Multiple redundant Wnt signaling components function in two processes during C. elegans vulval development

β-catenin-dependent Wnt signaling in C. elegans: teaching an old dog a new trick

Wnt signaling is an evolutionarily ancient pathway used to regulate many events during metazoan development. Genetic results from Caenorhabditis elegans more than a dozen years ago suggested that Wnt signaling in this nematode worm might be different than in vertebrates and Drosophila: the worm had a small number of Wnts, too many β-catenins, and some Wnt pathway components functioned in an opposite manner than in other species. Work over the ensuing years has clarified that C. elegans does possess a canonical Wnt/β-catenin signaling pathway similar to that in other metazoans, but that the majority of Wnt signaling in this species may proceed via a variant Wnt/β-catenin signaling pathway that uses some new components (mitogen-activated protein kinase signaling enzymes), and in which some conserved pathway components (β-catenin, T-cell factor [TCF]) are used in new and interesting ways. This review summarizes our current understanding of the canonical and novel TCF/β-catenin-dependent signaling pathways in C. elegans.

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.

The Wnt/beta-catenin asymmetry pathway patterns the atonal ortholog lin-32 to diversify cell fate in a Caenorhabditis elegans sensory lineage

Each sensory ray of the Caenorhabditis elegans male tail comprises three distinct neuroglial cell types. These three cells descend from a single progenitor, the ray precursor cell, through several rounds of asymmetric division called the ray sublineage. Ray development requires the conserved atonal-family bHLH gene lin-32, which specifies the ray neuroblast and promotes the differentiation of its progeny. However, the mechanisms that allocate specific cell fates among these progeny are unknown. Here we show that the distribution of LIN-32 during the ray sublineage is markedly asymmetric, localizing to anterior daughter cells in two successive cell divisions. The anterior-posterior patterning of LIN-32 expression and of differentiated ray neuroglial fates is brought about by the Wnt/β-catenin asymmetry pathway, including the Wnt ligand LIN-44, its receptor LIN-17, and downstream components LIT-1 (NLK), SYS-1 (β-catenin), and POP-1 (TCF). LIN-32 asymmetry itself has an important role in patterning ray cell fates, because the failure to silence lin-32 expression in posterior cells disrupts development of this branch of the ray sublineage. Together, our results illustrate a mechanism whereby the regulated function of a proneural-class gene in a single neural lineage can both specify a neural precursor and actively pattern the fates of its progeny. Moreover, they reveal a central role for the Wnt/β-catenin asymmetry pathway in patterning neural and glial fates in a simple sensory lineage.

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.

VANG-1 and PRKL-1 cooperate to negatively regulate neurite formation in Caenorhabditis elegans

Neuritogenesis is a critical early step in the development and maturation of neurons and neuronal circuits. While extracellular directional cues are known to specify the site and orientation of nascent neurite formation in vivo, little is known about the genetic pathways that block inappropriate neurite emergence in order to maintain proper neuronal polarity. Here we report that the Caenorhabditis elegans orthologues of Van Gogh (vang-1), Prickle (prkl-1), and Dishevelled (dsh-1), core components of planar cell polarity (PCP) signaling, are required in a subset of peripheral motor neurons to restrict neurite emergence to a specific organ axis. In loss-of-function mutants, neurons display supernumerary neurites that extend inappropriately along the orthogonal anteroposterior (A/P) body axis. We show that autonomous and non-autonomous gene activities are required early and persistently to inhibit the formation or consolidation of growth cone protrusions directed away from organ precursor cells. Furthermore, prkl-1 overexpression is sufficient to suppress neurite formation and reorient neuronal polarity in a vang-1– and dsh-1–dependent manner. Our findings suggest a novel role for a PCP–like pathway in maintaining polarized neuronal morphology by inhibiting neuronal responses to extrinsic or intrinsic cues that would otherwise promote extraneous neurite formation.

