The membrane-cytoskeleton interface: the role of dystrophin and utrophin

Label-free identification of myopathological features with coherent anti-Stokes Raman scattering

INTRODUCTION

The aim of this study was the label-free identification of distinct myopathological features with coherent anti-Stokes Raman scattering (CARS) imaging, which leaves the sample intact for further analysis.

METHODS

The protein distribution was determined without labels by CARS at 2,930 cm^-1 and was compared with the results of standard histological staining.

RESULTS

CARS imaging allowed the visualization of glycogen accumulation in glycogen storage disease type 5 (McArdle disease) and of internal nuclei in centronuclear myopathy. CARS identified an inhomogeneous protein distribution within muscle fibers in sporadic inclusion body myositis that was not shown with standard staining. In Duchenne muscular dystrophy, evidence for a higher protein content at the border of hypercontracted fibers was detected.

DISCUSSION

CARS enables the label-free identification of distinct myopathological features, possibly paving the way for subsequent proteomic, metabolic, and genomic analyses. Muscle Nerve 58: 457-460, 2018.

The dystrophin isoform Dp71eΔ71 is involved in neurite outgrowth and neuronal differentiation of PC12 cells

The Dp71 protein is the most abundant dystrophin in the central nervous system (CNS). Several dystrophin Dp71 isoforms have been described and are classified into three groups, each with a different C-terminal end. However, the functions of Dp71 isoforms remain unknown. In the present study, we analysed the effect of Dp71e_Δ71 overexpression on neuronal differentiation of PC12 Tet-On cells. Overexpression of dystrophin Dp71e_Δ71 stimulates neuronal differentiation, increasing the percentage of cells with neurites and neurite length. According to 2-DE analysis, Dp71e_Δ71 overexpression modified the protein expression profile of rat pheochromocytoma PC12 Tet-On cells that had been treated with neuronal growth factor (NGF) for nine days. Interestingly, all differentially expressed proteins were up-regulated compared to the control. The proteomic analysis showed that Dp71e_Δ71 increases the expression of proteins with important roles in the differentiation process, such as HspB1, S100A6, and K8 proteins involved in the cytoskeletal structure and HCNP protein involved in neurotransmitter synthesis. The expression of neuronal marker TH was also up-regulated. Mass spectrometry data are available via ProteomeXchange with identifier PXD009114. SIGNIFICANCE: This study is the first to explore the role of the specific isoform Dp71e_Δ71. The results obtained here support the hypothesis that the dystrophin Dp71e_Δ71 isoform has an important role in the neurite outgrowth by regulating the levels of proteins involved in the cytoskeletal structure, such as HspB1, S100A6, and K8, and in neurotransmitter synthesis, such as HCNP and TH, biological processes required to stimulate neuronal differentiation.

Gene Therapy for Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a relatively common inherited disorder caused by defective expression of the protein dystrophin. The most direct approach to treating this disease would be to restore dystrophin production in muscle. Recent progress has greatly increased the prospects for successful gene therapy of DMD, and here we summarize the most promising developments.

AREAS COVERED

Gene transfer using vectors derived from adeno-associated virus (AAV) has emerged as a promising method to restore dystrophin production in muscles bodywide, and represents a treatment option applicable to all DMD patients. Using information gleaned from PubMed searches of the literature, attendance at scientific conferences and results from our own lab, we provide an overview of the potential for gene therapy of DMD using AAV vectors including a summary of promising developments and issues that need to be resolved prior to large-scale therapeutic implementation.

EXPERT OPINION

Of the many approaches being pursued to treat DMD and BMD, gene therapy based on AAV-mediated delivery of microdystrophin is the most direct and promising method to treat the cause of the disorder. The major challenges to this approach are ensuring that microdystrophin can be delivered safely and efficiently without eliciting an immune response.

Missense mutation Lys18Asn in dystrophin that triggers X-linked dilated cardiomyopathy decreases protein stability, increases protein unfolding, and perturbs protein structure, but does not affect protein function

Genetic mutations in a vital muscle protein dystrophin trigger X-linked dilated cardiomyopathy (XLDCM). However, disease mechanisms at the fundamental protein level are not understood. Such molecular knowledge is essential for developing therapies for XLDCM. Our main objective is to understand the effect of disease-causing mutations on the structure and function of dystrophin. This study is on a missense mutation K18N. The K18N mutation occurs in the N-terminal actin binding domain (N-ABD). We created and expressed the wild-type (WT) N-ABD and its K18N mutant, and purified to homogeneity. Reversible folding experiments demonstrated that both mutant and WT did not aggregate upon refolding. Mutation did not affect the protein's overall secondary structure, as indicated by no changes in circular dichroism of the protein. However, the mutant is thermodynamically less stable than the WT (denaturant melts), and unfolds faster than the WT (stopped-flow kinetics). Despite having global secondary structure similar to that of the WT, mutant showed significant local structural changes at many amino acids when compared with the WT (heteronuclear NMR experiments). These structural changes indicate that the effect of mutation is propagated over long distances in the protein structure. Contrary to these structural and stability changes, the mutant had no significant effect on the actin-binding function as evident from co-sedimentation and depolymerization assays. These results summarize that the K18N mutation decreases thermodynamic stability, accelerates unfolding, perturbs protein structure, but does not affect the function. Therefore, K18N is a stability defect rather than a functional defect. Decrease in stability and increase in unfolding decrease the net population of dystrophin molecules available for function, which might trigger XLDCM. Consistently, XLDCM patients have decreased levels of dystrophin in cardiac muscle.

Utrophin A is essential in mediating the functional adaptations of mdx mouse muscle following chronic AMPK activation

Duchenne muscular dystrophy (DMD) is caused by the absence of dystrophin along muscle fibers. An attractive therapeutic avenue for DMD consists in the upregulation of utrophin A, a protein with high sequence identity and functional redundancy with dystrophin. Recent work has shown that pharmacological interventions that induce a muscle fiber shift toward a slower, more oxidative phenotype with increased expression of utrophin A confer morphological and functional improvements in mdx mice. Whether such improvements result from the increased expression of utrophin A per se or are linked to other beneficial adaptations associated with the slow, oxidative phenotype remain to be established. To address this central issue, we capitalized on the use of double knockout (dKO) mice, which are mdx mice also deficient in utrophin. We first compared expression of signaling molecules and markers of the slow, oxidative phenotype in muscles of mdx versus dKO mice and found that both strains exhibit similar phenotypes. Chronic activation of 5' adenosine monophosphate-activated protein kinase with 5-amino-4-imidazolecarboxamide riboside (AICAR) resulted in expression of a slower, more oxidative phenotype in both mdx and dKO mice. In mdx mice, this fiber type shift was accompanied by clear functional improvements that included reductions in central nucleation, IgM sarcoplasmic penetration and sarcolemmal damage resulting from eccentric contractions, as well as in increased grip strength. These important morphological and functional adaptations were not seen in AICAR-treated dKO mice. Our findings show the central role of utrophin A in mediating the functional benefits associated with expression of a slower, more oxidative phenotype in dystrophic animals.

