Developmental defects observed in hypomorphic anaphase-promoting complex mutants are linked to cell cycle abnormalities

C. elegans spermatocyte divisions show a weak spindle checkpoint response

Male meiotic division exhibits two consecutive chromosome separation events without apparent pausing. Several studies have shown that spermatocyte divisions are not stringently regulated as in mitotic cells. In this study, we investigated the role of the canonical spindle assembly (SAC) pathway in Caenorhabditis elegans spermatogenesis. We found the intensity of chromosome-associated outer kinetochore protein BUB-1 and SAC effector MDF-1 oscillates between the two divisions. However, the SAC target securin is degraded during the first division and remains undetectable for the second division. Inhibition of proteasome-dependent protein degradation did not affect the progression of the second division but stopped the first division at metaphase. Perturbation of spindle integrity did not affect the duration of meiosis II, and only slightly lengthened meiosis I. Our results demonstrate that male meiosis II is independent of SAC regulation, and male meiosis I exhibits only weak checkpoint response.

Securin Regulates the Spatiotemporal Dynamics of Separase

Separase is a key regulator of the metaphase to anaphase transition with multiple functions. Separase cleaves cohesin to allow chromosome segregation and localizes to vesicles to promote exocytosis in mid-anaphase. The anaphase promoting complex/cyclosome (APC/C) activates separase by ubiquitinating its inhibitory chaperone, securin, triggering its degradation. How this pathway controls the exocytic function of separase has not been investigated. During meiosis I, securin is degraded over several minutes, while separase rapidly relocalizes from kinetochore structures at the spindle and cortex to sites of action on chromosomes and vesicles at anaphase onset. The loss of cohesin coincides with the relocalization of separase to the chromosome midbivalent at anaphase onset. APC/C depletion prevents separase relocalization, while securin depletion causes precocious separase relocalization. Expression of non-degradable securin inhibits chromosome segregation, exocytosis, and separase localization to vesicles but not to the anaphase spindle. We conclude that APC/C mediated securin degradation controls separase localization. This spatiotemporal regulation will impact the effective local concentration of separase for more precise targeting of substrates in anaphase.

Ubiquitin ligases and a processive proteasome facilitate protein clearance during the oocyte-to-embryo transition in Caenorhabditis elegans

The ubiquitin-mediated degradation of oocyte translational regulatory proteins is a conserved feature of the oocyte-to-embryo transition. In the nematode Caenorhabditis elegans, multiple translational regulatory proteins, including the TRIM-NHL RNA-binding protein LIN-41/Trim71 and the Pumilio-family RNA-binding proteins PUF-3 and PUF-11, are degraded during the oocyte-to-embryo transition. Degradation of each protein requires activation of the M-phase cyclin-dependent kinase CDK-1, is largely complete by the end of the first meiotic division and does not require the anaphase-promoting complex. However, only LIN-41 degradation requires the F-box protein SEL-10/FBW7/Cdc4p, the substrate recognition subunit of an SCF-type E3 ubiquitin ligase. This finding suggests that PUF-3 and PUF-11, which localize to LIN-41-containing ribonucleoprotein particles, are independently degraded through the action of other factors and that the oocyte ribonucleoprotein particles are disassembled in a concerted fashion during the oocyte-to-embryo transition. We develop and test the hypothesis that PUF-3 and PUF-11 are targeted for degradation by the proteasome-associated HECT-type ubiquitin ligase ETC-1/UBE3C/Hul5, which is broadly expressed in C. elegans. We find that several GFP-tagged fusion proteins that are degraded during the oocyte-to-embryo transition, including fusions with PUF-3, PUF-11, LIN-41, IFY-1/Securin, and CYB-1/Cyclin B, are incompletely degraded when ETC-1 function is compromised. However, it is the fused GFP moiety that appears to be the critical determinant of this proteolysis defect. These findings are consistent with a conserved role for ETC-1 in promoting proteasome processivity and suggest that proteasomal processivity is an important element of the oocyte-to-embryo transition during which many key oocyte regulatory proteins are rapidly targeted for degradation.

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.

Aurora A depletion reveals centrosome-independent polarization mechanism in Caenorhabditis elegans

How living systems break symmetry in an organized manner is a fundamental question in biology. In wild-type Caenorhabditis elegans zygotes, symmetry breaking during anterior-posterior axis specification is guided by centrosomes, resulting in anterior-directed cortical flows and a single posterior PAR-2 domain. We uncover that C. elegans zygotes depleted of the Aurora A kinase AIR-1 or lacking centrosomes entirely usually establish two posterior PAR-2 domains, one at each pole. We demonstrate that AIR-1 prevents symmetry breaking early in the cell cycle, whereas centrosomal AIR-1 instructs polarity initiation thereafter. Using triangular microfabricated chambers, we establish that bipolarity of air-1(RNAi) embryos occurs effectively in a cell-shape and curvature-dependent manner. Furthermore, we develop an integrated physical description of symmetry breaking, wherein local PAR-2-dependent weakening of the actin cortex, together with mutual inhibition of anterior and posterior PAR proteins, provides a mechanism for spontaneous symmetry breaking without centrosomes.

Maternal MEMI Promotes Female Meiosis II in Response to Fertilization in Caenorhabditis elegans

In most animals, female meiosis completes only after fertilization. Sperm entry has been implicated in providing a signal for the initiation of the final meiotic processes; however, a maternal component required for this process has not been previously identified. We report the characterization of a novel family of three highly similar paralogs (memi-1, memi-2, memi-3) that encode oocyte-specific proteins. A hyper-morphic mutation memi-1(sb41) results in failure to exit female meiosis II properly; however, loss of all three paralogs results in a "skipped meiosis II" phenotype. Mutations that prevent fertilization, such as fer-1(hc1), also cause a skipped meiosis II phenotype, suggesting that the MEMI proteins represent a maternal component of a postfertilization signal that specifies the meiosis II program. MEMI proteins are degraded before mitosis and sensitive to ZYG-11, a substrate-specific adapter for cullin-based ubiquitin ligase activity, and the memi-1(sb41) mutation results in inappropriate persistence of the MEMI-1 protein into mitosis. Using an RNAi screen for suppressors of memi-1(sb41), we identified a sperm-specific PP1 phosphatase, GSP-3/4, as a putative sperm component of the MEMI pathway. We also found that MEMI and GSP-3/4 proteins can physically interact via co-immunoprecipitation. These results suggest that sperm-specific PP1 and maternal MEMI proteins act in the same pathway after fertilization to facilitate proper meiosis II and the transition into embryonic mitosis.

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.

High-throughput capturing and characterization of mutations in essential genes of Caenorhabditis elegans

BACKGROUND

Essential genes are critical for the development of all organisms and are associated with many human diseases. These genes have been a difficult category to study prior to the availability of balanced lethal strains. Despite the power of targeted mutagenesis, there are limitations in identifying mutations in essential genes. In this paper, we describe the identification of coding regions for essential genes mutated using forward genetic screens in Caenorhabditis elegans. The lethal mutations described here were isolated and maintained by a wild-type allele on a rescuing duplication.

