Zinc-dependent interaction between dishevelled and the Drosophila Wnt antagonist naked cuticle

Nkd1 functions downstream of Axin2 to attenuate Wnt signaling

Wnt signaling is a crucial developmental pathway involved in early development as well as stem-cell maintenance in adults and its misregulation leads to numerous diseases. Thus, understanding the regulation of this pathway becomes vitally important. Axin2 and Nkd1 are widely utilized negative feedback regulators in Wnt signaling where Axin2 functions to destabilize cytoplasmic β-catenin, and Nkd1 functions to inhibit the nuclear localization of β-catenin. Here, we set out to further understand how Axin2 and Nkd1 regulate Wnt signaling by creating axin2gh1/gh1, nkd1gh2/gh2 single mutants and axin2gh1/gh1;nkd1gh2/gh2 double mutant zebrafish using sgRNA/Cas9. All three Wnt regulator mutants were viable and had impaired heart looping, neuromast migration defects, and behavior abnormalities in common, but there were no signs of synergy in the axin2gh1/gh1;nkd1gh2/gh2 double mutants. Further, Wnt target gene expression by qRT-PCR and RNA-seq, and protein expression by mass spectrometry demonstrated that the double axin2gh1/gh1;nkd1gh2/gh2 mutant resembled the nkd1gh2/gh2 phenotype demonstrating that Nkd1 functions downstream of Axin2. In support of this, the data further demonstrates that Axin2 uniquely alters the properties of β-catenin-dependent transcription having novel readouts of Wnt activity compared with nkd1gh2/gh2 or the axin2gh1/gh1;nkd1gh2/gh2 double mutant. We also investigated the sensitivity of the Wnt regulator mutants to exacerbated Wnt signaling, where the single mutants displayed characteristic heightened Wnt sensitivity, resulting in an eyeless phenotype. Surprisingly, this phenotype was rescued in the double mutant, where we speculate that cross-talk between Wnt/β-catenin and Wnt/Planar Cell Polarity pathways could lead to altered Wnt signaling in some scenarios. Collectively, the data emphasizes both the commonality and the complexity in the feedback regulation of Wnt signaling.

Research on Mechanism of Nanometer Colloidal Particles of Prostate Specific Membrane Aptamer A10 (PSMA-a10)/TGX221 in Restraining the Transplantation Tumor of Rats with Prostate Cancer (PCa)

This study assesses the effect of nanometer colloidal particles of PSMA-a10/TGX22 1 on the transplantation tumor of rats with prostate cancer (PCa). 26 rats with PCa were divided into PPG group, exposed TGX221 group and nanometer TGX221 group followed by analysis of the growth of transplantation tumor, cell apoptotic rate by flow cytometry, and the expression of p110β and Bcl-2 by RT-PCR and Western Blot. Nanometer TGX221 group and exposed TGX221 group showed smaller tumor volume than in PPG group on day 13 after intervention with significant difference of tumor volume between nanometer TGX221 group and exposed TGX221 group on day 7. The inhibiting rate of tumor growth in exposed TGX221 group was less than that in nanometer TGX221 group. However, nanometer TGX221 group presented higher apoptotic rate than other two groups. The level of p110β and Bcl-2 in nanometer TGX221 group was declined gradually with significant differences compared to other two groups. In conclusion, the nanometer colloidal particles of PSMA-a10/TGX221 have promising tumor-targeted specificity possibly through inhibition of Bcl-2 and the activity of p110β, leading to restrained PCa growth.

Naked cuticle inhibits wingless signaling in Drosophila wing development

Wnt signaling is one of the major signaling pathways that regulate cell differentiation, tissue patterning and stem cell homeostasis and its dysfunction causes many human diseases, such as cancer. It is of tremendous interests to understand how Wnt signaling is regulated in a precise manner both temporally and spatially. Naked cuticle (Nkd) acts as a negative-feedback inhibitor for Wingless (Wg, a fly Wnt) signaling in Drosophila embryonic development. However, the role of Nkd remains controversial in later fly development, particularly on the canonical Wg pathway. In the present study, we show that nkd is essential for wing pattern formation, such that both gain and loss of nkd result in the disruption of Wg target expression in larvae stage and abnormal adult wing morphologies. Furthermore, we demonstrate that a thirty amino acid fragment in Nkd, identified previously in Wharton lab, is critical for the canonical Wg signaling, but is dispensable for Wg/planar cell polarity pathway. Putting aside the pleiotropic nature of nkd function, i.e. its role in the Decapentaplegic signaling, we conclude that Nkd universally inhibits the canonical Wg pathway across a life span of Drosophila development.

