Achieving immortality in the C. elegans germline

The C. elegans SET1 histone methyltransferase SET-2 is not required for transgenerational memory of silencing

The SET-2 /SET1 histone H3K4 methyltransferase and RNAi pathway components are required to maintain fertility across generations in C. elegans . SET-2 preserves the germline transcriptional program transgenerationally, and RNAi pathways rely on small RNAs to establish and maintain transgenerational gene silencing. We investigated whether the functionality of RNAi-induced transgenerational silencing and the composition of pools of endogenous small RNA are affected by the absence of SET-2 . Our results suggest that defects in RNAi pathways are not responsible for the transcriptional misregulation observed in the absence of SET-2 .

Genome-wide association and environmental suppression of the mortal germline phenotype of wild C. elegans

The animal germline lineage needs to be maintained along generations. However, some Caenorhabditis elegans wild isolates display a mortal germline phenotype, leading to sterility after several generations at 25°C. Using a genome-wide association approach, we detect a significant peak on chromosome III around 5 Mb, confirmed by introgressions. Thus, a seemingly deleterious genotype is maintained at intermediate frequency in the species. Environmental rescue is a likely explanation, and indeed associated bacteria and microsporidia suppress the phenotype of wild isolates as well as mutants in small RNA inheritance (nrde-2) and histone modifications (set-2). Escherichia coli strains of the K-12 lineage suppress the phenotype compared to B strains. By shifting a wild strain from E. coli K-12 to E. coli B, we find that memory of the suppressing condition is maintained over several generations. Thus, the mortal germline phenotype of wild C. elegans is in part revealed by laboratory conditions and may represent variation in epigenetic inheritance and environmental interactions. This study also points to the importance of non-genetic memory in the face of environmental variation.

The embryonic origin of primordial germ cells in the tardigrade Hypsibius exemplaris

Primordial germ cells (PGCs) give rise to gametes - cells necessary for the propagation and fertility of diverse organisms. Current understanding of PGC development is limited to the small number of organisms whose PGCs have been identified and studied. Expanding the field to include little-studied taxa and emerging model organisms is important to understand the full breadth of the evolution of PGC development. In the phylum Tardigrada, no early cell lineages have been identified to date using molecular markers. This includes the PGC lineage. Here, we describe PGC development in the model tardigrade Hypsibius exemplaris. The four earliest-internalizing cells (EICs) exhibit PGC-like behavior and nuclear morphology. The location of the EICs is enriched for mRNAs of conserved PGC markers wiwi1 (water bear piwi 1) and vasa. At early stages, both wiwi1 and vasa mRNAs are detectable uniformly in embryos, which suggests that these mRNAs do not serve as localized determinants for PGC specification. Only later are wiwi1 and vasa enriched in the EICs. Finally, we traced the cells that give rise to the four PGCs. Our results reveal the embryonic origin of the PGCs of H. exemplaris and provide the first molecular characterization of an early cell lineage in the tardigrade phylum. We anticipate that these observations will serve as a basis for characterizing the mechanisms of PGC development in this animal.

The Embryonic Origin of Primordial Germ Cells in the Tardigrade Hypsibius exemplaris

Primordial germ cells (PGCs) give rise to gametes â€" cells necessary for the propagation and fertility of diverse organisms. Current understanding of PGC development is limited to the small number of organisms whose PGCs have been identified and studied. Expanding the field to include little-studied taxa and emerging model organisms is important to understand the full breadth of the evolution of PGC development. In the phylum Tardigrada, no early cell lineages have been identified to date using molecular markers. This includes the PGC lineage. Here, we describe PGC development in the model tardigrade Hypsibius exemplaris . The four earliest-internalizing cells (EICs) exhibit PGC-like behavior and nuclear morphology. The location of the EICs is enriched for mRNAs of conserved PGC markers wiwi1 (water bear piwi 1) and vasa . At early stages, both wiwi1 and vasa mRNAs are detectable uniformly in embryos, which suggests that these mRNAs do not serve as localized determinants for PGC specification. Only later are wiwi1 and vasa enriched in the EICs. Finally, we traced the cells that give rise to the four PGCs. Our results reveal the embryonic origin of the PGCs of H. exemplaris and provide the first molecular characterization of an early cell lineage in the tardigrade phylum. We anticipate that these observations will serve as a basis for characterizing the mechanisms of PGC development in this animal.

