Nanotubes connecting B lymphocytes: High impact of differentiation-dependent lipid composition on their growth and mechanics

Molecular, morphological and functional properties of tunnelling nanotubes between normal and cancer urothelial cells: New insights from the in vitro model mimicking the situation after surgical removal of the urothelial tumor

Tunnelling nanotubes (TNTs) are membranous connections that represent a unique type of intercellular communication in different cell types. They are associated with cell physiology and cancer pathology. The possible existence of tunnelling nanotubes communication between urothelial cancer and normal cells has not yet been elucidated. Therefore, we analyzed TNTs formed by T24 cells (human invasive cancer urothelial cells) and normal porcine urothelial (NPU) cells, which serve as surrogate models for healthy human urothelial cells. Monocultures and cocultures of NPU and T24 cells were established and analyzed using live-cell imaging, optical tweezers, fluorescence microscopy, and scanning electron microscopy. TNTs of NPU cells differed significantly from tunnelling nanotubes of T24 cells in number, length, diameter, lipid composition, and elastic properties. Membrane domains enriched in cholesterol/sphingomyelin were present in tunnelling nanotubes of T24 cells but not in NPU cells. The tunnelling nanotubes in T24 cells were also easier to bend than the tunnelling nanotubes in NPU cells. The tunnelling nanotubes of both cell types were predominantly tricytoskeletal, and contained actin filaments, intermediate filaments, and microtubules, as well as the motor proteins myosin Va, dynein, and kinesin 5B. Mitochondria were transported within tunnelling nanotubes in living cells, and were colocalized with microtubules and the microtubule-associated protein dynamin 2. In cocultures, heterocellular tunnelling nanotubes were formed between NPU cells and T24 cells and vice versa. The presence of connexin 43 at the end of urothelial tunnelling nanotubes suggests a junctional connection and the involvement of tunnelling nanotube in signal transduction. In this study, we established a novel urothelial cancer-normal coculture model and showed cells in the minority tend to form tunnelling nanotubes with cells in the majority. The condition with cancer cells in the minority is an attractive model to mimic the situation after surgical resection with remaining cancer cells and may help to understand cancer progression and recurrence. Our results shed light on the biological activity of tunnelling nanotubes and have the potential to advance the search for anticancer drugs that target tunnelling nanotubes.

Selenoprotein I (selenoi) as a critical enzyme in the central nervous system

Selenoprotein I (selenoi) is a unique selenocysteine (Sec)-containing protein widely expressed throughout the body. Selenoi belongs to two different protein families: the selenoproteins that are characterized by a redox reactive Sec residue and the lipid phosphotransferases that contain the highly conserved cytidine diphosphate (CDP)-alcohol phosphotransferase motif. Selenoi catalyzes the third reaction of the CDP-ethanolamine branch of the Kennedy pathway within the endoplasmic reticulum membrane. This is not a redox reaction and does not directly involve the Sec residue, making selenoi quite distinct among selenoproteins. Selenoi is also unique among lipid phosphotransferases as the only family member containing a Sec residue near its C-terminus that serves an unknown function. The reaction catalyzed by selenoi involves the transfer of the ethanolamine phosphate group from CDP-ethanolamine to one of two lipid donors, 1,2-diacylglycerol (DAG) or 1-alkyl-2-acylglycerol (AAG), to produce PE or plasmanyl PE, respectively. Plasmanyl PE is subsequently converted to plasmenyl PE by plasmanylethanolamine desaturase. Both PE and plasmenyl PE are critical phospholipids in the central nervous system (CNS), as demonstrated through clinical studies involving SELENOI mutations as well as studies in cell lines and mice. Deletion of SELENOI in mice is embryonic lethal, while loss-of-function mutations in the human SELENOI gene have been found in rare cases leading to a form of hereditary spastic paraplegia (HSP). HSP is an upper motor disease characterized by spasticity of the lower limbs, which is often manifested with other symptoms such as impaired vision/hearing, ataxia, cognitive/intellectual impairment, and seizures. This article will summarize the current understanding of selenoi as a metabolic enzyme and discuss its role in the CNS physiology and pathophysiology.

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”.

