The chicken dystrophin cDNA: striking conservation of the C-terminal coding and 3' untranslated regions between man and chicken

Conserved non-coding elements: developmental gene regulation meets genome organization

Comparative genomics has revealed a class of non-protein-coding genomic sequences that display an extraordinary degree of conservation between two or more organisms, regularly exceeding that found within protein-coding exons. These elements, collectively referred to as conserved non-coding elements (CNEs), are non-randomly distributed across chromosomes and tend to cluster in the vicinity of genes with regulatory roles in multicellular development and differentiation. CNEs are organized into functional ensembles called genomic regulatory blocks–dense clusters of elements that collectively coordinate the expression of shared target genes, and whose span in many cases coincides with topologically associated domains. CNEs display sequence properties that set them apart from other sequences under constraint, and have recently been proposed as useful markers for the reconstruction of the evolutionary history of organisms. Disruption of several of these elements is known to contribute to diseases linked with development, and cancer. The emergence, evolutionary dynamics and functions of CNEs still remain poorly understood, and new approaches are required to enable comprehensive CNE identification and characterization. Here, we review current knowledge and identify challenges that need to be tackled to resolve the impasse in understanding extreme non-coding conservation.

Conserved regions of the DMD 3' UTR regulate translation and mRNA abundance in cultured myotubes

Duchenne muscular dystrophy (DMD), a severe muscle-wasting disease, is caused by mutations in the DMD gene, which encodes for the protein dystrophin. Its regulation is of therapeutic interest as even small changes in expression of functional dystrophin can significantly impact the severity of DMD. While tissue-specific distribution and transcriptional regulation of several DMD mRNA isoforms has been well characterized, the post-transcriptional regulation of dystrophin synthesis is not well understood. Here, we utilize qRTPCR and a quantitative dual-luciferase reporter assay to examine the effects of isoform specific DMD 5' UTRs and the highly conserved DMD 3' UTR on mRNA abundance and translational control of gene expression in C2C12 cells. The 5' UTRs were shown to initiate translation with low efficiency in both myoblasts and myotubes. Whereas, two large highly conserved elements in the 3' UTR, which overlap the previously described Lemaire A and D regions, increase mRNA levels and enhance translation upon differentiation of myoblasts into myotubes. The results presented here implicate an important role for DMD UTRs in dystrophin expression and delineate the cis-acting elements required for the myotube-specific regulation of steady-state mRNA levels and translational enhancer activity found in the DMD 3' UTR.

Assessment of dietary amino acid scarcity on growth and blood plasma proteome status of broiler chickens

Dietary Lys needs for chicks were studied. A titration diet consisting of progressive amounts of dietary Lys from 0.95% up to 1.40% was fed to broiler chicks from 0 to 18 d of age. Optimal dietary Lys level was calculated using regression analysis. Body weight gain and feed conversion were maximized at Lys levels of 1.24% (1.10% digestible) and 1.27% (1.13% digestible) of diet, respectively. Blood samples were then collected from 2 groups: birds fed the lowest Lys level and birds fed dietary Lys nearest the determined requirement level (1.25% Lys). Plasma was analyzed for protein spectra via mass spectrometry and then classified by their functional characteristics. The number of proteins was similar between the 2 samples, but there was a tendency toward increased peptides for specific proteins in plasma from chicks fed adequate Lys levels. Furthermore, after these proteins were classified, more muscle-related proteins were found in plasma samples of birds fed Lys-adequate diets. It would appear that an individual dietary amino acid deficiency does not necessarily translate into decreasing protein synthesis proportionate to body weight, but rather significant changes may be occurring within the types of proteins undergoing anabolism. In conclusion, results herein illustrate the potential for using functional genomics in nutritionally related responses of poultry.

The 3'-untranslated region of the dystrophin gene - conservation and consequences of loss

The 3'-untranslated region (3'UTR) of some vertebrate dystrophin genes shows an extraordinary degree and extent of conservation (better than that of many coding regions), a phenomenon that remains unexplained. We examine novel sequence and mutational data to explore the possible reasons for this. We show that loss of the human dystrophin 3'UTR is sufficient to cause Becker muscular dystrophy with pronounced reduction in dystrophin protein levels. The acquisition of dystrophin 3'UTR sequence from an amphibian and a cartilaginous fish allows us to refine previously identified functionally constrained regions which might account for the observed phenotype. These comprise (a) the open reading frame encoding the ancestral 'alternative' amphipathic C-terminal alpha-helix, normally removed from adult dystrophin by inclusion of a poorly conserved frameshifting penultimate exon, and (b) two highly conserved untranslated regions ('Lemaire A', 350 nucleotides and 'Lemaire D', 250 nucleotides) separated by a non-conserved 700-2000-nucleotide spacer. We consider the possibility that the 3'UTR may represent a significant target for pathogenic mutations.

