Metaphase to anaphase (mat) transition-defective mutants in Caenorhabditis elegans

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.

SIN-3 acts in distinct complexes to regulate the germline transcriptional program in Caenorhabditis elegans

The transcriptional co-regulator SIN3 influences gene expression through multiple interactions that include histone deacetylases. Haploinsufficiency and mutations in SIN3 are the underlying cause of Witteveen-Kolk syndrome and related intellectual disability and autism syndromes, emphasizing its key role in development. However, little is known about the diversity of its interactions and functions in developmental processes. Here, we show that loss of SIN-3, the single SIN3 homolog in Caenorhabditis elegans, results in maternal-effect sterility associated with de-regulation of the germline transcriptome, including de-silencing of X-linked genes. We identify at least two distinct SIN3 complexes containing specific histone deacetylases and show that they differentially contribute to fertility. Single-cell, single-molecule fluorescence in situ hybridization reveals that in sin-3 mutants the X chromosome becomes re-expressed prematurely and in a stochastic manner in individual germ cells, suggesting a role for SIN-3 in its silencing. Furthermore, we identify histone residues whose acetylation increases in the absence of SIN-3. Together, this work provides a powerful framework for the in vivo study of SIN3 and associated proteins.

Identification and sequencing of temperature sensitive alleles of the Anaphase Promoting Complex component mat-3 in C. elegans

The Anaphase Promoting Complex (APC) regulates the transition from metaphase to anaphase during cell division and is important to prevent defects in chromosome segregation. In a recent temperature sensitive genetic screen looking for further genes involved in fertilization, we isolated a new temperature sensitive allele of mat-3 (as49) . We also sequenced a previously identified mat-3 ( or344 ) allele that did not previously have an annotated sequence. We determined that the as49 allele was an Alanine to Threonine (A451T) mutation in the sixth exon and the or344 mutation was a Leucine to Phenylalanine (L474F) mutation in the seventh exon. These locations of the mutant alleles are consistent with other previously annotated alleles that displayed the same metaphase to anaphase transition defect phenotype and further reinforce the importance of the tetratricopeptide repeats to mediate protein interactions.

BUB-1-bound PLK-1 directs CDC-20 kinetochore recruitment to ensure timely embryonic mitoses

During mitosis, chromosomes assemble kinetochores to dynamically couple with spindle microtubules.1,2 Kinetochores also function as signaling hubs directing mitotic progression by recruiting and controlling the fate of the anaphase promoting complex/cyclosome (APC/C) activator CDC-20.3,4,5 Kinetochores either incorporate CDC-20 into checkpoint complexes that inhibit the APC/C or dephosphorylate CDC-20, which allows it to interact with and activate the APC/C.4,6 The importance of these two CDC-20 fates likely depends on the biological context. In human somatic cells, the major mechanism controlling mitotic progression is the spindle checkpoint. By contrast, progression through mitosis during the cell cycles of early embryos is largely checkpoint independent.7,8,9,10 Here, we first show that CDC-20 phosphoregulation controls mitotic duration in the C. elegans embryo and defines a checkpoint-independent temporal mitotic optimum for robust embryogenesis. CDC-20 phosphoregulation occurs at kinetochores and in the cytosol. At kinetochores, the flux of CDC-20 for local dephosphorylation requires an ABBA motif on BUB-1 that directly interfaces with the structured WD40 domain of CDC-20.6,11,12,13 We next show that a conserved "STP" motif in BUB-1 that docks the mitotic kinase PLK-114 is necessary for CDC-20 kinetochore recruitment and timely mitotic progression. The kinase activity of PLK-1 is required for CDC-20 to localize to kinetochores and phosphorylates the CDC-20-binding ABBA motif of BUB-1 to promote BUB-1-CDC-20 interaction and mitotic progression. Thus, the BUB-1-bound pool of PLK-1 ensures timely mitosis during embryonic cell cycles by promoting CDC-20 recruitment to the vicinity of kinetochore-localized phosphatase activity.

MEL-28/ELYS and CENP-C coordinately control outer kinetochore assembly and meiotic chromosome-microtubule interactions

During mitosis and meiosis in the majority of eukaryotes, centromeric chromatin comprised of CENP-A nucleosomes and their reader CENP-C recruits components of the outer kinetochore to build an interface with spindle microtubules.^1,2 One exception is C. elegans oocyte meiosis, where outer kinetochore proteins form cup-like structures on chromosomes independently of centromeric chromatin.³ Here, we show that the nucleoporin MEL-28 (ortholog of human ELYS) and CENP-C^HCP-4 act in parallel to recruit outer kinetochore components to oocyte meiotic chromosomes. Unexpectedly, co-inhibition of MEL-28 and CENP-C^HCP-4 resulted in chromosomes being expelled from the meiotic spindle prior to anaphase onset, a more severe phenotype than what was observed following ablation of the outer kinetochore.^4,5 This observation suggested that MEL-28 and the outer kinetochore independently link chromosomes to spindle microtubules. Consistent with this, the chromosome expulsion defect was observed following co-inhibition of MEL-28 and the microtubule-coupling KNL-1/MIS-12/NDC-80 (KMN) network of the outer kinetochore. Use of engineered mutants showed that MEL-28 acts in conjunction with the microtubule-binding NDC-80 complex to keep chromosomes within the oocyte meiotic spindle and that this function likely involves the Y-complex of nucleoporins that associate with MEL-28; by contrast, the ability to dock protein phosphatase 1, shared by MEL-28 and KNL-1, is not involved. These results highlight nuclear pore-independent functions for a conserved nucleoporin and explain two unusual features of oocyte meiotic chromosome segregation in C. elegans: centromeric chromatin-independent outer kinetochore assembly, and dispensability of the outer kinetochore for constraining chromosomes in the acentrosomal meiotic spindle.

