The maternal gene spn-4 encodes a predicted RRM protein required for mitotic spindle orientation and cell fate patterning in early C. elegans embryos.

C. elegans embryogenesis begins with a stereotyped sequence of asymmetric cell divisions that are largely responsible for establishing the nematode body plan. These early asymmetries are specified after fertilization by the widely conserved, cortically enriched PAR and PKC-3 proteins, which include three kinases and two PDZ domain proteins. During asymmetric cell divisions in the early embryo, centrosome pairs initially are positioned on transverse axes but then rotate to align with the anteroposterior embryonic axis. We show that rotation of the centrosomal/nuclear complex in an embryonic cell called P(1) requires a maternally expressed gene we name spn-4. The predicted SPN-4 protein contains a single RNA recognition motif (RRM), and belongs to a small subfamily of RRM proteins that includes one Drosophila and two human family members. Remarkably, in mutant embryos lacking spn-4 function the transversely oriented 'P(1)' mitotic spindle appears to re-specify the axis of cell polarity, and the division remains asymmetric. spn-4 also is required for other developmental processes, including the specification of mesendoderm, the restriction of mesectoderm fate to P(1) descendants, and germline quiescence during embryogenesis. We suggest that SPN-4 post-transcriptionally regulates the expression of multiple developmental regulators. Such SPN-4 targets might then act more specifically to generate a subset of the anterior-posterior asymmetries initially specified after fertilization by the more generally required PAR and PKC-3 proteins.

Restriction of mesendoderm to a single blastomere by the combined action of SKN-1 and a GSK-3beta homolog is mediated by MED-1 and -2 in C. elegans.

The endoderm and much of the mesoderm arise from the EMS cell in the four-cell C. elegans embryo. We report that the MED-1 and -2 GATA factors specify the entire fate of EMS, which otherwise produces two C-like mesectodermal progenitors. The meds are direct targets of the maternal SKN-1 transcription factor; however, their forced expression can direct SKN-1-independent reprogramming of non-EMS cells into mesendodermal progenitors. We find that SGG-1/GSK-3beta kinase acts both as a Wnt-dependent activator of endoderm in EMS and an apparently Wnt-independent repressor of the meds in the C lineage, indicating a dual role for this kinase in mesendoderm development. Our results suggest that a broad tissue territory, mesendoderm, in vertebrates has been confined to a single cell in nematodes through a common gene regulatory network.

Asymmetric cell division: fly neuroblast meets worm zygote.

Both Drosophila neuroblasts and Caenorhabditis elegans zygotes use a conserved protein complex to establish cell polarity and regulate spindle orientation. Mammalian epithelia also use this complex to regulate apical/basal polarity. Recent results have allowed us to compare the mechanisms regulating asymmetric cell division in Drosophila neuroblasts and the C. elegans zygote.

Cytokinesis in the C. elegans embryo: regulating contractile forces and a late role for the central spindle.

Genetic and molecular studies in the nematode Caenorhabditis elegans have identified multiple essential pathways that regulate and execute cytokinesis in early embryonic cells. These pathways influence both the microfilament cytoskeleton and the microtubule cytoskeleton. Microfilaments are enriched throughout the cell cortex at all times during the cell cycle in embryonic cells. Cortical microfilaments are required for multiple processes in embryonic cells, including polar body extrusion during meiosis, anterior-posterior axis specification by the sperm-donated microtubule-organizing center, and cytokinesis during mitosis. In addition to contractile apparatus proteins that are required positively for cleavage furrow ingression, the Nedd8 ubiquitin-like protein modification pathway negatively regulates contractile forces outside the cleavage furrow during cytokinesis. Another pathway that acts positively during cytokinesis involves the mitotic spindle. The central spindle, where anti-parallel non-kinetochore microtubules overlap and are cross-linked, is required for a late step in cytokinesis, and other pathway(s) involved in membrane addition during cytokinesis may also require the central spindle. The amenability of C. elegans to classical genetics, the ease of reducing gene function with RNA interference, the completion of the genome sequence, and the availability of transgenic GFP fusion proteins that render the cytoskeleton fluorescent, all serve to make the early worm embryo an especially promising system for further advances in the identification of cytokinesis pathways, and in defining their interactions.

