Cell to cell hijacking: Role of membrane nanotubes

Cell to Cell communications is the pivot for life processes. Any event that disrupts this leads to the loss of physiological function, eventually leading to cell death. Evolutionarily, cells developed an elaborate mechanism to undertake this paramount responsibility through cell surface glycocalyx, receptors, integrins, and other recognition molecules. Cells also exchange through local acting soluble mediators as well as through vesicles and exosomes. Recent development in this field led to the identification of a spectacular network of membrane process that seems to be the supremo of all that was known about cellular communications. These are called membrane nanotubes or tunneling nanotubes (TNT). Cellular communication can be subdivided into contact and contactless. The former provides more rapid and molecule transfer as compared to the latter. Tunneling nanotubes (TNTs) are a novel type of contact-based communication. TNTs are straight, thin membrane extensions connecting cells over long distances first reported in PC12 cells in 2004. TNT is believed to form from actin-based membrane protrusion. There are three different models of TNT formation. a>Protrusions from one cell grow and extend until it reaches the other cell, followed by a membrane fusion. b> Membrane protrusions grow from both cells until they meet and establish a connection c> TNT formation by cell dislodgement when cells migrate further apart from each other, and during this movement, TNT is formed. It is possible that all these three models may be operational depending on cell types and context. Tunneling nanotubes (TNT) are dynamic connections between cells, representing a novel route for cell-to-cell communication. TNT was reported in various cell types, like epithelial cells, neuronal cells, mesenchyma cells, and immune cells engaged in intercellular exchanges of molecules, subcellular organelles, and pathogen and viruses transport routes. TNT can extend up to 200 µm in length and about 50 nm to 1500 nm in diameter in macrophages. TNT can be established between similar cell types (homo-TNT) or between one cell type and another ( hetro TNT) and thus may be involved in the initiation and growth of cancer as well as dissemination of cancer cells. TNTs are also assumed to play a role in treatment resistance, e.g., in chemotherapy treatment of cancer. Recently, TNT has been used to hijack mitochondria from healthy cells by the cancer cells as a source of energy. TNT was also reported to transport miRNA and other RNA to the surrounding stroma creating an environment suitable for cancer growth. More research in this discipline is needed to understand the full function of these wonderful nanostructures.

14-3-3η Promotes Invadosome Formation via the FOXO3–Snail Axis in Rheumatoid Arthritis Fibroblast-like Synoviocytes

Erosive destruction of joint structures is a critical event in the progression of rheumatoid arthritis (RA), in which fibroblast-like synoviocytes (FLS) are the primary effectors. We previously reported that the ability of RA FLS to degrade extracellular matrix (ECM) components depends on the formation of actin-rich membrane protrusions, called invadosomes, through processes that remain elusive. 14-3-3η belongs to a family of scaffolding proteins involved in a wide range of cellular functions, and its expression is closely related to joint damage and disease activity in RA patients. In this study, we sought to assess the role of 14-3-3η in joint damage by examining its contribution to the invadosome formation phenotype of FLS. Using human primary FLS, we show that 14-3-3η expression is closely associated with their ability to form invadosomes. Furthermore, knockdown of 14-3-3η using shRNAs decreases the level of invadosome formation in RA FLS, whereas addition of the recombinant protein to FLS from healthy individuals promotes their formation. Mechanistic studies suggest that 14-3-3η regulates invadosome formation by increasing Snail expression, a mechanism that involves nuclear exclusion of the transcription repressor FOXO3. Our results implicate the 14-3-3η–FOXO3–Snail axis in promoting the aggressive ECM-degrading phenotype of RA FLS, and suggest a role for this scaffolding protein in cartilage degradation.

Microtopographical guidance of macropinocytic signaling patches

In fast-moving cells such as amoeba and immune cells, dendritic actin filaments are spatiotemporally regulated to shape large-scale plasma membrane protrusions. Despite their importance in migration, as well as in particle and liquid ingestion, how their dynamics are affected by micrometer-scale features of the contact surface is still poorly understood. Here, through quantitative image analysis of Dictyostelium on microfabricated surfaces, we show that there is a distinct mode of topographical guidance directed by the macropinocytic membrane cup. Unlike other topographical guidance known to date that depends on nanometer-scale curvature sensing protein or stress fibers, the macropinocytic membrane cup is driven by the Ras/PI3K/F-actin signaling patch and its dependency on the micrometer-scale topographical features, namely PI3K/F-actin–independent accumulation of Ras-GTP at the convex curved surface, PI3K-dependent patch propagation along the convex edge, and its actomyosin-dependent constriction at the concave edge. Mathematical model simulations demonstrate that the topographically dependent initiation, in combination with the mutually defining patch patterning and the membrane deformation, gives rise to the topographical guidance. Our results suggest that the macropinocytic cup is a self-enclosing structure that can support liquid ingestion by default; however, in the presence of structured surfaces, it is directed to faithfully trace bent and bifurcating ridges for particle ingestion and cell guidance.

