Muscle and neural isoforms of agrin increase utrophin expression in cultured myotubes via a transcriptional regulatory mechanism

Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

The absence of dystrophin in Duchenne muscular dystrophy disrupts the dystrophin-associated glycoprotein complex resulting in skeletal muscle fiber fragility and atrophy, associated with fibrosis as well as microtubule and neuromuscular junction disorganization. The specific, non-conventional cytoplasmic histone deacetylase 6 (HDAC6) was recently shown to regulate acetylcholine receptor distribution and muscle atrophy. Here, we report that administration of the HDAC6 selective inhibitor tubastatin A to the Duchenne muscular dystrophy, mdx mouse model increases muscle strength, improves microtubule, neuromuscular junction, and dystrophin-associated glycoprotein complex organization, and reduces muscle atrophy and fibrosis. Interestingly, we found that the beneficial effects of HDAC6 inhibition involve the downregulation of transforming growth factor beta signaling. By increasing Smad3 acetylation in the cytoplasm, HDAC6 inhibition reduces Smad2/3 phosphorylation, nuclear translocation, and transcriptional activity. These findings provide in vivo evidence that Smad3 is a new target of HDAC6 and implicate HDAC6 as a potential therapeutic target in Duchenne muscular dystrophy.

Promising therapeutic approaches of utrophin replacing dystrophin in the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a serious genetic neuromuscular rare disease that is prevalent and caused by the mutation/deletion of the X-linked DMD gene that encodes dystrophin. Utrophin is a dystrophin homologous protein on human chromosome 6. Dystrophin and utrophin are highly homologous. They can recruit many dystrophin-glycoprotein complex (DGC)-related proteins and co-localize at the sarcolemma in the early stage of human embryonic development. Moreover, utrophin is overexpressed naturally at the mature myofiber sarcolemma in DMD patients. Therefore, utrophin is considered the most promising homologous protein to replace dystrophin. This review summarizes various modulating drugs and gene therapy approaches for utrophin replacement. As a universal method to treat DMD disease, utrophin has a promising therapeutic prospect and deserves further investigation.

Recent insights into neuromuscular junction biology in Duchenne muscular dystrophy: Impacts, challenges, and opportunities

Duchenne muscular dystrophy (DMD) is the most common and relentless form of muscular dystrophy. The pleiotropic effects of dystrophin deficiency include remarkable impacts on neuromuscular junction (NMJ) structure and function. Some of these alterations contribute to the severe muscle wasting and weakness that distinguish DMD, while others attempt to compensate for them. Experimental approaches that correct NMJ biology in pre-clinical models of DMD attenuate disease progression and improve functional outcomes, which suggests that targeting the NMJ may be an effective therapeutic strategy for DMD patients. The objectives of this review are to 1) survey the distinctions in NMJ structure, function, and gene expression in the dystrophic context as compared to the healthy condition, and 2) summarize the efforts, opportunities and challenges to correct NMJ biology in DMD. This information will expand our basic understanding of neuromuscular biology and may be useful for designing novel NMJ-targeted drug or behavioural strategies to mitigate the dystrophic pathology and other disorders of the neuromuscular system.

Collagen VI is required for the structural and functional integrity of the neuromuscular junction

The synaptic cleft of the neuromuscular junction (NMJ) consists of a highly specialized extracellular matrix (ECM) involved in synapse maturation, in the juxtaposition of pre- to post-synaptic areas, and in ensuring proper synaptic transmission. Key components of synaptic ECM, such as collagen IV, perlecan and biglycan, are binding partners of one of the most abundant ECM protein of skeletal muscle, collagen VI (ColVI), previously never linked to NMJ. Here, we demonstrate that ColVI is itself a component of this specialized ECM and that it is required for the structural and functional integrity of NMJs. In vivo, ColVI deficiency causes fragmentation of acetylcholine receptor (AChR) clusters, with abnormal expression of NMJ-enriched proteins and re-expression of fetal AChRγ subunit, both in Col6a1 null mice and in patients affected by Ullrich congenital muscular dystrophy (UCMD), the most severe form of ColVI-related myopathies. Ex vivo muscle preparations from ColVI null mice revealed altered neuromuscular transmission, with electrophysiological defects and decreased safety factor (i.e., the excess current generated in response to a nerve impulse over that required to reach the action potential threshold). Moreover, in vitro studies in differentiated C2C12 myotubes showed the ability of ColVI to induce AChR clustering and synaptic gene expression. These findings reveal a novel role for ColVI at the NMJ and point to the involvement of NMJ defects in the etiopathology of ColVI-related myopathies.

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.

Soluble miniagrin enhances contractile function of engineered skeletal muscle

Neural agrin plays a pleiotropic role in skeletal muscle innervation and maturation, but its specific effects on the contractile function of aneural engineered muscle remain unknown. In this study, neonatal rat skeletal myoblasts cultured within 3-dimensional engineered muscle tissue constructs were treated with 10 nM soluble recombinant miniagrin and assessed using histological, biochemical, and functional assays. Depending on the treatment duration and onset time relative to the stage of myogenic differentiation, miniagrin was found to induce up to 1.7-fold increase in twitch and tetanus force amplitude. This effect was associated with the 2.3-fold up-regulation of dystrophin gene expression at 6 d after agrin removal and enhanced ACh receptor (AChR) cluster formation, but no change in cell number, expression of muscle myosin, or important aspects of intracellular Ca(2+) handling. In muscle constructs with endogenous ACh levels suppressed by the application of α-NETA, miniagrin increased AChR clustering and twitch force amplitude but failed to improve intracellular Ca(2+) handling and increase tetanus-to-twitch ratio. Overall, our studies suggest that besides its synaptogenic function that could promote integration of engineered muscle constructs in vivo, neural agrin can directly promote the contractile function of aneural engineered muscle via mechanisms distinct from those involving endogenous ACh.

