Sialic acids as receptor determinants for coronaviruses

In vitro antiviral activity of phlorotannins isolated from Ecklonia cava against porcine epidemic diarrhea coronavirus infection and hemagglutination

Despite the prepdominat agent causing severe entero-pathogenic diarrhea in swine, there are no effective therapeutical treatment of porcine epidemic diarrhea virus (PEDV). In this study, we evaluated the antiviral activity of five phlorotannins isolated from Ecklonia cava (E. cava) against PEDV. In vitro antiviral activity was tested using two different assay strategies: (1) blockage of the binding of virus to cells (simultaneous-treatment assay) and (2) inhibition of viral replication (post-treatment assay). In simultaneous-treatment assay, compounds 2-5 except compound 1 exhibited antiviral activities of a 50% inhibitory concentration (IC₅₀) with the ranging from 10.8 ± 1.4 to 22.5 ± 2.2 μM against PEDV. Compounds 1-5 were completely blocked binding of viral spike protein to sialic acids at less than 36.6 μM concentrations by hemagglutination inhibition. Moreover, compounds 4 and 5 of five phlorotannins inhibited viral replication with IC₅₀ values of 12.2 ± 2.8 and 14.6 ± 1.3 μM in the post-treatment assay, respectively. During virus replication steps, compounds 4 and 5 exhibited stronger inhibition of viral RNA and viral protein synthesis in late stages (18 and 24 h) than in early stages (6 and 12 h). Interestingly, compounds 4 and 5 inhibited both viral entry by hemagglutination inhibition and viral replication by inhibition of viral RNA and viral protein synthesis, but not viral protease. These results suggest that compounds isolated from E. cava have strong antiviral activity against PEDV, inhibiting viral entry and/or viral replication, and may be developed into natural therapeutic drugs against coronavirus infection.

Crystal structure of the receptor-binding domain from newly emerged Middle East respiratory syndrome coronavirus

The newly emerged Middle East respiratory syndrome coronavirus (MERS-CoV) has infected at least 77 people, with a fatality rate of more than 50%. Alarmingly, the virus demonstrates the capability of human-to-human transmission, raising the possibility of global spread and endangering world health and economy. Here we have identified the receptor-binding domain (RBD) from the MERS-CoV spike protein and determined its crystal structure. This study also presents a structural comparison of MERS-CoV RBD with other coronavirus RBDs, successfully positioning MERS-CoV on the landscape of coronavirus evolution and providing insights into receptor binding by MERS-CoV. Furthermore, we found that MERS-CoV RBD functions as an effective entry inhibitor of MERS-CoV. The identified MERS-CoV RBD may also serve as a potential candidate for MERS-CoV subunit vaccines. Overall, this study enhances our understanding of the evolution of coronavirus RBDs, provides insights into receptor recognition by MERS-CoV, and may help control the transmission of MERS-CoV in humans.

Quantum dot nanometal surface energy transfer based biosensing of sialic acid compositions and linkages in biological samples

Current methods for analyzing sialic acid diversity in modifications and linkages require multistep processing, derivatization, and chromatographic analyses. We here report a single-step optical method for identification and quantification of different compositions of sialoglycans on glycoproteins and in serum. This was achieved by measuring and quantifying nanometal surface energy transfer (NSET) signals between quantum dots and gold nanoparticles bound to specific sialic acid binding proteins (SBPs) and sialic acid moieties, respectively. The biosensing process is based on the NSET turn-on by external sialic acid species that compete for binding to the SBPs. Selectivity of the biosensor toward sialoglycans can be designed to detect the total amount, glycosylation linkages (α2-6 vs α2-3), and modifications (9-O-acetyl and N-glycolyl groups) in the samples. This nanobiosensor is a prototype expected to achieve limits of the detection down to the micromolar range for high-throughput quantification and analysis of different compositions of sialoglycans present in biological or biomedical samples.

Canine and feline parvoviruses preferentially recognize the non-human cell surface sialic acid N-glycolylneuraminic acid

Feline panleukopenia virus (FPV) is a pathogen whose canine-adapted form (canine parvovirus (CPV)) emerged in 1978. These viruses infect by binding host transferrin receptor type-1 (TfR), but also hemagglutinate erythrocytes. We show that hemagglutination involves selective recognition of the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) but not N-acetylneuraminic acid (Neu5Ac), which differs by only one oxygen atom from Neu5Gc. Overexpression of α2-6 sialyltransferase did not change binding, indicating that both α2-3 and α2-6 linkages are recognized. However, Neu5Gc expression on target cells did not enhance CPV or FPV infection in vitro. Thus, the conserved Neu5Gc-binding preference of these viruses likely plays a role in the natural history of the virus in vivo. Further studies must clarify relationships between virus infection and host Neu5Gc expression. As a first step, we show that transcripts of CMAH (which generates Neu5Gc from Neu5Ac) are at very low levels in Western dog breed cells.

Progress in understanding pathogenic mechanisms of porcine transmissible gastroenteritis virus

Porcine transmissible gastroenteritis virus (TG-EV) is an animal coronavirus that causes severe gastroenteritis in young TGEV-seronegative pigs. This review will focus on recent advances in research of the genomic structure, major structural proteins and their function, virus propagation and replication, virus receptors, genetics and pathogenic mechanisms of TGEV. These data will be helpful in understanding the molecular biological characteristics and genetic variation of TGEV and have important theoretical significance for the development of new vaccines and antiviral drugs.

