Long, delicate tubules are a new trade route for the intercellular exchange of goods, as shown by Amin Rustom, Hans-Hermann Gerdes (University of Heidelberg, Germany), and colleagues.
Rustom noticed these actin-rich extensions, which the group calls tunneling nanotubes (TNTs), while looking for secretory granules near the plasma membrane. The membrane dye that he used revealed long thin tubes, up to several cell diameters long but only ∼100 nm wide, linking some of the plated cells. “They are extremely sensitive,” says Gerdes. “Even light exposure disrupts them. Maybe because of [this sensitivity], this is the first time we realized these unique structures are there.” The group has seen the connectors in kidney and neurosecretory cell lines and primary neuroendocrine cultures, but other cell types could be similarly linked.
The TNTs are made when filopodia contact a distant cell. Once it becomes contiguous with the new partner, the extension establishes a one-way conveyor belt–like system through which one cell (probably the one that sent out the filopodia) dispatches endosomal-like vesicles to the other.
Smaller soluble molecules, however, are blocked from entry into the TNTs. The actin bundle at the base of the TNT, wrapped tightly by the surrounding membrane, may act like a plug to prevent passive diffusion of small molecules. If TNTs exist in vivo, then theoretically anything packaged into endosomes, such as morphogens or immunogenic components, could be used to signal between otherwise unconnected cells. ▪