Effective myotube formation in human adipose tissue-derived stem cells expressing dystrophin and myosin heavy chain by cellular fusion with mouse C2C12 myoblasts

Stem cell therapy for muscular dystrophies requires stem cells that are able to participate in the formation of new muscle fibers. However, the differentiation steps that are the most critical for this process are not clear. We investigated the myogenic phases of human adipose tissue-derived stem cells (hASCs) step by step and the capability of myotube formation according to the differentiation phase by cellular fusion with mouse myoblast C2C12 cells. In hASCs treated with 5-azacytidine and fibroblast growth factor-2 (FGF-2) for 1 day, the early differentiation step to express MyoD and myogenin was induced by FGF-2 treatment for 6 days. Dystrophin and myosin heavy chain (MyHC) expression was induced by hASC conditioned medium in the late differentiation step. Myotubes were observed only in hASCs undergoing the late differentiation step by cellular fusion with C2C12 cells. In contrast, hASCs that were normal or in the early stage were not involved in myotube formation. Our results indicate that stem cells expressing dystrophin and MyHC are more suitable for myotube formation by co-culture with myoblasts than normal or early differentiated stem cells expressing MyoD and myogenin.

Biglycan recruits utrophin to the sarcolemma and counters dystrophic pathology in mdx mice

Duchenne muscular dystrophy (DMD) is caused by mutations in dystrophin and the subsequent disruption of the dystrophin-associated protein complex (DAPC). Utrophin is a dystrophin homolog expressed at high levels in developing muscle that is an attractive target for DMD therapy. Here we show that the extracellular matrix protein biglycan regulates utrophin expression in immature muscle and that recombinant human biglycan (rhBGN) increases utrophin expression in cultured myotubes. Systemically delivered rhBGN up-regulates utrophin at the sarcolemma and reduces muscle pathology in the mdx mouse model of DMD. RhBGN treatment also improves muscle function as judged by reduced susceptibility to eccentric contraction-induced injury. Utrophin is required for the rhBGN therapeutic effect. Several lines of evidence indicate that biglycan acts by recruiting utrophin protein to the muscle membrane. RhBGN is well tolerated in animals dosed for as long as 3 months. We propose that rhBGN could be a therapy for DMD.

Subcutaneous injection, from birth, of epigallocatechin-3-gallate, a component of green tea, limits the onset of muscular dystrophy in mdx mice: a quantitative histological, immunohistochemical and electrophysiological study

Dystrophic muscles suffer from enhanced oxidative stress. We have investigated whether administration of an antioxidant, epigallocatechin-3-gallate (EGCG), a component of green tea, reduces their oxidative stress and pathophysiology in mdx mice, a mild phenotype model of human Duchenne-type muscular dystrophy. EGCG (5 mg/kg body weight in saline) was injected subcutaneously 4x a week into the backs of C57 normal and dystrophin-deficient mdx mice for 8 weeks after birth. Saline was injected into normal and mdx controls. EGCG had almost no observable effects on normal mice or on the body weights of mdx mice. In contrast, it produced the following improvements in the blood chemistry, muscle histology, and electrophysiology of the treated mdx mice. First, the activities of serum creatine kinase were reduced to normal levels. Second, the numbers of fluorescent lipofuscin granules per unit volume of soleus and diaphragm muscles were significantly decreased by about 50% compared to the numbers in the corresponding saline-treated controls. Third, in sections of diaphragm and soleus muscles, the relative area occupied by histologically normal muscle fibres increased significantly 1.5- to 2-fold whereas the relative areas of connective tissue and necrotic muscle fibres were substantially reduced. Fourth, the times for the maximum tetanic force of soleus muscles to fall by a half increased to almost normal values. Fifth, the amount of utrophin in diaphragm muscles increased significantly by 17%, partially compensating for the lack of dystrophin expression.

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.

Comparative proteomics profiling of a phospholamban mutant mouse model of dilated cardiomyopathy reveals progressive intracellular stress responses

Defective mobilization of Ca2+ by cardiomyocytes can lead to cardiac insufficiency, but the causative mechanisms leading to congestive heart failure (HF) remain unclear. In the present study we performed exhaustive global proteomics surveys of cardiac ventricle isolated from a mouse model of cardiomyopathy overexpressing a phospholamban mutant, R9C (PLN-R9C), and exhibiting impaired Ca2+ handling and death at 24 weeks and compared them with normal control littermates. The relative expression patterns of 6190 high confidence proteins were monitored by shotgun tandem mass spectrometry at 8, 16, and 24 weeks of disease progression. Significant differential abundance of 593 proteins was detected. These proteins mapped to select biological pathways such as endoplasmic reticulum stress response, cytoskeletal remodeling, and apoptosis and included known biomarkers of HF (e.g. brain natriuretic peptide/atrial natriuretic factor and angiotensin-converting enzyme) and other indicators of presymptomatic functional impairment. These altered proteomic profiles were concordant with cognate mRNA patterns recorded in parallel using high density mRNA microarrays, and top candidates were validated by RT-PCR and Western blotting. Mapping of our highest ranked proteins against a human diseased explant and to available data sets indicated that many of these proteins could serve as markers of disease. Indeed we showed that several of these proteins are detectable in mouse and human plasma and display differential abundance in the plasma of diseased mice and affected patients. These results offer a systems-wide perspective of the dynamic maladaptions associated with impaired Ca2+ homeostasis that perturb myocyte function and ultimately converge to cause HF.

PGC-1alpha regulates the neuromuscular junction program and ameliorates Duchenne muscular dystrophy

The coactivator PGC-1alpha mediates key responses of skeletal muscle to motor nerve activity. We show here that neuregulin-stimulated phosphorylation of PGC-1alpha and GA-binding protein (GABP) allows recruitment of PGC-1alpha to the GABP complex and enhances transcription of a broad neuromuscular junction gene program. Since a subset of genes controlled by PGC-1alpha and GABP is dysregulated in Duchenne muscular dystrophy (DMD), we examined the effects of transgenic PGC-1alpha in muscle of mdx mice. These animals show improvement in parameters characteristic of DMD, including muscle histology, running performance, and plasma creatine kinase levels. Thus, control of PGC-1alpha levels in skeletal muscle could represent a novel avenue to prevent or treat DMD.

