Ca2+-calmodulin binds to the carboxyl-terminal domain of dystrophin

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

Dystrophin- and Utrophin-Based Therapeutic Approaches for Treatment of Duchenne Muscular Dystrophy: A Comparative Review

Duchenne muscular dystrophy is a devastating disease that leads to progressive muscle loss and premature death. While medical management focuses mostly on symptomatic treatment, decades of research have resulted in first therapeutics able to restore the affected reading frame of dystrophin transcripts or induce synthesis of a truncated dystrophin protein from a vector, with other strategies based on gene therapy and cell signaling in preclinical or clinical development. Nevertheless, recent reports show that potentially therapeutic dystrophins can be immunogenic in patients. This raises the question of whether a dystrophin paralog, utrophin, could be a more suitable therapeutic protein. Here, we compare dystrophin and utrophin amino acid sequences and structures, combining published data with our extended in silico analyses. We then discuss these results in the context of therapeutic approaches for Duchenne muscular dystrophy. Specifically, we focus on strategies based on delivery of micro-dystrophin and micro-utrophin genes with recombinant adeno-associated viral vectors, exon skipping of the mutated dystrophin pre-mRNAs, reading through termination codons with small molecules that mask premature stop codons, dystrophin gene repair by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated genetic engineering, and increasing utrophin levels. Our analyses highlight the importance of various dystrophin and utrophin domains in Duchenne muscular dystrophy treatment, providing insights into designing novel therapeutic compounds with improved efficacy and decreased immunoreactivity. While the necessary actin and β-dystroglycan binding sites are present in both proteins, important functional distinctions can be identified in these domains and some other parts of truncated dystrophins might need redesigning due to their potentially immunogenic qualities. Alternatively, therapies based on utrophins might provide a safer and more effective approach.

Disrupted Calcium Homeostasis in Duchenne Muscular Dystrophy: A Common Mechanism behind Diverse Consequences

Duchenne muscular dystrophy (DMD) leads to disability and death in young men. This disease is caused by mutations in the DMD gene encoding diverse isoforms of dystrophin. Loss of full-length dystrophins is both necessary and sufficient for causing degeneration and wasting of striated muscles, neuropsychological impairment, and bone deformities. Among this spectrum of defects, abnormalities of calcium homeostasis are the common dystrophic feature. Given the fundamental role of Ca2+ in all cells, this biochemical alteration might be underlying all the DMD abnormalities. However, its mechanism is not completely understood. While abnormally elevated resting cytosolic Ca2+ concentration is found in all dystrophic cells, the aberrant mechanisms leading to that outcome have cell-specific components. We probe the diverse aspects of calcium response in various affected tissues. In skeletal muscles, cardiomyocytes, and neurons, dystrophin appears to serve as a scaffold for proteins engaged in calcium homeostasis, while its interactions with actin cytoskeleton influence endoplasmic reticulum organisation and motility. However, in myoblasts, lymphocytes, endotheliocytes, and mesenchymal and myogenic cells, calcium abnormalities cannot be clearly attributed to the loss of interaction between dystrophin and the calcium toolbox proteins. Nevertheless, DMD gene mutations in these cells lead to significant defects and the calcium anomalies are a symptom of the early developmental phase of this pathology. As the impaired calcium homeostasis appears to underpin multiple DMD abnormalities, understanding this alteration may lead to the development of new therapies. In fact, it appears possible to mitigate the impact of the abnormal calcium homeostasis and the dystrophic phenotype in the total absence of dystrophin. This opens new treatment avenues for this incurable disease.

Duchenne muscular dystrophy

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

Potential muscle activity disturbance in Penaeus monodon during Acute Hepatopancreatic Necrosis Disease (AHPND) infection: Inference through gene expression, calcium concentration, and MicroRNA

Global shrimp aquaculture farmers have suffered major economic losses due to disease outbreaks. A notable shrimp disease is Acute Hepatopancreatic Necrosis Disease (AHPND), which is caused by a new strain of Vibrio parahaemolyticus bacteria (Vp_AHPND) that mainly inhabits the shrimp gut and damages the hepatopancreas. Fewer studies have investigated whether this disease will affect shrimp muscle functioning or cause any muscle damage. We challenged Penaeus monodon shrimp with Vp_AHPND bacteria using an immersion method. Expression of Dystrophin gene, an important regulatory gene for maintenance of muscle integrity, was quantified from muscle samples using qRT-PCR. Additional verification was conducted by determining calcium concentration and bta-miR-4286 and dre-miR-107b miRNAs expression. P. monodon dystrophin gene demonstrated the highest expression level during AHPND infection when muscle calcium concentration was detected at its lowest level at 6 h post-infection (hpi). The highest muscle calcium concentration, determined at 36 hpi, was supported by higher bta-miR-4286 miRNA expression and lower dre-miR-107b miRNA expression in Vp_AHPND-infected samples compared to uninfected samples at the same time point. We deduced an interactive relationship between dystrophin gene expression, calcium concentration, and miRNA expression in P. monodon muscle tissues triggered by the invading Vp_AHPND bacterium.

