The Caenorhabditis elegans gene lin-44 controls the polarity of asymmetric cell divisions

Caenorhabditis elegans SEL-5/AAK1 regulates cell migration and cell outgrowth independently of its kinase activity

During Caenorhabditis elegans development, multiple cells migrate long distances or extend processes to reach their final position and/or attain proper shape. The Wnt signalling pathway stands out as one of the major coordinators of cell migration or cell outgrowth along the anterior-posterior body axis. The outcome of Wnt signalling is fine-tuned by various mechanisms including endocytosis. In this study, we show that SEL-5, the C. elegans orthologue of mammalian AP2-associated kinase AAK1, acts together with the retromer complex as a positive regulator of EGL-20/Wnt signalling during the migration of QL neuroblast daughter cells. At the same time, SEL-5 in cooperation with the retromer complex is also required during excretory canal cell outgrowth. Importantly, SEL-5 kinase activity is not required for its role in neuronal migration or excretory cell outgrowth, and neither of these processes is dependent on DPY-23/AP2M1 phosphorylation. We further establish that the Wnt proteins CWN-1 and CWN-2, together with the Frizzled receptor CFZ-2, positively regulate excretory cell outgrowth, while LIN-44/Wnt and LIN-17/Frizzled together generate a stop signal inhibiting its extension.

Mechanisms of lineage specification in Caenorhabditis elegans

The studies of cell fate and lineage specification are fundamental to our understanding of the development of multicellular organisms. Caenorhabditis elegans has been one of the premiere systems for studying cell fate specification mechanisms at single cell resolution, due to its transparent nature, the invariant cell lineage, and fixed number of somatic cells. We discuss the general themes and regulatory mechanisms that have emerged from these studies, with a focus on somatic lineages and cell fates. We next review the key factors and pathways that regulate the specification of discrete cells and lineages during embryogenesis and postembryonic development; we focus on transcription factors and include numerous lineage diagrams that depict the expression of key factors that specify embryonic founder cells and postembryonic blast cells, and the diverse somatic cell fates they generate. We end by discussing some future perspectives in cell and lineage specification.

Homologies and evolution of male tail characters in rhabditid and diplogastrid nematodes

A major question in evolutionary biology is how often the same developmental events, mechanisms and genes are reused in the recurrent evolution of similar phenotypes. If this happens frequently, it would suggest that evolution is often constrained by developmental genetic mechanisms. To help address this question, we used adherens junction staining and laser ablation to analyze the development underlying several features of nematode male tails have evolved recurrently. We find that recurrent evolution has sometimes employed similar developmental events (parallel evolution) and sometimes different events (convergent evolution). Specifically, phasmid position changed four times via cell migration and never by switches in cell lineage polarity; different genital papillae are missing in species with less than nine; and tail tip morphogenesis was gained at least twice (once with tail tip cell fusions and once without) and lost at least twice. As in previous analyses, we also find that genital papilla positions have shifted differently in different lineages relative to their conserved positions of origin in the lateral hypodermis. In particular, the v1 papilla homolog in diplogastrids has moved dorsally relative to the other v-papillae and lies posterior to the v2 papilla. The prevalence of recurrently evolved characters (homoplasy) suggests that caution should be exercised when using these characters for phylogenetic inference. On the other hand, because of their recurrent evolution, these characters provide good models for investigating how developmental and genetic systems may bias, constrain or allow phenotypic evolution.

GRDN-1/Girdin regulates dendrite morphogenesis and cilium position in two specialized sensory neuron types in C. elegans

Primary cilia are located at the dendritic tips of sensory neurons and house the molecular machinery necessary for detection and transduction of sensory stimuli. The mechanisms that coordinate dendrite extension with cilium position during sensory neuron development are not well understood. Here, we show that GRDN-1, the Caenorhabditis elegans ortholog of the highly conserved scaffold and signaling protein Girdin/GIV, regulates both cilium position and dendrite extension in the postembryonic AQR and PQR gas-sensing neurons. Mutations in grdn-1 disrupt dendrite outgrowth and mislocalize cilia to the soma or proximal axonal segments in AQR, and to a lesser extent, in PQR. GRDN-1 is localized to the basal body and regulates localization of HMR-1/Cadherin to the distal AQR dendrite. However, knockdown of HMR-1 and/or loss of SAX-7/LICAM, molecules previously implicated in sensory dendrite development in C. elegans, do not alter AQR dendrite morphology or cilium position. We find that GRDN-1 localization in AQR is regulated by UNC-116/Kinesin-1, and that correspondingly, unc-116 mutants exhibit severe AQR dendrite outgrowth and cilium positioning defects. In contrast, GRDN-1 and cilium localization in PQR is modulated by LIN-44/Wnt signaling. Together, these findings identify upstream regulators of GRDN-1, and describe new cell-specific roles for this multifunctional protein in sensory neuron development.

Wnt signalling in the development of axon, dendrites and synapses

Wnts are a highly conserved family of secreted glycoproteins that play essential roles in the morphogenesis and body patterning during the development of metazoan species. In recent years, mounting evidence has revealed important functions of Wnt signalling in diverse aspects of neural development, including neuronal polarization, guidance and branching of the axon and dendrites, as well as synapse formation and its structural remodelling. In contrast to Wnt signalling in cell proliferation and differentiation, which mostly acts through β-catenin-dependent pathways, Wnts engage a diverse array of non-transcriptional cascades in neuronal development, such as the planar cell polarity, cytoskeletal or calcium signalling pathways. In this review, we summarize recent advances in the mechanisms of Wnt signalling in the development of axon, dendrite and synapse formation.

From "the Worm" to "the Worms" and Back Again: The Evolutionary Developmental Biology of Nematodes

Since the earliest days of research on nematodes, scientists have noted the developmental and morphological variation that exists within and between species. As various cellular and developmental processes were revealed through intense focus on Caenorhabditis elegans, these comparative studies have expanded. Within the genus Caenorhabditis, they include characterization of intraspecific polymorphisms and comparisons of distinct species, all generally amenable to the same laboratory culture methods and supported by robust genomic and experimental tools. The C. elegans paradigm has also motivated studies with more distantly related nematodes and animals. Combined with improved phylogenies, this work has led to important insights about the evolution of nematode development. First, while many aspects of C. elegans development are representative of Caenorhabditis, and of terrestrial nematodes more generally, others vary in ways both obvious and cryptic. Second, the system has revealed several clear examples of developmental flexibility in achieving a particular trait. This includes developmental system drift, in which the developmental control of homologous traits has diverged in different lineages, and cases of convergent evolution. Overall, the wealth of information and experimental techniques developed in C. elegans is being leveraged to make nematodes a powerful system for evolutionary cellular and developmental biology.

Wnts Promote Synaptic Assembly Through T-Cell Specific Transcription Factors in Caenorhabditis elegans

Synapses are specialized neuronal connections essential for neuronal function. Defects in synaptic assembly or maintenance usually lead to various neurological disorders. Synaptic assembly is regulated by secreted molecules such as Wnts. Wnts are a large family of conserved glycosylated signaling molecules involved in many aspects of neural development and maintenance. However, the molecular mechanisms by which Wnts regulate synaptic assembly remain elusive due to the large number of ligands/receptors, the diversity of signaling cascades and the complexity of the nervous system. In this study, through genetic manipulation, we uncover that C. elegans Wnt-2 (CWN-2) is required for synaptic development. The CWN-2 signal is required during both embryonic and postembryonic development, in the nervous system and intestine, for promoting synaptic assembly. Furthermore, we provide genetic evidence for CWN-2 promoting synaptogenesis through the Frizzled receptor (FZD) CFZ-2, the Dishevelled (DVL) DSH-2, the β-catenin SYS-1 and the only T-cell specific transcription factor POP-1/TCF. Importantly, it is the first time to report the requirement of a TCF for presynaptic assembly. These findings expand our understanding of the synaptogenic mechanisms and may provide therapeutic insights into Wnt-related neurological disorders.

