism, physiology of mammals and their potential involvement in human pathology.

Mice deficient in group VIB phospholipase A2 (iPLA2gamma) exhibit relative resistance to obesity and metabolic abnormalities induced by a Western diet

Phospholipases A(2) (PLA(2)) play important roles in metabolic processes, and the Group VI PLA(2) family is comprised of intracellular enzymes that do not require Ca(2+) for catalysis. Mice deficient in Group VIA PLA(2) (iPLA(2)beta) develop more severe glucose intolerance than wild-type (WT) mice in response to dietary stress. Group VIB PLA(2) (iPLA(2)gamma) is a related enzyme distributed in membranous organelles, including mitochondria, and iPLA(2)gamma knockout (KO) mice exhibit altered mitochondrial morphology and function. We have compared metabolic responses of iPLA(2)gamma-KO and WT mice fed a Western diet (WD) with a high fat content. We find that KO mice are resistant to WD-induced increases in body weight and adiposity and in blood levels of cholesterol, glucose, and insulin, even though WT and KO mice exhibit similar food consumption and dietary fat digestion and absorption. KO mice are also relatively resistant to WD-induced insulin resistance, glucose intolerance, and altered patterns of fat vs. carbohydrate fuel utilization. KO skeletal muscle exhibits impaired mitochondrial beta-oxidation of fatty acids, as reflected by accumulation of larger amounts of long-chain acylcarnitine (LCAC) species in KO muscle and liver compared with WT in response to WD feeding. This is associated with increased urinary excretion of LCAC and much reduced deposition of triacylglycerols in liver by WD-fed KO compared with WT mice. The iPLA(2)gamma-deficient genotype thus results in a phenotype characterized by impaired mitochondrial oxidation of fatty acids and relative resistance to the metabolic abnormalities induced by WD.

Group VIA Ca2+-independent phospholipase A2 (iPLA2beta) and its role in beta-cell programmed cell death

Activation of phospholipases A(2) (PLA(2)s) leads to the generation of biologically active lipid mediators that can affect numerous cellular events. The Group VIA Ca(2+)-independent PLA(2), designated iPLA(2)beta, is active in the absence of Ca(2+), activated by ATP, and inhibited by the bromoenol lactone suicide inhibitor (BEL). Over the past 10-15 years, studies using BEL have demonstrated that iPLA(2)beta participates in various biological processes and the recent availability of mice in which iPLA(2)beta expression levels have been genetically-modified are extending these findings. Work in our laboratory suggests that iPLA(2)beta activates a unique signaling cascade that promotes beta-cell apoptosis. This pathway involves iPLA(2)beta dependent induction of neutral sphingomyelinase, production of ceramide, and activation of the intrinsic pathway of apoptosis. There is a growing body of literature supporting beta-cell apoptosis as a major contributor to the loss of beta-cell mass associated with the onset and progression of Type 1 and Type 2 diabetes mellitus. This underscores a need to gain a better understanding of the molecular mechanisms underlying beta-cell apoptosis so that improved treatments can be developed to prevent or delay the onset and progression of diabetes mellitus. Herein, we offer a general review of Group VIA Ca(2+)-independent PLA(2) (iPLA(2)beta) followed by a more focused discussion of its participation in beta-cell apoptosis. We suggest that iPLA(2)beta-derived products trigger pathways which can lead to beta-cell apoptosis during the development of diabetes.

Genetic ablation of calcium-independent phospholipase A2{gamma} leads to alterations in hippocampal cardiolipin content and molecular species distribution, mitochondrial degeneration, autophagy, and cognitive dysfunction

