Deciphering The Role of Huntingtin In Endosomal Functions

<p>Huntington's disease (HD), a neurodegenerative disorder, is a result of an abnormal expansion of CAG trinucleotide repeat encoding an extended polyglutamine (polyQ) tract in the coding region of Huntingtin (HTT) gene. While antisense oligonucleotide (ASO) strategies aimed at lowering mutant HTT levels seemed promising, recent clinical trials were unsuccessful due to worsening patient outcomes in the ASO treatment group compared to the placebo. This may be attributed to potential adverse effects resulting from decreased levels of normal HTT, which could play a pivotal role in brain functions and HD progression. The impact of reduced HTT levels on cellular functions and neurodegeneration largely remains elusive.</p> <p>Rab proteins, part of the Ras superfamily of small GTPases, govern cargo trafficking through endosomal pathways. Upon endocytosis into early endosomes regulated by Rab5, the endocytosed cargo can either be recycled back to the plasma membrane (via Rab4/Rab11) or degraded through the endosomal-lysosomal pathway (via Rab7). Disturbances in normal Rab functions could lead to abnormal cargo recycling or degradation, potentially contributing to neurodegeneration. HTT has been shown to interact with Rab5 on early endosomes, which direct cargo traffic towards recycling or degradation. However, the exact mechanism of how HTT regulates Rab5 and its downstream pathways is yet to be determined.</p> <p>My study involved in characterizing HTT homologue in <em>Drosophila</em>. By analyzing HTT knockout (KO) flies, I obtained in vivo evidence that HTT play a role in endolysosomal pathways orchestraed by Rab GTPases. In particular, I systematically tested HTT’s effect on the production of phosphatidylinositol phosphates, the sizes and maturation of endosomes and lysosomes such as lysosomal acidification, and the subcellular distribution of different endosomal markers. My findings support a critical role of HTT on endosome maturation, potentially by acting downstream of Rab5 and coordinating endosomal maturation, which in turn might regulating cellular cytoskeleton organization. Importantly, similar endosomal and cytoskeleton defects have also been observed in HTT-KO mammalian cell lines, supporting a conserved role of HTT in endosomal trafficking, a cellular homeostasis process already implicated in multiple neurodegenerative diseases.</p> <p>By elucidating the molecular mechanisms of normal HTT regulation on the endosomal pathway and the subsequent disruption caused by HTT depletion, we may identify new therapeutic targets to counter HD pathogenesis and reconsider the utility of ASO for reducing HTT levels.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation (PhD)

Year dc:date.available

2023

Authors dc:creator

• Yu, Yue
• <p>0000-0002-0860-6643</p>

Contributors dc:contributor

• Sheng Zhang, PhD
• Michael Beierlein, PhD
• Hugo Bellen, DVM, PhD

• Huntington's disease
• Huntingtin
• neurodegenerative disorder
• endosomal functions
• rab proteins
• Molecular and Cellular Neuroscience
• Nervous System Diseases

Repository record dc:identifier

https://digitalcommons.library.tmc.edu/utgsbs_dissertations/1307

OAI identifier oai:identifier

oai:digitalcommons.library.tmc.edu:utgsbs_dissertations-2364