A novel mechanism for modulating synaptic gene expression: differential localization of alpha-dystrobrevin transcripts in skeletal muscle

Α-Dystrobrevin-1 recruits Grb2 and α-catulin to organize neurotransmitter receptors at the neuromuscular junction

Neuromuscular junctions (NMJs), the synapses made by motor neurons on muscle fibers, form during embryonic development but undergo substantial remodeling postnatally. Several lines of evidence suggest that α-dystrobrevin, a component of the dystrophin-associated glycoprotein complex (DGC), is a crucial regulator of the remodeling process and that tyrosine phosphorylation of one isoform, α-dystrobrevin-1, is required for its function at synapses. We identified a functionally important phosphorylation site on α-dystrobrevin-1, generated phosphorylation-specific antibodies to it and used them to demonstrate dramatic increases in phosphorylation during the remodeling period, as well as in nerve-dependent regulation in adults. We then identified proteins that bind to this site in a phosphorylation-dependent manner and others that bind to α-dystrobrevin-1 in a phosphorylation-independent manner. They include multiple members of the DGC, as well as α-catulin, liprin-α1, Usp9x, PI3K, Arhgef5 and Grb2. Finally, we show that two interactors, α-catulin (phosphorylation independent) and Grb2 (phosphorylation dependent) are localized to NMJs in vivo, and that they are required for proper organization of neurotransmitter receptors on myotubes.

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).

Interaction of α-catulin with dystrobrevin contributes to integrity of dystrophin complex in muscle

The dystrophin complex is a multimolecular membrane-associated protein complex whose defects underlie many forms of muscular dystrophy. The dystrophin complex is postulated to function as a structural element that stabilizes the cell membrane by linking the contractile apparatus to the extracellular matrix. A better understanding of how this complex is organized and localized will improve our knowledge of the pathogenic mechanisms of diseases that involve the dystrophin complex. In a Caenorhabditis elegans genetic study, we demonstrate that CTN-1/α-catulin, a cytoskeletal protein, physically interacts with DYB-1/α-dystrobrevin (a component of the dystrophin complex) and that this interaction is critical for the localization of the dystrophin complex near dense bodies, structures analogous to mammalian costameres. We further show that in mouse α-catulin is localized at the sarcolemma and neuromuscular junctions and interacts with α-dystrobrevin and that the level of α-catulin is reduced in α-dystrobrevin-deficient mouse muscle. Intriguingly, in the skeletal muscle of mdx mice lacking dystrophin, we discover that the expression of α-catulin is increased, suggesting a compensatory role of α-catulin in dystrophic muscle. Together, our study demonstrates that the interaction between α-catulin and α-dystrobrevin is evolutionarily conserved in C. elegans and mammalian muscles and strongly suggests that this interaction contributes to the integrity of the dystrophin complex.

The ABCA1 cholesterol transporter associates with one of two distinct dystrophin-based scaffolds in Schwann cells

Cytoskeletal scaffolding complexes help organize specialized membrane domains with unique functions on the surface of cells. In this study, we define the scaffolding potential of the Schwann cell dystrophin glycoprotein complex (DGC) by establishing the presence of four syntrophin isoforms, (alpha1, beta1, beta2, and gamma2), and one dystrobrevin isoform, (alpha-dystrobrevin-1), in the abaxonal membrane. Furthermore, we demonstrate the existence of two separate DGCs in Schwann cells that divide the abaxonal membrane into spatially distinct domains, the DRP2/periaxin rich plaques and the Cajal bands that contain Dp116, utrophin, alpha-dystrobrevin-1 and four syntrophin isoforms. Finally, we show that the two different DGCs can scaffold unique accessory molecules in distinct areas of the Schwann cell membrane. Specifically, the cholesterol transporter ABCA1, associates with the Dp116/syntrophin complex in Cajal bands and is excluded from the DRP2/periaxin rich plaques.

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.