Genetic ablation of calcium-independent phospholipase A(2)gamma (iPLA(2)gamma) results in profound alterations in hippocampal phospholipid metabolism and mitochondrial phospholipid homeostasis resulting in enlarged and degenerating mitochondria leading to autophagy and cognitive dysfunction. Shotgun lipidomics demonstrated multiple alterations in hippocampal lipid metabolism in iPLA(2)gamma(-/-) mice including: 1) a markedly elevated hippocampal cardiolipin content with an altered molecular species composition characterized by a shift to shorter chain length molecular species; 2) alterations in both choline and ethanolamine glycerophospholipids, including a decreased plasmenylethanolamine content; 3) increased oxidized phosphatidylethanolamine molecular species; and 4) an increased content of ceramides. Electron microscopic examination demonstrated the presence of enlarged heteromorphic lamellar structures undergoing degeneration accompanied by the presence of ubiquitin positive spheroid inclusion bodies. Purification of these enlarged heteromorphic lamellar structures by buoyant density centrifugation and subsequent SDS-PAGE and proteomics identified them as degenerating mitochondria. Collectively, these results identify the obligatory role of iPLA(2)gamma in neuronal mitochondrial lipid metabolism and membrane structure demonstrating that iPLA(2)gamma loss of function results in a mitochondrial neurodegenerative disorder characterized by degenerating mitochondria, autophagy, and cognitive dysfunction.

Coupled calcium and zinc dyshomeostasis and oxidative stress in cardiac myocytes and mitochondria of rats with chronic aldosteronism

A dyshomeostasis of extra- and intracellular Ca(2+) and Zn(2+) occurs in rats receiving chronic aldosterone/salt treatment (ALDOST). Herein, we hypothesized that the dyshomeostasis of intracellular Ca(2+) and Zn(2+) is intrinsically coupled that alters the redox state of cardiac myocytes and mitochondria, with Ca(2+) serving as a pro-oxidant and Zn(2+) as an antioxidant. Toward this end, we harvested hearts from rats receiving 4 weeks of ALDOST alone or cotreatment with either spironolactone (Spiro), an aldosterone receptor antagonist, or amlodipine (Amlod), an L-type Ca(2+) channel blocker, and from age/sex-matched untreated controls. In each group, we monitored cardiomyocyte [Ca(2+)]i and [Zn(2+)]i and mitochondrial [Ca(2+)]m and [Zn(2+)]m; biomarkers of oxidative stress and antioxidant defenses; expression of Zn transporters, Zip1 and ZnT-1; metallothionein-1, a Zn(2+)-binding protein; and metal response element transcription factor-1, a [Zn(2+)]i sensor and regulator of antioxidant defenses. Compared with controls, at 4-week ALDOST, we found the following: (a) increased [Ca(2+)]i and [Zn(2+)]i, together with increased [Ca(2+)]m and [Zn(2+)]m, each of which could be prevented by Spiro and attenuated with Amlod; (b) increased levels of 3-nitrotyrosine and 4-hydroxy-2-nonenal in cardiomyocytes, together with increased H(2)O(2) production, malondialdehyde, and oxidized glutathione in mitochondria that were coincident with increased activities of Cu/Zn superoxide dismutase and glutathione peroxidase; and (c) increased expression of metallothionein-1, Zip1 and ZnT-1, and metal response element transcription factor-1, attenuated by Spiro. Thus, an intrinsically coupled dyshomeostasis of intracellular Ca(2+) and Zn(2+) occurs in cardiac myocytes and mitochondria in rats receiving ALDOST, where it serves to alter their redox state through a respective induction of oxidative stress and generation of antioxidant defenses. The importance of therapeutic strategies that can uncouple these two divalent cations and modulate their ratio in favor of sustained antioxidant defenses is therefore suggested.

Cardiolipin acts as a mitochondrial signalling platform to launch apoptosis

Cardiolipin (CL) is a unique anionic phospholipid specific to the mitochondria. CL influences the activity of electron transport chain enzyme complexes as well as members of the Bcl-2 family. Interactions between Bcl-2 family members and other pro-apoptotic enzymes have been shown to be crucial for the transduction of the apoptotic signalling cascades during programmed cell death. Targeting of tBid to the mitochondria, which is necessary for Bax/Bak oligomerization and cristae remodelling, is dependent on the exposure of CL at contact sites between the inner and outer mitochondrial membranes. Also, the mobilization of cytochrome c, another key apoptotic event, is tightly regulated by the oxidative state of cardiolipin. Moreover, CL has been shown to be essential for translocation and autoprocessing of caspase-8 on the mitochondria after death receptor stimulation. Deficiencies in CL inhibit the formation of tBid and prevent apoptosis by removing an essential activation platform for the autoprocessing of caspase-8. It is now apparent that CL acts as a crucial signalling platform from which it orchestrates apoptosis by integrating signals from a variety of death inducing proteins.

