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

Cellular sialoglycans are differentially required for endosomal and cell-surface entry of SARS-CoV-2 in lung cell lines

Cell entry of severe acute respiratory coronavirus-2 (SARS-CoV-2) and other CoVs can occur via two distinct routes. Following receptor binding by the spike glycoprotein, membrane fusion can be triggered by spike cleavage either at the cell surface in a transmembrane serine protease 2 (TMPRSS2)-dependent manner or within endosomes in a cathepsin-dependent manner. Cellular sialoglycans have been proposed to aid in CoV attachment and entry, although their functional contributions to each entry pathway are unknown. In this study, we used genetic and enzymatic approaches to deplete sialic acid from cell surfaces and compared the requirement for sialoglycans during endosomal and cell-surface CoV entry using lentiviral particles pseudotyped with the spike proteins of different sarbecoviruses. We show that entry of SARS-CoV-1, WIV1-CoV and WIV16-CoV, like the SARS-CoV-2 omicron variant, depends on endosomal cathepsins and requires cellular sialoglycans for entry. Ancestral SARS-CoV-2 and the delta variant can use either pathway for entry, but only require sialic acid for endosomal entry in cells lacking TMPRSS2. Binding of SARS-CoV-2 spike protein to cells did not require sialic acid, nor was sialic acid required for SARS-CoV-2 entry in TMRPSS2-expressing cells. These findings suggest that cellular sialoglycans are not strictly required for SARS-CoV-2 attachment, receptor binding or fusion, but rather promote endocytic entry of SARS-CoV-2 and related sarbecoviruses. In contrast, the requirement for sialic acid during entry of MERS-CoV pseudoparticles and authentic HCoV-OC43 was not affected by TMPRSS2 expression, consistent with a described role for sialic acid in merbecovirus and embecovirus cell attachment. Overall, these findings clarify the role of sialoglycans in SARS-CoV-2 entry and suggest that cellular sialoglycans mediate endosomal, but not cell-surface, SARS-CoV-2 entry.

Single-molecule imaging reveals allosteric stimulation of SARS-CoV-2 spike receptor binding domain by host sialic acid

Conformational dynamics of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike glycoprotein (S) mediate exposure of the binding site for the cellular receptor, angiotensin-converting enzyme 2 (ACE2). The N-terminal domain (NTD) of S binds terminal sialic acid (SA) moieties on the cell surface, but the functional role of this interaction in virus entry is unknown. Here, we report that NTD-SA interaction enhances both S-mediated virus attachment and ACE2 binding. Through single-molecule Förster resonance energy transfer imaging of individual S trimers, we demonstrate that SA binding to the NTD allosterically shifts the S conformational equilibrium, favoring enhanced exposure of the ACE2-binding site. Antibodies that target the NTD block SA binding, which contributes to their mechanism of neutralization. These findings inform on mechanisms of S activation at the cell surface.

Enhanced potency of an IgM-like nanobody targeting conserved epitope in SARS-CoV-2 spike N-terminal domain

Almost all the neutralizing antibodies targeting the receptor-binding domain (RBD) of spike (S) protein show weakened or lost efficacy against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged or emerging variants, such as Omicron and its sub-variants. This suggests that highly conserved epitopes are crucial for the development of neutralizing antibodies. Here, we present one nanobody, N235, displaying broad neutralization against the SARS-CoV-2 prototype and multiple variants, including the newly emerged Omicron and its sub-variants. Cryo-electron microscopy demonstrates N235 binds a novel, conserved, cryptic epitope in the N-terminal domain (NTD) of the S protein, which interferes with the RBD in the neighboring S protein. The neutralization mechanism interpreted via flow cytometry and Western blot shows that N235 appears to induce the S1 subunit shedding from the trimeric S complex. Furthermore, a nano-IgM construct (MN235), engineered by fusing N235 with the human IgM Fc region, displays prevention via inducing S1 shedding and cross-linking virus particles. Compared to N235, MN235 exhibits varied enhancement in neutralization against pseudotyped and authentic viruses in vitro. The intranasal administration of MN235 in low doses can effectively prevent the infection of Omicron sub-variant BA.1 and XBB in vivo, suggesting that it can be developed as a promising prophylactic antibody to cope with the ongoing and future infection.

Virological Traits of the SARS-CoV-2 BA.2.87.1 Lineage

Transmissibility and immune evasion of the recently emerged, highly mutated SARS-CoV-2 BA.2.87.1 are unknown. Here, we report that BA.2.87.1 efficiently enters human cells but is more sensitive to antibody-mediated neutralization than the currently dominating JN.1 variant. Acquisition of adaptive mutations might thus be needed for efficient spread in the population.

