Jonathon M. Tinsley

Utrophin-Dystrophin-Deficient Mice as a Model for Duchenne Muscular Dystrophy

The absence of dystrophin at the muscle membrane leads to Duchenne muscular dystrophy (DMD), a severe muscle-wasting disease that is inevitably fatal in early adulthood. In contrast, dystrophin-deficient mdx mice appear physically normal despite their underlying muscle pathology. We describe mice deficient for both dystrophin and the dystrophin-related protein utrophin. These mice show many signs typical of DMD in humans: they show severe progressive muscular dystrophy that results in premature death, they have ultrastructural neuromuscular and myotendinous junction abnormalities, and they aberrantly coexpress myosin heavy chain isoforms within a fiber. The data suggest that utrophin and dystrophin have complementing roles in normal functional or developmental pathways in muscle. Detailed study of these mice should provide novel insights into the pathogenesis of DMD and provide an improved model for rapid evaluation of gene therapy strategies.

Expression of full-length utrophin prevents muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is a lethal, progressive muscle wasting disease caused by a loss of sarcolemmal bound dystrophin, which results in the death of the muscle fiber leading to the gradual depletion of skeletal muscle. The molecular structure of dystrophin is very similar to that of the related protein utrophin. Utrophin is found in all tissues and is confined to the neuromuscular and myotendinous junctions in mature muscle. Sarcolemmal localization of a truncated utrophin transgene in the dystrophin-deficient mdx mouse significantly improves the dystrophic muscle phenotype. Therefore, upregulation of utrophin by drug therapy is a plausible therapeutic approach in the treatment of DMD. Here we demonstrate that expression of full-length utrophin in mdx mice prevents the development of muscular dystrophy. We assessed muscle morphology, fiber regeneration and mechanical properties (force development and resistance to stretch) of mdx and transgenic mdx skeletal and diaphragm muscle. The utrophin levels required in muscle are significantly less than the normal endogenous utrophin levels seen in lung and kidney, and we provide evidence that the pathology depends on the amount of utrophin expression. These results also have important implications for DMD therapies in which utrophin replacement is achieved by delivery using exogenous vectors.

Utrophin: A Structural and Functional Comparison to Dystrophin

Utrophin is an autosomally‐encoded homologue of dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene. Although, Utrophin is very similar in sequence to dystrophin and possesses many of the protein‐binding properties ascribed to dystrophin, both proteins are expressed in an apparently reciprocal manner and may be coordinately regulated. In normal skeletal muscle, Utrophin is found at the neuromuscular junction (NMJ) whereas dystrophin predominates at the sarcolemma. However, during development, and in some myopathies including DMD, utrophin is also found at the sarcolemma. This re‐distribution is often associated with a significant increase in the levels of utrophin. At the NMJ utrophin co‐localizes with the acetylcholine receptors (AChR) and may play a role in the stabilization of the synaptic cytoskeleton. Because utrophin and dystrophin are so similar, utrophin may be able to replace dystrophin in dystrophin deficient muscle. This review compares the structure and function of utrophin to dystrophin and discusses the rationale behind the use of utrophin as a potential therapeutic agent.

Daily Treatment with SMTC1100, a Novel Small Molecule Utrophin Upregulator, Dramatically Reduces the Dystrophic Symptoms in the mdx Mouse

Background Duchenne muscular dystrophy (DMD) is a lethal, progressive muscle wasting disease caused by a loss of sarcolemmal bound dystrophin, which results in the death of the muscle fibers leading to the gradual depletion of skeletal muscle. There is significant evidence demonstrating that increasing levels of the dystrophin-related protein, utrophin, in mouse models results in sarcolemmal bound utrophin and prevents the muscular dystrophy pathology. The aim of this work was to develop a small molecule which increases the levels of utrophin in muscle and thus has therapeutic potential. Methodology and Principal Findings We describe the in vivo activity of SMT C1100; the first orally bioavailable small molecule utrophin upregulator. Once-a-day daily-dosing with SMT C1100 reduces a number of the pathological effects of dystrophin deficiency. Treatment results in reduced pathology, better muscle physiology leading to an increase in overall strength, and an ability to resist fatigue after forced exercise; a surrogate for the six minute walk test currently recommended as the pivotal outcome measure in human trials for DMD. Conclusions and Significance This study demonstrates proof-of-principle for the use of in vitro screening methods in allowing identification of pharmacological agents for utrophin transcriptional upregulation. The best compound identified, SMT C1100, demonstrated significant disease modifying effects in DMD models. Our data warrant the full evaluation of this compound in clinical trials in DMD patients.

