Inhibition of Nucleolar Proteins in <italic>Caenorhabditis Elegans</Italic> Confers Enhanced Resistance to <italic>Salmonella Enterica</Italic> through a P53/<Italic>cep-1</Italic>-Dependent Mechanism
The relatively simple innate immune system of <italic>Caenorhabditis elegans</italic> and the number of traits that facilitate genetic and genomic analysis using this organism have nurtured rapid advances into the understanding of <italic>C. elegans</italic> innate immunity during the last few years. However, traditional methods of isolating and mapping <italic>C. elegans</italic> mutants exhibiting aberrant immune responses to pathogen infection are often labor intensive and time consuming. Therefore, a simple and rapid means of isolating and mapping <italic>C. elegans</italic> immune mutants will increase the number of mutants that can be studied. <italic>Salmonella enterica</italic>, as well as other bacterial pathogens, has been described to cause a significant distension of the <italic>C. elegans</italic> intestinal lumen, which correlates with death of the nematode. <italic>C. elegans</italic> mutants which exhibit a weakened immune response would therefore be expected to develop intestinal distension at an earlier time point than wild type. Likewise, mutants which exhibit an enhanced immune response would be expected to develop intestinal distension at a later time point than wild type. Taking advantage of this correlation, we designed a novel approach to isolating <italic>C. elegans</italic> mutants which exhibit aberrant immune responses to the bacterial pathogen, <italic>S. enterica</italic>. Furthermore, we validated and optimized the use of Amplifluor®, a high-throughput genotyping system, for use in <italic>C. elegans</italic> single nucleotide polymorphism (SNP) mapping.
To date, the only known negative regulators of innate immunity in <italic>C. elegans</italic> are dependent on the FOXO transcription factor, DAF-16 and regulate lifespan in addition to immunity. Therefore, we focused our efforts on identifying additional negative regulators of innate immunity by screening for mutants which display a reduced accumulation of <italic>S. enterica</italic> at a time point when wild-type nematodes are packed with bacteria. In a genetic screen for <italic>C. elegans</italic> mutants which display reduced accumulation of <italic>S. enterica</italic>/GFP, we identified a mutation in <italic>nol-6</italic>, a nucleolar protein containing a nucleolar RNA-associated protein (Nrap) domain which is conserved across eukaryotic organisms. <italic>nol-6</italic> is implicated in ribosomal RNA (rRNA) processing during the early stages of ribosome biogenesis. We show that knockdown of <italic>nol-6</italic> as well as other nucleolar genes leads to a reduction of pathogen accumulation and enhanced resistance to killing by pathogen. In addition, we demonstrate that enhanced resistance is dependent on p53/<italic>cep-1</italic>. Furthermore, microarray analysis shows a significant enrichment of upregulated genes that have previously been shown to be dependent on p53/<italic>cep-1</italic> for induction following ultraviolet radiation. These results represent the first evidence that <italic>C. elegans</italic> innate immunity is regulated by the nucleolus through a p53/<italic>cep-1</italic>-dependent mechanism.
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