par-1, a gene required for establishing polarity in C. elegans embryos, encodes a putative Ser/Thr kinase that is asymmetrically distributed

Single-stranded antisense siRNAs guide target RNA cleavage in RNAi

Small interfering RNAs (siRNAs) are the mediators of mRNA degradation in the process of RNA interference (RNAi). Here, we describe a human biochemical system that recapitulates siRNA-mediated target RNA degradation. By using affinity-tagged siRNAs, we demonstrate that a single-stranded siRNA resides in the RNA-induced silencing complex (RISC) together with eIF2C1 and/or eIF2C2 (human GERp95) Argonaute proteins. RISC is rapidly formed in HeLa cell cytoplasmic extract supplemented with 21 nt siRNA duplexes, but also by adding single-stranded antisense RNAs, which range in size between 19 and 29 nucleotides. Single-stranded antisense siRNAs are also effectively silencing genes in HeLa cells, especially when 5'-phosphorylated, and expand the repertoire of RNA reagents suitable for gene targeting.

Canonical and non-canonical Wnt signaling pathways in Caenorhabditis elegans: variations on a common signaling theme

Wnt glycoproteins are signaling molecules that control a wide range of developmental processes in organisms ranging from the simple metazoan Hydra to vertebrates. Wnt signaling also plays a key role in the development of the nematode C. elegans, and is involved in cell fate specification and determination of cell polarity and cell migration. Surprisingly, the first genetic studies of Wnt signaling in C. elegans revealed major differences with the established (canonical) Wnt signaling pathways of Drosophila and vertebrates. Thus, the Wnt-dependent induction of endoderm in the early embryo and the specification of several asymmetric cell divisions during larval development are mediated by as yet novel Wnt signaling pathways that repress, rather than activate the TCF/LEF-1 transcription factor POP-1. Recently, however, it has been shown that, in addition to these divergent Wnt pathways, C. elegans also has a canonical Wnt pathway that converts POP-1 into an activator and controls the expression of several homeobox genes. Interestingly, these different Wnt pathways use distinct beta-catenins to control POP-1 function: the endoderm induction pathway requires the beta-catenin WRM-1 and parallel input from a mitogen-activated kinase (MAPK) pathway to downregulate POP-1, whereas the canonical Wnt pathway employs the beta-catenin BAR-1 to activate Wnt target gene expression.

Heads or tails: cell polarity and axis formation in the early Caenorhabditis elegans embryo

In C. elegans, the first embryonic axis is established shortly after fertilization and requires both the microtubule and microfilament cytoskeleton. Cues from sperm-donated centrosomes result in a cascade of events that polarize the distribution of widely conserved PAR proteins at the cell cortex. The PAR proteins in turn polarize the cytoplasm and position mitotic spindles. Lessons learned from C. elegans should improve our understanding of how cells become polarized and divide asymmetrically during development.

Cell polarity: posterior Par-1 prevents proteolysis

The Par-1 kinase is required for anterior-posterior axis formation in Drosophila. New work has identified the posterior determinant, Oskar, as a Par-1 substrate. Phosphorylation stabilises Oskar, revealing a novel mechanism controlling its asymmetric distribution.

Rnai: a new technology in the post-genomic sequencing era

The complete genome sequences and huge numbers of predicted gene sequences from many complex organisms are available. Reverse-genetic analyses of these organisms will now be necessary to understand what all these genes are doing and how their functions interact. RNAi technology is very useful in this regard for studying gene function, not only in these model organisms, but also in the organisms previously considered not to be amenable to genetic analysis. This treatment reviews the discovery of RNAi, advances in the study of the mechanisms by which this material impinges on gene-product expressions, and technical improvements that will allow RNAi to be applied to a wide variety of organisms.

RNA interference

A conserved biological response to double-stranded RNA, known variously as RNA interference (RNAi) or post-transcriptional gene silencing, mediates resistance to both endogenous parasitic and exogenous pathogenic nucleic acids, and regulates the expression of protein-coding genes. RNAi has been cultivated as a means to manipulate gene expression experimentally and to probe gene function on a whole-genome scale.

