The extracellular processing of HIV-1 envelope glycoprotein gp160 by human plasmin

Furin-mediated protein processing in infectious diseases and cancer

Proteolytic cleavage regulates numerous processes in health and disease. One key player is the ubiquitously expressed serine protease furin, which cleaves a plethora of proteins at polybasic recognition motifs. Mammalian substrates of furin include cytokines, hormones, growth factors and receptors. Thus, it is not surprising that aberrant furin activity is associated with a variety of disorders including cancer. Furthermore, the enzymatic activity of furin is exploited by numerous viral and bacterial pathogens, thereby enhancing their virulence and spread. In this review, we describe the physiological and pathophysiological substrates of furin and discuss how dysregulation of a simple proteolytic cleavage event may promote infectious diseases and cancer. One major focus is the role of furin in viral glycoprotein maturation and pathogenicity. We also outline cellular mechanisms regulating the expression and activation of furin and summarise current approaches that target this protease for therapeutic intervention.

Mapping out the intricate relationship of the HIV envelope protein and the membrane environment

The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Human serum protein enhances HIV-1 replication and up-regulates the transcription factor AP-1

In vitro studies on HIV (HIV-1) replication and neutralization are usually performed in human cell cultures supplemented with FBS instead of human serum (HS). Here we show that in contrast to FBS, addition of increasing amounts of human serum from noninfected donors to the cell culture directly correlates with an increase in HIV-1 replication in vitro. This effect is independent of cell line, virus strain, or batch of pooled human serum used. We found that human serum affects viral transcription in a dose-dependent manner by activating the activator protein-1 (AP-1) member proteins c-FOS, JunD, and JunB in TZM-bl cells. Analysis of the human serum component responsible for this effect indicates that it is a protein having a molecular mass between 250 and 300 kDa. This serum protein, HIV-1 enhancing serum protein (HESP), might promote viral transcription in vivo and consequently play a role in disease progression.

Clostridium difficile spore-macrophage interactions: spore survival

BACKGROUND

Clostridium difficile is the main cause of nosocomial infections including antibiotic associated diarrhea, pseudomembranous colitis and toxic megacolon. During the course of Clostridium difficile infections (CDI), C. difficile undergoes sporulation and releases spores to the colonic environment. The elevated relapse rates of CDI suggest that C. difficile spores has a mechanism(s) to efficiently persist in the host colonic environment.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we provide evidence that C. difficile spores are well suited to survive the host's innate immune system. Electron microscopy results show that C. difficile spores are recognized by discrete patchy regions on the surface of macrophage Raw 264.7 cells, and phagocytosis was actin polymerization dependent. Fluorescence microscopy results show that >80% of Raw 264.7 cells had at least one C. difficile spore adhered, and that ∼60% of C. difficile spores were phagocytosed by Raw 264.7 cells. Strikingly, presence of complement decreased Raw 264.7 cells' ability to phagocytose C. difficile spores. Due to the ability of C. difficile spores to remain dormant inside Raw 264.7 cells, they were able to survive up to 72 h of macrophage infection. Interestingly, transmission electron micrographs showed interactions between the surface proteins of C. difficile spores and the phagosome membrane of Raw 264.7 cells. In addition, infection of Raw 264.7 cells with C. difficile spores for 48 h produced significant Raw 264.7 cell death as demonstrated by trypan blue assay, and nuclei staining by ethidium homodimer-1.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that despite efficient recognition and phagocytosis of C. difficile spores by Raw 264.7 cells, spores remain dormant and are able to survive and produce cytotoxic effects on Raw 264.7 cells.

Viral envelope protein folding and membrane hemifusion are enhanced by the conserved loop region of HIV-1 gp41

Fusion of human immunodeficiency virus (HIV-1) with target cells is mediated by the gp41 transmembrane envelope protein. The loop region within gp41 contains 2 crucial cysteines that play an unknown role in HIV-cell fusion. On the basis of cell-cell fusion assay, using human T-cell lines [Jurkat E6-1 and Jurkat HXBc2(4)], and virus-cell fusion assay, using fully infectious HIV-1 HXBc2 virus and TZM-bl human cell line, we provide evidence that the oxidation state of the disulfide bond within a loop domain peptide determines its activity. The oxidized (closed) form inhibits fusion, while the reduced (opened) form enhances hemifusion. These opposite activities reach 60% difference in viral fusion. Both forms of the loop domain interact with gp41: the opened form enhances gp41 folding into a bundle, whereas the closed form inhibits this folding. Therefore, the transformation of the cysteines from a reduced to an oxidized state enables the loop to convert from opened to closed conformations, which assists gp41 to fold and induces hemifusion. The significant conservation of the loop region within many envelope proteins suggests a general mechanism, which is exploited by viruses to enhance entry into their host cells.

