Distinct regions in the 3' untranslated region are responsible for targeting and stabilizing utrophin transcripts in skeletal muscle cells

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Anisomycin Activates Utrophin Upregulation Through a p38 Signaling Pathway

Duchenne muscular dystrophy is a recessive X‐linked disease characterized by progressive muscle wasting; cardiac or respiratory failure causes death in most patients by the third decade.  The disease is caused by mutations in the dystrophin gene that lead to a loss of functional dystrophin protein. Although there are currently few treatments for Duchenne muscular dystrophy, previous reports have shown that upregulating the dystrophin paralog utrophin in Duchenne muscular dystrophy mouse models is a promising therapeutic strategy. We conducted in silico mining of the Connectivity Map database for utrophin‐inducing agents, identifying the p38‐activating antibiotic anisomycin. Treatments of C2C12, undifferentiated murine myoblasts, and mdx primary myoblasts with anisomycin conferred increases in utrophin protein levels through p38 pathway activation.  Anisomycin also induced utrophin protein levels in the diaphragm of mdx mice.  Our study shows that repositioning small molecules such as anisomycin may prove to have Duchenne muscular dystrophy clinical utility.

Oral quercetin administration transiently protects respiratory function in dystrophin-deficient mice

KEY POINT

PGC-1α pathway activation has been shown to decrease disease severity and can be driven by quercetin. Oral quercetin supplementation protected respiratory function for 4-6 months during a 12 month dosing regimen. This transient protection was probably due to a failure to sustain elevated SIRT1 activity and downstream PGC-1α signalling. Quercetin supplementation may be a beneficial treatment as part of a cocktail provided continued SIRT1 activity elevation is achieved.

ABSTRACT

Duchenne muscular dystrophy (DMD) impacts 1 : 3500 boys and leads to muscle dysfunction culminating in death due to respiratory or cardiac failure. There is an urgent need for effective therapies with the potential for immediate application for this patient population. Quercetin, a flavonoid with an outstanding safety profile, may provide therapeutic relief to DMD patients as the wait for additional therapies continues. This study evaluated the capacity of orally administered quercetin (0.2%) in 2 month old mdx mice to improve respiratory function and end-point functional and histological outcomes in the diaphragm following 12 months of treatment. Respiratory function was protected for the first 4-6 months of treatment but appeared to become insensitive to quercetin thereafter. Consistent with this, end-point functional measures were decreased and histopathological measures were more severe in dystrophic muscle compared to C57 and similar between control-fed and quercetin-fed mdx mice. To better understand the transient nature of improved respiratory function, we measured PGC-1α pathway activity, which is suggested to be up-regulated by quercetin supplementation. This pathway was largely suppressed in dystrophic muscle compared to healthy muscle, and at the 14 month time point dietary quercetin enrichment did not increase expression of downstream effectors. These data support the efficacy of quercetin as an intervention for DMD in skeletal muscle, and also indicate the development of age-dependent quercetin insensitivity when continued supplementation fails to drive the PGC-1α pathway. Continued study is needed to determine if this is related to disease severity, age or other factors.

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.

Conserved regions of the DMD 3' UTR regulate translation and mRNA abundance in cultured myotubes

Duchenne muscular dystrophy (DMD), a severe muscle-wasting disease, is caused by mutations in the DMD gene, which encodes for the protein dystrophin. Its regulation is of therapeutic interest as even small changes in expression of functional dystrophin can significantly impact the severity of DMD. While tissue-specific distribution and transcriptional regulation of several DMD mRNA isoforms has been well characterized, the post-transcriptional regulation of dystrophin synthesis is not well understood. Here, we utilize qRTPCR and a quantitative dual-luciferase reporter assay to examine the effects of isoform specific DMD 5' UTRs and the highly conserved DMD 3' UTR on mRNA abundance and translational control of gene expression in C2C12 cells. The 5' UTRs were shown to initiate translation with low efficiency in both myoblasts and myotubes. Whereas, two large highly conserved elements in the 3' UTR, which overlap the previously described Lemaire A and D regions, increase mRNA levels and enhance translation upon differentiation of myoblasts into myotubes. The results presented here implicate an important role for DMD UTRs in dystrophin expression and delineate the cis-acting elements required for the myotube-specific regulation of steady-state mRNA levels and translational enhancer activity found in the DMD 3' UTR.

Converging pathways involving microRNA-206 and the RNA-binding protein KSRP control post-transcriptionally utrophin A expression in skeletal muscle

Several reports have previously highlighted the potential role of miR-206 in the post-transcriptional downregulation of utrophin A in cultured cells. Along those lines, we recently identified K-homology splicing regulator protein (KSRP) as an important negative regulator in the post-transcriptional control of utrophin A in skeletal muscle. We sought to determine whether these two pathways act together to downregulate utrophin A expression in skeletal muscle. Surprisingly, we discovered that miR-206 overexpression in cultured cells and dystrophic muscle fibers causes upregulation of endogenous utrophin A levels. We further show that this upregulation of utrophin A results from the binding of miR-206 to conserved sites located in the 3′-UTR (untranslated region) of KSRP, thus causing the subsequent inhibition of KSRP expression. This miR-206-mediated decrease in KSRP levels leads, in turn, to an increase in the expression of utrophin A due to a reduction in the activity of this destabilizing RNA-binding protein. Our work shows that miR-206 can oscillate between direct repression of utrophin A expression via its 3′-UTR and activation of its expression through decreased availability of KSRP and interactions with AU-rich elements located within the 3′-UTR of utrophin A. Our study thus reveals that two apparent negative post-transcriptional pathways can act distinctively as molecular switches causing repression or activation of utrophin A expression.

