The murine coronavirus hemagglutinin-esterase receptor-binding site: a major shift in ligand specificity through modest changes in architecture

First isolation of bovine coronavirus with a three-amino-acid deletion in the N gene causing severe respiratory and digestive disease in calve

Bovine coronavirus (BCoV), a persistent threat to global cattle industry, has caused significant economic losses worldwide. In this study, a viral strain was isolated from the intestinal content of a diseased calve, and identified by cytopathic effects observation, indirect immunofluorescence assay and electron microscopy. Results showed that BCoV NXWZ2310 belonging to the GIIb genotype and has a three-amino-acid deletion in the serine-rich region of the N gene. Importantly, the BCoV NXWZ2310 strain exhibited strong pathogenicity, causing nasal discharge and watery diarrhea in calves for 8 and 10 days, respectively. Viral shedding was detected in nasal, throat and rectal swabs at levels reaching 106.228 copies/mL, 105.0 copies /mL and 106.692 copies/mL, respectively. Pathological examination showed that NXWZ2310 resulted in parenchymal lesions of the pulmonary lobe and significant intestinal lesions. Both the lungs and intestines displayed marked microscopic lesions with clear viral antigens present. BCoV NXWZ2310 strain with N-gene deletion mutations, caused severe respiratory and digestive disease in calves. Therefore, effective strategies are needed for the prevention and control of this isolate.

Identification and Comparison of the Sialic Acid-Binding Domain Characteristics of Avian Coronavirus Infectious Bronchitis Virus Spike Protein

Infectious bronchitis virus (IBV) infections are initiated by the transmembrane spike (S) glycoprotein, which binds to host factors and fuses the viral and cell membranes. The N-terminal domain of the S1 subunit of IBV S protein binds to sialic acids, but the precise location of the sialic acid binding domain (SABD) and the role of the SABD in IBV-infected chickens remain unclear. Here, we identify the S1 N-terminal amino acid (aa) residues 19 to 227 (209 aa total) of IBV strains SD (GI-19) and GD (GI-7), and the corresponding region of M41 (GI-1), as the minimal SABD using truncated protein histochemistry and neuraminidase assays. Both α-2,3- and α-2,6-linked sialic acids on the surfaces of CEK cells can be used as attachment receptors by IBV, leading to increased infection efficiency. However, 9-O acetylation of the sialic acid glycerol side chain inhibits IBV S1 and SABD protein binding. We further constructed recombinant strains in which the S1 gene or the SABD in the GD and SD genomes were replaced with the corresponding region from M41 by reverse genetics. Infecting chickens with these viruses revealed that the virulence and nephrotropism of rSDM41-S1, rSDM41-206, rGDM41-S1, and rGDM41-206 strains were decreased to various degrees compared to their parental strains. A positive sera cross-neutralization test showed that the serotypes were changed for the recombinant viruses. Our results provide insight into IBV infection of host cells that may aid vaccine design. IMPORTANCE To date, only α-2,3-linked sialic acid has been identified as a potential host binding receptor for IBV. Here, we show the minimum region constituting the sialic acid binding domain (SABD) and the binding characteristics of the S1 subunit of spike (S) protein of IBV strains SD (GI-19), GD (GI-7), and M41 (GI-1) to various sialic acids. The 9-O acetylation modification partially inhibits IBV from binding to sialic acid, while the virus can also bind to sialic acid molecules linked to host cells through an α-2,6 linkage, serving as another receptor determinant. Substitution of the putative SABD from strain M41 into strains SD and GD resulted in reduced virulence, nephrotropism, and a serotype switch. These findings suggest that sialic acid binding has diversified during the evolution of γ-coronaviruses, impacting the biological properties of IBV strains. Our results offer insight into the mechanisms by which IBV invades host cells.

Virus diversity, wildlife-domestic animal circulation and potential zoonotic viruses of small mammals, pangolins and zoo animals

Wildlife is reservoir of emerging viruses. Here we identified 27 families of mammalian viruses from 1981 wild animals and 194 zoo animals collected from south China between 2015 and 2022, isolated and characterized the pathogenicity of eight viruses. Bats harbor high diversity of coronaviruses, picornaviruses and astroviruses, and a potentially novel genus of Bornaviridae. In addition to the reported SARSr-CoV-2 and HKU4-CoV-like viruses, picornavirus and respiroviruses also likely circulate between bats and pangolins. Pikas harbor a new clade of Embecovirus and a new genus of arenaviruses. Further, the potential cross-species transmission of RNA viruses (paramyxovirus and astrovirus) and DNA viruses (pseudorabies virus, porcine circovirus 2, porcine circovirus 3 and parvovirus) between wildlife and domestic animals was identified, complicating wildlife protection and the prevention and control of these diseases in domestic animals. This study provides a nuanced view of the frequency of host-jumping events, as well as assessments of zoonotic risk.

The Glycan-Binding Trait of the Sarbecovirus Spike N-Terminal Domain Reveals an Evolutionary Footprint

The spike protein on sarbecovirus virions contains two external, protruding domains: an N-terminal domain (NTD) with unclear function and a C-terminal domain (CTD) that binds the host receptor, allowing for viral entry and infection. While the CTD is well studied for therapeutic interventions, the role of the NTD is far less well understood for many coronaviruses. Here, we demonstrate that the spike NTD from SARS-CoV-2 and other sarbecoviruses binds to unidentified glycans in vitro similarly to other members of the Coronaviridae family. We also show that these spike NTD (S-NTD) proteins adhere to Calu3 cells, a human lung cell line, although the biological relevance of this is unclear. In contrast to what has been shown for Middle East respiratory syndrome coronavirus (MERS-CoV), which attaches sialic acids during cell entry, sialic acids present on Calu3 cells inhibited sarbecovirus infection. Therefore, while sarbecoviruses can interact with cell surface glycans similarly to other coronaviruses, their reliance on glycans for entry is different from that of other respiratory coronaviruses, suggesting sarbecoviruses and MERS-CoV have adapted to different cell types, tissues, or hosts during their divergent evolution. Our findings provide important clues for further exploring the biological functions of sarbecovirus glycan binding and adds to our growing understanding of the complex forces that shape coronavirus spike evolution. IMPORTANCE Spike N-terminal domains (S-NTD) of sarbecoviruses are highly diverse; however, their function remains largely understudied compared with the receptor-binding domains (RBD). Here, we show that sarbecovirus S-NTD can be phylogenetically clustered into five clades and exhibit various levels of glycan binding in vitro. We also show that, unlike some coronaviruses, including MERS-CoV, sialic acids present on the surface of Calu3, a human lung cell culture, inhibit SARS-CoV-2 and other sarbecoviruses. These results suggest that while glycan binding might be an ancestral trait conserved across different coronavirus families, the functional outcome during infection can vary, reflecting divergent viral evolution. Our results expand our knowledge on the biological functions of the S-NTD across diverse sarbecoviruses and provide insight on the evolutionary history of coronavirus spike.

