Adaptation of biological membranes to temperature: molecular species compositions of phosphatidylcholine and phosphatidylethanolamine in mitochondrial and microsomal membranes of liver from thermally-acclimated rainbow trout

Sexual dimorphism in the gonad lipidome of blue mussels (Mytilus sp.): New insights from a global lipidomics approach

Blue mussels (Mytilus sp.) are an economically important species for European aquaculture. Their importance as a food source is expected to increase in the coming net-zero society due to their low environmental footprint; however, their production is affected by anthropogenic stressors and climate change. During reproduction, lipids are key molecules for mussels as they are the main source of energy on which newly hatched embryos depend in the first days of their development. In this work, blue mussels of different origins are analysed, focusing on the differences in lipid composition between the ovary (BMO) and the testis (BMT). The lipidome of blue mussel gonads (BMG) is studied here by combining traditional lipid profiling methods, such as fatty acid and lipid class analysis, with untargeted liquid chromatography-mass spectrometry (LC-MS) lipidomics. The approach used here enabled the identification of 770 lipid molecules from 23 different lipid classes in BMG. BMT, which consists of billions of spermatocytes, had greater amounts of cell membrane and membrane lipid components such as FA18:0, C20 polyunsaturated fatty acids (PUFA), free sterols (ST), ceramide phosphoethanolamines (CerPE), ceramide aminoethylphosphonates (CAEP), cardiolipins (CL), glycerophosphocholines (PC), glycerophosphoethanolamines (PE) and glycerophosphoserines (PS). In BMO, saturated fatty acids (FA14:0 and FA16:0), monounsaturated fatty acids (MUFA) and other storage components such as C18-PUFA accumulated in triradylglycerolipids (TG) and alkyldiacylglycerols (neutral plasmalogens, TG O-), which, together with terpenes, wax esters and cholesterol esters, make up most of oocytes yolk reserves. BMO also had higher levels of ceramides (Cer) and generally alkyl/alkenyl glycerophospholipids (mainly plasmanyl/plasmenyl PC), suggesting a role for these lipids in vitellogenesis. Non-methylene interrupted dienoic fatty acids (NMID FA), typically found in plasmalogens, were the only membrane-forming PUFA predominantly detected in BMO. The results of this study are of great importance for clarifying the lipid composition of BMG and provide an important basis for future studies on the reproductive physiology of these organisms.

Miscibility Transition Temperature Scales with Growth Temperature in a Zebrafish Cell Line

Cells can alter the lipid content of their plasma membranes upon changes in their environment to maintain and adjust membrane function. Recent work suggests that some membrane functions arise because cellular plasma membranes are poised close to a miscibility transition under growth conditions. Here we report experiments utilizing giant plasma membrane vesicles (GPMVs) to explore how membrane transition temperature varies with growth temperature in a zebrafish cell line (ZF4) that can be adapted for growth between 20 and 32°C. We find that GPMV transition temperatures adjust to be 16.7 ± 1.2°C below growth temperature for four growth temperatures investigated and that adjustment occurs over roughly 2 days when temperature is abruptly lowered from 28 to 20°C. We also find that GPMVs have slightly different lipidomes when isolated from cells adapted for growth at 28 and 20°C. Similar to past work in vesicles derived from mammalian cells, fluctuating domains are observed in ZF4-derived GPMVs, consistent with their having critical membrane compositions. Taken together, these experimental results suggest that cells in culture biologically tune their membrane composition in a way that maintains specific proximity to a critical miscibility transition.

Seasonal influences on PCB retention and biotransformation in fish

There is extensive evidence that fish from waters with polychlorinated biphenyls (PCB)-contaminated sediments accumulate PCBs and related chemicals and that people who eat fish from contaminated waters have higher body burdens of PCBs and PCB metabolites than those who do not. PCBs and their metabolites are potentially toxic; thus, it is important to human health to understand the uptake, biotransformation, and elimination of PCBs in fish since these processes determine the extent of accumulation. The intestinal uptake of PCBs present in the diet of fish into fish tissues is a process that is influenced by the lipid composition of the diet. Biotransformation of PCBs in fish, as in mammals, facilitates elimination, although many PCB congeners are recalcitrant to biotransformation in fish and mammals. Sequential biotransformation of PCBs by cytochrome P450 and conjugation pathways is even less efficient in fish than in mammalian species, thus contributing to the retention of PCBs in fish tissues. A very important factor influencing overall PCB disposition in fish is water temperature. Seasonal changes in water temperature produce adaptive physiological and biochemical changes in fish. While uptake of PCBs from the diet is similar in fish acclimated to winter or summer temperatures, there is evidence that elimination of PCBs occurs much more slowly when the fish is acclimated at low temperatures than at warmer temperatures. Research to date suggests that the processes of elimination of PCBs are modulated by several factors in fish including seasonal changes in water temperature. Thus, the body burden of PCBs in fish from a contaminated location is likely to vary with season.

