Asymmetric segregation of PIE-1 in C. elegans is mediated by two complementary mechanisms that act through separate PIE-1 protein domains

Phospho-KNL-1 recognition by a TPR domain targets the BUB-1-BUB-3 complex to C. elegans kinetochores

During mitosis, the Bub1-Bub3 complex concentrates at kinetochores, the microtubule-coupling interfaces on chromosomes, where it contributes to spindle checkpoint activation, kinetochore-spindle microtubule interactions, and protection of centromeric cohesion. Bub1 has a conserved N-terminal tetratricopeptide repeat (TPR) domain followed by a binding motif for its conserved interactor Bub3. The current model for Bub1-Bub3 localization to kinetochores is that Bub3, along with its bound motif from Bub1, recognizes phosphorylated "MELT" motifs in the kinetochore scaffold protein Knl1. Motivated by the greater phenotypic severity of BUB-1 versus BUB-3 loss in C. elegans, we show that the BUB-1 TPR domain directly recognizes a distinct class of phosphorylated motifs in KNL-1 and that this interaction is essential for BUB-1-BUB-3 localization and function. BUB-3 recognition of phospho-MELT motifs additively contributes to drive super-stoichiometric accumulation of BUB-1-BUB-3 on its KNL-1 scaffold during mitotic entry. Bub1's TPR domain interacts with Knl1 in other species, suggesting that collaboration of TPR-dependent and Bub3-dependent interfaces in Bub1-Bub3 localization and functions may be conserved.

Germ fate determinants protect germ precursor cell division by reducing septin and anillin levels at the cell division plane

Animal cell cytokinesis, or the physical division of one cell into two, is thought to be driven by constriction of an actomyosin contractile ring at the division plane. The mechanisms underlying cell type-specific differences in cytokinesis remain unknown. Germ cells are totipotent cells that pass genetic information to the next generation. Previously, using formincyk-1(ts) mutant Caenorhabditis elegans 4-cell embryos, we found that the P2 germ precursor cell is protected from cytokinesis failure and can divide with greatly reduced F-actin levels at the cell division plane. Here, we identified two canonical germ fate determinants required for P2-specific cytokinetic protection: PIE-1 and POS-1. Neither has been implicated previously in cytokinesis. These germ fate determinants protect P2 cytokinesis by reducing the accumulation of septinUNC-59 and anillinANI-1 at the division plane, which here act as negative regulators of cytokinesis. These findings may provide insight into the regulation of cytokinesis in other cell types, especially in stem cells with high potency.

Patterning, regulation, and role of FoxO/DAF-16 in the early embryo

Insulin resistance and diabetes are associated with many health issues including higher rates of birth defects and miscarriage during pregnancy. Because insulin resistance and diabetes are both associated with obesity, which also affects fertility, the role of insulin signaling itself in embryo development is not well understood. A key downstream target of the insulin/insulin-like growth factor signaling (IIS) pathway is the forkhead family transcription factor FoxO (DAF-16 in C. elegans ). Here, we used quantitative live imaging to measure the patterning of endogenously tagged FoxO/DAF-16 in the early worm embryo. In 2-4-cell stage embryos, FoxO/DAF-16 initially localized uniformly to all cell nuclei, then became dramatically enriched in germ precursor cell nuclei beginning at the 8-cell stage. This nuclear enrichment in early germ precursor cells required germ fate specification, PI3K (AGE-1)- and PTEN (DAF-18)-mediated phospholipid regulation, and the deubiquitylase USP7 (MATH-33), yet was unexpectedly insulin receptor (DAF-2)- and AKT-independent. Functional analysis revealed that FoxO/DAF-16 acts as a cell cycle pacer for early cleavage divisions-without FoxO/DAF-16 cell cycles were shorter than in controls, especially in germ lineage cells. These results reveal the germ lineage specific patterning, upstream regulation, and cell cycle role for FoxO/DAF-16 during early C. elegans embryogenesis.

Phospho-KNL-1 recognition by a TPR domain targets the BUB-1-BUB-3 complex to C. elegans kinetochores

During mitosis, the Bub1-Bub3 complex concentrates at kinetochores, the microtubule-coupling interfaces on chromosomes, where it contributes to spindle checkpoint activation, kinetochore-spindle microtubule interactions, and protection of centromeric cohesion. Bub1 has a conserved N-terminal tetratricopeptide (TPR) domain followed by a binding motif for its conserved interactor Bub3. The current model for Bub1-Bub3 localization to kinetochores is that Bub3, along with its bound motif from Bub1, recognizes phosphorylated "MELT" motifs in the kinetochore scaffold protein Knl1. Motivated by the greater phenotypic severity of BUB-1 versus BUB-3 loss in C. elegans, we show that the BUB-1 TPR domain directly recognizes a distinct class of phosphorylated motifs in KNL-1 and that this interaction is essential for BUB-1-BUB-3 localization and function. BUB-3 recognition of phospho-MELT motifs additively contributes to drive super-stoichiometric accumulation of BUB-1-BUB-3 on its KNL-1 scaffold during mitotic entry. Bub1's TPR domain interacts with Knl1 in other species, suggesting that collaboration of TPR-dependent and Bub3-dependent interfaces in Bub1-Bub3 localization and functions may be conserved.

Germ fate determinants protect germ precursor cell division by restricting septin and anillin levels at the division plane

Animal cell cytokinesis, or the physical division of one cell into two, is thought to be driven by constriction of an actomyosin contractile ring at the division plane. The mechanisms underlying cell type-specific differences in cytokinesis remain unknown. Germ cells are totipotent cells that pass genetic information to the next generation. Previously, using formin cyk-1 (ts) mutant C. elegans embryos, we found that the P2 germ precursor cell is protected from cytokinesis failure and can divide without detectable F-actin at the division plane. Here, we identified two canonical germ fate determinants required for P2-specific cytokinetic protection: PIE-1 and POS-1. Neither has been implicated previously in cytokinesis. These germ fate determinants protect P2 cytokinesis by reducing the accumulation of septin UNC-59 and anillin ANI-1 at the division plane, which here act as negative regulators of cytokinesis. These findings may provide insight into cytokinetic regulation in other cell types, especially in stem cells with high potency.

