Inhibiting HCMV pUL89-C Endonuclease with Metal-Binding Compounds

Human cytomegalovirus (HCMV) infects individuals of all ages and establishes a lifelong latency. Current antiviral drugs are suboptimal in efficacy and safety and ineffective against resistant/refractory HCMV. Therefore, there is an unmet clinical need for efficacious, safe, and mechanistically novel HCMV drugs. The recent Food and Drug Administration (FDA) approval of letermovir (LTV) validated the HCMV terminase complex as a new target for antiviral development. LTV targets terminase subunit pUL56 but not the main endonuclease enzymatic function housed in the C terminus of subunit pUL89 (pUL89-C). Structurally and mechanistically, pUL89-C is an RNase H-like viral endonuclease entailing two divalent metal ions at the active site. In recent years, numerous studies have extensively explored pUL89-C inhibition using metal-chelating chemotypes, an approach previously used for inhibiting HIV ribonuclease H (RNase H) and integrase strand transfer (INST). Collectively, the work summarized herein validates the use of metal-binding scaffolds for designing potent and specific pUL89-C inhibitors.

Viral Nucleases

Nucleases are ubiquitous hydrolytic enzymes that cleave phosphodiester bond of DNA (DNases), RNA (RNases), or protein-RNA/DNA (phosphodiesterases), within the strand (endonucleases) or from the end (exonucleases) [...]

Repurposing N-hydroxy thienopyrimidine-2,4-diones (HtPD) as inhibitors of human cytomegalovirus pUL89 endonuclease: Synthesis and biological characterization

The terminase complex of human cytomegalovirus (HCMV) is required for viral genome packaging and cleavage. Critical to the terminase functions is a metal-dependent endonuclease at the C-terminus of pUL89 (pUL89-C). We have previously reported metal-chelating N-hydroxy thienopyrimidine-2,4-diones (HtPD) as inhibitors of human immunodeficiency virus 1 (HIV-1) RNase H. In the current work, we have synthesized new analogs and resynthesized known analogs of two isomeric HtPD subtypes, anti-HtPD (13), and syn-HtPD (14), and characterized them as inhibitors of pUL89-C. Remarkably, the vast majority of analogs strongly inhibited pUL89-C in the biochemical endonuclease assay, with IC50 values in the nM range. In the cell-based antiviral assay, a few analogs inhibited HCMV in low μM concentrations. Selected analogs were further characterized in a biophysical thermal shift assay (TSA) and in silico molecular docking, and the results support pUL89-C as the protein target of these inhibitors. Collectively, the biochemical, antiviral, biophysical, and in silico data reported herein indicate that the isomeric HtPD chemotypes 13-14 can serve as valuable chemical platforms for designing improved inhibitors of HCMV pUL89-C.

Zika Virus Inhibitors Based on a 1,3-Disubstituted 1H-Pyrazolo[3,4-d]pyrimidine-amine Scaffold

To search for Zika virus (ZIKV) antivirals, we have further explored previously reported 7H-pyrrolo[2,3-d]pyrimidines by examining an alternative substitution pattern of their central scaffold, leading to compound 5 with low micromolar antiviral activity. To circumvent the synthetic difficulties associated with compound 5, we have exploited a 1H-pyrazolo[3,4-d]pyrimidine scaffold and performed structure-activity relationship studies on its peripheral rings A and B. While ring B is less sensitive to structural modifications, an electron-withdrawing group at the para position of ring A is preferred for enhanced antiviral activity. Overall, we have not only discovered an alternative substitution pattern centered on a 1H-pyrazolo[3,4-d]pyrimidine scaffold but also generated anti-ZIKV compounds including 6 and 13, which possess low micromolar antiviral activity and relatively low cytotoxicity. These compounds represent new chemotypes that will be further optimized in our continued efforts to discover anti-ZIKV agents.

8-Hydroxy-1,6-naphthyridine-7-carboxamides as Inhibitors of Human Cytomegalovirus pUL89 Endonuclease

Human cytomegalovirus (HCMV) replication requires a metal-dependent endonuclease at the C-terminus of pUL89 (pUL89-C) for viral genome packaging and cleavage. We have previously shown that pUL89-C can be pharmacologically inhibited with designed metal-chelating compounds. We report herein the synthesis of a few 8-hydroxy-1,6-naphthyridine subtypes, including 5-chloro (subtype 15), 5-aryl (subtype 16), and 5-amino (subtype 17) variants. Analogs were studied for the inhibition of pUL89-C in a biochemical endonuclease assay, a biophysical thermal shift assay (TSA), in silico molecular docking, and for the antiviral potential against HCMV in cell-based assays. These studies identified eight analogs of 8-hydroxy-1,6-naphthyridine-7-carboxamide subtypes for further characterization, most of which inhibited pUL89-C with single-digit μM IC50 values, and conferred antiviral activity in μM range. TSA and molecular modeling of selected analogs corroborate their binding to pUL89-C. Collectively, our biochemical, antiviral, biophysical and in silico data suggest that 8-hydroxy-1,6-naphthyridine-7-carboxamide subtypes can be used for designing inhibitors of HCMV pUL89-C.

