Germ Granules Coordinate RNA-Based Epigenetic Inheritance Pathways

Compartmentalized localization of perinuclear proteins within germ granules in C. elegans

Germ granules, or nuage, are RNA-rich condensates that are often docked on the cytoplasmic surface of germline nuclei. C. elegans perinuclear germ granules are composed of multiple subcompartments, including P granules, Mutator foci, Z granules, SIMR foci, P -bodies, and E granules. Although many perinuclear proteins have been identified, their precise localization within the subcompartments of the germ granule is still unclear. Here, we systematically labeled perinuclear proteins with fluorescent tags via CRISPR-Cas9 technology. Using this nematode strain library, we identified a series of proteins localized in Z or E granules and extended the characterization of the D granule. Finally, we found that the LOTUS domain protein MIP-1/EGGD-1 regulated the multiphase organization of the germ granule. Overall, our work identified the germ-granule architecture and redefined the compartmental localization of perinuclear proteins. Additionally, the library of genetically modified nematode strains will facilitate research on C. elegans germ granules.

Intergenerational transport of double-stranded RNA in C. elegans can limit heritable epigenetic changes

RNAs in circulation carry sequence-specific regulatory information between cells in plant, animal, and host-pathogen systems. Such RNA can cross generational boundaries, as evidenced by somatic double-stranded RNA (dsRNA) in the nematode Caenorhabditis elegans silencing genes of matching sequence in progeny. Here we dissect the intergenerational path taken by dsRNA from parental circulation and discover that cytosolic import through the dsRNA importer SID-1 in the parental germline and/or developing progeny varies with developmental time and dsRNA substrates. Loss of SID-1 enhances initiation of heritable RNA silencing within the germline and causes changes in the expression of the sid-1-dependent gene sdg-1 that last for more than 100 generations after restoration of SID-1. The SDG-1 protein is enriched in perinuclear germ granules required for heritable RNA silencing but is expressed from a retrotransposon targeted by such silencing. This auto-inhibitory loop suggests how retrotransposons could persist by hosting genes that regulate their own silencing.

Caenorhabditis Elegans as a Model for Environmental Epigenetics

PURPOSE OF REVIEW

The burgeoning field of environmental epigenetics has revealed the malleability of the epigenome and uncovered numerous instances of its sensitivity to environmental influences; however, pinpointing specific mechanisms that tie together environmental triggers, epigenetic pathways, and organismal responses has proven difficult. This article describes how Caenorhabditis elegans can fill this gap, serving as a useful model for the discovery of molecular epigenetic mechanisms that are conserved in humans.

RECENT FINDINGS

Recent results show that environmental stressors such as methylmercury, arsenite, starvation, heat, bacterial infection, and mitochondrial inhibitors can all have profound effects on the epigenome, with some insults showing epigenetic and organismal effects for multiple generations. In some cases, the pathways connecting the stressor to epigenetic pathways and organismal responses have been elucidated. For example, a small RNA from the bacterial pathogen Pseudomonas aeruginosa induces transgenerational learned avoidance by activating the RNA interference PIWI-interacting RNA pathways across generations to downregulate, via Cer1 retrotransposon particles and histone methylation, maco-1, a gene that functions in sensory neurons to regulate chemotaxis. Mitochondrial inhibitors seem to have a profound effect on both the DNA methylation mark 6mA and histone methylation, and may act within mitochondrial DNA (mtDNA) to regulate mitochondrial stress response genes. Transgenerational transcriptional responses to alcohol have also been worked out at the single-nucleus resolution in C. elegans, demonstrating its utility when combined with modern sequencing technologies. These recent studies highlight how C. elegans can serve as a bridge between biochemical in vitro experiments and the more associative findings of epidemiological studies in humans to unveil possible mechanisms of environmental influence on the epigenome. The nematode is particularly well-suited to transgenerational experiments thanks to its rapid generation time and ability to self-fertilize. These studies have revealed connections between the various epigenetic mechanisms, and so studies in C. elegans that take advantage of recent advancements in sequencing technologies, including single-cell techniques, to gain unprecedented resolution of the whole epigenome across development and generations will be critical.

How germ granules promote germ cell fate

Germ cells are the only cells in the body capable of giving rise to a new organism, and this totipotency hinges on their ability to assemble membraneless germ granules. These specialized RNA and protein complexes are hallmarks of germ cells throughout their life cycle: as embryonic germ granules in late oocytes and zygotes, Balbiani bodies in immature oocytes, and nuage in maturing gametes. Decades of developmental, genetic and biochemical studies have identified protein and RNA constituents unique to germ granules and have implicated these in germ cell identity, genome integrity and gamete differentiation. Now, emerging research is defining germ granules as biomolecular condensates that achieve high molecular concentrations by phase separation, and it is assigning distinct roles to germ granules during different stages of germline development. This organization of the germ cell cytoplasm into cellular subcompartments seems to be critical not only for the flawless continuity through the germline life cycle within the developing organism but also for the success of the next generation.

Germ granule compartments coordinate specialized small RNA production

Germ granules are biomolecular condensates present in most animal germ cells. One function of germ granules is to help maintain germ cell totipotency by organizing mRNA regulatory machinery, including small RNA-based gene regulatory pathways. The C. elegans germ granule is compartmentalized into multiple subcompartments whose biological functions are largely unknown. Here, we identify an uncharted subcompartment of the C. elegans germ granule, which we term the E granule. The E granule is nonrandomly positioned within the germ granule. We identify five proteins that localize to the E granule, including the RNA-dependent RNA polymerase (RdRP) EGO-1, the Dicer-related helicase DRH-3, the Tudor domain-containing protein EKL-1, and two intrinsically disordered proteins, EGC-1 and ELLI-1. Localization of EGO-1 to the E granule enables synthesis of a specialized class of 22G RNAs, which derive exclusively from 5' regions of a subset of germline-expressed mRNAs. Defects in E granule assembly elicit disordered production of endogenous siRNAs, which disturbs fertility and the RNAi response. Our results define a distinct subcompartment of the C. elegans germ granule and suggest that one function of germ granule compartmentalization is to facilitate the localized production of specialized classes of small regulatory RNAs.