Staufen1 regulates diverse classes of mammalian transcripts

It is currently unknown how extensively the double-stranded RNA-binding protein Staufen (Stau)1 is utilized by mammalian cells to regulate gene expression. To date, Stau1 binding to the 3'-untranslated region (3'-UTR) of ADP ribosylation factor (ARF)1 mRNA has been shown to target ARF1 mRNA for Stau1-mediated mRNA decay (SMD). ARF1 SMD depends on translation and recruitment of the nonsense-mediated mRNA decay factor Upf1 to the ARF1 3'-UTR by Stau1. Here, we demonstrate that Stau1 binds to a complex structure within the ARF1 3'-UTR. We also use microarrays to show that 1.1 and 1.0% of the 11 569 HeLa-cell transcripts that were analyzed are upregulated and downregulated, respectively, at least two-fold upon Stau1 depletion in three independently performed experiments. We localize the Stau1 binding site to the 3'-UTR of four mRNAs that we define as natural SMD targets. Additionally, we provide evidence that the efficiency of SMD increases during the differentiation of C2C12 myoblasts to myotubes. We propose that Stau1 influences the expression of a wide variety of physiologic transcripts and metabolic pathways.

The proteome survey of an electricity-generating organ (Torpedo californica electric organ)

Torpedo californica is a species in class Chondrichthyes. Electric rays have evolved the electric organ, which is similar to the mammalian neuromuscular junction (NMJ). Here, we took a combined cDNA sequencing and proteomic approach to define the molecular constituents of the T. californica electric organ. For soluble proteins, 2-DE was used and 224 protein spots were mapped. Plasma membrane fractions were analyzed using the shotgun approach (LC-MS/MS). A Torpedo cDNA library was constructed and 607 cDNA clones were sequenced. Identification of electric organ proteins was done using cross-species comparisons, and a custom database was constructed from cDNA translations. We unambiguously identified 121 proteins and transcripts, 103 of which were novel additions to the existing databases of Torpedo fish. Fifteen proteins of known function, but not previously associated with either the electroplaque or NMJ, were present at high abundance. These included the heat shock and oxidative stress proteins, annexin V (calelectrin), and plectin 1. Most interesting were the unambiguous matches to 11 human ORFs of unknown function, including four potential RNA splicing proteins, a vacuolar sorting protein, and a tetraspanin containing protein. This analysis identified proteins that may play a role in the higher vertebrate neuromuscular junction or other electrical synapses.

Targeted inhibition of Ca2+/calmodulin signaling exacerbates the dystrophic phenotype in mdx mouse muscle

In this study, we crossbred mdx mice with transgenic mice expressing a small peptide inhibitor for calmodulin (CaM), known as the CaM-binding protein (CaMBP), driven by the slow fiber-specific troponin I slow promoter. This strategy allowed us to determine the impact of interfering with Ca(2+)/CaM-based signaling in dystrophin-deficient slow myofibers. Consistent with impairments in the Ca(2+)/CaM-regulated enzymes calcineurin and Ca(2+)/CaM-dependent kinase, the nuclear accumulation of nuclear factor of activated T-cell c1 and myocyte enhancer factor 2C was reduced in slow fibers from mdx/CaMBP mice. We also detected significant reductions in the levels of peroxisome proliferator gamma co-activator 1alpha and GA-binding protein alpha mRNAs in slow fiber-rich soleus muscles of mdx/CaMBP mice. In parallel, we observed significantly lower expression of myosin heavy chain I mRNA in mdx/CaMBP soleus muscles. This correlated with fiber-type shifts towards a faster phenotype. Examination of mdx/CaMBP slow muscle fibers revealed significant reductions in A-utrophin, a therapeutically relevant protein that can compensate for the lack of dystrophin in skeletal muscle. In accordance with lower levels of A-utrophin, we noted a clear exacerbation of the dystrophic phenotype in mdx/CaMBP slow fibers as exemplified by several pathological indices. These results firmly establish Ca(2+)/CaM-based signaling as key to regulating expression of A-utrophin in muscle. Furthermore, this study illustrates the therapeutic potential of using targets of Ca(2+)/CaM-based signaling as a strategy for treating Duchenne muscular dystrophy (DMD). Finally, our results further support the concept that strategies aimed at promoting the slow oxidative myofiber program in muscle may be effective in altering the relentless progression of DMD.

Utrophin upregulation for treating Duchenne or Becker muscular dystrophy: how close are we?

Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disorder for which there is currently no effective treatment. This disorder is caused by mutations or deletions in the gene encoding dystrophin that prevent expression of dystrophin at the sarcolemma. A promising pharmacological treatment for DMD aims to increase levels of utrophin, a homolog of dystrophin, in muscle fibers of affected patients to compensate for the absence of dystrophin. Here, we review recent developments in our understanding of the regulatory pathways that govern utrophin expression, and highlight studies that have used activators of these pathways to alleviate the dystrophic symptoms in DMD animal models. The results of these preclinical studies are promising and bring us closer to implementing appropriate utrophin-based drug therapies for DMD patients.

