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

Tunnelling nanotubes (TNTs) connect distant cells and mediate cargo transfer for intercellular communication in physiological and pathological contexts. How cells generate these actin-mediated protrusions to span lengths beyond those attainable by canonical filopodia remains unknown. Through a combination of micropatterning, microscopy, and optical tweezer-based approaches, we demonstrate that TNTs formed through the outward extension of actin achieve distances greater than the mean length of filopodia and that branched Arp2/3-dependent pathways attenuate the extent to which actin polymerizes in nanotubes, thus limiting their occurrence. Proteomic analysis using epidermal growth factor receptor kinase substrate 8 (Eps8) as a positive effector of TNTs showed that, upon Arp2/3 inhibition, proteins enhancing filament turnover and depolymerization were reduced and Eps8 instead exhibited heightened interactions with the inverted Bin/Amphiphysin/Rvs (I-BAR) domain protein IRSp53 that provides a direct connection with linear actin polymerases. Our data reveals how common protrusion players (Eps8 and IRSp53) form tunnelling nanotubes, and that when competing pathways overutilizing such proteins and monomeric actin in Arp2/3 networks are inhibited, processes promoting linear actin growth dominate to favour tunnelling nanotube formation.

Heparan sulfate proteoglycans: Mediators of cellular and molecular Alzheimer's disease pathogenic factors via tunnelling nanotubes?

Neurological disorders impact around one billion individuals globally (15 % approx.), with significant implications for disability and mortality with their impact in Australia currently amounts to 6.8 million deaths annually. Heparan sulfate proteoglycans (HSPGs) are complex extracellular molecules implicated in promoting Tau fibril formation resulting in Tau tangles, a hallmark of Alzheimer's disease (AD). HSPG-Tau protein interactions contribute to various AD stages via aggregation, toxicity, and clearance, largely via interactions with the glypican 1 and syndecan 3 core proteins. The tunnelling nanotubes (TNTs) pathway is emerging as a facilitator of intercellular molecule transport, including Tau and Amyloid ß proteins, across extensive distances. While current TNT-associated evidence primarily stems from cancer models, their role in Tau propagation and its effects on recipient cells remain unclear. This review explores the interplay of TNTs, HSPGs, and AD-related factors and proposes that HSPGs influence TNT formation in neurodegenerative conditions such as AD.

Horizontal mitochondrial transfer as a novel bioenergetic tool for mesenchymal stromal/stem cells: molecular mechanisms and therapeutic potential in a variety of diseases.

Intercellular mitochondrial transfer (MT) is a newly discovered form of cell-to-cell signalling involving the active incorporation of healthy mitochondria into stressed/injured recipient cells, contributing to the restoration of bioenergetic profile and cell viability, reduction of inflammatory processes and normalisation of calcium dynamics. Recent evidence has shown that MT can occur through multiple cellular structures and mechanisms: tunneling nanotubes (TNTs), via gap junctions (GJs), mediated by extracellular vesicles (EVs) and other mechanisms (cell fusion, mitochondrial extrusion and migrasome-mediated mitocytosis) and in different contexts, such as under physiological (tissue homeostasis and stemness maintenance) and pathological conditions (hypoxia, inflammation and cancer). As Mesenchimal Stromal/ Stem Cells (MSC)-mediated MT has emerged as a critical regulatory and restorative mechanism for cell and tissue regeneration and damage repair in recent years, its potential in stem cell therapy has received increasing attention. In particular, the potential therapeutic role of MSCs has been reported in several articles, suggesting that MSCs can enhance tissue repair after injury via MT and membrane vesicle release. For these reasons, in this review, we will discuss the different mechanisms of MSCs-mediated MT and therapeutic effects on different diseases such as neuronal, ischaemic, vascular and pulmonary diseases. Therefore, understanding the molecular and cellular mechanisms of MT and demonstrating its efficacy could be an important milestone that lays the foundation for future clinical trials.

Mesenchymal stem cells (MSCs) in Leber's hereditary optic neuropathy (LHON): a potential therapeutic approach for future.

