Antagonist activity of DWnt-4 and wingless in the Drosophila embryonic ventral ectoderm and in heterologous Xenopus assays

The Wnt Co-Receptor PTK7/Otk and Its Homolog Otk-2 in Neurogenesis and Patterning

Wnt signaling is a highly conserved metazoan pathway that plays a crucial role in cell fate determination and morphogenesis during development. Wnt ligands can induce disparate cellular responses. The exact mechanism behind these different outcomes is not fully understood but may be due to interactions with different receptors on the cell membrane. PTK7/Otk is a transmembrane receptor that is implicated in various developmental and physiological processes including cell polarity, cell migration, and invasion. Here, we examine two roles of Otk-1 and Otk-2 in patterning and neurogenesis. We find that Otk-1 is a positive regulator of signaling and Otk-2 functions as its inhibitor. We propose that PTK7/Otk functions in signaling, cell migration, and polarity contributing to the diversity of cellular responses seen in Wnt-mediated processes.

Regulation of Anisotropic Tissue Growth by Two Orthogonal Signaling Centers

The Drosophila wing has served as a paradigm to mechanistically characterize the role of morphogens in patterning and growth. Wingless (Wg) and Decapentaplegic (Dpp) are expressed in two orthogonal signaling centers, and their gradients organize patterning by regulating the expression of well-defined target genes. By contrast, graded activity of these morphogens is not an absolute requirement for wing growth. Despite their permissive role in regulating growth, here we show that Wg and Dpp are utilized in a non-interchangeable manner by the two existing orthogonal signaling centers to promote preferential growth along the two different axes of the developing wing. Our data indicate that these morphogens promote anisotropic growth by making use of distinct and non-interchangeable molecular mechanisms. Whereas Dpp drives growth along the anterior-posterior axis by maintaining Brinker levels below a growth-repressing threshold, Wg exerts its action along the proximal-distal axis through a double repression mechanism involving T cell factor (TCF).

Genome-Wide Identification and Expression Profiling of Wnt Family Genes in the Silkworm, Bombyx mori

Wnt is a family of conserved glycoproteins that participate in a variety of important biological processes including embryo development, cell proliferation and differentiation, and tissue regeneration. The Wnt family is a metazoan novelty found in all animal phyla. Studies have revealed that the number of Wnt genes varies among species, presumably due to reproduction and loss of genes during evolution. However, a comprehensive inventory of Wnt genes in Lepidoptera is lacking. In this study, we identified the repertoire of Wnt genes in the silkworm and seven other species of Lepidoptera and obtained eight Wnt genes (Wnt1, Wnt5–Wnt7, Wnt9–Wnt11, and WntA) in each species. Four of these Wnt genes are clustered in two orientations (5′-Wnt9-Wnt1-Wnt6-Wnt10-3′ and 5′-Wnt10-Wnt6-Wnt1-Wnt9-3′) in both moths and butterflies. Transcript analysis of Wnt in silkworm embryonic stages showed that each BmWnt gene had a unique expression pattern during embryological development. Analysis of a larval stage revealed differential expression of Wnt family members in diverse tissues. Our study provides an overview of the Wnt family in Lepidoptera and will inspire further functional study of the Wnt genes in the silkworm.

An embryonic system to assess direct and indirect Wnt transcriptional targets

During animal development, complex signals determine and organize a vast number of tissues using a very small number of signal transduction pathways. These developmental signaling pathways determine cell fates through a coordinated transcriptional response that remains poorly understood. The Wnt pathway is involved in a variety of these cellular functions, and its signals are transmitted in part through a β-catenin/TCF transcriptional complex. Here we report an in vivo Drosophila assay that can be used to distinguish between activation, de-repression and repression of transcriptional responses, separating upstream and downstream pathway activation and canonical/non-canonical Wnt signals in embryos. We find specific sets of genes downstream of both β-catenin and TCF with an additional group of genes regulated by Wnt, while the non-canonical Wnt4 regulates a separate cohort of genes. We correlate transcriptional changes with phenotypic outcomes of cell differentiation and embryo size, showing our model can be used to characterize developmental signaling compartmentalization in vivo.

