Translational repression of cyclin E prevents precocious mitosis and embryonic gene activation during C. elegans meiosis

C. elegans Germline as Three Distinct Tumor Models

Tumor cells display abnormal growth and division, avoiding the natural process of cell death. These cells can be benign (non-cancerous growth) or malignant (cancerous growth). Over the past few decades, numerous in vitro or in vivo tumor models have been employed to understand the molecular mechanisms associated with tumorigenesis in diverse regards. However, our comprehension of how non-tumor cells transform into tumor cells at molecular and cellular levels remains incomplete. The nematode C. elegans has emerged as an excellent model organism for exploring various phenomena, including tumorigenesis. Although C. elegans does not naturally develop cancer, it serves as a valuable platform for identifying oncogenes and the underlying mechanisms within a live organism. In this review, we describe three distinct germline tumor models in C. elegans, highlighting their associated mechanisms and related regulators: (1) ectopic proliferation due to aberrant activation of GLP-1/Notch signaling, (2) meiotic entry failure resulting from the loss of GLD-1/STAR RNA-binding protein, (3) spermatogenic dedifferentiation caused by the loss of PUF-8/PUF RNA-binding protein. Each model requires the mutations of specific genes (glp-1, gld-1, and puf-8) and operates through distinct molecular mechanisms. Despite these differences in the origins of tumorigenesis, the internal regulatory networks within each tumor model display shared features. Given the conservation of many of the regulators implicated in C. elegans tumorigenesis, it is proposed that these unique models hold significant potential for enhancing our comprehension of the broader control mechanisms governing tumorigenesis.

Characterization of the biochemical activity and tumor-promoting role of the dual protein methyltransferase METL-13/METTL13 in Caenorhabditis elegans

The methyltransferase-like protein 13 (METTL13) methylates the eukaryotic elongation factor 1 alpha (eEF1A) on two locations: the N-terminal amino group and lysine 55. The absence of this methylation leads to reduced protein synthesis and cell proliferation in human cancer cells. Previous studies showed that METTL13 is dispensable in non-transformed cells, making it potentially interesting for cancer therapy. However, METTL13 has not been examined yet in whole animals. Here, we used the nematode Caenorhabditis elegans as a simple model to assess the functions of METTL13. Using methyltransferase assays and mass spectrometry, we show that the C. elegans METTL13 (METL-13) methylates eEF1A (EEF-1A) in the same way as the human protein. Crucially, the cancer-promoting role of METL-13 is also conserved and depends on the methylation of EEF-1A, like in human cells. At the same time, METL-13 appears dispensable for animal growth, development, and stress responses. This makes C. elegans a convenient whole-animal model for studying METL13-dependent carcinogenesis without the complications of interfering with essential wild-type functions.

Redundant mechanisms regulating the proliferation vs. differentiation balance in the C. elegans germline

The proper production of gametes over an extended portion of the life of an organism is essential for a high level of fitness. The balance between germline stem cell (GSC) proliferation (self-renewal) and differentiation (production of gametes) must be tightly regulated to ensure proper gamete production and overall fitness. Therefore, organisms have evolved robust regulatory systems to control this balance. Here we discuss the redundancy in the regulatory system that controls the proliferation vs. differentiation balance in the C. elegans hermaphrodite germline, and how this redundancy may contribute to robustness. We focus on the various types of redundancy utilized to regulate this balance, as well as the approaches that have enabled these redundant mechanisms to be uncovered.

Identification of 14-3-3 proteins, Polo kinase, and RNA-binding protein Pes4 as key regulators of meiotic commitment in budding yeast

The initiation of the cell division process of meiosis requires exogenous signals that activate internal gene regulatory networks. Meiotic commitment ensures the irreversible continuation of meiosis, even upon withdrawal of the meiosis-inducing signals. A loss of meiotic commitment can cause highly abnormal polyploid cells and can ultimately lead to germ cell tumors. Despite the importance of meiotic commitment, only a few genes involved in commitment are known. In this study, we have discovered six new regulators of meiotic commitment in budding yeast: the Bcy1 protein involved in nutrient sensing, the meiosis-specific kinase Ime2, Polo kinase Cdc5, RNA-binding protein Pes4, and the 14-3-3 proteins Bmh1 and Bmh2. Decreased levels of these proteins cause a failure to establish or maintain meiotic commitment. Importantly, we found that Bmh1 and Bmh2 are involved in multiple processes throughout meiosis and in meiotic commitment. First, cells depleted of both Bmh1 and Bmh2 trigger the pachytene checkpoint, likely due to a role in DNA double-strand break repair. Second, Bmh1 interacts directly with the middle meiosis transcription factor Ndt80, and both Bmh1 and Bmh2 maintain Ndt80 levels. Third, Bmh1 and Bmh2 bind to Cdc5 and enhance its kinase activity. Finally, Bmh1 binds to Pes4, which regulates the timing of the translation of several mRNAs in meiosis II and is required to maintain meiotic commitment. Our results demonstrate that meiotic commitment is actively maintained throughout meiosis, with the 14-3-3 proteins and Polo kinase serving as key regulators of this developmental program.

Meiotic commitment: More than a transcriptional switch

Commitment to cellular differentiation programs can be controlled by self-activating transcription factors that trigger the expression of cell-type-specific genes. A new study shows that, although commitment to meiosis in yeast is controlled in this manner, additional signaling interactions promote the committed state.

Release of CHK-2 from PPM-1.D anchorage schedules meiotic entry

Transition from the stem/progenitor cell fate to meiosis is mediated by several redundant posttranscriptional regulatory pathways in Caenorhabditis elegans. Interfering with all three branches causes tumorous germ lines. SCFPROM-1 comprises one branch and mediates a scheduled degradation step at entry into meiosis. prom-1 mutants show defects in the timely initiation of meiotic prophase I events, resulting in high rates of embryonic lethality. Here, we identify the phosphatase PPM-1.D/Wip1 as crucial substrate for PROM-1. We report that PPM-1.D antagonizes CHK-2 kinase, a key regulator for meiotic prophase initiation, including DNA double-strand breaks, chromosome pairing, and synaptonemal complex formation. We propose that PPM-1.D controls the amount of active CHK-2 via both catalytic and noncatalytic activities; notably, noncatalytic regulation seems to be crucial at meiotic entry. PPM-1.D sequesters CHK-2 at the nuclear periphery, and programmed SCFPROM-1-mediated degradation of PPM-1.D liberates the kinase and promotes meiotic entry.

Neuronal regulated ire-1-dependent mRNA decay controls germline differentiation in Caenorhabditis elegans

Understanding the molecular events that regulate cell pluripotency versus acquisition of differentiated somatic cell fate is fundamentally important. Studies in Caenorhabditis elegans demonstrate that knockout of the germline-specific translation repressor gld-1 causes germ cells within tumorous gonads to form germline-derived teratoma. Previously we demonstrated that endoplasmic reticulum (ER) stress enhances this phenotype to suppress germline tumor progression(Levi-Ferber et al., 2015). Here, we identify a neuronal circuit that non-autonomously suppresses germline differentiation and show that it communicates with the gonad via the neurotransmitter serotonin to limit somatic differentiation of the tumorous germline. ER stress controls this circuit through regulated inositol requiring enzyme-1 (IRE-1)-dependent mRNA decay of transcripts encoding the neuropeptide FLP-6. Depletion of FLP-6 disrupts the circuit’s integrity and hence its ability to prevent somatic-fate acquisition by germline tumor cells. Our findings reveal mechanistically how ER stress enhances ectopic germline differentiation and demonstrate that regulated Ire1-dependent decay can affect animal physiology by controlling a specific neuronal circuit.

