Studies on thermal adaptation in Tetrahymena membrane lipids Modification of positional distribution of phospholipid acyl chains in plasma membranes, mitochondria and microsomes

Adaptive regulation of membrane lipids and fluidity during thermal acclimation in Tetrahymena

The free-living eukaryotic protozoan Tetrahymena is a potentially useful model for the thermoadaptive membrane regulation because of easy growth in the axenic culture, systematic isolation of subcellular organelles, and quick response to temperature stress. Exposure of Tetrahymena cells to the cold temperature induces marked alterations in the lipid composition and the physical properties (fluidity) of various membranes. The increase in fatty acid unsaturation of membrane phospholipids is required to preserve the proper fluidity. In this homeoviscous adaptive response, acyl-CoA desaturase plays a pivotal role and its activity is regulated by induction of the enzyme via transcriptional activation.

Studies on the subcellular distribution of 1-O-alkyl-2-acetyl-sn-glycero phosphocholine (PAF) and on the enzymic activities involved in its biosynthesis within the ciliate Tetrahymena pyriformis

The ciliated protozoan Tetrahymena pyriformis contains platelet-activating factor (PAF) as a physiological minor lipid. Its subcellular localization was found as follows: 13.7% in the pellicles, 24.9% in mitochondria, 56.5% in microsomes and 7.1% in the cytosol. Succinate dehydrogenase was used as marker enzyme. PAF remains cell-associated unless bovine serum albumin is included in the extracellular medium. In this case 15% of total PAF, portion comparable to that found in the pellicles, is released. Investigation of the principal enzymic activities involved in PAF formation showed that PAF-acetyltransferase (2.3.167) is totally absent from the protozoan. This means that the 'remodelling' pathway occurring in pro-inflammatory cells does not contribute in PAF formation in our system. A dithiothreitol (DTT)-insensitive CDPcholine phosphocholinetransferase activity involved in PAF biosynthesis is shown for the first time to be responsible for PAF production in T. pyriformis. It uses exogenous alkyl-acetyl-glycerol as substrate and is saturated over substrate concentration 250 microM. It can also use endogenous lipids as substrate. It is distributed mainly in mitochondria and microsomes, much less is found in the pellicles and it is totally absent from the cytosol. Its insensitivity to DTT, its selectivity to alkyl-acetyl-G and its different distribution compared to the enzymic activity involved in PC formation (EC 2.7.8.2) suggest that a different enzyme, specific for PAF formation (EC2.7.8.16) via the de novo pathway exists in the protozoan.

Rapid changes in the phospholipid composition of gill membranes during thermal acclimation of the rainbow trout, Salmo gairdneri

The phospholipid composition of gill tissue was determined in rainbow trout (Salmo gairdneri) undergoing thermal acclimation between 5 degrees C and 20 degrees C for a period of up to 28 days. Proportions of phosphatidylethanolamine (PE) and cardiolipin (CL) increased during cold acclimation and decreased during warm acclimation; proportions of phosphatidylcholine (PC) changed in the opposite direction (i.e., decreased during cold acclimation). In contrast, levels of phosphatidylserine, -inositol, and sphingomyelin did not vary significantly. Thermal modulation of headgroup composition occurred rapidly as reflected by changes in the ratio of PC-to-PE, which rose significantly from 2.40 +/- 0.09 to 2.92 +/- 0.09 within 72 h of transfer from 5 to 20 degrees C; adaptation to 5 degrees C was equally rapid. Proportions of PE changed more rapidly than those of PC during cold adaptation, whereas the opposite was true during warm acclimation. Both the time course and the direction of the observed changes in phospholipid composition suggest that such adjustments may contribute to the homeoviscous regulation of membrane properties, particularly during the initial stages of thermal adaptation.

