Conservation of large foci formation in arrested oocytes of Caenorhabditis nematodes

Cytoplasmic nucleoporin assemblage: the cellular artwork in physiology and disease

Nucleoporins, essential proteins building the nuclear pore, are pivotal for ensuring nucleocytoplasmic transport. While traditionally confined to the nuclear envelope, emerging evidence indicates their presence in various cytoplasmic structures, suggesting potential non-transport-related roles. This review consolidates findings on cytoplasmic nucleoporin assemblies across different states, including normal physiological conditions, stress, and pathology, exploring their structural organization, formation dynamics, and functional implications. We summarize the current knowledge and the latest concepts on the regulation of nucleoporin homeostasis, aiming to enhance our understanding of their unexpected roles in physiological and pathological processes.

The CCT chaperonin and actin modulate the ER and RNA-binding protein condensation during oogenesis and maintain translational repression of maternal mRNA and oocyte quality

The regulation of maternal mRNAs is essential for proper oogenesis, the production of viable gametes, and to avoid birth defects and infertility. Many oogenic RNA-binding proteins have been identified with roles in mRNA metabolism, some of which localize to dynamic ribonucleoprotein granules and others that appear dispersed. Here, we use a combination of in vitro condensation assays and the in vivo Caenorhabditis elegans oogenesis model to characterize the properties of the conserved KH-domain MEX-3 protein and to identify novel regulators of MEX-3 and three other translational regulators. We demonstrate that MEX-3 undergoes phase separation and appears to have intrinsic gel-like properties in vitro. We also identify novel roles for the chaperonin-containing tailless complex polypeptide 1 (CCT) chaperonin and actin in preventing ectopic RNA-binding protein condensates in maturing oocytes that appear to be independent of MEX-3 folding. The CCT chaperonin and actin also oppose the expansion of endoplasmic reticulum sheets that may promote ectopic condensation of RNA-binding proteins. These novel regulators of condensation are also required for the translational repression of maternal mRNA which is essential for oocyte quality and fertility. The identification of this regulatory network may also have implications for understanding the role of hMex3 phase transitions in cancer.

The CCT chaperonin and actin modulate the ER and RNA-binding protein condensation during oogenesis to maintain translational repression of maternal mRNA and oocyte quality

The regulation of maternal mRNAs is essential for proper oogenesis, the production of viable gametes, and to avoid birth defects and infertility. Many oogenic RNA-binding proteins have been identified with roles in mRNA metabolism, some of which localize to dynamic ribonucleoprotein granules and others that appear dispersed. Here, we use a combination of in vitro condensation assays and the in vivo C. elegans oogenesis model to determine the intrinsic properties of the conserved KH-domain MEX-3 protein and to identify novel regulators of MEX-3 and the Lsm protein, CAR-1. We demonstrate that MEX-3 undergoes liquid-liquid phase separation and appears to have intrinsic gel-like properties in vitro . We also identify novel roles for the CCT chaperonin and actin in preventing ectopic RNA-binding protein condensates in maturing oocytes that appear to be independent of MEX-3 folding. CCT and actin also oppose the expansion of ER sheets that may promote ectopic condensation of RNA-binding proteins that are associated with de-repression of maternal mRNA. This regulatory network is essential to preserve oocyte quality, prevent infertility, and may have implications for understanding the role of hMex3 phase transitions in cancer.

Significance statement

The molecular mechanisms that regulate phase transitions of oogenic RNA-binding proteins are critical to elucidate but are not fully understood.We identify novel regulators of RNA-binding protein phase transitions in maturing oocytes that are required to maintain translational repression of maternal mRNAs and oocyte quality.This study is the first to elucidate a regulatory network involving the CCT chaperonin, actin, and the ER for phase transitions of RNA-binding proteins during oogenesis. Our findings for the conserved MEX-3 protein may also be applicable to better understanding the role of hMex3 phase transitions in cancer.

Self-demixing of mRNA copies buffers mRNA:mRNA and mRNA:regulator stoichiometries

Cellular homeostasis requires the robust control of biomolecule concentrations, but how do millions of mRNAs coordinate their stoichiometries in the face of dynamic translational changes? Here, we identified a two-tiered mechanism controlling mRNA:mRNA and mRNA:protein stoichiometries where mRNAs super-assemble into condensates with buffering capacity and sorting selectivity through phase-transition mechanisms. Using C. elegans oogenesis arrest as a model, we investigated the transcriptome cytosolic reorganization through the sequencing of RNA super-assemblies coupled with single mRNA imaging. Tightly repressed mRNAs self-assembled into same-sequence nanoclusters that further co-assembled into multiphase condensates. mRNA self-sorting was concentration dependent, providing a self-buffering mechanism that is selective to sequence identity and controls mRNA:mRNA stoichiometries. The cooperative sharing of limiting translation repressors between clustered mRNAs prevented the disruption of mRNA:repressor stoichiometries in the cytosol. Robust control of mRNA:mRNA and mRNA:protein stoichiometries emerges from mRNA self-demixing and cooperative super-assembly into multiphase multiscale condensates with dynamic storage capacity.

