Homeoviscous Response Research Articles

Homeoviscous adaptation occurs with thermal acclimation in biological membranes from heart and gill, but not the brain, in the Antarctic fish Notothenia coriiceps.

As temperatures continue to rise, adjustments to biological membranes will be key for maintenance of function. It is largely unknown to what extent Antarctic notothenioids possess the capacity to remodel their biological membranes in response to thermal change. In this study, physical and biochemical properties were examined in membranes prepared from gill epithelia (plasma membranes), cardiac ventricles (microsomes, mitochondria), and brains (synaptic membranes, myelin, mitochondria) from Notothenia coriiceps following acclimation to 5°C (or held at ambient temperature, 0°C) for a minimum of 6weeks. Fluidity was measured between 0 and 30°C in all membranes, and polar lipid compositions and cholesterol contents were analyzed in a subset of biological membranes from all tissues. Osmotic permeability was measured in gills at 0 and 4°C. Gill plasma membranes, cardiac mitochondria, and cardiac microsomes displayed reduced fluidity following acclimation to 5°C, indicating compensation for elevated temperature. In contrast, no fluidity changes with acclimation were observed in any of the membranes prepared from brain. In all membranes, adjustments to the relative abundances of major phospholipid classes, and to the extent of fatty acid unsaturation, were undetectable following thermal acclimation. However, alterations in cholesterol contents and acyl chain length, consistent with the changes in fluidity, were observed in membranes from gill and cardiac tissue. Water permeability was reduced with 5°C acclimation in gills, indicating near-perfect homeostatic efficacy. Taken together, these results demonstrate a homeoviscous response in gill and cardiac membranes, and limited plasticity in membranes from the nervous system, in an Antarctic notothenioid.

Lipidome variations of deep-sea vent shrimps according to acclimation pressure: A homeoviscous response?

The present study questions the ability of marine fauna to modulate the molecular composition of their membrane lipids, as a function of environmental hydrostatic pressure conditions. Specific variations in the cell lipidome composition are presented for the deep-sea hydrothermal vent shrimp Mirocaris fortunata, acclimated at the laboratory for several months at both natural (18 MPa) and atmospheric pressures, but fed with the same diet. These animals display physiological capabilities of acclimation to pressure variations, possibly driven by a particular composition of their lipid species. In particular, it is shown that shrimps exposed to atmospheric pressure exhibit a significantly lower level of the mono-unsaturated fatty acid, vaccenic acid (C18:1n-7), compared to those maintained at natural pressure. The observed fatty acid variations are consistent with a homeoviscous response, i.e. a modulation of the lipidome composition, in response to physical constraints, in order to balance the effects of pressure on membrane order. Additionally, supervised multivariate data analysis was performed for complex lipids and revealed an increase in levels of sterols, sphingomyelin, and ether-phosphatidylethanolamine lipids for animals exposed to atmospheric pressure, with respect to natural pressure. These observations strongly suggest that lipidic domains are involved in the shrimp's response to experimental conditions, and their consistency with a homeoviscous response to pressure is discussed.

Effect of elevated temperature on membrane lipid saturation in Antarctic notothenioid fish.

Homeoviscous adaptation (HVA) is a key cellular response by which fish protect their membranes against thermal stress. We investigated evolutionary HVA (long time scale) in Antarctic and non-Antarctic fish. Membrane lipid composition was determined for four Perciformes fish: two closely related Antarctic notothenioid species (Trematomus bernacchii and Pagothenia borchgrevinki); a diversified related notothenioid Antarctic icefish (Chionodraco hamatus); and a New Zealand species (Notolabrus celidotus). The membrane lipid compositions were consistent across the three Antarctic species and these were significantly different from that of the New Zealand species. Furthermore, acclimatory HVA (short time periods with seasonal changes) was investigated to determine whether stenothermal Antarctic fish, which evolved in the cold, stable environment of the Southern Ocean, have lost the acclimatory capacity to modulate their membrane saturation states, making them vulnerable to anthropogenic global warming. We compared liver membrane lipid composition in two closely related Antarctic fish species acclimated at 0 °C (control temperature), 4 °C for a period of 14 days in T. bernacchii and 28 days for P. borchgrevinki, and 6 °C for 7 days in both species. Thermal acclimation at 4 °C did not result in changed membrane saturation states in either Antarctic species. Despite this, membrane functions were not compromised, as indicated by declining serum osmolality, implying positive compensation by enhanced hypo-osmoregulation. Increasing the temperature to 6 °C did not change the membrane lipids of P. borchgrevinki. However, in T. bernacchii, thermal acclimation at 6 °C resulted in an increase of membrane saturated fatty acids and a decline in unsaturated fatty acids. This is the first study to show a homeoviscous response to higher temperatures in an Antarctic fish, although for only one of the two species examined.

