Retrovirus-induced murine motor neuron disease: mapping the determinant of spongiform degeneration within the envelope gene

Proceedings of the inaugural Dark Genome Symposium: November 2022

In November 2022 the first Dark Genome Symposium was held in Boston, USA. The meeting was hosted by Rome Therapeutics and Enara Bio, two biotechnology companies working on translating our growing understanding of this vast genetic landscape into therapies for human disease. The spirit and ambition of the meeting was one of shared knowledge, looking to strengthen the network of researchers engaged in the field. The meeting opened with a welcome from Rosana Kapeller and Kevin Pojasek followed by a first session of field defining talks from key academics in the space. A series of panels, bringing together academia and industry views, were then convened covering a wide range of pertinent topics. Finally, Richard Young and David Ting gave their views on the future direction and promise for patient impact inherent in the growing understanding of the Dark Genome.

Retroviral Elements in Pathophysiology and as Therapeutic Targets for Amyotrophic Lateral Sclerosis

The study of the role of retroviruses in amyotrophic lateral sclerosis (ALS) dates back to the 1960s shortly after transposable elements themselves were first discovered. It was quickly realized that in wild mice both horizontal and vertical transmissions of retroviral elements were key to the development of an ALS-like syndrome leading to the postulate that endogenous retroviruses (ERVs) contribute significantly to the pathogenicity of this disease. Subsequent studies identified retroviral reverse transcriptase activity in brains of individuals with ALS from Guam. However, except for a single study from the former Soviet Union, ALS could not be transmitted to rhesus macaques. The discovery of an ALS-like syndrome in human immunodeficiency virus (HIV) and human T cell leukemia virus infected individuals led to renewed interest in the field and reverse transcriptase activity was found in the blood and cerebrospinal fluid of individuals with sporadic ALS. However, exogenous retroviruses could not be found in individuals with ALS which further reinforced the possibility of involvement of a human ERV (HERV). The first demonstration of the involvement of a HERV was the discovery of the activation of human endogenous retrovirus-K subtype HML-2 in the brains of individuals with ALS. The envelope protein of HML-2 is neurotoxic and transgenic animals expressing the envelope protein develop an ALS-like syndrome. Activation of HML-2 occurs in the context of generalized transposable element activation and is not specific for ALS. Individuals with HIV-associated ALS show a remarkable response to antiretroviral therapy; however, antiretroviral trials in ALS down-regulate HML-2 without ameliorating the disease. This highlights the need for specific drugs to be developed against HML-2 as a novel therapeutic target for ALS. Other approaches might include antisense oligonucleotides, shRNA targeted against the envelope gene or antibodies that can target the extracellular envelope protein. Future clinical trials in ALS should consider combination therapies to control these ERVs.

Human endogenous retrovirus K (HERV-K) env in neuronal extracellular vesicles: a new biomarker of motor neuron disease

Objective: Human endogenous retroviruses (HERVs) have been gradually confirmed to be involved in the onset and progression of motor neuron disease(MND). However, noninvasive detection of HERVs in the central nervous system is lacking. The aim of this study is to verify the relationship between the level of HERV-K env in neuronal extracellular vesicles in plasma and the onset and severity of MND.

Methods: We extracted neuronal extracellular vesicles from plasma of 39 MND patients and 30 age- and sex-matched controls, and detected HERV-K env in extracellular vesicles by an enzyme-linked immunosorbent assay (ELISA).

Results: Levels of HERV-K env in neuronal extracellular vesicles positively associated with range of lower motor neurons (LMNs) involved (1.66 ± 0.37 vs. 1.35 ± 0.34, p = 0.041), ALS phenotype (1.52 ± 0.31 vs. 1.24 ± 0.37, p = 0.013) and course of disease (1.83 ± 0.35 vs. 1.42 ± 0.22, p = 0.003), and increased in advanced-phase MND (definite and probable according to revised EI Escorial criteria) compared with early-phase MND (possible and lab-supported probable), albeit without very profound significance (1.52 ± 0.34 vs. 1.29 ± 0.36, p = 0.048).

Conclusions: In conclusion, levels of HERV-K env in neuronal extracellular vesicles extracted from plasma can be used as a noninvasive biomarker of severity of MND.

Pro-Inflammatory Signaling Upregulates a Neurotoxic Conotoxin-Like Protein Encrypted Within Human Endogenous Retrovirus-K

Motor neuron degeneration and spinal cord demyelination are hallmark pathological events in Amyotrophic Lateral Sclerosis (ALS). Endogenous retrovirus-K (ERVK) expression has an established association with ALS neuropathology, with murine modeling pointing to a role for the ERVK envelope (env) gene in disease processes. Here, we describe a novel viral protein cryptically encoded within the ERVK env transcript, which resembles two distinct cysteine-rich neurotoxic proteins: conotoxin proteins found in marine snails and the Human Immunodeficiency Virus (HIV) Tat protein. Consistent with Nuclear factor-kappa B (NF-κB)-induced retrotransposon expression, the ERVK conotoxin-like protein (CTXLP) is induced by inflammatory signaling. CTXLP is found in the nucleus, impacting innate immune gene expression and NF-κB p65 activity. Using human autopsy specimens from patients with ALS, we further showcase CTXLP expression in degenerating motor cortex and spinal cord tissues, concomitant with inflammation linked pathways, including enhancement of necroptosis marker mixed lineage kinase domain-like (MLKL) protein and oligodendrocyte maturation/myelination inhibitor Nogo-A. These findings identify CTXLP as a novel ERVK protein product, which may act as an effector in ALS neuropathology.

Human endogenous retrovirus-K contributes to motor neuron disease

The role of human endogenous retroviruses (HERVs) in disease pathogenesis is unclear. We show that HERV-K is activated in a subpopulation of patients with sporadic amyotrophic lateral sclerosis (ALS) and that its envelope (env) protein may contribute to neurodegeneration. The virus was expressed in cortical and spinal neurons of ALS patients, but not in neurons from control healthy individuals. Expression of HERV-K or its env protein in human neurons caused retraction and beading of neurites. Transgenic animals expressing the env gene developed progressive motor dysfunction accompanied by selective loss of volume of the motor cortex, decreased synaptic activity in pyramidal neurons, dendritic spine abnormalities, nucleolar dysfunction, and DNA damage. Injury to anterior horn cells in the spinal cord was manifested by muscle atrophy and pathological changes consistent with nerve fiber denervation and reinnervation. Expression of HERV-K was regulated by TAR (trans-activation responsive) DNA binding protein 43, which binds to the long terminal repeat region of the virus. Thus, HERV-K expression within neurons of patients with ALS may contribute to neurodegeneration and disease pathogenesis.

Splicing of Friend Murine Leukemia Virus env-mRNA Enhances Its Ability to Form Polysomes

Friend murine leukemia virus (MLV) belongs to the gamma retroviruses of the Retroviridae family. The positive-sense RNA of its genome contains a 5' long terminal repeat (LTR), 5' leader sequence, gag, pol, env, and 3' LTR. Transcription from proviral DNA begins from the R region of the 5' LTR and ends at the polyadenylation signal located at the R region of the other end of the 3' LTR. There is a 5' splice site in the 5' leader sequence and a 3' splice site at the 3' end of the pol region. Both full-length unspliced mRNAs and a singly spliced mRNA (env-mRNA) are produced in MLV-infected cells. The MLV Env protein plays important roles both in viral adsorption to host cells and in neuropathogenic disease in MLV-infected mice and rats. Understanding the regulatory mechanisms controlling Env expression is important for determining the functions of the Env protein. We have previously shown that splicing increases env-mRNA stability and translation efficiency. Generally, mRNA polysome formation correlates with translation efficiency. Therefore, here we investigated the effects of env-mRNA splicing on polysome formation to identify mechanisms for Env up-regulation due to splicing. We performed polysome profile analyses using Env-expression plasmids producing spliced or unspliced env-mRNA and showed that the former formed polysomes more efficiently than the latter. Thus, splicing of env-mRNA facilitated polysome formation, suggesting that this contributes to up-regulation of Env expression. We replaced the env region of the expression plasmids with a luciferase (luc) gene, and found that in this case both unspliced and spliced luc-mRNA formed polysomes to a similar extent. Thus, we conclude that whether mRNA polysome formation is affected by splicing depends on the structure of gene in question.