Recent Advances in the Genetic, Anatomical, and Environmental Regulation of the C. elegans Germ Line Progenitor Zone

The C. elegans germ line and its gonadal support cells are well studied from a developmental genetics standpoint and have revealed many foundational principles of stem cell niche biology. Among these are the observations that a niche-like cell supports a self-renewing stem cell population with multipotential, differentiating daughter cells. While genetic features that distinguish stem-like cells from their differentiating progeny have been defined, the mechanisms that structure these populations in the germ line have yet to be explained. The spatial restriction of Notch activation has emerged as an important genetic principle acting in the distal germ line. Synthesizing recent findings, I present a model in which the germ stem cell population of the C. elegans adult hermaphrodite can be recognized as two distinct anatomical and genetic populations. This review describes the recent progress that has been made in characterizing the undifferentiated germ cells and gonad anatomy, and presents open questions in the field and new directions for research to pursue.

Antitumoral Drug Potential of Tryptophan-Betaxanthin and Related Plant Betalains in the Caenorhabditis elegans Tumoral Model

Betalains are plants pigments identified as potent antioxidant molecules, naturally present in foods like beetroot and prickly pears. Although activities described for betalain-containing formulations include cancer prevention and treatment, the use of extracts instead of purified pigments has avoided the investigation of the real chemopreventive and chemotherapeutic potential of these phytochemicals. Three betalain-rich extracts and six individual pure betalains were used in this work to characterize the activity and to explore possible molecular mechanisms. The animal model Caenorhabditis elegans (tumoral strain JK1466) was used to evaluate the effect of betalains as chemotherapeutics drugs. An objective evaluation method of tumor growth in C. elegans has been developed to assess the possible antitumoral activity of the different treatments. This protocol allowed a fast and reliable screening of possible antitumoral drugs. Among the betalains tested, tryptophan-betaxanthin reduced tumor size by 56.4% and prolonged the animal’s lifespan by 9.3%, indicating high effectiveness and low toxicity. Structure–activity relationships are considered. Assays with mutant strains of C. elegans showed that the mechanism underlying these effects was the modulation of the DAF-16 transcription factor and the insulin signaling pathway. Our results indicate that tryptophan-betaxanthin and related betalains are strong candidates as antitumoral molecules in cancer treatment.

Stem cell niche exit in C. elegans via orientation and segregation of daughter cells by a cryptic cell outside the niche

Stem cells reside in and rely upon their niche to maintain stemness but must balance self-renewal with the production of daughters that leave the niche to differentiate. We discovered a mechanism of stem cell niche exit in the canonical C. elegans distal tip cell (DTC) germ stem cell niche mediated by previously unobserved, thin, membranous protrusions of the adjacent somatic gonad cell pair (Sh1). A disproportionate number of germ cell divisions were observed at the DTC-Sh1 interface. Stem-like and differentiating cell fates segregated across this boundary. Spindles polarized, pairs of daughter cells oriented between the DTC and Sh1, and Sh1 grew over the Sh1-facing daughter. Impeding Sh1 growth by RNAi to cofilin and Arp2/3 perturbed the DTC-Sh1 interface, reduced germ cell proliferation, and shifted a differentiation marker. Because Sh1 membrane protrusions eluded detection for decades, it is possible that similar structures actively regulate niche exit in other systems.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Biology of the Caenorhabditis elegans Germline Stem Cell System

Stem cell systems regulate tissue development and maintenance. The germline stem cell system is essential for animal reproduction, controlling both the timing and number of progeny through its influence on gamete production. In this review, we first draw general comparisons to stem cell systems in other organisms, and then present our current understanding of the germline stem cell system in Caenorhabditis elegans In contrast to stereotypic somatic development and cell number stasis of adult somatic cells in C. elegans, the germline stem cell system has a variable division pattern, and the system differs between larval development, early adult peak reproduction and age-related decline. We discuss the cell and developmental biology of the stem cell system and the Notch regulated genetic network that controls the key decision between the stem cell fate and meiotic development, as it occurs under optimal laboratory conditions in adult and larval stages. We then discuss alterations of the stem cell system in response to environmental perturbations and aging. A recurring distinction is between processes that control stem cell fate and those that control cell cycle regulation. C. elegans is a powerful model for understanding germline stem cells and stem cell biology.

Developmental Plasticity and Cellular Reprogramming in Caenorhabditis elegans

While Caenorhabditis elegans was originally regarded as a model for investigating determinate developmental programs, landmark studies have subsequently shown that the largely invariant pattern of development in the animal does not reflect irreversibility in rigidly fixed cell fates. Rather, cells at all stages of development, in both the soma and germline, have been shown to be capable of changing their fates through mutation or forced expression of fate-determining factors, as well as during the normal course of development. In this chapter, we review the basis for natural and induced cellular plasticity in C. elegans We describe the events that progressively restrict cellular differentiation during embryogenesis, starting with the multipotency-to-commitment transition (MCT) and subsequently through postembryonic development of the animal, and consider the range of molecular processes, including transcriptional and translational control systems, that contribute to cellular plasticity. These findings in the worm are discussed in the context of both classical and recent studies of cellular plasticity in vertebrate systems.

Developmental Control of the Cell Cycle: Insights from Caenorhabditis elegans

During animal development, a single fertilized egg forms a complete organism with tens to trillions of cells that encompass a large variety of cell types. Cell cycle regulation is therefore at the center of development and needs to be carried out in close coordination with cell differentiation, migration, and death, as well as tissue formation, morphogenesis, and homeostasis. The timing and frequency of cell divisions are controlled by complex combinations of external and cell-intrinsic signals that vary throughout development. Insight into how such controls determine in vivo cell division patterns has come from studies in various genetic model systems. The nematode Caenorhabditis elegans has only about 1000 somatic cells and approximately twice as many germ cells in the adult hermaphrodite. Despite the relatively small number of cells, C. elegans has diverse tissues, including intestine, nerves, striated and smooth muscle, and skin. C. elegans is unique as a model organism for studies of the cell cycle because the somatic cell lineage is invariant. Somatic cells divide at set times during development to produce daughter cells that adopt reproducible developmental fates. Studies in C. elegans have allowed the identification of conserved cell cycle regulators and provided insights into how cell cycle regulation varies between tissues. In this review, we focus on the regulation of the cell cycle in the context of C. elegans development, with reference to other systems, with the goal of better understanding how cell cycle regulation is linked to animal development in general.

Dynein Light Chain DLC-1 Facilitates the Function of the Germline Cell Fate Regulator GLD-1 in Caenorhabditis elegans

Developmental transitions of germ cells are often regulated at the level of post-transcriptional control of gene expression. In the Caenorhabditis elegans germline, stem and progenitor cells exit the proliferative phase and enter meiotic differentiation to form gametes essential for fertility. The RNA binding protein GLD-1 is a cell fate regulator that promotes meiosis and germ cell differentiation during development by binding to and repressing translation of target messenger RNAs. Here, we discovered that some GLD-1 functions are promoted by binding to DLC-1, a small protein that functions as an allosteric regulator of multisubunit protein complexes. We found that DLC-1 is required to regulate a subset of GLD-1 target messenger RNAs and that DLC-1 binding GLD-1 prevents ectopic germ cell proliferation and facilitates gametogenesis in vivo Additionally, our results reveal a new requirement for GLD-1 in the events of oogenesis leading to ovulation. DLC-1 contributes to GLD-1 function independent of its role as a light chain component of the dynein motor. Instead, we propose that DLC-1 promotes assembly of GLD-1 with other binding partners, which facilitates formation of regulatory ribonucleoprotein complexes and may direct GLD-1 target messenger RNA selectivity.

