DAF-16 and SMK-1 Contribute to Innate Immunity During Adulthood inCaenorhabditis elegans

Abstract

Aging is accompanied by a progressive decline in immune function termed “immunosenescence”. Deficient surveillance coupled with the impaired function of immune cells compromises host defense in older animals. The dynamic activity of regulatory modules that control immunity appears to underlie age-dependent modifications to the immune system. In the roundworm Caenorhabditis elegans levels of PMK-1 p38 MAP kinase diminish over time, reducing the expression of immune effectors that clear bacterial pathogens. Along with the PMK-1 pathway, innate immunity in C. elegans is regulated by the insulin signaling pathway. Here we asked whether DAF-16, a Forkhead box (FOXO) transcription factor whose activity is inhibited by insulin signaling, plays a role in host defense later in life. While in younger C. elegansDAF-16 is inactive unless stimulated by environmental insults, we found that even in the absence of acute stress the transcriptional activity of DAF-16 increases in an age-dependent manner. Beginning in the reproductive phase of adulthood, DAF-16 upregulates a subset of its transcriptional targets, including genes required to kill ingested microbes. Accordingly, DAF-16 has little to no role in larval immunity, but functions specifically during adulthood to confer resistance to bacterial pathogens. We found that DAF-16-mediated immunity in adults requires SMK-1, a regulatory subunit of the PP4 protein phosphatase complex. Our data suggest that as the function of one branch of the innate immune system of C. elegans (PMK-1) declines over time, DAF-16-mediated immunity ramps up to become the predominant means of protecting adults from infection, thus reconfiguring immunity later in life.

The PP2A/4/6 subfamily of phosphoprotein phosphatases regulates DAF-16 and confers resistance to environmental stress in postreproductive adult C. elegans

“…By Day 6 of adulthood, however, daf-16 knockdown resulted in a significant decrease in GFP expression ( Fig 2B). This suggests that DAF-16 becomes activated in an age-dependent manner, consistent with previous studies [8][9][10]. Recapitulating our prior observations, knocking down smk-1 also resulted in decreased reporter expression in Day 6 adults ( Fig 2B), and so together our results indicate that the transcriptional activity of DAF-16 during aging in non-stressed wildtype animals may be regulated by SMK-1.…”

“…This suggests that the immune protection conferred by DAF-16 during adulthood is associated with the intrinsic sustained increase in its transcriptional activity that begins during reproduction and persists into late adulthood. Consistent with this possibility, infection does not seem to induce DAF-16 activation in adult C. elegans [10,11].…”

“…During adulthood, the expression levels of the daf-16a splice isoform, and to an even greater extent the daf-16d isoform progressively increase [8]. This is coupled to increased expression of a subset of DAF-16 transcriptional targets in adults as compared to in larvae, indicating that DAF-16 is activated as C. elegans age [9,10].…”

“…This is coupled to increased expression of a subset of DAF-16 transcriptional targets in adults as compared to in larvae, indicating that DAF-16 is activated as C . elegans age [ 9 , 10 ].…”

Novel perspectives of target-binding by the evolutionarily conserved PP4 phosphatase

“…The Smk-1/DUF625 domain was first identified in the SMK-1 protein, a component of the IIs longevity pathway in C. elegans [ 31 ]. SMK-1 is the R3 subunit of PP4, which together with Daf-16 contributes to innate immunity in adult roundworms [ 42 ]. The SMK-1 domain is present in PP4R3 orthologues from yeast to humans and its primary sequence is highly conserved in Metazoa ( figure 1 c ).…”