Immune Cell Connection by Tunneling Nanotubes: The Impact of Intercellular Cross-Talk on the Immune Response and Its Therapeutic Applications

Direct intercellular communication is an important prerequisite for the development of multicellular organisms, the regeneration of tissue, and the maintenance of various physiological activities. Tunnel nanotubes (TNTs), which have diameters of approximately 50-1500 nm and lengths of up to several cell diameters, can connect cells over long distances and have emerged as one of the most important recently discovered types of efficient communication between cells. Moreover, TNTs can also directly transfer organelles, vehicles, proteins, genetic material, ions, and small molecules from one cell to adjacent and even distant cells. However, the mechanism of intercellular communication between various immune cells within the complex immune system has not been fully elucidated. Studies in the past decades have confirmed the existence of TNTs in many types of cells, especially in various kinds of immune cells. TNTs display different structural and functional characteristics between and within different immunocytes, playing a major role in the transmission of signals across various kinds of immune cells. In this review, we introduce the discovery and structure of TNTs, as well as their different functional properties within different immune cells. We also discuss the roles of TNTs in potentiating the immune response and their potential therapeutic applications.

Mycobacterium Lipids Modulate Host Cell Membrane Mechanics, Lipid Diffusivity, and Cytoskeleton in a Virulence-Selective Manner

Microbial lipids play a critical role in the pathogenesis of infectious diseases by modulating the host cell membrane properties, including lipid/protein diffusion and membrane organization. Mycobacterium tuberculosis (Mtb) synthesizes various chemically distinct lipids that are exposed on its outer membrane and interact with host cell membranes. However, the effects of the structurally diverse Mtb lipids on the host cell membrane properties to fine-tune the host cellular response remain unknown. In this study, we employed membrane biophysics and cell biology to assess the effects of different Mtb lipids on cell membrane mechanics, lipid diffusion, and the cytoskeleton of THP-1 macrophages. We found that Mtb lipids modulate macrophage membrane properties, actin cytoskeleton, and biochemical processes, such as protein phosphorylation and lipid peroxidation, in a virulence lipid-selective manner. These results emphasize that Mtb can fine-tune its interactions with the host cells governed by modulating the lipid profile on its surface. These observations provide a novel lipid-centric paradigm of Mtb pathogenesis that is amenable to pharmacological inhibition and could promote the development of robust biomarkers of Mtb infection and pathogenesis.

Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages

While tuberculosis (TB) is a risk factor in HIV-1-infected individuals, the mechanisms by which Mycobacterium tuberculosis (Mtb) worsens HIV-1 pathogenesis remain scarce. We showed that HIV-1 infection is exacerbated in macrophages exposed to TB-associated microenvironments due to tunneling nanotube (TNT) formation. To identify molecular factors associated with TNT function, we performed a transcriptomic analysis in these macrophages, and revealed the up-regulation of Siglec-1 receptor. Siglec-1 expression depends on Mtb-induced production of type I interferon (IFN-I). In co-infected non-human primates, Siglec-1 is highly expressed by alveolar macrophages, whose abundance correlates with pathology and activation of IFN-I/STAT1 pathway. Siglec-1 localizes mainly on microtubule-containing TNT that are long and carry HIV-1 cargo. Siglec-1 depletion decreases TNT length, diminishes HIV-1 capture and cell-to-cell transfer, and abrogates the exacerbation of HIV-1 infection induced by Mtb. Altogether, we uncover a deleterious role for Siglec-1 in TB-HIV-1 co-infection and open new avenues to understand TNT biology.

The Role of Glycosphingolipids in Immune Cell Functions

Glycosphingolipids (GSLs) exhibit a variety of functions in cellular differentiation and interaction. Also, they are known to play a role as receptors in pathogen invasion. A less well-explored feature is the role of GSLs in immune cell function which is the subject of this review article. Here we summarize knowledge on GSL expression patterns in different immune cells. We review the changes in GSL expression during immune cell development and differentiation, maturation, and activation. Furthermore, we review how immune cell GSLs impact membrane organization, molecular signaling, and trans-interactions in cellular cross-talk. Another aspect covered is the role of GSLs as targets of antibody-based immunity in cancer. We expect that recent advances in analytical and genome editing technologies will help in the coming years to further our knowledge on the role of GSLs as modulators of immune cell function.