Structural diversity despite strong evolutionary conservation in the 5'-untranslated region of the P-type dystrophin transcript

Analysis of the 5'-flanking regions of the Purkinje (P-) dystrophin genes and mRNAs in different species revealed strong sequence conservation but functional diversity. Multiple transcription initiation sites were identified in cerebella and muscles, tissues expressing P-dystrophin. The predominant initiation site was conserved, with another muscle-specific site located upstream. Despite sequence homology, significant tissue- and species-specific structural diversity in the P-type 5'-ends exists, including alternative splicing within the 5'-untranslated region combined with alternative splicing of intron 1. One amino terminus is conserved in mammals and, to a lesser extent, in chicken. However, alternative usage of ATG codons may result in a choice of N-termini or translation of short upstream ORFs in different species. Promoter activity of a fragment upstream of the cap site was shown by transient expression in myoblasts and in vivo following intramuscular injection. It is tissue- and developmentally regulated. Analysis of promoter deletions suggests the existence of negative regulatory elements in the proximal region.

Characteristics of skeletal muscle in mdx mutant mice

We review the extensive research conducted on the mdx mouse since 1987, when demonstration of the absence of dystrophin in mdx muscle led to X-chromosome-linked muscular dystrophy (mdx) being considered as a homolog of Duchenne muscular dystrophy. Certain results are contradictory. We consider most aspects of mdx skeletal muscle: (i) the distribution and roles of dystrophin, utrophin, and associated proteins; (ii) morphological characteristics of the skeletal muscle and hypotheses put forward to explain the regeneration characteristic of the mdx mouse; (iii) special features of the diaphragm; (iv) changes in basic fibroblast growth factor, ion flux, innervation, cytoskeleton, adhesive proteins, mastocytes, and metabolism; and (v) different lines of therapeutic research.

Early postnatal muscle contractile activity regulates the carbonic anhydrase phenotype of proprioceptive neurons in young and mature mice: evidence for a critical period in development

Carbonic anhydrase activity, a marker of mouse proprioceptive neurons in adult dorsal root ganglia, is first detectable in the perinatal period, increases until postnatal day 60 and remains stable in adulthood. The onset of carbonic anhydrase staining begins after the neurons have made connections with their targets suggesting that neuron-target interactions regulate carbonic anhydrase phenotype development. To examine this possibility, we first analysed carbonic anhydrase expression in mdx mice which are characterized by a massive but reversible degeneration of skeletal muscle concomitant with the carbonic anhydrase ontogenesis. Neuronal carbonic anhydrase expression in mdx mice stopped developing when the period of muscular degeneration-regeneration began. Furthermore this alteration persisted during adulthood. We then analysed carbonic anhydrase expression in fifth lumbar dorsal root ganglion of developing control mice before and after surgical procedures that might interfere with central and peripheral target influences on dorsal root ganglion neurons. Central disconnection (dorsal rhizotomy) did not affect the development of carbonic anhydrase activity. Disrupting neuron-peripheral target interactions by sciatic nerve transection or blocking muscle contraction by tenotomy stopped the development of neuronal carbonic anhydrase content. Finally, recovery was monitored following sciatic nerve crush. In adults, recovery of carbonic anhydrase activity was obtained after functional recuperation; similar manipulations during the first month of life induced irreversible alteration of the carbonic anhydrase phenotype. These results show that the development of carbonic anhydrase activity in proprioceptive neurons is regulated by neuron-muscle interactions (i.e. muscle contraction). They also provide evidence for a critical period in the development of the carbonic anhydrase phenotype. We suggest that these two mechanisms are responsible for the altered carbonic anhydrase phenotype of the dorsal root ganglion neurons in mdx mice, a model of human muscular dystrophy.

Electron microscopic evidence for a mucin-like region in chick muscle alpha-dystroglycan

alpha-Dystroglycan has been isolated from chicken cardiac muscle and its molecular weight was estimated to be approximately 135 kDa. The avian protein interacts with murine Engelbreth-Holm-Swarm (EHS) tumor laminin via interaction with the C-terminal LG4 and LG5 domains (fragment E3) of the laminin alpha-chain. This laminin binding is calcium-dependent and can be competed by heparin. Electron microscopy investigation on the shape of alpha-dystroglycan suggests that the core protein consists of two roughly globular domains connected by a segment which most likely corresponds to a mucin-like central region also predicted by sequence analysis on mammalian isoforms. This segment may act as a spacer in the dystrophin-associated glycoproteins complex exposing the N-terminal domain of alpha-dystroglycan to laminin in the extracellular space.

Characterization of the mammalian YAP (Yes-associated protein) gene and its role in defining a novel protein module, the WW domain

We report cDNA cloning and characterization of the human and mouse orthologs of the chicken YAP (Yes-associated protein) gene which encodes a novel protein that binds to the SH3 (Src homology 3) domain of the Yes proto-oncogene product. Sequence comparison between mouse, human, and chicken YAP proteins showed an inserted sequence in the mouse YAP that represented an imperfect repeat of an upstream sequence. Further analysis of this sequence revealed a putative protein module that is found in various structural, regulatory, and signaling molecules in yeast, nematode, and mammals including human dystrophin. Because one of the prominent features of this sequence motif is two tryptophans (W), we named it the WW domain (Bork, P., and Sudol, M. (1994) Trends Biochem. Sci. 19, 531-533). Since its delineation, more proteins have been shown to contain this domain, and we report here on the widespread distribution of the WW module and present a discussion of its possible function. We have also shown that the human YAP gene is well conserved among higher eukaryotes, but it may not be conserved in yeast. Its expression at the RNA level in adult human tissues is nearly ubiquitous, being relatively high in placenta, prostate, ovary, and testis, but is not detectable in peripheral blood leukocytes. Using fluorescence in situ hybridization on human metaphase chromosomes and by analyzing rodent-human hybrids by Southern blot hybridization and polymerase chain reaction amplification, we mapped the human YAP gene to chromosome band 11q13, a region to which the multiple endocrine neoplasia type 1 gene has been mapped.