The Caenorhabditis elegans gene ham-1 regulates daughter cell size asymmetry primarily in divisions that produce a small anterior daughter cell

C. elegans cell divisions that produce an apoptotic daughter cell exhibit Daughter Cell Size Asymmetry (DCSA), producing a larger surviving daughter cell and a smaller daughter cell fated to die. Genetic screens for mutants with defects in apoptosis identified several genes that are also required for the ability of these divisions to produce daughter cells that differ in size. One of these genes, ham-1, encodes a putative transcription factor that regulates a subset of the asymmetric cell divisions that produce an apoptotic daughter cell. In a survey of C. elegans divisions, we found that ham-1 mutations affect primarily anterior/posterior divisions that produce a small anterior daughter cell. The affected divisions include those that generate an apoptotic cell as well as those that generate two surviving cells. Our findings suggest that HAM-1 primarily promotes DCSA in a certain class of asymmetric divisions.

Size Matters: How C. elegans Asymmetric Divisions Regulate Apoptosis

Apoptosis is a form of programmed cell death used by metazoans to eliminate abnormal cells, control cell number, and shape the development of organs. The use of the nematode Caenorhabditis elegans as a model for the study of apoptosis has led to important insights into how cells die and how their corpses are removed. Eighty percent of these apoptotic cell deaths occur during nervous system development and in daughters of neuroblasts that divide asymmetrically. Pioneering work defined a conserved apoptosis pathway that is initiated in C. elegans by the BH3-only protein EGL-1 and that leads to the activation of the caspase CED-3. While the execution of the apoptotic fate is well understood, much less is known about the mechanisms that specify the apoptotic fate of particular cells. In some cells fated to die, this regulation occurs at the level of the egl-1 gene transcription, and investigators have identified several lineage-specific transcription factors that both positively and negatively regulate egl-1. In this review, we focus on a second set of molecules that appear to influence apoptosis by controlling the position of the cleavage plane in divisions that produce apoptotic cells.

A High-Throughput Small Molecule Screen for C. elegans Linker Cell Death Inhibitors

Programmed cell death is a ubiquitous process in metazoan development. Apoptosis, one cell death form, has been studied extensively. However, mutations inactivating key mammalian apoptosis regulators do not block most developmental cell culling, suggesting that other cell death pathways are likely important. Recent work in the nematode Caenorhabditis elegans identified a non-apoptotic cell death form mediating the demise of the male-specific linker cell. This cell death process (LCD, linker cell-type death) is morphologically conserved, and its molecular effectors also mediate axon degeneration in mammals and Drosophila. To develop reagents to manipulate LCD, we established a simple high-throughput screening protocol for interrogating the effects of small molecules on C. elegans linker cell death in vivo. From 23,797 compounds assayed, 11 reproducibly block linker cell death onset. Of these, five induce animal lethality, and six promote a reversible developmental delay. These results provide proof-of principle validation of our screening protocol, demonstrate that developmental progression is required for linker cell death, and suggest that larger scale screens may identify LCD-specific small-molecule regulators that target the LCD execution machinery.

Transcriptional control of non-apoptotic developmental cell death in C. elegans

Programmed cell death is an essential aspect of animal development. Mutations in vertebrate genes that mediate apoptosis only mildly perturb development, suggesting that other cell death modes likely have important roles. Linker cell-type death (LCD) is a morphologically conserved cell death form operating during the development of Caenorhabditis elegans and vertebrates. We recently described a molecular network governing LCD in C. elegans, delineating a key role for the transcription factor heat-shock factor 1 (HSF-1). Although HSF-1 functions to protect cells from stress in many settings by inducing expression of protein folding chaperones, it promotes LCD by inducing expression of the conserved E2 ubiquitin-conjugating enzyme LET-70/UBE2D2, which is not induced by stress. Following whole-genome RNA interference and candidate gene screens, we identified and characterized four conserved regulators required for LCD. Here we show that two of these, NOB-1/Hox and EOR-1/PLZF, act upstream of HSF-1, in the context of Wnt signaling. A third protein, NHR-67/TLX/NR2E1, also functions upstream of HSF-1, and has a separate activity that prevents precocious expression of HSF-1 transcriptional targets. We demonstrate that the SET-16/mixed lineage leukemia 3/4 (MLL3/4) chromatin regulation complex functions at the same step or downstream of HSF-1 to control LET-70/UBE2D2 expression. Our results identify conserved proteins governing LCD, and demonstrate that transcriptional regulators influence this process at multiple levels.

Identification of Wnt Pathway Target Genes Regulating the Division and Differentiation of Larval Seam Cells and Vulval Precursor Cells in Caenorhabditis elegans

The evolutionarily conserved Wnt/β-catenin signaling pathway plays a fundamental role during metazoan development, regulating numerous processes including cell fate specification, cell migration, and stem cell renewal. Wnt ligand binding leads to stabilization of the transcriptional effector β-catenin and upregulation of target gene expression to mediate a cellular response. During larval development of the nematode Caenorhabditis elegans, Wnt/β-catenin pathways act in fate specification of two hypodermal cell types, the ventral vulval precursor cells (VPCs) and the lateral seam cells. Because little is known about targets of the Wnt signaling pathways acting during larval VPC and seam cell differentiation, we sought to identify genes regulated by Wnt signaling in these two hypodermal cell types. We conditionally activated Wnt signaling in larval animals and performed cell type-specific "mRNA tagging" to enrich for VPC and seam cell-specific mRNAs, and then used microarray analysis to examine gene expression compared to control animals. Two hundred thirty-nine genes activated in response to Wnt signaling were identified, and we characterized 50 genes further. The majority of these genes are expressed in seam and/or vulval lineages during normal development, and reduction of function for nine genes caused defects in the proper division, fate specification, fate execution, or differentiation of seam cells and vulval cells. Therefore, the combination of these techniques was successful at identifying potential cell type-specific Wnt pathway target genes from a small number of cells and at increasing our knowledge of the specification and behavior of these C. elegans larval hypodermal cells.

The Tumor Suppressor BCL7B Functions in the Wnt Signaling Pathway

Human BCL7 gene family consists of BCL7A, BCL7B, and BCL7C. A number of clinical studies have reported that BCL7 family is involved in cancer incidence, progression, and development. Among them, BCL7B, located on chromosome 7q11.23, is one of the deleted genes in patients with Williams-Beuren syndrome. Although several studies have suggested that malignant diseases occurring in patients with Williams-Beuren syndrome are associated with aberrations in BCL7B, little is known regarding the function of this gene at the cellular level. In this study, we focused on bcl-7, which is the only homolog of BCL7 gene family in Caenorhabditis elegans, and analyzed bcl-7 deletion mutants. As a result, we found that bcl-7 is required for the asymmetric differentiation of epithelial seam cells, which have self-renewal properties as stem cells and divide asymmetrically through the WNT pathway. Distal tip cell development, which is regulated by the WNT pathway in Caenorhabditis elegans, was also affected in bcl-7-knockout mutants. Interestingly, bcl-7 mutants exhibited nuclear enlargement, reminiscent of the anaplastic features of malignant cells. Furthermore, in KATOIII human gastric cancer cells, BCL7B knockdown induced nuclear enlargement, promoted the multinuclei phenotype and suppressed cell death. In addition, this study showed that BCL7B negatively regulates the Wnt-signaling pathway and positively regulates the apoptotic pathway. Taken together, our data indicate that BCL7B/BCL-7 has some roles in maintaining the structure of nuclei and is involved in the modulation of multiple pathways, including Wnt and apoptosis. This study may implicate a risk of malignancies with BCL7B-deficiency, such as Williams-Beuren syndrome.