Development and applications of sialoglycan-recognizing probes (SGRPs) with defined specificities: exploring the dynamic mammalian sialoglycome

Glycans that are abundantly displayed on vertebrate cell surface and secreted molecules are often capped with terminal sialic acids (Sias). These diverse 9-carbon-backbone monosaccharides are involved in numerous intrinsic biological processes. They also interact with commensals and pathogens, while undergoing dynamic changes in time and space, often influenced by environmental conditions. However, most of this sialoglycan complexity and variation remains poorly characterized by conventional techniques, which often tend to destroy or overlook crucial aspects of Sia diversity and/or fail to elucidate native structures in biological systems, i.e. in the intact sialome. To date, in situ detection and analysis of sialoglycans has largely relied on the use of plant lectins, sialidases, or antibodies, whose preferences (with certain exceptions) are limited and/or uncertain. We took advantage of naturally evolved microbial molecules (bacterial adhesins, toxin subunits, and viral hemagglutinin-esterases) that recognize sialoglycans with defined specificity to delineate 9 classes of sialoglycan recognizing probes (SGRPs: SGRP1-SGRP9) that can be used to explore mammalian sialome changes in a simple and systematic manner, using techniques common in most laboratories. SGRP candidates with specificity defined by sialoglycan microarray studies were engineered as tagged probes, each with a corresponding nonbinding mutant probe as a simple and reliable negative control. The optimized panel of SGRPs can be used in methods commonly available in most bioscience labs, such as ELISA, western blot, flow cytometry, and histochemistry. To demonstrate the utility of this approach, we provide examples of sialoglycome differences in tissues from C57BL/6 wild-type mice and human-like Cmah-/- mice.

The structure of Phocaeicola vulgatus sialic acid acetylesterase

Sialic acids terminate many N- and O-glycans and are widely distributed on cell surfaces. There are a diverse range of enzymes which interact with these sugars throughout the tree of life. They can act as receptors for influenza and specific betacoronaviruses in viral binding and their cleavage is important in virion release. Sialic acids are also exploited by both commensal and pathogenic bacteria for nutrient acquisition. A common modification of sialic acid is 9-O-acetylation, which can limit the action of sialidases. Some bacteria, including human endosymbionts, employ esterases to overcome this modification. However, few bacterial sialic acid 9-O-acetylesterases (9-O-SAEs) have been structurally characterized. Here, the crystal structure of a 9-O-SAE from Phocaeicola vulgatus (PvSAE) is reported. The structure of PvSAE was determined to resolutions of 1.44 and 2.06 Å using crystals from two different crystallization conditions. Structural characterization revealed PvSAE to be a dimer with an SGNH fold, named after the conserved sequence motif of this family, and a Ser-His-Asp catalytic triad. These structures also reveal flexibility in the most N-terminal α-helix, which provides a barrier to active-site accessibility. Biochemical assays also show that PvSAE deacetylates both mucin and the acetylated chromophore para-nitrophenyl acetate. This structural and biochemical characterization of PvSAE furthers the understanding of 9-O-SAEs and may aid in the discovery of small molecules targeting this class of enzyme.

Mucins Inhibit Coronavirus Infection in a Glycan-Dependent Manner

Mucins are a diverse and heterogeneous family of glycoproteins that comprise the bulk of mucus and the epithelial glycocalyx. Mucins are intimately involved in viral transmission. Mucin and virus laden particles can be expelled from the mouth and nose to later infect others. Viruses must also penetrate the mucus layer before cell entry and replication. The role of mucins and their molecular structure have not been well-characterized in coronavirus transmission studies. Laboratory studies predicting high rates of fomite transmission have not translated to real-world infections, and mucins may be one culprit. Here, we probed both surface and direct contact transmission scenarios for their dependence on mucins and their structure. We utilized disease-causing, bovine-derived, human coronavirus OC43. We found that bovine mucins could inhibit the infection of live cells in a concentration- and glycan-dependent manner. The effects were observed in both mock fomite and direct contact transmission experiments and were not dependent upon surface material or time-on-surface. However, the effects were abrogated by removal of the glycans or in a cross-species infection scenario where bovine mucin could not inhibit the infection of a murine coronavirus. Together, our data indicate that the mucin molecular structure plays a complex and important role in host defense.

Evaluation of the Glycan-Binding and Esterase Activities of Hemagglutinin-Esterase-Fusion Glycoprotein from Influenza D Virus

Influenza D virus (IDV) is a new member of influenza virus that uses cattle as the primary reservoir and infects multiple agricultural animals. Similar to influenza C virus (ICV), IDV also has seven segments in its genome and has only one major surface glycoprotein, called the hemagglutinin-esterase-fusion (HEF) protein, for receptor-binding, receptor-destroying, and membrane fusion. HEF utilizes 9-O-acetylated sialic acids as its receptor and has both receptor binding and esterase activities, thus is a critical determinant of host tropism. Here, we summarize the methods to evaluate the glycan-binding and esterase activities of HEF in vitro. The glycan-bind property is monitored through glycan microarray, MDCK cell-binding assay, Hemagglutination assay, solid-phase lectin binding assay, and immunofluorescence of tissue sections, and its esterase property is analyzed via esterase enzymatic activity assay.

SDAV, the Rat Coronavirus-How Much Do We Know about It in the Light of Potential Zoonoses

Sialodacryoadenitis virus (SDAV) is known to be an etiological agent, causing infections in laboratory rats. Until now, its role has only been considered in studies on respiratory and salivary gland infections. The scant literature data, consisting mainly of papers from the last century, do not sufficiently address the topic of SDAV infections. The ongoing pandemic has demonstrated, once again, the role of the Coronaviridae family as extremely dangerous etiological agents of human zoonoses. The ability of coronaviruses to cross the species barrier and change to hosts commonly found in close proximity to humans highlights the need to characterize SDAV infections. The main host of the infection is the rat, as mentioned above. Rats inhabit large urban agglomerations, carrying a vast epidemic threat. Of the 2277 existing rodent species, 217 are reservoirs for 66 zoonotic diseases caused by viruses, bacteria, fungi, and protozoa. This review provides insight into the current state of knowledge of SDAV characteristics and its likely zoonotic potential.