Beyond the lipid hypothesis: mechanisms underlying phenotypic plasticity in inducible cold tolerance

The physiological adjustment of organisms in response to temperature variation is a crucial part of coping with environmental stress. An important component of the cold response is the increase in membrane lipid unsaturation, and this has been linked to an enhanced resistance to the debilitating or lethal effects of cold. Underpinning the lipid response is the upregulation of fatty acid desaturases (des), particularly those introducing double bonds at the 9-10 position of saturated fatty acids. For plants and microbes there is good genetic evidence that regulation of des genes, and the consequent changes in lipid saturation, are causally linked to generation of a cold-tolerant phenotype. In animals, however, supporting evidence is almost entirely limited to correlations of saturation with cold conditions. We describe our recent attempts to provide a direct test of this relationship by genetic manipulation of the nematode Caenorhabditis elegans. We show that this species displays a strong cold tolerant phenotype induced by prior conditioning to cold, and that this is directly linked to upregulated des activity. However, whilst genetic disruption of des activity and lipid unsaturation significantly reduced cold tolerance, animals retained a substantial component of their stress tolerant phenotype produced by cold conditioning. This indicates that mechanisms other than lipid unsaturation play an important role in cold adaptation.

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.

Phospholipid molecular species, beta-oxidation, desaturation and elongation of fatty acids in Atlantic salmon hepatocytes: effects of temperature and 3-thia fatty acids

We have investigated the effects of a 3-thia fatty acid (TTA) and of temperature on the fatty acid (FA) metabolism of Atlantic salmon (Salmo salar). One experiment investigated the activity of the peroxisomal beta-oxidation enzyme, acyl-CoA oxidase (ACO), and the incorporation of TTA into phospholipid (PL) molecular species. Salmon hepatocytes in culture were incubated either without TTA (control(spades)) or with 0.8 mM TTA (TTA(spades)) in a short term (48 h) temperature study at 5 degrees C and at 12 degrees C. TTA was incorporated into the four PL classes studied: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidylserine (PS). TTA was preferentially esterified with 18:1, 16:1, 20:4 and 22:6 in the PLs. Hepatocytes incubated with TTA had higher ACO activity at 5 degrees C than at 12 degrees C. In a second experiment salmon were fed a diet based on fish meal-fish oil without any TTA added (control) or a fish meal-fish oil diet supplemented with 0.6% TTA for 8 weeks at 12 degrees C and 20 weeks at 5 degrees C. At the end of the feeding trial, hepatocytes from fish acclimated to high or low temperatures were isolated from both dietary groups and incubated with either [1-(14)C]18:1 n-9 or [1-(14)C]20:4 n-3 at 5 degrees C or 12 degrees C. Radiolabelled 18:1 n-9 was mainly esterified into neutral lipids (NL), whereas [1-(14)C]20:4 n-3 was mainly esterified into PL at both temperatures. The rate of elongation of [1-(14)C]18:1 n-9 to 20:1 n-9 was twice as high in hepatocytes from fish fed the control diet than it was in hepatocytes from fish fed the TTA diet, at both temperatures. The amount of [1-(14)C]20:4 n-3 converted to 22:6 n-3 was approximately the same in hepatocytes from the two dietary groups, but there was a tendency to higher production of 22:6 n-3 at the lower temperature. Oxidation of [1-(14)C]18:1 n-9 to acid soluble products (ASP) and CO(2) was approximately 10-fold greater in hepatocytes kept at 5 degrees C than in those kept at 12 degrees C and the main oxidation products formed were acetate, oxaloacetate and malate.

Fat to the fire: the regulation of lipid oxidation with exercise and environmental stress

Lipids are an important fuel for submaximal aerobic exercise. The ways in which lipid oxidation is regulated during locomotion is an area of active investigation. Indeed, the integration between cellular regulation of lipid metabolism and whole-body exercise performance is a fascinating but often overlooked research area. Additionally, the interaction between environmental stress, exercise, and lipid oxidation has not been sufficiently examined. There are many functional and structural steps as fatty acids are mobilized, transported, and oxidized in working muscle, which may serve either as regulatory points for responding to acute or chronic stimuli or as raw material for natural selection. At the whole-animal level, the partitioning of lipids and carbohydrates across exercise intensities is remarkably similar among mammals, which suggests that there is conservation in regulatory mechanisms. Conversely, the proportions of circulatory and intramuscular fuels differ between species and across exercise intensities. Responses to acute and chronic environmental stress likely involve the interaction of genetic and nongenetic changes in the fatty acid pathway. Determining which of these factors help regulate the fatty acid pathway and what impact they have on whole-animal lipid oxidation and performance is an important area of future research. Using an integrative approach to complete the information loop from gene to physiological function provides the most powerful mode of analysis.

