Genetic regulation of entry into meiosis in Caenorhabditis elegans

GLD-2/RNP-8 cytoplasmic poly(A) polymerase is a broad-spectrum regulator of the oogenesis program

Regulated polyadenylation is a broadly conserved mechanism that controls key events during oogenesis. Pivotal to that mechanism is GLD-2, a catalytic subunit of cytoplasmic poly(A) polymerase (PAP). Caenorhabditis elegans GLD-2 forms an active PAP with multiple RNA-binding partners to regulate diverse aspects of germline and early embryonic development. One GLD-2 partner, RNP-8, was previously shown to influence oocyte fate specification. Here we use a genomic approach to identify transcripts selectively associated with both GLD-2 and RNP-8. Among the 335 GLD-2/RNP-8 potential targets, most were annotated as germline mRNAs and many as maternal mRNAs. These targets include gld-2 and rnp-8 themselves, suggesting autoregulation. Removal of either GLD-2 or RNP-8 resulted in shortened poly(A) tails and lowered abundance of four target mRNAs (oma-2, egg-1, pup-2, and tra-2); GLD-2 depletion also lowered the abundance of most GLD-2/RNP-8 putative target mRNAs when assayed on microarrays. Therefore, GLD-2/RNP-8 appears to polyadenylate and stabilize its target mRNAs. We also provide evidence that rnp-8 influences oocyte development; rnp-8 null mutants have more germ cell corpses and fewer oocytes than normal. Furthermore, RNP-8 appears to work synergistically with another GLD-2-binding partner, GLD-3, to ensure normal oogenesis. We propose that the GLD-2/RNP-8 enzyme is a broad-spectrum regulator of the oogenesis program that acts within an RNA regulatory network to specify and produce fully functional oocytes.

Cancer models in Caenorhabditis elegans

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

Soma-germline interactions that influence germline proliferation in Caenorhabditis elegans

Caenorhabditis elegans boasts a short lifecycle and high fecundity, two features that make it an attractive and powerful genetic model organism. Several recent studies indicate that germline proliferation, a prerequisite to optimal fecundity, is tightly controlled over the course of development. Cell proliferation control includes regulation of competence to proliferate, a poorly understood aspect of cell fate specification, as well as cell-cycle control. Furthermore, dynamic regulation of cell proliferation occurs in response to multiple external signals. The C. elegans germ line is proving a valuable model for linking genetic, developmental, systemic, and environmental control of cell proliferation. Here, we consider recent studies that contribute to our understanding of germ cell proliferation in C. elegans. We focus primarily on somatic control of germline proliferation, how it differs at different life stages, and how it can be altered in the context of the life cycle and changes in environmental status.

Caenorhabditis elegans as a model for stem cell biology

We review the application of Caenorhabditis elegans as a model system to understand key aspects of stem cell biology. The only bona fide stem cells in C. elegans are those of the germline, which serves as a valuable paradigm for understanding how stem-cell niches influence maintenance and differentiation of stem cells and how somatic differentiation is repressed during germline development. Somatic cells that share stem cell-like characteristics also provide insights into principles in stem-cell biology. The epidermal seam cell lineages lend clues to conserved mechanisms of self-renewal and expansion divisions. Principles of developmental plasticity and reprogramming relevant to stem-cell biology arise from studies of natural transdifferentiation and from analysis of early embryonic progenitors, which undergo a dramatic transition from a pluripotent, reprogrammable condition to a state of committed differentiation. The relevance of these developmental processes to our understanding of stem-cell biology in other organisms is discussed.

PRP-17 and the pre-mRNA splicing pathway are preferentially required for the proliferation versus meiotic development decision and germline sex determination in Caenorhabditis elegans

In C. elegans, the decision between germline stem cell proliferation and entry into meiosis is controlled by GLP-1 Notch signaling, which promotes proliferation through repression of the redundant GLD-1 and GLD-2 pathways that direct meiotic entry. We identify prp-17 as another gene functioning downstream of GLP-1 signaling that promotes meiotic entry, largely by acting on the GLD-1 pathway, and that also functions in female germline sex determination. PRP-17 is orthologous to the yeast and human pre-mRNA splicing factor PRP17/CDC40 and can rescue the temperature-sensitive lethality of yeast PRP17. This link to splicing led to an RNAi screen of predicted C. elegans splicing factors in sensitized genetic backgrounds. We found that many genes throughout the splicing cascade function in the proliferation/meiotic entry decision and germline sex determination indicating that splicing per se, rather than a novel function of a subset of splicing factors, is necessary for these processes.

Genome-wide analysis of mRNA targets for Caenorhabditis elegans FBF, a conserved stem cell regulator

Stem cells are essential for tissue generation during the development of multicellular creatures, and for tissue homeostasis in adults. The great therapeutic promise of stem cells makes understanding their regulation a high priority. PUF RNA-binding proteins have a conserved role in promoting self-renewal of germline stem cells. Here we use a genome-wide approach to identify putative target mRNAs for the Caenorhabditis elegans PUF protein known as FBF. We find that putative FBF targets represent approximately 7% of all protein-coding genes in C. elegans, implicating FBF as a broad-spectrum gene regulator. These putative FBF targets are enriched for regulators of meiotic entry and other components of the meiotic program as well as regulators of key developmental pathways. We suggest that these targets may be critical for FBF's role in stem cell maintenance. Comparison of likely FBF target mRNAs with putative PUF target mRNAs from Drosophila and humans reveals 40 shared targets, including several established stem cell regulators. We speculate that shared PUF targets represent part of a broadly used module of stem cell control.

Progression from a stem cell-like state to early differentiation in the C. elegans germ line

Controls of stem cell maintenance and early differentiation are known in several systems. However, the progression from stem cell self-renewal to overt signs of early differentiation is a poorly understood but important problem in stem cell biology. The Caenorhabditis elegans germ line provides a genetically defined model for studying that progression. In this system, a single-celled mesenchymal niche, the distal tip cell (DTC), employs GLP-1/Notch signaling and an RNA regulatory network to balance self-renewal and early differentiation within the "mitotic region," which continuously self-renews while generating new gametes. Here, we investigate germ cells in the mitotic region for their capacity to differentiate and their state of maturation. Two distinct pools emerge. The "distal pool" is maintained by the DTC in an essentially uniform and immature or "stem cell-like" state; the "proximal pool," by contrast, contains cells that are maturing toward early differentiation and are likely transit-amplifying cells. A rough estimate of pool sizes is 30-70 germ cells in the distal immature pool and approximately 150 in the proximal transit-amplifying pool. We present a simple model for how the network underlying the switch between self-renewal and early differentiation may be acting in these two pools. According to our model, the self-renewal mode of the network maintains the distal pool in an immature state, whereas the transition between self-renewal and early differentiation modes of the network underlies the graded maturation of germ cells in the proximal pool. We discuss implications of this model for controls of stem cells more broadly.