Ganglioside biochemistry

Gangliosides are sialic acid-containing glycosphingolipids. They occur especially on the cellular surfaces of neuronal cells, where they form a complex pattern, but are also found in many other cell types. The paper provides a general overview on their structures, occurrence, and metabolism. Key functional, biochemical, and pathobiochemical aspects are summarized.

Identification and characterization of a novel alpaca respiratory coronavirus most closely related to the human coronavirus 229E

In 2007, a novel coronavirus associated with an acute respiratory disease in alpacas (Alpaca Coronavirus, ACoV) was isolated. Full-length genomic sequencing of the ACoV demonstrated the genome to be consistent with other Alphacoronaviruses. A putative additional open-reading frame was identified between the nucleocapsid gene and 3'UTR. The ACoV was genetically most similar to the common human coronavirus (HCoV) 229E with 92.2% nucleotide identity over the entire genome. A comparison of spike gene sequences from ACoV and from HCoV-229E isolates recovered over a span of five decades showed the ACoV to be most similar to viruses isolated in the 1960's to early 1980's. The true origin of the ACoV is unknown, however a common ancestor between the ACoV and HCoV-229E appears to have existed prior to the 1960's, suggesting virus transmission, either as a zoonosis or anthroponosis, has occurred between alpacas and humans.

SARS-CoV and emergent coronaviruses: viral determinants of interspecies transmission

Most new emerging viruses are derived from strains circulating in zoonotic reservoirs. Coronaviruses, which had an established potential for cross-species transmission within domesticated animals, suddenly became relevant with the unexpected emergence of the highly pathogenic human SARS-CoV strain from zoonotic reservoirs in 2002. SARS-CoV infected approximately 8000 people worldwide before public health measures halted the epidemic. Supported by robust time-ordered sequence variation, structural biology, well-characterized patient pools, and biological data, the emergence of SARS-CoV represents one of the best-studied natural models of viral disease emergence from zoonotic sources. This review article summarizes previous and more recent advances into the molecular and structural characteristics, with particular emphasis on host–receptor interactions, that drove this remarkable virus disease outbreak in human populations.

Multifarious roles of sialic acids in immunity

Sialic acids are a diverse family of monosaccharides widely expressed on all cell surfaces of vertebrates and so-called "higher" invertebrates, and on certain bacteria that interact with vertebrates. This overview surveys examples of biological roles of sialic acids in immunity, with emphasis on an evolutionary perspective. Given the breadth of the subject, the treatment of individual topics is brief. Subjects discussed include biophysical effects regulation of factor H; modulation of leukocyte trafficking via selectins; Siglecs in immune cell activation; sialic acids as ligands for microbes; impact of microbial and endogenous sialidases on immune cell responses; pathogen molecular mimicry of host sialic acids; Siglec recognition of sialylated pathogens; bacteriophage recognition of microbial sialic acids; polysialic acid modulation of immune cells; sialic acids as pathogen decoys or biological masks; modulation of immunity by sialic acid O-acetylation; sialic acids as antigens and xeno-autoantigens; antisialoglycan antibodies in reproductive incompatibility; and sialic-acid-based blood groups.

Mechanisms of coronavirus cell entry mediated by the viral spike protein

Coronaviruses are enveloped positive-stranded RNA viruses that replicate in the cytoplasm. To deliver their nucleocapsid into the host cell, they rely on the fusion of their envelope with the host cell membrane. The spike glycoprotein (S) mediates virus entry and is a primary determinant of cell tropism and pathogenesis. It is classified as a class I fusion protein, and is responsible for binding to the receptor on the host cell as well as mediating the fusion of host and viral membranes—A process driven by major conformational changes of the S protein. This review discusses coronavirus entry mechanisms focusing on the different triggers used by coronaviruses to initiate the conformational change of the S protein: receptor binding, low pH exposure and proteolytic activation. We also highlight commonalities between coronavirus S proteins and other class I viral fusion proteins, as well as distinctive features that confer distinct tropism, pathogenicity and host interspecies transmission characteristics to coronaviruses.

Lipidomics of glycosphingolipids

Glycosphingolipids (GSLs) contain one or more sugars that are attached to a sphingolipid moiety, usually to a ceramide, but in rare cases also to a sphingoid base. A large structural heterogeneity results from differences in number, identity, linkage, and anomeric configuration of the carbohydrate residues, and also from structural differences within the hydrophobic part. GSLs form complex cell-type specific patterns, which change with the species, the cellular differentiation state, viral transformation, ontogenesis, and oncogenesis. Although GSL structures can be assigned to only a few series with a common carbohydrate core, their structural variety and the complex pattern are challenges for their elucidation and quantification by mass spectrometric techniques. We present a general overview of the application of lipidomics for GSL determination. This includes analytical procedures and instrumentation together with recent correlations of GSL molecular species with human diseases. Difficulties such as the structural complexity and the lack of standard substances for complex GSLs are discussed.

