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In sexually reproducing organisms maintenance of germ stem cell immortality is fundamental for transmitting genetic material to future generations. While previous research has mainly considered intrinsic regulatory mechanisms in the germline, our recent study has found a direct contribution of somatic cells in preserving germline immortality via the somatically expressed endoribonuclease ENDU-2 in Caenorhabditis elegans. We have identified ENDU-2 as a secreted protein that can be taken up by the germline. Here, we discuss how ENDU-2 might uncouple its RNA-binding and RNA-cleavage activities to control gene expression via either an endoribonuclease dependent or an independent way. We also speculate on a possible functional conservation of its mammalian homologs in mediating cell-cell communication as well as its potential significance in understanding human pathogenesis such as cancer development.

Multicellular organisms coordinate tissue specific responses to environmental information via both cell-autonomous and non-autonomous mechanisms. In addition to secreted ligands, recent reports implicated release of small RNAs in regulating gene expression across tissue boundaries. Here, we show that the conserved poly-U specific endoribonuclease ENDU-2 in C. elegans is secreted from the soma and taken-up by the germline to ensure germline immortality at elevated temperature. ENDU-2 binds to mature mRNAs and negatively regulates mRNA abundance both in the soma and the germline. While ENDU-2 promotes RNA decay in the soma directly via its endoribonuclease activity, ENDU-2 prevents misexpression of soma-specific genes in the germline and preserves germline immortality independent of its RNA-cleavage activity. In summary, our results suggest that the secreted RNase ENDU-2 regulates gene expression across tissue boundaries in response to temperature alterations and contributes to maintenance of stem cell immortality, probably via retaining a stem cell specific program of gene expression.

Kvainickas et al. establish that retromer, a coat complex that promotes endosomal recycling, is also required for the activation of the lysosomal mTORC1 complex. Mechanistically, retromer and the retromer-associated protein TBC1D5 separate mTORC1 and RAB7 microdomains on late endosomes, which is necessary for amino acid–mediated mTORC1 activation., Retromer is an evolutionarily conserved multiprotein complex that orchestrates the endocytic recycling of integral membrane proteins. Here, we demonstrate that retromer is also required to maintain lysosomal amino acid signaling through mTORC1 across species. Without retromer, amino acids no longer stimulate mTORC1 translocation to the lysosomal membrane, which leads to a loss of mTORC1 activity and increased induction of autophagy. Mechanistically, we show that its effect on mTORC1 activity is not linked to retromer’s role in the recycling of transmembrane proteins. Instead, retromer cooperates with the RAB7-GAP TBC1D5 to restrict late endosomal RAB7 into microdomains that are spatially separated from the amino acid–sensing domains. Upon loss of retromer, RAB7 expands into the ragulator-decorated amino acid–sensing domains and interferes with RAG-GTPase and mTORC1 recruitment. Depletion of retromer in Caenorhabditis elegans reduces mTORC1 signaling and extends the lifespan of the worms, confirming an evolutionarily conserved and unexpected role for retromer in the regulation of mTORC1 activity and longevity.

Macroautophagy/autophagy is an evolutionarily conserved cellular degradation and recycling process that is tightly regulated by external stimuli, diet, and stress. Our recent findings suggest that in C. elegans, a nutrient sensing pathway mediated by MTORC2 (mechanistic target of rapamycin kinase complex 2) and its downstream effector kinase SGK-1 (serum- and glucocorticoid-inducible kinase homolog 1) suppresses autophagy, involving mitophagy. Induced autophagy/mitophagy in MTORC2-deficient animals slows down development and impairs reproduction independently of the SGK-1 effectors DAF-16/FOXO and SKN-1/NFE2L2/NRF2. In this punctum, we discuss how TORC2-SGK-1 signaling might regulate autophagic turnover and its impact on mitochondrial homeostasis via linking mitochondria-derived reactive oxygen species (mtROS) production to mitophagic turnover.

Summary: Autophagy is stimulated by stress conditions and needs to be precisely tuned to ensure cellular homeostasis and organismal development and health. The kinase mechanistic target of rapamycin (mTOR) forms the enzymatic core of the highly conserved mTOR complexes mTORC1 and mTORC2. mTORC1 is a key inhibitor of autophagy, yet the function of mTORC2 in autophagy is controversial. We here show that inactivation of mTORC2 and its direct target serum- and glucocorticoid-inducible kinase 1 (SGK-1) potently induces autophagy and the autophagic degradation of mitochondria in C. elegans. Enhanced autophagy in mTORC2- or SGK-1-deficient animals contributes to their developmental and reproductive defects and is independent of the canonical SGK-1 effector DAF-16/FOXO. Importantly, we find that inactivation of mTORC2-SGK-1 signaling impairs mitochondrial homeostasis and triggers an increased release of mitochondria-derived reactive oxygen species (mtROS) to induce autophagy. Thus, mitochondrial stress couples reduced mTORC2 activity to enhanced autophagic turnover. : The multiprotein complex mTORC2 is a central regulator of development and metabolism. Aspernig et al. implicate C. elegans mTORC2 and its effector SGK-1 in mitochondrial homeostasis and report that an increased release of mitochondria-derived reactive oxygen species induces autophagy in mTORC2/SGK-1-deficient animals. Keywords: mammalian target of rapamycin, mTOR, mTORC2, serum glucocorticoid-regulated kinase 1, SGK-1, autophagy, mitophagy, mitochondria, reactive oxygen species, ROS

