Dystrophin and its isoforms

Current Outline of Exon Skipping Trials in Duchenne Muscular Dystrophy

Molecular treatments for Duchenne muscular dystrophy (DMD) are already in clinical practice. One particular means is exon skipping, an approach which has more than 15 years of background. There are several promising clinical trials based on earlier works. The aim is to be able to initiate the production of enough dystrophin to change the rate of progression and create a clinical shift towards the better. Some of these molecules already have received at least conditional approval by health authorities; however, we still need new accumulating data.

Synaptic alterations as a neurodevelopmental trait of Duchenne muscular dystrophy

Dystrophinopaties, e.g., Duchenne muscular dystrophy (DMD), Becker muscular dystrophy and X-linked dilated cardiomyopathy are inherited neuromuscular diseases, characterized by progressive muscular degeneration, which however associate with a significant impact on general system physiology. The more severe is the pathology and its diversified manifestations, the heavier are its effects on organs, systems, and tissues other than muscles (skeletal, cardiac and smooth muscles). All dystrophinopaties are characterized by mutations in a single gene located on the X chromosome encoding dystrophin (Dp427) and its shorter isoforms, but DMD is the most devasting: muscular degenerations manifests within the first 4 years of life, progressively affecting motility and other muscular functions, and leads to a fatal outcome between the 20s and 40s. To date, after years of studies on both DMD patients and animal models of the disease, it has been clearly demonstrated that a significant percentage of DMD patients are also afflicted by cognitive, neurological, and autonomic disorders, of varying degree of severity. The anatomical correlates underlying neural functional damages are established during embryonic development and the early stages of postnatal life, when brain circuits, sensory and motor connections are still maturing. The impact of the absence of Dp427 on the development, differentiation, and consolidation of specific cerebral circuits (hippocampus, cerebellum, prefrontal cortex, amygdala) is significant, and amplified by the frequent lack of one or more of its lower molecular mass isoforms. The most relevant aspect, which characterizes DMD-associated neurological disorders, is based on morpho-functional alterations of selective synaptic connections within the affected brain areas. This pathological feature correlates neurological conditions of DMD to other severe neurological disorders, such as schizophrenia, epilepsy and autistic spectrum disorders, among others. This review discusses the organization and the role of the dystrophin-dystroglycan complex in muscles and neurons, focusing on the neurological aspect of DMD and on the most relevant morphological and functional synaptic alterations, in both central and autonomic nervous systems, described in the pathology and its animal models.

Inward Rectifier Potassium Channels: Membrane Lipid-Dependent Mechanosensitive Gates in Brain Vascular Cells

Cerebral arteries contain two primary and interacting cell types, smooth muscle (SMCs) and endothelial cells (ECs), which are each capable of sensing particular hemodynamic forces to set basal tone and brain perfusion. These biomechanical stimuli help confer tone within arterial networks upon which local neurovascular stimuli function. Tone development is intimately tied to arterial membrane potential (VM) and changes in intracellular [Ca2+] driven by voltage-gated Ca2+ channels (VGCCs). Arterial VM is in turn set by the dynamic interplay among ion channel species, the strongly inward rectifying K+ (Kir) channel being of special interest. Kir2 channels possess a unique biophysical signature in that they strongly rectify, display negative slope conductance, respond to elevated extracellular K+ and are blocked by micromolar Ba2+. While functional Kir2 channels are expressed in both smooth muscle and endothelium, they lack classic regulatory control, thus are often viewed as a simple background conductance. Recent literature has provided new insight, with two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol, noted to (1) stabilize Kir2 channels in a preferred open or closed state, respectively, and (2) confer, in association with the cytoskeleton, caveolin-1 (Cav1) and syntrophin, hemodynamic sensitivity. It is these aspects of vascular Kir2 channels that will be the primary focus of this review.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Multiplex in situ hybridization within a single transcript: RNAscope reveals dystrophin mRNA dynamics

Dystrophin plays a vital role in maintaining muscle health, yet low mRNA expression, lengthy transcription time and the limitations of traditional in-situ hybridization (ISH) methodologies mean that the dynamics of dystrophin transcription remain poorly understood. RNAscope is highly sensitive ISH method that can be multiplexed, allowing detection of individual transcript molecules at sub-cellular resolution, with different target mRNAs assigned to distinct fluorophores. We instead multiplex within a single transcript, using probes targeted to the 5' and 3' regions of muscle dystrophin mRNA. Our approach shows this method can reveal transcriptional dynamics in health and disease, resolving both nascent myonuclear transcripts and exported mature mRNAs in quantitative fashion (with the latter absent in dystrophic muscle, yet restored following therapeutic intervention). We show that even in healthy muscle, immature dystrophin mRNA predominates (60-80% of total), with the surprising implication that the half-life of a mature transcript is markedly shorter than the time invested in transcription: at the transcript level, supply may exceed demand. Our findings provide unique spatiotemporal insight into the behaviour of this long transcript (with implications for therapeutic approaches), and further suggest this modified multiplex ISH approach is well-suited to long genes, offering a highly tractable means to reveal complex transcriptional dynamics.

Single-transcript multiplex in situ hybridisation reveals unique patterns of dystrophin isoform expression in the developing mammalian embryo

Background: The dystrophin gene has multiple isoforms: full-length dystrophin (dp427) is principally known for its expression in skeletal and cardiac muscle, but is also expressed in the brain, and several internal promoters give rise to shorter, N-terminally truncated isoforms with wider tissue expression patterns (dp260 in the retina, dp140 in the brain and dp71 in many tissues). These isoforms are believed to play unique cellular roles both during embryogenesis and in adulthood, but their shared sequence identity at both mRNA and protein levels makes study of distinct isoforms challenging by conventional methods. Methods: RNAscope is a novel in-situ hybridisation technique that offers single-transcript resolution and the ability to multiplex, with different target sequences assigned to distinct fluorophores. Using probes designed to different regions of the dystrophin transcript (targeting 5', central and 3' sequences of the long dp427 mRNA), we can simultaneously detect and distinguish multiple dystrophin mRNA isoforms at sub-cellular histological levels. We have used these probes in healthy and dystrophic canine embryos to gain unique insights into isoform expression and distribution in the developing mammal. Results: Dp427 is found in developing muscle as expected, apparently enriched at nascent myotendinous junctions. Endothelial and epithelial surfaces express dp71 only. Within the brain and spinal cord, all three isoforms are expressed in spatially distinct regions: dp71 predominates within proliferating germinal layer cells, dp140 within maturing, migrating cells and dp427 appears within more established cell populations. Dystrophin is also found within developing bones and teeth, something previously unreported, and our data suggests orchestrated involvement of multiple isoforms in formation of these tissues. Conclusions: Overall, shorter isoforms appear associated with proliferation and migration, and longer isoforms with terminal lineage commitment: we discuss the distinct structural contributions and transcriptional demands suggested by these findings.