The Dystrophin Complex: Structure, Function, and Implications for Therapy

The dystrophin complex stabilizes the plasma membrane of striated muscle cells. Loss of function mutations in the genes encoding dystrophin, or the associated proteins, trigger instability of the plasma membrane, and myofiber loss. Mutations in dystrophin have been extensively cataloged, providing remarkable structure-function correlation between predicted protein structure and clinical outcomes. These data have highlighted dystrophin regions necessary for in vivo function and fueled the design of viral vectors and now, exon skipping approaches for use in dystrophin restoration therapies. However, dystrophin restoration is likely more complex, owing to the role of the dystrophin complex as a broad cytoskeletal integrator. This review will focus on dystrophin restoration, with emphasis on the regions of dystrophin essential for interacting with its associated proteins and discuss the structural implications of these approaches.

Inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling mediates delayed myogenesis in Duchenne muscular dystrophy fetal muscle

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder characterized by muscle wasting and premature death. The defective gene is dystrophin, a structural protein, absence of which causes membrane fragility and myofiber necrosis. Several lines of evidence showed that in adult DMD patients dystrophin is involved in signaling pathways that regulate calcium homeostasis and differentiation programs. However, secondary aspects of the disease, such as inflammation and fibrosis development, might represent a bias in the analysis. Because fetal muscle is not influenced by gravity and does not suffer from mechanical load and/or inflammation, we investigated 12-week-old fetal DMD skeletal muscles, highlighting for the first time early alterations in signaling pathways mediated by the absence of dystrophin itself. We found that PLC/IP3/IP3R/Ryr1/Ca2+ signaling is widely active in fetal DMD skeletal muscles and, through the calcium-dependent PKCα protein, exerts a fundamental regulatory role in delaying myogenesis and in myofiber commitment. These data provide new insights into the origin of DMD pathology during muscle development.

Mechanical and non-mechanical functions of Dystrophin can prevent cardiac abnormalities in Drosophila

Dystrophin-deficiency causes cardiomyopathies and shortens the life expectancy of Duchenne and Becker muscular dystrophy patients. Restoring Dystrophin expression in the heart by gene transfer is a promising avenue to explore as a therapy. Truncated Dystrophin gene constructs have been engineered and shown to alleviate dystrophic skeletal muscle disease, but their potential in preventing the development of cardiomyopathy is not fully understood. In the present study, we found that either the mechanical or the signaling functions of Dystrophin were able to reduce the dilated heart phenotype of Dystrophin mutants in a Drosophila model. Our data suggest that Dystrophin retains some function in fly cardiomyocytes in the absence of a predicted mechanical link to the cytoskeleton. Interestingly, cardiac-specific manipulation of nitric oxide synthase expression also modulates cardiac function, which can in part be reversed by loss of Dystrophin function, further implying a signaling role of Dystrophin in the heart. These findings suggest that the signaling functions of Dystrophin protein are able to ameliorate the dilated cardiomyopathy, and thus might help to improve heart muscle function in micro-Dystrophin-based gene therapy approaches.

Calcium-binding proteins in skeletal muscles of the mdx mice: potential role in the pathogenesis of Duchenne muscular dystrophy

Duchenne muscular dystrophy is one of the most common hereditary diseases. Abnormal ion handling renders dystrophic muscle fibers more susceptible to necrosis and a rise in intracellular calcium is an important initiating event in dystrophic muscle pathogenesis. In the mdx mice, muscles are affected with different intensities and some muscles are spared. We investigated the levels of the calcium-binding proteins calsequestrin and calmodulin in the non-spared axial (sternomastoid and diaphragm), limb (tibialis anterior and soleus), cardiac and in the spared extraocular muscles (EOM) of control and mdx mice. Immunoblotting analysis showed a significant increase of the proteins in the spared mdx EOM and a significant decrease in the most affected diaphragm. Both proteins were comparable to the cardiac muscle controls. In limb and sternomastoid muscles, calmodulin and calsequestrin were affected differently. These results suggest that differential levels of the calcium-handling proteins may be involved in the pathogenesis of myonecrosis in mdx muscles. Understanding the signaling mechanisms involving Ca(2+)-calmodulin activation and calsequestrin expression may be a valuable way to develop new therapeutic approaches to the dystrophinopaties.

