Inception Mechanisms of Tunneling Nanotubes

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

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

Mechanical properties of intercellular tunneling nanotubes formed by different mechanisms

Tunneling nanotubes (TNTs) that connect cells have been recognized as a pathway for long-range intercellular transport of diverse cargoes, including viruses, lysosomes or other organelles, Ca2+ and electrical signals. TNTs can initially be formed from thin finger-like actin assembly-driven protrusions or cell contacts and dislodgment. However, it remains unclear whether the mechanical properties of TNTs formed by these two mechanisms are the same. Here, we developed novel microoperation methods to investigate the mechanical properties of TNTs in HEK293 cells, in which the TNTs form from thin finger-like actin assembly-driven protrusions and C2C12 cells, in which the TNTs form through contact and cell dislodgment. We found that TNTs formed by the two mechanisms represent elastic elements with similar tensile strength. In both the HEK and C2C12 cells, the tensile strength of TNTs exhibited a distinct size dependence on their lengths and diameters. Disturbing the cytoskeleton or removing extracellular Ca2+ also changed their tensile strength. In addition, the stiffening of the extracellular matrix (ECM) enhanced the length, diameter and tensile strength of TNTs both in both HEK and C2C12 cells. Finally, a theoretical model was established to reveal the changes in the TNT's mechanical properties with its length, diameter and individual tunneling nanotubes (iTNT) number. This work not only gains insights into the properties of TNTs but also helps understand the dynamics of various cells.

ROCK inhibitor enhances mitochondrial transfer via tunneling nanotubes in retinal pigment epithelium

Rationale: Tunnel nanotube (TNT)-mediated mitochondrial transport is crucial for the development and maintenance of multicellular organisms. Despite numerous studies highlighting the significance of this process in both physiological and pathological contexts, knowledge of the underlying mechanisms is still limited. This research focused on the role of the ROCK inhibitor Y-27632 in modulating TNT formation and mitochondrial transport in retinal pigment epithelial (RPE) cells. Methods: Two types of ARPE19 cells (a retinal pigment epithelial cell line) with distinct mitochondrial fluorescently labeled, were co-cultured and treated with ROCK inhibitor Y-27632. The formation of nanotubes and transport of mitochondria were assessed through cytoskeletal staining and live cell imaging. Mitochondrial dysfunction was induced by light damage to establish a model, while mitochondrial function was evaluated through measurement of oxygen consumption rate. The effects of Y-27632 on cytoskeletal and mitochondrial dynamics were further elucidated through detailed analysis. Results: Y-27632 treatment led to an increase in nanotube formation and enhanced mitochondrial transfer among ARPE19 cells, even following exposure to light-induced damage. Our analysis of cytoskeletal and mitochondrial distribution changes suggests that Y-27632 promotes nanotube-mediated mitochondrial transport by influencing cytoskeletal remodeling and mitochondrial movement. Conclusions: These results suggest that Y-27632 has the ability to enhance mitochondrial transfer via tunneling nanotubes in retinal pigment epithelium, and similarly predict that ROCK inhibitor can fulfill its therapeutic potential through promoting mitochondrial transport in the retinal pigment epithelium in the future.

Intercellular Mitochondrial Transfer: The Novel Therapeutic Mechanism for Diseases

Mitochondria, the dynamic organelles responsible for energy production and cellular metabolism, have the metabolic function of extracting energy from nutrients and synthesizing crucial metabolites. Nevertheless, recent research unveils that intercellular mitochondrial transfer by tunneling nanotubes, tumor microtubes, gap junction intercellular communication, extracellular vesicles, endocytosis and cell fusion may regulate mitochondrial function within recipient cells, potentially contributing to disease treatment, such as nonalcoholic steatohepatitis, glioblastoma, ischemic stroke, bladder cancer and neurodegenerative diseases. This review introduces the principal approaches to intercellular mitochondrial transfer and examines its role in various diseases. Furthermore, we provide a comprehensive overview of the inhibitors and activators of intercellular mitochondrial transfer, offering a unique perspective to illustrate the relationship between intercellular mitochondrial transfer and diseases.

Transfer of cardiomyocyte-derived extracellular vesicles to neighboring cardiac cells requires tunneling nanotubes during heart development

Rationale: Extracellular vesicles (EVs) are thought to mediate intercellular communication during development and disease. Yet, biological insight to intercellular EV transfer remains elusive, also in the heart, and is technically challenging to demonstrate. Here, we aimed to investigate biological transfer of cardiomyocyte-derived EVs in the neonatal heart. Methods: We exploited CD9 as a marker of EVs, and generated two lines of cardiomyocyte specific EV reporter mice: Tnnt2-Cre; double-floxed inverted CD9/EGFP and αMHC-MerCreMer; double-floxed inverted CD9/EGFP. The two mouse lines were utilized to determine whether developing cardiomyocytes transfer EVs to other cardiac cells (non-myocytes and cardiomyocytes) in vitro and in vivo and investigate the intercellular transport pathway of cardiomyocyte-derived EVs. Results: Genetic tagging of cardiomyocytes was confirmed in both reporter mouse lines and proof of concept in the postnatal heart showed that, a fraction of EGFP+/MYH1- non-myocytes exist firmly demonstrating in vivo cardiomyocyte-derived EV transfer. However, two sets of direct and indirect EGFP +/- cardiac cell co-cultures showed that cardiomyocyte-derived EGFP+ EV transfer requires cell-cell contact and that uptake of EGFP+ EVs from the medium is limited. The same was observed when co-cultiring with mouse macrophages. Further mechanistic insight showed that cardiomyocyte EV transfer occurs through type I tunneling nanotubes. Conclusion: While the current notion assumes that EVs are transferred through secretion to the surroundings, our data show that cardiomyocyte-derived EV transfer in the developing heart occurs through nanotubes between neighboring cells. Whether these data are fundamental and relate to adult hearts and other organs remains to be determined, but they imply that the normal developmental process of EV transfer goes through cell-cell contact rather than through the extracellular compartment.

