Interaction of CD82 tetraspanin proteins with HTLV-1 envelope glycoproteins inhibits cell-to-cell fusion and virus transmission

Hemolymph exosomes inhibit Spiroplasma eriocheiris infection by promoting Tetraspanin-mediated hemocyte phagocytosis in crab

Exosomes released from infected cells are thought to play an important role in the dissemination of pathogens, as well as in host-derived immune molecules during infection. As an intracellular pathogen, Spiroplasma eriocheiris is harmful to multiple crustaceans. However, the immune mechanism of exosomes during Spiroplasma infection has not been investigated. Here, we found exosomes derived from S. eriocheiris-infected crabs could facilitate phagocytosis and apoptosis of hemocytes, resulting in increased crab survival and suppression of Spiroplasma intracellular replication. Proteomic analysis revealed the altered abundance of EsTetraspanin may confer resistance to S. eriocheiris, possibly by mediating hemocyte phagocytosis in Eriocheir sinensis. Specifically, knockdown of EsTetraspanin in E. sinensis increased susceptibility to S. eriocheiris infection and displayed compromised phagocytic ability, whereas overexpression of EsTetraspanin in Drosophila S2 cells inhibited S. eriocheiris infection. Further, it was confirmed that intramuscular injection of recombinant LEL domain of EsTetraspanin reduced the mortality of S. eriocheiris-infected crabs. Blockade with anti-EsTetraspanin serum could exacerbate S. eriocheiris invasion of hemocytes and impair hemocyte phagocytic activity. Taken together, our findings prove for the first time that exosomes modulate phagocytosis to resist pathogenic infection in invertebrates, which is proposed to be mediated by exosomal Tetraspanin, supporting the development of preventative strategies against Spiroplasma infection.

HTLV-1 biofilm polarization maintained by tetraspanin CD82 is required for efficient viral transmission

IMPORTANCE

In the early stages of infection, human T-lymphotropic virus type 1 (HTLV-1) dissemination within its host is believed to rely mostly on cell-to-cell contacts. Past studies unveiled a novel mechanism of HTLV-1 intercellular transmission based on the remodeling of the host-cell extracellular matrix and the generation of cell-surface viral assemblies whose structure, composition, and function resemble bacterial biofilms. These polarized aggregates of infectious virions, identified as viral biofilms, allow the bulk delivery of viruses to target cells and may help to protect virions from immune attacks. However, viral biofilms' molecular and functional description is still in its infancy, although it is crucial to fully decipher retrovirus pathogenesis. Here, we explore the function of cellular tetraspanins (CD9, CD81, CD82) that we detect inside HTLV-1 particles within biofilms. Our results demonstrate specific roles for CD82 in the cell-surface distribution and intercellular transmission of HTLV-1 biofilms, which we document as two essential parameters for efficient viral transmission. At last, our findings indicate that N-glycosylation of cell-surface molecules, including CD82, is required for the polarization of HTLV-1 biofilms and for the efficient transmission of HTLV-1 between T-lymphocytes.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

Tetraspanins: useful multifunction proteins for the possible design and development of small-molecule therapeutic tools

Tetraspanins constitute a well-conserved superfamily of four-span small membrane proteins (TM4SF), with >30 members in humans, with important roles in numerous mechanisms of cell biology. Moreover, tetraspanins associate with either specific partner proteins or another tetraspanin, generating a network of interactions involved in cell and membrane compartmentalization and having a role in cellular development, proliferation, activation, motility, and membrane fusions. Therefore, tetraspanins are considered regulators of cellular signaling and are often depicted as 'molecular facilitators'. In view of these many physiological functions, it is likely that these molecules are important actors in pathological processes. In this review, we present the main characteristics of this superfamily, providing a more detailed description of some significant representatives and discuss their relevance as potential targets for the design and development of small-molecule therapeutics in different pathologies.

Tetraspanins are involved in Burkholderia pseudomallei-induced cell-to-cell fusion of phagocytic and non-phagocytic cells

Tetraspanins are four-span transmembrane proteins of host cells that facilitate infections by many pathogens. Burkholderia pseudomallei is an intracellular bacterium and the causative agent of melioidosis, a severe disease in tropical regions. This study investigated the role of tetraspanins in B. pseudomallei infection. We used flow cytometry to determine tetraspanins CD9, CD63, and CD81 expression on A549 and J774A.1 cells. Their roles in B. pseudomallei infection were investigated in vitro using monoclonal antibodies (MAbs) and recombinant large extracellular loop (EC2) proteins to pretreat cells before infection. Knockout of CD9 and CD81 in cells was performed using CRISPR Cas9 to confirm the role of tetraspanins. Pretreatment of A549 cells with MAb against CD9 and CD9-EC2 significantly enhanced B. pseudomallei internalization, but MAb against CD81 and CD81-EC2 inhibited MNGC formation. Reduction of MNGC formation was consistently observed in J774.A1 cells pretreated with MAbs specific to CD9 and CD81 and with CD9-EC2 and CD81-EC2. Data from knockout experiments confirmed that CD9 enhanced bacterial internalization and that CD81 inhibited MNGC formation. Our data indicate that tetraspanins are host cellular factors that mediated internalization and membrane fusion during B. pseudomallei infection. Tetraspanins may be the potential therapeutic targets for melioidosis.