BCL7B, a member of the human BCL7 gene family, is deleted in patients with Williams-Beuren syndrome. Although several clinical studies have suggested that malignant diseases occurring in patients with Williams-Beuren syndrome are associated with aberrations in BCL7B, little is known regarding the physiological function of this gene. Here, we show that bcl-7, the only homolog of BCL7 gene family in Caenorhabditis elegans, regulates asymmetric cell differentiation in somatic “stem-like” seam cells through at least the Wnt pathway and promotes the apoptotic pathway. In addition, bcl-7 deletion mutants show enlarged nuclei in epidermis and germ cells. Furthermore, in KATOIII human gastric cancer cells, BCL7B knockdown induces nuclear enlargement, as observed in Caenorhabditis elegans, and promotes the multinucleated phenotype, both of which are reminiscent of malignant diseases. BCL7B also negatively regulates the Wnt-signaling pathway and positively regulates the apoptotic pathway, similar to Caenorhabditis elegans. Altogether, this study may open the door for understanding the function of BCL7 family in cell differentiation and malignancies.

Function of the Ryk intracellular domain in C. elegans vulval development

BACKGROUND

Ryk is a subfamily of receptor tyrosine kinases, which along with Frizzled and Ror, function as Wnt receptors. Vertebrate Ryk intracellular domain (ICD) is released from the cell membrane by a proteolytic cleavage in the transmembrane region and localizes to the nucleus. In C. elegans, Ryk is encoded by the lin-18 gene and regulates the polarity of the P7.p vulval cell.

RESULTS

Based on Western blots, we were unable to detect the presence of the cleaved LIN-18 ICD fragment. Functional assays found that LIN-18 intracellular domain is not absolutely required for LIN-18 function, consistent with previous results. However, overexpression of the LIN-18 intracellular domain fragment (LIN-18ICD) weakly enhanced the phenotype of lin-18 loss-of-function mutants. Furthermore, this activity was specific to the serine-rich juxtamembrane region. We also found that the nuclear localization of LIN-18ICD fragment can be regulated by Wnt pathway components including CAM-1/Ror, and by PAR-5/14-3-3.

CONCLUSIONS

Release of LIN-18ICD by cleavage at the membrane is not the main mechanism of LIN-18 signaling in vulval cells. However, our results suggest that LIN-18 intracellular domain interacts with Wnt pathway components and a 14-3-3 protein and likely plays a minor role in LIN-18 signaling.

The development of sexual dimorphism: studies of the Caenorhabditis elegans male

Studies of the development of the Caenorhabditis elegans male have been carried out with the aim of understanding the basis of sexual dimorphism. Postembryonic development of the two C. elegans sexes differs extensively. Development along either the hermaphrodite or male pathway is specified initially by the X to autosome ratio. The regulatory events initiated by this ratio include a male-determining paracrine intercellular signal. Expression of this signal leads to different consequences in three regions of the body: the nongonadal soma, the somatic parts of the gonad, and the germ line. In the nongonadal soma, activity of the key Zn-finger transcription factor TRA-1 determines hermaphrodite development; in its absence, the male pathway is followed. Only a few genes directly regulated by TRA-1 are currently known, including members of the evolutionarily conserved, male-determining DM domain Zn-finger transcription factors. In the somatic parts of the gonad and germ line, absence of TRA-1 activity is not sufficient for full expression of the male pathway. Several additional transcription factors involved have been identified. In the germ line, regulatory genes for sperm development that act at the level of RNA in the cytoplasm play a prominent role.

Distinct DNA binding sites contribute to the TCF transcriptional switch in C. elegans and Drosophila

Regulation of gene expression by signaling pathways often occurs through a transcriptional switch, where the transcription factor responsible for signal-dependent gene activation represses the same targets in the absence of signaling. T-cell factors (TCFs) are transcription factors in the Wnt/ß-catenin pathway, which control numerous cell fate specification events in metazoans. The TCF transcriptional switch is mediated by many co-regulators that contribute to repression or activation of Wnt target genes. It is typically assumed that DNA recognition by TCFs is important for target gene location, but plays no role in the actual switch. TCF/Pangolin (the fly TCF) and some vertebrate TCF isoforms bind DNA through two distinct domains, a High Mobility Group (HMG) domain and a C-clamp, which recognize DNA motifs known as HMG and Helper sites, respectively. Here, we demonstrate that POP-1 (the C. elegans TCF) also activates target genes through HMG and Helper site interactions. Helper sites enhanced the ability of a synthetic enhancer to detect Wnt/ß-catenin signaling in several tissues and revealed an unsuspected role for POP-1 in regulating the C. elegans defecation cycle. Searching for HMG-Helper site clusters allowed the identification of a new POP-1 target gene active in the head muscles and gut. While Helper sites and the C-clamp are essential for activation of worm and fly Wnt targets, they are dispensable for TCF-dependent repression of targets in the absence of Wnt signaling. These data suggest that a fundamental change in TCF-DNA binding contributes to the transcriptional switch that occurs upon Wnt stimulation.

The DNA of cells must be correctly “read” so that the proper genes are expressed. Transcription factors are the primary “DNA readers”, and these proteins bind to specific DNA sequences. Using nematodes as a model system, we investigated the rules of DNA binding for a particular transcription factor, called POP-1, which mediates Wnt signaling, an important cell-cell communication pathway. In addition to its known DNA binding site, we found that POP-1 recognizes additional sequences, termed Helper sites, which are essential for activation of Wnt targets. We used this knowledge to discover that Wnt signaling is active in pacemaker cells in the nematode intestine, which control defecation, a rhythmic behavior with parallels to the vertebrate heartbeat. POP-1 has a dual role in regulating Wnt targets, repressing target genes in the absence of signaling and activating them upon signal stimulation. Surprisingly, we found that Helper sites are only required for activation and not repression, and that this is also the case in the fruit fly Drosophila. This work thus reveals an unexpected complexity in POP-1 DNA binding, which is likely to be relevant for its human counterparts, which play important roles in stem cell biology and cancer.