4,5-Dihydroxypyrimidine Methyl Carboxylates, Carboxylic Acids, and Carboxamides as Inhibitors of Human Cytomegalovirus pUL89 Endonuclease

Human cytomegalovirus (HCMV) terminase complex entails a metal-dependent endonuclease at the C-terminus of pUL89 (pUL89-C). We report herein the design, synthesis, and characterization of dihydroxypyrimidine (DHP) acid (14), methyl ester (13), and amide (15) subtypes as inhibitors of HCMV pUL89-C. All analogs synthesized were tested in an endonuclease assay and a thermal shift assay (TSA) and subjected to molecular docking to predict binding affinity. Although analogs inhibiting pUL89-C in the sub-μM range were identified from all three subtypes, acids (14) showed better overall potency, substantially larger thermal shift, and considerably better docking scores than esters (13) and amides (15). In the cell-based antiviral assay, six analogs inhibited HCMV with moderate activities (EC₅₀ = 14.4-22.8 μM). The acid subtype (14) showed good in vitro ADME properties, except for poor permeability. Overall, our data support the DHP acid subtype (14) as a valuable scaffold for developing antivirals targeting HCMV pUL89-C.

Discovery of N-benzyl hydroxypyridone carboxamides as a novel and potent antiviral chemotype against human cytomegalovirus (HCMV)

Current drugs for treating human cytomegalovirus (HCMV) infections are limited by resistance and treatment-associated toxicities. In developing mechanistically novel HCMV antivirals, we discovered an N-benzyl hydroxypyridone carboxamide antiviral hit (8a) inhibiting HCMV in submicromolar range. We describe herein the structure–activity relationship (SAR) for 8a, and the characterization of potent analogs for cytotoxicity/cytostatic property, the preliminary mechanism of action, and the absorption, distribution, metabolism and excretion (ADME) properties. The SAR revealed a few pharmacophore features conferring optimal antiviral profile, including the 5-OH, the N-1 benzyl, at least one –CH₂− in the linker, and a di-halogen substituted phenyl ring in the amide moiety. In the end, we identified numerous analogs with sub-micromolar antiviral potency and good selectivity index. The preliminary mechanism of action characterization used a pUL89-C biochemical endonuclease assay, a virus entry assay, a time-of-addition assay, and a compound withdrawal assay. ADME profiling measuring aqueous solubility, plasma and liver microsomal stability, and parallel artificial membrane permeability assay (PAMPA) permeability demonstrated largely favorable drug-like properties. Together, these studies validate the N-benzyl hydroxypyridone carboxamide as a viable chemotype for potent and mechanistically distinct antivirals against HCMV.

4,7-Disubstituted 7H-Pyrrolo[2,3-d]pyrimidines and Their Analogs as Antiviral Agents against Zika Virus

Discovery of compound 1 as a Zika virus (ZIKV) inhibitor has prompted us to investigate its 7H-pyrrolo[2,3-d]pyrimidine scaffold, revealing structural features that elicit antiviral activity. Furthermore, we have demonstrated that 9H-purine or 1H-pyrazolo[3,4-d]pyrimidine can serve as an alternative core structure. Overall, we have identified 4,7-disubstituted 7H-pyrrolo[2,3-d]pyrimidines and their analogs including compounds 1, 8 and 11 as promising antiviral agents against flaviviruses ZIKV and dengue virus (DENV). While the molecular target of these compounds is yet to be elucidated, 4,7-disubstituted 7H-pyrrolo[2,3-d]pyrimidines and their analogs are new chemotypes in the design of small molecules against flaviviruses, an important group of human pathogens.

Broad-Spectrum Antiviral Strategies and Nucleoside Analogues

The emergence or re-emergence of viruses with epidemic and/or pandemic potential, such as Ebola, Zika, Middle East Respiratory Syndrome (MERS-CoV), Severe Acute Respiratory Syndrome Coronavirus 1 and 2 (SARS and SARS-CoV-2) viruses, or new strains of influenza represents significant human health threats due to the absence of available treatments. Vaccines represent a key answer to control these viruses. However, in the case of a public health emergency, vaccine development, safety, and partial efficacy concerns may hinder their prompt deployment. Thus, developing broad-spectrum antiviral molecules for a fast response is essential to face an outbreak crisis as well as for bioweapon countermeasures. So far, broad-spectrum antivirals include two main categories: the family of drugs targeting the host-cell machinery essential for virus infection and replication, and the family of drugs directly targeting viruses. Among the molecules directly targeting viruses, nucleoside analogues form an essential class of broad-spectrum antiviral drugs. In this review, we will discuss the interest for broad-spectrum antiviral strategies and their limitations, with an emphasis on virus-targeted, broad-spectrum, antiviral nucleoside analogues and their mechanisms of action.

Anti-HCMV activity by an irreversible p97 inhibitor LC-1310

The AAA+ (ATPase associated with various cellular activities) protein p97, also called valosin-containing protein, is a hexameric ring ATPase and uses ATP hydrolysis to unfold or extract proteins from biological complexes. Many cellular processes are affected by p97 including ER-associated degradation, DNA damage response, cell signaling (NF-κB), cell cycle progression, autophagy, and others. Not surprisingly, with its role in many fundamental cellular processes, p97 function is important for the replication of many viruses. We tested irreversible p97-targeting compounds for their ability to inhibit the replication of multiple viruses compared to the known p97 inhibitors NMS-873 and CB-5083. Our results indicate that overall cellular toxicity for p97 compounds provides a challenge for antivirals targeting p97. However, we identified one compound with sub-micromolar activity against human cytomegalovirus and improved cell viability to provide evidence for the potential of irreversible p97 inhibitors as antivirals.

