Developmental and tissue-specific regulation of mouse dystrophin: the embryonic isoform in muscular dystrophy

Increased susceptibility to ATP via alteration of P2X receptor function in dystrophic mdx mouse muscle cells

Pathological cellular hallmarks of Duchenne muscular dystrophy (DMD) include, among others, abnormal calcium homeostasis. Changes in the expression of specific receptors for extracellular ATP in dystrophic muscle have been recently documented: here, we demonstrate that at the earliest, myoblast stage of developing dystrophic muscle a purinergic dystrophic phenotype arises. In myoblasts of a dystrophin-negative muscle cell line established from the mdx mouse model of DMD but not in normal myoblasts, exposure to extracellular ATP triggered a strong increase in cytoplasmic Ca2+ concentrations. Influx of extracellular Ca2+ was stimulated by ATP and BzATP and inhibited by zinc, Coomassie Brilliant Blue-G, and KN-62, demonstrating activation of P2X7 receptors. Significant expression of P2X4 and P2X7 proteins was immunodetected in dystrophic myoblasts. Therefore, full-length dystrophin appears, surprisingly, to play an important role in myoblasts in controlling responses to ATP. Our results suggest that altered function of P2X receptors may be an important contributor to pathogenic Ca2+ entry in dystrophic mouse muscle and may have implications for the pathogenesis of muscular dystrophies. Treatments aiming at inhibition of specific ATP receptors could be of a potential therapeutic benefit.

Differential expression of syntrophins and analysis of alternatively spliced dystrophin transcripts in the mouse brain

Expression of syntrophin genes, encoding members of the dystrophin-associated protein complex, was studied in the mouse brain. In the hippocampal formation there is distinctive co-localization of specific syntrophins with certain dystrophin isoforms in neurons, e.g. alpha1-syntrophin with the C-dystrophin in CA regions and beta2-syntrophin with the G-dystrophin in the dentate gyrus. Expression of the alpha1-syntrophin is predominant in CA regions and the olfactory bulb and it is also present in the cerebral cortex and the dentate gyrus. The beta2-syntrophin mRNA is most abundant in the dentate gyrus and is also evident in the pituitary, the cerebral cortex and in Ammon's horn and in traces in the caudate putamen. The choroid plexus was labelled by both alpha1- and beta2-syntrophin-specific probes. The expression of syntrophins in the brain correlates with expression of dystrophins and dystroglycan. There are brain areas such as the cerebral cortex where several different syntrophins and dystrophins are expressed together. Syntrophin expression co-localizes with utrophin in the choroid plexus and caudate putamen. Finally, no syntrophin was detected in the cerebellar Purkinje cells where the specific dystrophin isoform (P-type) is present. This specific distribution of syntrophins in the brain is particularly interesting, as muscle syntrophin interacts with neuronal nitric oxide synthase. This may suggest that the dystrophin-associated protein complex may be involved in synaptic organisation and signal transduction machinery in both muscle and neurons. The dystrophin isoform, with exons 71-74 spliced out and hence lacking syntrophin binding sites, had been believed to be predominant in the brain, but our analyses using in situ hybridization, S1 nuclease protection and the semi-quantitative polymerase chain reaction revealed that this alternatively spliced mRNA is a minor, low abundance form in the brain.

Expression and regulation of the dystrophin Purkinje promoter in human skeletal muscle, heart, and brain

