Evidence for a utrophin-glycoprotein complex in cultured cell lines and a possible role in cell adhesion

Abstract

Dystrophin, the defective protein in patients with the X-linked muscle wasting disorders Duchenne and Becker muscular dystrophies [ 11, is attached to an oligomeric complex which spans the sarcolemma of skeletal muscle 121. The complex is divided into two groups of proteins. The dystroglycan complex is comprised ofthe 43DAG and 156DAG proteins and is found in all tissues. The sarcoglycan complex is found only in skeletal muscle and is comprised of three proteins, SODAG, A3b and 35DAG. Only thc 43DAG protein binds directly to dystrophin. In addition, two cytoplasmic proteins 59DAP (syntrophin [ 3 ] ) and A0 also bind to dystrophin but appear not to be attached to the sarcoglycan and dystroglyean complexes 141.The autosomal homologue of dystrophin, utrophin, has becn shown to be closely related to dystrophin in sequence and structure. The homology is especially great in the dystroglycan binding region, the cysteine-rich and C-terminal (CRCT) domains 151. Utrophin is more widely distributed than dystrophin and is found in most tissues [6] and in many cell lines which lack dystrophin where it is associated with the membrane [71. I n normal skeletal muscle, utrophin is found primarily at the neuromuscular and myotendinous junctions. However, in dystrophic muscle, elevated levels ol utrophin are found throughout the sarcolemma 18.91. In md.r mouse muscle [ l o ] and rabbit sciatic nerve [ 1 I] it has been demonstrated that utrophin associates with a "dystrophin-glycoprotein"4ke complex.The N-terminal region of dystrophin binds actin at the intracellular surface of the sarcolemma [ 121, while the 156kd glycoprotein binds laminin in the muscle fibre basal lamina [ 13,141. This link is thought to be essential for the maintenance of membrane integrity and in preventing the disruption of the sarcolemma seen in DMD patients. Most cultured cells, when grown on extracellular matrix (ECM) proteins, form structures called focal contacts [15,16] where bundles of actin filaments terminate and interact with the ECM via the plasma membrane [ 151. Dystrophin has been identified at focal contacts in cultured Xenopus muscle [ 171, and has the same distribution as the focal contact proteins, vinculin and a-actinin in skeletal muscle [ 181, but not in smooth muscle [ 191. 100 90 80 70 60 50 40 30 20 10 n ., G S L F H P + Figure 1. Analysis of 43DAG and syntrophin content of cultured cell lines. Protein concentration was measured using densitometer readings from the photograph negatives of the Western blots. The cell lines are rat C6 glioma (G), SWA rat Schwannoma cells (S), rat L6 myoblast cell line (L), human skin fibroblasts (F), HeLa cells (H), a mouse monocyte-macrophage cell line P388D1 (P) and S p y 0 mouse myeloma cells (Sp).In figure 1 we demonstrate for the first time that cultured cell lines produce the dystrophin-glycoprotein complex components 43DAG and syntrophin. The presence of 43DAG and syntrophin means there is the potential for utrophin 10 form part of an oligomeric complex similar to the dystrophin-glycoprotein seen in...

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