A beta-catenin-independent dorsalization pathway activated by Axin/JNK signaling and antagonized by aida

Unraveling the roles of endoplasmic reticulum-associated degradation in metabolic disorders

Misfolded proteins retained in the endoplasmic reticulum cause many human diseases. ER-associated degradation (ERAD) is one of the protein quality and quantity control system located at ER, which is responsible for translocating the misfolded proteins or properly folded but excess proteins out of the ER for proteasomal degradation. Recent studies have revealed that mice with ERAD deficiency in specific cell types exhibit impaired metabolism homeostasis and metabolic diseases. Here, we highlight the ERAD physiological functions in metabolic disorders in a substrate-dependent and cell type-specific manner.

In vivo genome-editing screen identifies tumor suppressor genes that cooperate with Trp53 loss during mammary tumorigenesis

Breast cancer is a heterogeneous disease that comprises multiple histological and molecular subtypes. To gain insight into mutations that drive breast tumorigenesis, we describe a pipeline for the identification and validation of tumor suppressor genes. Based on an in vivo genome‐wide CRISPR/Cas9 screen in Trp53^+/– heterozygous mice, we identified tumor suppressor genes that included the scaffold protein Axin1, the protein kinase A regulatory subunit gene Prkar1a, as well as the proof‐of‐concept genes Pten, Nf1, and Trp53 itself. Ex vivo editing of primary mammary epithelial organoids was performed to further interrogate the roles of Axin1 and Prkar1a. Increased proliferation and profound changes in mammary organoid morphology were observed for Axin1/Trp53 and Prkar1a/Trp53 double mutants compared to Pten/Trp53 double mutants. Furthermore, direct in vivo genome editing via intraductal injection of lentiviruses engineered to express dual short‐guide RNAs revealed that mutagenesis of Trp53 and either Prkar1a, Axin1, or Pten markedly accelerated tumor development compared to Trp53‐only mutants. This proof‐of‐principle study highlights the application of in vivo CRISPR/Cas9 editing for uncovering cooperativity between defects in tumor suppressor genes that elicit mammary tumorigenesis.

Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte regeneration in the zebrafish heart

Zebrafish can achieve scar-free healing of heart injuries, and robustly replace all cardiomyocytes lost to injury via dedifferentiation and proliferation of mature cardiomyocytes. Previous studies suggested that Wnt/β-catenin signaling is active in the injured zebrafish heart, where it induces fibrosis and prevents cardiomyocyte cell cycling. Here, via targeting the destruction complex of the Wnt/β-catenin pathway with pharmacological and genetic tools, we demonstrate that Wnt/β-catenin activity is required for cardiomyocyte proliferation and dedifferentiation, as well as for maturation of the scar during regeneration. Using cardiomyocyte-specific conditional inhibition of the pathway, we show that Wnt/β-catenin signaling acts cell-autonomously to promote cardiomyocyte proliferation. Our results stand in contrast to previous reports and rather support a model in which Wnt/β-catenin signaling plays a positive role during heart regeneration in zebrafish.

Size matters: Large copy number losses in Hirschsprung disease patients reveal genes involved in enteric nervous system development

Hirschsprung disease (HSCR) is a complex genetic disease characterized by absence of ganglia in the intestine. HSCR etiology can be explained by a unique combination of genetic alterations: rare coding variants, predisposing haplotypes and Copy Number Variation (CNV). Approximately 18% of patients have additional anatomical malformations or neurological symptoms (HSCR-AAM). Pinpointing the responsible culprits within a CNV is challenging as often many genes are affected. Therefore, we selected candidate genes based on gene enrichment strategies using mouse enteric nervous system transcriptomes and constraint metrics. Next, we used a zebrafish model to investigate whether loss of these genes affects enteric neuron development in vivo.

This study included three groups of patients, two groups without coding variants in disease associated genes: HSCR-AAM and HSCR patients without associated anomalies (HSCR-isolated). The third group consisted of all HSCR patients in which a confirmed pathogenic rare coding variant was identified. We compared these patient groups to unaffected controls. Predisposing haplotypes were determined, confirming that every HSCR subgroup had increased contributions of predisposing haplotypes, but their contribution was highest in isolated HSCR patients without RET coding variants. CNV profiling proved that specifically HSCR-AAM patients had larger Copy Number (CN) losses. Gene enrichment strategies using mouse enteric nervous system transcriptomes and constraint metrics were used to determine plausible candidate genes located within CN losses. Validation in zebrafish using CRISPR/Cas9 targeting confirmed the contribution of UFD1L, TBX2, SLC8A1, and MAPK8 to ENS development. In addition, we revealed epistasis between reduced Ret and Gnl1 expression and between reduced Ret and Tubb5 expression in vivo. Rare large CN losses—often de novo—contribute to HSCR in HSCR-AAM patients. We proved the involvement of six genes in enteric nervous system development and Hirschsprung disease.

Hirschsprung disease is a congenital disorder characterized by the absence of intestinal neurons in the distal part of the intestine. It is a complex genetic disorder in which multiple variations in our genome combined, result in disease. One of these variations are Copy Number Variations (CNVs): large segments of our genome that are duplicated or deleted. Patients often have Hirschsprung disease without other symptoms. However, a proportion of patients has additional associated anatomical malformations and neurological symptoms. We found that CNVs, present in patients with associated anomalies, are more often larger compared to unaffected controls or Hirschsprung patients without other symptoms. Furthermore, Copy Number (CN) losses are enriched for constrained coding regions (CCR; genes usually not impacted by genomic alterations in unaffected controls) of which the expression is higher in the developing intestinal neurons compared to the intestine. We modelled loss of these candidate genes in zebrafish by disrupting the zebrafish orthologues by genome editing. For several genes this resulted in changes in intestinal neuron development, reminiscent of HSCR observed in patients. The results presented here highlight the importance of Copy Number profiling, zebrafish validation and evaluating all CCR expressed in developing intestinal neurons during diagnostic evaluation.

Axin Family of Scaffolding Proteins in Development: Lessons from C. elegans

Scaffold proteins serve important roles in cellular signaling by integrating inputs from multiple signaling molecules to regulate downstream effectors that, in turn, carry out specific biological functions. One such protein, Axin, represents a major evolutionarily conserved scaffold protein in metazoans that participates in the WNT pathway and other pathways to regulate diverse cellular processes. This review summarizes the vast amount of literature on the regulation and functions of the Axin family of genes in eukaryotes, with a specific focus on Caenorhabditis elegans development. By combining early studies with recent findings, the review is aimed to serve as an updated reference for the roles of Axin in C. elegans and other model systems.

