Renal concentrating defect in mice lacking group IV cytosolic phospholipase A(2)

Effect of very long-term storage and multiple freeze and thaw cycles on 11-dehydro-thromboxane-B2 and 8-iso-prostaglandin F2α, levels in human urine samples by validated enzyme immunoassays

Biological samples are often frozen and stored for years and/or thawed multiple times, thus assessing their stability on long-term storage and repeated freeze-thaw cycles is crucial. The study aims were to assess:-the long-term stability of two major enzymatic and non-enzymatic metabolites of arachidonic acid, i.e. urinary 11-dehydro-thromboxane-(Tx) B2, 8-iso-prostaglandin (PG)F2α, and creatinine in frozen urine samples;-the effect of multiple freeze-thaw cycles. Seven-hundred and three urine samples measured in previously-published studies, stored at -40 °C, and measured for a second time for 11-dehydro-TxB2 (n = 677) and/or 8-iso-PGF2α (n = 114) and/or creatinine (n = 610) were stable over 10 years and the 2 measurements were highly correlated (all rho = 0.99, P < 0.0001). Urine samples underwent 10 sequential freeze-thaw cycles, with and without the antioxidant 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (10 mM); urinary 11-dehydro-TxB2 and creatinine were stable across all cycles (11-dehydro-TxB2: 100.4 ± 21%; creatinine: 101 ± 7% of baseline at cycle ten; n = 17), while 8-iso-PGF2α significantly increased by cycle 6 (151 ± 22% of baseline at cycle ten, n = 17, P < 0.05) together with hydrogen peroxide only in the absence of antioxidant. Arachidonic acid metabolites and creatinine appear stable in human urines stored at -40 °C over 10 years. Multiple freeze-thaw cycles increase urinary 8-iso-PGF2α in urine samples without antioxidants. These data are relevant for studies using urine samples stored over long-term and/or undergoing multiple freezing-thawing.

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.

Segregated functions of two cytosolic phospholipase A2 isoforms (cPLA2α and cPLA2ε) in lipid mediator generation

Among the phospholipase A₂ (PLA₂) superfamily, group IVA cytosolic PLA₂ (cPLA₂α) is currently attracting much attention as a central regulator of arachidonic acid (AA) metabolism linked to eicosanoid biosynthesis. Following cell activation, cPLA₂α selectively releases AA, a precursor of a variety of eicosanoids, from phospholipids in perinuclear membrane compartments. cPLA₂α-null mice display various phenotypes that could be largely explained by reduced eicosanoid signaling. In contrast, group IVE cPLA₂ε, another member of the cPLA₂ family, acts as a Ca²⁺-dependent N-acyltransferase rather than a PLA₂, thereby regulating the biosynthesis of N-acylethanolamines (NAEs), a unique class of lipid mediators with an anti-inflammatory effect. In response to Ca²⁺ signaling, cPLA₂ε translocates to phosphatidylserine-rich organelle membranes in the endocytic/recycling pathway. In vivo, cPLA₂ε is induced in keratinocytes of psoriatic skin, and its genetic deletion exacerbates psoriatic inflammation due to a marked reduction of NAE-related lipids. cPLA₂ε also contributes to NAE generation in several if not all mouse tissues. Thus, the two members of the cPLA₂ family, cPLA₂α and cPLA₂ε, catalyze distinct enzymatic reactions to mobilize distinct sets of lipid mediators, thereby differently regulating pathophysiological events in health and disease. Such segregation of the cPLA₂α-eicosanoid and cPLA₂ε-NAE pathways represents a new paradigm of research on PLA₂s and lipid mediators.

Phospholipases: An Overview

Phospholipases are lipolytic enzymes that hydrolyze phospholipid substrates at specific ester bonds. Phospholipases are widespread in nature and play very diverse roles from aggression in snake venom to signal transduction, lipid mediator production, and metabolite digestion in humans. Phospholipases vary considerably in structure, function, regulation, and mode of action. Tremendous advances in understanding the structure and function of phospholipases have occurred in the last decades. This introductory chapter is aimed at providing a general framework of the current understanding of phospholipases and a discussion of their mechanisms of action and emerging biological functions.

Outside the mainstream: novel collecting duct proteins regulating water balance

Body water balance is critical to survival and, therefore, very tightly regulated by the hypothalamus and kidney. A key mechanism involved in this process, the arginine vasopressin-mediated phosphorylation and apical membrane insertion of aquaporin 2 in the collecting duct, has been extensively studied; however, with the increased availability of conditional knockout animals, several novel collecting duct proteins have recently been implicated in water homeostasis. In this Mini-Review, we briefly discuss these novel proteins and their roles in the regulation of water homeostasis.

Antidiuretic Action of Collecting Duct (Pro)Renin Receptor Downstream of Vasopressin and PGE2 Receptor EP4

Within the kidney, the (pro)renin receptor (PRR) is predominantly expressed in the collecting duct (CD), particularly in intercalated cells, and it is regulated by the PGE₂ receptor EP₄ Notably, EP₄ also controls urinary concentration through regulation of aquaporin 2 (AQP2). Here, we tested the hypothesis that sequential activation of EP₄ and PRR determines AQP2 expression in the CD, thus mediating the antidiuretic action of vasopressin (AVP). Water deprivation (WD) elevated renal PRR expression and urinary soluble PRR excretion in rats. Intrarenal infusion of a PRR decoy peptide, PRO20, or an EP₄ antagonist partially prevented the decrease in urine volume and the increase in urine osmolality and AQP2 expression induced by 48-hour WD. In primary cultures of rat inner medullary CD cells, AQP2 expression induced by AVP treatment for 24 hours depended on sequential activation of the EP₄ receptor and PRR. Additionally, mice lacking PRR in the CD exhibited increased urine volume and decreased urine osmolality under basal conditions and impaired urine concentrating capability accompanied by severe volume loss and a dangerous level of plasma hyperosmolality after WD. Together, these results suggest a previously undescribed linear AVP/PGE₂/EP₄/PRR pathway in the CD for regulation of AQP2 expression and urine concentrating capability.