Evidence for a common evolutionary origin of coronavirus spike protein receptor-binding subunits

Among different coronavirus genera, the receptor-binding S1 subunits of their spike proteins differ in primary, secondary, and tertiary structures. This study identified shared structural topologies (connectivity of secondary structural elements) in S1 domains of different coronavirus genera. The results suggest that coronavirus S1 subunits share a common evolutionary origin but have attained diverse sequences and structures following extensive divergent evolution. The results also increase understanding of the structures and functions of coronavirus S1 domains whose tertiary structures are currently unknown.

The sialic acid binding activity of the S protein facilitates infection by porcine transmissible gastroenteritis coronavirus

BACKGROUND

Transmissible gastroenteritis virus (TGEV) has a sialic acid binding activity that is believed to be important for enteropathogenicity, but that has so far appeared to be dispensable for infection of cultured cells. The aims of this study were to determine the effect of sialic acid binding for the infection of cultured cells under unfavorable conditions, and comparison of TGEV strains and mutants, as well as the avian coronavirus IBV concerning their dependence on the sialic acid binding activity.

METHODS

The infectivity of different viruses was analyzed by a plaque assay after adsorption times of 5, 20, and 60 min. Prior to infection, cultured cells were either treated with neuraminidase to deplete sialic acids from the cell surface, or mock-treated. In a second approach, pre-treatment of the virus with porcine intestinal mucin was performed, followed by the plaque assay after a 5 min adsorption time. A student's t-test was used to verify the significance of the results.

RESULTS

Desialylation of cells only had a minor effect on the infection by TGEV strain Purdue 46 when an adsorption period of 60 min was allowed for initiation of infection. However, when the adsorption time was reduced to 5 min the infectivity on desialylated cells decreased by more than 60%. A TGEV PUR46 mutant (HAD3) deficient in sialic acid binding showed a 77% lower titer than the parental virus after a 5 min adsorption time. After an adsorption time of 60 min the titer of HAD3 was 58% lower than that of TGEV PUR46. Another TGEV strain, TGEV Miller, and IBV Beaudette showed a reduction in infectivity after neuraminidase treatment of the cultured cells irrespective of the virion adsorption time.

CONCLUSIONS

Our results suggest that the sialic acid binding activity facilitates the infection by TGEV under unfavorable environmental conditions. The dependence on the sialic acid binding activity for an efficient infection differs in the analyzed TGEV strains.

Binding of avian coronavirus spike proteins to host factors reflects virus tropism and pathogenicity

The binding of viruses to host cells is the first step in determining tropism and pathogenicity. While avian infectious bronchitis coronavirus (IBV) infection and avian influenza A virus (IAV) infection both depend on α2,3-linked sialic acids, the host tropism of IBV is restricted compared to that of IAV. Here we investigated whether the interaction between the viral attachment proteins and the host could explain these differences by using recombinant spike domains (S1) of IBV strains with different pathogenicities, as well as the hemagglutinin (HA) protein of IAV H5N1. Protein histochemistry showed that S1 of IBV strain M41 and HA of IAV subtype H5N1 displayed sialic acid-dependent binding to chicken respiratory tract tissue. However, while HA bound with high avidity to a broad range of α2,3-linked sialylated glycans, M41 S1 recognized only one particular α2,3-linked disialoside in a glycan array. When comparing the binding of recombinant IBV S1 proteins derived from IBV strains with known differences in tissue tropism and pathogenicity, we observed that while M41 S1 displayed binding to cilia and goblet cells of the chicken respiratory tract, S1 derived from the vaccine strain H120 or the nonvirulent Beaudette strain had reduced or no binding to chicken tissues, respectively, in agreement with the reduced abilities of these viruses to replicate in vivo. While the S1 protein derived from the nephropathogenic IBV strain B1648 also hardly displayed binding to respiratory tract cells, distinct binding to kidney cells was observed, but only after the removal of sialic acid from S1. In conclusion, our data demonstrate that the attachment patterns of the IBV S proteins correlate with the tropisms and pathogenicities of the corresponding viruses.

Crystal structure of mouse coronavirus receptor-binding domain complexed with its murine receptor

Coronaviruses have evolved diverse mechanisms to recognize different receptors for their cross-species transmission and host-range expansion. Mouse hepatitis coronavirus (MHV) uses the N-terminal domain (NTD) of its spike protein as its receptor-binding domain. Here we present the crystal structure of MHV NTD complexed with its receptor murine carcinoembryonic antigen-related cell adhesion molecule 1a (mCEACAM1a). Unexpectedly, MHV NTD contains a core structure that has the same β-sandwich fold as human galectins (S-lectins) and additional structural motifs that bind to the N-terminal Ig-like domain of mCEACAM1a. Despite its galectin fold, MHV NTD does not bind sugars, but instead binds mCEACAM1a through exclusive protein-protein interactions. Critical contacts at the interface have been confirmed by mutagenesis, providing a structural basis for viral and host specificities of coronavirus/CEACAM1 interactions. Sugar-binding assays reveal that galectin-like NTDs of some coronaviruses such as human coronavirus OC43 and bovine coronavirus bind sugars. Structural analysis and mutagenesis localize the sugar-binding site in coronavirus NTDs to be above the β-sandwich core. We propose that coronavirus NTDs originated from a host galectin and retained sugar-binding functions in some contemporary coronaviruses, but evolved new structural features in MHV for mCEACAM1a binding.

