Reactivated endogenous retroviruses promote protein aggregate spreading

Prion-like spreading of protein misfolding is characteristic for neurodegenerative diseases, but the exact mechanisms of intercellular protein aggregate dissemination remain unresolved. Evidence accumulates that endogenous retroviruses, remnants of viral germline infections that are normally epigenetically silenced, become upregulated in neurodegenerative diseases such as amyotrophic lateral sclerosis and tauopathies. Here we uncover that activation of endogenous retroviruses affects prion-like spreading of proteopathic seeds. To identify changes in the proteome of donor cells that might contribute to protein aggregate spreading, we performed mass spectrometry analyses of total cell lysates and donor EV fractions using N2a cells expressing HA epitope-tagged Sup35 NM prion protein at early (P07) and late passages (P16) post cryopreservation. Among the proteins increased in donor cells and EVs upon prolonged culture, we identified mouse endogenous MLV retrovirus proteins to be highly increased.