Changes in thermal phase transition of various membranes during temperature acclimation in Tetrahymena

A Method for High-Throughput Measurements of Viscosity in Sub-micrometer-Sized Membrane Systems

To unravel the underlying principles of membrane adaptation in small systems like bacterial cells, robust approaches to characterize membrane fluidity are needed. Currently available relevant methods require advanced instrumentation and are not suitable for high-throughput settings needed to elucidate the biochemical pathways involved in adaptation. We developed a fast, robust, and financially accessible quantitative method to measure the microviscosity of lipid membranes in bulk suspension using a commercially available plate reader. Our approach, which is suitable for high-throughput screening, is based on the simultaneous measurements of absorbance and fluorescence emission of a viscosity-sensitive fluorescent dye, 9-(2,2-dicyanovinyl)julolidine (DCVJ), incorporated into a lipid membrane. We validated our method using artificial membranes with various lipid compositions over a range of temperatures and observed values that were in good agreement with previously published results. Using our approach, we were able to detect a lipid phase transition in the ruminant pathogen Mycoplasma mycoides.

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.

Molecular cloning of delta 9 fatty acid desaturase from the protozoan Tetrahymena thermophila and its mRNA expression during thermal membrane adaptation

In response to a decrease in its growth temperature, the protozoan Tetrahymena is known to increase the level of unsaturated fatty acids in its membrane phospholipids so as to maintain the correct physical state (fluidity) of the membranes. In this organism, synthesis of unsaturated fatty acids is initiated by delta 9 acyl-CoA desaturase. Our previous studies have shown that, during cold adaptation, the activity of microsomal palmitoyl- and stearoyl-CoA desaturase increases, reaching a maximal level at 2 h after a temperature down-shift to 15 degrees C. Two hypotheses have been proposed to explain this increase in desaturase activity: (1) self-regulation via a direct effect of reduced membrane fluidity, and (2) induction of desaturase mRNA. However, the precise mechanism is not clearly understood. In order to obtain further insight into the mechanism of regulation of the desaturase, we have isolated a gene that encodes delta 9 fatty acid desaturase from T. thermophila and examined its expression during cold adaptation. The nucleotide sequence indicates that the 1.4 kbp gene encodes a polypeptide of 292 amino acid residues which shows marked sequence similarity to delta 9 acyl-CoA desaturases from other sources, e.g. rat, mouse, Amblyomma americanum and Saccharomyces cerevisiae. This protein has three histidine-cluster motifs (one HXXXXH and two HXXHH), and two hydrophobic regions which are conserved among delta 9 acyl-CoA desaturases. The level of desaturase mRNA was sensitive to decreasing the temperature of the culture media, and was close to maximal immediately after the temperature was shifted down from 35 degrees C to 15 degrees C (0.8 degrees C/min). Thereafter, the amount of mRNA gradually decreased with time, but remained above the control level for at least 5 h. Furthermore, during the course of the cooling process to 15 degrees C, the increased expression of desaturase mRNA became evident at 27 degrees C. Nuclear run-on analysis and actinomycin D chase experiments revealed that the elevation of the mRNA level was due to increases in both transcription and mRNA stability. These results suggest that the enhanced desaturase activity is controlled, at least in part, at the transcriptional level.

Changes in lipid fluidity and fatty acid composition with altered culture temperature in Tetrahymena pyriformis-NT1

Cultures of T. pyriformis-NT1 were grown at 20 degrees C (Tg 20 degrees C) and 38 degrees C (Tg 38 degrees C). G.L.C. analysis and D.P.H. fluorescence polarization measurements in extracted phospholipids indicated that there was increased saturation of fatty acids and relatively reduced fluidity as growth temperature was increased. Breakpoints occurred in the Arrhenius plots of fluorescence polarization at 16 degrees C for Tg 38 degrees C total extracted phospholipids and 9 degrees C for Tg 20 degrees C lipids.

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.