Germline stem cell integrity and quiescence are controlled by an AMPK-dependent neuronal trafficking pathway

Abstract

During periods of energetic stress, Caenorhabditis elegans can execute a developmentally quiescent stage called “dauer”, during which all germline stem cells undergo a G2 cell cycle arrest. In animals that lack AMP-activated protein kinase (AMPK) signalling, the germ cells fail to arrest, undergo uncontrolled proliferation, and lose their reproductive capacity upon recovery from this quiescent stage. These germline defects are accompanied by, and likely result from, an altered chromatin landscape and gene expression program. Through genetic analysis we identified an allele of tbc-7, a predicted RabGAP protein that functions in the neurons, which when compromised, suppresses the germline hyperplasia in the dauer larvae, as well as the post-dauer sterility and somatic defects characteristic of AMPK mutants. This mutation also corrects the abundance and aberrant distribution of transcriptionally activating and repressive chromatin marks in animals that otherwise lack all AMPK signalling. We identified RAB-7 as one of the potential RAB proteins that is modulated by tbc-7 and show that the activity of RAB-7 is critical for the maintenance of germ cell integrity during the dauer stage. We reveal that TBC-7 is regulated by AMPK through two mechanisms when the animals enter the dauer stage. Acutely, the AMPK-mediated phosphorylation of TBC-7 reduces its activity, potentially by autoinhibition, thereby preventing the inactivation of RAB-7. In the more long term, AMPK regulates the miRNAs mir-1 and mir-44 to attenuate tbc-7 expression. Consistent with this, animals lacking mir-1 and mir-44 are post-dauer sterile, phenocopying the germline defects of AMPK mutants. Altogether, we have uncovered an AMPK-dependent and microRNA-regulated cellular trafficking pathway that is initiated in the neurons, and is critical to control germline gene expression cell non-autonomously in response to adverse environmental conditions.

AMPK regulates small RNA pathway prevalence to mediate soma-to-germ line communication and establish germline stem cell quiescence

“…We recently showed that the compromise of the RabGAP tbc-7 suppresses the germline hyperplasia and post-dauer sterility of AMPK mutants (Wong et al, 2023). To determine if the relationship between the partial Rde phenotype and the post-dauer sterility observed in AMPK mutants are linked, we compared the RNAi sensitivity of aak(0) animals with that of aak(0); tbc-7 mutants that exhibit a restoration of post-dauer fertility.…”

“…These results confirm that the defects caused by loss of AMPK signalling can be corrected by shifting resources to the miRNA biogenesis pathway at the onset of the dauer stage. Increasing the levels of a single pre-miRNA can have a global impact on the miRNA expression landscape; variations in rescue penetrance can then be attributed to the identity of the individual miRNA expressed and to the specific regulatory role it may hold (Wong et al, 2023).…”

“…Upon activation, AMPK acts in somatic cells to influence the production of small RNAs that mediate a state of quiescence in germline stem cells during periods of stress (Kadekar & Roy, 2019;Wong et al, 2023). However, it remains unclear how AMPK interacts with small RNA pathways to mediate this effect, or how the regulation of small RNA production in the soma can promote the proper adaptation of germline stem cells to a given challenge.…”

Neuronal exosomes transport a miRNA/RISC cargo to preserve germline stem cell integrity during energy stress

“…The AMP-activated protein kinase (AMPK) and its upstream activator, the LKB1 tumor-suppressor orthologue par-4, are essential to establish this quiescence. Not surprisingly, mutants that lack all AMPK signalling are completely sterile following recovery from this stage (2,3).…”

“…As animals enter the dauer stage, neuronally-expressed AMPK targets a RabGAP protein to ensure that the conserved GTPase RAB-7 remains GTP-bound (3). RAB-7 is essential for early-to-late endosome maturation and the generation of exosomes (4)(5)(6).…”