Integrative analysis of vascular endothelial cell genomic features identifies AIDA as a coronary artery disease candidate gene

BACKGROUND

Genome-wide association studies (GWAS) have identified hundreds of loci associated with coronary artery disease (CAD) and blood pressure (BP) or hypertension. Many of these loci are not linked to traditional risk factors, nor do they include obvious candidate genes, complicating their functional characterization. We hypothesize that many GWAS loci associated with vascular diseases modulate endothelial functions. Endothelial cells play critical roles in regulating vascular homeostasis, such as roles in forming a selective barrier, inflammation, hemostasis, and vascular tone, and endothelial dysfunction is a hallmark of atherosclerosis and hypertension. To test this hypothesis, we generate an integrated map of gene expression, open chromatin region, and 3D interactions in resting and TNFα-treated human endothelial cells.

RESULTS

We show that genetic variants associated with CAD and BP are enriched in open chromatin regions identified in endothelial cells. We identify physical loops by Hi-C and link open chromatin peaks that include CAD or BP SNPs with the promoters of genes expressed in endothelial cells. This analysis highlights 991 combinations of open chromatin regions and gene promoters that map to 38 CAD and 92 BP GWAS loci. We validate one CAD locus, by engineering a deletion of the TNFα-sensitive regulatory element using CRISPR/Cas9 and measure the effect on the expression of the novel CAD candidate gene AIDA.

CONCLUSIONS

Our data support an important role played by genetic variants acting in the vascular endothelium to modulate inter-individual risk in CAD and hypertension.

Divergent Axin and GSK-3 paralogs in the beta-catenin destruction complexes of tapeworms

The Wnt/beta-catenin pathway has many key roles in the development of animals, including a conserved and central role in the specification of the primary (antero-posterior) body axis. The posterior expression of Wnt ligands and the anterior expression of secreted Wnt inhibitors are known to be conserved during the larval metamorphosis of tapeworms. However, their downstream signaling components for Wnt/beta-catenin signaling have not been characterized. In this work, we have studied the core components of the beta-catenin destruction complex of the human pathogen Echinococcus multilocularis, the causative agent of alveolar echinococcosis. We focused on two Axin paralogs that are conserved in tapeworms and other flatworm parasites. Despite their divergent sequences, both Axins could robustly interact with one E. multilocularis beta-catenin paralog and limited its accumulation in a heterologous mammalian expression system. Similarly to what has been described in planarians (free-living flatworms), other beta-catenin paralogs showed limited or no interaction with either Axin and are unlikely to function as effectors in Wnt signaling. Additionally, both Axins interacted with three divergent GSK-3 paralogs that are conserved in free-living and parasitic flatworms. Axin paralogs have highly segregated expression patterns along the antero-posterior axis in the tapeworms E. multilocularis and Hymenolepis microstoma, indicating that different beta-catenin destruction complexes may operate in different regions during their larval metamorphosis.

IRF5 regulates unique subset of genes in dendritic cells during West Nile virus infection

Pathogen recognition receptor (PRR) signaling is critical for triggering innate immune activation and the expression of immune response genes, including genes that impart restriction against virus replication. RIG-I-like receptors and TLRs are PRRs that signal immune activation and drive the expression of antiviral genes and the production of type I IFN leading to induction of IFN-stimulated genes, in part through the interferon regulatory factor (IRF) family of transcription factors. Previous studies with West Nile virus (WNV) showed that IRF3 and IRF7 regulate IFN expression in fibroblasts and neurons, whereas macrophages and dendritic cells (DCs) retained the ability to induce IFN-β in the absence of IRF3 and IRF7 in a manner implicating IRF5 in PRR signaling actions. Here we assessed the contribution of IRF5 to immune gene induction in response to WNV infection in DCs. We examined IRF5-dependent gene expression and found that loss of IRF5 in mice resulted in modest and subtle changes in the expression of WNV-regulated genes. Anti-IRF5 chromatin immunoprecipitation with next-generation sequencing of genomic DNA coupled with mRNA analysis revealed unique IRF5 binding motifs within the mouse genome that are distinct from the canonical IRF binding motif and that link with IRF5-target gene expression. Using integrative bioinformatics analyses, we identified new IRF5 primary target genes in DCs in response to virus infection. This study provides novel insights into the distinct and unique innate immune and immune gene regulatory program directed by IRF5.

AXIN deficiency in human and mouse hepatocytes induces hepatocellular carcinoma in the absence of β-catenin activation

BACKGROUND & AIMS

The Wnt/β-catenin pathway is the most frequently deregulated pathway in hepatocellular carcinoma (HCC). Inactivating mutations of the gene encoding AXIN1, a known negative regulator of the Wnt/β-catenin signaling pathway, are observed in about 10% of HCCs. Whole-genome studies usually place HCC with AXIN1 mutations and CTNNB1 mutations in the group of tumors with Wnt/β-catenin activated program. However, it has been shown that HCCs with activating CTNNB1 mutations form a group of HCCs, with a different histology, prognosis and genomic signature to those with inactivating biallelic AXIN1 mutations. We aimed to elucidate the relationship between CTNNB1 mutations, AXIN1 mutations and the activation level of the Wnt/β-catenin program.

METHODS

We evaluated two independent human HCC datasets for the expression of a 23-β-catenin target genes program. We modeled Axin1 loss of function tumorigenesis in two engineered mouse models and performed gene expression profiling.

RESULTS

Based on gene expression, we defined three levels of β-catenin program activation: strong, weak or no activation. While more than 80% CTNNB1-mutated tumors were found in the strong or in the weak activation program, most of the AXIN1-mutated tumors (>70%) were found in the subgroup with no activation. We validated this result by demonstrating that mice with a hepatocyte specific AXIN1 deletion developed HCC in the absence of β-catenin induction. We defined a 329-gene signature common in human and mouse AXIN1 mutated HCC that is highly enriched in Notch and YAP oncogenic signatures.

CONCLUSIONS

AXIN1-mutated HCCs occur independently of the Wnt/β-catenin pathway and involve Notch and YAP pathways. These pathways constitute potentially interesting targets for the treatment of HCC caused by AXIN1 mutations.