Cloning and characterization of human and mouse SNRK sucrose non-fermenting protein (SNF-1)-related kinases

We previously isolated, from the earliest population of CD34+ hematopoietic progenitors that form in the aorta of the human embryo, a partial DNA complementary to RNA (cDNA) sequence that was later identified as the human homologue of rat sucrose non-fermenting protein (SNF-1) related kinase (rSNRK), a novel SNF-1-related kinase previously characterized in the rat. In the present study we report the cloning of the complete human SNF-1 related kinase (hSNRK) cDNA and show that the gene spans 39.8 kb at region 3p21 and contains six exons. Recombinant expression of the hSNRK coding sequence in Escherichia coli led to the production of a functional protein kinase of 85 kDa. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of hSNRK expression in fetal CD34+ hematopoietic progenitors revealed its continuous expression throughout human development with higher levels in highly dividing CD34+ CD38+ cells compared to quiescent CD34+ CD38- cells. This observation, together with the expression of hSNRK in numerous human leukemic cell lines, may reflect an implication of hSNRK protein in hematopoietic cell proliferation or differentiation. In the mouse, the SNRK cDNA is 4.6-kb-long and encodes a protein of 748 amino acids with a predicted molecular mass of 81,930 Da. The proteins from human, rat and mouse are strongly conserved and are characterized by the presence of a serine/threonine kinase catalytic domain, a bipartite nuclear targeting signal and an ubiquitin-associated domain. In situ hybridization and RT-PCR analysis of the pattern of mSNRK expression in the mouse reveals that it is temporally and spatially regulated during embryogenesis, and widespread expressed in adult tissues.

Microtubule capture by the cleavage apparatus is required for proper spindle positioning in yeast

Cell division is the result of two major cytoskeletal events: partition of the chromatids by the mitotic spindle and cleavage of the cell by the cytokinetic apparatus. Spatial coordination of these events ensures that each daughter cell inherits a nucleus. Here we show that, in budding yeast, capture and shrinkage of astral microtubules at the bud neck is required to position the spindle relative to the cleavage apparatus. Capture required the septins and the microtubule-associated protein Kar9. Like Kar9-defective cells, cells lacking the septin ring failed to position their spindle correctly and showed an increased frequency of nuclear missegregation. Microtubule attachment at the bud neck was followed by shrinkage and a pulling action on the spindle. Enhancement of microtubule shrinkage at the bud neck required the Par-1-related, septin-dependent kinases (SDK) Hsl1 and Gin4. Neither the formin Bnr1 nor the actomyosin contractile ring was required for either microtubule capture or microtubule shrinkage. Together, our results indicate that septins and septin-dependent kinases may coordinate microtubule and actin functions in cell division.

Mechanisms of spindle positioning: focus on flies and worms

Accurate spindle positioning is crucial for spatial control of cell division. During metazoan development, coordination between polarity cues and spindle position also ensures correct segregation of cell fate determinants. Converging evidence indicates that spindle positioning is achieved through interactions between cortical anchors and the plus ends of microtubules, generating pulling forces acting on spindle poles. This article discusses recent findings that indicate how this mechanism might be used for spindle positioning during Drosophila and Caenorhabditis elegans development.

Opinion: Building epithelial architecture: insights from three-dimensional culture models

How do individual cells organize into multicellular tissues? Here, we propose that the morphogenetic behaviour of epithelial cells is guided by two distinct elements: an intrinsic differentiation programme that drives formation of a lumen-enclosing monolayer, and a growth factor-induced, transient de-differentiation that allows this monolayer to be remodelled.

HcSTK, a Caenorhabditis elegans PAR-1 homologue from the parasitic nematode, Haemonchus contortus

A putative serine/threonine protein kinase (HcSTK) from the parasitic nematode Haemonchus contortus was characterised at the mRNA and amino acid levels. HcSTK displays a high level of identity (85-93% in the catalytic domain) with proteins of the PAR-1/MARK serine/threonine protein kinase (STK) subfamily, which represent signal transduction molecules involved in establishing and maintaining polarity in proliferating and differentiating cells. The transcript of hcstk is expressed in different developmental stages (second-, third-, fourth-stage larvae and adults) and various organs (muscle, intestine and reproductive) of H. contortus. In addition, there are several isoforms which appear to relate to a single gene. The expression profile of hcstk is similar to that of Caenorhabditis elegans PAR-1, and the level of sequence identity among members of the PAR-1/MARK STK subfamily, representing a range of species of vertebrates (e.g. humans and rodents), invertebrates (e.g. insects and C. elegans) and yeast, suggests that HcSTK may be involved in a conserved signal transduction pathway.

