Synapse-specific gene expression at the neuromuscular junction

Regulation of Gene expression at the neuromuscular Junction

Gene expression in skeletal muscle is profoundly changed upon innervation. 50 years of research on the neuromuscular system have greatly increased our understanding of the mechanisms underlying these changes. By controlling the expression and the activity of key transcription factors, nerve-evoked electrical activity in the muscle fiber positively and negatively regulates the expression of hundreds of genes. Innervation also compartmentalizes gene expression into synaptic and extra-synaptic regions of muscle fibers. In addition, electrically-evoked, release of several factors (e.g. Agrin, Neuregulin, Wnt ligands) induce the clustering of synaptic proteins and of a few muscle nuclei. The sub-synaptic nuclei acquire a particular chromatin organization and develop a specific gene expression program dedicated to building and maintaining a functional neuromuscular synapse. Deciphering synapse-specific, transcriptional regulation started with the identification of the N-box, a six base pair element present in the promoters of the acetylcholine δ and ε subunits. Most genes with synapse-specific expression turned out to contain at least one N-box in their promoters. The N-box is a response element for the synaptic signals Agrin and Neuregulins as well as a binding site for transcription factors of the Ets family. The Ets transcription factors GABP and Erm are implicated in the activation of post-synaptic genes via the N-box. In muscle fibers, Erm expression is restricted to the NMJ whereas GABP is expressed in all muscle nuclei but phosphorylated and activated by the JNK and ERK signaling pathways in response to Agrin and Neuregulins. Post-synaptic gene expression also correlates with chromatin modifications at the genomic level as evidenced by the strong enrichment of decondensed chromatin and acetylated histones in sub-synaptic nuclei. Here we discuss these transcriptional pathways for synaptic specialization at NMJs.

MuSk function during health and disease

The receptor tyrosine kinase MuSK (muscle-specific kinase) is the key signaling molecule during the formation of a mature and functional neuromuscular junction (NMJ). Signal transduction events downstream of MuSK activation induce both pre- and postsynaptic differentiation, which, most prominently, includes the clustering of acetylcholine receptors (AChRs) at synaptic sites. MuSK activation requires a complex interplay between its co-receptor Lrp4 (low-density lipoprotein receptor-related protein-4), the motor neuron-derived heparan-sulfate proteoglycan Agrin and the intracellular adaptor protein Dok-7. A tight regulation of MuSK kinase activity is crucial for proper NMJ development. Defects in MuSK signaling are the cause of muscle weakness as reported in congenital myasthenic syndromes and myasthenia gravis. This review focuses on recent structure-based analyses of MuSK, Agrin, Lrp4 and Dok-7 interactions and their function during MuSK activation. Conclusions about the regulation of the MuSK kinase that were derived from molecular structures will be highlighted. In addition, the role of MuSK during development and disease will be discussed.

Correct expression and localization of collagen XIII are crucial for the normal formation and function of the neuromuscular system

Transmembrane collagen XIII has been linked to maturation of the musculoskeletal system. Its absence in mice (Col13a1^-/- ) results in impaired neuromuscular junction (NMJ) differentiation and function, while transgenic overexpression (Col13a1^oe ) leads to abnormally high bone mass. Similarly, loss-of-function mutations in COL13A1 in humans produce muscle weakness, decreased motor synapse function and mild dysmorphic skeletal features. Here, analysis of the exogenous overexpression of collagen XIII in various muscles revealed highly increased transcript and protein levels, especially in the diaphragm. Unexpectedly, the main location of exogenous collagen XIII in the muscle was extrasynaptic, in fibroblast-like cells, while some motor synapses were devoid of collagen XIII, possibly due to a dominant negative effect. Concomitantly, phenotypical changes in the NMJs of the Col13a1^oe mice partly resembled those previously observed in Col13a1^-/- mice. Namely, the overall increase in collagen XIII expression in the muscle produced both pre- and postsynaptic abnormalities at the NMJ, especially in the diaphragm. We discovered delayed and compromised acetylcholine receptor (AChR) clustering, axonal neurofilament aggregation, patchy acetylcholine vesicle (AChV) accumulation, disrupted adhesion of the nerve and muscle, Schwann cell invagination and altered evoked synaptic function. Furthermore, the patterns of the nerve trunks and AChR clusters in the diaphragm were broader in the adult muscles, and already prenatally in the Col13a1^oe mice, suggesting collagen XIII involvement in the development of the neuromuscular system. Overall, these results confirm the role of collagen XIII at the neuromuscular synapses and highlight the importance of its correct expression and localization for motor synapse formation and function.