One-Pot Synthesis of 1-Hydroxyacridones from para-Quinols and ortho-Methoxycarbonylaryl Isocyanates

A variety of substituted acridones were synthesized via a one-pot, metal-free cascade reaction. In this event, the DBU-mediated addition between quinols and ortho-methoxycarbonylaryl isocyanates formed a bicyclic oxazolidinone, followed by a sequence of intramolecular condensation, tautomerization, and decarboxylation, which led to the formation of acridones. The acridones showed mild activity against the human cytomegalovirus.

Design and synthesis of an activity-based protein profiling probe derived from cinnamic hydroxamic acid

In our continued effort to discover new anti-hepatitis C virus (HCV) agents, we validated the anti-replicon activity of compound 1, a potent and selective anti-HCV hydroxamic acid recently reported by us. Generally favorable physicochemical and in vitro absorption, distribution, metabolism, and excretion (ADME) properties exhibited by 1 made it an ideal parent compound from which activity-based protein profiling (ABPP) probe 3 was designed and synthesized. Evaluation of probe 3 revealed that it possessed necessary anti-HCV activity and selectivity. Therefore, we have successfully obtained compound 3 as a suitable ABPP probe to identify potential molecular targets of compound 1. Probe 3 and its improved analogs are expected to join a growing list of ABPP probes that have made important contributions to not only the studies of biochemical and cellular functions but also discovery of selective inhibitors of protein targets.

5'-Silylated 3'-1,2,3-triazolyl Thymidine Analogues as Inhibitors of West Nile Virus and Dengue Virus

West Nile virus (WNV) and Dengue virus (DENV) are important human pathogens for which there are presently no vaccine or specific antivirals. We report herein a 5'-silylated nucleoside scaffold derived from 3'-azidothymidine (AZT) consistently and selectively inhibiting WNV and DENV at low micromolar concentrations. Further synthesis of various triazole bioisosteres demonstrated clear structure-activity relationships (SARs) in which the antiviral activity against WNV and DENV hinges largely on both the 5'-silyl group and the substituent of 3'-triazole or its bioisosteres. Particularly interesting is the 5' silyl group which turns on the antiviral activity against WNV and DENV while abrogating the previously reported antiviral potency against human immunodeficiency virus (HIV-1). The antiviral activity was confirmed through a plaque assay where viral titer reduction was observed in the presence of selected compounds. Molecular modeling and competitive S-adenosyl-l-methionine (SAM) binding assay suggest that these compounds likely confer antiviral activity via binding to methyltransferase (MTase).

Guidelines for the use of cell lines in biomedical research

Cell-line misidentification and contamination with microorganisms, such as mycoplasma, together with instability, both genetic and phenotypic, are among the problems that continue to affect cell culture. Many of these problems are avoidable with the necessary foresight, and these Guidelines have been prepared to provide those new to the field and others engaged in teaching and instruction with the information necessary to increase their awareness of the problems and to enable them to deal with them effectively. The Guidelines cover areas such as development, acquisition, authentication, cryopreservation, transfer of cell lines between laboratories, microbial contamination, characterisation, instability and misidentification. Advice is also given on complying with current legal and ethical requirements when deriving cell lines from human and animal tissues, the selection and maintenance of equipment and how to deal with problems that may arise.

Fluorescent peptide sensors for tyrosylprotein sulfotransferase activity

Tyrosine sulfurylation is a post-translational modification important for protein-protein interactions in the extracellular space that are instrumental in cell adhesion, cell signaling, immune responses, and pathogen recognition of host cells. Tyrosine sulfurylation is catalyzed by the tyrosylprotein sulfotransferases (TPSTs), and in humans there are two isoforms: hTPST1 and hTPST2. The study of hTPST function and the development of small molecule probes to examine the role of hTPSTs in cell biology have been delayed by the absence of a continuous direct assay for hTPST activity. We have developed a fluorescent peptide-based assay to directly monitor tyrosine sulfurylation in real time. TPST-mediated tyrosine sulfurylation of the peptides disrupts fluorophore quenching and results in increased fluorescence emission. The assay can be used to study TPST enzymatic activity, and we show that recombinant hTPSTs are active in the absence of divalent metal ions and that optimal activity is at pH 6.0. We further show that the assay can also be used to identify inhibitors of tyrosine sulfurylation. A clear understanding of hTPST function in normal cell biology and in disease states will require the identification of small molecule inhibitors or probes to modulate enzymatic activity, and our results will facilitate that process.

Synthesis and antiviral evaluation of 4′-(1,2,3-triazol-1-yl)thymidines

The Cu(i)-catalyzed azide–alkyne cycloaddition (CuAAC) of 4′-azidothymidine (5) generated a series of 1,2,3-triazole analogues (9) with moderate anti-HIV activities, while a similar cycloaddition reaction catalyzed by Ru(ii) failed.