NKD2 mediates stimulation-dependent ORAI1 trafficking to augment Ca2+ entry in T cells

Sustained activation of the Ca²⁺-release-activated Ca²⁺ (CRAC) channel is pivotal for effector T cell responses. The mechanisms underlying this sustainability remain poorly understood. We find that plasma membrane localization of ORAI1, the pore subunit of CRAC channels, is limited in effector T cells, with a significant fraction trapped in intracellular vesicles. From a targeted screen, we identify an essential component of ORAI1⁺ vesicles, naked cuticle homolog 2 (NKD2). Mechanistically, NKD2, an adaptor molecule activated by signaling pathways downstream of T cell receptors, orchestrates trafficking and insertion of ORAI1⁺ vesicles to the plasma membrane. Together, our findings suggest that T cell receptor (TCR)-stimulation-dependent insertion of ORAI1 into the plasma membrane is essential for sustained Ca²⁺ signaling and cytokine production in T cells.

Feedback control of Wnt signaling based on ultrastable histidine cluster co-aggregation between Naked/NKD and Axin

Feedback control is a universal feature of cell signaling pathways. Naked/NKD is a widely conserved feedback regulator of Wnt signaling which controls animal development and tissue homeostasis. Naked/NKD destabilizes Dishevelled, which assembles Wnt signalosomes to inhibit the β-catenin destruction complex via recruitment of Axin. Here, we discover that the molecular mechanism underlying Naked/NKD function relies on its assembly into ultra-stable decameric core aggregates via its conserved C-terminal histidine cluster (HisC). HisC aggregation is facilitated by Dishevelled and depends on accumulation of Naked/NKD during prolonged Wnt stimulation. Naked/NKD HisC cores co-aggregate with a conserved histidine cluster within Axin, to destabilize it along with Dishevelled, possibly via the autophagy receptor p62, which binds to HisC aggregates. Consistent with this, attenuated Wnt responses are observed in CRISPR-engineered flies and human epithelial cells whose Naked/NKD HisC has been deleted. Thus, HisC aggregation by Naked/NKD provides context-dependent feedback control of prolonged Wnt responses.

The Nkd EF-hand domain modulates divergent wnt signaling outputs in zebrafish

Wnt proteins regulate diverse biological responses by initiating two general outcomes: β-catenin-dependent transcription and β-catenin-independent activation of signaling cascades, the latter including modulation of calcium and regulation of cytoskeletal dynamics (Planar Cell Polarity, PCP). It has been difficult to elucidate the mechanisms by which Wnt signals are directed to effect one or the other outcome due to shared signaling proteins between the β-catenin-dependent and -independent pathways, such as the Dishevelled binding protein Naked. While all Naked paralogs contain a putative calcium-binding domain, the EF-Hand, Drosophila Naked does not bind calcium. Here we find a lineage-specific evolutionary change within the Drosophila Naked EF-hand that is not shared with other insects or vertebrates. We demonstrate the necessary role of the EF-hand for Nkd localization changes in calcium fluxing cells and using in vivo assays, we identify a role for the zebrafish Naked EF-hand in PCP but not in β-catenin antagonism. In contrast, Drosophila-like Nkd does not function in PCP, but is a robust antagonist of Wnt/β-catenin signaling. This work reveals that the zebrafish Nkd1 EF-hand is essential to balance Wnt signaling inputs and modulate the appropriate outputs, while the Drosophila-like EF-Hand primarily functions in β-catenin signaling.

Wnt ligand-dependent activation of the negative feedback regulator Nkd1

Nkd1, a negative feedback regulator of the Wnt pathway, localizes with Dvl2 to the putative Wnt signalosome, where it becomes activated by Wnt. Activated Nkd1 moves away from the membrane to become more cytosolic, where it interacts with β-catenin to prevent nuclear accumulation.

