A 1.6-kilobase-pair fragment in the genome of the ts1 mutant of Moloney murine leukemia virus TB that is associated with temperature sensitivity, nonprocessing of Pr80env, and paralytogenesis

MoMuLV-ts-1: A Unique Mouse Model of Retrovirus-Induced Lymphoma Transmitted by Breast Milk

Our laboratory has developed a murine model of lymphoma via breast milk transmission of MoMuLV-ts-1 (Moloney murine leukemia virus-temperature sensitive mutant-1). Uninfected offspring suckled from infected surrogate mothers become infected and develop lymphoma. Multiple gene integration sites of ts-1 into the infected mouse genome including tacc3, aurka, ndel1, tpx2, p53, and rhamm were identified, and mRNA expressions were quantitated. These genes produce centrosomal proteins, which may be involved in abnormal chromosomal segregation leading to aneuploidy or multiploidy, thus causing lymphoma. Since there is no report to date on this retroviral model leading to centrosomal abnormality, and causing lymphoma development, this is a valuable and unique model to study the centrosomal involvement in lymphomagenesis.

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.

Vertical Transmission of a Murine Retrovirus, ts1

Mechanism of maternal retroviral transmission remains an unsolved problem. The current investigation is a part of our ongoing research on vertical transmission of MoMuLV-TB ts1 in BALB/c mice. A total of 270 adult mice and 165 fetuses were used. Forty-four experimental mice were injected with 0.1 mL of 4.0 x 10(6) ffu/mL of ts1 virus at 72 h after birth; 24 controls were injected with DMEM. Almost half of the females went through two rounds of pregnancies. In the first round, 135 experimental and 57 control pups were produced. Forty-three experimental and 20 control pups were followed until they developed clinical symptoms. The second round of pregnancy produced a total of 46 mid-gestational and 119 full-term fetuses. PCR, and light and electron microscopy were performed to evaluate viral transmission. Overall, 99% vertical transmission occurred in pups of infected mothers. Twelve percent of mid-gestational and 39% full-term fetuses were PCR positive. We have established that, if mothers are infected with ts1 virus at 72 h after birth, then nearly 100% vertical transmission occurs, via in utero, intrapartum, or breast milk. Thirty-nine percent transmission occurred in utero alone. This is an excellent model to study the transplacental and post-gestational transmission of retroviruses, such as ts1.

A small animal model for mother-to-fetus transmission of ts1, a murine retrovirus

Infection with a murine retrovirus, MoMuLV-TB, ts1 in BALB/c mice has been established as a small animal model for retroviral neurodegenerative disease as shown with infections such as HIV. However, mother-to-pup transmission has never been demonstrated in this model. The current investigation examines vertical transmission of ts1 in this mouse model. A total of 15 females were used to produce 59 pups (16 were used for control, and 43 were used as experimental animals). For experiment 1, 24 5-day-old mice were injected with [0.2 mL of 2.0 x 10(6) ffu/mL ts1] virus. For experiment 2, 19 48-h-old mice were injected with [0.1 mL of 4 x 10(6) ffu/mL ts1] virus. Control groups were injected with DMEM only. PCR and electron microscopy were performed to determine the presence of virus. All mice from experiment 1 injected with ts1 showed viral infection, and retained 100% reproductive capacity. Three out of 102 pups produced by these infected females were infected with ts1. Nine percent of the pups from experiment 2 injected with ts1 retained normal reproductive capacity, and two out of eight (25%) pups had viral infection. Vertical transmission of this unique retrovirus occurs and is dependent, in part, on the timing of maternal infection.

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.

Sequence analysis of a neuropathogenic variant of Moloney murine leukemia virus ts1: evidence for recombination

The nucleotide sequence of the complete genome of ts1, a neuropathogenic variant of Moloney murine leukemia virus, has been determined. Although it is possible that the genome of ts1 accumulated point mutations during reverse transcription, our results now indicate that some of these mutations may have resulted from recombination.

Differential glycosylation of the Cas-Br-E env protein is associated with retrovirus-induced spongiform neurodegeneration

