Human immunodeficiency virus type 1 Gag contains a dileucine-like motif that regulates association with multivesicular bodies

Phosphatidylinositol-3-phosphate mediates Arc capsid secretion through the multivesicular body pathway

Activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) is an immediate early gene that plays a vital role in learning and memory. Arc protein has structural and functional properties similar to viral Group-specific antigen (Gag) protein and mediates the intercellular RNA transfer through virus-like capsids. However, the regulators and secretion pathway through which Arc capsids maneuver cargos are unclear. Here, we identified that phosphatidylinositol-3-phosphate (PI3P) mediates Arc capsid assembly and secretion through the endosomal-multivesicular body (MVB) pathway. Indeed, reconstituted Arc protein preferably binds to PI3P. In HEK293T cells, Arc forms puncta that colocalize with FYVE, an endosomal PI3P marker, as well as Rab5 and CD63, early endosomal and MVB markers, respectively. Superresolution imaging resolves Arc accumulates within the intraluminal vesicles of MVB. CRISPR double knockout of RalA and RalB, crucial GTPases for MVB biogenesis and exocytosis, severely reduces the Arc-mediated RNA transfer efficiency. RalA/B double knockdown in cultured rat cortical neurons increases the percentage of mature dendritic spines. Intake of extracellular vesicles purified from Arc-expressing wild-type, but not RalA/B double knockdown, cells in mouse cortical neurons reduces their surface GlutA1 levels. These results suggest that unlike the HIV Gag, whose membrane targeting requires interaction with plasma-membrane-specific phosphatidyl inositol (4,5) bisphosphate (PI(4,5)P2), the assembly of Arc capsids is mediated by PI3P at endocytic membranes. Understanding Arc's secretion pathway helps gain insights into its role in intercellular cargo transfer and highlights the commonality and distinction of trafficking mechanisms between structurally resembled capsid proteins.

Surface functionalization of virus-like particles via bioorthogonal click reactions for enhanced cell-specific targeting

Surface functionalization of nano drug carriers allows for precise delivery of therapeutic molecules to the target site. This technique involves attaching targeting molecules to the nanoparticle surface, facilitating selective interaction. In this study, we engineered virus-like particles (VLPs) to enhance their targeting capabilities. Azide groups incorporated on the lipid membranes of VLPs enabled bioorthogonal click reactions for conjugation with cycloalkyne-bearing molecules, providing efficient conjugation with high specificity. HIV-1 Gag VLPs were chosen due to their envelope, which allows host membrane component incorporation, and the Gag protein, which serves as a recognition motif for human T cells. This combination, along with antibody-mediated targeting, addresses the limitations of intracellular delivery to T cells, which typically exhibit low uptake of exogenous materials. The selective uptake of azide VLPs by CD3-positive T cells was evaluated in a co-culture system. Even without antibody conjugation, VLP uptake was enhanced in T cells, indicating their intrinsic targeting potential. Antibody conjugation further amplified this effect, demonstrating the synergistic benefits of the combined targeting approach. Our study shows that recombinant production of azide functionalized VLPs results in engineered nanoparticles that can be easily modified using bioorthogonal click reactions, providing high specificity and versatility for conjugation with various molecules, making it applicable to a wide range of biological products.

Phosphatidylinositol 3-phosphate mediates Arc capsids secretion through the multivesicular body pathway

Activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) is an immediate early gene that plays a vital role in learning and memory. The recent discovery that Arc mediates the inter-neuronal RNA transfer implies its role in regulating neuronal functions across long distances. Arc protein has structural and functional properties similar to viral Group-specific antigen (Gag). By assembling into high-order, virus-like capsids, Arc mediates the intercellular RNA transfer. However, the exact secretion pathway through which Arc capsids maneuver cargos is unclear. Here, we identified that Arc capsids assemble and secrete through the endosomal-multivesicular body (MVB) pathway. Arc's endosomal entry is likely mediated by phosphatidylinositol-3-phosphate (PI3P). Indeed, reconstituted Arc protein preferably binds to PI3P. In mammalian cells, Arc forms puncta that colocalizes with FYVE, an endosomal PI3P marker, and competitive binding to PI3P via prolonged FYVE expression reduces the average number of Arc puncta per cell. Overexpression of MTMR1, a PI3P phosphatase, significantly reduces Arc capsid secretion. Arc capsids secrete through the endosomal-MVB axis as extracellular vesicles. Live-cell imaging shows that fluorescently labeled Arc primarily colocalizes Rab5 and CD63, early endosomal and MVB markers, respectively. Superresolution imaging resolves Arc accumulates within the intraluminal vesicles of MVB. CRISPR double knockout of RalA and RalB, crucial GTPases for MVB biogenesis and exocytosis, severely reduces Arc-mediated RNA transfer efficiency. These results suggest that, unlike the Human Immunodeficiency Virus Gag, which assembles on and bud off from the plasma membrane, Arc capsids assemble at the endocytic membranes of the endosomal-MVB pathway mediated by PI3P. Understanding Arc's secretion pathway helps gain insights into its role in intercellular cargo transfer and highlights the commonality and distinction of trafficking mechanisms between structurally resembled capsid proteins.

Potential use of the cholesterol transfer inhibitor U18666A as an antiviral drug for research on various viral infections

Cholesterol plays critical functions in arranging the biophysical attributes of proteins and lipids in the plasma membrane. For various viruses, an association with cholesterol for virus entrance and/or morphogenesis has been demonstrated. Therefore, the lipid metabolic pathways and the combination of membranes could be targeted to selectively suppress the virus replication steps as a basis for antiviral treatment. U18666A is a cationic amphiphilic drug (CAD) that affects intracellular transport and cholesterol production. A robust tool for investigating lysosomal cholesterol transfer and Ebola virus infection is an androstenolone derived termed U18666A that suppresses three enzymes in the cholesterol biosynthesis mechanism. In addition, U18666A inhibited low-density lipoprotein (LDL)-induced downregulation of LDL receptor and triggered lysosomal aggregation of cholesterol. According to reports, U18666A inhibits the reproduction of baculoviruses, filoviruses, hepatitis, coronaviruses, pseudorabies, HIV, influenza, and flaviviruses, as well as chikungunya and flaviviruses. U18666A-treated viral infections may act as a novel in vitro model system to elucidate the cholesterol mechanism of several viral infections. In this article, we discuss the mechanism and function of U18666A as a potent tool for studying cholesterol mechanisms in various viral infections.

Quantification of phosphoinositides reveals strong enrichment of PIP2 in HIV-1 compared to producer cell membranes

Human immunodeficiency virus type 1 (HIV-1) acquires its lipid envelope during budding from the plasma membrane of the host cell. Various studies indicated that HIV-1 membranes differ from producer cell plasma membranes, suggesting budding from specialized membrane microdomains. The phosphoinositide PI(4,5)P₂ has been of particular interest since PI(4,5)P₂ is needed to recruit the viral structural polyprotein Gag to the plasma membrane and thus facilitates viral morphogenesis. While there is evidence for an enrichment of PIP₂ in HIV-1, fully quantitative analysis of all phosphoinositides remains technically challenging and therefore has not been reported, yet. Here, we present a comprehensive analysis of the lipid content of HIV-1 and of plasma membranes from infected and non-infected producer cells, resulting in a total of 478 quantified lipid compounds, including molecular species distribution of 25 different lipid classes. Quantitative analyses of phosphoinositides revealed strong enrichment of PIP₂, but also of PIP₃, in the viral compared to the producer cell plasma membrane. We calculated an average of ca. 8,000 PIP₂ molecules per HIV-1 particle, three times more than Gag. We speculate that the high density of PIP₂ at the HIV-1 assembly site is mediated by transient interactions with viral Gag polyproteins, facilitating PIP₂ concentration in this microdomain. These results are consistent with our previous observation that PIP₂ is not only required for recruiting, but also for stably maintaining Gag at the plasma membrane. We believe that this quantitative analysis of the molecular anatomy of the HIV-1 lipid envelope may serve as standard reference for future investigations.

