The dystrophin superfamily: variability and complexity

The membrane-cytoskeleton interface: the role of dystrophin and utrophin

Recent studies with transgenic animals have considerably advanced our knowledge of the roles of dystrophin and utrophin in both muscle and non-muscle tissues. Rigorous analyses of the roles of the various mdx mutations in mice, as well as the use of artificial transgenes in an mdx background, are beginning to define the functional importance of various regions of the dystrophin protein in normal muscle. Furthermore, recent biochemical analyses have revealed new insights into the role and organization of dystrophin at the membrane-cytoskeleton interface. Transgenic approaches have also revealed surprising and encouraging results with respect to utrophin. Against expectations, the long-awaited utrophin knockout mice have a remarkably mild phenotype with only subtle changes in neuromuscular junction architecture. On the other hand, mdx mice transgenic for a mini-utrophin construct showed rescue of the muscular dystrophy phenotype, clearly an encouraging finding with obvious therapeutic possibilities. These and other recent findings are discussed in the context of the structure and function of dystrophin and utrophin at the membrane-cytoskeleton interface.

Conservation of a putative AP1 binding site and complete homology to a fetal brain EST in a region upstream of the core muscle promoter in the human dystrophin gene

A region of 744 basepairs (bp) upstream of the muscular dystrophin promoter (UMDP) was amplified by inverse-polymerase chain reaction (PCR), cloned and sequenced. Analysis of this sequence for the presence of putative transcriptional control elements identified several similarities with known cis-acting sequence motifs including two MyoD and two Ap1 motifs. One of these Ap1 motifs was found to be completely conserved within an otherwise highly variable region among five primate species. Complete homology to a human fetal brain expressed sequence tag (EST) was also observed over 201 bp at the 5' end of the UMDP region. Northern blot analysis using a radiolabelled EST probe identified a 1 kb mRNA expressed in human placenta and at lower levels in the heart. These results raise the possibility that additional transcriptional regulatory elements are located upstream of the core muscle promoter, and provide the first evidence for the existence of a gene that overlaps the human dystrophin gene.

Contractile properties of diaphragm muscle segments from old mdx and old transgenic mdx mice

Diaphragm muscles of young (4- to 6-mo-old) mdx mice show severe fiber necrosis and have normalized forces and powers 60 and 46% of the values for control C57BL/10 mice. In contrast, microinjection of mdx mouse embryos with a truncated dystrophin minigene has produced young transgenic mdx (tg-mdx) mice with a level of dystrophin expression and structural and functional properties of diaphragm muscle strips measured in vitro not different from those of control mice. Whether dystrophin expression and functional corrections persist for the life span of these animals is not know. We tested the null hypothesis that, in old (24 mo) tg-mdx mice, dystrophin expression is adequate and diaphragm muscle strips have forces and powers not different from values for diaphragm muscle strips from young tg-mdx mice or control mice. Compared with control values, diaphragm muscle strips from old mdx mice had normalized forces and powers of 48 and 31%, respectively. Expression of dystrophin persisted in diaphragm muscles of old tg-mdx mice, and functional properties were not different from diaphragm muscles of young tg-mdx or young or old control mice. These results suggest that, with a transgenic animal approach, dystrophin expression and functional corrections persist for the life span of the animals.

The molecular basis for cross-reaction of an anti-dystrophin antibody with alpha-actinin

The epitope recognised by the anti-dystrophin monoclonal antibodies MANDYS141 and MANDYS142 has been characterised using a phage display peptide library and a bacteriophage lambda cDNA library. Using a phage display library of random 15-mer peptides, the epitope recognised by the two antibodies was identified as EEXF. A lambda gt11 clone obtained by screening a human muscle cDNA library was shown to contain part of the out-of-frame human mitochondrial succinyl CoA synthetase (alpha-subunit) cDNA sequence which contains the sequence EEPL, suggesting a minimum requirement of EEXF/L for antibody binding. The sequence EEDF is located in the helical rod region of dystrophin and the N-terminal domain of alpha-actinin; this may explain why native dystrophin is not detected, since the alpha-helical, coiled-coil folding of the rod region of dystrophin may obscure the epitope in the native protein. The antibody cross-reaction between dystrophin and alpha-actinin is likely to be fortuitous and not due to any structural homology that exists between these two members of the spectrin superfamily.

Expression of a new M(r) 70-kDa dystrophin-related protein in the axon of peripheral nerves from Torpedo marmorata

By comparison with localizations of dystrophin family products in rabbit peripheral nerves, we investigated the potential existence and distribution of similar products in peripheral nerves from Torpedo marmorata. In immunofluorescence studies, a specific set of monoclonal antibodies directed against dystrophin family proteins clearly stained a thin rim surrounding each Schwann cell-axon unit both in T. marmorata and rabbit peripheral nerves. In contrast when using the dystrophin/utrophin monoclonal H'3E7 antibody, we found a clear difference between rabbit and T. marmorata peripheral nerves according to fluorescent labeling detected within Torpedo nerve axons. Further differences were noted following western blot analyses of T. marmorata peripheral nerve extracts, highlighting the presence of a new and specific M(r) 70-kDa protein band belonging to the dystrophin family, which is localized within axons in addition to: (1) an M(r)400-kDa protein band detected with dystrophin/utrophin antibodies; and (2) an M(r) 116-kDa doublet protein band corresponding to Dp116 and Up116 isoforms. All of these products, detected according to the specificities of the monoclonal antibodies used, are discussed in terms of their potential identities as short and long dystrophin or utrophin mammalian products.

