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

Expression of Dystrophin Dp71 Splice Variants Is Temporally Regulated During Rodent Brain Development

Dystrophin Dp71 is the major product of the Duchenne muscular dystrophy (DMD) gene in the brain, and its loss in DMD patients and mouse models leads to cognitive impairments. Dp71 is expressed as a range of proteins generated by alternative splicing of exons 71 to 74 and 78, classified in the main Dp71d and Dp71f groups that contain specific C-terminal ends. However, it is unknown whether each isoform has a specific role in distinct cell types, brain regions, and/or stages of brain development. In the present study, we characterized the expression of Dp71 isoforms during fetal (E10.5, E15.5) and postnatal (P1, P7, P14, P21 and P60) mouse and rat brain development. We finely quantified the expression of several Dp71 transcripts by RT-PCR and cloning assays in samples from whole-brain and distinct brain structures. The following Dp71 transcripts were detected: Dp71d, Dp71d∆71, Dp71d∆74, Dp71d∆71,74, Dp71d∆71-74, Dp71f, Dp71f∆71, Dp71f∆74, Dp71f∆71,74, and Dp71fΔ71-74. We found that the Dp71f isoform is the main transcript expressed at E10.5 (> 80%), while its expression is then progressively reduced and replaced by the expression of isoforms of the Dp71d group from E15.5 to postnatal and adult ages. This major finding was confirmed by third-generation nanopore sequencing. In addition, we found that the level of expression of specific Dp71 isoforms varies as a function of postnatal stages and brain structure. Our results suggest that Dp71 isoforms have different and complementary roles during embryonic and postnatal brain development, likely taking part in a variety of maturation processes in distinct cell types.

The BALB/c.mdx62 mouse exhibits a dystrophic muscle pathology and is a model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating monogenic skeletal muscle-wasting disorder. Although many pharmacological and genetic interventions have been reported in preclinical studies, few have progressed to clinical trials with meaningful benefit. Identifying therapeutic potential can be limited by availability of suitable preclinical mouse models. More rigorous testing across models with varied background strains and mutations can identify treatments for clinical success. Here, we report the generation of a DMD mouse model with a CRISPR-induced deletion within exon 62 of the dystrophin gene (Dmd) and the first generated in BALB/c mice. Analysis of mice at 3, 6 and 12 months of age confirmed loss of expression of the dystrophin protein isoform Dp427 and resultant dystrophic pathology in limb muscles and the diaphragm, with evidence of centrally nucleated fibers, increased inflammatory markers and fibrosis, progressive decline in muscle function, and compromised trabecular bone development. The BALB/c.mdx62 mouse is a novel model of DMD with associated variations in the immune response and muscle phenotype, compared with those of existing models. It represents an important addition to the preclinical model toolbox for developing therapeutic strategies.

The unconditioned fear response in vertebrates deficient in dystrophin

Dystrophin loss due to mutations in the Duchenne muscular dystrophy (DMD) gene is associated with a wide spectrum of neurocognitive comorbidities, including an aberrant unconditioned fear response to stressful/threat stimuli. Dystrophin-deficient animal models of DMD demonstrate enhanced stress reactivity that manifests as sustained periods of immobility. When the threat is repetitive or severe in nature, dystrophinopathy phenotypes can be exacerbated and even cause sudden death. Thus, it is apparent that enhanced sensitivity to stressful/threat stimuli in dystrophin-deficient vertebrates is a legitimate cause of concern for patients with DMD that could impact neurocognition and pathophysiology. This review discusses our current understanding of the mechanisms and consequences of the hypersensitive fear response in preclinical models of DMD and the potential challenges facing clinical translatability.

Dp71 Point Mutations Induce Protein Aggregation, Loss of Nuclear Lamina Integrity and Impaired Braf35 and Ibraf Function in Neuronal Cells

Dystrophin Dp71 is the most abundant product of the Duchenne muscular dystrophy gene in the nervous system, and mutations impairing its function have been associated with the neurodevelopmental symptoms present in a third of DMD patients. Dp71 is required for the clustering of neurotransmitter receptors and the neuronal differentiation of cultured cells; nonetheless, its precise role in neuronal cells remains to be poorly understood. In this study, we analyzed the effect of two pathogenic DMD gene point mutations on the Dp71 function in neurons. We engineered C272Y and E299del mutations to express GFP-tagged Dp71 protein variants in N1E-115 and SH-SY5Y neuronal cells. Unexpectedly, the ectopic expression of Dp71 mutants resulted in protein aggregation, which may be mechanistically caused by the effect of the mutations on Dp71 structure, as predicted by protein modeling and molecular dynamics simulations. Interestingly, Dp71 mutant variants acquired a dominant negative function that, in turn, dramatically impaired the distribution of different Dp71 protein partners, including β-dystroglycan, nuclear lamins A/C and B1, the high-mobility group (HMG)-containing protein (BRAF35) and the BRAF35-family-member inhibitor of BRAF35 (iBRAF). Further analysis of Dp71 mutants provided evidence showing a role for Dp71 in modulating both heterochromatin marker H3K9me2 organization and the neuronal genes’ expression, via its interaction with iBRAF and BRAF5.

Generation of dystrophin short product-specific tag-insertion mouse: distinct Dp71 glycoprotein complexes at inhibitory postsynapse and glia limitans

Duchenne muscular dystrophy (DMD), the most severe form of dystrophinopathies, is a fatal X-linked recessive neuromuscular disorder characterized by progressive muscle degeneration and various extents of intellectual disabilities. Physiological and pathological roles of the responsible gene, dystrophin, in the brain remain elusive due to the presence of multiple dystrophin products, mainly full-length dystrophin, Dp427, and the short product, Dp71. In this study, we generated a Dp71-specific hemagglutinin (HA) peptide tag-insertion mice to enable specific detection of intrinsic Dp71 expression by anti-HA-tag antibodies. Immunohistochemical detections in the transgenic mice demonstrated Dp71 expression not only at the blood-brain barrier, where astrocytic endfeet surround the microvessels, but also at the inhibitory postsynapse of hippocampal dentate granule neurons. Interestingly, hippocampal cornu ammonis (CA)1 pyramidal neurons were negative for Dp71, although Dp427 detected by anti-dystrophin antibody was clearly present at the inhibitory postsynapse, suggesting cell-type dependent dystrophin expressions. Precise examination using the primary hippocampal culture validated exclusive localization of Dp71 at the inhibitory postsynaptic compartment but not at the excitatory synapse in neurons. We further performed interactome analysis and found that Dp71 formed distinct molecular complexes, i.e. synapse-associated Dp71 interacted with dystroglycan (Dg) and dystrobrevinβ (Dtnb), whereas glia-associated Dp71 did with Dg and dystrobrevinα (Dtna). Thus, our data indicate that Dp71 and its binding partners are relevant to the inhibitory postsynaptic function of hippocampal granule neurons and the novel Dp71-transgenic mouse provides a valuable tool to understand precise physiological expressions and functions of Dp71 and its interaction proteins in vivo and in vitro.

