CaM kinase II-dependent suppression of nicotinic acetylcholine receptor delta-subunit promoter activity

In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle

Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.

Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture

Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.

Sodium nitrate decreases agrin-induced acetylcholine receptor clustering

BACKGROUND

Humans are exposed to nitrate predominantly through diet with peak plasma concentrations within an hour after ingestion, but additional exposure is obtained from the environment, and minimally through de novo synthesis. Higher nitrate consumption has been associated with methemoglobinemia, spontaneous abortions, atherosclerosis, myocardial ischemia, septic and distressed lung, inflammatory bowel disease, amyotrophic lateral sclerosis, and neural tube defects. However, skeletal muscle development has not been examined.

METHODS

C2C12 skeletal muscle cell cultures were maintained, myoblasts were fused into myotubes, and then cultures were exposed to motor neuron derived agrin to enhance acetylcholine receptor (AChR) clustering. Untreated cultures were compared with cultures exposed to sodium nitrate at concentrations ranging from 10 ng/mL-100 μg/mL.

RESULTS

The results reported here demonstrate that 1 μg/mL sodium nitrate was sufficient to decrease the frequency of agrin-induced AChR clustering without affecting myotube formation. In addition, concentrations of sodium nitrate of 1 μg/mL or 100 μg/mL decreased gene expression of the myogenic transcription factor myogenin and AChR in correlation with the agrin-induced AChR clustering data.

CONCLUSIONS

These results reveal that sodium nitrate decreases the frequency of agrin-induced AChR clustering by a mechanism that includes myogenin and AChR gene expression. As a consequence sodium nitrate may pose a risk for skeletal muscle development and subsequent neuromuscular synapse formation in humans.

Molecular mechanisms underlying maturation and maintenance of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ), a peripheral synapse formed between motoneuron and skeletal muscle, is characterized by a protracted postnatal period of maturation and life-long maintenance. In neuromuscular disorders such as congenital myasthenic syndromes (CMSs), disruptions of NMJ maturation and/or maintenance are frequently observed. In particular, defective neuromuscular transmission associated with structural and molecular abnormalities at the pre- and postsynaptic membranes, as well as at the synaptic cleft, has been reported in these patients. Here, we review recent advances in the understanding of molecular and cellular events that mediate NMJ maturation and maintenance. The underlying regulatory mechanisms, including key molecular regulators at the presynaptic nerve terminal, synaptic cleft, and postsynaptic muscle membrane, are discussed.

Molecular control of neuromuscular junction development

Skeletal muscle innervation is a multi-step process leading to the neuromuscular junction (NMJ) apparatus formation. The transmission of the signal from nerve to muscle occurs at the NMJ level. The molecular mechanism that orchestrates the organization and functioning of synapses is highly complex, and it has not been completely elucidated so far. Neuromuscular junctions are assembled on the muscle fibers at very precise locations called end plates (EP). Acetylcholine receptor (AChR) clusterization at the end plates is required for an accurate synaptic transmission. This review will focus on some mechanisms responsible for accomplishing the correct distribution of AChRs at the synapses. Recent evidences support the concept that a dual transcriptional control of AChR genes in subsynaptic and extrasynaptic nuclei is crucial for AChR clusterization. Moreover, new players have been discovered in the agrin-MuSK pathway, the master organizer of postsynaptical differentiation. Mutations in this pathway cause neuromuscular congenital disorders. Alterations of the postynaptic apparatus are also present in physiological conditions characterized by skeletal muscle wasting. Indeed, recent evidences demonstrate how NMJ misfunctioning has a crucial role at the onset of age-associated sarcopenia.

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.

To build a synapse: signaling pathways in neuromuscular junction assembly

Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and skeletal muscle fibers and that exhibits a high degree of subcellular specialization. Aided by genetic techniques and suitable animal models, studies in the past decade have brought significant progress in identifying NMJ components and assembly mechanisms. This review highlights recent advances in the study of NMJ development, focusing on signaling pathways that are activated by diffusible cues, which shed light on synaptogenesis in the brain and contribute to a better understanding of muscular dystrophy.

