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

Bovine spongiform encephalopathy infection alters endogenous retrovirus expression in distinct brain regions of cynomolgus macaques (Macaca fascicularis)

Background

Prion diseases such as bovine spongiform encephalopathies (BSE) are transmissible neurodegenerative diseases which are presumably caused by an infectious conformational isoform of the cellular prion protein. Previous work has provided evidence that in murine prion disease the endogenous retrovirus (ERV) expression is altered in the brain. To determine if prion-induced changes in ERV expression are a general phenomenon we used a non-human primate model for prion disease.

Results

Cynomolgus macaques (Macaca fasicularis) were infected intracerebrally with BSE-positive brain stem material from cattle and allowed to develop prion disease. Brain tissue from the basis pontis and vermis cerebelli of the six animals and the same regions from four healthy controls were subjected to ERV expression profiling using a retrovirus-specific microarray and quantitative real-time PCR. We could show that Class I gammaretroviruses HERV-E4-1, ERV-9, and MacERV-4 increase expression in BSE-infected macaques. In a second approach, we analysed ERV-K-(HML-2) RNA and protein expression in extracts from the same cynomolgus macaques. Here we found a significant downregulation of both, the macaque ERV-K-(HML-2) Gag protein and RNA in the frontal/parietal cortex of BSE-infected macaques.

Conclusions

We provide evidence that dysregulation of ERVs in response to BSE-infection can be detected on both, the RNA and the protein level. To our knowledge, this is the first report on the differential expression of ERV-derived structural proteins in prion disorders. Our findings suggest that endogenous retroviruses may induce or exacerbate the pathological consequences of prion-associated neurodegeneration.

Prion infection influences murine endogenous retrovirus expression in neuronal cells

Prions as causative agents of transmissible spongiform encephalopathies have been well investigated in experimental and modelling work. However, little is known about the molecular pathogenesis of prion-induced encephalopathies, the role of co-factors, and the interaction of prions with cellular components. We investigated the influence of prion infection on expression of murine endogenous retroviruses (ERVs), which compose approximately 10% of the mouse genome. Hypothalamic neuronal cells (GT1) and neuroblastoma cells (N2a) were examined. Both cell lines can be persistently infected with mouse adapted prion strains, i.e., RML. Using a mammalian retrovirus-specific DNA microarray and quantitative PCR methods, we compared the expression profiles of ERVs in prion-infected, uninfected, and anti-prion compound-treated murine neuronal cell lines, including clonal cell populations. The results suggest that prion infection influences ERV expression in neuronal cell lines, that this influence is cell line-specific, ERV-specific, and responsive to anti-prion compound treatment.

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.

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

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

Involvement of temperature sensitive syncytium inducing VSV or defective retroviruses in the development of spongiform encephalopathies

Various enveloped viruses can induce syncytia in competent cells. Some temperature-sensitive mutants can express the trans-membrane viral G antigen under non-permissive conditions. The G antigen can then migrate at long distances, engulfing thousands of cells without producing any virus. When a temperature-sensitive vesicular stomatitis virus (VSV) infects a sensitive host, and under the condition that the G antigen is expressed, spongiosis can be induced in the central nervous system in the absence of detectable virus multiplication. We postulate that such a mechanism might be observed with various enveloped viruses, as recently illustrated with knock-out mice experimentally infected with defective murine leukemia virus (MULV).