The lipid composition of hypodermal membranes from the blue crab (Callinectes sapidus) changes during the molt cycle and alters hypodermal calcium permeability

Crustaceans are covered by a cuticle that does not grow. In order for an individual to grow, the cuticle must periodically be shed (ecdysis). Replacement of the old cuticle with a new one depends on processes that require precise timing and control, yet the nature and location of these controls remain unclear. A candidate site for them is within the hypodermal microvilli. These cellular structures extend through pore canals deep into the acellular cuticular matrix. Changes in the lipid composition of hypodermal microvilli could modulate water and ion fluxes and enzyme activities during critical stages of the molt cycle; however, the lipid composition of these structures has not been assessed during the molt cycle. Data presented here show that phospholipids isolated from hypodermal microvilli of Callinectes sapidus initially have elevated levels of n-6 fatty acids that decline steadily beginning just after ecdysis. Experiments with liposomes reveal that n-6 fatty acids decrease the calcium permeability of membranes, suggesting that the initially elevated levels in the cuticle may function to reduce calcium flux from the cuticle into the hypodermis. In addition, the ratio of cholesterol to phospholipid and the proportion of oleic acid in membrane phospholipids are maximal at 6 h post-ecdysis. It is known that changes in cholesterol and oleic acid content alter membrane permeability to water. It is, therefore possible that water flux through hypodermal membranes is also modulated in the early post-molt cuticle. Changes in microvillar lipid composition might serve importantly to control biomineralization in the post-ecdysal cuticle.

Membrane lipids and sodium pumps of cattle and crocodiles: an experimental test of the membrane pacemaker theory of metabolism

The influence of membrane lipid composition on the molecular activity of a major membrane protein (the sodium pump) was examined as a test of the membrane pacemaker theory of metabolism. Microsomal membranes from the kidneys of cattle (Bos taurus) and crocodiles (Crocodylus porosus) were found to possess similar sodium pump concentrations, but cattle membranes showed a four- to fivefold higher enzyme (Na(+)-K(+)-ATPase) activity when measured at 37 degrees C. The molecular activity of the sodium pumps (ATP/min) from both species was fully recoverable when delipidated pumps were reconstituted with membrane from the original source (same species). The results of experiments involving species membrane crossovers showed cattle sodium pump molecular activity to progressively decrease from 3,245 to 1,953 (P < 0.005) to 1,031 (P < 0.003) ATP/min when subjected to two cycles of delipidation and reconstitution with crocodile membrane as a lipid source. In contrast, the molecular activity of crocodile sodium pumps progressively increased from 729 to 908 (P < 0.01) to 1,476 (P = 0.01) ATP/min when subjected to two cycles of delipidation and reconstitution with cattle membrane as a lipid source. The lipid composition of the two membrane preparations showed similar levels of saturated ( approximately 31-34%) and monounsaturated ( approximately 23-25%) fatty acids. Cattle membrane had fourfold more n-3 polyunsaturated fatty acids (11.2 vs. 2.9%) but had a reduced n-6 polyunsaturate content (29 vs. 43%). The results support the membrane pacemaker theory of metabolism and suggest membrane lipids and their polyunsaturates play a significant role in determining the molecular activity of the sodium pump.

Thermal acclimation of surfactant secretion and its regulation by adrenergic and cholinergic agonists in type II cells isolated from warm-active and torpid golden-mantled ground squirrels, Spermophilus lateralis

Homeothermic mammals experience pulmonary surfactant dysfunction with relatively small fluctuations in body temperature. However, ground squirrels survive dramatic changes in body temperature during hibernation, when body temperature drops from 37 degrees C to 0-5 degrees C during prolonged torpor bouts. Using type II cells isolated from both warm-active and torpid squirrels, we determined the effect of assay temperature, autonomic agonists and torpor on surfactant secretion. Basal secretion was significantly higher in type II cells isolated from torpid squirrels compared with warm-active squirrels when assayed at the body temperature of the animal from which they were isolated (4 degrees C and 37 degrees C, respectively). A change in assay temperature significantly decreased surfactant secretion. However, the change in secretory rate between 37 degrees C and 4 degrees C was less than expected if due to temperature alone (Q(10) range=0.8-1.2). Therefore, the surfactant secretory pathway in squirrel type II cells demonstrates some temperature insensitivity. When incubated at the body temperature of the animal from which the cells were isolated, the adrenergic agonist, isoproterenol, significantly increased surfactant secretion in both warm-active and torpid squirrel type II cells. However, the cholinergic agonist, carbamylcholine chloride, only increased secretion in torpid squirrel type II cells when incubated at 4 degrees C. Torpor did not affect basal cAMP production from isolated type II cells. However, the production of cAMP appears to be upregulated in response to isoproterenol in torpid squirrel type II cells. Thus, at the cellular level, both the secretory and regulatory pathways involved in surfactant secretion are thermally insensitive. Upregulating basal secretion and increasing the sensitivity of type II cells to cholinergic stimulation may be adaptative characteristics of torpor that enable type II cells to function effectively at 0-5 degrees C.

