Effects of the imino sugar N-butyldeoxynojirimycin on the N-glycosylation of recombinant gp120

Expanding horizons of iminosugars as broad-spectrum anti-virals: mechanism, efficacy and novel developments

Iminosugars, a class of polyhydroxylated cyclic alkaloids with intriguing properties, hold promising therapeutic potentials against a broad spectrum of enveloped viruses, including DENV, HCV, HIV, and influenza viruses. Mechanistically, iminosugars act as the competitive inhibitors of host endoplasmic reticular α-glucosidases I and II to  disrupt the proper folding of viral nascent glycoproteins, which thereby exerts antiviral effects. Remarkably, the glycoproteins of many enveloped viruses are significantly more dependent on the calnexin pathway of the protein folding than most host glycoproteins. Therefore, extensive interests and efforts have been devoted to exploit iminosugars as broad-spectrum antiviral agents. This review provides the summary and insights into the recent advancements in the development of novel iminosugars as effective and selective antiviral agents against a variety of enveloped viruses, as well as the understandings of their antiviral mechanisms.

Tetrahydropyridines’ Stereoselective Formation, How Lockdown Assisted in the Identification of the Features of Its Mechanism

The multicomponent reaction of aldehydes, cyano-containing C-H acids, esters of 3-oxocarboxylic acid and ammonium acetate led to unexpected results. The boiling of starting materials in methanol for one to two hours resulted in the formation of polysubstituted 1,4,5,6-tetrahydropyridines with two or three stereogenic centers. During the 2020 lockdown, we obtained key intermediates of this six-step domino reaction. A number of fast and slow reactions occurred during the prolonged stirring of the reaction mass at rt. Sequence: 1. Knoevenagel condensation; 2. Michael addition; 3. Mannich reaction; 4. cyclization—fast reactions and cyclization of the product polysubstituted 2-hydroxypiperidine—was isolated after 40 min stirring at rt. Further monitoring proved the slow dehydration of 2-hydroxypiperidine to obtain 3,4,5,6-tetrahydropyridine after 7 days. Then, four-month isomerization occurred with 1,4,5,6-tetrahydropyridine formation. All reactions were stereoselective. Key intermediates and products structures were verified by X-ray diffraction analysis. Additionally, we specified conditions for the selective intermediates’ preparation.

Small molecule inhibitors of mammalian glycosylation

Glycans are one of the fundamental biopolymers encountered in living systems. Compared to polynucleotide and polypeptide biosynthesis, polysaccharide biosynthesis is a uniquely combinatorial process to which interdependent enzymes with seemingly broad specificities contribute. The resulting intracellular cell surface, and secreted glycans play key roles in health and disease, from embryogenesis to cancer progression. The study and modulation of glycans in cell and organismal biology is aided by small molecule inhibitors of the enzymes involved in glycan biosynthesis. In this review, we survey the arsenal of currently available inhibitors, focusing on agents which have been independently validated in diverse systems. We highlight the utility of these inhibitors and drawbacks to their use, emphasizing the need for innovation for basic research as well as for therapeutic applications.

The Role of Glycosphingolipids in Immune Cell Functions

Glycosphingolipids (GSLs) exhibit a variety of functions in cellular differentiation and interaction. Also, they are known to play a role as receptors in pathogen invasion. A less well-explored feature is the role of GSLs in immune cell function which is the subject of this review article. Here we summarize knowledge on GSL expression patterns in different immune cells. We review the changes in GSL expression during immune cell development and differentiation, maturation, and activation. Furthermore, we review how immune cell GSLs impact membrane organization, molecular signaling, and trans-interactions in cellular cross-talk. Another aspect covered is the role of GSLs as targets of antibody-based immunity in cancer. We expect that recent advances in analytical and genome editing technologies will help in the coming years to further our knowledge on the role of GSLs as modulators of immune cell function.