BACKGROUND: Optic neuropathy has become a new typical syndromic multi-system disease that leads to optic atrophy. This review discusses potential treatments and advances of Leber's hereditary optic neuropathy (LHON), a sporadic genetic disorder. LHON is caused due to slight mutations in mitochondria leading to mitochondrial dysfunction, causing vision loss. There are no current significant treatments that have been proven to work for LHON. METHODS: However, extensive review was carried out on capable studies that have shown potential treatment sensory systems and are being evaluated currently. Some of these studies are in clinical trials, whereas other ones are still being planned. Here, we focus more on treatment based on mesenchymal stem cells-mediated mitochondrial transfer via various techniques. We discuss different mitochondrial transfer modes and possible ways to understand the mitochondria transfer technique's phenotypic characteristics. CONCLUSION: It is clearly understood that transfer of healthy mitochondria from MSC to target cell would regulate the range of reactive oxygen species and ATP'S, which are majorly responsible for mutation upon irregulating. Therefore, mitochondrial transfer is suggested and discussed in this review with various aspects. The graphical abstract represents different means of mitochondrial transport like (a) Tunnelling nanotubules, (b) Extracellular vesicles, (c) Cell fusion and (d) Gap junctions. In (a) Tunnelling nanotubules, the signalling pathways TNF- α/TNF αip2 and NFkB/TNF αep2 are responsible for forming tunnels. Also, Miro protein acts as cargo for the transport of mitochondria with myosin's help in the presence of RhoGTPases [35]. In (b) Extracellular vesicles, the RhoA ARF6 contributes to Actin/Cytoskeletal rearrangement leading to the shedding of microvesicles. Coming to (c) Cell fusion when there is a high amount of ATP, the cells tend to fuse when in close proximity leading to the transfer of mitochondria via EFF-1/HAP2 [48]. In (d) Gap Junctions, Connexin43 is responsible for the intracellular channel in the presence of more ATP [86].

Mitochondria transfer from early stages of erythroblasts to their macrophage niche via tunnelling nanotubes.

Adult erythropoiesis entails a series of well-coordinated events that produce mature red blood cells. One of such events is the mitochondria clearance that occurs cell-autonomously via autophagy-dependent mechanisms. Interestingly, recent studies have shown mitochondria transfer activities between various cell types. In the context of erythropoiesis, macrophages are known to interact closely with the early stages of erythroblasts to provide a specialized niche, termed erythroblastic islands (EBI). However, whether mitochondria transfer can occur in the EBI niche has not been explored. Here, we report that mitochondria transfer in the EBI niche occurs in vivo. We observed mitochondria transfer activities from the early stages of erythroblasts to macrophages in the reconstituted in vitro murine EBI via different modes, including tunnelling nanotubes (TNT). Moreover, we demonstrated that Wiskott-Aldrich syndrome protein (WASp) in macrophages mediates TNT formation and mitochondria transfer via the modulation of F-actin filamentation, thus promoting mitochondria clearance from erythroid cells, to potentially enhance their differentiation. Taken together, our findings provide novel insight into the mitochondria clearance machineries that mediate erythroid maturation.

Emerging role of contact-mediated cell communication in tissue development and diseases.

Cells of multicellular organisms are in continuous conversation with the neighbouring cells. The sender cells signal the receiver cells to influence their behaviour in transport, metabolism, motility, division, and growth. How cells communicate with each other can be categorized by biochemical signalling processes, which can be characterised by the distance between the sender cell and the receiver cell. Existing classifications describe autocrine signals as those where the sender cell is identical to the receiver cell. Complementary to this scenario, paracrine signalling describes signalling between a sender cell and a different receiver cell. Finally, juxtacrine signalling describes the exchange of information between adjacent cells by direct cell contact, whereas endocrine signalling describes the exchange of information, e.g., by hormones between distant cells or even organs through the bloodstream. In the last two decades, however, an unexpected communication mechanism has been identified which uses cell protrusions to exchange chemical signals by direct contact over long distances. These signalling protrusions can deliver signals in both ways, from sender to receiver and vice versa. We are starting to understand the morphology and function of these signalling protrusions in many tissues and this accumulation of findings forces us to revise our view of contact-dependent cell communication. In this review, we will focus on the two main categories of signalling protrusions, cytonemes and tunnelling nanotubes. These signalling protrusions emerge as essential structural components of a vibrant communication network in the development and tissue homeostasis of any multicellular organism.

SIRT1-mediated tunnelling nanotubes may be a potential intervention target for arsenic-induced hepatocyte senescence and liver damage.