A robust neuromuscular system protects rat and human skeletal muscle from sarcopenia

Declining muscle mass and function is one of the main drivers of loss of independence in the elderly. Sarcopenia is associated with numerous cellular and endocrine perturbations, and it remains challenging to identify those changes that play a causal role and could serve as targets for therapeutic intervention. In this study, we uncovered a remarkable differential susceptibility of certain muscles to age-related decline. Aging rats specifically lose muscle mass and function in the hindlimbs, but not in the forelimbs. By performing a comprehensive comparative analysis of these muscles, we demonstrate that regional susceptibility to sarcopenia is dependent on neuromuscular junction fragmentation, loss of motoneuron innervation, and reduced excitability. Remarkably, muscle loss in elderly humans also differs in vastus lateralis and tibialis anterior muscles in direct relation to neuromuscular dysfunction. By comparing gene expression in susceptible and non-susceptible muscles, we identified a specific transcriptomic signature of neuromuscular impairment. Importantly, differential molecular profiling of the associated peripheral nerves revealed fundamental changes in cholesterol biosynthetic pathways. Altogether our results provide compelling evidence that susceptibility to sarcopenia is tightly linked to neuromuscular decline in rats and humans, and identify dysregulation of sterol metabolism in the peripheral nervous system as an early event in this process.

Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency

The collagen ColQ anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction (NMJ). It also binds MuSK and perlecan/dystroglycan, 2 signaling platforms of the postsynaptic domain. Mutations in ColQ cause a congenital myasthenic syndrome (CMS) with AChE deficiency. Because the absence of AChE does not fully explain the complexity of the syndrome and there is no curative treatment for the disease, we explored additional potential targets of ColQ by conducting a large genetic screening of ColQ-deficient mice, a model for CMS with AChE deficiency, and analyzed their NMJ and muscle phenotypes. We demonstrated that ColQ controls the development and the maturation of the postsynaptic domain by regulating synaptic gene expression. Notably, ColQ deficiency leads to an up-regulation of the 5 subunits of the nicotinic acetylcholine receptor (AChR), leading to mixed mature and immature AChRs at the NMJ of adult mice. ColQ also regulates the expression of extracellular matrix (ECM) components. However, whereas the ECM mRNAs were down-regulated in vitro, compensation seemed to occur in vivo to maintain normal levels of these mRNAs. Finally, ColQ deficiency leads to a general atrophic phenotype and hypoplasia that affect fast muscles. This study points to new specific hallmarks for this CMS.-Sigoillot, S. M., Bourgeois, F., Karmouch, J., Molgó, J., Dobbertin, A., Chevalier, C., Houlgatte, R., Léger, J., Legay, C. Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency.

Physiological and Pathophysiological Insights of Nav1.4 and Nav1.5 Comparison

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases.

HuR Mediates Changes in the Stability of AChR β-Subunit mRNAs after Skeletal Muscle Denervation

Acetylcholine receptors (AChRs) are heteromeric membrane proteins essential for neurotransmission at the neuromuscular junction. Previous work showed that muscle denervation increases expression of AChR mRNAs due to transcriptional activation of AChR subunit genes. However, it remains possible that post-transcriptional mechanisms are also involved in controlling the levels of AChR mRNAs following denervation. We examined whether post-transcriptional events indeed regulate AChR β-subunit mRNAs in response to denervation. First, in vitro stability assays revealed that the half-life of AChR β-subunit mRNAs was increased in the presence of denervated muscle protein extracts. A bioinformatics analysis revealed the existence of a conserved AU-rich element (ARE) in the 3'-untranslated region (UTR) of AChR β-subunit mRNA. Furthermore, denervation of mouse muscle injected with a luciferase reporter construct containing the AChR β-subunit 3'UTR, caused an increase in luciferase activity. By contrast, mutation of this ARE prevented this increase. We also observed that denervation increased expression of the RNA-binding protein human antigen R (HuR) and induced its translocation to the cytoplasm. Importantly, HuR binds to endogenous AChR β-subunit transcripts in cultured myotubes and in vivo, and this binding is increased in denervated versus innervated muscles. Finally, p38 MAPK, a pathway known to activate HuR, was induced following denervation as a result of MKK3/6 activation and a decrease in MKP-1 expression, thereby leading to an increase in the stability of AChR β-subunit transcripts. Together, these results demonstrate the important contribution of post-transcriptional events in regulating AChR β-subunit mRNAs and point toward a central role for HuR in mediating synaptic gene expression.