The effect of acclimation temperature on the fusion kinetics of lipid vesicles derived from endoplasmic reticulum membranes of rainbow trout (Oncorhynchus mykiss) liver

Membrane fusion is an obligatory step in many vital cellular processes. The well-established enrichment of bilayer-destabilizing lipids in membranes of poikilotherms subjected to growth at low temperatures leads to the prediction that such membranes will possess a greater propensity to undergo fusion. This hypothesis was explicitly tested in the present study by determining the kinetics of fusion between small unilamellar vesicles (SUVs) prepared from endoplasmic reticulum (ER) membranes of thermally-acclimated (to 5 and 20 degrees C) rainbow trout (Oncorhynchus mykiss) liver and bovine brain phosphatidylserine (BBPS). At temperatures above 10 degrees C, ER vesicles from 5 degrees C-acclimated trout, fused more rapidly and to a greater extent with BBPS vesicles (by average factors of 1.25- and 1.45-fold, respectively) than ER vesicles of 20 degrees C-acclimated trout. At temperatures >35 degrees C, apparent fusion rates declined while the extent of fusion increased in both acclimation groups. Fusion kinetics were found to be well correlated with and limited by the physical properties and phase state of the BBPS vesicles. These results indicate that dynamic attributes of biological membranes, such as the propensity to undergo fusion, are of potential regulatory significance and are partially conserved when growth or environmental temperature changes.

The curvature and cholesterol content of phospholipid bilayers alter the transbilayer distribution of specific molecular species of phosphatidylethanolamine

The curvature, cholesterol content, and transbilayer distribution of phospholipids significantly influence the functional properties of cellular membranes, yet little is known of how these parameters interact. In this study, the transbilayer distribution of phosphatidylethanolamine (PE) is determined in vesicles with large (98 nm) and small (19 nm) radii of curvature and with different proportions of PE, phosphatidylcholine, and cholesterol. It was found that the mean diameters of both types of vesicles were not influenced by their lipid composition, and that the amino-reactive compound 2,4,6-trinitrobenzenesulphonic acid (TNBS) was unable to cross the bilayer of either type of vesicle. When large vesicles were treated with TNBS, approximately 40% of the total membrane PE was derivatized; in the small vesicles 55% reacted. These values are interpreted as representing the percentage of total membrane PE residing in the outer leaflet of the vesicle bilayer. The large vesicles likely contained approximately 20% of the total membrane lipid as internal membranes. Therefore, in both types of vesicles, PE as a phospholipid class was randomly distributed between the inner and outer leaflets of the bilayer. The proportion of total PE residing in the outer leaflet was unaffected by changes in either the cholesterol or PE content of the vesicles. However, the transbilayer distributions of individual molecular species of PE were not random, and were significantly influenced by radius of curvature, membrane cholesterol content, or both. For example, palmitate- and docosahexaenoate-containing species of PE were preferentially located in the outer leaflet of the bilayer. Membrane cholesterol content affected the transbilayer distributions of stearate-, oleate-, and linoleate-containing species. The transbilayer distributions of palmitate-, docosahexaenoate-, and stearate-containing species were significantly influenced by membrane curvature, but only in the presence of high levels of cholesterol. Thus, differences in membrane curvature and cholesterol content alter the array of PE molecules present on the surfaces of phospholipid bilayers. In cells and organelles, these differences could have profound effects on a number of critical membrane functions and processes.

Is fluid-phase endocytosis conserved in hepatocytes of species acclimated and adapted to different temperatures?

Our primary objective was to determine if rates of fluid-phase endocytosis (FPE) were conserved in hepatocytes from organisms acclimated and adapted to different temperatures. To this aim, the fluorescent dye Lucifer yellow was employed to measure FPE at different assay temperatures (AT) in hepatocytes from 5 degrees C- and 20 degrees C-acclimated trout, Oncorhynchus mykiss (at 5 and 20 degrees C AT), 22 degrees C- and 35 degrees C-acclimated tilapia, Oreochromis nilotica (at 22 and 35 degrees C AT), and the Sprague-Dawley rat (at 10, 20, and 37 degrees C AT). FPE was also studied in rats fed a long-chain polyunsaturated fatty acid (PUFA)-enriched diet (at 10 degrees C AT). Despite being temperature dependent, endocytic rates (values in pl. cell(-1). h(-1)) in both species of fish were compensated after a period of acclimation. For example, in 20 degrees C-acclimated trout, the rate of endocytosis declined from 1.84 to 1.07 when the AT was reduced from 20 to 5 degrees C; however, after a period of acclimation at 5 degrees C, the rate (at 5 degrees C AT) was largely restored (1.80) and almost perfectly compensated (95%). In tilapia, endocytic rates were also temperature compensated, although only partially (36%). Relatively similar rates obtained at 5 degrees C in 5 degrees C-acclimated trout (1.8), at 20 degrees C in 20 degrees C-acclimated trout (1.84), and at 22 degrees C in 22 degrees C-acclimated tilapia (2.2) suggest that endocytic rates are somewhat conserved in these two species of fish. In contrast, the rate in rat measured at 37 degrees C (16.83) was severalfold greater than in fish at their respective body temperatures. A role for lipids in determining rates of endocytosis was supported by data obtained at 10 degrees C in hepatocytes isolated from rats fed a long-chain PUFA-enriched diet: endocytic rates were higher (5.35 pl. cell(-1). h(-1)) than those of rats fed a standard chow diet (2.33 pl. cell(-1). h(-1)). The conservation of endocytic rates in fish may be related to their ability to conserve other membrane characteristics (i.e., order or phase behavior) by restructuring their membrane lipid composition or by modulating the activities of proteins that regulate endocytosis and membrane traffic, whereas the lack of conservation between fish and rat may be due to differences in metabolic rate.