Association of aciculin with dystrophin and utrophin

Aciculin is a recently identified 60-kDa cytoskeletal protein, highly homologous to the glycolytic enzyme phosphoglucomutase type 1, (Belkin, A. M., Klimanskaya, I. V., Lukashev, M. E., Lilley, K., Critchley, D., and Koteliansky, V. E. (1994) J. Cell Sci. 107, 159-173). Aciculin expression in skeletal muscle is developmentally regulated, and this protein is particularly enriched at cell-matrix adherens junctions of muscle cells (Belkin, A. M., and Burridge, K. (1994) J. Cell Sci. 107, 1993-2003). The purpose of our study was to identify cytoskeletal protein(s) interacting with aciculin in various cell types. Using immunoprecipitation from cell lysates of metabolically labeled differentiating C2C12 muscle cells with anti-aciculin-specific antibodies, we detected a high molecular weight band (M(r) approximately 400,000), consistently coprecipitating with aciculin. We showed that this 400 kDa band comigrated with dystrophin and immunoblotted with anti-dystrophin antibodies. The association between aciculin and dystrophin in C2C12 cells was shown to resist Triton X-100 extraction and the majority of the complex could be extracted only in the presence of ionic detergents. In the reverse immunoprecipitation experiments, aciculin was detected in the precipitates with different anti-dystrophin antibodies. Immunodepletion experiments with lysates of metabolically labeled C2C12 myotubes showed that aciculin is a major dystrophin-associated protein in cultured skeletal muscle cells. Double immunostaining of differentiating and mature C2C12 myotubes with antibodies against aciculin and dystrophin revealed precise colocalization of these two cytoskeletal proteins throughout the process of myodifferentiation in culture. In skeletal muscle tissue, both proteins are concentrated at the sarcolemma and at myotendinous junctions. In contrast, utrophin, an autosomal homologue of dystrophin, was not codistributed with aciculin in muscle cell cultures and in skeletal muscle tissues. Analytical gel filtration experiments with purified aciculin and dystrophin showed interaction of these proteins in vitro, indicating that their association in skeletal muscle is due to direct binding. Whereas dystrophin was shown to be a major aciculin-associated protein in skeletal muscle, immunoblotting of anti-aciculin immunoprecipitates with antibodies against utrophin showed that aciculin is associated with utrophin in cultured A7r5 smooth muscle cells and REF52 fibroblasts. Immunodepletion experiments performed with lysates of metabolically labeled A7r5 cells demonstrated that aciculin is a major utrophin-binding protein in this cell type. Taken together, our data show that aciculin is a novel dystrophin- and utrophin-binding protein. Association of aciculin with dystrophin (utrophin) in various cell types might provide an additional cytoskeletal-matrix transmembrane link at sites where actin filaments terminate at the plasma membrane.

Expression and subcellular localization of dystrophin in skeletal, cardiac and smooth muscles during the human development

Dystrophin, the product of the DMD gene, is present in all muscle types in normal individuals. Its function has yet to be elucidated, but its absence or the presence of a truncated version of the protein is responsible for the appearance of Duchenne and Becker muscular dystrophies. Using monoclonal antibodies raised against distinct regions of the dystrophin protein, we have examined its expression and subcellular distribution during the human development in skeletal and smooth muscles. We show that both dystrophin expression and its association to the plasma membrane take place earlier in cardiac and smooth muscles (8 weeks of gestation) than in skeletal muscle. In skeletal muscle, dystrophin is first detected in the cytoplasm, and progressively localizes to the plasma membrane from 10 weeks onwards. Since we have obtained marked differences in staining when using antibodies against either a central region of the protein or the C-terminal part, we suggest that different fetal and adult dystrophin isoforms are expressed, probably differing in their C-terminal domain. These findings are discussed in the context of the pathology of Duchenne muscular dystrophy.

Microlesions and polymorphisms in the Duchenne/Becker muscular dystrophy gene

One third of mutations responsible for Duchenne or Becker muscular dystrophy (DMD/BMD) represent point mutations or other small sequence alterations not readily detectable by Southern blot analysis or multiplex amplification. Here, we report results of a comprehensive point mutation search that yielded seven new sequence variations and one novel polymorphism. We also summarize known mutations, polymorphisms and other small nucleotide variations in the DMD gene. To date, 12 nonsense mutations, two missense mutations, six microdeletions and one microinsertion have been reported in the coding sequence and a further six mutations in splice sites all of which were made responsible for the disease. Twelve polymorphisms with frequencies suitable for diagnostic purposes have been detected. A further 28 differences from the published sequence of the coding sequence or the promoter region are described.

