Transportin 1 colocalization with Fused in Sarcoma (FUS) inclusions is not characteristic for amyotrophic lateral sclerosis‐FUS confirming disrupted nuclear import of mutant FUS and distinguishing it from frontotemporal lobar degeneration with FUS inclusions

C. Troakes, T. Hortobágyi, C. Vance, S. Al‐Sarraj, B. Rogelj and C. E. Shaw (2013) Neuropathology and Applied Neurobiology 39, 553–561 Transportin 1 colocalization with Fused in Sarcoma (FUS) inclusions is not characteristic for amyotrophic lateral sclerosis‐FUS confirming disrupted nuclear import of mutant FUS and distinguishing it from frontotemporal lobar degeneration with FUS inclusions Aims: Transportin 1 (TNPO 1) is an abundant component of the Fused in Sarcoma (FUS)‐immunopositive inclusions seen in a subgroup of frontotemporal lobar degeneration (FTLD‐FUS). TNPO 1 has been shown to bind to the C‐terminal nuclear localizing signal (NLS) of FUS and mediate its nuclear import. Amyotrophic lateral sclerosis (ALS)‐linked C‐terminal mutants disrupt TNPO 1 binding to the NLS and impair nuclear import in cell culture. If this held true for human ALS then we predicted that FUS inclusions in patients with C‐terminal FUS mutations would not colocalize with TNPO 1. Methods: Expression of TNPO 1 and colocalization with FUS was studied in the frontal cortex of FTLD‐FUS (n = 3) and brain and spinal cord of ALS‐FUS (n = 3), ALS‐C9orf72 (n = 3), sporadic ALS (n = 7) and controls (n = 7). Expression levels and detergent solubility of TNPO 1 was measured by Western blot. Results: Aggregates of TNPO 1 were abundant and colocalized with FUS inclusions in the cortex of all FTLD‐FUS cases. In contrast, no TNPO 1‐positive aggregates or FUS colocalization was evident in two‐thirds, ALS‐FUS cases and was rare in one ALS‐FUS case. Nor were they present in C9orf72 or sporadic ALS. No increase in the levels of TNPO 1 was seen in Western blots of spinal cord tissues from all ALS cases compared with controls. Conclusions: These findings confirm that C‐terminal FUS mutations prevent TNPO 1 binding to the NLS, inhibiting nuclear import and promoting cytoplasmic aggregation. The presence of TNPO 1 in wild‐type FUS aggregates in FTLD‐FUS distinguishes the two pathologies and implicates different disease mechanisms.