Intra-host diversities of the receptor-binding domain of stork faeces-derived avian H5N1 viruses and its significance as predicted by molecular dynamic simulation

Virus evolution facilitates the emergence of viruses with unpredictable impacts on human health. This study investigated intra-host variations of the receptor-binding domain (RBD) of the haemagglutinin (HA) gene of the avian H5N1 viruses obtained from the 2004 and 2005 epidemics. The results showed that the mutation frequency of the RBD ranged from 0.3 to 0.6 %. The mutations generated one consensus and several minor populations. The consensus population of the 2004 epidemic was transmitted to the 2005 outbreak with increased frequency (39 and 45 %, respectively). Molecular dynamics simulation was applied to predict the significance of the variants. The results revealed that the consensus sequence (E218K/V248I) interacted unstably with sialic acid (SA) with an α2,6 linkage (SAα2,6Gal). Although the mutated K140R/E218K/V248I and Y191C/E218K/V248I sequences decreased the HA binding capacity to α2,3-linked SA, they were shown to bind α2,6-linked SA with increased affinity. Moreover, the substitutions at aa 140 and 191 were positive-selection sites. These data suggest that the K140R and Y191C mutations may represent a step towards human adaptation of the avian H5N1 virus

Rapid Estimation of Binding Activity of Influenza Virus Hemagglutinin to Human and Avian Receptors

A critical step for avian influenza viruses to infect human hosts and cause epidemics or pandemics is acquisition of the ability of the viral hemagglutinin (HA) to bind to human receptors. However, current global influenza surveillance does not monitor HA binding specificity due to a lack of rapid and reliable assays. Here we report a computational method that uses an effective scoring function to quantify HA-receptor binding activities with high accuracy and speed. Application of this method reveals receptor specificity changes and its temporal relationship with antigenicity changes during the evolution of human H3N2 viruses. The method predicts that two amino acid differences at 222 and 225 between HAs of A/Fujian/411/02 and A/Panama/2007/99 viruses account for their differences in binding to both avian and human receptors; this prediction was verified experimentally. The new computational method could provide an urgently needed tool for rapid and large-scale analysis of HA receptor specificities for global influenza surveillance.National Key Project (2008ZX10004-013)National Institutes of Health (U.S.) (grant AI07443)Singapore-MIT Alliance for Research and TechnologyMassachusetts Institute of Technology. International Science and Technology Initiatives Global Seed FundNational Basic Research Program (973 Program) (2009CB918503)National Basic Research Program (973 Program) (2006CB911002

Acquisition of Human-Type Receptor Binding Specificity by New H5N1 Influenza Virus Sublineages during Their Emergence in Birds in Egypt

Highly pathogenic avian influenza A virus subtype H5N1 is currently widespread in Asia, Europe, and Africa, with 60% mortality in humans. In particular, since 2009 Egypt has unexpectedly had the highest number of human cases of H5N1 virus infection, with more than 50% of the cases worldwide, but the basis for this high incidence has not been elucidated. A change in receptor binding affinity of the viral hemagglutinin (HA) from α2,3- to α2,6-linked sialic acid (SA) is thought to be necessary for H5N1 virus to become pandemic. In this study, we conducted a phylogenetic analysis of H5N1 viruses isolated between 2006 and 2009 in Egypt. The phylogenetic results showed that recent human isolates clustered disproportionally into several new H5 sublineages suggesting that their HAs have changed their receptor specificity. Using reverse genetics, we found that these H5 sublineages have acquired an enhanced binding affinity for α2,6 SA in combination with residual affinity for α2,3 SA, and identified the amino acid mutations that produced this new receptor specificity. Recombinant H5N1 viruses with a single mutation at HA residue 192 or a double mutation at HA residues 129 and 151 had increased attachment to and infectivity in the human lower respiratory tract but not in the larynx. These findings correlated with enhanced virulence of the mutant viruses in mice. Interestingly, these H5 viruses, with increased affinity to α2,6 SA, emerged during viral diversification in bird populations and subsequently spread to humans. Our findings suggested that emergence of new H5 sublineages with α2,6 SA specificity caused a subsequent increase in human H5N1 influenza virus infections in Egypt, and provided data for understanding the virus's pandemic potential