Calcium-calmodulin kinase II is the common factor in calcium-dependent cardiac expression and secretion of A- and B-type natriuretic peptides

Peptides derived from the precursor of A- and B-type natriuretic peptides (ANP and BNP) are powerful clinical markers of cardiac hypertrophy and dysfunction. It is known that many stimuli affecting the intracellular calcium concentration also induce ANP and BNP secretion. It was our intention to study the mechanisms by which calcium regulates the secretion of ANP and BNP. The effects of pacing and calcium-calmodulin kinase II activity on natriuretic peptide secretion were studied in isolated perfused rat atria and cultured rat neonatal cardiomyocytes. In isolated rat atrium pacing induced an increase in diastolic, systolic, and averaged intracellular free calcium concentration and a frequency-dependent increase in the secretion of both ANP and BNP. The molar ratio of the secreted natriuretic peptides (ANP to BNP) remained nearly constant ( approximately 1000) at all the pacing frequencies tested (1, 3, 6, and 8 Hz). Calmodulin kinase II inhibitor KN-93 (3 mum) did not affect intracellular free calcium concentration but showed a frequency-dependent inhibitory effect on ANP and BNP secretion without a change in ANP to BNP ratio. In the neonatal cardiomyocytes, KN-93 (3 mum) suppressed the secretion and gene expression of both ANP and BNP. Overexpression of constitutively active (T286D) or nuclear (delta(B)) calcium-calmodulin kinase II induced an increase in ANP and BNP gene expression. The results indicate that the calcium-dependent secretion and gene expression of A- and B-type natriuretic peptides are similarly regulated by calmodulin kinase II-dependent mechanisms. This is a plausible mechanism contributing to exercise-induced natriuretic peptide secretion and the augmented secretion in heart dysfunction due to impaired calcium handling.

Activity-dependent gene regulation in skeletal muscle is mediated by a histone deacetylase (HDAC)-Dach2-myogenin signal transduction cascade

Muscle activity contributes to muscle development and function largely by means of regulated gene expression. Many genes crucial to neuromuscular synapse formation, such as MuSK and nAChRs, are induced before muscle innervation or after muscle denervation, and this induction requires expression of the E-box binding, basic helix-loop-helix muscle-specific transcription factor, myogenin (Mgn). The mechanism by which muscle activity is coupled to gene expression is poorly defined. Here we report that inhibition of histone deacetylase (HDAC) activity attenuates the induction of activity-regulated genes in aneural myotubes and adult denervated muscle. The effect of HDAC inhibitors requires new protein synthesis, suggesting HDACs may regulate the expression of a Mgn transcriptional repressor. We identified Dach2 as a Mgn transcriptional repressor whose expression is dramatically reduced in an HDAC-dependent manner in developing aneural myotubes or adult denervated muscle. Dach2 overexpression in denervated muscle suppressed Mgn, nAChR, and MuSK gene induction, whereas Dach2 knockdown induced Mgn gene expression in innervated muscle and relieved Mgn promoter inhibition by HDAC inhibitors. Thus, a HDAC-Dach2-myogenin signaling pathway has been identified to decode nerve activity and control muscle gene expression in developing and adult skeletal muscle.

The regulation of acetylcholinesterase by cis-elements within intron I in cultured contracting myotubes

