Long-distance calls between cells connected by tunneling nanotubules

Cooperation of Various Cytoskeletal Components Orchestrates Intercellular Spread of Mitochondria between B-Lymphoma Cells through Tunnelling Nanotubes

Membrane nanotubes (NTs) are dynamic communication channels connecting spatially separated cells even over long distances and promoting the transport of different cellular cargos. NTs are also involved in the intercellular spread of different pathogens and the deterioration of some neurological disorders. Transport processes via NTs may be controlled by cytoskeletal elements. NTs are frequently observed membrane projections in numerous mammalian cell lines, including various immune cells, but their functional significance in the 'antibody factory' B cells is poorly elucidated. Here, we report that as active channels, NTs of B-lymphoma cells can mediate bidirectional mitochondrial transport, promoted by the cooperation of two different cytoskeletal motor proteins, kinesin along microtubules and myosin VI along actin, and bidirectional transport processes are also supported by the heterogeneous arrangement of the main cytoskeletal filament systems of the NTs. We revealed that despite NTs and axons being different cell extensions, the mitochondrial transport they mediate may exhibit significant similarities. Furthermore, we found that microtubules may improve the stability and lifespan of B-lymphoma-cell NTs, while F-actin strengthens NTs by providing a structural framework for them. Our results may contribute to a better understanding of the regulation of the major cells of humoral immune response to infections.

Tunneling nanotubes: The intercellular conduits contributing to cancer pathogenesis and its therapy

Tunneling nanotubes (TNTs) are intercellular conduits which meet the communication needs of non-adjacent cells situated in the same tissue but at distances up to a few hundred microns. TNTs are unique type of membrane protrusion which contain F-actin and freely hover over substratum in the extracellular space to connect the distant cells. TNTs, known to form through actin remodeling mechanisms, are intercellular bridges that connect cytoplasm of two cells, and facilitate the transfer of organelles, molecules, and pathogens among the cells. In tumor microenvironment, TNTs act as communication channel among cancer, normal, and immune cells to facilitate the transfer of calcium waves, mitochondria, lysosomes, and proteins, which in turn contribute to the survival, metastasis, and chemo-resistance in cancer cells. Recently, TNTs were shown to mediate the transfer of nanoparticles, drugs, and viruses between cells, suggesting that TNTs could be exploited as a potential route for delivery of anti-cancer agents and oncolytic viruses to the target cells. The present review discusses the emerging concepts and role of TNTs in the context of chemo- and radio-resistance with implications in the cancer therapy.

Mitochondrial transfer in hematological malignancies

Mitochondria are energy-generated organelles and take an important part in biological metabolism. Mitochondria could be transferred between cells, which serves as a new intercellular communication. Mitochondrial transfer improves mitochondrial defects, restores the biological functions of recipient cells, and maintains the high metabolic requirements of tumor cells as well as drug resistance. In recent years, it has been reported mitochondrial transfer between cells of bone marrow microenvironment and hematological malignant cells play a critical role in the disease progression and resistance during chemotherapy. In this review, we discuss the patterns and mechanisms on mitochondrial transfer and their engagement in different pathophysiological contexts and outline the latest knowledge on intercellular transport of mitochondria in hematological malignancies. Besides, we briefly outline the drug resistance mechanisms caused by mitochondrial transfer in cells during chemotherapy. Our review demonstrates a theoretical basis for mitochondrial transfer as a prospective therapeutic target to increase the treatment efficiency in hematological malignancies and improve the prognosis of patients.

Hunting for the cause: Evidence for prion-like mechanisms in Huntington’s disease

The hypothesis that pathogenic protein aggregates associated with neurodegenerative diseases spread from cell-to-cell in the brain in a manner akin to infectious prions has gained substantial momentum due to an explosion of research in the past 10–15 years. Here, we review current evidence supporting the existence of prion-like mechanisms in Huntington’s disease (HD), an autosomal dominant neurodegenerative disease caused by expansion of a CAG repeat tract in exon 1 of the huntingtin (HTT) gene. We summarize information gained from human studies and in vivo and in vitro models of HD that strongly support prion-like features of the mutant HTT (mHTT) protein, including potential involvement of molecular features of mHTT seeds, synaptic structures and connectivity, endocytic and exocytic mechanisms, tunneling nanotubes, and nonneuronal cells in mHTT propagation in the brain. We discuss mechanisms by which mHTT aggregate spreading and neurotoxicity could be causally linked and the potential benefits of targeting prion-like mechanisms in the search for new disease-modifying therapies for HD and other fatal neurodegenerative diseases.