Dystrophin mRNA transcripts from the P (Purkinje) promoter were shown to be differentially expressed in human skeletal muscle, heart, and brain. The expression pattern was characteristic of tissue type and developmental stage. Polymerase chain reaction (PCR) analysis of the P promoter transcripts in adult skeletal muscle and adult brain identified two alternatively spliced sequences, one that encodes a full-length dystrophin mRNA and a second that transcribes a termination codon 27 nucleotides (8 amino acids) after the ATG initiation site. Alternative splicing of this truncated coding transcript was developmentally regulated, and it was expressed as the major form in adult cortical brain and adult heart. The biological significance of this peptide remains unclear. The full-length transcript was the major form in fetal cortical brain and adult skeletal muscle. Ribonuclease protection assay demonstrated that as much as 20% of dystrophin transcription in normal adult skeletal muscle was derived from the full-length transcript from the P promoter. In contrast, adult heart did not express significant levels of P promoter derived transcripts. Thus, transcripts from the P promoter were found to be developmentally regulated in the brain, and its activity was differentially expressed in skeletal versus cardiac muscle tissues. These data show that the P promoter transcript displays a broader scope of expression, regulation, and complexity than previously appreciated.

Absence of dystrophin and spectrin in regenerating muscle fibers from Becker dystrophy patients

We studied muscle biopsies from 36 Becker muscular dystrophy patients, and correlated dystrophin negative fibers with regenerating and degenerating myofibers. Dystrophin immunohistochemistry was used to identify dystrophin-negative and dystrophin-positive fibers. Immunohistochemical staining for fetal myosin and acid ATPase identified regenerating fibers, and calcium glioxalate and beta-spectrin staining identified necrotic fibers. All Becker biopsies contained detectable dystrophin in the majority of muscle fibers. 13 cases (36%) showed no dystrophin negative fibers, 9 cases (25%) showed a generalized, markedly decreased immunostaining pattern, and 14 cases (39%) showed a subset of dystrophin negative fibers (0.3-8% of total). Most dystrophin-negative fibers in Becker muscle were judged to be in the process of regeneration, and not in degeneration. No correlation was observed between the age of the patients and number of dystrophin negative fibers. We conclude that the absence of dystrophin and spectrin labeling in some BMD myofibers is associated with regeneration, probably due to incomplete expression of dystrophin secondary to myofibers immaturity. Our results might be explained by a developmental delayed expression of these two proteins, or by abnormal assembling in membrane's components during regeneration in dystrophy. Furthermore, our results rationalize the recently reported finding of some dystrophin-negative fibers in polymyositis.

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 related proteins

During the past year significant progress has been made in understanding how dystrophin deficiency leads to muscle cell necrosis in Duchenne muscular dystrophy and Becker muscular dystrophy. Dystrophin interacts with a glycoprotein complex spanning the muscle sarcolemma, effectively linking the actin cytoskeleton to the extracellular matrix. The carboxyl terminus of dystrophin is required for glycoprotein binding. Interestingly, at least three mRNAs transcribed from the distal end of the DMD gene in tissues other than muscle have been shown to encode this domain. Deficiency of a second component of the dystrophin-associated glycoprotein complex has been shown to occur in another muscle-wasting disorder, severe childhood autosomal recessive muscular dystrophy. Sequence analysis of the entire cDNA for the autosomal dystrophin-related protein utrophin has shown that dystrophin and utrophin are closely related. Furthermore, both of these proteins have been shown to bind to the same or a similar glycoprotein complex in muscle.

Dystrophin-positive myotubes in innervated muscle cultures from Duchenne and Becker muscular dystrophy patients

Nerve-muscle co-cultures from five Duchenne muscular dystrophy (DMD) patients and one Becker (BMD) patient, were studied by immunocytochemistry with antibodies against different portions of dystrophin. Four DMD patients had a deletion in the dystrophin gene. Some dystrophin-positive myotubes were detected in a few samples of all DMD cases. PCR amplification of exon 8 of the dystrophin gene ruled out a contamination from rat spinal cord during innervation. Our results in three DMD cases, may be explained by a clonal selection of dystrophin-positive fibers observed in muscle biopsies, while in the other two cases, a "frame-restoring" mutation might account for the presence of dystrophin-positive myotubes. The possible expression of "dystrophin-related protein" or dystrophin immature isoform was considered. In the BMD case an abnormal truncated dystrophin was found in innervated muscle cultures, as well as in muscle biopsy.

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.

