Tunneling nanotube

A tunneling nanotube (TNT) or membrane nanotube is a term that has been applied to protrusions that extend from the plasma membrane which enable different animal cells to touch over long distances, sometimes over 100 μm between T cells. Two types of structures have been called nanotubes. The first type are less than 0.7 micrometers in diameter, contain actin and carry portions of plasma membrane between cells in both directions. The second type are larger (>0.7 μm), contain both actin and microtubules, and can carry components of the cytoplasm such as vesicles and organelles between cells, including whole mitochondria. The diameter of TNTs ranges from 50 to 200 nm and they can reach lengths of several cell diameters. These structures may be involved in cell-to-cell communication, transfer of nucleic acids such as mRNA and miRNA between cells in culture or in a tissue, and the spread of pathogens or toxins such as HIV and prions. TNTs have observed lifetimes ranging from a few minutes up to several hours, and several proteins have been implicated in their formation or inhibition. Membrane nanotubes were first described in a 1999 Cell article examining the development of Drosophila melanogaster wing imaginal discs. More recently, a Science article published in 2004 described structures that connected various types of immune cells together, as well as connections between cells in tissue culture. Since these publications, more TNT-like structures have been recorded, containing varying levels of F-actin, microtubules and other components, but remaining relatively homogenous in terms of composition. Several mechanisms may be involved in nanotube formation. These include molecular controls as well as cell-to-cell interactions. Two primary mechanisms for TNT formation have been proposed. The first involves cytoplasmic protrusions extending from one cell to another, where they fuse with the membrane of the target cell. The other is that, as two previously connected cells move away from one another, TNTs remain as bridges between the two cells.