Stability of a biomembrane tube covered with proteins

Membrane tubes are essential structural features in cells that facilitate biomaterial transport and inter- and intracellular signaling. The shape of these tubes can be regulated by the proteins that surround and adhere to them. We study the stability of a biomembrane tube coated with proteins by combining linear stability analysis, out-of-equilibrium hydrodynamic calculations, and numerical solutions of a Helfrich-like membrane model. Our analysis demonstrates that both long- and short-wavelength perturbations can destabilize the tubes. Numerical simulations confirm the derived linear stability criteria and yield the nonlinearly perturbed vesicle shapes. Our study highlights the interplay between membrane shape and protein density, where the shape instability concurs with a redistribution of proteins into a banded pattern.

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.

TFAM-mediated intercellular lipid droplet transfer promotes cadmium-induced mice nonalcoholic fatty liver disease

Cadmium (Cd) is an important environmental pollutant. Herein, we discovered a new way of lipid accumulation, where lipid droplets can be transferred across cells. In this study, mice and AML12 cells were used to establish models of Cd poisoning. After Cd treatment, the level of TFAM was reduced, thereby regulating the reconstitution of the cytosolic actin filament network. MYH9 is a myosin involved in cell polarization, migration, and movement of helper organelles. Rab18 is a member of the Rab GTPase family, which localizes to lipid droplets and regulates lipid drop dynamics. In this study, we found that Cd increases the interaction between MYH9 and Rab18. However, TFAM overexpression alleviated the increase in Cd-induced interaction between MYH9 and Rab18, thereby reducing the transfer of intercellular lipid droplets and the accumulation of intracellular lipids. Through a co-culture system, we found that the transferred lipid droplets can act as a signal to form an inflammatory storm-like effect, and ACSL4 can act as an effector to transfer lipid droplets and promote lipid accumulation in surrounding cells. These results suggest that TFAM can be used as a new therapeutic target for Cd-induced lipid accumulation in the liver.

Rescue of mitochondrial import failure by intercellular organellar transfer

Mitochondria are the powerhouses of eukaryotic cells, composed mostly of nuclear-encoded proteins imported from the cytosol. Thus, problems with the import machinery will disrupt their regenerative capacity and the cell's energy supplies - particularly troublesome for energy-demanding cells of nervous tissue and muscle. Unsurprisingly then, import breakdown is implicated in disease. Here, we explore the consequences of import failure in mammalian cells; wherein, blocking the import machinery impacts mitochondrial ultra-structure and dynamics, but, surprisingly, does not affect import. Our data are consistent with a response involving intercellular mitochondrial transport via tunnelling nanotubes to import healthy mitochondria and jettison those with blocked import sites. These observations support the existence of a widespread mechanism for the rescue of mitochondrial dysfunction.

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.

Fine-Tuning the Nanostructured Titanium Oxide Surface for Selective Biological Response

Cardiovascular diseases are a pre-eminent global cause of mortality in the modern world. Typically, surgical intervention with implantable medical devices such as cardiovascular stents is deployed to reinstate unobstructed blood flow. Unfortunately, existing stent materials frequently induce restenosis and thrombosis, necessitating the development of superior biomaterials. These biomaterials should inhibit platelet adhesion (mitigating stent-induced thrombosis) and smooth muscle cell proliferation (minimizing restenosis) while enhancing endothelial cell proliferation at the same time. To optimize the surface properties of Ti6Al4V medical implants, we investigated two surface treatment procedures: gaseous plasma treatment and hydrothermal treatment. We analyzed these modified surfaces through scanning electron microscopy (SEM), water contact angle analysis (WCA), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analysis. Additionally, we assessed in vitro biological responses, including platelet adhesion and activation, as well as endothelial and smooth muscle cell proliferation. Herein, we report the influence of pre/post oxygen plasma treatment on titanium oxide layer formation via a hydrothermal technique. Our results indicate that alterations in the titanium oxide layer and surface nanotopography significantly influence cell interactions. This work offers promising insights into designing multifunctional biomaterial surfaces that selectively promote specific cell types' proliferation─which is a crucial advancement in next-generation vascular implants.

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.

Multicellularity and the Need for Communication-A Systematic Overview on (Algal) Plasmodesmata and Other Types of Symplasmic Cell Connections

In the evolution of eukaryotes, the transition from unicellular to simple multicellular organisms has happened multiple times. For the development of complex multicellularity, characterized by sophisticated body plans and division of labor between specialized cells, symplasmic intercellular communication is supposed to be indispensable. We review the diversity of symplasmic connectivity among the eukaryotes and distinguish between distinct types of non-plasmodesmatal connections, plasmodesmata-like structures, and 'canonical' plasmodesmata on the basis of developmental, structural, and functional criteria. Focusing on the occurrence of plasmodesmata (-like) structures in extant taxa of fungi, brown algae (Phaeophyceae), green algae (Chlorophyta), and streptophyte algae, we present a detailed critical update on the available literature which is adapted to the present classification of these taxa and may serve as a tool for future work. From the data, we conclude that, actually, development of complex multicellularity correlates with symplasmic connectivity in many algal taxa, but there might be alternative routes. Furthermore, we deduce a four-step process towards the evolution of canonical plasmodesmata and demonstrate similarity of plasmodesmata in streptophyte algae and land plants with respect to the occurrence of an ER component. Finally, we discuss the urgent need for functional investigations and molecular work on cell connections in algal organisms.