C. elegans GATA factors EGL-18 and ELT-6 function downstream of Wnt signaling to maintain the progenitor fate during larval asymmetric divisions of the seam cells

The C. elegans seam cells are lateral epithelial cells arrayed in a single line from anterior to posterior that divide in an asymmetric, stem cell-like manner during larval development. These asymmetric divisions are regulated by Wnt signaling; in most divisions, the posterior daughter in which the Wnt pathway is activated maintains the progenitor seam fate, while the anterior daughter in which the Wnt pathway is not activated adopts a differentiated hypodermal fate. Using mRNA tagging and microarray analysis, we identified the functionally redundant GATA factor genes egl-18 and elt-6 as Wnt pathway targets in the larval seam cells. EGL-18 and ELT-6 have previously been shown to be required for initial seam cell specification in the embryo. We show that in larval seam cell asymmetric divisions, EGL-18 is expressed strongly in the posterior seam-fated daughter. egl-18 and elt-6 are necessary for larval seam cell specification, and for hypodermal to seam cell fate transformations induced by ectopic Wnt pathway overactivation. The TCF homolog POP-1 binds a site in the egl-18 promoter in vitro, and this site is necessary for robust seam cell expression in vivo. Finally, larval overexpression of EGL-18 is sufficient to drive expression of a seam marker in other hypodermal cells in wild-type animals, and in anterior hypodermal-fated daughters in a Wnt pathway-sensitized background. These data suggest that two GATA factors that are required for seam cell specification in the embryo independently of Wnt signaling are reused downstream of Wnt signaling to maintain the progenitor fate during stem cell-like divisions in larval development.

Deconvolving the roles of Wnt ligands and receptors in sensing and amplification

Establishment of cell polarity is crucial for many biological processes including cell migration and asymmetric cell division. The establishment of cell polarity consists of two sequential processes: an external gradient is first sensed and then the resulting signal is amplified and maintained by intracellular signaling networks usually using positive feedback regulation. Generally, these two processes are intertwined and it is challenging to determine which proteins contribute to the sensing or amplification process, particularly in multicellular organisms. Here, we integrated phenomenological modeling with quantitative single-cell measurements to separate the sensing and amplification components of Wnt ligands and receptors during establishment of polarity of the Caenorhabditis elegans P cells. By systematically exploring how P-cell polarity is altered in Wnt ligand and receptor mutants, we inferred that ligands predominantly affect the sensing process, whereas receptors are needed for both sensing and amplification. This integrated approach is generally applicable to other systems and will facilitate decoupling of the different layers of signal sensing and amplification.

Wnt signaling regulates acetylcholine receptor translocation and synaptic plasticity in the adult nervous system

The adult nervous system is plastic, allowing us to learn, remember, and forget. Experience-dependent plasticity occurs at synapses--the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt-signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In Caenorhabditis elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/α7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/α7 to synapses in Wnt-signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/α7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors.

Involvement of Wnt signaling pathways in the metamorphosis of the bryozoan Bugula neritina

In this study, we analyzed the metamorphosis of the marine bryozoan Bugula neritina. We observed the morphogenesis of the ancestrula. We defined three distinct pre-ancestrula stages based on the anatomy of the developing polypide and the overall morphology of pre-ancestrula. We then used an annotation based enrichment analysis tool to analyze the B. neritina transcriptome and identified over-representation of genes related to Wnt signaling pathways, suggesting its involvement in metamorphosis. Finally, we studied the temporal-spatial gene expression studies of several Wnt pathway genes. We found that one of the Wnt ligand, BnWnt10, was expressed spatially opposite to the Wnt antagonist BnsFRP within the blastemas, which is the presumptive polypide. Down-stream components of the canonical Wnt signaling pathway were exclusively expressed in the blastemas. Bnβcatenin and BnFz5/8 were exclusively expressed in the blastemas throughout the metamorphosis. Based on the genes expression patterns, we propose that BnWnt10 and BnsFRP may relate to the patterning of the polypide, in which the two genes served as positional signals and contributed to the polarization of the blastemas. Another Wnt ligand, BnWnt6, was expressed in the apical part of the pre-ancestrula epidermis. Overall, our findings suggest that the Wnt signaling pathway may be important to the pattern formation of polypide and the development of epidermis.

Multiple Wnts redundantly control polarity orientation in Caenorhabditis elegans epithelial stem cells

During development, cell polarization is often coordinated to harmonize tissue patterning and morphogenesis. However, how extrinsic signals synchronize cell polarization is not understood. In Caenorhabditis elegans, most mitotic cells are polarized along the anterior-posterior axis and divide asymmetrically. Although this process is regulated by a Wnt-signaling pathway, Wnts functioning in cell polarity have been demonstrated in only a few cells. We analyzed how Wnts control cell polarity, using compound Wnt mutants, including animals with mutations in all five Wnt genes. We found that somatic gonadal precursor cells (SGPs) are properly polarized and oriented in quintuple Wnt mutants, suggesting Wnts are dispensable for the SGPs' polarity, which instead requires signals from the germ cells. Thus, signals from the germ cells organize the C. elegans somatic gonad. In contrast, in compound but not single Wnt mutants, most of the six seam cells, V1–V6 (which are epithelial stem cells), retain their polarization, but their polar orientation becomes random, indicating that it is redundantly regulated by multiple Wnt genes. In contrast, in animals in which the functions of three Wnt receptors (LIN-17, MOM-5, and CAM-1) are disrupted—the stem cells are not polarized and divide symmetrically—suggesting that the Wnt receptors are essential for generating polarity and that they function even in the absence of Wnts. All the seam cells except V5 were polarized properly by a single Wnt gene expressed at the cell's anterior or posterior. The ectopic expression of posteriorly expressed Wnts in an anterior region and vice versa rescued polarity defects in compound Wnt mutants, raising two possibilities: one, Wnts permissively control the orientation of polarity; or two, Wnt functions are instructive, but which orientation they specify is determined by the cells that express them. Our results provide a paradigm for understanding how cell polarity is coordinated by extrinsic signals.

Proper functions and development of organs often require the synchronized polarization of entire cell groups. How cells coordinate their polarity is poorly understood. One plausible model is that individual cells recognize extrinsic signal gradients that orient their polarity, although this has not been shown in any organism. In particular, although Wnt signaling is important for cell polarization, and Wnt signal gradients are important for the coordinated specification of cell fates, the Wnts' involvement in orienting cell polarity is unclear. In the nematode Caenorhabditis elegans, most asymmetrically dividing mitotic cells are polarized in the same anterior-posterior orientation. Here we show that multiple Wnt proteins redundantly control the proper orientation of cell polarity, but not for polarization per se, in a group of epithelial stem cells. In contrast, Wnt receptors are indispensable for cells to adopt a polarized phenotype. Most stem cells are properly oriented by Wnt genes that are expressed either at their anterior or posterior side. Surprisingly, Wnt signals can properly orient stem cell polarity, even when their source is changed from anterior to posterior or vice versa. Our results suggest the presence of novel mechanisms by which Wnt genes orient cell polarity.

LIN-44/Wnt directs dendrite outgrowth through LIN-17/Frizzled in C. elegans Neurons

Nervous system function requires proper development of two functional and morphological domains of neurons, axons and dendrites. Although both these domains are equally important for signal transmission, our understanding of dendrite development remains relatively poor. Here, we show that in C. elegans the Wnt ligand, LIN-44, and its Frizzled receptor, LIN-17, regulate dendrite development of the PQR oxygen sensory neuron. In lin-44 and lin-17 mutants, PQR dendrites fail to form, display stunted growth, or are misrouted. Manipulation of temporal and spatial expression of LIN-44, combined with cell-ablation experiments, indicates that this molecule is patterned during embryogenesis and acts as an attractive cue to define the site from which the dendrite emerges. Genetic interaction between lin-44 and lin-17 suggests that the LIN-44 signal is transmitted through the LIN-17 receptor, which acts cell autonomously in PQR. Furthermore, we provide evidence that LIN-17 interacts with another Wnt molecule, EGL-20, and functions in parallel to MIG-1/Frizzled in this process. Taken together, our results reveal a crucial role for Wnt and Frizzled molecules in regulating dendrite development in vivo.

