Muscular dystrophy begins early in embryonic development deriving from stem cell loss and disrupted skeletal muscle formation

Abstract

SUMMARYExamination of embryonic myogenesis of two distinct, but functionally related, skeletal muscle dystrophy mutants (mdx and cav-3 -/-) establishes for the first time that key elements of the pathology of Duchenne muscular dystrophy (DMD) and limb-girdle muscular dystrophy type 1C (LGMD1c) originate in the disruption of the embryonic cardiac and skeletal muscle patterning processes. Disruption of myogenesis occurs earlier in mdx mutants, which lack a functional form of dystrophin, than in cav-3 -/-mutants, which lack the Cav3 gene that encodes the protein caveolin-3; this finding is consistent with the milder phenotype of LGMD-1c, a condition caused by mutations in Cav3, and the earlier [embryonic day (E)9.5] expression of dystrophin. Myogenesis is severely disrupted in mdx embryos, which display developmental delays; myotube morphology and displacement defects; and aberrant stem cell behaviour. In addition, the caveolin-3 protein is elevated in mdx embryos. Both cav-3 -/-and mdx mutants (from E15.5 and E11.5, respectively) exhibit hyperproliferation and apoptosis of Myf5-positive embryonic myoblasts; attrition of Pax7-positive myoblasts in situ; and depletion of total Pax7 protein in late gestation. Furthermore, both cav-3 -/-and mdx mutants have cardiac defects. In cav-3 -/-mutants, there is a more restricted phenotype comprising hypaxial muscle defects, an excess of malformed hypertrophic myotubes, a twofold increase in myonuclei, and reduced fast myosin heavy chain (FMyHC) content. Several mdx mutant embryo pathologies, including myotube hypotrophy, reduced myotube numbers and increased FMyHC, have reciprocity with cav-3 -/-mutants. In double mutant (mdxcav-3) embryos that are deficient in dystrophin (mdx) and heterozygous for caveolin-3 (cav-3), whereby caveolin-3 is reduced to 50% of wild-type (WT) levels, these phenotypes are severely exacerbated: intercostal muscle fibre density is reduced by 71%, and Pax7-positive cells are depleted entirely from the lower limbs and severely attenuated elsewhere; these data suggest a compensatory rather than a contributory role for the elevated caveolin-3 levels that are found in mdx embryos. These data establish a key role for dystrophin in early muscle formation and demonstrate that caveolin-3 and dystrophin are essential for correct fibre-type specification and emergent stem cell function. These data plug a significant gap in the natural history of muscular dystrophy and will be invaluable in establishing an earlier diagnosis for DMD/LGMD and in designing earlier treatment protocols, leading to better clinical outcome for these patients.

FHL1 mutations that cause clinically distinct human myopathies form protein aggregates and impair myoblast differentiation

“…Myoblast fusion is an essential step in muscle growth during development and for the regeneration of mature muscle (Pavlath and Horsley, 2003). Impaired myoblast fusion is observed in several myopathies (de Luna et al, 2006;Merrick et al, 2009;Vesa et al, 2009). Relative to wild-type HA-FHL1, which enhanced myoblast differentiation, the expression of RBM, SPM or XMPMA mutant HA-FHL1 did not promote increased myoblast fusion and myotube size in vitro.…”

Newborn screening for Duchenne muscular dystrophy gains support

“…Regarding the progression of DMD and its early detection, Mendell points to research showing that it begins early in embryonic development due to stem cell loss and disrupted skeletal muscle formation (Merrick et al, 2009 While Mendell's Ohio-based study produced exciting results, it isn't large enough to convince SACHDNC, Dr. Howell notes. "In newborn screening, 37,000 is small.…”

New factors contributing to dynamic calcium regulation in the skeletal muscle triad—a crowded place

“…Myotubes can be easily used for genetic modifications in knock-down and silencing studies for tubular signalling proteins (Lyfenko and Dirksen 2008), but myotubes do not necessarily compare to adult fibers due to maturation plasticity (Merrick et al 2009). For example, sarcolemmal mechanosensitive channel open probabilities decline substantially from myotubes to adult fibers (Franco-Obregon and Lansman 1994), and dedifferentiating cultured adult fibers show spontaneous elementary calcium-release events that can be blocked by DHPR antagonists (Brown et al 2007).…”