Heat Shock Induces Rapid Dephosphorylation of τ in Both Female and Male Rats Followed by Hyperphosphorylation Only in Female Rats: Implications for Alzheimer's Disease

Female and male 2–3‐month‐old Sprague‐Dawley rats were heat shocked at 42°C for 15 min. At 0, 3, 6, 12, and 24 h after heat shock, qualitative and quantitative immunoblot analysis of cerebral extracts and immunohistochemistry were performed using monoclonal anti‐τ antibodies that recognize nonphosphorylated (Tau‐1), phosphorylated (PHF‐1), and phosphate‐independent (Tau‐5 and Tau‐46) epitopes. At 0 h after heat shock, there was dephosphorylation of τ in both female and male rats as evidenced by (1) accentuation and attenuation of τ isoforms recognized by Tau‐1 and PHF‐1, respectively, and recognition of additional τ polypeptides by Tau‐1, Tau‐5, and Tau‐46 but not PHF‐1; (2) significant increase in the nonphosphorylated Tau‐1 epitope with resultant decrease in the ratio of total (phosphorylated plus nonphosphorylated) τ to nonphosphorylated τ; and (3) dephosphorylation of the Tau‐1 epitope in the somatodendritic compartment. By 6 h after heat shock, there was progressive hyperphosphorylation of τ in female but not male rats exemplified by (1) upward gel mobility shift recognized by PHF‐1, Tau‐5, and Tau‐46, and by Tau‐1 after dephosphorylation; (2) significant increase in the ratio of total τ to nonphosphorylated τ; and (3) rephosphorylation of the Tau‐1 epitope in the somatodendritic compartment. Two‐dimensional electrophoresis showed shifts to basic and acidic τ polypeptides at 0 and 6 h after heat shock, respectively. Hyperphosphorylation of τ also occurred after multiple heat‐shock episodes. Microtubules were present at 6 h after heat shock. There were no differences between control and heat‐shocked rats in extracts from peripheral nerves. Thus, we now have a simple rat model to study within 6 h the processes of dephosphorylation and hyperphosphorylation of τ, which are altered in Alzheimer's disease.