Neurons are among the most morphologically complex cells in the body. Early in development, newly born neurons project one or more processes called neurites that will eventually mature into axons and dendrites. While the genetic determinants that promote neurite emergence along specific trajectories are beginning to be elucidated, the cellular and molecular pathways that prevent inappropriate neurite formation to maintain proper neuronal morphology and prevent superfluous connections are largely unknown. Van Gogh and Prickle dependent-PCP signaling is a well-established regulator of cellular polarity especially along the surface of epithelial cells. In this study, we show that a conserved PCP–like pathway consisting of VANG-1/Van Gogh, PRKL-1/Prickle, and DSH-1/Dishevelled is involved in maintaining the polarized morphology of a subset of neurons in the nematode C. elegans. In particular, we show that loss of PRKL-1 results in neurons with too many neurites while PRKL-1 overexpression results in too few neurites. Our findings suggest that mechanisms that specifically block inappropriate neurite formation may be required to ensure proper neuronal connectivity in higher organisms.

Antagonism of LIN-17/Frizzled and LIN-18/Ryk in nematode vulva induction reveals evolutionary alterations in core developmental pathways

Most diversity in animals and plants results from the modification of already existing structures. Many organ systems, for example, are permanently modified during evolution to create developmental and morphological diversity, but little is known about the evolution of the underlying developmental mechanisms. The theory of developmental systems drift proposes that the development of conserved morphological structures can involve large-scale modifications in their regulatory mechanisms. We test this hypothesis by comparing vulva induction in two genetically tractable nematodes, Caenorhabditis elegans and Pristionchus pacificus. Previous work indicated that the vulva is induced by epidermal growth factor (EGF)/RAS and WNT signaling in Caenorhabditis and Pristionchus, respectively. Here, we show that the evolution of vulva induction involves major molecular alterations and that this shift of signaling pathways involves a novel wiring of WNT signaling and the acquisition of novel domains in otherwise conserved receptors in Pristionchus vulva induction. First, Ppa-LIN-17/Frizzled acts as an antagonist of WNT signaling and suppresses the ligand Ppa-EGL-20 by ligand sequestration. Second, Ppa-LIN-18/Ryk transmits WNT signaling and requires inhibitory SH3 domain binding motifs, unknown from Cel-LIN-18/Ryk. Third, Ppa-LIN-18/Ryk signaling involves Axin and β-catenin and Ppa-axl-1/Axin is epistatic to Ppa-lin-18/Ryk. These results confirm developmental system drift as an important theory for the evolution of organ systems and they highlight the significance of protein modularity in signal transduction and the dynamics of signaling networks.

Diversity of biological form in animals can be generated by the modification of already existing developmental and morphological structures. One major challenge in evolutionary biology is to identify the molecular and genetic changes associated with such morphological modifications. A decade ago, the theory of developmental systems drift was proposed arguing that large-scale changes in regulatory mechanisms can underlie the development of conserved morphological structures. Our work supports this hypothesis by comparing the development of the egg-laying organ between Caenorhabditis elegans and Pristionchus pacificus, two nematode species that have significantly different mechanisms of vulva induction despite their morphological similarity. Our studies in P. pacificus reveal major molecular alterations of signaling pathways that involve first, a novel wiring and second, the acquisition of novel protein domains in otherwise conserved receptors in WNT signaling. We show that all Wnt signaling molecules analyzed are conserved in sequence, but crucial receptor molecules have acquired novel small peptides that allow new regulatory linkages. The independent evolution of small protein domains in otherwise conserved proteins increases the evolutionary freedom of signaling pathways and developmental networks. Thus, our analysis of a developmental process that follows developmental system drift highlights the significance of protein modularity in signal transduction.

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.

Conservation, loss, and redeployment of Wnt ligands in protostomes: implications for understanding the evolution of segment formation

BACKGROUND

The Wnt genes encode secreted glycoprotein ligands that regulate a wide range of developmental processes, including axis elongation and segmentation. There are thirteen subfamilies of Wnt genes in metazoans and this gene diversity appeared early in animal evolution. The loss of Wnt subfamilies appears to be common in insects, but little is known about the Wnt repertoire in other arthropods, and moreover the expression and function of these genes have only been investigated in a few protostomes outside the relatively Wnt-poor model species Drosophila melanogaster and Caenorhabditis elegans. To investigate the evolution of this important gene family more broadly in protostomes, we surveyed the Wnt gene diversity in the crustacean Daphnia pulex, the chelicerates Ixodes scapularis and Achaearanea tepidariorum, the myriapod Glomeris marginata and the annelid Platynereis dumerilii. We also characterised Wnt gene expression in the latter three species, and further investigated expression of these genes in the beetle Tribolium castaneum.