Cytosolic Phospholipase A2α Is Essential for Renal Dysfunction and End-Organ Damage Associated With Angiotensin II-Induced Hypertension

BACKGROUND

The kidney plays an important role in regulating blood pressure (BP). cPLA2α in the kidney is activated by various agents including angiotensin II (Ang II) and selectively releases arachidonic acid (AA) from tissue lipids, generating pro- and antihypertensive eicosanoids. Since activation of cPLA2α is the rate-limiting step in AA release, this study was conducted to determine its contribution to renal dysfunction and end-organ damage associated with Ang II-induced hypertension.

METHODS

cPLA2α(+/+) and cPLA2α(-/-) mice were infused with Ang II (700 ng/ kg/min) or its vehicle for 13 days. Mice were placed in metabolic cages to monitor their food and water intake, and urine was collected and its volume was measured. Doppler imaging was performed to assess renal hemodynamics. On the 13th day of Ang II infusion, mice were sacrificed and their tissues and blood collected for further analysis.

RESULTS

Ang II increased renal vascular resistance, water intake, and urine output and Na(+) excretion, decreased urine osmolality, and produced proteinuria in cPLA2α(+/+) mice. Ang II also caused accumulation of F4/80(+) macrophages and CD3(+) T cells and renal fibrosis, and increased oxidative stress in the kidneys of cPLA2α(+/+) mice. All these effects of Ang II were minimized in cPLA2α(-/-) mice.

CONCLUSION

cPLA2α contributes to renal dysfunction, inflammation, and end-organ damage, most likely via the action of pro-hypertensive eicosanoids and increased oxidative stress associated with Ang II-induced hypertension. Thus, cPLA2α could serve as a potential therapeutic target for treating renal dysfunction and end-organ damage in hypertension.

Cytosolic phospholipase A₂: physiological function and role in disease

The group IV phospholipase A2 (PLA2) family is comprised of six intracellular enzymes (GIVA, -B, -C, -D, -E, and -F) commonly referred to as cytosolic PLA2 (cPLA2)α, -β, -γ, -δ, -ε, and -ζ. They contain a Ser-Asp catalytic dyad and all except cPLA2γ have a C2 domain, but differences in their catalytic activities and subcellular localization suggest unique regulation and function. With the exception of cPLA2α, the focus of this review, little is known about the in vivo function of group IV enzymes. cPLA2α catalyzes the hydrolysis of phospholipids to arachidonic acid and lysophospholipids that are precursors of numerous bioactive lipids. The regulation of cPLA2α is complex, involving transcriptional and posttranslational processes, particularly increases in calcium and phosphorylation. cPLA2α is a highly conserved widely expressed enzyme that promotes lipid mediator production in human and rodent cells from a variety of tissues. The diverse bioactive lipids produced as a result of cPLA2α activation regulate normal physiological processes and disease pathogenesis in many organ systems, as shown using cPLA2α KO mice. However, humans recently identified with cPLA2α deficiency exhibit more pronounced effects on health than observed in mice lacking cPLA2α, indicating that much remains to be learned about this interesting enzyme.

Cytosolic phospholipase A2α is critical for angiotensin II-induced hypertension and associated cardiovascular pathophysiology

Angiotensin II activates cytosolic phospholipase A(2)α (cPLA2α) and releases arachidonic acid from tissue phospholipids, which mediate or modulate ≥1 cardiovascular effects of angiotensin II and has been implicated in hypertension. Because arachidonic acid release is the rate limiting step in eicosanoid production, cPLA2α might play a central role in the development of angiotensin II-induced hypertension. To test this hypothesis, we investigated the effect of angiotensin II infusion for 13 days by micro-osmotic pumps on systolic blood pressure and associated pathogenesis in wild type (cPLA2α(+/+)) and cPLA2α(-/-) mice. Angiotensin II-induced increase in systolic blood pressure in cPLA2α(+/+) mice was abolished in cPLA2α(-/-) mice; increased systolic blood pressure was also abolished by the arachidonic acid metabolism inhibitor, 5,8,11,14-eicosatetraynoic acid in cPLA2α(+/+) mice. Angiotensin II in cPLA2α(+/+) mice increased cardiac cPLA2 activity and urinary eicosanoid excretion, decreased cardiac output, caused cardiovascular remodeling with endothelial dysfunction, and increased vascular reactivity in cPLA2α(+/+) mice; these changes were diminished in cPLA2α(-/-) mice. Angiotensin II also increased cardiac infiltration of F4/80(+) macrophages and CD3(+) T lymphocytes, cardiovascular oxidative stress, expression of endoplasmic reticulum stress markers p58(IPK), and CHOP in cPLA2α(+/+) but not cPLA2α(-/-) mice. Angiotensin II increased cardiac activity of ERK1/2 and cSrc in cPLA2α(+/+) but not cPLA2α(-/-) mice. These data suggest that angiotensin II-induced hypertension and associated cardiovascular pathophysiological changes are mediated by cPLA2α activation, most likely through the release of arachidonic acid and generation of eicosanoids with predominant prohypertensive effects and activation of ≥1 signaling molecules, including ERK1/2 and cSrc.