Initiation of Meiotic Development Is Controlled by Three Post-transcriptional Pathways in Caenorhabditis elegans

A major event in germline development is the transition from stem/progenitor cells to entry into meiosis and gametogenesis. This transition requires downregulation of mitotic cell cycle activity and upregulation of processes associated with meiosis. We identify the Caenorhabditis elegans SCFPROM-1 E3 ubiquitin-ligase complex as functioning to downregulate mitotic cell cycle protein levels including cyclin E, WAPL-1, and KNL-2 at meiotic entry and, independently, promoting homologous chromosome pairing as a positive regulator of the CHK-2 kinase. SCFPROM-1 is thus a novel regulator of meiotic entry, coordinating downregulation of mitotic cell cycle proteins and promoting homolog pairing. We further show that SCFPROM-1 functions redundantly, in parallel to the previously described GLD-1 and GLD-2 meiotic entry pathways, downstream of and inhibited by GLP-1 Notch signaling, which specifies the stem cell fate. Accordingly, C. elegans employs three post-transcriptional pathways, SCFPROM-1-mediated protein degradation, GLD-1-mediated translational repression, and GLD-2-mediated translational activation, to control and coordinate the initiation of meiotic development.

The CSR-1 endogenous RNAi pathway ensures accurate transcriptional reprogramming during the oocyte-to-embryo transition in Caenorhabditis elegans

Endogenous RNAi (endoRNAi) is a conserved mechanism for fine-tuning gene expression. In the nematode Caenorhabditis elegans, several endoRNAi pathways are required for the successful development of reproductive cells. The CSR-1 endoRNAi pathway promotes germ cell development, primarily by facilitating the expression of germline genes. In this study, we report a novel function for the CSR-1 pathway in preventing premature activation of embryonic transcription in the developing oocytes, which is accompanied by a general Pol II activation. This CSR-1 function requires its RNase activity, suggesting that, by controlling the levels of maternal mRNAs, CSR-1-dependent endoRNAi contributes to an orderly reprogramming of transcription during the oocyte-to-embryo transition.

During the oocyte-to-embryo transition, the control of development is transferred from the mother to the embryo. A key event during this transition is the transcriptional activation of the embryonic genome, which is tightly controlled. Here, by using the nematode C. elegans, we uncover a role for endogenous RNA interference in this process. We demonstrate that a specific endoRNAi pathway, employing the Argonaute protein CSR-1, functions as a break on gene-specific, and potentially global, activation of embryonic transcription in the developing oocytes. Our findings reveal a new layer of control over the transcriptional reprogramming during the oocyte-to-embryo transition, raising questions about its potential conservation in mammalian development.

Early experiences mediate distinct adult gene expression and reproductive programs in Caenorhabditis elegans

Environmental stress during early development in animals can have profound effects on adult phenotypes via programmed changes in gene expression. Using the nematode C. elegans, we demonstrated previously that adults retain a cellular memory of their developmental experience that is manifested by differences in gene expression and life history traits; however, the sophistication of this system in response to different environmental stresses, and how it dictates phenotypic plasticity in adults that contribute to increased fitness in response to distinct environmental challenges, was unknown. Using transcriptional profiling, we show here that C. elegans adults indeed retain distinct cellular memories of different environmental conditions. We identified approximately 500 genes in adults that entered dauer due to starvation that exhibit significant opposite (“seesaw”) transcriptional phenotypes compared to adults that entered dauer due to crowding, and are distinct from animals that bypassed dauer. Moreover, we show that two-thirds of the genes in the genome experience a 2-fold or greater seesaw trend in gene expression, and based upon the direction of change, are enriched in large, tightly linked regions on different chromosomes. Importantly, these transcriptional programs correspond to significant changes in brood size depending on the experienced stress. In addition, we demonstrate that while the observed seesaw gene expression changes occur in both somatic and germline tissue, only starvation-induced changes require a functional GLP-4 protein necessary for germline development, and both programs require the Argonaute CSR-1. Thus, our results suggest that signaling between the soma and the germ line can generate phenotypic plasticity as a result of early environmental experience, and likely contribute to increased fitness in adverse conditions and the evolution of the C. elegans genome.

Environmental stress during early development in animals can have profound effects on adult behavior and physiology due to programmed changes in gene expression. However, whether different stresses result in distinct changes in traits that allow stressed animals to better survive and reproduce in future adverse conditions is largely unknown. Using the animal model system, C. elegans, we show that adults that experienced starvation exhibit opposite (“seesaw”) genome-wide gene expression changes compared to adults that experienced crowding, and are distinct from animals that experienced favorable conditions. Genes that are similarly up- or downregulated due to either starvation or crowding are located in clusters on the same chromosomes. Importantly, these gene expression changes of differently-stressed animals result in corresponding changes in progeny number, a life history trait of evolutionary significance. These distinct gene expression programs require different signaling pathways that communicate across somatic and germline tissue types. Thus, different environmental stresses experienced early in development induce distinct signaling mechanisms to result in changes in gene expression and reproduction in adults, and likely contribute to increased survival in future adverse conditions.

Cell cycle controls stress response and longevity in C. elegans

Recent studies have revealed a variety of genes and mechanisms that influence the rate of aging progression. In this study, we identified cell cycle factors as potent regulators of health and longevity in C. elegans. Focusing on the cyclin-dependent kinase 2 (cdk-2) and cyclin E (cye-1), we show that inhibition of cell cycle genes leads to tolerance towards environmental stress and longevity. The reproductive system is known as a key regulator of longevity in C. elegans. We uncovered the gonad as the central organ mediating the effects of cell cycle inhibition on lifespan. In particular, the proliferating germ cells were essential for conferring longevity. Steroid hormone signaling and the FOXO transcription factor DAF-16 were required for longevity associated with cell cycle inhibition. Furthermore, we discovered that SKN-1 (ortholog of mammalian Nrf proteins) activates protective gene expression and induces longevity when cell cycle genes are inactivated. We conclude that both, germline absence and inhibition through impairment of cell cycle machinery results in longevity through similar pathways. In addition, our studies suggest further roles of cell cycle genes beyond cell cycle progression and support the recently described connection of SKN-1/Nrf to signals deriving from the germline.

Cell Fate Maintenance and Reprogramming During the Oocyte-to-Embryo Transition

This chapter reviews our current understanding of the mechanisms that regulate reprogramming during the oocyte-to-embryo transition (OET). There are two major events reshaping the transcriptome during OET. One is the clearance of maternal transcripts in the early embryo, extensively reviewed by others. The other event, which is the focus of this chapter, is the embryonic (or zygotic) genome activation (EGA). The mechanisms controlling EGA can be broadly divided into transcriptional and posttranscriptional. The former includes the regulation of the basal transcription machinery, the regulation by specific transcription factors and chromatin modifications. The latter is performed mostly via specific RNA-binding proteins (RBPs). Different animal models have been used to decipher the regulation of EGA. These models are often biased for the specific type of regulation, which is why we discuss the models ranging from invertebrates to mammals. Whether these biases stem from incomplete understanding of EGA in these models, or reflect evolutionarily distinct solutions to EGA regulation, is a key unresolved problem in developmental biology. As the mechanisms controlling developmental reprogramming can, and in some cases have been shown to, function in differentiated cells subjected to induced reprogramming, our understanding of EGA regulation may have implications for the efficiency of induced reprogramming and, thus, for regenerative medicine.

Meioc maintains an extended meiotic prophase I in mice

The meiosis-specific chromosomal events of homolog pairing, synapsis, and recombination occur over an extended meiotic prophase I that is many times longer than prophase of mitosis. Here we show that, in mice, maintenance of an extended meiotic prophase I requires the gene Meioc, a germ-cell specific factor conserved in most metazoans. In mice, Meioc is expressed in male and female germ cells upon initiation of and throughout meiotic prophase I. Mouse germ cells lacking Meioc initiate meiosis: they undergo pre-meiotic DNA replication, they express proteins involved in synapsis and recombination, and a subset of cells progress as far as the zygotene stage of prophase I. However, cells in early meiotic prophase—as early as the preleptotene stage—proceed to condense their chromosomes and assemble a spindle, as if having progressed to metaphase. Meioc-deficient spermatocytes that have initiated synapsis mis-express CYCLIN A2, which is normally expressed in mitotic spermatogonia, suggesting a failure to properly transition to a meiotic cell cycle program. MEIOC interacts with YTHDC2, and the two proteins pull-down an overlapping set of mitosis-associated transcripts. We conclude that when the meiotic chromosomal program is initiated, Meioc is simultaneously induced so as to extend meiotic prophase. Specifically, MEIOC, together with YTHDC2, promotes a meiotic (as opposed to mitotic) cell cycle program via post-transcriptional control of their target transcripts.

