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Walter Birchmeier

Publications and source records attributed to Walter Birchmeier.

At least 19 recordsLinked to original sources
PubMed2004

The neuronal scaffold protein Shank3 mediates signaling and biological function of the receptor tyrosine kinase Ret in epithelial cells.

Shank proteins, initially also described as ProSAP proteins, are scaffolding adaptors that have been previously shown to integrate neurotransmitter receptors into the cortical cytoskeleton at postsynaptic densities. We show here that Shank proteins are also crucial in receptor tyrosine kinase signaling. The PDZ domain-containing Shank3 protein was found to represent a novel interaction partner of the receptor tyrosine kinase Ret, which binds specifically to a PDZ-binding motif present in the Ret9 but not in the Ret51 isoform. Furthermore, we show that Ret9 but not Ret51 induces epithelial cells to form branched tubular structures in three-dimensional cultures in a Shank3-dependent manner. Ret9 but not Ret51 has been previously shown to be required for kidney development. Shank3 protein mediates sustained Erk-MAPK and PI3K signaling, which is crucial for tubule formation, through recruitment of the adaptor protein Grb2. These results demonstrate that the Shank3 adaptor protein can mediate cellular signaling, and provide a molecular mechanism for the biological divergence between the Ret9 and Ret51 isoform.

Adaptor Proteins, Signal Transducing↗
PubMed2004

Mutations in the desmosomal protein plakophilin-2 are common in arrhythmogenic right ventricular cardiomyopathy.

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is associated with fibrofatty replacement of cardiac myocytes, ventricular tachyarrhythmias and sudden cardiac death. In 32 of 120 unrelated individuals with ARVC, we identified heterozygous mutations in PKP2, which encodes plakophilin-2, an essential armadillo-repeat protein of the cardiac desmosome. In two kindreds with ARVC, disease was incompletely penetrant in most carriers of PKP2 mutations.

Adolescent↗
PubMed2004

Requirement of plakophilin 2 for heart morphogenesis and cardiac junction formation.

Plakophilins are proteins of the armadillo family that function in embryonic development and in the adult, and when mutated can cause disease. We have ablated the plakophilin 2 gene in mice. The resulting mutant mice exhibit lethal alterations in heart morphogenesis and stability at mid-gestation (E10.5-E11), characterized by reduced trabeculation, disarrayed cytoskeleton, ruptures of cardiac walls, and blood leakage into the pericardiac cavity. In the absence of plakophilin 2, the cytoskeletal linker protein desmoplakin dissociates from the plaques of the adhering junctions that connect the cardiomyocytes and forms granular aggregates in the cytoplasm. By contrast, embryonic epithelia show normal junctions. Thus, we conclude that plakophilin 2 is important for the assembly of junctional proteins and represents an essential morphogenic factor and architectural component of the heart.

Alleles↗
PubMed2004

Essential role of BCL9-2 in the switch between beta-catenin's adhesive and transcriptional functions.

beta-Catenin controls both cadherin-mediated cell adhesion and activation of Wnt target genes. We demonstrate here that the beta-catenin-binding protein BCL9-2, a homolog of the human proto-oncogene product BCL9, induces epithelial-mesenchymal transitions of nontransformed cells and increases beta-catenin-dependent transcription. RNA interference of BCL9-2 in carcinoma cells induces an epithelial phenotype and translocates beta-catenin from the nucleus to the cell membrane. The switch between beta-catenin's adhesive and transcriptional functions is modulated by phosphorylation of Tyr 142 of beta-catenin, which favors BCL9-2 binding and precludes interaction with alpha-catenin. During zebrafish embryogenesis, BCL9-2 acts in the Wnt8-signaling pathway and regulates mesoderm patterning.

Animals↗
PubMed2004

Mutations and elevated transcriptional activity of conductin (AXIN2) in hepatoblastomas.