Lipids in Exosome Biology

Extracellular vesicles (EVs), and exosomes in particular, were initially considered as "garbage bags" for secretion of undesired cellular components. This view has changed considerably over the last two decades, and exosomes have now emerged as important organelles controlling cell-to-cell signaling. They are present in biological fluids and have important roles in the communication between cells in physiological and pathological processes. They are envisioned for clinical use as carriers of biomarkers, therapeutic targets, and vehicles for drug delivery. Important efforts are being made to characterize the contents of these vesicles and to understand the mechanisms that govern their biogenesis and modes of action. This chapter aims to recapitulate the place given to lipids in our understanding of exosome biology. Besides their structural role and their function as carriers, certain lipids and lipid-modifying enzymes seem to exert privileged functions in this mode of cellular communication. By extension, the use of selective "lipid inhibitors" might turn out to be interesting modulators of exosomal-based cell signaling.

Morphology and ultrastructure of retrovirus particles

Retrovirus morphogenesis entails assembly of Gag proteins and the viral genome on the host plasma membrane, acquisition of the viral membrane and envelope proteins through budding, and formation of the core through the maturation process. Although in both immature and mature retroviruses, Gag and capsid proteins are organized as paracrystalline structures, the curvatures of these protein arrays are evidently not uniform within one or among all virus particles. The heterogeneity of retroviruses poses significant challenges to studying the protein contacts within the Gag and capsid lattices. This review focuses on current understanding of the molecular organization of retroviruses derived from the sub-nanometer structures of immature virus particles, helical capsid protein assemblies and soluble envelope protein complexes. These studies provide insight into the molecular elements that maintain the stability, flexibility and infectivity of virus particles. Also reviewed are morphological studies of retrovirus budding, maturation, infection and cell-cell transmission, which inform the structural transformation of the viruses and the cells during infection and viral transmission, and lead to better understanding of the interplay between the functioning viral proteins and the host cell.

Role of Gag and lipids during HIV-1 assembly in CD4(+) T cells and macrophages

HIV-1 is an RNA enveloped virus that preferentially infects CD4⁺ T lymphocytes and also macrophages. In CD4⁺ T cells, HIV-1 mainly buds from the host cell plasma membrane. The viral Gag polyprotein targets the plasma membrane and is the orchestrator of the HIV assembly as its expression is sufficient to promote the formation of virus-like particles carrying a lipidic envelope derived from the host cell membrane. Certain lipids are enriched in the viral membrane and are thought to play a key role in the assembly process and the envelop composition. A large body of work performed on infected CD4⁺ T cells has provided important knowledge about the assembly process and the membrane virus lipid composition. While HIV assembly and budding in macrophages is thought to follow the same general Gag-driven mechanism as in T-lymphocytes, the HIV cycle in macrophage exhibits specific features. In these cells, new virions bud from the limiting membrane of seemingly intracellular compartments, where they accumulate while remaining infectious. These structures are now often referred to as Virus Containing Compartments (VCCs). Recent studies suggest that VCCs represent intracellularly sequestered regions of the plasma membrane, but their precise nature remains elusive. The proteomic and lipidomic characterization of virions produced by T cells or macrophages has highlighted the similarity between their composition and that of the plasma membrane of producer cells, as well as their enrichment in acidic lipids, some components of raft lipids and in tetraspanin-enriched microdomains. It is likely that Gag promotes the coalescence of these components into an assembly platform from which viral budding takes place. How Gag exactly interacts with membrane lipids and what are the mechanisms involved in the interaction between the different membrane nanodomains within the assembly platform remains unclear. Here we review recent literature regarding the role of Gag and lipids on HIV-1 assembly in CD4⁺ T cells and macrophages.

Interrelationship between cytoplasmic retroviral Gag concentration and Gag-membrane association

The early events in the retrovirus assembly pathway, particularly the timing and nature of Gag translocation from the site of protein translation to the inner leaflet of the plasma membrane, are poorly understood. We have investigated the interrelationship between cytoplasmic Gag concentration and plasma membrane association using complementary live-cell biophysical fluorescence techniques in real time with both human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) Gag proteins. In particular, dual-color, z-scan fluorescence fluctuation spectroscopy in conjunction with total internal reflection fluorescence and conventional, epi-illumination imaging were utilized. Our results demonstrate that HTLV-1 Gag is capable of membrane targeting and particle assembly at low (i.e., nanomolar) cytoplasmic concentrations and that there is a critical threshold concentration (approaching micromolar) prior to the observation of HIV-1 Gag associated with the plasma membrane. These observations imply fundamental differences between HIV-1 and HTLV-1 Gag trafficking and membrane association.

Evaluation of the ultrastructure of the small intestine of hiv infected children by transmission and scanning electronic microscopy

OBJECTIVES

To describe HIV children's small intestinal ultrastructural findings.

METHODS

Descriptive, observational study of small intestine biopsies performed between August 1994 and May 1995 at São Paulo, SP, Brazil. This material pertained to 11 HIV infected children and was stored in a laboratory in paraffin blocks. Scanning and transmission electronic microscopy were used to view those intestine samples and ultrastructural findings were described by analyzing digitalized photos of this material. Ethical Committee approval was obtained.

RESULTS

In most samples scanning microscopy showed various degrees of shortening and decreasing number of microvilli and also completes effacements in some areas. Derangement of the enterocytes was seen frequently and sometimes cells well defined borders limits seemed to be loosened. In some areas a mucous-fibrin like membrane with variable thickness and extension appeared to partially or totally coat the epithelial surface. Fat drops were present in the intestinal lumen in various samples and a bacterium morphologically resembling bacilli was seen in two occasions. Scanning microscopy confirmed transmission microscopy microvilli findings and also showed little "tufts" of those structures. In addition, it showed an increased number of vacuoles and multivesicular bodies inside various enterocytes, an increased presence of intraepithelial lymphocytes, mitochondrial vacuolization and basement membrane enlargement in the majority of samples analyzed. However, some samples exhibited normal aspect.

CONCLUSIONS

Our study showed the common occurrence of various important intestinal ultrastructural alterations with variable degrees among HIV infected children, some of them in our knowledge not described before.

