Principal among the immune regulators in C. elegans
is the NSY-1/SEK-1/PMK-1 MAP kinase pathway, which was identified in a forward genetic screen for mutants with enhanced susceptibility to infection with the Gram-negative bacteria Pseudomonas aeruginosa [18
]. This pathway is orthologous to the ASK1 (MAP kinase kinase kinase)/MKK3/6 (MAP kinase kinase)/p38 (MAP kinase) pathway in mammals, and its identification in C. elegans
provided an important clue about the evolutionary origins of innate immunity. Activation of this signaling cassette is complex. An ortholog of mammalian SARM called Toll-interleukin-1 receptor (TIR-1) [19
], and the protein kinases Cδ (PKCδ) [23
] and D (PKD) [24
] act upstream of NSY-1. A recent study found that a signaling module formed by the G protein alpha subunit (Gqα) and the signal transducer phospholipase Cβ (PLCβ) modulate the activity of the p38 MAP kinase cassette within the intestine [25••
]. Interestingly, however, stimulation of the p38 MAP kinase cassette occurs in a manner independent of the single TLR homolog in C. elegans
). Thus, dissection of the p38 MAP kinase cassette enables analyses of immune mechanisms that are important in the absence of TLR signaling.
The p38 MAP kinase pathway acts cell autonomously in the intestinal epithelium [26
] to coordinate defense against a wide variety of ingested pathogens. C. elegans
carrying loss-of-function mutations in pmk-1
are hypersusceptible to infection with the Gram-negative pathogens P. aeruginosa
], Salmonella enterica
], Yersinia pestis
] and Serratia marcescens
]; the Gram-positive pathogens Enterococcus faecalis
] and Staphylococcus aureus
]; and the fungus Candida albicans
]. Moreover, activity of the p38 MAP kinase PMK-1 declines with age and was recently shown to underlie the increased susceptibility to bacterial killing that occurs in older C. elegans
]. Troemel et al.
used global transcriptional profiling analyses of nematodes growing under normal laboratory conditions to show that PMK-1 regulates the expression of putative antimicrobial effectors, including ShK toxins, C-type lectins and genes carrying a CUB-like domain [27
], in the absence of pathogen challenge. This has been termed ‘basal regulation,’ to distinguish it from the induction of immune effectors that occurs during challenge with a pathogen. In addition, a variety of genes that are induced during pathogen attack by diverse pathogens require PMK-1 [18
]; however, the full spectrum of genes that are activated by pathogens in a PMK-1-dependent manner has not been determined. Interestingly, transcriptional profiling experiments have demonstrated that the immune effectors upregulated by divergent pathogens, including several genes that require PMK-1, are largely non-overlapping [27
]. These data suggest that the PMK-1 cascade coordinates the induction of multiple immune effectors that differ depending on the infecting organism.
Work by Shivers et al.
has recently shed light on a mechanism downstream of PMK-1 that accounts for part of the immune specificity mediated by this protein [30••
]. Using an approach that highlights some of the advantages of working with a model genetic host such as C. elegans
, these researchers fused the promoter for a PMK-1-dependent putative antimicrobial peptide with the gene encoding GFP and integrated the array into the C. elegans
genome, thereby creating an in vivo
sensor for the transcriptional activation of this gene. By conducting a forward genetic screen for C. elegans
mutants that were both hypersusceptible to P. aeruginosa
infection and exhibited diminished expression of this transcriptional reporter, they uncovered mutations in each of the four genes in the p38 MAP kinase cassette (TIR-1, NSY-1, SEK-1 and PMK-1) and also in ATF-7, a transcription factor orthologous to mammalian ATF2/ATF7, which did not previously have a described immune function in C. elegans
. Subsequent characterization of ATF-7 in the C. elegans
antibacterial immune response revealed that it functions as a repressor of p38 MAP kinase PMK-1-dependent genes in C. elegans
when worms are feeding on E. coli
. However, ATF-7 switches to become a transcriptional activator of immune response genes when it is directly phosphorylated by PMK-1 during P. aeruginosa
infection. Interestingly, it seems that ATF-7 is not a positive regulator of resistance to E. faecalis
, despite the fact that PMK-1 is required for normal defense against this Gram-positive pathogen. These data suggest that there are PMK-1-dependent signaling regulators that are downstream of PMK-1 and independent of ATF-7 that are differentially activated during E. faecalis
infection. How one pathway coordinates such disparate outputs remains an open and very interesting question.