Critical role for the kinesin KIF3A in the HIV life cycle in primary human macrophages

Capsid-host interactions for HIV-1 ingress

The HIV-1 capsid, composed of approximately 1,200 copies of the capsid protein, encases genomic RNA alongside viral nucleocapsid, reverse transcriptase, and integrase proteins. After cell entry, the capsid interacts with a myriad of host factors to traverse the cell cytoplasm, pass through the nuclear pore complex (NPC), and then traffic to chromosomal sites for viral DNA integration. Integration may very well require the dissolution of the capsid, but where and when this uncoating event occurs remains hotly debated. Based on size constraints, a long-prevailing view was that uncoating preceded nuclear transport, but recent research has indicated that the capsid may remain largely intact during nuclear import, with perhaps some structural remodeling required for NPC traversal. Completion of reverse transcription in the nucleus may further aid capsid uncoating. One canonical type of host factor, typified by CPSF6, leverages a Phe-Gly (FG) motif to bind capsid. Recent research has shown these peptides reside amid prion-like domains (PrLDs), which are stretches of protein sequence devoid of charged residues. Intermolecular PrLD interactions along the exterior of the capsid shell impart avid host factor binding for productive HIV-1 infection. Herein we overview capsid-host interactions implicated in HIV-1 ingress and discuss important research questions moving forward. Highlighting clinical relevance, the long-acting ultrapotent inhibitor lenacapavir, which engages the same capsid binding pocket as FG host factors, was recently approved to treat people living with HIV.

The two-step cargo recognition mechanism of heterotrimeric kinesin

Kinesin-driven intracellular transport is essential for various cell biological events and thus plays a crucial role in many pathological processes. However, little is known about the molecular basis of the specific and dynamic cargo-binding mechanism of kinesins. Here, an integrated structural analysis of the KIF3/KAP3 and KIF3/KAP3-APC complexes unveils the mechanism by which KIF3/KAP3 can dynamically grasp APC in a two-step manner, which suggests kinesin-cargo recognition dynamics composed of cargo loading, locking, and release. Our finding is the first demonstration of the two-step cargo recognition and stabilization mechanism of kinesins, which provides novel insights into the intracellular trafficking machinery.

HIV-1 promotes ubiquitination of the amyloidogenic C-terminal fragment of APP to support viral replication

HIV-1 replication in macrophages and microglia involves intracellular assembly and budding into modified subsets of multivesicular bodies (MVBs), which support both viral persistence and spread. However, the cellular factors that regulate HIV-1's vesicular replication remain poorly understood. Recently, amyloid precursor protein (APP) was identified as an inhibitor of HIV-1 replication in macrophages and microglia via an unknown mechanism. Here, we show that entry of HIV-1 Gag into MVBs is blocked by the amyloidogenic C-terminal fragment of APP, "C99", but not by the non-amyloidogenic product, "C83". To counter this, Gag promotes multi-site ubiquitination of C99 which controls both exocytic sorting of MVBs and further processing of C99 into toxic amyloids. Processing of C99, entry of Gag into MVBs and release of infectious virus could be suppressed by expressing ubiquitination-defective C99 or by γ-secretase inhibitor treatment, suggesting that APP's amyloidogenic pathway functions to sense and suppress HIV-1 replication in macrophages and microglia.

Microtubules and viral infection

Microtubules (MTs) form rapidly adaptable, complex intracellular networks of filaments that not only provide structural support, but also form the tracks along which motors traffic macromolecular cargos to specific sub-cellular sites. These dynamic arrays play a central role in regulating various cellular processes including cell shape and motility as well as cell division and polarization. Given their complex organization and functional importance, MT arrays are carefully controlled by many highly specialized proteins that regulate the nucleation of MT filaments at distinct sites, their dynamic growth and stability, and their engagement with other subcellular structures and cargoes destined for transport. This review focuses on recent advances in our understanding of how MTs and their regulatory proteins function, including their active targeting and exploitation, during infection by viruses that utilize a wide variety of replication strategies that occur within different cellular sub-compartments or regions of the cell.

Getting on the right track: Interactions between viruses and the cytoskeletal motor proteins

The cytoskeleton is an essential component of the cell and it is involved in multiple physiological functions, including intracellular organization and transport. It is composed of three main families of proteinaceous filaments; microtubules, actin filaments and intermediate filaments and their accessory proteins. Motor proteins, which comprise the dynein, kinesin and myosin superfamilies, are a remarkable group of accessory proteins that mainly mediate the intracellular transport of cargoes along with the cytoskeleton. Like other cellular structures and pathways, viruses can exploit the cytoskeleton to promote different steps of their life cycle through associations with motor proteins. The complexity of the cytoskeleton and the differences among viruses, however, has led to a wide diversity of interactions, which in most cases remain poorly understood. Unveiling the details of these interactions is necessary not only for a better comprehension of specific infections, but may also reveal new potential drug targets to fight dreadful diseases such as rabies disease and acquired immunodeficiency syndrome (AIDS). In this review, we describe a few examples of the mechanisms that some human viruses, that is, rabies virus, adenovirus, herpes simplex virus, human immunodeficiency virus, influenza A virus and papillomavirus, have developed to hijack dyneins, kinesins and myosins.