Multigenerational downregulation of insulin/IGF-1 signaling in adulthood improves lineage survival, reproduction, and fitness in Caenorhabditis elegans supporting the developmental theory of ageing

Adulthood-only downregulation of insulin/IGF-1 signaling (IIS), an evolutionarily conserved pathway regulating resource allocation between somatic maintenance and reproduction, increases life span without fecundity cost in the nematode, Caenorhabditis elegans. However, long-term multigenerational effects of reduced IIS remain unexplored and are proposed to carry costs for offspring quality. To test this hypothesis, we ran a mutation accumulation (MA) experiment and downregulated IIS in half of the 400 MA lines by silencing daf-2 gene expression using RNA interference (RNAi) across 40 generations. Contrary to the prediction, adulthood-only daf-2 RNAi reduced extinction of MA lines both under UV-induced and spontaneous MA. Fitness of the surviving UV-induced MA lines was higher under daf-2 RNAi. Reduced IIS increased intergenerational F1 offspring fitness under UV stress but had no quantifiable transgenerational effects. Functional hrde-1 was required for the benefits of multigenerational daf-2 RNAi. Overall, we found net benefit to fitness from multigenerational reduction of IIS and the benefits became more apparent under stress. Because reduced daf-2 expression during development carries fitness costs, we suggest that our findings are best explained by the developmental theory of ageing, which maintains that the decline in the force of selection with age results in poorly regulated gene expression in adulthood.

RG/RGG repeats in the C. elegans homologs of Nucleolin and GAR1 contribute to sub-nucleolar phase separation

The intrinsically disordered RG/RGG repeat domain is found in several nucleolar and P-granule proteins, but how it influences their phase separation into biomolecular condensates is unclear. We survey all RG/RGG repeats in C. elegans and uncover nucleolar and P-granule-specific RG/RGG motifs. An uncharacterized protein, K07H8.10, contains the longest nucleolar-like RG/RGG domain in C. elegans. Domain and sequence similarity, as well as nucleolar localization, reveals K07H8.10 (NUCL-1) to be the homolog of Nucleolin, a protein conserved across animals, plants, and fungi, but previously thought to be absent in nematodes. Deleting the RG/RGG repeats within endogenous NUCL-1 and a second nucleolar protein, GARR-1 (GAR1), demonstrates these domains are dispensable for nucleolar accumulation. Instead, their RG/RGG repeats contribute to the phase separation of proteins into nucleolar sub-compartments. Despite this common RG/RGG repeat function, only removal of the GARR-1 RG/RGG domain affects worm fertility and development, decoupling precise sub-nucleolar structure from nucleolar function.

Mutation, selection, and the prevalence of the Caenorhabditis elegans heat-sensitive mortal germline phenotype

Caenorhabditis elegans strains with the heat-sensitive mortal germline phenotype become progressively sterile over the course of a few tens of generations when maintained at temperatures near the upper range of C. elegans' tolerance. Mortal germline is transgenerationally heritable, and proximately under epigenetic control. Previous studies have suggested that mortal germline presents a relatively large mutational target and that mortal germline is not uncommon in natural populations of C. elegans. The mortal germline phenotype is not monolithic. Some strains exhibit a strong mortal germline phenotype, in which individuals invariably become sterile over a few generations, whereas other strains show a weaker (less penetrant) phenotype in which the onset of sterility is slower and more stochastic. We present results in which we (1) quantify the rate of mutation to the mortal germline phenotype and (2) quantify the frequency of mortal germline in a collection of 95 wild isolates. Over the course of ∼16,000 meioses, we detected one mutation to a strong mortal germline phenotype, resulting in a point estimate of the mutation rate UMrt≈ 6×10-5/genome/generation. We detected no mutations to a weak mortal germline phenotype. Six out of 95 wild isolates have a strong mortal germline phenotype, and although quantification of the weak mortal germline phenotype is inexact, the weak mortal germline phenotype is not rare in nature. We estimate a strength of selection against mutations conferring the strong mortal germline phenotype s¯≈0.1%, similar to selection against mutations affecting competitive fitness. The appreciable frequency of weak mortal germline variants in nature combined with the low mutation rate suggests that mortal germline may be maintained by balancing selection.

C. elegans as a model organism to study female reproductive health

Female reproductive health has been historically understudied and underfunded. Here, we present the advantages of using a free-living nematode, Caenorhabditis elegans, as an animal system to study fundamental aspects of female reproductive health. C. elegans is a powerful high-throughput model organism that shares key genetic and physiological similarities with humans. In this review, we highlight areas of pressing medical and biological importance in the 21st century within the context of female reproductive health. These include the decline in female reproductive capacity with increasing chronological age, reproductive dysfunction arising from toxic environmental insults, and cancers of the reproductive system. C. elegans has been instrumental in uncovering mechanistic insights underlying these processes, and has been valuable for developing and testing therapeutics to combat them. Adopting a convenient model organism such as C. elegans for studying reproductive health will encourage further research into this field, and broaden opportunities for making advancements into evolutionarily conserved mechanisms that control reproductive function.

