Rapid tau protein dephosphorylation and differential rephosphorylation during cardiac arrest-induced cerebral ischemia and reperfusion

The effects of cerebral ischemia/reperfusion on phosphorylation of microtub ule-associated tau proteins were assessed in a canine model of cardiac arre st. As tau proteins are phosphorylated by kinases involved in different tra nsduction signal pathways, their phosphorylation state is an excellent mark er of neuronal homeostasis and microtubule dynamics. Canine brain tau prote ins were characterized by immunoblotting using phosphorylation-dependent an tibodies and antisera raised against different amino- and carboxy-terminal tau sequences. The present study reports a complete dephosphorylation of ta u proteins during ischemia, which is shown by a higher electrophoretic mobi lity and the almost (if not total) disappearance of phosphorylation-depende nt monoclonal antibody labeling. After 2-hour restoration of spontaneous ci rculation, a decrease in the electrophoretic mobility was observed, and aft er 24 hours of reperfusion, a full restoration of the phosphorylation was v isualized using phosphorylation-dependent monoclonal antibodies directed ag ainst Ser/Thr-Pro sites. However, one particular phosphorylation site invol ved in tau binding to microtubules, located on Ser(262/356), was never full y significantly rephosphorylated, suggesting that microtubule metabolism wa s still affected after 24 hours of reperfusion. Thus, the sequential and di fferential recovery of tau phosphorylation after ischemia followed by reper fusion is a useful marker with which to monitor neuronal integrity after br ain ischemia.