KIF3A inhibits nasopharyngeal carcinoma proliferation, migration and invasion by interacting with β-catenin to suppress its nuclear accumulation

Nasopharyngeal carcinoma (NPC) is a malignant epithelial tumor prevalent in southern China and Southeast Asia. Previous studies have shown that Kinesin Family Member 3A (KIF3A) plays a critical role in the oncogenesis of various cancer types. However, the role of KIF3A in NPC tumorigenesis and the mechanism underlying its function have not been reported. In this study, we found that KIF3A was significantly downregulated in NPC cells and tissues, and KIF3A expression in NPC patients was associated with tumor stage and was positively corrected with overall survival. In vitro and in vivo experiments indicated that overexpression of KIF3A inhibited NPC cell proliferation, migration, and invasion. Mechanistic studies found that KIF3A bound β-catenin and attenuated β-catenin aggregation in the nucleus. Moreover, rescue experiments demonstrated that the inhibitory effect of KIF3A on NPC proliferation, migration and invasion was partially dependent on β-catenin. Taken together, our data suggest that KIF3A interacts with β-catenin and attenuates NPC proliferation, migration, and invasion by suppressing the intranuclear aggregation of β-catenin. KIF3A may be a promising therapeutic target of patients with NPC.

Post-translational modifications and stabilization of microtubules regulate transport of viral factors during infections

Tubulin post-translational modifications (PTMs) constitute a source of diversity for microtubule (MT) functions, in addition to the different isotypes of α and β-tubulin acting as building blocks of MTs. Also, MT-associated proteins (MAPs) confer different characteristics to MTs. The combination of all these factors regulates the stability of these structures that act as rails to transport organelles within the cell, facilitating the association of motor complexes. All these functions are involved in crucial cellular processes in most cell types, ranging from spindle formation in mitosis to the defense against incoming cellular threats during phagocytosis mediated by immune cells. The regulation of MT dynamics through tubulin PTMs has evolved to depend on many different factors that act in a complex orchestrated manner. These tightly regulated processes are particularly relevant during the induction of effective immune responses against pathogens. Viruses have proved not only to hijack MTs and MAPs in order to favor an efficient infection, but also to induce certain PTMs that improve their cellular spread and lead to secondary consequences of viral processes. In this review, we offer a perspective on relevant MT-related elements exploited by viruses.

Macrophage Cell-Cell Interactions Promoting HIV-1 Infection

Many pathogens infect macrophages as part of their intracellular life cycle. This is particularly true for viruses, of which HIV-1 is one of the best studied. HIV-1 infection of macrophages has important consequences for viral persistence and pathogenesis, but the mechanisms of macrophage infection remain to be fully elucidated. Despite expressing viral entry receptors, macrophages are inefficiently infected by cell-free HIV-1 virions, whereas direct cell-cell spread is more efficient. Different modes of cell-cell spread have been described, including the uptake by macrophages of infected T cells and the fusion of infected T cells with macrophages, both leading to macrophage infection. Cell-cell spread can also transmit HIV-1 between macrophages and from macrophages to T cells. Here, we describe the current state of the field concerning the cell-cell spread of HIV-1 to and from macrophages, discuss mechanisms, and highlight potential in vivo relevance.

Expression and potential functions of KIF3A/3B to promote nuclear reshaping and tail formation during Larimichthys polyactis spermiogenesis

KIF3A and KIF3B are homologous motor subunits of the Kinesin II protein family. KIF3A, KIF3B, and KAP3 form a heterotrimeric complex and play a significant role in spermatogenesis. Here, we first cloned full-length kif3a/3b cDNAs from Larimichthys polyactis. Lp-kif3a/3b are highly related to their homologs in other animals. The proteins are composed of three domains, an N-terminal head domain, a central stalk domain, and a C-terminus tail domain. Lp-kif3a/3b mRNAs were found to be ubiquitously expressed in the examined tissues, with high expression in the testis. Fluorescence in situ hybridization (FISH) was used to analyze the expression of Lp-kif3a/3b mRNAs during spermiogenesis. The results showed that Lp-kif3a/3b mRNAs had similar expression pattern and were continuously expressed during spermiogenesis. From middle spermatid to mature sperm, Lp-kif3a/3b mRNAs gradually localized to the side of the spermatid where the midpiece and tail form. In addition, we used immunofluorescence (IF) to observe that Lp-KIF3A protein co-localizes with tubulin during spermiogenesis. In early spermatid, Lp-KIF3A protein and microtubule signals were randomly distributed in the cytoplasm. In middle spermatid, however, the protein was detected primarily around the nucleus. In late spermatid, the protein migrated primarily to one side of the nucleus where the tail forms. In mature sperm, Lp-KIF3A and microtubules accumulated in the midpiece. Moreover, Lp-KIF3A co-localized with the mitochondria. In mature sperm, Lp-KIF3A and mitochondria were present in the midpiece. Therefore, Lp-KIF3A/KIF3B may be involved in spermiogenesis in L. polyactis, particularly during nuclear reshaping and tail formation.