Formation of molecular species of mitochondrial cardiolipin. 1. A novel transacylation mechanism to shuttle fatty acids between sn-1 and sn-2 positions of multiple phospholipid species

Mitochondrial cardiolipin undergoes extensive remodeling of its acyl groups to generate uniformly substituted species, such as tetralinoleoyl-cardiolipin, but the mechanism of this remodeling has not been elucidated, except for the fact that it requires tafazzin. Here we show that purified recombinant Drosophila tafazzin exchanges acyl groups between cardiolipin and phosphatidylcholine by a combination of forward and reverse transacylations. The acyl exchange is possible in the absence of phospholipase A(2) because it requires only trace amounts of lysophospholipids. We show that purified tafazzin reacts with various phospholipid classes and with various acyl groups both in sn-1 and sn-2 position. Expression studies in Sf9 insect cells suggest that the effect of tafazzin on cardiolipin species is dependent on the cellular environment and not on enzymatic substrate specificity. Our data demonstrate that tafazzin catalyzes general acyl exchange between phospholipids, which raises the question whether pattern formation in cardiolipin is the result of the equilibrium distribution of acyl groups between multiple phospholipid species.

Lipid droplet biogenesis induced by stress involves triacylglycerol synthesis that depends on group VIA phospholipase A2

This work investigates the metabolic origin of triacylglycerol (TAG) formed during lipid droplet (LD) biogenesis induced by stress. Cytotoxic inhibitors of fatty acid synthase induced TAG synthesis and LD biogenesis in CHO-K1 cells, in the absence of external sources of fatty acids. TAG synthesis was required for LD biogenesis and was sensitive to inhibition and down-regulation of the expression of group VIA phospholipase A(2) (iPLA(2)-VIA). Induction of stress with acidic pH, C(2)-ceramide, tunicamycin, or deprivation of glucose also stimulated TAG synthesis and LD formation in a manner dependent on iPLA(2)-VIA. Overexpression of the enzyme enhanced TAG synthesis from endogenous fatty acids and LD occurrence. During stress, LD biogenesis but not TAG synthesis required phosphorylation and activation of group IVA PLA(2) (cPLA(2)alpha). The results demonstrate that iPLA(2)-VIA provides fatty acids for TAG synthesis while cPLA(2)alpha allows LD biogenesis. LD biogenesis during stress may be a survival strategy, recycling structural phospholipids into energy-generating substrates.

Eicosanoid signalling pathways in the heart

Myocardial phospholipids serve as primary reservoirs of arachidonic acid (AA), which is liberated through the rate-determining hydrolytic action of cardiac phospholipases A2 (PLA2s). A predominant PLA2 in myocardium is calcium-independent phospholipase A2beta (iPLA2beta), which, through its calmodulin (CaM) and ATP-binding domains, is regulated by alterations in local cellular Ca2+ concentrations and cardiac bioenergetic status, respectively. Importantly, iPLA2beta has been demonstrated to be activated by ischaemia through elevation of the concentration of myocardial fatty acyl-CoA, which abrogates Ca2+/CaM-mediated inhibition of iPLA2beta. AA released by PLA2-catalysed hydrolysis of phospholipids serves as a precursor for eicosanoids generated by pathways dependent on cyclooxygenases (COX), lipoxygenases (LOX), and cytochromes P450 (CYP). Eicosanoids initiate and propagate diverse signalling cascades, primarily through their interaction with cellular receptors and ion channels. However, during pathologic states such as ischaemia or congestive heart failure, eicosanoids contribute to multiple maladaptive changes including inflammation, alterations of cellular growth programmes, and activation of multiple transcriptional events leading to the deleterious sequelae of these pathologic states. This review summarizes the central roles of myocardial PLA(2)s in eicosanoid signalling in the heart, the major COX, LOX, and CYP pathways of eicosanoid generation in the myocardium, and the effects of important eicosanoids on receptor-, ion channel-, and transcription-mediated processes that facilitate cardiac hypertrophy, mediate ischaemic preconditioning, and precipitate arrhythmogenesis in response to pathologic stimuli.