Hepatoblastoma (HB) is the most frequent malignant liver tumour of childhood. Most HBs develop sporadically but their incidence is highly elevated in patients with familial adenomatous polyposis coli (FAP). These patients carry germline mutations in the adenomatous polyposis coli (APC) tumour suppressor gene. APC forms a multi-protein complex involved in the WNT signalling pathway that controls the stability of beta-catenin, the central effector in this cascade. Whereas APC mutations are rare in sporadic HBs, a high frequency of beta-catenin mutations leading to overactivation of WNT signalling was previously found in these tumours. This pathway is negatively controlled by conductin (axin2), representing a further partner in this signalling complex. To investigate whether alterations in conductin may also be involved in the pathogenesis of sporadic HBs, 37 HBs and five HB cell lines were screened for mutations using single-strand conformation polymorphism (SSCP) analysis, reverse transcription-polymerase chain reaction (RT-PCR), and direct sequencing. In two cases, larger deletions (52 and 1624 bp) leading to frameshifts were found. In addition, one HB carried a somatic point mutation. Expression analysis by competitive RT-PCR in HBs revealed up-regulation of conductin mRNA compared with adjacent liver samples. This mRNA overexpression resulted in increased conductin protein levels demonstrated by western blot analysis. Tumours with activating beta-catenin mutations revealed higher levels of conductin mRNA transcripts. This finding indicates that conductin is a direct target gene of WNT signalling in HBs, as has been demonstrated in other tissues. In summary, conductin mutations may represent an alternative mechanism leading to activation of WNT signalling in HBs. The overexpression of conductin mRNA in HBs reflects activation of the WNT pathway because conductin represents a target gene of WNT signalling in liver tissue.

Axin Protein↗
PubMed2004

Making tubes: step by step.

Branched hollow tubes form the architectural basis of many mammalian organs. The growth factor HGF/SF and its receptor, the Met receptor tyrosine kinase, stimulate epithelial cells to undergo tubulogenesis in vitro. In this issue of Developmental Cell, O'Brien et al. (2004) look at temporal regulation and the role of two HGF/SF effectors, the ERK 1/2 MAP kinases and matrix metalloproteases, in this process.

Animals↗
PubMed2004

The docking protein Gab1 is an essential component of an indirect mechanism for fibroblast growth factor stimulation of the phosphatidylinositol 3-kinase/Akt antiapoptotic pathway.

The docking protein Gab1 has been implicated as a mediator of multiple signaling pathways that are activated by a variety of receptor tyrosine kinases and cytokines. We have previously proposed that fibroblast growth factor 1 (FGF1) stimulation of tyrosine phosphorylation of Gab1 and recruitment of phosphatidylinositol (PI) 3-kinase are mediated by an indirect mechanism in which the docking protein fibroblast receptor substrate 2alpha (FRS2alpha) plays a critical role. In this report, we explore the role of Gab1 in FGF1 signaling by using mouse embryo fibroblasts (MEFs) derived from Gab1(-/-) or FRS2alpha(-/-) mice. We demonstrate that Gab1 is essential for FGF1 stimulation of both PI 3-kinase and the antiapoptotic protein kinase Akt, while FGF1-induced mitogen-activated protein kinase (MAPK) stimulation is not affected by Gab1 deficiency. To test the indirect mechanism for FGF1 stimulation of PI 3-kinase and Akt, we use a chimeric docking protein composed of the membrane targeting signal and the phosphotyrosine-binding domain of FRS2alpha fused to the C-terminal portion of Gab1, the region including the binding sites for the complement of signaling proteins that are recruited by Gab1. We demonstrate that expression of the chimeric docking protein in Gab1(-/-) MEFs rescues PI 3-kinase and the Akt responses, while expression of the chimeric docking protein in FRS2alpha(-/-) MEFs rescues stimulation of both Akt and MAPK. These experiments underscore the essential role of Gab1 in FGF1 stimulation of the PI 3-kinase/Akt signaling pathway and provide further support for the indirect mechanism for FGF1 stimulation of PI 3-kinase involving regulated assembly of a multiprotein complex.

Adaptor Proteins, Signal Transducing↗
PubMed2004

Rap1 regulates the formation of E-cadherin-based cell-cell contacts.

In epithelial tissues, cells are linked to their neighbors through specialized cell-cell adhesion proteins. E-cadherin is one of the most important membrane proteins for the establishment of intimate cell-cell contacts, but the molecular mechanism by which it is recruited to contact sites is largely unknown. We report here that the cytoplasmic domain of E-cadherin interacts with C3G, a guanine nucleotide exchange factor for Rap1. In epithelial cell cultures, ligation of the extracellular domain of E-cadherin enhances Rap1 activity, which in turn is necessary for the proper targeting of E-cadherin molecules to maturing cell-cell contacts. Furthermore, our data suggest that Cdc42 functions downstream of Rap1 in this process. We conclude that Rap1 plays a vital role in the establishment of E-cadherin-based cell-cell adhesion.