Kinesin-2 heterodimerization alters entry into a processive run along the microtubule but not stepping within the run

Heterodimeric KIF3AC and KIF3AB, two members of the mammalian kinesin-2 family, generate force for microtubule plus end-directed cargo transport. However, the advantage of heterodimeric kinesins over homodimeric ones is not well-understood. We showed previously that microtubule association for entry into a processive run was similar in rate for KIF3AC and KIF3AB at ∼7.0 μm^-1 s^-1 Yet, for engineered homodimers of KIF3AA and KIF3BB, this constant is significantly faster at 11 μm^-1 s^-1 and much slower for KIF3CC at 2.1 μm^-1 s^-1 These results led us to hypothesize that heterodimerization of KIF3A with KIF3C and KIF3A with KIF3B altered the intrinsic catalytic properties of each motor domain. Here, we tested this hypothesis by using presteady-state stopped-flow kinetics and mathematical modeling. Surprisingly, the modeling revealed that the catalytic properties of KIF3A and KIF3B/C were altered upon microtubule binding, yet each motor domain retained its relative intrinsic kinetics for ADP release and subsequent ATP binding and the nucleotide-promoted transitions thereafter. These results are consistent with the interpretation that for KIF3AB, each motor head is catalytically similar and therefore each step is approximately equivalent. In contrast, for KIF3AC the results predict that the processive steps will alternate between a fast step for KIF3A followed by a slow step for KIF3C resulting in asymmetric stepping during a processive run. This study reveals the impact of heterodimerization of the motor polypeptides for microtubule association to start the processive run and the fundamental differences in the motile properties of KIF3C compared with KIF3A and KIF3B.

Myeloid Cell Interaction with HIV: A Complex Relationship

Cells of the myeloid lineage, particularly macrophages, serve as primary hosts for HIV in vivo, along with CD4 T lymphocytes. Macrophages are present in virtually every tissue of the organism, including locations with negligible T cell colonization, such as the brain, where HIV-mediated inflammation may lead to pathological sequelae. Moreover, infected macrophages are present in multiple other tissues. Recent evidence obtained in humanized mice and macaque models highlighted the capacity of macrophages to sustain HIV replication in vivo in the absence of T cells. Combined with the known resistance of the macrophage to the cytopathic effects of HIV infection, such data bring a renewed interest in this cell type both as a vehicle for viral spread as well as a viral reservoir. While our understanding of key processes of HIV infection of macrophages is far from complete, recent years have nevertheless brought important insight into the uniqueness of the macrophage infection. Productive infection of macrophages by HIV can occur by different routes including from phagocytosis of infected T cells. In macrophages, HIV assembles and buds into a peculiar plasma membrane-connected compartment that preexists to the infection. While the function of such compartment remains elusive, it supposedly allows for the persistence of infectious viral particles over extended periods of time and may play a role on viral transmission. As cells of the innate immune system, macrophages have the capacity to detect and respond to viral components. Recent data suggest that such sensing may occur at multiple steps of the viral cycle and impact subsequent viral spread. We aim to provide an overview of the HIV-macrophage interaction along the multiple stages of the viral life cycle, extending when pertinent such observations to additional myeloid cell types such as dendritic cells or blood monocytes.

Mathematical modeling and quantitative analysis of HIV-1 Gag trafficking and polymerization

Gag, as the major structural protein of HIV-1, is necessary for the assembly of the HIV-1 sphere shell. An in-depth understanding of its trafficking and polymerization is important for gaining further insights into the mechanisms of HIV-1 replication and the design of antiviral drugs. We developed a mathematical model to simulate two biophysical processes, specifically Gag monomer and dimer transport in the cytoplasm and the polymerization of monomers to form a hexamer underneath the plasma membrane. Using experimental data, an optimization approach was utilized to identify the model parameters, and the identifiability and sensitivity of these parameters were then analyzed. Using our model, we analyzed the weight of the pathways involved in the polymerization reactions and concluded that the predominant pathways for the formation of a hexamer might be the polymerization of two monomers to form a dimer, the polymerization of a dimer and a monomer to form a trimer, and the polymerization of two trimers to form a hexamer. We then deduced that the dimer and trimer intermediates might be crucial in hexamer formation. We also explored four theoretical combined methods for Gag suppression, and hypothesized that the N-terminal glycine residue of the MA domain of Gag might be a promising drug target. This work serves as a guide for future theoretical and experimental efforts aiming to understand HIV-1 Gag trafficking and polymerization, and might help accelerate the efficiency of anti-AIDS drug design.

The human immunodeficiency virus (HIV-1) is a retrovirus that causes acquired immunodeficiency syndrome (AIDS), an infectious disease with high annual mortality. Gag protein is the major structural protein of HIV-1 and can self-assemble into the HIV-1 sphere shell. Therefore, an in-depth understanding of Gag protein trafficking and polymerization is important for gaining further insights into the mechanisms of HIV-1 replication and the design of antiviral drugs. Through mathematical modeling, optimization and quantitative analysis, we hypothesized the budding and release time of virus-like particles and revealed that the dimer and trimer intermediates might be crucial in hexamer formation. We also concluded that the predominant pathways in hexamer formation might be the polymerization of two monomers to form a dimer, the polymerization of a dimer and a monomer to form a trimer, and the polymerization of two trimers to form a hexamer. Our analysis also suggested that the N-terminal glycine residue of the MA domain of Gag might be a promising drug target. These results serve as a guide for further theoretical and experimental efforts aiming to understand HIV-1 Gag trafficking and polymerization and could aid anti-AIDS drug design.