Differential expression of Dp71 and Dp40 isoforms in proliferating and differentiated neural stem cells: Identification of Dp40 splicing variants

Dp71 and Dp40 are the main products of the DMD gene in the central nervous system, and they are developmentally regulated from the early stages of embryonic development to adulthood. To further study the roles of Dp71 and Dp40 during cell proliferation and neural differentiation, we analyzed Dp71/Dp40 isoform expression at the mRNA level by RT-PCR assays to identify alternative splicing (AS) in the isoforms expressed in rat neural stem/progenitor cells (NSPCs) and in differentiated cells (neurons and glia). We found that proliferating NSPCs expressed Dp71d, Dp71dΔ71, Dp71f, Dp71fΔ71, Dp71dΔ74 and Dp40, as well as two Dp40 isoforms: Dp40Δ63,64 and Dp40Δ64-67. In differentiated cells we also found the expression of Dp71d, Dp71dΔ71, Dp71f, Dp71fΔ71 and Dp40. However, the expression frequencies were different in both stages. In addition, in differentiated cells, we found Dp71fΔ71-74, and interestingly, we did not find the expression of Dp71dΔ74 or the newly identified Dp40 isoforms. In this work we show that NSPC differentiation is accompanied by changes in Dp71/Dp40 isoform expression, suggesting different roles for these isoforms in NSPCs proliferation and neuronal differentiation, and we describe, for the first time, alternative splicing of Dp40.

Duchenne muscular dystrophy

Duchenne muscular dystrophy is a severe, progressive, muscle-wasting disease that leads to difficulties with movement and, eventually, to the need for assisted ventilation and premature death. The disease is caused by mutations in DMD (encoding dystrophin) that abolish the production of dystrophin in muscle. Muscles without dystrophin are more sensitive to damage, resulting in progressive loss of muscle tissue and function, in addition to cardiomyopathy. Recent studies have greatly deepened our understanding of the primary and secondary pathogenetic mechanisms. Guidelines for the multidisciplinary care for Duchenne muscular dystrophy that address obtaining a genetic diagnosis and managing the various aspects of the disease have been established. In addition, a number of therapies that aim to restore the missing dystrophin protein or address secondary pathology have received regulatory approval and many others are in clinical development.

Rescue of Defective Electroretinographic Responses in Dp71-Null Mice With AAV-Mediated Reexpression of Dp71

Purpose

To study the potential effect of a gene therapy, designed to rescue the expression of dystrophin Dp71 in the retinas of Dp71-null mice, on retinal physiology.

Methods

We recorded electroretinograms (ERGs) in Dp71-null and wild-type littermate mice. In dark-adapted eyes, responses to flashes of several strengths were measured. In addition, flash responses on a 25-candela/square meters background were measured. On- and Off-mediated responses to sawtooth stimuli and responses to photopic sine-wave modulation (3–30 Hz) were also recorded. After establishing the ERG phenotype, the ShH10-GFP adeno-associated virus (AAV), which has been previously shown to target specifically Müller glial cells (MGCs), was delivered intravitreously with or without (sham therapy) the Dp71 coding sequence under control of a CBA promoter. ERG recordings were repeated three months after treatment. Real-time quantitative PCR and Western blotting analyses were performed in order to quantify Dp71 expression in the retinas.

Results

Dp71-null mice displayed reduced b-waves in dark- and light-adapted flash ERGs and smaller response amplitudes to photopic rapid-on sawtooth modulation and to sine-wave stimuli. Three months after intravitreal injections of the ShH10-GFP-2A-Dp71 AAV vector, ERG responses were completely recovered in treated eyes of Dp71-null mice. The functional rescue was associated with an overexpression of Dp71 in treated retinas.

Conclusions

The present results show successful functional recovery accompanying the reexpression of Dp71. In addition, this experimental model sheds light on MGCs influencing ERG components, since previous reports showed that aquaporin 4 and Kir4.1 channels were mislocated in MGCs of Dp71-null mice, while their distribution could be normalized following intravitreal delivery of the same ShH10-GFP-2A-Dp71 vector.

Dystrophin Dp71 and the Neuropathophysiology of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by frameshift mutations in the DMD gene that prevent the body-wide translation of its protein product, dystrophin. Besides a severe muscle phenotype, cognitive impairment and neuropsychiatric symptoms are prevalent. Dystrophin protein 71 (Dp71) is the major DMD gene product expressed in the brain and mutations affecting its expression are associated with the DMD neuropsychiatric syndrome. As with dystrophin in muscle, Dp71 localises to dystrophin-associated protein complexes in the brain. However, unlike in skeletal muscle; in the brain, Dp71 is alternatively spliced to produce many isoforms with differential subcellular localisations and diverse cellular functions. These include neuronal differentiation, adhesion, cell division and excitatory synapse organisation as well as nuclear functions such as nuclear scaffolding and DNA repair. In this review, we first describe brain involvement in DMD and the abnormalities observed in the DMD brain. We then review the gene expression, RNA processing and functions of Dp71. We review genotype-phenotype correlations and discuss emerging cellular/tissue evidence for the involvement of Dp71 in the neuropathophysiology of DMD. The literature suggests changes observed in the DMD brain are neurodevelopmental in origin and that their risk and severity is associated with a cumulative loss of distal DMD gene products such as Dp71. The high risk of neuropsychiatric syndromes in Duchenne patients warrants early intervention to achieve the best possible quality of life. Unravelling the function and pathophysiological significance of dystrophin in the brain has become a high research priority to inform the development of brain-targeting treatments for Duchenne.

The polyaromatic hydrocarbon β-naphthoflavone alters binding of YY1, Sp1, and Sp3 transcription factors to the Dp71 promoter in hepatic cells

The smallest product of the Duchenne muscular dystrophy gene, dystrophin (Dp)71, is ubiquitously expressed in nonmuscle tissues. We previously showed that Dp71 expression in hepatic cells is modulated in part by stimulating factor 1 (Sp1), stimulating protein 3 (Sp3), and yin yang 1 (YY1) transcription factors, and that the polyaromatic hydrocarbon, β-naphthoflavone (β‑NF), downregulates Dp71 expression. The aim of the present study was to determine whether β‑NF represses Dp71 expression by altering mRNA stability or its promoter activity. Reverse transcription‑quantitative polymerase chain reaction was used to measure half‑life mRNA levels in β‑NF‑treated cells exposed to actinomycin D, an inhibitor of transcription, for 0, 4, 8, 12 and 16 h. Transient transfections with a plasmid carrying the Dp71 basal promoter fused to luciferase reporter gene were carried out in control and β‑NF‑treated cells. Electrophoretic mobility shift assays (EMSAs) were performed with labeled probes, corresponding to Dp71 promoter sequences, and nuclear extracts of control and β‑NF‑treated cells. To the best of our knowledge, the results demonstrated for the first time that this negative regulation takes place at the promoter level rather than the mRNA stability level. Interestingly, using EMSAs, β‑NF reduced binding of YY1, Sp1, and Sp3 to the Dp71 promoter. It also suggests that β‑NF may modulate the expression of other genes regulated by these transcription factors. In conclusion, β‑NF represses Dp71 expression in hepatic cells by altering binding of YY1, Sp1, and Sp3 to the Dp71 promoter.

Humanizing the mdx mouse model of DMD: the long and the short of it

Duchenne muscular dystrophy (DMD) is a common fatal heritable myopathy, with cardiorespiratory failure occurring by the third decade of life. There is no specific treatment for DMD cardiomyopathy, in large part due to a lack of understanding of the mechanisms underlying the cardiac failure. Mdx mice, which have the same dystrophin mutation as human patients, are of limited use, as they do not develop early dilated cardiomyopathy as seen in patients. Here we summarize the usefulness of the various commonly used DMD mouse models, highlight a model with shortened telomeres like humans, and identify directions that warrant further investigation.

