Neural stem cells as tools for understanding retroviral neuropathogenesis

Ecotropic Murine Leukemia Virus Infection of Glial Progenitors Interferes with Oligodendrocyte Differentiation: Implications for Neurovirulence

Certain murine leukemia viruses (MLVs) are capable of inducing fatal progressive spongiform motor neuron disease in mice that is largely mediated by viral Env glycoprotein expression within central nervous system (CNS) glia. While the etiologic mechanisms and the glial subtypes involved remain unresolved, infection of NG2 glia was recently observed to correlate spatially and temporally with altered neuronal physiology and spongiogenesis. Since one role of NG2 cells is to serve as oligodendrocyte (OL) progenitor cells (OPCs), we examined here whether their infection by neurovirulent (FrCasE) or nonneurovirulent (Fr57E) ecotropic MLVs influenced their viability and/or differentiation. Here, we demonstrate that OPCs, but not OLs, are major CNS targets of both FrCasE and Fr57E. We also show that MLV infection of neural progenitor cells (NPCs) in culture did not affect survival, proliferation, or OPC progenitor marker expression but suppressed certain glial differentiation markers. Assessment of glial differentiation in vivo using transplanted transgenic NPCs showed that, while MLVs did not affect cellular engraftment or survival, they did inhibit OL differentiation, irrespective of MLV neurovirulence. In addition, in chimeric brains, where FrCasE-infected NPC transplants caused neurodegeneration, the transplanted NPCs proliferated. These results suggest that MLV infection is not directly cytotoxic to OPCs but rather acts to interfere with OL differentiation. Since both FrCasE and Fr57E viruses restrict OL differentiation but only FrCasE induces overt neurodegeneration, restriction of OL maturation alone cannot account for neuropathogenesis. Instead neurodegeneration may involve a two-hit scenario where interference with OPC differentiation combined with glial Env-induced neuronal hyperexcitability precipitates disease.

IMPORTANCE

A variety of human and animal retroviruses are capable of causing central nervous system (CNS) neurodegeneration manifested as motor and cognitive deficits. These retroviruses infect a variety of CNS cell types; however, the specific role each cell type plays in neuropathogenesis remains to be established. The NG2 glia, whose CNS functions are only now emerging, are a newly appreciated viral target in murine leukemia virus (MLV)-induced neurodegeneration. Since one role of NG2 glia is that of oligodendrocyte progenitor cells (OPCs), we investigated here whether their infection by the neurovirulent MLV FrCasE contributed to neurodegeneration by affecting OPC viability and/or development. Our results show that both neurovirulent and nonneurovirulent MLVs interfere with oligodendrocyte differentiation. Thus, NG2 glial infection could contribute to neurodegeneration by preventing myelin formation and/or repair and by suspending OPCs in a state of persistent susceptibility to excitotoxic insult mediated by neurovirulent virus effects on other glial subtypes.

Retrovirus-induced spongiform neurodegeneration is mediated by unique central nervous system viral targeting and expression of env alone

Certain murine leukemia viruses (MLVs) can induce progressive noninflammatory spongiform neurodegeneration similar to that caused by prions. The primary MLV determinants responsible have been mapped to within the env gene; however, it has remained unclear how env mediates disease, whether non-Env viral components are required, and what central nervous system (CNS) cells constitute the critical CNS targets. To address these questions, we examined the effect of transplanting engraftable C17.2 neural stem cells engineered to pseudotype, disseminate, and trans-complement neurovirulent (CasBrE, CasE, and CasES) or non-neurovirulent (Friend and SFF-FE) env sequences (SU or SU/TM) within the CNS using either the "non-neurovirulent" amphotropic helper virus, 4070A, or pgag-polgpt (a nonpackaged vector encoding Gag-Pol). These studies revealed that acute MLV-induced spongiosis results from two separable activities of Env. First, Env causes neuropathology through unique viral targeting within the CNS, which was efficiently mediated by ecotropic Envs (CasBrE and Friend), but not 4070A amphotropic Env. Second, Env induces spongiosis through a toxin activity that is MLV-receptor independent and does not require the coexpression of other viral structural proteins. CasBrE and 4070A Envs possess the toxin activity, whereas Friend Env does not. Although the identity of the critical viral target cell(s) remains unresolved, our results appear to exclude microglia and oligodendrocyte lineage cells, while implicating viral entry into susceptible neurons. Thus, MLV-induced disease parallels prionopathies in that a single protein, Env, mediates both the CNS targeting and the toxicity of the infectious agent that manifests itself as progressive vacuolar neurodegeneration.

Oligodendrocytes are a major target of the toxicity of spongiogenic murine retroviruses

The neurovirulent retroviruses FrCasE and Moloney MLV-ts1 cause noninflammatory spongiform neurodegeneration in mice, manifested clinically by progressive spasticity and paralysis. Neurons have been thought to be the primary target of toxicity of these viruses. However the neurons themselves appear not to be infected, and the possible indirect mechanisms driving the neuronal toxicity have remained enigmatic. Here we have re-examined the cells that are damaged by these viruses, using lineage-specific markers. Surprisingly, these cells expressed the basic helix-loop-helix transcription factor Olig2, placing them in the oligodendrocyte lineage. Olig2+ cells were found to be infected, and many of these cells exhibited focal cytoplasmic vacuolation, suggesting that infection by spongiogenic retroviruses is directly toxic to these cells. As cytoplasmic vacuolation progressed, however, signs of viral protein expression appeared to wane, although residual viral RNA was detectable by in situ hybridization. Cells with the most advanced cytoplasmic effacement expressed the C/EBP-homologous protein (CHOP). This protein is up-regulated as a late event in a cellular response termed the integrated stress response. This observation may link the cellular pathology observed in the brain with cellular stress responses known to be induced by these viruses. The relevance of these observations to oligodendropathy in humans is discussed.

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.

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.

Reexamination of amphotropic murine leukemia virus neurovirulence: neural stem cell-mediated microglial infection fails to induce acute neurodegeneration

The 4070A amphotropic murine leukemia virus (A-MuLV) has been variably reported to harbor neurovirulence determinants within its env gene. In this report we reexamined this issue by applying two approaches previously demonstrated to amplify murine leukemia virus neurovirulence. The first approach involved introducing the 4070A env gene into the background of Friend virus clone FB29 to enhance peripheral virus replication kinetics and central nervous system entry. The resulting chimeric virus, FrAmE, exhibited widespread vascular infection throughout the central nervous system (CNS); however, parenchymal infection was quite limited. Neither clinical neurological signs nor spongiform neurological changes accompanied FrAmE CNS infection. To overcome this CNS entry limitation, 4070A and FrAmE were delivered directly into the CNS via transplantation of infected C17.2 neural stem cells (NSCs). Significantly, NSC dissemination of either 4070A or FrAmE resulted in widespread, high-level amphotropic virus expression within the CNS parenchyma, including the infection of microglia, the critical target required for inducing neurodegeneration. Despite the extensive CNS infection, no associated clinical neurological signs or acute neuropathological changes were observed. Interestingly, we observed the frequent appearance of circulating polytropic (MCF) virus in the serum of amphotropic virus-infected animals. However, neither peripheral inoculation of an amphotropic/MCF virus mixture nor transplantation of NSCs expressing both amphotropic and MCF viruses induced acute clinical neurological signs or spongiform neuropathology. Thus, the results generated in this study suggest that the 4070A env gene is not inherently neurovirulent. However, the frequent appearance of endogenous MCF viruses suggests the possibility that the interactions of amphotropic viruses with endogenous retroviral elements could contribute to the development of retrovirus-induced neurodegenerative disease.

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.