Microtubule Regulation and Function during Virus Infection

Microtubules (MTs) form a rapidly adaptable network of filaments that radiate throughout the cell. These dynamic arrays facilitate a wide range of cellular processes, including the capture, transport, and spatial organization of cargos and organelles, as well as changes in cell shape, polarity, and motility. Nucleating from MT-organizing centers, including but by no means limited to the centrosome, MTs undergo rapid transitions through phases of growth, pause, and catastrophe, continuously exploring and adapting to the intracellular environment. Subsets of MTs can become stabilized in response to environmental cues, acquiring distinguishing posttranslational modifications and performing discrete functions as specialized tracks for cargo trafficking. The dynamic behavior and organization of the MT array is regulated by MT-associated proteins (MAPs), which include a subset of highly specialized plus-end-tracking proteins (+TIPs) that respond to signaling cues to alter MT behavior. As pathogenic cargos, viruses require MTs to transport to and from their intracellular sites of replication. While interactions with and functions for MT motor proteins are well characterized and extensively reviewed for many viruses, this review focuses on MT filaments themselves. Changes in the spatial organization and dynamics of the MT array, mediated by virus- or host-induced changes to MT regulatory proteins, not only play a central role in the intracellular transport of virus particles but also regulate a wider range of processes critical to the outcome of infection.

Retroviruses and microtubule-associated motor proteins

Retroviruses are obligate intracellular parasites of eukaryotic cells. After reverse transcription, the viral DNA contained in the preintegration complex is delivered to the nucleus of the host cell, where it integrates. Before reaching the nucleus, the incoming particle and the preintegration complex must travel throughout the cytoplasm. Likewise, the newly synthesized viral proteins and viral particles must transit the cytoplasm during exit. The cytoplasm is a crowded environment, and simple diffusion is difficult. Therefore, viruses have evolved to utilize the cellular mechanisms of movement through the cytoplasm, where microtubules are the roads, and the ATP-dependent motors dynein and kinesin are the vehicles for retrograde and anterograde trafficking. This review will focus on how different retroviruses (Mazon-Pfizer monkey virus, prototype foamy virus, bovine immunodeficiency virus, human immunodeficiency virus type 1, and murine leukemia virus) have subjugated the microtubule-associated motor proteins for viral replication. Although there have been advances in our understanding of how retroviruses move along microtubules, the strategies are different among them. Thus, a better understanding of the mechanisms used by each retrovirus to functionally subvert microtubule motor proteins will provide important clues in the design of new antiretroviral drugs that can specifically disrupt intracellular viral trafficking.

Antigen Export Reduces Antigen Presentation and Limits T Cell Control of M. tuberculosis

Persistence of Mycobacterium tuberculosis results from bacterial strategies that manipulate host adaptive immune responses. Infected dendritic cells (DCs) transport M. tuberculosis to local lymph nodes but activate CD4 T cells poorly, suggesting bacterial manipulation of antigen presentation. However, M. tuberculosis antigens are also exported from infected DCs and taken up and presented by uninfected DCs, possibly overcoming this blockade of antigen presentation by infected cells. Here we show that the first stage of this antigen transfer, antigen export, benefits M. tuberculosis by diverting bacterial proteins from the antigen presentation pathway. Kinesin-2 is required for antigen export and depletion of this microtubule-based motor increases activation of antigen-specific CD4 T cells by infected cells and improves control of intracellular infection. Thus, although antigen transfer enables presentation by bystander cells, it does not compensate for reduced antigen presentation by infected cells and represents a bacterial strategy for CD4 T cell evasion.

Molecular Determinants Directing HIV-1 Gag Assembly to Virus-Containing Compartments in Primary Macrophages

The subcellular sites of HIV-1 assembly, determined by the localization of the structural protein Gag, vary in a cell-type-dependent manner. In T cells and transformed cell lines used as model systems, HIV-1 assembles at the plasma membrane (PM). The binding and localization of HIV-1 Gag to the PM are mediated by the interaction between the matrix (MA) domain, specifically the highly basic region, and a PM-specific acidic phospholipid, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2]. In primary macrophages, prominent accumulation of assembling or assembled particles is found in the virus-containing compartments (VCCs), which largely consist of convoluted invaginations of the PM. To elucidate the molecular mechanism of HIV-1 Gag targeting to the VCCs, we examined the impact of overexpression of polyphosphoinositide 5-phosphatase IV (5ptaseIV), which depletes cellular PI(4,5)P2, in primary macrophages. We found that the VCC localization and virus release of HIV-1 are severely impaired upon 5ptaseIV overexpression, suggesting an important role for the MA-PI(4,5)P2 interaction in HIV-1 assembly in primary macrophages. However, our analysis of HIV-1 Gag derivatives with MA changes showed that this interaction contributes to Gag membrane binding but is dispensable for specific targeting of Gag to the VCCs per se We further determined that deletion of the NC domain abolishes VCC-specific localization of HIV-1 Gag. Notably, HIV-1 Gag localized efficiently to the VCCs when the NC domain was replaced with a leucine zipper dimerization motif that promotes Gag multimerization. Altogether, our data revealed that targeting of HIV-1 Gag to the VCCs requires NC-dependent multimerization.