Coordinated maintenance of H3K36/K27 methylation by histone demethylases preserves germ cell identity and immortality

Germ cells have evolved unique mechanisms to ensure the transmission of genetically and nongenetically encoded information, whose alteration compromises germ cell immortality. Chromatin factors play fundamental roles in these mechanisms. H3K36 and H3K27 methyltransferases shape and propagate a pattern of histone methylation essential for C. elegans germ cell maintenance, but the role of respective histone demethylases remains unexplored. Here, we show that jmjd-5 regulates H3K36me2 and H3K27me3 levels, preserves germline immortality, and protects germ cell identity by controlling gene expression. The transcriptional and biological effects of jmjd-5 loss can be hindered by the removal of H3K27demethylases, indicating that H3K36/K27 demethylases act in a transcriptional framework and promote the balance between H3K36 and H3K27 methylation required for germ cell immortality. Furthermore, we find that in wild-type, but not in jmjd-5 mutants, alterations of H3K36 methylation and transcription occur at high temperature, suggesting a role for jmjd-5 in adaptation to environmental changes.

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jmjd-5 is required for germ cell immortality at high temperature

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jmjd-5 sustains the expression of germline genes and represses somatic fate

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Mutations in jmjd-5 result in a global increase of H3K36me2 and H3K27me3

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Ablation of H3K27 demethylases counteracts the effects of jmjd-5 mutations

Novel LOTUS-domain proteins are organizational hubs that recruit C. elegans Vasa to germ granules

We describe MIP-1 and MIP-2, novel paralogous C. elegans germ granule components that interact with the intrinsically disordered MEG-3 protein. These proteins promote P granule condensation, form granules independently of MEG-3 in the postembryonic germ line, and balance each other in regulating P granule growth and localization. MIP-1 and MIP-2 each contain two LOTUS domains and intrinsically disordered regions and form homo- and heterodimers. They bind and anchor the Vasa homolog GLH-1 within P granules and are jointly required for coalescence of MEG-3, GLH-1, and PGL proteins. Animals lacking MIP-1 and MIP-2 show temperature-sensitive embryonic lethality, sterility, and mortal germ lines. Germline phenotypes include defects in stem cell self-renewal, meiotic progression, and gamete differentiation. We propose that these proteins serve as scaffolds and organizing centers for ribonucleoprotein networks within P granules that help recruit and balance essential RNA processing machinery to regulate key developmental transitions in the germ line.

An age-downregulated ribosomal RpS28 protein variant regulates the muscle proteome

Recent evidence indicates that the composition of the ribosome is heterogeneous and that multiple types of specialized ribosomes regulate the synthesis of specific protein subsets. In Drosophila, we find that expression of the ribosomal RpS28 protein variants RpS28a and RpS28-like preferentially occurs in the germline, a tissue resistant to aging and that it significantly declines in skeletal muscle during aging. Muscle-specific overexpression of RpS28a at levels similar to those seen in the germline decreases early mortality and promotes the synthesis of a subset of proteins with known anti-aging roles, some of which have preferential expression in the germline. These findings indicate a contribution of specialized ribosomal proteins to the regulation of the muscle proteome during aging.

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

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

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.

Emerging Roles for Chromo Domain Proteins in Genome Organization and Cell Fate in C. elegans

In most eukaryotes, the genome is packaged with histones and other proteins to form chromatin. One of the major mechanisms for chromatin regulation is through post-translational modification of histone proteins. Recognition of these modifications by effector proteins, often dubbed histone “readers,” provides a link between the chromatin landscape and gene regulation. The diversity of histone reader proteins for each modification provides an added layer of regulatory complexity. In this review, we will focus on the roles of chromatin organization modifier (chromo) domain containing proteins in the model nematode, Caenorhabditis elegans. An amenability to genetic and cell biological approaches, well-studied development and a short life cycle make C. elegans a powerful system to investigate the diversity of chromo domain protein functions in metazoans. We will highlight recent insights into the roles of chromo domain proteins in the regulation of heterochromatin and the spatial conformation of the genome as well as their functions in cell fate, fertility, small RNA pathways and transgenerational epigenetic inheritance. The spectrum of different chromatin readers may represent a layer of regulation that integrates chromatin landscape, genome organization and gene expression.

