Opportunities and Challenges in Tunneling Nanotubes Research: How Far from Clinical Application?

Targeting specific brain districts for advanced nanotherapies: A review from the perspective of precision nanomedicine

Numerous studies are focused on nanoparticle penetration into the brain functionalizing them with ligands useful to cross the blood-brain barrier. However, cell targeting is also crucial, given that cerebral pathologies frequently affect specific brain cells or areas. Functionalize nanoparticles with the most appropriate targeting elements, tailor their physical parameters, and consider the brain's complex anatomy are essential aspects for precise therapy and diagnosis. In this review, we addressed the state of the art on targeted nanoparticles for drug delivery in diseased brain regions, outlining progress, limitations, and ongoing challenges. We also provide a summary and overview of general design principles that can be applied to nanotherapies, considering the areas and cell types affected by the most common brain disorders. We then emphasize lingering uncertainties that hinder the translational possibilities of nanotherapies for clinical use. Finally, we offer suggestions for continuing preclinical investigations to enhance the overall effectiveness of precision nanomedicine in addressing neurological conditions. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease Therapeutic Approaches and Drug Discovery > Emerging Technologies.

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.

Breast Cancers That Disseminate to Bone Marrow Acquire Aggressive Phenotypes through CX43-related Tumor-Stroma Tunnels

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

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

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

Tunneling Nanotubes 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.

The direct transfer approach for transcellular drug delivery

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

Targeting chemoresistance and mitochondria-dependent metabolic reprogramming in acute myeloid leukemia

Chemoresistance often complicates the management of cancer, as noted in the instance of acute myeloid leukemia (AML). Mitochondrial function is considered important for the viability of AML blasts and appears to also modulate chemoresistance. As mitochondrial metabolism is aberrant in AML, any distinct pathways could be directly targeted to impact both cell viability and chemoresistance. Therefore, identifying and targeting those precise rogue elements of mitochondrial metabolism could be a valid therapeutic strategy in leukemia. Here, we review the evidence for abnormalities in mitochondria metabolic processes in AML cells, that likely impact chemoresistance. We further address several therapeutic approaches targeting isocitrate dehydrogenase 2 (IDH2), CD39, nicotinamide phosphoribosyl transferase (NAMPT), electron transport chain (ETC) complex in AML and also consider the roles of mesenchymal stromal cells. We propose the term "mitotherapy" to collectively refer to such regimens that attempt to override mitochondria-mediated metabolic reprogramming, as used by cancer cells. Mounting evidence suggests that mitotherapy could provide a complementary strategy to overcome chemoresistance in liquid cancers, as well as in solid tumors.

PROM1-mediated cell signal transduction in cancer stem cells and hepatocytes

Prominin-1 (PROM1), also called CD133, is a penta-span transmembrane protein that is localized in membrane protrusions, such as microvilli and filopodia. It is known to be expressed in cancer stem cells and various progenitor cells of bone marrow, liver, kidney, and intestine. Accumulating evidence has revealed that PROM1 has multiple functions in various organs, such as eye, tooth, peripheral nerve, and liver, associating with various molecular protein partners. PROM1 regulates PKA-induced gluconeogenesis, TGFβ-induced fibrosis, and IL-6-induced regeneration in the liver, associating with Radixin, SMAD7, and GP130, respectively. In addition, PROM1 is necessary to maintain cancer stem cell properties by activating PI3K and β-Catenin. PROM1-deficienct mice also show distinct phenotypes in eyes, brain, peripheral nerves, and tooth. Here, we discuss recent findings of PROM1-mediated signal transduction. [BMB Reports 2023; 56(2): 65-70].

Morphological Characteristics of Young and Old Murine Hematopoietic Stem Cell Niches, as Modeled In Vitro

The hematopoietic stem cell (HSC) niche undergoes detrimental changes with age. The molecular differences between young and old niches are well studied and understood; however, young and old niches have not yet been extensively characterized in terms of morphology. In the present work, a 2D stromal model of young and old HSC niches isolated from bone marrow was investigated using light and scanning electron microscopy (SEM) to characterize cell density after one, two, or three weeks of culturing, cell shape, and cell surface morphological features. Our work is aimed at identifying morphological differences between young and old niche cells that could be used to discriminate between their respective murine HSC niches. The results show several age-specific morphological characteristics. The old niches differ from the young ones in terms of lower cell proliferating capacity, increased cell size with a flattened appearance, increased number of adipocytes, and the presence of tunneling nanotubes. In addition, proliferating cell clusters are present in the young niches but not in the old niches. Together, these characteristics could be used as a relatively simple and reliable tool to discriminate between young and old murine HSC niches and as a complementary approach to imaging with specific cellular markers.

