HIV-1 Nef inhibits ruffles, induces filopodia, and modulates migration of infected lymphocytes

An Enzyme-Responsive Self-Immolative Recognition Marker for Manipulating Cell-Cell Interactions

The development of innovative strategies for cell membranes engineering is of prime interest to explore and manipulate cell-cell interactions. Herein, an enzyme-sensitive recognition marker that can be introduced on cell surface via bioorthogonal chemistry is designed. Once functionalized in this fashion, the cells gain the ability to assemble with cell partners coated with the complementary marker through non-covalent click chemistry. The artificial cell adhesion induces natural biological processes associated with cell proximity such as inhibiting cancer cell proliferation and migration. On the other hand, the enzymatic activation of the stimuli-responsive marker triggers the disassembly of cells, thereby restoring the tumor cell proliferation and migration rates. Thus, the study shows that the ready-to-use complementary markers are valuable tools for controlling the formation and the breaking of bonds between cells, offering an easy way to investigate biological processes associated to cell proximity.

Imaging flow cytometry of tumoroids: A new method for studying GPCR expression

Fluorescence confocal microscopy is commonly used to analyze the regulation membrane proteins expression such as G protein-coupled receptors (GPCRs). With this approach, the internal movement of GPCRs within the cell can be observed with a high degree of resolution. However, these microscopy techniques led to complex and time-consuming analysis and did not allow a large population of events to be sampled. A recent approach termed imaging flow cytometry (IFC), which combines flow cytometry and fluorescence microscopy, had two main advantages to study the regulation of GPCRs expression such as orexins receptors (OXRs): the ability (1) to analyze large numbers of cells and; (2) to visualize cell integrity and fluorescent markers localization. Here, we compare these two technologies using the orexin A (OxA) ligand coupled to rhodamine (OxA-rho) to investigate anti-tumoral OX1R expression in human digestive cancers. IFC has been adapted for cancer epithelial adherent cells and also to 3D cell culture tumoroids which partially mimic tumoral structures. In the absence of specific antibody, expression of OX1R is examined in the presence of OxA-rho. 2D-culture of colon cancer cells HT-29 exhibits a maximum level of OX1R internalization induced by OxA with 19% ± 3% colocalizing to early endosomes. In 3D-culture of HT-29 cells, internalization of OX1R/OxA-rho reached its maximum at 60 min, with 30.7% ± 6.4% of OX1R colocalizing with early endosomes. This is the first application of IFC to the analysis of the expression of a native GPCR, OX1R, in both 2D and 3D cultures of adherent cancer cells.

HIV-1 Reverse Transcriptase Expression in HPV16-Infected Epidermoid Carcinoma Cells Alters E6 Expression and Cellular Metabolism, and Induces a Hybrid Epithelial/Mesenchymal Cell Phenotype

The high incidence of epithelial malignancies in HIV-1 infected individuals is associated with co-infection with oncogenic viruses, such as high-risk human papillomaviruses (HR HPVs), mostly HPV16. The molecular mechanisms underlying the HIV-1-associated increase in epithelial malignancies are not fully understood. A collaboration between HIV-1 and HR HPVs in the malignant transformation of epithelial cells has long been anticipated. Here, we delineated the effects of HIV-1 reverse transcriptase on the in vitro and in vivo properties of HPV16-infected cervical cancer cells. A human cervical carcinoma cell line infected with HPV16 (Ca Ski) was made to express HIV-1 reverse transcriptase (RT) by lentiviral transduction. The levels of the mRNA of the E6 isoforms and of the factors characteristic to the epithelial/mesenchymal transition were assessed by real-time RT-PCR. The parameters of glycolysis and mitochondrial respiration were determined using Seahorse technology. RT expressing Ca Ski subclones were assessed for the capacity to form tumors in nude mice. RT expression increased the expression of the E6*I isoform, modulated the expression of E-CADHERIN and VIMENTIN, indicating the presence of a hybrid epithelial/mesenchymal phenotype, enhanced glycolysis, and inhibited mitochondrial respiration. In addition, the expression of RT induced phenotypic alterations impacting cell motility, clonogenic activity, and the capacity of Ca Ski cells to form tumors in nude mice. These findings suggest that HIV-RT, a multifunctional protein, affects HPV16-induced oncogenesis, which is achieved through modulation of the expression of the E6 oncoprotein. These results highlight a complex interplay between HIV antigens and HPV oncoproteins potentiating the malignant transformation of epithelial cells.

A History and Atlas of the Human CD4+ T Helper Cell

CD4+ T cells have orchestrated and regulated immunity since the introduction of jawed vertebrates, yet our understanding of CD4+ T cell evolution, development, and cellular physiology has only begun to be unearthed in the past few decades. Discoveries of genetic diseases that ablate this cellular population have provided insight into their critical functions while transcriptomics, proteomics, and high-resolution microscopy have recently revealed new insights into CD4+ T cell anatomy and physiology. This article compiles historical, microscopic, and multi-omics data that can be used as a reference atlas and index to dissect cellular physiology within these influential cells and further understand pathologies like HIV infection that inflict human CD4+ T cells.

Novel role of HIV-1 Nef in regulating the ubiquitination of cellular proteins

Our recent data established that HIV-1 Nef is pivotal in determining the fate of cellular proteins by modulating ubiquitination. However, it is unknown which proteins are ubiquitinated in the presence of Nef, a question critical for understanding the proliferation/restriction strategies of HIV-1 in infected cells. To identify cellular proteins ubiquitinated by Nef, we conducted a proteomic analysis of cellular proteins in the presence and absence of Nef. Proteomic analysis in HEK293T cells indicated that 93 proteins were upregulated and 232 were downregulated in their ubiquitination status by Nef. Computational analysis classified these proteins based on molecular function, biological process, subcellular localization, and biological pathway. Of those proteins, we found a majority of molecular functions to be involved in binding and catalytic activity. With respect to biological processes, a significant portion of the proteins identified were related to cellular and metabolic processes. Subcellular localization analysis showed the bulk of proteins to be localized to the cytosol and cytosolic compartments, which is consistent with the known function and location of Nef during HIV-1 infection. As for biological pathways, the wide range of affected proteins was denoted by the multiple modes to fulfill function, as distinguished from a strictly singular means, which was not detected. Among these ubiquitinated proteins, six were found to directly interact with Nef, wherein two were upregulated and four downregulated. We also identified 14 proteins involved in protein stability through directly participating in the Ubiquitin Proteasome System (UPS)-mediated proteasomal degradation pathway. Of those proteins, we found six upregulated and eight downregulated. Taken together, these analyses indicate that HIV-1 Nef is integral to regulating the stability of various cellular proteins via modulating ubiquitination. The molecular mechanisms directing Nef-triggered regulation of cellular protein ubiquitination are currently under investigation.

Induction of Filopodia During Cytomegalovirus Entry Into Human Iris Stromal Cells

Many viruses exploit thin projections of filopodia for cell entry and cell-to-cell spread. Using primary cultures of human iris stromal (HIS) cells derived from human eye donors, we report a significant increase in filopodia formation during human cytomegalovirus (HCMV) infection. Using confocal microscopy, we observed a large number of virions being frequently associated along the filopodia prior to cell infection. Depolymerization of actin filaments resulted in a significant inhibition of HCMV entry into HIS cell. Our results further revealed that the transient expression of HCMV envelope glycoprotein B (gB) triggers the induction of the filopodial system. Since gB is known to bind the diverse chains of heparan sulfate (HS), a comparative study was performed to evaluate the gB-mediated filopodial induction in cells expressing either wild-type HS and/or 3-O sulfated HS (3-OS HS). We found that cells co-expressing HCMV gB together with the 3-O sulfotranseferase-3 (3-OST-3) enzyme had a much higher and robust filopodia induction compared to cells co-expressing gB with wild-type HS. The above results were further verified by pre-treating HIS cells with anti-3-OS HS (G2) peptide and/or heparinase-I before challenging with HCMV infection, which resulted in a significant loss in the filopodial counts as well as decreased viral infectivity. Taken together, our findings highlight that HCMV entry into HIS cells actively modulates the actin cytoskeleton via coordinated actions possibly between gB and the 3-OS HS receptor to influence viral infectivity.

