Genetic and functional linkage of Kir5.1 and Kir2.1 channel subunits

We have identified several cDNAs for the human Kir5.1 subunit of inwardly rectifying K(+) channels. Alternative splicing of exon 3 and the usage of two alternative polyadenylation sites contribute to cDNA diversity. The hKir5.1 gene KCNJ16 is assigned to chromosomal region 17q23.1-24.2, and is separated by only 34 kb from the hKir2.1 gene (KCNJ2). In the brain, Kir5.1 mRNA is restricted to the evolutionary older parts of the hindbrain, midbrain and diencephalon and overlaps with Kir2.1 in the superior/inferior colliculus and the pontine region. In the kidney Kir5.1 and Kir2.1 are colocalized in the proximal tubule. When expressed in Xenopus oocytes, Kir5.1 is efficiently targeted to the cell surface and forms electrically silent channels together with Kir2.1, thus negatively controlling Kir2.1 channel activity in native cells.     

Genistein and tyrphostin AG556 inhibit inwardly-rectifying Kir2.1 channels expressed in HEK 293 cells via protein tyrosine kinase inhibition

Previous studies reported the controversial effects that protein tyrosine kinase (PTK) inhibition could induce an up-regulation or down-regulation of Kir2.1 current. The present study investigates how the recombinant human Kir2.1 channels are regulated by PTKs using whole-cell patch voltage-clamp, immunoprecipitation and Western blot, and mutagenesis approaches. We found that hKir2.1 current was reversibly inhibited by the broad spectrum PTK inhibitor genistein and the highly selective EGFR (epidermal growth factor receptor) kinase inhibitor AG556 in a concentration-dependent manner. The inhibition of hKir2.1 channels by genistein or AG556 was countered by the protein tyrosine phosphatase (PTP) inhibitor orthovanadate. Immunoprecipitation and Western blot analysis revealed that tyrosine phosphorylation level of Kir2.1 channels was reduced by genistein or AG556, and the reduction was significantly antagonized by orthovanadate. The mutation of Y242 dramatically reduced the inhibitory response to AG556. The results obtained in this study demonstrate that hKir2.1 channels are down-regulated by PTK inhibition, suggesting that EGFR kinase participates in the modulation of human cardiac excitability.     

Structure, tissue distribution, and pharmacological characterization of Xenopus orexins

We isolated the Xenopus gene encoding prepro-orexin to predict the structures of orexins in submammalian chordates. Putative mature Xenopus orexin-A and -B are highly similar to each mammalian counterpart. Especially, the C-terminal 10 residues were highly conserved among these species and isopeptides. Immunohistochemical examination of Xenopus brain revealed that orexin-containing neurons were highly specifically localized in the ventral hypothalamic nucleus. A rich network of immunoreactive fibers was found in various regions of the Xenopus brain. The distribution was similar to that of mammalian orexins. Xenopus orexin-A and -B specifically bind and activate human orexin receptors expressed in Chinese hamster ovary cells. Of interest, Xenopus orexin-B had several-fold higher affinity to human OX2R compared with human orexins. These results suggest that Xenopus orexin-B might be a useful pharmacological tool as an OX2R selective high-affinity agonist.     

Transit defect of potassium-chloride Co-transporter 3 is a major pathogenic mechanism in hereditary motor and sensory neuropathy with agenesis of the corpus callosum

Missense and protein-truncating mutations of the human potassium-chloride co-transporter 3 gene (KCC3) cause hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), which is a severe neurodegenerative disease characterized by axonal dysfunction and neurodevelopmental defects. We previously reported that KCC3-truncating mutations disrupt brain-type creatine kinase-dependent activation of the co-transporter through the loss of its last 140 amino acids. Here, we report a novel and more distal HMSN/ACC-truncating mutation (3402C → T; R1134X) that eliminates only the last 17 residues of the protein. This small truncation disrupts the interaction with brain-type creatine kinase in mammalian cells but also affects plasma membrane localization of the mutant transporter. Although it is not truncated, the previously reported HMSN/ACC-causing 619C → T (R207C) missense mutation also leads to KCC3 loss of function in Xenopus oocyte flux assay. Immunodetection in Xenopus oocytes and in mammalian cultured cells revealed a decreased amount of R207C at the plasma membrane, with significant retention of the mutant proteins in the endoplasmic reticulum. In mammalian cells, curcumin partially corrected these mutant protein mislocalizations, with more protein reaching the plasma membrane. These findings suggest that mis-trafficking of mutant protein is an important pathophysiological feature of HMSN/ACC causative KCC3 mutations.     

