Pathological and genetic analysis of the degenerating muscle (dmu) mouse: a new allele of Scn8a

Here, we describe a novel spontaneous autosomal recessive mutation in the mouse that is characterized by skeletal and cardiac muscle degeneration. We have named this mutant degenerating muscle (dmu). At birth, mutant mice are indistinguishable from their normal littermates. Thereafter, the disease progresses rapidly and a phenotype is first observed at approximately 11 days after birth; the dmu mice are weak and have great difficulty in moving. The principal cause of the lack of mobility is muscle atrophy and wasting in the hindquarters. Affected mice die at or around the time of weaning of unknown causes. Histopathological observations and ultrastructural analysis revealed muscle degeneration in both skeletal and cardiac muscle, but no abnormalities in sciatic nerves. Using linkage analysis, we have mapped the dmu locus to the distal portion of mouse chromosome 15 in a region syntenic to human chromosome 12q13. Interestingly, scapuloperoneal muscular dystrophy (SPMD) in humans has been linked to this region. SPMD patients with associated cardiomyopathy have also been described in the past. Initial analysis of candidate genes on mouse chromosome 15 reveal that although intact transcripts for Scn8a, the gene encoding the sodium channel 8a subunit, are present in dmu mice, their levels are dramatically reduced. Furthermore, genetic complementation crosses between dmu and med (mutation in Scn8a) mice revealed that they are allelic. Our results suggest that at least a portion of the dmu phenotype is caused by a down-regulation of Scn8a, making dmu a new allele of Scn8a.

The dystroglycan complex is necessary for stabilization of acetylcholine receptor clusters at neuromuscular junctions and formation of the synaptic basement membrane

The dystrophin-associated protein (DAP) complex spans the sarcolemmal membrane linking the cytoskeleton to the basement membrane surrounding each myofiber. Defects in the DAP complex have been linked previously to a variety of muscular dystrophies. Other evidence points to a role for the DAP complex in formation of nerve-muscle synapses. We show that myotubes differentiated from dystroglycan-/- embryonic stem cells are responsive to agrin, but produce acetylcholine receptor (AChR) clusters which are two to three times larger in area, about half as dense, and significantly less stable than those on dystroglycan+/+ myotubes. AChRs at neuromuscular junctions are similarly affected in dystroglycan-deficient chimeric mice and there is a coordinate increase in nerve terminal size at these junctions. In culture and in vivo the absence of dystroglycan disrupts the localization to AChR clusters of laminin, perlecan, and acetylcholinesterase (AChE), but not rapsyn or agrin. Treatment of myotubes in culture with laminin induces AChR clusters on dystroglycan+/+, but not -/- myotubes. These results suggest that dystroglycan is essential for the assembly of a synaptic basement membrane, most notably by localizing AChE through its binding to perlecan. In addition, they suggest that dystroglycan functions in the organization and stabilization of AChR clusters, which appear to be mediated through its binding of laminin.

Chimaeric mice deficient in dystroglycans develop muscular dystrophy and have disrupted myoneural synapses

Mutations in the dystrophin gene (DMD) and in genes encoding several dystrophin-associated proteins result in Duchenne and other forms of muscular dystrophy. alpha-Dystroglycan (Dg) binds to laminins in the basement membrane surrounding each myofibre and docks with beta-Dg, a transmembrane protein, which in turn interacts with dystrophin or utrophin in the subplasmalemmal cytoskeleton. alpha- and beta-Dgs are thought to form the functional core of a larger complex of proteins extending from the basement membrane to the intracellular cytoskeleton, which serves as a superstructure necessary for sarcolemmal integrity. Dgs have also been implicated in the formation of synaptic densities of acetylcholine receptors (AChRs) on skeletal muscle. Here we report that chimaeric mice generated with ES cells targeted for both Dg alleles have skeletal muscles essentially devoid of Dgs and develop a progressive muscle pathology with changes emblematic of muscular dystrophies in humans. In addition, many neuromuscular junctions are disrupted in these mice. The ultrastructure of basement membranes and the deposition of laminin within them, however, appears unaffected in Dg-deficient muscles. We conclude that Dgs are necessary for myofibre survival and synapse differentiation or stability, but not for the formation of the muscle basement membrane, and that Dgs may have more than a purely structural function in maintaining muscle integrity.