DAF-2 and ERK couple nutrient availability to meiotic progression during Caenorhabditis elegans oogenesis

An RNAi screen of Rab GTPase genes in C. elegans reveals that somatic cells of the reproductive system depend on rab-1 for morphogenesis but not stem cell niche maintenance

Membrane trafficking is a crucial function of all cells and is regulated at multiple levels from vesicle formation, packaging, and localization to fusion, exocytosis, and endocytosis. Rab GTPase proteins are core regulators of eukaryotic membrane trafficking, but developmental roles of specific Rab GTPases are less well characterized, potentially because of their essentiality for basic cellular function. C. elegans gonad development entails the coordination of cell growth, proliferation, and migration-processes in which membrane trafficking is known to be required. Here we take an organ-focused approach to Rab GTPase function in vivo to assess the roles of Rab genes in reproductive system development. We performed a whole-body RNAi screen of the entire Rab family in C. elegans to uncover Rabs essential for gonad development. Notable gonad defects resulted from RNAi knockdown of rab-1, the key regulator of ER-Golgi trafficking. We then examined the effects of tissue-specific RNAi knockdown of rab-1 in somatic reproductive system and germline cells. We interrogated the dual functions of the distal tip cell (DTC) as both a leader cell of gonad organogenesis and the germline stem cell niche. We find that rab-1 functions cell-autonomously and non-cell-autonomously to regulate both somatic gonad and germline development. Gonad migration, elongation, and gamete differentiation-but surprisingly not germline stem niche function-are highly sensitive to rab-1 RNAi.

Bidirectional transfer of a small membrane-impermeable molecule between the Caenorhabditis elegans intestine and germline

The extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) is a positive regulator of cell proliferation often upregulated in cancer. Its Caenorhabditis elegans ortholog MPK-1 stimulates germline stem cell (GSC) proliferation nonautonomously from the intestine or somatic gonad. How MPK-1 can perform this task from either of these two tissues however remains unclear. We reasoned that somatic MPK-1 activity could lead to the generation of proproliferative small molecules that could transfer from the intestine and/or somatic gonad to the germline. Here, in support of this hypothesis, we demonstrate that a significant fraction of the small membrane-impermeable fluorescent molecule, 5-carboxyfluorescein, transfers to the germline after its microinjection in the animal's intestine. The larger part of this transfer targets oocytes and requires the germline receptor mediated endocytosis 2 (RME-2) yolk receptor. A minor quantity of the dye is however distributed independently from RME-2 and more widely in the animal, including the distal germline, gonadal sheath, coelomocytes, and hypodermis. We further show that the intestine-to-germline transfer efficiency of this RME-2 independent fraction does not vary together with GSC proliferation rates or MPK-1 activity. Therefore, if germline proliferation was influenced by small membrane-impermeable molecules generated in the intestine, it is unlikely that proliferation would be regulated at the level of molecule transfer rate. Finally, we show that conversely, a similar fraction of germline injected 5-carboxyfluorescein transfers to the intestine, demonstrating transfer bidirectionality. Altogether, our results establish the possibility of an intestine-to-germline signaling axis mediated by small membrane-impermeable molecules that could promote GSC proliferation cell nonautonomously downstream of MPK-1 activity.

A non-canonical role of somatic Cyclin D/CYD-1 in oogenesis and in maintenance of reproductive fidelity, dependent on the FOXO/DAF-16 activation state

For the optimal survival of a species, an organism coordinates its reproductive decisions with the nutrient availability of its niche. Thus, nutrient-sensing pathways like insulin-IGF-1 signaling (IIS) play an important role in modulating cell division, oogenesis, and reproductive aging. Lowering of the IIS leads to the activation of the downstream FOXO transcription factor (TF) DAF-16 in Caenorhabditis elegans which promotes oocyte quality and delays reproductive aging. However, less is known about how the IIS axis responds to changes in cell cycle proteins, particularly in the somatic tissues. Here, we show a new aspect of the regulation of the germline by this nutrient-sensing axis. First, we show that the canonical G1-S cyclin, Cyclin D/CYD-1, regulates reproductive fidelity from the uterine tissue of wild-type worms. Then, we show that knocking down cyd-1 in the uterine tissue of an IIS receptor mutant arrests oogenesis at the pachytene stage of meiosis-1 in a DAF-16-dependent manner. We observe activated DAF-16-dependent deterioration of the somatic gonadal tissues like the sheath cells, and transcriptional de-regulation of the sperm-to-oocyte switch genes which may be the underlying reason for the absence of oogenesis. Deleting DAF-16 releases the arrest and leads to restoration of the somatic gonad but poor-quality oocytes are produced. Together, our study reveals the unrecognized cell non-autonomous interaction of Cyclin D/CYD-1 and FOXO/DAF-16 in the regulation of oogenesis and reproductive fidelity.

Non-cell-autonomous regulation of germline proteostasis by insulin/IGF-1 signaling-induced dietary peptide uptake via PEPT-1

Gametogenesis involves active protein synthesis and is proposed to rely on proteostasis. Our previous work in C. elegans indicates that germline development requires coordinated activities of insulin/IGF-1 signaling (IIS) and HSF-1, the central regulator of the heat shock response. However, the downstream mechanisms were not identified. Here, we show that depletion of HSF-1 from germ cells impairs chaperone gene expression, causing protein degradation and aggregation and, consequently, reduced fecundity and gamete quality. Conversely, reduced IIS confers germ cell resilience to HSF-1 depletion-induced protein folding defects and various proteotoxic stresses. Surprisingly, this effect was not mediated by an enhanced stress response, which underlies longevity in low IIS conditions, but by reduced ribosome biogenesis and translation rate. We found that IIS activates the expression of intestinal peptide transporter PEPT-1 by alleviating its repression by FOXO/DAF-16, allowing dietary proteins to be efficiently incorporated into an amino acid pool that fuels germline protein synthesis. Our data suggest this non-cell-autonomous pathway is critical for proteostasis regulation during gametogenesis.

Actomyosin-mediated apical constriction promotes physiological germ cell death in C. elegans

Germ cell apoptosis in Caenorhabditis elegans hermaphrodites is a physiological process eliminating around 60% of all cells in meiotic prophase to maintain tissue homeostasis. In contrast to programmed cell death in the C. elegans soma, the selection of germ cells undergoing apoptosis is stochastic. By live-tracking individual germ cells at the pachytene stage, we found that germ cells smaller than their neighbors are selectively eliminated through apoptosis before differentiating into oocytes. Thus, cell size is a strong predictor of physiological germ cell death. The RAS/MAPK and ECT/RHO/ROCK pathways together regulate germ cell size by controlling actomyosin constriction at the apical rachis bridges, which are cellular openings connecting the syncytial germ cells to a shared cytoplasmic core. Enhancing apical constriction reduces germ cell size and increases the rate of cell death while inhibiting the actomyosin network in the germ cells prevents their death. We propose that actomyosin contractility at the rachis bridges of the syncytial germ cells amplifies intrinsic disparities in cell size. Through this mechanism, the animals can adjust the balance between physiological germ cell death and oocyte differentiation.

Rbpms2 promotes female fate upstream of the nutrient sensing Gator2 complex component Mios

Reproductive success relies on proper establishment and maintenance of biological sex. In many animals, including mammals, the primary gonad is initially ovary biased. We previously showed the RNA binding protein (RNAbp), Rbpms2, is required for ovary fate in zebrafish. Here, we identified Rbpms2 targets in oocytes (Rbpms2-bound oocyte RNAs; rboRNAs). We identify Rbpms2 as a translational regulator of rboRNAs, which include testis factors and ribosome biogenesis factors. Further, genetic analyses indicate that Rbpms2 promotes nucleolar amplification via the mTorc1 signaling pathway, specifically through the mTorc1-activating Gap activity towards Rags 2 (Gator2) component, Missing oocyte (Mios). Cumulatively, our findings indicate that early gonocytes are in a dual poised, bipotential state in which Rbpms2 acts as a binary fate-switch. Specifically, Rbpms2 represses testis factors and promotes oocyte factors to promote oocyte progression through an essential Gator2-mediated checkpoint, thereby integrating regulation of sexual differentiation factors and nutritional availability pathways in zebrafish oogenesis.