Syncoilin upregulation in muscle of patients with neuromuscular disease

Syncoilin may have a role in linking the desmin-associated intermediate filament network of the muscle fiber with the dystrophin-associated protein complex (DAPC). We have evaluated syncoilin in a range of neuromuscular disorders including Duchenne and Becker muscular dystrophy, central core disease, congenital muscular dystrophies, and neurogenic disorders. Our results show that syncoilin immunolabeling is not only altered in muscle fibers with alterations in the DAPC but also in response to a variety of genetic defects, including those associated with proteins of the extracellular matrix and the intracellular Ca2+-release channel (ryanodine receptor). The pattern of syncoilin immunolabeling in these diseases appeared to reflect a rearrangement of the intermediate filament-associated cytoskeleton that characterizes both muscle fiber development and conditions in which the cytoskeletal organization of the muscle fiber is significantly affected. These observations raise the possibility that mutations in the gene encoding for syncoilin may underlie some forms of muscle disease.

Muscle regeneration in dystrophin-deficient mdx mice studied by gene expression profiling

Background

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is lethal. In contrast, dystrophin-deficient mdx mice recover due to effective regeneration of affected muscle tissue. To characterize the molecular processes associated with regeneration, we compared gene expression levels in hindlimb muscle tissue of mdx and control mice at 9 timepoints, ranging from 1–20 weeks of age.

Results

Out of 7776 genes, 1735 were differentially expressed between mdx and control muscle at at least one timepoint (p < 0.05 after Bonferroni correction). We found that genes coding for components of the dystrophin-associated glycoprotein complex are generally downregulated in the mdx mouse. Based on functional characteristics such as membrane localization, signal transduction, and transcriptional activation, 166 differentially expressed genes with possible functions in regeneration were analyzed in more detail. The majority of these genes peak at the age of 8 weeks, where the regeneration activity is maximal. The following pathways are activated, as shown by upregulation of multiple members per signalling pathway: the Notch-Delta pathway that plays a role in the activation of satellite cells, and the Bmp15 and Neuregulin 3 signalling pathways that may regulate proliferation and differentiation of satellite cells. In DMD patients, only few of the identified regeneration-associated genes were found activated, indicating less efficient regeneration processes in humans.

Conclusion

Based on the observed expression profiles, we describe a model for muscle regeneration in mdx mice, which may provide new leads for development of DMD therapies based on the improvement of muscle regeneration efficacy.

Molecular heterogeneity of the dystrophin-associated protein complex in the mouse kidney nephron: differential alterations in the absence of utrophin and dystrophin

The dystrophin-associated protein complex (DPC) consisting of syntrophin, dystrobrevin, and dystroglycan isoforms is associated either with dystrophin or its homolog utrophin. It is present not only in muscle cells, but also in numerous tissues, including kidney, liver, and brain. Using high-resolution immunofluorescence imaging and Western blotting, we have investigated the effects of utrophin and dystrophin gene deletion on the formation and membrane anchoring of the DPC in kidney epithelial cells, which co-express utrophin and low levels of the C-terminal dystrophin isoform Dp71. We show that multiple, molecularly distinct DPCs co-exist in the nephron; these DPCs have a segment-specific distribution and are only partially associated with utrophin in the basal membrane of tubular epithelial cells. In utrophin-deficient mice, a selective reduction of beta2-syntrophin has been observed in medullary tubular segments, whereas alpha1-syntrophin and beta1-syntrophin are retained, concomintant with an upregulation of beta-dystroglycan, beta-dystrobrevin, and Dp71. These findings suggest that beta2-syntrophin is dependent on utrophin for association with the DPC, and that loss of utrophin is partially compensated by Dp71, allowing the preservation of the DPC in kidney epithelial cells. This hypothesis is confirmed by the almost complete loss of all DPC proteins examined in mice lacking full-length utrophin and all C-terminal dystrophin isoforms (utrophin(0/0)/mdx(3Cv)). The DPC thus critically depends on these proteins for assembly and/or membrane localization in kidney epithelial cells.

