Tunnelling nanotube formation is driven by Eps8/IRSp53-dependent linear actin polymerization

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.

Beyond synapses: cytoplasmic connections in brain function and evolution

Following Ramón y Cajal's groundbreaking contributions to the identification of synapses, research in neuroscience predominantly focused on their pivotal role in neural communication (the neuron doctrine), overlooking an intriguing possibility suggested by Golgi of non-synaptic interactions among neural cells. Recent studies across species have unveiled the existence of direct cellular communication through modalities such as intercellular bridges (IBs) formed during incomplete cytokinesis, de novo tunnelling nanotubes (TNTs), and cytoplasmic connections arising from cell-cell fusion. In this review, we delve into these non-synaptic modes of communication between neural cells, describing their morphological features and functional significance. Notably, we discuss recent in vivo findings in ctenophores and in mice which offer fresh insights into the evolutionary functions of these intercellular connections. These findings underscore the need to consider the roles of cytoplasmic connections in neural cell communication during brain development and in pathophysiological conditions. This review highlights the importance of investigating these non-synaptic communication pathways to improve our understanding of neural communication and evolution in metazoans.

Induction of tunnelling nanotube-like structures by influenza A viruses requires the onset of apoptosis

As well as spreading through virions, influenza A viruses (IAVs) can evade antiviral drugs and neutralising antibodies by spreading directly from cell to cell. In cell culture this can occur by the induction of intercellular membrane connections known as tunnelling nanotube-like structures (TLSs), which are capable of trafficking the viral genome between cells. Here, we showed that TLSs are formed by IAV infected cells in vivo, and then used in vitro models to ask how IAVs induce their formation. We found that TLS formation is not induced by cytokine signalling from infected to uninfected cells, but induction does require intracellular IAV replication. IAV replication can form filamentous virions which have structural similarities to TLSs, but we found that TLS induction is independent of virion morphology. We therefore looked at the intracellular responses to infection and found that the induction of TLSs correlated with the induction of apoptosis. Furthermore, the ability of IAVs to drive TLS formation can be modulated by chemically inhibiting, or inducing apoptosis. Finally, we found that inhibiting apoptosis, which prevents IAVs from inducing TLSs, lead to a significant reduction in the ability of IAVs to directly spread between cells. Our results, which suggest that IAVs can control their ability to spread directly from cell to cell by driving infected cells into apoptosis, identifies a new way in which a virus can manipulate its host to evade antiviral immune responses.

Slik sculpts the plasma membrane into cytonemes to control cell-cell communication

Cytonemes are signaling filopodia that facilitate long-range cell-cell communication by forming synapses between cells. Initially discovered in Drosophila for transporting morphogens during embryogenesis, they have since been identified in mammalian cells and implicated in carcinogenesis. Despite their importance, mechanisms controlling cytoneme biogenesis remain elusive. Here, we demonstrate that the Ser/Thr kinase Slik drives remote cell proliferation by promoting cytoneme formation. This function depends on the coiled-coil domain of Slik (SlikCCD), which directly sculpts membranes into tubules. Importantly, Slik plays opposing roles in cytoneme biogenesis: its membrane-sculpting activity promotes cytoneme formation, but this is counteracted by its kinase activity, which enhances actin association with the plasma membrane via Moesin phosphorylation. In vivo, SlikCCD enhances cytoneme formation in one epithelial layer of the wing disc to promote cell proliferation in an adjacent layer. Finally, this function relies on the STRIPAK complex, which controls cytoneme formation and governs proliferation at a distance by regulating Slik association with the plasma membrane. Our study unveils an unexpected structural role of a kinase in sculpting membranes, crucial for cytoneme-mediated control of cell proliferation.

Tunneling nanotube-like structures regulate distant cellular interactions during heart formation

In the developing mammalian heart, the endocardium and the myocardium are separated by so-called cardiac jelly. Communication between the endocardium and the myocardium is essential for cardiac morphogenesis. How membrane-localized receptors and ligands achieve interaction across the cardiac jelly is not understood. Working in developing mouse cardiac morphogenesis models, we used a variety of cellular, imaging, and genetic approaches to elucidate this question. We found that myocardium and endocardium interacted directly through microstructures termed tunneling nanotube-like structures (TNTLs). TNTLs extended from cardiomyocytes (CMs) to contact endocardial cells (ECs) directly. TNTLs transported cytoplasmic proteins, transduced signals between CMs and ECs, and initiated myocardial growth toward the heart lumen to form ventricular trabeculae-like structures. Loss of TNTLs disturbed signaling interactions and, subsequently, ventricular patterning.

