The role of basal and myogenic factors in the transcriptional activation of utrophin promoter A: implications for therapeutic up-regulation in Duchenne muscular dystrophy

Endogenous bioluminescent reporters reveal a sustained increase in utrophin gene expression upon EZH2 and ERK1/2 inhibition

Duchenne muscular dystrophy (DMD) is an X-linked disorder caused by loss of function mutations in the dystrophin gene (Dmd), resulting in progressive muscle weakening. Here we modelled the longitudinal expression of endogenous Dmd, and its paralogue Utrn, in mice and in myoblasts by generating bespoke bioluminescent gene reporters. As utrophin can partially compensate for Dmd-deficiency, these reporters were used as tools to ask whether chromatin-modifying drugs can enhance Utrn expression in developing muscle. Myoblasts treated with different PRC2 inhibitors showed significant increases in Utrn transcripts and bioluminescent signals, and these responses were independently verified by conditional Ezh2 deletion. Inhibition of ERK1/2 signalling provoked an additional increase in Utrn expression that was also seen in Dmd-mutant cells, and maintained as myoblasts differentiate. These data reveal PRC2 and ERK1/2 to be negative regulators of Utrn expression and provide specialised molecular imaging tools to monitor utrophin expression as a therapeutic strategy for DMD.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Identification of therapeutics that target eEF1A2 and upregulate utrophin A translation in dystrophic muscles

Up-regulation of utrophin in muscles represents a promising therapeutic strategy for the treatment of Duchenne Muscular Dystrophy. We previously demonstrated that eEF1A2 associates with the 5’UTR of utrophin A to promote IRES-dependent translation. Here, we examine whether eEF1A2 directly regulates utrophin A expression and identify via an ELISA-based high-throughput screen, FDA-approved drugs that upregulate both eEF1A2 and utrophin A. Our results show that transient overexpression of eEF1A2 in mouse muscles causes an increase in IRES-mediated translation of utrophin A. Through the assessment of our screen, we reveal 7 classes of FDA-approved drugs that increase eEF1A2 and utrophin A protein levels. Treatment of mdx mice with the 2 top leads results in multiple improvements of the dystrophic phenotype. Here, we report that IRES-mediated translation of utrophin A via eEF1A2 is a critical mechanism of regulating utrophin A expression and reveal the potential of repurposed drugs for treating DMD via this pathway.

One potential approach for the treatment of Duchenne muscular dysrophy is to increase expression of the dystrophin homolog utrophin. Here, the authors show that eEF1A2 regulates utrophin expression, and show that 2 FDA-approved drugs upregulate eEIF1A2 and utrophin level in mice, leading to improvement of the dystrophic phenotype.

Utrophin haploinsufficiency does not worsen the functional performance, resistance to eccentric contractions and force production of dystrophic mice

The lack of dystrophin in Duchenne muscular dystrophy (DMD) compromises the integrity and function of muscle fibers. Skeletal muscles, except the diaphragm, do not undergo progressive degeneration in adult mdx mice due to compensatory mechanisms, including structural protein upregulation. New mouse models, including utrophin haploinsufficient mdx (mdx/utrn+/-) mice, may better recapitulate DMD. Our goal was to determine whether mdx/utrn+/- worsens the mdx phenotype and to characterize the course of the disease on muscle function and contractility at 1, 2, and 5 months of age, which encompass all stages of development relevant to DMD therapy. The functional performances of mdx/utrn+/- mice showed that they are not more affected than mdx/utrn+/+ mice based on downhill treadmill running parameters and subsequent recovery measured by open-field voluntary activity. WT mice ran the entire distance (450 m) on the treadmill, with an additional 561 m during the 4 h of open-field while mdx/utrn+/+ and mdx/utrn+/- mice completed, respectively, 236 m and 273 m on the treadmill and 341 m and 287 m during the open-field period. In addition, isolated ex vivo contractile properties and repeated eccentric contractions showed that mdx/utrn+/- does not significantly worsen the function of dystrophic EDL muscles, which are mainly composed of fast-twitch fibers that are preferentially affected in DMD. Twitch, absolute tetanic, and specific tetanic forces were very similar in dystrophic EDL muscles from mdx/utrn+/+ and mdx utrn+/- mice at 1, 2, and 5 months of age. Five-month-old mdx/utrn+/+ and mdx/utrn+/- mice lost roughly 50% of their force due to repeated eccentric contractions. Thus, histological, morphological, biochemical functional and contractile observations showed that utrophin haploinsufficiency has a very limited impact on mdx mice.