Gangliosides and CD82 inhibit the motility of colon cancer by downregulating the phosphorylation of EGFR at different tyrosine sites and signaling pathways

Previous studies have shown that (GM3), a ganglioside, suppresses hepatoma cell motility and migration by inhibiting phosphorylation of EGFR and the activity of the PI3K/AKT signaling pathway. Therefore, the aim of the present study was to investigate whether the combined treatment of CD82 with gangliosides can exert a synergistic inhibitory effect on cell motility and migration. Epidermal growth factor receptor (EGFR) signaling was studied for its role in the mechanism through which CD82 and gangliosides synergistically inhibit the motility and migration of SW620 human colon adenocarcinoma cells. GM3 and/or GM2 treatment, and/or overexpression of CD82 was performed in SW620 cells. High-performance thin layer chromatography, reverse transcription-quantitative PCR, western blotting and flow cytometry assays were used to confirm the content changes of GM2, GM3 and CD82. In addition, the phosphorylation of EGFR, MAPK and Akt were evaluated by western blot analysis. SW620 cell motility was investigated using wound healing analysis and chemotaxis migration assay. The combination of GM3 and GM2 with CD82 was found to markedly suppress EGF-stimulated SW620 cell motility compared with the individual factors or combination of GM2 or GM3 with CD82 by inhibiting the phosphorylation of EGFR. The results suggested that CD82 in combination with either GM2 or GM3 can exert a synergistic inhibitory effect on cell motility and migration; however, the synergistic mechanisms elicited by GM2 or GM3 with CD82 differ.

Monocyte Subsets Have Distinct Patterns of Tetraspanin Expression and Different Capacities to Form Multinucleate Giant Cells

Monocytes are able to undergo homotypic fusion to produce different types of multinucleated giant cells, such as Langhans giant cells in response to M. tuberculosis infection or foreign body giant cells in response to implanted biomaterials. Monocyte fusion is highly coordinated and complex, with various soluble, intracellular, and cell-surface components mediating different stages of the process. Tetraspanins, such as CD9, CD63, and CD81, are known to be involved in cell:cell fusion and have been suggested to play a role in regulating homotypic monocyte fusion. However, peripheral human monocytes are not homogenous: they exist as a heterogeneous population consisting of three subsets, classical (CD14⁺⁺CD16^-), intermediate (CD14⁺⁺CD16⁺), and non-classical (CD14⁺CD16⁺), at steady state. During infection with mycobacteria, the circulating populations of intermediate and non-classical monocytes increase, suggesting they may play a role in the disease outcome. Human monocytes were separated into subsets and then induced to fuse using concanavalin A. The intermediate monocytes were able to fuse faster and form significantly larger giant cells than the other subsets. When antibodies targeting tetraspanins were added, the intermediate monocytes responded to anti-CD63 by forming smaller giant cells, suggesting an involvement of tetraspanins in fusion for at least this subset. However, the expression of fusion-associated tetraspanins on monocyte subsets did not correlate with the extent of fusion or with the inhibition by tetraspanin antibody. We also identified a CD9^High and a CD9^Low monocyte population within the classical subset. The CD9^High classical monocytes expressed higher levels of tetraspanin CD151 compared to CD9^Low classical monocytes but the CD9^High classical subset did not exhibit greater potential to fuse and the role of these cells in immunity remains unknown. With the exception of dendrocyte-expressed seven transmembrane protein, which was expressed at higher levels on the intermediate monocyte subset, the expression of fusion-related proteins between the subsets did not clearly correlate with their ability to fuse. We also did not observe any clear correlation between giant cell formation and the expression of pro-inflammatory or fusogenic cytokines. Although tetraspanin expression appears to be important for the fusion of intermediate monocytes, the control of multinucleate giant cell formation remains obscure.

The tetraspanin CD9 facilitates MERS-coronavirus entry by scaffolding host cell receptors and proteases

Infection by enveloped coronaviruses (CoVs) initiates with viral spike (S) proteins binding to cellular receptors, and is followed by proteolytic cleavage of receptor-bound S proteins, which prompts S protein-mediated virus-cell membrane fusion. Infection therefore requires close proximity of receptors and proteases. We considered whether tetraspanins, scaffolding proteins known to facilitate CoV infections, hold receptors and proteases together on cell membranes. Using knockout cell lines, we found that the tetraspanin CD9, but not the tetraspanin CD81, formed cell-surface complexes of dipeptidyl peptidase 4 (DPP4), the MERS-CoV receptor, and the type II transmembrane serine protease (TTSP) member TMPRSS2, a CoV-activating protease. This CD9-facilitated condensation of receptors and proteases allowed MERS-CoV pseudoviruses to enter cells rapidly and efficiently. Without CD9, MERS-CoV viruses were not activated by TTSPs, and they trafficked into endosomes to be cleaved much later and less efficiently by cathepsins. Thus, we identified DPP4:CD9:TTSP as the protein complexes necessary for early, efficient MERS-CoV entry. To evaluate the importance of these complexes in an in vivo CoV infection model, we used recombinant Adenovirus 5 (rAd5) vectors to express human DPP4 in mouse lungs, thereby sensitizing the animals to MERS-CoV infection. When the rAd5-hDPP4 vectors co-expressed small RNAs silencing Cd9 or Tmprss2, the animals were significantly less susceptible, indicating that CD9 and TMPRSS2 facilitated robust in vivo MERS-CoV infection of mouse lungs. Furthermore, the S proteins of virulent mouse-adapted MERS-CoVs acquired a CD9-dependent cell entry character, suggesting that CD9 is a selective agent in the evolution of CoV virulence.

