Identification of cathepsin B as a mediator of neuronal death induced by Aβ-activated microglial cells using a functional genomics approach

Alzheimer's disease is a progressive neurodegenerative disease characterized by senile plaques, neurofibrillary tangles, dystrophic neurites, and reactive glial cells. Activated microglia are found to be intimately associated with senile plaques and may play a central role in mediating chronic inflammatory conditions in Alzheimer's disease. Activation of cultured murine microglial BV2 cells by freshly sonicated A beta 42 results in the secretion of neurotoxic factors that kill primary cultured neurons. To understand molecular pathways underlying Abeta- induced microglial activation, we analyzed the expression levels of transcripts isolated from Abeta42- activated BV2 cells using high density filter arrays. The analysis of these arrays identified 554 genes that are transcriptionally up- regulated by Abeta42 in a statistically significant manner. Quantitative reverse transcription-PCR was used to confirm the regulation of a subset of genes, including cysteine proteases cathepsin B and cathepsin L, tissue inhibitor of matrix metalloproteinase 2, cytochrome c oxidase, and allograft inflammatory factor 1. Small interfering RNA- mediated silencing of the cathepsin B gene in A beta- activated BV2 cells diminished the microglial activation- mediated neurotoxicity. Moreover, CA- 074, a specific cathepsin B inhibitor, also abolished the neurotoxic effects caused by A beta 42- activated BV2 cells. Our results suggest an essential role for secreted cathepsin B in neuronal death mediated by Abeta-activated inflammatory response.