The PTK7-related transmembrane proteins off-track and off-track 2 are co-receptors for Drosophila Wnt2 required for male fertility

Wnt proteins regulate many developmental processes and are required for tissue homeostasis in adult animals. The cellular responses to Wnts are manifold and are determined by the respective Wnt ligand and its specific receptor complex in the plasma membrane. Wnt receptor complexes contain a member of the Frizzled family of serpentine receptors and a co-receptor, which commonly is a single-pass transmembrane protein. Vertebrate protein tyrosine kinase 7 (PTK7) was identified as a Wnt co-receptor required for control of planar cell polarity (PCP) in frogs and mice. We found that flies homozygous for a complete knock-out of the Drosophila PTK7 homolog off track (otk) are viable and fertile and do not show PCP phenotypes. We discovered an otk paralog (otk2, CG8964), which is co-expressed with otk throughout embryonic and larval development. Otk and Otk2 bind to each other and form complexes with Frizzled, Frizzled2 and Wnt2, pointing to a function as Wnt co-receptors. Flies lacking both otk and otk2 are viable but male sterile due to defective morphogenesis of the ejaculatory duct. Overexpression of Otk causes female sterility due to malformation of the oviduct, indicating that Otk and Otk2 are specifically involved in the sexually dimorphic development of the genital tract.

Wnts are secreted, growth factor-like proteins that are important for the development of many tissues and organs in animals. They are also required in adult animals and humans for controlling the balance between growth and differentiation. Wnts are bound at the cell surface by Wnt receptors, which are dimers composed of a Frizzled protein and a co-receptor. Here we have analyzed the Drosophila Wnt co-receptors Off-track (Otk) and Off-track 2 (Otk2), which are closely related to vertebrate Protein tyrosine kinase 7 (PTK7). We found that in contrast to PTK7 in mice and frogs, which controls planar cell polarity (PCP), Otk and Otk2 together are needed in males for development of the ejaculatory duct, a tube-like organ that transports the mature sperm. Our data furthermore indicate that Otk and Otk2 are co-receptors for Wnt2. The sterile phenotype of Wnt2 mutant males is not identical to that of otk, otk2 double mutants, so additional Wnts may be involved in this process. Interestingly, the function of Wnt2 in male fertility appears to be evolutionarily conserved, because male mice mutant for Wnt7A, the vertebrate homolog of Drosophila Wnt2, are sterile due to abnormal development of the vas deferens, which corresponds to the fly ejaculatory duct.

Canonical Wnt signaling in the visceral muscle is required for left-right asymmetric development of the Drosophila midgut

Many animals develop left-right (LR) asymmetry in their internal organs. The mechanisms of LR asymmetric development are evolutionarily divergent, and are poorly understood in invertebrates. Therefore, we studied the genetic pathway of LR asymmetric development in Drosophila. Drosophila has several organs that show directional and stereotypic LR asymmetry, including the embryonic gut, which is the first organ to develop LR asymmetry during Drosophila development. In this study, we found that genes encoding components of the Wnt-signaling pathway are required for LR asymmetric development of the anterior part of the embryonic midgut (AMG). frizzled 2 (fz2) and Wnt4, which encode a receptor and ligand of Wnt signaling, respectively, were required for the LR asymmetric development of the AMG. arrow (arr), an ortholog of the mammalian gene encoding low-density lipoprotein receptor-related protein 5/6, which is a co-receptor of the Wnt-signaling pathway, was also essential for LR asymmetric development of the AMG. These results are the first demonstration that Wnt signaling contributes to LR asymmetric development in invertebrates, as it does in vertebrates. The AMG consists of visceral muscle and an epithelial tube. Our genetic analyses revealed that Wnt signaling in the visceral muscle but not the epithelium of the midgut is required for the AMG to develop its normal laterality. Furthermore, fz2 and Wnt4 were expressed in the visceral muscles of the midgut. Consistent with these results, we observed that the LR asymmetric rearrangement of the visceral muscle cells, the first visible asymmetry of the developing AMG, did not occur in embryos lacking Wnt4 expression. Our results also suggest that canonical Wnt/β-catenin signaling, but not non-canonical Wnt signaling, is responsible for the LR asymmetric development of the AMG. Canonical Wnt/β-catenin signaling is reported to have important roles in LR asymmetric development in zebrafish. Thus, the contribution of canonical Wnt/β-catenin signaling to LR asymmetric development may be an evolutionarily conserved feature between vertebrates and invertebrates.

