Investigating olfactory deficits as a preclinical marker of Parkinson's disease

Parkinson’s disease is the fastest growing neurological condition in the world, expected to affect more than 12 million people globally by 2040. Despite centuries of ground-breaking research, the diagnosis of Parkinson’s disease is unreliable and there are no disease-modifying therapeutics. This may be attributable to a focus on the movement disorders associated with disease, which present only after substantial neurodegeneration has occurred. This results in a late, inaccurate diagnosis which is problematic for the development and utilisation of neuroprotective therapies which need to be administered early. Although there has been recognition of non-motor aspects of Parkinson’s disease as early as 1817 by James Parkinson himself, there is no standardised utility of non-motor symptoms in the identification and monitoring of disease. The concurrence of non-motor symptoms in early Parkinson’s disease has great promise in identifying people at risk of developing the clinical disease, especially in those with REM sleep behaviour disorder, however these assays still require further development. The most prevalent non-motor symptom is a loss in the sense of smell (hyposmia) and understanding the pathobiology underlying this phenomenon may help to inform future diagnostic approaches through the development of novel biomarkers of disease. As such, the aim of this thesis was to test the utility of concurrent non-motor symptoms in a clinical setting, as well as investigate the underlying biology of the olfactory system in Parkinson’s disease using post-mortem human olfactory bulbs and tau knockout mice. The concurrence of non-motor symptoms was investigated in a cohort of participants presenting with multiple non-motor symptoms, including REM sleep behaviour disorder, hyposmia, and anxiety. Upon positron emission tomography scanning, it was demonstrated that the participants with concurrent non-motor symptoms had reduced vesicular monoamine transporter 2 binding in the caudate nucleus and the putamen in a degenerative pattern like that of Parkinson’s disease. Although these findings demonstrate the utility of non-motor symptoms to identify people with early nigrostriatal degeneration, understanding the underlying pathological changes will allow the development of more sensitive diagnostic tools. As such, human post-mortem olfactory bulbs from subjects with pathologically confirmed Parkinson’s disease and neurological controls were examined for changes in systems associated with Parkinson's disease, including protein clearance, the dopaminergic system, and metal homeostasis. The accumulation of monomeric alpha-synuclein was observed within the bulbs as determined by immunoblot and there were protein changes indicative of an environment under oxidative stress. Characterisation of the dopaminergic system demonstrated perturbations in catechol-O-methyltransferase-mediated dopamine breakdown. Finally, there was a loss of metal homeostasis as demonstrated by an accumulation of key metals, including iron, sodium, zinc, and lead. These findings warranted further investigation in an in vivo model. Tau knockout mice have been reported as an age-dependent model of Parkinson’s disease, and the next study of this thesis demonstrated that these mice develop an olfactory impairment at seven months of age, approximately five months before they develop a motor impairment, aligned with the presentation of symptoms in Parkinson’s disease. The mechanism(s) underlying the olfactory deficits of these animals was characterised to determine the congruency between the mice and the post-mortem tissue. Firstly, it was found that this olfactory deficit correlates with an accumulation of alpha-synuclein and autophagic impairment, in the olfactory bulb. Similar to the human bulbs, young tau knockout mice appear to have a deficit in catechol-O-methyltransferase dopamine breakdown as well as an accumulation of bulbar metals. These findings were further implicated in the functional olfactory deficit as treatment with the dopamine 2 receptor antagonist haloperidol, and the iron/copper chelator PBT434, resulted in a rescue of the hyposmic phenotype in young tau knockout mice. These findings implicate a loss of tau function as a contributor to disease pathways. This was further investigated using the mutant tau overexpressing mice, the rTg4510s, and like the tau knockout mice these animals demonstrate an age-dependent hyposmia. Together these studies have enabled the identification of potential mechanisms of olfactory impairment in Parkinson’s disease that warrant further scrutiny, including dopamine mis-metabolism and metal accumulation. Findings in the tau knockout mice add face validity to the model and support the utility of this animal in non-motor studies. These findings may help aid in the development of novel diagnostic approaches, as well as patient stratification strategies to overcome the current hurdles in clinical trials of neuroprotective therapies in Parkinson’s disease.