Role of Tunneling Nanotubes in the Nervous System

Cellular communication and the transfer of information from one cell to another is crucial for cell viability and homeostasis. During the last decade, tunneling nanotubes (TNTs) have attracted scientific attention, not only as a means of direct intercellular communication, but also as a possible system to transport biological cargo between distant cells. Peculiar TNT characteristics make them both able to increase cellular survival capacities, as well as a potential target of neurodegenerative disease progression. Despite TNT formation having been documented in a number of cell types, the exact mechanisms triggering their formation are still not completely known. In this review, we will summarize and highlight those studies focusing on TNT formation in the nervous system, as well as their role in neurodegenerative diseases. Moreover, we aim to stress some possible mechanisms and important proteins probably involved in TNT formation in the nervous system.

Spontaneous formation and growth kinetics of lipid nanotubules induced by passive nanoparticles

Lipid nanotubules (LNTs) are conduits that form on the membranes of cells and organelles, and they are ubiquitous in all forms of life from archaea and bacteria to plants and mammals. The formation, shape and dynamics of these LNTs are critical for cellular functions, supporting the transport of myriad cellular cargoes as well as communication within and between cells, and they are also widely believed to be responsible for exploitation of host cells by pathogens for the spread of infection and diseases. In vitro kinetic control of LNT formation can considerably enhance the scope of utilization of these structures for disease control and therapy. Here we report a new paradigm for spontaneous lipid nanotubulation, capturing the dynamical regimes of growth, stabilization and retraction of the tubes through the binding of synthetic nanoparticles on supported lipid bilayers (SLBs). The tubulation is determined by the spontaneous binding-unbinding of nanoparticles on the LNTs. The presented methodology could be used to rectify malfunctioning cellular tubules or to prevent the pathogenic spread of diseases through inhibition of cell-to-cell nanotubule formation.

Mechanical properties of tunneling nanotube and its mechanical stability in human embryonic kidney cells

Tunneling nanotubes (TNTs) are thin membrane tubular structures that interconnect physically separated cells. Growing evidence indicates that TNTs play unique roles in various diseases by facilitating intercellular transfer of signaling and organelles, suggesting TNTs as a potential target for disease treatment. The efficiency of TNT-dependent communication is largely determined by the number of TNTs between cells. Though TNTs are physically fragile structures, the mechanical properties of TNTs and the determinants of their mechanical stability are still unclear. Here, using atomic force microscope (AFM) and microfluidic techniques, we investigated the mechanical behavior and abundance of TNTs in human embryonic kidney (HEK293) cells upon the application of forces. AFM measurements demonstrate that TNTs are elastic structures with an apparent spring constant of 79.1 ± 16.2 pN/μm. The stiffness and membrane tension of TNTs increase by length. TNTs that elongate slower than 0.5 μm/min display higher mechanical stability, due to the growth rate of F-actin inside TNTs being limited at 0.26 μm/min. Importantly, by disturbing the cytoskeleton, membrane, or adhesion proteins of TNTs, we found that F-actin and cadherin connection dominantly determines the tensile strength and flexural strength of TNTs respectively. It may provide new clues for screening TNT-interfering drugs that alter the stability of TNTs.

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.