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.

Agrin induces association of Chrna1 mRNA and nicotinic acetylcholine receptor in C2C12 myotubes

In the mammalian central nervous system transcripts of certain synaptic components are localized near the synapse, allowing for rapid regulation of protein levels. Here we test whether an mRNA localization mechanism also exists in the postsynaptic specialization induced by agrin in C2C12 myotubes. RT-PCR showed that Chrna1 was co-purified with nicotinic acetylcholine receptor (AChR) isolated by affinity column or by ultracentrifugation. In addition, Stau1 was found to interact with Chrna1 mRNA, and knocking down of Stau1 by RNAi resulted in defective AChR clustering. These results suggest that mRNA localization also participates in the formation of mammalian neuromuscular junction (NMJ).

Drug discovery for Duchenne muscular dystrophy via utrophin promoter activation screening

BACKGROUND

Duchenne muscular dystrophy (DMD) is a devastating muscle wasting disease caused by mutations in dystrophin, a muscle cytoskeletal protein. Utrophin is a homologue of dystrophin that can functionally compensate for its absence when expressed at increased levels in the myofibre, as shown by studies in dystrophin-deficient mice. Utrophin upregulation is therefore a promising therapeutic approach for DMD. The use of a small, drug-like molecule to achieve utrophin upregulation offers obvious advantages in terms of delivery and bioavailability. Furthermore, much of the time and expense involved in the development of a new drug can be eliminated by screening molecules that are already approved for clinical use.

METHODOLOGY/PRINCIPAL FINDINGS

We developed and validated a cell-based, high-throughput screening assay for utrophin promoter activation, and used it to screen the Prestwick Chemical Library of marketed drugs and natural compounds. Initial screening produced 20 hit molecules, 14 of which exhibited dose-dependent activation of the utrophin promoter and were confirmed as hits. Independent validation demonstrated that one of these compounds, nabumetone, is able to upregulate endogenous utrophin mRNA and protein, in C2C12 muscle cells.

CONCLUSIONS/SIGNIFICANCE

We have developed a cell-based, high-throughput screening utrophin promoter assay. Using this assay, we identified and validated a utrophin promoter-activating drug, nabumetone, for which pharmacokinetics and safety in humans are already well described, and which represents a lead compound for utrophin upregulation as a therapy for DMD.

Tumor suppression by RNA from C/EBPβ 3'UTR through the inhibition of protein kinase Cε activity

BACKGROUND

Since the end of last century, RNAs from the 3'untranslated region (3'UTR) of several eukaryotic mRNAs have been found to exert tumor suppression activity when introduced into malignant cells independent of their whole mRNAs. In this study, we sought to determine the molecular mechanism of the tumor suppression activity of a short RNA from 3'UTR of C/EBPβ mRΝΑ (C/EBPβ 3'UTR RNA) in human hepatocarcinoma cells SMMC-7721.

METHODOLOGY/PRINCIPAL FINDINGS

By using Western blotting, immunocytochemistry, molecular beacon, confocal microscopy, protein kinase inhibitors and in vitro kinase assays, we found that, in the C/EBPβ 3'UTR-transfectant cells of SMMC-7721, the overexpressed C/EBPβ 3'UTR RNA induced reorganization of keratin 18 by binding to this keratin; that the C/EBPβ 3'UTR RNA also reduced phosphorylation and expression of keratin 18; and that the enzyme responsible for phosphorylating keratin 18 is protein kinase Cε. We then found that the C/EBPβ 3'UTR RNA directly inhibited the phosphorylating activity of protein kinase Cε; and that C/EBPβ 3'UTR RNA specifically bound with the protein kinase Cε-keratin 18 conjugate.

CONCLUSION/SIGNIFICANCE

Together, these facts suggest that the tumor suppression in SMMC-7721 by C/EBPβ 3'UTR RNA is due to the inhibition of protein kinase Cε activity through direct physical interaction between C/EBPβ 3'UTR RNA and protein kinase Cε. These facts indicate that the 3'UTR of some eukaryotic mRNAs may function as regulators for genes other than their own.

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.

