Dystrobrevin and dystrophin: an interaction through coiled-coil motifs

C. elegans models of neuromuscular diseases expedite translational research

The nematode Caenorhabditis elegans is a genetic model organism and the only animal with a complete nervous system wiring diagram. With only 302 neurons and 95 striated muscle cells, a rich array of mutants with defective locomotion and the facility for individual targeted gene knockdown by RNA interference, it lends itself to the exploration of gene function at nerve muscle junctions. With approximately 60% of human disease genes having a C. elegans homologue, there is growing interest in the deployment of lowcost, high-throughput, drug screens of nematode transgenic and mutant strains mimicking aspects of the pathology of devastating human neuromuscular disorders. Here we explore the contributions already made by C. elegans to our understanding of muscular dystrophies (Duchenne and Becker), spinal muscular atrophy, amyotrophic lateral sclerosis, Friedreich’s ataxia, inclusion body myositis and the prospects for contributions to other neuromuscular disorders. A bottleneck to low-cost, in vivo, large-scale chemical library screening for new candidate therapies has been rapid, automated, behavioural phenotyping. Recent progress in quantifying simple swimming (thrashing) movements is making such screening possible and is expediting the translation of drug candidates towards the clinic.

Profound human/mouse differences in alpha-dystrobrevin isoforms: a novel syntrophin-binding site and promoter missing in mouse and rat

Background

The dystrophin glycoprotein complex is disrupted in Duchenne muscular dystrophy and many other neuromuscular diseases. The principal heterodimeric partner of dystrophin at the heart of the dystrophin glycoprotein complex in the main clinically affected tissues (skeletal muscle, heart and brain) is its distant relative, α-dystrobrevin. The α-dystrobrevin gene is subject to complex transcriptional and post-transcriptional regulation, generating a substantial range of isoforms by alternative promoter use, alternative polyadenylation and alternative splicing. The choice of isoform is understood, amongst other things, to determine the stoichiometry of syntrophins (and their ligands) in the dystrophin glycoprotein complex.

Results

We show here that, contrary to the literature, most α-dystrobrevin genes, including that of humans, encode three distinct syntrophin-binding sites, rather than two, resulting in a greatly enhanced isoform repertoire. We compare in detail the quantitative tissue-specific expression pattern of human and mouse α-dystrobrevin isoforms, and show that two major gene features (the novel syntrophin-binding site-encoding exon and the internal promoter and first exon of brain-specific isoforms α-dystrobrevin-4 and -5) are present in most mammals but specifically ablated in mouse and rat.

Conclusion

Lineage-specific mutations in the murids mean that the mouse brain has fewer than half of the α-dystrobrevin isoforms found in the human brain. Our finding that there are likely to be fundamental functional differences between the α-dystrobrevins (and therefore the dystrophin glycoprotein complexes) of mice and humans raises questions about the current use of the mouse as the principal model animal for studying Duchenne muscular dystrophy and other related disorders, especially the neurological aspects thereof.

Cardiac consequences to skeletal muscle-centric therapeutics for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disease of muscle deterioration. Duchenne muscular dystrophy affects all striated muscles in the body, including the heart. Recent advances in palliative care, largely directed at improving respiratory function, have extended life but paradoxically further unmasked emergent heart disease in DMD patients. New experimental strategies have shown promise in restoring dystrophin in the skeletal muscles of dystrophin- deficient animals. These strategies often have little or no capacity for restitution of dystrophin in the hearts of these animals. This article draws on both clinical data and recent experimental data to posit that effective skeletal muscle restricted therapies for DMD will paradoxically heighten cardiomyopathy and heart failure in these patients.

Expression of synemin in the mouse spinal cord

beta-Synemin was previously identified as an alpha-dystrobrevin-interacting protein in muscle. To better understand its function in neural tissue, in situ and immunohistochemical analyses were performed to identify where the synemin isoforms are expressed in the spinal cord of C57BL/6 and dystrophin-deficient (mdx) C57BL/10 mice. These analyses show that synemin transcript and its encoded protein colocalize in the anterior horn cells and that no differences in synemin expression were found in nerve tissue from C57BL/6 or mdx mice. The expression of synemin mRNA and protein predominantly in the anterior horn cells suggests that synemin performs an essential function in those cells. Because synemin is more highly expressed in the midbrain and pons, its function in neurological cells was further pursued by identifying coexpressed proteins in cells from those regions of the brain. These results show that neurons that express synemin also express tryptophan hydroxylase-1, a marker of serotoninergic nerve fibers. Muscle Nerve, 2009.

A 2-bp deletion in exon 74 of the dystrophin gene does not clearly induce muscle weakness

Duchenne muscular dystrophy (DMD) is caused by mutation of the dystrophin gene. Cases of dystrophinopathy with a 2-bp deletion in the dystrophin gene commonly result in DMD. We report here a case of dystrophinopathy in a 9-years-old boy with a 2-bp deletion in exon 74 of the dystrophin gene; however, the boy had no clear clinical signs of muscle weakness. Immunohistochemical studies with N-terminal (DYS3) and rod-domain anti-dystrophin (DYS1) antibodies revealed that the dystrophin signals were weaker than in the control sample (non-dystrophinopathy) at the sarcolemma of myofibers, and the studies with C-terminus anti-dystrophin antibody (DYS2) were negative. Our patient's mutation is located between the binding sites of alpha-syntrophin and alpha-dystrobrevin. These results suggest that this mutation does not clearly induce muscle weakness at least through the age of 9 years.

Molecular and cellular adaptations to chronic myotendinous strain injury in mdx mice expressing a truncated dystrophin

Myotendinous strain injury is the most common injury of human skeletal muscles because the majority of muscle forces are transmitted through this region. Although the immediate response to strain injury is well characterized, the chronic response to myotendinous strain injury is less clear. Here we examined the molecular and cellular adaptations to chronic myotendinous strain injury in mdx mice expressing a microdystrophin transgene (microdystrophin(DeltaR4-R23)). We found that muscles with myotendinous strain injury had an increased expression of utrophin and alpha7-integrin together with the dramatic restructuring of peripheral myofibrils into concentric rings. The sarcolemma of the microdystrophin(DeltaR4-R23)/mdx gastrocnemius muscles was highly protected from experimental lengthening contractions, better than wild-type muscles. We also found a positive correlation between myotendinous strain injury and ringed fibers in the HSA(LR) (human skeletal actin, long repeat) mouse model of myotonic dystrophy. We suggest that changes in protein expression and the formation of rings are adaptations to myotendinous strain injury that help to prevent muscle necrosis and retain the function of necessary muscles during injury, ageing and disease.

Destabilization of the dystrophin-glycoprotein complex without functional deficits in alpha-dystrobrevin null muscle

α-Dystrobrevin is a component of the dystrophin-glycoprotein complex (DGC) and is thought to have both structural and signaling roles in skeletal muscle. Mice deficient for α-dystrobrevin (adbn^−/−) exhibit extensive myofiber degeneration and neuromuscular junction abnormalities. However, the biochemical stability of the DGC and the functional performance of adbn^−/− muscle have not been characterized. Here we show that the biochemical association between dystrophin and β-dystroglycan is compromised in adbn^−/− skeletal muscle, suggesting that α-dystrobrevin plays a structural role in stabilizing the DGC. However, despite muscle cell death and DGC destabilization, costamere organization and physiological performance is normal in adbn^−/− skeletal muscle. Our results demonstrate that myofiber degeneration alone does not cause functional deficits and suggests that more complex pathological factors contribute to the development of muscle weakness in muscular dystrophy.