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.

Subcellular patterns of SPE-6 localization reveal unexpected complexities in Caenorhabditis elegans sperm activation and sperm function

To acquire and maintain directed cell motility, Caenorhabditis elegans sperm must undergo extensive, regulated cellular remodeling, in the absence of new transcription or translation. To regulate sperm function, nematode sperm employ large numbers of protein kinases and phosphatases, including SPE-6, a member of C. elegans' highly expanded casein kinase 1 superfamily. SPE-6 functions during multiple steps of spermatogenesis, including functioning as a "brake" to prevent premature sperm activation in the absence of normal extracellular signals. Here, we describe the subcellular localization patterns of SPE-6 during wild-type C. elegans sperm development and in various sperm activation mutants. While other members of the sperm activation pathway associate with the plasma membrane or localize to the sperm's membranous organelles, SPE-6 surrounds the chromatin mass of unactivated sperm. During sperm activation by either of two semiautonomous signaling pathways, SPE-6 redistributes to the front, central region of the sperm's pseudopod. When disrupted by reduction-of-function alleles, SPE-6 protein is either diminished in a temperature-sensitive manner (hc187) or is mislocalized in a stage-specific manner (hc163). During the multistep process of sperm activation, SPE-6 is released from its perinuclear location after the spike stage in a process that does not require the fusion of membranous organelles with the plasma membrane. After activation, spermatozoa exhibit variable proportions of perinuclear and pseudopod-localized SPE-6, depending on their location within the female reproductive tract. These findings provide new insights regarding SPE-6's role in sperm activation and suggest that extracellular signals during sperm migration may further modulate SPE-6 localization and function.

APC/CFZR-1 regulates centrosomal ZYG-1 to limit centrosome number

Aberrant centrosome numbers are associated with human cancers. The levels of centrosome regulators positively correlate with centrosome number. Thus, tight control of centrosome protein levels is critical. In Caenorhabditis elegans, the anaphase-promoting complex/cyclosome and its co-activator FZR-1 (APC/CFZR-1), a ubiquitin ligase, negatively regulates centrosome assembly through SAS-5 degradation. In this study, we report the C. elegans ZYG-1 (Plk4 in humans) as a potential substrate of APC/CFZR-1. Inhibiting APC/CFZR-1 or mutating a ZYG-1 destruction (D)-box leads to elevated ZYG-1 levels at centrosomes, restoring bipolar spindles and embryonic viability to zyg-1 mutants, suggesting that APC/CFZR-1 influences centrosomal ZYG-1 via the D-box motif. We also show the Slimb/βTrCP-binding (SB) motif is critical for ZYG-1 degradation, substantiating a conserved mechanism by which ZYG-1/Plk4 stability is regulated by the SKP1-CUL1-F-box (Slimb/βTrCP)-protein complex (SCFSlimb/βTrCP)-dependent proteolysis via the conserved SB motif in C. elegans. Furthermore, we show that co-mutating ZYG-1 SB and D-box motifs stabilizes ZYG-1 in an additive manner, suggesting that the APC/CFZR-1 and SCFSlimb/βTrCP ubiquitin ligases function cooperatively for timely ZYG-1 destruction in C. elegans embryos where ZYG-1 activity remains at threshold level to ensure normal centrosome number.

The conserved molting/circadian rhythm regulator NHR-23/NR1F1 serves as an essential co-regulator of C. elegans spermatogenesis

In sexually reproducing metazoans, spermatogenesis is the process by which uncommitted germ cells give rise to haploid sperm. Work in model systems has revealed mechanisms controlling commitment to the sperm fate, but how this fate is subsequently executed remains less clear. While studying the well-established role of the conserved nuclear hormone receptor transcription factor, NHR-23/NR1F1, in regulating C. elegans molting, we discovered that NHR-23/NR1F1 is also constitutively expressed in developing primary spermatocytes and is a critical regulator of spermatogenesis. In this novel role, NHR-23/NR1F1 functions downstream of the canonical sex-determination pathway. Degron-mediated depletion of NHR-23/NR1F1 within hermaphrodite or male germlines causes sterility due to an absence of functional sperm, as depleted animals produce arrested primary spermatocytes rather than haploid sperm. These spermatocytes arrest in prometaphase I and fail to either progress to anaphase or attempt spermatid-residual body partitioning. They make sperm-specific membranous organelles but fail to assemble their major sperm protein into fibrous bodies. NHR-23/NR1F1 appears to function independently of the known SPE-44 gene regulatory network, revealing the existence of an NHR-23/NR1F1-mediated module that regulates the spermatogenesis program.

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.

The G2-to-M Transition Is Ensured by a Dual Mechanism that Protects Cyclin B from Degradation by Cdc20-Activated APC/C

In the eukaryotic cell cycle, a threshold level of cyclin B accumulation triggers the G2-to-M transition, and subsequent cyclin B destruction triggers mitotic exit. The anaphase-promoting complex/cyclosome (APC/C) is the E3 ubiquitin ligase that, together with its co-activator Cdc20, targets cyclin B for destruction during mitotic exit. Here, we show that two pathways act in concert to protect cyclin B from Cdc20-activated APC/C in G2, in order to enable cyclin B accumulation and the G2-to-M transition. The first pathway involves the Mad1-Mad2 spindle checkpoint complex, acting in a distinct manner from checkpoint signaling after mitotic entry but employing a common molecular mechanism-the promotion of Mad2-Cdc20 complex formation. The second pathway involves cyclin-dependent kinase phosphorylation of Cdc20, which is known to reduce Cdc20's affinity for the APC/C. Cooperation of these two mechanisms, which target distinct APC/C binding interfaces of Cdc20, enables cyclin B accumulation and the G2-to-M transition.