Polyadenylation of Friend murine leukemia virus env-mRNA is affected by its splicing

As splicing was previously found to be important for increasing Friend murine leukemia virus env-mRNA stability and translation, we investigated whether splicing of env-mRNA affected the poly(A) tail length using env expression vectors that yielded unspliced or spliced env-mRNA. Incomplete polyadenylation was detected in a fraction of the unspliced env-mRNA products in an env gene-dependent manner, showing that splicing of Friend murine leukemia virus plays an important role in the efficiency of complete polyadenylation of env-mRNA. These results suggested that the promotion of complete polyadenylation of env-mRNA by splicing might partially explain up-regulation of Env protein expression as a result of splicing.

A 38 nt region and its flanking sequences within gag of Friend murine leukemia virus are crucial for splicing at the correct 5' and 3' splice sites

The genome of the Friend murine leukemia virus (Fr-MLV) contains a 5' splice site (5'ss) located at 205 nt and a 3'ss located at 5489 nt. In our previous studies, it was shown that if the HindIII-BglII (879-1904 bp) fragment within gag is deleted from the proA8m1 vector, which carries the entire Fr-MLV sequence, then cryptic splicing of env-mRNA occurs. Here, attempts were made to identify the genomic segment(s) in this region that is/are essential to correct splicing. First, vectors with a serially truncated HindIII-BglII fragment were constructed. The vector, in which a 38 bp fragment (1612-1649 bp) is deleted or reversed in proA8m1, only produced splice variants. It was found that a 38 nt region within gag contains important elements that positively regulate splicing at the correct splice sites. Further analyses of a series of vectors carrying the 38 bp fragment and its flanking sequences showed that a region (1183-1611 nt) upstream of the 38 nt fragment also contains sequences that positively or negatively influence splicing at the correct splice sites. The SphI-NdeI (5140-5400 bp) fragment just upstream of the 3'ss was deleted from vectors that carried the 38 bp fragment and its flanking sequences, which yielded correctly spliced mRNA; interestingly, these deleted vectors showed cryptic splicing. These findings suggest that the 5140-5400 nt region located just upstream of the 3'ss is required for the splicing function of the 38 nt fragment and its flanking sequences.

Kinetic studies of the effect of a 17-nucleotide difference in the 0.3-kb region containing the R-U5-5' leader sequence of Friend murine leukemia virus on viral gene expression

In addition to the env gene, a 0.3-kb fragment containing the R-U5-5' leader sequence is essential for the induction of spongiform neurodegeneration by Friend murine leukemia virus (Fr-MLV) clone A8 and it also influences expression of the Env protein. Kinetic studies were carried out using two recombinant viruses, R7f, carrying the A8 0.3-kb fragment, and Rec5, carrying the 0.3-kb fragment of the non-neuropathogenic Fr-MLV clone 57. These analyses suggested that the 0.3-kb fragment influenced the expression level of the Env protein by regulating the amount of spliced env-mRNA rather than the amount of total viral mRNA or viral production.

Narrowing down the critical region within env gene for determining neuropathogenicity of murine leukemia virus A8

Friend murine leukemia virus clone A8 causes spongiform neurodegeneration in the rat brain, and the env gene of A8 is a primary determinant of neuropathogenicity. In order to narrow down the critical region within the env gene that determines neuropathogenicity, we constructed chimeric viruses having chimeric env between A8 and non-neuropathogenic 57 on the background of A8 virus. After replacement of the BamHI (at nucleotide 5715)-AgeI (at nucleotide 6322) fragment of A8 virus with the corresponding fragment of 57, neuropathogenicity was lost. In contrast, the chimeric viruses that have the BamHI (5715)-AgeI (6322) fragment of A8 induced spongiosis in 100% of infected rats at the same or slightly lower intensity than A8 virus. These results indicate that the BamHI (5715)-AgeI (6322) fragment of A8, which contains the signal sequence and the N-terminal half of RBD, is crucial for the induction of spongiform neurodegeneration. In the BamHI (5715)-AgeI (6322) fragment, seven amino acids differed between A8 and 57, one in the signal sequence and six in RBD, which suggests that these amino acids significantly contribute to the neuropathogenicity of A8.

Misfolding of CasBrE SU is reversed by interactions with 4070A Env: implications for gammaretroviral neuropathogenesis

Background

CasBrE is a neurovirulent murine leukemia virus (MLV) capable of inducing paralytic disease with associated spongiform neurodegeneration. The neurovirulence of this virus has been genetically mapped to the surface expressed subunit (SU) of the env gene. However, CasBrE SU synthesized in the absence of the transmembrane subunit (TM) does not retain ecotropic receptor binding activity, indicating that folding of the receptor binding domain (RBD) requires this domain. Using a neural stem cell (NSC) based viral trans complementation approach to examine whether misfolded CasBrE SU retained neurovirulence, we observed CasBrE SU interaction with the "non-neurovirulent" amphotropic helper virus, 4070A which restored functional activity of CasBrE SU.

Results

Herein, we show that infection of NSCs expressing CasBrE SU with 4070A (CasES+4070A-NSCs) resulted in the redistribution of CasBrE SU from a strictly secreted product to include retention on the plasma membrane. Cell surface cross-linking analysis suggested that CasBrE SU membrane localization was due to interactions with 4070A Env. Viral particles produced from CasES+4070A-NSCS contained both CasBrE and 4070A gp70 Env proteins. These particles displayed ecotropic receptor-mediated infection, but were still 100-fold less efficient than CasE+4070A-NSC virus. Infectious center analysis showed CasBrE SU ecotropic transduction efficiencies approaching those of NSCs expressing full length CasBrE Env (CasE; SU+TM). In addition, CasBrE SU-4070A Env interactions resulted in robust ecotropic superinfection interference indicating near native intracellular SU interaction with its receptor, mCAT-1.

Conclusions

In this report we provided evidence that 4070A Env and CasBrE SU physically interact within NSCs leading to CasBrE SU retention on the plasma membrane, incorporation into viral particles, restoration of mCAT-1 binding, and capacity for initiation of TM-mediated fusion events. Thus, heterotropic Env-SU interactions facilitates CasBrE SU folding events that restore Env activity. These findings are consistent with the idea that one protein conformation acts as a folding scaffold or nucleus for a second protein of similar primary structure, a process reminiscent of prion formation. The implication is that template-based protein folding may represent an inherent feature of neuropathogenic proteins that extends to retroviral Envs.

Elevated levels of transcripts encoding a human retroviral envelope protein (syncytin) in muscles from patients with motor neuron disease

Retroviral components of both exogenous and endogenous origins have been associated with nervous system diseases in both animals and humans. In the present study, the levels of transcripts from elements in the human endogenous retrovirus (HERV) W family were determined in muscle biopsies from patients with motor neuron disease (MND) and control subjects. Transcripts from the HERV-W element on chromosome 7q21.2 encoding syncytin and from the SOD1 gene were detected at elevated levels in biopsies from the most affected muscles from MND patients compared to biopsies from control individuals. According to a recent study, syncytin is expressed in microglia in normal brain and can be up-regulated in macrophages/microglia during inflammation. Although syncytin may have cytotoxic effects, it is therefore more likely that the present findings reflect a macrophage response in the muscles undergoing neurogenic atrophy than a primary pathogenetic event in MND.

The degree of folding instability of the envelope protein of a neurovirulent murine retrovirus correlates with the severity of the neurological disease

A small group of ecotropic murine retroviruses cause a spongiform neurodegenerative disease manifested by tremor, paralysis, and wasting. The neurovirulence of these viruses has long been known to be determined by the sequence of the viral envelope protein, although the nature of the neurotoxicity remains to be clarified. Studies on the neurovirulent viruses FrCas(NC) and Moloney murine leukemia virus ts1 indicate that the nascent envelope protein misfolds, is retained in the endoplasmic reticulum (ER), and induces an unfolded protein response. In the present study we constructed a series of viruses with chimeric envelope genes containing segments from virulent and avirulent retroviruses. Each of the viruses studied was highly neuroinvasive but differed in the severity of the neurological disease they induced. Only viruses that contained the receptor-binding domain (RBD) of the neurovirulent virus induced neurological disease. Likewise, only viruses containing the RBD of the neurovirulent virus exhibited increased binding of the ER chaperone BiP to the envelope precursor protein and induced the unfolded protein response. Thus, the RBD determined both neurovirulence and folding instability. Among viruses carrying the neurovirulent RBD, the severity of the disease was increased when envelope sequences from the neurovirulent virus outside the RBD were also present. Interestingly, these sequences appeared to further increase the degree of folding instability (BiP binding) of the viral envelope protein. These results provide strong support for the hypothesis that this spongiform neurodegenerative disease represents a virus-induced protein folding disorder.