HIV-1-Mediated Acceleration of Oncovirus-Related Non-AIDS-Defining Cancers

With the advent of combination antiretroviral therapy (cART), overall survival has been improved, and the incidence of acquired immunodeficiency syndrome (AIDS)-defining cancers has also been remarkably reduced. However, non-AIDS-defining cancers among human immunodeficiency virus-1 (HIV-1)-associated malignancies have increased significantly so that cancer is the leading cause of death in people living with HIV in certain highly developed countries, such as France. However, it is currently unknown how HIV-1 infection raises oncogenic virus-mediated cancer risks in the HIV-1 and oncogenic virus co-infected patients, and thus elucidation of the molecular mechanisms for how HIV-1 expedites the oncogenic viruses-triggered tumorigenesis in the co-infected hosts is imperative for developing therapeutics to cure or impede the carcinogenesis. Hence, this review is focused on HIV-1 and oncogenic virus co-infection-mediated molecular processes in the acceleration of non-AIDS-defining cancers.

Understanding the key functions of Myosins in viral infection

Myosins, a class of actin-based motor proteins existing in almost any organism, are originally considered only involved in driving muscle contraction, reshaping actin cytoskeleton, and anchoring or transporting cargoes, including protein complexes, organelles, vesicles. However, accumulating evidence reveals that myosins also play vital roles in viral infection, depending on viral species and infection stages. This review systemically summarizes the described various myosins, the performed functions, and the involved mechanisms or molecular pathways during viral infection. Meanwhile, the existing issues are also discussed. Additionally, the important technologies or agents, including siRNA, gene editing, and myosin inhibitors, would facilitate dissecting the actions and mechanisms for described and undescribed myosins, which could be adopted to prevent or control viral infection are also characterized.

Single-chain variable fragments of broadly neutralizing antibodies prevent HIV cell-cell transmission

Broadly neutralizing antibodies (bNAbs) are able to prevent HIV infection following passive administration. Single-chain variable fragments (scFv) may have advantages over IgG as their smaller size permits improved diffusion into mucosal tissues. We have previously shown that scFv of bNAbs retain significant breadth and potency against cell-free viral transmission in a TZM-bl assay. However, scFv have not been tested for their ability to block cell-cell transmission, a model in which full-sized bNAbs lose potency. We tested 4 scFv (CAP256.25, PGT121, 3BNC117 and 10E8v4) compared to IgG, in free-virus and cell-cell neutralization assays in A3.01 cells, against a panel of seven heterologous viruses. We show that free-virus neutralization titers in the TZM-bl and A3.01 assays were not significantly different, and confirm that scFv show a 1 to 32-fold reduction in activity in the cell-free model, compared to IgG. However, whereas IgG show 3.4 to 19-fold geometric mean potency loss in cell-cell neutralization compared to free-virus transmission, scFv had more comparable activity in the two assays, with only a 1.3 to 2.3-fold reduction. Geometric mean IC₅₀ of scFv for cell-cell transmission ranged from 0.65 μg/ml (10E8v4) to 2.3 μg/ml (3BNC117) with IgG and scFv neutralization showing similar potency against cell-associated transmission. Therefore, despite the reduced activity of scFv in cell-free assays, their retention of activity in the cell-cell format may make scFv useful for the prevention of both modes of transmission in HIV prevention studies. Importance Broadly neutralizing antibodies (bNAbs) are a major focus for passive immunization against HIV, with the recently concluded HVTN AMP (Antibody Mediated Protection) trial providing proof of concept. Most studies focus on cell-free HIV, however cell-associated virus may play a significant role in HIV infection, pathogenesis and latency. Single-chain variable fragments (scFv) of antibodies may have increased tissue penetration, and reduced immunogenicity. We previously demonstrated that scFv of four HIV-directed bNAbs (CAP256-VRC26.25, PGT121, 3BNC117 and 10E8v4) retain significant potency and breadth against cell-free HIV. As some bNAbs have been shown to lose potency against cell-associated virus, we investigated the ability of bNAb scFv to neutralize this mode of transmission. We demonstrate that unlike IgG, scFv of bNAbs are able to neutralize cell-free and cell-associated virus with similar potency. These scFv, which show functional activity in the therapeutic range, may therefore be suitable for further development as passive immunity for HIV prevention.

Cell-cell interactions via non-covalent click chemistry

Metabolic glycoengineering with unnatural sugars became a valuable tool for introducing recognition markers on the cell membranes via bioorthogonal chemistry. By using this strategy, we functionalized the surface of tumor and T cells using complementary artificial markers based on both β-cyclodextrins (β-CDs) and adamantyl trimers, respectively. Once tied on cell surfaces, the artificial markers induced cell-cell adhesion through non-covalent click chemistry. These unnatural interactions between A459 lung tumor cells and Jurkat T cells triggered the activation of natural killer (NK) cells thanks to the increased production of interleukin-2 (IL-2) in the vicinity of cancer cells, leading ultimately to their cytolysis. The ready-to-use surface markers designed in this study can be easily inserted on the membrane of a wide range of cells previously submitted to metabolic glycoengineering, thereby offering a simple way to investigate and manipulate intercellular interactions.

Apoptosis of Hepatocytes: Relevance for HIV-Infected Patients under Treatment

Due to medical advances over the past few decades, human immunodeficiency virus (HIV) infection, once a devastatingly mortal pandemic, has become a manageable chronic condition. However, available antiretroviral treatments (cART) cannot fully restore immune health and, consequently, a number of inflammation-associated and/or immunodeficiency complications have manifested themselves in treated HIV-infected patients. Among these chronic, non-AIDS (acquired immune deficiency syndrome)-related conditions, liver disease is one of the deadliest, proving to be fatal for 15–17% of these individuals. Aside from the presence of liver-related comorbidities, including metabolic disturbances and co-infections, HIV itself and the adverse effects of cART are the main factors that contribute to hepatic cell injury, inflammation, and fibrosis. Among the molecular mechanisms that are activated in the liver during HIV infection, apoptotic cell death of hepatocytes stands out as a key pathogenic player. In this review, we will discuss the evidence and potential mechanisms involved in the apoptosis of hepatocytes induced by HIV, HIV-encoded proteins, or cART. Some antiretroviral drugs, especially the older generation, can induce apoptosis of hepatic cells, which occurs through a variety of mechanisms, such as mitochondrial dysfunction, increased production of reactive oxygen species (ROS), and induction of endoplasmic reticulum (ER) stress and unfolded protein response (UPR), all of which ultimately lead to caspase activation and cell death.

Proteomes, kinases and signalling pathways in virus-induced filopodia, as potential antiviral therapeutics targets

Filopodia are thin finger-like protrusions at the surface of cells that are internally occupied with bundles of tightly parallel actin filaments. They play significant roles in cellular physiological processes, such as adhesion to extracellular matrix, guidance towards chemo-attractants and in wound healing. Filopodia were recently reported to play important roles in viral infection including initial viral attachment to host cells, cell surfing, viral trafficking, internalization, budding, virus release and spread to other cells in a form that would avoid the host immune system. The detailed virus-host protein interactions underlying most of these processes remain to be elucidated. This review will describe some reported virus-host protein interactions on filopodia with the aim of identifying potential new anti-virus therapeutic targets. Exploring this research area may lead to the development of novel classes of anti-viral therapeutics that can block signalling pathways used by the virus to trigger filopodia formation. Successful compounds would inhibit initial virus attachment, formation of filopodia, expression of putative virus binding protein, extracellular virus trafficking, and budding.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen–major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane–cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Bridging the Gap: Virus Long-Distance Spread via Tunneling Nanotubes

Tunneling nanotubes (TNTs) are actin-based intercellular conduits that connect distant cells and allow intercellular transfer of molecular information, including genetic information, proteins, lipids, and even organelles. Besides providing a means of intercellular communication, TNTs may also be hijacked by pathogens, particularly viruses, to facilitate their spread. Viruses of many different families, including retroviruses, herpesviruses, orthomyxoviruses, and several others have been reported to trigger the formation of TNTs or TNT-like structures in infected cells and use these structures to efficiently spread to uninfected cells. In the current review, we give an overview of the information that is currently available on viruses and TNT-like structures, and we discuss some of the standing questions in this field.

