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

Mitochondria to plasma membrane redox signaling is essential for fatty acid β-oxidation-driven insulin secretion

We asked whether acute redox signaling from mitochondria exists concomitantly to fatty acid- (FA-) stimulated insulin secretion (FASIS) at low glucose by pancreatic β-cells. We show that FA β-oxidation produces superoxide/H2O2, providing: i) mitochondria-to-plasma-membrane redox signaling, closing KATP-channels synergically with elevated ATP (substituting NADPH-oxidase-4-mediated H2O2-signaling upon glucose-stimulated insulin secretion); ii) activation of redox-sensitive phospholipase iPLA2γ/PNPLA8, cleaving mitochondrial FAs, enabling metabotropic GPR40 receptors to amplify insulin secretion (IS). At fasting glucose, palmitic acid stimulated IS in wt mice; palmitic, stearic, lauric, oleic, linoleic, and hexanoic acids also in perifused pancreatic islets (PIs), with suppressed 1st phases in iPLA2γ/PNPLA8-knockout mice/PIs. Extracellular/cytosolic H2O2-monitoring indicated knockout-independent redox signals, blocked by mitochondrial antioxidant SkQ1, etomoxir, CPT1 silencing, and catalase overexpression, all inhibiting FASIS, keeping ATP-sensitive K+-channels open, and diminishing cytosolic [Ca2+]-oscillations. FASIS in mice was a postprandially delayed physiological event. Redox signals of FA β-oxidation are thus documented, reaching the plasma membrane, essentially co-stimulating IS.

Knockdown of iPLA2γ enhances cisplatin-induced apoptosis by increasing ROS-dependent peroxidation of mitochondrial phospholipids in bladder cancer cells

Cisplatin (CDDP) is a platinum-based drug with anti-cancer activity and is widely used as a standard therapy for bladder cancer. It is well known that CDDP causes cell death by increasing the generation of reactive oxygen species (ROS) and lipid peroxidation, but the mechanism of its anti-cancer effects has not been fully elucidated. There are still some problems such as chemoresistance in CDDP therapy. In the present study, we found the expression of Ca2+-independent phospholipase A2γ (iPLA2γ), which has been reported to regulate cellular redox homeostasis by inhibiting lipid peroxide accumulation, in human bladder cancer tissues. Thus, we investigated the effect of iPLA2γ knockdown on CDDP-induced bladder cancer cell death. As a result, we found that iPLA2γ knockdown significantly enhanced CDDP-induced apoptosis, intracellular and mitochondrial ROS production, cytochrome c release and caspase activation in bladder cancer cells. Moreover, mitochondrial membrane potential was decreased and peroxidation of mitochondrial phospholipids was increased by iPLA2γ knockdown. It was also shown that co-treatment of bromoenol lactone, an iPLA2 inhibitor, increased CDDP-induced apoptosis. These results indicated that iPLA2γ plays an important role in protecting bladder cancer cells from CDDP-induced apoptosis, and that iPLA2γ inhibitors might represent a novel strategy in CDDP-based multi-drug therapy.

A patatin-like phospholipase is important for mitochondrial function in malaria parasites

IMPORTANCE

For their proliferation within red blood cells, malaria parasites depend on a functional electron transport chain (ETC) within their mitochondrion, which is the target of several antimalarial drugs. Here, we have used gene disruption to identify a patatin-like phospholipase, PfPNPLA2, as important for parasite replication and mitochondrial function in Plasmodium falciparum. Parasites lacking PfPNPLA2 show defects in their ETC and become hypersensitive to mitochondrion-targeting drugs. Furthermore, PfPNPLA2-deficient parasites show differences in the composition of their cardiolipins, a unique class of phospholipids with key roles in mitochondrial functions. Finally, we demonstrate that parasites devoid of PfPNPLA2 have a defect in gametocyte maturation, underlining the importance of a functional ETC for parasite transmission to the mosquito vector.

Genetic deletion of skeletal muscle iPLA2γ results in mitochondrial dysfunction, muscle atrophy and alterations in whole-body energy metabolism

Skeletal muscle is the major site of glucose utilization in mammals integrating serum glucose clearance with mitochondrial respiration. To mechanistically elucidate the roles of iPLA2γ in skeletal muscle mitochondria, we generated a skeletal muscle-specific calcium-independent phospholipase A2γ knockout (SKMiPLA2γKO) mouse. Genetic ablation of skeletal muscle iPLA2γ resulted in pronounced muscle weakness, muscle atrophy, and increased blood lactate resulting from defects in mitochondrial function impairing metabolic processing of pyruvate and resultant bioenergetic inefficiency. Mitochondria from SKMiPLA2γKO mice were dysmorphic displaying marked changes in size, shape, and interfibrillar juxtaposition. Mitochondrial respirometry demonstrated a marked impairment in respiratory efficiency with decreases in the mass and function of oxidative phosphorylation complexes and cytochrome c. Further, a pronounced decrease in mitochondrial membrane potential and remodeling of cardiolipin molecular species were prominent. Collectively, these alterations prevented body weight gain during high-fat feeding through enhanced glucose disposal without efficient capture of chemical energy thereby altering whole-body bioenergetics.

The critical role of cardiolipin in metazoan differentiation, development, and maturation

Cardiolipins are phospholipids that are central to proper mitochondrial functioning. Because mitochondria play crucial roles in differentiation, development, and maturation, we would also expect cardiolipin to play major roles in these processes. Indeed, cardiolipin has been implicated in the mechanism of three human diseases that affect young infants, implying developmental abnormalities. In this review, we will: (1) Review the biology of cardiolipin; (2) Outline the evidence for essential roles of cardiolipin during organismal development, including embryogenesis and cell maturation in vertebrate organisms; (3) Place the role(s) of cardiolipin during embryogenesis within the larger context of the roles of mitochondria in development; and (4) Suggest avenues for future research.

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.