PTK7/Otk interacts with Wnts and inhibits canonical Wnt signalling

Wnt signalling is an evolutionarily conserved pathway that directs cell-fate determination and morphogenesis during metazoan development. Wnt ligands are secreted glycoproteins that act at a distance causing a wide range of cellular responses from stem cell maintenance to cell death and cell proliferation. How Wnt ligands cause such disparate responses is not known, but one possibility is that different outcomes are due to different receptors. Here, we examine PTK7/Otk, a transmembrane receptor that controls a variety of developmental and physiological processes including the regulation of cell polarity, cell migration and invasion. PTK7/Otk co-precipitates canonical Wnt3a and Wnt8, indicating a role in Wnt signalling, but PTK7 inhibits rather than activates canonical Wnt activity in Xenopus, Drosophila and luciferase reporter assays. Loss of PTK7 function activates canonical Wnt signalling and epistasis experiments place PTK7 at the level of the Frizzled receptor. In Drosophila, Otk interacts with Wnt4 and opposes canonical Wnt signalling in embryonic patterning. We propose a model where PTK7/Otk functions in non-canonical Wnt signalling by turning off the canonical signalling branch.

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.

The function and evolution of Wnt genes in arthropods

Wnt signalling is required for a wide range of developmental processes, from cleavage to patterning and cell migration. There are 13 subfamilies of Wnt ligand genes and this diverse repertoire appeared very early in metazoan evolution. In this review, we first summarise the known Wnt gene repertoire in various arthropods. Insects appear to have lost several Wnt subfamilies, either generally, such as Wnt3, or in lineage specific patterns, for example, the loss of Wnt7 in Anopheles. In Drosophila and Acyrthosiphon, only seven and six Wnt subfamilies are represented, respectively; however, the finding of nine Wnt genes in Tribolium suggests that arthropods had a larger repertoire ancestrally. We then discuss what is currently known about the expression and developmental function of Wnt ligands in Drosophila and other insects in comparison to other arthropods, such as the spiders Achaearanea and Cupiennius. We conclude that studies of Wnt genes have given us much insight into the developmental roles of some of these ligands. However, given the frequent loss of Wnt genes in insects and the derived development of Drosophila, further studies of these important genes are required in a broader range of arthropods to fully understand their developmental function and evolution.

Wnt and calcium signaling: beta-catenin-independent pathways

Wnt signaling is a complex pathway in which beta-catenin is typically viewed as a central mediator. However, within the past 15 years, at least three Wnt-mediated pathways have been proposed that function independent of beta-catenin. One pathway involves activation of calcium/calmodulin-dependent kinase II (CamKII) and protein kinase C (PKC). Another includes recruitment of heterotrimeric GTP-binding proteins to activate phospholipase C (PLC) and phosphodiesterase (PDE). Lastly, a pathway similar to the planar cell polarity (PCP) pathway in Drosophila has been identified that activates the Jun-N-terminal kinase (JNK) and, perhaps, small GTP-binding proteins. Calcium has been implicated as an important second messenger in all of these pathways. This review will focus on the role of calcium in Wnt signaling and, as a consequence, provide a limited overview of beta-catenin-independent Wnt signaling.

