The effect of the pathological V72I, D109N and T190M missense mutations on the molecular structure of α-dystroglycan

Protective role for the N-terminal domain of α-dystroglycan in Influenza A virus proliferation

α-Dystroglycan (α-DG) is a highly glycosylated basement membrane receptor that is cleaved by the proprotein convertase furin, which releases its N-terminal domain (α-DGN). Before cleavage, α-DGN interacts with the glycosyltransferase LARGE1 and initiates functional O-glycosylation of the mucin-like domain of α-DG. Notably, α-DGN has been detected in a wide variety of human bodily fluids, but the physiological significance of secreted α-DGN remains unknown. Here, we show that mice lacking α-DGN exhibit significantly higher viral titers in the lungs after Influenza A virus (IAV) infection (strain A/Puerto Rico/8/1934 H1N1), suggesting an inability to control virus load. Consistent with this, overexpression of α-DGN before infection or intranasal treatment with recombinant α-DGN prior and during infection, significantly reduced IAV titers in the lungs of wild-type mice. Hemagglutination inhibition assays using recombinant α-DGN showed in vitro neutralization of IAV. Collectively, our results support a protective role for α-DGN in IAV proliferation.

A molecular overview of the primary dystroglycanopathies

Dystroglycan is a major non-integrin adhesion complex that connects the cytoskeleton to the surrounding basement membranes, thus providing stability to skeletal muscle. In Vertebrates, hypoglycosylation of α-dystroglycan has been strongly linked to muscular dystrophy phenotypes, some of which also show variable degrees of cognitive impairments, collectively termed dystroglycanopathies. Only a small number of mutations in the dystroglycan gene, leading to the so called primary dystroglycanopathies, has been described so far, as opposed to the ever-growing number of identified secondary or tertiary dystroglycanopathies (caused by genetic abnormalities in glycosyltransferases or in enzymes involved in the synthesis of the carbohydrate building blocks). The few mutations found within the autonomous N-terminal domain of α-dystroglycan seem to destabilise it to different degrees, without influencing the overall folding and targeting of the dystroglycan complex. On the contrary other mutations, some located at the α/β interface of the dystroglycan complex, seem to be able to interfere with its maturation, thus compromising its stability and eventually leading to the intracellular engulfment and/or partial or even total degradation of the dystroglycan uncleaved precursor.

A dystroglycan mutation (p.Cys667Phe) associated to muscle-eye-brain disease with multicystic leucodystrophy results in ER-retention of the mutant protein

Dystroglycan (DG) is a cell adhesion complex composed by two subunits, the highly glycosylated α-DG and the transmembrane β-DG. In skeletal muscle, DG is involved in dystroglycanopathies, a group of heterogeneous muscular dystrophies characterized by a reduced glycosylation of α-DG. The genes mutated in secondary dystroglycanopathies are involved in the synthesis of O-mannosyl glycans and in the O-mannosylation pathway of α-DG. Mutations in the DG gene (DAG1), causing primary dystroglycanopathies, destabilize the α-DG core protein influencing its binding to modifying enzymes. Recently, a homozygous mutation (p.Cys699Phe) hitting the β-DG ectodomain has been identified in a patient affected by muscle-eye-brain disease with multicystic leucodystrophy, suggesting that other mechanisms than hypoglycosylation of α-DG could be implicated in dystroglycanopathies. Herein, we have characterized the DG murine mutant counterpart by transfection in cellular systems and high-resolution microscopy. We observed that the mutation alters the DG processing leading to retention of its uncleaved precursor in the endoplasmic reticulum. Accordingly, small-angle X-ray scattering data, corroborated by biochemical and biophysical experiments, revealed that the mutation provokes an alteration in the β-DG ectodomain overall folding, resulting in disulfide-associated oligomerization. Our data provide the first evidence of a novel intracellular mechanism, featuring an anomalous endoplasmic reticulum-retention, underlying dystroglycanopathy.