Utrophin localization in normal and dystrophin-deficient heart

BACKGROUND

The localization of dystrophin at the sarcolemma of cardiac skeletal fibers and cardiac Purkinje fibers has been described. Dystrophin deficiency produces clinical manifestations of disease in skeletal muscles and hearts of patients with Duchenne and Becker muscular dystrophy. Utrophin (or dystrophin-related protein), a dystrophin homologous protein, was found to be expressed in fetal muscles and reexpressed in dystrophin-deficient skeletal muscle fibers. We therefore examined utrophin expression in normal and in dystrophin-deficient hearts.

METHODS AND RESULTS

The expression and subcellular distribution of utrophin was examined in cardiac muscle by immunoblot and immunofluorescence analysis in normal bovine heart compared with dystrophin. Utrophin expression was also examined in normal and dystrophin-deficient hearts of MDX mice. Three monoclonal antibodies reacting with dystrophin and utrophin solely or reacting with both proteins along with two polyclonal antibodies reacting with either utrophin or dystrophin and utrophin were tested. In normal bovine heart, utrophin was not expressed at the periphery of fibers but was strongly expressed in intercalated disks and in the cytoplasm of cardiac Purkinje fibers. In cardiocytes, utrophin was colocalized along transverse T tubules with dystrophin. Dystrophin was present at the periphery of cardiocytes and cardiac Purkinje fibers as well as in transverse T tubules but was absent or faintly expressed in intercalated disks. The results with monoclonal and polyclonal antibodies were identical. Western blot analysis revealed that the detected molecules corresponded only to a 400-kD protein band and not to possible shorter transcripts of utrophin or dystrophin (apo-utrophin or apo-dystrophin). In dystrophin-deficient hearts of MDX mice, utrophin alone was abundant but not organized in the same networklike distribution.

CONCLUSIONS

This first localization of utrophin in normal heart (in Purkinje fibers, transverse tubules, and intercalated disks) showed a distinct subcellular localization of this protein with dystrophin, suggesting an important function of this protein in intercellular communication. In dystrophin-deficient hearts of MDX mice, utrophin alone is overexpressed as in skeletal muscle sarcolemma, an area normally occupied by dystrophin but not organized in the same networklike distribution.

Beta heavy-spectrin has a restricted tissue and subcellular distribution during Drosophila embryogenesis

The components of the membrane skeleton play an important role in maintaining membrane structure during the dynamic changes in cell shape that characterize development. beta Heavy-spectrin is a unique beta-spectrin from Drosophila melanogaster that is closer in size (M(r) = 430 × 10(3)) to dystrophin than to other beta-spectrin members of the spectrin/alpha-actinin/dystrophin gene super-family. Here we establish that both the subcellular localization of the beta Heavy-spectrin protein and the tissue distribution of beta Heavy-spectrin transcript accumulation change dramatically during embryonic development. Maternally loaded protein is uniformly distributed around the plasma membrane of the egg. During cellularization it is associated with the invaginating furrow canals and in a region of the lateral membranes at the apices of the forming cells (apicolateral). During gastrulation the apicolateral staining remains and is joined by a new apical cap, or plate, of beta Heavy-spectrin in areas where morphogenetic movements occur. These locations include the ventral and cephalic furrows and the posterior midgut invagination. Thus, dynamic rearrangement of the subcellular distribution of the protein is precisely coordinated with changes in cell shape. Zygotic message and protein accumulate after the germ band is fully extended, in the musculature, epidermis, hindgut, and trachea of the developing embryo. beta Heavy-spectrin in the epidermis, hindgut, and trachea is apically localized, while the protein in the somatic and visceral musculature is not obviously polarized. The distribution of beta Heavy-spectrin suggests roles in establishing an apicolateral membrane domain that is known to be rich in intercellular junctions and in establishing a unique membrane domain associated with contractile processes.

Deletion analysis of the dystrophin-actin binding domain

Three sequence motifs at the N-terminus of dystrophin have previously been proposed to be important for binding to actin. By analyzing a series of purified bacterial fusion proteins deleted for each of these sites we have demonstrated that none of the three are critical for dystrophin-actin interactions. Instead, our data suggest that sequences in the N-terminal 90 amino acids of dystrophin, excluding a conserved KTFT motif, contain the major site for interaction with actin.

Molecular organization at the glycoprotein-complex-binding site of dystrophin. Three dystrophin-associated proteins bind directly to the carboxy-terminal portion of dystrophin

Direct interaction between the C-terminal portion of dystrophin and dystrophin-associated proteins was investigated. The binding of dystrophin to each protein was reconstituted by overlaying bacterially expressed dystrophin fusion proteins onto the blot membranes to which dystrophin-associated proteins were transferred after separation by SDS/PAGE with the following results. (a) Among the components of the glycoprotein complex which links dystrophin to the sarcolemma, a 43-kDa dystrophin-associated glycoprotein binds directly to dystrophin. Although at least one of the binding sites of this protein resides within the cysteine-rich domain of dystrophin, a contribution of additional amino acid residues within the first half of the C-terminal domain was also suggested for more secure binding. (b) Two other proteins also directly bind to dystrophin. Their binding sites are suggested to reside within the last half of the C-terminal domain which is alternatively spliced depending on the tissue type. Previously, based on the enzyme digestion experiments, we showed that the binding site for the glycoprotein complex on dystrophin is present within the cysteine-rich domain and the first half of the C-terminal domain [Suzuki, A., Yoshida, M., Yamamoto, H. & Ozawa, E. (1992) FEBS Lett. 308, 154-160]. Here, we have extended this work and found that the region which is involved in interaction with the complex is widely extended to the entire length of this part of the molecule. On the basis of the present results, we propose a model of molecular architecture at the binding site for the complex on dystrophin.