DOS-3 mediates cell-non-autonomous DAF-16/FOXO activity in antagonizing age-related loss of C. elegans germline stem/progenitor cells

Age-related depletion of stem cells causes tissue degeneration and failure to tissue regeneration, driving aging at the organismal level. Previously we reported a cell-non-autonomous DAF-16/FOXO activity in antagonizing the age-related loss of germline stem/progenitor cells (GSPCs) in C. elegans, indicating that regulation of stem cell aging occurs at the organ system level. Here we discover the molecular effector that links the cell-non-autonomous DAF-16/FOXO activity to GSPC maintenance over time by performing a tissue-specific DAF-16/FOXO transcriptome analysis. Our data show that dos-3, which encodes a non-canonical Notch ligand, is a direct transcriptional target of DAF-16/FOXO and mediates the effect of the cell-non-autonomous DAF-16/FOXO activity on GSPC maintenance through activating Notch signaling in the germ line. Importantly, expression of a human homologous protein can functionally substitute for DOS-3 in this scenario. As Notch signaling controls the specification of many tissue stem cells, similar mechanisms may exist in other aging stem cell systems.

Patterning, regulation, and role of FoxO/DAF-16 in the early embryo

Insulin resistance and diabetes are associated with many health issues including higher rates of birth defects and miscarriage during pregnancy. Because insulin resistance and diabetes are both associated with obesity, which also affects fertility, the role of insulin signaling itself in embryo development is not well understood. A key downstream target of the insulin/insulin-like growth factor signaling (IIS) pathway is the forkhead family transcription factor FoxO (DAF-16 in C. elegans ). Here, we used quantitative live imaging to measure the patterning of endogenously tagged FoxO/DAF-16 in the early worm embryo. In 2-4-cell stage embryos, FoxO/DAF-16 initially localized uniformly to all cell nuclei, then became dramatically enriched in germ precursor cell nuclei beginning at the 8-cell stage. This nuclear enrichment in early germ precursor cells required germ fate specification, PI3K (AGE-1)- and PTEN (DAF-18)-mediated phospholipid regulation, and the deubiquitylase USP7 (MATH-33), yet was unexpectedly insulin receptor (DAF-2)- and AKT-independent. Functional analysis revealed that FoxO/DAF-16 acts as a cell cycle pacer for early cleavage divisions-without FoxO/DAF-16 cell cycles were shorter than in controls, especially in germ lineage cells. These results reveal the germ lineage specific patterning, upstream regulation, and cell cycle role for FoxO/DAF-16 during early C. elegans embryogenesis.

Rbpms2 promotes female fate upstream of the nutrient sensing Gator2 complex component, Mios

Reproductive success relies on proper establishment and maintenance of biological sex. In many animals, including mammals, the primary gonad is initially ovary in character. We previously showed the RNA binding protein (RNAbp), Rbpms2, is required for ovary fate in zebrafish. Here, we identified Rbpms2 targets in oocytes (Rbpms2-bound oocyte RNAs; rboRNAs). We identify Rbpms2 as a translational regulator of rboRNAs, which include testis factors and ribosome biogenesis factors. Further, genetic analyses indicate that Rbpms2 promotes nucleolar amplification via the mTorc1 signaling pathway, specifically through the mTorc1-activating Gap activity towards Rags 2 (Gator2) component, Missing oocyte (Mios). Cumulatively, our findings indicate that early gonocytes are in a dual poised, bipotential state in which Rbpms2 acts as a binary fate-switch. Specifically, Rbpms2 represses testis factors and promotes oocyte factors to promote oocyte progression through an essential Gator2-mediated checkpoint, thereby integrating regulation of sexual differentiation factors and nutritional availability pathways in zebrafish oogenesis.

Two distinct mechanisms lead to either oocyte or spermatocyte decrease in C. elegans after whole developmental exposure to γ-rays

Wildlife is subject to various sources of pollution, including ionizing radiation. Adverse effects can impact the survival, growth, or reproduction of organisms, later affecting population dynamics. In invertebrates, reproduction, which directly impacts population dynamics, has been found to be the most radiosensitive endpoint. Understanding the underlying molecular pathways inducing this reproduction decrease can help to comprehend species-specific differences in radiosensitivity. From our previous studies, we found that decrease in reproduction is life stage dependent in the roundworm Caenorhabditis elegans, possibly resulting from an accumulation of damages during germ cell development and gamete differentiation. To go further, we used the same experimental design to assess more precisely the molecular determinants of reproductive toxicity, primarily decreases in gamete number. As before, worms were chronically exposed to 50 mGy·h-1 external gamma ionizing radiation throughout different developmental periods (namely embryogenesis, gametogenesis, and full development). To enable cross species extrapolation, conserved molecular pathways across invertebrates and vertebrates were analysed: apoptosis and MAP kinase Ras/ERK (MPK-1), both involved in reproduction and stress responses. Our results showed that these pathways are life-stage dependent, resulting from an accumulation of damages upon chronic exposure to IR throughout the life development. The Ras/ERK pathway was activated in our conditions in the pachytene region of the gonad where it regulates cell fate including apoptosis, but not in the ovulation zone, where it controls oocyte maturation and ovulation. Additionally, assessment of germ cell proliferation via Ras/ERK pathway showed no effect. Finally, a functional analysis of apoptosis revealed that while the decrease of the ovulation rate is caused by DNA-damaged induced apoptosis, this process does not occur in spermatocytes. Thus, sperm decrease seems to be mediated via another mechanism, probably a decrease in germ cell proliferation speed that needs further investigation to better characterize sex-specific responses to IR exposure. These results are of main importance to describe radio-induced reprotoxic effects and contribute as weight of evidence for the AOP #396 "Deposition of ionizing energy leads to population decline via impaired meiosis".

Specific sensory neurons and insulin-like peptides modulate food type-dependent oogenesis and fertilization in Caenorhabditis elegans

An animal's responses to environmental cues are critical for its reproductive program. Thus, a mechanism that allows the animal to sense and adjust to its environment should make for a more efficient reproductive physiology. Here, we demonstrate that in Caenorhabditis elegans specific sensory neurons influence onset of oogenesis through insulin signaling in response to food-derived cues. The chemosensory neurons ASJ modulate oogenesis onset through the insulin-like peptide (ILP) INS-6. In contrast, other sensory neurons, the olfactory neurons AWA, regulate food type-dependent differences in C. elegans fertilization rates, but not onset of oogenesis. AWA modulates fertilization rates at least partly in parallel to insulin receptor signaling, since the insulin receptor DAF-2 regulates fertilization independently of food type, which requires ILPs other than INS-6. Together our findings suggest that optimal reproduction requires the integration of diverse food-derived inputs through multiple neuronal signals acting on the C. elegans germline.

DNA damage signals from somatic uterine tissue arrest oogenesis through activated DAF-16

Germ line integrity is crucial for progeny fitness. Organisms deploy the DNA damage response (DDR) signaling to protect the germ line from genotoxic stress, facilitating the cell-cycle arrest of germ cells and DNA repair or their apoptosis. Cell-autonomous regulation of germ line quality in response to DNA damage is well studied; however, how quality is enforced cell non-autonomously on sensing somatic DNA damage is less known. Using Caenorhabditis elegans, we show that DDR disruption, only in the uterus, when insulin/IGF-1 signaling (IIS) is low, arrests oogenesis in the pachytene stage of meiosis I, in a FOXO/DAF-16 transcription factor-dependent manner. Without FOXO/DAF-16, germ cells of the IIS mutant escape the arrest to produce poor-quality oocytes, showing that the transcription factor imposes strict quality control during low IIS. Activated FOXO/DAF-16 senses DDR perturbations during low IIS to lower ERK/MPK-1 signaling below a threshold to promote germ line arrest. Altogether, we elucidate a new surveillance role for activated FOXO/DAF-16 that ensures optimal germ cell quality and progeny fitness in response to somatic DNA damage.

