The Proapoptotic Bcl-2 Protein Bax Plays an Important Role in the Pathogenesis of Reovirus Encephalitis

Abstract

Encephalitis induced by reovirus serotype 3 (T3) strains results from the apoptotic death of infected neurons. Extrinsic apoptotic signaling is activated in reovirus-infected neurons in vitro and in vivo, but the role of intrinsic apoptosis signaling during encephalitis is largely unknown. Bax plays a key role in intrinsic apoptotic signaling in neurons by allowing the release of mitochondrial cytochrome c. We found Bax activation and cytochrome c release in neurons following infection of neonatal mice with T3 reoviruses. Bax ؊/؊ mice infected with T3 Abney (T3A) have reduced central nervous system (CNS) tissue injury and decreased apoptosis, despite viral replication that is similar to that in wild-type (WT) Bax ؉/؉ mice. In contrast, in the heart, T3A-infected Bax ؊/؊ mice have viral growth, caspase activation, and injury comparable to those in WT mice, indicating that the role of Bax in pathogenesis is organ specific. Nonmyocarditic T3 Dearing (T3D)-infected Bax ؊/؊ mice had delayed disease and enhanced survival compared to WT mice. T3D-infected Bax ؊/؊ mice had significantly lower viral titers and levels of activated caspase 3 in the brain despite unaffected transneuronal spread of virus. Cytochrome c and Smac release occurred in some reovirus-infected neurons in the absence of Bax; however, this was clearly reduced compared to levels seen in Bax ؉/؉ wild-type mice, indicating that Bax is necessary for efficient activation of proapoptotic mitochondrial signaling in infected neurons. Our studies suggest that Bax is important for reovirus growth and pathogenesis in neurons and that the intrinsic pathway of apoptosis, mediated by Bax, is important for full expression of disease, CNS tissue injury, apoptosis, and viral growth in the CNS of reovirus-infected mice.

Cloning and characterization of Bax1 and Bax2 genes of Ctenopharyngodon idellus and evaluation of transcript expression in response to grass carp reovirus infection

“…It has been shown earlier that many viruses could activate Bax, such as influenza A virus, hepatitis C virus, and rotavirus (McLean et al 2009;Martin-Latil et al 2007;Benali-Furet et al 2005). In Bax -/-mice, reovirus-induced apoptosis and tissue injury were reduced compared with wild-type controls (Berens and Tylcr 2011). Knocking out of Bax in mouse embryonic fibroblasts prevented the apoptosis induced by influenza A virus and impaired viral replication (McLean et al 2009).…”

“…Knocking out of Bax in mouse embryonic fibroblasts prevented the apoptosis induced by influenza A virus and impaired viral replication (McLean et al 2009). The mechanisms underlying the capability of a variety of viruses to trigger Bax may be dependent on antiviral innate immunity pathways (Berens and Tylcr 2011). In the GCRV-infected grass carp, the immune responses are elicited in the gill, liver, spleen, and intestine (Shi et al 2014).…”

“…The extrinsic pathway is initiated when a death ligand binds to a death receptor on the cell surface, activating the expression of caspase-8 with subsequent activation of the downstream effector caspase-3, caspase-6, and caspase-7 (Jin and El-Deiry 2005;Kominsky et al 2002). In the intrinsic apoptotic pathway, caspase-8 activated by extrinsic pathway cleaves BH3-only Bcl-2 proteins, which can activate the proapoptotic Bcl-2 proteins, including Bax and Bcl-associated killer (Bak), to mediate the mitochondrial outer membrane permeabilization (MOMP), leading the release of cytochrome c (cyt c) into cytosol (Berens and Tylcr 2011;Kim et al 2009;Joza et al 2001). In CIK cells, GCRV-104 not only activates caspase-8 and downstream caspase-3 but also triggers intrinsic pathwayrelated caspase gene such as caspase-9, suggesting that the GCRV-induced apoptosis of CIK depends on caspase activation through both the intrinsic and extrinsic pathways (Jia et al 2014).…”

Apoptosis Induced by Mammalian Reovirus Is Beta Interferon (IFN) Independent and Enhanced by IFN Regulatory Factor 3- and NF-κB-Dependent Expression of Noxa

“…In cultured cells, reovirus apoptosis occurs independently of Bax and Bak, as levels of apoptosis in MEFs lacking Bax and Bak are equivalent to those in wild-type cells (89). However, in Bax-deficient mice, reovirus produces less apoptosis and tissue damage in the CNS than those observed for wild-type mice (9). This effect is organ specific, as levels of apoptosis in the heart do not differ between wild-type and Baxdeficient animals.…”

“…In keeping with the linkage between antiviral innate immunity and apoptotic cell death, both IRF-3 and NF-B are required for maximum levels of apoptosis in response to reovirus (20,35). Many cellular proapoptotic factors, including Bid, Bax, TRAIL, and Fas, also enhance apoptosis in reovirus-infected cells (9,15,18,24). However, mechanisms by which these proteins become activated, and the specific functions of IRF-3 and NF-B in this process, remain unclear.…”