Dystrophin and Spectrin, Two Highly Dissimilar Sisters of the Same Family

Dystrophin and Spectrin are two proteins essential for the organization of the cytoskeleton and for the stabilization of membrane cells. The comparison of these two sister proteins, and with the dystrophin homologue utrophin, enables us to emphasise that, despite a similar topology with common subdomains and a common structural basis of a three-helix coiled-coil, they show a large range of dissimilarities in terms of genetics, cell expression and higher level structural organisation. Interactions with cellular partners, including proteins and membrane phospholipids, also show both strikingly similar and very different behaviours. The differences between dystrophin and spectrin are also illustrated by the large variety of pathological anomalies emerging from the dysfunction or the absence of these proteins, showing that they are keystones in their function of providing a scaffold that sustains cell structure.

Overexpression of mutant dystrophin Dp71[INCREMENT]₇₈-₇₉ stimulates cell proliferation

Dp71 dystrophin is the main DMD gene product expressed in the central nervous system. Experiments using PC12 cells as a neuronal model have shown that Dp71 isoforms are involved in differentiation, adhesion, cell division, and nuclear architecture. To contribute to the knowledge of Dp71 domains function, we previously reported the isolation and partial characterization of the dystrophin Dp71[INCREMENT]78-79 (a mutant that lacks exons 71, 78, and 79), which stimulates the neuronal differentiation of PC12-C11 clone. In this article, we generated a doxycycline (Dox)-inducible expression system in PC12 Tet-On cells (B10 cells) to overexpress and control the transcription of Dp71[INCREMENT]78-79. Western blotting and confocal microscopy showed an increase in the amount of Dp71[INCREMENT]78-79 (217±75-fold) with the addition of Dox to growth medium. Cell proliferation assays and morphometric analyses demonstrated that Dp71[INCREMENT]78-79 increases the growth rate of B10 cells and reduces the nerve growth factor-neuronal differentiation. Western blotting analysis revealed an upregulation in the expression of proliferating cell nuclear antigen, focal adhesion kinase, and β-dystroglycan in B10 cells compared with control cells. Our results show that the inducible expression of Dp71[INCREMENT]78-79 increases the growth rate of PC12 Tet-On cells, suggesting a role of this protein in cell proliferation.

Transcription factors YY1, Sp1 and Sp3 modulate dystrophin Dp71 gene expression in hepatic cells

Dystrophin Dp71, the smallest product encoded by the Duchenne muscular dystrophy gene, is ubiquitously expressed in all non-muscle cells. Although Dp71 is involved in various cellular processes, the mechanisms underlying its expression have been little studied. In hepatic cells, Dp71 expression is down-regulated by the xenobiotic β-naphthoflavone. However, the effectors of this regulation remain unknown. In the present study we aimed at identifying DNA elements and transcription factors involved in Dp71 expression in hepatic cells. Relevant DNA elements on the Dp71 promoter were identified by comparing Dp71 5'-end flanking regions between species. The functionality of these elements was demonstrated by site-directed mutagenesis. Using EMSAs and ChIP, we showed that the Sp1 (specificity protein 1), Sp3 (specificity protein 3) and YY1 (Yin and Yang 1) transcription factors bind to the Dp71 promoter region. Knockdown of Sp1, Sp3 and YY1 in hepatic cells increased endogenous Dp71 expression, but reduced Dp71 promoter activity. In summary, Dp71 expression in hepatic cells is carried out, in part, by YY1-, Sp1- and Sp3-mediated transcription from the Dp71 promoter.

Nucleocytoplasmic shuttling of the Duchenne muscular dystrophy gene product dystrophin Dp71d is dependent on the importin α/β and CRM1 nuclear transporters and microtubule motor dynein

Even though the Duchenne muscular dystrophy (DMD) gene product Dystrophin Dp71d is involved in various key cellular processes through its role as a scaffold for structural and signalling proteins at the plasma membrane as well as the nuclear envelope, its subcellular trafficking is poorly understood. Here we map the nuclear import and export signals of Dp71d by truncation and point mutant analysis, showing for the first time that Dp71d shuttles between the nucleus and cytoplasm mediated by the conventional nuclear transporters, importin (IMP) α/β and the exportin CRM1. Binding was confirmed in cells using pull-downs, while in vitro binding assays showed direct, high affinity (apparent dissociation coefficient of c. 0.25nM) binding of Dp71d to IMPα/β. Interestingly, treatment of cells with the microtubule depolymerizing reagent nocodazole or the dynein inhibitor EHNA both decreased Dp71d nuclear localization, implying that Dp71d nuclear import may be facilitated by microtubules and the motor protein dynein. The role of Dp71d in the nucleus appears to relate in part to interaction with the nuclear envelope protein emerin, and maintenance of the integrity of the nuclear architecture. The clear implication is that Dp71d's previously unrecognised nuclear transport properties likely contribute to various, important physiological roles.

Characterization of 65 epitope-specific dystrophin monoclonal antibodies in canine and murine models of duchenne muscular dystrophy by immunostaining and western blot

Epitope-specific monoclonal antibodies can provide unique insights for studying cellular proteins. Dystrophin is one of the largest cytoskeleton proteins encoded by 79 exons. The absence of dystrophin results in Duchenne muscular dystrophy (DMD). Over the last two decades, dozens of exon-specific human dystrophin monoclonal antibodies have been developed and successfully used for DMD diagnosis. Unfortunately, the majority of these antibodies have not been thoroughly characterized in dystrophin-deficient dogs, an outstanding large animal model for translational research. To fill the gap, we performed a comprehensive study on 65 dystrophin monoclonal antibodies in normal and dystrophic dogs (heart and skeletal muscle) by immunofluorescence staining and western blot. For comparison, we also included striated muscles from normal BL10 and dystrophin-null mdx mice. Our analysis revealed distinctive species, tissue and assay-dependent recognition patterns of different antibodies. Importantly, we identified 15 antibodies that can consistently detect full-length canine dystrophin in both immunostaining and western blot. Our results will serve as an important reference for studying DMD in the canine model.

Monoclonal antibodies for clinical trials of Duchenne muscular dystrophy therapy

Most pathogenic mutations in Duchenne and Becker muscular dystrophies involve deletion of single or multiple exons from the dystrophin gene, so exon-specific monoclonal antibodies (mAbs) can be used to distinguish normal and mutant dystrophin proteins. In Duchenne therapy trials, mAbs can be used to identify or rule out dystrophin-positive "revertant" fibres, which have an internally-deleted dystrophin protein and which occur naturally in some Duchenne patients. Using phage-displayed peptide libraries, we now describe the new mapping of the binding sites of five dystrophin mAbs to a few amino-acids within single exons. The phage display method also confirmed previous mapping of MANEX1A (exon 1) and MANDRA1 (exon 77) by other methods. Of the 79 dystrophin exons, mAbs are now available against single exons 1, 6, 8, 12, 13, 14, 17, 21, 26, 28, 38, 41, 43, 44, 45, 46, 47, 50, 51, 58, 59, 62, 63, 75 and 77. Many have been used in clinical trials, as well as for diagnosis and studies of dystrophin isoforms.

Association between loss of dp140 and cognitive impairment in duchenne and becker dystrophies

The presence of variable degrees of non progressive cognitive impairment is recognized as a clinical feature of patients with Duchenne and Becker muscular dystrophies (DMD and BMD), but its pathogenesis still remains a matter of debate. A number of findings have proved that rearrangements located in the second part of the dystrophin ( DMD ) gene seem to be preferentially associated with cognitive impairment. Dp140 is a distal dystrophin isoform, mainly expressed during fetal brain development, whose role for neuropsychological functioning was suggested.