Imbalance between CaM kinase II and calcineurin activities impairs caffeine-induced calcium release in hypertrophic cardiomyocytes

Cardiac hypertrophy impairs Ca(2+) handling in the sarcoplasmic reticulum, thereby impairing cardiac contraction. To identify the mechanisms underlying impaired Ca(2+) release from the sarcoplasmic reticulum in hypertrophic cardiomyocytes, we assessed Ca(2+)-dependent signaling and the phosphorylation of phospholamban, which regulates Ca(2+) uptake during myocardial relaxation and is in turn regulated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and calcineurin. In cultured rat cardiomyocytes, treatment with endothelin-1, angiotensin II, and phenylephrine-induced hypertrophy and increased CaMKII autophosphorylation and calcineurin expression. The calcineurin level reached its maximum at 72h and remained elevated for at least 96h after endothelin-1 or angiotensin II treatment. By contrast, CaMKII autophosphorylation, phospholamban phosphorylation, and caffeine-induced Ca(2+) mobilization all peaked 48h after these treatments. By 96h after treatment, CaMKII autophosphorylation and phospholamban phosphorylation had returned to baseline, and caffeine-induced Ca(2+) mobilization was impaired relative to baseline. A similar biphasic change was observed in dystrophin levels in endothelin-1-induced hypertrophic cardiomyocytes, and treatment with the novel CaM antagonists DY-9760e and DY-9836 significantly inhibited the hypertrophy-induced dystrophin breakdown. Taken together, the abnormal Ca(2+) regulation in cardiomyocytes following hypertrophy is in part mediated by an imbalance in calcineurin and CaMKII activities, which leads to abnormal phospholamban activity.

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.

Acetylcholine Receptor Organization at the Dystrophic Extraocular Muscle Neuromuscular Junction

Spared extraocular muscles of dystrophic mice are not subjected to regeneration process and can be used to verify whether the lack of dystrophin per se could cause changes in acetylcholine receptor (AChR) distribution. In the present study, rectus and oblique (spared) and retractor bulbi (nonspared) muscles were dissected from adult control (C57Bl/10) and mdx mice. AChRs and nerve terminals were labeled with rhodamine-alpha-bungarotoxin and anti-NF200-IgG-FITC, respectively, and visualized by confocal microscopy. Rectus and oblique muscles presented 0.5% central nucleation, while retractor bulbi had central nucleation in 45% of muscle fibers. In mdx rectus, AChRs were distributed in branches in 99% of the junctions examined (n = 200), similar to that observed for controls. Nerve terminals covered the AChR branches in 100% of the junctions examined. In control retractor bulbi, AChRs were distributed in regular branches. In mdx retractor bulbi, multiple fragmented islands of receptors were seen in 56% of the endplates examined (n = 200). These results suggest that the lack of dystrophin per se does not influence the distribution of acetylcholine receptors at the neuromuscular junction of spared extraocular muscles.

Skeletrophin, a novel RING molecule controlled by the chromatin remodeling complex, is downregulated in malignant melanoma

Recent experiments have revealed that aberrant functionality of the chromatin remodeling complex is related to tumorigenicity in various malignant tumors. Skeletrophin is an actin-binding cytoskeleton-related molecule, which is induced by the overexpression of truncated human SWI1 (SMARCF1). Human SWI1 is a sub-unit of the chromatin remodeling complex and binds chromatin through its ARID (AT-rich interactive domain). Truncated SWI1 lacks one of the two glucocorticoid-receptor binding domains and inhibits the intact human SWI1 in a dominant negative manner. Skeletrophin, was therefore identified as a candidate molecule for the indication of change to a malignant phenotype due to the aberrant function of the chromatin remodeling complex. Surprisingly, the skeletrophin gene is located in 1p36.32, where the putative tumor suppressor gene of cutaneous malignant melanoma has long been postulated to be on. Cutaneous malignant melanoma is a highly aggressive tumor. To overcome the clinical problem of malignant melanoma and highly invasive and metastatic activity, it is important to unravel the molecular mechanism responsible for melanoma progression. Recent studies including those from our laboratories have elucidated that skeletrophin is a novel RING-HC type ubiquitin ligase and that the ubiquitin ligase pathway mediated by skeletrophin acts to oppose melanoma cell invasion. Here, we summarize the characterization of skeletrophin, with emphasis on its biological activity, the disruption of which is linked with melanoma progression.

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.

PRT1 of Arabidopsis is a ubiquitin protein ligase of the plant N-end rule pathway with specificity for aromatic amino-terminal residues

The gene PRT1 of Arabidopsis, encoding a 45-kD protein with two RING finger domains, is essential for the degradation of F-dihydrofolate reductase, a model substrate of the N-end rule pathway of protein degradation. We have determined the function of PRT1 by expression in yeast (Saccharomyces cerevisiae). PRT1 can act as a ubiquitin protein ligase in the heterologous host. The identified substrates of PRT1 have an aromatic residue at their amino-terminus, indicating that PRT1 mediates degradation of N-end rule substrates with aromatic termini but not of those with aliphatic or basic amino-termini. Expression of model substrates in mutant and wild-type plants confirmed this substrate specificity. A ligase activity exclusively devoted to aromatic amino-termini of the N-end rule pathway is apparently unique to plants. The results presented also imply that other known substrates of the plant N-end rule pathway are ubiquitylated by one or more different ubiquitin protein ligases.