Homeoviscous response of Clostridium pasteurianum to butanol toxicity during glycerol fermentation

Clostridium pasteurianum ATCC 6013 achieves high n-butanol production when glycerol is used as the sole carbon source. In this study, the homeoviscous membrane response of C. pasteurianum ATCC 6013 has been examined through n-butanol challenge experiments. Homeoviscous response is a critical aspect of n-butanol tolerance and has not been examined in detail for C. pasteurianum. Lipid membrane compositions were examined for glycerol fermentations with n-butanol production, and during cell growth in the absence of n-butanol production, using gas chromatography–mass spectrometry (GC–MS) and proton nuclear magnetic resonance (1H-NMR). Membrane stabilization due to homeoviscous response was further examined by surface pressure–area (π–A) analysis of membrane extract monolayers. C. pasteurianum was found to exert a homeoviscous response that was comprised of an increase lipid tail length and a decrease in the percentage of unsaturated fatty acids with increasing n-butanol challenge. This led to a more rigid or stable membrane that counteracted n-butanol fluidization. This is the first report on the changes in the membrane lipid composition during n-butanol production by C. pasteurianum ATCC 6013, which is a versatile microorganism that has the potential to be engineered as an industrial n-butanol producer using crude glycerol.

PCB-153 and temperature cause restructuring of goldfish membranes: Homeoviscous response to a chemical fluidiser

Ortho-substituted PCBs intercalate between membrane phospholipids similarly to cholesterol and increase fluidity. Ectothermic animals have a well-developed homeoviscous response to counter the fluidising effect of temperature and avoid the disruption of membrane proteins. However, it remains unknown whether chemical fluidisation can also activate a homeoviscous response or interfere with normal acclimation to temperature. The fatty acid composition and cholesterol content of membranes from gill, white muscle, liver, and brain was measured in goldfish exposed to 4 treatments in a 2×2 factorial design (acclimated to 5 or 20°C, and exposed or not to PCB-153). The expression of Δ6 and Δ9 desaturases was also measured in gill and liver because these enzymes modulate changes in membrane unsaturation. We hypothesised that thermal and chemical stress would cause similar adjustments in phospholipid unsaturation, membrane cholesterol, and desaturase expression. Results show that PCB-153 triggers a homeoviscous response by changing cholesterol content in liver (+51%) and brain (+216%), as well as the double bond index in gills (−17%). In response to higher temperature, the membranes of gill, muscle, and brain substitute polyunsaturated fatty acids such as arachidonate [20:4] and eicosadienoate [20:2] with saturated fatty acids such as palmitate [16:0] and stearate [18:0]. Each tissue has a distinct pattern of changes, suggesting that different local factors contribute to the stress response. It is also possible that the thermal tolerance of individual species influences the homeoviscous response because the changes observed in goldfish liver are not consistent with what has been reported for trout liver. No evidence supporting the activation of desaturase expression could be found. Overall, and contrary to expectation, modulating membrane cholesterol is the main mechanism used to cope with PCB-153, whereas changes in unsaturation dominate temperature acclimation. If also present in other species, these protective responses may prove particularly important for polar fish that face the combined effects of thermal stress from climate change and chemical stress from organochlorine deposition. This study is the first to show that in vivo exposure to a membrane fluidiser can cause a homeoviscous response in an ectothermic animal. We conclude that the homeostatic mechanisms that preserve normal membrane function vary: (1) with the nature of the stress that perturbs fluidity, (2) with local conditions within each tissue, and (3) possibly with the thermal tolerance of individual species. These complicating factors will have to be considered in future studies of homeoviscous adjustments.

Eicosapentaenoic acid regulates scallop (Placopecten magellanicus) membrane fluidity in response to cold.