Anti-SARS-CoV-2 Natural Products as Potentially Therapeutic Agents

Severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2), a β-coronavirus, is the cause of the recently emerged pandemic and worldwide outbreak of respiratory disease. Researchers exchange information on COVID-19 to enable collaborative searches. Although there is as yet no effective antiviral agent, like tamiflu against influenza, to block SARS-CoV-2 infection to its host cells, various candidates to mitigate or treat the disease are currently being investigated. Several drugs are being screened for the ability to block virus entry on cell surfaces and/or block intracellular replication in host cells. Vaccine development is being pursued, invoking a better elucidation of the life cycle of the virus. SARS-CoV-2 recognizes O-acetylated neuraminic acids and also several membrane proteins, such as ACE2, as the result of evolutionary switches of O-Ac SA recognition specificities. To provide information related to the current development of possible anti-SARS-COV-2 viral agents, the current review deals with the known inhibitory compounds with low molecular weight. The molecules are mainly derived from natural products of plant sources by screening or chemical synthesis via molecular simulations. Artificial intelligence-based computational simulation for drug designation and large-scale inhibitor screening have recently been performed. Structure-activity relationship of the anti-SARS-CoV-2 natural compounds is discussed.

The sialyl-O-acetylesterase NanS of Tannerella forsythia encompasses two catalytic modules with different regiospecificity for O7 and O9 of sialic acid

The periodontal pathogen Tannerella forsythia utilizes host sialic acids as a nutrient source. To also make O-acetylated sialyl residues susceptible to the action of its sialidase and sialic acid up-take system, Tannerella produces NanS, an O-acetylesterase with two putative catalytic domains. Here, we analyzed NanS by homology modeling, predicted a catalytic serine-histidine-aspartate triad for each catalytic domain and performed individual domain inactivation by single alanine exchanges of the triad nucleophiles S32 and S311. Subsequent functional analyses revealed that both domains possess sialyl-O-acetylesterase activity, but differ in their regioselectivity with respect to position O9 and O7 of sialic acid. The 7-O-acetylesterase activity inherent to the C-terminal domain of NanS is unique among sialyl-O-acetylesterases and fills the current gap in tools targeting 7-O-acetylation. Application of the O7-specific variant NanS-S32A allowed us to evidence the presence of cellular 7,9-di-O-acetylated sialoglycans by monitoring the gain in 9-O-acetylation upon selective removal of acetyl groups from O7. Moreover, we established de-7,9-O-acetylation by wild-type NanS as an easy and efficient method to validate the specific binding of three viral lectins commonly used for the recognition of (7),9-O-acetylated sialoglycans. Their binding critically depends on an acetyl group in O9, yet de-7,9-O-acetylation proved advantageous over de-9-O-acetylation as the additional removal of the 7-O-acetyl group eliminated ligand formation by 7,9-ester migration. Together, our data show that NanS gained dual functionality through recruitment of two esterase modules with complementary activities. This enables Tannerella to scavenge 7,9-di-O-acetylated sialyl residues and provides a novel, O7-specific tool for studying sialic acid O-acetylation.

Phylogenic analysis of coronavirus genome and molecular studies on potential anti-COVID-19 agents from selected FDA-approved drugs

The emergence of 2019 novel Coronavirus (COVID-19 or 2019-nCoV) has caused significant global morbidity and mortality with no consensus specific treatment. We tested the hypothesis that FDA-approved antiretrovirals, antibiotics, and antimalarials will effectively inhibit COVID-19 two major drug targets, coronavirus nucleocapsid protein (NP) and hemagglutinin-esterase (HE). To test this hypothesis, we carried out a phylogenic analysis of coronavirus genome to understand the origins of NP and HE, and also modeled the proteins before molecular docking, druglikeness, toxicity assessment, molecular dynamics simulation (MDS) and ligand-based pharmacophore modeling of the selected FDA-approved drugs. Our models for NP and HE had over 95% identity with templates 5EPW and 3CL5 respectively in the PDB database, with majority of the amino acids occupying acceptable regions. The active sites of the proteins contained conserved residues that were involved in ligand binding. Lopinavir and ritonavir possessed greater binding affinities for NP and HE relative to remdesivir, while levofloxacin and hydroxychloroquine were the most notable among the other classes of drugs. The Root Mean Square Deviation (RMSD), Root Mean Square Fluctuation (RMSF), Radius of gyration (Rg), and binding energy values obtained after 100 ns of MDS revealed good stability of these compounds in the binding sites of the proteins while important pharmacophore features were also identified. The study showed that COVID-19 likely originated from bat, owing to the over 90% genomic similarity observed, and that lopinavir, levofloxacin, and hydroxychloroquine might serve as potential anti-COVID-19 lead molecules for additional optimization and drug development for the treatment of COVID-19.Communicated by Ramaswamy H. Sarma.

Clinical and molecular aspects of veterinary coronaviruses

Coronaviruses are a large group of RNA viruses that infect a wide range of animal species. The replication strategy of coronaviruses involves recombination and mutation events that lead to the possibility of cross-species transmission. The high plasticity of the viral receptor due to a continuous modification of the host species habitat may be the cause of cross-species transmission that can turn into a threat to other species including the human population. The successive emergence of highly pathogenic coronaviruses such as the Severe Acute Respiratory Syndrome (SARS) in 2003, the Middle East Respiratory Syndrome Coronavirus in 2012, and the recent SARS-CoV-2 has incentivized a number of studies on the molecular basis of the coronavirus and its pathogenesis. The high degree of interrelatedness between humans and wild and domestic animals and the modification of animal habitats by human urbanization, has favored new viral spreads. Hence, knowledge on the main clinical signs of coronavirus infection in the different hosts and the distinctive molecular characteristics of each coronavirus is essential to prevent the emergence of new coronavirus diseases. The coronavirus infections routinely studied in veterinary medicine must be properly recognized and diagnosed not only to prevent animal disease but also to promote public health.