Variable loss of Kir4.1 channel function in SeSAME syndrome mutations

SeSAME syndrome is a complex disease characterized by seizures, sensorineural deafness, ataxia, mental retardation and electrolyte imbalance. Mutations in the inwardly rectifying potassium channel Kir4.1 (KCNJ10 gene) have been linked to this condition. Kir4.1 channels are weakly rectifying channels expressed in glia, kidney, cochlea and possibly other tissues. We determined the electrophysiological properties of SeSAME mutant channels after expression in transfected mammalian cells. We found that a majority of mutations (R297C, C140R, R199X, T164I) resulted in complete loss of Kir4.1 channel function while two mutations (R65P and A167V) produced partial loss of function. All mutant channels were rescued upon co-transfection of wild-type Kir4.1 but not Kir5.1 channels. Cell-surface biotinylation assays indicate significant plasma membrane expression of all mutant channels with exception of the non-sense mutant R199X. These results indicate the differential loss of Kir channel function among SeSAME syndrome mutations.     

Co-expression of human Kir3 subunits can yield channels with different functional properties

To date, no comprehensive study has been done on all combinations of the human homologues of the Kir3.0 channel family, and the human homologue of Kir3.3 has not yet been identified. To obtain support for the contention that most of the functional data on non-human Kir3.0 channels can be extrapolated to human channels, we have cloned the human homologues of the Kir3.0 family, including the yet unidentified human Kir3.3, and the human Kir4.1. The expression pattern of these channels in various human brain areas and peripheral tissues, analysed by Northern blot analysis, allows for the existence of various homomeric and heteromeric forms of human Kir3.0 channels. Expression studies of all possible combinations in Xenopus oocytes indicated that in homomeric Kir3.2c and heteromeric Kir3.1/3.2c channels mediate, in our studies, inward currents with largest amplitude of any other Kir3.0 channel combinations, followed by heteromeric Kir3.1/3.4 and homomeric Kir4.1 channels. Channel combinations which include Kir3.3 are detrimental to the formation of functional channels. The co-expression experiments with different Kir channel subunits indicate the selective formation of certain channel combinations, suggesting that channel specificity is not solely dependent on spatial and temporal regulation of Kir subunit expression.     

V-ATPase B1-subunit promoter drives expression of EGFP in intercalated cells of kidney, clear cells of epididymis and airway cells of lung in transgenic mice

Inward rectifier K⁺ channels (Kir) are a significant determinant of endothelial cell (EC) membrane potential, which plays an important role in endothelium-dependent vasodilatation. In the present study, several complementary strategies were applied to determine the Kir2 subunit composition of human aortic endothelial cells (HAECs). Expression levels of Kir2.1, Kir2.2, and Kir2.4 mRNA were similar, whereas Kir2.3 mRNA expression was significantly weaker. Western blot analysis showed clear Kir2.1 and Kir2.2 protein expression, but Kir2.3 protein was undetectable. Functional analysis of endothelial inward rectifier K⁺ current (I_K) demonstrated that 1) I_K current sensitivity to Ba²⁺ and pH were consistent with currents determined using Kir2.1 and Kir2.2 but not Kir2.3 and Kir2.4, and 2) unitary conductance distributions showed two prominent peaks corresponding to known unitary conductances of Kir2.1 and Kir2.2 channels with a ratio of 4:6. When HAECs were transfected with dominant-negative (dn)Kir2.x mutants, endogenous current was reduced 50% by dnKir2.1 and 85% by dnKir2.2, whereas no significant effect was observed with dnKir2.3 or dnKir2.4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K⁺ conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells. potassium channels; inward rectifier potassium channel     

Kir2.6 regulates the surface expression of Kir2.x inward rectifier potassium channels

