The expression and function of a group VIA calcium-independent phospholipase A2 (iPLA2beta) in beta-cells

The role of the PLA2G6 gene in neurodegenerative diseases

PLA2G6-associated neurodegeneration (PLAN) represents a continuum of clinically and genetically heterogeneous neurodegenerative disorders with overlapping features. Usually, it encompasses three autosomal recessive diseases, including infantile neuroaxonal dystrophy or neurodegeneration with brain iron accumulation (NBIA) 2A, atypical neuronal dystrophy with childhood-onset or NBIA2B, and adult-onset dystonia-parkinsonism form named PARK14, and possibly a certain subtype of hereditary spastic paraplegia. PLAN is caused by variants in the phospholipase A2 group VI gene (PLA2G6), which encodes an enzyme involved in membrane homeostasis, signal transduction, mitochondrial dysfunction, and α-synuclein aggregation. In this review, we discuss PLA2G6 gene structure and protein, functional findings, genetic deficiency models, various PLAN disease phenotypes, and study strategies in the future. Our primary aim is to provide an overview of genotype-phenotype correlations of PLAN subtypes and speculate on the role of PLA2G6 in potential mechanisms underlying these conditions.

Noncanonical Regulation of cAMP-Dependent Insulin Secretion and Its Implications in Type 2 Diabetes

Impaired glucose tolerance (IGT) and β-cell dysfunction in insulin resistance associated with obesity lead to type 2 diabetes (T2D). Glucose-stimulated insulin secretion (GSIS) from β-cells occurs via a canonical pathway that involves glucose metabolism, ATP generation, inactivation of K ATP channels, plasma membrane depolarization, and increases in cytosolic concentrations of [Ca 2+ ] c . However, optimal insulin secretion requires amplification of GSIS by increases in cyclic adenosine monophosphate (cAMP) signaling. The cAMP effectors protein kinase A (PKA) and exchange factor activated by cyclic-AMP (Epac) regulate membrane depolarization, gene expression, and trafficking and fusion of insulin granules to the plasma membrane for amplifying GSIS. The widely recognized lipid signaling generated within β-cells by the β-isoform of Ca 2+ -independent phospholipase A 2 enzyme (iPLA 2 β) participates in cAMP-stimulated insulin secretion (cSIS). Recent work has identified the role of a G-protein coupled receptor (GPCR) activated signaling by the complement 1q like-3 (C1ql3) secreted protein in inhibiting cSIS. In the IGT state, cSIS is attenuated, and the β-cell function is reduced. Interestingly, while β-cell-specific deletion of iPLA 2 β reduces cAMP-mediated amplification of GSIS, the loss of iPLA 2 β in macrophages (MØ) confers protection against the development of glucose intolerance associated with diet-induced obesity (DIO). In this article, we discuss canonical (glucose and cAMP) and novel noncanonical (iPLA 2 β and C1ql3) pathways and how they may affect β-cell (dys)function in the context of impaired glucose intolerance associated with obesity and T2D. In conclusion, we provide a perspective that in IGT states, targeting noncanonical pathways along with canonical pathways could be a more comprehensive approach for restoring β-cell function in T2D. © 2023 American Physiological Society. Compr Physiol 13:5023-5049, 2023.

Neurodegenerative Disorders: Spotlight on Sphingolipids

Neurodegenerative diseases are incurable diseases of the nervous system that lead to a progressive loss of brain areas and neuronal subtypes, which is associated with an increase in symptoms that can be linked to the affected brain areas. The key findings that appear in many neurodegenerative diseases are deposits of proteins and the damage of mitochondria, which mainly affect energy production and mitophagy. Several causative gene mutations have been identified in various neurodegenerative diseases; however, a large proportion are considered sporadic. In the last decade, studies linking lipids, and in particular sphingolipids, to neurodegenerative diseases have shown the importance of these sphingolipids in the underlying pathogenesis. Sphingolipids are bioactive lipids consisting of a sphingoid base linked to a fatty acid and a hydrophilic head group. They are involved in various cellular processes, such as cell growth, apoptosis, and autophagy, and are an essential component of the brain. In this review, we will cover key findings that demonstrate the relevance of sphingolipids in neurodegenerative diseases and will focus on neurodegeneration with brain iron accumulation and Parkinson’s disease.

