Investigating Tunneling Nanotubes in Cancer Cells: Guidelines for Structural and Functional Studies through Cell Imaging

Transforming the concept of connectivity: unveiling tunneling nanotube biology and their roles in brain development and neurodegeneration

Tunneling nanotubes (TNTs) are thin tubular membrane protrusions that connect distant cells, generating a complex cellular network. Over the past few decades, research on TNTs has provided important insights into their biology, including structural composition, formation mechanisms, modulators, and functionality. It has been discovered that TNTs allow cytoplasmic continuity between connected cells, facilitating fast intercellular communication via both passive and active exchange of materials. These features are pivotal in the nervous system, where rapid processing of inputs is physiologically required. TNTs have been implicated in the progression of neurodegenerative diseases and cancer in various in vitro models, and TNT-like structures have also been observed in the developing brain and in vivo. This highlights their significant role in pathophysiological processes. In this comprehensive review we aim to provide an extensive overview of TNTs, starting from key structural features and mechanisms of formation and describing the main experimental techniques used to detect these structures both in vitro and in vivo. We focus primarily on the nervous system, where the discovery of TNTs could prompt a reconsideration of the brain functioning as individual units (the neuronal theory of Cajal) versus neurons being physically connected, as Golgi believed. We illustrate the involvement of TNTs in brain development and neurodegenerative states and highlight the limitations and future research needs in this field.

Magnolol facilitates mitochondrial-peroxisome dysfunction and induces oxeiptosis in lung cancer cells following transfer via tunneling nanotubes

Recent therapeutic advances have improved survival rates for patients with lung cancer. However, drug resistance remains a significant challenge and is closely linked to mitochondrial alterations in lung cancer cells. Magnolol, a compound extracted from Magnolia officinalis, has anti-cancer properties. However, its impact on mitochondria in cancer cells remains poorly understood. This study aimed to explore the therapeutic potential of Magnolol in lung cancer and elucidate its effects on mitochondria in lung cancer cells. The effects of Magnolol on lung cancer were studied using xenograft mouse studies and in vitro analyses. Magnolol promotes the production of reactive oxygen species (ROS) and inhibits the antioxidant pathway in cancer cells. This disruption further impairs redox interactions between mitochondria and peroxisome, leading to mitochondrial dysfunction and mitocytosis. Additionally, Magnolol activates oxeiptosis, facilitating intercellular transport of damaged mitochondria and peroxisomes via tunneling nanotubes. The increased fusion of mitochondria may contribute to mitochondrial dysfunction, promote the accumulation of dysfunctional peroxisomes in recipient cells, and elevate ROS levels. In turn, this process enhances oxeiptosis, ultimately inhibiting tumour progression. These findings suggest that Magnolol may serve as a promising targeted therapeutic for disrupting mitochondrial function in lung cancer.

Bone marrow mesenchymal stem cells transport connexin43 via tunneling nanotubes to alleviate isopreterenol-induced myocardial hypertrophy

BACKGROUND

Paracrine signaling plays an important role in stem cell therapy. However, it alone cannot fully explain the therapeutic mechanisms of stem cell therapy in treating heart diseases. Recently, tunneling nanotubes (TNTs)-a novel type of long-distance intercellular connectional structure-have been identified between mesenchymal stem cells (MSCs) and cardiomyocytes (CMs). TNTs mediate the transmission of multiple signaling molecules, enabling cells to exert different biological functions. In the present study, we investigated the role of TNTs in MSC-based therapy for myocardial hypertrophy.

METHODS

MSCs And CMs were co Cultured for 24 h with or without isopreterenol (ISO) to induce myocardial hypertrophy. Confocal microscopy was used to quantify and analyze the number, morphology, composition, and cell source of TNTs between MSCs and CMs. the effects of ISO on CMs were assessed by comparing cell area (measured by confocal microscopy) and expression levels of hypertrophy Related genes (using qRT PCR) under co Culture and trans Well culture conditions. Flow cytometry was employed to assess the transfer of connexin43 (Cx43) from MSCs to CMs; lentivirus Mediated Cx43 overexpression and Cx43 siRNA were used to investigate the effects of Cx43 on ISO Induced myocardial hypertrophy.

RESULTS

ISO stimulation significantly increased the number, length, and thickness of TNTs between MSCs and CMs (Number: P < 0.05; length and thickness: P < 0.01). ISO also increased the proportion of TNTs containing microtubules and those derived from MSCs (P < 0.05). Co-culture conditions were more effective than trans-well culture in alleviating ISO-induced myocardial hypertrophy (P < 0.05). Furthermore, Cx43 was observed in TNTs, and ISO enhanced the transfer of Cx43-mCherry from MSCs to co-cultured CMs (P < 0.05). Overexpression of Cx43 in CMs alleviated myocardial hypertrophy, whereas knocking down of Cx43 in MSCs reduced their ability to alleviate myocardial hypertrophy (P < 0.05).

CONCLUSIONS

Our results demonstrate that ISO promotes the formation of TNTs, particularly between MSCs and CMs, and induces changes in the morphology of TNTs (thickening and lengthening). Additionally, MSCs transmitted Cx43 to CMs via TNTs, which contributes to the alleviation of ISO-induced myocardial hypertrophy. These results suggest that TNTs represent an important mechanism in MSC-mediated therapy for myocardial hypertrophy.