Single-transcript multiplex in situ hybridisation reveals unique patterns of dystrophin isoform expression in the developing mammalian embryo

Background: The dystrophin gene has multiple isoforms: full-length dystrophin (dp427) is principally known for its expression in skeletal and cardiac muscle, but is also expressed in the brain, and several internal promoters give rise to shorter, N-terminally truncated isoforms with wider tissue expression patterns (dp260 in the retina, dp140 in the brain and dp71 in many tissues). These isoforms are believed to play unique cellular roles both during embryogenesis and in adulthood, but their shared sequence identity at both mRNA and protein levels makes study of distinct isoforms challenging by conventional methods. Methods: RNAscope is a novel in-situ hybridisation technique that offers single-transcript resolution and the ability to multiplex, with different target sequences assigned to distinct fluorophores. Using probes designed to different regions of the dystrophin transcript (targeting 5', central and 3' sequences of the long dp427 mRNA), we can simultaneously detect and distinguish multiple dystrophin mRNA isoforms at sub-cellular histological levels. We have used these probes in healthy and dystrophic canine embryos to gain unique insights into isoform expression and distribution in the developing mammal. Results: Dp427 is found in developing muscle as expected, apparently enriched at nascent myotendinous junctions. Endothelial and epithelial surfaces express dp71 only. Within the brain and spinal cord, all three isoforms are expressed in spatially distinct regions: dp71 predominates within proliferating germinal layer cells, dp140 within maturing, migrating cells and dp427 appears within more established cell populations. Dystrophin is also found within developing bones and teeth, something previously unreported, and our data suggests orchestrated involvement of multiple isoforms in formation of these tissues. Conclusions: Overall, shorter isoforms appear associated with proliferation and migration, and longer isoforms with terminal lineage commitment: we discuss the distinct structural contributions and transcriptional demands suggested by these findings.

Functional and structural features of proteins associated with alternative splicing

The alternative splicing is a mechanism increasing the number of expressed proteins and a variety of these functions. We uncovered the protein domains most frequently lacked or occurred in the splice variants. Proteins presented by several isoforms participate in such processes as transcription regulation, immune response, etc. Our results displayed the association of alternative splicing with branched regulatory pathways. By considering the published data on the protein proteins encoded by the 18th human chromosome, we noted that alternative products display the differences in several functional features, such as phosphorylation, subcellular location, ligand specificity, protein-protein interactions, etc. The investigation of alternative variants referred to the protein kinase domain was performed by comparing the alternative sequences with 3D structures. It was shown that large enough insertions/deletions could be compatible with the kinase fold if they match between the conserved secondary structures. Using the 3D data on human proteins, we showed that conformational flexibility could accommodate fold alterations in splice variants. The investigations of structural and functional differences in splice isoforms are required to understand how to distinguish the isoforms expressed as functioning proteins from the non-realized transcripts. These studies allow filling the gap between genomic and proteomic data.

Heart transplantation in patients with dystrophinopathic cardiomyopathy: Review of the literature and personal series

Cardiomyopathy associated with dystrophinopathies [Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), X-linked dilated cardiomyopathy (XL-dCM) and cardiomyopathy of Duchenne/Becker (DMD/BMD) carriers] is an increasing recognized manifestation of these neuromuscular disorders and notably contributes to their morbidity and mortality. Dystrophinopathic cardiomyopathy (DCM) is the result of the dystrophin protein deficiency at the myocardium level, parallel to the deficiency occurring at the skeletal muscle level. It begins as a "presymptomatic" stage in the first decade of life and evolves in a stepwise manner toward pictures of overt cardiomyopathy (hypertrophic stage, arrhythmogenic stage and dilated cardiomyopathy). The final stage caused by the extensive loss of cardiomyocytes results in an irreversible cardiac failure, characterized by frequent episodes of acute congestive heart failure (CHF), despite a correct pharmacological treatment. The picture of a severe dilated cardiomyopathy with intractable heart failure is typical of BMD, XL-dCM and cardiomyopathy of DMD/BMD carriers, while it is less frequently observed in patients with DMD. Heart transplantation (HT) is the only curative therapy for patients with dystrophinopathic end-stage heart failure who remain symptomatic despite an optimal medical therapy. However, no definitive figures exist in literature concerning the number of patients with DCM transplanted, and their outcome. This overview is to summarize the clinical outcomes so far published on the topic, to report the personal series of dystrophinopathic patients receiving heart transplantation and finally to provide evidence that heart transplantation is a safe and effective treatment for selected patients with end-stage DCM.

Novel mutation in exon 56 of the dystrophin gene in a child with Duchenne muscular dystrophy

Duchenne type muscular dystrophy (DMD) is an allelic X-linked recessive disorder caused by mutations in the gene encoding dystrophin. Genotype analysis has shown that deletion mutations account for approximately 65% of all cases, and 5-10% are duplications, while the remaining 30% of affected individuals may have smaller mutations, including point mutations, small deletions or small insertions. In this study, we present the case of a 4-year-old boy with typical clinical features of DMD, who developed normally until the age of 2. However, at age 3 he presented his first symptom, a tendency to fall, had difficulty in rising from the floor and in walking on his toes. At age 4 he had a waddling gait and could no longer climb stairs. A physical examination revealed proximal muscle weakness, calf hypertrophy, deep tendon hyporflexia and a positive Gower's sign. To identify the disease-causing gene in the proband, all coding regions (exons 1-79) of the dystrophin gene were PCR-amplified and sequenced. A novel duplication (c.8284dupA) in exon 56 of the dystrophin gene was identified, which was predicted to generate a frameshift mutation and create a premature termination codon (p.Ile2762Asnfs*10). This mutation was further confirmed by single-strand conformation polymorphism (SSCP) analysis, which revealed an extra band found in exon 56 of the dystrophin in the proband; however, this was not present in his family members or in the 100 matched normal controls. The data presented in this study may aid in expanding the spectrum of mutations causing DMD. To our knowledge, we demonstrate for the first time that a small duplication mutation can cause severe DMD.