A mutational analysis of dishevelled in Drosophila defines novel domains in the dishevelled protein as well as novel suppressing alleles of axin

Drosophila dishevelled (dsh) functions in two pathways: it is necessary to transduce Wingless (Wg) signaling and it is required in planar cell polarity. To learn more about how Dsh can discriminate between these functions, we performed genetic screens to isolate additional dsh alleles and we examined the potential role of protein phosphorylation by site-directed mutagenesis. We identified two alleles with point mutations in the Dsh DEP domain that specifically disrupt planar polarity signaling. When positioned in the structure of the DEP domain, these mutations are located close to each other and to a previously identified planar polarity mutation. In addition to the requirement for the DEP domain, we found that a cluster of potential phosphorylation sites in a binding domain for the protein kinase PAR-1 is also essential for planar polarity signaling. To identify regions of dsh that are necessary for Wg signaling, we screened for mutations that modified a GMR-GAL4;UAS-dsh overexpression phenotype in the eye. We recovered many alleles of the transgene containing missense mutations, including mutations in the DIX domain and in the DEP domain, the latter group mapping separately from the planar polarity mutations. In addition, several transgenes had mutations within a domain containing a consensus sequence for an SH3-binding protein. We also recovered second-site-suppressing mutations in axin, mapping at a region that may specifically interact with overexpressed Dsh.

Ancient pathways programmed by small RNAs

Double-stranded RNA can now be used in a wide variety of eukaryotes to suppress the expression of virtually any gene, allowing the rapid analysis of that gene's function, a technique known as RNA interference. But how cells use the information in double-stranded RNA to suppress gene expression and why they contain the machinery to do so remain the subjects of intense scrutiny. Current evidence suggests that RNA interference and other "RNA silencing" phenomena reflect an elaborate cellular apparatus that eliminates abundant but defective messenger RNAs and defends against molecular parasites such as transposons and viruses.

Malignant worms: what cancer research can learn from C. elegans

Developmental processes in the nematode C. elegans are controlled by pathways of gene functions that are analogous to those used in mammals. Hence, genetic studies in C. elegans have helped build the frameworks for these regulatory pathways. Many homologs of human genes that are targets for mutation in cancer have been found to function at distinct steps within such genetic pathways. This way, studies in C. elegans have provided important clues about the functions of human oncogenes and tumor suppressors. Understanding how human cancer genes function and act in signaling cascades is of great importance. This information reveals what kind of molecular changes contribute to the process of cell transformation. Moreover, additional candidate oncogenes and tumor suppressors may be revealed by identifying the functional partners of genes with an established role in cancer. Furthermore, identifying a cascade of gene functions increases the number of potential targets for therapeutic intervention, as blocking either one of multiple genes may interfere with signal transduction through the pathway. Simultaneous approaches in a number of different model systems act synergistically in solving pathways of gene functions. By using multiple models, the field takes advantage of the strengths of each system and circumvents its limitations. As one of the most powerful genetic animal systems, C. elegans will continue to reveal new mammalian signaling components. In addition, now that the C. elegans genome sequence has been completed, an increasing number of researchers are likely to discover homologs of human disease genes in the nematode and to analyze gene function in the worm model. Combined with the great potential of this animal in drug screens, it is simple to predict that C. elegans will worm its way deeper and deeper into cancer research.

Par-1 regulates stability of the posterior determinant Oskar by phosphorylation

Par-1 kinase is critical for polarization of the Drosophila melanogaster oocyte and the one-cell Caenorhabditis elegans embryo. Although Par-1 localizes specifically to the posterior pole in both cells, neither its targets nor its function at the posterior pole have been elucidated. Here we show that Drosophila Par-1 phosphorylates the posterior determinant Oskar (Osk) and demonstrate genetically that Par-1 is required for accumulation of Osk protein. We show in cell-free extracts that Osk protein is intrinsically unstable and that it is stabilized after phosphorylation by Par-1. Our data indicate that posteriorly localized Par-1 regulates posterior patterning by stabilizing Osk.

Short hairpin RNAs (shRNAs) induce sequence-specific silencing in mammalian cells

RNA interference (RNAi) was first recognized in Caenorhabditis elegans as a biological response to exogenous double-stranded RNA (dsRNA), which induces sequence-specific gene silencing. RNAi represents a conserved regulatory motif, which is present in a wide range of eukaryotic organisms. Recently, we and others have shown that endogenously encoded triggers of gene silencing act through elements of the RNAi machinery to regulate the expression of protein-coding genes. These small temporal RNAs (stRNAs) are transcribed as short hairpin precursors (approximately 70 nt), processed into active, 21-nt RNAs by Dicer, and recognize target mRNAs via base-pairing interactions. Here, we show that short hairpin RNAs (shRNAs) can be engineered to suppress the expression of desired genes in cultured Drosophila and mammalian cells. shRNAs can be synthesized exogenously or can be transcribed from RNA polymerase III promoters in vivo, thus permitting the construction of continuous cell lines or transgenic animals in which RNAi enforces stable and heritable gene silencing.