Translational control in the C. elegans hermaphrodite germ line

The formation of a fully developed gamete from an undifferentiated germ cell requires progression through numerous developmental stages and cell fate decisions. The precise timing and level of gene expression guides cells through these stages. Translational regulation is highly utilized in the germ line of many species, including Caenorhabditis elegans , to regulate gene expression and ensure the proper formation of gametes. In this review, we discuss some of the developmental stages and cell fate decisions involved in the formation of functional gametes in the C. elegans germ line in which translational control has been implicated. These stages include the mitosis versus meiosis decision, the sperm/oocyte decision, and gamete maturation. We also discuss some of the techniques used to identify mRNA targets; the identification of these targets is necessary to clearly understand the role each RNA-binding protein plays in these decisions. Relatively few mRNA targets have been identified, thus providing a major focus for future research. Finally, we propose some reasons why translational control may be utilized so heavily in the germ line. Given that many species have this substantial reliance on translational regulation for the control of gene expression in the germ line, an understanding of translational regulation in the C. elegans germ line is likely to increase our understanding of gamete formation in general.

Mutations in Caenorhabditis elegans him-19 show meiotic defects that worsen with age

Faithful meiotic chromosome segregation requires pairing, synapsis and recombination of homologous chromosomes. In mammals, chromosomal non-disjunction increases with age. A mutation in Caenorhabditis eleganshim-19 mimics these age-dependent chromosome segregation defects and might therefore further our understanding of this phenomenon.

From a screen for meiotic Caenorhabditis elegans mutants based on high incidence of males, we identified a novel gene, him-19, with multiple functions in prophase of meiosis I. Mutant him-19(jf6) animals show a reduction in pairing of homologous chromosomes and subsequent bivalent formation. Consistently, synaptonemal complex formation is spatially restricted and possibly involves nonhomologous chromosomes. Also, foci of the recombination protein RAD-51 occur delayed or cease altogether. Ultimately, mutation of him-19 leads to chromosome missegregation and reduced offspring viability. The observed defects suggest that HIM-19 is important for both homology recognition and formation of meiotic DNA double-strand breaks. It therefore seems to be engaged in an early meiotic event, resembling in this respect the regulator kinase CHK-2. Most astonishingly, him-19(jf6) hermaphrodites display worsening of phenotypes with increasing age, whereas defects are more severe in female than in male meiosis. This finding is consistent with depletion of a him-19-dependent factor during the production of oocytes. Further characterization of him-19 could contribute to our understanding of age-dependent meiotic defects in humans.

C. elegans star proteins, GLD-1 and ASD-2, regulate specific RNA targets to control development

A comprehensive understanding of the C. elegans STAR proteins GLD-1 and ASD-2 is emerging from a combination of studies. Those employing genetic analysis reveal in vivo function, others involving biochemical approaches pursue the identification of mRNA targets through which these proteins act. Lastly, mechanistic studies provide the molecular pathway of target mRNA regulation.

Scratching the niche that controls Caenorhabditis elegans germline stem cells

The Caenorhabditis elegans gonad provides a well-defined model for a stem cell niche and its control of self-renewal and differentiation. The distal tip cell (DTC) forms a mesenchymal niche that controls germline stem cells (GSCs), both to generate the germline tissue during development and to maintain it during adulthood. The DTC uses GLP-1/Notch signaling to regulate GSCs; germ cells respond to Notch signaling with a network of RNA regulators to control the decision between self-renewal and entry into the meiotic cell cycle.

METT-10, a putative methyltransferase, inhibits germ cell proliferative fate in Caenorhabditis elegans

Germ-line stem cells are unique because they either self-renew through mitosis or, at a certain frequency, switch to meiosis and produce gametes. The switch from proliferation to meiosis is tightly regulated, and aberrations in switching result in either too little or too much proliferation. To understand the genetic basis of this regulation, we characterized loss-of-function mutations and a novel tumorous allele of Caenorhabditis elegans mett-10, which encodes a conserved putative methyltransferase. We show that METT-10 is a nuclear protein that acts in the germ line to inhibit the specification of germ-cell proliferative fate. METT-10 also promotes vulva, somatic gonad, and embryo development and ensures meiotic development of those germ cells that do differentiate. In addition, phenotypic analysis of a mett-10 null allele reveals that METT-10 enables mitotic cell cycle progression. The finding that METT-10 functions to inhibit germ-cell proliferative fate, despite promoting mitotic cell cycle progression of those germ cells that do proliferate, separates the specification of proliferative fate from its execution.

Antagonism between GLD-2 binding partners controls gamete sex

Cytoplasmic polyadenylation is a key mechanism of gene control. In Caenorhabditis elegans, GLD-2 and GLD-3 provide the catalytic and RNA-binding subunits, respectively, of a major cytoplasmic poly(A) polymerase (PAP). Here, we identify RNP-8 as a second GLD-2 partner. RNP-8 binds GLD-2 and stimulates GLD-2 activity to form a functional PAP, much like GLD-3. Moreover, GLD-2/RNP-8 and GLD-2/GLD-3 exist as separate complexes that form selectively during development, and RNP-8 and GLD-3 appear to have distinct RNA-binding specificities. Therefore, GLD-2 can form either of two discrete PAPs. In C. elegans hermaphrodites, gamete production begins with spermatogenesis and transitions later to oogenesis. We suggest that the combinatorial use of GLD-2 contributes to this transition, as GLD-2/GLD-3 promotes spermatogenesis, whereas GLD-2/RNP-8 specifies oogenesis. Indeed, RNP-8 and GLD-3 antagonize each other, as evidenced by genetic cosuppression and molecular competition for GLD-2 binding. We conclude that GLD-2 and its binding partners control gamete identity.