Meiosis is the specialized cell division that halves the genetic content of germ cells to produce haploid gametes. This reductive division is preceded by a preparative phase of the cell cycle, meiotic prophase I, during which several meiosis-specific chromosomal events occur. Across sexually reproducing organisms, prophase of meiosis I is dramatically longer than mitotic prophase. However, it was not known in mammals how and why meiotic prophase I is extended. We have identified a mouse mutant in which this extended prophase I is disrupted: germ cells lacking Meioc initiate meiosis, but prematurely proceed to metaphase. Mutant male meiotic germ cells mis-express a cell cycle regulator that is normally expressed in mitotic male germ cells, suggesting that Meioc is required for germ cells to properly transition to a meiotic cell cycle program. Biochemical analyses of proteins and transcripts that associate with MEIOC protein suggest that MEIOC may promote the transition from a mitotic to meiotic cell cycle program by post-transcriptionally regulating target transcripts. Our studies indicate that in mammals, as in other sexually reproducing organisms, meiotic prophase I must be extended to allow time for meiotic chromosomal events to reach completion.

Analysis of Cyclin E1 Functions in Porcine Preimplantation Embryonic Development by Fluorescence Microscopy

Cyclin E1 (CCNE1) is a core component of cell cycle regulation that drives the transition into the S phase. CCNE1 plays critical roles in cell cycle, cell proliferation, and cellular functions. However, the function of CCNE1 in early embryonic development is limited. In the present study, the function and expression of Ccne1 in porcine early parthenotes were examined. Immunostaining experiments showed that CCNE1 localized in the nucleus, starting at the four-cell stage. Knockdown of Ccne1 by double-stranded RNA resulted in the failure of blastocyst formation and induced blastocyst apoptosis. Ccne1 depletion increased expression of the pro-apoptotic gene Bax, and decreased the expression of Oct4 and the rate of inner cell mass (ICM)/trophectoderm formation. The results indicated that CCNE1 affects blastocyst formation by inducing cell apoptosis and ICM formation during porcine embryonic development.

Translational Control of Germ Cell Decisions

Germline poses unique challenges to gene expression control at the transcriptional level. While the embryonic germline maintains a global hold on new mRNA transcription, the female adult germline produces transcripts that are not translated into proteins until embryogenesis of subsequent generation. As a consequence, translational control plays a central role in governing various germ cell decisions including the formation of primordial germ cells, self-renewal/differentiation decisions in the adult germline, onset of gametogenesis and oocyte maturation. Mechanistically, several common themes such as asymmetric localization of mRNAs, conserved RNA-binding proteins that control translation by 3' UTR binding, translational activation by the cytoplasmic elongation of the polyA tail and the assembly of mRNA-protein complexes called mRNPs have emerged from the studies on Caenorhabditis elegans, Xenopus and Drosophila. How mRNPs assemble, what influences their dynamics, and how a particular 3' UTR-binding protein turns on the translation of certain mRNAs while turning off other mRNAs at the same time and space are key challenges for future work.

Analysis of the C. elegans Germline Stem Cell Pool

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

Increasing Notch signaling antagonizes PRC2-mediated silencing to promote reprograming of germ cells into neurons

Cell-fate reprograming is at the heart of development, yet very little is known about the molecular mechanisms promoting or inhibiting reprograming in intact organisms. In the C. elegans germline, reprograming germ cells into somatic cells requires chromatin perturbation. Here, we describe that such reprograming is facilitated by GLP-1/Notch signaling pathway. This is surprising, since this pathway is best known for maintaining undifferentiated germline stem cells/progenitors. Through a combination of genetics, tissue-specific transcriptome analysis, and functional studies of candidate genes, we uncovered a possible explanation for this unexpected role of GLP-1/Notch. We propose that GLP-1/Notch promotes reprograming by activating specific genes, silenced by the Polycomb repressive complex 2 (PRC2), and identify the conserved histone demethylase UTX-1 as a crucial GLP-1/Notch target facilitating reprograming. These findings have wide implications, ranging from development to diseases associated with abnormal Notch signaling.

DOI:http://dx.doi.org/10.7554/eLife.15477.001

The DNA in genes encodes the basic information needed to build an organism or control its day-to-day operations. Most cells in an organism contain the same genetic information, but different types of cell use the information differently. For example, many of the genes that are active in a muscle cell are different from those that are active in a skin cell.

These different patterns of gene activation largely determine a cell’s identity and are brought about by DNA-binding proteins or chemical modifications to the DNA (which are both forms of so-called epigenetic regulation). Nevertheless, cells occasionally change their identities – a phenomenon that is referred to as reprograming. This process allows tissues to be regenerated after wounding, but, due to technical difficulties, reprograming has been often studied in isolated cells grown in a dish.

Seelk, Adrian-Kalchhauser et al. set out to understand how being surrounded by intact tissue influences reprograming. The experiments made use of C. elegans worms, because disturbing how this worm’s DNA is packaged can trigger its cells to undergo reprograming. Seelk, Adrian-Kalchhauser et al. show that a signaling pathway that is found in many different animals enhances this kind of reprograming in C. elegans.

On the one hand, these findings help in understanding how epigenetic regulation can be altered by a specific tissue environment. On the other hand, the findings also suggest that abnormal signaling can result in altered epigenetic control of gene expression and lead to cells changing their identity. Indeed, increased signaling is linked to a major epigenetic mechanism seen in specific blood tumors, suggesting that the regulatory principles uncovered using this simple worm model could eventually provide insights into a human disease.

A future challenge will be to determine precisely how the studied signaling pathway interacts with the epigenetic regulator that controls reprograming. Understanding this interaction in molecular detail could help to devise strategies for controlling reprograming. These strategies could in turn lead to treatments for people with conditions that cause specific cells types to be lost, such as Alzheimer’s disease or injuries.

DOI:http://dx.doi.org/10.7554/eLife.15477.002

Germline Stem Cell Differentiation Entails Regional Control of Cell Fate Regulator GLD-1 in Caenorhabditis elegans

Germline stem cell differentiation in Caenorhabditis elegans is controlled by glp-1 Notch signaling. Cell fate regulator GLD-1 is sufficient to induce meiotic entry and expressed at a high level during meiotic prophase, inhibiting mitotic gene activity. glp-1 signaling and other regulators control GLD-1 levels post-transcriptionally (low in stem cells, high in meiotic prophase), but many aspects of GLD-1 regulation are uncharacterized, including the link between glp-1-mediated transcriptional control and post-transcriptional GLD-1 regulation. We established a sensitive assay to quantify GLD-1 levels across an ∼35-cell diameter field, where distal germline stem cells differentiate proximally into meiotic prophase cells in the adult C. elegans hermaphrodite, and applied the approach to mutants in known or proposed GLD-1 regulators. In wild-type GLD-1 levels elevated ∼20-fold in a sigmoidal pattern. We found that two direct transcriptional targets of glp-1 signaling, lst-1 and sygl-1, were individually required for repression of GLD-1. We determined that lst-1 and sygl-1 act in the same genetic pathway as known GLD-1 translational repressor fbf-1, while lst-1 also acts in parallel to fbf-1, linking glp-1-mediated transcriptional control and post-transcriptional GLD-1 repression. Additionally, we estimated the position in wild-type gonads where germ cells irreversibly commit to meiotic development based on GLD-1 levels in worms where glp-1 activity was manipulated to cause an irreversible fate switch. Analysis of known repressors and activators, as well as modeling the sigmoidal accumulation pattern, indicated that regulation of GLD-1 levels is largely regional, which we integrated with the current view of germline stem cell differentiation.