Spatial single-cell sequencing of meiosis I arrested oocytes indicates acquisition of maternal transcripts from the soma

Maternal RNAs are stored from minutes to decades in oocytes throughout meiosis I arrest in a transcriptionally quiescent state. Recent reports, however, propose a role for nascent transcription in arrested oocytes. Whether arrested oocytes launch nascent transcription in response to environmental or hormonal signals while maintaining the meiosis I arrest remains undetermined. We test this by integrating single-cell RNA sequencing, RNA velocity, and RNA fluorescence in situ hybridization on C. elegans meiosis I arrested oocytes. We identify transcripts that increase as the arrested meiosis I oocyte ages, but rule out extracellular signaling through ERK MAPK and nascent transcription as a mechanism for this increase. We report transcript acquisition from neighboring somatic cells as a mechanism of transcript increase during meiosis I arrest. These analyses provide a deeper view at single-cell resolution of the RNA landscape of a meiosis I arrested oocyte and as it prepares for oocyte maturation and fertilization.

Regulation of germline proteostasis by HSF1 and insulin/IGF-1 signaling

Protein homeostasis (proteostasis) is essential for cellular function and organismal health and requires the concerted actions of protein synthesis, folding, transport, and turnover. In sexually reproducing organisms, the immortal germline lineage passes genetic information across generations. Accumulating evidence indicates the importance of proteome integrity for germ cells as genome stability. As gametogenesis involves very active protein synthesis and is highly energy-demanding, it has unique requirements for proteostasis regulation and is sensitive to stress and nutrient availability. The heat shock factor 1 (HSF1), a key transcriptional regulator of cellular response to cytosolic and nuclear protein misfolding has evolutionarily conserved roles in germline development. Similarly, insulin/insulin-like growth factor-1 (IGF-1) signaling, a major nutrient-sensing pathway, impacts many aspects of gametogenesis. Here, we focus on HSF1 and IIS to review insights into their roles in germline proteostasis and discuss the implications on gamete quality control during stress and aging.

Analysis of the C. elegans Germline Stem Cell Pool

The Caenorhabditis elegans germline is an excellent model for studying the genetic and molecular regulation of stem cell self-renewal and progression of cells from a stem cell state to a differentiated state. The germline tissue is organized in an assembly line with the germline stem cell (GSC) pool at one end and differentiated gametes at the other. A simple mesenchymal niche caps the GSC pool and maintains GSCs in an undifferentiated state by signaling through the conserved Notch pathway. Notch signaling activates transcription of the key GSC regulators lst-1 and sygl-1 proteins in a gradient through the GSC pool. LST-1 and SYGL-1 proteins work with PUF RNA regulators in a self-renewal hub to maintain the GSC pool. In this chapter, we present methods for characterizing the C. elegans GSC pool and early stages of germ cell differentiation. The methods include examination of germlines in living and fixed worms, cell cycle analysis, and analysis of markers. We also discuss assays to separate mutant phenotypes that affect the stem cell vs. differentiation decision from those that affect germ cell processes more generally.

Innexin function dictates the spatial relationship between distal somatic cells in the Caenorhabditis elegans gonad without impacting the germline stem cell pool

Gap-junctional signaling mediates myriad cellular interactions in metazoans. Yet, how gap junctions control the positioning of cells in organs is not well understood. Innexins compose gap junctions in invertebrates and affect organ architecture. Here, we investigate the roles of gap-junctions in controlling distal somatic gonad architecture and its relationship to underlying germline stem cells in Caenorhabditis elegans. We show that a reduction of soma-germline gap-junctional activity causes displacement of distal sheath cells (Sh1) towards the distal end of the gonad. We confirm, by live imaging, transmission electron microscopy, and antibody staining, that bare regions-lacking somatic gonadal cell coverage of germ cells-are present between the distal tip cell (DTC) and Sh1, and we show that an innexin fusion protein used in a prior study encodes an antimorphic gap junction subunit that mispositions Sh1. We determine that, contrary to the model put forth in the prior study based on this fusion protein, Sh1 mispositioning does not markedly alter the position of the borders of the stem cell pool nor of the progenitor cell pool. Together, these results demonstrate that gap junctions can control the position of Sh1, but that Sh1 position is neither relevant for GLP-1/Notch signaling nor for the exit of germ cells from the stem cell pool.

Regulation of oocyte maturation: Role of conserved ERK signaling

During oogenesis, oocytes arrest at meiotic prophase I to acquire competencies for resuming meiosis, fertilization, and early embryonic development. Following this arrested period, oocytes resume meiosis in response to species-specific hormones, a process known as oocyte maturation, that precedes ovulation and fertilization. Involvement of endocrine and autocrine/paracrine factors and signaling events during maintenance of prophase I arrest, and resumption of meiosis is an area of active research. Studies in vertebrate and invertebrate model organisms have delineated the molecular determinants and signaling pathways that regulate oocyte maturation. Cell cycle regulators, such as cyclin-dependent kinase (CDK1), polo-like kinase (PLK1), Wee1/Myt1 kinase, and the phosphatase CDC25 play conserved roles during meiotic resumption. Extracellular signal-regulated kinase (ERK), on the other hand, while activated during oocyte maturation in all species, regulates both species-specific, as well as conserved events among different organisms. In this review, we synthesize the general signaling mechanisms and focus on conserved and distinct functions of ERK signaling pathway during oocyte maturation in mammals, non-mammalian vertebrates, and invertebrates such as Drosophila and Caenorhabditis elegans.

Reevaluation of the role of LIP-1 as an ERK/MPK-1 dual specificity phosphatase in the C. elegans germline

The RAS–ERK pathway is critical for metazoan development. In development, ERK activity is regulated by a balance of phosphorylation of ERK by MEK (MAPK kinase) and dephosphorylation by DUSPs (dual specificity phosphatases). LIP-1, a DUSP6/7 family member, was previously suggested to regulate MPK-1/ERK activity by dephosphorylating MPK-1 in the Caenorhabditis elegans germline, based on LIP-1's mutant phenotype in the germline and its DUSP role in vulval development. However, our investigations demonstrate that LIP-1 does not function as an MPK-1 DUSP in the germline and likely regulates germline functions through distinct targets. Our results present a cautionary note about misinterpreting similar mutant phenotypes caused by mutations in different genes and assuming that genes function similarly in different tissues.

The fidelity of a signaling pathway depends on its tight regulation in space and time. Extracellular signal-regulated kinase (ERK) controls wide-ranging cellular processes to promote organismal development and tissue homeostasis. ERK activation depends on a reversible dual phosphorylation on the TEY motif in its active site by ERK kinase (MEK) and dephosphorylation by DUSPs (dual specificity phosphatases). LIP-1, a DUSP6/7 homolog, was proposed to function as an ERK (MPK-1) DUSP in the Caenorhabditis elegans germline primarily because of its phenotype, which morphologically mimics that of a RAS/let-60 gain-of-function mutant (i.e., small oocyte phenotype). Our investigations, however, reveal that loss of lip-1 does not lead to an increase in MPK-1 activity in vivo. Instead, we show that loss of lip-1 leads to 1) a decrease in MPK-1 phosphorylation, 2) lower MPK-1 substrate phosphorylation, 3) phenocopy of mpk-1 reduction-of-function (rather than gain-of-function) allele, and 4) a failure to rescue mpk-1–dependent germline or fertility defects. Moreover, using diverse genetic mutants, we show that the small oocyte phenotype does not correlate with increased ectopic MPK-1 activity and that ectopic increase in MPK-1 phosphorylation does not necessarily result in a small oocyte phenotype. Together, these data demonstrate that LIP-1 does not function as an MPK-1 DUSP in the C. elegans germline. Our results caution against overinterpretation of the mechanistic underpinnings of orthologous phenotypes, since they may be a result of independent mechanisms, and provide a framework for characterizing the distinct molecular targets through which LIP-1 may mediate its several germline functions.

