Optical Dyes to Monitor Tension and Growth in Model Membranes

Abstract

Cells have been shown to dynamically regulate physical properties of the membrane in response to mechanical cues. In particular, membranes have been shown to undergo surface area changes in response to membrane tension, a process which plays a role in cellular growth, migration and signaling. The ability to simultaneously monitor membrane strain and membrane surface area changes would be beneficial in understanding this process, yet we lack robust methods to accomplish this. Here, we introduce an optical technique to monitor membrane tension and surface area changes in real time. Through the combinatorial use of two types of optical dyes, which localize to distinct regions of the bilayer membrane, we can monitor changes in lipid separation through the processes of membrane stretch and lipid uptake. Laurdan, a polarity-sensitive probe, is localized to the hydrophobic region of the membrane, while phospholipid-conjugated FRET pairs localize to the hydrophilic head region. These dyes act in concert to indicate changes in water permeation and phospholipid head separation, respectively, as membrane properties change. We observed linear shifts in fluorescence in response to applied tension, indicating the potential of this system as a membrane tension sensor. We then used these probes to observe the process of tension-induced lipid uptake, and discovered that membranes under strain take up excess lipid faster and in greater amounts. Ultimately, the combined use of these probes offers a more complete picture of membrane dynamics, and allows us to remotely track changes in tension and surface area in model membranes.