Abstract
Dementia is a devastating health, economic, and social problem in New Zealand and globally. The growing burden of dementia is exacerbated by limited understanding of its pathogenesis and a scarcity of preventative and therapeutic strategies. In particular, little is known about the underlying diseases that result in young-onset dementia (YOD; onset before 65 years of age), despite the unique and considerable burdens faced by these patients and their families. Tauopathies are a collection of neurodegenerative disorders characterised by the accumulation of pathological tau protein in the brain, which are the leading cause of dementia, irrespective of age of onset. Depending on the prominent neuropathologies, tauopathies are classed as whether pathological tau is the primary or secondary major disease hallmark. Recent evidence has implicated altered arginine metabolism in the pathogenesis of late-onset Alzheimer’s disease (LOAD), a secondary tauopathy. The key metabolic pathways of the amino acid L-arginine include nitric oxide (NO) production, the polyamine system, agmatine synthesis, the glutamine-glutamate cycle, and its interrelated urea cycle. Therefore, arginine metabolism is significantly involved in key neural processes such as cerebrovascular function, synaptic plasticity, microtubule stabilisation, neurotransmission, and ammonia detoxification. Tauopathy mouse models also demonstrate changes in aspects of arginine metabolism, suggesting these metabolic pathways could be a common alteration in tauopathies. This thesis aimed to further explore how arginine metabolism is altered in tauopathies, through the investigation of L-arginine metabolites and their interrelated metabolic pathways in the leading causes of YOD, early-onset Alzheimer’s disease (EOAD) and frontotemporal dementia (FTD), along with the PS19 tauopathy model.
Given the altered arginine metabolism in the LOAD brain, Experiment 1 systematically investigated L-arginine metabolites and related enzymes in post-mortem regional brain tissue from EOAD cases to assess if these changes are common to Alzheimer’s disease irrespective of age of disease-onset. L-ornithine levels were drastically reduced across the EOAD brain, along with putrescine. Furthermore, constitutive NO production was impaired in the EOAD brain in addition to alterations in the polyamine system, particularly the upregulation of the direct polyamine retroconversion pathway. Alongside elevated urea in the EOAD brain, several urea cycle enzymes were also upregulated. These findings demonstrate arginine metabolism, particularly the NO pathway, polyamine system, and urea cycle are altered in the EOAD brain.
To explore whether arginine metabolism is affected in wider tauopathies, Experiment 2 systematically investigated L-arginine metabolites and related enzymes in post-mortem regional brain tissue from FTD cases. Similar to EOAD, reduced L-ornithine and putrescine were the major L-arginine metabolite changes in the FTD brain, as well as downregulated constitutive NO production, alterations in the polyamine system synthetic and retroconversion pathways, upregulation of urea cycle enzymes and increased urea levels. Intriguingly, arginase II upregulation was also notable in the FTD brain, in contrast to its product L-ornithine. FTD is comprised of cases with underlying tau (primary tauopathies) or transactive response DNA-binding protein of 43 kDa (TDP-43) proteinopathies. Therefore, this experiment carried out a preliminary comparison between these FTD subtypes, which had a similar L-arginine metabolic profile, with greater disruption of the glutamine-glutamate cycle and polyamine retroconversion pathways in the TDP-43 cases and the arginase pathway in the tau cases. These findings demonstrate arginine metabolism is altered in the FTD brain irrespective of the underlying tau or TDP-43 proteinopathy.
As the post-mortem tissue used in the aforementioned experiments came from tauopathy cases at the end disease-stage, Experiment 3 carried out a time-course study with the PS19 tauopathy model to investigate how arginine metabolism changes in the brain and plasma with the development of pathological tau-driven neurodegeneration. Starting at a young age, PS19 mice displayed increased L-ornithine and putrescine levels in the brain, alongside upregulation of the arginine-citrulline cycle and arginase-ornithine decarboxylase pathway. Further dysregulation of the polyamine system was found progressively, with notably increased spermidine and decreased spermine alongside their associated synthetic and retroconversion pathways. Additionally, the brain glutamine-glutamate cycle was shifted to favour glutamine production and constitutive NO production was impaired in the oldest PS19 mice. Fewer changes were seen in the plasma, with age-dependent alterations in spermine, and at the oldest age increased glutamine but reduced urea levels. These findings demonstrate alterations in these arginine metabolic pathways during the accumulation of pathological tau and development of neurodegeneration.
This thesis has made a significant contribution to the understanding of the underlying processes in tauopathies, that for the first time, demonstrates arginine metabolism is altered in EOAD, FTD, and consequently YOD and primary tauopathies. These findings indicate altered arginine metabolism could serve as a common pathological mechanism in the pathogenesis of tauopathies or even neurodegenerative diseases as whole. Taken together with the PS19 tauopathy model time-course, alterations in arginine metabolism could be early events in the development of tauopathies and therefore are potentially significant targets for the development of improved diagnostic biomarkers and curative therapies for these devastating diseases.