Protease-resistant cell meshworks: An indication of membrane nanotube-based syncytia formation

Cell biology considers most animal tissues as assemblies of "individual" cells that rely on different contact-dependent communication mechanisms, including synapses, gap junctions or - a recent awareness - membrane nano- and microtubes. However, by protease-mediated singularization of dense 2D/ 3D cell cultures and tissue explants, we show here that cell collectives stay connected via a continuous meshwork of F-actin-based membrane tubes, resembling tunneling nanotube (TNT)-based networks observed between dispersed cell cultures. Fusion of respective tubes was accompanied by the ingrowth of microtubules and the invasion of mitochondria and lysosomes. Remarkably, in homology to the plasmodesmata-based plant symplast, we found evidence for expanded, membrane-based syncytia in animal tissues by observing dye transfer among the highly interlinked cells. This approach allows for the first time to visualize and quantify membrane continuity-based connections among densely packed cells and to assess their potential physiological and pathological impact closer to the in vivo situation.

Live cell superresolution-structured illumination microscopy imaging analysis of the intercellular transport of microvesicles and costimulatory proteins via nanotubes between immune cells

Membrane nanotubes are transient long-distance connections between cells that can facilitate intercellular communication. These tethers can form spontaneously between many cell types, including cells of the immune and nervous systems. Traffic of viral proteins, vesicles, calcium ions, mRNA, miRNA, mitochondria, lysosomes and membrane proteins/raft domains have all been reported so far via the open ended tunneling nanotubes (TNTs). Recently we reported on existence of plasma membrane derived GM₁/GM₃ ganglioside enriched microvesicles and costimulatory proteins in nanotubes connecting B lymphocytes, the way they are formed and transported across TNTs, however, still remained unclear. Here, using live cell confocal and Structured Illumination (SR-SIM) superresolution imaging, we show that B cells respond to bacterial (Cholera) toxin challenge by their subsequent internalization followed by rapid formation of intracellular microvesicles (MVs). These MVs are then transported between adjacent B cells via nanotubes. Selective transport-inhibition analysis of two abundant motor proteins in these cell types demonstrated that actin-based non-muscle myosin 2A dominantly mediates intercellular MV-transport via TNTs, in contrast to the microtubule-based dynein, as shown by the unchanged transport after inhibition of the latter. As suggested by SR-SIM images of GFP-CD86 transfected macrophages, these costimulatory molecules may be transferred by unusually shaped MVs through thick TNTs connecting macrophages. In contrast, in B cell cultures the same GFP-CD86 is dominantly transported along the membrane wall of TNTs. Such intercellular molecule-exchange can consequently improve the efficiency of antigen-dependent T cell activation, especially in macrophages with weak costimulator expression and T cell activation capacity. Such improved T cell activating potential of these two cell types may result in a more efficient cellular immune response and formation of immunological memory. The results also highlight the power of superresolution microscopy to uncover so far hidden structural details of biological processes, such as microvesicle formation and transport.

Alterations in the properties of the cell membrane due to glycosphingolipid accumulation in a model of Gaucher disease

Gaucher disease is a lysosomal storage disease characterized by the malfunction of glucocerebrosidase resulting in the accumulation of glucosylceramide and other sphingolipids in certain cells. Although the disease symptoms are usually attributed to the storage of undigested substrate in lysosomes, here we show that glycosphingolipids accumulating in the plasma membrane cause profound changes in the properties of the membrane. The fluidity of the sphingolipid-enriched membrane decreased accompanied by the enlargement of raft-like ordered membrane domains. The mobility of non-raft proteins and lipids was severely restricted, while raft-resident components were only mildly affected. The rate of endocytosis of transferrin receptor, a non-raft protein, was significantly retarded in Gaucher cells, while the endocytosis of the raft-associated GM1 ganglioside was unaffected. Interferon-γ-induced STAT1 phosphorylation was also significantly inhibited in Gaucher cells. Atomic force microscopy revealed that sphingolipid accumulation was associated with a more compliant membrane capable of producing an increased number of nanotubes. The results imply that glycosphingolipid accumulation in the plasma membrane has significant effects on membrane properties, which may be important in the pathogenesis of Gaucher disease.