The Lysophospholipase PNPLA7 Controls Hepatic Choline and Methionine Metabolism

The in vivo roles of lysophospholipase, which cleaves a fatty acyl ester of lysophospholipid, remained unclear. Recently, we have unraveled a previously unrecognized physiological role of the lysophospholipase PNPLA7, a member of the Ca2+-independent phospholipase A2 (iPLA2) family, as a key regulator of the production of glycerophosphocholine (GPC), a precursor of endogenous choline, whose methyl groups are preferentially fluxed into the methionine cycle in the liver. PNPLA7 deficiency in mice markedly decreases hepatic GPC, choline, and several metabolites related to choline/methionine metabolism, leading to various symptoms reminiscent of methionine shortage. Overall metabolic alterations in the liver of Pnpla7-null mice in vivo largely recapitulate those in methionine-deprived hepatocytes in vitro. Reduction of the methyl donor S-adenosylmethionine (SAM) after methionine deprivation decreases the methylation of the PNPLA7 gene promoter, relieves PNPLA7 expression, and thereby increases GPC and choline levels, likely as a compensatory adaptation. In line with the view that SAM prevents the development of liver cancer, the expression of PNPLA7, as well as several enzymes in the choline/methionine metabolism, is reduced in human hepatocellular carcinoma. These findings uncover an unexplored role of a lysophospholipase in hepatic phospholipid catabolism coupled with choline/methionine metabolism.

Hepatic phosphatidylcholine catabolism driven by PNPLA7 and PNPLA8 supplies endogenous choline to replenish the methionine cycle with methyl groups

Choline supplies methyl groups for regeneration of methionine and the methyl donor S-adenosylmethionine in the liver. Here, we report that the catabolism of membrane phosphatidylcholine (PC) into water-soluble glycerophosphocholine (GPC) by the phospholipase/lysophospholipase PNPLA8-PNPLA7 axis enables endogenous choline stored in hepatic PC to be utilized in methyl metabolism. PNPLA7-deficient mice show marked decreases in hepatic GPC, choline, and several metabolites related to the methionine cycle, accompanied by various signs of methionine insufficiency, including growth retardation, hypoglycemia, hypolipidemia, increased energy consumption, reduced adiposity, increased fibroblast growth factor 21 (FGF21), and an altered histone/DNA methylation landscape. Moreover, PNPLA8-deficient mice recapitulate most of these phenotypes. In contrast to wild-type mice fed a methionine/choline-deficient diet, both knockout strains display decreased hepatic triglyceride, likely via reductions of lipogenesis and GPC-derived glycerol flux. Collectively, our findings highlight the biological importance of phospholipid catabolism driven by PNPLA8/PNPLA7 in methyl group flux and triglyceride synthesis in the liver.

Restoring mitochondrial cardiolipin homeostasis reduces cell death and promotes recovery after spinal cord injury

Alterations in phospholipids have long been associated with spinal cord injury (SCI). However, their specific roles and signaling cascades in mediating cell death and tissue repair remain unclear. Here we investigated whether alterations of cardiolipin (CL), a family of mitochondrion-specific phospholipids, play a crucial role in mitochondrial dysfunction and neuronal death following SCI. Lipidomic analysis was used to determine the profile of CL alteration in the adult rat spinal cord following a moderate contusive SCI at the 10th thoracic (T10) level. Cellular, molecular, and genetic assessments were performed to determine whether CL alterations mediate mitochondrial dysfunction and neuronal death after SCI, and, if so, whether reversing CL alteration leads to neuroprotection after SCI. Using lipidomic analysis, we uncovered CL alterations at an early stage of SCI. Over 50 distinct CL species were identified, of which 50% showed significantly decreased abundance after SCI. The decreased CL species contained mainly polyunsaturated fatty acids that are highly susceptible to peroxidation. In parallel, 4-HNE, a lipid peroxidation marker, significantly increased after SCI. We found that mitochondrial oxidative stress not only induced CL oxidation, but also resulted in CL loss by activating cPLA₂ to hydrolyze CL. CL alterations induced mitochondrial dysfunction and neuronal death. Remarkably, pharmacologic inhibition of CL alterations with XJB-5-131, a novel mitochondria-targeted electron and reactive oxygen species scavenger, reduced cell death, tissue damage and ameliorated motor deficits after SCI in adult rats. These findings suggest that CL alteration could be a novel mechanism that mediates injury-induced neuronal death, and a potential therapeutic target for ameliorating secondary SCI.

Cardiolipin Regulates Mitochondrial Ultrastructure and Function in Mammalian Cells

Cardiolipin (CL) is a unique, tetra-acylated diphosphatidylglycerol lipid that mainly localizes in the inner mitochondria membrane (IMM) in mammalian cells and plays a central role in regulating mitochondrial architecture and functioning. A deficiency of CL biosynthesis and remodeling perturbs mitochondrial functioning and ultrastructure. Clinical and experimental studies on human patients and animal models have also provided compelling evidence that an abnormal CL content, acyl chain composition, localization, and level of oxidation may be directly linked to multiple diseases, including cardiomyopathy, neuronal dysfunction, immune cell defects, and metabolic disorders. The central role of CL in regulating the pathogenesis and progression of these diseases has attracted increasing attention in recent years. In this review, we focus on the advances in our understanding of the physiological roles of CL biosynthesis and remodeling from human patients and mouse models, and we provide an overview of the potential mechanism by which CL regulates the mitochondrial architecture and functioning.

SILAC-based quantitative proteomics to investigate the eicosanoid associated inflammatory response in activated macrophages

BACKGROUND

Macrophages play a central role in inflammation by phagocytosing invading pathogens, apoptotic cells and debris, as well as mediating repair of tissues damaged by trauma. In order to do this, these dynamic cells generate a variety of inflammatory mediators including eicosanoids such as prostaglandins, leukotrienes and hydroxyeicosatraenoic acids (HETEs) that are formed through the cyclooxygenase, lipoxygenase and cytochrome P450 pathways. The ability to examine the effects of eicosanoid production at the protein level is therefore critical to understanding the mechanisms associated with macrophage activation.

RESULTS

This study presents a stable isotope labelling with amino acids in cell culture (SILAC) -based proteomics strategy to quantify the changes in macrophage protein abundance following inflammatory stimulation with Kdo2-lipid A and ATP, with a focus on eicosanoid metabolism and regulation. Detailed gene ontology analysis, at the protein level, revealed several key pathways with a decrease in expression in response to macrophage activation, which included a promotion of macrophage polarisation and dynamic changes to energy requirements, transcription and translation. These findings suggest that, whilst there is evidence for the induction of a pro-inflammatory response in the form of prostaglandin secretion, there is also metabolic reprogramming along with a change in cell polarisation towards a reduced pro-inflammatory phenotype.

CONCLUSIONS

Advanced quantitative proteomics in conjunction with functional pathway network analysis is a useful tool to investigate the molecular pathways involved in inflammation.

