Structural analysis of receptor engagement and antigenic drift within the BA.2 spike protein

Determinants of susceptibility to SARS-CoV-2 infection in murine ACE2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) utilizes angiotensin-converting enzyme 2 (ACE2) as a receptor to enter host cells, and primary receptor recognition of the spike protein is a major determinant of the host range of SARS-CoV-2. Since the emergence of SARS-CoV-2, a considerable number of variants have emerged. However, the determinants of host tropism of SARS-CoV-2 remain elusive. We conducted infection assays with chimeric recombinant SARS-CoV-2 carrying the spike protein from 10 viral variants, assessing their entry efficiency using mammalian ACE2 orthologs from species that have close contact with humans. We found that only murine ACE2 exhibited different susceptibilities to infection with the SARS-CoV-2 variants. Moreover, we revealed that the mutation N501Y in the viral spike protein has a crucial role in determining the infectivity of cells expressing murine ACE2 and of mice in vivo. Next, we identified six amino acid substitutions at 24, 30, 31, 82, 83, and 353 in murine ACE2 that allowed for viral entry of the variants to which murine ACE2 was previously resistant. Furthermore, we showed that ACE2 from a species closely related to mice, Mus caroli, is capable of supporting entry of the viral variants that could not use murine ACE2. These results suggest that few ACE2 orthologs have different susceptibility to infection with SARS-CoV-2 variants as observed for murine ACE2. Collectively, our study reveals critical amino acids in ACE2 and the SARS-CoV-2 spike protein that are involved in the host tropism of SARS-CoV-2, shedding light on interspecies susceptibility to infection.IMPORTANCESARS-CoV-2 can infect many species besides humans, leading to the evolution of the virus and adaptation to other animal hosts, which could trigger a new COVID-19 wave. The SARS-CoV-2 spike protein utilizes ACE2 as a receptor for entry into host cells. The interaction of ACE2 with the spike protein determines the host range of SARS-CoV-2. In this study, using chimeric viruses carrying the spike protein of SARS-CoV-2 variants to infect cells expressing different ACE2 orthologs from species humans come in close contact with, we confirmed murine ACE2 alone showed different susceptibility to the variants. We identified residues in murine ACE2 and the viral spike that restrict viral entry. Furthermore, an ACE2 ortholog from a species genetically close to mice mediated entry of SARS-CoV-2 variants incapable of infecting mice. This research highlights the uniquely limited susceptibility of mice to different SARS-CoV-2 variants and provides invaluable insights into the host tropism of SARS-CoV-2.

Mechanistic study of the transmission pattern of the SARS-CoV-2 omicron variant

The omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) characterized by 30 mutations in its spike protein, has rapidly spread worldwide since November 2021, significantly exacerbating the ongoing COVID-19 pandemic. In order to investigate the relationship between these mutations and the variant's high transmissibility, we conducted a systematic analysis of the mutational effect on spike-angiotensin-converting enzyme-2 (ACE2) interactions and explored the structural/energy correlation of key mutations, utilizing a reliable coarse-grained model. Our study extended beyond the receptor-binding domain (RBD) of spike trimer through comprehensive modeling of the full-length spike trimer rather than just the RBD. Our free-energy calculation revealed that the enhanced binding affinity between the spike protein and the ACE2 receptor is correlated with the increased structural stability of the isolated spike protein, thus explaining the omicron variant's heightened transmissibility. The conclusion was supported by our experimental analyses involving the expression and purification of the full-length spike trimer. Furthermore, the energy decomposition analysis established those electrostatic interactions make major contributions to this effect. We categorized the mutations into four groups and established an analytical framework that can be employed in studying future mutations. Additionally, our calculations rationalized the reduced affinity of the omicron variant towards most available therapeutic neutralizing antibodies, when compared with the wild type. By providing concrete experimental data and offering a solid explanation, this study contributes to a better understanding of the relationship between theories and observations and lays the foundation for future investigations.