Gene expression profiling of microglia infected by a highly neurovirulent murine leukemia virus: implications for neuropathogenesis

BACKGROUND

Certain murine leukemia viruses (MLVs) are capable of inducing progressive spongiform motor neuron disease in susceptible mice upon infection of the central nervous system (CNS). The major CNS parenchymal target of these neurovirulent retroviruses (NVs) are the microglia, whose infection is largely coincident with neuropathological changes. Despite this close association, the role of microglial infection in disease induction is still unknown. In this paper, we investigate the interaction of the highly virulent MLV, FrCasE, with microglia ex vivo to evaluate whether infection induces specific changes that could account for neurodegeneration. Specifically, we compared microglia infected with FrCasE, a related non-neurovirulent virus (NN) F43/Fr57E, or mock-infected, both at a basic virological level, and at the level of cellular gene expression using quantitative real time RT-PCR (qRT-PCR) and Afffymetrix 430A mouse gene chips.

RESULTS

Basic virological comparison of NN, NV, and mock-infected microglia in culture did not reveal differences in virus expression that provided insight into neuropathogenesis. Therefore, microglial analysis was extended to ER stress gene induction based on previous experiments demonstrating ER stress induction in NV-infected mouse brains and cultured fibroblasts. Analysis of message levels for the ER stress genes BiP (grp78), CHOP (Gadd153), calreticulin, and grp58 in cultured microglia, and BiP and CHOP in microglia enriched fractions from infected mouse brains, indicated that FrCasE infection did not induce these ER stress genes either in vitro or in vivo. To broadly identify physiological changes resulting from NV infection of microglia in vitro, we undertook a gene array screen of more than 14,000 well-characterized murine genes and expressed sequence tags (ESTs). This analysis revealed only a small set of gene expression changes between infected and uninfected cells (<18). Remarkably, gene array comparison of NN- and NV-infected microglia revealed only 3 apparent gene expression differences. Validation experiments for these genes by Taqman real-time RT-PCR indicated that only single Ig IL-1 receptor related protein (SIGIRR) transcript was consistently altered in culture; however, SIGIRR changes were not observed in enriched microglial fractions from infected brains.

CONCLUSION

The results from this study indicate that infection of microglia by the highly neurovirulent virus, FrCasE, does not induce overt physiological changes in this cell type when assessed ex vivo. In particular, NV does not induce microglial ER stress and thus, FrCasE-associated CNS ER stress likely results from NV interactions with another cell type or from neurodegeneration directly. The lack of NV-induced microglial gene expression changes suggests that FrCasE either affects properties unique to microglia in situ, alters the expression of microglial genes not represented in this survey, or affects microglial cellular processes at a post-transcriptional level. Alternatively, NV-infected microglia may simply serve as an unaffected conduit for persistent dissemination of virus to other neural cells where they produce acute neuropathogenic effects.

The Other Transmissible Spongiform Encephalopathies

Murine leukemia viruses may produce encephalopathies that have the same characteristics as those induced by infectious proteins or prions: neuronal loss, astrocytosis, and absence of inflammatory response. The pathogenic mechanism is still poorly understood but it seems that it involves the envelope proteins (Env), which may be misprocessed in the cell, giving rise to pathogenic isoforms that trigger oxidative damage. Env may also affect the cytokine pattern in the central nervous system and thus, induce encephalopathy.

Bystander killing during avian leukosis virus subgroup B infection requires TVB(S3) signaling

Cell killing by avian leukosis virus subgroup B (ALV-B) in cultures has been extensively studied, but the molecular basis of this process has not been established. Here we show that superinfection, which has been linked to cell killing by ALV-B, plays no crucial role in cell death induction. Instead, we show that signaling by the ALV-B receptor, TVB(S3), a member of the tumor necrosis factor receptor family, is essential for ALV-B-mediated cell death. TVB(S3) activated caspase-dependent apoptosis during ALV-B infection. Strikingly, apoptosis induction occurred predominantly in uninfected cells, while ALV-B-infected cells were protected against cell death. This bystander killing phenomenon was reproduced in a virus-free system by cocultivating ALV-B Env-expressing cells with TVB(S3)-expressing cells. Taken together, our results indicated that ALV-B-mediated apoptosis is triggered by ALV-B Env-TVB(S3) interactions.

Protection against murine leukemia virus-induced spongiform myeloencephalopathy in mice overexpressing Bcl-2 but not in mice deficient for interleukin-6, inducible nitric oxide synthetase, ICE, Fas, Fas ligand, or TNF-R1 genes

Some murine leukemia viruses (MuLVs), among them Cas-Br-E and ts-1 MuLVs, are neurovirulent, inducing spongiform myeloencephalopathy and hind limb paralysis in susceptible mice. It has been shown that the env gene of these viruses harbors the determinant of neurovirulence. It appears that neuronal loss occurs by an indirect mechanism, since the target motor neurons have not been found to be infected. However, the pathogenesis of the disease remains unclear. Several lymphokines, cytokines, and other cellular effectors have been found to be aberrantly expressed in the brains of infected mice, but whether these are required for the development of the neurodegenerative lesions is not known. In an effort to identify the specific effectors which are indeed required for the initiation and/or development of spongiform myeloencephalopathy, we inoculated gene-deficient (knockout [KO]) mice with ts-1 MuLV. We show here that interleukin-6 (IL-6), inducible nitric oxide synthetase (iNOS), ICE, Fas, Fas ligand (FasL), and TNF-R1 KO mice still develop signs of disease. However, transgenic mice overexpressing Bcl-2 in neurons (NSE/Bcl-2) were largely protected from hind limb paralysis and had less-severe spongiform lesions. These results indicate that motor neuron death occurs in this disease at least in part by a Bcl-2-inhibitable pathway not requiring the ICE, iNOS, Fas/FasL, TNF-R1, and IL-6 gene products.

10A1-MuLV but not the related amphotropic 4070A MuLV is highly neurovirulent: importance of sequences upstream of the structural Gag coding region

Recombinants of Moloney murine leukemia virus (MoMuLV) with either an amphotropic (MoAmphoV) or 10A1-tropic host range (Mo10A1V) induce a spongiform neurodegenerative disease in susceptible mice. To test whether MoMuLV -derived sequences are required for induction of neuropathology, mice were inoculated with either the original 10A1 or the amphotropic (4070A) MuLV isolate. Strikingly, wild-type 10A1 was more neurovirulent than Mo10A1V, inducing severe neurological clinical symptoms with a median latency of 99 days in 100% of infected mice. In contrast, no motor disturbances were detected in any of the 4070A-infected mice, although limited central nervous system lesions were observed. A viral determinant conferring high neurovirulence to 10A1 was mapped to a region encompassing the first 676 bases of the viral genome, including the U5 LTR and encoding the amino-terminus of glycosylated Gag (glycoGag). In contrast to studies with the highly neurovirulent CasFr(KP) virus, an inverse correlation between surface expression levels of glycoGag and neurovirulence was not observed; however, this does not rule out a common underlying mechanism regulating virus pathogenicity.

Human immunodeficiency virus type 1 envelope-mediated neuronal death: uncoupling of viral replication and neurotoxicity

Although brain tissue from patients with human immunodeficiency virus (HIV) and/or AIDS is consistently infected by HIV type 1 (HIV-1), only 20 to 30% of patients exhibit clinical or neuropathological evidence of brain injury. Extensive HIV-1 sequence diversity is present in the brain, which may account in part for the variability in the occurrence of HIV-induced brain disease. Neurological injury caused by HIV-1 is mediated directly by neurotoxic viral proteins or indirectly through excess production of host molecules by infected or activated glial cells. To elucidate the relationship between HIV-1 infection and neuronal death, we examined the neurotoxic effects of supernatants from human 293T cells or macrophages expressing recombinant HIV-1 virions or gp120 proteins containing the V1V3 or C2V3 envelope region from non-clade B, brain-derived HIV-1 sequences. Neurotoxicity was measured separately as apoptosis or total neuronal death, with apoptosis representing 30 to 80% of the total neuron death observed, depending on the individual virus. In addition, neurotoxicity was dependent on expression of HIV-1 gp120 and could be blocked by anti-gp120 antibodies, as well as by antibodies to the human CCR5 and CXCR4 chemokine receptors. Despite extensive sequence diversity in the recombinant envelope region (V1V3 or C2V3), there was limited variation in the neurotoxicity induced by supernatants from transfected 293T cells. Conversely, supernatants from infected macrophages caused a broader range of neurotoxicity levels that depended on each virus and was independent of the replicative ability of the virus. These findings underscore the importance of HIV-1 envelope protein expression in neurotoxic pathways associated with HIV-induced brain disease and highlight the envelope as a target for neuroprotective therapeutic interventions.

