Migrasome formation is initiated preferentially in tubular junctions by alternations of membrane tension or intracellular pressure

Migrasomes, the transient vesicle-like cellular organelles, arise on the retraction fibers (RFs), the branched tubular extensions of the plasma membrane generated during cell migration. Migrasomes form in two steps: a local RF swelling is followed by a protein-dependent stabilization of the emerging spherical bulge. Here we approached experimentally and theoretically the previously unaddressed mechanism of the initial RF swelling. We hypothesized that the swelling can be driven by alterations of the generic mechanical factors, the RFs luminal pressure and membrane tension. To examine the effects of pressure, we exposed migrating RF-producing cells to a hypotonic medium and observed the formation of migrasome-like bulges with a preferential location in the RF branching sites. To test the results of tension variations, we developed a biomimetic system of three membrane tubules connected by a junction and subjected to controlled membrane tension. An abrupt increase of tension resulted in a migrasome-like bulge formation in the junction and in the tubular regions. Following the formation, the tubules bulges moved toward and merged with the junctional bulge. To understand the physical forces behind the observations, we considered theoretically the mechanical energy of a membrane system consisting of a three-way tubular junction with emerging tubular arms connected to a membrane reservoir. The energy minimization predicted the membrane bulging, preferably, in the junction site as a result of both an increase in the luminal pressure and an abrupt rise of the membrane tension. We discuss the common physical background of the two phenomena.