Local transcriptional control of utrophin expression at the neuromuscular synapse

Synapse-specific and neuregulin-induced transcription require an ets site that binds GABPalpha/GABPbeta

Localization of acetylcholine receptors (AChRs) to neuromuscular synapses is mediated by multiple pathways. Agrin, which is the signal for one pathway, stimulates a redistribution of previously unlocalized AChRs to synaptic sites. The signal for a second pathway is not known, but this signal stimulates selective transcription of AChR genes in myofiber nuclei located near the synaptic site. Neuregulin (NRG) is a good candidate for the extracellular signal that induces synapse-specific gene expression, since NRG is concentrated at synaptic sites and activates AChR gene expression in cultured muscle cells. Previous studies have demonstrated that 181 bp of 5' flanking DNA from the AChR delta-subunit gene are sufficient to confer synapse-specific transcription in transgenic mice and NRG responsiveness in cultured muscle cells, but the critical sequences within this cis-acting regulatory region have not been identified. We transfected AChR delta-subunit-hGH gene fusions into a muscle cell line, and we show that a potential binding site for Ets proteins is required for NRG-induced gene expression. Furthermore, we produced transgenic mice carrying AChR delta-subunit-hGH gene fusions with a mutation in this NRG-response element (NRE), and we show that this NRE is necessary for synapse-specific transcription in mice. The NRE binds proteins in myotube nuclear extracts, and nucleotides that are important for NRG responsiveness are likewise critical for formation of the protein-DNA complex. This complex contains GABPalpha, an Ets protein, and GABPbeta, a protein that lacks an Ets domain but dimerizes with GABPalpha, because formation of the protein-DNA complex is inhibited by antibodies to either GABPalpha or GABPbeta. These results demonstrate that synapse-specific and NRG-induced gene expression require an Ets-binding site and suggest that GABPalpha/GABPbeta mediates the transcriptional response of the AChR delta-subunit gene to synaptic signals, including NRG.

Molecular mechanisms and putative signalling events controlling utrophin expression in mammalian skeletal muscle fibres

The absence of full-length dystrophin molecules in skeletal muscle fibres results in the most severe form of muscular dystrophy, the Duchenne form (DMD). Several years ago, an autosomal homologue to dystrophin, termed utrophin, was identified. Although utrophin is expressed along the sarcolemma in developing, regenerating and DMD muscles, it nonetheless accumulates at the postsynaptic membrane of the neuromuscular junction in both normal and DMD adult muscle fibres. Due to the high degree of sequence identity between dystrophin and utrophin, it has been previously suggested that utrophin could in fact functionally compensate for the lack of dystrophin. Recent studies using transgenic mouse model systems have directly tested this hypothesis and revealed that upregulation of utrophin throughout dystrophic muscle fibres represents indeed, a viable approach for the treatment of DMD. Current studies are therefore focusing on the elucidation of the various regulatory mechanisms presiding over expression of utrophin in muscle fibres in attempts to ultimately identify small molecules which could systematically increase utrophin levels in extrasynaptic compartments of dystrophic muscle fibres. This review presents some of the recent data relevant for our understanding of the transcriptional regulatory mechanisms involved in maintaining expression of utrophin at the neuromuscular junction. In addition, the contribution of specific cues originating from motoneurons and the putative involvement of signalling events are also discussed.

The sparing of extraocular muscle in dystrophinopathy is lost in mice lacking utrophin and dystrophin

The extraocular muscles are one of few skeletal muscles that are structurally and functionally intact in Duchenne muscular dystrophy. Little is known about the mechanisms responsible for differential sparing or targeting of muscle groups in neuromuscular disease. One hypothesis is that constitutive or adaptive properties of the unique extraocular muscle phenotype may underlie their protection in dystrophinopathy. We assessed the status of extraocular muscles in the mdx mouse model of muscular dystrophy. Mice showed mild pathology in accessory extraocular muscles, but no signs of pathology were evident in the principal extraocular muscles at any age. By immunoblotting, the extraocular muscles of mdx mice exhibited increased levels of a dystrophin analog, dystrophin-related protein or utrophin. These data suggest, but do not provide mechanistic evidence, that utrophin mediates eye muscle protection. To examine a potential causal relationship, knockout mouse models were used to determine whether eye muscle sparing could be reversed. Mice lacking expression of utrophin alone, like the dystrophin-deficient mdx mouse, showed no pathological alterations in extraocular muscle. However, mice deficient in both utrophin and dystrophin exhibited severe changes in both the accessory and principal extraocular muscles, with the eye muscles affected more adversely than other skeletal muscles. Selected extraocular muscle fiber types still remained spared, suggesting the operation of an alternative mechanism for muscle sparing in these fiber types. We propose that an endogenous upregulation of utrophin is mechanistic in protecting extraocular muscle in dystrophinopathy. Moreover, data lend support to the hypothesis that interventions designed to increase utrophin levels may ameliorate the pathology in other skeletal muscles in Duchenne muscular dystrophy.

