Loss of cytomegalovirus infectivity after treatment with castanospermine or related plant alkaloids correlates with aberrant glycoprotein synthesis

Characterization of pseudotyped vesicular stomatitis virus bearing the heartland virus envelope glycoprotein

The heartland virus (HRTV) is a novel phlebovirus that causes severe infections in the USA and closely related to the severe fever thrombocytopenia syndrome virus (SFTSV), a causative agent for SFTS in Asia. The entry mechanisms of HRTV remain unclear. Here, we developed the pseudotyped vesicular stomatitis virus bearing the HRTV glycoprotein (GP) (HRTVpv), and the antigenicity and the entry mechanisms of HRTV were analyzed. HRTVpv was neutralized by anti-SFTSV Gc antibody, but not the anti-SFTSV Gn antibodies. Entry of HRTVpv to cells was inhibited by bafilomycin A1 and dynasore, and but it was enhanced in cells overexpressed with C-type lectins. Production of infectious HRTVpv and SFTSVpv was reduced by Nn-DNJ, α-glucosidase inhibitor. The entry of HRTV occurs via pH- and dynamin-dependent endocytosis. Furthermore, Nn-DNJ may be a possible therapeutic agent against HRTV and SFTSV.

Iminosugars With Endoplasmic Reticulum α-Glucosidase Inhibitor Activity Inhibit ZIKV Replication and Reverse Cytopathogenicity in vitro

Zika virus (ZIKV), a vector-borne virus of the family Flaviviridae, continues to spread and remains a significant global public health threat. Currently, there are no approved vaccines or antivirals against ZIKV. We investigated the anti-ZIKV ability of three iminosugars with endoplasmic reticulum α-glucosidase inhibitor (ER-AGI) activity, namely deoxynojirimycin (DNJ), castanospermine, and celgosivir. None of the three iminosugars showed any significant cytotoxicity in Vero or human microglia CHME3 cells when applied for 72 h at concentrations up to 100 μM. Iminosugar treatment of Vero or CHME3 cells prior to ZIKV infection resulted in significant inhibition of ZIKV replication over 48 h. Reduction in ZIKV replication in iminosugar-treated cells was not associated with any significant change in the expression levels of key antiviral genes. Following infection with three different strains of ZIKV, iminosugar-treated Vero or CHME3 cells showed no cell death, whereas vehicle-treated control cells exhibited 50–60% cell death at 72 h post-infection (hpi). While there was no significant difference in apoptosis between iminosugar-treated and control cells, iminosugar-treated cells exhibited a substantial reduction of necrosis at 72 hpi following ZIKV infection. In summary, iminosugars with ER-AGI activity inhibit ZIKV replication and significantly reduce necrosis without altering the antiviral gene expression and apoptosis of infected human cells. The results of this study strongly suggest that iminosugars are promising anti-ZIKV antiviral agents and such warrant further in vivo studies.

Mechanisms of Antiviral Activity of Iminosugars Against Dengue Virus

The antiviral mechanism of action of iminosugars against many enveloped viruses, including dengue virus (DENV), HIV, influenza and hepatitis C virus, is believed to be mediated by inducing misfolding of viral N-linked glycoproteins through inhibition of host endoplasmic reticulum-resident α-glucosidase enzymes. This leads to reduced secretion and/or infectivity of virions and hence lower viral titres, both in vitro and in vivo. Free oligosaccharide analysis from iminosugar-treated cells shows that antiviral activity correlates with production of mono- and tri-glucosylated sugars, indicative of inhibition of ER α-glucosidases. We demonstrate that glucose-mimicking iminosugars inhibit isolated glycoprotein and glycolipid processing enzymes and that this inhibition also occurs in primary cells treated with these drugs. Galactose-mimicking iminosugars that have been tested do not inhibit glycoprotein processing but do inhibit glycolipid processing, and are not antiviral against DENV. By comparison, the antiviral activity of glucose-mimetic iminosugars that inhibit endoplasmic reticulum-resident α-glucosidases, but not glycolipid processing, demonstrates that inhibition of α-glucosidases is responsible for iminosugar antiviral activity against DENV. This monograph will review the investigations of many researchers into the mechanisms of action of iminosugars and the contribution of our current understanding of these mechanisms for optimising clinical delivery of iminosugars. The effects of iminosugars on enzymes other than glucosidases, the induction of ER stress and viral receptors will be also put into context. Data suggest that inhibition of α-glucosidases results in inhibited release of virus and is the primary antiviral mechanism of action of iminosugars against DENV.

