Friday, 08 September 2017 16:46

New tools to investigate influenza virus targets

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Published in mBiosphere

Part of what determines susceptibility to viral infection is the ability of the virus to attach to a host cell. For influenza virus, the virus binds to a sugar called sialic acid with its hemagglutinin (HA) glycoprotein; sialic acids play a role in viral release when cleaved by the neuraminidase (NA) glycoprotein (sialic acids are N- or O-substituted derivatives of neuraminic acid). These sialic acid residues decorate many different cellular proteins to modify the protein function, and the sialic acids themselves can have different chemical modifications that influence these functional modifications.

What sialic modifications best facilitate influenza virus attachment? This question, recently addressed in an mSphere report, will help scientists to understand both which host species are susceptible and how viral infection proceeds within different species. One of the most common sialic acid forms, N-acetylneuraminic acid (Neu5Ac), is also one of the preferred influenza receptors, but Neu5Ac itself can have further modification through O-acetylation. How do modifications affect influenza interactions? To address this question, a scientific team led by first author Brian Wasik and senior scientist Colin Parrish used sialoglycan-recognizing probes (SGRPs) that recognize different O-acetylated forms of sialic acid.

mSphere: Distribution of O-Acetylationed Sialic Acids among Target Host Tissues for Influenza Virus

Influenza SA 3Presence of different O-acetyl sialic acid modifications in human (HEK-293, A549), canine (MDCK) and equine (NBL-6) cell lines. Source.

The researchers generated the probes by recombinantly fusing an immunoglobulin Fc domain to a viral hemagglutinin esterase protein that interacts with specific sialic modifications. The esterase activity was inactivated, giving the scientists the ability to interrogate a specific sialic acid modification on a variety of cell and tissue types using modified immunostaining (see right) and immunohistochemistry techniques. Similar tools using plant lectins (carbohydrate-interacting proteins) were generated to study the sialic acid linkage patterns.

The researchers used these tools on cell lines and tissue samples from humans and animals. These modification patterns are important to understand the infection process in natural influenza hosts such as people and birds, and are also important to determine how closely animal models of infection mimic those of natural hosts. The summary of the survey results (see table, right) makes it easy to see similarities in sialic acid acetylation patterns between natural hosts like humans and pigs. 

Influenza SA 2Table summarizing sialic acid residue modifications and linkages in different species. Source.

Sialic acid linkages also play an important role in influenza susceptibility, with human and avian influenza virus susceptibility associated with a2-6 or a2-3 linkages, respectively. The distribution of linkages varied across different host species, with pigs (a natural influenza host) and ferrets (a model used in many lab studies) displaying a pattern similar to that in humans, which varied from the pattern observed in birds and dogs.

Understanding the preferred binding sugar of influenza virus strains will help to explain its species restriction and the evolutionary pressures on influenza HA, as viral variants that can bind different sialic acids may be selected for, thereby changing viral antigenicity. Potential applications go beyond understanding infection: imagine treating the newly infected or exposed with “decoy’ sugars” that could interact with the virus before it bound its cellular ligand. In the future, tools like these to detect sialic acid modifications could be applied to many viruses that use sugar modifications as their host cell receptor, expanding our understanding of host susceptibility. 

Last modified on Friday, 08 September 2017 17:19
Julie Wolf

ASM Communications Social Media Specialist Julie Wolf spent her research career focused on medical mycology and infectious disease. Broadly interested in microbiology and scientific communication, she has taught at Long Island University and the community biolab Genspace and has written for the Scientista Foundation and Scholastic’s Science World magazine. Follow her on Twitter for more ASM and Microbiology highlights at @JulieMarieWolf.