Role of dystrophin isoforms and associated proteins in muscular dystrophy (review).

The membrane cytoskeletal component dystrophin and its associated glycoproteins play a central role in the molecular pathogenesis of several muscular dystrophies, i.e. Duchenne/Becker muscular dystrophy, congenital muscular dystrophy and various forms of limb-girdle muscular dystrophy. Although the most frequent of these disorders, Duchenne muscular dystrophy, is mainly recognized as a disease of skeletal muscle fibers, pathophysiological changes also involve the heart and diaphragm, as well as the peripheral and central nervous system. Thus current research efforts into the elucidation of the molecular mechanisms underlying these genetic diseases are not only directed towards studying skeletal muscle necrosis but also investigate abnormalities of heart and brain dystrophin-glycoprotein complexes in cardiomyopathy and brain deficiencies associated with muscular dystrophy. Furthermore, many isoforms of dystrophin and dystrophin-associated components have been identified in various non-muscle tissues and their function(s) are mostly unknown. With respect to skeletal muscle fibers, the characterization of new dystrophin-associated proteins, such as dystrobrevin, sarcospan and the syntrophins, led to a modified model of the spatial configuration of the dystrophin-glycoprotein complex. However, it is generally accepted now that beta-dystroglycan forms the plasmalemma-spanning linkage between dystrophin and the laminin-binding protein alpha-dystroglycan and that this complex is associated with the sarcoglycan subcomplex of sarcolemmal glycoproteins.

Decreased expression of brain beta-dystroglycan in Duchenne muscular dystrophy but not in the mdx animal model.

Abnormalities in the muscle dystrophin-glycoprotein complex are implicated in the molecular pathogenesis of various neuromuscular disorders. Weakening of the trans-sarcolemmal linkage between the actin membrane-cytoskeleton and the extracellular matrix appears to trigger destabilization of the muscle cell periphery. In addition to muscular weakness, one-third of patients suffering from Duchenne muscular dystrophy exhibit mental retardation. Since little is known about the pathophysiology of brain abnormalities in these patients, we investigated the fate of the most abundant dystrophin-associated protein, beta-dystroglycan, in the central nervous system. It was found to be present throughout all normal brain regions studied. In contrast, this glycoprotein was greatly reduced in brain microsomes derived from Duchenne specimens, while it is of normal abundance in the brain from the dystrophic animal model mdx. Deficiency in brain beta-dystroglycan might render nervous tissue more susceptible to cellular disturbances and this may result in cognitive impairment in some Duchenne patients.

Oligomerization of beta-dystroglycan in rabbit diaphragm and brain as revealed by chemical crosslinking.

The surface component beta-dystroglycan is a member of the dystrophin-glycoprotein complex providing a trans-sarcolemmal linkage between the actin membrane cytoskeleton and the extracellular matrix component laminin-alpha2. Although abnormalities in this complex are involved in the pathophysiology of various neuromuscular disorders, little is known about the organization of dystrophin-associated glycoproteins in diaphragm and brain. We therefore investigated the oligomerization of beta-dystroglycan and its connection with the most abundant dystrophin homologues in these two tissues. Employing detergent solubilization and alkaline extraction procedures of native membranes, it was confirmed that beta-dystroglycan behaves like an integral surface molecule as predicted by its cDNA sequence. Immunoblot analysis following chemical crosslinking of native membranes showed that beta-dystroglycan has a tendency to form high-molecular-mass complexes. Within these crosslinkable complexes, immuno-reactive overlaps were observed between beta-dystroglycan, alpha-dystroglycan, laminin and 427 kDa dystrophin in diaphragm and skeletal muscle. In synaptosomes, the major brain dystrophin isoform Dp116 also exhibited an immuno-reactive overlap with members of the dystroglycan complex. These findings demonstrate that beta-dystroglycan does not exist as a monomer in native membranes and imply that certain dystrophin isoforms and dystrophin-associated components interact with this surface protein in diaphragm and brain as has been previously shown for skeletal and heart muscle.

Rabbit brain and muscle isoforms containing the carboxy-terminal domain of 427 kDa skeletal muscle dystrophin exhibit similar biochemical properties.

The membrane cytoskeletal component dystrophin, which is the protein product of the human Duchenne muscular dystrophy gene, exists in manifold isoforms. Using immunoblot analysis with an antibody to the carboxy-terminal domain of the 427 kDa skeletal muscle dystrophin, we investigated the membrane cytoskeletal properties of dystrophin isoforms from rabbit skeletal muscle, heart and brain. All isoforms identified, including the abundant brain isoforms Dp116 which lacks the amino-terminal actin-binding domain of 427 kDa dystrophin, exhibited similar biochemical properties, i.e. insolubility in non-ionic detergent but extraction from the membrane with alkaline solutions. In muscle, beta-dystroglycan was found to be more tightly associated with dystrophin than alpha-sarcoglycan. These findings agree with the proposed structure of identified muscle and brain dystrophin isoforms and are also consistent with the current model of the dystrophin-glycoprotein complex.

Environmental print as strategy for developmental literacy of young atypical children.

12 children with handicapping problems were taught various strategies during a two-year period. These programs, designed to enrich their appreciation and awareness of the sounds of letters and how they are used, increased the children's attention span, parental participation with children and teacher, and promoted active involvement in reading.