Intercellular conduits in tumours: the new social network

Invisible Bridges: Unveiling the Role and Prospects of Tunneling Nanotubes in Cancer Therapy

Tunneling nanotubes (TNTs) are essential intercellular communication channels that significantly impact cancer pathophysiology, affecting tumor progression and resistance. This review methodically examines the mechanisms of TNTs formation, their structural characteristics, and their functional roles in material and signal transmission between cells. Highlighting their regulatory functions within the tumor microenvironment, TNTs are crucial for modulating cell survival, proliferation, drug resistance, and immune evasion. The review critically evaluates the therapeutic potential of TNTs, focusing on their applications in targeted drug delivery and gene therapy. It also proposes future research directions to thoroughly understand TNTs biogenesis, identify cell-specific molecular targets, and develop advanced technologies for the real-time monitoring of TNTs. By integrating insights from molecular biology, nanotechnology, and immunology, this review highlights the transformative potential of TNTs in advancing cancer treatment strategies.

TNTdetect.AI: A Deep Learning Model for Automated Detection and Counting of Tunneling Nanotubes in Microscopy Images

BACKGROUND

Tunneling nanotubes (TNTs) are cellular structures connecting cell membranes and mediating intercellular communication. TNTs are manually identified and counted by a trained investigator; however, this process is time-intensive. We therefore sought to develop an automated approach for quantitative analysis of TNTs.

METHODS

We used a convolutional neural network (U-Net) deep learning model to segment phase contrast microscopy images of both cancer and non-cancer cells. Our method was composed of preprocessing and model development. We developed a new preprocessing method to label TNTs on a pixel-wise basis. Two sequential models were employed to detect TNTs. First, we identified the regions of images with TNTs by implementing a classification algorithm. Second, we fed parts of the image classified as TNT-containing into a modified U-Net model to estimate TNTs on a pixel-wise basis.

RESULTS

The algorithm detected 49.9% of human expert-identified TNTs, counted TNTs, and calculated the number of TNTs per cell, or TNT-to-cell ratio (TCR); it detected TNTs that were not originally detected by the experts. The model had 0.41 precision, 0.26 recall, and 0.32 f-1 score on a test dataset. The predicted and true TCRs were not significantly different across the training and test datasets (p = 0.78).

CONCLUSIONS

Our automated approach labeled and detected TNTs and cells imaged in culture, resulting in comparable TCRs to those determined by human experts. Future studies will aim to improve on the accuracy, precision, and recall of the algorithm.

Tunneling nanotubes, TNT, communicate glioblastoma with surrounding non-tumor astrocytes to adapt them to hypoxic and metabolic tumor conditions

Cell-to-cell communication is essential for the development and proper function of multicellular systems. We and others demonstrated that tunneling nanotubes (TNT) proliferate in several pathological conditions such as HIV, cancer, and neurodegenerative diseases. However, the nature, function, and contribution of TNT to cancer pathogenesis are poorly understood. Our analyses demonstrate that TNT structures are induced between glioblastoma (GBM) cells and surrounding non-tumor astrocytes to transfer tumor-derived mitochondria. The mitochondrial transfer mediated by TNT resulted in the adaptation of non-tumor astrocytes to tumor-like metabolism and hypoxia conditions. In conclusion, TNT are an efficient cell-to-cell communication system used by cancer cells to adapt the microenvironment to the invasive nature of the tumor.

Immune Cell Connection by Tunneling Nanotubes: The Impact of Intercellular Cross-Talk on the Immune Response and Its Therapeutic Applications

Direct intercellular communication is an important prerequisite for the development of multicellular organisms, the regeneration of tissue, and the maintenance of various physiological activities. Tunnel nanotubes (TNTs), which have diameters of approximately 50-1500 nm and lengths of up to several cell diameters, can connect cells over long distances and have emerged as one of the most important recently discovered types of efficient communication between cells. Moreover, TNTs can also directly transfer organelles, vehicles, proteins, genetic material, ions, and small molecules from one cell to adjacent and even distant cells. However, the mechanism of intercellular communication between various immune cells within the complex immune system has not been fully elucidated. Studies in the past decades have confirmed the existence of TNTs in many types of cells, especially in various kinds of immune cells. TNTs display different structural and functional characteristics between and within different immunocytes, playing a major role in the transmission of signals across various kinds of immune cells. In this review, we introduce the discovery and structure of TNTs, as well as their different functional properties within different immune cells. We also discuss the roles of TNTs in potentiating the immune response and their potential therapeutic applications.