Cardiac immune cell infiltration associates with abnormal lipid metabolism

CD36 mediates the uptake of long-chain fatty acids (FAs), a major energy substrate for the myocardium. Under excessive FA supply, CD36 can cause cardiac lipid accumulation and inflammation while its deletion reduces heart FA uptake and lipid content and increases glucose utilization. As a result, CD36 was proposed as a therapeutic target for obesity-associated heart disease. However, more recent reports have shown that CD36 deficiency suppresses myocardial flexibility in fuel preference between glucose and FAs, impairing tissue energy balance, while CD36 absence in tissue macrophages reduces efferocytosis and myocardial repair after injury. In line with the latter homeostatic functions, we had previously reported that CD36^-/- mice have chronic subclinical inflammation. Lipids are important for the maintenance of tissue homeostasis and there is limited information on heart lipid metabolism in CD36 deficiency. Here, we document in the hearts of unchallenged CD36^-/- mice abnormalities in the metabolism of triglycerides, plasmalogens, cardiolipins, acylcarnitines, and arachidonic acid, and the altered remodeling of these lipids in response to an overnight fast. The hearts were examined for evidence of inflammation by monitoring the presence of neutrophils and pro-inflammatory monocytes/macrophages using the respective positron emission tomography (PET) tracers, ⁶⁴Cu-AMD3100 and ⁶⁸Ga-DOTA-ECL1i. We detected significant immune cell infiltration in unchallenged CD36^-/- hearts as compared with controls and immune infiltration was also observed in hearts of mice with cardiomyocyte-specific CD36 deficiency. Together, the data show that the CD36^-/- heart is in a non-homeostatic state that could compromise its stress response. Non-invasive immune cell monitoring in humans with partial or total CD36 deficiency could help evaluate the risk of impaired heart remodeling and disease.

Current Knowledge on Mammalian Phospholipase A1, Brief History, Structures, Biochemical and Pathophysiological Roles

Phospholipase A₁ (PLA₁) is an enzyme that cleaves an ester bond at the sn-1 position of glycerophospholipids, producing a free fatty acid and a lysophospholipid. PLA₁ activities have been detected both extracellularly and intracellularly, which are well conserved in higher eukaryotes, including fish and mammals. All extracellular PLA₁s belong to the lipase family. In addition to PLA₁ activity, most mammalian extracellular PLA₁s exhibit lipase activity to hydrolyze triacylglycerol, cleaving the fatty acid and contributing to its absorption into the intestinal tract and tissues. Some extracellular PLA₁s exhibit PLA₁ activities specific to phosphatidic acid (PA) or phosphatidylserine (PS) and serve to produce lysophospholipid mediators such as lysophosphatidic acid (LPA) and lysophosphatidylserine (LysoPS). A high level of PLA₁ activity has been detected in the cytosol fractions, where PA-PLA₁/DDHD1/iPLA₁ was responsible for the activity. Many homologs of PA-PLA₁ and PLA₂ have been shown to exhibit PLA₁ activity. Although much has been learned about the pathophysiological roles of PLA₁ molecules through studies of knockout mice and human genetic diseases, many questions regarding their biochemical properties, including their genuine in vivo substrate, remain elusive.

Mechano-energetic aspects of Barth syndrome

Energy-demanding organs like the heart are strongly dependent on oxidative phosphorylation in mitochondria. Oxidative phosphorylation is governed by the respiratory chain located in the inner mitochondrial membrane. The inner mitochondrial membrane is the only cellular membrane with significant amounts of the phospholipid cardiolipin, and cardiolipin was found to directly interact with a number of essential protein complexes, including respiratory chain complexes I to V. An inherited defect in the biogenesis of cardiolipin causes Barth syndrome, which is associated with cardiomyopathy, skeletal myopathy, neutropenia and growth retardation. Energy conversion is dependent on reducing equivalents, which are replenished by oxidative metabolism in the Krebs cycle. Cardiolipin deficiency in Barth syndrome also affects Krebs cycle activity, metabolite transport and mitochondrial morphology. During excitation-contraction coupling, calcium (Ca²⁺ ) released from the sarcoplasmic reticulum drives sarcomeric contraction. At the same time, Ca²⁺ influx into mitochondria drives the activation of Krebs cycle dehydrogenases and the regeneration of reducing equivalents. Reducing equivalents are essential not only for energy conversion, but also for maintaining a redox buffer, which is required to detoxify reactive oxygen species (ROS). Defects in CL may also affect Ca²⁺ uptake into mitochondria and thereby hamper energy supply and demand matching, but also detoxification of ROS. Here, we review the impact of cardiolipin deficiency on mitochondrial function in Barth syndrome and discuss potential therapeutic strategies.

Studying Lipid-Related Pathophysiology Using the Yeast Model

Saccharomyces cerevisiae, commonly known as baker's yeast, is one of the most comprehensively studied model organisms in science. Yeast has been used to study a wide variety of human diseases, and the yeast model system has proved to be an especially amenable tool for the study of lipids and lipid-related pathophysiologies, a topic that has gained considerable attention in recent years. This review focuses on how yeast has contributed to our understanding of the mitochondrial phospholipid cardiolipin (CL) and its role in Barth syndrome (BTHS), a genetic disorder characterized by partial or complete loss of function of the CL remodeling enzyme tafazzin. Defective tafazzin causes perturbation of CL metabolism, resulting in many downstream cellular consequences and clinical pathologies that are discussed herein. The influence of yeast research in the lipid-related pathophysiologies of Alzheimer's and Parkinson's diseases is also summarized.

Histopathology is required to identify and characterize myopathies in high-throughput phenotype screening of genetically engineered mice

The development of mouse models that replicate the genetic and pathological features of human disease is important in preclinical research because these types of models enable the completion of meaningful pharmacokinetic, safety, and efficacy studies. Numerous relevant mouse models of human disease have been discovered in high-throughput screening programs, but there are important specific phenotypes revealed by histopathology that are not reliably detected by any other physiological or behavioral screening tests. As part of comprehensive phenotypic analyses of over 4000 knockout (KO) mice, histopathology identified 12 lines of KO mice with lesions indicative of an autosomal recessive myopathy. This report includes a brief summary of histological and other findings in these 12 lines. Notably, the inverted screen test detected muscle weakness in only 4 of these 12 lines (Scyl1, Plpp7, Chkb, and Asnsd1), all 4 of which have been previously recognized and published. In contrast, 6 of 8 KO lines showing negative or inconclusive findings on the inverted screen test (Plppr2, Pnpla7, Tenm1, Srpk3, Sidt2, Yif1b, Mrs2, and Pnpla2) had not been previously identified as having myopathies. These findings support the need to include histopathology in phenotype screening protocols in order to identify novel genetic myopathies that are not clinically evident or not detected by the inverted screen test.