Alternative utrophin mRNAs contribute to phenotypic differences between dystrophin-deficient mice and Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal disorder caused by absence of functional dystrophin protein. Compensation in dystrophin‐deficient (mdx) mice may be achieved by overexpression of its fetal paralogue, utrophin. Strategies to increase utrophin levels by stimulating promoter activity using small compounds are therefore a promising pharmacological approach. Here, we characterise similarities and differences existing within the mouse and human utrophin locus to assist in high‐throughput screening for potential utrophin modulator drugs. We identified five novel 5′‐utrophin isoforms (A′,B′,C,D and F) in adult and embryonic tissue. As the more efficient utrophin‐based response in mdx skeletal muscle appears to involve independent transcriptional activation of conserved, myogenic isoforms (A′ and F), elevating their paralogues in DMD patients is an encouraging therapeutic strategy.

Combinatorial therapeutic activation with heparin and AICAR stimulates additive effects on utrophin A expression in dystrophic muscles

Upregulation of utrophin A is an attractive therapeutic strategy for treating Duchenne muscular dystrophy (DMD). Over the years, several studies revealed that utrophin A is regulated by multiple transcriptional and post-transcriptional mechanisms, and that pharmacological modulation of these pathways stimulates utrophin A expression in dystrophic muscle. In particular, we recently showed that activation of p38 signaling causes an increase in the levels of utrophin A mRNAs and protein by decreasing the functional availability of the destabilizing RNA-binding protein called K-homology splicing regulatory protein, thereby resulting in increases in the stability of existing mRNAs. Here, we treated 6-week-old mdx mice for 4 weeks with the clinically used anticoagulant drug heparin known to activate p38 mitogen-activated protein kinase, and determined the impact of this pharmacological intervention on the dystrophic phenotype. Our results show that heparin treatment of mdx mice caused a significant ∼1.5- to 3-fold increase in utrophin A expression in diaphragm, extensor digitorum longus and tibialis anterior (TA) muscles. In agreement with these findings, heparin-treated diaphragm and TA muscle fibers showed an accumulation of utrophin A and β-dystroglycan along their sarcolemma and displayed improved morphology and structural integrity. Moreover, combinatorial drug treatment using both heparin and 5-amino-4-imidazolecarboxamide riboside (AICAR), the latter targeting 5' adenosine monophosphate-activated protein kinase and the transcriptional activation of utrophin A, caused an additive effect on utrophin A expression in dystrophic muscle. These findings establish that heparin is a relevant therapeutic agent for treating DMD, and illustrate that combinatorial treatment of heparin with AICAR may serve as an effective strategy to further increase utrophin A expression in dystrophic muscle via activation of distinct signaling pathways.

Cis-Acting Sequence Elements and Upstream Open Reading Frame in Mouse Utrophin-A 5'-UTR Repress Cap-Dependent Translation

Utrophin, the autosomal homologue of dystrophin can functionally compensate for dystrophin deficiency. Utrophin upregulation could therefore be a therapeutic strategy in Duchenne Muscular Dystrophy (DMD) that arises from mutation in dystrophin gene. In contrast to its transcriptional regulation, mechanisms operating at post-transcriptional level of utrophin expression have not been well documented. Although utrophin-A 5'-UTR has been reported with internal ribosome entry site (IRES), its inhibitory effect on translation is also evident. In the present study we therefore aimed to compare relative contribution of cap-independent and cap-dependent translation with mouse utrophin-A 5'-UTR through m7G-capped and A-capped mRNA transfection based reporter assay. Our results demonstrate that cap-independent translation with utrophin-A 5'-UTR is not as strong as viral IRES. However, cap-independent mode has significant contribution as cap-dependent translation is severely repressed with utrophin-A 5'-UTR. We further identified two sequence elements and one upstream open reading frame in utrophin-A 5'-UTR responsible for repression. The repressor elements in utrophin-A 5'-UTR may be targeted for utrophin upregulation.