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.

Peering into tunneling nanotubes-The path forward

The identification of Tunneling Nanotubes (TNTs) and TNT-like structures signified a critical turning point in the field of cell-cell communication. With hypothesized roles in development and disease progression, TNTs' ability to transport biological cargo between distant cells has elevated these structures to a unique and privileged position among other mechanisms of intercellular communication. However, the field faces numerous challenges-some of the most pressing issues being the demonstration of TNTs in vivo and understanding how they form and function. Another stumbling block is represented by the vast disparity in structures classified as TNTs. In order to address this ambiguity, we propose a clear nomenclature and provide a comprehensive overview of the existing knowledge concerning TNTs. We also discuss their structure, formation-related pathways, biological function, as well as their proposed role in disease. Furthermore, we pinpoint gaps and dichotomies found across the field and highlight unexplored research avenues. Lastly, we review the methods employed to date and suggest the application of new technologies to better understand these elusive biological structures.

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.

Tunneling Nanotubes and the Eye: Intercellular Communication and Implications for Ocular Health and Disease

Cellular communication is an essential process for the development and maintenance of all tissues including the eye. Recently, a new method of cellular communication has been described, which relies on formation of tubules, called tunneling nanotubes (TNTs). These structures connect the cytoplasm of adjacent cells and allow the direct transport of cellular cargo between cells without the need for secretion into the extracellular milieu. TNTs may be an important mechanism for signaling between cells that reside long distances from each other or for cells in aqueous environments, where diffusion-based signaling is challenging. Given the wide range of cargoes transported, such as lysosomes, endosomes, mitochondria, viruses, and miRNAs, TNTs may play a role in normal homeostatic processes in the eye as well as function in ocular disease. This review will describe TNT cellular communication in ocular cell cultures and the mammalian eye in vivo, the role of TNTs in mitochondrial transport with an emphasis on mitochondrial eye diseases, and molecules involved in TNT biogenesis and their function in eyes, and finally, we will describe TNT formation in inflammation, cancer, and stem cells, focusing on pathological processes of particular interest to vision scientists.

Tunneling Nanotubes and Tumor Microtubes in Cancer

Intercellular communication among cancer cells and their microenvironment is crucial to disease progression. The mechanisms by which communication occurs between distant cells in a tumor matrix remain poorly understood. In the last two decades, experimental evidence from different groups proved the existence of thin membranous tubes that interconnect cells, named tunneling nanotubes, tumor microtubes, cytonemes or membrane bridges. These highly dynamic membrane protrusions are conduits for direct cell-to-cell communication, particularly for intercellular signaling and transport of cellular cargo over long distances. Tunneling nanotubes and tumor microtubes may play an important role in the pathogenesis of cancer. They may contribute to the resistance of tumor cells against treatments such as surgery, radio- and chemotherapy. In this review, we present the current knowledge about the structure and function of tunneling nanotubes and tumor microtubes in cancer and discuss the therapeutic potential of membrane tubes in cancer treatment.

Polyplexes Are Endocytosed by and Trafficked within Filopodia

The improvement of nonviral gene therapies relies to a large extent on understanding many fundamental physical and biological properties of these systems. This includes interactions of synthetic delivery systems with the cell and mechanisms of trafficking delivery vehicles, which remain poorly understood on both the extra- and intracellular levels. In this study, the mechanisms of cellular internalization and trafficking of polymer-based nanoparticle complexes consisting of polycations and nucleic acids, termed polyplexes, have been observed in detail at the cellular level. For the first time evidence has been obtained that filopodia, actin projections that radiate out from the surface of cells, serve as a route for the direct endocytosis of polyplexes. Confocal microscopy images demonstrated that filopodia on HeLa cells detect external polyplexes and extend into the extracellular milieu to internalize these particles. Polyplexes are observed to be internalized into membrane-bound vesicles (i.e., clathrin-coated pits and caveolae) directly within filopodial projections and are subsequently transported along actin to the main cell body for potential delivery of the nucleic acids to the nucleus. The kinetics and speed of polyplex trafficking have also been measured. The polyplex-loaded vesicles were also discovered to traffic between two cells within filopodial bridges. These findings provide novel insight into the early events of cellular contact with polyplexes through filopodial-based interactions in addition to endocytic vesicle trafficking-an important fundamental discovery to enable advancement of nonviral gene editing, nucleic acid therapies, and biomedical materials.