IMPORTANCE

In T cells and model cell lines, HIV-1 Gag localizes to the PM in a manner dependent on the MA-PI(4,5)P2 interaction. On the other hand, in primary macrophages, HIV-1 Gag localizes to convoluted intracellular membrane structures termed virus-containing compartments (VCCs). Although these compartments have been known for decades, and despite the implication of viruses in VCCs being involved in virus reservoir maintenance and spread, the viral determinant(s) that promotes Gag targeting to VCCs is unknown. In this study, we found that the MA-PI(4,5)P2 interaction facilitates efficient Gag membrane binding in macrophages but is not essential for Gag targeting to VCCs. Rather, our results revealed that NC-dependent multimerization promotes VCC targeting. Our findings highlight the differential roles played by MA and NC in HIV-1 Gag membrane binding and targeting and suggest a multimerization-dependent mechanism for Gag trafficking in primary macrophages similar to that for Gag localization to uropods in polarized T cells.

Human Immunodeficiency Virus Type 2 (HIV-2) Gag Is Trafficked in an AP-3 and AP-5 Dependent Manner

Although human immunodeficiency virus (HIV) types 1 and 2 are closely related lentiviruses with similar replication cycles, HIV-2 infection is associated with slower progression to AIDS, a higher proportion of long term non-progressors, and lower rates of transmission than HIV-1, likely as a consequence of a lower viral load during HIV-2 infection. A mechanistic explanation for the differential viral load remains unclear but knowledge of differences in particle production between HIV-1 and HIV-2 may help to shed light on this issue. In contrast to HIV-1, little is known about the assembly of HIV-2 particles, and the trafficking of HIV-2 Gag, the structural component of the virus, within cells. We have established that HIV-2 Gag accumulates in intracellular CD63 positive compartments, from which it may be delivered or recycled to the cell surface, or degraded. HIV-2 particle release was dependent on the adaptor protein complex AP-3 and the newly identified AP-5 complex, but much less so on AP-1. In contrast, HIV-1 particle release required AP-1 and AP-3, but not AP-5. AP-2, an essential component of clathrin-mediated endocytosis, which was previously shown to be inhibitory to HIV-1 particle release, had no effect on HIV-2. The differential requirement for adaptor protein complexes confirmed that HIV-1 and HIV-2 Gag have distinct cellular trafficking pathways, and that HIV-2 particles may be more susceptible to degradation prior to release.

siRNA Screen Identifies Trafficking Host Factors that Modulate Alphavirus Infection

Little is known about the repertoire of cellular factors involved in the replication of pathogenic alphaviruses. To uncover molecular regulators of alphavirus infection, and to identify candidate drug targets, we performed a high-content imaging-based siRNA screen. We revealed an actin-remodeling pathway involving Rac1, PIP5K1- α, and Arp3, as essential for infection by pathogenic alphaviruses. Infection causes cellular actin rearrangements into large bundles of actin filaments termed actin foci. Actin foci are generated late in infection concomitantly with alphavirus envelope (E2) expression and are dependent on the activities of Rac1 and Arp3. E2 associates with actin in alphavirus-infected cells and co-localizes with Rac1-PIP5K1-α along actin filaments in the context of actin foci. Finally, Rac1, Arp3, and actin polymerization inhibitors interfere with E2 trafficking from the trans-Golgi network to the cell surface, suggesting a plausible model in which transport of E2 to the cell surface is mediated via Rac1- and Arp3-dependent actin remodeling.

Potent in vitro antiviral activity of Cistus incanus extract against HIV and Filoviruses targets viral envelope proteins

Novel therapeutic options are urgently needed to improve global treatment of virus infections. Herbal products with confirmed clinical safety features are attractive starting material for the identification of new antiviral activities. Here we demonstrate that Cistus incanus (Ci) herbal products inhibit human immunodeficiency virus (HIV) infections in vitro. Ci extract inhibited clinical HIV-1 and HIV-2 isolates, and, importantly, a virus isolate with multiple drug resistances, confirming broad anti-HIV activity. Antiviral activity was highly selective for virus particles, preventing primary attachment of the virus to the cell surface and viral envelope proteins from binding to heparin. Bioassay-guided fractionation indicated that Ci extract contains numerous antiviral compounds and therefore has favorably low propensity to induce virus resistance. Indeed, no resistant viruses emerged during 24 weeks of continuous propagation of the virus in the presence of Ci extracts. Finally, Ci extracts also inhibited infection by virus particles pseudotyped with Ebola and Marburg virus envelope proteins, indicating that antiviral activity of Ci extract extends to emerging viral pathogens. These results demonstrate that Ci extracts show potent and broad in vitro antiviral activity against viruses that cause life-threatening diseases in humans and are promising sources of agents that target virus particles.

Extracellular ATP induces the rapid release of HIV-1 from virus containing compartments of human macrophages