DNA replication defects delay cell division and disrupt cell polarity in early Caenorhabditis elegans embryos.

In early Caenorhabditis elegans embryos, asymmetric cell divisions produce descendants with asynchronous cell cycle times. To investigate the relationship between cell cycle regulation and pattern formation, we have identified a collection of embryonic-lethal mutants in which cell divisions are delayed and cell fate patterns are abnormal. In div (for division delayed) mutant embryos, embryonic cell divisions are delayed but remain asynchronous. Some div mutants produce well-differentiated cell types, but they frequently lack the endodermal and mesodermal cell fates normally specified by a transcriptional activator called SKN-1. We show that mislocalization of PIE-1, a negative regulator of SKN-1, prevents the specification of endoderm and mesoderm in div-1 mutant embryos. In addition to defects in the normally asymmetric distribution of PIE-1, div mutants also exhibit other losses of asymmetry during early embryonic cleavages. The daughters of normally asymmetric divisions are nearly equal in size, and cytoplasmic P-granules are not properly localized to germline precursors in div mutant embryos. Thus the proper timing of cell division appears to be important for multiple aspects of asymmetric cell division. One div gene, div-1, encodes the B subunit of the DNA polymerase alpha-primase complex. Reducing the function of other DNA replication genes also results in a delayed division phenotype and embryonic lethality. Thus the other div genes we have identified are likely to encode additional components of the DNA replication machinery in C. elegans.

The aurora-related kinase AIR-2 recruits ZEN-4/CeMKLP1 to the mitotic spindle at metaphase and is required for cytokinesis.

BACKGROUND: The Aurora/Ipl1p-related kinase AIR-2 is required for mitotic chromosome segregation and cytokinesis in early Caenorhabditis elegans embryos. Previous studies have relied on non-conditional mutations or RNA-mediated interference (RNAi) to inactivate AIR-2. It has therefore not been possible to determine whether AIR-2 functions directly in cytokinesis or if the cleavage defect results indirectly from the failure to segregate DNA. One intriguing hypothesis is that AIR-2 acts to localize the mitotic kinesin-like protein ZEN-4 (also known as CeMKLP1), which later functions in cytokinesis. RESULTS: Using conditional alleles, we established that AIR-2 is required at metaphase or early anaphase for normal segregation of chromosomes, localization of ZEN-4, and cytokinesis. ZEN-4 is first required late in cytokinesis, and also functions to maintain cell separation through much of the subsequent interphase. DNA segregation defects alone were not sufficient to disrupt cytokinesis in other mutants, suggesting that AIR-2 acts specifically during cytokinesis through ZEN-4. AIR-2 and ZEN-4 shared similar genetic interactions with the formin homology (FH) protein CYK-1, suggesting that AIR-2 and ZEN-4 function in a single pathway, in parallel to a contractile ring pathway that includes CYK-1. Using in vitro co-immunoprecipitation experiments, we found that AIR-2 and ZEN-4 interact directly. CONCLUSIONS: AIR-2 has two functions during mitosis: one in chromosome segregation, and a second, independent function in cytokinesis through ZEN-4. AIR-2 and ZEN-4 may act in parallel to a second pathway that includes CYK-1.

Embryonic polarity: protein stability in asymmetric cell division.

Recent work on pattern formation in Caenorhabditis elegans has uncovered a new mechanism of asymmetric cell division: the cytoplasm is polarized by cortical proteins, and this polarization then influences the stability of other maternally expressed proteins that in turn determine early embryonic cell fates.

