Temperature adaptation of biological membranes: differential homoeoviscous responses in brush-border and basolateral membranes of carp intestinal mucosa

Homologous Delta-12 Fatty Acid Desaturase (FAD2) Genes Affect Gene Expression and Linoleic Acid Levels in Lentinula edodes under Heat Stress

Delta-12 fatty acid desaturases (FAD2s) actively regulate stress responses and cell differentiation in living organisms. In this study, six homologous FAD2 genes were identified based on the genome sequence of Lentinula edodes. Then, the six FAD2 protein sequences were analyzed using bioinformatics tools, including ExPASy ProtParam, SignalP, TMHMM, and TargetP. These analyses were performed to predict the physical and chemical properties, signal peptides, and transmembrane and conserved domains of these proteins. The polypeptide sequences were aligned, and a maximum likelihood phylogenetic tree was constructed using MEGA 7.0 software to elucidate the phylogenetic relationships between homologous FAD2 sequences. The results demonstrated that the FAD2 proteins contained three conserved histidine-rich regions (HXXXH, HXXHH, and HXXHH), which included eight histidine residues. The linoleic acid content and FAD2 enzyme activity were further analyzed, and the levels in the mutagenic heat-tolerant strain 18N44 were lower than those in the wild-type strain 18. Interestingly, the expression levels of the FAD2-2 and FAD2-3 genes under heat stress in strain 18N44 were lower than those in strain 18. These findings indicated that FAD2-2 and FAD2-3 may play major roles in the synthesis of linoleic acid during heat stress.

Nervonic Acid (24:1n-9) is a Dominant Unsaturated Fatty Acid in the Intestinal Brush Border of Atlantic Cod

Atlantic cod is a coldwater teleost of commercial importance. The intestinal epithelium is a large organ in vertebrates serving an important role in nutrient selection and uptake as well as an immunological barrier. Here, we perform lipid and fatty acid analysis of the plasma membrane from the cod intestinal enterocytes after separation of the brush border membrane and the basolateral membrane fractions. Our results show that both membrane fractions contain an unusually high amount of cholesterol and glycolipids but low levels of glycerophospholipids compared with other reported studies on fish. Sphingomyelin was the dominant lipid in the brush border fraction and was also prominent in the basolateral fraction where phosphatidylcholine was the dominant glycerophospholipid. Furthermore, our results show a distinct difference in fatty acids content, where monounsaturated fatty acids (MUFA) were more abundant than polyunsaturated fatty acid (PUFA). Nervonic acid (24:1n-9) was a prominent fatty acid in the BBM at ~50% of the total MUFA. We hypothesize that the high cholesterol content and the presence of this rare fatty acid may serve to maintain membrane fluidity in the cold environment.

Cloning and characterization of a Δ9-desaturase gene of the Antarctic fish Chionodraco hamatus and Trematomus bernacchii

Chionodraco hamatus and Trematomus bernacchii are perciforms, members of the fish suborder Notothenioidei that live in the Antarctic Ocean and experience very cold and persistent environmental temperature. These fish have biochemical and molecular features that allow them to live at these extreme cold temperatures. Fine tuning of the level of unsaturated fatty acids content in membrane is a key mechanism of living organisms to adapt to cold and high temperatures. Desaturases are key enzymes that synthesize unsaturated fatty acyl-CoAs from saturated fatty acids. We cloned and sequenced a Δ(9)-desaturase gene and its cDNA of C. hamatus, and the cDNA of T. bernacchii. The coded proteins are virtually identical and share homology to other Δ(9)-desaturase fish sequences. These proteins contain, in the first trans-membrane domain, two cysteine residues that may form a disulfur bond present in the corresponding membrane region of Δ(9)-desaturase proteins of other Antarctic fish but not in Eleginops maclovinus that experiences higher environmental temperatures and in all other Δ(9)-desaturase genes of mammals present in data bases. C. hamatus Δ(9)-desaturase gene complements a Saccharomyces cerevisiae mutant lacking Δ(9)-desaturase (Ole1) gene. Analysis of sequence homology of the trans-membrane domains of Δ(9)-desaturase and the cytoplasmic region of the same proteins of Antarctic fish, non-Antarctic fish and mammals suggest that the significant differences found in the homologous sequences of the first trans-membrane domain may be due to the specific lipid content of their membrane.

