A gain-of-function mutation in oma-1, a C. elegans gene required for oocyte maturation, results in delayed degradation of maternal proteins and embryonic lethality

Specific sensory neurons and insulin-like peptides modulate food type-dependent oogenesis and fertilization in Caenorhabditis elegans

An animal's responses to environmental cues are critical for its reproductive program. Thus, a mechanism that allows the animal to sense and adjust to its environment should make for a more efficient reproductive physiology. Here, we demonstrate that in Caenorhabditis elegans specific sensory neurons influence onset of oogenesis through insulin signaling in response to food-derived cues. The chemosensory neurons ASJ modulate oogenesis onset through the insulin-like peptide (ILP) INS-6. In contrast, other sensory neurons, the olfactory neurons AWA, regulate food type-dependent differences in C. elegans fertilization rates, but not onset of oogenesis. AWA modulates fertilization rates at least partly in parallel to insulin receptor signaling, since the insulin receptor DAF-2 regulates fertilization independently of food type, which requires ILPs other than INS-6. Together our findings suggest that optimal reproduction requires the integration of diverse food-derived inputs through multiple neuronal signals acting on the C. elegans germline.

Nucleus-independent transgenerational small RNA inheritance in Caenorhabditis elegans

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

The APC/CFZY-1/Cdc20 Complex Coordinates With OMA-1 to Regulate the Oocyte-to-Embryo Transition in Caenorhabditis elegans

During oocyte maturation and the oocyte-to-embryo transition, key developmental regulators such as RNA-binding proteins coordinate translation of particular messenger RNA (mRNAs) and related developmental processes by binding to their cognate maternal mRNAs. In the nematode Caenorhabditis elegans, these processes are regulated by a set of CCCH zinc finger proteins. Oocyte maturation defective-1 (OMA-1) and OMA-2 are two functionally redundant CCCH zinc finger proteins that turnover rapidly during the first embryonic cell division. These turnovers are required for proper transition from oogenesis to embryogenesis. A gain-of-function mutant of OMA-1, oma-1(zu405), stabilizes and delays degradation of OMA-1, resulting in delayed turnover and mis-segregation of other cell fate determinants, which eventually causes embryonic lethality. We performed a large-scale forward genetic screen to identify suppressors of the oma-1(zu405) mutant. We show here that multiple alleles affecting functions of various anaphase promoting complex/cyclosome (APC/C) subunits, including MAT-1, MAT-2, MAT-3, EMB-30, and FZY-1, suppress the gain-of-function mutant of OMA-1. Transcriptome analysis suggested that overall transcription in early embryos occurred after introducing mutations in APC/C genes into the oma-1(zu405) mutant. Mutations in APC/C genes prevent OMA-1 enrichment in P granules and correct delayed degradation of downstream cell fate determinants including pharynx and intestine in excess-1 (PIE-1), posterior segregation-1 (POS-1), muscle excess-3 (MEX-3), and maternal effect germ-cell defective-1 (MEG-1). We demonstrated that only the activator FZY-1, but not FZR-1, is incorporated in the APC/C complex to regulate the oocyte-to-embryo transition. Our findings suggested a genetic relationship linking the APC/C complex and OMA-1, and support a model in which the APC/C complex promotes P granule accumulation and modifies RNA binding of OMA-1 to regulate the oocyte-to-embryo transition process.

piRNAs coordinate poly(UG) tailing to prevent aberrant and perpetual gene silencing

Noncoding RNAs have emerged as mediators of transgenerational epigenetic inheritance (TEI) in a number of organisms. A robust example of such RNA-directed TEI is the inheritance of gene-silencing states following RNA interference (RNAi) in the metazoan C. elegans. During RNAi inheritance, gene silencing is transmitted by a self-perpetuating cascade of siRNA-directed poly(UG) tailing of mRNA fragments (pUGylation), followed by siRNA synthesis from poly(UG)-tailed mRNA templates (termed pUG RNA/siRNA cycling). Despite the self-perpetuating nature of pUG RNA/siRNA cycling, RNAi inheritance is finite, suggesting that systems likely exist to prevent indefinite RNAi-triggered gene silencing. Here we show that, in the absence of Piwi-interacting RNAs (piRNAs), an animal-specific class of small noncoding RNA, RNAi-based gene silencing can become essentially permanent, lasting at near 100% penetrance for more than 5 years and hundreds of generations. This perpetual gene silencing is mediated by continuous pUG RNA/siRNA cycling. Further, we find that piRNAs coordinate endogenous RNAi pathways to prevent germline-expressed genes, which are not normally subjected to TEI, from entering a state of continual and irreversible epigenetic silencing also mediated by continuous maintenance of pUG RNA/siRNA cycling. Together, our results show that one function of C. elegans piRNAs is to insulate germline-expressed genes from aberrant and runaway inactivation by the pUG RNA/siRNA epigenetic inheritance system.

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

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

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

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

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

The molecular underpinnings of fertility: Genetic approaches in Caenorhabditis elegans

The study of mutations that impact fertility has a catch-22. Fertility mutants are often lost since they cannot simply be propagated and maintained. This has hindered progress in understanding the genetics of fertility. In mice, several molecules are found to be required for the interactions between the sperm and egg, with JUNO and IZUMO1 being the only known receptor pair on the egg and sperm surface, respectively. In Caenorhabditis elegans, a total of 12 proteins on the sperm or oocyte have been identified to mediate gamete interactions. Majority of these genes were identified through mutants isolated from genetic screens. In this review, we summarize the several key screening strategies that led to the identification of fertility mutants in C. elegans and provide a perspective about future research using genetic approaches. Recently, advancements in new technologies such as high-throughput sequencing and Crispr-based genome editing tools have accelerated the molecular, cell biological, and mechanistic analysis of fertility genes. We review how these valuable tools advance our understanding of the molecular underpinnings of fertilization. We draw parallels of the molecular mechanisms of fertilization between worms and mammals and argue that our work in C. elegans complements fertility research in humans and other species.