Down-regulation of a novel actin-binding molecule, skeletrophin, in malignant melanoma

In the present study, we cloned and characterized a novel actin-binding molecule, designated skeletrophin, from aggregated neuroblastoma cells. The putative amino acid sequence of human skeletrophin cDNA contained a cysteine-rich zinc-finger motif which was also found in dystrophin and five ankyrin repeats. Northern blot analysis revealed that the 3.2-kb skeletrophin mRNA was expressed in normal skeletal muscle, and to a lesser extent in heart, brain, and kidney. Specific antibody was prepared to human skeletrophin peptide, and a single protein band with an approximate molecular weight of 70 kd was detected in tissue extracts by immunoblotting using the antibody. To better understand the biological properties of skeletrophin, we used a yeast two-hybrid system to screen for molecules interacting with skeletrophin and found that skeletrophin bound to actin monomer. Co-immunoprecipitation experiments also demonstrated that skeletrophin was able to bind to actin monomer. Fluorescence in situ hybridization mapped the skeletrophin gene on human chromosome 1p36.2-36.3, in which putative tumor suppressor genes for malignant melanoma have been postulated to exist. We therefore immunohistochemically stained benign nevi and malignant melanoma tissues. Notably, 23 of 25 benign nevi expressed skeletrophin in cytoplasm, but 18 of 38 cases of primary skin melanoma appeared to lack skeletrophin expression. Treatment with a demethylating agent, 5'-aza-2-deoxycytidine, restored skeletrophin expression in cultured Mewo melanoma cells. The present findings suggest that skeletrophin may be a novel actin-binding cytoskeleton-related molecule, expression of which is silenced in a considerable number of melanoma specimens.

Force impairment in calpain 3-deficient mice is not correlated with mechanical disruption

Defects in human calpain 3 are responsible for limb-girdle muscular dystrophy type 2A, an autosomal-recessive disorder characterized mainly by late-onset proximal muscular atrophy. A corresponding murine model has previously been generated by gene targeting. In this report, muscular activity of calpain 3-deficient (capn3(-/-)) mice was evaluated at different ages. Growth curves showed a progressive global muscular atrophy. Histological examination throughout the lifespan of mice confirmed the dystrophic lesions. Whole animal tests showed only a mild significant impairment of the forelimbs. Studies of the mechanical properties of selected isolated fast- and slow-twitch muscles demonstrated that slow-twitch muscles were significantly weaker in capn3(-/-) mice than in wild-type mice. Three different tests showed that there was no membrane disruption, suggesting a nonmechanical etiology of capn3(-/-) mice dystrophy. These findings are consistent with a mechanism involving signaling systems.

CaMBOT: profiling and characterizing calmodulin-binding proteins

Calmodulin (CaM) is an essential calcium-binding protein that binds to and activates a diverse population of downstream targets (calmodulin-binding proteins; CaMBPs) that carry out its critical signalling functions. In spite of the central importance of CaM in Ca(2+)-mediated signal transduction pathways in all eukaryotes, many CaMBPs remain to be identified and characterized. SDS-PAGE followed by gel overlay with recombinant, metabolically radiolabelled CaM (Calmodulin-binding Overlay Technique, CaMBOT) is a valuable method for following behavioural, developmental, forensic and physiological changes in total CaMBP populations and to identify candidate CaMBPs for further study. CaMBOT has also been adapted to isolate cDNAs encoding novel CaMBPs in various organisms. Recently, the method was used to examine the CaMBP complement encoded by the Arabidopsis genome and to identify a new family of transcription activators. To add to its diversity, CaMBOT may be useful for finding target proteins for work on phytoremediation and for the screening of pharmaceuticals and toxic agents that, directly or indirectly, affect CaM and its target proteins. This review discusses all of these topics and the role of CaMBOT in characterizing a functional unit of the proteome-proteins regulated by calmodulin.

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.

Inhibition of dystroglycan binding to laminin disrupts the PI3K/AKT pathway and survival signaling in muscle cells

Dystroglycan is a component of the dystrophin-glycoprotein complex (DGC) in muscle and a cell surface receptor for laminin. Numerous muscular dystrophies are the result of disruption of proteins comprising the DGC, but the underlying pathogenetic mechanisms are unknown. Because apoptosis is an early feature of muscular dystrophy in vivo, and perturbation of cell-extracellular matrix associations is known to induce apoptosis, we investigated the role of dystroglycan-laminin interactions in the propagation and maintenance of cell survival signals in muscle cells. We found that disrupting the interaction between alpha-dystroglycan and the extracellular matrix protein laminin induces apoptosis in muscle cells. This increase in apoptosis is mediated in part by caspase activation and can be blocked by a caspase-3 inhibitor. We demonstrate a role for the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway in muscle cell-survival signaling using a pharmacological inhibitor of PI3K. Treatment with this inhibitor resulted in decreased phosphorylation of AKT and its downstream effector glycogen synthase kinase (GSK)-3beta and induced apoptosis in muscle cell cultures. Disruption of dystroglycan-laminin interactions resulted in decreased phosphorylation of AKT and GSK-3beta. Furthermore, activation of AKT prior to the disruption of dystroglycan-laminin protected the muscle cells from the induction of apoptosis. These results support a role for the PI3K/AKT pathway in the propagation of cell-survival signals mediated by the DGC and provide new insight into the molecular pathogenesis associated with the development of muscular dystrophies.