LAY SUMMARY

Liver cancer has a poor prognosis. Defining the molecular pathways involved is important for developing new therapeutic approaches. The Wnt/β-catenin pathway is the most frequently deregulated pathway in hepatocellular carcinoma (HCC). Mutations of AXIN1, a member of this pathway, represent about 10% of HCC mutations. Using both human HCC collections and engineered mouse models of liver cancers with AXIN1 mutation or deletion, we defined a common signature of liver tumors mutated for AXIN1 and demonstrate that these tumors occur independently of the activation of the Wnt/β-catenin pathway.

AIDA Selectively Mediates Downregulation of Fat Synthesis Enzymes by ERAD to Retard Intestinal Fat Absorption and Prevent Obesity

The efficiency of intestinal absorption of dietary fat constitutes a primary determinant accounting for individual vulnerability to obesity. However, how fat absorption is controlled and contributes to obesity remains unclear. Here, we show that inhibition of endoplasmic-reticulum-associated degradation (ERAD) increases the abundance of triacylglycerol synthesis enzymes and fat absorption in small intestine. The C2-domain protein AIDA acts as an essential factor for the E3-ligase HRD1 of ERAD to downregulate rate-limiting acyltransferases GPAT3, MOGAT2, and DGAT2. Aida^-/- mice, when grown in a thermal-neutral condition or fed high-fat diet, display increased intestinal fatty acid re-esterification, circulating and tissue triacylglycerol, accompanied with severely increased adiposity without enhancement of adipogenesis. Intestine-specific knockout of Aida largely phenocopies its whole-body knockout, strongly indicating that increased intestinal TAG synthesis is a primary impetus to obesity. The AIDA-mediated ERAD system may thus represent an anti-thrifty mechanism impinging on the enzymes for intestinal fat absorption and systemic fat storage.

Leucine repeat adaptor protein 1 interacts with Dishevelled to regulate gastrulation cell movements in zebrafish

Gastrulation is a fundamental morphogenetic event that requires polarised cell behaviours for coordinated asymmetric cell movements. Wnt/PCP signalling plays a critical role in this process. Dishevelled is an important conserved scaffold protein that relays Wnt/PCP signals from membrane receptors to the modulation of cytoskeleton organisation. However, it remains unclear how its activity is regulated for the activation of downstream effectors. Here, we report that Lurap1 is a Dishevelled-interacting protein that regulates Wnt/PCP signalling in convergence and extension movements during vertebrate gastrulation. Its loss-of-function leads to enhanced Dishevelled membrane localisation and increased JNK activity. In maternal-zygotic lurap1 mutant zebrafish embryos, cell polarity and directional movement are disrupted. Time-lapse analyses indicate that Lurap1, Dishevelled, and JNK functionally interact to orchestrate polarised cellular protrusive activity, and Lurap1 is required for coordinated centriole/MTOC positioning in movement cells. These findings demonstrate that Lurap1 functions to regulate cellular polarisation and motile behaviours during gastrulation movements.

Gastrulation is an early morphogenic event driven by coordinated asymmetric/polarised cell movements. Here, the authors show in zebrafish that Lurap1, a protein that interacts with Dishevelled, regulates Wnt and planar cell polarity, coordinating centriole positioning during convergence and extension.

Highly efficient one-step scarless protein tagging by type IIS restriction endonuclease-mediated precision cloning

Protein tagging with a wide variety of epitopes and/or fusion partners is used routinely to dissect protein function molecularly. Frequently, the required DNA subcloning is inefficient, especially in cases where multiple constructs are desired for a given protein with unique tags. Additionally, the generated clones have unwanted junction sequences introduced. To add versatile tags into the extracellular domain of the transmembrane protein THSD1, we developed a protein tagging technique that utilizes non-classical type IIS restriction enzymes that recognize non-palindromic DNA sequences and cleave outside of their recognition sites. Our results demonstrate that this method is highly efficient and can precisely fuse any tag into any position of a protein in a scarless manner. Moreover, this method is cost-efficient and adaptable because it uses commercially available type IIS restriction enzymes and is compatible with the traditional cloning system used by many labs. Therefore, precision tagging technology will benefit a number of researchers by providing an alternate method to integrate an array of tags into protein expression constructs.

WDR26 is a new partner of Axin1 in the canonical Wnt signaling pathway

The stability of β‐catenin is very important for canonical Wnt signaling. A protein complex including Axin/APC/GSK3β phosphorylates β‐catenin to be degraded by ubiquitination with β‐TrCP. In the recent study, we isolated WDR26, a protein that binds to Axin. Here, we found that WDR26 is a negative regulator of the canonical Wnt signaling pathway, and that WDR26 affected β‐catenin levels. In addition, WDR26/Axin binding is involved in the ubiquitination of β‐catenin. These results suggest that WDR26 plays a negative role in β‐catenin degradation in the Wnt signaling pathway.

Genome-Wide MicroRNA and Gene Analysis of Mesenchymal Stem Cell Chondrogenesis Identifies an Essential Role and Multiple Targets for miR-140-5p

microRNAs (miRNAs) are abundantly expressed in development where they are critical determinants of cell differentiation and phenotype. Accordingly miRNAs are essential for normal skeletal development and chondrogenesis in particular. However, the question of which miRNAs are specific to the chondrocyte phenotype has not been fully addressed. Using microarray analysis of miRNA expression during mesenchymal stem cell chondrogenic differentiation and detailed examination of the role of essential differentiation factors, such as SOX9, TGF-β, and the cell condensation phase, we characterize the repertoire of specific miRNAs involved in chondrocyte development, highlighting in particular miR-140 and miR-455. Further with the use of mRNA microarray data we integrate miRNA expression and mRNA expression during chondrogenesis to underline the particular importance of miR-140, especially the -5p strand. We provide a detailed identification and validation of direct targets of miR-140-5p in both chondrogenesis and adult chondrocytes with the use of microarray and 3'UTR analysis. This emphasizes the diverse array of targets and pathways regulated by miR-140-5p. We are also able to confirm previous experimentally identified targets but, additionally, identify a novel positive regulation of the Wnt signaling pathway by miR-140-5p. Wnt signaling has a complex role in chondrogenesis and skeletal development and these findings illustrate a previously unidentified role for miR-140-5p in regulation of Wnt signaling in these processes. Together these developments further highlight the role of miRNAs during chondrogenesis to improve our understanding of chondrocyte development and guide cartilage tissue engineering.