In Planta Preliminary Screening of ER Glycoprotein Folding Quality Control (ERQC) Modulators

Small molecule modulators of the Endoplasmic Reticulum glycoprotein folding quality control (ERQC) machinery have broad-spectrum antiviral activity against a number of enveloped viruses and have the potential to rescue secretion of misfolded but active glycoproteins in rare diseases. In vivo assays of candidate inhibitors in mammals are expensive and cannot be afforded at the preliminary stages of drug development programs. The strong conservation of the ERQC machinery across eukaryotes makes transgenic plants an attractive system for low-cost, easy and fast proof-of-concept screening of candidate ERQC inhibitors. The Arabidopsis thaliana immune response is mediated by glycoproteins, the folding of which is controlled by ERQC. We have used the plant response to bacterial peptides as a means of assaying an ERQC inhibitor in vivo. We show that the treatment of the plant with the iminosugar NB-DNJ, which is a known ER α-glucosidase inhibitor in mammals, influences the immune response of the plant to the bacterial peptide elf18 but not to the flagellin-derived flg22 peptide. In the NB-DNJ-treated plant, the responses to elf18 and flg22 treatments closely follow the ones observed for the ER α-glucosidase II impaired plant, At psl5-1. We propose Arabidopsis thaliana as a promising platform for the development of low-cost proof-of-concept in vivo ERQC modulation.

Iminosugar antivirals: the therapeutic sweet spot

Many viruses require the host endoplasmic reticulum protein-folding machinery in order to correctly fold one or more of their glycoproteins. Iminosugars with glucose stereochemistry target the glucosidases which are key for entry into the glycoprotein folding cycle. Viral glycoproteins are thus prevented from interacting with the protein-folding machinery leading to misfolding and an antiviral effect against a wide range of different viral families. As iminosugars target host enzymes, they should be refractory to mutations in the virus. Iminosugars therefore have great potential for development as broad-spectrum antiviral therapeutics. We outline the mechanism giving rise to the antiviral activity of iminosugars, the current progress in the development of iminosugar antivirals and future prospects for this field.

In vitro modulatory effects of functionalized pyrimidines and piperidine derivatives on Aryl hydrocarbon receptor (AhR) and glucocorticoid receptor (GR) activities

The development of biologically active molecules based on molecular recognition is an attractive and challenging task in medicinal chemistry and the molecules that can activate/deactivate certain receptors are of great medical interest. In this contribution, selected pyrimidine/piperidine derivatives were synthesized and tested for the ability to activate/deactivate Aryl hydrocarbon receptor (AhR) and Glucocorticoid receptor (GR). Tested compounds are shown to activate the receptors but to much lesser extent than positive controls, dioxin and dexamethasone for Ahr and GR, respectively. However, some of them antagonized the positive controls action. Although further in vivo studies are needed to fully characterize the bioactivities of these compounds, the reported in vitro evidences demonstrate that they might be used as the modulators of AhR and GR activities.

Ester Prodrugs of IHVR-19029 with Enhanced Oral Exposure and Prevention of Gastrointestinal Glucosidase Interaction

IHVR-19029 (6) is a lead endoplasmic reticulum α-glucosidases I and II inhibitor, which efficiently protected mice from lethal Ebola and Marburg virus infections via injection route, but suffered from low bioavailability and off-target interactions with gut glucosidases when administered orally. In an effort to improve efficacious exposure levels and avoid side effects, we designed and synthesized ester prodrugs. Not only were the prodrugs stable in simulated gastric and intestinal fluids and were inactive against glucosidases but they also exhibited antiviral activities against dengue virus infection in a cell based assay. Further in vitro evaluation showed that the bioconversion of the prodrugs is species dependent: in mice, the prodrugs were converted to 6 in the plasma and liver; while in human, the conversion occurred mainly in liver. An in vivo pharmacokinetic study in mice demonstrated that the tetrabutyrate prodrug 8 achieved the most improved overall exposure of 6 upon both oral and intravenous administration.

