#18,252
While HPAI H5 remains primarily an avian-adapted influenza virus, recent events suggest it may be getting closer to adapting to mammalian physiology, and that understandably has a lot of people concerned.
Yesterday, in Preprint: Large-Scale Computational Modeling of H5 Influenza Variants Against HA1-Neutralizing Antibodies, we looked at a study that cautioned: `These results indicate that the virus is well on its way to moving from epidemic to pandemic status in the near future.'
Still, the HPAI H5 virus faces some formidable obstacles before it can present a genuine pandemic threat. High among them; improving its ability to bind to mammalian alpha 2,6 receptor cells, and increasing its replication in the human respiratory tract.
Over the years we've looked at numerous studies on the types of amino-acid changes that can facilitate mammalian adaptation, including:
Substitutions, such as PB2(E627K), were rapidly selected upon infection of humans with avian H5N1 or H7N9 influenza viruses, adapting the viral polymerase for the shorter mammalian ANP32A
In 2019, Barclay et al. returned with:
aWe last looked ANP32 six weeks ago, in Preprint: An Emerging PB2-627 Polymorphism Increases the Pandemic Potential of Avian Influenza.
Today we've another study, published by researchers at EMBL (European Molecular Biology Laboratory) in Nature Communications today; one that is about as complicated as its title and abstract suggests.
This research is very much a work in progress, and there is a lot more to learn. Hopefully, by better understanding these processes, we can identify worrisome changes in the virus and perhaps, even develop better antiviral drugs.
Structures of influenza A and B replication complexes give insight into avian to human host adaptation and reveal a role of ANP32 as an electrostatic chaperone for the apo-polymeraseBenoît Arragain, Tim Krischuns, Martin Pelosse, Petra Drncova, Martin Blackledge,
Nadia Naffakh & Stephen Cusack
Nature Communications volume 15, Article number: 6910 (2024)
Abstract
Replication of influenza viral RNA depends on at least two viral polymerases, a parental replicase and an encapsidase, and cellular factor ANP32. ANP32 comprises an LRR domain and a long C-terminal low complexity acidic region (LCAR).
Here we present evidence suggesting that ANP32 is recruited to the replication complex as an electrostatic chaperone that stabilises the encapsidase moiety within apo-polymerase symmetric dimers that are distinct for influenza A and B polymerases. The ANP32 bound encapsidase, then forms the asymmetric replication complex with the replicase, which is embedded in a parental ribonucleoprotein particle (RNP). Cryo-EM structures reveal the architecture of the influenza A and B replication complexes and the likely trajectory of the nascent RNA product into the encapsidase. The cryo-EM map of the FluB replication complex shows extra density attributable to the ANP32 LCAR wrapping around and stabilising the apo-encapsidase conformation. These structures give new insight into the various mutations that adapt avian strain polymerases to use the distinct ANP32 in mammalian cells.
Today's full study runs 20 pages, and those who wish to read it are advised to pack a lunch. Fortunately, we do have the following press release that summarizes its findings. I've reproduced a few excerpts, but you'll want to follow the link to read it in its entirety.
A new publication from the Cusack group sheds light on how a key avian influenza virus enzyme can mutate to allow the virus to replicate in mammals
Summary
- The avian influenza virus needs to mutate to cross the species barrier and to infect and replicate within mammalian cells.
- The Cusack group from EMBL Grenoble has deciphered the structure of the avian influenza virus’s polymerase when it interacts with a human protein essential for the virus to replicate within the cell.
- The structure of this replication complex, published in Nature Communications, provides important information about the mutations that avian influenza polymerase must undergo to adapt to mammals, including humans.
- These results can help scientists monitor the evolution and adaptability of bird flu strains, such as H5N1 or H7N9, towards infecting other species.
In recent years, public health measures, surveillance, and vaccination have helped bring about significant progress in reducing the impact of seasonal flu epidemics, caused by human influenza viruses A and B. However, a possible outbreak of avian influenza A (commonly known as ‘bird flu’) in mammals, including humans, poses a significant threat to public health.