The wild mouse ecotropic retrovirus, Cas-Br-E, induces progressive, noninflammatory spongiform neurodegenerative disease in susceptible mice. Functional genetic analysis of the Cas-Br-E genome indicates that neurovirulence maps to the env gene, which encodes the surface glycoprotein responsible for binding and fusion of virus to host cells. To understand how the envelope protein might be involved in the induction of disease, we examined the regional and temporal expression of Cas-Br-E Env protein in the central nervous systems (CNS) of mice infected with the highly neurovirulent chimeric virus FrCas(E). We observed that multiple isoforms of Cas-Br-E Env were expressed in the CNS, with different brain regions exhibiting unique patterns of processed Env glycoprotein. Specifically, the expression of gp70 correlated with regions showing microglial infection and spongiform neurodegeneration. In contrast, regions high in neuronal infection and without neurodegenerative changes (the cerebellum and olfactory bulb) were characterized by a gp65 Env protein isoform. Sedimentation analysis of brain region extracts indicated that gp65 rather than gp70 was incorporated into virions. Biochemical analysis of the Cas-Br-E Env isoforms indicated that they result from differential processing of N-linked sugars. Taken together, these results indicate that differential posttranslational modification of the Cas-Br-E Env is associated with a failure to incorporate certain Env isoforms into virions in vivo, suggesting that defective viral assembly may be associated with the induction of spongiform neurodegeneration.

Neural stem cells as engraftable packaging lines can mediate gene delivery to microglia: evidence from studying retroviral env-related neurodegeneration

The induction of spongiform myeloencephalopathy by murine leukemia viruses is mediated primarily by infection of central nervous system (CNS) microglia. In this regard, we have previously shown that CasBrE-induced disease requires late, rather than early, virus replication events in microglial cells (W. P. Lynch et al., J. Virol. 70:8896-8907, 1996). Furthermore, neurodegeneration requires the presence of unique sequences within the viral env gene. Thus, the neurodegeneration-inducing events could result from microglial expression of retroviral envelope protein alone or from the interaction of envelope protein with other viral structural proteins in the virus assembly and maturation process. To distinguish between these possible mechanisms of disease induction, we engineered the engraftable neural stem cell line C17-2 into packaging/producer cells in order to deliver the neurovirulent CasBrE env gene to endogenous CNS cells. This strategy resulted in significant CasBrE env expression within CNS microglia without the appearance of replication competent virus. CasBrE envelope expression within microglia was accompanied by increased expression of activation markers F4/80 and Mac-1 (CD11b) but failed to induce spongiform neurodegenerative changes. These results suggest that envelope expression alone within microglia is not sufficient to induce neurodegeneration. Rather, microglia-mediated disease appears to require neurovirulent Env protein interaction with other viral proteins during assembly or maturation. More broadly, the results presented here prove the efficacy of a novel method by which neural stem cell biology may be harnessed for genetically manipulating the CNS, not only for studying neurodegeneration but also as a paradigm for the disseminated distribution of retroviral vector-transduced genes.

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.

Two separate envelope regions influence induction of brain disease by a polytropic murine retrovirus (FMCF98)

The major determinants involved in neurological disease induction by polytropic murine leukemia virus FMCF98 are encoded by the envelope gene. To map these determinants further, we produced four chimeras which contained neurovirulent FMCF98 envelope sequences combined with envelope sequences from the closely related nonneurovirulent polytropic virus FMCF54. Surprisingly, two chimeric viruses containing completely separate envelope regions from FMCF98 could both induce neurological disease. Clinical signs caused by both neurovirulent chimeras appeared to be indistinguishable from those caused by FMCF98, although the incubation periods were longer. One neurovirulence determinant mapped to the N-terminal portion of gp7O, which contains the VRA and VRB receptor-binding regions, while the other determinant mapped downstream of both of the variable regions. Western blot (immunoblot) analyses and immunohistochemical staining of tissue sections indicated that the variations in neurovirulence of these viruses could not be explained by differences in either the quantitative level or the location of virus expression in the brain.

ts1-Induced spongiform encephalomyelopathy: physical forms of high-mobility DNA in spinal cord tissues of paralyzed mice are products of premature termination of reverse transcription

ts1 is a temperature-sensitive mutant of Moloney murine leukemia virus that causes hind-limb paralysis in mice. In tissues of the central nervous systems of paralyzed moribund FVB/N mice, a major component of the unintegrated viral DNA of ts1 consists of highly mobile physical forms of viral-specific DNA (HM DNA). Previous studies with ecotropic virus-specific polarity probes showed that the gp70-coding region of the env gene in the HM DNA was minus-sense single-stranded DNA. The physical forms of the HM DNA have now been characterized in more detail with additional ecotropic virus-specific probes that hybridized to the p15E-coding region of the env gene and two locations within the U3 region of the long terminal repeat. Two major classes of HM DNA were found: class I molecules consist of short minus-sense single-stranded DNA; class II molecules are partial DNA duplexes that are longer than the class I molecules. The two classes of HM DNA molecules are intermediate products of reverse transcription of the viral RNA of ts1. Since tissues that are infected with cytopathic retroviruses may contain high levels of unintegrated viral DNA, the HM DNA may have a role in inducing neurodegeneration in the central nervous systems of mice that are infected with ts1.