Harnessing the power of genetics: fast forward genetics in Caenorhabditis elegans

Forward genetics is a powerful tool to unravel molecular mechanisms of diverse biological processes. The success of genetic screens primarily relies on the ease of genetic manipulation of an organism and the availability of a plethora of genetic tools. The roundworm Caenorhabditis elegans has been one of the favorite models for genetic studies due to its hermaphroditic lifestyle, ease of maintenance, and availability of various genetic manipulation tools. The strength of C. elegans genetics is highlighted by the leading role of this organism in the discovery of several conserved biological processes. In this review, the principles and strategies for forward genetics in C. elegans are discussed. Further, the recent advancements that have drastically accelerated the otherwise time-consuming process of mutation identification, making forward genetic screens a method of choice for understanding biological functions, are discussed. The emphasis of the review has been on providing practical and conceptual pointers for designing genetic screens that will identify mutations, specifically disrupting the biological processes of interest.

p53 inhibits the proliferation of male germline stem cells from dairy goat cultured on poly-L-lysine

Male germline stem cells (mGSCs) can transmit genetic materials to the next generation and dedifferentiate into pluripotent stem cells. However, in livestock, mGSC lines are difficult to establish, because of the factors that affect their isolation and culture. The extracellular matrix serves as a substrate for attachment and affects the fate of these stem cells. Poly-L-lysine (PL), an extracellular matrix of choice, inhibits and/or kills cancer cells, and promotes the attachment of stem cells in culture. However, how it affects the characteristics and potentials of these stem cells in culture needs to be elucidated. Here, we isolated, enriched and cultured dairy goat mGSCs on five types of extracellular matrices. To explore the best extracellular matrix to use for culturing them, the characteristics and proliferation ability of the cells were determined. Results showed that the cells shared several characteristics with previously reported mGSCs, including the poor effect of PL on their proliferative and colony-forming abilities. Further examination showed upregulation of p53 expression in these cells, which could be inhibiting their proliferation. When a p53 inhibitor was included in the culture medium, it was confirmed to be responsible for the inhibition of proliferation in mGSCs. Optimal concentration of the inhibitor in the culture of these cells was 5 µM. Furthermore, addition of the p53 inhibitor increased the expression of the markers of self-renewal and cell cycle in goat mGSCs. In summary, suppressing p53 is beneficial for the proliferation of dairy goat mGSCs, cultured on PL.

Transgenerational Sterility of Piwi Mutants Represents a Dynamic Form of Adult Reproductive Diapause

Environmental stress can induce adult reproductive diapause, a state of developmental arrest that temporarily suspends reproduction. Deficiency for C. elegans Piwi protein PRG-1 results in strains that reproduce for many generations but then become sterile. We found that sterile-generation prg-1/Piwi mutants typically displayed pronounced germ cell atrophy as L4 larvae matured into 1-day-old adults. Atrophied germlines spontaneously reproliferated across the first days of adulthood, and this was accompanied by fertility for day 2–4 adults. Sterile day 5 prg-1 mutant adults remained sterile indefinitely, but providing an alternative food source could restore their fertility. Our data imply that late-generation prg-1 mutants experience a dynamic form of adult reproductive diapause, promoted by stress response, cell death, and RNAi pathways, where delayed fertility and reproductive quiescence represent parallel adaptive developmental outcomes. This may occur in response to a form of “heritable stress” that is transmitted by gametes and epigenetic in nature.

The balance of poly(U) polymerase activity ensures germline identity, survival and development in Caenorhabditis elegans

Poly(U) polymerases (PUPs) catalyze 3' uridylation of mRNAs and small RNAs, a modification often correlating with decreased RNA stability. We have investigated the importance of three proteins with in vitro PUP activity, PUP-1/CDE-1, PUP-2 and PUP-3, in C. elegans germline development. Genetic analysis indicates that PUP-1/CDE-1 and PUP-2 are developmentally redundant under conditions of temperature stress during which they ensure germline viability and development. Multiple lines of evidence indicate that pup-1/-2 double mutant germ cells fail to maintain their identity as distinct from soma. Consistent with phenotypic data, PUP-1 and PUP-2 are expressed in embryonic germ cell precursors and throughout germline development. The developmental importance of PUP activity is presumably in regulating gene expression as both a direct and indirect consequence of modifying target RNAs. PUP-3 is significantly overexpressed in the pup-1/-2 germline, and loss of pup-3 function partially suppresses pup-1/-2 germline defects. We conclude that one major function of PUP-1/-2 is to limit PUP-3 expression. Overall, the balance of PUP-1, PUP-2 and PUP-3 activities appears to ensure proper germline development.