A Ticket to Ride: The Implications of Direct Intercellular Communication via Tunneling Nanotubes in Peritoneal and Other Invasive Malignancies

It is well established that the role of the tumor microenvironment (TME) in cancer progression and therapeutic resistance is crucial, but many of the underlying mechanisms are still being elucidated. Even with better understanding of molecular oncology and identification of genomic drivers of these processes, there has been a relative lag in identifying and appreciating the cellular drivers of both invasion and resistance. Intercellular communication is a vital process that unifies and synchronizes the diverse components of the tumoral infrastructure. Elucidation of the role of extracellular vesicles (EVs) over the past decade has cast a brighter light on this field. And yet even with this advance, in addition to diffusible soluble factor-mediated paracrine and endocrine cell communication as well as EVs, additional niches of intratumoral communication are filled by other modes of intercellular transfer. Tunneling nanotubes (TNTs), tumor microtubes (TMs), and other similar intercellular channels are long filamentous actin-based cellular conduits (in most epithelial cancer cell types, ~15-500 µm in length; 50–1000+ nm in width). They extend and form direct connections between distant cells, serving as conduits for direct intercellular transfer of cell cargo, such as mitochondria, exosomes, and microRNAs; however, many of their functional roles in mediating tumor growth remain unknown. These conduits literally create a physical bridge to create a syncytial network of dispersed cells amidst the intercellular stroma-rich matrix. Emerging evidence suggests that they provide a cellular mechanism for induction and emergence of drug resistance and contribute to increased invasive and metastatic potential. They have been imaged in vitro and also in vivo and ex vivo in tumors from human patients as well as animal models, thus not only proving their existence in the TME, but opening further speculation about their exact role in the dynamic niche of tumor ecosystems. TNT cellular networks are upregulated between cancer and stromal cells under hypoxic and other conditions of physiologic and metabolic stress. Furthermore, they can connect malignant cells to benign cells, including vascular endothelial cells. The field of investigation of TNT-mediated tumor-stromal, and tumor-tumor, cell-cell communication is gaining momentum. The mixture of conditions in the microenvironment exemplified by hypoxia-induced ovarian cancer TNTs playing a crucial role in tumor growth, as just one example, is a potential avenue of investigation that will uncover their role in relation to other known factors, including EVs. If the role of cancer heterocellular signaling via TNTs in the TME is proven to be crucial, then disrupting formation and maintenance of TNTs represents a novel therapeutic approach for ovarian and other similarly invasive peritoneal cancers.

Cx43 and Associated Cell Signaling Pathways Regulate Tunneling Nanotubes in Breast Cancer Cells

Connexin 43 (Cx43) forms gap junctions that mediate the direct intercellular diffusion of ions and small molecules between adjacent cells. Cx43 displays both pro- and anti-tumorigenic properties, but the mechanisms underlying these characteristics are not fully understood. Tunneling nanotubes (TNTs) are long and thin membrane projections that connect cells, facilitating the exchange of not only small molecules, but also larger proteins, organelles, bacteria, and viruses. Typically, TNTs exhibit increased formation under conditions of cellular stress and are more prominent in cancer cells, where they are generally thought to be pro-metastatic and to provide growth and survival advantages. Cx43 has been described in TNTs, where it is thought to regulate small molecule diffusion through gap junctions. Here, we developed a high-fidelity CRISPR/Cas9 system to knockout (KO) Cx43. We found that the loss of Cx43 expression was associated with significantly reduced TNT length and number in breast cancer cell lines. Notably, secreted factors present in conditioned medium stimulated TNTs more potently when derived from Cx43-expressing cells than from KO cells. Moreover, TNT formation was significantly induced by the inhibition of several key cancer signaling pathways that both regulate Cx43 and are regulated by Cx43, including RhoA kinase (ROCK), protein kinase A (PKA), focal adhesion kinase (FAK), and p38. Intriguingly, the drug-induced stimulation of TNTs was more potent in Cx43 KO cells than in wild-type (WT) cells. In conclusion, this work describes a novel non-canonical role for Cx43 in regulating TNTs, identifies key cancer signaling pathways that regulate TNTs in this setting, and provides mechanistic insight into a pro-tumorigenic role of Cx43 in cancer.