Naturally Occurring Pyrrolizidines: Inhibition of α-Glucosidase 1 and Anti-HIV Activity of One Stereoisomer

Alexine, a naturally occurring pyrrolizidine alkaloid, isolated from Alexa leiopetala, and four stereoisomers, isolated from Castanospermum australe, were investigated for inhibitory activity against the growth of HIV-1. Only treatment with the 7,7a-diepialexine restricted virus growth (IC50 0.38 mm) although it was less active than the indolizidine alkaloid castanospermine (IC50 0.02 mm). The antiviral effects of 7,7a-diepialexine, like castanospermine, correlated with the inhibitory activity against purified pig kidney α-glucosidase 1 of the glycoprotein processing enzymes and the reduced cleavage of the precursor HIV-1 glycoprotein gp160.

Action of Celgosivir (6 O-Butanoyl Castanospermine) against the Pestivirus BVDV: Implications for the Treatment of Hepatitis C

α-Glucosidase I inhibitors have been shown to inhibit the replication of a broad range of enveloped viruses by preventing the correct folding of their envelope glycoproteins. This study assesses the potential of 6 O-butanoyl castanospermine (cel-gosivir) as a treatment for hepatitis C virus (HCV). In the absence of an adequate culture system for HCV, the closely related virus, bovine viral diarrhoea virus (BVDV), was used as a surrogate model. Using both a plaque assay and a cytopathic effect assay, celgosivir (IC50 16 and 47 μM respectively) was shown to be more potent than N-nonyl DNJ (105 and 74 μM), castanospermine (110 and 367 μM) and N-butyl DNJ (>250 and 550 μM). Of the α-glucosi-dase inhibitors tested, only N-nonyl DNJ showed evidence of toxicity (CC50 ≥120 μM). Two-way combinations of interferon-α, ribavirin and either celgosivir or castanospermine demonstrated that each could enhance the antiviral efficacy of the others, either additively or synergistically. The observation that the number of viral genomes released from BVDV-infected cells was inhibited by either castanospermine or celgosivir in parallel with the number of infectious units was taken as confirmation that these α-glucosidase I inhibitors block the production or release of flavivirus particles.

Antiviral drug discovery: broad-spectrum drugs from nature

Covering: up to April 2014. The development of drugs with broad-spectrum antiviral activities is a long pursued goal in drug discovery. It has been shown that blocking co-opted host-factors abrogates the replication of many viruses, yet the development of such host-targeting drugs has been met with scepticism mainly due to toxicity issues and poor translation to in vivo models. With the advent of new and more powerful screening assays and prediction tools, the idea of a drug that can efficiently treat a wide range of viral infections by blocking specific host functions has re-bloomed. Here we critically review the state-of-the-art in broad-spectrum antiviral drug discovery. We discuss putative targets and treatment strategies, with particular focus on natural products as promising starting points for antiviral lead development.

Inhibition of endoplasmic reticulum-resident glucosidases impairs severe acute respiratory syndrome coronavirus and human coronavirus NL63 spike protein-mediated entry by altering the glycan processing of angiotensin I-converting enzyme 2

Endoplasmic reticulum (ER)-resident glucosidases I and II sequentially trim the three terminal glucose moieties on the N-linked glycans attached to nascent glycoproteins. These reactions are the first steps of N-linked glycan processing and are essential for proper folding and function of many glycoproteins. Because most of the viral envelope glycoproteins contain N-linked glycans, inhibition of ER glucosidases with derivatives of 1-deoxynojirimycin, i.e., iminosugars, efficiently disrupts the morphogenesis of a broad spectrum of enveloped viruses. However, like viral envelope proteins, the cellular receptors of many viruses are also glycoproteins. It is therefore possible that inhibition of ER glucosidases not only compromises virion production but also disrupts expression and function of viral receptors and thus inhibits virus entry into host cells. Indeed, we demonstrate here that iminosugar treatment altered the N-linked glycan structure of angiotensin I-converting enzyme 2 (ACE2), which did not affect its expression on the cell surface or its binding of the severe acute respiratory syndrome coronavirus (SARS-CoV) spike glycoprotein. However, alteration of N-linked glycans of ACE2 impaired its ability to support the transduction of SARS-CoV and human coronavirus NL63 (HCoV-NL63) spike glycoprotein-pseudotyped lentiviral particles by disruption of the viral envelope protein-triggered membrane fusion. Hence, in addition to reducing the production of infectious virions, inhibition of ER glucosidases also impairs the entry of selected viruses via a post-receptor-binding mechanism.