SIGNIFICANCE STATEMENT

Muscle denervation is a convenient model to examine expression of genes encoding proteins of the neuromuscular junction, especially acetylcholine receptors (AChRs). Despite the accepted model of AChR regulation, which implicates transcriptional mechanisms, it remains plausible that such events cannot fully account for changes in AChR expression following denervation. We show that denervation increases expression of the RNA-binding protein HuR, which in turn, causes an increase in the stability of AChR β-subunit mRNAs in denervated muscle. Our findings demonstrate for the first time the contribution of post-transcriptional events in controlling AChR expression in skeletal muscle, and points toward a central role for HuR in mediating synaptic development while also paving the way for developing RNA-based therapeutics for neuromuscular diseases.

Mechanisms controlling neuromuscular junction stability

The neuromuscular junction (NMJ) is the synaptic connection between motor neurons and muscle fibers. It is involved in crucial processes such as body movements and breathing. Its proper development requires the guidance of motor axons toward their specific targets, the development of multi-innervated myofibers, and a selective synapse stabilization. It first consists of the removal of excessive motor axons on myofibers, going from multi-innervation to a single innervation of each myofiber. Whereas guidance cues of motor axons toward their specific muscular targets are well characterized, only few molecular and cellular cues have been reported as clues for selecting and stabilizing specific neuromuscular junctions. We will first provide a brief summary on NMJ development. We will then review molecular cues that are involved in NMJ stabilization, in both pre- and post-synaptic compartments, considering motor neurons and Schwann cells on the one hand, and muscle on the other hand. We will provide links with pathologies and highlight advances that can be brought both by basic research on NMJ development and clinical data resulting from the analyses of neurodegeneration of synaptic connections to obtain a better understanding of this process. The goal of this review is to highlight the findings toward understanding the roles of poly- or single-innervations and the underlying mechanisms of NMJ stabilization.

Functions of the podocyte proteins nephrin and Neph3 and the transcriptional regulation of their genes

Nephrin and Neph-family proteins [Neph1–3 (nephrin-like 1–3)] belong to the immunoglobulin superfamily of cell-adhesion receptors and are expressed in the glomerular podocytes. Both nephrin and Neph-family members function in cell adhesion and signalling, and thus regulate the structure and function of podocytes and maintain normal glomerular ultrafiltration. The expression of nephrin and Neph3 is altered in human proteinuric diseases emphasizing the importance of studying the transcriptional regulation of the nephrin and Neph3 genes NPHS1 (nephrosis 1, congenital, Finnish type) and KIRREL2 (kin of IRRE-like 2) respectively. The nephrin and Neph3 genes form a bidirectional gene pair, and they share transcriptional regulatory mechanisms. In the present review, we summarize the current knowledge of the functions of nephrin and Neph-family proteins and transcription factors and agents that control nephrin and Neph3 gene expression.

Regulation of nephrin gene by the Ets transcription factor, GA-binding protein

BACKGROUND

Transcription factor GA-binding protein (GABP) is suggested to be involved in the formation of the neuromuscular junctions by regulating the transcription of synapse genes. Interestingly, neurons and podocytes share molecular and functional similarities that led us to investigate the expression and function of GABP in podocytes and its role in transcriptional regulation of nephrin, the key molecule of the podocyte slit diaphragm that is essential for normal glomerular ultrafiltration.

METHODS

The expression and localization of GABP in the rat and human kidney as well as in human embryonic kidney A293 cells and undifferentiated and differentiated human podocytes were analysed by immunoblotting and immunostaining. The role of GABP in activating the nephrin promoter was investigated by reporter gene assay and site-directed mutagenesis of the GABP-binding elements, and the interaction of GABP with the nephrin promoter was analysed by chromatin immunoprecipitation. The function of GABP in podocytes was studied by knocking down GABPα in differentiated human podocytes using lentiviral shRNA targeting GABPα.