The aims of the current study were to explore the possible association between cognitive impairment and DNA mutations affecting the regulatory regions of Dp140, as well as to compare the neuropsychological functioning of patients affected with DMD and Intermediate muscular dystrophy (IMD) with those affected by Becker muscular dystrophy (BMD). Fiftythree patients genetically diagnosed with DMD, IMD and BMD, subdivided according to sites of mutations along the DMD gene, underwent a neuropsychological assessment, evaluating their general cognitive abilities, verbal memory, attention and executive functions. Twenty patients with mutations, terminating in exon 44 or starting at exon 45 were tested by polymerase chain reaction (PCR) amplification of microsatellites STR44, SK12, SK21 and P20 DXS269, in order to evaluate the integrity of the Dp140 promoter region.

According to our statistical results, there was not a significant difference in terms of general intelligence between the allelic forms of the disease, a higher frequency of mental retardation was observed in DMD patients. The patients with BMD had better results on tests, measuring long-term verbal learning memory and executive functions. We found that patients lacking Dp140 performed more poorly on all neuropsychological tests compared to those with preserved Dp140. Overall, our findings suggest that the loss of Dp140 is associated with a higher risk of intellectual impairment among patients with dystrophinopathies and highlights the possible role of this distal isoform in normal cognitive development.

Assessment of the structural and functional impact of in-frame mutations of the DMD gene, using the tools included in the eDystrophin online database

Background

Dystrophin is a large essential protein of skeletal and heart muscle. It is a filamentous scaffolding protein with numerous binding domains. Mutations in the DMD gene, which encodes dystrophin, mostly result in the deletion of one or several exons and cause Duchenne (DMD) and Becker (BMD) muscular dystrophies. The most common DMD mutations are frameshift mutations resulting in an absence of dystrophin from tissues. In-frame DMD mutations are less frequent and result in a protein with partial wild-type dystrophin function. The aim of this study was to highlight structural and functional modifications of dystrophin caused by in-frame mutations.

Methods and results

We developed a dedicated database for dystrophin, the eDystrophin database. It contains 209 different non frame-shifting mutations found in 945 patients from a French cohort and previous studies. Bioinformatics tools provide models of the three-dimensional structure of the protein at deletion sites, making it possible to determine whether the mutated protein retains the typical filamentous structure of dystrophin. An analysis of the structure of mutated dystrophin molecules showed that hybrid repeats were reconstituted at the deletion site in some cases. These hybrid repeats harbored the typical triple coiled-coil structure of native repeats, which may be correlated with better function in muscle cells.

Conclusion

This new database focuses on the dystrophin protein and its modification due to in-frame deletions in BMD patients. The observation of hybrid repeat reconstitution in some cases provides insight into phenotype-genotype correlations in dystrophin diseases and possible strategies for gene therapy. The eDystrophin database is freely available: http://edystrophin.genouest.org/.

Characterization of Dp71Δ(78-79), a novel dystrophin mutant that stimulates PC12 cell differentiation

Dp71 has an important role in the central nervous system. To better understand the function of Dp71 domains in neuronal differentiation, PC12 cells were stably transfected with a dystrophin mutant, Dp71Δ(78-79) , which lacks exons 78 and 79. Based on the percentage of cells bearing neurites and neurite length analyses, we found that cells stably expressing Dp71Δ(78-79) (PC12-C11) differentiate more efficiently than non-transfected cells. While wild-type cells reach their maximum differentiation 9-12 days after initiating the differentiation process, the PC12-C11 cells reach differentiation in 4-6 days. Protein expression analysis showed a down-regulation of Dp71a and an up-regulation of Dp71ab and/or Up71, β-dystroglycan and neuron-specific enolase in undifferentiated and in neural growth factor differentiated PC12-C11 cells. No change was observed in the expression of Grb2 and Up400. The subcellular localization of Dp71Δ(78-79) was in the cell periphery, and there was no change in localization during the differentiation process, which was also observed throughout the neurite extensions.

Knockdown of dystrophin Dp71 impairs PC12 cells cycle: localization in the spindle and cytokinesis structures implies a role for Dp71 in cell division

The function of dystrophin Dp71 in neuronal cells remains to be established. Previously, we revealed the involvement of this protein in both nerve growth factor (NGF)-induced neuronal differentiation and cell adhesion by isolation and characterization of PC12 neuronal cells with depleted levels of Dp71. In this work, a novel phenotype of Dp71-knockdown cells was characterized, which is their delayed growth rate. Cell cycle analyses revealed an altered behavior of Dp71-depleted cells, which consists of a delay in G0/G1 transition and an increase in apoptosis during nocodazole-induced mitotic arrest. Dp71 associates with lamin B1 and β-dystroglycan, proteins involved in aspects of the cell division cycle; therefore, we compared the distribution of Dp71 with that of lamin B1 and β-dystroglycan in PC12 cells at mitosis and cytokinesis by means of immunofluorescence and confocal microscopy analysis. All of these three proteins exhibited a similar immunostaining pattern, localized at mitotic spindle, cleavage furrow, and midbody. It is noteworthy that a drastic decreased staining in mitotic spindle, cleavage furrow, and midbody was observed for both lamin B1 and β-dystroglycan in Dp71-depleted cells. Furthermore, we demonstrated the interaction of Dp71 with lamin B1 in PC12 cells by immunoprecipitation and pull-down assays, and importantly, we revealed that knockdown of Dp71 expression caused a marked reduction in lamin B1 levels and altered localization of the nuclear envelope protein emerin. Our data indicate that Dp71 is a component of the mitotic spindle and cytokinesis multi-protein apparatuses that might modulate the cell division cycle by affecting lamin B1 and β-dystroglycan levels.

Brain metabolite composition in relation to cognitive function and dystrophin mutations in boys with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a hereditary X-linked recessive disorder affecting the synthesis of dystrophin, a protein essential for structural stability in muscle. Dystrophin also occurs in the central nervous system, particularly in the neocortex, hippocampus and cerebellum. Quantitative metabolic analysis by localized (1) H MRS was performed in the cerebellum (12 patients and 15 controls) and a temporo-parietal location (eight patients and 15 controls) in patients with DMD and healthy controls to investigate possible metabolic differences. In addition, the site of individual mutations on the dystrophin gene was analyzed and neuropsychological cognitive functions were examined. Cognitive deficits in the patient group were found in line with earlier investigations, mainly concerning verbal short-term memory, visuo-spatial long-term memory and verbal fluency, but also the full-scale IQ. Causal mutations were identified in all patients with DMD. Quantitative MRS showed consistent choline deficits, in both cerebellar white matter and temporo-parietal cortex, as well as small, but significant, metabolic abnormalities for glutamate and total N-acetyl compounds in the temporo-parietal region. Compartment water analysis did not reveal any abnormalities. In healthy subjects, choline levels were age related in the cerebellum. The choline deficit contrasts with earlier findings in DMD, where a surplus of choline was postulated for the cerebellum. In patients, total N-acetyl compounds in the temporo-parietal region were related to verbal IQ and verbal short-term memory. However, choline, the putative main metabolic abnormality, was not found to be associated with cognitive deficits. Furthermore, in contrast with the cognitive performance, the metabolic brain composition did not depend significantly on whether or not gene mutations concerned the expression of the dystrophin isoform Dp140, leading to the conclusion that the effect of the missing Dp140 isoform on cognitive performance is not mediated through the observed metabolite composition, or is caused by local effects beyond the resolution accessible to MRS investigations.