Choice between 1- and 2-furrow cytokinesis in Caenorhabditis elegans embryos with tripolar spindles

Excessive centrosomes often lead to multipolar spindles, and thus probably to multipolar mitosis and aneuploidy. In Caenorhabditis elegans, ∼70% of the paternal emb-27APC6 mutant embryonic cells contained more than two centrosomes and formed multipolar spindles. However, only ~30% of the cells with tripolar spindles formed two cytokinetic furrows. The rest formed one furrow, similar to normal cells. To investigate the mechanism via which cells avoid forming two cytokinetic furrows even with a tripolar spindle, we conducted live-cell imaging in emb-27APC6 mutant cells. We observed that the chromatids were aligned on only two of the three sides of the tripolar spindle, and the angle of the tripolar spindle relative to the long axis of the cell correlated with the number of cytokinetic furrows. Our numerical modeling showed that the combination of cell shape, cortical pulling forces, and heterogeneity of centrosome size determines whether cells with a tripolar spindle form one or two cytokinetic furrows.

Regulated Activation of the PAR Polarity Network Ensures a Timely and Specific Response to Spatial Cues

How do cells polarize at the correct time and in response to the correct cues? In the C. elegans zygote, the timing and geometry of polarization rely on a single dominant cue-the sperm centrosome-that matures at the end of meiosis and specifies the nascent posterior. Polarization requires that the conserved PAR proteins, which specify polarity in the zygote, be poised to respond to the centrosome. Yet, how and when PAR proteins achieve this unpolarized, but responsive, state is unknown. We show that oocyte maturation initiates a fertilization-independent PAR activation program. PAR proteins are initially not competent to polarize but gradually acquire this ability following oocyte maturation. Surprisingly, this program allows symmetry breaking even in unfertilized oocytes lacking centrosomes. Thus, if PAR proteins can respond to multiple polarizing cues, how is specificity for the centrosome achieved? Specificity is enforced by Polo-like and Aurora kinases (PLK-1 and AIR-1 in C. elegans), which impose a delay in the activation of the PAR network so that it coincides with maturation of the centrosome cue. This delay suppresses polarization by non-centrosomal cues, which can otherwise trigger premature polarization and multiple or reversed polarity domains. Taken together, these findings identify a regulatory program that enforces proper polarization by synchronizing PAR network activation with cell cycle progression, thereby ensuring that PAR proteins respond specifically to the correct cue. Temporal control of polarity network activity is likely to be a common strategy to ensure robust, dynamic, and specific polarization in response to developmentally deployed cues.

Developmental Control of the Cell Cycle: Insights from Caenorhabditis elegans

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

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.

MIB-1 Is Required for Spermatogenesis and Facilitates LIN-12 and GLP-1 Activity in Caenorhabditis elegans

Covalent attachment of ubiquitin to substrate proteins changes their function or marks them for proteolysis, and the specificity of ubiquitin attachment is mediated by the numerous E3 ligases encoded by animals. Mind Bomb is an essential E3 ligase during Notch pathway signaling in insects and vertebrates. While Caenorhabditis elegans encodes a Mind Bomb homolog (mib-1), it has never been recovered in the extensive Notch suppressor/enhancer screens that have identified numerous pathway components. Here, we show that C. elegans mib-1 null mutants have a spermatogenesis-defective phenotype that results in a heterogeneous mixture of arrested spermatocytes, defective spermatids, and motility-impaired spermatozoa. mib-1 mutants also have chromosome segregation defects during meiosis, molecular null mutants are intrinsically temperature-sensitive, and many mib-1 spermatids contain large amounts of tubulin. These phenotypic features are similar to the endogenous RNA intereference (RNAi) mutants, but mib-1 mutants do not affect RNAi. MIB-1 protein is expressed throughout the germ line with peak expression in spermatocytes followed by segregation into the residual body during spermatid formation. C. elegans mib-1 expression, while upregulated during spermatogenesis, also occurs somatically, including in vulva precursor cells. Here, we show that mib-1 mutants suppress both lin-12 and glp-1 (C. elegans Notch) gain-of-function mutants, restoring anchor cell formation and a functional vulva to the former and partly restoring oocyte production to the latter. However, suppressed hermaphrodites are only observed when grown at 25°, and they are self-sterile. This probably explains why mib-1 was not previously recovered as a Notch pathway component in suppressor/enhancer selection experiments.

Sex- and Gamete-Specific Patterns of X Chromosome Segregation in a Trioecious Nematode

Three key steps in meiosis allow diploid organisms to produce haploid gametes: (1) homologous chromosomes (homologs) pair and undergo crossovers; (2) homologs segregate to opposite poles; and (3) sister chromatids segregate to opposite poles. The XX/XO sex determination system found in many nematodes [1] facilitates the study of meiosis because variation is easily recognized [2-4]. Here we show that meiotic segregation of X chromosomes in the trioecious nematode Auanema rhodensis [5] varies according to sex (hermaphrodite, female, or male) and type of gametogenesis (oogenesis or spermatogenesis). In this species, XO males exclusively produce X-bearing sperm [6, 7]. The unpaired X precociously separates into sister chromatids, which co-segregate with the autosome set to generate a functional haplo-X sperm. The other set of autosomes is discarded into a residual body. Here we explore the X chromosome behavior in female and hermaphrodite meioses. Whereas X chromosomes segregate following the canonical pattern during XX female oogenesis to yield haplo-X oocytes, during XX hermaphrodite oogenesis they segregate to the first polar body to yield nullo-X oocytes. Thus, crosses between XX hermaphrodites and males yield exclusively male progeny. During hermaphrodite spermatogenesis, the sister chromatids of the X chromosomes separate during meiosis I, and homologous X chromatids segregate to the functional sperm to create diplo-X sperm. Given these intra-species, intra-individual, and intra-gametogenesis variations in the meiotic program, A. rhodensis is an ideal model for studying the plasticity of meiosis and how it can be modulated.

