Nuclear condensates formed by truncated mutant NEK1s impede ribosomal RNA biogenesis and drive motor dysfunction

NIMA-related kinase 1 (NEK1), a serine/threonine kinase, is a risk variant in amyotrophic lateral sclerosis (ALS). While the full-length NEK1 protein is involved in various cellular processes, such as DNA damage response and microtubule stability, the pathological role of truncated mutant NEK1s in ALS remains elusive. Here, we demonstrated that three truncated forms of NEK1 derived from ALS-related NEK1 nonsense mutations translocated from cytoplasm to the nucleus, exhibited obvious enrichment in the nucleoli, and formed liquid-like nucleoplasmic foci, which are prone to undergo liquid-liquid phase separation in cells and in vitro. Mechanistically, the truncated NEK1s interacted with numerous nucleolar proteins, thereby impairing ribosomal RNA biogenesis and translation. Transgenic flies expressing truncated mutant NEK1s displayed motor dysfunction and reduced survival length. These findings offer new insight in which truncated mutant NEK1s cause cell toxicity via interfering with the phase behavior of nucleolar protein FBL and provide a gain-of-function mechanism in ALS pathogenesis involving NEK1.