Exon-specific dystrophin antibodies for studies of Duchenne muscular dystrophy

Exon-specific anti-dystrophin antibodies are used to monitor the success of treatments for Duchenne muscular dystrophy that aim to restore the missing dystrophin protein. Dystrophin is a large cytoskeletal protein encoded by 79 exons and expressed mainly in muscle. Most cases of Duchenne and Becker muscular dystrophies are caused by genetic deletion of one or more exons. In-frame deletions permit some synthesis of internally-deleted dystrophin and cause the milder Becker form, while out-of-frame deletions in the severe Duchenne form result in early stop-codons and no functional dystrophin synthesis. In this study, we describe the production of ten new monoclonal antibodies against a rod region encoded by exons 55–59 and their mapping to specific dystrophin exons, thus filling a major gap in the spectrum of available antibodies. The antibodies have already been applied in a published clinical trial of a drug treatment for Duchenne muscular dystrophy.

Induction of dystrophin Dp71 expression during neuronal differentiation: opposite roles of Sp1 and AP2alpha in Dp71 promoter activity

In this study, we delineated the molecular mechanisms that modulate Dp71 expression during neuronal differentiation, using the N1E-115 cell line. We demonstrated that Dp71 expression is up-regulated in response to cAMP-mediated neuronal differentiation of these cells, and that this induction is controlled at promoter level. Functional deletion analysis of the Dp71 promoter revealed that a 5'-flanking 159-bp DNA fragment that contains Sp1 and AP2 binding sites is necessary and sufficient for basal expression of this TATA-less promoter, as well as for its induction during neuronal differentiation. Electrophoretic mobility shift and chromatin immunoprecipitation assays revealed that Sp1 and AP2alpha bind to their respective DNA elements within the Dp71 basal promoter. Overall, mutagenesis assays on the Sp1 and AP2 binding sites, over-expression of Sp1 and AP2alpha, as well as knock-down experiments on Sp1 and AP2alpha gene expression established that Dp71 basal expression is controlled by the combined action of Sp1 and AP2alpha, which act as activator and repressor, respectively. Furthermore, we demonstrated that induction of Dp71 expression in differentiated cells is the result of the maintenance of positive regulation exerted by Sp1, as well as of the loss of AP2alpha binding, which ultimately releases the promoter from repression.

Dystrophin Dp71 is critical for stability of the DAPs in the nucleus of PC12 cells

We have adopted the PC12 cell line as in vitro cell model for studying Dp71 function in neuronal cells. These cells express a cytoplasmic (Dp71f) and a nuclear (Dp71d) isoform of Dp71 as well as various dystrophin-associated proteins (DAPs). In this study, we revealed by confocal microscopy analysis and Western blotting evaluation of cell fractions the presence of different DAPs (beta-dystroglycan, beta-dystrobrevin, epsilon-sarcoglycan and gamma1-syntrophin) in the nucleus of PC12 cells. Furthermore, we established by immunoprecipitation assays that Dp71d and the DAPs form a dystrophin-associated protein complex (DAPC) in the nucleus. Interestingly, depletion of Dp71 by antisense treatment (antisense-Dp71 cells) provoked a drastic reduction of nuclear DAPs, which indicates that Dp71d is critical for DAPs stability within the nucleus. Although Up71, the utrophin gene product homologous to Dp71, exhibited increased expression in the antisense-Dp71 cells, its scarce nuclear levels makes unlikely that could compensate for Dp71 nuclear deficiency.

Insertion of the IL1RAPL1 gene into the duplication junction of the dystrophin gene

Duplications of one or more exons of the dystrophin gene are the second most common mutation in dystrophinopathies. Even though duplications are suggested to occur with greater complexity than thought earlier, they have been considered an intragenic event. Here, we report the insertion of a part of the IL1RAPL1 (interleukin-1 receptor accessory protein-like 1) gene into the duplication junction site. When the actual exon junction was examined in 15 duplication mutations in the dystrophin gene by analyzing dystrophin mRNA, one patient was found to have an unknown 621 bp insertion at the junction of duplication of exons from 56 to 62. Unexpectedly, the inserted sequence was found completely identical to sequences of exons 3-5 of the IL1RAPL1 gene that is nearly 100 kb distal from the dystrophin gene. Accordingly, the insertion of IL1RAPL1 exons 3-5 between dystrophin exons 62 and 56 was confirmed at the genomic sequence level. One junction between the IL1RAPL1 intron 5 and dystrophin intron 55 was localized within an Alu sequence. These results showed that a fragment of the IL1RAPL1 gene was inserted into the duplication junction of the dystrophin gene in the same direction as the dystrophin gene. This suggests the novel possibility of co-occurrence of complex genomic rearrangements in dystrophinopathy.

Analysis of Dp71 contribution in the severity of mental retardation through comparison of Duchenne and Becker patients differing by mutation consequences on Dp71 expression

The presence of variable degrees of cognitive impairment, extending from severe mental retardation to specific deficits, in patients with dystrophinopathies is a well-recognized problem. However, molecular basis underlying mental retardation and its severity remain poorly understood and still a matter of debate. Here, we report one of the largest study based on the comparison of clinical, cognitive, molecular and expression data in a large cohort of 81 patients affected with Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) bearing mutations predicted to affect either all dystrophin products, including Dp71 or all dystrophin products, except Dp71. In addition to the consistent data defining molecular basis underlying mental retardation in DMD, we show that BMD patients with MR have mutations that significantly affect Dp71 expression or with mutations located in exons 75 and 76. We also show that mutations upstream to exon 62, with DMD phenotype, predicted to lead to a loss-of-function of all dystrophin products, except Dp71 isoform, are associated, predominantly, with normal or borderline cognitive performances. Altogether, these reliable phenotype-genotype correlations in combination with Dp71 mRNA and protein expression studies, strongly indicate that loss-of-function of all dystrophin products is systematically associated with severe form of MR, and Dp71 deficit is a factor that contributes in the severity of MR and may account for a shift of 2 SD downward of the intelligence quotient.

Dp71f modulates GSK3-beta recruitment to the beta1-integrin adhesion complex

Previously, it was shown that Dp71f binds to the beta1-integrin adhesion complex to modulate PC12 cell adhesion. The absence of Dp71f led to a failure in the beta1-integrin adhesion complex formation. One of the structural proteins which links the beta1-integrin cytoplasmic domain to the actin cytoskeleton is ILK. GSK3-beta is an ILK substrate and the carboxi-terminal region of dystrophin 427 is a substrate for hierarchical phosphorylation by GSK3-beta. Dp71f contains the carboxi-terminal domain present in dystrophin 427. By using co-immunoprecipitation assays, in the present work it is demonstrated that in the neuronal PC12 cell line an interaction between Dp71f and GSK3-beta occurs. This interaction was corroborated by in vitro pulldown assays. We show that GSK3-beta is recruited to the beta1-integrin complex and that a reduced expression of Dp71f induces a reduced GSK3-beta recruitment to the beta1-integrin complex. In addition, the present work establishes that adhesion of PC12 cells to laminin does not influence the phosphorylation status of Dp71f.

Synemin expression in brain

beta-Synemin has been identified as an alpha-dystrobrevin-interacting protein in human muscle, although at least two synemin transcripts are expressed in brain. To understand synemin's function in neural tissue, in situ and immunohistochemical analyses were performed to identify where alpha- and beta-synemin are expressed in the brain of C57BL/6 and mdx (dystrophin null) mice. This analysis shows that the alpha- and beta-synemin transcripts and their encoded proteins colocalize in neurons, especially in the midbrain and pons. Since alpha-dystrobrevin-1 and synemin do not colocalize in brain as in muscle, this suggests that another member of the dystrophin-associated protein complex might interact with synemin in brain. In support of this, synemin mRNA expression was decreased in mdx mice, suggesting that synemin transcription is linked to dystrophin expression. Our findings show where synemin is expressed in brain and allow one to speculate with regard to its function in neural tissue.