Progesterone and a phospholipase inhibitor increase the endosomal bis(monoacylglycero)phosphate content and block HIV viral particle intercellular transmission

Progesterone, the cationic amphiphile U18666A and a phospholipase inhibitor (Methyl Arachidonyl Fluoro Phosphonate, MAFP) inhibited by 70%-90% HIV production in viral reservoir cells, i.e. human THP-1 monocytes and monocyte-derived macrophages (MDM). These compounds triggered an inhibition of fluid phase endocytosis (macropinocytosis) and modified cellular lipid homeostasis since endosomes accumulated filipin-stained sterols and Bis(Monoacylglycero)Phosphate (BMP). BMP was quantified using a new cytometry procedure and was increased by 1.25 times with MAFP, 1.7 times with U18666A and 2.5 times with progesterone. MAFP but not progesterone or U18666A inhibited the hydrolysis of BMP by the Pancreatic Lipase Related Protein 2 (PLRP2) as shown by in-vitro experiments. The possible role of sterol transporters in steroid-mediated BMP increase is discussed. Electron microscopy showed the accumulation of viral particles either into large intracellular viral-containing compartments or outside the cells, indicating that endosomal accumulation of BMP could block intracellular biogenesis of viral particles while inhibition of macropinocytosis would prevent viral particle uptake. This is the first report linking BMP metabolism with a natural steroid such as progesterone or with involvement of a phospholipase A1 activity. BMP cellular content could be used as a biomarker for efficient anti-viral drugs.

Major histocompatibility complex class-II molecules promote targeting of human immunodeficiency virus type 1 virions in late endosomes by enhancing internalization of nascent particles from the plasma membrane

Productive assembly of human immunodeficiency virus type 1 (HIV-1) takes place, primarily, at the plasma membrane. However, depending on the cell types, a significant proportion of nascent virus particles are internalized and routed to late endosomes. We previously reported that expression of human leucocyte antigen (HLA)-DR promoted a redistribution of Gag in late endosomes and an increased detection of mature virions in these compartments in HeLa and human embryonic kidney 293T model cell lines. Although this redistribution of Gag resulted in a marked decrease of HIV-1 release, the underlying mechanism remained undefined. Here, we provide evidence that expression of HLA-DR at the cell surface induces a redistribution of mature Gag products into late endosomes by enhancing nascent HIV-1 particle internalization from the plasma membrane through a process that relies on the presence of intact HLA-DR α and β-chain cytosolic tails. These findings raise the possibility that major histocompatibility complex class-II molecules might influence endocytic events at the plasma membrane and as a result promote endocytosis of progeny HIV-1 particles.

Maturation of the Gag core decreases the stability of retroviral lipid membranes

To better understand how detergents disrupt enveloped viruses, we monitored the biophysical stability of murine leukemia virus (MLV) virus-like particles (VLPs) against a panel of commonly used detergents using real-time biosensor measurements. Although exposure to many detergents, such as Triton X-100 and Empigen, results in lysis of VLP membranes, VLPs appeared resistant to complete membrane lysis by a significant number of detergents, including Tween 20, Tween 80, Lubrol, and Saponin. VLPs maintained their structural integrity after exposure to Tween 20 at concentrations up to 500-fold above its CMC. Remarkably, VLPs containing immature cores composed of unprocessed (uncleaved) Gag polyprotein were significantly more resistant to detergent lysis than VLPs with mature cores. Although the maturity of retroviral Gag is known to influence the stability of the protein core structure itself, our studies suggest that the maturity of the Gag core also influences the stability of the lipid bilayer surrounding the core.

Characterization of cytoplasmic Gag-gag interactions by dual-color z-scan fluorescence fluctuation spectroscopy

Fluorescence fluctuation spectroscopy (FFS) quantifies the interactions of fluorescently-labeled proteins inside living cells by brightness analysis. However, the study of cytoplasmic proteins that interact with the plasma membrane is challenging with FFS. If the cytoplasmic section is thinner than the axial size of the observation volume, cytoplasmic and membrane-bound proteins are coexcited, which leads to brightness artifacts. This brightness bias, if not recognized, leads to erroneous interpretation of the data. We have overcome this challenge by introducing dual-color z-scan FFS and the addition of a distinctly colored reference protein. Here, we apply this technique to study the cytoplasmic interactions of the Gag proteins from human immunodeficiency virus type 1 (HIV-1) and human T-lymphotropic virus type 1 (HTLV-1). The Gag protein plays a crucial role in the assembly of retroviruses and is found in both membrane and cytoplasm. Dual-color z-scans demonstrate that brightness artifacts are caused by a dim nonpunctate membrane-bound fraction of Gag. We perform an unbiased brightness characterization of cytoplasmic Gag by avoiding the membrane-bound fraction and reveal previously unknown differences in the behavior of the two retroviral Gag species. HIV-1 Gag exhibits concentration-dependent oligomerization in the cytoplasm, whereas HTLV-1 Gag lacks significant cytoplasmic Gag-Gag interactions.

New insights into HTLV-1 particle structure, assembly, and Gag-Gag interactions in living cells

Human T-cell leukemia virus type 1 (HTLV-1) has a reputation for being extremely difficult to study in cell culture. The challenges in propagating HTLV-1 has prevented a rigorous analysis of how these viruses replicate in cells, including the detailed steps involved in virus assembly. The details for how retrovirus particle assembly occurs are poorly understood, even for other more tractable retroviral systems. Recent studies on HTLV-1 using state-of-the-art cryo-electron microscopy and fluorescence-based biophysical approaches explored questions related to HTLV-1 particle size, Gag stoichiometry in virions, and Gag-Gag interactions in living cells. These results provided new and exciting insights into fundamental aspects of HTLV-1 particle assembly-which are distinct from those of other retroviruses, including HIV-1. The application of these and other novel biophysical approaches promise to provide exciting new insights into HTLV-1 replication.

Activation of the inositol (1,4,5)-triphosphate calcium gate receptor is required for HIV-1 Gag release

The structural precursor polyprotein, Gag, encoded by all retroviruses, including the human immunodeficiency virus type 1 (HIV-1), is necessary and sufficient for the assembly and release of particles that morphologically resemble immature virus particles. Previous studies have shown that the addition of Ca(2+) to cells expressing Gag enhances virus particle production. However, no specific cellular factor has been implicated as mediator of Ca(2+) provision. The inositol (1,4,5)-triphosphate receptor (IP3R) gates intracellular Ca(2+) stores. Following activation by binding of its ligand, IP3, it releases Ca(2+) from the stores. We demonstrate here that IP3R function is required for efficient release of HIV-1 virus particles. Depletion of IP3R by small interfering RNA, sequestration of its activating ligand by expression of a mutated fragment of IP3R that binds IP3 with very high affinity, or blocking formation of the ligand by inhibiting phospholipase C-mediated hydrolysis of the precursor, phosphatidylinositol-4,5-biphosphate, inhibited Gag particle release. These disruptions, as well as interference with ligand-receptor interaction using antibody targeted to the ligand-binding site on IP3R, blocked plasma membrane accumulation of Gag. These findings identify IP3R as a new determinant in HIV-1 trafficking during Gag assembly and introduce IP3R-regulated Ca(2+) signaling as a potential novel cofactor in viral particle release.