Two conserved regulatory cytoplasmic poly(A) polymerases, GLD-4 and GLD-2, regulate meiotic progression in C. elegans

Translational regulation is heavily employed during developmental processes to control the timely accumulation of proteins independently of gene transcription. In particular, mRNA poly(A) tail metabolism in the cytoplasm is a key determinant for balancing an mRNA's translational output and its decay rate. Noncanonical poly(A) polymerases (PAPs), such as germline development defective-2 (GLD-2), can mediate poly(A) tail extension. Little is known about the regulation and functional complexity of cytoplasmic PAPs. Here we report the discovery of Caenorhabditis elegans GLD-4, a cytoplasmic PAP present in P granules that is orthologous to Trf4/5p from budding yeast. GLD-4 enzymatic activity is enhanced by its interaction with GLS-1, a protein associated with the RNA-binding protein GLD-3. GLD-4 is predominantly expressed in germ cells, and its activity is essential for early meiotic progression of male and female gametes in the absence of GLD-2. For commitment into female meiosis, both PAPs converge on at least one common target mRNA-i.e., gld-1 mRNA-and, as a consequence, counteract the repressive action of two PUF proteins and the putative deadenylase CCR-4. Together our findings suggest that two different cytoplasmic PAPs stabilize and translationally activate several meiotic mRNAs to provide a strong fail-safe mechanism for early meiotic progression.

FBF and its dual control of gld-1 expression in the Caenorhabditis elegans germline

FBF, a PUF RNA-binding protein, is a key regulator of the mitosis/meiosis decision in the Caenorhabditis elegans germline. Genetically, FBF has a dual role in this decision: it maintains germ cells in mitosis, but it also facilitates entry into meiosis. In this article, we explore the molecular basis of that dual role. Previous work showed that FBF downregulates gld-1 expression to promote mitosis and that the GLD-2 poly(A) polymerase upregulates gld-1 expression to reinforce the decision to enter meiosis. Here we ask whether FBF can act as both a negative regulator and a positive regulator of gld-1 expression and also investigate its molecular mechanisms of control. We first show that FBF co-immunoprecipitates with gld-1 mRNA, a result that complements previous evidence that FBF directly controls gld-1 mRNA. Then we show that FBF represses gld-1 expression, that FBF physically interacts with the CCF-1/Pop2p deadenylase and can stimulate deadenylation in vitro, and that CCF-1 is partially responsible for maintaining low GLD-1 in the mitotic region. Finally, we show that FBF can elevate gld-1 expression, that FBF physically interacts with the GLD-2 poly(A) polymerase, and that FBF can enhance GLD-2 poly(A) polymerase activity in vitro. We propose that FBF can affect polyadenylation either negatively by its CCF-1 interaction or positively by its GLD-2 interaction.

A mutation in teg-4, which encodes a protein homologous to the SAP130 pre-mRNA splicing factor, disrupts the balance between proliferation and differentiation in the C. elegans germ line

Dividing stem cells can give rise to two types of daughter cells; self-renewing cells that have virtually the same properties as the parent cell, and differentiating cells that will eventually form part of a tissue. The Caenorhabditis elegans germ line serves as a model to study how the balance between these two types of daughter cells is maintained. A mutation in teg-4 causes over-proliferation of the stem cells, thereby disrupting the balance between proliferation and differentiation. We have cloned teg-4 and found it to encode a protein homologous to the highly conserved splicing factor subunit 3 of SF3b. Our allele of teg-4 partially reduces TEG-4 function. In an effort to determine how teg-4 functions in controlling stem cell proliferation, we have performed genetic epistasis analysis with known factors controlling stem cell proliferation. We found that teg-4 is synthetic tumorous with genes in both major redundant genetic pathways that function downstream of GLP-1/Notch signaling to control the balance between proliferation and differentiation. Therefore, teg-4 is unlikely to function specifically in either of these two genetic pathways. Further, the synthetic tumorous phenotype seen with one of the genes from these pathways is epistatic to glp-1, indicating that teg-4 functions downstream of glp-1, likely as a positive regulator of meiotic entry. We propose that a reduction in teg-4 activity reduces the splicing efficiency of targets involved in controlling the balance between proliferation and differentiation. This results in a shift in the balance towards proliferation, eventually forming a germline tumor.

Purpose and regulation of stem cells: a systems-biology view from the Caenorhabditis elegans germ line

Stem cells are expected to play a key role in the development and maintenance of organisms, and hold great therapeutic promises. However, a number of questions must be answered to achieve an understanding of stem cells and put them to use. Here I review some of these questions, and how they relate to the model system provided by the Caenorhabditis elegans germ line, which is exceptional in its thorough genetic characterization and experimental accessibility under in vivo conditions. A fundamental question is how to define a stem cell; different definitions can be adopted that capture different features of interest. In the C. elegans germ line, stem cells can be defined by cell lineage or by cell commitment ('commitment' must itself be carefully defined). These definitions are associated with two other important questions about stem cells: their functions (which must be addressed following a systems approach, based on an evolutionary perspective) and their regulation. I review possible functions and their evolutionary groundings, including genome maintenance and powerful regulation of cell proliferation and differentiation, and possible regulatory mechanisms, including asymmetrical division and control of transit amplification by a developmental timer. I draw parallels between Drosophila and C. elegans germline stem cells; such parallels raise intriguing questions about Drosophila stem cells. I conclude by showing that the C. elegans germ line bears similarities with a number of other stem cell systems, which underscores its relevance to the understanding of stem cells.

C. elegans RNA-binding proteins PUF-8 and MEX-3 function redundantly to promote germline stem cell mitosis

Maintenance of mitotically cycling germline stem cells (GSCs) is vital for continuous production of gametes. In worms and insects, signaling from surrounding somatic cells play an essential role in the maintenance of GSCs by preventing premature differentiation. In addition, germ cell proteins such as the Drosophila Pumilio and Caenorhabditis elegans FBF, both members of the PUF family translational regulators, contribute to GSC maintenance. FBF functions by suppressing GLD-1, which promotes meiotic entry. However, factors that directly promote GSC proliferation, rather than prevent differentiation, are not known. Here we show that PUF-8, another C. elegans member of the PUF family and MEX-3, a KH domain translational regulator, function redundantly to promote GSC mitosis. We find that PUF-8 protein is highly enriched in mitotic germ cells, which is similar to the expression pattern of MEX-3 described earlier. The puf-8(−) mex-3(−) double mutant gonads contain far fewer germ cells than both single mutants and wild-type. While these cells lack mitotic, meiotic and sperm markers, they retain the germ cell-specific P granules, and are capable of gametogenesis if GLP-1, which normally blocks meiotic entry, is removed. Significantly, we find that at least one of these two proteins is essential for germ cell proliferation even in meiotic entry-defective mutants, which otherwise produce germ cell tumors. We conclude PUF-8 and MEX-3 contribute to GSC maintenance by promoting mitotic proliferation rather than by blocking meiotic entry.