Dendritic cell maturation, but not type I interferon exposure, restricts infection by HTLV-1, and viral transmission to T-cells

Human T lymphotropic Virus type 1 (HTLV-1) is the etiological agent of Adult T cell Leukemia/Lymphoma (ATLL) and HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP). Both CD4+ T-cells and dendritic cells (DCs) infected with HTLV-1 are found in peripheral blood from HTLV-1 carriers. We previously demonstrated that monocyte-derived IL-4 DCs are more susceptible to HTLV-1 infection than autologous primary T-cells, suggesting that DC infection precedes T-cell infection. However, during blood transmission, breast-feeding or sexual transmission, HTLV-1 may encounter different DC subsets present in the blood, the intestinal or genital mucosa respectively. These different contacts may impact HTLV-1 ability to infect DCs and its subsequent transfer to T-cells. Using in vitro monocyte-derived IL-4 DCs, TGF-β DCs and IFN-α DCs that mimic DCs contacting HTLV-1 in vivo, we show here that despite their increased ability to capture HTLV-1 virions, IFN-α DCs restrict HTLV-1 productive infection. Surprisingly, we then demonstrate that it is not due to the antiviral activity of type-I interferon produced by IFN-α DCs, but that it is likely to be linked to a distinct trafficking route of HTLV-1 in IL-4 DCs vs. IFN-α DCs. Finally, we demonstrate that, in contrast to IL-4 DCs, IFN-α DCs are impaired in their capacity to transfer HTLV-1 to CD4 T-cells, both after viral capture and trans-infection and after their productive infection. In conclusion, the nature of the DCs encountered by HTLV-1 upon primo-infection and the viral trafficking route through the vesicular pathway of these cells determine the efficiency of viral transmission to T-cells, which may condition the fate of infection.

Molecular Mechanisms of HTLV-1 Cell-to-Cell Transmission

The tumorvirus human T-cell lymphotropic virus type 1 (HTLV-1), a member of the delta-retrovirus family, is transmitted via cell-containing body fluids such as blood products, semen, and breast milk. In vivo, HTLV-1 preferentially infects CD4⁺ T-cells, and to a lesser extent, CD8⁺ T-cells, dendritic cells, and monocytes. Efficient infection of CD4⁺ T-cells requires cell-cell contacts while cell-free virus transmission is inefficient. Two types of cell-cell contacts have been described to be critical for HTLV-1 transmission, tight junctions and cellular conduits. Further, two non-exclusive mechanisms of virus transmission at cell-cell contacts have been proposed: (1) polarized budding of HTLV-1 into synaptic clefts; and (2) cell surface transfer of viral biofilms at virological synapses. In contrast to CD4⁺ T-cells, dendritic cells can be infected cell-free and, to a greater extent, via viral biofilms in vitro. Cell-to-cell transmission of HTLV-1 requires a coordinated action of steps in the virus infectious cycle with events in the cell-cell adhesion process; therefore, virus propagation from cell-to-cell depends on specific interactions between cellular and viral proteins. Here, we review the molecular mechanisms of HTLV-1 transmission with a focus on the HTLV-1-encoded proteins Tax and p8, their impact on host cell factors mediating cell-cell contacts, cytoskeletal remodeling, and thus, virus propagation.

[Intercellular transmission of HTLV-1: not all mechanisms have been revealed]

HTLV-1 is the etiological agent of Adult T cell Leukemia/Lymphoma (ATLL) and of HTLV-1-Associated Myelopathy/tropical spastic paraparesis (HAM/TSP). It is mainly detected in CD4+ lymphocytes in vivo, but proviral genomes have also been detected although less frequently, in CD8+ T lymphocytes, B lymphocytes, monocytes, macrophages, dendritic cells and other non-lymphoid cells. Virus spread is highly dependent on cell-cell contact. This mode of transmission is correlated with an increased ability of infected cells to migrate, a property linked to cytoskeleton reorganization induced by the viral Tax protein. Cell-to-cell transmission relies on at least three non-exclusive molecular pathways. First, a specialized area, the "virological synapse'' (VS) promotes direct transmission of budding HTLV-1 particles into a synaptic cleft formed between infected and uninfected cells. Second, HTLV-1 particles accumulate at the plasma membrane of infected cells in a biofilm-like extracellular viral assembly that resembles a bacterial biofilm. Viral biofilm is rapidly transmitted to uninfected cells when infected cells contact target cells. Finally, membrane extensions called inter-cellular conduits facilitate HTLV-1 proteins transfer from infected to uninfected target cells, and may stabilize cell-cell contacts. The aim of this review is to summarize the molecular mechanisms of these HTLV-1 transmission pathways.

Cellular Factors Involved in HTLV-1 Entry and Pathogenicit

Human T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia (ATL) and HTLV-1 – associated myelopathy and tropical spastic paraparesis (HAM/TSP). HTLV-1 has a preferential tropism for CD4 T cells in healthy carriers and ATL patients, while both CD4 and CD8 T cells serve as viral reservoirs in HAM/TSP patients. HTLV-1 has also been detected other cell types, including monocytes, endothelial cells, and dendritic cells. In contrast to the limited cell tropism of HTLV-1 in vivo, the HTLV receptor appears to be expressed in almost all human or animal cell lines. It remains to be examined whether this cell tropism is determined by host factors or by HTLV-1 heterogeneity. Unlike most retroviruses, cell-free virions of HTLV-1 are very poorly infectious. The lack of completely HTLV-1-resistant cells and the low infectivity of HTLV-1 have hampered research on the HTLV entry receptor. Entry of HTLV-1 into target cells is thought to involve interactions between the env (Env) glycoproteins, a surface glycoprotein (surface unit), and a transmembrane glycoprotein. Recent studies have shown that glucose transporter GLUT1, heparan sulfate proteoglycans (HSPGs), and neuropilin-1 (NRP-1) are the three proteins important for the entry of HTLV-1. Studies using adherent cell lines have shown that GLUT1 can function as a receptor for HTLV. HSPGs are required for efficient entry of HTLV-1 into primary CD4 T cells. NRP-1 is expressed in most established cell lines. Further studies have shown that these three molecules work together to promote HTLV-1 binding to cells and fusion of viral and cell membranes. The virus could first contact with HSPGs and then form complexes with NRP-1, followed by association with GLUT1. It remains to be determined whether these three molecules can explain HTLV-1 cell tropism. It also remains to be more definitively proven that these molecules are sufficient to permit HTLV-1 entry into completely HTLV-1-resistant cells.