Age-associated anatomical and physiological alterations in Caenorhabditis elegans

Since its introduction by Sydney Brenner, Caenorhabditis elegans has become a widely studied organism. Given its highly significant properties, including transparency, short lifespan, self-fertilization, high reproductive yield and ease in manipulation and genetic modifications, the nematode has contributed to the elucidation of several fundamental aspects of biology, such as development and ageing. Moreover, it has been extensively used as a platform for the modelling of ageing-associated human disorders, especially those related to neurodegeneration. The use of C. elegans for such purposes requires, and at the same time promotes the investigation of its normal ageing process. In this review we aim to summarize the major organismal alterations during normal worm ageing, in terms of morphology and functionality.

Nucleoporin foci are stress-sensitive condensates dispensable for C. elegans nuclear pore assembly

Nucleoporins (Nups) assemble nuclear pores that form the permeability barrier between nucleoplasm and cytoplasm. Nucleoporins also localize in cytoplasmic foci proposed to function as pore pre-assembly intermediates. Here, we characterize the composition and incidence of cytoplasmic Nup foci in an intact animal, C. elegans. We find that, in young non-stressed animals, Nup foci only appear in developing sperm, oocytes and embryos, tissues that express high levels of nucleoporins. The foci are condensates of highly cohesive FG repeat-containing nucleoporins (FG-Nups), which are maintained near their solubility limit in the cytoplasm by posttranslational modifications and chaperone activity. Only a minor fraction of FG-Nup molecules concentrate in Nup foci, which dissolve during M phase and are dispensable for nuclear pore assembly. Nucleoporin condensation is enhanced by stress and advancing age, and overexpression of a single FG-Nup in post-mitotic neurons is sufficient to induce ectopic condensation and organismal paralysis. We speculate that Nup foci are non-essential and potentially toxic condensates whose assembly is actively suppressed in healthy cells.

RNA granules: functional compartments or incidental condensates?

RNA granules are mesoscale assemblies that form in the absence of limiting membranes. RNA granules contain factors for RNA biogenesis and turnover and are often assumed to represent specialized compartments for RNA biochemistry. Recent evidence suggests that RNA granules assemble by phase separation of subsoluble ribonucleoprotein (RNP) complexes that partially demix from the cytoplasm or nucleoplasm. We explore the possibility that some RNA granules are nonessential condensation by-products that arise when RNP complexes exceed their solubility limit as a consequence of cellular activity, stress, or aging. We describe the use of evolutionary and mutational analyses and single-molecule techniques to distinguish functional RNA granules from "incidental condensates."

The GATA factor ELT-3 specifies endoderm in Caenorhabditis angaria in an ancestral gene network

Endoderm specification in Caenorhabditis elegans occurs through a network in which maternally provided SKN-1/Nrf, with additional input from POP-1/TCF, activates the GATA factor cascade MED-1,2→END-1,3→ELT-2,7. Orthologues of the MED, END and ELT-7 factors are found only among nematodes closely related to C. elegans, raising the question of how gut is specified in their absence in more distant species in the genus. We find that the C. angaria, C. portoensis and C. monodelphis orthologues of the GATA factor gene elt-3 are expressed in the early E lineage, just before their elt-2 orthologues. In C. angaria, Can-pop-1(RNAi), Can-elt-3(RNAi) and a Can-elt-3 null mutation result in a penetrant ‘gutless’ phenotype. Can-pop-1 is necessary for Can-elt-3 activation, showing that it acts upstream. Forced early E lineage expression of Can-elt-3 in C. elegans can direct the expression of a Can-elt-2 transgene and rescue an elt-7 end-1 end-3; elt-2 quadruple mutant strain to viability. Our results demonstrate an ancestral mechanism for gut specification and differentiation in Caenorhabditis involving a simpler POP-1→ELT-3→ELT-2 gene network.

Oocyte Quiescence: From Formation to Awakening

Decades of work using various model organisms have resulted in an exciting and emerging field of oocyte maturation. High levels of insulin and active mammalian target of rapamycin signals, indicative of a good nutritional environment, and hormones such as gonadotrophin, indicative of the growth of the organism, work together to control oocyte maturation to ensure that reproduction happens at the right timing under the right conditions. In the wild, animals often face serious challenges to maintain oocyte quiescence under long-term unfavorable conditions in the absence of mates or food. Failure to maintain oocyte quiescence will result in activation of oocytes at the wrong time and thus lead to exhaustion of the oocyte pool and sterility of the organism. In this review, we discuss the shared mechanisms in oocyte quiescence and awakening and a conserved role of noradrenergic signals in maintenance of the quiescent oocyte pool under unfavorable conditions in simple model organisms.

The endogenous mex-3 3´UTR is required for germline repression and contributes to optimal fecundity in C. elegans

RNA regulation is essential to successful reproduction. Messenger RNAs delivered from parent to progeny govern early embryonic development. RNA-binding proteins (RBPs) are the key effectors of this process, regulating the translation and stability of parental transcripts to control cell fate specification events prior to zygotic gene activation. The KH-domain RBP MEX-3 is conserved from nematode to human. It was first discovered in Caenorhabditis elegans, where it is essential for anterior cell fate and embryo viability. Here, we show that loss of the endogenous mex-3 3´UTR disrupts its germline expression pattern. An allelic series of 3´UTR deletion variants identify repressing regions of the UTR and demonstrate that repression is not precisely coupled to reproductive success. We also show that several RBPs regulate mex-3 mRNA through its 3´UTR to define its unique germline spatiotemporal expression pattern. Additionally, we find that both poly(A) tail length control and the translation initiation factor IFE-3 contribute to its expression pattern. Together, our results establish the importance of the mex-3 3´UTR to reproductive health and its expression in the germline. Our results suggest that additional mechanisms control MEX-3 function when 3´UTR regulation is compromised.