Host Receptors of Influenza Viruses and Coronaviruses-Molecular Mechanisms of Recognition

Among the four genera of influenza viruses (IVs) and the four genera of coronaviruses (CoVs), zoonotic αIV and βCoV have occasionally caused airborne epidemic outbreaks in humans, who are immunologically naïve, and the outbreaks have resulted in high fatality rates as well as social and economic disruption and losses. The most devasting influenza A virus (IAV) in αIV, pandemic H1N1 in 1918, which caused at least 40 million deaths from about 500 million cases of infection, was the first recorded emergence of IAVs in humans. Usually, a novel human-adapted virus replaces the preexisting human-adapted virus. Interestingly, two IAV subtypes, A/H3N2/1968 and A/H1N1/2009 variants, and two lineages of influenza B viruses (IBV) in βIV, B/Yamagata and B/Victoria lineage-like viruses, remain seasonally detectable in humans. Both influenza C viruses (ICVs) in γIV and four human CoVs, HCoV-229E and HCoV-NL63 in αCoV and HCoV-OC43 and HCoV-HKU1 in βCoV, usually cause mild respiratory infections. Much attention has been given to CoVs since the global epidemic outbreaks of βSARS-CoV in 2002-2004 and βMERS-CoV from 2012 to present. βSARS-CoV-2, which is causing the ongoing COVID-19 pandemic that has resulted in 890,392 deaths from about 27 million cases of infection as of 8 September 2020, has provoked worldwide investigations of CoVs. With the aim of developing efficient strategies for controlling virus outbreaks and recurrences of seasonal virus variants, here we overview the structures, diversities, host ranges and host receptors of all IVs and CoVs and critically review current knowledge of receptor binding specificity of spike glycoproteins, which mediates infection, of IVs and of zoonotic, pandemic and seasonal CoVs.

An emerging novel bovine coronavirus with a 4-amino-acid insertion in the receptor-binding domain of the hemagglutinin-esterase gene

The hemagglutinin-esterase (HE) protein of betacoronavirus lineage A is a secondary receptor in the infection process and is involved in the emergence of new betacoronavirus genotypes with altered host specificity and tissue tropism. We previously reported a novel recombinant bovine coronavirus (BCoV) strain that was circulating in dairy cattle in China, but this virus was not successfully isolated, and the genetic characteristics of BCoV are still largely unknown. In this study, 20 diarrheic faecal samples were collected from a farm in Liaoning province that had an outbreak of calf diarrhea (≤ 3 months of age) in November 2018, and all of the samples tested positive for BCoV by RT-PCR. In addition, a BCoV strain with a recombinant HE (designated as SWUN/A1/2018) and another BCoV strain with a recombinant HE containing an insertion (designated as SWUN/A10/2018) were successfully isolated in cell culture (TCID50: 10^4.25/mL and 10^4.73/mL, respectively). Unexpectedly, we identified the emergence of a novel BCoV variant characterized by a 12-nt bovine gene insertion in the receptor-binding domain in a natural recombinant HE gene, suggesting a novel evolutionary pattern in BCoV.

Coronavirus hemagglutinin-esterase and spike proteins coevolve for functional balance and optimal virion avidity

Human coronaviruses OC43 and HKU1 are respiratory pathogens of zoonotic origin that have gained worldwide distribution. OC43 apparently emerged from a bovine coronavirus (BCoV) spillover. All three viruses attach to 9-O-acetylated sialoglycans via spike protein S with hemagglutinin-esterase (HE) acting as a receptor-destroying enzyme. In BCoV, an HE lectin domain promotes esterase activity toward clustered substrates. OC43 and HKU1, however, lost HE lectin function as an adaptation to humans. Replaying OC43 evolution, we knocked out BCoV HE lectin function and performed forced evolution-population dynamics analysis. Loss of HE receptor binding selected for second-site mutations in S, decreasing S binding affinity by orders of magnitude. Irreversible HE mutations led to cooperativity in virus swarms with low-affinity S minority variants sustaining propagation of high-affinity majority phenotypes. Salvageable HE mutations induced successive second-site substitutions in both S and HE. Apparently, S and HE are functionally interdependent and coevolve to optimize the balance between attachment and release. This mechanism of glycan-based receptor usage, entailing a concerted, fine-tuned activity of two envelope protein species, is unique among CoVs, but reminiscent of that of influenza A viruses. Apparently, general principles fundamental to virion-sialoglycan interactions prompted convergent evolution of two important groups of human and animal pathogens.

Cryo-EM structure of coronavirus-HKU1 haemagglutinin esterase reveals architectural changes arising from prolonged circulation in humans

The human betacoronaviruses HKU1 and OC43 (subgenus Embecovirus) arose from separate zoonotic introductions, OC43 relatively recently and HKU1 apparently much longer ago. Embecovirus particles contain two surface projections called spike (S) and haemagglutinin-esterase (HE), with S mediating receptor binding and membrane fusion, and HE acting as a receptor-destroying enzyme. Together, they promote dynamic virion attachment to glycan-based receptors, specifically 9-O-acetylated sialic acid. Here we present the cryo-EM structure of the ~80 kDa, heavily glycosylated HKU1 HE at 3.4 Å resolution. Comparison with existing HE structures reveals a drastically truncated lectin domain, incompatible with sialic acid binding, but with the structure and function of the esterase domain left intact. Cryo-EM and mass spectrometry analysis reveals a putative glycan shield on the now redundant lectin domain. The findings further our insight into the evolution and host adaptation of human embecoviruses, and demonstrate the utility of cryo-EM for studying small, heavily glycosylated proteins.

SARS-CoV-2 Evolutionary Adaptation toward Host Entry and Recognition of Receptor O-Acetyl Sialylation in Virus-Host Interaction

The recently emerged SARS-CoV-2 is the cause of the global health crisis of the coronavirus disease 2019 (COVID-19) pandemic. No evidence is yet available for CoV infection into hosts upon zoonotic disease outbreak, although the CoV epidemy resembles influenza viruses, which use sialic acid (SA). Currently, information on SARS-CoV-2 and its receptors is limited. O-acetylated SAs interact with the lectin-like spike glycoprotein of SARS CoV-2 for the initial attachment of viruses to enter into the host cells. SARS-CoV-2 hemagglutinin-esterase (HE) acts as the classical glycan-binding lectin and receptor-degrading enzyme. Most β-CoVs recognize 9-O-acetyl-SAs but switched to recognizing the 4-O-acetyl-SA form during evolution of CoVs. Type I HE is specific for the 9-O-Ac-SAs and type II HE is specific for 4-O-Ac-SAs. The SA-binding shift proceeds through quasi-synchronous adaptations of the SA-recognition sites of the lectin and esterase domains. The molecular switching of HE acquisition of 4-O-acetyl binding from 9-O-acetyl SA binding is caused by protein–carbohydrate interaction (PCI) or lectin–carbohydrate interaction (LCI). The HE gene was transmitted to a β-CoV lineage A progenitor by horizontal gene transfer from a 9-O-Ac-SA–specific HEF, as in influenza virus C/D. HE acquisition, and expansion takes place by cross-species transmission over HE evolution. This reflects viral evolutionary adaptation to host SA-containing glycans. Therefore, CoV HE receptor switching precedes virus evolution driven by the SA-glycan diversity of the hosts. The PCI or LCI stereochemistry potentiates the SA–ligand switch by a simple conformational shift of the lectin and esterase domains. Therefore, examination of new emerging viruses can lead to better understanding of virus evolution toward transitional host tropism. A clear example of HE gene transfer is found in the BCoV HE, which prefers 7,9-di-O-Ac-SAs, which is also known to be a target of the bovine torovirus HE. A more exciting case of such a switching event occurs in the murine CoVs, with the example of the β-CoV lineage A type binding with two different subtypes of the typical 9-O-Ac-SA (type I) and the exclusive 4-O-Ac-SA (type II) attachment factors. The protein structure data for type II HE also imply the virus switching to binding 4-O acetyl SA from 9-O acetyl SA. Principles of the protein–glycan interaction and PCI stereochemistry potentiate the SA–ligand switch via simple conformational shifts of the lectin and esterase domains. Thus, our understanding of natural adaptation can be specified to how carbohydrate/glycan-recognizing proteins/molecules contribute to virus evolution toward host tropism. Under the current circumstances where reliable antiviral therapeutics or vaccination tools are lacking, several trials are underway to examine viral agents. As expected, structural and non-structural proteins of SARS-CoV-2 are currently being targeted for viral therapeutic designation and development. However, the modern global society needs SARS-CoV-2 preventive and therapeutic drugs for infected patients. In this review, the structure and sialobiology of SARS-CoV-2 are discussed in order to encourage and activate public research on glycan-specific interaction-based drug creation in the near future.