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

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

Multiple functions of DYRK2 in cancer and tissue development

Dual-specificity tyrosine-regulated kinases (DYRKs) are evolutionarily conserved from yeast to mammals. Accumulating studies have revealed that DYRKs have important roles in regulation of the cell cycle and survival. DYRK2, a member of the class II DYRK family protein, is a key regulator of p53, and phosphorylates it at Ser46 to induce apoptosis in response to DNA damage. Moreover, recent studies have uncovered that DYRK2 regulates G1/S transition, epithelial-mesenchymal-transition, and stemness in human cancer cells. DYRK2 also appears to have roles in tissue development in lower eukaryotes. Thus, the elucidation of mechanisms for DYRK2 during mammalian tissue development will promote the understanding of cell differentiation, tissue homeostasis, and congenital diseases as well as cancer. In this review, we discuss the roles of DYRK2 in tumor cells. Moreover, we focus on DYRK2-dependent developmental mechanisms in several species including fly (Drosophila), worm (Caenorhabditis elegans), zebrafish (Danio rerio), and mammals.

Identifying candidate genes associated with sperm morphology abnormalities using weighted single-step GWAS in a Duroc boar population

Artificial insemination (AI) has been used as a routine technology globally in the pig production industry since 1930. One of the preferable advantages of AI technology is that the semen of elite boars can be disseminated to the commercial sow population rapidly. Understanding the genetic background of semen traits may help in developing genetic improvement programs of boars by including these traits into the selection index. In this study, we utilized weighted single-step genome-wide association study (wssGWAS) to identify genetic regions and further candidate genes associated with sperm morphology abnormalities (proximal droplet, distal droplet, bent tail, coiled tail, and distal midpiece reflex) in a Duroc boar population. Several genomic regions explained 2.76%-9.22% of the genetic variances for sperm morphology abnormalities were identified. The first three detected QTL regions together explained about 7.65%-25.10% of the total genetic variances of the studied traits. Several genes were detected and considered as candidate genes for each of the traits under study: coiled tail, HOOK1, ARSA, SYCE3, SOD3, GMNN, RBPJ, STIL, and FGF1; bent tail, FGF1, ADIPOR1, ARPC5, FGFR3, PANX1, IZUMO1R, ANKRD49, and GAL; proximal droplet, NSF, WNT3, WNT9B, LYZL6, FGFR1OP, RNASET2, FYN, LRRC6, EPC1, DICER1, FNDC3A, and PFN1; distal droplet, ARSA, SYCE3, MOV10L1, CBR1, KDM6B, TP53, PTBP2, UBR7, KIF18A, ADAM15, FAAH, TEKT3, and SRD5A1; and distal midpiece reflex, OMA1, PFN1, PELP1, BMP2, GPR18, TM9SF2, and SPIN1. GO and KEGG enrichment analysis revealed the potential function of the identified candidate genes in spermatogenesis, testis functioning, and boar spermatozoa plasma membrane activating and maintenance. In conclusion, we detected candidate genes associated with the coiled tail, bent tail, proximal droplet, distal droplet, and distal midpiece reflex in a Duroc boar population using wssGWAS. Overall, these novel results reflect the polygenic genetic architecture of the studied sperm morphology abnormality traits, which may provide knowledge for conducting genomic selection on these traits. The detected genetic regions can be used in developing trait-specific marker assisted selection models by assigning higher genetic variances to these regions.

Multi-modal regulation of C. elegans hermaphrodite spermatogenesis by the GLD-1-FOG-2 complex

Proper germ cell sex determination in Caenorhabditis nematodes requires a network of RNA-binding proteins (RBPs) and their target mRNAs. In some species, changes in this network enabled limited XX spermatogenesis, and thus self-fertility. In C. elegans, one of these selfing species, the global sex-determining gene tra-2 is regulated in germ cells by a conserved RBP, GLD-1, via the 3' untranslated region (3'UTR) of its transcript. A C. elegans-specific GLD-1 cofactor, FOG-2, is also required for hermaphrodite sperm fate, but how it modifies GLD-1 function is unknown. Germline feminization in gld-1 and fog-2 null mutants has been interpreted as due to cell-autonomous elevation of TRA-2 translation. Consistent with the proposed role of FOG-2 in translational control, the abundance of nearly all GLD-1 target mRNAs (including tra-2) is unchanged in fog-2 mutants. Epitope tagging reveals abundant TRA-2 expression in somatic tissues, but an undetectably low level in wild-type germ cells. Loss of gld-1 function elevates germline TRA-2 expression to detectable levels, but loss of fog-2 function does not. A simple quantitative model of tra-2 activity constrained by these results can successfully sort genotypes into normal or feminized groups. Surprisingly, fog-2 and gld-1 activity enable the sperm fate even when GLD-1 cannot bind to the tra-2 3' UTR. This suggests the GLD-1-FOG-2 complex regulates uncharacterized sites within tra-2, or other mRNA targets. Finally, we quantify the RNA-binding capacities of dominant missense alleles of GLD-1 that act genetically as "hyper-repressors" of tra-2 activity. These variants bind RNA more weakly in vitro than does wild-type GLD-1. These results indicate that gld-1 and fog-2 regulate germline sex via multiple interactions, and that our understanding of the control and evolution of germ cell sex determination in the C. elegans hermaphrodite is far from complete.

Transgenerational Epigenetic Inheritance Is Negatively Regulated by the HERI-1 Chromodomain Protein

Transgenerational epigenetic inheritance (TEI) is the inheritance of epigenetic information for two or more generations. In most cases, TEI is limited to a small number of generations (two to three). The short-term nature of TEI could be set by innate biochemical limitations to TEI or by genetically encoded systems that actively limit TEI. In Caenorhabditis elegans, double-stranded RNA (dsRNA)-mediated gene silencing [RNAi (RNA interference)] can be inherited (termed RNAi inheritance or RNA-directed TEI). To identify systems that might actively limit RNA-directed TEI, we conducted a forward genetic screen for factors whose mutation enhanced RNAi inheritance. This screen identified the gene heritable enhancer of RNAi (heri-1), whose mutation causes RNAi inheritance to last longer (> 20 generations) than normal. heri-1 encodes a protein with a chromodomain, and a kinase homology domain that is expressed in germ cells and localizes to nuclei. In C. elegans, a nuclear branch of the RNAi pathway [termed the nuclear RNAi or NRDE (nuclear RNA defective) pathway] promotes RNAi inheritance. We find that heri-1(-) animals have defects in spermatogenesis that are suppressible by mutations in the nuclear RNAi Argonaute (Ago) HRDE-1, suggesting that HERI-1 might normally act in sperm progenitor cells to limit nuclear RNAi and/or RNAi inheritance. Consistent with this idea, we find that the NRDE nuclear RNAi pathway is hyperresponsive to experimental RNAi treatments in heri-1 mutant animals. Interestingly, HERI-1 binds to genes targeted by RNAi, suggesting that HERI-1 may have a direct role in limiting nuclear RNAi and, therefore, RNAi inheritance. Finally, the recruitment of HERI-1 to chromatin depends upon the same factors that drive cotranscriptional gene silencing, suggesting that the generational perdurance of RNAi inheritance in C. elegans may be set by competing pro- and antisilencing outputs of the nuclear RNAi machinery.