Unveiling cellular communications through rapid pan-membrane-protein labeling

Dynamic protein distribution within and across the plasma membrane is pivotal in regulating cell communication. However, rapid, high-density labeling methods for multiplexed live imaging across diverse cell types remain scarce. Here, we demonstrate N-hydroxysuccinimide (NHS)-ester-based amine crosslinking of fluorescent dyes to uniformly label live mammalian cell surface proteins. Using model cell systems, we capture previously elusive membrane topology and cell-cell interactions. Live imaging shows transient membrane protein accumulation at cell-cell contacts and bidirectional migration patterns guided by membrane fibers in DC2.4 dendritic cells. Multiplexed superresolution imaging reveals the biogenesis of membrane tunneling nanotubes that facilitate intercellular transfer in DC2.4 cells, and caveolin 1-dependent endocytosis of insulin receptors in HEK293T cells. 3D superresolution imaging reveals membrane topology remodeling in response to stimulation, generation of microvesicles, and phagocytic activities in Jurkat T cells. Furthermore, NHS-labeling remains stable in vivo, enabling visualization of intercellular transfer among splenocytes using a T cell lymphoma mouse model.

Breast cancers that disseminate to bone marrow acquire aggressive phenotypes through CX43-related tumor-stroma tunnels

Estrogen receptor-positive (ER+) breast cancer commonly disseminates to bone marrow, where interactions with mesenchymal stromal cells (MSCs) shape disease trajectory. We modeled these interactions with tumor-MSC co-cultures and used an integrated transcriptome-proteome-network-analyses workflow to identify a comprehensive catalog of contact-induced changes. Conditioned media from MSCs failed to recapitulate genes and proteins, some borrowed and others tumor-intrinsic, induced in cancer cells by direct contact. Protein-protein interaction networks revealed the rich connectome between "borrowed" and "intrinsic" components. Bioinformatics prioritized one of the borrowed components, CCDC88A/GIV, a multi-modular metastasis-related protein that has recently been implicated in driving a hallmark of cancer, growth signaling autonomy. MSCs transferred GIV protein to ER+ breast cancer cells (that lack GIV) through tunnelling nanotubes via connexin (Cx)43-facilitated intercellular transport. Reinstating GIV alone in GIV-negative breast cancer cells reproduced approximately 20% of both the borrowed and the intrinsic gene induction patterns from contact co-cultures; conferred resistance to anti-estrogen drugs; and enhanced tumor dissemination. Findings provide a multiomic insight into MSC→tumor cell intercellular transport and validate how transport of one such candidate, GIV, from the haves (MSCs) to have-nots (ER+ breast cancer) orchestrates aggressive disease states.

Tumor Cell Communications as Promising Supramolecular Targets for Cancer Chemotherapy: A Possible Strategy

Fifty-two years have passed since President Nixon launched the "War on Cancer". Despite unparalleled efforts and funds allocated worldwide, the outlined goals were not achieved because cancer treatment approaches such as chemotherapy, radiation therapy, hormonal and targeted therapies have not fully met the expectations. Based on the recent literature, a new direction in cancer therapy can be proposed which targets connections between cancer cells and their microenvironment by chemical means. Cancer-stromal synapses such as immunological synapses between cancer and immune cells provide an attractive target for this approach. Such synapses form ligand-receptor clusters on the interface of the interacting cells. They share a common property of involving intercellular clusters of spatially proximate and cooperatively acting proteins. Synapses provide the space for the focused intercellular signaling molecules exchange. Thus, the disassembly of cancer-stromal synapses may potentially cause the collapse of various tumors. Additionally, the clustered arrangement of synapse components offers opportunities to enhance treatment safety and precision by using targeted crosslinking chemical agents which may inactivate cancer synapses even in reduced concentrations. Furthermore, attaching a cleavable cell-permeable toxic agent(s) to a crosslinker may further enhance the anti-cancer effect of such therapeutics. The highlighted approach promises to be universal, relatively simple and cost-efficient. We also hope that, unlike chemotherapeutic and immune drugs that interact with a single target, by using supramolecular large clusters that include many different components as a target, the emergence of a resistance characteristic of chemo- and immunotherapy is extremely unlikely.

Combining sophisticated fast FLIM, confocal microscopy, and STED nanoscopy for live-cell imaging of tunneling nanotubes

Cell-to-cell communication via tunneling nanotubes (TNTs) is a challenging topic with a growing interest. In this work, we proposed several innovative tools that use red/near-infrared dye labeling and employ lifetime-based imaging strategies to investigate the dynamics of TNTs in a living mesothelial H28 cell line that exhibits spontaneously TNT1 and TNT2 subtypes. Thanks to a fluorescence lifetime imaging microscopy module being integrated into confocal microscopy and stimulated emission depletion nanoscopy, we applied lifetime imaging, lifetime dye unmixing, and lifetime denoising techniques to perform multiplexing experiments and time-lapses of tens of minutes, revealing therefore structural and functional characteristics of living TNTs that were preserved from light exposure. In these conditions, vesicle-like structures, and tubular- and round-shaped mitochondria were identified within living TNT1. In addition, mitochondrial dynamic studies revealed linear and stepwise mitochondrial migrations, bidirectional movements, transient backtracking, and fission events in TNT1. Transfer of Nile Red-positive puncta via both TNT1 and TNT2 was also detected between living H28 cells.