Calcium misregulation and the pathogenesis of muscular dystrophy

Although the exact nature of the relationship between calcium and the pathogenesis of Duchenne muscular dystrophy (DMD) is not fully understood, this is an important issue which has been addressed in several recent reviews (Alderton and Steinhardt, 2000a, Gailly, 2002, Allen et al., 2005). A key question when trying to understand the cellular basis of DMD is how the absence or low level of expression of dystrophin, a cytoskeletal protein, results in the slow but progressive necrosis of muscle fibres. Although loss of cytoskeletal and sarcolemmal integrity which results from the absence of dystrophin clearly plays a key role in the pathogenesis associated with DMD, a number of lines of evidence also establish a role for misregulation of calcium ions in the DMD pathology, particularly in the cytoplasmic space just under the sarcolemma. A number of calcium-permeable channels have been identified which can exhibit greater activity in dystrophic muscle cells, and exIsting evidence suggests that these may represent different variants of the same channel type (perhaps the transient receptor potential channel, TRPC). In addition, a prominent role for calcium-activated proteases in the DMD pathology has been established, as well as modulation of other intracellular regulatory proteins and signaling pathways. Whether dystrophin and its associated proteins have a direct role in the regulation of calcium ions, calcium channels or intracellular calcium stores, or indirectly alters calcium regulation through enhancement of membrane tearing, remains unclear. Here we focus on areas of consensus or divergence amongst the existing literature, and propose areas where future research would be especially valuable.

Characterization of a novel Dp71 dystrophin-associated protein complex (DAPC) present in the nucleus of HeLa cells: members of the nuclear DAPC associate with the nuclear matrix

Dystrophin is an essential component in the assembly and maintenance of the dystrophin-associated protein complex (DAPC), which includes members of the dystroglycan, syntrophin, sarcoglycan and dystrobrevin protein families. Distinctive complexes have been described in the cell membrane of different tissues and cultured cells. In this work, we report the identification and characterization of a novel DAPC present in the nuclei of HeLa cells, which contains dystrophin Dp71 as a key component. Using confocal microscopy and cell fractionation analyses, we found the presence of Dp71, beta-sarcoglycan, beta-dystroglycan, alpha- and beta-syntrophin, alpha1- and beta-dystrobrevin and nNOS in the nuclei of HeLa cells. Furthermore, we demonstrated by co-immunoprecipitation experiments that most of these proteins form a complex in the nuclear compartment. Next, we analyze the possible association of the nuclear DAPC with the nuclear matrix. We found the presence of Dp71, beta-dystroglycan, nNOS, beta-sarcoglycan, alpha/beta syntrophin, alpha1-dystrobrevin and beta-dystrobrevin in the nuclear matrix protein fractions and in situ nuclear matrix preparations from HeLa cells. Moreover, we found that Dp71, beta-dystroglycan and beta-dystrobrevin co-immunoprecipitated with the nuclear matrix proteins lamin B1 and actin. The association of members of the nuclear DAPC with the nuclear matrix indicates that they may work as scaffolding proteins involved in nuclear architecture.

Dystrophin Dp71f associates with the beta1-integrin adhesion complex to modulate PC12 cell adhesion

Dystrophin Dp71 is the main product of the Duchenne muscular dystrophy gene in the brain; however, its function is unknown. To study the role of Dp71 in neuronal cells, we previously generated by antisense treatment PC12 neuronal cell clones with decreased Dp71 expression (antisense-Dp71 cells). PC12 cells express two different splicing isoforms of Dp71, a cytoplasmic variant called Dp71f and a nuclear isoform called Dp71d. We previously reported that antisense-Dp71 cells display deficient adhesion to substrate and reduced immunostaining of beta1-integrin in the cell area contacting the substrate. In this study, we isolated additional antisense-Dp71 clones to analyze in detail the potential involvement of Dp71f isoform with the beta1-integrin adhesion system of PC12 cells. Immunofluorescence analyses as well as immunoprecipitation assays demonstrated that the PC12 cell beta1-integrin adhesion complex is composed of beta1-integrin, talin, paxillin, alpha-actinin, FAK and actin. In addition, our results showed that Dp71f associates with most of the beta1-integrin complex components (beta1-integrin, FAK, alpha-actinin, talin and actin). In the antisense-Dp71 cells, the deficiency of Dp71 provokes a significant reduction of the beta1-integrin adhesion complex and, consequently, the deficient adhesion of these cells to laminin. In vitro binding experiments confirmed the interaction of Dp71f with FAK and beta1-integrin. Our data indicate that Dp71f is a structural component of the beta1-integrin adhesion complex of PC12 cells that modulates PC12 cell adhesion by conferring proper complex assembly and/or maintenance.

Phosphorylation of dystrophin Dp71d by Ca2+/calmodulin-dependent protein kinase II modulates the Dp71d nuclear localization in PC12 cells

We have shown that the splicing isoform of Dp71 (Dp71d) localizes to the nucleus of PC12 cells, an established cell line derived from a rat pheochromocytoma; however, the mechanisms governing its nuclear localization are unknown. As protein phosphorylation modulates the nuclear import of proteins, and as Dp71d presents several potential sites for phosphorylation, we analyzed whether Dp71d is phosphorylated in PC12 cells and the role of phosphorylation on its nuclear localization. We demonstrated that Dp71d is phosphorylated under basal conditions at serine and threonine residues by endogenous protein kinases. Dp71d phosphorylation was activated by 2-O-tetradecanoyl phorbol 13-acetate (TPA), but this effect was blocked by EGTA. Supporting the role of intracellular calcium on Dp71d phosphorylation, we observed that the stimulation of calcium influx by cell depolarization increased Dp71d phosphorylation, and that the calcium-calmodulin inhibitor N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7) blocked such induction. The blocking action of bisindolylmaleimide I (Bis I), a specific inhibitor for Ca2+/diacylglicerol-dependent protein kinase (PKC), on Dp71d phosphorylation suggested the participation of PKC in this event. In addition, transfection experiments with Ca2+/calmodulin-dependent protein kinase II (CaMKII) expression vectors as well as the use of KN-62, a CaMKII-specific inhibitor, demonstrated that CaMKII is also involved in Dp71d phosphorylation. Stimulation of Dp71d phosphorylation by cell depolarization and/or the overexpression of CaMKII favored the Dp71d nuclear accumulation. Overall, our results indicate that CAMKII-mediated Dp71d phosphorylation modulates its nuclear localization.

Role of dystrophins and utrophins in platelet adhesion process

Platelets are crucial at the site of vascular injury, adhering to the sub-endothelial matrix through receptors on their surface, leading to cell activation and aggregation to form a haemostatic plug. Platelets display focal adhesions as well as stress fibres to contract and facilitate expulsion of growth and pro-coagulant factors contained in the granules and to constrict the clot. The interaction of F-actin with different actin-binding proteins determines the properties and composition of the focal adhesions. Recently, we demonstrated the presence of dystrophin-associated protein complex corresponding to short dystrophin isoforms (Dp71d and Dp71) and the uthophin gene family (Up400 and Up71), which promote shape change, adhesion, aggregation, and granule centralisation. To elucidate participation of both complexes during the platelet adhesion process, their potential association with integrin beta-1 fraction and the focal adhesion system (alpha-actinin, vinculin and talin) was evaluated by immunofluorescence and immunoprecipitation assays. It was shown that the short dystrophin-associated protein complex participated in stress fibre assembly and in centralisation of cytoplasmic granules, while the utrophin-associated protein complex assembled and regulated focal adhesions. The simultaneous presence of dystrophin and utrophin complexes indicates complementary structural and signalling mechanisms to the actin network, improving the platelet haemostatic role.

