Nucleolar stress induces nucleolar stress body formation 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 identify a subnuclear organelle named the nucleolar stress body (NoSB), the formation of which is induced by the inhibition of rRNA transcription or inactivation of rRNA processing and maturation in C. elegans. NoSB does not colocalize with other previously described subnuclear organelles. We conduct forward genetic screening and identify a 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 increases nosr-1 expression. By using transcriptome analysis of wild-type animals subjected to different nucleolar stress conditions and nosr-1 mutants, we identify 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 conclude 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.

rRNA intermediates coordinate the formation of nucleolar vacuoles in C. elegans.

The nucleolus is the most prominent membraneless organelle within the nucleus. How the nucleolar structure is regulated is poorly understood. Here, we identified two types of nucleoli in C. elegans. Type I nucleoli are spherical and do not have visible nucleolar vacuoles (NoVs), and rRNA transcription and processing factors are evenly distributed throughout the nucleolus. Type II nucleoli contain vacuoles, and rRNA transcription and processing factors exclusively accumulate in the periphery rim. The NoV contains nucleoplasmic proteins and is capable of exchanging contents with the nucleoplasm. The high-order structure of the nucleolus is dynamically regulated in C. elegans. Faithful rRNA processing is important to prohibit NoVs. The depletion of 27SA2 rRNA processing factors resulted in NoV formation. The inhibition of RNA polymerase I (RNAPI) transcription and depletion of two conserved nucleolar factors, nucleolin and fibrillarin, prohibits the formation of NoVs. This finding provides a mechanism to coordinate structure maintenance and gene expression.

H3K9me1/2 methylation limits the lifespan of daf-2 mutants in C. elegans.

Histone methylation plays crucial roles in the development, gene regulation, and maintenance of stem cell pluripotency in mammals. Recent work shows that histone methylation is associated with aging, yet the underlying mechanism remains unclear. In this work, we identified a class of putative histone 3 lysine 9 mono/dimethyltransferase genes (met-2, set-6, set-19, set-20, set-21, set-32, and set-33), mutations in which induce synergistic lifespan extension in the long-lived DAF-2 (insulin growth factor 1 [IGF-1] receptor) mutant in Caenorhabditis elegans. These putative histone methyltransferase plus daf-2 double mutants not only exhibited an average lifespan nearly three times that of wild-type animals and a maximal lifespan of approximately 100 days, but also significantly increased resistance to oxidative and heat stress. Synergistic lifespan extension depends on the transcription factor DAF-16 (FOXO). mRNA-seq experiments revealed that the mRNA levels of DAF-16 Class I genes, which are activated by DAF-16, were further elevated in the daf-2;set double mutants. Among these genes, tts-1, F35E8.7, ins-35, nhr-62, sod-3, asm-2, and Y39G8B.7 are required for the lifespan extension of the daf-2;set-21 double mutant. In addition, treating daf-2 animals with the H3K9me1/2 methyltransferase G9a inhibitor also extends lifespan and increases stress resistance. Therefore, investigation of DAF-2 and H3K9me1/2 deficiency-mediated synergistic longevity will contribute to a better understanding of the molecular mechanisms of aging and therapeutic applications.

Structural recognition of the mRNA 3' UTR by PUF-8 restricts the lifespan of C. elegans.

The molecular mechanisms of aging are unsolved fundamental biological questions. Caenorhabditis elegans is an ideal model organism for investigating aging. PUF-8, a PUF (Pumilio and FBF) protein in C. elegans, is crucial for germline development through binding with the 3' untranslated regions (3' UTR) in the target mRNAs. Recently, PUF-8 was reported to alter mitochondrial dynamics and mitophagy by regulating MFF-1, a mitochondrial fission factor, and subsequently regulated longevity. Here, we determined the crystal structure of the PUF domain of PUF-8 with an RNA substrate. Mutagenesis experiments were performed to alter PUF-8 recognition of its target mRNAs. Those mutations reduced the fertility and extended the lifespan of C. elegans. Deep sequencing of total mRNAs from wild-type and puf-8 mutant worms as well as in vivo RNA Crosslinking and Immunoprecipitation (CLIP) experiments identified six PUF-8 regulated genes, which contain at least one PUF-binding element (PBE) at the 3' UTR. One of the six genes, pqm-1, is crucial for lipid storage and aging process. Knockdown of pqm-1 could revert the lifespan extension of puf-8 mutant animals. We conclude that PUF-8 regulate the lifespan of C. elegans may not only via MFF but also via modulating pqm-1-related pathways.

Functional Proteomics Identifies a PICS Complex Required for piRNA Maturation and Chromosome Segregation.

piRNAs play significant roles in suppressing transposons and nonself nucleic acids, maintaining genome integrity, and defending against viral infections. In C. elegans, piRNA precursors are transcribed in the nucleus and are subjected to a number of processing and maturation steps. The biogenesis of piRNAs is not fully understood. We use functional proteomics in C. elegans and identify a piRNA biogenesis and chromosome segregation (PICS) complex. The PICS complex contains TOFU-6, PID-1, PICS-1, TOST-1, and ERH-2, which exhibit dynamic localization among different subcellular compartments. In the germlines, the PICS complex contains TOFU-6/PICS-1/ERH-2/PID-1, is largely concentrated at the perinuclear granule zone, and engages in piRNA processing. During embryogenesis, the TOFU-6/PICS-1/ERH-2/TOST-1 complex accumulates in the nucleus and plays essential roles in chromosome segregation. The functions of these factors in mediating chromosome segregation are independent of piRNA production. We speculate that differential compositions of PICS factors may help cells coordinate distinct cellular processes.

A Cytoplasmic Argonaute Protein Promotes the Inheritance of RNAi.

RNAi-elicited gene silencing is heritable and can persist for multiple generations after its initial induction in C. elegans. However, the mechanism by which parental-acquired trait-specific information from RNAi is inherited by the progenies is not fully understood. Here, we identified a cytoplasmic Argonaute protein, WAGO-4, necessary for the inheritance of RNAi. WAGO-4 exhibits asymmetrical translocation to the germline during early embryogenesis, accumulates at the perinuclear foci in the germline, and is required for the inheritance of exogenous RNAi targeting both germline- and soma-expressed genes. WAGO-4 binds to 22G-RNAs and their mRNA targets. Interestingly, WAGO-4-associated endogenous 22G-RNAs target the same cohort of germline genes as CSR-1 and contain untemplated addition of uracil at the 3' ends. The poly(U) polymerase CDE-1 is required for the untemplated uridylation of 22G-RNAs and inheritance of RNAi. Therefore, we conclude that, in addition to the nuclear RNAi pathway, the cytoplasmic RNAi machinery also promotes RNAi inheritance.