Pathological and Therapeutic Significance of Tumor-Derived Extracellular Vesicles in Cancer Cell Migration and Metastasis

The infiltration of primary tumors and metastasis formation at distant sites strongly impact the prognosis and the quality of life of cancer patients. Current therapies including surgery, radiotherapy, and chemotherapy are limited in targeting the complex cell migration mechanisms responsible for cancer cell invasiveness and metastasis. A better understanding of these mechanisms and the development of new therapies are urgently needed. Extracellular vesicles (EVs) are lipid-enveloped particles involved in inter-tissue and inter-cell communication. This review article focuses on the impact of EVs released by tumor cells, specifically on cancer cell migration and metastasis. We first introduce cell migration processes and EV subtypes, and we give an overview of how tumor-derived EVs (TDEVs) may impact cancer cell migration. Then, we discuss ongoing EV-based cancer therapeutic approaches, including the inhibition of general EV-related mechanisms as well as the use of EVs for anti-cancer drug delivery, focusing on the harnessing of TDEVs. We propose a protein-EV shuttle as a route alternative to secretion or cell membrane binding, influencing downstream signaling and the final effect on target cells, with strong implications in tumorigenesis. Finally, we highlight the pitfalls and limitations of therapeutic EV exploitation that must be overcome to realize the promise of EVs for cancer therapy.

Herpes Simplex Virus Type 1 Infection Induces the Formation of Tunneling Nanotubes

Herpes simplex virus type 1 (HSV-1) is human specific virus. The intercellular transmission of HSV-1 is essential in its pathogenesis. The tunneling nanotube (TNT), a new mode connecting distant cells, has been found to play an important role in the spread of various viruses like human immunodeficiency virus (HIV) and influenza virus. However, whether HSV-1 can be transmitted through TNTs has not been confirmed. The purpose of this study was to clarify this, and further to determine the effect of inhibiting the actin-related protein 2/3 (Arp2/3) complex on the intercellular transmission of HSV-1. A scanning electron microscope and fluorescence microscope detected the formation of TNTs between HSV-1 infected cells. Envelope glycoprotein D (gD) and envelope glycoprotein E (gE) of HSV-1 and viral particles were observed in TNTs. Treatment with CK666, an inhibitor of the Arp2/3 complex, reduced the number of TNTs by approximately 40-80%. At the same time, the DNA level of HSV-1 in cells and the number of plaque formation units (PFU) were also reduced by nearly 30%. These findings indicated that TNT contributes to HSV-1 transmission and that the inhibition of the Arp2/3 complex could impair HSV-1 transmission, which not only provides a novel insight into the transmission mode of HSV-1, but also a putative new antiviral target.

Morphological and functional alterations in hemocytes of Mytilus galloprovincialis exposed in high-impact anthropogenic sites

The work aimed to study the induction of morphological alterations in M. galloprovincialis in the field and its suitability to be integrated into a sensitive, simple, and cost-effective cell-based multimarker approach for the detection of the stress status induced by pollution in coastal marine environments in view of ecotoxicological biomonitoring and assessment application. Cellular morphometric alterations was paralleled by the analysis of standardized biomarkers such as lysosomal membrane destabilization, and genotoxocity biomarkers such as micronuclei and binuclated cells frequencies were investigated. The study was carried out by means of a transplanting experiment in the field, using caged organisms from an initial population exposed in the field in two multi-impacted coastal sites of the central Mediterranean area, Bagnoli in the eastern Tyrrhenian Sea and Augusta-Melilli-Priolo in the western Ionian Sea. Capo Miseno (NA) for the Tyrrhenian area and Brucoli (ME) for the Ionian area were chosen as control sites. Hemocyte enlargement and filopodial elongation increased frequencies were observed in organisms exposed to the impacted sites. These morphometric alterations showed strong agreement with the lysosomal membrane destabilization and biomarkers of genotoxicity, suggesting their usefulness in detecting the pollutant-induced stress syndrome related to genotoxic damage.

Aging Hallmarks and the Role of Oxidative Stress

Aging is a complex biological process accompanied by a progressive decline in the physical function of the organism and an increased risk of age-related chronic diseases such as cardiovascular diseases, cancer, and neurodegenerative diseases. Studies have established that there exist nine hallmarks of the aging process, including (i) telomere shortening, (ii) genomic instability, (iii) epigenetic modifications, (iv) mitochondrial dysfunction, (v) loss of proteostasis, (vi) dysregulated nutrient sensing, (vii) stem cell exhaustion, (viii) cellular senescence, and (ix) altered cellular communication. All these alterations have been linked to sustained systemic inflammation, and these mechanisms contribute to the aging process in timing not clearly determined yet. Nevertheless, mitochondrial dysfunction is one of the most important mechanisms contributing to the aging process. Mitochondria is the primary endogenous source of reactive oxygen species (ROS). During the aging process, there is a decline in ATP production and elevated ROS production together with a decline in the antioxidant defense. Elevated ROS levels can cause oxidative stress and severe damage to the cell, organelle membranes, DNA, lipids, and proteins. This damage contributes to the aging phenotype. In this review, we summarize recent advances in the mechanisms of aging with an emphasis on mitochondrial dysfunction and ROS production.

Mitochondrial Transplantation in Mitochondrial Medicine: Current Challenges and Future Perspectives

Mitochondrial diseases (MDs) are inherited genetic conditions characterized by pathogenic mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). Current therapies are still far from being fully effective and from covering the broad spectrum of mutations in mtDNA. For example, unlike heteroplasmic conditions, MDs caused by homoplasmic mtDNA mutations do not yet benefit from advances in molecular approaches. An attractive method of providing dysfunctional cells and/or tissues with healthy mitochondria is mitochondrial transplantation. In this review, we discuss what is known about intercellular transfer of mitochondria and the methods used to transfer mitochondria both in vitro and in vivo, and we provide an outlook on future therapeutic applications. Overall, the transfer of healthy mitochondria containing wild-type mtDNA copies could induce a heteroplasmic shift even when homoplasmic mtDNA variants are present, with the aim of attenuating or preventing the progression of pathological clinical phenotypes. In summary, mitochondrial transplantation is a challenging but potentially ground-breaking option for the treatment of various mitochondrial pathologies, although several questions remain to be addressed before its application in mitochondrial medicine.

