The neurotoxic effects of Manganese and the novel protective role of Drp1 in the autophagy pathway

Abstract

The precise pathological mechanism of Manganese (Mn) is not yet known however prior evidence suggests a prominent role of mitochondrial dysfunction. Relevant to Parkinson’s disease, Mn is known to bind to α-synuclein and increase its aggregation. Autophagy is also implicated in neurodegeneration however little is known about the effects of Mn on this pathway. The aim of this thesis was to investigate the effects of Mn on autophagy, α-synuclein aggregation and mitochondrial function in multiple paradigms. The effects of inhibiting the mitochondrial fission protein Drp1 were also assessed in these cellular contexts following Mn exposure. Acute exposure to sub-lethal Mn concentrations, in vitro and in vivo, caused an inhibition of autophagy flux in the absence of mitochondrial dysfunction. Despite this, Drp1 inhibition improved Mn-induced autophagy impairment and reduced cell death. In a cellular model of α-synuclein overexpression, in addition to the effect on autophagy, Mn also potentiated α-synuclein aggregation; rescued by Drp1 inhibition. This was observed at a later timepoint, suggesting that α-synuclein aggregation could be a consequence of impaired autophagy. The effects of chronic Mn accumulation on neurodegenerative pathology were assessed in six month old transgenic SLC39A14 knockout mice. Accumulated levels of the autophagy markers, LC3 and p62, were identified in both DAergic and GABAergic neurons, indicative of impaired autophagy. These neurons exhibited normal mitochondrial morphology, corroborating results from previous chapters. No alterations in striatal TH density or microglial activation within the substantia nigra were observed at this timepoint, however increased astrocyte activation was identified. This study is the first to examine the effects of Mn exposure on mitochondrial function, autophagy flux and α-synuclein aggregation concurrently. Results demonstrate that the autophagy pathway is primarily affected, followed by α-synuclein aggregation whereas mitochondrial function remains unaffected. Despite this, the inhibition of Drp1 ameliorated the effects of Mn on these pathways. The discovery that the pathogenic and protective mechanisms of Mn and Drp1 inhibition, respectively, intersect at the autophagic pathway is novel and suggestive of a therapeutic target.