A deficiency of lunatic fringe is associated with cystic dilation of the rete testis

Lunatic fringe belongs to a family of beta1-3 N-acetyltransferases that modulate the affinity of the Notch receptors for their ligands through the elongation of O-fucose moieties on their extracellular domain. A role for Notch signaling in vertebrate fertility has been predicted by the intricate expression of the Notch receptors and their ligands in the oocyte and granulosa cells of the ovary and the spermatozoa and Sertoli cells of the testis. It has been demonstrated that disruption of Notch signaling by inactivation of lunatic fringe led to infertility associated with pleiotropic defects in follicle development and meiotic maturation of oocytes. Lunatic fringe null males were found to be subfertile. Here, we report that gene expression data demonstrate that fringe and Notch signaling genes are expressed in the developing testis and the intratesticular ductal tract, predicting roles for this pathway during embryonic gonadogenesis and spermatogenesis. Spermatogenesis was not impaired in the majority of the lunatic fringe null males; however, spermatozoa were unilaterally absent in the epididymis of many mice. Histological and immunohistochemical analysis of these testes revealed the development of unilateral cystic dilation of the rete testis. Tracer dye experiments confirm a block in the connection between the rete testis and the efferent ducts. Further, the dye studies demonstrated that many lunatic fringe mutant males had partial blocks of the connection between the rete testis and the efferent ducts bilaterally.

Proteasomal regulation of the proliferation vs. meiotic entry decision in the Caenorhabditis elegans germ line

Reproductive fitness in many animals relies upon a tight balance between the number of cells that proliferate in the germ line and the number of cells that enter meiosis and differentiate as gametes. In the Caenorhabditis elegans germ line, the GLP-1/Notch signaling pathway controls this balance between proliferation and meiotic entry. Here we describe the identification of the proteasome as an additional regulator of this balance. We show that a decrease in proteasome activity, through either genetic mutation or RNAi to core components of the proteasome, shifts this balance toward excess germ-line proliferation. We further demonstrate that there are likely two or more proteasome targets that contribute to excess germ-line proliferation when proteasome activity is reduced. One of these targets is likely a component or regulator of the Notch-signaling pathway, while the other functions on one of the two major redundant genetic pathways downstream of GLP-1/Notch signaling. We propose a model in which the proteasome degrades proteins that are necessary for proliferation as cells switch from proliferation to meiotic entry.

Wispy, the Drosophila homolog of GLD-2, is required during oogenesis and egg activation

Egg activation is the process that modifies mature, arrested oocytes so that embryo development can proceed. One key aspect of egg activation is the cytoplasmic polyadenylation of certain maternal mRNAs to permit or enhance their translation. wispy (wisp) maternal-effect mutations in Drosophila block development during the egg-to-embryo transition. We show here that the wisp gene encodes a member of the GLD-2 family of cytoplasmic poly(A) polymerases (PAPs). The WISP protein is required for poly(A) tail elongation of bicoid, Toll, and torso mRNAs upon egg activation. In Drosophila, WISP and Smaug (SMG) have previously been reported to be required to trigger the destabilization of maternal mRNAs during egg activation. SMG is the major regulator of this activity. We report here that SMG is still translated in activated eggs from wisp mutant mothers, indicating that WISP does not regulate mRNA stability by controlling the translation of smg mRNA. We have also analyzed in detail the very early developmental arrest associated with wisp mutations. Pronuclear migration does not occur in activated eggs laid by wisp mutant females. Finally, we find that WISP function is also needed during oogenesis to regulate the poly(A) tail length of dmos during oocyte maturation and to maintain a high level of active (phospho-) mitogen-activated protein kinases (MAPKs).

Drosophila follicle cells: morphogenesis in an eggshell

Epithelial morphogenesis is important for organogenesis and pivotal for carcinogenesis, but mechanisms that control it are poorly understood. The Drosophila follicular epithelium is a genetically tractable model to understand these mechanisms in vivo. This epithelium of follicle cells encases germline cells to create an egg. In this review, we summarize progress toward understanding mechanisms that maintain the epithelium or permit migrations essential for oogenesis. Cell-cell communication is important, but the same signals are used repeatedly to control distinct events. Understanding intrinsic mechanisms that alter responses to developmental signals will be important to understand regulation of cell shape and organization.

FLI-1 Flightless-1 and LET-60 Ras control germ line morphogenesis in C. elegans

Background

In the C. elegans germ line, syncytial germ line nuclei are arranged at the cortex of the germ line as they exit mitosis and enter meiosis, forming a nucleus-free core of germ line cytoplasm called the rachis. Molecular mechanisms of rachis formation and germ line organization are not well understood.

Results

Mutations in the fli-1 gene disrupt rachis organization without affecting meiotic differentiation, a phenotype in C. elegans referred to here as the germ line morphogenesis (Glm) phenotype. In fli-1 mutants, chains of meiotic germ nuclei spanned the rachis and were partially enveloped by invaginations of germ line plasma membrane, similar to nuclei at the cortex. Extensions of the somatic sheath cells that surround the germ line protruded deep inside the rachis and were associated with displaced nuclei in fli-1 mutants. fli-1 encodes a molecule with leucine-rich repeats and gelsolin repeats similar to Drosophila flightless 1 and human Fliih, which have been shown to act as cytoplasmic actin regulators as well as nuclear transcriptional regulators. Mutations in let-60 Ras, previously implicated in germ line development, were found to cause the Glm phenotype. Constitutively-active LET-60 partially rescued the fli-1 Glm phenotype, suggesting that LET-60 Ras and FLI-1 might act together to control germ line morphogenesis.

Conclusion

FLI-1 controls germ line morphogenesis and rachis organization, a process about which little is known at the molecular level. The LET-60 Ras GTPase might act with FLI-1 to control germ line morphogenesis.

