Aberrant neuromuscular junctions and delayed terminal muscle fiber maturation in α-dystroglycanopathies

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Abstract

Recent studies have revealed an association between post-translational modification of α-dystroglycan (α-DG) and certain congenital muscular dystrophies known as secondary α-dystroglycanopathies (α-DGpathies). Fukuyama-type congenital muscular dystrophy (FCMD) is classified as a secondary α-DGpathy because the responsible gene, fukutin, is a putative glycosyltransferase for α-DG. To investigate the pathophysiology of secondary α-DGpathies, we profiled gene expression in skeletal muscle from FCMD patients. cDNA microarray analysis and quantitative real-time polymerase chain reaction showed that expression of developmentally regulated genes, including myosin heavy chain (MYH) and myogenic transcription factors (MRF4, myogenin and MyoD), in FCMD muscle fibers is inconsistent with dystrophy and active muscle regeneration, instead more of implicating maturational arrest. FCMD skeletal muscle contained mainly immature type 2C fibers positive for immature-type MYH. These characteristics are distinct from Duchenne muscular dystrophy, suggesting that another mechanism in addition to dystrophy accounts for the FCMD skeletal muscle lesion. Immunohistochemical analysis revealed morphologically aberrant neuromuscular junctions (NMJs) lacking MRF4 co-localization. Hypoglycosylated α-DG indicated a lack of aggregation, and acetylcholine receptor (AChR) clustering was compromised in FCMD and the myodystrophy mouse, another model of secondary α-DGpathy. Electron microscopy showed aberrant NMJs and neural terminals, as well as myotubes with maturational defects. Functional analysis of NMJs of α-DGpathy showed decreased miniature endplate potential and higher sensitivities to d-Tubocurarine, suggesting aberrant or collapsed formation of NMJs. Because α-DG aggregation and subsequent clustering of AChR are crucial for NMJ formation, hypoglycosylation of α-DG results in aberrant NMJ formation and delayed muscle terminal maturation in secondary α-DGpathies. Although severe necrotic degeneration or wasting of skeletal muscle fibers is the main cause of congenital muscular dystrophies, maturational delay of muscle fibers also underlies the etiology of secondary α-DGpathies.

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