Syncrip facilitates porcine parvovirus viral dna replication through the alternative splicing of ns1 mrna to promote ns2 mrna formation



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PPV.NS

Discussion
PPV is a major causative agent of stillbirth, mummifica-
tion, and embryonic death in swine [
1
]. The PPV genome 
encodes four nonstructural proteins NS1, NS2, NS3 and 
late SAT protein [
35

36
]. NS1 protein has helicase and 
nickase activities [
37

38
], and can induce apoptosis and 
cell cycle arrest [
39
]. NS2 protein can block the expres-
sion of IFN-β induced by ds-RNA [
9
], and associates 
with the nuclear egress of progeny virions efficiently 
[
40
]. A late non-structural protein SAT is expressed from 
the same mRNA as VP2, which can contribute to viral 
spreading by inducing irreversible endoplasmic reticu-
lum stress in the progress of PPV infection [
2
].
Alternative splicing process of Parvovirus pre-mRNA 
plays a key role in regulating the expression of viral pro-
teins [
25

41
]. Mature PPV NS2 mRNA is completely 
located in NS1 mRNA, which is formed by alternative 
splicing of NS1 mRNA. SYNCRIP has been reported as 
a splicing factor to be involved in post-transcriptional 
[
42

43
], miRNA exosomal sorting process [
21
], syn-
aptic protein mRNA expression [
11
] and RNA virus 
replication [
26

27
]. SYNCRIP, a member of heteroge-
neous nuclear ribonucleoproteins (hnRNP), is highly 
conserved among different species. SYNCRIP, which is 
a cytoplasm RNA-binding protein contains three typi-
cal domains, RRM1 domain, RRM2 domain and RRM3 
domain. Porcine SYNCRIP is encoded by the porcine 
SYNCRIP gene and possesses 99.84% identity to human 
SYNCRIP (GenBank: NM006372). In this study, we first 
reported that NS1 mRNA could directly interact with 
SYNCRIP, and found that SYNCRIP could be detected 
in most tissues of pigs, such as the spleen, lung, kidney, 
ovary and uterus. The expression of porcine SYNCRIP 
was upregulated with PPV infection levels. Impor-
tantly, we found that SYNCRIP knockout significantly 
impaired PPV replication. This result agrees with the 
previous observations that SYNCRIP is involved in 
MHV and HCV RNA replication [
26

27
].
Previous studies have reported that some RNA bind-
ing proteins are involved in alternative splicing either 
viral RNA or host cell RNA. HnRNP K can interact 
with influenza virulence factor NS1 binding protein 
(NS1-BP) to promote the splicing of the viral M1 RNA 
into M2 RNA [
44
], also modulate CD44 alternative 
splicing during the epithelial mesenchymal transition 
[
45
], and regulate the neuronal differentiation factor 
TRF2 alternative splicing. RBM38 is an essential host 
factor of B19V pre-mRNA splicing and it is important 
for the expression of the non-structural 11 kDa protein 
[
24
]. Because the hnRNP family has a highly conserved 
RNA recognition region, we speculated that SYNCRIP 
may directly interact with PPV virulence factor NS1 
mRNA and regulate NS1 expression. When recombi-
nant plasmids (pEGFP-NS1, pEGFP-NS2, pEGFP-VP1 
and pEGFP-VP2) were transfected into PK-15
SYNCRIP

PK-15 and PK-15
V
cells, the level of NS1 protein and 
mRNA were decreased in PK-15
SYNCRIP
cells, compared 
to that in PK-15 or PK-15
V
control cells, but the levels 
of VP1 and VP2 were not decreased in PK-15
SYNCRIP
cells, compared to those in PK-15 or PK-15
V
control 
cells. In addition, we noted that NS1 mRNA was signifi-
cantly decreased in the PPV-infected PK-15
SYNCRIP
cells 
relative to PK-15 and PK-15
V
cells. Furthermore, NS2 
mutant’s replication rate significantly decreased com-
pared to parental PPV. These results suggest that 


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Chen et al. Vet Res (2021) 52:73 

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