Dystrophin deletions and cognitive impairment in Duchenne/Becker muscular dystrophy

Analyses of deletions in the dystrophin gene and of cognitive status were performed on patients with Duchenne (DMD) or Becker (BMD) muscular dystrophy in order to find a correlation between both features. Molecular study by multiplex and simplex PCR of dystrophin exons led to the identification of 51 deletions in 126 unrelated patients. Most of them were frameshift, in full agreement with severe clinical symptoms, three patients with a BMD-like phenotype had in-frame mutations. Deletions were localized with reference to the different dystrophin isoform sequences and were clustered in two main areas, 5' and central+ 3' end of the gene. Cognitive abilities were tested in 47 out of 51 patients with identified mutations, 23 of them being mentally impaired. Comparison of molecular and neuropsychological features showed that deletions localized in central and 3' parts of the gene (18 out of 23) are preferentially associated with mental impairment. Fourteen of them were found in the regulatory and coding sequences for the three CNS specific carboxy terminal isoforms. Therefore, though mutations with variable locations may lead to cognitive impairment, our results show that deletions in the distal portion of the gene are basically related to mental retardation.

Dystrophins, utrophins, and associated scaffolding complexes: role in mammalian brain and implications for therapeutic strategies

Two decades of molecular, cellular, and functional studies considerably increased our understanding of dystrophins function and unveiled the complex etiology of the cognitive deficits in Duchenne muscular dystrophy (DMD), which involves altered expression of several dystrophin-gene products in brain. Dystrophins are normally part of critical cytoskeleton-associated membrane-bound molecular scaffolds involved in the clustering of receptors, ion channels, and signaling proteins that contribute to synapse physiology and blood-brain barrier function. The utrophin gene also drives brain expression of several paralogs proteins, which cellular expression and biological roles remain to be elucidated. Here we review the structural and functional properties of dystrophins and utrophins in brain, the consequences of dystrophins loss-of-function as revealed by numerous studies in mouse models of DMD, and we discuss future challenges and putative therapeutic strategies that may compensate for the cognitive impairment in DMD based on experimental manipulation of dystrophins and/or utrophins brain expression.

Genetic isolation and characterization of a splicing mutant of zebrafish dystrophin

Sapje-like (sap(cl100)) was one of eight potential zebrafish muscle mutants isolated as part of an early-pressure screen of 500 families. This mutant shows a muscle tearing phenotype similar to sapje (dys-/-) and both mutants fail to genetically complement suggesting they have a mutation in the same gene. Protein analysis confirms a lack of dystrophin in developing sapje-like embryos. Sequence analysis of the sapje-like dystrophin mRNA shows that exon 62 is missing in the dystrophin transcript causing exon 63 to be translated out of frame terminating translation at a premature stop codon at the end of exon 63. Sequence analysis of sapje-like genomic DNA identified a mutation in the donor splice junction at the end of dystrophin exon 62. This mutation is similar to splicing mutations associated with human forms of Duchenne Muscular Dystrophy. Sapje-like is the first zebrafish dystrophin splicing mutant identified to date and represents a novel disease model which can be used in future studies to identify therapeutic compounds for treating diseases caused by splicing defects.

Selective vacuolar degeneration in dystrophin-deficient canine Purkinje fibers despite preservation of dystrophin-associated proteins with overexpression of Dp71

BACKGROUND

Respiratory support therapy significantly improves life span in patients with Duchenne muscular dystrophy; cardiac-related fatalities, including lethal arrhythmias, then become a crucial issue. It is therefore important to more thoroughly understand cardiac involvement, especially pathology of the conduction system, in the larger Duchenne muscular dystrophy animal models such as dystrophic dogs.

METHODS AND RESULTS

When 10 dogs with canine X-linked muscular dystrophy in Japan (CXMD(J)) were examined at the age of 1 to 13 months, dystrophic changes of the ventricular myocardium were not evident; however, Purkinje fibers showed remarkable vacuolar degeneration as early as 4 months of age. The degeneration of CXMD(J) Purkinje fibers was coincident with overexpression of Dp71 at the sarcolemma and translocation of mu-calpain to the cell periphery near the sarcolemma or in the vacuoles. Immunoblotting of the microdissected fraction showed that mu-calpain-sensitive proteins such as desmin and cardiac troponin-I or -T were selectively degraded in the CXMD(J) Purkinje fibers. Utrophin was highly upregulated in the earlier stage of CXMD(J) Purkinje fibers, but the expression was dislocated when vacuolar degeneration was recognized at 4 months of age. Nevertheless, the expression of dystrophin-associated proteins alpha-, beta-, gamma-, and delta-sarcoglycans and beta-dystroglycan was well maintained at the sarcolemma of Purkinje fibers.

CONCLUSIONS

Selective vacuolar degeneration of Purkinje fibers was found in the early stages of dystrophin deficiency. Dislocation of utrophin besides upregulation of Dp71 can be involved with this pathology. The degeneration of Purkinje fibers can be associated with the distinct deep Q waves in ECG and fatal arrhythmia seen in dystrophin deficiency.

The mouse dystrophin muscle promoter/enhancer drives expression of mini-dystrophin in transgenic mdx mice and rescues the dystrophy in these mice

Successful gene therapy for Duchenne muscular dystrophy (DMD) requires the restoration of dystrophin protein in skeletal muscles. To achieve this goal, appropriate regulatory elements that impart tissue-specific transgene expression need to be identified. Currently, most muscle-directed gene therapy studies utilize the muscle creatine kinase promoter. We have previously described a muscle enhancer element (mDME-1) derived from the mouse dystrophin gene that increases transcription from the mouse dystrophin muscle promoter. Here, we explore the use of this native mouse dystrophin muscle promoter/enhancer to drive expression of a human dystrophin minigene in transgenic mice. We show that the dystrophin promoter can provide tissue-specific transgene expression and that the mini-dystrophin protein is expressed at the sarcolemma of skeletal muscles from mdx mice, where it restores the dystrophin-associated glycoprotein complex. The level of transgene expression obtained is sufficient to protect mdx muscles from the morphological and physiological symptoms of muscular dystrophy, as well as from exercise-induced damage. Therefore, the dystrophin muscle promoter/enhancer sequence represents an alternative for use in gene therapy vectors for the treatment of DMD.

Cerebellar synaptic defects and abnormal motor behavior in mice lacking alpha- and beta-dystrobrevin

The dystrobrevins (alphaDB and betaDB) bind directly to dystrophin and are components of a transmembrane dystrophin-glycoprotein complex (DGC) that links the cytoskeleton to extracellular proteins in many tissues. We show here that alphaDB, betaDB, and dystrophin are all concentrated at a discrete subset of inhibitory synapses on the somata and dendrites of cerebellar Purkinje cells. Dystrophin is depleted from these synapses in mice lacking both alphaDB and betaDB, and DBs are depleted from these synapses in mice lacking dystrophin. In dystrophin mutants and alphaDB,betaDB double mutants, the size and number of GABA receptor clusters are decreased at cerebellar inhibitory synapses, and sensorimotor behaviors that reflect cerebellar function are perturbed. Synaptic and behavioral abnormalities are minimal in mice lacking either alphaDB or betaDB. Together, our results show that the DGC is required for proper maturation and function of a subset of inhibitory synapses, that DB is a key component of this DGC, and that interference with this DGC leads to behavioral abnormalities. We suggest that motor deficits in muscular dystrophy patients, which are their cardinal symptoms, may reflect not only peripheral derangements but also CNS defects.