Tunneling nanotubes: The transport highway for astrocyte-neuron communication in the central nervous system

Tunneling nanotubes (TNTs) have emerged as pivotal structures for intercellular communication, enabling the transfer of cellular components across distant cells. Their involvement in neurological disorders has attracted considerable scientific interest. This review delineates the functions of TNTs within the central nervous system, examining their role in the transmission of bioenergetic substrates, and signaling molecules, and their multifaceted impact on both physiological and pathological processes, with an emphasis on neurodegenerative diseases. The review highlights the selectivity and specificity of TNTs as dedicated pathways for intercellular cargo delivery, particularly under stress conditions that provoke increased TNT formation. The potential of TNTs as therapeutic targets is explored in depth. We pay particular attention to the interactions between astrocytes and neurons mediated by TNTs, which are fundamental to brain architecture and function. Dysfunctions in these interactions are implicated in the spread of protein aggregates and mitochondrial anomalies, contributing to the pathogenesis of neurodegenerative diseases. The review culminates with a synthesis of the current understanding of TNT biology and identifies research gaps, advocating for intensified exploration into TNTs as a promising therapeutic frontier.

High-Resolution Microscopic Characterization of Tunneling Nanotubes in Living U87 MG and LN229 Glioblastoma Cells

Tunneling nanotubes (TNTs) are fine, nanometer-sized membrane connections between distant cells that provide an efficient communication tool for cellular organization. TNTs are thought to play a critical role in cellular behavior, particularly in cancer cells. The treatment of aggressive cancers such as glioblastoma remains challenging due to their high potential for developing therapy resistance, high infiltration rates, uncontrolled cell growth, and other aggressive features. A better understanding of the cellular organization via cellular communication through TNTs could help to find new therapeutic approaches. In this study, we investigate the properties of TNTs in two glioblastoma cell lines, U87 MG and LN229, including measurements of their diameter by high-resolution live-cell stimulated emission depletion (STED) microscopy and an analysis of their length, morphology, lifetime, and formation by live-cell confocal microscopy. In addition, we discuss how these fine compounds can ideally be studied microscopically. In particular, we show which membrane-labeling method is suitable for studying TNTs in glioblastoma cells and demonstrate that live-cell studies should be preferred to explore the role of TNTs in cellular behavior. Our observations on TNT formation in glioblastoma cells suggest that TNTs could be involved in cell migration and serve as guidance.

Tunneling Nanotubes: The Cables for Viral Spread and Beyond

Multicellular organisms require cell-to-cell communication to maintain homeostasis and thrive. For cells to communicate, a network of filamentous, actin-rich tunneling nanotubes (TNTs) plays a pivotal role in facilitating efficient cell-to-cell communication by connecting the cytoplasm of adjacent or distant cells. Substantial documentation indicates that diverse cell types employ TNTs in a sophisticated and intricately organized fashion for both long and short-distance communication. Paradoxically, several pathogens, including viruses, exploit the structural integrity of TNTs to facilitate viral entry and rapid cell-to-cell spread. These pathogens utilize a "surfing" mechanism or intracellular transport along TNTs to bypass high-traffic cellular regions and evade immune surveillance and neutralization. Although TNTs are present across various cell types in healthy tissue, their magnitude is increased in the presence of viruses. This heightened induction significantly amplifies the role of TNTs in exacerbating disease manifestations, severity, and subsequent complications. Despite significant advancements in TNT research within the realm of infectious diseases, further studies are imperative to gain a precise understanding of TNTs' roles in diverse pathological conditions. Such investigations are essential for the development of novel therapeutic strategies aimed at leveraging TNT-associated mechanisms for clinical applications. In this chapter, we emphasize the significance of TNTs in the life cycle of viruses, showcasing the potential for a targeted approach to impede virus-host cell interactions during the initial stages of viral infections. This approach holds promise for intervention and prevention strategies.

The direct transfer approach for transcellular drug delivery

A promising paradigm for drug administration that has garnered increasing attention in recent years is the direct transfer (DT) of nanoparticles for transcellular drug delivery. DT requires direct cell-cell contact and facilitates unidirectional and bidirectional matter exchange between neighboring cells. Consequently, DT enables fast and deep penetration of drugs into the targeted tissues. This comprehensive review discusses the direct transfer concept, which can be delineated into the following three distinct modalities: membrane contact-direct transfer, gap junction-mediated direct transfer (GJ-DT), and tunneling nanotubes-mediated direct transfer (TNTs-DT). Further, the intercellular structures for each modality of direct transfer and their respective merits and demerits are summarized. The review also discusses the recent progress on the drugs or drug delivery systems that could activate DT.

