The pathogenesis of murine retroviral infection of the central nervous system

Rebound from Inhibition: Self-Correction against Neurodegeneration?

Neural networks play a critical role in establishing constraints on excitability in the central nervous system. Several recent studies have suggested that network dysfunction in the brain and spinal cord are compromised following insult by a neurodegenerative trigger and might precede eventual neuronal loss and neurological impairment. Early intervention of network excitability and plasticity might therefore be critical in resetting hyperexcitability and preventing later neuronal damage. Here, the behavior of neurons that generate burst firing upon recovery from inhibitory input or intrinsic membrane hyperpolarization (rebound neurons) is examined in the context of neural networks that underlie rhythmic activity observed in areas of the brain and spinal cord that are vulnerable to neurodegeneration. In a non-inflammatory rodent model of spongiform neurodegenerative disease triggered by retrovirus infection of glia, rebound neurons are particularly vulnerable to neurodegeneration, likely due to an inherently low calcium buffering capacity. The dysfunction of rebound neurons translates into a dysfunction of rhythmic neural circuits, compromising normal neurological function and leading to eventual morbidity. Understanding how virus infection of glia can mediate dysfunction of rebound neurons, induce hyperexcitability and loss of rhythmic function, pathologic features observed in neurodegenerative disorders ranging from epilepsy to motor neuron disease, might therefore suggest a common pathway for early therapeutic intervention.

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.

A viral non-coding region determining neuropathogenicity of murine leukemia virus A8 is responsible for envelope protein expression in the rat brain

Friend murine leukemia virus clone A8 causes spongiform neurodegeneration in the rat brain. A 0.3-kb fragment containing the R-U5-5' leader sequence of A8 is required in addition to the A8-env gene to induce spongiosis. The A8-env gene is a primary determinant of neuropathogenicity. Comparative studies of the neuropathogenic virus R7f, which carries the 0.3-kb fragment of A8 and A8-env on the background of the non-neuropathogenic clone 57, and the non-neuropathogenic virus Rec5, which carries A8-env on the background of 57, showed that the 0.3-kb fragment of A8 was responsible for increasing the ratio of Env/Gag expression in the brain, but not in the spleen.

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.

A 0.3-kb fragment containing the R-U5-5' leader sequence is essential for the induction of spongiform neurodegeneration by A8 murine leukemia virus

Friend murine leukemia virus (Fr-MLV) clone A8 causes spongiform neurodegeneration in the rat brain. The A8-env gene is a primary determinant of neuropathogenicity, and the 1.5-kb ClaI-HindIII fragment containing the LTR and 5' leader from A8 are additionally required for spongiosis. After replacement of the A8 enhancer region of the neuropathogenic chimera with the enhancer region of non-neuropathogenic 57, viral titer in the brain was reduced by two orders of magnitude. However, the A8 enhancer region was not responsible for the induction of spongiosis. The region responsible for neuropathogenesis was located in the 0.3-kb KpnI-AatII fragment of A8 containing the R-U5-5' leader. The chimeric virus possessing this 0.3-kb fragment of A8 and the A8-env in the 57 background induced a high rate of spongiform neurodegeneration within 7 weeks (9/9 of infected rats). Studies using cultured cells suggest that the 0.3-kb fragment influences the expression of Env protein. Furthermore, these neuropathogenic chimerae, despite low viral replication in the brain, exhibited a stronger expression of Env protein compared with that of non-neuropathogenic viruses.

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.

Disparate regions of envelope protein regulate syncytium formation versus spongiform encephalopathy in neurological disease induced by murine leukemia virus TR

The murine leukemia virus (MLV) TR1.3 provides an excellent model to study the wide range of retrovirus-induced central nervous system (CNS) pathology and disease. TR1.3 rapidly induces thrombotic events in brain microvessels and causes cell-specific syncytium formation of brain capillary endothelial cells (BCEC). A single amino acid substitution, W102G, in the MLV envelope protein (Env) regulates the pathogenic effects. The role of Env in determining this disease phenotype compared to the induction of spongiform encephalomyelitis with a longer latency, as seen in several other MLV and in human retroviruses, was determined by studying in vitro-attenuated TR1.3. Virus cloned from this selection, termed TRM, induced progressive neurological disease characterized by ataxia and paralysis and the appearance of spongiform neurodegeneration throughout the brain stem and spinal cord. This disease was associated with virus replication in both BCEC and highly ramified glial cells. TRM did not induce syncytium formation, either in vivo or in vitro. Sequence and mutational analyses demonstrated that TRM contained a reversion of Env G102W but that neurological disease mapped to the single amino acid substitution Env S159P. The results demonstrate that single nucleotide changes within disparate regions of Env control dramatically different CNS disease patterns.

