Endocytosis in plant–microbe interactions

Plants encounter throughout their life all kinds of microorganisms, such as bacteria, fungi, or oomycetes, with either friendly or unfriendly intentions. During evolution, plants have developed a wide range of defense mechanisms against attackers. In return, adapted microbes have developed strategies to overcome the plant lines of defense, some of these microbes engaging in mutualistic or parasitic endosymbioses. By sensing microbe presence and activating signaling cascades, the plasma membrane through its dynamics plays a crucial role in the ongoing molecular dialogue between plants and microbes. This review describes the contribution of endocytosis to different aspects of plant–microbe interactions, microbe recognition and development of a basal immune response, and colonization of plant cells by endosymbionts. The putative endocytic routes for the entry of microbe molecules or microbes themselves are explored with a special emphasis on clathrin-mediated endocytosis. Finally, we evaluate recent findings that suggest a link between the compartmentalization of plant plasma membrane into microdomains and endocytosis.     

Cell signalling and Trypanosoma cruzi invasion

Mammalian cell invasion by the protozoan pathogen Trypanosoma cruzi is critical to its survival in the host. To promote its entry into a wide variety of non-professional phagocytic cells, infective trypomastigotes exploit an arsenal of heterogenous surface glycoproteins, secreted proteases and signalling agonists to actively manipulate multiple host cell signalling pathways. Signals initiated in the parasite upon contact with mammalian cells also function as critical regulators of the invasion process. Whereas the full spectrum of cellular responses modulated by T. cruzi is not yet known, mounting evidence suggests that these pathways impinge on a number of cellular processes, in particular the ubiquitous wound-repair mechanism exploited for lysosome-mediated parasite entry. Furthermore, differential engagement of host cell signalling pathways in a cell type-specific manner and modulation of host cell gene expression by T. cruzi are becoming recognized as essential determinants of infectivity and intracellular survival by this pathogen.     

The molecular basis of HIV entry

Infection by HIV starts when the virus attaches to a susceptible cell. For viral replication to continue, the viral envelope must fuse with a cellular membrane, thereby delivering the viral core to the cytoplasm, where the RNA genome is reverse-transcribed. The key players in this entry by fusion are the envelope glycoprotein, on the viral side, and CD4 and a co-receptor, CCR5 or CXCR4, on the cellular side. Here, the interplay of these molecules is reviewed from cell-biological, structural, mechanistic, and modelling-based perspectives. Hypotheses are evaluated regarding the cellular compartment for entry, the transfer of virus through direct cell-to-cell contact, the sequence of molecular events, and the number of molecules involved on each side of the virus-cell divide. An emerging theme is the heterogeneity among the entry mediators on both sides, a diversity that affects the efficacy of entry inhibitors, be they small-molecule ligands, peptides or neutralizing antibodies. These insights inform rational strategies for therapy as well as vaccination.     

CD44 binds to the Shigella IpaB protein and participates in bacterial invasion of epithelial cells

Shigella entry into epithelial cells is characterized by a transient reorganization of the host cell cytoskeleton at the site of bacterial interaction with the cell membrane, which leads to bacterial engulfment in a macropinocytic process. Using affinity chromatography on HeLa cell extracts, we show here that the hyaluronan receptor CD44 associates with IpaB, a Shigella protein that is secreted upon cell contact. Overlay and solid-phase assays indicated that IpaB binds directly to the extracellular domain of CD44; binding is saturable and inhibitable, with a half- maximal inhibitory concentration of 175 nM. Immunoprecipitation experiments showed that IpaB associates with CD44 during Shigella entry. CD44 is recruited at bacterial entry sites and localizes at the plasma membrane of cellular extensions induced by Shigella . Pretreatment of cells with an anti-CD44 monoclonal antibody resulted in inhibition of Shigella -induced cytoskeletal reorganization, as well as inhibition of bacterial entry, whereas transfection of CD44 in cells that are deficient for CD44 results in increased bacterial binding to cells and internalization. The IpaB–CD44 interaction appears to be required for Shigella invasion by initiating the early steps of the entry process.     