RESULTS

We found that Daphnia and Platynereis both contain twelve Wnt subfamilies demonstrating that the common ancestors of arthropods, ecdysozoans and protostomes possessed all members of all Wnt subfamilies except Wnt3. Furthermore, although there is striking loss of Wnt genes in insects, other arthropods have maintained greater Wnt gene diversity. The expression of many Wnt genes overlap in segmentally reiterated patterns and in the segment addition zone, and while these patterns can be relatively conserved among arthropods and the annelid, there have also been changes in the expression of some Wnt genes in the course of protostome evolution. Nevertheless, our results strongly support the parasegment as the primary segmental unit in arthropods, and suggest further similarities between segmental and parasegmental regulation by Wnt genes in annelids and arthropods respectively.

CONCLUSIONS

Despite frequent losses of Wnt gene subfamilies in lineages such as insects, nematodes and leeches, most protostomes have probably maintained much of their ancestral repertoire of twelve Wnt genes. The maintenance of a large set of these ligands could be in part due to their combinatorial activity in various tissues rather than functional redundancy. The activity of such Wnt 'landscapes' as opposed to the function of individual ligands could explain the patterns of conservation and redeployment of these genes in important developmental processes across metazoans. This requires further analysis of the expression and function of these genes in a wider range of taxa.

kin-19/casein kinase Iα has dual functions in regulating asymmetric division and terminal differentiation in C. elegans epidermal stem cells

Casein Kinase I (CKI) is a conserved component of the Wnt signaling pathway, which regulates cell fate determination in metazoans. We show that post-embryonic asymmetric division and fate specification of C. elegans epidermal stem cells are controlled by a non-canonical Wnt/β-catenin signaling pathway, involving the β-catenins WRM-1 and SYS-1, and that C. elegans kin-19/CKIα functions in this pathway. Furthermore, we find that kin-19 is the only member of the Wnt asymmetry pathway that functions with, or in parallel to, the heterochronic temporal patterning pathway to control withdrawal from self-renewal and subsequent terminal differentiation of epidermal stem cells. We show that, except in the case of kin-19, the Wnt asymmetry pathway and the heterochronic pathway function separately and in parallel to control different aspects of epidermal stem cell fate specification. However, given the function of kin-19/CKIα in both pathways, and that CKI, Wnt signaling pathway and heterochronic pathway genes are widely conserved in animals, our findings suggest that CKIα may function as a regulatory hub through which asymmetric division and terminal differentiation are coordinated in adult stem cells of vertebrates.

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.

Repression of Wnt signaling by a Fer-type nonreceptor tyrosine kinase

The Wnt signaling pathway must be properly modulated to ensure an appropriate output: pathological conditions result from either insufficient or excessive levels of Wnt signal. For example, hyperactivation of the Wnt pathway is associated with various cancers and subnormal Wnt signaling can lead to increased invasiveness of tumor cells. We found that the Caenorhabditis elegans ortholog of the Fer nonreceptor tyrosine kinase, FRK-1, limits Wnt signaling by preventing the adhesion complex-associated β-catenin, HMP-2, from participating in Wnt-dependent specification of the endoderm during embryogenesis. Removal of FRK-1 function results in relocalization of HMP-2 to the nucleus of epidermal cells, and allows it to substitute for WRM-1, the nuclear β-catenin that normally transduces the Wnt signal during endoderm development. APR-1, the C. elegans APC ortholog, is similarly required to prevent HMP-2 relocalization and keeps it from participating in Wnt signal transduction; this finding partially explains the paradoxical observation that APR-1 acts either negatively or positively in Wnt signaling, depending on context. The apparent hyperactivation of the Wnt response in the absence of FRK-1 leads to hyperproliferation in the endoderm, as is also seen when WRM-1 is overexpressed in wild-type embryos. The specification and proliferation activities of Wnt signaling are separable: although the Tcf/Lef factor POP-1 acts in Wnt-dependent endoderm specification, it is not apparently required for hyperproliferation resulting from excessive Wnt signaling. These findings highlight a role for a Fer-type kinase in setting the proper levels of Wnt signaling and demonstrate the importance of this modulation in ensuring appropriate cell division.