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.

Analysis of whole-genome re-sequencing data of ducks reveals a diverse demographic history and extensive gene flow between Southeast/South Asian and Chinese populations

BACKGROUND

The most prolific duck genetic resource in the world is located in Southeast/South Asia but little is known about the domestication and complex histories of these duck populations.

RESULTS

Based on whole-genome resequencing data of 78 ducks (Anas platyrhynchos) and 31 published whole-genome duck sequences, we detected three geographic distinct genetic groups, including local Chinese, wild, and local Southeast/South Asian populations. We inferred the demographic history of these duck populations with different geographical distributions and found that the Chinese and Southeast/South Asian ducks shared similar demographic features. The Chinese domestic ducks experienced the strongest population bottleneck caused by domestication and the last glacial maximum (LGM) period, whereas the Chinese wild ducks experienced a relatively weak bottleneck caused by domestication only. Furthermore, the bottleneck was more severe in the local Southeast/South Asian populations than in the local Chinese populations, which resulted in a smaller effective population size for the former (7100-11,900). We show that extensive gene flow has occurred between the Southeast/South Asian and Chinese populations, and between the Southeast Asian and South Asian populations. Prolonged gene flow was detected between the Guangxi population from China and its neighboring Southeast/South Asian populations. In addition, based on multiple statistical approaches, we identified a genomic region that included three genes (PNPLA8, THAP5, and DNAJB9) on duck chromosome 1 with a high probability of gene flow between the Guangxi and Southeast/South Asian populations. Finally, we detected strong signatures of selection in genes that are involved in signaling pathways of the nervous system development (e.g., ADCYAP1R1 and PDC) and in genes that are associated with morphological traits such as cell growth (e.g., IGF1R).

CONCLUSIONS

Our findings provide valuable information for a better understanding of the domestication and demographic history of the duck, and of the gene flow between local duck populations from Southeast/South Asia and China.

Updating Phospholipase A2 Biology

The phospholipase A2 (PLA2) superfamily contains more than 50 enzymes in mammals that are subdivided into several distinct families on a structural and biochemical basis. In principle, PLA2 has the capacity to hydrolyze the sn-2 position of glycerophospholipids to release fatty acids and lysophospholipids, yet several enzymes in this superfamily catalyze other reactions rather than or in addition to the PLA2 reaction. PLA2 enzymes play crucial roles in not only the production of lipid mediators, but also membrane remodeling, bioenergetics, and body surface barrier, thereby participating in a number of biological events. Accordingly, disturbance of PLA2-regulated lipid metabolism is often associated with various diseases. This review updates the current state of understanding of the classification, enzymatic properties, and biological functions of various enzymes belonging to the PLA2 superfamily, focusing particularly on the novel roles of PLA2s in vivo.

Phospholipase A2 superfamily in cancer

Phospholipase A2 enzymes (PLA₂s) comprise a superfamily that is generally divided into six subfamilies known as cytosolic PLA₂s (cPLA₂s), calcium-independent PLA₂s (iPLA₂s), secreted PLA₂s (sPLA₂s), lysosomal PLA₂s, platelet-activating factor (PAF) acetylhydrolases, and adipose specific PLA₂s. Each subfamily consists of several isozymes that possess PLA₂ activity. The first three PLA₂ subfamilies play important roles in inflammation-related diseases and cancer. In this review, the roles of well-studied enzymes sPLA₂-IIA, cPLA₂α and iPLA₂β in carcinogenesis and cancer development were discussed. sPLA₂-IIA seems to play conflicting roles and can act as a tumor suppressor or a tumor promoter according to the cancer type, but cPLA₂α and iPLA₂β play protumorigenic role in most cancers. The mechanisms of PLA₂-mediated signal transduction and crosstalk between cancer cells and endothelial cells in the tumor microenvironment are described. Moreover, the mechanisms by which PLA₂s mediate lipid reprogramming and glycerophospholipid remodeling in cancer cells are illustrated. PLA₂s as the upstream regulators of the arachidonic acid cascade are generally high expressed and activated in various cancers. Therefore, they can be considered as potential pharmacological targets and biomarkers in cancer. The detailed information summarized in this review may aid in understanding the roles of PLA₂s in cancer, and provide new clues for the development of novel agents and strategies for tumor prevention and treatment.

Mitochondrial diseases: expanding the diagnosis in the era of genetic testing

Mitochondrial diseases are clinically and genetically heterogeneous. These diseases were initially described a little over three decades ago. Limited diagnostic tools created disease descriptions based on clinical, biochemical analytes, neuroimaging, and muscle biopsy findings. This diagnostic mechanism continued to evolve detection of inherited oxidative phosphorylation disorders and expanded discovery of mitochondrial physiology over the next two decades. Limited genetic testing hampered the definitive diagnostic identification and breadth of diseases. Over the last decade, the development and incorporation of massive parallel sequencing has identified approximately 300 genes involved in mitochondrial disease. Gene testing has enlarged our understanding of how genetic defects lead to cellular dysfunction and disease. These findings have expanded the understanding of how mechanisms of mitochondrial physiology can induce dysfunction and disease, but the complete collection of disease-causing gene variants remains incomplete. This article reviews the developments in disease gene discovery and the incorporation of gene findings with mitochondrial physiology. This understanding is critical to the development of targeted therapies.

Gene Deletion of Calcium-Independent Phospholipase A2γ (iPLA2γ) Suppresses Adipogenic Differentiation of Mouse Embryonic Fibroblasts

Adipogenic differentiation is a complex process by which fibroblast-like undifferentiated cells are converted into cells that accumulate lipid droplets. We here investigated the effect of gene deletion of calcium-independent phospholipase A₂γ (iPLA₂γ), a membrane-bound PLA₂ enzyme, on adipogenic differentiation in mice. Since iPLA₂γ knockout (KO) mice showed reduced fat volume and weight, we prepared mouse embryonic fibroblasts (MEF) from wild-type (WT) and iPLA₂γ KO mice and examined the effect of iPLA₂γ deletion on in vitro adipogenic differentiation. iPLA₂γ increased during adipogenic differentiation in WT mouse-derived MEFs, and the differentiation was partially abolished in iPLA₂γ KO-derived MEFs. In KO-derived MEFs, the inductions of peroxisome proliferator activator receptor γ (PPARγ) and CAAT/enhancer-binding protein α (C/EBPα) were also reduced during adipogenic differentiation, and the reductions in PPARγ and C/EBPα expressions and the defect in adipogenesis were restored by treatment with troglitazone, a PPARγ ligand. These results indicate that iPLA₂γ might play a critical role in adipogenic differentiation by regulating PPARγ expression.