Activation of p38 signaling increases utrophin A expression in skeletal muscle via the RNA-binding protein KSRP and inhibition of AU-rich element-mediated mRNA decay: implications for novel DMD therapeutics

Several therapeutic approaches are currently being developed for Duchenne muscular dystrophy (DMD) including upregulating the levels of endogenous utrophin A in dystrophic fibers. Here, we examined the role of post-transcriptional mechanisms in controlling utrophin A expression in skeletal muscle. We show that activation of p38 leads to an increase in utrophin A independently of a transcriptional induction. Rather, p38 controls the levels of utrophin A mRNA by extending the half-life of transcripts via AU-rich elements (AREs). This mechanism critically depends on a decrease in the functional availability of KSRP, an RNA-binding protein known to promote decay of ARE-containing transcripts. In vitro and in vivo binding studies revealed that KSRP interacts with specific AREs located within the utrophin A 3' UTR. Electroporation experiments to knockdown KSRP led to an increase in utrophin A in wild-type and mdx mouse muscles. In pre-clinical studies, treatment of mdx mice with heparin, an activator of p38, causes a pronounced increase in utrophin A in diaphragm muscle fibers. Together, these studies identify a pathway that culminates in the post-transcriptional regulation of utrophin A through increases in mRNA stability. Furthermore, our results constitute proof-of-principle showing that pharmacological activation of p38 may prove beneficial as a novel therapeutic approach for DMD.

Duchenne muscular dystrophy drug discovery - the application of utrophin promoter activation screening

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a devastating genetic muscle wasting disease caused by mutations in the DMD gene that in turn lead to an absence of dystrophin. Currently, there is no definitive therapy for DMD. Gene- and cell-based therapies designed to replace dystrophin have met some degree of success, as have strategies that seek to improve the dystrophic pathology independent of dystrophin.

AREAS COVERED

In this review the authors focus on utrophin promoter activation-based strategies and their implications on potential therapeutics for DMD. These strategies in common are designed to identify drugs/small molecules that can activate the utrophin promoter and would allow the functional substitution of dystrophin by upregulating utrophin expression in dystrophic muscle. The authors provide an overview of utrophin biology with a focus on regulation of the utrophin promoter and discuss current attempts in identifying utrophin promoter-activating molecules using high-throughput screening (HTS).

EXPERT OPINION

The characterisation of utrophin promoter regulatory mechanisms coupled with advances in HTS have allowed researchers to undertake screens and identify a number of promising lead compounds that may prove useful for DMD. In principle, these pharmacological compounds offer significant advantages from a translational viewpoint for developing DMD therapeutics.

A cell-based high-throughput screening assay for posttranscriptional utrophin upregulation

Duchenne muscular dystrophy (DMD) is a devastating muscle-wasting disease caused by mutations in the dystrophin gene. Utrophin is a homologue of dystrophin that can compensate for its absence when overexpressed in DMD animal models. Utrophin upregulation is therefore a promising therapeutic approach for DMD. Utrophin is regulated at both transcriptional and posttranscriptional levels. Transcriptional regulation has been studied extensively, and assays have been described for the identification of utrophin promoter-targeting molecules. However, despite the profound impact that posttranscriptional regulation has on utrophin expression, screening assays have not yet been described that could be used to discover pharmaceuticals targeting this key phase of regulation. We describe the development and validation of a muscle cell line-based assay in which a stably expressed luciferase coding sequence is flanked by the utrophin 5'- and 3'-untranslated regions (UTRs). The assay was validated using the posttranscriptional regulation of utrophin by miR-206. The assay has a Z' of 0.7, indicating robust performance in high-throughput format. This assay can be used to study utrophin regulatory mechanisms or to screen chemical libraries for compounds that upregulate utrophin posttranscriptionally via its UTRs. Compounds identified via this assay, used alone or in a synergistic combination with utrophin promoter-targeting molecules, would be predicted to have therapeutic potential for DMD.

Detection of interacting transcription factors in human tissues using predicted DNA binding affinity

Background

Tissue-specific gene expression is generally regulated by combinatorial interactions among transcription factors (TFs) which bind to the DNA. Despite this known fact, previous discoveries of the mechanism that controls gene expression usually consider only a single TF.

Results

We provide a prediction of interacting TFs in 22 human tissues based on their DNA-binding affinity in promoter regions. We analyze all possible pairs of 130 vertebrate TFs from the JASPAR database. First, all human promoter regions are scanned for single TF-DNA binding affinities with TRAP and for each TF a ranked list of all promoters ordered by the binding affinity is created. We then study the similarity of the ranked lists and detect candidates for TF-TF interaction by applying a partial independence test for multiway contingency tables. Our candidates are validated by both known protein-protein interactions (PPIs) and known gene regulation mechanisms in the selected tissue. We find that the known PPIs are significantly enriched in the groups of our predicted TF-TF interactions (2 and 7 times more common than expected by chance). In addition, the predicted interacting TFs for studied tissues (liver, muscle, hematopoietic stem cell) are supported in literature to be active regulators or to be expressed in the corresponding tissue.