Caenorhabditis elegans germ granules accumulate hundreds of low translation mRNAs with no systematic preference for germ cell fate regulators

In animals with germ plasm, embryonic germline precursors inherit germ granules, condensates proposed to regulate mRNAs coding for germ cell fate determinants. In Caenorhabditis elegans, mRNAs are recruited to germ granules by MEG-3, a sequence non-specific RNA-binding protein that forms stabilizing interfacial clusters on germ granules. Using fluorescence in situ hybridization, we confirmed that 441 MEG-3-bound transcripts are distributed in a pattern consistent with enrichment in germ granules. Thirteen are related to transcripts reported in germ granules in Drosophila or Nasonia. The majority, however, are low-translation maternal transcripts required for embryogenesis that are not maintained preferentially in the nascent germline. Granule enrichment raises the concentration of certain transcripts in germ plasm but is not essential to regulate mRNA translation or stability. Our findings suggest that only a minority of germ granule-associated transcripts contribute to germ cell fate in C. elegans and that the vast majority function as non-specific scaffolds for MEG-3.

Heritable epigenetic changes are constrained by the dynamics of regulatory architectures

Interacting molecules create regulatory architectures that can persist despite turnover of molecules. Although epigenetic changes occur within the context of such architectures, there is limited understanding of how they can influence the heritability of changes. Here, I develop criteria for the heritability of regulatory architectures and use quantitative simulations of interacting regulators parsed as entities, their sensors, and the sensed properties to analyze how architectures influence heritable epigenetic changes. Information contained in regulatory architectures grows rapidly with the number of interacting molecules and its transmission requires positive feedback loops. While these architectures can recover after many epigenetic perturbations, some resulting changes can become permanently heritable. Architectures that are otherwise unstable can become heritable through periodic interactions with external regulators, which suggests that mortal somatic lineages with cells that reproducibly interact with the immortal germ lineage could make a wider variety of architectures heritable. Differential inhibition of the positive feedback loops that transmit regulatory architectures across generations can explain the gene-specific differences in heritable RNA silencing observed in the nematode Caenorhabditis elegans. More broadly, these results provide a foundation for analyzing the inheritance of epigenetic changes within the context of the regulatory architectures implemented using diverse molecules in different living systems.

Phase Separation as a Driver of Stem Cell Organization and Function during Development

A properly organized subcellular composition is essential to cell function. The canonical organizing principle within eukaryotic cells involves membrane-bound organelles; yet, such structures do not fully explain cellular complexity. Furthermore, discrete non-membrane-bound structures have been known for over a century. Liquid-liquid phase separation (LLPS) has emerged as a ubiquitous mode of cellular organization without the need for formal lipid membranes, with an ever-expanding and diverse list of cellular functions that appear to be regulated by this process. In comparison to traditional organelles, LLPS can occur across wider spatial and temporal scales and involves more distinct protein and RNA complexes. In this review, we discuss the impacts of LLPS on the organization of stem cells and their function during development. Specifically, the roles of LLPS in developmental signaling pathways, chromatin organization, and gene expression will be detailed, as well as its impacts on essential processes of asymmetric cell division. We will also discuss how the dynamic and regulated nature of LLPS may afford stem cells an adaptable mode of organization throughout the developmental time to control cell fate. Finally, we will discuss how aberrant LLPS in these processes may contribute to developmental defects and disease.

Heritable epigenetic changes are constrained by the dynamics of regulatory architectures

Interacting molecules create regulatory architectures that can persist despite turnover of molecules. Although epigenetic changes occur within the context of such architectures, there is limited understanding of how they can influence the heritability of changes. Here I develop criteria for the heritability of regulatory architectures and use quantitative simulations of interacting regulators parsed as entities, their sensors and the sensed properties to analyze how architectures influence heritable epigenetic changes. Information contained in regulatory architectures grows rapidly with the number of interacting molecules and its transmission requires positive feedback loops. While these architectures can recover after many epigenetic perturbations, some resulting changes can become permanently heritable. Such stable changes can (1) alter steady-state levels while preserving the architecture, (2) induce different architectures that persist for many generations, or (3) collapse the entire architecture. Architectures that are otherwise unstable can become heritable through periodic interactions with external regulators, which suggests that the evolution of mortal somatic lineages with cells that reproducibly interact with the immortal germ lineage could make a wider variety of regulatory architectures heritable. Differential inhibition of the positive feedback loops that transmit regulatory architectures across generations can explain the gene-specific differences in heritable RNA silencing observed in the nematode C. elegans, which range from permanent silencing to recovery from silencing within a few generations and subsequent resistance to silencing. More broadly, these results provide a foundation for analyzing the inheritance of epigenetic changes within the context of the regulatory architectures implemented using diverse molecules in different living systems.

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.

Nucleus-independent transgenerational small RNA inheritance in Caenorhabditis elegans

In Caenorhabditis elegans worms, epigenetic information transmits transgenerationally. Still, it is unknown whether the effects transfer to the next generation inside or outside of the nucleus. Here, we use the tractability of gene-specific double-stranded RNA-induced silencing to demonstrate that RNA interference can be inherited independently of any nuclear factors via mothers that are genetically engineered to transmit only their ooplasm but not the oocytes' nuclei to the next generation. We characterize the mechanisms and, using RNA sequencing, chimeric worms, and sequence polymorphism between different isolates, identify endogenous small RNAs which, similarly to exogenous siRNAs, are inherited in a nucleus-independent manner. From a historical perspective, these results might be regarded as partial vindication of discredited cytoplasmic inheritance theories from the 19th century, such as Darwin's "pangenesis" theory.

