The CHIP-LMO7-BAG5 complex controls tau clearance and yields repurposed and newly designed therapeutic candidates for Alzheimer's disease

Technical Summary: The CHIP-LMO7-BAG5 Complex as a Therapeutic Target for Tauopathies

Problem Statement
Tauopathies, including Alzheimer's disease (AD), are characterized by the pathological accumulation of tau protein, leading to neurodegeneration and cognitive decline. A critical challenge in treating these conditions is identifying mechanisms that effectively regulate tau clearance in vivo without inducing toxicity. While the E3 ubiquitin ligase CHIP (Carboxyl terminus of Hsc70-interacting protein) is known to promote tau degradation, its activity is tightly regulated by cochaperones. The specific molecular mechanisms governing the inhibition of CHIP in the context of tau homeostasis, and the potential to pharmacologically target this regulation, remained to be fully elucidated.

Methodology
The study employed a multi-disciplinary approach combining cross-species genetic screens, structural biology, and pharmacological validation:

1. Genetic Screening: Researchers conducted cross-species genetic screens to identify novel regulators of tau levels in vivo, leading to the discovery of LMO7.
2. Mechanistic Characterization: The study utilized in vivo models to analyze the interaction between LMO7, CHIP, and BAG5. Specifically, adult-onset LMO7 knockdown was performed in tauopathy mice to assess effects on CHIP activity, tau levels, gliosis, and memory function.
3. Structural Modeling and Virtual Screening: Leveraging AlphaFold-derived structural models of the CHIP-LMO7-BAG5 complex, the team performed AI-based virtual screening. This guided the design of a CHIP-derived competing peptide intended to disrupt the inhibitory complex.
4. Drug Repurposing and Optimization: The virtual screening pipeline was used to identify existing FDA-approved drugs capable of disrupting the complex. Additionally, Mycophenolic acid derivatives were optimized to create new candidates (e.g., MPA-2).
5. Therapeutic Validation: The efficacy of the designed peptide (via viral delivery) and the identified small molecules (Telmisartan and MPA-2, via oral administration) was tested in tauopathy mouse models to evaluate their ability to reduce pathogenic tau and improve cognitive outcomes.

Key Contributions

• Identification of LMO7: The paper establishes LMO7 as a critical in vivo regulator of tau levels.
• Elucidation of the Inhibitory Mechanism: The study defines a specific molecular mechanism where LMO7 bridges the E3 ligase CHIP and its inhibitory cochaperone BAG5. This complex formation suppresses CHIP's ligase activity, thereby limiting tau ubiquitination and degradation.
• Therapeutic Strategy Development: The work demonstrates that disrupting the CHIP-LMO7-BAG5 interaction is a viable strategy to enhance tau clearance. This was achieved through two distinct avenues:
  1. Peptide Therapy: A CHIP-derived competing peptide designed based on structural models.
  2. Small Molecule Repurposing and Design: The identification of FDA-approved drugs (Telmisartan) and the development of optimized derivatives (MPA-2) that disrupt the complex.

Results

• Genetic Manipulation: Adult LMO7 knockdown in tauopathy mice successfully enhanced CHIP activity. This resulted in a reduction of both total and phosphorylated tau levels, a decrease in gliosis, and a significant rescue of memory deficits.
• Pharmacological Intervention:
  • Viral delivery of the designed competing peptide reduced pathogenic tau levels without observed toxicity.
  • Oral administration of Telmisartan and the optimized derivative MPA-2 similarly reduced pathogenic tau in vivo.
  • Crucially, these interventions demonstrated efficacy without the toxicity often associated with aggressive tau clearance strategies.

Significance
The paper claims to establish the CHIP-LMO7-BAG5 complex as a "druggable node" in tau homeostasis. By demonstrating that disrupting this complex enhances tau clearance and rescues cognitive deficits in mouse models, the study provides a mechanistic foundation for new therapeutic approaches. The findings suggest that repurposing existing drugs like Telmisartan and utilizing newly designed candidates like MPA-2 offers a promising path for treating tauopathies, specifically Alzheimer's disease and STUB1-associated ataxias. The work bridges the gap between genetic discovery and translational medicine by validating both peptide and small-molecule strategies derived from structural insights.