Cholesterol synthesis inhibitor U18666A and the role of sterol metabolism and trafficking in numerous pathophysiological processes

The multiple actions of U18666A have enabled major discoveries in lipid research and contributed to understanding the pathophysiology of multiple diseases. This review describes these advances and the utility of U18666A as a tool in lipid research. Harry Rudney's recognition that U18666A inhibited oxidosqualene cyclase led him to discover a pathway for formation of polar sterols that he proved to be important regulators of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase. Laura Liscum's recognition that U18666A inhibited the egress of cholesterol from late endosomes and lysosomes led to greatly improved perspective on the major pathways of intracellular cholesterol trafficking. The inhibition of cholesterol trafficking by U18666A mimicked the loss of functional Niemann-Pick type C protein responsible for NPC disease and thus provided a model for this disorder. U18666A subsequently became a tool for assessing the importance of molecular trafficking through the lysosomal pathway in other conditions such as atherosclerosis, Alzheimer's disease, and prion infections. U18666A also provided animal models for two important disorders: petite mal (absence) epilepsy and cataracts. This was the first chronic model of absence epilepsy. U18666A is also being used to address the role of oxidative stress in apoptosis. How can one molecule have so many effects? Perhaps because of its structure as an amphipathic cationic amine it can interact and inhibit diverse proteins. Restricting the availability of cholesterol for membrane formation through inhibition of cholesterol synthesis and intracellular trafficking could also be a mechanism for broadly affecting many processes. Another possibility is that through intercalation into membrane U18666A can alter membrane order and therefore the function of resident proteins. The similarity of the effects of natural and enantiomeric U18666A on cells and the capacity of intercalated U18666A to increase membrane order are arguments in favor of this possibility.

Cell biology of HIV-1 infection of macrophages

HIV infection of macrophages is a critically important component of viral pathogenesis and progression to AIDS. Although the virus follows the same life cycle in macrophages and T lymphocytes, several aspects of the virus-host relationship are unique to macrophage infection. Examples of these are the long-term persistence of productive infection, sustained by the absence of cell death, and the ability of progeny virus to bud into and accumulate in endocytic compartments designated multivesicular bodies (MVBs). Recently, the hypothesis that viral exploitation of the macrophage endocytic machinery is responsible for perpetuating the chronic state of infection unique to this cell type has been challenged in several independent studies employing a variety of experimental strategies. This review examines the evidence supporting and refuting the canonical hypothesis and highlights recently identified cellular factors that may contribute to the unique aspects of the HIV-macrophage interaction.

Differential control of CXCR4 and CD4 downregulation by HIV-1 Gag

Background

The ESCRT (endosomal sorting complex required for transport) machinery functions to sort cellular receptors into the lumen of the multivesicular body (MVB) prior to lysosomal degradation. ESCRT components can also be recruited by enveloped viruses to sites of viral assembly where they have been proposed to mediate viral egress. For example, HIV-1 budding is dependent on Gag-mediated recruitment of the cellular ESCRTs-I, -III, AIP1/Alix and Vps4 proteins. Viral recruitment of ESCRT proteins could therefore impact on host cell processes such as receptor downregulation.

Results

Here we show that downregulation of the HIV-1 co-receptor, CXCR4, by its ligand SDF-1, is ESCRT-I dependent. Expression of HIV-1 Gag attenuated downregulation of CXCR4, resulting in accumulation of undegraded receptors within intracellular compartments. The effect of Gag was dependent on an ESCRT-I interacting motif within the C-terminal p6 region of Gag. In contrast, PMA-induced downregulation of the HIV-1 receptor CD4 was independent of ESCRT-I and Vps4; HIV-1 Gag had no effect on this process.

Conclusion

These results establish that the HIV-1 receptor, CD4, and co-receptor, CXCR4 are differentially regulated by ESCRT proteins. HIV-1 Gag selectively modulates protein sorting at the MVB, interfering with ESCRT-I dependent but not ESCRT-I independent processes.

HIV-1 replication in dendritic cells occurs through a tetraspanin-containing compartment enriched in AP-3

Dendritic cells (DC) are crucial components of the early events of HIV infection. Dendritic cells capture and internalize HIV at mucosal surfaces and efficiently transfer the virus to CD4+ T cells in trans through infectious synapses (trans-infection pathway). Alternatively, HIV-1 replicates in DC (R5-HIV-1) (cis-infection pathway). Here, we analyzed HIV trafficking in DC during the trans-infection pathway as well as the cis-infection pathway. Confocal immunofluorescence microscopy demonstrated that after capture by DC, R5-HIV-1 and HIV-1 pseudotyped with vesicular stomatitis virus protein G colocalized in a viral compartment enriched in tetraspanins including CD81, CD82 and CD9, although at different levels, indicating a role of the viral envelope in targeting to the tetraspanin-rich compartment. Replication of R5-HIV-1 in DC (cis-infection pathway) also led to the accumulation, in an envelope-independent manner, of mature viral particles in a tetraspanin-rich compartment. A fraction of the HIV-1-containing compartments appeared directly accessible from the cell surface. In sharp contrast with the trans-infection pathway, the delta-subunit of the adaptor protein 3 (AP-3) complex was enriched on the HIV-1-containing compartment during R5-HIV-1 replication in DC (cis-infection pathway). Downregulation of AP-3 delta-adaptin reduced significantly viral particle release from HIV-1-infected DC. Together, these studies demonstrate a role for AP-3 in HIV replication in a tetraspanin-rich compartment in DC and contribute to the elucidation of the trafficking pathways required for DC-T cell transfer of HIV-1 infection, a critical step during the early events of HIV infection.

Analysis of human immunodeficiency virus matrix domain replacements

The matrix (MA) domain of the HIV-1 structural precursor Gag (PrGag) protein targets PrGag proteins to membrane assembly sites, and facilitates incorporation of envelope proteins into virions. To evaluate the specific requirements for the MA membrane-binding domain (MBD) in HIV-1 assembly and replication, we examined viruses in which MA was replaced by alternative MBDs. Results demonstrated that the pleckstrin homology domains of AKT protein kinase and phospholipase C delta1 efficiently directed the assembly and release of virus-like particles (VLPs) from cells expressing chimeric proteins. VLP assembly and release also were mediated in a phorbol ester-dependent fashion by the cysteine-rich binding domain of phosphokinase Cgamma. Although alternative MBDs promoted VLP assembly and release, the viruses were not infectious. Notably, PrGag processing was reduced, while cleavage of GagPol precursors resulted in the accumulation of Pol-derived intermediates within virions. Our results indicate that the HIV-1 assembly machinery is flexible with regard to its means of membrane association, but that alternative MBDs can interfere with the elaboration of infectious virus cores.

Productive human immunodeficiency virus type 1 assembly takes place at the plasma membrane

Gag proteins are necessary and sufficient to direct human immunodeficiency virus type 1 (HIV-1) particle assembly and budding. Recent evidence suggests that Gag targeting to late endosomal/multivesicular body (LE/MVB) compartments occurs prior to viral particle budding at the plasma membrane (PM). However, the route that Gag follows before reaching its steady-state destinations still remains a subject of debate. Using a subcellular fractionation method that separates PM from LE/MVB combined with pulse-chase labeling, we analyzed Gag trafficking in HIV-1-producing HEK 293T cells. Our results reveal that the majority of newly synthesized Gag is primarily targeted to the PM. While PM-targeted Gag was efficiently released, a significant fraction of the remaining cell surface-associated Gag was found to be subsequently internalized to LE/MVB, where it accumulated, thus accounting for the majority of LE/MVB-associated Gag. Importantly, this accumulation of Gag in LE/MVB was found to be cholesterol dependent since it was sensitive to the sterol-binding drugs filipin and methyl-beta-cyclodextrin. These results point towards the PM as being the primary site of productive HIV-1 assembly in cells that also support Gag accumulation in intracellular compartments.

