Amiloride derivatives block ion channel activity and enhancement of virus-like particle budding caused by HIV-1 protein Vpu

Viral proteins function as ion channels

Viral ion channels are short membrane proteins with 50–120 amino acids and play an important role either in regulating virus replication, such as virus entry, assembly and release or modulating the electrochemical balance in the subcellular compartments of host cells. This review summarizes the recent advances in viral encoded ion channel proteins (or viroporins), including PBCV-1 KcV, influenza M2, HIV-1 Vpu, HCV p7, picornavirus 2B, and coronavirus E and 3a. We focus on their function and mechanisms, and also discuss viral ion channel protein serving as a potential drug target.

The Vpu protein: new concepts in virus release and CD4 down-modulation

Human immunodeficiency virus type 1 (HIV-1) and several simian immunodeficiency viruses (SIV) encode for a transmembrane protein known as Vpu (viral protein U). While one of the smallest of the HIV-1 proteins, it has two important functions within virus-infected cells. The first of these functions is the down-regulation of the CD4 receptor to prevent its interaction with the HIV-1 envelope glycoprotein. Vpu interacts with the CD4 receptor in the rough endoplasmic reticulum (RER), resulting in its re-translocation across the RER and subsequent degradation via the proteasomal pathway. The second major function of the Vpu protein is to facilitate release of virus from infected cells. Previous studies have shown that virus release is cell type specific, suggesting that certain cells may express a restriction factor that inhibits virus release in the absence of Vpu. Recently, bone marrow stromal antigen 2 (BST-2/HM1.24/CD317/tetherin) has been identified as this factor. This review will focus on new findings within the last four years on the role of Vpu in CD4 down-regulation and the restriction of virus release from cells. We will relate these findings to virus pathogenesis and propose questions regarding BST-2 as a restriction factor.

Ligand-protein docking studies of potential HIV-1 drug compounds using the algorithm FlexX

Four compounds are docked to a pentameric bundle representing the transmembrane part of the Vpu protein from HIV-1. Employing the docking algorithm FlexX, their free energy of binding is estimated leading to the conclusion that potential drug candidates need to form H-bonds either with neighbouring or with n + 2 helices at the site of the serines within the bundle.

Novel approaches to inhibiting HIV-1 replication

Considerable success has been achieved in the treatment of HIV-1 infection, and more than two-dozen antiretroviral drugs are available targeting several distinct steps in the viral replication cycle. However, resistance to these compounds emerges readily, even in the context of combination therapy. Drug toxicity, adverse drug-drug interactions, and accompanying poor patient adherence can also lead to treatment failure. These considerations make continued development of novel antiretroviral therapeutics necessary. In this article, we highlight a number of steps in the HIV-1 replication cycle that represent promising targets for drug discovery. These include lipid raft microdomains, the RNase H activity of the viral enzyme reverse transcriptase, uncoating of the viral core, host cell machinery involved in the integration of the viral DNA into host cell chromatin, virus assembly, maturation, and budding, and the functions of several viral accessory proteins. We discuss the relevant molecular and cell biology, and describe progress to date in developing inhibitors against these novel targets. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

Antiviral efficacy of the novel compound BIT225 against HIV-1 release from human macrophages

Building on previous findings that amiloride analogues inhibit HIV-1 replication in monocyte-derived macrophages (MDM), Biotron Limited has generated a library of over 300 small-molecule compounds with significant improvements in anti-HIV-1 activity. Our lead compound, BIT225, blocks Vpu ion channel activity and also shows anti-HIV-1 activity, with a 50% effective concentration of 2.25+/-0.23 microM (mean+/-the standard error) and minimal in vitro toxicity (50% toxic concentration, 284 microM) in infected MDM, resulting in a selectivity index of 126. In this study, we define the antiretroviral efficacy of BIT225 activity in macrophages, which are important drug targets because cells of the monocyte lineage are key reservoirs of HIV-1, disseminating virus to the peripheral tissues as they differentiate into macrophages. In assays with acutely and chronically HIV-1Ba-L-infected MDM, BIT225 resulted in significant reductions in viral integration and virus release as measured by real-time PCR and a reverse transcriptase (RT) activity assay at various stages of monocyte-to-macrophage differentiation. Further, the TZM-bl assay showed that the de novo virus produced at low levels in the presence of BIT225 was less infectious than virus produced in the absence of the compound. No antiviral activity was observed in MDM chronically infected with HIV-2, which lacks Vpu, confirming our initial targeting of and screening against this viral protein. The activity of BIT225 is post-virus integration, with no direct effects on the HIV-1 enzymes RT and protease. The findings of this study suggest that BIT225 is a late-phase inhibitor of the viral life cycle, targeting Vpu, and is a drug capable of significantly inhibiting HIV-1 release from both acute and chronically infected macrophages.

Therapies for coronaviruses. Part 2: Inhibitors of intracellular life cycle

BACKGROUND

Severe acute respiratory syndrome (SARS) coronavirus emerged from an animal reservoir in 2002 and has the potential to reemerge, as shown by the occurrence of non-laboratory-associated new cases in the winter of 2003. In the absence of a vaccine, broad spectrum anticoronaviral medications are needed.

OBJECTIVE

Anticoronavirals targeting viral entry were reviewed in part I. Here we review anticoronaviral therapies directed against the intracellular life cycle, with an emphasis on allowed patents and pending patents.

METHOD

The published literature, in particular, patent publications is searched for relevant documents. The information is organized and critiqued.

