Tissue-selective expression of alpha-dystrobrevin is determined by multiple promoters.
alpha-Dystrobrevin, the mammalian orthologue of the Torpedo 87-kDa postsynaptic protein, is a dystrophin-associated and dystrophin-related protein. Knockout of the gene in the mouse results in muscular dystrophy. The control of the alpha-dystrobrevin gene in the various tissues is therefore of interest. Multiple dystrobrevin isoforms differing in their domain content are generated by alternative splicing of a single gene. The data presented here demonstrate that expression of alpha-dystrobrevin from three promoters, that are active in a tissue-selective manner, also plays a role in the function of the protein in different tissues. The most proximal promoter A is active in brain and to a lesser extent in lung, whereas the most distal promoter B, which possesses several Sp1 binding sites, is restricted to brain. Promoter C, which contains multiple consensus myogenic binding sites, is up-regulated during in vitro myoblast differentiation. Interestingly, the organization and the activity of the alpha-dystrobrevin promoters is reminiscent of those in the dystrophin gene. Taken together we suggest that the multipromoter system, distributed over a region of 270 kilobases at the 5'-end of the alpha-dystrobrevin gene, has been developed to allow the regulation of this gene in different cell types and/or different developmental stages. (+info)
A PDZ-containing scaffold related to the dystrophin complex at the basolateral membrane of epithelial cells.
Membrane scaffolding complexes are key features of many cell types, serving as specialized links between the extracellular matrix and the actin cytoskeleton. An important scaffold in skeletal muscle is the dystrophin-associated protein complex. One of the proteins bound directly to dystrophin is syntrophin, a modular protein comprised entirely of interaction motifs, including PDZ (protein domain named for PSD-95, discs large, ZO-1) and pleckstrin homology (PH) domains. In skeletal muscle, the syntrophin PDZ domain recruits sodium channels and signaling molecules, such as neuronal nitric oxide synthase, to the dystrophin complex. In epithelia, we identified a variation of the dystrophin complex, in which syntrophin, and the dystrophin homologues, utrophin and dystrobrevin, are restricted to the basolateral membrane. We used exogenously expressed green fluorescent protein (GFP)-tagged fusion proteins to determine which domains of syntrophin are responsible for its polarized localization. GFP-tagged full-length syntrophin targeted to the basolateral membrane, but individual domains remained in the cytoplasm. In contrast, the second PH domain tandemly linked to a highly conserved, COOH-terminal region was sufficient for basolateral membrane targeting and association with utrophin. The results suggest an interaction between syntrophin and utrophin that leaves the PDZ domain of syntrophin available to recruit additional proteins to the epithelial basolateral membrane. The assembly of multiprotein signaling complexes at sites of membrane specialization may be a widespread function of dystrophin-related protein complexes. (+info)
Unexpected modes of PDZ domain scaffolding revealed by structure of nNOS-syntrophin complex.
The PDZ protein interaction domain of neuronal nitric oxide synthase (nNOS) can heterodimerize with the PDZ domains of postsynaptic density protein 95 and syntrophin through interactions that are not mediated by recognition of a typical carboxyl-terminal motif. The nNOS-syntrophin PDZ complex structure revealed that the domains interact in an unusual linear head-to-tail arrangement. The nNOS PDZ domain has two opposite interaction surfaces-one face has the canonical peptide binding groove, whereas the other has a beta-hairpin "finger." This nNOS beta finger docks in the syntrophin peptide binding groove, mimicking a peptide ligand, except that a sharp beta turn replaces the normally required carboxyl terminus. This structure explains how PDZ domains can participate in diverse interaction modes to assemble protein networks. (+info)
Simultaneous analysis of expression of the three myotonic dystrophy locus genes in adult skeletal muscle samples: the CTG expansion correlates inversely with DMPK and 59 expression levels, but not DMAHP levels.
The causative mutation in the majority of cases of myotonic dystrophy has been shown to be the expansion of a CTG trinucleotide repeat, but the mechanism(s) by which this repeat leads to the very complex symptomatology in this disorder remains controversial. We have developed a highly sensitive and quantifiable assay, based on competitive RT-PCR, to test the hypothesis that the expansion disrupts the expression of the genes in its immediate vicinity, DMPK, 59 and DMAHP. In order to avoid cell culture-induced artifacts we performed these experiments using adult skeletal muscle biopsy samples and analysed total cytoplasmic poly(A)+mRNA levels for each gene simultaneously, as this is more physiologically relevant than allele-specific levels. There was considerable overlap between the expression levels of the three genes in myotonic dystrophy patient samples and samples from control individuals. However, in the myotonic dystrophy samples we detected a strong inverse correlation between the repeat size and the levels of expression of DMPK and 59. This is the first report of a possible effect of the CTG expansion on gene 59. Our results indicate that whilst a simple dosage model of gene expression in the presence of the mutation is unlikely to be sufficient in itself to explain the complex molecular pathology in this disease, the repeat expansion may be a significant modifier of the expression of these two genes. (+info)
Myotonic dystrophy is associated with a reduced level of RNA from the DMWD allele adjacent to the expanded repeat.