C. elegans Heterochromatin Factor SET-32 Plays an Essential Role in Transgenerational Establishment of Nuclear RNAi-Mediated Epigenetic Silencing

The dynamic process by which nuclear RNAi engages a transcriptionally active target, before the repressive state is stably established, remains largely a mystery. Here, we found that the onset of exogenous dsRNA-induced nuclear RNAi in C. elegans is a transgenerational process, and it requires a putative histone methyltransferase (HMT), SET-32. By developing a CRISPR-based genetic approach, we found that silencing establishment at the endogenous targets of germline nuclear RNAi also requires SET-32. Although SET-32 and two H3K9 HMTs, MET-2 and SET-25, are dispensable for the maintenance of silencing, they do contribute to transcriptional repression in mutants that lack the germline nuclear Argonaute protein HRDE-1, suggesting a conditional role of heterochromatin in the maintenance phase. Our study indicates that (1) establishment and maintenance of siRNA-guided transcriptional repression are two distinct processes with different genetic requirements and (2) the rate-limiting step of the establishment phase is a transgenerational, chromatin-based process.

Caenorhabditis elegans DLC-1 associates with ribonucleoprotein complexes to promote mRNA regulation

Ribonucleoprotein complexes, which contain mRNAs and their regulator proteins, carry out post-transcriptional control of gene expression. The function of many RNA-binding proteins depends on their association with cofactors. Here, we use a genomic approach to identify transcripts associated with DLC-1, a protein previously identified as a cofactor of two unrelated RNA-binding proteins that act in the Caenorhabditis elegans germline. Among the 2732 potential DLC-1 targets, most are germline mRNAs associated with oogenesis. Removal of DLC-1 affects expression of its targets expressed in the oocytes, meg-1 and meg-3. We propose that DLC-1 acts as a cofactor for multiple ribonucleoprotein complexes, including the ones regulating gene expression during oogenesis.

Multiple Mechanisms Inactivate the LIN-41 RNA-Binding Protein To Ensure a Robust Oocyte-to-Embryo Transition in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, the conserved LIN-41 RNA-binding protein is a translational repressor that coordinately controls oocyte growth and meiotic maturation. LIN-41 exerts these effects, at least in part, by preventing the premature activation of the cyclin-dependent kinase CDK-1 Here we investigate the mechanism by which LIN-41 is rapidly eliminated upon the onset of meiotic maturation. Elimination of LIN-41 requires the activities of CDK-1 and multiple SCF (Skp1, Cul1, and F-box protein)-type E3 ubiquitin ligase subunits, including the conserved substrate adaptor protein SEL-10/Fbw7/Cdc4, suggesting that LIN-41 is a target of ubiquitin-mediated protein degradation. Within the LIN-41 protein, two nonoverlapping regions, Deg-A and Deg-B, are individually necessary for LIN-41 degradation; both contain several potential phosphodegron sequences, and at least one of these sequences is required for LIN-41 degradation. Finally, Deg-A and Deg-B are sufficient, in combination, to mediate SEL-10-dependent degradation when transplanted into a different oocyte protein. Although LIN-41 is a potent inhibitor of protein translation and M phase entry, the failure to eliminate LIN-41 from early embryos does not result in the continued translational repression of LIN-41 oocyte messenger RNA targets. Based on these observations, we propose a model for the elimination of LIN-41 by the SEL-10 E3 ubiquitin ligase and suggest that LIN-41 is inactivated before it is degraded. Furthermore, we provide evidence that another RNA-binding protein, the GLD-1 tumor suppressor, is regulated similarly. Redundant mechanisms to extinguish translational repression by RNA-binding proteins may both control and provide robustness to irreversible developmental transitions, including meiotic maturation and the oocyte-to-embryo transition.

Spatiotemporal regulation of liquid-like condensates in epigenetic inheritance

Non-membrane bound organelles such as nucleoli, processing bodies, cajal bodies, and germ granules form via spontaneous self-assembly of specific proteins and RNAs. How these biomolecular condensates form and interact are poorly understood. Here we identify two proteins, ZNFX-1 and WAGO-4, that localize to C. elegans germ granules (P granules) in early germline blastomeres. Later in germline development, ZNFX-1/WAGO-4 separate from P granules to define an independent liquid-like condensate that we term the Z granule. In adult germ cells, Z granules assemble into ordered tri-condensate assemblages with P granules and Mutator foci, which we term the PZM granule. Finally, we show that one biological function of ZNFX-1 and WAGO-4 is to interact with silencing RNAs in the C. elegans germline to direct transgenerational epigenetic inheritance (TEI). We speculate that the temporal and spatial ordering of liquid droplet organelles may help cells organize and coordinate the complex RNA processing pathways underlying gene regulatory systems, such as RNA-directed TEI.

The Coding Regions of Germline mRNAs Confer Sensitivity to Argonaute Regulation in C. elegans

Protein-coding genes undergo a wide array of regulatory interactions with factors that engage non-coding regions. Open reading frames (ORFs), in contrast, are thought to be constrained by coding function, precluding a major role in gene regulation. Here, we explore Piwi-interacting (pi)RNA-mediated transgene silencing in C. elegans and show that marked differences in the sensitivity to piRNA silencing map to the endogenous sequences within transgene ORFs. Artificially increasing piRNA targeting within the ORF of a resistant transgene can lead to a partial yet stable reduction in expression, revealing that piRNAs not only silence but can also “tune” gene expression. Our findings support a model that involves a temporal element to mRNA regulation by germline Argonautes, likely prior to translation, and suggest that piRNAs afford incremental control of germline mRNA expression by targeting the body of the mRNA, including the coding region.

Intergenerational Transmission of Gene Regulatory Information in Caenorhabditis elegans

Epigenetic mechanisms can stably maintain gene expression states even after the initiating conditions have changed. Often epigenetic information is transmitted only to daughter cells, but evidence is emerging, in both vertebrate and invertebrate systems, for transgenerational epigenetic inheritance (TEI), the transmission of epigenetic gene regulatory information across generations. Each new description of TEI helps uncover the properties, molecular mechanisms and biological roles for TEI. The nematode Caenorhabditis elegans has been particularly instrumental in the effort to understand TEI, as multiple environmental and genetic triggers can initiate an epigenetic signal that can alter the expression of both transgenes and endogenous loci. Here, we review recent studies of TEI in C. elegans.