The cold but not hard fats in ectotherms: consequences of lipid restructuring on susceptibility of biological membranes to peroxidation, a review

The production of reactive oxygen species is a regular feature of life in the presence of oxygen. Some reactive oxygen species possess sufficient energy to initiate lipid peroxidation in biological membranes, self-propagating reactions with the potential to damage membranes by altering their physical properties and ultimately their function. Two of the most prominent patterns of lipid restructuring in membranes of ectotherms involve contents of polyunsaturated fatty acids and ratios of the abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine. Since polyunsaturated fatty acids and phosphatidylethanolamine are particularly vulnerable to oxidation, it is likely that higher contents of these lipids at low body temperature elevate the inherent susceptibility of membranes to lipid peroxidation. Although membranes from animals living at low body temperatures may be more prone to oxidation, the generation of reactive oxygen species and lipid peroxidation are sensitive to temperature. These scenarios raise the possibility that membrane susceptibility to lipid peroxidation is conserved at physiological temperatures. Reduced levels of polyunsaturated fatty acids and phosphatidylethanolamine may protect membranes at warm temperatures from deleterious oxidations when rates of reactive oxygen species production and lipid peroxidation are relatively high. At low temperatures, enhanced susceptibility may ensure sufficient lipid peroxidation for cellular processes that require lipid oxidation products.

An explicit test of the phospholipid saturation hypothesis of acquired cold tolerance in Caenorhabditis elegans

Protection of poikilothermic animals from seasonal cold is widely regarded as being causally linked to changes in the unsaturation of membrane phospholipids, yet in animals this proposition remains formally untested. We have now achieved this by the genetic manipulation of lipid biosynthesis of Caenorhabditis elegans independent of temperature. Worms transferred from 25 degrees C to 10 degrees C develop over several days a much-increased tolerance of lethal cold (0 degrees C) and also an increased phospholipid unsaturation, as in higher animal models. Of the three C. elegans Delta9-desaturases, transcript levels of fat-7 only were up-regulated by cold transfer. RNAi suppression of fat-7 caused the induction of fat-5 desaturase, so to control desaturase expression we combined RNAi of fat-7 with a fat-5 knockout. These fat-5/fat-7 manipulated worms displayed the expected negative linear relationship between lipid saturation and cold tolerance at 0 degrees C, an outcome confirmed by dietary rescue. However, this change in lipid saturation explains just 16% of the observed difference between cold tolerance of animals held at 25 degrees C and 10 degrees C. Thus, although the manipulated lipid saturation affects the tolerable thermal window, and altered Delta9-desaturase expression accounts for cold-induced lipid adjustments, the effect is relatively small and none of the lipid manipulations were sufficient to convert worms between fully cold-sensitive and fully cold-tolerant states. Critically, transfer of 10 degrees C-acclimated worms back to 25 degrees C led to them restoring the usual cold-sensitive phenotype within 24 h despite retaining a lipid profile characteristic of 10 degrees C worms. Other nonlipid mechanisms of acquired cold protection clearly dominate inducible cold tolerance.

Rhodamine 123 permeability through the catfish intestinal wall: Relationship to thermal acclimation and acute temperature change

Temperature is known to influence xenobiotic retention in fish. The effect of acute and acclimatory temperature change upon Rhodamine 123 (Rho123) permeability through an in vitro catfish multi-segment (3) everted sac intestinal wall model was examined in a 9 cell matrix of acclimation and assay temperatures (10, 20 and 30 degrees C). Changes in Rho123 permeability were examined in context with membrane fluidity, xenobiotic solubility and intestinal morphology. When assayed at the acclimation temperature greater Rho123 permeability was noted at warmer acclimation temperatures for the proximal and middle intestinal segments, while the distal segment exhibited little change and apparent compensation across temperatures. Rho123 permeability was increased as assay temperatures were elevated above the acclimation temperature for most comparisons. Cold acclimation significantly increased total intestinal length (43.2%) and proximal intestine weights while total body weights did not differ. Brush border membranes (BBM) increased fluidity with increased assay temperatures, however, composite anisotropy lines were not significantly different between acclimation treatments. In an additive manner, the membrane probe DPH exhibited increased solubility in BBM with increases in acclimation and assay temperatures. Compositely, these results suggest that acclimation and acute temperature change may differentially influence xenobiotic permeability among intestinal segments with interacting mechanisms.