Neurons have distinct compartments, which include axons and dendrites. Both of these compartments are essential for communication between neurons, as signals are received by dendrites and transmitted by axons. Although dendrites are vital for neural connectivity, very little is known about how they are formed. Here, we have investigated how dendrites develop in vivo by examining an oxygen sensory neuron (PQR) in the nematode C. elegans. Using a genetic approach, we have discovered that Wnt proteins, a group of highly conserved secreted morphogens, interact with their canonical Frizzled receptors to control the development of the PQR dendrite. We show that Wnt molecules act as attractive signals to determine the initiation and direction of dendrite outgrowth. Interestingly, Wnt proteins act specifically on the dendrite without affecting the axon, suggesting that outgrowth of the dendrite can be regulated by distinct processes that are independent of axon formation. We predict that similar mechanisms may be in place in other species owing to the conserved roles of Wnt and Frizzled molecules in development.

A SNX3-dependent retromer pathway mediates retrograde transport of the Wnt sorting receptor Wntless and is required for Wnt secretion

Wnt proteins are lipid-modified glycoproteins that play a central role in development, adult tissue homeostasis and disease. Secretion of Wnt proteins is mediated by the Wnt-binding protein Wntless (Wls), which transports Wnt from the Golgi network to the cell surface for release. It has recently been shown that recycling of Wls through a retromer-dependent endosome-to-Golgi trafficking pathway is required for efficient Wnt secretion, but the mechanism of this retrograde transport pathway is poorly understood. Here, we report that Wls recycling is mediated through a retromer pathway that is independent of the retromer sorting nexins SNX1-SNX2 and SNX5-SNX6. We have found that the unrelated sorting nexin, SNX3, has an evolutionarily conserved function in Wls recycling and Wnt secretion and show that SNX3 interacts directly with the cargo-selective subcomplex of the retromer to sort Wls into a morphologically distinct retrieval pathway. These results demonstrate that SNX3 is part of an alternative retromer pathway that functionally separates the retrograde transport of Wls from other retromer cargo.

Neuroblast migration along the anteroposterior axis of C. elegans is controlled by opposing gradients of Wnts and a secreted Frizzled-related protein

The migration of neuroblasts along the anteroposterior body axis of C. elegans is controlled by multiple Wnts that act partially redundantly to guide cells to their precisely defined final destinations. How positional information is specified by this system is, however, still largely unknown. Here, we used a novel fluorescent in situ hybridization methods to generate a quantitative spatiotemporal expression map of the C. elegans Wnt genes. We found that the five Wnt genes are expressed in a series of partially overlapping domains along the anteroposterior axis, with a predominant expression in the posterior half of the body. Furthermore, we show that a secreted Frizzled-related protein is expressed at the anterior end of the body axis, where it inhibits Wnt signaling to control neuroblast migration. Our findings reveal that a system of regionalized Wnt gene expression and anterior Wnt inhibition guides the highly stereotypic migration of neuroblasts in C. elegans. Opposing expression of Wnts and Wnt inhibitors has been observed in basal metazoans and in the vertebrate neurectoderm. Our results in C. elegans support the notion that a system of posterior Wnt signaling and anterior Wnt inhibition is an evolutionarily conserved principle of primary body axis specification.

Cell identification and cell lineage analysis

Caenorhabditis elegans is uniquely suited to the analysis of cell lineage patterns. C. elegans has a small number of somatic cells whose position and morphology are almost invariant from animal to animal. Because C. elegans is virtually transparent, cells can be identified in live animals using a simple bright-field microscopy technique, Nomarski differential interference contrast (DIC), or by expression of transgenic fluorescent reporter genes. The small size and rapid development of C. elegans mean that animals can develop while under continuous observation, allowing cell lineages to be analyzed throughout embryonic and postembryonic development. Embryonic cell lineages can also be traced semiautomatically using timelapse imaging of GFP-labeled nuclei. Analysis of mutant cell lineages remains important for defining the roles of developmental control genes.

Wnt signaling controls the stem cell-like asymmetric division of the epithelial seam cells during C. elegans larval development

Metazoan stem cells repopulate tissues during adult life by dividing asymmetrically to generate another stem cell and a cell that terminally differentiates. Wnt signaling regulates the division pattern of stem cells in flies and vertebrates. While the short-lived nematode C. elegans has no adult somatic stem cells, the lateral epithelial seam cells divide in a stem cell-like manner in each larval stage, usually generating a posterior daughter that retains the seam cell fate and an anterior daughter that terminally differentiates. We show that while wild-type adult animals have 16 seam cells per side, animals with reduced function of the TCF homolog POP-1 have as many as 67 seam cells, and animals with reduced function of the β-catenins SYS-1 and WRM-1 have as few as three. Analysis of seam cell division patterns showed alterations in their stem cell-like divisions in the L2-L4 stages: reduced Wnt signaling caused both daughters to adopt non-seam fates, while activated Wnt signaling caused both daughters to adopt the seam fate. Therefore, our results indicate that Wnt signaling globally regulates the asymmetric, stem cell-like division of most or all somatic seam cells during C. elegans larval development, and that Wnt pathway regulation of stem cell-like behavior is conserved in nematodes.

Re-programming of C. elegans male epidermal precursor fates by Wnt, Hox, and LIN-12/Notch activities

In Caenorhabditiselegans males, different subsets of ventral epidermal precursor (Pn.p) cells adopt distinct fates in a position-specific manner: three posterior cells, P(9-11).p, comprise the hook sensillum competence group (HCG) with three potential fates (1 degrees , 2 degrees , or 3 degrees ), while eight anterior cells, P(1-8).p, fuse with the hyp7 epidermal syncytium. Here we show that activation of the canonical BAR-1 beta-catenin pathway of Wnt signaling alters the competence of P(3-8).p and specifies ectopic HCG-like fates. This fate transformation requires the Hox gene mab-5. In addition, misexpression of mab-5 in P(1-8).p is sufficient to establish HCG competence among these cells, as well as to generate ectopic HCG fates in combination with LIN-12 or EGF signaling. While increased Wnt signaling induces predominantly 1 degrees HCG fates, increased LIN-12 or EGF signaling in combination with MAB-5 overexpression promotes 2 degrees HCG fates in anterior Pn.p cells, suggesting distinctive functions of Wnt, LIN-12, and EGF signaling in specification of HCG fates. Lastly, wild-type mab-5 function is necessary for normal P(9-11).p fate specification, indicating that regulation of ectopic HCG fate formation revealed in anterior Pn.p cells reflect mechanisms of pattern formation during normal hook development.

The role of C. elegans Ena/VASP homolog UNC-34 in neuronal polarity and motility

Ena/VASP proteins mediate the effects of guidance cues on the actin cytoskeleton. The single C. elegans homolog of the Ena/VASP family of proteins, UNC-34, is required for the migrations of cells and growth cones. Here we show that unc-34 mutant alleles also interact genetically with Wnt mutants to reveal a role for unc-34 in the establishment of neuronal polarity along the C. elegans anterior-posterior axis. Our mutant analysis shows that eliminating UNC-34 function results in neuronal migration and polarity phenotypes that are enhanced at higher temperatures, revealing a heat-sensitive process that is normally masked by the presence of UNC-34. Finally, we show that the UNC-34 protein is expressed broadly and accumulates in axons and at the apical junctions of epithelial cells. While most mutants lacked detectable UNC-34, three unc-34 mutants that contained missense mutations in the EVH1 domain produced full-length UNC-34 that failed to localize to apical junctions and axons, supporting the role for the EVH1 domain in localizing Ena/VASP family members.