Dystrophin-glycoprotein complex: its role in the molecular pathogenesis of muscular dystrophies

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is associated with a large oligomeric complex of sarcolemmal glycoproteins, including dystroglycan which provides a linkage to the extracellular matrix component, laminin. In patients with DMD, the absence of dystrophin leads to the loss in all of the dystrophin-associated proteins, causing the disruption of the linkage between the subsarcolemmal cytoskeleton and the extracellular matrix. This may render the sarcolemma vulnerable to physical stress. These recent developments in the research concerning the function of the dystrophin-glycoprotein complex pave a way for the better understanding of the pathogenesis of muscular dystrophies.

Expression of the transcripts initiated in the 62nd intron of the dystrophin gene

The pattern of expression of two distal transcripts initiated in the 62nd intron of the dystrophin gene was investigated under different circumstances; (i) during the development of different rat tissues these transcripts and Dp71, a protein encoded by one of them, increased with brain development and decreased with muscle development; (ii) in cultured glial and neuronal cells, the distal promoter was coactivated with tissue-specific upstream promoters, the muscle-type promoter in glial cells and the brain-type promoter in neuronal cells, which suggests that activity of the upstream promoter does not interfere with activity of the distal promoter; (iii) in lymphoblasts of DMD patients with various deletions of the dystrophin gene, the most distal of which included the 56th intron, the production of the distal transcript was not perturbed.

Dystrophin and dystrophin-related protein (utrophin) distribution in normal and dystrophin-deficient skeletal muscles

The respective localizations of dystrophin and dystrophin-related protein (DRP or utrophin) along the sarcolemmal membrane and at the neuromuscular junctions (NMJs) in normal and dystrophin-deficient skeletal muscles, were determined using confocal laser microscopy. The analysis was prompted by the recent availability of a new anti-utrophin mAb [Bewick et al. NeuroReport 1992; 3:857-860] and different mAbs that react with dystrphin or both dystrophin and utrophin. In dystrophin-deficient muscles, utrophin was expressed and detectable over large subcellular areas normally occupied by dystrophin along the sarcolemmal membranes and at the NMJs. Utrophin was expressed in a non-uniform, discontinuous way on the sarcolemmal membrane in dystrophin-deficient skeletal muscles, similar to dystrophin in normal muscle fibres. The respective distributions of both related muscle proteins and their positions relative to the alpha-bungarotoxin acetylcholine (ACh) receptor marker were determined. Double-staining experiments and superimposition of the confocal images showed that utrophin was more closely associated with ACh receptors than dystrophin at the NMJs in normal muscles. Utrophin distribution consequently differed from that of dystrophin.

Dystrophin and dystrophin-related protein expression in Torpedo marmorata electric organ

The presence of different dystrophin-related protein forms was investigated in electric organ as compared to cardiac, white or red skeletal muscles from Torpedo marmorata. Two strategies were followed. First, we used specific C-terminal dystrophin and dystrophin-related protein monoclonal antibodies which we characterized in the present study. 400 kDa protein bands were detected in the tissues mentioned above with both specific types of antibodies. Second, we produced monoclonal antibodies raised against a dystrophin-enriched preparation from T. marmorata electric organ. Western blot and immunofluorescence analyses showed the tissue specificity of T. marmorata antibodies and allowed us to classify them as types I, II and III. Vessel walls and neuromuscular junctions were labeled with T. marmorata type II and III antibodies in human muscles (skeletal and smooth). Both approaches demonstrated that the T. marmorata electric organ contained different proteins related with dystrophin: a dystrophin form, a dystrophin-related protein form and a dystrophin-related protein isoform, homologous to the dystrophin-related protein present in muscle vessel walls and at the neuromuscular junctions of human tissues. The presence of dystrophin and dystrophin-related protein is finally discussed relative to their functions and organ specificities.

Monoclonal antibodies targeted against the C-terminal domain of dystrophin or utrophin

The structure-function relationships of dystrophin, a protein which is absent or defective in patients with Duchenne or Becker muscular dystrophies, and utrophin can only be compared if specific antibodies are produced. We expressed C-terminal parts of dystrophin and utrophin in expression vectors. Mice were immunized with recombinant proteins and 26 monoclonal antibodies were produced and analyzed. Their respective epitopes were determined using other overlapping recombinant products. We observed antibody specificity towards 400 kDa dystrophin and/or utrophin protein bands, either by Western blot analysis or immunodetection in human skeletal (quadriceps) and smooth (uterus) muscles. These antibodies have been used to compare the relative abundance of both dystrophin and utrophin relative to the structures analyzed.