Precise trafficking, localization, and activity of inward rectifier potassium Kir2 channels are important for shaping the electrical response of skeletal muscle. However, how coordinated trafficking occurs to target sites remains unclear. Kir2 channels are tetrameric assemblies of Kir2.x subunits. By immunocytochemistry we show that endogenous Kir2.1 and Kir2.2 are localized at the plasma membrane and T-tubules in rodent skeletal muscle. Recently, a new subunit, Kir2.6, present in human skeletal muscle, was identified as a gene in which mutations confer susceptibility to thyrotoxic hypokalemic periodic paralysis. Here we characterize the trafficking and interaction of wild type Kir2.6 with other Kir2.x in COS-1 cells and skeletal muscle in vivo. Immunocytochemical and electrophysiological data demonstrate that Kir2.6 is largely retained in the endoplasmic reticulum, despite high sequence identity with Kir2.2 and conserved endoplasmic reticulum and Golgi trafficking motifs shared with Kir2.1 and Kir2.2. We identify amino acids responsible for the trafficking differences of Kir2.6. Significantly, we show that Kir2.6 subunits can coassemble with Kir2.1 and Kir2.2 in vitro and in vivo. Notably, this interaction limits the surface expression of both Kir2.1 and Kir2.2. We provide evidence that Kir2.6 functions as a dominant negative, in which incorporation of Kir2.6 as a subunit in a Kir2 channel heterotetramer reduces the abundance of Kir2 channels on the plasma membrane.     

Zacopride selectively activates the Kir2.1 channel via a PKA signaling pathway in rat cardiomyocytes

We recently reported that zacopride is a selective inward rectifier potassium current (IK1 ) channel agonist, suppressing ventricular arrhythmias without affecting atrial arrhythmias. The present study aimed to investigate the unique pharmacological properties of zacopride. The whole-cell patch-clamp technique was used to study IK1 currents in rat atrial myocytes and Kir2.x currents in human embryonic kidney (HEK)-293 cells transfected with inward rectifier potassium channel (Kir)2.1, Kir2.2, Kir2.3, or mutated Kir2.1 (at phosphorylation site S425L). Western immunoblots were performed to estimate the relative protein expression levels of Kir2.x in rat atria and ventricles. Results showed that zacopride did not affect the IK1 and transmembrane potential of atrial myocytes. In HEK293 cells, zacopride increased Kir2.1 homomeric channels by 40.7%±9.7% at 50 mV, but did not affect Kir2.2 and Kir2.3 homomeric channels, and Kir2.1-Kir2.2, Kir2.1-Kir2.3 and Kir2.2-Kir2.3 heteromeric channels. Western immunoblots showed that similar levels of Kir2.3 protein were expressed in rat atria and ventricles, but atrial Kir2.1 protein level was only 25% of that measured in the ventricle. In addition, 5-hydroxytryptamine (5-HT) 3 receptor was undetectable, whereas 5-HT 4 receptor was weakly expressed in HEK293 cells. The Kir2.1-activating effect of zacopride in these cells was abolished by inhibition of protein kinase A (PKA), but not PKC or PKG. Furthermore, zacopride did not activate the mutant Kir2.1 channel in HEK293 cells but selectively activated the Kir2.1 homomeric channel via a PKA-dependent pathway, independent to that of the 5-HT receptor.     

Regulation of inward rectifier K+ channels by shift of intracellular pH dependence

The mechanistic link between mitochondrial metabolism and inward rectifier K+ channel activity was investigated by studying the effects of a mitochondrial inhibitor, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) on inward rectifiers of the Kir2 subfamily expressed in Xenopus oocytes, using two-electrode voltage-clamp, patch-clamp, and intracellular pH recording. FCCP inhibited Kir2.2 and Kir2.3 currents and decreased intracellular pH, but the pH change was too small to account for the inhibitory effect by itself. However, pre-incubation of oocytes with imidazole prevented both the pH decrease and the inhibition of Kir2.2 and Kir2.3 currents by FCCP. The pH dependence of Kir2.2 was shifted to higher pH in membrane patches from FCCP-treated oocytes compared to control oocytes. Therefore, the inhibition of Kir2.2 by FCCP may involve a combination of intracellular acidification and a shift in the intracellular pH dependence of these channels. To investigate the sensitivity of heteromeric channels to FCCP, we studied its effect on currents expressed by heteromeric tandem dimer constructs. While Kir2.1 homomeric channels were insensitive to FCCP, both Kir2.1-Kir2.2 and Kir2.1-Kir2.3 heterotetrameric channels were inhibited. These data support the notion that mitochondrial dysfunction causes inhibition of heteromeric inward rectifier K+ channels. The reduction of inward rectifier K+ channel activity observed in heart failure and ischemia may result from the mitochondrial dysfunction that occurs in these conditions.     

