Nucleolar stress induced the formation of a nucleolar stress body via the NOSR-1/NUMR-1 axis in Caenorhabditis elegans

Environmental stimuli not only alter gene expression profiles but also induce structural changes in cells. How distinct nuclear bodies respond to cellular stress is poorly understood. Here, we identified a new subnuclear organelle named the nucleolar stress body (NoSB), the formation of which was induced by the inhibition of rRNA transcription or inactivation of rRNA processing and maturation in C. elegans. NoSB did not colocalize with other previously described subnuclear organelles. We conducted forward genetic screening and identified a new bZIP transcription factor, named nucleolar stress response-1 (NOSR-1), that is required for NoSB formation. The inhibition of rRNA transcription or inactivation of rRNA processing and maturation increased nosr-1 expression. By using transcriptome analysis of wild-type animals subjected to different nucleolar stress conditions and nosr-1 mutants, we identified that the SR-like protein NUMR-1 (nuclear localized metal responsive) is the target of NOSR-1. Interestingly, NUMR-1 is a component of NoSB and itself per se is required for the formation of NoSB. We concluded that the NOSR-1/NUMR-1 axis likely responds to nucleolar stress and mediates downstream stress-responsive transcription programs and subnuclear morphology alterations in C. elegans.