Dystrophin and utrophin "double knockout" dystrophic mice exhibit a spectrum of degenerative musculoskeletal abnormalities

Duchenne muscular dystrophy (DMD) is a degenerative muscle disorder characterized by the lack of dystrophin expression at the sarcolemma of muscle fibers. In addition, DMD patients acquire osteopenia, fragility fractures, and scoliosis indicating that a deficiency in skeletal homeostasis coexists but little is known about the effects of DMD on bone and other connective tissues within the musculoskeletal system. Recent evidence has emerged implicating adult stem cell dysfunction in DMD myopathogenesis. Given the common mesenchymal origin of muscle and bone, we sought to investigate bone and other musculoskeletal tissues in a DMD mouse model. Here, we report that dystrophin-utrophin double knockout (dko) mice exhibit a spectrum of degenerative changes, outside skeletal muscle, in bone, articular cartilage, and intervertebral discs, in addition to reduced lifespan, muscle degeneration, spinal deformity, and cardiomyopathy previously reported. We also report these mice to have a reduced capacity for bone healing and exhibit spontaneous heterotopic ossification in the hind limb muscles. Therefore, we propose the dko mouse as a model for premature musculoskeletal aging and posit that a similar phenomenon may occur in patients with DMD.

Gene replacement therapies for duchenne muscular dystrophy using adeno-associated viral vectors

The muscular dystrophies collectively represent a major health challenge, as few significant treatment options currently exist for any of these disorders. Recent years have witnessed a proliferation of novel approaches to therapy, spanning increased testing of existing and new pharmaceuticals, DNA delivery (both anti-sense oligonucleotides and plasmid DNA), gene therapies and stem cell technologies. While none of these has reached the point of being used in clinical practice, all show promise for being able to impact different types of muscular dystrophies. Our group has focused on developing direct gene replacement strategies to treat recessively inherited forms of muscular dystrophy, particularly Duchenne and Becker muscular dystrophy (DMD/BMD). Both forms of dystrophy are caused by mutations in the dystrophin gene and all cases can in theory be treated by gene replacement using synthetic forms of the dystrophin gene. The major challenges for success of this approach are the development of a suitable gene delivery shuttle, generating a suitable gene expression cassette able to be carried by such a shuttle, and achieving safe and effective delivery without elicitation of a destructive immune response. This review summarizes the current state of the art in terms of using adeno-associated viral vectors to deliver synthetic dystrophin genes for the purpose of developing gene therapy for DMD.

A molecular trajectory of α-actinin activation

The mechanisms by which living cells respond to mechanical stimuli are not yet fully understood. It has been suggested that mechanosensing proteins play an important role in mechanotransduction because their binding affinities are directly affected by the external stress. α-Actinin is an actin cross-linker and may act as a mechanosensor in adhesion sites. Its interaction with vinculin is suggested to be mechanically regulated. In this study, the free energy of activation is explored using the umbrella sampling method. An activation trajectory is generated in which α-actinin's vinculin-binding site swings out of the rod domain, leading to approximately an 8 kcal/mol free energy release. The activation trajectory reveals several local and global conformational changes along the activation pathway accompanied by the breakage of a number of key interactions stabilizing the inhibited structure. These results may shed light on the role of α-actinin in cellular mechanotransduction and focal adhesion formation.

Fetal microchimeric cells in a fetus-treats-its-mother paradigm do not contribute to dystrophin production in serially parous mdx females

Throughout every pregnancy, genetically distinct fetal microchimeric stem/progenitor cells (FMCs) engraft in the mother, persist long after delivery, and may home to damaged maternal tissues. Phenotypically normal fetal lymphoid progenitors have been described to develop in immunodeficient mothers in a fetus-treats-its-mother paradigm. Since stem cells contribute to muscle repair, we assessed this paradigm in the mdx mouse model of Duchenne muscular dystrophy. mdx females were bred serially to either ROSAeGFP males or mdx males to obtain postpartum microchimeras that received either wild-type FMCs or dystrophin-deficient FMCs through serial gestations. To enhance regeneration, notexin was injected into the tibialis anterior of postpartum mice. FMCs were detected by qPCR at a higher frequency in injected compared to noninjected side muscle (P=0.02). However, the number of dystrophin-positive fibers was similar in mothers delivering wild-type compared to mdx pups. In addition, there was no correlation between FMC detection and percentage dystrophin, and no GFP+ve FMCs were identified that expressed dystrophin. In 10/11 animals, GFP+ve FMCs were detected by immunohistochemistry, of which 60% expressed CD45 with 96% outside the basal lamina defining myofiber contours. Finally we confirmed lack of FMC contribution to statellite cells in postpartum mdx females mated with Myf5-LacZ males. We conclude that the FMC contribution to regenerating muscles is insufficient to have a functional impact.

Tissue expression and actin binding of a novel N-terminal utrophin isoform

Utrophin and dystrophin present two large proteins that link the intracellular actin cytoskeleton to the extracellular matrix via the C-terminal-associated protein complex. Here we describe a novel short N-terminal isoform of utrophin and its protein product in various rat tissues (N-utro, 62 kDa, amino acids 1–539, comprising the actin-binding domain plus the first two spectrin repeats). Using different N-terminal recombinant utrophin fragments, we show that actin binding exhibits pronounced negative cooperativity (affinity constants K₁ = ∼5 × 10⁶ and K₂ = ∼1 × 10⁵M⁻¹) and is Ca²⁺-insensitive. Expression of the different fragments in COS7 cells and in myotubes indicates that the actin-binding domain alone binds exlusively to actin filaments. The recombinant N-utro analogue binds in vitro to actin and in the cells associates to the membranes. The results indicate that N-utro may be responsible for the anchoring of the cortical actin cytoskeleton to the membranes in muscle and other tissues.