Bacillus anthracis interacts with plasmin(ogen) to evade C3b-dependent innate immunity

The causative agent of anthrax, Bacillus anthracis, is capable of circumventing the humoral and innate immune defense of the host and modulating the blood chemistry in circulation to initiate a productive infection. It has been shown that the pathogen employs a number of strategies against immune cells using secreted pathogenic factors such as toxins. However, interference of B. anthracis with the innate immune system through specific interaction of the spore surface with host proteins such as the complement system has heretofore attracted little attention. In order to assess the mechanisms by which B. anthracis evades the defense system, we employed a proteomic analysis to identify human serum proteins interacting with B. anthracis spores, and found that plasminogen (PLG) is a major surface-bound protein. PLG efficiently bound to spores in a lysine- and exosporium-dependent manner. We identified α-enolase and elongation factor tu as PLG receptors. PLG-bound spores were capable of exhibiting anti-opsonic properties by cleaving C3b molecules in vitro and in rabbit bronchoalveolar lavage fluid, resulting in a decrease in macrophage phagocytosis. Our findings represent a step forward in understanding the mechanisms involved in the evasion of innate immunity by B. anthracis through recruitment of PLG resulting in the enhancement of anti-complement and anti-opsonization properties of the pathogen.

Insights into the mechanism of HIV-1 envelope induced membrane fusion as revealed by its inhibitory peptides

HIV-1 fusion with its target cells is mediated by the glycoprotein 41 (gp41) transmembrane subunit of the viral envelope glycoprotein (ENV). The current models propose that gp41 undergoes several conformational changes between the apposing viral and cell membranes to facilitate fusion. In this review we focus on the progress that has been made in revealing the dynamic role of the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) regions within gp41 to the fusion process. The involvement of these regions in the formation of the gp41 pre-hairpin and hairpin conformations during an ongoing fusion event was mainly discovered by their derived inhibitory peptides. For example, the core structure within the hairpin conformation in a dynamic fusion event is suggested to be larger than its high resolution structure and its minimal boundaries were determined in situ. Also, inhibitory peptides helped reveal the dual contribution of the NHR to the fusion process. Finally, we will also discuss several developments in peptide design that has led to a deeper understanding of the mechanism of viral membrane fusion.

In vitro identification of novel plasminogen-binding receptors of the pathogen Leptospira interrogans

BACKGROUND

Leptospirosis is a multisystem disease caused by pathogenic strains of the genus Leptospira. We have reported that Leptospira are able to bind plasminogen (PLG), to generate active plasmin in the presence of activator, and to degrade purified extracellular matrix fibronectin.

METHODOLOGY/PRINCIPAL FINDINGS

We have now cloned, expressed and purified 14 leptospiral recombinant proteins. The proteins were confirmed to be surface exposed by immunofluorescence microscopy and were evaluated for their ability to bind plasminogen (PLG). We identified eight as PLG-binding proteins, including the major outer membrane protein LipL32, the previously published rLIC12730, rLIC10494, Lp29, Lp49, LipL40 and MPL36, and one novel leptospiral protein, rLIC12238. Bound PLG could be converted to plasmin by the addition of urokinase-type PLG activator (uPA), showing specific proteolytic activity, as assessed by its reaction with the chromogenic plasmin substrate, D-Val-Leu-Lys 4-nitroanilide dihydrochloride. The addition of the lysine analog 6-aminocaproic acid (ACA) inhibited the protein-PLG interaction, thus strongly suggesting the involvement of lysine residues in plasminogen binding. The binding of leptospiral surface proteins to PLG was specific, dose-dependent and saturable. PLG and collagen type IV competed with LipL32 protein for the same binding site, whereas separate binding sites were observed for plasma fibronectin.

CONCLUSIONS/SIGNIFICANCE

PLG-binding/activation through the proteins/receptors on the surface of Leptospira could help the bacteria to specifically overcome tissue barriers, facilitating its spread throughout the host.

