A soma-to-germline transformation in long-lived Caenorhabditis elegans mutants

Abstract

Unlike the soma, which ages during the lifespan of multicellular organisms, the germ line traces an essentially immortal lineage. Genomic instability in somatic cells increases with age, and this decline in somatic maintenance might be regulated to facilitate resource reallocation towards reproduction at the expense of cellular senescence. Here we show that Caenorhabditis elegans mutants with increased longevity exhibit a soma-to-germline transformation of gene expression programs normally limited to the germ line. Decreased insulin-like signalling causes the somatic misexpression of the germline-limited pie-1 and pgl family of genes in intestinal and ectodermal tissues. The forkhead boxO1A (FOXO) transcription factor DAF-16, the major transcriptional effector of insulin-like signalling, regulates pie-1 expression by directly binding to the pie-1 promoter. The somatic tissues of insulin-like mutants are more germline-like and protected from genotoxic stress. Gene inactivation of components of the cytosolic chaperonin complex that induce increased longevity also causes somatic misexpression of PGL-1. These results indicate that the acquisition of germline characteristics by the somatic cells of C. elegans mutants with increased longevity contributes to their increased health and survival.

Fig. 1. Mutations in the insulin/IGF-like signaling pathway cause soma-to-germline transformation

(a) Schematic of the C. elegans germline (b) wild type L2/L3 larvae express pie-1p::gfp::pgl-1 only in the germline (region between dashed lines marks background autofluorescence), (c) in perinuclear structures (arrows). (d) daf-2(e1370) and (e) age-1(mg305) mutations cause somatic expression of pie-1p::gfp::pgl-1 in the intestine and hypodermis in dauers and (f) daf-2(e1370) L3/L4 larvae. (g) SDS-PAGE resolved whole worm lysates of wild type, daf-2(e1370), and age-1(mg305) animals harboring the pie-1p::gfp::pgl-1 reporter. GFP::PGL-1 (Arrow). *, unknown crossreacting protein. L1, Larval stage 1, L3, Larval stage 3, D, dauer, L4, Larval stage 4.

Fig. 2. DAF-16 regulates the expression of pie-1

(a) Schematic of the pie-1 promoter. (b) A ³²P labeled region of the pie-1 promoter (EMSA probe, lane 1) can recruit purified recombinant DAF-16-FLAG (Lane 2). The bound DAF-16 protein can be supershifted with αFLAG antibodies (lane 3). Binding is competed away with a cold dsDNA oligo containing DAF-16 binding/associated elements (DBE/DAE, lane 4). (c) DAF-16 is enriched on the promoter of pie-1 in daf-2 RNAi treated worms. Chromatin IP (ChIP) was performed using α-GFP antibodies in wild type (untagged) and a somatic exclusive daf-16::gfp strain (somatic DAF-16::GFP). Binding of two non-DAF-16-associated control regions (ChrIV, ChrV), two DAF-16-associated promoter regions (sod-3, mtl-1), and two regions in the pie-1 promoter (pie-1 A, pie-1 B) was measured from the IP. Error bars represent S.E.M.

Fig. 3. The somatic misexpression of germline-specific genes in insulin-like signaling mutants contributes to their increased longevity

Wild type (blue) or two hypomorphic insulin-like receptor mutants daf-2(e1368) (red) and daf-2(e1370) (yellow) animals were fed RNAi clones targeting germline-specific genes that are misexpressed in somatic cells and two known soma-germline specificity regulators only in adulthood. RNAi depletion of germline-restricted genes in wild type animals increases lifespan compared to vector control while inactivation in daf-2 mutants shortens the lifespan of these normally long-lived animals. RNAi targeting daf-2 (increased lifespan) or daf-16 (decreased lifespan) were used as controls and lifespan was normalized to animals fed vector control RNAi.

Fig. 4. The cytosolic chaperonin complex regulates the expression of PGL-1 in somatic cells

PGL-1 immunostaining is restricted to perinuclear structures of meiotic and mitotic germ cells in animals fed vector control (top panel). Black arrow points to four intestinal nuclei lacking PGL-1 expression. RNAi targeting cct-6 causes somatic PGL-1 expression in intestinal (middle panel) and hypodermal cells(bottom panel). White arrows indicate perinuclear PGL-1 localization.

Fig. 5. Model for the regulation of germline gene expression in the soma by C. elegans longevity regulators

Insulin-like signaling negatively regulates the FoxO transcription factor DAF-16. DAF-16 positively regulates the expression of pie-1 and germline specific genes in somatic tissues. The cytosolic chaperonin complex (cct) negatively regulates the activity of SKN-1 and the somatic expression of PGL-1. Solid lines represent experimentally observed regulation. Dashed lines indicate potential regulation