Distal transcript of the dystrophin gene initiated from an alternative first exon and encoding a 75-kDa protein widely distributed in nonmuscle tissues

A transcript generated by the distal part of the Duchenne Muscular Dystrophy (DMD) gene was initially detected in cells where the full size 14-kilobase (kb) messenger RNA is not found at a significant level. This transcript, approximately 4.5 kb long, corresponds to the cysteine-rich and carboxyl-terminal domains of dystrophin. It begins with a novel 80- to 100-nucleotide exon containing an ATG start site for a new coding sequence of 17 nucleotides in-frame with the consecutive dystrophin cDNA sequence from exon 63. This result suggests the existence of a third promoter that would be localized about 8 kilobases upstream from exon 63 of the DMD gene. The distal transcript is widely distributed but is absent in adult skeletal and myometrial muscle. It is much more abundant in fetal tissues. With an antibody directed against the dystrophin carboxyl terminus, the protein corresponding to this transcript was detected as a 70- to 75-kDa entity on Western blots. It was found in all tissues analyzed except in skeletal muscle. It was not found in lymphoblastoid cells from a Duchenne patient with a complete deletion of the dystrophin gene. The role and subcellular localization of this protein is not known. It may explain extramuscular symptoms exhibited by some Duchenne patients.

Characterisation of dystrophin in fetuses at risk for Duchenne muscular dystrophy

Dystrophin, the product of the Duchenne muscular dystrophy (DMD) gene, was studied in muscle from 16 human fetuses at risk for the disease. Eleven high risk (greater than 95% probability) and 5 low-risk (less than 25% probability) fetuses were studied with antibodies raised to different regions of the protein. All low-risk fetuses showed a similar pattern to that of normal fetuses of a comparable age: using Western blot analysis, a protein was detected of similar size and abundance to that of normal fetuses (i.e. smaller molecular weight than that of adult muscle); immunocytochemistry showed uniform sarcolemmal staining in fetuses older than 18 weeks gestation and differential staining of myotubes at different stages of development (distinguished by size) in younger fetuses (less than 15 weeks gestation). In contrast, Western blot analysis of high-risk fetuses detected low levels of dystrophin in 4 cases; 7 fetuses had no detectable protein. Immunocytochemistry with some dystrophin antibodies showed weak staining of the sarcolemma and around central nuclei in younger fetuses; in older fetuses there was little sarcolemmal staining with any antibody other than occasional positive fibres. These results indicate that careful study of dystrophin in fetuses at risk for DMD can be used to establish the clinical phenotype and provide additional information for future family counselling.

Experimental regeneration in canine muscular dystrophy--1. Immunocytochemical evaluation of dystrophin and beta-spectrin expression

The expression of dystrophin and beta-spectrin was examined from 1 to 56 days in regenerating muscle fibres in normal and dystrophic dogs, following necrosis induced by the venom of Notechis scutatis. Normal and dystrophic dog muscle regenerated at an equal rate and new myotubes were present in both at the periphery of necrotic fibres by 3 days. In normal dogs dystrophin was detected in the sarcoplasm of the regenerating fibres by 3 days and was localized to the plasma membrane by 4 days. The localization of dystrophin is independent of beta-spectrin and was detected before beta-spectrin, which was not observed until 5-6 days. Normal peripheral labelling of both was restored by 14 days in normal dogs. Normal beta-spectrin labelling of regenerating dystrophic fibres was also restored by 14 days and is not dependent on the presence of dystrophin in dystrophic dogs. A proportion of regenerating fibres in normal and dystrophic dogs showed weak immunolabelling of beta-spectrin prior to 14 days. This is a feature of immature muscle fibres. Antibodies to different domains of dystrophin bound to the periphery and sarcoplasm of regenerating fibres in dystrophic dogs, particularly during the first 7 days of regeneration, but the fluorescence was less intense than in normal dogs. Weak labelling with antibodies corresponding to the C-terminus of the rod domain of dystrophin persisted on dystrophic regenerating fibres up to 21 days. This may relate to developmental isoforms of dystrophin.