title: Temperature-dependent changes in plasma-membrane lipid order and the phagocytotic activity of the amoeba Acanthamoeba castellanii are closely correlated.
abstract: The relationship between temperature-dependent changes in phagocytotic activity of Acanthamoeba castellanii and the fatty acid composition and physical properties of plasma membrane-enriched fractions were determined in cultures acclimated to 30 degrees C and 15 degrees C. Chilling (from 30 degrees C to 15 degrees C) had a very pronounced short-term inhibitory effect on phagocytosis only in stationary-phase cultures, which displayed a low degree of fatty acid unsaturation. A subsequent increase in phagocytosis by these cells was correlated with a low-temperature-induced increase in fatty acid unsaturation (shown previously [Jones, Lloyd and Harwood (1993) Biochem. J. 296, 183-188] to be due to n-6 desaturase induction). Plasma membrane-enriched fractions from 15 degrees C-acclimated cells also showed a marked increase in the relative proportion of polyunsaturated fatty acids. Steady-state fluorescence depolarization studies, using the membrane probe diphenylhexatriene, revealed increases in plasma membrane order with decreasing assay temperature. Over the upper assay-temperature range (25-40 degrees C), fluorescence anisotropy values were higher in membranes from 30 degrees C-acclimated cells; a 3.3 degrees C relative displacement of plots indicated that temperature-induced changes in membrane lipid composition compensated for approx. 22% of the ordering effect of low temperature. Changes in the temperature-dependence of fluorescence anisotropy, possibly corresponding to lateral phase separations or alterations in other bulk physical properties of membranes, occurred between 20 and 25 degrees C in membranes from 30 degrees C-acclimated cells and between 15 and 20 degrees C in membranes from 15 degrees C-acclimated cells. Fluorescence anisotropy plots were superimposed at assay temperatures between 5 and 15 degrees C. Short-term phagocytotic rates in whole cells decreased with assay temperature. Arrhenius discontinuities in rates of phagocytosis occurred at approx. 25.0 degrees C and 17.5 degrees C in 30 degrees C- and 15 degrees C-acclimated cells respectively, and in each case were thus within the temperature ranges of slope-change in the corresponding fluorescence anisotropy plots. The results show a direct correlation between plasma membrane fatty acid unsaturation, membrane physical properties and phagocytotic activity in A. castellanii. Therefore, a specific integrated physiological process has been correlated with fatty acid desaturase induction for the first time.
authors.affiliation: School of Molecular and Medical Biosciences, School of Pure and Applied Biology, University of Wales College of Cardiff, U.K.
authors.forename: S V
authors.initials: SV
authors.lastname: Avery
authors.forename: D
authors.initials: D
authors.lastname: Lloyd
authors.forename: J L
authors.initials: JL
authors.lastname: Harwood
mesh_terms: D000048:Acanthamoeba / Q000737:chemistry* / Q000502:physiology*; D000818:Animals; D002462:Cell Membrane / Q000737:chemistry / Q000502:physiology; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003080:Cold Temperature; D004161:Diphenylhexatriene; D044943:Fatty Acid Desaturases / Q000378:metabolism; D005227:Fatty Acids / Q000032:analysis*; D005231:Fatty Acids, Unsaturated / Q000032:analysis / Q000737:chemistry; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D008563:Membrane Lipids / Q000032:analysis* / Q000737:chemistry; D010587:Phagocytosis / Q000502:physiology*
chemical_list: D005227:Fatty Acids; D005231:Fatty Acids, Unsaturated; D005456:Fluorescent Dyes; D008563:Membrane Lipids; D004161:Diphenylhexatriene; D044943:Fatty Acid Desaturases