Distinct Roles for Sialoside and Protein Receptors in Coronavirus Infection

Coronaviruses (CoVs) are common human and animal pathogens that can transmit zoonotically and cause severe respiratory disease syndromes. CoV infection requires spike proteins, which bind viruses to host cell receptors and catalyze virus-cell membrane fusion. Several CoV strains have spike proteins with two receptor-binding domains, an S1A that engages host sialic acids and an S1B that recognizes host transmembrane proteins. As this bivalent binding may enable broad zoonotic CoV infection, we aimed to identify roles for each receptor in distinct infection stages. Focusing on two betacoronaviruses, murine JHM-CoV and human Middle East respiratory syndrome coronavirus (MERS-CoV), we found that virus particle binding to cells was mediated by sialic acids; however, the transmembrane protein receptors were required for a subsequent virus infection. These results favored a two-step process in which viruses first adhere to sialic acids and then require subsequent engagement with protein receptors during infectious cell entry. However, sialic acids sufficiently facilitated the later stages of virus spread through cell-cell membrane fusion, without requiring protein receptors. This virus spread in the absence of the prototype protein receptors was increased by adaptive S1A mutations. Overall, these findings reveal roles for sialic acids in virus-cell binding, viral spike protein-directed cell-cell fusion, and resultant spread of CoV infections.IMPORTANCE CoVs can transmit from animals to humans to cause serious disease. This zoonotic transmission uses spike proteins, which bind CoVs to cells with two receptor-binding domains. Here, we identified the roles for the two binding processes in the CoV infection process. Binding to sialic acids promoted infection and also supported the intercellular expansion of CoV infections through syncytial development. Adaptive mutations in the sialic acid-binding spike domains increased the intercellular expansion process. These findings raise the possibility that the lectin-like properties of many CoVs contribute to facile zoonotic transmission and intercellular spread within infected organisms.

Post-Glycosylation Modification of Sialic Acid and Its Role in Virus Pathogenesis

Sialic acids are a family of nine carbon keto-aldononulosonic acids presented at the terminal ends of glycans on cellular membranes. α-Linked sialoglycoconjugates often undergo post-glycosylation modifications, among which O-acetylation of N-acetyl neuraminic acid (Neu5Ac) is the most common in mammalian cells. Isoforms of sialic acid are critical determinants of virus pathogenesis. To date, the focus of viral receptor-mediated attachment has been on Neu5Ac. O-Acetylated Neu5Acs have been largely ignored as receptor determinants of virus pathogenesis, although it is ubiquitous across species. Significantly, the array of structures resulting from site-specific O-acetylation by sialic acid O-acetyltransferases (SOATs) provides a means to examine specificity of viral binding to host cells. Specifically, C₄ O-acetylated Neu5Ac can influence virus pathogenicity. However, the biological implications of only O-acetylated Neu5Ac at C_7-9 have been explored extensively. This review will highlight the biological significance, extraction methods, and synthetic modifications of C₄ O-acetylated Neu5Ac that may provide value in therapeutic developments and targets to prevent virus related diseases.

Structural basis for human coronavirus attachment to sialic acid receptors

Coronaviruses cause respiratory tract infections in humans and outbreaks of deadly pneumonia worldwide. Infections are initiated by the transmembrane spike (S) glycoprotein, which binds to host receptors and fuses the viral and cellular membranes. To understand the molecular basis of coronavirus attachment to oligosaccharide receptors, we determined cryo-EM structures of coronavirus OC43 S glycoprotein trimer in isolation and in complex with a 9-O-acetylated sialic acid. We show that the ligand binds with fast kinetics to a surface-exposed groove and that interactions at the identified site are essential for S-mediated viral entry into host cells, but free monosaccharide does not trigger fusogenic conformational changes. The receptor-interacting site is conserved in all coronavirus S glycoproteins that engage 9-O-acetyl-sialogycans, with an architecture similar to those of the ligand-binding pockets of coronavirus hemagglutinin esterases and influenza virus C/D hemagglutinin-esterase fusion glycoproteins. Our results demonstrate these viruses evolved similar strategies to engage sialoglycans at the surface of target cells.

Structural and functional analyses reveal how 9-O-acetyl sialic acid is recognized by the human coronavirus OC43 S glycoprotein and how this interaction promotes viral entry.

Prevalence of a novel bovine coronavirus strain with a recombinant hemagglutinin/esterase gene in dairy calves in China

Bovine coronavirus (BCoV) is the causative agent of diarrhoea in newborn calves, winter dysentery in adult cattle and respiratory tract illnesses in cattle across the world. In this study, a total of 190 faecal samples from dairy calves with diarrhoea were collected from 14 farms in six Chinese provinces, and BCoV was detected in 18.95% (36/190) of the samples by reverse transcriptase polymerase chain reaction. Full-length spike, hemagglutinin/esterase (HE), nucleocapsid and transmembrane genes were simultaneously cloned from 13 clinical samples (eight farms in four provinces), and most of the BCoV strains showed a unique evolutionary pattern based on the phylogenetic analysis of these genes. Interesting, 10 of the 13 strains were identified as HE recombinant strains, and these strains had experienced the same recombination event and carried the same recombination sites located between the esterase and lectin domain. They also shared an identical aa variant (F181V) in the R2-loop. Moreover, 9/10 strains displayed another identical aa variant (P, S158A) in the adjacent R1-loop of the HE gene, which differs from the other available BCoV HE sequences in the GenBank database. Our results showed that BCoV is widely circulating in dairy cattle in China, contributing to the diagnosis and control of dairy calves diarrhoea. Furthermore, a BCoV strain that carries a recombinant HE gene has spread in dairy calves in China. To the best of our knowledge, this is the first description of an HE recombination event occurring in BCoV; this is also the first description of the molecular prevalence of BCoV in China. Our findings will enhance current understanding about the genetic evolution of BCoV.