The lipid core of a biological membrane requires a certain degree of structural rigidity, but it must also be sufficiently fluid to permit lateral movement of the constituent lipids and embedded proteins. Ectotherms can counteract the ordering effects of reduced temperature by changing the structure of their membranes, a process known as homeoviscous adaptation (1). Although the content of unsaturated fatty acids in the membranes of ectothermic animals is generally known to increase in response to cold (2), no clear and direct relationship between unsaturated fatty acids and membrane fluidity has been established in marine organisms. For example, phospholipid molecular species containing docosahexaenoic acid (22:6 3) are believed to be important in controlling finfish membrane fluidity (3–6), but a direct correlation between 22:6 3 and membrane fluidity has not been found (4, 5, 7, 8). In contrast, we show here a simple but very strong relationship between fluidity and a single polyunsaturated fatty acid, eicosapentaenoic acid (20:5 3), in gill membranes from a marine bivalve mollusc, the sea scallop Placopecten magellanicus. Phospholipids are the main structural elements of biological membranes, and their physical characteristics are key determinants of membrane structure and function. Many vital cell activities that depend on the optimal functioning of membranes are therefore sensitive to the chemistry of the membrane lipids (9) and to environmental conditions, such as temperature and pressure, that perturb the phase behavior and dynamics of lipids in membranes (10). Under extreme or variable conditions, organisms can exploit the tremendous chemical diversity among membrane lipids to defend the physical properties of the membrane (10). Thus in ectotherms, where changes in temperature cause important membrane perturbations, the usual adaptive response includes a modification of lipid composition (11). Sessile animals living in Newfoundland waters must maintain membrane structure and function in the face of extreme cold in deep waters (as low as 1.4°C) or seasonally highly variable conditions in surface waters (as much as 22°C in 6 months) (12). In the present study, we exposed sea scallops to a 10°C decrease in temperature for up to 3 weeks and then examined the relationship between the fatty acid composition of branchial phospholipids and membrane fluidity. Vesicles were prepared from the gills of scallops acclimated to temperatures of 15 and 5°C. After three weeks of thermal acclimation, the structural order of the phospholipids was measured by electron spin resonance (ESR) spectroscopy at five temperatures (0–20°C) that span the physiological range of Placopecten magellanicus (Fig. 1). The vesicles prepared from gills of 5°C-acclimated scallops were significantly (ANCOVA, P 0.03) less ordered than vesicles from 15°C-acclimated scallops. Temperature acclimation had shifted the order parameter curve 1–2°C toward lower assay temperatures, giving a homeoviscous efficacy (13) of 14%. Such a partial adjustment towards an ideal or complete homeoviscous response has also been found in crabs (14) and crayfish (15). In these invertebrates, the costs of perfect compensation may be too high, or the benefits too low. On the other hand, the ESR measurements in this study were made with the spin probe 5-doxyl stearic acid, reflecting the homeoviscous response in the outer region of the purified lipid bilayer. It is possible that the response deeper in the bilayer, in the actual region of alkenyl chain unsaturation, would have been greater (16). Received 30 November 2001; accepted 22 March 2002. 1 Present address: Department of Physiology and Experimental Medicine, The George Washington University Medical Center, Ross Hall Room 402, 2300 Eye Street, NW, Washington, DC 20037. 2 To whom correspondence should be addressed. E-mail: cparrish@ mun.ca Reference: Biol. Bull. 202: 201–203. (June 2002)

No evidence for homeoviscous adaptation in a heterothermic tissue: tuna heat exchangers.

Many poikilotherms are known to adjust the membrane composition of their cells in response to a temperature change so that membrane fluidity, and therefore function, is conserved. Such compensatory changes in membrane composition are considered "homeoviscous adaptations." In this study, we examined a heterothermic tissue, the visceral rete mirabile of the bluefin tuna, for evidence of homeoviscous adaptation. We measured the proportions of phospholipid fatty acids and phospholipid head groups as a function of position along the rete thermal gradient, which has been estimated to be approximately 10 degrees C. We found no effect of position along the rete on the composition of either phospholipid fatty acids or head groups. Our results were unexpected in light of our previous demonstration of compensation of metabolic enzyme activity in the same tissue. The lack of evidence for a homeoviscous response may be due to the fluctuating nature of the thermal gradient along the visceral retia; i.e., membranes may be adapted to a eurythermal existence rather than being fine-tuned to a particular temperature.

Cholesterol content of trout plasma membranes varies with acclimation temperature.