Cardiolipin remodeling in Barth syndrome and other hereditary cardiomyopathies

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

12-LOX catalyzes the oxidation of 2-arachidonoyl-lysolipids in platelets generating eicosanoid-lysolipids that are attenuated by iPLA2γ knockout

The canonical pathway of eicosanoid production in most mammalian cells is initiated by phospholipase A2-mediated release of arachidonic acid, followed by its enzymatic oxidation resulting in a vast array of eicosanoid products. However, recent work has demonstrated that the major phospholipase in mitochondria, iPLA2γ (patatin-like phospholipase domain containing 8 (PNPLA8)), possesses sn-1 specificity, with polyunsaturated fatty acids at the sn-2 position generating polyunsaturated sn-2-acyl lysophospholipids. Through strategic chemical derivatization, chiral chromatographic separation, and multistage tandem MS, here we first demonstrate that human platelet-type 12-lipoxygenase (12-LOX) can directly catalyze the regioselective and stereospecific oxidation of 2-arachidonoyl-lysophosphatidylcholine (2-AA-LPC) and 2-arachidonoyl-lysophosphatidylethanolamine (2-AA-LPE). Next, we identified these two eicosanoid-lysophospholipids in murine myocardium and in isolated platelets. Moreover, we observed robust increases in 2-AA-LPC, 2-AA-LPE, and their downstream 12-LOX oxidation products, 12(S)-HETE-LPC and 12(S)-HETE-LPE, in calcium ionophore (A23187)-stimulated murine platelets. Mechanistically, genetic ablation of iPLA2γ markedly decreased the calcium-stimulated production of 2-AA-LPC, 2-AA-LPE, and 12-HETE-lysophospholipids in mouse platelets. Importantly, a potent and selective 12-LOX inhibitor, ML355, significantly inhibited the production of 12-HETE-LPC and 12-HETE-LPE in activated platelets. Furthermore, we found that aging is accompanied by significant changes in 12-HETE-LPC in murine serum that were also markedly attenuated by iPLA2γ genetic ablation. Collectively, these results identify previously unknown iPLA2γ-initiated signaling pathways mediated by direct 12-LOX oxidation of 2-AA-LPC and 2-AA-LPE. This oxidation generates previously unrecognized eicosanoid-lysophospholipids that may serve as biomarkers for age-related diseases and could potentially be used as targets in therapeutic interventions.

Hypothesis: Chemical activity regulates and coordinates the processes maintaining glycerophospholipid homeostasis in mammalian cells

Mammalian cells maintain the complex glycerophospholipid (GPL) class compositions of their various membranes within close limits because this is essential to their well‐being or viability. Surprisingly, however, it is still not understood how those compositions are maintained except that GPL synthesis and degradation are closely coordinated. Here, we hypothesize that abrupt changes in the chemical activity of the individual GPL classes coordinate synthesis and degradation as well other the homeostatic processes. We have previously proposed that only a limited number of “allowed” or “optimal” GPL class compositions exist in cellular membranes because those compositions are energetically more favorable than others, that is, they represent local free energy minima (Somerharju et al 2009, Biochim. Biophys. Acta 1788, 12‐23). This model, however, could not satisfactorily explain how the “optimal” compositions are sensed by the key homeostatic enzymes, that is, rate‐limiting synthetizing enzymes and homeostatic phospholipases. We now hypothesize that when the mole fraction of a GPL class exceeds an optimal value, its chemical activity abruptly increases which (a) increases its propensity to efflux from the membrane thus making it susceptible for hydrolysis by homeostatic phospholipases; (b) increases its potency to inhibit its own biosynthesis via a feedback mechanism; (c) enhances its conversion to another glycerophospholipid class via a novel process termed “head group remodeling” or (d) enhances its translocation to other subcellular membranes. In summary, abrupt change in the chemical activity of the individual GPL classes is proposed to regulate and coordinate those four processes maintaining GPL class homeostasis in mammalian cells.

Genetic Ablation of Calcium-independent Phospholipase A2γ Exacerbates Glomerular Injury in Adriamycin Nephrosis in Mice

Genetic ablation of calcium-independent phospholipase A₂γ (iPLA₂γ) in mice results in marked damage of mitochondria and enhanced autophagy in glomerular visceral epithelial cells (GECs) or podocytes. The present study addresses the role of iPLA₂γ in glomerular injury. In adriamycin nephrosis, deletion of iPLA₂γ exacerbated albuminuria and reduced podocyte number. Glomerular LC3-II increased and p62 decreased in adriamycin-treated iPLA₂γ knockout (KO) mice, compared with treated control, in keeping with increased autophagy in KO. iPLA₂γ KO GECs in culture also demonstrated increased autophagy, compared with control GECs. iPLA₂γ KO GECs showed a reduced oxygen consumption rate and increased phosphorylation of AMP kinase (pAMPK), consistent with mitochondrial dysfunction. Adriamycin further stimulated pAMPK and autophagy. After co-transfection of GECs with mito-YFP (to label mitochondria) and RFP-LC3 (to label autophagosomes), or RFP-LAMP1 (to label lysosomes), there was greater colocalization of mito-YFP with RFP-LC3-II and with RFP-LAMP1 in iPLA₂γ KO GECs, compared with WT, indicating enhanced mitophagy in KO. Adriamycin increased mitophagy in WT cells. Thus, iPLA₂γ has a cytoprotective function in the normal glomerulus and in glomerulopathy, as deletion of iPLA₂γ leads to mitochondrial damage and impaired energy homeostasis, as well as autophagy and mitophagy.

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.