An adenovirus traffic update: from receptor engagement to the nuclear pore

Adenoviruses have a bipolar nature: they are ubiquitous pathogens that occasionally cause life-threatening diseases or they can be engineered into powerful gene transfer vectors. The goal of this article is to summarize the most recent advances in adenovirus receptor engagement, internalization, endosomal maturation, endosomal escape and trafficking to the nuclear pore. A better understanding of this initial part of the adenovirus lifecycle may identify new mechanistic-based treatments for adenovirus-induced diseases and help in the engineering of more efficient vectors.

Sialic acid and sialyltransferase activity in serum and tissues of dogs with mammary tumors

In humans, the glycosylation pattern of serum and of membrane glycoproteins is associated with invasiveness of tumors: specifically, α2,6-sialylation and α2,3-sialylation are associated with metastasizing and nonmetastasizing tumors, respectively. In turn, the type of sialylation depends on the activity of α2,6 or α2,3 sialyltransferase (ST) enzymes. Because of the high prevalence of metastasizing tumors with biological behavior similar to the human counterpart, female dogs with metastasizing neoplasms could provide a good animal model for investigating the potential roles of sialic acid (Sia) and ST enzymes in the pathogenesis of metastatic tumors. The aims of this study were (1) to validate a solid-phase method based on lectin staining of serum and tissue homogenates to investigate sialylation and ST activity and (2) to compare the results obtained with this method and with lectin staining and to collect preliminary information on sialylation and ST activity in dogs with (n = 8) and without (n = 8) mammary tumors. The data recorded in healthy dogs revealed that serum and tissue glycoproteins are prevalently characterized by a α2,6 sialylation, but ST-α2,3 seems to be the most active enzyme in both samples. Sia-α2,3 and ST-α2,3 activity decreases in serum and tissues of dogs with tumors, especially in a dog with metastasis, suggesting that the equilibrium between ST-α2,6 and ST-α2,3 activity shifts toward the former, as reported in humans.

The sialome--far more than the sum of its parts

The glycome is defined as the glycan repertoire of cells, tissues, and organisms, as found under specified conditions. The vastly diverse glycome is generated by a nontemplate driven biosynthesis, which is indirectly encoded in the genome, and very dynamic. Due to this overwhelming diversity, glycomic analysis must be approached at different hierarchical levels of complexity. In this review five such levels of complexity and the experimental approaches used for analysis at each level are discussed for a subclass of the glycome: the sialome. The sialome, in analogy to the canopy of a forest, covers the cell membrane with diverse array of complex sialylated structures. Sialome complexity includes modification of sialic acid core structure (the leaves and flowers), the linkage to the underlying sugar (the stems), the identity, and arrangement of the underlying glycans (the branches), the structural attributes of the underlying glycans (the trees), and finally, the spatial organization of the sialoglycans in relation to components of the intact cell surface (the forest). Understanding the full complexity of the sialome thus requires combined analyses at multiple levels, that is, the sialome is far more than the sum of its parts.

Attachment of mouse hepatitis virus to O-acetylated sialic acid is mediated by hemagglutinin-esterase and not by the spike protein

The members of Betacoronavirus phylocluster A possess two types of surface projections, one comprised of the spike protein (S) and the other of hemagglutinin-esterase (HE). Purportedly, these viruses bind to O-acetylated sialic acids (O-Ac-Sias) primarily through S, with HE serving merely as receptor-destroying enzyme. Here, we show that, in apparent contrast to human and ungulate host range variants of Betacoronavirus-1, murine coronaviruses actually bind to O-Ac-Sias via HE exclusively. Apparently, expansion of group A betacoronaviruses into new hosts and niches was accompanied by changes in HE ligand and substrate preference and in the roles of HE and S in Sia receptor usage.

[Variation of coronavirus tropism]

Viral tropism is defined as the group of target cells which can be infected by this virus. Knowing viral tropism is knowing the target organ, and the animal species used as host. The variation of tropism allow for virus to evolve, cross species barriers, and infect a new host. Coronavirus family is a very large group of viruses which infect birds and mammals. These RNA viruses can rapidly evolve. In the history of coronaviruses, several examples of variations of tropism have been described, and have as consequences the emergence of a new infection (coronavirus associated with SARS, porcine respiratory coronavirus), or a new expression of the clinical presentation of the infection (fatal infectious peritonitis in cats and ferrets).

Feline lectin activity is critical for the cellular entry of feline infectious peritonitis virus

Feline infectious peritonitis is a lethal disease of felids caused by systemic infection with a feline coronavirus. Here, we report identification and analysis of the feline homologue to the human lectin DC-SIGN and show that it is a coreceptor for virulent strains of serotype 1 and serotype 2 feline coronaviruses.