Myotonic dystrophy is caused by the expansion of a CTG repeat sequence. The mechanism by which this expanded repeat produces the pathophysiology of myotonic dystrophy is not clear. It has been shown previously that expansion of the repeat produces allele-specific effects on transcripts from two genes, DMPK and SIX5. We have examined the effect of repeat expansion on the level of RNA from a third gene, DMWD. We have identified a polymorphism in this gene and developed a quantitative allele-specific assay for DMWD RNA levels, which we have applied to nuclear and cytoplasmic fractions of RNA from DM cell lines. We have found that the level of the DM-associated allele in the cytoplasm of DM cell lines is reduced by 20-50% compared with the wild-type allele, similar to the level of reduction found for SIX5 in allele-specific analysis. However, no such reduction is observed in RNA from the nuclear fraction of DM cell lines. This may reflect the complex nature of processing transcriptional units at the DM locus. (+info)
Different dystrophin-like complexes are expressed in neurons and glia.
Duchenne muscular dystrophy is a fatal muscle disease that is often associated with cognitive impairment. Accordingly, dystrophin is found at the muscle sarcolemma and at postsynaptic sites in neurons. In muscle, dystrophin forms part of a membrane-spanning complex, the dystrophin-associated protein complex (DPC). Whereas the composition of the DPC in muscle is well documented, the existence of a similar complex in brain remains largely unknown. To determine the composition of DPC-like complexes in brain, we have examined the molecular associations and distribution of the dystrobrevins, a widely expressed family of dystrophin-associated proteins, some of which are components of the muscle DPC. beta-Dystrobrevin is found in neurons and is highly enriched in postsynaptic densities (PSDs). Furthermore, beta-dystrobrevin forms a specific complex with dystrophin and syntrophin. By contrast, alpha-dystrobrevin-1 is found in perivascular astrocytes and Bergmann glia, and is not PSD-enriched. alpha-Dystrobrevin-1 is associated with Dp71, utrophin, and syntrophin. In the brains of mice that lack dystrophin and Dp71, the dystrobrevin-syntrophin complexes are still formed, whereas in dystrophin-deficient muscle, the assembly of the DPC is disrupted. Thus, despite the similarity in primary sequence, alpha- and beta-dystrobrevin are differentially distributed in the brain where they form separate DPC-like complexes. (+info)
In vitro interactions of Caenorhabditis elegans dystrophin with dystrobrevin and syntrophin.
Dystrophin, the product of the gene mutated in Duchenne muscular dystrophy (DMD) is bound by its C-terminus to a protein complex including the related protein dystrobrevin. Both proteins contain a putative coiled-coil domain consisting of two alpha-helices. It has been reported that the two proteins bind to each other by the first one of the two alpha-helices. We have revisited this question using the Caenorhabditis elegans homologs of dystrophin and dystrobrevin. In vitro interaction occurs through the more conserved second helix. We propose a new model of dystrophin interactions with associated proteins. (+info)
Prevention of the dystrophic phenotype in dystrophin/utrophin-deficient muscle following adenovirus-mediated transfer of a utrophin minigene.
Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disorder caused by the lack of a subsarcolemmal protein, dystrophin. We have previously shown that the dystrophin-related protein, utrophin is able to compensate for the lack of dystrophin in the mdx mouse, the mouse model for DMD. Here, we explore whether utrophin delivered to the limb muscle of dystrophin/utrophin-deficient double knockout (dko) neonatal mice can protect the muscle from subsequent dystrophic damage. Utrophin delivery may avoid the potential problems of an immune response associated with the delivery of dystrophin to a previously dystrophin-deficient host. Dko muscle (tibialis anterior) was injected with a first generation recombinant adenovirus containing a utrophin minigene. Up to 95% of the fibres continued expressing the minigene 30 days after injection. Expression of utrophin caused a marked reduction from 80% centrally nucleated fibres (CNFs) in the uninjected dko TA to 12% in the injected dko TA. Within the region of the TA expressing the utrophin minigene, a significant decrease in the prevelance of necrosis was noted. These results demonstrate that the utrophin minigene delivered using an adenoviral vector is able to afford protection to the dystrophin/utrophin-deficient muscle of the dko mouse. Gene Therapy (2000) 7, 201-204. (+info)