Intercellular communications play a major role in tissue homeostasis and responses to external cues. Novel structures for this communication have recently been described. These tunneling nanotubes (TNTs) consist of thin-extended membrane protrusions that connect cells together. TNTs allow the cell-to-cell transfer of various cellular components, including proteins, RNAs, viruses, and organelles, such as mitochondria. Mesenchymal stem cells (MSCs) are both naturally present and recruited to many different tissues where their interaction with resident cells via secreted factors has been largely documented. Their immunosuppressive and repairing capacities constitute the basis for many current clinical trials. MSCs recruited to the tumor microenvironment also play an important role in tumor progression and resistance to therapy. MSCs are now the focus of intense scrutiny due to their capacity to form TNTs and transfer mitochondria to target cells, either in normal physiological or in pathological conditions, leading to changes in cell energy metabolism and functions, as described in this review.
Cell communication is essential for tissue homeostasis, specific cell functions, and response to external cues. Indeed, during development and self-repair, tissues constantly need to adapt to changing biological conditions in order to reach physiological homeostasis. For this, their constituting cells constantly interact with target cells that reside in their close vicinity or alternatively, they can reach out to cells much further away, without necessarily involving the close-by surrounding cells. This cell-to-cell communication can be achieved by various processes including diffusible factors like cytokines and chemokines, secreted microvesicles, or direct passage through gap junctions. Long-distance diffusible factors can target different cell types, depending on the expression, by these cells, of the relevant receptors.
Another impressive means of communication cells devised to allow long-distance cell-to-cell contacts are the formation of tunneling nanotubes (TNTs) between these cells, as initially reported in the rat pheochromocytoma- (PC12-) derived cells and in immune cells [
Tunneling nanotube (TNT). Tunneling nanotubes can connect many different cells together, using cytoskeleton actin microfilaments, microtubules, or both. TNTs allow the trafficking, from donor to recipient cells, of cargoes including organelles, proteins, miRNAs, and ions.
In the past few years, a number of studies reported this capacity of cells, from an ever increasing number of cell types, to connect to one another. Interestingly, these TNTs also allow the trafficking of a number of different cargos between the connected cells, therefore increasing the combinatorial complexity of these cell-to-cell connections and their biological outcome, as summarized in Table
Authors | TNT donor cells | TNT receiver cells | Transported cargoes | References |
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Onfelt et al. (2004) | Human NK cells | Human EBV-transformed human B cells | GFP-tagged cell surface class I MHC | [ |
Human macrophages | Same cells | |||
Human EBV-transformed human B cells | Same cells | |||
Murine J774 macrophages | Same cells | |||
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Rustom et al. (2004) | Rat pheochromocytoma PC12 | Same cells | Microvesicles | [ |
Human embryonic kidney (HEK) | Same cells | Organelles | ||
Normal rat kidney (NRK) | Same cells | |||
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Castro et al. (2005) | Colon carcinoma cell line SW620 | Same cells | ND | [ |
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Koyanagi et al. (2005) | Human endothelial progenitor (EPC) | Neonatal rat cardiomyocytes (CM) | Mitochondria | [ |
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Watkins et al. (2005) | Human dendritic cells | Same cells and THP-1 cells | Calcium flux | [ |
Human THP-1 monocytes | Same cells | Major histocompatibility proteins (MHC class I) | ||
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Chinnery et al. (2008) | Murine MHC class II dendritic cells | Same cells | ND | [ |
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Gurke et al. (2008) | Normal rat kidney cells (NRK) | Same cells | Endocytic organelles | [ |
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Onfelt et al. (2006) | Human macrophages | Same cells | Bacteria | [ |
Mitochondria | ||||
Vesicles (endosomes, lysosomes) | ||||
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Sowinski et al. (2008) | Jurkat T cells | Same cells and primary T cells | HIV viral particles | [ |
Primary T cells | Same cells | |||