Mechanisms of host receptor adaptation by severe acute respiratory syndrome coronavirus

The severe acute respiratory syndrome coronavirus (SARS-CoV) from palm civets has twice evolved the capacity to infect humans by gaining binding affinity for human receptor angiotensin-converting enzyme 2 (ACE2). Numerous mutations have been identified in the receptor-binding domain (RBD) of different SARS-CoV strains isolated from humans or civets. Why these mutations were naturally selected or how SARS-CoV evolved to adapt to different host receptors has been poorly understood, presenting evolutionary and epidemic conundrums. In this study, we investigated the impact of these mutations on receptor recognition, an important determinant of SARS-CoV infection and pathogenesis. Using a combination of biochemical, functional, and crystallographic approaches, we elucidated the molecular and structural mechanisms of each of these naturally selected RBD mutations. These mutations either strengthen favorable interactions or reduce unfavorable interactions with two virus-binding hot spots on ACE2, and by doing so, they enhance viral interactions with either human (hACE2) or civet (cACE2) ACE2. Therefore, these mutations were viral adaptations to either hACE2 or cACE2. To corroborate the above analysis, we designed and characterized two optimized RBDs. The human-optimized RBD contains all of the hACE2-adapted residues (Phe-442, Phe-472, Asn-479, Asp-480, and Thr-487) and possesses exceptionally high affinity for hACE2 but relative low affinity for cACE2. The civet-optimized RBD contains all of the cACE2-adapted residues (Tyr-442, Pro-472, Arg-479, Gly-480, and Thr-487) and possesses exceptionally high affinity for cACE2 and also substantial affinity for hACE2. These results not only illustrate the detailed mechanisms of host receptor adaptation by SARS-CoV but also provide a molecular and structural basis for tracking future SARS-CoV evolution in animals.

The design, synthesis and biological evaluations of C-6 or C-7 substituted 2-hydroxyisoquinoline-1,3-diones as inhibitors of hepatitis C virus

C7-Substituted 2-hydroxyisoquinoline-1,3-diones inhibit the strand transfer of HIV integrase (IN) and the reverse-transcriptase-associated ribonuclease H (RNH). Hepatitis C virus (HCV) NS5B polymerase shares a similar active site fold to RNH and IN, suggesting that N-hydroxyimides could be useful inhibitor scaffolds of HCV via targeting the NS5B. Herein we describe the design, chemical synthesis, replicon and biochemical assays, and molecular docking of C-6 or C-7 aryl substituted 2-hydroxyisoquinoline-1,3-diones as novel HCV inhibitors. The synthesis involved an improved and clean cyclization method, which allowed the convenient preparation of various analogs. Biological studies revealed that the C-6 analogs, a previously unknown chemotype, consistently inhibit both HCV replicon and recombinant NS5B at low micromolar range. Molecular modeling studies suggest that these inhibitors may bind to the NS5B active site.

Mutagenic analysis of herpes simplex virus type 1 glycoprotein L reveals the importance of an arginine-rich region for function

Herpes simplex virus type 1 (HSV-1) glycoproteins H and L (gH and gL) are required for virus-induced membrane fusion. Expression of gH at the virion or infected cell surface is mediated by the chaperone-like activity of gL. We have previously shown that a region between amino acids 155 and 161 is critical for gL chaperone-like activity. Here, we conducted Ala substitution mutagenesis of residues in this region and found that substitution of Cys160, Arg156, Arg158, or Arg156/158/159 with Ala resulted in a gL mutant that bound gH but displayed a reduced ability in gH trafficking and membrane fusion. Substitution of Arg156 with another positively charged amino acid, Lys, restored function. Substitution of Arg158 with Lys restored function in gH trafficking and cell fusion but not virus entry. These results indicate that an arginine-rich region of gL is critical for function.

Herpes simplex virus type 1 mediates fusion through a hemifusion intermediate by sequential activity of glycoproteins D, H, L, and B

Virus-induced membrane fusion can be subdivided into three phases defined by studies of class I and class II fusion proteins. During Phase I, two membranes are brought into close apposition. Phase II marks the mixing of the outer membrane leaflets leading to formation of a hemifusion intermediate. A fusion pore stably forms and expands in Phase III, thereby completing the fusion process. Herpes simplex virus type 1 (HSV-1) requires four glycoproteins to complete membrane fusion, but none has been defined as class I or II. Therefore, we investigated whether HSV-1-induced membrane fusion occurred following the same general phases as those described for class I and II proteins. In this study we demonstrate that glycoprotein D (gD) and the glycoprotein H and glycoprotein L complex (gHL) mediated lipid mixing indicative of hemifusion. However, content mixing and full fusion required glycoprotein B (gB) to be present along with gD and gHL. Our results indicate that, like class I and II fusion proteins, fusion mediated by HSV-1 glycoproteins occurred through a hemifusion intermediate. In addition, both gB and gHL are probably directly involved in the fusion process. From this, we propose a sequential model for fusion via HSV-1 glycoproteins whereby gD is required for Phase I, gHL is required for Phase II, and gB is required for Phase III. We further propose that glycoprotein H and gB are likely to function sequentially to promote membrane fusion in other herpesviruses such as Epstein-Barr virus and human herpesvirus 8.