Dystroglycan is involved in skin morphogenesis downstream of the Notch signaling pathway

Dystroglycan (Dg) is a transmembrane protein involved both in the assembly and maintenance of basement membrane structures essential for tissue morphogenesis, and the transmission of signals across the plasma membrane. We used a morpholino knockdown approach to investigate the function of Dg during Xenopus laevis skin morphogenesis. The loss of Dg disrupts epidermal differentiation by affecting the intercalation of multiciliated cells, deposition of laminin, and organization of fibronectin in the extracellular matrix (ECM). Depletion of Dg also affects cell-cell adhesion, as shown by the reduction of E-cadherin expression at the intercellular contacts, without affecting the distribution of β(1) integrins. This was associated with a decrease of cell proliferation, a disruption of multiciliated-cell intercalation, and the down-regulation of the transcription factor P63, a marker of differentiated epidermis. In addition, we demonstrated that inhibition or activation of the Notch pathway prevents and promotes transcription of X-dg. Our study showed for the first time in vivo that Dg, in addition to organizing laminin in the ECM, also acts as a key signaling component in the Notch pathway.

Biomechanics of the sarcolemma and costameres in single skeletal muscle fibers from normal and dystrophin-null mice

We studied the biomechanical properties of the sarcolemma and its links through costameres to the contractile apparatus in single mammalian myofibers of Extensor digitorum longus muscles isolated from wild (WT) and dystrophin-null (mdx) mice. Suction pressures (P) applied through a pipette to the sarcolemma generated a bleb, the height of which increased with increasing P. Larger increases in P broke the connections between the sarcolemma and myofibrils and eventually caused the sarcolemma to burst. We used the values of P at which these changes occurred to estimate the tensions and stiffness of the system and its individual elements. Tensions of the whole system and the sarcolemma, as well as the maximal tension sustained by the costameres, were all significantly lower (1.8-3.3 fold) in muscles of mdx mice compared to WT. Values of P at which separation and bursting occurred, as well as the stiffness of the whole system and of the isolated sarcolemma, were ~2-fold lower in mdx than in WT. Our results indicate that the absence of dystrophin reduces muscle stiffness, increases sarcolemmal deformability, and compromises the mechanical stability of costameres and their connections to nearby myofibrils.

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

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

Dystroglycan versatility in cell adhesion: a tale of multiple motifs

Dystroglycan is a ubiquitously expressed heterodimeric adhesion receptor. The extracellular α-subunit makes connections with a number of laminin G domain ligands including laminins, agrin and perlecan in the extracellular matrix and the transmembrane β-subunit makes connections to the actin filament network via cytoskeletal linkers including dystrophin, utrophin, ezrin and plectin, depending on context. Originally discovered as part of the dystrophin glycoprotein complex of skeletal muscle, dystroglycan is an important adhesion molecule and signalling scaffold in a multitude of cell types and tissues and is involved in several diseases. Dystroglycan has emerged as a multifunctional adhesion platform with many interacting partners associating with its short unstructured cytoplasmic domain. Two particular hotspots are the cytoplasmic juxtamembrane region and at the very carboxy terminus of dystroglycan. Regions which between them have several overlapping functions: in the juxtamembrane region; a nuclear localisation signal, ezrin/radixin/moesin protein, rapsyn and ERK MAP Kinase binding function, and at the C terminus a regulatory tyrosine governing WW, SH2 and SH3 domain interactions. We will discuss the binding partners for these motifs and how their interactions and regulation can modulate the involvement of dystroglycan in a range of different adhesion structures and functions depending on context. Thus dystroglycan presents as a multifunctional scaffold involved in adhesion and adhesion-mediated signalling with its functions under exquisite spatio-temporal regulation.

TAPP2 links phosphoinositide 3-kinase signaling to B-cell adhesion through interaction with the cytoskeletal protein utrophin: expression of a novel cell adhesion-promoting complex in B-cell leukemia

Tandem pleckstrin homology domain proteins (TAPPs) are recruited to the plasma membrane via binding to phosphoinositides produced by phosphoinositide 3-kinases (PI3Ks). Whereas PI3Ks are critical for B-cell activation, the functions of TAPP proteins in B cells are unknown. We have identified 40 potential interaction partners of TAPP2 in B cells, including proteins involved in cytoskeletal rearrangement, signal transduction and endocytic trafficking. The association of TAPP2 with the cytoskeletal proteins utrophin and syntrophin was confirmed by Western blotting. We found that TAPP2, syntrophin, and utrophin are coexpressed in normal human B cells and B-chronic lymphocytic leukemia (B-CLL) cells. TAPP2 and syntrophin expression in B-CLL was variable from patient to patient, with significantly higher expression in the more aggressive disease subset identified by zeta-chain-associated protein kinase of 70 kDa (ZAP70) expression and unmutated immunoglobulin heavy chain (IgH) genes. We examined whether TAPP can regulate cell adhesion, a known function of utrophin/syntrophin in other cell types. Expression of membrane-targeted TAPP2 enhanced B-cell adhesion to fibronectin and laminin, whereas PH domain-mutant TAPP2 inhibited adhesion. siRNA knockdown of TAPP2 or utrophin, or treatment with PI3K inhibitors, significantly inhibited adhesion. These findings identify TAPP2 as a novel link between PI3K signaling and the cytoskeleton with potential relevance for leukemia progression.