β-naphthoflavone represses dystrophin Dp71 expression in Hepa-1 cells

Dystrophin Dp71 is expressed in hepatic tissue; however, its function in this tissue remains unknown. The Dp71 promoter sequence contains conserved CACGC motifs, which constitute the invariant core sequence of xenobiotic-regulatory elements. These elements function as target sites for the aryl hydrocarbon receptor/aryl hydrocarbon nuclear translocator (Ahr/ARNT) in genes regulated by this transcription factor. Thus, Dp71 expression in hepatic cells would be regulated by Ahr signaling. In this study, the effect of the xenobiotics beta-Naphthoflavone (betaNF), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and Benzo[a]Pyrene (BaP) on Dp71 expression was analyzed in Hepa-1 cells. It was demonstrated that betaNF, but not BaP or TCDD, represses Dp71 expression at both transcriptional and translational levels. To test directly the involvement of the Ahr signaling in the negative regulation of Dp71, we analyzed the effect of betaNF on Dp71 expression in the liver of wild type (Ahr+/+) and AHR-null (Ahr-/-) mice. The Dp71 mRNA repression, caused by the betaNF treatment, was also found in the liver tissue of wild type mice; however, such negative effect was reversed in the liver of AHR-null mice, which supports the participation of the Ahr signaling in Dp71 downregulation. Modulation of Dp71 expression by betaNF may represent a novel mechanism of Ahr action.

The Drosophila homologue of the dystrophin gene - introns containing promoters are the major contributors to the large size of the gene

We show that the drosophila gene encoding the dystrophin-like protein (DLP) is as complex as the mammalian dystrophin gene. Three 5' promoters and three internal promoters regulate the expression of three full-length and three truncated products, respectively. The existence of this complex gene structure in such evolutionary remote organisms suggests that both types of products have diverse important functions. The promoters of both the DLP gene and the mammalian dystrophin gene are located in very large introns. These introns contribute significantly to the large size of the genes. The possible relevance of the conservation of the large size of introns containing promoters to the regulation of promoter activity is discussed.

Molecular cloning and protein expression of Duchenne muscular dystrophy gene products in porcine retina

Due to the difference between rodent and human retinal circuitry, we characterize a new animal model of retinal perturbation in neurotransmission in Duchenne Muscular Dystrophy (DMD) patients. We investigated the expression and localization of dystrophin proteins and dystrophin associated proteins in porcine retina by reverse transcription polymerase chain reaction, Western blot analysis and immunohistochemistry. Homologues of human DMD gene products and alternative spliced isoforms of Dp71 were identified. We observed that dystrophins were expressed in the outer plexiform layer, around blood vessels and at the inner limiting membrane as previously described in human and mouse retinae. Moreover, by double immunostaining we showed that beta-dystroglycan co-localizes with dystrophin in the outer plexiform layer whereas alpha1-syntrophin labeling differs from that for dystrophins. Using confocal laser microscopy we observed that dystrophins labeling co-localizes with pre- and post-synaptic cell markers in the outer plexiform layer. We suggest that porcine retina constitutes a good model to study the role of dystrophins in retinal neurotransmission and should be used to investigate the physiological roles of dystrophins in signal transduction.

Dystrophin Dp71 in PC12 cell adhesion

Previously, we reported that PC12 cells with decreased Dp71 expression (antisense-Dp71 cells) display deficient nerve-growth-factor-induced neurite outgrowth. In this study, we show that disturbed neurite outgrowth of antisense-Dp71 cells is accompanied by decreased adhesion activity on laminin, collagen and fibronectin. In wild-type cells, the immunostaining of Dp71 and beta1-integrin overlaps in the basal area contacting the substrate, but staining of both proteins decrease in the antisense-Dp71 cells. Morphology of antisense-Dp71 cells at the electron microscopic level is characterized by the lack of filopodia, cellular projections involved in adhesion. Our findings suggest that Dp71 is required for the efficient PC12 cell attachment to beta1-integrin-dependent substrata and that decreased adhesion activity of the antisense-Dp71 cells could determine their deficiency to extend neurites.

Dystrophin Dp71 Expression Is Down-regulated during Myogenesis

Dp71 expression is present in myoblasts but declines during myogenesis to avoid interfering with the function of dystrophin, the predominant Duchenne muscular dystrophy gene product in differentiated muscle fibers. To elucidate the transcriptional regulatory mechanisms operating on the developmentally regulated expression of Dp71, we analyzed the Dp71 expression and promoter activity during myogenesis of the C2C12 cells. We demonstrated that the cellular content of Dp71 transcript and protein decrease in myotubes as a consequence of the negative regulation that the differentiation stimulus exerts on the Dp71 promoter. Promoter deletion analysis showed that the 224-bp 5'-flanking region, which contains several Sp-binding sites (Sp-A to Sp-D), is responsible for the Dp71 promoter basal activity in myoblasts as well as for down-regulation of the promoter in differentiated cells. Electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that Sp1 and Sp3 transcription factors specifically bind to the Sp-binding sites in the minimal Dp71 promoter region. Site-directed mutagenesis assay revealed that Sp-A is the most important binding site for the proximal Dp71 promoter activity. Additionally, cotransfection of the promoter construct with Sp1- and Sp3-expressing vectors into Drosophila SL2 cells, which lack endogenous Sp family, confirmed that these proteins activate specifically the minimal Dp71 promoter. Endogenous Sp1 and Sp3 proteins were detected only in myoblasts and not in myotubes, which indicates that the lack of these factors causes down-regulation of the Dp71 promoter activity in differentiated cells. In corroboration, efficient promoter activity was restored in differentiated muscle cells by exogenous expression of Sp1 and Sp3.

Computational analysis suggests that alternative first exons are involved in tissue-specific transcription in rice (Oryza sativa)

MOTIVATION

Transcription start site selection and alternative splicing greatly contribute to diversifying gene expression. Recent studies have revealed the existence of alternative first exons, but most have involved mammalian genes, and as yet the regulation of usage of alternative first exons has not been clarified, especially in plants.

RESULTS

We systematically identified putative alternative first exon transcripts in rice, verified the candidates using RT-PCR, and searched for the promoter elements that might regulate the alternative first exons. As a result, we detected a number of unreported alternative first exons, some of which are regulated in a tissue-specific manner.

SUPPLEMENTARY INFORMATION

http://www.bioinfo.sfc.keio.ac.jp/research/intron.