Pluripotent Stem Cell Derived Neurons as In Vitro Models for Studying Autosomal Recessive Parkinson's Disease (ARPD): PLA2G6 and Other Gene Loci

Parkinson's disease (PD) is a neurodegenerative motor disorder which is largely sporadic; however, some familial forms have been identified. Genetic PD can be inherited by autosomal, dominant or recessive mutations. While the dominant mutations mirror the prototype of PD with adult-onset and L-dopa-responsive cases, autosomal recessive PD (ARPD) exhibit atypical phenotypes with additional clinical manifestations. Young-onset PD is also very common with mutations in recessive gene loci. The main genes associated with ARPD are Parkin, PINK1, DJ-1, ATP13A2, FBXO7 and PLA2G6. Calcium dyshomeostasis is a mainstay in all types of PD, be it genetic or sporadic. Intriguingly, calcium imbalances manifesting as altered Store-Operated Calcium Entry (SOCE) is suggested in PLA2G6-linked PARK 14 PD. The common pathways underlying ARPD pathology, including mitochondrial abnormalities and autophagic dysfunction, can be investigated ex vivo using induced pluripotent stem cell (iPSC) technology and are discussed here. PD pathophysiology is not faithfully replicated by animal models, and, therefore, nigral dopaminergic neurons generated from iPSC serve as improved human cellular models. With no cure to date and treatments aiming at symptomatic relief, these in vitro models derived through midbrain floor-plate induction provide a platform to understand the molecular and biochemical pathways underlying PD etiology in a patient-specific manner.

The structure of iPLA2β reveals dimeric active sites and suggests mechanisms of regulation and localization

Calcium-independent phospholipase A₂β (iPLA₂β) regulates important physiological processes including inflammation, calcium homeostasis and apoptosis. It is genetically linked to neurodegenerative disorders including Parkinson’s disease. Despite its known enzymatic activity, the mechanisms underlying iPLA₂β-induced pathologic phenotypes remain poorly understood. Here, we present a crystal structure of iPLA₂β that significantly revises existing mechanistic models. The catalytic domains form a tight dimer. They are surrounded by ankyrin repeat domains that adopt an outwardly flared orientation, poised to interact with membrane proteins. The closely integrated active sites are positioned for cooperative activation and internal transacylation. The structure and additional solution studies suggest that both catalytic domains can be bound and allosterically inhibited by a single calmodulin. These features suggest mechanisms of iPLA₂β cellular localization and activity regulation, providing a basis for inhibitor development. Furthermore, the structure provides a framework to investigate the role of neurodegenerative mutations and the function of iPLA₂β in the brain.

Calcium-independent phospholipase A₂β (iPLA₂β) is involved in many physiological and pathological processes but the underlying mechanisms are largely unknown. Here, the authors present the structure of dimeric iPLA₂β, providing insights into the regulation of its activity and cellular localization.

Pla2g6 Deficiency in Zebrafish Leads to Dopaminergic Cell Death, Axonal Degeneration, Increased β-Synuclein Expression, and Defects in Brain Functions and Pathways

This study aimed to gain insights into the pathophysiology underlying PLA2G6-associated neurodegeneration that is implicated in three different neurological disorders, suggesting that other, unknown genetic or environmental factors might contribute to its wide phenotypic expression. To accomplish this, we downregulated the function of pla2g6 in the zebrafish nervous system, performed parkinsonism-related phenotypic characterization, and determined the effects of gene regulation upon the loss of pla2g6 function by using RNA sequencing and downstream analyses. Pla2g6 deficiency resulted in axonal degeneration, dopaminergic and motor neuron cell loss, and increased β-synuclein expression. We also observed that many of the identified, differentially expressed genes were implicated in other brain disorders, which might explain the variable phenotypic expression of pla2g6-associated disease, and found that top enriched canonical pathways included those already known or suggested to play a major role in the pathogenesis of Parkinson's disease. Our data support that pla2g6 is relevant for cranial motor development with significant implications in the pathophysiology underlying Parkinson's disease.