ZIF-1-mediated degradation of zinc finger proteins in the Caenorhabditis elegans germ line

Rapid and conditional protein depletion is the gold standard genetic tool for deciphering the molecular basis of developmental processes. Previously, we showed that by conditionally expressing the E3 ligase substrate adaptor ZIF-1 in Caenorhabditis elegans somatic cells, proteins tagged with the first CCCH Zn finger 1 (ZF1) domain from the germline regulator PIE-1 degrade rapidly, resulting in loss-of-function phenotypes. The described role of ZIF-1 is to clear PIE-1 and several other CCCH Zn finger proteins from early somatic cells, helping to enrich them in germline precursor cells. Here, we show that proteins tagged with the PIE-1 ZF1 domain are subsequently cleared from primordial germ cells (PGCs) in embryos and from undifferentiated germ cells in larvae and adults by ZIF-1. We harness germline ZIF-1 activity to degrade a ZF1-tagged fusion protein from PGCs and show that its depletion produces phenotypes equivalent to those of a null mutation. Our findings reveal that ZIF-1 transitions from degrading CCCH Zn finger proteins in somatic cells to clearing them from undifferentiated germ cells, and that ZIF-1 activity can be harnessed as a new genetic tool to study the early germline.

ZIF-1-mediated degradation of endogenous and heterologous zinc finger proteins in the C. elegans germ line

Rapid and conditional protein depletion is the gold standard genetic tool for deciphering the molecular basis of developmental processes. Previously, we showed that by conditionally expressing the E3 ligase substrate adaptor ZIF-1 in Caenorhabditis elegans somatic cells, proteins tagged with the first CCCH Zn finger (ZF1) domain from the germline regulator PIE-1 degrade rapidly, resulting in loss-of-function phenotypes. The described role of ZIF-1 is to clear PIE-1 and several other CCCH Zn finger proteins from early somatic cells, helping to enrich them in germline precursor cells. Here, we show that proteins tagged with the PIE-1 ZF1 domain are subsequently cleared from primordial germ cells in embryos and from undifferentiated germ cells in larvae and adults by ZIF-1. We harness germline ZIF-1 activity to degrade a ZF1-tagged heterologous protein from PGCs and show that its depletion produces phenotypes equivalent to those of a null mutation. Our findings reveal that ZIF-1 switches roles from degrading CCCH Zn finger proteins in somatic cells to clearing them from undifferentiated germ cells, and that ZIF-1 activity can be harnessed as a new genetic tool to study the early germ line.

Mitochondrial genome recovery by ATFS-1 is essential for development after starvation

Nutrient availability regulates the C. elegans life cycle as well as mitochondrial physiology. Food deprivation significantly reduces mitochondrial genome (mtDNA) numbers and leads to aging-related phenotypes. Here we show that the bZIP (basic leucine zipper) protein ATFS-1, a mediator of the mitochondrial unfolded protein response (UPRmt), is required to promote growth and establish a functional germline after prolonged starvation. We find that recovery of mtDNA copy numbers and development after starvation requires mitochondrion-localized ATFS-1 but not its nuclear transcription activity. We also find that the insulin-like receptor DAF-2 functions upstream of ATFS-1 to modulate mtDNA content. We show that reducing DAF-2 activity represses ATFS-1 nuclear function while causing an increase in mtDNA content, partly mediated by mitochondrion-localized ATFS-1. Our data indicate the importance of the UPRmt in recovering mitochondrial mass and suggest that atfs-1-dependent mtDNA replication precedes mitochondrial network expansion after starvation.

PIE-1 SUMOylation promotes germline fates and piRNA-dependent silencing in C. elegans

Germlines shape and balance heredity, integrating and regulating information from both parental and foreign sources. Insights into how germlines handle information have come from the study of factors that specify or maintain the germline fate. In early Caenorhabditis elegans embryos, the CCCH zinc finger protein PIE-1 localizes to the germline where it prevents somatic differentiation programs. Here, we show that PIE-1 also functions in the meiotic ovary where it becomes SUMOylated and engages the small ubiquitin-like modifier (SUMO)-conjugating machinery. Using whole-SUMO-proteome mass spectrometry, we identify HDAC SUMOylation as a target of PIE-1. Our analyses of genetic interactions between pie-1 and SUMO pathway mutants suggest that PIE-1 engages the SUMO machinery both to preserve the germline fate in the embryo and to promote Argonaute-mediated surveillance in the adult germline.

A polarity pathway for exocyst-dependent intracellular tube extension

Lumen extension in intracellular tubes can occur when vesicles fuse with an invading apical membrane. Within the Caenorhabditis elegans excretory cell, which forms an intracellular tube, the exocyst vesicle-tethering complex is enriched at the lumenal membrane and is required for its outgrowth, suggesting that exocyst-targeted vesicles extend the lumen. Here, we identify a pathway that promotes intracellular tube extension by enriching the exocyst at the lumenal membrane. We show that PAR-6 and PKC-3/aPKC concentrate at the lumenal membrane and promote lumen extension. Using acute protein depletion, we find that PAR-6 is required for exocyst membrane recruitment, whereas PAR-3, which can recruit the exocyst in mammals, appears dispensable for exocyst localization and lumen extension. Finally, we show that CDC-42 and RhoGEF EXC-5/FGD regulate lumen extension by recruiting PAR-6 and PKC-3 to the lumenal membrane. Our findings reveal a pathway that connects CDC-42, PAR proteins, and the exocyst to extend intracellular tubes.

The developmental and physiological roles of phagocytosis in Caenorhabditis elegans

Phagocytosis is an essential process by which cellular debris and pathogens are cleared from the environment. Cells extend their plasma membrane to engulf objects and contain them within a limiting membrane for isolation from the cytosol or for intracellular degradation in phagolysosomes. The basic mechanisms of phagocytosis and intracellular clearance are well conserved between animals. Indeed, much of our understanding is derived from studies on the nematode worm, Caenorhabditis elegans. Here, we review the latest progress in understanding the mechanisms and functions of phagocytic clearance from C. elegans studies. In particular, we highlight new insights into phagocytic signaling pathways, phagosome formation and phagolysosome resolution, as well as the challenges in studying these cyclic processes.