RESULTS

We applied whole genome sequencing to identify the causative molecular lesion resulting in lethality in existing C. elegans mutant strains. These strains are balanced and can be easily maintained for subsequent characterization. Our method can be effectively used to analyze mutations in a large number of essential genes. We describe here the identification of 64 essential genes in a region of chromosome I covered by the duplication sDp2. Of these, 42 are nonsense mutations, six are splice signal mutations, one deletion, and 15 are non-synonymous mutations. Many of the essential genes in this region function in cell cycle, transcriptional regulation, and RNA processing.

CONCLUSIONS

The essential genes identified here are represented by mutant strains, many of which have more than one mutant allele. The genetic resource can be utilized to further our understanding of essential gene function and will be applicable to the study of C. elegans development, conserved cellular function, and ultimately lead to improved human health.

The ubiquitin proteasome system in Caenorhabditis elegans and its regulation

Protein degradation constitutes a major cellular function that is responsible for maintenance of the normal cellular physiology either through the degradation of normal proteins or through the elimination of damaged proteins. The Ubiquitin–Proteasome System (UPS)¹ is one of the main proteolytic systems that orchestrate protein degradation. Given that up- and down- regulation of the UPS system has been shown to occur in various normal (such as ageing) and pathological (such as neurodegenerative diseases) processes, the exogenous modulation of the UPS function and activity holds promise of (a) developing new therapeutic interventions against various diseases and (b) establishing strategies to maintain cellular homeostasis. Since the proteasome genes are evolutionarily conserved, their role can be dissected in simple model organisms, such as the nematode, Caenorhabditis elegans. In this review, we survey findings on the redox regulation of the UPS in C. elegans showing that the nematode is an instrumental tool in the identification of major players in the UPS pathway. Moreover, we specifically discuss UPS-related genes that have been modulated in the nematode and in human cells and have resulted in similar effects thus further exhibiting the value of this model in the study of the UPS.

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UPS is one of the main proteolytic systems that orchestrate protein degradation.

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Proteasome function can be dissected in Caenorhabditis elegans.

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Nematodes can be used in the identification of major players in the UPS pathway.

The Anaphase-Promoting Complex (APC) ubiquitin ligase regulates GABA transmission at the C. elegans neuromuscular junction

Regulation of both excitatory and inhibitory synaptic transmission is critical for proper nervous system function. Aberrant synaptic signaling, including altered excitatory to inhibitory balance, is observed in numerous neurological diseases. The ubiquitin enzyme system controls the abundance of many synaptic proteins and thus plays a key role in regulating synaptic transmission. The Anaphase-Promoting Complex (APC) is a multi-subunit ubiquitin ligase that was originally discovered as a key regulator of protein turnover during the cell cycle. More recently, the APC has been shown to function in postmitotic neurons, where it regulates diverse processes such as synapse development and synaptic transmission at glutamatergic synapses. Here we report that the APC regulates synaptic GABA signaling by acting in motor neurons to control the balance of excitatory (acetylcholine) to inhibitory (GABA) transmission at the Caenorhabditis elegans neuromuscular junction (NMJ). Loss-of-function mutants in multiple APC subunits have increased muscle excitation at the NMJ; this phenotype is rescued by expression of the missing subunit in GABA neurons. Quantitative imaging and electrophysiological analyses indicate that APC mutants have decreased GABA release but normal cholinergic transmission. Consistent with this, APC mutants exhibit convulsions in a seizure assay sensitive to reductions in GABA signaling. Previous studies in other systems showed that the APC can negatively regulate the levels of the active zone protein SYD-2 Liprin-α. Similarly, we found that SYD-2 accumulates in APC mutants at GABAergic presynaptic sites. Finally, we found that the APC subunit EMB-27 CDC16 can localize to presynapses in GABA neurons. Together, our data suggest a model in which the APC acts at GABAergic presynapses to promote GABA release and inhibit muscle excitation. These findings are the first evidence that the APC regulates transmission at inhibitory synapses and have implications for understanding nervous system pathologies, such as epilepsy, that are characterized by misregulated GABA signaling.

Phospho-regulation pathways during egg activation in Drosophila melanogaster

Egg activation is the series of events that transition a mature oocyte to an egg capable of supporting embryogenesis. Increasing evidence points toward phosphorylation as a critical regulator of these events. We used Drosophila melanogaster to investigate the relationship between known egg activation genes and phosphorylation changes that occur upon egg activation. Using the phosphorylation states of four proteins-Giant Nuclei, Young Arrest, Spindly, and Vap-33-1-as molecular markers, we showed that the egg activation genes sarah, CanB2, and cortex are required for the phospho-regulation of multiple proteins. We show that an additional egg activation gene, prage, regulates the phosphorylation state of a subset of these proteins. Finally, we show that Sarah and calcineurin are required for the Anaphase Promoting Complex/Cyclosome (APC/C)-dependent degradation of Cortex following egg activation. From these data, we present a model in which Sarah, through the activation of calcineurin, positively regulates the APC/C at the time of egg activation, which leads to a change in phosphorylation state of numerous downstream proteins.

HECT-E3 ligase ETC-1 regulates securin and cyclin B1 cytoplasmic abundance to promote timely anaphase during meiosis in C. elegans

The anaphase inhibitor securin plays a crucial role in regulating the timing of sister chromatid separation during mitosis. When sister chromatid pairs become bioriented, the E3 ligase anaphase promoting complex/cyclosome (APC/C) ubiquitylates securin for proteolysis, triggering sister chromatid separation. Securin is also implicated in regulating meiotic progression. Securin protein levels change sharply during cell cycle progression, enabling its timely action. To understand the mechanism underlying the tightly regulated dynamics of securin, we analyzed the subcellular localization of the securin IFY-1 during C. elegans development. IFY-1 was highly expressed in the cytoplasm of germ cells. The cytoplasmic level of IFY-1 declined immediately following meiosis I division and remained low during meiosis II and following mitoses. We identified a C. elegans homolog of another type of E3 ligase, UBE3C, designated ETC-1, as a regulator of the cytoplasmic IFY-1 level. RNAi-mediated depletion of ETC-1 stabilized IFY-1 and CYB-1 (cyclin B1) in post-meiosis I embryos. ETC-1 knockdown in a reduced APC function background caused an embryonic lethal phenotype. In vitro, ETC-1 ubiquitylates IFY-1 and CYB-1 in the presence of the E2 enzyme UBC-18, which functions in pharyngeal development. Genetic analysis revealed that UBC-18 plays a distinct role together with ETC-1 in regulating the cytoplasmic level of IFY-1 during meiosis. Our study reports a novel mechanism, mediated by ETC-1, that co-operates with APC/C to maintain the meiotic arrest required for proper cell cycle timing during reproduction.