Phospholipase A2 as a Molecular Determinant of Store-Operated Calcium Entry

Activation of phospholipases A2 (PLA2) leads to the generation of biologically active lipid products that can affect numerous cellular events. Ca(2+)-independent PLA2 (iPLA2), also called group VI phospholipase A2, is one of the main types forming the superfamily of PLA2. Beside of its role in phospholipid remodeling, iPLA2 has been involved in intracellular Ca(2+) homeostasis regulation. Several studies proposed iPLA2 as an essential molecular player of store operated Ca(2+) entry (SOCE) in a large number of excitable and non-excitable cells. iPLA2 activation releases lysophosphatidyl products, which were suggested as agonists of store operated calcium channels (SOCC) and other TRP channels. Herein, we will review the important role of iPLA2 on the intracellular Ca(2+) handling focusing on its role in SOCE regulation and its implication in physiological and/or pathological processes.

Calcium-independent phospholipases A2 in murine osteoblastic cells and their inhibition by bromoenol lactone: impact on arachidonate dynamics and prostaglandin synthesis

CONTEXT

Bromoenol lactone (BEL) is an inhibitor of group VI phospholipases (iPLA2s), but has been shown to have severe side effects.

OBJECTIVE

iPLA2 characterization in osteoblasts and effect of BEL on prostaglandin (PG) E2 formation.

METHODS

iPLA2 expression: RT-PCR, Western Blotting. PGE2 formation: GC-MS after stimulation, treatment with inhibitors or gene silencing. Arachidonate (AA) reacylation into phospholipids, inhibitor reaction products, PGHS-1 modification proteomic analysis: HR-LC-MS/MS. AA accumulation: (14)C-AA.

RESULTS

iPLA2ß and iPLA2γ were expressed and functionally active. BEL inhibition up to 20 μM caused AA accumulation and enhanced PGE2 formation, followed by a decrease at higher concentrations. BEL reacted with intracellular cysteine and GSH leading to GSH depletion and oxidative stress.

DISCUSSION

Initial PGE2 enhancement after BEL inhibition is due to iPLA2-independent accumulation of AA. GSH depletion caused by high BEL concentrations is responsible for the decrease in PGE2 production.

CONCLUSION

BEL must be used with caution in a cellular environment due to conditions of extreme oxidative stress.

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.

Signaling properties of 4-hydroxyalkenals formed by lipid peroxidation in diabetes

Peroxidation of polyunsaturated fatty acids is intensified in cells subjected to oxidative stress and results in the generation of various bioactive compounds, of which 4-hydroxyalkenals are prominent. During the progression of type 2 diabetes mellitus, the ensuing hyperglycemia promotes the generation of reactive oxygen species (ROS) that contribute to the development of diabetic complications. It has been suggested that ROS-induced lipid peroxidation and the resulting 4-hydroxyalkenals markedly contribute to the development and progression of these pathologies. Recent findings, however, also suggest that noncytotoxic levels of 4-hydroxyalkenals play important signaling functions in the early phase of diabetes and act as hormetic factors to induce adaptive and protective responses in cells, enabling them to function in the hyperglycemic milieu. Our studies and others' have proposed such regulatory functions for 4-hydroxynonenal and 4-hydroxydodecadienal in insulin secreting β-cells and vascular endothelial cells, respectively. This review presents and discusses the mechanisms regulating the generation of 4-hydroxyalkenals under high glucose conditions and the molecular interactions underlying the reciprocal transition from hormetic to cytotoxic agents.

Recessive truncating NALCN mutation in infantile neuroaxonal dystrophy with facial dysmorphism

BACKGROUND

Infantile neuroaxonal dystrophy (INAD) is a recessive disease that results in total neurological degeneration and death in childhood. PLA2G6 mutation is the underlying genetic defect, but rare genetic heterogeneity has been demonstrated. One of the five families we studied did not link to PLA2G6 locus, and in the family one of the two affected siblings additionally had atypical features including facial dysmorphism, pectus carinatum, scoliosis, pes varus, zygodactyly and bilateral cryptorchidism as well as cerebellar atrophy, as previously reported.