Arsenic, a widespread environmental poison, can cause significant liver damage upon exposure. Mitochondria are the most sensitive organelles to external factors. Dysfunctional mitochondria play a crucial role in cellular senescence and liver damage. Tunnelling nanotubes (TNTs), membrane structures formed between cells, with fibrous actin (F-actin) serving as the scaffold, facilitate mitochondrial transfer between cells. Notably, TNTs mediate the delivery of healthy mitochondria to damaged cells, thereby mitigating cellular damage. Although limited studies have suggested that F-actin may be modulated by the longevity gene SIRT1, the association between arsenic-induced liver damage and this mechanism remains unexplored. The findings of the current study indicate that arsenic suppresses SIRT1 and F-actin in the rat liver and MIHA cells, impeding the formation of TNTs and mitochondrial transfer between MIHA cells, thereby playing a pivotal role in mitochondrial dysfunction, cellular senescence and liver damage induced by arsenic. Notably, increasing SIRT1 levels effectively mitigated liver mitochondrial dysfunction and cellular senescence triggered by arsenic, highlighting SIRT1's crucial regulatory function. This research provides novel insights into the mechanisms underlying arsenic-induced liver damage, paving the way for the development of targeted preventive and therapeutic drugs to address arsenic-induced liver damage.

Orchestrating Blood Flow in the Retina: Interpericyte Tunnelling Nanotube Communication.

The retina transforms light into electrical signals, which are sent to the brain via the optic nerve to form our visual perception. This complex signal processing is performed by the retinal neuron and requires a significant amount of energy. Since neurons are unable to store energy, they must obtain glucose and oxygen from the bloodstream to produce energy to match metabolic needs. This process is called neurovascular coupling (NVC), and it is based on a precise mechanism that is not totally understood. The discovery of fine tubular processes termed tunnelling nanotubes (TNTs) set a new type of cell-to-cell communication. TNTs are extensions of the cellular membrane that allow the transfer of material between connected cells. Recently, they have been reported in the brain and retina of living mice, where they connect pericytes, which are vascular mural cells that regulate vessel diameter. Accordingly, these TNTs were termed interpericyte tunnelling nanotubes (IPTNTs), which showed a vital role in blood delivery and NVC. In this chapter, we review the involvement of TNTs in NVC and discuss their implications in retinal neurodegeneration.

SIRT-1/RHOT-1/PGC-1α loop modulates mitochondrial biogenesis and transfer to offer resilience following endovascular stem cell therapy in ischemic stroke.

Current clinical interventions for stroke majorly involve thrombolysis or thrombectomy, however, cessation of the progressive deleterious cellular cascades post-stroke and long-term neuroprotection are yet to be explored. Mitochondria are highly vulnerable organelles and their dysfunction is one of the detrimental consequences following stroke. Mitochondria dysregulation activate unfavourable cellular events over a period of time that leads to the collapse of neuronal machinery in the brain. Hence, strategies to protect and replenish mitochondria in injured neurons may be useful and needs to be explored. Stem cell therapy in ischemic stroke holds a great promise. Past studies have shown beneficial outcomes of endovascularly delivered stem cells in both pre-clinical and clinical settings. Intra-arterial (IA) administration can provide more cells to the stroke foci and affected brain regions than intravenous administration. Supplying new mitochondria to the stroke-compromised neurons either in the core or penumbra by infused stem cells can help increase their survival and longevity. Previously, our lab has demonstrated that IA 1*105 mesenchymal stem cells (MSCs) in rats were safe, efficacious and rendered neuroprotection by regulating neuronal calcineurin, modulating sirtuin1(SIRT-1) mediated inflammasome signaling, ameliorating endoplasmic reticulum-stress, alleviation of post-stroke edema and reducing cellular apoptosis. To explore further, our present study aims to investigate the potential of IA MSCs in protecting and replenishing mitochondria in the injured neurons post-stroke and thereof involvement of SIRT-1/RHOT-1/PGC-1α loop towards mitochondria transfer, biogenesis, and neuroprotection. This study will open new avenues for using stem cells for ischemic stroke in clinics as one of the future adjunctive therapies.

Mitochondrial transfer from mesenchymal stem cells: Mechanisms and functions.

Mesenchymal stem cells based therapy has been used in clinic for almost 20 years and has shown encouraging effects in treating a wide range of diseases. However, the underlying mechanism is far more complicated than it was previously assumed. Mitochondria transfer is one way that recently found to be employed by mesenchymal stem cells to exert its biological effects. As one way of exchanging mitochondrial components, mitochondria transfer determines both mesenchymal stem cells and recipient cell fates. In this review, we describe the factors that contribute to MSCs-MT. Then, the routes and mechanisms of MSCs-MT are summarized to provide a theoretical basis for MSCs therapy. Besides, the advantages and disadvantages of MSCs-MT in clinical application are analyzed.