Conserved regulation of MAP kinase expression by PUF RNA-binding proteins

Mitogen-activated protein kinase (MAPK) and PUF (for Pumilio and FBF [fem-3 binding factor]) RNA-binding proteins control many cellular processes critical for animal development and tissue homeostasis. In the present work, we report that PUF proteins act directly on MAPK/ERK-encoding mRNAs to downregulate their expression in both the Caenorhabditis elegans germline and human embryonic stem cells. In C. elegans, FBF/PUF binds regulatory elements in the mpk-1 3′ untranslated region (3′ UTR) and coprecipitates with mpk-1 mRNA; moreover, mpk-1 expression increases dramatically in FBF mutants. In human embryonic stem cells, PUM2/PUF binds 3′UTR elements in both Erk2 and p38α mRNAs, and PUM2 represses reporter constructs carrying either Erk2 or p38α 3′ UTRs. Therefore, the PUF control of MAPK expression is conserved. Its biological function was explored in nematodes, where FBF promotes the self-renewal of germline stem cells, and MPK-1 promotes oocyte maturation and germ cell apoptosis. We found that FBF acts redundantly with LIP-1, the C. elegans homolog of MAPK phosphatase (MKP), to restrict MAPK activity and prevent apoptosis. In mammals, activated MAPK can promote apoptosis of cancer cells and restrict stem cell self-renewal, and MKP is upregulated in cancer cells. We propose that the dual negative regulation of MAPK by both PUF repression and MKP inhibition may be a conserved mechanism that influences both stem cell maintenance and tumor progression.

The mitogen-activated protein (MAP) kinase (MAPK) enzyme is crucial for regulation of both stem cell maintenance and tumorigenesis. Two conserved controls of MAPK include its activation by RAS signaling and a kinase cascade as well as its inactivation by MAPK phosphatases (MKPs). We identify a third mode of conserved MAPK regulation. We demonstrate that PUF (for Pumilio and FBF [fem-3 binding factor]) RNA-binding proteins repress mRNAs encoding MAPK enzymes in both the Caenorhabditis elegans germline and human embryonic stem cells. PUF proteins have emerged as conserved regulators of germline stem cells in C. elegans, Drosophila, and probably vertebrates. Their molecular mode of action relies on binding to sequence elements in the 3′ untranslated region of target mRNAs. We report that PUF proteins bind and repress mRNAs encoding C. elegans MPK-1 as well as human ERK2 and p38α. We also report that PUF repression and MKP inactivation function redundantly in the C. elegans germline to restrict MPK-1/MAPK activity and prevent germ cell apoptosis. We suggest that this dual regulation of MAPK activity by PUF and MKP proteins may be a conserved mechanism for the control of growth and differentiation during animal development and tissue homeostasis.

Non-canonical poly(A) polymerase in mammalian gametogenesis

Polyadenylation of mRNA precursors initially occurs in the nucleus of eukaryotic cells, and the polyadenylated mRNAs are then transported into the cytoplasm. Because the length of the poly(A) tail is implicated in various aspects of mRNA metabolism, including the transport into the cytoplasm, stability, and translational control, processing of mRNA precursors at the 3'-end is important for post-transcriptional gene regulation. In particular, the lengthening, maintenance, and shortening of poly(A) tails in the cytoplasm are all essential for modulation of gametogenesis. Here we focus on the functional roles of mouse Tpap and Gld-2 in spermatogenesis and oocyte maturation, respectively.

Xenopus Rbm9 is a novel interactor of XGld2 in the cytoplasmic polyadenylation complex

During early development, control of the poly(A) tail length by cytoplasmic polyadenylation is critical for the regulation of specific mRNA expression. Gld2, an atypical poly(A) polymerase, is involved in cytoplasmic polyadenylation in Xenopus oocytes. In this study, a new XGld2-interacting protein was identified: Xenopus RNA-binding motif protein 9 (XRbm9). This RNA-binding protein is exclusively expressed in the cytoplasm of Xenopus oocytes and interacts directly with XGld2. It is shown that XRbm9 belongs to the cytoplasmic polyadenylation complex, together with cytoplasmic polyadenylation element-binding protein (CPEB), cleavage and polyadenylation specificity factor (CPSF) and XGld2. In addition, tethered XRbm9 stimulates the translation of a reporter mRNA. The function of XGld2 in stage VI oocytes was also analysed. The injection of XGld2 antibody into oocytes inhibited polyadenylation, showing that endogenous XGld2 is required for cytoplasmic polyadenylation. Unexpectedly, XGld2 and CPEB antibody injections also led to an acceleration of meiotic maturation, suggesting that XGld2 is part of a masking complex with CPEB and is associated with repressed mRNAs in oocytes.

Analysis of the C. elegans germline stem cell region

We present methods for characterizing the mitotic and early meiotic regions of the Caenorhabditis elegans germline. The methods include examination of germlines in living and fixed worms, cell cycle analysis, analysis of markers, and initial characterization of mutants that affect germline proliferation.

Controls of germline stem cells, entry into meiosis, and the sperm/oocyte decision in Caenorhabditis elegans

The Caenorhabditis elegans germ line provides an exceptional model for analysis of the molecular controls governing stem cell maintenance, the cell cycle transition from mitosis to meiosis, and the choice of sexual identity-sperm or oocyte. Germline stem cells are maintained in an undifferentiated state within a well-defined niche formed by a single somatic cell, the distal tip cell (DTC). In both sexes, the DTC employs GLP-1/Notch signaling and FBF/PUF RNA-binding proteins to maintain stem cells and promote mitotic divisions, three additional RNA regulators (GLD-1/quaking, GLD-2/poly(A) polymerase, and GLD-3/Bicaudal-C) control entry into meiosis, and FOG-1/CPEB and FOG-3/Tob proteins govern sperm specification. These key regulators are part of a robust regulatory network that controls germ cell proliferation, stem cell maintenance, and sex determination. Parallels with controls in other organisms include the use of PUF proteins for stem cell maintenance and the prominence of mRNA regulation for the control of germline development.

Caenorhabditis elegans germ line: a model for stem cell biology

Like many stem cell systems, the Caenorhabditis elegans germ line contains a self-renewing germ cell population that is maintained by a niche. Although the exact cellular mechanism for self-renewal is not yet known, three recent studies shed considerable light on the cell cycle behavior of germ cells, including a support for significant and plastic renewal potential. This review brings together the results of the three recent cell-based studies, places them in the context of previous work, and discusses future perspectives for the field.