Quantum information teleportation through biological wires,gravitational micro-bio-holes and holographic micro-bio-systems: A hypothesis

Biological systems like cells, bacteria, chloroplasts and other micro-organisms could exchange quantum particles like electrons, photons and gravitational waves and have large distant information teleportation. This is because that their DNAs and membranes are formed from quantum particles like electrons and protons and by their motions, some currents and waves are emerged. These waves have the main role in information teleportation. There are different methods which could be used for quantum information teleportation in biological system. Some of these mechanisms are: 1. Microbes, micro-bubbles and some other biological molecules like to form some biological lines specially near the cellular gates. Also, some biological lines may be formed between two cells. These biological lines could play the role of wires which transmit information from a place to another one. For example, some signatures of this quantum information teleportation could be seen in biological lines which are emerged near the plant cell walls or gates or close to chloroplasts. Chloroplasts shoot some spinors which maybe confined within the micro-bubbles or absorb by microbes. These bubbles and microbes may join to each other and form some biological lines which may be strengthen from a plant cell to another. These biological lines could be seen near the plant cell walls or on a metal which connects two parts of a leaf. 2. Some another signatures of “quantum photon exchange or quantum information teleportation” could be seen between microbes under the objective lenses and macro-objects on the eye lenses of a light microscope. It seems that as microscope make big images from microbes for us, produce small pictures of macro-objects for microbes such as they could diagnose them and interact with them. This property could be used in controlling microbes. 3. Another way for controlling microbes is using of virtual shapes which are induced by a special light source. For example, using a multi-gonal lamp, one can induce multi-gonal shape within the micro-bubbles. Also, this special lamp could force microbes and micro-bubbles to build multi-gonal colonies on a metal-glass slide. Maybe, by using this property, one can build a light source with the shape of anti-microbial matter and induce anti-microbial property within micro-bubbles. 4. Another main way for quantum teleportation is using of gravitational holes which may be emerged by increasing concentration of microbes and heavy cells in some points. These holes absorb microbes and micro-bubbles and conduct them to the heavy cells. Usually, there are some white holes near these dark holes which as a proposal, one can assume that these white holes are another end of gravitational holes and emit photons which are entered from dark end. 5. And finally, a very main mechanism for quantum information teleportation with microbes and controlling them is using of a holography and inducing virtual microbes and biological molecules in biological systems. For example, by a combinations of two lights with different colors under a light microscope in a dark room, one may induce some non-virtual microbes in biological systems such as each microbe interacts with a virtual microbe. This is because that light waves take photos of microbes, collide with lenses of microscopes and return to the slide and form virtual microbes or biological molecules. This technique could be used in curing diseases. Although, results of our experiments show the correctness of these mechanisms and theories, however, for the moment, we propose them only as a proposal and hypothesis and hope that other scientists do similar experiments. Also, some of our experiments may be at preliminary stages; however they could be used as a hypothesis, proposal and guidance.     

Assembly of the Murine Leukemia Virus Is Directed towards Sites of Cell–Cell Contact

We have investigated the underlying mechanism by which direct cell–cell contact enhances the efficiency of cell-to-cell transmission of retroviruses. Applying 4D imaging to a model retrovirus, the murine leukemia virus, we directly monitor and quantify sequential assembly, release, and transmission events for individual viral particles as they happen in living cells. We demonstrate that de novo assembly is highly polarized towards zones of cell–cell contact. Viruses assembled approximately 10-fold more frequently at zones of cell contact with no change in assembly kinetics. Gag proteins were drawn to adhesive zones formed by viral Env glycoprotein and its cognate receptor to promote virus assembly at cell–cell contact. This process was dependent on the cytoplasmic tail of viral Env. Env lacking the cytoplasmic tail while still allowing for contact formation, failed to direct virus assembly towards contact sites. Our data describe a novel role for the viral Env glycoprotein in establishing cell–cell adhesion and polarization of assembly prior to becoming a fusion protein to allow virus entry into cells.     

Assembly of the Murine Leukemia Virus Is Directed towards Sites of Cell–Cell Contact

We have investigated the underlying mechanism by which direct cell–cell contact enhances the efficiency of cell-to-cell transmission of retroviruses. Applying 4D imaging to a model retrovirus, the murine leukemia virus, we directly monitor and quantify sequential assembly, release, and transmission events for individual viral particles as they happen in living cells. We demonstrate that de novo assembly is highly polarized towards zones of cell–cell contact. Viruses assembled approximately 10-fold more frequently at zones of cell contact with no change in assembly kinetics. Gag proteins were drawn to adhesive zones formed by viral Env glycoprotein and its cognate receptor to promote virus assembly at cell–cell contact. This process was dependent on the cytoplasmic tail of viral Env. Env lacking the cytoplasmic tail while still allowing for contact formation, failed to direct virus assembly towards contact sites. Our data describe a novel role for the viral Env glycoprotein in establishing cell–cell adhesion and polarization of assembly prior to becoming a fusion protein to allow virus entry into cells.     

