DNA damage in germ cells induces an innate immune response that triggers systemic stress resistance

Abstract

DNA damage responses have been well characterized with regard to their cell-autonomous checkpoint functions leading to cell cycle arrest, senescence and apoptosis. In contrast, systemic responses to tissue-specific genome instability remain poorly understood. In adult Caenorhabditis elegans worms germ cells undergo mitotic and meiotic cell divisions, whereas somatic tissues are entirely post-mitotic. Consequently, DNA damage checkpoints function specifically in the germ line, whereas somatic tissues in adult C. elegans are highly radio-resistant. Some DNA repair systems such as global-genome nucleotide excision repair (GG-NER) remove lesions specifically in germ cells. Here we investigated how genome instability in germ cells affects somatic tissues in C. elegans. We show that exogenous and endogenous DNA damage in germ cells evokes elevated resistance to heat and oxidative stress. The somatic stress resistance is mediated by the ERK MAP kinase MPK-1 in germ cells that triggers the induction of putative secreted peptides associated with innate immunity. The innate immune response leads to activation of the ubiquitin-proteasome system (UPS) in somatic tissues, which confers enhanced proteostasis and systemic stress resistance. We propose that elevated systemic stress resistance promotes endurance of somatic tissues to allow delay of progeny production when germ cells are genomically compromised.

Figure 1. DNA damage in the germline leads to somatic stress resistance

(A) Worms were UVB-treated at the L4 stage and 48 hours later exposed to heat shock (HS) at 35°C (A) or 5mM paraquat (B). (C) Wt worms were treated at L4 stage with UVB (520mJ/cm²), IR (90Gy) or HU (25mM) and exposed to heat shock as in (A). (D-F) Heat shock at 35°C was applied on day two of adulthood. (G) Wt and glp-1 mutants were treated as described in (A). (H) L4 worms were treated with UVB (520mJ/cm²) and live progeny was monitored from the time of exposure until the indicated days. Total offspring: untreated 263 (SD±12), UV-treated 256 (SD±6) viable offspring per worm. (Error bars=SD. n≥100 for each experimental condition (A-G); n=10 (H). *=P<0.05; ***=P<0.0001; log rank analysis (A-G) and two-tailed t-test (H)).

Figure 2. Stress resistance induced by germline DNA damage is mediated through MPK-1

(A) L4 larvae were exposed to 520mJ/cm² UVB or 90Gy IR and subjected to heat stress as described in Fig.1. (B) Worms were grown at 25°C and IR treated on day 1 of adulthood. Respective phospho-ERK signal was compared to tubulin. (Error bars=SD. In A, B n≥100 for each experimental condition; ***=P<0.0001, log rank test).

Figure 3. Somatic stress resistance upon DNA damage in germ cells is mediated through MPK-1 induced functional innate immune response

(A) Pearson correlation analysis of significantly induced genes upon UV treatment with transcriptomes upon IR, pathogen infections (MN= M. nematophilum, PA= P. aeruginosa, BS=B. subtilis), and pmk-1 mutants. (B) Gene expression was assayed by qPCR 6h post UV and IR treatment. (C) Germlines were dissected 1-2h post UVB (520mJ/cm²) or IR (90Gy) for qPCR analysis. (D) L4 larvae were treated with 90Gy IR or placed for 10h on B. subtilis, and 24h later exposed to P. aeruginosa. (E) Young adults were treated with 90Gy IR or P. aeruginosa for 4h, and 24h later exposed to heat stress. (Error bars=SD. n≥100 for each experimental condition (D, E); ***=P<0.0001, log rank test).

Figure 4. Innate immune responses trigger UPS activation to confer systemic stress resistance

(A, B) sur-5::UbV-GFP reporter expression was assessed 24h post 90Gy at L4 stage. (C) Early L4 sur-5::UbV-GFP reporter worms and sur-5::UbV^K29/48R-GFP transgenic worms were fed for 12h with RNAi against proteasomal subunits and after treated and analyzed as in (B). OP50 bacteria served as control. (D) Worms were fed RNAi as in (C), treated with UVB (520mJ/cm²) or IR (90Gy) and exposed to heat stress as described in Fig. 1. (Error bars=SD. n≥100 for each experimental condition (D); ***=P<0.0001, log rank analysis). (E) sur-5::UbV-GFP and sur-5::UbV^K29/48R-GFP reporter worms were treated with 520 mJ/cm² UVB or fed with B. subtilis at L4 stage for indicated times and protein levels analysed on day 1 of adulthood. (F) The “germline DNA damage-induced systemic stress response” (GDISR) is triggered by MPK-1 activity in germ cells while intestinal pathogen infection activates stress resistance through PMK-1. Both MAPKs induce putative secreted immune peptides that we suggest to mediate systemic stress resistance by activation of the UPS. We propose that UPS activity in turn enhances proteostasis to elevate somatic endurance in the presence of genomically compromised germ cells or pathogen infection.