Blocking the large extracellular loop (LEL) domain of FcTetraspanin-3 could inhibit the infection of white spot syndrome virus (WSSV) in Chinese shrimp, Fenneropenaeus chinensis

Tetraspanins belong to the transmembrane 4 superfamily (TM(4)SF), which span the cell membrane 4 times and act as bridges or connectors. Increasing evidences have shown that tetraspanins play important role in virus infection. The large extracellular loop (LEL) of a tetraspanin is considered as a possible target of some virus. Tetraspanins are widely found in invertebrates, but the functional roles of most invertebrate tetraspanins have remained unknown. Recently, a tetraspanin, called FcTetraspanin-3, was cloned from the cDNA library of Chinese shrimp, Fenneropenaeus chinensis. The FcTetraspanin-3 constitutive expression in all examined tissues and the expression of the gene were highly induced in hepatopancreas, lymphoid organ and intestine by white spot syndrome virus (WSSV) challenge. In this study, we expressed and purified the recombinant peptide containing the LEL domain of FcTetraspanin-3, and produced the anti-LEL polyclone antibody. The expression of FcTetraspanin-3 was observed by real-time PCR and Western blot. Also, the localization of FcTetraspanin-3-positive cells in intestine and hepatopancreas were revealed by immunofluorescence. The results of anti-LEL antibody blocking experiments shown that the antibody can significantly reduce the mortality of shrimp challenged by WSSV. Additionally, dsRNA interference was utilized to examine the functional role of FcTetraspanin-3 in response to WSSV infection, and a sensible decrease of the viral copy number in the tetraspanin knockdown shrimp. These results suggested the blocking of LEL domain of FcTetraspanin-3 could inhibit the infection of WSSV. FcTetraspanin-3 might play an important role in response to WSSV infection, and the LEL domain of FcTetraspanin-3 might mediate the entry of WSSV.

Significance of palmitoylation of CD81 on its association with tetraspanin-enriched microdomains and mediating hepatitis C virus cell entry

CD81, a co-receptor for hepatitis C virus (HCV), is a member of the tetraspanin superfamily and is heavily palmitoylated in the juxtamembrane cysteine residues. Palmitoylation plays an important role in protein-protein interactions and association with cholesterol-rich domains of membranes. In this study, Huh7 cells expressing wild-type or palmitoylation-defective CD81 were generated to analyze whether palmitoylation of CD81 is involved in HCV cell entry. Our data showed that de-palmitoylation of CD81 dramatically reduced its association with tetraspanin CD151, but did not influence CD81 partition in detergent-resistant membranes. Moreover, de-palmitoylated CD81 decreased the host cell susceptibility to HCV. Notably, CD151-specific antibodies and siRNA inhibited HCV cell entry, and detachment of CD81 with CD151 decreased the lateral movement of virus particle/CD81 complex to areas of cell-cell contact. These results suggest that palmitoylation of CD81 should facilitate HCV entry, at least in part, by regulating the association of CD81 with tetraspanin-enriched microdomains.

Role of O-glycosylation and expression of CD43 and CD45 on the surfaces of effector T cells in human T cell leukemia virus type 1 cell-to-cell infection

We used replication-dependent retroviral vectors to identify cell surface antigens involved in the cell-to-cell transmission of human T cell leukemia virus type 1 (HTLV-1). We generated monoclonal antibodies (MAbs) against Jurkat T cells and selected several IgM MAbs that strongly inhibited HTLV-1 but not human immune deficiency virus type 1 (HIV-1) cell-to-cell infection. These MAbs recognized the so-called Tn antigen (GalNAcα1-O-Ser/Thr) that arises on Jurkat cells from a mutation in the T-synthase-specific chaperone Cosmc and the consequent loss of O-glycan elongation. Anti-Tn MAbs precipitated two major O-glycan carrier proteins, CD43 and CD45, and caused a strong aggregation of Jurkat cells. The restoration of O-glycosylation in Jurkat cells by stably transducing the wild-type Cosmc gene resulted in a 3- to 4-fold increase in the level of surface expression of CD43 and enhanced HTLV-1 transmission 10-fold in comparison to that of parental cells. The short hairpin RNA (shRNA) knockdown of CD43 or CD45 expression in Jurkat-Cosmc, HBP-ALL, and CEM T cells decreased HTLV-1 infection severalfold. The knockdown of CD45 in Jurkat cells severely reduced both HTLV-1 and HIV-1 infections, but Cosmc coexpression partially rescued infection. HTLV-1 proteins, which assembled in small patches on Jurkat cells, formed large clusters on the surface of Jurkat-Cosmc cells. These data indicate that large aggregates of HTLV-1 assemblies are more infectious than multiple clustered virions. We suggest that heavily O-glycosylated CD43 and CD45 molecules render cells less adhesive, prevent inappropriate cell-cell contacts, and favor the assembly of HTLV-1 particles into large, highly infectious structures on the surface of T cells.

Retroviral matrix and lipids, the intimate interaction

Retroviruses are enveloped viruses that assemble on the inner leaflet of cellular membranes. Improving biophysical techniques has recently unveiled many molecular aspects of the interaction between the retroviral structural protein Gag and the cellular membrane lipids. This interaction is driven by the N-terminal matrix domain of the protein, which probably undergoes important structural modifications during this process, and could induce membrane lipid distribution changes as well. This review aims at describing the molecular events occurring during MA-membrane interaction, and pointing out their consequences in terms of viral assembly. The striking conservation of the matrix membrane binding mode among retroviruses indicates that this particular step is most probably a relevant target for antiviral research.

