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2015 ; 89
(11
): 6067-79
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Influenza A Virus Panhandle Structure Is Directly Involved in RIG-I Activation
and Interferon Induction
#MMPMID25810557
Liu G
; Park HS
; Pyo HM
; Liu Q
; Zhou Y
J Virol
2015[Jun]; 89
(11
): 6067-79
PMID25810557
show ga
Retinoic acid-inducible gene I (RIG-I) is an important innate immune sensor that
recognizes viral RNA in the cytoplasm. Its nonself recognition largely depends on
the unique RNA structures imposed by viral RNA. The panhandle structure residing
in the influenza A virus (IAV) genome, whose primary function is to serve as the
viral promoter for transcription and replication, has been proposed to be a RIG-I
agonist. However, this has never been proved experimentally. Here, we employed
multiple approaches to determine if the IAV panhandle structure is directly
involved in RIG-I activation and type I interferon (IFN) induction. First, in
porcine alveolar macrophages, we demonstrated that the viral genomic coding
region is dispensable for RIG-I-dependent IFN induction. Second, using in
vitro-synthesized hairpin RNA, we showed that the IAV panhandle structure could
directly bind to RIG-I and stimulate IFN production. Furthermore, we investigated
the contributions of the wobble base pairs, mismatch, and unpaired nucleotides
within the wild-type panhandle structure to RIG-I activation. Elimination of
these destabilizing elements within the panhandle structure promoted RIG-I
activation and IFN induction. Given the function of the panhandle structure as
the viral promoter, we further monitored the promoter activity of these panhandle
variants and found that viral replication was moderately affected, whereas viral
transcription was impaired dramatically. In all, our results indicate that the
IAV panhandle promoter region adopts a nucleotide composition that is optimal for
balanced viral RNA synthesis and suboptimal for RIG-I activation. IMPORTANCE: The
IAV genomic panhandle structure has been proposed to be an RIG-I agonist due to
its partial complementarity; however, this has not been experimentally confirmed.
Here, we provide direct evidence that the IAV panhandle structure is competent
in, and sufficient for, RIG-I activation and IFN induction. By constructing
panhandle variants with increased complementarity, we demonstrated that the
wild-type panhandle structure could be modified to enhance RIG-I activation and
IFN induction. These panhandle variants posed moderate influence on viral
replication but dramatic impairment of viral transcription. These results
indicate that the IAV panhandle promoter region adopts a nucleotide composition
to achieve optimal balance of viral RNA synthesis and suboptimal RIG-I
activation. Our results highlight the multifunctional role of the IAV panhandle
promoter region in the virus life cycle and offer novel insights into the
development of antiviral agents aiming to boost RIG-I signaling or virus
attenuation by manipulating this conserved region.