Genetics of kidney disease and related cardiometabolic phenotypes in Zuni Indians: the Zuni Kidney Project

The objective of this study is to identify genetic factors associated with chronic kidney disease (CKD) and related cardiometabolic phenotypes among participants of the Genetics of Kidney Disease in Zuni Indians study. The study was conducted as a community-based participatory research project in the Zuni Indians, a small endogamous tribe in rural New Mexico. We recruited 998 members from 28 extended multigenerational families, ascertained through probands with CKD who had at least one sibling with CKD. We used the Illumina Infinium Human1M-Duo version 3.0 BeadChips to type 1.1 million single nucleotide polymorphisms (SNPs). Prevalence estimates for CKD, hyperuricemia, diabetes, and hypertension were 24%, 30%, 17% and 34%, respectively. We found a significant (p < 1.58 × 10^-7) association for a SNP in a novel gene for serum creatinine (PTPLAD2). We replicated significant associations for genes with serum uric acid (SLC2A9), triglyceride levels (APOA1, BUD13, ZNF259), and total cholesterol (PVRL2). We found novel suggestive associations (p < 1.58 × 10^-6) for SNPs in genes with systolic (OLFML2B), and diastolic blood pressure (NFIA). We identified a series of genes associated with CKD and related cardiometabolic phenotypes among Zuni Indians, a population with a high prevalence of kidney disease. Illuminating genetic variations that modulate the risk for these disorders may ultimately provide a basis for novel preventive strategies and therapeutic interventions.

Bleeding diathesis and gastro-duodenal ulcers in inherited cytosolic phospholipase-A2 alpha deficiency

Arachidonic acid (AA), when cleaved from phospholipids by cytosolic phospholipase A2 alpha (cPLA2a), generates eicosanoids, with pro-hemostatic, pro-inflammatory, vasoactive and gastro-protective functions. We describe a patient (27-year-old man) and his twin-sister with early-onset bleeding diathesis and recurrent gastro-intestinal (GI) ulcers. Platelet aggregation/δ-granules secretion by collagen was impaired, but normal by AA; serum levels of thromboxane (Tx) B2 and 12-hydroxyeicosatetraenoic acid, and urinary levels of 11-dehydro-TxB2 were extremely low. Patients were homozygous for 1723G>C transition in PLA2G4A gene, which changed the codon for Asp575 to His. GI ulcers affected 5/14 heterozygous (< 40 years) and 1/16 wild-type homozygous (> 60 years) family members; none had bleeding diathesis. The proband, his sister and mother also had mildly reduced factor XI levels. Platelet messenger RNA expression did not differ among subjects with different PLA2G4A genotypes. Conversely, platelet cPLA2a was undetectable by Western Blotting in the proband and his sister, and decreased in 1723G>C heterozygous subjects, suggesting that the variant is transcribed, but not translated or translated into an unstable protein. We described a syndromic form of deficiency of cPLA2a , characterised by recurrent GI ulcers and bleeding diathesis, associated with mild inherited deficiency of factor XI. Unlike other reported patients with cPLA2a deficiency, these patients had extremely low levels of platelet TxA2 biosynthesis.

Effects of Schizolobium parahyba extract on experimental Bothrops venom-induced acute kidney injury

BACKGROUND

Venom-induced acute kidney injury (AKI) is a frequent complication of Bothrops snakebite with relevant morbidity and mortality. The aim of this study was to assess the effects of Schizolobium parahyba (SP) extract, a natural medicine with presumed anti-Bothrops venom effects, in an experimental model of Bothrops jararaca venom (BV)-induced AKI.

METHODOLOGY

Groups of 8 to 10 rats received infusions of 0.9% saline (control, C), SP 2 mg/kg, BV 0.25 mg/kg and BV immediately followed by SP (treatment, T) in the doses already described. After the respective infusions, animals were assessed for their glomerular filtration rate (GFR, inulin clearance), renal blood flow (RBF, Doppler), blood pressure (BP, intra-arterial transducer), renal vascular resistance (RVR), urinary osmolality (UO, freezing point), urinary neutrophil gelatinase-associated lipocalin (NGAL, enzyme-linked immunosorbent assay [ELISA]), lactate dehydrogenase (LDH, kinetic method), hematocrit (Hct, microhematocrit), fibrinogen (Fi, Klauss modified) and blinded renal histology (acute tubular necrosis score).

PRINCIPAL FINDINGS

BV caused significant decreases in GFR, RBF, UO, HcT and Fi; significant increases in RVR, NGAL and LDH; and acute tubular necrosis. SP did not prevent these changes; instead, it caused a significant decrease in GFR when used alone.

CONCLUSION

SP administered simultaneously with BV, in an approximate 10∶1 concentration, did not prevent BV-induced AKI, hemolysis and fibrinogen consumption. SP used alone caused a decrease in GFR.

Group VIA phospholipase A2 is a target for vasopressin signaling in the thick ascending limb

Na+-K+-2Cl− cotransporter (NKCC2)-mediated NaCl reabsorption in the thick ascending limb (TAL) is stimulated by AVP via V2 receptor/PKA/cAMP signaling. This process is antagonized by locally produced eicosanoids such as 20-HETE or prostaglandin E2, which are synthesized in a phospholipase A2-dependent reaction cascade. Using microarray-based gene expression analysis, we found evidence for an AVP-dependent downregulation of the calcium-independent isoform of PLA2, iPLA2β, in the outer medulla of rats. In the present study, we therefore examined the contribution of iPLA2β to NKCC2 regulation. Immunoreactive iPLA2β protein was detected in cultured mTAL cells as well as in the entire TAL of rodents and humans with the exception of the macula densa. Administration of the V2 receptor-selective agonist desmopressin (5 ng/h; 3 days) to AVP-deficient diabetes insipidus rats increased outer medullary phosphorylated NKCC2 (pNKCC2) levels more than twofold in association with a marked reduction in iPLA2β abundance (−65%; P < 0.05), thus confirming microarray results. Inhibition of iPLA2β in Sprague-Dawley rats with FKGK 11 (0.5 μM) or in mTAL cells with FKGK 11 (10 μM) or ( S)-bromoenol lactone (5 μM) for 1 h markedly increased pNKCC2 levels without affecting total NKCC2 expression. Collectively, these data indicate that iPLA2β acts as an inhibitory modulator of NKCC2 activity and suggest that downregulation of iPLA2β may be a relevant step in AVP-mediated urine concentration.