The requirements for herpes simplex virus type 1 cell-cell spread via nectin-1 parallel those for virus entry

Herpes simplex virus type 1 (HSV-1) spreads from an infected cell to an uninfected cell by virus entry, virus-induced cell fusion, and cell-cell spread. The three forms of virus spread require the viral proteins gB, gD, and gH-gL, as well as a cellular gD receptor. The mutual requirement for the fusion glycoproteins and gD receptor suggests that virus entry, cell fusion, and cell-cell spread occur by a similar mechanism. The goals of this study were to examine the role of the nectin-1alpha transmembrane domain and cytoplasmic tail in cell-cell spread and to obtain a better understanding of the receptor-dependent events occurring at the plasma membrane during cell-cell spread. We determined that an intact nectin-1alpha V-like domain was required for cell-cell spread, while a membrane-spanning domain and cytoplasmic tail were not. Chimeric forms of nectin-1 that were non-functional for virus entry did not mediate cell-cell spread regardless of whether they could mediate cell fusion. Also, cell-cell spread of syncytial isolates was dependent upon nectin-1alpha expression and occurred through a nectin-1-dependent mechanism. Taken together, our results indicate that nectin-1-dependent events occurring at the plasma membrane during cell-cell spread were equivalent to those for virus entry.

Herpes simplex virus type 1 glycoprotein L mutants that fail to promote trafficking of glycoprotein H and fail to function in fusion can induce binding of glycoprotein L-dependent anti-glycoprotein H antibodies

The herpes simplex virus type 1 (HSV-1) glycoproteins H (gH) and L (gL) form a heterodimer and efficient expression of gH at the virion or cell surface is dependent upon gL. Five carboxy-terminal deletion mutants of gL were created and their ability to interact with and mediate cell-surface expression of gH, to promote binding of gL-dependent anti-gH antibodies and to contribute to cell fusion was analysed. All of the gL mutants bound gH, but only two mutants, containing the amino-terminal 161 or 168 aa of gL, mediated cell-surface expression of gH, and only gL161 and gL168 functioned in cell fusion. The binding of gL to gH, therefore, was not sufficient to ensure gH cell-surface expression and it was not possible to separate the gH-trafficking role of gL from gL function in fusion. Co-expression of gH with any gL mutant conferred binding of the anti-gH mAbs 53S and LP11. If the acquisition of 53S and LP11 binding to gH reflects a gL-induced conformational change, such a change is not sufficient to mediate trafficking of the gH-gL heterodimer.

The nectin-1alpha transmembrane domain, but not the cytoplasmic tail, influences cell fusion induced by HSV-1 glycoproteins

Nectin-1 is a receptor for herpes simplex virus (HSV), a member of the immunoglobulin superfamily, and a cellular adhesion molecule. To study domains of nectin-1alpha involved in cell fusion, we measured the ability of nectin-1alpha/nectin-2alpha chimeras, nectin-1alpha/CD4 chimeras, and transmembrane domain and cytoplasmic tail mutants of nectin-1alpha to promote cell fusion induced by HSV-1 glycoproteins. Our results demonstrate that only chimeras and mutants containing the entire V-like domain and a link to the plasma membrane conferred cell-fusion activity. The transmembrane domain and cytoplasmic tail of nectin-1 were not required for any viral receptor or cell adhesion function tested. Cellular cytoplasmic factors that bind to the nectin-1alpha cytoplasmic tail, therefore, did not influence virus entry or cell fusion. Interestingly, the efficiency of cell fusion was reduced when membrane-spanning domains of nectin-1alpha and gD were replaced by glycosylphosphatidylinositol tethers, indicating that transmembrane domains may play a modulatory role in the gD/nectin-1alpha interaction in fusion.

Evidence that use of an inactivated equine herpesvirus vaccine induces serum cytotoxicity affecting the equine arteritis virus neutralisation test

Several laboratories worldwide have recently experienced problems related to serum cytotoxicity with the equine arteritis virus (EAV) neutralisation test (VN) when using Office International des Epizooties (OIE) reference laboratory prescribed rabbit kidney (RK-13) indicator cells. Cytotoxicity can be mistaken for viral cytopathic effect and has led to increasing difficulties in test interpretation, consequently causing disruption to both equine breeding and disease surveillance. Results from experimental and field-derived data suggest that this serum cytotoxicity is associated with use of a tissue-culture-derived equine herpesvirus vaccine, probably manifested through a vaccine-induced anti-cellular antibody response directed against RK-13 cells. Two alternative EAV VN methods were shown to significantly reduce the effects of cytotoxicity (from 73 to <5% prevalence) among vaccinated horses but did not completely eliminate the problem. Use of ELISA-based tests, which are not affected by serum cytotoxicity but which are not currently recognised as international standards, should be evaluated as a useful backup in screening equine sera for EAV VN antibodies.

Fusion activity of lipid-anchored envelope glycoproteins of herpes simplex virus type 1

Expression of the herpes simplex virus type 1 (HSV-1) glycoproteins gB, gD, gH, and gL is necessary and sufficient to cause cell fusion. To identify the requirements for a membrane-spanning domain in HSV-1 glycoprotein-induced cell fusion, we created gB, gD, and gH mutants with transmembrane and cytoplasmic domains replaced by a glycosylphosphatidylinositol (gpi)-addition sequence. The corresponding gBgpi, gDgpi, and gHgpi proteins were expressed with wild-type efficiency at the cell surface and were linked to the plasma membrane via a gpi anchor. The gDgpi mutant promoted cell fusion near wild-type gD levels when co-expressed with gB, gH, and gL in a cell-mixing fusion assay, indicating that the gD transmembrane and cytoplasmic domains were not required for fusion activity. A plasma membrane link was required for fusion because a gD mutant lacking a transmembrane and cytoplasmic domain was nonfunctional for fusion. The gDgpi mutant was also able to cooperate with wild-type gB, gH, and gL to form syncytia, albeit at a size smaller than those formed in the wild-type situation. The gBgpi and gHgpi mutants were unable to promote fusion when expressed with the other wild-type viral glycoproteins, highlighting the requirement of the specific transmembrane and cytoplasmic domains for gB and gH function.