Treatment with tumor-treating fields (TTFields) suppresses intercellular tunneling nanotube formation in vitro and upregulates immuno-oncologic biomarkers in vivo in malignant mesothelioma

Disruption of intercellular communication within tumors is emerging as a novel potential strategy for cancer-directed therapy. Tumor-Treating Fields (TTFields) therapy is a treatment modality that has itself emerged over the past decade in active clinical use for patients with glioblastoma and malignant mesothelioma, based on the principle of using low-intensity alternating electric fields to disrupt microtubules in cancer cells undergoing mitosis. There is a need to identify other cellular and molecular effects of this treatment approach that could explain reported increased overall survival when TTFields are added to standard systemic agents. Tunneling nanotube (TNTs) are cell-contact-dependent filamentous-actin-based cellular protrusions that can connect two or more cells at long-range. They are upregulated in cancer, facilitating cell growth, differentiation, and in the case of invasive cancer phenotypes, a more chemoresistant phenotype. To determine whether TNTs present a potential therapeutic target for TTFields, we applied TTFields to malignant pleural mesothelioma (MPM) cells forming TNTs in vitro. TTFields at 1.0 V/cm significantly suppressed TNT formation in biphasic subtype MPM, but not sarcomatoid MPM, independent of effects on cell number. TTFields did not significantly affect function of TNTs assessed by measuring intercellular transport of mitochondrial cargo via intact TNTs. We further leveraged a spatial transcriptomic approach to characterize TTFields-induced changes to molecular profiles in vivo using an animal model of MPM. We discovered TTFields induced upregulation of immuno-oncologic biomarkers with simultaneous downregulation of pathways associated with cell hyperproliferation, invasion, and other critical regulators of oncogenic growth. Several molecular classes and pathways coincide with markers that we and others have found to be differentially expressed in cancer cell TNTs, including MPM specifically. We visualized short TNTs in the dense stromatous tumor material selected as regions of interest for spatial genomic assessment. Superimposing these regions of interest from spatial genomics over the plane of TNT clusters imaged in intact tissue is a new method that we designate Spatial Profiling of Tunneling nanoTubes (SPOTT). In sum, these results position TNTs as potential therapeutic targets for TTFields-directed cancer treatment strategies. We also identified the ability of TTFields to remodel the tumor microenvironment landscape at the molecular level, thereby presenting a potential novel strategy for converting tumors at the cellular level from 'cold' to 'hot' for potential response to immunotherapeutic drugs.

Mitochondrial DNA-targeted therapy: A novel approach to combat cancer

Mitochondrial DNA (mtDNA) encodes proteins and RNAs that are essential for mitochondrial function and cellular homeostasis, and participates in important processes of cellular bioenergetics and metabolism. Alterations in mtDNA are associated with various diseases, especially cancers, and are considered as biomarkers for some types of tumors. Moreover, mtDNA alterations have been found to affect the proliferation, progression and metastasis of cancer cells, as well as their interactions with the immune system and the tumor microenvironment (TME). The important role of mtDNA in cancer development makes it a significant target for cancer treatment. In recent years, many novel therapeutic methods targeting mtDNA have emerged. In this study, we first discussed how cancerogenesis is triggered by mtDNA mutations, including alterations in gene copy number, aberrant gene expression and epigenetic modifications. Then, we described in detail the mechanisms underlying the interactions between mtDNA and the extramitochondrial environment, which are crucial for understanding the efficacy and safety of mtDNA-targeted therapy. Next, we provided a comprehensive overview of the recent progress in cancer therapy strategies that target mtDNA. We classified them into two categories based on their mechanisms of action: indirect and direct targeting strategies. Indirect targeting strategies aimed to induce mtDNA damage and dysfunction by modulating pathways that are involved in mtDNA stability and integrity, while direct targeting strategies utilized molecules that can selectively bind to or cleave mtDNA to achieve the therapeutic efficacy. This study highlights the importance of mtDNA-targeted therapy in cancer treatment, and will provide insights for future research and development of targeted drugs and therapeutic strategies.

Hematopoietic stem and progenitors cells gene editing: Beyond blood disorders

Lessons learned from decades-long practice in the transplantation of hematopoietic stem and progenitor cells (HSPCs) to treat severe inherited disorders or cancer, have set the stage for the current ex vivo gene therapies using autologous gene-modified hematopoietic stem and progenitor cells that have treated so far, hundreds of patients with monogenic disorders. With increased knowledge of hematopoietic stem and progenitor cell biology, improved modalities for patient conditioning and with the emergence of new gene editing technologies, a new era of hematopoietic stem and progenitor cell-based gene therapies is poised to emerge. Gene editing has the potential to restore physiological expression of a mutated gene, or to insert a functional gene in a precise locus with reduced off-target activity and toxicity. Advances in patient conditioning has reduced treatment toxicities and may improve the engraftment of gene-modified cells and specific progeny. Thanks to these improvements, new potential treatments of various blood- or immune disorders as well as other inherited diseases will continue to emerge. In the present review, the most recent advances in hematopoietic stem and progenitor cell gene editing will be reported, with a focus on how this approach could be a promising solution to treat non-blood-related inherited disorders and the mechanisms behind the therapeutic actions discussed.