Assessment of the potential for host-targeted iminosugars UV-4 and UV-5 activity against filovirus infections in vitro and in vivo

Iminosugars are host-directed antivirals with broad-spectrum activity. The iminosugar, N-butyl-deoxynojirimycin (NB-DNJ or Miglustat^®), is used in humans for treatment of Gaucher's disease and has mild antiviral properties. More potent analogs of NB-DNJ have been generated and have demonstrated activity against a variety of viruses including flaviviruses, influenza, herpesviruses and filoviruses. In the current study, a panel of analogs based on NB-DNJ was analyzed for activity against Ebola (EBOV) and Marburg viruses (MARV). The antiviral activity of NB-DNJ (UV-1), UV-2, UV-3, UV-4 and UV-5 against both EBOV and MARV was demonstrated in Vero cells. Subsequent studies to examine the activity of UV-4 and UV-5 using rodent models of EBOV and MARV were performed. In vivo efficacy studies provided inconsistent data following treatment with iminosugars using filovirus mouse models. A tolerability study in nonhuman primates demonstrated that UV-4 could be administered at much higher dose levels than rodents. Since UV-4 was active in vitro, had been demonstrated to be active against influenza and dengue in vivo, and was being tested in a Phase 1 clinical trial, a small proof-of-concept nonhuman primate trial was performed to determine whether this antiviral candidate could provide clinical benefit to EBOV-infected individuals. Administration of UV-4B did not provide a clinical or survival benefit to macaques infected with EBOV-Makona; however, dosing of animals was not optimal in this study. Efficacy may be improved by thrice daily dosing (e.g. by nasogastric tube feeding) to match the efficacious dosing regimens demonstrated against dengue and influenza viruses.

Synthesis of functionalized dihydro-2-oxypyrroles and tetrahydropyridines using 2,6-pyridinedicarboxylic acid as an efficient and mild organocatalyst

Simple and mild one-pot protocols for the synthesis of diversely functionalized dihydro-2-oxypyrroles and tetrahydropyridines using catalytic amounts of 2,6-PDCA.

A Novel Iminosugar UV-12 with Activity against the Diverse Viruses Influenza and Dengue (Novel Iminosugar Antiviral for Influenza and Dengue)

Iminosugars are capable of targeting the life cycles of multiple viruses by blocking host endoplasmic reticulum α-glucosidase enzymes that are required for competent replication of a variety of enveloped, glycosylated viruses. Iminosugars as a class are approved for use in humans with diseases such as diabetes and Gaucher’s disease, providing evidence for safety of this class of compounds. The in vitro antiviral activity of iminosugars has been described in several publications with a subset of these demonstrating in vivo activity against flaviviruses, herpesviruses, retroviruses and filoviruses. Although there is compelling non-clinical in vivo evidence of antiviral efficacy, the efficacy of iminosugars as antivirals has yet to be demonstrated in humans. In the current study, we report a novel iminosugar, UV-12, which has efficacy against dengue and influenza in mouse models. UV-12 exhibits drug-like properties including oral bioavailability and good safety profile in mice and guinea pigs. UV-12 is an example of an iminosugar with activity against multiple virus families that should be investigated in further safety and efficacy studies and demonstrates potential value of this drug class as antiviral therapeutics.

Glycoprotein misfolding in the endoplasmic reticulum: identification of released oligosaccharides reveals a second ER-associated degradation pathway for Golgi-retrieved proteins

Endoplasmic reticulum-associated degradation (ERAD) is a key cellular process whereby misfolded proteins are removed from the endoplasmic reticulum (ER) for subsequent degradation by the ubiquitin/proteasome system. In the present work, analysis of the released, free oligosaccharides (FOS) derived from all glycoproteins undergoing ERAD, has allowed a global estimation of the mechanisms of this pathway rather than following model proteins through degradative routes. Examining the FOS produced in endomannosidase-compromised cells following α-glucosidase inhibition has revealed a mechanism for clearing Golgi-retrieved glycoproteins that have failed to enter the ER quality control cycle. The Glc3Man7GlcNAc2 FOS species has been shown to be produced in the ER lumen by a mechanism involving a peptide: N-glycanase-like activity, and its production was sensitive to disruption of Golgi-ER trafficking. The detection of this oligosaccharide was unaffected by the overexpression of EDEM1 or cytosolic mannosidase, both of which increased the production of previously characterised cytosolically localised FOS. The lumenal FOS identified are therefore distinct in their production and regulation compared to FOS produced by the conventional route of misfolded glycoproteins directly removed from the ER. The production of such lumenal FOS is indicative of a novel degradative route for cellular glycoproteins that may exist under certain conditions.