Efficient generation of transgenic reporter strains and analysis of expression patterns in Caenorhabditis elegans using library MosSCI

BACKGROUND

In C. elegans, germline development and early embryogenesis rely on posttranscriptional regulation of maternally transcribed mRNAs. In many cases, the 3' untranslated region (UTR) is sufficient to govern the expression patterns of these transcripts. Several RNA-binding proteins are required to regulate maternal mRNAs through the 3'UTR. Despite intensive efforts to map RNA-binding protein-mRNA interactions in vivo, the biological impact of most binding events remains unknown. Reporter studies using single copy integrated transgenes are essential to evaluate the functional consequences of interactions between RNA-binding proteins and their associated mRNAs.

RESULTS

In this report, we present an efficient method of generating reporter strains with improved throughput by using a library variant of MosSCI transgenesis. Furthermore, using RNA interference, we identify the suite of RNA-binding proteins that control the expression pattern of five different maternal mRNAs.

CONCLUSIONS

The results provide a generalizable and efficient strategy to assess the functional relevance of protein-RNA interactions in vivo, and reveal new regulatory connections between key RNA-binding proteins and their maternal mRNA targets. Developmental Dynamics 245:925-936, 2016. © 2016 Wiley Periodicals, Inc.

Genomic Analyses of Sperm Fate Regulator Targets Reveal a Common Set of Oogenic mRNAs in Caenorhabditis elegans

Germ cell specification as sperm or oocyte is an ancient cell fate decision, but its molecular regulation is poorly understood. In Caenorhabditis elegans, the FOG-1 and FOG-3 proteins behave genetically as terminal regulators of sperm fate specification. Both are homologous to well-established RNA regulators, suggesting that FOG-1 and FOG-3 specify the sperm fate post-transcriptionally. We predicted that FOG-1 and FOG-3, as terminal regulators of the sperm fate, might regulate a battery of gamete-specific differentiation genes. Here we test that prediction by exploring on a genomic scale the messenger RNAs (mRNAs) associated with FOG-1 and FOG-3. Immunoprecipitation of the proteins and their associated mRNAs from spermatogenic germlines identifies 81 FOG-1 and 722 FOG-3 putative targets. Importantly, almost all FOG-1 targets are also FOG-3 targets, and these common targets are strongly biased for oogenic mRNAs. The discovery of common target mRNAs suggested that FOG-1 and FOG-3 work together. Consistent with that idea, we find that FOG-1 and FOG-3 proteins co-immunoprecipitate from both intact nematodes and mammalian tissue culture cells and that they colocalize in germ cells. Taking our results together, we propose a model in which FOG-1 and FOG-3 work in a complex to repress oogenic transcripts and thereby promote the sperm fate.

CDK-1 and two B-type cyclins promote PAR-6 stabilization during polarization of the early C. elegans embryo

In the C. elegans embryo, formation of an antero-posterior axis of polarity relies on signaling by the conserved PAR proteins, which localize asymmetrically in two mutually exclusive groups at the embryonic cortex. Depletion of any PAR protein causes a loss of polarity and embryonic lethality. A genome-wide RNAi screen previously identified two B-type cyclins, cyb-2.1 and cyb-2.2, as suppressors of par-2(it5ts) lethality. We found that the loss of cyb-2.1 or cyb-2.2 suppressed the lethality and polarity defects of par-2(it5ts) mutants and that these cyclins act in cell polarity with their cyclin-dependent kinase partner, CDK-1. Interestingly, cyb-2.1; cyb-2.2 double mutants did not show defects in cell cycle progression or timing of polarity establishment, suggesting that they regulate polarity independently of their typical role in cell cycle progression. Loss of both cyclin genes or of cdk-1 resulted in a decrease in PAR-6 levels in the embryo. Furthermore, the activity of the cullin CUL-2 was required to achieve suppression of par-2 lethality when both cyclins were absent. Our results support a model in which CYB-2.1/2/CDK-1 antagonize CUL-2 activity to promote stabilization of PAR-6 levels during polarization of the early C. elegans embryo. They also suggest that CYB-2.1 and CYB-2.2 contribute to the coupling of cell cycle progression and asymmetric segregation of cell fate determinants.

E3 ubiquitin ligases promote progression of differentiation during C. elegans embryogenesis

Regulated choice between cell fate maintenance and differentiation provides decision points in development to progress toward more restricted cell fates or to maintain the current one. Caenorhabditis elegans embryogenesis follows an invariant cell lineage where cell fate is generally more restricted upon each cell division. EMS is a progenitor cell in the four-cell embryo that gives rise to the endomesoderm. We recently found that when ubiquitin-mediated protein degradation is compromised, the anterior daughter of EMS, namely MS, reiterates the EMS fate. This observation demonstrates an essential function of ubiquitin-mediated protein degradation in driving the progression of EMS-to-MS differentiation. Here we report a genome-wide screen of the ubiquitin pathway and extensive lineage analyses. The results suggest a broad role of E3 ligases in driving differentiation progression. First, we identified three substrate-binding proteins for two Cullin-RING ubiquitin ligase (CRL) E3 complexes that promote the progression from the EMS fate to MS, namely LIN-23/β-TrCP and FBXB-3 for the CRL1/SCF complex and ZYG-11/ZYG-11B for the CRL2 complex. Genetic analyses suggest these E3 ligases function through a multifunctional protein OMA-1 and the endomesoderm lineage specifier SKN-1 to drive differentiation. Second, we found that depletion of components of the CRL1/SCF complex induces fate reiteration in all major founder cell lineages. These data suggest that regulated choice between self-renewal and differentiation is widespread during C. elegans embryogenesis as in organisms with regulative development, and ubiquitin-mediated protein degradation drives the choice towards differentiation. Finally, bioinformatic analysis of time series gene expression data showed that expression of E3 genes is transiently enriched during time windows of developmental stage transitions. Transcription factors show similar enrichment, but not other classes of regulatory genes. Based on these findings we propose that ubiquitin-mediated protein degradation, like many transcription factors, function broadly as regulators driving developmental progression during embryogenesis in C. elegans.

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.