A case of Duchenne muscular dystrophy with truncated dystrophin. Significance of a cysteine-rich domain for functional expression of dystrophin protein

A Duchenne muscular dystrophy case showed truncated dystrophin (320 kDa) with an isoelectric point slightly shifted towards a more alkaline pH. From the polymerase chain reaction and immunochemical analysis data, the expressed dystrophin protein was predicted to lack the portion comprising the tail of the rod-like domain, the cysteine-rich domain, and the head of the C-terminal domain. These results indicated the functional importance of the cysteine-rich domain in the dystrophin protein.

The complete amino acid sequence for brain beta spectrin (beta fodrin): relationship to globin sequences

The amino acid sequence of mouse brain beta spectrin (beta fodrin), deduced from the nucleotide sequence of complementary DNA clones, reveals that this non-erythroid beta spectrin comprises 2363 residues, with a molecular weight of 274,449 Da. Brain beta spectrin contains three structural domains and we suggest the position of several functional domains including f-actin, synapsin I, ankyrin and spectrin self association sites. Analysis of deduced amino acid sequences indicated striking homology and similar structural characteristics of brain beta spectrin repeats beta 11 and beta 12 to globins. In vitro analysis has demonstrated that heme is capable of specific attachment to brain spectrin, suggesting possible new functions in electron transfer, oxygen binding, nitric oxide binding or heme scavenging.

The 87K postsynaptic membrane protein from Torpedo is a protein-tyrosine kinase substrate homologous to dystrophin

Postsynaptic peripheral membrane proteins at the neuromuscular junction have been proposed to participate in the immobilization of the nicotinic acetylcholine receptor at the synapse. An 87 kd cytoplasmic peripheral membrane protein has been demonstrated to colocalize with the nicotinic acetylcholine receptor in the Torpedo electric organ and at the mammalian neuromuscular junction. We have cloned the cDNA encoding the 87K protein from Torpedo electric organ, and the predicted protein sequence is homologous to the C-terminal domains of dystrophin, the protein product of the Duchenne muscular dystrophy gene. The 87K protein displays a restricted pattern of expression detected only in electric organ, brain, and skeletal muscle. Analysis of the in vitro and in vivo phosphorylation of the 87K protein indicates that it is multiply phosphorylated on serine, threonine, and tyrosine residues. The 87K protein is in a complex with other proteins associated with the postsynaptic membrane, including dystrophin and a 58 kd protein. These results suggest that the 87K protein is involved in the formation and stability of synapses and is regulated by protein phosphorylation.

Deficiency of dystrophin-associated proteins: a common mechanism leading to muscle cell necrosis in severe childhood muscular dystrophies

Dystrophin is a large cytoskeletal protein encoded by the Duchenne muscular dystrophy (DMD) gene. Dystrophin is associated with a large oligomeric complex of sarcolemmal glycoproteins, including the novel laminin-binding glycoprotein called dystroglycan, which provides a linkage to the extracellular matrix. In DMD, the absence of dystrophin leads to a drastic reduction in all of the dystrophin-associated proteins. In severe childhood autosomal recessive muscular dystrophy with DMD-like phenotype (SCARMD), a specific deficiency of the 50 kDa dystrophin-associated glycoprotein is found. Thus, the disruption/dysfunction of the dystrophin-glycoprotein complex due to the deficiency of one or more of the dystrophin-associated proteins is presumed to cause the disruption of the linkage between the subsarcolemmal cytoskeleton and the extracellular matrix. This may render muscle cells susceptible to necrosis in two forms of severe childhood muscular dystrophy, DMD and SCARMD.

PCR analysis of muscular dystrophy in mdx mice

PCR amplification has enabled a variety of studies to be performed on the murine dystrophin transcripts. Figure 7.12 displays a summary of the features of the murine dystrophin mRNA that have been described in this article. The location of the mutation in the original mdx mouse is indicated, as are the different spliced forms of the dystrophin transcript. Also shown are the location of various PCR primer binding sites that were used to deduce the alternative splicing pattern of the gene. It is likely that conventional cloning efforts aimed at identifying the variety of dystrophin spliced forms would have taken years to perform, particularly since several of the isoforms are expressed at levels significantly below the estimated 0.02% of total mRNA that dystrophin represents in skeletal muscle (Hoffman et al., 1987a, b). Amplification of dystrophin mRNA simplifies scanning methods for the identification of DNA sequence variations. Attempts to re-isolate and sequence the 14 kb cDNA to determine the mutation in separate strains of mdx mice are not likely to be time or cost effective. PCR enables these types of questions to be answered in a relatively short period of time, and similar types of analyses can be applied to human DMD tissues. Knowledge of the transcript diversity displayed by the dystrophin gene will enable the role of these separate isoforms to be addressed. Despite considerable effort by a variety of laboratories over the last five years, the precise functional role played by dystrophin remains unclear, and it can only be assumed that the separate isoforms act to modulate the functional role of dystrophin in separate tissues or in response to differing physiological states. PCR amplification of the dystrophin isoforms has enabled the variable regions of the transcript to be subcloned (Bies et al., 1992). These clones have been used to reintroduce the variable regions into full-length mini-gene expression vectors, which are currently being tested for functional activity through the generation of transgenic mdx mice. The transgenic mice can be easily identified through the PCR-ASO assays described in this article, and the reverse transcriptase PCR assays will enable a detailed analysis of the expression pattern of the introduced mini-genes. It is hoped that such analyses will further attempts to determine the feasibility of using gene therapy as a treatment for DMD/BMD.