Small molecule inhibitors of ER α-glucosidases are active against multiple hemorrhagic fever viruses

Host cellular endoplasmic reticulum α-glucosidases I and II are essential for the maturation of viral glycosylated envelope proteins that use the calnexin mediated folding pathway. Inhibition of these glycan processing enzymes leads to the misfolding and degradation of these viral glycoproteins and subsequent reduction in virion secretion. We previously reported that, CM-10-18, an imino sugar α-glucosidase inhibitor, efficiently protected the lethality of dengue virus infection of mice. In the current study, through an extensive structure-activity relationship study, we have identified three CM-10-18 derivatives that demonstrated superior in vitro antiviral activity against representative viruses from four viral families causing hemorrhagic fever. Moreover, the three novel imino sugars significantly reduced the mortality of two of the most pathogenic hemorrhagic fever viruses, Marburg virus and Ebola virus, in mice. Our study thus proves the concept that imino sugars are promising drug candidates for the management of viral hemorrhagic fever caused by variety of viruses.

Influenza A virus entry inhibitors targeting the hemagglutinin

Influenza A virus (IAV) has caused seasonal influenza epidemics and influenza pandemics, which resulted in serious threat to public health and socioeconomic impacts. Until now, only 5 drugs belong to two categories are used for prophylaxis and treatment of IAV infection. Hemagglutinin (HA), the envelope glycoprotein of IAV, plays a critical role in viral binding, fusion and entry. Therefore, HA is an attractive target for developing anti‑IAV drugs to block the entry step of IAV infection. Here we reviewed the recent progress in the study of conformational changes of HA during viral fusion process and the development of HA-based IAV entry inhibitors, which may provide a new choice for controlling future influenza pandemics.

One-Pot Three-Component Synthesis of Highly Substituted Piperidines Using 1-Methyl-2-Oxopyrrolidinium Hydrogen Sulfate

1-Methyl-2-oxopyrrolidinium hydrogen sulfate ([Hpyro][HSO4]) is used as an ionic liquid catalyst for the one-pot three-component synthesis of highly substituted piperidines from aromatic aldehydes, anilines and β–ketoesters in refluxing ethanol. This homogeneous catalyst procedure has the advantages of easy work-up, no need for column chromatography, simple and readily available precursors, and good to high yields.

Aza-Claisen rearrangement of 2-C-hydroxymethyl glycals as a versatile strategy towards synthesis of isofagomine and related biologically important azasugars

Synthesis of isofagomine has been achieved by implementation of aza-Claisen rearrangement of 2-C-hydroxymethyl glycals as a key step. The above rearrangement has also been utilized in the synthesis of biologically important polyhydroxylated piperidine frameworks such as isogalactofagomine, ent-isogalactofagomine and their analogues and some other azasugars as glycosidase inhibitors.

Variable loop glycan dependency of the broad and potent HIV-1-neutralizing antibodies PG9 and PG16

The HIV-1-specific antibodies PG9 and PG16 show marked cross-isolate neutralization breadth and potency. Antibody neutralization has been shown to be dependent on the presence of N-linked glycosylation at position 160 in gp120. We show here that (i) the loss of several key glycosylation sites in the V1, V2, and V3 loops; (ii) the generation of pseudoviruses in the presence of various glycosidase inhibitors; and (iii) the growth of pseudoviruses in a mutant cell line (GnT1(-/-)) that alters envelope glycosylation patterns all have significant effects on the sensitivity of virus to neutralization by PG9 and PG16. However, the interaction of antibody is not inhibited by sugar monosaccharides corresponding to those found in glycans on the HIV surface. We show that some of the glycosylation effects described are isolate dependent and others are universal and can be used as diagnostic for the presence of PG9 and PG16-like antibodies in the sera of HIV-1-infected patients. The results suggest that PG9 and PG16 recognize a conformational epitope that is dependent on glycosylation at specific variable loop N-linked sites. This information may be valuable for the design of immunogens to elicit PG9 and PG16-like antibodies, as well as constructs for cocrystallization studies.