Hematopoietic stem and progenitors cells gene editing: Beyond blood disorders

Lessons learned from decades-long practice in the transplantation of hematopoietic stem and progenitor cells (HSPCs) to treat severe inherited disorders or cancer, have set the stage for the current ex vivo gene therapies using autologous gene-modified hematopoietic stem and progenitor cells that have treated so far, hundreds of patients with monogenic disorders. With increased knowledge of hematopoietic stem and progenitor cell biology, improved modalities for patient conditioning and with the emergence of new gene editing technologies, a new era of hematopoietic stem and progenitor cell-based gene therapies is poised to emerge. Gene editing has the potential to restore physiological expression of a mutated gene, or to insert a functional gene in a precise locus with reduced off-target activity and toxicity. Advances in patient conditioning has reduced treatment toxicities and may improve the engraftment of gene-modified cells and specific progeny. Thanks to these improvements, new potential treatments of various blood- or immune disorders as well as other inherited diseases will continue to emerge. In the present review, the most recent advances in hematopoietic stem and progenitor cell gene editing will be reported, with a focus on how this approach could be a promising solution to treat non-blood-related inherited disorders and the mechanisms behind the therapeutic actions discussed.

Topological Connection between Vesicles and Nanotubes in Single-Molecule Lipid Membranes Driven by Head-Tail Interactions

Lipid nanotube-vesicle networks are important channels for intercellular communication and transport of matter. Experimentally observed in neighboring mammalian cells but also reproduced in model membrane systems, a broad consensus exists on their formation and stability. Lipid membranes must be composed of at least two molecular components, each stabilizing low (generally a phospholipid) and high curvatures. Strong anisotropy or enhanced conical shape of the second amphiphile is crucial for the formation of nanotunnels. Anisotropic driving forces generally favor nanotube protrusions from vesicles. In this work, we report the unique case of topologically connected nanotubes-vesicles obtained in the absence of directional forces, in single-molecule membranes, composed of an anisotropic bolaform glucolipid, above its melting temperature, T_m. Cryo-TEM and fluorescence confocal microscopy show the interconnection between vesicles and nanotubes in a single-phase region, between 60 and 90 °C under diluted conditions. Solid-state NMR demonstrates that the glucolipid can assume two distinct configurations, head-head and head-tail. These arrangements, seemingly of comparable energy above the T_m, could explain the existence and stability of the topologically connected vesicles and nanotubes, which are generally not observed for classical single-molecule phospholipid-based membranes above their T_m.

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.

Mitochondrial transplantation as a promising therapy for mitochondrial diseases

Mitochondrial diseases are a group of inherited or acquired metabolic disorders caused by mitochondrial dysfunction which may affect almost all the organs in the body and present at any age. However, no satisfactory therapeutic strategies have been available for mitochondrial diseases so far. Mitochondrial transplantation is a burgeoning approach for treatment of mitochondrial diseases by recovery of dysfunctional mitochondria in defective cells using isolated functional mitochondria. Many models of mitochondrial transplantation in cells, animals, and patients have proved effective via various routes of mitochondrial delivery. This review presents different techniques used in mitochondrial isolation and delivery, mechanisms of mitochondrial internalization and consequences of mitochondrial transplantation, along with challenges for clinical application. Despite some unknowns and challenges, mitochondrial transplantation would provide an innovative approach for mitochondrial medicine.

Extracellular Vesicles and Membrane Protrusions in Developmental Signaling

During embryonic development, cells communicate with each other to determine cell fate, guide migration, and shape morphogenesis. While the relevant secreted factors and their downstream target genes have been characterized extensively, how these signals travel between embryonic cells is still emerging. Evidence is accumulating that extracellular vesicles (EVs), which are well defined in cell culture and cancer, offer a crucial means of communication in embryos. Moreover, the release and/or reception of EVs is often facilitated by fine cellular protrusions, which have a history of study in development. However, due in part to the complexities of identifying fragile nanometer-scale extracellular structures within the three-dimensional embryonic environment, the nomenclature of developmental EVs and protrusions can be ambiguous, confounding progress. In this review, we provide a robust guide to categorizing these structures in order to enable comparisons between developmental systems and stages. Then, we discuss existing evidence supporting a role for EVs and fine cellular protrusions throughout development.

Miro proteins and their role in mitochondrial transfer in cancer and beyond

Mitochondria are organelles essential for tumor cell proliferation and metastasis. Although their main cellular function, generation of energy in the form of ATP is dispensable for cancer cells, their capability to drive their adaptation to stress originating from tumor microenvironment makes them a plausible therapeutic target. Recent research has revealed that cancer cells with damaged oxidative phosphorylation import healthy (functional) mitochondria from surrounding stromal cells to drive pyrimidine synthesis and cell proliferation. Furthermore, it has been shown that energetically competent mitochondria are fundamental for tumor cell migration, invasion and metastasis. The spatial positioning and transport of mitochondria involves Miro proteins from a subfamily of small GTPases, localized in outer mitochondrial membrane. Miro proteins are involved in the structure of the MICOS complex, connecting outer and inner-mitochondrial membrane; in mitochondria-ER communication; Ca²⁺ metabolism; and in the recycling of damaged organelles via mitophagy. The most important role of Miro is regulation of mitochondrial movement and distribution within (and between) cells, acting as an adaptor linking organelles to cytoskeleton-associated motor proteins. In this review, we discuss the function of Miro proteins in various modes of intercellular mitochondrial transfer, emphasizing the structure and dynamics of tunneling nanotubes, the most common transfer modality. We summarize the evidence for and propose possible roles of Miro proteins in nanotube-mediated transfer as well as in cancer cell migration and metastasis, both processes being tightly connected to cytoskeleton-driven mitochondrial movement and positioning.