Chlamydia trachomatis Cell-to-Cell Spread through Tunneling Nanotubes

Tunneling nanotubes (TNTs) are transient cellular connections that consist of dynamic membrane protrusions. They play an important role in cell-to-cell communication and mediate the intercellular exchanges of molecules and organelles. TNTs can form between different cell types and may contribute to the spread of pathogens by serving as cytoplasmic corridors. We demonstrate that Chlamydia (C.) trachomatis-infected human embryonic kidney (HEK) 293 cells and other cells form TNT-like structures through which reticulate bodies (RBs) pass into uninfected cells. Observed TNTs have a life span of 1 to 5 h and contain microtubules, which are essential for chlamydial transfer. They can bridge distances of up to 50 μm between connecting neighboring cells. Consistent with the biological role for TNTs, we show that C. trachomatis spread also occurs under conditions in which the extracellular route of chlamydial entry into host cells is blocked. Based on our findings, we propose that TNTs play a critical role in the direct, cell-to-cell transmission of chlamydia. IMPORTANCE Intracellular bacterial pathogens often undergo a life cycle in which they parasitize infected host cells in membranous vacuoles. Two pathways have been described by which chlamydia can exit infected host cells: lytic cell destruction or exit via extrusion formation. Whether direct, cell-to-cell contact may also play a role in the spread of infection is unknown. Tunneling nanotubes (TNTs) interconnect the cytoplasm of adjacent cells to mediate efficient communication and the exchange of material between them. We used Chlamydia trachomatis and immortalized cells to analyze whether TNTs mediate bacterial transmission from an infected donor to uninfected acceptor cells. We show that chlamydia-infected cells build TNTs through which the intracellular reticulate bodies (RBs) of the chlamydia can pass into uninfected neighboring cells. Our study contributes to the understanding of the function of TNTs in the cell-to-cell transmission of intracellular pathogens and provides new insights into the strategies by which chlamydia spreads among multicellular tissues.

Tunneling nanotubes and mesenchymal stem cells: New insights into the role of melatonin in neuronal recovery

Efficient cell-to-cell communication is essential for tissue development, homeostasis, and the maintenance of cellular functions after injury. Tunneling nanotubes (TNTs) have emerged as a new important method of cell-to-cell communication. TNTs are primarily established between stressed and unstressed cells and can transport a variety of cellular components. Mitochondria are important trafficked entities through TNTs. Transcellular mitochondria transfer permits the incorporation of healthy mitochondria into the endogenous network of recipient cells, changing the bioenergetic profile and other functional properties of the recipient and may allow the recipient cells to recuperate from apoptotic processes and return to a normal operating state. Mesenchymal cells (MSCs) can form TNTs and transfer mitochondria and other constituents to target cells. This occurs under both physiological and pathological conditions, leading to changes in cellular energy metabolism and functions. This review summarizes the newly described capacity of melatonin to improve mitochondrial fusion/fission dynamics and promote TNT formation. This new evidence suggests that melatonin's protective effects could be attributed to its ability to prevent mitochondrial damage in injured cells, reduce senescence, and promote anastasis, a natural cell recovery phenomenon that rescues cells from the brink of death. The modulation of these new routes of intercellular communication by melatonin could play a key role in increasing the therapeutic potential of MSCs.

Miro proteins and their role in mitochondrial transfer in cancer and beyond

Mitochondria are organelles essential for tumor cell proliferation and metastasis. Although their main cellular function, generation of energy in the form of ATP is dispensable for cancer cells, their capability to drive their adaptation to stress originating from tumor microenvironment makes them a plausible therapeutic target. Recent research has revealed that cancer cells with damaged oxidative phosphorylation import healthy (functional) mitochondria from surrounding stromal cells to drive pyrimidine synthesis and cell proliferation. Furthermore, it has been shown that energetically competent mitochondria are fundamental for tumor cell migration, invasion and metastasis. The spatial positioning and transport of mitochondria involves Miro proteins from a subfamily of small GTPases, localized in outer mitochondrial membrane. Miro proteins are involved in the structure of the MICOS complex, connecting outer and inner-mitochondrial membrane; in mitochondria-ER communication; Ca²⁺ metabolism; and in the recycling of damaged organelles via mitophagy. The most important role of Miro is regulation of mitochondrial movement and distribution within (and between) cells, acting as an adaptor linking organelles to cytoskeleton-associated motor proteins. In this review, we discuss the function of Miro proteins in various modes of intercellular mitochondrial transfer, emphasizing the structure and dynamics of tunneling nanotubes, the most common transfer modality. We summarize the evidence for and propose possible roles of Miro proteins in nanotube-mediated transfer as well as in cancer cell migration and metastasis, both processes being tightly connected to cytoskeleton-driven mitochondrial movement and positioning.

Identification and Characterization of Tunneling Nanotubes Involved in Human Mast Cell FcεRI-Mediated Apoptosis of Cancer Cells

Simple Summary

Mast cells (MCs) are ubiquitously found in most tissues and in and around tumors. Their role in cancer pathogenesis remains an open area of investigation, and their interactions with tumor cells has not been explored. Here, a novel mechanism of communication between human MCs and tumor cells involving tunneling nanotubes (TnT) and other membrane structures is described. The formation of these communication structures is dependent on MC receptors interacting with tumor antigens through tumor-specific immunoglobulins and results in tumor-killing mediators from MC entering the tumor cells. This mechanism underlying the MC killing of tumor cells has important implications in understanding cancer pathogenesis.