Phospholipases: an overview

Phospholipids are present in all living organisms. They are a major component of all biological membranes, along with glycolipids and cholesterol. Enzymes aimed at cleaving the various bonds in phospholipids, namely phospholipases, are consequently widespread in nature, playing very diverse roles from aggression in snake venom to signal transduction, lipid mediators production, and digestion in humans. Although all phospholipases target phospholipids as substrates, they vary in the site of action on the phospholipids molecules, physiological function, mode of action, and their regulation. Significant studies on phospholipases characterization, physiological role, and industrial potential have been conducted worldwide. Some of them have been directed for biotechnological advances, such as gene discovery and functional enhancement by protein engineering. Others reported phospholipases as virulence factors and major causes of pathophysiological effects. In this introductory chapter, we provide brief details of different phospholipases.

COX-2 disruption leads to increased central vasopressin stores and impaired urine concentrating ability in mice

It was hypothesized that cyclooxygenase-2 (COX-2) activity promotes urine concentrating ability through stimulation of vasopressin (AVP) release after water deprivation (WD). COX-2-deficient (COX-2(-/-), C57BL/6) and wild-type (WT) mice were water deprived for 24 h, and water balance, central AVP mRNA and peptide level, AVP plasma concentration, and AVP-regulated renal transport protein abundances were measured. In male COX-2(-/-), basal urine output and water intake were elevated while urine osmolality was decreased compared with WT. Water deprivation resulted in lower urine osmolality, higher plasma osmolality in COX-2(-/-) mice irrespective of gender. Hypothalamic AVP mRNA level increased and was unchanged between COX-2(-/-) and WT after WD. AVP peptide content was higher in COX-2(-/-) compared with WT. At baseline, plasma AVP concentration was elevated in conscious chronically catheterized COX-2(-/-) mice, but after WD plasma AVP was unchanged between COX-2(-/-) and WT mice (43 ± 11 vs. 70 ± 16 pg/ml). Renal V2 receptor abundance was downregulated in COX-2(-/-) mice. Medullary interstitial osmolality increased and did not differ between COX-2(-/-) and WT after WD. Aquaporin-2 (AQP2; cortex-outer medulla), AQP3 (all regions), and UT-A1 (inner medulla) protein abundances were elevated in COX-2(-/-) at baseline and further increased after WD. COX-2(-/-) mice had elevated plasma urea and creatinine and accumulation of small subcapsular glomeruli. In conclusion, hypothalamic COX-2 activity is not necessary for enhanced AVP expression and secretion in response to water deprivation. Renal medullary COX-2 activity negatively regulates AQP2 and -3. The urine concentrating defect in COX-2(-/-) is likely caused by developmental glomerular injury and not dysregulation of AVP or collecting duct aquaporins.

cPLA(2)α gene activation by IL-1β is dependent on an upstream kinase pathway, enzymatic activation and downstream 15-lipoxygenase activity: a positive feedback loop

Cytosolic phospholipase A(2)α (cPLA(2)α) is the most widely studied member of the Group IV PLA(2) family. The enzyme is Ca(2+)-dependent with specificity for phospholipid substrates containing arachidonic acid. As the pinnacle of the arachidonic acid pathway, cPLA(2)α has a primary role in the biosynthesis of a diverse family of eicosanoid metabolites, with potent physiological, inflammatory and pathological consequences. cPLA(2)α activity is regulated by pro-inflammatory stimuli through pathways involving increased intracellular Ca(2+) levels, phosphorylation coupled to increased enzymatic activity and de novo gene transcription. This study addresses the signal transduction pathways for protein phosphorylation and gene induction following IL-1β stimulation in human fetal lung fibroblasts. Our results utilizing both inhibitors and kinase-deficient cells demonstrate that cPLA(2)α is phosphorylated within 10min of IL-1β treatment, an event requiring p38 MAPK as well as the upstream kinase, MKK3/MKK6. Inhibition of p38 MAPK also blocks the phosphorylation of a downstream, nuclear kinase, MSK-1. Our results further demonstrate that the activities of both cPLA(2)α and a downstream lipoxygenase (15-LOX2) are required for IL-1β-dependent induction of cPLA(2)α mRNA expression. Overall, these data support an MKK3/MKK6→p38 MAPK→MSK-1→cPLA(2)α→15-LOX2-dependent, positive feedback loop where a protein's enzymatic activity is required to regulate its own gene induction by a pro-inflammatory stimulus.

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

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

Phospholipase A(2) is required for PIN-FORMED protein trafficking to the plasma membrane in the Arabidopsis root

Phospholipase A(2) (PLA(2)), which hydrolyzes a fatty acyl chain of membrane phospholipids, has been implicated in several biological processes in plants. However, its role in intracellular trafficking in plants has yet to be studied. Here, using pharmacological and genetic approaches, the root hair bioassay system, and PIN-FORMED (PIN) auxin efflux transporters as molecular markers, we demonstrate that plant PLA(2)s are required for PIN protein trafficking to the plasma membrane (PM) in the Arabidopsis thaliana root. PLA(2)alpha, a PLA(2) isoform, colocalized with the Golgi marker. Impairments of PLA(2) function by PLA(2)alpha mutation, PLA(2)-RNA interference (RNAi), or PLA(2) inhibitor treatments significantly disrupted the PM localization of PINs, causing internal PIN compartments to form. Conversely, supplementation with lysophosphatidylethanolamine (the PLA(2) hydrolytic product) restored the PM localization of PINs in the pla(2)alpha mutant and the ONO-RS-082-treated seedling. Suppression of PLA(2) activity by the inhibitor promoted accumulation of trans-Golgi network vesicles. Root hair-specific PIN overexpression (PINox) lines grew very short root hairs, most likely due to reduced auxin levels in root hair cells, but PLA(2) inhibitor treatments, PLA(2)alpha mutation, or PLA(2)-RNAi restored the root hair growth of PINox lines by disrupting the PM localization of PINs, thus reducing auxin efflux. These results suggest that PLA(2), likely acting in Golgi-related compartments, modulates the trafficking of PIN proteins.