Wnt signaling and the polarity of the primary body axis

How animals establish and pattern the primary body axis is one of the most fundamental problems in biology. Data from diverse deuterostomes (frog, fish, mouse, and amphioxus) and from planarians (protostomes) suggest that Wnt signaling through beta-catenin controls posterior identity during body plan formation in most bilaterally symmetric animals. Wnt signaling also influences primary axis polarity of pre-bilaterian animals, indicating that an axial patterning role for Wnt signaling predates the evolution of bilaterally symmetric animals. The use of posterior Wnt signaling and anterior Wnt inhibition might be a unifying principle of body plan development in most animals.

The roles of EGF and Wnt signaling during patterning of the C. elegans Bgamma/delta Equivalence Group

BACKGROUND

During development, different signaling pathways interact to specify cell fate by regulating transcription factors necessary for fate specification and morphogenesis. In Caenorhabditis elegans, the EGF-Ras and Wnt signaling pathways have been shown to interact to specify cell fate in three equivalence groups: the vulval precursor cells (VPCs), the hook competence group (HCG) and P11/12. In the VPCs, HCG and P11/12 pair, EGF and Wnt signaling positively regulate different Hox genes, each of which also functions during fate specification. In the male, EGF-Ras signaling is required to specify the Bgamma fate within the Bgamma/delta equivalence pair, while Notch signaling is required for Bdelta fate specification. In addition, TGF-beta signaling by dbl-1/dpp controls ceh-13/labial/Hox1 expression in Bgamma.

RESULTS

We show that EGF-Ras signaling is required for Bgamma expression of ceh-13/labial/Hox1. The transcription factors lin-1/ETS and lin-31/Forkhead, functioning downstream of the EGF pathway, as well as sur-2/MED23 (a component of the Mediator complex) also control ceh-13 expression in Bgamma. In addition, our results indicate that lin-44/Wnt, mom-2/Wnt and lin-17/Fz are necessary to maintain the division of Bgamma along a longitudinal axis. We also show that dbl-1/dpp acts either in parallel or downstream of EGF pathway to regulate ceh-13/Hox1 expression in Bgamma. Lastly, we find that a dbl-1/dpp null mutation did not cause any vulval or P12 defects and did not enhance vulval and P12 defects of reduction-of-function mutations of components of the EGF pathway.

CONCLUSIONS

ceh-13/labial/Hox1 expression in Bgamma is regulated by the EGF pathway and downstream factors lin-1/ETS lin-31/Forkhead and sur-2/MED23. Wnt signaling is required for proper Bgamma division, perhaps to orient the Bgamma mitotic spindle. Our results suggest that dbl-1/dpp is not required for VPC and P12 specification, highlighting another difference among these EGF-dependent equivalence groups.

A wound-induced Wnt expression program controls planarian regeneration polarity

Regeneration requires specification of the identity of new tissues to be made. Whether this process relies only on intrinsic regulative properties of regenerating tissues or whether wound signaling provides input into tissue repatterning is not known. The head-versus-tail regeneration polarity decision in planarians, which requires Wnt signaling, provides a paradigm to study the process of tissue identity specification during regeneration. The Smed-wntP-1 gene is required for regeneration polarity and is expressed at the posterior pole of intact animals. Surprisingly, wntP-1 was expressed at both anterior- and posterior-facing wounds rapidly after wounding. wntP-1 expression was induced by all types of wounds examined, regardless of whether wounding prompted tail regeneration. Regeneration polarity was found to require new expression of wntP-1. Inhibition of the wntP-2 gene enhanced the polarity phenotype due to wntP-1 inhibition, with new expression of wntP-2 in regeneration occurring subsequent to expression of wntP-1 and localized only to posterior-facing wounds. New expression of wntP-2 required wound-induced wntP-1. Finally, wntP-1 and wntP-2 expression changes occurred even in the absence of neoblast stem cells, which are required for regeneration, suggesting that the role of these genes in polarity is independent of and instructive for tail formation. These data indicate that wound-induced input is involved in resetting the normal polarized features of the body axis during regeneration.

HOM-C genes, Wnt signaling and axial patterning in the C. elegans posterior ventral epidermis

Wnt signaling and HOM-C/Hox genes pattern cell fate along the anterior/posterior axis in many animals. In general, Wnt signaling participates in establishing the anterior/posterior axis, whereas HOM-C genes confer regional identities to cells along the axis. However, recent work in non-bilaterial metazoans suggests that the ancestral patterning system relied on Wnts, with a later co-option of HOM-C genes to replace Wnts in regional patterning. Here we provide direct experimental support for this model from C. elegans, where a regional Wnt patterning system is uncovered in HOM-C gene mutants. Anterior/posterior patterning of P11/P12 cell fate in the C. elegans tail is normally dependent on the HOM-C gene egl-5/Abdominal-B. If the HOM-C gene mab-5/fushi tarazu is also mutant, however, a Wnt signal can promote P12 fate in the absence of egl-5. Furthermore, transcription of egl-5 in the P12.pa cell is influenced by an autoregulatory element that is essential in wild type, but not in mab-5 egl-5 double mutants, identifying regulatory parallels between P12 cell fate specification and egl-5 transcriptional regulation in the P12 lineage. Together, our results identify complex regulatory relationships among signaling pathways and HOM-C genes, and uncover a layering of patterning systems that may reflect their evolutionary history.

A beta-catenin-dependent Wnt pathway mediates anteroposterior axon guidance in C. elegans motor neurons

Background

Wnts are secreted glycoproteins that regulate diverse aspects of development, including cell proliferation, cell fate specification and differentiation. More recently, Wnts have been shown to direct axon guidance in vertebrates, flies and worms. However, little is known about the intracellular signaling pathways downstream of Wnts in axon guidance.

Methodology/Principal Findings

Here we show that the posterior C. elegans Wnt protein LIN-44 repels the axons of the adjacent D-type motor neurons by activating its receptor LIN-17/Frizzled on the neurons. Moreover, mutations in mig-5/Disheveled, gsk-3, pry-1/Axin, bar-1/β-catenin and pop-1/TCF, also cause disrupted D-type axon pathfinding. Reduced BAR-1/β-catenin activity in D-type axons leads to undergrowth of axons, while stabilization of BAR-1/β-catenin in a lin-23/SCF^β-TrCP mutant results in an overextension phenotype.

Conclusions/Significance

Together, our data provide evidence that Wnt-mediated axon guidance can be transduced through a β-catenin-dependent pathway.