Development of physical forms of unintegrated retroviral DNA in mouse spinal cord tissue during ts1-induced spongiform encephalomyelopathy: elevated levels of a novel single-stranded form in paralyzed mice

ts1 is a murine leukemia virus that causes rapidly evolving hindlimb paralysis in susceptible strains of mice. Following perinatal infection, three physical forms of unintegrated viral DNA were detected in the spinal cord by Southern blot hybridization. Linear and supercoiled closed-circle viral double-stranded DNAs were detected in both the central nervous system and non-central nervous system tissues. An elevated level of a novel minus-sense single-stranded form of viral DNA, which had a very high mobility in agarose gels, was correlated with the onset of symptoms of paralysis. As the severity of paralysis progressed, the level of this single-stranded form increased rapidly, with the highest level in the spinal cords of moribund mice. Since the virulence of a number of cytopathic retroviruses has been associated with the presence of increased amounts of unintegrated viral DNA in the tissues of the infected hosts, this novel form of highly mobile unintegrated single-stranded DNA may have a role in the neuropathogenesis of ts1.

Viral load and its relationship to quinolinic acid, TNF alpha, and IL-6 levels in the CNS of retroviral infected mice

Mouse models of infection of the central nervous system (CNS) have been used to study retroviral-induced neurologic disease. Ecotropic-neurotropic murine leukemia virus (MuLV) infection of susceptible neonatal mice causes a neurologic disease characterized by progressive hindlimb paralysis. The lesions consist of chronic noninflammatory spongiform change predominantly involving brainstem and spinal cord. Two molecularly cloned strains of MuLV, ts-1, a temperature-sensitive mutant of Moloney MuLV, and pNE-8, derived from a feral mouse isolate Cas-Br-E, were used in this study. Infected mice were sacrificed at regular intervals postinoculation throughout the time-course of disease. The neuropathology was evaluated using standard histological and immunohistopathological techniques. Tissue concentrations of viral proteins and potentially cytotoxic factors were compared with the histopathology in select regions of the CNS. Areas of extensive vacuolation with neuronal and oligodendroglial infection were observed in spinal cord, brainstem, and cerebellum. High titers of infectious virus were observed within CNS lesions, whereas low titers were observed in morphologically uninvolved areas. Western blot analysis revealed abundant production of viral envelope proteins, which correlated well with infectious virus titers. Serum quinolinic acid (QUIN) concentrations in both groups of noninfected and infected mice were similar. However, CNS tissue concentrations of QUIN, TNF alpha, and IL-6 in ts-1 infected mice were significantly higher than in pNE-8 infected or noninfected mice. The difference in concentration of these factors may be the result of greater activation of macrophages/microglia in ts-1 infected mice. During murine retroviral encephalitis, CNS damage may be mediated by direct infection of CNS cells and may be enhanced by indirect effects of neurotoxic factors possibly secreted by infected/activated macrophages.

Microglial infection by a neurovirulent murine retrovirus results in defective processing of envelope protein and intracellular budding of virus particles

The observation of murine retrovirus infection of microglial cells in brain regions expressing spongiform neurodegenerative changes suggests that these cells may play an important role in pathogenesis. To evaluate this potential in vitro, murine microglial cells were infected in mixed glial cultures with the highly neurovirulent murine retrovirus, FrCasE. The microglia were then isolated from the mixed cultures on the basis of their differential adherence and shown to be approximately 98% pure. The infected microglia expressed viral envelope protein at the plasma membrane, while viral budding was primarily intracellular. Evaluation of the viral envelope protein by immunoblotting indicated that the immunoreactive species produced was exclusively a 90-kDa precursor protein. Very little of the envelope protein was associated with particles released from these cells, and viral titers in the culture supernatant were low. Interestingly, these cells were still capable of infecting permissive target cells when seeded as infectious centers. This partially defective infection of microglial cells suggests a potential cellular means by which a neurovirulent retrovirus could disrupt normal microglia and in turn central nervous system motor system functioning.

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.

Murine retrovirus-induced spongiform encephalopathy: disease expression is dependent on postnatal development of the central nervous system