Managing available resources is a key necessity of each organism to cope with the environment. The nematode C. elegans responds to nutritional deprivation or harsh environmental conditions with a multitude of developmental adaptations, among them a starvation-induced quiescence at early larval development (L1). daf-18, the C. elegans homolog of the human tumor suppressor gene PTEN, is essential for the maintenance of survival and germline stem cell arrest during the L1 diapause. We show here that daf-18 mutants, independently to their failure to maintain G2 arrest of the primordial germ cells, develop a gonad phenotype after refeeding. This highly penetrant gonadal phenotype is further enhanced by a mutation in shc-1, encoding a protein homologous to the human adaptor ShcA. Features of this phenotype are a tumor-like phenotype encompassing hyper-proliferation of germ cell nuclei and disruption/invasion of the basement membrane surrounding the gonad. The penetrance of this phenotype is reduced by decreasing starvation temperature. In addition, it is also ameliorated in a dose-dependent way by exposure to the antibiotic doxycyclin either during starvation or during subsequent refeeding. Since, in eukaryotic cells, doxycyclin specifically blocks mitochondrial translation, our results suggest that daf-18 and shc-1;daf-18 mutants fail to adapt mitochondrial activity to reduced nutritional availability during early larval developing.

In Caenorhabditis elegans (C. elegans), the promotion of longevity by the transcription factor DAF-16 requires reduced insulin/IGF receptor (IIR) signaling or the ablation of the germline, although the reason for the negative impact of germ cells is unknown. FOXO/DAF-16 activity inhibits germline proliferation in both daf-2 mutants and gld-1 tumors. In contrast to its function as a germline tumor suppressor, we now provide evidence that somatic DAF-16 in the presence of IIR signaling can also result in tumorigenic activity, which counteracts robust lifespan extension. In contrast to the cell-autonomous IIR signaling, which is required for larval germline proliferation, activation of DAF-16 in the hypodermis results in hyperplasia of the germline and disruption of the surrounding basement membrane. SHC-1 adaptor protein and AKT-1 kinase antagonize, whereas AKT-2 and SGK-1 kinases promote, this cell-nonautonomous DAF-16 function. Our data suggest that a functional balance of DAF-16 activities in different tissues determines longevity and reveals a novel, cell-nonautonomous role of FOXO/DAF-16 to affect stem cells., Author Summary Previous studies have shown that DAF–16/FOXO transcription factor promotes longevity and stress resistance and inhibits tumor progression in the absence of insulin signaling. Here we show that active DAF-16 in the epidermis can shorten lifespan by promoting a tumorous germline phenotype. In contrast to the known inhibitory effect of insulin signaling upon DAF-16, an active insulin and PI3K signaling are required for DAF-16–mediated signaling to the germline. In addition, AKT-1– and SHC-1–mediated JNK signaling antagonize AKT-2 and SGK-1 to affect the reproductive system. This is to our knowledge the first report about a detrimental effect of DAF-16 on lifespan. Furthermore it emphasizes that DAF-16 activity is highly dependent on the cellular context and communication between different tissues.

Correlative evidence links stress, accumulation of oxidative cellular damage, and aging in several species. Genetic studies in species ranging from yeast to mammals revealed several pathways regulating stress response and life span, including caloric intake, mitochondrial respiration, insulin/IGF-1 (IIS), and JNK (c-Jun N-terminal kinase) signaling. How IIS and JNK signaling cross-talk to defend against diverse stressors contributing to aging is of critical importance but, so far, only poorly understood. In this study, we demonstrate that the adaptor protein SHC-1, the Caenorhabditis elegans homolog of human p52Shc, coordinates mechanisms of stress response and aging. Using genetic and biochemical approaches, we discover that SHC-1 not only opposes IIS but also activates JNK signaling. Loss of shc-1 function results in accelerated aging and enhanced sensitivity to heat, oxidative stress, and heavy metals, whereas expression of human p52Shc rescues the shc-1 mutant phenotype. SHC-1 acts upstream of the insulin/IGF receptor DAF-2 and the PI3 kinase AGE-1 and directly interacts with DAF-2. Moreover, SHC-1 activates JNK signaling by binding to MEK-1 kinase. Both aspects converge on controlling the nuclear translocation and activation of the FOXO transcription factor DAF-16. Our findings establish C. elegans SHC-1 as a critical scaffold that directly cross-connects the two parallel JNK and IIS pathways and help to explain how these signaling cascades cooperate to ascertain normal stress response and life span in C. elegans.