Plasminogen acquisition and activation at the surface of leptospira species lead to fibronectin degradation

Pathogenic Leptospira species are the etiological agents of leptospirosis, a widespread disease of human and veterinary concern. In this study, we report that Leptospira species are capable of binding plasminogen (PLG) in vitro. The binding to the leptospiral surface was demonstrated by indirect immunofluorescence confocal microscopy with living bacteria. The PLG binding to the bacteria seems to occur via lysine residues because the ligation is inhibited by addition of the lysine analog 6-aminocaproic acid. Exogenously provided urokinase-type PLG activator (uPA) converts surface-bound PLG into enzymatically active plasmin, as evaluated by the reaction with the chromogenic plasmin substrate d-Val-Leu-Lys 4-nitroanilide dihydrochloridein. The PLG activation system on the surface of Leptospira is PLG dose dependent and does not cause injury to the organism, as cellular growth in culture was not impaired. The generation of active plasmin within Leptospira was observed with several nonvirulent high-passage strains and with the nonpathogenic saprophytic organism Leptospira biflexa. Statistically significant higher activation of plasmin was detected with a low-passage infectious strain of Leptospira. Plasmin-coated virulent Leptospira interrogans bacteria were capable of degrading purified extracellular matrix fibronectin. The breakdown of fibronectin was not observed with untreated bacteria. Our data provide for the first time in vitro evidence for the generation of active plasmin on the surface of Leptospira, a step that may contribute to leptospiral invasiveness.

Enhancing the proteolytic maturation of human immunodeficiency virus type 1 envelope glycoproteins

In virus-infected cells, the envelope glycoprotein (Env) precursor, gp160, of human immunodeficiency virus type 1 is cleaved by cellular proteases into a fusion-competent gp120-gp41 heterodimer in which the two subunits are noncovalently associated. However, cleavage can be inefficient when recombinant Env is expressed at high levels, either as a full-length gp160 or as a soluble gp140 truncated immediately N-terminal to the transmembrane domain. We have explored several methods for obtaining fully cleaved Env for use as a vaccine antigen. We tested whether purified Env could be enzymatically digested with purified protease in vitro. Plasmin efficiently cleaved the Env precursor but also cut at a second site in gp120, most probably the V3 loop. In contrast, a soluble form of furin was specific for the gp120-gp41 cleavage site but cleaved inefficiently. Coexpression of Env with the full-length or soluble form of furin enhanced Env cleavage but also reduced Env expression. When the Env cleavage site (REKR) was mutated in order to see if its use by cellular proteases could be enhanced, several mutants were found to be processed more efficiently than the wild-type protein. The optimal cleavage site sequences were RRRRRR, RRRRKR, and RRRKKR. These mutations did not significantly alter the capacity of the Env protein to mediate fusion, so they have not radically perturbed Env structure. Furthermore, unlike that of wild-type Env, expression of the cleavage site mutants was not significantly reduced by furin coexpression. Coexpression of Env cleavage site mutants and furin is therefore a useful method for obtaining high-level expression of processed Env.

Pharmacophore determination of a gp120 C terminal-derived anti-HIV peptide construct interfering with membrane fusion suggesting that processing of the gp120 C terminus is a prelude to fusion

A multiple antigen peptide [CLIV; (PTKAKRR1VVQREKR2)4-K2-K-betaA] from the C terminus of the gp120 subunit of HIV Env inhibits Env-mediated cell-to-cell fusion through direct interference with the process (Virology 2000;273:169). We have examined various CLIV analogs using a cell-to-cell fusion assay, receptor binding assays, and molecular modeling to further address the characteristics of the peptide responsible for its anti-HIV activity. We show that (1) CLIV does not interfere with Env binding to CD4 and does not interact with the binding site of Env on CXCR4; (2) CLIV does not inhibit protease activities already reported to play a role in fusion; and (3) the pharmacophore is composed of cleavage site1 with amino acid residues at its C terminal end. Based on our data and on the literature, we propose that CLIV interferes with processing of the gp120 C terminus at site1 by the lymphocyte surface after CD4 binding. Our hypothesis implies that the cleavage region of Env is submitted to a stepwise processing including the known intracellular cleavage of gp160 at site2 in order to set the activation of the fusion peptide and a yet unexplored cleavage at site1 by the target cell surface that triggers fusion.

Inhibition of HIV-2(ROD) replication in a lymphoblastoid cell line by the alpha1-antitrypsin Portland variant (alpha1-PDX) and the decRVKRcmk peptide: comparison with HIV-1(LAI)