Use of chimeric nectin-1(HveC)-related receptors to demonstrate that ability to bind alphaherpesvirus gD is not necessarily sufficient for viral entry

Human nectin-1 (HveC, Prr1), a member of the immunoglobulin superfamily and a receptor for the entry of herpes simplex viruses 1 and 2 (HSV-1, HSV-2), pseudorabies virus (PRV), and bovine herpesvirus 1 (BHV-1), binds to viral gD. For HSV-1, HSV-2, and PRV, the gD-binding region of nectin-1 has been localized to the N-terminal V-like domain. To determine whether the two C-like domains of nectin-1 influenced gD binding and entry activity, genes encoding chimeric proteins were constructed. Portions of nectin-1 were replaced with homologous regions from nectin-2 (HveB, Prr2), a related protein with ability to mediate the entry of PRV, HSV-2, and Rid mutants of HSV-1, but not HSV-1 or BHV-1. Also, one or more domains of nectin-1 were fused to the two membrane-proximal Ig domains of CD4, a protein with no herpesvirus entry or gD-binding activity. The chimeric proteins were expressed in Chinese hamster ovary cells, which normally lack alphaherpesvirus entry receptors, and detected on the cell surface by one or more anti-nectin-1 monoclonal antibodies. One chimeric protein (nectin-1 amino acids 1-124 fused to CD4) failed to bind to soluble forms of HSV-1, HSV-2, PRV, and BHV-1 gD and, as expected, also failed to mediate entry of the viruses from which these gDs were derived. The other chimeric receptors bound all forms of gD. Some mediated the entry of all the viruses tested but others mediated entry of some but not all the viruses. We conclude that binding of gD to the nectin-1 V domain is not sufficient for entry activity, that there are structural requirements for entry activity independent of gD binding, and that these requirements are different for the several alphaherpesviruses that can use nectin-1 as a receptor.

Contributions of gD receptors and glycosaminoglycan sulfation to cell fusion mediated by herpes simplex virus 1

Two cell surface proteins (nectin-1/HveC and nectin-2/HveB) shown previously to serve as receptors for the entry of herpes simplex virus 1 (HSV-1) wild-type and/or mutant strains were found to serve also as receptors for HSV-1-induced cell fusion. Transfection with genomic DNA from a syncytial HSV-1 strain encoding wild-type gD resulted in fusion of Chinese hamster ovary (CHO) cells expressing nectin-1 but not of cells expressing nectin-2. In contrast, transfection with DNA from a related HSV-1 strain encoding the mutant Rid1 form of gD resulted in fusion of CHO cells expressing either receptor but not of control cells. These results are consistent with the ability of each receptor to mediate entry of viruses expressing wild-type or Rid1 gD and with results obtained previously with HVEM (HveA), a third HSV-l entry receptor. Undersulfation of GAGs in receptor-expressing cell lines predictably reduced susceptibility to HSV-l infection. In contrast, susceptibility to cell fusion mediated by HVEM or nectin-1 was not reduced. Undersulfation of GAGs partially inhibited cell fusion mediated by nectin-2. We conclude that HSV-1-induced cell fusion requires a gD-binding entry receptor, that ability of an HSV-1 strain to use HVEM, nectin-2 or nectin-1 for cell fusion depends on the allele of gD expressed and that GAGs may influence cell fusion, dependent on the gD-binding receptor used, but are less important for cell fusion mediated by HVEM, nectin-2 or nectin-l than for viral entry.

Organization of the rat Tage4 gene and herpesvirus entry activity of the encoded protein

The Tage4 gene (Tumor-Associated Glycoprotein E4) is a member of the immunoglobulin superfamily overexpressed in rat colon tumors and Min mouse intestinal adenomas. The Tage4 cDNA presents approximately 60% identity with the human CD155, a member of the immunoglobulin superfamily coding for a transmembrane protein capable of serving as an entry receptor for poliovirus, porcine pseudorabies virus and bovine herpesvirus 1. We determined the structure of the Tage4 gene. This gene covers approximately 15 kb and is composed of eight exons and seven introns. We also isolated approximately 2 kb of the 5' flanking region of the Tage4 gene and demonstrated the existence of closely clustered transcription start sites. No splicing variant was identified by RT-PCR indicating that the Tage4 gene is transcribed as a unique mRNA. Finally, the protein encoded by the Tage4 gene was tested for ability to mediate entry of several viruses. These structural and functional features of the rat Tage4 gene were compared to those of the human CD155 gene. The results indicated that the Tage4 gene is probably orthologous to the gene for CD155.

