Role of amphipathic helix of a herpesviral protein in membrane deformation and T cell receptor downregulation

Antigen B from Echinococcus granulosus enters mammalian cells by endocytic pathways

Background

Cystic hydatid disease is a zoonosis caused by the larval stage (hydatid) of Echinococcus granulosus (Cestoda, Taeniidae). The hydatid develops in the viscera of intermediate host as a unilocular structure filled by the hydatid fluid, which contains parasitic excretory/secretory products. The lipoprotein Antigen B (AgB) is the major component of E. granulosus metacestode hydatid fluid. Functionally, AgB has been implicated in immunomodulation and lipid transport. However, the mechanisms underlying AgB functions are not completely known.

Methodology/Principal findings

In this study, we investigated AgB interactions with different mammalian cell types and the pathways involved in its internalization. AgB uptake was observed in four different cell lines, NIH-3T3, A549, J774 and RH. Inhibition of caveolae/raft-mediated endocytosis causes about 50 and 69% decrease in AgB internalization by RH and A549 cells, respectively. Interestingly, AgB colocalized with the raft endocytic marker, but also showed a partial colocalization with the clathrin endocytic marker. Finally, AgB colocalized with an endolysosomal tracker, providing evidence for a possible AgB destination after endocytosis.

Conclusions/Significance

The results indicate that caveolae/raft-mediated endocytosis is the main route to AgB internalization, and that a clathrin-mediated entry may also occur at a lower frequency. A possible fate for AgB after endocytosis seems to be the endolysosomal system. Cellular internalization and further access to subcellular compartments could be a requirement for AgB functions as a lipid carrier and/or immunomodulatory molecule, contributing to create a more permissive microenvironment to metacestode development and survival.

Antigen B (AgB) is an oligomeric lipoprotein highly abundant in Echinococcus granulosus hydatid fluid. AgB has already been characterized as an immunomodulatory protein, capable of inducing a permissive immune response to parasite development. Also, an important role in lipid acquisition is attributed to AgB, because it has been found associated to different classes of host lipids. However, the mechanisms of interaction employed by AgB to perform its functions remain undetermined. In this study, we demonstrate that mammalian cells are able to internalize E. granulosus AgB in culture and found that specific mechanisms of endocytosis are involved. Our results extend the understanding of AgB biological role indicating cellular internalization as a mechanism of interaction, which in turn, may represent a target to intervention.

Structure of a lipid A phosphoethanolamine transferase suggests how conformational changes govern substrate binding

Multidrug-resistant (MDR) gram-negative bacteria have increased the prevalence of fatal sepsis in modern times. Colistin is a cationic antimicrobial peptide (CAMP) antibiotic that permeabilizes the bacterial outer membrane (OM) and has been used to treat these infections. The OM outer leaflet is comprised of endotoxin containing lipid A, which can be modified to increase resistance to CAMPs and prevent clearance by the innate immune response. One type of lipid A modification involves the addition of phosphoethanolamine to the 1 and 4' headgroup positions by phosphoethanolamine transferases. Previous structural work on a truncated form of this enzyme suggested that the full-length protein was required for correct lipid substrate binding and catalysis. We now report the crystal structure of a full-length lipid A phosphoethanolamine transferase from Neisseria meningitidis, determined to 2.75-Å resolution. The structure reveals a previously uncharacterized helical membrane domain and a periplasmic facing soluble domain. The domains are linked by a helix that runs along the membrane surface interacting with the phospholipid head groups. Two helices located in a periplasmic loop between two transmembrane helices contain conserved charged residues and are implicated in substrate binding. Intrinsic fluorescence, limited proteolysis, and molecular dynamics studies suggest the protein may sample different conformational states to enable the binding of two very different- sized lipid substrates. These results provide insights into the mechanism of endotoxin modification and will aid a structure-guided rational drug design approach to treating multidrug-resistant bacterial infections.

The Modulation of Apoptotic Pathways by Gammaherpesviruses

Apoptosis or programmed cell death is a tightly regulated process fundamental for cellular development and elimination of damaged or infected cells during the maintenance of cellular homeostasis. It is also an important cellular defense mechanism against viral invasion. In many instances, abnormal regulation of apoptosis has been associated with a number of diseases, including cancer development. Following infection of host cells, persistent and oncogenic viruses such as the members of the Gammaherpesvirus family employ a number of different mechanisms to avoid the host cell’s “burglar” alarm and to alter the extrinsic and intrinsic apoptotic pathways by either deregulating the expressions of cellular signaling genes or by encoding the viral homologs of cellular genes. In this review, we summarize the recent findings on how gammaherpesviruses inhibit cellular apoptosis via virus-encoded proteins by mediating modification of numerous signal transduction pathways. We also list the key viral anti-apoptotic proteins that could be exploited as effective targets for novel antiviral therapies in order to stimulate apoptosis in different types of cancer cells.