In vivo monitoring of mRNA movement in Drosophila body wall muscle cells reveals the presence of myofiber domains

BACKGROUND

In skeletal muscle each muscle cell, commonly called myofiber, is actually a large syncytium containing numerous nuclei. Experiments in fixed myofibers show that mRNAs remain localized around the nuclei in which they are produced.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we generated transgenic flies that allowed us to investigate the movement of mRNAs in body wall myofibers of living Drosophila embryos. We determined the dynamic properties of GFP-tagged mRNAs using in vivo confocal imaging and photobleaching techniques and found that the GFP-tagged mRNAs are not free to move throughout myofibers. The restricted movement indicated that body wall myofibers consist of three domains. The exchange of mRNAs between the domains is relatively slow, but the GFP-tagged mRNAs move rapidly within these domains. One domain is located at the centre of the cell and is surrounded by nuclei while the other two domains are located at either end of the fiber. To move between these domains mRNAs have to travel past centrally located nuclei.

CONCLUSIONS/SIGNIFICANCE

These data suggest that the domains made visible in our experiments result from prolonged interactions with as yet undefined structures close to the nuclei that prevent GFP-tagged mRNAs from rapidly moving between the domains. This could be of significant importance for the treatment of myopathies using regenerative cell-based therapies.

Identification of a putative network of actin-associated cytoskeletal proteins in glomerular podocytes defined by co-purified mRNAs

The glomerular podocyte is a highly specialized and polarized kidney cell type that contains major processes and foot processes that extend from the cell body. Foot processes from adjacent podocytes form interdigitations with those of adjacent cells, thereby creating an essential intercellular junctional domain of the renal filtration barrier known as the slit diaphragm. Interesting parallels have been drawn between the slit diaphragm and other sites of cell-cell contact by polarized cells. Notably mutations in several genes encoding proteins localized to the foot processes can lead to proteinuria and kidney failure. Mutations in the Wilm's tumor gene (WT1) can also lead to kidney disease and one isoform of WT1, WT1(+KTS), has been proposed to regulate gene expression post-transcriptionally. We originally sought to identify mRNAs associated with WT1(+KTS) through an RNA immunoprecipitation and microarray approach, hypothesizing that the proteins encoded by these mRNAs might be important for podocyte morphology and function. We identified a subset of mRNAs that were remarkably enriched for transcripts encoding actin-binding proteins and other cytoskeletal proteins including several that are localized at or near the slit diaphragm. Interestingly, these mRNAs included those of α-actinin-4 and non-muscle myosin IIA that are mutated in genetic forms of kidney disease. However, isolation of the mRNAs occurred independently of the expression of WT1, suggesting that the identified mRNAs were serendipitously co-purified on the basis of co-association in a common subcellular fraction. Mass spectroscopy revealed that other components of the actin cytoskeleton co-purified with these mRNAs, namely actin, tubulin, and elongation factor 1α. We propose that these mRNAs encode a number of proteins that comprise a highly specialized protein interactome underlying the slit diaphragm. Collectively, these gene products and their interactions may prove to be important for the structural integrity of the actin cytoskeleton in podocytes as well as other polarized cell types.

MicroRNA-206: the skeletal muscle-specific myomiR

MicroRNAs (miRNAs) are a class of non-coding RNAs involved in post-transcriptional gene silencing. A small number of striated muscle-specific miRNAs have been identified and shown to have an important role in myogenesis, embryonic muscle growth and cardiac function and hypertrophy. One of these myomiRs (myo=muscle+miR=miRNA), miR-206, is unique in that it is only expressed in skeletal muscle. The purpose of this review is to discuss what is currently known about miR-206 and its function in myogenesis as well as propose potential new roles for miR-206 in skeletal muscle biology. The review is also intended to serve as a comprehensive resource for miR-206 with the hope of encouraging further research on the role of miR-206 in skeletal muscle.

Downstream utrophin enhancer is required for expression of utrophin in skeletal muscle

BACKGROUND

Duchenne muscular dystrophy is caused by the absence of the muscle cytoskeletal protein dystrophin. Utrophin is an autosomal homologue of dystrophin, and overexpression of utrophin is expected to compensate for the dystrophin deficit. We previously reported that the 5.4-kb 5'-flanking region of the utrophin gene containing the A-utrophin core promoter did not drive transgene expression in heart and skeletal muscle. To clarify the regulatory mechanism of utrophin expression, we generated a nuclear localization signal-tagged LacZ transgenic (Tg) mouse, in which the LacZ gene was driven by the 129-bp downstream utrophin enhancer (DUE) and the 5.4-kb 5'-flanking region of the utrophin promoter.

METHODS

Two Tg lines were established. The levels of transgene mRNA expression in several tissues were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitative RT-PCR. Cryosections of several tissues were stained with haematoxylin and eosin and X-gal.

RESULTS

The transgene expression patterns were consistent with endogenous utrophin in several tissues including heart and skeletal muscle. Transgene expression was also up-regulated more in regenerating muscle than in nonregenerating muscle. Moreover, utrophin expression was augmented in the skeletal muscle of DUE Tg/dystrophin-deficient mdx mice through cross-breeding experiments. We finally established cultures of primary myogenic cells from this Tg mouse and found that utrophin up-regulation during muscle differentiation depends on the DUE motif.