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Bukoreshtliev et al. (2009) | PC12 cells | PC12 cells | Intracellular organelle transfer | [ |
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Eugenin et al. (2009) | Human macrophages | Same cells | HIV viral particles | [ |
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Plotnikov et al. (2010) | Human mesenchymal multipotent stromal cells (MMSC) | Rat renal tubular cells (RTC) | Mitochondria | [ |
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Acquistapace et al. (2011) | Human mesenchymal stem cells (MSCs) | Cardiomyocytes | Mitochondria and intracellular material | [ |
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Domhan et al. (2011) | Human proximal tubular epithelial cells (RPTEC) | Same cells | Microvesicles | [ |
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Wang et al. (2011) | Rat hippocampal astrocytes | Same cells and rat hippocampal neurons | Endoplasmic reticulum | [ |
Rat hippocampal neurons | Same cells and rat hippocampal astrocytes | Mitochondria | ||
Golgi fragments | ||||
Endosomes | ||||
Amyloid |
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Yasuda et al. (2011) | Human umbilical vein endothelial cells (HUVEC) | Stressed HUVEC | Lysosomes | [ |
Mitochondria | ||||
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Islam et al. (2012) | Murine MSCs | Murine alveoli | Mitochondria | [ |
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Lou et al. (2012) | Human primary cancer cells | Same cells | Mitochondria | [ |
Human mesothelial lines (MSTO-211H, VAMT, H-Meso) | Same cells | |||
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Schiller et al. 2012 | HeLa | Same cells | Transmembrane HLA-A2-EGFP protein | [ |
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Vallabhaneni et al. (2012) | Human MSCs | Human vascular smooth muscle cells (VSMCs) | Mitochondria | [ |
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Wittig et al. (2012) | Human retinal pigment epithelial (ARP-19) cells | Same cells | [ | |
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Costanzo et al. (2013) | CAD cells | Same cells and with transfected CADs | Htt aggregates | [ |
Primary cerebellar granule neurons (CGNs) | Same cells and with transfected CGNs | |||
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Pasquier et al. (2013) | Human mesenchymal stem cells (MSCs) | Same cells and ovarian and cancer cell lines | Mitochondria | [ |
Human endothelial cells (HECs) | Same cells and ovarian and cancer cell lines | |||
Human ovarian cancer cells (SKOV3, OVCAR3, HTB-161) | Same cells | |||
Human breast cancer cells (MDA-MB231 and MCF7) | Same cells | |||
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Rainy et al. (2013) | Human B cells | Human T cells | Plasma membrane-associatedproteins (H-Ras) | [ |
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Ady et al. (2014) | VAMT (sarcomatoid mesothelioma cell line) | Same cells | ND | [ |
H2052 (mesothelioma cell line) | Same cells | |||
MSTO-211H (derived from mesothelioma patient) | Same cells | |||
Met5A (immortalized mesothelioma cell line) | Same cells | |||
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Ahmad et al. (2014) | Murine MSCs | Murine lung epithelial cells | Mitochondria | [ |
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Liu et al. (2014) | Human MSCs | Human umbilical vein endothelial cell (HUVEC) | Mitochondria | [ |
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Thayanithy et al. (2014) | Murine osteosarcoma K7M2 cells | Same cells and MC3T3 murine osteoblasts | MicroRNAs (miR-199a) | [ |
SKOV3 ovarian cancer cells | Nonmalignant ovarian epithelial cells | |||
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Thayanithy et al. (2014) | Human biphasic mesothelioma MSTO-211H cells | Same cells | Exosomes from other cells | [ |
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Biran et al. (2015) | Oncogene or DNA damage-induced senescent cells | NK cells | Proteins | [ |
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Burtey et al. (2015) | HeLa | NRK fibroblasts | Tf-R (transferrin receptor), endosomes | [ |
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Caicedo et al. (2015) | Human mesenchymal stem cells (MSCs) | Human breast cancer cell line MDA-MB-231 | Mitochondria | [ |
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Polak et al. (2015) | Bidirectional: human MSCs to human acute lymphoblastic leukemia cells (BCP-ALL cell line)
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ND | [ | |
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Wang and Gerdes (2015) | PC12 cells (−/+ultraviolet light treatment) | PC12 cells (−/+ultraviolet light treatment) | Mitochondria | [ |