An NF-kappa B-dependent survival pathway protects against cell death induced by TVB receptors for avian leukosis viruses

TVB receptors are death receptors of the tumor necrosis factor receptor (TNFR) family and serve as cellular receptors for cytopathic subgroups B and D and noncytopathic subgroup E of the avian leukosis viruses (ALVs). Although TVB is essential for ALV-B-mediated cell death, binding of the ALV-B envelope protein to its cognate receptor TVB activates cell death only in the presence of protein biosynthesis inhibitors, which presumably block the expression of protective factors. In the case of TNFR-1, the main antiapoptotic pathway depends upon nuclear factor kappa B (NF-kappa B)-activated survival factors. Here we show that overexpression of TVB receptors in human 293 cells activates NF-kappa B via a mechanism involving the cytoplasmic death domains of these receptors. NF-kappa B is also activated upon binding of a soluble ALV-B or ALV-E surface envelope-immunoglobulin fusion protein to the cognate TVB receptors and by ALV-B infection of a chicken embryo fibroblast cell line (DF1). Importantly, the cycloheximide requirement for TVB-dependent cell death was overcome by the expression of a transdominant form of I kappa B-alpha, and downregulation of NF-kappa B by the immunomodulator pyrrolidinedithiocarbamate enhanced the cytopathogenicity of ALV-B. These results demonstrate that TVB receptors trigger NF-kappa B-dependent gene expression and that NF-kappa B-regulated survival factors can protect against virus-induced cell death.

Increased expression of MIP-1 alpha and MIP-1 beta mRNAs in the brain correlates spatially and temporally with the spongiform neurodegeneration induced by a murine oncornavirus

The chimeric murine oncornavirus FrCas(E) causes a rapidly progressive paralytic disease associated with spongiform neurodegeneration throughout the neuroaxis. Neurovirulence is determined by the sequence of the viral envelope gene and by the capacity of the virus to infect microglia. The neurocytopathic effect of this virus appears to be indirect, since the cells which degenerate are not infected. In the present study we have examined the possible role of inflammatory responses in this disease and have used as a control the virus F43. F43 is an highly neuroinvasive but avirulent virus which differs from FrCas(E) only in 3' pol and env sequences. Like FrCas(E), F43 infects large numbers of microglial cells, but it does not induce spongiform neurodegeneration. RNAase protection assays were used to detect differential expression of genes encoding a variety of cytokines, chemokines, and inflammatory cell-specific markers. Tumor necrosis factor alpha (TNF-alpha) and TNF-beta mRNAs were upregulated in advanced stages of disease but not early, even in regions with prominent spongiosis. Surprisingly there was no evidence for upregulation of the cytokines interleukin-1 alpha (IL-1 alpha), IL-1 beta, and IL-6 or of the microglial marker F4/80 at any stage of this disease. In contrast, increased levels of the beta-chemokines MIP-1 alpha and -beta were seen early in the disease and were concentrated in regions of the brain rich in spongiosis, and the magnitude of responses was similar to that observed in the brains of mice injected with the glutamatergic neurotoxin ibotenic acid. MIP-1alpha and MIP-1beta mRNAs were also upregulated in F43-inoculated mice, but the responses were three- to fivefold lower and occurred later in the course of infection than was observed in FrCas(E)-inoculated mice. These results suggest that the robust increase in expression of MIP-1 alpha and MIP-1 beta in the brain represents a correlate of neurovirulence in this disease, whereas the TNF responses are likely secondary events.

TVB receptors for cytopathic and noncytopathic subgroups of avian leukosis viruses are functional death receptors

The identification of TVB(S3), a cellular receptor for the cytopathic subgroups B and D of avian leukosis virus (ALV-B and ALV-D), as a tumor necrosis factor receptor-related death receptor with a cytoplasmic death domain, provides a compelling argument that viral Env-receptor interactions are linked to cell death (4). However, other TVB proteins have been described that appear to have similar death domains but are cellular receptors for the noncytopathic subgroup E of ALV (ALV-E): TVB(T), a turkey subgroup E-specific ALV receptor, and TVB(S1), a chicken receptor for subgroups B, D, and E ALV. To begin to understand the role of TVB receptors in the cytopathic effects associated with infection by specific ALV subgroups, we asked whether binding of a soluble ALV-E surface envelope protein (SU) to its receptor can lead to cell death. Here we report that ALV-E SU-receptor interactions can induce apoptosis in quail or turkey cells. We also show directly that TVB(S1) and TVB(T) are functional death receptors that can trigger cell death by apoptosis via a mechanism involving their cytoplasmic death domains and activation of the caspase pathway. These data demonstrate that ALV-B and ALV-E use functional death receptors to enter cells, and it remains to be determined why only subgroups B and D viral infections lead specifically to cell death.

Lentivirus infection in the brain induces matrix metalloproteinase expression: role of envelope diversity

Infection of the brain by lentiviruses, including human immunodeficiency virus (HIV) and feline immunodeficiency virus (FIV), causes inflammation and results in neurodegeneration. Molecular diversity within the lentivirus envelope gene has been implicated in the regulation of cell tropism and the host response to infection. Here, we examine the hypothesis that envelope sequence diversity modulates the expression of host molecules implicated in lentivirus-induced brain disease, including matrix metalloproteinases (MMP) and related transcription factors. Infection of primary macrophages by chimeric HIV clones containing brain-derived envelope fragments from patients with HIV-associated dementia (HAD) or nondemented AIDS patients (HIV-ND) showed that MMP-2 and -9 levels in conditioned media were significantly higher for the HAD clones. Similarly, STAT-1 and JAK-1 levels were higher in macrophages infected by HAD clones. Infections of primary feline macrophages by the neurovirulent FIV strain (V(1)CSF), the less neurovirulent strain (Petaluma), and a chimera containing the V(1)CSF envelope in a Petaluma background (FIV-Ch) revealed that MMP-2 and -9 levels were significantly higher in conditioned media from V(1)CSF- and FIV-Ch-infected macrophages, which was associated with increased intracellular STAT-1 and JAK-1 levels. The STAT-1 inhibitor fludarabine significantly reduced MMP-2 expression, but not MMP-9 expression, in FIV-infected macrophages. Analysis of MMP mRNA and protein levels in brain samples from HIV-infected persons or FIV-infected cats showed that MMP-2 and -9 levels were significantly increased in lentivirus-infected brains compared to those of uninfected controls. Elevated MMP expression was accompanied by significant increases in STAT-1 and JAK-1 mRNA and protein levels in the same brain samples. The present findings indicate that two lentiviruses, HIV and FIV, have common mechanisms of MMP-2 and -9 induction, which is modulated in part by envelope sequence diversity and the STAT-1/JAK-1 signaling pathway.