Plectin 1 links intermediate filaments to costameric sarcolemma through β-synemin, α-dystrobrevin and actin

In skeletal muscles, the sarcolemma is possibly stabilized and protected against contraction-imposed stress by intermediate filaments (IFs) tethered to costameric sarcolemma. Although there is emerging evidence that plectin links IFs to costameres through dystrophin-glycoprotein complexes (DGC), the molecular organization from plectin to costameres still remains unclear. Here, we show that plectin 1, a plectin isoform expressed in skeletal muscle, can interact with β-synemin, actin and a DGC component, α-dystrobrevin, in vitro. Ultrastructurally, β-synemin molecules appear to be incorporated into costameric dense plaques, where they seem to serve as actin-associated proteins rather than IF proteins. In fact, they can bind actin and α-dystrobrevin in vitro. Moreover, in vivo immunoprecipitation analyses demonstrated that β-synemin- and plectin-immune complexes from lysates of muscle light microsomes contained α-dystrobrevin, dystrophin, nonmuscle actin, metavinculin, plectin and β-synemin. These findings suggest a model in which plectin 1 interacts with DGC and integrin complexes directly, or indirectly through nonmuscle actin and β-synemin within costameres. The DGC and integrin complexes would cooperate to stabilize and fortify the sarcolemma by linking the basement membrane to IFs through plectin 1, β-synemin and actin. Besides, the two complexes, together with plectin and IFs, might have their own functions as platforms for distinct signal transduction.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Schistosome dystrophin and dystrobrevin proteins contain large insertions

Dystrophin and dystrobrevin are distantly related proteins that form a heterodimeric membrane-bound complex in all metazoa so far studied. The signature feature of the dystrophin/dystrobrevin/dystrotelin superfamily is a highly conserved cluster of domains whose integrity and size are maintained in all known members- more than 60 sequences so far reported. We were therefore surprised to find that the analogous regions of the Schistosoma mansoni proteins bear multiple large insertions amounting to 100% (dystrophin) and 35% (dystrobrevin) of their expected size. We isolated orthologous sequences from Schistosoma haematobium and Schistosoma bovis, and found that the insertions have a 10-fold higher rate of change (Ka/Ks value) than their respective host sequences, suggesting a much lower degree of functional constraint. A survey of known S. mansoni protein sequences shows that only two other proteins (both acetylcholine receptors) share these properties. We discuss the implications of the positions and sizes of these insertions for the structure and function of the dystrophins and dystrobrevins.

Dystrobrevin and dystrophin family gene expression in zebrafish

Dystrophin/dystrobrevin superfamily proteins play structural and signalling roles at the plasma membrane of many cell types. Defects in them or the associated multiprotein complex cause a range of neuromuscular disorders. Members of the dystrophin branch of the family form heterodimers with members of the dystrobrevin branch, mediated by their coiled-coil domains. To determine which combinations of these proteins might interact during embryonic development, we set out to characterise the gene expression pattern of dystrophin and dystrobrevin family members in zebrafish. gamma-dystrobrevin (dtng), a novel dystrobrevin recently identified in fish, is the predominant form of dystrobrevin in embryonic development. Dtng and dmd (dystrophin) have similar spatial and temporal expression patterns in muscle, where transcripts are localized to the ends of differentiated fibres at the somite borders. Dtng is expressed in the notochord while dmd is expressed in the chordo-neural hinge and then in floor plate and hypochord. In addition, dtng is dynamically expressed in rhombomeres 2 and 4-6 of the hindbrain and in the ventral midbrain. alpha-dystrobrevin (dtna) is expressed widely in the brain with particularly strong expression in the hypothalamus and the telencephalon; drp2 is also expressed widely in the brain. Utrophin expression is found in early pronephros and lateral line development and utrophin and dystrophin are both expressed later in the gut. beta-dystrobrevin (dtnb) is expressed in the pronephric duct and widely at low levels. In summary, we find clear instances of co-expression of dystrophin and dystrobrevin family members in muscle, brain and pronephric duct development and many examples of strong and specific expression of members of one family but not the other, an intriguing finding given the presumed heterodimeric state of these molecules.

Beta-synemin expression in cardiotoxin-injected rat skeletal muscle

Background

β-synemin was originally identified in humans as an α-dystrobrevin-binding protein through a yeast two-hybrid screen using an amino acid sequence derived from exons 1 through 16 of α-dystrobrevin, a region common to both α-dystrobrevin-1 and -2. α-Dystrobrevin-1 and -2 are both expressed in muscle and co-localization experiments have determined which isoform preferentially functions with β-synemin in vivo. The aim of our study is to show whether each α-dystrobrevin isoform has the same affinity for β-synemin or whether one of the isoforms preferentially functions with β-synemin in muscle.

Methods

The two α-dystrobrevin isoforms (-1 and -2) and β-synemin were localized in regenerating rat tibialis anterior muscle using immunoprecipitation, immunohistochemical and immunoblot analyses. Immunoprecipitation and co-localization studies for α-dystrobrevin and β-synemin were performed in regenerating muscle following cardiotoxin injection. Protein expression was then compared to that of developing rat muscle using immunoblot analysis.

Results

With an anti-α-dystrobrevin antibody, β-synemin co-immunoprecipitated with α-dystrobrevin whereas with an anti-β-synemin antibody, α-dystrobrevin-1 (rather than the -2 isoform) preferentially co-immunoprecipitated with β-synemin. Immunohistochemical experiments show that β-synemin and α-dystrobrevin co-localize in rat skeletal muscle. In regenerating muscle, β-synemin is first expressed at the sarcolemma and in the cytoplasm at day 5 following cardiotoxin injection. Similarly, β-synemin and α-dystrobrevin-1 are detected by immunoblot analysis as weak bands by day 7. In contrast, immunoblot analysis shows that α-dystrobrevin-2 is expressed as early as 1 day post-injection in regenerating muscle. These results are similar to that of developing muscle. For example, in embryonic rats, immunoblot analysis shows that β-synemin and α-dystrobevin-1 are weakly expressed in developing lower limb muscle at 5 days post-birth, while α-dystrobrevin-2 is detectable before birth in 20-day post-fertilization embryos.

Conclusion

Our results clearly show that β-synemin expression correlates with that of α-dystrobrevin-1, suggesting that β-synemin preferentially functions with α-dystrobrevin-1 in vivo and that these proteins are likely to function coordinately to play a vital role in developing and regenerating muscle.