Conclusions

The findings from this study indicate that tissue-specific gene expression is regulated by one or two central regulators and a large number of TFs interacting with these central hubs. Our results are in agreement with recent experimental studies.

Translational regulation of utrophin by miRNAs

BACKGROUND

Utrophin is the autosomal homolog of dystrophin, the product of the Duchenne Muscular Dystrophy (DMD) locus. Its regulation is of therapeutic interest as its overexpression can compensate for dystrophin's absence in animal models of DMD. The tissue distribution and transcriptional regulation of utrophin have been characterized extensively, and more recently translational control mechanisms that may underlie its complex expression patterns have begun to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

Using a variety of bioinformatic, molecular and cell biology techniques, we show that the muscle isoform utrophin-A is predominantly suppressed at the translational level in C2C12 myoblasts. The extent of translational inhibition is estimated to be ~99% in C2C12 cells and is mediated by both the 5'- and 3'-UTRs of the utrophin-A mRNA. In this study we identify five miRNAs (let-7c, miR-150, miR-196b, miR-296-5p, miR-133b) that mediate the repression, and confirm repression by the previously identified miR-206. We demonstrate that this translational repression can be overcome by blocking the actions of miRNAs, resulting in an increased level of utrophin protein in C2C12 cells.

CONCLUSIONS/SIGNIFICANCE

The present study has identified key inhibitory mechanisms featuring miRNAs that regulate utrophin expression, and demonstrated that these mechanisms can be targeted to increase endogenous utrophin expression in cultured muscle cells. We suggest that miRNA-mediated inhibitory mechanisms could be targeted by methods similar to those described here as a novel strategy to increase utrophin expression as a therapy for DMD.

Progress in therapy for Duchenne muscular dystrophy

Duchenne muscular dystrophy is a devastating muscular dystrophy of childhood. Mutations in the dystrophin gene destroy the link between the internal muscle filaments and the extracellular matrix, resulting in severe muscle weakness and progressive muscle wasting. There is currently no cure and, whilst palliative treatment has improved, affected boys are normally confined to a wheelchair by 12 years of age and die from respiratory or cardiac complications in their twenties or thirties. Therapies currently being developed include mutation-specific treatments, DNA- and cell-based therapies, and drugs which aim to modulate cellular pathways or gene expression. This review aims to provide an overview of the different therapeutic approaches aimed at reconstructing the dystrophin-associated protein complex, including restoration of dystrophin expression and upregulation of the functional homologue, utrophin.

Targeting artificial transcription factors to the utrophin A promoter: effects on dystrophic pathology and muscle function

Duchenne muscular dystrophy is caused by a genetic defect in the dystrophin gene. The absence of dystrophin results in muscle fiber necrosis and regeneration, leading to progressive muscle fiber loss. Utrophin is a close analogue of dystrophin. A substantial, ectopic expression of utrophin in the extrasynaptic sarcolemma of dystrophin-deficient muscle fibers can prevent deleterious effects of dystrophin deficiency. An alternative approach for the extrasynaptic up-regulation of utrophin involves the augmentation of utrophin transcription via the endogenous utrophin A promoter using custom-designed transcriptional activator proteins with zinc finger (ZFP) motifs. We tested a panel of custom-designed ZFP for their ability to activate the utrophin A promoter. Expression of one such ZFP efficiently increased, in a time-dependent manner, utrophin transcript and protein levels both in vitro and in vivo. In dystrophic mouse (mdx) muscles, administration of adenoviral vectors expressing this ZFP led to significant enhancement of muscle function with decreased necrosis, restoration of the dystrophin-associated proteins, and improved resistance to eccentric contractions. These studies provide evidence that specifically designed ZFPs can act as strong transcriptional activators of the utrophin A promoter. These may thus serve as attractive therapeutic agents for dystrophin deficiency states such as Duchenne muscular dystrophy.

Modulation of utrophin A mRNA stability in fast versus slow muscles via an AU-rich element and calcineurin signaling

We examined the role of post-transcriptional mechanisms in controlling utrophin A mRNA expression in slow versus fast skeletal muscles. First, we determined that the half-life of utrophin A mRNA is significantly shorter in the presence of proteins isolated from fast muscles. Direct plasmid injection experiments using reporter constructs containing the full-length or truncated variants of the utrophin 3'UTR into slow soleus and fast extensor digitorum longus muscles revealed that a region of 265 nucleotides is sufficient to confer lower levels of reporter mRNA in fast muscles. Further analysis of this region uncovered a conserved AU-rich element (ARE) that suppresses expression of reporter mRNAs in cultured muscle cells. Moreover, stability of reporter mRNAs fused to the utrophin full-length 3'UTR was lower in the presence of fast muscle protein extracts. This destabilization effect seen in vivo was lost upon deletion of the conserved ARE. Finally, we observed that calcineurin signaling affects utrophin A mRNA stability through the conserved ARE. These results indicate that ARE-mediated mRNA decay is a key mechanism that regulates expression of utrophin A mRNA in slow muscle fibers. This is the first demonstration of ARE-mediated mRNA decay regulating the expression of a gene associated with the slow myogenic program.