Wnt and EGF pathways act together to induce C. elegans male hook development

Comparative studies of vulva development between Caenorhabditis elegans and other nematode species have provided some insight into the evolution of patterning networks. However, molecular genetic details are available only in C. elegans and Pristionchus pacificus. To extend our knowledge on the evolution of patterning networks, we studied the C. elegans male hook competence group (HCG), an equivalence group that has similar developmental origins to the vulval precursor cells (VPCs), which generate the vulva in the hermaphrodite. Similar to VPC fate specification, each HCG cell adopts one of three fates (1 degree, 2 degrees, 3 degrees), and 2 degrees HCG fate specification is mediated by LIN-12/Notch. We show that 2 degrees HCG specification depends on the presence of a cell with the 1 degree fate. We also provide evidence that Wnt signaling via the Frizzled-like Wnt receptor LIN-17 acts to specify the 1 degree and 2 degrees HCG fate. A requirement for EGF signaling during 1 degree fate specification is seen only when LIN-17 activity is compromised. In addition, activation of the EGF pathway decreases dependence on LIN-17 and causes ectopic hook development. Our results suggest that WNT plays a more significant role than EGF signaling in specifying HCG fates, whereas in VPC specification EGF signaling is the major inductive signal. Nonetheless, the overall logic is similar in the VPCs and the HCG: EGF and/or WNT induce a 1 degree lineage, and LIN-12/NOTCH induces a 2 degrees lineage. Wnt signaling is also required for execution of the 1 degree and 2 degrees HCG lineages. lin-17 and bar-1/beta-catenin are preferentially expressed in the presumptive 1 degree cell P11.p. The dynamic subcellular localization of BAR-1-GFP in P11.p is concordant with the timing of HCG fate determination.

Complex network of Wnt signaling regulates neuronal migrations during Caenorhabditis elegans development

Members of the Wnt family of secreted glycoproteins regulate many developmental processes, including cell migration. We and others have previously shown that the Wnts egl-20, cwn-1, and cwn-2 are required for cell migration and axon guidance. However, the roles in cell migration of all of the Caenorhabditis elegans Wnt genes and their candidate receptors have not been explored fully. We have extended our analysis to include all C. elegans Wnts and six candidate Wnt receptors: four Frizzleds, the sole Ryk family receptor LIN-18, and the Ror receptor tyrosine kinase CAM-1. We show that three of the Wnts, CWN-1, CWN-2, and EGL-20, play major roles in directing cell migrations and that all five Wnts direct specific cell migrations either by acting redundantly or by antagonizing each other's function. We report that all four Frizzleds function to direct Q-descendant cell migrations, but only a subset of the putative Wnt receptors function in directing migrations of other cells. Finally, we find striking differences between the phenotypes of the Wnt quintuple and Frizzled quadruple mutants.

Positive and negative regulatory inputs restrict pax-6/vab-3 transcription to sensory organ precursors in Caenorhabditis elegans

The Pax-6 gene encodes a transcription factor essential for the development of eyes and other sensory organs in species ranging from planaria to mice. Because Pax-6 activity can be both necessary and sufficient for eye organogenesis, much work has focused on PAX-6 function and regulation of target genes. However, less is known about the genetic mechanisms that establish the Pax-6 expression pattern. We have utilized Caenorhabditis elegans as a relatively simple model system to characterize the regulation of Pax-6 transcription in sensory organ precursors. In C. elegans males, two sensory mating structures, the copulatory spicules and the post-cloacal sensilla, are formed from stereotyped divisions of the two post-embryonic blast cells, B.a and Y.p, respectively. A C. elegans pax-6 transcript, vab-3, is necessary for the development of these sensory structures. Using a green fluorescent protein (GFP)-based vab-3 transcriptional reporter, we show that expression is restricted to the sensory organ lineages of B.a and Y.p. Transcription of vab-3 in the tail region of the worm requires the Abdominal B homeobox gene, egl-5. Opposing this activation, a transcription factor cascade and a Wnt signaling pathway each act to restrict vab-3 expression to the appropriate cell lineages. Thus we have identified multiple genetic pathways that act to restrict pax-6/vab-3 gene expression to the sensory organ precursor cells.

Analysis of Wnt signaling during Caenorhabditis elegans postembryonic development

Wnts play a central role in the development of many cells and tissue types in all species studied to date. Like many other extracellular signaling pathways, secreted Wnt proteins are involved in many different processes; in C. elegans these include: cell proliferation, differentiation, cell migration, control of cell polarity, axon outgrowth and control of the stem cell niche. Perturbations in Wnt signaling are also key factors in cancer formation, and therefore of interest to oncobiologists. Wnts are secreted glycoproteins, which bind to Frizzled transmembrane receptors and signal either through, or independently of beta-catenin. Both beta-catenin-dependant (Wnt/beta-catenin) and -independent pathways function during postembryonic development in C. elegans and allow Wnt researchers to explore aspects of Wnt signaling both in common with other organisms and unique to the nematode. Chapter 9 in Volume 2 discusses various processes controlled by Wnt signaling during C. elegans embryonic development; this chapter discusses Wnt controlled processes that occur during postembryonic development, including an overview of methods used to observe their function.

Wnt signal from multiple tissues and lin-3/EGF signal from the gonad maintain vulval precursor cell competence in Caenorhabditis elegans

The Caenorhabditis elegans vulva has been a valuable paradigm for defining components of signaling pathways and elucidating how signaling events are coordinated to generate a developmental pattern. Vulval precursor cells (VPCs) are induced to adopt vulval fates in the third larval stage by LIN-3, an EGF-like signal produced by the gonad. Competence to respond to the inductive signal requires that the VPCs do not fuse to the major hypodermal syncytium, hyp7. We found that two Wnt-encoding genes, cwn-1 and egl-20, play a major role in preventing fusion of VPCs with hyp7 in the second larval stage. By using tissue-specific rescue of mig-14/Wntless, which is required for the production of Wnt ligands, we found that Wnt signal produced by multiple tissues, including neurons and muscles, promotes or maintains VPC competence before vulval induction. In addition, through laser ablation and genetic analysis, we provide evidence that LIN-3 signal from the gonad also plays a significant role in preventing VPCs from fusing with hyp7. We propose that Wnt signaling plays a permissive role in preventing VPCs from fusing with hyp7 and reevaluate the roles of Wnt and LIN-3/EGF signaling in competence and induction.

The Flightless I homolog, fli-1, regulates anterior/posterior polarity, asymmetric cell division and ovulation during Caenorhabditis elegans development

Flightless I (Fli I) is an evolutionarily conserved member of the gelsolin family, containing actin-binding and severing activity in vitro. The physiological function of Fli I during animal development remains largely undefined. In this study, we reveal a key role of the Caenorhabditis elegans Fli I homolog, fli-1, in specifying asymmetric cell division and in establishing anterior-posterior polarity in the zygote. The fli-1 gene also regulates the cytokinesis of somatic cells and the development of germline and interacts with the phosphoinositol-signaling pathway in the regulation of ovulation. The fli-1 reporter gene shows that the localization of FLI-1 coincides with actin-rich regions and that the actin cytoskeleton is impaired in many tissues in the fli-1 mutants. Furthermore, the function of fli-1 in C. elegans can be functionally substituted by the Drosophila Fli I. Our studies demonstrate that fli-1 plays an important role in regulating the actin-dependent events during C. elegans development.

The C. elegans CBFbeta homolog, BRO-1, regulates the proliferation, differentiation and specification of the stem cell-like seam cell lineages

The RUNX/CBFbeta heterodimeric transcription factor plays an important role in regulating cell proliferation and differentiation in a variety of developmental contexts. Aberrant function of Runx and CBFbeta has been causally related to the development of various diseases, including acute myeloid leukemia, gastric cancer and cleidocranial dysplasia. The underlying mechanism of the RUNX/CBFbeta complex in regulation of cell proliferation is still poorly defined. In this study, we demonstrate that the Caenorhabditis elegans CBFbeta homolog, bro-1, is essential for the proliferation, differentiation and specification of a row of stem cell-like lineages, called seam cells. BRO-1 forms complex with the C. elegans RUNX homolog, RNT-1, and augments the DNA-binding activity of RNT-1. The RNT-1/BRO-1 complex directly interacts with the C. elegans Groucho homolog, UNC-37, whose loss of function mutations display similar defects in the proliferation of seam cells as those of bro-1 and rnt-1 mutants. Additionally, the defects in seam cell division in bro-1 mutants are substantially rescued by the inactivation of the negative regulators of the G1 to S phase cell cycle progression, including the lin-35 Rb, fzr-1 Cdh1 and cki-1 CIP homologs. Our studies indicate that the transcriptional repression activity of the RNT-1/BRO-1 complex regulates the G1 to S cell cycle progression during seam cell division.

