Role of Selenoprotein P in brain zinc homeostasis

Abstract

Selenoprotein P (Sepp1) is a selenium-rich antioxidant protein involved in extracellular transport of selenium (Se). Sepp1 also has metal binding properties. Zinc (Zn2+) is an essential micronutrient that is released from terminals in the brain that utilize the neurotransmitter, glutamate. Both Zn2+ and Se are necessary for proper brain function. However, intracellular Zn2+ accumulation can contribute to neurotoxicity, and extracellular Zn2+ can promote aggregation of amyloid-beta to form brain plaques during development of Alzheimer's disease (AD). Through metal column purification, we confirmed Sepp1's ability to bind Zn2+ as well as other biometals including Co2+ and Ni2+. We investigated the role of Sepp1 in Zn2+ regulation by examining Zn2+ levels in wildtype (WT) and Sepp1 knockout (Sepp1-/-) mice. Zinc-N-(6-methoxy-8-quinolyl)-ptoluenesulphonamide (TSQ) staining revealed increased levels of intracellular Zn2+ in the Sepp1-/-hippocampus, the region of the brain that is crucial to memory formation, compared to the WT mice. Mass spectrometry analysis of freshly frozen brain samples demonstrated a marked increase in total brain Zn2+ levels in the Sepp1-/-mice. Additionally, levels of key Zn2+-regulating proteins in the brain are affected by the absence of Sepp1, possibly in response to the elevated Zn2+ content. However, live Zn2+ imaging of hippocampal slices with a selective extracellular fluorescent Zn2+ indicator (Fluozin-3) showed that Sepp1-/-mice have impaired Zn2+ release in response KCl-induced neuron depolarization, which may result in memory impairments. Taken together, our findings reveal that Sepp1 plays a crucial role in the maintenance of Zn2+ homeostasis in the hippocampus and for proper brain function. The identification of a naturally occurring Zn2+-chelator regulated by dietary selenium may significantly contribute to the treatment and prevention of AD.