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.

Nucleomorphin. A novel, acidic, nuclear calmodulin-binding protein from dictyostelium that regulates nuclear number

Probing of Dictyostelium discoideum cell extracts after SDS-PAGE using (35)S-recombinant calmodulin (CaM) as a probe has revealed approximately three-dozen Ca(2+)-dependent calmodulin binding proteins. Here, we report the molecular cloning, expression, and subcellular localization of a gene encoding a novel calmodulin-binding protein (CaMBP); we have called nucleomorphin, from D. discoideum. A lambdaZAP cDNA expression library of cells from multicellular development was screened using a recombinant calmodulin probe ((35)S-VU1-CaM). The open reading frame of 1119 nucleotides encodes a polypeptide of 340 amino acids with a calculated molecular mass of 38.7 kDa and is constitutively expressed throughout the Dictyostelium life cycle. Nucleomorphin contains a highly acidic glutamic/aspartic acid inverted repeat (DEED) with significant similarity to the conserved nucleoplasmin domain and a putative transmembrane domain in the carboxyl-terminal region. Southern blotting reveals that nucleomorphin exists as a single copy gene. Using gel overlay assays and CaM-agarose we show that bacterially expressed nucleomorphin binds to bovine CaM in a Ca(2+)-dependent manner. Amino-terminal fusion to the green fluorescence protein (GFP) showed that GFP-NumA localized to the nucleus as distinct arc-like patterns similar to heterochromatin regions. GFP-NumA lacking the acidic DEED repeat still showed arc-like accumulations at the nuclear periphery, but the number of nuclei in these cells was increased markedly compared with control cells. Cells expressing GFP-NumA lacking the transmembrane domain localized to the nuclear periphery but did not affect nuclear number or gross morphology. Nucleomorphin is the first nuclear CaMBP to be identified in Dictyostelium. Furthermore, these data present the first identification of a member of the nucleoplasmin family as a calmodulin-binding protein and suggest nucleomorphin has a role in nuclear structure in Dictyostelium.

Function and genetics of dystrophin and dystrophin-related proteins in muscle

The X-linked muscle-wasting disease Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin. There is currently no effective treatment for the disease; however, the complex molecular pathology of this disorder is now being unravelled. Dystrophin is located at the muscle sarcolemma in a membrane-spanning protein complex that connects the cytoskeleton to the basal lamina. Mutations in many components of the dystrophin protein complex cause other forms of autosomally inherited muscular dystrophy, indicating the importance of this complex in normal muscle function. Although the precise function of dystrophin is unknown, the lack of protein causes membrane destabilization and the activation of multiple pathophysiological processes, many of which converge on alterations in intracellular calcium handling. Dystrophin is also the prototype of a family of dystrophin-related proteins, many of which are found in muscle. This family includes utrophin and alpha-dystrobrevin, which are involved in the maintenance of the neuromuscular junction architecture and in muscle homeostasis. New insights into the pathophysiology of dystrophic muscle, the identification of compensating proteins, and the discovery of new binding partners are paving the way for novel therapeutic strategies to treat this fatal muscle disease. This review discusses the role of the dystrophin complex and protein family in muscle and describes the physiological processes that are affected in Duchenne muscular dystrophy.

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 dystrophin-glycoprotein complex, cellular signaling, and the regulation of cell survival in the muscular dystrophies

Mutations of different components of the dystrophin-glycoprotein complex (DGC) cause muscular dystrophies that vary in terms of severity, age of onset, and selective involvement of muscle groups. Although the primary pathogenetic processes in the muscular dystrophies have clearly been identified as apoptotic and necrotic muscle cell death, the pathogenetic mechanisms that lead to cell death remain to be determined. Studies of components of the DGC in muscle and in nonmuscle tissues have revealed that the DGC is undoubtedly a multifunctional complex and a highly dynamic structure, in contrast to the unidimensional concept of the DGC as a mechanical component in the cell. Analysis of the DGC reveals compelling analogies to two other membrane-associated protein complexes, namely integrins and caveolins. Each of these complexes mediates signal transduction cascades in the cell, and disruption of each complex causes muscular dystrophies. The signal transduction cascades associated with the DGC, like those associated with integrins and caveolins, play important roles in cell survival signaling, cellular defense mechanisms, and regulation of the balance between cell survival and cell death. This review focuses on the functional components of the DGC, highlighting the evidence of their participation in cellular signaling processes important for cell survival. Elucidating the link between these functional components and the pathogenetic processes leading to cell death is the foremost challenge to understanding the mechanisms of disease expression in the muscular dystrophies due to defects in the DGC.

