Antagonism of the interferon-induced OAS-RNase L pathway by murine coronavirus ns2 protein is required for virus replication and liver pathology

Abstract

Many viruses induce hepatitis in humans, highlighting the need to understand the underlying mechanisms of virus-induced liver pathology. The murine coronavirus, mouse hepatitis virus (MHV), causes acute hepatitis in its natural host and provides a useful model for understanding virus interaction with liver cells. The MHV accessory protein, ns2, antagonizes the type I interferon response and promotes hepatitis. We show that ns2 has 2',5'-phosphodiesterase activity, which blocks the interferon inducible 2',5'-oligoadenylate synthetase (OAS)-RNase L pathway to facilitate hepatitis development. Ns2 cleaves 2',5'-oligoadenylate, the product of OAS, to prevent activation of the cellular endoribonuclease RNase L and consequently block viral RNA degradation. An ns2 mutant virus was unable to replicate in the liver or induce hepatitis in wild-type mice, but was highly pathogenic in RNase L deficient mice. Thus, RNase L is a critical cellular factor for protection against viral infection of the liver and the resulting hepatitis.

Graphical abstract

Figure 1

The Interferon-Induced OAS-RNase L Pathway After infection, viral RNA is detected by pattern recognition receptors RIG-I and MDA5, resulting in the induction of IFN-α/β, which in turn induces ISGs, including OAS. OAS is activated by dsRNA to produce 2-5A, which activates RNase L. RNase L degrades cellular and viral RNA producing more RNA that is recognized by MDA5 and RIG-I, resulting in enhanced IFN induction. 2′-PDE cleaves 2-5A and inhibits the activation of RNase L. MHV ns2, like the cellular enzyme 2′-PDE, is a 2′,5′-phosphodiesterase. OAS, 2′-5′-oligoadenylate synthetase; 2-5A, 2′,5′ oligoadenylate; 2′-PDE, 2′-phosphodiesterase.

Figure 2

MHV Replication Is Not Inhibited by RNase L Signaling (A–C) BMMs from B6 or RNase L^−/− mice were infected with MHV-A59, SeV, or EMCV (1 PFU/cell). At the indicated time points, RNA was isolated from cell lysates and viral mRNA expression quantified by qRT-PCR (n = 3). Relative viral mRNA expression levels were expressed as 2^−ΔCT, relative to β-actin mRNA [ΔC_T = C_{T(gene of interest)} – C_T(β-actin)]. (D) Cells infected with MHV-A59 were lysed by freeze-thawing and titers determined by plaque assay (n = 3). See also Table S1.

Figure 3

The MHV ns2 Mutant Replicates to the Same Extent as WT Virus in BMM from RNase L^−/− but Not PKR^−/− mice (A and B) BMMs, derived from RNase L^−/− or PKR^−/− mice were infected (0.01 PFU/cell). At the indicated times, titers of viruses in the cell lysates combined with supernatants were determined by plaque assay (n = 3). (C) BMMs derived from B6 or RNase L^−/− mice were infected (MOI = 0.01 PFU/cell). At 48 hr p.i., the cells were stained with anti-MHV N antibody and Hoechst. The scale bar represents 100 μm. See also Figure S1.

Figure 4

MHV ns2 Protein Inhibits Ribosomal RNA Degradation as well as Accumulation of 2-5A in Infected BMMs and in 293T Cells Induced with Poly(I:C) (A) B6 BMMs were infected (1 PFU/cell). At the indicated times, RNA was purified and analyzed on a Bioanalyzer. The positions of 18S and 28S rRNA are indicated. (B) 293T cells were transfected with pCAGGS (pC), pC-ns2, or pC-ns2-H126R and 24 hr later were transfected with 10 μg/ml poly(I:C). Four hours later, RNA was purified and analyzed on a Bioanalyzer. (C and D) 2-5A was isolated from virus-infected BMMs described in (A) derived from B6 mice at 6 and 9 hr p.i (C) or RNase L^−/− mice at 9 hr p.i. (D) and analyzed by FRET. Error bars represent the SEM (n = 3). Asterisks indicate that differences are statistically significant (^∗p < 0.05 and^∗∗∗p < 0.001). (E) 2-5A from transfected 293T cells described in (B) was isolated and quantified by FRET. Error bars represent the SEM (n = 3). Asterisks indicate that differences are statistically significant (^∗∗∗p < 0.001). See also Figure S2.

Figure 5

MHV ns2 Cleaves (2′-5′)p₃A₃ into AMP and ATP In Vitro (A–C) Purified (2′-5′)p₃A₃ was incubated with 10 μg/ml of purified MHV ns2, ns2-H126R, or murine 2′-PDE. At the times indicated, the reactions were stopped; (2′-5′)p₃A₃ and its degradation products were analyzed by HPLC. (D) The amount of uncleaved (2′-5′)p₃A₃ remaining was determined by FRET. Error bars represent the SEM (n = 3).

Figure 6

MHV ns2 Enhances Virus Replication In Vivo by Inhibiting RNase L Signaling (A) Four-week-old B6 or RNase L^−/− mice were inoculated intrahepatically with A59 or ns2-H126R (200 PFU/mouse). At 3, 5, and 7 d.p.i., organs were harvested and homogenized, and virus titers were determined. The dashed line represents the limit of detection and the error bars represent the SEM (n = 5). Asterisks indicate that differences are statistically significant (^∗p < 0.05 and ^∗∗∗p < 0.001). (B) Liver sections from mice sacrificed at 5 d.p.i. were stained with anti-MHV N monoclonal antibody. The scale bar represents 200 μm. Sections are representative of two sections from each of three animals from each group. See also Figure S3.

Figure 7

MHV ns2 Facilitates the Induction of Viral Hepatitis in vivo (A) Livers sections from mock or virus-infected mice sacrificed at 5 and 7 d.p.i. were stained with hematoxylin and eosin (H&E). Arrows show areas of necrosis. The scale bar represents 500 μm. Sections are representative of two sections from three animals from each group. (B) Liver sections from mice sacrificed at 5 d.p.i. were stained with H&E for detection of inflammation or with anti-caspase-3 antibody for detection of apoptosis. The scale bars represent 100 μm. Sections are representative of two sections from three animals from each group. (C) Livers were harvested from mock- or virus-infected mice (n = 5) at 3, 5, and 7 d.p.i., and RNA was isolated. TNF-α and IFN-γ mRNAs were quantified by qRT-PCR, normalized to β-actin mRNA level, and expressed as the fold change relative to mock infected with the formula 2^−Δ[ΔCT].The error bars represents the SEM. Asterisks indicate that differences are statistically significant (^∗p < 0.05). See also Table S1.