MCF7 Spheroid Development: New Insight about Spatio/Temporal Arrangements of TNTs, Amyloid Fibrils, Cell Connections, and Cellular Bridges

Human breast adenocarcinoma cells (MCF7) grow in three-dimensional culture as spheroids that represent the structural complexity of avascular tumors. Therefore, spheroids offer a powerful tool for studying cancer development, aggressiveness, and drug resistance. Notwithstanding the large amount of data regarding the formation of MCF7 spheroids, a detailed description of the morpho-functional changes during their aggregation and maturation is still lacking. In this study, in addition to the already established role of gap junctions, we show evidence of tunneling nanotube (TNT) formation, amyloid fibril production, and opening of large stable cellular bridges, thus reporting the sequential events leading to MCF7 spheroid formation. The variation in cell phenotypes, sustained by dynamic expression of multiple proteins, leads to complex networking among cells similar to the sequence of morphogenetic steps occurring in embryogenesis/organogenesis. On the basis of the observation that early events in spheroid formation are strictly linked to the redox homeostasis, which in turn regulate amyloidogenesis, we show that the administration of N-acetyl-l-cysteine (NAC), a reactive oxygen species (ROS) scavenger that reduces the capability of cells to produce amyloid fibrils, significantly affects their ability to aggregate. Moreover, cells aggregation events, which exploit the intrinsic adhesiveness of amyloid fibrils, significantly decrease following the administration during the early aggregation phase of neutral endopeptidase (NEP), an amyloid degrading enzyme.

Modeling membrane nanotube morphology: the role of heterogeneity in composition and material properties

Membrane nanotubes are dynamic structures that may connect cells over long distances. Nanotubes are typically thin cylindrical tubes, but they may occasionally have a beaded architecture along the tube. In this paper, we study the role of membrane mechanics in governing the architecture of these tubes and show that the formation of bead-like structures along the nanotubes can result from local heterogeneities in the membrane either due to protein aggregation or due to membrane composition. We present numerical results that predict how membrane properties, protein density, and local tension compete to create a phase space that governs the morphology of a nanotube. We also find that there exists a discontinuity in the energy that impedes two beads from fusing. These results suggest that the membrane-protein interaction, membrane composition, and membrane tension closely govern the tube radius, number of beads, and the bead morphology.

Tunneling nanotubes: A bridge for heterogeneity in glioblastoma and a new therapeutic target?

BACKGROUND

The concept of tumour heterogeneity is not novel but is fast becoming a paradigm by which to explain part of the highly recalcitrant nature of aggressive malignant tumours. Glioblastoma is a prime example of such difficult-to-treat, invasive, and incurable malignancies. With the advent of the post-genomic age and increased access to next-generation sequencing technologies, numerous publications have described the presence and extent of intratumoural and intertumoural heterogeneity present in glioblastoma. Moreover, there have been numerous reports more directly correlating the heterogeneity of glioblastoma to its refractory, reoccurring, and inevitably terminal nature. It is therefore prudent to consider the different forms of heterogeneity seen in glioblastoma and how to harness this understanding to better strategize novel therapeutic approaches. One of the most central questions of tumour heterogeneity is how these numerous different cell types (both tumour and non-tumour) in the tumour mass communicate.

RECENT FINDINGS

This chapter provides a brief review on the variable heterogeneity of glioblastoma, with a focus on cellular heterogeneity and on modalities of communication that can induce further molecular diversity within the complex and ever-evolving tumour microenvironment. We provide particular emphasis on the emerging role of actin-based cellular conduits called tunnelling nanotubes (TNTs) and tumour microtubes (TMs) and outline the perceived current problems in the field that need to be resolved before pharmacological targeting of TNTs can become a reality.

CONCLUSIONS

We conclude that TNTs and TMs provide a new and exciting avenue for the therapeutic targeting of glioblastoma and that numerous inroads have already made into TNT and TM biology. However, to target TMs and TNTs, several advances must be made before this aim can become a reality.

Novel approaches for glioblastoma treatment: Focus on tumor heterogeneity, treatment resistance, and computational tools

BACKGROUND

Glioblastoma (GBM) is a highly aggressive primary brain tumor. Currently, the suggested line of action is the surgical resection followed by radiotherapy and treatment with the adjuvant temozolomide, a DNA alkylating agent. However, the ability of tumor cells to deeply infiltrate the surrounding tissue makes complete resection quite impossible, and, in consequence, the probability of tumor recurrence is high, and the prognosis is not positive. GBM is highly heterogeneous and adapts to treatment in most individuals. Nevertheless, these mechanisms of adaption are unknown.