Hybrid spectrin type repeats produced by exon-skipping in dystrophin

Dystrophin is the protein whose defect underlies Duchenne Muscular Dystrophy, DMD, a common (1:3500 male births) and fatal condition in which muscle tissue deteriorates leading to death in the second or third decade of life. Dystrophin is coded for by the largest human gene, and one of the most complex. It is translated from at least 7 distinct promoters, with the largest transcripts (which are the ones involved in DMD) containing 79 exons over >2.5 Mbp [K.F. O'Brien, L.M. Kunkel, Dystrophin and muscular dystrophy: past, present, and future, Mol. Genet. Metab. 74 (2001) 75-88, H.M. Sadoulet-Puccio, L.M. Kunkel, Dystrophin and its isoforms, Brain Pathol. 6 (1996) 25-35]. Exacerbating this complexity, it has recently been shown that dystrophin is subject to extensive alternative RNA processing, potentially producing a wide variety dystrophin variants [M. Sironi, R. Cagliani, U. Pozzoli, A. Bardoni, G.P. Comi, R. Giorda, N. Bresolin, The dystrophin gene is alternatively spliced throughout its coding sequence FEBS Lett 517 (2002) 163-166]. The structure of the dystrophin protein is highly modular, with the most common module being a motif termed the spectrin type repeat, or STR, of which there are 24. Each STR is roughly coded for by two exons, and the most common type of multiple exon-skipping events start and end at introns in the middle of STRs [R.G. Roberts, A.J. Coffey, M. Bobrow, D.R. Bentley, Exon structure of the human dystrophin gene Genomics 16 (1993) 536-538, M. Koenig, L.M. Kunkel, Detailed analysis of the repeat domain of dystrophin reveals four potential hinge segments that may confer flexibility, J. Biol. Chem. 265 (1990) 4560-4566]. This would produce fractional STR modules, however, the concept of STRs as proteins domains makes the viability of such fractional motifs questionable. However, certain of these events produce pairs of potentially complementary fractional domain that might reassemble into a hybrid STR motif. We have constructed model fragment corresponding to one such exon-skipping event, and show that the hybrid STR so produced is viable, and furthermore that some of the properties of the protein containing it differ substantially of the native, un-skipped parent.

Disruption of DMD and deletion of ACSL4 causing developmental delay, hypotonia, and multiple congenital anomalies

We have studied a male patient with significant developmental delay, growth failure, hypotonia, girdle weakness, microcephaly, and multiple congenital anomalies including atrial (ASD) and ventricular (VSD) septal defects. Detailed cytogenetic and molecular analyses revealed three de novo X chromosome aberrations and a karyotype 46,Y,der(X)inv(X) (p11.4q11.2)inv(X)(q11.2q21.32 approximately q22.2)del(X)(q22.3q22.3) was determined. The three X chromosome aberrations in the patient include: a pericentric inversion (inv 1) that disrupted the Duchenne muscular dystrophy (DMD) gene, dystrophin, at Xp11.4; an Xq11.2q21.32 approximately q22.2 paracentric inversion (inv 2) putatively affecting no genes; and an interstitial deletion at Xq22.3 that results in functional nullisomy of several known genes, including a gene previously associated with X-linked nonsyndromic mental retardation, acyl-CoA synthetase long chain family member 4 (ACSL4). These findings suggest that the disruption of DMD and the absence of ACSL4 in the patient are responsible for neuromuscular disease and cognitive impairment.

A new model mouse for Duchenne muscular dystrophy produced by 2.4 Mb deletion of dystrophin gene using Cre-loxP recombination system

Duchenne muscular dystrophy (DMD) is caused by mutation in the 2.4-Mb dystrophin (DMD) gene . This gene encodes a number of tissue-specific isoforms of dystrophin generated by transcription from at least seven promoters and also by alternative splicing. We deleted entire genomic region of the DMD gene on mouse chromosome X using a Cre-loxP recombination system. Introduction of a loxP site in dystrophin's first and last exon by homologous recombination in mouse embryonic stem (ES) cells generated "DMD-floxed" (flanked by loxP sites) ES cells, which we subjected to Cre-mediated excision leading to establishment of "DMD-null" ES cell lines. The DMD-null mice produced from the DMD-null ES cells were viable but displayed severe muscular hypertrophy and dystrophy. In addition to the muscular impairment, the DMD-null mouse exhibited some behavioral abnormality and male sterility. The DMD-floxed mice produced from the DMD-floxed ES cells were viable, phenotypically normal, and were born with the expected Mendelian frequency, despite the absence of brain (cortical)-type dystrophin (Dp427c) expression. Since production of multiple dystrophin isoforms due to alternative splicing or exon skipping is totally prevented in the DMD-null mouse, these new mutants will provide an improved model system for functional studies of dystrophin and its isoforms.

Molecular heterogeneity of the dystrophin-associated protein complex in the mouse kidney nephron: differential alterations in the absence of utrophin and dystrophin

The dystrophin-associated protein complex (DPC) consisting of syntrophin, dystrobrevin, and dystroglycan isoforms is associated either with dystrophin or its homolog utrophin. It is present not only in muscle cells, but also in numerous tissues, including kidney, liver, and brain. Using high-resolution immunofluorescence imaging and Western blotting, we have investigated the effects of utrophin and dystrophin gene deletion on the formation and membrane anchoring of the DPC in kidney epithelial cells, which co-express utrophin and low levels of the C-terminal dystrophin isoform Dp71. We show that multiple, molecularly distinct DPCs co-exist in the nephron; these DPCs have a segment-specific distribution and are only partially associated with utrophin in the basal membrane of tubular epithelial cells. In utrophin-deficient mice, a selective reduction of beta2-syntrophin has been observed in medullary tubular segments, whereas alpha1-syntrophin and beta1-syntrophin are retained, concomintant with an upregulation of beta-dystroglycan, beta-dystrobrevin, and Dp71. These findings suggest that beta2-syntrophin is dependent on utrophin for association with the DPC, and that loss of utrophin is partially compensated by Dp71, allowing the preservation of the DPC in kidney epithelial cells. This hypothesis is confirmed by the almost complete loss of all DPC proteins examined in mice lacking full-length utrophin and all C-terminal dystrophin isoforms (utrophin(0/0)/mdx(3Cv)). The DPC thus critically depends on these proteins for assembly and/or membrane localization in kidney epithelial cells.