Conformational change of Dishevelled plays a key regulatory role in the Wnt signaling pathways

The intracellular signaling molecule Dishevelled (Dvl) mediates canonical and non-canonical Wnt signaling via its PDZ domain. Different pathways diverge at this point by a mechanism that remains unclear. Here we show that the peptide-binding pocket of the Dvl PDZ domain can be occupied by Dvl's own highly conserved C-terminus, inducing a closed conformation. In Xenopus, Wnt-regulated convergent extension (CE) is readily affected by Dvl mutants unable to form the closed conformation than by wild-type Dvl. We also demonstrate that while Dvl cooperates with other Wnt pathway elements to activate canonical Wnt signaling, the open conformation of Dvl more effectively activates Jun N-terminal kinase (JNK). These results suggest that together with other players in the Wnt signaling pathway, the conformational change of Dvl regulates Wnt stimulated JNK activity in the non-canonical Wnt signaling.

DOI:http://dx.doi.org/10.7554/eLife.08142.001

The development of an animal embryo depends on a number of signaling pathways that pass information from the outside of the cell to the inside. These pathways include Wnt signaling, which also regulates cell growth. The pathways must be precisely controlled; abnormal Wnt activity has been implicated in several human diseases, ranging from heart disease to cancer.

Wnt signaling is complex, and actually comprises two major pathways: the canonical pathway (which depends on a protein called β-catenin) and the PCP pathway (which doesn't depend on β-catenin). Both pathways are triggered when Wnt molecules bind to receptors on the outside of the cell. These receptors pass the signal into the cell and to a protein called ‘Dishevelled’ (or ‘Dvl’ for short). This protein then passes the signal on through either the canonical or PCP pathway. Nevertheless it is not clear how the Dishevelled protein can direct the signal specifically down either one of these pathways.

Lee et al. now show that the Dishevelled protein can take on at least two different shapes. When it is ‘closed’, one end of the protein is tucked inside a pocket elsewhere on the protein's surface. But when Dishevelled is ‘open’, this end of the protein moves out of this pocket. Further experiments using frogs (called Xenopus, which are commonly used in research) reveal that mutant versions of Dishevelled that were unable to take on the closed form strongly affected an aspect of the frog's development that involves the PCP pathway.

Lee et al. then demonstrate that while Dishevelled cooperates with several other Wnt pathway components to activate the canonical pathway, the open form of Dishevelled activates the PCP pathway. The next challenge following on from this work is to find out how Wnt molecules binding to the receptor trigger the shape change in Dishevelled.

DOI:http://dx.doi.org/10.7554/eLife.08142.002

Long-chain Acyl-CoA synthetase 4A regulates Smad activity and dorsoventral patterning in the zebrafish embryo

Long-chain polyunsaturated fatty acids (LC-PUFA) and their metabolites are critical players in cell biology and embryonic development. Here we show that long-chain acyl-CoA synthetase 4a (Acsl4a), an LC-PUFA activating enzyme, is essential for proper patterning of the zebrafish dorsoventral axis. Loss of Acsl4a results in dorsalized embryos due to attenuated bone morphogenetic protein (Bmp) signaling. We demonstrate that Acsl4a modulates the activity of Smad transcription factors, the downstream mediators of Bmp signaling. Acsl4a promotes the inhibition of p38 mitogen-activated protein kinase and the Akt-mediated inhibition of glycogen synthase kinase 3, critical inhibitors of Smad activity. Consequently, introduction of a constitutively active Akt can rescue the dorsalized phenotype of Acsl4a-deficient embryos. Our results reveal a critical role for Acsl4a in modulating Bmp-Smad activity and provide a potential avenue for LC-PUFAs to influence a variety of developmental processes.

Candidate disease gene prediction using Gentrepid: application to a genome-wide association study on coronary artery disease

Current single-locus-based analyses and candidate disease gene prediction methodologies used in genome-wide association studies (GWAS) do not capitalize on the wealth of the underlying genetic data, nor functional data available from molecular biology. Here, we analyzed GWAS data from the Wellcome Trust Case Control Consortium (WTCCC) on coronary artery disease (CAD). Gentrepid uses a multiple-locus-based approach, drawing on protein pathway- or domain-based data to make predictions. Known disease genes may be used as additional information (seeded method) or predictions can be based entirely on GWAS single nucleotide polymorphisms (SNPs) (ab initio method). We looked in detail at specific predictions made by Gentrepid for CAD and compared these with known genetic data and the scientific literature. Gentrepid was able to extract known disease genes from the candidate search space and predict plausible novel disease genes from both known and novel WTCCC-implicated loci. The disease gene candidates are consistent with known biological information. The results demonstrate that this computational approach is feasible and a valuable discovery tool for geneticists.

Disabled-2 (Dab2) inhibits Wnt/β-catenin signalling by binding LRP6 and promoting its internalization through clathrin

Wnt signalling requires caveolin-dependent endocytic uptake of the Fz/LRP6 receptor complex. The tumour suppressor Disabled-2 inhibits Wnt signalling by sequestering CK2-phosphorylated LRP6 into an alternative clathrin-dependent endocytic pathway.

Canonical Wnt signalling requires caveolin-dependent internalization of low-density lipoprotein receptor-related protein 6 (LRP6). Here we report that the tumour suppressor and endocytic adaptor disabled-2 (Dab2), previously described as an inhibitor of Wnt/β-catenin signalling, selectively recruits LRP6 to the clathrin-dependent endocytic route, thereby sequestering it from caveolin-mediated endocytosis. Wnt stimulation induces the casein kinase 2 (CK2)-dependent phosphorylation of LRP6 at S1579, promoting its binding to Dab2 and internalization with clathrin. LRP6 receptor mutant (S1579A), deficient in CK2-mediated phosphorylation and Dab2 binding, fails to associate with clathrin, and thus escapes the inhibitory effects of Dab2 on Wnt/β-catenin signalling. Our data suggest that the S1579 site of LRP6 is a negative regulatory point during LRP6-mediated dorsoventral patterning in zebrafish and in allograft mouse tumour models. We conclude that the tumour suppressor functions of Dab2 involve modulation of canonical Wnt signalling by regulating the endocytic fate of the LRP6 receptor.