Localization and age-dependent expression of the inward rectifier K+ channel subunit Kir 5.1 in a mammalian reproductive system.

Kir 5.1 is a member of the inward rectifier potassium channel superfamily which does not form functional channels when expressed by itself in Xenopus laevis oocytes. rt-PCR reveals high levels of Kir 5.1 mRNA expression in testis but the function of this channel remains unknown. To determine the cell-specific expression of this channel in the testis we raised a polyclonal antibody against an external epitope of Kir 5.1 and tested its specificity in Xenopus oocytes expressing several cloned Kir subunits. Strong immunoreactivity for Kir 5.1 was found in seminiferous tubules of rat testis and, particularly, in spermatogonia, primary and secondary spermatocytes, spermatids and in the head and body of spermatozoa. The intensity of Kir 5.1 immunofluorescence, quantified using laser scanning microscopy, increased with age at every stage in the development of sperm from spermatogonia and reached a peak in 60-day-old rats. In contrast, the immunofluorescence decreased in 90-day-old animals and was detected mostly in spermatozoa. The results demonstrate that Kir 5.1 expression in the testis is localised to cells involved in spermatogenesis, showing a temporal pattern of expression during sexual maturity.     

Molecular Mechanism of Inward Rectifier Potassium Channel 2.3 Regulation by Tax-Interacting Protein-1

Inwardly rectifying potassium channel 2.3 (Kir2.3) is specifically targeted on the basolateral membranes of epithelial and neuronal cells, and it thus plays an important role in maintaining potassium homeostasis. Tax-interacting protein-1 (TIP-1), an atypical PDZ-domain-containing protein, binds to Kir2.3 with a high affinity, causing the intracellular accumulation of Kir2.3 in cultured epithelial cells. However, the molecular basis of the TIP-1/Kir2.3 interaction is still poorly understood. Here, we present the crystal structure of TIP-1 in complex with the C-terminal Kir2.3-peptide (residues 436-445) to reveal the molecular details of the interaction between them. Moreover, isothermal titration calorimetry experiments show that the C-terminal Kir2.3-peptide binds much more strongly to TIP-1 than to mammalian Lin-7, indicating that TIP-1 can compete with mammalian Lin-7 to uncouple Kir2.3 from its basolateral membrane anchoring complex. We further show that the phosphorylation/dephosphorylation of Ser443 within the C-terminal Kir2.3 PDZ-binding motif RRESAI dynamically regulates the Kir2.3/TIP-1 association in heterologous HEK293T cells. These data suggest that TIP-1 may act as an important regulator for the endocytic pathway of Kir2.3.     

Identification and pharmacological correction of a membrane trafficking defect associated with a mutation in the sulfonylurea receptor causing familial hyperinsulinism.

Persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is a genetic disorder characterized by excess secretion of insulin and hypoglycemia. In most patients, the disease is caused by mutations in sulfonylurea receptor-1 (SUR1), which, in association with Kir6.2, constitutes the functional ATP-sensitive potassium (K(ATP)) channel of the pancreatic beta-cell. Previous studies reported that coexpression of the PHHI mutant R1394H-SUR1 with Kir6.2 in COS cells produces no functional channels. To investigate if the loss of function could be due to impaired trafficking of mutant channels to the cell membrane, we have cotransfected wild-type and mutant SUR1 subunits with Kir6.2 into HEK293 cells and examined their cellular localization by immunofluorescent staining. Our results show that unlike the wild-type subunits, which showed fluorescence at the cell surface, the mutant subunits displayed fluorescence in punctate structures. Co-immunostaining with antibodies against organelle-specific marker proteins identified these structures as the trans-Golgi network. Limited localization in clathrin-positive, but transferrin receptor-negative vesicles was also observed. The post-endoplasmic reticulum localization suggests that the mutation does not impair the folding and assembly of the channels so as to cause its retention by the endoplasmic reticulum. Diazoxide, a K(ATP) channel opener drug that is used in the treatment of PHHI, restored the surface expression in a manner that could be prevented by the channel blocker glibenclamide. When expressed in Xenopus oocytes, R1394H-SUR1 formed functional channels with Kir6.2, indicating that the primary consequence of the mutation is impairment of trafficking rather than function. Thus, our data uncover a novel mechanism underlying the therapeutic action of diazoxide in the treatment of PHHI, i.e. its ability to recruit channels to the membrane. Furthermore, this is the first report to describe a trafficking disorder effecting retention of mutant proteins in the trans-Golgi network.     