Mouse alpha1-syntrophin binding to Grb2: further evidence of a role for syntrophin in cell signaling

Syntrophins have been proposed to serve as adapter proteins. Syntrophins are found in the dystrophin glycoprotein complex (DGC); defects in the constituents of this complex are linked to various muscular dystrophies. Blot overlay experiments demonstrate that alpha-dystroglycan, beta-dystroglycan, and syntrophins all bind Grb2, the growth factor receptor bound adapter protein. Mouse alpha1-syntrophin sequences were produced as chimeric fusion proteins in bacteria and found to also bind Grb2 in a Ca2+-independent manner. This binding was localized to the proline rich sequences adjacent to and overlapping with the N-terminal pleckstrin homology domain (PH1). Grb2 bound syntrophin with an apparent KD of 563 +/- 15 nM. Grb2-C-SH3 domain bound syntrophin with slightly higher affinity than Grb2-N-SH3 domain. Crk-L, an SH2/SH3 protein of similar domain structure but different specificity, does not bind these syntrophin sequences.

BIG: a calossin-like protein required for polar auxin transport in Arabidopsis

Polar auxin transport is crucial for the regulation of auxin action and required for some light-regulated responses during plant development. We have found that two mutants of Arabidopsis-doc1, which displays altered expression of light-regulated genes, and tir3, known for its reduced auxin transport-have similar defects and define mutations in a single gene that we have renamed BIG. BIG is very similar to the Drosophila gene Calossin/Pushover, a member of a gene family also present in Caenorhabditis elegans and human genomes. The protein encoded by BIG is extraordinary in size, 560 kD, and contains several putative Zn-finger domains. Expression-profiling experiments indicate that altered expression of multiple light-regulated genes in doc1 mutants can be suppressed by elevated levels of auxin caused by overexpression of an auxin biosynthetic gene, suggesting that normal auxin distribution is required to maintain low-level expression of these genes in the dark. Double mutants of tir3 with the auxin mutants pin1, pid, and axr1 display severe defects in auxin-dependent growth of the inflorescence. Chemical inhibitors of auxin transport change the intracellular localization of the auxin efflux carrier PIN1 in doc1/tir3 mutants, supporting the idea that BIG is required for normal auxin efflux.

Extraocular muscle is spared despite the absence of an intact sarcoglycan complex in gamma- or delta-sarcoglycan-deficient mice

Models of the dystrophin-glycoprotein complex do not reconcile the novel sparing of extraocular muscle in muscular dystrophy. Extraocular muscle sparing in Duchenne muscular dystrophy implies the existence of adaptive properties in these muscles that may extend protection to other neuromuscular diseases. We studied the extraocular muscle morphology and dystrophin-glycoprotein complex organization in murine targeted deletion of the gamma-sarcoglycan (gsg(-/-)) and delta-sarcoglycan (dsg(-/-)) genes, two models of autosomal recessive limb girdle muscular dystrophy. In contrast to limb and diaphragm, the principal extraocular muscles were intact in gsg(-/-) and dsg(-/-) mice. However, central nucleated, presumptive regenerative, fibers were seen in the accessory extraocular muscles (retractor bulbi, levator palpebrae superioris) of both strains. Skeletal muscles of gsg(-/-) mice exhibited in vivo Evans Blue dye permeability, while the principal extraocular muscles did not. Disruption of gamma-sarcoglycan produced secondary displacement of alpha- and beta-sarcoglycans in the extraocular muscles. The intensity of immunofluorescence for dystrophin and alpha- and beta-dystroglycan also appeared to be slightly reduced. Utrophin localization was unchanged. The finding that sarcoglycan disruption was insufficient to elicit alterations in extraocular muscle suggests that loss of mechanical stability and increased sarcolemmal permeability are not inevitable consequences of mutations that disrupt the dystrophin-glycoprotein complex organization and must be accounted for in models of muscular dystrophy.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Contribution of the different modules in the utrophin carboxy-terminal region to the formation and regulation of the DAP complex