Animals↗
PubMed2003

Role of beta-catenin in synaptic vesicle localization and presynaptic assembly.

Cadherins and catenins are thought to promote adhesion between pre and postsynaptic elements in the brain. Here we show a role for beta-catenin in localizing the reserved pool of vesicles at presynaptic sites. Deletion of beta-catenin in hippocampal pyramidal neurons in vivo resulted in a reduction in the number of reserved pool vesicles per synapse and an impaired response to prolonged repetitive stimulation. This corresponded to a dispersion of vesicles along the axon in cultured neurons. Interestingly, these effects are not due to beta-catenin's involvement in cadherin-mediated adhesion or wnt signaling. Instead, beta-catenin modulates vesicle localization via its PDZ binding domain to recruit PDZ proteins such as Veli to cadherin at synapses. This study defines a specific role for cadherins and catenins in synapse organization beyond their roles in mediating cell adhesion.

Animals↗
PubMed2003

Deletion of beta-catenin impairs T cell development.

T cells encounter two main checkpoints during development in the thymus. These checkpoints are critically dependent on signals derived from the thymic microenvironment as well as from the pre-T cell receptor (pre-TCR) and the alphabeta TCR. Here we show that T cell-specific deletion of beta-catenin impaired T cell development at the beta-selection checkpoint, leading to a substantial decrease in splenic T cells. In addition, beta-catenin also seemed to be a target of TCR-CD3 signals in thymocytes and mature T cells. These data indicate that beta-catenin-mediated signals are required for normal T cell development.

Animals↗
PubMed2003

Genetic interaction between Wnt/beta-catenin and BMP receptor signaling during formation of the AER and the dorsal-ventral axis in the limb.

By conditional gene ablation in mice, we found that beta-catenin, an essential downstream effector of canonical Wnt signaling, is a key regulator of formation of the apical ectodermal ridge (AER) and of the dorsal-ventral axis of the limbs. By generation of compound mutants, we also show that beta-catenin acts downstream of the BMP receptor IA in AER induction, but upstream or parallel in dorsal-ventral patterning. Thus, AER formation and dorsal-ventral patterning of limbs are tightly controlled by an intricate interplay between Wnt/beta-catenin and BMP receptor signaling.

Animals↗
PubMed2003

beta-Catenin is required for specification of proximal/distal cell fate during lung morphogenesis.

The lungs are divided, both structurally and functionally, into two distinct components, the proximal airways, which conduct air, and the peripheral airways, which mediate gas exchange. The mechanisms that control the specification of these two structures during lung development are currently unknown. Here we show that beta-catenin signaling is required for the formation of the distal, but not the proximal, airways. When the gene for beta-catenin was conditionally excised in epithelial cells of the developing mouse lung prior to embryonic day 14.5, the proximal lung tubules grew and differentiated appropriately. The mice, however, died at birth because of respiratory failure. Analysis of the lungs by in situ hybridization and immunohistochemistry, using molecular markers of the epithelial and mesenchymal components of both proximal and peripheral airways, showed that the lungs were composed primarily of proximal airways. These observations establish, for the first time, both the sites and timing of specification of the proximal and peripheral airways in the developing lung, and that beta-catenin is one of the essential components of this specification.

Animals↗
PubMed2003

beta-Catenin signals regulate cell growth and the balance between progenitor cell expansion and differentiation in the nervous system.

beta-Catenin is an essential component of the canonical Wnt signaling system that controls decisive steps in development. We employed here two conditional beta-catenin mutant alleles to alter beta-catenin signaling in the central nervous system of mice: one allele to ablate beta-catenin and the second allele to express a constitutively active beta-catenin. The tissue mass of the spinal cord and brain is reduced after ablation of beta-catenin, and the neuronal precursor population is not maintained. In contrast, the spinal cord and brain of mice that express activated beta-catenin is much enlarged in mass, and the neuronal precursor population is increased in size. beta-Catenin signals are thus essential for the maintenance of proliferation of neuronal progenitors, controlling the size of the progenitor pool, and impinging on the decision of neuronal progenitors to proliferate or to differentiate.