APC/CFZR-1 Controls SAS-5 Levels To Regulate Centrosome Duplication in Caenorhabditis elegans

As the primary microtubule-organizing center, centrosomes play a key role in establishing mitotic bipolar spindles that secure correct transmission of genomic content. For the fidelity of cell division, centrosome number must be strictly controlled by duplicating only once per cell cycle. Proper levels of centrosome proteins are shown to be critical for normal centrosome number and function. Overexpressing core centrosome factors leads to extra centrosomes, while depleting these factors results in centrosome duplication failure. In this regard, protein turnover by the ubiquitin-proteasome system provides a vital mechanism for the regulation of centrosome protein levels. Here, we report that FZR-1, the Caenorhabditis elegans homolog of Cdh1/Hct1/Fzr, a coactivator of the anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase, functions as a negative regulator of centrosome duplication in the C. elegans embryo. During mitotic cell division in the early embryo, FZR-1 is associated with centrosomes and enriched at nuclei. Loss of fzr-1 function restores centrosome duplication and embryonic viability to the hypomorphic zyg-1(it25) mutant, in part, through elevated levels of SAS-5 at centrosomes. Our data suggest that the APC/C^FZR-1 regulates SAS-5 levels by directly recognizing the conserved KEN-box motif, contributing to proper centrosome duplication. Together, our work shows that FZR-1 plays a conserved role in regulating centrosome duplication in C. elegans.

MIP-MAP: High-Throughput Mapping of Caenorhabditis elegans Temperature-Sensitive Mutants via Molecular Inversion Probes

Mutants remain a powerful means for dissecting gene function in model organisms such as Caenorhabditis elegans. Massively parallel sequencing has simplified the detection of variants after mutagenesis but determining precisely which change is responsible for phenotypic perturbation remains a key step. Genetic mapping paradigms in C. elegans rely on bulk segregant populations produced by crosses with the problematic Hawaiian wild isolate and an excess of redundant information from whole-genome sequencing (WGS). To increase the repertoire of available mutants and to simplify identification of the causal change, we performed WGS on 173 temperature-sensitive (TS) lethal mutants and devised a novel mapping method. The mapping method uses molecular inversion probes (MIP-MAP) in a targeted sequencing approach to genetic mapping, and replaces the Hawaiian strain with a Million Mutation Project strain with high genomic and phenotypic similarity to the laboratory wild-type strain N2. We validated MIP-MAP on a subset of the TS mutants using a competitive selection approach to produce TS candidate mapping intervals with a mean size < 3 Mb. MIP-MAP successfully uses a non-Hawaiian mapping strain and multiplexed libraries are sequenced at a fraction of the cost of WGS mapping approaches. Our mapping results suggest that the collection of TS mutants contains a diverse library of TS alleles for genes essential to development and reproduction. MIP-MAP is a robust method to genetically map mutations in both viable and essential genes and should be adaptable to other organisms. It may also simplify tracking of individual genotypes within population mixtures.

Caenorhabditis elegans oocytes detect meiotic errors in the absence of canonical end-on kinetochore attachments

During mitosis, cells monitor kinetochore–microtubule attachments as a means of detecting errors. Although end-on attachments have not been observed in Caenorhabditis elegans oocytes, Davis-Roca et al. now report that these cells alter key aspects of anaphase progression in the presence of meiotic defects, revealing a new strategy for error detection during cell division.

Mitotically dividing cells use a surveillance mechanism, the spindle assembly checkpoint, that monitors the attachment of spindle microtubules to kinetochores as a means of detecting errors. However, end-on kinetochore attachments have not been observed in Caenorhabditis elegans oocytes and chromosomes instead associate with lateral microtubule bundles; whether errors can be sensed in this context is not known. Here, we show that C. elegans oocytes delay key events in anaphase, including AIR-2/Aurora B relocalization to the microtubules, in response to a variety of meiotic defects, demonstrating that errors can be detected in these cells and revealing a mechanism that regulates anaphase progression. This mechanism does not appear to rely on several components of the spindle assembly checkpoint but does require the kinetochore, as depleting kinetochore components prevents the error-induced anaphase delays. These findings therefore suggest that in this system, kinetochores could be involved in sensing meiotic errors using an unconventional mechanism that does not use canonical end-on attachments.

Inhibition of ectopic microtubule assembly by the kinesin-13 KLP-7 prevents chromosome segregation and cytokinesis defects in oocytes

In most species, oocytes lack centrosomes. Accurate meiotic spindle assembly and chromosome segregation - essential to prevent miscarriage or developmental defects - thus occur through atypical mechanisms that are not well characterized. Using quantitative in vitro and in vivo functional assays in the C. elegans oocyte, we provide novel evidence that the kinesin-13 KLP-7 promotes destabilization of the whole cellular microtubule network. By counteracting ectopic microtubule assembly and disorganization of the microtubule network, this function is strictly required for spindle organization, chromosome segregation and cytokinesis in meiotic cells. Strikingly, when centrosome activity was experimentally reduced, the absence of KLP-7 or the mammalian kinesin-13 protein MCAK (KIF2C) also resulted in ectopic microtubule asters during mitosis in C. elegans zygotes or HeLa cells, respectively. Our results highlight the general function of kinesin-13 microtubule depolymerases in preventing ectopic, spontaneous microtubule assembly when centrosome activity is defective or absent, which would otherwise lead to spindle microtubule disorganization and aneuploidy.