Golgi-associated cPLA2alpha regulates endothelial cell-cell junction integrity by controlling the trafficking of transmembrane junction proteins

In endothelial cells specifically, cPLA2alpha translocates from the cytoplasm to the Golgi complex in response to cell confluence. Considering the link between confluence and cell-cell junction formation, and the emerging role of cPLA2alpha in intracellular trafficking, we tested whether Golgi-associated cPLA2alpha is involved in the trafficking of junction proteins. Here, we show that the redistribution of cPLA2alpha from the cytoplasm to the Golgi correlates with adherens junction maturation and occurs before tight junction formation. Disruption of adherens junctions using a blocking anti-VE-cadherin antibody reverses the association of cPLA2alpha with the Golgi. Silencing of cPLA2alpha and inhibition of cPLA2alpha enzymatic activity using various inhibitors result in the diminished presence of the transmembrane junction proteins VE-cadherin, occludin, and claudin-5 at cell-cell contacts, and in their accumulation at the Golgi. Altogether, our data support the idea that VE-cadherin triggers the relocation of cPLA2alpha to the Golgi and that in turn, Golgi-associated cPLA2alpha regulates the transport of transmembrane junction proteins through or from the Golgi, thereby controlling the integrity of endothelial cell-cell junctions.

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.

V2 vasopressin receptor deficiency causes changes in expression and function of renal and hypothalamic components involved in electrolyte and water homeostasis

Polyuria, hypernatremia, and hypovolemia are the major clinical signs of inherited nephrogenic diabetes insipidus (NDI). Hypernatremia is commonly considered a secondary sign caused by the net loss of water due to insufficient insertion of aquaporin-2 water channels into the apical membrane of the collecting duct cells. In the present study, we employed transcriptome-wide expression analysis to study gene expression in V2 vasopressin receptor (Avpr2)-deficient mice, an animal model for X-linked NDI. Gene expression changes in NDI mice indicate increased proximal tubular sodium reabsorption. Expression of several key genes including Na+-K+-ATPase and carbonic anhydrases was increased at the mRNA levels and accompanied by enhanced enzyme activities. In addition, altered expression was also observed for components of the eicosanoid and thyroid hormone pathways, including cyclooxygenases and deiodinases, in both kidney and hypothalamus. These effects are likely to contribute to the clinical NDI phenotype. Finally, our data highlight the involvement of the renin-angiotensin-aldosterone system in NDI pathophysiology and provide clues to explain the effectiveness of diuretics and indomethacin in the treatment of NDI.

Roles of ERK and cPLA2 in the angiotensin II-mediated biphasic regulation of Na+-HCO3(-) transport

Regulation of renal proximal transport by angiotensin II (Ang II) is biphasic: low concentrations (picomolar to nanomolar) stimulate reabsorption, but higher concentrations (nanomolar to micromolar) inhibit reabsorption. Traditionally, the stimulatory effect has been attributed to activation of protein kinase C and/or a decrease in intracellular cAMP, whereas the inhibitory action has been attributed to the activation of phospholipase A2 (PLA2) and the subsequent release of arachidonic acid. The Ang II receptor subtype responsible for these effects and the intracellular signaling pathways involved are not completely understood. We isolated proximal tubules from wild-type, Ang II type 1A receptor (AT1A)-deficient, and group IVA cytosolic phospholipase A2 (cPLA2alpha)-deficient mice, and compared their responses to Ang II. In wild-type mice, we found that the stimulatory and inhibitory effects of Ang II on Na+-HCO3(-) cotransporter activity are both AT1-mediated but that ERK activation only plays a role in the former. The stimulatory effect of Ang II was also observed in AT1A-deficient mice, suggesting that this occurs through AT1B. In contrast, the inhibitory effects of Ang II appeared to be mediated by cPLA2alpha activation because high-concentration Ang II stimulated Na+-HCO3(-) cotransporter activity when cPLA2alpha activity was abrogated by pharmacological means or genetic knockout. Consistent with this observation, we found that activation of the cPLA2alpha/P450 pathway suppressed ERK activation. We conclude that Ang II activates ERK and cPLA2alpha in a concentration-dependent manner via AT1, and that the balance between ERK and cPLA2alpha activities determines the ultimate response to Ang II in intact proximal tubules.

Overexpression of PKD2 in the mouse is associated with renal tubulopathy

Polycystin-2 (PC-2), a cation channel of the Trp family, is involved in autosomal dominant polycystic kidney disease (ADPKD) type 2 (ADPKD2). This protein has recently been localized to the primary cilium where its channel function seems to be involved in a mechanosensory phenomenon. However, its biological function is not totally understood, especially in tubule formation. In the present paper, we describe a mouse model for human PC-2 overexpression, obtained by inserting a human bacterial artificial chromosome (BAC) containing the PKD2 gene. Three lines were generated, expressing different levels of PKD2. One line, PKD2-Y, has been explored in more detail and we will present physiological and molecular exploration of these transgenic animals. Our data demonstrate that transgenic animals older than 12 months present tubulopathy with proteinuria and failure to concentrate urine. Moreover, the kidney cortex has been found disorganized. Finally, we observe that extracellular matrix protein expression is downregulated in these animals. In conclusion, overexpression of human PKD2 leads to anomalies in tubular function, probably due to abnormalities in tubule morphogenesis.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Function, activity, and membrane targeting of cytosolic phospholipase A(2)zeta in mouse lung fibroblasts