Natural Genetic Variation in a Multigenerational Phenotype in C. elegans

Although heredity mostly relies on the transmission of DNA sequence, additional molecular and cellular features are heritable across several generations. In the nematode Caenorhabditis elegans, insights into such unconventional inheritance result from two lines of work. First, the mortal germline (Mrt) phenotype was defined as a multigenerational phenotype whereby a selfing lineage becomes sterile after several generations, implying multigenerational memory [1, 2]. Second, certain RNAi effects are heritable over several generations in the absence of the initial trigger [3-5]. Both lines of work converged when the subset of Mrt mutants that are heat sensitive were found to closely correspond to mutants defective in the RNAi-inheritance machinery, including histone modifiers [6-9]. Here, we report the surprising finding that several C. elegans wild isolates display a heat-sensitive mortal germline phenotype in laboratory conditions: upon chronic exposure to higher temperatures, such as 25°C, lines reproducibly become sterile after several generations. This phenomenon is reversible, as it can be suppressed by temperature alternations at each generation, suggesting a non-genetic basis for the sterility. We tested whether natural variation in the temperature-induced Mrt phenotype was of genetic nature by building recombinant inbred lines between the isolates MY10 (Mrt) and JU1395 (non-Mrt). Using bulk segregant analysis, we detected two quantitative trait loci. After further recombinant mapping and genome editing, we identified the major causal locus as a polymorphism in the set-24 gene, encoding a SET- and SPK-domain protein. We conclude that C. elegans natural populations may harbor natural genetic variation in epigenetic inheritance phenomena.

MiR-302 enhances the viability and stemness of male germline stem cells

MicroRNAs were reported to be able to regulate mGSCs' self-renewal through post-transcriptional inhibition of gene expression. miR-302 worked as one important microRNA family existed mainly in human ESCs, and its role in mGSCs has not been reported yet. To elucidate the role of miR-302 in dairy goat mGSCs, the expression profile of miR-302 was explored through qPCR and FISH. Furthermore, to detect the function of miR-302, the expression vector containing miR-302 was transfected into mGSCs, and then, the cell cycle, the cell apoptosis and the genes associated with mGSCs' self-renewal and differentiation were examined. The results showed that miR-302 expressed in testis moderately and located on the basement of seminiferous tubes which shared the same location as mGSCs. Transfection of the vector containing miR-302 fragment into the immortalized mGSCs obviously enhanced the cell proliferation ability and the attachment ability, also, promoted the expression level of CD49f and OCT4. Also, miR-302 reduced the cell apoptosis and downregulated the expression of P21. miR-302 sustained mGSCs' proliferation in vitro.

VIG-1 is required for maintenance of genome stability in Caenorhabditis elegans

To explore the function of VIG-1 in Caenorhabditis elegans, we analyzed the phenotypes of two vig-1 deletion mutants: vig-1(tm3383) and vig-1(ok2536). Both vig-1 mutants exhibited phenotypes associated with genome instability, such as a high incidence of males (Him) and increased embryonic lethality. These phenotypes became more evident in succeeding generations, implying that the germline of vig-1 accumulates DNA damage over generations. To examine whether vig-1 causes a defect in the DNA damage response, we treated worms with UV or camptothecin, a specific topoisomerase I inhibitor. We observed that the embryonic survival of the vig-1 mutants was reduced compared with that of the wild-type worms. Our results thus suggest that VIG-1 is required for maintaining genome stability in response to endogenous and exogenous genotoxic stresses.

MORC-1 Integrates Nuclear RNAi and Transgenerational Chromatin Architecture to Promote Germline Immortality

Germline-expressed endogenous small interfering RNAs (endo-siRNAs) transmit multigenerational epigenetic information to ensure fertility in subsequent generations. In Caenorhabditis elegans, nuclear RNAi ensures robust inheritance of endo-siRNAs and deposition of repressive H3K9me3 marks at target loci. How target silencing is maintained in subsequent generations is poorly understood. We discovered that morc-1 is essential for transgenerational fertility and acts as an effector of endo-siRNAs. Unexpectedly, morc-1 is dispensable for siRNA inheritance but is required for target silencing and maintenance of siRNA-dependent chromatin organization. A forward genetic screen identified mutations in met-1, which encodes an H3K36 methyltransferase, as potent suppressors of morc-1(-) and nuclear RNAi mutant phenotypes. Further analysis of nuclear RNAi and morc-1(-) mutants revealed a progressive, met-1-dependent enrichment of H3K36me3, suggesting that robust fertility requires repression of MET-1 activity at nuclear RNAi targets. Without MORC-1 and nuclear RNAi, MET-1-mediated encroachment of euchromatin leads to detrimental decondensation of germline chromatin and germline mortality.

