Characteristic Features of FUS Inclusions in Spinal Motor Neurons of Sporadic Amyotrophic Lateral Sclerosis

Alterations of RNA metabolism caused by mutations in RNA-binding protein genes, such as transactivating DNA-binding protein-43 (TDP-43) and fused in sarcoma (FUS), have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Unlike the accumulation of TDP43, which is accepted as a pathological hall mark of sporadic ALS (sALS), FUS pathology in sALS is still under debate. Although immunoreactive inclusions of FUS have been detected in sALS patients previously, the technical limitation of signal detection, including the necessity of specific antigen retrieval, restricts our understanding of FUS-associated ALS pathology. In this study, we applied a novel detection method using a conventional antigen retrieval technique with Sudan Black B treatment to identify FUS-positive inclusions in sALS patients. We classified pathological motor neurons into 5 different categories according to the different aggregation characteristics of FUS and TDP-43. Although the granular type was more dominant for inclusions with TDP-43, the skein-like type was more often observed in FUS-positive inclusions, suggesting that these 2 proteins undergo independent aggregation processes. Moreover, neurons harboring FUS-positive inclusions demonstrated substantially reduced expression levels of dynactin-1, a retrograde motor protein, indicating that perturbation of nucleocytoplasmic transport is associated with the formation of cytoplasmic inclusions of FUS in sALS.