Conserved mechanism of Wnt signaling function in the specification of vulval precursor fates in C. elegans and C. briggsae

The C. elegans hermaphrodite vulva serves as a paradigm for understanding how signaling pathways control organ formation. Previous studies have shown that Wnt signaling plays important roles in vulval development. To understand the function and evolution of Wnt signaling in Caenorhabditis nematodes we focused on C. briggsae, a species that is substantially divergent from C. elegans in terms of the evolutionary time scale yet shares almost identical morphology. We isolated mutants in C. briggsae that display multiple pseudo-vulvae resulting from ectopic VPC induction. We cloned one of these loci and found that it encodes an Axin homolog, Cbr-PRY-1. Our genetic studies revealed that Cbr-pry-1 functions upstream of the canonical Wnt pathway components Cbr-bar-1 (beta-catenin) and Cbr-pop-1(tcf/lef) as well as the Hox target Cbr-lin-39 (Dfd/Scr). We further characterized the pry-1 vulval phenotype in C. briggsae and C. elegans using 8 cell fate markers, cell ablation, and genetic interaction approaches. Our results show that ectopically induced VPCs in pry-1 mutants adopt 2° fates independently of the gonad-derived inductive and LIN-12/Notch-mediated lateral signaling pathways. We also found that Cbr-pry-1 mutants frequently show a failure of P7.p induction. A similar, albeit low penetrant, defect is also observed in C. elegans pry-1 mutants. The genetic analysis of the P7.p induction defect revealed that it was caused by altered regulation of lin-12 and its transcriptional target lip-1 (MAP kinase phosphatase). Thus, our results provide evidence for LIN-12/Notch-dependent and independent roles of Wnt signaling in promoting 2 degrees VPC fates in both nematode species.

Cross-talk between canonical Wnt signaling and the sirtuin-FoxO longevity pathway to protect against muscular pathology induced by mutant PABPN1 expression in C. elegans

Developmental pathways may be play a role in adult cell survival. However, whether they interact with longevity/cell survival pathways to confer protection against disease-associated proteotoxicity remains largely unknown. We previously reported that the inhibition of key longevity modulators such as the deacetylase sir-2.1/SIRT1 (Sir2) and its target daf-16/FoxO protects transgenics nematodes from muscle cell decline and abnormal motility produced by the expression of mutant (polyalanine-expanded) PABPN1, the oculopharyngeal muscular dystrophy (OPMD) protein. Here, we report that canonical Wnt signaling (i) modulates muscular pathology in mutant PABPN1 nematodes, and (ii) cooperates with the Sir2-FoxO longevity pathway to confer protection against mutant PABPN1 toxicity at the cellular and behavioral levels. Mutant PABPN1 toxicity was modified by genes along the canonical Wnt pathway, several of which depend on daf-16 for activity. ss-catenin and pop-1/TCF RNAi suppressed the protection from mutant PABPN1 confered by loss-of-function mutations in sir-2.1 and daf-16. Moreover, the aggravation of muscle cell pathology by increased sir-2.1 dosage was reversed by ss-catenin and pop-1 RNAi. The chemical inhibition of GSK-3ss, a repressor of ss-catenin activity, protected against mutant PABPN1 toxicity in a daf-16-dependent manner, which is consistent with a cross-talk between ss-catenin signaling and Sir2-FoxO signaling in protecting from mutant PABPN1 toxicity. Our data reveal that canonical Wnt signaling and Sir2-FoxO signaling interact to modulate diseased muscle survival, and indicate that GSK-3ss inhibitors and sirtuin inhibitors both have therapeutic potential for muscle protection in OPMD.