In this report, we have examined the role of central nervous system (CNS) development in the pathogenesis of neurodegenerative disease induced by murine retroviruses. This was accomplished by comparing the effect of delivering viruses, with either severe or marginal neurovirulence (J. L. Portis, S. Czub, C. F. Garon, and F. J. McAtee, J. Virol. 64:1648-1656, 1990), during the midgestational development of the mouse (gestation days 9 to 10). Midgestation inoculation of the marginally neurovirulent virus, 15-1, resulted in high level CNS infection, as determined by viral DNA and protein analysis. The high-level infection resulted in rapid, severe disease with 100% incidence and an average clinical onset on postnatal day 17 (P17). The disease onset was comparable to that observed for the highly neurovirulent virus, FrCasE, when inoculated neonatally (onset ca. P16). To evaluate whether disease could be induced even earlier in CNS development, FrCasE was inoculated during midgestation. Surprisingly, neither clinical nor histological manifestations of CNS disease were accelerated but rather appeared at the same developmental time as seen for neonatally inoculated animals (onset of neuropathology at ca. P10; onset of clinical disease at ca. P15). CNS infection, on the other hand, occurred at earlier times (< P0), at higher levels, and with a broader distribution than in neonatally inoculated animals. No infection of the neurons which ultimately degenerate was observed in any regimen of virus inoculation. It was observed, however, that the gp70 viral envelope protein from the CNS showed an increase mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis compared with the envelope protein from infected spleens or purified virions. These results indicate that a postnatal developmental event must occur to allow the presence of a neurovirulent virus to precipitate spongiform degeneration and that an altered envelope protein may be participating in the process.

Oligomerization and transport of the envelope protein of Moloney murine leukemia virus-TB and of ts1, a neurovirulent temperature-sensitive mutant of MoMuLV-TB

Ts1, a temperature-sensitive mutant of Moloney murine leukemia virus-TB (MoMuLV-TB), causes a progressive hindlimb paralytic disease in susceptible strains of mice. Previously, it has been shown that a single amino acid substitution, Val-25----Ile in gPr80env, is responsible for the temperature sensitivity, inefficient transport, and processing of gPr80env at the restrictive temperature and the neurovirulence of ts1. Since the neurovirulence of ts1 is associated with inefficient transport and processing of gPr80env and since in other systems involving viral envelope proteins it has been shown that correct folding and oligomerization of envelope monomers are required for efficient transport, we have investigated the ability of gPr80env derived from either wild-type MoMuLV-TB or ts1 to associate into oligomeric complexes. In these experiments, we establish that at both the restrictive and the nonrestrictive temperatures gPr80env molecules derived from MoMuLV-TB associate to form oligomeric complexes and these oligomers are most likely trimers. gPr80env molecules derived from ts1 also oligomerize at both temperatures; however, at the restrictive temperature, most of the molecules within the trimeric complexes remain as gPr80env and are not processed to gp70 and Prp15E. These results indicate that lack of oligomerization of gPr80env is not responsible for the transport defect of ts1. Therefore, by interacting specifically with critical sites within target cells, oligomers of mutant gPr80env rather than "tangles" of monomeric viral envelope proteins may be involved in the neurodegenerative disorder produced by ts1.

Murine retrovirus-induced spongiform encephalopathy: productive infection of microglia and cerebellar neurons in accelerated CNS disease

We have examined the pathological lesions and sites of infection in mice inoculated with a highly neurovirulent recombinant wild mouse ecotropic retrovirus (FrCasE). The spongiform lesions appeared initially as swollen postsynaptic neuronal processes, progressing to swelling in neuronal cell bodies, all in the absence of detectable gliosis. Infection of neurons in regions of vacuolation was not detected. However, high level infection of cerebellar granule neurons was observed in the absence of cytopathology, wherein viral protein was found associated with both axons and dendrites. Infection of ramified and amoeboid microglial cells was associated with cytopathology in the brain stem, and endothelial cell-pericyte infection was found throughout the CNS. No evidence of defective retroviral expression was observed. These results are consistent with an indirect mechanism of retrovirus-induced neuropathology.

Substitution of leucine for isoleucine in a sequence highly conserved among retroviral envelope surface glycoproteins attenuates the lytic effect of the Friend murine leukemia virus

Friend murine leukemia virus is a replication-competent retrovirus that contains no oncogene and that exerts lytic and leukemogenic properties. Thus, newborn mice inoculated with Friend murine leukemia virus develop severe early hemolytic anemia before appearance of erythroleukemia. To identify the retroviral determinants regulating these effects, we used chimeric infectious constructions and site-directed point mutations between a virulent Friend murine leukemia virus strain and a naturally occurring variant attenuated in lytic and leukemogenic effects. We found that severe hemolytic anemia was always associated with higher numbers of blood reticulocytes with budding retroviral particles. Furthermore, a remarkably conservative leucine to isoleucine change in the extracellular SU component of the retroviral envelope was sufficient to attenuate this lytic effect. Also, this leucine at position 348 of the envelope precursor protein was located within the only stretch of five amino acids that is conserved in the extracellular SU component of all murine, feline, and primate type C and type D retroviral envelopes. This observation suggested an important structural function for this yet undescribed conserved sequence of the envelope. Lastly, we observed that lytic and leukemogenic effects were attenuated by a deletion of a second repeat in the transcriptional enhancer region of the viral long terminal repeats of the variant strain.

