Neurotoxicity induces cleavage of p35 to p25 by calpain

Cyclin-dependent kinase 5 (cdk5) and its neuron-specific activator p35 are required for neurite outgrowth and cortical lamination(1-3). Proteolytic cl eavage of p35 produces p25, which accumulates in the brains of patients wit h Alzheimer's disease(4). Conversion of p35 to p25 causes prolonged activat ion and mislocalization of cdk5. Consequently, the p25/cdk5 kinase hyperpho sphorylates tau, disrupts the cytoskeleton and promotes the death (apoptosi s) of primary neurons. Here we describe the mechanism of conversion of p35 to p25. In cultured primary cortical neurons, excitotoxins, hypoxic stress and calcium influx induce the production of p25. In fresh brain lysates, ad dition of calcium can stimulate cleavage of p35 to p25. Specific inhibitors of calpain, a calcium-dependent cysteine protease, effectively inhibit the calcium-induced cleavage of p35. In vitro, calpain directly cleaves p35 to release a fragment with relative molecular mass 25,000. The sequence of th e calpain cleavage product corresponds precisely to that of p25. Applicatio n of the amyloid beta-peptide A beta(1-42) induces the conversion of p35 to p25 in primary cortical neurons. Furthermore, inhibition of cdk5 or calpai n activity reduces cell death in A beta-treated cortical neurons. These obs ervations indicate that cleavage of p35 to p25 by calpain may be involved i n the pathogenesis of Alzheimer's disease.