DAF-18/PTEN inhibits germline zygotic gene activation during primordial germ cell quiescence

Abstract

Quiescence, an actively-maintained reversible state of cell cycle arrest, is not well understood. PTEN is one of the most frequently lost tumor suppressors in human cancers and regulates quiescence of stem cells and cancer cells. The sole PTEN ortholog in Caenorhabditis elegans is daf-18. In a C. elegans loss-of-function mutant for daf-18, primordial germ cells (PGCs) divide inappropriately in L1 larvae hatched into starvation conditions, in a TOR-dependent manner. Here, we further investigated the role of daf-18 in maintaining PGC quiescence in L1 starvation. We found that maternal or zygotic daf-18 is sufficient to maintain cell cycle quiescence, that daf-18 acts in the germ line and soma, and that daf-18 affects timing of PGC divisions in fed animals. Importantly, our results also implicate daf-18 in repression of germline zygotic gene activation, though not in germline fate specification. However, TOR is less important to germline zygotic gene expression, suggesting that in the absence of food, daf-18/PTEN prevents inappropriate germline zygotic gene activation and cell division by distinct mechanisms.

Alternative somatic and germline gene-regulatory strategies during starvation-induced developmental arrest

“…We analyzed a putative null mutant, cec-4(ok314), shown by Belew et al to disrupt PGC chromatin compaction. We found that the mutant PGCs do not proliferate during L1 starvation (data not shown), suggesting that transcriptional activation is not sufficient for PGC proliferation, though it is necessary ( Fry et al, 2021 ). However, the mutant has reduced brood size overall, and the effect is particularly pronounced following 8 days of L1 starvation ( Figure 7A ), revealing a significant consequence of transcriptional activation in starved PGCs.…”

“…Three of our six largest clusters (clusters 2, 4, and 6) are expressed at relatively low levels upon hatching, exhibit peak expression levels beyond 4 days of starvation, and either maintain or increase expression levels up to 12 days of starvation. These clusters are enriched with genes expressed in several tissues related to reproduction, including “germline,” “gonad primordium,” and “reproductive system” ( Figure 3A ) ( Angeles-Albores et al, 2016 ), which was surprising given that transcription is repressed in germ cells in the first few hours of L1 arrest ( Belew et al, 2021 ; Fry et al, 2021 ). We directly assessed if genes typically expressed in the soma or germline exhibit distinct expression patterns.…”

“…We determined expression profiles of known targets (direct and indirect) of critical regulators to shed light on when they are most active. We focused our analysis on DAF-16/FoxO, DAF-18/PTEN, LIN-35/Rb, and HLH-30/TFEB because loss of each severely compromises starvation survival and genome-wide expression data for each mutant in L1 arrest are available ( Cui et al, 2013 ; Tepper et al, 2013 ; Kaplan et al, 2015 ; Murphy et al, 2019 ; Fry et al, 2021 ). Positively regulated targets (genes down-regulated in the mutant) are expressed at their highest levels at different times after hatching, with DAF-16, DAF-18, and some HLH-30 targets peaking between 6 and 12 h of arrest, and LIN-35 and other HLH-30 targets peaking between 2 and 4 days of L1 arrest ( Figures 2C – 2F ), consistent with relatively early and late function, respectively.…”

“…Upon hatching, L1 larvae have 558 cells, two of which are the PGCs, Z2 and Z3. Z2 and Z3 are transcriptionally repressed during L1 arrest, which depends on chromatin compaction genes CEC-4 and HPL-2/HP1 as well as the kinase AMPK and phosphatase DAF-18/PTEN ( Demoinet et al, 2017 ; Belew et al, 2021 ; Fry et al, 2021 ). However, the physiological significance of germline transcriptional repression during L1 arrest, and whether this repression is maintained throughout arrest, has not been addressed.…”

Germline stem cells in human

“…Knowledge about how PGCs are induced comes largely from studies in model mammals, including nonhuman primates, 86 , 87 , 89 – 95 mice, 12 , 83 , 85 , 96 – 108 and pigs. 109 – 118 Studies in the other group of model organisms, including chicken, 119 – 126 zebrafish, 63 , 64 , 66 , 67 , 127 medaka, 128 – 136 frogs, 55 – 57 , 137 Drosophila, 138 – 146 and Caenorhabditis elegans, 147 – 156 provide abundant information about PGC specification by preformation or maternally inherited determinants. The PGCs in these two groups of organisms show a similarity that is the presence of some kind of aggregate of electron-dense, basophilic bodies containing the proteins and RNAs, (e.g., VASA, NANOS, and MAGO) in their cytoplasm.…”