Truncated dystrophins can influence neuromuscular synapse structure

Duchenne muscular dystrophy (DMD) is characterized by muscle degeneration and structural defects in the neuromuscular synapse that are caused by mutations in dystrophin. Whether aberrant neuromuscular synapse structure is an indirect consequence of muscle degeneration or a direct result of loss of dystrophin function is not known. Rational design of truncated dystrophins has enabled the design of expression cassettes highly effective at preventing muscle degeneration in mouse models of DMD using gene therapy. Here we examined the functional capacity of a minidystrophin (minidysGFP) and a microdystrophin (microdystrophin(DeltaR4-R23)) transgene on the maturation and maintenance of neuromuscular junctions (NMJ) in mdx mice. We found that minidysGFP prevents fragmentation and the loss of postsynaptic folds at the NMJ. In contrast, microdystrophin (DeltaR4-R23) was unable to prevent synapse fragmentation in the limb muscles despite preventing muscle degeneration, although fragmentation was observed to temporally correlate with the formation of ringed fibers. Surprisingly, microdystrophin(DeltaR4-R23) increased the length of synaptic folds in the diaphragm muscles of mdx mice independent of muscle degeneration or the formation of ringed fibers. We also demonstrate that the number and depth of synaptic folds influences the density of voltage-gated sodium channels at the neuromuscular synapse in mdx, microdystrophin(DeltaR4-R23)/mdx and mdx:utrophin double knockout mice. Together, these data suggest that maintenance of the neuromuscular synapse is governed through its lateral association with the muscle cytoskeleton, and that dystrophin has a direct role in promoting the maturation of synaptic folds to allow more sodium channels into the junction.

ZZ domain of dystrophin and utrophin: topology and mapping of a beta-dystroglycan interaction site

Dystrophin forms part of a vital link between actin cytoskeleton and extracellular matrix via the transmembrane adhesion receptor dystroglycan. Dystrophin and its autosomal homologue utrophin interact with beta-dystroglycan via their highly conserved C-terminal cysteine-rich regions, comprising the WW domain (protein-protein interaction domain containing two conserved tryptophan residues), EF hand and ZZ domains. The EF hand region stabilizes the WW domain providing the main interaction site between dystrophin or utrophin and dystroglycan. The ZZ domain, containing a predicted zinc finger motif, stabilizes the WW and EF hand domains and strengthens the overall interaction between dystrophin or utrophin and beta-dystroglycan. Using bacterially expressed ZZ domain, we demonstrate a conformational effect of zinc binding to the ZZ domain, and identify two zinc-binding regions within the ZZ domain by SPOTs overlay assays. Epitope mapping of the dystrophin ZZ domain was carried out with new monoclonal antibodies by ELISA, overlay assay and immunohistochemistry. One monoclonal antibody defined a discrete region of the ZZ domain that interacts with beta-dystroglycan. The epitope was localized to the conformationally sensitive second zinc-binding site in the ZZ domain. Our results suggest that residues 3326-3332 of dystrophin form a crucial part of the contact region between dystrophin and beta-dystroglycan and provide new insight into ZZ domain organization and function.

Sparks, signals and shock absorbers: how dystrophin loss causes muscular dystrophy

The dystrophin-glycoprotein complex (DGC) can be considered as a specialized adhesion complex, linking the extracellular matrix to the actin cytoskeleton, primarily in muscle cells. Mutations in several components of the DGC lead to its partial or total loss, resulting in various forms of muscular dystrophy. These typically manifest as progressive wasting diseases with loss of muscle integrity. Debate is ongoing about the precise function of the DGC: initially a strictly mechanical role was proposed but it has been suggested that there is aberrant calcium handling in muscular dystrophy and, more recently, changes in MAP kinase and GTPase signalling have been implicated in the aetiology of the disease. Here, we discuss new and interesting developments in these aspects of DGC function and attempt to rationalize the mechanical, calcium and signalling hypotheses to provide a unifying hypothesis of the underlying process of muscular dystrophy.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

ZZ domain of CBP: an unusual zinc finger fold in a protein interaction module

CREB-binding protein (CBP) is a large, multi-domain protein that provides a multitude of binding sites for transcriptional coactivators. The site of interaction of the tumor suppressor p53 and the oncoprotein E1A with CBP/p300 has been identified with the third cysteine-histidine-rich (CH3) domain, which incorporates two zinc-binding motifs, ZZ and TAZ2. We show that these two domains fold independently and do not interact in solution. Our experiments demonstrate conclusively that the interaction of p53 and E1A with the CH3 domain resides exclusively in the TAZ2 domain, with no contribution from the ZZ domain. We report also the three-dimensional solution structure of the ZZ domain of murine CBP. The 52 residue ZZ domain contains two twisted antiparallel beta-sheets and a short alpha-helix, and binds two zinc ions. The identity of the zinc coordinating ligands was resolved unambiguously using NMR spectroscopy of the ZZ domain substituted with (113)Cd. One zinc ion is coordinated tetrahedrally via two CXXC motifs to four cysteine side-chains, and the second zinc ion is coordinated tetrahedrally by a third CXXC motif, together with an unusual HXH motif coordinating via the N(epsilon2) atom of His40 and the N(delta1) atom of His-42. The first zinc cluster of the ZZ domain is strictly conserved, whereas the second zinc cluster shows variability in the position of the two histidine residues, reflecting the wide variety of molecules that incorporate ZZ domains. The structure of the ZZ domain shows that it belongs to the family of cross-brace zinc finger motifs that include the PHD, RING, and FYVE domains; however, its biological function is unclear. Mapping of the positions of conserved residues onto the calculated structures reveals a face containing exposed aromatic and hydrophobic side-chains, while the opposite face contains a series of conserved charged or hydrophilic groups. These homologies suggest that the ZZ domain is involved in ligand binding or molecular scaffolding, with specificity provided by the variability of the sequence that contains the helix in the murine CPB ZZ domain structure.

Absence of Dp71 in mdx3cv mouse spermatozoa alters flagellar morphology and the distribution of ion channels and nNOS

In muscle, the absence of dystrophin alters the dystrophin-associated protein complex (DAPC), which is involved in the clustering and anchoring of signaling proteins and ion and water channels. Here we show that mice spermatozoa express only dystrophin Dp71 and utrophin Up71. The purpose of this study was to explore the effect of the absence of Dp71 on the morphology and membrane distribution of members of the DAPC, ion channels and signaling proteins of spermatozoa obtained from dystrophic mutant mdx3cv mice. Our work indicates that although the absence of Dp71 results in a dramatic decrease in beta-dystroglycan, it induces membrane redistribution and an increase in the total level of alpha-syntrophin, voltage-dependent Na+ (micro1) and K+ (Kv1.1) channels and neural nitric oxide synthase (nNOS). The short utrophin (Up71) was upregulated and redistributed in the spermatozoa of mdx3cv mice. A significant increase in abnormal flagella morphology was observed in the absence of Dp71, which was partially corrected when the plasma membrane was eliminated by detergent treatment. Our observations point to a new phenotype associated with the absence of Dp71. Abnormal flagellar structure and altered distribution of ion channels and signaling proteins may be responsible for the fertility problems of mdx3cv mice.