The mouse dystrophin muscle enhancer-1 imparts skeletal muscle, but not cardiac muscle, expression onto the dystrophin Purkinje promoter in transgenic mice

A subset of patients harboring mutations in the dystrophin gene suffer from X-linked dilated cardiomyopathy (XLCM), a familial heart disease that is not accompanied by any clinical signs of skeletal muscle myopathy. As the muscle (M) isoform of dystrophin is not expressed in these patients, the absence of skeletal muscle symptoms has been attributed to expression of the brain (B) and cerebellar Purkinje (CP) isoforms of dystrophin in skeletal, but not cardiac, muscles of XLCM patients. The compensatory mechanism of dystrophin B and CP promoter upregulation is not known but it has been suggested that the dystrophin muscle enhancer from intron 1, DME-1, may be important in this activity. Previous studies have shown that the presence of the DME-1 is essential for a significant increase in dystrophin B and CP promoter activity in skeletal muscle cells in culture. Here, we demonstrate that the mouse dystrophin CP promoter drives expression of a lacZ reporter gene specifically to the cerebellar Purkinje cell layer but not to skeletal or cardiac muscle of transgenic mice. However, if the mouse counterpart of DME-1 is present in the transgene construct, the dystrophin CP promoter is now activated in skeletal muscle, but not in cardiac muscle. Our findings provide in vivo evidence for the importance of the dystrophin muscle enhancer sequences in activating the dystrophin CP promoter in skeletal muscle. Furthermore, they provide support for the model in which muscle enhancers, like DME-1, activate the dystrophin B and CP promoters in skeletal muscle, but not in cardiac muscle, of XLCM patients.

Dystrophin and mutations: one gene, several proteins, multiple phenotypes

A large and complex gene on the X chromosome encodes dystrophin. Many mutations have been described in this gene, most of which affect the expression of the muscle isoform, the best-known protein product of this locus. These mutations result in the Duchenne and Becker muscular dystrophies (DMD and BMD). However, there are several other tissue specific isoforms of dystrophin, some exclusively or predominantly expressed in the brain or the retina. Mutations affecting the correct expression of these tissue-specific isoforms have been associated with the CNS involvement common in DMD. Rare mutations also account for the allelic disorder X-linked dilated cardiomyopathy, in which dystrophin expression or function is affected mostly or exclusively in the heart. Genotype definition of the dystrophin gene in patients with dystrophinopathies has taught us much about functionally important domains of the protein itself and has provided insights into several regulatory mechanisms governing the gene expression profile. Here, we focus on current understanding of the genotype-phenotype relation for mutations in the dystrophin gene and their implications for gene functions.

Target selection for antisense oligonucleotide induced exon skipping in the dystrophin gene

BACKGROUND

Duchenne muscular dystrophy (DMD) is an X-linked recessive muscle wasting disorder characterised by the absence of the protein dystrophin. Antisense oligonucleotides have been used to re-direct dystrophin pre-mRNA processing by blocking sequences crucial to pre-mRNA splicing, thereby inducing skipping of specific exons. We wished to determine which splicing motifs are most amenable as targets for antisense oligonucleotide induction of efficient and specific skipping of selected exons.

METHODS

Antisense oligonucleotides were directed at regions of dystrophin exon 19 involved in pre-mRNA splicing, including the donor and acceptor splice sites and the exon splicing enhancer (ESE). Cultured myotubes were transfected with antisense oligonucleotides at various concentrations and studies undertaken to determine both specificity and efficiency of induced exon 19 skipping.

RESULTS

Antisense oligonucleotides as small as 12 nucleotides targeting the ESE induced consistent and specific skipping of only exon 19 in both human and normal and mdx mouse myotubes. Antisense oligonucleotides directed at the donor and acceptor splice sites also induced specific exon 19 skipping while mismatched antisense oligonucleotides could only induce skipping when delivered at higher concentrations. No other dystrophin exons were removed from the mature mRNA as a consequence of these antisense oligonucleotides treatments.

CONCLUSIONS

Antisense oligonucleotides directed at the ESE tended to be marginally more efficient than those which targeted the donor or acceptor splice sites, based on their ability to induce specific skipping at lower concentrations. The specificity of exon removal does not appear to be a function of target selection, but may reflect the combination of the splicing motifs and position of that exon in the pre-mRNA.

Na(+)/Ca(2+) exchange in human myotubes: intracellular calcium rises in response to external sodium depletion are enhanced in DMD

This study aims to investigate the sodium/calcium exchanger expression in human co-cultured skeletal muscle cells and to compare the effects of Na(+)/Ca(2+) exchange activity in normal and dystrophic (Duchenne's muscular dystrophy) human co-cultured myotubes. For this purpose, variations of intracellular calcium concentration ([Ca(2+)](int)) were monitored, as the variations of the fluorescence ratio of indo-1 probe, in response to external sodium depletion. External sodium withdrawal induced [Ca(2+)](int) rises within several seconds in both normal and Duchenne's muscular dystrophy myotubes. These Na(+)-free-induced [Ca(2+)](int) elevations were attributed to the reverse mode of the Na(+)/Ca(2+) exchange mechanism since the phenomenon was dependent on extracellular calcium concentration ([Ca(2+)](ext)), and since it was sensitive to external Ni(2+) ions. Amplitudes of Na(+)-free-induced [Ca(2+)](int) rises were significantly greater in Duchenne's muscular dystrophy cells than in normal ones. Such a difference disappeared when the sarcoplasmic reticulum was pharmacologically blocked, suggesting that the reverse mode of the Na(+)/Ca(2+) exchange mechanism was able to generate enhanced calcium-induced calcium-release in Duchenne's muscular dystrophy myotubes. Immunostaining images of Na(+)/Ca(2+) exchanger (NCX) isoforms, obtained by confocal microscopy, revealed the presence of NCX1 and NCX3 at the sarcolemmal level of both normal and Duchenne's muscular dystrophy myotubes. No differences were observed in the location of NCX isoforms expression between normal and Duchenne's muscular dystrophy co-cultured myotubes.