RECENT FINDINGS

In this review, we will discuss the recent discoveries in molecular and cellular heterogeneity, mechanisms of therapeutic resistance, and new technological approaches to identify new treatments for GBM. The combination of biology and computer resources allow the use of algorithms to apply artificial intelligence and machine learning approaches to identify potential therapeutic pathways and to identify new drug candidates.

CONCLUSION

These new approaches will generate a better understanding of GBM pathogenesis and will result in novel treatments to reduce or block the devastating consequences of brain cancers.

Live cell superresolution-structured illumination microscopy imaging analysis of the intercellular transport of microvesicles and costimulatory proteins via nanotubes between immune cells

Membrane nanotubes are transient long-distance connections between cells that can facilitate intercellular communication. These tethers can form spontaneously between many cell types, including cells of the immune and nervous systems. Traffic of viral proteins, vesicles, calcium ions, mRNA, miRNA, mitochondria, lysosomes and membrane proteins/raft domains have all been reported so far via the open ended tunneling nanotubes (TNTs). Recently we reported on existence of plasma membrane derived GM₁/GM₃ ganglioside enriched microvesicles and costimulatory proteins in nanotubes connecting B lymphocytes, the way they are formed and transported across TNTs, however, still remained unclear. Here, using live cell confocal and Structured Illumination (SR-SIM) superresolution imaging, we show that B cells respond to bacterial (Cholera) toxin challenge by their subsequent internalization followed by rapid formation of intracellular microvesicles (MVs). These MVs are then transported between adjacent B cells via nanotubes. Selective transport-inhibition analysis of two abundant motor proteins in these cell types demonstrated that actin-based non-muscle myosin 2A dominantly mediates intercellular MV-transport via TNTs, in contrast to the microtubule-based dynein, as shown by the unchanged transport after inhibition of the latter. As suggested by SR-SIM images of GFP-CD86 transfected macrophages, these costimulatory molecules may be transferred by unusually shaped MVs through thick TNTs connecting macrophages. In contrast, in B cell cultures the same GFP-CD86 is dominantly transported along the membrane wall of TNTs. Such intercellular molecule-exchange can consequently improve the efficiency of antigen-dependent T cell activation, especially in macrophages with weak costimulator expression and T cell activation capacity. Such improved T cell activating potential of these two cell types may result in a more efficient cellular immune response and formation of immunological memory. The results also highlight the power of superresolution microscopy to uncover so far hidden structural details of biological processes, such as microvesicle formation and transport.

The Novel Roles of Connexin Channels and Tunneling Nanotubes in Cancer Pathogenesis

Neoplastic growth and cellular differentiation are critical hallmarks of tumor development. It is well established that cell-to-cell communication between tumor cells and "normal" surrounding cells regulates tumor differentiation and proliferation, aggressiveness, and resistance to treatment. Nevertheless, the mechanisms that result in tumor growth and spread as well as the adaptation of healthy surrounding cells to the tumor environment are poorly understood. A major component of these communication systems is composed of connexin (Cx)-containing channels including gap junctions (GJs), tunneling nanotubes (TNTs), and hemichannels (HCs). There are hundreds of reports about the role of Cx-containing channels in the pathogenesis of cancer, and most of them demonstrate a downregulation of these proteins. Nonetheless, new data demonstrate that a localized communication via Cx-containing GJs, HCs, and TNTs plays a key role in tumor growth, differentiation, and resistance to therapies. Moreover, the type and downstream effects of signals communicated between the different populations of tumor cells are still unknown. However, new approaches such as artificial intelligence (AI) and machine learning (ML) could provide new insights into these signals communicated between connected cells. We propose that the identification and characterization of these new communication systems and their associated signaling could provide new targets to prevent or reduce the devastating consequences of cancer.