Trans-cellular tunnels induced by the fungal pathogen Candida albicans facilitate invasion through successive epithelial cells without host damage

The opportunistic fungal pathogen Candida albicans is normally commensal, residing in the mucosa of most healthy individuals. In susceptible hosts, its filamentous hyphal form can invade epithelial layers leading to superficial or severe systemic infection. Although invasion is mainly intracellular, it causes no apparent damage to host cells at early stages of infection. Here, we investigate C. albicans invasion in vitro using live-cell imaging and the damage-sensitive reporter galectin-3. Quantitative single cell analysis shows that invasion can result in host membrane breaching at different stages and host cell death, or in traversal of host cells without membrane breaching. Membrane labelling and three-dimensional 'volume' electron microscopy reveal that hyphae can traverse several host cells within trans-cellular tunnels that are progressively remodelled and may undergo 'inflations' linked to host glycogen stores. Thus, C. albicans early invasion of epithelial tissues can lead to either host membrane breaching or trans-cellular tunnelling.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Tunneling nanotubes and related structures: molecular mechanisms of formation and function

Tunneling nanotubes (TNTs) are F-actin-based, membrane-enclosed tubular connections between animal cells that transport a variety of cellular cargo. Over the last 15 years since their discovery, TNTs have come to be recognized as key players in normal cell communication and organism development, and are also exploited for the spread of various microbial pathogens and major diseases like cancer and neurodegenerative disorders. TNTs have also been proposed as modalities for disseminating therapeutic drugs between cells. Despite the rapidly expanding and wide-ranging relevance of these structures in both health and disease, there is a glaring dearth of molecular mechanistic knowledge regarding the formation and function of these important but enigmatic structures. A series of fundamental steps are essential for the formation of functional nanotubes. The spatiotemporally controlled and directed modulation of cortical actin dynamics would be required to ensure outward F-actin polymerization. Local plasma membrane deformation to impart negative curvature and membrane addition at a rate commensurate with F-actin polymerization would enable outward TNT elongation. Extrinsic tactic cues, along with cognate intrinsic signaling, would be required to guide and stabilize the elongating TNT towards its intended target, followed by membrane fusion to create a functional TNT. Selected cargoes must be transported between connected cells through the action of molecular motors, before the TNT is retracted or destroyed. This review summarizes the current understanding of the molecular mechanisms regulating these steps, also highlighting areas that deserve future attention.

A Monte Carlo study of giant vesicle morphologies in nonequilibrium environments

It is known that giant vesicles undergo dynamic morphological changes when exposed to a detergent. The solubilization process may take multiple pathways. In this work, we identify lipid vesicle shape dynamics before the solubilization of 1,2-dioleoyl-sn-glycero-3-phosphocholine giant vesicles with Triton X-100 (TR) detergent. The violent lipid vesicle dynamics was observed with laser confocal scanning microscopy and was qualitatively explained via a numerical simulation. A three-dimensional Monte Carlo scheme was constructed that emulated the nonequilibrium conditions at the beginning stages of solubilization, accounting for a gradual addition of TR detergent molecules into the lipid bilayers. We suggest that the main driving factor for morphology change in lipid vesicles is the associative tendency of the TR molecules, which induces spontaneous curvature of the detergent inclusions, an intrinsic consequence of their molecular shape. The majority of the observed lipid vesicle shapes in the experiments were found to correspond very well to the numerically calculated shapes in the phase space of possible solutions. The results give an insight into the early stages of lipid vesicle solubilization by amphiphilic molecules, which is nonequilibrium in nature and very difficult to study.

Reactive Oxygen Species in Acute Lymphoblastic Leukaemia: Reducing Radicals to Refine Responses

Acute lymphoblastic leukaemia (ALL) is the most common cancer diagnosed in children and adolescents. Approximately 70% of patients survive >5-years following diagnosis, however, for those that fail upfront therapies, survival is poor. Reactive oxygen species (ROS) are elevated in a range of cancers and are emerging as significant contributors to the leukaemogenesis of ALL. ROS modulate the function of signalling proteins through oxidation of cysteine residues, as well as promote genomic instability by damaging DNA, to promote chemotherapy resistance. Current therapeutic approaches exploit the pro-oxidant intracellular environment of malignant B and T lymphoblasts to cause irreversible DNA damage and cell death, however these strategies impact normal haematopoiesis and lead to long lasting side-effects. Therapies suppressing ROS production, especially those targeting ROS producing enzymes such as the NADPH oxidases (NOXs), are emerging alternatives to treat cancers and may be exploited to improve the ALL treatment. Here, we discuss the roles that ROS play in normal haematopoiesis and in ALL. We explore the molecular mechanisms underpinning overproduction of ROS in ALL, and their roles in disease progression and drug resistance. Finally, we examine strategies to target ROS production, with a specific focus on the NOX enzymes, to improve the treatment of ALL.

Post-bifurcation behaviour of elasto-capillary necking and bulging in soft tubes

Previous linear bifurcation analyses have evidenced that an axially stretched soft cylindrical tube may develop an infinite-wavelength (localized) instability when one or both of its lateral surfaces are under sufficient surface tension. Phase transition interpretations have also highlighted that the tube admits a final evolved ‘two-phase’ state. How the localized instability initiates and evolves into the final ‘two-phase’ state is still a matter of contention, and this is the focus of the current study. Through a weakly nonlinear analysis conducted for a general material model, the initial sub-critical bifurcation solution is found to be localized bulging or necking depending on whether the axial stretch is greater or less than a certain threshold value. At this threshold value, an exceptionally super-critical kink-wave solution arises in place of localization. A thorough interpretation of the anticipated post-bifurcation behaviour based on our theoretical results is also given, and this is supported by finite-element method simulations.