Impact of genetic background and experimental reproducibility on identifying chemical compounds with robust longevity effects

Limiting the debilitating consequences of ageing is a major medical challenge of our time. Robust pharmacological interventions that promote healthy ageing across diverse genetic backgrounds may engage conserved longevity pathways. Here we report results from the Caenorhabditis Intervention Testing Program in assessing longevity variation across 22 Caenorhabditis strains spanning 3 species, using multiple replicates collected across three independent laboratories. Reproducibility between test sites is high, whereas individual trial reproducibility is relatively low. Of ten pro-longevity chemicals tested, six significantly extend lifespan in at least one strain. Three reported dietary restriction mimetics are mainly effective across C. elegans strains, indicating species and strain-specific responses. In contrast, the amyloid dye ThioflavinT is both potent and robust across the strains. Our results highlight promising pharmacological leads and demonstrate the importance of assessing lifespans of discrete cohorts across repeat studies to capture biological variation in the search for reproducible ageing interventions.

Remodelling germ cells by intercellular cannibalism

Work from the early 1980s reported strange lobes protruding from Caenorhabditis elegans germ cell precursors. However, the fate and potential significance of these lobes remained unexplored for decades. Now, neighbouring endodermal cells are shown to sever and digest these lobes, in an unexpected process of 'intercellular cannibalism', which could play an important part in regulating primordial germ cells.

High telomerase is a hallmark of undifferentiated spermatogonia and is required for maintenance of male germline stem cells

Telomerase inactivation causes loss of the male germline in worms, fish, and mice, indicating a conserved dependence on telomere maintenance in this cell lineage. Here, using telomerase reverse transcriptase (Tert) reporter mice, we found that very high telomerase expression is a hallmark of undifferentiated spermatogonia, the mitotic population where germline stem cells reside. We exploited these high telomerase levels as a basis for purifying undifferentiated spermatogonia using fluorescence-activated cell sorting. Telomerase levels in undifferentiated spermatogonia and embryonic stem cells are comparable and much greater than in somatic progenitor compartments. Within the germline, we uncovered an unanticipated gradient of telomerase activity that also enables isolation of more mature populations. Transcriptomic comparisons of Tert(High) undifferentiated spermatogonia and Tert(Low) differentiated spermatogonia by RNA sequencing reveals marked differences in cell cycle and key molecular features of each compartment. Transplantation studies show that germline stem cell activity is confined to the Tert(High) cKit(-) population. Telomere shortening in telomerase knockout strains causes depletion of undifferentiated spermatogonia and eventual loss of all germ cells after undifferentiated spermatogonia drop below a critical threshold. These data reveal that high telomerase expression is a fundamental characteristic of germline stem cells, thus explaining the broad dependence on telomerase for germline immortality in metazoans.

Reduced insulin/IGF-1 signaling restores germ cell immortality to Caenorhabditis elegans Piwi mutants

Defects in the Piwi/piRNA pathway lead to transposon desilencing and immediate sterility in many organisms. We found that the C. elegans Piwi mutant prg-1 became sterile after growth for many generations. This phenotype did not occur for RNAi mutants with strong transposon-silencing defects and was separable from the role of PRG-1 in transgene silencing. Brief periods of starvation extended the transgenerational lifespan of prg-1 mutants by stimulating the DAF-16/FOXO longevity transcription factor. Constitutive activation of DAF-16 via reduced daf-2 insulin/IGF-1 signaling immortalized prg-1 strains via RNAi proteins and histone H3 lysine 4 demethylases. In late-generation prg-1 mutants, desilencing of repetitive segments of the genome occurred, and silencing of repetitive loci was restored in prg-1; daf-2 mutants. This study reveals an unexpected interface between aging and transgenerational maintenance of germ cells, where somatic longevity is coupled to a genome-silencing pathway that promotes germ cell immortality in parallel to the Piwi/piRNA system.

H3K4 demethylase activities repress proliferative and postmitotic aging

Homeostasis of postmitotic and proliferating cells is maintained by pathways that repress stress. We found that the Caenorhabditis elegans histone 3 lysine 4 (H3K4) demethylases RBR-2 and SPR-5 promoted postmitotic longevity of stress-resistant daf-2 adults, altered pools of methylated H3K4, and promoted silencing of some daf-2 target genes. In addition, RBR-2 and SPR-5 were required for germ cell immortality at a high temperature. Transgenerational proliferative aging was enhanced for spr-5; rbr-2 double mutants, suggesting that these histone demethylases may function sequentially to promote germ cell immortality by targeting distinct H3K4 methyl marks. RBR-2 did not play a comparable role in the maintenance of quiescent germ cells in dauer larvae, implying that it represses stress that occurs as a consequence of germ cell proliferation, rather than stress that accumulates in nondividing cells. We propose that H3K4 demethylase activities promote the maintenance of chromatin states during stressful growth conditions, thereby repressing postmitotic aging of somatic cells as well as proliferative aging of germ cells.

