Valosin-containing protein (VCP) is a hexameric ATPase associated with diverse cellular activities. Genetic mutations in VCP are associated with several forms of muscular and neuronal degeneration, including amyotrophic lateral sclerosis (ALS). Moreover, VCP mediates UV-induced proteolysis of RNA polymerase II (RNAPII), but little is known about the effects of VCP mutations on the transcriptional machinery. Here, we used silica particle-assisted chromatin enrichment and mass spectrometry to study proteins co-localized with RNAPII in precursor neurons differentiated from VCP-mutant or control induced pluripotent stem cells. Remarkably, we observed diminished RNAPII binding of proteins involved in transcription elongation and mRNA splicing in mutant cells. One of these is SART3, a recycling factor of the splicing machinery, whose knockdown leads to perturbed intron retention in several ALS-associated genes. Additional reduced proteins are RBM45, EIF5A and RNF220, mutations in which are associated with various neurodegenerative disorders and are linked to TDP-43 aggregation. Conversely, we observed increased RNAPII binding of heat shock proteins such as HSPB1. Together, these findings shed light on how transcription and splicing machinery are impaired by VCP mutations, which might contribute to aberrant alternative splicing and proteinopathy in neurodegeneration.

Mutations causing amyotrophic lateral sclerosis (ALS) clearly implicate ubiquitously expressed and predominantly nuclear RNA binding proteins, which form pathological cytoplasmic inclusions in this context. However, the possibility that wild-type RNA binding proteins mislocalize without necessarily becoming constituents of cytoplasmic inclusions themselves remains relatively unexplored. We hypothesized that nuclear-to-cytoplasmic mislocalization of the RNA binding protein fused in sarcoma (FUS), in an unaggregated state, may occur more widely in ALS than previously recognized. To address this hypothesis, we analysed motor neurons from a human ALS induced-pluripotent stem cell model caused by the VCP mutation. Additionally, we examined mouse transgenic models and post-mortem tissue from human sporadic ALS cases. We report nuclear-to-cytoplasmic mislocalization of FUS in both VCP-mutation related ALS and, crucially, in sporadic ALS spinal cord tissue from multiple cases. Furthermore, we provide evidence that FUS protein binds to an aberrantly retained intron within the SFPQ transcript, which is exported from the nucleus into the cytoplasm. Collectively, these data support a model for ALS pathogenesis whereby aberrant intron retention in SFPQ transcripts contributes to FUS mislocalization through their direct interaction and nuclear export. In summary, we report widespread mislocalization of the FUS protein in ALS and propose a putative underlying mechanism for this process.