NF-κB and IRF1 Induce Endogenous Retrovirus K Expression via Interferon-Stimulated Response Elements in Its 5' Long Terminal Repeat.

UNLABELLED: Thousands of endogenous retroviruses (ERV), viral fossils of ancient germ line infections, reside within the human genome. Evidence of ERV activity has been observed widely in both health and disease. While this is most often cited as a bystander effect of cell culture or disease states, it is unclear which signals control ERV transcription. Bioinformatic analysis suggests that the viral promoter of endogenous retrovirus K (ERVK) is responsive to inflammatory transcription factors. Here we show that one reason for ERVK upregulation in amyotrophic lateral sclerosis (ALS) is the presence of functional interferon-stimulated response elements (ISREs) in the viral promoter. Transcription factor overexpression assays revealed independent and synergistic upregulation of ERVK by interferon regulatory factor 1 (IRF1) and NF-κB isoforms. Tumor necrosis factor alpha (TNF-α) and LIGHT cytokine treatments of human astrocytes and neurons enhanced ERVK transcription and protein levels through IRF1 and NF-κB binding to the ISREs. We further show that in ALS brain tissue, neuronal ERVK reactivation is associated with the nuclear translocation of IRF1 and NF-κB isoforms p50 and p65. ERVK overexpression can cause motor neuron pathology in murine models. Our results implicate neuroinflammation as a key trigger of ERVK provirus reactivation in ALS. These molecular mechanisms may also extend to the pathobiology of other ERVK-associated inflammatory diseases, such as cancers, HIV infection, rheumatoid arthritis, and schizophrenia. IMPORTANCE: It has been well established that inflammatory signaling pathways in ALS converge at NF-κB to promote neuronal damage. Our findings suggest that inflammation-driven IRF1 and NF-κB activity promotes ERVK reactivation in neurons of the motor cortex in ALS. Thus, quenching ERVK activity through antiretroviral or immunomodulatory regimens may hinder virus-mediated neuropathology and improve the symptoms of ALS or other ERVK-associated diseases.

TDP-43 regulates endogenous retrovirus-K viral protein accumulation.

The concomitant expression of neuronal TAR DNA binding protein 43 (TDP-43) and human endogenous retrovirus-K (ERVK) is a hallmark of ALS. Since the involvement of TDP-43 in retrovirus replication remains controversial, we sought to evaluate whether TDP-43 exerts an effect on ERVK expression. In this study, TDP-43 bound the ERVK promoter in the context of inflammation or proteasome inhibition, with no effect on ERVK transcription. However, over-expression of ALS-associated aggregating forms of TDP-43, but not wild-type TDP-43, significantly enhanced ERVK viral protein accumulation. Human astrocytes and neurons further demonstrated cell-type specific differences in their ability to express and clear ERVK proteins during inflammation and proteasome inhibition. Astrocytes, but not neurons, were able to clear excess ERVK proteins through stress granule formation and autophagy. In vitro findings were validated in autopsy motor cortex tissue from patients with ALS and neuro-normal controls. We further confirmed marked enhancement of ERVK in cortical neurons of patients with ALS. Despite evidence of enhanced stress granule and autophagic response in ALS cortical neurons, these cells failed to clear excess ERVK protein accumulation. This highlights how multiple cellular pathways, in conjunction with disease-associated mutations, can converge to modulate the expression and clearance of viral gene products from genomic elements such as ERVK. In ALS, ERVK protein aggregation is a novel aspect of TDP-43 misregulation contributing towards the pathology of this neurodegenerative disease.

Pro-Inflammatory Signaling Upregulates a Neurotoxic Conotoxin-Like Protein Encrypted Within Human Endogenous Retrovirus-K.

Motor neuron degeneration and spinal cord demyelination are hallmark pathological events in Amyotrophic Lateral Sclerosis (ALS). Endogenous retrovirus-K (ERVK) expression has an established association with ALS neuropathology, with murine modeling pointing to a role for the ERVK envelope (env) gene in disease processes. Here, we describe a novel viral protein cryptically encoded within the ERVK env transcript, which resembles two distinct cysteine-rich neurotoxic proteins: conotoxin proteins found in marine snails and the Human Immunodeficiency Virus (HIV) Tat protein. Consistent with Nuclear factor-kappa B (NF-κB)-induced retrotransposon expression, the ERVK conotoxin-like protein (CTXLP) is induced by inflammatory signaling. CTXLP is found in the nucleus, impacting innate immune gene expression and NF-κB p65 activity. Using human autopsy specimens from patients with ALS, we further showcase CTXLP expression in degenerating motor cortex and spinal cord tissues, concomitant with inflammation linked pathways, including enhancement of necroptosis marker mixed lineage kinase domain-like (MLKL) protein and oligodendrocyte maturation/myelination inhibitor Nogo-A. These findings identify CTXLP as a novel ERVK protein product, which may act as an effector in ALS neuropathology.