https://link.springer.com/article/10.1186/1742-2094-11-24
Background
Japanese
encephalitis virus (JEV) infection leads to Japanese encephalitis (JE)
in humans. JEV is transmitted through mosquitoes and maintained in a
zoonotic cycle. This cycle involves pigs as the major reservoir, water
birds as carriers and mosquitoes as vectors. JEV invasion into the
central nervous system (CNS) may occur via antipodal transport of
virions or through the vascular endothelial cells. Microglial cells get
activated in response to pathogenic insults. JEV infection induces the
innate immune response and triggers the production of type I
interferons. The signaling pathway of type I interferon production is
regulated by a number of molecules. TRIM proteins are known to regulate
the expression of interferons; however, the involvement of TRIM genes
and their underlying mechanism during JEV infection are not known.
Japanese encephalitis virus (JEV), a flavivirus with
single-stranded RNA, is the leading cause of viral encephalitis in most
of southeast Asian countries. JEV is transmitted through mosquitoes and
maintained in a zoonotic cycle. This cycle involves pigs as the major
reservoir/amplifying host, water birds as carriers and mosquitoes as
vectors [1]. The estimated worldwide annual incidence of Japanese encephalitis (JE) is about 45,000 human cases and 10,000 deaths [2].
JE leads to long-term neurological damage and significant mortality
among children. Approximately 25% of encephalitis patients die, while
about 50% of the survivors develop permanent neurologic and/or
psychiatric sequelae [1].
The flaviviruses are known to induce proinflammatory response in CNS after infection.
A
key step toward induction of innate immunity against viral infections,
including JEV, is the production of type I interferons. The presence of
virus is sensed by pattern recognition receptors (PRRs) such as
Toll-like receptors (TLRs) and RIG-I (retinoic acid-inducible gene
1)-like receptors (RLRs) [3, 4].
The engagement of these (receptors) through pathogen molecular patterns
can lead to the production of various cytokines and chemokines and
other proinflammatory factors. The key regulators of the induction of
type I IFNs during viral infections are RIG-I and MDA5 (melanoma
differentiation-associated protein 5) [5, 6, 7, 8, 9, 10].
These are known to interact with MAVS (mitochondrial antiviral
signaling protein), which leads to downstream activation of various
kinases such as TBK1/IKKε (TANK-binding kinase 1/I kappa B Kinase-ε),
which in turn lead to phosphorylation and activation of various
transcription factors to induce IFN-β and IFN-α [11, 12, 13].
The production of type I interferons is crucial for generating
antiviral response against viruses. Production of interferons is
mediated by various transcription factors such as interferon regulatory
factors (IRF). Among the IRF family members, IRF-3 has been well
documented to play a role in expression of type I interferons in
response to viral infections. Phosphorylation of IRF-3 leads to
activation, dimerization and nuclear translocation, ultimately leading
to the transcription and production of IFN-β. IFN-β further initiates a
cascade of signaling events mediated by IRF-7 and IRF-5 resulting in the
production of IFN-γ and activation of various interferon-stimulated
genes (ISGs) [8, 14].
The
TRIM family (tripartite-motif family) of proteins has been reported for
their roles in regulating the innate immune response to viral
infections [15]. TRIM proteins are structurally characterized by a RING domain, a B-box domain and a coiled-coil domain [16, 17].
Functionally, most TRIMs are E3 ubiquitin ligases, where RING domains
have ubiquitin ligase activity, while the b-Box domains have interacting
motifs. TRIM proteins have been reported for their roles in cellular
processes such as cell differentiation, transcriptional regulation,
signaling cascades and apoptosis [15, 18, 19]. Many TRIM proteins play important roles in antiviral activities [20].
TRIM5 and TRIM22 are known to restrict HIV replication, while TRIM19
has been reported to restrict VSV and herpes simplex virus (HSV)
replication [21, 22, 23, 24].
TRIM21 has been known to play a crucial role in regulating type I
interferon production, but its role during viral infections is not well
understood [25, 26]. TRIM21 interacts and ubiquitinates IRF-3, IRF-7 and IRF-8 [27].
Due to such interactions, TRIM21 has been implicated in regulating type
I interferon signaling directly by modulating the upstream
transcription factors. TRIM21 is part of the RoSSA ribonucleoprotein,
which includes a single polypeptide and one of four small RNA molecules.
TRIM21 has been reported to recognize and degrade viruses in the
cytoplasm by binding to antibody-coated virions [28].
This
is the first report showing the role of TRIM21 in modulating the type I
interferon response upon JEV infection in human microglial cells. We
have demonstrated that induction of TRIM21 during JEV infection is a
compensatory mechanism to downregulate the type I interferon production
mediated by IRF-3. TRIM21 overexpression leads to downregulation of
JEV-mediated activation of IRF-3 and downstream IFN-β production,
whereas silencing of TRIM21 results in facilitation of JEV-mediated
activation of IRF-3 and upregulation of IFN-β production. We thereby
report the inhibitory role of TRIM21 on IFN-β production during JEV
infection in human microglial cells.
