- Title
- The deregulation of Fyn kinase in Alzheimer's Disease
- Creator
- Taleski, Goce
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Sporadic Alzheimer's Disease (AD) is the prevailing form of dementia worldwide. Neuropathologically, this debilitating disorder is characterised by deposition of senile amyloid beta (Aβ) plaques and Taucontaining neurofibrillary tangles (NFTs). Aβ peptides are produced from the proteolytic processing of amyloid precursor protein (APP). There is strong experimental evidence that pathological accumulation of Aβ oligomers triggers a degenerative cascade mediated by Fyn kinase and abnormally phosphorylated Tau (p-Tau) proteins; this ultimately promotes synaptic and memory deficits in AD. The deregulation of Tau also prompts its self-aggregation into paired helical filaments that can aggregate further into NFTs. Significant clinical failures of Aβ-centric therapeutics for AD have recently shifted the industry towards development of Tau-targeting approaches that are currently under clinical investigation. However, the sole targeting of either Aβ or p-Tau may fail to address the underlying disease mechanisms, established risk factors, and complexity of this disorder. In an attempt to disentangle some of these issues, we considered the contribution of prevalent modifiable risk factors associated with AD, more specifically, low plasma folate levels and elevated plasma total homocysteine levels. Folate and homocysteine play a key role in one-carbon metabolism, a series of metabolic pathways that control cellular methylation potential. The integrity of one-carbon metabolism is critical for formation of methylation-dependent Protein Phosphatase 2A (PP2A) enzymes, which are major signalling mediators that become downregulated in AD-affected brain regions. Methylated PP2A holoenzymes are the predominant phosphatases that dephosphorylate p-Tau at many Ser/Thr epitopes. They also dephosphorylate APP phosphorylated at the Thr668 residue, which is linked to enhanced APP amyloidogenic processing. Here, we first demonstrated that both APP phosphorylation at Thr668 and Tau phosphorylation at AD-like epitopes are enhanced in various brain regions from mouse models with disturbances to one-carbon metabolism. These changes were associated with a concomitant reduction in methylated PP2A enzyme levels and increased activity of the Tau/APP kinase, GSK-3β. These findings support the involvement of altered one-carbon metabolism in AD pathogenic processes. Since Fyn kinase regulates both APP and Tau via Tyrosine (Tyr) phosphorylation, and becomes aberrantly activated in AD, we also deemed it important to address the impact of one-carbon metabolism and PP2A methylation on Fyn regulation. Our studies show that one-carbon metabolism and/or methylated PP2A enzymes regulate the distinct subcellular distribution of Fyn in neuronal cellular and ex vivo models. Fyn compartmentalisation was closely associated with its function in neurite outgrowth. Furthermore, we found that Fyn interacted with methylated PP2A enzymes, which may play a role in its targeting. In mice with altered one-carbon metabolism, the net activity and total protein expression levels of Fyn were dramatically altered in a brain region-specific manner; notably, Fyn was hyperactive in the cortex of these mice. Moreover, under conditions of increased Fyn signalling, we identified site-specific Tyr phosphorylation of PP2A catalytic subunit (PP2Ac), which could have downstream ramifications for both APP and Tau regulation. Tyr phosphorylation of PP2Ac influenced physiological Fyn targeting and Fyn-dependent neurite outgrowth in N2a neuroblastoma cells. Together, our findings support the existence of a reciprocal functional interaction between Fyn and PP2A. Lastly, we demonstrated that a methyl group donor-based intervention ameliorated behavioural impairments, improved spatial memory, and reduced brain p-Tau accumulation in an AD-like mouse model. The mechanisms of action of this intervention likely included Fyn downregulation and the upregulation of methylated PP2A enzymes. Thus, the collective experimental work presented in this thesis unveils novel regulatory mechanisms of significance to AD pathogenesis. Besides opening up new avenues for AD research, our findings should also guide the development of rational therapeutics for delaying AD onset and/or disease progression.
- Subject
- Alzheimer's Disease; Fyn kinase; dementia; disease progression
- Identifier
- http://hdl.handle.net/1959.13/1432915
- Identifier
- uon:39133
- Rights
- Copyright 2021 Goce Taleski
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 13 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 412 KB | Adobe Acrobat PDF | View Details Download |