Centrosome Aurora A gradient ensures single polarity axis in C. elegans embryos

Cellular asymmetries are vital for generating cell fate diversity during development and in stem cells. In the newly fertilized Caenorhabditis elegans embryo, centrosomes are responsible for polarity establishment, i.e. anterior-posterior body axis formation. The signal for polarity originates from the centrosomes and is transmitted to the cell cortex, where it disassembles the actomyosin network. This event leads to symmetry breaking and the establishment of distinct domains of evolutionarily conserved PAR proteins. However, the identity of an essential component that localizes to the centrosomes and promotes symmetry breaking was unknown. Recent work has uncovered that the loss of Aurora A kinase (AIR-1 in C. elegans and hereafter referred to as Aurora A) in the one-cell embryo disrupts stereotypical actomyosin-based cortical flows that occur at the time of polarity establishment. This misregulation of actomyosin flow dynamics results in the occurrence of two polarity axes. Notably, the role of Aurora A in ensuring a single polarity axis is independent of its well-established function in centrosome maturation. The mechanism by which Aurora A directs symmetry breaking is likely through direct regulation of Rho-dependent contractility. In this mini-review, we will discuss the unconventional role of Aurora A kinase in polarity establishment in C. elegans embryos and propose a refined model of centrosome-dependent symmetry breaking.

The Role of pkc-3 and Genetic Suppressors in Caenorhabditis elegans Epithelial Cell Junction Formation

Epithelial cells form intercellular junctions to strengthen cell-cell adhesion and limit diffusion, allowing epithelia to function as dynamic tissues and barriers separating internal and external environments. Junctions form as epithelial cells differentiate; clusters of junction proteins first concentrate apically, then mature into continuous junctional belts that encircle and connect each cell. In mammals and Drosophila, atypical protein kinase C (aPKC) is required for junction maturation, although how it contributes to this process is poorly understood. A role for the Caenorhabditis elegans aPKC homolog PKC-3 in junction formation has not been described previously. Here, we show that PKC-3 is essential for junction maturation as epithelia first differentiate. Using a temperature-sensitive allele of pkc-3 that causes junction breaks in the spermatheca and leads to sterility, we identify intragenic and extragenic suppressors that render pkc-3 mutants fertile. Intragenic suppressors include an unanticipated stop-to-stop mutation in the pkc-3 gene, providing evidence for the importance of stop codon identity in gene activity. One extragenic pkc-3 suppressor is a loss-of-function allele of the lethal(2) giant larvae homolog lgl-1, which antagonizes aPKC within epithelia of Drosophila and mammals, but was not known previously to function in C. elegans epithelia. Finally, two extragenic suppressors are loss-of-function alleles of sups-1-a previously uncharacterized gene. We show that SUPS-1 is an apical extracellular matrix protein expressed in epidermal cells, suggesting that it nonautonomously regulates junction formation in the spermatheca. These findings establish a foundation for dissecting the role of PKC-3 and interacting genes in epithelial junction maturation.

Centrosome Aurora A regulates RhoGEF ECT-2 localisation and ensures a single PAR-2 polarity axis in C. elegans embryos

Proper establishment of cell polarity is essential for development. In the one-cell C. elegans embryo, a centrosome-localised signal provides spatial information for polarity establishment. It is hypothesised that this signal causes local inhibition of the cortical actomyosin network, and breaks symmetry to direct partitioning of the PAR proteins. However, the molecular nature of the centrosomal signal that triggers cortical anisotropy in the actomyosin network to promote polarity establishment remains elusive. Here, we discover that depletion of Aurora A kinase (AIR-1 in C. elegans) causes pronounced cortical contractions on the embryo surface, and this creates more than one PAR-2 polarity axis. This function of AIR-1 appears to be independent of its role in microtubule nucleation. Importantly, upon AIR-1 depletion, centrosome positioning becomes dispensable in dictating the PAR-2 axis. Moreover, we uncovered that a Rho GEF, ECT-2, acts downstream of AIR-1 in regulating contractility and PAR-2 localisation, and, notably, AIR-1 depletion influences ECT-2 cortical localisation. Overall, this study provides a novel insight into how an evolutionarily conserved centrosome Aurora A kinase inhibits promiscuous PAR-2 domain formation to ensure singularity in the polarity establishment axis.

The Caenorhabditis elegans Transgenic Toolbox

The power of any genetic model organism is derived, in part, from the ease with which gene expression can be manipulated. The short generation time and invariant developmental lineage have made Caenorhabditis elegans very useful for understanding, e.g., developmental programs, basic cell biology, neurobiology, and aging. Over the last decade, the C. elegans transgenic toolbox has expanded considerably, with the addition of a variety of methods to control expression and modify genes with unprecedented resolution. Here, we provide a comprehensive overview of transgenic methods in C. elegans, with an emphasis on recent advances in transposon-mediated transgenesis, CRISPR/Cas9 gene editing, conditional gene and protein inactivation, and bipartite systems for temporal and spatial control of expression.

MRG-1 is required for both chromatin-based transcriptional silencing and genomic integrity of primordial germ cells in Caenorhabditis elegans

In Caenorhabditis elegans, germline cells remain transcriptionally silenced during embryogenesis. The transcriptional silencing is achieved by two different mechanisms: One is the inhibition of RNA polymerase II in P2-P4 cells at the establishment stage, and another is chromatin-based silencing in two primordial germ cells (PGCs) at the maintenance stage; however, the molecular mechanism underlying chromatin-based silencing is less understood. We investigated the role of the chromodomain protein MRG-1, which is an essential maternal factor for germline development, in transcriptional silencing in PGCs. PGCs lacking maternal MRG-1 showed increased levels of two histone modifications (H3K4me2 and H4K16ac), which are epigenetic markers for active transcription, and precocious activation of germline promoters. Loss of MES-4, a H3K36 methyltransferase, also caused similar derepression of the germline genes in PGCs, suggesting that both MRG-1 and MES-4 function in chromatin-based silencing in PGCs. In addition, the mrg-1 null mutant showed abnormal chromosome structures and a decrease in homologous recombinase RAD-51 foci in PGCs, but the mes-4 null mutant did not show such phenotypes. Taken together, we propose that MRG-1 has two distinct functions: chromatin-based transcriptional silencing and preserving genomic integrity at the maintenance stage of PGCs.