Neuropathology of experimental autoimmune encephalomyelitis modified by retroviral infection

The A8 virus is a molecular clone of the neuropathogenic FrC6 virus derived from the Friend murine leukemia virus (F-MuLV). To elucidate the effects of A8 virus-infection on immune-mediated diseases in the central nervous system, we investigated the development of acute and monophasic experimental autoimmune encephalomyelitis (EAE) in A8 virus-infected Lewis rats. In EAE rats after A8 virus infection (A8-EAE), many inflammatory cells were found in the gray matter including the frontal lobe, where almost no inflammatory cells were found in rats with EAE alone. The modified distribution of inflammatory cells was not dependent on the ages of A8 virus-infected rats, although the frequency of the modified distribution was reduced in older rats. The chimeric virus Rec2, which contains the pol and env genes of 57 virus on the background of A8 and does not induce spongiform degeneration in the CNS, caused the same distributional modification of inflammatory cells in the rats with EAE as in A8-EAE rats. Furthermore, the incidence and intensity of spongiform degeneration, thymoma and splenomegaly caused by A8 virus were reduced by the induction of EAE.

Differential expression of intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 in chronic murine retroviral encephalitis

The cell adhesion molecules, intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1, are important mediators of immune interactions within the central nervous system (CNS). A wide variety of pro-inflammatory insults to the brain, including viral infection, result in upregulation of these molecules on brain endothelial cells, astrocytes, and microglia. This study investigated the expression of ICAM-1 and VCAM-1 in chronic encephalitis induced by infection with a temperature sensitive (ts-1) strain of Moloney murine leukaemia virus (MoMuLV), an ecotropic murine retrovirus. During the late stages of disease, viral antigen was present in both endothelial cells and microglia, but not astrocytes, in regions of spongiform change and gliosis. In these areas, ICAM-1 staining was detected on activated microglia, but not on endothelial cells or astrocytes. In contrast, no cells showed increased VCAM-1 expression in the CNS. These findings demonstrate that there is cell-specific, differential expression of these adhesion molecules in ts-1 retroviral encephalitis. The lack of endothelial cell expression correlates with the characteristic lack of lymphocytic infiltrate in this chronic retroviral encephalitis and suggests that increased microglial ICAM-1 expression may play a role in the pathogenesis of MoMuLV (ts-1)-mediated neurodegeneration.

Contribution of virus-receptor interaction to distinct viral proliferation of neuropathogenic and nonneuropathogenic murine leukemia viruses in rat glial cells

The efficiency of receptor-mediated entry of pseudotyped virus carrying the surface protein (SU) of clone A8, a neuropathogenic variant of Friend murine leukemia virus (FrMLV), to rat glial cell line F10 was 1 order of magnitude greater than that of pseudotyped virus carrying SU of nonneuropathogenic FrMLV clone 57. Introduction of the gene coding for ecotropic MLV receptor on F10 cells (F10-ecoR) into SIRC cells, which are naturally resistant to FrMLV infection, also revealed the difference in receptor recognition between the A8 and the 57 viruses. Our results show that the difference in receptor utilization between A8-SU and 57-SU only partially explains the 3-order-of-magnitude difference in proliferation between A8 and 57 viruses in F10 cells.

Possible retroviral origin of prion disease: could prion disease be reconsidered as a preleukemia syndrome?