Mammalian patatin domain containing proteins: a family with diverse lipolytic activities involved in multiple biological functions

The human genome expresses nine patatin-like phospholipase domain containing proteins (PNPLA1-9). Members of this family share a protein domain discovered initially in patatin, the most abundant protein of the potato tuber. Patatin is a lipid hydrolase with an unusual folding topology that differs from classical lipases. Mammalian PNPLAs include lipid hydrolases with specificities for diverse substrates such as triacylglycerols, phospholipids, and retinol esters. Analysis of induced mutant mouse models and the clinical phenotype of patients with mutations revealed important insights into the physiological role of several members of the PNPLA family. This review aims to summarize current knowledge of PNPLA proteins and to document their emerging importance in lipid and energy homeostasis.

Tryptase activates calcium-independent phospholipase A2 and releases PGE2 in airway epithelial cells

Human small airway epithelial cells (HSAEC) form the boundary between the external environmental allergens and the internal lung milieu. Mast cells are present in human lung tissue interspersed within the pulmonary epithelium and can secrete a host of pre- and newly formed mediators from their granules, which may propagate small airway inflammation. In this study, tryptase stimulation of HSAEC increased membrane-associated, calcium-independent phospholipase A(2)gamma (iPLA(2)gamma) activity, resulting in increased arachidonic acid and PGE(2) release. These responses were inhibited by pretreating HSAEC with the iPLA(2)-selective inhibitor bromoenol lactone. The tryptase-stimulated PGE(2) production was inhibited by treating HSAEC with the cyclooxygenase (COX)-1-selective inhibitor SC-560 and the nonselective COX inhibitor aspirin but not by the COX-2-selective inhibitor CAY10404, indicating that the early release of arachidonic acid is metabolized by constitutive COX-1 to form PGE(2) in tryptase-stimulated HSAEC. Additionally, platelet-activating factor production and neutrophil adherence to tryptase-stimulated HSAEC was also increased. This complex response can set up a cascade of inflammatory mediator production in small airways. We speculate that selective inhibition of iPLA(2)gamma-mediated phospholipid hydrolysis may prove beneficial in inflammatory airway diseases.

Calcium-independent phospholipase A2 in rabbit ventricular myocytes

We have previously reported that the majority of phospholipase A2 (PLA2) activity in rabbit ventricular myocytes is membrane-associated, calcium-independent (iPLA2), selective for arachidonylated plasmalogen phospholipids and inhibited by the iPLA2-selective inhibitor bromoenol lactone (BEL). Here, we identified the presence of iPLA2 in rabbit ventricular myocytes, determined the full length sequences for rabbit iPLA2beta and iPLA2gamma and compared their homology to the human isoforms. Rabbit iPLA2beta encoded a protein with a predicated molecular mass of 74 kDa that is 91% identical to the human iPLA2beta short isoform. Full length iPLA2gamma protein has a predicated molecular mass of 88 kDa and is 88% identical to the human isoform. Immunoblot analysis of iPLA2beta and gamma in membrane and cytosolic fractions from rabbit and human cardiac myocytes demonstrated a similar pattern of distribution with both isoforms present in the membrane fraction, but no detectable protein in the cytosol. Membrane-associated iPLA2 activity was inhibited preferentially by the R enantiomer of bromoenol lactone [(R)-BEL], indicating that the majority of activity is due to iPLA2gamma.