Isolation and characterization of a novel v-SNARE family protein that interacts with a calcium-dependent protein kinase from the common ice plant, Mesembryanthemum crystallinum

McCPK1 (Mesembryanthemum crystallinum calcium-dependent protein kinase 1) mRNA expression is transiently salinity- and dehydrationstress responsive. The enzyme also undergoes dynamic subcellular localization changes in response to these same stresses. Using the yeast-two hybrid system, we have isolated and characterized a M. crystallinum CPK1 Adaptor Protein 2 (McCAP2). We show that McCPK1 interacts with the C-terminal, coiled-coil containing region of McCAP2 in the yeast two-hybrid system. This interaction was confirmed in vitro between the purified recombinant forms of each of the proteins and in vivo by coimmunoprecipitation experiments from plant extracts. McCAP2, however, was not a substrate for McCPK1. Computational threading analysis suggested that McCAP2 is a member of a novel family of proteins with unknown function also found in rice and Arabidopsis. These proteins contain coiled-coil spectrin repeat domains present in the syntaxin super-family that participate in vesicular and protein trafficking. Consistent with the interaction data, subcellular localization and fractionation studies showed that McCAP2 colocalizes with McCPK1 to vesicular structures located on the actin cytoskeleton and within the endoplasmic reticulum in cells subjected to low humidity stress. McCAP2 also colocalizes with AtVTIl1a, an Arabidopsis v-SNARE [vesicle-soluble N-ethyl maleimide-sensitive factor (NSF) attachment protein (SNAP) receptor] present in the trans-Golgi network (TGN) and prevacuolar compartments (PVCs). Both interaction and subcellular localization studies suggest that McCAP2 may possibly serve as an adaptor protein responsible for vesicle-mediated trafficking of McCPK1 to or from the plasma membrane along actin microfilaments of the cytoskeleton.

The dystrotelin, dystrophin and dystrobrevin superfamily: new paralogues and old isoforms

BACKGROUND

Dystrophins and dystrobrevins are distantly related proteins with important but poorly understood roles in the function of metazoan muscular and neuronal tissues. Defects in them and their associated proteins cause a range of neuromuscular disorders. Members of this superfamily have been discovered in a relatively serendipitous way; we set out to compile a comprehensive description of dystrophin- and dystrobrevin-related sequences from available metazoan genome sequences, validated in representative organisms by RT-PCR, or acquired de novo from key species.

RESULTS

Features of the superfamily revealed by our survey include: a) Dystrotelin, an entirely novel branch of the superfamily, present in most vertebrates examined. Dystrotelin is expressed in the central nervous system, and is a possible orthologue of Drosophila DAH. We describe the preliminary characterisation of its function, evolution and expression. b) A novel vertebrate member of the dystrobrevin family, gamma-dystrobrevin, an ancient branch now extant only in fish, but probably present in our own ancestors. Like dystrophin, zebrafish gamma-dystrobrevin mRNA is localised to myosepta. c) The extent of conservation of alternative splicing and alternative promoter use in the dystrophin and dystrobrevin genes; alternative splicing of dystrophin exons 73 and 78 and alpha-dystrobrevin exon 13 are conserved across vertebrates, as are the use of the Dp116, Dp71 and G-utrophin promoters; the Dp260 and Dp140 promoters are tetrapod innovations. d) The evolution of the unique N-terminus of DRP2 and its relationship to Dp116 and G-utrophin. e) A C-terminally truncated common ancestor of dystrophin and utrophin in cyclostomes. f) A severely restricted repertoire of dystrophin complex components in ascidians.

CONCLUSION

We have refined our understanding of the evolutionary history and isoform diversity of the five previously reported vertebrate superfamily members and describe two novel members, dystrotelin and gamma-dystrobrevin. Dystrotelins, dystrophins and dystrobrevins are roughly equally related to each other. Vertebrates therefore have a repertoire of seven superfamily members (three dystrophins, three dystrobevins, and one dystrotelin), with one lost in tetrapods. Most invertebrates studied have one member from each branch. Although the basic shared function which is implied by the common architecture of these distantly related proteins remains unclear, it clearly permeates metazoan biology.

Regulation of capacitative calcium entries by α1‐syntrophin: association of TRPC1 with dystrophin complex and the PDZ domain of α1‐syntrophin

Calcium mishandling in Duchenne dystrophic muscle suggested that dystrophin, a membrane-associated cytoskeleton protein, might regulate calcium signaling cascade such as calcium influx pathway. It was previously shown that abnormal calcium entries involve uncontrolled stretch-activated currents and store-operated Ca2+ currents supported by TRPC1 channels. Moreover, our recent work demonstrated that reintroduction of minidystrophin in dystrophic myotubes restores normal capacitative calcium entries (CCEs). However, until now, no molecular link between the dystrophin complex and calcium entry channels has been described. This study is the first to show by coimmunoprecipitation assays the molecular association of TRPC1 with dystrophin and alpha1-syntrophin in muscle cells. TRPC1 was also associated with alpha1-syntrophin in dystrophic muscle cells independently of dystrophin. Furthermore, glutathione S-transferase (GST) pull-down assays showed that TRPC1 binds to the alpha1-syntrophin PDZ domain. Transfected recombinant alpha1-syntrophin formed a complex with TRPC1 channels and restored normal CCEs in dystrophic muscle cells. We suggest that normal regulation of CCEs in skeletal muscle depends on the association between TRPC1 channels and alpha1-syntrophin that may anchor the store-operated channels to the dystrophin-associated protein complex (DAPC). The loss of this molecular association could participate in the calcium alterations observed in dystrophic muscle cells. This study provides a new model for the regulation of calcium influx by interaction with the scaffold of the DAPC in muscle cells.

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.

Complete deletion of all alpha-dystrobrevin isoforms does not reveal new neuromuscular junction phenotype

The dystrophin glycoprotein complex (DGC) is critical for muscle stability, and mutations in DGC proteins lead to muscular dystrophy. The DGC also contributes to the maturation and maintenance of the neuromuscular junction (NMJ). The gene encoding the DGC protein alpha-dystrobrevin undergoes alternative splicing to produce at least five known isoforms. Isoform-specific antibody staining and reverse transcription PCR in mutant mice with a deletion of exon 3 of the alpha-dystrobrevin gene suggested the existence of a remaining synaptic isoform, which might be compensating for alpha-dystrobrevin function. To test this possibility and to more completely understand the synaptic function of alpha-dystrobrevin, we used a two-step homologous recombination strategy combined with in vivo Cre-mediated excision to generate mice with a large deletion of the alpha-dystrobrevin gene to disrupt all isoforms. However, these mice did not exhibit a more severe NMJ phenotype than that observed in the exon 3-deleted mice. Nonetheless, these mice not only eliminate possible compensation by remaining isoforms of alpha-dystrobrevin, but also offer a conditional allele that could be used to identify tissue-specific and developmental functions of alpha-dystrobrevin. This work also demonstrates a successful strategy to achieve deletion of a large genomic sequence, which can be a valuable tool for functional studies of genes encoding multiple isoforms that span a large genomic region.

Sarcolemmal damage in dystrophin deficiency is modulated by synergistic interactions between mechanical and oxidative/nitrosative stresses

Dystrophin deficiency is the cause of Duchenne muscular dystrophy, but the precise physiological basis for muscle necrosis remains unclear. To determine whether dystrophin-deficient muscles are abnormally susceptible to oxidative and nitric oxide (NO)-driven tissue stress, a hindlimb ischemia/reperfusion (I/R) model was used. Dystrophic mdx mice exhibited abnormally high levels of lipid peroxidation and protein nitration, which were preceded by exaggerated NO production during ischemia. Visualization of NO with the fluorescent probe 4,5-diaminofluorescein diacetate suggested that excess NO production during ischemia occurred within a subset of mdx fibers. In mdx muscles only, prior exposure to I/R dramatically increased the level of sarcolemmal damage resulting from stretch-mediated mechanical stress, indicating greatly exacerbated hyperfragility of the dystrophic fiber membrane. Treatment with NO synthase inhibitors (l-N(G)-nitroarginine methyl ester hydrochloride or 7-nitroindazol) effectively blocked the synergistic interaction between I/R and mechanical stress-mediated sarcolemmal damage under these conditions. Taken together, our findings provide direct ex-perimental evidence that several prevailing hy-potheses regarding the cause of muscle fiber damage in dystrophin-deficient muscle can be integrated into a common pathophysiological framework involving interactions between oxidative stress, ab-normal NO regulation, and hyperfragility of the sarcolemma.