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) initiates eicosanoid production; however, this pathway is not completely ablated in cPLA(2)alpha(-/-) lung fibroblasts stimulated with A23187 or serum. cPLA(2)alpha(+/+) fibroblasts preferentially released arachidonic acid, but A23187-stimulated cPLA(2)alpha(-/-) fibroblasts nonspecifically released multiple fatty acids. Arachidonic acid release from cPLA(2) alpha(-/-) fibroblasts was inhibited by the cPLA(2)alpha inhibitors pyrrolidine-2 (IC(50), 0.03 microM) and Wyeth-1 (IC(50), 0.1 microM), implicating another C2 domain-containing group IV PLA(2). cPLA(2) alpha(-/-) fibroblasts contain cPLA(2)beta and cPLA(2)zeta but not cPLA(2)epsilon or cPLA(2)delta. Purified cPLA(2)zeta exhibited much higher lysophospholipase and PLA(2) activity than cPLA(2)beta and was potently inhibited by pyrrolidine-2 and Wyeth-1, which did not inhibit cPLA(2)beta. In contrast to cPLA(2)beta, cPLA(2)zeta expressed in Sf9 cells mediated A23187-induced arachidonic acid release, which was inhibited by pyrrolidine-2 and Wyeth-1. cPLA(2)zeta exhibits specific activity, inhibitor sensitivity, and low micromolar calcium dependence similar to cPLA(2)alpha and has been identified as the PLA(2) responsible for calcium-induced fatty acid release and prostaglandin E(2) production from cPLA(2) alpha(-/-) lung fibroblasts. In response to ionomycin, EGFP-cPLA(2)zeta translocated to ruffles and dynamic vesicular structures, whereas EGFP-cPLA(2)alpha translocated to the Golgi and endoplasmic reticulum, suggesting distinct mechanisms of regulation for the two enzymes.

Role of Microsomal Prostaglandin E Synthase 1 in the Kidney

Prostaglandin E(2) (PGE(2)) is one of the most ubiquitous prostanoids in the kidney, where it may influence a wide range of physiologic functions. PGE(2) is generated through enzymatic metabolism of prostanoid endoperoxides by specific PGE synthases (PGES). Several putative PGES have been identified and cloned, including the membrane-associated, inducible microsomal PGES1 (mPGES1), which is expressed in the kidney. To evaluate the physiologic role of mPGES1 in the kidney, mice with targeted disruption of mPges1 gene were studied, with a focus on responses where PGE(2) has been implicated, including urinary concentration, regulation of blood pressure, and response to a loop diuretic. The absence of mPGES1 was associated with a 50% decrease in basal excretion of PGE(2) in urine (P < 0.001). In female but not male mPGES1-deficient mice, there was a reciprocal increase in basal excretion of other prostanoids. Nonetheless, urinary osmolalities were similar in mPges1(+/+) and mPges1(-/-) mice at baseline and after 12 h of water deprivation. Likewise, there were no differences in blood pressure between mPGES1-deficient and wild-type mice on control or high- or low-salt diets. The furosemide-induced increase in urinary PGE(2) excretion that was seen in wild-type mice was attenuated in mPGES1-deficient mice. However, furosemide-associated diuresis was reduced only in male, not female, mPGES1-deficient mice. Stimulation of renin by furosemide was not affected by mPGES1 deficiency. These data suggest that mPGES1 contributes to basal synthesis of PGE(2), but there are other pathways that lead to renal PGE(2) synthesis. Moreover, there are significant gender differences in physiologic contributions of mPGES1 to control kidney function.

Biochemical properties and pathophysiological roles of cytosolic phospholipase A2s

Phospholipase A(2) (PLA(2)) (EC 3.1.1.4) catalyzes hydrolysis of the sn-2 ester bond of glycerophospholipids. The enzyme is essential for the production of two classes of lipid mediators, fatty acid metabolites and lysophospholipid-related lipids, as well as being involved in the remodeling of membrane phospholipids. Among many mammalian PLA(2)s, cytosolic PLA(2)alpha (cPLA(2)alpha) plays a critical role in various physiological and pathophysiological conditions through generating lipid mediators. Here, we summarize the in vivo significance of cPLA(2)alpha, revealed from the phenotypes of cPLA(2)alpha-null mice, and properties of newly discovered cPLA(2) family enzymes. We also briefly introduce a quantitative lipidomics strategy using liquid chromatography-mass spectrometry, a powerful tool for the comprehensive analysis of lipid mediators.

Roles of cPLA2alpha and arachidonic acid in cancer

Phospholipase A(2)s (PLA(2)s) are key enzymes that catalyze the hydrolysis of membrane phospholipids to release bioactive lipids that play an important role in normal cellular homeostasis. Under certain circumstances, disrupted production of key lipid mediators may adversely impact physiological processes, leading to pathological conditions such as inflammation and cancer. In particular, cytosolic PLA(2)alpha (cPLA(2)alpha) has a high selectivity for liberating arachidonic acid (AA) that is subsequently metabolized by a panel of downstream enzymes for eicosanoid production. Although concentrations of free AA are maintained at low levels in resting cells, alterations in AA production, often resulting from dysregulation of cPLA(2)alpha activity, are observed in transformed cells. In this review, we summarize recent evidence that cPLA(2)alpha plays a role in the pathogenesis of many human cancers. Much of this evidence has been accumulated from functional studies using cPLA(2)alpha-deficient mice, as well as mechanistic studies in cell culture. We also discuss the potential contribution of cPLA(2)alpha and AA to apoptosis, and the regulatory mechanisms leading to aberrant expression of cPLA(2)alpha.

Cytosolic phospholipase A 2 : Biochemical properties and physiological roles

Phosphatidylcholine (PC) is a major constituent of biological membranes and a component of serum lipoproteins and pulmonary surfactants. The PC and other glycerophospholipid compositions of membranes change dynamically through stimulus-dependent and independent pathways, principally by the action of two different types of enzymes; phospholipase A2 [EC 3.1.1.4] and acyl-CoA:lysophospholipid acyltransferase [EC 2.3.1.23]. Phospholipase A2 is a key enzyme that catalyzes deacylation of the sn-2 position of glycerophospholipids. This enzyme is critical in the remodeling of membrane lipids and formation of two subclasses of lipid mediators, fatty acid derivatives and lysophospholipids. Among many different subtypes of phospholipase A2 enzymes, we found that cytosolic phospholipase A2alpha (cPLA2alpha) is important in various pathological and physiological responses. Here, we summarize the phenotypes resulting from genetic ablation of cPLA2alpha, and the properties of newly discovered enzymes in the cPLA2 family. Comprehensive analysis of lipid mediators using liquid chromatography-tandem mass spectrometry (LC-MS/MS) is useful for understanding the roles of individual mediators in physiological and pathological processes.