RESULTS

GABP is expressed in the nuclei in rat and human glomeruli. In addition, in A293 cells and undifferentiated and differentiated human podocytes, GABP highly enriches in the nucleus. GABP activates and binds nephrin proximal promoter and Ets sites are essential for this activity. Knock-down of GABPα stimulates apoptosis in cultured podocytes.

CONCLUSIONS

The results show that GABP is expressed in podocytes and is involved in the regulation of nephrin gene expression. Furthermore, GABP may be important in the maintenance of podocyte function by regulating apoptosis.

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).

Upregulation of brain utrophin does not rescue behavioral alterations in dystrophin-deficient mice

Dystrophin, the protein responsible for X-linked Duchenne muscular dystrophy (DMD), is normally expressed in both muscle and brain, which explains that its loss also leads to cognitive deficits. The utrophin protein, an autosomal homolog, is a natural candidate for dystrophin replacement in patients. Pharmacological upregulation of endogenous utrophin improves muscle physiology in dystrophin-deficient mdx mice, and represents a potential therapeutic tool that has the advantage of allowing delivery to various organs following peripheral injections. Whether this could alleviate cognitive deficits, however, has not been explored. Here, we first investigated basal expression of all utrophins and dystrophins in the brain of mdx mice and found no evidence for spontaneous compensation by utrophins. Then, we show that systemic chronic, spaced injections of arginine butyrate (AB) alleviate muscle alterations and upregulate utrophin expression in the adult brain of mdx mice. AB selectively upregulated brain utrophin Up395, while reducing expression of Up113 and Up71. This, however, was not associated with a significant improvement of behavioral functions typically affected in mdx mice, which include exploration, emotional reactivity, spatial and fear memories. We suggest that AB did not overcome behavioral and cognitive dysfunctions because the regional and cellular expression of utrophins did not coincide with dystrophin expression in untreated mice, nor did it in AB-treated mice. While treatments based on the modulation of utrophin may alleviate DMD phenotypes in certain organs and tissues that coexpress dystrophins and utrophins in the same cells, improvement of cognitive functions would likely require acting on specific dystrophin-dependent mechanisms.

Myogenin regulates denervation-dependent muscle atrophy in mouse soleus muscle

Muscle inactivity due to injury or disease results in muscle atrophy. The molecular mechanisms contributing to muscle atrophy are poorly understood. However, it is clear that expression of atrophy-related genes, like Atrogin-1 and MuRF-1, are intimately tied to loss of muscle mass. When these atrophy-related genes are knocked out, inactive muscles retain mass. Muscle denervation stimulates muscle atrophy and Myogenin (Myog) is a muscle-specific transcription factor that is highly induced following muscle denervation. To investigate if Myog contributes to muscle atrophy, we have taken advantage of conditional Myog null mice. We show that in the denervated soleus muscle Myog expression contributes to reduced muscle force, mass, and cross-sectional area. We found that Myog mediates these effects, at least in part, by regulating expression of the Atrogin-1 and MuRF-1 genes. Indeed Myog over-expression in innervated muscle stimulates Atrogin-1 gene expression and Myog over-expression stimulates Atrogin-1 promoter activity. Thus, Myog and the signaling cascades regulating its induction following muscle denervation may represent novel targets for therapies aimed at reducing denervation-induced muscle atrophy.

Skeletal muscle IP3R1 receptors amplify physiological and pathological synaptic calcium signals

