The SAT Protein of Porcine Parvovirus Accelerates Viral Spreading through Induction of Irreversible Endoplasmic Reticulum Stress

Abstract

The SAT protein (SATp) of porcine parvovirus (PPV) accumulates in the endoplasmic reticulum (ER), and SAT deletion induces the slow-spreading phenotype. The in vitro comparison of the wild-type Kresse strain and its SAT knockout (SAT Ϫ ) mutant revealed that prolonged cell integrity and late viral release are responsible for the slower spreading of the SAT Ϫ virus. During PPV infection, regardless of the presence or absence of SATp, the expression of downstream ER stress response proteins (Xbp1 and CHOP) was induced. However, in the absence of SATp, significant differences in the quantity and the localization of CHOP were detected, suggesting a role of SATp in the induction of irreversible ER stress in infected cells. The involvement of the induction of irreversible ER stress in porcine testis (PT) cell necrosis and viral egress was confirmed by treatment of infected cells by ER stressinducing chemicals (MG132, dithiothreitol, and thapsigargin), which accelerated the egress and spreading of both the wild-type and the SAT Ϫ viruses. UV stress induction had no beneficial effect on PPV infection, underscoring the specificity of ER stress pathways in the process. However, induction of CHOP and its nuclear translocation cannot alone be responsible for the biological effect of SAT, since nuclear CHOP could not complement the lack of SAT in a coexpression experiment. IMPORTANCE SATp is encoded by an alternative open reading frame of the PPV genome. Earlier we showed that SATp of the attenuated PPV NADL-2 strain accumulates in the ER and accelerates virus release and spreading. Our present work revealed that slow spreading is a general feature of SAT Ϫ PPVs and is the consequence of prolonged cell integrity. PPV infection induced ER stress in infected cells regardless of the presence of SATp, as demonstrated by the morphological changes of the ER and expression of the stress response proteins Xbp1 and CHOP. However, the presence of SATp made the ER stress more severe and accelerated cell death during infection, as shown by the higher rate of expression of CHOP and alteration of the localization of CHOP. The beneficial effect of irreversible ER stress on PPV spread was confirmed by treatment of infected cells with ER stress-inducing chemicals.KEYWORDS CHOP, ER stress, SAT, Xbp1, alternative ORF, parvovirus, protoparvovirus, viral egress P orcine parvovirus (PPV) belongs to the species Ungulate protoparvovirus 1 in the genus Protoparvovirus. It is the causative agent of the stillbirth, mummification, embryonic death, infertility (SMEDI) syndrome in swine (1).A highly pathogenic strain with broad tissue tropism, strain Kresse, was isolated in the 1980s (2, 3). Kresse strain-infected piglets showed atypical signs of PPV infection, including necrotic lesions (4) on the lips, snout, tongue, and foot.The PPV single-stranded genome contains two major open reading frames (ORFs)

Porcine parvovirus replication is suppressed by activation of the PERK signaling pathway and endoplasmic reticulum stress-mediated apoptosis

“…However, a study of vesicular stomatitis virus demonstrated that ER stress activation inhibited viral infection (Baltzis et al, 2004). Previous studies have shown that treatment with the ER stress inducer MG132 inhibited infection by PPV strain NADL-2, similar to the results observed with PPV strain Kresse infection at a low MOI (Boisvert et al, 2010;Meszaros et al, 2017). Meszaros et al also showed that the specific ER stress inducer Tg inhibited infection by PPV strain Kresse.…”

“…The early stage of infection with the attenuated PPV NADL-2 strain was almost completely abolished by the ER inducer MG132 (Boisvert et al, 2010). A similar inhibitory effect of MG132 treatment of infection with PPV strain Kresse was detected at a low MOI (Meszaros et al, 2017). These data suggested that ER stress has both negative and positive effects on PPV replication related to the strain and dose of the virus.…”

“…Previous studies have shown that PPV infection can induce ER stress (Meszaros et al, 2017;Zadori et al, 2005). However, little is known about the potential mechanisms involved.…”

“…During viral infection, a large number of viral proteins are synthesized in the ER, which destroys the cellular translation machinery, resulting in the induction of ER stress and subsequent facilitation of viral infection (Zhang and Wang, 2012). It was reported that the SAT protein (SATp) of the attenuated PPV strains NADL-2 and Kresse accumulates in the ER and that the SAT deletion mutant of both of these strains induces a slow-spreading phenotype (Meszaros et al, 2017;Zadori et al, 2005). ER stress activation promoted viral egress of porcine testis (PT) fibroblasts infected with the Kresse strain of PPV at a high multiplicity of infection (MOI) (Meszaros et al, 2017).…”

“…It was reported that the SAT protein (SATp) of the attenuated PPV strains NADL-2 and Kresse accumulates in the ER and that the SAT deletion mutant of both of these strains induces a slow-spreading phenotype (Meszaros et al, 2017;Zadori et al, 2005). ER stress activation promoted viral egress of porcine testis (PT) fibroblasts infected with the Kresse strain of PPV at a high multiplicity of infection (MOI) (Meszaros et al, 2017). The early stage of infection with the attenuated PPV NADL-2 strain was almost completely abolished by the ER inducer MG132 (Boisvert et al, 2010).…”