Misregulation of Wnt signaling is at the root of many diseases, most notably colorectal cancer, and although we understand the activation of the pathway, we have a very poor understanding of the circumstances under which Wnt signaling turns itself off. There are numerous negative feedback regulators of Wnt signaling, but two stand out as constitutive and obligate Wnt-induced regulators: Axin2 and Nkd1. Whereas Axin2 behaves similarly to Axin in the destruction complex, Nkd1 is more enigmatic. Here we use zebrafish blastula cells that are responsive Wnt signaling to demonstrate that Nkd1 activity is specifically dependent on Wnt ligand activation of the receptor. Furthermore, our results support the hypothesis that Nkd1 is recruited to the Wnt signalosome with Dvl2, where it becomes activated to move into the cytoplasm to interact with β-catenin, inhibiting its nuclear accumulation. Comparison of these results with Nkd function in Drosophila generates a unified and conserved model for the role of this negative feedback regulator in the modulation of Wnt signaling.

Down-regulation of NKD1 increases the invasive potential of non-small-cell lung cancer and correlates with a poor prognosis

Background

As a negative modulator of the canonical Wnt signaling pathway, Naked1 (NKD1) is widely expressed in many normal tissues. However, the expression pattern and clinicopathological significance of NKD1 in patients with non-small-cell lung cancer (NSCLC) is still unclear.

Methods

Immunohistochemical studies were performed on 35 cases of normal lung tissues and 100 cases of NSCLC, including 66 cases with complete follow-up records. The NKD1 protein and mRNA expressions were detected by western blot and Real-time PCR, respectively. To examine the effect of NKD1 on the invasiveness of lung cancer cells, NKD1 was down-regulated by siRNA in lung cancer cell lines and the invasive ability was then evaluated by the Matrigel invasion assay. In addition, the expressions of Dishevelled-1 and β-catenin proteins, as well as MMP mRNA were also examined in NKD1 knockdown cells.

Results

In 35 fresh lung cancer tissues examined, 27(79%) of them exhibited lower levels of NKD1 protein in comparison with their corresponding normal tissue (P = 0.009). However, the NKD1 mRNA level was significantly higher in cancerous lung tissues, compared with the adjacent normal tissues. In 100 NSCLC tissues, NKD1 was significantly lower in 78 cases (78%) than in the normal specimens, determined by immunohistochemical staining. The reduced NKD1 expression was correlated with histological type (P = 0.003), poor differentiation (P = 0.004), lymph node metastasis (P = 0.013), TNM stage (P = 0.002) and poor survival (62.88 ± 3.23 versus 23.61 ± 2.18 months, P = 0.03). In addition, NKD1 knockdown could up-regulate Dishevelled-1 and β-catenin protein levels, as well as increased MMP-7 transcription and the invasive ability of lung cancer cells. Furthermore, when the NKD1-knockdown cells were treated with Dishevelled-1 antibody, their invasive potential was significantly reduced.

Conclusion

NKD1 protein is reduced but NKD1 mRNA is elevated in NSCLC. Reduced NKD1 protein expression correlates with a poor prognosis in NSCLC. NKD1 might inhibit the activity of the canonical Wnt pathway through Dishevelled-1.

Naked1 antagonizes Wnt signaling by preventing nuclear accumulation of β-catenin

Cyto-nuclear shuttling of β-catenin is at the epicenter of the canonical Wnt pathway and mutations in genes that result in excessive nuclear accumulation of β-catenin are the driving force behind the initiation of many cancers. Recently, Naked Cuticle homolog 1 (Nkd1) has been identified as a Wnt-induced intracellular negative regulator of canonical Wnt signaling. The current model suggests that Nkd1 acts between Disheveled (Dvl) and β-catenin. Here, we employ the zebrafish embryo to characterize the cellular and biochemical role of Nkd1 in vivo. We demonstrate that Nkd1 binds to β-catenin and prevents its nuclear accumulation. We also show that this interaction is conserved in mammalian cultured cells. Further, we demonstrate that Nkd1 function is dependent on its interaction with the cell membrane. Given the conserved nature of Nkd1, our results shed light on the negative feedback regulation of Wnt signaling through the Nkd1-mediated negative control of nuclear accumulation of β-catenin.