Long-term expression of full-length human dystrophin in transgenic mdx mice expressing internally deleted human dystrophins

One of the possible therapies for Duchenne muscular dystrophy (DMD) is the introduction of a functional copy of the dystrophin gene into the patient. For this approach to be effective, therapeutic levels and long-term expression of the protein need to be achieved. However, immune responses to the newly expressed dystrophin have been predicted, particularly in DMD patients who express no dystrophin or only very truncated versions. In a previous study, we demonstrated a strong humoral and cytotoxic immune response to human dystrophin in the mdx mouse. However, the mdx mouse was tolerant to murine dystrophin, possibly due to the endogenous expression of dystrophin in revertant fibres or the other nonmuscle dystrophin isoforms. In the present study, we delivered human and murine dystrophin plasmids by electrotransfer after hyaluronidase pretreatment to increase gene transfer efficiencies. Tolerance to murine dystrophin was still seen with this improved gene delivery. Tolerance to exogenous recombinant full-length human dystrophin was seen in mdx transgenic lines expressing internally deleted versions of human dystrophin. These results suggest that the presence of revertant fibres may prevent the development of serious immune responses in patients undergoing dystrophin gene therapy.

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.

Loss of dystrophin causes aberrant mechanotransduction in skeletal muscle fibers

Dystrophin is a cytoskeletal protein found at the inner surface of skeletal and cardiac muscle fibers. We hypothesize that deficiency of dystrophin increases muscle compliance and causes an aberrant mechanotransduction in muscle fibers. To test this hypothesis, we measured the length-tension relationships and determined intracellular signaling leading to the activation of mitogen-activated protein (MAP) kinases in diaphragm muscle fibers from dystrophin-deficient mdx mice. Compared with controls, length-tension curves of the mdx mice were shifted to the right. A higher level of activation of extracellular signal-regulated kinase 1/2 (ERK1/2) but not c-Jun N-terminal kinase-1 or p38 MAP kinase was observed in the mdx muscle compared with the normal muscle in response to mechanical stretch. Removal of Ca2+ from the medium inhibited stretch-induced ERK1/2 activation only in mdx muscle fibers but not in the normal fibers. Conversely, pretreatment with TMB-8 (an antagonist of intracellular Ca2+ blocked the mechanical stretch-induced ERK1/2 activation in normal but not in mdx muscle fibers. Pretreatment of muscle with nifedipine (L-type calcium channel antagonist) marginally decreased the activation of ERK1/2 in normal or mdx muscle whereas pretreatment with gadolinium (III) chloride (an inhibitor of stretch-activated channels) only blocked the activation of ERK1/2 in mdx muscle, with no significant effect on normal muscle. A higher basal level of activation of activator protein-1 (AP-1) transcription factor was observed in dystrophin-deficient diaphragm, which was further augmented by mechanical stretch. Mechanical stretch-induced activation of AP-1 was decreased by pretreatment of muscle fibers with PD98059 (ERK1/2 inhibitor) and removal of Ca2+ ions from incubation medium. Our results show that dystrophin is a load-bearing element and its deficiency leads to loss of muscle stiffness and aberrant mechanotransduction in skeletal muscle fibers.

Localizing synaptic mRNAs at the neuromuscular junction: it takes more than transcription

The neuromuscular junction has been used for several decades as an excellent model system to examine the cellular and molecular events involved in the formation and maintenance of a differentiated chemical synapse. In this context, several laboratories have focused their efforts over the last 15 years on the important contribution of transcriptional mechanisms to the regulation of the development and plasticity of the postsynaptic apparatus in muscle fibers. Converging lines of evidence now indicate that post-transcriptional events, operating at the level of mRNA stability and targeting, are likely to also play key roles at the neuromuscular junction. Here, we present the recent findings highlighting the role of these additional molecular events and extend our review to include data showing that post-transcriptional events are also important in the control of the expression of genes encoding synaptic proteins in muscle cells placed under different conditions. Finally, we discuss the possibility that mis-regulation of post-transcriptional events can occur in certain neuromuscular diseases and cause abnormalities of the neuromuscular junction.

Partitioning of exogenous delta-tocopherol between the triacylglycerol and membrane lipid fractions of chicken muscle

The partitioning of exogenous delta-tocopherol, added dissolved in ethanol, between the neutral triacylglycerols and membranes of chicken leg muscles was investigated. The two lipid fractions were separated using differential ultracentrifugation techniques. Triacylglycerols were obtained after high-speed centrifugation of the minced muscle at 130000 g for 30 min. Membranes were collected from a muscle-buffer homogenate (pH 7.5) between 10000 g for 20 min and 130000 g for 30 min. The triacylglycerols collected represented from 15 to 80% of the total triacylglycerols of the minced muscle, the yields increasing with increasing muscle triacylglycerol content. The phospholipids in the isolated membrane fraction represented from 20 to 35% of the total phospholipids of the muscle. At low muscle total lipid contents (3-5%), the added delta-tocopherol was present in approximately the same concentration in both muscle lipid fractions. At higher total lipid contents, achieved by adding exogenous triacylglycerols, the delta-tocopherol concentration in the membranes increased relative to that in the triacylglycerols.

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.

Disruption of DAG1 in differentiated skeletal muscle reveals a role for dystroglycan in muscle regeneration

Striated muscle-specific disruption of the dystroglycan (DAG1) gene results in loss of the dystrophin-glycoprotein complex in differentiated muscle and a remarkably mild muscular dystrophy with hypertrophy and without tissue fibrosis. We find that satellite cells, expressing dystroglycan, support continued efficient regeneration of skeletal muscle along with transient expression of dystroglycan in regenerating muscle fibers. We demonstrate a similar phenomenon of reexpression of functional dystroglycan in regenerating muscle fibers in a mild form of human muscular dystrophy caused by disruption of posttranslational dystroglycan processing. Thus, maintenance of regenerative capacity by satellite cells expressing dystroglycan is likely responsible for mild disease progression in mice and possibly humans. Therefore, inadequate repair of skeletal muscle by satellite cells represents an important mechanism affecting the pathogenesis of muscular dystrophy.