The Axin/TNKS complex interacts with KIF3A and is required for insulin-stimulated GLUT4 translocation

Insulin-stimulated glucose uptake by the glucose transporter GLUT4 plays a central role in whole-body glucose homeostasis, dysregulation of which leads to type 2 diabetes. However, the molecular components and mechanisms regulating insulin-stimulated glucose uptake remain largely unclear. Here, we demonstrate that Axin interacts with the ADP-ribosylase tankyrase 2 (TNKS2) and the kinesin motor protein KIF3A, forming a ternary complex crucial for GLUT4 translocation in response to insulin. Specific knockdown of the individual components of the complex attenuated insulin-stimulated GLUT4 translocation to the plasma membrane. Importantly, TNKS2(-/-) mice exhibit reduced insulin sensitivity and higher blood glucose levels when re-fed after fasting. Mechanistically, we demonstrate that in the absence of insulin, Axin, TNKS and KIF3A are co-localized with GLUT4 on the trans-Golgi network. Insulin treatment suppresses the ADP-ribosylase activity of TNKS, leading to a reduction in ADP ribosylation and ubiquitination of both Axin and TNKS, and a concurrent stabilization of the complex. Inhibition of Akt, the major effector kinase of insulin signaling, abrogates the insulin-mediated complex stabilization. We have thus elucidated a new protein complex that is directly associated with the motor protein kinesin in insulin-stimulated GLUT4 translocation.

Cdk5-mediated phosphorylation of Axin directs axon formation during cerebral cortex development

Axon formation is critical for the establishment of connections between neurons, which is a prerequisite for the development of neural circuitry. Kinases such as cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase-3β (GSK-3β), have been implicated to regulate axon outgrowth. Nonetheless, the in vivo roles of these kinases in axon development and the underlying signaling mechanisms remain essentially unknown. We report here that Cdk5 is important for axon formation in mouse cerebral cortex through regulating the functions of axis inhibitor (Axin), a scaffold protein of the canonical Wnt pathway. Knockdown of Axin in utero abolishes the formation and projection of axons. Importantly, Axin is phosphorylated by Cdk5, and this phosphorylation facilitates the interaction of Axin with GSK-3β, resulting in inhibition of GSK-3β activity and dephosphorylation of its substrate collapsin response mediator protein-2 (CRMP-2), a microtubule-associated protein. Specifically, both phosphorylation of Axin and its interaction with GSK-3β are critically required for axon formation in mouse cortex development. Together, our findings reveal a new regulatory mechanism of axon formation through Cdk5-dependent phosphorylation of Axin.

Distinct signaling properties of mitogen-activated protein kinase kinases 4 (MKK4) and 7 (MKK7) in embryonic stem cell (ESC) differentiation

Signal transduction pathways are integral components of the developmental regulatory network that guides progressive cell fate determination. MKK4 and MKK7 are upstream kinases of the mitogen-activated protein kinases (MAPKs), responsible for channeling physiological and environmental signals to their cellular responses. Both kinases are essential for survival of mouse embryos, but because of embryonic lethality, their precise developmental roles remain largely unknown. Using gene knock-out mouse ESCs, we studied the roles of MKK4 and MKK7 in differentiation in vitro. While MKK4 and MKK7 were dispensable for ESC self-renewal and pluripotency maintenance, they exhibited unique signaling and functional properties in differentiation. MKK4 and MKK7 complemented each other in activation of the JNK-c-Jun cascades and loss of both led to senescence upon cell differentiation. On the other hand, MKK4 and MKK7 had opposite effects on activation of the p38 cascades during differentiation. Specifically, MKK7 reduced p38 activation, while Mkk7(-/-) ESCs had elevated phosphorylation of MKK4, p38, and ATF2, and increased MEF2C expression. Consequently, Mkk7(-/-) ESCs had higher expression of MHC and MLC and enhanced formation of contractile cardiomyocytes. In contrast, MKK4 was required for p38 activation and Mkk4(-/-) ESCs exhibited diminished p-ATF2 and MEF2C expression, resulting in impaired MHC induction and defective cardiomyocyte differentiation. Exogenous MKK4 expression partially restored the ability of Mkk4(-/-) ESCs to differentiate into cardiomyocytes. Our results uncover complementary and interdependent roles of MKK4 and MKK7 in development, and identify the essential requirement for MKK4 in p38 activation and cardiomyocyte differentiation.

Twist controls skeletal development and dorsoventral patterning by regulating runx2 in zebrafish

Background

Twist1a and twist1b are the principal components of twists that negatively regulate a number of cellular signaling events. Expression of runx2 and downstream targets is essential for skeletal development and ventral organizer formation and specification in early vertebrate embryos, but what controls ventral activity of maternal runx2 and how twists function in zebrafish embryogenesis still remain unclear.

Methodology/Principal Findings

By studying the loss of twist induced by injection of morpholino-oligonucleotide in zebrafish, we found that twist1a and twist1b, but not twist2 or twist3, were required for proper skeletal development and dorsoventral patterning in early embryos. Overexpression of twist1a or twist1b following mRNA injection resulted in deteriorated skeletal development and formation of typical dorsalized embryos, whereas knockdown of twist1a and twist1b led to the formation of abnormal embryos with enhanced skeletal formation and typical ventralized patterning. Overexpression of twist1a or twist1b decreased the expression of runx2b, whereas twist1a and twist1b knockdown increased runx2b expression. We have further demonstrated that phenotypes induced by twist1a and twist1b knockdown were rescued by runx2b knockdown.

Conclusions/Significance

Together, these results suggest that twist1a and twist1b control skeletal development and dorsoventral patterning by regulating runx2b in zebrafish and provide potential targets for the treatment of diseases or syndromes associated with decreased skeletal development.

A novel oncoprotein Pirh2: rising from the shadow of MDM2

Pirh2 (p53-induced RING-H2) is an E3 ubiquitin ligase that can target p53 for degradation and thereby repress a diverse group of biological activities regulated by p53. Notably, Pirh2, rather than MDM2, is the primary degrader of active p53 under conditions of DNA damage. Moreover, Pirh2 is highly expressed in multiple cancer cell lines regardless of p53 status. Recent research has shown that Pirh2 is involved in many signalling pathways related to the genesis and evolution of cancer. This review aims to summarize a comprehensive picture of the role of Pirh2 in cellular processes and its significance to tumorigenesis. Furthermore, this review focuses on its potential role as a cancer therapeutic target.