METHODS

Sural biopsy was investigated by electron microscopy. PLA2G6 was screened for mutations by Sanger sequencing. In the mutation-free family, candidate disease loci were found via linkage analysis using data from single nucleotide polymorphism genome scans. Exome sequencing was applied to find the variants at the loci.

RESULTS

PLA2G6 mutations were identified in four families including the one with an unusually severe phenotype that led to death within the first 2 years of life. In the remaining family, seven candidate loci totalling 15.2 Mb were found and a homozygous truncating mutation p.Q642X was identified in NALCN at 13q32.3. The patients are around 20-years-old.

CONCLUSIONS

NALCN is the gene responsible for INAD with facial dysmorphism. The patients have lived to adulthood despite severe growth and neuromotor retardation. NALCN forms a voltage-independent ion channel with a role in the regulation of neuronal excitability. Our findings broaden the spectrum of genes associated with neuroaxonal dystrophy. Testing infants with idiopathic severe growth retardation and neurodegeneration for NALCN mutations could benefit families.

Group VIA PLA2 (iPLA2β) is activated upstream of p38 mitogen-activated protein kinase (MAPK) in pancreatic islet β-cell signaling

Group VIA phospholipase A(2) (iPLA(2)β) in pancreatic islet β-cells participates in glucose-stimulated insulin secretion and sarco(endo)plasmic reticulum ATPase (SERCA) inhibitor-induced apoptosis, and both are attenuated by pharmacologic or genetic reductions in iPLA(2)β activity and amplified by iPLA(2)β overexpression. While exploring signaling events that occur downstream of iPLA(2)β activation, we found that p38 MAPK is activated by phosphorylation in INS-1 insulinoma cells and mouse pancreatic islets, that this increases with iPLA(2)β expression level, and that it is stimulated by the iPLA(2)β reaction product arachidonic acid. The insulin secretagogue D-glucose also stimulates β-cell p38 MAPK phosphorylation, and this is prevented by the iPLA(2)β inhibitor bromoenol lactone. Insulin secretion induced by d-glucose and forskolin is amplified by overexpressing iPLA(2)β in INS-1 cells and in mouse islets, and the p38 MAPK inhibitor PD169316 prevents both responses. The SERCA inhibitor thapsigargin also stimulates phosphorylation of both β-cell MAPK kinase isoforms and p38 MAPK, and bromoenol lactone prevents both events. Others have reported that iPLA(2)β products activate Rho family G-proteins that promote MAPK kinase activation via a mechanism inhibited by Clostridium difficile toxin B, which we find to inhibit thapsigargin-induced β-cell p38 MAPK phosphorylation. Thapsigargin-induced β-cell apoptosis and ceramide generation are also prevented by the p38 MAPK inhibitor PD169316. These observations indicate that p38 MAPK is activated downstream of iPLA(2)β in β-cells incubated with insulin secretagogues or thapsigargin, that this requires prior iPLA(2)β activation, and that p38 MAPK is involved in the β-cell functional responses of insulin secretion and apoptosis in which iPLA(2)β participates.

Epac2-dependent rap1 activation and the control of islet insulin secretion by glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) binds its Class II G protein-coupled receptor to stimulate cyclic adenosine monophosphate (cAMP) production and to potentiate the glucose metabolism-dependent secretion of insulin from pancreatic β cells located within the islets of Langerhans. Prior clinical studies demonstrate that this cAMP-mediated action of GLP-1 to potentiate glucose-stimulated insulin secretion (GSIS) is of major therapeutic importance when evaluating the abilities of GLP-1 receptor (GLP-1R) agonists to lower levels of blood glucose in type 2 diabetic subjects. Surprisingly, recent in vitro studies of human or rodent islets of Langerhans provide evidence for the existence of a noncanonical mechanism of β cell cAMP signal transduction, one that may explain how GLP-1R agonists potentiate GSIS. What these studies demonstrate is that a cAMP-regulated guanine nucleotide exchange factor designated as Epac2 couples β cell cAMP production to the protein kinase A-independent stimulation of insulin exocytosis. Provided here is an overview of the Epac2 signal transduction system in β cells, with special emphasis on Rap1, a Ras-related GTPase that is an established target of Epac2.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

