Our data reveals that many C. elegans longevity mutants exhibit a soma-to-germline transformation that contributes to their enhanced survival. The somatic protection stemming from the soma-to-germline transformation likely plays a role both during the pre-reproductive dauer diapause stage, where C. elegans can survive for months, and post-reproductive adulthood to promote longevity. This phenotype although surprising makes intuitive sense as germ cells employ protective mechanisms to ensure genetic integrity. The recruitment of these normally germline-protective pathways in somatic tissues provides a mechanism for the finding that long-lived mutants exhibit resistance to genotoxic stress. Germ cells are tremendously resistant to numerous stresses, which renders them immortal and capable of regeneration on a scale not shared by somatic tissues. We hypothesize that the transformation of somatic cells to a more germ cell-like state provides increased genomic stability and may be an important pathway facilitating lifespan extension.
Alternatively, misregulation of germline genes in the soma may result in the ability of these post-mitotic cells to reinstate a proliferative state. chk-1
and other checkpoint proteins regulate postmitotic cell survival38
. In the embryo the CHK-1 pathway is suppressed so that cell divisions can occur properly. If in the insulin-like signaling mutants the somatic tissues were more proliferative this could also result in increased lifespan. Intriguingly, smk-1/rad-2
regulates the silencing of chk-1
and SMK-1 is a component of the insulin-like signaling longevity pathway39, 40
. A return to a replicatively competent state could also explain the observed stability of the somatic nuclear pore complex in the insulin signaling mutants if the subunits are being replaced with new copies, which does not occur in non-proliferative cell types41
. This idea fits with the observation that RNAi toward the nuclear pore component dct-3/hTpr
increases lifespan in wild type animals but decreases the fitness of daf-2/InsR
, similar to our analysis of the germline misexpressed genes.
The fact that the most potent transcriptional regulator of the lifespan extension phenotypes in insulin-like signaling mutants, daf-16/FoxO
, mediates the soma-to-germline transformation points to the importance of this phenotype in the regulation of lifespan and stress resistance14, 15
. We demonstrate here by EMSA and ChIP that DAF-16/FoxO can bind to the pie-1
promoter in somatic cells. The in vivo
binding is weaker than previously described target promoters of sod-3
, however, this level of binding correlates with a lower induction of pie-1
mRNA compared to the canonical targets (Fig. S5
). A similar level of enrichment of DAF-16 on the pie
-1 promoter in a strain expressing DAF-16::mCHERRY in the hypodermis, muscle, and neurons but not intestine points to these cell types as the key regulatory tissues (Table S3
. In conjunction with our qRT-PCR and genetic epistasis analyses we have uncovered that DAF-16/FoxO is an important direct regulator of pie-1
expression and modulator of the soma-to-germline transformation phenotype.
Our data support a model where the somatic tissues of insulin-like signaling mutants display transcriptional dysregulation to become more germ cell-like. Unlike the cct
RNAi treated animals, the detection of misexpressed germline proteins in the insulin-like signaling mutants has been complicated. However, germ cells are also under tight translational control, which may explain the lowered level of germline proteins46
. This added layer of expression control might be important corollary for the longevity phenotype as canonical synMuvB mutants such as mep-1, lin-15b
strongly misexpress germline proteins in the soma5
and have general sickness and a short lifespan (data not shown, Fig. S11
Similar to the longevity and soma-to-germline transformation, the enhanced RNAi phenotype of the insulin-like signaling mutants is dependent on DAF-16/FoxO3
. In addition, loss of the DAF-16 transcriptional target gene zfp-1/AF10
can suppress the multivulval phenotype of synMuv mutants and partially suppress the lethality of mep-1
. Moreover, synMuvB mutants transcriptionally dysregulate the gonad distal tip cell (DTC) marker lag-2
in other somatic tissues, which is partially dependent upon zfp-1/AF10
. Finally, zfp-1
has been shown to be required for the ability of C. elegans
to perform RNAi47
. Taken together these data suggest that soma-to-germline transformation, enhanced RNAi, and longevity share common regulatory mechanisms.
The idea that somatic tissues have the potential of adopting a more germ cell-like character provides a means of dissecting the dichotomy between the instability of somatic cells and the immortality of germline stem cells. Uncovering the mechanisms that mediate this transformation phenotype will provide insight into the pathways that maintain germ cell viability, cell fate specificity and increased survival stemming from the acquisition of germline quality by somatic cells.
Given that protection of the germline is an evolutionarily shared trait across species, it will be interesting to investigate if this is a broadly conserved mechanism of modulating lifespan. The idea that somatic cells maintain the potential to reacquire pathways lost during differentiation is tantalizing and may facilitate the elucidation of therapeutics to assist in cellular repair and possibly regeneration.