Identification and characterization of a novel isoform of fused in sarcoma and its role in amyotrophic lateral sclerosis

The fused in sarcoma (FUS) gene is mutated in 4% of familial cases of Amyotrophic lateral sclerosis (ALS) and 1% of sporadic cases. Moreover, FUS immunoreactive inclusions are present in both ALS patients. Importantly, mutations in FUS account for a particularly aggressive, juvenile form of the disease. Hence, elucidating the pathological role of FUS in ALS is crucial for understanding the disease. While the mechanisms underlying neurodegeneration in ALS are not fully understood, defects in alternative splicing have been previously associated with ALS disease pathology. Additionally, alternative spliced forms of proteins linked to ALS, have also beenimplicated. This thesis focuses on the study of a newly identified, extracellular isoform of FUS, which was termed 'FUSEC'. FUSEC has a unique N-terminus sequence but shares its C-terminus with canonical FUS. For FUS, the most abundant ALS mutations are enriched at the C-terminus. Here it was confirmed FUSEC expression at the mRNA and protein level. Furthermore, it was demonstrated it is N-glycosylated mostly with complex N-glycans, and this regulates its secretion. Also it was studied its intracellular localization and its potential normal functions. Finally, the role of FUSEC in ALS was examined. FUSEC bearing ALS mutations demonstrated inclusion formation, induction of ER stress and Golgi fragmentation and finally apoptosis, key cellular features of ALS. The discovery of a new alternatively spliced isoform of FUS provides novel insights into the pathogenic mechanisms implicated in ALS.