Direct cell contact influences bone marrow mesenchymal stem cell fate

Adult bone marrow-derived mesenchymal stem cells (MSC) can differentiate into various cell types of mesenchymal origin, but mechanisms regulating such cellular changes are unclear. We have conducted co-culture experiments to examine whether mesenchymal stem cell differentiation is influenced by indirect or direct contact with differentiated cells. Cultured adult mesenchymal stem cells showed some characteristics of synthetic state vascular smooth muscle cells (SMC). When co-cultured with vascular endothelial cells (EC) without cell contact, they exhibited abundant well-organised smooth muscle alpha-actin (alpha-actin) filaments. Direct co-culture with endothelial cells resulted in increased smooth muscle alpha-actin mRNA and protein, yet also comprehensive disruption of smooth muscle alpha-actin filament organisation. In order to assess whether these cell contact effects on mesenchymal stem cells were cell type specific, we also analysed direct co-cultures of mesenchymal stem cells with dermal fibroblasts. However, these experiments were characterised by the appearance of abundant spindle-shaped myofibroblast-like cells containing organised smooth muscle alpha-actin filaments. Thus, direct contact with distinct differentiated cells may be a critical determinant of mesenchymal stem cell fate in blood vessels and other connective tissues.     

Impact of Photocatalysis on Fungal Cells: Depiction of Cellular and Molecular Effects on Saccharomyces cerevisiae

We have investigated the antimicrobial effects of photocatalysis on the yeast model Saccharomyces cerevisiae. To accurately study the antimicrobial mechanisms of the photocatalytic process, we focused our investigations on two questions: the entry of the nanoparticles in treated cells and the fate of the intracellular environment. Transmission electronic microscopy did not reveal any entry of nanoparticles within the cells, even for long exposure times, despite degradation of the cell wall space and deconstruction of cellular compartments. In contrast to proteins located at the periphery of the cells, intracellular proteins did not disappear uniformly. Disappearance or persistence of proteins from the pool of oxidized intracellular isoforms was not correlated to their functions. Altogether, our data suggested that photocatalysis induces the establishment of an intracellular oxidative environment. This hypothesis was sustained by the detection of an increased level of superoxide ions (O₂°⁻) in treated cells and by greater cell cultivability for cells expressing oxidant stress response genes during photocatalytic exposure. The increase in intracellular ROS, which was not connected to the entry of nanoparticles within the cells or to a direct contact with the plasma membrane, could be the result of an imbalance in redox status amplified by chain reactions. Moreover, we expanded our study to other yeast and filamentous fungi and pointed out that, in contrast to the laboratory model S. cerevisiae, some environmental strains are very resistant to photocatalysis. This could be related to the cell wall composition and structure.     

Micro-managing the executioner: pathogen targeting of mitochondria

Eukaryotic cell viability is largely regulated at the level of mitochondria, with cell death executed by endogenous proteins that act to increase the permeability of the inner and/or outer membranes of these organelles. The gastric pathogen, Helicobacter pylori, can mimic this mechanism by producing the pro-apoptotic toxin, VacA, which was recently demonstrated to (i) localize to mitochondria within epithelial cells, (ii) rapidly transport into mitochondria in vitro, and (iii) induce changes consistent with permeabilization of mitochondrial membranes by a mechanism dependent on cellular entry and toxin membrane channel activity. The targeting of mitochondrial membranes is emerging as a strategy used by pathogenic microbes to control cell viability while circumventing upstream pathways and checkpoints of cell death.     

Virus entry paradigms

Viruses, despite being relatively simple in structure and composition, have evolved to exploit complex cellular processes for their replication in the host cell. After binding to their specific receptor on the cell surface, viruses (or viral genomes) have to enter cells to initiate a productive infection. Though the entry processes of many enveloped viruses is well understood, that of most non-enveloped viruses still remains unresolved. Recent studies have shown that compared to direct fusion at the plasma membrane, endocytosis is more often the preferred means of entry into the target cell. Receptor-mediated endocytic pathways such as the dynamin-dependent clathrin and caveolar pathways are well characterized as viral entry portals. However, many viruses are able to utilize multiple uptake pathways. Fluid phase uptake, though relatively non-specific in terms of its cargo, potentially aids viral infection by its ability to intersect with the endocytic pathway. In fact, many viruses despite using specialized pathways for entry are still able to generate productive infection via fluid phase uptake. Macropinocytosis, a major fluid uptake pathway found in epithelial cells and fibroblasts, is stimulated by growth factor receptors. Many viruses can induce these signaling cascades in cells leading to macropinocytosis. Though endocytic trafficking is utilized by both enveloped and non-enveloped viruses, key differences lie in the way membranes are traversed to deposit the viral genome at its site of replication. This review will discuss recent developments in the rapidly evolving field of viral entry.     