A neurodegenerative mitochondrial disease phenotype due to biallelic loss-of-function variants in PNPLA8 encoding calcium-independent phospholipase A2γ

Animal studies have demonstrated the critical roles of the patatin-like protein family plays in cellular growth, lipid homeostasis, and second messenger signaling the nervous system. Of the nine known calcium-independent phospholipase A2γ, only PNPLA2, PNLPA6, PNPLA9 and most recently a single patient with PNPLA8 are associated with mitochondrial-related neurodegeneration. Using whole exome sequencing, we report two unrelated individuals with variable but similar clinical features of microcephaly, severe global developmental delay, spasticity, lactic acidosis, and progressive cerebellar atrophy with biallelic loss-of-function variants in PNPLA8.

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.

The role of mitochondrial cardiolipin in heart function and its implication in cardiac disease

Mitochondria play an essential role in the energy metabolism of the heart. Many of the essential functions are associated with mitochondrial membranes and oxidative phosphorylation driven by the respiratory chain. Mitochondrial membranes are unique in the cell as they contain the phospholipid cardiolipin. The important role of cardiolipin in cardiovascular health is highlighted by several cardiac diseases, in which cardiolipin plays a fundamental role. Barth syndrome, Sengers syndrome, and Dilated cardiomyopathy with ataxia (DCMA) are genetic disorders, which affect cardiolipin biosynthesis. Other cardiovascular diseases including ischemia/reperfusion injury and heart failure are also associated with changes in the cardiolipin pool. Here, we summarize molecular functions of cardiolipin in mitochondrial biogenesis and morphology. We highlight the role of cardiolipin for the respiratory chain, metabolite carriers, and mitochondrial metabolism and describe links to apoptosis and mitochondria specific autophagy (mitophagy) with possible implications in cardiac disease.

Lysosomal phospholipase A2

Lysosomal phospholipase A2 (PLA2G15) is a ubiquitous enzyme uniquely characterized by a subcellular localization to the lysosome and late endosome. PLA2G15 has an acidic pH optimum, is calcium independent, and acts as a transacylase in the presence of N-acetyl-sphingosine as an acceptor. Recent studies aided by the delineation of the crystal structure of PLA2G15 have clarified further the catalytic mechanism, sn-1 versus sn-2 specificity, and the basis whereby cationic amphiphilic drugs inhibit its activity. PLA2G15 has recently been shown to hydrolyze short chain oxidized phospholipids which access the catalytic site directly based on their aqueous solubility. Studies on the PLA2G15 null mouse suggest a role for the enzyme in the catabolism of pulmonary surfactant. PLA2G15 may also have a role in host defense and in the processing of lipid antigens for presentation by CD1 proteins. This article is part of a Special Issue entitled Novel functions of phospholipase A2 Guest Editors: Makoto Murakami and Gerard Lambeau.

"HETE"ing up mitochondria in human heart failure

A decade of research has established the phospholipase iPLA₂γ as being involved in cardiomyocyte dysfunction and necrosis leading to heart failure, but the mechanisms by which iPLA₂γ acts and its interaction with the mitochondrial permeability transition pore (mPTP) that is critical for cardiac homeostasis are unclear. New investigations by Moon et al. demonstrate that mitochondria in failing hearts undergo dynamic shifts in PLA₂ isoform expression, leading to a redistribution of eicosanoid composition that contributes to pathologic mPTP opening.

Cold-Induced Thermogenesis Depends on ATGL-Mediated Lipolysis in Cardiac Muscle, but Not Brown Adipose Tissue

Fatty acids (FAs) activate and fuel UCP1-mediated non-shivering thermogenesis (NST) in brown adipose tissue (BAT). Release of FAs from intracellular fat stores by adipose triglyceride lipase (ATGL) is considered a key step in NST. Accordingly, the severe cold intolerance of global ATGL knockout (AKO) mice has been attributed to defective BAT lipolysis. Here we show that this conclusion is incorrect. We demonstrate that although the BAT-specific loss of ATGL impairs BAT lipolysis and alters BAT morphology, it does not compromise the β₃-adrenergic thermogenic response or cold-induced NST. Instead, NST depends on nutrient supply or lipolysis in white adipose tissue during fasting, suggesting that circulating energy substrates are sufficient to fuel NST. Cold intolerance in AKO mice is not caused by BAT dysfunction as previously suspected but by severe cardiomyopathy. We conclude that functional NST requires adequate substrate supply and cardiac function, but does not depend on ATGL-mediated lipolysis in BAT.

Role of Cardiolipin in Mitochondrial Signaling Pathways

The phospholipid cardiolipin (CL) is an essential constituent of mitochondrial membranes and plays a role in many mitochondrial processes, including respiration and energy conversion. Pathological changes in CL amount or species composition can have deleterious consequences for mitochondrial function and trigger the production of reactive oxygen species. Signaling networks monitor mitochondrial function and trigger an adequate cellular response. Here, we summarize the role of CL in cellular signaling pathways and focus on tissues with high-energy demand, like the heart. CL itself was recently identified as a precursor for the formation of lipid mediators. We highlight the concept of CL as a signaling platform. CL is exposed to the outer mitochondrial membrane upon mitochondrial stress and CL domains serve as a binding site in many cellular signaling events. During mitophagy, CL interacts with essential players of mitophagy like Beclin 1 and recruits the autophagic machinery by its interaction with LC3. Apoptotic signaling pathways require CL as a binding platform to recruit apoptotic factors such as tBid, Bax, caspase-8. CL required for the activation of the inflammasome and plays a role in inflammatory signaling. As changes in CL species composition has been observed in many diseases, the signaling pathways described here may play a general role in pathology.

Antioxidant tempol suppresses heart cytosolic phospholipase A2α stimulated by chronic intermittent hypoxia

Adaptation to chronic intermittent hypoxia (CIH) is associated with reactive oxygen species (ROS) generation implicated in the improved cardiac tolerance against acute ischemia–reperfusion injury. Phospholipases A2(PLA2s) play an important role in cardiomyocyte phospholipid metabolism influencing membrane homeostasis. Here we aimed to determine the effect of CIH (7000 m, 8 h/day, 5 weeks) on the expression of cytosolic PLA2(cPLA2α), its phosphorylated form (p-cPLA2α), calcium-independent (iPLA2), and secretory (sPLA2IIA) at protein and mRNA levels, as well as fatty acids (FA) profile in left ventricular myocardium of adult male Wistar rats. Chronic administration of antioxidant tempol was used to verify the ROS involvement in CIH effect on PLA2s expression and phospholipid FA remodeling. While CIH did not affect PLA2s mRNA levels, it increased the total cPLA2α protein in cytosol and membranes (by 191% and 38%, respectively) and p-cPLA2α (by 23%) in membranes. On the contrary, both iPLA2and sPLA2IIA were downregulated by CIH. CIH further decreased phospholipid n-6 polyunsaturated FA (PUFA) and increased n-3 PUFA proportion. Tempol treatment prevented only CIH-induced cPLA2α up-regulation and its phosphorylation on Ser505. Our results show that CIH diversely affect myocardial PLA2s and suggest that ROS are responsible for the activation of cPLA2α under these conditions.