RNA interference: mechanisms and applications

RNA interference (RNAi) is a phenomenon induced by double-stranded RNA (dsRNA) in which gene expression is inhibited through specific degradation of mRNA. The mechanism involves conversion of dsRNA into short RNAs that direct ribonucleases to homologous mRNA targets. This process is related to normal defence against viruses and mobilisation of transposons. Treatment with dsRNA has become an important method for analysing gene functions in invertebrate organisms. RNAi has also been demonstrated in several vertebrate species but with lower efficiency. Development of procedures for in vivo production of dsRNA may provide efficient tools for tissue- and stage-specific gene targeting.

Synaptogenesis: insights from worm and fly

Synapse formation is the ultimate step in wiring a nervous system. Synapses are remarkably diverse in size and shape, and are regulated dynamically. Recently, live observations combined with ultrastructural analysis have revealed many details of the cellular interactions that precede synapse formation. Genetic screens in Caenorhabditis elegans and Drosophila have implicated signaling pathways that may involve small G-proteins, ubiquitin-mediated protein degradation and selective cell adhesion in target recognition, synaptic assembly and growth.

Regulation of chemosensory receptor expression and sensory signaling by the KIN-29 Ser/Thr kinase

Sensory signals regulate multiple developmental and behavioral circuits in C. elegans, providing a genetically tractable system in which to investigate the mechanisms underlying the acquisition and integration of sensory information. kin-29 mutants are defective in the expression of a set of chemoreceptor genes, and exhibit characteristics associated with altered sensory signaling, including increased lifespan, decreased body size, and deregulated entry into the dauer developmental stage. kin-29 encodes a Ser/Thr kinase with similarity to the MARK and AMPK/SNF1 family of kinases. We show that KIN-29 acts cell-autonomously and non-cell-autonomously in sensory neurons to regulate chemoreceptor expression, body size, and the dauer decision, suggesting that kin-29 function is essential for the correct acquisition and transduction of sensory information.

Cell cycle regulation of pEg3, a new Xenopus protein kinase of the KIN1/PAR-1/MARK family

We report the characterization of pEg3, a Xenopus protein kinase related to members of the KIN1/PAR-1/MARK family. The founding members of this newly emerging kinase family were shown to be involved in the establishment of cell polarity and both microtubule dynamic and cytoskeleton organization. Sequence analyses suggest that pEg3 and related protein kinases in human, mouse, and Caenorhabditis elegans might constitute a distinct group in this family. pEg3 is encoded by a maternal mRNA, polyadenylated in unfertilized eggs and specifically deadenylated in embryos. In addition to an increase in expression, we have shown that pEg3 is phosphorylated during oocyte maturation. Phosphorylation of pEg3 is cell cycle dependent during Xenopus early embryogenesis and in synchronized cultured XL2 cells. In embryos, the kinase activity of pEg3 is correlated to its phosphorylation state and is maximum during mitosis. Using Xenopus egg extracts we demonstrated that phosphorylation occurs at least in the noncatalytic domain of the kinase, suggesting that this domain might be important for pEg3 function.

The Caenorhabditis elegans EGL-26 protein mediates vulval cell morphogenesis

In screens for Caenorhabditis elegans mutants defective in vulval morphogenesis, we isolated multiple mutants in which the uterus and the vulva fail to make a functional connection, resulting in an egg-laying defective phenotype. Two of these connection of gonad defective (Cog) mutants carry alleles of the egl-26 gene. We demonstrate that vulval lineages in egl-26 mutant animals are normal, but one vulval cell, vulF, adopts an abnormal morphology. This results in formation of an abnormally thick layer of vulval tissue at the apex of the vulva and a physical blockage of the exit to the vulva from the uterus. egl-26 was cloned and is predicted to encode a novel protein. Mosaic analysis indicates that egl-26 activity is required in the primary vulval lineage for vulF morphogenesis. Expression of a functional translational fusion of EGL-26 to GFP was observed within the primary vulval lineage only in vulE, which neighbors vulF. EGL-26 is localized at the apical edge of the vulE cell. It is thus possible that vulE acts to instruct morphological changes in the neighboring cell, vulF, in an interaction mediated by EGL-26.

An overview of C. Elegans trafficking mutants

It is almost 40 years since Sydney Brenner introduced Caenorhabditis elegans as a model genetic system. During that time mutants with defects in intracellular trafficking have been identified in a diverse range of screens for abnormalities. This should, of course, come as no surprise as it is hard to imagine any biological process in which the regulated movement of vesicles within the cells is not critical at some step. Almost all of these genes have mammalian homologs, and yet the role of many of these homologs has not been investigated. Perhaps the protein that regulates your favorite trafficking step has already been identified in C. elegans? Here I provide a brief overview of those trafficking mutants identified in C. elegans and where you can read more about them.