Leite D, Pilkington GJ. Pre-Clinical Drug Testing in 2D and 3D Human In Vitro Models of Glioblastoma Incorporating Non-Neoplastic Astrocytes: Tunneling Nano Tubules and Mitochondrial Transfer Modulates Cell Behavior and Therapeutic Respons

The role of astrocytes in the glioblastoma (GBM) microenvironment is poorly understood; particularly with regard to cell invasion and drug resistance. To assess this role of astrocytes in GBMs we established an all human 2D co-culture model and a 3D hyaluronic acid-gelatin based hydrogel model (HyStem™-HP) with different ratios of GBM cells to astrocytes. A contact co-culture of fluorescently labelled GBM cells and astrocytes showed that the latter promotes tumour growth and migration of GBM cells. Notably, the presence of non-neoplastic astrocytes in direct contact, even in low amounts in co-culture, elicited drug resistance in GBM. Recent studies showed that non-neoplastic cells can transfer mitochondria along tunneling nanotubes (TNT) and rescue damaged target cancer cells. In these studies, we explored TNT formation and mitochondrial transfer using 2D and 3D in vitro co-culture models of GBM and astrocytes. TNT formation occurs in glial fibrillary acidic protein (GFAP) positive "reactive" astrocytes after 48 h co-culture and the increase of TNT formations was greater in 3D hyaluronic acid-gelatin based hydrogel models. This study shows that human astrocytes in the tumour microenvironment, both in 2D and 3D in vitro co-culture models, could form TNT connections with GBM cells. We postulate that the association on TNT delivery non-neoplastic mitochondria via a TNT connection may be related to GBM drug response as well as proliferation and migration.

Intercellular transfer of HLA-G: its potential in cancer immunology

Intercellular protein transfer between cancer cells and immune cells is a very common phenomenon that can affect different stages of host antitumor immune responses. HLA-G, a non-classical HLA class I antigen, has been observed to be widely expressed in various malignancies, and its immune-suppressive functions have been well recognised. HLA-G expression in cancer cells can directly mediate immune tolerance by interacting with inhibitory receptors such as ILT2 and ILT4 expressed on immune cells. Moreover, a network of multiple directional intercellular transfers of HLA-G among cancer cells and immune cells through trogocytosis, exosomes and tunnelling nanotubes provides malignant cells with an alternative ploy for antigen sharing and induces more complex heterogeneity, to modulate immune responses, ultimately leading to immune evasion, therapy resistance, disease progression and poor clinical outcome. Herein, we discuss the relative aspects of the intercellular transfer of HLA-G between tumor cells and immune cells and its potential use in tumor immunology research and translational cancer therapy.

Perspective on nanochannels as cellular mediators in different disease conditions

Tunnelling nanotubes (TNTs), also known as membrane nanochannels, are actin-based structures that facilitate cytoplasmic connections for rapid intercellular transfer of signals, organelles and membrane components. These dynamic TNTs can form de novo in animal cells and establish complex intercellular networks between distant cells up to 150 μm apart. Within the last decade, TNTs have been discovered in different cell types including tumor cells, macrophages, monocytes, endothelial cells and T cells. It has also been further elucidated that these nanotubes play a vital role in diseased conditions such as cancer, where TNT formation occurs at a higher pace and is used for rapid intercellular modulation of chemo-resistance. Viruses such as HIV, HSV and prions also hijack the existing TNT connections between host cells for rapid transmission and evasion of the host immune responses. The following review aims to describe the heterogeneity of TNTs, their role in different tissues and disease conditions in order to enhance our understanding on how these nanotubes can be used as a target for therapies.

