1.L.1 The Tunneling Nanotube (TNT) Family

Animal cells have evolved different mechanisms to communicate with one another. In 2004, a new route of cell-to-cell communication mediated by tunneling nanotubes (TNT) was reported (Rustom et al., 2004). These membranous cell bridges form de novo between cells and mediate the intercellular transfer of organelles, plasma membrane components and cytoplasmic molecules. The characterization of TNT-like bridges from several cell types revealed variations in the cytoskeletal composition as well as in the modality by which they interconnect cells, suggesting that different subclasses may exist (Gerdes and Carvalho, 2008). Furthermore, the growing number of cell types for which TNT-like structures were detected, supports the view that they represent a general mechanism for functional connectivity between cells (Gerdes et al., 2007).

Gurke et al. (2008) discovered that TNTs facilitate the intercellular transport of various cellular components. The subsequent identification of TNT-like structures in numerous cell types revealed some structural diversity. At the same time it emerged that the direct transfer of cargo between cells is a common functional property, suggesting a general role of TNT-like structures in selective, long-range cell-to-cell communication.

Rustom (2009) noted that TNTs may mediate the intercellular spread of diverse pathogens. This phenomenon shows striking similarities to plant tissues, where cells are connected via membranous channels, called plasmodesmata. Hase et al. (2009) have provided evidence that the protein, M-Sec, promotes membrane nanotube formation by interacting with Ral and the exocyst complex.

Wang et al. (2010) noted that TNTs contain F-actin and mediate the bidirectional spread of electrical signals between TNT-connected normal rat kidney cells over distances of 10 to 70 μm. Similar results were obtained for other cell types, suggesting that electrical coupling via TNTs may be a widespread characteristic of animal cells. Strength of electrical coupling depended on the length and number of TNT connections. Several lines of evidence implicate a role for gap junctions in this long-distance electrical coupling: punctate connexin 43 immunoreactivity was frequently detected at one end of TNTs, and electrical coupling was voltage-sensitive and inhibited by meclofenamic acid, a gap-junction blocker. Cell types lacking gap junctions did not show TNT-dependent electrical coupling, which suggests that TNT-mediated electrical signals are transmitted through gap junctions at a membrane interface between the TNT and one cell of the connected pair.

A wide variety of cell types, including immune cells, interact via transient, long-distance membrane connections via tunnelling nanotubes (TNTs). Considerable heterogeneity in their structures, modes of formation and functional properties have emerged, suggesting the existence of distinct subclasses (Sowinski et al., 2011). Open-ended tunneling nanotubes allow for the trafficking of cytoplasmic materials (e.g. endocytic vesicles) and the transmission of calcium signals. Closed-ended membrane nanotubes do not connect the cytoplasms of two interacting cells, possibly because a junction exists within the nanotube or where the nanotube meets a cell body. Live cell imaging suggested that membrane nanotubes between T cells could present a novel route for HIV-1 transmission.

Inflammatory and leukemic cells and some viral infected lymphocytes exhibit TNTs. They may provide a mechanism for viral transmission, not only enhancing the transmission efficiency but also mediating the escape from antibody neutralization, leading persistent infection. Poly synapse, multi-tunneling nanotube, flower-like structures on cell surfaces have been observed, for example, in EB virus infected human leukemic cells (Wu et al. 2010).  

TNTs are thin protrusions of the plasma membrane that allow direct physical connections of the plasma membranes between cells. The proposed functions for TNTs include the cell-to-cell transfer of large cellular structures such as membrane vesicles and organelles, as well as signal transduction molecules. TNT and TNT-related structures are thought to facilitate the intercellular spreading of virus and/or pathogenic proteins. Despite their contribution to normal cellular functions and importance in pathological conditions, M-Sec (also called TNFaip2) is a key molecule for TNT formation (Kimura et al. 2012). In cooperation with the RalA small GTPase and the exocyst complex, M-Sec can induce the formation of functional TNTs, indicating that the remodeling of the actin cytoskeleton and vesicle trafficking are involved in M-Sec-mediated TNT formation. 

Conductive carbon nanotube (CNT) membranes with hydrophobic pores can be positively or negatively charged and are consequently capable of regulating the transport of nanoparticles across their channels by 'opening' or 'closing' them. The switch between penetration and rejection of nanoparticles through/by CNT membranes is of high efficiency and allows dynamic control. The underlying mechanism responsible for the controlled transmembrane transport is that CNT pore channels with different polarities can prompt or prevent the formation of their non-covalent interactions with charged nanoparticles, resulting in their rejection or penetration through the CNT membranes (Wei et al. 2015). 

Quinn et al. 2016 discovered tunneling nanotube connections between myocytes and nonmyocytes in cardiac scar border tissue. Their results provided direct electrophysiological evidence of heterocellular electrical coupling in native myocardium and identified tunneling nanotubes as a probable mediator of electrical coupling localized to myocyte-nonmyocyte contacts in the heart.

RalGPS2 or of its PH-domain increases the number and the length of nanotubes, while the knock-down of RalGPS2 caused a strong reduction of these structures. Using a series of RalA mutants impaired in the interaction with different downstream components (Sec5, Exo84, RalBP1) D'Aloia et al. 2018 demonstrated that the interaction of RalA with Sec5 is required for TNTs formation. Further, RalGPS2 interacts with the transmembrane MHC class III protein leukocyte specific transcript 1 (LST1) and RalA, leading to the formation of a complex which promotes TNTs generation.

The reactions catalyzed by TNTs are:

Ions, molecules and organelles (cell1) ⇌ Ions, molecules and organelles (cell2)