Variation in structural motifs within SARS-related coronavirus spike proteins

SARS-CoV-2 is the third known coronavirus (CoV) that has crossed the animal-human barrier in the last two decades. However, little structural information exists related to the close genetic species within the SARS-related coronaviruses. Here, we present three novel SARS-related CoV spike protein structures solved by single particle cryo-electron microscopy analysis derived from bat (bat SL-CoV WIV1) and civet (cCoV-SZ3, cCoV-007) hosts. We report complex glycan trees that decorate the glycoproteins and density for water molecules which facilitated modeling of the water molecule coordination networks within structurally important regions. We note structural conservation of the fatty acid binding pocket and presence of a linoleic acid molecule which are associated with stabilization of the receptor binding domains in the "down" conformation. Additionally, the N-terminal biliverdin binding pocket is occupied by a density in all the structures. Finally, we analyzed structural differences in a loop of the receptor binding motif between coronaviruses known to infect humans and the animal coronaviruses described in this study, which regulate binding to the human angiotensin converting enzyme 2 receptor. This study offers a structural framework to evaluate the close relatives of SARS-CoV-2, the ability to inform pandemic prevention, and aid in the development of pan-neutralizing treatments.

Two Receptor Binding Strategy of SARS-CoV-2 Is Mediated by Both the N-Terminal and Receptor-Binding Spike Domain

It is not well understood why severe acute respiratory syndrome (SARS)-CoV-2 spreads much faster than other β-coronaviruses such as SARS-CoV and Middle East respiratory syndrome (MERS)-CoV. In a previous publication, we predicted the binding of the N-terminal domain (NTD) of SARS-CoV-2 spike to sialic acids (SAs). Here, we experimentally validate this interaction and present simulations that reveal a second possible interaction between SAs and the spike protein via a binding site located in the receptor-binding domain (RBD). The predictions from molecular-dynamics simulations and the previously-published 2D-Zernike binding-site recognition approach were validated through flow-induced dispersion analysis (FIDA)─which reveals the capability of the SARS-CoV-2 spike to bind to SA-containing (glyco)lipid vesicles, and flow-cytometry measurements─which show that spike binding is strongly decreased upon inhibition of SA expression on the membranes of angiotensin converting enzyme-2 (ACE2)-expressing HEK cells. Our analyses reveal that the SA binding of the NTD and RBD strongly enhances the infection-inducing ACE2 binding. Altogether, our work provides in silico, in vitro, and cellular evidence that the SARS-CoV-2 virus utilizes a two-receptor (SA and ACE2) strategy. This allows the SARS-CoV-2 spike to use SA moieties on the cell membrane as a binding anchor, which increases the residence time of the virus on the cell surface and aids in the binding of the main receptor, ACE2, via 2D diffusion.

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.

ACE2-Independent Bat Sarbecovirus Entry and Replication in Human and Bat Cells

Hundreds of sarbecoviruses have been found in bats, but only a fraction of them have the ability to infect cells using angiotensin-converting enzyme 2 (ACE2), the receptor for SARS-CoV and -2. To date, only ACE2-dependent sarbecoviruses have been isolated from field samples or grown in the laboratory. ACE2-independent sarbecoviruses, comprising the majority of the subgenus, have not been propagated in any type of cell culture, as the factors and conditions needed for their replication are completely unknown. Given the significant zoonotic threat posed by sarbecoviruses, cell culture models and in vitro tools are urgently needed to study the rest of this subgenus. We previously showed that the exogenous protease trypsin could facilitate cell entry of viral-like particles pseudotyped with spike protein from some of the ACE2-independent sarbecoviruses. Here, we tested if these conditions were sufficient to support bona fide viral replication using recombinant bat sarbecoviruses. In the presence of trypsin, some of the spike proteins from clade 2 viruses were capable of supporting bat sarbecovirus infection and replication in human and bat cells. Protease experiments showed a specific viral dependence on high levels of trypsin, as TMPRSS2 and furin had no effect on clade 2 virus entry. These results shed light on how sarbecoviruses transmit and coexist in their natural hosts, provide key insights for future efforts to isolate and grow these viruses from field samples, and further underscore the need for broadly protective, universal coronavirus vaccines. IMPORTANCE Our studies demonstrate that some unexplored sarbecoviruses are capable of replicating in human and bat cells in an ACE2-independent way but need a high trypsin environment. We found that trypsin is not compensated by other known proteases involved in some coronavirus entry. This work provides important information that the trypsin-dependent entry may be a widely employed mechanism for coronaviruses and will help for further understanding the biological features of the less-studied viruses.

Involvement of sialoglycans in SARS-COV-2 infection: Opportunities and challenges for glyco-based inhibitors

Viral infections have been the causes of global pandemics, including the ongoing coronavirus disease 2019, which prompted the investigation into the infection mechanisms to find treatment and aid the vaccine design. Betacoronaviruses use spike glycoprotein on their surface to bind to host receptors, aiding their host attachment and cell fusion. Protein-glycan interaction has been implicated in the viral entry mechanism of many viruses and has recently been shown in SARS-CoV-2. Here, we reviewed the current knowledge on protein-glycan interactions that facilitate SARS-CoV-2 host entry, with special interest in sialoglycans present on both the virions and host cell surfaces. We also analyze how such information provides opportunities and challenges in glyco-based inhibitors.