Eicosanoid Signaling in Insect Immunology: New Genes and Unresolved Issues

This paper is focused on eicosanoid signaling in insect immunology. We begin with eicosanoid biosynthesis through the actions of phospholipase A₂, responsible for hydrolyzing the C18 polyunsaturated fatty acid, linoleic acid (18:2n-6), from cellular phospholipids, which is subsequently converted into arachidonic acid (AA; 20:4n-6) via elongases and desaturases. The synthesized AA is then oxygenated into one of three groups of eicosanoids, prostaglandins (PGs), epoxyeicosatrienoic acids (EETs) and lipoxygenase products. We mark the distinction between mammalian cyclooxygenases and insect peroxynectins, both of which convert AA into PGs. One PG, PGI₂ (also called prostacyclin), is newly discovered in insects, as a negative regulator of immune reactions and a positive signal in juvenile development. Two new elements of insect PG biology are a PG dehydrogenase and a PG reductase, both of which enact necessary PG catabolism. EETs, which are produced from AA via cytochrome P450s, also act in immune signaling, acting as pro-inflammatory signals. Eicosanoids signal a wide range of cellular immune reactions to infections, invasions and wounding, including nodulation, cell spreading, hemocyte migration and releasing prophenoloxidase from oenocytoids, a class of lepidopteran hemocytes. We briefly review the relatively scant knowledge on insect PG receptors and note PGs also act in gut immunity and in humoral immunity. Detailed new information on PG actions in mosquito immunity against the malarial agent, Plasmodium berghei, has recently emerged and we treat this exciting new work. The new findings on eicosanoid actions in insect immunity have emerged from a very broad range of research at the genetic, cellular and organismal levels, all taking place at the international level.

FATTY ACID DESATURASE5 Is Required to Induce Autoimmune Responses in Gigantic Chloroplast Mutants of Arabidopsis

Chloroplasts mediate genetically controlled cell death via chloroplast-to-nucleus retrograde signaling. To decipher the mechanism, we examined chloroplast-linked lesion-mimic mutants of Arabidopsis (Arabidopsis thaliana) deficient in plastid division, thereby developing gigantic chloroplasts (GCs). These GC mutants, including crumpled leaf (crl), constitutively express immune-related genes and show light-dependent localized cell death (LCD), mirroring typical autoimmune responses. Our reverse genetic approach excludes any potential role of immune/stress hormones in triggering LCD. Instead, transcriptome and in silico analyses suggest that reactive electrophile species (RES) generated via oxidation of polyunsaturated fatty acids (PUFAs) or lipid peroxidation-driven signaling may induce LCD. Consistent with these results, the one of the suppressors of crl, dubbed spcrl4, contains a causative mutation in the nuclear gene encoding chloroplast-localized FATTY ACID DESATURASE5 (FAD5) that catalyzes the conversion of palmitic acid (16:0) to palmitoleic acid (16:1). The loss of FAD5 in the crl mutant might attenuate the levels of RES and/or lipid peroxidation due to the reduced levels of palmitic acid-driven PUFAs, which are prime targets of reactive oxygen species. The fact that fad5 also compromises the expression of immune-related genes and the development of LCD in other GC mutants substantiates the presence of an intrinsic retrograde signaling pathway, priming the autoimmune responses in a FAD5-dependent manner.

A genome-wide survey of CD4(+) lymphocyte regulatory genetic variants identifies novel asthma genes

BACKGROUND

Genome-wide association studies have yet to identify the majority of genetic variants involved in asthma. We hypothesized that expression quantitative trait locus (eQTL) mapping can identify novel asthma genes by enabling prioritization of putative functional variants for association testing.

OBJECTIVE

We evaluated 6706 cis-acting expression-associated variants (eSNPs) identified through a genome-wide eQTL survey of CD4(+) lymphocytes for association with asthma.

METHODS

eSNPs were tested for association with asthma in 359 asthmatic patients and 846 control subjects from the Childhood Asthma Management Program, with verification by using family-based testing. Significant associations were tested for replication in 579 parent-child trios with asthma from Costa Rica. Further functional validation was performed by using formaldehyde-assisted isolation of regulatory elements (FAIRE) quantitative PCR and chromatin immunoprecipitation PCR in lung-derived epithelial cell lines (Beas-2B and A549) and Jurkat cells, a leukemia cell line derived from T lymphocytes.