Cellular and Molecular Networking Within the Ecosystem of Cancer Cell Communication via Tunneling Nanotubes

Intercellular communication is vital to the ecosystem of cancer cell organization and invasion. Identification of key cellular cargo and their varied modes of transport are important considerations in understanding the basic mechanisms of cancer cell growth. Gap junctions, exosomes, and apoptotic bodies play key roles as physical modalities in mediating intercellular transport. Tunneling nanotubes (TNTs)-narrow actin-based cytoplasmic extensions-are unique structures that facilitate direct, long distance cell-to-cell transport of cargo, including microRNAs, mitochondria, and a variety of other sub cellular components. The transport of cargo via TNTs occurs between malignant and stromal cells and can lead to changes in gene regulation that propagate the cancer phenotype. More notably, the transfer of these varied molecules almost invariably plays a critical role in the communication between cancer cells themselves in an effort to resist death by chemotherapy and promote the growth and metastases of the primary oncogenic cell. The more traditional definition of "Systems Biology" is the computational and mathematical modeling of complex biological systems. The concept, however, is now used more widely in biology for a variety of contexts, including interdisciplinary fields of study that focus on complex interactions within biological systems and how these interactions give rise to the function and behavior of such systems. In fact, it is imperative to understand and reconstruct components in their native context rather than examining them separately. The long-term objective of evaluating cancer ecosystems in their proper context is to better diagnose, classify, and more accurately predict the outcome of cancer treatment. Communication is essential for the advancement and evolution of the tumor ecosystem. This interplay results in cancer progression. As key mediators of intercellular communication within the tumor ecosystem, TNTs are the central topic of this article.

Intercellular Spread of Protein Aggregates in Neurodegenerative Disease

Most neurodegenerative diseases are characterized by the accumulation of protein aggregates, some of which are toxic to cells. Mounting evidence demonstrates that in several diseases, protein aggregates can pass from neuron to neuron along connected networks, although the role of this spreading phenomenon in disease pathogenesis is not completely understood. Here we briefly review the molecular and histopathological features of protein aggregation in neurodegenerative disease, we summarize the evidence for release of proteins from donor cells into the extracellular space, and we highlight some other mechanisms by which protein aggregates might be transmitted to recipient cells. We also discuss the evidence that supports a role for spreading of protein aggregates in neurodegenerative disease pathogenesis and some limitations of this model. Finally, we consider potential therapeutic strategies to target spreading of protein aggregates in the treatment of neurodegenerative diseases.

Chemotherapy-Induced Tunneling Nanotubes Mediate Intercellular Drug Efflux in Pancreatic Cancer

Intercellular communication plays a critical role in the ever-evolving landscape of invasive cancers. Recent studies have elucidated the potential role of tunneling nanotubes (TNTs) in this function. TNTs are long, filamentous, actin-based cell protrusions that mediate direct cell-to-cell communication between malignant cells. In this study, we investigated the formation of TNTs in response to variable concentrations of the chemotherapeutic drug doxorubicin, which is used extensively in the treatment of cancer patients. Doxorubicin stimulated an increased formation of TNTs in pancreatic cancer cells, and this occurred in a dose-dependent fashion. Furthermore, TNTs facilitated the intercellular redistribution of this drug between connected cells in both pancreatic and ovarian cancer systems in vitro. To provide supportive evidence for the relevance of TNTs in pancreatic cancer in vivo, we performed multiphoton fluorescence microscopy and imaged TNTs in tumor specimens resected from three human patients with pancreatic adenocarcinoma, and one with neuroendocrine carcinoma. In sum, TNT formation was upregulated in aggressive forms of pancreatic carcinoma, was further stimulated after chemotherapy exposure, and acted as a novel method for drug efflux. These findings implicate TNTs as a potential novel mechanism of drug resistance in chemorefractory forms of cancer.

Imaging Tunneling Membrane Tubes Elucidates Cell Communication in Tumors

Intercellular communication is a vital yet underdeveloped aspect of cancer pathobiology. This Opinion article reviews the importance and challenges of microscopic imaging of tunneling nanotubes (TNTs) in the complex tumor microenvironment. The use of advanced microscopy to characterize TNTs in vitro and ex vivo, and related extensions called tumor microtubes (TMs) reported in gliomas in vivo, has propelled this field forward. This topic is important because the identification of TNTs and TMs fills the gap in our knowledge of how cancer cells communicate at long range in vivo, inducing intratumor heterogeneity and resistance to treatment. Here we discuss the concept that TNTs/TMs fill an important niche in the ever-changing microenvironment and the role of advanced microscopic imaging to elucidate that niche.