Abstract

Mast cells (MCs) are found in practically all tissues where they participate in innate and adaptive immune responses. They are also found in and around tumors, yet their interactions with cancer cells and the resulting impact on cancer cell growth and metastasis are not well understood. In this study, we examined a novel mechanism of IgE-FcεRI-mediated, intercellular communication between human adipose-derived mast cells (ADMC) and cancer cells. The formation of heterotypic tunneling nanotubes (TnT) and membrane structures between MCs and tumor cells in vitro was examined using microscopy and a diverse array of molecule-specific indicator dyes. We show that several MC-specific structures are dependent on the specific interactions between human tumor IgE-sensitized MCs and antigens on the tumor cell surface. The formation of TnT, membrane blebs and other MC-specific structures paralleled FcεRI-degranulation occurring within 30 min and persisting for up to 24 h. The TnT-specific adhesion of FcεRI-activated MCs to tumor cells was characterized by the transport of the MC granule content into the tumor cells, including tryptase and TNF-α. This interaction led to apoptosis of the tumor cells, which differs from previous studies examining tissue cells within the cancer microenvironment. The formation of heterotypic TnT results in stimulation of an invasive tumor cell phenotype and increased tumor cell invasion and chemoresistance of the cancer cells. These studies describe a heretofore-unrecognized mechanism underlying IgE-mediated interactions and FcεRI-activated MC-mediated killing of tumor cells through the formation of TnT.

Fluorescent Artificial Antigens Revealed Extended Membrane Networks Utilized by Live Dendritic Cells for Antigen Uptake

Dendritic cells (DCs) can infiltrate tight junctions of the epithelium to collect remote antigens during immune surveillance. While elongated membrane structures represent a plausible structure to perform this task, their functional mechanisms remain elusive owing to the lack of high-resolution characterizations in live DCs. Here, we developed fluorescent artificial antigens (FAAs) based on quantum dots coated with polyacrylic acid. Single-particle tracking of FAAs enables us to superresolve the membrane fiber network responsible for the antigen uptake. Using the DC2.4 cell line as a model system, we discovered the extensive membrane network approaching 200 μm in length with tunnel-like cavities about 150 nm in width. The membrane fiber network also contained heterogeneous circular migrasomes. Disconnecting the membrane network from the cell body decreased the intracellular FAA density. Our study enables mechanistic investigations of DC membrane networks and nanocarriers that target this mechanism.

Tunneling Nanotubes between Cells Migrating in ECM Mimicking Fibrous Environments

Simple Summary

Tumor cells grow, spread, and invade in a three-dimensional manner, but most experimental approaches in cancer cell biology focus on the behavior of cells in two-dimensional spaces. Patterns of cell invasion and spread in 2D may not accurately depict cell behavior in 3D tumors. We cultivated and investigated malignant mesothelioma cells on a 3D scaffold composed of nanofibers positioned in parallel and crosshatched network configurations to overcome this barrier. We focused on studying long extensions called tunneling nanotubes (TNTs) protruding from cells and connecting with other cells, which have been shown to transmit signals from cell to cell and extend into the tumor microenvironments in a 3D manner. This manuscript describes the biophysics of the formation and function of cancer cell TNTs. These findings will impact the research community by accurately assessing how TNTs affect cancer cell invasion and migration in their natural 3D microenvironment using a novel bioengineered platform.

Abstract

Tunneling nanotubes (TNTs) comprise a unique class of actin-rich nanoscale membranous protrusions. They enable long-distance intercellular communication and may play an integral role in tumor formation, progression, and drug resistance. TNTs are three-dimensional, but nearly all studies have investigated them using two-dimensional cell culture models. Here, we applied a unique 3D culture platform consisting of crosshatched and aligned fibers to fabricate synthetic suspended scaffolds that mimic the native fibrillar architecture of tumoral extracellular matrix (ECM) to characterize TNT formation and function in its native state. TNTs are upregulated in malignant mesothelioma; we used this model to analyze the biophysical properties of TNTs in this 3D setting, including cell migration in relation to TNT dynamics, rate of TNT-mediated intercellular transport of cargo, and conformation of TNT-forming cells. We found that highly migratory elongated cells on aligned fibers formed significantly longer but fewer TNTs than uniformly spread cells on crossing fibers. We developed new quantitative metrics for the classification of TNT morphologies based on shape and cytoskeletal content using confocal microscopy. In sum, our strategy for culturing cells in ECM-mimicking bioengineered scaffolds provides a new approach for accurate biophysical and biologic assessment of TNT formation and structure in native fibrous microenvironments.