RESULTS

Cis-acting eSNPs demonstrated associations with asthma in both cohorts. We confirmed the previously reported association of ORMDL3/GSDMB variants with asthma (combined P = 2.9 × 10(-8)). Reproducible associations were also observed for eSNPs in 3 additional genes: fatty acid desaturase 2 (FADS2; P = .002), N-acetyl-α-D-galactosaminidase (NAGA; P = .0002), and Factor XIII, A1 (F13A1; P = .0001). Subsequently, we demonstrated that FADS2 mRNA is increased in CD4(+) lymphocytes in asthmatic patients and that the associated eSNPs reside within DNA segments with histone modifications that denote open chromatin status and confer enhancer activity.

CONCLUSIONS

Our results demonstrate the utility of eQTL mapping in the identification of novel asthma genes and provide evidence for the importance of FADS2, NAGA, and F13A1 in the pathogenesis of asthma.

Isolation and analysis of components of CD95 (APO-1/Fas) death-inducing signaling complex

CD95 (APO-1/Fas) is an apoptosis-inducing receptor belonging to the tumor necrosis factor receptor superfamily. Multimerization of CD95 leads to instant recruitment of the signaling molecules FADD and caspase-8 to the activated receptor forming the death-inducing signaling complex (DISC). DISC formation is the first essential step of CD95 signaling and results in activation of caspase-8 starting a signaling cascade that leads to apoptosis. Here we describe a method for analyzing the CD95 DISC. The method is based on coimmunoprecipitation of the signaling molecules with the activated CD95 receptor followed by Western blot detection of associated molecules. Therefore, this method can analyze the very first signaling events during CD95-mediated apoptosis.

Subcellular distribution of rat liver NADPH-2,4-dienoyl-CoA reductase

The subcellular distribution of 2,4-dienoyl-CoA reductase (EC. 1.3.1.34, DCR) in rat liver was studied biochemically and immunochemically after the induction of clofibrate. DCR activity was mainly detected in the mitochondrial fraction by sucrose density gradient centrifugation in the livers of both normal and clofibrate-treated rats. It was also shown that the polyclonal antibody against purified DCR detected the enzyme in the mitochondrial fraction. However, the antibodies, which were affinity purified using the purified mitochondrial DCR or were epitope-selected using the fusion-polypeptide expressed from the mitochondrial cDNA clone (lambda gt11-RDR181), were cross-reacted with the peroxisomal DCR. These results suggest that peroxisomal DCR is immunochemically indistinguishable from mitochondrial DCR.

The known purified mammalian 2,4-dienoyl-CoA reductases are mitochondrial isoenzymes

The aim of this work was to determine the subcellular location of mammalian 2,4-dienoyl-CoA reductase, a key enzyme for degradation of polyunsaturated fatty acids by beta-oxidation. The enzyme was purified according to Kimura et al. (J Biochem 96:1463, 1984), and antibodies were raised in rabbits. Monospecific antibodies were obtained via purification on an affinity column. Immunoblotting of isolated rat liver mitochondria and peroxisomes with the monospecific reductase antibody showed that the antigen was located only in mitochondria. Immunocytochemical experiments with liver tissue, using the protein A-gold labeling technique, confirmed this result. The similarity of their characteristics suggests that the purified reductases described in the literature are the same isoenzyme. Consequently, since the rat enzyme was localized here to the mitochondria, purification and characterization of peroxisomal mammalian reductases remain to be achieved in the future. In addition, a significant induction also of mitochondrial reductase by clofibrate was observed in the immunoblotting experiments.

Reaction of electron-transfer flavoprotein ubiquinone oxidoreductase with the mitochondrial respiratory chain

Submitochondrial particles catalyze the reduction of electron-transfer flavoprotein (ETF) by NADH and succinate under anaerobic conditions in reactions that are totally inhibited by rotenone and thenoyl trifluoroacetone, respectively. The particles also catalyze the ATP-dependent reduction of NAD+ by enzymatically reduced ETF. The latter reaction is inhibited by rotenone and carbonyl cyanide chlorophenylhydrazone and all three reactions are inhibited by antibody to electrontransfer flavoprotein-ubiquinone oxidoreductase (ETF-QO). These observations indicated that ETF-QO reacts with the pool of ubiquinone that is reduced by NADH and succinic dehydrogenases. Consistent with this hypothesis, NADH- and succinic-ETF reductase activities are inhibited 99% in ubiquinone-depleted particles, and reincorporation of exogenous ubiquinone restores at least 90% of these activities. Reduction of the bc1 complex by ETF and acyl CoA oxidase activity are also inhibited by antibody to ETF-QO. Myxothiazole and antimycin which inhibit the quinonol oxidation and quinone reduction sites, respectively, in the bc1 complex also inhibit electron transport from ETF-QO through the complex according to current models of the Q-cycle (Rich, P.R. (1986) J. Bioenerg. Biomembranes 18, 145-156). The results show that ETF-QO is an obligatory component of the electron transport pathway between ETF and the ubiquinone pool and suggest a mechanism for the steady-state turnover of ETF-QO.