Loss of JNK2 increases intestinal tumor susceptibility in Apc1638+/- mice with dietary modulation

A recent study has shown that c-Jun NH2-terminal kinases (JNKs) 2 interacts with and inhibits β-catenin signaling in vitro. To determine the role of genetic interaction between JNK2 and β-catenin in vivo and to elucidate JNK2-mediated intestinal carcinogenesis, we crossed the JNK2-/- mice with Apc1638+/- mice that carry inactivated Apc allele and develop intestinal tumor due to β-catenin activation. We found that the introduction of mutant JNK2 into Apc1638+/- mice did not increase intestinal tumorigenesis when the mice were fed a defined AIN-76A control diet. However, loss of JNK2 significantly increased animal body weight in the Apc/JNK2+/- and Apc/JNK2-/- mice. Surprisingly, JNK2 loss was synergistic with a Western-style high-risk diet (high fat and phosphate and low calcium and vitamin D) to accelerate intestinal tumorigenesis. Tumor number increased to 3.56 from 1.89 (on AIN-76A diet) in the Apc/JNK2+/- mice (P<0.01) and increased to 4.14 from 1.92 (on AIN-76A diet) in the Apc/JNK2-/- mice (P<0.01) although there was a slight increase of tumor formation in Apc/JNK2+/+ mice. Intestinal tumorigenesis in Apc/JNK2 double-mutant mice with high-risk diet modulation was associated with β-catenin signaling, peroxisome proliferator-activated receptor-γ and inflammation pathway. Collectively, we concluded that JNK2 may function in controlling fat metabolism and loss of JNK2 increases the risk of obesity, the latter synergizes with high-fat diet to increase intestinal tumor susceptibility. This data strongly suggests the importance of JNK2 in intestinal carcinogenesis and the importance of dietary manipulation for cancer prevention in the population whose JNK2 is inactivated.

Negative regulation of wnt11 expression by Jnk signaling during zebrafish gastrulation

Stress-induced Sapk/Jnk signaling is involved in cell survival and apoptosis. Recent studies have increased our understanding of the physiological roles of Jnk signaling in embryonic development. However, still unclear is the precise function of Jnk signaling during gastrulation, a critical step in the establishment of the vertebrate body plan. Here we use morpholino-mediated knockdown of the zebrafish orthologs of the Jnk activators Mkk4 and Mkk7 to examine the effect of Jnk signaling abrogation on early vertebrate embryogenesis. Depletion of zebrafish Mkk4b led to abnormal convergent extension (CE) during gastrulation, whereas Mkk7 morphants exhibited defective somitogenesis. Surprisingly, Mkk4b morphants displayed marked upregulation of wnt11, which is the triggering ligand of CE and stimulates Jnk activation via the non-canonical Wnt pathway. Conversely, ectopic activation of Jnk signaling by overexpression of an active form of Mkk4b led to wnt11 downregulation. Mosaic lineage tracing studies revealed that Mkk4b-Jnk signaling suppressed wnt11 expression in a non-cell-autonomous manner. These findings provide the first evidence that wnt11 itself is a downstream target of the Jnk cascade in the non-canonical Wnt pathway. Our work demonstrates that Jnk activation is indispensable for multiple steps during vertebrate body plan formation. Furthermore, non-canonical Wnt signaling may coordinate vertebrate CE movements by triggering Jnk activation that represses the expression of the CE-triggering ligand wnt11.

The protein stability of Axin, a negative regulator of Wnt signaling, is regulated by Smad ubiquitination regulatory factor 2 (Smurf2)

Axin is a negative regulator of Wnt/β-catenin signaling via regulating the level of β-catenin, which is a key effector molecule. Therefore, controlling the level of Axin is a critical step for the regulation of Wnt/β-catenin signaling. It has been shown that ubiquitination-mediated proteasomal degradation may play a critical role in the regulation of Axin; however, the E3 ubiquitin ligase(s), which attaches ubiquitin to a target protein in combination with an E2 ubiquitin-conjugating enzyme, for Axin has not yet been identified. Here, we show that Smurf2 is an E3 ubiquitin ligase for Axin. Transient expression of Smurf2 down-regulated the level of Axin and increased the ubiquitination of Axin. Conversely, shRNA specific to Smurf2 blocked Axin ubiquitination. Essential domains of Axin responsible for Smurf2 interaction as well as Smurf2-mediated down-regulation and ubiquitination were identified. In vitro ubiquitination assays followed by analysis using mass spectroscopy revealed that Smurf2 specifically ubiquitinylated Lys(505) of Axin and that the Axin(K505R) mutant resisted degradation. Knockdown of endogenous Smurf2 increased the level of endogenous Axin and resulted in reduced β-catenin/Tcf reporter activity. Overall, our data strongly suggest that Smurf2 is a genuine E3 ligase for Axin.

Identification of novel families and classification of the C2 domain superfamily elucidate the origin and evolution of membrane targeting activities in eukaryotes

Eukaryotes contain an elaborate membrane system, which bounds the cell itself, nuclei, organelles and transient intracellular structures, such as vesicles. The emergence of this system was marked by an expansion of a number of structurally distinct classes of lipid-binding domains that could throw light on the early evolution of eukaryotic membranes. The C2 domain is a useful model to understand these events because it is one of the most prevalent eukaryotic lipid-binding domains deployed in diverse functional contexts. Most studies have concentrated on C2 domains prototyped by those in protein kinase C (PKC-C2) isoforms that bind lipid in a calcium-dependent manner. While two other distinct families of C2 domains, namely those in PI3K-C2 and PTEN-C2 are also recognized, a complete picture of evolutionary relationships within the C2 domain superfamily is lacking. We systematically studied this superfamily using sequence profile searches, phylogenetic and phyletic-pattern analysis and structure-prediction. Consequently, we identified several distinct families of C2 domains including those respectively typified by C2 domains in the Aida (axin interactor, dorsalization associated) proteins, B9 proteins (e.g. Mks1 (Xbx-7), Stumpy (Tza-1) and Tza-2) involved in centrosome migration and ciliogenesis, Dock180/Zizimin proteins which are Rac/CDC42 GDP exchange factors, the EEIG1/Sym-3, EHBP1 and plant RPG/PMI1 proteins involved in endocytotic recycling and organellar positioning and an apicomplexan family. We present evidence that the last eukaryotic common ancestor (LECA) contained at least 10 C2 domains belonging to 6 well-defined families. Further, we suggest that this pre-LECA diversification was linked to the emergence of several quintessentially eukaryotic structures, such as membrane repair and vesicular trafficking system, anchoring of the actin and tubulin cytoskeleton to the plasma and vesicular membranes, localization of small GTPases to membranes and lipid-based signal transduction. Subsequent lineage-specific expansions of Zizimin-type C2 domains and functionally linked CDC42/Rac GTPases occurred independently in eukaryotes that evolved active amoeboid motility. While two lipid-binding regions are likely to be shared by majority of C2 domains, the actual constellation of lipid-binding residues (predominantly basic) are distinct in each family potentially reflective of the functional and biochemical diversity of these domains. Importantly, we show that the calcium-dependent membrane interaction is a derived feature limited to the PKC-C2 domains. Our identification of novel C2 domains offers new insights into interaction between both the microtubular and microfilament cytoskeleton and cellular membranes.