Alleles↗
PubMed2003

Helicobacter pylori CagA protein targets the c-Met receptor and enhances the motogenic response.

Infection with the human microbial pathogen Helicobacter pylori is assumed to lead to invasive gastric cancer. We find that H. pylori activates the hepatocyte growth factor/scatter factor receptor c-Met, which is involved in invasive growth of tumor cells. The H. pylori effector protein CagA intracellularly targets the c-Met receptor and promotes cellular processes leading to a forceful motogenic response. CagA could represent a bacterial adaptor protein that associates with phospholipase Cgamma but not Grb2-associated binder 1 or growth factor receptor-bound protein 2. The H. pylori-induced motogenic response is suppressed and blocked by the inhibition of PLCgamma and of MAPK, respectively. Thus, upon translocation, CagA modulates cellular functions by deregulating c-Met receptor signaling. The activation of the motogenic response in H. pylori-infected epithelial cells suggests that CagA could be involved in tumor progression.

Adaptor Proteins, Signal Transducing↗
PubMed2003

How to make tubes: signaling by the Met receptor tyrosine kinase.

Hepatocyte growth factor/scatter factor (HGF/SF), acting through the receptor tyrosine kinase Met, stimulates cells derived from a variety of different organs to form elongated hollow tubules when grown in three-dimensional gels. In vivo data also indicate a role for HGF/SF and Met in tubule formation during liver and kidney regeneration and mammary gland formation. Activation of Met results in the recruitment of a myriad of signal transducers that regulate dissociation of adherens junctions and the stimulation of cellular motility, survival, proliferation and morphogenesis during tubule formation. Among these many signal transducers, the Gab1 adaptor protein and its effector, the SHP2 tyrosine phosphatase, have been found to be crucial for tubulogenesis and for the sustained stimulation of the ERK/MAP kinase pathway. Here, we discuss the contribution of these and other signaling pathways and the role of HGF/SF and Met in the formation of epithelial cell tubules both in vitro in branching-morphogenesis assays and in vivo during organogenesis.

Animals↗
PubMed2003

Beta-catenin regulates Cripto- and Wnt3-dependent gene expression programs in mouse axis and mesoderm formation.

Gene expression profiling of beta-catenin, Cripto and Wnt3 mutant mouse embryos has been used to characterise the genetic networks that regulate early embryonic development. We have defined genes whose expression is regulated by beta-catenin during formation of the anteroposterior axis and the mesoderm, and have identified Cripto, which encodes a Nodal co-receptor, as a primary target of beta-catenin signals both in embryogenesis as well as in colon carcinoma cell lines and tissues. We have also defined groups of genes regulated by Wnt3/beta-catenin signalling during primitive streak and mesoderm formation. Our data assign a key role to beta-catenin upstream of two distinct gene expression programs during anteroposterior axis and mesoderm formation.

Animals↗
PubMed2002

The ankyrin repeat protein Diversin recruits Casein kinase Iepsilon to the beta-catenin degradation complex and acts in both canonical Wnt and Wnt/JNK signaling.

Wnt signals control decisive steps in development and can induce the formation of tumors. Canonical Wnt signals control the formation of the embryonic axis, and are mediated by stabilization and interaction of beta-catenin with Lef/Tcf transcription factors. An alternative branch of the Wnt pathway uses JNK to establish planar cell polarity in Drosophila and gastrulation movements in vertebrates. We describe here the vertebrate protein Diversin that interacts with two components of the canonical Wnt pathway, Casein kinase Iepsilon (CKIepsilon) and Axin/Conductin. Diversin recruits CKIepsilon to the beta-catenin degradation complex that consists of Axin/Conductin and GSK3beta and allows efficient phosphorylation of beta-catenin, thereby inhibiting beta-catenin/Tcf signals. Morpholino-based gene ablation in zebrafish shows that Diversin is crucial for axis formation, which depends on beta-catenin signaling. Diversin is also involved in JNK activation and gastrulation movements in zebrafish. Diversin is distantly related to Diego of Drosophila, which functions only in the pathway that controls planar cell polarity. Our data show that Diversin is an essential component of the Wnt-signaling pathway and acts as a molecular switch, which suppresses Wnt signals mediated by the canonical beta-catenin pathway and stimulates signaling via JNK.

Animals↗