Microtubule depolymerization and tau phosphorylation

Inge Grundke-Iqbal and Khalid Iqbal found a connection between microtubule associated tau and Alzheimer's disease. They described that abnormally phosphorylated tau is a component of the paired helical filaments found in the disease. Afterwards they described that tau hyperphosphorylation prevents microtubule assembly. Now trying to complement the relationship between microtubules and tau phosphorylation, we have commented on the effect of microtubule disassembly on tau phosphorylation. In this study, we investigated the role of microtubule depolymerization induced by nocodazole on tau phosphorylation in human neuroblastoma SH-SY5Y cells. Our results indicate that nocodazole provokes tau phosphorylation mediated by GSK3, as determined by using AT-8 or Tau-1 antibodies. Interestingly, total GSK3β and GSK3β phosphorylation on Ser-9 are not altered during nocodazole treatment. In addition, microtubule stabilization with taxol had similar effects, likely because taxol and tau compete for the same binding sites on microtubules, and in the presence of taxol, tau could be detached from microtubules. Thus, unbound tau from microtubles can be phosphorylated by GSK3, even if the activity of GSK3 is not altered, probably because tau unbound to microtubules could be a better substrate for the kinase than microtubule-associated tau. These findings suggest that microtubule depolymerization can be a primary event in neurodegenerative disorders like Alzheimer's disease and that tau phosphorylation takes place afterwards.