Shotgun lipidomics reveals the temporally dependent, highly diversified cardiolipin profile in the mammalian brain: temporally coordinated postnatal diversification of cardiolipin molecular species with neuronal remodeling

Large-scale neuronal remodeling through apoptosis occurs shortly after birth in all known mammalian species. Apoptosis, in large part, depends upon critical interactions between mitochondrial membranes and cytochrome c. Herein, we examined the hypothesis that the large-scale reorganization of neuronal circuitry after birth is accompanied by profound alterations in cardiolipin (CL) content and molecular species distribution. During embryonic development, over 100 CL molecular species were identified and quantitated in murine neuronal tissues. The embryonic CL profile was notable for the presence of abundant amounts of relatively short aliphatic chains (e.g., palmitoleic and oleic acids). In sharp contrast, after birth, the CL profile contained a remarkably complex repertoire of CL molecular species, in which the signaling fatty acids (i.e., arachidonic and docosahexaenoic acids) were markedly increased. These results identify the rapid remodeling of CL in the perinatal period with resultant alterations in the physical properties of the mitochondrial membrane. The complex distribution of aliphatic chains in the neuronal CL pool is separate and distinct from that in other organs (e.g., heart, liver, etc.), where CL molecular species contain predominantly only one major type of aliphatic chain (e.g., linoleic acid). Analyses of mRNA levels by real-time quantitative polymerase chain reactions suggested that the alterations in CL content were due to the combined effects of both attenuation of de novo CL biosynthesis and decreased remodeling of CL. Collectively, these results provide a new perspective on the complexity of CL in neuronal signaling, mitochondrial bioenergetics, and apoptosis.

Cardiolipin synthesis for the assembly of bacterial and mitochondrial membranes

In this article, the formation of prokaryotic and eukaryotic cardiolipin is reviewed in light of its biological function. I begin with a detailed account of the structure of cardiolipin, its stereochemistry, and the resulting physical properties, and I present structural analogs of cardiolipin that occur in some organisms. Then I continue to discuss i) the de novo formation of cardiolipin, ii) its acyl remodeling, iii) the assembly of cardiolipin into biological membranes, and iv) the degradation of cardiolipin, which may be involved in apoptosis and mitochondrial fusion. Thus, this article covers the entire metabolic cycle of this unique phospholipid. It is shown that mitochondria produce cardiolipin species with a high degree of structural uniformity and molecular symmetry, among which there is often a dominant form with four identical acyl chains. The subsequent assembly of cardiolipin into functional membranes is largely unknown, but the analysis of crystal structures of membrane proteins has revealed a first glimpse into the underlying principles of cardiolipin-protein interactions. Disturbances of cardiolipin metabolism are crucial in the pathophysiology of human Barth syndrome and perhaps also play a role in diabetes and ischemic heart disease.

Genetic ablation of calcium-independent phospholipase A2gamma leads to alterations in mitochondrial lipid metabolism and function resulting in a deficient mitochondrial bioenergetic phenotype

Previously, we identified a novel calcium-independent phospholipase, designated calcium-independent phospholipase A(2) gamma (iPLA(2)gamma), which possesses dual mitochondrial and peroxisomal subcellular localization signals. To identify the roles of iPLA(2)gamma in cellular bioenergetics, we generated mice null for the iPLA(2)gamma gene by eliminating the active site of the enzyme through homologous recombination. Mice null for iPLA(2)gamma display multiple bioenergetic dysfunctional phenotypes, including 1) growth retardation, 2) cold intolerance, 3) reduced exercise endurance, 4) greatly increased mortality from cardiac stress after transverse aortic constriction, 5) abnormal mitochondrial function with a 65% decrease in ascorbate-induced Complex IV-mediated oxygen consumption, and 6) a reduction in myocardial cardiolipin content accompanied by an altered cardiolipin molecular species composition. We conclude that iPLA(2)gamma is essential for maintaining efficient bioenergetic mitochondrial function through tailoring mitochondrial membrane lipid metabolism and composition.