Calcineurin-NFAT signaling, together with GABP and peroxisome PGC-1α, drives utrophin gene expression at the neuromuscular junction

We examined whether calcineurin-NFAT (nuclear factors of activated T cells) signaling plays a role in specifically directing the expression of utrophin in the synaptic compartment of muscle fibers. Immunofluorescence experiments revealed the accumulation of components of the calcineurin-NFAT signaling cascade within the postsynaptic membrane domain of the neuromuscular junction. RT-PCR analysis using synaptic vs. extrasynaptic regions of muscle fibers confirmed these findings by showing an accumulation of calcineurin transcripts within the synaptic compartment. We also examined the effect of calcineurin on utrophin gene expression. Pharmacological inhibition of calcineurin in mice with either cyclosporin A or FK506 resulted in a marked decrease in utrophin A expression at synaptic sites, whereas constitutive activation of calcineurin had the opposite effect. Mutation of the previously identified NFAT binding site in the utrophin A promoter region, followed by direct gene transfer studies in mouse muscle, led to an inhibition in the synaptic expression of a lacZ reporter gene construct. Transfection assays performed with cultured myogenic cells indicated that calcineurin acted additively with GA binding protein (GABP) to transactivate utrophin A gene expression. Because both GABP- and calcineurin-mediated pathways are targeted by peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), we examined whether this coactivator contributes to utrophin gene expression. In vitro and in vivo transfection experiments showed that PGC-1α alone induces transcription from the utrophin A promoter. Interestingly, this induction is largely potentiated by coexpression of PGC-1α with GABP. Together, these studies indicate that the synaptic expression of utrophin is also driven by calcineurin-NFAT signaling and occurs in conjunction with signaling events that involve GABP and PGC-1α.

The utrophin A 5'-untranslated region confers internal ribosome entry site-mediated translational control during regeneration of skeletal muscle fibers

Utrophin up-regulation in muscle fibers of Duchenne muscular dystrophy patients represents a potential therapeutic strategy. It is thus important to delineate the regulatory events presiding over utrophin in muscle in attempts to develop pharmacological interventions aimed at increasing utrophin expression. A number of studies have now shown that under several experimental conditions, the abundance of utrophin is increased without a corresponding elevation in its mRNA. Here, we examine whether utrophin expression is regulated at the translational level in regenerating muscle fibers. Treatment of mouse tibialis anterior muscles with cardiotoxin to induce muscle degeneration/regeneration led to a large (approximately 14-fold) increase in the levels of utrophin A with a modest change in expression of its transcript (40%). Isolation of the mouse utrophin A 5'-untranslated region (UTR) revealed that it is relatively long with a predicted high degree of secondary structure. In control muscles, the 5'-UTR of utrophin A caused an inhibition upon translation of a reporter protein. Strikingly, this inhibition was removed during regeneration, indicating that expression of utrophin A in regenerating muscles is translationally regulated via its 5'-UTR. Using bicistronic reporter vectors, we observed that this translational effect involves an internal ribosome entry site in the utrophin A 5'-UTR. Thus, internal ribosome entry site-mediated translation of utrophin A can, at least partially, account for the discordant expression of utrophin A protein and transcript in regenerating muscle. These findings provide a novel target for up-regulating levels of utrophin A in Duchenne muscular dystrophy muscle fibers via pharmacological interventions.

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.

Molecular, cellular, and pharmacological therapies for Duchenne/Becker muscular dystrophies

Although the molecular defect causing Duchenne/Becker muscular dystrophy (DMD/BMD) was identified nearly 20 years ago, the development of effective therapeutic strategies has nonetheless remained a daunting challenge. Over the years, a variety of different approaches have been explored in an effort to compensate for the lack of the DMD gene product called dystrophin. This review not only presents some of the most promising molecular, cellular, and pharmacological strategies but also highlights some issues that need to be addressed before considering their implementation. Specifically, we describe current strategies being developed to exogenously deliver healthy copies of the dystrophin gene to dystrophic muscles. We present the findings of several studies that have focused on repairing the mutant dystrophin gene using various approaches. We include a discussion of cell-based therapies that capitalize on the use of myoblast or stem cell transfer. Finally, we summarize the results of several studies that may eventually lead to the development of appropriate drug-based therapies. In this context, we review our current knowledge of the mechanisms regulating expression of utrophin, the autosomal homologue of dystrophin. Given the complexity associated with the dystrophic phenotype, it appears likely that a combinatorial approach involving different therapeutic strategies will be necessary for the appropriate management and eventual treatment of this devastating neuromuscular disease.

The synaptic muscle-specific kinase (MuSK) complex: New partners, new functions

The muscle-specific kinase MuSK is part of an agrin receptor complex that stimulates tyrosine phosphorylation and drives clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane at the vertebrate neuromuscular junction (NMJ). MuSK also regulates synaptic gene transcription in subsynaptic nuclei. Over the past few years, decisive progress has been made in the identification of MuSK effectors, helping to understand its function in the formation of the NMJ. Similarly to AChR, MuSK and several of its partners are the target of mutations responsible for diseases of the NMJ, such as congenital myasthenic syndromes. This minireview will focus on the multiple MuSK effectors so far identified that place MuSK at the center of a multifunctional signaling complex involved in the organization of the NMJ and associated disorders.

Sarcolipin retention in the endoplasmic reticulum depends on its C-terminal RSYQY sequence and its interaction with sarco(endo)plasmic Ca(2+)-ATPases

Sarcolipin (SLN) and phospholamban (PLN) are effective inhibitors of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). These homologous proteins differ at their N and C termini: the C-terminal Met-Leu-Leu in PLN is replaced by Arg-Ser-Tyr-Gln-Tyr in SLN. The role of the C-terminal sequence of SLN tagged N-terminally with the FLAG epitope (NF-SLN) in endoplasmic reticulum (ER) retention was investigated by transfecting human embryonic kidney-293 cells with cDNAs encoding NF-SLN or a series of NF-SLN mutants in which C-terminal amino acids were deleted progressively. Immunofluorescence and immunoblotting of transfected cells by using anti-FLAG antibodies indicated that NF-SLN and PLN tagged at its N terminus with the FLAG epitope, even when overexpressed, were restricted to the ER. However, C-terminal truncation deletions of SLN, which lacked RSYQY, were not localized to ER and did not inhibit Ca(2+)-dependent Ca2+ uptake by SERCA. The shortest deletion constructs, NF-SLN 1-22 and NF-SLN 1-23, did not express stable protein products. However, all NF-SLN cDNA constructs, including NF-SLN 1-22 and NF-SLN 1-23, were expressed stably and localized to the ER when they were coexpressed with SERCA2a. These results show that NF-SLN subcellular distribution depends on SERCA coexpression and on its luminal, C-terminal RSYQY sequence. By using immunoprecipitation and MS, glucose-regulated protein 78/BiP and glucose-regulated protein 94 were identified as proteins that interact with NF-SLN through the RSYQY sequence. Thus, in the absence of SERCA, retention of NF-SLN in the ER is mediated through its association with other components through the C-terminal RSYQY sequence.