The onset of spontaneous contraction in rat primary muscle cultures coincides with an increase in acetylcholinesterase (AChE) activity. In order to establish whether contractile activity modulates the rate of AChE transcript synthesis, and what elements of the gene are determinant, we examined the promoter and intron I in contracting muscle cultures. Ache genomic fragments attached to a luciferase reporter were transfected into muscle cultures that were either electrically stimulated or paralyzed with tetrodotoxin to enhance or inhibit contractions, respectively. Cultures transfected with intron I-containing constructs showed a 2-fold increase in luciferase activity following electrical stimulation, compared to tetrodotoxin treatment, suggesting that this region contains elements responding to contractile activity. Deleting a 780 bp distal region within intron I, containing an N-box element at +890 bp, or introducing a 2-bp mutation within its core sequence, eliminated the contraction-induced response. In contrast, mutating an N-box element at +822 bp had no effect on the response. Furthermore, co-transfecting a dominant negative GA-binding protein (GABP), a transcription factor known to selectively bind N-box elements, reduced the stimulation-mediated increase. Our results suggest that the N-box within intron I at +890 bp is a regulatory element important in the transcriptional response of Ache to contractile activity in muscle.

Myogenin-dependent nAChR clustering in aneural myotubes

During development of the neuromuscular junction, nerve-derived agrin and the cell substrate laminin stimulate postsynaptic nAChR clustering. This clustering is dependent on activation of the tyrosine kinase, MuSK, which signals receptor clustering via a rapsyn-dependent mechanism. Myogenin is a muscle-specific transcription factor that controls myoblast differentiation and nAChR gene expression. Here, we used RNA interference to investigate if myogenin is also necessary for nAChR clustering. We find that myogenin expression is essential for robust nAChR clustering and cannot be compensated by the muscle regulatory factors MyoD, myf5, and MRF4. In addition, we show that clustering cannot be rescued in myogenin-depleted myotubes by simply overexpressing the essential clustering molecules MuSK, rapsyn, and nAChRs. These data suggest that myogenin controls the expression of molecules crucial to nAChR clustering in addition to its role in regulating nAChR gene expression.

Characterization of a muscle-specific enhancer in human MuSK promoter reveals the essential role of myogenin in controlling activity-dependent gene regulation

Neuromuscular synaptogenesis is initiated by the release of agrin from motor neurons and the activation of the receptor tyrosine kinase, MuSK, in the postsynaptic membrane. MuSK gene expression is regulated by nerve-derived agrin and muscle activity. Agrin stimulates synapse-specific MuSK gene expression by activating GABP(alphabeta) transcription factors in endplate-associated myonuclei. In contrast, the mechanism by which muscle activity regulates MuSK gene expression is not known. We report on a 60-bp MuSK enhancer that confers promoter regulation by muscle differentiation, changes in intracellular calcium, and muscle activity. Within this enhancer, we identified a single E-box that is essential for this regulation. This E-box binds myogenin, and we showed that myogenin is necessary for not only MuSK but also nAChR gene regulation by muscle activity. Surprisingly, the same E-box functions in vivo to mediate muscle-specific and differentiation-dependent gene induction in zebrafish, suggesting an evolutionary conserved mechanism of regulation of synaptic protein gene expression.

Melatonin modulates rat myotube-acetylcholine receptors by inhibiting calmodulin

Melatonin, the pineal gland hormone, modulates alpha-bungarotoxin sensitive nicotinic acetylcholine receptors in sympathetic nerve terminals, cerebellum and chick retina imposing a diurnal variation in functional responses [Markus, R.P., Zago, W.M., Carneiro, R.C., 1996. Melatonin modulation of presynaptic nicotinic acetylcholine receptors in the rat vas deferens. J. Pharmacol. Exp. Ther. 279, 18-22; Markus, R.P., Santos, J.M., Zago, W., Reno, L.A., 2003. Melatonin nocturnal surge modulates nicotinic receptors and nicotine-induced [3HI] glutamate release in rat cerebellum slices. J. Pharmacol. Exp. Ther. 305, 525-530; Sampaio, L.F.S., Hamassaki-Britto, D.E., Markus, R.P., 2005. Influence of melatonin on the development of functional nicotinic acetylcholine receptors in cultured chick retinal cells. Braz. J. Med. Biol. Res. 38, 603-613]. Here we show that in rat myotubes forskolin and melatonin reduced the number of nicotinic acetylcholine receptors expressed in plasma membrane. In addition, these cells expressed melatonin MT1 receptors, which are known to be coupled to G(i)-protein. However, the pharmacological profile of melatonin analogs regarding the reduction in cyclic AMP accumulation and number of nicotinic acetylcholine receptors did not point to a mechanism mediated by activation of G(i)-protein coupled receptors. On the other hand, calmidazolium, a classical inhibitor of calmodulin, reduced in a similar manner both effects. Considering that one isoform of adenylyl cyclase present in rat myotubes is regulated by Ca2+/calmodulin, we propose that melatonin modulates the number of nicotinic acetylcholine receptors via reduction in cyclic AMP accumulation.