In sexually reproducing organisms, germ cells undergo meiosis and differentiate to form oocytes or sperm. Coordination of this process requires a gene regulatory program that acts while the genome is undergoing chromatin condensation. As such, RNA regulatory pathways are an important contributor. The germline of the nematode Caenorhabditis elegans is a suitable model system to study germ cell differentiation. Several RNA-binding proteins (RBPs) coordinate each transition in the germline such as the transition from mitosis to meiosis. MEX-3 is a conserved RNA-binding protein found in most animals including humans. In C. elegans, MEX-3 displays a highly restricted pattern of expression. Here, we define the importance of the 3´UTR in regulating MEX-3 expression pattern in vivo and characterize the RNA-binding proteins involved in this regulation. Our results show that deleting various mex-3 3´UTR regions alter the pattern of expression in the germline in various ways. These mutations also reduced—but did not eliminate—reproductive capacity. Finally, we demonstrate that multiple post-transcriptional mechanisms control MEX-3 levels in different domains of the germline. Our data suggest that coordination of MEX-3 activity requires multiple layers of regulation to ensure reproductive robustness.

Germ cell ribonucleoprotein granules in different clades of life: From insects to mammals

Ribonucleoprotein (RNP) granules are no newcomers in biology. Found in all life forms, ranging across taxa, these membrane-less "organelles" have been classified into different categories based on their composition, structure, behavior, function, and localization. Broadly, they can be listed as stress granules (SGs), processing bodies (PBs), neuronal granules (NGs), and germ cell granules (GCGs). Keeping in line with the topic of this review, RNP granules present in the germ cells have been implicated in a wide range of cellular functions including cellular specification, differentiation, proliferation, and so forth. The mechanisms used by them can be diverse and many of them remain partly obscure and active areas of research. GCGs can be of different types in different organisms and at different stages of development, with multiple types coexisting in the same cell. In this review, the different known subcategories of GCGs have been studied with respect to five distinct model organisms, namely, Drosophila, Caenorhabditis elegans, Xenopus, Zebrafish, and mammals. Of them, the cytoplasmic polar granules in Drosophila, P granules in C. elegans, balbiani body in Xenopus and Zebrafish, and chromatoid bodies in mammals have been specifically emphasized upon. A descriptive account of the same has been provided along with insights into our current understanding of their functional significance with respect to cellular events relating to different developmental and reproductive processes. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization RNA in Disease and Development > RNA in Disease.

Vertical transmission in Caenorhabditis nematodes of RNA molecules encoding a viral RNA-dependent RNA polymerase

In organisms composed of a single cell, RNAs of viral origin may be transmitted to daughter cells at cell division without passing through an extracellular virion stage. These RNAs usually encode an RNA-dependent RNA polymerase that enables their replication. For some of these agents, such as Narnaviruses, no capsid protein is expressed, and thus, they are called capsidless viruses. Here, we identify putative capsidless viral RNAs in animals, in nematodes closely related to the model organism Caenorhabditis elegans. We show that these RNAs are transmitted vertically through the host germline. Our work provides evidence that animal cells harbor capsidless viruses.

Here, we report on the discovery in Caenorhabditis nematodes of multiple vertically transmitted RNAs coding for putative RNA-dependent RNA polymerases. Their sequences share similarity to distinct RNA viruses, including bunyaviruses, narnaviruses, and sobemoviruses. The sequences are present exclusively as RNA and are not found in DNA form. The RNAs persist in progeny after bleach treatment of adult animals, indicating vertical transmission of the RNAs. We tested one of the infected strains for transmission to an uninfected strain and found that mating of infected animals with uninfected animals resulted in infected progeny. By in situ hybridization, we detected several of these RNAs in the cytoplasm of the male and female germline of the nematode host. The Caenorhabditis hosts were found defective in degrading exogenous double-stranded RNAs, which may explain retention of viral-like RNAs. Strikingly, one strain, QG551, harbored three distinct virus-like RNA elements. Specific patterns of small RNAs complementary to the different viral-like RNAs were observed, suggesting that the different RNAs are differentially recognized by the RNA interference (RNAi) machinery. While vertical transmission of viruses in the family Narnaviridae, which are known as capsidless viruses, has been described in fungi, these observations provide evidence that multicellular animal cells harbor similar viruses.

Germ Cell Responses to Stress: The Role of RNP Granules

The ability to respond to stress is critical to survival for animals. While stress responses have been studied at both organismal and cellular levels, less attention has been given to the effect of stress on the germ line. Effective germ line adaptations to stress are essential to the propagation of a species. Recent studies suggest that germ cells share some cellular responses to stress with somatic cells, including the assembly of RNP granules, but may also have unique requirements. One fundamental difference between oocytes and sperm, as well as most somatic cells, is the long lifespan of oocytes. Since women are born with all of their eggs, oocytes must maintain their cellular quality over decades prior to fertilization. This prolonged meiotic arrest is one type of stress that eventually contributes to decreased fertility in older women. Germ cell responses to nutritional stress and heat stress have also been well-characterized using model systems. Here we review our current understanding of how germ cells respond to stress, with an emphasis on the dynamic assembly of RNP granules that may be adaptive. We compare and contrast stress responses of male gametes with those of female gametes, and discuss how the dynamic cellular remodeling of the germ line can impact the regulation of gene expression. We also discuss the implications of recent in vitro studies of ribonucleoprotein granule assembly on our understanding of germ line responses to stress, and the gaps that remain in our understanding of the function of RNP granules during stress.