A SUMO-Dependent Protein Network Regulates Chromosome Congression during Oocyte Meiosis

During Caenorhabditis elegans oocyte meiosis, a multi-protein ring complex (RC) localized between homologous chromosomes, promotes chromosome congression through the action of the chromokinesin KLP-19. While some RC components are known, the mechanism of RC assembly has remained obscure. We show that SUMO E3 ligase GEI-17/PIAS is required for KLP-19 recruitment to the RC, and proteomic analysis identified KLP-19 as a SUMO substrate in vivo. In vitro analysis revealed that KLP-19 is efficiently sumoylated in a GEI-17-dependent manner, while GEI-17 undergoes extensive auto-sumoylation. GEI-17 and another RC component, the kinase BUB-1, contain functional SUMO interaction motifs (SIMs), allowing them to recruit SUMO modified proteins, including KLP-19, into the RC. Thus, dynamic SUMO modification and the presence of SIMs in RC components generate a SUMO-SIM network that facilitates assembly of the RC. Our results highlight the importance of SUMO-SIM networks in regulating the assembly of dynamic protein complexes.

LIN-23, an E3 Ubiquitin Ligase Component, Is Required for the Repression of CDC-25.2 Activity during Intestinal Development in Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) utilizes two different cell-cycle modes, binucleations during the L1 larval stage and endoreduplications at four larval moltings, for its postembryonic intestinal development. Previous genetic studies indicated that CDC-25.2 is specifically required for binucleations at the L1 larval stage and is repressed before endoreduplications. Furthermore, LIN-23, the C. elegans β-TrCP ortholog, appears to function as a repressor of CDC-25.2 to prevent excess intestinal divisions. We previously reported that intestinal hyperplasia in lin-23(e1883) mutants was effectively suppressed by the RNAi depletion of cdc-25.2. Nevertheless, LIN-23 targeting CDC-25.2 for ubiquitination as a component of E3 ubiquitin ligase has not yet been tested. In this study, LIN-23 is shown to be the major E3 ubiquitin ligase component, recognizing CDC-25.2 to repress their activities for proper transition of cell-cycle modes during the C. elegans postembryonic intestinal development. In addition, for the first time that LIN-23 physically interacts with both CDC-25.1 and CDC-25.2 and facilitates ubiquitination for timely regulation of their activities during the intestinal development.

Regulation of the MEI-1/MEI-2 Microtubule-Severing Katanin Complex in Early Caenorhabditis elegans Development

After fertilization, rapid changes of the Caenorhabditis elegans cytoskeleton occur in the transition from meiosis to mitosis, requiring precise regulation. The MEI-1/MEI-2 katanin microtubule-severing complex is essential for meiotic spindle formation but must be quickly inactivated to allow for proper formation of the mitotic spindle. MEI-1/MEI-2 inactivation is dependent on multiple redundant pathways. The primary pathway employs the MEL-26 substrate adaptor for the CUL-3/cullin-based E3 ubiquitin ligase, which targets MEI-1 for proteosomal degradation. Here, we used quantitative antibody staining to measure MEI-1 levels to determine how other genes implicated in MEI-1 regulation act relative to CUL-3/MEL-26. The anaphase-promoting complex/cyclosome, APC/C, the DYRK (Dual-specificity tyrosine-regulated kinase), MBK-2, and the CUL-2-based E3 ubiquitin ligase act together to degrade MEI-1, in parallel to MEL-26/CUL-3. CUL-2 is known to keep MEL-26 low during meiosis, so CUL-2 apparently changes its target from MEL-26 in meiosis to MEI-1 in mitosis. RFL-1, an activator of cullin E3 ubiquitin ligases, activates CUL-2 but not CUL-3 for MEI-1 elimination. HECD-1 (HECT/Homologous to the E6AP carboxyl terminus domain) E3 ligase acts as a MEI-1 activator in meiosis but functions as an inhibitor during mitosis, without affecting levels of MEI-1 or MEI-2. Our results highlight the multiple layers of MEI-1 regulation that are required during the switch from the meiotic to mitotic modes of cell division.

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.

Identification of Conserved MEL-28/ELYS Domains with Essential Roles in Nuclear Assembly and Chromosome Segregation

Nucleoporins are the constituents of nuclear pore complexes (NPCs) and are essential regulators of nucleocytoplasmic transport, gene expression and genome stability. The nucleoporin MEL-28/ELYS plays a critical role in post-mitotic NPC reassembly through recruitment of the NUP107-160 subcomplex, and is required for correct segregation of mitotic chromosomes. Here we present a systematic functional and structural analysis of MEL-28 in C. elegans early development and human ELYS in cultured cells. We have identified functional domains responsible for nuclear envelope and kinetochore localization, chromatin binding, mitotic spindle matrix association and chromosome segregation. Surprisingly, we found that perturbations to MEL-28’s conserved AT-hook domain do not affect MEL-28 localization although they disrupt MEL-28 function and delay cell cycle progression in a DNA damage checkpoint-dependent manner. Our analyses also uncover a novel meiotic role of MEL-28. Together, these results show that MEL-28 has conserved structural domains that are essential for its fundamental roles in NPC assembly and chromosome segregation.

Most animal cells have a nucleus that contains the genetic material: the chromosomes. The nucleus is enclosed by the nuclear envelope, which provides a physical barrier between the chromosomes and the surrounding cytoplasm, and enables precisely controlled transport of proteins into and out of the nucleus. Transport occurs through nuclear pore complexes, which consist of multiple copies of ~30 different proteins called nucleoporins. Although the composition of nuclear pore complexes is known, the mechanisms of their assembly and function are still unclear. We have analyzed the nucleoporin MEL-28/ELYS through a systematic dissection of functional domains both in the nematode Caenorhabditis elegans and in human cells. Interestingly, MEL-28/ELYS localizes not only to nuclear pore complexes, but is also associated with chromosomal structures known as kinetochores during cell division. Our studies have revealed that even small perturbations in MEL-28/ELYS can have dramatic consequences on nuclear pore complex assembly as well as on separation of chromosomes during cell division. Surprisingly, inhibition of MEL-28/ELYS causes cell-cycle delay, suggesting activation of a cellular surveillance system for chromosomal damages. Finally, we conclude that the structural domains of MEL-28/ELYS are conserved from nematodes to humans.