The Role of Rho-GTPases and actin polymerization during Macrophage Tunneling Nanotube Biogenesis

Macrophage interactions with other cells, either locally or at distances, are imperative in both normal and pathological conditions. While soluble means of communication can transmit signals between different cells, it does not account for all long distance macrophage interactions. Recently described tunneling nanotubes (TNTs) are membranous channels that connect cells together and allow for transfer of signals, vesicles, and organelles. However, very little is known about the mechanism by which these structures are formed. Here we investigated the signaling pathways involved in TNT formation by macrophages using multiple imaging techniques including super-resolution microscopy (3D-SIM) and live-cell imaging including the use of FRET-based Rho GTPase biosensors. We found that formation of TNTs required the activity and differential localization of Cdc42 and Rac1. The downstream Rho GTPase effectors mediating actin polymerization through Arp2/3 nucleation, Wiskott-Aldrich syndrome protein (WASP) and WASP family verprolin-homologous 2 (WAVE2) proteins are also important, and both pathways act together during TNT biogenesis. Finally, TNT function as measured by transfer of cellular material between cells was reduced following depletion of a single factor demonstrating the importance of these factors in TNTs. Given that the characterization of TNT formation is still unclear in the field; this study provides new insights and would enhance the understanding of TNT formation towards investigating new markers.

Differential identity of Filopodia and Tunneling Nanotubes revealed by the opposite functions of actin regulatory complexes

Tunneling Nanotubes (TNTs) are actin enriched filopodia-like protrusions that play a pivotal role in long-range intercellular communication. Different pathogens use TNT-like structures as "freeways" to propagate across cells. TNTs are also implicated in cancer and neurodegenerative diseases, making them promising therapeutic targets. Understanding the mechanism of their formation, and their relation with filopodia is of fundamental importance to uncover their physiological function, particularly since filopodia, differently from TNTs, are not able to mediate transfer of cargo between distant cells. Here we studied different regulatory complexes of actin, which play a role in the formation of both these structures. We demonstrate that the filopodia-promoting CDC42/IRSp53/VASP network negatively regulates TNT formation and impairs TNT-mediated intercellular vesicle transfer. Conversely, elevation of Eps8, an actin regulatory protein that inhibits the extension of filopodia in neurons, increases TNT formation. Notably, Eps8-mediated TNT induction requires Eps8 bundling but not its capping activity. Thus, despite their structural similarities, filopodia and TNTs form through distinct molecular mechanisms. Our results further suggest that a switch in the molecular composition in common actin regulatory complexes is critical in driving the formation of either type of membrane protrusion.

Lost in translation: applying 2D intercellular communication via tunneling nanotubes in cell culture to physiologically relevant 3D microenvironments

Tunneling nanotubes (TNTs) are membranous conduits for direct cell-to-cell communication. Until the past decade, little had been known about their composite structure, function, and mechanisms of action in both normal physiologic conditions as well as in disease states. Now TNTs are attracting increasing interest for their key role(s) in the pathogenesis of disease, including neurodegenerative disorders, inflammatory and infectious diseases, and cancer. The field of TNT biology is still in its infancy, but inroads have been made in determining potential mechanisms and function of these remarkable structures. For example, TNTs function as critical conduits for cellular exchange of information; thus, in cancer, they may play an important role in critical pathophysiologic features of the disease, including cellular invasion, metastasis, and emergence of chemotherapy drug resistance. Although the TNT field is still in a nascent stage, we propose that TNTs can be investigated as novel targets for drug-based treatment of cancer and other diseases.

Mitotic cells form actin-based bridges with adjacent cells to provide intercellular communication during rounding

In order to achieve accurate chromosome segregation, eukaryotic cells undergo a dramatic change in morphology to obtain a spherical shape during mitosis. Interphase cells communicate directly with each other by exchanging ions and small molecules via gap junctions, which have important roles in controlling cell growth and differentiation. As cells round up during mitosis, the gap junctional communication between mitotic cells and adjacent interphase cells ceases. Whether mitotic cells use alternative mechanisms for mediating direct cell-cell communication during rounding is currently unknown. Here, we have studied the mechanisms involved in the remodeling of gap junctions during mitosis. We further demonstrate that mitotic cells are able to form actin-based plasma membrane bridges with adjacent cells during rounding. These structures, termed “mitotic nanotubes,” were found to be involved in mediating the transport of cytoplasm, including Rab11-positive vesicles, between mitotic cells and adjacent cells. Moreover, a subpool of the gap-junction channel protein connexin43 localized in these intercellular bridges during mitosis. Collectively, the data provide new insights into the mechanisms involved in the remodeling of gap junctions during mitosis and identify actin-based plasma membrane bridges as a novel means of communication between mitotic cells and adjacent cells during rounding.