Microarray analysis of rat brain gene expression after chronic administration of sodium valproate

Valproic acid has been used to treat mania and bipolar disorder, but its mechanism of action is not agreed on. We used rat genome U34A Affymetrix oligonucleotide microarrays, containing 8799 known probesets, to determine the effect of 30-day daily intraperitoneal administration of valproate (200mg/kg) on rat brain gene expression. We found 87 down-regulated genes and 34 up-regulated genes of at least a 1.4-fold change in valproate-treated compared to control rats. The experiments were done on five independent samples for each group, each in duplicate. The genes affected are known to be involved in a variety of pathways, including synaptic transmission, ion channels and transport, G-protein signaling, lipid, glucose and amino-acid metabolism, transcriptional and translational regulation, phosphoinositol cycle, protein kinases and phosphatases, and apoptosis. Our results suggest that the therapeutic effect of valproate may involve the modulation of multiple signaling pathways.

Regulatory mechanism and physiological role of cytosolic phospholipase A2

Cytosolic phospholipase A2alpha (cPLA2alpha) preferentially hydrolyzes phospholipids containing arachidonic acid and plays a key role in the biosynthesis of eicosanoids. This review discusses the essential features of cPLA2alpha regulation and addresses new insights into the functional properties of this enzyme. Full activation of the enzyme requires Ca2+ binding to an N-terminal C2 domain and phosphorylation on serine residues. Ca2+ binding induces translocation of cPLA2alpha from the cytosol to the perinuclear membranes. Serine phosphorylation is mediated by mitogen-activated protein kinases (MAPKs), Ca2+/calmodulin-dependent protein kinase II, and MAPK-interacting kinase Mnk1. Interaction with proteins and lipids, which include vimentin, annexins, NADPH oxidase, phosphatidylcholine, phosphatidylinositol 4,5-bisphosphate (PIP2), and ceramide-1-phosphate, can also modulate the activity of cPLA2alpha. Recent evidence has established the physiological and pathological roles of cPLA2alpha using cPLA2alpha knockout mice. This enzyme has been implicated in fertility, striated muscle growth, renal concentration, postischemic brain injury, arthritis, inflammatory bone resorption, intestinal polyposis, pulmonary fibrosis, acute respiratory distress syndrome, and autoimmune encephalomyelitis. Now novel three paralogs, cPLA2beta, cPLA2gamma, and cPLA2delta, have been identified in humans. cPLA2gamma is distinct from others in that it is farnesylated and lacks the C2 domain. Biological roles for these new enzymes have not yet been defined.

Cardiovascular and renal phenotyping of genetically modified mice: a challenge for traditional physiology

1. The advent of techniques to genetically modify experimental animals and produce directed mutations in both a conditional and tissue-specific manner has dramatically opened up new fields for physiologists in cardiovascular and renal research. 2. A consequence of altering the genetic background of mice is the difficulty in predicting the phenotypic outcome of the genetic mutation. We therefore suggest that physiologists may need to change their current experimental paradigms to face this new era. Hence, our aim is to propose a complementary research philosophy for physiologists working in the post-genomic era. That is, instead of using strictly hypothesis-driven research philosophies, one will have to perform screening studies of mutant mice, within a field of interest, to find valuable phenotypes. Once a relevant phenotype is found, in-depth studies of the underlying mechanisms should be performed. These follow-up studies should be performed using a traditional hypothesis-driven research philosophy. 3. The rapidly increasing availability of mutated mouse models of human disease also necessitates the development of techniques to characterize these various mouse phenotypes. In particular, the miniaturization and refinement of techniques currently used to study the renal and cardiovascular system in larger animals will be discussed in the present review. Hence, we aim to outline what techniques are currently available and should be present in a laboratory to screen and study renal and cardiovascular phenotypes in genetically modified mice, with particular emphasis on methodologies used in the intact, conscious animal.

Aquaporin water channel genes are differentially expressed and regulated by ovarian steroids during the periimplantation period in the mouse

The periimplantation period is marked by edematous changes in the uterus. In the mouse, increased uterine vascular permeability occurs in response to estrogen and certain vasoactive mediators, but the mechanisms that regulate fluid transport during implantation are not fully understood. Aquaporins (AQPs) are a family of membrane channel proteins that facilitate bulk water transport. To assess their role in implantation, we examined the expression of AQPs 0-9 in the mouse uterus on d 1-8 of pregnancy. Our results show distinct uterine expression patterns for AQP1, AQP4, and AQP5. AQP1 is localized to the inner circular myometrium throughout the periimplantation period. AQP4 is highly expressed in the luminal epithelium on d 1 of pregnancy but barely detectable at the time of implantation. AQP5 is expressed at low levels in the glandular epithelium during early pregnancy but is markedly increased on d 5. By immunohistochemistry, AQP5 is localized in the basolateral region of the uterine glands. Treatment of adult ovariectomized mice with replacement steroids demonstrates an estrogen-induced shift in AQP1 signals from the myometrium to the uterine stromal vasculature, suggesting a role in uterine fluid imbibition. In contrast, AQP5 is induced only in estrogen-treated, progesterone-primed uteri. We also observed expression of AQP8 in the inner-cell mass and AQP9 in the mural trophectoderm of the implanting blastocyst. Collectively, these results suggest that members of the AQP family are involved in embryo and uterine fluid homeostasis during implantation.