Modulation of bacterial entry into epithelial cells by association between vinculin and the Shigella IpaA invasin.

Shigella flexneri is the causative agent of bacillary dysentery in humans. Shigella invasion of epithelial cells is characterized by cytoskeletal rearrangements and formation of cellular projections engulfing the bacterium in a macropinocytic process. We show here that vinculin, a protein involved in linking actin filaments to the plasma membrane, is a direct target of Shigella during cell invasion. IpaA, a Shigella protein secreted upon cell contact, rapidly associates with vinculin during bacterial invasion. Although defective for cell entry, an ipaA mutant is still able to induce foci of actin polymerization, but differs from wild-type Shigella in its ability to recruit vinculin and alpha-actinin. Presumably, IpaA-vinculin interaction initiates the formation of focal adhesion-like structures required for efficient invasion.     

Peptide entry inhibitors of enveloped viruses: The importance of interfacial hydrophobicity

There are many peptides known that inhibit the entry of enveloped viruses into cells, including one peptide that is successfully being used in the clinic as a drug. In this review, we discuss the discovery, antiviral activity and mechanism of action of such peptides. While peptide entry inhibitors have been discovered by a wide variety of approaches (structure-based, accidental, intentional, rational and brute force) we show here that they share a common physical chemical property: they are at least somewhat hydrophobic and/or amphipathic and have a propensity to interact with membrane interfaces. We propose that this propensity drives a shared mechanism of action for many peptide entry inhibitors, involving direct interactions with viral and cellular membranes, as well as interactions with the complex hydrophobic protein/lipid interfaces that are exposed, at least transiently, during virus–cell fusion. By interacting simultaneously with the membrane interfaces and other critical hydrophobic surfaces, we hypothesize that peptide entry inhibitors can act by changing the physical chemistry of the membranes, and the fusion protein interfaces bridging them, and by doing so interfere with the fusion of cellular and viral membranes. Based on this idea, we propose that an approach that focuses on the interfacial hydrophobicity of putative entry inhibitors could lead to the efficient discovery of novel, broad-spectrum viral entry inhibitors. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.     

Contact response of cells can mediate morphogenetic pattern formation

Theories of morphogenetic pattern formation have included Turing's chemical prepatterns, mechanochemical interactions, cell sorting, and other mechanisms involving guided motion or signalling of cells. Many of these theories presuppose long-range cellular communication or other controls such as chemical concentration fields. However, the possibility that direct interactions between cells can lead to order and structure has not been seriously investigated in mathematical models. In this paper we consider this possibility, with emphasis on cells that reorient and align with each other when they come into contact. We show that such contact responses can account for the formation of multicellular patterns called parallel arrays. These patterns typically occur in tissue cultures of fibroblasts, and consist of clusters of cells sharing a common axis of orientation. Using predictions of a mathematical model and computer simulations of cell motion and interactions we show that contact responses alone, in the absence of other global controls, can promote the formation of these patterns. We suggest other situations in which patterns may result from direct cellular communication. Previous theories of morphogenesis are briefly reviewed and compared with this proposed mechanism.     

Emerging roles for ubiquitin in adenovirus cell entry

Adenovirus relies on numerous interactions between viral and host cell proteins to efficiently enter cells. Undoubtedly, post-translational modifications of host and cellular proteins can impact the efficiency of this cell entry process. Ubiquitylation, once simply thought of as a modification targeting proteins for proteasomal degradation, is now known to regulate protein trafficking within cells, protein-protein interactions and cell signalling pathways. Accumulating evidence suggests that protein ubiquitylation can influence all stages of the life cycle of other viruses such as cell entry, replication and egress. Until recently, the influence of ubiquitylation has only been documented during adenovirus replication. This review highlights the most recent evidence demonstrating direct engagement of host ubiquitylation and SUMOylation machinery by adenovirus during cell entry. Additionally, potential roles for host protein ubiquitylation and the potential for adenovirus regulation of host ubiquitylation machinery during cell entry are discussed.     