Coordinating cell proliferation and differentiation: Antagonism between cell cycle regulators and cell type-specific gene expression

Cell proliferation and differentiation show a remarkable inverse relationship. Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and permanent exit from the division cycle. Mechanistic insight in the temporal coordination between cell cycle exit and differentiation has come from studies of cells in culture and genetic animal models. As initially described for skeletal muscle differentiation, temporal coordination involves mutual antagonism between cyclin-dependent kinases that promote cell cycle entry and transcription factors that induce tissue-specific gene expression. Recent insights highlight the contribution of chromatin-regulating complexes that act in conjunction with the transcription factors and determine their activity. In particular SWI/SNF chromatin remodelers contribute to dual regulation of cell cycle and tissue-specific gene expression during terminal differentiation. We review the concerted regulation of the cell cycle and cell type-specific transcription, and discuss common mutations in human cancer that emphasize the clinical importance of proliferation versus differentiation control.

Repression of somatic cell fate in the germline

Germ cells must transmit genetic information across generations, and produce gametes while also maintaining the potential to form all cell types after fertilization. Preventing the activation of somatic programs is, therefore, crucial to the maintenance of germ cell identity. Studies in Caenorhabditis elegans, Drosophila melanogaster, and mouse have revealed both similarities and differences in how somatic gene expression is repressed in germ cells, thereby preventing their conversion into somatic tissues. This review will focus on recent developments in our understanding of how global or gene-specific transcriptional repression, chromatin regulation, and translational repression operate in the germline to maintain germ cell identity and repress somatic differentiation programs.

Specifying and protecting germ cell fate

Germ cells are the special cells in the body that undergo meiosis to generate gametes and subsequently entire new organisms after fertilization, a process that continues generation after generation. Recent studies have expanded our understanding of the factors and mechanisms that specify germ cell fate, including the partitioning of maternally supplied 'germ plasm', inheritance of epigenetic memory and expression of transcription factors crucial for primordial germ cell (PGC) development. Even after PGCs are specified, germline fate is labile and thus requires protective mechanisms, such as global transcriptional repression, chromatin state alteration and translation of only germline-appropriate transcripts. Findings from diverse species continue to provide insights into the shared and divergent needs of these special reproductive cells.

Control of Caenorhabditis elegans germ-line stem-cell cycling speed meets requirements of design to minimize mutation accumulation

Background

Stem cells are thought to play a critical role in minimizing the accumulation of mutations, but it is not clear which strategies they follow to fulfill that performance objective. Slow cycling of stem cells provides a simple strategy that can minimize cell pedigree depth and thereby minimize the accumulation of replication-dependent mutations. Although the power of this strategy was recognized early on, a quantitative assessment of whether and how it is employed by biological systems is missing.

Results

Here we address this problem using a simple self-renewing organ – the C. elegans gonad – whose overall organization is shared with many self-renewing organs. Computational simulations of mutation accumulation characterize a tradeoff between fast development and low mutation accumulation, and show that slow-cycling stem cells allow for an advantageous compromise to be reached. This compromise is such that worm germ-line stem cells should cycle more slowly than their differentiating counterparts, but only by a modest amount. Experimental measurements of cell cycle lengths derived using a new, quantitative technique are consistent with these predictions.

Conclusions

Our findings shed light both on design principles that underlie the role of stem cells in delaying aging and on evolutionary forces that shape stem-cell gene regulatory networks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0148-y) contains supplementary material, which is available to authorized users.

Caenorhabditis elegans as a model for cancer research

The term cancer describes a group of multifaceted diseases characterized by an intricate pathophysiology. Despite significant advances in the fight against cancer, it remains a key public health concern and burden on societies worldwide. Elucidation of key molecular and cellular mechanisms of oncogenic diseases will facilitate the development of better intervention strategies to counter or prevent tumor development. In vivo and in vitro models have long been used to delineate distinct biological processes involved in cancer such as apoptosis, proliferation, angiogenesis, invasion, metastasis, genome instability, and metabolism. In this review, we introduce Caenorhabditis elegans as an emerging animal model for systematic dissection of the molecular basis of tumorigenesis, focusing on the well-established processes of apoptosis and autophagy. Additionally, we propose that C. elegans can be used to advance our understanding of cancer progression, such as deregulation of energy metabolism, stem cell reprogramming, and host-microflora interactions.

The TRIM-NHL protein LIN-41 and the OMA RNA-binding proteins antagonistically control the prophase-to-metaphase transition and growth of Caenorhabditis elegans oocytes

In many animals, oocytes enter meiosis early in their development but arrest in meiotic prophase I. Oocyte growth, which occurs during this arrest period, enables the acquisition of meiotic competence and the capacity to produce healthy progeny. Meiotic resumption, or meiotic maturation, involves the transition to metaphase I (M phase) and is regulated by intercellular signaling and cyclin-dependent kinase activation. Premature meiotic maturation would be predicted to diminish fertility as the timing of this event, which normally occurs after oocyte growth is complete, is crucial. In the accompanying article in this issue, we identify the highly conserved TRIM-NHL protein LIN-41 as a translational repressor that copurifies with OMA-1 and OMA-2, RNA-binding proteins redundantly required for normal oocyte growth and meiotic maturation. In this article, we show that LIN-41 enables the production of high-quality oocytes and plays an essential role in controlling and coordinating oocyte growth and meiotic maturation. lin-41 null mutants display a striking defect that is specific to oogenesis: pachytene-stage cells cellularize prematurely and fail to progress to diplotene. Instead, these cells activate CDK-1, enter M phase, assemble spindles, and attempt to segregate chromosomes. Translational derepression of the CDK-1 activator CDC-25.3 appears to contribute to premature M-phase entry in lin-41 mutant oocytes. Genetic and phenotypic analyses indicate that LIN-41 and OMA-1/2 exhibit an antagonistic relationship, and we suggest that translational regulation by these proteins could be important for controlling and coordinating oocyte growth and meiotic maturation.

An RNAi-based suppressor screen identifies interactors of the Myt1 ortholog of Caenorhabditis elegans

Oocyte maturation in all species is controlled by a protein complex termed the maturation promoting factor (MPF). MPF comprises a cyclin-dependent kinase (CDK) and its partner cyclin, and it is regulated by dueling regulatory phosphorylation events on the CDK. In Caenorhabditis elegans, the Wee1/Myt1 ortholog WEE-1.3 provides the inhibitory phosphorylations on CDK-1 that keep MPF inactive and halt meiosis. Prior work has shown that depletion of WEE-1.3 in C. elegans results in precocious oocyte maturation in vivo and a highly penetrant infertility phenotype. This study sought to further define the precocious maturation phenotype and to identify novel interactors with WEE-1.3. We found that WEE-1.3 is expressed throughout the germline and in developing embryos in a perinuclear pattern, and demonstrated that oocytes in WEE-1.3–depleted germlines have begun to transcribe embryonic genes and exhibit inappropriate expression of proteins normally restricted to fertilized eggs. In addition, we performed an RNAi suppressor screen of the infertile phenotype to identify novel factors that, when co-depleted with WEE-1.3, restore fertility to these animals. We screened ∼1900 essential genes by RNAi feeding and identified 44 (∼2% of the tested genes) that are suppressors of the WEE-1.3 depletion phenotype. The suppressors include many previously unidentified players in the meiotic cell cycle and represent a pool of potential WEE-1.3 interacting proteins that function during C. elegans oocyte maturation and zygotic development.