At least four non-env factors that reside in the LTR, in the 5'-non-coding region, in gag and in part of pol affect neuropathogenicity of PVC-441 murine leukemia virus (MuLV)

PVC-441 murine leukemia virus (MuLV) is neuropathogenic in F344 rats. Recently, an infectious DNA clone was isolated and its nucleotide sequence was determined (J. Virol. 72: 3423-3426. 1998). To identify the viral determinants of neuropathogenicity of the molecularly cloned PVC-441 MuLV, chimeras were constructed between PVC-441 MuLV and F-MuLV clones at appropriate restriction enzyme sites that divide the viral genome approximately in LTR-non-coding, gag-, pol-, and env-gene regions. Results indicated that the LTR-non-coding and the gag-gene regions of PVC-441 MuLV affected independently the neuropathogenicity in combination with the env gene region as evidenced clinically and pathologically. Studies on the distribution of vacuolar degeneration suggested that the pons and cervical spinal cord areas were the primary targets and the large brain was the latest target of PVC-441 MuLV. Further studies with chimeric viruses that were formed in the LTR-non-coding and the gag gene regions revealed that at least four factors affected the neuropathogenicity of PVC-441 MuLV. Two factors were found in the U3, and R-U5-5'-non-coding regions, and at least two factors in the gag gene region that contained the N-terminal part of the pol gene. Among these factors, at least two factors seemed to be 'cis-acting' from each other

Brain infection by neuroinvasive but avirulent murine oncornaviruses

The chimeric murine oncornavirus FrCas(E) causes a rapidly progressive noninflammatory spongiform encephalomyelopathy after neonatal inoculation. The virus was constructed by the introduction of pol-env sequences from the wild mouse virus CasBrE into the genome of a neuroinvasive but nonneurovirulent strain of Friend murine leukemia virus (FMuLV), FB29. Although the brain infection by FrCas(E) as well as that by other neurovirulent murine retroviruses has been described in detail, little attention has been paid to the neuroinvasive but nonneurovirulent viruses. The purpose of the present study was to compare brain infection by FrCas(E) with that by FB29 and another nonneurovirulent virus, F43, which contains pol-env sequences from FMuLV 57. Both FB29 and F43 infected the same spectrum of cell types in the brain as that infected by FrCas(E), including endothelial cells, microglia, and populations of neurons which divide postnatally. Viral burdens achieved by the two nonneurovirulent viruses in the brain were actually higher than that of FrCas(E). The widespread infection of microglia by the two nonneurovirulent viruses is notable because it is infection of these cells by FrCas(E) which is thought to be a critical determinant of its neuropathogenicity. These results indicate that although the sequence of the envelope gene determines neurovirulence, this effect appears to operate through a mechanism which does not influence either viral tropism or viral burden in the brain. Although all three viruses exhibited similar tropism for granule neurons in the cerebellar cortex, there was a striking difference in the distribution of envelope proteins in those cells in vivo. The FrCas(E) envelope protein accumulated in terminal axons, whereas those of FB29 and F43 remained predominantly in the cell bodies. These observations suggest that differences in the intracellular sorting of these proteins may exist and that these differences appear to correlate with neurovirulence.

Increased neurovirulence of polytropic mouse retroviruses delivered by inoculation of brain with infected neural stem cells

Following intraperitoneal (IP) inoculation of neonatal mice, the polytropic recombinant murine leukemia virus (MuLV), Fr98, induces a severe brain disease characterized by ataxia, seizures and death. In contrast, no apparent clinical neurological disease is seen after IP infection with Fr54, a polytropic MuLV differing from Fr98 in its envelope gene sequences. In the brain both Fr98 and Fr54 infect primarily capillary endothelial cells and microglia. However, the level of microglial infection by Fr98 is twofold higher than by Fr54, which might account for the difference in neurovirulence. In the present study, in order to test directly whether an increase in the number of microglia infected by Fr54 would be sufficient to induce clinical disease, we attempted to increase the level of Fr54 in the brain by changing the route of infection. After intraventricular inoculation with Fr54-infected neural stem cells (clone C17.2), a well-established vehicle for delivery of viruses and genes to the brain, mice became ataxic and died 4 weeks postinfection. In these mice induction of brain disease was correlated with a higher level of viral antigen in the cerebrum and an increase in the number of infected microglial cells in all brain regions examined compared with mice inoculated IP. In contrast, mice inoculated with neural stem cells infected with an ecotropic nonneurovirulent murine leukemia virus, FB29, developed no clinical disease in spite of evidence for widespread infection of microglia in brain. Since the main differences between Fr54 and FB29 are in the SU (gp70) region of the envelope gene, this region is most likely to account for the differences in induction of CNS disease seen in the current experiments.

Genetic control and dynamics of the cellular immune response to the human T-cell leukaemia virus, HTLV-I

About 1% of people infected with the human T-cell leukaemia virus, type 1 (HTLV-I) develop a disabling chronic inflammatory disease of the central nervous system known as HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Patients with HAM/TSP have a vigorous immune response to HTLV-I, and it has been widely suggested that this immune response, particularly the HTLV-I-specific cytotoxic T-lymphocyte (CTL) response, causes the tissue damage that is seen in HAM/TSP. In this paper we summarize recent evidence that a strong CTL response to HTLV-I does in fact protect against HAM/TSP by reducing the proviral load of HTLV-I. We conclude that HTLV-I is persistently replicating at a high level, despite the relative constancy of its genome sequence. These results imply that antiretroviral drugs could reduce the risk of HAM/TSP by reducing the viral load, and that an effective anti-HTLV-I vaccine should elicit a strong CTL response to the virus. The dynamic nature of the infection also has implications for the epidemiology and the evolution of HTLV-I.

Nuclear factors that bind to the U3 region of two murine myeloid leukemia-inducing retroviruses, Cas-Br-E and Graffi

Cas-Br-E and Graffi are two myeloid leukemia-inducing murine viruses. Cas-Br-E induces, in NIH-Swiss mice, mostly non-T, non-B leukemia composed of very immature cells with no specific characteristics (Bergeron et al. (1993). Leukemia 7, 954-962). The Graffi murine leukemia virus causes exclusively myeloid leukemia, but the tumor cells are clearly of granulocytic nature (Ru et al. (1993). J. Virol. 67, 4722). We were interested to understand the role of the long terminal repeat (LTR) U3 region in the myeloid specificity of these two retroviruses. We used DNase I footprinting and gel mobility shift assays to identify a number of protein binding sites within Cas-Br-E and Graffi U3 regions. The pattern of protected regions is highly similar for the two viruses. Some factors identified in other murine leukemia viruses, like the core binding factor, also bind to Cas-Br-E and Graffi LTR; however, other binding sites seem specific for these two viruses. Only one difference between them was noted, at the 5' end of the U3 region. Transcriptional activity of both LTRs was also analyzed in various cell lines and compared with other murine leukemia viruses. The results show a slight myeloid specificity for the two LTRs, and indicate that the Graffi enhancer is quite strong in a broad range of cell types.

Neuronal death induced by brain-derived human immunodeficiency virus type 1 envelope genes differs between demented and nondemented AIDS patients

Human immunodeficiency virus type 1 (HIV-1) infection of the brain results in viral replication primarily in macrophages and microglia. Despite frequent detection of viral genome and proteins in the brains of AIDS patients with and without HIV dementia, only 20% of AIDS patients become demented. To investigate the role of viral envelope gene variation in the occurrence of dementia, we examined regions of variability in the viral envelope gene isolated from brains of AIDS patients. Brain-derived HIV-1 V1-V2 envelope sequences from seven demented and six nondemented AIDS patients displayed significant sequence differences between clinical groups, and by phylogenetic analysis, sequences from the demented group showed clustering. Infectious recombinant viruses containing brain-derived V3 sequences from both clinical groups were macrophagetropic, and viruses containing brain-derived V1, V2, and V3 sequences from both clinical groups spread efficiently in macrophages. In an indirect in vitro neurotoxicity assay using supernatant fluid from HIV-1-infected macrophages, recombinant viruses from demented patients induced greater neuronal death than viruses from nondemented patients. Thus, the HIV-1 envelope diversity observed in these patient groups appeared to influence the release of neurotoxic molecules from macrophages and might account in part for the variability in occurrence of dementia in AIDS patients.

Amphotropic murine leukemia viruses induce spongiform encephalomyelopathy

Recombinants of amphotropic murine leukemia virus (A-MuLV) have found widespread use in retroviral vector systems due to their ability to efficiently and stably infect cells of several different species, including human. Previous work has shown that replication-competent recombinants containing the amphotropic env gene, encoding the major SU envelope glycoprotein that determines host tropism, induce lymphomas in vivo. We show here that these viruses also induce a spongiform encephalomyelopathy in mice inoculated perinatally. This fatal central nervous system disease is characterized by noninflammatory spongiform lesions of nerve and glial cells and their processes, and is associated with moderate astro- and microgliosis. The first clinical symptoms are ataxia, tremor, and spasticity, progressing to complete tetraparesis and incontinence, and finally death of the animal. Sequences within the amphotropic env gene are necessary for disease induction. Coinfection of A-MuLV recombinants with nonneuropathogenic ecotropic or polytropic MuLV drastically increases the incidence, degree, and distribution of the neurodegenerative disorder. The consequence of these results in view of the use of A-MuLV recombinants in the clinic is discussed.