Zebrafish Nkd1 promotes Dvl degradation and is required for left-right patterning

The establishment of the left-right (LR) axis in zebrafish embryos relies on signals from the dorsal forerunner cells (DFC) and the Kupffer's vesicle (KV). While the Wnt signaling network influences many aspects of embryonic development, its precise role in LR patterning is still unclear. One branch of the Wnt network leads to stabilization of β-catenin and activation of downstream target genes. Other Wnt ligands appear to act independently of β-catenin to modulate calcium release and influence cell polarity. Central to regulation of β-catenin and coordination of convergent extension (CE) movements is Dishevelled (Dvl). Naked Cuticle (Nkd) binds Dvl and modulates β-catenin-dependent and independent Wnt signaling. Here, we analyze the expression patterns of three zebrafish Nkd homologs and find enriched expression of nkd1 in DFCs and KV. Dvl is degraded upon Nkd1 overexpression in zebrafish. Knockdown of Nkd1 specifically in the DFC results in β-catenin nuclear localization and transcriptional activation as well as alterations to DFC migration, KV formation, ciliogenesis and LR patterning. Furthermore, we identify asymmetric expression of the Nodal antagonist charon around the KV and show that Nkd1 knockdown impacts asymmetric charon expression. Our findings show that Nkd1 acts as a β-catenin antagonist in the DFCs necessary for LR patterning.

Dishevelled: The hub of Wnt signaling

Wnt signaling controls a variety of developmental and homeostatic events. As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) protein relays Wnt signals from receptors to downstream effectors. In the canonical Wnt pathway that depends on the nuclear translocation of beta-catenin, Dvl is recruited by the receptor Frizzled and prevents the constitutive destruction of cytosolic beta-catenin. In the non-canonical Wnt pathways such as Wnt-Frizzled/PCP (planar cell polarity) signaling, Dvl signals via the Daam1-RhoA axis and the Rac1 axis. In addition, Dvl plays important roles in Wnt-GSK3beta-microtubule signaling, Wnt-calcium signaling, Wnt-RYK signaling, Wnt-atypical PKC signaling, etc. Dvl also functions to mediate receptor endocytosis. To fulfill its multifaceted functions, it is not surprising that Dvl associates with various kinds of proteins. Its activity is also modulated dynamically by phosphorylation, ubiquitination and degradation. In this review, we summarize the current understanding of Dvl functions in Wnt signal transduction and its biological functions in mouse development, and also discuss the molecular mechanisms of its actions.

Mutations in the human naked cuticle homolog NKD1 found in colorectal cancer alter Wnt/Dvl/beta-catenin signaling

Background

Mutation of Wnt signal antagonists Apc or Axin activates β-catenin signaling in many cancers including the majority of human colorectal adenocarcinomas. The phenotype of apc or axin mutation in the fruit fly Drosophila melanogaster is strikingly similar to that caused by mutation in the segment-polarity gene, naked cuticle (nkd). Nkd inhibits Wnt signaling by binding to the Dishevelled (Dsh/Dvl) family of scaffold proteins that link Wnt receptor activation to β-catenin accumulation and TCF-dependent transcription, but human NKD genes have yet to be directly implicated in cancer.

Methodology/Principal Findings

We identify for the first time mutations in NKD1 - one of two human nkd homologs - in a subset of DNA mismatch repair-deficient colorectal tumors that are not known to harbor mutations in other Wnt-pathway genes. The mutant Nkd1 proteins are defective at inhibiting Wnt signaling; in addition, the mutant Nkd1 proteins stabilize β-catenin and promote cell proliferation, in part due to a reduced ability of each mutant Nkd1 protein to bind and destabilize Dvl proteins.

Conclusions/Significance

Our data raise the hypothesis that specific NKD1 mutations promote Wnt-dependent tumorigenesis in a subset of DNA mismatch-repair-deficient colorectal adenocarcinomas and possibly other Wnt-signal driven human cancers.

Application of an integrated physical and functional screening approach to identify inhibitors of the Wnt pathway

Large-scale proteomic approaches have been used to study signaling pathways. However, identification of biologically relevant hits from a single screen remains challenging due to limitations inherent in each individual approach. To overcome these limitations, we implemented an integrated, multi-dimensional approach and used it to identify Wnt pathway modulators. The LUMIER protein-protein interaction mapping method was used in conjunction with two functional screens that examined the effect of overexpression and siRNA-mediated gene knockdown on Wnt signaling. Meta-analysis of the three data sets yielded a combined pathway score (CPS) for each tested component, a value reflecting the likelihood that an individual protein is a Wnt pathway regulator. We characterized the role of two proteins with high CPSs, Ube2m and Nkd1. We show that Ube2m interacts with and modulates beta-catenin stability, and that the antagonistic effect of Nkd1 on Wnt signaling requires interaction with Axin, itself a negative pathway regulator. Thus, integrated physical and functional mapping in mammalian cells can identify signaling components with high confidence and provides unanticipated insights into pathway regulators.