Control of planar cell polarity by interaction of DWnt4 and four-jointed

The Drosophila eye and the wing display specific planar cell polarity. Although Frizzled (Fz) signaling has been implicated in the establishment of ommatidial and wing hair polarity, evidence for the Wnt gene function has been limited. Here we examined the function of a Drosophila homolog of Wnt4 (DWnt4) in the control of planar polarity. We show that DWnt4 mRNA and protein are preferentially expressed in the ventral region of eye disc. DWnt4 mutant eyes show polarity reversals mostly in the ventral domain, consistent with the ventral expression of DWnt4. Ectopic expression of DWnt4 in the dorsoventral (DV) polar margins is insufficient to induce ommatidial polarity but becomes inductive when coexpressed with Four-jointed (Fj). Similarly, DWnt4 and Fj result in synergistic induction of hair polarity toward the source of expression in the wing. Consistent with genetic interaction, we provide evidence for direct interaction of DWnt4 and Fj transmembrane protein. The extracellular domain of Fj is required for direct binding to DWnt4 and for the induction of hair polarity. In contrast to the synergy between DWnt4 and Fj, DWnt4 antagonizes the polarizing effect of Fz. Our results suggest that DWnt4 is involved in ommatidial polarity signaling in the ventral region of the eye and its function is mediated by interacting with Fj.

Wnt signalling pathway: a new target for the treatment of osteoporosis

The prevention and treatment of osteoporosis traditionally involves the use of antiresorptive agents that target osteoclast function. Antiresorptive therapy is not associated with a significant increase in bone mass and, thus, only partially reduces the risk of fractures. For that reason, the search for anabolic agents, which target osteoblast function, represents an urgent medical need. The first approved bone anabolic drug for the treatment of osteoporosis was teriparatide (human parathyroid hormone 1-34). Recently, both human genetics and animal studies have pointed out the role of the Wnt/LRP5 pathway as a major regulator of bone mass accrual. Wnts are secreted glycoproteins that bind to receptor complexes including low-density lipoprotein receptor-related protein (LRP)-5/6 and Frizzled proteins. A subsequent intracellular cascade of events stabilises beta-catenin, leading to its translocation into the nucleus where, associated with Tcf/Lef transcription factors, it triggers gene expression. The existence of many potential pharmacological targets in this pathway makes it attractive for bone anabolic drug discovery.

Identification of a novel target of D/V signaling in Drosophila wing disc: Wg-independent function of the organizer

Growth and patterning during Drosophila wing development are mediated by signaling from its dorso-ventral (D/V) organizer. Wingless is expressed in the D/V boundary and functions as a morphogen to activate target genes at a distance. Wingless pathway and thereby D/V signaling is negatively regulated by the homeotic gene Ultrabithorax (Ubx) to mediate haltere development. In an enhancer-trap screen to identify genes that show differential expression between wing and haltere discs, we identified CG32062, which codes for a RNA-binding protein. In wing discs, CG32062 is expressed only in non-D/V cells. CG32062 expression in non-D/V cells is dependent on Notch-mediated signaling from the D/V boundary. However, CG32062 expression is independent of Wingless function, thus providing evidence for a second long-range signaling mechanism of the D/V organizer. In haltere discs, CG32062 is negatively regulated by Ubx. The non-cell autonomous nature of Ubx-mediated repression of CG32062 expression suggests that the novel component of D/V signaling is also negatively regulated during haltere specification.

Multiple mechanisms for Wnt11-mediated repression of the canonical Wnt signaling pathway