Rhes protein transits from neuron to neuron and facilitates mutant huntingtin spreading in the brain

Rhes (RASD2) is a thyroid hormone-induced gene that regulates striatal motor activity and promotes neurodegeneration in Huntington disease (HD) and tauopathy. Rhes moves and transports the HD protein, polyglutamine-expanded huntingtin (mHTT), via tunneling nanotube (TNT)-like membranous protrusions between cultured neurons. However, similar intercellular Rhes transportation in the intact brain was unknown. Here, we report that Rhes induces TNT-like protrusions in the striatal medium spiny neurons (MSNs) and transported between dopamine-1 receptor (D1R)-MSNs and D2R-MSNs of intact striatum and organotypic brain slices. Notably, mHTT is robustly transported within the striatum and from the striatum to the cortical areas in the brain, and Rhes deletion diminishes such transport. Moreover, Rhes moves to the cortical regions following restricted expression in the MSNs of the striatum. Thus, Rhes is a first striatum-enriched protein demonstrated to move and transport mHTT between neurons and brain regions, providing new insights into interneuronal protein transport in the brain.

Tunneling Nanotubes: A New Target for Nanomedicine?

Tunneling nanotubes (TNTs), discovered in 2004, are thin, long protrusions between cells utilized for intercellular transfer and communication. These newly discovered structures have been demonstrated to play a crucial role in homeostasis, but also in the spreading of diseases, infections, and metastases. Gaining much interest in the medical research field, TNTs have been shown to transport nanomedicines (NMeds) between cells. NMeds have been studied thanks to their advantageous features in terms of reduced toxicity of drugs, enhanced solubility, protection of the payload, prolonged release, and more interestingly, cell-targeted delivery. Nevertheless, their transfer between cells via TNTs makes their true fate unknown. If better understood, TNTs could help control NMed delivery. In fact, TNTs can represent the possibility both to improve the biodistribution of NMeds throughout a diseased tissue by increasing their formation, or to minimize their formation to block the transfer of dangerous material. To date, few studies have investigated the interaction between NMeds and TNTs. In this work, we will explain what TNTs are and how they form and then review what has been published regarding their potential use in nanomedicine research. We will highlight possible future approaches to better exploit TNT intercellular communication in the field of nanomedicine.

Specialized Intercellular Communications via Tunnelling Nanotubes in Acute and Chronic Leukemia

Simple Summary

Tunneling nanotubes (TNTs) are cytoplasmic channels which regulate the contacts between cells and allow the transfer of several elements, including ions, mitochondria, microvesicles, exosomes, lysosomes, proteins, and microRNAs. Through this transport, TNTs are implicated in different physiological and pathological phenomena, such as immune response, cell proliferation and differentiation, embryogenesis, programmed cell death, and angiogenesis. TNTs can promote cancer progression, transferring substances capable of altering apoptotic dynamics, modifying the metabolism and energy balance, inducing changes in immunosurveillance, or affecting the response to chemotherapy. In this review, we evaluated their influence on hematologic malignancies’ progression and resistance to therapies, focusing on acute and chronic myeloid and acute lymphoid leukemia.

Abstract

Effectual cell-to-cell communication is essential to the development and differentiation of organisms, the preservation of tissue tasks, and the synchronization of their different physiological actions, but also to the proliferation and metastasis of tumor cells. Tunneling nanotubes (TNTs) are membrane-enclosed tubular connections between cells that carry a multiplicity of cellular loads, such as exosomes, non-coding RNAs, mitochondria, and proteins, and they have been identified as the main participants in healthy and tumoral cell communication. TNTs have been described in numerous tumors in in vitro, ex vivo, and in vivo models favoring the onset and progression of tumors. Tumor cells utilize TNT-like membranous channels to transfer information between themselves or with the tumoral milieu. As a result, tumor cells attain novel capabilities, such as the increased capacity of metastasis, metabolic plasticity, angiogenic aptitude, and chemoresistance, promoting tumor severity. Here, we review the morphological and operational characteristics of TNTs and their influence on hematologic malignancies’ progression and resistance to therapies, focusing on acute and chronic myeloid and acute lymphoid leukemia. Finally, we examine the prospects and challenges for TNTs as a therapeutic approach for hematologic diseases by examining the development of efficient and safe drugs targeting TNTs.

Tunneling nanotubes and related structures: molecular mechanisms of formation and function

Tunneling nanotubes (TNTs) are F-actin-based, membrane-enclosed tubular connections between animal cells that transport a variety of cellular cargo. Over the last 15 years since their discovery, TNTs have come to be recognized as key players in normal cell communication and organism development, and are also exploited for the spread of various microbial pathogens and major diseases like cancer and neurodegenerative disorders. TNTs have also been proposed as modalities for disseminating therapeutic drugs between cells. Despite the rapidly expanding and wide-ranging relevance of these structures in both health and disease, there is a glaring dearth of molecular mechanistic knowledge regarding the formation and function of these important but enigmatic structures. A series of fundamental steps are essential for the formation of functional nanotubes. The spatiotemporally controlled and directed modulation of cortical actin dynamics would be required to ensure outward F-actin polymerization. Local plasma membrane deformation to impart negative curvature and membrane addition at a rate commensurate with F-actin polymerization would enable outward TNT elongation. Extrinsic tactic cues, along with cognate intrinsic signaling, would be required to guide and stabilize the elongating TNT towards its intended target, followed by membrane fusion to create a functional TNT. Selected cargoes must be transported between connected cells through the action of molecular motors, before the TNT is retracted or destroyed. This review summarizes the current understanding of the molecular mechanisms regulating these steps, also highlighting areas that deserve future attention.