The regulation of Dkk1 expression during embryonic development

During embryogenesis, the Dkk1 mediated Wnt inhibition controls the spatiotemporal dynamics of cell fate determination, cell differentiation and cell death. Furthermore, the Dkk1 dose is critical for the normal Wnt homeostasis, as alteration of the Dkk1 activity is associated with various diseases. We investigated the regulation of Dkk1 expression during embryonic development. We identified nine conserved non-coding elements (CNEs), located 3' to the Dkk1 locus. Analyses of the regulatory potential revealed that four of these CNEs in combination drive reporter expression very similar to Dkk1 expression in several organs of transgenic embryos. We extended the knowledge of Dkk1 expression during hypophysis, external genitalia and kidney development, suggesting so far to unexplored functions of Dkk1 during the development of these organs. Characterization of the regulatory potential of four individual CNEs revealed that each of these promotes Dkk1 expression in brain and kidney. In combination, two enhancers are responsible for expression in the pituitary and the genital tubercle. Furthermore, individual CNEs mediates craniofacial, optic cup and limb specific Dkk1 regulation. Our study substantially improves the knowledge of Dkk1 regulation during embryonic development and thus might be of high relevance for therapeutic approaches.

The Wnt pathway controls cell death engulfment, spindle orientation, and migration through CED-10/Rac

Wnt signalling pathways have extremely diverse functions in animals, including induction of cell fates or tumours, guidance of cell movements during gastrulation, and the induction of cell polarity. Wnt can induce polar changes in cellular morphology by a remodelling of the cytoskeleton. However, how activation of the Frizzled receptor induces cytoskeleton rearrangement is not well understood. We show, by an in depth 4-D microscopy analysis, that the Caenorhabditis elegans Wnt pathway signals to CED-10/Rac via two separate branches to regulate modulation of the cytoskeleton in different cellular situations. Apoptotic cell clearance and migration of the distal tip cell require the MOM-5/Fz receptor, GSK-3 kinase, and APC/APR-1, which activate the CED-2/5/12 branch of the engulfment machinery. MOM-5 (Frizzled) thus can function as an engulfment receptor in C. elegans. Our epistatic analyses also suggest that the two partially redundant signalling pathways defined earlier for engulfment may act in a single pathway in early embryos. By contrast, rearrangement of mitotic spindles requires the MOM-5/Fz receptor, GSK-3 kinase, and beta-catenins, but not the downstream factors LIT-1/NLK or POP-1/Tcf. Taken together, our results indicate that in multiple developmental processes, CED-10/Rac can link polar signals mediated by the Wnt pathway to rearrangements of the cytoskeleton.

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.

Wnt-Ror signaling to SIA and SIB neurons directs anterior axon guidance and nerve ring placement in C. elegans

Wnt signaling through Frizzled proteins guides posterior cells and axons in C. elegans into different spatial domains. Here we demonstrate an essential role for Wnt signaling through Ror tyrosine kinase homologs in the most prominent anterior neuropil, the nerve ring. A genetic screen uncovered cwn-2, the C. elegans homolog of Wnt5, as a regulator of nerve ring placement. In cwn-2 mutants, all neuronal structures in and around the nerve ring are shifted to an abnormal anterior position. cwn-2 is required at the time of nerve ring formation; it is expressed by cells posterior of the nerve ring, but its precise site of expression is not critical for its function. In nerve ring development, cwn-2 acts primarily through the Wnt receptor CAM-1 (Ror), together with the Frizzled protein MIG-1, with parallel roles for the Frizzled protein CFZ-2. The identification of CAM-1 as a CWN-2 receptor contrasts with CAM-1 action as a non-receptor in other C. elegans Wnt pathways. Cell-specific rescue of cam-1 and cell ablation experiments reveal a crucial role for the SIA and SIB neurons in positioning the nerve ring, linking Wnt signaling to specific cells that organize the anterior nervous system.

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.