CONCLUSIONS

Our results showed that DUE is indispensable for utrophin expression in skeletal muscle and heart, and primary myogenic cells from this Tg mice provide a high through-put screening system for drugs that up-regulate utrophin expression.

cis-acting sequences and trans-acting factors in the localization of mRNA for mitochondrial ribosomal proteins

mRNA localization is a conserved post-transcriptional process crucial for a variety of systems. Although several mechanisms have been identified, emerging evidence suggests that most transcripts reach the protein functional site by moving along cytoskeleton elements. We demonstrated previously that mRNA for mitochondrial ribosomal proteins are asymmetrically distributed in the cytoplasm, and that localization in the proximity of mitochondria is mediated by the 3'-UTR. Here we show by biochemical analysis that these mRNA transcripts are associated with the cytoskeleton through the microtubule network. Cytoskeleton association is functional for their intracellular localization near the mitochondrion, and the 3'-UTR is involved in this cytoskeleton-dependent localization. To identify the minimal elements required for localization, we generated DNA constructs containing, downstream from the GFP gene, deletion mutants of mitochondrial ribosomal protein S12 3'-UTR, and expressed them in HeLa cells. RT-PCR analysis showed that the localization signals responsible for mRNA localization are located in the first 154 nucleotides. RNA pull-down assays, mass spectrometry, and RNP immunoprecipitation assay experiments, demonstrated that mitochondrial ribosomal protein S12 3'-UTR interacts specifically with TRAP1 (tumor necrosis factor receptor-associated protein1), hnRNPM4 (heterogeneous nuclear ribonucleoprotein M4), Hsp70 and Hsp60 (heat shock proteins 70 and 60), and alpha-tubulin in vitro and in vivo.

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.

Conservative sequences in 3'UTR of TCRzeta mRNA regulate TCRzeta in SLE T cells

We have demonstrated that T-cell receptor zeta (zeta) mRNA with a 562-bp deleted alternatively spliced 3'-untranslated region (3'UTR) observed in T cells of patients with systemic lupus erythematosus (SLE) can lead to a reduction in zeta and TCR/CD3 (J. Immunol., 2003 & 2005). To determine the region in zeta mRNA 3'UTR for the regulation of zeta, zeta mRNA with 3'UTR truncations ligated into pDON-AI was used to infect murine T-cell hybridoma MA5.8 cells, which do not contain zeta. As a Western blot analysis demonstrated the importance of the regions from +871 to +950, containing conservative sequence 1 (CS1), and +1070 to +1136, containing CS2, for the production of zeta, we constructed MA5.8 mutants carrying zeta mRNA 3'UTR with deletions of these regions (DeltaCS1 and DeltaCS2 mutants). Western blot and FACS analyses showed significant reduction in the cell surface zeta and TCR/CD3 in both these mutants, and IL-2 production was decreased, compared with MA5.8 cells transfected with wild-type zeta mRNA. Furthermore, real-time PCR demonstrated the instability of zeta mRNA with 3'UTR deletions in these MA5.8 mutants. In conclusion, CS1 and CS2 may be responsible for the regulation of zeta and TCR/CD3 through the stability of zeta mRNA in SLE T cells.

MicroRNA-206 is overexpressed in the diaphragm but not the hindlimb muscle of mdx mouse

MicroRNAs are highly conserved, noncoding RNAs involved in posttranscriptional gene silencing. MicroRNAs have been shown to be involved in a range of biological processes, including myogenesis and muscle regeneration. The objective of this study was to test the hypothesis that microRNA expression is altered in dystrophic muscle, with the greatest change occurring, of the muscles examined, in the diaphragm. The expression of the muscle-enriched microRNAs was determined in the soleus, plantaris, and diaphragm muscles of control and dystrophin-deficient ( mdx) mice by semiquantitative PCR. In the soleus and plantaris, expression of the mature microRNA 133a (miR-133a) and miR-206, respectively, was decreased by ∼25%, whereas in the diaphragm, miR-206 expression increased by 4.5-fold relative to control. The increased expression of miR-206 in the mdx diaphragm was paralleled by a 4.4-fold increase in primary miRNA-206 (pri-miRNA-206) transcript level. Expression of Myod1 was elevated 2.7-fold only in the mdx diaphragm, consistent with an earlier finding demonstrating Myod1 can activate pri-miRNA-206 transcription. Transcript levels of Drosha and Dicer, major components of microRNA biogenesis pathway, were unchanged in mdx muscle, suggesting the pathway is not altered under dystrophic conditions. Previous in vitro analysis found miR-206 was capable of repressing utrophin expression; however, under dystrophic conditions, both utrophin transcript and protein levels were significantly increased by 69% and 3.9-fold, respectively, a finding inconsistent with microRNA regulation. These results are the first to report alterations in expression of muscle-enriched microRNAs in skeletal muscle of the mdx mouse, suggesting microRNAs may have a role in the pathophysiology of muscular dystrophy.

Viral-mediated gene therapy for the muscular dystrophies: successes, limitations and recent advances

Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.