Colloquium paper: uniquely human evolution of sialic acid genetics and biology

Darwinian evolution of humans from our common ancestors with nonhuman primates involved many gene-environment interactions at the population level, and the resulting human-specific genetic changes must contribute to the "Human Condition." Recent data indicate that the biology of sialic acids (which directly involves less than 60 genes) shows more than 10 uniquely human genetic changes in comparison with our closest evolutionary relatives. Known outcomes are tissue-specific changes in abundant cell-surface glycans, changes in specificity and/or expression of multiple proteins that recognize these glycans, and novel pathogen regimes. Specific events include Alu-mediated inactivation of the CMAH gene, resulting in loss of synthesis of the Sia N-glycolylneuraminic acid (Neu5Gc) and increase in expression of the precursor N-acetylneuraminic acid (Neu5Ac); increased expression of alpha2-6-linked Sias (likely because of changed expression of ST6GALI); and multiple changes in SIGLEC genes encoding Sia-recognizing Ig-like lectins (Siglecs). The last includes binding specificity changes (in Siglecs -5, -7, -9, -11, and -12); expression pattern changes (in Siglecs -1, -5, -6, and -11); gene conversion (SIGLEC11); and deletion or pseudogenization (SIGLEC13, SIGLEC14, and SIGLEC16). A nongenetic outcome of the CMAH mutation is human metabolic incorporation of foreign dietary Neu5Gc, in the face of circulating anti-Neu5Gc antibodies, generating a novel "xeno-auto-antigen" situation. Taken together, these data suggest that both the genes associated with Sia biology and the related impacts of the environment comprise a relative "hot spot" of genetic and physiological changes in human evolution, with implications for uniquely human features both in health and disease.

Inhibition of porcine transmissible gastroenteritis virus (TGEV) replication in mini-pigs by shRNA

Transmissible gastroenteritis virus (TGEV) is the causative agent of porcine transmissible gastroenteritis (TGE), characterized by high mortality and severely retarded growth in piglets that dramatically affects the porcine industry. Previously, we have identified two shRNA-expressing plasmids pEGFP-U6/P1 and pEGFP-U6/P2 that target RNA-dependent RNA polymerase (RdRP) gene of TGEV with more than 95% of virus inhibition in vitro. In this study, inhibition of the TGEV replication by pEGFP-U6/P1 and pEGFP-U6/P2 was tested in mini-pigs. SPF mini-pigs at 25 days old were injected with the shRNA-expressing plasmids and then infected with TGEV. The results from the analyses of clinical signs, histopathology, indirect immunofluorescence (IIF) and RT-PCR show that the two shRNA-expressing plasmids could significantly decrease the quantity of TGEV in different organs and protect mini-pigs from TGEV infection. These findings illustrate the prospect for TGEV-specific shRNAs to be new anti-TGEV agents.

Contribution of the porcine aminopeptidase N (CD13) receptor density to porcine epidemic diarrhea virus infection

Porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis virus (TGEV), which belong to group 1 coronaviruses, are important viral pathogens in pigs causing lethal diarrhea. As with the other members in the group 1, theses viruses are also known to use the host aminopeptidase N (APN) as the major receptor for cell entry. Remarkably, it was found that they utilize distinct cultured cell lines for in vitro virus propagation, since PEDV could not be replicated in swine testis (ST) cells expressing native porcine APN (pAPN), which are highly susceptible to TGEV. To explain the mechanism causing this discrimination, we postulated that there may be a correlation between the pAPN expression level and PEDV infection. As a first step toward understanding the role of cellular receptor density in PEDV replication, therefore, sub-lines of ST cells stably overexpressing recombinant pAPN were generated. We initially confirmed that the control ST cells do express relatively low levels of endogenous pAPN. In contrast, in the engineered stable cell lines, a high level of recombinant pAPN expression was demonstrated. The introduction of a pAPN gene into nonpermissive ST cells was further found to be fully sufficient to support productive infection, revealing that constitutive overexpression of pAPN can directly rescue PEDV multiplication. We further assessed whether the pAPN enzymatic function is relevant to PEDV infection. The enzymatic active motif-null mutant pAPN still retained the ability to exert its receptor activity and consequently, to cause infectious virus production. Moreover, the only APN inhibitor blocking the protease activity site had no obvious negative effect on viral infection, indicating that the enzymatic role of APN is dispensable for the process of virus replication. Taken together, our results suggest that pAPN receptor density appears to be an important factor in contributing to efficient PEDV infection.

Virus entry paradigms

Viruses, despite being relatively simple in structure and composition, have evolved to exploit complex cellular processes for their replication in the host cell. After binding to their specific receptor on the cell surface, viruses (or viral genomes) have to enter cells to initiate a productive infection. Though the entry processes of many enveloped viruses is well understood, that of most non-enveloped viruses still remains unresolved. Recent studies have shown that compared to direct fusion at the plasma membrane, endocytosis is more often the preferred means of entry into the target cell. Receptor-mediated endocytic pathways such as the dynamin-dependent clathrin and caveolar pathways are well characterized as viral entry portals. However, many viruses are able to utilize multiple uptake pathways. Fluid phase uptake, though relatively non-specific in terms of its cargo, potentially aids viral infection by its ability to intersect with the endocytic pathway. In fact, many viruses despite using specialized pathways for entry are still able to generate productive infection via fluid phase uptake. Macropinocytosis, a major fluid uptake pathway found in epithelial cells and fibroblasts, is stimulated by growth factor receptors. Many viruses can induce these signaling cascades in cells leading to macropinocytosis. Though endocytic trafficking is utilized by both enveloped and non-enveloped viruses, key differences lie in the way membranes are traversed to deposit the viral genome at its site of replication. This review will discuss recent developments in the rapidly evolving field of viral entry.

