Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Neuroscience

Supervisor

Prado, Marco A.M.

2nd Supervisor

Prado, Vania F.

Co-Supervisor

Abstract

In neurodegenerative diseases, certain proteins misfold and form toxic aggregates that cause brain matter atrophy, leading to decline in motor and/or cognitive functions. To maintain cellular proteostasis and survival, molecular chaperones regulate protein maturation and help to prevent aberrant protein aggregation. The molecular chaperone Hsp90 regulates hundreds of proteins and interestingly, several of those are misfolded in neurodegenerative diseases. Stress inducible-phosphoprotein-1 (STI1, STIP1), an Hsp90 co-chaperone, orchestrates client protein transfer between chaperones Hsp70 and Hsp90 through physical interactions with both chaperones. Notably, previous work in yeast, worms, and mouse neurons all showed that STI1 protects organisms against stressors and amyloid-like proteotoxicity in vitro. However, the physiological roles of STI1 during aging, and whether STI1 can modulate proteotoxicity and aggregation in mammals is unknown. In this dissertation, we explore whether decreased or increased STI1 levels in mice, can modulate aging and proteostasis responses to misfolded protein stress. Our hypothesis is that modifying intracellular and extracellular levels of STI1, in vivo, affects neuronal resilience during aging, disturbs Hsp90 chaperone machinery function, and modulates levels of protein misfolding and aggregation.

We reveal that STI1 knockdown in mice reduces Hsp90 machinery function, and that mice present with age-dependent decline in neuronal resilience in the hippocampus, resulting in memory impairments. Unexpectedly, we find that overexpressing STI1 in an AD mouse model accelerates insoluble Aβ aggregation, resulting in greater levels of neurodegeneration and cognitive impairments. Likewise, STI1 overexpression augments α-synuclein accumulation in a mouse model of synucleinopathy, however, knocking down STI1 attenuated α-synuclein aggregation, improving motor performance. Notably, in both models of proteinopathies, STI1 colocalized with protein aggregates, likely modulating STI1 functions. Since our results suggest that reducing STI1 would be favourable for decreasing protein aggregation, we generated STI1 conditional knockdown mice, to establish whether reducing STI1 after development is tolerable, and indeed, we found that to be the case. Overall, in this thesis we provide a greater understanding of mammalian STI1 in neuronal resilience and protein misfolding in vivo and discover STI1 as a potential therapeutic target for treating protein misfolding diseases in the brain.

Summary for Lay Audience

In the brains of Alzheimer’s and Lewy Body Dementia patients, proteins such as Aβ and α-synuclein acquire an improper shape, causing them to clump together and kill brain cells. This cell loss results in memory impairments and motor difficulties, leading to poor quality of life, and eventually death. Notably, during aging there is a decline in the efficiency of cellular quality control systems, making this an important process to study for possible therapeutic interventions in protein misfolding diseases. Hsp90, a major player in this machinery, is regulated by the protein STI1, and STI1 works with Hsp90 to fold and refold proteins. Interestingly, STI1 also protects brain cell cultures against toxic proteins, however, it is unknown what roles STI1 plays in brain aging and in preventing harmful protein clumping in mammalian models. This thesis addresses whether changing the amounts of STI1, using genetic techniques, will protect nerve cells from dying, and reduce harmful protein clumping in the brain.

We find that genetic reduction of STI1, in mice, impairs the quality control machinery, and causes brain cell loss, and memory impairments if it occurs during embryonic development. Since STI1 seems to safeguard brain cells, we tested whether overexpressing STI1 would be protective against toxic protein accumulation in a mouse model of Alzheimer’s disease. Contrary to our predictions, mice with more STI1 had significantly more protein aggregates in their brains, which led to greater amounts of brain cell loss, and cognitive impairments. We followed up these studies by increasing or decreasing STI1 in mouse models of synucleinopathies. Similarly, synucleinopathy mice with more STI1, had more harmful misfolded proteins, whereas mice with less STI1 had less aggregates. Since decreasing STI1 throughout life had negative consequences for aging but reduced malevolent protein clumping in models of neurodegenerative diseases, we tested whether decreasing STI1 during adulthood would impact behaviour and cognition. Reduction in STI1 during adulthood was not detrimental, indicating that targeting STI1 to treat protein clumping in neurodegenerative diseases would be possible. The knowledge gained from my research reveals the STI1-Hsp90 machinery as a potential therapeutic target, which could be useful in preventing or slowing neurodegenerative disease progression.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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