Control of oocyte growth and meiotic maturation in Caenorhabditis elegans

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

Developmental genetics of secretory vesicle acidification during Caenorhabditis elegans spermatogenesis

Secretory vesicles are used during spermatogenesis to deliver proteins to the cell surface. In Caenorhabditis elegans, secretory membranous organelles (MO) fuse with the plasma membrane to transform spermatids into fertilization-competent spermatozoa. We show that, like the acrosomal vesicle of mammalian sperm, MOs undergo acidification during development. Treatment of spermatids with the V-ATPase inhibitor bafilomycin blocks both MO acidification and formation of functional spermatozoa. There are several spermatogenesis-defective mutants that cause defects in MO morphogenesis, including spe-5. We determined that spe-5, which is on chromosome I, encodes one of two V-ATPase B paralogous subunits. The spe-5 null mutant is viable but sterile because it forms arrested, multi-nucleate spermatocytes. Immunofluorescence with a SPE-5-specific monoclonal antibody shows that SPE-5 expression begins in spermatocytes and is found in all subsequent stages of spermatogenesis. Most SPE-5 is discarded into the residual body during spermatid budding, but a small amount remains in budded spermatids where it localizes to MOs as a discrete dot. The other V-ATPase B subunit is encoded by vha-12, which is located on the X chromosome. Usually, spe-5 mutants are self-sterile in a wild-type vha-12 background. However, an extrachromosomal transgene containing wild-type vha-12 driven by its own promoter allows spe-5 mutant hermaphrodites to produce progeny, indicating that VHA-12 can at least partially substitute for SPE-5. Others have shown that the X chromosome is transcriptionally silent in the male germline, so expression of the autosomally located spe-5 gene ensures that a V-ATPase B subunit is present during spermatogenesis.

Protein phosphatase 5 is a negative regulator of separase function during cortical granule exocytosis in C. elegans

Mutations in the Caenorhabditis elegans separase gene, sep-1, are embryonic lethal. Newly fertilized mutant embryos have defects in polar body extrusion, fail to undergo cortical granule exocytosis, and subsequently fail to complete cytokinesis. Chromosome nondisjunction during the meiotic divisions is readily apparent after depletion of sep-1 by RNAi treatment, but much less so in hypomorphic mutant embryos. To identify factors that influence the activity of separase in cortical granule exocytosis and cytokinesis, we carried out a genetic suppressor screen. A mutation in the protein phosphatase 5 (pph-5) gene was identified as an extragenic suppressor of sep-1. This mutation suppressed the phenotypes of hypomorphic separase mutants but not RNAi depleted animals. Depletion of pph-5 caused no phenotypes on its own, but was effective in restoring localization of mutant separase to vesicles and suppressing cortical granule exocytosis and cytokinesis phenotypes. The identification of PPH-5 as a suppressor of separase suggests that a new phospho-regulatory pathway plays an important role in regulating anaphase functions of separase.

emb-1 encodes the APC16 subunit of the Caenorhabditis elegans anaphase-promoting complex

In the nematode Caenorhabditis elegans, temperature-sensitive mutants of emb-1 arrest as one-cell embryos in metaphase of meiosis I in a manner that is indistinguishable from embryos that have been depleted of known subunits of the anaphase-promoting complex or cyclosome (APC/C). Here we show that the emb-1 phenotype is enhanced in double mutant combinations with known APC/C subunits and suppressed in double mutant combinations with known APC/C suppressors. In addition to its meiotic function, emb-1 is required for mitotic proliferation of the germline. These studies reveal that emb-1 encodes K10D2.4, a homolog of the small, recently discovered APC/C subunit, APC16.

Caenorhabditis elegans cyclin B3 is required for multiple mitotic processes including alleviation of a spindle checkpoint-dependent block in anaphase chromosome segregation

The master regulators of the cell cycle are cyclin-dependent kinases (Cdks), which influence the function of a myriad of proteins via phosphorylation. Mitotic Cdk1 is activated by A-type, as well as B1- and B2-type, cyclins. However, the role of a third, conserved cyclin B family member, cyclin B3, is less well defined. Here, we show that Caenorhabditis elegans CYB-3 has essential and distinct functions from cyclin B1 and B2 in the early embryo. CYB-3 is required for the timely execution of a number of cell cycle events including completion of the MII meiotic division of the oocyte nucleus, pronuclear migration, centrosome maturation, mitotic chromosome condensation and congression, and, most strikingly, progression through the metaphase-to-anaphase transition. Our experiments reveal that the extended metaphase delay in CYB-3–depleted embryos is dependent on an intact spindle assembly checkpoint (SAC) and results in salient defects in the architecture of holocentric metaphase chromosomes. Furthermore, genetically increasing or decreasing dynein activity results in the respective suppression or enhancement of CYB-3–dependent defects in cell cycle progression. Altogether, these data reveal that CYB-3 plays a unique, essential role in the cell cycle including promoting mitotic dynein functionality and alleviation of a SAC–dependent block in anaphase chromosome segregation.

Every time a cell divides in two, the genetic material, DNA, is copied; each copied chromosome is referred to as a pair of sister chromatids. Each chromatid must be cleanly separated from its sister so that each daughter cell inherits the same DNA complement as the starting cell. The mitotic spindle is a cellular machine that physically separates the sister chromatids from one another. The chromatids are attached to the spindle at kinetochores, which are structures built at specific sites (centromeres) on each chromatid. The cell monitors the attachment of each chromatid and blocks their separation until they are all properly attached. This process is called the spindle assembly checkpoint (SAC). Here we report that loss of an evolutionarily conserved cell cycle regulator, Cyclin B3/CYB-3, results in an unusual and strikingly persistent SAC–dependent delay in sister chromatid separation. Furthermore, CYB-3 promotes the activity of a cellular motor, dynein, in this and other mitotic processes. Altogether, our results indicate that Cyclin B3 genetically interacts with mitotic dynein and is absolutely required to satisfy a SAC–dependent inhibition in sister chromatid separation.

Functional redundancy of paralogs of an anaphase promoting complex/cyclosome subunit in Caenorhabditis elegans meiosis

The anaphase promoting complex/cyclosome (APC/C) mediates the metaphase-to-anaphase transition by instructing the ubiquitination and turnover of key proteins at this stage of the cell cycle. We have recovered a gain-of-function allele in an APC5 subunit of the anaphase promoting complex/cyclosome. This finding led us to investigate further the role of APC5 in Caenorhabditis elegans, which contains two APC5 paralogs. We have shown that these two paralogs, such-1 and gfi-3, are coexpressed in the germline but have nonoverlapping expression patterns in other tissues. Depletion of such-1 or gfi-3 alone does not have a notable effect on the meiotic divisions; however, codepletion of these two factors results in meiotic arrest. In sum, the two C. elegans APC5 paralogs have a redundant function during the meiotic divisions.

Mutual antagonism between the anaphase promoting complex and the spindle assembly checkpoint contributes to mitotic timing in Caenorhabditis elegans

The anaphase promoting complex/cyclosome (APC/C) triggers the separation of sister chromatids and exit from mitosis across eukaryotic evolution. The APC/C is inhibited by the spindle assembly checkpoint (SAC) until all chromosomes have achieved bipolar attachment, but whether the APC/C reciprocally regulates the SAC is less understood. Here, we report the characterization of a novel allele of the APC5 component SUCH-1 in Caenorhabditis elegans. We find that some such-1(t1668) embryos lack paternally contributed DNA and centrioles and assemble a monopolar spindle in the one-cell stage. Importantly, we show that mitosis is drastically prolonged in these embryos, as well as in embryos that are otherwise compromised for APC/C function and assemble a monopolar spindle. This increased duration of mitosis is dependent on the SAC, since inactivation of the SAC components MDF-1/MAD1 or MDF-2/MAD2 rescues proper timing in these embryos. Moreover, partial depletion of the E1 enzyme uba-1 significantly increases mitosis duration upon monopolar spindle assembly. Taken together, our findings raise the possibility that the APC/C negatively regulates the SAC and, therefore, that the SAC and the APC/C have a mutual antagonistic relationship in C. elegans embryos.