From carbohydrate leads to glycomimetic drugs

Carbohydrates are the most abundant natural products. Besides their role in metabolism and as structural building blocks, they are fundamental constituents of every cell surface, where they are involved in vital cellular recognition processes. Carbohydrates are a relatively untapped source of new drugs and therefore offer exciting new therapeutic opportunities. Advances in the functional understanding of carbohydrate-protein interactions have enabled the development of a new class of small-molecule drugs, known as glycomimetics. These compounds mimic the bioactive function of carbohydrates and address the drawbacks of carbohydrate leads, namely their low activity and insufficient drug-like properties. Here, we examine examples of approved carbohydrate-derived drugs, discuss the potential of carbohydrate-binding proteins as new drug targets (focusing on the lectin families) and consider ways to overcome the challenges of developing this unique class of novel therapeutics.

Total sialic acid: an acute phase reactant in cats with a possible role in feline coronavirus infection

The aims of this study were to validate a colorimetric method to measure total sialic acid (TSA) in feline serum and to investigate the serum concentration of TSA in clinically healthy cats seronegative (n = 9) and seropositive (n = 48) for feline coronavirus (FCoV), and in cats affected by feline infectious peritonitis (FIP, n = 28), tumors (n = 20), or inflammation (n = 16). The correlation between TSA and alpha(1)-acid glycoprotein (AGP) was also investigated. The method employed in this study is precise and accurate at TSA levels (in mg/L) commonly encountered in feline serum. No significant differences between seropositive (385.6 +/- 192.2 mg/L) and seronegative (433.5 +/- 179.0 mg/L) cats were detectable, suggesting that the simple infection by FCoVs does not influence TSA levels. Compared with seropositive controls, the concentration of TSA was higher in cats with FIP (556.7 +/- 268.3 mg/L, P = 0.003), tumors (522.5 +/- 294.4 mg/L, P = 0.028), and inflammation (546.8 +/- 208.3 mg/L, P = 0.018). The discriminating power of TSA for FIP is moderate (area under the ROC curve = 0.65) and the likelihood ratio is higher than 3.0 only at high TSA levels. Consequently, TSA could support a diagnosis of FIP only at extremely high serum concentration (> 800 mg/L) or when the pre-test probability of FIP is high. No correlations were found between the TSA and AGP concentrations in cats with FIP, suggesting that sialylated proteins other than AGP are present. Both the antibody titre and the degree of AGP sialylation were negatively correlated with TSA levels, suggesting that increased TSA may contribute to reduce the burden of FCoVs.

[Human coronaviruses]

Coronaviruses are a large group of viruses and infect a lot of species of mammals and birds. Five coronaviruses currently infect humans: HCoVs 229E and OC43, identified in the 1960s, SARS-CoV identified in March 2003 during the SARS epidemic, and the HCoVs NL63 and HKU1, identified in 2004 and 2005 respectively. The genome of the coronaviruses is a linear, non-segmented, positive-sense single-stranded RNA molecule of approximately 30kb. The evolution of these viruses occurs through some features: the generation of multiple mutants during the replication resulting on a quasispecies structure of the viral population, the demonstrated ability of coronaviruses to establish persistent infections, the flexibility of the genome due to a high frequency of homologue or heterologue recombinations, the ability to jump barrier species and to adapt to the new environment. Two epidemiologic pictures of HCoV infections have to be distinguished: as suggested by recent studies, HCoVs except SARS-CoV, are distributed worldwide and cocirculate during seasonal outbreaks. The distribution of the different HCoV species varies according to the geographic area and season. In contrast, the SARS-CoV is responsible of the first emerging infectious disease of this millennium, infecting more than 8000 people between November 2002 and July 2003. Its circulation has been stopped by drastic public health policy. Human coronaviruses may be also involved in enteric and neurologic diseases. The detection of these viruses is difficult and mainly based on molecular assays (RT-PCR). There is no established specific therapy to date.

Ganglioside-linked terminal sialic acid moieties on murine macrophages function as attachment receptors for murine noroviruses

Noroviruses are the major cause of nonbacterial gastroenteritis in humans. However, little is known regarding the norovirus life cycle, including cell binding and entry. In contrast to human noroviruses, the recently discovered murine norovirus 1 (MNV-1) readily infects murine macrophages and dendritic cells in culture. Many viruses, including the related feline calicivirus, use terminal sialic acids (SA) as receptors for infection. Therefore, we tested whether SA moieties play a role during MNV-1 infection of murine macrophages. Competition with SA-binding lectins and neuraminidase treatment led to a reduction in MNV-1 binding and infection in cultured and primary murine macrophages, suggesting a role for SA during the initial steps of the MNV-1 life cycle. Because SA moieties can be attached to glycolipids (i.e., gangliosides), we next determined whether MNV-1 uses gangliosides during infection. The gangliosides GD1a, GM1, and asialo-GM1 (GA1) are natural components of murine macrophages. MNV-1 bound to ganglioside GD1a, which is characterized by an SA on the terminal galactose, but not to GM1 or asialo-GM1 in an enzyme-linked immunosorbent assay. The depletion of gangliosides using an inhibitor of glycosylceramide synthase (d-threo-P4) led to a reduction of MNV-1 binding and infection in cultured and primary murine macrophages. This defect was specifically rescued by the addition of GD1a. A similar phenotype was observed for MNV field strains WU11 (GV/WU11/2005/USA) and S99 (GV/Berlin/2006/DE). In conclusion, our data indicate that MNV can use terminal SA on gangliosides as attachment receptors during binding to murine macrophages.

