Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.577932
Title: Synucleins in the midbrain dopaminergic system : the role in health and disease
Author: Connor-Robson, Natalie
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2013
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Abstract:
Synucleinopathies are a group of diseases characterised by the presence of insoluble aggregated forms of α-synuclein. The most common of these diseases is Parkinson’s disease (PD) which affects approximately 1% of the UK population over the age of 60. Alpha-synuclein has also been linked to the disease through familial mutations and genome wide association studies as well as by its presence in sporadic cases. Although solid evidence exists for a role of α-synuclein in PD, it remains unclear as to how this protein exerts its toxicity on neurons and exactly how this leads to the cell death characteristic of this neurodegenerative disease. Alpha-synuclein belongs to a family of three proteins which also includes β- and γ-synuclein. These three proteins are highly homologous and evolutionarily conserved, however none of them have a well defined function. Evidence suggests a role for these proteins in synaptic vesicle dynamics but a more specific function remains to be unveiled. However, due to the considerable degree of homology across these three proteins, knockout models have been considered to allow functional compensation of the missing synuclein protein through one of the remaining family members. This has hindered studies from elucidating not only the role of α-synuclein but also β- and γ-synuclein. To overcome this problem triple synuclein knockout mice have been produced and characterised, as described in this thesis. As expected studies of these animals revealed no alterations in the number of dopaminergic neurons in either the substantia nigra pars compacta or ventral tegmental area. Despite this, a significant deficit in striatal dopamine concentrations was detected, regardless of the fact that the levels and function of tyrosine hydroxylase being normal. As well as this triple synuclein null mice were demonstrated to be hyperdopaminergic through various behavioural tests. Work employing physcostimulants and, through a collaboration, using fast scan cyclic voltametry suggested a role for these proteins in normal dopamine release dynamics at the level of the synaptic vesicle. A previous body of work has indicated that the loss of α- and/or γ-synuclein is able to provide a degree of resistance against the toxic affects of the dopaminergic neurotoxin MPTP. It was therefore hypothesised that the triple synuclein null animals would also display resistance to this toxin. However, these animals were shown to be more sensitive than wild type controls. Importantly it was apparent that animals lacking β-synuclein alone or in combination with other synucleins were the most sensitive to this toxin. Further work revealed a significant deficit in the ability of triple synuclein null mice to store dopamine in their synaptic vesicles. This may explain the sensitivity to MPP+, the active metabolite of MPTP, due to the fact it cannot be efficiently stored in synaptic vesicles, which restricts the toxins access to the mitochondria where it normally inhibits complex I, thus leading to cell death. When recombinant β-synuclein was reintroduced the deficit in synaptic vesicle dopamine uptake could be restored. However, β-synuclein can not do this alone and requires incubation with cytosolic factors, suggesting it acts as a chaperone in this role. This may explain why lines of synuclein null mice that specifically have the absence of β-synuclein apparently fair least well when exposed to MPTP. Finally, in order to assess the extent to which a loss of function role of α-synuclein leads to pathological alteration at the synapse an entirely novel conditional α-synuclein knockout mouse model was produced. Currently no ideal model exists to answer this question as conventional knockout models are based on the knockout of the protein in development. This may allow functional compensatory mechanisms to be established which can be overcome with a conditional knockout approach. As well as this it is important to assess this loss in an aged nervous system, as PD is a disease of aging. It is likely that, as α-synuclein forms insoluble Lewy bodies and undergoes abnormal posttranslational modifications, the amount of normally functioning protein at the synapse is depleted, therefore allowing a loss of function effect to develop. It is hoped this model will allow new insight into the early disease process. Overall this work further contributes to a body of evidence that suggests the synucleins play an important role in synaptic dopamine handling, particularly at the synaptic vesicle level. It is hoped that the newly established conditional α-synuclein knockout model will produce a new perspective on the loss of function role of α-synuclein in early disease development, an avenue that has yet to be fully explored.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.577932  DOI: Not available
Keywords: QH301 Biology ; QH426 Genetics ; R Medicine (General)
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