M-Sec induced by HTLV-1 mediates an efficient viral transmission

Human T-cell leukemia virus type 1 (HTLV-1) infects target cells primarily through cell-to-cell routes. Here, we provide evidence that cellular protein M-Sec plays a critical role in this process. When purified and briefly cultured, CD4+ T cells of HTLV-1 carriers, but not of HTLV-1- individuals, expressed M-Sec. The viral protein Tax was revealed to mediate M-Sec induction. Knockdown or pharmacological inhibition of M-Sec reduced viral infection in multiple co-culture conditions. Furthermore, M-Sec knockdown reduced the number of proviral copies in the tissues of a mouse model of HTLV-1 infection. Phenotypically, M-Sec knockdown or inhibition reduced not only plasma membrane protrusions and migratory activity of cells, but also large clusters of Gag, a viral structural protein required for the formation of viral particles. Taken together, these results suggest that M-Sec induced by Tax mediates an efficient cell-to-cell viral infection, which is likely due to enhanced membrane protrusions, cell migration, and the clustering of Gag.

In-depth characterization of the chikungunya virus replication cycle

The chikungunya virus has spread globally with a remarkably high attack rate. Infection causes arthralgic sequelae that can last for years. Nevertheless, there are no specific drugs or vaccines to contain the virus. Understanding the biology of the virus, such as its replication cycle, is a powerful tool to identify new drugs and comprehend virus-host interactions. Even though the chikungunya virus has been known for a long time (first described in 1952), many aspects of the replication cycle remain unclear. Furthermore, part of the cycle is based on observations of other alphaviruses. In this study, we used electron and scanning microscopy, as well as biological assays, to analyze and investigate the stages of the chikungunya virus replication cycle. Based on our data, we found other infection cellular activities than those usually described for the chikungunya virus replication cycle, i.e. we show particles enveloping intracellularly without budding in a membrane-delimited morphogenesis area; and we also observed virion release by membrane protrusions. Our work provides novel details regarding the biology of chikungunya virus and fills gaps in our knowledge of its replication cycle. These findings may contribute to a better understanding of virus-host interactions and support the development of antivirals. IMPORTANCE The understanding of virus biology is essential to containing virus dissemination, and exploring the virus replication cycle is a powerful tool to do this. There are many points in the biology of the chikungunya virus that need to be clarified, especially regarding its replication cycle. Our incomplete understanding of chikungunya virus infection stages is based on studies with other alphaviruses. We systematized the chikungunya virus replication cycle using microscopic imaging in the order of infection stages: entry, replication, protein synthesis, assembly/morphogenesis, and release. The imaging evidence shows novel points in the replication cycle of enveloping without budding, as well as particle release by cell membrane protrusion.

On the Role of Electrostatic Repulsion in Topological Defect-Driven Membrane Fission

Within a modified Langevin Poisson–Boltzmann model of electric double layers, we derived an analytical expression for osmotic pressure between two charged surfaces. The orientational ordering of the water dipoles as well as the space dependencies of electric potentials, electric fields, and osmotic pressure between two charged spheres were taken into account in the model. Thus, we were able to capture the interaction between the parent cell and connected daughter vesicle or the interactions between neighbouring beads in necklace-like membrane protrusions. The predicted repulsion between them can facilitate the topological antidefect-driven fission of membrane daughter vesicles and the fission of beads of undulated membrane protrusions.

A Computational Modeling of Invadopodia Protrusion Into an Extracellular Matrix Fiber Network

Invadopodia are dynamic actin-rich membrane protrusions that have been implicated in cancer cell invasion and metastasis. In addition, invasiveness of cancer cells is strongly correlated with invadopodia formation, which are observed during extravasation and colonization of metastatic cancer cells at secondary sites. However, quantitative understanding of the interaction of invadopodia with extracellular matrix (ECM) is lacking, and how invadopodia protrusion speed is associated with the frequency of protrusion-retraction cycles remains unknown. Here, we present a computational framework for the characterization of invadopodia protrusions which allows two way interactions between intracellular branched actin network and ECM fibers network. We have applied this approach to predicting the invasiveness of cancer cells by computationally knocking out actin-crosslinking molecules, such as α-actinin, filamin and fascin. The resulting simulations reveal distinct invadopodia dynamics with cycles of protrusion and retraction. Specifically, we found that 1) increasing accumulation of MT1-MMP at tips of invadopodia as the duration of protrusive phase is increased, and 2) the movement of nucleus toward the leading edge of the cell becomes unstable as duration of the retractile phase (or myosin turnover time) is longer than 1 min.

Comparative analysis of the MyTH4-FERM myosins reveals insights into the determinants of actin track selection in polarized epithelia

MyTH4-FERM (MF) myosins evolved to play a role in the creation and function of a variety of actin-based membrane protrusions that extend from cells. Here, we performed an analysis of the MF myosins, Myo7A, Myo7B, and Myo10, to gain insight into how they select for their preferred actin networks. Using enterocytes that create spatially separated actin tracks in the form of apical microvilli and basal filopodia, we show that actin track selection is principally guided by the mode of oligomerization of the myosin along with the identity of the motor domain, with little influence from the specific composition of the lever arm. Chimeric variants of Myo7A and Myo7B fused to a leucine zipper parallel dimerization sequence in place of their native tails both selected apical microvilli as their tracks, while a truncated Myo10 used its native antiparallel coiled-coil to traffic to the tips of filopodia. Swapping lever arms between the Class 7 and 10 myosins did not change actin track preference. Surprisingly, fusing the motor-neck region of Myo10 to a leucine zipper or oligomerization sequences derived from the Myo7A and Myo7B cargo proteins USH1G and ANKS4B, respectively, re-encoded the actin track usage of Myo10 to apical microvilli with significant efficiency.