Plasma membrane is the site of productive HIV-1 particle assembly

Recently proposed models that have gained wide acceptance posit that HIV-1 virion morphogenesis is initiated by targeting the major structural protein (Gag) to late endosomal membranes. Thereafter, late endosome-based secretory pathways are thought to deliver Gag or assembled virions to the plasma membrane (PM) and extracellular milieu. We present several findings that are inconsistent with this model. Specifically, we demonstrate that HIV-1 Gag is delivered to the PM, and virions are efficiently released into the extracellular medium, when late endosome motility is abolished. Furthermore, we show that HIV-1 virions are efficiently released when assembly is rationally targeted to the PM, but not when targeted to late endosomes. Recently synthesized Gag first accumulates and assembles at the PM, but a proportion is subsequently internalized via endocytosis or phagocytosis, thus accounting for observations of endosomal localization. We conclude that HIV-1 assembly is initiated and completed at the PM, and not at endosomal membranes.

HIV-1 matrix protein: a mysterious regulator of the viral life cycle

Significant progress has been achieved in the last few years concerning the human immunodeficiency virus (HIV-1) life cycle, mostly in the fields of cellular receptors for the virus, virus assembly and budding of virus particles from the cell surface. Meanwhile, some aspects, such as postentry events, virus maturation and the regulatory role of individual viral proteins remain poorly defined. This review summarizes some recent findings concerning the role of Gag Pr55 and its proteolytic processing in the HIV-1 life cycle with particular emphasis on the functions of matrix protein p17 (MA), the protein which plays a key role in regulation of the early and late steps of viral morphogenesis. Based on our recent observations, the possibility is discussed that two subsets of MA exist, one cleaved from the Gag precursor in the host cell (cMA), and the other cleaved in the virions (vMA). It is suggested that two MA fractions possess diverse functions and are involved in different stages of virus morphogenesis as key regulators of the viral life cycle.

Human immunodeficiency virus type 1 matrix protein assembles on membranes as a hexamer

The membrane-binding matrix (MA) domain of the human immunodeficiency virus type 1 (HIV-1) structural precursor Gag (PrGag) protein oligomerizes in solution as a trimer and crystallizes in three dimensions as a trimer unit. A number of models have been proposed to explain how MA trimers might align with respect to PrGag capsid (CA) N-terminal domains (NTDs), which assemble hexagonal lattices. We have examined the binding of naturally myristoylated HIV-1 matrix (MyrMA) and matrix plus capsid (MyrMACA) proteins on membranes in vitro. Unexpectedly, MyrMA and MyrMACA proteins both assembled hexagonal cage lattices on phosphatidylserine-cholesterol membranes. Membrane-bound MyrMA proteins did not organize into trimer units but, rather, organized into hexamer rings. Our results yield a model in which MA domains stack directly above NTD hexamers in immature particles, and they have implications for HIV assembly and interactions between MA and the viral membrane glycoproteins.

Rab-GTPases: dual roles in vesicular transport and viral replication

Productive viral infection is dependent upon post-entry migration of viruses/viral components to sites within a host cell that complement viral deficiencies. Delivery of virions or component proteins to appropriate sites within an infected cell is critical for completing successive stages in viral replication, including release into the cytoplasm, uncoating, genome replication, viral gene expression, assembly and budding. Vesicular transport is essential for steady-state cellular trafficking of membrane-associated proteins. Rab GTPases and their associated effectors are key regulators of vesicular transport pathways. In recent years, Rab proteins have been implicated in the endocytic or exocytic sorting of component viral proteins or intact viruses, most of which are known to be membrane-encapsulated and enveloped. This review will discuss the current understanding of how Rab GTPases and their effectors may regulate individual vesicular transport steps, and detail emerging discoveries examining how specific Rabs and effectors support viral replication.

Major histocompatibility complex class II molecules promote human immunodeficiency virus type 1 assembly and budding to late endosomal/multivesicular body compartments

Human immunodeficiency virus type 1 (HIV-1) assembly, budding, and release occur mostly at the plasma membrane in T lymphocytes as well as in established nonlymphoid cell lines, while in macrophages these processes occur primarily in intracellular compartments that harbor late endosomal/multivesicular body (LE/MVB) markers, including human leukocyte antigen DR (HLA-DR). Major histocompatibility complex class II molecules (MHC-II), which are expressed in macrophages and activated T cells, have been previously reported to induce the formation of multilaminar and multivesicular endocytic MHC-II-like structures analogous to MVB upon their expression in HEK 293 cells. Here, we have examined the role of MHC-II in HIV-1 Gag targeting as well as in virus assembly and release. Expression of HLA-DR in nonlymphoid cell lines induced a relocation of Gag to intracellular compartments that harbored LE/MVB markers and increased the accumulation of viral particles assembling intracellularly. Consequently, viral production and release from the cell surface was found to be substantially decreased in HLA-DR-expressing cells. This process was specific, since it was not observed with HLA-DR molecules lacking their cytoplasmic tails, nor with structurally related but functionally distinct MHC-II molecules such as HLA-DM or HLA-DO. Importantly, virus released intracellularly in HLA-DR-expressing cells retained infectivity. Overall, these results suggest a role of MHC-II molecules in promoting HIV-1 assembly and budding to LE/MVB and raise the possibility that this activity might be part of a normal pathway of virus production in cell types physiologically expressing MHC-II molecules, such as macrophages.

Mutation of dileucine-like motifs in the human immunodeficiency virus type 1 capsid disrupts virus assembly, gag-gag interactions, gag-membrane binding, and virion maturation

The human immunodeficiency virus type 1 (HIV-1) Gag precursor protein Pr55(Gag) drives the assembly and release of virus-like particles in the infected cell. The capsid (CA) domain of Gag plays an important role in these processes by promoting Gag-Gag interactions during assembly. The C-terminal domain (CTD) of CA contains two dileucine-like motifs (L189/L190 and I201/L202) implicated in regulating the localization of Gag to multivesicular bodies (MVBs). These dileucine-like motifs are located in the vicinity of the CTD dimer interface, a region of CA critical for Gag-Gag interactions during virus assembly and CA-CA interactions during core formation. To study the importance of the CA dileucine-like motifs in various aspects of HIV-1 replication, we introduced a series of mutations into these motifs in the context of a full-length, infectious HIV-1 molecular clone. CA mutants LL189,190AA and IL201,202AA were both severely impaired in virus particle production because of a variety of defects in the binding of Gag to membrane, Gag multimerization, and CA folding. In contrast to the model suggesting that the CA dileucine-like motifs regulate MVB targeting, the IL201,202AA mutation did not alter Gag localization to the MVB in either HeLa cells or macrophages. Revertants of single-amino-acid substitution mutants were obtained that no longer contained dileucine-like motifs but were nevertheless fully replication competent. The varied phenotypes of the mutants reported here provide novel insights into the interplay among Gag multimerization, membrane binding, virus assembly, CA dimerization, particle maturation, and virion infectivity.