Involvement of cholesterol in thermally induced restructuring of biological membranes was investigated in several tissues of rainbow trout (Oncorhynchus mykiss). Cholesterol-rich plasma membranes (PM) were isolated from erythrocytes, liver, kidney, and gill of fish acclimated to 5 and 20 degrees C. Mean PM cholesterol-to-phospholipid molar ratios (C/P) from warm-acclimated animals were significantly higher than those of cold-acclimated fish in liver (0.26 vs. 0.18; P < 0.01), kidney (0.49 vs. 0.40; P < 0.02), and gill (0.66 vs. 0.60; P < 0.05); erythrocyte C/P did not differ significantly with acclimation temperature (0.28 vs. 0.25; P = 0.25). In light of the ordering effects of cholesterol on fluid-phase membranes, these results are consistent with a role for cholesterol in the homeoviscous response of some poikilotherm PMs. Tissue differences in both PM cholesterol levels and the magnitude of thermally evoked cholesterol changes may reflect tissue-specific membrane functions. Lower PM C/P of trout tissues relative to corresponding data available for homeotherms also support a possible evolutionary relationship between cholesterol content and thermal adaptation of the PM.

Investigation of acetone-butanol-ethanol (ABE) fermentation by fluorescence

Photophysical techniques have potential for the development of optical sensors in monitoring and controlling fermentors. In the particular case of acetone-butanol-ethanol (ABE) fermentation, carried out by bacteria of the speciesClostridium acetobutylicum, we have developed two studies based on fluorescence spectroscopy. First, we measured the intrinsic fluorescence of NADH related to bacteria metabolism, leading to a linear relationship between the NADH specific fluorescence and the specific rate of butyric acid production. At the same time, we have correlated enzymatic activities (acetate kinase, butyrate kinase, acetoacetate decarboxylase) with NADH specific fluorescence. Second, we studied the fluorescence polarization of extrinsic DPH (1,6-diphenyl-1,3,5-hexatriene) related to membrane fluidity. A simultaneous increase in both DPH anisotropy (order parameter increase) and butanol production is observed. Even though these results seem contradictory, because of the well-known fluidizing effect of butanol on lipids, they can be explained by a homeoviscous response ofC. acetobutylicum to the presence of butanol during fermentation. Thus the apparent changes in fluidity could be the result of the adaptative membrane alteration.

Ontogeny of the capacity for homeoviscous adaptation in white sturgeon (Acipenser transmontanus)

AbstractMost fish species tolerate a narrower range of temperatures during their early life history. To ascertain whether this might be related to development of the capacity to regulate membrane fluidity (homeoviscous adaptation), we examined the fatty acid composition of tissue phospholipids from developing white sturgeon eggs, embryos, and prefeeding larvae incubated at temperatures both within the range of natural spawning temperatures (14°), and above (20°) and below (8°). Mortality was highest at low temperature, with the greatest losses prior to neurulation. High temperature also resulted in increased mortality relative to 14°, with peak mortality between neurulation and hatch. At a constant temperature of 14°, significant changes in the fatty acid composition of tissue phospholipids were observed during development, but the patterns of change were altered by development at low and high temperature. Contrary to homeoviscous theory, embryos incubated at low temperature exhibited a decline in the ratio of unsaturated to saturated fatty acids (U/S ratio). In contrast, posthatch larvae exposed to low and high temperature exhibited, respectively, higher and lower U/S ratios than larvae maintained at 14°, implying that the capacity for homeoviscous adaptation is acquired at the time of hatch. Although the lack of a homeoviscous response by embryos may account for low survival at temperature extremes, the specific influences of the observed changes in phospholipid fatty acid composition on the biophysical properties of cell membranes remain to be determined. © 1993 Wiley‐Liss, Inc.

Species-Specific Responses of Membranes and the Na++ K+Pump to Temperature Change in the Kidney of Two Species of Freshwater Fish, Roach (Rutilus rutilus) and Arctic Char (Salvelinus alpinus)

A protocol has been developed for the separation of brush-border membrane (BBM) and basolateral membrane (BLM) fractions from the kidneys of the roach (Rutilus rutilus) and the Arctic char (Salvelinus alpinus), based on differential precipitation and differential centrifugation. Steady state polarization fluorescence was used to measure membrane fluidity in BBM and BLM from the kidneys of cold- and warm-acclimated fish. The homeoviscous response (%HR) in the BLM of cold-acclimated S. alpinus was about four fold higher (78.2%) than in the BLM of cold-acclimated R. rutilus. No %HR was shown by the BBM of R. rutilus, but a fairly strong one (54.3%) by the BBM of S. alpinus. Acclimation temperature had no effect on thermal stabilit, of the (Na⁺ + K⁺)-ATPase from kidney microsomes of S. alpinus, but the preincubation temperature that resulted in 50% inactivation $(LT_{50})$ of the same enzyme was significantly (> 2° C) lower in coldthan in warm-acclimated R. rutilus. No changes or only slight changes of ther...