Autosomal recessive spastic ataxia of Charlevoix Saguenay (ARSACS) is an early onset neurodegenerative disorder resulting from mutations in the SACS gene that encodes the protein sacsin. Sacsin is a 520kDa multi-domain protein localised at the cytosolic face of the outer mitochondrial membrane with suggested roles in proteostasis and most recently in the regulation of mitochondrial morphology. An excessively interconnected mitochondrial network was observed as a consequence of reduced levels of sacsin protein following SACS knockdown in neuroblastoma cells as well as in an ARSACS patient carrying the common Quebec homozygous SACS mutation 8844delT. Moreover, it was suggested that sacsin has a role in mitochondrial fission as it was found to interact with mitochondrial fission protein Dynamin related protein 1 (Drp1). The aim of this thesis was to explore sacsin’s role in the regulation of mitochondrial morphology and dynamics in non-Quebec ARSACS patients and sacsin knockdown fibroblasts. This study shows that loss of sacsin function promotes a more interconnected mitochondrial network in non-Quebec ARSACS patients and in sacsin knockdown fibroblasts. Moreover, recruitment of the essential mitochondrial fission protein Drp1 to the mitochondria was significantly reduced in ARSACS patient cells and in sacsin knockdown fibroblasts. This reduced recruitment of Drp1 to mitochondria also occurred when cells were treated to induce mitochondrial fission. Furthermore, both the size and intensity of Drp1 foci localised to the mitochondria were significantly reduced in both sacsin knockdown and patient fibroblasts. Finally, reduced ATP production, decreased respiratory capacity of mitochondria and an increase in mitochondrial reactive oxygen species demonstrated impaired mitochondrial function in ARSACS patient and sacsin knockdown fibroblasts. These results suggest a role for sacsin in the stabilisation or recruitment of cytoplasmic Drp1 to prospective sites of mitochondrial fission similar to that observed by other mitochondrial fission accessory proteins.

Sacsin, which is mutated in the neurodegenerative disease Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS), is a 520 kDa modular protein with regions of homology to molecular chaperones and domains linking to the ubiquitin proteasome system. This suggests a role in proteostasis. Previously, sacsin has been shown to partially localise with mitochondria, and loss of sacsin results in elongated and dysfunctional mitochondria. Moreover, alterations in neurofilaments have recently been reported in a mouse model of ARSACS. Despite these findings, pathophysiological mechanisms of ARSACS are poorly understood. The aim of this thesis was to elucidate the cellular role of sacsin by determining how loss of its function leads to the observed mitochondrial and intermediate filament defects. This hoped to shed light on the mechanism of disease in ARSACS. The results indicate that the mitochondrial elongation seen in ARSACS is likely due to reduced mitochondrial localisation of the essential fission factor DRP1. This may be mediated by loss of function of a complex involving sacsin and dynactin-6, a subunit of the dynein-dynactin motor complex, which has previously been shown to be required for DRP1 mitochondrial recruitment. DRP1-mediated mitochondrial fission is necessary for mitochondrial quality control; hence a disruption to mitochondrial quality control is likely to occur in sacsin deficient cells, which may explain the mitochondrial dysfunction in ARSACS. Furthermore, sacsin null cells display a dramatic collapse and perinuclear bundling of the vimentin intermediate filament network. This is coupled with the displacement of cellular organelles, particularly mitochondria, early endosomes and the Golgi, which accumulate at the periphery of the vimentin bundle. These are characteristic features of aggresome formation, indicating an aggregation of misfolded protein, which occurs due to disrupted proteostasis. Further supporting this, the proteostasis components ubiquitin, HSP70, LAMP2 and p62 are recruited to the perinuclear vimentin bundles. In summary, the findings of this thesis indicate a role for sacsin in mitochondrial and protein quality control, the dysfunction of which is likely to be particularly detrimental in neurons. Mitochondrial dysfunction along with protein misfolding and aggregation are implicated in many neurodegenerative diseases, and ARSACS is no exception.