Polyunsaturated fatty acids enhance the heat induced stress response in rainbow trout (Oncorhynchus mykiss) leukocytes

The heat shock response has been studied extensively, yet the molecular signals that trigger the response remain elusive. The dogma of the heat shock response contends that denatured proteins initiate the response, but evidence is accumulating to point to a more complex system in which at least more than one signal is involved in this process. Thermal stress initiates changes in cellular phospholipid membrane physical state, which when acted upon by phospholipases may release lipid mediators that could serve as triggering signals during the heat shock response. We have examined the heat shock response in freshly isolated leukocytes from the pronephros of rainbow trout (Oncorhynchus mykiss). In this study, we show that leukocytes isolated from rainbow trout acclimated to 5 or 19 degrees C express elevated levels of heat shock protein 70 (hsp70) mRNA when heat shocked at 5 degrees C above their respective acclimation temperature and supplementation with exogenous docosahexaenoic acid or arachidonic acid followed by heat shock enhanced levels of hsp70 mRNA. The time course for docosahexaenoic acid induced enhancement of hsp70 mRNA was accelerated compared with heat shock alone, and staurosporine inhibited the docosahexaenoic acid induced increase of hsp70 mRNA. We also provide evidence that phospholipase A2 is involved in the heat shock response.

Heterothermal acclimation: an experimental paradigm for studying the control of thermal acclimation in crabs

A method for the study of the control of the attainment of thermal acclimation has been applied to the crabs, Cancer pagurus and Carcinus maenas. Crabs were heterothermally acclimated by using an anterior-posterior partition between two compartments, one at 8 degrees C and the other at 22 degrees C. One compartment held a three-quarter section of the crab including the central nervous system (CNS), eye stalks, and ipsilateral legs; the other held a quarter section including the contralateral legs. Criteria used to assess the acclimation responses were comparisons of muscle plasma membrane fatty acid composition and "fluidity." In both species, the major fatty acids of phosphatidylcholine were 16:0, 18:1, 20:5, and 22:6, whereas phosphatidylethanolamine contained significantly less 16:0 but more 18:0; these fatty acids comprised 80% of the total. Differences in fatty acid composition were demonstrated between fractions obtained from the ipsilateral and contralateral legs from the same heterothermally acclimated individual. In all acclimation states (except 22CNS, phosphatidylcholine fraction), membrane lipid saturation was significantly increased with acclimation at 22 degrees as compared with 8 degrees C. Membrane fluidity was determined by using 1,3-diphenyl-1,3,5 hexatriene (DPH) fluorescence polarization. In both species, membranes from legs held at 8 degrees were more fluid than from legs held at 22 degrees C irrespective of the acclimation temperature of the CNS. Heterothermal acclimation demonstrated that leg muscle membrane composition and fluidity respond primarily to local temperature and were not predominately under central direction. The responses between 8 degrees C- and 22 degrees C-acclimated legs were more pronounced when the CNS was cold-acclimated, so a central influence cannot be excluded.

Influences of subzero thermal acclimation on mitochondrial membrane composition of temperate zone marine bivalve mollusks

The phospholipid and phospholipid fatty acid composition of gill mitochondrial membranes from two temperate zone marine bivalve mollusks, the quahog, Mercenaria mercenaria, and the American oyster, Crassostrea virginica, were examined after acclimation to 12 and -1 degree C. Cardiolipin (CL) was the only phospholipid with proportions altered upon acclimation to -1 degree C, increasing 188% in the mitochondrial membranes of M. mercenaria. Although the ratio of bilayer stabilizing to destabilizing lipids is frequently associated with cold acclimation in ectothermic species, no change was found in this ratio in either of the species. Polyunsaturated fatty acids (PUFA) were found only to increase in C. virginica with cold acclimation, with total n-3 PUFA increasing in the phospholipid phosphatidylethanolamine, total n-6 PUFA increasing in CL, and total PUFA increasing in phosphatidylinositol. Monounsaturated fatty acids, not PUFA, were found to have increased in M. mercenaria, with 18:1 n-9 increasing by 150% in CL, and 20:1 increasing in both CL and phosphatidylcholine, by 146 and 192%, respectively. These manipulations of membrane phospholipid and fatty acid composition may represent an attempt by these species to help maintain membrane function at low temperatures.