RNA-specific ribonucleotidyl transferases

RNA-specific nucleotidyl transferases (rNTrs) are a diverse family of template-independent polymerases that add ribonucleotides to the 3'-ends of RNA molecules. All rNTrs share a related active-site architecture first described for DNA polymerase beta and a catalytic mechanism conserved among DNA and RNA polymerases. The best known examples are the nuclear poly(A) polymerases involved in the 3'-end processing of eukaryotic messenger RNA precursors and the ubiquitous CCA-adding enzymes that complete the 3'-ends of tRNA molecules. In recent years, a growing number of new enzymes have been added to the list that now includes the "noncanonical" poly(A) polymerases involved in RNA quality control or in the readenylation of dormant messenger RNAs in the cytoplasm. Other members of the group are terminal uridylyl transferases adding single or multiple UMP residues in RNA-editing reactions or upon the maturation of small RNAs and poly(U) polymerases, the substrates of which are still not known. 2'-5'Oligo(A) synthetases differ from the other rNTrs by synthesizing oligonucleotides with 2'-5'-phosphodiester bonds de novo.

The Bro1-domain protein, EGO-2, promotes Notch signaling in Caenorhabditis elegans

In Caenorhabditis elegans, as in other animals, Notch-type signaling mediates numerous inductive events during development. The mechanism of Notch-type signaling involves proteolytic cleavage of the receptor and subsequent transport of the receptor intracellular domain to the nucleus, where it acts as a transcriptional regulator. Notch-type signaling activity is modulated by post-translational modifications and endocytosis of ligand and receptor. We previously identified the ego-2 (enhancer of glp-1) gene as a positive regulator of germline proliferation that interacts genetically with the GLP-1/Notch signaling pathway in the germline. Here, we show that ego-2 positively regulates signaling in various tissues via both GLP-1 and the second C. elegans Notch-type receptor, LIN-12. ego-2 activity also promotes aspects of development not known to require GLP-1 or LIN-12. The EGO-2 protein contains a Bro1 domain, which is known in other systems to localize to certain endosomal compartments. EGO-2 activity in the soma promotes GLP-1 signaling in the germline, consistent with a role for EGO-2 in production of active ligand. Another C. elegans Bro1-domain protein, ALX-1, is known to interact physically with LIN-12/Notch. We document a complex phenotypic interaction between ego-2 and alx-1, consistent with their relationship being antagonistic with respect to some developmental processes and agonistic with respect to others.

A family of poly(U) polymerases

The GLD-2 family of poly(A) polymerases add successive AMP monomers to the 3' end of specific RNAs, forming a poly(A) tail. Here, we identify a new group of GLD-2-related nucleotidyl transferases from Arabidopsis, Schizosaccharomyces pombe, Caenorhabditis elegans, and humans. Like GLD-2, these enzymes are template independent and add nucleotides to the 3' end of an RNA substrate. However, these new enzymes, which we refer to as poly(U) polymerases, add poly(U) rather than poly(A) to their RNA substrates.

Developmental expression of FOG-1/CPEB protein and its control in the Caenorhabditis elegans hermaphrodite germ line

The specification of a germ cell as sperm or oocyte and determination of cell number remain unsolved questions in developmental biology. This paper examines Caenorhabditis elegans FOG-1, a CPEB-related RNA-binding protein that controls the sperm fate. We find that abundant FOG-1 protein is observed transiently in germ cells just prior to their expression of an early sperm-differentiation marker. As the germline tissue elongates, abundant FOG-1 appears more and more distally as sperm become specified, but disappears when the germ line switches to oogenesis. This dynamic pattern is controlled by both globally acting and germline-specific sex-determining regulators. Importantly, the extent of FOG-1 expression corresponds roughly to sperm number in wild-type and mutants, altering sperm number. By contrast, three other key regulators of the sperm/oocyte decision do not similarly correspond to sperm number. We suggest that FOG-1 is precisely modulated in both time and space to specify sperm fate and control sperm number.

The Cid1 family of non-canonical poly(A) polymerases

Polyadenylation is an essential processing step for most eukaryotic mRNAs. In the nucleus, poly(A) polymerase adds poly(A) tails to mRNA 3' ends, contributing to their export, stability and translatability. Recently, a novel class of non-canonical poly(A) polymerases was discovered in yeast, worms and vertebrates. Different members of the Cid1 family, named after its founding member in the fission yeast Schizosaccharomyces pombe, are localized in the nucleus and the cytoplasm and are thought to target specific RNAs for polyadenylation. Polyadenylation of a target RNA by a Cid1-like poly(A) polymerase can lead to its degradation or stabilization, depending on the enzyme involved. Cid1-like proteins have important roles in diverse biological processes, including RNA surveillance pathways, DNA integrity checkpoint responses and RNAi-dependent heterochromatin formation.

The GLD-2 poly(A) polymerase activates gld-1 mRNA in the Caenorhabditis elegans germ line

mRNA regulation is crucial for many aspects of metazoan development and physiology, including regulation of stem cells and synaptic plasticity. In the nematode germ line, RNA regulators control stem cell maintenance, the sperm/oocyte decision, and progression through meiosis. Of particular importance to this work are three GLD (germ-line development) regulatory proteins, each of which promotes entry into the meiotic cell cycle: GLD-1 is a STAR/Quaking translational repressor, GLD-2 is a cytoplasmic poly(A) polymerase, and GLD-3 is a homolog of Bicaudal-C. Here we report that the gld-1 mRNA is a direct target of the GLD-2 poly(A) polymerase: polyadenylation of gld-1 mRNA depends on GLD-2, the abundance of GLD-1 protein is dependent on GLD-2, and the gld-1 mRNA coimmunoprecipitates with both GLD-2 and GLD-3 proteins. We suggest that the GLD-2 poly(A) polymerase enhances entry into the meiotic cell cycle at least in part by activating GLD-1 expression. The importance of this conclusion is twofold. First, the activation of gld-1 mRNA by GLD-2 identifies a positive regulatory step that reinforces the decision to enter the meiotic cell cycle. Second, gld-1 mRNA is initially repressed by FBF (for fem-3 binding factor) to maintain stem cells but then becomes activated by the GLD-2 poly(A) polymerase once stem cells begin to make the transition into the meiotic cell cycle. Therefore, a molecular switch regulates gld-1 mRNA activity to accomplish the transition from mitosis to meiosis.