Global characterization of the oocyte-to-embryo transition in Caenorhabditis elegans uncovers a novel mRNA clearance mechanism

The oocyte-to-embryo transition (OET) is thought to be mainly driven by post-transcriptional gene regulation. However, expression of both RNAs and proteins during the OET has not been comprehensively assayed. Furthermore, specific molecular mechanisms that regulate gene expression during OET are largely unknown. Here, we quantify and analyze transcriptome-wide, expression of mRNAs and thousands of proteins in Caenorhabditis elegans oocytes, 1-cell, and 2-cell embryos. This represents a first comprehensive gene expression atlas during the OET in animals. We discovered a first wave of degradation in which thousands of mRNAs are cleared shortly after fertilization. Sequence analysis revealed a statistically highly significant presence of a polyC motif in the 3' untranslated regions of most of these degraded mRNAs. Transgenic reporter assays demonstrated that this polyC motif is required and sufficient for mRNA degradation after fertilization. We show that orthologs of human polyC-binding protein specifically bind this motif. Our data suggest a mechanism in which the polyC motif and binding partners direct degradation of maternal mRNAs. Our data also indicate that endogenous siRNAs but not miRNAs promote mRNA clearance during the OET.

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.

Fertilization-induced K63-linked ubiquitylation mediates clearance of maternal membrane proteins

In Caenorhabditis elegans, fertilization triggers endocytosis and rapid turnover of maternal surface membrane proteins in lysosomes, although the precise mechanism of this inducible endocytosis is unknown. We found that high levels of K63-linked ubiquitin chains transiently accumulated on endosomes upon fertilization. Endocytosis and the endosomal accumulation of ubiquitin were both regulated downstream of the anaphase-promoting complex, which drives the oocyte's meiotic cell cycle after fertilization. The clearance of maternal membrane proteins and the accumulation of K63-linked ubiquitin on endosomes depended on UBC-13 and UEV-1, which function as an E2 complex that specifically mediates chain elongation of K63-linked polyubiquitin. CAV-1-GFP, an endocytic cargo protein, was modified with K63-linked polyubiquitin in a UBC-13/UEV-1-dependent manner. In ubc-13 or uev-1 mutants, CAV-1-GFP and other membrane proteins were internalized from the plasma membrane normally after fertilization. However, they were not efficiently targeted to the multivesicular body (MVB) pathway but recycled to the cell surface. Our results suggest that UBC-13-dependent K63-linked ubiquitylation is required for proper MVB sorting rather than for internalization. These results also demonstrate a developmentally controlled function of K63-linked ubiquitylation.

The C. elegans CSR-1 argonaute pathway counteracts epigenetic silencing to promote germline gene expression

Organisms can develop adaptive sequence-specific immunity by reexpressing pathogen-specific small RNAs that guide gene silencing. For example, the C. elegans PIWI-Argonaute/piwi-interacting RNA (piRNA) pathway recruits RNA-dependent RNA polymerase (RdRP) to foreign sequences to amplify a transgenerational small-RNA-induced epigenetic silencing signal (termed RNAe). Here, we provide evidence that, in addition to an adaptive memory of silenced sequences, C. elegans can also develop an opposing adaptive memory of expressed/self-mRNAs. We refer to this mechanism, which can prevent or reverse RNAe, as RNA-induced epigenetic gene activation (RNAa). We show that CSR-1, which engages RdRP-amplified small RNAs complementary to germline-expressed mRNAs, is required for RNAa. We show that a transgene with RNAa activity also exhibits accumulation of cognate CSR-1 small RNAs. Our findings suggest that C. elegans adaptively acquires and maintains a transgenerational CSR-1 memory that recognizes and protects self-mRNAs, allowing piRNAs to recognize foreign sequences innately, without the need for prior exposure

De novo inference of systems-level mechanistic models of development from live-imaging-based phenotype analysis

Elucidation of complex phenotypes for mechanistic insights presents a significant challenge in systems biology. We report a strategy to automatically infer mechanistic models of cell fate differentiation based on live-imaging data. We use cell lineage tracing and combinations of tissue-specific marker expression to assay progenitor cell fate and detect fate changes upon genetic perturbation. Based on the cellular phenotypes, we further construct a model for how fate differentiation progresses in progenitor cells and predict cell-specific gene modules and cell-to-cell signaling events that regulate the series of fate choices. We validate our approach in C. elegans embryogenesis by perturbing 20 genes in over 300 embryos. The result not only recapitulates current knowledge but also provides insights into gene function and regulated fate choice, including an unexpected self-renewal. Our study provides a powerful approach for automated and quantitative interpretation of complex in vivo information.

Regulation of maternal Wnt mRNA translation in C. elegans embryos

The restricted spatiotemporal translation of maternal mRNAs, which is crucial for correct cell fate specification in early C. elegans embryos, is regulated primarily through the 3'UTR. Although genetic screens have identified many maternally expressed cell fate-controlling RNA-binding proteins (RBPs), their in vivo targets and the mechanism(s) by which they regulate these targets are less clear. These RBPs are translated in oocytes and localize to one or a few blastomeres in a spatially and temporally dynamic fashion unique for each protein and each blastomere. Here, we characterize the translational regulation of maternally supplied mom-2 mRNA, which encodes a Wnt ligand essential for two separate cell-cell interactions in early embryos. A GFP reporter that includes only the mom-2 3'UTR is translationally repressed properly in oocytes and early embryos, and then correctly translated only in the known Wnt signaling cells. We show that the spatiotemporal translation pattern of this reporter is regulated combinatorially by a set of nine maternally supplied RBPs. These nine proteins all directly bind the mom-2 3'UTR in vitro and function as positive or negative regulators of mom-2 translation in vivo. The net translational readout for the mom-2 3'UTR reporter is determined by competitive binding between positive- and negative-acting RBPs for the 3'UTR, along with the distinct spatiotemporal localization patterns of these regulators. We propose that the 3'UTR of maternal mRNAs contains a combinatorial code that determines the topography of associated RBPs, integrating positive and negative translational inputs.

RNA recognition by the Caenorhabditis elegans oocyte maturation determinant OMA-1

Maternally supplied mRNAs encode proteins that pattern early embryos in many species. In the nematode Caenorhabditis elegans, a suite of RNA-binding proteins regulates expression of maternal mRNAs during oogenesis, the oocyte to embryo transition, and early embryogenesis. To understand how these RNA-binding proteins contribute to development, it is necessary to determine how they select specific mRNA targets for regulation. OMA-1 and OMA-2 are redundant proteins required for oocyte maturation--an essential part of meiosis that prepares oocytes for fertilization. Both proteins have CCCH type tandem zinc finger RNA-binding domains. Here, we define the RNA binding specificity of OMA-1 and demonstrate that OMA-1/2 are required to repress the expression of a glp-1 3'-UTR reporter in developing oocytes. OMA-1 binds with high affinity to a conserved region of the glp-1 3'-UTR previously shown to interact with POS-1 and GLD-1, RNA-binding proteins required for glp-1 reporter repression in the posterior of fertilized embryos. Our results reveal that OMA-1 is a sequence-specific RNA-binding protein required to repress expression of maternal transcripts during oogenesis and suggest that interplay between OMA-1 and other factors for overlapping binding sites helps to coordinate the transition from oocyte to embryo.