Current concepts regarding the HTLV-1 receptor complex

The identity of the Human T lymphotropic Virus type 1 (HTLV-1) receptor remained an unsolved puzzle for two decades, until the recent demonstration that three molecules, Glucose Transporter 1, Neuropilin-1 and Heparan Sulfate Proteoglycans are involved in HTLV-1 binding and entry. Despite these advances, several questions remain unanswered, including the precise role of each of these molecules during virus entry. In light of the most recent data, we propose a model of the HTLV-1 receptor complex and discuss its potential impact on HTLV-1 infection.

Strategies for targeting tetraspanin proteins: potential therapeutic applications in microbial infections

The identification of novel targets and strategies for therapy of microbial infections is an area of intensive research due to the failure of conventional vaccines or antibiotics to combat both newly emerging diseases (e.g. viruses such as severe acute respiratory syndrome (SARS) and new influenza strains, and antibiotic-resistant bacteria) and entrenched, pandemic diseases exemplified by HIV. One clear approach to this problem is to target processes of the host organism rather than the microbe. Recent data have indicated that members of the tetraspanin superfamily, proteins with a widespread distribution in eukaryotic organisms and 33 members in humans, may provide such an approach.

Tetraspanins traverse the membrane four times, but are distinguished from other four-pass membrane proteins by the presence of conserved charged residues in the transmembrane domains and a defining ‘signature’ motif in the larger of the two extracellular domains (the EC2). They characteristically form promiscuous associations with one another and with other membrane proteins and lipids to generate a specialized type of microdomain: the tetraspanin-enriched microdomain (TEM). TEMs are integral to the main role of tetraspanins as ‘molecular organizers’ involved in functions such as membrane trafficking, cell-cell fusion, motility, and signaling. Increasing evidence demonstrates that tetraspanins are used by intracellular pathogens as a means of entering and replicating within human cells. Although previous investigations focused mainly on viruses such as hepatitis C and HIV, it is now becoming clear that other microbes associate with tetraspanins, using TEMs as a ‘gateway’ to infection.

In this article we review the properties and functions of tetraspanins/TEMs that are relevant to infective processes and discuss the accumulating evidence that shows how different pathogens exploit these properties in infection and in the pathogenesis of disease. We then investigate the novel and exciting possibilities of targeting tetraspanins for the treatment of infectious disease, using specific antibodies, recombinant EC2 domains, small-molecule mimetics, and small interfering RNA. Such therapies, directed at host-cell molecules, may provide alternative options for combating fast-mutating or newly emerging pathogens, where conventional approaches face difficulties.

Cell-to-Cell Spread of Retroviruses

Viruses from several families use direct cell-to-cell infection to disseminate between cells. Retroviruses are a relatively recent addition to this list, and appear to spread cell-to-cell by induction of multimolecular complexes termed virological synapses that assemble at the interface between infected and receptor-expressing target cells. Over the past five years, detailed insight into the cellular and molecular basis of virological synapse-mediated retroviral cell-to-cell spread has been obtained, but important questions and controversies have been raised that remain to be resolved. This review will focus on recent advances in the field with emphasis on areas in which work still needs to be done.

Opposing effects of a tyrosine-based sorting motif and a PDZ-binding motif regulate human T-lymphotropic virus type 1 envelope trafficking

Human T-lymphotropic virus type 1 (HTLV-1) envelope (Env) glycoprotein mediates binding of the virus to its receptor on the surface of target cells and subsequent fusion of virus and cell membranes. To better understand the mechanisms that control HTLV-1 Env trafficking and activity, we have examined two protein-protein interaction motifs in the cytoplasmic domain of Env. One is the sequence YSLI, which matches the consensus YXXPhi motifs that are known to interact with various adaptor protein complexes; the other is the sequence ESSL at the C terminus of Env, which matches the consensus PDZ-binding motif. We show here that mutations that destroy the YXXPhi motif increased Env expression on the cell surface and increased cell-cell fusion activity. In contrast, mutation of the PDZ-binding motif greatly diminished Env expression in cells, which could be restored to wild-type levels either by mutating the YXXPhi motif or by silencing AP2 and AP3, suggesting that interactions with PDZ proteins oppose an Env degradation pathway mediated by AP2 and AP3. Silencing of the PDZ protein hDlg1 did not affect Env expression, suggesting that hDlg1 is not a binding partner for Env. Substitution of the YSLI sequence in HTLV-1 Env with YXXPhi elements from other cell or virus membrane-spanning proteins resulted in alterations in Env accumulation in cells, incorporation into virions, and virion infectivity. Env variants containing YXXPhi motifs that are predicted to have high-affinity interaction with AP2 accumulated to lower steady-state levels. Interestingly, mutations that destroy the YXXPhi motif resulted in viruses that were not infectious by cell-free or cell-associated routes of infection. Unlike YXXPhi, the function of the PDZ-binding motif manifests itself only in the producer cells; AP2 silencing restored the incorporation of PDZ-deficient Env into virus-like particles (VLPs) and the infectivity of these VLPs to wild-type levels.

The roles of tetraspanins in HIV-1 replication

Tetraspanins are small integral membrane proteins that are known to control a variety of cellular processes, including signaling, migration and cell-cell fusion. Research over the past few years established that they are also regulators of various steps in the HIV-1 replication cycle, but the mechanisms through which these proteins either enhance or repress virus spread remain largely unknown.