Reinvestigation of the dysbindin subunit of BLOC-1 (biogenesis of lysosome-related organelles complex-1) as a dystrobrevin-binding protein

Dysbindin was identified as a dystrobrevin-binding protein potentially involved in the pathogenesis of muscular dystrophy. Subsequently, genetic studies have implicated variants of the human dysbindin-encoding gene, DTNBP1, in the pathogeneses of Hermansky-Pudlak syndrome and schizophrenia. The protein is a stable component of a multisubunit complex termed BLOC-1 (biogenesis of lysosome-related organelles complex-1). In the present study, the significance of the dystrobrevin-dysbindin interaction for BLOC-1 function was examined. Yeast two-hybrid analyses, and binding assays using recombinant proteins, demonstrated direct interaction involving coiled-coil-forming regions in both dysbindin and the dystrobrevins. However, recombinant proteins bearing the coiled-coil-forming regions of the dystrobrevins failed to bind endogenous BLOC-1 from HeLa cells or mouse brain or muscle, under conditions in which they bound the Dp71 isoform of dystrophin. Immunoprecipitation of endogenous dysbindin from brain or muscle resulted in robust co-immunoprecipitation of the pallidin subunit of BLOC-1 but no specific co-immunoprecipitation of dystrobrevin isoforms. Within BLOC-1, dysbindin is engaged in interactions with three other subunits, named pallidin, snapin and muted. We herein provide evidence that the same 69-residue region of dysbindin that is sufficient for dystrobrevin binding in vitro also contains the binding sites for pallidin and snapin, and at least part of the muted-binding interface. Functional, histological and immunohistochemical analyses failed to detect any sign of muscle pathology in BLOC-1-deficient, homozygous pallid mice. Taken together, these results suggest that dysbindin assembled into BLOC-1 is not a physiological binding partner of the dystrobrevins, likely due to engagement of its dystrobrevin-binding region in interactions with other subunits.

Dissecting the signaling and mechanical functions of the dystrophin-glycoprotein complex

Duchenne muscular dystrophy is a severe disorder caused by mutations in the dystrophin gene. Dystrophin is required for assembly of the dystrophin-glycoprotein complex and provides a mechanically strong link between the cytoskeleton and the extracellular matrix. Several proteins in the complex also participate in signaling cascades, but the relationship between these signaling and mechanical functions in the development of muscular dystrophy is unclear. To explore the mechanisms of myofiber necrosis in dystrophin-deficient muscle, we tested the hypothesis that restoration of this complex without a link to the cytoskeleton ameliorates dystrophic pathology. Transgenic mice were generated that express Dp116, a non-muscle isoform of dystrophin that assembles the dystrophin-glycoprotein complex, in muscles of dystrophin-deficient mdx4cv mice. However, the phenotype of these mice was more severe than in controls. Displacement of utrophin by Dp116 correlated with the severity of dystrophy in different muscle groups. Comparison with other transgenic lines demonstrated that parts of the dystrophin central rod domain were required to localize neuronal nitric oxide synthase to the sarcolemma, but this was not correlated with presence or extent of dystrophy. Our results suggest that mechanical destabilization, rather than signaling dysfunction, is the primary cause of myofiber necrosis in dystrophin-deficient muscle.

Sarcolemmopathy: muscular dystrophies with cell membrane defects

In this article, we review the molecular pathology of muscular dystrophies caused by defects of proteins located within or near cell membranes. These disorders include Bethlem myopathy, merosinopathy, dystrophinopathy, sarcoglycanopathies, integrinopathy, dysferlinopathy and caveolinopathy. We refer to these diseases collectively as sarcolemmopathy. Here, we describe the biological functions of these proteins in the context of muscular contractions and their roles in the infrastructure of muscle; defects of muscle infrastructures cause those diseases. As an example, in dystrophinopathy, cell membranes have mechanical defects due to the absence of dystrophin. Cracks of the cell membrane induced by muscle contraction may allow the influx and efflux of substances that trigger muscle cell degeneration. However, such cracks may be resealed on relaxation. In addition, dystrophinopathy causes secondary defects of various dystrophin-associated proteins suggesting that defects in cell signaling participate in the pathologic process. With regard to other sarcolemmopathies, we discuss pathological mechanisms based on available data.

A highly functional mini-dystrophin/GFP fusion gene for cell and gene therapy studies of Duchenne muscular dystrophy

A promising approach for treating Duchenne muscular dystrophy (DMD) is by autologous cell transplantation of myogenic stem cells transduced with a therapeutic expression cassette. Development of this method has been hampered by a low frequency of cellular engraftment, the difficulty of tracing transplanted cells, the rapid loss of autologous cells carrying marker genes that are unable to halt muscle necrosis and the difficulty of stable transfer of a large dystrophin gene into myogenic stem cells. We engineered a 5.7 kb miniDys-GFP fusion gene by replacing the dystrophin C-terminal domain (DeltaCT) with an eGFP coding sequence and removing much of the dystrophin central rod domain (DeltaH2-R19). In a transgenic mdx(4Cv) mouse expressing the miniDys-GFP fusion protein under the control of a skeletal muscle-specific promoter, the green fusion protein localized on the sarcolemma, where it assembled the dystrophin-glycoprotein complex and completely prevented the development of dystrophy in transgenic mdx(4Cv) muscles. When myogenic and other stem cells from these mice were transplanted into mdx(4Cv) recipients, donor cells can be readily identified in skeletal muscle by direct green fluorescence or by using antibodies against GFP or dystrophin. In mdx(4Cv) mice reconstituted with bone marrow cells from the transgenic mice, we monitored engraftment in various muscle groups and found the number of miniDys-GFP(+) fibers increased with time. We suggest that these transgenic mdx(4Cv) mice are highly useful for developing autologous cell therapies for DMD.

C-terminal-truncated microdystrophin recruits dystrobrevin and syntrophin to the dystrophin-associated glycoprotein complex and reduces muscular dystrophy in symptomatic utrophin/dystrophin double-knockout mice

C-terminal-truncated (DeltaC) microdystrophin is being developed for Duchenne muscular dystrophy gene therapy. Encouraging results have been achieved in the mdx mouse model. Unfortunately, mdx mice do not display the same phenotype as human patients. Evaluating DeltaC microdystrophin in a symptomatic model will be of significant relevance to human trials. Utrophin/dystrophin double-knockout (u-dko) mice were developed to model severe dystrophic changes in human patients. In this study we evaluated the therapeutic effect of the DeltaR4-R23/DeltaC microdystrophin gene (DeltaR4/DeltaC) after serotype-6 adeno-associated virus-mediated gene transfer in neonatal u-dko muscle. At 2 months after gene transfer, the percentage of centrally nucleated myofiber was reduced from 89.2 to 3.4% and muscle weight was normalized. Furthermore, we have demonstrated for the first time that DeltaC microdystrophin can eliminate interstitial fibrosis and macrophage infiltration and restore dystrobrevin and syntrophin to the dystrophin-associated glycoprotein complex. Interestingly neuronal nitric oxide synthase was not restored. The most impressive results were achieved in muscle force measurement. Neonatal gene therapy increased twitch- and tetanic-specific force. It also brought the response to eccentric contraction-induced injury to the normal range. In summary, our results suggest that the DeltaR4/DeltaC microgene holds great promise in preventing muscular dystrophy.

