Protein X of Borna Disease Virus Inhibits Apoptosis and Promotes Viral Persistence in the Central Nervous Systems of Newborn-Infected Rats

Abstract

Borna disease virus (BDV) is a neurotropic member of the order Mononegavirales with noncytolytic replication and obligatory persistence in cultured cells and animals. Here we show that the accessory protein X of BDV represents the first mitochondrion-localized protein of an RNA virus that inhibits rather than promotes apoptosis induction. Rat C6 astroglioma cells persistently infected with wild-type BDV were significantly more resistant to death receptor-dependent and -independent apoptotic stimuli than uninfected cells or cells infected with a BDV mutant expressing reduced amounts of X. Confocal microscopy demonstrated that X colocalizes with mitochondria and expression of X from plasmid DNA rendered human 293T and mouse L929 cells resistant to apoptosis induction. A recombinant virus encoding a mutant X protein unable to associate with mitochondria (BDV-X A6A7 ) failed to block apoptosis in C6 cells. Furthermore, Lewis rats neonatally infected with BDV-X A6A7 developed severe neurological symptoms and died around day 30 postinfection, whereas all animals infected with wild-type BDV remained healthy and became persistently infected. TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) staining revealed a significant increase in the number of apoptotic cells in the brain of BDV-X A6A7 -infected animals, whereas the numbers of CD3 ؉ T lymphocytes were comparable to those detected in animals infected with wild-type BDV. Our data thus indicate that inhibition of apoptosis by X promotes noncytolytic viral persistence and is required for the survival of cells in the central nervous system of BDV-infected animals.

Epidemiology and host spectrum of Borna disease virus infections

“…In addition to these most abundant proteins, the BDV genome encodes protein p10 (X), matrix protein p16 (M), glycoprotein p57 (G, gp94 when glycosylated) and RNA polymerase (L) (Walker et al, 2000; reviewed by Lipkin & Briese, 2007;Tomonaga et al, 2002). Polymerase activity is important in the adaptation of BDV to new hosts (Ackermann et al, 2007) and the X protein regulates viral polymerase activity and inhibits apoptosis, being essential for host survival (Poenisch et al, 2009). BDV uniquely limits its genome amplification by trimming the genome, which may favour non-cytolytic virus persistence and evasion of the antiviral host response (Habjan et al, 2008;Schneider et al, 2005).…”

A viral peptide that targets mitochondria protects against neuronal degeneration in models of Parkinson’s disease

“…To dissect the mechanism supporting the neuroprotective properties of BDV and to test the role of the X protein in axonal protection, we next compared the protective effects of a recombinant wild-type (wt) BDV (BDV-X wt ) with a recombinant BDV in which the X protein had been mutated to abrogate its mitochondrial localization (BDV-X A6A7 ) 10 . Consistent with the results shown above, infection with BDV-X wt strongly protected against rotenoneinduced axonal fragmentation.…”

“…This non-structural protein was initially described as an important regulator of viral RNA synthesis and of polymerase complex assembly in the nucleus of infected cells [11][12][13] . Later, it was shown that X represents the first mitochondrion-localized protein of an RNA virus that inhibits rather than promotes induction of apoptosis, thereby favouring non-cytolytic viral persistence 10 and escape to mitochondrial antiviral signalling protein-mediated host cell defence 14 . It was therefore tempting to investigate whether the X protein could protect neurons against neurodegenerative insults.…”

“…BDV non-cytolytic replication depends on the expression of an 87 amino acids viral protein called X 10 . This non-structural protein was initially described as an important regulator of viral RNA synthesis and of polymerase complex assembly in the nucleus of infected cells [11][12][13] .…”

Second-site mutations in Borna disease virus overexpressing viral accessory protein X

“…X accumulates in the nucleus of infected cells, where it regulates the activity of the viral polymerase complex through interactions with the viral P protein (Poenisch et al, 2004;Schwardt et al, 2005;Wolff et al, 2000). A fraction of X associates with mitochondria and suppresses virus-induced apoptosis of infected cells (Poenisch et al, 2009). The X protein is mainly synthesized from a 0.8 kb mRNA that contains three overlapping open reading frames.…”