A link between endoplasmic reticulum stress-induced β-cell apoptosis and the group VIA Ca2+-independent phospholipase A2 (iPLA2β)

Endoplasmic reticulum (ER) stress is becoming recognized as an important contributing factor in various diseases, including diabetes mellitus. Prolonged ER stress can cause β-cell apoptosis; however, the underlying mechanism(s) that contribute to this process are not well understood. Early reports suggested that arachidonic acid metabolites and a Ca(2+)-independent phospholipase A(2) (iPLA(2)) activity play a role in β-cell apoptosis. The PLA(2) family of enzymes catalyse the hydrolysis of the sn-2 substituent (i.e. arachidonic acid) of membrane phospholipids. In light of our findings that the pancreatic islet β-cells are enriched in arachidonate-containing phospholipids and express the group VIA iPLA(2)β, we considered the possibility that iPLA(2)β participates in ER stress-induced β-cell apoptosis. Our work revealed a novel mechanism, involving ceramide generation and triggering of mitochondrial abnormalities, by which iPLA(2)β participates in the β-cell apoptosis process. Here, we review our evidence linking ER stress, β-cell apoptosis and iPLA(2)β. Continued studies in this area will increase our understanding of the contribution of iPLA(2)β to the evolution of diabetes mellitus and will further our knowledge of factors that influence β-cell health in diabetes mellitus and identify potential targets for future therapeutic interventions to prevent β-cell death.

Effects of endoplasmic reticulum stress on group VIA phospholipase A2 in beta cells include tyrosine phosphorylation and increased association with calnexin

The Group VIA phospholipase A(2) (iPLA(2)β) hydrolyzes glycerophospholipids at the sn-2-position to yield a free fatty acid and a 2-lysophospholipid, and iPLA(2)β has been reported to participate in apoptosis, phospholipid remodeling, insulin secretion, transcriptional regulation, and other processes. Induction of endoplasmic reticulum (ER) stress in β-cells and vascular myocytes with SERCA inhibitors activates iPLA(2)β, resulting in hydrolysis of arachidonic acid from membrane phospholipids, by a mechanism that is not well understood. Regulatory proteins interact with iPLA(2)β, including the Ca(2+)/calmodulin-dependent protein kinase IIβ, and we have characterized the iPLA(2)β interactome further using affinity capture and LC/electrospray ionization/MS/MS. An iPLA(2)β-FLAG fusion protein was expressed in an INS-1 insulinoma cell line and then adsorbed to an anti-FLAG matrix after cell lysis. iPLA(2)β and any associated proteins were then displaced with FLAG peptide and analyzed by SDS-PAGE. Gel sections were digested with trypsin, and the resultant peptide mixtures were analyzed by LC/MS/MS with database searching. This identified 37 proteins that associate with iPLA(2)β, and nearly half of them reside in ER or mitochondria. They include the ER chaperone calnexin, whose association with iPLA(2)β increases upon induction of ER stress. Phosphorylation of iPLA(2)β at Tyr(616) also occurs upon induction of ER stress, and the phosphoprotein associates with calnexin. The activity of iPLA(2)β in vitro increases upon co-incubation with calnexin, and overexpression of calnexin in INS-1 cells results in augmentation of ER stress-induced, iPLA(2)β-catalyzed hydrolysis of arachidonic acid from membrane phospholipids, reflecting the functional significance of the interaction. Similar results were obtained with mouse pancreatic islets.

[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.