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

The metaphase to anaphase transition is a critical stage of the eukaryotic cell cycle, and, thus, it is highly regulated. Errors during this transition can lead to chromosome segregation defects and death of the organism. In genetic screens for temperature-sensitive maternal effect embryonic lethal (Mel) mutants, we have identified 32 mutants in the nematode Caenorhabditis elegans in which fertilized embryos arrest as one-cell embryos. In these mutant embryos, the oocyte chromosomes arrest in metaphase of meiosis I without transitioning to anaphase or producing polar bodies. An additional block in M phase exit is evidenced by the failure to form pronuclei and the persistence of phosphohistone H3 and MPM-2 antibody staining. Spermatocyte meiosis is also perturbed; primary spermatocytes arrest in metaphase of meiosis I and fail to produce secondary spermatocytes. Analogous mitotic defects cause M phase delays in mitotic germline proliferation. We have named this class of mutants "mat" for metaphase to anaphase transition defective. These mutants, representing six different complementation groups, all map near genes that encode subunits of the anaphase promoting complex or cyclosome, and, here, we show that one of the genes, emb-27, encodes the C. elegans CDC16 ortholog.

Wnt signalling in Caenorhabditis elegans: regulating repressors and polarizing the cytoskeleton.

Wnt proteins are secreted, cysteine-rich glycoprotein ligands with numerous roles during animal development. Recent studies of endoderm induction during embryogenesis in the nematode Caenorhabditis elegans challenge the prevailing view that Wnt signalling specifies cell fate by converting transcriptional repressors into activators. Instead, a mitogen-activated protein kinase (MAPK)-related pathway converges with Wnt signalling in C. elegans to relieve transcriptional repression. Furthermore, Wnt signalling induces endoderm in part by aligning the mitotic spindle in a responding cell along the anterior-posterior body axis. To orient mitotic spindles, Wnt signalling might directly target the cytoskeleton, prior to any regulation of gene transcription in responding cells.

Cell division: plant-like properties of animal cell cytokinesis.

Recent evidence that a syntaxin is required for cytokinesis in Caenorhabditis elegans embryos suggests that the mechanism of cell division in plant and animal cells may be more similar than previously imagined.

Wnt pathway components orient a mitotic spindle in the early Caenorhabditis elegans embryo without requiring gene transcription in the responding cell.

In a four-cell-stage Caenorhabditis elegans embryo, Wnt signaling polarizes an endoderm precursor called EMS. The polarization of this cell orients its mitotic spindle in addition to inducing endodermal fate in one daughter cell. Reducing the function of Wnt pathway genes, including a newly identified GSK-3beta homolog called gsk-3, disrupts endoderm induction, whereas only a subset of these genes is required for proper EMS mitotic spindle orientation. Wnt pathway genes thought to act downstream of gsk-3 appear not to be required for spindle orientation, suggesting that gsk-3 represents a branch point in the control of endoderm induction and spindle orientation. Orientation of the mitotic spindle does not require gene transcription in EMS, suggesting that Wnt signaling may directly target the cytoskeleton in a responding cell.

Cell polarity in the early Caenorhabditis elegans embryo.

Beginning with the first mitotic division in a Caenorhabditis elegans embryo, asymmetric cleavages establish much of the body plan. Although they share a common axis of polarity, at least three kinds of asymmetric cell division occur: two are under intrinsic control, while a third requires an inductive signal and may operate repeatedly throughout development.

The nonmuscle myosin regulatory light chain gene mlc-4 is required for cytokinesis, anterior-posterior polarity, and body morphology during Caenorhabditis elegans embryogenesis.