Identification of functional domains in sarcoglycans essential for their interaction and plasma membrane targeting

Mutations in sarcoglycans have been reported to cause autosomal-recessive limb-girdle muscular dystrophies. In skeletal and cardiac muscle, sarcoglycans are assembled into a complex on the sarcolemma from four subunits (alpha, beta, gamma, delta). In this report, we present a detailed structural analysis of sarcoglycans using deletion study, limited proteolysis and co-immunoprecipitation. Our results indicate that the extracellular regions of sarcoglycans consist of distinctive functional domains connected by proteinase K-sensitive sites. The N-terminal half domains are required for sarcoglycan interaction. The C-terminal half domains of beta-, gamma- and delta-sarcoglycan consist of a cysteine-rich motif and a previously unrecognized conserved sequence, both of which are essential for plasma membrane localization. Using a heterologous expression system, we demonstrate that missense sarcoglycan mutations affect sarcoglycan complex assembly and/or localization to the cell surface. Our data suggest that the formation of a stable complex is necessary but not sufficient for plasma membrane targeting. Finally, we provide evidence that the beta/delta-sarcoglycan core can associate with the C-terminus of dystrophin. Our results therefore generate important information on the structure of the sarcoglycan complex and the molecular mechanisms underlying the effects of various sarcoglycan mutations in muscular dystrophies.

Potassium channel Kir4.1 macromolecular complex in retinal glial cells

A major role for Müller cells in the retina is to buffer changes in the extracellular K+ concentration ([K+]o) resulting from light-evoked neuronal activity. The primary K+ conductance in Müller cells is the inwardly rectifying K+ channel Kir4.1. Since this channel is constitutively active, K+ can enter or exit Müller cells depending on the state of the [K+]o. This process of [K+]o buffering by Müller cells ("K+ siphoning") is enhanced by the precise accumulation of these K+ channels at discrete subdomains of Müller cell membranes. Specifically, Kir4.1 is localized to the perivascular processes of Müller cells in animals with vascular retinas and to the endfeet of Müller cells in all species examined. The water channel aquaporin-4 (AQP4) also appears to be important for [K+]o buffering and is expressed in Müller cells in a very similar subcellular distribution pattern to that of Kir4.1. To gain a better understanding of how Müller cells selectively target K+ and water channels to discrete membrane subdomains, we addressed the question of whether Kir4.1 and AQP4 associate with the dystrophin-glycoprotein complex (DGC) in the mammalian retina. Immunoprecipitation (IP) experiments were utilized to show that Kir4.1 and AQP4 are associated with DGC proteins in rat retina. Furthermore, AQP4 was also shown to co-precipitate with Kir4.1, suggesting that both channels are tethered together by the DGC in Müller cells. This work further defines a subcellular localization mechanism in Müller cells that facilitates [K+]o buffering in the retina.

Molecular Basis of Dystrobrevin Interaction with Kinesin Heavy Chain: Structural Determinants of their Binding

Dystrobrevins are a family of widely expressed dystrophin-associated proteins that comprises alpha and beta isoforms and displays significant sequence homology with several protein-binding domains of the dystrophin C-terminal region. The complex distribution of the multiple dystrobrevin isoforms suggests that the variability of their composition may be important in mediating their function. We have recently identified kinesin as a novel dystrobrevin-interacting protein and localized the dystrobrevin-binding site on the cargo-binding domain of neuronal kinesin heavy chain (Kif5A). In the present study, we assessed the kinetics of the dystrobrevin-Kif5A interaction by quantitative pull-down assay and surface plasmon resonance (SPR) analysis and found that beta-dystrobrevin binds to kinesin with high affinity (K(D) approximately 40 nM). Comparison of the sensorgrams obtained with alpha and beta-dystrobrevin at the same concentration of analyte showed a lower affinity of alpha compared to that of beta-dystrobrevin, despite their functional domain homology and about 70% sequence identity. Analysis of the contribution of single dystrobrevin domains to the interaction revealed that the deletion of either the ZZ domain or the coiled-coil region decreased the kinetics of the interaction, suggesting that the tertiary structure of dystrobrevin may play a role in regulating the interaction of dystrobrevin with kinesin. In order to understand if structural changes induced by post-translational modifications could affect dystrobrevin affinity for kinesin, we phosphorylated beta-dystrobrevin in vitro and found that it showed reduced binding capacity towards kinesin. The interaction between the adaptor/scaffolding protein dystrobrevin and the motor protein kinesin may play a role in the transport and targeting of components of the dystrophin-associated protein complex to specific sites in the cell, with the differences in the binding properties of dystrobrevin isoforms reflecting their functional diversity within the same cell type. Phosphorylation events could have a regulatory role in this context.

Muscle regeneration in dystrophin-deficient mdx mice studied by gene expression profiling

Background

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is lethal. In contrast, dystrophin-deficient mdx mice recover due to effective regeneration of affected muscle tissue. To characterize the molecular processes associated with regeneration, we compared gene expression levels in hindlimb muscle tissue of mdx and control mice at 9 timepoints, ranging from 1–20 weeks of age.

Results

Out of 7776 genes, 1735 were differentially expressed between mdx and control muscle at at least one timepoint (p < 0.05 after Bonferroni correction). We found that genes coding for components of the dystrophin-associated glycoprotein complex are generally downregulated in the mdx mouse. Based on functional characteristics such as membrane localization, signal transduction, and transcriptional activation, 166 differentially expressed genes with possible functions in regeneration were analyzed in more detail. The majority of these genes peak at the age of 8 weeks, where the regeneration activity is maximal. The following pathways are activated, as shown by upregulation of multiple members per signalling pathway: the Notch-Delta pathway that plays a role in the activation of satellite cells, and the Bmp15 and Neuregulin 3 signalling pathways that may regulate proliferation and differentiation of satellite cells. In DMD patients, only few of the identified regeneration-associated genes were found activated, indicating less efficient regeneration processes in humans.

Conclusion

Based on the observed expression profiles, we describe a model for muscle regeneration in mdx mice, which may provide new leads for development of DMD therapies based on the improvement of muscle regeneration efficacy.

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.

