The study of the role of retroviruses in amyotrophic lateral sclerosis (ALS) dates back to the 1960s shortly after transposable elements themselves were first discovered. It was quickly realized that in wild mice both horizontal and vertical transmissions of retroviral elements were key to the development of an ALS-like syndrome leading to the postulate that endogenous retroviruses (ERVs) contribute significantly to the pathogenicity of this disease. Subsequent studies identified retroviral reverse transcriptase activity in brains of individuals with ALS from Guam. However, except for a single study from the former Soviet Union, ALS could not be transmitted to rhesus macaques. The discovery of an ALS-like syndrome in human immunodeficiency virus (HIV) and human T cell leukemia virus infected individuals led to renewed interest in the field and reverse transcriptase activity was found in the blood and cerebrospinal fluid of individuals with sporadic ALS. However, exogenous retroviruses could not be found in individuals with ALS which further reinforced the possibility of involvement of a human ERV (HERV). The first demonstration of the involvement of a HERV was the discovery of the activation of human endogenous retrovirus-K subtype HML-2 in the brains of individuals with ALS. The envelope protein of HML-2 is neurotoxic and transgenic animals expressing the envelope protein develop an ALS-like syndrome. Activation of HML-2 occurs in the context of generalized transposable element activation and is not specific for ALS. Individuals with HIV-associated ALS show a remarkable response to antiretroviral therapy; however, antiretroviral trials in ALS down-regulate HML-2 without ameliorating the disease. This highlights the need for specific drugs to be developed against HML-2 as a novel therapeutic target for ALS. Other approaches might include antisense oligonucleotides, shRNA targeted against the envelope gene or antibodies that can target the extracellular envelope protein. Future clinical trials in ALS should consider combination therapies to control these ERVs.

Purpose of the review Persistent infections capable of causing central nervous system (CNS) complications months or years after the initial infection represent a major public health concern. This concern is particularly relevant considering the ongoing coronavirus disease 2019 pandemic, where the long-term neurological effects are still being recognized. Recent findings Viral infections are a risk factor for the development of neurodegenerative diseases. In this paper, we provide an in-depth exploration of the prevalent known and suspected persistent pathogens and their epidemiological and mechanistic links to later development of CNS disease. We examine the pathogenic mechanisms involved, including direct viral damage and indirect immune dysregulation, while also addressing the challenges associated with detecting persistent pathogens. Summary Viral encephalitis has been closely associated with the later development of neurodegenerative diseases and persistent viral infections of the CNS can result in severe and debilitating symptoms. Further, persistent infections may result in the development of autoreactive lymphocytes and autoimmune mediated tissue damage. Diagnosis of persistent viral infections of the CNS remains challenging and treatment options are limited. The development of additional testing modalities as well as novel antiviral agents and vaccines against these persistent infections remains a crucial research goal.