The cell adhesion molecule "CAR" and sialic acid on human erythrocytes influence adenovirus in vivo biodistribution

Although it has been known for 50 years that adenoviruses (Ads) interact with erythrocytes ex vivo, the molecular and structural basis for this interaction, which has been serendipitously exploited for diagnostic tests, is unknown. In this study, we characterized the interaction between erythrocytes and unrelated Ad serotypes, human 5 (HAd5) and 37 (HAd37), and canine 2 (CAV-2). While these serotypes agglutinate human erythrocytes, they use different receptors, have different tropisms and/or infect different species. Using molecular, biochemical, structural and transgenic animal-based analyses, we found that the primary erythrocyte interaction domain for HAd37 is its sialic acid binding site, while CAV-2 binding depends on at least three factors: electrostatic interactions, sialic acid binding and, unexpectedly, binding to the coxsackievirus and adenovirus receptor (CAR) on human erythrocytes. We show that the presence of CAR on erythrocytes leads to prolonged in vivo blood half-life and significantly reduced liver infection when a CAR-tropic Ad is injected intravenously. This study provides i) a molecular and structural rationale for Ad-erythrocyte interactions, ii) a basis to improve vector-mediated gene transfer and iii) a mechanism that may explain the biodistribution and pathogenic inconsistencies found between human and animal models.

Identification of major histocompatibility complex class I C molecule as an attachment factor that facilitates coronavirus HKU1 spike-mediated infection

Human coronavirus HKU1 (HCoV-HKU1) is a recently discovered human coronavirus associated with respiratory tract infections worldwide. In this study, we have identified the major histocompatibility complex class I C molecule (HLA-C) as an attachment factor in facilitating HCoV-HKU1 spike (S)-mediated infection. HCoV-HKU1 S pseudotyped virus was assembled using a human immunodeficiency virus type 1-derived reporter virus harboring the human codon-optimized spike of HCoV-HKU1. We identified human alveolar epithelial A549 cells as the most susceptible cell line among those tested to infection by HCoV-HKU1 S pseudotypes. A549 cells were shown to bind purified soluble HCoV-HKU1 S(1-600) glycopeptide. To search for the functional receptor for HCoV-HKU1, an A549 cDNA expression library was constructed and transduced into the nonpermissive, baby hamster kidney cells line BHK-21. Transduced cells that bind soluble HCoV-HKU1 S(1-600) glycoprotein with C-terminal FLAG were sorted. Sequencing of two independent clones revealed cDNA inserts encoding HLA-C. Inhibition of HLA-C expression or function by RNAi silencing and anti-HLA-C antibody decreased HCoV-HKU1 S pseudotyped virus infection of A549 cells by 62 to 65%, whereas pretreatment of cells with neuraminidase decreased such infection by only 13%. When HLA-C was constitutively expressed in another nonpermissive cell line, NIH-3T3, quantitative PCR showed that the binding of HCoV-HKU1 S pseudotyped virus to cell surfaces was increased by 200-fold, but the cells remained nonsusceptible to HCoV-HKU1 S pseudotyped virus infection. Our data suggest that HLA-C is involved in the attachment of HCoV-HKU1 to A549 cells and is a potential candidate to facilitate cell entry. However, other unknown surface proteins on A549 cells may be concomitantly utilized by S glycoprotein of HCoV-HKU1 during viral entry. Further studies are required to elucidate other putative receptors or coreceptors for HCoV-HKU1 and the mechanism of HCoV-HKU1 S-mediated cell entry.

Association between faecal shedding of feline coronavirus and serum alpha1-acid glycoprotein sialylation

The sialylation pattern of serum alpha1-acid glycoprotein (AGP) in non-symptomatic cats infected by feline coronavirus (FCoV) and its possible relationship with the amount of FCoVs shed in faeces were investigated. Blood from three specific pathogen-free cats (group A) and from 10 non-symptomatic FCoV-positive cats from catteries with low (group B, three cats) or high (group C, seven cats) levels of faecal shedding were collected monthly. AGP was purified from serum and Western blotting followed by lectin-staining of alpha(2,3)-linked and alpha(2,6)-linked sialic acid. Faecal shedding was quantified in group C by quantitative polymerase chain reaction. Variations of AGP sialylation were recorded only in cats from group C, on which viral shedding peaked before the occurrence of feline infectious peritonitis (FIP) in the cattery, and decreased 1 month later, when serum AGP had an increase of alpha(2,3)-linked sialic acid. These results suggest that hypersialylation of AGP may be involved in host-virus interactions.

Multiple changes in sialic acid biology during human evolution

Humans are genetically very similar to “great apes”, (chimpanzees, bonobos, gorillas and orangutans), our closest evolutionary relatives. We have discovered multiple genetic and biochemical differences between humans and these other hominids, in relation to sialic acids and in Siglecs (Sia-recognizing Ig superfamily lectins). An inactivating mutation in the CMAH gene eliminated human expression of N-glycolylneuraminic acid (Neu5Gc) a major sialic acid in “great apes”. Additional human-specific changes have been found, affecting at least 10 of the <60 genes known to be involved in the biology of sialic acids. There are potential implications for unique features of humans, as well as for human susceptibility or resistance to disease. Additionally, metabolic incorporation of Neu5Gc from animal-derived materials occurs into biotherapeutic molecules and cellular preparations - and into human tissues from dietary sources, particularly red meat and milk products. As humans also have varying and sometime high levels of circulating anti-Neu5Gc antibodies, there are implications for biotechnology products, and for some human diseases associated with chronic inflammation.