Cortical beta-catenin and APC regulate asymmetric nuclear beta-catenin localization during asymmetric cell division in C. elegans

In C. elegans, Wnt signaling regulates a number of asymmetric cell divisions. During telophase, WRM-1/beta-catenin localizes asymmetrically to the anterior cortex and the posterior daughter's nucleus. However, cortical WRM-1's functions are not known. Here, we use a membrane-targeted form of WRM-1 to show that cortical WRM-1 inhibits Wnt signaling and the nuclear localization of WRM-1. These functions are mediated by APR-1/APC, which regulates WRM-1 nuclear export. We also show that APR-1 as well as PRY-1/Axin and Dishevelled homologs localize asymmetrically to the cortex. Our results suggest a model in which cortical WRM-1 recruits APR-1 to the anterior cortex before and during division, and the cortical APR-1 stimulates WRM-1 export from the anterior nucleus at telophase. Because beta-catenin and APC are localized to the cortex in many cell types in different species, our results suggest that these cortical proteins may regulate asymmetric divisions or Wnt signaling in other organisms as well.

Reciprocal asymmetry of SYS-1/beta-catenin and POP-1/TCF controls asymmetric divisions in Caenorhabditis elegans

beta-Catenins are conserved regulators of metazoan development that function with TCF DNA-binding proteins to activate transcription. In Caenorhabditis elegans, SYS-1/beta-catenin and POP-1/TCF regulate several asymmetric divisions, including that of the somatic gonadal precursor cell (SGP). In the distal but not the proximal SGP daughter, SYS-1/beta-catenin and POP-1/TCF transcriptionally activate ceh-22 to specify the distal fate. Here, we investigate the distribution of SYS-1/beta-catenin and its regulation. Using a rescuing transgene, VNS::SYS-1, which fuses VENUS fluorescent protein to SYS-1, we find more VNS::SYS-1 in distal than proximal SGP daughters, a phenomenon we call "SYS-1 asymmetry." In addition, SYS-1 asymmetry is seen in many other tissues, consistent with the idea that SYS-1 regulates asymmetric divisions broadly during C. elegans development. In particular, SYS-1 is more abundant in E than MS, and SYS-1 is critical for the endodermal fate. In all cases, SYS-1 is reciprocal to POP-1 asymmetry: cells with higher SYS-1 have lower POP-1, and vice versa. SYS-1 asymmetry is controlled posttranslationally and relies on frizzled and dishevelled homologs, which also control POP-1 asymmetry. Therefore, upstream regulators modulate the SYS-1 to POP-1 ratio by increasing SYS-1 and decreasing POP-1 within the same cell. By contrast, SYS-1 asymmetry does not rely on WRM-1, which appears specialized for POP-1 asymmetry. We suggest a two-pronged pathway for control of SYS-1:POP-1, which can robustly accomplish differential gene expression in daughters of an asymmetric cell division.

Asymmetric localizations of LIN-17/Fz and MIG-5/Dsh are involved in the asymmetric B cell division in C. elegans

LIN-44/Wnt and LIN-17/Frizzled (Fz) function in a planar cell polarity (PCP)-like pathway to regulate the asymmetric B cell division in Caenorhabditis elegans. We observed asymmetric localization of LIN-17/Frizzled (Fz) and MIG-5/Dishevelled (Dsh) during the B cell division. LIN-17::GFP was asymmetrically localized within the B cell prior to and after the B cell division and correlated with B cell polarity. Asymmetric localization of LIN-17::GFP was dependent upon LIN-44/Wnt and MIG-5/Dsh function. The LIN-17 transmembrane domain and a portion of the cysteine-rich domain (CRD) were required for LIN-17 function and asymmetric distribution to the B cell daughters, while the conserved KTXXXW motif was only required for function. MIG-5::GFP was also asymmetrically localized within the B cell prior to and after the B cell division in a LIN-17- and LIN-44-dependent manner. Functions of the MIG-5 DEP, PDZ and DIX domains were also conserved. Thus, a novel PCP-like pathway, in which LIN-17 and MIG-5 are asymmetrically localized, is involved in the regulation of B cell polarity.

Wnt Gradient Formation Requires Retromer Function in Wnt-Producing Cells

Wnt proteins function as morphogens that can form long-range concentration gradients to pattern developing tissues. Here, we show that the retromer, a multiprotein complex involved in intracellular protein trafficking, is required for long-range signaling of the Caenorhabditis elegans Wnt ortholog EGL-20. The retromer functions in EGL-20–producing cells to allow the formation of an EGL-20 gradient along the anteroposterior axis. This function is evolutionarily conserved, because Wnt target gene expression is also impaired in the absence of the retromer complex in vertebrates. These results demonstrate that the ability of Wnt to regulate long-range patterning events is dependent on a critical and conserved function of the retromer complex within Wnt-producing cells.

Wnt signals can function as positional cues in establishing cell polarity

Wnt signaling plays important roles in cell polarization in diverse organisms, and loss of cell polarity is an early event in tumorigenesis caused by mutations in Wnt pathway genes. Despite this, the precise roles of Wnt proteins in cell polarization have remained elusive. In no organism has it been shown that the asymmetric position of a Wnt signal is essential to establishing a cell's polarity. Attempts to test this by ubiquitous expression of Wnt genes have suggested that Wnt signals might act only as permissive factors in cell polarization. Here we find, by using cell manipulations and ectopic gene expression in C. elegans, that the position from which Wnt signals are presented can determine the polarity of both embryonic and postembryonic cells. Furthermore, the position from which a Wnt signal is presented can determine the polarity of Frizzled receptor localization, suggesting that the polarizing effect of Wnt is likely to be direct. These results demonstrate that Wnt proteins can function as positional cues in establishing cell polarity.

Wnt signals and frizzled activity orient anterior-posterior axon outgrowth in C. elegans

Secreted proteins of the Wnt family affect axon guidance, asymmetric cell division, and cell fate. We show here that C. elegans Wnts acting through Frizzled receptors can shape axon and dendrite trajectories by reversing the anterior-posterior polarity of neurons. In lin-44/Wnt and lin-17/Frizzled mutants, the polarity of the PLM mechanosensory neuron is reversed along the body axis: the long PLM process, PLM growth cone, and synapses are posterior to its cell body instead of anterior. Similarly, the polarity of the ALM mechanosensory neuron is reversed in cwn-1 egl-20 Wnt double mutants, suggesting that different Wnt signals regulate neuronal polarity at different anterior-posterior positions. LIN-17 protein is asymmetrically localized to the posterior process of PLM in a lin-44-dependent manner, indicating that Wnt signaling redistributes LIN-17 in PLM. In this context, Wnts appear to function not as instructive growth cone attractants or repellents, but as organizers of neuronal polarity.