Duchenne muscular dystrophy: current knowledge, treatment, and future prospects

The cloning of the dystrophin gene has led to major advances in the understanding of the molecular genetic basis of Duchenne, Becker, and other muscular dystrophies associated with mutations in genes encoding members of the dystrophin-associated glycoprotein complex. The recent introduction of pharmaceutical agents such as prednisone has shown great promise in delaying the progression of Duchenne muscular dystrophy but there remains a need to develop more long-term therapeutic interventions. Knowledge of the nature of the dystrophin gene and the glycoprotein complex has led many researchers to think that somatic gene replacement represents the most promising approach to treatment. The potential use of this strategy has been shown in the mdx mouse model of Duchenne muscular dystrophy, where germ line gene transfer of either a full-length or a smaller Becker-type dystrophin minigene prevents necrosis and restores normal muscle function.

Sarcolemmal proteins and the spectrum of limb-girdle muscular dystrophies

Proteins of the sarcolemma are of crucial importance for the pathogenesis of muscular dystrophies. This update focuses on the dystrophin-associated proteins including the dystroglycan and sarcoglycan complexes, caveolin-3, dysferlin, and the extracellular matrix component collagen type VI. The molecular findings are correlated with some of the clinical phenotypes that are part of the limb-girdle muscular dystrophy spectrum, including fukutin-related proteinopathy (LGMD 21), the sarcoglycanopathies (LGMD 2C-F), caveolinopathy (LGMD 1C), dysferlinopathy (LGMD 2B), and finally Bethlem myopathy. Although recent progress has been tremendous, much remains to be learned about the pathophysiological consequences caused by a deficiency of any one of these components.

Dystrophin expression in muscle following gene transfer with a fully deleted ("gutted") adenovirus is markedly improved by trans-acting adenoviral gene products

Helper-dependent adenoviruses (HDAd) are Ad vectors lacking all or most viral genes. They hold great promise for gene therapy of diseases such as Duchenne muscular dystrophy (DMD), because they are less immunogenic than E1/E3-deleted Ad (first-generation Ad or FGAd) and can carry the full-length (Fl) dystrophin (dys) cDNA (12 kb). We have compared the transgene expression of a HDAd (HDAdCMVDysFl) and a FGAd (FGAdCMV-dys) in cell culture (HeLa, C2C12 myotubes) and in the muscle of mdx mice (the mouse model for DMD). Both vectors encoded dystrophin regulated by the same cytomegalovirus (CMV) promoter. We demonstrate that the amount of dystrophin expressed was significantly higher after gene transfer with FGAdCMV-dys compared to HDAdCMVDysFl both in vitro and in vivo. However, gene transfer with HDAdCMVDysFl in the presence of a FGAd resulted in a significant increase of dystrophin expression indicating that gene products synthesized by the FGAd increase, in trans, the amount of dystrophin produced. This enhancement occurred in cell culture and after gene transfer in the muscle of mdx mice and dystrophic golden retriever (GRMD) dogs, another animal model for DMD. The E4 region of Ad is required for the enhancement, because no increase of dystrophin expression from HDAdCMVDysFl was observed in the presence of an E1/E4-deleted Ad in vitro and in vivo. The characterization of these enhancing gene products followed by their inclusion into an HDAd may be required to produce sufficient dystrophin to mitigate the pathology of DMD by HDAd-mediated gene transfer.

Dystrophin and muscular dystrophy: past, present, and future

Duchenne muscular dystrophy was described in the medical literature in the early 1850s but the molecular basis of the disease was not determined until the late 1980s. The cloning of dystrophin led to the identification of a large complex of proteins that plays an important, although not yet well understood, role in muscle biology. Concomitant with the elucidation of the function of dystrophin and its associated proteins has been the pursuit of therapeutic options for muscular dystrophy. Although there is still no cure for this disorder, great advances are being made in the areas of gene introduction and cell transplant therapy.

Dystroglycan distribution in adult mouse brain: a light and electron microscopy study

Dystroglycan, originally identified in muscle as a component of the dystrophin-associated glycoprotein complex, is a ubiquitously expressed cell-surface receptor that forms a transmembrane link between the extracellular matrix and the cytoskeleton. It contains two subunits, alpha and beta, formed by proteolytic cleavage of a common precursor. In the brain, different neuronal subtypes and glial cells may express dystroglycan in complex with distinct cytoplasmic proteins such as dystrophin, utrophin and their truncated forms. To examine the distribution of dystroglycan in adult mouse brain, we raised antibodies against the recombinant amino- and carboxyl-terminal domains of alpha-dystroglycan. On western blot, the antibodies recognized specifically alpha-dystroglycan in cerebellar extracts. Using light microscopy, alpha-dystroglycan was found in neurons of the cerebral cortex, hippocampus, olfactory bulb, basal ganglia, thalamus, hypothalamus, brainstem and cerebellum, where dystrophin and its truncated isoforms are also known to be present. Electron microscopy revealed that alpha-dystroglycan immunoreactivity was preferentially associated with the postsynaptic specializations. Dystroglycan immunostaining was also detected in perivascular astrocytes and in those facing the pia mater, where utrophin and dystrophin truncated isoforms are present. The cell body and endfeet of astrocytes around blood vessels and the endothelial cells at the blood-brain barrier also expressed dystroglycan. From these data, we suggest that dystroglycan, by bridging the extracellular matrix and the cytoskeleton, may play an important functional role at specialized intercellular contacts, synapses and the blood-brain barrier, whose structural and functional organization strictly depend on the integrity of the extracellular matrix-cytoskeleton linkage.

Genomic organization and single-nucleotide polymorphism map of desmuslin, a novel intermediate filament protein on chromosome 15q26.3

BACKGROUND

Desmuslin is an alpha-dystrobrevin-interacting protein expressed primarily in heart and skeletal muscle. The desmuslin protein interacts with and is closely related to desmin, a protein encoded by a locus mutated in some forms of hereditary distal myopathy. As a muscle-specific intermediate filament protein, desmuslin is also a candidate for myopathies of unknown etiology.

RESULTS

The desmuslin gene was localized to chromosome 15q26.3 by electronic screening of the human DNA sequence database. Primer pairs were designed to amplify the 5 exons of the desmuslin gene in 11 overlapping DNA segments. The desmuslin gene was screened for mutations in 71 patients with various forms of myopathy for which there was no known cause. In this analysis, 10 common and 2 rare amino acid altering single-nucleotide polymorphisms were identified, all of which were seen in a control population of individuals thus making these unlikely causes of the phenotype. Interestingly, one of the single-nucleotide polymorphisms found in a patient resulted in a premature stop codon in the first exon. The nonsense mutation was also detected in the patient's unaffected father and one unaffected control; it was detected in 0.44% (2/454) of unrelated chromosomes and is therefore predicted to have a homozygous frequency of 0.002%.

CONCLUSION

No causative mutations were found in the desmuslin gene. However, the single-nucleotide polymorphisms mapped in this study represent a well-mapped group that can be used for disequilibrium studies of this region of chromosome 15q26.3.