The Caenorhabditis elegans par-5 gene encodes a 14-3-3 protein required for cellular asymmetry in the early embryo

The establishment of anterior-posterior polarity in the Caenorhabditis elegans embryo requires the activity of the maternally expressed par genes. We report the identification and analysis of a new par gene, par-5. We show that par-5 is required for asynchrony and asymmetry in the first embryonic cell divisions, normal pseudocleavage, normal cleavage spindle orientation at the two-cell stage, and localization of P granules and MEX-5 during the first and subsequent cell cycles. Furthermore, par-5 activity is required in the first cell cycle for the asymmetric cortical localization of PAR-1 and PAR-2 to the posterior, and PAR-3, PAR-6, and PKC-3 to the anterior. When PAR-5 is reduced by mutation or by RNA interference, these proteins spread around the cortex of the one-cell embryo and partially overlap. We have shown by sequence analysis of par-5 mutants and by RNA interference that the par-5 gene is the same as the ftt-1 gene, and encodes a 14-3-3 protein. The PAR-5 14-3-3 protein is present in gonads, oocytes, and early embryos, but is not asymmetrically distributed. Our analysis indicates that the par-5 14-3-3 gene plays a crucial role in the early events leading to polarization of the C. elegans zygote.

Bazooka and atypical protein kinase C are required to regulate oocyte differentiation in the Drosophila ovary

The par genes, identified by their role in the establishment of anterior-posterior polarity in the Caenorhabditis elegans zygote, subsequently have been shown to regulate cellular polarity in diverse cell types by means of an evolutionarily conserved protein complex including PAR-3, PAR-6, and atypical protein kinase C (aPKC). The Drosophila homologs of par-1, par-3 (bazooka, baz), par-6 (DmPar-6), and pkc-3 (Drosophila aPKC, DaPKC) each are known to play conserved roles in the generation of cell polarity in the germ line as well as in epithelial and neural precursor cells within the embryo. In light of this functional conservation, we examined the potential role of baz and DaPKC in the regulation of oocyte polarity. Our analyses reveal germ-line autonomous roles for baz and DaPKC in the establishment of initial anterior-posterior polarity within germ-line cysts and maintenance of oocyte cell fate. Germ-line clonal analyses indicate both proteins are essential for two key aspects of oocyte determination: the posterior translocation of oocyte specification factors and the posterior establishment of the microtubule organizing center within the presumptive oocyte. We demonstrate BAZ and DaPKC colocalize to belt-like structures between germarial cyst cells. However, in contrast to their regulatory relationship in the Drosophila and C. elegans embryos, these proteins are not mutually dependent for their germ-line localization, nor is either protein specifically required for PAR-1 localization to the fusome. Therefore, whereas BAZ, DaPKC, and PAR-1 are functionally conserved in establishing oocyte polarity, the regulatory relationships among these genes are not well conserved, indicating these molecules function differently in different cellular contexts.

RNAi As a tool for understanding germline development in Caenorhabditis elegans: uses and cautions

RNA-mediated genetic interference (RNAi) has become a very useful tool for analyzing gene function in development and other processes. RNAi can be used as a complement to traditional genetic studies or as a primary means of determining biological function. However, the efficacy of RNAi depends on a variety of factors that the researcher must take into consideration. This review focuses on germline development in the nematode, Caenorhabditis elegans, and discusses the uses and limitations of RNAi in providing new information about gene function as well as the possible endogenous role RNAi plays in germline physiology.

Of flies and men--studying human disease in Drosophila

During the past year, the Drosophila genome has been sequenced. More than 60% of genes implicated in human disease have Drosophila orthologues. Developments in RNA-mediated interference and homologous recombination have made 'reverse genetics' feasible in Drosophila. Conventional Drosophila genetics is being used increasingly to place human disease genes of unknown function in the context of functional pathways.

Intercellular junctions and cellular polarity: the PAR-aPKC complex, a conserved core cassette playing fundamental roles in cell polarity

Two PDZ-domain-containing adapter-like proteins, PAR-3 and PAR-6, and a protein kinase, atypical protein kinase C (PKC), cooperate together to establish cell polarity in a variety of biological contexts. These include asymmetric cell division in early Caenorhabditis elegans embryo and Drosophila neuroblasts, as well as the establishment and maintenance of apical-basal polarity in Drosophila and mammalian epithelial cells. Recent studies on the role of this PAR-aPKC complex in epithelial cell polarization provide new insights into the molecular basis of epithelial junctional formation and cell polarity.