C. elegans germ granules sculpt both germline and somatic RNAome

Germ granules are membrane-less organelles essential for small RNA biogenesis and germline development. Among the conserved properties of germ granules is their association with the nuclear membrane. Recent studies demonstrated that LOTUS domain proteins, EGGD-1 and EGGD-2 (also known as MIP-1 and MIP-2 respectively), promote the formation of perinuclear germ granules in C. elegans. This finding presents a unique opportunity to evaluate the significance of perinuclear localization of germ granules. Here we show that loss of eggd-1 causes the coalescence of germ granules and formation of abnormal cytoplasmic aggregates. Impairment of perinuclear granules affects certain germline classes of small RNAs including Piwi-interacting RNAs. Transcriptome profiling reveals overexpression of spermatogenic and cuticle-related genes in eggd-1 hermaphrodites. We further demonstrate that disruption of germ granules activates HLH-30-mediated transcriptional program in somatic tissues. Collectively, our findings underscore the essential role of EGGD-1 in germ granule organization and reveal an unexpected germ granule-to-soma communication.

Expansion microscopy reveals subdomains in C. elegans germ granules

Light and electron microscopy techniques have been indispensable in the identification and characterization of liquid-liquid phase separation membraneless organelles. However, for complex membraneless organelles such as the perinuclear germ granule in C. elegans, our understanding of how the intact organelle is regulated is hampered by (1) technical limitations in confocal fluorescence imaging for the simultaneous examination of multiple granule protein markers and (2) inaccessibility of electron microscopy. We take advantage of the newly developed super resolution method of expansion microscopy (ExM) and in situ staining of the whole proteome to examine the C. elegans germ granule, the P granule. We show that in small RNA pathway mutants, the P granule is smaller compared with WT animals. Furthermore, we investigate the relationship between the P granule and two other germ granules, Mutator foci and Z granule, and show that they are located within the same protein-dense regions while occupying distinct subdomains within this ultrastructure. This study will serve as an important tool in our understanding of germ granule biology and the biological role of liquid-liquid phase separation.

The third barrier to transgenerational inheritance in animals: somatic epigenetic resetting

After early controversy, it is now increasingly clear that acquired responses to environmental factors may perpetuate across multiple generations-a phenomenon termed transgenerational epigenetic inheritance (TEI). Experiments with Caenorhabditis elegans, which exhibits robust heritable epigenetic effects, demonstrated small RNAs as key factors of TEI. Here, we discuss three major barriers to TEI in animals, two of which, the "Weismann barrier" and germline epigenetic reprogramming, have been known for decades. These are thought to effectively prevent TEI in mammals but not to the same extent in C. elegans. We argue that a third barrier-that we termed "somatic epigenetic resetting"-may further inhibit TEI and, unlike the other two, restricts TEI in C. elegans as well. While epigenetic information can overcome the Weismann barrier and transmit from the soma to the germline, it usually cannot "travel back" directly from the germline to the soma in subsequent generations. Nevertheless, heritable germline memory may still influence the animal's physiology by indirectly modifying gene expression in somatic tissues.

Hypoxia induces transgenerational epigenetic inheritance of small RNAs

Animals sense and adapt to decreased oxygen availability, but whether and how hypoxia exposure in ancestors can elicit phenotypic consequences in normoxia-reared descendants are unclear. We show that hypoxia educes an intergenerational reduction in lipids and a transgenerational reduction in fertility in the nematode Caenorhabditis elegans. The transmission of these epigenetic phenotypes is dependent on repressive histone-modifying enzymes and the argonaute HRDE-1. Feeding naive C. elegans small RNAs extracted from hypoxia-treated worms is sufficient to induce a fertility defect. Furthermore, the endogenous small interfering RNA F44E5.4/5 is upregulated intergenerationally in response to hypoxia, and soaking naive normoxia-reared C. elegans with F44E5.4/5 double-stranded RNA (dsRNA) is sufficient to induce an intergenerational fertility defect. Finally, we demonstrate that labeled F44E5.4/5 dsRNA is itself transmitted from parents to children. Our results suggest that small RNAs respond to the environment and are sufficient to transmit non-genetic information from parents to their naive children.

Specialized germline P-bodies are required to specify germ cell fate in Caenorhabditis elegans embryos

In animals with germ plasm, specification of the germline involves 'germ granules', cytoplasmic condensates that enrich maternal transcripts in the germline founder cells. In Caenorhabditis elegans embryos, P granules enrich maternal transcripts, but surprisingly P granules are not essential for germ cell fate specification. Here, we describe a second condensate in the C. elegans germ plasm. Like canonical P-bodies found in somatic cells, 'germline P-bodies' contain regulators of mRNA decapping and deadenylation and, in addition, the intrinsically-disordered proteins MEG-1 and MEG-2 and the TIS11-family RNA-binding protein POS-1. Embryos lacking meg-1 and meg-2 do not stabilize P-body components, misregulate POS-1 targets, mis-specify the germline founder cell and do not develop a germline. Our findings suggest that specification of the germ line involves at least two distinct condensates that independently enrich and regulate maternal mRNAs in the germline founder cells. This article has an associated 'The people behind the papers' interview.

Acute and inherited piRNA-mediated silencing in a rde-3 ribonucleotidyltransferase mutant

We recently developed a piRNA-based silencing assay (piRNAi) to study small-RNA mediated epigenetic silencing: acute gene silencing is induced by synthetic piRNAs expressed from extra-chromosomal array and transgenerational inheritance can be quantified after array loss. The assay allows inheritance assays by injecting piRNAs directly into mutant animals and targeting endogenous genes ( e.g. , him-5 and him-8 ) with obvious phenotypes (increased male frequency). Here we demonstrate the piRNAi assay by quantifying acute and inherited silencing in the ribonucleotidyltransferase rde-3 (ne3370) mutant. In the absence of rde-3, acute silencing was reduced but still detectable, whereas inherited silencing was abolished.