Lipid mediators in health and disease: enzymes and receptors as therapeutic targets for the regulation of immunity and inflammation

Prostaglandins, leukotrienes, platelet-activating factor, lysophosphatidic acid, sphingosine 1-phosphate, and endocannabinoids, collectively referred to as lipid mediators, play pivotal roles in immune regulation and self-defense, and in the maintenance of homeostasis in living systems. They are produced by multistep enzymatic pathways, which are initiated by the de-esterification of membrane phospholipids by phospholipase A2s or sphingo-myelinase. Lipid mediators exert their biological effects by binding to cognate receptors, which are members of the G protein-coupled receptor superfamily. The synthesis of the lipid mediators and subsequent induction of receptor activity is tightly regulated under normal physiological conditions, and enzyme and/or receptor dysfunction can lead to a variety of disease conditions. Thus, the manipulation of lipid mediator signaling, through either enzyme inhibitors or receptor antagonists and agonists, has great potential as a therapeutic approach to disease. In this review, I summarize our current state of knowledge of the synthesis of lipid mediators and the function of their cognate receptors, and discuss the effects of genetic or pharmacological ablation of enzyme or receptor function on various pathophysiological processes.

Intracellular- and extracellular-derived Ca(2+) influence phospholipase A(2)-mediated fatty acid release from brain phospholipids

Docosahexaenoic acid (DHA) and arachidonic acid (AA) are found in high concentrations in brain cell membranes and are important for brain function and structure. Studies suggest that AA and DHA are hydrolyzed selectively from the sn-2 position of synaptic membrane phospholipids by Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) and Ca(2+)-independent phospholipase A(2) (iPLA(2)), respectively, resulting in increased levels of the unesterified fatty acids and lysophospholipids. Cell studies also suggest that AA and DHA release depend on increased concentrations of Ca(2+), even though iPLA(2) has been thought to be Ca(2+)-independent. The source of Ca(2+) for activation of cPLA(2) is largely extracellular, whereas Ca(2+) released from the endoplasmic reticulum can activate iPLA(2) by a number of mechanisms. This review focuses on the role of Ca(2+) in modulating cPLA(2) and iPLA(2) activities in different conditions. Furthermore, a model is suggested in which neurotransmitters regulate the activity of these enzymes and thus the balanced and localized release of AA and DHA from phospholipid in the brain, depending on the primary source of the Ca(2+) signal.

Calcium-independent phospholipase A2 (iPLA2 beta)-mediated ceramide generation plays a key role in the cross-talk between the endoplasmic reticulum (ER) and mitochondria during ER stress-induced insulin-secreting cell apoptosis

Endoplasmic reticulum (ER) stress induces INS-1 cell apoptosis by a pathway involving Ca(2+)-independent phospholipase A(2) (iPLA(2)beta)-mediated ceramide generation, but the mechanism by which iPLA(2)beta and ceramides contribute to apoptosis is not well understood. We report here that both caspase-12 and caspase-3 are activated in INS-1 cells following induction of ER stress with thapsigargin, but only caspase-3 cleavage is amplified in iPLA(2)beta overexpressing INS-1 cells (OE), relative to empty vector-transfected cells, and is suppressed by iPLA(2)beta inhibition. ER stress also led to the release of cytochrome c and Smac and, unexpectedly, their accumulation in the cytosol is amplified in OE cells. These findings raise the likelihood that iPLA(2)beta participates in ER stress-induced apoptosis by activating the intrinsic apoptotic pathway. Consistent with this possibility, we find that ER stress promotes iPLA(2)beta accumulation in the mitochondria, opening of mitochondrial permeability transition pore, and loss in mitochondrial membrane potential (Delta Psi) in INS-1 cells and that these changes are amplified in OE cells. ER stress also led to greater ceramide generation in ER and mitochondria fractions of OE cells. Exposure to ceramide alone induces loss in Delta Psi and apoptosis and these are suppressed by forskolin. ER stress-induced mitochondrial dysfunction and apoptosis are also inhibited by forskolin, as well as by inactivation of iPLA(2)beta or NSMase, suggesting that iPLA(2)beta-mediated generation of ceramides via sphingomyelin hydrolysis during ER stress affect the mitochondria. In support, inhibition of iPLA(2)beta or NSMase prevents cytochrome c release. Collectively, our findings indicate that the iPLA(2)beta-ceramide axis plays a critical role in activating the mitochondrial apoptotic pathway in insulin-secreting cells during ER stress.