Http://C. elegans: mining the functional genomic landscape

Caenorhabditis elegans is a powerful animal model for the study of functional genomics. The completed and well-annotated DNA sequence is available and a systematic study of gene function by RNA-interference-mediated knockdown of every gene is in progress. Full-genome DNA microarrays and DNA chips can be used to determine expression changes at different stages of development and in different mutant backgrounds, and a protein-interaction map based on the yeast two-hybrid approach is in progress. These high-capacity approaches to studying gene function will provide new insights into invertebrate and vertebrate biology.

zyg-8, a gene required for spindle positioning in C. elegans, encodes a doublecortin-related kinase that promotes microtubule assembly

Proper spindle positioning is essential for spatial control of cell division. Here, we show that zyg-8 plays a key role in spindle positioning during asymmetric division of one-cell stage C. elegans embryos by promoting microtubule assembly during anaphase. ZYG-8 harbors a kinase domain and a domain related to Doublecortin, a microtubule-associated protein (MAP) affected in patients with neuronal migration disorders. Sequencing of zyg-8 mutant alleles demonstrates that both domains are essential for function. ZYG-8 binds to microtubules in vitro, colocalizes with microtubules in vivo, and promotes stabilization of microtubules to drug or cold depolymerization in COS-7 cells. Our findings demonstrate that ZYG-8 is a MAP crucial for proper spindle positioning in C. elegans, and indicate that the function of the Doublecortin domain in modulating microtubule dynamics is conserved across metazoan evolution.

Specification of germ cell fates by FOG-3 has been conserved during nematode evolution

Rapid changes in sexual traits are ubiquitous in evolution. To analyze this phenomenon, we are studying species of the genus Caenorhabditis. These animals use one of two different mating systems-male/hermaphroditic, like the model organism Caenorhabditis elegans, or male/female, like C. remanei. Since hermaphrodites are essentially females that produce sperm for self-fertilization, elucidating the control of cell fate in the germ line in each species could provide the key to understanding how these mating systems evolved. In C. elegans, FOG-3 is required to specify that germ cells become sperm. Thus, we cloned its homologs from both C. remanei and C. briggsae. Each species produces a single homolog of FOG-3, and RNA-mediated interference indicates that FOG-3 functions in each species to specify that germ cells develop as sperm rather than as oocytes. What factors account for the different mating systems? Northern analyses and RT-PCR data reveal that the expression of fog-3 is always correlated with spermatogenesis. Since the promoters for all three fog-3 genes contain binding sites for the transcription factor TRA-1A and are capable of driving expression of fog-3 in C. elegans hermaphrodites, we propose that alterations in the upstream sex-determination pathway, perhaps acting through TRA-1A, allow spermatogenesis in C. elegans and C. briggsae XX larvae but not in C. remanei.

Axis determination in C. elegans: initiating and transducing polarity

The anterior-posterior axis in Caenorhabditis elegans is determined by the sperm and leads to the asymmetric localisation of PAR (partitioning-defective) proteins, which are critical for polarity. New findings demonstrate that sperm asters play a critical role and suggest models for how PAR asymmetry is established. In addition, studies of blastomere fate determination and heterotrimeric G proteins have started to uncover how initial polarity may be translated into the asymmetric distribution of maternal proteins and the control of spindle position.

PAR-1 is a Dishevelled-associated kinase and a positive regulator of Wnt signalling

Wnt signalling regulates beta-catenin-dependent developmental processes through the Dishevelled protein (Dsh). Dsh regulates two distinct pathways, one mediated by beta-catenin and the other by Jun kinase (JNK). We have purified a Dsh-associated kinase from Drosophila that encodes a homologue of Caenorhabditis elegans PAR-1, a known determinant of polarity during asymmetric cell divisions. Treating cells with Wnt increases endogenous PAR-1 activity coincident with Dsh phosphorylation. PAR-1 potentiates Wnt activation of the beta-catenin pathway but blocks the JNK pathway. Suppressing endogenous PAR-1 function inhibits Wnt signalling through beta-catenin in mammalian cells, and Xenopus and Drosophila embryos. PAR-1 seems to be a positive regulator of the beta-catenin pathway and an inhibitor of the JNK pathway. These findings show that PAR-1, a regulator of polarity, is also a modulator of Wnt-beta-catenin signalling, indicating a link between two important developmental pathways.