GLH/VASA helicases promote germ granule formation to ensure the fidelity of piRNA-mediated transcriptome surveillance

piRNAs function as guardians of the genome by silencing non-self nucleic acids and transposable elements in animals. Many piRNA factors are enriched in perinuclear germ granules, but whether their localization is required for piRNA biogenesis or function is not known. Here we show that GLH/VASA helicase mutants exhibit defects in forming perinuclear condensates containing PIWI and other small RNA cofactors. These mutant animals produce largely normal levels of piRNA but are defective in triggering piRNA silencing. Strikingly, while many piRNA targets are activated in GLH mutants, we observe that hundreds of endogenous genes are aberrantly silenced by piRNAs. This defect in self versus non-self recognition is also observed in other mutants where perinuclear germ granules are disrupted. Together, our results argue that perinuclear germ granules function critically to promote the fidelity of piRNA-based transcriptome surveillance in C. elegans and preserve self versus non-self distinction.

Phase separated, membrane-less germ granules preserve fertility and cellular function in animal germ cells. Here the authors show that loss of germ granules impacts piRNA pathway fidelity in the recognition of self and non-self nucleic acids.

A family of C. elegans VASA homologs control Argonaute pathway specificity and promote transgenerational silencing

Germline Argonautes direct transcriptome surveillance within perinuclear membraneless organelles called nuage. In C. elegans, a family of Vasa-related Germ Line Helicase (GLH) proteins localize in and promote the formation of nuage. Previous studies have implicated GLH proteins in inherited silencing, but direct roles in small-RNA production, Argonaute binding, or mRNA targeting have not been identified. Here we show that GLH proteins compete with each other to control Argonaute pathway specificity, bind directly to Argonaute target mRNAs, and promote the amplification of small RNAs required for transgenerational inheritance. We show that the ATPase cycle of GLH-1 regulates direct binding to the Argonaute WAGO-1, which engages amplified small RNAs. Our findings support a dynamic and direct role for GLH proteins in inherited silencing beyond their role as structural components of nuage.

It’s Just a Phase: Exploring the Relationship Between mRNA, Biomolecular Condensates, and Translational Control

Cells spatially organize their molecular components to carry out fundamental biological processes and guide proper development. The spatial organization of RNA within the cell can both promote and result from gene expression regulatory control. Recent studies have demonstrated diverse associations between RNA spatial patterning and translation regulatory control. One form of patterning, compartmentalization in biomolecular condensates, has been of particular interest. Generally, transcripts associated with cytoplasmic biomolecular condensates—such as germ granules, stress granules, and P-bodies—are linked with low translational status. However, recent studies have identified new biomolecular condensates with diverse roles associated with active translation. This review outlines RNA compartmentalization in various condensates that occur in association with repressed or active translational states, highlights recent findings in well-studied condensates, and explores novel condensate behaviors.

Inheritance and maintenance of small RNA-mediated epigenetic effects

Heritable traits are predominantly encoded within genomic DNA, but it is now appreciated that epigenetic information is also inherited through DNA methylation, histone modifications, and small RNAs. Several examples of transgenerational epigenetic inheritance of traits have been documented in plants and animals. These include even the inheritance of traits acquired through the soma during the life of an organism, implicating the transfer of epigenetic information via the germline to the next generation. Small RNAs appear to play a significant role in carrying epigenetic information across generations. This review focuses on how epigenetic information in the form of small RNAs is transmitted from the germline to the embryos through the gametes. We also consider how inherited epigenetic information is maintained across generations in a small RNA-dependent and independent manner. Finally, we discuss how epigenetic traits acquired from the soma can be inherited through small RNAs.

Germ granules and gene regulation in the Caenorhabditis elegans germline

The transparency of Caenorhabditis elegans provides a unique window to observe and study the function of germ granules. Germ granules are specialized ribonucleoprotein (RNP) assemblies specific to the germline cytoplasm, and they are largely conserved across Metazoa. Within the germline cytoplasm, they are positioned to regulate mRNA abundance, translation, small RNA production, and cytoplasmic inheritance to help specify and maintain germline identity across generations. Here we provide an overview of germ granules and focus on the significance of more recent observations that describe how they further demix into sub-granules, each with unique compositions and functions.

Transgenerational inheritance of sexual attractiveness via small RNAs enhances evolvability in C. elegans

It is unknown whether transient transgenerational epigenetic responses to environmental challenges affect the process of evolution, which typically unfolds over many generations. Here, we show that in C. elegans, inherited small RNAs control genetic variation by regulating the crucial decision of whether to self-fertilize or outcross. We found that under stressful temperatures, younger hermaphrodites secrete a male-attracting pheromone. Attractiveness transmits transgenerationally to unstressed progeny via heritable small RNAs and the Argonaute Heritable RNAi Deficient-1 (HRDE-1). We identified an endogenous small interfering RNA pathway, enriched in endo-siRNAs that target sperm genes, that transgenerationally regulates sexual attraction, male prevalence, and outcrossing rates. Multigenerational mating competition experiments and mathematical simulations revealed that over generations, animals that inherit attractiveness mate more and their alleles spread in the population. We propose that the sperm serves as a "stress-sensor" that, via small RNA inheritance, promotes outcrossing in challenging environments when increasing genetic variation is advantageous.

piRNAs initiate transcriptional silencing of spermatogenic genes during C. elegans germline development

Eukaryotic genomes harbor invading transposable elements that are silenced by PIWI-interacting RNAs (piRNAs) to maintain genome integrity in animal germ cells. However, whether piRNAs also regulate endogenous gene expression programs remains unclear. Here, we show that C. elegans piRNAs trigger the transcriptional silencing of hundreds of spermatogenic genes during spermatogenesis, promoting sperm differentiation and function. This silencing signal requires piRNA-dependent small RNA biogenesis and loading into downstream nuclear effectors, which correlates with the dynamic reorganization of two distinct perinuclear biomolecular condensates present in germ cells. In addition, the silencing capacity of piRNAs is temporally counteracted by the Argonaute CSR-1, which targets and licenses spermatogenic gene transcription. The spatial and temporal overlap between these opposing small RNA pathways contributes to setting up the timing of the spermatogenic differentiation program. Thus, our work identifies a prominent role for piRNAs as direct regulators of endogenous transcriptional programs during germline development and gamete differentiation.

