A shift to 50°C provokes death in distinct ways for glucose- and oleate-grown cells of Yarrowia lipolytica
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Based on the observation that shocks provoked by heat or amphiphilic compounds present some similarities, this work aims at studying whether cells grown on oleate (amphiphilic pre-stress) acquire a tolerance to heat shock. In rich media, changing glucose for oleate significantly enhanced the cell resistance to the shock, however, cells grown on a minimal oleate medium lost their ability to grow on agar with the same kinetic than glucose-grown cells (more than 7-log decrease in 18 min compared with 3-log for oleate-grown cells). Despite this difference in kinetics, the sequence of events was similar for oleate-grown cells maintained at 50°C with a (1) loss of ability to form colonies at 27°C, (2) loss of membrane integrity and (3) lysis (observed only for some minimal-oleate-grown cells). Glucose-grown cells underwent different changes. Their membranes, which were less fluid, lost their integrity as well and cells were rapidly inactivated. But, surprisingly, their nuclear DNA was not stained by propidium iodide and other cationic fluorescent DNA-specific probes but became stainable by hydrophobic ones. Moreover, they underwent a dramatic increase in membrane viscosity. The evolution of lipid bodies during the heat shock depended also on the growth medium. In glucose-grown cells, they seemed to coalesce with the nuclear membrane whereas for oleate-grown cells, they coalesced together forming big droplets which could be released in the medium. In some rare cases of oleate-grown cells, lipid bodies were fragmented and occupied all the cell volume. These results show that heat triggers programmed cell death with uncommon hallmarks for glucose-grown cells and necrosis for methyl-oleate-grown cells.
KeywordsHeat Membrane integrity Membrane fluidity Fluorescence Yarrowia lipolytica Programmed cell death Lipid Oleate Chromatin changes
This work was partially funded by the French Agency for Research (programme Transaronat). A grant was given to Ta by the French Ministry for Research and to Phan-Thi by the Agence Universitaire de la Francophonie. An important part of the work was carried out in the Spectral Imaging Platform of IFR 92. The authors are thankful to Christine Bernard-Rojas for technical help.
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