Germ granule dysfunction is a hallmark and mirror of Piwi mutant sterility

Abstract

In several species, Piwi/piRNA genome silencing defects cause immediate sterility that correlates with transposon expression and transposon-induced genomic instability. In C. elegans, mutations in the Piwi-related gene (prg-1) and other piRNA deficient mutants cause a transgenerational decline in fertility over a period of several generations. Here we show that the sterility of late generation piRNA mutants correlates poorly with increases in DNA damage signaling. Instead, sterile individuals consistently exhibit altered perinuclear germ granules. We show that disruption of germ granules does not activate transposon expression but induces multiple phenotypes found in sterile prg-1 pathway mutants. Furthermore, loss of the germ granule component pgl-1 enhances prg-1 mutant infertility. Environmental restoration of germ granule function for sterile pgl-1 mutants restores their fertility. We propose that Piwi mutant sterility is a reproductive arrest phenotype that is characterized by perturbed germ granule structure and is phenocopied by germ granule dysfunction, independent of genomic instability.

Dual roles for piRNAs in promoting and preventing gene silencing in C. elegans

“…Next, we singled 10 animals from each population of wild-type and prg-1 mutant animals (as well as mut-14 smut-1 and prg-1; mut-14 smut-1, discussed below) and propagated them without starvation for 65 generations. Propagation across many generations in the presence of food leads to progressive germline dysfunction and loss of fertility in prg-1 mutants, which we also observed in this experiment (Heestand et al, 2018;Simon et al, 2014;Spichal et al, 2021). At 50 generations, an arbitrary time point chosen because at this point the individual prg-1 lines had diverged considerably in health, as discussed below, we again sequenced small RNAs and mRNAs from the 3 surviving prg-1 mutant lines and 3 of the wild-type lines harvested as adults.…”

“…In the nematode Caenorhabditis elegans , loss of piRNAs does not lead to immediate sterility, but instead causes a gradual loss of fertility over numerous generations such that the germline loses its immortal nature ( Simon et al, 2014 ). As sterility ensues in piRNA mutants, the germ granules involved in RNA surveillance collapse ( Spichal et al, 2021 ). However, the sterility of piRNA mutants is not clearly linked to transposon activation and genomic instability ( Barucci et al, 2020 ; Reed et al, 2020 ; Simon et al, 2014 ; Spichal et al, 2021 ).…”

Two isoforms of the essential C. elegans Argonaute CSR-1 differentially regulate sperm and oocyte fertility

“…In fact, we did not detect mature spermatids in the majority of individuals examined from this strain. In parallel, we examined the number of germ granules (as measured by counting the number of ZNFX-1 and PGL-1 foci) present in quadruple ago mutants vs. wild-type worms at 25°C, because germ granule dysfunction has been associated with a transgenerational loss of fertility in hermaphrodites ( 80 ) (Figure 9E ). However, we did not detect any differences in the number of ZNFX-1 or PGL-1 foci, suggesting that defects in germ granule homeostasis were not at the root of the loss of fertility in the quadruple spermatogenesis ago mutant.…”

Nuage condensates: accelerators or circuit breakers for sRNA silencing pathways?

“…These observations suggest sRNA amplification mechanisms become dangerously promiscuous in mutants lacking piRNAs, and consistent with this, prg-1 mutants become completely sterile over successive generations (Barrucci et al, 2020;Wahba et al, 2021). Interestingly, in prg-1 mutants, the secondary Argonaute WAGO-1 mislocalizes to the cytoplasm (Barucci et al, 2020) and late generation prg-1 mutants frequently loose PGL-1 localization to P granules (Spichal et al, 2021). One possibility, therefore, is that lack of piRNAs/PRG-1 degrades nuage organization over time leading to inappropriate mixing of Argonautes and sRNA amplification machineries in the cytoplasm.…”