HIV type 1 (HIV-1) infects CD4(+) T lymphocytes and tissue macrophages. Infected macrophages differ from T cells in terms of decreased to absent cytopathicity and for active accumulation of new progeny HIV-1 virions in virus-containing compartments (VCC). For these reasons, infected macrophages are believed to act as "Trojan horses" carrying infectious particles to be released on cell necrosis or functional stimulation. Here we explored the hypothesis that extracellular ATP (eATP) could represent a microenvironmental signal potentially affecting virion release from VCC of infected macrophages. Indeed, eATP triggered the rapid release of infectious HIV-1 from primary human monocyte-derived macrophages (MDM) acutely infected with the CCR5-dependent HIV-1 strain. A similar phenomenon was observed in chronically infected promonocytic U1 cells differentiated to macrophage-like cells (D-U1) by costimulation with phorbol esters and urokinase-type plasminogen activator. Worthy of note, eATP did not cause necrotic, apoptotic, or pyroptotic cell death, and its effect on HIV-1 release was suppressed by Imipramine (an antidepressant agent known to inhibit microvesicle formation by interfering with membrane-associated acid sphingomyelinase). Virion release was not triggered by oxidized ATP, whereas the effect of eATP was inhibited by a specific inhibitor of the P2X7 receptor (P2X7R). Thus, eATP triggered the discharge of virions actively accumulating in VCC of infected macrophages via interaction with the P2X7R in the absence of significant cytopathicity. These findings suggest that the microvesicle pathway and P2X7R could represent exploitable targets for interfering with the VCC-associated reservoir of infectious HIV-1 virions in tissue macrophages.

HIV-1 capsids bind and exploit the kinesin-1 adaptor FEZ1 for inward movement to the nucleus

Intracellular transport of cargos, including many viruses, involves directed movement on microtubules mediated by motor proteins. Although a number of viruses bind motors of opposing directionality, how they associate with and control these motors to accomplish directed movement remains poorly understood. Here we show that human immunodeficiency virus type 1 (HIV-1) associates with the kinesin-1 adaptor protein, Fasiculation and Elongation Factor zeta 1 (FEZ1). RNAi-mediated FEZ1 depletion blocks early infection, with virus particles exhibiting bi-directional motility but no net movement to the nucleus. Furthermore, both dynein and kinesin-1 motors are required for HIV-1 trafficking to the nucleus. Finally, the ability of exogenously expressed FEZ1 to promote early HIV-1 infection requires binding to kinesin-1. Our findings demonstrate that opposing motors both contribute to early HIV-1 movement and identify the kinesin-1 adaptor, FEZ1 as a capsid-associated host regulator of this process usurped by HIV-1 to accomplish net inward movement towards the nucleus.

Sorting of small infectious virus particles by flow virometry reveals distinct infectivity profiles

The nature and concentration of lipids and proteins at the surface of viruses are essential parameters for determining particle infectiveness. Historically, averaged bulk analysis of viral particles has been the primary method to quantitatively investigate these parameters, though this neglects heterogeneity within populations. Here we analyze the properties of Junin virus particles using a sensitive flow virometry assay and further sort virions, while conserving their infectivness. This method allows us to characterize the relationship between infectivity, virus size, and RNA content and to compare particles secreted by Vero cells with those from physiologically relevant human primary macrophages. Our study highlights significant differences in particle infectivity according to its nature, the type of producer cells and the lipid membrane composition at the budding site. Together, our results present the flow virometry assay as a powerful and versatile tool to define virus particle profiles.

HIV-1 replication in human immune cells is independent of TAR DNA binding protein 43 (TDP-43) expression

The TAR DNA binding protein (TDP-43) was originally identified as a host cell factor binding to the HIV-1 LTR and thereby suppressing HIV-1 transcription and gene expression (Ou et al., J.Virol. 1995, 69(6):3584). TDP-43 is a global regulator of transcription, can influence RNA metabolism in many different ways and is ubiquitously expressed. Thus, TDP-43 could be a major factor restricting HIV-1 replication at the level of LTR transcription and gene expression. These facts prompted us to revisit the role of TDP-43 for HIV-1 replication. We utilized established HIV-1 cell culture systems as well as primary cell models and performed a comprehensive analysis of TDP-43 function and investigated its putative impact on HIV-1 gene expression. In HIV-1 infected cells TDP-43 was neither degraded nor sequestered from the nucleus. Furthermore, TDP-43 overexpression as well as siRNA mediated knockdown did not affect HIV-1 gene expression and virus production in T cells and macrophages. In summary, our experiments argue against a restricting role of TDP-43 during HIV-1 replication in immune cells.

C11ORF24 is a novel type I membrane protein that cycles between the Golgi apparatus and the plasma membrane in Rab6-positive vesicles

The Golgi apparatus is an intracellular compartment necessary for post-translational modification, sorting and transport of proteins. It plays a key role in mitotic entry through the Golgi mitotic checkpoint. In order to identify new proteins involved in the Golgi mitotic checkpoint, we combine the results of a knockdown screen for mitotic phenotypes and a localization screen. Using this approach, we identify a new Golgi protein C11ORF24 (NP_071733.1). We show that C11ORF24 has a signal peptide at the N-terminus and a transmembrane domain in the C-terminal region. C11ORF24 is localized on the Golgi apparatus and on the trans-Golgi network. A large part of the protein is present in the lumen of the Golgi apparatus whereas only a short tail extends into the cytosol. This cytosolic tail is well conserved in evolution. By FRAP experiments we show that the dynamics of C11ORF24 in the Golgi membrane are coherent with the presence of a transmembrane domain in the protein. C11ORF24 is not only present on the Golgi apparatus but also cycles to the plasma membrane via endosomes in a pH sensitive manner. Moreover, via video-microscopy studies we show that C11ORF24 is found on transport intermediates and is colocalized with the small GTPase RAB6, a GTPase involved in anterograde transport from the Golgi to the plasma membrane. Knocking down C11ORF24 does not lead to a mitotic phenotype or an intracellular transport defect in our hands. All together, these data suggest that C11ORF24 is present on the Golgi apparatus, transported to the plasma membrane and cycles back through the endosomes by way of RAB6 positive carriers.