Mutations in KCNJ13 cause autosomal-dominant snowflake vitreoretinal degeneration

Snowflake vitreoretinal degeneration (SVD, MIM 193230) is a developmental and progressive hereditary eye disorder that affects multiple tissues within the eye. Diagnostic features of SVD include fibrillar degeneration of the vitreous humor, early-onset cataract, minute crystalline deposits in the neurosensory retina, and retinal detachment. A genome-wide scan previously localized the genetic locus for SVD to a 20 Mb region flanked by D2S2158 and D2S2202. This region contains 59 genes, of which 20 were sequenced, disclosing a heterozygous mutation (484C > T, R162W) in KCNJ13, member 13 of subfamily J of the potassium inwardly rectifying channel family in all affected individuals. The mutation in KCNJ13, the gene encoding Kir7.1, was not present in unaffected family members and 210 control individuals. Kir7.1 localized to human retina and retinal pigment epithelium and was especially prevalent in the internal limiting membrane adjacent to the vitreous body. Molecular modeling of this mutation predicted disruption of the structure of the potassium channel in the closed state located immediately adjacent to the cell-membrane inner boundary. Functionally, unlike wild-type Kir7.1 whose overexpression in CHO-K1 cells line produces highly selective potassium current, overexpression of R162W mutant Kir7.1 produces a nonselective cation current that depolarizes transfected cells and increases their fragility. These results indicate that the KCNJ13 R162W mutation can cause SVD and further show that vitreoretinal degeneration can arise through mutations in genes whose products are not structural components of the vitreous.     

Requirement of CDC45 for Postimplantation Mouse Development

CDC45 is required for the initiation of DNA replication in Saccharomyces cerevisiae and functions as a DNA polymerase alpha loading factor in Xenopus, but its role in mammalian DNA replication is unknown. To investigate the genetic and physiological functions of CDC45, we used a gene targeting strategy to generate mice lacking a functional CDC45 gene. Homozygous mutant mice lacking a functional CDC45 gene underwent uterine implantation and induced uterine decidualization but did not develop substantially thereafter. Detailed analysis of CDC45 null embryos cultured in vitro revealed impaired proliferation of the inner cell mass. These findings make CDC45 the only putative replication factor experimentally proven to be essential for mammalian development. The CDC45 gene localizes to human chromosome 22q11.2 in the DiGeorge syndrome critical region (DGCR). Almost 90% of individuals with congenital cardiac and craniofacial defects have a monoallelic deletion in the DGCR that includes CDC45. We report here that heterozygous mutant mice develop into adulthood without any apparent abnormalities, so that it is unlikely that hemizygosity of CDC45 alone is responsible for the cardiac and craniofacial defects in the congenital syndromes.     

A recombinant inwardly rectifying potassium channel coupled to GTP- binding proteins

GTP-binding (G) proteins have been shown to mediate activation of inwardly rectifying potassium (K+) channels in cardiac, neuronal and neuroendocrine cells. Here, we report functional expression of a recombinant inwardly rectifying channel which we call KGP (or hpKir3.4), to signify that it is K+ selective, G-protein-gated and isolated from human pancreas. KGP expression in Xenopus oocytes resulted in sizeable basal (or agonist-independent) currents while coexpression with a G-protein-linked receptor, yielded additional agonist-induced currents. Coexpression of KGP and hGIRK1 (a human brain homolog of GIRK1/Kir3.1) produced much larger basal currents than those observed with KGP or hGIRK1 alone, and upon coexpression with receptor, similarly large agonist-induced currents could be obtained. Pertussis toxin treatment significantly diminished agonist-dependent currents due to either KGP or KGP/hGIRK1 expression. Interestingly, PTX also significantly reduced basal KGP or KGP/hGIRK1 currents, suggesting that basal activity is largely the result of G-protein gating as well. When the two channels were coexpressed with receptor, the relative increase in current elicited by agonist was similar whether KGP and hGIRK1 were expressed alone or together. When in vitro translated or when expressed in Xenopus oocytes or CHO mammalian cells, KGP gave rise to a nonglycosylated 45-kD protein. Antibodies directed against either KGP or hGIRK1 coprecipitated both proteins coexpressed in oocytes, providing evidence for the heteromeric assembly of the two channels and suggesting that the current potentiation seen with coexpression of the two channel subunits is due to specific interactions between them. An endogenous oocyte protein similar in size to KGP was also coprecipitated with hGIRK1.     

