Excitotoxin-induced neuronal death is associated with response of a unique intracellular aspartic proteinase, cathepsin E

Excitotoxicity produced by excessive stimulation of glutamate receptors is known to lead to neuronal lesion and death. Here, we demonstrate that quantitative and qualitative changes in cathepsin E (CE) gene products are associated with execution of the excitotoxic neuronal death. Intracerebroventricular injection of kainate (KA) resulted in marked elevation of both mRNA and protein levels of CE in the rat hippocampal CA3 region, where the enzyme was mainly found in vulnerable neurons and activated microglia. Northern blot analysis showed that the size of the transcript for CE was identical with that normally expressed in rat spleen. Immunoblot analysis, however, revealed the predominant occurrence of the highly modified CE species, besides the mature CE. This polypeptide was distinct from the mature CE in molecular masses (106 vs. 82 kDa) and pI (6.4-7.3 vs. 5.0-5.5) and showed resistance to conversion into the enzymatically active form by acid treatment. Consistent with these in vivo results, administration of glutamate to primary cultured rat hippocampal neurons resulted in a marked expression of this novel CE species. These data indicate that excessive stimulation of glutamate receptors causes induction of the CE gene response followed by persistent expression of CE in the novel form, besides its mature form, predominantly in the hippocampal neurons.