Extracellular Vesicles, Tunneling Nanotubes, and Cellular Interplay: Synergies and Missing Links

The process of intercellular communication seems to have been a highly conserved evolutionary process. Higher eukaryotes use several means of intercellular communication to address both the changing physiological demands of the body and to fight against diseases. In recent years, there has been an increasing interest in understanding how cell-derived nanovesicles, known as extracellular vesicles (EVs), can function as normal paracrine mediators of intercellular communication, but can also elicit disease progression and may be used for innovative therapies. Over the last decade, a large body of evidence has accumulated to show that cells use cytoplasmic extensions comprising open-ended channels called tunneling nanotubes (TNTs) to connect cells at a long distance and facilitate the exchange of cytoplasmic material. TNTs are a different means of communication to classical gap junctions or cell fusions; since they are characterized by long distance bridging that transfers cytoplasmic organelles and intracellular vesicles between cells and represent the process of heteroplasmy. The role of EVs in cell communication is relatively well-understood, but how TNTs fit into this process is just emerging. The aim of this review is to describe the relationship between TNTs and EVs, and to discuss the synergies between these two crucial processes in the context of normal cellular cross-talk, physiological roles, modulation of immune responses, development of diseases, and their combinatory effects in tissue repair. At the present time this review appears to be the first summary of the implications of the overlapping roles of TNTs and EVs. We believe that a better appreciation of these parallel processes will improve our understanding on how these nanoscale conduits can be utilized as novel tools for targeted therapies.

The ins and outs of eukaryotic viruses: Knowledge base and ontology of a viral infection

Viruses are genetically diverse, infect a wide range of tissues and host cells and follow unique processes for replicating themselves. All these processes were investigated and indexed in ViralZone knowledge base. To facilitate standardizing data, a simple ontology of viral life-cycle terms was developed to provide a common vocabulary for annotating data sets. New terminology was developed to address unique viral replication cycle processes, and existing terminology was modified and adapted. The virus life-cycle is classically described by schematic pictures. Using this ontology, it can be represented by a combination of successive terms: “entry”, “latency”, “transcription”, “replication” and “exit”. Each of these parts is broken down into discrete steps. For example Zika virus “entry” is broken down in successive steps: “Attachment”, “Apoptotic mimicry”, “Viral endocytosis/ macropinocytosis”, “Fusion with host endosomal membrane”, “Viral factory”. To demonstrate the utility of a standard ontology for virus biology, this work was completed by annotating virus data in the ViralZone, UniProtKB and Gene Ontology databases.

Influenza virus exploits tunneling nanotubes for cell-to-cell spread

Tunneling nanotubes (TNTs) represent a novel route of intercellular communication. While previous work has shown that TNTs facilitate the exchange of viral or prion proteins from infected to naïve cells, it is not clear whether the viral genome is also transferred via this mechanism and further, whether transfer via this route can result in productive replication of the infectious agents in the recipient cell. Here we present evidence that lung epithelial cells are connected by TNTs, and in spite of the presence of neutralizing antibodies and an antiviral agent, Oseltamivir, influenza virus can exploit these networks to transfer viral proteins and genome from the infected to naïve cell, resulting in productive viral replication in the naïve cells. These observations indicate that influenza viruses can spread using these intercellular networks that connect epithelial cells, evading immune and antiviral defenses and provide an explanation for the incidence of influenza infections even in influenza-immune individuals and vaccine failures.

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.

Intercellular communication in malignant pleural mesothelioma: properties of tunneling nanotubes

Malignant pleural mesothelioma is a particularly aggressive and locally invasive malignancy with a poor prognosis despite advances in understanding of cancer cell biology and development of new therapies. At the cellular level, cultured mesothelioma cells present a mesenchymal appearance and a strong capacity for local cellular invasion. One important but underexplored area of mesothelioma cell biology is intercellular communication. Our group has previously characterized in multiple histological subtypes of mesothelioma a unique cellular protrusion known as tunneling nanotubes (TnTs). TnTs are long, actin filament-based, narrow cytoplasmic extensions that are non-adherent when cultured in vitro and are capable of shuttling cellular cargo between connected cells. Our prior work confirmed the presence of nanotube structures in tumors resected from patients with human mesothelioma. In our current study, we quantified the number of TnTs/cell among various mesothelioma subtypes and normal mesothelial cells using confocal microscopic techniques. We also examined changes in TnT length over time in comparison to cell proliferation. We further examined potential approaches to the in vivo study of TnTs in animal models of cancer. We have developed novel approaches to study TnTs in aggressive solid tumor malignancies and define fundamental characteristics of TnTs in malignant mesothelioma. There is mounting evidence that TnTs play an important role in intercellular communication in mesothelioma and thus merit further investigation of their role in vivo.