Post-transcriptional RNA regulons affecting cell cycle and proliferation

The cellular growth cycle is initiated and maintained by punctual, yet agile, regulatory events involving modifications of cell cycle proteins as well as coordinated gene expression to support cyclic checkpoint decisions. Recent evidence indicates that post-transcriptional partitioning of messenger RNA subsets by RNA-binding proteins help physically localize, temporally coordinate, and efficiently translate cell cycle proteins. This dynamic organization of mRNAs encoding cell cycle components contributes to the overall economy of the cell cycle consistent with the post-transcriptional RNA regulon model of gene expression. This review examines several recent studies demonstrating the coordination of mRNA subsets encoding cell cycle proteins during nuclear export and subsequent coupling to protein synthesis, and discusses evidence for mRNA coordination of p53 targets and the DNA damage response pathway. We consider how these observations may connect to upstream and downstream post-transcriptional coordination and coupling of splicing, export, localization, and translation. Published examples from yeast, nematode, insect, and mammalian systems are discussed, and we consider genetic evidence supporting the conclusion that dysregulation of RNA regulons may promote pathogenic states of growth such as carcinogenesis.

GLD-4-mediated translational activation regulates the size of the proliferative germ cell pool in the adult C. elegans germ line

To avoid organ dysfunction as a consequence of tissue diminution or tumorous growth, a tight balance between cell proliferation and differentiation is maintained in metazoans. However, cell-intrinsic gene expression mechanisms controlling adult tissue homeostasis remain poorly understood. By focusing on the adult Caenorhabditis elegans reproductive tissue, we show that translational activation of mRNAs is a fundamental mechanism to maintain tissue homeostasis. Our genetic experiments identified the Trf4/5-type cytoplasmic poly(A) polymerase (cytoPAP) GLD-4 and its enzymatic activator GLS-1 to perform a dual role in regulating the size of the proliferative zone. Consistent with a ubiquitous expression of GLD-4 cytoPAP in proliferative germ cells, its genetic activity is required to maintain a robust proliferative adult germ cell pool, presumably by regulating many mRNA targets encoding proliferation-promoting factors. Based on translational reporters and endogenous protein expression analyses, we found that gld-4 activity promotes GLP-1/Notch receptor expression, an essential factor of continued germ cell proliferation. RNA-protein interaction assays documented also a physical association of the GLD-4/GLS-1 cytoPAP complex with glp-1 mRNA, and ribosomal fractionation studies established that GLD-4 cytoPAP activity facilitates translational efficiency of glp-1 mRNA. Moreover, we found that in proliferative cells the differentiation-promoting factor, GLD-2 cytoPAP, is translationally repressed by the stem cell factor and PUF-type RNA-binding protein, FBF. This suggests that cytoPAP-mediated translational activation of proliferation-promoting factors, paired with PUF-mediated translational repression of differentiation factors, forms a translational control circuit that expands the proliferative germ cell pool. Our additional genetic experiments uncovered that the GLD-4/GLS-1 cytoPAP complex promotes also differentiation, forming a redundant translational circuit with GLD-2 cytoPAP and the translational repressor GLD-1 to restrict proliferation. Together with previous findings, our combined data reveals two interconnected translational activation/repression circuitries of broadly conserved RNA regulators that maintain the balance between adult germ cell proliferation and differentiation.

Throughout adulthood, animal tissue homeostasis requires adult stem cell activities. A tight balance between self-renewal and differentiation protects against tissue overgrowth or loss. This balance is strongly influenced by niche-mediated signaling pathways that primarily trigger a transcriptional response in stem cells to promote self-renewal/proliferation. However, the cell-intrinsic mechanisms that modulate signaling pathways to promote proliferation or differentiation are poorly understood. Recently, post-transcriptional mRNA regulation emerged in diverse germline stem cell systems as an important gene expression mechanism, primarily preventing the protein synthesis of factors that promote the switch to differentiation. In the adult C. elegans germ line, this study finds that the evolutionarily conserved cytoplasmic poly(A) polymerase, GLD-4, plays an crucial role in maintaining a healthy balance between proliferation and differentiation forces. This is in part due to translational activation of the mRNA that encodes the germ cell-expressed Notch signaling receptor, an essential regulator of proliferation. Moreover, GLD-4 activity is part of a redundant genetic network downstream of Notch that, together with several other conserved mRNA regulators, promotes differentiation onset. Given the widespread expression of these conserved RNA regulators in metazoans, cell fate balances that are reinforced by translational activation and repression circuitries may therefore be a general mechanism of adult tissue maintenance.

The TRIM-NHL protein LIN-41 controls the onset of developmental plasticity in Caenorhabditis elegans

The mechanisms controlling cell fate determination and reprogramming are fundamental for development. A profound reprogramming, allowing the production of pluripotent cells in early embryos, takes place during the oocyte-to-embryo transition. To understand how the oocyte reprogramming potential is controlled, we sought Caenorhabditis elegans mutants in which embryonic transcription is initiated precociously in germ cells. This screen identified LIN-41, a TRIM-NHL protein and a component of the somatic heterochronic pathway, as a temporal regulator of pluripotency in the germline. We found that LIN-41 is expressed in the cytoplasm of developing oocytes, which, in lin-41 mutants, acquire pluripotent characteristics of embryonic cells and form teratomas. To understand LIN-41 function in the germline, we conducted structure-function studies. In contrast to other TRIM-NHL proteins, we found that LIN-41 is unlikely to function as an E3 ubiquitin ligase. Similar to other TRIM-NHL proteins, the somatic function of LIN-41 is thought to involve mRNA regulation. Surprisingly, we found that mutations predicted to disrupt the association of LIN-41 with mRNA, which otherwise compromise LIN-41 function in the heterochronic pathway in the soma, have only minor effects in the germline. Similarly, LIN-41-mediated repression of a key somatic mRNA target is dispensable for the germline function. Thus, LIN-41 appears to function in the germline and the soma via different molecular mechanisms. These studies provide the first insight into the mechanism inhibiting the onset of embryonic differentiation in developing oocytes, which is required to ensure a successful transition between generations.

Reprogramming into a naïve, pluripotent state during the oocyte-to-embryo transition is directed by the oocyte cytoplasm. To understand how this reprogramming is controlled, we searched for C. elegans mutants in which the activation of embryonic genome, a landmark event demarcating the switch from a germline- to embryo-specific transcription, is initiated precociously in germ cells. This screen identified a novel function for LIN-41, a member of the TRIM-NHL protein family, in preventing a premature onset of embryonic-like differentiation and teratoma formation in developing oocytes, thus ensuring a successful passage between generations. This is the first example of such a regulator in cells that are poised for embryonic development. Interestingly, the majority of molecular “roadblocks” to reprograming that have been identified so far are epigenetic regulators. However, we propose that, at least in germ cells, LIN-41-like regulators may fulfill an analogous role in the cytoplasm, which has possible implications for the generation of human pluripotent stem cells.

Structural and functional implications of the QUA2 domain on RNA recognition by GLD-1

The STAR family comprises ribonucleic acid (RNA)-binding proteins that play key roles in RNA-regulatory processes. RNA recognition is achieved by a KH domain with an additional α-helix (QUA2) that seems to extend the RNA-binding surface to six nucleotides for SF1 (Homo sapiens) and seven nucleotides for GLD-1 (Caenorhabditis elegans). To understand the structural basis of this probable difference in specificity, we determined the solution structure of GLD-1 KH-QUA2 with the complete consensus sequence identified in the tra-2 gene. Compared to SF1, the GLD-1 KH-QUA2 interface adopts a different conformation resulting indeed in an additional sequence-specific binding pocket for a uracil at the 5'end. The functional relevance of this binding pocket is emphasized by our bioinformatics analysis showing that GLD-1 binding sites with this 5'end uracil are more predictive for the functional response of the messenger RNAs to gld-1 knockout. We further reveal the importance of the KH-QUA2 interface in vitro and that its alteration in vivo affects the level of translational repression dependent on the sequence of the GLD-1 binding motif. In conclusion, we demonstrate that the QUA2 domain distinguishes GLD-1 from other members of the STAR family and contributes more generally to the modulation of RNA-binding affinity and specificity of KH domain containing proteins.