PIE-1 Translation in the Germline Lineage Contributes to PIE-1 Asymmetry in the Early Caenorhabditis elegans Embryo

In the C. elegans embryo, the germline lineage is established through successive asymmetric cell divisions that each generate a somatic and a germline daughter cell. PIE-1 is an essential maternal factor that is enriched in embryonic germline cells and is required for germline specification. We estimated the absolute concentration of PIE-1::GFP in germline cells and find that PIE-1::GFP concentration increases by roughly 4.5 fold, from 92 nM to 424 nM, between the 1 and 4-cell stages. Previous studies have shown that the preferential inheritance of PIE-1 by germline daughter cells and the degradation of PIE-1 in somatic cells are important for PIE-1 enrichment in germline cells. In this study, we provide evidence that the preferential translation of maternal PIE-1::GFP transcripts in the germline also contributes to PIE-1::GFP enrichment. Through an RNAi screen, we identified Y14 and MAG-1 (Drosophila tsunagi and mago nashi) as regulators of embryonic PIE-1::GFP levels. We show that Y14 and MAG-1 do not regulate PIE-1 degradation, segregation or synthesis in the early embryo, but do regulate the concentration of maternally-deposited PIE-1::GFP. Taken together, or findings point to an important role for translational control in the regulation of PIE-1 levels in the germline lineage.

Spindle assembly checkpoint strength is linked to cell fate in the Caenorhabditis elegans embryo

The spindle assembly checkpoint (SAC) is a conserved mitotic regulator that preserves genome stability by monitoring kinetochore-microtubule attachments and blocking anaphase onset until chromosome biorientation is achieved. Despite its central role in maintaining mitotic fidelity, the ability of the SAC to delay mitotic exit in the presence of kinetochore-microtubule attachment defects (SAC "strength") appears to vary widely. How different cellular aspects drive this variation remains largely unknown. Here we show that SAC strength is correlated with cell fate during development of Caenorhabditis elegans embryos, with germline-fated cells experiencing longer mitotic delays upon spindle perturbation than somatic cells. These differences are entirely dependent on an intact checkpoint and only partially attributable to differences in cell size. In two-cell embryos, cell size accounts for half of the difference in SAC strength between the larger somatic AB and the smaller germline P₁ blastomeres. The remaining difference requires asymmetric cytoplasmic partitioning downstream of PAR polarity proteins, suggesting that checkpoint-regulating factors are distributed asymmetrically during early germ cell divisions. Our results indicate that SAC activity is linked to cell fate and reveal a hitherto unknown interaction between asymmetric cell division and the SAC.

Polo-like Kinase Couples Cytoplasmic Protein Gradients in the C. elegans Zygote

Intracellular protein gradients underlie essential cellular and developmental processes, but the mechanisms by which they are established are incompletely understood. During the asymmetric division of the C. elegans zygote, the RNA-binding protein MEX-5 forms an anterior-rich cytoplasmic gradient that causes the RNA-binding protein POS-1 to form an opposing, posterior-rich gradient. We demonstrate that the polo-like kinase PLK-1 mediates the repulsive coupling between MEX-5 and POS-1 by increasing the mobility of POS-1 in the anterior. PLK-1 is enriched in the anterior cytoplasm and phosphorylates POS-1, which is both necessary and sufficient to increase POS-1 mobility. Regulation of POS-1 mobility depends on both the interaction between PLK-1 and MEX-5 and between MEX-5 and RNA, suggesting that MEX-5 may recruit PLK-1 to RNA in the anterior. The low concentration of MEX-5/PLK-1 in the posterior cytoplasm provides a permissive environment for the retention of POS-1, which depends on POS-1 RNA binding. Our findings describe a novel reaction/diffusion mechanism in which the asymmetric distribution of cytoplasmic PLK-1 couples two RNA-binding protein gradients, thereby partitioning the cytoplasm.

Cdc42 regulates junctional actin but not cell polarization in the Caenorhabditis elegans epidermis

During morphogenesis, adherens junctions (AJs) remodel to allow changes in cell shape and position while preserving adhesion. Here, we examine the function of Rho guanosine triphosphatase CDC-42 in AJ formation and regulation during Caenorhabditis elegans embryo elongation, a process driven by asymmetric epidermal cell shape changes. cdc-42 mutant embryos arrest during elongation with epidermal ruptures. Unexpectedly, we find using time-lapse fluorescence imaging that cdc-42 is not required for epidermal cell polarization or junction assembly, but rather is needed for proper junctional actin regulation during elongation. We show that the RhoGAP PAC-1/ARHGAP21 inhibits CDC-42 activity at AJs, and loss of PAC-1 or the interacting linker protein PICC-1/CCDC85A-C blocks elongation in embryos with compromised AJ function. pac-1 embryos exhibit dynamic accumulations of junctional F-actin and an increase in AJ protein levels. Our findings identify a previously unrecognized molecular mechanism for inhibiting junctional CDC-42 to control actin organization and AJ protein levels during epithelial morphogenesis.

A toolkit for GFP-mediated tissue-specific protein degradation in C. elegans

Proteins that are essential for embryo production, cell division and early embryonic events are frequently reused later in embryogenesis, during organismal development or in the adult. Examining protein function across these different biological contexts requires tissue-specific perturbation. Here, we describe a method that uses expression of a fusion between a GFP-targeting nanobody and a SOCS-box containing ubiquitin ligase adaptor to target GFP-tagged proteins for degradation. When combined with endogenous locus GFP tagging by CRISPR-Cas9 or with rescue of a null mutant with a GFP fusion, this approach enables routine and efficient tissue-specific protein ablation. We show that this approach works in multiple tissues - the epidermis, intestine, body wall muscle, ciliated sensory neurons and touch receptor neurons - where it recapitulates expected loss-of-function mutant phenotypes. The transgene toolkit and the strain set described here will complement existing approaches to enable routine analysis of the tissue-specific roles of C. elegans proteins.

