But such membrane structures are not limited to the cell’s outer boundary. They also serve various purposes inside, and even outside, the cell. And new research is helping us to understand better some of these roles. For instance, as a recent research paper explains, membrane structures can be used to form long, thin nanotubes used by immune cells to reach out and destroy harmful cells:
Membrane nanotubes are membranous tethers that physically link cell bodies over long distances. Here, we present evidence that nanotubes allow human natural killer (NK) cells to interact functionally with target cells over long distances. Nanotubes were formed when NK cells contacted target cells and moved apart. The frequency of nanotube formation was dependent on the number of receptor/ligand interactions and increased on NK cell activation. Most importantly, NK cell nanotubes contained a submicron scale junction where proteins accumulated, including DAP10, the signaling adaptor that associates with the activating receptor NKG2D, and MHC class I chain-related protein A (MICA), a cognate ligand for NKG2D, as occurs at close intercellular synapses between NK cells and target cells.
As described above, like the cell membrane, these membrane nanotubes include a variety of specialized, critical molecular machines.
And why are such nanotubes needed? The researchers hypothesized that such nanotubes might be used to maintain contact with target cells that move around too much:
It is well established that T or NK cells receive a “stop” signal when activated by a target or antigen-presenting cell. However, target cells would not receive an equivalent stop signal; therefore, particularly motile target cells may be able to move away from cytolytic NK or T cells before an effector response has been realized. Hence, one speculative role for nanotubes could be to facilitate cytolytic cells being able to sustain an interaction with target cells that are particularly motile (e.g., other lymphocytes).
More research is required to understand these nanotubes better, but what we do understand reveals a most interesting story. How curious it is that the nanotubes are able to recruit important molecular machines in order to function. And how strange that the nanotubes deploy when required and successfully kill the target cell. Are we really to believe that such structures and functions arose via a sequence of mutations? And that each mutation was random with respect to the need and the final design? And this in spite of the fact that no such sequence of mutations is actually known to us? In fact this seems to be yet another example of nature not cooperating with evolutionary expectations.