The effect of a noncanonical Wnt, Wnt11, on canonical Wnt signaling stimulated by Wnt1 and activated forms of LRP5 (low density lipoprotein receptor-related protein-5), Dishevelled1 (Dvl1), and beta-catenin was examined in NIH3T3 cells and P19 embryonic carcinoma cells. Wnt11 repressed Wnt1-mediated activation of LEF-1 reporter activity in both cell lines. However, Wnt11 was unable to inhibit canonical signaling activated by LRP5, Dvl1, or beta-catenin in NIH3T3 cells, although it could in P19 cells. In addition, Wnt11-mediated inhibition of canonical signaling in NIH3T3 cells is ligand-specific; Wnt11 could effectively repress canonical signaling activated by Wnt1, Wnt3, or Wnt3a but not by Wnt7a or Wnt7b. Co-culture experiments with NIH3T3 cells showed that the co-expression of Wnt11 with Wnt1 was not an essential requirement for the inhibition, suggesting receptor competition as a possible mechanism. Moreover, in both cell types, elevation of intracellular Ca(2+) levels, which can result from Wnt11 treatment, led to the inhibition of canonical signaling. This result suggests that Wnt11 might not be able to signal in NIH3T3. Furthermore, P19 cells were found to express both endogenous canonical Wnts and Wnt11. Knockdown of Wnt11 expression using siRNA resulted in increased LEF-1 reporter activity, thus indicating that Wnt11-mediated suppression of canonical signaling exists in vivo.

Wnt-5/pipetail functions in vertebrate axis formation as a negative regulator of Wnt/beta-catenin activity

We provide genetic evidence defining a role for noncanonical Wnt function in vertebrate axis formation. In zebrafish, misexpression of Wnt-4, -5, and -11 stimulates calcium (Ca2+) release, defining the Wnt/Ca2+ class. We describe genetic interaction between two Wnt/Ca2+ members, Wnt-5 (pipetail) and Wnt-11 (silberblick), and a reduction of Ca2+ release in Wnt-5/pipetail. Embryos genetically depleted of both maternal and zygotic Wnt-5 product exhibit cell movement defects as well as hyperdorsalization and axis-duplication phenotypes. The dorsalized phenotypes result from increased beta-catenin accumulation and activation of downstream genes. The Wnt-5 loss-of-function defect is consistent with Ca2+ modulation having an antagonistic interaction with Wnt/beta-catenin signaling.

Wnt-mediated axon guidance via the Drosophila Derailed receptor

In nervous systems with bilateral symmetry, many neurons project axons across the midline to the opposite side. In each segment of the Drosophila embryonic nervous system, axons that display this projection pattern choose one of two distinct tracts: the anterior or posterior commissure. Commissure choice is controlled by Derailed, an atypical receptor tyrosine kinase expressed on axons projecting in the anterior commissure. Here we show that Derailed keeps these axons out of the posterior commissure by acting as a receptor for Wnt5, a member of the Wnt family of secreted signalling molecules. Our results reveal an unexpected role in axon guidance for a Wnt family member, and show that the Derailed receptor is an essential component of Wnt signalling in these guidance events.

The TAK1-NLK mitogen-activated protein kinase cascade functions in the Wnt-5a/Ca(2+) pathway to antagonize Wnt/beta-catenin signaling

Wnt signaling controls a variety of developmental processes. The canonical Wnt/beta-catenin pathway functions to stabilize beta-catenin, and the noncanonical Wnt/Ca(2+) pathway activates Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). In addition, the Wnt/Ca(2+) pathway activated by Wnt-5a antagonizes the Wnt/beta-catenin pathway via an unknown mechanism. The mitogen-activated protein kinase (MAPK) pathway composed of TAK1 MAPK kinase kinase and NLK MAPK also negatively regulates the canonical Wnt/beta-catenin signaling pathway. Here we show that activation of CaMKII induces stimulation of the TAK1-NLK pathway. Overexpression of Wnt-5a in HEK293 cells activates NLK through TAK1. Furthermore, by using a chimeric receptor (beta(2)AR-Rfz-2) containing the ligand-binding and transmembrane segments from the beta(2)-adrenergic receptor (beta(2)AR) and the cytoplasmic domains from rat Frizzled-2 (Rfz-2), stimulation with the beta-adrenergic agonist isoproterenol activates activities of endogenous CaMKII, TAK1, and NLK and inhibits beta-catenin-induced transcriptional activation. These results suggest that the TAK1-NLK MAPK cascade is activated by the noncanonical Wnt-5a/Ca(2+) pathway and antagonizes canonical Wnt/beta-catenin signaling.