RESULTS/CONCLUSION

Many promising anticoronaviral strategies are identified. Monoclonal antibodies, protease inhibitors, interferon-based drugs and nucleic-acid based antivirals are most advanced, each having its own advantages and disadvantages. A multi-pronged approach, keeping all venues open, is advocated.

Viral channel-forming proteins

Channel-forming proteins are found in a number of viral genomes. In some cases, their role in the viral life cycle is well understood, in some cases it needs still to be elucidated. A common theme is that their mode of action involves a change of electrochemical or proton gradient across the lipid membrane which modulates the viral or cellular activity. Blocking these proteins can be a suitable therapeutic strategy as for some viruses this may be "lethal." Besides the many biological relevant questions still to be answered, there are also many open questions concerning the biophysical side as well as structural information and the mechanism of function on a molecular level. The immanent biophysical issues are addressed and the work in the field is summarized.

Interactions of viral protein U (Vpu) with cellular factors

Viral protein U (Vpu) is encoded by one of several accessory genes of HIV-1. Like the accessory gene products Vif and Vpr, Vpu targets host proteins such as CD4 for degradation via the recruitment of cellular multi-subunit ubiquitin ligases. Vpu also forms ion channels in cellular membranes. Through one or both of these attributes, Vpu antagonizes host cell factors that restrict the release of progeny virions from infected cells. A key target of Vpu has recently been identified as the interferon-induced transmembrane protein BST-2/CD317 (tetherin), which restricts viral replication by retaining nascent virions on the cell surface. The counteraction of this host defense allows Vpu to be considered an antagonist of the innate immune response to viral infection.

Requirements of the membrane proximal tyrosine and dileucine-based sorting signals for efficient transport of the subtype C Vpu protein to the plasma membrane and in virus release

Previously, we showed that the Vpu protein from HIV-1 subtype C is more efficiently transported to the cell surface than the well studied subtype B Vpu (Pacyniak et al., 2005) and that a SHIV expressing the subtype C Vpu exhibited a decreased rate of CD4+ T cell loss following inoculation in macaques (Hill et al., 2008). In this study, we examined the role of overlapping tyrosine-based (YXXPhi) and dileucine-based ([D/E]XXXL[L/I]) motifs in the membrane proximal region of the subtype C Vpu (EYRKLL) in Vpu intracellular transport, CD4 surface expression and virus release from the cell surface. We constructed three site-directed mutants of the subtype C vpu and fused these genes to the gene for enhanced green fluorescent protein (EGFP). The first mutation made altered the tyrosine (EARKLL; VpuSCEGFPY35A), the second altered the dileucine motif (EYRKLG; VpuSCEGFPL39G), and the third contained both amino acid substitutions (EARKLG; VpuSCEGFPYL35,39AG) in this region of the Vpu protein. The VpuSCEGFPY35A protein was transported to the cell surface similar to the unmodified VpuSCEGFP1 while VpuSCEGFPL39G was expressed at the cell surface at significantly reduced levels. The VpuSCEGFPYL35,39AG was found to have an intermediate level of cell surface expression. All three mutant Vpu proteins were analyzed for the ability to prevent cell surface expression of CD4. We found that both single mutants did not significantly effect CD4 surface expression while the double mutant (VpuSCEGFPYL35,39AG) was significantly less efficient at preventing cell surface CD4 expression. Chimeric simian human immunodeficiency viruses were constructed with these mutations in vpu (SHIVSCVpuY35A, SHIVSCVpuL39G and SHIVSCVpuYL35,39AG). Our results indicate that SHIVSCVpuL39G replicated much more efficiently and was much more cytopathic than SHIVSCVpu. In contrast, SHIVSCVpuY35A and SHIVSCVpuYL35,39AG replicated less efficiently when compared to the parental SHIVSCVpu. Taken together, these results show for the first time that the membrane proximal tyrosine-based sorting motif in the cytoplasmic domain of Vpu is essential for efficient virus release. These results also indicate that the dileucine-based sorting motif affects the intracellular trafficking of subtype C Vpu proteins, virus replication, and release.

Modulation of influenza virus replication by alteration of sodium ion transport and protein kinase C activity

In recent years, increasing levels of resistance to the four FDA-approved anti-influenza virus drugs have been described and vaccine manufacturers have experienced demands that exceed their capacity. This situation underlines the urgent need for novel antivirals as well as innovations in vaccine production in preparation for the next influenza epidemic. Here we report the development of a cell-based high-throughput screen which we have used for the identification of compounds that modulate influenza virus growth either negatively or positively. We screened a library of compounds with known biological activity and identified distinct groups of inhibitors and enhancers that target sodium channels or protein kinase C (PKC). We confirmed these results in viral growth assays and find that treatment with a sodium channel opener or PKC inhibitor significantly reduces viral replication. In contrast, inhibition of sodium channels or activation of PKC leads to enhanced virus production in tissue culture. These diametrically opposing effects strongly support a role for PKC activity and the regulation of Na⁺ currents in influenza virus replication and both may serve as targets for antiviral drugs. Furthermore, we raise the possibility that compounds that result in increased viral titers may be beneficial for boosting the production of tissue culture-grown influenza vaccines.