Detecting and quantifying stress granules in tissues of multicellular organisms with the Obj.MPP analysis tool

Stress granules (SGs) are macromolecular assemblies induced by stress and composed of proteins and mRNAs stalled in translation initiation. SGs play an important role in the response to stress and in the modulation of signaling pathways. Furthermore, these structures are related to the pathological ribonucleoprotein (RNP) aggregates found in neurodegenerative disease contexts, highlighting the need to understand how they are formed and recycled in normal and pathological contexts. Although genetically tractable multicellular organisms have been key in identifying modifiers of RNP aggregate toxicity, in vivo analysis of SG properties and regulation has lagged behind, largely due to the difficulty of detecting SG from images of intact tissues. Here, we describe the object detector software Obj.MPP and show how it overcomes the limits of classical object analyzers to extract the properties of SGs from wide-field and confocal images of Caenorhabditis elegans and Drosophila tissues, respectively. We demonstrate that Obj.MPP enables the identification of genes modulating the assembly of endogenous and pathological SGs, and thus that it will be useful in the context of future genetic screens and in vivo studies.

Remodeling of the endoplasmic reticulum in Caenorhabditis elegans oocytes is regulated by CGH-1

A key aspect of development in all metazoans is remodeling at the cellular level. During the development of gametes, remodeling occurs throughout the germ line. When Caenorhabditis elegans hermaphrodites become depleted of sperm after 4 days of adulthood, significant cellular remodeling occurs within the meiotically-arrested oocytes, including the formation of ribonucleoprotein granules. Since major remodeling of the endoplasmic reticulum (ER) occurs in early embryos, we investigated the extent of ER remodeling in meiotically-arrested oocytes. We found, using a combination of fluorescence reporters and transmission electron microscopy, that the ER in arrested oocytes accumulates in patches and sheets that are enriched at the cortex. Our findings suggest this remodeling is not due to simple displacement by large amounts of yolk that accumulate in arrested oocytes, and instead may be genetically regulated. We further identified the Ddx6 RNA helicase, CGH-1, as a key regulator of ER in the germ line. In cgh-1(tn691) oocytes, we detected cortical ER patches as well as aberrant granules of the RNA-binding proteins, PAB-1, MEX-3, and CGH-1. Taken together, our results suggest the possibility that the spatial organization of RNA binding proteins may regulate the translation of mRNAs associated with the ER that in turn, controls the organization of the ER in the adult germ line.

Control of oocyte meiotic maturation in C. elegans

In virtually all sexually reproducing animals, oocytes arrest in meiotic prophase and resume meiosis in a conserved biological process called meiotic maturation. Meiotic arrest enables oocytes, which are amongst the largest cells in an organism, to grow and accumulate the necessary cellular constituents required to support embryonic development. Oocyte arrest can be maintained for a prolonged period, up to 50 years in humans, and defects in the meiotic maturation process interfere with the faithful segregation of meiotic chromosomes, representing the leading cause of human birth defects and female infertility. Hormonal signaling and interactions with somatic cells of the gonad control the timing of oocyte meiotic maturation. Signaling activates the CDK1/cyclin B kinase, which plays a central role in regulating the nuclear and cytoplasmic events of meiotic maturation. Nuclear maturation encompasses nuclear envelope breakdown, meiotic spindle assembly, and chromosome segregation whereas cytoplasmic maturation involves major changes in oocyte protein translation and cytoplasmic organelles and is less well understood. Classically, meiotic maturation has been studied in organisms with large oocytes to facilitate biochemical analysis. Recently, the nematode Caenorhabditis elegans is emerging as a genetic paradigm for studying the regulation of oocyte meiotic maturation. Studies in this system have revealed conceptual, anatomical, and molecular links to oocytes in all animals including humans. This review focuses on the signaling mechanisms required to control oocyte growth and meiotic maturation in C. elegans and discusses how the downstream regulation of protein translation coordinates the completion of meiosis and the oocyte-to-embryo transition.

From "the Worm" to "the Worms" and Back Again: The Evolutionary Developmental Biology of Nematodes

Since the earliest days of research on nematodes, scientists have noted the developmental and morphological variation that exists within and between species. As various cellular and developmental processes were revealed through intense focus on Caenorhabditis elegans, these comparative studies have expanded. Within the genus Caenorhabditis, they include characterization of intraspecific polymorphisms and comparisons of distinct species, all generally amenable to the same laboratory culture methods and supported by robust genomic and experimental tools. The C. elegans paradigm has also motivated studies with more distantly related nematodes and animals. Combined with improved phylogenies, this work has led to important insights about the evolution of nematode development. First, while many aspects of C. elegans development are representative of Caenorhabditis, and of terrestrial nematodes more generally, others vary in ways both obvious and cryptic. Second, the system has revealed several clear examples of developmental flexibility in achieving a particular trait. This includes developmental system drift, in which the developmental control of homologous traits has diverged in different lineages, and cases of convergent evolution. Overall, the wealth of information and experimental techniques developed in C. elegans is being leveraged to make nematodes a powerful system for evolutionary cellular and developmental biology.

