Modulating FKBP5/FKBP51 and autophagy lowers HTT (huntingtin) levels.

ABSTRACT

Current disease-modifying therapies for Huntington disease (HD) focus on lowering mutant HTT (huntingtin; mHTT) levels, and the immunosuppressant drug rapamycin is an intriguing therapeutic for aging and neurological disorders. Rapamycin interacts with FKBP1A/FKBP12 and FKBP5/FKBP51, inhibiting the MTORC1 complex and increasing cellular clearance mechanisms. Whether the levels of FKBP (FK506 binding protein) family members are altered in HD models and if these proteins are potential therapeutic targets for HD have not been investigated. Here, we found levels of FKBP5 are significantly reduced in HD R6/2 and zQ175 mouse models and human HD isogenic neural stem cells and medium spiny neurons derived from induced pluripotent stem cells. Moreover, FKBP5 interacts and colocalizes with HTT in the striatum and cortex of zQ175 mice and controls. Importantly, when we decreased FKBP5 levels or activity by genetic or pharmacological approaches, we observed reduced levels of mHTT in our isogenic human HD stem cell model. Decreasing FKBP5 levels by siRNA or pharmacological inhibition increased LC3-II levels and macroautophagic/autophagic flux, suggesting autophagic cellular clearance mechanisms are responsible for mHTT lowering. Unlike rapamycin, the effect of pharmacological inhibition with SAFit2, an inhibitor of FKBP5, is MTOR independent. Further, in vivo treatment for 2 weeks with SAFit2, results in reduced HTT levels in both HD R6/2 and zQ175 mouse models. Our studies establish FKBP5 as a protein involved in the pathogenesis of HD and identify FKBP5 as a potential therapeutic target for HD.Abbreviations ACTB/ß-actin actin beta; AD Alzheimer disease; BafA1 bafilomycin A1; BCA bicinchoninic acid; BBB blood brain barrier; BSA bovine serum albumin; CoIP co-immunoprecipitation; DMSO dimethyl sulfoxide; DTT dithiothreitol; FKBPs FK506 binding proteins; HD Huntington disease; HTT huntingtin; iPSC induced pluripotent stem cells; MAP1LC3/LC3microtubule associated protein 1 light chain 3; MAPT/tau microtubule associated protein tau; MES 2-ethanesulfonic acid; MOPS 3-(N-morphorlino)propanesulfonic acid); MSN medium spiny neurons; mHTT mutant huntingtin; MTOR mechanistic target of rapamycin kinase; NSC neural stem cells; ON overnight; PD Parkinson disease; PPIase peptidyl-prolyl cis/trans-isomerases; polyQ polyglutamine; PPP1R1B/DARPP-32 protein phosphatase 1 regulatory inhibitor subunit 1B; PTSD post-traumatic stress disorder; RT room temperature; SQSTM1/p62 sequestosome 1; SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis; TBSTTris-buffered saline, 0.1% Tween 20; TUBA tubulin; ULK1 unc-51 like autophagy activating kinase 1; VCL vinculin; WT littermate controls.