Opposing Wnt pathways orient cell polarity during organogenesis

The orientation of asymmetric cell division contributes to the organization of cells within a tissue or organ. For example, mirror-image symmetry of the C. elegans vulva is achieved by the opposite division orientation of the vulval precursor cells (VPCs) flanking the axis of symmetry. We characterized the molecular mechanisms contributing to this division pattern. Wnts MOM-2 and LIN-44 are expressed at the axis of symmetry and orient the VPCs toward the center. These Wnts act via Fz/LIN-17 and Ryk/LIN-18, which control beta-catenin localization and activate gene transcription. In addition, VPCs on both sides of the axis of symmetry possess a uniform underlying "ground" polarity, established by the instructive activity of Wnt/EGL-20. EGL-20 establishes ground polarity via a novel type of signaling involving the Ror receptor tyrosine kinase CAM-1 and the planar cell polarity component Van Gogh/VANG-1. Thus, tissue polarity is determined by the integration of multiple Wnt pathways.

Wnt signaling in Pristionchus pacificus gonadal arm extension and the evolution of organ shape

Changes in organ morphology have been essential to the evolution of novel body forms and in permitting organisms to invade new ecological niches. Changes in the arrangement of cells and tissues and in the regulation of morphological movements are fundamental to evolutionary transitions of organ shape and function. However, little is known about the genetic and developmental control of these changes. We use interspecific differences in the migration and extension of the nematode hermaphrodite gonadal arms to study the generation of morphological novelty. We show that the extending Pristionchus pacificus gonadal arms display a ventral migration that is unique to the Diplogastridae in comparison to the Rhabditidae, including Caenorhabditis elegans, and other nematodes. This results in the distal gonad residing along the ventral side of the body in P. pacificus in contrast to lying on the dorsal side of the body as in C. elegans. We show that at the cellular level this morphogenetic movement is regulated by signals from the developing vulva and the sister gonadal arm. We further show that in P. pacificus Wnt signaling is essential for this regulation. We show genetic and molecular evidence that suggest the Wnt ligands Ppa-mom-2 and Ppa-cwn-2 are components of the signaling mechanism. Supporting these findings, the hermaphrodite gonad of Ppa-bar-1 mutant animals mimics the shape of the C. elegans hermaphrodite gonad; the arms fail to extend ventrally. Thus, this genetic analysis of gonad migration provides insight into the mechanisms underlying the generation of morphological novelty and organ shape.

The long and the short of Wnt signaling in C. elegans

The simplicity of C. elegans makes it an outstanding system to study the role of Wnt signaling in development. Many asymmetric cell divisions in C. elegans require the Wnt/beta-catenin asymmetry pathway. Recent studies confirm that SYS-1 is a structurally and functionally divergent beta-catenin, and implicate lipids and retrograde trafficking in maintenance of WRM-1/beta-catenin asymmetry. Wnts also regulate short-range events such as spindle rotation and gastrulation, and a PCP-like pathway regulates asymmetric divisions. Long-range, cell non-autonomous Wnt signals regulate vulval induction. Both short-range and long-range Wnt signal s are regulated by recycling of MIG-14/Wntless via the retromer complex. These studies indicate that C. elegans continues to be useful for identifying new, conserved mechanisms underlying Wnt signaling in metazoans.

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.

Wnt signaling induces vulva development in the nematode Pristionchus pacificus

The Caenorhabditis elegans vulva is induced by a member of the epidermal growth factor (EGF) family that is expressed in the gonadal anchor cell, representing a prime example of signaling processes in animal development. Comparative studies indicated that vulva induction has changed rapidly during evolution. However, nothing was known about the molecular mechanisms underlying these differences. By analyzing deletion mutants in five Wnt pathway genes, we show that Wnt signaling induces vulva formation in Pristionchus pacificus. A Ppa-bar-1/beta-catenin deletion is completely vulvaless. Several Wnt ligands and receptors act redundantly in vulva induction, and Ppa-egl-20/Wnt; Ppa-mom-2/Wnt; Ppa-lin-18/Ryk triple mutants are strongly vulvaless. Wnt ligands are differentially expressed in the somatic gonad, the anchor cell, and the posterior body region, respectively. In contrast, previous studies indicated that Ppa-lin-17, one of the Frizzled-type receptors, has a negative role in vulva formation. We found that mutations in Ppa-bar-1 and Ppa-egl-20 suppress the phenotype of Ppa-lin-17. Thus, an unexpected complexity of Wnt signaling is involved in vulva induction and vulva repression in P. pacificus. This study provides the first molecular identification of the inductive vulva signal in a nematode other than Caenorhabditis.

