TNT‐Induced Phagocytosis: Tunneling Nanotubes Mediate the Transfer of Pro‐Phagocytic Signals From Apoptotic to Viable Cells

Abstract

The exposure of phosphatidylserine (PS) on the surface membrane of apoptotic cells triggers the recruitment of phagocytic receptors and subsequently results in uptake by phagocytes. Here we describe how apoptotic cells can use intercellular membrane nanotubes to transfer exposed PS to neighboring viable cells, and thus deposit an “eat‐me” tag on the viable cells. Tunneling nanotubes (TNTs) connected UV‐treated apoptotic rat pheochromocytoma PC12 cells with neighboring untreated cells. These TNTs were composed of PS‐exposed plasma membrane and facilitated the transfer of the membrane from apoptotic to viable cells. Other pro‐phagocytic signals, such as oxidized phospholipids and calreticulin, were also transferred to viable cells. In addition, anti‐phagocytic signal CD47 presenting on the plasma membrane of viable cells was masked by the transferred PS‐membrane. Confocal imaging revealed an increase of phagocytosis of viable PC12 cells by murine RAW264.7 macrophages when the viable PC12 cells were cocultured with UV‐treated PC12 cells. Treatment with 50 nM cytochalasin D would abolish TNTs and correspondingly inhibit this phagocytosis of the viable cells. Our study indicates that exposed‐PS membrane is delivered from apoptotic to viable cells through TNTs. This transferred membrane may act as a pro‐phagocytic signal for macrophages to induce phagocytosis of viable cells in a situation where they are in the vicinity of apoptotic cells. J. Cell. Physiol. 232: 2271–2279, 2017. © 2016 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals Inc.

Specialized Intercellular Communications via Cytonemes and Nanotubes

“…In addition, T lymphocytes transfer the FAS ligand to one another through TNTs, resulting in cell death (Arkwright et al 2010). Further evidence suggests that apoptotic cells transfer phosphatidylserine (PS) containing membranes to other cells through TNTs (Bittins & Wang 2017). These PS membranes trigger a phagocytic response, indicating that cells undergoing cell death can actively use TNTs to recruit macrophages to promote their own clearing.…”

Opportunities and Challenges in Tunneling Nanotubes Research: How Far from Clinical Application?

“…Numerous studies over the past ten years have shown that TNTs are widely present in various cell types [ 5 , 6 ]. More importantly, TNTs enable the transfer of small molecules, proteins, vesicles, and organelles between cells [ 2 , 7 , 8 , 9 , 10 , 11 ]. In 2010, we discovered that TNTs mediated depolarization coupling in non-neuronal cells, indicating that TNTs facilitate electrical signal transduction in addition to material transport [ 12 ].…”

Perspectives of cellular communication through tunneling nanotubes in cancer cells and the connection to radiation effects

“…The involvement of TNTs in apoptosis signalling was found in T cells, in which death signals are propagated by TNTs and immune stimulation leads to an enhanced TNT formation among cells. In fact, it was shown that phagocytosis signals are transferred from apoptotic to viable cells in order to help the immune system to detect a damaged cellular region [67]. Furthermore, it was reported that activated natural killer cells more frequently form TNTs and connected target cells are more often lysed than unconnected [43].…”