Fission yeast Cid12 has dual functions in chromosome segregation and checkpoint control

Fission yeast Cid12 is a member of the Cid1 family of specialized poly(A) polymerases. Like cells lacking cid1, cid12Delta mutants were shown to have checkpoint defects when DNA replication was inhibited. Here, we show that Cid12 is also required for faithful chromosome segregation and that mutation of amino acid residues predicted to be essential for poly(A) polymerase activity resulted in loss of Cid12 function in vivo. Cells lacking Cid12 had an increased chromosome segregation failure rate due to precocious loss of sister chromatid cohesion at the centromere but not along the chromosome arms. In keeping with a recently described function for Cid12 in RNA interference (RNAi)-mediated heterochromatin assembly, this was accompanied by an accumulation of polyadenylated transcripts corresponding to naturally silenced repeat elements within heterochromatic domains, with consequent defects in centromeric gene silencing. These cells also suffered increased meiotic defects, and their viability was dependent on the spindle checkpoint protein Bub1. To account for the effects of Cid12 on various aspects of DNA metabolism, including chromosome segregation and the checkpoint control, we suggest that Cid12 has dual functions in RNAi silencing and regulating mRNA stability.

C. elegans CPB-3 interacts with DAZ-1 and functions in multiple steps of germline development

Cytoplasmic polyadenylation element-binding proteins (CPEBs) are well-conserved RNA-binding proteins, which regulate mRNA translation mainly through control of poly(A) elongation. Here, we show that CPB-3, one of the four CPEB homologs in C. elegans, positively regulates multiple aspects of oocyte production. CPB-3 protein was highly expressed in early meiotic regions of the hermaphrodite gonad. Worms deficient in cpb-3 were apparently impaired in germ cell proliferation and differentiation including sperm/oocyte switching and progression of female meiosis. We also show that cpb-3 is likely to promote the meiotic entry in parallel with gld-3, a component of one of the redundant but essential genetic pathways for the entry to and progression through meiosis. Taken together, CPEB appears to have a conserved role in the early phase of meiosis and in the sperm/oocyte specification, in addition to its reported function during meiotic progression.

Asymmetric and symmetric stem-cell divisions in development and cancer

Much has been made of the idea that asymmetric cell division is a defining characteristic of stem cells that enables them to simultaneously perpetuate themselves (self-renew) and generate differentiated progeny. Yet many stem cells can divide symmetrically, particularly when they are expanding in number during development or after injury. Thus, asymmetric division is not necessary for stem-cell identity but rather is a tool that stem cells can use to maintain appropriate numbers of progeny. The facultative use of symmetric or asymmetric divisions by stem cells may be a key adaptation that is crucial for adult regenerative capacity.

Stress-induced germ cell apoptosis by a p53 independent pathway in Caenorhabditis elegans

In Caenorhabditis elegans, several distinct apoptosis pathways have been characterized in the germline. The physiological pathway is though to eliminate excess germ cells during oogenesis to maintain gonad homeostasis and it is activated by unknown mechanisms. The DNA damage-induced germ cell apoptosis occurs in response to genotoxic agents and involves the proteins EGL-1 and CED-13, and the DNA damage response protein p53. Germ cell apoptosis can also be induced in response to pathogen infection through an EGL-1 dependent pathway. To gain insight into the mechanism and functions of germ cell apoptosis, we investigated whether and how other forms of stress induce this cell death. We found that oxidative, osmotic, heat shock and starvation stresses induce germ cell apoptosis through a p53 and EGL-1 independent pathway. We also learned that the MAPK kinases MEK-1 and SEK-1, and the p53 antagonist protein ABL-1, are essential for stress-induced germ cell apoptosis. We conclude that in C. elegans responses to various stresses that do not involve genotoxicity include an increase in germ cell apoptosis through the physiological pathway.

Cellular analyses of the mitotic region in the Caenorhabditis elegans adult germ line

The Caenorhabditis elegans germ line provides a model for understanding how signaling from a stem cell niche promotes continued mitotic divisions at the expense of differentiation. Here we report cellular analyses designed to identify germline stem cells within the germline mitotic region of adult hermaphrodites. Our results support several conclusions. First, all germ cells within the mitotic region are actively cycling, as visualized by bromodeoxyuridine (BrdU) labeling. No quiescent cells were found. Second, germ cells in the mitotic region lose BrdU label uniformly, either by movement of labeled cells into the meiotic region or by dilution, probably due to replication. No label-retaining cells were found in the mitotic region. Third, the distal tip cell niche extends processes that nearly encircle adjacent germ cells, a phenomenon that is likely to anchor the distal-most germ cells within the niche. Fourth, germline mitoses are not oriented reproducibly, even within the immediate confines of the niche. We propose that germ cells in the distal-most rows of the mitotic region serve as stem cells and more proximal germ cells embark on the path to differentiation. We also propose that C. elegans adult germline stem cells are maintained by proximity to the niche rather than by programmed asymmetric divisions.

Requirement of fission yeast Cid14 in polyadenylation of rRNAs

Polyadenylation in eukaryotes is conventionally associated with increased nuclear export, translation, and stability of mRNAs. In contrast, recent studies suggest that the Trf4 and Trf5 proteins, members of a widespread family of noncanonical poly(A) polymerases, share an essential function in Saccharomyces cerevisiae that involves polyadenylation of nuclear RNAs as part of a pathway of exosome-mediated RNA turnover. Substrates for this pathway include aberrantly modified tRNAs and precursors of snoRNAs and rRNAs. Here we show that Cid14 is a Trf4/5 functional homolog in the distantly related fission yeast Schizosaccharomyces pombe. Unlike trf4 trf5 double mutants, cells lacking Cid14 are viable, though they suffer an increased frequency of chromosome missegregation. The Cid14 protein is constitutively nucleolar and is required for normal nucleolar structure. A minor population of polyadenylated rRNAs was identified. These RNAs accumulated in an exosome mutant, and their presence was largely dependent on Cid14, in line with a role for Cid14 in rRNA degradation. Surprisingly, both fully processed 25S rRNA and rRNA processing intermediates appear to be channeled into this pathway. Our data suggest that additional substrates may include the mRNAs of genes involved in meiotic regulation. Polyadenylation-assisted nuclear RNA turnover is therefore likely to be a common eukaryotic mechanism affecting diverse biological processes.

