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GDP-mannose-pyrophosphorylase-B (GMPPB) facilitates the generation of GDP-mannose, a sugar donor required for glycosylation. GMPPB defects cause muscle disease due to hypoglycosylation of [alpha]-dystroglycan ([alpha]-DG). AlphaDG is part of a protein complex, which links the extracellular matrix with the cytoskeleton, thus stabilizing myofibers. Mutations of the catalytically inactive homolog GMPPA cause alacrima, achalasia, and mental retardation syndrome (AAMR syndrome), which also involves muscle weakness. Here, we showed that Gmppa-KO mice recapitulated cognitive and motor deficits. As structural correlates, we found cortical layering defects, progressive neuron loss, and myopathic alterations. Increased GDP-mannose levels in skeletal muscle and in vitro assays identified GMPPA as an allosteric feedback inhibitor of GMPPB. Thus, its disruption enhanced mannose incorporation into glycoproteins, including [alpha]-DG in mice and humans. This increased [alpha]-DG turnover and thereby lowered [alpha]-DG abundance. In mice, dietary mannose restriction beginning after weaning corrected [alpha]-DG hyperglycosylation and abundance, normalized skeletal muscle morphology, and prevented neuron degeneration and the development of motor deficits. Cortical layering and cognitive performance, however, were not improved. We thus identified GMPPA defects as the first congenital disorder of glycosylation characterized by [alpha]-DG hyperglycosylation, to our knowledge, and we have unraveled underlying disease mechanisms and identified potential dietary treatment options., Introduction Cell surface proteins are important for the attachment of a cell to the extracellular matrix (ECM). They are modified by post-translational covalent attachment of branched carbohydrate oligomers. N-linked oligosaccharides [...]

The Fra3B locus on chromosome 3p14.2 targeting the fragile histidine triad (Fhit) gene represents one of the most common fragile sites of the human genome and is associated with early preneoplastic and malignant disorders in multiple human tumors. Fhit was classified as a tumor suppressor; however, the molecular mechanisms of its function are not well established. Here, we report that Fhit associates with the lymphoid enhancer-binding factor 1/T cell factor/[beta]-catenin complex by directly binding to [beta]-catenin, a major player in the canonical Wnt pathway that is deregulated in numerous forms of human cancer. In binding to the [beta]-catenin C-terminal domain, Fhit represses transcription of target genes such as cyclin D1, axin2, MMP-14, and survivin. Knockdown of Fhit reversed this effect, whereas this reversal was not detectable when [beta]-catenin was knocked down simultaneously. The Fhit enzymatic activity as a diadenosine-polyphosphate hydrolase is not required for the down-regulation of [beta]-catenin-mediated transcription as examined with an enzymatic inactive Fhit-H96N protein. ChIPs revealed recruitment of Fhit/[beta]-catenin complexes to target gene promoters. In soft agar assays Fhit and [beta]-catenin are involved in regulation of anchorage-independent growth. These observations assign to the tumor suppressor Fhit an unexpected role in the regulation of [beta]-catenin-mediated gene transcription. histidine triad | T cell factor | lymphoid enhancer-binding factor 1

Author(s): Jung Hwa Kim [1, 6]; Bogyou Kim [1, 6]; Ling Cai [2, 6]; Hee June Choi [1]; Kenneth A. Ohgi [2]; Chris Tran [3]; Charlie Chen [3]; Chin Ha [...]

[Beta]-catenin, the vertebrate homolog of the Drosophila Armadillo protein, has been shown to have dual cellular functions, as a component of both the cadherincatenin cell adhesion complex and the Wnt signaling pathway. At Wnt signaling, [Beta]-catenin becomes stabilized in the cytoplasm and subsequently available for interaction with transcription factors of the lymphocyte enhancer factor-1/T-cell factor family, resulting in a nuclear localization of [Beta]-catenin. Although [Beta]-catenin does not bind DNA directly, its carboxyland amino-terminal regions exhibit a transactivating activity still not well understood molecularly. Here we report the identification of an interaction partner of [Beta]-catenin, a nuclear protein designated Pontin52. Pontin52 binds [Beta]-catenin in the region of Armadillo repeats 2-5 and, more importantly, also binds the TATA box binding protein. We provide evidence for an in vivo multiprotein complex composed of Pontin52, [Beta]-catenin, and lymphocyte enhancer factor-1/T-cell factor. Our results suggest involvement of Pontin52 in the nuclear function of [Beta]-catenin.

Catenins are proteins associated with the cytoplasmic domain of cadherins, a family of transmembrane cell adhesion molecules. The cadherin-catenin adhesion system is involved in morphogenesis during development and in the maintenance of the integrity of different tissue types. Using a gene trap strategy, we have isolated a mouse mutation for the gene encoding the [Alpha]-E-catenin. This form of the [Alpha]-catenin appears frequently coexpressed with E-cadherin in epithelial cell types. The mutation obtained eliminates the carboxyl-terminal third of the protein but nevertheless provokes a complete loss-of-function phenotype. Homozygous mutants show disruption of the trophoblast epithelium (the first differentiated embryonic tissue), and development is consequently blocked at the blastocyst stage. This phenotype parallels the defects observed in E-cadherin mutant embryos. Our results show the requirement of the [Alpha]-E-catenin carboxy terminus for its function and represent evidence of the role of the [Alpha]-E-catenin in vivo, identifying this molecule as the natural partner of the E-cadherin in trophoblast epithelium.

Plakoglobin is the only component common to both the desmosomal plaque and the cadherin-catenin cell adhesion complex in the adherens junction. It is highly homologous to vertebrate [Beta]-catenin and to Drosophila armadillo protein and may - like these proteins - be also involved in signaling pathways. To analyze the role of plakoglobin during mouse development we inactivated the plakoglobin gene by homologous recombination in embryonic stem cells and generated transgenic mice. Plakoglobin null-mutant embryos died from Embryonic Day 10.5 onward, due to severe heart defects. Some mutant embryos developed further, especially on a C57BL/6 genetic background, and died around birth, presumably due to cardiac dysfunction, and with skin blistering and subcorneal acantholysis. Ultrastructural analysis revealed that here desmosomes were greatly reduced in number and structurally altered. Thus, using reversed genetics we demonstrate that plakoglobin is an essential structural component for desmosome function. The skin phenotype in plakoglobin-deficient mice is reminiscent of the human blistering disease, epidermolytic hyperkeratosis.