Phosphorylation of the carboxyl-terminal region of dystrophin

Dystrophin is a protein product of the gene responsible for Duchenne and Becker muscular dystrophy. The protein is localized to the inner surface of sarcolemma and is associated with a group of membrane (glyco)proteins. Dystrophin links cytoskeletal actins via the dystrophin-associated protein complex to extracellular matrix protein, laminin. This structural organization implicates the role of dystrophin in stabilizing the sarcolemma of muscle fibers. Precisely how dystrophin functions is far from clear. The presence of an array of isoforms of the C-terminal region of dystrophin suggests that dystrophin may have functions other than structural. In agreement, many potential phosphorylation sites are found in the C-terminal region of dystrophin, and the C-terminal region of dystrophin is phosphorylated both in vitro and in vivo by many protein kinases, including MAP kinase, p34cdc2 kinase, CaM kinase, and casein kinase, and is dephosphorylated by calcineurin. The C-terminal domain of dystrophin is also a substrate for hierarchical phosphorylation by casein kinase-2 and GSK-3. These observations, in accordance with the finding that the cysteine-rich region binds to Ca2+, Zn2+, and calmodulin, suggest an active involvement of dystrophin in transducing signals across muscle sarcolemma. Phosphorylation-dephosphorylation of the C-terminal region of dystrophin may play a role in regulating dystrophin-protein interactions and (or) transducing signal from the extracellular matrix via the dystrophin molecule to the cytoskeleton.

Presence of long and short dystrophin and/or utrophin products in Torpedo marmorata peripheral nerves

Peripheral nerves from rabbit and Torpedo marmorata were comparatively analyzed for the presence of short dystrophin products. Western blot analyses of Torpedo marmorata peripheral nerve extracts revealed the existence of three proteins belonging to the dystrophin family: a M(r) 400 kDa protein band detected with dystrophin/utrophin, dystrophin-specific and Torpedo utrophin-specific antibodies, a molecule identified as Dp116 and, for the first time at the protein level, a new protein probably corresponding to Up116. All of these products were carefully identified according to the specificities of the monoclonal antibodies used. In immunofluorescence studies, clear staining of the thin rim surrounding each Schwann cell-axon unit was observed in both Torpedo marmorata and rabbit peripheral nerves, showing colocalization of all of these molecules. Their potential functions were discussed in comparison to similar products found in rabbit peripheral nerves.

Down-regulation of mitochondrial mRNAs in the mdx mouse model for Duchenne muscular dystrophy

In our search for genes up- or down-regulated genes in the mdx mouse model for Duchenne muscular dystrophy, we isolated a down-regulated mitochondrial DNA clone. In addition to this clone, all protein-coding mitochondrial genes tested had tissue-specific and age independent down-regulated expression. This implied mechanisms at the RNA level since no change in the mitochondrial DNA contents were detected. Cytochrome c oxidase activity showed the same range of down-regulated expression. These data provide a molecular basis for energetic metabolism modifications in mdx mice.

Dystrophin-related protein in the platelet membrane skeleton. Integrin-induced change in detergent-insolubility and cleavage by calpain in aggregating platelets

The platelet membrane is lined with a membrane skeleton that associates with transmembrane adhesion receptors and is thought to play a role in regulating the stability of the membrane, distribution and function of adhesive receptors, and adhesive receptor-induced transmembrane signaling. When platelets are lysed with Triton X-100, cytoplasmic actin filaments can be sedimented by centrifugation at low g-forces (15,600 x g) but the membrane skeleton requires 100,000 x g. The present study shows that DRP (dystrophin-related protein) sediments from lysed platelets along with membrane skeleton proteins. Sedimentation results from association with the membrane skeleton because DRP was released into the detergent-soluble fraction when actin filaments were depolymerized. Interaction of fibrinogen with the integrin alpha IIb beta 3 induces platelet aggregation, transmembrane signaling, and the formation of integrin-rich cytoskeletal complexes that can be sedimented from detergent lysates at low g-forces. Like other membrane skeleton proteins, DRP redistributed from the high-speed pellet to the integrin-rich low-speed pellet of aggregating platelets. One of the signaling enzymes that is activated following alpha IIb beta 3-ligand interactions in a platelet aggregate is calpain; DRP was cleaved by calpain to generate an approximately 140-kDa fragment that remained associated with the low-speed detergent-insoluble fraction. These studies show that DRP is part of the platelet membrane skeleton and indicate that DRP participates in the cytoskeletal reorganizations resulting from signal transmission between extracellular adhesive ligand and the interior of the cell.