Adenovirus-Mediated Utrophin Gene Transfer Mitigates the Dystrophic Phenotype of mdx Mouse Muscles

Utrophin is a close homolog of dystrophin, the protein whose mutations cause Duchenne muscular dystrophy (DMD). Utrophin is present at low levels in normal and dystrophic muscle, whereas dystrophin is largely absent in DMD. In such cases, the replacement of dystrophin using a utrophin gene transfer strategy could be more advantageous because utrophin would not be a neoantigen. To establish if adenovirus (AV)-mediated utrophin gene transfer is a possible option for the treatment of DMD, an AV vector expressing a shortened version of utrophin (AdCMV-Utr) was constructed. The effect of utrophin overexpression was investigated following intramuscular injection of this AV into mdx mice, the mouse model of DMD. When the tibialis anterior (TA) muscles of 3- to 5-day-old animals were injected with 5 microl of AdCMV-Utr (7.0 x 10(11) virus/ml), an average of 32% of fibers were transduced and the transduction level remained stable for at least 60 days. The presence of utrophin restored the normal histochemical pattern of the dystrophin-associated protein complex at the cell surface and resulted in a reduction in the number of centrally nucleated fibers. The transduced fibers were largely impermeable to the tracer dye Evans blue, suggesting that utrophin protects the surface membrane from breakage. In vitro measurements of the force decline in response to high-stress eccentric contractions demonstrated that the muscles overexpressing utrophin were more resistant to mechanical stress-induced injury. Taken together, these data indicate that AV-mediated utrophin gene transfer can correct various aspects of the dystrophic phenotype. However, a progressive reduction in the number of transduced fibers was observed when the TA muscles of 30- to 45-day-old mice were injected with 25 microl of AdCMV-Utr. This reduction coincides with a humoral response to the AV and transgene, which consists of a hybrid mouse-human cDNA.

Muscle and Neural Isoforms of Agrin Increase Utrophin Expression in Cultured Myotubes via a Transcriptional Regulatory Mechanism

Duchenne muscular dystrophy is a prevalent X-linked neuromuscular disease for which there is currently no cure. Recently, it was demonstrated in a transgenic mouse model that utrophin could functionally compensate for the lack of dystrophin and alleviate the muscle pathology (Tinsley, J. M., Potter, A. C., Phelps, S. R., Fisher, R., Trickett, J. I., and Davies, K. E. (1996) Nature 384, 349–353). In this context, it thus becomes essential to determine the cellular and molecular mechanisms presiding over utrophin expression in attempts to overexpress the endogenous gene product throughout skeletal muscle fibers. In a recent study, we showed that the nerve exerts a profound influence on utrophin gene expression and postulated that nerve-derived trophic factors mediate the local transcriptional activation of the utrophin gene within nuclei located in the postsynaptic sarcoplasm (Gramolini, A. O., Dennis, C. L., Tinsley, J. M., Robertson, G. S., Davies, K. E, Cartaud, J., and Jasmin, B. J. (1997)J. Biol. Chem. 272, 8117–8120). In the present study, we have therefore focused on the effect of agrin on utrophin expression in cultured C2 myotubes. In response to Torpedo-, muscle-, or nerve-derived agrin, we observed a significant 2-fold increase in utrophin mRNAs. By contrast, CGRP treatment failed to affect expression of utrophin transcripts. Western blotting experiments also revealed that the increase in utrophin mRNAs was accompanied by an increase in the levels of utrophin. To determine whether these changes were caused by parallel increases in the transcriptional activity of the utrophin gene, we transfected muscle cells with a 1.3-kilobase pair utrophin promoter-reporter (nlsLacZ) gene construct and treated them with agrin for 24–48 h. Under these conditions, both muscle- and nerve-derived agrin increased the activity of β-galactosidase, indicating that agrin treatment led, directly or indirectly, to the transcriptional activation of the utrophin gene. Furthermore, this increase in transcriptional activity in response to agrin resulted from a greater number of myonuclei expressing the 1.3-kilobase pair utrophin promoter-nlsLacZ construct. Deletion of 800 base pairs 5′ from this fragment decreased the basal levels of nlsLacZ expression and abolished the sensitivity of the utrophin promoter to exogenously applied agrin. In addition, site-directed mutagenesis of an N-box motif contained within this 800-base pair fragment demonstrated its essential contribution in this regulatory mechanism. Finally, direct gene transfer studies performed in vivo further revealed the importance of this DNA element for the synapse-specific expression of the utrophin gene along multinucleated muscle fibers. These data show that both muscle and neural isoforms of agrin can regulate expression of the utrophin gene and further indicate that agrin is not only involved in the mechanisms leading to the formation of clusters containing presynthesized synaptic molecules but that it can also participate in the local regulation of genes encoding synaptic proteins. Together, these observations are therefore relevant for our basic understanding of the events involved in the assembly and maintenance of the postsynaptic membrane domain of the neuromuscular junction and for the potential use of utrophin as a therapeutic strategy to counteract the effects of Duchenne muscular dystrophy.