Phospholipase A2 (PLA2) enzymes in membrane trafficking: mediators of membrane shape and function

Since the mid-1990s, there have been tremendous advances in our understanding of the roles that lipid-modifying enzymes play in various intracellular membrane trafficking events. Phospholipases represent the largest group of lipid-modifying enzymes and accordingly display a wide range of functions. The largest class of phospholipases are the phospholipase A(2) (PLA2) enzymes, and these have been most extensively studied for their roles in the generation lipid signaling molecules, e.g. arachidonic acid. In recent years, however, cytoplasmic PLA2 enzymes have also become increasingly associated with various intracellular trafficking events, such as the formation of membrane tubules from the Golgi complex and endosomes, and membrane fusion events in the secretory and endocytic pathways. Moreover, the ability of cytoplasmic PLA2 enzymes to directly affect the structure and function of membranes by altering membrane curvature suggests novel functional roles for these enzymes. This review will focus on the role of cytoplasmic PLA2 enzymes in intracellular membrane trafficking and the mechanisms by which they influence membrane structure and function.

20-HETE and furosemide-induced natriuresis in salt-sensitive essential hypertension

Cyclooxygenase metabolites of arachidonic acid modulate the natriuretic effect of furosemide. It is not known whether 20-HETE, a monooxygenase metabolite of arachidonic acid that also inhibits sodium transport, participates in the action of furosemide. We measured urine sodium (UNaV) and 20-HETE during furosemide diuresis (40 mg three times over 12 hours) in 12 salt-sensitive (SS) and 11 salt-resistant (SR), salt-replete hypertensive subjects (126+/-24 mmol/24 hours positive sodium balance produced by 160-mmol-sodium diet and 2 L saline infusion). Individual systolic blood pressure decreases from the salt-replete to the salt-depleted state were the index of salt-sensitivity. SS had low plasma renin with blunted responses to changes in salt balance, inappropriate plasma aldosterone, and an increased aldosterone/renin ratio. UNaV by furosemide was less in SS (263+/-25 mmol/12 hours) than in SR (351+/-25 mmol/12 hours, P<0.02) patients. 20-HETE was not different between SS and SR patients before (1.92+/-0.38 versus 1.37+/-0.34 microg/h) or after furosemide (1.52+/-0.27 versus 2.01+/-0.40 microg/h), but furosemide changed 20-HETE excretion in opposite direction in SR (0.63+/-0.26) versus SS (-0.40+/-0.17, P<0.005) patients. In all patients together, %Delta20-HETE by furosemide correlated with %DeltaUNaV (r=0.56, P<0.01) and negatively with salt-sensitivity of blood pressure (r=-0.55, P<0.01). In SS, Delta20-HETE by furosemide correlated with Deltaaldosterone/renin ratio (r=0.60, P<0.05), whereas 20-HETE during furosemide had a negative correlation with body mass index (r=-0.73, P<0.01). Our data suggest that 20-HETE modulates the natriuretic response to furosemide, and impaired natriuresis of SS involves a mechanism that alters the 20-HETE response to furosemide and is linked to salt-sensitivity of blood pressure.

Discrete role for cytosolic phospholipase A(2)alpha in platelets: studies using single and double mutant mice of cytosolic and group IIA secretory phospholipase A(2)

Among several different types of phospholipase A(2) (PLA(2)), cytosolic PLA(2) (cPLA(2))alpha and group IIA (IIA) secretory PLA(2) (sPLA(2)) have been studied intensively. To determine the discrete roles of cPLA(2)alpha in platelets, we generated two sets of genetically engineered mice (cPLA(2)alpha(-/-)/sPLA(2)-IIA(-/-) and cPLA(2)alpha(-/-)/sPLA(2)-IIA(+/+)) and compared their platelet function with their respective wild-type C57BL/6J mice (cPLA(2)alpha(+/+)/sPLA(2)-IIA(-/-)) and C3H/HeN (cPLA(2)alpha(+/+)/sPLA(2)-IIA(+/+)). We found that cPLA(2)alpha is needed for the production of the vast majority of thromboxane (TX)A(2) with collagen stimulation of platelets. In cPLA(2)alpha-deficient mice, however, platelet aggregation in vitro is only fractionally decreased because small amounts of TX produced by redundant phospholipase enzymes sufficiently preserve aggregation. In comparison, adenosine triphosphate activation of platelets appears wholly independent of cPLA(2)alpha and sPLA(2)-IIA for aggregation or the production of TX, indicating that these phospholipases are specifically linked to collagen receptors. However, the lack of high levels of TX limiting vasoconstriction explains the in vivo effects seen: increased bleeding times and protection from thromboembolism. Thus, cPLA(2)alpha plays a discrete role in the collagen-stimulated production of TX and its inhibition has a therapeutic potential against thromboembolism, with potentially limited bleeding expected.

Roles for cytosolic phospholipase A2alpha as revealed by gene-targeted mice

Cytosolic phospholipase A2alpha (cPLA2alpha) has unique characteristics among phospholipase A2 (PLA2) family members. Under regulation by intracellular signaling system, cytosolic phospholipase A2alpha liberates arachidonic acid that can be metabolized by downstream enzymes to generate prostaglandins (PGs) and leukotrienes (LTs). Mice deficient in this enzyme have been generated by gene-targeting techniques. Cytosolic phospholipase A2alpha-deficient mice have a normal appearance and grow normally. Close examinations have revealed a renal concentration defect and intestinal ulcerative lesions. There may also be other disadvantages that are not manifested in well-regulated housing conditions. Although female mice are fertile, they become pregnant less frequently and have small litter sizes; moreover, impaired parturition results in few surviving pups. Primary cultured cells prepared from cytosolic phospholipase A2alpha-deficient mice produce significantly smaller amounts of prostaglandins and leukotrienes. Various disease models such as anaphylaxis, acute lung injury, brain injury induced by ischemia/reperfusion and neurotoxin, and polyposis have been investigated. In all these settings, cytosolic phospholipase A2alpha-deficient mice show significantly milder phenotypes. The mechanisms by which deficiencies of this enzyme exert protective effects may differ, but, a cytosolic phospholipase A2alpha inhibitor could have a wide spectrum of clinical targets. Specific functions of cytosolic phospholipase A2alpha have been clearly demonstrated using the gene-targeted mice. Also, comparisons with mice in which related enzymes and receptors have been manipulated using genetic technologies provide further insights into roles of lipid mediators in physiology and pathology.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.