Bazooka and PAR-6 are required with PAR-1 for the maintenance of oocyte fate in Drosophila

The anterior-posterior axis of C. elegans is defined by the asymmetric division of the one-cell zygote, and this is controlled by the PAR proteins, including PAR-3 and PAR-6, which form a complex at the anterior of the cell, and PAR-1, which localizes at the posterior [1-4]. PAR-1 plays a similar role in axis formation in Drosophila: the protein localizes to the posterior of the oocyte and is necessary for the localization of the posterior and germline determinants [5, 6]. PAR-1 has recently been shown to have an earlier function in oogenesis, where it is required for the maintenance of oocyte fate and the posterior localization of oocyte-specific markers [7, 8]. Here, we show that the homologs of PAR-3 (Bazooka) and PAR-6 are also required to maintain oocyte fate. Germline clones of mutants in either gene give rise to egg chambers that develop 16 nurse cells and no oocyte. Furthermore, oocyte-specific factors, such as Orb protein and the centrosomes, still localize to one cell but fail to move from the anterior to the posterior cortex. Thus, PAR-1, Bazooka, and PAR-6 are required for the earliest polarity in the oocyte, providing the first example in Drosophila where the three homologs function in the same process. Although these PAR proteins therefore seem to play a conserved role in early anterior-posterior polarity in C. elegans and Drosophila, the relationships between them are different, as the localization of PAR-1 does not require Bazooka or PAR-6 in Drosophila, as it does in the worm.

Spindle dynamics and the role of gamma-tubulin in early Caenorhabditis elegans embryos

gamma-Tubulin is a ubiquitous and highly conserved component of centrosomes in eukaryotic cells. Genetic and biochemical studies have demonstrated that gamma-tubulin functions as part of a complex to nucleate microtubule polymerization from centrosomes. We show that, as in other organisms, Caenorhabditis elegans gamma-tubulin is concentrated in centrosomes. To study centrosome dynamics in embryos, we generated transgenic worms that express GFP::gamma-tubulin or GFP::beta-tubulin in the maternal germ line and early embryos. Multiphoton microscopy of embryos produced by these worms revealed the time course of daughter centrosome appearance and growth and the differential behavior of centrosomes destined for germ line and somatic blastomeres. To study the role of gamma-tubulin in nucleation and organization of spindle microtubules, we used RNA interference (RNAi) to deplete C. elegans embryos of gamma-tubulin. gamma-Tubulin (RNAi) embryos failed in chromosome segregation, but surprisingly, they contained extensive microtubule arrays. Moderately affected embryos contained bipolar spindles with dense and long astral microtubule arrays but with poorly organized kinetochore and interpolar microtubules. Severely affected embryos contained collapsed spindles with numerous long astral microtubules. Our results suggest that gamma-tubulin is not absolutely required for microtubule nucleation in C. elegans but is required for the normal organization and function of kinetochore and interpolar microtubules.

Evidence that RME-1, a conserved C. elegans EH-domain protein, functions in endocytic recycling

In genetic screens for new endocytosis genes in Caenorhabditis elegans we identified RME-1, a member of a conserved class of Eps15-homology (EH)-domain proteins. Here we show that RME-1 is associated with the periphery of endocytic organelles, which is consistent with a direct role in endocytic transport. Endocytic defects in rme-1 mutants indicate that the protein is likely to have a function in endocytic recycling. Evidence from studies of mammalian RME-1 also points to a function for RME-1 in recycling, specifically in the exit of membrane proteins from recycling endosomes. These studies show a conserved function in endocytic recycling for the RME-1 family of EH proteins.

Pod-2, along with pod-1, defines a new class of genes required for polarity in the early Caenorhabditis elegans embryo

The asymmetric division of the one-cell Caenorhabditis elegans zygote gives rise to two cells of different size and fate, thereby establishing the animal's anterior--posterior (a-p) axis. Through genetics, a number of genes required for this polarity have been characterized, but many components remain unidentified. Recently, our laboratory discovered a mutation in the pod-1 gene (for polarity and osmotic defective) that uniquely perturbed polarity and osmotic protection. Here, we describe a new C. elegans polarity gene identified during screens for conditional embryonic lethals. Embryos in which this gene has been mutated show a loss of physical and developmental asymmetries in the one-cell embryo, including the mislocalization of PAR and POD-1 proteins required for early polarity. Furthermore, mutant embryos are osmotically sensitive, allowing us to designate this gene pod-2. Thus, pod-2, along with pod-1, defines a new class of C. elegans polarity genes. Genetic analyses indicate that pod-2 functions in the same pathway as pod-1. Temperature-shift studies indicate that pod-2 is required during oogenesis, indicating that aspects of embryonic polarization may precede fertilization. pod-2 mutant embryos also exhibit a unique germline inheritance defect in which germline identity localizes to the wrong spot in the one-cell embryo and is therefore inherited by the wrong cell at the four-cell stage. Our data suggest that pod-2 may be required to properly position an a-p polarity cue.