C. elegans RNA-binding protein GLD-1 recognizes its multiple targets using sequence, context, and structural information to repress translation

Caenorhabditis elegans GLD-1, a maxi-KH motif containing RNA-binding protein, has various functions mainly during female germ cell development, suggesting that it likely controls the expression of a selective group of maternal mRNAs. To gain an insight into how GLD-1 specifically recognizes these mRNA targets, we identified 38 biochemically proven GLD-1 binding regions from multiple mRNA targets that are among over 100 putative targets co-immunoprecipitated with GLD-1. The sequence information of these regions revealed three over-represented and phylogenetically conserved sequence motifs. We found that two of the motifs, one of which is novel, are important for GLD-1 binding in several GLD-1 binding regions but not in other regions. Further analyses indicate that the importance of one of the sequence motifs is dependent on two aspects: (1) surrounding sequence information, likely acting as an accessory feature for GLD-1 to efficiently select the sequence motif and (2) RNA secondary structural environment where the sequence motif resides, which likely provides "binding-site accessibility" for GLD-1 to effectively recognize its targets. Our data suggest some mRNAs recruit GLD-1 by a distinct mechanism, which involves more than one sequence motif that needs to be embedded in the correct context and structural environment.

CDK phosphorylation of SLD-2 is required for replication initiation and germline development in C. elegans

Timely phosphorylation of SLD-2 by CDK is essential for proper replication initiation and cell proliferation in the germline of C. elegans.

Cyclin-dependent kinase (CDK) plays a vital role in proliferation control across eukaryotes. Despite this, how CDK mediates cell cycle and developmental transitions in metazoa is poorly understood. In this paper, we identify orthologues of Sld2, a CDK target that is important for DNA replication in yeast, and characterize SLD-2 in the nematode worm Caenorhabditis elegans. We demonstrate that SLD-2 is required for replication initiation and the nuclear retention of a critical component of the replicative helicase CDC-45 in embryos. SLD-2 is a CDK target in vivo, and phosphorylation regulates the interaction with another replication factor, MUS-101. By mutation of the CDK sites in sld-2, we show that CDK phosphorylation of SLD-2 is essential in C. elegans. Finally, using a phosphomimicking sld-2 mutant, we demonstrate that timely CDK phosphorylation of SLD-2 is an important control mechanism to allow normal proliferation in the germline. These results determine an essential function of CDK in metazoa and identify a developmental role for regulated SLD-2 phosphorylation.

LET-418/Mi2 and SPR-5/LSD1 cooperatively prevent somatic reprogramming of C. elegans germline stem cells

Throughout their journey to forming new individuals, germline stem cells must remain totipotent, particularly by maintaining a specific chromatin structure. However, the place epigenetic factors occupy in this process remains elusive. So far, “sensitization” of chromatin by modulation of histone arrangement and/or content was believed to facilitate transcription-factor-induced germ cell reprogramming. Here, we demonstrate that the combined reduction of two epigenetic factors suffices to reprogram C. elegans germ cells. The histone H3K4 demethylase SPR-5/LSD1 and the chromatin remodeler LET-418/Mi2 function together in an early process to maintain germ cell status and act as a barrier to block precocious differentiation. This epigenetic barrier is capable of limiting COMPASS-mediated H3K4 methylation, because elevated H3K4me3 levels correlate with germ cell reprogramming in spr-5; let-418 mutants. Interestingly, germ cells deficient for spr-5 and let-418 mainly reprogram as neurons, suggesting that neuronal fate might be the first to be derepressed in early embryogenesis.

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SPR-5/LSD1 and LET-418/Mi2 interact to jointly control germ cell status

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C. elegans germ cells reprogram as neurons in spr-5 let-418 mutants

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SPR-5 and LET-418 counteract COMPASS-dependent H3K4 methylation in the germline

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High H3K4me3 levels in germ cells correlate with somatic reprogramming

Divide and differentiate: CDK/Cyclins and the art of development

The elegant choreography of metazoan development demands exquisite regulation of cell-division timing, orientation, and asymmetry. In this review, we discuss studies in Drosophila and C. elegans that reveal how the cell cycle machinery, comprised of cyclin-dependent kinase (CDK) and cyclins functions as a master regulator of development. We provide examples of how CDK/cyclins: (1) regulate the asymmetric localization and timely destruction of cell fate determinants; (2) couple signaling to the control of cell division orientation; and (3) maintain mitotic zones for stem cell proliferation. These studies illustrate how the core cell cycle machinery should be viewed not merely as an engine that drives the cell cycle forward, but rather as a dynamic regulator that integrates the cell-division cycle with cellular differentiation, ensuring the coherent and faithful execution of developmental programs.

Ovarian germline stem cells: an unlimited source of oocytes?

While there has been progress in directing the development of embryonic stem cells and induced pluripotent stem cells toward a germ cell state, their ability to serve as a source of functional oocytes in a clinically relevant model or situation has yet to be established. Recent studies suggest that the adult mammalian ovary is not endowed with a finite number of oocytes, but instead possesses stem cells that contribute to their renewal. The ability to isolate and promote the growth and development of such ovarian germline stem cells (GSCs) would provide a novel means to treat infertility in women. Although such ovarian GSCs are well characterized in nonmammalian model organisms, the findings that support the existence of adult ovarian GSCs in mammals have been met with considerable evidence that disputes their existence. This review details the lessons provided by model organisms that successfully utilize ovarian GSCs to allow for a continual and high level of female germ cell production throughout their life, with a specific focus on the cellular mechanisms involved in GSC self-renewal and oocyte development. Such an overview of the role that oogonial stem cells play in maintaining fertility in nonmammalian species serves as a backdrop for the data generated to date that supports or disputes the existence of GSCs in mammals as well as the future of this area of research in terms of its potential for any application in reproductive medicine.

RNA-binding protein GLD-1/quaking genetically interacts with the mir-35 and the let-7 miRNA pathways in Caenorhabditis elegans

Messenger RNA translation is regulated by RNA-binding proteins and small non-coding RNAs called microRNAs. Even though we know the majority of RNA-binding proteins and microRNAs that regulate messenger RNA expression, evidence of interactions between the two remain elusive. The role of the RNA-binding protein GLD-1 as a translational repressor is well studied during Caenorhabditis elegans germline development and maintenance. Possible functions of GLD-1 during somatic development and the mechanism of how GLD-1 acts as a translational repressor are not known. Its human homologue, quaking (QKI), is essential for embryonic development. Here, we report that the RNA-binding protein GLD-1 in C. elegans affects multiple microRNA pathways and interacts with proteins required for microRNA function. Using genome-wide RNAi screening, we found that nhl-2 and vig-1, two known modulators of miRNA function, genetically interact with GLD-1. gld-1 mutations enhance multiple phenotypes conferred by mir-35 and let-7 family mutants during somatic development. We used stable isotope labelling with amino acids in cell culture to globally analyse the changes in the proteome conferred by let-7 and gld-1 during animal development. We identified the histone mRNA-binding protein CDL-1 to be, in part, responsible for the phenotypes observed in let-7 and gld-1 mutants. The link between GLD-1 and miRNA-mediated gene regulation is further supported by its biochemical interaction with ALG-1, CGH-1 and PAB-1, proteins implicated in miRNA regulation. Overall, we have uncovered genetic and biochemical interactions between GLD-1 and miRNA pathways.