Mapping of a major osteomagenic determinant of murine leukemia virus RFB-14 to non-long terminal repeat sequences

Certain isolates of murine leukemia viruses (MuLVs) have, apart from a leukemogenic potential, the capability of inducing diseases of nonhematopoietic tissues in susceptible strains of mice. We have reported on the molecular cloning of a bone-tumorigenic virus, RFB-14 MuLV, which was found to induce benign bone tumors, osteomas, with 100% incidence in mice of the CBA/Ca strain (L. Pedersen, W. Behnisch, J. Schmidt, A. Luz, F. S. Pedersen, V. Erfle, and P. G. Strauss, J. Virol. 66:6186-6190, 1992). In order to analyze the bone tumor-inducing phenotype of RFB-14 MuLV, we have studied the pathogenic potential of recombinant viruses between RFB-14 and the nonosteomagenic, highly leukemogenic SL3-3 MuLV. The recombinants were constructed so as to reveal whether a major determinant of osteomagenicity maps to sequences within or outside the long terminal repeats (LTR). Our data show that a major determinant of the osteoma-inducing potential of RFB-14 MuLV maps to the non-LTR region of the genome. Furthermore, we demonstrate that a strong determinant of leukemogenicity is harbored by the non-LTR region of SL3-3 MuLV.

The prion protein gene is dispensable for the development of spongiform myeloencephalopathy induced by the neurovirulent Cas-Br-E murine leukemia virus

The Cas-Br-E murine leukemia virus (MuLV) induces paralysis in susceptible mice that is accompanied by a severe spongiform myeloencephalopathy. These neurodegenerative lesions are very similar to those observed in prion diseases. To determine whether the prion protein gene (Prn-p) product was a downstream effector of this neurovirulent MuLV, we inoculated Prn-p(-/-) knockout homozygote and control heterozygote or wild-type mice with this retrovirus. All groups developed typical paralysis and spongiform encephalopathy, and no differences in clinical or histological phenotypes were observed between these groups. These results indicate that the Cas-Br-E MuLV does not require the prion protein to induce lesions. Thus, MuLV and prion proteins may induce a very similar disease through distinct pathways, or the viral Env protein, which harbors the primary determinant of pathogenicity, may act in a common pathway but downstream of the prion protein.

Late virus replication events in microglia are required for neurovirulent retrovirus-induced spongiform neurodegeneration: evidence from neural progenitor-derived chimeric mouse brains

CasBrE is a neurovirulent murine retrovirus which induces a spongiform myeloencephalopathy in susceptible mice. Genetic mapping studies have indicated that sequences responsible for neurovirulence reside within the env gene. To address the question of direct envelope protein neuroxicity in the central nervous system (CNS), we have generated chimeric mice expressing the CasBrE envelope protein in cells of neuroectodermal origin. Specifically, the multipotent neural progenitor cell line C17.2 was engineered to express the CasBrE env gene as either gp70/p15E (CasE) or gp70 alone (CasES). CasE expression in these cells resulted in complete (>10(5)) interference of superinfection with Friend murine leukemia virus clone FB29, whereas CasES expression resulted in a 1.8-log-unit decrease in FB29 titer. Introduction of these envelope-expressing C17.2 cells into the brains of highly susceptible IRW mice resulted in significant engraftment as integral cytoarchitecturally correct components of the CNS. Despite high-level envelope protein expression from the engrafted cells, no evidence of spongiform neurodegeneration was observed. To examine whether early virus replication events were necessary for pathogenesis, C17.2 cells expressing whole virus were transplanted into mice in which virus replication in the host was specifically restricted by Fv-1 to preintegration events. Again, significant C17.2 cell engraftment and infectious virus expression failed to precipitate spongiform lesions. In contrast, transplantation of virus-expressing C17.2 progenitor cells in the absence of the Fv-1 restriction resulted in extensive spongiform neurodegeneration by 2 weeks postengraftment. Cytological examination indicated that infection had spread beyond the engrafted cells, and in particular to host microglia. Spongiform neuropathology in these animals was directly correlated with CasBrE env expression in microglia rather than expression from neural progenitor cells. These results suggest that the envelope protein of CasBrE is not itself neurotoxic but that virus infectious events beyond binding and fusion in microglia are necessary for the induction of CNS disease.

CAR1, a TNFR-related protein, is a cellular receptor for cytopathic avian leukosis-sarcoma viruses and mediates apoptosis

Viral envelope (Env)-receptor interactions have been implicated in the cell death associated with infection by subgroups B and D avian leukosis-sarcoma viruses (ALVs). A chicken protein, CAR1, was identified that permitted infection of mammalian cells by these viral subgroups. CAR1 bound to a viral Env fusion protein, comprising an ALV-B surface Env protein and the Fc region of an immunoglobulin, indicating that it is a specific viral receptor. CAR1 contains two extracellular cysteine-rich domains characteristic of the TNFR family and a cytoplasmic region strikingly similar to the death domain of TNFR1 and Fas, implicating this receptor in cell killing. Chicken embryo fibroblasts susceptible to ALV-B infection and transfected quail QT6 cells expressing CAR1 underwent apoptosis in response to the Env-Ig fusion protein, demonstrating that this cytopathic ALV receptor can mediate cell death.

Human T-cell-leukemia virus type I in post-transfusional spastic paraparesis: complete proviral sequence from uncultured blood cells

Human T-cell-leukemia virus type I (HTLV-I) is the causative agent of adult T-cell leukemia/lymphoma (ATL) and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). The different disease outcome may be attributable to subtle mutations leading to modification of viral tropism or infectivity. Initial attempts found a very high level of sequence conservation among all HTLV-I strains. However, only one complete proviral DNA sequence is reported from a TSP/HAM patient, with a provirus derived from immortalized lymphocytes, which might be expected to be a leukemogenic variant rather than a neurotropic one. We cloned and sequenced a complete HTLV-I provirus (HTLV-IBoi) derived from the uncultured lymphocytes of a sub-acute post-transfusional TSP/HAM patient with clonal integration of HTLV-I. HTLV-IBoi proviral genome is 9033 bp long, and its overall genetic organization is similar to that of the prototype HTLV-I(ATK), without major deletions or insertions. No premature termination codon was found in the 4 open reading frames of the pX region. Divergence at the nucleotide level of HTLV-IBoi from the reported full-length HTLV-I varies from 1 to 9.4%, and indicates that it corresponds to a cosmopolitan genotype. This study did not identify specific sequences associated with neurotropic strains.

Immunogenic determinants of a neuropathogenic murine leukemia virus

Previous studies of Cas-Br-M murine leukemia virus (MuLV) (Cas-MuLV) infection demonstrated that cytotoxic T cells (CTL) of the CD8+ phenotype play a role in resistance to the neuropathogenic effects of the virus in NFS/N mice. In the current study, we sought to identify the Cas-MuLV epitopes that are immunogenic for the CTL response. Infection of adult NFS/N mice with a well-characterized neuropathogenic variant of Friend MuLV, PVC-211 MuLV (PVC-MuLV), was not immunogenic for MuLV-specific CTL. Therefore, we constructed chimeric viruses between Cas-MuLV and PVC-MuLV. Infectious chimeras contained the Cas-MuLV env gene on a PVC-MuLV background (PVC-CasenvMuLV) and the PVC-MuLV env gene on a Cas-MuLV background (Cas-PVCenvMuLV). Cas-MuLV-specific CTL were found following inoculation of both the chimeric viruses and the parental Cas-MuLV but not the parental PVC-MuLV, despite evidence of antibody responses to both parental and chimeric MuLV. CTL generated in response to infection with PVC-CasenvMuLV and Cas-PVCenvMuLV were exclusively of the CD8+ phenotype. These results indicate that both the env and gag-pol regions of Cas-MuLV express epitopes that are immunogenic for CTL.

