GMPPA defects cause a neuromuscular disorder with α-dystroglycan hyperglycosylation
Patricia Franzka, Henriette Henze, M. Juliane Jung, Svenja Caren Schüler, Sonnhild Mittag, Karina Biskup, Lutz Liebmann, Takfarinas Kentache, José Morales, Braulio Martínez, Istvan Katona, Tanja Herrmann, Antje-Kathrin Huebner, J. Christopher Hennings, Susann Groth, Lennart Gresing, Rüdiger Horstkorte, Thorsten Marquardt, Joachim Weis, Christoph Kaether, Osvaldo M. Mutchinick, Alessandro Ori, Otmar Huber, Véronique Blanchard, Julia von Maltzahn, Christian A. Hübner
Patricia Franzka, Henriette Henze, M. Juliane Jung, Svenja Caren Schüler, Sonnhild Mittag, Karina Biskup, Lutz Liebmann, Takfarinas Kentache, José Morales, Braulio Martínez, Istvan Katona, Tanja Herrmann, Antje-Kathrin Huebner, J. Christopher Hennings, Susann Groth, Lennart Gresing, Rüdiger Horstkorte, Thorsten Marquardt, Joachim Weis, Christoph Kaether, Osvaldo M. Mutchinick, Alessandro Ori, Otmar Huber, Véronique Blanchard, Julia von Maltzahn, Christian A. Hübner
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Research Article Muscle biology

GMPPA defects cause a neuromuscular disorder with α-dystroglycan hyperglycosylation

Abstract

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 α-dystroglycan (α-DG). Alpha-DG 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 α-DG in mice and humans. This increased α-DG turnover and thereby lowered α-DG abundance. In mice, dietary mannose restriction beginning after weaning corrected α-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 α-DG hyperglycosylation, to our knowledge, and we have unraveled underlying disease mechanisms and identified potential dietary treatment options.

Authors

Patricia Franzka, Henriette Henze, M. Juliane Jung, Svenja Caren Schüler, Sonnhild Mittag, Karina Biskup, Lutz Liebmann, Takfarinas Kentache, José Morales, Braulio Martínez, Istvan Katona, Tanja Herrmann, Antje-Kathrin Huebner, J. Christopher Hennings, Susann Groth, Lennart Gresing, Rüdiger Horstkorte, Thorsten Marquardt, Joachim Weis, Christoph Kaether, Osvaldo M. Mutchinick, Alessandro Ori, Otmar Huber, Véronique Blanchard, Julia von Maltzahn, Christian A. Hübner

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Figure 6

GMPPA knockdown in myoblasts increases α-DG turnover and causes myotube degeneration.

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GMPPA knockdown in myoblasts increases α-DG turnover and causes myotube ...
(A) Immunoblot analyses confirmed efficient knockdown of either dystroglycan (siDag1) or GMPPA. Compared with controls (siScr), α-DG stability was decreased in the presence of cycloheximide (CHX) upon GMPPA knockdown. Ubiquitin served as a control for efficient CHX treatment. GAPDH served as loading control (n = 3 experiments; 1-way ANOVA with Bonferroni post hoc test). (B) GMPPA knockdown did not affect the differentiation of primary myoblasts (n = 3 experiments; 1-way ANOVA with Bonferroni post hoc test). Experimental design, representative images of primary mouse myoblasts stained for myogenin (MYOG) or myosin heavy chain (MYHC), and quantification of the fusion index. (C) Knockdown of GMPPA did not affect the differentiation of the myoblast cell line C2C12 (n = 3 experiments; 1-way ANOVA with Bonferroni post hoc test). Experimental design, representative images stained for MYOG and MYHC, and quantification of the myotube diameter. (D) The size of C2C12-derived myotubes was altered upon knockdown of GMPPA (n = 3 experiments; 1-way ANOVA with Bonferroni post hoc test). Experimental design, representative images of differentiated C2C12 cells, and quantification of myotube diameter. Nuclei are stained in blue. Scale bars (B–D): 70 μm. Quantitative data are presented as mean ± SEM with individual data points. *P < 0.05; **P < 0.005; ***P < 0.0005.