Kainate induces AKT, ERK and cdk5/GSK3β pathway deregulation, phosphorylates tau protein in mouse hippocampus

Acute treatment with kainate 30 mg/kg (KA) produced behavioral alterations and reactive gliosis. However, it did not produce major death of mouse hippocampal neurons, indicating that concentrations were not cytotoxic. KA caused rapid and temporal Erk phosphorylation (at 6 h) and Akt dephosphorylation (1–3 days). Concomitantly, the activation of GSK3β was increased 1–3 days after KA. After 7 days, a reduction in GSK3β activation was observed. Caspase-3 activity increased, but to a lesser extent than calpain activation (measured by fluorimetry and calpain-cleaved α-spectrin). As calpain is involved in cdk5 activation, and cdk5 is related to GSK3β, the cdk5/p25 pathway was examined. Results showed that the p25/p35 ratio in KA-injected mice for 3 days was 73.6% higher than control levels. However, no changes in cdk5 expression were detected. Both Western blot and immunohistochemistry against p-Tau(Thr231) indicated an increase at this phosphorylated site of tau protein. Indeed an increase in p-Tau(Ser199) and p-Tau(Ser396) was observed by Western blot. Our results demonstrate that tau hyperphosphorylation, induced by KA, is due to an increase in GSK3β/cdk5 activity in combination with an inactivation of Akt. This indicates that the calpain/cdk5 pathway for tau phosphorylation has a potential role in delayed apoptotic death evoked by excitotoxicity. Moreover, the subsequent activation of caspase and calpain proteases leads to dephosphorylation of tau, thus increasing microtubular destructuration. Taken together, our results provide new insights in the activation of several kinase-pathways implicated in cytoskeletal alterations that are a common feature of neurodegenerative diseases.