Multifunctional Roles of the N-Terminal Region of HIV-1SF2Nef Are Mediated by Three Independent Protein Interaction Sites

HIV-1 Nef promotes virus spread and disease progression by altering host cell transport and signaling processes through interaction with multiple host cell proteins. The N-terminal region in HIV-1 Nef encompassing residues 12 to 39 has been implicated in many Nef activities, including disruption of CD4 T lymphocyte polarization and homing to lymph nodes, antagonism of SERINC5 restriction to virion infectivity, downregulation of cell surface CD4 and major histocompatibility complex class I (MHC-I), release of Nef-containing extracellular vesicles, and phosphorylation of Nef by recruitment of the Nef-associated kinase complex (NAKC). How this region mediates these pleiotropic functions is unclear. Characterization of a panel of alanine mutants spanning the N-terminal region to identify specific functional determinants revealed this region to be dispensable for effects of Nef from HIV-1 strain SF2 (HIV-1_SF2Nef) on T cell actin organization and chemotaxis, retargeting of the host cell kinase Lck to the trans-Golgi network, and incorporation of Nef into extracellular vesicles. MHC-I downmodulation was specific to residue M20, and inhibition of T cell polarization by Nef required the integrity of the entire region. In contrast, downmodulation of cell surface CD4 and SERINC5 antagonism were mediated by a specific motif encompassing residues 32 to 39 that was also essential for efficient HIV replication in primary CD4 T lymphocytes. Finally, Nef phosphorylation via association with the NAKC was mediated by two EP repeats within residues 24 to 29 but was dispensable for other functions. These results identify the N-terminal region as a multifunctional interaction module for at least three different host cell ligands that mediate independent functions of HIV-1_SF2Nef to facilitate immune evasion and virus spread.IMPORTANCE HIV-1 Nef critically determines virus spread and disease progression in infected individuals by acting as a protein interaction adaptor via incompletely defined mechanisms and ligands. Residues 12 to 39 near the N terminus of Nef have been described as an interaction platform for the Nef-associated kinase complex (NAKC) and were recently identified as essential determinants for a broad range of Nef activities. Here, we report a systematic mapping of this amino acid stretch that revealed the presence of three independent interaction motifs with specific ligands and activities. While downmodulation of cell surface MHC-I depends on M20, two EP repeats are the minimal binding site for the NAKC, and residues 32 to 39 mediate antagonism of the host cell restriction factor SERINC5 as well as downmodulation of cell surface CD4. These results reveal that the N-terminal region of HIV-1_SF2Nef is a versatile and multifunctional protein interaction module that exerts essential functions of the pathogenicity factor via independent mechanisms.

Infusion of HIV-1 Nef-expressing astrocytes into the rat hippocampus induces enteropathy and interstitial pneumonitis and increases blood-brain-barrier permeability

Even though HIV-1 replication can be suppressed by combination antiretroviral therapy (cART) inflammatory processes still occur, contributing to comorbidities. Comorbidities are attributed to variety of factors, including HIV-1 mediated inflammation. Several HIV-1 proteins mediate central nervous system (CNS) inflammation, including Nef. Nef is an early HIV-1 protein, toxic to neurons and glia and is sufficient to cause learning impairment similar to some deficits observed in HIV-1 associated neurocognitive disorders. To determine whether hippocampal Nef expression by astrocytes contributes to comorbidities, specifically peripheral inflammation, we infused Sprague Dawley rats with GFP- (control) or Nef-transfected astrocytes into the right hippocampus. Brain, lung, and ileum were collected postmortem for the measurement of inflammatory markers. Increased blood-brain-barrier permeability and serum IL-1β levels were detected in the Nef-treated rats. The lungs of Nef-treated rats demonstrated leukocyte infiltration, macrophage upregulation, and enhanced vascular permeability. Ileal tissue showed reactive follicular lymphoid hyperplasia, increased permeability and macrophage infiltration. The intracerebroventricular application of IL-1 receptor antagonist reduced infiltration of immune cells into ileum and lung, indicating the important role of IL-1β in mediating the spread of inflammation from the brain to other tissues. This suggests that localized expression of a single viral protein, HIV-1 Nef, can contribute to a broader inflammatory response by upregulation of IL-1β. Further, these results suggest that Nef contributes to the chronic inflammation seen in HIV patients, even in those whose viremia is controlled by cART.

HIV-1 Nef Targets HDAC6 to Assure Viral Production and Virus Infection

HIV Nef is a central auxiliary protein in HIV infection and pathogenesis. Our results indicate that HDAC6 promotes the aggresome/autophagic degradation of the viral polyprotein Pr55Gag to inhibit HIV-1 production. Nef counteracts this antiviral activity of HDAC6 by inducing its degradation and subsequently stabilizing Pr55Gag and Vif viral proteins. Nef appears to neutralize HDAC6 by an acidic/endosomal-lysosomal processing and does not need the downregulation function, since data obtained with the non-associated cell-surface Nef-G2A mutant - the cytoplasmic location of HDAC6 - together with studies with chemical inhibitors and other Nef mutants, point to this direction. Hence, the polyproline rich region P72xxP75 (69-77 aa) and the di-Leucin motif in the Nef-ExxxLL160-165 sequence of Nef, appear to be responsible for HDAC6 clearance and, therefore, required for this novel Nef proviral function. Nef and Nef-G2A co-immunoprecipitate with HDAC6, whereas the Nef-PPAA mutant showed a reduced interaction with the anti-HIV-1 enzyme. Thus, the P72xxP75 motif appears to be responsible, directly or indirectly, for the interaction of Nef with HDAC6. Remarkably, by neutralizing HDAC6, Nef assures Pr55Gag location and aggregation at plasma membrane, as observed by TIRFM, promotes viral egress, and enhances the infectivity of viral particles. Consequently, our results suggest that HDAC6 acts as an anti-HIV-1 restriction factor, limiting viral production and infection by targeting Pr55Gag and Vif. This function is counteracted by functional HIV-1 Nef, in order to assure viral production and infection capacities. The interplay between HIV-1 Nef and cellular HDAC6 may determine viral infection and pathogenesis, representing both molecules as key targets to battling HIV.

Remodeling of the Actin Network Associated with the Non-Structural Protein 1 (NS1) of West Nile Virus and Formation of NS1-Containing Tunneling Nanotubes

The cellular response to the recombinant NS1 protein of West Nile virus (NS1^WNV) was studied using three different cell types: Vero E6 simian epithelial cells, SH-SY5Y human neuroblastoma cells, and U-87MG human astrocytoma cells. Cells were exposed to two different forms of NS1^WNV: (i) the exogenous secreted form, sNS1^WNV, added to the extracellular milieu; and (ii) the endogenous NS1^WNV, the intracellular form expressed in plasmid-transfected cells. The cell attachment and uptake of sNS1^WNV varied with the cell type and were only detectable in Vero E6 and SH-SY5Y cells. Addition of sNS1^WNV to the cell culture medium resulted in significant remodeling of the actin filament network in Vero E6 cells. This effect was not observed in SH-SY5Y and U-87MG cells, implying that the cellular uptake of sNS1^WNV and actin network remodeling were dependent on cell type. In the three cell types, NS1^WNV-expressing cells formed filamentous projections reminiscent of tunneling nanotubes (TNTs). These TNT-like projections were found to contain actin and NS1^WNV proteins. Interestingly, similar actin-rich, TNT-like filaments containing NS1^WNV and the viral envelope glycoprotein E^WNV were also observed in WNV-infected Vero E6 cells.

Zearalenone inhibits T cell chemotaxis by inhibiting cell adhesion and migration related proteins

Zearalenone (ZEA) is a phenolic resorcylic acid lactone mycotoxin produced by several Fusarium species that grow on temperate and tropical crops. The number of reports documenting the immunotoxic effects of ZEA is increasing, but the underlying mechanism is not clear. The purpose of this study was to investigate the effects of ZEA on T cell chemotaxis and evaluate changes in adhesion and migration proteins associated with this process. Specifically, T cells were isolated from BALB/C mouse splenic lymphocytes, activated by concanavalin A (Con A), and then exposed to different concentrations of ZEA. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used observe the ultrastructural changes inside the cell and on the cell surface, respectively. The transwell migration assay was used to evaluate the effect of ZEA on T cell chemotaxis in the presence of CCL19 or CCL21. A confocal 3D laser was used to capture the morphology of perforated cells and western blot was used to detect the expression of proteins associated with cell migration and adhesion. Additionally, we used flow cytometry to examine the expression of chemokine receptors on T cells. Finally, the chemokine (RANTES and MIP-1α) levels secreted by T cells were assessed using cytometric bead array. Overall, our data showed that treatment with ZEA caused ultrastructural damage on the surface as well as inside of T cells. Moreover, ZEA inhibited T cell chemotaxis which was mediated by CCL19 or CCL21 and disrupted the balance of T cell subtypes. The expression of T cell adhesion and migration proteins was also inhibited by ZEA. The expression of T cell chemokine receptor as well as secretion of RANTES and MIP-1α by T cells was suppressed after ZEA treatment. In summary, our results indicate that ZEA reduced the chemotactic effect of T cells mediated by chemokines, which was likely linked to the inhibition of T cell motility and accompanied by decreased expression of adhesion and migration proteins.