Mitochondrial membrane composition of two Arctic marine bivalve mollusks, Serripes groenlandicus and Mya truncata

The phospholipid and fatty acid composition of gill mitochondria membranes from two Arctic marine bivalve mollusks, Mya truncata and Serripes groenlandicus, were examined. These animals were collected from the Arctic Ocean, where waters remain below 0 degrees C throughout the year. In both species, the primary membrane phospholipids were phosphatidylcholine, and phosphatidylethanolamine. Although a low ratio of bilayer-stabilizing phospholipids to bilayer-destabilizing phospholipids is frequently associated with cold acclimation in temperate species, this ratio is very different between the two species. The monounsaturated fatty acid 20:1 was abundant in the membranes of both Arctic species equaling 13.0% of the fatty acid composition in S. groenlandicus, and 17.7% in M. truncata. Polyunsaturated fatty acids were relatively low in the Arctic species, equaling 35.9% of total membrane fatty acids compared to that of temperate zone mollusks. It is suggested that monoenes are common in the tissues of Arctic species since they play a role in maintaining membrane function at subzero temperatures.

Thermal acclimation of phase behavior in plasma membrane lipids of rainbow trout hepatocytes

The fluorescent probes laurdan (6-dodecanoyl-2-dimethylaminonapthalene) and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl] dipalmitoyl-L-alpha-phosphatidylethanolamine (NBD-PE) in addition to Fourier transform infrared spectroscopy (FTIR) were employed to measure the phase behavior and physical properties of hepatocyte plasma membranes isolated from the livers of thermally acclimated (5 and 20 degreesC) rainbow trout (Oncorhynchus mykiss). The primary objective was to determine the extent to which the phase behavior of membrane lipids is conserved at different growth temperatures. Arrhenius plots of laurdan-generalized polarization revealed a single discontinuity believed to reflect either the onset of the gel-fluid phase transition or the formation of gel phase microdomains, and this discontinuity occurred at significantly higher temperatures in membranes of 20 degrees C (13.2 +/- 0.7 degrees C)- than 5 degrees C (7.2 +/- 0.1 degrees C)-acclimated trout. Similarly, acclimation from 5 to 20 degrees C increased both the onset temperature (from 2.0 +/- 0.3 to 7.2 +/- 0.6 degrees C) and the thermal range (from 10.9 +/- 0.5 to 16.0 +/- 1.0) of the gel-fluid transition as assessed by FTIR. The gel-fluid transition midpoint (approximately -2 degrees C) and completion temperatures (-9 degrees C) were unchanged by thermal acclimation. The anisotropy of NBD-PE fluorescence displayed a distinct minimum in membranes of both warm- and cold-acclimated trout (reflecting alterations in lipid packing that in pure lipid membranes ultimately lead to the formation of nonlamellar phases) in the range of 56-58 degrees C; only membranes of 5 degrees C-acclimated trout displayed an additional minimum at significantly lower temperatures (24.5 +/- 1.7 degrees C). Collectively, these data suggest that the regulation of both the temperature at which gel phase lipids begin to form in response to cooling as well as the propensity of membrane lipids to form nonlamellar phases at higher temperatures may be key features of membrane organization subject to adaptive regulation.

Regulation of membrane lipid bilayer structure during seasonal variation: a study on the brain membranes of Clarias batrachus

(1) A significant seasonal variation in the membrane fluidity (as sensed by DPH-fluorescence polarization), membrane lipid components (phospholipid and neutral lipid), fatty acid composition of membrane phospholipid (phosphatidylcholine, phosphatidylethanolamine and sphingomyelin), positional distribution of fatty acids at Sn-1 and Sn-2 position of phosphatidyl-choline and -ethanolamine is noticed in the brain membranes (myelin, synaptosomes, and mitochondria) of a tropical air breathing teleost, Clarias batrachus. (2) A 'partial compensation' of membrane fluidity during seasonal adaptation is observed in myelin and mitochondria membrane fractions. Synaptosomes membrane fraction exhibits a different response. Depletion (about 15-70%) of membrane lipid components (phospholipid, cholesterol, diacylglycerol and triacylglycerol) per unit of membrane protein is the characteristic feature of summer adaptation. An increase (about 20-100%) in the level of oleic acid and decrease (about 20-60%) in the level of stearic acid are almost common features in membrane phospholipid fractions of winter-adapted Clarias (3) From the tissue slice experiment it is evident that there is an activation of cellular phospholipase A2 at lower growth temperature and of cellular phospholipase A1 at higher growth temperature and this suggests the reorganization of molecular architecture of the membrane during seasonal adaptation. (4) Accumulation of oleic acid in Sn-1 position and polyunsaturated fatty acids in Sn-2 position of phosphatidylcholine and -ethanolamine during winter indicates an increase in the concentration of 1-monoenoic, 2-polyenoic molecular species of phospholipid in order to maintain the stability of membrane lipid bilayer.