Signal Transduction Pathways in Ageing

It is now widely recognised that ageing and its associated functional decline are regulated by a wide range of molecules that fit into specific cellular pathways. Here, we describe several of the evolutionary conserved cellular signalling pathways that govern organismal ageing and discuss how their identification, and work on the individual molecules that contribute to them, has aided in the design of therapeutic strategies to alleviate the adverse effects of ageing and age-related disease.

Biphasic adaptation to osmotic stress in the C. elegans germ line

Cells respond to environmental stress in multiple ways. In the germ line, heat shock and nutritive stress trigger the assembly of large ribonucleoprotein (RNP) granules via liquid-liquid phase separation (LLPS). The RNP granules are hypothesized to maintain the quality of oocytes during stress. The goal of this study was to investigate the cellular response to glucose in the germ line and determine if it is an osmotic stress response. We found that exposure to 500 mM glucose induces the assembly of RNP granules in the germ line within 1 h. Interestingly, the RNP granules are maintained for up to 3 h; however, they dissociate after longer periods of stress. The RNP granules include processing body and stress granule proteins, suggesting shared functions. Based on several lines of evidence, the germ line response to glucose largely appears to be an osmotic stress response, thus identifying osmotic stress as a trigger of LLPS. Although RNP granules are not maintained beyond 3 h of osmotic stress, the quality of oocytes does not appear to decrease after longer periods of stress, suggesting a secondary adaptation in the germ line. We used an indirect marker of glycerol and observed high levels after 5 and 20 h of glucose exposure. Moreover, in gpdh-1;gpdh-2 germ lines, glycerol levels are reduced concomitant with RNP granules being maintained for an extended period. We speculate that increased glycerol levels may function as a secondary osmoregulatory adaptive response in the germ line, following a primary response of RNP granule assembly.

Run-on of germline apoptosis promotes gonad senescence in C. elegans

Aging (senescence) includes causal mechanisms (etiologies) of late-life disease, which remain poorly understood. According to the recently proposed hyperfunction theory, based on the older theory of antagonistic pleiotropy, senescent pathologies can arise from futile, post-reproductive run-on of processes that in early life promote fitness. Here we apply this idea to investigate the etiology of senescent pathologies in the reproductive system of Caenorhabditis elegans hermaphrodites, particularly distal gonad degeneration and disintegration. Hermaphrodite germ cells frequently undergo "physiological" (non-damage-induced) apoptosis (PA) to provision growing oocytes. Run-on of such PA is a potential cause of age-related gonad degeneration. We document the continuation of germline apoptosis in later life, and report that genetically blocking or increasing PA retards or accelerates degeneration, respectively. In wild-type males, which lack germ line apoptosis, gonad disintegration does not occur. However, mutational induction of PA in males does not lead to gonad disintegration. These results suggest that as germ-cell proliferation rate declines markedly in aging hermaphrodites (but not males), run-on of PA becomes a pathogenic mechanism that promotes gonad degeneration. This illustrates how hyperfunction, or non-adaptive run-on in later life of a process that promotes fitness in early life, can promote atrophic senescent pathology in C. elegans.

MDL-1, a growth- and tumor-suppressor, slows aging and prevents germline hyperplasia and hypertrophy in C. elegans

In C. elegans, increased lifespan in daf-2 insulin/IGF-1 receptor mutants is accompanied by up-regulation of the MDL-1 Mad basic helix-loop-helix leucine zipper transcription factor. Here we describe the role of mdl-1 in C. elegans germline proliferation and aging. The deletion allele mdl-1(tm311) shortened lifespan, and did so significantly more so in long-lived daf-2 mutants implying that mdl-1(+) contributes to effects of daf-2 on lifespan. mdl-1 mutant hermaphrodites also lay increased numbers of unfertilized oocytes. During aging, unfertilized oocytes in the uterus develop into tumors, whose development was accelerated by mdl-1(tm311). Opposite phenotypes were seen in daf-2 mutants, i.e. mdl-1 and daf-2 mutant germlines are hyperplastic and hypoplastic, respectively. Thus, MDL-1, like its mammalian orthologs, is an inhibitor of cell proliferation and growth that slows progression of an age-related pathology in C. elegans (uterine tumors). In addition, intestine-limited rescue of mdl-1 increased lifespan but not to wild type levels. Thus, mdl-1 likely acts both in the intestine and the germline to influence age-related mortality.

The DEAD Box RNA helicase VBH-1 is a new player in the stress response in C. elegans

For several years, DEAD box RNA helicase Vasa (DDX4) has been used as a bona fide germline marker in different organisms. C. elegans VBH-1 is a close homolog of the Vasa protein, which plays an important role in gametogenesis, germ cell survival and embryonic development. Here, we show that VBH-1 protects nematodes from heat shock and oxidative stress. Using the germline-defective mutant glp-4(bn2) we found that a potential somatic expression of vbh-1 might be important for stress survival. We also show that the VBH-1 paralog LAF-1 is important for stress survival, although this protein is not redundant with its counterpart. Furthermore, we observed that the mRNAs of the heat shock proteins hsp-1 and sip-1 are downregulated when vbh-1 or laf-1 are silenced. Previously, we reported that in C. elegans, VBH-1 was primarily expressed in P granules of germ cells and in the cytoplasm of all blastomeres. Here we show that during stress, VBH-1 co-localizes with CGH-1 in large aggregates in the gonad core and oocytes; however, VBH-1 aggregates do not overlap with CGH-1 foci in early embryos under the same conditions. These data demonstrate that, in addition to the previously described role for this protein in the germline, VBH-1 plays an important role during the stress response in C. elegans through the potential direct or indirect regulation of stress response mRNAs.