EGG-4 and EGG-5 Link Events of the Oocyte-to-Embryo Transition with Meiotic Progression in C. elegans

The molecular underpinnings of the oocyte-to-embryo transition are poorly understood. Here we show that two protein tyrosine phosphatase-like (PTPL) family proteins, EGG-4 and EGG-5, are required for key events of the oocyte-to-embryo transition in Caenorhabditis elegans. The predicted EGG-4 and EGG-5 amino acid sequences are 99% identical and their functions are redundant. In embryos lacking EGG-4 and EGG-5, we observe defects in meiosis, polar body formation, the block to polyspermy, F-actin dynamics, and eggshell deposition. During oogenesis, EGG-4 and EGG-5 assemble at the oocyte cortex with the previously identified regulators or effectors of the oocyte-to-embryo transition EGG-3, CHS-1, and MBK-2 [1, 2]. All of these molecules share a complex interdependence with regards to their dynamics and subcellular localization. Shortly after fertilization, EGG-4 and EGG-5 are required to properly coordinate a redistribution of CHS-1 and EGG-3 away from the cortex during meiotic anaphase I. Therefore, EGG-4 and EGG-5 are not only required for critical events of the oocyte-to-embryo transition but also link the dynamics of the regulatory machinery with the advancing cell cycle.

Embryogenesis: Degenerate phosphatases control the oocyte-to-embryo transition

The oocyte-to-embryo transition requires drastic reorganizations within a short timeframe. Recent studies show that, in the nematode Caenorhabditis elegans, phosphotyrosine-binding pseudo-phosphatases are key regulators of this critical developmental transition.

Kinesin-1 and cytoplasmic dynein act sequentially to move the meiotic spindle to the oocyte cortex in Caenorhabditis elegans

During female meiosis in animals, the meiotic spindle is attached to the egg cortex by one pole during anaphase to allow selective disposal of half the chromosomes in a polar body. In Caenorhabditis elegans, this anaphase spindle position is achieved sequentially through kinesin-1-dependent early translocation followed by anaphase-promoting complex (APC)-dependent spindle rotation. Partial depletion of cytoplasmic dynein heavy chain by RNA interference blocked spindle rotation without affecting early translocation. Dynein depletion also blocked the APC-dependent late translocation that occurs in kinesin-1-depleted embryos. Time-lapse imaging of green fluorescent protein-tagged dynein heavy chain as well as immunofluorescence with dynein-specific antibodies revealed that dynein starts to accumulate at spindle poles just before the initiation of rotation or late translocation. Accumulation of dynein at poles was kinesin-1 independent and APC dependent, just like dynein driven spindle movements. This represents a case of kinesin-1/dynein coordination in which these two motors of opposite polarity act sequentially and independently on a cargo to move it in the same direction.

MEX-5 asymmetry in one-cell C. elegans embryos requires PAR-4- and PAR-1-dependent phosphorylation

Anteroposterior polarity in early C. elegans embryos is required for the specification of somatic and germline lineages, and is initiated by a sperm-induced reorganization of the cortical cytoskeleton and PAR polarity proteins. Through mechanisms that are not understood, the kinases PAR-1 and PAR-4, and other PAR proteins cause the cytoplasmic zinc finger protein MEX-5 to accumulate asymmetrically in the anterior half of the one-cell embryo. We show that MEX-5 asymmetry requires neither vectorial transport to the anterior, nor protein degradation in the posterior. MEX-5 has a restricted mobility before fertilization and in the anterior of one-cell embryos. However, MEX-5 mobility in the posterior increases as asymmetry develops, presumably allowing accumulation in the anterior. The MEX-5 zinc fingers and a small, C-terminal domain are essential for asymmetry; the zinc fingers restrict MEX-5 mobility, and the C-terminal domain is required for the increase in posterior mobility. We show that a crucial residue in the C-terminus, Ser 458, is phosphorylated in vivo. PAR-1 and PAR-4 kinase activities are required for the phosphorylation of S458, providing a link between PAR polarity proteins and the cytoplasmic asymmetry of MEX-5.

E1 ubiquitin-activating enzyme UBA-1 plays multiple roles throughout C. elegans development

Poly-ubiquitination of target proteins typically marks them for destruction via the proteasome and provides an essential mechanism for the dynamic control of protein levels. The E1 ubiquitin-activating enzyme lies at the apex of the ubiquitination cascade, and its activity is necessary for all subsequent steps in the reaction. We have isolated a temperature-sensitive mutation in the Caenorhabditis elegans uba-1 gene, which encodes the sole E1 enzyme in this organism. Manipulation of UBA-1 activity at different developmental stages reveals a variety of functions for ubiquitination, including novel roles in sperm fertility, control of body size, and sex-specific development. Levels of ubiquitin conjugates are substantially reduced in the mutant, consistent with reduced E1 activity. The uba-1 mutation causes delays in meiotic progression in the early embryo, a process that is known to be regulated by ubiquitin-mediated proteolysis. The uba-1 mutation also demonstrates synthetic lethal interactions with alleles of the anaphase-promoting complex, an E3 ubiquitin ligase. The uba-1 mutation provides a sensitized genetic background for identifying new in vivo functions for downstream components of the ubiquitin enzyme cascade, and it is one of the first conditional mutations reported for the essential E1 enzyme in a metazoan animal model.

Symmetrically dividing cell specific division axes alteration observed in proteasome depleted C. elegans embryo

A fertilised Caenorhabditis elegans embryo shows an invariable pattern of cell division and forms a multicellular body where each cell locates to a defined position. Mitotic spindle orientation is determined by several preceding events including the migration of duplicated centrosomes on a nucleus and the rotation of nuclear-centrosome complex. Cell polarity is the dominant force driving nuclear-centrosome rotation and setting the mitotic spindle axis in parallel with the polarity axis during asymmetric cell division. It is reasonable that there is no nuclear-centrosome rotation in symmetrically dividing blastomeres, but the mechanism(s) which suppress rotation in these cells have been proposed because the rotations occur in some polarity defect embryos. Here we show the nuclear-centrosome rotation can be induced by depletion of RPN-2, a regulatory subunit of the proteasome. In these embryos, cell polarity is established normally and both asymmetrically and symmetrically dividing cells are generated through asymmetric cell divisions. The nuclear-centrosome rotations occurred normally in the asymmetrically dividing cell lineage, but also induced in symmetrically dividing daughter cells. Interestingly, we identified RPN-2 as a binding protein of PKC-3, one of critical elements for establishing cell polarity during early asymmetric cell divisions. In addition to asymmetrically dividing cells, PKC-3 is also expressed in symmetrically dividing cells and a role to suppress nuclear-centrosome rotation has been anticipated. Our data suggest that the expression of RPN-2 is involved in the mechanism to suppress nuclear-centrosome rotation in symmetrically dividing cells and it may work in cooperation with PKC-3.

