Tunneling nanotubes: A novel pharmacological target for neurodegenerative diseases?

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.

Structural and material basis of neuron-glioma interactions

The intricate interplay between neurons and gliomas has emerged as an important area of investigation in glioma biology. Accumulating evidence underscores that structural and material alterations constitute the fundamental basis of neuron‒glioma interactions and their pathological consequences. This review comprehensively examines the mechanisms underlying these interactions, with a particular emphasis on specialized structures that facilitate neuron‒glioma communication, including synapses, cell surface ion channels, and tumor microtubules (TMs). In addition to classical neurotransmitters, we highlight the exchange of cytokines, proteins, and extracellular vesicles (EVs) between these cell types. By synthesizing current research findings, this review establishes a conceptual framework for developing innovative therapeutic strategies targeting neuron‒glioma interfaces, offering new perspectives for glioma treatment approaches.

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

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

Tunneling Nanotube: An Enticing Cell-Cell Communication in the Nervous System

The field of neuroscience is rapidly progressing, continuously uncovering new insights and discoveries. Among the areas that have shown immense potential in research, tunneling nanotubes (TNTs) have emerged as a promising subject of study. These minute structures act as conduits for the transfer of cellular materials between cells, representing a mechanism of communication that holds great significance. In particular, the interplay facilitated by TNTs among various cell types within the brain, including neurons, astrocytes, oligodendrocytes, glial cells, and microglia, can be essential for the normal development and optimal functioning of this complex organ. The involvement of TNTs in neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease, and Parkinson's disease, has attracted significant attention. These disorders are characterized by the progressive degeneration of neurons and the subsequent decline in brain function. Studies have predicted that TNTs likely play critical roles in the propagation and spread of pathological factors, contributing to the advancement of these diseases. Thus, there is a growing interest in understanding the precise functions and mechanisms of TNTs within the nervous system. This review article, based on our recent work on Rhes-mediated TNTs, aims to explore the functions of TNTs within the brain and investigate their implications for neurodegenerative diseases. Using the knowledge gained from studying TNTs could offer novel opportunities for designing targeted treatments that can stop the progression of neurodegenerative disorders.

Revealing the structure and organization of intercellular tunneling nanotubes (TNTs) by STORM imaging

Tunneling nanotubes (TNTs) are nanoscale, actin-rich, transient intercellular tubes for cell-to-cell communication, which transport various cargoes between distant cells. The structural complexity and spatial organization of the involved components of TNTs remain unknown. In this work, the STORM super-resolution imaging technique was applied to elucidate the structural organization of microfilaments and microtubules in intercellular TNTs at the nanometer scale. Our results reveal different distributions of microfilaments and intertwined structures of microtubules in TNTs, which promote the knowledge of TNT communications.

SARS-CoV-2: A Master of Immune Evasion

Viruses and their hosts have coevolved for a long time. This coevolution places both the pathogen and the human immune system under selective pressure; on the one hand, the immune system has evolved to combat viruses and virally infected cells, while viruses have developed sophisticated mechanisms to escape recognition and destruction by the immune system. SARS-CoV-2, the pathogen that is causing the current COVID-19 pandemic, has shown a remarkable ability to escape antibody neutralization, putting vaccine efficacy at risk. One of the virus's immune evasion strategies is mitochondrial sabotage: by causing reactive oxygen species (ROS) production, mitochondrial physiology is impaired, and the interferon antiviral response is suppressed. Seminal studies have identified an intra-cytoplasmatic pathway for viral infection, which occurs through the construction of tunneling nanotubes (TNTs), hence enhancing infection and avoiding immune surveillance. Another method of evading immune monitoring is the disruption of the antigen presentation. In this scenario, SARS-CoV-2 infection reduces MHC-I molecule expression: SARS-CoV-2's open reading frames (ORF 6 and ORF 8) produce viral proteins that specifically downregulate MHC-I molecules. All of these strategies are also exploited by other viruses to elude immune detection and should be studied in depth to improve the effectiveness of future antiviral treatments. Compared to the Wuhan strain or the Delta variant, Omicron has developed mutations that have impaired its ability to generate syncytia, thus reducing its pathogenicity. Conversely, other mutations have allowed it to escape antibody neutralization and preventing cellular immune recognition, making it the most contagious and evasive variant to date.

Tunneling Nanotube-Mediated Communication: A Mechanism of Intercellular Nucleic Acid Transfer

Tunneling nanotubes (TNTs) are thin, F-actin-based membranous protrusions that connect distant cells and can provide e a novel mechanism for intercellular communication. By establishing cytoplasmic continuity between interconnected cells, TNTs enable the bidirectional transfer of nuclear and cytoplasmic cargo, including organelles, nucleic acids, drugs, and pathogenic molecules. TNT-mediated nucleic acid transfer provides a unique opportunity for donor cells to directly alter the genome, transcriptome, and metabolome of recipient cells. TNTs have been reported to transport DNA, mitochondrial DNA, mRNA, viral RNA, and non-coding RNAs, such as miRNA and siRNA. This mechanism of transfer is observed in physiological as well as pathological conditions, and has been implicated in the progression of disease. Herein, we provide a concise overview of TNTs' structure, mechanisms of biogenesis, and the functional effects of TNT-mediated intercellular transfer of nucleic acid cargo. Furthermore, we highlight the potential translational applications of TNT-mediated nucleic acid transfer in cancer, immunity, and neurological diseases.

Tunneling Nanotubes: A New Target for Nanomedicine?

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