Abstract
Membrane tension modulates the morphology of plasma-membrane tubular protrusions in cells but is difficult to measure. Here, we propose to use microscopy imaging to assess the membrane tension. We report direct measurement of membrane nanotube diameters with unprecedented resolution using stimulated emission depletion (STED) microscopy. For this purpose, we integrated an optical tweezers setup in a commercial microscope equipped for STED imaging and established micropipette aspiration of giant vesicles. Membrane nanotubes were pulled from the vesicles at specific membrane tension imposed by the aspiration pipet. Tube diameters calculated from the applied tension using the membrane curvature elasticity model are in excellent agreement with data measured directly with STED. Our approach can be extended to cellular membranes and will then allow us to estimate the mechanical membrane tension within the force-induced nanotubes.
Highlights
Cellular membranes are found to attain a multitude of morphologies and often exhibit highly curved segments with certain functionality
Trapping potential in the outer region of the imaging field could be affected by spherical aberration of the microscope objective
We have shown for the first time that super-resolution microscopy like stimulated emission depletion (STED) can be used to directly measure the membrane nanotube diameter
Summary
Cellular membranes are found to attain a multitude of morphologies and often exhibit highly curved segments with certain functionality. Highly curved membrane nanotubes are involved in several cellular functions such as cell migration,[1] signaling,[2] remote communication and motility,[3] and cell spreading.[4] Tunneling membrane nanotubes play an important role in transfer of cellular content (small molecules, proteins, prions, viral particles, vesicles, and organelles) in a variety of cell types[5−9] as well as electrical signals.[10] During migration, tubular membrane protrusions ( referred to as retracting fibers) are formed behind the migrating cell and are responsible for releasing cellular content.[11] In all of these examples, when not supported by the underlying substrate, membrane shape is modulated by membrane tension which affects the membrane surface area and morphology.[12,13] Membrane tension provides a link between membrane mechanics, morphology, as well as mechanical transduction in the cell, for example, via tensionsensitive membrane channels. How cellular tension is regulated and mechanobiological cues are perceived by the cell is poorly understood.[14]
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