CD36-specific antibodies block release of HIV-1 from infected primary macrophages and its transmission to T cells

HIV-1-infected macrophages likely represent viral reservoirs, as they accumulate newly formed virions in internal virus-containing compartments (VCCs). However, the nature and biogenesis of VCCs remain poorly defined. We show that upon HIV-1 infection of primary human macrophages, Gag is recruited to preexisting compartments containing the scavenger receptor CD36, which then become VCCs. Silencing of CD36 in HIV-1-infected macrophages decreases the amount of virions released. Strikingly, soluble anti-CD36 antibodies, but not the natural ligands of CD36, inhibit release of virions from HIV-1-infected macrophages and the transmission of virus to CD4(+) T cells. The effect of the antibodies is potent, rapid, and induces the retention of virions within VCCs. Ectopic expression of CD36 in HeLa cells renders them susceptible to the inhibitory effect of the anti-CD36 mAb upon HIV-1 infection. We show that the anti-CD36 mAb inhibits HIV-1 release by clustering newly formed virions at their site of budding, and that signaling via CD36 is not required. Thus, HIV-1 reservoirs in macrophages may be tackled therapeutically using anti-CD36 antibodies to prevent viral dissemination.

HIV trafficking in host cells: motors wanted!

Throughout the viral replication cycle, viral proteins, complexes, and particles need to be transported within host cells. These transport events are dependent on the host cell cytoskeleton and molecular motors. However, the mechanisms by which virus is trafficked along cytoskeleton filaments and how molecular motors are recruited and regulated to guarantee successful integration of the viral genome and production of new viruses has only recently begun to be understood. Recent studies on HIV have identified specific molecular motors involved in the trafficking of these viral particles. Here we review recent literature on the transport of HIV components in the cell, provide evidence for the identity and role of molecular motors in this process, and highlight how these trafficking events may be related to those occurring with other viruses.

Organization and regulation of intracellular plasma membrane-connected HIV-1 assembly compartments in macrophages

BACKGROUND

In HIV-1-infected human monocyte-derived macrophages (MDMs), virus particles assemble primarily on intracellularly sequestered plasma membrane domains termed intracellular plasma membrane-connected compartments (IPMCs). Despite their clear role in virus formation, little is known of the organization, composition, dynamics or function of these compartments.

RESULTS

We have used amphipathic membrane dyes to reveal the complex three-dimensional structure of IPMCs in whole MDMs and to visualize connections between IPMCs and the cell surface. The observation of similar IPMC structures in both infected and uninfected cells indicates that these compartments are not induced by virus infection, but are present constitutively in MDMs. By expressing a phospholipase Cδ pleckstrin homology domain linked to green fluorescent protein, we demonstrate that IPMCs contain phosphatidylinositol 4,5-bisphosphate. Live cell imaging of cells expressing this probe shows that IPMCs are dynamic, but relatively stable, sub-domains of the plasma membrane. As recent electron microscopy studies indicated that portions of IPMCs are coated with β2 integrin-containing focal adhesion-like complexes linked to actin, we investigated whether the actin cytoskeleton is required for the organization of IPMCs. In MDMs treated with the actin polymerization inhibitor latrunculin, the normally compact IPMCs dispersed into smaller structures that remained connected to the plasma membrane. Moreover, latrunculin enhanced the release of preformed, mature HIV-1 particles from infected MDMs.

CONCLUSIONS

IPMCs are constitutive features of MDMs that are continuous with the plasma membrane and are used as unique sites for the assembly of new virions following infection by HIV-1. A functionally intact actin cytoskeleton is required to maintain the organization of the IPMCs and, in HIV-1-infected cells, perturbation of the actin cytoskeleton influences both the organization of the compartment and the release of sequestered virus.

Dynamics of HIV-containing compartments in macrophages reveal sequestration of virions and transient surface connections

During HIV pathogenesis, infected macrophages behave as “viral reservoirs” that accumulate and retain virions within dedicated internal Virus-Containing Compartments (VCCs). The nature of VCCs remains ill characterized and controversial. Using wild-type HIV-1 and a replication-competent HIV-1 carrying GFP internal to the Gag precursor, we analyzed the biogenesis and evolution of VCCs in primary human macrophages. VCCs appear roughly 14 hours after viral protein synthesis is detected, initially contain few motile viral particles, and then mature to fill up with virions that become packed and immobile. The amount of intracellular Gag, the proportion of dense VCCs, and the density of viral particles in their lumen increased with time post-infection. In contrast, the secretion of virions, their infectivity and their transmission to T cells decreased overtime, suggesting that HIV-infected macrophages tend to pack and retain newly formed virions into dense compartments. A minor proportion of VCCs remains connected to the plasma membrane overtime. Surprisingly, live cell imaging combined with correlative light and electron microscopy revealed that such connections can be transient, highlighting their dynamic nature. Together, our results shed light on the late phases of the HIV-1 cycle and reveal some of its macrophage specific features.