Using RNA-mediated genetic interference in a phenotypic screen, we identified a conserved nonmuscle myosin II regulatory light chain gene in Caenorhabditis elegans, which we name mlc-4. Maternally supplied mlc-4 function is required for cytokinesis during both meiosis and mitosis and for establishment of anterior-posterior (a-p) asymmetries after fertilization. Reducing the function of mlc-4 or nmy-2, a nonmuscle myosin II gene, also leads to a loss of polarized cytoplasmic flow in the C. elegans zygote, supporting models in which cytoplasmic flow may be required to establish a-p differences. Germline P granule localization at the time of cytoplasmic flow is also lost in these embryos, although P granules do become localized to the posterior pole after the first mitosis. This result suggests that a mechanism other than cytoplasmic flow or mlc-4/nmy-2 activity can generate some a-p asymmetries in the C. elegans zygote. By isolating a deletion allele, we show that removing zygotic mlc-4 function results in an elongation phenotype during embryogenesis. An mlc-4/green fluorescent protein transgene is expressed in lateral rows of hypodermal cells and these cells fail to properly change shape in mlc-4 mutant animals during elongation.

The TAK1-NLK-MAPK-related pathway antagonizes signalling between beta-catenin and transcription factor TCF.

The Wnt signalling pathway regulates many developmental processes through a complex of beta-catenin and the T-cell factor/lymphoid enhancer factor (TCF/LEF) family of high-mobility-group transcription factors. Wnt stabilizes cytosolic beta-catenin, which then binds to TCF and activates gene transcription. This signalling cascade is conserved in vertebrates, Drosophila and Caenorhabditis elegans. In C. elegans, the proteins MOM-4 and LIT-1 regulate Wnt signalling to polarize responding cells during embryogenesis. MOM-4 and LIT-1 are homologous to TAK1 (a kinase activated by transforming growth factor-beta) mitogen-activated protein-kinase-kinase kinase (MAP3K) and MAP kinase (MAPK)-related NEMO-like kinase (NLK), respectively, in mammalian cells. These results raise the possibility that TAK1 and NLK are also involved in Wnt signalling in mammalian cells. Here we show that TAK1 activation stimulates NLK activity and downregulates transcriptional activation mediated by beta-catenin and TCF. Injection of NLK suppresses the induction of axis duplication by microinjected beta-catenin in Xenopus embryos. NLK phosphorylates TCF/LEF factors and inhibits the interaction of the beta-catenin-TCF complex with DNA. Thus, the TAK1-NLK-MAPK-like pathway negatively regulates the Wnt signalling pathway.

MAP kinase and Wnt pathways converge to downregulate an HMG-domain repressor in Caenorhabditis elegans.

The signalling protein Wnt regulates transcription factors containing high-mobility-group (HMG) domains to direct decisions on cell fate during animal development. In Caenorhabditis elegans, the HMG-domain-containing repressor POP-1 distinguishes the fates of anterior daughter cells from their posterior sisters throughout development, and Wnt signalling downregulates POP-1 activity in one posterior daughter cell called E. Here we show that the genes mom-4 and lit-1 are also required to downregulate POP-1, not only in E but also in other posterior daughter cells. Consistent with action in a common pathway, mom-4 and lit-1 exhibit similar mutant phenotypes and encode components of the mitogen-activated protein kinase (MAPK) pathway that are homologous to vertebrate transforming-growth-factor-beta-activated kinase (TAK1) and NEMO-like kinase (NLK), respectively. Furthermore, MOM-4 and TAK1 bind related proteins that promote their kinase activities. We conclude that a MAPK-related pathway cooperates with Wnt signal transduction to downregulate POP-1 activity. These functions are likely to be conserved in vertebrates, as TAK1 and NLK can downregulate HMG-domain-containing proteins related to POP-1.

cyk-1: a C. elegans FH gene required for a late step in embryonic cytokinesis.