Construction and characterization of expression systems for the env gene of ts1, a mutant of Moloney murine leukemia virus-TB

A temperature-sensitive mutant of the Moloney murine leukemia virus-TB, ts1, causes hindlimb paralysis and immunodeficiency in mice. At the restrictive temperature, the envelope precursor polyprotein, gPr80env, is inefficiently processed intracellularly, and this is associated with the neurovirulence of ts1. To test the hypothesis that expression of the envelope proteins of ts1 alone without infectious virus production can induce paralysis, it is necessary to use either transmissible retroviral expression vectors or microinjection of eukaryotic gene expression plasmid to introduce the env gene of ts1 into germlines of mice. In this study, we have constructed three retrovirus vectors and three gene expression plasmids, all of which contain the env gene of ts1. By comparing the different expression systems, we found that one construct, pts1-env(F) can express the envelope proteins at a level comparable to the level expressed in ts1-infected cells. Furthermore, the expressed envelope proteins of pts1-env(F)-transfected cells possess the phenotypes of the proteins expressed by the env gene of ts1.

High susceptibility of FVB/N mice to the paralytic disease induced by ts1, a mutant of Moloney murine leukemia virus TB

The ts1 mutant of Moloney murine leukemia virus TB causes a degenerative neurologic and immunologic disease in susceptible strains of mice. This disease syndrome is characterized by development of spongiform encephalomyelopathy resulting in hindlimb paralysis, generalized bodywasting, and marked thymic atrophy associated with immune deficiency. The viral genetic determinants responsible for hindlimb paralysis in BALB/c and CFW/D mice have been localized to two point mutations in the env gene: one results in a Val-25----IIe substitution in the envelope precursor polyprotein gPr80env and the other, in an Arg-430----Lys substitution in the gp70. In this report we present studies showing that FVB/N mice were highly susceptible to ts1 and exhibited the shortest and most uniform latency period of all the murine strains tested. In addition, we have found that, unlike in CFW/D and BALB/c mice, only the Val-25----IIe substitution in the gPr80env is required to induce hindlimb paralysis in FVB/N mice. Our studies show that there was enhanced replication of ts1 in all tissues of FVB/N mice and that the virus titer in the spinal cord was more than 10-fold higher in FVB/N than in BALB/c mice by 30 days postinoculation, when the clinical signs of paralysis became evident in FVB/N mice. Apparently, other host factors that do not require the Arg-430----Lys substitution allowed high levels of viral replication within the central nervous system of FVB/N mice. These results, together with the finding that 100% of FVB/N mice that were inoculated with ts1 at 5 days of age developed hindlimb paralysis at 30-60 days postinoculation, whereas only 33% of 5-day-old BALB/c mice developed hindlimb paralysis with a much longer latency period, suggest that subtle virus-host interactions determine the incidence, the latency period, and the severity of the disease caused by ts1.

Site-directed mutagenesis of the codon for Ile-25 in gPr80env alters the neurovirulence of ts1, a mutant of Moloney murine leukemia virus TB

ts1, a spontaneous temperature-sensitive mutant of Moloney murine leukemia virus TB, causes hind-limb paralysis in mice. A Val-25----Ile substitution in gPr80env is responsible for temperature sensitivity, inefficient processing of gPr80env, and neurovirulence. In this study, the Ile-25 in gPr80env was replaced with Thr, Ala, Leu, Gly, and Glu by site-directed mutagenesis of the codon for Ile-25 to generate a new set of mutant viruses, i.e., ts1-T, -A, -L, -G, and -E, respectively. The phenotypic characteristics of these mutant viruses differed from those of ts1. For each mutant, the degree of temperature sensitivity was correlated with the degree of inefficient processing of gPr80env, and the following rank order was observed for both parameters: ts1-E greater than ts1-G greater than ts1-L greater than ts1-A greater than ts1 greater than ts1-T. In FVB/N mice, mutant viruses of low and intermediate temperature sensitivity and inefficiency in processing of gPr80env were neurovirulent and consistently caused mutant-specific disease profiles: ts1-T caused severe whole-body tremor, ts1-A generally caused hind-limb paralysis, and ts1-L generally caused a delayed-onset paraparesis. By 150 days postinfection, FVB/N mice that were infected with ts1-G and -E, mutants of high temperature sensitivity and inefficiency in processing of gPr80env, had lymphoid leukemia instead of a neurological disease. These results suggest that the dynamics of gPr80env processing are important in determining the neurovirulent phenotype in vivo.