Cryptic splicing involving the splice site mutation in the canine model of Duchenne muscular dystrophy

Golden retriever muscular dystrophy arises from a mutation in the acceptor splice site of intron 6 of the dystrophin gene. Skipping of exon 7 disrupts the mRNA reading frame and results in premature termination of translation. We are using this animal model to evaluate treatments for Duchenne muscular dystrophy, including gene repair induced by chimeric oligonucleotides. After injection of golden retriever muscular dystrophy (GRMD) muscle with a chimeric oligonucleotide to repair the lesion, immunostaining revealed a modest increase in the number of dystrophin-positive fibres at the injection sites. Dystrophin gene transcripts containing exon 7 were detected by reverse transcription-polymerase chain reaction, suggesting that low levels of splice site correction may have occurred. However, DNA sequencing of these apparently normal dystrophin gene transcripts revealed that the first five bases of exon 7 were missing. It will be important to be aware of this phenomenon with respect to further gene correction studies in the canine model.

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.

Alterations in dystrophin and utrophin expression parallel the reorganization of GABAergic synapses in a mouse model of temporal lobe epilepsy

Dystrophin and its autosomal homologue utrophin are coexpressed in muscle cells, and utrophin is functionally able to replace dystrophin in models of Duchenne muscular dystrophy. In brain, the two proteins are expressed differentially, suggesting distinct functional roles. Dystrophin is associated with postsynaptic GABA(A) receptors in hippocampus, cortex and cerebellum, whereas utrophin is present extrasynaptically, notably in large brainstem neurons. Here, the regulation of dystrophin and utrophin was investigated in a model of temporal lobe epilepsy. Adult mice were injected unilaterally with kainic acid into the dorsal hippocampus to induce loss of pyramidal cells and hypertrophy of dentate gyrus (DG) granule cells, as described (Suzuki, F., Junier, M.P., Guilhem, D., Sorensen, J.C. & Onteniente, B. (1995) Neuroscience, 64, 665--674.). These morphological changes were associated with an increase in postsynaptic GABA(A)-receptors in the ipsilateral DG, as demonstrated by a parallel increase in punctate immunoreactivity to GABA(A)-receptor alpha 2 subunit, gephyrin and dystrophin in the molecular layer. Thus, both dystrophin and gephyrin were involved in postsynaptic clustering of GABA(A) receptors. A transient induction of utrophin was seen at the onset of degeneration in CA1 and CA3 pyramidal cells and in the hilus. Most strikingly, however, utrophin immunoreactivity appeared in the granule cell layer of the DG and became very strong in hypertrophic granule cells 1--2 months post-kainate treatment. These results suggest that utrophin provides structural support of neuronal membranes, whereas dystrophin is a component of GABAergic synapses.

Syne-1, a dystrophin- and Klarsicht-related protein associated with synaptic nuclei at the neuromuscular junction

We describe a novel protein, Syne-1, that is associated with nuclear envelopes in skeletal, cardiac, and smooth muscle cells. Syne-1 contains multiple spectrin repeats similar to those found in dystrophin and utrophin, as well as a domain homologous to the carboxyl-terminal of Klarsicht, a protein associated with nuclei and required for a subset of nuclear migrations in Drosophila. In adult skeletal muscle fibers, levels of Syne-1 are highest in the nuclei that lie beneath the postsynaptic membrane at the neuromuscular junction. These nuclei are transcriptionally specialized, expressing genes for synaptic components at higher levels than extrasynaptic nuclei in the same cytoplasm. Syne-1 is the first protein found to be selectively associated with synaptic nuclei. Syne-1 becomes concentrated in synaptic nuclei postnatally. It remains synaptically enriched following denervation or degeneration/regeneration, and is also present at high levels in the central nuclei of dystrophic myotubes. The location and structure of Syne-1 suggest that it may participate in the migration of myonuclei in myotubes and/or their anchoring at the postsynaptic apparatus. Finally, we identify a homologous gene, syne-2, that is expressed in an overlapping but distinct pattern.

Differential expression of utrophin and dystrophin in CNS neurons: an in situ hybridization and immunohistochemical study

The cellular distribution of utrophin, the autosomal homologue of dystrophin, was investigated in developing and adult rat and mouse brain by in situ hybridization and immunohistochemistry. Digoxigenin-labeled cRNA probes complementary to N-terminal, rod-domain, and C-terminal encoding sequences of utrophin were used to differentiate between full-length and short C-terminal isoforms. Largely overlapping distribution patterns were seen for the three probes in neurons of cerebral cortex, accessory olfactory bulb, and several sensory and motor brainstem nuclei as well as in blood vessels, pia mater, and choroid plexus. The C-terminal probe was detected in addition in the main olfactory bulb, striatum, thalamic reticular nucleus, and hypothalamus, suggesting a selective expression of G-utrophin in these neurons. Western blot analysis with isoform-specific antisera confirmed the expression of both full-length and G-utrophin in brain. Immunohistochemically, only full-length utrophin was detected in neurons, in close association with the plasma membrane. In addition, intense staining was seen in blood vessels, meninges, and choroid plexus, selectively localized in the basolateral membrane of immunopositive epithelial cells. The expression pattern of utrophin was already established at early postnatal stages and did not change thereafter. Double-labeling analysis revealed that utrophin and dystrophin are differentially expressed on the cellular and subcellular levels in juvenile and adult brain. Likewise, in mice lacking full-length dystrophin isoforms (mdx mice), no change in utrophin expression and distribution could be detected in brain, although utrophin was markedly up-regulated in muscle cells. These results suggest that utrophin and dystrophin are independently regulated and have distinct functional roles in CNS neurons.

Identification and characterisation of transcript and protein of a new short N-terminal utrophin isoform

Dystrophin and utrophin are known to link the intracellular cytoskeleton to the extracellular matrix via a transmembraneous glycoprotein complex. Four short C-terminal isoforms (Dp71, Dp116, Dp140, and Dp260) are described for dystrophin and three for utrophin (Up71, Up113, and Up140). We describe here for the first time the existence of a 3.7-kb transcript and a 62-kDa protein in C6 glioma cells representing a short N-terminal isoform unique for utrophin (N-utrophin). More than 20 clones covering the entire coding region of utrophin were isolated from a rat C6 glioma cell cDNA library. Two clones were found to code for a protein with 539 amino acids. Its sequence is identical to that of the full-length utrophin, except for the last residue where Cys is replaced by Val. This isoform contains the actin binding domain (consisting of two calponin homology subdomains), followed by two spectrin-like repeats. A recombinant fragment corresponding to N-utrophin binds to F-actin in vitro with an equilibrium constant (affinity) K of 4.5 x 10(5) M(-1) and a stoichiometry of one fragment per around five actin monomers. Immunocytochemical staining of C6 glioma cells with antisera specific for different utrophin regions localised full-length utrophin in the submembraneous cortical actin layer as revealed by confocal microscopy. A distinct staining pattern for the N-utrophin was not detectable, although it was expected to localise at the actin stress fibers. It is assumed that it co-localises via the two spectrin-like repeats with the full-length utrophin at the cell membrane.