Identification of an intracellular trafficking and assembly pathway for HIV-1 gag

Retroviral Gag proteins are membrane-bound polyproteins that are necessary and sufficient for virus-like particle (VLP) formation. It is not known how Gag traffics through the cell or how the site of particle production is determined. Here we use two techniques, biarsenical/tetracysteine (TC) labeling and release from a cycloheximide block, to follow the trafficking of newly synthesized HIV-1 Gag. Gag first appears diffusely distributed in the cytosol, accumulates in perinuclear clusters, passes transiently through a multivesicular body (MVB)-like compartment, and then travels to the plasma membrane (PM). Sequential passage of Gag through these temporal intermediates was confirmed by live cell imaging. Induction of a transient rise in cytoplasmic calcium increased the amounts of Gag, Gag assembly intermediates and VLPs in MVBs, and resulted in a dramatic increase in VLP release. These results define an intracellular trafficking pathway for HIV-1 Gag that uses perinuclear compartments and the MVB as trafficking intermediates. We propose that the regulation of Gag association with MVB-like compartments regulates the site of HIV-1 budding and particle formation.

Mapping of tetraspanin-enriched microdomains that can function as gateways for HIV-1

Specific spatial arrangements of proteins and lipids are central to the coordination of many biological processes. Tetraspanins have been proposed to laterally organize cellular membranes via specific associations with each other and with distinct integrins. Here, we reveal the presence of tetraspanin-enriched microdomains (TEMs) containing the tetraspanins CD9, CD63, CD81, and CD82 at the plasma membrane. Fluorescence and immunoelectron microscopic analyses document that the surface of HeLa cells is covered by several hundred TEMs, each extending over a few hundred nanometers and containing predominantly two or more tetraspanins. Further, we reveal that the human immunodeficiency virus type 1 (HIV-1) Gag protein, which directs viral assembly and release, accumulates at surface TEMs together with the HIV-1 envelope glycoprotein. TSG101 and VPS28, components of the mammalian ESCRT1 (endosomal sorting complex required for transport), which is part of the cellular extravesiculation machinery critical for HIV-1 budding, are also recruited to cell surface TEMs upon virus expression, suggesting that HIV-1 egress can be gated through these newly mapped microdomains.

HIV-1 Vpu promotes release and prevents endocytosis of nascent retrovirus particles from the plasma membrane

The human immunodeficiency virus (HIV) type-1 viral protein U (Vpu) protein enhances the release of diverse retroviruses from human, but not monkey, cells and is thought to do so by ablating a dominant restriction to particle release. Here, we determined how Vpu expression affects the subcellular distribution of HIV-1 and murine leukemia virus (MLV) Gag proteins in human cells where Vpu is, or is not, required for efficient particle release. In HeLa cells, where Vpu enhances HIV-1 and MLV release approximately 10-fold, concentrations of HIV-1 Gag and MLV Gag fused to cyan fluorescent protein (CFP) were initially detected at the plasma membrane, but then accumulated over time in early and late endosomes. Endosomal accumulation of Gag-CFP was prevented by Vpu expression and, importantly, inhibition of plasma membrane to early endosome transport by dominant negative mutants of Rab5a, dynamin, and EPS-15. Additionally, accumulation of both HIV and MLV Gag in endosomes required a functional late-budding domain. In human HOS cells, where HIV-1 and MLV release was efficient even in the absence of Vpu, Gag proteins were localized predominantly at the plasma membrane, irrespective of Vpu expression or manipulation of endocytic transport. While these data indicated that Vpu inhibits nascent virion endocytosis, Vpu did not affect transferrin endocytosis. Moreover, inhibition of endocytosis did not restore Vpu-defective HIV-1 release in HeLa cells, but instead resulted in accumulation of mature virions that could be released from the cell surface by protease treatment. Thus, these findings suggest that a specific activity that is present in HeLa cells, but not in HOS cells, and is counteracted by Vpu, traps assembled retrovirus particles at the cell surface. This entrapment leads to subsequent endocytosis by a Rab5a- and clathrin-dependent mechanism and intracellular sequestration of virions in endosomes.

Viral particles of the endogenous retrovirus ZAM from Drosophila melanogaster use a pre-existing endosome/exosome pathway for transfer to the oocyte

Background

Retroviruses have evolved various mechanisms to optimize their transfer to new target cells via late endosomes. Here, we analyzed the transfer of ZAM, a retroelement from Drosophila melanogaster, from ovarian follicle cells to the oocyte at stage 9–10 of oogenesis, when an active yolk transfer is occurring between these two cell types.

Results

Combining genetic and microscopic approaches, we show that a functional secretory apparatus is required to tether ZAM to endosomal vesicles and to direct its transport to the apical side of follicle cells. There, ZAM egress requires an intact follicular epithelium communicating with the oocyte. When gap junctions are inhibited or yolk receptors mutated, ZAM particles fail to sort out the follicle cells.

Conclusion

Overall, our results indicate that retrotransposons do not exclusively perform intracellular replication cycles but may usurp exosomal/endosomal traffic to be routed from one cell to another.

Binding of the C-terminal domain of the HIV-1 capsid protein to lipid membranes: a biophysical characterization

The capsid protein, CA, of HIV-1 forms a capsid that surrounds the viral genome. However, recent studies have shown that an important proportion of the CA molecule does not form part of this capsid, and its location and function are still unknown. In the present work we show, by using fluorescence, differential scanning calorimetry and Fourier-transform infrared spectroscopy, that the C-terminal region of CA, CA-C, is able to bind lipid vesicles in vitro in a peripheral fashion. CA-C had a greater affinity for negatively charged lipids (phosphatidic acid and phosphatidylserine) than for zwitterionic lipids [PC/Cho/SM (equimolar mixture of phosphatidylcholine, cholesterol and sphingomyelin) and phosphatidylcholine]. The interaction of CA-C with lipid membranes was supported by theoretical studies, which predicted that different regions, occurring close in the three-dimensional CA-C structure, were responsible for the binding. These results show the flexibility of CA-C to undergo conformational rearrangements in the presence of different binding partners. We hypothesize that the CA molecules that do not form part of the mature capsid might be involved in lipid-binding interactions in the inner leaflet of the virion envelope.

A mutation in the human immunodeficiency virus type 1 Gag protein destabilizes the interaction of the envelope protein subunits gp120 and gp41

The Gag protein of human immunodeficiency virus type 1 (HIV-1) associates with the envelope protein complex during virus assembly. The available evidence indicates that this interaction involves recognition of the gp41 cytoplasmic tail (CT) by the matrix protein (MA) region of Pr55(Gag). Here we show that substitution of Asp for Leu at position 49 (L49D) in MA results in a specific reduction in particle-associated gp120 without affecting the levels of gp41. Mutant virions were markedly reduced in single-cycle infectivity despite a relatively modest defect in fusion with target cells. Studies with HIV-1 particles containing decreased levels of envelope proteins suggested that the L49D mutation also inhibits a postentry step in infection. Truncation of the gp41 tail, or pseudotyping by vesicular stomatitis virus glycoprotein, restored both the fusion and infectivity of L49D mutant virions to wild-type levels. Truncation of gp41 also resulted in equivalent levels of gp120 on particles with and without the MA mutation and enhanced the replication of the L49D mutant virus in T cells. The impaired fusion and infectivity of L49D mutant particles were also complemented by a single point mutation in the gp41 CT that disrupted the tyrosine-containing endocytic motif. Our results suggest that an altered interaction between the MA domain of Gag and the gp41 cytoplasmic tail leads to dissociation of gp120 from gp41 during HIV-1 particle assembly, thus resulting in impaired fusion and infectivity.

