Cholesterol uptake is dependent on membrane fluidity in mycoplasmas

Area of Science:

• Microbiology
• Cell Biology
• Biochemistry

Background:

• Mycoplasmas, including Acholeplasma laidlawii, are bacteria lacking cell walls, making their cell membranes crucial for structural integrity and function.
• Cell membrane fluidity, influenced by lipid composition and temperature, plays a vital role in cellular processes like nutrient uptake.
• Cholesterol is an essential component of many eukaryotic cell membranes, but its role and incorporation mechanisms in prokaryotes like mycoplasmas are of particular interest.

Purpose of the Study:

• To investigate the effect of temperature-induced phase transitions of the cell membrane lipid bilayer on exogenous cholesterol incorporation in Acholeplasma laidlawii.
• To compare cholesterol uptake rates in mycoplasma cells with different fatty acid enrichments (elaidate vs. oleate) under varying temperature conditions.

Main Methods:

• Acholeplasma laidlawii cells were enriched with either elaidate or oleate.
• Cells were cultured and subjected to a temperature shift-down from 37°C to 4°C.
• Exogenous cholesterol incorporation into the cell membrane was monitored and quantified at different temperatures.

Main Results:

• Transferring elaidate-enriched A. laidlawii cells to 4°C drastically reduced exogenous cholesterol incorporation, indicating membrane arrest.
• Oleate-enriched A. laidlawii cells showed a slower rate of cholesterol uptake upon temperature shift-down compared to elaidate-enriched cells.
• Cholesterol incorporation was significantly faster when the mycoplasma cell membrane lipid bilayer was in the liquid-crystalline state (higher temperature) than in the gel state (lower temperature).

Conclusions:

• The rate of exogenous cholesterol incorporation into living mycoplasma cell membranes is highly dependent on the physical state of the lipid bilayer.
• A liquid-crystalline membrane state facilitates rapid cholesterol uptake, whereas a gel state severely impedes it.
• These findings underscore the importance of membrane fluidity and lipid phase behavior in regulating membrane composition and function in prokaryotes.