Role of molecular species catabolism in the temperature-induced restructuring of phosphatidylcholines in liver microsomes of thermally-acclimated rainbow trout (Oncorhynchus mykiss)

Most previous studies of the temperature-induced restructuring of phospholipid molecular species composition have examined steps in the biosynthesis of phospholipids to explain the accumulation of unsaturated fatty acids in membranes of cold-acclimated poikilotherms. In contrast, the present study explores the role of phospholipases in this restructuring process by determining the rates of degradation of specific molecular species of phosphatidylcholine, using enzymes (microsomes) freshly isolated from the liver of rainbow trout. (Oncorhynchus mykiss) acclimated to either 5° or 20°C. The substrate preparation employed to assay phospholipase activity possessed a range of molecular species, all radiolabeled with 1-(14)C-palmitic acid at thesn-1 position, similar to that present in native trout liver microsomes. After defined periods of incubation (120 and 240 min at 5°C; 60 and 120 min at 20°C), phospholipids were extracted from the reaction mixture and the distribution of radioactivity among the molecular species of phosphatidylcholine was determined by HPLC/liquid scintillation counting. In general, molecular species catabolism was not significantly influenced by either assay or acclimation temperature. Only in 20°C-acclimated fish did a reduction in assay temperature (from to 20 to 5°C) result in significantly increased proportions of radioactivity being recovered in one polyunsaturated fatty acid-containing species (16:0/22:6-PC). It is concluded: 1) that phospholipase specificity, assayed under conditions approximating thosein situ, is not significantly influenced by temperature; and 2), that the increased proportions of unsaturated fatty acid-containing molecular species of phosphatidylcholine observed at low temperatures must reflect the specificity of biosynthetic rather than degradative processes.

Effects of temperature and dietary n-3 and n-6 fatty acids on endocytic processes in isolated rainbow trout (Oncorhynchus mykiss, Walbaum) hepatocytes

Effects of different incubation temperatures (2, 8, 14 and 20°C) and hepatocyte membrane fatty acid composition on the rate of internalization and lysosomal degradation of the ligand, mannosylated albumin, that is taken up by receptor-mediated endocytosis, were investigated in rainbow trout (Oncorhynchus mykiss, Walbaum). The fish were kept at a water temperature ranging from 9 to 14°C and fed pelleted diets coated with either capelin oil (control), EPA/DHA-concentrate (rich in n-3 polyunsaturated fatty acids) or soybean oil (rich in n-6 unsaturated fatty acids) for at least 3 months prior to sampling. The endocytic uptake mediated by the mannose receptor was very efficient at all temperatures studied. Lysosomal degradation, on the other hand, came to a halt below 8°C. The activation energies for uptake and degradation were 54.6 and 164.2 kJ/mol respectively. No negative effects of increased amounts of either n-3 or n-6 fatty acids were observed on the endocytic parameters studied. On the contrary, multivariate analysis indicated a positive relationship between high levels of n-6 fatty acids and low unsaturation index in the phosphatidylcholine (PC) fraction of the hepatocytes and the internalization rate of 2°C, meaning that the rate of receptor-mediated endocytosis may be affected by membrane fatty acid composition.

Direct effects of temperature on phospholipid and polyunsaturated fatty acid metabolism in isolated brain cells from rainbow trout, Oncorhynchus mykiss

1. The direct effects of temperature on the metabolism of [1-14C]18:2(n-6), [1-14C]18:3(n-3), [1-14C]20:4(n-6) and [1-14C]20:5(n-3) were studied in isolated brain cells from rainbow trout, Oncorhynchus mykiss. 2. Recovery of radioactivity from all the polyunsaturated fatty acids (PUFA) in total lipid was significantly greater at 5 and 15 degrees C than at 25 degrees C. 3. The lower incubation temperatures decreased the relative net incorporation of all the 14C-labelled PUFA into phosphatidylcholine (PC) and increased the relative incorporation of the PUFA into the other phosphoglycerides, especially phosphatidylethanolamine (PE). 4. The effects on PC were generally more significant between 25 and 15 degrees C, whereas the effects on PE were generally significant both between 25 and 15 degrees C and between 15 and 5 degrees C. 5. This suggests that the lysophospholipid acyltransferases responsible for the incorporation of PUFA into different phosphoglycerides may have differential sensitivities to temperature. 6. In contrast, the acyltransferase activities showed fatty acyl preferences that were independent of temperature. 7. Although a trend towards decreased activity at 5 degrees C was apparent, temperature generally had little significant effect on the relative percentages of the PUFA metabolized via the desaturase pathways.

Cultured human skin fibroblasts modify their plasma membrane lipid composition and fluidity according to growth temperature suggesting homeoviscous adaptation at hypothermic (30 degrees C) but not at hyperthermic (40 degrees C) temperatures