A Val-25-to-Ile substitution in the envelope precursor polyprotein, gPr80env, is responsible for the temperature sensitivity, inefficient processing of gPr80env, and neurovirulence of ts1, a mutant of Moloney murine leukemia virus TB

ts1 is a neurovirulent spontaneous temperature-sensitive mutant of Moloney murine leukemia virus TB which causes hindlimb paralysis in mice. Previously, it had been shown that the temperature-sensitive defect resided in the env gene. At the restrictive temperature, the envelope precursor polyprotein, gPr80env, is inefficiently processed intracellularly into two cleavage products, gp70 and Prp15E. This inefficient processing of gPr80env is correlated with neurovirulence. In this study, it was shown that a single amino acid substitution, Val-25----Ile in gPr80env, is responsible for the temperature sensitivity, inefficient processing of gPr80env at the restrictive temperature, and neurovirulence of ts1. At the restrictive temperature, a steady-state level of nonprocessed, endoglycosidase H-sensitive gPr80env remained in the endoplasmic reticulum of cells infected by ts1, but no endoglycosidase H-resistant gPr80env and only trace amounts of gp70 were detected in the infected cells. Since the host cell-encoded processing protease resides in the cis cisternae of the Golgi apparatus, inefficient processing of gPr80env at the restrictive temperature is most likely due to inefficient transport of gPr80env from the endoplasmic reticulum to the cis cisternae of the Golgi apparatus rather than due to misfolded gPr80env being a poor substrate for the processing protease at the restrictive temperature.

ts1, a mutant of Moloney murine leukemia virus-TB, causes both immunodeficiency and neurologic disorders in BALB/c mice

BALB/c mice infected with ts1, a mutant of Moloney murine leukemia virus-TB, develop generalized body wasting, profound neurologic disorders, severe thymic atrophy and lymphopenia due to destruction of T lymphocytes and drastic immunodeficiency. ts1 was found not only able to infect T lymphocytes but also to impair their function. In addition, ts1 also infects and induces syncyntia formation in macrophages. The genetic determinant(s) responsible for ts1's ability to induce immunodeficiency has been localized to the env gene.

The reduced virulence of the thymotropic Moloney murine leukemia virus derivative MoMuLV-TB is mapped to 11 mutations within the U3 region of the long terminal repeat

Chimeric constructs were generated by exchanging genomic fragments between the potent T-cell lymphoma inducer Moloney murine leukemia virus (MoMuLV) and its derivative MoMuLV-TB, which induces T-cell lymphoma after a relatively longer latent period. Analysis of the T-cell lymphoma-inducing potential of the hybrid viruses that were obtained localized the primary determinant critical to efficient T-cell lymphoma induction to the MoMuLV ClaI-XbaI fragment which comprises 48 nucleotides (nt) of p15E, p2E, the 3'-noncoding sequence, and 298 nt of U3. The 438-base-pair ClaI-XbaI fragments of MoMuLV and MoMuLV-TB differed in only 11 nt. Nine mutations were found within the enhancer. These mutations occurred within the two CORE, the two GRE-LVa, and two of the four NF1 nuclear factor-binding motifs. MoMuLV-TB replicated better than MoMuLV in thymus-bone marrow (TB) cells, a cultured cell line of lymphoid origin. In addition, MoMuLV-TB and NwtTB-2, a recombinant virus with the ClaI-SmaI fragment of MoMuLV-TB in a MoMuLV background, replicated in thymocytes as efficiently as did MoMuLV or TBNwt-2, the reciprocal recombinant virus, with the ClaI-SmaI fragment of MoMuLV in a MoMuLV-TB background. Like NwtTB-4, a recombinant virus with the ClaI-XbaI fragment of MoMuLV-TB in a MoMuLV background, NwtTB-2 induced lymphoma after a long latent period. The finding given above suggests that thymotropism is not the only factor that determines the T-cell lymphoma-inducing potential of MoMuLV. It appears likely that mutations in one or more of the MoMuLV-TB nuclear factor-binding motifs may have altered the interaction of the enhancer with specific nuclear factors; this, in turn, may affect the T-cell lymphoma-inducing potential of MoMuLV-TB.

Identification of point mutations in the envelope gene of Moloney murine leukemia virus TB temperature-sensitive paralytogenic mutant ts1: molecular determinants for neurovirulence

ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, induces hind-limb paralysis in mice. The DNA of both the ts1 and Moloney murine leukemia virus TB env genes has been sequenced, and the encoded amino acid sequences have been deduced from the DNA sequences. Four amino acids in the ts1 envelope protein have been identified which may be responsible for the ts1 phenotype, which includes temperature sensitivity, nonprocessing of Pr80env, and neurovirulence.

