Hsp22 with an N-Terminal Domain Truncation Mediates a Reduction in Tau Protein Levels

Oolonghomobisflavans from Camellia sinensis disaggregate tau fibrils across Alzheimer's disease models

Alzheimer's disease (AD) is a common debilitating neurodegenerative disease with limited treatment options. Amyloid-β (Aβ) and tau fibrils are well-established hallmarks of AD, which can induce oxidative stress, neuronal cell death, and are linked to disease pathology. Here, we describe the effects of Oolonghomobisflavan A (OFA) and Oolonghomobisflavan B (OFB) on tau fibril disaggregation and prionogenic seeding. Transcriptomic analysis of OF-treated animals reveals the induction of a proteostasis-enhancing and health-promoting signature. OFA treatment reduced the burden of Tau protein aggregation in a C. elegans model expressing pathogenic human tau ("hTau-expressing") and promoted Tau disaggregation and inhibited seeding in assays using ex vivo brain-derived paired helical filament tau protein fibrils from Alzheimer's disease brain donors. Correspondingly, treatment with OF improved multiple fitness and aging-related health parameters in the hTau-expressing C. elegans model, including reproductive output, muscle function, and importantly, reversed the shortened lifespan stemming from pathogenic Tau expression. Collectively, this study provides new evidence supporting the neuroprotective effects of OFs and reveal a new therapeutic strategy for targeting AD and other neurodegenerative diseases characterized by tauopathy.

DnaJs are enriched in tau regulators

The aberrant accumulation of tau protein is implicated as a pathogenic factor in many neurodegenerative diseases. Tau seeding may underlie its predictable spread in these diseases. Molecular chaperones can modulate tau pathology, but their effects have mainly been studied in isolation. This study employed a semi-high throughput assay to identify molecular chaperones influencing tau seeding using Tau RD P301S FRET Biosensor cells, which express a portion of tau containing the frontotemporal dementia-related P301S tau mutation fused to a FRET biosensor. Approximately fifty chaperones from five major families were screened using live cell imaging to monitor FRET-positive tau seeding. Among the tested chaperones, five exhibited significant effects on tau in the primary screen. Notably, three of these were from the DnaJ family. In subsequent studies, overexpression of DnaJA2, DnaJB1, and DnaJB6b resulted in significant reductions in tau levels. Knockdown experiments by shRNA revealed an inverse correlation between DnaJB1 and DnaJB6b with tau levels. DnaJB6b overexpression, specifically, reduced total tau levels in a cellular model with a pre-existing pool of tau, partially through enhanced proteasomal degradation. Further, DnaJB6b interacted with tau complexes. These findings highlight the potent chaperone activity within the DnaJ family, particularly DnaJB6b, towards tau.

Compilation of reported protein changes in the brain in Alzheimer's disease

Proteomic studies of human Alzheimer's disease brain tissue have potential to identify protein changes that drive disease, and to identify new drug targets. Here, we analyse 38 published Alzheimer's disease proteomic studies, generating a map of protein changes in human brain tissue across thirteen brain regions, three disease stages (preclinical Alzheimer's disease, mild cognitive impairment, advanced Alzheimer's disease), and proteins enriched in amyloid plaques, neurofibrillary tangles, and cerebral amyloid angiopathy. Our dataset is compiled into a searchable database (NeuroPro). We found 848 proteins were consistently altered in 5 or more studies. Comparison of protein changes in early-stage and advanced Alzheimer's disease revealed proteins associated with synapse, vesicle, and lysosomal pathways show change early in disease, but widespread changes in mitochondrial associated protein expression change are only seen in advanced Alzheimer's disease. Protein changes were similar for brain regions considered vulnerable and regions considered resistant. This resource provides insight into Alzheimer's disease brain protein changes and highlights proteins of interest for further study.

