Numerous signaling pathways involved in mediating C. elegans innate immunity have been identified. Our lab has previously shown the C. elegans Sma/Mab TGF-beta signaling pathway to regulate immunity against both Gram-negative and Gram-positive pathogens. Using a candidate gene approach, we identified myocyte enhancer factor 2 (MEF-2) as a participant in C. elegans defense but only against Gram-negative bacteria such as Pseudomonas aeruginosa and Salmonella typhimurium. In order to reveal the pathways in which MEF-2 exerts its immune function, we analyzed the pathogen susceptibility of double mutants of
mef-2 and genes shown to interact with
mef-2 in other organisms. MEF-2 has been shown to interact with Smad2, a component of the vertebrate TGF-beta pathway. The susceptibility of C. elegans
mef-2(
gv1);
sma-6(
wk7) double mutants to P. aeruginosa is indistinguishable from that of
sma-6 alone, suggesting that these genes are likely to function in the same immune pathway. As
sma-6 mutants are more susceptible to P. aeruginosa than
mef-2 mutants, SMA proteins probably regulate additional mediators of immunity, consistent with their role in defense against both Gram-negative and positive pathogens. Calcineurin activates MEF-2 in vertebrates via dephosphorylation. C. elegans mutants in calcineurin subunit A,
tax-6, display enhanced susceptibility to P. aeruginosa. As
mef-2(
gv1);
tax-6(
p675) double mutants phenocopy the enhanced susceptibility of each single mutant, it is likely these genes also function in the same defense pathway. Histone deacetylase 4 (HDAC4) deactivates MEF2 in vertebrates, repressing its transcriptional activity, and C. elegans HDA-4 interacts with C. elegans MEF-2 in vitro. Consistent with its role as a negative regulator of
mef-2,
hda-4 mutants are more resistant to P. aeruginosa infection. To elucidate possible mechanisms of defense by MEF-2, proliferation of S. typhimurium within infected
mef-2 mutants was assessed. Interestingly, increased susceptibility is not associated with a defect in clearing bacteria as the load of Salmonella in
mef-2 animals was not significantly different from that of N2. This result indicates that MEF-2 does not directly impact bacterial proliferation, but instead regulates the hosts survival upon infection. Currently, we are using whole genome microarray analysis to identify downstream targets of MEF-2 which are involved in the response against P. aeruginosa and S. typhimurium. Additionally, we are assessing the possibility of conservation of the role of
mef-2 in Drosophila immunity. Progress in these areas will be presented.