The oocyte-to-embryo transition

The oocyte-to-embryo transition refers to the process whereby a fully grown, relatively quiescent oocyte undergoes maturation, fertilization, and is converted into a developmentally active, mitotically dividing embryo, arguably one of the most dramatic transitions in biology. This transition occurs very rapidly in Caenorhabditis elegans, with fertilization of a new oocyte occurring every 23 min and the first mitotic division occurring 45 min later. Molecular events regulating this transition must be very precisely timed. This chapter reviews our current understanding of the coordinated temporal regulation of different events during this transition. We divide the oocyte-to-embryo transition into a number of component processes, which are coordinated primarily through the MBK-2 kinase, whose activation is intimately tied to completion of meiosis, and the OMA-1/OMA-2 proteins, whose expression and functions span multiple processes during this transition. The oocyte-to-embryo transition occurs in the absence of de novo transcription, and all the factors required for the process, whether mRNA or protein, are already present within the oocyte. Therefore, all regulation of this transition is posttranscriptional. The combination of asymmetric partitioning of maternal factors, protein modification-mediated functional switching, protein degradation, and highly regulated translational repression ensure a smooth oocyte-to-embryo transition. We will highlight protein degradation and translational repression, two posttranscriptional processes which play particularly critical roles in this transition.

piRNAs initiate an epigenetic memory of nonself RNA in the C. elegans germline

Organisms employ a fascinating array of strategies to silence invasive nucleic acids such as transposons and viruses. Although evidence exists for several pathways that detect foreign sequences, including pathways that sense copy number, unpaired DNA, or aberrant RNA (e.g., dsRNA), in many cases, the mechanisms used to distinguish "self" from "nonself" nucleic acids remain mysterious. Here, we describe an RNA-induced epigenetic silencing pathway that permanently silences single-copy transgenes. We show that the Piwi Argonaute PRG-1 and its genomically encoded piRNA cofactors initiate permanent silencing, and maintenance depends on chromatin factors and the WAGO Argonaute pathway. Our findings support a model in which PRG-1 scans for foreign sequences and two other Argonaute pathways serve as epigenetic memories of "self" and "nonself" RNAs. These findings suggest how organisms can utilize RNAi-related mechanisms to detect foreign sequences not by any molecular signature, but by comparing the foreign sequence to a memory of previous gene expression.

The NRF2-related interactome and regulome contain multifunctional proteins and fine-tuned autoregulatory loops

NRF2 is a well-known, master transcription factor (TF) of oxidative and xenobiotic stress responses. Recent studies uncovered an even wider regulatory role of NRF2 influencing carcinogenesis, inflammation and neurodegeneration. Prompted by these advances here we present a systems-level resource for NRF2 interactome and regulome that includes 289 protein-protein, 7469 TF-DNA and 85 miRNA interactions. As systems-level examples of NRF2-related signaling we identified regulatory loops of NRF2 interacting proteins (e.g., JNK1 and CBP) and a fine-tuned regulatory system, where 35 TFs regulated by NRF2 influence 63 miRNAs that down-regulate NRF2. The presented network and the uncovered regulatory loops may facilitate the development of efficient, NRF2-based therapeutic agents.

Multiple RNA-binding proteins function combinatorially to control the soma-restricted expression pattern of the E3 ligase subunit ZIF-1

In C. elegans embryos, transcriptional repression in germline blastomeres requires PIE-1 protein. Germline blastomere-specific localization of PIE-1 depends, in part, upon regulated degradation of PIE-1 in somatic cells. We and others have shown that the temporal and spatial regulation of PIE-1 degradation is controlled by translation of the substrate-binding subunit, ZIF-1, of an E3 ligase. We now show that ZIF-1 expression in embryos is regulated by five maternally-supplied RNA-binding proteins. POS-1, MEX-3, and SPN-4 function as repressors of ZIF-1 expression, whereas MEX-5 and MEX-6 antagonize this repression. All five proteins bind directly to the zif-1 3' UTR in vitro. We show that, in vivo, POS-1 and MEX-5/6 have antagonistic roles in ZIF-1 expression. In vitro, they bind to a common region of the zif-1 3' UTR, with MEX-5 binding impeding that by POS-1. The region of the zif-1 3' UTR bound by MEX-5/6 also partially overlaps with that bound by MEX-3, consistent with their antagonistic functions on ZIF-1 expression in vivo. Whereas both MEX-3 and SPN-4 repress ZIF-1 expression, neither protein alone appears to be sufficient, suggesting that they function together in ZIF-1 repression. We propose that MEX-3 and SPN-4 repress ZIF-1 expression exclusively in 1- and 2-cell embryos, the only period during embryogenesis when these two proteins co-localize. As the embryo divides, ZIF-1 continues to be repressed in germline blastomeres by POS-1, a germline blastomere-specific protein. MEX-5/6 antagonize repression by POS-1 and MEX-3, enabling ZIF-1 expression in somatic blastomeres. We propose that ZIF-1 expression results from a net summation of complex positive and negative translational regulation by 3' UTR-binding proteins, with expression in a specific blastomere dependent upon the precise combination of these proteins in that cell.

New insights into the mechanism of fertilization in nematodes

Fertilization results from the fusion of male and female gametes in all sexually reproducing organisms. Much of nematode fertility work was focused on Caenorhabditis elegans and Ascaris suum. The C. elegans hermaphrodite produces a limited number of sperm initially and then commits to the exclusive production of oocytes. The postmeiotic differentiation called spermiogenesis converts sessile spermatids into motile spermatozoa. The motility of spermatozoa depends on dynamic assembly and disassembly of a major sperm protein-based cytoskeleton uniquely found in nematodes. Both self-derived and male-derived spermatozoa are stored in spermatheca, the site of fertilization in hermaphrodites. The oocyte is arrested in meiotic prophase I until a sperm-derived signal relieves the inhibition allowing the meiotic maturation to occur. Oocyte undergoes meiotic maturation, enters into spermatheca, gets fertilized, completes meiosis, and exits into uterus as a zygote. This review focuses on our current understanding of the events around fertilization in nematodes.