We investigated the effects of alpha1-antitrypsine Portland variant (alpha1-PDX) and decanoylRVKRchloromethylketone (decRVKRcmk) on HIV-2(ROD) replication in the Jurkat lymphoblastoid cell line. To this end, cells were stably transfected with the alpha1-PDX (J-PDX) and used as targets for HIV-2(ROD) infection. Controls were prepared with the empty vector (J-pcDNA3). HIV-2(ROD) and HIV-1(LAI) replications were significantly inhibited and delayed in the presence of the alpha1-PDX protein. When decRVKRcmk was used at 35 microM, inhibition rates were 70-80% for HIV-2(ROD) and HIV-1(LAI), while total inhibition occurred at 70 microM. Control peptides consisting of decanoylRVKR and acetylYVADcmk had no effect. In the presence of the alpha1-PDX or the decRVKRcmk at 35 microM, the infectivity of HIV-2(ROD) and HIV-1(LAI) produced was 3-4-fold lower. Both molecules inhibited syncytium formation by HIV-2(ROD) and HIV-1(LAI) to a considerable extent. Finally, the inhibition of viral replication was correlated with the ability of the decRVKRcmk at 35 and 70 microM and of the alpha1-PDX, to inhibit the processing of envelope glycoprotein precursors. The alpha1-PDX protein and the decRVKRcmk peptide at 35 microM inhibited HIV-2 and HIV-1 to a similar level suggesting that identical or closely related endoproteases are involved in the maturation of their envelope glycoprotein precursors into surface and transmembrane glycoproteins. The significant inhibition observed with alpha1-PDX indicates that furin or furin-like endoproteases appear to play a major role in the maturation process.

Purification and characterization of a Ca2+-independent endoprotease activity from peripheral blood lymphocytes: involvement in HIV-1 gp160 maturation

We have analyzed the calcium requirement for HIV-1 gp160 processing in cultured nonlymphoid (CV-1 and HeLa-CD4) and human-lymphoid [Jurkat, Molt-4 and peripheral blood lymphocytes (PBMCs)] cells. The processing of gp160 in these cells, infected with recombinant vaccinia virus encoding the gp160 gene, was only partially affected by intracellular calcium depletion induced by the calcium ionophore A23187 and calcium chelator EGTA. These observations prompted us to purify the Ca(2+)-independent gp160 processing enzyme from natural targets of HIV-1 PBMCs. The endoprotease was purified to homogeneity by the use of four chromatography fractionation steps and the constant detection of the Ca(2+)-independent activity at each one of them. The enzyme was believed to be a membrane-associated heteromeric 120-kDa protein composed of three subunits of 66, 32, and 24 kDa. It was found to be specifically inhibited by substrate analogues, decanoyl-RVKR-chloromethyl ketone, and serine protease inhibitors including diisopropyl fluorophosphate (DFP) and TLCK. In contrast, no effect was observed with reducing agents including 2-beta-mercaptoethanol, N-ethylmaleimide, L-cysteine, and dithiothreitol. There were significant similarities between inhibition profiles of the purified enzyme in vitro and those of the endogenous endoprotease(s) in cell culture experiments. Therefore, the selectivity of purified endoprotease for the gp160 cleavage site, its requirement for additional residues around this consensus sequence, and its isolation from natural targets of HIV-1, made it a good candidate in the gp160 maturation process. We provide more direct and supporting evidence that HIV-1 gp160 maturation may involve at least two families of divergent endoproteases according to calcium dependence.

Maturation of HIV envelope glycoprotein precursors by cellular endoproteases

The entry of enveloped viruses into its host cells is a crucial step for the propagation of viral infection. The envelope glycoprotein complex controls viral tropism and promotes the membrane fusion process. The surface glycoproteins of enveloped viruses are synthesized as inactive precursors and sorted through the constitutive secretory pathway of the infected cells. To be infectious, most of the viruses require viral envelope glycoprotein maturation by host cell endoproteases. In spite of the strong variability of primary sequences observed within different viral envelope glycoproteins, the endoproteolytical cleavage occurs mainly in a highly conserved domain at the carboxy terminus of the basic consensus sequence (Arg-X-Lys/Arg-Arg downward arrow). The same consensus sequence is recognized by the kexin/subtilisin-like serine proteinases (so called convertases) in many cellular substrates such as prohormones, proprotein of receptors, plasma proteins, growth factors and bacterial toxins. Therefore, several groups of investigators have evaluated the implication of convertases in viral envelope glycoprotein cleavage. Using the vaccinia virus overexpression system, furin was first shown to mediate the proteolytic maturation of both human immunodeficiency virus (HIV-1) and influenza virus envelope glycoproteins. In vitro studies demonstrated that purified convertases directly and specifically cleave viral envelope glycoproteins. Although these studies suggested the participation of several enzymes belonging to the convertases family, recent data suggest that other protease families may also participate in the HIV envelope glycoprotein processing. Their role in the physiological maturation process is still hypothetical and the molecular mechanism of the cleavage is not well documented. Crystallization of the hemagglutinin precursor (HA0) of influenza virus allowed further understanding of the molecular interaction between viral precursors and the cellular endoproteases. Furthermore, relationships between differential pathogenicity of influenza strains and their susceptibility to cleavage are molecularly funded. Here we review the most recent data and recent insights demonstrating the crucial role played by this activation step in virus infectivity. We discuss the cellular endoproteases that are implicated in HIV gp160 endoproteolytical maturation into gp120 and gp41.