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.

Ste20-like kinase SLK displays myofiber type specificity and is involved in C2C12 myoblast differentiation

Cell growth and terminal differentiation are controlled by complex signaling cascades that regulate the expression of specific subsets of genes implicated in cell fate and morphogenic processes. We have recently cloned and characterized a novel Ste20-like kinase termed SLK that is associated with adhesion structures during cell adhesion and spreading. However, the specific function of SLK is poorly understood. To gain further insight into the role of SLK, we have characterized its activity, expression, and distribution in skeletal muscle and during the in vitro differentiation of C2C12 myoblasts. Although SLK is expressed ubiquitously in adult tissues, our results show that it is predominantly expressed in muscle masses during development. Furthermore, SLK activity is upregulated during the differentiation of C2C12 myoblasts. In addition, we have found that SLK localizes presynaptically at neuromuscular junctions and that it is preferentially expressed in types I and IIA myofibers at major myofibrillar striations. Supporting a role in myoblast function and differentiation, SLK expression is induced in Myf5- and Pax7-positive activated satellite cells during regeneration and expression of dominant negative SLK in C2C12 cultures impairs myoblast fusion, suggesting a role for SLK in muscle cell differentiation.

Therapeutic Strategies for Duchenne and Becker Dystrophies

Duchenne muscular dystrophy (DMD), a severe X-linked genetic disease affecting one in 3500 boys, is the most common myopathy in children. DMD is due to a lack of dystrophin, a submembrane protein of the cytoskeleton, which leads to the progressive degeneration of skeletal, cardiac, and smooth muscle tissue. A milder form of the disease, Becker muscular dystrophy (BMD), is characterized by the presence of a semifunctional truncated dystrophin, or reduced levels of full-length dystrophin. DMD is the focus of three different supportive or therapeutic approaches: gene therapy, cell therapy, and drug therapy. Here we consider these approaches in terms of three potential goals: improvement of dystrophic phenotype, expression of dystrophin, and overexpression of utrophin. Utrophin exhibits 80% homology with dystrophin and is able to perform similar functions. Pharmacological strategies designed to overexpress utrophin appear promising and may circumvent many obstacles to gene and cell-based therapies.

Kinase- and rapsyn-independent activities of the muscle-specific kinase (MuSK)

The muscle-specific receptor tyrosine kinase (MuSK) is co-localized with nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane of the skeletal neuromuscular junction, and is required for all known aspects of postsynaptic differentiation. Studies in vitro have shown that Z(+)-agrin, a nerve-derived proteoglycan, activates MuSK's kinase activity to promote clustering of AChRs and MuSK itself with a cytoplasmic, receptor-associated protein, rapsyn. These studies, however, have used soluble forms of agrin, whereas agrin is cell- or matrix-attached in vivo. We show here that immobilized (particle- or cell-attached) agrin but not soluble agrin is able to aggregate MuSK in the absence of rapsyn and that this aggregation does not require MuSK's kinase activity but does require MuSK's cytoplasmic domain. Moreover, immobilized agrin can promote clustering of AChRs by a mechanism that requires MuSK and rapsyn but does not require MuSK's kinase activity. These results imply that rapsyn and signaling components activated by MuSK kinase may be dispensable for some early aspects of postsynaptic differentiation.

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.

Localization of the RNA-binding proteins Staufen1 and Staufen2 at the mammalian neuromuscular junction

Staufen is an RNA-binding protein, first identified for its role in oogenesis and CNS development in Drosophila. Two mammalian homologs of Staufen have been identified and shown to bind double-stranded RNA and tubulin, and to function in the somatodendritic transport of mRNA in neurons. Here, we examined whether Staufen proteins are expressed in skeletal muscle in relation to the neuromuscular junction. Immunofluorescence experiments revealed that Staufen1 (Stau1) and Staufen2 (Stau2) accumulate preferentially within the postsynaptic sarcoplasm of muscle fibers as well as at newly formed ectopic synapses. Western blot analyses showed that the levels of Stau1 and Stau2 are greater in slow muscles than in fast-twitch muscles. Muscle denervation induced a significant increase in the expression of Stau1 and Stau2 in the extrasynaptic compartment of both fast and slow muscles. Consistent with these observations, we also demonstrated that expression of Stau1 and Stau2 is increased during myogenic differentiation and that treatment of myotubes with agrin and neuregulin induces a further increase in the expression of both Staufen proteins. We propose that Stau1 and Stau2 are key components of the postsynaptic apparatus in muscle, and that they contribute to the maturation and plasticity of the neuromuscular junction.

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.