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Zhu et al. (2015) | CAD neuronal cells | Same cells | Prions | [ |
Lysosomes | ||||
Early endosomes | ||||
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Hashimoto et al. (2016) | Monocyte-derived macrophages | Same cells | HIV-1 | [ |
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Hayakawa et al. (2016) | Astrocytes | Neurons | Mitochondria | [ |
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Jackson et al. (2016) | Human MSCs | Human monocyte-derived macrophages | Mitochondria | [ |
Murine alveolar macrophages | ||||
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Lu et al. (2016) | Bladder cancer cells | Same cells | Mitochondria | [ |
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Moschoi et al. (2016) | BM-MSCs | Acute myeloid leukemia cells | Mitochondria | [ |
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Tardivel et al. (2016) | Neurons | Neurons | Tau protein | [ |
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Victoria et al. (2016) | Astrocytes | Neurons | Prions | [ |
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Zhang et al. (2016) | iPSC-MSCs and BM-MSCs | Cardiomyocytes | Mitochondria | [ |
Cells involved in connections through nanotubes can be of the same or of different types. Many cell types appear endowed with the capacity to form TNTs with one another. TNTs were observed among rat pheochromocytoma (PC12) cells [
Cells of the immune system, notably macrophages, dendritic cells (DCs), NK, and B cells, extensively use TNTs to communicate [
As it will be further described below, mesenchymal stem cells (MSCs) actively use TNTs to deliver cargos to renal tubular cells [
Formation of TNTs has been observed for a number of cancer cells, either connecting cancer cells together or connecting cancer cells with normal stromal cells, notably mesenchymal stem cells (MSCs). TNT formation was described in a diversity of different cancer cell types, including malignant mesothelial cells [
The nanotubes formed between these different cell types do share some features, notably a continuity in cell membrane and cytoplasm between the connected cells, allowing the trafficking of biological cargos. However, with the accumulation of new TNT-related data, it appears that these structures will have properties, concerning the connecting modes, cargos transported, cytoskeleton-based molecular motors, and biological outcome that will underline the specificity of each cell system.
Organelles such as mitochondria have now been described as trafficking entities in the tunneling nanotubes connecting many different cells types including renal proximal tubular epithelial cells (RPTEC) [
MSCs were shown to share mitochondria through a TNT-mediated process with number of target cells. These target cells include cardiomyocytes [
Mitochondrial trafficking from MSCs to MDA-MB-231 breast cancer cells. (a) MSC mitochondrial network. MSCs were labeled by MitoTracker Deep Red FM and Green CellTracker CMFDA. Scale bars, 10
On a technical point of view, detection of the transfer of mitochondria from donor to target cells is often performed based on the imaging of mitochondria prelabeled with fluorescently dyes such as MitoTrackers as shown in numerous reports, including [
As it will be discussed later in the review, this mitochondrial trafficking leads to notable effects in the target cells as mitochondria are involved in multiple cellular functions including the biosynthesis of ATP, through the electron transport chain, or that of lipids and amino acids. In addition, mitochondria are now recognized as signaling entities that can induce cell events such as autophagy and apoptosis.
Among organelles, lysosomes were also found to be transferred between progenitor and senescent endothelial cells and this transfer [
Viruses also display the capacity to be transmitted through TNTs. This was shown for HIV, between infected T cells and noninfected T cells, thus eliminating the need for the infected cells to release a fully mature HIV virus in order to infect the neighboring cells [
TNTs also constitute a route for the transfer of microRNAs between cells, as shown for miR-19a among K7M2 murine osteosarcoma cells, [
Two major processes have been proposed for the formation of TNTs. Cells can extend filopodia-like protrusions that, in contact with target cells, can undergo plasma membrane fusions. Alternatively, cells that were initially in close contact with one another can move apart, remaining bound by the extending tunneling nanotube structure. Several proteins have now been identified for their role in nanotube formation, for the functional connection between the two interacting cells, and for the cargo trafficking within the connecting TNTs.