Exploring the human mesenchymal stem cell tubule communication network through electron microscopy

Cells use several mechanisms to transfer information to other cells. In this study, we describe micro/nanotubular connections and exosome-like tubule fragments in multipotent mesenchymal stem cells (MSCs) from human arteries. Scanning and transmission electron microscopy allowed characterization of sinusoidal microtubular projections (700 nm average size, 200 µm average length, with bulging mitochondria and actin microfilaments); short, uniform, variously shaped nanotubular projections (100 nm, bidirectional communication); and tubule fragments (50 nm). This is the first study demonstrating that MSCs from human arteries constitutively interact through an articulate and dynamic tubule network allowing long-range cell to cell communication.

Nonviral gene delivery vectors use syndecan-dependent transport mechanisms in filopodia to reach the cell surface

Lipoplexes and polyplexes, that is, assemblies of cationic lipids and polymers with nucleic acids, respectively, are popular nanocarriers for delivery of genes or siRNA into cells for therapeutic or cell biological purposes. Although endocytosis represents a major mechanism for their cellular entry, very little is known about parameters that govern early events in the initial interaction of such delivery devices with the cell surface. Here, we demonstrate that prior to entry, poly- and lipoplexes are captured by thin, actin-rich filopodial extensions, protruding from the cell surface. Subsequent additional recruitment and local clustering of filopodia-localized syndecans, presumably driven by multivalent interactions with the polycationic nanocarriers, appear instrumental in their processing to the cell body. Detailed microscopic analyses reveal that the latter relies on either directional surfing along or retraction of the filopodia. By interfering with actin polymerization or inhibiting the motor protein myosin II, localized at the base of filopodia, our data reveal that the binding of the nanocarriers to and subsequent clustering of syndecans initiates actin retrograde flow, which moves the syndecan-bound nanocarriers to the cell body. At the present experimental conditions, inhibition of this process inhibits nanocarrier-mediated transfection by 50-90%. The present findings add novel insight to our understanding of the mechanism of nanocarrier-cell surface interaction, which may be instrumental in further improving delivery efficiency. In addition, the current experimental approach may also be of relevance to improving our understanding of cellular infection by viruses and pathogenic bacteria, given a striking parallel in filopodia-mediated processing of these infectious particles and nanocarriers.

NK cells: immune cross-talk and therapeutic implications

Increased evidence of cross-talk between NK cells and other immune cells has enhanced the possibilities of exploiting the interplay between the activation and inhibition of NK cells for immunotherapeutic purposes. The battery of receptors possessed by NK cells help them to efficiently detect aberrant and infected cells and embark on the signaling pathways necessary to eliminate them. Endogenous expansion of NK cells and their effector mechanisms are under exploration for enhancing adoptive immunotherapy prospects in combination with immunostimulatory and cell-death-sensitizing treatments against cancer, viral infections and other pathophysiological autoimmune conditions. Various modes of NK cell manipulation are being undertaken to overcome issues such as relapse and graft rejections associated with adoptive immunotherapy. While tracing the remarkable properties of NK cells and the major developments in this field, we highlight the role of immune cooperativity in the betterment of current immunotherapeutic approaches.

To gate, or not to gate: regulatory mechanisms for intercellular protein transport and virus movement in plants

Cell-to-cell signal transduction is vital for orchestrating the whole-body physiology of multi-cellular organisms, and many endogenous macromolecules, proteins, and nucleic acids function as such transported signals. In plants, many of these molecules are transported through plasmodesmata (Pd), the cell wall-spanning channel structures that interconnect plant cells. Furthermore, Pd also act as conduits for cell-to-cell movement of most plant viruses that have evolved to pirate these channels to spread the infection. Pd transport is presumed to be highly selective, and only a limited repertoire of molecules is transported through these channels. Recent studies have begun to unravel mechanisms that actively regulate the opening of the Pd channel to allow traffic. This macromolecular transport between cells comprises two consecutive steps: intracellular targeting to Pd and translocation through the channel to the adjacent cell. Here, we review the current knowledge of molecular species that are transported though Pd and the mechanisms that control this traffic. Generally, Pd traffic can occur by passive diffusion through the trans-Pd cytoplasm or through the membrane/lumen of the trans-Pd ER, or by active transport that includes protein-protein interactions. It is this latter mode of Pd transport that is involved in intercellular traffic of most signal molecules and is regulated by distinct and sometimes interdependent mechanisms, which represent the focus of this article.