HIV-1 Balances the Fitness Costs and Benefits of Disrupting the Host Cell Actin Cytoskeleton Early after Mucosal Transmission

HIV-1 primarily infects T lymphocytes and uses these motile cells as migratory vehicles for effective dissemination in the host. Paradoxically, the virus at the same time disrupts multiple cellular processes underlying lymphocyte motility, seemingly counterproductive to rapid systemic infection. Here we show by intravital microscopy in humanized mice that perturbation of the actin cytoskeleton via the lentiviral protein Nef, and not changes to chemokine receptor expression or function, is the dominant cause of dysregulated infected T cell motility in lymphoid tissue by preventing stable cellular polarization required for fast migration. Accordingly, disrupting the Nef hydrophobic patch that facilitates actin cytoskeletal perturbation initially accelerates systemic viral dissemination after female genital transmission. However, the same feature of Nef was subsequently critical for viral persistence in immune-competent hosts. Therefore, a highly conserved activity of lentiviral Nef proteins has dual effects and imposes both fitness costs and benefits on the virus at different stages of infection.

The C Terminus of Rotavirus VP4 Protein Contains an Actin Binding Domain Which Requires Cooperation with the Coiled-Coil Domain for Actin Remodeling

The interactions between viruses and actin cytoskeleton have been widely studied. We showed that rotaviruses remodel microfilaments in intestinal cells and demonstrated that this was due to the VP4 spike protein. Microfilaments mainly occur in the apical domain of infected polarized enterocytes and favor the polarized apical exit of viral progeny. The present work aims at the identification of molecular determinants of actin-VP4 interactions. We used various deletion mutants of VP4 that were transfected into Cos-7 cells and analyzed interactions by immunofluorescence confocal microscopy. It has been established that the C-terminal part of VP4 is embedded within viral particles when rotavirus assembles. The use of specific monoclonal antibodies demonstrated that VP4 is expressed in different forms in infected cells: classically as spike on the outer layer of virus particles, but also as free soluble protein in the cytosol. The C terminus of free VP4 was identified as interacting with actin microfilaments. The VP4 actin binding domain is unable to promote microfilament remodeling by itself; the coiled-coil domain is also required in this process. This actin-binding domain was shown to dominate a previously identified peroxisomal targeting signal, located in the three last amino acids of VP4. The newly identified actin-binding domain is highly conserved in rotavirus strains from species A, B, and C, suggesting that actin binding and remodeling is a general strategy for rotavirus exit. This provides a novel mechanism of protein-protein interactions, not involving cell signaling pathways, to facilitate rotavirus exit.IMPORTANCE Rotaviruses are causal agents of acute infantile viral diarrhea. In intestinal cells, in vitro as well as in vivo, virus assembly and exit do not imply cell lysis but rely on an active process in which the cytoskeleton plays a major role. We describe here a novel molecular mechanism by which the rotavirus spike protein VP4 drives actin remodeling. This relies on the fact that VP4 occurs in different forms. Besides its structural function within the virion, a large proportion of VP4 is expressed as free protein. Here, we show that free VP4 possesses a functional actin-binding domain. This domain, in coordination with a coiled-coil domain, promotes actin cytoskeleton remodeling, thereby providing the capacity to destabilize the cell membrane and allow efficient rotavirus exit.

Myosin-X is essential to the intercellular spread of HIV-1 Nef through tunneling nanotubes

Tunneling nanotubes (TNTs) are intercellular structures that allow for the passage of vesicles, organelles, genomic material, pathogenic proteins and pathogens. The unconventional actin molecular motor protein Myosin-X (Myo10) is a known inducer of TNTs in neuronal cells, yet its role in other cell types has not been examined. The Nef HIV-1 accessory protein is critical for HIV-1 pathogenesis and can self-disseminate in culture via TNTs. Understanding its intercellular spreading mechanism could reveal ways to control its damaging effects during HIV-1 infection. Our goal in this study was to characterize the intercellular transport mechanism of Nef from macrophages to T cells. We demonstrate that Nef increases TNTs in a Myo10-dependent manner in macrophages and observed the transfer of Nef via TNTs from macrophages to T cells. To quantify this transfer mechanism, we established an indirect flow cytometry assay. Since Nef expression in T cells down-regulates the surface receptor CD4, we correlated the decrease in CD4 to the transfer of Nef between these cells. Thus, we co-cultured macrophages expressing varying levels of Nef with a T cell line expressing high levels of CD4 and quantified the changes in CD4 surface expression resulting from Nef transfer. We demonstrate that Nef transfer occurs via a cell-to-cell dependent mechanism that directly correlates with the presence of Myo10-dependent TNTs. Thus, we show that Nef can regulate Myo10 expression, thereby inducing TNT formation, resulting in its own transfer from macrophages to T cells. In addition, we demonstrate that up-regulation of Myo10 induced by Nef also occurs in human monocyte derived macrophages during HIV-1 infection.

HIV-1 Nef Hijacks Lck and Rac1 Endosomal Traffic To Dually Modulate Signaling-Mediated and Actin Cytoskeleton-Mediated T Cell Functions

Endosomal traffic of TCR and signaling molecules regulates immunological synapse formation and T cell activation. We recently showed that Rab11 endosomes regulate the subcellular localization of the tyrosine kinase Lck and of the GTPase Rac1 and control their functions in TCR signaling and actin cytoskeleton remodeling. HIV-1 infection of T cells alters their endosomal traffic, activation capacity, and actin cytoskeleton organization. The viral protein Nef is pivotal for these modifications. We hypothesized that HIV-1 Nef could jointly alter Lck and Rac1 endosomal traffic and concomitantly modulate their functions. In this study, we show that HIV-1 infection of human T cells sequesters both Lck and Rac1 in a pericentrosomal compartment in an Nef-dependent manner. Strikingly, the Nef-induced Lck compartment contains signaling-competent forms (phosphorylated on key Tyr residues) of Lck and some of its downstream effectors, TCRζ, ZAP70, SLP76, and Vav1, avoiding the proximal LAT adaptor. Importantly, Nef-induced concentration of signaling molecules was concomitant with the upregulation of several early and late T cell activation genes. Moreover, preventing the concentration of the Nef-induced Lck compartment by depleting the Rab11 effector FIP3 counteracted Nef-induced gene expression upregulation. In addition, Nef extensively sequesters Rac1 and downregulates Rac1-dependent actin cytoskeleton remodeling, thus reducing T cell spreading. Therefore, by modifying their endosomal traffic, Nef hijacks signaling and actin cytoskeleton regulators to dually modulate their functional outputs. Our data shed new light into the molecular mechanisms that modify T cell physiology during HIV-1 infection.