Neurotropic retroviruses of wild mice and macaques

A neurotropic retrovirus causes a naturally occurring lower-limb paralysis in wild mice, characterized by a noninflammatory spongiform change located primarily in the lower spinal cord. The causative agent is an ecotropic murine leukemia virus, unique to certain wild mice in southern California. The disease is readily transmitted to newborn susceptible laboratory mice. The paralytogenic property is attributed to direct viral injury to motor neurons and glial cells and is associated with unique amino acids in the murine leukemia virus envelope gp70. This murine model may have relevance to both human T-lymphotropic virus type I, and human immunodeficiency virus infection of human brain. It presents a practical model for testing antiviral agents aimed at retrovirus infection of the mammalian central nervous system. Simian acquired immunodeficiency syndrome type D retrovirus causes a silent infection of the brain in infected macaques. Viral nucleic acids are detected in the brain parenchyma in the absence of viral antigen, neurological symptoms, and neuropathology. Infected choroid plexus epithelial cells are the source of cell-free virus in the cerebrospinal fluid of viremic monkeys. This model adds yet another example of retroviral infection of the central nervous system and points to the choroid plexus as a potential source of infectious virus.

Natural variants of human immunodeficiency virus from patients with neurological disorders do not kill T4+ cells

Human immunodeficiency virus (HIV) has selective T4-cell tropism and is cytocidal to cells with the helper-inducer phenotype. Central nervous system dysfunctions can complicate full-blown acquired immunodeficiency syndrome (AIDS) but can also be present either in isolation or in the context of AIDS-related complex. Remarkably bland histopathological findings have been reported in some patients with AIDS dementia in the presence of severe clinical dysfunction. Thus, to understand the cytopathic properties of HIV, we recovered five viral isolates from 4 patients with neurological symptoms of AIDS and identified them as HIVs. The replication and cytocidal properties of these isolates were compared with lymphadenopathy-associated virus in vitro. All five isolates exhibited replication efficiency equivalent to lymphadenopathy-associated virus, but four isolates did not kill CD4 (T4+) cells. These findings provide evidence for the existence of replication-competent noncytocidal natural variants of HIV and raise the possibility that, in some AIDS patients, neurological disorders might be caused by HIV variants that are noncytocidal to T4 cells.

Non-cytocidal natural variants of human immunodeficiency virus isolated from AIDS patients with neurological disorders

To understand the mechanism of HIV-mediated neuropathology five viral isolates were obtained from four AIDS cases with central nervous system manifestations as the primary involvement. The isolates were identified as HIVs by antigenic cross-reactivity and nucleic acid hybridizations to HIV-specific antibodies and DNA probes. The replication and cytopathic properties of these isolates were studied and compared with lymphadenopathy-associated virus (HIVLAV). All isolates had replication competence equivalent to LAV, but four isolates did not kill T4 (CD-4) cells. This isolation of non-cytocidal natural variants of HIV raises the possibility that in some AIDS cases the neurological disorders might be due to HIV variants that are non-cytocidal to T4 cells. The results also indicate that virus replication and cytotoxicity are not always concordant functions in HIV.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Cas-Br-E murine leukemia virus: sequencing of the paralytogenic region of its genome and derivation of specific probes to study its origin and the structure of its recombinant genomes in leukemic tissues

The ecotropic Cas-Br-E murine leukemia virus (MuLV) and its molecularly cloned derivative pBR-NE-8 MuLV are capable of inducing hind-limb paralysis and leukemia after inoculation into susceptible mice. T1 oligonucleotide fingerprinting, molecular hybridization, and restriction enzyme analysis previously showed that the env gene of Cas-Br-E MuLV diverged the most from that of other ecotropic MuLVs. To analyze proviruses in leukemic tissues, we derived DNA probes specific to Cas-Br-E sequences: two from the env region and one from the U3 long terminal repeat. With these probes, we found that this virus induced clonal (or oligoclonal) tumors and we documented the presence of typical mink cell focus-forming-type proviruses in leukemic tissues and the possible presence of other recombinant MuLV proviruses. Since the region harboring the determinant of paralysis was mapped within the pol-env region of the virus (L. DesGroseillers, M. Barrette, and P. Jolicoeur, J. Virol. 52:356-363, 1984), we performed the complete nucleotide sequence of this region covering the 3' end of the genome. We compared the deduced amino acid sequences of the pol carboxy-terminal domain and of the env gene products with those of other nonparalytogenic, ecotropic, and mink cell focus-forming MuLVs. This amino acid comparison revealed that this part of the pol gene product and the p15E diverged very little from homologous proteins of other MuLVs. However, the Cas-Br-E gp70 sequence was found to be quite divergent from that of other MuLVs, and the amino acid changes were distributed all along the protein. Therefore, gp70 remains the best candidate for harboring the determinant of paralysis.

