Protopine promotes the proteasomal degradation of pathological tau in Alzheimer's disease models via HDAC6 inhibition

•The natural alkaloid protopine is brain-permeable, which improves memory functions in 3xTg-AD and P301S tau transgenic models of AD.•Protopine specifically reduces sarkosyl-insoluble pathological tau in AD mice models.•Protopine directly binds and inhibits HDAC6, thus promoting the ubiquitination and degradation of pathological tau via the ubiquitin-proteasomal system. Collective evidences have indicated that intracellular accumulation of hyperphosphorylated tau forms neurofibrillary tangles in the brain, which impairs memory, cognition and affects social activities in Alzheimer's disease (AD). To investigate the tau-reducing, and memory-enhancing properties of protopine (PRO), a natural alkaloid isolated from Chinese herbal medicine Corydalis yanhusuo (Yanhusuo in Chinese). By using Histone deacetylase 6 (HDAC6) profiling and immunoprecipitation assays, we assessed that PRO mediated the heat shock protein 90 (HSP90) chaperonic activities for the degradation of pathological tau in AD cell culture models. To study the efficacy of PRO in vivo, we employed 3xTg-AD and P301S tau mice models. Liquid chromatography/quadrupole time-of-flight mass spectrometry was used to analyze the pharmacokinetic profile of PRO. Seven-month-old 3xTg-AD mice and 1.5-month-old P301S mice were administered PRO (1 and 2.5 mg/kg) orally every day. Morris water maze, contextual fear conditioning and rotarod assays were applied for studying memory functions. Sarkosyl differential centrifugation was used to analyze soluble and insoluble tau. Immunohistochemical analysis were performed to determine tau deposits in AD mice's brain sections. Molecular docking, binding affinity studies and primary cell culture studies were performed to demonstrate the mechanism of action of PRO in silico and in vitro. Our pharmacokinetic profiling demonstrated that PRO significantly entered the brain at a concentration of 289.47 ng/g, and specifically attenuated tau pathology, improved learning and memory functions in both 3xTg-AD and P301S mice. Docking, binding affinity studies, and fluorometric assays demonstrated that PRO directly bound to the catalytic domain 1 (CD1) of HDAC6 and down-regulated its activity. In primary cortical neurons, PRO enhanced acetylation of α-tubulin, indicating HDAC6 inhibition. Meanwhile, PRO promoted the ubiquitination of tau and recruited heat shock protein 70 (HSP70) and heat shock cognate complex 71 (HSC70) for the degradation of pathological tau via the ubiquitin-protea