Cellular expression of alphaherpesvirus gD interferes with entry of homologous and heterologous alphaherpesviruses by blocking access to a shared gD receptor

Several human and animal alphaherpesviruses can enter cells via human herpesvirus entry mediator C (HveC), a receptor for viral glycoprotein D (gD). In previous studies with cells expressing unknown entry mediators, cellular expression of alphaherpesvirus gD was shown to inhibit entry of the homologous virus and sometimes also of heterologous alphaherpesviruses. To investigate the mechanism of gD-mediated interference and the basis for cross-interference among alphaherpesviruses, HveC was expressed in cells as the sole entry mediator, in the presence or absence of one of the gDs encoded by herpes simplex virus type 1, pseudorabies virus, or bovine herpesvirus type 1. Cells expressing HveC alone were highly susceptible to entry of all three viruses, whereas cells coexpressing HveC and any one of the gDs were at least partially resistant to infection by each virus. Coexpression of gD with HveC did not cause reduced levels of cell-surface HveC but the HveC had reduced ability to bind to exogenous gD. Coimmunoprecipitation experiments revealed that HveC was complexed with gD in lysates of cells expressing both. Thus, cellular expression of gD can interfere with alphaherpesvirus entry by blocking ligand-binding sites of the gD receptor(s) used for entry and cross-interference can occur because different forms of alphaherpesvirus gD can compete for shared entry receptors.

The first immunoglobulin-like domain of HveC is sufficient to bind herpes simplex virus gD with full affinity, while the third domain is involved in oligomerization of HveC

The human herpesvirus entry mediator C (HveC/PRR1) is a member of the immunoglobulin family used as a cellular receptor by the alphaherpesviruses herpes simplex virus (HSV), pseudorabies virus, and bovine herpesvirus type 1. We previously demonstrated direct binding of the purified HveC ectodomain to purified HSV type 1 (HSV-1) and HSV-2 glycoprotein D (gD). Here, using a baculovirus expression system, we constructed and purified truncated forms of the receptor containing one [HveC(143t)], two [HveC(245t)], or all three immunoglobulin-like domains [HveC(346t)] of the extracellular region. All three constructs were equally able to compete with HveC(346t) for gD binding. The variable domain bound to virions and blocked HSV infection as well as HveC(346t). Thus, all of the binding to the receptor occurs within the first immunoglobulin-like domain, or V-domain, of HveC. These data confirm and extend those of Cocchi et al. (F. Cocchi, M. Lopez, L. Menotti, M. Aoubala, P. Dubreuil, and G. Campadelli-Fiume, Proc. Natl. Acad. Sci. USA 95:15700, 1998). Using biosensor analysis, we measured the affinity of binding of gD from HSV strains KOS and rid1 to two forms of HveC. Soluble gDs from the KOS strain of HSV-1 had the same affinity for HveC(346t) and HveC(143t). The mutant gD(rid1t) had an increased affinity for HveC(346t) and HveC(143t) due to a faster rate of complex formation. Interestingly, we found that HveC(346t) was a tetramer in solution, whereas HveC(143t) and HveC(245t) formed dimers, suggesting a role for the third immunoglobulin-like domain of HveC in oligomerization. In addition, the stoichiometry between gD and HveC appeared to be influenced by the level of HveC oligomerization.

Herpes simplex virus glycoprotein D can bind to poliovirus receptor-related protein 1 or herpesvirus entry mediator, two structurally unrelated mediators of virus entry

Several cell membrane proteins have been identified as herpes simplex virus (HSV) entry mediators (Hve). HveA (formerly HVEM) is a member of the tumor necrosis factor receptor family, whereas the poliovirus receptor-related proteins 1 and 2 (PRR1 and PRR2, renamed HveC and HveB) belong to the immunoglobulin superfamily. Here we show that a truncated form of HveC directly binds to HSV glycoprotein D (gD) in solution and at the surface of virions. This interaction is dependent on the native conformation of gD but independent of its N-linked glycosylation. Complex formation between soluble gD and HveC appears to involve one or two gD molecules for one HveC protein. Since HveA also mediates HSV entry by interacting with gD, we compared both structurally unrelated receptors for their binding to gD. Analyses of several gD variants indicated that structure and accessibility of the N-terminal domain of gD, essential for HveA binding, was not necessary for HveC interaction. Mutations in functional regions II, III, and IV of gD had similar effects on binding to either HveC or HveA. Competition assays with neutralizing anti-gD monoclonal antibodies (MAbs) showed that MAbs from group Ib prevented HveC and HveA binding to virions. However, group Ia MAbs blocked HveC but not HveA binding, and conversely, group VII MAbs blocked HveA but not HveC binding. Thus, we propose that HSV entry can be mediated by two structurally unrelated gD receptors through related but not identical binding with gD.

A cell surface protein with herpesvirus entry activity (HveB) confers susceptibility to infection by mutants of herpes simplex virus type 1, herpes simplex virus type 2, and pseudorabies virus

Certain mutant strains of herpes simplex virus type 1 (HSV-1) are unable to infect cells in which entry is dependent on HVEM, the previously described herpesvirus entry mediator designated here as herpesvirus entry protein A (HveA). These mutant viruses can infect other cells where entry is apparently dependent on other co-receptors. The mutant virus HSV-1(KOS)Rid1 was used to screen a human cDNA expression library for ability of transfected plasmids to convert resistant Chinese hamster ovary cells to susceptibility to virus entry. A plasmid expressing the previously described poliovirus receptor-related protein 2 (Prr2) was isolated on the basis of this activity. This protein, designated here as HveB, was shown to mediate the entry of three mutant HSV-1 strains that cannot use HVEM as co-receptor, but not wild-type HSV-1 strains. HveB also mediated the entry of HSV-2 and pseudorabies virus but not bovine herpesvirus type 1. HveB was expressed in some human neuronal cell lines, fibroblastic cells, keratinocytes, and primary activated T lymphocytes. Antibodies specific for HveB blocked infection of HveB-expressing CHO cells and a human fibroblastic cell strain HEL299. Differences in ability of HSV-1 and HSV-2 strains to use HveB for entry should influence the types of cells that can be infected and thereby account in part for serotype and strain differences in tissue tropism and pathogenicity.

