GMPPA defects cause a neuromuscular disorder with α-dystroglycan hyperglycosylation
Patricia Franzka, … , Julia von Maltzahn, Christian A. Hübner
Patricia Franzka, … , Julia von Maltzahn, Christian A. Hübner
Published March 23, 2021
Citation Information: J Clin Invest. 2021;131(9):e139076. https://doi.org/10.1172/JCI139076.
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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 4

GMPPA is an allosteric inhibitor of GMPPB.

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GMPPA is an allosteric inhibitor of GMPPB. (A) GDP-mannose was increased...
(A) GDP-mannose was increased in skeletal muscles of 8-month-old KO mice (n = 3 mice per group; unpaired 2-tailed Student’s t test). (B) Overexpressed GMPPA-Myc and GMPPB-FLAG coprecipitated. (C) Upon heterologous expression with GMPPB, the N-terminal GMPPA variant G182D was detected but not the C-terminal variant T334P. (D) GMPPA devoid of the C-terminal aa 206-420 (ΔCT) did not interact with GMPPB; GMPPA devoid of the N-terminal aa 1-205 (ΔNT) still coprecipitated with GMPPB. (E) Pulldown of recombinant GST-GMPPA WT, -GMPPA N182D, -GMPPA T334P, and MBP-GMPPB. (F) Pulldown of recombinant MBP-GMPPA deletion constructs and GST-GMPPB. (G) Red signals indicate proximity of GMPPA and GMPPB of 40nm or less in WT but not in KO skeletal muscle in proximity ligation assays (PLAs). Scale bars: 5 μm. Additional controls are shown in Supplemental Figure 3. (H) Recombinant purified MBP-GMPPB showed enzymatic activity, which was inhibited by an equal amount of recombinant GST-GMPPA in the presence of 80 μM GDP-mannose (n = 3 experiments; 1-way ANOVA with Bonferroni post hoc test). (I) Inhibition of GMPPB activity by GMPPA WT increased with higher GDP-mannose concentrations (n = 3 experiments; 2-way ANOVA with Bonferroni post hoc test). Controls are shown in Supplemental Figure 3. (J) The enzymatic activity of recombinant GMPPB (20 ng/μL) was inhibited by increasing amounts of recombinant GMPPA WT but not by disease-associated variants N182D and T334P (n = 3 experiments; 2-way ANOVA with Bonferroni post hoc test). (K) The enzymatic activity of recombinant GMPPB was inhibited in a GDP-mannose–dependent manner in the presence of recombinant GMPPA WT, but not in the presence of GMPPA N182D and T334P (n = 3 experiments; 2-way ANOVA with Bonferroni post hoc test). Quantitative data are presented as mean ± SEM with individual data points. *P < 0.05; **P < 0.005; ***P < 0.0005.