Targeting p62 Palmitoylation as a Potential Therapeutic Strategy towards Huntington Disease

Abstract

Huntington’s Disease (HD) is a neurodegenerative disorder caused by a mutation in the huntingtin gene leading to the accumulation of mutant huntingtin (mHTT) protein and disruption of many cellular pathways causing neuronal cell death. Lowering mHTT levels has become a primary therapeutic strategy. Autophagy, an intracellular clearance pathway is disrupted in HD at various stages including the cargo-loading step. mHTT aggregation in HD is believed to stem, partly, from the cargo-loading defect during autophagy resulting in empty autophagosomes. p62 is an autophagy receptor responsible for delivering misfolded proteins and aggregates to autophagosomes and eventually lysosomes for degradation. We have identified that p62 is palmitoylated. Palmitoylation is a critical post-translational modification required for protein trafficking and membrane associations. The dynamic nature of palmitoylation may allow for the re-localization of toxic proteins to the autophagosomes. Unpublished data by Dr. Martin and colleagues has shown that palmitoylation of p62 is significantly reduced in the cortex of HD patients and the YAC128 HD mouse models and may be the cause of the cargo-loading defect of autophagosomes in HD. We predict that rescuing p62 palmitoylation will improve cargo loading and promote the clearance of toxic aggregates. A high-throughput screen of FDA-approved drugs previously identified Vorinostat, a histone deacetylase inhibitor, that can penetrate the blood-brain barrier and may increase palmitoylation. The goal of this project was to Investigate the effect of Vorinostat on the palmitoylation of p62 as well as its localization to the autophagosomes and lysosomes. Through the work described in this thesis, I have confirmed that palmitoylation of p62 increases in response to Vorinostat treatment with a concomitant increase in localization to the autophagosomes and the lysosomes. Additionally, Vorinostat improves the localization of HTT to the lysosomes. This study demonstrates that an increase in palmitoylation of p62 results in the inhibition of its acetylation providing insights into a potential mechanism of p62 regulation by Vorinostat. Understanding the underlying mechanism of this regulation will be integral in identifying new therapeutic targets for HD. Subsequently, the enzymes that affect protein palmitoylation were explored leading to the identification of a few candidates that may be regulating p62 palmitoylation. Identification of these enzymes may provide a therapeutic target for rescuing p62 palmitoylation. The investigations thus far suggest that Vorinostat is capable of rescuing p62 palmitoylation as well as improving the targeting of p62 and HTT to the lysosomes which may improve autophagy and have a therapeutic potential in HD.