The induction of neuronal death by up-regulated microglial cathepsin H in LPS-induced neuroinflammation

Background: Neuroinflammation is a hallmark that leads to selective neuronal loss and/or dysfunction in neurodegenerative disorders. Microglia-derived lysosomal cathepsins are increasingly recognized as important inflammatory mediators to trigger signaling pathways that aggravate neuroinflammation. However, cathepsin H (Cat H), a cysteine protease, has been far less studied in neuroinflammation, compared to cathepsins B, D, L, and S. The expression patterns and functional roles of Cat H in the brain in neuroinflammation remain unknown. Methods: C57BL/6J mice were intraperitoneally injected with either 0.9% saline or lipopolysaccharide (LPS, 5 mg/kg). Immunohistochemistry (IHC) and in situ hybridization (ISH) were used to analyze expression and localization of Cat H in the brain. Nitrite assay was used to examine microglial activation in vitro; ELISA was used to determine the release of Cat H and proinflammatory cytokines (TNF-alpha, IL-1 beta, IL-6, IFN-gamma). Cat H activity was analyzed by cellular Cat H assay kit. Flow cytometry and in situ cell death detection were used to investigate neuronal death. Data were evaluated for statistical significance with one-way ANOVA and t test. Results: Cat H mRNA was only present in perivascular microglia and non-parenchymal sites under normal conditions. After LPS injection, Cat H mRNA expression in activated microglia in different brain regions was increased. Twenty-four hours after LPS injection, Cat H mRNA expression was maximal in SNr; 72 h later, it peaked in cerebral cortex and hippocampus then decreased and maintained at a low level. The expression of Cat H protein exhibited the similar alterations after LPS injection. In vitro, inflammatory stimulation (LPS, TNF-alpha, IL-1 beta, IL-6, and IFN-gamma) increased the release and activity of Cat H in microglia. Conversely, addition of Cat H to microglia promoted the production and release of NO, IL-1 beta, and IFN-gamma which could be prevented by neutralizing antibody. Further, addition of Cat H to Neuro2a cells induced neuronal death. Conclusions: Taken together, these data indicate that the up-regulated microglial Cat H expression, release, and activity in the brain lead to neuronal death in neuroinflammation. The functional link of Cat H with microglial activation might contribute to the initiation and maintenance of microglia-driven chronic neuroinflammation.