Stability and translation of TCR zeta mRNA are regulated by the adenosine-uridine-rich elements in splice-deleted 3' untranslated region of zeta-chain

Systemic lupus erythematosus (SLE) T cells display reduced expression of TCR zeta protein. Recently, we reported that in SLE T cells, the residual TCR zeta protein is predominantly derived from an alternatively spliced form that undergoes splice deletion of 562 nt (from 672 to 1233 bases) within the 3' untranslated region (UTR) of TCR zeta mRNA. The stability and translation of the alternatively spliced form of TCR zeta mRNA are low compared with that of the wild-type TCR zeta mRNA. We report that two adenosine-uridine-rich sequence elements (AREs), defined by the splice-deleted 3' UTR region, but not an ARE located upstream are responsible for securing TCR zeta mRNA stability and translation. The stabilizing effect of the splice-deleted region-defined AREs extended to the luciferase mRNA and was not cell type-specific. The findings demonstrate distinct sequences within the splice-deleted region 672 to 1233 of the 3' UTR, which regulate the transcription, mRNA stability, and translation of TCR zeta mRNA. The absence of these sequences represents a molecular mechanism that contributes to altered TCR zeta-chain expression in lupus.

Annexin A2 recognises a specific region in the 3'-UTR of its cognate messenger RNA

Annexin A2 is a multifunctional Ca(2+)- and lipid-binding protein. We previously showed that a distinct pool of cellular Annexin A2 associates with mRNP complexes or polysomes associated with the cytoskeleton. Here we report in vitro and in vivo experiments showing that Annexin A2 present in this subset of mRNP complexes interacts with its cognate mRNA and c-myc mRNA, but not with beta(2)-microglobulin mRNA translated on membrane-bound polysomes. The protein recognises sequence elements within the untranslated regions, but not within the coding region, of its cognate mRNA. Alignment of the Annexin A2-binding 3'-untranslated regions of annexin A2 mRNA from several species reveals a five nucleotide consensus sequence 5'-AA(C/G)(A/U)G. The Annexin A2-interacting region of the 3'-untranslated region can be mapped to a sequence of about 100 nucleotides containing two repeats of the consensus sequence. The binding elements appear to involve both single and double stranded regions, indicating that a specific higher order mRNA structure is required for binding to Annexin A2. We suggest that this type of interaction is representative for a group of mRNAs translated on cytoskeleton-bound polysomes.

The 3'-untranslated region directs ribosomal protein-encoding mRNAs to specific cytoplasmic regions

mRNA localization is a conserved post-transcriptional process crucial for a variety of systems. We have analyzed the subcellular distribution of mRNAs encoding human cytosolic and mitochondrial ribosomal proteins. Biochemical fractionation experiments showed that the transcripts for cytosolic ribosomal proteins associate preferentially with the cytoskeleton via actin microfilaments. Transfection in HeLa cells of a GFP reporter construct containing the cytosolic ribosomal protein L4 3'-UTR showed that the 3'-UTR is necessary for the association of the transcript to the cytoskeleton. Using confocal analysis we demonstrate that the chimeric transcript is specifically associated with the perinuclear cytoskeleton. We also show that mRNA for mitochondrial ribosomal protein S12 is asymmetrically distributed in the cytoplasm. In fact, this transcript was localized mainly in the proximity of mitochondria, and the localization was 3'-UTR-dependent. In summary, ribosomal protein mRNAs constitute a new class of localized transcripts that share a common localization mechanism.

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.

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.

The utrophin promoter A drives high expression of the transgenic LacZ gene in liver, testis, colon, submandibular gland, and small intestine

BACKGROUND

Duchenne muscular dystrophy (DMD) is caused by the absence of the muscle cytoskeletal protein dystrophin. Utrophin is an autosomal homologue of dystrophin, and overexpression of the protein is expected to compensate for the defect of dystrophin. The utrophin gene has two promoters, A and B, and promoter A of the utrophin gene is a possible target of pharmacological interventions for DMD because A-utrophin is up-regulated in dystrophin-deficient mdx skeletal and cardiac muscles. To investigate the utrophin promoter A activity in vivo, we generated nuclear localization signal-tagged LacZ transgenic mice, where the LacZ gene was driven by the 5-kb flanking region of the A-utrophin gene.

METHODS

Four transgenic lines were established by mating four independent founders with C57BL/6J mice. The levels of mRNA for beta-galactosidase in several tissues were examined by RT-PCR. Cryosections from several tissues were stained with hematoxylin and eosin (H&E) and with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal).

RESULTS

The 5-kb upstream region of the A-utrophin gene showed high transcriptional activity in liver, testis, colon, submandibular gland, and small intestine, consistent with the endogenous expression of utrophin protein. Surprisingly, the levels of both beta-gal protein and mRNA for the transgene in cardiac and skeletal muscles were extremely low, even in nuclei near the neuromuscular junctions. These results indicate that the regulation of the utrophin gene in striated muscle is different from that in non-muscle tissues.

CONCLUSIONS

Our results clearly showed that the utrophin A promoter is not sufficient to drive expression in muscle, but other regulatory elements are required.