Human coronaviruses OC43 and HKU1 bind to 9-O-acetylated sialic acids via a conserved receptor-binding site in spike protein domain A

Human betacoronaviruses OC43 and HKU1 are endemic respiratory pathogens and, while related, originated from independent zoonotic introductions. OC43 is in fact a host-range variant of the species Betacoronavirus-1, and more closely related to bovine coronavirus (BCoV)-its presumptive ancestor-and porcine hemagglutinating encephalomyelitis virus (PHEV). The β1-coronaviruses (β1CoVs) and HKU1 employ glycan-based receptors carrying 9-O-acetylated sialic acid (9-O-Ac-Sia). Receptor binding is mediated by spike protein S, the main determinant of coronavirus host specificity. For BCoV, a crystal structure for the receptor-binding domain S1^A is available and for HKU1 a cryoelectron microscopy structure of the complete S ectodomain. However, the location of the receptor-binding site (RBS), arguably the single-most important piece of information, is unknown. Here we solved the 3.0-Å crystal structure of PHEV S1^A We then took a comparative structural analysis approach to map the β1CoV S RBS, using the general design of 9-O-Ac-Sia-binding sites as blueprint, backed-up by automated ligand docking, structure-guided mutagenesis of OC43, BCoV, and PHEV S1^A, and infectivity assays with BCoV-S-pseudotyped vesicular stomatitis viruses. The RBS is not exclusive to OC43 and related animal viruses, but is apparently conserved and functional also in HKU1 S1^A The binding affinity of the HKU1 S RBS toward short sialoglycans is significantly lower than that of OC43, which we attribute to differences in local architecture and accessibility, and which may be indicative for differences between the two viruses in receptor fine-specificity. Our findings challenge reports that would map the OC43 RBS elsewhere in S1^A and that of HKU1 in domain S1^B.

Betacoronavirus Adaptation to Humans Involved Progressive Loss of Hemagglutinin-Esterase Lectin Activity

Human beta1-coronavirus (β1CoV) OC43 emerged relatively recently through a single zoonotic introduction. Like related animal β1CoVs, OC43 uses 9-O-acetylated sialic acid as receptor determinant. β1CoV receptor binding is typically controlled by attachment/fusion spike protein S and receptor-binding/receptor-destroying hemagglutinin-esterase protein HE. We show that following OC43’s introduction into humans, HE-mediated receptor binding was selected against and ultimately lost through progressive accumulation of mutations in the HE lectin domain. Consequently, virion-associated receptor-destroying activity toward multivalent glycoconjugates was reduced and altered such that some clustered receptor populations are no longer cleaved. Loss of HE lectin function was also observed for another respiratory human coronavirus, HKU1. This thus appears to be an adaptation to the sialoglycome of the human respiratory tract and for replication in human airways. The findings suggest that the dynamics of virion-glycan interactions contribute to host tropism. Our observations are relevant also to other human respiratory viruses of zoonotic origin, particularly influenza A virus.

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Adaption of coronaviruses OC43 and HKU1 to humans involved loss of HE lectin function

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OC43 HE receptor binding site was lost via progressive accumulation of mutations

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Loss of HE receptor binding alters sialate-9-O-acetylesterase receptor destroying activity

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Balance of receptor binding and receptor destruction contributes to host tropism

Structure of the infectious salmon anemia virus receptor complex illustrates a unique binding strategy for attachment

Orthomyxoviruses are an important family of RNA viruses, which include the various influenza viruses. Despite global efforts to eradicate orthomyxoviral pathogens, these infections remain pervasive. One such orthomyxovirus, infectious salmon anemia virus (ISAV), spreads easily throughout farmed and wild salmonids, constituting a significant economic burden. ISAV entry requires the interplay of the virion-attached hemagglutinin-esterase and fusion glycoproteins. Preventing infections will rely on improved understanding of ISAV entry. Here, we present the crystal structures of ISAV hemagglutinin-esterase unbound and complexed with receptor. Several distinctive features observed in ISAV HE are not seen in any other viral glycoprotein. The structures reveal a unique mode of receptor binding that is dependent on the oligomeric assembly of hemagglutinin-esterase. Importantly, ISAV hemagglutinin-esterase receptor engagement does not initiate conformational rearrangements, suggesting a distinct viral entry mechanism. This work improves our understanding of ISAV pathogenesis and expands our knowledge on the overall diversity of viral glycoprotein-mediated entry mechanisms. Finally, it provides an atomic-resolution model of the primary neutralizing antigen critical for vaccine development.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Effective one-pot multienzyme (OPME) synthesis of monotreme milk oligosaccharides and other sialosides containing 4-O-acetyl sialic acid

A facile one-pot two-enzyme chemoenzymatic approach has been established for the gram (Neu4,5Ac2α3Lac, 1.33 g) and preparative scale (Neu4,5Ac2α3LNnT) synthesis of monotreme milk oligosaccharides. Other O-acetyl-5-N-acetylneuraminic acid (Neu4,5Ac2)- or 4-O-acetyl-5-N-glycolylneuraminic acid (Neu4Ac5Gc) -containing α2-3-sialosides have also been synthesized in the preparative scale. Used as an effective probe, Neu4,5Ac2α3GalβpNP was found to be a suitable substrate by human influenza A viruses but not bacterial sialidases.

Effects of Sialic Acid Modifications on Virus Binding and Infection

Sialic acids (Sias) are abundantly displayed on the surfaces of vertebrate cells, and particularly on all mucosal surfaces. Sias interact with microbes of many types, and are the targets of specific recognition by many different viruses. They may mediate virus binding and infection of cells, or alternatively can act as decoy receptors that bind virions and block virus infection. These nine-carbon backbone monosaccharides naturally occur in many different modified forms, and are attached to underlying glycans through varied linkages, creating significant diversity in the pathogen receptor forms. Here we review the current knowledge regarding the distribution of modified Sias in different vertebrate hosts, tissues, and cells, their effects on viral pathogens where those have been examined, and outline unresolved questions.