Glycan receptor binding determinants of Influenza A virus hemagglutinin

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biological Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.An understanding of the factors involved in the human adaptation of influenza A viruses is critical for various aspects of influenza preparedness, including the development of appropriate surveillance measures, preventive strategies and effective treatments. A key step in influenza human adaptation is the acquisition of mutations in the viral coat glycoprotein, hemagglutinin (HA), which changes its binding specificity towards glycan receptors in the human upper respiratory epithelia (referred to as human receptors). In this thesis, determinants that mediate changes in HA-glycan receptor binding specificity are investigated, with focus on the molecular environments within and surrounding the glycan receptor binding site (RBS) of HA. The glycan receptor binding properties of HA from different influenza subtypes (H1N1, H2N2, H3N2 and H5N1) are studied using a combination of approaches including dose-dependent glycan binding, human tissue staining and structural modeling. Using these complementary analyses, it is shown in this thesis that the molecular interactions between amino acids in and proximal to the RBS (referred to as amino acid interaction networks), including those between the RBS and glycosylation at sites proximal to the RBS, and interactions between the RBS and the glycan receptor together govern the high affinity binding of HA to human receptors. The thesis is divided into three sections. First, the evolution of glycan receptor binding specificity of recent human-adapted H3 strains such as A/Fujian/411/02 and A/Panama/2007/99 is investigated, with implications on vaccine production in chicken eggs. Second, the determinants of glycan receptor binding affinity of potentially pandemic avian viruses is studied in the context of the recently circulating H2 A/Chicken/Pennsylvania/2004 and the highly pathogenic H5 A/Vietnam/1203/2004. Here it is shown that mutations which cause human adaptation of H2 do not increase human receptor binding affinity in H5, and the importance of amino acid interaction networks is implicated. Third, determinants that govern the high affinity human receptor binding of pandemic influenza HAs is investigated using the prototypic 1918 H1N1 HA as a model system. The roles of amino acid interaction networks and the molecular interactions between the RBS and glycosylation at sites proximal to the RBS in contributing to the high affinity human receptor binding of 1918 H1N HA are investigated. The approaches presented in this thesis to systematically investigate molecular interactions between HA and glycan receptors that impinge on quantitative HA-glycan receptor binding affinity offer a new angle towards studying determinants of human receptor binding specificity and affinity of influenza A virus HAs.by Xiaoying Koh.Ph.D

Characterization of mutations in the receptor binding site of influenza A viruses determining virus host, tissue, and cell tropisms using systems biology approaches

Influenza A viruses (IAVs) cause occasional pandemics and seasonal epidemics, thus presenting continuous challenges to public health. Vaccination is the primary strategy for the prevention and control of influenza outbreaks. The antigenicity matched high-yield seed strain is critical for the success of influenza vaccine. Currently, there are several limitations for the influenza vaccine manufacture: 1) the conventional methods for generating such strains are time consuming; 2) egg-based vaccines, the predominant production platform, have several disadvantages including the emergence of viral antigenic variants that can be induced during egg passage; 3) vaccine seed viruses often do not grow efficiently in mammalian cell lines. Previous studies suggested that mutations in the receptor binding site (RBS) that locates at the globular head of the HA1 can change IAVs’ binding specificity, antigenicity, and yield and thus RBS would be an potential target for engineering vaccine seed strain. However, systematic analysis of the mutations on RBS affecting those viral phenotypes is lacking. Specifically, this dissertation has following aims: Firstly, we developed a novel method to rapidly generate high-yield candidate vaccine strains by integrating error-prone PCR, site-directed mutagenesis strategies, and reverse genetics. The error-prone PCR- based reverse genetic system could also be applied to gain-ofunction studies for influenza virus and other pathogens; Secondly, in this dissertation, we identified an Y161F mutation in the hemagglutinin (HA) that enhanced the infectivity and thermostability of virus without changing its original antigenic properties which would prompted the development of cell-based vaccines; Thirdly, the molecular mechanisms underlying host adaption of equine-origin influenza A(H3N8) virus from horses to dogs are unknown. This dissertation identified that a substitution of W222L in the HA of the equine-origin A(H3N8) virus facilitated its host adaption to dogs. This mutation increased binding avidity of the virus specifically to sialyl Lewis X motifs, which were found abundantly in the submucosal glands of dog trachea but not in equine trachea. To summary, this dissertation investigated the role of RBS in IAVs biology and expanded the current knowledge toward IAV vaccine strain engineering, IAV host adaption and evolution