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.

Orai, STIM1 and iPLA2beta: a view from a different perspective

The mechanism of store-operated Ca(2+) entry (SOCE) remains one of the intriguing mysteries in the field of Ca(2+) signalling. Recent discoveries have resulted in the molecular identification of STIM1 as a Ca(2+) sensor in endoplasmic reticulum, Orai1 (CRACM1) as a plasma membrane channel that is activated by the store-operated pathway, and iPLA(2)beta as an essential component of signal transduction from the stores to the plasma membrane channels. Numerous studies have confirmed that molecular knock-down of any one of these three molecules impair SOCE in a wide variety of cell types, but their mutual relations are far from being understood. This report will focus on the functional roles of Orai1, STIM1 and iPLA(2)beta, and will address some specific questions about Orai1 and TRPC1, and their relation to SOC channels in excitable and non-excitable cells. Also, it will analyse the novel role of STIM1 as a trigger for CIF production, and the complex relationship between STIM1 and Orai1 expression, puncta formation and SOCE activation. It will highlight some of the most recent findings that may challenge simple conformational coupling models of SOCE, and will offer some new perspectives on the complex relationships between Orai1, STIM1 and iPLA(2)beta in the SOCE pathway.

Age-related changes in bone morphology are accelerated in group VIA phospholipase A2 (iPLA2beta)-null mice

Phospholipases A(2) (PLA(2)) hydrolyze the sn-2 fatty acid substituent, such as arachidonic acid, from phospholipids, and arachidonate metabolites are recognized mediators of bone modeling. We have previously generated knockout (KO) mice lacking the group VIA PLA(2) (iPLA(2)beta), which participates in a variety of signaling events; iPLA(2)beta mRNA is expressed in bones of wild-type (WT) but not KO mice. Cortical bone size, trabecular bone volume, bone mineralizing surfaces, and bone strength are similar in WT and KO mice at 3 months and decline with age in both groups, but the decreases are more pronounced in KO mice. The lower bone mass phenotype observed in KO mice is not associated with an increase in osteoclast abundance/activity or a decrease in osteoblast density, but is accompanied by an increase in bone marrow fat. Relative to WT mice, undifferentiated bone marrow stromal cells (BMSCs) from KO mice express higher levels of PPAR-gamma and lower levels of Runx2 mRNA, and this correlates with increased adipogenesis and decreased osteogenesis in BMSCs from these mice. In summary, our studies indicate that age-related losses in bone mass and strength are accelerated in iPLA(2)beta-null mice. Because adipocytes and osteoblasts share a common mesenchymal stem cell origin, our findings suggest that absence of iPLA(2)beta causes abnormalities in osteoblast function and BMSC differentiation and identify a previously unrecognized role of iPLA(2)beta in bone formation.

Induction of group VIA phospholipase A2activity during in vitro ischemia in C2C12 myotubes is associated with changes in the level of its splice variants

The involvement of group VI Ca2+-independent PLA2s (iPLA2-VI) in in vitro ischemia [oxygen and glucose deprivation (OGD)] in mouse C2C12 myotubes was investigated. OGD induced a time-dependent (0–6 h) increase in bromoenol lactone (BEL)-sensitive iPLA2activity, which was suppressed by specific short interfering (si)RNA knockdown of iPLA2-VIA. OGD was associated with an increase in iPLA2-VIA protein levels, whereas mRNA levels were unchanged. The levels of iPLA2-VIB mRNA and protein were not increased by OGD. RT-PCR and Western blot analysis identified a mouse iPLA2-VIA homolog to catalytically inactive 50-kDa iPLA2-VIA-ankyrin variants previously identified in humans. Both the mRNA and protein levels of this ∼50-kDa variant were reduced significantly within 1 h following OGD. In C2C12 myoblasts, iPLA2-VIA seemed to predominantly reside at the endoplasmatic reticulum, where it accumulated further during OGD. A time-dependent reduction in cell viability during the early OGD period (3 h) was partially prevented by iPLA2-VIA knockdown or pharmacological inhibition (10 μM BEL), whereas iPLA2-VIA overexpression had no effect on cell viability. Taken together, these data demonstrate that OGD in C2C12 myotubes is associated with an increase in iPLA2-VIA activity that decreases cell viability. iPLA2-VIA activation may be modulated by changes in the levels of active and inactive iPLA2-VIA isoforms.