Thermally induced changes in lipid composition of raft and non-raft regions of hepatocyte plasma membranes of rainbow trout

In poikilotherms, increases in plasma membrane (PM) cholesterol and an increase in the degree of lipid acyl chain saturation commonly accompany an increase in growth temperature. This has typically been interpreted in terms of membrane fluidity/order homeostasis, but these changes would also be expected to stabilize the structure of PM rafts against thermal perturbation. Rafts are microdomains that organize the molecules of many signaling cascades and are formed as a result of interactions between lipids with saturated acyl chains and cholesterol. No study to date has examined the thermally induced compositional changes of raft and non-raft regions of the PM separately. In this study we have measured the phospholipid class composition and fatty acid composition of raft-enriched (raft) and raft-depleted PM (RDPM) of hepatocytes from trout Oncorhynchus mykiss acclimated to 5 degrees C and 20 degrees C. In the raft, warm acclimation was associated with a reduction in the proportion of phosphatidylcholine from 56% to 30% while phosphatidylserine and phosphatidylinositol each increased from 8% to approximately 20% of the total phospholipid. Additionally, there were significantly fewer unsaturated fatty acids in the raft lipids from warm-acclimated (61%) than from the cold-acclimated trout (68%). In contrast, there were no significant changes in phospholipid class or acyl chain unsaturation in the RDPM. These data suggest that changes in raft lipid composition, rather than the PM as a whole, are particularly important during thermal acclimation.

Post-genomic approaches to understanding the mechanisms of environmentally induced phenotypic plasticity

Post-genomic techniques offer new and detailed insights into the mechanisms underpinning all biological processes, including phenotypic plasticity and environmentally relevant phenotypes. Although they require access to genomic resources it is now possible to create these for species of comparative or environmental interest even within a modest research project. Here we describe an open transcript screen for genes responding to environmental cold that might account for the acquired cold-specific phenotype in all its complex manifestations. Construction of a cDNA microarray led to a survey of transcript expression levels in seven tissues of carp, as a function of time,and three different extents of cooling. The resulting data delineated a common stress response found in all tissues that comprises genes involved in cellular homeostasis, including energy charge, ATP turnover, protein turnover and stress protein production. These genes respond to kinds of perturbation other than cold and probably form part of a more general stress response common to other species. We also defined tissue-specific response patterns of transcript regulation whose main characteristics were investigated by a profiling technique based on categorisation of gene function. These genes underpin the highly tissue-specific pattern of physiological adaptations observed in the cold-acclimated fish. As a result we have identified a large number of candidate gene targets with which to investigate adaptive responses to environmental challenge.

Membrane order conservation in raft and non-raft regions of hepatocyte plasma membranes from thermally acclimated rainbow trout

Homeoviscous adaptation (HVA), the thermal conservation of membrane fluidity/order at different body temperatures, has been observed to varying degrees in different membranes. However, HVA has not been studied in raft and non-raft regions of the plasma membrane (PM) separately. Rafts are ordered PM microdomains implicated in signal transduction, membrane traffic and cholesterol homeostasis. Using infrared spectroscopy, we measured order in raft-enriched PM (raft) and raft-depleted PM (RDPM) isolated from hepatocytes of rainbow trout (Oncorhynchus mykiss) acclimated to 5 and 20 degrees C. We found approximately 130% and 90% order compensation in raft and RDPM, respectively, suggesting their independent regulation. Raft was more ordered than RDPM in the warm-acclimated trout, a difference fully explained by a 58% enrichment of cholesterol, compared to RPDM. Unexpectedly, raft and RDPM from cold-acclimated trout did not differ in cholesterol content or order. Freezing the membrane samples during preparation had no effect on order. Treatment with cyclodextrin depleted cholesterol by 36%, 56%, and 55%, producing significant decreases in order in raft and RDPM from warm-acclimated trout and RDPM from cold-acclimated trout, respectively. However, a 69% depletion of cholesterol from raft from cold-acclimated trout had no significant effect on order. This result, and the lack of a difference in order between raft and RDPM, suggests that raft and non-raft PM in cold-acclimated trout are not spatially segregated by phase separation due to cholesterol.

Specific binding of the endocytosis tracer horseradish peroxidase to intestinal fatty acid-binding protein (I-FABP) in apical membranes of carp enterocytes

In a previous study we had demonstrated that a 15-kDa protein present in carp intestinal brush-border membrane vesicles (BBMV) was able to bind the endocytosis tracer horseradish peroxidase (HRP) with high specificity. Here we show that this protein corresponds to a peripheral membrane protein, identified by partial amino acid sequence analysis as the intestinal fatty acid-binding protein (I-FABP), a member of the small cytosolic fatty acid binding protein family (FABPs). The presence of I-FABP and its HRP-binding activity was demonstrated both in the cytosolic and membrane-associated fractions of intestinal mucosa by Western and ligand blot analyses, respectively. Also, both fractions displayed significant capacity to bind [(3)H]palmitic acid, a known ligand for I-FABP. Immunohistochemical analysis showed that I-FABP localizes both in the cytosol and in the brush-border membranes of epithelial cells. Taken together the unusual extra-cellular localization of I-FABP as well as its ability to interact with HRP suggests a novel function for this protein in the intestinal mucosa.