The Anaphase-Promoting Complex (APC) ubiquitin ligase affects chemosensory behavior in C. elegans

The regulation of fundamental aspects of neurobiological function has been linked to the ubiquitin signaling system (USS), which regulates the degradation and activity of proteins and is catalyzed by E1, E2, and E3 enzymes. The Anaphase-Promoting Complex (APC) is a multi-subunit E3 ubiquitin ligase that controls diverse developmental and signaling processes in post-mitotic neurons; however, potential roles for the APC in sensory function have yet to be explored. In this study, we examined the effect of the APC ubiquitin ligase on chemosensation in Caenorhabditis elegans by testing chemotaxis to the volatile odorants, diacetyl, pyrazine, and isoamyl alcohol, to which wild-type worms are attracted. Animals with loss of function mutations in either of two alleles (g48 and ye143) of the gene encoding the APC subunit EMB-27 APC6 showed increased chemotaxis towards diacetyl and pyrazine, odorants sensed by AWA neurons, but exhibited normal chemotaxis to isoamyl alcohol, which is sensed by AWC neurons. The statistically significant increase in chemotaxis in the emb-27 APC6 mutants suggests that the APC inhibits AWA-mediated chemosensation in C. elegans. Increased chemotaxis to pyrazine was also seen with mutants lacking another essential APC subunit, MAT-2 APC1; however, mat-2 APC1 mutants exhibited wild type responses to diacetyl. The difference in responsiveness of these two APC subunit mutants may be due to differential strength of these hypomorphic alleles or may indicate the presence of functional sub-complexes of the APC at work in this process. These findings are the first evidence for APC-mediated regulation of chemosensation and lay the groundwork for further studies aimed at identifying the expression levels, function, and targets of the APC in specific sensory neurons. Because of the similarity between human and C. elegans nervous systems, the role of the APC in sensory neurons may also advance our understanding of human sensory function and disease.

Spindle assembly checkpoint proteins regulate and monitor meiotic synapsis in C. elegans

Homologue synapsis is required for meiotic chromosome segregation, but how synapsis is initiated between chromosomes is poorly understood. In Caenorhabditis elegans, synapsis and a checkpoint that monitors synapsis depend on pairing centers (PCs), cis-acting loci that interact with nuclear envelope proteins, such as SUN-1, to access cytoplasmic microtubules. Here, we report that spindle assembly checkpoint (SAC) components MAD-1, MAD-2, and BUB-3 are required to negatively regulate synapsis and promote the synapsis checkpoint response. Both of these roles are independent of a conserved component of the anaphase-promoting complex, indicating a unique role for these proteins in meiotic prophase. MAD-1 and MAD-2 localize to the periphery of meiotic nuclei and interact with SUN-1, suggesting a role at PCs. Consistent with this idea, MAD-1 and BUB-3 require full PC function to inhibit synapsis. We propose that SAC proteins monitor the stability of pairing, or tension, between homologues to regulate synapsis and elicit a checkpoint response.

Kinetochore-independent chromosome segregation driven by lateral microtubule bundles

During cell division, chromosomes attach to spindle microtubules at sites called kinetochores, and force generated at the kinetochore-microtubule interface is the main driver of chromosome movement. Surprisingly, kinetochores are not required for chromosome segregation on acentrosomal spindles in Caenorhabditis elegans oocytes, but the mechanism driving chromosomes apart in their absence is not understood. In this study, we show that lateral microtubule–chromosome associations established during prometaphase remain intact during anaphase to facilitate separation, defining a novel form of kinetochore-independent segregation. Chromosome dynamics during congression and segregation are controlled by opposing forces; plus-end directed forces are mediated by a protein complex that forms a ring around the chromosome center and dynein on chromosome arms provides a minus-end force. At anaphase onset, ring removal shifts the balance between these forces, triggering poleward movement along lateral microtubule bundles. This represents an elegant strategy for controlling chromosomal movements during cell division distinct from the canonical kinetochore-driven mechanism.

DOI:http://dx.doi.org/10.7554/eLife.06462.001

An animal's genetic material is packaged into structures called chromosomes. Most animals have two sets of chromosomes: one from each parent. Sperm and egg cells must contain half the number of chromosomes compared to other cells in the body, so that when they fuse, the resulting embryo receives a full complement of chromosomes.

Egg and sperm cells are made via a type of cell division called meiosis. In meiosis, the genetic material of a cell is copied once but then the cell divides twice. Therefore, at the end of the two divisions, the resulting sperm or egg cells contain half the number of chromosomes as the original cell. During cell division, the genetic material is separated by a structure called the spindle apparatus.

The spindle is made of protein filaments called microtubules. At each end of the spindle, there is a cluster of microtubule ends, known as a ‘pole’. The other ends of the microtubules extend out towards the center of the spindle, where they overlap with the microtubules from the opposite pole. The chromosomes line up in the center of the spindle and then the chromosomes are separated, with half moving to one spindle pole, and half to the other. In most forms of cell division, the microtubules attach to the chromosomes via sites called kinetochores. However, it was recently discovered that kinetochores are not required to separate chromosomes to make egg cells in the worm C. elegans, suggesting that these chromosomes associate with the spindle in a different way.

Muscat, Torre-Santiago et al. have now used high-resolution imaging to look at this chromosome separation process in more detail and to figure out how the chromosomes separate when C. elegans forms egg cells. The experiments revealed that the chromosomes move within the spindle along parallel microtubule bundles, much like trains moving along a track. The chromosomes are moved into position at the center of the spindle by a ring-shaped group (or ‘complex’) of proteins that forms around the center of each chromosome. The protein complex comes off the chromosomes as they separate, and a motor protein called dynein walks along the microtubules to pull the separated chromosomes to the poles.