A maternally expressed Caenorhabditis elegans gene called cyk-1 is required for polar body extrusion during meiosis and for a late step in cytokinesis during embryonic mitosis. Other microfilament- and microtubule-dependent processes appear normal in cyk-1 mutant embryos, indicating that cyk-1 regulates a specific subset of cytoskeletal functions. Because cytokinesis initiates normally and cleavage furrows ingress extensively in cyk-1 mutant embryos, we propose that the wild-type cyk-1 gene is required for a late step in cytokinesis. Cleavage furrows regress after completion of mitosis in cyk-1 mutants, leaving multiple nuclei in a single cell. Positional cloning and sequence analysis of the cyk-1 gene reveal that it encodes an FH protein, a newly defined family of proteins that appear to interact with the cytoskeleton during cytokinesis and in the regulation of cell polarity. Consistent with cyk-1 function being required for a late step in embryonic cytokinesis, we show that the CYK-1 protein co-localizes with actin microfilaments as a ring at the leading edge of the cleavage furrow, but only after extensive furrow ingression. We discuss our findings in the context of other studies suggesting that FH genes in yeast and insects function early in cytokinesis to assemble a cleavage furrow.

A new DNA-binding motif in the Skn-1 binding domain-DNA complex.

The DNA-binding domain of Skn-1, a developmental transcription factor that specifies mesoderm in C. elegans, is shown by X-ray crystallography to have a novel fold in which a compact, monomeric, four-helix unit organizes two DNA-contact elements. At the C-terminus, a helix extends from the domain to occupy the major groove of DNA in a manner similar to bZip proteins. Skn-1, however, lacks the leucine zipper found in all bZips. Additional contacts with the DNA are made by a short basic segment at the N-terminus of the domain, reminiscent of the 'homeodomain arm'.

Maternal control of pattern formation in early Caenorhabditis elegans embryos.

Genetic screens for recessive, maternal-effect, embryonic-lethal mutations have identified about 25 genes that control early steps of pattern formation in the nematode Caenorhabditis elegans. These maternal genes are discussed as belonging to one of three groups. The par group genes establish and maintain polarity in the one-cell zygote in response to sperm entry, defining an anterior/posterior body axis at least in part through interactions with the cyto-skeleton mediated by cortically localized proteins. Blastomere identity group genes act down-stream of the par group to specify the identities of individual embryonic cells, or blastomeres, using both cell autonomous and non-cell autonomous mechanisms. Requirements for the blastomere identity genes are consistent with previous studies suggesting that early asymmetric cleavages in the C. elegans embryo generate six "founder" cells that account for much of the C. elegans body plan. Intermediate group genes, most recently identified, may link the establishment of polarity in the zygote by par group genes to the localization of blastomere identity group gene functions. This review summarizes the known requirements for the members of each group, although it seems clear that additional regulatory genes controlling pattern formation in the early embryo have yet to be identified. An emerging challenge is to link the function of the genes in these three groups into interacting pathways that can account for the specification of the six founder cell identities in the early embryo, five of which produce somatic cell types and one of which produces the germline.

The maternal par genes and the segregation of cell fate specification activities in early Caenorhabditis elegans embryos.

After fertilization in C. elegans, activities encoded by the maternally expressed par genes appear to establish cellular and embryonic polarity. Loss-of-function mutations in the par genes disrupt anterior-posterior (a-p) asymmetries in early embryos and result in highly abnormal patterns of cell fate. Little is known about how the early asymmetry defects are related to the cell fate patterning defects in par mutant embryos, or about how the par gene products affect the localization and activities of developmental regulators known to specify the cell fate patterns made by individual blastomeres. Examples of such regulators of blastomere identity include the maternal proteins MEX-3 and GLP-1, expressed at high levels anteriorly, and SKN-1 and PAL-1, expressed at high levels posteriorly in early embryos. To better define par gene functions, we examined the expression patterns of MEX-3, PAL-1 and SKN-1, and we analyzed mex-3, pal-1, skn-1 and glp-1 activities in par mutant embryos. We have found that mutational inactivation of each par gene results in a unique phenotype, but in no case do we observe a complete loss of a-p asymmetry. We conclude that no one par gene is required for all a-p asymmetry and we suggest that, in some cases, the par genes act independently of each other to control cell fate patterning and polarity. Finally, we discuss the implications of our findings for understanding how the initial establishment of polarity in the zygote by the par gene products leads to the proper localization of more specifically acting regulators of blastomere identity.