Congenital Myasthenic Syndromes: Multiple Molecular Targets at the Neuromuscular Junction

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A synaptic CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

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.

Tyrosine-phosphorylated and nonphosphorylated isoforms of alpha-dystrobrevin: roles in skeletal muscle and its neuromuscular and myotendinous junctions

alpha-Dystrobrevin (DB), a cytoplasmic component of the dystrophin-glycoprotein complex, is found throughout the sarcolemma of muscle cells. Mice lacking alphaDB exhibit muscular dystrophy, defects in maturation of neuromuscular junctions (NMJs) and, as shown here, abnormal myotendinous junctions (MTJs). In normal muscle, alternative splicing produces two main alphaDB isoforms, alphaDB1 and alphaDB2, with common NH2-terminal but distinct COOH-terminal domains. alphaDB1, whose COOH-terminal extension can be tyrosine phosphorylated, is concentrated at the NMJs and MTJs. alphaDB2, which is not tyrosine phosphorylated, is the predominant isoform in extrajunctional regions, and is also present at NMJs and MTJs. Transgenic expression of either isoform in alphaDB-/- mice prevented muscle fiber degeneration; however, only alphaDB1 completely corrected defects at the NMJs (abnormal acetylcholine receptor patterning, rapid turnover, and low density) and MTJs (shortened junctional folds). Site-directed mutagenesis revealed that the effectiveness of alphaDB1 in stabilizing the NMJ depends in part on its ability to serve as a tyrosine kinase substrate. Thus, alphaDB1 phosphorylation may be a key regulatory point for synaptic remodeling. More generally, alphaDB may play multiple roles in muscle by means of differential distribution of isoforms with distinct signaling or structural properties.

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.

Congenital myasthenic syndromes: progress over the past decade

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic basal lamina, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A basal lamina CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

Characterization of human alpha-dystrobrevin isoforms in HL-60 human promyelocytic leukemia cells undergoing granulocytic differentiation

The biochemical properties and spatial localization of the protein alpha-dystrobrevin and other isoforms were investigated in cells of the human promyelocytic leukemia line HL-60 granulocytic differentiation as induced by retinoic acid (RA). Alpha-dystrobrevin was detected both in the cytosol and the nuclei of these cells, and a short isoform (gamma-dystrobrevin) was modified by tyrosine phosphorylation soon after the onset of the RA-triggered differentiation. Varying patterns of distribution of alpha-dystrobrevin and its isoforms could be discerned in HL-60 promyelocytes, RA-differentiated mature granulocytes, and human neutrophils. Moreover, the gamma-dystrobrevin isoform was found in association with actin and myosin light chain. The results provide new information about potential involvement of alpha-dystrobrevin and its splice isoforms in signal transduction in myeloid cells during induction of granulocytic differentiation and/or at the commitment stage of differentiation or phagocytic cells.

Molecular characterization of a metastatic neuroendocrine cell cancer arising in the prostates of transgenic mice