Mouse neuropathogenic poliovirus strains cause damage in the central nervous system distinct from poliomyelitis

Poliomyelitis as a consequence of poliovirus infection is observed only in primates. Despite a host range restricted to primates, experimental infection of rodents with certain genetically well defined poliovirus strains produces neurological disease. The outcome of infection of mice with mouse-adapted poliovirus strains has been described previously mainly in terms of paralysis and death, and it was generally assumed that these strains produce the same disease syndromes in normal mice and in mice transgenic for the human poliovirus receptor (hPVR-tg mice). We report a comparison of the clinical course and the histopathological features of neurological disease resulting from intracerebral virus inoculation in normal mice with those of murine poliomyelitis in hPVR-tg mice. The consistent pattern of clinical deficits in poliomyelitic transgenic mice contrasted with highly variable neurologic disease that developed in mice infected with different mouse-adapted polioviruses. Histopathological analysis showed a diffuse encephalomyelitis induced by specific poliovirus serotype 2 isolates in normal mice, that affected neuronal cell populations without discrimination, whereas in hPVR-tg animals, damage was restricted to spinal motor neurons. Mouse neurovirulent strains of poliovirus type 2 differed from mouse neurovirulent poliovirus type 1 derivatives in their ability to induce CNS lesions. Our findings indicate that the characteristic clinical appearance and highly specific histopathological features of poliomyelitis are mediated by the hPVR.(ABSTRACT TRUNCATED AT 250 WORDS)

A point mutation in the env gene of a murine leukemia virus induces syncytium formation and neurologic disease

TR1.3 is a Friend-related murine leukemia virus that has been shown to cause intracerebral hemorrhages and neurologic disease due to infection and subsequent cytopathology of cerebral vessel endothelium. A striking feature of this pathology is the formation of endothelial cell syncytia. The pathogenesis of this disease has now been mapped to a single amino acid substitution of tryptophan to glycine in the variable region of the envelope protein. This same mutation enabled TR1.3 to form syncytia and retard cell proliferation in vitro in the SC-1 mouse embryoblast line but did not affect the pH dependence of viral entry. These results demonstrate that subtle molecular changes in retroviral env genes can induce both syncytium formation and overt clinical disease.

Identification of human T-cell lymphotropic virus type I 21-base-pair repeat-specific and glial cell-specific DNA-protein complexes

The human T-cell lymphotropic virus type I (HTLV-I)-encoded protein, Tax, is capable of trans-activating HTLV-I transcription by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21-bp sequence. There is no evidence that purified Tax can bind to DNA in the absence of cellular factors, suggesting that Tax most likely regulates transcription via interaction with cellular factors. Since HTLV-I is a documented agent of adult T-cell leukemia and tropical spastic paraparesis, disorders of the immune and nervous systems, respectively, characterization of cellular factors of lymphoid and neuroglial origin which interact with the 21-bp repeat elements is essential to understanding of the mechanisms involved in basal and Tax-mediated transcription in cells of immune and nervous system origin. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected both 21-bp repeat-specific and glial cell-specific DNA-protein complexes. Several 21-bp repeat-specific DNA-protein complexes were detected when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9), neuronal (IMR-32 and SK-N-MC), and glial (U-373 MG, Hs683, and U-118) origin were used in reactions with each of the three 21-bp repeat elements. In addition, a glial cell-specific DNA-protein complex was detected when nuclear extracts derived from U-373 MG, Hs683, and U-118 glial cell lines reacted with the promoter-distal and central 21-bp repeat elements. Furthermore, EMS analyses performed with nuclear extracts derived from lymphocytic and glial cell origin and a 223-bp fragment of the HTLV-I long terminal repeat encompassing the three 21-bp repeat elements (designated Tax-responsive elements 1 and 2, TRE-1/-2) have also resulted in the detection of glial cell type-specific DNA-protein complexes. Competition EMS analyses with oligonucleotides containing transcription factor binding site sequences indicate the involvement of a cyclic AMP response element binding protein in the formation of DNA-protein complexes which form with all three 21-bp repeat elements and the glial cell-specific DNA-protein complex as well as the involvement of Sp1 or an Sp1-related factor in the formation of the 21-bp repeat III-specific DNA-protein complexes.

Identification of a sequence in the unique 5' open reading frame of the gene encoding glycosylated Gag which influences the incubation period of neurodegenerative disease induced by a murine retrovirus

Neonatal inoculation of the wild-mouse ecotropic retrovirus CasBrE (clone 15-1) causes a noninflammatory spongiform neurodegenerative disease with an incubation period of > or = 6 months. Introduction of sequences from Friend murine leukemia virus (clone FB29) into the genome of CasBrE results in a marked shortening of the incubation period. The FB29 sequences which influence the incubation period were previously localized to the 5' leader sequence of the viral genome (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). In the current study, we constructed a series of chimeric viruses consisting of the genome of CasBrE containing various segments of the leader sequence from FB29. A 41-nucleotide element (positions 481 through 521) near the 3' end of the leader was found to have a strong influence on the incubation period. This element influenced the kinetics of virus replication and/or spread in nonneuronal tissues, a property which was shown previously to determine the extent of central nervous system infection (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). Curiously, this sequence had no demonstrable effect on virus replication in vitro in a fibroblastic cell line from Mus dunni. This segment encodes 14 of the unique 88-amino-acid N terminus of pr75gag, the precursor of a glycosylated form of the gag polyprotein which is expressed at the cell surface. Previous in vitro studies of mutants of Moloney murine leukemia virus lacking expression of glycosylated Gag failed to reveal a function for this protein in virus replication. We mutated the Kozak consensus sequence around the initiation codon for this protein in the chimeric virus CasFrKP, a virus which induces neurologic disease with a short (18- to 23-day) incubation period. M. dunni cells infected with the mutants lacked detectable cell surface Gag, but, compared with CasFrKP, no effect on replication kinetics in vitro was observed. In contrast, there was a marked slowing of the replication kinetics in vivo and a dramatic attenuation of neurovirulence. These studies indicate that glycosylated Gag has an important function in virus replication and/or spread in the mouse and further suggest that the sequence of its N terminus is a critical, though likely indirect, determinant of neurovirulence.

Identification of the infected target cell type in spongiform myeloencephalopathy induced by the neurotropic Cas-Br-E murine leukemia virus

The Cas-Br-E murine leukemia virus (MuLV) induces a progressive hindlimb paralysis accompanied by a spongiform myeloencephalopathy in susceptible mice. In order to better understand the pathological process leading to these neurodegenerative lesions, we have investigated the nature of the cell type(s) infected by the virus during the course of the disease in CFW/D and SWR/J mice. For this purpose, we used in situ hybridization with virus-specific probes in combination with cell-type-specific histochemical (lectin) and immunological markers as well as morphological assessment. In the early stage of infection, endothelial cells represented the main cell type expressing viral RNA in the central nervous system (CNS). With disease progression and the appearance of lesions, microglial cells became the major cell type infected, accounting for up to 65% of the total infected cell population in diseased areas. Morphologically, these cells appeared activated and were frequently found in clusters. Infection and activation of microglial cells were almost exclusively restricted to diseased regions of the CNS. Neurons in diseased regions were not discernibly infected with virus at either early or late times of disease progression. Similarly, the proportion of infected astrocytes was typically < 1%. Although some endothelial cells and oligodendrocytes were infected by the virus, their infection was not limited to diseased CNS regions. These results are consistent with a model of indirect motor neuron degeneration, subsequent to the infection of nonneuronal CNS cells and especially of microglial cells. Infected microglial cells may play a role in the disease process by releasing not only virions or viral env-gene-encoded gp70 proteins but also other factors which may be directly or indirectly toxic to neurons. Parallels between microglial cell infection by MuLV and by lentiviruses, and specifically by human immunodeficiency virus, are discussed.

