beta-Sarcoglycan, one of the subunits of the sarcoglycan complex, is a transmembranous glycoprotein which associates with dystrophin and is the molecule responsible for beta-sarcoglycanopathy, a Duchenne-like autosomal recessive muscular dystrophy. To develop an animal model of beta-sarcoglycanopathy and to clarify the role of beta-sarcoglycan in the pathogenesis of the muscle degeneration in vivo, we developed beta-sarcoglycan-deficient mice using a gene targeting technique. beta-Sarcoglycan-deficient mice (BSG(-)(/-)mice) exhibited progressive muscular dystrophy with extensive degeneration and regeneration. The BSG(-)(/-)mice also exhibited muscular hypertrophy characteristic of beta-sarcoglycanopathy. Immunohistochemical and immunoblot analyses of BSG(-)(/-)mice demonstrated that deficiency of beta-sarcoglycan also caused loss of all of the other sarcoglycans as well as of sarcospan in the sarcolemma. On the other hand, laminin-alpha2, alpha- and beta-dystroglycan and dystrophin were still present in the sarcolemma. However, the dystrophin-dystroglycan complex in BSG(-)(/-)mice was unstable compared with that in the wild-type mice. Our data suggest that loss of the sarcoglycan complex and sarcospan alone is sufficient to cause muscular dystrophy, that beta-sarcoglycan is an important protein for formation of the sarcoglycan complex associated with sarcospan and that the role of the sarcoglycan complex and sarcospan may be to strengthen the dystrophin axis connecting the basement membrane with the cytoskeleton.

The sarcoglycan complex consists of four transmembrane protein subunits. Mutation of any one of the genes encoding these four subunits causes complete loss or marked decrease in expression of the whole complex, resulting in the phenotype of Duchenne-like autosomal recessive muscular dystrophy, termed sarcoglycanopathy. As the basis for understanding this process, we examined how the sarcoglycan complex is formed and associates with other proteins during myogenic differentiation, using a myogenic cell line. Accumulation of the sarcoglycan subunits and formation of the sarcoglycan complex were accomplished with myotube formation. In protein transport inhibition experiments with blefeldin A, we found that the sarcoglycan complex is formed in the endoplasmic reticulum and then associates with the dystroglycan complex and sarcospan en route from the Golgi apparatus to the cell surface. In early myotubes, limited kinds of incomplete sarcoglycan complexes were observed. Their analyses would provide information on the possible patterns of formation of the sarcoglycan complex.Keywords: C2C12 cell; complex formation; differentiation; dystrophin-associated protein; sarcoglycanopathy.Dystrophin is a long and slender protein that is present on the cytoplasmic surface of the cell membrane. It connects via its N-terminal region with actin filaments in the membrane skeleton network and via its C-terminal region with the transmembranous dystroglycan (DG) complex, which in turn binds to laminin, a major component of the basal lamina. Thus, dystrophin and the DG complex comprise an axis that fixes the basal lamina to the membrane skeleton network, thereby, protecting the cell membrane (reviewed in [1]). Loss of dystrophin results in the severely debilitating muscular disorder termed Duchenne muscular dystrophy.The DG complex is composed of two subunits: aDG and bDG. It binds to the membrane-integrated sarcoglycan (SG) complex, composed of four subunits (aSG, bSG, gSG and dSG), each of which has a single membrane-spanning region. Interestingly, mutation of any one of the SG genes gives rise to sarcoglycanopathy, the clinical phenotype of which closely resembles that of X-linked recessive Duchenne muscular dystrophy, but which is transmitted in an autosomal recessive manner (reviewed in [2]). In the muscles of patients with sarcoglycanopathy, besides the defective subunit being absent, the other subunits of the SG complex are also absent or greatly reduced [3]. Thus, mutation of even one of the four genes results in the loss of expression of the whole SG complex [4±8]. Defective expression of the whole SG complex has been suggested as being the primary cause of all the four types of sarcoglycanopathy. This is the reason why mutation in any of the four SG genes causes the same pathologic phenotype. The central question is how a defect in a single subunit results in the loss of expression of the whole complex. In order to clarify this, we must first understand how the SG subunits assemble in a normal muscle.Recently Holt & Campbell [9...

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We found a novel dystrophin-associated protein (DAP) exhibiting almost the same mobility as y-sarcoglycan on SDS-PAGE. This novel DAP with basic charge is separated from y-sarcoglycan by 2-dimensional PAGE or de-/V-glycosylation followed by SDS-PAGE. This DAP is most likely the rabbit homologue of "8-sarcoglycan", the y-sarcoglycan-like protein identified previously |Nigro et al. (1996) Hum. Mol. Genet. 5, 1179-1186], since an internal amino acid sequence from the DAP matched the predicted amino acid sequence of "human 8-sarcoglycan" within the limits of species difference and this DAP was recognized by anti-"5-sarcoglycan" antibody. The DAP was found to be contained in the sarcoglycan fraction which was prepared by treatment of the dystrophin-DAP complex with n-octyl ß-D-glucoside and crosslinked with ß-and/or y-sarcoglycan by a chemical crosslinker, dithiobis(succinimidyl propionate). Therefore, we concluded that the DAP is the fourth component of the sarcoglycan complex.