Utrophin mRNA expression in muscle is not restricted to the neuromuscular junction

Utrophin is normally present exclusively in synaptic regions of skeletal muscle fibers, although it is expressed extrasynaptically in certain pathological situations, where it has been proposed to compensate for the absence of dystrophin in Duchenne muscular dystrophy patients and mdx mice. Recently there have been conflicting reports regarding the preferential expression of utrophin mRNA at the neuromuscular junction. Using in situ hybridization with RNA probes, we show a clear accumulation of autoradiographic labeling at more than 90% of neuromuscular junctions (identified by histochemical demonstration of cholinesterase activity). The intensity of this labeling is proportional to the number of junctional myonuclei in the section. Some clusters of labeling were found associated with nonmuscle nuclei (e.g., blood vessels, nerves), where utrophin is present. In addition, labeling for utrophin mRNA was associated with about 25% of extrajunctional myonuclei, where the protein is not present. The mean labeling per nucleus at junctional myonuclei was at least 10 times greater than at extrajunctional myonuclei. We discuss the possible regulatory mechanisms involved in the heterogeneous expression of utrophin mRNA in skeletal muscle.

beta-Spectrin is colocalized with both voltage-gated sodium channels and ankyrinG at the adult rat neuromuscular junction

Voltage-gated sodium channels (VGSCs) are concentrated in the depths of the postsynaptic folds at mammalian neuromuscular junctions (NMJs) where they facilitate action potential generation during neuromuscular transmission. At the nodes of Ranvier and the axon hillocks of central neurons, VGSCs are associated with the cytoskeletal proteins, beta-spectrin and ankyrin, which may help to maintain the high local density of VGSCs. Here we show in skeletal muscle, using immunofluorescence, that beta-spectrin is precisely colocalized with both VGSCs and ankyrinG, the nodal isoform of ankyrin. In en face views of rat NMJs, acetylcholine receptors (AChRs), and utrophin immunolabeling are organized in distinctive linear arrays corresponding to the crests of the postsynaptic folds. In contrast, beta-spectrin, VGSCs, and ankyrinG have a punctate distribution that extends laterally beyond the AChRs, consistent with a localization in the depths of the folds. Double antibody labeling shows that beta-spectrin is precisely colocalized with both VGSCs and ankyrinG at the NMJ. Furthermore, quantification of immunofluorescence in labeled transverse sections reveals that beta-spectrin is also concentrated in perijunctional regions, in parallel with an increase in labeling of VGSCs and ankyrinG, but not of dystrophin. These observations suggest that interactions with beta-spectrin and ankyrinG help to maintain the concentration of VGSCs at the NMJ and that a common mechanism exists throughout the nervous system for clustering VGSCs at a high density.

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 beta-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.

An improved method for the simultaneous demonstration of mRNA and esterase activity at the human neuromuscular junction

The aim of this study was to develop a simple means of studying the distribution of mRNA coding for post-synaptic proteins at the human neuromuscular junction. A reliable method by which to identify the junctions in tissue sections after in situ hybridization was essential. A method is described for combining the histochemical demonstration of esterase activity at the neuromuscular junction with autoradiographic localization of mRNA by in situ hybridization in the same cryostat section of skeletal muscle. The indigogenic esterase method of Strum and Hall-Craggs (1982) was modified in such a way that it is able to survive the multiple steps involved in in situ hybridization and autoradiography. The protocol is simple and reproducible and has been used successfully on sections of both rat and human skeletal muscle. To demonstrate the method, sections were reacted to reveal esterase activity and were then processed for in situ hybridization using a 35S-labelled probe specific for the epsilon-subunit of the acetylcholine receptor. The reaction product was retained after the lengthy in situ hybridization and autoradiographic procedures. To our knowledge, this is the first demonstration of acetylcholine receptor mRNA by in situ hybridization at human neuromuscular junctions.

The membrane-cytoskeleton interface: the role of dystrophin and utrophin

Recent studies with transgenic animals have considerably advanced our knowledge of the roles of dystrophin and utrophin in both muscle and non-muscle tissues. Rigorous analyses of the roles of the various mdx mutations in mice, as well as the use of artificial transgenes in an mdx background, are beginning to define the functional importance of various regions of the dystrophin protein in normal muscle. Furthermore, recent biochemical analyses have revealed new insights into the role and organization of dystrophin at the membrane-cytoskeleton interface. Transgenic approaches have also revealed surprising and encouraging results with respect to utrophin. Against expectations, the long-awaited utrophin knockout mice have a remarkably mild phenotype with only subtle changes in neuromuscular junction architecture. On the other hand, mdx mice transgenic for a mini-utrophin construct showed rescue of the muscular dystrophy phenotype, clearly an encouraging finding with obvious therapeutic possibilities. These and other recent findings are discussed in the context of the structure and function of dystrophin and utrophin at the membrane-cytoskeleton interface.

Duchenne muscular dystrophy and the neuromuscular junction: the utrophin link

Although the precise function of utrophin at the postsynaptic membrane of the neuromuscular junction still remains unclear, despite recent genetic 'knockout' experiments, a separate study in a transgenic mouse model system for Duchenne muscular dystrophy (DMD) has nonetheless shown that overexpression of utrophin into extrasynaptic regions of muscle fibers can functionally compensate for the lack of dystrophin and alleviate the muscle pathology. In this context, the next step is to identify the mechanisms presiding over expression of utrophin at the neuromuscular synapse in attempts to induce its expression throughout DMD muscle fibers. In fact, additional studies have shown that an important DNA element contained with the utrophin promoter may confer synapse-specific expression to the utrophin gene. Identification of the events culminating in the transactivation of the utrophin gene within synaptic myonuclei should provide important cues for the development of an effective therapeutic strategy for DMD.