Role of complex carbohydrates in human immunodeficiency virus type 1 infection and resistance to antibody neutralization

Complex N-glycans flank the receptor binding sites of the outer domain of HIV-1 gp120, ostensibly forming a protective "fence" against antibodies. Here, we investigated the effects of rebuilding this fence with smaller glycoforms by expressing HIV-1 pseudovirions from a primary isolate in a human cell line lacking N-acetylglucosamine transferase I (GnTI), the enzyme that initiates the conversion of oligomannose N-glycans into complex N-glycans. Thus, complex glycans, including those that surround the receptor binding sites, are replaced by fully trimmed oligomannose stumps. Conversely, the untrimmed oligomannoses of the silent domain of gp120 are likely to remain unchanged. For comparison, we produced a mutant virus lacking a complex N-glycan of the V3 loop (N301Q). Both variants exhibited increased sensitivities to V3 loop-specific monoclonal antibodies (MAbs) and soluble CD4. The N301Q virus was also sensitive to "nonneutralizing" MAbs targeting the primary and secondary receptor binding sites. Endoglycosidase H treatment resulted in the removal of outer domain glycans from the GnTI- but not the parent Env trimers, and this was associated with a rapid and complete loss in infectivity. Nevertheless, the glycan-depleted trimers could still bind to soluble receptor and coreceptor analogs, suggesting a block in post-receptor binding conformational changes necessary for fusion. Collectively, our data show that the antennae of complex N-glycans serve to protect the V3 loop and CD4 binding site, while N-glycan stems regulate native trimer conformation, such that their removal can lead to global changes in neutralization sensitivity and, in extreme cases, an inability to complete the conformational rearrangements necessary for infection.

The role of glycosylation and domain interactions in the thermal stability of human angiotensin-converting enzyme

The N and C domains of somatic angiotensin-converting enzyme (sACE) differ in terms of their substrate specificity, inhibitor profiling, chloride dependency and thermal stability. The C domain is thermally less stable than sACE or the N domain. Since both domains are heavily glycosylated, the effect of glycosylation on their thermal stability was investigated by assessing their catalytic and physicochemical properties. Testis ACE (tACE) expressed in mammalian cells, mammalian cells in the presence of a glucosidase inhibitor and insect cells yielded proteins with altered catalytic and physicochemical properties, indicating that the more complex glycans confer greater thermal stabilization. Furthermore, a decrease in tACE and N-domain N-glycans using site-directed mutagenesis decreased their thermal stability, suggesting that certain N-glycans have an important effect on the protein's thermodynamic properties. Evaluation of the thermal stability of sACE domain swopover and domain duplication mutants, together with sACE expressed in insect cells, showed that the C domain contained in sACE is less dependent on glycosylation for thermal stabilization than a single C domain, indicating that stabilizing interactions between the two domains contribute to the thermal stability of sACE and are decreased in a C-domain-duplicating mutant.

Glucosylated free oligosaccharides are biomarkers of endoplasmic- reticulum α-glucosidase inhibition

The inhibition of ER (endoplasmic reticulum) α-glucosidases I and II by imino sugars, including NB-DNJ (N-butyl-deoxynojirimycin), causes the retention of glucose residues on N-linked oligosaccharides. Therefore, normal glycoprotein trafficking and processing through the glycosylation pathway is abrogated and glycoproteins are directed to undergo ERAD (ER-associated degradation), a consequence of which is the production of cytosolic FOS (free oligosaccharides). Following treatment with NB-DNJ, FOS were extracted from cells, murine tissues and human plasma and urine. Improved protocols for analysis were developed using ion-exchange chromatography followed by fluorescent labelling with 2-AA (2-aminobenzoic acid) and purification by lectin-affinity chromatography. Separation of 2-AA-labelled FOS by HPLC provided a rapid and sensitive method that enabled the detection of all FOS species resulting from the degradation of glycoproteins exported from the ER. The generation of oligosaccharides derived from glucosylated protein degradation was rapid, reversible, and time- and inhibitor concentration-dependent in cultured cells and in vivo. Long-term inhibition in cultured cells and in vivo indicated a slow rate of clearance of glucosylated FOS. In mouse and human urine, glucosylated FOS were detected as a result of transrenal excretion and provide unique and quantifiable biomarkers of ER-glucosidase inhibition.