COILED-COIL REGIONS IN THE CARBOXY-TERMINAL DOMAINS OF DYSTROPHIN AND RELATED PROTEINS : POTENTIALS FOR PROTEIN-PROTEIN INTERACTIONS

Dystrophin, the protein product of the DMD gene, is a component of the muscle-membrane cytoskeleton. Mutations in the DMD gene result in the allelic myopathies Duchenne and Becket muscular dystrophy (DMD and BMD). The majority of mutations causing DMD and BMD are large intragenic deletions. The spectrum of these deletions can be used to explain the differences in the clinical severity between DMD and BMD patients. In general, mutations causing BMD, the milder disorder, maintain the translational reading frame of dystrophin and result in near normal amounts of an internally truncated protein. By contrast, deletions causing DMD disrupt the reading frame of dystrophin, often resulting in premature translational termination. Thus, DMD patients often produce little or no detectable dystrophin.

Utrophin: A potential replacement for dystrophin?

This paper reviews the evidence that utrophin, the autosomally encoded protein related to dystrophin, may be capable of performing the same cellular functions as dystrophin. If this is the case, it may be possible to modify the regulation of utrophin expression as an alternative route to dystrophin gene therapy for sufferers of DMD and/or BMD.

Local transcriptional control of utrophin expression at the neuromuscular synapse.

Recently, the use of a transgenic mouse model system for Duchenne muscular dystrophy has demonstrated the ability of utrophin to functionally replace dystrophin and alleviate the muscle pathology (see Tinsley, J. M., Potter, A. C., Phelps, S. R., Fisher, R., Trickett, J. I., and Davies, K. E. (1996) Nature 384, 349–353). However, there is currently a clear lack of information concerning the regulatory mechanisms presiding over utrophin expression during normal myogenesis and synaptogenesis. Using in situ hybridization, we show that utrophin mRNAs selectively accumulate within the postsynaptic sarcoplasm of adult muscle fibers. In addition, we demonstrate that a 1.3-kilobase fragment of the human utrophin promoter is sufficient to confer synapse-specific expression to a reporter gene. Deletion of 800 base pairs from this promoter fragment reduces the overall expression of the reporter gene and abolishes its synapse-specific expression. Finally, we also show that utrophin is present at the postsynaptic membrane of ectopic synapses induced to form at sites distant from the original neuromuscular junctions. Taken together, these results indicate that nerve-derived factors regulate locally the transcriptional activation of the utrophin gene in skeletal muscle fibers and that myonuclei located in extrasynaptic regions are capable of expressing utrophin upon receiving appropriate neuronal cues.

Dystrophin and dystrophin-related proteins: a review of protein and RNA studies

The analysis of dystrophin gene expression has led to the identification of multiple transcripts and varying isoforms. The data indicate that transcription of the dystrophin gene occurs from several promoters, which involves developmental and tissue-dependent regulation. These discoveries have complicated the interpretation of immunolocalization studies, although there is a strong correlation between the amount and size of dystrophin and the severity of the clinical phenotype. The importance of using protein-specific antibodies for dystrophin analysis has been underscored by the identification of a protein, designated utrophin, which exhibits significant sequence homology with dystrophin. This review addresses the recent studies of dystrophin and utrophin expression in an attempt to illustrate the transcriptional diversity of these large genes and the localization of their protein products within various tissues.