Tunneling nanotubes (TNT): A potential mechanism for intercellular HIV trafficking

Cell-to-cell communication coordinates the development of multicellular systems, and is mediated by soluble factors, gap junctions and the recently described tunneling nanotubes (TNT). Both TNT and gap junctions facilitate the transfer of intracellular mediators between the cytoplasm of connected cells. We recently described that HIV induced the formation of TNT in human primary macrophages in correlation with viral replication. Based on these results we hypothesized that during HIV infection, TNTs are hijacked by HIV to spread infection. TNT like structures may be a novel mechanism of amplification of HIV infection. Our findings and those of others require further investigation to identify the specific mechanisms by which pathogens use TNT.

Cellular bridges: Routes for intercellular communication and cell migration

Cell-to-cell communication is the basis of all biology in multicellular organisms, allowing evolution of complex forms and viability in dynamic environments. Though biochemical interactions occur over distances, physical continuity remains the most direct means of cellular interactions. Cellular bridging through thin cytoplasmic channels-plasmodesmata in plants and tunneling nanotubes in animals-creates direct routes for transfer of signals and components, even pathogens, between cells. Recently, two new cellular connections, designated epithelial (EP) bridges, were discovered and found to be structurally distinct from other cellular channels. The first EP bridge type facilitates material transport between cells similar to plasmodesmata and tunneling nanotubes, the second EP bridge type mediates migration of cells between EP cell masses representing a novel form of cell migration. Here, we compare the structures and functions of EP bridges with other cellular channels and discuss biochemical and cellular interactions involved in EP bridge formation. Potential roles for EP bridges in health and disease are also presented.

Mesenchymal stem cells rescue cardiomyoblasts from cell death in an in vitro ischemia model via direct cell-to-cell connections

Background

Bone marrow derived mesenchymal stem cells (MSCs) are promising candidates for cell based therapies in myocardial infarction. However, the exact underlying cellular mechanisms are still not fully understood. Our aim was to explore the possible role of direct cell-to-cell interaction between ischemic H9c2 cardiomyoblasts and normal MSCs. Using an in vitro ischemia model of 150 minutes of oxygen glucose deprivation we investigated cell viability and cell interactions with confocal microscopy and flow cytometry.

Results

Our model revealed that adding normal MSCs to the ischemic cell population significantly decreased the ratio of dead H9c2 cells (H9c2 only: 0.85 ± 0.086 vs. H9c2+MSCs: 0.16 ± 0.035). This effect was dependent on direct cell-to-cell contact since co-cultivation with MSCs cultured in cell inserts did not exert the same beneficial effect (ratio of dead H9c2 cells: 0.90 ± 0.055). Confocal microscopy revealed that cardiomyoblasts and MSCs frequently formed 200-500 nm wide intercellular connections and cell fusion rarely occurred between these cells.

Conclusion

Based on these results we hypothesize that mesenchymal stem cells may reduce the number of dead cardiomyoblasts after ischemic damage via direct cell-to-cell interactions and intercellular tubular connections may play an important role in these processes.

Novel mechanisms of central nervous system damage in HIV infection

Human immunodeficiency virus-1 infection of the central nervous system is an early event after primary infection, resulting in motor and cognitive defects in a significant number of individuals despite successful antiretroviral therapy. The pathology of the infected brain is characterized by enhanced leukocyte infiltration, microglial activation and nodules, aberrant expression of inflammatory factors, neuronal dysregulation and loss, and blood–brain barrier disruption. Months to years following the primary infection, these central nervous system insults result in a spectrum of motor and cognitive dysfunction, ranging from mild impairment to frank dementia. The mechanisms that mediate impairment are still not fully defined. In this review we discuss the cellular and molecular mechanisms that facilitate impairment and new data that implicate intercellular communication systems, gap junctions and tunneling nanotubes, as mediators of human immunodeficiency virus-1 toxicity and infection within the central nervous system. These data suggest potential targets for novel therapeutics.

Tubular bridges for bronchial epithelial cell migration and communication

BACKGROUND

Biological processes from embryogenesis to tumorigenesis rely on the coordinated coalescence of cells and synchronized cell-to-cell communication. Intercellular signaling enables cell masses to communicate through endocrine pathways at a distance or by direct contact over shorter dimensions. Cellular bridges, the longest direct connections between cells, facilitate transfer of cellular signals and components over hundreds of microns in vitro and in vivo.