Molecular mechanisms of transgenerational epigenetic inheritance

Increasing evidence indicates that non-DNA sequence-based epigenetic information can be inherited across several generations in organisms ranging from yeast to plants to humans. This raises the possibility of heritable 'epimutations' contributing to heritable phenotypic variation and thus to evolution. Recent work has shed light on both the signals that underpin these epimutations, including DNA methylation, histone modifications and non-coding RNAs, and the mechanisms by which they are transmitted across generations at the molecular level. These mechanisms can vary greatly among species and have a more limited effect in mammals than in plants and other animal species. Nevertheless, common principles are emerging, with transmission occurring either via direct replicative mechanisms or indirect reconstruction of the signal in subsequent generations. As these processes become clearer we continue to improve our understanding of the distinctive features and relative contribution of DNA sequence and epigenetic variation to heritable differences in phenotype.

Nuage condensates: accelerators or circuit breakers for sRNA silencing pathways?

Nuage are RNA-rich condensates that assemble around the nuclei of developing germ cells. Many proteins required for the biogenesis and function of silencing small RNAs (sRNAs) enrich in nuage, and it is often assumed that nuage is the cellular site where sRNAs are synthesized and encounter target transcripts for silencing. Using C. elegans as a model, we examine the complex multicondensate architecture of nuage and review evidence for compartmentalization of silencing pathways. We consider the possibility that nuage condensates balance the activity of competing sRNA pathways and serve to limit, rather than enhance, sRNA amplification to protect transcripts from dangerous runaway silencing.

Pathological phase transitions in ALS-FTD impair dynamic RNA-protein granules

The genetics of human disease serves as a robust and unbiased source of insight into human biology, both revealing fundamental cellular processes and exposing the vulnerabilities associated with their dysfunction. Over the last decade, the genetics of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) have epitomized this concept, as studies of ALS-FTD-causing mutations have yielded fundamental discoveries regarding the role of biomolecular condensation in organizing cellular contents while implicating disturbances in condensate dynamics as central drivers of neurodegeneration. Here we review this genetic evidence, highlight its intersection with patient pathology, and discuss how studies in model systems have revealed a role for aberrant condensation in neuronal dysfunction and death. We detail how multiple, distinct types of disease-causing mutations promote pathological phase transitions that disturb the dynamics and function of ribonucleoprotein (RNP) granules. Dysfunction of RNP granules causes pleiotropic defects in RNA metabolism and can drive the evolution of these structures to end-stage pathological inclusions characteristic of ALS-FTD. We propose that aberrant phase transitions of these complex condensates in cells provide a parsimonious explanation for the widespread cellular abnormalities observed in ALS as well as certain histopathological features that characterize late-stage disease.

The Dynamic Regulation of mRNA Translation and Ribosome Biogenesis During Germ Cell Development and Reproductive Aging

The regulation of mRNA translation, both globally and at the level of individual transcripts, plays a central role in the development and function of germ cells across species. Genetic studies using flies, worms, zebrafish and mice have highlighted the importance of specific RNA binding proteins in driving various aspects of germ cell formation and function. Many of these mRNA binding proteins, including Pumilio, Nanos, Vasa and Dazl have been conserved through evolution, specifically mark germ cells, and carry out similar functions across species. These proteins typically influence mRNA translation by binding to specific elements within the 3′ untranslated region (UTR) of target messages. Emerging evidence indicates that the global regulation of mRNA translation also plays an important role in germ cell development. For example, ribosome biogenesis is often regulated in a stage specific manner during gametogenesis. Moreover, oocytes need to produce and store a sufficient number of ribosomes to support the development of the early embryo until the initiation of zygotic transcription. Accumulating evidence indicates that disruption of mRNA translation regulatory mechanisms likely contributes to infertility and reproductive aging in humans. These findings highlight the importance of gaining further insights into the mechanisms that control mRNA translation within germ cells. Future work in this area will likely have important impacts beyond germ cell biology.

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

Small RNAs in epigenetic inheritance: from mechanisms to trait transmission

Inherited information is transmitted to progeny primarily by the genome through the gametes. However, in recent years, epigenetic inheritance has been demonstrated in several organisms, including animals. Although it is clear that certain post‐translational histone modifications, DNA methylation, and noncoding RNAs regulate epigenetic inheritance, the molecular mechanisms responsible for epigenetic inheritance are incompletely understood. This review focuses on the role of small RNAs in transmitting epigenetic information across generations in animals. Examples of documented cases of transgenerational epigenetic inheritance are discussed, from the silencing of transgenes to the inheritance of complex traits, such as fertility, stress responses, infections, and behavior. Experimental evidence supporting the idea that small RNAs are epigenetic molecules capable of transmitting traits across generations is highlighted, focusing on the mechanisms by which small RNAs achieve such a function. Just as the role of small RNAs in epigenetic processes is redefining the concept of inheritance, so too our understanding of the molecular pathways and mechanisms that govern epigenetic inheritance in animals is radically changing.