Activity-dependent gene regulation in conditionally-immortalized muscle precursor cell lines

Skeletal muscle contractile activity has been implicated in many aspects of muscle cell differentiation and maturation. Much of the research in this area has depended upon costly and labor-intensive cultures of isolated primary muscle cells because widely available immortalized muscle cell lines often do not display a high level of either spontaneous or stimulated contractile activity. We sought to develop conditionally-immortalized skeletal muscle cell lines that would provide a source of myofibers that exhibit robust spontaneous contractile activity similar to primary muscle cultures. Using a tetracycline-regulated retroviral vector expressing a temperature-sensitive T-antigen to infect primary myoblasts, we isolated individual clonal muscle precursor cell lines that have characteristics of activated satellite cells during growth and rapidly differentiate into mature myotubes with spontaneous contractile activity after culture in non-transformation-permissive conditions. Comparison of these cell lines (known as rat myoblast-like tetracycline (RMT) cell lines) to primary cell cultures revealed that they share a wide variety of morphological, physiological, and biochemical characteristics. Most importantly, the time-course and extent of activity-dependent gene regulation observed in primary cell culture for all genes tested, including subunits of the nicotinic acetylcholine receptor (nAChR), muscle specific kinase (MuSK), and myogenin, is reproduced in RMT lines. These immortalized cell lines are a useful alternative to primary cultures for studying muscle differentiation and molecular and physiological aspects of electrical activity in muscle fibers.

Accelerated response of the myogenin gene to denervation in mutant mice lacking phosphorylation of myogenin at threonine 87

Gene expression in skeletal muscle is regulated by a family of myogenic basic helix-loop-helix (bHLH) proteins. The binding of these bHLH proteins, notably MyoD and myogenin, to E-boxes in their own regulatory regions is blocked by protein kinase C (PKC)-mediated phosphorylation of a single threonine residue in their basic region. Because electrical stimulation increases PKC activity in skeletal muscle, these data have led to an attractive model suggesting that electrical activity suppresses gene expression by stimulating phosphorylation of this critical threonine residue in myogenic bHLH proteins. We show that electrical activity stimulates phosphorylation of myogenin at threonine 87 (T87) in vivo and that calmodulin-dependent kinase II (CaMKII), as well as PKC, catalyzes this reaction in vitro. We find that phosphorylation of myogenin at T87 is dispensable for skeletal muscle development. We show, however, that the decrease in myogenin (myg) expression following innervation is delayed and that the increase in expression following denervation is accelerated in mutant mice lacking phosphorylation of myogenin at T87. These data indicate that two distinct innervation-dependent mechanisms restrain myogenin activity: an inactivation mechanism mediated by phosphorylation of myogenin at T87, and a second, novel regulatory mechanism that regulates myg gene activity independently of T87 phosphorylation.

