Dysregulation of actin dynamics in amyotrophic lateral sclerosis

Mutations in the C9ORF72 (chromosome 9 open reading frame 72) gene account for 40% of familial cases of Amyotrophic lateral sclerosis (ALS), and pathological forms of TDP-43 in motor neurons are present in almost all cases of ALS. Currently, there is no effective treatment for this disorder. Therefore, given their importance in ALS, understanding the pathological roles of C9ORF72 and TDP-43 is crucial for developing effective therapeutic strategies. The mechanisms underlying neurodegeneration in ALS are still not fully understood. Whilst defects in cytoskeletal organisation and cytoskeletal proteins have been previously associated with ALS, the role of actin filaments and actin binding proteins in ALS has not been previously examined. The overall aim of the studies described in this thesis was to examine the regulation of actin dynamics and actin binding proteins in C9ORF72 and TDP-43 related ALS. Firstly, the normal cellular function of C9ORF72 was examined in Chapter 3. Here it was demonstrated that C9ORF72 is an actin binding protein that regulates actin dynamics. Furthermore, the uDENN domain of C9ORF72 is required for this activity via cofilin-mediated signalling mechanisms. These studies therefore provide novel insights into the normal cellular function of C9ORF72. Secondly, in Chapter 4, it was demonstrated that actin dynamics is disturbed by pathological forms of TDP-43 in cells expressing cytoplasmic TDP-43, in the TDP-43 rNLS mice model and in sporadic ALS (SALS) patient tissues. These studies therefore demonstrate that TDP-43 pathology is associated with increased actin polymerisation, possibly mediated by a direct interaction between actin and TDP-43. Moreover, actin polymerisation causes both mis-localisation of TDP-43 to the cytoplasm and stress granule formation, implying that actin polymerisation induces pathological events relevant to ALS. Finally, the regulation of actin-binding and regulatory proteins in the TDP-43 rNLS mice model and in SALS patients was examined in Chapter 5. Cofilin phosphorylation and profilin-1 expression was enhanced in SALS patients. Furthermore, increased levels of Rac1/cdc42 and Limk1 phosphorylation were also detected, thus providing mechanistic insights into these observations. Similarly, increased levels of profilin-1 and cofilin phosphorylation correlated with disease course pathologically and phenotypically in TDP-43 rNLS mice, consistent with the findings obtained in SALS patients. While actin related proteins; Arp2, Arp3 and ARPC3 were decreased at a later stage in TDP-43 rNLS mice. In conclusion, this thesis provides novel insights into the mechanisms of neurodegeneration in ALS. Importantly, it identifies dysregulation of actin dynamics and actin binding proteins as novel disease mechanisms in both C9ORF72 and TDP-43 associated ALS.