Functional and structural investigation of a broadly neutralizing SARS-CoV-2 antibody

Since its emergence, SARS-CoV-2 has been continuously evolving, hampering the effectiveness of current vaccines against COVID-19. mAbs can be used to treat patients at risk of severe COVID-19. Thus, the development of broadly protective mAbs and an understanding of the underlying protective mechanisms are of great importance. Here, we isolated mAbs from donors with breakthrough infection with Omicron subvariants using a single-B cell screening platform. We identified a mAb, O5C2, which possesses broad-spectrum neutralization and antibody-dependent cell-mediated cytotoxic activities against SARS-CoV-2 variants, including EG.5.1. Single-particle analysis by cryo-electron microscopy revealed that O5C2 targeted an unusually large epitope within the receptor-binding domain of spike protein that overlapped with the angiotensin-converting enzyme 2 binding interface. Furthermore, O5C2 effectively protected against BA.5 Omicron infection in vivo by mediating changes in transcriptomes enriched in genes involved in apoptosis and interferon responses. Our findings provide insights into the development of pan-protective mAbs against SARS-CoV-2.

Altered receptor binding, antibody evasion and retention of T cell recognition by the SARS-CoV-2 XBB.1.5 spike protein

The XBB.1.5 variant of SARS-CoV-2 has rapidly achieved global dominance and exhibits a high growth advantage over previous variants. Preliminary reports suggest that the success of XBB.1.5 stems from mutations within its spike glycoprotein, causing immune evasion and enhanced receptor binding. We present receptor binding studies that demonstrate retention of binding contacts with the human ACE2 receptor and a striking decrease in binding to mouse ACE2 due to the revertant R493Q mutation. Despite extensive evasion of antibody binding, we highlight a region on the XBB.1.5 spike protein receptor binding domain (RBD) that is recognized by serum antibodies from a donor with hybrid immunity, collected prior to the emergence of the XBB.1.5 variant. T cell assays reveal high frequencies of XBB.1.5 spike-specific CD4+ and CD8+ T cells amongst donors with hybrid immunity, with the CD4+ T cells skewed towards a Th1 cell phenotype and having attenuated effector cytokine secretion as compared to ancestral spike protein-specific cells. Thus, while the XBB.1.5 variant has retained efficient human receptor binding and gained antigenic alterations, it remains susceptible to recognition by T cells induced via vaccination and previous infection.

Deciphering the free energy landscapes of SARS-CoV-2 wild type and Omicron variant interacting with human ACE2

The binding of the receptor binding domain (RBD) of the SARS-CoV-2 spike protein to the host cell receptor angiotensin-converting enzyme 2 (ACE2) is the first step in human viral infection. Therefore, understanding the mechanism of interaction between RBD and ACE2 at the molecular level is critical for the prevention of COVID-19, as more variants of concern, such as Omicron, appear. Recently, atomic force microscopy has been applied to characterize the free energy landscape of the RBD-ACE2 complex, including estimation of the distance between the transition state and the bound state, xu. Here, using a coarse-grained model and replica-exchange umbrella sampling, we studied the free energy landscape of both the wild type and Omicron subvariants BA.1 and XBB.1.5 interacting with ACE2. In agreement with experiment, we find that the wild type and Omicron subvariants have similar xu values, but Omicron binds ACE2 more strongly than the wild type, having a lower dissociation constant KD.

ACE2-using merbecoviruses: Further evidence of convergent evolution of ACE2 recognition by NeoCoV and other MERS-CoV related viruses

Angiotensin-converting enzyme 2 (ACE2) was recognized as an entry receptor shared by coronaviruses from Sarbecovirus and Setracovirus subgenera, including three human coronaviruses: SARS-CoV, SARS-CoV-2, and NL63. We recently disclosed that NeoCoV and three other merbecoviruses (PDF-2180, MOW15-22, PnNL 2018B), which are MERS-CoV relatives found in African and European bats, also utilize ACE2 as their functional receptors through unique receptor binding mechanisms. This unexpected receptor usage assumes significance, particularly in light of the prior recognition of Dipeptidyl peptidase-4 (DPP4) as the only known protein receptor for merbecoviruses. In contrast to other ACE2-using coronaviruses, NeoCoV and PDF-2180 engage a distinct and relatively compact binding surface on ACE2, facilitated by protein-glycan interactions, which is demonstrated by the Cryo-EM structures of the receptor binding domains (RBDs) of these viruses in complex with a bat ACE2 orthologue. These findings further support the hypothesis that phylogenetically distant coronaviruses, characterized by distinct RBD structures, can independently evolve to acquire ACE2 affinity during inter-species transmission and adaptive evolution. To date, these viruses have exhibited limited efficiency in entering human cells, although single mutations like T510F in NeoCoV can overcome the incompatibility with human ACE2. In this review, we present a comprehensive overview of ACE2-using merbecoviruses, summarize our current knowledge regarding receptor usage and host tropism determination, and deliberate on potential strategies for prevention and intervention, with the goal of mitigating potential future outbreaks caused by spillover of these viruses.

