Mapping of TNTs using Correlative Cryo-Electron Microscopy Reveals a Novel Structure

Abstract

The harmonious orchestration of intercellular communication is essential for multicellular organisms. One mechanism by which cells communicate is through long, actin-rich membranous protrusions, called tunneling nanotubes, that allow for the intercellular transport of various cargoes, including viruses, organelles, and proteins between the cytoplasm of distant cells in vitro and in vivo. Over the last decade, studies have focused on their functional role but information regarding their structure and the differences with other cellular protrusions such as filopodia, is still lacking.Here, we report the structural characterization of tunneling nanotubes using correlative light-and cryo-electron microscopy approaches. We demonstrate their structural identity compared to filopodia by showing that they are comprised of a bundle of functional individual Tunneling Nanotubes containing membrane-bound compartments and allowing organelle transfer. effectively improved our understanding of these novel structures and underscored their role in cellto-cell communication, facilitating the bi-and uni-directional transfer of compounds between cells, including: organelles, pathogens, ions, genetic material, and misfolded proteins 5 . Altogether, in vitro and in vivo evidence has shown that TNTs can be involved in many different processes such as stem cell differentiation, tissue regeneration, neurodegenerative disorders, immune response, and cancer 2,6-11 .Although these in vitro and in vivo studies have been informative, the structural complexity of TNTs remains largely unknown. As a result, TNTs have been regarded with skepticism by one part of the scientific community 5,12 . Two major issues that remain to be clarified are whether these protrusions are different from other previously studied cellular processes such as filopodia 13 and whether their function in allowing the exchange of cargos between distant cells is due to direct communication between the cytoplasm of distant cells or to a classic exo-endocytosis process or a trogocytosis event 14 .Addressing these questions has been difficult due to considerable technical challenges in preserving the ultrastructure of TNTs for electron microscopy (EM) studies. To date, the ultrastructure of TNTs using Scanning and Transmission EM (SEM and TEM, respectively) has only been analyzed in a handful of articles 1,15-18 .Although very similar under fluorescence microscopy (FM), TNT formation appears to be oppositely regulated by the same actin modifiers that act on filopodia 19 . Furthermore, filopodia have not been shown to allow cargo transfer 13,20,21 . Thus, we hypothesize that TNTs might be different organelles from filopodia and display structural differences in morphology and actin architecture.In order to compare the ultrastructure and actin architecture of TNTs and filopodia at high resolution and ensure that the structures identified by TEM/SEM represent the functional units observed by FM, we employed a combination of live imaging, correlative light-and cryo-electron tomog...

Fine intercellular connections in development: TNTs, cytonemes, or intercellular bridges?

“…The recent use of correlative light and cryo-electron microscopy has provided several structural details about TNT morphology, that suggest that the terms «thin» and «thick» TNTs should be used with caution [31]. In this study, TNTs were preserved closer to their native status owing to fixation by rapid freezing.…”

“…Moreover, cryo-electron tomography in neuronal cells showed that both thin and thicker iTNTs contained only actin and not microtubules [31], differently from what was proposed as a distinctive feature of thick TNTs in macrophages [35]. Of specific interest, mitochondria were shown to be transported within iTNTs presumably on actin rails, in a tubulin-independent mechanism.…”

“…The average diameter of iTNTs of about 120 nm and such close-packing of acting bundles did not seem to impede iTNTs to transfer vesicular compartments and even mitochondria within their lumen, as the membrane was often observed to bulge to accommodate the passage of the vesicle (Figure 1). Furthermore, cryoelectron tomography demonstrated the existence of thin short filaments labeled by N-cadherin antibodies, that appeared to connect iTNTs between each other, possibly for holding them in a bundle and conferring higher mechanical stability [31] (Figure 1). In addition to iTNT bundles, single thicker TNTs (600-900 nm in diameter) were also observed.…”

“…Of specific interest, mitochondria were shown to be transported within iTNTs presumably on actin rails, in a tubulin-independent mechanism. This suggests that TNTs do not necessarily need microtubules to allow the transfer of larger cargoes, such as vesicles and organelles [31].…”

“…Considering the recent identification of the unique structure of TNTs formed between neuronal cells in culture [31], in this review we will specifically focus on the differences between TNTs and other intercellular structures (e.g. intercellular bridges and cytonemes) both from the structural and functional point of view.…”