Selenoprotein I is essential for proper central nervous system development: A study on ethanolamine phospholipid deficiency in oligodendrocyte development and function

Abstract

Selenoprotein I (SELENOI; EPT1) is an endoplasmic reticulum resident ethanolamine phosphotransferase that catalyzes the final reaction of the ethanolamine branch of the Kennedy Pathway of phospholipid synthesis, in which phosphatidylethanolamine (PE) and plasmenyl-PE are produced. PE is a major component in mammalian cellular membranes and plays a key role in membrane architecture while also serving as a precursor for biologically active molecules. Plasmenyl-PE is enriched in nervous tissues, particularly in myelin, serving a critical role in a variety of biological functions including neuroprotection through scavenging reactive oxygen species (ROS). Rare mutations in SELENOI have been shown to be associated with a complex form of hereditary spastic paraplegia (HSP), a large group of neurodevelopmental and/or neurodegenerative disorders of multigenetic origin that results in spasticity and weakness in the lower limbs. Patients deficient of SELENOI suffer from a myriad of debilitating symptoms including severely delayed growth, cerebral and cerebellar atrophy, delayed motor development, lower limb spasticity, and hypomyelination. We developed a murine central nervous system-specific SELENOI conditional knockout (cKO) and observed profound deficits in tests of motor coordination accompanied by hypomyelination and reactive gliosis in regions of the corticospinal tract. Lipidomic and flow cytometric analyses of whole brains revealed altered lipid profile, elevated lipid peroxidation, and reduced percentage of myelinating oligodendrocytes. These findings suggested that cells of the oligodendroglial lineage are most vulnerable to the loss of SELENOI. During proliferation and differentiation, oligodendrocyte progenitor cells (OPCs) increase their uptake of iron which leads to elevated levels of ROS and, consequently, lipid peroxidation. The intracellular accumulation of iron and lipid peroxidation are two hallmarks of ferroptosis, a non-apoptotic cell death pathway. We, therefore, hypothesized that loss of SELENOI-derived plasmenyl-PE in OPCs results in increased vulnerability to lipid peroxidation and ferroptosis. We isolated SELENOI cKO OPCs and measured their sensitivity and response to ferroptosis induction. We then evaluated their differentiation and proliferation capacities ex vivo. We found that SELENOI cKO OPCs exhibited a smaller increase in iron uptake and greater increase in lipid peroxidation levels in response to ferroptosis induction compared to WT controls. Notably, we observed no effect on cell viability. Furthermore, we noted a decreased differentiation capacity and an increased rate of proliferation accompanied by downregulation of cell cycle exit and neuronal differentiation protein 1 (CEND1) in these OPCs. These data collectively suggest that SELENOI cKO OPCs evade ferroptosis by mitigating overwhelming levels of lipid peroxidation through restricting iron uptake, and that the resultant iron uptake restriction may negatively influence factors that drive differentiation and suppress proliferation, such as CEND1. Further investigation is needed to discern the precise molecular pathways through which SELENOI exerts its effects on these processes, as understanding these mechanisms could open new avenues for therapeutic interventions in myelination disorders.