Migrasome formation is initiated preferentially in tubular junctions by membrane tension.

Migrasomes, the vesicle-like membrane microstructures, arise on the retraction fibers (RFs), the branched nanotubules pulled out of cell plasma membranes during cell migration and shaped by membrane tension. Migrasomes form in two steps: a local RF bulging is followed by a protein-dependent stabilization of the emerging spherical bulge. Here, we addressed theoretically and experimentally the previously unexplored mechanism of bulging of membrane tubular systems. We assumed that the bulging could be driven by increases in membrane tension and experimentally verified this hypothesis in live-cell and biomimetic systems. We exposed RF-generating live cells to a hypotonic medium, which produced water flows into the cells and a related increase in the membrane tension. We observed the formation of migrasome-like bulges with a preferential location in the RF branching sites. Next, we developed a biomimetic system of three membrane tubules pulled out of a giant plasma membrane vesicle (GPMV), connected by a junction, and subjected to pulling forces controlled by the GPMV membrane tension. An abrupt increase in the GPMV tension resulted in the generation of migrasome-like bulges mainly in the junctions. To understand the physical forces behind these observations, we considered theoretically the mechanical energy of a membrane system consisting of a three-way tubular junction with emerging tubular arms subjected to membrane tension. Substantiating our experimental observations, the energy minimization predicted a tension increase to drive the formation of membrane bulges, preferably in the junction site, independently of the way of the tension application. We generalized the model to derive universal criteria of bulging in branched membrane tubules.