Extraocular muscle is spared despite the absence of an intact sarcoglycan complex in γ- or δ-sarcoglycan-deficient mice

Models of the dystrophin-glycoprotein complex do not reconcile the novel sparing of extraocular muscle in muscular dystrophy. Extraocular muscle sparing in Duchenne muscular dystrophy implies the existence of adaptive properties in these muscles that may extend protection to other neuromuscular diseases. We studied the extraocular muscle morphology and dystrophin-glycoprotein complex organization in murine targeted deletion of the γ-sarcoglycan (gsg^-/-) and δ-sarcoglycan (dsg^-/-) genes, two models of autosomal recessive limb girdle muscular dystrophy. In contrast to limb and diaphragm, the principal extraocular muscles were intact in gsg^-/- and dsg^-/- mice. However, central nucleated, presumptive regenerative, fibers were seen in the accessory extraocular muscles (retractor bulbi, levator palpebrae superioris) of both strains. Skeletal muscles of gsg^-/- mice exhibited in vivo Evans Blue dye permeability, while the principal extraocular muscles did not. Disruption of γ-sarcoglycan produced secondary displacement of α- and β-sarcoglycans in the extraocular muscles. The intensity of immunofluorescence for dystrophin and α- and β-dystroglycan also appeared to be slightly reduced. Utrophin localization was unchanged. The finding that sarcoglycan disruption was insufficient to elicit alterations in extraocular muscle suggests that loss of mechanical stability and increased sarcolemmal permeability are not inevitable consequences of mutations that disrupt the dystrophin-glycoprotein complex organization and must be accounted for in models of muscular dystrophy.