HERV-K (HML-2) envelope protein induces mitochondrial depolarization and neurotoxicity via endolysosome iron dyshomeostasis

Human endogenous retroviruses (HERVs) are associated with the pathogenesis of amyotrophic lateral sclerosis (ALS); a disease characterized by motor neuron degeneration and cell death. The HERV-K subtype HML-2 envelope protein (HERV-K Env) is expressed in brain, spinal cord and cerebrospinal fluid of people living with ALS and through CD98 receptor-linked interactions causes neurodegeneration. HERV-K Env-induced increases in oxidative stress are implicated in the pathogenesis of ALS, and ferrous iron (Fe2+) generates reactive oxygen species (ROS). Endolysosome stores of Fe2+ are central to iron trafficking and endolysosome de-acidification releases Fe2+ into the cytoplasm. Because HERV-K Env is an arginine-rich protein that is likely endocytosed and arginine is a pH-elevating amino acid, it was important to determine HERV-K Env effects on endolysosome pH and whether HERV-K Env-induced neurotoxicity is downstream of Fe2+ released from endolysosomes. Here, we showed using SH-SY5Y human neuroblastoma cells and primary cultures of human cortical neurons (HCNs, information on age and sex was not available) that HERV-K Env (1) is endocytosed via CD98 receptors, (2) concentration-dependently de-acidified endolysosomes, (3) decreased endolysosome Fe2+ concentrations, (4) increased cytosolic and mitochondrial Fe2+ and ROS levels, (5) depolarized mitochondrial membrane potential, and (6) induced cell death; effects blocked by an antibody against the CD98 receptor and by the endolysosome iron chelator deferoxamine. Thus, HERV-K Env-induced increases in cytosolic and mitochondrial Fe2+ and ROS as well as cell death appear to be mechanistically caused by HERV-K Env endocytosis, endolysosome de-acidification, and endolysosome Fe2+ efflux into the cytoplasm.Significance Statement Human endogenous retroviruses (HERVs) represent 8% of genomic DNA, play roles in brain development, and are silenced in adult brain. HML-2 envelope-protein (HERV-K Env) activation is found in brain, spinal cord, and cerebrospinal fluid in an amyotrophic lateral sclerosis (ALS) subgroup and is implicated in ALS pathogenesis. HERV-K Env-induced neurotoxicity increases reactive oxygen species (ROS) levels, but by unclear mechanisms. We showed that HERV-K Env is endocytosed, de-acidified endolysosomes, decreased endolysosome Fe2+ levels, increased cytosol and mitochondria Fe2+ and ROS levels, depolarized mitochondrial membrane potential, and induced cell death; effects blocked by a CD98 receptor antibody and the endolysosome iron-chelator deferoxamine. Identifying mechanisms of HERV-K Env-induced toxicity may identify targets against which therapeutics can be developed in HERV-K Env-associated diseases.