Entry of alphaherpesviruses mediated by poliovirus receptor-related protein 1 and poliovirus receptor

A human member of the immunoglobulin superfamily was shown to mediate entry of several alphaherpesviruses, including herpes simplex viruses (HSV) 1 and 2, porcine pseudorabies virus (PRV), and bovine herpesvirus 1 (BHV-1). This membrane glycoprotein is poliovirus receptor-related protein 1 (Prr1), designated here as HveC. Incubation of HSV-1 with a secreted form of HveC inhibited subsequent infection of a variety of cell lines, suggesting that HveC interacts directly with the virus. Poliovirus receptor (Pvr) itself mediated entry of PRV and BHV-1 but not of the HSV strains tested. HveC was expressed in human cells of epithelial and neuronal origin; it is the prime candidate for the coreceptor that allows both HSV-1 and HSV-2 to infect epithelial cells on mucosal surfaces and spread to cells of the nervous system.

HIV-1 particle release mediated by Vpu is distinct from that mediated by p6

Vpu and the C-terminal peptide of Gag (p6) are both HIV-1-encoded proteins that augment the release of virus particles from cells. We examined the functional relationship between these proteins and their activities during particle release. Our results indicate that efficient HIV-1 particle release from HeLa and Jurkat cells depends on the presence of Vpu. However, Vpu is dispensable for efficient release from Cos cells. In contrast, p6 is required for efficient release from Cos cells but not from Jurkat or HeLa cells. These data suggest that Vpu and p6 have distinct activities in virus exit from different cell lines. Intracellular proteolytic processing of Gag precursor protein is more complete in Cos cells than in HeLa cells. However, this processing has little or no effect on Vpu- or p6-mediated particle release. p6 is required for incorporation of yet another virus protein (Vpr) into cells but our data suggest that Vpr plays no role in p6-dependent particle release. Vpu also facilitates the degradation of CD4 in virus producing cells but, in contrast to particle release, the ability of Vpu to facilitate the degradation of CD4 is not cell line-dependent.

Cell type-dependence for Vpu function

Human immunodeficiency virus type 1 Vpu has been shown to facilitate virus release from HeLa cells. We demonstrated that Vpu expression is not required for efficient virus release from Cos 1 and CV-1 cells. A yeast GAL4 transcriptional activation system was used to screen for cellular proteins that may interact with Vpu. One such protein was identified which we provisionally designate "Vpu interactive protein" or VIP.

Human immunodeficiency virus type 1 Vpu has a CD4- and an envelope glycoprotein-independent function

Vpu is a 16-kDa membrane-associated phosphoprotein that is expressed from the same, singly spliced message as the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein precursor, gp160. Previous studies suggest that Vpu functions in the late stages of viral replication, possibly in virus egression from the cell. Recently, it has been demonstrated that Vpu functions to allow gp160 to be more efficiently processed by disrupting CD4-gp160 complexes generated by transfection of HeLa cells. We show here that the lack of expression of intact Vpu results in a 90% reduction in infectious virus produced over a single round of replication from HeLa cells in the absence of CD4 expression. This reduction persists when HIV-1 particles are pseudotyped with the HIV-2 or amphotropic murine leukemia virus envelope glycoprotein. Pulse-chase analysis of HIV-1 capsid protein (p24) in the absence of CD4 and envelope glycoprotein demonstrates that the rate of virus release is reduced when Vpu is not expressed. Our findings indicate that Vpu has a function involving particle release not dependent on CD4 or envelope glycoprotein expression.

The successful medical management of gastric outflow obstruction associated with the use of non-steroidal anti-inflammatory drugs in the elderly

One hundred and nineteen consecutive elderly patients with endoscopically diagnosed peptic ulceration were reviewed. Associated gastric outflow obstruction was present in 10.1%. The presenting clinical features differed significantly from typical younger patients and most (11/12) were taking non-steroidal anti-inflammatory drugs, suggesting a possible role for these agents in the pathogenesis of gastric outflow obstruction. These elderly patients have been successfully managed by medical therapy alone.

Autoimmune haemolytic anaemia associated with malignant peritoneal mesothelioma

A 57-year-old man with a history of pulmonary asbestosis was incidentally found to have benign mesothelial hyperplasia of the peritoneum at hernia repair. Five months later he developed a Coombs positive haemolytic anaemia of the IgG-C3d type caused by non specific IgG antibodies. At that time no underlying cause for the anaemia was found. The anaemia responded to steroids, but remained steroid dependent. Six months after the diagnosis of the anaemia, a malignant peritoneal mesothelioma was found at laparotomy. The association between malignant mesothelioma and autoimmune haemolytic anaemia has been reported on one previous occasion. The description of a second case suggests that the association is not purely coincidental and that malignant mesothelioma should be added to the list of solid tumours that can be associated with autoimmune haemolytic anaemia. The finding of red blood cells coated with IgG and C3d in this as well as in other cases adds further evidence to the hypothesis that a quinidine type mechanism of haemolysis might be responsible for Coombs positive haemolytic anaemia associated with solid tumours.