The British Medical Journal (BMJ) recently published a preliminary report on the endpoint of a five-year campaign by independent researchers of the Cochrane’s Acute Respiratory Infections Group and the BMJ that casts doubt on the effectiveness of the antiviral drug Tamiflu (oseltamivir) manufactured by the multinational pharmaceutical firm Roche.

Which makes highly topical the discovery of a protein produced by the influenza A virus helps it outwit one of our body’s natural defence mechanisms by scientists at The University of Texas at Austin. The UT researchers say that this protein could potentially make good target for new antiviral drugs directed against the influenza A virus.

A UT release notes that better antiviral drugs could help the millions of people annually infected by flu, which kills up to 500,000 people each year globally.

The researchers note that an influenza virus uses a portion of the host’s cellular machinery to create copies of itself, known as replication, when it infects a human cell. In this study, they discovered that the above mentioned protein produced by human body cells, DDX21, blocks this replication process, and have also revealed through their research that NS1, a protein created by the virus, effectively inhibits DDX21 while promoting viral replication.

Image caption: This region of the NS1 viral protein binds the host protein DDX21, making it a potential target for new antivirals against the flu virus

Influenza viral RNA synthesis is catalyzed by the viral polymerase containing PA, PB1, and PB2 proteins, and the coauthors show that DDX21 RNA helicase inhibits influenza virus RNA synthesis by binding PB1 and preventing polymerase assembly. The viral NS1 protein overcomes this restriction by interacting with DDX21 and releasing PB1.

“If you could figure out how to stop NS1 from binding to DDX21, you could stop the virus cold,” says Robert Krug, a professor in the College of Natural Sciences at The University of Texas at Austin and corresponding author on the study, published this week in the journal Cell Host and Microbe.

Dr. Krug says in the UT release that in addition to countering the body’s defence mechanisms, the viral NS1 protein actually performs other important roles for the virus, such as inhibiting the host’s synthesis of interferon, a key antiviral protein.

“It means that if you could block that NS1 function, you’d be blocking not only its interaction with DDX21 but many other important functions, so it’s a great target,” Dr. Krug observes.

The paper titled “Cellular DDX21 RNA Helicase Inhibits Influenza A Virus Replication but Is Counteracted by the Viral NS1 Protein”, (Cell Host and Microbe April 9, 2014 DOI: http://dx.doi.org/10.1016/j.chom.2014.03.002) coauthored by Guifang Chen, Chien-Hung Liu, Ligang Zhou, and Robert M. Krug, all from The University of Texas at Austin, notes that “Influenza A virus RNA synthesis is catalyzed by the viral polymerase comprised of the PA, PB1, and PB2 proteins.”

Dr. Krug and his team discovered that the viral NS1 protein is often associated, or bound together, with the host DDX21 protein in infected human body cells. To understand what role DDX21 might play in virus replication, the researchers used a technique called siRNA gene silencing to knock down the production of DDX21 in infected cells. When they did, virus replication increased 30 fold.

“That told us that DDX21 is a host restriction factor, that it inhibits replication,” says Dr. Krug. “That was the key to understanding what was happening. It was an exciting moment.

Next, the researchers discovered that DDX21 blocks replication by binding to a protein that the virus needs to replicate, called PB1. Finally, they discovered that NS1 binds to DDX21 and makes PB1 available again for replication. This result confirmed that NS1 was indeed the countermeasure used by the virus to get around the body’s natural defence mechanism.

Highlights of the UT researcher’s findings are 1) that DDX21 inhibits influenza viral RNA synthesis by binding the PB1 polymerase subunit; 2) Viral NS1 protein binds DDX21 and displaces PB1 to restore viral RNA synthesis; 3)DDX21 binds to a region of NS1 that also participates in other crucial functions; and 4) A virus mutant whose NS1 does not bind DDX21 exhibits depressed late-gene expression.

In the release, it is noted that, “The need for new antiviral drugs that would be effective against the influenza virus is great, especially in light of the BMJ report on Tamiflu, of which millions of doses have been stockpiled at massive cost by governments around the world as a potential foil against influenza pandemics. Because flu vaccines are not 100 percent effective, antiviral drugs play an important role in fast-spreading epidemics. Yet influenza A viruses are developing resistance to antiviral drugs currently in use.”

The major focus of the Krug laboratory at UT is the regulation of the processing and nuclear export of cellular and viral mRNAs in virus-infected cells, and the structure and function of viral proteins that regulate this mRNA processing.

This research centers on the NS1 protein encoded by influenza A viruses (NS1A protein), which is a unique regulator of several cellular post-transcriptional events in both the nucleus and cytoplasm.

More about the background to Tamiflu and open data can be found in this BMJ feature (BMJ 2012;345:e7303) and accompanying editorial (BMJ 2012;345:e7304).

Sources:
The University of Texas at Austin
Cell Host and Microbe
The British Medical Journal (BMJ)

Image Credits:
The University of Texas at Austin