A Genetic Cascade of let-7-ncl-1-fib-1 Modulates Nucleolar Size and rRNA Pool in Caenorhabditis elegans

Abstract

Ribosome biogenesis takes place in the nucleolus, the size of which is often coordinated with cell growth and development. However, how metazoans control nucleolar size remains largely unknown. Caenorhabditis elegans provides a good model to address this question owing to distinct tissue distribution of nucleolar sizes and a mutant, ncl-1, which exhibits larger nucleoli than wild-type worms. Here, through a series of loss-of-function analyses, we report that the nucleolar size is regulated by a circuitry composed of microRNA let-7, translation repressor NCL-1, and a major nucleolar pre-rRNA processing protein FIB-1/fibrillarin. In cooperation with RNA binding proteins PUF and NOS, NCL-1 suppressed the translation of FIB-1/fibrillarin, while let-7 targeted the 3’UTR of ncl-1 and inhibited its expression. Consequently, the abundance of FIB-1 is tightly controlled and correlated with the nucleolar size. Together, our findings highlight a novel genetic cascade by which post-transcriptional regulators interplay in developmental control of nucleolar size and function.

Cell Biology of theCaenorhabditis elegansNucleus

“…Downregulation of ncl-1 results not only in enlarged nucleoli but also in enlarged cell and body size (Frank and Roth 1998), suggesting that NCL-1 may be a key regulator in determining cell and organismal growth. The expression of ncl-1 is regulated by the let-7 miRNA (Yi et al 2015), a heterochronic gene encoding a 21-base miRNA needed for correct temporal regulation of development (Reinhart et al 2000, reviewed in Moss 2007. NCL-1, in turn, regulates the C. elegans fib-1, which codes for the C. elegans homolog of fibrillarin involved in pre-rRNA processing.…”

“…NCL-1, in turn, regulates the C. elegans fib-1, which codes for the C. elegans homolog of fibrillarin involved in pre-rRNA processing. The levels of FIB-1 affect nucleolar size (Yi et al 2015). Thus, in C. elegans, nucleolar size appears to be regulated, at least in part, through pathways that oversee the temporal execution of development.…”

Nucleolar control by a non‐apoptotic p53‐caspases‐deubiquitinylase axis promotes resistance to bacterial infection

“…30,31 To screen for an additional suppressor of nucleolar size in C. elegans, we employed a transgenic worm strain, ncl-1(e1942);cguIs1[Pfib-1::fib-1::gfp], that expresses ectopic fibrillarin fused with GFP as a reporter in the ncl-1 mutant background. 30,31 To screen for an additional suppressor of nucleolar size in C. elegans, we employed a transgenic worm strain, ncl-1(e1942);cguIs1[Pfib-1::fib-1::gfp], that expresses ectopic fibrillarin fused with GFP as a reporter in the ncl-1 mutant background.…”

“…31 In this context, any changes in the nucleolar structure are presumably independent of NCL-1. Since the amount of FIB-1 is correlated with the size of the nucleolus, 31 this increase in the FIB-1::GFP reporter expression consistently observed in both genetic contexts of cep-1 loss, even in the absence of NCL-1, is highly suggestive of a NCL-1-independent role of p53/CEP-1 in nucleolar size regulation. Intriguingly, knockdown of cep-1 expression in this transgenic worm via RNA interference (RNAi) feeding resulted in further elevation of FIB-1::GFP reporter protein levels.…”

“…The dominant role of NCL-1 in nucleolar suppression has been demonstrated through phenotypic characterization of the ncl-1 mutant, which exhibits gain or enlargement of the nucleolar structure. 30,31 To screen for an additional suppressor of nucleolar size in C. elegans, we employed a transgenic worm strain, ncl-1(e1942);cguIs1[Pfib-1::fib-1::gfp], that expresses ectopic fibrillarin fused with GFP as a reporter in the ncl-1 mutant background. 31 In this context, any changes in the nucleolar structure are presumably independent of NCL-1.…”