The syntrophin-dystrobrevin subcomplex in human neuromuscular disorders

The syntrophins and alpha-dystrobrevin form a subcomplex with dystrophin at the skeletal muscle membrane, and are also highly concentrated at the neuromuscular synapse. Here we demonstrate that the different syntrophins and alpha-dystrobrevin isoforms have distinct expression patterns during human skeletal muscle development, and are differentially affected by loss of dystrophin anchorage and denervation in human neuromuscular disease. During normal fetal development, and in Duchenne muscular dystrophy and denervation disorders, alpha1-syntrophin and alpha-dystrobrevin are absent or markedly reduced at the sarcolemmal membrane. beta1-Syntrophin is the predominant syntrophin isoform expressed at the muscle membrane during development, and it undergoes upregulation in response to loss of alpha1-syntrophin in Duchenne muscular dystrophy and in denervation. Upregulation of beta1-syntrophin in neuromuscular disorders is associated with re-expression of the fetal nicotinic acetylcholine receptor gamma-subunit, cardiac actin, and neonatal myosin, suggesting reversion of muscle fibers to an immature phenotype. We show that denervation specifically affects expression of the syntrophin-dystrobrevin subcomplex and does not affect levels or localization of other members of the dystrophin-associated protein complex. Our results confirm that dystrophin is required for anchorage of the syntrophin-dystrobrevin subcomplex and suggest that expression of the syntrophin-dystrobrevin complex may be independently regulated through neuromuscular transmission.

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.

Formation of multiple complexes between beta-dystroglycan and dystrophin family products

Beta-dystroglycan is expressed in a wide variety of tissues and has generally been reported with an Mr of 43 kDa, sometimes accompanied with a 31 kDa protein assumed to be a truncated product. This molecule was recently identified as the anomalous beta-dystroglycan expressed in various carcinoma cell lines. We produced and characterized a G5 polyclonal antibody specific to beta-dystroglycan that is directed against the C-terminal portion of the molecule. We provide evidence that beta-dystroglycan may vary in size and properties by studying different Xenopus tissues. Besides normal beta-dystroglycan with an Mr of 43 kDa in smooth and cardiac muscle and sciatic nerve extracts, we found it in skeletal muscle and brain proteins with an Mr of 38 and 65 kDa, respectively. Glycosylation properties and proteolytic susceptibilities of these different beta-dystroglycans are analysed and compared in this work. Crosslinking experiments with various beta-dystroglycan preparations obtained from skeletal and cardiac muscles and brain gave rise to specific new covalent products with Mr of 125 kDa (doublet band), or 120 and 130 kDa, or 140 and 240 kDa, respectively. We provide evidence, using various similar beta-dystroglycan preparations, that the immunoprecipitation procedure with G5 specific polyclonal antibody allows consistent pelleting of various dystrophin-family isoforms. Skeletal muscles from Xenopus reveals the presence of two distinct beta-dystroglycan complexes, one with dystrophin and another one which involves alpha-dystrobrevin. Cardiac muscle and brain from Xenopus are shown to contain three beta-dystroglycan complexes related to various dystrophin-family isoforms. Dystrophin or alpha-dystrobrevin or Dp71 were found in cardiac muscle and dystrophin or Dp180 or Up71 in brain. This variability in the relationship between beta-dystroglycan and dystrophin-family isoforms suggests that each protein--currently known as dystrophin associated protein--could not be present in each of these complexes.

?-synemin localizes to regions of high stress in human skeletal myofibers

Synemin is an intermediate filament protein shown previously to interact with alpha-dystrobrevin and desmin. Immunoblot analysis detects a beta-synemin protein of 170 kDa in human skeletal muscle and an alpha-synemin protein of 225 kDa in monkey brain. Low-resolution immunohistochemical analysis localizes beta-synemin within muscle along the sarcolemma, whereas confocal microscopic analysis further refines localization to the costamere and muscle Z-lines. In addition to these locations, beta-synemin is also enriched at the neuromuscular and myotendinous junctions, other regions that undergo high stress during myofiber contraction. Based on its localization and its expression pattern, it is proposed that beta-synemin functions as a structural protein involved in maintaining muscle integrity through its interactions with alpha-dystrobrevin, desmin, and other structural proteins.

Anchoring of aquaporin-4 in brain: Molecular mechanisms and implications for the physiology and pathophysiology of water transport

Astrocytes show an enrichment of aquaporin-4 (AQP4) in those parts of the plasma membrane that are apposed to pial or perivascular basal laminae. This observation begged the following questions: 1, What are the molecular mechanisms that are responsible for the site specific anchoring of AQP4? 2, What are the physiological and pathophysiological roles of the AQP4 pools at these specialized membrane domains? Recent studies suggest that the site specific anchoring depends on the dystrophin complex. Further, alpha-syntrophin (a member of the dystrophin complex) is required to maintain a polarized expression of AQP4 in the perivascular membranes. Hence transgenic mice deficient in alpha-syntrophin provided a model where the perivascular pool of AQP4 could be removed for assessment of its functional roles. Data suggest that the perivascular pool of AQP4 plays a role in edema formation and that this pool (through its serial coupling with the AQP4 pools in other astrocyte membranes) is involved in K(+) siphoning. In the cerebral cortex, the astrocyte membrane domain contacting the pial basal lamina differs from the perivascular membrane domain in regard to the mechanisms for AQP anchoring. Thus deletion of alpha-syntrophin causes only a 50% loss of AQP4 from the former membrane (compared with a 90% loss in the latter), pointing to the existence of additional anchoring proteins. We will also discuss the subcellular distribution and anchoring of AQP4 in the other cell types that express this protein: endothelial cells, ependymal cells, and the specialized astrocytes of the osmosensitive organs.

Distribution and Localization of Vinculin-Talin-Integrin System and Dystrophin-Glycoprotein Complex in Human Skeletal Muscle

The vinculin-talin-integrin system and the dystrophin-glycoprotein complex (DGC) are two protein systems with structural and signaling functions, allowing interaction between muscle fibers and extracellular matrix. Although numerous studies have been conducted on these systems, their localization and distribution patterns along the nonjunctional sarcolemma are not clear. On this basis, we carried out an indirect immunofluorescence study on the vastus lateralis muscle of human adults not affected by neuromuscular diseases to better define these patterns. Our results showed that all tested proteins of the two systems have a costameric distribution; all tested proteins of the two systems colocalize with each other (about 90-95% of the cases); only alpha-sarcoglycan in a few cases (about 6%) does not colocalize with other proteins; in about 9-10% of the cases, dystrophin and beta-dystroglycan colocalize partially with other proteins; all tested proteins can be localized in different fibers, both in the region of the sarcolemma over I or A bands. The colocalization between the vinculin-talin-integrin and DGC systems may imply their functional interaction involving the structural aspect, by providing a stronger adhesion between sarcolemma and extracellular matrix in well-defined regions of the muscle fiber. Besides, their colocalization may suggest the existence of a mechanism of mutual modulation of the transmitted signals. This reciprocal control may determine, in different conditions, the prevalence of one system over another with a consequent transmission of different messages to the sarcolemma-associated cytoskeleton.