Sialic acids (Sias) are components of cell-surface glycoproteins and glycolipids, as well as secreted glycoproteins and milk oligosaccharides. Sias play important roles in cell signaling, development, and host–pathogen interactions. Cellular enzymes can modify Sias, yet how modifications vary between tissues and hosts has not been fully elucidated.

Many viruses use Sias as receptors, with different modifications aiding or inhibiting virus infection. How modified Sias influence viral protein evolution and determine host/tissue tropism are poorly understood, and are important areas of research.

New advances in molecular glycobiology using pathogen proteins to detect varied forms allows for improved study of modified Sias that have otherwise proven difficult to isolate. This opens new avenues of inquiry for virology, as well as host interactions with bacterial and eukaryotic pathogens.

Coronavirus receptor switch explained from the stereochemistry of protein-carbohydrate interactions and a single mutation

Hemagglutinin-esterases (HEs) are bimodular envelope proteins of orthomyxoviruses, toroviruses, and coronaviruses with a carbohydrate-binding "lectin" domain appended to a receptor-destroying sialate-O-acetylesterase ("esterase"). In concert, these domains facilitate dynamic virion attachment to cell-surface sialoglycans. Most HEs (type I) target 9-O-acetylated sialic acids (9-O-Ac-Sias), but one group of coronaviruses switched to using 4-O-Ac-Sias instead (type II). This specificity shift required quasisynchronous adaptations in the Sia-binding sites of both lectin and esterase domains. Previously, a partially disordered crystal structure of a type II HE revealed how the shift in lectin ligand specificity was achieved. How the switch in esterase substrate specificity was realized remained unresolved, however. Here, we present a complete structure of a type II HE with a receptor analog in the catalytic site and identify the mutations underlying the 9-O- to 4-O-Ac-Sia substrate switch. We show that (i) common principles pertaining to the stereochemistry of protein-carbohydrate interactions were at the core of the transition in lectin ligand and esterase substrate specificity; (ii) in consequence, the switch in O-Ac-Sia specificity could be readily accomplished via convergent intramolecular coevolution with only modest architectural changes in lectin and esterase domains; and (iii) a single, inconspicuous Ala-to-Ser substitution in the catalytic site was key to the emergence of the type II HEs. Our findings provide fundamental insights into how proteins "see" sugars and how this affects protein and virus evolution.

An Open Receptor-Binding Cavity of Hemagglutinin-Esterase-Fusion Glycoprotein from Newly-Identified Influenza D Virus: Basis for Its Broad Cell Tropism

Influenza viruses cause seasonal flu each year and pandemics or epidemic sporadically, posing a major threat to public health. Recently, a new influenza D virus (IDV) was isolated from pigs and cattle. Here, we reveal that the IDV utilizes 9-O-acetylated sialic acids as its receptor for virus entry. Then, we determined the crystal structures of hemagglutinin-esterase-fusion glycoprotein (HEF) of IDV both in its free form and in complex with the receptor and enzymatic substrate analogs. The IDV HEF shows an extremely similar structural fold as the human-infecting influenza C virus (ICV) HEF. However, IDV HEF has an open receptor-binding cavity to accommodate diverse extended glycan moieties. This structural difference provides an explanation for the phenomenon that the IDV has a broad cell tropism. As IDV HEF is structurally and functionally similar to ICV HEF, our findings highlight the potential threat of the virus to public health.

Complexity and Diversity of the Mammalian Sialome Revealed by Nidovirus Virolectins

Sialic acids (Sias), 9-carbon-backbone sugars, are among the most complex and versatile molecules of life. As terminal residues of glycans on proteins and lipids, Sias are key elements of glycotopes of both cellular and microbial lectins and thus act as important molecular tags in cell recognition and signaling events. Their functions in such interactions can be regulated by post-synthetic modifications, the most common of which is differential Sia-O-acetylation (O-Ac-Sias). The biology of O-Ac-Sias remains mostly unexplored, largely because of limitations associated with their specific in situ detection. Here, we show that dual-function hemagglutinin-esterase envelope proteins of nidoviruses distinguish between a variety of closely related O-Ac-Sias. By using soluble forms of hemagglutinin-esterases as lectins and sialate-O-acetylesterases, we demonstrate differential expression of distinct O-Ac-sialoglycan populations in an organ-, tissue- and cell-specific fashion. Our findings indicate that programmed Sia-O-acetylation/de-O-acetylation may be critical to key aspects of cell development, homeostasis, and/or function.

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Virolectins detect and distinguish between closely related O-Ac-Sias in situ

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O-Ac-sialoglycans occur in nature in a diversity not appreciated so far

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O-Ac-Sias are differentially expressed in a species-, tissue-, and cell-specific fashion

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There is extensive cell-to-cell variability in O-Ac-Sia expression in vivo and in vitro

Human Coronavirus HKU1 Spike Protein Uses O-Acetylated Sialic Acid as an Attachment Receptor Determinant and Employs Hemagglutinin-Esterase Protein as a Receptor-Destroying Enzyme

Human coronavirus (hCoV) HKU1 is one of six hCoVs identified to date and the only one with an unidentified cellular receptor. hCoV-HKU1 encodes a hemagglutinin-esterase (HE) protein that is unique to the group a betacoronaviruses (group 2a). The function of HKU1-HE remains largely undetermined. In this study, we examined binding of the S1 domain of hCoV-HKU1 spike to a panel of cells and found that the S1 could specifically bind on the cell surface of a human rhabdomyosarcoma cell line, RD. Pretreatment of RD cells with neuraminidase (NA) and trypsin greatly reduced the binding, suggesting that the binding was mediated by sialic acids on glycoproteins. However, unlike other group 2a CoVs, e.g., hCoV-OC43, for which 9-O-acetylated sialic acid (9-O-Ac-Sia) serves as a receptor determinant, HKU1-S1 bound with neither 9-O-Ac-Sia-containing glycoprotein(s) nor rat and mouse erythrocytes. Nonetheless, the HKU1-HE was similar to OC43-HE, also possessed sialate-O-acetylesterase activity, and acted as a receptor-destroying enzyme (RDE) capable of eliminating the binding of HKU1-S1 to RD cells, whereas the O-acetylesterase-inactive HKU1-HE mutant lost this capacity. Using primary human ciliated airway epithelial (HAE) cell cultures, the only in vitro replication model for hCoV-HKU1 infection, we confirmed that pretreatment of HAE cells with HE but not the enzymatically inactive mutant blocked hCoV-HKU1 infection. These results demonstrate that hCoV-HKU1 exploits O-Ac-Sia as a cellular attachment receptor determinant to initiate the infection of host cells and that its HE protein possesses the corresponding sialate-O-acetylesterase RDE activity.