DWnt4 regulates cell movement and focal adhesion kinase during Drosophila ovarian morphogenesis

Cell motility is regulated by extracellular cues and by intracellular factors that accumulate at sites of contact between cells and the extracellular matrix. One of these factors, focal adhesion kinase (FAK), regulates the cycle of focal adhesion formation and disassembly that is required for cell movement to occur. Recently, Wnt signaling has also been implicated in the control of cell movement in vertebrates, but the mechanism through which Wnt proteins influence motility is unclear. We demonstrate that Drosphila Wnt4 is required for cell movement and FAK regulation during ovarian morphogenesis. Dfrizzled2, Disheveled, and protein kinase C are also required. The DWnt4 cell motility pathway is distinct from both the canonical Wnt pathway and the planar polarity pathway. Our data suggest that DWnt4 facilitates motility through regulation of focal adhesions.

Seven Wnt homologues in Drosophila: a case study of the developing tracheae

Sequencing of the Drosophila genome has revealed that there are "silent" homologues of many important genes-family members that were not detected by classic genetic approaches. Why have so many homologues been conserved during evolution? Perhaps each one has a different but important function in every system. Perhaps each one works independently in a different part of the body. Or, perhaps some are redundant. Here, we take one well known gene family and analyze how the individual members contribute to the making of one system, the tracheae. There are seven DWnt genes in the Drosophila genome, including wingless (wg). The wg gene helps to pattern the developing trachea but is not responsible for all Wnt functions there. We test each one of the seven DWnts in several ways and find evidence that wg and DWnt2 can function in the developing trachea: when both genes are removed together, the phenotype is identical or very similar to that observed when the Wnt pathway is shut down. DWnt2 is expressed near the tracheal cells in the embryo in a different pattern to wg but is also transduced through the canonical Wnt pathway. We find that the seven DWnt genes vary in their effectiveness in specific tissues, such as the tracheae, and, moreover, the epidermis and the tracheae respond to DWnt2 and Wg differently. We suggest that the main advantage of retaining a number of similar genes is that it allows more subtle forms of control and more flexibility during evolution.

DWnt4 and wingless elicit similar cellular responses during imaginal development

Wnt genes encode evolutionarily conserved secreted proteins that provide critical functions during development. Although Wnt proteins share highly conserved features, they also show sequence divergence, which almost certainly contributes to the variety of their signaling activities. We previously reported that DWnt4 and wingless (wg), two divergent clustered Wnt genes, can have either antagonist or distinct functions during Drosophila embryogenesis. Here we provide evidence that both genes can elicit similar cellular responses during imaginal development. Ectopic expression of DWnt4 along the anterior/posterior (A/P) boundary of imaginal discs alters morphogenesis of adult appendages. In the wing disc, DWnt4 phenocopies ectopic Wg activity by inducing notum to wing transformation, suggesting similar signaling capabilities of both molecules. In support of this, we demonstrate that DWnt4 can rescue wg loss-of-function phenotypes in the antenna and haltere and is able to substitute for Wg in wing field specification. We also show that both genes are transcribed in overlapping domains in imaginal discs, suggesting that DWnt4 may cooperate with wg during limb patterning.

The Wnt/Ca2+ pathway: a new vertebrate Wnt signaling pathway takes shape

Members of the vertebrate Wnt family have been subdivided into two functional classes according to their biological activities. Some Wnts signal through the canonical Wnt-1/wingless pathway by stabilizing cytoplasmic beta-catenin. By contrast other Wnts stimulate intracellular Ca2+ release and activate two kinases, CamKII and PKC, in a G-protein-dependent manner. Moreover, putative Wnt receptors belonging to the Frizzled gene family have been identified that preferentially couple to the two prospective pathways in the absence of ectopic Wnt ligand and that might account for the signaling specificity of the Wnt pathways. As Ca2+ release was the first described feature of the noncanonical pathway, and as Ca2+ probably plays a key role in the activation of CamKII and PKC, we have named this Wnt pathway the Wnt/Ca2+ pathway.