Amiloride derivatives inhibit coxsackievirus B3 RNA replication

Amiloride derivatives are known blockers of the cellular Na(+)/H(+) exchanger and the epithelial Na(+) channel. More recent studies demonstrate that they also inhibit ion channels formed by a number of viral proteins. We previously reported that 5-(N-ethyl-N-isopropyl)amiloride (EIPA) modestly inhibits intracellular replication and, to a larger extent, release of human rhinovirus 2 (HRV2) (E. V. Gazina, D. N. Harrison, M. Jefferies, H. Tan, D. Williams, D. A. Anderson and S. Petrou, Antiviral Res. 67:98-106, 2005). Here, we demonstrate that amiloride and EIPA strongly inhibit coxsackievirus B3 (CVB3) RNA replication and do not inhibit CVB3 release, in contrast to our previous findings on HRV2. Passaging of plasmid-derived CVB3 in the presence of amiloride generated mutant viruses with amino acid substitutions in position 299 or 372 of the CVB3 polymerase. Introduction of either of these mutations into the CVB3 plasmid produced resistance to amiloride and EIPA, suggesting that they act as inhibitors of CVB3 polymerase, a novel mechanism of antiviral activity for these compounds.

Modulation of the severe CD4+ T-cell loss caused by a pathogenic simian-human immunodeficiency virus by replacement of the subtype B vpu with the vpu from a subtype C HIV-1 clinical isolate

Previously, we showed that the Vpu protein from subtype C human immunodeficiency virus type 1 (HIV-1) was efficiently targeted to the cell surface, suggesting that this protein has biological properties that differ from the well-studied subtype B Vpu protein. In this study, we have further analyzed the biological properties of the subtype C Vpu protein. Flow cytometric analysis revealed that the subtype B Vpu (strain HXB2) was more efficient at down-regulating CD4 surface expression than the Vpu proteins from four subtype C clinical isolates. We constructed a simian-human immunodeficiency virus virus, designated as SHIV(SCVpu), in which the subtype B vpu gene from the pathogenic SHIV(KU-1bMC33) was substituted with the vpu from a clinical isolate of subtype C HIV-1 (strain C.96.BW16B01). Cell culture studies revealed that SHIV(SCVpu) replicated with slightly reduced kinetics when compared with the parental SHIV(KU-1bMC33) and that the viral Env and Gag precursor proteins were synthesized and processed similarly compared to the parental SHIV(KU-1bMC33). To determine if substitution of the subtype C Vpu protein affected the pathogenesis of the virus, three pig-tailed macaques were inoculated with SHIV(SCVpu) and circulating CD4+ T-cell levels and viral loads were monitored for up to 44 weeks. Our results show that SHIV(SCVpu) caused a more gradual decline in the rate of CD4+ T cells in pig-tailed macaques compared to those inoculated with parental subtype B SHIV(KU-1bMC33). These results show for the first time that different Vpu proteins of HIV-1 can influence the rate at which CD4+ T-cell loss occurs in the SHIV/pig-tailed macaque model.

Conformational changes induced by a single amino acid substitution in the trans-membrane domain of Vpu: implications for HIV-1 susceptibility to channel blocking drugs

The channel-forming trans-membrane domain of Vpu (Vpu TM) from HIV-1 is known to enhance virion release from the infected cells and is a potential target for ion-channel blockers. The substitution of alanine at position 18 by a histidine (A18H) has been shown to render HIV-1 infections susceptible to rimantadine, a channel blocker of M2 protein from the influenza virus. In order to describe the influence of the mutation on the structure and rimantadine susceptibility of Vpu, we determined the structure of A18H Vpu TM, and compared it to those of wild-type Vpu TM and M2 TM. Both isotropic and orientationally dependent NMR frequencies of the backbone amide resonance of His18 were perturbed by rimantadine, and those of Ile15 and Trp22 were also affected, suggesting that His18 is the key residue for rimantadine binding and that residues located on the same face of the TM helix are also involved. A18H Vpu TM has an ideal, straight alpha-helix spanning residues 6-27 with an average tilt angle of 41 degrees in C14 phospholipid bicelles, indicating that the tilt angle is increased by 11 degrees compared to that of wild-type Vpu TM. The longer helix formed by the A18H mutation has a larger tilt angle to compensate for the hydrophobic mismatch with the length of the phospholipids in the bilayer. These results demonstrate that the local change of the primary structure plays an important role in secondary and tertiary structures of Vpu TM in lipid bilayers and affects its ability to interact with channel blockers.

Drug–protein interaction with Vpu from HIV-1: proposing binding sites for amiloride and one of its derivatives

Vpu is an 81-amino-acid auxiliary protein of the genome of HIV-1. It is proposed that one of its roles is to enhance particle release by self-assembling to form water-filled channels enabling the flux of ions at the site of the plasma membrane of the infected cell. Hexamethylene amiloride has been shown to block Vpu channel activity when the protein is reconstituted into lipid bilayers. In a docking approach with monomeric, pentameric and hexameric bundle models of Vpu corresponding to the transmembrane part of the protein, a putative binding site of hexamethylene amiloride is proposed and is compared with the site for the nonpotent amiloride. The binding mode for both ligands is achieved by optimizing hydrogen bond interactions with serines. Binding energies and binding constants are the lowest for protonated hexamethylene amiloride in the pentameric bundle.