Nuclear localization and dominant-negative suppression by a mutant SKCa3 N-terminal channel fragment identified in a patient with schizophrenia

The small conductance calcium-activated K+ channel gene SKCa3/KCNN3 maps to 1q21, a region strongly linked to schizophrenia. Recently, a 4-base pair deletion in SKCa3 was reported in a patient with schizophrenia, which truncates the protein at the end of the N-terminal cytoplasmic region (SKCa3Delta). We generated a green fluorescent protein-SKCa3 N-terminal construct (SKCa3-1/285) that is identical to SKCa3Delta except for the last two residues. Using confocal microscopy we demonstrate that SKCa3-1/285 localizes rapidly and exclusively to the nucleus of mammalian cells like several other pathogenic polyglutamine-containing proteins. This nuclear targeting is mediated in part by two polybasic sequences present at the C-terminal end of SKCa3-1/285. In contrast, full-length SKCa3, SKCa2, and IKCa1 polypeptides are all excluded from the nucleus and express as functional channels. When overexpressed in human Jurkat T cells, SKCa3-1/285 can suppress endogenous SKCa2 currents but not voltage-gated K+ currents. This dominant-negative suppression is most likely mediated through the co-assembly of SKCa3-1/285 with native subunits and the formation of non-functional tetramers. The nuclear localization of SKCa3-1/285 may alter neuronal architecture, and its ability to dominantly suppress endogenous small conductance K(Ca) currents may affect patterns of neuronal firing. Together, these two effects may play a part in the pathogenesis of schizophrenia and other neuropsychiatric disorders.     

Identification and preliminary function study of Xenopus laevis DRR1 gene

Xenopus laevis has recently been determined as a novel study platform of gene function. In this study, we cloned Xenopus DRR1 (xDRR1), which is homologous to human down-regulated in renal carcinoma (DRR1) gene. Bioinformatics analysis for DRR1 indicated that xDRR1 shared 74% identity with human DRR1 and 66% with mouse DRR1, and the phlogenetic tree of DRR1 protein was summarized. The xDRR1 gene locates in nuclei determined by transfecting A549 cells with the recombinant plasmid pEGFP-N1/xDRR1. RT-PCR analysis revealed that xDRR1 gene was expressed in all stages of early embryo development and all kinds of detected tissues, and whole-mount in situ hybridization showed xDRR1 was mainly present along ectoderm and mesoderm. Furthermore, xDRR1 expression could suppress A549 cell growth by transfecting with plasmid pcDNA3.1(+)/xDRR1. xDRR1 probably plays important roles involving in cell growth regulation and Xenopus embryo development.     

Wnt11-R, a protein closely related to mammalian Wnt11, is required for heart morphogenesis in Xenopus

Wnt11 is a secreted protein that signals through the non-canonical planar cell polarity pathway and is a potent modulator of cell behavior and movement. In human, mouse, and chicken, there is a single Wnt11 gene, but in zebrafish and Xenopus, there are two genes related to Wnt11. The originally characterized Xenopus Wnt11 gene is expressed during early embryonic development and has a critical role in regulation of gastrulation movements. We have identified a second Xenopus Wnt11-Related gene (Wnt11-R) that is expressed after gastrulation. Sequence comparison suggests that Xenopus Wnt11-R, not Wnt11, is the ortholog of mammalian and chicken Wnt11. Xenopus Wnt11-R is expressed in neural tissue, dorsal mesenchyme derived from the dermatome region of the somites, the brachial arches, and the muscle layer of the heart, similar to the expression patterns reported for mouse and chicken Wnt11. Xenopus Wnt11-R exhibits biological properties similar to those previously described for Xenopus Wnt11, in particular the ability to activate Jun-N-terminal kinase (JNK) and to induce myocardial marker expression in ventral marginal zone (VMZ) explants. Morpholino inhibition experiments demonstrate, however, that Wnt11-R is not required for cardiac differentiation, but functions in regulation of cardiac morphogenesis. Embryos with reduced Wnt11-R activity exhibit aberrant cell-cell contacts within the myocardial wall and defects in fusion of the nascent heart tube.