Ets-2 repressor factor silences extrasynaptic utrophin by N-box mediated repression in skeletal muscle

Utrophin is the autosomal homologue of dystrophin, the protein product of the Duchenne's muscular dystrophy (DMD) locus. Utrophin expression is temporally and spatially regulated being developmentally down-regulated perinatally and enriched at neuromuscular junctions (NMJs) in adult muscle. Synaptic localization of utrophin occurs in part by heregulin-mediated extracellular signal-regulated kinase (ERK)-phosphorylation, leading to binding of GABPalpha/beta to the N-box/EBS and activation of the major utrophin promoter-A expressed in myofibers. However, molecular mechanisms contributing to concurrent extrasynaptic silencing that must occur to achieve NMJ localization are unknown. We demonstrate that the Ets-2 repressor factor (ERF) represses extrasynaptic utrophin-A in muscle. Gel shift and chromatin immunoprecipitation studies demonstrated physical association of ERF with the utrophin-A promoter N-box/EBS site. ERF overexpression repressed utrophin-A promoter activity; conversely, small interfering RNA-mediated ERF knockdown enhanced promoter activity as well as endogenous utrophin mRNA levels in cultured muscle cells in vitro. Laser-capture microscopy of tibialis anterior NMJ and extrasynaptic transcriptomes and gene transfer studies provide spatial and direct evidence, respectively, for ERF-mediated utrophin repression in vivo. Together, these studies suggest "repressing repressors" as a potential strategy for achieving utrophin up-regulation in DMD, and they provide a model for utrophin-A regulation in muscle.

Musculoskeletal gene therapy and its potential use in the treatment of complicated musculoskeletal infection

Tissue repair is a major issue in orthopedics. Many musculoskeletal tissues, including cartilage, meniscus, and the anterior cruciate ligament, heal poorly after injury. Recent studies have led to the identification of numerous growth factors and other gene products that can promote the regeneration of damaged musculoskeletal tissues. In the last century, the discovery and evolving use of antibiotics has significantly decreased the prevalence and severity of infectious diseases. In many orthopedic scenarios, however, treatment of infections can be difficult, and often involves a prolonged course of antibiotics with concomitant surgical interventions and loss of tissue. Although studies have demonstrated the successful transfer of target genes and the associated manipulation of the musculoskeletal tissue environment, researchers have made few attempts designed to use gene therapy to treat infectious musculoskeletal diseases in animal models. Before it is possible to use gene-based approaches to treat such diseases effectively, researchers must perform more studies to investigate the potential problems that may arise when using gene therapy in an infectious environment.

Myogenin-dependent nAChR clustering in aneural myotubes

During development of the neuromuscular junction, nerve-derived agrin and the cell substrate laminin stimulate postsynaptic nAChR clustering. This clustering is dependent on activation of the tyrosine kinase, MuSK, which signals receptor clustering via a rapsyn-dependent mechanism. Myogenin is a muscle-specific transcription factor that controls myoblast differentiation and nAChR gene expression. Here, we used RNA interference to investigate if myogenin is also necessary for nAChR clustering. We find that myogenin expression is essential for robust nAChR clustering and cannot be compensated by the muscle regulatory factors MyoD, myf5, and MRF4. In addition, we show that clustering cannot be rescued in myogenin-depleted myotubes by simply overexpressing the essential clustering molecules MuSK, rapsyn, and nAChRs. These data suggest that myogenin controls the expression of molecules crucial to nAChR clustering in addition to its role in regulating nAChR gene expression.

