Human endogenous retroviruses (HERVs) constitute 8% of the human genome and have been implicated in both health and disease. Increased HERV gene activity occurs in immunologically activated glia, although the consequences of HERV expression in the nervous system remain uncertain. Here, we report that the HERV-W encoded glycoprotein syncytin is upregulated in glial cells within acute demyelinating lesions of multiple sclerosis patients. Syncytin expression in astrocytes induced the release of redox reactants, which were cytotoxic to oligodendrocytes. Syncytin-mediated neuroinflammation and death of oligodendrocytes, with the ensuing neurobehavioral deficits, were prevented by the antioxidant ferulic acid in a mouse model of multiple sclerosis. Thus, syncytin's proinflammatory properties in the nervous system demonstrate a novel role for an endogenous retrovirus protein, which may be a target for therapeutic intervention.
To generate an extensive set of subgenomic (sg) mRNAs, nidoviruses (arteriviruses and coronaviruses) use a mechanism of discontinuous transcription. During this process, mRNAs are generated that represent the genomic 5 sequence, the so-called leader RNA, fused at specific positions to different 3 regions of the genome. The fusion of the leader to the mRNA bodies occurs at a short, conserved sequence element, the transcription-regulating sequence (TRS), which precedes every transcription unit in the genome and is also present at the 3 end of the leader sequence. Here, we have used site-directed mutagenesis of the infectious cDNA clone of the arterivirus equine arteritis virus to show that sg mRNA synthesis requires a base-pairing interaction between the leader TRS and the complement of a body TRS in the viral negative strand. Mutagenesis of the body TRS of equine arteritis virus RNA7 reduced sg RNA7 transcription severely or abolished it completely. Mutations in the leader TRS dramatically influenced the synthesis of all sg mRNAs. The construction of double mutants in which a mutant leader TRS was combined with the corresponding mutant RNA7 body TRS resulted in the specific restoration of mRNA7 synthesis. The analysis of the mRNA leader-body junctions of a number of mutants with partial transcriptional activity provided support for a mechanism of discontinuous minus-strand transcription that resembles similarity-assisted, copy-choice RNA recombination.T ranscriptional regulation is one of the key processes in the controlled expression of genetic information. In general, the transcription of eukaryotic DNA genomes is regulated in the nucleus, where it is subject to pre-, co-, and posttranscriptional control mechanisms (1, 2). In contrast, the transcription of eukaryotic positive-strand RNA [(ϩ)RNA] virus genomes occurs in the cytoplasm and relies on the process of RNAdependent RNA synthesis. In some cases, the genome is the only viral mRNA and gene expression is regulated solely at the (post-)translational level, primarily by the synthesis and controlled processing of precursor polyproteins. Alternatively, regulation may occur at the transcriptional level, either by segmentation of the genome or by the generation of one or multiple subgenomic (sg) mRNAs. The latter strategy usually involves the recognition of internal promoter sequences by the viral RNAdependent RNA polymerase (RdRp) complex, a process that resembles the recognition of DNA promoters by DNAdependent RNA polymerases (3, 4).Nidoviruses (arteriviruses and coronaviruses) are mammalian (ϩ)RNA viruses that appear to have evolved the use of sg mRNAs to regulate the expression of their polycistronic genome (5, 6). The nidovirus replicase is expressed from the genomic RNA as a polyprotein, but the structural proteins are translated from a set of six to eight sg mRNAs (Fig. 1). A key feature of these sg transcripts is the fact that their 5Ј and 3Ј terminal sequences are identical to those of the genome. This nested set structure results from a fusion of the ...