Cytoplasmic location of linoleoyl-CoA desaturase in microsomal membranes of rat liver

The intramembrane localization of linoleoyl-CoA desaturase in rat liver microsomes was examined by various methods, such as digestion by proteases, effect of detergents, and inhibition by the antibodies against purified terminal desaturase. Exposure of the desaturase on the surface of microsomal vesicles was suggested by the fact that the enzyme activity in the intact microsomes was susceptible to tryptic digestion, and considerably inhibited by anti-desaturase antibodies. When microsomes were previously treated with trypsin, the enzyme became more susceptible to the antibodies. Furthermore, it was demonstrated that the protein fragments cleaved from microsomal membranes by tryptic digestion formed a single precipitin line with the antibodies by the double-immunodiffusion test. These findings suggest the presence of linoleoyl-CoA desaturase on the cytoplasmic surface in the endoplasmic reticulum, since tryptic digestion liberates only the protein components situated on the surface area of membranes. In addition, desaturase activity in the intact microsomes was not stimulated by addition of the detergent, indicating the further outside location of the active site of the enzyme in microsomal vesicles. The pretreatment of microsomes with a low concentration (0.05%) of sodium deoxycholate, which destroys the permeability barrier for macromolecules without membrane disassembly, did not increase the susceptibility to tryptic digestion and the antibodies. These results show that linoleoyl-CoA desaturase is not present in a latent state in the membrane.

[Immunological specificity and cytoplasmic location of delta 6-desaturase in microsomal membrane]

The enzymatic properties of the three types of microsomal acyl-CoA desaturases, delta 6-, delta 9- and delta 5-desaturase, were immunologically compared using a monospecific antibody raised against the purified linoleoyl-CoA desaturase (delta 6-desaturase). By the double immunodiffusion technique, the anti-delta 6-desaturase antibody showed a single precipitin line to the purified delta 6-desaturase and microsomes treated with Triton X-100, but no line was observed with the partially purified delta 9-desaturase. The antibody even inhibited definitely delta 6-desaturase activity in microsomes, but neither stearoyl-CoA (delta 9-) nor eicosatrienoic acid (delta 5-) desaturations were inhibited. By these immunological investigations it was confirmed that terminal delta 6-desaturase is different enzyme from desaturases delta 9- and delta 5. The intramembrane localization of delta 6-desaturase in rat liver microsomes was examined by various methods, such as digestion by proteases, effect of detergents and inhibition by the antibodies against purified terminal desaturase. Exposure of the desaturase on the surface of microsomal vesicles was suggested by the fact that the enzyme activity in the intact microsomes was susceptible to tryptic digestion and considerably inhibited by anti-desaturase antibodies. When microsomes were previously treated with trypsin, the enzyme became more susceptible to the antibodies. Furthermore, it was demonstrated that the protein fragments cleaved from microsomal membranes by tryptic digestion formed a single precipitin line with the antibodies by the double immunodiffusion test. These findings suggest the presence of delta 6-desaturase on the cytoplasmic surface in the endoplasmic reticulum, since tryptic digestion liberates only the protein components situated on the surface area of membranes. In addition, desaturase activity in the intact microsomes was not stimulated by addition of the detergent, indicating the further outside location of the active site of the enzyme in microsomal vesicles. The previous exposure of microsomes to a low concentration (0.05%) of sodium deoxycholate, which destroys the permeability barrier for macromolecules whichout membrane disassembly, did not increase the susceptibility to tryptic digestion and the antibodies. These results show that delta 6-desaturase is not present in a latent state in the membrane.

Immunochemical evidence for the enzymatic difference of delta 6-desaturase from delta 9- and delta 5-desaturase in rat liver microsomes

The enzymatic properties of the three types of microsomal acyl-CoA desaturases, delta 6-, delta 9- and delta 5-desaturases, were immunologically compared using a monospecific antibody raised against the purified linoleoyl-CoA desaturase (delta 6-desaturase). By the double immunodiffusion technique, the anti-delta 6-desaturase antibody showed a single precipitin line to the purified delta 6-desaturase and microsomes treated with Triton X-100, but no line was observed with the partially purified delta 9-desaturase. The antibody even inhibited definitely delta 6-desaturase activity in microsomes, but neither stearoyl-CoA (delta 9-) nor eicosatrienoic acid (delta 5-) desaturations were inhibited. By these immunological investigations it was confirmed that terminal delta 6-desaturase is different enzyme from desaturases delta 9- and delta 5.