Muscat, Torre-Santiago et al.'s findings thus show that meiosis in C. elegans during the production of egg cells works in a very different way to other types of cell division. In the future, it will be important to understand how dynein and the ring-shaped complex are regulated, as this may shed light on what causes mistakes in the separation of genetic material during meiosis, which can lead to infertility, miscarriages, and birth defects in humans and other animals.

DOI:http://dx.doi.org/10.7554/eLife.06462.002

DNA damage response and spindle assembly checkpoint function throughout the cell cycle to ensure genomic integrity

Errors in replication or segregation lead to DNA damage, mutations, and aneuploidies. Consequently, cells monitor these events and delay progression through the cell cycle so repair precedes division. The DNA damage response (DDR), which monitors DNA integrity, and the spindle assembly checkpoint (SAC), which responds to defects in spindle attachment/tension during metaphase of mitosis and meiosis, are critical for preventing genome instability. Here we show that the DDR and SAC function together throughout the cell cycle to ensure genome integrity in C. elegans germ cells. Metaphase defects result in enrichment of SAC and DDR components to chromatin, and both SAC and DDR are required for metaphase delays. During persistent metaphase arrest following establishment of bi-oriented chromosomes, stability of the metaphase plate is compromised in the absence of DDR kinases ATR or CHK1 or SAC components, MAD1/MAD2, suggesting SAC functions in metaphase beyond its interactions with APC activator CDC20. In response to DNA damage, MAD2 and the histone variant CENPA become enriched at the nuclear periphery in a DDR-dependent manner. Further, depletion of either MAD1 or CENPA results in loss of peripherally associated damaged DNA. In contrast to a SAC-insensitive CDC20 mutant, germ cells deficient for SAC or CENPA cannot efficiently repair DNA damage, suggesting that SAC mediates DNA repair through CENPA interactions with the nuclear periphery. We also show that replication perturbations result in relocalization of MAD1/MAD2 in human cells, suggesting that the role of SAC in DNA repair is conserved.

Checkpoints are surveillance pathways that monitor and correct cellular errors to ensure that the genome is transmitted intact through cell division; defects in checkpoints lead to human disease such as cancer. Two major checkpoint pathways that have been extensively studied are the DNA damage response and the spindle assembly checkpoint. As their names imply, they have been thought to monitor distinct chromosomal events during the cell cycle. Here, we used C. elegans proliferating germ cells and human cells to investigate the role of these checkpoints when either DNA is damaged or the spindle is perturbed. We discovered that these checkpoints function together in response to these different perturbations to ensure genome integrity. Our studies have important implications for cancer treatments, as many cancer chemotherapies target one of these checkpoint pathways without consideration for the effect on the other pathway.

A semi-dominant mutation in the general splicing factor SF3a66 causes anterior-posterior axis reversal in one-cell stage C. elegans embryos

Establishment of anterior-posterior polarity in one-cell stage Caenorhabditis elegans embryos depends in part on astral microtubules. As the zygote enters mitosis, these microtubules promote the establishment of a posterior pole by binding to and protecting a cytoplasmic pool of the posterior polarity protein PAR-2 from phosphorylation by the cortically localized anterior polarity protein PKC-3. Prior to activation of the sperm aster, the oocyte Meiosis I and II spindles assemble and function, usually at the future anterior pole, but these meiotic spindle microtubules fail to establish posterior polarity through PAR-2. Here we show that a semi-dominant mutation in the general splicing factor SF3a66 can lead to a reversed axis of AP polarity that depends on PAR-2 and possibly on close proximity of oocyte meiotic spindles with the cell cortex. One possible explanation is that reduced levels of PKC-3, due to a general splicing defect, can result in axis reversal due to a failure to prevent oocyte meiotic spindle microtubules from interfering with AP axis formation.

C. elegans as a model for membrane traffic

The counterbalancing action of the endocytosis and secretory pathways maintains a dynamic equilibrium that regulates the composition of the plasma membrane, allowing it to maintain homeostasis and to change rapidly in response to alterations in the extracellular environment and/or intracellular metabolism. These pathways are intimately integrated with intercellular signaling systems and play critical roles in all cells. Studies in Caenorhabditis elegans have revealed diverse roles of membrane trafficking in physiology and development and have also provided molecular insight into the fundamental mechanisms that direct cargo sorting, vesicle budding, and membrane fisson and fusion. In this review, we summarize progress in understanding membrane trafficking mechanisms derived from work in C. elegans, focusing mainly on work done in non-neuronal cell-types, especially the germline, early embryo, coelomocytes, and intestine.

Identification of suppressors of mbk-2/DYRK by whole-genome sequencing

Screening for suppressor mutations is a powerful method to isolate genes that function in a common pathway or process. Because suppressor mutations often do not have phenotypes on their own, cloning of suppressor loci can be challenging. A method combining whole-genome sequencing (WGS) and single nucleotide polymorphism (SNP) mapping (WGS/SNP mapping) was developed to identify mutations with visible phenotypes in C. elegans. We show here that WGS/SNP mapping is an efficient method to map suppressor mutations without the need for previous phenotypic characterization. Using RNA-mediated interference to test candidate loci identified by WGS/SNP mapping, we identified 10 extragenic and six intragenic suppressors of mbk-2, a DYRK family kinase required for the transition from oocyte to zygote. Remarkably, seven suppressors are mutations in cell-cycle regulators that extend the timing of the oocyte-to-zygote transition.