Modeling the binding specificity of the RNA-binding protein GLD-1 suggests a function of coding region-located sites in translational repression

To understand the function of the hundreds of RNA-binding proteins (RBPs) that are encoded in animal genomes it is important to identify their target RNAs. Although it is generally accepted that the binding specificity of an RBP is well described in terms of the nucleotide sequence of its binding sites, other factors such as the structural accessibility of binding sites or their clustering, to enable binding of RBP multimers, are also believed to play a role. Here we focus on GLD-1, a translational regulator of Caenorhabditis elegans, whose binding specificity and targets have been studied with a variety of methods such as CLIP (cross-linking and immunoprecipitation), RIP-Chip (microarray measurement of RNAs associated with an immunoprecipitated protein), profiling of polysome-associated mRNAs and biophysical determination of binding affinities of GLD-1 for short nucleotide sequences. We show that a simple biophysical model explains the binding of GLD-1 to mRNA targets to a large extent, and that taking into account the accessibility of putative target sites significantly improves the prediction of GLD-1 binding, particularly due to a more accurate prediction of binding in transcript coding regions. Relating GLD-1 binding to translational repression and stabilization of its target transcripts we find that binding sites along the entire transcripts contribute to functional responses, and that CDS-located sites contribute most to translational repression. Finally, biophysical measurements of GLD-1 affinity for a small number of oligonucleotides appear to allow an accurate reconstruction of the sequence specificity of the protein. This approach can be applied to uncover the specificity and function of other RBPs.

Cdk1 modulation ensures the coordination of cell-cycle events during the switch from meiotic prophase to mitosis

BACKGROUND

Budding yeast cells that enter the developmental path of meiosis do not commit to finishing meiosis until after prophase I and the realization of such meiosis-specific events as pairing of homologous chromosomes and initiation of recombination. If the meiosis-inducing signal is withdrawn prior to commitment, cells exit meiosis and return to mitosis. The timing of this transition poses a singular problem for maintaining genome integrity. Cells in meiotic prophase have already replicated their DNA, but they have not undergone the morphological changes intrinsic to mitosis, including budding. Successful re-entry into mitosis requires that these cells bud but not rereplicate their DNA, reversing the normal order of mitosis. This study focuses on the cellular mechanisms that permit this dramatically altered order of cell-cycle events.

RESULTS

By developing a microfluidics assay to monitor individual cells, we show that the successful transition from meiotic prophase to mitosis requires the modulation of Cdk1 activity to coordinate cell-cycle events. The S. cerevisiae Wee1 homolog Swe1 prevents the formation of multinucleate cells by restraining M phase CDK activity to allow bud formation prior to nuclear division. The remaining S phase CDK activity promotes bud formation and prevents origin licensing so that DNA cannot rereplicate between bud formation and nuclear division. Once a bud has formed, M phase CDK drives cells through a normal mitotic division.

CONCLUSIONS

Our study uncovers the essential requirement of Swe1 to modulate CDK activity to coordinate cell-cycle events and maintain genome integrity during the transition from meiotic prophase to mitosis.

Sister chromatid segregation in meiosis II: deprotection through phosphorylation

Meiotic divisions (meiosis I and II) are specialized cell divisions to generate haploid gametes. The first meiotic division with the separation of chromosomes is named reductional division. The second division, which takes place immediately after meiosis I without intervening S-phase, is equational, with the separation of sister chromatids, similar to mitosis. This meiotic segregation pattern requires the two-step removal of the cohesin complex holding sister chromatids together: cohesin is removed from chromosome arms that have been subjected to homologous recombination in meiosis I and from the centromere region in meiosis II. Cohesin in the centromere region is protected from removal in meiosis I, but this protection has to be removed--deprotected--for sister chromatid segregation in meiosis II. Whereas the mechanisms of cohesin protection are quite well understood, the mechanisms of deprotection have been largely unknown until recently. In this review I summarize our current knowledge on cohesin deprotection.

CRL2(LRR-1) E3-ligase regulates proliferation and progression through meiosis in the Caenorhabditis elegans germline

The ubiquitin-proteolytic system controls the stability of proteins in space and time. In this study, using a temperature-sensitive mutant allele of the cul-2 gene, we show that CRL2(LRR-1) (CUL-2 RING E3 ubiquitin-ligase and the Leucine Rich Repeat 1 substrate recognition subunit) acts at multiple levels to control germline development. CRL2(LRR-1) promotes germ cell proliferation by counteracting the DNA replication ATL-1 checkpoint pathway. CRL2(LRR-1) also participates in the mitotic proliferation/meiotic entry decision, presumably controlling the stability of meiotic promoting factors in the mitotic zone of the germline. Finally, CRL2(LRR-1) inhibits the first steps of meiotic prophase by targeting in mitotic germ cells degradation of the HORMA domain-containing protein HTP-3, required for loading synaptonemal complex components onto meiotic chromosomes. Given its widespread evolutionary conservation, CUL-2 may similarly regulate germline development in other organisms as well.

Stem cell proliferation versus meiotic fate decision in Caenorhabditis elegans

The C. elegans germ line has emerged as an important model for -understanding how a stem cell population is maintained throughout the life of the animal while still producing the gametes necessary for propagation of the species. The stem cell population in the adult hermaphrodite is relatively large, with stem cells giving rise to daughters that appear intrinsically equivalent; however, some of the daughters retain the proliferative fate while others enter meiotic prophase. While machinery exists for cells to progress through the mitotic cell cycle and machinery exists for cells to progress through meiotic prophase, central to understanding germ line development is identifying the genes and regulatory processes that determine whether the mitotic cell cycle or meiotic prophase machinery will be utilized; in other words, the genes that regulate the switch of germ cells from the proliferative stem cell fate to the meiotic development fate. Whether a germ cell self-renews or enters meiotic prophase is largely determined by its proximity to the distal tip cell (DTC), which is the somatic niche cell that caps the distal end of the gonad. Germ cells close to the DTC have high levels of GLP-1 Notch signaling, which promotes the proliferative fate, while cells further from the DTC have high activity levels of the GLD-1 and GLD-2 redundant RNA regulatory pathways, as well as a third uncharacterized pathway, each of which direct cells to enter meiotic prophase. Other factors and pathways modulate this core genetic pathway, or work in parallel to it, presumably to ensure that a tight balance is maintained between proliferation and meiotic entry.

Germline stem cells and their regulation in the nematode Caenorhabditis elegans

C. elegans germline stem cells exist within a stem cell pool that is maintained by a single-celled mesenchymal niche and Notch signaling. Downstream of Notch signaling, a regulatory network governs stem cells and differentiation. Central to that network is the FBF RNA-binding protein, a member of the widely conserved PUF family that functions by either of two broadly conserved mechanisms to repress its target mRNAs. Without FBF, germline stem cells do not proliferate and they do not maintain their naïve, undifferentiated state. Therefore, FBF is a pivotal regulator of germline self-renewal. Validated FBF targets include several key differentiation regulators as well as a major cell cycle regulator. A genomic analysis identifies many other developmental and cell cycle regulators as likely FBF targets and suggests that FBF is a broad-spectrum regulator of the genome with >1,000 targets. A comparison of the FBF target list with similar lists for human PUF proteins, PUM1 and PUM2, reveals ∼200 shared targets. The FBF hub works within a network controlling self-renewal vs. differentiation. This network consists of classical developmental cell fate regulators and classical cell cycle regulators. Recent results have begun to integrate developmental and cell cycle regulation within the network. The molecular dynamics of the network remain a challenge for the future, but models are proposed. We suggest that molecular controls of C. elegans germline stem cells provide an important model for controls of stem cells more broadly.

Translational control in the Caenorhabditis elegans germ line

Translational control is a prevalent form of gene expression regulation in the Caenorhabditis elegans germ line. Linking the amount of protein synthesis to mRNA quantity and translational accessibility in the cell cytoplasm provides unique advantages over DNA-based controls for developing germ cells. This mode of gene expression is especially exploited in germ cell fate decisions and during oogenesis, when the developing oocytes stockpile hundreds of different mRNAs required for early embryogenesis. Consequently, a dense web of RNA regulators, consisting of diverse RNA-binding proteins and RNA-modifying enzymes, control the translatability of entire mRNA expression programs. These RNA regulatory networks are tightly coupled to germ cell developmental progression and are themselves under translational control. The underlying molecular mechanisms and RNA codes embedded in the mRNA molecules are beginning to be understood. Hence, the C. elegans germ line offers fertile grounds for discovering post-transcriptional mRNA regulatory mechanisms and emerges as great model for a systems level understanding of translational control during development.