Tunneling nanotubes enable intercellular transfer in zebrafish embryos

Tunneling nanotubes (TNTs) are thin intercellular connections that facilitate the transport of diverse cargoes, ranging from ions to organelles. While TNT studies have predominantly been conducted in cell cultures, the existence of open-ended TNTs within live organisms remains unverified. Despite the observation of intercellular connections during embryonic development across various species, their functional role in facilitating material transfer between connected cells has not been confirmed. In this study, we performed mosaic labeling of gastrula cells in zebrafish embryos to demonstrate the coexistence of TNT-like structures alongside other cellular protrusions. These embryonic TNT-like connections exhibited a morphology similar to that of TNTs described in cell culture, appeared to have similar formation mechanisms, and could be induced by Eps8 overexpression and CK666 treatment. Most notably, we demonstrated their capability to transfer both soluble cargoes and organelles, thus confirming their open-endedness. This study demonstrates the existence of functional, open-ended TNTs in a living embryo.

Balancing limited resources in actin network competition

In cells, multiple actin networks coexist in a dynamic manner. These networks compete for a common pool of actin monomers and actin-binding proteins. Interestingly, all of these networks manage to coexist despite the strong competition for resources. Moreover, the coexistence of networks with various strengths is key to cell adaptation to external changes. However, a comprehensive view of how these networks coexist in this competitive environment, where resources are limited, is still lacking. To address this question, we used a reconstituted system, in closed microwells, consisting of beads propelled by actin polymerization or micropatterns functionalized with lipids capable of initiating polymerization close to a membrane. This system enabled us to build dynamic actin architectures, competing for a limited pool of proteins, over a period of hours. We demonstrated the importance of protein turnover for the coexistence of actin networks, showing that it ensures resource distribution between weak and strong networks. However, when competition becomes too intense, turnover alone is insufficient, leading to a selection process that favors the strongest networks. Consequently, we emphasize the importance of competition strength, which is defined by the turnover rate, the amount of available protein, and the number of competing structures. More generally, this work illustrates how turnover allows biological populations with various competition strengths to coexist despite resource constraints.

CK-666 exerts anticancer effects by regulating autophagy, tunneling nanotubes and extracellular vesicles formation

CK-666, an inhibitor of the actin-related protein complex 2/3 (Arp2/3), can suppress lamellipodia formation and cell migration. However, research on its application in tumor therapy is still limited. Using RNA-seq, we clustered and analyzed the functions of differentially expressed mRNAs in CK-666-treated tumor cells. Interestingly, the differentially expressed genes related to CK-666 were closely associated with exosomes and autophagy. Through Western blot, we confirmed that CK-666 promoted the high expression of exosome and autophagy markers in tumor cells. Transmission electron microscopy results indicated the appearance of extracellular vesicles larger than exosomes. Scanning electron microscopy findings revealed that CK-666 inhibited the formation of intercellular tunneling nanotubes (TNTs). Fluorescent staining further revealed that CK-666 induced the formation and secretion of CD63-positive vesicles within the tunnels of retraction fibers (RFs). In vitro experiments verified that CK-666 preferentially inhibited fibroblasts in 3D tumorspheres. In the tumor 3D-Histoculture Drug Response Assay (3D-HDRA), it was found that CK-666 could suppress the activity of isolated tumor tissues. Moreover, our study discovered that the combination of CK-666 and docetaxel (DTX) significantly enhanced DTX sensitivity. In summary, our results suggest that CK-666 may play an oncogenic role by regulating autophagy, TNTs, and extracellular vesicles formation.

Intercellular nanotube-mediated mitochondrial transfer enhances T cell metabolic fitness and antitumor efficacy

Mitochondrial loss and dysfunction drive T cell exhaustion, representing major barriers to successful T cell-based immunotherapies. Here, we describe an innovative platform to supply exogenous mitochondria to T cells, overcoming these limitations. We found that bone marrow stromal cells establish nanotubular connections with T cells and leverage these intercellular highways to transplant stromal cell mitochondria into CD8+ T cells. Optimal mitochondrial transfer required Talin 2 on both donor and recipient cells. CD8+ T cells with donated mitochondria displayed enhanced mitochondrial respiration and spare respiratory capacity. When transferred into tumor-bearing hosts, these supercharged T cells expanded more robustly, infiltrated the tumor more efficiently, and exhibited fewer signs of exhaustion compared with T cells that did not take up mitochondria. As a result, mitochondria-boosted CD8+ T cells mediated superior antitumor responses, prolonging animal survival. These findings establish intercellular mitochondrial transfer as a prototype of organelle medicine, opening avenues to next-generation cell therapies.