Drosophila Naked cuticle (Nkd) engages the nuclear import adaptor Importin-alpha3 to antagonize Wnt/beta-catenin signaling

Precise control of Wnt/beta-catenin signaling is critical for animal development, stem cell renewal, and prevention of disease. In the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene limits signaling by the Wnt ligand Wingless (Wg) during embryo segmentation. Nkd is an intracellular protein that is composed of separable membrane- and nuclear-localization sequences (NLS) as well as a conserved EF-hand motif that binds the Wnt receptor-associated scaffold protein Dishevelled (Dsh), but the mechanism by which Nkd inhibits Wnt signaling remains a mystery. Here we identify a second NLS in Nkd that is required for full activity and that binds to the canonical nuclear import adaptor Importin-alpha3. The Nkd NLS is similar to the Importin-alpha3-binding NLS in the Drosophila heat-shock transcription factor (dHSF), and each Importin-alpha3-binding NLS required intact basic residues in similar positions for nuclear import and protein function. Our results provide further support for the hypothesis that Nkd inhibits nuclear step(s) in Wnt/beta-catenin signaling and broaden our understanding of signaling pathways that engage the nuclear import machinery.

Cell-autonomous, myristyl-independent activity of the Drosophila Wnt/Wingless antagonist Naked cuticle (Nkd)

Robust animal development, tissue homeostasis, and stem cell renewal requires precise control of the Wnt/beta-catenin signaling axis. In the embryo of the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene attenuates signaling by the Wnt ligand Wingless (Wg) during segmentation. nkd mutants have been reported to exhibit abnormalities in wg transcription, Wg protein distribution and/or transport, and the intracellular response to Wg, but the relationship between each alteration and the molecular mechanism of Nkd action remains unclear. In addition, whether Nkd acts in a cell-autonomous or nonautonomous fashion in the embryo is not known. Mammalian Nkd homologs have N-terminal consensus sequences that direct the post-translational addition of a lipophilic myristoyl moiety, but fly and mosquito Nkd, while sharing N-terminal sequence homology, lack a myristoylation consensus sequence. Here we provide evidence that fly Nkd acts cell-autonomously in the embryo, with its N-terminus able to confer unique functional properties and membrane association that cannot be mimicked in vivo by heterologous myristoylation consensus sequences. In conjunction with our recent observation that Nkd requires nuclear localization for function, our data suggest that Nkd acts at more than one subcellular location within signal-receiving cells to attenuate Wg signaling.

Naked2 acts as a cargo recognition and targeting protein to ensure proper delivery and fusion of TGF-alpha containing exocytic vesicles at the lower lateral membrane of polarized MDCK cells

Transforming growth factor-alpha (TGF-alpha) is the major autocrine EGF receptor ligand in vivo. In polarized epithelial cells, proTGF-alpha is synthesized and then delivered to the basolateral cell surface. We previously reported that Naked2 interacts with basolateral sorting determinants in the cytoplasmic tail of a Golgi-processed form of TGF-alpha and that TGF-alpha is not detected at the basolateral surface of Madin-Darby canine kidney (MDCK) cells expressing myristoylation-deficient (G2A) Naked2. By high-resolution microscopy, we now show that wild-type, but not G2A, Naked2-associated vesicles fuse at the plasma membrane. We further demonstrate that Naked2-associated vesicles are delivered to the lower lateral membrane of polarized MDCK cells independent of mu1B adaptin. We identify a basolateral targeting segment within Naked2; residues 1-173 redirect NHERF-1 from the apical cytoplasm to the basolateral membrane, and internal deletion of residues 37-104 results in apical mislocalization of Naked2 and TGF-alpha. Short hairpin RNA knockdown of Naked2 leads to a dramatic reduction in the 16-kDa cell surface isoform of TGF-alpha and increased cytosolic TGF-alpha immunoreactivity. We propose that Naked2 acts as a cargo recognition and targeting (CaRT) protein to ensure proper delivery, tethering, and fusion of TGF-alpha-containing vesicles to a distinct region at the basolateral surface of polarized epithelial cells.