Molecular cloning and sequence analysis of stearoyl-CoA desaturase in milkfish, Chanos chanos

Stearoyl-CoA desaturase (EC 1.14.99.5) is a key enzyme in the biosynthesis of polyunsaturated fatty acids and the maintenance of the homeoviscous fluidity of biological membranes. The stearoyl-CoA desaturase cDNA in milkfish (Chanos chanos) was cloned by RT-PCR and RACE, and it was compared with the stearoyl-CoA desaturase in cold-tolerant teleosts, common carp and grass carp. Nucleotide sequence analysis revealed that the cDNA clone has a 972-bp open reading frame encoding 323 amino acid residues. Alignments of the deduced amino acid sequence showed that the milkfish stearoyl-CoA desaturase shares 79% and 75% identity with common carp and grass carp, and 63%-64% with other vertebrates such as sheep, hamsters, rats, mice, and humans. Like common carp and grass carp, the deduced amino acid sequence in milkfish well conserves three histidine cluster motifs (one HXXXXH and two HXXHH) that are essential for catalysis of stearoyl-CoA desaturase activity. However, RT-PCR analysis showed that stearoyl-CoA desaturase expression in milkfish is detected in the tissues of liver, muscle, kidney, brain, and gill, and more expression sites were found in milkfish than in common carp and grass carp. Phylogenic relationships among the deduced stearoyl-CoA desaturase amino acid sequence in milkfish and those in other vertebrates showed that the milkfish stearoyl-CoA desaturase amino acid sequence is phylogenetically closer to those of common carp and grass carp than to other higher vertebrates.

Horseradish peroxidase binding to intestinal brush-border membranes of Cyprinus carpio. Identification of a putative receptor

Morphologic studies have shown that the classic endocytosis tracer horseradish peroxidase (HRP) is actively internalized by vesicular transport in the carp intestine, suggesting the existence of specific binding sites in the apical membrane of enterocytes. The aim of the present study was to develop an in vitro binding assay using isolated carp intestinal brush-border membranes (BBM) to demonstrate and characterize these specific HRP binding sites. The results obtained show that HRP binding to BBM exhibits a saturable mode and high affinity (K(d) = 22 nM). In addition, HRP binding sites are highly enriched in BBM compared to basolateral membranes. On the other hand, HRP interaction with these sites is apparently of an ionic character because binding increased concomitantly with decreasing NaCl concentrations in the assay, reaching a maximum in the absence of NaCl. Other proteins that are also internalized in carp intestine did not significantly inhibit HRP binding to BBM. A lectin-type of interaction was discarded because neither manan nor ovoalbumin inhibited HRP binding. Proteinase K treatment of BBM reduced HRP binding by 70%, suggesting a proteic nature for this binding site. Finally, ligand blotting assays showed that HRP binds specifically to a 15.3-kDa protein. Taken together, these results are consistent with the existence of a functional receptor for HRP in carp intestinal mucosa that could mediate its internalization.

Lipid compositional correlates of temperature-adaptive interspecific differences in membrane physical structure

Teleost species from cold environments possess more disordered brain synaptic membranes than species from warm habitats, thereby providing equivalent physical structures at their respective habitat temperatures. We have related this adaptive interspecific biophysical response to the fatty acid composition of brain membranes from 17 teleost species obtained from Antarctic, temperate and semi-tropical waters, as well as from rat and turkey as representative homeotherms. Cold-adaptive increases in membrane disorder (determined by fluorescence anisotropy with diphenylhexatriene as probe) were correlated with large and linear increases in the proportion of unsaturated fatty acids, from 35 to 60 % in phosphatidylcholine (PtdCho) and from 55 to 85 % in phosphatidylethanolamine (PtdEth). For PtdCho, the cold-adaptive increase in unsaturation was associated almost entirely with increased proportions (from 7 to 40 %) of polyunsaturated fatty acids (PUFAs), with mono-unsaturates (MUFAs) providing an approximately constant proportion in all species. Exactly opposite effects were evident for phosphatidylethanolamine (PtdEth). Thus, the compositional adaptation for PtdCho occurred largely by exchange of polyunsaturated and mono-unsaturated fatty acid in the sn-2 position, whilst for PtdEth it involved exchanges between saturates and mono-unsaturates at the sn-1 position. This difference may be related to the different molecular shapes of the two phosphoglycerides and the need to maintain the balance between bilayer-stabilising and -destabilising tendencies. This comparative study provides a more comprehensive view of the compositional adjustments that accompany and perhaps account for temperature-adaptive interspecific differences in membrane physical structure.

Dietary n-3 long-chain polyunsaturated fatty acid deprivation, tissue lipid composition, ex vivo prostaglandin production, and stress tolerance in juvenile Dover sole (Solea solea L.)

