Membrane Remodeling Without Energy

Cell membranes instantly and spontaneously form reservoirs to accommodate sudden changes in membrane area.

AsianScientist (Jul. 16, 2015) – The cell membrane is rapidly remodeled in response to physical stress through a purely mechanical process that does not require additional energy. These results have been published in Nature Communications.

Cells need to expand, stretch or move during critical physiological processes like development, breathing or wound healing. While doing so, cells frequently change their shape or size and such changes must be accommodated by the membrane that enwraps the cell. However, cell membranes are stiff, inelastic structures, and therefore the mechanism by which they accommodate these changes remains unclear.

Membrane area can be increased or decreased to accommodate cell shape changes by processes that allow cargo such as proteins and macromolecules to enter or leave the cell via the cell membrane. When small areas of membrane engulf the cargo like a bubble and enter the cell, the membrane area is reduced. When similar vesicles merge with the membrane to release cargo, the membrane area is increased.

Known as endocytosis and exocytosis respectively, these are active, energy dependent processes that are supported by the cytoskeleton, a network of filamentous proteins distributed throughout the cell. However, the time scales of these processes vary from seconds to minutes and therefore cannot fully account for the instant and continuous membrane remodeling required for many physiological processes.

In the current study, a team of researchers led by Dr. Nils Gauthier from the Mechanobiology Institute (MBI) at the National University of Singapore (NUS) showed that even before the onset of active membrane remodeling, membranes instantly respond to the physical forces through a passive, mechanical process.

This discovery was made when two types of mechanical stimuli were used to induce cell shape changes. Specifically, the cells were stretched by submitting them to a fixed amount of linear strain, or swollen by being exposed to a hypo-osmotic medium.

In both cases, a rapid release of tension, either by stretch release or re-exposure to isotonic medium respectively, resulted in the formation of numerous, small, membrane reservoirs which were quickly resorbed. The nature of membrane reservoirs appeared different depending on the type of mechanical stimuli, for instance, tubular invaginations resulted from stretch release whereas osmotic shock caused larger vacuole-like dilations.

The formation of these membrane reservoirs indicated the activation of a rapid membrane remodeling process in response to stress. The authors observed that this is a passive process as it did not require energy or the cytoskeleton and resembled the response of a synthetic lipid bilayer to similar mechanical stimuli.

This study reveals the existence of a mechanical membrane remodeling process that can instantly respond to cell shape changes. Membrane reservoirs are created in order to prepare the cell membrane for a rapid increase in membrane area. This explains how large membrane requirements for certain cellular processes like cell spreading can be rapidly met.

As reservoirs can store and release membrane upon subsequent stretch cycles, they may function to support the constant requirement of membrane remodeling in vital processes where cells undergo continual cycles of stretch and relaxation, such as breathing or as the heart beats.

The article can be found at: Kosmalska et al. (2015) Physical Principles of Membrane Remodelling During Cell Mechanoadaptation.

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Source: Mechanobiology Institute.
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