Ca(2+) release from internal stores is critical for mediating both normal and pathological intracellular Ca(2+) signaling. Recent studies suggest that the inositol 1,4,5-triphosphate (IP(3)) receptor mediates Ca(2+) release from internal stores upon cholinergic activation of the neuromuscular junction (NMJ) in both physiological and pathological conditions. Here, we report that the type I IP(3) receptor (IP(3)R(1))-mediated Ca(2+) release plays a crucial role in synaptic gene expression, development, and neuromuscular transmission, as well as mediating degeneration during excessive cholinergic activation. We found that IP(3)R(1)-mediated Ca(2+) release plays a key role in early development of the NMJ, homeostatic regulation of neuromuscular transmission, and synaptic gene expression. Reducing IP(3)R(1)-mediated Ca(2+) release via siRNA knockdown or IP(3)R blockers in C2C12 cells decreased calpain activity and prevented agonist-induced acetylcholine receptor (AChR) cluster dispersal. In fully developed NMJ in adult muscle, IP(3)R(1) knockdown or blockade effectively increased synaptic strength at presynaptic and postsynaptic sites by increasing both quantal release and expression of AChR subunits and other NMJ-specific genes in a pattern resembling muscle denervation. Moreover, in two mouse models of cholinergic overactivity and NMJ Ca(2+) overload, anti-cholinesterase toxicity and the slow-channel myasthenic syndrome (SCS), IP(3)R(1) knockdown eliminated NMJ Ca(2+) overload, pathological activation of calpain and caspase proteases, and markers of DNA damage at subsynaptic nuclei, and improved both neuromuscular transmission and clinical measures of motor function. Thus, blockade or genetic silencing of muscle IP(3)R(1) may be an effective and well tolerated therapeutic strategy in SCS and other conditions of excitotoxicity or Ca(2+) overload.

Distinct roles for classical nuclear import receptors in the growth of multinucleated muscle cells

Proper muscle function is dependent on spatial and temporal control of gene expression in myofibers. Myofibers are multinucleated cells that are formed, repaired and maintained by the process of myogenesis in which progenitor myoblasts proliferate, differentiate and fuse. Gene expression is dependent upon proteins that require facilitated nuclear import, however little is known about the regulation of nucleocytoplasmic transport during the formation of myofibers. We analyzed the role of karyopherin alpha (KPNA), a key classical nuclear import receptor, during myogenesis. We established that five karyopherin alpha paralogs are expressed by primary mouse myoblasts in vitro and that their steady-state levels increase in multinucleated myotubes, suggesting a global increase in demand for classical nuclear import during myogenesis. We used siRNA-mediated knockdown to identify paralog-specific roles for KPNA1 and KPNA2 during myogenesis. KPNA1 knockdown increased myoblast proliferation, whereas KPNA2 knockdown decreased proliferation. In contrast, no proliferation defect was observed with KPNA4 knockdown. Only knockdown of KPNA2 decreased myotube growth. These results identify distinct pathways involved in myoblast proliferation and myotube growth that rely on specific nuclear import receptors suggesting that regulation of classical nuclear import pathways likely plays a critical role in controlling gene expression in skeletal muscle.

Chromatin modifications that support acetylcholine receptor gene activation are established during muscle cell determination and differentiation

Localization of acetylcholine receptors (AChRs) to the postsynaptic region of muscle is mediated in part by transcriptional mechanisms. An important way of regulating transcription is through targeting histone modifications on chromatin to distinct gene loci. Using chromatin immunoprecipitation, we examined the developmental regulation of certain histone modifications at the AChR epsilon subunit locus, including methylations at lysine residues K4 and K27 and acetylations at K9 and K14. We modeled various stages of muscle development in cell culture, including pre-determined cells, committed but undifferentiated myoblasts, and differentiated myotubes, and modeled synaptic myotube nuclei by stimulating myotubes with neuregulin (NRG) 1. We found that a pattern of histone modifications associated with transcriptional activation is targeted to the AChR epsilon subunit locus in myotubes prior to stimulation with NRG1 and does not change upon addition of NRG1. Instead, we found that during muscle cell determination and differentiation, specific histone modifications are targeted to the AChR epsilon subunit locus. Within the gene, at K4, dimethylation is induced during muscle cell determination, while trimethylation is induced during differentiation. At K27, loss of trimethylation and appearance of monomethylation occurs during determination and differentiation. In addition, in a region upstream of the gene, K4 di- and trimethylation, and K9/14 acetylation are induced in a distinct developmental pattern, which may reflect a functional regulatory element. These results suggest synaptic signaling does not directly target histone modifications but rather the histone modification pattern necessary for transcriptional activation is previously established in a series of steps during muscle development.