Imino sugar glucosidase inhibitors as broadly active anti-filovirus agents

Ebola virus and Marburg virus are members of the family of Filoviridae and are etiological agents of a deadly hemorrhagic fever disease. The clinical symptoms of Ebola and Marburg hemorrhagic fevers are difficult to distinguish and there are currently no specific antiviral therapies against either of the viruses. Therefore, a drug that is safe and effective against both would be an enormous breakthrough. We and others have shown that the folding of the glycoproteins of many enveloped viruses, including the filoviruses, is far more dependent upon the calnexin pathway of protein folding than are most host glycoproteins. Drugs that inhibit this pathway would be expected to be selectively antiviral. Indeed, as we summarize in this review, imino sugars that are competitive inhibitors of the host endoplasmic reticular α-glucosidases I and II, which are enzymes that process N-glycan on nascent glycoproteins and thereby inhibit calnexin binding to the nascent glycoproteins, have been shown to have antiviral activity against a number of enveloped viruses including filoviruses. In this review, we describe the state of development of imino sugars for use against the filoviruses, and provide an explanation for the basis of their antiviral activity as well as limitations.

Antiviral therapies targeting host ER alpha-glucosidases: current status and future directions

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ER α-glucosidases are essential host factors for the morphogenesis of many enveloped viruses.

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Imino sugars are competitive inhibitors of the ER α-glucosidases I and II.

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Broad-spectrum antiviral efficacies of imino sugars have been demonstrated in vitro, and in vivo.

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Strategies for development of potent and specific ER α-glucosidase inhibitors have been proposed.

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Targeting glucosidase is promising for viral hemorrhagic fever and respiratory infections.

Endoplasmic reticulum (ER)-resident α-glucosidases I and II sequentially trim the three terminal glucose moieties on N-linked glycans attached to nascent glycoproteins. These reactions are the first steps of N-linked glycan processing and are essential for proper folding and function of many glycoproteins. Because most viral envelope glycoproteins contain N-linked glycans, inhibition of ER α-glucosidases with derivatives of 1-deoxynojirimycin (DNJ) or castanospermine (CAST), two well-studied pharmacophores of α-glucosidase inhibitors, efficiently disrupts the morphogenesis of a broad spectrum of enveloped viruses. Moreover, both DNJ and CAST derivatives have been demonstrated to prevent the death of mice infected with several distinct flaviviruses and filoviruses and suppress the multiplication of several other species of viruses in infected animals. N-Butyl derivative of DNJ (NB-DNJ) and 6 O-bytanoyl prodrug of CAST (Bu-CAST) have been evaluated in human clinical trials for their antiviral activities against human immunodeficiency virus and hepatitis C virus, and there is an ongoing trial of treating dengue patients with Bu-CAST. This article summarizes the current status of ER α-glucosidase-targeted antiviral therapy and proposes strategies for development of more efficacious and specific ER α-glucosidase inhibitors as broad-spectrum, drug resistance-refractory antiviral therapeutics. These host function-targeted, broad-spectrum antiviral agents do not rely on time-consuming etiologic diagnosis, and should therefore be particularly promising in the management of viral hemorrhagic fever and respiratory tract viral infections, medical conditions that can be caused by many different enveloped RNA viruses, with a short window for medical intervention.

Inhibition of α-glucosidase and α-amylase by diaryl derivatives of imidazole-thione and 1,2,4-triazole-thiol

The in vivo and in vitro effects of 4,5-diphenylimidazole-2-thione (1), 4,5-diphenyl-1,2,4-triazole-3-thiol (2) and 5-(2-hydroxyphenyl)-4-phenyl-1,2,4-triazole-3-thiol (3) on α-glucosidase and α-amylase were investigated. The in vivo inhibition has been found to be dose-dependent and to occur at a value less than LD50. The in vitro treatment of the enzymes by 4,5-diphenylimidazole-2-thione exhibited a reversible inhibition of the non-competitive type with Ki value of 3.5 and 6.5×10(-5) M for α-glucosidase and α-amylase, respectively. 4,5-diphenyl-1,2,4-triazole-3-thione showed a reversible inhibition of the competitive and non-competitive types, with Ki value of 10(-5) M magnitude, for α-glucosidase and α-amylase. On the other hand, 5-(o-hydroxyphenyl)-4-phenyl-1,2,4-triazole-3-thione did not display an inhibitory effect towards α-amylase but showed a potent inhibition of the competitive type for hepatic α-glucosidase with 10(-5) M magnitude of Ki value.