End-of-life targeted degradation of DAF-2 insulin/IGF-1 receptor promotes longevity free from growth-related pathologies

Preferably, lifespan-extending therapies should work when applied late in life without causing undesired pathologies. Reducing insulin/insulin-like growth factor (IGF)-1 signaling (IIS) increases lifespan across species, but the effects of reduced IIS interventions in extreme geriatric ages remains unknown. Using the nematode Caenorhabditis elegans, we engineered the conditional depletion of the DAF-2/insulin/IGF-1 transmembrane receptor using an auxin-inducible degradation (AID) system. This allowed for the temporal and spatial reduction in DAF-2 protein levels at time points after which interventions such as RNAi become ineffective. Using this system, we found that AID-mediated depletion of DAF-2 protein surpasses the longevity of daf-2 mutants. Depletion of DAF-2 during early adulthood resulted in multiple adverse phenotypes, including growth retardation, germline shrinkage, egg retention, and reduced brood size. By contrast, AID-mediated depletion of DAF-2 post-reproduction, or specifically in the intestine in early adulthood, resulted in an extension of lifespan without these deleterious effects. Strikingly, at geriatric ages, when 75% of the population had died, AID-mediated depletion of DAF-2 protein resulted in a doubling in lifespan. Thus, we provide a proof-of-concept that even close to the end of an individual’s lifespan, it is possible to slow aging and promote longevity.

The goal of geroscience, or research into old age, is to promote health during old age, and thus, to increase lifespan. In the body, the groups of biochemical reactions, or ‘pathways’, that allow an organism to sense nutrients, and regulate growth and stress, play major roles in ensuring healthy aging. Indeed, organisms that do not produce a working version of the insulin/IGF-1 receptor, a protein involved in one such pathway, show increased lifespan. In the worm Caenorhabditis elegans, mutations in the insulin/IGF-1 receptor can even double their lifespan. However, it is unclear whether this increase can be achieved once the organism has reached old age.

To answer this question, Venz et al. genetically engineered the nematode worm C. elegans so that they could trigger the rapid degradation of the insulin/IGF-1 receptor either in the entire organism or in a specific tissue. Venz et al. started by aging several C. elegans worms for three weeks, until about 75% had died. At this point, they triggered the degradation of the insulin/IGF-1 receptor in some of the remaining worms, keeping the rest untreated as a control for the experiment.

The results showed that the untreated worms died within a few days, while worms in which the insulin/IGF-1 receptor had been degraded lived for almost one more month. This demonstrates that it is possible to double the lifespan of an organism at the very end of life.

Venz et al.’s findings suggest that it is possible to make interventions to extend an organism’s lifespan near the end of life that are as effective as if they were performed when the organism was younger. This sparks new questions regarding the quality of this lifespan extension: do the worms become younger with the intervention, or is aging simply slowed down?

Insulin/IGF-1 signaling and heat stress differentially regulate HSF1 activities in germline development

Germline development is sensitive to nutrient availability and environmental perturbation. Heat shock transcription factor 1 (HSF1), a key transcription factor driving the cellular heat shock response (HSR), is also involved in gametogenesis. The precise function of HSF1 (HSF-1 in C. elegans) and its regulation in germline development are poorly understood. Using the auxin-inducible degron system in C. elegans, we uncovered a role of HSF-1 in progenitor cell proliferation and early meiosis and identified a compact but important transcriptional program of HSF-1 in germline development. Interestingly, heat stress only induces the canonical HSR in a subset of germ cells but impairs HSF-1 binding at its developmental targets. Conversely, insulin/insulin growth factor 1 (IGF-1) signaling dictates the requirement for HSF-1 in germline development and functions through repressing FOXO/DAF-16 in the soma to activate HSF-1 in germ cells. We propose that this non-cell-autonomous mechanism couples nutrient-sensing insulin/IGF-1 signaling to HSF-1 activation to support homeostasis in rapid germline growth.

Germline Stem and Progenitor Cell Aging in C. elegans

Like many animals and humans, reproduction in the nematode C. elegans declines with age. This decline is the cumulative result of age-related changes in several steps of germline function, many of which are highly accessible for experimental investigation in this short-lived model organism. Here we review recent work showing that a very early and major contributing step to reproductive decline is the depletion of the germline stem and progenitor cell pool. Since many cellular and molecular aspects of stem cell biology and aging are conserved across animals, understanding mechanisms of age-related decline of germline stem and progenitor cells in C. elegans has broad implications for aging stem cells, germline stem cells, and reproductive aging.

An interview with Swathi Arur

Swathi Arur is an Associate Professor for the Department of Genetics at the MD Anderson Cancer Center, USA, where she uses multidisciplinary approaches to understand female germline development and fertility. She has received numerous accolades, including the MD Anderson Distinguished Research Faculty Mentor Award in 2017. In 2020, she was elected to the American Association for the Advancement of Science (AAAS). Swathi joined the team at Development as an Academic Editor in 2020, and we met with her over Zoom to hear more about her life, her career and her love for C. elegans.

Starvation Responses Throughout the Caenorhabditiselegans Life Cycle

Caenorhabditis elegans survives on ephemeral food sources in the wild, and the species has a variety of adaptive responses to starvation. These features of its life history make the worm a powerful model for studying developmental, behavioral, and metabolic starvation responses. Starvation resistance is fundamental to life in the wild, and it is relevant to aging and common diseases such as cancer and diabetes. Worms respond to acute starvation at different times in the life cycle by arresting development and altering gene expression and metabolism. They also anticipate starvation during early larval development, engaging an alternative developmental program resulting in dauer diapause. By arresting development, these responses postpone growth and reproduction until feeding resumes. A common set of signaling pathways mediates systemic regulation of development in each context but with important distinctions. Several aspects of behavior, including feeding, foraging, taxis, egg laying, sleep, and associative learning, are also affected by starvation. A variety of conserved signaling, gene regulatory, and metabolic mechanisms support adaptation to starvation. Early life starvation can have persistent effects on adults and their descendants. With its short generation time, C. elegans is an ideal model for studying maternal provisioning, transgenerational epigenetic inheritance, and developmental origins of adult health and disease in humans. This review provides a comprehensive overview of starvation responses throughout the C. elegans life cycle.

The molecular tug of war between immunity and fertility: Emergence of conserved signaling pathways and regulatory mechanisms

Reproduction and immunity are energy intensive, intimately linked processes in most organisms. In women, pregnancy is associated with widespread immunological adaptations that alter immunity to many diseases, whereas, immune dysfunction has emerged as a major cause for infertility in both men and women. Deciphering the molecular bases of this dynamic association is inherently challenging in mammals. This relationship has been traditionally studied in fast-living, invertebrate species, often in the context of resource allocation between life history traits. More recently, these studies have advanced our understanding of the mechanistic underpinnings of the immunity-fertility dialogue. Here, we review the molecular connections between reproduction and immunity from the perspective of human pregnancy to mechanistic discoveries in laboratory organisms. We focus particularly on recent invertebrate studies identifying conserved signaling pathways and transcription factors that regulate resource allocation and shape the balance between reproductive status and immune health.

Two classes of active transcription sites and their roles in developmental regulation

The expression of genes encoding powerful developmental regulators is exquisitely controlled, often at multiple levels. Here, we investigate developmental expression of three conserved genes, Caenorhabditis elegans mpk-1, lag-1, and lag-3/sel-8, which encode homologs of ERK/MAPK and core components of the Notch-dependent transcription complex, respectively. We use single-molecule FISH (smFISH) and MATLAB to visualize and quantify nuclear nascent transcripts and cytoplasmic mRNAs as a function of position along the germline developmental axis. Using differentially labeled probes, one spanning an exceptionally long first intron and the other spanning exons, we identify two classes of active transcription sites (ATS). The iATS class, for "incomplete" ATS, harbors only partial nascent transcripts; the cATS class, for "complete" ATS, harbors full-length nascent transcripts. Remarkably, the frequencies of iATS and cATS are patterned along the germline axis. For example, most mpk-1 ATS are iATS in hermaphrodite germline stem cells, but most are cATS in differentiating stem cell daughters. Thus, mpk-1 ATS class frequencies switch in a graded manner as stem cell daughters begin differentiation. Importantly, the patterns of ATS class frequency are gene-, stage-, and sex-specific, and cATS frequency strongly correlates with transcriptional output. Although the molecular mechanism underlying ATS classes is not understood, their primary difference is the extent of transcriptional progression. To generate only partial nascent transcripts in iATS, progression must be slowed, paused, or aborted midway through the gene. We propose that regulation of ATS class can be a critical mode of developmental gene regulation.