Lentivirus degradation and DC-SIGN expression by human platelets and megakaryocytes

BACKGROUND AND AIM

As platelets are able to endocytose human immunodeficiency virus (HIV), we have investigated the fate of lentiviruses when endocytosed by human platelets and megakaryocytes (MK), and have characterized a specific receptor directly involved in this function.

METHODS

Genetically modified (non-replicative) lentiviruses with an HIV envelope (HIV-e) or with a vesicular stomatitis virus protein G envelope (VSV-e) were alternatively used and their interaction with platelets and MK analyzed by electron microscopy (EM) and immunoEM.

RESULTS

When incubated with platelets, HIV-e and VSV-e lentiviruses were internalized in specific endocytic vesicles and trafficked to the surface connected canalicular system (SCCS). Double immunolabeling for the viral P24 core protein and alpha-granule markers showed that lentiviruses were degraded in the SCCS after contact with alpha-granule proteins. In culture MK, lentiviruses were found in endocytic vesicles and accumulated in acid phosphatase-containing multivesicular bodies (MVB). The expression of the pathogen receptor dendritic cell-specific ICAM-grabbing non-integrin (DC-SIGN) was then demonstrated in platelets by flow cytometry, immunoEM and Western blot. Anti-DC-SIGN antibodies decreased HIV-e lentivirus internalization by platelets, showing that the receptor is functional. Specific signals for DC-SIGN protein and mRNA were also found in MK.

CONCLUSION

This study indicates that platelets and MK can internalize lentiviruses in a pathway, which either provide a shelter to lentiviral particles or alternatively disrupts viral integrity. The receptor DC-SIGN is involved in this function.

Intracellular Versus Cell Surface Assembly of Retroviral Pseudotypes Is Determined by the Cellular Localization of the Viral Glycoprotein, Its Capacity to Interact with Gag, and the Expression of the Nef Protein

Retroviral Gag and Env glycoproteins (GPs) are expressed from distinct cellular areas and need to encounter to interact and assemble infectious particles. Retroviral particles may also incorporate GPs derived from other enveloped viruses via active or passive mechanisms, a process known as "pseudotyping." To further understand the mechanisms of pseudotyping, we have investigated the capacity of murine leukemia virus (MLV) or lentivirus core particles to recruit GPs derived from different virus families: the G protein of vesicular stomatitis virus (VSV-G), the hemagglutinin from an influenza virus, the E1E2 glycoproteins of hepatitis C virus (HCV-E1E2), and the retroviral Env glycoproteins of MLV and RD114 cat endogenous virus. The parameters that influenced the incorporation of viral GPs onto retroviral core particles were (i) the intrinsic cell localization properties of both viral GP and retroviral core proteins, (ii) the ability of the viral GP to interact with the retroviral core, and (iii) the expression of the lentiviral Nef protein. Whereas the hemagglutinin and VSV-G glycoproteins were recruited by MLV and lentivirus core proteins at the cell surface, the HCV and MLV GPs were most likely recruited in late endosomes. In addition, whereas these glycoproteins could be passively incorporated on either retrovirus type, the MLV GP was also actively recruited by MLV core proteins, which, through interactions with the cytoplasmic tail of the latter GP, induced its localization to late endosomal vesicles. Finally, the expression of Nef proteins specifically enhanced the incorporation of the retroviral GPs by increasing their localization in late endosomes.

Rab9 GTPase is required for replication of human immunodeficiency virus type 1, filoviruses, and measles virus

Rab proteins and their effectors facilitate vesicular transport by tethering donor vesicles to their respective target membranes. By using gene trap insertional mutagenesis, we identified Rab9, which mediates late-endosome-to-trans-Golgi-network trafficking, among several candidate host genes whose disruption allowed the survival of Marburg virus-infected cells, suggesting that Rab9 is utilized in Marburg replication. Although Rab9 has not been implicated in human immunodeficiency virus (HIV) replication, previous reports suggested that the late endosome is an initiation site for HIV assembly and that TIP47-dependent trafficking out of the late endosome to the trans-Golgi network facilitates the sorting of HIV Env into virions budding at the plasma membrane. We examined the role of Rab9 in the life cycles of HIV and several unrelated viruses, using small interfering RNA (siRNA) to silence Rab9 expression before viral infection. Silencing Rab9 expression dramatically inhibited HIV replication, as did silencing the host genes encoding TIP47, p40, and PIKfyve, which also facilitate late-endosome-to-trans-Golgi vesicular transport. In addition, silencing studies revealed that HIV replication was dependent on the expression of Rab11A, which mediates trans-Golgi-to-plasma-membrane transport, and that increased HIV Gag was sequestered in a CD63+ endocytic compartment in a cell line stably expressing Rab9 siRNA. Replication of the enveloped Ebola, Marburg, and measles viruses was inhibited with Rab9 siRNA, although the non-enveloped reovirus was insensitive to Rab9 silencing. These results suggest that Rab9 is an important cellular target for inhibiting diverse viruses and help to define a late-endosome-to-plasma-membrane vesicular transport pathway important in viral assembly.

Interaction of HIV-1 Gag with the clathrin-associated adaptor AP-2

The envelope glycoprotein (Env) of HIV-1 interacts with the clathrin-associated adaptor complex AP-2 during the late phase of the viral replication cycle. Upon its synthesis, Env, therefore, is retrieved from the cellular surface unless internalization is inhibited by viral Gag. Here we demonstrate that not only Env, but also HIV-1 Gag, specifically binds to AP-2. Gag-AP-2 association was found to depend on tyrosine residue 132 and valine residue 135 at the matrix-capsid junction in the Gag polyprotein. Results of a morphological analysis of viral egress from cells expressing dominant-negative AP-2 suggest an involvement of AP-2 in confining HIV-1 exit to distinct microdomains. Further, particle release from AP-2-mutant cells was enhanced compared to release from wild-type cells but the infectivity of virus released from these cells was moderately reduced. Together these data attribute a role to the AP-2 complex in the regulation of HIV-1 assembly/release.

Statins Could Be Used to Control Replication of Some Viruses, Including HIV-1

Statins are mainly known for their plasma cholesterol-lowering properties and are widely used for the prevention of cardiovascular diseases. They however also exert pleiotropic effects through a variety of mechanisms, among which several immunosuppressive effects that are unrelated to their cholesterol-lowering activity. Interestingly, there has been recent evidence of antiviral effects, including preliminary studies on the efficacy of statins against HIV-1. This paper more particularly focuses on the specific inhibition of the binding of leukocyte function-associated antigen-1 (LFA-1) to intercellular adhesion molecule (ICAM-1) by statins, independently of the inhibition of HMGCoA reductase. Targeting the statin-binding site within LFA-1 or regulating LFA-1 affinity by inhibiting prenylation of the small GTPases could prove useful to treat inflammatory, autoimmune diseases and possibly viral infections, including HIV-1.