Moesin controls cell-cell fusion and osteoclast function

Cell-cell fusion is an evolutionarily conserved process that is essential for many functions, including fertilisation and the formation of placenta, muscle and osteoclasts, multinucleated cells that are unique in their ability to resorb bone. The mechanisms of osteoclast multinucleation involve dynamic interactions between the actin cytoskeleton and the plasma membrane that are still poorly characterized. Here, we found that moesin, a cytoskeletal linker protein member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role in both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin inhibition favors their ability to fuse into multinucleated osteoclasts. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances the formation of tunneling nanotubes (TNTs), F-actin-based intercellular bridges that we reveal here to trigger cell-cell fusion. Moesin also controls HIV-1- and inflammation-induced cell fusion. In addition, moesin regulates the formation of the sealing zone, the adhesive structure determining osteoclast bone resorption area, and thus controls bone degradation, via a β3-integrin/RhoA/SLK pathway. Supporting our results, moesin - deficient mice present a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of cell-cell fusion and osteoclast biology, opening new opportunities to specifically target osteoclast activity in bone disease therapy.

Astroglia proliferate upon the biogenesis of tunneling nanotubes via α-synuclein dependent transient nuclear translocation of focal adhesion kinase

Astroglia play crucial neuroprotective roles by internalizing pathogenic aggregates and facilitating their degradation. Here, we show that α-SYN protofibril-induced organelle toxicities and reactive oxygen species (ROS) cause premature cellular senescence in astrocytes and astrocyte-derived cancer cells, resulting in a transient increase in the biogenesis of tunneling nanotubes (TNTs). TNT-biogenesis and TNT-mediated cell-to-cell transfer lead to clearance of α-SYN-induced organelle toxicities, reduction in cellular ROS levels, and reversal of cellular senescence. Enhanced cell proliferation is seen in the post-recovered cells after recovering from α-SYN-induced organelle toxicities. Further, we show that α-SYN-induced senescence promotes the transient localization of focal adhesion kinase (FAK) in the nucleus. FAK-mediated regulation of Rho-associated kinases plays a significant role in the biogenesis of TNTs and their subsequent proliferation. Our study emphasizes that TNT biogenesis has a potential role in the clearance of α-SYN-induced cellular toxicities, the consequences of which cause enhanced proliferation in the post-recovered astroglia cells.

The cellular protrusions for inter-cellular material transfer: similarities between filopodia, cytonemes, tunneling nanotubes, viruses, and extracellular vesicles

Extracellular vesicles (EVs) are crucial for transferring bioactive materials between cells and play vital roles in both health and diseases. Cellular protrusions, including filopodia and microvilli, are generated by the bending of the plasma membrane and are considered to be rigid structures facilitating various cellular functions, such as cell migration, adhesion, and environment sensing. Compelling evidence suggests that these protrusions are dynamic and flexible structures that can serve as sources of a new class of EVs, highlighting the unique role they play in intercellular material transfer. Cytonemes are specialized filopodia protrusions that make direct contact with neighboring cells, mediating the transfer of bioactive materials between cells through their tips. In some cases, these tips fuse with the plasma membrane of neighboring cells, creating tunneling nanotubes that directly connect the cytosols of the adjacent cells. Additionally, virus particles can be released from infected cells through small bud-like of plasma membrane protrusions. These different types of protrusions, which can transfer bioactive materials, share common protein components, including I-BAR domain-containing proteins, actin cytoskeleton, and their regulatory proteins. The dynamic and flexible nature of these protrusions highlights their importance in cellular communication and material transfer within the body, including development, cancer progression, and other diseases.