APC/CFzr/Cdh1 promotes cell cycle progression during the Drosophila endocycle

The endocycle is a commonly observed variant cell cycle in which cells undergo repeated rounds of DNA replication with no intervening mitosis. How the cell cycle machinery is modified to transform a mitotic cycle into endocycle has long been a matter of interest. In both plants and animals, the transition from the mitotic cycle to the endocycle requires Fzr/Cdh1, a positive regulator of the Anaphase-Promoting Complex/Cyclosome (APC/C). However, because many of its targets are transcriptionally downregulated upon entry into the endocycle, it remains unclear whether the APC/C functions beyond the mitotic/endocycle boundary. Here, we report that APC/C Fzr/Cdh1 activity is required to promote the G/S oscillation of the Drosophila endocycle. We demonstrate that compromising APC/C activity, after cells have entered the endocycle, inhibits DNA replication and results in the accumulation of multiple APC/C targets, including the mitotic cyclins and Geminin. Notably, our data suggest that the activity of APC/C Fzr/Cdh1 during the endocycle is not continuous but is cyclic, as demonstrated by the APC/C-dependent oscillation of the pre-replication complex component Orc1. Taken together, our data suggest a model in which the cyclic activity of APC/C Fzr/Cdh1 during the Drosophila endocycle is driven by the periodic inhibition of Fzr/Cdh1 by Cyclin E/Cdk2. We propose that, as is observed in mitotic cycles, during endocycles, APC/C Fzr/Cdh1 functions to reduce the levels of the mitotic cyclins and Geminin in order to facilitate the relicensing of DNA replication origins and cell cycle progression.

EGG-3 regulates cell-surface and cortex rearrangements during egg activation in Caenorhabditis elegans

Fertilization triggers egg activation and converts the egg into a developing embryo. The events of this egg-to-embryo transition typically include the resumption of meiosis, the reorganization of the cortical actin cytoskeleton, and the remodeling of the oocyte surface. The factors that regulate sperm-dependent egg-activation events are not well understood. Caenorhabditis elegans EGG-3, a member of the protein tyrosine phosphatase-like (PTPL) family, is essential for regulating cell-surface and cortex rearrangements during egg activation in response to sperm entry. Although fertilization occurred normally in egg-3 mutants, the polarized dispersal of F-actin is altered, a chitin eggshell is not formed, and no polar bodies are produced. EGG-3 is associated with the oocyte plasma membrane in a pattern that is similar to CHS-1 and MBK-2. CHS-1 is required for eggshell deposition, whereas MBK-2 is required for the degradation of maternal proteins during the egg-to-embryo transition. The localization of CHS-1 and EGG-3 are interdependent and both genes were required for the proper localization of MBK-2 in oocytes. Therefore, EGG-3 plays a central role in egg activation by influencing polarized F-actin dynamics and the localization or activity of molecules that are directly involved in executing the egg-to-embryo transition.

The AAA-ATPase FIGL-1 controls mitotic progression, and its levels are regulated by the CUL-3MEL-26 E3 ligase in the C. elegans germ line

Members of the AAA-ATPase (ATPases associated with diverse cellular activities) family use the energy from ATP hydrolysis to disrupt protein complexes involved in many cellular processes. Here, we report that FIGL-1 (Fidgetin-like 1), the single Caenorhabditis elegans homolog of mammalian fidgetin and fidgetin-like 1 AAA-ATPases, controls progression through mitosis in the germ line and the early embryo. Loss of figl-1 function leads to the accumulation of mitotic nuclei in the proliferative zone of the germ line, resulting in sterility owing to depletion of germ cells. Like the AAA-ATPase MEI-1 (also known as katanin), FIGL-1 interacts with microtubules and with MEL-26, a specificity factor of CUL-3-based E3 ligases involved in targeting proteins for ubiquitin-dependent degradation by the 26S proteasome. In the germ line, FIGL-1 is enriched in nuclei of mitotic cells, but it disappears at the transition into meiosis. Conversely, MEL-26 expression is low in nuclei of the mitotic zone and induced during meiosis. FIGL-1 accumulates in the germ line and spreads to the meiotic zone after inactivation of mel-26 or cul-3 in vivo. We conclude that degradation of FIGL-1 by the CUL-3MEL-26 E3 ligase spatially restricts FIGL-1 function to mitotic cells, where it is required for correct progression through mitosis.

Cortical granule exocytosis in C. elegans is regulated by cell cycle components including separase

In many organisms, cortical granules undergo exocytosis following fertilization, releasing cargo proteins that modify the extracellular covering of the zygote. We identified cortical granules in Caenorhabditis elegans and have found that degranulation occurs in a wave that initiates in the vicinity of the meiotic spindle during anaphase I. Previous studies identified genes that confer an embryonic osmotic sensitivity phenotype, thought to result from abnormal eggshell formation. Many of these genes are components of the cell cycle machinery. When we suppressed expression of several of these genes by RNAi, we observed that cortical granule trafficking was disrupted and the eggshell did not form properly. We conclude that osmotic sensitivity phenotypes occur because of defects in trafficking of cortical granules and the subsequent formation of an impermeable eggshell. We identified separase as a key cell cycle component that is required for degranulation. Separase localized to cortically located filamentous structures in prometaphase I upon oocyte maturation. After fertilization, separase disappeared from these structures and appeared on cortical granules by anaphase I. RNAi of sep-1 inhibited degranulation in addition to causing extensive chromosomal segregation failures. Although the temperature-sensitive sep-1(e2406) allele exhibited similar inhibition of degranulation, it had minimal effects on chromosome segregation. These observations lead us to speculate that SEP-1 has two separable yet coordinated functions: to regulate cortical granule exocytosis and to mediate chromosome separation.

Regulation of MBK-2/Dyrk Kinase by Dynamic Cortical Anchoring during the Oocyte-to-Zygote Transition

BACKGROUND

Successful transition from oocyte to zygote depends on the timely degradation of oocyte proteins to prepare for embryonic development. In C. elegans, degradation of the oocyte protein MEI-1 depends on MBK-2, a kinase that phosphorylates MEI-1 shortly after fertilization during the second meiotic division.

RESULTS

Here we report that precise timing of MEI-1 phosphorylation depends on the cell cycle-regulated release of MBK-2 from the cortex. Prior to the meiotic divisions, MBK-2 is tethered at the cortex by EGG-3, an oocyte protein required for egg activation (see [1], accompanying paper in this issue). During the meiotic divisions, EGG-3 is internalized and degraded in an APC/C (anaphase-promoting complex/cyclosome)-dependent manner. EGG-3 internalization and degradation correlate with MBK-2 release from the cortex and MEI-1 phosphorylation in the cytoplasm. In an egg-3 mutant, MEI-1 is phosphorylated and degraded prematurely.

CONCLUSION

We suggest that successful transition from an oocyte to a zygote depends on the cell cycle-regulated relocalization of key regulators from the cortex to the cytoplasm of the egg.

Identification of the C. elegans anaphase promoting complex subunit Cdc26 by phenotypic profiling and functional rescue in yeast

Background

RNA interference coupled with videorecording of C. elegans embryos is a powerful method for identifying genes involved in cell division processes. Here we present a functional analysis of the gene B0511.9, previously identified as a candidate cell polarity gene in an RNAi videorecording screen of chromosome I embryonic lethal genes.