Modulating p56Lck in T-Cells by a Chimeric Peptide Comprising Two Functionally Different Motifs of Tip from Herpesvirus saimiri

The Lck interacting protein Tip of Herpesvirus saimiri is responsible for T-cell transformation both in vitro and in vivo. Here we designed the chimeric peptide hTip-CSKH, comprising the Lck specific interacting motif CSKH of Tip and its hydrophobic transmembrane sequence (hTip), the latter as a vector targeting lipid rafts. We found that hTip-CSKH can induce a fivefold increase in proliferation of human and Aotus sp. T-cells. Costimulation with PMA did not enhance this proliferation rate, suggesting that hTip-CSKH is sufficient and independent of further PKC stimulation. We also found that human Lck phosphorylation was increased earlier after stimulation when T-cells were incubated previously with hTip-CSKH, supporting a strong signalling and proliferative effect of the chimeric peptide. Additionally, Lck downstream signalling was evident with hTip-CSKH but not with control peptides. Importantly, hTip-CSKH could be identified in heavy lipid rafts membrane fractions, a compartment where important T-cell signalling molecules (LAT, Ras, and Lck) are present during T-cell activation. Interestingly, hTip-CSKH was inhibitory to Jurkat cells, in total agreement with the different signalling pathways and activation requirements of this leukemic cell line. These results provide the basis for the development of new compounds capable of modulating therapeutic targets present in lipid rafts.

A membrane proximal helix in the cytosolic domain of the human APP interacting protein LR11/SorLA deforms liposomes

Over the last decade, compelling evidence has linked the development of Alzheimer's disease (AD) to defective intracellular trafficking of the amyloid precursor protein (APP). Faulty APP trafficking results in an overproduction of Aβ peptides, which is generally agreed to be the primary cause of AD-related pathogenesis. LR11 (SorLA), a type I transmembrane sorting receptor, has emerged as a key regulator of APP trafficking and processing. It directly interacts with APP and diverts it away from amyloidogenic processing. The 54-residue cytosolic domain of LR11 is essential for its proper intracellular localization and trafficking which, in turn, determines the fate of APP. Here, we have found a surprising membrane-proximal amphipathic helix in the cytosolic domain of LR11. Moreover, a peptide corresponding to this region folds into an α-helical structure in the presence of liposomes and transforms liposomes to small vesicles and tubule-like particles. We postulate that this amphipathic helix may contribute to the dynamic remodeling of membrane structure and facilitate LR11 intracellular transport.

The arginine-rich N-terminal domain of ROP18 is necessary for vacuole targeting and virulence of Toxoplasma gondii

Toxoplasma gondii uses specialized secretory organelles called rhoptries to deliver virulence determinants into the host cell during parasite invasion. One such determinant called rhoptry protein 18 (ROP18) is a polymorphic serine/threonine kinase that phosphorylates host targets to modulate acute virulence. Following secretion into the host cell, ROP18 traffics to the parasitophorous vacuole membrane (PVM) where it is tethered to the cytosolic face of this host-pathogen interface. However, the functional consequences of PVM association are not known. In this report, we show that ROP18 mutants altered in an arginine-rich domain upstream of the kinase domain fail to associate to the PVM following secretion from rhoptries. During infection, host cells upregulate immunity-related GTPases that localize to and destroy the PVM surrounding the parasites. ROP18 disarms this host innate immune pathway by phosphorylating IRGs in a critical GTPase domain and preventing loading on the PVM. Vacuole-targeting mutants of ROP18 failed to phosphorylate Irga6 and were unable to divert IRGs from the PVM, despite retaining intrinsic kinase activity. As a consequence, these mutants were avirulent in a mouse model of acute toxoplasmosis. Thus, the association of ROP18 with the PVM, mediated by its N-terminal arginine-rich domain, is critical to its function as a virulence determinant.