Mammalian cell metabolism is responding to changes in temperature. Body temperature is regulated around 37 degrees C, but temperatures of exposed skin areas may vary between 20 degrees C and 40 degrees C for extended periods of time without apparent disturbance of adequate cellular functions. Cellular membrane functions are depending from temperatures but also from their lipid environment, which is a major component of membrane fluidity. Temperature-induced changes of membrane fluidity may be counterbalanced by adaptive modification of membrane lipids. Temperature-dependent changes of whole cell- and of purified membrane lipids and possible homeoviscous adaptation of membrane fluidity have been studied in human skin fibroblasts cultured at 30 degrees C, 37 degrees C, and 40 degrees C for ten days. Membrane anisotropy was measured by polarized fluorescence spectroscopy using TMA-DPH for superficial and DPH for deeper membrane layers. Human fibroblasts were able to adapt themselves to hypothermic temperatures (30 degrees C) by modifying the fluidity of the deeper apolar regions of the plasma membranes as reported by changes of fluorescence anisotropy due to appropriate changes of their plasma membrane lipid composition. This could not be shown for the whole cells. At 40 degrees C growth temperature, adaptive changes of the membrane lipid composition, except for some changes in fatty acid compositions, were not seen. Independent from the changes of the membrane lipid composition, the fluorescence anisotropy of the more superficial membrane layers (TMA-DPH) increased in cells growing at 30 degrees C and decreased in cells growing at 40 degrees C.

Thermal adaptation in biological membranes: functional significance of changes in phospholipid molecular species composition

Quantities of 1-palmitoyl 2-docosahexaenoyl phosphatidylcholine (16:0/22:6-PC or PDPC) increase from 24 to 40 weight percent as a consequence of cold acclimation in mitochondrial membranes of rainbow trout liver (J. Comp. Physiol. 156, 665-674, 1986). The present study was undertaken to assess the impact of such a large change in the proportions of a single molecular species on the fluidity, lateral packing (as sensed by phospholipase A2), and permeability of biological membranes. These properties were examined in multilamellar liposomes prepared from binary mixtures of dipalmitoyl phosphatidylcholine (DPPC) and PDPC in proportions increasing from 10 to 40 mole% PDPC. Glucose permeability was positively correlated with both assay temperature and PDPC content. The temperature dependence of Na+ permeability declined steadily as the mole fraction of PDPC increased; consequently, sodium permeability was positively correlated with PDPC content at 5 degrees C, but inversely correlated at 20 degrees C. Phospholipase A2 activity was independent of both assay temperature and vesicle composition. Vesicles of all compositions displayed a single transition in the temperature dependence of 1,6 diphenyl-1,3,5-hexatriene (DPH) fluorescence polarization, which shifted to lower temperature and broadened as proportions of PDPC increased. At temperatures below the transition, fluidity was positively correlated with the mole fraction of PDPC, but interfacial and deeper regions of the bilayer were affected differently by variations in PDPC content. Nonelectrolyte permeability was the only index of membrane structure or function to be significantly correlated with the fluidity of the bilayer interior. The tendencies of PDPC to both fluidize the membrane and to reduce the temperature sensitivity of electrolyte permeation may promote the adaptation of membrane function to low temperature.

Distribution of omega-3 and omega-6 fatty acids in the ether- and ester-linked phosphoglycerides from tissues of the rainbow trout, Salmo gairdneri

1. Data presented here demonstrate that polyunsaturated fatty acids in the phospholipids of rainbow trout tissues are compartmentalized differently than in mammalian tissues. 2. We have determined the distribution of omega-3 (n-3) and omega-6 (n-6) fatty acids in the alkyl-, alk-1-enyl-, and diacyl- subclasses of phosphatidylcholines (PC), phosphatidyl-ethanolamines (PE), phosphatidylinositols (PI), and phosphatidylserines (PS) from gill, kidney and spleen of rainbow trout. 3. Alkyl-linked PC and alk-1-enyl-linked PE were the most abundant ether-containing phospholipids, amounting to 10-15% of each class; no ether-linked PI or PS was detected. 4. C20:4 n-6 was found in high concentrations only in PI; the n-3 fatty acids were found in highest concentration in the ether-linked phospholipids as compared with the diacyl subclasses and C20:5 n-3 was especially prevalent in 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine and C22:6 n-3 was prevalent in PS.

Differences in the association of ribosomes with heavy rough and light rough endoplasmic reticulum membranes of MPC-11 cells

The treatment of total endoplasmic reticulum membranes of mouse plasmacytoma cells with EDTA resulted in an abolition of the heavy rough (HR) subfraction, while there was a large increase in smooth (S) membranes. When HR and light rough (LR) endoplasmic reticulum membranes were treated individually with EDTA and re-centrifuged on discontinuous sucrose gradients it was observed that HR were converted into S membranes, i.e. membranes virtually devoid of ribosomes. LR membranes were not affected to the same extent but there was a shift to a somewhat lower density. A quantitation of ribosomes released by EDTA showed that 95% of 60 S and 72% of 40 S subunits were removed from HR membranes while for LR membranes the corresponding values were 8.5 and 22.6% respectively. Ratios of radioactivity to absorbance at 260 nm calculated for 40 S and 60 S subunits isolated from HR and LR membranes show that 60 S subunits from LR membranes, in contrast to those from HR membranes, equilibrate only slowly with the free pool of ribosomal subunits. The results indicate that the ribosomes associated with HR membranes are 'loosely bound' and those with LR membranes 'tightly bound'. When poly(A)-containing mRNA isolated from HR and LR membranes was translated in vitro and the products analysed for light-chain immunoglobulin content, it was found that the HR fraction was enriched in light-chain mRNA.