Tetraspanins regulate cell-to-cell transmission of HIV-1

BACKGROUND

The presence of the tetraspanins CD9, CD63, CD81 and CD82 at HIV-1 budding sites, at the virological synapse (VS), and their enrichment in HIV-1 virions has been well-documented, but it remained unclear if these proteins play a role in the late phase of the viral replication cycle. Here we used overexpression and knockdown approaches to address this question.

RESULTS

Neither ablation of CD9, CD63 and/or CD81, nor overexpression of these tetraspanins was found to affect the efficiency of virus release. However, confirming recently reported data, tetraspanin overexpression in virus-producing cells resulted in the release of virions with substantially reduced infectivity. We also investigated the roles of these tetraspanins in cell-to-cell transmission of HIV-1. Overexpression of CD9 and CD63 led to reduced cell-to-cell transmission of this virus. Interestingly, in knockdown experiments we found that ablation of CD63, CD9 and/or CD81 had no effect on cell-free infectivity. However, knockdown of CD81, but not CD9 and CD63, enhanced productive particle transmission to target cells, suggesting additional roles for tetraspanins in the transmission process. Finally, tetraspanins were found to be downregulated in HIV-1-infected T lymphocytes, suggesting that HIV-1 modulates the levels of these proteins in order to maximize the efficiency of its transmission within the host.

CONCLUSION

Altogether, these results establish an active role of tetraspanins in HIV-1 producer cells.

Formation of syncytia is repressed by tetraspanins in human immunodeficiency virus type 1-producing cells

In vitro propagation studies have established that human immunodeficiency virus type 1 (HIV-1) is most efficiently transmitted at the virological synapse that forms between producer and target cells. Despite the presence of the viral envelope glycoprotein (Env) and CD4 and chemokine receptors at the respective surfaces, producer and target cells usually do not fuse with each other but disengage after the viral particles have been delivered, consistent with the idea that syncytia, at least in vitro, are not required for HIV-1 spread. Here, we tested whether tetraspanins, which are well known regulators of cellular membrane fusion processes that are enriched at HIV-1 exit sites, regulate syncytium formation. We found that overexpression of tetraspanins in producer cells leads to reduced syncytium formation, while downregulation has the opposite effect. Further, we document that repression of Env-induced cell-cell fusion by tetraspanins depends on the presence of viral Gag, and we demonstrate that fusion repression requires the recruitment of Env by Gag to tetraspanin-enriched microdomains (TEMs). However, sensitivity to fusion repression by tetraspanins varied for different viral strains, despite comparable recruitment of their Envs to TEMs. Overall, these data establish tetraspanins as negative regulators of HIV-1-induced cell-cell fusion, and they start delineating the requirements for this regulation.

Targeting of tetraspanin proteins--potential benefits and strategies

The tetraspanin transmembrane proteins have emerged as key players in malignancy, the immune system, during fertilization and infectious disease processes. Tetraspanins engage in a wide range of specific molecular interactions, occurring through the formation of tetraspanin-enriched microdomains (TEMs). TEMs therefore serve as a starting point for understanding how tetraspanins affect cell signalling, adhesion, morphology, motility, fusion and virus infection. An abundance of recent evidence suggests that targeting tetraspanins, for example, by monoclonal antibodies, soluble large-loop proteins or RNAi technology, should be therapeutically beneficial.

CD81 is a central regulator of cellular events required for hepatitis C virus infection of human hepatocytes

Infection with hepatitis C virus (HCV) is still a major public health problem, and the events leading to hepatocyte infection are not yet fully understood. Combining confocal microscopy with biochemical analysis and studies of infection requirements using pharmacological inhibitors and small interfering RNAs, we show here that engagement of CD81 activates the Rho GTPase family members Rac, Rho, and Cdc42 and that the block of these signaling pathways drastically reduces HCV infectivity. Activation of Rho GTPases mediates actin-dependent relocalization of the HCV E2/CD81 complex to cell-cell contact areas where CD81 comes into contact with the tight-junction proteins occludin, ZO-1, and claudin-1, which was recently described as an HCV coreceptor. Finally, we show that CD81 engagement activates the Raf/MEK/ERK signaling cascade and that this pathway affects postentry events of the virus life cycle. In conclusion, we describe a range of cellular events that are manipulated by HCV to coordinate interactions with its multiple coreceptors and to establish productive infections and find that CD81 is a central regulator of these events.

Tetraspanins CD9 and CD81 modulate HIV-1-induced membrane fusion

Protein organization on the membrane of target cells may modulate HIV-1 transmission. Since the tetraspanin CD81 is associated to CD4, the receptor of HIV-1 envelope protein (Env; gp120/gp41), we have explored the possibility that this molecule may modulate the initial steps of HIV-1 infection. On the other hand, CD81 belongs to the tetraspanin family, which has been described as organizers of protein microdomains on the plasma membrane. Therefore, the role of CD81 and other related tetraspanin, CD9, on the cell-to-cell fusion process mediated by HIV-1 was studied. We found that anti-tetraspanin Abs enhanced the syncytia formation induced by HIV-1 envelope proteins and viral entry in human T lymphoblasts. In addition, anti-CD81 Abs triggered its clustering in patches, where CD4 and CXCR4 were included. Moreover, the knocking down of CD81 and CD9 expression resulted in an increase in syncytia formation and viral entry. Accordingly, overexpression of CD81 and CD9 rendered cells less susceptible to Env-mediated syncytia formation. These data indicate that CD9 and CD81 have an important role in membrane fusion induced by HIV-1 envelope.