Membrane nanotubes are ancient machinery for cell-to-cell communication and transport. Their interference with the immune system

Nanotubular connections between mammalian cell types came into the focus only two decades ago, when “live cell super-resolution imaging” was introduced. Observations of these long-time overlooked structures led to understanding mechanisms of their growth/withdrawal and exploring some key genetic and signaling factors behind their formation. Unbelievable level of multiple supportive collaboration between tumor cells undergoing cytotoxic chemotherapy, cross-feeding” between independent bacterial strains or “cross-dressing” collaboration of immune cells promoting cellular immune response, all via nanotubes, have been explored recently. Key factors and "calling signals" determining the spatial directionality of their growth and their overall in vivo significance, however, still remained debated. Interestingly, prokaryotes, including even ancient archaebacteria, also seem to use such NT connections for intercellular communication. Herein, we will give a brief overview of current knowledge of membrane nanotubes and depict a simple model about their possible “historical role”.

Cell-to-Cell Transmission of Turkey Herpesvirus in Chicken Embryo Cells via Tunneling Nanotubes

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus that causes immunosuppression, T cell lymphomas, and neuropathic disease in infected chickens. To protect chickens from MDV infection, an avirulent live vaccine of turkey herpesvirus (HVT) has been successfully used in chickens worldwide. Many vaccine manufacturers have used chicken embryo fibroblast (CEF) cells to produce the HVT vaccine. Generally, it has been suggested that HVT is a highly cell-associated herpesvirus that spread via cell-to-cell contact, but it is unclear how HVT is transmitted from infected cells to uninfected target cells. Here, we show via immunofluorescence analysis that nanotubes containing the actin cytoskeleton and HVT antigens from infected CEF cells were observed to contact neighboring cells. When the infected cells were treated with inhibitors for actin polymerization or depolymerization, the formation and extension of the nanotubes from infected cells were greatly inhibited and the intercellular contact was abolished, leading to a drastic reduction in plaque formation and viral titers of the cell-associated virus. Our data indicate that cell-to-cell contacts via nanotubes composed of actin filaments are essential for efficient viral spreading and replication. This finding might contribute to the further improvement of efficient HVT vaccine production.

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.

The Curious Strategy of Multipartite Viruses

Multipartite virus genomes are composed of several segments, each packaged in a distinct viral particle. Although this puzzling genome architecture is found in ∼17% of known viral species, its distribution among hosts or among distinct types of genome-composing nucleic acid remains poorly understood. No convincing advantage of multipartitism has been identified, yet the maintenance of genomic integrity appears problematic. Here we review recent studies shedding light on these issues. Multipartite viruses rapidly modify the copy number of each segment/gene from one host species to another, a putative benefit if host switches are common. One multipartite virus functions in a multicellular way: The segments do not all need to be present in the same cell and can functionally complement across cells, maintaining genome integrity within hosts. The genomic integrity maintenance during host-to-host transmission needs further elucidation. These features challenge several virology foundations and could apply to other multicomponent viral systems.

Miro proteins connect mitochondrial function and intercellular transport

Mitochondria are organelles present in most eukaryotic cells, where they play major and multifaceted roles. The classical notion of the main mitochondrial function as the powerhouse of the cell per se has been complemented by recent discoveries pointing to mitochondria as organelles affecting a number of other auxiliary processes. They go beyond the classical energy provision via acting as a relay point of many catabolic and anabolic processes, to signaling pathways critically affecting cell growth by their implication in de novo pyrimidine synthesis. These additional roles further underscore the importance of mitochondrial homeostasis in various tissues, where its deregulation promotes a number of pathologies. While it has long been known that mitochondria can move within a cell to sites where they are needed, recent research has uncovered that mitochondria can also move between cells. While this intriguing field of research is only emerging, it is clear that mobilization of mitochondria requires a complex apparatus that critically involves mitochondrial proteins of the Miro family, whose role goes beyond the mitochondrial transfer, as will be covered in this review.

Role of Tunneling Nanotubes in Viral Infection, Neurodegenerative Disease, and Cancer

The network of tunneling nanotubes (TNTs) represents the filamentous (F)-actin rich tubular structure which is connected to the cytoplasm of the adjacent and or distant cells to mediate efficient cell-to-cell communication. They are long cytoplasmic bridges with an extraordinary ability to perform diverse array of function ranging from maintaining cellular physiology and cell survival to promoting immune surveillance. Ironically, TNTs are now widely documented to promote the spread of various pathogens including viruses either during early or late phase of their lifecycle. In addition, TNTs have also been associated with multiple pathologies in a complex multicellular environment. While the recent work from multiple laboratories has elucidated the role of TNTs in cellular communication and maintenance of homeostasis, this review focuses on their exploitation by the diverse group of viruses such as retroviruses, herpesviruses, influenza A, human metapneumovirus and SARS CoV-2 to promote viral entry, virus trafficking and cell-to-cell spread. The later process may aggravate disease severity and the associated complications due to widespread dissemination of the viruses to multiple organ system as observed in current coronavirus disease 2019 (COVID-19) patients. In addition, the TNT-mediated intracellular spread can be protective to the viruses from the circulating immune surveillance and possible neutralization activity present in the extracellular matrix. This review further highlights the relevance of TNTs in ocular and cardiac tissues including neurodegenerative diseases, chemotherapeutic resistance, and cancer pathogenesis. Taken together, we suggest that effective therapies should consider precise targeting of TNTs in several diseases including virus infections.

Membrane shape remodeling by protein crowding

The steric repulsion between proteins on biological membranes is one of the most generic mechanisms that cause membrane shape changes. We present a minimal model in which a spontaneous curvature is induced by asymmetric protein crowding. Our results show that the interplay between the induced spontaneous curvature and the membrane tension determines the energy-minimizing shapes, which describes the wide range of experimentally observed membrane shapes, i.e., flat membranes, spherical vesicles, elongated tubular protrusions, and pearling structures. Moreover, the model gives precise predictions on how membrane shape changes by protein crowding can be tuned by controlling the protein size, the density of proteins, and the size of the crowded domain.