Results

Whereas weak RNAi inhibition of B0511.9 causes embryonic cell polarity defects, strong inhibition causes embryos to arrest in metaphase of meiosis I. The range of defects induced by RNAi of B0511.9 is strikingly similar to those displayed by mutants of anaphase-promoting complex/cyclosome (APC/C) components. Although similarity searches did not reveal any obvious homologue of B0511.9 in the non-redundant protein database, we found that the N-terminus shares a conserved sequence pattern with the N-terminus of the small budding yeast APC/C subunit Cdc26 and its orthologues from a variety of other organisms. Furthermore, we show that B0511.9 robustly complements the temperature-sensitive growth defect of a yeast cdc26Δ mutant.

Conclusion

These data demonstrate that B0511.9 encodes the C. elegans APC/C subunit CDC-26.

The C. elegans anaphase promoting complex and MBK-2/DYRK kinase act redundantly with CUL-3/MEL-26 ubiquitin ligase to degrade MEI-1 microtubule-severing activity after meiosis

The C. elegans embryo supports both meiotic and mitotic spindles, requiring careful regulation of components specific to each spindle type. The MEI-1/katanin microtubule-severing complex is required for meiosis but must be inactivated prior to mitosis. Downregulation of MEI-1 depends on MEL-26, which binds MEI-1, targeting it for degradation by the CUL-3 E3 ubiquitin ligase complex. Here we report that other protein degradation pathways, involving the anaphase promoting complex (APC) and the MBK-2/DYRK kinase, act in parallel to MEL-26 to inactivate MEI-1. At 25 degrees all mel-26(null) embryos die due to persistence of MEI-1 into mitosis, but at 15 degrees a significant portion of embryos hatch due to lower levels of ectopic MEI-1, suggesting that a redundant pathway also regulates MEI-1 degradation at 15 degrees. Previously the MBK-2/DYRK kinase was suggested to trigger MEL-26 mediated MEI-1 degradation. However, mbk-2 enhances the incomplete lethality of mel-26(null) at 15 degrees, arguing that MEL-26 acts in parallel to MBK-2. APC mutants behave similarly. In mel-26 embryos, ectopic MEI-1 remains until the onset of gastrulation, but in mbk-2; apc embryos, MEI-1 only persists through the first mitosis. We propose that mbk-2 and apc couple the initial phase of MEI-1 degradation to meiotic exit, after which MEL-26 completes MEI-1 degradation.

Suppressors of spindle checkpoint defect (such) mutants identify new mdf-1/MAD1 interactors in Caenorhabditis elegans

The spindle assembly checkpoint (SAC) governs the timing of metaphase-to-anaphase transition and is essential for genome stability. The Caenorhabditis elegans mutant strain gk2 carries a deletion within the mdf-1/MAD1 gene that results in death of the homozygous strain after two or three generations. Here we describe 11 suppressors of the mdf-1(gk2) lethality, 10 identified in an ethyl methanesulfonate (EMS) mutagenesis screen and 1 isolated using the dog-1(gk10) (deletions of guanine-rich DNA) mutator strain. Using time-lapse imaging of early embryonic cells and germline mitotic division, we demonstrate that there are two classes of suppressors. Eight suppressors compensate for the loss of the checkpoint by delaying mitotic progression, which coincides with securin (IFY-1/Pds1) accumulation; three suppressors have normal IFY-1/Pds1 levels and normal anaphase onset. Furthermore, in the class of suppressors with delayed mitotic progression, we have identified four alleles of known suppressors emb-30/APC4 and fzy-1/CDC20, which are components of the anaphase-promoting complex/cyclosome (APC/C). In addition, we have identified another APC/C component capable of bypassing the checkpoint requirement that has not previously been described in C. elegans. The such-1/APC5-like mutation, h1960, significantly delays anaphase onset both in germline and in early embryonic cells.

The anaphase-promoting complex is required in both dividing and quiescent cells during zebrafish development

The anaphase-promoting complex/cyclosome (APC/C) regulates multiple stages of the cell cycle, most prominently mitosis. We describe zebrafish with mutations in two APC/C subunits, Cdc16 and Cdc26, whose phenotypes reveal a multifaceted set of defects resulting from the gradual depletion of the APC/C. First, loss of the APC/C in dividing cells results in mitotic arrest, followed by apoptosis. This defect becomes detectable in different organs at different larval ages, because the subunits of the APC/C are maternally deposited, are unusually stable, and are depleted at uneven rates in different tissues. Second, loss of the APC/C in quiescent or differentiated cells results in improper re-entry into the cell cycle, again in an apparently tissue-specific manner. This study is the first demonstration of both functions of the APC/C in a vertebrate organism and also provides an illustration of the surprisingly complex effects that essential, maternally supplied factors can have on the growing animal over a period of 10 days or longer.

Components of the spindle assembly checkpoint regulate the anaphase-promoting complex during meiosis in Caenorhabditis elegans

Temperature-sensitive mutations in subunits of the Caenorhabditis elegans anaphase-promoting complex (APC) arrest at metaphase of meiosis I at the restrictive temperature. Embryos depleted of the APC co-activator FZY-1 by RNAi also arrest at this stage. To identify regulators and potential substrates of the APC, we performed a genetic suppressor screen with a weak allele of the APC subunit MAT-3/CDC23/APC8, whose defects are specific to meiosis. Twenty-seven suppressors that resulted in embryonic viability and larval development at the restrictive temperature were isolated. We have identified the molecular lesions in 18 of these suppressors, which correspond to five genes. In addition to a single intragenic suppressor, we found mutations in the APC co-activator fzy-1 and in three spindle assembly checkpoint genes, mdf-1, mdf-2, and mdf-3/san-1, orthologs of Mad1, Mad2, and Mad3, respectively. Reduction-of-function alleles of mdf-2 and mdf-3 suppress APC mutants and exhibit pleiotropic phenotypes in an otherwise wild-type background. Analysis of a single separation-of-function allele of mdf-1 suggests that MDF-1 has a dual role during development. These studies provide evidence that components of the spindle assembly checkpoint may regulate the metaphase-to-anaphase transition in the absence of spindle damage during C. elegans meiosis.

The eggshell is required for meiotic fidelity, polar-body extrusion and polarization of the C. elegans embryo

Background

Fertilization restores the diploid state and begins the process by which the single-cell oocyte is converted into a polarized, multicellular organism. In the nematode, Caenorhabditis elegans, two of the earliest events following fertilization are secretion of the chitinous eggshell and completion of meiosis, and in this report we demonstrate that the eggshell is essential for multiple developmental events at the one-cell stage.

Results

We show that the GLD (Germline differentiation abnormal)-1-regulated hexosamine pathway enzyme, glucosamine-6-phosphate N-acetyltransferase (GNA)-2, is required for synthesis of uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc), the substrate for eggshell chitin synthesis by chitin synthase-1 (CHS-1). Furthermore, while chs-1(RNAi) or combined RNAi with the chitin-binding proteins, CEJ-1 and B0280.5, does not interfere with normal meiotic timing, lagging chromosomes are observed at meiosis, and polar-body extrusion fails. We also demonstrate that chitin, and either CEJ-1 or B0280.5, are essential for the osmotic/permeability barrier and for movement of the sperm pronucleus/centrosome complex to the cortex, which is associated with the initiation of polarization.

Conclusion

Our results indicate that the eggshell is required in single-cell C. elegans development, playing an essential role in multiple actin-dependent early events. Furthermore, the earliest meiotic roles precede osmotic barrier formation, indicating that the role of the eggshell is not limited to generation of the osmotic barrier.