β-Dystrobrevin interacts directly with kinesin heavy chain in brain

β-Dystrobrevin, a member of the dystrobrevin protein family, is a dystrophin-related and -associated protein restricted to non-muscle tissues and is highly expressed in kidney, liver and brain. Dystrobrevins are now thought to play an important role in intracellular signal transduction, in addition to providing a membrane scaffold in muscle, but the precise role of β-dystrobrevin has not yet been determined. To study β-dystrobrevin's function in brain, we used the yeast two-hybrid approach to look for interacting proteins. Four overlapping clones were identified that encoded Kif5A, a neuronal member of the Kif5 family of proteins that consists of the heavy chains of conventional kinesin. A direct interaction of β-dystrobrevin with Kif5A was confirmed by in vitro and in vivo association assays. Co-immunoprecipitation with a monoclonal kinesin heavy chain antibody precipitated both α- and β-dystrobrevin, indicating that this interaction is not restricted to the β-dystrobrevin isoform. The site for Kif5A binding to β-dystrobrevin was localized in a carboxyl-terminal region that seems to be important in heavy chain-mediated kinesin interactions and is highly homologous in all three Kif5 isoforms, Kif5A, Kif5B and Kif5C. Pull-down and immunofluorescence experiments also showed a direct interaction between β-dystrobrevin and Kif5B. Our findings suggest a novel function for dystrobrevin as a motor protein receptor that might play a major role in the transport of components of the dystrophin-associated protein complex to specific sites in the cell.

The stn-1 syntrophin gene of C.elegans is functionally related to dystrophin and dystrobrevin

Syntrophins are a family of PDZ domain-containing adaptor proteins required for receptor localization. Syntrophins are also associated with the dystrophin complex in muscles. We report here the molecular and functional characterization of the Caenorhabditis elegans gene stn-1 (F30A10.8), which encodes a syntrophin with homology to vertebrate alpha and beta-syntrophins. stn-1 is expressed in neurons and in muscles of C.elegans. stn-1 mutants resemble dystrophin (dys-1) and dystrobrevin (dyb-1) mutants: they are hyperactive, bend their heads when they move forward, tend to hypercontract, and are hypersensitive to the acetylcholinesterase inhibitor aldicarb. These phenotypes are suppressed when stn-1 is expressed under the control of a muscular promoter, indicating that they are caused by the absence of stn-1 in muscles. These results suggest that the role of syntrophin is linked to dystrophin function in C.elegans.

DAMAGE, a novel alpha-dystrobrevin-associated MAGE protein in dystrophin complexes

Mice rendered null for alpha-dystrobrevin, a component of the dystrophin complex, have muscular dystrophy, despite the fact that the sarcolemma remains relatively intact (Grady, R. M., Grange, R. W., Lau, K. S., Maimone, M. M., Nichol, M. C., Stull, J. T., and Sanes, J. R. (1999) Nat. Cell Biol. 1, 215-220) Thus, alpha-dystrobrevin may serve a signaling function that is important for the maintenance of muscle integrity. We have identified a new dystrobrevin-associated protein, DAMAGE, that may play a signaling role in brain, muscle, and peripheral nerve. In humans, DAMAGE is encoded by an intronless gene located at chromosome Xq13.1, a locus that contains genes involved in mental retardation. DAMAGE associates directly with alpha-dystrobrevin, as shown by yeast two-hybrid, and co-immunoprecipitates with the dystrobrevin-syntrophin complex from brain. This co-immunoprecipitation is dependent on the presence of alpha-dystrobrevin but not beta-dystrobrevin. The DAMAGE protein contains a potential nuclear localization signal, 30 12-amino acid repeats, and two MAGE homology domains. The domain structure of DAMAGE is similar to that of NRAGE, a MAGE protein that mediates p75 neurotrophin receptor signaling and neuronal apoptosis (Salehi, A. H., Roux, P. P., Kubu, C. J., Zeindler, C., Bhakar, A., Tannis, L. L., Verdi, J. M., and Barker, P. A. (2000) Neuron 27, 279-288). DAMAGE is highly expressed in brain and is present in the cell bodies and dendrites of hippocampal and Purkinje neurons. In skeletal muscle, DAMAGE is at the postsynaptic membrane and is associated with a subset of myonuclei. DAMAGE is also expressed in peripheral nerve, where it localizes along with other members of the dystrophin complex to the perineurium and myelin. These results expand the role of dystrobrevin and the dystrophin complex in membrane signaling and disease.

Deficiency of the syntrophins and alpha-dystrobrevin in patients with inherited myopathy

The syntrophins and dystrobrevins are members of the dystrophin-associated protein complex, and are thought to function as modular adaptors for signalling proteins recruited to the sarcolemmal membrane. We have characterised the expression of the syntrophins (alpha-, beta1-, and beta2-) and alpha-dystrobrevin by immunohistochemistry in normal human muscle and in biopsies from 162 patients with myopathies of unknown aetiology (with normal staining for dystrophin and other dystrophin-associated proteins). Unlike mice, beta2-syntrophin is expressed at the sarcolemma in post-natal human skeletal muscle. Deficiency of alpha-dystrobrevin +/- beta2-syntrophin was present in 16/162 (10%) patients, compared to age-matched controls. All patients presented with congenital-onset hypotonia and weakness, although there was variability in clinical severity. Two major clinical patterns emerged: patients with deficiency of beta2-syntrophin and alpha-dystrobrevin presented with severe congenital weakness and died in the first year of life, and two patients with deficiency of alpha-dystrobrevin had congenital muscular dystrophy with complete external ophthalmoplegia. We have sequenced the coding regions of alpha-dystrobrevin and beta2-syntrophin in these patients, and identified a new isoform of dystrobrevin, but have not identified any mutations. This suggests that disease causing mutations occur outside the coding region of these genes, in gene(s) encoding other components of the syntrophin-dystrobrevin subcomplex, or in gene(s) responsible for their post-translational modification and normal localisation.

The expression of alpha-dystrobrevin and dystrophin during skeletal muscle regeneration

The expression of alpha-dystrobrevin and dystrophin in rat tibialis anterior muscles was chronologically evaluated during a cycle of regeneration after myonecrosis induced by the injection of cardiotoxin. In immunohistochemical studies, alpha-dystrobrevin and dystrophin were first stained weakly at the sarcolemma of some regenerating muscle fibers on day 5. On day 7, alpha-dystrobrevin was still stained weakly, whereas dystrophin was stained conspicuously. After day 10, alpha-dystrobrevin and dystrophin were both stained conspicuously on almost all regenerating muscle fibers. In the Western blot analysis, alpha-dystrobrevin and dystrophin were first detected as visible bands on days 5 and 7, respectively. The bands of alpha-dystrobrevin and dystrophin both darkened sequentially up to day 10. The protein levels based on the densitometrical analysis of the bands on each day were converted to the percentage of the protein level on day 28, which was taken as 100%. The sequential line based on these data showed that alpha-dystrobrevin and dystrophin reached 50% of the protein level on day 28 by 6.6 and 5.3 days, respectively. These data provide evidence that alpha-dystrobrevin regenerates more slowly than dystrophin in skeletal muscle.

Tyrosine-phosphorylated and nonphosphorylated isoforms of alpha-dystrobrevin: roles in skeletal muscle and its neuromuscular and myotendinous junctions

alpha-Dystrobrevin (DB), a cytoplasmic component of the dystrophin-glycoprotein complex, is found throughout the sarcolemma of muscle cells. Mice lacking alphaDB exhibit muscular dystrophy, defects in maturation of neuromuscular junctions (NMJs) and, as shown here, abnormal myotendinous junctions (MTJs). In normal muscle, alternative splicing produces two main alphaDB isoforms, alphaDB1 and alphaDB2, with common NH2-terminal but distinct COOH-terminal domains. alphaDB1, whose COOH-terminal extension can be tyrosine phosphorylated, is concentrated at the NMJs and MTJs. alphaDB2, which is not tyrosine phosphorylated, is the predominant isoform in extrajunctional regions, and is also present at NMJs and MTJs. Transgenic expression of either isoform in alphaDB-/- mice prevented muscle fiber degeneration; however, only alphaDB1 completely corrected defects at the NMJs (abnormal acetylcholine receptor patterning, rapid turnover, and low density) and MTJs (shortened junctional folds). Site-directed mutagenesis revealed that the effectiveness of alphaDB1 in stabilizing the NMJ depends in part on its ability to serve as a tyrosine kinase substrate. Thus, alphaDB1 phosphorylation may be a key regulatory point for synaptic remodeling. More generally, alphaDB may play multiple roles in muscle by means of differential distribution of isoforms with distinct signaling or structural properties.