Zebrafish Naked1 and Naked2 antagonize both canonical and non-canonical Wnt signaling

Wnt signaling controls a wide range of developmental processes and its aberrant regulation can lead to disease. To better understand the regulation of this pathway, we identified zebrafish homologues of Naked Cuticle (Nkd), Nkd1 and Nkd2, which have previously been shown to inhibit canonical Wnt/beta-catenin signaling. Zebrafish nkd1 expression increases substantially after the mid-blastula transition in a pattern mirroring that of activated canonical Wnt/beta-catenin signaling, being expressed in both the ventrolateral blastoderm margin and also in the axial mesendoderm. In contrast, zebrafish nkd2 is maternally and ubiquitously expressed. Overexpression of Nkd1 or Nkd2a suppressed canonical Wnt/beta-catenin signaling at multiple stages of early zebrafish development and also exacerbated the cyclopia and axial mesendoderm convergence and extension (C&E) defect in the non-canonical Wnt/PCP mutant silberblick (slb/wnt11). Thus, Nkds are sufficient to antagonize both canonical and non-canonical Wnt signaling. Reducing Nkd function using antisense morpholino oligonucleotides resulted in increased expression of canonical Wnt/beta-catenin target genes. Finally, reducing Nkd1 function in slb mutants suppressed the axial mesendoderm C&E defect. These data indicate that zebrafish Nkd1 and Nkd2 function to limit both canonical and non-canonical Wnt signaling.

Structural studies of human Naked2: a biologically active intrinsically unstructured protein

Naked1 and 2 are two mammalian orthologs of Naked Cuticle, a canonical Wnt signaling antagonist in Drosophila. Naked2, but not Naked1, interacts with transforming growth factor-alpha (TGFalpha) and escorts TGFalpha-containing vesicles to the basolateral membrane of polarized epithelial cells. Full-length Naked2 is poorly soluble. Since most functional domains, including the Dishevelled binding region, EF-hand, vesicle recognition, and membrane targeting motifs, reside in the N-terminal half of the protein, we expressed and purified the first 217 residues of human Naked2 and performed a functional analysis of this fragment. Its circular dichroism (CD) and nuclear magnetic resonance (NMR) spectra showed no evidence of secondary and/or tertiary structure. The fragment did not bind calcium or zinc. These results indicate that the N-terminal half of Naked2 behaves as an intrinsically unstructured protein.

An unconventional nuclear localization motif is crucial for function of the Drosophila Wnt/wingless antagonist Naked cuticle

Wnt/beta-catenin signals orchestrate cell fate and behavior throughout the animal kingdom. Aberrant Wnt signaling impacts nearly the entire spectrum of human disease, including birth defects, cancer, and osteoporosis. If Wnt signaling is to be effectively manipulated for therapeutic advantage, we first must understand how Wnt signals are normally controlled. Naked cuticle (Nkd) is a novel and evolutionarily conserved inducible antagonist of Wnt/beta-catenin signaling that is crucial for segmentation in the model genetic organism, the fruit fly Drosophila melanogaster. Nkd can bind and inhibit the Wnt signal transducer Dishevelled (Dsh), but the mechanism by which Nkd limits Wnt signaling in the fly embryo is not understood. Here we show that nkd mutants exhibit elevated levels of the beta-catenin homolog Armadillo but no alteration in Dsh abundance or distribution. In the fly embryo, Nkd and Dsh are predominantly cytoplasmic, although a recent report suggests that vertebrate Dsh requires nuclear localization for activity in gain-of-function assays. While Dsh-binding regions of Nkd contribute to its activity, we identify a conserved 30-amino-acid motif, separable from Dsh-binding regions, that is essential for Nkd function and nuclear localization. Replacement of the 30-aa motif with a conventional nuclear localization sequence rescued a small fraction of nkd mutant animals to adulthood. Our studies suggest that Nkd targets Dsh-dependent signal transduction steps in both cytoplasmic and nuclear compartments of cells receiving the Wnt signal.