ERK phosphorylates chromosomal axis component HORMA domain protein HTP-1 to regulate oocyte numbers

Oocyte numbers, a critical determinant of female reproductive fitness, are highly regulated, yet the mechanisms underlying this regulation remain largely undefined. In the Caenorhabditis elegans gonad, RAS/extracellular signal-regulated kinase (ERK) signaling regulates oocyte numbers; mechanisms are unknown. We show that the RAS/ERK pathway phosphorylates meiotic chromosome axis protein HTP-1 at serine-325 to control chromosome dynamics and regulate oocyte number. Phosphorylated HTP-1(S325) accumulates in vivo in an ERK-dependent manner in early-mid pachytene stage germ cells and is necessary for synaptonemal complex extension and/or maintenance. Lack of HTP-1 phosphorylation leads to asynapsis and persistence of meiotic double-strand breaks, causing delayed meiotic progression and reduced oocyte number. In contrast, early onset of ERK activation causes precocious meiotic progression, resulting in increased oocyte number, which is reversed by removal of HTP-1 phosphorylation. The RAS/ERK/HTP-1 signaling cascade thus functions to monitor formation and maintenance of synapsis for timely resolution of double-strand breaks, oocyte production, and reproductive fitness.

Nutrient status shapes selfish mitochondrial genome dynamics across different levels of selection

Cooperation and cheating are widespread evolutionary strategies. While cheating confers an advantage to individual entities within a group, competition between groups favors cooperation. Selfish or cheater mitochondrial DNA (mtDNA) proliferates within hosts while being selected against at the level of host fitness. How does environment shape cheater dynamics across different selection levels? Focusing on food availability, we address this question using heteroplasmic Caenorhabditis elegans. We find that the proliferation of selfish mtDNA within hosts depends on nutrient status stimulating mtDNA biogenesis in the developing germline. Interestingly, mtDNA biogenesis is not sufficient for this proliferation, which also requires the stress-response transcription factor FoxO/DAF-16. At the level of host fitness, FoxO/DAF-16 also prevents food scarcity from accelerating the selection against selfish mtDNA. This suggests that the ability to cope with nutrient stress can promote host tolerance of cheaters. Our study delineates environmental effects on selfish mtDNA dynamics at different levels of selection.

Glucose and cholesterol induce abnormal cell divisions via DAF-12 and MPK-1 in C. elegans

People exposed to starvation have a high risk of developing cancer later in life, and prior studies have shown these individuals have high insulin and cholesterol levels and are sensitive to glucose. Using C. elegans as a model, we found that glucose and cholesterol can promote survival and cause starved L1 diapause worms to undergo abnormal neuronal cell divisions. Starvation has also been shown to promote long-term survival; however, we found that the functions of glucose and cholesterol in relation to these cell divisions are distinct from their effects on survival. We demonstrate that glucose functions in a DAF-16/FOXO-independent IIS pathway to activate the MAPK ontogenetic signaling to induce neuronal Q-cell divisions, and cholesterol works through DAF-12/steroidogenic pathways to promote these cell divisions. daf-12 and mpk-1/MAPK mutants suppress the function of glucose and cholesterol in these divisions, and a fully functioning dpMPK-1 requires the steroid hormone receptor DAF-12 for these divisions to occur. These afflictions also can be passed on to the immediate progeny. This work indicates a possible link between glucose and cholesterol in starved animals and an increased risk of cancer.

Transcriptome changes reveal the genetic mechanisms of the reproductive plasticity of workers in lower termites

Background

The reproductive plasticity of termite workers provides colonies with tremendous flexibility to respond to environmental changes, which is the basis for evolutionary and ecological success. Although it is known that all colony members share the same genetic background and that differences in castes are caused by differences in gene expression, the pattern of the specific expression of genes involved in the differentiation of workers into reproductives remains unclear. In this study, the isolated workers of Reticulitermes labralis developed into reproductives, and then comparative transcriptomes were used for the first time to reveal the molecular mechanisms underlying the reproductive plasticity of workers.

Results

We identified 38,070 differentially expressed genes and found a pattern of gene expression involved in the differentiation of the workers into reproductives. 12, 543 genes were specifically upregulated in the isolated workers. Twenty-five signal transduction pathways classified into environmental information processing were related to the differentiation of workers into reproductives. Ras functions as a signalling switch regulates the reproductive plasticity of workers. The catalase gene which is related to longevity was up-regulated in reproductives.

Conclusion

We demonstrate that workers leaving the natal colony can induce the expression of stage-specific genes in the workers, which leads to the differentiation of workers into reproductives and suggests that the signal transduction along the Ras-MAPK pathway crucially controls the reproductive plasticity of the workers. This study also provides an important model for revealing the molecular mechanism of longevity changes.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6037-y) contains supplementary material, which is available to authorized users.

MPK-1/ERK pathway regulates DNA damage response during development through DAF-16/FOXO

Ultraviolet (UV) induces distorting lesions to the DNA that can lead to stalling of the RNA polymerase II (RNAP II) and that are removed by transcription-coupled nucleotide excision repair (TC-NER). In humans, mutations in the TC-NER genes CSA and CSB lead to severe postnatal developmental defects in Cockayne syndrome patients. In Caenorhabditis elegans, mutations in the TC-NER genes csa-1 and csb-1, lead to developmental growth arrest upon UV treatment. We conducted a genetic suppressor screen in the nematode to identify mutations that could suppress the developmental defects in csb-1 mutants. We found that mutations in the ERK1/2 MAP kinase mpk-1 alleviate the developmental retardation in TC-NER mutants, while constitutive activation of the RAS-MAPK pathway exacerbates the DNA damage-induced growth arrest. We show that MPK-1 act via insulin/insulin-like signaling pathway and regulates the FOXO transcription factor DAF-16 to mediate the developmental DNA damage response.

Fasting prevents hypoxia-induced defects of proteostasis in C. elegans

Low oxygen conditions (hypoxia) can impair essential physiological processes and cause cellular damage and death. We have shown that specific hypoxic conditions disrupt protein homeostasis in C. elegans, leading to protein aggregation and proteotoxicity. Here, we show that nutritional cues regulate this effect of hypoxia on proteostasis. Animals fasted prior to hypoxic exposure develop dramatically fewer polyglutamine protein aggregates compared to their fed counterparts, indicating that the effect of hypoxia is abrogated. Fasting also reduced the hypoxia-induced exaggeration of proteostasis defects in animals that express Aβ_1–42 and in animals with a temperature-sensitive mutation in dyn-1, suggesting that this effect was not specific to polyglutamine proteins. Our data also demonstrate that the nutritional environment experienced at the onset of hypoxia dictates at least some aspects of the physiological response to hypoxia. We further demonstrate that the insulin/IGF-like signaling pathway plays a role in mediating the protective effects of fasting in hypoxia. Animals with mutations in daf-2, the C. elegans insulin-like receptor, display wild-type levels of hypoxia-induced protein aggregation upon exposure to hypoxia when fed, but are not protected by fasting. DAF-2 acts independently of the FOXO transcription factor, DAF-16, to mediate the protective effects of fasting. These results suggest a non-canonical role for the insulin/IGF-like signaling pathway in coordinating the effects of hypoxia and nutritional state on proteostasis.