Translation repressors, an RNA helicase, and developmental cues control RNP phase transitions during early development

Like membranous organelles, large-scale coassembly of macromolecules can organize functions in cells. Ribonucleoproteins (RNPs) can form liquid or solid aggregates, but control and consequences of these RNP states in living, developing tissue are poorly understood. Here, we show that regulated RNP factor interactions drive transitions among diffuse, semiliquid, or solid states to modulate RNP sorting and exchange in the Caenorhabditis elegans oocyte cytoplasm. Translation repressors induce an intrinsic capacity of RNP components to coassemble into either large semiliquids or solid lattices, whereas a conserved RNA helicase prevents polymerization into nondynamic solids. Developmental cues dramatically alter both fluidity and sorting within large RNP assemblies, inducing a transition from RNP segregation in quiescent oocytes to dynamic exchange in the early embryo. Therefore, large-scale organization of gene expression extends to the cytoplasm, where regulation of supramolecular states imparts specific patterns of RNP dynamics.

Effects of stress and aging on ribonucleoprotein assembly and function in the germ line

In a variety of cell types, ribonucleoprotein (RNP) complexes play critical roles in regulating RNA metabolism. The germ line contains RNPs found also in somatic cells, such as processing (P) bodies and stress granules, as well as several RNPs unique to the germ line, including germ granules, nuage, Balbiani bodies, P granules, U bodies, and sponge bodies. Recent advances have identified a conserved response of germ line RNPs to environmental stresses such as nutritional stress and heat shock. The RNPs increase significantly in size based on cytology; their morphology and subcellular localization changes, and their composition changes. These dynamic changes are reversible when stresses diminish, and similar changes occur in response to aging or extended meiotic arrest prior to fertilization of oocytes. Intriguing correlations exist between the dynamics of the RNPs and the microtubule cytoskeleton and its motor proteins, suggesting a possible mechanism for the assembly and dissociation of the large RNP granules. Similarly, coordinated changes of the nuclear membrane and endoplasmic reticulum may also help unravel the regulatory mechanisms of RNP dynamics. Based on their composition, the RNPs are thought to regulate mRNA decay and/or translation, and initial support for some of these roles is now at hand. Ultimately, the question of why RNP remodeling occurs to such a large extent during a variety of stresses and aging remains to be fully answered, but a current attractive hypothesis is that the plasticity promotes the maintenance of oocyte quality.

Alternative Perspectives on Aging in Caenorhabditis elegans: Reactive Oxygen Species or Hyperfunction?

SIGNIFICANCE

The biological mechanisms at the heart of the aging process are a long-standing mystery. An influential theory has it that aging is the result of an accumulation of molecular damage, caused in particular by reactive oxygen species produced by mitochondria. This theory also predicts that processes that protect against oxidative damage (involving detoxification, repair, and turnover) protect against aging and increase lifespan.

RECENT ADVANCES

However, recent tests of the oxidative damage theory, many using the short-lived nematode worm Caenorhabditis elegans, have often failed to support the theory. This motivates consideration of alternative models. One new theory, conceived by M.V. Blagosklonny, proposes that aging is caused by hyperfunction, that is, overactivity during adulthood of processes (particularly biosynthetic) that contribute to development and reproduction. Such hyperfunction can lead to hypertrophy-associated pathologies, which cause the age increase in death.

CRITICAL ISSUES

Here we assess whether the hyperfunction theory is at all consistent with what is known about C. elegans aging, and conclude that it is. In particular, during adulthood, C. elegans shows a number of changes that may reflect pathology and/or hyperfunction. Such changes seem to contribute to death, at least in some cases (e.g., yolk accumulation).

FUTURE DIRECTIONS

Our assessment suggests that the hyperfunction theory is a plausible alternative to the molecular damage theory to explain aging in C. elegans.

Control of oocyte growth and meiotic maturation in Caenorhabditis elegans

In sexually reproducing animals, oocytes arrest at diplotene or diakinesis and resume meiosis (meiotic maturation) in response to hormones. Chromosome segregation errors in female meiosis I are the leading cause of human birth defects, and age-related changes in the hormonal environment of the ovary are a suggested cause. Caenorhabditis elegans is emerging as a genetic paradigm for studying hormonal control of meiotic maturation. The meiotic maturation processes in C. elegans and mammals share a number of biological and molecular similarities. Major sperm protein (MSP) and luteinizing hormone (LH), though unrelated in sequence, both trigger meiotic resumption using somatic Gα(s)-adenylate cyclase pathways and soma-germline gap-junctional communication. At a molecular level, the oocyte responses apparently involve the control of conserved protein kinase pathways and post-transcriptional gene regulation in the oocyte. At a cellular level, the responses include cortical cytoskeletal rearrangement, nuclear envelope breakdown, assembly of the acentriolar meiotic spindle, chromosome segregation, and likely changes important for fertilization and the oocyte-to-embryo transition. This chapter focuses on signaling mechanisms required for oocyte growth and meiotic maturation in C. elegans and discusses how these mechanisms coordinate the completion of meiosis and the oocyte-to-embryo transition.