Antiviral effects of an iminosugar derivative on flavivirus infections

Endoplasmic reticulum (ER) alpha-glucosidase inhibitors, which block the trimming step of N-linked glycosylation, have been shown to eliminate the production of several ER-budding viruses. Here we investigated the effects of one such inhibitor, N-nonyl-deoxynojirimycin (NN-DNJ), a 9-carbon alkyl iminosugar derivative, on infection by Japanese encephalitis virus (JEV) and dengue virus serotype 2 (DEN-2). In the presence of NN-DNJ, JEV and DEN-2 infections were suppressed in a dose-dependent manner. This inhibitory effect appeared to influence DEN-2 infection more than JEV infection, since lower concentrations of NN-DNJ substantially blocked DEN-2 replication. Secretion of the flaviviral glycoproteins E and NS1 was greatly reduced, and levels of DEN-2 viral RNA replication measured by fluorogenic reverse transcription-PCR were also decreased, by NN-DNJ. Notably, the viral glycoproteins, prM, E, and NS1 were found to associate transiently with the ER chaperone calnexin, and this interaction was affected by NN-DNJ, suggesting a potential role of calnexin in the folding of flaviviral glycoproteins. Additionally, in a mouse model of lethal challenge by JEV infection, oral delivery of NN-DNJ reduced the mortality rate. These findings show that NN-DNJ has an antiviral effect on flavivirus infection, likely through interference with virus replication at the posttranslational modification level, occurring mainly in the ER.

Targeting glycosylation as a therapeutic approach

Increased understanding of the role of protein- and lipid-linked carbohydrates in a wide range of biological processes has led to interest in drugs that target the enzymes involved in glycosylation. But given the importance of carbohydrates in fundamental cellular processes such as protein folding, therapeutic strategies that modulate, rather than ablate, the activity of enzymes involved in glycosylation are likely to be a necessity. Two such approaches that use imino sugars to affect glycosylation enzymes now show considerable promise in the treatment of viral infections, such as hepatitis B, and glucosphingolipid storage disorders, such as Gaucher disease.

Polyhydroxylated alkaloids -- natural occurrence and therapeutic applications

Over one hundred polyhydroxylated alkaloids have been isolated from plants and micro-organisms. These alkaloids can be potent and highly selective glycosidase inhibitors and are arousing great interest as tools to study cellular recognition and as potential therapeutic agents. However, only three of the natural products so far have been widely studied for therapeutic potential due largely to the limited commercial availability of the other compounds.

The heterodimeric structure of glucosidase II is required for its activity, solubility, and localization in vivo

Glucosidase II is an ER heterodimeric enzyme that cleaves sequentially the two innermost alpha-1,3-linked glucose residues from N-linked oligosaccharides on nascent glycoproteins. This processing allows the binding and release of monoglucosylated (Glc(1)Man(9)GlcNAc(2)) glycoproteins with calnexin and calreticulin, the lectin-like chaperones of the endoplasmic reticulum. We have isolated two cDNA isoforms of the human alpha subunit (alpha1 and alpha2) differing by a 66 bp stretch, and a cDNA for the corresponding beta subunit. The alpha1 and alpha2 forms have distinct mobilities on SDS-PAGE and are expressed in most of the cell lines we have tested, but were absent from the glucosidase II-deficient cell line PHA(R) 2.7. Using COS7 cells, the coexpression of the beta subunit with the catalytic alpha subunit was found to be essential for enzymatic activity, solubilization, and/or stability, and ER retention of the alpha/beta complex. Transfected cell extracts expressing either alpha1 or alpha2 forms with the beta subunit showed similar activities, while mutating( )the nucleophile (D542N) predicted from the glycoside hydrolase Family 31 active site consensus sequence abolished enzymatic activity. In order to compare the kinetic parameters of both alpha1/beta and alpha2/beta forms of human glucosidase II the protein was expressed with the baculovirus expression system. Expression of the human alpha or beta subunit alone led to the formation of active human/insect heteroenzymes, demonstrating functional complementation by the endogenous insect glucosidase II subunits. The activity of both forms of recombinant human glucosidase II was examined with a p-nitrophenyl alpha-D-glucopyranoside substrate, and a two binding site kinetic model for this substrate was shown. The K(M1-2) values and apparent K(i1-2 )for deoxynojirimycin and castanospermine were determined and found to be identical for both isoforms suggesting they have similar catalysis and inhibition characteristics. The substrate specificities of both isoforms using the physiological oligosaccharides were assessed and found to be similar.

