Although regarded as a part of the "Junk DNA", ERVs have been shown to play unique functional roles in cellular biology and are implicated in a wide range of diseases like cancer, autoimmune disease, and neurodegeneration [
48‐
51]. We first compared the expression profiles of our old world monkeys (
Macaca fascicularis) to existing human profiles [
41,
46,
52]. Beside a broad number of similarities, we identified differences in the expression profiles between humans and macaques when comparing Class II betaretroviruses. The HERV-K-(HML-2, -6, and -9) that are strongly expressed in human brain are at the detection limit in macaque brain.
Using our non-human primate model we next examined the regulation of endogenous retrovirus expression upon BSE-infection. To date, prion-infected macaques have not been used for analysis of ERV-expression. Differential expression of ERVs upon prion disease have been previously reported in other species, including humans, though brain material has not been examined [
28]. Here we provide evidence that ERVs are differentially expressed in an experimentally controlled macaque model of BSE-infection which is highly relevant for human CJD. We have identified the upregulation of ERVs E4-1, MacERV-4, and ERV-9 by ERV-specific microarray. Interestingly, ERV-9 has also been shown to be upregulated in cerebral spinal fluid of sCJD patients [
28]. Thus, the macaque-adapted BSE and human sCJD profiles show common ERV-regulation patterns in response to prion infection irrespective of the analysed sample types (brain or cerebrospinal fluid).
We could not judge whether the regulation of ERVs is cause or consequence of prion infection. Although many
in vivo studies have been performed, the mechanism connecting prion deposition and neuronal loss has not been determined in detail. The spatial distribution of PrP
Sc-aggregates in different brain regions is not always coincident with neuronal decay in these areas. Thus it appears that neurodegeneration itself could be promoted by biological pathways that are more complex than protein aggregation, possibly by analogous mechanisms as described for neurotropic retroviruses. Indeed, it is known that some retroviruses can induce neurodegeneration in the absence of infectious prions. Different classes of leukemia retroviruses, including CasBrE, 10A1-MuLV, ts1MoMuLV-TB and MoAmphoV induce fatal spongiform encephalomyelopathy [
35‐
37]. Symptoms include tremor, wasting and paraparesis that is caused by neuronal loss, spongiform lesions and astrogliosis, which is intriguingly similar to prion disease pathology [
53]. All four retroviruses are classified as gammaretroviruses, the same subgroup as the here described BSE-upregulated MacERV-4, ERV-9 and HERV-E. Thus, changes in ERV expression may induce or exacerbate the pathological consequences of neurodegeneration.
Upregulation of endogenous retroviruses can also potentially induce neurodegeneration. It has been shown that the regulatory LTR-region of HERV-E regulates the expression of the Opitz-syndrome associated
mid1 gene [
54] that is essential for
vermis cerebelli development [
55]. It has been proposed that overexpression of
mid1 leads to disturbance in microtubule-homeostasis, comparable to the tau-aggregation induced neuronal loss in Alzheimer's disease [
56,
57]. Thus upregulation of HERV-E in BSE-infected macaques may also contribute to neuronal decay similar to the above mechanism. This suggests that induction of retroviral elements may be a consequence of prion infection, and a downstream mechanism of neurodegeneration [
36]. This, however, does not imply that endogenous retroviruses can cause prion disease. The appearance of viral particles in prion-infected cells [
58,
59] has been discussed controversially. Interestingly, recent work has identified PrP
C as an integral component of the Human Immunodeficiency Virus-1 (HIV-1) [
60]. After cells are infected with HIV-1, the virus replicates, assembles and buds preferentially from the lipid rafts [
61] presumably including the GPI-anchored PrP
C into the surface of the HIV-1 particles. In addition, HIV-1 Gag and Env colocalize with PrP
C in infected T-cell lines [
62]. Furthermore, expression of HIV-1
gag increases the susceptibility to and sustains the prion infection in cell culture [
26,
63]. Thus spreading of prion infection from one cell to another may not be restricted to exosomal vesicle transfer [
64], but may also be triggered by Gag of endogenous origin, such as MacERV-K-(HML-2) Gag. In line with this, murine N2a cells express, produce, and release MuLV (murine-leukaemia virus) particles of endogenous origin, called NeRV (neuroblastoma endogenous retrovirus) [
65]. Infection of N2a cells with prions leads to the excretion of exosomes that harbor PrP
C or infectious PrP
Sc. Intriguingly, antibodies against PrP
C could label both, exosomes and infectious virions [
66]. This suggests that intercellular trafficking of prions could at least partially be mediated by hitchhiking on endogenous retroviral particles. This is in line with earlier observations that PrP can interact with retroviral RNA [
67‐
69] eventually resulting in the formation of active nucleoprotein structures [
70] that include Gag. Using specific antibodies we could show that macERV-K-(HML-2) Gag protein is expressed in the brains of cynomolgus macaques. To our knowledge this is the first time that a structural element of an endogenous retrovirus has been detected in the primate central nervous system. The functional role of the protein in neuronal physiology is unknown. Interestingly, among the many endogenous retroviral subfamilies, HERV-K-(HML-2) can produce viral particles [
71‐
74]. It remains unclear whether macERV (HML-2) Gag can be excreted by neurons. However, active release of HERV-K-(HML-2) Gag containing retrovirus-like particles have been described in Tera-1 [
47] cells and were also found in blood plasma of patients [
75,
76]. Taken together, the detection of macERV-K-(HML-2) protein in the frontal/parietal cortex indicates that it may have a physiological role in the brain. Furthermore, finding that HERV-K-(HML-2) Gag protein and RNA is downregulated in BSE-infected macaques suggests that this role may be connected to neuronal survival. Further studies will be necessary to determine the mechanism and function of HERV-K HML-2 Gag downregulation.