Viral determinants of simian immunodeficiency virus (SIV) virulence in rhesus macaques assessed by using attenuated and pathogenic molecular clones of SIVmac

To identify viral determinants of simian immunodeficiency virus (SIV) virulence, two pairs of reciprocal recombinants constructed from a pathogenic (SIVmac239) and a nonpathogenic (SIVmac1A11) molecular clone of SIV were tested in rhesus macaques. A large 6.2-kb fragment containing gag, pol, env, and the regulatory genes from each of the cloned (parental) viruses was exchanged to produce one pair of recombinant viruses (designated SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11gag-env/239 to indicate the genetic origins of the 5'/internal/3' regions, respectively, of the virus). A smaller 1.4-kb fragment containing the external env domain of each of the parental viruses was exchanged to create the second pair (SIVmac1A11/239env/1A11 and SIVmac239/1A11env/239) of recombinant viruses. Each of the two parental and four recombinant viruses was inoculated intravenously into four rhesus macaques, and all 24 animals were viremic by 4 weeks postinoculation (p.i.). Virus could not be isolated from peripheral blood mononuclear cells (PBMC) of any animals infected with SIVmac1A11 after 6 weeks p.i. but was consistently isolated from all macaques inoculated with SIVmac239 for 92 weeks p.i. Virus isolation was variable from animals infected with recombinant viruses; SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11env/239 were isolated most frequently. Animals inoculated with SIVmac239 had 10 to 100 times more virus-infected PBMC than those infected with recombinant viruses. Three animals infected with SIVmac239 died with simian AIDS (SAIDS) during the 2-year observation period after inoculation, and the fourth SIVmac239-infected animal had clinical signs of SAIDS. Two animals infected with recombinant viruses died with SAIDS; one was infected with SIVmac239/1A11gag-env/239, and the other was infected with SIVmac1A11/239gag-env/1A11. The remaining 18 macaques remained healthy by 2 years p.i., and 13 were aviremic. One year after inoculation, peripheral lymph nodes of some of these healthy, aviremic animals harbored infected cells. All animals seroconverted within the first few weeks of infection, and the magnitude of antibody response to SIV was proportional to the levels and duration of viremia. Virus-suppressive PBMC were detected within 2 to 4 weeks p.i. in all animals but tended to decline as viremia disappeared. There was no association of levels of cell-mediated virus-suppressive activity and either virus load or disease progression. Taken together, these results indicate that differences in more than one region of the viral genome are responsible for the lack of virulence of SIVmac1A11.

Intracerebral hemorrhages and syncytium formation induced by endothelial cell infection with a murine leukemia virus

The mechanisms of endothelial cell damage that lead to cerebral hemorrhage are not completely understood. In this study, a cloned murine retrovirus, TR1.3, that uniformly induced stroke in neonatal BALB/c mice is described. Restriction digest mapping suggests that TR1.3 is part of the Friend murine leukemia virus (FMuLV) family. However, unlike mice exposed to other FMuLVs, mice infected with TR1.3 virus developed tremors and seizures within 8 to 18 days postinoculation. This was uniformly followed by paralysis and death within 1 to 2 days. Postmortem examination of TR1.3-inoculated mice revealed edematous brain tissue with large areas of intracerebral hemorrhage. Histologic analysis revealed prominent small vessel pathology including syncytium formation of endothelial cells. Immunohistochemical analysis of frozen brain sections using double fluorescence staining demonstrated that TR1.3 virus specifically infected small vessel endothelial cells. Although infection of vessel endothelial cells was detected in several organs, only brain endothelial cells displayed viral infection associated with hemorrhage. The primary determinant of TR1.3-induced neuropathogenicity was found to reside within a 3.0-kb fragment containing the 3' end of the pol gene, the env gene, and the U3 region of the long terminal repeat. The restricted tropism and acute pathogenicity of this cloned murine retrovirus provide a model for studying virus-induced stroke and for elucidating the mechanisms involved in syncytium formation by retroviruses in vivo.

Sequences responsible for the distinctive hemolytic potentials of Friend and Moloney murine leukemia viruses are dispersed but confined to the psi-gag-PR region

Friend and Moloney murine leukemia viruses (F- and M-MuLV) induce distinct diseases in hematopoietic tissues following inoculation of newborn mice of susceptible strains. F-MuLV induces erythroleukemia preceded by severe early hemolytic anemia; M-MuLV induces thymomas and only very mild hemolysis. The major viral determinant of severe early hemolytic anemia residues in the env gene, but sequences located outside this gene can modulate this effect. By means of genetic chimeras of F- and M-MuLV, we have found that although they are confined to the 5' portion of the env gene intron, sequences that determine the distinctive hemolytic potentials of F- and M-MuLV are widely distributed over a region spanning the RNA encapsidation domain, the gag gene, and the portion of the pol gene encoding the viral protease. Within this large region, two fragments of M-MuLV, a 1.3-kb region encoding the matrix, pp12, and capsid proteins and a 0.8-kb region encoding the nucleocapsid and the viral protease, were capable, individually, of partially attenuating the capacity of F-MuLV for induction of severe early hemolytic anemia. In association, these two fragments conferred complete attenuation. Moreover, a second pair of adjacent fragments within this large region appeared to behave cooperatively to confer complete attenuation; a 0.36-kb region roughly corresponding to the encapsidation domain, although not detectably altering hemolytic potential on its own, deepened the attenuation conferred by the adjacent 1.3-kb region. Whether capable of inducing severe early hemolytic anemia or not and despite different efficiencies of induction of recombinant polytropic viruses, all chimeric viruses retained the erythroleukemogenicity of the F-MuLV parent.

Distinct superinfection interference properties yet similar receptor utilization by cytopathic and noncytopathic feline leukemia viruses

Cell killing by cytopathic retroviruses is often associated with a delay or failure in the establishment of superinfection interference. Superinfection has been observed during T-cell killing and fatal immunodeficiency disease induction by the feline leukemia virus (FeLV) chimera FeLV-FAIDS-EECC, containing the surface envelope glycoprotein (SU) of FeLV-FAIDS clone 61C. We demonstrate here that 61C SU has a defect that results in a nearly complete failure to establish superinfection interference against homologous virus challenge. This failure was evident only in feline T (FeT) cell clones expressing envelope protein, not in the rare cells that have survived cytopathic infection to become chronically infected. The regions of SU responsible for this defect were the same as those previously identified as responsible for T-cell killing. The superinfection interference properties of a noncytophatic molecular clone, FeLV-FAIDS-61E, were different in that 61E established interference to homologous virus challenge, both in SU-expressing cell clones and in chronically infected cells. Neither 61E nor EECC established interference against heterologous virus challenge. Viruses expressing chimeric SU proteins displayed varied and intermediate interference properties. Purified 61E and 61C SU competed for binding sites on FeT cell surfaces, and purified 61E SU blocked infection of virus bearing 61E or 61C SU. In addition, purified 61E and 61C SU each coprecipitated 70-kDa FeT cell surface proteins. Our data are consistent with the hypothesis that there are multiple cellular components necessary for 61E and 61C attachment to and penetration of FeT cells, a primary receptor that is utilized by both 61E and 61C, and secondary receptors that are likely to be virus specific.

Viral determinants that control the neuropathogenicity of PVC-211 murine leukemia virus in vivo determine brain capillary endothelial cell tropism of the virus in vitro

PVC-211 murine leukemia virus (MuLV) is a neuropathogenic, weakly leukemogenic variant of the nonneuropathogenic, highly leukemogenic Friend MuLV (F-MuLV). Chimeric viruses constructed from PVC-211 MuLV clone 3d and F-MuLV clone 57 indicate that the env gene of PVC-211 MuLV contains the determinant(s) responsible for pathological changes in the central nervous system. However, sequences within the 5' one-third (AatII-EcoRI region) of the PVC-211 MuLV genome, which include the 5' leader sequence, the gag gene, and the 5' quarter of the pol gene, are also needed in conjunction with the env gene determinant(s) to cause clinically evident neurological disease in the majority of virus-infected animals after a short latency. In the presence of the AatII-EcoRI region of the PVC-211 MuLV genome, the PVC-211 MuLV env gene sequences encoding the amino-terminal half of the SU protein, which contains the receptor-binding region of the protein, were sufficient to cause rapidly progressive neurological disease. When PVC-211 MuLV, F-MuLV, and various chimeric viruses were tested for their ability to replicate in cultured brain capillary endothelial cells (BCEC), the primary site of PVC-211 MuLV replication within the central nervous system, there was a direct correlation between the replication efficiency of a virus in BCEC in vitro and its ability to cause neurological disease in vivo. This observation indicates that the sequences in PVC-211 MuLV that render it neuropathogenic affect its replication in BCEC and suggests that rapid and efficient replication of the virus in BCEC is crucial for the pathological changes in the central nervous system that result in development of neurological disease.

Neurological disease induced in transgenic mice expressing the env gene of the Cas-Br-E murine retrovirus

The Cas-Br-E murine leukemia virus induces a spongiform myeloencephalopathy in susceptible mice. We constructed transgenic mice harboring either the viral genome (in a replication-defective form) or only its env gene. Low levels of expression of either transgene resulted in mild neuropathology and/or signs of neurological disease in more than half of these mice. These results indicate that the disease can occur in the absence of virus replication and strongly suggest that the env gp70/p15E complex is sufficient to induce disease.