The inner loop of tetraspanins CD82 and CD81 mediates interactions with human T cell lymphotrophic virus type 1 Gag protein

The tetraspanin superfamily proteins play important roles in organizing membrane protein complexes, modulating integrin function, and controlling T cell adhesion. Tetraspanins such as CD82 contain two extracellular loops with its N terminus, C terminus, and inner loop exposed to the cytoplasm. The matrix (MA) domain of human T cell lymphotrophic virus, type 1 (HTLV-1), Gag interacts with the cytoplasmic face of the plasma membrane and is concentrated at tetraspanin-enriched microdomains. To understand the basis of this association, we generated site-directed mutations in the various domains of CD82 and used coimmunoprecipitation and colocalization approaches to examine interactions with HTLV-1 MA. The large extracellular loop of CD82, which is important for interactions with integrins, was not required for the association with HTLV-1 MA. The cytoplasmic N terminus and C terminus of CD82 were also dispensable for CD82-MA interactions. In contrast, mutations of conserved amino acids in the inner loop of CD82 or of palmitoylated cysteines that flank the inner loop diminished CD82 association with MA. HTLV-1 MA also interacted with the inner loop of CD81. Thus, association of HTLV-1 Gag with tetraspanin-enriched microdomains is mediated by the inner loops of CD81 and CD82.

Neuropilin-1 is involved in human T-cell lymphotropic virus type 1 entry

Human T-cell lymphotropic virus type 1 (HTLV-1) is transmitted through a viral synapse and enters target cells via interaction with the glucose transporter GLUT1. Here, we show that Neuropilin-1 (NRP1), the receptor for semaphorin-3A and VEGF-A165 and a member of the immune synapse, is also a physical and functional partner of HTLV-1 envelope (Env) proteins. HTLV-1 Env and NRP1 complexes are formed in cotransfected cells, and endogenous NRP1 contributes to the binding of HTLV-1 Env to target cells. NRP1 overexpression increases HTLV-1 Env-dependent syncytium formation. Moreover, overexpression of NRP1 increases both HTLV-1 and HTLV-2 Env-dependent infection, whereas down-regulation of endogenous NRP1 has the opposite effect. Finally, overexpressed GLUT1, NRP1, and Env form ternary complexes in transfected cells, and endogenous NRP1 and GLUT1 colocalize in membrane junctions formed between uninfected and HTLV-1-infected T cells. These data show that NRP1 is involved in HTLV-1 and HTLV-2 entry, suggesting that the HTLV receptor has a multicomponent nature.

Complete predicted three-dimensional structure of the facilitator transmembrane protein and hepatitis C virus receptor CD81: conserved and variable structural domains in the tetraspanin superfamily

Tetraspanins are a superfamily of transmembrane proteins implicated in cellular development, motility, and activation through their interactions with a large range of proteins and with specific membrane microdomains. The complete three-dimensional structure of the tetraspanin CD81 has been predicted by molecular modeling and from the crystallographic structure of the EC2 large extracellular domain. Periodicity of sequence conservation, homology modeling, secondary structure prediction, and protein docking were used. The transmembrane domain appears organized as a four-stranded left-handed coiled coil directly connecting to two helices of the EC2. A smaller extracellular loop EC1 contains a small largely hydrophobic beta-strand that packs in a conserved hydrophobic groove of the EC2. The palmitoylable intracellular N-terminal segment forms an amphipathic membrane-parallel helix. Structural variability occurs mainly in an hypervariable subdomain of the EC2 and in intracellular regions. Therefore, the variable interaction selectivity of tetraspanins originates both from sequence variability within structurally conserved domains and from the occurrence of small structurally variable domains. In CD81 and other tetraspanins, the numerous membrane-exposed aromatic residues are asymmetrically clustered and protrude on one side of the transmembrane domain. This may represent a functional specialization of these two sides for interactions with cholesterol, proteins, or membrane microdomains.

HTLV-1 Gag protein associates with CD82 tetraspanin microdomains at the plasma membrane

We examined the association of HTLV-1 Gag with tetraspanin-enriched microdomains in the plasma membrane. Immunofluorescent staining and confocal image analysis showed that HTLV-1 Gag protein colocalized with CD82 and other tetraspanins at the plasma membrane of T cells. HTLV-1 Gag, which is associated with the inner surface of the plasma membrane, was concentrated to the patches formed by antibody-mediated cross-linking of CD82 on the cell surface. Also, CD82 and HTLV-1 Gag rapidly segregated to the immune synapse that is formed between Raji B cells and Jurkat T cells in the presence of bacterial superantigen. CD82, which was immunoprecipitated from cell extracts prepared in Brij97 detergent conditions, was associated with the matrix (MA) protein. Stable interaction of MA and CD82 in Brij97-disrupted cell extracts required Gag multimerization and proteolytic processing. The form of MA that coimmunoprecipitated with CD82 was a cysteine-linked homodimer. The viral envelope glycoprotein was not required for the association of Gag with CD82-enriched membrane regions. In contrast to HTLV-1, HIV-1 Gag did not colocalize, cosegregate, or coimmunoprecipitate with CD82. Our data suggest that once at the plasma membrane, HTLV-1 virion components associate with CD82-containing microdomains, which may facilitate the mobilization of nascent virions to sites of intercellular adhesion.

HTLV-1 tropism and envelope receptor

The identification of CD4 as the primary receptor for HIV followed shortly after the discovery of the virus, but the HTLV receptor remained long elusive, until its recent identification as the GLUT1 glucose transporter. In the present review, we describe the status of the literature that surrounded this discovery as well as the in vitro and in vivo observations that led to the identification of GLUT1. Also, we will explore a few tracks to conciliate the in vitro and in vivo data on HTLV-1 tropism within the context of the HTLV literature that has accumulated over the past 25 years. A close examination of these data leads us to conclude that the preferential detection of HTLV-1 in CD4+ T lymphocyte subsets in vivo, even in the absence of leukemia, is not likely to be directly related to envelope receptor interactions, but rather to an array of postentry selection bottlenecks in vivo. Furthermore, we propose that infection of other hematopoietic and nonhematopoietic cells is likely to take place during the lifetime of an individual, with a burst early during the infection.