The carboxy-terminal region of utrophin, like the homologous proteins dystrophin, Drp2 and dystrobrevins, contains structural domains frequently involved in protein-protein interaction. These domains (WW, EF hands, ZZ and H1-H2) mediate recognition and binding to a multicomponent complex of proteins, also known as dystrophin-associated proteins (DAPs) for their association with dystrophin, the product of the gene, mutated in Duchenne muscular dystrophy. We have exploited phage display and in vitro binding assays to study the recognition specificity of the different domains of the utrophin carboxy-terminus. We found that none of the carboxy-terminal domains of utrophin, when isolated from its structural context, selects specific ligand peptides from a phage-displayed peptide library. By contrast, panning with an extended region containing the WW, EF hands, and ZZ domain defines the consensus binding motif, PPxY which is also found in beta-dystroglycan, a component of the DAP complex that interacts with utrophin in several tissues. WW-mediated binding to PPxY peptides and to beta-dystroglycan requires the presence of the EF hands and ZZ domain. When the ZZ domain is either deleted or engaged in binding to calmodulin, the utrophin beta-dystroglycan complex cannot be formed. These findings suggest a potential regulatory mechanism by means of which the attachment of utrophin to the DAP complex can be modulated by the Ca(2+)-dependent binding of calmodulin. The remaining two motifs found in the carboxy-terminus (H1-H2) mediate the formation of utrophin-dystrobrevin hybrids but do not select ligands in a repertoire of random nonapeptides.

Sarcospan-deficient mice maintain normal muscle function

Sarcospan is an integral membrane component of the dystrophin-glycoprotein complex (DGC) found at the sarcolemma of striated and smooth muscle. The DGC plays important roles in muscle function and viability as evidenced by defects in components of the DGC, which cause muscular dystrophy. Sarcospan is unique among the components of the complex in that it contains four transmembrane domains with intracellular N- and C-terminal domains and is a member of the tetraspan superfamily of proteins. Sarcospan is tightly linked to the sarcoglycans, and together these proteins form a subcomplex within the DGC. Stable expression of sarcospan at the sarcolemma is dependent upon expression of the sarcoglycans. Here we describe the generation and analysis of mice carrying a null mutation in the Sspn gene. Surprisingly, the Sspn-deficient muscle maintains expression of other components of the DGC at the sarcolemma, and no gross histological abnormalities of muscle from the mice are observed. The Sspn-deficient muscle maintains sarcolemmal integrity as determined by serum creatine kinase and Evans blue uptake assays, and the Sspn-deficient muscle maintains normal force and power generation capabilities. These data suggest either that sarcospan is not required for normal DGC function or that the Sspn-deficient muscle is compensating for the absence of sarcospan, perhaps by utilizing another protein to carry out its function.

Phosphorylation of dystrophin and alpha-syntrophin by Ca(2+)-calmodulin dependent protein kinase II

A Ca(2+)-calmodulin dependent protein kinase activity (DGC-PK) was previously shown to associate with skeletal muscle dystrophin glycoprotein complex (DGC) preparations, and phosphorylate dystrophin and a protein with the same electrophoretic mobility as alpha-syntrophin (R. Madhavan, H.W. Jarrett, Biochemistry 33 (1994) 5797-5804). Here, we show that DGC-PK and Ca(2+)-calmodulin dependent protein kinase II (CaM kinase II) phosphorylate a common site (RSDS(3616)) within the dystrophin C terminal domain that fits the consensus CaM kinase II phosphorylation motif (R/KXXS/T). Furthermore, both kinase activities phosphorylate exactly the same three fusion proteins (dystrophin fusions DysS7 and DysS9, and the syntrophin fusion) out of a panel of eight fusion proteins (representing nearly 100% of syntrophin and 80% of dystrophin protein sequences), demonstrating that DGC-PK and CaM kinase II have the same substrate specificity. Complementing these results, anti-CaM kinase II antibodies specifically stained purified DGC immobilized on nitrocellulose membranes. Renaturation of electrophoretically resolved DGC proteins revealed a single protein kinase band (M(r) approximately 60,000) that, like CaM kinase II, underwent Ca(2+)-calmodulin dependent autophosphorylation. Based on these observations, we conclude DGC-PK represents a dystrophin-/syntrophin-phosphorylating skeletal muscle isoform of CaM kinase II. We also show that phosphorylation of the dystrophin C terminal domain sequences inhibits their syntrophin binding in vitro, suggesting a regulatory role for phosphorylation.

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

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

Apocalmodulin

Intracellular Ca2+ is normally maintained at submicromolar levels but increases during many forms of cellular stimulation. This increased Ca2+ binds to receptor proteins such as calmodulin (CaM) and alters the cell's metabolism and physiology. Calcium-CaM binds to target proteins and alters their function in such a way as to transduce the Ca2+ signal. Calcium-free or apocalmodulin (ApoCaM) binds to other proteins and has other specific effects. Apocalmodulin has roles in the cell that apparently do not require the ability to bind Ca2+ at all, and these roles appear to be essential for life. Apocalmodulin differs from Ca2+-CaM in its tertiary structure. It binds target proteins differently, utilizing different binding motifs such as the IQ motif and noncontiguous binding sites. Other kinds of binding potentially await discovery. The ApoCaM-binding proteins are a diverse group of at least 15 proteins including enzymes, actin-binding proteins, as well as cytoskeletal and other membrane proteins, including receptors and ion channels. Much of the cellular CaM is bound in a Ca2+-independent manner to membrane structures within the cell, and the proportion bound changes with cell growth and density, suggesting it may be a storage form. Apocalmodulin remains tightly bound to other proteins as subunits and probably hastens the response of these proteins to Ca2+. The overall picture that emerges is that CaM cycles between its Ca2+-bound and Ca2+-free states and in each state binds to different proteins and performs essential functions. Although much of the research focus has been on the roles of Ca2+-CaM, the roles of ApoCaM are equally vital but less well understood.