Identification of HSPB8 modulators counteracting misfolded protein accumulation in neurodegenerative diseases

AIMS

The small Heat Shock Protein B8 (HSPB8) is the core component of the chaperone-assisted selective autophagy (CASA) complex. This complex selectively targets, transports, and tags misfolded proteins for their recognition by autophagic receptors and insertion into autophagosome for clearance. CASA is essential to maintain intracellular proteostasis, especially in heart, muscle, and brain often exposed to various types of cell stresses. In neurons, HSPB8 protects against neurotoxicity caused by misfolded proteins in several models of neurodegenerative diseases; by facilitating autophagy, HSPB8 assists misfolded protein degradation also counteracting proteasome overwhelming and inhibition.

MATERIALS AND METHODS

To enhance HSPB8 protective activity, we screened a library of approximately 120,000 small molecules to identify compounds capable of increasing HSPB8 gene transcription, translation, or protein stability. We found 83 compounds active in preliminary dose-response assays and further classified them in 19 chemical classes by medicinal chemists' visual inspection. Of these 19 prototypes, 14 induced HSPB8 mRNA and protein levels in SH-SY5Y cells.

KEY FINDINGS

Out of these 14, 3 successfully reduced the aggregation propensity of a disease-associated mutant misfolded Superoxide Dismutase 1 (SOD1) protein in a flow cytometry-based "aggregation assay" [Flow cytometric analysis of Inclusions and Trafficking" (FloIT)] and induced the expression (mRNA and protein) of some autophagy receptors. Notably, the 3 hits were inactive in HSPB8-depleted cells, confirming that their protective activity is mediated by and requires HSPB8.

SIGNIFICANCE

Thus, these compounds may be highly relevant for a therapeutic approach in several human disorders, including neurodegenerative diseases, in which enhancement of CASA exerts beneficial activities.

Chaperoning activity of the cyclophilin family prevents tau aggregation

Tauopathies, such as Alzheimer's disease, are characterized by the misfolding and progressive accumulation of the microtubule associated protein tau. Chaperones, tasked with maintaining protein homeostasis, can become imbalanced with age and contribute to the progression of neurodegenerative disease. Cyclophilins are a promising pool of underinvestigated chaperones with peptidyl-prolyl isomerase activity that may play protective roles in regulating tau aggregation. Using a Thioflavin T fluorescence-based assay to monitor in vitro tau aggregation, all eight cyclophilins, which include PPIA to PPIH prevent tau aggregation, with PPIB, PPIC, PPID, and PPIH showing the greatest inhibition. The low thermal stability of PPID and the strong heparin binding of PPIB undermines the simplistic interpretation of reduced tau aggregation. In a cellular model of tau accumulation, all cyclophilins, except PPID and PPIH, reduce insoluble tau. PPIB, PPIC, PPIE, and PPIF also reduce soluble tau levels with PPIC exclusively protecting cells from tau seeding. Overall, this study demonstrates cyclophilins prevent tau fibril formation and many reduce cellular insoluble tau accumulation with PPIC having the greatest potential as a molecular tool to mitigate tau seeding and accumulation.

Small Heat Shock Protein 22 Improves Cognition and Learning in the Tauopathic Brain

The microtubule-associated protein tau pathologically accumulates and aggregates in Alzheimer's disease (AD) and other tauopathies, leading to cognitive dysfunction and neuronal loss. Molecular chaperones, like small heat-shock proteins (sHsps), can help deter the accumulation of misfolded proteins, such as tau. Here, we tested the hypothesis that the overexpression of wild-type Hsp22 (wtHsp22) and its phosphomimetic (S24,57D) Hsp22 mutant (mtHsp22) could slow tau accumulation and preserve memory in a murine model of tauopathy, rTg4510. Our results show that Hsp22 protected against deficits in synaptic plasticity and cognition in the tauopathic brain. However, we did not detect a significant change in tau phosphorylation or levels in these mice. This led us to hypothesize that the functional benefit was realized through the restoration of dysfunctional pathways in hippocampi of tau transgenic mice since no significant benefit was measured in non-transgenic mice expressing wtHsp22 or mtHsp22. To identify these pathways, we performed mass spectrometry of tissue lysates from the injection site. Overall, our data reveal that Hsp22 overexpression in neurons promotes synaptic plasticity by regulating canonical pathways and upstream regulators that have been characterized as potential AD markers and synaptogenesis regulators, like EIF4E and NFKBIA.