The puromycin-sensitive aminopeptidase PAM-1 is required for meiotic exit and anteroposterior polarity in the one-cell Caenorhabditis elegans embryo

In the nematode Caenorhabditis elegans, sperm entry into the oocyte triggers the completion of meiosis and the establishment of the embryonic anteroposterior (AP) axis. How the early embryo makes the transition from a meiotic to a mitotic zygote and coordinates cell cycle changes with axis formation remains unclear. We have discovered roles for the C. elegans puromycin-sensitive aminopeptidase PAM-1 in both cell cycle progression and AP axis formation, further implicating proteolytic regulation in these processes. pam-1 mutant embryos exhibit a delay in exit from meiosis: thus, this peptidase is required for progression to mitotic interphase. In addition, the centrosomes associated with the sperm pronucleus fail to closely associate with the posterior cortex in pam-1 mutants, and the AP axis is not specified. The meiotic exit and polarity defects are separable, as inactivation of the B-type cyclin CYB-3 in pam-1 mutants rescues the meiotic exit delay but not the polarity defects. Thus PAM-1 may regulate CYB-3 during meiotic exit but presumably targets other protein(s) to regulate polarity. We also show that the pam-1 gene is expressed both maternally and paternally, providing additional evidence that sperm-donated gene products have important roles during early embryogenesis in C. elegans. The degradation of proteins through ubiquitin-mediated proteolysis has been previously shown to regulate the cell cycle and AP axis formation in the C. elegans zygote. Our analysis of PAM-1 requirements shows that a puromycin-sensitive aminopeptidase is also required for proteolytic regulation of the oocyte to embryo transition.

Cyclin-dependent kinases in C. elegans

Cell division is an inherent part of organismal development, and defects in this process can lead to developmental abnormalities as well as cancerous growth. In past decades, much of the basic cell-cycle machinery has been identified, and a major challenge in coming years will be to understand the complex interplay between cell division and multicellular development. Inevitably, this requires the use of more complex multicellular model systems. The small nematode Caenorhabditis elegans is an excellent model system to study the regulation of cell division in a multicellular organism, and is poised to make important contributions to this field. The past decade has already seen a surge in cell-cycle research in C. elegans, yielding information on the function of many basic cell-cycle regulators, and making inroads into the developmental control of cell division. This review focuses on the in vivo roles of cyclin-dependent kinases in C. elegans, and highlights novel findings implicating CDKs in coupling development to cell-cycle progression.

Start me up: Cell signaling and the journey from oocyte to embryo inC. elegans

Intercellular communication plays a pivotal role in regulating and coordinating oocyte meiosis and fertilization, key triggers for embryonic development. The nematode Caenorhabaditis elegans has emerged as an important experimental paradigm for exploring these fundamental reproductive processes and their regulation. The oocytes of most animal species arrest during meiotic prophase and complete meiosis in response to intercellular signaling in the process of meiotic maturation. Oocyte meiotic maturation is defined by the transition between diakinesis and metaphase of meiosis I and is accompanied by nuclear envelope breakdown and meiotic spindle assembly. As such, the meiotic maturation process is essential for completing meiosis and a prerequisite for successful fertilization. In C. elegans, the processes of meiotic maturation, ovulation, and fertilization are temporally coupled: sperm utilize the major sperm protein as a hormone to trigger oocyte meiotic maturation, and, in turn, the maturing oocyte signals its own ovulation, leading to fertilization. The powerful genetic screens possible in C. elegans have led to the identification of several sperm cell surface proteins that are required for the interaction and fusion of gametes at fertilization. The study of these proteins provides fundamental insights into fertilization mechanisms, their role in speciation, and their potential conservation across phyla. Signaling processes sparked by fertilization are required for meiotic chromosome segregation and initiating the embryonic program. Here we review recent advances in understanding how signaling mechanisms contribute to the oocyte-to-embryo transition in C. elegans.

Degrade to create: developmental requirements for ubiquitin-mediated proteolysis during earlyC. elegansembryogenesis

The ubiquitin protein conjugation system tags proteins with the small polypeptide ubiquitin. Most poly-ubiquitinated proteins are recognized and degraded by the proteasome, a large multi-subunit protease. Ubiquitin-dependent protein degradation is used as a regulatory tool for many essential processes, the best studied of which is eukaryotic cell cycle progression. More recently, genetic studies in C. elegans have identified multiple roles for the ubiquitin system in early development, where ubiquitin-dependent protein degradation governs such diverse events as passage through meiosis, cytoskeletal regulation and cell fate determination.

ARI-1, an RBR family ubiquitin-ligase, functions with UBC-18 to regulate pharyngeal development in C. elegans

The LIN-35 retinoblastoma protein homolog and the ubiquitin-conjugating enzyme UBC-18 function redundantly to control an early step of pharyngeal morphogenesis in C. elegans. In order to identify ubiquitin-ligases acting downstream of UBC-18, we carried out a two-hybrid screen using UBC-18 as the bait molecule. Our screen identified three putative ubiquitin-ligases, one of which, ARI-1, showed genetic interactions leading to defective pharyngeal development that were identical to that previously observed for UBC-18. ARI-1 is a member of the RBR family of ubiquitin-ligases and contains a C-terminal motif that places it within the highly conserved Ariadne subfamily of RBR ligases. Our analyses indicate that ARI-1 is the principal Ariadne family member in C. elegans that is involved in the control of pharyngeal development with UBC-18. Using GFP reporters, we find that ARI-1 is expressed dynamically in a wide range of tissues including muscles and neurons during embryonic and postembryonic development. We also provide evidence that dsRNA species containing 14 or fewer base pairs of contiguous identity with closely related mRNAs are sufficient to mediate off-target silencing in C. elegans.

The C. elegans DYRK Kinase MBK-2 Marks Oocyte Proteins for Degradation in Response to Meiotic Maturation

The oocyte-to-embryo transition transforms a differentiated germ cell into a totipotent zygote capable of somatic development. In C. elegans, several oocyte proteins, including the meiotic katanin subunit MEI-1 and the oocyte maturation protein OMA-1, must be degraded during this transition . Degradation of MEI-1 and OMA-1 requires the dual-specificity YAK-1-related (DYRK) kinase MBK-2 . Here, we demonstrate that MBK-2 directly phosphorylates MEI-1 and OMA-1 in vitro and that this activity is essential for degradation in vivo. Phosphorylation of MEI-1 by MBK-2 reaches maximal levels after the meiotic divisions, immediately preceding MEI-1 degradation. MEI-1 phosphorylation and degradation still occur in spe-9 eggs, which undergo meiotic maturation and exit in the absence of fertilization . In contrast, MEI-1 phosphorylation and degradation are blocked in cell-cycle mutants that arrest during the meiotic divisions, and are accelerated in wee-1.3(RNAi) oocytes, which prematurely enter meiotic M phase (A. Golden, personal communication). A GFP:MBK-2 fusion relocalizes from the cortex to the cytoplasm during the meiotic divisions, and this relocalization also depends on cell-cycle progression. Our findings suggest that regulators of meiotic M phase activate a remodeling program, independently of fertilization, to prepare eggs for embryogenesis.