GSK3beta is involved in JNK2-mediated beta-catenin inhibition

Background

We have recently reported that mitogen-activated protein kinase (MAPK) JNK1 downregulates β-catenin signaling and plays a critical role in regulating intestinal homeostasis and in suppressing tumor formation. This study was designed to determine whether JNK2, another MAPK, has similar and/or different functions in the regulation of β-catenin signaling.

Methodology and Principal Findings

We used an in vitro system with manipulation of JNK2 and β-catenin expression and found that activated JNK2 increased GSK3β activity and inhibited β-catenin expression and transcriptional activity. However, JNK2-mediated downregulation of β-catenin was blocked by the proteasome inhibitor MG132 and GSK3β inhibitor lithium chloride. Moreover, targeted mutations at GSK3β phosphorylation sites (Ser33 and Ser37) of β-catenin abrogated JNK2-mediated suppression of β-catenin. In vivo studies further revealed that JNK2 deficiency led to upregulation of β-catenin and increase of GSK3-β phosphorylation in JNK2-/- mouse intestinal epithelial cells. Additionally, physical interaction and co-localization among JNK2, β-catenin and GSK3β were observed by immunoprecipitation, mammalian two-hybridization assay and confocal microscopy, respectively.

Conclusion and Significance

In general, our data suggested that JNK2, like JNK1, interacts with and suppresses β-catenin signaling in vitro and in vivo, in which GSK3β plays a key role, although previous studies have shown distinct functions of JNK1 and JNK2. Our study also provides a novel insight into the crosstalk between Wnt/β-catenin and MAPK JNKs signaling.

Dab2 stabilizes Axin and attenuates Wnt/beta-catenin signaling by preventing protein phosphatase 1 (PP1)-Axin interactions

Wnt/beta-catenin signaling plays a pivotal role in modulating cellular proliferation, differentiation, tissue organization and embryonic development. Earlier, we found that the endocytic adaptor disabled-2 (Dab2) could attenuate Wnt/beta-catenin signaling by stabilizing Axin and preventing its translocation to the membrane. Recently, protein phosphatase 1 (PP1) has been shown to interact with, and dephosphorylate Axin, leading to its destabilization. Here, we show that Dab2 functions upstream of PP1 to block the interaction between Axin and PP1, inhibiting Axin dephosphorylation and thereby stabilizing its expression, ultimately leading to inhibition of Wnt/beta-catenin. We show that Dab2 acts as a competitive inhibitor of PP1 by binding to the same C-terminal domain of Axin. Both PP1 and Axin bind to the N-terminus of Dab2 and a Dab2 truncation mutant consisting of the N-terminal phosphotyrosine binding domain blocks PP1-Axin interactions and inhibits Wnt signaling. We confirm the inhibitory effect of Dab2 on Wnt/beta-catenin signaling in zebrafish embryos, showing that its ectopic expression phenocopies Axin overexpression resulting in altered dorsoventral patterning. We conclude that Dab2 stabilizes Axin and attenuates Wnt/beta-catenin signaling by preventing PP1 from binding Axin.

Asymmetric syntheses and Wnt signal inhibitory activity of melleumin A and four analogues of melleumins A and B

Guided by nature: A flexible and epimerization-free approach for the asymmetric syntheses of melleumin A and four analogues of melleumins A and B was developed, which allowed confirming the stereochemistry at C-4 of melleumin A, and revealed that the unnatural 4-epi-melleumin B possesses a modest inhibitory activity on Wnt signaling. The first total synthesis of melleumin A and four analogues of melleumins A and B is described. The N-acyl L-Thr-Gly/beta-hydroxy-gamma-amino acid coupling/macrolactamization strategy allowed the efficient assembly of the three segments being free of epimerization. While the Jouin-Castro method with minor modification allows a rapid entrance to the key syn-beta-hydroxy-gamma-amino acid segment, required for the synthesis of melleumin A, an extension of our malimide-based methodology using a changed N-protecting group affords a flexible access to several anti-beta-hydroxy-gamma-amino acids, and hence analogues of melleumins A and B. Among them, unnatural 4-epi-melleumin B (2 a) exhibits a modest inhibitory activity on Wnt signaling. The total synthesis of melleumin A allowed confirmation of its full structure.

Selection of suitable housekeeping genes for expression analysis in glioblastoma using quantitative RT-PCR

Background

Considering the broad variation in the expression of housekeeping genes among tissues and experimental situations, studies using quantitative RT-PCR require strict definition of adequate endogenous controls. For glioblastoma, the most common type of tumor in the central nervous system, there was no previous report regarding this issue.

Results

Here we show that amongst seven frequently used housekeeping genes TBP and HPRT1 are adequate references for glioblastoma gene expression analysis. Evaluation of the expression levels of 12 target genes utilizing different endogenous controls revealed that the normalization method applied might introduce errors in the estimation of relative quantities. Genes presenting expression levels which do not significantly differ between tumor and normal tissues can be considered either increased or decreased if unsuitable reference genes are applied. Most importantly, genes showing significant differences in expression levels between tumor and normal tissues can be missed. We also demonstrated that the Holliday Junction Recognizing Protein, a novel DNA repair protein over expressed in lung cancer, is extremely over-expressed in glioblastoma, with a median change of about 134 fold.

Conclusion

Altogether, our data show the relevance of previous validation of candidate control genes for each experimental model and indicate TBP plus HPRT1 as suitable references for studies on glioblastoma gene expression.

