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 for amyotrophic lateral sclerosis (ALS). While the full-length NEK1 is involved in diverse cellular processes, such as DNA damage response and microtubule stability, the pathogenic mechanism of NEK1 nonsense mutations in ALS remains elusive. Here, we demonstrate that three truncated forms of NEK1 derived from ALS-related NEK1 nonsense mutations translocate from the cytoplasm to the nucleus, exhibit nucleolar localization, and simultaneously form liquid-like nucleoplasmic foci. In contrast to the diffuse cytoplasmic distribution of wild-type NEK1, these nuclear-localized truncated mutants are prone to undergo liquid-liquid phase separation both in cells and in vitro. Mechanistically, the truncated NEK1s interact with the nucleolar protein FBL, thereby impairing ribosomal RNA biogenesis and translation. Transgenic flies expressing truncated mutant NEK1s display motor dysfunction and reduced survival length, and a knock-in transgenic mouse model expressing ALS-related NEK1 mutant similarly exhibits motor deficits accompanied by ribosomal RNA dysregulation. These findings suggest that ALS-related NEK1 mutants expressing truncated forms of NEK1 cause cell toxicity by interfering with ribosomal RNA metabolism and reveal a gain-of-function mechanism in ALS pathogenesis involving NEK1.