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 C. elegans ROR receptor tyrosine kinase, CAM-1, non-autonomously inhibits the Wnt pathway

Inhibitors of Wnt signaling promote normal development and prevent cancer by restraining when and where the Wnt pathway is activated. ROR proteins, a class of Wnt-binding receptor tyrosine kinases, inhibit Wnt signaling by an unknown mechanism. To clarify how RORs inhibit the Wnt pathway, we examined the relationship between Wnts and the sole C. elegans ROR homolog, cam-1, during C. elegans vulval development, a Wnt-regulated process. We found that loss and overexpression of cam-1 causes reciprocal defects in Wnt-mediated cell-fate specification. Our molecular and genetic analyses revealed that the CAM-1 extracellular domain (ECD) is sufficient to non-autonomously antagonize multiple Wnts, suggesting that the CAM-1/ROR ECD sequesters Wnts. A sequestration model is supported by our findings that the CAM-1 ECD binds to several Wnts in vitro. These results demonstrate how ROR proteins help to refine the spatial pattern of Wnt activity in a complex multicellular environment.

Migration of neuronal cells along the anterior–posterior body axis of C. elegans: Wnts are in control

Migrating neuronal cells are directed to their final positions by an array of guidance cues. It has been shown that guidance molecules such as UNC-6/Netrin and SLT-1/Slit play a major role in controlling cell and axon migrations along the dorsal-ventral body axis. Much less is known, however, about the mechanisms that mediate migration along the anterior-posterior (AP) body axis. Recent research in Caenorhabditis elegans has uncovered an important role of the Wnt family of signalling molecules in controlling AP-directed neuronal cell migration and polarity. A common theme that emerges from these studies is that multiple Wnt proteins function in parallel as instructive cues or permissive signals to control neuronal patterning along this major body axis.

Two functionally distinct Axin-like proteins regulate canonical Wnt signaling in C. elegans

Axin is a central component of the canonical Wnt signaling pathway that interacts with the adenomatous polyposis coli protein APC and the kinase GSK3beta to downregulate the effector beta-catenin. In the nematode Caenorhabditis elegans, canonical Wnt signaling is negatively regulated by the highly divergent Axin ortholog PRY-1. Mutation of pry-1 leads to constitutive activation of BAR-1/beta-catenin-dependent Wnt signaling and results in a range of developmental defects. The pry-1 null phenotype is however not fully penetrant, indicating that additional factors may partially compensate for PRY-1 function. Here, we report the cloning and functional analysis of a second Axin-like protein, which we named AXL-1. We show that despite considerable sequence divergence with PRY-1 and other Axin family members, AXL-1 is a functional Axin ortholog. AXL-1 functions redundantly with PRY-1 in negatively regulating BAR-1/beta-catenin signaling in the developing vulva and the Q neuroblast lineage. In addition, AXL-1 functions independently of PRY-1 in negatively regulating canonical Wnt signaling during excretory cell development. In contrast to vertebrate Axin and the related protein Conductin, AXL-1 and PRY-1 are not functionally equivalent. We conclude that Axin function in C. elegans is divided over two different Axin orthologs that have specific functions in negatively regulating canonical Wnt signaling.

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.

Mechanisms and evolution of environmental responses in Caenorhabditis elegans

We review mechanistic and evolutionary aspects of interactions between the model organism Caenorhabditis elegans and its environment. In particular, we focus on environmental effects affecting developmental mechanisms. We describe natural and laboratory environments of C. elegans and provide an overview of the different environmental responses of this organism. We then show how two developmental processes respond to changes in the environment. First, we discuss the development of alternative juvenile stages, the dauer and non-dauer larva. This example illustrates how development responds to variation in the environment to generate complex phenotypic variation. Second, we discuss the development of the C. elegans vulva. This example illustrates how development responds to variation in the environment while generating an invariant final phenotype.