An essential role for the piRNA pathway in regulating the ribosomal RNA pool in C. elegans

Piwi-interacting RNAs (piRNAs) are RNA effectors with key roles in maintaining genome integrity and promoting fertility in metazoans. In Caenorhabditis elegans loss of piRNAs leads to a transgenerational sterility phenotype. The plethora of piRNAs and their ability to silence transcripts with imperfect complementarity have raised several (non-exclusive) models for the underlying drivers of sterility. Here, we report the extranuclear and transferable nature of the sterility driver, its suppression via mutations disrupting the endogenous RNAi and poly-uridylation machinery, and copy-number amplification at the ribosomal DNA locus. In piRNA-deficient animals, several small interfering RNA (siRNA) populations become increasingly overabundant in the generations preceding loss of germline function, including ribosomal siRNAs (risiRNAs). A concomitant increase in uridylated sense rRNA fragments suggests that poly-uridylation may potentiate RNAi-mediated gene silencing of rRNAs. We conclude that loss of the piRNA machinery allows for unchecked amplification of siRNA populations, originating from abundant highly structured RNAs, to deleterious levels.

Mating can initiate stable RNA silencing that overcomes epigenetic recovery

Stable epigenetic changes appear uncommon, suggesting that changes typically dissipate or are repaired. Changes that stably alter gene expression across generations presumably require particular conditions that are currently unknown. Here we report that a minimal combination of cis-regulatory sequences can support permanent RNA silencing of a single-copy transgene and its derivatives in C. elegans simply upon mating. Mating disrupts competing RNA-based mechanisms to initiate silencing that can last for >300 generations. This stable silencing requires components of the small RNA pathway and can silence homologous sequences in trans. While animals do not recover from mating-induced silencing, they often recover from and become resistant to trans silencing. Recovery is also observed in most cases when double-stranded RNA is used to silence the same coding sequence in different regulatory contexts that drive germline expression. Therefore, we propose that regulatory features can evolve to oppose permanent and potentially maladaptive responses to transient change.

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.

The role of stress-activated RNA-protein granules in surviving adversity

Severe environmental stress can trigger a plethora of physiological changes and, in the process, significant cytoplasmic reorganization. Stress-activated RNA-protein granules have been implicated in this cellular overhaul by sequestering pre-existing mRNAs and influencing their fates during and after stress acclimation. While the composition and dynamics of stress-activated granule formation has been well studied, their function and impact on RNA-cargo has remained murky. Several recent studies challenge the view that these granules degrade and silence mRNAs present at the onset of stress and instead suggest new roles for these structures in mRNA storage, transit, and inheritance. Here we discuss recent evidence for revised models of stress-activated granule functions and the role of these granules in stress survival and recovery.

Small RNAs and chromatin in the multigenerational epigenetic landscape of Caenorhabditis elegans

For decades, it was thought that the only heritable information transmitted from one individual to another was that encoded in the DNA sequence. However, it has become increasingly clear that this is not the case and that the transmission of molecules from within the cytoplasm of the gamete also plays a significant role in heritability. The roundworm, Caenorhabditis elegans, has emerged as one of the leading model organisms in which to study the mechanisms of transgenerational epigenetic inheritance (TEI). Collaborative efforts over the past few years have revealed that RNA molecules play a critical role in transmitting transgenerational responses, but precisely how they do so is as yet uncertain. In addition, the role of histone modifications in epigenetic inheritance is increasingly apparent, and RNA and histones interact in a way that we do not yet fully understand. Furthermore, both exogenous and endogenous RNA molecules, as well as other environmental triggers, are able to induce heritable epigenetic changes that affect transcription across the genome. In most cases, these epigenetic changes last only for a handful of generations, but occasionally can be maintained much longer: perhaps indefinitely. In this review, we discuss the current understanding of the role of RNA and histones in TEI, as well as making clear the gaps in our knowledge. We also speculate on the evolutionary implications of epigenetic inheritance, particularly in the context of a short-lived, clonally propagating species. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Stress resets ancestral heritable small RNA responses

Transgenerational inheritance of small RNAs challenges basic concepts of heredity. In Caenorhabditis elegans nematodes, small RNAs are transmitted across generations to establish a transgenerational memory trace of ancestral environments and distinguish self-genes from non-self-elements. Carryover of aberrant heritable small RNA responses was shown to be maladaptive and to lead to sterility. Here, we show that various types of stress (starvation, high temperatures, and high osmolarity) induce resetting of ancestral small RNA responses and a genome-wide reduction in heritable small RNA levels. We found that mutants that are defective in various stress pathways exhibit irregular RNAi inheritance dynamics even in the absence of stress. Moreover, we discovered that resetting of ancestral RNAi responses is specifically orchestrated by factors that function in the p38 MAPK pathway and the transcription factor SKN-1/Nrf2. Stress-dependent termination of small RNA inheritance could protect from run-on of environment-irrelevant heritable gene regulation.

Biological implications of decapping: beyond bulk mRNA decay

It is well established that mRNA steady-state levels do not directly correlate with transcription rate. This is attributed to the multiple post-transcriptional mechanisms, which control both mRNA turnover and translation within eukaryotic cells. One such mechanism is the removal of the 5' end cap structure of RNAs (decapping). This 5' cap plays a fundamental role in cellular functions related to mRNA processing, transport, translation, quality control, and decay, while its chemical modifications influence the fate of cytoplasmic mRNAs. Decapping is a highly controlled process, performed by multiple decapping enzymes, and regulated by complex cellular networks. In this review, we provide an updated synopsis of 5' end modifications and functions, and give an overview of mRNA decapping enzymes, presenting their enzymatic properties. Focusing on DCP2 decapping enzyme, a major component on the 5'-3' mRNA decay pathway, we describe cis-elements and trans-acting factors that affect its activity, substrate specificity, and cellular localization. Finally, we discuss current knowledge on the biological functions of mRNA decapping and decay factors, highlighting the major questions that remain to be addressed.