Muscular dystrophies, the cytoskeleton and cell adhesion

Muscular dystrophies are associated with mutations in genes encoding several classes of proteins. These range from extracellular matrix and integral membrane proteins to cytoskeletal proteins, but also include a heterogeneous group of proteins including proteases, nuclear proteins, and signalling molecules. Muscular dystrophy phenotypes have also become evident in studies on various knockout mice defective in proteins not previously considered or known to be mutated in muscular dystrophies. Some unifying themes are beginning to emerge from all of these data. This review will consider recent advances in our understanding of the molecules involved and bring together data that suggest a role for the cytoskeleton and cell adhesion in muscular dystrophies.

Utrophin binds laterally along actin filaments and can couple costameric actin with sarcolemma when overexpressed in dystrophin-deficient muscle

Dystrophin is widely thought to mechanically link the cortical cytoskeleton with the muscle sarcolemma. Although the dystrophin homolog utrophin can functionally compensate for dystrophin in mice, recent studies question whether utrophin can bind laterally along actin filaments and anchor filaments to the sarcolemma. Herein, we have expressed full-length recombinant utrophin and show that the purified protein is fully soluble with a native molecular weight and molecular dimensions indicative of monomers. We demonstrate that like dystrophin, utrophin can form an extensive lateral association with actin filaments and protect actin filaments from depolymerization in vitro. However, utrophin binds laterally along actin filaments through contribution of acidic spectrin-like repeats rather than the cluster of basic repeats used by dystrophin. We also show that the defective linkage between costameric actin filaments and the sarcolemma in dystrophin-deficient mdx muscle is rescued by overexpression of utrophin. Our results demonstrate that utrophin and dystrophin are functionally interchangeable actin binding proteins, but that the molecular epitopes important for filament binding differ between the two proteins. More generally, our results raise the possibility that spectrin-like repeats may enable some members of the plakin family of cytolinkers to laterally bind and stabilize actin filaments.

The WW domain: linking cell signalling to the membrane cytoskeleton

The WW domain is one of the smallest yet most versatile protein-protein interaction modules. The ability of this simple domain to interact with a number of proline-containing ligands has resulted in a great deal of functional diversity. Most recently it has been shown that WW domain interactions can also be differentially regulated by tyrosine phosphorylation. Here we briefly review WW domain ligands and structure in comparison to SH3 domain ligands and structure and discuss recent findings with regard to the regulation of WW domain interactions by phosphorylation. In particular we describe the potential for differential binding of the b-dystroglycan WW domain ligand by dystrophin or caveolin-3 in skeletal muscle and show how this could act as a switch to alter the relative affinity of the muscle dystroglycan complex for caveolin-3 or dystrophin and utrophin.

The spectrin repeat: a structural platform for cytoskeletal protein assemblies

Spectrin repeats are three-helix bundle structures which occur in a large number of diverse proteins, either as single copies or in tandem arrangements of multiple repeats. They can serve structural purposes, by coordination of cytoskeletal interactions with high spatial precision, as well as a 'switchboard' for interactions with multiple proteins with a more regulatory role. We describe the structure of the alpha-actinin spectrin repeats as a prototypical example, their assembly in a defined antiparallel dimer, and the interactions of spectrin repeats with multiple other proteins. The alpha-actinin rod domain shares several features common to other spectrin repeats. (1) The rod domain forms a rigid connection between two actin-binding domains positioned at the two ends of the alpha-actinin dimer. The exact distance and rigidity are important, for example, for organizing the muscle Z-line and maintaining its architecture during muscle contraction. (2) The spectrin repeats of alpha-actinin have evolved to make tight antiparallel homodimer contacts. (3) The spectrin repeats are important interaction sites for multiple structural and signalling proteins. The interactions of spectrin repeats are, however, diverse and defy any simple classification of their preferred interaction sites, which is possible for other domains (e.g. src-homology domains 3 or 2). Nevertheless, the binding properties of the repeats perform important roles in the biology of the proteins where they are found, and lead to the assembly of complex, multiprotein structures involved both in cytoskeletal architecture as well as in forming large signal transduction complexes.

Towards a complete atomic structure of spectrin family proteins

The spectrin family of proteins represents a discrete group of cytoskeletal proteins comprising principally alpha-actinin, spectrin, dystrophin, and homologues and isoforms. They all share three main structural and functional motifs, namely, the spectrin repeat, EF-hands, and a CH domain-containing actin-binding domain. These proteins are variously involved in organisation of the actin cytoskeleton, membrane cytoskeleton architecture, cell adhesion, and contractile apparatus. The highly modular nature of these molecules has been a hindrance to the determination of their complete structures due to the inherent flexibility imparted on the proteins, but has also been an asset, inasmuch as the individual modules were of a size amenable to structural analysis by both crystallographic and NMR approaches. Representative structures of all the major domains shared by spectrin family proteins have now been solved at atomic resolution, including in some cases multiple domains from several family members. High-resolution structures, coupled with lower resolution methods to determine the overall molecular shape of these proteins, allow us for the first time to build complete atomic structures of the spectrin family of proteins.

The interaction of dystrophin with beta-dystroglycan is regulated by tyrosine phosphorylation

Dystrophin and the dystrophin-associated protein complex (DAPC) have recently been implicated in cell signalling events. These proteins are ideally placed to transduce signals from the extracellular matrix (ECM) to the cytoskeleton. Here we show that beta-dystroglycan is tyrosine-phosphorylated in C2/C4 mouse myotubes. Tyrosine phosphorylation was detected by mobility shifts on SDS-polyacrylamide gels (SDS-PAGE) and confirmed by immunoprecipitation and two-dimensional gel electrophoresis. The potential functional significance of this tyrosine phosphorylation was investigated using peptide 'SPOTs' assays. Phosphorylation of tyrosine in the 15 most C-terminal amino acids of beta-dystroglycan disrupts its interaction with dystrophin. The tyrosine residue in beta-dystroglycan's WW-binding motif PPPY appears to be the most crucial in disrupting the beta-dystroglycan-dystrophin interaction. beta-dystroglycan forms the essential link between dystrophin and the rest of the DAPC. This regulation by tyrosine phosphorylation may have implications in the pathogenesis and treatment of Duchenne's muscular dystrophy (DMD).