Reprogramming and the mammalian germline: the Weismann barrier revisited

The germline represents a unique cell type that can transmit genetic material to the next generation. During early embryonic development, somatic cells give rise to a small population of cells known as germ cells, which eventually differentiate into mature gametes. Germ cells undergo a process of removing and resetting relevant epigenetic information, mainly by DNA demethylation. This extensive epigenetic reprogramming leads to the conversion of germ cells into immortal cells that can pass on the genome to the next generation. In the absence of germline-specific reprogramming, germ cells would preserve the old, parental epigenetic memory, which would prevent the transfer of heritable information to the offspring. On the contrary, somatic cells cannot reset epigenetic information by preserving the full methylation pattern on imprinting genes. In this review, we focus on the capacity of germ cells and somatic cells (soma) to transfer genetic information to the next generation, and thus revisit the Weismann theory of heredity.

Caenorhabditis elegans chromatin-associated proteins SET-2 and ASH-2 are differentially required for histone H3 Lys 4 methylation in embryos and adult germ cells

Methylation of histone H3 lysine 4 (H3K4me), a mark associated with gene activation, is mediated by SET1 and the related mixed lineage leukemia (MLL) histone methyltransferases (HMTs) across species. Mammals contain seven H3K4 HMTs, Set1A, Set1B, and MLL1-MLL5. The activity of SET1 and MLL proteins relies on protein-protein interactions within large multisubunit complexes that include three core components: RbBP5, Ash2L, and WDR5. It remains unclear how the composition and specificity of these complexes varies between cell types and during development. Caenorhabditis elegans contains one SET1 protein, SET-2, one MLL-like protein, SET-16, and single homologs of RbBP5, Ash2L, and WDR5. Here we show that SET-2 is responsible for the majority of bulk H3K4 methylation at all developmental stages. However, SET-2 and absent, small, or homeotic discs 2 (ASH-2) are differentially required for tri- and dimethylation of H3K4 (H3K4me3 and -me2) in embryos and adult germ cells. In embryos, whereas efficient H3K4me3 requires both SET-2 and ASH-2, H3K4me2 relies mostly on ASH-2. In adult germ cells by contrast, SET-2 serves a major role whereas ASH-2 is dispensable for H3K4me3 and most H3K4me2. Loss of SET-2 results in progressive sterility over several generations, suggesting an important function in the maintenance of a functional germ line. This study demonstrates that individual subunits of SET1-related complexes can show tissue specificity and developmental regulation and establishes C. elegans as a model to study SET1-related complexes in a multicellular organism.

Structural maintenance of chromosomes (SMC) proteins promote homolog-independent recombination repair in meiosis crucial for germ cell genomic stability

In meiosis, programmed DNA breaks repaired by homologous recombination (HR) can be processed into inter-homolog crossovers that promote the accurate segregation of chromosomes. In general, more programmed DNA double-strand breaks (DSBs) are formed than the number of inter-homolog crossovers, and the excess DSBs must be repaired to maintain genomic stability. Sister-chromatid (inter-sister) recombination is postulated to be important for the completion of meiotic DSB repair. However, this hypothesis is difficult to test because of limited experimental means to disrupt inter-sister and not inter-homolog HR in meiosis. We find that the conserved Structural Maintenance of Chromosomes (SMC) 5 and 6 proteins in Caenorhabditis elegans are required for the successful completion of meiotic homologous recombination repair, yet they appeared to be dispensable for accurate chromosome segregation in meiosis. Mutations in the smc-5 and smc-6 genes induced chromosome fragments and dismorphology. Chromosome fragments associated with HR defects have only been reported in mutants, which have disrupted inter-homolog crossover. Surprisingly, the smc-5 and smc-6 mutations did not disrupt the formation of chiasmata, the cytologically visible linkages between homologous chromosomes formed from meiotic inter-homolog crossovers. The mutant fragmentation defect appeared to be preferentially enhanced by the disruptions of inter-homolog recombination but not by the disruptions of inter-sister recombination. Based on these findings, we propose that the C. elegans SMC-5/6 proteins are required in meiosis for the processing of homolog-independent, presumably sister-chromatid-mediated, recombination repair. Together, these results demonstrate that the successful completion of homolog-independent recombination is crucial for germ cell genomic stability.