Similarities and differences between the Wnt and reelin pathways in the forming brain

One of the key features in development is the reutilization of successful signaling pathways. Here, we emphasize the involvement of the Wnt pathway, one of the five kinds of signal transduction pathway predominating early embryonic development of all animals, in regulating the formation of brain structure. We discuss the interrelationships between the Wnt and reelin pathways in the regulation of cortical layering. We summarize data emphasizing key molecules, which, when mutated, result in abnormal brain development. This integrated view, which is based on conservation of pathways, reveals the relative position of participants in the pathway, points to control mechanisms, and allows raising testable working hypotheses. Nevertheless, although signaling pathways are highly conserved from flies to humans, the overall morphology is not. We propose that future studies directed at understanding of diversification will provide fruitful insights on mammalian brain formation.

PR72, a novel regulator of Wnt signaling required for Naked cuticle function

The Wnt signaling cascade is a central regulator of cell fate determination during embryonic development, whose deregulation contributes to oncogenesis. Naked cuticle is the first Wnt-induced antagonist found in this pathway, establishing a negative-feedback loop that limits the Wnt signal required for early segmentation. In addition, Naked cuticle is proposed to function as a switch, acting to restrict classical Wnt signaling and to activate a second Wnt signaling pathway that controls planar cell polarity during gastrulation movements in vertebrates. Little is known about the biochemical function of Naked cuticle or its regulation. Here we report that PR72, a Protein Phosphatase type 2A regulatory subunit of unknown function, interacts both physically and functionally with Naked cuticle. We show that PR72, like Naked cuticle, acts as a negative regulator of the classical Wnt signaling cascade, establishing PR72 as a novel regulator of the Wnt signaling pathway. Our data provide evidence that the inhibitory effect of Naked cuticle on Wnt signaling depends on the presence of PR72, both in mammalian cell culture and in Xenopus embryos. Moreover, PR72 is required during early embryonic development to regulate cell morphogenetic movements during body axis formation.

A targeted mutation of Nkd1 impairs mouse spermatogenesis

Nkd1 is an antagonist of the canonical Wnt/beta-catenin signaling pathway. The EF-hand motif of Nkd1 is required for its inhibitory function. Early studies suggested that Nkd1 might play important roles in mouse embryonic development and tumorigenesis. We constructed Nkd1(-/-) mice whose Nkd1 protein lacked the EF-hand and was unable to inhibit Wnt/beta-catenin signaling. The homozygotes were viable and grew normally, but their fertility in males was reduced. In wild-type adult testes, Nkd1 mRNA was expressed more abundantly in the elongating spermatids than in the round spermatids. Lack of EF-hand caused reductions in the testis weight and sperm count by 30 and 60%, respectively. During testis development, Nkd1 mRNA expression started at the 25th day after birth, coincident with the onset of Wnt1 expression. Nuclear localization of beta-catenin increased in the elongating spermatids, suggesting that the mutant Nkd1 failed to inhibit the Wnt/beta-catenin pathway. These results suggest that deletion of the EF-hand from Nkd1 reduces the number of the elongating spermatids at haploid stage. In contrast, the mutant Nkd1 did not affect intestinal polyposis in Apc(Delta716) mice.

Myristoylated Naked2 escorts transforming growth factor alpha to the basolateral plasma membrane of polarized epithelial cells

The epidermal growth factor receptor ligands transforming growth factor alpha (TGF alpha) and amphiregulin are delivered to the basolateral surface of polarized epithelial cells where they are cleaved by TACE/ADAM17. Basolateral sorting information resides in their cytoplasmic tail domains, but tail-interacting proteins required for basolateral trafficking have not been identified. Naked (NKD)1 and NKD2 are mammalian homologs of Drosophila Naked Cuticle, which negatively regulates canonical Wnt signaling by binding Dishevelled. We present evidence that NKD2, but not NKD1, binds to basolateral sorting motifs in the cytoplasmic tail of TGF alpha. Processing and cell-surface delivery of TGF alpha are accelerated in NKD2-overexpressing Madin-Darby canine kidney cells. NKD2 is myristoylated on glycine, the second residue. On expression of myristoylation-defective (G2A) NKD2, neither NKD2 nor TGF alpha appears at the basolateral plasma membrane of polarized Madin-Darby canine kidney cells; however, membrane staining for TGF alpha is restored on silencing expression of this mutant NKD2. Amphiregulin does not interact with NKD2 and retains its basolateral localization in G2A-NKD2-expressing cells, as do Na(+), K(+) ATPase alpha 1 and E-cadherin. These data identify an unexpected function for NKD2, i.e., myristoylation-dependent escort of TGF alpha to the basolateral plasma membrane of polarized epithelial cells.