The genetics and cell biology of fertilization

Although the general events surrounding fertilization in many species are well described, the molecular underpinnings of fertilization are still poorly understood. Caenorhabditis elegans has emerged as a powerful model system for addressing the molecular and cell biological mechanism of fertilization. A primary advantage is the ability to isolate and propagate mutants that effect gametes and no other cells. This chapter provides conceptual guidelines for the identification, maintenance, and experimental approaches for the study fertility mutants.

EGG molecules couple the oocyte-to-embryo transition with cell cycle progression

The oocyte-to-embryo transition is a precisely coordinated process in which an oocyte becomes fertilized and transitions to an embryonic program of events. The molecules involved in this process have not been well studied. Recently, a group of interacting molecules in C. elegans have been described as coordinating the oocyte-to-embryo transition with the advancement of the cell cycle. Genes egg-3, egg-4, and egg-5 represent a small class of regulatory molecules known as protein-tyrosine phosphase-like proteins, which can bind phosphorylated substrates and act as scaffolding molecules or inhibitors. These genes are responsible for coupling the movements and activities of regulatory kinase mbk-2 with advancement of the cell cycle during the oocyte-to-embryo transition.

zif-1 translational repression defines a second, mutually exclusive OMA function in germline transcriptional repression

Specification of primordial germ cells requires global repression of transcription. In C. elegans, primordial germ cells are generated through four rounds of asymmetric divisions, starting from the zygote P0, each producing a transcriptionally repressed germline blastomere (P1-P4). Repression in P2-P4 requires PIE-1, which is provided maternally in oocytes and segregated to all germline blastomeres. We have shown previously that OMA-1 and OMA-2 repress global transcription in P0 and P1 by sequestering TAF-4, an essential component of TFIID. Soon after the first mitotic cycle, OMA proteins undergo developmentally regulated degradation. Here, we show that OMA proteins also repress transcription in P2-P4 indirectly, through a completely different mechanism that operates in oocytes. OMA proteins bind to both the 3' UTR of the zif-1 transcript and the eIF4E-binding protein, SPN-2, repressing translation of zif-1 mRNA in oocytes. zif-1 encodes the substrate-binding subunit of the E3 ligase for PIE-1 degradation. Inhibition of zif-1 translation in oocytes ensures high PIE-1 levels in oocytes and germline blastomeres. The two OMA protein functions are strictly regulated in both space and time by MBK-2, a kinase activated following fertilization. Phosphorylation by MBK-2 facilitates the binding of OMA proteins to TAF-4 and simultaneously inactivates their function in repressing zif-1 translation. Phosphorylation of OMA proteins displaces SPN-2 from the zif-1 3' UTR, releasing translational repression. We propose that MBK-2 phosphorylation serves as a developmental switch, converting OMA proteins from specific translational repressors in oocytes to global transcriptional repressors in embryos, together effectively repressing transcription in all germline blastomeres.

Cell fate specification in the C. elegans embryo

Cell specification requires that particular subsets of cells adopt unique expression patterns that ultimately define the fates of their descendants. In C. elegans, cell fate specification involves the combinatorial action of multiple signals that produce activation of a small number of "blastomere specification" factors. These initiate expression of gene regulatory networks that drive development forward, leading to activation of "tissue specification" factors. In this review, the C. elegans embryo is considered as a model system for studies of cell specification. The techniques used to study cell fate in this species, and the themes that have emerged, are described.

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.

Roles of the Wnt effector POP-1/TCF in the C. elegans endomesoderm specification gene network

In C. elegans the 4-cell stage blastomere EMS is an endomesodermal precursor. Its anterior daughter, MS, makes primarily mesodermal cells, while its posterior daughter E generates the entire intestine. The gene regulatory network underlying specification of MS and E has been the subject of study for more than 15 years. A key component of the specification of the two cells is the involvement of the Wnt/beta-catenin asymmetry pathway, which through its nuclear effector POP-1, specifies MS and E as different from each other. Loss of pop-1 function results in the mis-specification of MS as an E-like cell, because POP-1 directly represses the end-1 and end-3 genes in MS, which would otherwise promote an endoderm fate. A long-standing question has been whether POP-1 plays a role in specifying MS fate beyond repression of endoderm fate. This question has been difficult to ask because the only chromosomal lesions that remove both end-1 and end-3 are large deletions removing hundreds of genes. Here, we report the construction of bona fide end-1 end-3 double mutants. In embryos lacking activity of end-1, end-3 and pop-1 together, we find that MS fate is partially restored, while E expresses early markers of MS fate and adopts characteristics of both MS and C. Our results suggest that POP-1 is not critical for MS specification beyond repression of endoderm specification, and reveal that Wnt-modified POP-1 and END-1/3 further reinforce E specification by repressing MS fate in E. By comparison, a previous work suggested that in the related nematode C. briggsae, Cb-POP-1 is not required to repress endoderm specification in MS, in direct contrast with Ce-POP-1, but is critical for repression of MS fate in E. The findings reported here shed new light on the flexibility of combinatorial control mechanisms in endomesoderm specification in Caenorhabditis.

An eIF4E-binding protein regulates katanin protein levels in C. elegans embryos

In Caenorhabditis elegans, the MEI-1-katanin microtubule-severing complex is required for meiosis, but must be down-regulated during the transition to embryogenesis to prevent defects in mitosis. A cullin-dependent degradation pathway for MEI-1 protein has been well documented. In this paper, we report that translational repression may also play a role in MEI-1 down-regulation. Reduction of spn-2 function results in spindle orientation defects due to ectopic MEI-1 expression during embryonic mitosis. MEL-26, which is both required for MEI-1 degradation and is itself a target of the cullin degradation pathway, is present at normal levels in spn-2 mutant embryos, suggesting that the degradation pathway is functional. Cloning of spn-2 reveals that it encodes an eIF4E-binding protein that localizes to the cytoplasm and to ribonucleoprotein particles called P granules. SPN-2 binds to the RNA-binding protein OMA-1, which in turn binds to the mei-1 3' untranslated region. Thus, our results suggest that SPN-2 functions as an eIF4E-binding protein to negatively regulate translation of mei-1.