Correlated NOS-Imu and myf5 expression by satellite cells in mdx mouse muscle regeneration during NOS manipulation and deflazacort treatment

Satellite cells, muscle precursor cells in skeletal muscle, are normally quiescent and become activated by disease or injury. A lack of dystrophin and changes in the expression or activity of neuronal nitric oxide synthase (NOS-I) affect the timing of activation in vivo. Nitric oxide synthase inhibition delays muscle repair in normal mice, and worsens muscular dystrophy in the mdx mouse, a genetic homologue of Duchenne muscular dystrophy. However, the potential role of activation and repair events mediated by nitric oxide in determining the outcome of steroid or other treatments for muscular dystrophy is not clear. We tested the hypothesis that the extent of repair in dystrophic muscles of mdx mice is partly dependent on NOS-Imu expression and activity. Myotube formation in regenerating muscle was promoted by deflazacort treatment of mdx dystrophic mice (P<0.05), and improved by combination with the nitric oxide synthase substrate, L-arginine, especially in the diaphragm. NOS-Imu mRNA expression and activity were present in satellite cells and very new myotubes of regenerating and dystrophic muscle. Deflazacort treatment resulted in increased NOS-Imu expression in regenerating muscles in a strong and specific correlation with myf5 expression (r=0.95, P<0.01), a marker for muscle repair. Nitric oxide synthase inhibition prevented the deflazacort-induced rise in NOS-Imu and myf5 expression in the diaphragm without affecting the diameter of non-regenerating fibres. These in vivo studies suggest that gains in NOS-Imu expression and nitric oxide synthase activity in satellite cells can increase the extent and speed of repair, even in the absence of dystrophin in muscle fibres. NOS-Imu may be a useful therapeutic target to augment the effects of steroidal or other treatments of muscular dystrophy.

A novel pathway for MuSK to induce key genes in neuromuscular synapse formation

At the developing neuromuscular junction the Agrin receptor MuSK is the central organizer of subsynaptic differentiation induced by Agrin from the nerve. The expression of musk itself is also regulated by the nerve, but the mechanisms involved are not known. Here, we analyzed the activation of a musk promoter reporter construct in muscle fibers in vivo and in cultured myotubes, using transfection of multiple combinations of expression vectors for potential signaling components. We show that neuronal Agrin by activating MuSK regulates the expression of musk via two pathways: the Agrin-induced assembly of muscle-derived neuregulin (NRG)-1/ErbB, the pathway thought to regulate acetylcholine receptor (AChR) expression at the synapse, and via a direct shunt involving Agrin-induced activation of Rac. Both pathways converge onto the same regulatory element in the musk promoter that is also thought to confer synapse-specific expression to AChR subunit genes. In this way, a positive feedback signaling loop is established that maintains musk expression at the synapse when impulse transmission becomes functional. The same pathways are used to regulate synaptic expression of AChR epsilon. We propose that the novel pathway stabilizes the synapse early in development, whereas the NRG/ErbB pathway supports maintenance of the mature synapse.

MAPK signal transduction pathway mediates agrin effects on neurite elongation in cultured hippocampal neurons

We have previously shown that agrin regulates the rates of axonal and dendritic elongation by modulating the expression of microtubule-associated proteins in cultured hippocampal neurons. However, the mechanisms by which agrin-induced signals are propagated to the nucleus where they can lead to the phosphorylation, and hence the activation, of transcription factors, are not known. In the present study, we identified downstream elements that play essential roles in the agrin-signaling pathway in developing central neurons. Our results indicate that agrin induces the combined activation of the extracellular signal-regulated kinases (ERK1/ERK2) and p38 in central neurons. In addition, they showed that PD98059 and SB202190, synthetic inhibitors of ERK1/ERK2 and p38 respectively, prevented the changes in the rate of neurite elongation induced by agrin in cultured hippocampal neurons. Collectively, these results suggest that agrin might modulate the expression of neuron-specific genes involved in neurite elongation by inducing CREB phosphorylation through the activation of the MAPK signal transduction pathway in cultured hippocampal neurons.

Cloning, expression, and genomic organization of mouse mp29 gene

Human p29 has been demonstrated in the yeast two-hybrid method and in vitro GST pull-down assay to associate with GCIP, a cyclin D interacting protein. In this study, we describe the cloning and genomic structure of the mouse homologue, mp29. The overall mouse mp29 amino acid sequence is highly identical (91%) to human p29. Polyclonal antibody against mp29 was raised and the subcellular localization of mp29 was identified to be in the nucleus. Genomic clones containing mp29 gene were isolated and this gene was divided into seven exons spanning 9kb of genomic DNA. The transcription initiation site of mp29 gene was determined to be 94bp upstream of the translation initiation codon and the first 140bp proximal TATA-less promoter region is required to activate minimal transcription of mouse mp29 gene in mammalian cells.

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.

Agrin in the CNS: a protein in search of a function?

The extracellular matrix molecule agrin mediates the motor neuron induced accumulation of acetylcholine receptors (AChR) at the neuromuscular junction. Agrin is also present in the CNS. However, while its spatiotemporal pattern of expression is consistent with a function in neuron-neuron synapse formation, it also suggests a role for agrin in other aspects of neural tissue morphogenesis. Here we review the data supporting these synaptic and non-synaptic functions of agrin in the CNS. The results of studies aimed at identifying a neuronal receptor for agrin (NRA) and its associated signal transduction pathways are examined. Possible roles for agrin in the etiology of diseases affecting the brain are also discussed.

A newly identified chromosomal microdeletion and an N-box mutation of the AChR epsilon gene cause a congenital myasthenic syndrome

Congenital myasthenic syndromes (CMSs) are frequently caused by mutations of the coding region of the acetylcholine receptor epsilon subunit (AChRepsilon) gene leading to a reduced expression of the acetylcholine receptor (AChR) at the postsynaptic membrane. Two recent observations have linked two different N-box mutations of the human AChRepsilon promoter to a clinical CMS phenotype. N-boxes are regulatory sequence elements of mammalian promoters that confer synapse-specific expression of several genes, including the AChR subunit genes. Here, we report on a novel point mutation (epsilon-154G-->A) in the N-box of the AChRepsilon promoter in a German CMS pedigree. Semiquantitative analysis of AChRepsilon mRNA levels in the patient's muscle indicated significantly impaired AChRepsilon expression. We provide additional evidence of a pathogenic role for this mutation using the mutated promoter (epsilon-154G-->A) driving a heterologous gene (luciferase) in rat skeletal muscle. We show that agrin-induced gene expression is significantly reduced by the N-box mutant (mt) compared with the wild-type (wt) promoter. Refined haplotype analysis and direct sequencing revealed maternal inheritance of the mutant AChRepsilon promoter (epsilon-154G-->A) together with paternal inheritance of a chromosomal microdeletion (Delta1290 bp) encompassing the promoter and the first two exons of the AChRepsilon gene in the index patient. In conclusion, we provide genetic and functional evidence that a mutation of the AChRepsilon subunit promoter (epsilon-154G-->A) causes CMS due to the reduction of gene expression in skeletal muscle. Moreover, this is the first report of a chromosomal microdeletion affecting an AChR gene. This type of mutation may be missed in standard screening techniques of CMS patients.