Characterization of a progressive neurodegenerative disease induced by a temperature-sensitive Moloney murine leukemia virus infection

A progressive neurodegenerative disease occurred following infection of mice with a temperature-sensitive (ts) isolate of Moloney (Mo) murine leukemia virus (MuLV), ts Mo BA-1 MuLV. This NB-tropic ecotropic MuLV, which was ts for a late function, induced a syndrome of tremor, weakness of the hind limbs, and spasticity following infection of several strains of laboratory neonatal mice, including NFS, C3H/He, CBA, SJL, and BALB/c. The latent period of 8 to 16 weeks was considerably longer than that observed for the acute paralytic diseases observed following neonatal infection with other ts Mo-MuLV, rat-passaged Friend MuLV, and some wild mouse ecotropic MuLVs. Spongiform pathology without inflammation and degeneration of neurons devoid of budding virions occurred in the cerebellar grey matter, brain stem, and upper spinal cord; but lower spinal cord anterior horn cells were less obviously affected than in other MuLV-associated neuroparalytic syndromes. ts Mo BA-1 MuLV differed from other ts Mo-MuLV mutants that are capable of inducing a neuroparalytic syndrome in that while infected nervous system tissue contained high levels of MuLV p30 and gp70, no evidence of precursor accumulation or abnormal processing of MuLV p30 or gp70 could be demonstrated. The localization of virus within the nervous system suggests that direct neuronal infection may not be the etiologic mechanism in this MuLV-induced neurodegenerative disease.

The neurovirulent determinants of ts1, a paralytogenic mutant of Moloney murine leukemia virus TB, are localized in at least two functionally distinct regions of the genome

To better understand the molecular mechanism involved in retrovirus ts1-induced paralytic disease in mice, we constructed a panel of recombinant viruses between ts1 and the wild-type viruses Moloney murine leukemia virus (MoMuLV) and MoMuLV-TB, a strain of MoMuLV. These recombinant viruses were constructed in an attempt to identify the sequence(s) in the genome of ts1 which contains the critical mutation(s) responsible for the neurovirulence of ts1. Two functionally distinct sequences in the genome of ts1 were found to be responsible for its paralytogenic ability. One of these sequences, the 0.77-kilobase-pair XbaI-BamHI (nucleotides 5765 to 6537) fragment which encodes the 5' half of gp70 and 11 base pairs upstream of the env gene coding sequence, determines the inability of ts1 to process Pr80env. The other sequence, the 2.30-kilobase-pair BamHI-PstI (nucleotides 538 to 8264 and 1 to 567) fragment, which comprises nearly two-thirds of the env gene, the long terminal repeat, and the 5' noncoding sequence, determines the enhanced neurotropism of ts1. Replacement of any one of these two regions with the homologous region from either one of the two wild-type viruses resulted in recombinant viruses which either totally failed to induce paralysis or induced a greatly attenuated form of paresis in some of the infected mice.

ts1, a Paralytogenic mutant of Moloney murine leukemia virus TB, has an enhanced ability to replicate in the central nervous system and primary nerve cell culture

A temperature-sensitive mutant of Moloney murine leukemia virus TB (MoMuLV-TB), ts1, which is defective in intracellular processing of envelope precursor protein (Pr80env), also possesses the ability to induce hind-limb paralysis in infected mice. To investigate whether ts1 has acquired neurotropism and to determine to what extent it can replicate in the central nervous system, we compared viral titers in the spleen, plasma, spinal cord, and brain throughout the course of infection of mice infected with ts1 and parental wild-type (wt) MoMuLV-TB. In both the ts1- and wt-inoculated mice, the concentrations of infectious virus recovered from the plasma and spleen increased rapidly and reached a plateau by 10 days postinfection (p.i.). In contrast, virus concentrations in the spinal cord and brain of ts1-inoculated mice increased gradually and reached a titer comparable to that in the spleen and exceeding that in the plasma only at 25 to 30 days p.i. At this time, the virus titer was approximately 200X greater in ts1-infected spinal cord tissue and approximately 20X greater in ts1-infected brain tissue than in the same wt-infected tissues. Paralysis became evident at 25 to 30 days p.i. in ts1-inoculated mice, whereas the wt-inoculated mice were normal. In addition, a substantial amount of Pr80env was detected in the spinal cords of ts1-inoculated mice compared with that found in the spinal cords of wt-inoculated mice. The infectious virus isolated from ts1-infected nerve tissue was found to possess the characteristic phenotype of the ts1 virus. Microscopic lesions of ts1-inoculated mice at 30 days p.i. consisted of vacuolar degeneration of motor neurons and spongy change of white matter in the brain stem and spinal cord. Similar but less severe lesions were observed in wt-inoculated mice. With primary cultures of central nervous system tissue we showed that ts1 can infect and replicate in both neuron and glial cells. In contrast, although wt MoMuLV-TB replicated in glial cell-rich culture, viral replication was barely detectable in neuron-rich culture.