Molecular Relay Stations in Membrane Nanotubes: IRSp53 Involved in Actin-Based Force Generation

Membrane nanotubes are cell protrusions that grow to tens of micrometres and functionally connect cells. Actin filaments are semi-flexible polymers, and their polymerisation provides force for the formation and growth of membrane nanotubes. The molecular bases for the provision of appropriate force through such long distances are not yet clear. Actin filament bundles are likely involved in these processes; however, even actin bundles weaken when growing over long distances, and there must be a mechanism for their regeneration along the nanotubes. We investigated the possibility of the formation of periodic molecular relay stations along membrane nanotubes by describing the interactions of actin with full-length IRSp53 protein and its N-terminal I-BAR domain. We concluded that I-BAR is involved in the early phase of the formation of cell projections, while IRSp53 is also important for the elongation of protrusions. Considering that IRSp53 binds to the membrane along the nanotubes and nucleates actin polymerisation, we propose that, in membrane nanotubes, IRSp53 establishes molecular relay stations for actin polymerisation and, as a result, supports the generation of force required for the growth of nanotubes.

Live cell superresolution-structured illumination microscopy imaging analysis of the intercellular transport of microvesicles and costimulatory proteins via nanotubes between immune cells

Membrane nanotubes are transient long-distance connections between cells that can facilitate intercellular communication. These tethers can form spontaneously between many cell types, including cells of the immune and nervous systems. Traffic of viral proteins, vesicles, calcium ions, mRNA, miRNA, mitochondria, lysosomes and membrane proteins/raft domains have all been reported so far via the open ended tunneling nanotubes (TNTs). Recently we reported on existence of plasma membrane derived GM₁/GM₃ ganglioside enriched microvesicles and costimulatory proteins in nanotubes connecting B lymphocytes, the way they are formed and transported across TNTs, however, still remained unclear. Here, using live cell confocal and Structured Illumination (SR-SIM) superresolution imaging, we show that B cells respond to bacterial (Cholera) toxin challenge by their subsequent internalization followed by rapid formation of intracellular microvesicles (MVs). These MVs are then transported between adjacent B cells via nanotubes. Selective transport-inhibition analysis of two abundant motor proteins in these cell types demonstrated that actin-based non-muscle myosin 2A dominantly mediates intercellular MV-transport via TNTs, in contrast to the microtubule-based dynein, as shown by the unchanged transport after inhibition of the latter. As suggested by SR-SIM images of GFP-CD86 transfected macrophages, these costimulatory molecules may be transferred by unusually shaped MVs through thick TNTs connecting macrophages. In contrast, in B cell cultures the same GFP-CD86 is dominantly transported along the membrane wall of TNTs. Such intercellular molecule-exchange can consequently improve the efficiency of antigen-dependent T cell activation, especially in macrophages with weak costimulator expression and T cell activation capacity. Such improved T cell activating potential of these two cell types may result in a more efficient cellular immune response and formation of immunological memory. The results also highlight the power of superresolution microscopy to uncover so far hidden structural details of biological processes, such as microvesicle formation and transport.