When blood flow to various parts of the body becomes restricted, those tissues suffer from a lack of oxygen, a condition called hypoxia. Hypoxia can cause cellular damage and death, as in stroke and cardiovascular disease. We have found that in the model organism C. elegans (a roundworm) specific concentrations of hypoxia cause aggregation of polyglutamine proteins–the same kind of proteins that are found in an aggregated state in the neurodegenerative disorder Huntington’s disease. Here, we show that that worms can be protected from hypoxia-induced protein aggregation if they are fasted (removed from their food source) prior to experiencing hypoxia. Furthermore, we show that the insulin receptor is required for this protection. The insulin receptor is responsible for detecting insulin, a hormone that is released after feeding. Worms with a nonfunctional version of the insulin receptor displayed hypoxia-induced protein aggregation despite being fasted before the hypoxic exposure. Our results highlight a new role for the insulin signaling pathway in coordinating the effects of both hypoxia and nutritional state on protein aggregation.

Apoptosis contributes to protect germ cells from the oogenic germline starvation response but is not essential for the gonad shrinking or recovery observed during adult reproductive diapause in C. elegans

When C. elegans hermaphrodites are deprived of food during the mid-L4 larval stage and throughout adulthood, they enter an alternative stage termed "adult reproductive diapause (ARD)" in which they halt reproduction and extend their lifespan. During ARD, germ cell proliferation stops; oogenesis is slowed; and the gonad shrinks progressively, which has been described as the "oogenic germline starvation response". Upon refeeding, the shrunken gonad is regenerated, and animals recover fertility and live out their remaining lifespan. Little is known about the effects of ARD on oocyte quality after ARD. Thus, the aim of this study was to determine how oocyte quality is affected after ARD by measuring brood size and embryonic lethality as a reflection of defective oocyte production. We found that ARD affects reproductive capacity. The oogenic germline starvation response protects oogenic germ cells by slowing oogenesis to prevent prolonged arrest in diakinesis. In contrast to a previous report, we found that germ cell apoptosis is not the cause of gonad shrinkage; instead, we propose that ovulation contributes to gonad shrinkage during the oogenic germline starvation response. We show that germ cell apoptosis increases and continues during ARD via lin-35/Rb and an unknown mechanism. Although apoptosis contributes to maintain germ cell quality during ARD, we demonstrated that apoptosis is not essential to preserve animal fertility. Finally, we show that IIS signaling inactivation partially participates in the oogenic germline starvation response.

The DNA damage checkpoint and the spindle position checkpoint: guardians of meiotic commitment

Exogenous signals induce cells to enter the specialized cell division process of meiosis, which produces haploid gametes from diploid progenitor cells. Once cells initiate the meiotic divisions, it is imperative that they complete meiosis. Inappropriate exit from meiosis and entrance into mitosis can create polyploid cells and can lead to germline tumors. Saccharomyces cerevisiae cells enter meiosis when starved of nutrients but can return to mitosis if provided nutrient-rich medium before a defined commitment point. Once past the meiotic commitment point in prometaphase I, cells stay committed to meiosis even in the presence of a mitosis-inducing signal. Recent research investigated the maintenance of meiotic commitment in budding yeast and found that two checkpoints that do not normally function in meiosis I, the DNA damage checkpoint and the spindle position checkpoint, have crucial functions in maintaining meiotic commitment. Here, we review these findings and discuss how the mitosis-inducing signal of nutrient-rich medium could activate these two checkpoints in meiosis to prevent inappropriate meiotic exit.

C. elegans PTEN and AMPK block neuroblast divisions by inhibiting a BMP-insulin-PP2A-MAPK pathway

Caenorhabditis elegans that hatch in the absence of food stop their postembryonic development in a process called L1 arrest. Intriguingly, we find that the postembryonic Q neuroblasts divide and migrate during L1 arrest in mutants that have lost the energy sensor AMP-activated protein kinase (AMPK) or the insulin/IGF-1 signaling (IIS) negative regulator DAF-18/PTEN. We report that DBL-1/BMP works upstream of IIS to promote agonistic insulin-like peptides during L1 arrest. However, the abnormal Q cell divisions that occur during L1 arrest use a novel branch of the IIS pathway that is independent of the terminal transcription factor DAF-16/FOXO. Using genetic epistasis and drug interactions we show that AMPK functions downstream of, or in parallel with DAF-18/PTEN and IIS to inhibit PP2A function. Further, we show that PP2A regulates the abnormal Q cell divisions by activating the MPK-1/ERK signaling pathway via LIN-45/RAF, independently of LET-60/RAS. PP2A acts as a tumor suppressor in many oncogenic signaling cascades. Our work demonstrates a new role for PP2A that is needed to induce neuroblast divisions during starvation and is regulated by both insulin and AMPK.

The insulin/IGF signaling cascade modulates SUMOylation to regulate aging and proteostasis in Caenorhabditis elegans

Although aging-regulating pathways were discovered a few decades ago, it is not entirely clear how their activities are orchestrated, to govern lifespan and proteostasis at the organismal level. Here, we utilized the nematode Caenorhabditis elegans to examine whether the alteration of aging, by reducing the activity of the Insulin/IGF signaling (IIS) cascade, affects protein SUMOylation. We found that IIS activity promotes the SUMOylation of the germline protein, CAR-1, thereby shortening lifespan and impairing proteostasis. In contrast, the expression of mutated CAR-1, that cannot be SUMOylated at residue 185, extends lifespan and enhances proteostasis. A mechanistic analysis indicated that CAR-1 mediates its aging-altering functions, at least partially, through the notch-like receptor glp-1. Our findings unveil a novel regulatory axis in which SUMOylation is utilized to integrate the aging-controlling functions of the IIS and of the germline and provide new insights into the roles of SUMOylation in the regulation of organismal aging.

Aging may seem inescapable, but there are many factors, from diet to genetic mutations, that can affect this process. In fact, scientists have started to uncover the mechanisms that control and influence this slow decline. For example, in the small worm Caenorhabditis elegans, removing the germs cells – which give rise to eggs – extends the lifespan. Similarly, interfering with the activity of the Insulin/IGF-1 signaling (IIS) pathway leads to a longer life for the animals. However, it is unclear whether these two mechanisms work together, or if they operate in parallel.

To explore this, Moll, Roitenberg et al. first looked at how the IIS pathway regulates a type of protein modification known as SUMOylation in C. elegans. Reducing the activity of the IIS pathway slowed down aging in the worms. It also decreased the levels of SUMOylation of certain proteins, including CAR-1, which is found in the structures that produce germ cells. Further experiments showed that stopping the SUMOylation of CAR-1 extended the lifespan of the animals. In fact, replacing the protein with a mutated version of CAR-1 that cannot accept the SUMO element makes the worms live longer and resist a toxic protein that causes Alzheimer’s disease in humans. These results therefore show that, in C. elegans, the IIS pathway and a mechanism that involves CAR-1 in germ cells work together to determine the pace of aging. Further studies are now needed to dissect how the IIS pathway influences SUMOylation, and whether the findings hold true in mammals.

Spatiotemporal Gene Expression Analysis of the Caenorhabditis elegans Germline Uncovers a Syncytial Expression Switch

Developmental programs are executed by tightly controlled gene regulatory pathways. Here, we combined the unique sample retrieval capacity afforded by laser capture microscopy with analysis of mRNA abundance by CEL-Seq (cell expression by linear amplification and sequencing) to generate a spatiotemporal gene expression map of the Caenorhabditis elegans syncytial germline from adult hermaphrodites and males. We found that over 6000 genes exhibit spatiotemporally dynamic expression patterns throughout the hermaphrodite germline, with two dominant groups of genes exhibiting reciprocal shifts in expression at late pachytene during meiotic prophase I. We found a strong correlation between restricted spatiotemporal expression and known developmental and cellular processes, indicating that these gene expression changes may be an important driver of germ cell progression. Analysis of the male gonad revealed a shift in gene expression at early pachytene and upregulation of subsets of genes following the meiotic divisions, specifically in early and late spermatids, mostly transcribed from the X chromosome. We observed that while the X chromosome is silenced throughout the first half of the gonad, some genes escape this control and are highly expressed throughout the germline. Although we found a strong correlation between the expression of genes corresponding to CSR-1-interacting 22G-RNAs during germ cell progression, we also found that a large fraction of genes may bypass the need for CSR-1-mediated germline licensing. Taken together, these findings suggest the existence of mechanisms that enable a shift in gene expression during prophase I to promote germ cell progression.