IMPORTANCE

Human coronaviruses (hCoV) are important human respiratory pathogens. Among the six hCoVs identified to date, only hCoV-HKU1 has no defined cellular receptor. It is also unclear whether hemagglutinin-esterase (HE) protein plays a role in viral entry. In this study, we found that, similarly to other members of the group 2a CoVs, sialic acid moieties on glycoproteins are critical receptor determinants for the hCoV-HKU1 infection. Interestingly, the virus seems to employ a type of sialic acid different from those employed by other group 2a CoVs. In addition, we determined that the HKU1-HE protein is an O-acetylesterase and acts as a receptor-destroying enzyme (RDE) for hCoV-HKU1. This is the first study to demonstrate that hCoV-HKU1 uses certain types of O-acetylated sialic acid residues on glycoproteins to initiate the infection of host cells and that the HKU1-HE protein possesses sialate-O-acetylesterase RDE activity.

Accessory proteins of SARS-CoV and other coronaviruses

The huge RNA genome of SARS coronavirus comprises a number of open reading frames that code for a total of eight accessory proteins. Although none of these are essential for virus replication, some appear to have a role in virus pathogenesis. Notably, some SARS-CoV accessory proteins have been shown to modulate the interferon signaling pathways and the production of pro-inflammatory cytokines. The structural information on these proteins is also limited, with only two (p7a and p9b) having their structures determined by X-ray crystallography. This review makes an attempt to summarize the published knowledge on SARS-CoV accessory proteins, with an emphasis on their involvement in virus-host interaction. The accessory proteins of other coronaviruses are also briefly discussed. This paper forms part of a series of invited articles in Antiviral Research on "From SARS to MERS: 10 years of research on highly pathogenic human coronaviruses" (see Introduction by Hilgenfeld and Peiris (2013)).

Glycan Engagement by Viruses: Receptor Switches and Specificity

A large number of viruses, including many human pathogens, bind cell-surface glycans during the initial steps of infection. Viral glycan receptors such as glycosaminoglycans and sialic acid-containing carbohydrates are often negatively charged, but neutral glycans such as histo-blood group antigens can also function as receptors. The engagement of glycans facilitates attachment and entry and, consequently, is often a key determinant of the host range, tissue tropism, pathogenicity, and transmissibility of viruses. Here, we review current knowledge about virus-glycan interactions using representative crystal structures of viral attachment proteins in complex with glycans. We illuminate the determinants of specificity utilized by different glycan-binding viruses and explore the potential of these interactions for switching receptor specificities within or even between glycan classes. A detailed understanding of these parameters is important for the prediction of binding sites where structural information is not available, and is invaluable for the development of antiviral therapeutics.

Coronavirus entry and release in polarized epithelial cells: a review

Most coronaviruses cause respiratory or intestinal infections in their animal or human host. Hence, their interaction with polarized epithelial cells plays a critical role in the onset and outcome of infection. In this paper, we review the knowledge regarding the entry and release of coronaviruses, with particular emphasis on the severe acute respiratory syndrome and Middle East respiratory syndrome coronaviruses. As these viruses approach the epithelial surfaces from the apical side, it is not surprising that coronavirus cell receptors are exposed primarily on the apical domain of polarized epithelial cells. With respect to release, all possibilities appear to occur. Thus, most coronaviruses exit through the apical surface, several through the basolateral one, although the Middle East respiratory syndrome coronavirus appears to use both sides. These observations help us understand the local or systematic spread of the infection within its host as well as the spread of the virus within the host population. Copyright © 2014 John Wiley & Sons, Ltd.

Lineage specific antigenic differences in porcine torovirus hemagglutinin-esterase (PToV-HE) protein

Hemagglutinin-esterases (HE) are viral envelope proteins present in some members from the toro-, corona- and orthomyxovirus families, all related with enteric and/or respiratory tract infections. HE proteins mediate reversible binding to sialic acid receptor determinants, very abundant glycan residues in the enteric and respiratory tracts. The role of the HE protein during the torovirus infection cycle remains unknown, although it is believed to be important in the natural infection process. The phylogenetic analysis of HE coding sequences from porcine torovirus (PToV) field strains revealed the existence of two distinct HE lineages. In a previous study, PToV virus strains with HE proteins from the two lineages were found coexisting in a pig herd, and they were even obtained from the same animal at two consecutive sampling time points. In this work, we report antigenic differences between the two HE lineages, and discuss the possible implications that the coexistence of viruses belonging to both lineages might have on the spread and sustainment of PToV infection in the farms.

Isolation of a novel swine influenza virus from Oklahoma in 2011 which is distantly related to human influenza C viruses

Of the Orthomyxoviridae family of viruses, only influenza A viruses are thought to exist as multiple subtypes and has non-human maintenance hosts. In April 2011, nasal swabs were collected for virus isolation from pigs exhibiting influenza-like illness. Subsequent electron microscopic, biochemical, and genetic studies identified an orthomyxovirus with seven RNA segments exhibiting approximately 50% overall amino acid identity to human influenza C virus. Based on its genetic organizational similarities to influenza C viruses this virus has been provisionally designated C/Oklahoma/1334/2011 (C/OK). Phylogenetic analysis of the predicted viral proteins found that the divergence between C/OK and human influenza C viruses was similar to that observed between influenza A and B viruses. No cross reactivity was observed between C/OK and human influenza C viruses using hemagglutination inhibition (HI) assays. Additionally, screening of pig and human serum samples found that 9.5% and 1.3%, respectively, of individuals had measurable HI antibody titers to C/OK virus. C/OK virus was able to infect both ferrets and pigs and transmit to naive animals by direct contact. Cell culture studies showed that C/OK virus displayed a broader cellular tropism than a human influenza C virus. The observed difference in cellular tropism was further supported by structural analysis showing that hemagglutinin esterase (HE) proteins between two viruses have conserved enzymatic but divergent receptor-binding sites. These results suggest that C/OK virus represents a new subtype of influenza C viruses that currently circulates in pigs that has not been recognized previously. The presence of multiple subtypes of co-circulating influenza C viruses raises the possibility of reassortment and antigenic shift as mechanisms of influenza C virus evolution.

Exploring the effect of sialic acid orientation on ligand-receptor interactions

Here, we present the synthesis of two sialo-micelles to validate the significance of sialic acid orientation during specific carbohydrate-protein and carbohydrate-carbohydrate interactions. Our data clearly suggest that orientation of carboxylic acid and glycerol side chains of sialic acid moieties exert fine tuning of ligand-receptor interactions.