Suspended animation, diapause and quiescence: arresting the cell cycle in C. elegans

Developing organisms require nutrients to support cell division vital for growth and development. An adaptation to stress, used by many organisms, is to reversibly enter an arrested state by reducing energy-requiring processes, such as development and cell division. This "wait it out" approach to survive stress until the environment is conductive for growth and development is used by many metazoans. Much is known about the molecular regulation of cell division, metazoan development and responses to environmental stress. However, how these biological processes intersect is less understood. Here, we review studies conducted in Caenorhabditis elegans that investigate how stresses such as oxygen deprivation (hypoxia and anoxia), exogenous chemicals or starvation affect cellular processes in the embryo, larvae or adult germline. Using C. elegans to identify how stress signals biological arrest can help in our understanding of evolutionary pressures as well as human health-related issues.

New insights into the regulation of RNP granule assembly in oocytes

In a variety of cell types in plants, animals, and fungi, ribonucleoprotein (RNP) complexes play critical roles in regulating RNA metabolism. These RNP granules include processing bodies and stress granules that are found broadly across cell types, as well as RNP granules unique to the germline, such as P granules, polar granules, sponge bodies, and germinal granules. This review focuses on RNP granules localized in oocytes of the major model systems, Caenorhabditis elegans, Drosophila, Xenopus, mouse, and zebrafish. The signature families of proteins within oocyte RNPs include Vasa and other RNA-binding proteins, decapping activators and enzymes, Argonaute family proteins, and translation initiation complex proteins. This review describes the many recent insights into the dynamics and functions of RNP granules, including their roles in mRNA degradation, mRNA localization, translational regulation, and fertility. The roles of the cytoskeleton and cell organelles in regulating RNP granule assembly are also discussed.

Assembly of RNP granules in stressed and aging oocytes requires nucleoporins and is coordinated with nuclear membrane blebbing

Protective cellular responses to stress and aging in the germline are essential for perpetuation of a species; however, relatively few studies have focused on how germ cells respond to stress and aging. We have previously shown that large ribonucleoprotein (RNP) complexes assemble in oocytes of Caenorhabditis during extended meiotic arrest or after environmental stress. Here we explore the regulation of these dynamic RNPs and demonstrate their assembly is coordinated with dramatic, nuclear membrane blebbing in oocytes. Our ultrastructural analyses reveal distinct changes in the endoplasmic reticulum, and the first evidence for the assembly of stacked annulate lamellae in Caenorhabditis. We further show several nucleoporins are required for the complete assembly of RNP granules, and a disruption in RNP granule assembly coupled with a low frequency of nuclear blebbing in arrested oocytes negatively impacts embryonic viability. Our observations support a model where nuclear membrane blebbing is required to increase the trafficking of nucleoporins to the cell cortex in stressed or meiotically arrested cells and to facilitate the recruitment of RNA and protein components of RNPs into large complexes. These new insights may have general implications for better understanding how germ cells preserve their integrity when fertilization is delayed and how cells respond to stress.

C. elegans Dicer interacts with the P-granule component GLH-1 and both regulate germline RNPs

P granules, ribonucleoprotein (RNP) complexes specific to the cytoplasmic side of the nuclear pores of Caenorhabditis elegans germ cells, are implicated in post-transcriptional control of maternally-transcribed mRNAs. Here we show a relationship in C. elegans of Dicer, the riboendonuclease processing enzyme of the RNA interference and microRNA pathways, with GLH-1, a germline-specific RNA helicase and a constitutive component of P granules. Based on results from GST-pull-downs and immunoprecipitations, GLH-1 binds DCR-1 and this binding does not require RNA. Both GLH-1 protein and glh-1 mRNA levels are reduced in the dcr-1(ok247) null mutant background; conversely, a reduction of DCR-1 protein is observed in the glh-1(gk100) deletion strain. Thus, in the C. elegans germline, DCR-1 and GLH-1 are interdependent. In addition, evidence indicates that DCR-1 protein levels, like those of GLH-1, are likely regulated by the Jun N-terminal kinase (JNK), KGB-1. In adult germ cells, DCR-1 is found in uniformly-distributed, small puncta both throughout the cytoplasm and the nucleus, on the inner side of nuclear pores, and associated with P granules. In arrested oocytes, GLH-1 and DCR-1 re-localize to cytoplasmic and cortically-distributed RNP granules and are necessary to recruit other components to these complexes. We predict that the GLH-1/DCR-1 complex may function in the transport, deposition, or regulation of maternally-transcribed mRNAs and their associated miRNAs.

TGF-β and insulin signaling regulate reproductive aging via oocyte and germline quality maintenance

Reproductive cessation is perhaps the earliest aging phenotype that humans experience. Similarly, reproduction of Caenorhabditis elegans ceases in mid-adulthood. Although somatic aging has been studied in both worms and humans, mechanisms regulating reproductive aging are not yet understood. Here, we show that TGF-β Sma/Mab and Insulin/IGF-1 signaling regulate C. elegans reproductive aging by modulating multiple aspects of the reproductive process, including embryo integrity, oocyte fertilizability, chromosome segregation fidelity, DNA damage resistance, and oocyte and germline morphology. TGF-β activity regulates reproductive span and germline/oocyte quality noncell-autonomously and is temporally and transcriptionally separable from its regulation of growth. Chromosome segregation, cell cycle, and DNA damage response genes are upregulated in TGF-β mutant oocytes, decline in aged mammalian oocytes, and are critical for oocyte quality maintenance. Our data suggest that C. elegans and humans share many aspects of reproductive aging, including the correlation between reproductive aging and declining oocyte quality and mechanisms determining oocyte quality.