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.

Barth syndrome cardiomyopathy

Barth syndrome (BTHS) is an inherited form of cardiomyopathy, caused by a mutation within the gene encoding the mitochondrial transacylase tafazzin. Tafazzin is involved in the biosynthesis of the unique phospholipid cardiolipin (CL), which is almost exclusively found in mitochondrial membranes. CL directly interacts with a number of essential protein complexes in the mitochondrial membranes including the respiratory chain, mitochondrial metabolite carriers, and proteins, involved in shaping mitochondrial morphology. Here we describe, how in BTHS CL deficiency causes changes in the morphology of mitochondria, structural changes in the respiratory chain, decreased respiration, and increased generation of reactive oxygen species. A large number of cellular and animal models for BTHS have been established to elucidate how mitochondrial dysfunction induces sarcomere disorganization and reduced contractility, resulting in dilated cardiomyopathy in vivo.

Lipoquality control by phospholipase A2 enzymes

The phospholipase A₂ (PLA₂) family comprises a group of lipolytic enzymes that typically hydrolyze the sn-2 position of glycerophospholipids to give rise to fatty acids and lysophospholipids. The mammalian genome encodes more than 50 PLA₂s or related enzymes, which are classified into several subfamilies on the basis of their structures and functions. From a general viewpoint, the PLA₂ family has mainly been implicated in signal transduction, producing bioactive lipid mediators derived from fatty acids and lysophospholipids. Recent evidence indicates that PLA₂s also contribute to phospholipid remodeling for membrane homeostasis or energy production for fatty acid β-oxidation. Accordingly, PLA₂ enzymes can be regarded as one of the key regulators of the quality of lipids, which I herein refer to as lipoquality. Disturbance of PLA₂-regulated lipoquality hampers tissue and cellular homeostasis and can be linked to various diseases. Here I overview the current state of understanding of the classification, enzymatic properties, and physiological functions of the PLA₂ family.

Lipidomics for studying metabolism

Many thousands of lipid species exist and their metabolism is interwoven via numerous pathways and networks. These networks can also change in response to cellular environment alterations, such as exercise or development of a disease. Measuring such alterations and understanding the pathways involved is crucial to fully understand cellular metabolism. Such demands have catalysed the emergence of lipidomics, which enables the large-scale study of lipids using the principles of analytical chemistry. Mass spectrometry, largely due to its analytical power and rapid development of new instruments and techniques, has been widely used in lipidomics and greatly accelerated advances in the field. This Review provides an introduction to lipidomics and describes some common, but important, cellular metabolic networks that can aid our understanding of metabolic pathways. Some representative applications of lipidomics for studying lipid metabolism and metabolic diseases are highlighted, as well as future applications for the use of lipidomics in studying metabolic pathways.

SREBP-2/PNPLA8 axis improves non-alcoholic fatty liver disease through activation of autophagy

Dysregulated autophagy is associated with steatosis and non-alcoholic fatty liver disease (NAFLD), however the mechanisms connecting them remain poorly understand. Here, we show that co-administration of lovastatin and ezetimibe (L/E) significantly reverses hepatic triglyceride accumulation concomitant with an increase in SREBP-2 driven autophagy in mice fed a high-fat diet (HFD). We further show that the statin mediated increase in SREBP-2 directly activates expression of patatin-like phospholipase domain-containing enzyme 8 (PNPLA8) gene, and PNPLA8 associates with autophagosomes and is associated with a decrease in cellular triglyceride. Moreover, we show that over-expression of PNPLA8 dramatically decreases hepatic steatosis through increased autophagy in hepatocytes of HFD-fed mice. Live-cell imaging analyses also reveal that PNPLA8 dynamically interacts with LC3 and we suggest that the SREBP-2/PNPLA8 axis represents a novel regulatory mechanism for lipid homeostasis. These data provide a possible mechanism for the reported beneficial effects of statins for decreasing hepatic triglyceride levels in NAFLD patients.

Cyclooxygenase-2 Mediated Oxidation of 2-Arachidonoyl-Lysophospholipids Identifies Unknown Lipid Signaling Pathways

Eicosanoid lipids play important roles in cellular signaling as second messengers in inflammation, immune response, vascular tone, and the CNS. Biosynthesis of eicosanoid lipids proceeds via hydrolysis of esterified arachidonic acid from phospholipids followed by oxidation of the released arachidonic acid by a variety of enzymes including cyclooxygenases (COX). Herein, we demonstrate the remarkable ability of COX-2, but not COX-1, to directly oxidize 2-arachidonoyl-lysolipids, resulting in the generation of previously unknown classes of eicosanoid-lysolipids, and provide evidence that intracellular lipases can release eicosanoids from their eicosanoid-lysolipid precursors. Importantly, genetic ablation of a phospholipase, iPLA₂γ, significantly reduced the amounts of these eicosanoid-lysolipids in murine hepatic tissue and fibroblasts. Furthermore, calcium stimulation of wild-type murine lung fibroblasts produced robust increases in these eicosanoid-lysolipids, which were markedly attenuated in iPLA₂γ^-/- fibroblasts. Collectively, these results identify an iPLA₂γ-initiated pathway generating new classes of lipid metabolites with potential signaling functions resulting from the direct COX-2 catalyzed oxidation of 2-arachidonoyl-lysolipids.

Intramitochondrial phospholipid trafficking

Mitochondrial functions and architecture rely on a defined lipid composition of their outer and inner membranes, which are characterized by a high content of non-bilayer phospholipids such as cardiolipin (CL) and phosphatidylethanolamine (PE). Mitochondrial membrane lipids are synthesized in the endoplasmic reticulum (ER) or within mitochondria from ER-derived precursor lipids, are asymmetrically distributed within mitochondria and can relocate in response to cellular stress. Maintenance of lipid homeostasis thus requires multiple lipid transport processes to be orchestrated within mitochondria. Recent findings identified members of the Ups/PRELI family as specific lipid transfer proteins in mitochondria that shuttle phospholipids between mitochondrial membranes. They cooperate with membrane organizing proteins that preserve the spatial organization of mitochondrial membranes and the formation of membrane contact sites, unravelling an intimate crosstalk of membrane lipid transport and homeostasis with the structural organization of mitochondria. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

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.