C-terminal truncated dystrophin identified in skeletal muscle of an asymptomatic boy with a novel nonsense mutation of the dystrophin gene

Mutations that cause premature stop codons in the dystrophin gene lead to a complete loss of dystrophin from skeletal muscle, resulting in severe Duchenne muscular dystrophy. Here, a C-terminally truncated dystrophin resulting from a novel nonsense mutation is shown for the first time to be localized to the muscle plasma membrane. An asymptomatic 8-y-old boy was examined for dystrophin in skeletal muscle because of high serum creatine kinase activity. Remarkably, no dystrophin labeling was seen with an MAb against the C-terminal domain, suggesting the presence of an early stop codon in the dystrophin gene. Labeling with an antibody specific to the N-terminal domain, however, revealed weak, patchy, and discontinuous staining, suggesting limited production of a truncated form of the protein. Molecular analysis revealed a novel nonsense mutation (Q3625X) as a result of a single nucleotide change in the patient's genomic DNA (C10873T), leaving 1.6% of dystrophin gene product unsynthesized at the C terminus. Dystrophin mRNA analysis did not show rescue of the nonsense mutation as a result of exon-skipping by an alternative splicing mechanism. This is the first report of an asymptomatic dystrophinopathy with a nonsense mutation in the dystrophin gene.

Identification and characterization of epsilon-sarcoglycans in the central nervous system

Alpha-, beta-, gamma-, and delta-sarcoglycans (SGs) are transmembrane glycoprotein components of the dystrophin-associated protein (DAP) complex, which is critical for the stability of the striated muscle cell membrane. Epsilon-SG was found as a homologue of alpha-SG, but unlike other SG members, it is ubiquitously expressed in various tissues as well as in striated muscle. Moreover, mutations in the epsilon-SG gene cause myoclonus-dystonia, indicating the importance of epsilon-SG for the function in the central nervous system. To gain insight into the role of epsilon-SG, its expression and subcellular distribution in mouse tissues and especially in the mouse brain were investigated. Analysis by reverse transcription-polymerase chain reaction showed four splice variants of epsilon-SG transcripts in the mouse brain, two of which are major transcript forms. One is a conventional form including exon 8 (epsilon-SG1), and the other is a novel form excluding exon 8 but including a previously unknown exon, 11b (epsilon-SG2). Immunoblot analysis using various mouse tissues indicated a broad expression pattern for epsilon-SG1, but epsilon-SG2 was expressed exclusively in the brain. Therefore, both epsilon-SG isoforms coexist in various regions of the brain. Furthermore, these isoforms were found in neuronal cells using immunohistochemical analysis. Subcellular fractionation of brain homogenates, however, indicated that epsilon-SG1 and epsilon-SG2 are relatively enriched in post- and pre-synaptic membrane fractions, respectively. These results suggest that the two epsilon-SG isoforms might play different roles in synaptic functions of the central nervous system.

Dystrophin Dp71 is required for neurite outgrowth in PC12 cells

To determine the role of Dp71 in neuronal cells, we generated PC12 cell lines in which Dp71 protein levels were controlled by stable transfection with either antisense or sense constructs. Cells expressing the antisense Dp71 RNA (antisense-Dp71 cells) contained reduced amounts of the two endogenous Dp71 isoforms. Antisense-Dp71 cells exhibited a marked suppression of neurite outgrowth upon the induction with NGF or dibutyryl cyclic AMP. Early responses to NGF-induced neuronal differentiation, such as the cessation of cell division and the activation of ERK1/2 proteins, were normal in the antisense-Dp71 cells. On contrary, the induction of MAP2, a late differentiation marker, was disturbed in these cells. Additionally, the deficiency of Dp71 correlated with an altered expression of the dystrophin-associated protein complex (DAPC) members alpha and beta dystrobrevins. Our results indicate that normal expression of Dp71 is essential for neurite outgrowth in PC12 cells and constitute the first direct evidence implicating Dp71 in a neuronal function.

Dystrophin isoform Dp7l is present in lamellipodia and focal complexes in human astrocytoma cells U-373 MG

Dp71 is the most abundant product of the dmd gene in the brain. There are at least 2 isoforms derived from alternative splicing of exon 78 (Dp71d, which contains exon 78 and Dp71f, the spliced isoform) but the precise localization and function of each isoform is still unknown. In the present study, we demonstrate by RT-PCR that the Dp71f isoform is present in an astrocytoma cell line U-373 MG, and its subcellular localization was determined in the cytoplasm, particularly in perinuclear areas, with lower amounts towards the periphery but increasing in the leader borders of lamellipodia and focal complexes. Double labeling indirect immunofluorescence showed that Dp71f colocalized with actin-like beta-dystroglycan and beta-1 integrin. We also demonstrated by triple labeling that Dp71f was colocalized with actin and two members of integrin complexes, alpha-actinin and vinculin, in focal complexes. Ventral plasma membranes were enriched and in those containing focal complex proteins, we found colocalization of Dp71f, actin and vinculin. It is concluded that U-373 MG cells express Dp71f as part of lamellipodia and focal complex proteins, and possibly connected via distroglycan complexes to integrin complexes.

Identification of Dp71 isoforms in the platelet membrane cytoskeleton. Potential role in thrombin-mediated platelet adhesion

Utrophin is a component of the platelet membrane cytoskeleton and participates in cytoskeletal reorganization (Earnest, J. P., Santos, G. F., Zuerbig, S., and Fox, J. E. B. (1995) J. Biol. Chem. 270, 27259-27265). Although platelets do not contain dystrophin, the identification of smaller C-terminal isoforms of dystrophin, including Dp71, which are expressed in a wide range of nonmuscle tissues and cell lines, has not been investigated. In this report, we have identified Dp71 protein variants of 55-60 kDa (designated Dp71Delta(110)) in the membrane cytoskeleton of human platelets. Both Dp71Delta(110) and utrophin sediment from lysed platelets along with the high speed detergent-insoluble pellet, which contains components of the membrane cytoskeleton. Like the membrane cytoskeletal proteins vinculin and spectrin, Dp71Delta(110) and utrophin redistributed from the high speed detergent-insoluble pellet to the integrin-rich low speed pellet of thrombin-stimulated platelets. Immunoelectron microscopy provided further evidence that Dp71Delta(110) was localized to the submembranous cytoskeleton. In addition to Dp71Delta(110), platelets contained several components of the dystrophin-associated protein complex, including beta-dystroglycan and syntrophin. To better understand the potential function of Dp71Delta(110), collagen adhesion assays were performed on platelets isolated from wild-type or Dp71-deficient (mdx(3cv)) mice. Adhesion to collagen in response to thrombin was significantly decreased in platelets isolated from mdx(3cv) mice, compared with wild-type platelets. Collectively, our results provide evidence that Dp71Delta(110) is a component of the platelet membrane cytoskeleton, is involved in cytoskeletal reorganization and/or signaling, and plays a role in thrombin-mediated platelet adhesion.

The role of utrophin in the potential therapy of Duchenne muscular dystrophy

Duchenne muscular dystrophy is an X-linked recessive muscle wasting disease caused by the absence of the muscle cytoskeletal protein, dystrophin. Dystrophin is a member of the spectrin superfamily of proteins and is closely related in sequence similarity and functional motifs to three proteins that constitute the dystrophin related protein family, including the autosomal homologue, utrophin. An alternative strategy circumventing many problems associated with somatic gene therapies for Duchenne muscular dystrophy has arisen from the demonstration that utrophin can functionally substitute for dystrophin and its over-expression in muscles of dystrophin-null transgenic mice completely prevents the phenotype arising from dystrophin deficiency. One potential approach to increase utrophin levels in muscle for possible therapeutic purpose in humans is to increase expression of the utrophin gene at a transcriptional level via promoter activation. This has lead to an interest in the identification and manipulation of important regulatory regions and/or molecules that increase the expression of utrophin and their delivery to dystrophin-deficient tissue. As pre-existing cellular mechanisms are utilized, this approach would avoid many problems associated with conventional gene therapies.