UNDERSTANDING GLYCOSIDE HYDROLASE PROCESSIVITY FOR IMPROVED BIOMASS CONVERSION

In nature, organisms secrete synergistic enzyme cocktails to deconstruct crystalline polysaccharides, such as cellulose and chitin, to soluble sugars. The cocktails consist of multiple classes of processive and non-processive glycoside hydrolases (GH) that aid in substrate accessibility and reduce product inhibition. Processive GHs attach to chain ends and hydrolyze many glycosidic linkages in sequence to produce disaccharide units before dissociation, and as such, are responsible for the majority of hydrolytic bond cleavages. Accordingly, processive GHs are targets for activity improvements towards efficient and economical biomass conversion. However, the mechanism and factors responsible for processivity are still not understood completely at the molecular level. Specifically, the relationship between processive GH function and the enzyme active site topology and chemical composition has yet to be elucidated. Using molecular simulation and free energy calculations, this work presents a molecular-level understanding of the protein-carbohydrate interactions governing processive GHs, which will facilitate rational design of GHs for enhanced biomass conversion. We hypothesize that processive GHs, having long tunnels or deep active site clefts, will allow more amino acids to interact with the ligand and exhibit strong ligand binding and low substrate dissociation rate constants; whereas non-processive enzymes, having more open tunnels or clefts, will exhibit comparatively weak binding and high dissociation rate constants. Moreover, the ligand binding free energy of a processive enzyme must also be more thermodynamically favorable than the work required to decrystallize a polymer from the substrate matrix. We selected the Serratia marcescens Family 18 chitinase model system, including processive chitinases, ChiA and ChiB, and a non-processive chitinase, ChiC, to test our hypotheses. We find that processive ChiA and ChiB exhibit ligand binding free energies that are more thermodynamically favorable than the work to decrystallize a chito-oligosaccharide from the crystalline chitin surface, which is essential for forward processive movement. The non-processive ChiC binds chito-oligosaccharides with a free energy that is significantly less favorable than the work of decrystallization. In general, our findings suggest that processive GH function necessitates tight binding within the enzyme active site. We also observed that aromatic and polar residues close to the catalytic center of ChiA and ChiB have a greater effect on ligand binding and processivity than the residues at the entrance or exit of the cleft. Mutation of active site aromatic and polar residues generally resulted in reduction in processivity and substantial reduction in substrate binding. Overall, our work demonstrates the existence of a fundamental relationship between ligand binding free energy and processive GH active site characteristics

Predicting structural and energetic effects of mutations at protein-protein interfaces

Understanding the structural, dynamical and energetic basis of protein-protein interactions (PPIs) is key for a number of research disciplines. Predicting which sites in PPIs show potential for modulation with binding free energy (ΔG) calculations allows experimental work to be targeted and inhibitors to be rationally designed. However, PPIs remain a challenging target for computational free energy calculations due to their large and complex interfaces. A number of different methods for predicting ΔG from molecular dynamics simulations have been developed, yet each suffers from unique problems in its potential for widespread implementation across PPIs. This thesis initially evaluates the efficacy of the existing MM-PB/GBSA free energy calculation techniques and notes a niche for the improvement of the methods’ predictive power. This is followed by the development of a new computational method for predicting the effects of any PPI interface mutation, which we term Mutational Locally Enhanced Sampling (MULES). MULES generates atomistic molecular dynamics trajectories of native and mutant protein complexes simultaneously. These trajectories are then used to calculate relative binding free energies (ΔΔG) between the two complexes, investigating both structural and energetic effects of individual amino acids at an interface. In principle MULES allows the effect of any mutation to be calculated. Initially tested against a prototypical set of mutations with experimentally measured ΔΔG, MULES showed significantly improved accuracy in ΔΔG prediction and high precision and speed compared to existing methods. The approach was further validated on a large and diverse dataset of approximately 60 individual mutations, comparing results to experimental data and other computational predictions. Validation provided additional evidence for the improved accuracy, precision, speed and particularly versatility of the technique, but also identified areas for improvement. The successes and limitations of MULES discovered here will be of interest to the protein design, drug discovery and computational chemical biology communities