Functional analysis of microRNA pathway genes in the somatic gonad and germ cells during ovulation in C. elegans

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play critical roles in animal development and physiology, though functions for most miRNAs remain unknown. Worms with reduced miRNA biogenesis due to loss of Drosha or Pasha/DGCR8 activity are sterile and fail to ovulate, indicating that miRNAs are required for the process of oocyte maturation and ovulation. Starting with this penetrant sterile phenotype and using new strains created to perform tissue specific RNAi, we characterized the roles of the C. elegans Pasha, pash-1, and two miRNA-specific Argonautes, alg-1 and alg-2, in somatic gonad cells and in germ cells in the regulation of ovulation. Conditional loss of pash-1 activity resulted in a reduced rate of ovulation and in basal and ovulatory sheath contractions. Similarly, knockdown of miRNA-specific Argonautes in the cells of the somatic gonad by tissue-specific RNAi results in a reduction of the ovulation rate and in basal and ovulatory sheath contractions. Reduced miRNA pathway gene activity resulted in a range of defects, including oocytes that were pinched upon entry of the oocyte into the distal end of the spermatheca in about 42% of the ovulation events observed following alg-1 RNAi. This phenotype was not observed on worms exposed to control RNAi. In contrast, knockdown of alg-1 and alg-2 in germ cells results in few defects in oocyte maturation and ovulation. These data identify specific steps in the process of ovulation that require miRNA pathway gene activity in the somatic gonad cells.

Plant Development: Suppression the Key to Asymmetric Cell Fate

A new study shows that SPEECHLESS determines cell fate in the stomatal lineage but is inherited equally by daughter cells following an asymmetric cell division. The polarity determinant BASL acts as a MAPK scaffold, targeting SPEECHLESS for degradation in the larger daughter cell.

Alternative splicing of U2AF1 reveals a shared repression mechanism for duplicated exons

The auxiliary factor of U2 small nuclear ribonucleoprotein (U2AF) facilitates branch point (BP) recognition and formation of lariat introns. The gene for the 35-kD subunit of U2AF gives rise to two protein isoforms (termed U2AF35a and U2AF35b) that are encoded by alternatively spliced exons 3 and Ab, respectively. The splicing recognition sequences of exon 3 are less favorable than exon Ab, yet U2AF35a expression is higher than U2AF35b across tissues. We show that U2AF35b repression is facilitated by weak, closely spaced BPs next to a long polypyrimidine tract of exon Ab. Each BP lacked canonical uridines at position -2 relative to the BP adenines, with efficient U2 base-pairing interactions predicted only for shifted registers reminiscent of programmed ribosomal frameshifting. The BP cluster was compensated by interactions involving unpaired cytosines in an upstream, EvoFold-predicted stem loop (termed ESL) that binds FUBP1/2. Exon Ab inclusion correlated with predicted free energies of mutant ESLs, suggesting that the ESL operates as a conserved rheostat between long inverted repeats upstream of each exon. The isoform-specific U2AF35 expression was U2AF65-dependent, required interactions between the U2AF-homology motif (UHM) and the α6 helix of U2AF35, and was fine-tuned by exon Ab/3 variants. Finally, we identify tandem homologous exons regulated by U2AF and show that their preferential responses to U2AF65-related proteins and SRSF3 are associated with unpaired pre-mRNA segments upstream of U2AF-repressed 3′ss. These results provide new insights into tissue-specific subfunctionalization of duplicated exons in vertebrate evolution and expand the repertoire of exon repression mechanisms that control alternative splicing.

C. elegans midbodies are released, phagocytosed and undergo LC3-dependent degradation independent of macroautophagy

In animals, the midbody coordinates the end of cytokinesis when daughter cells separate through abscission. The midbody was thought to be sequestered by macroautophagy, but recent evidence suggests that midbodies are primarily released and phagocytosed. It was unknown, however, whether autophagy proteins play a role in midbody phagosome degradation. Using a protein degradation assay, we show that midbodies are released in Caenorhabditiselegans. Released midbodies are known to be internalized by actin-driven phagocytosis, which we show requires the RAB-5 GTPase to localize the class III phosphoinositide 3-kinase (PI3K) complex at the cortex. Autophagy-associated proteins, including the Beclin 1 homolog BEC-1 and the Atg8/LC3-family members LGG-1 and LGG-2, localize around the midbody phagosome and are required for midbody degradation. In contrast, proteins required specifically for macroautophagy, such as UNC-51 and EPG-8 (homologous to ULK1/Atg1 and Atg14, respectively) are not required for midbody degradation. These data suggest that the C. elegans midbody is degraded by LC3-associated phagocytosis (LAP), not macroautophagy. Our findings reconcile the two prevailing models on the role of phagocytic and autophagy proteins, establishing a new non-canonical role for autophagy proteins in midbody degradation.

Summary: Autophagy proteins are required for the degradation of midbodies. In C. elegans, Atg8/LC3-family proteins act during phagosome maturation rather than during macroautophagy.

Three Residues Make an Evolutionary Switch for Folding and RNA-Destabilizing Activity in the TTP Family of Proteins

Tristetraprolin (TTP) binds to mRNA transcripts to promote their degradation. The TTP protein family in humans includes two other proteins, TIS11b and TIS11d. All three proteins contain a highly homologous RNA binding domain (RBD) that consists of two CCCH zinc fingers (ZFs). Both ZFs are folded in the absence of RNA in TIS11d and TIS11b. In TTP, however, only ZF1 adopts a stable fold. The focus of this study is to understand the origin and biological significance of the structural differences of the RBD. We identified three residues that affect the affinity for the structural Zn(2+) and determine the folding of ZF2 in the absence of RNA. We observed that the mRNA destabilizing activity of TTP was increased when the partially disordered RBD of TTP was replaced with the fully structured RBD of TIS11d, indicating that differences in the folded state of the RBD affect the activity of the proteins in the cell.

Restricted distribution of mrg-1 mRNA in C. elegans primordial germ cells through germ granule-independent regulation

The chromodomain protein MRG-1 is an essential maternal factor for proper germline development that protects germ cells from cell death in C. elegans. Unlike germ granules, which are exclusively segregated to the germline blastomeres at each cell division from the first cleavage of the embryo, MRG-1 is abundant in all cells in early embryos and is then gradually restricted to the primordial germ cells (PGCs) by the morphogenesis stage. Here, we show that this characteristic spatiotemporal expression pattern is dictated by the mrg-1 3'UTR and is differentially regulated at the RNA level between germline and somatic cells. Asymmetric segregation of germ granules is not necessary to localize MRG-1 to the PGCs. We found that MES-4, an essential chromatin regulator in germ cells, also accumulates in the PGCs in a germ granule-independent manner. We propose that C.elegans PGCs have a novel mechanism to accumulate at least some chromatin-associated proteins that are essential for germline immortality.