RNA recognition by the embryonic cell fate determinant and germline totipotency factor MEX-3

Totipotent stem cells have the potential to differentiate into every cell type. Renewal of totipotent stem cells in the germline and cellular differentiation during early embryogenesis rely upon posttranscriptional regulatory mechanisms. The Caenorhabditis elegans RNA binding protein, MEX-3, plays a key role in both processes. MEX-3 is a maternally-supplied factor that controls the RNA metabolism of transcripts encoding critical cell fate determinants. However, the nucleotide sequence specificity and requirements of MEX-3 mRNA recognition remain unclear. Only a few candidate regulatory targets have been identified, and the full extent of the network of MEX-3 targets is not known. Here, we define the consensus sequence required for MEX-3 RNA recognition and demonstrate that this element is required for MEX-3 dependent regulation of gene expression in live worms. Based on this work, we identify several candidate MEX-3 targets that help explain its dual role in regulating germline stem cell totipotency and embryonic cell fate specification.

A germline-specific isoform of eIF4E (IFE-1) is required for efficient translation of stored mRNAs and maturation of both oocytes and sperm

Fertility and embryonic viability are measures of efficient germ cell growth and development. During oogenesis and spermatogenesis, new proteins are required for both mitotic expansion and differentiation. Qualitative and quantitative changes in protein synthesis occur by translational control of mRNAs, mediated in part by eIF4E, which binds the mRNAs 5' cap. IFE-1 is one of five eIF4E isoforms identified in C. elegans. IFE-1 is expressed primarily in the germ line and associates with P granules, large mRNPs that store mRNAs. We isolated a strain that lacks IFE-1 [ife-1(bn127)] and demonstrated that the translation of several maternal mRNAs (pos-1, pal-1, mex-1 and oma-1) was inefficient relative to that in wild-type worms. At 25 degrees C, ife-1(bn127) spermatocytes failed in cytokinesis, prematurely expressed the pro-apoptotic protein CED-4/Apaf-1, and accumulated as multinucleate cells unable to mature to spermatids. A modest defect in oocyte development was also observed. Oocytes progressed normally through mitosis and meiosis, but subsequent production of competent oocytes became limiting, even in the presence of wild-type sperm. Combined gametogenesis defects decreased worm fertility by 80% at 20 degrees C; ife-1 worms were completely sterile at 25 degrees C. Thus, IFE-1 plays independent roles in late oogenesis and spermatogenesis through selective translation of germline-specific mRNAs.

Maternal mRNAs are regulated by diverse P body-related mRNP granules during early Caenorhabditis elegans development

Processing bodies (P bodies) are conserved mRNA-protein (mRNP) granules that are thought to be cytoplasmic centers for mRNA repression and degradation. However, their specific functions in vivo remain poorly understood. We find that repressed maternal mRNAs and their regulators localize to P body-like mRNP granules in the Caenorhabditis elegans germ line. Surprisingly, several distinct types of regulated granules form during oocyte and embryo development. 3' untranslated region elements direct mRNA targeting to one of these granule classes. The P body factor CAR-1/Rap55 promotes association of repressed mRNA with granules and contributes to repression of Notch/glp-1 mRNA. However, CAR-1 controls Notch/glp-1 only during late oogenesis, where it functions with the RNA-binding regulators PUF-5, PUF-6, and PUF-7. The P body protein CGH-1/Rck/Dhh1 differs from CAR-1 in control of granule morphology and promotes mRNP stability in arrested oocytes. Therefore, a system of diverse and regulated RNP granules elicits stage-specific functions that ensure proper mRNA control during early development.

Large P body-like RNPs form in C. elegans oocytes in response to arrested ovulation, heat shock, osmotic stress, and anoxia and are regulated by the major sperm protein pathway

As Caenorhabditis elegans hermaphrodites age, sperm become depleted, ovulation arrests, and oocytes accumulate in the gonad arm. Large ribonucleoprotein (RNP) foci form in these arrested oocytes that contain RNA-binding proteins and translationally masked maternal mRNAs. Within 65 min of mating, the RNP foci dissociate and fertilization proceeds. The majority of arrested oocytes with foci result in viable embryos upon fertilization, suggesting that foci are not deleterious to oocyte function. We have determined that foci formation is not strictly a function of aging, and the somatic, ceh-18, branch of the major sperm protein pathway regulates the formation and dissociation of oocyte foci. Our hypothesis for the function of oocyte RNP foci is similar to the RNA-related functions of processing bodies (P bodies) and stress granules; here, we show three orthologs of P body proteins, DCP-2, CAR-1 and CGH-1, and two markers of stress granules, poly (A) binding protein (PABP) and TIA-1, appear to be present in the oocyte RNP foci. Our results are the first in vivo demonstration linking components of P bodies and stress granules in the germ line of a metazoan. Furthermore, our data demonstrate that formation of oocyte RNP foci is inducible in non-arrested oocytes by heat shock, osmotic stress, or anoxia, similar to the induction of stress granules in mammalian cells and P bodies in yeast. These data suggest commonalities between oocytes undergoing delayed fertilization and cells that are stressed environmentally, as to how they modulate mRNAs and regulate translation.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.