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.

On the Role of Curved Membrane Nanodomains, and Passive and Active Skeleton Forces in the Determination of Cell Shape and Membrane Budding

Biological membranes are composed of isotropic and anisotropic curved nanodomains. Anisotropic membrane components, such as Bin/Amphiphysin/Rvs (BAR) superfamily protein domains, could trigger/facilitate the growth of membrane tubular protrusions, while isotropic curved nanodomains may induce undulated (necklace-like) membrane protrusions. We review the role of isotropic and anisotropic membrane nanodomains in stability of tubular and undulated membrane structures generated or stabilized by cyto- or membrane-skeleton. We also describe the theory of spontaneous self-assembly of isotropic curved membrane nanodomains and derive the critical concentration above which the spontaneous necklace-like membrane protrusion growth is favorable. We show that the actin cytoskeleton growth inside the vesicle or cell can change its equilibrium shape, induce higher degree of segregation of membrane nanodomains or even alter the average orientation angle of anisotropic nanodomains such as BAR domains. These effects may indicate whether the actin cytoskeleton role is only to stabilize membrane protrusions or to generate them by stretching the vesicle membrane. Furthermore, we demonstrate that by taking into account the in-plane orientational ordering of anisotropic membrane nanodomains, direct interactions between them and the extrinsic (deviatoric) curvature elasticity, it is possible to explain the experimentally observed stability of oblate (discocyte) shapes of red blood cells in a broad interval of cell reduced volume. Finally, we present results of numerical calculations and Monte-Carlo simulations which indicate that the active forces of membrane skeleton and cytoskeleton applied to plasma membrane may considerably influence cell shape and membrane budding.

Pathogenic Stress Induces Human Monocyte to Express an Extracellular Web of Tunneling Nanotubes

Actin-based tunneling nanotubes are a means of intercellular communication between remote cells. In the last decade, this type of nanotube was described in a wide variety of cell types and it became widely accepted that communication through these nanotubes is related to response to environmental changes. Few reports, however, are available regarding the expression of similar nanotubes in vivo or in primary cells. Moreover, the functional significance of this intercellular communication for health and disease is largely unknown. In this context, and as a first step in unraveling these questions, we examined the formation of similar nanotubes in primary peripheral human monocytes. To that end, we combined the use of a live cell imaging system along with advanced methods of fluorescent and scanning electron microscopy. This experimental approach reveals for the first time that the bacterial lipopolysaccharide endotoxin induces a transient expression of an unexpected abundance of actin-based tunneling nanotubes associated with vesicles. In addition, it was found that a similar response can be achieved by treating human monocytes with various bacterial and yeast membrane components, as well as with a viral component analog. In all these cases, this response is mediated by distinct complexes of toll-like receptors. Therefore, we suggest that the observed phenomena are related to a broad type of monocyte pathogen response, and raise the possibility that the phenomena described above may be involved in many clinical situations related to inflammation as a new topic of study.

Functional and Morphological Changes Induced in Mytilus Hemocytes by Selected Nanoparticles

Nanoparticles (NPs) show various properties depending on their composition, size, and surface coating, which shape their interactions with biological systems. In particular, NPs have been shown to interact with immune cells, that represent a sensitive surveillance system of external and internal stimuli. In this light, in vitro models represent useful tools for investigating nano-bio-interactions in immune cells of different organisms, including invertebrates. In this work, the effects of selected types of NPs with different core composition, size and functionalization (custom-made PVP-AuNP and commercial nanopolystyrenes PS-NH2 and PS-COOH) were investigated in the hemocytes of the marine bivalve Mytilus galloprovincialis. The role of exposure medium was evaluated using either artificial seawater (ASW) or hemolymph serum (HS). Hemocyte morphology was investigated by scanning electron microscopy (SEM) and different functional parameters (lysosomal membrane stability, phagocytosis, and lysozyme release) were evaluated. The results show distinct morphological and functional changes induced in mussel hemocytes depending on the NP type and exposure medium. Mussel hemocytes may represent a powerful alternative in vitro model for a rapid pre-screening strategy for NPs, whose utilization will contribute to the understanding of the possible impact of environmental exposure to NPs in marine invertebrates.

In vitro rescue immature oocytes - a literature review

Immature oocytes retrieved from in vitro fertilization (IVF) and clinical in vitro maturation (IVM) is a common problem, especially in patients with advanced age, poor ovarian response (POR), or polycystic ovary syndrome (PCOS). Considering there is no common name to describe this group of oocytes, we suggest naming all of immature oocytes retrieved from IVF and clinical IVM cycles as 'Medical Unusable Oocytes' (MUO) as none of them will be used for subsequent treatment and will eventually be discarded. Scientists attempt to improve the clinical utilization rate of MUO instead of discarding them. Rescue IVM and mitochondria supplementation may be available approaches to mature MUO. We propose a specific definition of rescue IVM, namely the cultivation and maturation of immature oocytes in vitro collected from IVF cycles with human chorionic gonadotropin (hCG) trigger. Rescue IVM is usually mixed up with clinical IVM. Clarification of the differences between rescue IVM and clinical IVM is necessary. This manuscript aims to clarify the rather confusing IVM procedures and review existing methods of improving rescue IVM, currently available information on the success rate, and explore the future possibility of rescue IVM serving as a promising tool in reproductive medicine.