Coupling between cytoplasmic concentration gradients through local control of protein mobility in the Caenorhabditis elegans zygote

Cell polarity is characterized by the asymmetric distribution of factors at the cell cortex and in the cytoplasm. Although mechanisms that establish cortical asymmetries have been characterized, less is known about how persistent cytoplasmic asymmetries are generated. During the asymmetric division of the Caenorhabditis elegans zygote, the PAR proteins orchestrate the segregation of the cytoplasmic RNA-binding proteins MEX-5/6 to the anterior cytoplasm and PIE-1, POS-1, and MEX-1 to the posterior cytoplasm. In this study, we find that MEX-5/6 control the segregation of GFP::PIE-1, GFP::POS-1, and GFP::MEX-1 by locally increasing their mobility in the anterior cytoplasm. Remarkably, PIE-1, POS-1, and MEX-1 form gradients with distinct strengths, which correlates with differences in their responsiveness to MEX-5/6. We show that MEX-5/6 act downstream of the polarity regulators PAR-1 and PAR-3 and in a concentration-dependent manner to increase the mobility of GFP::PIE-1. These findings suggest that the MEX-5/6 concentration gradients are directly coupled to the establishment of posterior-rich PIE-1, POS-1, and MEX-1 concentration gradients via the formation of anterior-fast, posterior-slow mobility gradients.

Mitotic chromosome length scales in response to both cell and nuclear size

Quantitative analysis relating cell, nuclear, and chromosome size in C. elegans embryos predicts two levels of chromosome length regulation in response to both cell and nuclear size.

Multicellular development requires that cells reduce in size as a result of consecutive cell divisions without increase in embryo volume. To maintain cellular integrity, organelle size adapts to cell size throughout development. During mitosis, the longest chromosome arm must be shorter than half of the mitotic spindle for proper chromosome segregation. Using high-resolution time-lapse microscopy of living Caenorhabditis elegans embryos, we have quantified the relation between cell size and chromosome length. In control embryos, chromosome length scaled to cell size. Artificial reduction of cell size resulted in a shortening of chromosome length, following a trend predicted by measurements from control embryos. Disturbing the RAN (Ras-related nuclear protein)-GTP gradient decoupled nuclear size from cell size and resulted in chromosome scaling to nuclear size rather than cell size; smaller nuclei contained shorter chromosomes independent of cell size. In sum, quantitative analysis relating cell, nuclear, and chromosome size predicts two levels of chromosome length regulation: one through cell size and a second in response to nuclear size.

Cytoplasmic localization and asymmetric division in the early embryo of Caenorhabditis elegans

During the initial cleavages of the Caenorhabditis elegans embryo, a series of rapid and invariant asymmetric cell divisions pattern the fate, size, and position of four somatic blastomeres and a single germline blastomere. These asymmetric divisions are orchestrated by a collection of maternally deposited factors that are initially symmetrically distributed in the newly fertilized embryo. Maturation of the sperm-derived centrosome in the posterior cytoplasm breaks this symmetry by triggering a dramatic and highly stereotyped partitioning of these maternal factors. A network of conserved cell polarity regulators, the PAR proteins, form distinct anterior and posterior domains at the cell cortex. From these domains, the PAR proteins direct the segregation of somatic and germline factors into opposing regions of the cytoplasm such that, upon cell division, they are preferentially inherited by the somatic blastomere or the germline blastomere, respectively. The segregation of these factors is controlled, at least in part, by a series of reaction-diffusion mechanisms that are asymmetrically deployed along the anterior/posterior axis. The characterization of these mechanisms has important implications for our understanding of how cells are polarized and how spatial organization is generated in the cytoplasm. For further resources related to this article, please visit the WIREs website.

A novel function for the Caenorhabditis elegans torsin OOC-5 in nucleoporin localization and nuclear import

Mutation in the human AAA+ protein torsinA leads to DYT1 dystonia. Loss of a Caenorhabditis elegans torsin, OOC-5, leads to defects in nucleoporin localization and nuclear import, a novel phenotype for a torsin mutant. NE ultrastructural defects similar to those in mouse and fly torsin mutants are also found, showing conservation of function.

Torsin proteins are AAA+ ATPases that localize to the endoplasmic reticular/nuclear envelope (ER/NE) lumen. A mutation that markedly impairs torsinA function causes the CNS disorder DYT1 dystonia. Abnormalities of NE membranes have been linked to torsinA loss of function and the pathogenesis of DYT1 dystonia, leading us to investigate the role of the Caenorhabditis elegans torsinA homologue OOC-5 at the NE. We report a novel role for torsin in nuclear pore biology. In ooc-5–mutant germ cell nuclei, nucleoporins (Nups) were mislocalized in large plaques beginning at meiotic entry and persisted throughout meiosis. Moreover, the KASH protein ZYG-12 was mislocalized in ooc-5 gonads. Nups were mislocalized in adult intestinal nuclei and in embryos from mutant mothers. EM analysis revealed vesicle-like structures in the perinuclear space of intestinal and germ cell nuclei, similar to defects reported in torsin-mutant flies and mice. Consistent with a functional disruption of Nups, ooc-5–mutant embryos displayed impaired nuclear import kinetics, although the nuclear pore-size exclusion barrier was maintained. Our data are the first to demonstrate a requirement for a torsin for normal Nup localization and function and suggest that these functions are likely conserved.

Repurposing an endogenous degradation system for rapid and targeted depletion of C. elegans proteins

The capability to conditionally inactivate gene function is essential for understanding the molecular basis of development. In gene and mRNA targeting approaches, protein products can perdure, complicating genetic analysis. Current methods for selective protein degradation require drug treatment or take hours for protein removal, limiting their utility in studying rapid developmental processes in vivo. Here, we repurpose an endogenous protein degradation system to rapidly remove targeted C. elegans proteins. We show that upon expression of the E3 ubiquitin ligase substrate-recognition subunit ZIF-1, proteins tagged with the ZF1 zinc-finger domain can be quickly degraded in all somatic cell types examined with temporal and spatial control. We demonstrate that genes can be engineered to become conditional loss-of-function alleles by introducing sequences encoding the ZF1 tag into endogenous loci. Finally, we use ZF1 tagging to establish the site of cdc-42 gene function during a cell invasion event. ZF1 tagging provides a powerful new tool for the analysis of dynamic developmental events.