Entry of feline calicivirus is dependent on clathrin-mediated endocytosis and acidification in endosomes

Feline calicivirus is a major causative agent of respiratory disease in cats. It is also one of the few cultivatable members of Caliciviridae. We have examined the entry process of feline calicivirus (FCV). An earlier study demonstrated that acidification in endosomes may be required. We have confirmed this observation and expanded upon it, demonstrating, using drugs to inhibit the various endocytic pathways and dominant-negative mutants, that FCV infects cells via clathrin-mediated endocytosis. We have also observed that FCV permeabilizes cell membranes early during infection to allow the co-entry of toxins such as alpha-sarcin. Inhibitors of endosome acidification such as chloroquine and bafilomycin A1 blocked this permeabilization event, demonstrating that acidification is required for uncoating of the genome and access to the cytoplasm.

Hexamethylene amiloride blocks E protein ion channels and inhibits coronavirus replication

All coronaviruses encode a small hydrophobic envelope (E) protein, which mediates viral assembly and morphogenesis by an unknown mechanism. We have previously shown that the E protein from Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) forms cation-selective ion channels in planar lipid bilayers (Wilson, L., McKinlay, C., Gage, P., Ewart, G., 2004. SARS coronavirus E protein forms cation-selective ion channels. Virology 330(1), 322–331). We now report that three other E proteins also form cation-selective ion channels. These E proteins were from coronaviruses representative of taxonomic groups 1–3: human coronavirus 229E (HCoV-229E), mouse hepatitis virus (MHV), and infectious bronchitis virus (IBV), respectively. It appears, therefore, that coronavirus E proteins in general, belong to the virus ion channels family. Hexamethylene amiloride (HMA) – an inhibitor of the HIV-1 Vpu virus ion channel – inhibited the HCoV-229E and MHV E protein ion channel conductance in bilayers and also inhibited replication of the parent coronaviruses in cultured cells, as determined by plaque assay. Conversely, HMA had no antiviral effect on a recombinant MHV with the entire coding region of E protein deleted (MHVΔE). Taken together, the data provide evidence of a link between inhibition of E protein ion channel activity and the antiviral activity of HMA.

A single amino acid substitution within the transmembrane domain of the human immunodeficiency virus type 1 Vpu protein renders simian–human immunodeficiency virus (SHIVKU-1bMC33) susceptible to rimantadine

Previous studies from our laboratory have shown that the transmembrane domain (TM) of the Vpu protein of human immunodeficiency virus type 1 (HIV-1) contributes to the pathogenesis of SHIV(KU-1bMC33) in macaques and that the TM domain of Vpu could be replaced with the M2 protein viroporin from influenza A virus. Recently, we showed that the replacement of the TM domain of Vpu with that of the M2 protein of influenza A virus resulted in a virus (SHIV(M2)) that was sensitive to rimantadine [Hout, D.R., Gomez, M.L., Pacyniak, E., Gomez, L.M., Inbody, S.H., Mulcahy, E.R., Culley, N., Pinson, D.M., Powers, M.F., Wong, S.W., Stephens, E.B., 2006. Substitution of the transmembrane domain of Vpu in simian human immunodeficiency virus (SHIV(KU-1bMC33)) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques. Virology 344, 541-558]. Based on previous studies of the M2 protein which have shown that the His-X-X-X-Trp motif within the M2 is essential to the function of the M2 proton channel, we have constructed a novel SHIV in which the alanine at position 19 of the TM domain was replaced with a histidine residue resulting in the motif His-Ile-Leu-Val-Trp. The SHIV(VpuA19H) replicated with similar kinetics as the parental SHIV(KU-1bMC33) and pulse-chase analysis revealed that the processing of viral proteins was similar to SHIV(KU-1bMC33). This SHIV(VpuA19H) virus was found to be more sensitive to the M2 ion channel blocker rimantadine than SHIV(M2). Electron microscopic examination of SHIV(VpuA19H)-infected cells treated with rimantadine revealed an accumulation of viral particles at the cell surface and within intracellular vesicles, which was similar to that previously observed to SHIV(M2)-infected cells treated with rimantadine. These data indicate that the Vpu protein of HIV-1 can be converted into a rimantadine-sensitive ion channel with the alteration of one amino acid and provide additional evidence that drugs targeting the Vpu TM/ion channel can be effective anti-HIV-1 drugs.

Recruitment of the adaptor protein 2 complex by the human immunodeficiency virus type 2 envelope protein is necessary for high levels of virus release

The envelope (Env) protein of human immunodeficiency virus type 2 (HIV-2) and the HIV-1 Vpu protein stimulate the release of retroviral particles from human cells that restrict virus production, an activity that we call the enhancement of virus release (EVR). We have previously shown that two separate domains in the HIV-2 envelope protein are required for this activity: a glycine-tyrosine-x-x-hydrophobic (GYxxtheta) motif in the cytoplasmic tail and an unmapped region in the ectodomain of the protein. We here report that the cellular partner of the GYxxtheta motif is the adaptor protein complex AP-2. The mutation of this motif or the depletion of AP-2 by RNA interference abrogated EVR activity and changed the cellular distribution of the Env from a predominantly punctate pattern to a more diffuse distribution. Since the L domain of equine infectious anemia virus (EIAV) contains a Yxxtheta motif that interacts with AP-2, we used both wild-type and L domain-defective particles of HIV-1 and EIAV to examine whether the HIV-2 Env EVR function was analogous to L domain activity. We observed that the production of all particles was stimulated by HIV-2 Env or Vpu, suggesting that the L domain and EVR activities play independent roles in the release of retroviruses. Interestingly, we found that the cytoplasmic tail of the murine leukemia virus (MLV) Env could functionally substitute for the HIV-2 Env tail, but it did so in a manner that did not require a Yxxtheta motif or AP-2. The cellular distribution of the chimeric HIV-2/MLV Env was significantly less punctate than the wild-type Env, although confocal analysis revealed an overlap in the steady-state locations of the two proteins. Taken together, these data suggest that the essential GYxxtheta motif in the HIV-2 Env tail recruits AP-2 in order to direct Env to a cellular pathway or location that is necessary for its ability to enhance virus release but that an alternate mechanism provided by the MLV Env tail can functionally substitute.