Larval Dover sole fed an Artemia diet supplemented with n-3 long-chain (C20 + C22) polyunsaturated fatty acids (PUFA) are known to be more resistant to low-temperature injury. Here we explore the relationship between tissue fatty acid composition and tolerance of stressful environmental conditions over the larval and early juvenile periods. Artemia nauplii supplemented with n-3 long-chain PUFA-deficient and PUFA-enriched oil emulsions were fed to two groups of larvae. Whole body tissue samples from the resulting PUFA-deficient and -enriched juveniles possessed 12.1 and 21.9% n-3 long-chain PUFA, respectively. These differences were at the expense of C18 PUFA, while proportions of saturated fatty acids, monounsaturated fatty acids, and total PUFA were unaffected. Brain and eye tissues from the PUFA-deficient fish contained lower levels of 22:6n-3, known to be important for optimal nervous system function, incorporating instead a range of fatty acids of lower unsaturation. PUFA-deprived juveniles showed substantially greater mortality when exposed to a combination of low temperature and low salinity, as well as to high temperature and to hypoxia. After adaptation to the different diets, both dietary groups were fed a common formulated feed high in n-3 long-chain PUFA. Tissue PUFA in both groups progressively increased to the same high value, with a consequent loss of the differences in cold-susceptibility. These correlated changes support a link between dietary manipulation of n-3 long-chain PUFA and development of a stress-sensitive phenotype. PUFA deprivation had no detectable effect upon static hydrocarbon order of purified brain membranes (as assessed by fluorescence polarization) but was associated with an increase in the whole-body content of prostaglandins. We conclude that susceptibility to environmental stress is responsive to dietary n-3 long-chain PUFA manipulation, possibly due to altered tissue development or the overproduction of eicosanoids.

Identification and characterization of an animal delta(12) fatty acid desaturase gene by heterologous expression in Saccharomyces cerevisiae

We have cloned a Caenorhabditis elegans cDNA encoding a Delta12 fatty acid desaturase and demonstrated its activity by heterologous expression in Saccharomyces cerevisiae. The predicted protein is highly homologous both to the cloned plant genes with similar function and to the published sequence of the C. elegans omega-3 fatty acid desaturase. In addition, it conforms to the structural constraints expected of a membrane-bound fatty acid desaturase including the canonical histidine-rich regions. This is the first report of a cloned animal Delta(12) desaturase gene. Expression of this cDNA in yeast resulted in the accumulation of 16:2 and 18:2 (linoleic) acids. The increase of membrane fluidity brought about by this change in unsaturation was measured. The production of polyunsaturated fatty acids in yeast cells and the concomitant increase in membrane fluidity was correlated with a modest increase in growth rate at low temperature and with increased resistance to ethanol and oxidative stress.

Thermal thresholds of lipid restructuring and delta(9)-desaturase expression in the liver of carp (Cyprinus carpio L.)

Cold acclimation induces a transient enzymatic activation of the acyl CoA-(Δ)(9)-desaturase in carp liver. We have determined thresholds for two underlying mechanisms; namely, the activation of latent enzyme and the induced synthesis of new desaturase. Carp were progressively cooled from 30 degrees C to 23, 17 and 10 degrees C, where they were held for up to 5 days. Endoplasmic reticulum phospholipids showed substantial changes in fatty acid composition, with linear decreases in the proportion of saturates with temperature over the full range of cooling (11.3 % in phosphatidylcholine and 15.8 % in phosphatidylethanolamine). In the phosphatidyl-ethanolamine fraction, this was linked to increased proportions of monoenes, particularly 20:1(n-9). Modest cooling to 23 degrees C on day 1 induced a 2.5-fold transient increase in delta(9)-desaturase activity without any change in the amount of desaturase protein or transcript. Further cooling to 17 degrees C induced a greater and more sustained increase in desaturase activity, reaching sevenfold on day 5, with a 10- to 20-fold increase in the amount of desaturase transcript. Extreme cooling to 10 degrees C led to a very large, but transient, 40- to 50-fold increase in desaturase transcript amounts, a modest 40–50 % increase in desaturase protein but no further increase in activity over that observed at 17 degrees C. These results distinguish at least three mechanisms involved in cold-induced lipid restructuring; the activation of latent desaturase observed with gentle cooling, the induction of desaturase gene transcription and, finally, a third unidentified lipid compensatory mechanism that occurs with extreme cooling. The complex nature of cold-induced lipid restructuring also involves changes in the activity of other biosynthetic enzymes, including elongase and positional- and phospholipid-specific acyltransferases.