Polarized exocyst-mediated vesicle fusion directs intracellular lumenogenesis within the C. elegans excretory cell

Lumenogenesis of small seamless tubes occurs through intracellular membrane growth and directed vesicle fusion events. Within the Caenorhabditis elegans excretory cell, which forms seamless intracellular tubes (canals) that mediate osmoregulation, lumens grow in length and diameter when vesicles fuse with the expanding lumenal surface. Here, we show that lumenal vesicle fusion depends on the small GTPase RAL-1, which localizes to vesicles and acts through the exocyst vesicle-tethering complex. Loss of either the exocyst or RAL-1 prevents excretory canal lumen extension. Within the excretory canal and other polarized cells, the exocyst co-localizes with the PAR polarity proteins PAR-3, PAR-6 and PKC-3. Using early embryonic cells to determine the functional relationships between the exocyst and PAR proteins, we show that RAL-1 recruits the exocyst to the membrane, while PAR proteins concentrate membrane-localized exocyst proteins to a polarized domain. These findings reveal that RAL-1 and the exocyst direct the polarized vesicle fusion events required for intracellular lumenogenesis of the excretory cell, suggesting mechanistic similarities in the formation of topologically distinct multicellular and intracellular lumens.

The biology of the germ line in echinoderms

The formation of the germ line in an embryo marks a fresh round of reproductive potential. The developmental stage and location within the embryo where the primordial germ cells (PGCs) form, however, differs markedly among species. In many animals, the germ line is formed by an inherited mechanism, in which molecules made and selectively partitioned within the oocyte drive the early development of cells that acquire this material to a germ-line fate. In contrast, the germ line of other animals is fated by an inductive mechanism that involves signaling between cells that directs this specialized fate. In this review, we explore the mechanisms of germ-line determination in echinoderms, an early-branching sister group to the chordates. One member of the phylum, sea urchins, appears to use an inherited mechanism of germ-line formation, whereas their relatives, the sea stars, appear to use an inductive mechanism. We first integrate the experimental results currently available for germ-line determination in the sea urchin, for which considerable new information is available, and then broaden the investigation to the lesser-known mechanisms in sea stars and other echinoderms. Even with this limited insight, it appears that sea stars, and perhaps the majority of the echinoderm taxon, rely on inductive mechanisms for germ-line fate determination. This enables a strongly contrasted picture for germ-line determination in this phylum, but one for which transitions between different modes of germ-line determination might now be experimentally addressed.

Protection of germline gene expression by the C. elegans Argonaute CSR-1

In Caenorhabditis elegans, the Piwi-interacting small RNA (piRNA)-mediated germline surveillance system encodes more than 30,000 unique 21-nucleotide piRNAs, which silence a variety of foreign nucleic acids. What mechanisms allow endogenous germline-expressed transcripts to evade silencing by the piRNA pathway? One likely candidate in a protective mechanism is the Argonaute CSR-1, which interacts with 22G-small RNAs that are antisense to nearly all germline-expressed genes. Here, we use an in vivo RNA tethering assay to demonstrate that the recruitment of CSR-1 to a transcript licenses expression of the transcript, protecting it from piRNA-mediated silencing. Licensing occurs mainly at the level of transcription, as we observe changes in pre-mRNA levels consistent with transcriptional activation when CSR-1 is tethered. Furthermore, the recruitment of CSR-1 to a previously silenced locus transcriptionally activates its expression. Together, these results demonstrate a rare positive role for an endogenous Argonaute pathway in heritably licensing and protecting germline transcripts.

Regulation of maternal Wnt mRNA translation in C. elegans embryos

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

The oocyte-to-embryo transition

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

Cell polarity and asymmetric cell division: the C. elegans early embryo

Cell polarity is crucial for many functions including cell migration, tissue organization and asymmetric cell division. In animal cells, cell polarity is controlled by the highly conserved PAR (PARtitioning defective) proteins. par genes have been identified in Caenorhabditis elegans in screens for maternal lethal mutations that disrupt cytoplasmic partitioning and asymmetric division. Although PAR proteins were identified more than 20 years ago, our understanding on how they regulate polarity and how they are regulated is still incomplete. In this chapter we review our knowledge of the processes of cell polarity establishment and maintenance, and asymmetric cell division in the early C. elegans embryo. We discuss recent findings that highlight new players in cell polarity and/or reveal the molecular details on how PAR proteins regulate polarity processes.

A specific set of exon junction complex subunits is required for the nuclear retention of unspliced RNAs in Caenorhabditis elegans

The exon junction complex (EJC) is highly conserved in many organisms and is involved in various steps of mRNA metabolism. During the course of investigating the role of EJC in the germ line sex determination of the nematode Caenorhabditis elegans, we found that depletion of one of the three core subunits (Y14, MAG-1, and eukaryotic translation initiation factor 4III [eIF4AIII]) or one auxiliary subunit (UAP56) of EJC resulted in the cytoplasmic leakage of unspliced RNAs from almost all of the C. elegans protein-coding genes examined thus far. This leakage was also observed with the depletion of several splicing factors, including SF3b, IBP160, and PRP19, all of which genetically interacted with Y14. We also found that Y14 physically interacts with both pre-mRNA and spliceosomal U snRNAs, especially U2 snRNA, and that the interaction was abolished when both IBP160 and PRP19 were depleted. Our results strongly suggest that a specific set of EJC subunits is recruited onto introns and interacts with components of the spliceosome, including U2 snRNP, to provide a critical signal for the surveillance and nuclear retention of unspliced RNAs in C. elegans.

Inactivation of conserved C. elegans genes engages pathogen- and xenobiotic-associated defenses

The nematode C. elegans is attracted to nutritious bacteria and is repelled by pathogens and toxins. Here we show that RNAi and toxin-mediated disruption of core cellular activities, including translation, respiration, and protein turnover, stimulate behavioral avoidance of normally attractive bacteria. RNAi of these and other essential processes induces expression of detoxification and innate immune effectors, even in the absence of toxins or pathogens. Disruption of core processes in non-neuronal tissues was sufficient to stimulate aversion behavior, revealing a neuroendocrine axis of control that additionally required serotonergic and Jnk kinase signaling pathways. We propose that surveillance pathways overseeing core cellular activities allow animals to detect invading pathogens that deploy toxins and virulence factors to undermine vital host functions. Variation in cellular surveillance and endocrine pathways controlling behavior, detoxification, and immunity selected by past toxin or microbial interactions could underlie aberrant responses to foods, medicines, and microbes.