Modulation of membrane fluidity in living protoplasts of Nicotiana plumbaginifolia by catalytic hydrogenation

A homogeneous water-soluble Ru catalyst, has been incorporated into mesophyll protoplasts isolated from Nicotiana plumbaginifolia leaves. In the presence of hydrogen gas this complex causes an extensive loss of unsaturated fatty acid bonds and a concomitant increase in microviscosity of the cellular membranes. Although the gradual reduction of the level of unsaturation, per se, is accompanied by considerable cell damage, there is an optimum reaction time where approximately 50% of the protoplasts are still living and about 20% of the double bounds initially present in fatty acyl residues have undergone hydrogenation. The possible mechanism of the self regulatory process competing with the hydrogenation in the early stages of the reaction is also discussed.

Thermal adaptation of Tetrahymena membranes with special reference to mitochondria. II. Preferential interaction of cardiolipin with specific molecular species of phospholipid

A specific effect of cardiolipin on fluidity of mitochondrial membranes was demonstrated in Tetrahymena cells acclimated to a lower temperature in the previous report (Yamauchi, T., Ohki, K., Maruyama, H. and Nozawa, Y. (1981) Biochim. Biophys. Acta 649, 385-392). This study was further confirmed by the experiment using fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH). Anisotropy of DPH for microsomal and pellicular total lipids from Tetrahymena cells showed that membrane fluidity of these lipids increased gradually as the cells were incubated at 15 degrees C after the shift down of growth temperature from 39 degrees C. However, membrane fluidity of mitochondrial total lipids was kept constant up to 10 h. This finding is compatible with the result obtained using spin probe in the previous report. Additionally, the break-point temperature of DPH anisotropy was not changed in mitochondrial lipids whereas those temperatures in pellicular and microsomal lipids lowered during the incubation at 15 degrees C. Interaction between cardiolipins and various phospholipids, which were isolated from Tetrahymena cells grown at 39 degrees C or 15 degrees C and synthesized chemically, was investigated extensively using a spin labeling technique. The addition of cardiolipins from Tetrahymena cells grown at either 39 degrees C or 15 degrees C did not change the membrane fluidity (measured at 15 degrees C) of phosphatidylcholine from whole cells grown at 39 degrees C. On the other hand, both cardiolipins of 39 degrees C-grown and 15 degrees C-grown cells decreased the membrane fluidity of phosphatidylcholine from Tetrahymena cells grown at 15 degrees C. The same results were obtained for phosphatidylcholines of mitochondria and microsomes. Membrane fluidity of phosphatidylethanolamine, isolated from cells grown at 15 degrees C, was reduced to a small extent by Tetrahymena cardiolipin whereas that of 39 degrees C-grown cells was not changed. Representative molecular species of phosphatidylcholines of cells grown at 39 degrees C and 15 degrees C were synthesized chemically; 1-palmitoyl-2-oleoylphosphatidylcholine for 39 degrees C-grown cells and dipalmitoleoylphosphatidylcholine for 15 degrees C-grown ones. By the addition of Tetrahymena cardiolipin, the membrane fluidity of 1-palmitoyl-2-oleoylphosphatidylcholine was not changed but that of dipalmitoleoylphosphatidylcholine was decreased markedly. These phenomena were caused by Tetrahymena cardiolipin. However, bovine heart cardiolipin, which has a different composition of fatty acyl chains from the Tetrahymena one, exerted only a small effect.

Cold stress induces in situ phospholipid molecular species changes in cell surface membranes

The structural organization of Tetrahymena pyriformis is such that its cilia are remote from the main centers of lipid metabolism. As a result, the ciliary membrane lipid composition of cells exposed to low-temperature stress is initially unaffected by the significant metabolic changes induced in microsomal membranes. Nevertheless, changes in the ciliary membrane lipid composition can be detected during the first 4 h of cold exposure. A combination of in vivo and in vitro experiments has provided strong evidence for a substantial retailoring of ciliary phospholipid molecular species in situ in the absence of any importation of lipids from the cell interior or change in overall ciliary fatty acid composition. The mechanism responsible for the ciliary lipid changes is independent of the one(s) triggering internal acclimation responses. Our observations establish for the first time that chilling stress can simultaneously induce separate and distinctive lipid modification responses in different parts of a cell. This finding could be important in identifying the molecular 'sensor' capable of actuating stress-induced lipid changes.