Lamin A/C-mediated neuromuscular junction defects in Emery-Dreifuss muscular dystrophy

The LMNA gene encodes lamins A and C, two intermediate filament-type proteins that are important determinants of interphase nuclear architecture. Mutations in LMNA lead to a wide spectrum of human diseases including autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD), which affects skeletal and cardiac muscle. The cellular mechanisms by which mutations in LMNA cause disease have been elusive. Here, we demonstrate that defects in neuromuscular junctions (NMJs) are part of the disease mechanism in AD-EDMD. Two AD-EDMD mouse models show innervation defects including misexpression of electrical activity–dependent genes and altered epigenetic chromatin modifications. Synaptic nuclei are not properly recruited to the NMJ because of mislocalization of nuclear envelope components. AD-EDMD patients with LMNA mutations show the same cellular defects as the AD-EDMD mouse models. These results suggest that lamin A/C–mediated NMJ defects contribute to the AD-EDMD disease phenotype and provide insights into the cellular and molecular mechanisms for the muscle-specific phenotype of AD-EDMD.

Basic helix-loop-helix factors recruit nuclear factor I to enhance expression of the NaV 1.4 Na+ channel gene

We have previously shown that the basic helix-loop-helix (bHLH) transcription factors coordinate Na(V) 1.4 Na(+) channel gene expression in skeletal muscle, but the identity of the co-factors they direct is unknown. Using C2C12 muscle cells as a model system, we test the hypothesis that the bHLH factors counteract negative regulation exerted through a repressor E box (-90/-85) by recruiting positive-acting transcription factors to the nucleotides (-135/-57) surrounding the repressor E box. We used electrophoretic mobility shift assays to identify candidate factors that bound the repressor E box or these adjacent regions. Repressor E box-binding factors included the known transcription factor, ZEB/AREB6, and a novel repressor E box-binding factor designated REB. Mutations of the repressor E box that interfere with the binding of these factors prevented repression. The transcription factor, nuclear factor I (NFI), bound immediately upstream and downstream of the repressor E box. Mutation of the NFI-binding sites diminished the ability of myogenin and MRF4 to counteract repression. Based on these observations we suggest that bHLH factors recruit NFI to enhance skeletal muscle Na(+) channel expression.

Heregulin-induced epigenetic regulation of the utrophin-A promoter

Utrophin is the autosomal homolog of dystrophin, the product of the Duchenne's muscular dystrophy (DMD) locus. Utrophin is of therapeutic interest since its over-expression can compensate dystrophin's absence. Utrophin is enriched at neuromuscular junctions due to heregulin-mediated utrophin-A promoter activation. We demonstrate that heregulin activated MSK1/2 and phosphorylated histone H3 at serine 10 in cultured C2C12 muscle cells, in an ERK-dependent manner. MSK1/2 inhibition suppressed heregulin-mediated utrophin-A activation. MSK1 over-expression potentiated heregulin-mediated utrophin-A activation and chromatin remodeling at the utrophin-A promoter. These results identify MSK1/2 as key effectors modulating utrophin-A expression as well as identify novel targets for DMD therapy.

Postsynaptic chromatin is under neural control at the neuromuscular junction

In adult skeletal muscle, the nicotinic acetylcholine receptor (AChR) specifically accumulates at the neuromuscular junction, to allow neurotransmission. This clustering is paralleled by a compartmentalization of AChR genes expression to subsynaptic nuclei, which acquire a unique gene expression program and a specific morphology in response to neural cues. Our results demonstrate that neural agrin-dependent reprogramming of myonuclei involves chromatin remodelling, histone hyperacetylation and histone hyperphosphorylation. Activation of AChR genes in subsynaptic nuclei is mediated by the transcription factor GABP. Here we demonstrate that upon activation, GABP recruits the histone acetyl transferase (HAT) p300 on the AChR epsilon subunit promoter, whereas it rather recruits the histone deacetylase HDAC1 when the promoter is not activated. Moreover, the HAT activity of p300 is required in vivo for AChR expression. GABP therefore couples chromatin hyperacetylation and AChR activation by neural factors in subsynaptic nuclei.