Alpha-glucosidase inhibitors as potential broad based anti-viral agents

N-Linked oligosaccharides play many roles in the fate and functions of glycoproteins. One function is to assist in the folding of proteins by mediating interactions of the lectin-like chaperone proteins calnexin and calreticulin with nascent glycoproteins. These interactions can be prevented by inhibitors of the alpha-glucosidases and this causes some proteins to be misfolded and retained within the endoplasmic reticulum. In human immunodeficiency virus (HIV) and hepatitis B virus (HBV) the misfolding of key viral envelope glycoproteins interferes with the viral life cycle. It has been demonstrated in an animal model of chronic HBV that glucosidase inhibitors can alter glycosylation and have anti-viral activity. As the mechanism of action of alpha-glucosidase inhibitors is the induction of misfolded or otherwise defective viral glycoproteins, such inhibitors may be useful therapeutics for many viruses, especially those which bud from the endoplasmic reticulum (where protein folding takes place). For example bovine viral diarrhea virus, a pestivirus akin to hepatitis C virus, is also extremely sensitive to glucosidase inhibition.

Chapter 1 Allelochemical Properties or the Raison D'être of Alkaloids

This chapter provides evidence that alkaloids are not waste products or functionless molecules as formerly assumed, but rather defense compounds employed by plants for survival against herbivores and against microorganisms and competing plants. These molecules were developed during evolution through natural selection in that they fit many important molecular targets, often receptors, of cells, which are seen in molecules that mimic endogenous neurotransmitters. The chapter discusses that microorganisms and herbivores rely on plants as a food source. Since both have survived, there must be mechanisms of adaptations toward the defensive chemistry of plants. Many herbivores have evolved strategies to avoid the extremely toxic plants and prefer the less toxic ones. Many herbivores have potent mechanisms to detoxify xenobiotics, which allow the exploitation of at least the less toxic plants. In insects, many specialists evolved that are adapted to the defense chemicals of their host plant, in that they accumulate these compounds and exploit them for their own defense. Alkaloids function as defense molecules against insect predators in the examples studied, and this is further support for the hypothesis that the same compound also serves for chemical defense in the host plant. It needs more experimental data to understand fully the intricate interconnections between plants, their alkaloids, and herbivores, microorganisms, and other plants.

Analysis of human antibody responses to human cytomegalovirus envelope glycoproteins found in two families of disulfide linked glycoprotein complexes designated gC-I and gC-II

Human antibody responses to human cytomegalovirus (HCMV) envelope glycoproteins were analyzed using immunoaffinity purified glycoproteins and Western blotting. Two families of disulfide linked glycoprotein complexes, designated gC-I and gC-II, were isolated. These complexes were reduced and their individual glycoproteins separated by polyacrylamide gel electrophoresis prior to electroblotting. The reactivity of adult convalescent sera with individual glycoproteins was compared to that of sera from congenitally infected infants. All sera tested reacted with a 52,000 molecular weight glycoprotein from these complexes, but only 75% reacted with a 93,000 to 130,000 molecular weight glycoprotein from gC-I complexes. Most adult convalescent sera reacted with glycoproteins from gC-II complexes. However, 14 of 16 infant sera did not have high enough levels of gC-II antibodies to give a positive reaction with Western blotting. A longitudinal study was done with several infants and their mothers. These studies indicated a failure by the infants and their mothers to develop detectable levels of gC-II antibodies months to years after the initial infection or after repeated stimulation with HCMV due to persistent infection. The inability of these infants to develop significant levels of gC-II antibodies was not due to an inability to respond to viral glycoproteins since they had antibodies to gC-I glycoproteins. We also determined that the strains of HCMV infecting some of these infants expressed gC-II glycoproteins. Thus, the lack of response by these infants was not due to lack of expression of gC-II glycoproteins by their infecting strain.