Amantadine Inhibits the Function of an Ion Channel Encoded by Gb Virus B, but Fails to Inhibit Virus Replication

A chemically synthesized peptide representing the C-terminal subunit (p13-C) of the p13 protein of GB virus B (GBV-B), the most closely related virus to hepatitis C virus (HCV) showed ion channel activity in artificial lipid bilayers. The channels had a variable conductance and were more permeable to potassium ions than to chloride ions. Amantadine but not hexam-ethylene amiloride (HMA) inhibited the ion channel function of p13-C in the lipid membranes. However, neither agent was able to inhibit the replication and secretion of GBV-B from virus-infected cultured marmoset hepatocytes, which were harvested from a marmoset that was infected in vivo or inhibit replication after in vitro infection of naive hepatocytes. These data suggest that the GBV-B ion channel, contrary to the data derived from the lipid membranes, is either resistant to amantadine or that virus replication and secretion are independent of ion channel function. As the p7 protein of HCV also has ion channel activity that is apparently resistant to amantadine in vivo, the former possibility is most likely. Ion channels are likely to have an important role in the life cycle of many viruses and compounds that block these channels may prove to be useful antiviral agents.

Targeted infection of HIV-1 Env expressing cells by HIV(CD4/CXCR4) vectors reveals a potential new rationale for HIV-1 mediated down-modulation of CD4

Background

Efficient targeted gene transfer and cell type specific transgene expression are important for the safe and effective expression of transgenes in vivo. Enveloped viral vectors allow insertion of exogenous membrane proteins into their envelopes, which could potentially aid in the targeted transduction of specific cell types. Our goal was to specifically target cells that express the T cell tropic HIV-1 envelope protein (Env) using the highly specific interaction of Env with its cellular receptor (CD4) inserted into the envelope of an HIV-1-based viral vector.

Results

To generate HIV-1-based vectors carrying the CD4 molecule in their envelope, the CD4 ectodomain was fused to diverse membrane anchors and inserted together with the HIV-1 coreceptor CXCR4 into the envelopes of HIV-1 vector particles. Independent of the type of CD4 anchor, all chimeric CD4 proteins inserted into HIV-1 vector envelopes and the resultant HIV(CD4/CXCR4) particles were able to selectively confer neomycin resistance to cells expressing the fusogenic T cell tropic HIV-1 Env protein. Unexpectedly, in the absence of Env on the target cells, all vector particles carrying the CD4 ectodomain anchored in their envelope adhered to various cell types without infecting these cells. This cell adhesion was very avid. It was independent of the presence of Env on the target cell, the type of CD4 anchor or the presence of CXCR4 on the particle. In mixed cell populations with defined ratios of Env⁺/Env^- cells, the targeted transduction of Env⁺ cells by HIV(CD4/CXCR4) particles was diminished in proportion to the number of Env^- cells.

Conclusion

Vector diversion caused by a strong, non-selective cell binding of CD4⁺-vector particles effectively prevents the targeted transduction of HIV-1 Env expressing cells in mixed cell populations. This Env-independent cell adhesion severely limits the effective use of targeted HIV(CD4/CXCR4) vectors designed to interfere with HIV-1 replication in vivo. Importantly, the existence of this newly described and remarkably strong CD4-dependent cell adhesion suggests that the multiple viral efforts to reduce CD4 cell surface expression may, in part, be to prevent cell adhesion to non-target cells and thereby to increase the infectivity of viral progeny. Preventing CD4 down-modulation by HIV-1 might be an effective component of a multi-faceted antiviral strategy.

Ion transport blockers inhibit human rhinovirus 2 release

Picornavirus replication causes leakage of cytoplasmic K+ and an influx of Na+ and Ca2+. In this study, we have explored the possibility that a blockade of Ca2+ and Na+ influx would reduce rhinovirus production and/or release. The Ca2+-channel blockers, verapamil and diltiazem, as well as the blocker of Na+/H+ exchange and the epithelial Na+ channel, EIPA, inhibited both virus production and release. The effect on virus release was more pronounced than the effect on production, thus raising the possibility that rhinovirus release may serve as a target for antiviral agents. Unexpectedly, our results also showed that the antiviral activity of the Ca2+-channel blockers was not due to the block of Ca2+ influx. Similarly, the antiviral activity of EIPA appeared to be unrelated to the blockade of cellular Na+/H+ exchanger or the epithelial Na+ channel. Potential alternative mechanisms of the antiviral activity of these compounds are discussed.