How do histone modifications contribute to transgenerational epigenetic inheritance in C. elegans?

Gene regulatory information can be inherited between generations in a phenomenon termed transgenerational epigenetic inheritance (TEI). While examples of TEI in many animals accumulate, the nematode Caenorhabditis elegans has proven particularly useful in investigating the underlying molecular mechanisms of this phenomenon. In C. elegans and other animals, the modification of histone proteins has emerged as a potential carrier and effector of transgenerational epigenetic information. In this review, we explore the contribution of histone modifications to TEI in C. elegans. We describe the role of repressive histone marks, histone methyltransferases, and associated chromatin factors in heritable gene silencing, and discuss recent developments and unanswered questions in how these factors integrate with other known TEI mechanisms. We also review the transgenerational effects of the manipulation of histone modifications on germline health and longevity.

C. elegans germ granules require both assembly and localized regulators for mRNA repression

Cytoplasmic RNA-protein (RNP) granules have diverse biophysical properties, from liquid to solid, and play enigmatic roles in RNA metabolism. Nematode P granules are paradigmatic liquid droplet granules and central to germ cell development. Here we analyze a key P granule scaffolding protein, PGL-1, to investigate the functional relationship between P granule assembly and function. Using a protein-RNA tethering assay, we find that reporter mRNA expression is repressed when recruited to PGL-1. We determine the crystal structure of the PGL-1 N-terminal region to 1.5 Å, discover its dimerization, and identify key residues at the dimer interface. Mutations of those interface residues prevent P granule assembly in vivo, de-repress PGL-1 tethered mRNA, and reduce fertility. Therefore, PGL-1 dimerization lies at the heart of both P granule assembly and function. Finally, we identify the P granule-associated Argonaute WAGO-1 as crucial for repression of PGL-1 tethered mRNA. We conclude that P granule function requires both assembly and localized regulators.

Connecting the Dots: Linking Caenorhabditis elegans Small RNA Pathways and Germ Granules

Germ granules are non-membrane bound, phase-separated organelles, composed of RNAs and proteins. Germ granules are present only within the germ cells of animals, including model systems such as Caenorhabditis elegans, Drosophila, mice, and zebrafish, where they play critical roles in specifying the germ lineage, the inheritance of epigenetic information, and post-transcriptional gene regulation. Across species, conserved germ granule proteins reflect these essential functions. A significant proportion of proteins that localize to germ granules are components of RNA metabolism and small RNA (sRNA) gene regulatory pathways. Here we synthesize our current knowledge of the roles that germ granules and their components play in sRNA pathway functions, transgenerational inheritance, and fertility in the C. elegans germline.

ZSP-1 is a Z granule surface protein required for Z granule fluidity and germline immortality in Caenorhabditis elegans

Germ granules are biomolecular condensates that form in germ cells of all/most animals, where they regulate mRNA expression to promote germ cell function and totipotency. In the adult Caenorhabditis elegans germ cell, these granules are composed of at least four distinct sub-compartments, one of which is the Z granule. To better understand the role of the Z granule in germ cell biology, we conducted a genetic screen for genes specifically required for Z granule assembly or morphology. Here, we show that zsp-1, which encodes a low-complexity/polyampholyte-domain protein, is required for Z granule homeostasis. ZSP-1 localizes to the outer surface of Z granules. In the absence of ZSP-1, Z granules swell to an abnormal size, fail to segregate with germline blastomeres during development, and lose their liquid-like character. Finally, ZSP-1 promotes piRNA- and siRNA-directed gene regulation and germline immortality. Our data suggest that Z granules coordinate small RNA-based gene regulation to promote germ cell function and that ZSP-1 helps/is need to maintain Z granule morphology and liquidity.

Germ Granules Allow Transmission of Small RNA-Based Parental Responses in the "Germ Plasm"

In the recent decade small RNA-based inheritance has been implicated in a variety of transmitted physiological responses to the environment. In Caenorhabditis elegans, heritable small RNAs rely on RNA-dependent RNA polymerases, RNA-processing machinery, chromatin modifiers, and argonauts for their biogenesis and gene-regulatory effects. Importantly, many of these factors reside in evolutionary conserved germ granules that are required for maintaining germ cell identity and gene expression. Recent literature demonstrated that transient disturbance to the stability of the germ granules leads to changes in the pools of heritable small RNAs and the physiology of the progeny. In this piece, we discuss the heritable consequences of transient destabilization of germ granules and elaborate on the various small RNA-related processes that act in the germ granules. We further propose that germ granules may serve as environment sensors that translate environmental changes to inheritable small RNA-based responses.

PLP-1 is essential for germ cell development and germline gene silencing in Caenorhabditis elegans

The germline genome is guarded against invading foreign genetic elements by small RNA-dependent gene-silencing pathways. Components of these pathways localize to, or form distinct aggregates in the vicinity of, germ granules. These components and their dynamics in and out of granules are currently being intensively studied. Here, we report the identification of PLP-1, a Caenorhabditis elegans protein related to the human single-stranded nucleic acid-binding protein Pur-alpha, as a component of germ granules in C. elegans We show that PLP-1 is essential for silencing different types of transgenes in the germ line and for suppressing the expression of several endogenous genes controlled by the germline gene-silencing pathways. Our results reveal that PLP-1 functions downstream of small RNA biogenesis during initiation of gene silencing. Based on these results and the earlier findings that Pur-alpha proteins interact with both RNA and protein, we propose that PLP-1 couples certain RNAs with their protein partners in the silencing complex. PLP-1 orthologs localized on RNA granules may similarly contribute to germline gene silencing in other organisms.