Ubiquitin-proteasome system in spermatogenesis

Spermatogenesis represents a complex succession of cell division and differentiation events resulting in the continuous formation of spermatozoa. Such a complex program requires precise expression of enzymes and structural proteins which is effected not only by regulation of gene transcription and translation, but also by targeted protein degradation. In this chapter, we review current knowledge about the role of the ubiquitin-proteasome system in spermatogenesis, describing both proteolytic and non-proteolytic functions of ubiquitination. Ubiquitination plays essential roles in the establishment of both spermatogonial stem cells and differentiating spermatogonia from gonocytes. It also plays critical roles in several key processes during meiosis such as genetic recombination and sex chromosome silencing. Finally, in spermiogenesis, we summarize current knowledge of the role of the ubiquitin-proteasome system in nucleosome removal and establishment of key structures in the mature spermatid. Many mechanisms remain to be precisely defined, but present knowledge indicates that research in this area has significant potential to translate into benefits that will address problems in both human and animal reproduction.

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.

Confocal imaging of the microtubule cytoskeleton in C. elegans embryos and germ cells

The microtubule cytoskeleton plays important roles in a number of cellular processes including cell division, establishing and maintaining cell architecture and polarity, and intracellular trafficking. The identification and characterization of factors required for the proper functioning of the microtubule cytoskeleton have been aided by approaches that combine sensitive and rapid methods for high-resolution optical imaging, such as confocal microscopy, with the powerful genetics available in model organisms. Here we present methods for confocal imaging of live and fixed tissues of the nematode C. elegans, a model organism that has been employed with great success to study the microtubule cytoskeleton and its roles in cell division and cell polarity.

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.

Dynamic regulation of autophagy and endocytosis for cell remodeling during early development

Fertilization triggers cell remodeling from each gamete to a totipotent zygote. Using Caenorhabditis elegans as a model system, it has been revealed that lysosomal degradation pathways play important roles in cellular remodeling during this developmental transition. Endocytosis and autophagy, two pathways leading to the lysosomes, are highly upregulated during this period. A subset of maternal membrane proteins is selectively endocytosed and degraded in the lysosomes before the first mitotic cell division. Autophagy is also induced shortly after fertilization and executes the degradation of paternally inherited embryonic organelles, e.g. mitochondria and membranous organelles. This mechanism underlies the maternal inheritance of the mitochondrial genome. Autophagy is also required for the removal of extra P-granule (germ granules in C. elegans) components in somatic cells of early embryos and thereby for the specific distribution of P-granules to germ cells. This review focuses on recent advances in the study of the physiological roles and mechanisms of lysosomal pathways during early development in C. elegans.

Control of oocyte growth and meiotic maturation in Caenorhabditis elegans

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

Rab6 is required for the exocytosis of cortical granules and the recruitment of separase to the granules during the oocyte-to-embryo transition in Caenorhabditis elegans

Remodeling of the embryo surface after fertilization is mediated by the exocytosis of cortical granules derived from the Golgi complex. This process is essential for oocyte-to-embryo transition in many species. However, how the fertilization signal reaches the cortical granules for their timely exocytosis is largely unknown. In Caenorhabditis elegans, the recruitment of separase, a downstream effector of the fertilization signal, to the cortical granules is essential for exocytosis because separase is required for membrane fusion. However, the molecule that recruits separase to the cortical granules remains unidentified. In this study, we found that Rab6, a Golgi-associated GTPase, is essential to recruit separase to the cortical granules in C. elegans embryos. Knockdown of the rab-6.1 gene, a Rab6 homolog in C. elegans, resulted in failure of the membrane fusion step of cortical granule exocytosis. Using a transgenic strain that expresses GFP-fused RAB-6.1, we found that RAB-6.1 temporarily co-localized with separase on the cortical granules for a few minutes and then was dispersed in the cytoplasm concomitantly with membrane fusion. We found that RAB-6.1, as well as cyclin-dependent kinase (CDK)-1 and anaphase promoting complex/cyclosome (APC/C), was required to recruit separase to the cortical granules. RAB-6.1 was not required for the chromosome segregation process, unlike CDK-1, APC/C and SEP-1. The results indicate that RAB-6.1 is required specifically for the membrane fusion step of exocytosis and for the recruitment of separase to the granules. Thus, RAB-6.1 is an important molecule for the timely exocytosis of the cortical granules during 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.

The near demise and subsequent revival of classical genetics for investigating Caenorhabditis elegans embryogenesis: RNAi meets next-generation DNA sequencing

Molecular genetic investigation of the early Caenorhabditis elegans embryo has contributed substantially to the discovery and general understanding of the genes, pathways, and mechanisms that regulate and execute developmental and cell biological processes. Initially, worm geneticists relied exclusively on a classical genetics approach, isolating mutants with interesting phenotypes after mutagenesis and then determining the identity of the affected genes. Subsequently, the discovery of RNA interference (RNAi) led to a much greater reliance on a reverse genetics approach: reducing the function of known genes with RNAi and then observing the phenotypic consequences. Now the advent of next-generation DNA sequencing technologies and the ensuing ease and affordability of whole-genome sequencing are reviving the use of classical genetics to investigate early C. elegans embryogenesis.

Immunofluorescence microscopy

Immunofluorescence microscopy is a powerful technique that is widely used by researchers to assess both the localization and endogenous expression levels of their favorite proteins. The application of this approach to C. elegans, however, requires special methods to overcome the diffusion barrier of a dense, collagen-based outer cuticle. This chapter outlines several alternative fixation and permeabilization strategies for overcoming this problem and for producing robust immunohistochemical staining of both whole animals and freeze-fractured samples. In addition, we provide an accounting of widely used antibody reagents available to the research community. We also describe several approaches aimed at reducing non-specific background often associated with immunohistochemical studies. Finally, we discuss a variety of approaches to raise antisera directed against C. elegans antigens.