Mitochondrial acyltransferases and glycerophospholipid metabolism

Our understanding of the synthesis and remodeling of mitochondrial phospholipids remains incomplete. Two isoforms of glycerol-3-phosphate acyltransferase (GPAT1 and 2) and two isoforms of acylglycerol-3-phosphate acyltransferase (AGPAT4 and 5) are located on the outer mitochondrial membrane, suggesting that both lysophosphatidic acid and phosphatidic acid are synthesized in situ for de novo glycerolipid biosynthesis. However, it is believed that the phosphatidic acid substrate for cardiolipin and phosphatidylethanolamine biosynthesis is produced at the endoplasmic reticulum whereas the phosphatidic acid synthesized in the mitochondria must be transferred to the endoplasmic reticulum before it undergoes additional steps to form the mature phospholipids that are trafficked back to the mitochondria. It is unclear whether mitochondrial phospholipids are remodeled by mitochondrial acyltransferases or whether lysophospholipids must return to the endoplasmic reticulum or to the mitochondrial associated membrane for reesterification. In this review we will focus on the few glycerolipid acyltransferases that are known to be mitochondrial. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Molecular species composition of plant cardiolipin determined by liquid chromatography mass spectrometry

Cardiolipin (CL), an anionic phospholipid of the inner mitochondrial membrane, provides essential functions for stabilizing respiratory complexes and is involved in mitochondrial morphogenesis and programmed cell death in animals. The role of CL and its metabolism in plants are less well understood. The measurement of CL in plants, including its molecular species composition, is hampered by the fact that CL is of extremely low abundance, and that plants contain large amounts of interfering compounds including galactolipids, neutral lipids, and pigments. We used solid phase extraction by anion exchange chromatography to purify CL from crude plant lipid extracts. LC/MS was used to determine the content and molecular species composition of CL. Thus, up to 23 different molecular species of CL were detected in different plant species, including Arabidopsis, mung bean, spinach, barley, and tobacco. Similar to animals, plant CL is dominated by highly unsaturated species, mostly containing linoleic and linolenic acid. During phosphate deprivation or exposure to an extended dark period, the amount of CL decreased in Arabidopsis, accompanied with an increased degree in unsaturation. The mechanism of CL remodeling during stress, and the function of highly unsaturated CL molecular species, remains to be defined.

Genetic Ablation of Calcium-independent Phospholipase A2γ Induces Glomerular Injury in Mice

Glomerular visceral epithelial cells (podocytes) play a critical role in the maintenance of glomerular permselectivity. Podocyte injury, manifesting as proteinuria, is the cause of many glomerular diseases. We reported previously that calcium-independent phospholipase A2γ (iPLA2γ) is cytoprotective against complement-mediated glomerular epithelial cell injury. Studies in iPLA2γ KO mice have demonstrated an important role for iPLA2γ in mitochondrial lipid turnover, membrane structure, and metabolism. The aim of the present study was to employ iPLA2γ KO mice to better understand the role of iPLA2γ in normal glomerular and podocyte function as well as in glomerular injury. We show that deletion of iPLA2γ did not cause detectable albuminuria; however, it resulted in mitochondrial structural abnormalities and enhanced autophagy in podocytes as well as loss of podocytes in aging KO mice. Moreover, after induction of anti-glomerular basement membrane nephritis in young mice, iPLA2γ KO mice exhibited significantly increased levels of albuminuria, podocyte injury, and loss of podocytes compared with wild type. Thus, iPLA2γ has a protective functional role in the normal glomerulus and in glomerulonephritis. Understanding the role of iPLA2γ in glomerular pathophysiology provides opportunities for the development of novel therapeutic approaches to glomerular injury and proteinuria.

Confirmation of a non-synonymous SNP in PNPLA8 as a candidate causal mutation for Weaver syndrome in Brown Swiss cattle

Background

Bovine progressive degenerative myeloencephalopathy (Weaver syndrome) is a neurodegenerative disorder in Brown Swiss cattle that is characterized by progressive hind leg weakness and ataxia, while sensorium and spinal reflexes remain unaffected. Although the causal mutation has not been identified yet, an indirect genetic test based on six microsatellite markers and consequent exclusion of Weaver carriers from breeding have led to the complete absence of new cases for over two decades. Evaluation of disease status by imputation of 41 diagnostic single nucleotide polymorphisms (SNPs) and a common haplotype published in 2013 identified several suspected carriers in the current breeding population, which suggests a higher frequency of the Weaver allele than anticipated. In order to prevent the reemergence of the disease, this study aimed at mapping the gene that underlies Weaver syndrome and thus at providing the basis for direct genetic testing and monitoring of today’s Braunvieh/Brown Swiss herds.

Results

Combined linkage/linkage disequilibrium mapping on Bos taurus chromosome (BTA) 4 based on Illumina Bovine SNP50 genotypes of 43 Weaver-affected, 31 Weaver carrier and 86 Weaver-free animals resulted in a maximum likelihood ratio test statistic value at position 49,812,384 bp. The confidence interval (0.853 Mb) determined by the 2-LOD drop-off method was contained within a 1.72-Mb segment of extended homozygosity. Exploitation of whole-genome sequence data from two official Weaver carriers and 1145 other bulls that were sequenced in Run4 of the 1000 bull genomes project showed that only a non-synonymous SNP (rs800397662) within the PNPLA8 gene at position 49,878,773 bp was concordant with the Weaver carrier status. Targeted SNP genotyping confirmed this SNP as a candidate causal mutation for Weaver syndrome. Genotyping for the candidate causal mutation in a random sample of 2334 current Braunvieh animals suggested a frequency of the Weaver allele of 0.26 %.

Conclusions

Through combined use of exhaustive sequencing data and SNP genotyping results, we were able to provide evidence that supports the non-synonymous mutation at position 49,878,773 bp as the most likely causal mutation for Weaver syndrome. Further studies are needed to uncover the exact mechanisms that underlie this syndrome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-016-0201-5) contains supplementary material, which is available to authorized users.

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

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