Intracellular calcium transients in skeletal muscle cells initiate phenotypic adaptations via activation of calcineurin and its effector nuclear factor of activated t-cells (NFAT). Furthermore, endogenous production of nitric oxide (NO) via calcium-calmodulin-dependent NO synthase (NOS) is involved in skeletal muscle phenotypic plasticity. Here, we provide evidence that NO enhances calcium-dependent nuclear accumulation and transcriptional activity of NFAT and induces phosphorylation of glycogen synthase kinase-3beta (GSK-3beta) in C2C12 myotubes. The calcium ionophore A23187 (1 microM for 9 h) or thapsigargin (2 microM for 4 h) increased NFAT transcriptional activity by seven- and fourfold, respectively, in myotubes transiently transfected with an NFAT-dependent reporter plasmid (pNFAT-luc, Stratagene). Cotreatment with the NOS-inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 5 mM) or the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM) prevented the calcium effects on NFAT activity. The NO donor diethylenetriamine-NONO (DETA-NO; 10 microM) augmented the effects of A23187 on NFAT-dependent transcription. Similarly, A23187 (0.4 microM for 4 h) caused nuclear accumulation of NFAT and increased phosphorylation (i.e., inactivation) of GSK-3beta, whereas cotreatment with L-NAME or ODQ inhibited these responses. Finally, the NO donor 3-(2-hydroxy-2-nitroso-1-propylhydrazino)-1-propanamine (PAPA-NO; 1 microM for 1 h) increased phosphorylation of GSK-3beta in a manner dependent on guanylate cyclase activity. We conclude that NOS activity mediates calcium-induced phosphorylation of GSK-3beta and activation of NFAT-dependent transcription in myotubes. Furthermore, these effects of NO are guanylate cyclase-dependent.

Two promoters in the distal half of the Duchenne Muscular Dystrophy gene drive transcription of mRNAs which have novel first exons and encode the shortened forms of dystrophin, apo-dystrophin-1 (Dp71) and apo-dystrophin-2 (Dp116). Apo-dystrophin-1 has a G + C rich promoter and is expressed in a wide range of cell types, whilst apo-dystrophin-2 is confined to peripheral nerve and brain. We have isolated and sequenced the unique 5' exon of rat apo-dystrophin-2 mRNA. Conceptual translation of this sequence indicates that apo-dystrophin-2 contains a unique 23 amino acid terminal peptide. Using specific probes derived from sequences at the 5' ends of apo-dystrophin-1 and apo-dystrophin-2 we have determined the expression of these two mRNAs during mouse embryonic development by RNA in situ hybridization. In contrast to full-length dystrophin, neither of these short dystrophin transcripts appear before organogenesis is well established. Apo-dystrophin-1 mRNA is detected in midline cells of the ventral neural tube and later, in the ependymal cells lining the ventricles of the brain. These results suggest that apo-dystrophin-1 mRNA is associated with glial cells in the CNS. Apo-dystrophin-1 transcripts are also abundant in the teeth primordia throughout their development. In contrast apo-dystrophin-2 mRNA is largely undetectable during development, although transcripts are seen in the newborn brain. Western blots of late human fetal tissue extracts confirm that apo-dystrophin-2 is most abundant in brain and analysis of RNA and protein in cultured cell lines reveal expression of apo-dystrophin-1 and apo-dystrophin-2 in glioma cells.

All previous studies of the localization of utrophin (the dystrophin-related protein) in muscle and other tissues have been performed only with antibodies against the C-terminal region of the protein. Since several short forms of dystrophin, the apo-dystrophins, are produced from the 3' end of the dystrophin gene, there is a possibility that similar short forms of utrophin exist and that these could be responsible for some of the many different localizations of 'utrophin' in muscle. We have produced a new panel of 15 mAbs against the N-terminal region of utrophin and we have used it together with mAbs against the C-terminal region to show that full-length utrophin is present at neuromuscular junctions, in nerves, blood vessels and capillaries in normal muscle and in the sarcolemma of patients with muscular dystrophy and dermatomyositis. However, two of the 15 mAbs also recognised rat/mouse utrophin and both of these detected an additional 62 kDa protein on Western blots of rat C6 glioma cells. This potential 62 kDa 'apo-utrophin' was not detected in human cerebral cortex, in rat Schwannoma cells nor in any of the non-nerve cells and tissues tested.

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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