HIV-1 Nef is expressed in astrocytes, but a contribution to neuropathogenesis and the development of HIV-associated dementia (HAD) remains uncertain. To determine the neuropathogenic actions of the HIV-1 Nef protein, the brain-derived (YU-2) and blood-derived (NL4-3) Nef proteins were expressed in neural cells using an alphavirus vector, which resulted in astrocyte death (P < 0.001). Supernatants from Nef-expressing astrocytes also caused neuronal death, suggesting the release of neurotoxic molecules by astrocytes. Analysis of pro-inflammatory gene induction in astrocytes expressing Nef revealed increased IP-10 mRNA expression (4000-fold) that was Nef sequence dependent. Recombinant IP-10 caused selective cell death in neurons (P < 0.001) but not astrocytes, and the cytotoxicity of supernatant from astrocytes expressing Nef YU-2 was blocked by an antibody directed against the chemokine receptor CXCR3 (P < 0.001). SCID/NOD mice implanted with a Nef YU-2-expressing vector displayed abnormal motor behavior (P < 0.05), neuroinflammation, and neuronal loss relative to controls. Analysis of mRNA levels in brains from patients with HAD also revealed increased expression of IP-10 (P < 0.05), which was confirmed by immunoreactivity detected principally in astrocytes. Phylogenetic and protein structure analyses of Nef sequences derived from HIV/AIDS patients with and without HAD suggested viral evolution toward a neurotropic Nef protein. These results indicate that HIV-1 Nef contributes to neuropathogenesis by directly causing astrocyte death together with indirect neuronal death through the cytotoxic actions of IP-10 on neurons. Furthermore, Nef molecular diversity was evident in brain tissue among patients with neurological disease and which may influence IP-10 production by astrocytes.
West Nile virus (WNV) infection causes neurological disease at all levels of the neural axis, accompanied by neuroinflammation and neuronal loss, although the underlying mechanisms remain uncertain. Given the substantial activation of neuroinflammatory pathways observed in WNV infection, we hypothesized that WNV-mediated neuroinflammation and cell death occurred through WNV infection of both glia and neurons, which was driven in part by WNV capsid protein expression. Analysis of autopsied neural tissues from humans with WNV encephalomyelitis (WNVE) revealed WNV infection of both neurons and glia. Upregulation of proinflammatory genes, CXCL10, interleukin-1, and indolamine-2,3-deoxygenase with concurrent suppression of the protective astrocytespecific endoplasmic reticulum stress sensor gene, OASIS (for old astrocyte specifically induced substance), was evident in WNVE patients compared to non-WNVE controls. These findings were supported by increased ex vivo expression of these proinflammatory genes in glia infected by WNV-NY99. WNV infection caused endoplasmic reticulum stress gene induction and apoptosis in neurons but did not affect glial viability. WNV-infected astrocytic cells secreted cytotoxic factors, which caused neuronal apoptosis. The expression of the WNV-NY99 capsid protein in neurons and glia by a Sindbis virus-derived vector (SINrep5-WNVc) caused neuronal death and the release of neurotoxic factors by infected astrocytes, coupled with proinflammatory gene induction and suppression of OASIS. Striatal implantation of SINrep5-WNV C induced neuroinflammation in rats, together with the induction of CXCL10 and diminished OASIS expression, compared to controls. Moreover, magnetic resonance neuroimaging showed edema and tissue injury in the vicinity of the SINrep5-WNVc implantation site compared to controls, which was complemented by neurobehavioral abnormalities in the SINrep5-WNVc-implanted animals. These studies underscore the important interactions between the WNV capsid protein and neuroinflammation in the pathogenesis of WNV-induced neurological disorders.In North America, widespread West Nile virus (WNV) infection was first recognized in 1999 during an outbreak of viral encephalitis in New York City (32). Infection by WNV causes a spectrum of neurological disorders and ensuing death in a subset of infected individuals (24,26). WNV belongs to the flavivirus family (reviewed in references 11, 14, and 34), which are enveloped viruses with a genome consisting of one 10-to 11-kb single-stranded RNA molecule of positive-strand mRNA polarity. WNV genomic RNA contains one large open reading frame, which is translated into a single polypeptide and cleaved by viral and cellular proteases into three structural proteins and several nonstructural (NS) proteins (11, 34). The WNV structural proteins include the capsid (C) protein, the small transmembrane protein (M and its precursor preM), and the surface or envelope glycoprotein (E), which are all involved in pathogenesis. It is clear that infection of the centr...
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