Immune system dysfunction and autoimmune disease in mice lacking Emk (Par-1) protein kinase

Emk is a serine/threonine protein kinase implicated in regulating polarity, cell cycle progression, and microtubule dynamics. To delineate the role of Emk in development and adult tissues, mice lacking Emk were generated by targeted gene disruption. Emk(-/-) mice displayed growth retardation and immune cell dysfunction. Although B- and T-cell development were normal, CD4(+)T cells lacking Emk exhibited a marked upregulation of the memory marker CD44/pgp-1 and produced more gamma interferon and interleukin-4 on stimulation through the T-cell receptor in vitro. In addition, B-cell responses to T-cell-dependent and -independent antigen challenge were altered in vivo. As Emk(-/-) animals aged, they developed splenomegaly, lymphadenopathy, membranoproliferative glomerulonephritis, and lymphocytic infiltrates in the lungs, parotid glands and kidneys. Taken together, these results demonstrate that the Emk protein kinase is essential for maintaining immune system homeostasis and that loss of Emk may contribute to autoimmune disease in mammals.

CDC-42 regulates PAR protein localization and function to control cellular and embryonic polarity in C. elegans

BACKGROUND

The polarization of the anterior-posterior axis (A-P) of the Caenorhabditis elegans zygote depends on the activity of the par genes and the presence of intact microfilaments. Functional links between the PAR proteins and the cytoskeleton, however, have not been fully explored. It has recently been shown that in mammalian cells, some PAR homologs form a complex with activated Cdc42, a Rho GTPase that is implicated in the control of actin organization and cellular polarity. A role for Cdc42 in the establishment of embryonic polarity in C. elegans has not been described.

RESULTS

To investigate the function of Cdc42 in the control of cellular and embryonic polarity in C. elegans, we used RNA-mediated interference (RNAi) to inhibit cdc-42 activity in the early embryo. Here, we demonstrate that RNAi of cdc-42 disrupts manifestations of polarity in the early embryo, that these phenotypes depend on par-2 and par-3 gene function, and that cdc-42 is required for the localization of the PAR proteins.

CONCLUSIONS

Our genetic analysis of the regulatory relationships between cdc-42 and the par genes demonstrates that Cdc42 organizes embryonic polarity by controlling the localization and activity of the PAR proteins. Combined with the recent biochemical analysis of their mammalian homologs, these results simultaneously identify both a regulator of the PAR proteins, activated Cdc42, and effectors for Cdc42, the PAR complex.

CDC-42 controls early cell polarity and spindle orientation in C. elegans

BACKGROUND

Generation of asymmetry in the one-cell embryo of C. elegans establishes the anterior--posterior axis (A-P), and is necessary for the proper identity of early blastomeres. Conserved PAR proteins are asymmetrically distributed and are required for the generation of this early asymmetry. The small G protein Cdc42 is a key regulator of polarity in other systems, and recently it has been shown to interact with the mammalian homolog of PAR-6. The function of Cdc42 in C. elegans had not yet been investigated, however.

RESULTS

Here, we show that C. elegans cdc-42 plays an essential role in the polarity of the one-cell embryo and the proper localization of PAR proteins. Inhibition of cdc-42 using RNA interference results in embryos with a phenotype that is nearly identical to par-3, par-6, and pkc-3 mutants, and asymmetric localization of these and other PAR proteins is lost. We further show that C. elegans CDC-42 physically interacts with PAR-6 in a yeast two-hybrid system, consistent with data on the interaction of human homologs.

CONCLUSIONS

Our results show that CDC-42 acts in concert with the PAR proteins to control the polarity of the C. elegans embryo, and provide evidence that the interaction of CDC-42 and the PAR-3/PAR-6/PKC-3 complex has been evolutionarily conserved as a functional unit.

PAR-1 is required for the maintenance of oocyte fate in Drosophila

The PAR-1 kinase is required for the posterior localisation of the germline determinants in C. elegans and Drosophila, and localises to the posterior of the zygote and the oocyte in each case. We show that Drosophila PAR-1 is also required much earlier in oogenesis for the selection of one cell in a germline cyst to become the oocyte. Although the initial steps in oocyte determination are delayed, three markers for oocyte identity, the synaptonemal complex, the centrosomes and Orb protein, still become restricted to one cell in mutant clones. However, the centrosomes and Orb protein fail to translocate from the anterior to the posterior cortex of the presumptive oocyte in region 3 of the germarium, and the cell exits meiosis and becomes a nurse cell. Furthermore, markers for the minus ends of the microtubules also fail to move from the anterior to the posterior of the oocyte in mutant clones. Thus, PAR-1 is required for the maintenance of oocyte identity, and plays a role in microtubule-dependent localisation within the oocyte at two stages of oogenesis. Finally, we show that PAR-1 localises on the fusome, and provides a link between the asymmetry of the fusome and the selection of the oocyte.