The role of connexin 43 (Cx43) gap junction marker has been documented for different cell systems. In the murine model of LPS-induced acute lung injury, gap junctions between the instillated murine bone marrow stromal cells (BMSCs) and the pulmonary alveolar epithelial cells depended on the expression of Cx43 by both cell types and occurred at sites of high-Cx43 expression. Cx43 was therefore proposed as essential for BMSC attachment to the alveolar cells, leading to the generation of TNTs between these cells [
The role of M-Sec/TNFaip2 and the exocyst complex has also been put forward in different studies [
Another small GTPase, Cdc42, was found to play a dual role in TNT formation. Cdc42 was demonstrated to play a role in the TNT elongation process in the Raw264.7 macrophage cell [
The mitochondrial trafficking within these TNTs can rely on the Rho GTPase Miro1 (also called RhoT1/2), as shown for the transfer of mitochondria from mesenchymal stem cells to damaged alveolar epithelial cells in mouse models of airway injury [
Other cytoskeleton motors can allow the transport of small molecules and organelles within the cells. In addition to the kinesin motor, the cytoplasmic dynein also moves along microtubules while, on the other hand, the family of myosins are actin-based cytoskeleton motors [
Depending both on the types of cells connected and cargos transported, it is likely that TNTs will rely on different types of cytoskeletons, that is, microfilaments and/or microtubules, and therefore on different cytoskeleton motors to support the trafficking of these cargos. For instance, the protein Tau was reported to associate with both microtubules and the actin network and to contribute to the formation of TNTs, bridging neurons together [
Altogether, the diversity of factors involved in the formation of the TNTs and of the cargoes trafficking within these TNTs points to the complexity of the whole process of TNT-mediated cell-to-cell communication. New paradigms will be needed to allow to predict which cargoes might be transferred, using what type of cytoskeletal motor, for any given couple of cell types.
The formation of TNTs, as tested in 2D in vitro cultures, was observed to be controlled by several factors including serum and glucose concentrations, viral infection, or exposure to therapeutic agents, as detailed further below. Beyond the fact that this information is important to design experimental settings and collect in vitro data on TNTs, it also gives clues about how TNT formation might be regulated in vivo, by nutrient supply, infection, or therapy, and thus contribute to our understanding of the holistic organism responses.
In vitro, low-serum (2.5% FBS) and high-glucose concentrations (50 mM) were found to stimulate TNT formation, as observed between murine K7M2 osteosarcoma cells and MC3T3 osteoblast cells [
Cellular stress caused, for instance, by HIV infection in human macrophages was demonstrated to increase the number (but not the length) of TNTs formed by these macrophages towards other macrophages, in correlation with viral replication [
Mesenchymal stem cells (MSCs) are characterized by their multilineage differentiation capacity, notably into osteocytes, adipocytes, and chondrocytes [
MSCs are attracted and activated by cytokines such as IFN-
The tumor microenvironment is known to play an important role in tumor progression, metastasis, and resistance to therapy [
MSCs interact with other cells, reprograming their function through the secretion of small molecules like growth factors, chemokines, cytokines, and molecular mediators (bioactive lipids, nucleotides, among others). The human mesenchymal stem cells (MSCs) have been shown to display the ability to connect to target cells through tunneling nanotubes and to transfer the mitochondria through these TNTs. Prockop laboratory observed for the first time that functional mitochondria could be transferred between MSCs to tumor cells [
Mitochondria can also be transported from MSCs to the other cells by microvesicles (MVs). MSC mitochondria can be taken by arrestin domain-containing protein 1-mediated microvesicles (ARMMs) that range from 0.1 to 1