The tetraspanin web modulates immune-signalling complexes

The tetraspanin web represents a new concept of molecular interactions in the immune system. Whereas most surface immune-modulating molecules involve receptor-ligand interactions, tetraspanins associate with partner proteins and facilitate their lateral positioning in the membrane. Moreover, the same tetraspanin molecule can associate with different proteins depending on the cell type. Most importantly, members of this family tend to associate with each other, together with their partners, in membrane microdomains that provide a scaffold for the transmission of external stimuli to intracellular-signalling components.

Tetraspanin CD82 controls the association of cholesterol-dependent microdomains with the actin cytoskeleton in T lymphocytes: relevance to co-stimulation

T-cell activation is initiated by the concerted engagement of the T-cell receptor and different co-stimulatory molecules, and requires cytoskeleton-dependent membrane dynamics. Here, we have studied the relationships between tetraspanins, cytoskeleton and raft microdomains, and their relevance in T-cell signaling. Localization studies and density-gradient flotation experiments indicate that part of tetraspanins localizes in raft microdomains linked to the actin cytoskeleton. First, partial coalescence of lipid raft is triggered by tetraspanin cross-linking and results in large caps in which F-actin also concentrates. Second, the amount of tetraspanins, which are recovered in the cholesterol-dependent insoluble fractions of low and intermediate density, and which appears to be membrane vesicles by electron microscopy, is under cytoskeletal influence. Disruption of actin filaments enhances the amount of tetraspanins recovered in typical raft fractions, whereas F-actin-stabilizing agents induce the opposite effect. Our data also reveal that CD82 constitutes a link between raft domains and the actin cytoskeleton, which is functionally relevant. First, tetraspanin signaling induces a selective translocation of CD82 from detergent-resistant membrane fractions to the cytoskeleton-associated pellet. Second, all functional effects linked to CD82 engagement, such as adhesion to culture plates, formation of actin bundles and early events of tyrosine phosphorylation, are abolished, or strongly reduced, by cholesterol depletion. We also show that dynamic relocalization of CD82 and F-actin at the periphery of the immune synapse is induced upon contact of T cells with antigen-presenting cells. This suggests that the tetraspanin web might participate in the membrane dynamics required for proper T-cell signaling. More generally, the interaction of tetraspanins with raft domains and with the actin cytoskeleton might relate with their role in many cellular functions as membrane organizers.

Evidence that the transmembrane domain proximal region of the human T-cell leukemia virus type 1 fusion glycoprotein gp21 has distinct roles in the prefusion and fusion-activated states

To investigate the structural context of the fusion peptide region in human T-cell leukemia virus type 1 gp21, maltose-binding protein (MBP) was used as an N-terminal solubilization partner for the entire gp21 ectodomain (residues 313-445) and C-terminally truncated ectodomain fragments. The bacterial expression of the MBP/gp21 chimeras resulted in soluble trimers containing intramonomer disulfide bonds. Detergents blocked the proteolytic cleavage of fusion peptide residues in the MBP/gp21-(313-425) chimera, indicating that the fusion peptide is available for interaction with detergent despite the presence of an N-terminal MBP domain. Limited proteolysis experiments indicated that the transmembrane domain proximal sequence Thr(425)-Ala(439) protects fusion peptide residues from chymotrypsin. MBP/gp21 chimera stability therefore depends on a functional interaction between N-terminal and transmembrane domain proximal regions in a gp21 helical hairpin structure. In addition, thermal aggregation experiments indicated that the Thr(425)-Ser(436) sequence confers stability to the fusion peptide-containing MBP/gp21 chimeras. The functional role of the transmembrane domain proximal sequence was assessed by alanine-scanning mutagenesis of the full-length envelope glycoprotein, with 11 of 12 single alanine substitutions resulting in 1.5- to 4.5-fold enhancements in cell-cell fusion activity. By contrast, single alanine substitutions in MBP/gp21 did not significantly alter chimera stability, indicating that multiple residues within the transmembrane domain proximal region and the fusion peptide and adjacent glycine-rich segment contribute to stability, thereby mitigating the potential effects of the substitutions. The fusion-enhancing effects of the substitutions are therefore likely to be caused by alteration of the prefusion complex. Our observations suggest that the function of the transmembrane domain proximal sequence in the prefusion envelope glycoprotein is distinct from its role in stabilizing the fusion peptide region in the fusion-activated helical hairpin conformation of gp21.

Structure of the tetraspanin main extracellular domain. A partially conserved fold with a structurally variable domain insertion

The tetraspanin family of membrane glycoproteins is involved in the regulation of cellular development, proliferation, activation, and mobility. We have attempted to predict the structural features of the large extracellular domain of tetraspanins (EC2), which is very important in determining their functional specificity. The tetraspanin EC2 is composed of two subdomains: a conserved three-helix subdomain and a variable secondary structure subdomain inserted within the conserved subdomain. The occurrence of key disulphide bridges and other invariant residues leads to a conserved relative topology of both subdomains and also suggests a structural classification of tetraspanins. Using the CD81 EC2 structure as a template, the structures of two other EC2s were predicted by homology modeling and indicate a conserved shape, in which the variable subdomain is located at one side of the structure. The conserved and variable subdomains might contain sites that correspond, respectively, to common and specific interactions of tetraspanins. The tetraspanin EC2 seems to correspond to a new scheme of fold conservation/variability among proteins, namely the insertion of a structurally variable subdomain within an otherwise conserved fold.