Structure and expression of fatty acid desaturases

Fatty acid desaturases are enzymes that introduce double bonds into fatty acyl chains. They are present in all groups of organisms, i.e., bacteria, fungi, plants and animals, and play a key role in the maintenance of the proper structure and functioning of biological membranes. The desaturases are characterized by the presence of three conserved histidine tracks which are presumed to compose the Fe-binding active centers of the enzymes. Recent findings on the structure and expression of different types of fatty acid desaturase in cyanobacteria, plants and animals are reviewed in this article. Roles of individual desaturases in temperature acclimation and principles of regulation of the desaturase genes are discussed.

Catalytic hydrogenation of polyunsaturated biological membranes: effects on membrane fatty acid composition and physical properties

The relationship between phospholipid saturation and membrane physical structure in a complex, highly polyunsaturated biological membrane (trout liver microsomes) has been studied by the graded and specific hydrogenation of polyunsaturated fatty acids. The homogeneous catalyst Pd(QS)2 caused rapid and effective hydrogenation, increasing the proportion of saturated fatty acids from 20-30% up to 60%, without loss or fragmentation. Long chain, polyunsaturated fatty acids (20:5 omega 3, 22:6 omega 3) were rapidly converted to a large number of partially hydrogenated isomers, and ultimately to the fully saturated C20 or C22 fatty acids. C18 mono- and di-unsaturates showed slower rates of hydrogenation. Increased saturation was closely associated with an increased membrane physical order as determined by the fluorescence anisotropy probe, 1,6-diphenyl-1,3,5-hexatriene. However, extensive hydrogenation led to highly ordered membranes exhibiting a gel-liquid crystalline phase transition between 30 and 60 degrees C. Polyunsaturated membranes can thus be converted into partially or substantially saturated membranes with measurable phase structure without direct alteration of other membrane components. This offers a less equivocal means of assessing the influence of polyunsaturation upon membrane structure and function.

Effects of membrane fatty acids on thermal and oxidative injury in the human premonocytic line U937

Heat shock (HS) proteins (HSP) function as molecular chaperones and protect cells from thermal and oxidative injury. The signals leading to HSP synthesis, i.e. the "cellular thermometer(s)," are still a matter of debate. In the human premonocytic line U937, we investigated the effects of specific modification of membrane fatty acid (FA) composition by incubation with various saturated and unsaturated fatty acids (UFA) on the HS response and on hydrogen peroxide (H2O2)-induced cell death. FA readily incorporated into U937 cell membranes. UFA did not modulate the HS response but potentiated H2O2-mediated damage, while pre-exposure to HS protected the UFA-treated cells from this increased H2O2 toxicity.

An isothermal induction of delta 9-desaturase in cultured carp hepatocytes

Cold exposure of carp leads to the induced activity of the hepatic delta 9-desaturase (Schünke, M. and Wodtke, E. (1983) Biochim. Biophys. Acta 734, 70-75). We have investigated the controlled expression of this enzyme using isolated carp hepatocytes. Culture at 30 degrees C, of cells isolated from 30 degrees C-acclimated carp. resulted in an 8-13-fold increase in desaturase-specific activity over 4 days, whilst another enzyme of intermediary metabolism, glucose-6-phosphatase, decreased by more than 60%. This desaturase induction was associated with a loss of intracellular lipid vesicles and with increases in the levels of oleic acid of membrane phosphoglycerides and corresponding decreases in 22:6(n - 3). Supplementation of cultures with oleic acid and with polyunsaturated fatty acids did not cause any reduction in the desaturase induction. The level of immunodetectable desaturase protein increased during culture at 30 degrees C and a desaturase mRNA was detected after 2 days of culture by Northern analysis. These results suggest that in vitro culture leads to an increased synthesis of desaturase protein by means of activated gene transcription. Significantly, transfer of cultures of 30 degrees C-acclimated hepatocytes to 10 degrees C resulted in a smaller induction of desaturase activity; thus cold transfer of cells in itself did not induce hepatocyte desaturase activity as does whole animal cooling. This suggests either that cold induction of desaturase activity in vivo involves systemic control or that the conditions imposed by culture prevent cold induction.