Function of membrane rafts in viral lifecycles and host cellular response

Membrane rafts are small (10–200 nm) sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Membrane rafts play an important role in viral infection cycles and viral virulence. Viruses are divided into four main classes, enveloped DNA virus, enveloped RNA virus, nonenveloped DNA virus, and nonenveloped RNA virus. General virus infection cycle is also classified into two sections, the early stage (entry process) and the late stage (assembly, budding, and release processes of virus particles). In the viral cycle, membrane rafts act as a scaffold of many cellular signal transductions, which are associated with symptoms caused by viral infections. In this paper, we describe the functions of membrane rafts in viral lifecycles and host cellular response according to each virus classification, each stage of the virus lifecycle, and each virus-induced signal transduction.

Bilateral inhibition of HAUSP deubiquitinase by a viral interferon regulatory factor protein

Herpesvirus-associated ubiquitin-specific protease (HAUSP) regulates the stability of p53 and the p53-binding protein MDM2, implicating HAUSP as a therapeutic target for tuning p53-mediated antitumor activity. Here we report the structural analysis of HAUSP with Kaposi's sarcoma-associated herpesvirus viral interferon (IFN) regulatory factor 4 (vIRF4) and the discovery of two vIRF4-derived peptides, vif1 and vif2, as potent and selective HAUSP antagonists. This analysis reveals a bilateral belt-type interaction that results in inhibition of HAUSP. The vif1 peptide binds the HAUSP TRAF domain, competitively blocking substrate binding, whereas the vif2 peptide binds both the HAUSP TRAF and catalytic domains, robustly suppressing its deubiquitination activity. Peptide treatments comprehensively blocked HAUSP, leading to p53-dependent cell-cycle arrest and apoptosis in culture and to tumor regression in xenograft mouse model. Thus, the virus has developed a unique strategy to target the HAUSP-MDM2-p53 pathway, and these virus-derived short peptides represent biologically active HAUSP antagonists.

Activation of the STAT6 transcription factor in Jurkat T-cells by the herpesvirus saimiri Tip protein

Herpesvirus saimiri (HVS), a T-lymphotropic monkey herpesvirus, induces fulminant T-cell lymphoma in non-natural primate hosts. In addition, it can immortalize human T-cells in vitro. HVS tyrosine kinase-interacting protein (Tip) is an essential viral gene required for T-cell transformation both in vitro and in vivo. In this study, we found that Tip interacts with the STAT6 transcription factor and induces phosphorylation of STAT6 in T-cells. The interaction with STAT6 requires the Tyr(127) residue and Lck-binding domain of Tip, which are indispensable for interleukin (IL)-2-independent T-cell transformation by HVS. It was also demonstrated that Tip induces nuclear translocation of STAT6, as well as activation of STAT6-dependent transcription in Jurkat T-cells. Interestingly, the phosphorylated STAT6 mainly colocalized with vesicles containing Tip within T-cells, but was barely detectable in the nucleus. However, nuclear translocation of phospho-STAT6 and transcriptional activation of STAT6 by IL-4 stimulation were not affected significantly in T-cells expressing Tip. Collectively, these findings suggest that the constitutive activation of STAT6 by Tip in T-cells may contribute to IL-2-independent T-cell transformation by HVS.

Inhibition of retromer activity by herpesvirus saimiri tip leads to CD4 downregulation and efficient T cell transformation

The mammalian retromer is an evolutionally conserved protein complex composed of a vacuolar protein sorting trimer (Vps 26/29/35) that participates in cargo recognition and a sorting nexin (SNX) dimer that binds to endosomal membranes. The retromer plays an important role in efficient retrograde transport for endosome-to-Golgi retrieval of the cation-independent mannose-6-phosphate receptor (CI-MPR), a receptor for lysosomal hydrolases, and other endosomal proteins. This ultimately contributes to the control of cell growth, cell adhesion, and cell migration. The herpesvirus saimiri (HVS) tyrosine kinase-interacting protein (Tip), required for the immortalization of primary T lymphocytes, targets cellular signaling molecules, including Lck tyrosine kinases and the p80 endosomal trafficking protein. Despite the pronounced effects of HVS Tip on T cell signal transduction, the details of its activity on T cell immortalization remain elusive. Here, we report that the amino-terminal conserved, glutamate-rich sequence of Tip specifically interacts with the retromer subunit Vps35 and that this interaction not only causes the redistribution of Vps35 from the early endosome to the lysosome but also drastically inhibits retromer activity, as measured by decreased levels of CI-MPR and lower activities of cellular lysosomal hydrolases. Physiologically, the inhibition of intracellular retromer activity by Tip is ultimately linked to the downregulation of CD4 surface expression and to the efficient in vitro immortalization of primary human T cells to interleukin-2 (IL-2)-independent permanent growth. Therefore, HVS Tip uniquely targets the retromer complex to impair the intracellular trafficking functions of infected cells, ultimately contributing to efficient T cell transformation.