Substitution of the transmembrane domain of Vpu in simian-human immunodeficiency virus (SHIVKU1bMC33) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques

The Vpu protein of human immunodeficiency virus type 1 has been shown to shunt the CD4 receptor molecule to the proteasome for degradation and to enhance virus release from infected cells. The exact mechanism by which the Vpu protein enhances virus release is currently unknown but some investigators have shown that this function is associated with the transmembrane domain and potential ion channel properties. In this study, we determined if the transmembrane domain of Vpu could be functionally substituted with that of the prototypical viroporin, the M2 protein of influenza A virus. We constructed chimeric vpu gene in which the transmembrane domain of Vpu was replaced with that of the M2 protein of influenza. This chimeric vpu gene was substituted for the vpu gene in the genome of a pathogenic simian human immunodeficiency virus, SHIVKU-1bMC33. The resulting virus, SHIVM2, synthesized a Vpu protein that had a slightly different Mr compared to the parental SHIVKU-1bMC33, reflecting the different sizes of the two Vpu proteins. The SHIVM2 was shown to replicate with slightly reduced kinetics when compared to the parental SHIVKU-1bMC33 but electron microscopy revealed that the site of maturation was similar to the parental virus SHIVKU1bMC33. We show that the replication and spread of SHIVM2 could be blocked with the antiviral drug rimantadine, which is known to target the M2 ion channel. Our results indicate a dose dependent inhibition of SHIVM2 with 100 microM rimantadine resulting in a >95% decrease in p27 released into the culture medium. Rimantadine did not affect the replication of the parental SHIVKU-1bMC33. Examination of SHIVM2-infected cells treated with 50 microM rimantadine revealed numerous viral particles associated with the cell plasma membrane and within intracytoplasmic vesicles, which is similar to HIV-1 mutants lacking a functional vpu. To determine if SHIVM2 was as pathogenic as the parental SHIVKU-1bMC33 virus, two pig-tailed macaques were inoculated and followed for up to 8 months. Both pig-tailed macaques developed severe CD4+ T cell loss within 1 month of inoculation, high viral loads, and histological lesions consistent with lymphoid depletion similar to the parental SHIVKU-1bMC33. Taken together, these results indicate for the first time that the TM domain of the Vpu protein can be functionally substituted with the TM of M2 of influenza A virus, and shows that compounds that target the TM domain of Vpu protein of HIV-1 could serve as novel anti-HIV-1 drugs.

Scrambling of the amino acids within the transmembrane domain of Vpu results in a simian-human immunodeficiency virus (SHIVTM) that is less pathogenic for pig-tailed macaques

Previous studies have shown that the transmembrane (TM) domain of the subtype B Vpu enhances virion release from cells and some studies have shown that this domain may form an oligomeric structure with properties of an ion channel. To date, no studies have been performed to assess the role of this domain in virus pathogenesis in a macaque model of disease. Using a pathogenic molecular clone of simian human immunodeficiency virus (SHIVKU-1bMC33), we have generated a novel virus in which the transmembrane domain of the Vpu protein was scrambled but maintained hydrophobic in nature (SHIVTM), which presumably would disrupt any ion channel TM properties of this protein. Vectors expressing the Vpu as a fusion protein with the enhanced green fluorescent protein (VpuTMEGFP) indicate that it was transported to the same intracellular compartment as the unmodified Vpu protein but did not down-regulate cell surface expression of CD4. To assess the pathogenicity of SHIVTM, three pig-tailed macaques were inoculated with the SHIVTM and monitored for 6-8 months for CD4+ T cell levels, viral loads and the stability of the sequence of the vpu gene. Our results indicated that unlike the parental SHIVKU-1bMC33, inoculation of macaques with SHIVTM did not cause a severe CD4+ T cell loss over the course of their infections. Sequence analysis of the vpu gene analyzed from sequential PBMC samples derived from macaques revealed that the scrambled TM was stable during the course of infection. At necropsy, examination of tissues revealed low viral loads and none of the pathology commonly observed in lymphoid and non-lymphoid tissues following inoculation with the pathogenic parental SHIVKU-1bMC33 virus. Thus, these results show for the first time that the TM domain of Vpu contributes to the pathogenicity of SHIVKU-1bMC33 in pig-tailed macaques.

Identification of a region within the cytoplasmic domain of the subtype B Vpu protein of human immunodeficiency virus type 1 (HIV-1) that is responsible for retention in the golgi complex and its absence in the Vpu protein from a subtype C HIV-1

The structure of the Vpu protein of human immunodeficiency virus type 1 (HIV-1) is composed of a short Nterminal domain (NTD), a transmembrane domain (TM), and a cytoplasmic domain (CD). Previous studies have shown that the Vpu protein from subtype B HIV-1 is transported predominantly to the rough endoplasmic reticulum (RER)/Golgi complex compartments of the cell and is not incorporated into virions. Using a previously described VpuEGFP reporter system in which the Vpu protein was fused to the gene for enhanced green fluorescent protein (EGFP), we showed that the subtype B Vpu fusion protein was localized to the RER/Golgi region of the cell, similar to the native protein. In the present study, we show that fusion of the subtype C Vpu to EGFP results in a fusion protein that is transported to the cell surface. Using this reporter system, chimeric Vpu proteins in which the CD of the subtype B and C proteins were exchanged showed that the CD was sufficient for targeting the subtype B protein to the Golgi complex of the cell. Following identification of the cytoplasmic domain as being responsible for intracellular targeting, we then generated a series of mutants in which 13, 23, 31, 38, 51, and 56 amino acids were deleted from the cytoplasmic domain of subtype B Vpu. These deletion mutants were analyzed by SDS-PAGE for size, for membrane localization, and intracellular localization by confocal fluorescence microscopy. Our results indicate that the mutant with the carboxyl-terminal 13 amino acids deleted was still localized to the Golgi complex but mutants with 23, 31, 38, 51, and 56 amino acids from the carboxyl-terminus of the subtype B Vpu were transported to the cell surface. These results suggest that a signal for the retention of the subtype B Vpu within the Golgi complex resides in the second alpha-helical domain.