Mitochondria, in an isolated form, can also be internalized by cells, notably cardiomyocytes [
Islam and colleagues demonstrated the transfer of mitochondria in vivo from MSCs to pulmonary alveolar epithelial cells in a murine model of lipopolysaccharide- (LPS-) induced acute lung injury [
Lung alveolar macrophages were also shown to acquire MSC mitochondria, which lead to an enhancement of their phagocytic activity and, thus, contributed to the MSC antimicrobial effect in a murine model of
The transfer to target cells of mitochondria, isolated beforehand from cells, was also demonstrated in vivo in a rabbit model of regional ischemia [
The transfer of MSC mitochondria to A549
The acquisition of human vascular smooth muscle cell (VSMC) mitochondria by human MSCs resulted in the increase of MSC proliferation rate [
In a rabbit ischemia model, the injection of autologous mitochondria at the site of ischemia resulted in their internalization by cardiomyocytes and in increased cell survival [
Tunneling nanotubes appear henceforth to constitute a widespread means of communication between cells that can lay close-by or far apart. This communication process is used by many cell types, allowing the trafficking of many different cargoes between these cells. This TNT-mediated cell-to-cell exchange can contribute to the cell homeostasis, to the spontaneous tissue repair, to the spreading of pathologies, and to the resistance to therapies.
As detailed in this review, mesenchymal stem cells are particularly prone to establish these TNT connections with target cells. Numerous studies reported and characterized effects that the mitochondrial trafficking in these TNTs can have on the target cells be at the metabolic or functional levels. On a therapeutic point of view, at a first glance, these effects can be beneficial, when they lead for instance to tissue repair, but also detrimental, when they contribute to acquired resistance to therapy. Obviously, further work will be necessary to find the tools to enhance the first while hindering the second. The fact that mitochondria can be transferred spontaneously between cells or from preparation of mitochondria, isolated beforehand, will obviously open new paradigms for the available options to treat patients.
Acute lymphoblastic leukemia
Acute myeloid leukemia
Cytarabine or cytosine arabinoside
Adipose tissue-derived mesenchymal stem cells
Bone marrow-derived mesenchymal stem cells
Cardiomyocytes
Chronic obstructive pulmonary disease
Connexin 43
Dendritic cells
Doxorubicin
Endothelial progenitor cells
Etoposide
Fetal bovine serum
Guanosine triphosphatase
Human immunodeficiency virus
Human microvascular endothelial cells
Intercellular adhesion molecule 1
Indoleamine 2,3-dioxygenase
Inducible nitric oxide synthase
Insulin receptor substrate of 53 kDa
Leukemia-initiating cell
Lipopolysaccharide
Laryngeal squamous cell carcinoma
Leukocyte-specific transcript 1
Melanoma cell adhesion molecule
Mesenchymal stem cells
Mitochondrially encoded cytochrome c oxidase II
Mitochondrial DNA
Microvesicles
Myosin-X
Natural killer
Nonobese diabetic
Normal rat kidney
NOD scid gamma
O-GlcNAc transferase
Rat pheochromocytoma
Prostaglandin E2
Renal proximal tubular epithelial cells
Single-nucleotide polymorphism
Tumor necrosis factor, alpha-induced protein 2
Tunneling nanotube
TNF-
Vasodilator-stimulated phosphoprotein
Vascular endothelial growth factor
Vincristine
Vascular smooth muscle cell.
The authors declare that there is no conflict of interest regarding the publication of this paper.
The authors would like to thank Ricardo Vasquez (Universidad San Francisco de Quito) for the design of the nanotube figure. The authors would like to thank the Montpellier RIO imaging facility (MRI) for providing adequate environment for confocal microscopy. Marie-Luce Vignais is a staff scientist from the National Center for Scientific Research (CNRS). This work was supported by grants from the Agence Nationale pour la Recherche MITOSTEM and the Ligue Contre le Cancer-Comité de l’Aude.