Intracellular lipid flux and membrane microdomains as organizing principles in inflammatory cell signaling

Lipid rafts and caveolae play a pivotal role in organization of signaling by TLR4 and several other immune receptors. Beyond the simple cataloguing of signaling events compartmentalized by these membrane microdomains, recent studies have revealed the surprisingly central importance of dynamic remodeling of membrane lipid domains to immune signaling. Simple interventions upon membrane lipid, such as changes in cholesterol loading or crosslinking of raft lipids, are sufficient to induce micrometer-scale reordering of membranes and their protein cargo with consequent signal transduction. In this review, using TLR signaling in the macrophage as a central focus, we discuss emerging evidence that environmental and genetic perturbations of membrane lipid regulate protein signaling, illustrate how homeostatic flow of cholesterol and other lipids through rafts regulates the innate immune response, and highlight recent attempts to harness these insights toward therapeutic development.

Exosomes/microvesicles: mediators of cancer-associated immunosuppressive microenvironments

Cancer cells, both in vivo and in vitro, have been demonstrated to release membranous structures, defined as microvesicles or exosomes, consisting of an array of macromolecules derived from the originating cells, including proteins, lipids, and nucleic acids. While only recently have the roles of these vesicular components in intercellular communication become elucidated, significant evidence has demonstrated that tumor exosomes can exert a broad array of detrimental effects on the immune system-ranging from apoptosis of activated cytotoxic T cells to impairment of monocyte differentiation into dendritic cells, to induction of myeloid-suppressive cells and T regulatory cells. Immunosuppressive exosomes of tumor origin can be found within neoplastic lesions and in biologic fluids from cancer patients, implying a potential role of these pathways in in vivo tumor progression and systemic paraneoplastic syndromes. Through the expression of molecules involved in angiogenesis promotion, stromal remodeling, signaling pathway activation through growth factor/receptor transfer, chemoresistance, and genetic intercellular exchange, tumor exosomes could represent a central mediator of the tumor microenvironment. By understanding the nature of these tumor-derived exosomes/microvesicles and their roles in mediating cancer progression and modulating the host immune response will significantly impact therapeutic approaches targeting exosomes.

Influenza virus assembly and budding

Influenza A virus causes seasonal epidemics, sporadic pandemics and is a significant global health burden. Influenza virus is an enveloped virus that contains a segmented negative strand RNA genome. Assembly and budding of progeny influenza virions is a complex, multi-step process that occurs in lipid raft domains on the apical membrane of infected cells. The viral proteins hemagglutinin (HA) and neuraminidase (NA) are targeted to lipid rafts, causing the coalescence and enlargement of the raft domains. This clustering of HA and NA may cause a deformation of the membrane and the initiation of the virus budding event. M1 is then thought to bind to the cytoplasmic tails of HA and NA where it can then polymerize and form the interior structure of the emerging virion. M1, bound to the cytoplasmic tails of HA and NA, additionally serves as a docking site for the recruitment of the viral RNPs and may mediate the recruitment of M2 to the site of virus budding. M2 initially stabilizes the site of budding, possibly enabling the polymerization of the matrix protein and the formation of filamentous virions. Subsequently, M2 is able to alter membrane curvature at the neck of the budding virus, causing membrane scission and the release of the progeny virion. This review investigates the latest research on influenza virus budding in an attempt to provide a step-by-step analysis of the assembly and budding processes for influenza viruses.

Membrane curvature during peroxisome fission requires Pex11

Pex11p is required for peroxisome proliferation. This study demonstrates that the N-terminus of Pex11p forms an amphipathic helix that generates membrane curvature required for peroxisome fission.

Pex11 is a key player in peroxisome proliferation, but the molecular mechanisms of its function are still unknown. Here, we show that Pex11 contains a conserved sequence at the N-terminus that can adopt the structure of an amphipathic helix. Using Penicillium chrysogenum Pex11, we show that this amphipathic helix, termed Pex11-Amph, associates with liposomes in vitro. This interaction is especially evident when negatively charged liposomes are used with a phospholipid content resembling that of peroxisomal membranes. Binding of Pex11-Amph to negatively charged membrane vesicles resulted in strong tubulation. This tubulation of vesicles was also observed when the entire soluble N-terminal domain of Pex11 was used. Using mutant peptides, we demonstrate that maintaining the amphipathic properties of Pex11-Amph in conjunction with retaining its α-helical structure are crucial for its function. We show that the membrane remodelling capacity of the amphipathic helix in Pex11 is conserved from yeast to man. Finally, we demonstrate that mutations abolishing the membrane remodelling activity of the Pex11-Amph domain also hamper the function of full-length Pex11 in peroxisome fission in vivo.