Membrane lipid perturbation modifies the set point of the temperature of heat shock response in yeast

Addition of a saturated fatty acid (SFA) induced a strong increase in heat shock (HS) mRNA transcription when cells were heat-shocked at 37 degrees C, whereas treatment with an unsaturated fatty acid (UFA) reduced or eliminated the level of HS gene transcription at 37 degrees C. Transcription of the delta 9-desaturase gene (Ole1) of Histoplasma capsulatum, whose gene product is responsible for the synthesis of UFA, is up-regulated in a temperature-sensitive strain. We show that when the L8-14C mutant of Saccharomyces cerevisiae, which has a disrupted Ole1 gene, is complemented with its own Ole1 coding region under control of its own promoter or Ole1 promoters of H. capsulatum, the level of HS gene transcription depends on the activity of the promoters. Fluorescence anisotropy of mitochondrial membranes of completed strains corresponded to the different activity of the Ole1 promoter used. We propose that the SFA/UFA ratio and perturbation of membrane lipoprotein complexes are involved in the perception of rapid temperature changes and under HS conditions disturbance of the preexisting membrane physical state causes transduction of a signal that induces transcription of HS genes.

Molecular architecture and biophysical properties of phospholipids during thermal adaptation in fish: an experimental and model study

Phospholipids from livers of carps (Cyprinus carpio L.) adapted to winter (5 degrees C) and summer (25 degrees C) temperatures were isolated, and the fatty acid composition of total phospholipids, as well as molecular species composition of diacyl phosphatidylcholines and ethanolamines, were determined. Order parameter of 5-doxyl stearic acid and steady-state fluorescence anisotropy of different anthroyloxy fatty acids--[2-, 12(N-9-anthroyloxy)stearic acid and 16(N-9-anthroyloxy)palmitic acid--embedded in native and synthetic (16:0/16:0, 16:0/22:6, 18:0/22:6, 18:1/22:6, 20:4/20:4, 22:6/22:6 phosphatidylcholines and 16:0/18:1, 18:1/22:6 phosphatidylethanolamines) phospholipid vesicles was also determined between -30 and 30 degrees C and 5 and 30 degrees C, respectively. There is an accumulation of 1-monoenoic, 2-polyenoic diacyl phosphatidylcholine and ethanolamine with a concomitant reduction of 1-stearoyl,2-docosahexaenoyl species in the cold-adapted state. Despite a 30% accumulation of long-chain polyunsaturated fatty acids in phospholipids in cold, there is only a 5 degrees C downshift in the solid-gel to liquid-crystalline phase transition temperature (-8 vs. -13 degrees C). Vesicles from total phospholipids of cold-adapted fish proved to be more disordered in all segments than from the warm-adapted ones when assayed using 2,12-(N-9-anthroyloxy)stearic and 16-(N-9-anthroyloxy)palmitic acid. Vesicles made from purified phosphatidylcholines showed the same pattern, but they were more disordered than the corresponding total phospholipids. This could be modelled using mixed phospholipid vesicles made of synthetic 16:0/22:6 phosphatidylcholine (75%) and either 18:1/22:6 phosphatidylethanolamine (25%) vs. 16:0/18:1 phosphatidylethanolamine (25%) and comparison of the anisotropy parameters of 100% 16:0/22:6 and 100% 18:1/22:6 phosphatidylcholine vesicles. Mixing either 16:0/18:1 (25%) or 18:1/22:6 (25%) phosphatidylethanolamines to 18:0/22:6 (75%) phosphatidylcholine shifted down or up, respectively, the transition temperature of vesicles compared to 100% 18:0/22:6 vesicles assayed by electron spin resonance spectroscopy using 5-doxylstearic acid. It is concluded that it is not the gross amount of long-chain polyunsaturated fatty acids in phospholipids, but rather their specific combination with cis delta 9 monounsaturated fatty acids in the position sn-1, especially in phosphatidylethanolamines, that is important in determining the physical properties of biomembranes in relation to adaptational temperature.

Thermal acclimation and dietary lipids alter the composition, but not fluidity, of trout sperm plasma membrane

The effect of a long-term adaptation of rainbow trout to 8 and 18 degrees C combined with a corn oil- or a fish oil-supplemented diet on the characteristics of the spermatozoan plasma membrane was investigated. The experiment lasted up to 22 mon during which spermatozoa were collected from the mature males. Spermatozoan plasma membranes were isolated by nitrogen cavitation, and the cholesterol content, phospholipid composition and fatty acid pattern were investigated. Membrane viscosity was assessed on whole cells by electron spin resonance using spin-labeled phospholipids. Neither diet nor rearing temperature influenced the cholesterol content of the plasma membrane nor the phospholipid class distribution. The rearing temperature of the broodstock only slightly affected the phospholipid fatty acids. A minor decrease in 18:0 and increase in monounsaturated fatty acids was observed for the cold-adapted fish. These modifications were not sufficient to affect membrane fluidity, and we conclude that trout spermatozoa do not display any homeoviscous adaptations in these conditions. On the contrary, the dietary fatty acid intake greatly modified the fatty acid profile of plasma membrane phospholipids. The fish oil-fed trout displayed a much higher n-3/n-6 fatty acid ratio than did the corn oil-fed ones, but the 22:6n-3 levels remained unchanged. Modifications in plasma membrane composition by the diet were obtained although neither of the two diets was deficient in essential fatty acids. The enrichment in n-3 fatty acids, however, did not affect plasma membrane fluidity which was unchanged by the diets.

