Role of calpain and caspase in β-amyloid-induced cell death in rat primary septal cultured neurons

The invariant characteristic features associated with Alzheimer's disease (AD) brain include the presence of extracellular neuritic plaques composed of amyloid β (Aβ) peptide, intracellular neurofibrillary tangles containing hyper-phosphorylated tau protein and the loss of basal forebrain cholinergic neurons. Studies of the pathological changes that characterize AD and several other lines of evidence indicate that in vivo accumulation of Aβ1–42 may initiate the process of neurodegeneration observed in AD brains. However, the cause of degeneration of the basal forebrain cholinergic neurons and their association to Aβ peptides or phosphorylated tau protein have not been clearly established. In the present study, using rat primary septal cultures, we have shown that Aβ1–42, in a time (1–48 h) and concentration (0.01–20 μM)-dependent manner, induce toxicity in cultured neurons. Subsequently, we have demonstrated that Aβ toxicity is mediated via activation of cysteine proteases, i.e., calpain and caspase, and proteolytic breakdown of their downstream substrates tau, microtubule-associated protein-2 and αII-spectrin. Additionally, Aβ-treatment was found to induce phosphorylation of tau protein along with decreased levels of phospho-Akt and phospho-Ser9glycogen synthase kinase-3β. Exposure to specific inhibitors of caspase or calpain can partially protect cultured neurons against Aβ-induced toxicity but their effects are not found to be additive. These results, taken together, suggest that Aβ peptide can induce toxicity in rat septal cultured neurons by activating multiple intracellular signaling molecules. Additionally, evidence that inhibitors of caspase and calpains can partially protect the cultured basal forebrain neurons raised the possibility that their inhibitors could be of therapeutic relevance in the treatment of AD pathology.