The RNA-binding protein HuR binds to acetylcholinesterase transcripts and regulates their expression in differentiating skeletal muscle cells

During myogenic differentiation, acetylcholinesterase (AChE) transcript levels are known to increase dramatically. Although this increase can be attributed in part to increased transcriptional activity, posttranscriptional mechanisms have also been implicated in the high levels of AChE mRNA in myotubes. In this study, we observed that transfection of a luciferase reporter construct containing the full-length AChE 3'-untranslated region (UTR) resulted in significantly higher (5-fold) luciferase activity in differentiated myotubes versus myoblasts. RNA-electrophoretic mobility shift assays (REMSAs) performed with a full-length AChE 3'-UTR probe and the AU-rich element revealed that the intensity of RNA-binding protein complexes increased as myogenic differentiation proceeded. Using several complementary approaches including supershift REMSA, mRNA-binding protein pull-down assays, and immunoprecipitation followed by reverse transcription-PCR, we found that the mRNA-stabilizing protein HuR interacts directly with AChE transcripts. Stable overexpression of HuR in C2C12 cells increased the expression of endogenous AChE transcripts as well as that of the luciferase reporter construct containing the AChE 3'-UTR. In vitro stability assays performed with protein extracts from these cells versus controls resulted in a slower rate of AChE mRNA decay. The down-regulation of HuR expression mediated through small interfering RNA further confirmed the role of HuR in the regulation of AChE mRNA levels. Taken together, these studies demonstrate that HuR interacts with the AChE 3'-UTR to regulate posttranscriptionally the expression of AChE mRNA during myogenic differentiation.

Factors Associated with Induced Chronic Inflammation in mdx Skeletal Muscle Cause Posttranslational Stabilization and Augmentation of Extrasynaptic Sarcolemmal Utrophin

Chronic inflammation in tibialis anterior muscles of mdx mice was produced by a single injection of a recombinant adenovirus vector (AV) expressing an immunogenic beta-galactosidase (beta-gal). In regions of intense beta-gal staining, mononuclear infiltrates abounded, and muscle fibers showed strong extrasynaptic utrophin immunostaining, restoration of dystrophin-associated protein complex, and a marked reduction of the prevalence of centronucleation. Immunoblot analysis confirmed an increase of endogenous utrophin without an increase of the mRNA of the major muscle isoform utrA. Significantly better maximal tetanic force values were demonstrated in the inflammatory versus control mdx muscles. The resistance to lengthening contraction- induced damage was also significantly increased in the former. In muscles of mice lacking TNF-alpha gene, AV vector did not induce inflammation and extrajunctional utrophin increase did not occur. In the inflammatory mdx muscles, proteolytic activity of calcium-activated calpain was reduced, and in mdx myotubes in vitro, incubation with NO donors also reduced calpain-mediated utrophin proteolysis. Since utrophin was shown to be a natural substrate of calpain and known inhibitors of calpain in cultured mdx myotubes increased utrophin levels, the above results were consistent with the following conclusions: (1) extrasynaptic utrophin increase is mainly responsible for the antidystrophic effect; (2) extrasynaptic utrophin increase is a result of posttranscriptional mechanism(s) related to proinflammatory factors; and (3) reduction of endogenous muscle calpain activity by inflammatory cytokines has an important role in the stabilization and increase of the extrasynaptic utrophin.

Evolution of pathological changes in the gastrocnemius of the mdx mice correlate with utrophin and beta-dystroglycan expression

Utrophin can function in muscle as a substitute for dystrophin and its over-expression has been used successfully to ameliorate mdx muscle pathology. Despite of this fact, there are no detailed studies on the expression of endogenous skeletal muscle utrophin- and dystrophin-associated glycoproteins throughout the life span of mdx mice. We have monitored, sequentially, the expression of matrix metalloproteinase-9 (MMP-9), myosin heavy chain, utrophin and beta-dystroglycan, as well as the mRNA expression of utrophin and of structurally related proteins, in mdx and control mice. We found an inverse relationship between concentration of muscle utrophin and abundance of groups of degenerative-regenerative fibers and of MMP-9 expression. There was also temporal correlation between the decline of utrophin at 15 days of age and the onset of muscle necrosis. Conversely, reappearance of utrophin, with a peak around 2 months of age, was followed by a progressive decline of necrosis. A lineal correlation between utrophin and beta-dystroglycan levels, not seen in controls, indicates that improvement of mdx is due to utrophin binding to dystrophin-associated glycoproteins. Utrophin and other structurally related protein transcripts were not up-regulated, suggesting a post-transcriptional regulation for utrophin in skeletal muscle.