Gene regulatory network interactions in sea urchin endomesoderm induction

A major goal of contemporary studies of embryonic development is to understand large sets of regulatory changes that accompany the phenomenon of embryonic induction. The highly resolved sea urchin pregastrular endomesoderm-gene regulatory network (EM-GRN) provides a unique framework to study the global regulatory interactions underlying endomesoderm induction. Vegetal micromeres of the sea urchin embryo constitute a classic endomesoderm signaling center, whose potential to induce archenteron formation from presumptive ectoderm was demonstrated almost a century ago. In this work, we ectopically activate the primary mesenchyme cell-GRN (PMC-GRN) that operates in micromere progeny by misexpressing the micromere determinant Pmar1 and identify the responding EM-GRN that is induced in animal blastomeres. Using localized loss-of -function analyses in conjunction with expression of endo16, the molecular definition of micromere-dependent endomesoderm specification, we show that the TGFbeta cytokine, ActivinB, is an essential component of this induction in blastomeres that emit this signal, as well as in cells that respond to it. We report that normal pregastrular endomesoderm specification requires activation of the Pmar1-inducible subset of the EM-GRN by the same cytokine, strongly suggesting that early micromere-mediated endomesoderm specification, which regulates timely gastrulation in the sea urchin embryo, is also ActivinB dependent. This study unexpectedly uncovers the existence of an additional uncharacterized micromere signal to endomesoderm progenitors, significantly revising existing models. In one of the first network-level characterizations of an intercellular inductive phenomenon, we describe an important in vivo model of the requirement of ActivinB signaling in the earliest steps of embryonic endomesoderm progenitor specification.

Co-opted JNK/SAPK signaling in Wnt/beta-catenin-induced tumorigenesis

Aberrant stimulation of the canonical Wnt pathway induces mammary tumorigenesis in mice. It has been well documented that two types of tumors, adenocarcinoma and adenocarcinoma with squamous metaplasia, develop in these mutants. However, the molecular mechanism underlying the induction of squamous transdifferentiation remains largely unknown. Here, we show that JNK/SAPK signaling plays an important role in Wnt-dependent mammary development and malignant transformation. The JNK/SAPK pathway is stimulated in pregnancy-mediated lobulo-alveolar morphogenesis, a process highly dependent on Wnt/beta-catenin signaling. Strong elevations of JNK/SAPK signaling are associated with squamous metaplasia of the Wnt-induced adenocarcinoma. Reconstitution of beta-catenin and JNK/SAPK signaling activities also promotes expression of the squamous cell marker in cultured epithelial cells. Furthermore, a synergistic activation of these two pathways can be identified in the malignant squamous cells of human endometrial and lung cancers. This is potentially a significant discovery in modern cancer therapy because of the effectiveness of an angiogenesis inhibitor, Avastin, for the treatment of adenocarcinoma, but not squamous cell carcinoma, in human lung cancers. Our finding may improve the usage of biomarkers to distinguish these two poorly differentiated tumor types, sharing similar histologic features.

c-Jun N-terminal kinase 1 interacts with and negatively regulates Wnt/beta-catenin signaling through GSK3beta pathway

Increasing evidence shows that there is an interaction between mitogen-activated protein kinase and Wnt signaling and that their interaction plays important roles in a variety of cellular processes. However, how the two signaling interacts is not clear. In this study, we found that beta-catenin expression was strikingly increased in the intestinal normal mucosa and tumors of c-Jun N-terminal kinase (JNK) 1-deficient mice by immunohistochemical staining and that both beta-catenin expression and transcriptional activity were significantly upregulated in JNK1-deficient mouse embryonic fibroblasts. However, active JNK1 significantly inhibited beta-catenin expression and suppressed beta-catenin-mediated transcriptional activity by enhancing glycogen synthase kinase 3beta (GSK3beta) activity. But beta-catenin inhibition was significantly reduced by GSK3beta RNA interference or GSK3beta inhibitor lithium chloride and proteasome inhibitor MG132. Further, mutant beta-catenin at the phosphorylation sites of Ser33 and Ser37 by GSK3beta was resistant to activated JNK1-induced beta-catenin degradation. Moreover, the physical interaction between JNK1 and beta-catenin was detected by immunoprecipitation, and their colocalization was seen in cellular nuclei and cytoplasm. Taken together, our data provide direct evidence that JNK1 interacts with and negatively regulates beta-catenin signaling through GSK3beta pathway and that the beta-catenin alteration is probably responsible for the intestinal tumor formation in JNK1-deficient mice.

CDK5 activator p35 downregulates E-cadherin precursor independently of CDK5

Dysfunction of E-cadherins often results in metastasis of cancerous cells. Here we show that p35, a critical regulator of cyclin-dependent kinase 5 (CDK5), specifically depletes the precursor form of E-cadherin, but not the mature form, by using a precursor-specific antibody. Most intriguingly, this downregulation of precursor E-cadherin by p35 is unequivocally independent of CDK5. Moreover, we found that p35 forms complexes with E-cadherin proteins. We also found that p35 co-expression can target E-cadherin to lysosomes and that p35-triggered disappearance of E-cadherin precursor can be blocked specifically by lysosomal protease inhibitors, indicating that p35 induces endocytosis and subsequent degradation of precursor E-cadherin.

Protein encoded by the Axin(Fu) allele effectively down-regulates Wnt signaling but exerts a dominant negative effect on c-Jun N-terminal kinase signaling

Axin plays an architectural role in many important signaling pathways that control various aspects of development and tumorigenesis, including the Wnt, transforming growth factor-beta, MAP kinase pathways, as well as p53 activation cascades. It is encoded by the mouse Fused (Fu) locus; the Axin(Fu) allele is caused by insertion of an IAP transposon. Axin(Fu/Fu) mice display varying phenotypes ranging from embryonic lethality to relatively normal adulthood with kinky tails. However, the protein product(s) has not been identified or characterized. In the present study, we conducted immunoprecipitation using brain extracts from the Axin(Fu) mice with specific antibodies against different regions of Axin and found that a truncated Axin containing amino acids 1-596 (designated as Axin(Fu-NT)) and the full-length complement of Axin (Axin(WT)) can both be generated from the Axin(Fu) allele. When tested for functionality changes, Axin(Fu-NT) was found to abolish Axin-mediated activation of JNK, which plays a critical role in dorsoventral patterning. Together with a proteomics approach, we found that Axin(Fu-NT) contains a previously uncharacterized dimerization domain and can form a heterodimeric interaction with Axin(WT). The Axin(Fu-NT)/Axin(WT) is not conducive to JNK activation, providing a molecular explanation for the dominant negative effect of Axin(Fu-NT) on JNK activation by wild-type Axin. Importantly, Axin(Fu-NT) exhibits no difference in the inhibition of Wnt signaling compared with Axin(WT) as determined by reporter gene assays, interaction with key Wnt regulators, and expression of Wnt marker genes in zebrafish embryos, suggesting that altered JNK signaling contributes, at least in part, to the developmental defects seen in Axin(Fu) mice.