Signature molecules expressed differentially in a liver disease stage-specific manner by HIV-1 and HCV co-infection

To elucidate HIV-1 co-infection-induced acceleration of HCV liver disease and identify stage-specific molecular signatures, we applied a new high-resolution molecular screen, the Affymetrix GeneChip Human Transcriptome Array (HTA2.0), to HCV-mono- and HIV/HCV-co-infected liver specimens from subjects with early and advanced disease. Out of 67,528 well-annotated genes, we have analyzed the functional and statistical significance of 75 and 28 genes expressed differentially between early and advanced stages of HCV mono- and HIV/HCV co-infected patient liver samples, respectively. We also evaluated the expression of 25 and 17 genes between early stages of mono- and co-infected liver tissues and between advanced stages of mono- and co-infected patient's samples, respectively. Based on our analysis of fold-change in gene expression as a function of disease stage (i.e., early vs. advanced), coupled with consideration of the known relevant functions of these genes, we focused on four candidate genes, ACSL4, GNMT, IFI27, and miR122, which are expressed stage-specifically in HCV mono- and HIV-1/HCV co-infective liver disease and are known to play a pivotal role in regulating HCV-mediated hepatocellular carcinoma (HCC). Our qRT-PCR analysis of the four genes in patient liver specimens supported the microarray data. Protein products of each gene were detected in the endoplasmic reticulum (ER) where HCV replication takes place, and the genes' expression significantly altered replicability of HCV in the subgenomic replicon harboring regulatory genes of the JFH1 strain of HCV in Huh7.5.1. With respect to three well-known transferrable HIV-1 viral elements-Env, Nef, and Tat-Nef uniquely augmented replicon expression, while Tat, but not the others, substantially modulated expression of the candidate genes in hepatocytic cells. Combinatorial expression of these cellular and viral genes in the replicon cells further altered replicon expression. Taken together, these results showed that HIV-1 viral proteins can exacerbate liver pathology in the co-infected patients by disparate molecular mechanisms-directly or indirectly dysregulating HCV replication, even if lack of association of HCV load and end-stage liver disease in hemophilic patients were reported, and modulating expression of hepatocellular genes critical for disease progression. These findings also provide major insights into development of stage-specific hepatocellular biomarkers for improved diagnosis and prognosis of HCV-mediated liver disease.

Bone degradation machinery of osteoclasts: An HIV-1 target that contributes to bone loss

Bone deficits are frequent in HIV-1-infected patients. We report here that osteoclasts, the cells specialized in bone resorption, are infected by HIV-1 in vivo in humanized mice and ex vivo in human joint biopsies. In vitro, infection of human osteoclasts occurs at different stages of osteoclastogenesis via cell-free viruses and, more efficiently, by transfer from infected T cells. HIV-1 infection markedly enhances adhesion and osteolytic activity of human osteoclasts by modifying the structure and function of the sealing zone, the osteoclast-specific bone degradation machinery. Indeed, the sealing zone is broader due to F-actin enrichment of its basal units (i.e., the podosomes). The viral protein Nef is involved in all HIV-1-induced effects partly through the activation of Src, a regulator of podosomes and of their assembly as a sealing zone. Supporting these results, Nef-transgenic mice exhibit an increased osteoclast density and bone defects, and osteoclasts derived from these animals display high osteolytic activity. Altogether, our study evidences osteoclasts as host cells for HIV-1 and their pathological contribution to bone disorders induced by this virus, in part via Nef.

All-Round Manipulation of the Actin Cytoskeleton by HIV

While significant progress has been made in terms of human immunodeficiency virus (HIV) therapy, treatment does not represent a cure and remains inaccessible to many people living with HIV. Continued mechanistic research into the viral life cycle and its intersection with many aspects of cellular biology are not only fundamental in the continued fight against HIV, but also provide many key observations of the workings of our immune system. Decades of HIV research have testified to the integral role of the actin cytoskeleton in both establishing and spreading the infection. Here, we review how the virus uses different strategies to manipulate cellular actin networks and increase the efficiency of various stages of its life cycle. While some HIV proteins seem able to bind to actin filaments directly, subversion of the cytoskeleton occurs indirectly by exploiting the power of actin regulatory proteins, which are corrupted at multiple levels. Furthermore, this manipulation is not restricted to a discrete class of proteins, but rather extends throughout all layers of the cytoskeleton. We discuss prominent examples of actin regulators that are exploited, neutralized or hijacked by the virus, and address how their coordinated deregulation can lead to changes in cellular behavior that promote viral spreading.

Metabolic control of the scaffold protein TKS5 in tissue-invasive, proinflammatory T cells

Pathogenic T cells in individuals with rheumatoid arthritis (RA) infiltrate non-lymphoid tissue sites, maneuver through extracellular matrix and form lasting inflammatory microstructures. Here we found that RA T cells abundantly express the podosome scaffolding protein TKS5, which enables them to form tissue-invasive membrane structures. TKS5 overexpression was regulated by the intracellular metabolic environment of RA T cells-specifically, by reduced glycolytic flux that led to deficiencies in ATP and pyruvate. ATPlopyruvatelo conditions triggered fatty acid biosynthesis and the formation of cytoplasmic lipid droplets. Restoration of pyruvate production or inhibition of fatty acid synthesis corrected the tissue-invasiveness of RA T cells in vivo and reversed their proarthritogenic behavior. Thus, metabolic control of T cell locomotion provides new opportunities to interfere with T cell invasion into specific tissue sites.

Exosomes surf on filopodia to enter cells at endocytic hot spots, traffic within endosomes, and are targeted to the ER

Exosomes are nanovesicles released by virtually all cells, which act as intercellular messengers by transfer of protein, lipid, and RNA cargo. Their quantitative efficiency, routes of cell uptake, and subcellular fate within recipient cells remain elusive. We quantitatively characterize exosome cell uptake, which saturates with dose and time and reaches near 100% transduction efficiency at picomolar concentrations. Highly reminiscent of pathogenic bacteria and viruses, exosomes are recruited as single vesicles to the cell body by surfing on filopodia as well as filopodia grabbing and pulling motions to reach endocytic hot spots at the filopodial base. After internalization, exosomes shuttle within endocytic vesicles to scan the endoplasmic reticulum before being sorted into the lysosome as their final intracellular destination. Our data quantify and explain the efficiency of exosome internalization by recipient cells, establish a new parallel between exosome and virus host cell interaction, and suggest unanticipated routes of subcellular cargo delivery.

D186/D190 is an allele-dependent determinant of HIV-1 Nef function

The HIV-1 pathogenesis factor Nef interacts with numerous ligands to affect cellular vesicular transport, signal transduction and cytoskeletal dynamics. While most Nef functions depend on multivalent protein interaction motifs, disrupting actin dynamics requires a motif that specifically recruits the host kinase PAK2. An adjacent aspartate was recently predicted to mediate Nef-β-catenin interactions. We report here that β-catenin can be co-immunoprecipitated with Nef.GFP from Jurkat T cell lysates. This association is conserved among lentiviral Nef proteins but does not involve classical Nef protein interaction motifs, including the critical aspartate. While aspartate-to-alanine mutations impaired cell surface receptor downregulation and interference with actin dynamics and cell motility by HIV-1 NA7 Nef, analogous mutations did not affect HIV-1 SF2 Nef function. These allelic differences were determined by a proximal lysine/arginine polymorphism. These results emphasize differences between Nef alleles regarding the functional role of individual residues and underscore the need for allele-specific structure-function analyses.

Exosomes and nanotubes: Control of immune cell communication

Cell-cell communication is critical to coordinate the activity and behavior of a multicellular organism. The cells of the immune system not only must communicate with similar cells, but also with many other cell types in the body. Therefore, the cells of the immune system have evolved multiple ways to communicate. Exosomes and tunneling nanotubes (TNTs) are two means of communication used by immune cells that contribute to immune functions. Exosomes are small membrane vesicles secreted by most cell types that can mediate intercellular communication and in the immune system they are proposed to play a role in antigen presentation and modulation of gene expression. TNTs are membranous structures that mediate direct cell-cell contact over several cell diameters in length (and possibly longer) and facilitate the interaction and/or the transfer of signals, material and other cellular organelles between connected cells. Recent studies have revealed additional, but sometimes conflicting, structural and functional features of both exosomes and TNTs. Despite the new and exciting information in exosome and TNT composition, origin and in vitro function, biologically significant functions are still being investigated and determined. In this review, we discuss the current field regarding exosomes and TNTs in immune cells providing evaluation and perspectives of the current literature.