The GTPase RhoA increases utrophin expression and stability, as well as its localization at the plasma membrane

The Rho family of small GTPases are signalling molecules involved in cytoskeleton remodelling and gene transcription. Their activities are important for many cellular processes, including myogenesis. In particular, RhoA positively regulates skeletal-muscle differentiation. We report in the present study that the active form of RhoA increases the expression of utrophin, the autosomal homologue of dystrophin in the mouse C2C12 and rat L8 myoblastic cell lines. Even though this RhoA-dependent utrophin increase is higher in proliferating myoblasts, it is maintained during myogenic differentiation. This occurs via two mechanisms: (i) transcriptional activation of the utrophin promoter A and (ii) post-translational stabilization of utrophin. In addition, RhoA increases plasma-membrane localization of utrophin. Thus RhoA activation up-regulates utrophin levels and enhances its localization at the plasma membrane.

Interferon regulatory factor-1 is prerequisite to the constitutive expression and IFN-gamma-induced upregulation of B7-H1 (CD274)

Majority of cancer cells upregulate co-inhibitory molecule B7-H1 which confers resistance to anti-tumor immunity, allowing cancers to escape from host immune surveillance. We addressed the molecular mechanism underlying the regulation of cancer-associated B7-H1 expression in response to interferon-gamma (IFN-gamma). Using promoter constructs in luciferase assay, the region between 202 and 320 bp from the translational start site is responsible for B7-H1 expression. Electrophoretic mobility shift assay, site-directed mutagenesis and knockdown experiment using siRNA revealed that interferon regulatory factor-1 (IRF-1) is primarily responsible for the constitutive B7-H1 expression as well as for the IFN-gamma-mediated B7-H1 upregulation in a human lung cancer cell line A549. Additionally, AG490, a Janus activated kinase/signal transducer and activator of transcription inhibitor, greatly abolished the responsiveness of A549 cells to IFN-gamma by reducing the IRF-1 transcription. Our findings support a critical role of IRF-1 in the regulation of constitutive and IFN-gamma-induced expression of B7-H1 in cancer cells.

A 1.3 kb promoter fragment confers spatial and temporal expression of utrophin A mRNA in mouse skeletal muscle fibers

Upregulation of utrophin in muscle is currently being examined as a potential therapy for Duchenne muscular dystrophy patients. In this context, we generated transgenic mice harboring a 1.3 kb human utrophin A promoter fragment driving expression of the lacZ gene. Characterization of reporter expression during postnatal muscle development revealed that the levels and localization of beta-galactosidase parallel expression of utrophin A transcripts. Moreover, we noted that the utrophin A promoter is more active in slow soleus muscles. Additionally, expression of the reporter gene was regulated during muscle regeneration in a manner similar to utrophin A transcripts. Together, these results show that the utrophin A promoter-lacZ construct mirrors expression of utrophin A mRNAs indicating that this utrophin A promoter fragment confers temporal and spatial patterns of expression in skeletal muscle. This transgenic mouse will be valuable as an in vivo model for developing and testing molecules aimed at increasing utrophin A expression.

Okadaic acid augments utrophin in myogenic cells

Duchenne muscular dystrophy is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A potential therapeutic approach is to increase utrophin levels in muscle. One way to achieve this aim is to increase the expression of the utrophin gene at a transcriptional level via promoter activation. In this study, we have shown that utrophin A mRNA levels can be induced by okadaic acid in murine myogenic C2C12 cells. We have found that a utrophin A promoter reporter can be induced by Sp1 in C2C12 myoblasts, but not in myotubes. This activation can be enhanced by okadaic acid treatment. Our data suggest that this induction is due to Sp1 phosphorylation during myogenesis and thus, utrophin expression in muscle could be regulated by treatment with phosphatase inhibitors. Control of utrophin promoter activation could then be used to increase the expression of utrophin, and thus ameliorate the symptoms of Duchenne muscular dystrophy.

Advances in Duchenne muscular dystrophy gene therapy

Since the initial characterization of the genetic defect for Duchenne muscular dystrophy, much effort has been expended in attempts to develop a therapy for this devastating childhood disease. Gene therapy was the obvious answer but, initially, the dystrophin gene and its product seemed too large and complex for this approach. However, our increasing knowledge of the organization of the gene and the role of dystrophin in muscle function has indicated ways to manipulate them both. Gene therapy for Duchenne muscular dystrophy now seems to be in reach.