Sperm and oocytes are essential for the faithful transmission of genetic information during sexual reproduction. As germ cells mature into sperm and oocytes, DNA double-strand breaks (DSBs) are deliberately created on each chromosome and a subset of DSBs is repaired to form meiotic crossovers between homologous chromosomes. Because germ cells must undergo this programmed process of deliberate DNA damage and repair, identifying repair factors active in germ cells and determining the requirement of their functions in meiotic DSB repair are important first steps in understanding infertility and developmental disorders caused by defective sperm and oocytes. In this manuscript, we find that the evolutionarily conserved SMC-5 and SMC-6 proteins fulfill a critical role in preserving genomic stability in germ cells in C. elegans. Our findings further describe the genetic mechanisms by which the C. elegans SMC-5/6 proteins function in meiotic DSB repair. These data reveal that inter-sister homologous recombination, a repair mechanism thought to function as a back-up repair method in meiosis, serves a more significant role in normal meiosis than was previously appreciated.

Cancer models in Caenorhabditis elegans

Although now dogma, the idea that nonvertebrate organisms such as yeast, worms, and flies could inform, and in some cases even revolutionize, our understanding of oncogenesis in humans was not immediately obvious. Aided by the conservative nature of evolution and the persistence of a cohort of devoted researchers, the role of model organisms as a key tool in solving the cancer problem has, however, become widely accepted. In this review, we focus on the nematode Caenorhabditis elegans and its diverse and sometimes surprising contributions to our understanding of the tumorigenic process. Specifically, we discuss findings in the worm that address a well-defined set of processes known to be deregulated in cancer cells including cell cycle progression, growth factor signaling, terminal differentiation, apoptosis, the maintenance of genome stability, and developmental mechanisms relevant to invasion and metastasis.

Genome integrity is regulated by the Caenorhabditis elegans Rad51D homolog rfs-1

Multiple mechanisms ensure genome maintenance through DNA damage repair, suppression of transposition, and telomere length regulation. The mortal germline (Mrt) mutants in Caenorhabditis elegans are defective in maintaining genome integrity, resulting in a progressive loss of fertility over many generations. Here I show that the high incidence of males (him)-15 locus, defined by the deficiency eDf25, is allelic to rfs-1, the sole rad-51 paralog group member in C. elegans. The rfs-1/eDf25 mutant displays a Mrt phenotype and mutant animals manifest features of chromosome fusions prior to the onset of sterility. Unlike other Mrt genes, rfs-1 manifests fluctuations in telomere lengths and functions independently of telomerase. These data suggest that rfs-1 is a novel regulator of genome stability.

Aging and the germ line: where mortality and immortality meet

Germ cells are highly specialized cells that form gametes, and they are the only cells within an organism that contribute genes to offspring. Germline stem cells (GSCs) sustain gamete production, both oogenesis (egg production) and spermatogenesis (sperm production), in many organisms. Since the genetic information contained within germ cells is passed from generation to generation, the germ line is often referred to as immortal. Therefore, it is possible that germ cells possess unique strategies to protect and transmit the genetic information contained within them indefinitely. However, aging often leads to a dramatic decrease in gamete production and fecundity. In addition, single gene mutations affecting longevity often have a converse effect on reproduction. Recent studies examining age-related changes in GSC number and activity, as well as changes to the stem cell microenvironment, provide insights into the mechanisms underlying the observed reduction in gametogenesis over the lifetime of an organism.

Uncoupling of pathways that promote postmitotic life span and apoptosis from replicative immortality of Caenorhabditis elegans germ cells

A dichotomy exists between germ and somatic cells in most organisms, such that somatic cell lineages proliferate for a single generation, whereas the germ cell lineage has the capacity to proliferate from one generation to the next, indefinitely. Several theories have been proposed to explain the unlimited replicative life span of germ cells, including the elimination of damaged germ cells by apoptosis or expression of high levels of gene products that prevent aging in somatic cells. These theories were tested in the nematode Caenorhabditis elegans by examining the consequences of eliminating either apoptosis or the daf-16, daf-18 or sir-2.1 genes that promote longevity of postmitotic somatic cells. However, germ cells of strains deficient for these activities displayed an unlimited proliferative capacity. Thus, C. elegans germ cells retain their youthful character via alternative pathways that prevent or eliminate damage that accumulates as a consequence of cell proliferation.

Genetics of proliferative aging

Human lifespan is limited by aging of both mitotic and post-mitotic cells. These two forms of aging may occur by distinct or overlapping mechanisms. Telomere erosion has been shown to limit the proliferative lifespan of human somatic cells. Other vertebrates, such as mice, possess robust telomerase activity in most cell types and their somatic cells display finite replicative lifespans as a consequence of other forms of macromolecular damage. Genetic analysis in humans, mice and yeast has provided clues regarding pathways that may affect a cell's replicative lifespan. In addition, analysis of the means by which germ cells maintain their effervescent character may provide a deeper understanding of how replicative aging occurs in somatic cells.