Functional domains within the human immunodeficiency virus type 2 envelope protein required to enhance virus production

Primate lentiviruses code for a protein that stimulates virus production. In human immunodeficiency virus type 1 (HIV-1), the activity is provided by the accessory protein, Vpu, while in HIV-2 and simian immunodeficiency virus it is a property of the envelope (Env) glycoprotein. Using a group of diverse retroviruses and cell types, we have confirmed the functional equivalence of the two proteins. However, despite these similarities, the two proteins have markedly different functional domains. While the Vpu activity is associated primarily with its membrane-spanning region, we have determined that the HIV-2 Env activity requires both the cytoplasmic tail and ectodomain of the protein, with the membrane-spanning domain being less important. Within the Env cytoplasmic tail, we further defined the necessary sequence as a membrane-proximal tyrosine-based motif. Providing the two Env regions separately as distinct CD8 chimeric proteins did not increase virus release. This suggests that the two domains must be either contained within a single protein or closely associated within a multiprotein oligomer, such as the Env trimer, in order to function. Finally, we observed that wild-type levels of incorporation of the HIV-2 Env into budding viruses were not required for this activity.

Dengue Virus M Protein C-Terminal Peptide (DVM-C) Forms Ion Channels

A chemically synthesized peptide consisting of the C-terminus of the M protein of the Dengue virus type 1 strain Singapore S275/90 (DVM-C) produced ion channel activity in artificial lipid bilayers. The channels had a variable conductance and were more permeable to sodium and potassium ions than to chloride ions and more permeable to chloride ions than to calcium ions. Hexamethylene amiloride (100 microM) and amantadine (10 microM), blocked channels formed by DVM-C. Ion channels may play an important role in the life cycle of many viruses and drugs that block these channels may prove to be useful antiviral agents.

Channel Activity of a Viral Transmembrane Peptide in Micro-BLMs: Vpu1-32from HIV-1

We report for the first time on pore-suspending lipid bilayers, which we call micro-black lipid membranes (micro-BLMs), based on a highly ordered macroporous silicon array. Micro-BLMs were established by first functionalizing the backside porous silicon surface with gold and then chemisorbing 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol followed by spreading 1,2-diphytanoyl-sn-glycero-3-phosphocholine dissolved in n-decane. Impedance spectroscopy revealed the formation of single lipid bilayers confirmed by a mean specific capacitance of 0.6 +/- 0.2 microF/cm2. Membrane resistances were in the G omega-regime and beyond. The potential of the system for single channel recordings was demonstrated by inserting the transmembrane domain of the HIV-1 accessory peptide Vpu(1-32), which forms helix bundles with characteristic opening states. We elucidated different amilorides as potential drugs to inhibit channel activity of Vpu.

Fusion of the upstream vpu sequences to the env of simian human immunodeficiency virus (SHIV(KU-1bMC33)) results in the synthesis of two envelope precursor proteins, increased numbers of virus particles associated with the cell surface and is pathogenic for pig-tailed macaques

Previous studies have shown that the gene coding for the Vpu protein of the human immunodeficiency virus type 1 (HIV-1) is 5' to the env gene, is in a different reading frame, and overlaps the env by 90 nucleotides. In this study, we examined the processing of the Env protein as well as the maturation and infectivity of a virus (SHIV(Vpenv)) in which a single nucleotide was removed at the vpu-env junction, fusing the first 162 bases of vpu to the env ORF. Pulse-chase analysis revealed that SHIV(Vpenv)-infected cells gave rise to two precursor glycoprotein species (gp160 and gp175). Immune precipitation results also revealed that an anti-Vpu serum could immune precipitate the gp175 precursor, suggesting that the amino-terminal Vpu sequence was fused to the Env protein. Growth curves revealed that the SHIV(Vpenv)-inoculated cultures released approximately three times more p27 into the culture medium than parental SHIV(KU-1bMC33). Electron microscopy revealed that while both viruses matured at the cell plasma membrane, significantly higher quantities of virus particles were cell associated on SHIV(Vpenv)-infected cells compared to cultures inoculated with parental SHIV(KU-1bMC33). Furthermore, virus was observed maturing into intracellular vesicles of SHIV(Vpenv)-infected cells. To assess the pathogenicity of SHIV(Vpenv), three pig-tailed macaques were inoculated with the SHIV(Vpenv) and monitored for 6 months for CD4(+) T cell levels, viral loads, and the stability of the deletion at the vpu-env junction. Our results indicated that SHIV(Vpenv) caused a severe CD4(+) T cell loss in all three macaques within weeks of inoculation. Sequence analysis of the vpu gene analyzed from sequential PBMC samples derived from macaques revealed that this mutation was stable during the period of rapid CD4(+) T cell loss. Sequence analysis showed that with increasing time of infection, the one base pair deletion was repaired in all three macaques inoculated with SHIV(Vpenv) with the reversion occurring at 10 weeks in macaque CT1G and at 12 weeks in macaque CP3R and CT1R. These results indicate that fusion of the first 54 amino acids of Vpu to Env results in intracellular maturation of virus, and accumulation of virus within intracellular vesicles as well as on the cell plasma membrane. Our results indicate that while fusion of the vpu gene to env results in a virus that is still pathogenic for pig-tailed macaques, there is a selective pressure to maintain the vpu and env genes in separate reading frames.