The Cusack group at EMBL Grenoble studies the replication process of influenza viruses. A new study from this group sheds light on the different mutations that the avian influenza virus can undergo to be able to replicate in mammalian cells.
Some avian influenza strains can cause severe disease and mortality. Fortunately, significant biological differences between birds and mammals normally prevent avian influenza from spreading from birds to other species. To infect mammals, the avian influenza virus must mutate to overcome two main barriers: the ability to enter the cell and to replicate within that cell. To cause an epidemic or pandemic, it must also acquire the ability to be transmitted between humans.
However, sporadic contamination of wild and domestic mammals by bird flu is becoming increasingly common. Of particular concern is the recent unexpected infection of dairy cows in the USA by an avian H5N1 strain, which risks becoming endemic in cattle. This might facilitate adaptation to humans, and indeed, a few cases of transmission to humans have been reported, so far resulting in only mild symptoms.
At the heart of this process is the polymerase, an enzyme that orchestrates the virus’s replication inside host cells. This flexible protein can rearrange itself according to the different functions it performs during infection. These include transcription – copying the viral RNA into messenger RNA to make viral proteins – and replication – making copies of the viral RNA to package into new viruses.
Viral replication is a complex process to study because it involves two viral polymerases and a host cell protein – ANP32. Together, these three proteins form the replication complex, a molecular machine that carries out replication. ANP32 is known as a ‘chaperone’, meaning that it acts as a stabiliser for certain cellular proteins. It can do this thanks to a key structure – its long acidic tail. In 2015, it was discovered that ANP32 is critical for influenza virus replication, but its function was not fully understood.
The results of the new study, published in the journal Nature Communications, show that ANP32 acts as a bridge between the two viral polymerases – called replicase and encapsidase. The names reflect the two distinct conformations taken up by the polymerases to perform two different functions – creating copies of the viral RNA (replicase) and packaging the copy inside a protective coating with ANP32’s help (encapsidase).
Through its tail, ANP32 acts as a stabiliser for the replication complex, allowing it to form within the host cell. Interestingly, the ANP32 tail differs between birds and mammals, although the core of the protein remains very similar. This biological difference explains why the avian influenza virus does not replicate easily in mammals and humans.
“The key difference between avian and human ANP32 is a 33-amino-acid insertion in the avian tail, and the polymerase has to adapt to this difference,” explained Benoît Arragain, a postdoctoral fellow in the Cusack group and first author of the publication. “For the avian-adapted polymerase to replicate in human cells, it must acquire certain mutations to be able to use human ANP32.”
To better understand this process, Arragain and his collaborators obtained the structure of the replicase and encapsidase conformations of a human-adapted avian influenza polymerase (from strain H7N9) while they were interacting with human ANP32. This structure gives detailed information about which amino acids are important in forming the replication complex and which mutations could allow the avian influenza polymerase to adapt to mammalian cells.
To obtain these results, Arragain carried out in vitro experiments at EMBL Grenoble, using the Eukaryotic Expression Facility, the ISBG biophysical platform, and the cryo-electron microscopy platform available through the Partnership for Structural Biology. “We also collaborated with the Naffakh group at the Institut Pasteur, who carried out cellular experiments,” added Arragain. “In addition, we obtained the structure of the human type B influenza replication complex, which is similar to that of influenza A. The cellular experiments confirmed our structural data.”
These new insights into the influenza replication complex can be used to study polymerase mutations in other similar strains of the avian influenza virus. It is therefore possible to use the structure obtained from the H7N9 strain and adapt it to other strains such as H5N1.
While the differences in avian and mammalian ANP32 constitute a `species barrier' of sorts, it is not an insurmountable one. Adaptive mutations in the avian polymerase (typically PB2/E627K, D701N or Q591R - and probably others) have been shown to allow the virus to use the mammalian ANP32 for replication.
Fortunately - as least so far - these mutations tend to appear only sporadically, and have not become `fixed' in the wild type virus. Of course, how long that luck will last is unknown.
Those with good memories will recall we looked at research into the potential of editing the ANP32 gene in poultry to make them more resistant to avian influenza viruses last October, in Nature: Creating Resistance to Avian Influenza Infection Through Genome Editing of the ANP32 Gene Family.