An overview on tumor treating fields (TTFields) technology as a new potential subsidiary biophysical treatment for COVID-19

COVID-19 pandemic situation has affected millions of people with tens of thousands of deaths worldwide. Despite all efforts for finding drugs or vaccines, the key role for the survival of patients is still related to the immune system. Therefore, improving the efficacy and the functionality of the immune system of COVID-19 patients is very crucial. The potential new, non-invasive, FDA-approved biophysical technology that could be considered in this regard is tumor treating fields (TTFields) based on an alternating electric field has great biological effects. TTFields have significant effects in improving the functionality of dendritic cell, and cytotoxic T-cells, and these cells have a major role in defense against viral infection. Hence, applying TTFields could help COVID-19 patients against infection. Additionally, TTFields can reduce viral genomic replication, by reducing the expressions of some of the vital members of DNA replication complex genes from the minichromosome maintenance family (MCMs). These genes not only are involved in DNA replication but it has also been proven that they have a crucial role in viral replication. Also, TTFields suppress the formation of the network of tunneling nanotubes (TNTs) which is knows as filamentous (F)-actin-rich tubular structures. TNTs have a critical role in promoting the spread of viruses through improving viral entry and acting as a protective agent for viral components from immune cells and even pharmaceuticals. Moreover, TTFields enhance autophagy which leads to apoptosis of virally infected cells. Thus, it can be speculated that using TTFields may prove to be a promising approach as a subsidiary treatment of COVID-19.

The 3.0 Cell Communication: New Insights in the Usefulness of Tunneling Nanotubes for Glioblastoma Treatment

Simple Summary

Communication between cells helps tumors acquire resistance to chemotherapy and makes the struggle against cancer more challenging. Tunneling nanotubes (TNTs) are long channels able to connect both nearby and distant cells, contributing to a more malignant phenotype. This finding might be useful in designing novel strategies of drug delivery exploiting these systems of connection. This would be particularly important to reach tumor niches, where glioblastoma stem cells proliferate and provoke immune escape, thereby increasing metastatic potential and tumor recurrence a few months after surgical resection of the primary mass. Along with the direct inhibition of TNT formation, TNT analysis, and targeting strategies might be useful in providing innovative tools for the treatment of this tumor.

Abstract

Glioblastoma (GBM) is a particularly challenging brain tumor characterized by a heterogeneous, complex, and multicellular microenvironment, which represents a strategic network for treatment escape. Furthermore, the presence of GBM stem cells (GSCs) seems to contribute to GBM recurrence after surgery, and chemo- and/or radiotherapy. In this context, intercellular communication modalities play key roles in driving GBM therapy resistance. The presence of tunneling nanotubes (TNTs), long membranous open-ended channels connecting distant cells, has been observed in several types of cancer, where they emerge to steer a more malignant phenotype. Here, we discuss the current knowledge about the formation of TNTs between different cellular types in the GBM microenvironment and their potential role in tumor progression and recurrence. Particularly, we highlight two prospective strategies targeting TNTs as possible therapeutics: (i) the inhibition of TNT formation and (ii) a boost in drug delivery between cells through these channels. The latter may require future studies to design drug delivery systems that are exchangeable through TNTs, thus allowing for access to distant tumor niches that are involved in tumor immune escape, maintenance of GSC plasticity, and increases in metastatic potential.

Secretome and Tunneling Nanotubes: A Multilevel Network for Long Range Intercellular Communication between Endothelial Cells and Distant Cells

As a cellular interface between the blood and tissues, the endothelial cell (EC) monolayer is involved in the control of key functions including vascular tone, permeability and homeostasis, leucocyte trafficking and hemostasis. EC regulatory functions require long-distance communications between ECs, circulating hematopoietic cells and other vascular cells for efficient adjusting thrombosis, angiogenesis, inflammation, infection and immunity. This intercellular crosstalk operates through the extracellular space and is orchestrated in part by the secretory pathway and the exocytosis of Weibel Palade Bodies (WPBs), secretory granules and extracellular vesicles (EVs). WPBs and secretory granules allow both immediate release and regulated exocytosis of messengers such as cytokines, chemokines, extracellular membrane proteins, coagulation or growth factors. The ectodomain shedding of transmembrane protein further provide the release of both receptor and ligands with key regulatory activities on target cells. Thin tubular membranous channels termed tunneling nanotubes (TNTs) may also connect EC with distant cells. EVs, in particular exosomes, and TNTs may contain and transfer different biomolecules (e.g., signaling mediators, proteins, lipids, and microRNAs) or pathogens and have emerged as a major triggers of horizontal intercellular transfer of information.