Researchers from Massachusetts General Hospital (MGH) and the University of California, Riverside, have shown for the first time that RNA interference (RNAi) -- an antiviral mechanism known to be used by plants and lower organisms -- is active in the response of human cells to some important viruses. In their report receiving advance online publication in Nature Microbiology, the investigators document both the production of RNAi molecules in human cells infected with the influenza A virus and the suppression of RNAi defense by a viral protein known to block the process in a common animal model.
"Viruses are the most abundant infectious agents and are a constant threat to human health," says Kate Jeffrey, PhD, of the Gastrointestinal Unit in the MGH Department of Medicine, co-corresponding author of the paper. "Vaccines are somewhat effective but can have limited use when viruses like influenza rapidly mutate from year to year. Identifying therapeutic targets within patients that could help them fight off an infection is a critical strategy for combating the spread of common, often-dangerous viruses."
First described in the 1990s -- a discovery that led to the 2006 Nobel Prize -- RNAi is a process by which organisms suppress the expression of target genes through the action of small RNA segments that bind to corresponding gene sequences. Not only is RNAi used to regulate gene expression within an organism, it also can combat viral infection by silencing the activity of viral genes required for the pathogen's replication.
Whether or not RNAi contributes to antiviral defense in mammals has been uncertain. The only previous demonstration -- by researchers led by Shou-Wei Ding, PhD, a professor of Plant Pathology and Microbiology at UC Riverside and co-corresponding author of the current study -- was done in embryonic stem cells and in newborn mice. Ding has been studying antiviral RNAi for more than two decades and also was the first to describe the action of the influenza virus protein NS1 in blocking RNAi in fruit flies. His team collaborated with investigators from Jeffrey's laboratory to investigate whether or not an RNAi response is induced in human and mouse cells infected with the influenza virus, one of many important viruses using RNA as its genetic material.
Their experiments verified that influenza-A-infected mature human cells do generate the small RNA segments used in RNAi but that virally-produced NS1 blocks the processing of those molecules into the complexes that bind to and silence their target genes. If cells were infected with an influenza A mutant lacking NS1, they proceeded to produce large number of the molecular complexes required for RNAi, which include a protein called Argonaute that slices through the target gene.
Experiments in cells with an inactivated form of Argonaute -- which contributes only to the antiviral and not the gene regulation activity of RNAi -- confirmed that they were observing an antiviral RNAi response. The observation that a viral protein called VP35, which is used by the Ebola and Marburg viruses to suppress RNAi, suggests that RNAi may also be active against those dangerous pathogens and other viruses that utilized RNA as their genetic code or in their replication cycle.
"We now need to assess more directly the role of antiviral RNAi in human infectious diseases caused by RNA viruses -- which include Ebola, West Nile and Zika along with influenza -- and how harnessing or boosting the antiviral RNAi response could be used to reduce the severity of these infections," says Jeffrey, who is an assistant professor of Medicine at Harvard Medical School. "Bringing the expertise of Dr. Ding's team, which specializes in the RNAi biology of lower organisms, together with my group that specializes in mammalian immunology was a perfect match." The teams will continue to work together to investigate some of these questions.
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