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.

Modulation of the pancreatic islet beta-cell-delayed rectifier potassium channel Kv2.1 by the polyunsaturated fatty acid arachidonate

Glucose stimulates both insulin secretion and hydrolysis of arachidonic acid (AA) esterified in membrane phospholipids of pancreatic islet beta-cells, and these processes are amplified by muscarinic agonists. Here we demonstrate that nonesterified AA regulates the biophysical activity of the pancreatic islet beta-cell-delayed rectifier channel, Kv2.1. Recordings of Kv2.1 currents from INS-1 insulinoma cells incubated with AA (5 mum) and subjected to graded degrees of depolarization exhibit a significantly shorter time-to-peak current interval than do control cells. AA causes a rapid decay and reduced peak conductance of delayed rectifier currents from INS-1 cells and from primary beta-cells isolated from mouse, rat, and human pancreatic islets. Stimulating mouse islets with AA results in a significant increase in the frequency of glucose-induced [Ca(2+)] oscillations, which is an expected effect of Kv2.1 channel blockade. Stimulation with concentrations of glucose and carbachol that accelerate hydrolysis of endogenous AA from islet phosphoplipids also results in accelerated Kv2.1 inactivation and a shorter time-to-peak current interval. Group VIA phospholipase A(2) (iPLA(2)beta) hydrolyzes beta-cell membrane phospholipids to release nonesterified fatty acids, including AA, and inhibiting iPLA(2)beta prevents the muscarinic agonist-induced accelerated Kv2.1 inactivation. Furthermore, glucose and carbachol do not significantly affect Kv2.1 inactivation in beta-cells from iPLA(2)beta(-/-) mice. Stably transfected INS-1 cells that overexpress iPLA(2)beta hydrolyze phospholipids more rapidly than control INS-1 cells and also exhibit an increase in the inactivation rate of the delayed rectifier currents. These results suggest that Kv2.1 currents could be dynamically modulated in the pancreatic islet beta-cell by phospholipase-catalyzed hydrolysis of membrane phospholipids to yield non-esterified fatty acids, such as AA, that facilitate Ca(2+) entry and insulin secretion.

Activation mechanism for CRAC current and store-operated Ca2+ entry: calcium influx factor and Ca2+-independent phospholipase A2beta-mediated pathway

Here we tested the role of calcium influx factor (CIF) and calcium-independent phospholipase A2 (iPLA2) in activation of Ca2+ release-activated Ca2+ (CRAC) channels and store-operated Ca2+ entry in rat basophilic leukemia (RBL-2H3) cells. We demonstrate that 1) endogenous CIF production may be triggered by Ca2+ release (net loss) as well as by simple buffering of free Ca2+ within the stores, 2) a specific 82-kDa variant of iPLA2beta and its corresponding activity are present in membrane fraction of RBL cells, 3) exogenous CIF (extracted from other species) mimics the effects of endogenous CIF and activates iPLA2beta when applied to cell homogenates but not intact cells, 4) activation of ICRAC can be triggered in resting RBL cells by dialysis with exogenous CIF, 5) molecular or functional inhibition of iPLA2beta prevents activation of ICRAC, which could be rescued by cell dialysis with a human recombinant iPLA2beta, 6) dependence of ICRAC on intracellular pH strictly follows pH dependence of iPLA2beta activity, and 7) (S)-BEL, a chiral enantiomer of suicidal substrate specific for iPLA2beta, could be effectively used for pharmacological inhibition of ICRAC and store-operated Ca2+ entry. These findings validate and significantly advance our understanding of the CIF-iPLA2-dependent mechanism of activation of ICRAC and store-operated Ca2+ entry.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.