Tunneling nanotubes and tumor microtubes-Emerging data on their roles in intercellular communication and pathophysiology: Summary of an International FASEB Catalyst Conference October 2023

In the past decade, there has been a steady rise in interest in studying novel cellular extensions and their potential roles in facilitating human diseases, including neurologic diseases, viral infectious diseases, cancer, and others. One of the exciting new aspects of this field is improved characterization and understanding of the functions and potential mechanisms of tunneling nanotubes (TNTs), which are actin-based filamentous protrusions that are structurally distinct from filopodia. TNTs form and connect cells at long distance and serve as direct conduits for intercellular communication in a wide range of cell types in vitro and in vivo. More researchers are entering this field and investigating the role of TNTs in mediating cancer cell invasion and drug resistance, cellular transfer of proteins, RNA or organelles, and intercellular spread of infectious agents, such as viruses, bacteria, and prions. Even further, the elucidation of highly functional membrane tubes called "tumor microtubes" (TMs) in incurable gliomas has further paved a new path for understanding how and why the tumor type is highly invasive at the cellular level and also resistant to standard therapies. Due to the wide-ranging and rapidly growing applicability of TNTs and TMs in pathophysiology across the spectrum of biology, it has become vital to bring researchers in the field together to discuss advances and the future of research in this important niche of protrusion biology.

Tunneling Nanotubes in Myeloid Cells: Perspectives for Health and Infectious Diseases

Tunneling nanotubes (TNTs) are cellular connections, which represent a novel route for cell-to-cell communication. Strong evidence points to a role for TNTs in the intercellular transfer of signals, molecules, organelles, and pathogens, involving them in many cellular functions. In myeloid cells (e.g., monocytes/macrophages, dendritic cells, and osteoclasts), intercellular communication via TNT contributes to their differentiation and immune functions, by favoring material and pathogen transfer, as well as cell fusion. This chapter addresses the complexity of the definition and characterization of TNTs in myeloid cells, the different processes involved in their formation, their existence in vivo, and finally their function(s) in health and infectious diseases, with the example of HIV-1 infection.

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.

Tunneling Nanotubes in the Brain

Tunneling nanotubes (TNTs) have emerged as intriguing structures facilitating intercellular communications across diverse cell types, which are integral to several biological processes, as well as participating in various disease progression. This review provides an in-depth analysis of TNTs, elucidating their structural characteristics and functional roles, with a particular focus on their significance within the brain environment and their implications in neurological and neurodegenerative disorders. We explore the interplay between TNTs and neurological diseases, offering potential mechanistic insights into disease progression, while also highlighting their potential as viable therapeutic targets. Additionally, we address the significant challenges associated with studying TNTs, from technical limitations to their investigation in complex biological systems. By addressing some of these challenges, this review aims to pave the way for further exploration into TNTs, establishing them as a central focus in advancing our understanding of neurodegenerative disorders.

Identification and Characterization of Tunneling Nanotubes for Intercellular Trafficking

Tunneling nanotubes (TNTs) are thin membranous channels providing a direct cytoplasmic connection between remote cells. They are commonly observed in different cell cultures and increasing evidence supports their role in intercellular communication, and pathogen and amyloid protein transfer. However, the study of TNTs presents several pitfalls (e.g., difficulty in preserving such delicate structures, possible confusion with other protrusions, structural and functional heterogeneity, etc.) and therefore requires thoroughly designed approaches. The methods described in this protocol represent a guideline for the characterization of TNTs (or TNT-like structures) in cell culture. Specifically, optimized protocols to (1) identify TNTs and the cytoskeletal elements present inside them; (2) evaluate TNT frequency in cell culture; (3) unambiguously distinguish them from other cellular connections or protrusions; (4) monitor their formation in living cells; (5) characterize TNTs by a micropatterning approach; and (6) investigate TNT ultrastructure by cryo-EM are provided. Finally, this article describes how to assess TNT-mediated cell-to-cell transfer of cellular components, which is a fundamental criterion for identifying functional TNTs. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Identification of tunneling nanotubes Alternate Protocol 1: Identifying the cytoskeletal elements present in tunneling nanotubes Alternate Protocol 2: Distinguishing tunneling nanotubes from intercellular bridges formed during cell division Basic Protocol 2: Deciphering tunneling nanotube formation and lifetime by live fluorescent microscopy Alternate Protocol 3: Deciphering tunneling nanotube formation using a live-compatible dye Basic Protocol 3: Assessing tunneling nanotubes functionality in intercellular transfer Alternate Protocol 4: Flow cytometry approach to quantify the rate of vesicle or mitochondria transfer Support Protocol: Controls to support TNT-mediated transfer Basic Protocol 4: Studies of tunneling nanotubes by cell micropatterning Basic Protocol 5: Characterization of the ultrastructure of tunneling nanotubes by cryo-EM.