Dystrophin and dystrophin-related proteins: a review of protein and RNA studies

The analysis of dystrophin gene expression has led to the identification of multiple transcripts and varying isoforms. The data indicate that transcription of the dystrophin gene occurs from several promoters, which involves developmental and tissue-dependent regulation. These discoveries have complicated the interpretation of immunolocalization studies, although there is a strong correlation between the amount and size of dystrophin and the severity of the clinical phenotype. The importance of using protein-specific antibodies for dystrophin analysis has been underscored by the identification of a protein, designated utrophin, which exhibits significant sequence homology with dystrophin. This review addresses the recent studies of dystrophin and utrophin expression in an attempt to illustrate the transcriptional diversity of these large genes and the localization of their protein products within various tissues.

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

Construction of dystrophin fusion proteins to raise targeted antibodies to different epitopes

For the study of the structure and function relationship of dystrophin, defective in DMD, and for diagnostic purposes it is important to dispose of antibodies against different parts of the protein. We have made five different constructs for the expression of fusion proteins containing parts of the four domains of dystrophin. Two different recombinant expression vectors, pATH2 and pEX1, were used. Rabbits were immunized with the fusion products and several polyclonal antibodies were raised. At a later stage, monoclonal antibodies were also raised to some of the fusion proteins. One polyclonal antibody, named P20 AB, is directed against the region covering amino acid sequence 1749-2248 or the nucleotide sequence 5456-6953 of the mRNA, which corresponds to the major deletion-prone region of the DMD gene. We show the particular value, sensitivity and specificity of the P20 AB in dystrophin analysis.

Glycoprotein-binding site of dystrophin is confined to the cysteine-rich domain and the first half of the carboxy-terminal domain

Dystrophin, a protein product of the Duchenne muscular dystrophy gene, is thought to associate with the muscle membrane by way of a glycoprotein complex which was co-purified with dystrophin. Here, we firstly demonstrate direct biochemical evidence for association of the carboxy-terminal region of dystrophin with the glycoprotein complex. The binding site is found to lie further inward than previously expected and confined to the cysteine-rich domain and the first half of the carboxy-terminal domain. Since this portion corresponds well to the region that, when missing, results in severe phenotypes, our finding may provide a molecular basis of the disease.

Determination of the exon structure of the distal portion of the dystrophin gene by vectorette PCR

The structure of the 3' one-third of the dystrophin gene has not previously been established. We have used vectorette PCR on a yeast artificial chromosome containing part of the human dystrophin gene to determine that there are 20 exons in this region and to characterize adjacent intron sequences of each one. Combined with previous information on the remainder of the gene, this study shows that the coding sequence is distributed between 79 exons. We have used PCR between exons to measure the distances that separate the more closely clustered exons. Vectorette PCR products were used as probes on Southern blots to assign all the 3' exons to genomic HindIII fragments that are commonly detected in the analysis of dystrophin gene deletions. The results will be useful for determining the effect of genomic deletions on the translational reading frame, for setting up genomic PCR assays to confirm point mutations, for analyzing splice site mutations, and for investigating potential cis-acting elements involved in tissue-specific alternative splicing. Vectorette PCR using primers derived from cDNA sequence represents an efficient and widely applicable method for establishing gene structure and obtaining intron sequence flanking exons, starting from a genomic clone and a cDNA sequence.

Proteolytic susceptibility of the central domain in chicken gizzard and skeletal muscle dystrophins

We investigated proteolytic susceptibility of the central domain in dystrophin molecules from chicken smooth and skeletal muscles. Dystrophin-enriched preparations from both muscles were made as described in Pons et al. (Proc. Natl. Acad. Sci. USA (1990) 87, 7851-7855). These preparations contained other protein components in addition to dystrophin. Three enzymes (Staphylococcus aureus proteinase, chymotrypsin and trypsin) having different proteolytic specificities were used. Time-courses of proteinase degradation were examined by the Western immunoblot technique using a specific polyclonal serum directed against a fragment (residues 1173-1728) of the dystrophin central domain. We observed accumulation of some major proteinase-resistant fragments, in the 110-160 kDa range originating from that central region of the molecule. Cleavage patterns of the smooth and skeletal muscle preparations were quite similar, but molecular weights of the breakdown products differed slightly. Interpretation of the results was based on two predictive structural models of the dystrophin central domain (Koenig and Kunkel (1990) J. Biol. Chem. 265, 4560-4566 and Cross et al. (1990) FEBS Lett. 262, 87-90). Skip residues at the end of repeat 13 (around the 1740th residue of the dystrophin amino acid sequence), as hypothesized in the Cross model, constitute probably the most sensitive site within the dystrophin central domain for any exogenous (or even endogenous) proteinase. Variations observed between dystrophins from skeletal and smooth muscles also suggest that the structures of both dystrophins differ slightly even within the dystrophin central domain. This precise identification of proteinase-resistant dystrophin fragments of variable lengths is a first step towards further physicochemical studies on the very large and rare dystrophin molecule.