An E-cadherin-mediated hitchhiking mechanism for C. elegans germ cell internalization during gastrulation

Gastrulation movements place endodermal precursors, mesodermal precursors and primordial germ cells (PGCs) into the interior of the embryo. Somatic cell gastrulation movements are regulated by transcription factors that also control cell fate, coupling cell identity and position. By contrast, PGCs in many species are transcriptionally quiescent, suggesting that they might use alternative gastrulation strategies. Here, we show that C. elegans PGCs internalize by attaching to internal endodermal cells, which undergo morphogenetic movements that pull the PGCs into the embryo. We show that PGCs enrich HMR-1/E-cadherin at their surfaces to stick to endoderm. HMR-1 expression in PGCs is necessary and sufficient to ensure internalization, suggesting that HMR-1 can promote PGC-endoderm adhesion through a mechanism other than homotypic trans interactions between the two cell groups. Finally, we demonstrate that the hmr-1 3' untranslated region promotes increased HMR-1 translation in PGCs. Our findings reveal that quiescent PGCs employ a post-transcriptionally regulated hitchhiking mechanism to internalize during gastrulation, and demonstrate a morphogenetic role for the conserved association of PGCs with the endoderm.

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.

Characterization of DeltaN-Zfp36l2 mutant associated with arrest of early embryonic development and female infertility

The zinc finger protein 36-like 2, Zfp36l2, has been implicated in female mouse infertility, because an amino-terminal truncation mutation (ΔN-Zfp36l2) leads to two-cell stage arrest of embryos derived from the homozygous mutant female gamete. Zfp36l2 is a member of the tristetraprolin (TTP) family of CCCH tandem zinc finger proteins that can bind to transcripts containing AU-rich elements (ARE), resulting in deadenylation and destabilization of these transcripts. I show here that the mouse Zfp36l2 is composed of two exons and a single intron, encoding a polypeptide of 484 amino acids. I observed that ΔN-Zfp36l2 protein is similar to both wild-type Zfp36l2 and TTP (Zfp36) in that it shuttles between the cytoplasm and nucleus, binds to RNAs containing AREs, and promotes deadenylation of a model ARE transcript in a cell-based co-transfection assay. Surprisingly, in contrast to TTP, Zfp36l2 mRNA and protein were rapidly down-regulated upon LPS exposure in bone marrow-derived macrophages. The ΔN-Zfp36l2 protein was substantially more resistant to stimulus-induced down-regulation than the WT. I postulate that the embryonic arrest linked to the ΔN-Zfp36l2 truncation might be related to its resistance to stimulus-induced down-regulation.

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

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

Dissection of the NUP107 nuclear pore subcomplex reveals a novel interaction with spindle assembly checkpoint protein MAD1 in Caenorhabditis elegans

Nuclear pore complex assembly and kinetochore function depend on the NUP107 subcomplex, but the roles of each of its nine constituents are unknown. NUP107 itself is shown to be dispensable for NPC assembly but needed for proper localization of kinetochore protein NUF2 and Aurora B kinase. Moreover, a novel interaction is found with SAC protein MAD1.

Nuclear pore complexes consist of several subcomplexes. The NUP107 complex is important for nucleocytoplasmic transport, nuclear envelope assembly, and kinetochore function. However, the underlying molecular mechanisms and the roles of individual complex members remain elusive. We report the first description of a genetic disruption of NUP107 in a metazoan. Caenorhabditis elegans NUP107/npp-5 mutants display temperature-dependent lethality. Surprisingly, NPP-5 is dispensable for incorporation of most nucleoporins into nuclear pores and for nuclear protein import. In contrast, NPP-5 is essential for proper kinetochore localization of NUP133/NPP-15, another NUP107 complex member, whereas recruitment of NUP96/NPP-10C and ELYS/MEL-28 is NPP-5 independent. We found that kinetochore protein NUF2/HIM-10 and Aurora B/AIR-2 kinase are less abundant on mitotic chromatin upon NPP-5 depletion. npp-5 mutants are hypersensitive to anoxia, suggesting that the spindle assembly checkpoint (SAC) is compromised. Indeed, NPP-5 interacts genetically and physically with SAC protein MAD1/MDF-1, whose nuclear envelope accumulation requires NPP-5. Thus our results strengthen the emerging connection between nuclear pore proteins and chromosome segregation.

PUF-8 suppresses the somatic transcription factor PAL-1 expression in C. elegans germline stem cells

RNA-binding proteins of the PUF family are well conserved post-transcriptional regulators that control a variety of developmental processes. The C. elegans protein PUF-8 is essential for several aspects of germ cell development including the maintenance of germline stem cells (GSCs). To explore the molecular mechanisms underlying its function, we have identified 160 germline-expressed mRNAs as potential targets of PUF-8. We generated GFP::H2B-3' UTR fusions for 17 mRNAs to assay their post-transcriptional regulation in germ cells. Twelve transgenes were not expressed in the mitotic germ cells, and depletion of PUF-8 led to misexpression of six of them in these cells. In contrast, the expression of 3' UTR fusion of hip-1, which encodes the HSP-70 interacting protein, was dependent on PUF-8. These results indicate that PUF-8 may regulate the expression of its targets both negatively as well as positively. We investigated the PUF-8-mediated post-transcriptional control of one mRNA, namely pal-1, which encodes a homeodomain transcription factor responsible for muscle development. Our results show that PUF-8 binds in vitro to specific sequences within pal-1 3' UTR that are critical for post-transcriptional suppression in GSCs. Removal of PUF-8 resulted in PAL-1 misexpression, and PAL-1-dependent misexpression of the myogenic promoter HLH-1 in germ cells. We propose that PUF-8 protects GSCs from the influence of somatic differentiation factors such as PAL-1, which are produced in the maternal germline but meant for embryogenesis.