Bi-directional exchange of membrane components occurs during co-culture of mesenchymal stem cells and nucleus pulposus cells

Mesenchymal stem cell (MSC)-based therapies have been proposed as novel treatments for intervertebral disc (IVD) degeneration. We have previously demonstrated that when MSCs are co-cultured with nucleus pulposus (NP) cells with direct cell-cell contact, they differentiate along the NP lineage and simultaneously stimulate the degenerate NP cell population to regain a normal (non-degenerate) phenotype, an effect which requires cell-cell communication. However, the mechanisms by which NP cells and MSCs interact in this system are currently unclear. Thus, in this study we investigated a range of potential mechanisms for exchange of cellular components or information that may direct these changes, including cell fusion, gap-junctional communication and exchange of membrane components by direct transfer or via microvesicle formation. Flow cytometry of fluorescently labeled MSCs and NP cells revealed evidence of some cell fusion and formation of gapjunctions, although at the three timepoints studied these phenomena were detectable only in a small proportion of cells. While these mechanisms may play a role in cell-cell communication, the data suggests they are not the predominant mechanism of interaction. However, flow cytometry of fluorescently dual-labeled cells showed that extensive bi-directional transfer of membrane components is operational during direct co-culture of MSCs and NP cells. Furthermore, there was also evidence for secretion and internalization of membrane-bound microvesicles by both cell types. Thus, this study highlights bi-directional intercellular transfer of membrane components as a possible mechanism of cellular communication between MSC and NP cells.     

Human T-cell leukemia virus type 1 receptor expression among syncytium-resistant cell lines revealed by a novel surface glycoprotein-immunoadhesin

The envelope glycoproteins of human T-cell leukemia virus type 1 (HTLV-1) perform functions that are crucial for virus entry into cells. The surface glycoprotein (SU) is responsible for viral recognition of, and binding to, target cells through its interaction with an unknown cell surface receptor. To facilitate molecular analysis of the receptor-binding properties of SU and to characterize the cellular receptor employed by HTLV-1, we have expressed a recombinant SU fused to the Fc domain of human immunoglobulin G. Here, we demonstrate that this novel SU-immunoadhesin retains both the biochemical properties of Fc and the receptor-binding specificity of the HTLV-1 SU. We use this SU-immunoadhesin to demonstrate, by direct cell surface binding assays, that the receptor used by HTLV-1 has been conserved through vertebrate evolution. Moreover, using murine-human somatic cell hybrids we provide data that do not support the previously assigned location for the HTLV-1 receptor on human chromosome 17. Most importantly, we show that many cell lines that are resistant to HTLV-1 envelope-mediated infection and syncytium formation express functional receptors that are recognized by the HTLV-1 SU. Based on our results, we suggest that for some HTLV-1-resistant cell lines the block to viral entry occurs at a late post-receptor-binding step of the entry process. Our findings will be of value in developing new strategies to identify the cellular receptor used by HTLV-1.     

Plant–microbe interactions in the apoplast: Communication at the plant cell wall

The apoplast is a continuous plant compartment that connects cells between tissues and organs and is one of the first sites of interaction between plants and microbes. The plant cell wall occupies most of the apoplast and is composed of polysaccharides and associated proteins and ions. This dynamic part of the cell constitutes an essential physical barrier and a source of nutrients for the microbe. At the same time, the plant cell wall serves important functions in the interkingdom detection, recognition, and response to other organisms. Thus, both plant and microbe modify the plant cell wall and its environment in versatile ways to benefit from the interaction. We discuss here crucial processes occurring at the plant cell wall during the contact and communication between microbe and plant. Finally, we argue that these local and dynamic changes need to be considered to fully understand plant–microbe interactions.

Plants and microbes modify the host cell wall to benefit from the interaction by altering its properties, which are defined by the biochemistry of its polysaccharides, regulated by cell wall ions and proteins.     

Getting to the roots of parasitism by nematodes

Most phytoparasitic nematodes infect plant roots and some species have evolved sophisticated interactive relationships with host cells to sustain a sedentary parasitic habit. The recent isolation of multiple 'parasitism genes' expressed specifically within the esophageal gland cells of sedentary phytonematodes suggests that an arsenal of different secreted parasitism proteins from the nematode might have direct effects on recipient host cells. These include cell wall modifications and potential interactions with signal transduction receptors in the extracellular space, as well as direct introduction of proteins into host cells that might influence cellular metabolism, the cell cycle, selective protein degradation, a localized defense response and regulatory activity within the host cell nucleus.