The SCHOOL of nature: IV. Learning from viruses

During the co-evolution of viruses and their hosts, the latter have equipped themselves with an elaborate immune system to defend themselves from the invading viruses. In order to establish a successful infection, replicate and persist in the host, viruses have evolved numerous strategies to counter and evade host antiviral immune responses as well as exploit them for productive viral replication. These strategies include those that modulate signaling mediated by cell surface receptors. Despite tremendous advancement in recent years, the exact molecular mechanisms underlying these critical points in viral pathogenesis remain unknown. In this work, based on a novel platform of receptor signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) platform, I suggest specific mechanisms used by different viruses such as human immunodeficiency virus (HIV), cytomegalovirus (CMV), severe acute respiratory syndrome coronavirus, human herpesvirus 6 and others, to modulate receptor signaling. I also use the example of HIV and CMV to illustrate how two unrelated enveloped viruses use a similar SCHOOL mechanism to modulate the host immune response mediated by two functionally different receptors: T cell antigen receptor and natural killer cell receptor, NKp30. This suggests that it is very likely that similar general mechanisms can be or are used by other viral and possibly non-viral pathogens. Learning from viruses how to target cell surface receptors not only helps us understand viral strategies to escape from the host immune surveillance, but also provides novel avenues in rational drug design and the development of new therapies for immune disorders.

T-cell antigen receptor (TCR) transmembrane peptides: A new paradigm for the treatment of autoimmune diseases

Cell surface membranes are generally considered as inert and hydrophobic providing a stable physical barrier that anchor proteins and maintain cellular homeostasis between the intra- and the extra-cellular environment. The integral proteins that transverse membranes do so once or multiple times and can function alone or as part of a larger complex. Far from being inert, there is a multiplicity of biophysical factors that drive protein-protein and protein-lipid interactions within membranes that are being increasingly recognised as very important for cellular function. Unravelling these "hot-spots" on the contact surface of transmembrane (TM) proteins and targeting peptides to these sites to interrupt the cohesive interaction between the proteins provides both an enormous challenge and a huge therapeutic potential that as yet remains unrecognized. Indeed, with biopharmaceutical research on the rise, TM peptides may prove a useful innovation. Using the T-cell antigen receptor (TCR) as a model system of multi-subunits interacting at the TM via electrostatic charges the potential for peptides as therapeutic agents to interfere with normal immune responses is discussed. The principles of such can be extended to other similar receptor systems including those involved in cancer or infection.

New therapeutic strategies targeting transmembrane signal transduction in the immune system

Single-chain receptors and multi-chain immune recognition receptors (SRs and MIRRs, respectively) represent families of structurally related but functionally different surface receptors expressed on different cells. In contrast to SRs, a distinctive and common structural characteristic of MIRR family members is that the extracellular recognition domains and intracellular signaling domains are located on separate subunits. How extracellular ligand binding triggers MIRRs and initiates intracellular signal transduction processes is not clear. A novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, suggests that the homooligomerization of receptor intracellular signaling domains represents a necessary and sufficient condition for receptor triggering. In this review, I demonstrate striking similarities between a consensus model of SR signaling and the SCHOOL model of MIRR signaling and show how these models, together with the lessons learned from viral pathogenesis, provide a molecular basis for novel pharmacological approaches targeting inter- and intrareceptor transmembrane interactions as universal therapeutic targets for a diverse variety of immune and other disorders.

Influenza virus m2 ion channel protein is necessary for filamentous virion formation

Influenza A virus buds from cells as spherical (approximately 100-nm diameter) and filamentous (approximately 100 nm x 2 to 20 microm) virions. Previous work has determined that the matrix protein (M1) confers the ability of the virus to form filaments; however, additional work has suggested that the influenza virus M2 integral membrane protein also plays a role in viral filament formation. In examining the role of the M2 protein in filament formation, we observed that the cytoplasmic tail of M2 contains several sites that are essential for filament formation. Additionally, whereas M2 is a nonraft protein, expression of other viral proteins in the context of influenza virus infection leads to the colocalization of M2 with sites of virus budding and lipid raft domains. We found that an amphipathic helix located within the M2 cytoplasmic tail is able to bind cholesterol, and we speculate that M2 cholesterol binding is essential for both filament formation and the stability of existing viral filaments.