HIV-1 Infection of T Lymphocytes and Macrophages Affects Their Migration via Nef

The human immunodeficiency virus (HIV-1) disseminates in the body and is found in several organs and tissues. Although HIV-1 mainly targets both CD4⁺ T lymphocytes and macrophages, it has contrasting effects between these cell populations. HIV-1 infection namely reduces the viability of CD4⁺ T cells, whereas infected macrophages are long-lived. In addition, the migration of T cells is reduced by the infection, whereas HIV-1 differentially modulates the migration modes of macrophages. In 2-dimensions (2D) assays, infected macrophages are less motile compared to the control counterparts. In 3D environments, macrophages use two migration modes that are dependent on the matrix architecture: amoeboid and mesenchymal migration. HIV-1-infected macrophages exhibit a reduced amoeboid migration but an enhanced mesenchymal migration, via the viral protein Nef. Indeed, the mesenchymal migration involves podosomes, and Nef stabilizes these cell structures through the activation of the tyrosine kinase Hck, which in turn phosphorylates the Wiskott–Aldrich syndrome protein (WASP). WASP is a key player in actin remodeling and cell migration. The reprogramed motility of infected macrophages observed in vitro correlates in vivo with enhanced macrophage infiltration in experimental tumors in Nef-transgenic mice compared to control mice. In conclusion, HIV infection of host target cells modifies their migration capacity; we infer that HIV-1 enhances virus spreading in confined environments by reducing T cells migration, and facilitates virus dissemination into different organs and tissues of the human body by enhancing macrophage mesenchymal migration.

Virological Mechanisms in the Coinfection between HIV and HCV

Due to shared transmission routes, coinfection with Hepatitis C Virus (HCV) is common in patients infected by Human Immunodeficiency Virus (HIV). The immune-pathogenesis of liver disease in HIV/HCV coinfected patients is a multifactorial process. Several studies demonstrated that HIV worsens the course of HCV infection, increasing the risk of cirrhosis and hepatocellular carcinoma. Also, HCV might increase immunological defects due to HIV and risk of comorbidities. A specific cross-talk among HIV and HCV proteins in coinfected patients modulates the natural history, the immune responses, and the life cycle of both viruses. These effects are mediated by immune mechanisms and by a cross-talk between the two viruses which could interfere with host defense mechanisms. In this review, we focus on some virological/immunological mechanisms of the pathogenetic interactions between HIV and HCV in the human host.

Association with PAK2 Enables Functional Interactions of Lentiviral Nef Proteins with the Exocyst Complex

Human immunodeficiency virus type 1 (HIV-1) Nef enhances virus replication and contributes to immune evasion in vivo, but the underlying molecular mechanisms remain incompletely defined. Nef interferes with host cell actin dynamics to restrict T lymphocyte responses to chemokine stimulation and T cell receptor engagement. This relies on the assembly of a labile multiprotein complex including the host kinase PAK2 that Nef usurps to phosphorylate and inactivate the actin-severing factor cofilin. Components of the exocyst complex (EXOC), an octameric protein complex involved in vesicular transport and actin remodeling, were recently reported to interact with Nef via the same molecular surface that mediates PAK2 association. Exploring the functional relevance of EXOC in Nef-PAK2 complex assembly/function, we found Nef-EXOC interactions to be specifically mediated by the PAK2 interface of Nef, to occur in infected human T lymphocytes, and to be conserved among lentiviral Nef proteins. In turn, EXOC was dispensable for direct downstream effector functions of Nef-associated PAK2. Surprisingly, PAK2 was essential for Nef-EXOC association, which required a functional Rac1/Cdc42 binding site but not the catalytic activity of PAK2. EXOC was dispensable for Nef functions in vesicular transport but critical for inhibition of actin remodeling and proximal signaling upon T cell receptor engagement. Thus, Nef exploits PAK2 in a stepwise mechanism in which its kinase activity cooperates with an adaptor function for EXOC to inhibit host cell actin dynamics.

IMPORTANCE

Human immunodeficiency virus type 1 (HIV-1) Nef contributes to AIDS pathogenesis, but the underlying molecular mechanisms remain incompletely understood. An important aspect of Nef function is to facilitate virus replication by disrupting T lymphocyte actin dynamics in response to stimulation via its association with the host cell kinase PAK2. We report here that the molecular surface of Nef for PAK2 association also mediates interaction of Nef with EXOC and establish that PAK2 provides an essential adaptor function for the subsequent formation of Nef-EXOC complexes. PAK2 and EXOC specifically cooperate in the inhibition of actin dynamics and proximal signaling induced by T cell receptor engagement by Nef. These results establish EXOC as a functionally relevant Nef interaction partner, emphasize the suitability of the PAK2 interaction surface for future therapeutic interference with Nef function, and show that such strategies need to target activity-independent PAK2 functions.

SAMHD1 Inhibits LINE-1 Retrotransposition by Promoting Stress Granule Formation

The SAM domain and HD domain containing protein 1 (SAMHD1) inhibits retroviruses, DNA viruses and long interspersed element 1 (LINE-1). Given that in dividing cells, SAMHD1 loses its antiviral function yet still potently restricts LINE-1, we propose that, instead of blocking viral DNA synthesis by virtue of its dNTP triphosphohydrolase activity, SAMHD1 may exploit a different mechanism to control LINE-1. Here, we report a new activity of SAMHD1 in promoting cellular stress granule assembly, which correlates with increased phosphorylation of eIF2α and diminished eIF4A/eIF4G interaction. This function of SAMHD1 enhances sequestration of LINE-1 RNP in stress granules and consequent blockade to LINE-1 retrotransposition. In support of this new mechanism of action, depletion of stress granule marker proteins G3BP1 or TIA1 abrogates stress granule formation and overcomes SAMHD1 inhibition of LINE-1. Together, these data reveal a new mechanism for SAMHD1 to control LINE-1 by activating cellular stress granule pathway.

Long interspersed element 1 (LINE-1 or L1) comprises 17% of human genome, and has played a major role in shaping the evolution of human genome. Approximately 100 copies of LINE-1 are still active in an average individual genome. Movement of these LINE-1 sequences to new loci in the genome has the potential of causing sporadic cases of disease. Among the multi-layered mechanisms by which the host controls LINE-1 activity is a group of host restriction factors including APOBEC3 proteins. SAMHD1 was known for the association of its mutations with the Aicardi-Goutieres syndrome (AGS), a congenital autoimmune disease. SAMHD1 was recently reported as a host restriction factor that inhibits a number of retroviruses and DNA viruses including human immunodeficiency virus type 1 (HIV-1) and herpes simplex virus 1 (HSV-1). Here, we demonstrate that SAMHD1 inhibits LINE-1 retrotransposition through promoting the sequestration of LINE-1 RNP within the cytoplasmic stress granules. SAMHD1 promotes the formation of large stress granules by inducing phosphorylation of eIF2α and disrupting eIF4A/eIF4G interaction. This is the first report describing the role of SAMHD1 in modulating the formation of stress granules. We envision that this function of SAMHD1 not only contributes to the inhibition of LINE-1, but also the restriction of various viruses.

Involvement of the Rac1-IRSp53-Wave2-Arp2/3 Signaling Pathway in HIV-1 Gag Particle Release in CD4 T Cells

During HIV-1 assembly, the Gag viral proteins are targeted and assemble at the inner leaflet of the cell plasma membrane. This process could modulate the cortical actin cytoskeleton, located underneath the plasma membrane, since actin dynamics are able to promote localized membrane reorganization. In addition, activated small Rho GTPases are known for regulating actin dynamics and membrane remodeling. Therefore, the modulation of such Rho GTPase activity and of F-actin by the Gag protein during virus particle formation was considered. Here, we studied the implication of the main Rac1, Cdc42, and RhoA small GTPases, and some of their effectors, in this process. The effect of small interfering RNA (siRNA)-mediated Rho GTPases and silencing of their effectors on Gag localization, Gag membrane attachment, and virus-like particle production was analyzed by immunofluorescence coupled to confocal microscopy, membrane flotation assays, and immunoblot assays, respectively. In parallel, the effect of Gag expression on the Rac1 activation level was monitored by G-LISA, and the intracellular F-actin content in T cells was monitored by flow cytometry and fluorescence microscopy. Our results revealed the involvement of activated Rac1 and of the IRSp53-Wave2-Arp2/3 signaling pathway in HIV-1 Gag membrane localization and particle release in T cells as well as a role for actin branching and polymerization, and this was solely dependent on the Gag viral protein. In conclusion, our results highlight a new role for the Rac1-IRSp53-Wave2-Arp2/3 signaling pathway in the late steps of HIV-1 replication in CD4 T lymphocytes.