Development of a human mitochondria-focused cDNA microarray (hMitChip) and validation in skeletal muscle cells: implications for pharmaco- and mitogenomics

Mitochondrial research has influenced our understanding of human evolution, physiology and pathophysiology. Mitochondria, intracellular organelles widely known as 'energy factories' of the cell, also play fundamental roles in intermediary metabolism, steroid hormone and heme biosyntheses, calcium signaling, generation of radical oxygen species, and apoptosis. Mitochondria possess a distinct DNA (mitochondrial DNA); yet, the vast majority of mitochondrial proteins are encoded by the nuclear DNA. Mitochondria-related genetic defects have been described in a variety of mostly rare, often fatal, primary mitochondrial disorders; furthermore, they are increasingly reported in association with many common morbid conditions, such as cancer, obesity, diabetes and neurodegenerative disorders, although their role remains unclear. This study describes the creation of a human mitochondria-focused cDNA microarray (hMitChip) and its validation in human skeletal muscle cells treated with glucocorticoids. We suggest that hMitChip is a reliable and novel tool that will prove useful for systematically studying the contribution of mitochondrial genomics to human health and disease.

The role of neuregulin-ErbB4 interactions on the proliferation and organization of cells in the subventricular zone

Coordinated regulation of neuronal progenitor differentiation in the subventricular zone (SVZ) is a fundamental feature of adult neurogenesis. However, the molecular control of this process remains mostly undeciphered. Here, we investigate the role of neuregulins (NRGs) in this process and show that a NRG receptor, ErbB4, is primarily expressed by polysialylated neural cell adhesion molecule immature neuroblasts but is also detected in a subset of GFAP+ astroglial cells, ependymal cells, and Dlx2+ precursors in the SVZ. Of the NRG ligands, both NRG1 and -2 are expressed by immature polysialylated neural cell adhesion molecule neuroblasts in the SVZ. NRG2 is also expressed by some of the GFAP+ putative stem cells lining the ventricles. Infusion of exogenous NRG1 leads to rapid aggregation of Dlx2+ cells in the SVZ and affects the initiation and maintenance of organized neuroblast migration from the SVZ toward the olfactory bulb. In contrast, the infusion of NRG2 increased the number of Sox2 and GFAP+ precursors in the SVZ. An outcome of this NRG2 effect is an increase in the number of newly generated migrating neuroblasts in the rostral migratory stream and GABAergic interneurons in the olfactory bulb. The analysis of conditional null mice that lack NRG receptor, ErbB4, in the nervous system revealed that the observed activities of NRG2 require ErbB4 activation. These results indicate that different NRG ligands affect distinct populations of differentiating neural precursors in the neurogenic regions of the mature forebrain. Furthermore, these studies identify NRG2 as a factor capable of promoting SVZ proliferation, leading to the formation of new neurons in vivo.

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

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

A 1.3 kb promoter fragment confers spatial and temporal expression of utrophin A mRNA in mouse skeletal muscle fibers

Upregulation of utrophin in muscle is currently being examined as a potential therapy for Duchenne muscular dystrophy patients. In this context, we generated transgenic mice harboring a 1.3 kb human utrophin A promoter fragment driving expression of the lacZ gene. Characterization of reporter expression during postnatal muscle development revealed that the levels and localization of beta-galactosidase parallel expression of utrophin A transcripts. Moreover, we noted that the utrophin A promoter is more active in slow soleus muscles. Additionally, expression of the reporter gene was regulated during muscle regeneration in a manner similar to utrophin A transcripts. Together, these results show that the utrophin A promoter-lacZ construct mirrors expression of utrophin A mRNAs indicating that this utrophin A promoter fragment confers temporal and spatial patterns of expression in skeletal muscle. This transgenic mouse will be valuable as an in vivo model for developing and testing molecules aimed at increasing utrophin A expression.

Spectrins and the Golgi

Several isoforms of spectrin membrane skeleton proteins have been localized to the Golgi complex. Golgi-specific membrane skeleton proteins associate with the Golgi as a detergent-resistant cytoskeletal structure that likely undergoes a dynamic assembly process that accommodates Golgi membrane dynamics. This review discusses the potential roles for this molecule in Golgi functions. In particular, it will focus on a recently identified distant cousin to conventional erythroid spectrin variously named Syne-1, Nesprin, myne, Enaptin, MSP-300, and Ank-1. Syne-1 has the novel ability to bind to both the Golgi and the nuclear envelope, a property that raises several intriguing and novel insights into Golgi structure and function. These include (1) the facilitation of interactions between Golgi and transitional ER sites on the nuclear envelope of muscle cells, and (2) an ability to impart localized specificity to the secretory pathway within large multinucleate syncytia such as skeletal muscle fibers.