Macrophages archive HIV-1 virions for dissemination in trans

Viruses have evolved various strategies in order to persist within the host. To date, most information on mechanisms of HIV-1 persistence has been derived from studies with lymphocytes, but there is little information regarding mechanisms that govern HIV-1 persistence in macrophages. It has previously been demonstrated that virus assembly in macrophages occurs in cytoplasmic vesicles, which exhibit the characteristics of multivesicular bodies or late endosomes. The infectious stability of virions that assemble intracellularly in macrophages has not been evaluated. We demonstrate that virions assembling intracellularly in primary macrophages retain infectivity for extended intervals. Infectious virus was recovered directly from cytoplasmic lysates of macrophages and could be transmitted from macrophages to peripheral blood lymphocytes in trans 6 weeks after ongoing viral replication was blocked. Cell-associated virus decayed significantly from 1 to 2 weeks post infection, but decreased minimally thereafter. The persistence of intracellular virions did not require the viral accessory proteins Vpu or Nef. The stable sequestration of infectious virions within cytoplasmic compartments of macrophages may represent an additional mechanism for viral persistence in HIV-1-infected individuals.

Dynamic fluorescent imaging of human immunodeficiency virus type 1 gag in live cells by biarsenical labeling

Human immunodeficiency virus type 1 (HIV-1) Gag is the primary structural protein of the virus and is sufficient for particle formation. We utilized the recently developed biarsenical-labeling method to dynamically observe HIV-1 Gag within live cells by adding a tetracysteine tag (C-C-P-G-C-C) to the C terminus of Gag in both Pr55Gag expression and full-length proviral constructs. Membrane-permeable biarsenical compounds FlAsH and ReAsH covalently bond to this tetracysteine sequence and specifically fluoresce, effectively labeling Gag in the cell. Biarsenical labeling readily and specifically detected a tetracysteine-tagged HIV-1 Gag protein (Gag-TC) in HeLa, Mel JuSo, and Jurkat T cells by deconvolution fluorescence microscopy. Gag-TC was localized primarily at or near the plasma membrane in all cell types examined. Fluorescent two-color analysis of Gag-TC in HeLa cells revealed that nascent Gag was present mostly at the plasma membrane in distinct regions. Intracellular imaging of a Gag-TC myristylation mutant observed a diffuse signal throughout the cell, consistent with the role of myristylation in Gag localization to the plasma membrane. In contrast, mutation of the L-domain core sequence did not appreciably alter the localization of Gag, suggesting that the PTAP L domain functions at the site of budding rather than as a targeting signal. Taken together, our results show that Gag concentrates in specific plasma membrane areas rapidly after translation and demonstrate the utility of biarsenical labeling for visualizing the dynamic localization of Gag.

AP-3 directs the intracellular trafficking of HIV-1 Gag and plays a key role in particle assembly

Gag proteins direct the process of retroviral particle assembly and form the major protein constituents of the viral core. The matrix region of the HIV-1 Gag polyprotein plays a critical role in the transport of Gag to the plasma membrane assembly site. Recent evidence indicates that Gag trafficking to late endosomal compartments, including multivesicular bodies, occurs prior to viral particle budding from the plasma membrane. Here we demonstrate that the matrix region of HIV-1 Gag interacts directly with the delta subunit of the AP-3 complex, and that this interaction plays an important functional role in particle assembly. Disruption of this interaction eliminated Gag trafficking to multivesicular bodies and diminished HIV particle formation. These studies illuminate an early step in retroviral particle assembly and provide evidence that the trafficking of Gag to late endosomes is part of a productive particle assembly pathway.

Retrovirus budding

The release of retrovirus particles from the infected cell is greatly stimulated by short motifs, known as "late" or "L" domains, present within the Gag precursor protein. Three distinct classes of L domains have been identified; these bear the core sequence: Pro-Thr/Ser-Ala-Pro [P(T/S)AP], Pro-Pro-x-Tyr (PPxY), or Tyr-Pro-x-Leu (YPxL). A number of recent studies have demonstrated that L domains function by interacting with components of the machinery responsible for sorting cellular proteins into the multivesicular body (MVB) pathway. This review traces the history of L domain discovery and characterization, and highlights the relationship between L domain activity, retrovirus release, and the host endosomal sorting machinery.

Endosomal membrane traffic: convergence point targeted by Mycobacterium tuberculosis and HIV

Inhibition of phagolysosome biogenesis in infected macrophages is a classical pathogenesis determinant of Mycobacterium tuberculosis. In this review we primarily cover the cellular mechanisms of M. tuberculosis phagosome maturation arrest. A detailed picture is beginning to emerge, involving regulators of membrane trafficking in mammalian cells and phagosomal interactions with endosomal organelles and the trans-Golgi network. We also present a hypothesis that overlaps may exist between the mycobacterial interference with the host cell membrane trafficking processes and the targeting of the late endosomal sorting machinery by HIV during viral budding in macrophages. We propose that interference with the endosomal sorting machinery contributes to the synergism between the two significant human diseases--AIDS and tuberculosis.

Expression of human immunodeficiency virus type 1 gag modulates ligand-induced downregulation of EGF receptor

Many enveloped viruses use the ESCRT proteins of the cellular vacuolar protein sorting pathway for efficient egress from the cell. Recruitment of the ESCRT proteins by human immunodeficiency virus type 1 (HIV-1) Gag is required for HIV-1 particle budding and egress. ESCRT proteins normally function at endosomal membranes, where they facilitate the downregulation of mitogen-activated receptors such as EGF receptor (EGFR) through multivesicular body biogenesis. It is not known whether the Gag-mediated recruitment of ESCRT proteins functionally depletes the pool of these molecules that is available for the downregulation of EGFR. Here we show that the expression of HIV-1 Gag decreases the rate of EGFR downregulation, as assessed by decreases in the rates of (125)I-EGF and EGFR degradation. The effect of Gag was dependent on the presence of the TSG101 binding motif (PTAP) within the Gag C-terminal p6 domain. Cells expressing HIV-1 Gag retained more EGFR in late endosomes. This effect occurred when Gag was expressed alone from a heterologous promoter and when Gag expression was driven by the HIV-1 long terminal repeat within pHXB2DeltaBalD25S, a noninfectious lentiviral vector. Gag-expressing cells exhibited higher levels of activated mitogen-activated protein kinase for longer times after EGF addition than did cells that did not express HIV-1 Gag. These results indicate that HIV-1 Gag can impinge upon the functioning of the cellular vacuolar protein sorting pathway and reveal yet another facet of the intricate effects of HIV-1 infection on host cell physiology.

Phosphatidylinositol (4,5) bisphosphate regulates HIV-1 Gag targeting to the plasma membrane

A critical early event in the HIV type 1 (HIV-1) particle assembly pathway is the targeting of the Gag protein to the site of virus assembly. In many cell types, assembly takes place predominantly at the plasma membrane. Cellular factors that regulate Gag targeting remain undefined. The phosphoinositide phosphatidylinositol (4,5) bisphosphate [PI(4,5)P2] controls the plasma membrane localization of a number of cellular proteins. To explore the possibility that this lipid may be involved in Gag targeting and virus particle production, we overexpressed phosphoinositide 5-phosphatase IV, an enzyme that depletes cellular PI(4,5)P2, or overexpressed a constitutively active form of Arf6 (Arf6/Q67L), which induces the formation of PI(4,5)P2-enriched endosomal structures. Both approaches severely reduced virus production. Upon 5-phosphatase IV overexpression, Gag was no longer localized on the plasma membrane but instead was retargeted to late endosomes. Strikingly, in cells expressing Arf6/Q67L, Gag was redirected to the PI(4,5)P2-enriched vesicles and HIV-1 virions budded into these vesicles. These results demonstrate that PI(4,5)P2 plays a key role in Gag targeting to the plasma membrane and thus serves as a cellular determinant of HIV-1 particle production.