Crystal structure of the alpha-actinin rod reveals an extensive torsional twist

BACKGROUND

Alpha-actinin is a ubiquitously expressed protein found in numerous actin structures. It consists of an N-terminal actin binding domain, a central rod domain, and a C-terminal domain and functions as a homodimer to cross-link actin filaments. The rod domain determines the distance between cross-linked actin filaments and also serves as an interaction site for several cytoskeletal and signaling proteins.

RESULTS

We report here the crystal structure of the alpha-actinin rod. The structure is a twisted antiparallel dimer that contains a conserved acidic surface.

CONCLUSIONS

The novel features revealed by the structure allow prediction of the orientation of parallel and antiparallel cross-linked actin filaments in relation to alpha-actinin. The conserved acidic surface is a possible interaction site for several cytoplasmic tails of transmembrane proteins involved in the recruitment of alpha-actinin to the plasma membrane.

Epitopes in the interacting regions of beta-dystroglycan (PPxY motif) and dystrophin (WW domain)

The dystroglycan gene produces two products from a single mRNA, the extracellular alpha-dystroglycan and the transmembrane beta-dystroglycan. The Duchenne muscular dystrophy protein, dystrophin, associates with the muscle membrane via beta-dystroglycan, the WW domain of dystrophin interacting with a PPxY motif in beta-dystroglycan. A panel of four monoclonal antibodies (MANDAG1-4) was produced using the last 16 amino acids of beta-dystroglycan as immunogen. The mAbs recognized a 43 kDa band on Western blots of all cells and tissues tested and stained the sarcolemma in immunohistochemistry of skeletal muscle over a wide range of animal species. A monoclonal antibody (mAb) against the WW domain of dystrophin, MANHINGE4A, produced using a 16-mer synthetic peptide, recognized dystrophin on Western blots and also stained the sarcolemma. We have identified the precise sequences recognized by the mAbs using a phage-displayed random 15-mer peptide library. A 7-amino-acid consensus sequence SPPPYVP involved in binding all four beta-dystroglycan mAbs was identified by sequencing 17 different peptides selected from the library. PPY were the most important residues for three mAbs, but PxxVP were essential residues for a fourth mAb, MANDAG2. By sequencing five different random peptides from the library, the epitope on dystrophin recognized by mAb MANHINGE4A was identified as PWxRA in the first beta-strand of the WW domain, with the W and R residues invariably present. A recent three-dimensional structure confirms that the two epitopes are adjacent in the dystrophin-dystroglycan complex, highlighting the question of how the two interacting motifs can also be accessible to antibodies during immunolocalization in situ.

Specific disruption of a schwann cell dystrophin-related protein complex in a demyelinating neuropathy

Dystroglycan-dystrophin complexes are believed to have structural and signaling functions by linking extracellular matrix proteins to the cytoskeleton and cortical signaling molecules. Here we characterize a dystroglycan-dystrophin-related protein 2 (DRP2) complex at the surface of myelin-forming Schwann cells. The complex is clustered by the interaction of DRP2 with L-periaxin, a homodimeric PDZ domain-containing protein. In the absence of L-periaxin, DRP2 is mislocalized and depleted, although other dystrophin family proteins are unaffected. Disruption of the DRP2-dystroglycan complex is followed by hypermyelination and destabilization of the Schwann cell-axon unit in Prx(-/-) mice. Hence, the DRP2-dystroglycan complex likely has a distinct function in the terminal stages of PNS myelinogenesis, possibly in the regulation of myelin thickness.

Brain dystrophin-glycoprotein complex: persistent expression of beta-dystroglycan, impaired oligomerization of Dp71 and up-regulation of utrophins in animal models of muscular dystrophy

BACKGROUND

Aside from muscle, brain is also a major expression site for dystrophin, the protein whose abnormal expression is responsible for Duchenne muscular dystrophy. Cognitive impairments are frequently associated with this genetic disease, we therefore studied the fate of brain and skeletal muscle dystrophins and dystroglycans in dystrophic animal models.

RESULTS

All dystrophin-associated glycoproteins investigated were reduced in dystrophic muscle fibres. In Dp427-deficient mdx brain and Dp71-deficient mdx-3cv brain, the expression of alpha-dystroglycan and laminin was reduced, utrophin isoforms were up-regulated and beta-dystroglycan was not affected. Immunofluorescence localization of beta-dystroglycan in comparison with glial, endothelial and neuronal cell markers revealed co-localization of von Willebrand factor with beta-dystroglycan. Its expression at the endothelial-glial interface was preserved in dystrophin isoform-deficient brain from mdx and mdx-3cv mice. In addition, chemical crosslinking revealed that the Dp71 isoform exists in mdx brain predominantly as a monomer.

CONCLUSIONS

This suggests an association of beta-dystroglycan with membranes at the vascular-glial interface in the forebrain. In contrast to dystrophic skeletal muscle fibres, dystrophin deficiency does not trigger a reduction of all dystroglycans in the brain, and utrophins may partially compensate for the lack of brain dystrophins. Abnormal oligomerization of the dystrophin isoform Dp71 might be involved in the pathophysiological mechanisms underlying abnormal brain functions.

The complexities of dystroglycan

The notion of dystroglycan as a simple laminin-binding receptor is increasingly being challenged. New roles and new binding partners are continually emerging. Recent structural advances have provided exciting new insights into the precise molecular interactions between dystroglycan and other key components of the dystroglycan complex. Coupled with an increasing understanding of dystroglycan function at the molecular level, we are finally beginning to probe the complexities of dystroglycan, not only in disease, but in development, adhesion and signalling.

Immune responses to dystropin: implications for gene therapy of Duchenne muscular dystrophy

Introduction of dystrophin by gene transfer into the dystrophic muscles of Duchenne muscular dystrophy (DMD) patients has the possibility of triggering an immune response as many patients will not have been exposed to some (or all) of the epitopes of dystrophin. This could in turn lead to cytotoxic destruction of transfected muscle fibres. We assessed such concerns in the dystrophin-deficient mdx mouse using plasmid DNA as the gene transfer system. This avoids complications associated with the administration of viral proteins. Gene transfer of cDNAs encoding mouse full-length or a truncated minidystrophin did not evoke either a humoral or cytotoxic immune response. Mdx mice may be tolerant due to the presence of rare 'revertant' dystrophin-positive fibres in their skeletal muscles. In contrast, gene transfer of human full-length or minidystrophin provoked both humoral and cytotoxic responses leading to destruction of the transfected fibres. These experiments demonstrate the potential risk of deleterious effects following gene therapy in DMD patients and lead us to suggest that patients enrolled in gene therapy trials should ideally have small, preferably point, mutations and evidence of 'revertant' dystrophin-positive muscle fibres.