Potential new anti-human immunodeficiency virus type 1 compounds depress virus replication in cultured human macrophages

We report that the amiloride analogues 5-(N,N-hexamethylene)amiloride and 5-(N,N-dimethyl)amiloride inhibit, at micromolar concentrations, the replication of human immunodeficiency virus type 1 (HIV-1) in cultured human blood monocyte-derived macrophages. These compounds also inhibit the in vitro activities of the HIV-1 Vpu protein and might represent lead compounds for a new class of anti-HIV-1 drugs.

Cation-selective ion channels formed by p7 of hepatitis C virus are blocked by hexamethylene amiloride

A 63 residue peptide, p7, encoded by hepatitis C virus was synthesised and tested for ion channel activity in lipid bilayer membranes. Ion channels formed by p7 had a variable conductance: some channels had conductances as low as 14 pS. The reversal potential of currents flowing through the channels formed by p7 showed that they were permeable to potassium and sodium ions and less permeable to calcium ions. Addition of Ca(2+) to solutions made channels formed by p7 less potassium- or sodium-selective. Hexamethylene amiloride, a drug previously shown to block ion channels formed by Vpu encoded by HIV-1, blocked channels formed by p7. In view of the increasing number of peptides encoded by viruses that have been shown to form ion channels, it is suggested that ion channels may play an important role in the life cycle of many viruses and that drugs that block these channels may prove to be useful antiviral agents.

Interaction of amiloride and one of its derivatives with Vpu from HIV-1: a molecular dynamics simulation

Vpu is an 81-residue membrane protein, with a single transmembrane segment that is encoded by HIV-1 and is involved in the enhancement of virion release via formation of an ion channel. Cyclohexamethylene amiloride (Hma) has been shown to inhibit ion channel activity. In the present 12-ns simulation study a putative binding site of Hma blockers in a pentameric model bundle built of parallel aligned helices of the first 32 residues of Vpu was found near Ser-23. Hma orientates along the channel axis with its alkyl ring pointing inside the pore, which leads to a blockage of the pore.

Chemical synthesis and single channel properties of tetrameric and pentameric TASPs (template-assembled synthetic proteins) derived from the transmembrane domain of HIV virus protein u (Vpu)

Vpu, an 81-residue membrane protein encoded by the genome of HIV-1, is involved in CD4 degradation and facilitates virion budding from infected cells. The latter activity requires an intact transmembrane (TM) domain; however, the mechanism remains unclear. Vpu forms ion channels, an activity linked to the TM domain and envisioned to arise by oligomerization. The precise number of Vpu monomers that structure the channel is not yet known. To address this issue, we have synthesized tetrameric and pentameric proteins consisting of a carrier template to which four or five peptides corresponding to the TM domain of Vpu are attached. Ketoxime-forming chemoselective ligation efficiently ligated four and five copies, respectively, of the linear transmembrane peptide that was solubilized by the addition of a cleavable polyethylene glycol-polyamide auxiliary to a template. Purified tetrameric and pentameric proteins, denoted as T(4)Vpu and T(5)Vpu, exhibit the predicted mass as determined by MS analysis and fold with a high helical content as evidenced by CD. Both T(4)Vpu and T(5)Vpu, after reconstitution in lipid bilayers, form discrete ion channels of distinct conductance and high propensity to be open. The most frequent openings have a single channel conductance of 42 +/- 5 pS for T(4)Vpu and 76 +/- 5 pS for T(5)Vpu in 0.5m KCl. These findings validate the notion that the channels formed by Vpu result from the self-assembly of monomers. We conclude that a five-helix bundle of the TM of Vpu may approximate the structural motif underlying the oligomeric state of the conductive channel.

Vpu from HIV-1 on an atomic scale: experiments and computer simulations

Vpu is an 81 amino acid protein encoded by HIV-1. Its role is to amplify viral release by two mechanisms: (i) docking to CD4 with the consequence of targeting CD4 for ubiquitine-mediated degradation, and (ii) formation of ion channels to enhance particle release. The intensive research on its in vivo function, combined with structural investigations, makes this viral membrane protein one of the better characterised membrane proteins. The wealth of structural information enables the use of computational methods to elucidate the mechanisms of function on an atomic scale. The discovery of Vpu and the development of structural models in a chronological order is summarised and first efforts on investigating the mechanics are outlined.

Viroporins

Viroporins are a group of proteins that participate in several viral functions, including the promotion of release of viral particles from cells. These proteins also affect cellular functions, including the cell vesicle system, glycoprotein trafficking and membrane permeability. Viroporins are not essential for the replication of viruses, but their presence enhances virus growth. Comprising some 60-120 amino acids, viroporins have a hydrophobic transmembrane domain that interacts with and expands the lipid bilayer. Some viroporins also contain other motifs, such as basic amino acid residues or a domain rich in aromatic amino acids that confers on the protein the ability to interact with the interfacial lipid bilayer. Viroporin oligomerization gives rise to hydrophilic pores at the membranes of virus-infected cells. As the list of known viroporins steadily grows, recent research efforts focus on deciphering the actions of the viroporins poliovirus 2B, alphavirus 6K, HIV-1 Vpu and influenza virus M2. All these proteins can enhance the passage of ions and small molecules through membranes depending on their concentration gradient. Future work will lengthen the list of viroporins and will provide a deeper understanding of their mechanisms of action.