Abstract
Free radical-mediated oxidative damage to proteins, lipids, and DNA occurs in neurons during acute brain injuries and in neurodegenerative disorders. Membrane lipid peroxidation contributes to neuronal dysfunction and death, in part by disrupting neuronal ion homeostasis and cellular bioenergetics. Emerging findings suggest that 4-hydroxynonenal (HNE), an aldehyde produced during lipid peroxidation, impairs the function of various proteins involved in neuronal homeostasis. Here we tested the hypothesis that HNE impairs the cellular system that removes damaged proteins and organelles, the autophagy-lysosome pathway in rat primary cortical neurons. We found that HNE, at a concentration that causes apoptosis over a 48-72 h period, increases protein levels of LC3 II and p62 and within 1 and 4 h of exposure, respectively; LC3 II and p62 immunoreactive puncta were observed in the cytoplasm of HNE-treated neurons at 6 h. The extent of up-regulation of p62 and LC3 II in response to HNE was not affected by co-treatment with the lysosome inhibitor bafilomycin A1, suggesting that the effects of HNE on autophagy were secondary to lysosome inhibition. Indeed, we found that neurons exposed to HNE exhibit elevated pH levels, and decreased protein substrate hydrolysis and cathepsin B activity. Neurons exposed to HNE also exhibited the accumulation of K63-linked polyubiquitinated proteins, which are substrates targeted for lysosomal degradation. Moreover, we found that the levels of LAMP2a and constitutively active heat-shock protein 70, and numbers of LAMP2a-positive lysosomes, are decreased in neurons exposed to HNE. Our findings demonstrate that the lipid peroxidation product HNE causes early impairment of lysosomes which may contribute to the accumulation of damaged and dysfunctional proteins and organelles and consequent neuronal death. Because impaired lysosome function is increasingly recognized as an early event in the neuronal death that occurs in neurodegenerative disorders, our findings suggest a role for HNE in such lysosomal dysfunction.