Dual mechanisms of translation initiation of the full-length HIV-1 mRNA contribute to gag synthesis

The precursor group-specific antigen (pr55^Gag) is central to HIV-1 assembly. Its expression alone is sufficient to assemble into virus-like particles. It also selects the genomic RNA for encapsidation and is involved in several important virus-host interactions for viral assembly and restriction, making its synthesis essential for aspects of viral replication. Here, we show that the initiation of translation of the HIV-1 genomic RNA is mediated through both a cap-dependent and an internal ribosome entry site (IRES)-mediated mechanisms. In support of this notion, pr55^Gag synthesis was maintained at 70% when cap-dependent translation initiation was blocked by the expression of eIF4G- and PABP targeting viral proteases in two in vitro systems and in HIV-1-expressing cells directly infected with poliovirus. While our data reveal that IRES-dependent translation of the viral genomic RNA ensures pr55^Gag expression, the synthesis of other HIV-1 proteins, including that of pr160^Gag/Pol, Vpr and Tat is suppressed early during progressive poliovirus infection. The data presented herein implies that the unspliced HIV-1 genomic RNA utilizes both cap-dependent and IRES-dependent translation initiation to supply pr55^Gag for virus assembly and production.

Kinesin-2: a family of heterotrimeric and homodimeric motors with diverse intracellular transport functions

Kinesin-2 was first purified as a heterotrimeric, anterograde, microtubule-based motor consisting of two distinct kinesin-related subunits and a novel associated protein (KAP) that is currently best known for its role in intraflagellar transport and ciliogenesis. Subsequent work, however, has revealed diversity in the oligomeric state of different kinesin-2 motors owing to the combinatorial heterodimerization of its subunits and the coexistence of both heterotrimeric and homodimeric kinesin-2 motors in some cells. Although the functional significance of the homo- versus heteromeric organization of kinesin-2 motor subunits and the role of KAP remain uncertain, functional studies suggest that cooperation between different types of kinesin-2 motors or between kinesin-2 and a member of a different motor family can generate diverse patterns of anterograde intracellular transport. Moreover, despite being restricted to ciliated eukaryotes, kinesin-2 motors are now known to drive diverse transport events outside cilia. Here, I review the organization, assembly, phylogeny, biological functions, and motility mechanism of this diverse family of intracellular transport motors.

The HIV-1-containing macrophage compartment: a perfect cellular niche?

Macrophages are a major target of HIV-1 infection and are believed to act as viral reservoirs and mediators of HIV-1-associated neurological damage. These pathological roles may be associated with the ability of the virus to assemble and accumulate in apparently intracellular compartments in macrophages. These so-called virus-containing compartments were initially thought to be late endosomes or multivesicular bodies, but it has since been shown that they are distinct structures that have complex three-dimensional morphology, a unique set of protein markers, and features such as a near-neutral pH and frequent connections to the extracellular milieu. These features appear to protect HIV-1 from hostile elements both within and outside the cell. This review discusses the cellular and molecular characteristics of HIV-1-containing compartments in macrophages and how they offer a safe haven for the virus, with important consequences for pathogenesis.

Clustering and mobility of HIV-1 Env at viral assembly sites predict its propensity to induce cell-cell fusion

HIV-1 Env mediates virus attachment to and fusion with target cell membranes, and yet, while Env is still situated at the plasma membrane of the producer cell and before its incorporation into newly formed particles, Env already interacts with the viral receptor CD4 on target cells, thus enabling the formation of transient cell contacts that facilitate the transmission of viral particles. During this first encounter with the receptor, Env must not induce membrane fusion, as this would prevent the producer cell and the target cell from separating upon virus transmission, but how Env's fusion activity is controlled remains unclear. To gain a better understanding of the Env regulation that precedes viral transmission, we examined the nanoscale organization of Env at the surface of producer cells. Utilizing superresolution microscopy (stochastic optical reconstruction microscopy [STORM]) and fluorescence recovery after photobleaching (FRAP), we quantitatively assessed the clustering and dynamics of Env upon its arrival at the plasma membrane. We found that Gag assembly induced the aggregation of small Env clusters into larger domains and that these domains were completely immobile. Truncation of the cytoplasmic tail (CT) of Env abrogated Gag's ability to induce Env clustering and restored Env mobility at assembly sites, both of which correlated with increased Env-induced fusion of infected and uninfected cells. Hence, while Env trapping by Gag secures Env incorporation into viral particles, Env clustering and its sequestration at assembly sites likely also leads to the repression of its fusion function, and thus, by preventing the formation of syncytia, Gag helps to secure efficient transfer of viral particles to target cells.

Viral interactions with microtubules: orchestrators of host cell biology?

Viral interaction with the microtubule (MT) cytoskeleton is critical to infection by many viruses. Most data regarding virus–MT interaction indicate key roles in the subcellular transport of virions/viral genomic material to sites of replication, assembly and egress. However, the MT cytoskeleton orchestrates diverse processes in addition to subcellular cargo transport, including regulation of signaling pathways, cell survival and mitosis, suggesting that viruses, expert manipulators of the host cell, may use the virus–MT interface to control multiple aspects of cell biology. Several lines of evidence support this idea, indicating that specific viral proteins can modify MT dynamics and/or structure and regulate processes such as apoptosis and innate immune signaling through MT-dependent mechanisms. Here, the authors review general aspects of virus–MT interactions, with emphasis on viral mechanisms that modify MT dynamics and functions to affect processes beyond virion transport. The emerging importance of discrete viral protein–MT interactions in pathogenic processes indicates that these interfaces may represent new targets for future therapeutics and vaccine development.