Pharmacological manipulation of Hsp90 and its effects on phospho-tau species in an AD-relevant cell-based model

Abstract

Hyperphosphorylated tau is one of the hallmarks of Alzheimer's disease and other related dementias. Removal of phospho-tau species occurs by chaperone-mediated folding and degradation and the molecular chaperone Hsp90 is one of the key members of the chaperone machinery involved in the turnover of tau in the cell. Recent research provides a rationale for the therapeutic inhibition of Hsp90 which leads to the clearance of abnormal tau species. We have studied the effects of Hsp90 inhibitors on AD-relevant phospho-tau species and client proteins in a cell-based model of AD. SK-N-AS cells were transiently transfected with tau and treated with different Hsp90 inhibitors. Effects on tau phosphorylation on epitopes implicated in AD and Hsp70 induction were measured using an AlphaScreen assay. The effect of Hsp90 inhibition on client proteins was measured by western blot analysis and densitometry. Efficacy of Hsp90 inhibition was measured by Hsp70 induction. Inhibition of Hsp90 causes a decrease of phospho-epitopes pS202, pS235 and pS396 without causing a major change in total tau levels. N-terminal inhibitors were shown to cause a much higher induction of Hsp70 compared to C-terminal inhibitors. The client proteins p25/p35, cdk5, GSK3β and c-Abl were affected differentially by N- and C-terminal Hsp90 inhibitors. C-terminal inhibitors do not affect the stability of Hsp90 client proteins to the same extent as N-terminal inhibitors. Hsp90 inhibition can potentially provide an alternative mechanism to kinase inhibition for tackling tau hyperphosphorylation in AD. Further studies of small molecule Hsp90 inhibitors may bring value for the future treatment of AD.