Insulin-like growth factor I plasmid therapy promotes in vivo angiogenesis

Angiogenesis, the formation of neovessels from the endothelium of preexisting vessels, is stimulated by soluble angiogenic factors. Insulin-like growth factor I (IGF-I) stimulates myogenesis and induces nerve regeneration after injury, and it has been shown to stimulate angiogenesis. However, the in vivo angiogenic effects of IGF-I in regenerating and diabetic muscle have yet to be described. Therefore, we studied the effects of human IGF-I (hIGF-I) delivered by a plasmid-mediated therapy on angiogenesis in mouse models of these two conditions. Plasmid hIGF-I was delivered to the injured tibialis muscle by direct intramuscular injection followed by electroporation. Initial experiments compared two muscle-specific hIGF-I-expressing constructs containing either a skeletal actin 3'UTR (pAV2001) or a human growth hormone (GH) 3'UTR (pAV2002). Skeletal actin 3'UTR mediates sequestration of hIGF-I in the muscle and was more active, while the GH 3'UTR mediated release of IGF-I into the circulation. Treatment of regenerating muscle with pAV2001 and sequestration of IGF-I in muscle led to increased expression of vascular endothelial growth factor (VEGF) and VEGF receptors fetal liver kinase-1 and FmS-like tyrosine kinase receptor-1, as well as platelet endothelial cell adhesion molecule-1, on endothelial cells. These results indicate that IGF-I can amplify angiogenic responses in regenerating muscle. In a mouse diabetic model, plasmid-mediated IGF-I therapy reversed diabetic microangiopathy, as shown by increased angiogenesis and arterial flow as analyzed by Doppler imaging. These studies show that plasmid IGF-I delivery and sequestration in muscle can augment angiogenesis in regenerating muscle and increase blood flow and angiogenesis in the diabetic limb.

AU-Rich Elements in the Collagenase 3 mRNA Mediate Stabilization of the Transcript by Cortisol in Osteoblasts

Collagenase 3 degrades collagen fibrils and is necessary for bone resorption. Cortisol increases collagenase 3 mRNA in osteoblasts by stabilizing collagenase 3 transcripts. To understand mechanisms involved, we used RNA electrophoretic mobility shift assay and RNA turnover studies. Cortisol increased the binding of Ob cell cytosolic extracts to AU-rich sequences in the collagenase 3 3'-untranslated region (UTR). No cortisol-dependent protein complexes were formed with the coding region or the 5'-UTR. Functional assays, using transient transfections of CMV-driven c-fos collagenase 3'-UTR chimeric constructs, demonstrated that the 3'-UTR of collagenase 3 stabilizes c-fos mRNA in transcriptionally arrested Ob cells, cortisol prolongs the transcript half-life, and mutations of AU-rich sequences destabilize c-fos transcripts precluding the cortisol effect. Purification of osteoblast cytosolic extracts by ultracentrifugation, ion exchange, and RNA affinity chromatography, and polyacrylamide gel electrophoresis followed by mass spectroscopy identified specific proteins. RNA gel mobility supershift assays demonstrated that vinculin and far upstream element (FUSE)-binding protein 2 interacted with collagenase 3 3'-UTR sequences, and RNA interference demonstrated these proteins altered collagenase mRNA stability. In conclusion, AU-rich sequences of the 3'-UTR of collagenase 3 and vinculin and FUSE-binding protein 2 regulate collagenase mRNA stability in osteoblasts.

The Arabidopsis cytoskeletal genome

In the past decade the first Arabidopsis genes encoding cytoskeletal proteins were identified. A few dozen genes in the actin and tubulin cytoskeletal systems have been characterized thoroughly, including gene families encoding actins, profilins, actin depolymerizing factors, α-tubulins, and β-tubulins. Conventional molecular genetics have shown these family members to be differentially expressed at the temporal and spatial levels with an ancient split separating those genes expressed in vegetative tissues from those expressed in reproductive tissues. A few members of other cytoskeletal gene families have also been partially characterized, including an actin-related protein, annexins, fimbrins, kinesins, myosins, and villins. In the year 2001 the Arabidopsis genome sequence was completed. Based on sequence homology with well-characterized animal, fungal, and protist sequences, we find candidate cytoskeletal genes in the Arabidopsis database: more than 150 actin-binding proteins (ABPs), including monomer binding, capping, cross-linking, attachment, and motor proteins; more than 200 microtubule-associated proteins (MAPs); and, surprisingly, 10 to 40 potential intermediate filament (IF) proteins. Most of these sequences are uncharacterized and were not identified as related to cytoskeletal proteins. Several Arabidopsis ABPs, MAPs, and IF proteins are represented by individual genes and most were represented as as small gene families. However, several classes of cytoskeletal genes including myosin, eEF1α, CLIP, tea1, and kinesin are part of large gene families with 20 to 70 potential gene members each. This treasure trove of data provides an unprecedented opportunity to make rapid advances in understanding the complex plant cytoskeletal proteome. However, the functional analysis of these proposed cytoskeletal proteins and their mutants will require detailed analysis at the cell biological, molecular genetic, and biochemical levels. New approaches will be needed to move more efficiently and rapidly from this mass of DNA sequence to functional studies on cytoskeletal proteins.