Concepts and functions of small RNA pathways in C. elegans

A diversity of gene regulatory mechanisms drives the changes in gene expression required for animal development. Here, we discuss the developmental roles of a class of gene regulatory factors composed of a core protein subunit of the Argonaute family and a 21-26-nucleotide RNA cofactor. These represent ancient regulatory complexes, originally evolved to repress genomic parasites such as transposons, viruses and retroviruses. However, over the course of evolution, small RNA-guided pathways have expanded and diversified, and they play multiple roles across all eukaryotes. Pertinent to this review, Argonaute and small RNA-mediated regulation has acquired numerous functions that affect all aspects of animal life. The regulatory function is provided by the Argonaute protein and its interactors, while the small RNA provides target specificity, guiding the Argonaute to a complementary RNA. C. elegans has 19 different, functional Argonautes, defining distinct yet interconnected pathways. Each Argonaute binds a relatively well-defined class of small RNA with distinct molecular properties. A broad classification of animal small RNA pathways distinguishes between two groups: (i) the microRNA pathway is involved in repressing relatively specific endogenous genes and (ii) the other small RNA pathways, which effectively act as a genomic immune system to primarily repress expression of foreign or "non-self" RNA while maintaining correct endogenous gene expression. microRNAs play prominent direct roles in all developmental stages, adult physiology and lifespan. The other small RNA pathways act primarily in the germline, but their impact extends far beyond, into embryogenesis and adult physiology, and even to subsequent generations. Here, we review the mechanisms and developmental functions of the diverse small RNA pathways of C. elegans.

Phase Separation in Germ Cells and Development

The animal germline is an immortal cell lineage that gives rise to eggs and/or sperm each generation. Fusion of an egg and sperm, or fertilization, sets off a cascade of developmental events capable of producing an array of different cell types and body plans. How germ cells develop, function, and eventually give rise to entirely new organisms is an important question in biology. A growing body of evidence suggests that phase separation events likely play a significant and multifaceted role in germ cells and development. Here, we discuss the organization, dynamics, and potential functions of phase-separated compartments in germ cells and examine the various ways in which phase separation might contribute to the development of multicellular organisms.

DEPS-1 is required for piRNA-dependent silencing and PIWI condensate organisation in Caenorhabditis elegans

Membraneless organelles are sites for RNA biology including small non-coding RNA (ncRNA) mediated gene silencing. How small ncRNAs utilise phase separated environments for their function is unclear. We investigated how the PIWI-interacting RNA (piRNA) pathway engages with the membraneless organelle P granule in Caenorhabditis elegans. Proteomic analysis of the PIWI protein PRG-1 reveals an interaction with the constitutive P granule protein DEPS-1. DEPS-1 is not required for piRNA biogenesis but piRNA-dependent silencing: deps-1 mutants fail to produce the secondary endo-siRNAs required for the silencing of piRNA targets. We identify a motif on DEPS-1 which mediates a direct interaction with PRG-1. DEPS-1 and PRG-1 form intertwining clusters to build elongated condensates in vivo which are dependent on the Piwi-interacting motif of DEPS-1. Additionally, we identify EDG-1 as an interactor of DEPS-1 and PRG-1. Our study reveals how specific protein-protein interactions drive the spatial organisation and piRNA-dependent silencing within membraneless organelles.

poly(UG)-tailed RNAs in genome protection and epigenetic inheritance

Mobile genetic elements threaten genome integrity in all organisms. MUT-2/RDE-3 is a ribonucleotidyltransferase required for transposon silencing and RNA interference (RNAi) in C. elegans^1–4. When tethered to RNAs in heterologous expression systems, RDE-3 can add long stretches of alternating non-templated uridine (U) and guanosine (G) ribonucleotides to the 3’ termini of these RNAs (poly(UG) or pUG tails)⁵. Here we show that, in its natural context in C. elegans, RDE-3 adds pUG tails to targets of RNAi, as well as to transposon RNAs. pUG tails with more than 16 perfectly alternating 3’ U and G nucleotides convert RNA fragments into agents of gene silencing. pUG tails promote gene silencing by recruiting RNA-dependent RNA polymerases (RdRPs), which use pUG-tailed RNAs (pUG RNAs) as templates to synthesize small interfering RNAs (siRNAs). Our results show that cycles of pUG RNA–templated siRNA synthesis and siRNA-directed mRNA pUGylation underlie dsRNA-directed transgenerational epigenetic inheritance in the C. elegans germline. We speculate that this pUG RNA/siRNA silencing loop allows parents to inoculate progeny against the expression of unwanted or parasitic genetic elements

Defend Thyself and Thy Offspring

Three recent studies (Dodson and Kennedy, 2019; Lev et al., 2019; Ouyang et al., 2019) reveal that germ granule formation is necessary to protect germline-expressed genes from improper small RNA-mediated silencing. Loss of this protection leads to accumulation of small RNAs, impacting gene expression in multiple subsequent generations.

The PIWI protein Aubergine recruits eIF3 to activate translation in the germ plasm

Piwi-interacting RNAs (piRNAs) and PIWI proteins are essential in germ cells to repress transposons and regulate mRNAs. In Drosophila, piRNAs bound to the PIWI protein Aubergine (Aub) are transferred maternally to the embryo and regulate maternal mRNA stability through two opposite roles. They target mRNAs by incomplete base pairing, leading to their destabilization in the soma and stabilization in the germ plasm. Here, we report a function of Aub in translation. Aub is required for translational activation of nanos mRNA, a key determinant of the germ plasm. Aub physically interacts with the poly(A)-binding protein (PABP) and the translation initiation factor eIF3. Polysome gradient profiling reveals the role of Aub at the initiation step of translation. In the germ plasm, PABP and eIF3d assemble in foci that surround Aub-containing germ granules, and Aub acts with eIF3d to promote nanos translation. These results identify translational activation as a new mode of mRNA regulation by Aub, highlighting the versatility of PIWI proteins in mRNA regulation.