A retroviral etiology might explain why amyloid plaque and/or spongiosis are or are not associated with neuronal death in prion diseases. While retroviral genes themselves may be responsible for neuronal death, a retrovirus may also cause mutations in cellular genes. Hence, the prion gene may be altered by a retrovirus in the same way as a cellular proto-oncogene is altered to produce an oncogene, either by transduction or by integration of the provirus in its vicinity. In both cases, the resulting abnormal prion protein, acting as a catalyst, may induce the formation of amyloid plaques. In addition, a wild type retrovirus may recombine to the vesicular stomatitis virus (VSV) to give rise to a pseudotyped retrovirus able to induce spongiosis. It is reported here that in scrapie, a blood monocytoid cell proliferates in vitro. If confirmed in other species, this raises the question of the potential link between prion disease and leukemia. Indeed neurovirulent strains of murine leukemia virus, a slow acting retrovirus, are known to induce spongiform encephalopathies. A preliminary attempt to purify reverse transcriptase by chromatography, using the classical protocol, failed because of the presence of a prion-like protein secreted by the blood mononuclear cells which stuck to the phosphocellulose column. Therefore, if a retrovirus is present in prion diseases, it would be evidenced only in animals developing the disease in the absence of prion protein. From this point of view, mice obtained in 1997 by the group of D. Dormont in France, offer a unique opportunity to test the retroviral hypothesis.

Mapping of a neurovirulence determinant within the envelope protein of a polytropic murine retrovirus: induction of central nervous system disease by low levels of virus

Murine leukemia virus (MuLV) clone Fr98 is a recombinant polytropic virus that causes neurological disease characterized by ataxia in susceptible mouse strains. The envelope gene of Fr98 has been previously shown to encode at least two separate neurovirulence determinants. In the present study, the determinant encoded within the EcoRI/AvrII fragment of the envelope gene was further defined. In these experiments, neurovirulence was associated with a change from a serine to an arginine at position 195 and a glycine to an alanine at position 198 within the envelope protein. Neurovirulent and nonvirulent virus clones, which differed only at these two amino acid residues, showed no difference in the type or location of cells infected. Furthermore, equivalent levels of viral p30 capsid protein were detected in the brains of mice infected with either the neurovirulent or nonvirulent virus clones. These results were consistent with the interpretation that the envelope protein of the neurovirulent virus differed from that of the nonvirulent virus by having a greater toxic effect on central nervous system function.

Neurologic disease induced by polytropic murine retroviruses: neurovirulence determined by efficiency of spread to microglial cells

Several murine leukemia viruses (MuLV) induce neurologic disease in susceptible mice. To identify features of central nervous system (CNS) infection that correlate with neurovirulence, we compared two neurovirulent MuLV, Fr98 and Fr98/SE, with a nonneurovirulent MuLV, Fr54. All three viruses utilize the polytropic receptor and are coisogenic, each containing a different envelope gene within a common genetic background. Both Fr98 and Fr98/SE induce a clinical neurologic disease characterized by hyperexcitability and ataxia yet differ in incubation period, 16 to 30 and 30 to 60 days, respectively. Fr54 infects the CNS but fails to induce clinical signs of neurologic disease. In this study, we compared the histopathology, regional virus distribution, and cell tropism in the brain, as well as the relative CNS viral burdens. All three viruses induced similar histopathologic effects, characterized by intense reactive astrogliosis and microglial activation associated with minimal vacuolar degeneration. The infected target cells for each virus consisted primarily of endothelial and microglial cells, with rare oligodendrocytes. Infection localized predominantly in white matter tracts of the cerebellum, internal capsule, and corpus callosum. The only feature that correlated with relative neurovirulence was viral burden as measured by both viral CA protein expression in cerebellar homogenates and quantification of infected cells. Interestingly, Fr54 (nonneurovirulent) and Fr98/SE (slow disease) had similar viral burdens at 3 weeks postinoculation, suggesting that they entered the brain with comparable efficiencies. However, spread of Fr98/SE within the brain thereafter exceeded that of Fr54, reaching levels of viral burden comparable to that seen for Fr98 (rapid disease) at 3 weeks. These results suggest that the determinants of neurovirulence in the envelope gene may influence the efficiency of virus spread within the brain and that a critical number of infected cells may be required for induction of clinical neurologic disease.

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.

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.

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.

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.