Identifying mechanisms responsible for ubiquitylation of TAR DNA-binding protein 43 (TDP-43) in ALS pathogenesis.

The recently identified mutations in the CCNF gene are a novel cause of ALS/FTD. CCNF encodes Cyclin F, an E3 ubiquitin ligase that forms a part of a SCF complex that binds to proteins for ubiquitylation and degradation by the UPS. We focused on the p.Ser621Gly (S621G) missense mutation that segregated across multiple generations in an Australian family with ALS/FTD. Experimental expression of the CCNFS621G mutation led to defective protein degradation and signature features of ALS pathogenesis in vitro. We investigated the effect of the CCNFS621G mutation in neuronal cells and determined that the Lys48-specific ubiquitylation of protein substrates was elevated compared to the CCNFWT control. This was reflected in the expression of RRM2 (a known Cyclin F substrate) that was ubiquitylated mostly with Lys48-ubiquitin in CCNFS621G-transfected cells while RRM2 in the CCNFWT control lysates were ubiquitylated with both Lys48- and Lys63-ubiquitin. Additionally, we also identified that Ser621 as a phosphorylation site that when occupied by phosphate, reduced the Lys48-ubiquitylation activity of the SCF(Cyclin F) complex by ~0.74-fold (n=3, p<0.01) compared to the CCNFS621G mutant indicating that the Ser621 phosphorylation site is a key regulator of Cyclin F E3 ligase activity. It is postulated that the formation of TDP-43 aggregates is a key step in ALS pathogenesis despite the mechanisms being poorly understood. We have evidence to indicate that Cyclin F and TDP-43 associate as binding partners to form inclusions within neuronal cells. These findings highlight new mechanisms of Cyclin F for sustaining neuronal cell homeostasis.