The features and functions of prostatic neuroendocrine (NE) cells remain ill-defined. Neuroendocrine differentiation (NED) in adenocarcinoma of the human prostate (CaP) is associated with more aggressive disease, but the underlying mediators are poorly understood. We examined these issues in transgenic mice that utilize regulatory elements from the cryptdin-2 gene (Defcr2) to express simian virus 40 large T antigen (TAg) in prostatic NE cells. CR2-TAg mice develop prostatic intraepithelial neoplasia at 8 weeks of age, 1 week after the onset of TAg expression. An invasive phase follows 2-4 weeks later, with lymph node, liver, lung, brain, and bone metastases appearing within 16 weeks. DNA microarray studies revealed 122 mRNAs that were increased >/=2-fold in duplicate assays of 16-week-old CR2-TAg versus normal prostates. Thirty two transcripts encode proteins associated with neurons and endocrine cells (e.g. basic helix loop helix, SRY-related high mobility group box and sine-oculis homeobox transcription factors, Hu RNA-binding proteins, neuronatin, Racgap1, collapsin response mediator protein-1, synaptotagmin-1, proprotein convertase, and secretogranins). Follow-up studies of candidate mediators and biomarkers of differentiation/growth in the microarray data set involved real time quantitative reverse transcriptase-PCR assays of laser capture microdissected NE cells from CR2-TAg prostates plus liver metastases, and immunohistochemical comparisons of transgenic mouse prostates and 35 human CaP samples. Our findings include (a) expression of the bHLH mouse achaete-scute homolog (mASH1) in normal and CR2-TAg NE cells and foci of NED in human CaP, (b) glutamic acid decarboxylase and its product (gamma-aminobutyric acid) in neoplastic NE cells juxtaposed next to cohorts of normal gamma-aminobutyric acid receptor expressing secretory cells (a potential route for paracrine interactions between these two epithelial lineages), and (c) aromatic l-amino-acid decarboxylase, but not its dopamine/serotonin products, in CR2-TAg NE cells and NED. These results underscore the value of CR2-TAg mice for characterizing normal NE cell biology and tumorigenesis.

Expression of mutant Ets protein at the neuromuscular synapse causes alterations in morphology and gene expression

The localized transcription of several muscle genes at the motor endplate is controlled by the Ets transcription factor GABP. To evaluate directly its contribution to the formation of the neuromuscular junction, we generated transgenic mice expressing a general Ets dominant-negative mutant specifically in skeletal muscle. Quantitative RT-PCR analysis demonstrated that the expression of genes containing an Ets-binding site was severely affected in the mutant mice. Conversely, the expression of other synaptic genes, including MuSK and Rapsyn, was unchanged. In these animals, muscles expressing the mutant transcription factor developed normally, but examination of the post-synaptic morphology revealed marked alterations of both the primary gutters and secondary folds of the neuromuscular junction. Our results demonstrate that Ets transcription factors are crucial for the normal formation of the neuromuscular junction. They further show that Ets-independent mechanisms control the synaptic expression of a distinct set of synaptic genes.

The spectrum of congenital myasthenic syndromes

The past decade saw remarkable advances in defining the molecular and genetic basis of the congenital myasthenic syndromes. These advances would not have been possible without antecedent clinical observations, electrophysiologic analysis, and careful morphologic studies that pointed to candidate genes or proteins. For example, a kinetic abnormality of the acetylcholine receptor (AChR) detected at the single channel level pointed to a kinetic mutation in an AChR subunit; endplate AChR deficiency suggested mutations residing in an AChR subunit or in rapsyn; absence of acetylcholinesterase (AChE) from the endplate predicted mutations in the catalytic or collagen-tailed subunit of this enzyme; and a history of abrupt episodes of apnea associated with a stimulation dependent decrease of endplate potentials and currents implicated proteins concerned with ACh resynthesis or vesicular filling. Discovery of mutations in endplate-specific proteins also prompted expression studies that afforded proof of pathogenicity, provided clues for rational therapy, lead to precise structure function correlations, and highlighted functionally significant residues or molecular domains that previous systematic mutagenesis studies had failed to detect. An overview of the spectrum of the congenital myasthenic syndromes suggests that most are caused by mutations in AChR subunits, and particularly in the epsilon subunit. Future studies will likely uncover new types of CMS that reside in molecules governing quantal release, organization of the synaptic basal lamina, and expression and aggregation of AChR on the postsynaptic junctional folds.

Dystrobrevin dynamics in muscle-cell signalling: a possible target for therapeutic intervention in Duchenne muscular dystrophy?