Gene therapy for muscular dystrophy - a review of promising progress

This review considers the most significant progress in developing genetic therapeutic interventions for Duchenne muscular dystrophy (DMD), in order to illustrate the challenges facing gene therapy of all the muscular dystrophies (MDs). While in vivo repair of genetic mutations affecting muscle remains inefficient, the most promising interventions focus on supplementation of minimally sized transgenes encoding the abnormal muscle proteins. Intramuscular injection of recombinant viral vectors carrying therapeutic transgenes has yielded the most impressive amelioration of dystrophic muscle pathology in animal models, although achieving lasting, body-wide transgene delivery in the absence of a significant immunological reaction remains unrealised. Ex vivo correction and engraftment of muscle precursor and stem cell populations remains a potential (albeit presently less efficient) alternative to in vivo interventions. Recent advances provide encouraging evidence for the feasibility of genetic therapy of the MDs, but further evaluation of interventions in larger animal models and ultimately human trials is necessary to identify the most promising techniques for development.

Expression of utrophin A mRNA correlates with the oxidative capacity of skeletal muscle fiber types and is regulated by calcineurin/NFAT signaling

Utrophin levels have recently been shown to be more abundant in slow vs. fast muscles, but the nature of the molecular events underlying this difference remains to be fully elucidated. Here, we determined whether this difference is due to the expression of utrophin A or B, and examined whether transcriptional regulatory mechanisms are also involved. Immunofluorescence experiments revealed that slower fibers contain significantly more utrophin A in extrasynaptic regions as compared with fast fibers. Single-fiber RT-PCR analysis demonstrated that expression of utrophin A transcripts correlates with the oxidative capacity of muscle fibers, with cells expressing myosin heavy chain I and IIa demonstrating the highest levels. Functional muscle overload, which stimulates expression of a slower, more oxidative phenotype, induced a significant increase in utrophin A mRNA levels. Because calcineurin has been implicated in controlling this slower, high oxidative myofiber program, we examined expression of utrophin A transcripts in muscles having altered calcineurin activity. Calcineurin inhibition resulted in an 80% decrease in utrophin A mRNA levels. Conversely, muscles from transgenic mice expressing an active form of calcineurin displayed higher levels of utrophin A transcripts. Electrophoretic mobility shift and supershift assays revealed the presence of a nuclear factor of activated T cells (NFAT) binding site in the utrophin A promoter. Transfection and direct gene transfer studies showed that active forms of calcineurin or nuclear NFATc1 transactivate the utrophin A promoter. Together, these results indicate that expression of utrophin A is related to the oxidative capacity of muscle fibers, and implicate calcineurin and its effector NFAT in this mechanism.

Pharmacological strategies for muscular dystrophy

Duchenne muscular dystrophy (DMD) is a fatal, genetic disorder whose relentless progression underscores the urgency for developing a cure. Although Duchenne initiated clinical trials roughly 150 years ago, therapies for DMD remain supportive rather than curative. A paradigm shift towards developing rational therapeutic strategies occurred with identification of the DMD gene. Gene- and cell-based therapies designed to replace the missing gene and/or dystrophin protein have achieved varying degrees of success. However, pharmacological strategies not designed to replace dystrophin per se appear promising, and can circumvent many hurdles hampering gene- and cell-based therapy. Here, we will review present pharmacological strategies, in particular those dealing with functional substitution of dystrophin by utrophin and enhancing muscle progenitor commitment by myostatin blockade, with a view toward facilitating drug discovery for DMD.

Ets, Ap-1 and GATA factor families regulate the utrophin B promoter: potential regulatory mechanisms for endothelial-specific expression

Duchenne muscular dystrophy is caused by dystrophin deficiency, which can be prevented in the mdx mouse model by over-expression of an autosomal homologue, utrophin. Utrophin has two characterised full-length promoters, A and B. No data are available on the transcriptional regulation of B utrophin, which has been recently localised to the endothelium. Similar to characterised endothelial promoters, Ets and Ap-1 individually trans-activate the human B core promoter. Synergistic activation by GATA-2 and c-jun to the order of 20-fold was observed.

A- and B-utrophin have different expression patterns and are differentially up-regulated in mdx muscle

Duchenne muscular dystrophy (DMD) is a fatal childhood disease caused by mutations that abolish the expression of dystrophin in muscle. Utrophin is a paralogue of dystrophin and can functionally replace it in skeletal muscle. A method to induce utrophin up-regulation in muscle should therefore be therapeutically useful in DMD. We have previously shown that there are two full-length utrophin mRNA species: A and B. Here we describe the generation and characterization of antibodies specific to A- and B-utrophin. We show that both mRNA isoforms are translated into full-length proteins, which have very different expression patterns. B-utrophin is expressed in vascular endothelial cells; A-utrophin is expressed at the neuromuscular junction, choroid plexus, pia mater, and renal glomerulus. We have analyzed the expression of A- and B-utrophin protein and RNA in dystrophin-deficient tissues. We conclude that (i) the previously described expression patterns of utrophin represent a composite of A- and B-utrophin, (ii) A- but not B-utrophin is up-regulated in dystrophin-deficient striated muscle, and (iii) this up-regulation occurs post-transcriptionally with an additional transcriptional component in skeletal muscle. These results have important implications for understanding the biology of utrophin and are crucial for future studies aiming to effect its therapeutic up-regulation in DMD patients.