Quantitative analysis of germline mitosis in adult C. elegans

Certain aspects of the distal gonad of C. elegans are comparable to niche/stem cell systems in other organisms. The distal tip cell (DTC) caps a blind-ended tube; only the distal germ cells maintain proliferation in response to signaling from the DTC via the GLP-1/Notch signaling pathway in the germ line. Fruitful comparison between this system and other stem cell systems is limited by a lack of basic information regarding germ cell division behavior in C. elegans. Here, we explore the spatial pattern of cell division frequency in the adult C. elegans germ line relative to distance from the distal tip. We mapped the positions of actively dividing germline nuclei in over 600 fixed gonad preparations including the wild type and a gain-of-function ligand-responsive GLP-1 receptor mutant with an extended mitotic zone. One particularly surprising observation from these data is that the frequency of cell divisions is lower in distal-most cells-cells that directly contact the distal tip cell body-relative to cells further proximal, a difference that persists in the gain-of-function GLP-1 mutant. These results suggest that cell division frequency in the distal-most cells may be suppressed or otherwise controlled in a complex manner. Further, our data suggest that the presence of an active cell division influences the probability of observing simultaneous cell divisions in the same gonad arm, and that simultaneous divisions tend to cluster spatially. We speculate that this system behaves similarly to niche/stem cell/transit amplifying cell systems in other organisms.

Translational regulators maintain totipotency in the Caenorhabditis elegans germline

The molecular mechanisms that maintain totipotency of the germline are not well understood. Here, we show that two conserved translational regulators, MEX-3 and GLD-1, are essential for maintaining totipotency in the Caenorhabditis elegans germline. In mex-3 gld-1 mutants, germ cells transdifferentiate into various somatic cell types such as muscles or neurons. Our findings implicate RNA regulation in the maintenance of totipotency, suggest that multiple mechanisms maintain totipotency at different stages of germline development, and establish a genetically tractable model for studying the development of teratomas.

Molecular mechanisms of germline stem cell regulation

Germline stem cells (GSCs), which can self-renew and generate differentiated progeny, are unique stem cells in that they are solely dedicated to reproduction and transmit genetic information from generation to generation. Through the use of genetic techniques in Drosophila, Caenorhabditis elegans, and mouse, exciting progress has been made in understanding molecular mechanisms underlying interactions between stem cells and niches. The knowledge gained from studying GSCs has provided an intellectual framework for defining niches and molecular regulatory mechanisms for other adult stem cells. In this review, we summarize recent progress and discuss conserved mechanisms underlying GSC self-renewal and differentiation by comparing three GSC systems. Because GSCs and other adult stem cells share "stemness," we hope this review will help define fundamental principles of stem cell regulation and provide further guidance for future studies of other adult stem cells.

C. elegans cell cycles: invariance and stem cell divisions

The adult Caenorhabditis elegans nematode, a small roundworm, has a precisely defined number of somatic cells that create organs that are also found in larger animals, including intestine, muscles, skin, an excretory system and a primitive brain. Every cell has a defined role in this sophisticated, but tiny animal. Therefore, stringent control of the cell cycle is required to produce the almost invariant cell lineage that generates the C. elegans somatic body plan. The proliferation of germ cells is regulated differently, and occurs within a stem cell niche.

Possible role of mouse poly(A) polymerase mGLD-2 during oocyte maturation

Cytoplasmic polyadenylation of mRNAs is involved in post-transcriptional regulation of genes, including translational activation. In addition to yeast Cid1 and Cid13 and mouse TPAP, GLD-2 has been recently identified as a cytoplasmic poly(A) polymerase in Caenorhabditis elegans and Xenopus oocytes. In this study, we have characterized mouse GLD-2, mGLD-2, in adult tissues, meiotically maturing oocytes, and NIH3T3 cultured cells. mGLD-2 was ubiquitously present in all tissues and cells tested. mGLD-2 was localized in the nucleus as well as in the cytoplasm of somatic, testicular, and cultured cells. Transfection of expression plasmids encoding mGLD-2 and the mutant proteins into NIH3T3 cells revealed that a 17-residue sequence in the N-terminal region of mGLD-2 probably acts as a localization signal required for the transport into the nucleus. Analysis of reverse transcriptase-polymerase chain reaction indicated the presence of mGLD-2 mRNA in the oocytes throughout meiotic maturation. However, 54-kDa mGLD-2 was found in the oocytes only at the metaphases I and II after germinal vesicle breakdown, presumably due to translational control. When mGLD-2 synthesis was artificially inhibited and enhanced by injection of double-stranded and polyadenylated RNAs into the germinal vesicle-stage oocytes, respectively, oocyte maturation was significantly arrested at the metaphase-I stage. These results suggest that mGLD-2 may act in the ooplasm on the progression of metaphase I to metaphase II during oocyte maturation.

Cyclin D regulation of a sexually dimorphic asymmetric cell division

The C. elegans somatic gonadal precursor cell (SGP) divides asymmetrically to establish gonad-specific coordinates in both sexes. In addition, the SGP division is sexually dimorphic and initiates sex-specific programs of gonadogenesis. Wnt/MAPK signaling determines the gonadal axes, and the FKH-6 transcription factor specifies the male mode of SGP division. In this paper, we demonstrate that C. elegans cyclin D controls POP-1/TCF asymmetry in the SGP daughters as well as fkh-6 and rnr expression in the SGPs. Although cyclin D mutants have delayed SGP divisions, the cyclin D defects are not mimicked by other methods of retarding the SGP division. We find that EFL-1/E2F has an antagonistic effect on fkh-6 expression and gonadogenesis, which is relieved by cyclin D activity. We propose that cyclin D and other canonical regulators of the G1/S transition coordinate key regulators of axis formation and sex determination with cell cycle progression to achieve the sexually dimorphic SGP asymmetric division.

Requirement for P granules and meiosis for accumulation of the germline RNA helicase CGH-1

In Caenorhabditis elegans, lack of the conserved germline RNA helicase CGH-1 causes infertility and excessive levels of physiological germline apoptosis, a process that normally claims about half of all developing oocytes. In yeast the CGH-1 ortholog is a key component of degradative "processing (P) bodies," which may share some properties with germline protein-RNA complexes such as P granules. During oogenesis CGH-1 associates with P granules, but also accumulates to high levels in additional cytoplasmic particles. Here we show that appropriate levels and localization of CGH-1 depends on some P granule components and on mechanisms that establish meiotic development. At the same time, germ cell death is not increased by various abnormalities in P granules or meiosis. We conclude that in developing oocytes CGH-1 particles accumulate specifically in response to meiotic development and have distinct functions from P granules, and may be dynamic protein-mRNA structures.