The dystrophin-protein complex forms one of the connections between the extracellular matrix and the cytoskeleton of muscle. This link is disrupted in patients with Duchenne and Becker muscular dystrophies. Dystrobrevin is a component of the dystrophin-protein complex that binds to the C-terminus of dystrophin and also to syntrophin. As its name suggests, dystrobrevin is a relative of dystrophin participating in similar intermolecular interactions. Dystrobrevin-deficient mice have a form of muscular dystrophy that leaves the sarcolemma and dystrophin-protein complex intact but affects an as yet unidentified signalling pathway in muscle. Given that the up-regulation of several genes has a beneficial effect on the muscle in some dystrophic mouse models, alpha-dystrobrevin has a number of properties that might be protective in muscular dystrophy. This article discusses the function of dystrobrevin in muscle and reviews its suitability as a therapeutic target for treating patients with Duchenne and Becker muscular dystrophies.

Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome

Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes. However, in a subset of patients with EP AChR deficiency, the genetic defect has remained elusive. Rapsyn, a 43-kDa postsynaptic protein, plays an essential role in the clustering of AChR at the EP. Seven tetratricopeptide repeats (TPRs) of rapsyn subserve self-association, a coiled-coil domain binds to AChR, and a RING-H2 domain associates with beta-dystroglycan and links rapsyn to the subsynaptic cytoskeleton. Rapsyn self-association precedes recruitment of AChR to rapsyn clusters. In four patients with EP AChR deficiency but with no mutations in AChR subunits, we identify three recessive rapsyn mutations: one patient carries L14P in TPR1 and N88K in TPR3; two are homozygous for N88K; and one carries N88K and 553ins5, which frameshifts in TPR5. EP studies in each case show decreased staining for rapsyn and AChR, as well as impaired postsynaptic morphological development. Expression studies in HEK cells indicate that none of the mutations hinders rapsyn self-association but that all three diminish coclustering of AChR with rapsyn.

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.

Congenital myasthenic syndromes: genetic defects of the neuromuscular junction

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or the resynthesis of ACh. Insufficient resynthesis of ACh is now known to be caused by mutations that reduce the expression, catalytic efficiency, or both of choline acetyltransferase. The synaptic CMS are caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent ColQ from associating with catalytic subunits or from insertion into the synaptic basal lamina. With one exception, postsynaptic CMS identified to date are associated with a kinetic abnormality or decreased expression of the acetylcholine receptor (AChR). Numerous mutations have now been identified in subunits of AChR that alter the kinetics or surface expression of the receptor. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most mutations that reduce surface expression of AChR reside in the receptor's epsilon subunit and are partially compensated by residual expression of the fetal-type gamma subunit. Null mutations in both alleles of other AChR subunits are likely lethal, owing to absence of a substituting subunit.

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.

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

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

Syncoilin, a novel member of the intermediate filament superfamily that interacts with alpha-dystrobrevin in skeletal muscle

Dystrophin coordinates the assembly of a complex of structural and signaling proteins that are required for normal muscle function. A key component of the dystrophin protein complex is alpha-dystrobrevin, a dystrophin-associated protein whose absence results in neuromuscular junction defects and muscular dystrophy. To gain further insights into the role of alpha-dystrobrevin in skeletal muscle, we used the yeast two-hybrid system to identify a novel alpha-dystrobrevin-binding partner called syncoilin. Syncoilin is a new member of the intermediate filament superfamily and is highly expressed in skeletal and cardiac muscle. In normal skeletal muscle, syncoilin is concentrated at the neuromuscular junction, where it colocalizes and coimmunoprecipitates with alpha-dystrobrevin-1. Expression studies in mammalian cells demonstrate that, while alpha-dystrobrevin and syncoilin associate directly, overexpression of syncoilin does not result in the self-assembly of intermediate filaments. Finally, unlike many components of the dystrophin protein complex, we show that syncoilin expression is up-regulated in dystrophin-deficient muscle. These data suggest that alpha-dystrobrevin provides a link between the dystrophin protein complex and the intermediate filament network at the neuromuscular junction, which may be important for the maintenance and maturation of the synapse.