TCR zeta mRNA with an alternatively spliced 3'-untranslated region detected in systemic lupus erythematosus patients leads to the down-regulation of TCR zeta and TCR/CD3 complex

The reduction or absence of TCR zeta-chain (zeta) expression in systemic lupus erythematosus (SLE) patients is thought to be related to the pathogenesis of SLE. Recently, we reported the predominant expression of zeta mRNA containing an alternatively spliced 3'-untranslated region (3'UTR; zetamRNA/as-3'UTR) and a reduction in the expression of zeta mRNA containing the wild-type 3'UTR (zetamRNA/w-3'UTR) in T cells from SLE patients. Here we show that AS3'UTR mutants (MA5.8 cells deficient in zeta protein that have been transfected with zetamRNA/as-3'UTR) exhibit a reduction in the expression of TCR/CD3 complex and zeta protein on their cell surface as well as a reduction in the production of IL-2 after stimulation with anti-CD3 Ab compared with that in wild-type 3'UTR mutants (MA5.8 cells transfected with zetamRNA/w-3'UTR). Furthermore, the real-time PCR analyses demonstrated that the half-life of zetamRNA/as-3'UTR in AS3'UTR mutants (3 h) was much shorter than that of zetamRNA/w-3'UTR in wild-type 3'UTR mutants (15 h). Thus, the lower stability of zetamRNA/as-3'UTR, which is predominant in SLE T cells, may be responsible for the reduced expression of the TCR/CD3 complex, including zeta protein, in SLE T cells.

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.

Transcriptional activation of the utrophin promoter B by a constitutively active Ets-transcription factor

Duchenne muscular dystrophy is an X-linked genetic disease caused by the absence of functional dystrophin. Pharmacological upregulation of utrophin, the autosomal homologue of dystrophin, offers a potential therapeutic approach to treat Duchenne patients. Full-length utrophin mRNA is transcribed from two alternative promoters, called A and B. In contrast to the utrophin promoter A, little is known about the factors regulating the activity of the utrophin promoter B. Computer analysis of this second promoter revealed the presence of several conserved binding motives for Ets-transcription factors. Using electrotransfer of cDNA into mouse muscles, we demonstrate that a genetically modified beta-subunit of the Ets-transcription factor GA-binding protein potently activates a utrophin promoter B reporter construct in innervated muscle fibers in vivo. These results make the GA-binding protein and the signaling cascade regulating its activity in muscle cells, potential targets for the pharmacological modulation of utrophin expression in Duchenne patients.

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

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

Glucocorticoid-mediated regulation of utrophin levels in human muscle fibers

Previous studies on transgenic mice indicate that upregulation of utrophin protein may offer a potential treatment strategy for Duchenne muscular dystrophy. We have analyzed the effect of the glucocorticoid 6alpha-methylprednisolone-21 sodium succinate on utrophin protein levels, using a cell-based assay with differentiated human myotubes, derived from biopsies of healthy individuals or Duchenne muscular dystrophy patients. We found that within 5-7 days 6alpha-methylprednisolone-21 sodium succinate increases utrophin protein up to approximately 40% in both normal and dystrophin-deficient myotubes compared to untreated control cultures. When analyzed in promoter-reporter assays 6alpha-methylprednisolone-21 sodium succinate activated a utrophin promoter A-fragment but did not activate a utrophin promoter B-fragment. Surprisingly, endogenous levels of utrophin mRNA in 6alpha-methylprednisolone-21 sodium succinate-treated muscle cells were unaltered indicating that the utrophin-inducing effect of glucocorticoids may be a result of post-transcriptional mechanisms. We have also analyzed 66 glucocorticoids for their effect on utrophin protein levels and found that glucocorticoids in general are able to induce utrophin protein in human myotubes.

Targeted trafficking of neurotransmitter receptors to synaptic sites

Emerging data are sheding light on the critical task for synapses to locally control the production of neurotransmitter receptors ultimately leading to receptor accumulation and modulation at postsynaptic sites. By analogy with the epithelial-cell paradigm, the postsynaptic compartment may be regarded as a polarized domain favoring the selective recruitment and retention of newly delivered receptors at synaptic sites. Targeted delivery of receptors to synaptic sites is facilitated by a local organization of the exocytic pathway, likely resulting from spatial cues triggered by the nerve. This review focuses on the various mechanisms responsible for regulation of receptor assembly and trafficking. A particular emphasis is given to the role of synaptic anchoring and scaffolding proteins in the sorting and routing of their receptor companion along the exocytic pathway. Other cellular components such as lipidic microdomains, the docking and fusion machinery, and the cytoskeleton also contribute to the dynamics of receptor trafficking at the synapse.

The 3'-untranslated region of the dystrophin gene - conservation and consequences of loss

The 3'-untranslated region (3'UTR) of some vertebrate dystrophin genes shows an extraordinary degree and extent of conservation (better than that of many coding regions), a phenomenon that remains unexplained. We examine novel sequence and mutational data to explore the possible reasons for this. We show that loss of the human dystrophin 3'UTR is sufficient to cause Becker muscular dystrophy with pronounced reduction in dystrophin protein levels. The acquisition of dystrophin 3'UTR sequence from an amphibian and a cartilaginous fish allows us to refine previously identified functionally constrained regions which might account for the observed phenotype. These comprise (a) the open reading frame encoding the ancestral 'alternative' amphipathic C-terminal alpha-helix, normally removed from adult dystrophin by inclusion of a poorly conserved frameshifting penultimate exon, and (b) two highly conserved untranslated regions ('Lemaire A', 350 nucleotides and 'Lemaire D', 250 nucleotides) separated by a non-conserved 700-2000-nucleotide spacer. We consider the possibility that the 3'UTR may represent a significant target for pathogenic mutations.

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.

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.