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.

Muscular nitric oxide synthase (muNOS) and utrophin

Duchenne muscular dystrophy (DMD), the severe X-linked recessive disorder which results in progressive muscle degeneration, is due to a lack of dystrophin, a membrane cytoskeletal protein. Three types of treatment are envisaged: pharmacological (glucocorticoid), myoblast transplantation, and gene therapy. An alternative to the pharmacological approach is to compensate for dystrophin loss by the upregulation of another cytoskeletal protein, utrophin. Utrophin and dystrophin are part of a complex of proteins and glycoproteins, which links the basal lamina to the cytoskeleton, thus ensuring the stability of the muscle membrane. One protein of the complex, syntrophin, is associated with a muscular isoform of the neuronal nitric oxide synthase (nNOS). We have demonstrated an overexpression of utrophin, visualised by immunofluorescence and quantified by Western blotting, in normal myotubes and in mdx (the animal model of DMD) myotubes, as in normal (C57) and mdx mice, both treated with nitric oxide (NO) donor or L-arginine, the NOS substrate. There is evidence that utrophin may be capable of performing the same cellular functions as dystrophin and may functionally compensate for its lack. Thus, we propose to use NO donors, as palliative treatment of Duchenne and Becker muscular dystrophies, pending, or in combination with, gene and/or cellular therapy. Discussion has focussed on the various isoforms of NOS that could be implicated in the regeneration process. Dystrophic and healthy muscles respond to treatment, suggesting that although NOS is delocalised in the cytoplasm in the case of DMD, it conserves substantial activity. eNOS present in mitochondria and iNOS present in cytoplasm and the neuromuscular junction could also be activated. Lastly, production of NO by endothelial NOS of the capillaries would also be beneficial through increased supply of metabolites and oxygen to the muscles.

The agrin/muscle-specific kinase pathway: new targets for autoimmune and genetic disorders at the neuromuscular junction

The increasing understanding of the structural complexity of the neuromuscular junction (NMJ), and the processes that are important in its development, suggests many possible new disease targets. Here, we summarize briefly the genetic and autoimmune disorders that affect neuromuscular transmission, and the identified targets, including new evidence that antibodies to muscle-specific receptor tyrosine kinase (MuSK) are involved in the pathogenesis of acetylcholine receptor (AChR) antibody-negative myasthenia gravis. We then review the development of the NMJ, focusing on the important roles of nerve-derived agrin and MuSK in clustering of AChRs and other essential components of the NMJ.

Agrin regulates neuronal responses to excitatory neurotransmitters in vitro and in vivo

Agrin mediates motor neuron-induced differentiation of the postsynaptic apparatus of the neuromuscular junction but its function in brain remains unknown. Here we report that expression of c-fos, induced by activation of nicotinic or glutamatergic receptors, was significantly lower in cortical neurons cultured from agrin-deficient mutant mouse embryos compared to wildtype. Agrin-deficient neurons also exhibited increased resistance to excitotoxic injury. Treatment with recombinant agrin restored glutamate-induced c-fos expression and excitotoxicity of the agrin-deficient neurons to near wild-type levels, confirming the agrin dependence of the phenotype. The observation that c-fos induction by activation of voltage-gated Ca2+ channels is also reduced in agrin-deficient neurons raises the possibility that agrin may play a wider role by regulating responses to Ca(2+)-mediated signals. Consistent with the decline in response of cultured mutant neurons to glutamate, decreases in kainic acid-induced seizure and mortality were observed in adult agrin heterozygous mice. Together, these data demonstrate that agrin plays an important role in defining neuronal responses to excitatory neurotransmitters both in vitro and in vivo.