Feeding state regulates pheromone-mediated avoidance behavior via the insulin signaling pathway in Caenorhabditis elegans

Animals change sensory responses and their eventual behaviors, depending on their internal metabolic status and external food availability. However, the mechanisms underlying feeding state-dependent behavioral changes remain undefined. Previous studies have shown that Caenorhabditis elegans hermaphrodite exhibits avoidance behaviors to acute exposure of a pheromone, ascr#3 (asc-ΔC9, C9). Here, we show that the ascr#3 avoidance behavior is modulated by feeding state via the insulin signaling pathway. Starvation increases ascr#3 avoidance behavior, and loss-of-function mutations in daf-2 insulin-like receptor gene dampen this starvation-induced ascr#3 avoidance behavior. DAF-2 and its downstream signaling molecules, including the DAF-16 FOXO transcription factor, act in the ascr#3-sensing ADL neurons to regulate synaptic transmission to downstream target neurons, including the AVA command interneurons. Moreover, we found that starvation decreases the secretion of INS-18 insulin-like peptides from the intestine, which antagonizes DAF-2 function in the ADL neurons. Altogether, this study provides insights about the molecular communication between intestine and sensory neurons delivering hunger message to sensory neurons, which regulates avoidance behavior from pheromones to facilitate survival chance.

CDK-11-Cyclin L is required for gametogenesis and fertility in C. elegans

CDK11, a member of the cyclin-dependent kinase family, has been implicated in a diverse array of functions including transcription, RNA processing, sister chromatid cohesion, spindle assembly, centriole duplication and apoptosis. Despite its involvement in many essential functions, little is known about the requirements for CDK11 and its partner Cyclin L in a developing multicellular organism. Here we investigate the function of CDK11 and Cyclin L during development of the nematode Caenorhabditis elegans. Worms express two CDK11 proteins encoded by distinct loci: CDK-11.1 is essential for normal male and female fertility and is broadly expressed in the nuclei of somatic and germ line cells, while CDK-11.2 is nonessential and is enriched in hermaphrodite germ line nuclei beginning in mid pachytene. Hermaphrodites lacking CDK-11.1 develop normally but possess fewer mature sperm and oocytes and do not fully activate the RAS-ERK pathway that is required for oocyte production in response to environmental cues. Most of the sperm and eggs that are produced in cdk-11.1 null animals appear to complete development normally but fail to engage in sperm-oocyte signaling suggesting that CDK-11.1 is needed at multiple points in gametogenesis. Finally, we find that CDK-11.1 and CDK-11.2 function redundantly during embryonic and postembryonic development and likely do so in association with Cyclin L. Our results thus define multiple requirements for CDK-11-Cyclin L during animal development.

Regulation of reproduction and longevity by nutrient-sensing pathways

Nutrients are necessary for life, as they are a crucial requirement for biological processes including reproduction, somatic growth, and tissue maintenance. Therefore, signaling systems involved in detecting and interpreting nutrient or energy levels-most notably, the insulin/insulin-like growth factor 1 (IGF-1) signaling pathway, mechanistic target of rapamycin (mTOR), and adenosine monophosphate-activated protein kinase (AMPK)-play important roles in regulating physiological decisions to reproduce, grow, and age. In this review, we discuss the connections between reproductive senescence and somatic aging and give an overview of the involvement of nutrient-sensing pathways in controlling both reproductive function and lifespan. Although the molecular mechanisms that affect these processes can be influenced by distinct tissue-, temporal-, and pathway-specific signaling events, the progression of reproductive aging and somatic aging is systemically coordinated by integrated nutrient-sensing signaling pathways regulating somatic tissue maintenance in conjunction with reproductive capacity.

Signaling-Mediated Regulation of Meiotic Prophase I and Transition During Oogenesis

Generation of healthy oocytes requires coordinated regulation of multiple cellular events and signaling pathways. Oocytes undergo a unique developmental growth and differentiation pattern interspersed with long periods of arrest. Oocytes from almost all species arrest in prophase I of oogenesis that allows for long period of growth and differentiation essential for normal oocyte development. Depending on species, oocytes that transit from prophase I to meiosis I also arrest at meiosis I for fairly long periods of time and then undergo a second arrest at meiosis II that is completed upon fertilization. While there are species-specific differences in C. elegans, D. melanogaster, and mammalian oocytes in stages of prophase I, meiosis I, or meiosis II arrest, in all cases cell signaling pathways coordinate the developmental events controlling oocyte growth and differentiation to regulate these crucial phases of transition. In particular, the ERK MAP kinase signaling pathway, cyclic AMP second messengers, and the cell cycle regulators CDK1/cyclin B are key signaling pathways that seem evolutionarily conserved in their control of oocyte growth and meiotic maturation across species. Here, I identify the common themes and differences in the regulation of key meiotic events during oocyte growth and maturation.

Conserved insulin signaling in the regulation of oocyte growth, development, and maturation

Insulin signaling regulates various aspects of physiology, such as glucose homeostasis and aging, and is a key determinant of female reproduction in metazoans. That insulin signaling is crucial for female reproductive health is clear from clinical data linking hyperinsulinemic and hypoinsulinemic condition with certain types of ovarian dysfunction, such as altered steroidogenesis, polycystic ovary syndrome, and infertility. Thus, understanding the signaling mechanisms that underlie the control of insulin-mediated ovarian development is important for the accurate diagnosis of and intervention for female infertility. Studies of invertebrate and vertebrate model systems have revealed the molecular determinants that transduce insulin signaling as well as which biological processes are regulated by the insulin-signaling pathway. The molecular determinants of the insulin-signaling pathway, from the insulin receptor to its downstream signaling components, are structurally and functionally conserved across evolution, from worms to mammals-yet, physiological differences in signaling still exist. Insulin signaling acts cooperatively with gonadotropins in mammals and lower vertebrates to mediate various aspects of ovarian development, mainly owing to evolution of the endocrine system in vertebrates. In contrast, insulin signaling in Drosophila and Caenorhabditis elegans directly regulates oocyte growth and maturation. In this review, we compare and contrast insulin-mediated regulation of ovarian functions in mammals, lower vertebrates, C. elegans, and Drosophila, and highlight conserved signaling pathways and regulatory mechanisms in general while illustrating insulin's unique role in specific reproductive processes.

DAF-18/PTEN signals through AAK-1/AMPK to inhibit MPK-1/MAPK in feedback control of germline stem cell proliferation

Under replete growth conditions, abundant nutrient uptake leads to the systemic activation of insulin/IGF-1 signalling (IIS) and the promotion of stem cell growth/proliferation. Activated IIS can stimulate the ERK/MAPK pathway, the activation of which also supports optimal stem cell proliferation in various systems. Stem cell proliferation rates can further be locally refined to meet the resident tissue's need for differentiated progeny. We have recently shown that the accumulation of mature oocytes in the C. elegans germ line, through DAF-18/PTEN, inhibits adult germline stem cell (GSC) proliferation, despite high systemic IIS activation. We show here that this feedback occurs through a novel cryptic signalling pathway that requires PAR-4/LKB1, AAK-1/AMPK and PAR-5/14-3-3 to inhibit the activity of MPK-1/MAPK, antagonize IIS, and inhibit both GSC proliferation and the production of additional oocytes. Interestingly, our results imply that DAF-18/PTEN, through PAR-4/LKB1, can activate AAK-1/AMPK in the absence of apparent energy stress. As all components are conserved, similar signalling cascades may regulate stem cell activities in other organisms and be widely implicated in cancer.

