HAP40 is a conserved central regulator of Huntingtin and a potential modulator of Huntington's disease pathogenesis

Area of Science:

• Molecular Biology
• Neuroscience
• Genetics

Background:

• Huntingtin (HTT) protein dysfunction is implicated in Huntington's disease (HD) pathogenesis.
• The in vivo regulation and physiological roles of HTT remain incompletely understood.
• HAP40, an HTT-associated protein, has unclear physiological and pathological significance in HD.

Purpose of the Study:

• To investigate the in vivo regulation of HTT.
• To characterize the functional and physical interactions between HTT and HAP40.
• To determine HAP40's role in HTT stability and HD pathogenesis.

Main Methods:

• Proteomic analysis to identify HTT binding partners.
• Functional studies in Drosophila and human cell lines.
• HTT and HAP40 protein stability assays (depletion, overexpression, half-life measurements).
• Analysis of HAP40 in HD models (mouse and human cells).
• Assessment of HAP40's effect on mutant HTT toxicity in Drosophila models.

Main Results:

• HAP40 identified as a conserved binding partner of HTT in flies and human cells.
• HAP40 is crucial for HTT stability; HAP40 depletion reduces HTT levels, while HAP40 overexpression increases HTT half-life.
• HTT absence leads to HAP40 degradation, suggesting a reciprocal regulatory relationship.
• HAP40 interaction with HTT is unaffected by polyglutamine expansion; HAP40 does not abnormally accumulate in HD cells.
• HAP40 partially modulates full-length mutant HTT neurodegeneration in Drosophila but not exon 1 fragment toxicity.

Conclusions:

• HAP40 is a central, positive regulator of endogenous HTT stability and function.
• A conserved mechanism governing HTT stability and in vivo function is uncovered.
• HAP40's role in modulating HTT's function, stability, and mutant HTT toxicity implicates it as a potential HD pathogenesis modulator.