Multiple regulatory events controlling the expression and localization of utrophin in skeletal muscle fibers: insights into a therapeutic strategy for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most prevalent inherited muscle disease and results from mutations/deletions in the X-linked dystrophin gene. Although several approaches have been envisaged to counteract the effects of this progressive disease, there is currently no cure available. One strategy consists in utilizing a protein normally expressed in DMD muscle which, once expressed at appropriate levels and at the correct subcellular location, could compensate for the lack of dystrophin. A candidate for such a role is the dystrophin-related protein now referred to as utrophin. In contrast to dystrophin, which is expressed along the length of healthy muscle fibers, utrophin accumulates at the neuromuscular junction in both normal and DMD fibers. Several years ago, we began a series of experiments to determine the mechanisms responsible for the expression of utrophin at the neuromuscular synapse. Initially, we showed that utrophin transcripts accumulate preferentially within the postsynaptic sarcoplasm. To determine whether selective transcription of the utrophin gene accounts for this synaptic accumulation of utrophin mRNAs, we injected several utrophin promoter-reporter constructs directly into mouse muscle and demonstrated the preferential synaptic expression of the reporter gene. These results suggested that local transcriptional activation of the utrophin gene is responsible for the accumulation of utrophin mRNAs at the neuromuscular junction. In these studies, we also demonstrated that an N-box motif contained within the utrophin promoter plays a critical role in directing the synapse-specific expression of the utrophin gene. Additionally, our studies have shown that the ets-factors GABP alpha and beta are part of a protein complex that can bind to the N-box motif to transactivate the gene in muscle cells in culture and in vivo. In these experiments, we also noted that the nerve-derived trophic factors agrin and ARIA/heregulin regulate expression of utrophin via the activation of GABP alpha and beta which in turn, transactivate the utrophin gene via the N-box motif. Although these studies demonstrate that transcriptional activation can regulate utrophin mRNA levels, it is possible that additional mechanisms are also involved. In particular, the association of mRNAs with cytoskeletal elements and RNA-binding proteins may contribute to the accumulation of utrophin transcripts within the postsynaptic sarcoplasm. In recent studies, we have begun to examine this and we have now identified specific regions within the 3' untranslated region that are necessary for targeting and stabilizing utrophin mRNAs in skeletal muscle cells. A series of in vivo studies have also led us to conclude that post-transcriptional mechanisms are indeed important in regulating the abundance of utrophin transcripts in muscle. Together, these studies should lead to the identification of cis- and trans-acting elements regulating transcription of the utrophin gene as well as the stability and targeting of its mRNA in muscle cells. The results should therefore, identify specific targets that may become important in designing specific pharmacological interventions directed at increasing the expression of utrophin into extrasynaptic regions of DMD muscle fibers. In addition, these findings will contribute to our basic understanding of the cellular and molecular events involved in the formation, maintenance and plasticity of the neuromuscular synapse.

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

Duchenne muscular dystrophy (DMD) is an X-linked recessive muscle wasting disease caused by the absence of a muscle cytoskeletal protein, dystrophin. Utrophin is the autosomal homologue of dystrophin. We previously demonstrated that overexpression of utrophin in the muscles of dystrophin-null transgenic mice completely prevented the phenotype arising from dystrophin deficiency. Two independently regulated promoters control utrophin expression and the upstream promoter (promoter A) is synaptically regulated in muscle. In this study, we have investigated basal regulation and myogenic induction of promoter A. Interactions between Ap2 and Sp1 and their cognate DNA motifs are critical for basal transcription from the minimal promoter region. During differentiation of C2C12 myoblasts in vitro, a 2-fold increase in A-utrophin mRNA level was observed. Expression of a reporter gene, whose transcription was driven by a 1.3 kb promoter A fragment, paralleled expression of the endogenous transcript. Myogenic induction mapped to a conserved upstream muscle-specific E-box, which was shown to bind myogenic regulatory factors, transactivating the promoter up to 18-fold in transient assays. This study provides a basis for further understanding the regulatory mechanisms that control utrophin expression in muscle and may facilitate the development of reagents to effect therapeutic up-regulation of utrophin in DMD.

Expression of ciliary neurotrophic factor receptor in myasthenia gravis

In myasthenia gravis, high anti-nicotinic receptor (AChR) antibody titers are not always associated with severity of the disease, suggesting the involvement of other pathological effectors. We showed that ciliary neurotrophic factor receptor (CNTFR) gene expression was higher in muscle biopsy tissue from severely affected MG patients regardless of anti-nAChR antibody titer. This increase was also triggered in vitro by a seric factor from MG patients. CNTFR protein expression was decreased in muscles from seropositive MG patients only.Altogether, our data indicate that the alteration of CNTFR expression in some MG patients could contribute to the severity of the disease in a subgroup of patients.

Increased expression of utrophin in a slow vs. a fast muscle involves posttranscriptional events

In addition to showing differences in the levels of contractile proteins and metabolic enzymes, fast and slow muscles also differ in their expression profile of structural and synaptic proteins. Because utrophin is a structural protein expressed at the neuromuscular junction, we hypothesize that its expression may be different between fast and slow muscles. Western blots showed that, compared with fast extensor digitorum longus (EDL) muscles, slow soleus muscles contain significantly more utrophin. Quantitative RT-PCR revealed that this difference is accompanied by a parallel increase in the expression of utrophin transcripts. Interestingly, the higher levels of utrophin and its mRNA appear to occur in extrasynaptic regions of muscle fibers as shown by immunofluorescence and in situ hybridization experiments. Furthermore, nuclear run-on assays showed that the rate of transcription of the utrophin gene was nearly identical between EDL and soleus muscles, indicating that increased mRNA stability accounts for the higher levels of utrophin in slow muscles. Direct plasmid injections of reporter gene constructs showed that cis-acting elements contained within the utrophin 3'-untranslated region (3'-UTR) confer greater stability to chimeric LacZ transcripts in soleus muscles. Finally, we observed a clear difference between EDL and soleus muscles in the abundance of RNA-binding proteins interacting with the utrophin 3'-UTR. Together, these findings highlight the contribution of posttranscriptional events in regulating the expression of utrophin in muscle.

Agrin differentially regulates the rates of axonal and dendritic elongation in cultured hippocampal neurons

In the present study, we examined the role of agrin in axonal and dendritic elongation in central neurons. Dissociated hippocampal neurons were grown in the presence of either recombinant agrin or antisense oligonucleotides designed to block agrin expression. Our results indicate that agrin differentially regulates axonal and dendritic growth. Recombinant agrin decreased the rate of elongation of main axons but induced the formation of axonal branches. On the other hand, agrin induced both dendritic elongation and dendritic branching. Conversely, cultured hippocampal neurons depleted of agrin extended longer, nonbranched axons and shorter dendrites when compared with controls. These changes in the rates of neurite elongation and branching were paralleled by changes in the composition of the cytoskeleton. In the presence of agrin, there was an upregulation of the expression of microtubule-associated proteins MAP1B, MAP2, and tau. In contrast, a downregulation of the expression of these MAPs was detected in agrin-depleted cells. Taken collectively, these results suggest an important role for agrin as a trigger of the transcription of neuro-specific genes involved in neurite elongation and branching in central neurons.