Centrosome, the Newly Identified Passenger through Tunneling Nanotubes, Increases Binucleation and Proliferation Marker in Receiving Cells

Type 1 tunneling nanotubes (TNTs-1) are long, cytoplasmic protrusions containing actin, microtubules and intermediate filaments that provide a bi-directional road for the transport of various components between distant cells. TNT-1 formation is accompanied by dramatic cytoskeletal reorganization offering mechanical support for intercellular communication. Although the centrosome is the major microtubule nucleating center and also a signaling hub, the relationship between the centrosome and TNTs-1 is still unexplored. We provide here the first evidence of centrosome localization and orientation towards the TNTs-1 protrusion site, which is implicated in TNT-1 formation. We also envision a model whereby synchronized reorientation of the Golgi apparatus along with the centrosome towards TNTs-1 ensures effective polarized trafficking through TNTs-1. Furthermore, using immunohistochemistry and live imaging, we observed for the first time the movement of an extra centrosome within TNTs-1. In this regard, we hypothesize a novel role for TNTs-1 as a critical pathway serving to displace extra centrosomes and potentially to either protect malignant cells against aberrant centrosome amplification or contribute to altering cells in the tumor environment. Indeed, we have observed the increase in binucleation and proliferation markers in receiving cells. The fact that the centrosome can be both as the base and the user of TNTs-1 offers new perspectives and new opportunities to follow in order to improve our knowledge of the pathophysiological mechanisms under TNT control.

Role of Tunneling Nanotubes in Viral Infection, Neurodegenerative Disease, and Cancer

The network of tunneling nanotubes (TNTs) represents the filamentous (F)-actin rich tubular structure which is connected to the cytoplasm of the adjacent and or distant cells to mediate efficient cell-to-cell communication. They are long cytoplasmic bridges with an extraordinary ability to perform diverse array of function ranging from maintaining cellular physiology and cell survival to promoting immune surveillance. Ironically, TNTs are now widely documented to promote the spread of various pathogens including viruses either during early or late phase of their lifecycle. In addition, TNTs have also been associated with multiple pathologies in a complex multicellular environment. While the recent work from multiple laboratories has elucidated the role of TNTs in cellular communication and maintenance of homeostasis, this review focuses on their exploitation by the diverse group of viruses such as retroviruses, herpesviruses, influenza A, human metapneumovirus and SARS CoV-2 to promote viral entry, virus trafficking and cell-to-cell spread. The later process may aggravate disease severity and the associated complications due to widespread dissemination of the viruses to multiple organ system as observed in current coronavirus disease 2019 (COVID-19) patients. In addition, the TNT-mediated intracellular spread can be protective to the viruses from the circulating immune surveillance and possible neutralization activity present in the extracellular matrix. This review further highlights the relevance of TNTs in ocular and cardiac tissues including neurodegenerative diseases, chemotherapeutic resistance, and cancer pathogenesis. Taken together, we suggest that effective therapies should consider precise targeting of TNTs in several diseases including virus infections.

Tunneling nanotubes: Reshaping connectivity

Tunneling nanotubes (TNTs), open membranous channels between connected cells, represent a novel direct way of communication between distant cells for the diffusion of various cellular material, including survival or death signals, genetic material, organelles, and pathogens. Their discovery prompted us to review our understanding of many physiological and pathological processes involving cellular communication but also allowed us to discover new mechanisms of communication at a distance. While this has enriched the field, it has also generated some confusion, as different TNT-like protrusions have been described, and it is not clear whether they have the same structure-function. Most studies have been based on low-resolution imaging methods, and one of the major problems is the inconsistency in demonstrating the capacity of these various connections to transfer material between cells belonging to different populations. This brief review examines the fundamental properties of TNTs. In adult tissues, TNTs are stimulated by different diseases, stresses, and inflammatory signals. 'Moreover', based on the similarity of the processes of development of synaptic spines and TNT formation, we argue that TNTs in the brain predate synaptic transmission, being instrumental in the orchestration of the immature neuronal circuit.

Direct Transfer of Mesoporous Silica Nanoparticles between Macrophages and Cancer Cells

Macrophages line the walls of microvasculature, extending processes into the blood flow to capture foreign invaders, including nano-scale materials. Using mesoporous silica nanoparticles (MSNs) as a model nano-scale system, we show the interplay between macrophages and MSNs from initial uptake to intercellular trafficking to neighboring cells along microtubules. The nature of cytoplasmic bridges between cells and their role in the cell-to-cell transfer of nano-scale materials is examined, as is the ability of macrophages to function as carriers of nanomaterials to cancer cells. Both direct administration of nanoparticles and adoptive transfer of nanoparticle-loaded splenocytes in mice resulted in abundant localization of nanomaterials within macrophages 24 h post-injection, predominately in the liver. While heterotypic, trans-species nanomaterial transfer from murine macrophages to human HeLa cervical cancer cells or A549 lung cancer cells was robust, transfer to syngeneic 4T1 breast cancer cells was not detected in vitro or in vivo. Cellular connections and nanomaterial transfer in vivo were rich among immune cells, facilitating coordinated immune responses.