Relocalization of neuronal nitric oxide synthase (nNOS) as a marker for complete restoration of the dystrophin associated protein complex in skeletal muscle

A lack of effective treatments for Duchenne muscular dystrophy, a fatal X-linked myopathy, has focused attention on the possibility of gene therapy. The aim of the gene therapy approach is the restoration of the dystrophin associated complex of proteins, one member of which is neuronal nitric oxide synthase, an important enzyme in signal transduction. Transgenic mdx mice and plasmid gene transfer of both human and murine recombinant dystrophins was used to assess whether nNOS could be restored to the sarcolemma following dystrophin gene transfer at a variety of levels of expression. Murine revertant fibres and human patients with different dystrophin deletions were used to assess the relationship between exon deletion and loss of neuronal nitric oxide synthase localization to the sarcolemma. We demonstrate that the domain encoded by exons 45-48 is required for localization of neuronal nitric oxide synthase to the sarcolemma. On the basis of these observations we suggest that neuronal nitric oxide synthase is a useful marker for complete restoration of the dystrophin associated complex and should be used as one of the criteria for selecting the recombinant molecule to be used for gene therapy in Duchenne muscular dystrophy.

The postsynaptic density and dendritic raft localization of PSD-Zip70, which contains an N-myristoylation sequence and leucine-zipper motifs

The postsynaptic site of the excitatory synapse, which is composed of the postsynaptic density (PSD) attached to the postsynaptic membrane, is a center for synaptic plasticity. To reveal the molecular organization and functional regulation of the postsynaptic site, we cloned a 70 kDa protein that is concentrated in PSDs using a monoclonal antibody against the PSD. This protein, named PSD-Zip70, is highly homologous to the human FEZ1/LZTS1 gene product. PSD-Zip70 contains an N-myristoylation consensus sequence, a polybasic cluster in the N-terminal region and four leucine-zipper motifs in the C-terminal region. Light and electron microscopy showed that this protein was localized to the dendritic spines, especially in the PSD and the postsynaptic membrane. Fractionation of the synaptic plasma membrane demonstrated that PSD-Zip70 was localized to the PSD and the dendritic raft. In Madin-Darby canine kidney (MDCK) cells, exogenous PSD-Zip70 was targeted to the apical plasma membrane of microvilli, and its N-myristoylation was necessary for this targeting. In hippocampal neurons, N-myristoylation was also required for the membrane localization and the C-terminal region was critically involved in the synaptic targeting. These results suggest that PSD-Zip70 may be involved in the dynamic properties of the structure and function of the postsynaptic site.

Molecular pathophysiology of myofiber injury in deficiencies of the dystrophin-glycoprotein complex

Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin, a 427 kd protein normally found at the cytoplasmic face of the sarcolemma. In normal muscle, dystrophin is associated with a multimolecular glycoprotein complex. Primary mutations in the genes encoding members of this glycoprotein complex are also associated with muscular dystrophy. The dystrophin-glycoprotein complex provides a physical linkage between the internal cytoskeleton of myofibers and the extracellular matrix, but the precise functions of the dystrophin-glycoprotein complex remain uncertain. In this review, five potential pathogenetic mechanisms implicated in the initiation of myofiber injury in dystrophin-glycoprotein complex deficiencies are discussed: (1) mechanical weakening of the sarcolemma, (2) inappropriate calcium influx, (3) aberrant cell signaling, (4) increased oxidative stress, and (5) recurrent muscle ischemia. Particular emphasis is placed on the multifunctional nature of the dystrophin-glycoprotein complex and the fact that the above mechanisms are in no way mutually exclusive and may interact with one another to a significant degree.

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.

Dystrobrevin dynamics in muscle-cell signalling: a possible target for therapeutic intervention in Duchenne muscular dystrophy?

The dystrophin-protein complex forms one of the connections between the extracellular matrix and the cytoskeleton of muscle. This link is disrupted in patients with Duchenne and Becker muscular dystrophies. Dystrobrevin is a component of the dystrophin-protein complex that binds to the C-terminus of dystrophin and also to syntrophin. As its name suggests, dystrobrevin is a relative of dystrophin participating in similar intermolecular interactions. Dystrobrevin-deficient mice have a form of muscular dystrophy that leaves the sarcolemma and dystrophin-protein complex intact but affects an as yet unidentified signalling pathway in muscle. Given that the up-regulation of several genes has a beneficial effect on the muscle in some dystrophic mouse models, alpha-dystrobrevin has a number of properties that might be protective in muscular dystrophy. This article discusses the function of dystrobrevin in muscle and reviews its suitability as a therapeutic target for treating patients with Duchenne and Becker muscular dystrophies.

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.

Genetic evidence for a dystrophin-glycoprotein complex (DGC) in Caenorhabditis elegans

Dystrophin is the product of the gene mutated in Duchenne muscular dystrophy (DMD). Neither the function of dystrophin nor the physiopathology of the disease have been clearly established so far. In mammals, the dystrophin-glycoprotein complex (DGC) includes dystrophin, as well as transmembrane and cytoplasmic proteins. Since Caenorhabditis elegans possesses a dystrophin-like gene (dys-1), we investigated whether homologues of the DGC members could also be found in the C. elegans genome. Conserved homologues were found for dystroglycan, delta/gamma-sarcoglycan and syntrophin. Divergent but related proteins were found for alpha- and beta-sarcoglycans. No sarcospan counterpart was found. The expression of the conserved homologues was inactivated using the RNA interference technique. Phenotypes similar to that of dys-1 were obtained, both in the wild-type background and in combination with other mutations. These results strongly suggest that a protein complex comprising functional analogies with the DGC exists in C. elegans.

Muscular dystrophies involving the dystrophin-glycoprotein complex: an overview of current mouse models

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that connects the cytoskeleton of a muscle fiber to its surrounding extracellular matrix. Mutations in the DGC disrupt the complex and lead to muscular dystrophy. There are a few naturally occurring animal models of DGC-associated muscular dystrophy (e.g. the dystrophin-deficient mdx mouse, dystrophic golden retriever dog, HFMD cat and the delta-sarcoglycan-deficient BIO 14.6 cardiomyopathic hamster) that share common genetic protein abnormalities similar to those of the human disease. However, the naturally occurring animal models only partially resemble human disease. In addition, no naturally occurring mouse models associated with loss of other DGC components are available. This has encouraged the generation of genetically engineered mouse models for DGC-linked muscular dystrophy. Not only have analyses of these mice led to a significant improvement in our understanding of the pathogenetic mechanisms for the development of muscular dystrophy, but they will also be immensely valuable tools for the development of novel therapeutic approaches for these incapacitating diseases.