METHODOLOGY/PRINCIPAL FINDINGS

Using various cellular imaging techniques on human tissue cultures, we identified two types of tubular, bronchial epithelial (EP) connections, up to a millimeter in length, designated EP bridges. Structurally distinct from other cellular connections, the first type of EP bridge may mediate transport of cellular material between cells, while the second type of EP bridge is functionally distinct from all other cellular connections by mediating migration of epithelial cells between EP masses. Morphological and biochemical interactions with other cell types differentially regulated the nuclear factor-kappaB and cyclooxygenase inflammatory pathways, resulting in increased levels of inflammatory molecules that impeded EP bridge formation. Pharmacologic inhibition of these inflammatory pathways caused increased morphological and mobility changes stimulating the biogenesis of EP bridges, in part through the upregulation of reactive oxygen species pathways.

CONCLUSIONS/SIGNIFICANCE

EP bridge formation appears to be a normal response of EP physiology in vitro, which is differentially inhibited by inflammatory cellular pathways depending upon the morphological and biochemical interactions between EP cells and other cell types. These tubular EP conduits may represent an ultra long-range form of direct intercellular communication and a completely new mechanism of tissue-mediated cell migration.

Mechanisms and functions for the duration of intercellular contacts made by lymphocytes

Communication across intercellular contacts is central to establishing appropriate innate and adaptive immune responses. Recent imaging of lymphocyte interactions suggests that a complex orchestration of cell-cell contact times is a key correlate to establishing appropriate immune responses. Here I review the molecular and cellular processes that influence the duration of intercellular contacts, including integrin activation and dynamic changes in membrane morphology. I discuss how these processes can be regulated, for example, by the balance of activating and inhibitory receptor signals, and how they can establish the appropriate outcome for individual cell-cell interactions.

Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages: a potential mechanism for intercellular HIV trafficking

Cell to cell communication is essential for the organization/coordination of multicellular systems and cellular development. Cellular communication is mediated by soluble factors, including growth factors, neurotransmitters, cytokines/chemokines, gap junctions, and the recently described tunneling nanotubes (TNT). TNT are long cytoplasmatic bridges that enable long range directed communication between cells. The proposed function for TNT is the cell-to-cell transfer of large cellular structures such as vesicles and organelles. We demonstrate that HIV-infection of human macrophages results in an increased number of TNT, and show HIV particles within these structures. We propose that HIV "highjacks" TNT communication to spread HIV through an intercellular route between communicated cells, contributing to the pathogenesis of AIDS.

Cell-to-cell cross-talk between mesenchymal stem cells and cardiomyocytes in co-culture

The goals of the study were: (1) to explore the communication between human mesenchymal stem cells (MSC) and rat cardiac myocytes resulting in differentiation of the stem cells and, (2) to evaluate the role of mitochondria in it. Light and fluorescence microscopy as well as scanning electron microscopy revealed that after co-cultivation, cells formed intercellular contacts and transient exchange with cytosolic elements could be observed. The transport of cytosolic entity had no specific direction. Noticeably, mitochondria also could be transferred to the recipient cells in a unidirectional fashion (towards cardiomyocytes only). Transmission electron microscopy revealed significant variability in both the diameter of intercellular contacting tubes and their shape. Inside of these nanotubes mitochondria-resembling structures were identified. Moreover, after co-cultivation with cardiomyocytes, expression of human-specific myosin was revealed in MSC. Thus, we speculate that: (1) transport of intracellular elements to MSC possibly can determine the direction of their differentiation and, (2) mitochondria may be involved in the mechanism of the stem cell differentiation. It looks plausible that mitochondrial transfer to recipient cardiomyocytes may be involved in the mechanism of failed myocardium repair after stem cells transplantation.

TGFbeta as a potential mediator of progesterone action in the mammary gland of pregnancy

The molecular mechanisms controlling the onset of copious milk secretion are only now beginning to be elucidated. We have known for nearly four decades that progesterone suppresses milk secretion during pregnancy, and that the fall in progesterone near parturition is necessary for secretory activation. Similarly, we've known for 15 years that transforming growth factor beta (TGFbeta) also suppresses milk secretion. Yet no formal link between the two has ever been established. This work aims to review the evidence for and against a link between progesterone and TGFbeta, raise unanswered questions, and to propose further lines of research.