Neuronal differentiation in C. elegans

The small size and defined connectivity of the C. elegans nervous system and the amenability of this species to systematic functional screens have continued to yield new insights into neuronal differentiation. Many aspects of C. elegans neuronal development resemble those of other more complex neurons. The basic cellular machinery of synaptic transmission is highly conserved. Recent work has begun to unveil the roles of proteoglycans in axon guidance and branching, and of the extracellular matrix in neuronal process maintenance. The importance of ubiquitin-mediated protein turnover in neuronal differentiation is revealed by the identification of new and conserved pathways that promote the organization and function of the synapse.

Caenorhabditis elegans lin-35/Rb, efl-1/E2F and other synthetic multivulva genes negatively regulate the anaphase-promoting complex gene mat-3/APC8

Retinoblastoma (Rb)/E2F complexes repress expression of many genes important for G(1)-to-S transition, but also appear to regulate gene expression at other stages of the cell cycle. In C. elegans, lin-35/Rb and other synthetic Multivulva (SynMuv) group B genes function redundantly with other sets of genes to regulate G(1)/S progression, vulval and pharyngeal differentiation, and other unknown processes required for viability. Here we show that lin-35/Rb, efl-1/E2F, and other SynMuv B genes negatively regulate a component of the anaphase-promoting complex or cyclosome (APC/C). The APC/C is a multisubunit complex that promotes metaphase-to-anaphase progression and G(1) arrest by targeting different substrates for ubiquitination and proteasome-mediated destruction. The C. elegans APC/C gene mat-3/APC8 has been defined by temperature-sensitive embryonic lethal alleles that strongly affect germline meiosis and mitosis but only weakly affect somatic development. We describe severe nonconditional mat-3 alleles and a hypomorphic viable allele (ku233), all of which affect postembryonic cell divisions including those of the vulval lineage. The ku233 lesion is located outside of the mat-3 coding region and reduces mat-3 mRNA expression. Loss-of-function alleles of lin-35/Rb and other SynMuv B genes suppress mat-3(ku233) defects by restoring mat-3 mRNA to wild-type levels. Therefore, Rb/E2F complexes appear to repress mat-3 expression.

PAR proteins and the establishment of cell polarity duringC. elegans development

Cells become polarized to develop functional specializations and to distribute developmental determinants unequally during division. Studies that began in the nematode C. elegans have identified a group of largely conserved proteins, called PAR proteins, that play key roles in the polarization of many different cell types. During initial stages of cell polarization, certain PAR proteins become distributed asymmetrically along the cell cortex and subsequently direct the localization and/or activity of other proteins. Here I discuss recent findings on how PAR proteins become and remain asymmetric in three different contexts during C. elegans development: anterior-posterior polarization of the one-cell embryo, apicobasal polarization of non-epithelial early embryonic cells, and apicobasal polarization of epithelial cells. Although polarity within each of these cell types requires PAR proteins, the cues and regulators of PAR asymmetry can differ.

Caenorhabditis elegans UBC-2 functions with the anaphase-promoting complex but also has other activities

The anaphase-promoting complex or cyclosome (APC/C) is a multi-subunit ubiquitin ligase that regulates the eukaryotic cell cycle. APC/C belongs to the RING finger class of ubiquitin ligases that function by interacting with a ubiquitin-conjugating enzyme (Ubc), thus inciting the Ubc to transfer ubiquitin onto a target protein. Extensive studies with APC/C in other organisms have identified several possible Ubcs that might function as partners for APC/C. This report presents phenotypic and biochemical evidence showing that, in Caenorhabditis elegans, UBC-2 interacts specifically with the APC/C. This conclusion is based on three lines of evidence: first, the RNAi phenotype of ubc-2 is indistinguishable from RNAi phenotypes of APC/C subunits; second, RNAi of ubc-2 but not other Ubcs enhances the phenotype of hypomorphic APC/C mutants; third, purified UBC-2 and APC-11, the RING finger subunit of the APC/C, show robust ubiquitination activity in in vitro assays. APC-11 interaction is specific for UBC-2 as ubiquitination is not seen when APC-11 is combined other C. elegans Ubcs. As expected from the Ubc that functions with the APC/C, ubc-2(RNAi) produces metaphase blocks in both mitotic germ cells and in meiotic divisions of post-fertilization oocytes. In addition, ubc-2(RNAi) results in two germline phenotypes that appear to be unrelated to the APC/C: an expanded transition zone indicative of a pre-pachytene meiotic arrest and endo-reduplicated oocytes indicative of a problem in ovulation or oocyte-soma interactions.

A clean start: degradation of maternal proteins at the oocyte-to-embryo transition

In many organisms, the transition from oocyte to embryo occurs in the absence of mRNA transcription. Therefore, early developmental programs rely on maternal mRNAs and proteins that are synthesized during oogenesis. The regulated translation of maternal RNAs is essential for the proper deployment of regulatory factors during early embryogenesis. Recent studies suggest that the degradation of maternal proteins by the ubiquitin-proteasome pathway is also crucial for the oocyte-to-embryo transition. In this article, we explore the hypothesis that the coordinated degradation of germline proteins is essential for remodeling the oocyte into a totipotent zygote that is capable of somatic development.

Zyg-11 and cul-2 regulate progression through meiosis II and polarity establishment in C. elegans

The mechanisms that ensure coupling between meiotic cell cycle progression and subsequent developmental events, including specification of embryonic axes, are poorly understood. Here, we establish that zyg-11 and the cullin cul-2 promote the metaphase-to-anaphase transition and M phase exit at meiosis II in Caenorhabditis elegans. Our results indicate that ZYG-11 acts with a CUL-2-based E3 ligase that is essential at meiosis II and that functions redundantly with the anaphase-promoting complex/cyclosome at meiosis I. Our data also indicate that delayed M phase exit in zyg-11(RNAi) embryos is due to accumulation of the B type cyclin CYB-3. We demonstrate that PAR proteins and P granules become polarized in an inverted manner during the meiosis II delay resulting from zyg-11 or cul-2 inactivation, and that zyg-11 and cul-2 can regulate polarity establishment independently of a role in cell cycle progression. Furthermore, we find that microtubules appear dispensable for ectopic polarity during the meiosis II delay in zyg-11(RNAi) embryos, as well as for AP polarity during the first mitotic cell cycle in wild-type embryos. Our findings suggest a model in which a CUL-2-based E3 ligase promotes cell cycle progression and prevents polarity establishment during meiosis II, and in which the centrosome acts as a cue to polarize the embryo along the AP axis after exit from the meiotic cell cycle.

Cell Polarity and the Cytoskeleton in theCaenorhabditis ElegansZygote

The anterior-posterior axis of the Caenorhabditis elegans zygote forms shortly after fertilization when the sperm pronucleus and its associated centrosomal asters provide a cue that establishes the anterior-posterior (AP) body axis. In response to this cue, the microfilament cytoskeleton polarizes the distribution of a group of widely conserved, cortically localized regulators called the PAR proteins, which are required for the first mitotic division to be asymmetric. These asymmetries include a posterior displacement of the first mitotic spindle and the differential segregation of cell-fate determinants to the anterior and posterior daughters produced by the first cleavage of the zygote. Here we review recent advances in our understanding of the mechanisms that polarize the one-cell zygote to generate an AP axis of asymmetry.