Evolution of the SARS-CoV-2 Omicron spike

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant of concern, first identified in November 2021, rapidly spread worldwide and diversified into several subvariants. The Omicron spike (S) protein accumulated an unprecedented number of sequence changes relative to previous variants. In this review, we discuss how Omicron S protein structural features modulate host cell receptor binding, virus entry, and immune evasion and highlight how these structural features differentiate Omicron from previous variants. We also examine how key structural properties track across the still-evolving Omicron subvariants and the importance of continuing surveillance of the S protein sequence evolution over time.

Determinants of species-specific utilization of ACE2 by human and animal coronaviruses

Utilization of human ACE2 allowed several bat coronaviruses (CoVs), including the causative agent of COVID-19, to infect humans directly or via intermediate hosts. However, the determinants of species-specific differences in ACE2 usage and the frequency of the ability of animal CoVs to use human ACE2 are poorly understood. Here we applied VSV pseudoviruses to analyze the ability of Spike proteins from 26 human or animal CoVs to use ACE2 receptors across nine reservoir, potential intermediate and human hosts. We show that SARS-CoV-2 Omicron variants evolved towards more efficient ACE2 usage but mutation of R493Q in BA.4/5 and XBB Spike proteins disrupts utilization of ACE2 from Greater horseshoe bats. Variations in ACE2 residues 31, 41 and 354 govern species-specific differences in usage by coronaviral Spike proteins. Mutation of T403R allows the RaTG13 bat CoV Spike to efficiently use all ACE2 orthologs for viral entry. Sera from COVID-19 vaccinated individuals neutralize the Spike proteins of various bat Sarbecoviruses. Our results define determinants of ACE2 receptor usage of diverse CoVs and suggest that COVID-19 vaccination may protect against future zoonoses of bat coronaviruses.

Antigenic evolution of SARS coronavirus 2

SARS coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, emerged in China in December 2019. Vaccines developed were very effective initially, however, the virus has shown remarkable evolution with multiple variants spreading globally over the last three years. Nowadays, newly emerging Omicron lineages are gaining substitutions at a fast rate, resulting in escape from neutralization by antibodies that target the Spike protein. Tools to map the impact of substitutions on the further antigenic evolution of SARS-CoV-2, such as antigenic cartography, may be helpful to update SARS-CoV-2 vaccines. In this review, we focus on the antigenic evolution of SARS-CoV-2, highlighting the impact of Spike protein substitutions individually and in combination on immune escape.

Structural Basis for the Enhanced Infectivity and Immune Evasion of Omicron Subvariants

The Omicron variants of SARS-CoV-2 have emerged as the dominant strains worldwide, causing the COVID-19 pandemic. Each Omicron subvariant contains at least 30 mutations on the spike protein (S protein) compared to the original wild-type (WT) strain. Here we report the cryo-EM structures of the trimeric S proteins from the BA.1, BA.2, BA.3, and BA.4/BA.5 subvariants, with BA.4 and BA.5 sharing the same S protein mutations, each in complex with the surface receptor ACE2. All three receptor-binding domains of the S protein from BA.2 and BA.4/BA.5 are "up", while the BA.1 S protein has two "up" and one "down". The BA.3 S protein displays increased heterogeneity, with the majority in the all "up" RBD state. The different conformations preferences of the S protein are consistent with their varied transmissibility. By analyzing the position of the glycan modification on Asn343, which is located at the S309 epitopes, we have uncovered the underlying immune evasion mechanism of the Omicron subvariants. Our findings provide a molecular basis of high infectivity and immune evasion of Omicron subvariants, thereby offering insights into potential therapeutic interventions against SARS-CoV-2 variants.