Calcium homeostasis and ultrastructural studies in a patient with limb girdle muscular dystrophy type 2C

There is increasing evidence that gamma-sarcoglycan is absent and other sarcoglycans are reduced in patients with the limb-girdle muscular dystrophy type 2C (LGMD2C) form of severe childhood autosomal recessive muscular dystrophy. In the present investigation, we combined microspectrofluorimetry and electron microscopy techniques to investigate the physiological function and the ultrastructure of control and LGMD2C myotubes. Results obtained from Ca2+ measurements showed that the resting level of the cytosolic free calcium ([Ca2+ ]i ) in control myotubes was 73+/-3.4 nmol/l (mean+/-se, n=35) and in LGMD2C myotubes was 69+/-4 nmol/l (n=44). Carbachol (CCh, 10 micromol/l ) induced a 335+/-10 nmol/l (n=8) rise in [Ca2+ ]i in control myotubes and 531.9+/-32 nmol/l (n=23) in LGMD2C myotubes. Similarly, elevations of [Ca2+ ]i by 35 mmol/l K+ were 324+/-32 nmol/l (n=8) in control myotubes and 442.8+/-24 nmol/l (n=22) in LGMD2C myotubes. Caffeine (10 mmol/l) activated similar [Ca2+]i peaks in control and LGMD2C myotubes but induced a biphasic response in LGMD2C in four out of 12 myotubes and only a monophasic response in control myotubes. The ultrastructural results showed that the plasma membrane was abnormally indented and convoluted in both the LGMD2C biopsy and the LGMD2C cultured myotubes. It is suggested that the reduction in components of the dystrophin-glycoprotein complex results in the instability and an increase in the surface area of the plasma membrane, which may result in a higher population of Ca2+ channels in the LGMD2C myotubes.

alpha-actinin-2 is a new component of the dystrophin-glycoprotein complex

The human skeletal muscle yeast two-hybrid cDNA library was screened with the carboxyl-terminal region (the last 200 amino acids) of dystrophin. Two interacting clones were identified corresponding to alpha-actinin-2 and actin. Interactions between alpha-actinin, actin, and dystrophin were confirmed by the ligand-blotting technique, by colocalization of dystrophin and alpha-actinin-2 to the isolated skeletal muscle sarcolemmal vesicles and to the plasma membranes isolated from C2C12 myoblasts, and by indirect immunolocalization of dystrophin and alpha-actinin-2 in skeletal muscle cells. This is the first identification of a direct interaction between alpha-actinin, actin, and the carboxyl-terminal region of dystrophin. We propose that dystrophin forms lateral, multicontact association with actin and that binding of alpha-actinin-2 to the carboxyl-terminus of dystrophin is the communication link between the integrins and the dystrophin/dystrophin-glycoprotein complex.

Inducible nitric oxide synthase requires both the canonical calmodulin-binding domain and additional sequences in order to bind calmodulin and produce nitric oxide in the absence of free Ca2+

All three mammalian isoforms of nitric oxide synthase (NOS) must bind calmodulin (CaM) for enzymatic activity. Only NOS2 (the inducible isoform, iNOS) does so at the low levels of free Ca2+ in resting cells and when almost all Ca2+ is chelated in cell-free preparations. To test directly whether the predicted CaM-binding region of mouse NOS2 accounts for its Ca2+ independence, we prepared chimeric NOS's in which mouse NOS2 residues 503-532 were reciprocally exchanged with the corresponding residues 725-754 of rat NOS1 (neuronal NOS). Unlike either parent, both chimeras required an intermediate level of free Ca2+ to bind CaM and generate NO. In cell lysates, the concentration of Ca2+ necessary for half-maximal activity (EC50) was approximately 0 for NOS2, 200-300 n for NOS1, and 7-10 n for the chimeras. Results were similar when the region exchanged was enlarged by 7-8 residues toward the amino terminus. In contrast, when the carboxyl-terminal half of NOS2 (residues 454-1144) was replaced with that of NOS1 (residues 675-1429), the resulting chimera resembled NOS1 (EC50, 200-300 n free Ca2+). Truncation analysis suggested that NOS2 residues within the sequence 484-726 were required for Ca2+-independent CaM-binding. Thus, both the canonical CaM-binding domain and additional residues within the region 484-726 are necessary for NOS2's ability to bind CaM and produce NO when Ca2+ levels approach zero.