Nucleolin regulates 14-3-3ζ mRNA and promotes cofilin phosphorylation to induce tunneling nanotube formation

Tunneling nanotubes (TNTs) mediate intercellular communication between animal cells in health and disease, but the mechanisms of their biogenesis and function are poorly understood. Here we report that the RNA-binding protein (RBP) nucleolin, which interacts with the known TNT-inducing protein MSec, is essential for TNT formation in mammalian cells. Nucleolin, through its RNA-binding domains (RBDs), binds to and maintains the cytosolic levels of 14-3-3ζ mRNA, and is, therefore, required for TNT formation. A specific region of the 3'-untranslated region (UTR) of the 14-3-3ζ mRNA is likely to be involved in its regulation by nucleolin. Functional complementation experiments suggest that nucleolin and 14-3-3ζ form a linear signaling axis that promotes the phosphorylation and inactivation of the F-actin depolymerization factor cofilin to induce TNT formation. MSec also similarly inactivates cofilin, but potentiates TNT formation independent of the nucleolin-14-3-3ζ axis, despite biochemically interacting with both proteins. We show that 14-3-3ζ and nucleolin are required for the formation of TNTs between primary mouse neurons and astrocytes and in multiple other mammalian cell types. We also report that the Caenorhabditis elegans orthologs of 14-3-3ζ and MSec regulate the size and architecture of the TNT-like cellular protrusions of the distal tip cell (DTC), the germline stem cell niche in the gonad. Our study demonstrates a novel and potentially conserved mRNA-guided mechanism of TNT formation through the maintenance of cellular 14-3-3ζ mRNA levels by the RBP nucleolin.

Biological lipid nanotubes and their potential role in evolution

The membrane of cells and organelles are highly deformable fluid interfaces, and can take on a multitude of shapes. One distinctive and particularly interesting property of biological membranes is their ability to from long and uniform nanotubes. These nanoconduits are surprisingly omnipresent in all domains of life, from archaea, bacteria, to plants and mammals. Some of these tubes have been known for a century, while others were only recently discovered. Their designations are different in different branches of biology, e.g. they are called stromule in plants and tunneling nanotubes in mammals. The mechanical transformation of flat membranes to tubes involves typically a combination of membrane anchoring and external forces, leading to a pulling action that results in very rapid membrane nanotube formation - micrometer long tubes can form in a matter of seconds. Their radius is set by a mechanical balance of tension and bending forces. There also exists a large class of membrane nanotubes that form due to curvature inducing molecules. It seems plausible that nanotube formation and functionality in plants and animals may have been inherited from their bacterial ancestors during endosymbiotic evolution. Here we attempt to connect observations of nanotubes in different branches of biology, and outline their similarities and differences with the aim of providing a perspective on their joint functions and evolutionary origin.

Visualization and quantification of dynamic intercellular coupling in human embryonic stem cells using single cell sonoporation

Gap junctions (GJs), which are proteinaceous channels, couple adjacent cells by permitting direct exchange of intracellular molecules with low molecular weights. GJ intercellular communication (GJIC) plays a critical role in regulating behaviors of human embryonic stem cells (hESCs), affecting their proliferation and differentiation. Here we report a novel use of sonoporation that enables single cell intracellular dye loading and dynamic visualization/quantification of GJIC in hESC colonies. By applying a short ultrasound pulse to excite single microbubbles tethered to cell membranes, a transient pore on the cell membrane (sonoporation) is generated which allows intracellular loading of dye molecules and influx of Ca²⁺ into single hESCs. We employ live imaging for continuous visualization of intercellular dye transfer and Ca²⁺ diffusion in hESC colonies. We quantify cell–cell permeability based on dye diffusion using mass transport models. Our results reveal heterogeneous intercellular connectivity and a variety of spatiotemporal characteristics of intercellular Ca²⁺ waves in hESC colonies induced by sonoporation of single cells.

Novel Intrinsic Mechanisms of Active Drug Extrusion at the Blood-Brain Barrier: Potential Targets for Enhancing Drug Delivery to the Brain?

The blood-brain barrier (BBB) limits the pharmacotherapy of several brain disorders. In addition to the structural and metabolic characteristics of the BBB, the ATP-driven, drug efflux transporter P-glycoprotein (Pgp) is a selective gatekeeper of the BBB; thus, it is a primary hindrance to drug delivery into the brain. Here, we review the complex regulation of Pgp expression and functional activity at the BBB with an emphasis on recent studies from our laboratory. In addition to traditional processes such as transcriptional regulation and posttranscriptional or posttranslational modification of Pgp expression and functionality, novel mechanisms such as intra- and intercellular Pgp trafficking and intracellular Pgp-mediated lysosomal sequestration in BBB endothelial cells with subsequent disposal by blood neutrophils are discussed. These intrinsic mechanisms of active drug extrusion at the BBB are potential therapeutic targets that could be used to modulate P-glycoprotein activity in the treatment of brain diseases and enhance drug delivery to the brain.

Direct Transfer of Mesoporous Silica Nanoparticles between Macrophages and Cancer Cells

Macrophages line the walls of microvasculature, extending processes into the blood flow to capture foreign invaders, including nano-scale materials. Using mesoporous silica nanoparticles (MSNs) as a model nano-scale system, we show the interplay between macrophages and MSNs from initial uptake to intercellular trafficking to neighboring cells along microtubules. The nature of cytoplasmic bridges between cells and their role in the cell-to-cell transfer of nano-scale materials is examined, as is the ability of macrophages to function as carriers of nanomaterials to cancer cells. Both direct administration of nanoparticles and adoptive transfer of nanoparticle-loaded splenocytes in mice resulted in abundant localization of nanomaterials within macrophages 24 h post-injection, predominately in the liver. While heterotypic, trans-species nanomaterial transfer from murine macrophages to human HeLa cervical cancer cells or A549 lung cancer cells was robust, transfer to syngeneic 4T1 breast cancer cells was not detected in vitro or in vivo. Cellular connections and nanomaterial transfer in vivo were rich among immune cells, facilitating coordinated immune responses.