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.

Experimental and therapeutic approaches to muscular dystrophies

PURPOSE OF REVIEW

Most patients suffering from muscular dystrophies can now obtain a precise diagnosis of their underlying molecular defect, but no efficient treatment to prevent disability and death. This review summarizes recent progress towards developing efficient treatments for these severe diseases.

RECENT FINDINGS

Different levels of progress have been achieved in three main approaches: gene therapy, cell therapy and pharmacological therapy. Gene therapy has progressed by improving different vectors for gene delivery. Adenoviruses (mainly high capacity versions) and adeno-associated viruses were the most explored viral vectors. Progress was made in understanding the factors needed for an efficient transfection of muscle. An understanding of protein structure and function in muscular dystrophies has allowed elegant examples of protein engineering as a way of gene therapy. Non-viral vectors for gene transfer, targeted gene modification and transcription modulation have also been explored recently. Cell therapy (myogenic-cell transplantation) progressed in understanding myoblast transplantation in primates for human applications, evaluating protocols for the control of graft rejection, understanding the biology of donor myogenic cells, and searching for alternative sources of donor cells. Three clinical trials using pharmacological approaches (anabolic agents and gentamicin) show very poor or negative results. Other pharmacological approaches (upregulation of alternative therapeutic proteins) are still being researched in mice.

SUMMARY

This panoply of experimental approaches covered all the current possibilities of attacking the problem of treating muscular dystrophies. It is expected that one or more will progress to provide efficient tools for the ultimate clinical goal: to prolong function and life in severe muscular dystrophy patients.

Modular organization of phylogenetically conserved domains controlling developmental regulation of the human skeletal myosin heavy chain gene family

The mammalian skeletal myosin heavy chain locus is composed of a six-membered family of tandemly linked genes whose complex regulation plays a central role in striated muscle development and diversification. We have used publicly available genomic DNA sequences to provide a theoretical foundation for an experimental analysis of transcriptional regulation among the six promoters at this locus. After reconstruction of annotated drafts of the human and murine loci from fragmented DNA sequences, phylogenetic footprint analysis of each of the six promoters using standard and Bayesian alignment algorithms revealed unexpected patterns of DNA sequence conservation among orthologous and paralogous gene pairs. The conserved domains within 2.0 kilobases of each transcriptional start site are rich in putative muscle-specific transcription factor binding sites. Experiments based on plasmid transfection in vitro and electroporation in vivo validated several predictions of the bioinformatic analysis, yielding a picture of synergistic interaction between proximal and distal promoter elements in controlling developmental stage-specific gene activation. Of particular interest for future studies of heterologous gene expression is a 650-base pair construct containing modules from the proximal and distal human embryonic myosin heavy chain promoter that drives extraordinarily powerful transcription during muscle differentiation in vitro.

Sp1 and the ets-related transcription factor complex GABP alpha/beta functionally cooperate to activate the utrophin promoter

Duchenne muscular dystrophy (DMD) is a fatal neuromuscular disease caused by the absence of dystrophin. Utrophin is the autosomal homolog of dystrophin and capable of compensating for the absence of dystrophin, when overexpressed. In skeletal muscle, utrophin plays an important role in the formation of neuromuscular junctions. This selective enrichment occurs, in part by transcriptional regulation of the utrophin gene at the sub-synaptic nuclei in muscle. Utrophin's complex transcriptional regulation is not yet fully understood, however, GABP alpha / beta has recently been shown to bind the N box and activate the utrophin promoter in response to heregulin. In this study, we show that the transcription factor Sp1 binds and activates the utrophin promoter in Drosophila S2 cells as well as define a Sp1 response element. We demonstrate that heregulin treatment of cultured muscle cells activates the ERK pathway and phosphorylates serine residue(s) in the consensus ERK recognition site of Sp1. Finally, Sp1 is shown to functionally cooperate with GABP alpha / beta and cause a 58-fold increase of de novo utrophin promoter transcription. Taken together, these findings help define mechanisms used for transcriptional regulation of utrophin expression as well as identify new targets for achieving potentially therapeutic upregulation of utrophin in DMD.