Mutated FUS in familial amyotrophic lateral sclerosis involves multiple hnRNPs in the formation of neuronal cytoplasmic inclusions.

Fused in sarcoma (FUS), coded by FUS, is a heterogeneous nuclear ribonucleoprotein (hnRNP). FUS mutations are among the major mutations in familial amyotrophic lateral sclerosis (ALS-FUS: ALS6). The pathological hallmarks of ALS-FUS are FUS-positive neuronal cytoplasmic inclusions (NCI). We examined various hnRNPs in FUS NCIs in the hippocampus in ALS-FUS cases with different FUS mutations (Case 1, H517P; Case 2, R521C). We also examined TDP43-positive NCIs in sporadic ALS hippocampi. Immunohistochemistry was performed using primary antibodies against FUS, p-TDP43, TDP43, hnRNPA1, hnRNPD, PCBP1, PCBP2, and p62. Numerous FUS inclusions were found in the hippocampal granule and pyramidal cell layers. Double immunofluorescence revealed colocalization of FUS and p-TDP43, and FUS and PCBP2 (p-TDP43/FUS: 64.3%, PCBP2/FUS: 23.9%). Colocalization of FUS and PCBP1, however, was rare (PCBP1/FUS: 7.6%). In the hippocampi of patients with sporadic ALS, no colocalization was observed between TDP43-positive inclusions and other hnRNPs. This is the first study to show that FUS inclusions colocalize with other hnRNPs, such as TDP43, PCBP2, and PCBP1. These findings suggest that in ALS-FUS, FUS inclusions are the initiators, followed by alterations of multiple other hnRNPs, resulting in impaired RNA metabolism.