Expression and localization of utrophin in differentiating PC12 cells

To ascertain the role of utrophin in cultured neuronal cells, we investigated its expression and distribution along the NGF-induced differentiation of PC12 cells grown on different substrata. Utrophin mRNA was measured by RT-PCR assay and utrophin protein was quantified by immunoblot analysis. The distribution of utrophin and beta-dystroglycan was analyzed by confocal microscopy. We demonstrate that utrophin protein was increased 4-fold during differentiation of cells grown laminin. Concomitant with this up-regulation, utrophin was enriched at the growth cones in differentiating cells, where it co-localizes with beta-dystroglycan. These data suggest the presence of a utrophin-beta-dystroglycan complex in PC12 cells that participates in the formation and/or stabilization of the growth cone-extracellular matrix adhesion.

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

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

Dystrophin binding to nonmuscle actin

We purified actin from bovine brain by DNase I affinity chromatography in order to compare the binding of dystrophin to muscle actin with its binding to nonmuscle actin. While both beta- and gamma-nonmuscle actins are expressed in brain, Western blot analysis with isoform-specific antibodies indicated that our purified brain actin was exclusively the gamma-isoform. The recombinant amino-terminal, actin-binding domain of dystrophin bound to muscle and brain actin in a saturable manner (approximately 1 mol/mol actin) with similar Kd values of 13.7+/-3.5 and 10.6+/-3.7 microM, respectively. We further demonstrate that intact dystrophin in the dystrophin-glycoprotein complex bound with equal avidity to muscle and brain F-actin. These data argue that a preferential binding of dystrophin to nonmuscle actin is not the basis for its targeting to the muscle cell plasmalemma but do support the hypothesis that dystrophin is capable of interacting with filamentous actin in nonmuscle tissues.

Biochemical characterisation of the actin-binding properties of utrophin

Utrophin is a large ubiquitously expressed cytoskeletal protein that is important for maturation of vertebrate neuromuscular junctions. It is highly homologous to dystrophin, the protein defective in Duchenne and Becker muscular dystrophies. Utrophin binds to the actin cytoskeleton via an N-terminal actin-binding domain, which is related to the actin-binding domains of members of the spectrin superfamily of proteins. We have determined the actin-binding properties of this utrophin domain and investigated its binding site on F-actin. An F-actin cosedimentation assay confirmed that the domain binds more tightly to beta-F-actin than to alpha-F-actin and that the full-length utrophin domain binds more tightly to both actin isoforms than a truncated construct, lacking a characteristic utrophin N-terminal extension. Both domain constructs exist in solution as compact monomers and bind to actin as 1:1 complexes. Analysis of the products of partial proteolysis of the domain in the presence of F-actin showed that the N-terminal extension was protected by binding to actin. The actin isoform dependence of utrophin binding could reflect differences at the N-termini of the actin isoforms, thus localising the utrophin-binding site on actin. The involvement of the actin N-terminus in utrophin binding was also supported by competition binding assays using myosin subfragment S1, which also binds F-actin near its N-terminus. Cross-linking studies suggested that utrophin contacts two actin monomers in the actin filament as does myosin S1. These biochemical approaches complement our structural studies and facilitate characterisation of the actin-binding properties of the utrophin actin-binding domain.

The artificial zinc finger coding gene 'Jazz' binds the utrophin promoter and activates transcription

Up-regulation of utrophin gene expression is recognized as a plausible therapeutic approach in the treatment of Duchenne muscular dystrophy (DMD). We have designed and engineered new zinc finger-based transcription factors capable of binding and activating transcription from the promoter of the dystrophin-related gene, utrophin. Using the recognition 'code' that proposes specific rules between zinc finger primary structure and potential DNA binding sites, we engineered a new gene named 'Jazz' that encodes for a three-zinc finger peptide. Jazz belongs to the Cys2-His2 zinc finger type and was engineered to target the nine base pair DNA sequence: 5'-GCT-GCT-GCG-3', present in the promoter region of both the human and mouse utrophin gene. The entire zinc finger alpha-helix region, containing the amino acid positions that are crucial for DNA binding, was specifically chosen on the basis of the contacts more frequently represented in the available list of the 'code'. Here we demonstrate that Jazz protein binds specifically to the double-stranded DNA target, with a dissociation constant of about 32 nM. Band shift and super-shift experiments confirmed the high affinity and specificity of Jazz protein for its DNA target. Moreover, we show that chimeric proteins, named Gal4-Jazz and Sp1-Jazz, are able to drive the transcription of a test gene from the human utrophin promoter.

Adhesion-dependent tyrosine phosphorylation of (beta)-dystroglycan regulates its interaction with utrophin

Many cell adhesion-dependent processes are regulated by tyrosine phosphorylation. In order to investigate the role of tyrosine phosphorylation of the utrophin-dystroglycan complex we treated suspended or adherent cultures of HeLa cells with peroxyvanadate and immunoprecipitated (beta)-dystroglycan and utrophin from cell extracts. Western blotting of (β)-dystroglycan and utrophin revealed adhesion- and peroxyvanadate-dependent mobility shifts which were recognised by anti-phospho-tyrosine antibodies. Using maltose binding protein fusion constructs to the carboxy-terminal domains of utrophin we were able to demonstrate specific interactions between the WW, EF and ZZ domains of utrophin and (beta)-dystroglycan by co-immunoprecipitation with endogenous (beta)-dystroglycan. In extracts from cells treated with peroxyvanadate, where endogenous (beta)-dystroglycan was tyrosine phosphorylated, (beta)-dystroglycan was no longer co-immunoprecipitated with utrophin fusion constructs. Peptide ‘SPOTs’ assays confirmed that tyrosine phosphorylation of (beta)-dystroglycan regulated the binding of utrophin. The phosphorylated tyrosine was identified as Y(892) in the (beta)-dystroglycan WW domain binding motif PPxY thus demonstrating the physiological regulation of the (beta)-dystroglycan/utrophin interaction by adhesion-dependent tyrosine phosphorylation.