IMPORTANCE

During HIV-1 assembly, the Gag proteins are targeted and assembled at the inner leaflet of the host cell plasma membrane. Gag interacts with specific membrane phospholipids that can also modulate the regulation of cortical actin cytoskeleton dynamics. Actin dynamics can promote localized membrane reorganization and thus can be involved in facilitating Gag assembly and particle formation. Activated small Rho GTPases and effectors are regulators of actin dynamics and membrane remodeling. We thus studied the effects of the Rac1, Cdc42, and RhoA GTPases and their specific effectors on HIV-1 Gag membrane localization and viral particle release in T cells. Our results show that activated Rac1 and the IRSp53-Wave2-Arp2/3 signaling pathway are involved in Gag plasma membrane localization and viral particle production. This work uncovers a role for cortical actin through the activation of Rac1 and the IRSp53/Wave2 signaling pathway in HIV-1 particle formation in CD4 T lymphocytes.

HIV-1 Nef and Vpu Interfere with L-Selectin (CD62L) Cell Surface Expression To Inhibit Adhesion and Signaling in Infected CD4+ T Lymphocytes

Leukocyte recirculation between blood and lymphoid tissues is required for the generation and maintenance of immune responses against pathogens and is crucially controlled by the L-selectin (CD62L) leukocyte homing receptor. CD62L has adhesion and signaling functions and initiates the capture and rolling on the vascular endothelium of cells entering peripheral lymph nodes. This study reveals that CD62L is strongly downregulated on primary CD4(+) T lymphocytes upon infection with human immunodeficiency virus type 1 (HIV-1). Reduced cell surface CD62L expression was attributable to the Nef and Vpu viral proteins and not due to increased shedding via matrix metalloproteases. Both Nef and Vpu associated with and sequestered CD62L in perinuclear compartments, thereby impeding CD62L transport to the plasma membrane. In addition, Nef decreased total CD62L protein levels. Importantly, infection with wild-type, but not Nef- and Vpu-deficient, HIV-1 inhibited the capacity of primary CD4(+) T lymphocytes to adhere to immobilized fibronectin in response to CD62L ligation. Moreover, HIV-1 infection impaired the signaling pathways and costimulatory signals triggered in primary CD4(+) T cells by CD62L ligation. We propose that HIV-1 dysregulates CD62L expression to interfere with the trafficking and activation of infected T cells. Altogether, this novel HIV-1 function could contribute to virus dissemination and evasion of host immune responses.

IMPORTANCE

L-selectin (CD62L) is an adhesion molecule that mediates the first steps of leukocyte homing to peripheral lymph nodes, thus crucially controlling the initiation and maintenance of immune responses to pathogens. Here, we report that CD62L is downmodulated on the surfaces of HIV-1-infected T cells through the activities of two viral proteins, Nef and Vpu, that prevent newly synthesized CD62L molecules from reaching the plasma membrane. We provide evidence that CD62L downregulation on HIV-1-infected primary T cells results in impaired adhesion and signaling functions upon CD62L triggering. Removal of cell surface CD62L may predictably keep HIV-1-infected cells away from lymph nodes, the privileged sites of both viral replication and immune response activation, with important consequences, such as systemic viral spread and evasion of host immune surveillance. Altogether, we propose that Nef- and Vpu-mediated subversion of CD62L function could represent a novel determinant of HIV-1 pathogenesis.

Intracellular Nef detected in peripheral blood mononuclear cells from HIV patients

We report here the novel finding that HIV-negative factor (Nef) protein is present in considerable numbers of peripheral blood mononuclear cells (PBMCs) from viremic HIV-infected patients not on antiretroviral therapy (ART) and also in patients receiving virologically suppressive ART, though to a smaller degree. Interestingly, these Nef-positive PBMCs constitute predominantly uninfected bystander cells. These results may explain systemic pathology in HIV patients, even in those receiving ART.

HIV-1 reprograms the migration of macrophages

Macrophages are motile leukocytes, targeted by HIV-1, thought to play a critical role in host dissemination of the virus. However, whether infection impacts their migration capacity remains unknown. We show that 2-dimensional migration and the 3-dimensional (3D) amoeboid migration mode of HIV-1-infected human monocyte-derived macrophages were inhibited, whereas the 3D mesenchymal migration was enhanced. The viral protein Nef was necessary and sufficient for all HIV-1-mediated effects on migration. In Nef transgenic mice, tissue infiltration of macrophages was increased in a tumor model and in several tissues at steady state, suggesting a dominant role for mesenchymal migration in vivo. The mesenchymal motility involves matrix proteolysis and podosomes, cell structures constitutive of monocyte-derived cells. Focusing on the mechanisms used by HIV-1 Nef to control the mesenchymal migration, we show that the stability, size, and proteolytic function of podosomes are increased via the phagocyte-specific kinase Hck and Wiskott-Aldrich syndrome protein (WASP), 2 major regulators of podosomes. In conclusion, HIV-1 reprograms macrophage migration, which likely explains macrophage accumulation in several patient tissues, which is a key step for virus spreading and pathogenesis. Moreover, Nef points out podosomes and the Hck/WASP signaling pathway as good candidates to control tissue infiltration of macrophages, a detrimental phenomenon in several diseases.

HIV-1 Nef and Vpu are functionally redundant broad-spectrum modulators of cell surface receptors, including tetraspanins

HIV-1 Nef and Vpu are thought to optimize virus replication in the infected host, at least in part via their ability to interfere with vesicular host cell trafficking. Despite the use of distinct molecular mechanisms, Nef and Vpu share specificity for some molecules such as CD4 and major histocompatibility complex class I (MHC-I), while disruption of intracellular transport of the host cell restriction factor CD317/tetherin represents a specialized activity of Vpu not exerted by HIV-1 Nef. To establish a profile of host cell receptors whose intracellular transport is affected by Nef, Vpu, or both, we comprehensively analyzed the effect of these accessory viral proteins on cell surface receptor levels on A3.01 T lymphocytes. Thirty-six out of 105 detectable receptors were significantly downregulated by HIV-1 Nef, revealing a previously unappreciated scope with which HIV-1 Nef remodels the cell surface of infected cells. Remarkably, the effects of HIV-1 Vpu on host cell receptor exposure largely matched those of HIV-1 Nef in breadth and specificity (32 of 105, all also targeted by Nef), even though the magnitude was generally less pronounced. Of particular note, cell surface exposure of all members of the tetraspanin (TSPAN) protein family analyzed was reduced by both Nef and Vpu, and the viral proteins triggered the enrichment of TSPANs in a perinuclear area of the cell. While Vpu displayed significant colocalization and physical association with TSPANs, interactions of Nef with TSPANs were less robust. TSPANs thus emerge as a major target of deregulation in host cell vesicular transport by HIV-1 Nef and Vpu. The conservation of this activity in two independent accessory proteins suggests its importance for the spread of HIV-1 in the infected host.

IMPORTANCE

In this paper, we define that HIV-1 Nef and Vpu display a surprising functional overlap and affect the cell surface exposure of a previously unexpected breadth of cellular receptors. Our analyses furthermore identify the tetraspanin protein family as a previously unrecognized target of Nef and Vpu activity. These findings have implications for the interpretation of effects detected for these accessory gene products on individual host cell receptors and illustrate the coevolution of Nef and Vpu function.

Adding new dimensions: towards an integrative understanding of HIV-1 spread

In vitro studies in primary or immortalized cells continue to be used to elucidate the essential principles that govern the interactions between HIV-1 and isolated target cells. However, until recently, substantial technical barriers prevented this information from being efficiently translated to the more complex scenario of HIV-1 spread in the host in vivo, which has limited our understanding of the impact of host physiological parameters on the spread of HIV-1. In this Review, we discuss the recent development of imaging approaches to visualize HIV-1 spread and the adaptation of these approaches to organotypic ex vivo models and animal models. We focus on new concepts, including the mechanisms and in vivo relevance of cell-cell transmission for HIV-1 spread and the function of the HIV-1 pathogenesis factor Nef, which have emerged from the application of these integrative approaches in complex cell systems.