Structural order of membranes and composition of phospholipids in fish brain cells during thermal acclimatization

A comparison of the structural orders of membranes of a mixed brain-cell population isolated from Cyprinus carpio L. acclimated to either summer (23-25 degrees C) or winter (5 degrees C) revealed a high degree of compensation (80%) for temperature, as assayed by electron spin resonance spectroscopy. The cells rapidly forget their thermal history and adjust the physical properties of the membranes when shifted to the other extreme of temperature either in vivo or in vitro. Phospholipids separated from both types of animals exhibit only around 10% compensation. Arachidonic and docosahexaenoic acids are the major polyunsaturated fatty acids in the brains, but the fatty acid composition of the brain total phospholipids does not vary with adaptation to temperature. Separation of phosphatidylcholines and phosphatidylethanolamines into molecular species revealed a 2- to 3-fold accumulation of 18:1/22:6, 18:1/20:4, and 18:1/18:1 species in the latter; 18:0/22:6 showed an opposite tendency. Molecular species composition of phosphatidylcholines did not vary with the temperature. The same trends of changes were seen with brains of freshwater fish from subtropical (Catla catla L.) or boreal (Acerina cernua) regions. It is concluded that the gross amount of docosahexaenoic acid (22:6) plays only a minor role in adjusting the membrane physical properties to temperature. Factors other than lipids might be involved in the adaptation processes. Due to their specific molecular architecture, molecules such as 18:1/22:6, 18:1/20:4, or 18:1/18:1 phosphatidylethanolamine might prevent the contraction of membranes in the cold and may provide an environment for some other components involved in the temperature regulation of physical properties of nerve cell membranes.

A near perfect temperature adaptation of bilayer order in vertebrate brain membranes

The bilayer order of a brain synaptic membrane fraction from a number of fish, mammalian and avian species have been compared in relation to their respective body temperatures using steady-state and time-resolved fluorescence anisotropy techniques. Fluorescence anisotropy for both 1,6-diphenyl-1,3,5-hexatriene and trans-parinaric acid increased in the order: antarctic Notothenia, trout, perch, cichlid, rat and starling, this also being the order of increasing body temperature. This suggests that cold-adapted fish species possess more disordered brain membranes than warm-adapted fish species, and mammals and birds membranes were more ordered than fish membranes. Comparison of temperature profiles for both fluorescence probes showed that fish species display similar anisotropies, and by inference bilayer order, to mammals and birds when measured at their respective body temperatures. Time-resolved analysis showed that the interspecific differences in (P2) order parameter was consistently related to body temperature whilst the rotational diffusion coefficient was not. These results suggest that brain membrane order is highly conserved within the vertebrates despite large differences in thermal habits and phylogenetic position. Polar fish species have by far the lowest bilayer order indicating that invasion of extreme cold habitats involved an adaptive decrease in bilayer order and conversely adoption of a high body temperature by mammals involved an adaptive increase in bilayer order. The conservation of membrane static order for these species at their respective body temperatures indicates a regulatory control of this aspect of membrane hydrocarbon structure and the functional importance of this structure.

Rapid cold-induced changes of membrane order and delta 9-desaturase activity in endoplasmic reticulum of carp liver: a time-course study of thermal acclimation

The membrane order of liver endoplasmic reticulum (ER) membranes of 10 degrees C- and 30 degrees C-acclimated carp has been compared using the fluorescence polarization technique with DPH as probe. Membranes from cold-acclimated fish displayed lower polarizations than corresponding membranes from warm-acclimated fish, the difference compensating for 34-50% of the direct effects of temperature upon polarization. The changes in delta 9-desaturase activity and fluorescence polarization of DPH in ER membranes have been monitored as a function of time during cold acclimation of 30 degrees C-acclimated carp. Cooling was achieved in three stages over 48 h. Desaturase activity in both rough and smooth ER showed a rapid increase in activity for the first three days followed by a decline on day 4 and a second increase up to day 10. Polarization of DPH (measured at 10 degrees C) was rapidly reduced on cooling with no further change after day 4. The halftime for change in polarization and for the first desaturase induction were both approx. 2 days although large changes in polarization were evident within 24 h after the onset of cooling. During the cooling phases the daily changes in DPH polarization were quantitatively related to increments in desaturase capacity. The second desaturase induction had no effect upon membrane structure, at least as indicated by the polarization technique.