Large-scale sorting of C. elegans embryos reveals the dynamics of small RNA expression

Caenorhabditis elegans is one of the most prominent model systems for embryogenesis, but collecting many precisely staged embryos has been impractical. Thus, early C. elegans embryogenesis has not been amenable to most high-throughput genomics or biochemistry assays. To overcome this problem, we devised a method to collect staged C. elegans embryos by fluorescence-activated cell sorting (eFACS). In a proof-of-principle experiment, we found that a single eFACS run routinely yielded tens of thousands of almost perfectly staged 1-cell stage embryos. As the earliest embryonic events are driven by posttranscriptional regulation, we combined eFACS with second-generation sequencing to profile the embryonic expression of small, noncoding RNAs. We discovered complex and orchestrated changes in the expression between and within almost all classes of small RNAs, including microRNAs and 26G-RNAs, during embryogenesis.

Transcription regulatory networks in Caenorhabditis elegans inferred through reverse-engineering of gene expression profiles constitute biological hypotheses for metazoan development

Differential gene expression governs the development, function and pathology of multicellular organisms. Transcription regulatory networks study differential gene expression at a systems level by mapping the interactions between regulatory proteins and target genes. While microarray transcription profiles are the most abundant data for gene expression, it remains challenging to correctly infer the underlying transcription regulatory networks. The reverse-engineering algorithm LeMoNe (learning module networks) uses gene expression profiles to extract ensemble transcription regulatory networks of coexpression modules and their prioritized regulators. Here we apply LeMoNe to a compendium of microarray studies of the worm Caenorhabditis elegans. We obtain 248 modules with a regulation program for 5020 genes and 426 regulators and a total of 24 012 predicted transcription regulatory interactions. Through GO enrichment analysis, comparison with the gene-gene association network WormNet and integration of other biological data, we show that LeMoNe identifies functionally coherent coexpression modules and prioritizes regulators that relate to similar biological processes as the module genes. Furthermore, we can predict new functional relationships for uncharacterized genes and regulators. Based on modules involved in molting, meiosis and oogenesis, ciliated sensory neurons and mitochondrial metabolism, we illustrate the value of LeMoNe as a biological hypothesis generator for differential gene expression in greater detail. In conclusion, through reverse-engineering of C. elegans expression data, we obtained transcription regulatory networks that can provide further insight into metazoan development.

The NK-2 class homeodomain factor CEH-51 and the T-box factor TBX-35 have overlapping function in C. elegans mesoderm development

The C. elegans MS blastomere, born at the 7-cell stage of embryogenesis, generates primarily mesodermal cell types, including pharynx cells, body muscles and coelomocytes. A presumptive null mutation in the T-box factor gene tbx-35, a target of the MED-1 and MED-2 divergent GATA factors, was previously found to result in a profound decrease in the production of MS-derived tissues, although the tbx-35(-) embryonic arrest phenotype was variable. We report here that the NK-2 class homeobox gene ceh-51 is a direct target of TBX-35 and at least one other factor, and that CEH-51 and TBX-35 share functions. Embryos homozygous for a ceh-51 null mutation arrest as larvae with pharynx and muscle defects, although these tissues appear to be specified correctly. Loss of tbx-35 and ceh-51 together results in a synergistic phenotype resembling loss of med-1 and med-2. Overexpression of ceh-51 causes embryonic arrest and generation of ectopic body muscle and coelomocytes. Our data show that TBX-35 and CEH-51 have overlapping function in MS lineage development. As T-box regulators and NK-2 homeodomain factors are both important for heart development in Drosophila and vertebrates, our results suggest that these regulators function in a similar manner in C. elegans to specify a major precursor of mesoderm.

Maternally encoded stem-loop-binding protein is degraded in 2-cell mouse embryos by the co-ordinated activity of two separately regulated pathways

Oocytes accumulate mRNAs and proteins that direct early embryonic development. Although subsequent development requires the timely degradation of these maternal products, little is known of the underlying mechanisms. The stem-loop-binding protein (SLBP), which regulates the stability and translation of mRNAs encoding histones and is synthesized during S-phase and degraded during G2 in somatic cells, accumulates during oogenesis. Maternal SLBP is required for mouse embryos to develop beyond the 2-cell stage, but must be degraded to allow the cell-cycle-regulated expression of somatic cells to be established. We report that the quantity of maternal SLBP changes little following fertilization until 44-52 h post-hCG, corresponding to mid-/late G2 of the 2-cell stage, when it decreases by 75%. Efficient degradation requires two pathways. The first requires activity of cyclin-dependent kinases (cdk) and embryonic transcription, preferentially targets nuclear SLBP, and likely corresponds to the pathway that degrades SLBP at G2 in somatic cells. The second does not require cdk activity or transcription and becomes active at 44-52 h post-hCG independently of cell-cycle progression to mid-/late G2, but is not solely regulated by the time elapsed since hCG injection. Thus, the co-ordinated activity of two separately regulated pathways eliminates maternally encoded SLBP from early mouse embryos.

Suppressors of the cdc-25.1(gf)-associated intestinal hyperplasia reveal important maternal roles for prp-8 and a subset of splicing factors in C. elegans

The maternal contribution of gene products enables embryos to initiate their developmental program in the absence of zygotic gene expression. In Caenorhabditis elegans, maternal CDC-25.1 levels are tightly regulated to promote early cell divisions, while stabilization of this phosphatase by gain-of-function mutations gives rise to intestinal-specific hyperplasia. To identify regulators of CDC-25.1 levels and/or function, we performed a modifier screen of the cdc-25.1(gf)-dependent hyperplasia. One of the isolated suppressor mutants possesses a donor splice site mutation in prp-8, a key splicing factor of the U5-specific snRNP. prp-8(rr40) produces aberrant prp-8 splice variants that generate C-terminal truncations at the expense of wild-type prp-8. Levels of maternal transcripts are reduced, including cdc-25.1, while zygotic transcripts appear unperturbed, suggesting a germ-line-specific role for this splicing factor in regulating the splicing, and consequently, the steady-state levels of maternal transcripts. Using a novel feeding RNAi strategy we found that only a subset of splicing factors suppress cdc-25.1(gf), suggesting that they too may play specific roles in germ-line spliceosome function. In humans, mutations in the corresponding hPrp8 C-terminal domain result in retinitis pigmentosa, a retinal-specific disorder. Intriguingly, despite affecting the general splicing apparatus, both human and C. elegans show tissue-specific defects resulting from mutations in this key splicing component. Our findings suggest that in addition to its important regulatory function in the C. elegans germ line, prp-8(rr40) may provide further insight into the etiology of this splicing-associated human disorder.