Actin and neurofilament binding domain of brain spectrin beta subunit

Tryptic digestion of brain spectrin generates a number of fragments from alpha and beta subunits; when these fragments are incubated with F-actin or neurofilament light subunit, four of them with molecular masses below 30 kDa sediment with the cytoskeleton structures. A selective purification of these fragments by ammonium sulfate fractionation and butyl-Sepharose chromatography has been achieved. Two fragments with molecular masses of 28 and 23 kDa bind to F-actin. Native brain spectrin causes half-maximal inhibition of the association at a concentration of 3 microM. Protein sequencing indicates that the actin-binding domain is contained in the beta subunit, in a stretch of amino acids at the N terminus from Ala43 (28-kDa fragment) or from Met104 (23-kDa fragment) and terminate probably at the C-terminal Lys288 or Lys284. Amino acids are numbered by reference to the sequence of the Drosophila beta subunit. The 28-kDa fragment also binds to the low-molecular-mass subunit of neurofilaments; brain spectrin heterodimer disrupts this binding. Hence, spectrin binds to F-actin and to neurofilaments via a common binding domain.

A homologue of dystrophin is expressed at the blood vessel membrane of DMD and BMD patients: immunological evidence

Muscles from Becker muscular dystrophy (BMD) and Duchenne muscular dystrophy (DMD) patients were analysed using monoclonal and polyclonal antibodies raised against different regions of the dystrophin molecule. On blot, two of the antibodies detected a protein of Mr 400K in muscle extracts from all patients, including a BMD patient with a deletion which spanned more than 40% of the central rod domain of the Xp21 encoded dystrophin. Immunocytochemical labelling of tissue sections from the same patients showed that the same two antibodies labelled a protein at the surface membrane of smooth muscle fibers in blood vessels of both BMD and DMD muscles. Thus we have demonstrated a 400K blood vessel-associated protein, which is immunologically homologous with dystrophin, for at least two epitopes from the carboxy terminal and the central rod domains must be encoded by another gene than the dystrophin gene.

Becker muscular dystrophy patient with a large intragenic dystrophin deletion: implications for functional minigenes and gene therapy

The genetic defects responsible for the allelic disorders of BMD and the more severe DMD have been shown to be mutations within the dystrophin gene, which encodes a 14 kb transcript. We describe here a BMD patient who belongs to a small class of subjects with large in frame deletions of the dystrophin gene that remove apparently dispensable coding sequence, thereby producing functional truncated dystrophin. The in vitro reconstruction of these deletion derivatives of full length dystrophin transcripts should enable higher efficiency transfection of human muscle or murine germline cells using retroviral based vectors, compared with the full length transcript. This capability offers a means of examining retroviral mediated transfer as a potential therapeutic strategy in severely affected DMD patients.

Is the carboxyl-terminus of dystrophin required for membrane association? A novel, severe case of Duchenne muscular dystrophy

Duchenne muscular dystrophy is a lethal X-linked recessive disorder caused by the deficiency of a component of the muscle fiber membrane cytoskeleton called dystrophin. Becker muscular dystrophy, a clinically milder disorder, results from dystrophin abnormalities rather than deficiency. We identified the first patient who is clearly an exception to these established clinical and biochemical correlates. The patient described clinically had particularly severe Duchenne dystrophy. Biochemically, his muscle contained substantial amounts of abnormal dystrophin (Becker-like). Characterization of the dystrophin protein and gene revealed a unique intragenic gene deletion resulting in a dystrophin protein missing the carboxyl-terminal domain. This patient's dystrophin seemed to have a deleterious "dominant" effect on his muscle: The presence of this abnormal protein was more damaging to the myofibers than the absence of dystrophin would have been. This patient challenges the current hypothesis that dystrophin associates with the plasma membrane solely via its carboxyl-terminus, yet supports the hypothesis that an intact carboxyl-terminus is crucial for correct dystrophin function.

Point mutation in the human dystrophin gene: identification through western blot analysis

Using antibodies directed against the amino-terminus of dystrophin, we identified a truncated protein in a Duchenne muscular dystrophy patient. Antibodies directed against the carboxy-terminus failed to identify any cross-reactive material, a result consistent with premature termination of dystrophin translation. The estimated molecular mass of 126 kDa predicted the approximate location of the mutation in the mRNA and in the gene. Sequencing of cloned PCR products from patient muscle cDNA revealed a nonsense mutation, which was confirmed by direct sequencing of amplified patient genomic DNA. The mutation, a G to T transversion, at position 3714 changes a glutamic acid codon to an Amber stop codon. Translation of mRNA containing this mutation would be expected to result in a truncated protein with a molecular mass of 133 kDa, in close agreement with the 126 kDa estimated by Western blot analysis. This is the first reported case of a point mutation in this very large human gene.