Thrombin downregulates muscle acetylcholine receptors via an IP3 signaling pathway by activating its G-protein-coupled protease-activated receptor-1

Regulation of thrombin activity may be required during skeletal muscle differentiation since the thrombin tissue inhibitor protease nexin-1 appears at the myotube stage before being localized at the neuromuscular synapse. Here, we have used a model of rat fetal myotube primary cultures to study the effect of thrombin on acetylcholine receptor (AChR) expression, which is enhanced at the myotube stage. Our results show that thrombin decreases both the number of surface AChRs (AChRn) and AChR alpha-subunit gene expression. Using the agonist peptide SFLLRN, we establish that the AChRn decrease is mediated by the G protein-coupled thrombin receptor "protease-activated receptor-1" (PAR-1). Moreover, the specific thrombin inhibitor hirudin increases AChRn by inhibiting the thrombin intrinsically present in the cultures. We further demonstrate that the activation of PAR-1 by thrombin induces intracellular calcium movements that are blocked by 2-APB, an inhibitor of inositol 1,4,5-triphosphate (IP3)-induced calcium release. These calcium signals are more intense in nuclei than in the cytoplasm and are consistent with the intracellular distribution of IP3 receptor that we find in the cytoplasm in a cross-striated pattern and at a high level in the nuclear envelope zone. Finally, we show that the blockade of these IP3-induced calcium signals by 2-APB prevents the AChRn decrease induced by thrombin. Our results thus demonstrate that thrombin downregulates AChR expression by activating PAR-1 and that this effect is mediated via an IP3 signaling pathway.

Protein kinase C and calcium/calmodulin-activated protein kinase II (CaMK II) suppress nicotinic acetylcholine receptor gene expression in mammalian muscle. A specific role for CaMK II in activity-dependent gene expression

Nicotinic acetylcholine receptor (nAChR) gene expression is regulated by both muscle activity and increased intracellular calcium. This regulation is an important developmental event that rids receptors from the extrajunctional region of the developing muscle fiber. In avian muscle, it has been proposed that muscle activity suppresses nAChR gene expression via calcium-activated protein kinase C (PKC)-dependent phosphorylation of the myogenic transcription factor, myogenin. Here, we examined the role that PKC and other kinases play in mediating calcium- and activity-dependent suppression of nAChR genes in rat primary myotubes. We found that although activated PKC could regulate nAChR promoter activity and transiently suppressed both nAChR and myogenin gene expression, it did not appear to be required for calcium- or activity-dependent control of nAChR gene expression in mammalian muscle. Neither depletion of PKC from myotubes nor specific pharmacological inhibition of PKC blocked the suppression of nAChR gene expression produced by calcium or muscle depolarization. In contrast, we provide evidence that calcium/calmodulin-activated protein kinase II participates in mediating the effects of muscle depolarization on nAChR and myogenin gene expression.

Interaction of the myogenic determination factor myogenin with E12 and a DNA target: mechanism and kinetics

The myogenic determination factors MyoD, myogenin, myf5, and MRF4 are members of the basic helix-loop-helix (bHLH) family of transcription factors and crucial agents of myogenesis. The bHLH regions of these proteins enable them to dimerize with E proteins, another class of the bHLH family, and to bind a specific DNA element known as an E box (CANNTG consensus sequence), which results in the activation of muscle-specific gene expression. As a model for such assembly of the myogenic determination factor/E protein-DNA ternary complex, we have studied the physiologically relevant association of myogenin, E12, and the 3' E box of the acetylcholine receptor (AChR) alpha-subunit gene enhancer. Using the technique of electrophoretic mobility shift assay combined with order-of-addition and time-course experiments, we find that heterodimerization of myogenin with E12 occurs prior to DNA-binding. In addition, we deduce the dissociation (Kd) and rate (k) constants for each step in the formation of the myogenin/E12-DNA ternary complex. Kinetic simulations indicate that at 37 degrees C myogenin and E12 heterodimerize with a Kd of 36 microM (k(on) of 573 M(-1) x s(-1) and k(off )of 0.0205 x s(-1)), and that subsequently the heterodimer binds the AChR alpha-subunit gene enhancer 3' E box with a Kd of 8.8 nM (with possible k(on) and k(off) values ranging from 1.0x10(8) to 14.1x10(8) M(-1) x s(-1), and 0.875 to 12.3 s(-1), respectively).