Influenza C virus and bovine coronavirus esterase reveal a similar catalytic mechanism: new insights for drug discovery

Both, the influenza C (INF-C) virus haemagglutinin esterase fusion and bovine coronavirus (BCoV) haemagglutinin esterase surface glycoproteins exhibit a lectin binding capability and a receptor-destroying 9-O-acetyl esterase activity that recognise 9-O-acetyl-N-acetylneuraminic acid (Neu5,9Ac(2))-containing glycans. Here we report nuclear magnetic resonance and molecular modelling studies on the 9-O-acetyl esterase showing that the alpha-configured Neu5,9Ac(2) is strictly preferred by the INF-C and BCoV esterases. Interestingly, we have discovered that the INF-C esterase function releases acetate independently of the chemical nature of the aglycon moiety, whereas subtle differences in substrate recognition were found for BCoV esterase. Analysis of the apo and complexed X-ray crystal structure of INF-C esterase revealed that binding of 9-O-acetylated N-acetylneuraminic acids is a dynamic process that involves conformational rearrangement of serine-57 in the esterase active site. This study provides valuable insights towards the design of drugs to combat INF-C virus and coronavirus infections causing outbreaks of upper respiratory infections and severe diarrhea in calves, respectively.

The feline acute phase reaction

The acute phase reaction (APR) is a response to potentially pathogenic stimuli. It begins with the release of interleukin (IL)-1, IL-6 and tumour necrosis factor (TNF)-α from inflammatory cells. These cytokines induce fever, leucocytosis and release of serum acute phase proteins (APPs). In this review, the characteristics of the feline APR are described. In cats with inflammatory conditions, fever is a common finding, with leucocytosis due to the release of cells from the marginal pool, followed by activation of myelopoiesis. Because excitement frequently causes leucocytosis in cats, a diagnosis of inflammation should therefore be supported by additional findings such as the presence of toxic neutrophils. The major APPs are serum amyloid A and α₁-acid glycoprotein (AGP), which both increase a few hours after the inflammatory stimulus and remain elevated for as long as the inflammation persists. AGP plays an important role in the diagnosis of feline infectious peritonitis (FIP) and may also be useful also in studies of FIP pathogenesis.

Feline aminopeptidase N is not a functional receptor for avian infectious bronchitis virus

Background

Coronaviruses are an important cause of infectious diseases in humans, including severe acute respiratory syndrome (SARS), and have the continued potential for emergence from animal species. A major factor in the host range of a coronavirus is its receptor utilization on host cells. In many cases, coronavirus-receptor interactions are well understood. However, a notable exception is the receptor utilization by group 3 coronaviruses, including avian infectious bronchitis virus (IBV). Feline aminopeptidase N (fAPN) serves as a functional receptor for most group 1 coronaviruses including feline infectious peritonitis virus (FIPV), canine coronavirus, transmissible gastroenteritis virus (TGEV), and human coronavirus 229E (HCoV-229E). A recent report has also suggested a role for fAPN during IBV entry (Miguel B, Pharr GT, Wang C: The role of feline aminopeptidase N as a receptor for infectious bronchitis virus. Brief review. Arch Virol 2002, 147:2047–2056.

Results

Here we show that, whereas both transient transfection and constitutive expression of fAPN on BHK-21 cells can rescue FIPV and TGEV infection in non-permissive BHK cells, fAPN expression does not rescue infection by the prototype IBV strain Mass41. To account for the previous suggestion that fAPN could serve as an IBV receptor, we show that feline cells can be infected with the prototype strain of IBV (Mass 41), but with low susceptibility compared to primary chick kidney cells. We also show that BHK-21 cells are slightly susceptible to certain IBV strains, including Ark99, Ark_DPI, CA99, and Iowa97 (<0.01% efficiency), but this level of infection is not increased by fAPN expression.

Conclusion

We conclude that fAPN is not a functional receptor for IBV, the identity of which is currently under investigation.

Effective inhibition of porcine transmissible gastroenteritis virus replication in ST cells by shRNAs targeting RNA-dependent RNA polymerase gene

Transmissible gastroenteritis virus (TGEV) is identified as one of the most important pathogenic agents during swine enteric infection, leading to high mortality in neonatal pigs and severe annual economic loss in swine-producing areas. Up to date, various vaccines developed against TGEV still need to be improved. To exploit the possibility of using RNA interference (RNAi) as a strategy against TGEV infection, two shRNA-expressing plasmids (pEGFP-U6/P1 and pEGFP-U6/P2) targeting the RNA-dependent RNA polymerase (RdRp) gene of TGEV were constructed and transfected into swine testicular (ST) cells. The cytopathic effect (CPE) and MTS assays demonstrated that both shRNAs were capable of protecting cells against TGEV invasion with very high specificity and efficiency. A real-time quantitative RT-PCR further confirmed that the amounts of viral RNAs in cell cultures pre-transfected with the two plasmids were reduced by 95.2% and up to 100%, respectively. Our results suggest that RNAi might be a promising new strategy against TGEV infection.