Insulin-like signalling to the maternal germline controls progeny response to osmotic stress

In 1893 August Weismann proposed that information about the environment could not pass from somatic cells to germ cells, a hypothesis now known as the Weismann barrier. However, recent studies have indicated that parental exposure to environmental stress can modify progeny physiology and that parental stress can contribute to progeny disorders. The mechanisms regulating these phenomena are poorly understood. We report that the nematode Caenorhabditis elegans can protect itself from osmotic stress by entering a state of arrested development and can protect its progeny from osmotic stress by increasing the expression of the glycerol biosynthetic enzyme GPDH-2 in progeny. Both of these protective mechanisms are regulated by insulin-like signalling: insulin-like signalling to the intestine regulates developmental arrest, while insulin-like signalling to the maternal germline regulates glycerol metabolism in progeny. Thus, there is a heritable link between insulin-like signalling to the maternal germline and progeny metabolism and gene expression. We speculate that analogous modulation of insulin-like signalling to the germline is responsible for effects of the maternal environment on human diseases that involve insulin signalling, such as obesity and type-2 diabetes.

Discovering Functional ERK Substrates Regulating Caenorhabditis elegans Germline Development

The Rat Sarcoma (RAS) GTPAse-mediated extracellular signal-regulated kinase (ERK) pathway regulates multiple biological processes across metazoans. In particular during Caenorhabditis elegans oogenesis, ERK signaling has been shown to regulate over seven distinct biological processes in a temporal and sequential manner. To fully elucidate how ERK signaling cascade orchestrates these different biological processes in vivo, identification of the direct functional substrates of the pathway is critical. This chapter describes the methods that were used to identify ERK substrates in a global manner and study their functions in the germline. These approaches can also be generally applied to study ERK-dependent biological processes in other systems.

Control of Germline Stem Cell Lineages by Diet and Physiology

Tight coupling of reproduction to environmental factors and physiological status is key to long-term species survival. In particular, highly conserved pathways modulate germline stem cell lineages according to nutrient availability. This chapter focuses on recent in vivo studies in genetic model organisms that shed light on how diet-dependent signals control the proliferation, maintenance, and survival of adult germline stem cells and their progeny. These signaling pathways can operate intrinsically in the germ line, modulate the niche, or act through intermediate organs to influence stem cells and their differentiating progeny. In addition to illustrating the extent of dietary regulation of reproduction, findings from these studies have implications for fertility during aging or disease states.

Analysis of the C. elegans Germline Stem Cell Pool

The Caenorhabditis elegans germline is an excellent model for studying the regulation of a pool of stem cells and progression of cells from a stem cell state to a differentiated state. At the tissue level, the germline is organized in an assembly line with the germline stem cell (GSC) pool at one end and differentiated cells at the other. A simple mesenchymal niche caps the GSC region of the germline and maintains GSCs in an undifferentiated state by signaling through the conserved Notch pathway. Downstream of Notch signaling, key regulators include novel LST-1 and SYGL-1 proteins and a network of RNA regulatory proteins. In this chapter we present methods for characterizing the C. elegans GSC pool and early germ cell differentiation. The methods include examination of the germline in living and fixed worms, cell cycle analysis, and analysis of markers. We also discuss assays to separate mutants that affect the stem cell vs. differentiation decision from those that affect germ cell processes more generally.

Spatial and Temporal Analysis of Active ERK in the C. elegans Germline

The evolutionarily conserved extracellular signal transducing RTK-RAS-ERK pathway is an important kinase-signaling cascade that controls multiple cellular and developmental processes principally via activation of ERK, the terminal kinase of the pathway. Tight regulation of ERK activity is essential for normal development and homeostasis; overly active ERK results in excessive cellular proliferation, while underactive ERK causes cell death. C. elegans is a powerful model system that has helped characterize the function and regulation of RTK-RAS-ERK signaling pathway during development. In particular, the RTK-RAS-ERK pathway is essential for C. elegans germline development, which is the focus of this method. Using antibodies specific to the active, diphosphorylated form of ERK (dpERK), the stereotypical localization pattern can be visualized within the germline. Because this pattern is both spatially and temporally controlled, the ability to reproducibly assay dpERK is useful to identify regulators of the pathway that affect dpERK signal duration and amplitude and thus germline development. Here we demonstrate how to successfully dissect, stain, and image dpERK within the C. elegans gonad. This method can be adapted for spatial localization of any signaling or structural protein in the C. elegans gonad, provided an antibody compatible with immunofluorescence is available.

Maternal Diet and Insulin-Like Signaling Control Intergenerational Plasticity of Progeny Size and Starvation Resistance

Maternal effects of environmental conditions produce intergenerational phenotypic plasticity. Adaptive value of these effects depends on appropriate anticipation of environmental conditions in the next generation, and mismatch between conditions may contribute to disease. However, regulation of intergenerational plasticity is poorly understood. Dietary restriction (DR) delays aging but maternal effects have not been investigated. We demonstrate maternal effects of DR in the roundworm C. elegans. Worms cultured in DR produce fewer but larger progeny. Nutrient availability is assessed in late larvae and young adults, rather than affecting a set point in young larvae, and maternal age independently affects progeny size. Reduced signaling through the insulin-like receptor daf-2/InsR in the maternal soma causes constitutively large progeny, and its effector daf-16/FoxO is required for this effect. nhr-49/Hnf4, pha-4/FoxA, and skn-1/Nrf also regulate progeny-size plasticity. Genetic analysis suggests that insulin-like signaling controls progeny size in part through regulation of nhr-49/Hnf4, and that pha-4/FoxA and skn-1/Nrf function in parallel to insulin-like signaling and nhr-49/Hnf4. Furthermore, progeny of DR worms are buffered from adverse consequences of early-larval starvation, growing faster and producing more offspring than progeny of worms fed ad libitum. These results suggest a fitness advantage when mothers and their progeny experience nutrient stress, compared to an environmental mismatch where only progeny are stressed. This work reveals maternal provisioning as an organismal response to DR, demonstrates potentially adaptive intergenerational phenotypic plasticity, and identifies conserved pathways mediating these effects.

Information from a mother’s environment can be transmitted to her offspring. In theory, the way mothers provision offspring can be beneficial or pathological depending on whether the environments of the mother and her offspring match. We find that roundworms fed a restricted diet produce fewer but larger offspring. These offspring recover better from starvation, growing faster and having increased fertility. Thus, we find that worms are more likely to thrive after early-life starvation if their mothers have been preconditioned with limited nutrient availability. We describe a genetic network that mediates effects of a mother’s diet on the size and starvation resistance of her offspring. The same genes required to extend the lifespan of worms fed a restricted diet are also required for the differential maternal provisioning we describe. In particular, insulin-like signaling, pha-4/FoxA, skn-1/Nrf, and nhr-49/Hnf4 function in the mother to transmit information about her diet to her offspring. Our work underscores the impact of maternal diet on reproductive health, with consequences for offspring physiology. The conserved genetic network controlling such effects of diet across generations is likely relevant to human diseases related to nutrient sensing and storage.

A Transport Model for Estimating the Time Course of ERK Activation in the C. elegans Germline

The Caenorhabditis elegans germline is a well-studied model system for investigating the control of cell fate by signaling pathways. Cell signals at the distal tip of the germline promote cell proliferation; just before the loop, signals couple cell maturation to organism-level nutrient status; at the proximal end of the germline, signals coordinate oocyte maturation and fertilization in the presence of sperm. The latter two events require dual phosphorylation and activation of ERK, the effector molecule of the Ras/MAPK cascade. In C. elegans, ERK is known as MPK-1. At this point, none of today's methods for real-time monitoring of dually phosphorylated MPK-1 are working in the germline. Consequently, quantitative understanding of the MPK-1-dependent processes during germline development is limited. Here, we make a step toward advancing this understanding using a model-based framework that reconstructs the time course of MPK-1 activation from a snapshot of a fixed germline. Our approach builds on a number of recent studies for estimating temporal dynamics from fixed organisms, but takes advantage of the anatomy of the germline to simplify the analysis. Our model predicts that the MPK-1 signal turns on ∼30 h into germ cell progression and peaks ∼7 h later.