HIV-1 Nef is transferred from expressing T cells to hepatocytic cells through conduits and enhances HCV replication

HIV-1 infection enhances HCV replication and as a consequence accelerates HCV-mediated hepatocellular carcinoma (HCC). However, the precise molecular mechanism by which this takes place is currently unknown. Our data showed that infectious HIV-1 failed to replicate in human hepatocytic cell lines. No discernible virus replication was observed, even when the cell lines transfected with HIV-1 proviral DNA were co-cultured with Jurkat T cells, indicating that the problem of liver deterioration in the co-infected patient is not due to the replication of HIV-1 in the hepatocytes of the HCV infected host. Instead, HIV-1 Nef protein was transferred from nef-expressing T cells to hepatocytic cells through conduits, wherein up to 16% (average 10%) of the cells harbored the transferred Nef, when the hepatocytic cells were co-cultured with nef-expressing Jurkat cells for 24 h. Further, Nef altered the size and numbers of lipid droplets (LD), and consistently up-regulated HCV replication by 1.5∼2.5 fold in the target subgenomic replicon cells, which is remarkable in relation to the initially indolent viral replication. Nef also dramatically augmented reactive oxygen species (ROS) production and enhanced ethanol-mediated up-regulation of HCV replication so as to accelerate HCC. Taken together, these data indicate that HIV-1 Nef is a critical element in accelerating progression of liver pathogenesis via enhancing HCV replication and coordinating modulation of key intra- and extra-cellular molecules for liver decay.

HIV-1 vaginal transmission: cell-free or cell-associated virus?

The vast majority of new HIV infections in male-to-female transmission occurs through semen, where HIV-1 is present in two different forms: as free and as cell-associated virus. In the female lower genital tract, semen mixes with female genital secretions that contain various factors, some of which facilitate or inhibit HIV-1 transmission. Next, HIV-1 crosses the genital epithelia, reaches the regional lymph nodes, and disseminates through the female host. Cervico-vaginal mucosa contains multiple barriers, resulting in a low probability of vaginal transmission. However, in some cases, HIV-1 is able to break these barriers. Although the exact mechanisms of how these barriers function remain unclear, their levels of efficiency against cell-free and cell-associated HIV-1 are different, and both cell-free and cell-associated virions seem to use different strategies to overcome these barriers. Understanding the basic mechanisms of HIV-1 vaginal transmission is required for the development of new antiviral strategies to contain HIV-1 epidemics.

Rho'ing in and out of cells: viral interactions with Rho GTPase signaling

Rho GTPases are key regulators of actin and microtubule dynamics and organization. Increasing evidence shows that many viruses have evolved diverse interactions with Rho GTPase signaling and manipulate them for their own benefit. In this review, we discuss how Rho GTPase signaling interferes with many steps in the viral replication cycle, especially entry, replication, and spread. Seen the diversity between viruses, it is not surprising that there is considerable variability in viral interactions with Rho GTPase signaling. However, several largely common effects on Rho GTPases and actin architecture and microtubule dynamics have been reported. For some of these processes, the molecular signaling and biological consequences are well documented while for others we just begin to understand them. A better knowledge and identification of common threads in the different viral interactions with Rho GTPase signaling and their ultimate consequences for virus and host may pave the way toward the development of new antiviral drugs that may target different viruses.

Long-distance relationships: do membrane nanotubes regulate cell-cell communication and disease progression?

Metazoan cells rapidly exchange signals at tight cell-cell interfaces, including synapses and gap junctions. Advances in imaging recently exposed a third mode of intercellular cross-talk mediated by thin, actin-containing membrane extensions broadly known as "membrane" or "tunneling" nanotubes. An explosion of research suggests diverse functions for nanotubular superhighways, including cell-cell electrical coupling, calcium signaling, small-molecule exchange, and, remarkably, the transfer of bulky cargoes, including organelles or pathogenic agents. Despite great enthusiasm for all things nanotubular and their potential roles in cell signaling and pathogenesis, key questions remain regarding the mechanisms by which these structures regulate directional cell-cell exchange; how these linkages are formed and between which cells and, critically, whether nanotubes are as prevalent in vivo as they appear to be in the incubator.

Efficiency of cell-free and cell-associated virus in mucosal transmission of human immunodeficiency virus type 1 and simian immunodeficiency virus

Effective strategies are needed to block mucosal transmission of human immunodeficiency virus type 1 (HIV-1). Here, we address a crucial question in HIV-1 pathogenesis: whether infected donor mononuclear cells or cell-free virus plays the more important role in initiating mucosal infection by HIV-1. This distinction is critical, as effective strategies for blocking cell-free and cell-associated virus transmission may be different. We describe a novel ex vivo model system that utilizes sealed human colonic mucosa explants and demonstrate in both the ex vivo model and in vivo using the rectal challenge model in rhesus monkeys that HIV-1-infected lymphocytes can transmit infection across the mucosa more efficiently than cell-free virus. These findings may have significant implications for our understanding of the pathogenesis of mucosal transmission of HIV-1 and for the development of strategies to prevent HIV-1 transmission.

HIV-1 Nef promotes the localization of Gag to the cell membrane and facilitates viral cell-to-cell transfer

Background

Newly synthesized HIV-1 particles assemble at the plasma membrane of infected cells, before being released as free virions or being transferred through direct cell-to-cell contacts to neighboring cells. Localization of HIV-1 Gag precursor at the cell membrane is necessary and sufficient to trigger viral assembly, whereas the GagPol precursor is additionally required to generate a fully matured virion. HIV-1 Nef is an accessory protein that optimizes viral replication through partly defined mechanisms. Whether Nef modulates Gag and/or GagPol localization and assembly at the membrane and facilitates viral cell-to-cell transfer has not been extensively characterized so far.

Results

We report that Nef increases the total amount of Gag proteins present in infected cells, and promotes Gag localization at the cell membrane. Moreover, the processing of p55 into p24 is improved in the presence of Nef. We also examined the effect of Nef during HIV-1 cell-to-cell transfer. We show that without Nef, viral transfer through direct contacts between infected cells and target cells is impaired. With a nef-deleted virus, the number of HIV-1 positive target cells after a short 2h co-culture is reduced, and viral material transferred to uninfected cells is less matured. At later time points, this defect is associated with a reduction in the productive infection of new target cells.

Conclusions

Our results highlight a previously unappreciated role of Nef during the viral replication cycle. Nef promotes HIV-1 Gag membrane localization and processing, and facilitates viral cell-to-cell transfer.

Regulation of actin dynamics by protein kinase R control of gelsolin enforces basal innate immune defense

Primary resistance to pathogens is reliant on both basal and inducible immune defenses. To date, research has focused upon inducible innate immune responses. In contrast to resistance via cytokine induction, basal defense mechanisms are less evident. Here we showed that the antiviral protein kinase R (PKR) inhibited the key actin-modifying protein gelsolin to regulate actin dynamics and control cytoskeletal cellular functions under homeostatic conditions. Through this mechanism, PKR controlled fundamental innate immune, actin-dependent processes that included membrane ruffling and particle engulfment. Accordingly, PKR counteracted viral entry into the cell. These findings identify a layer of host resistance, showing that the regulation of actin-modifying proteins during the innate immune response bolsters first-line defense against intracellular pathogens and has a sustained effect on virus production. Moreover, these data provide proof of principle for a concept in which the cell cytoskeleton could be targeted to elicit broad antiviral protection.

Pathways of cell-cell transmission of HTLV-1

The deltaretroviruses human T cell lymphotropic virus type 1 (HTLV-1) and human T cell lymphotropic virus type 2 (HTLV-2) have long been believed to differ from retroviruses in other genera by their mode of transmission. While other retroviruses were thought to primarily spread by producing cell-free particles that diffuse through extracellular fluids prior to binding to and infecting target cells, HTLV-1 and HTLV-2 were believed to transmit the virus solely by cell–cell interactions. This difference in transmission was believed to reflect the fact that, relative to other retroviruses, the cell-free virions produced by HTLV-infected cells are very poorly infectious. Since HTLV-1 and HTLV-2 are primarily found in T cells in the peripheral blood, spread of these viruses was believed to occur between infected and uninfected, T cells, although little was known about the cellular and viral proteins involved in this interaction. Recent studies have revealed that the method of transmission of HTLV is not unique: other retroviruses including human immunodeficiency virus (HIV) are also transmitted from cell-to-cell, and this method is dramatically more efficient than cell-free transmission. Moreover, cell–cell transmission of HTLV-1, as well as HIV, can occur following interactions between dendritic cells and T cells, as well as between T cells. Conversely, other studies have shown that cell-free HTLV-1 is not as poorly infectious as previously thought, since it is capable of infecting certain cell types. Here we summarize the recent insights about the mechanisms of cell–cell transmission of HTLV-1 and other retroviruses. We also review in vitro and in vivo studies of infection and discuss how these finding may relate to the spread of HTLV-1 between individuals.