Brain dystrophin, neurogenetics and mental retardation

Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product, dystrophin, that exhibit nervous system developmental, regional and cell-type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling complexes whose composition is unique to each cellular and developmental context. Through additional interactions with a variety of proteins of the extracellular matrix, plasma membrane, cytoskeleton and distinct intracellular compartments, brain dystrophin acquires the capability to participate in the modulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and selected areas of the embryonic and postnatal neuraxis, and may regulate distinct aspects of neurogenesis, neuronal migration and cellular differentiation. By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experimental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regional cellular signal integration. At a systems level, dystrophin may regulate essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appears to impair intracellular calcium homeostasis and to disrupt multiple protein-protein interactions that normally promote information transfer and signal integration from the extracellular environment to the nucleus within regulated microdomains. In DMD/BMD, the individual profiles of cognitive and behavioral deficits, mental retardation and other phenotypic variations appear to depend on complex profiles of transcriptional regulation associated with individual dystrophin mutations that result in the corresponding presence or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional regulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationships between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with mental retardation.

The neurobiology of duchenne muscular dystrophy: learning lessons from muscle?

Several forms of inherited muscular dystrophy are associated with brain abnormalities and cognitive impairment. One of the most common and severe of these diseases is Duchenne muscular dystrophy (DMD). Dystrophin, the product of the DMD gene, is found in neurones, where it is associated with the postsynaptic membrane. Cognitive impairment in individuals with DMD is thought to be due to an abnormality in the neuronal membrane that is caused by lack of dystrophin. Recent experimental evidence has provided valuable clues in our understanding of the complex molecular neurobiology of muscular dystrophy.

Short communication: altered synaptic clustering of GABAA receptors in mice lacking dystrophin (mdx mice)

Dystrophin is selectively localized in the postsynaptic density of neurons in cerebral cortex, hippocampus and cerebellum. Here, we show by double-immunofluorescence staining that dystrophin is extensively colocalized with GABAA receptor subunit clusters in these brain regions. To determine the relevance of this observation, we investigated in mdx mice, which provide a model of Duchenne muscular dystrophy, whether the absence of dystrophin affects the synaptic clustering of GABAA receptors. A marked reduction in the number of clusters immunoreactive for the alpha1 and alpha2 subunits was observed in, respectively, cerebellum and hippocampus of mdx mice, but not in striatum, which is normally devoid of dystrophin. Furthermore, these alterations were not accompanied by a change in gephyrin staining, although gephyrin is colocalized with the majority of GABAA receptor clusters in these regions. These results indicate that dystrophin may play an important role in the clustering or stabilization of GABAA receptors in a subset of central inhibitory synapses. These deficits may underlie the cognitive impairment seen in Duchenne patients.

Unusual 5' transcript complexity of plectin isoforms: novel tissue-specific exons modulate actin binding activity

Plectin, the most versatile cytolinker identified to date, has essential functions in maintaining the mechanical integrity of skin, skeletal muscle and heart, as indicated by analyses of plectin-deficient mice and humans. Expression of plectin in a vast variety of tissues and cell types, combined with a large number of different binding partners identified at the molecular level, calls for complex mechanisms regulating gene transcription and expression of the protein. To investigate these mechanisms, we analyzed the transcript diversity and genomic organization of the murine plectin gene and found a remarkable complexity of its 5'-end structure. An unusually high number of 14 alternatively spliced exons, 11 of them directly splicing into plectin exon 2, were identified. Analysis of their tissue distribution revealed that expression of a few of them is restricted to tissues such as brain, or skeletal muscle and heart. In addition, we found two short exons tissue-specifically spliced into a highly conserved set of exons encoding the N-terminal actin binding domain (ABD), common to plectin and the superfamily of spectrin/dystrophin-type actin binding proteins. Using recombinant proteins we show that a novel ABD version contained in the muscle-specific isoform of plectin exhibits significantly higher actin binding activity than other splice forms. This fine tuning mechanism based on alternative splicing is likely to optimize the proposed biological role of plectin as a cytolinker opposing intense mechanical forces in tissues like striated muscle.

Role for alpha-dystrobrevin in the pathogenesis of dystrophin-dependent muscular dystrophies

A dystrophin-containing glycoprotein complex (DGC) links the basal lamina surrounding each muscle fibre to the fibre's cytoskeleton, providing both structural support and a scaffold for signalling molecules. Mutations in genes encoding several DGC components disrupt the complex and lead to muscular dystrophy. Here we show that mice deficient in alpha-dystrobrevin, a cytoplasmic protein of the DGC, exhibit skeletal and cardiac myopathies. Analysis of double and triple mutants indicates that alpha-dystrobrevin acts largely through the DGC. Structural components of the DGC are retained in the absence of alpha-dystrobrevin, but a DGC-associated signalling protein, nitric oxide synthase, is displaced from the membrane and nitric-oxide-mediated signalling is impaired. These results indicate that both signalling and structural functions of the DGC are required for muscle stability, and implicate alpha-dystrobrevin in the former.

Behavioral characterization of mdx3cv mice deficient in C-terminal dystrophins

Cognitive deficits are frequently associated with Duchenne muscular dystrophy (DMD). They might be due to a deficiency in the brain isoforms of the 427 kDa full-length dystrophin, and/or to altered expression of other C-terminal dystrophin-gene products (Dp71, Dp140) also found in brain. Mdx mice, which only lack full-length dystrophin in both muscle and brain, were previously shown to have moderate learning and memory deficits. In the present study, we investigated behavioral responses in mdx3cv mutants, which have altered expression of all the dystrophin-gene products. Contrary to the original mdx mice, mdx3cv mice showed enhanced anxiety-related behaviors and reduced locomotion as compared to control mice. Although those perturbations might be related to the lack in C-terminal dystrophins, they do not seem sufficient to induce strong learning deficits in this mutant. Indeed, we showed that mdx3cv mice may display similar or weaker deficits during the learning of a bar-pressing task, as compared to mdx mice. The relevance of the mdx3cv mutant as a model to study the cognitive deficits associated with DMD is discussed.