Human Endogenous Retrovirus K Envelope in Spinal Fluid of Amyotrophic Lateral Sclerosis Is Toxic

OBJECTIVE: Human endogenous retroviruses have been implicated in neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). Expression of human endogenous retrovirus K (HERV‐K) subtype HML‐2 envelope (Env) in human neuronal cultures and in transgenic mice results in neurotoxicity and neurodegeneration, and mice expressing HML‐2 Env display behavioral and neuromuscular characteristics resembling ALS. This study aims to characterize the neurotoxic properties of HML‐2 Env. METHODS: Env neurotoxicity was detected in ALS cerebrospinal fluid and confirmed using recombinant Env protein in a cell‐based assay and a mouse model. The mechanism of neurotoxicity was assessed with immunoprecipitation followed by mass spectrometry and Western blot, and by screening a panel of inhibitors. RESULTS: We observed that recombinant HML‐2 Env protein caused neurotoxicity resulting in neuronal cell death, retraction of neurites, and decreased neuronal electrical activity. Injection of the Env protein into the brains of mice also resulted in neuronal cell death. HML‐2 Env protein was also found in the cerebrospinal fluid of patients with sporadic ALS. The neurotoxic properties of the Env and the cerebrospinal fluid could be rescued with the anti‐Env antibody. The Env was found to bind to CD98HC complexed to β1 integrin on the neuronal cell surface. Using a panel of compounds to screen for their ability to block Env‐induced neurotoxicity, we found that several compounds were protective and are linked to the β1 integrin pathway. INTERPRETATION: HERV‐K Env is released extracellularly in ALS and causes neurotoxicity via a novel mechanism. Present results pave the way for new treatment strategies in sporadic ALS. ANN NEUROL 2022;92:545–561