Calcium influx blocks the skeletal muscle acetylcholine receptor alpha-subunit gene in vivo

The histone deacetylase HDAC4 connects neural activity to muscle transcriptional reprogramming

Neural activity actively regulates muscle gene expression. This regulation is crucial for specifying muscle functionality and synaptic protein expression. How neural activity is relayed into nuclei and connected to the muscle transcriptional machinery, however, is not known. Here we identify the histone deacetylase HDAC4 as the critical linker connecting neural activity to muscle transcription. We found that HDAC4 is normally concentrated at the neuromuscular junction (NMJ), where nerve innervates muscle. Remarkably, reduced neural input by surgical denervation or neuromuscular diseases dissociates HDAC4 from the NMJ and dramatically induces its expression, leading to robust HDAC4 nuclear accumulation. We present evidence that nuclear accumulated HDAC4 is responsible for the coordinated induction of synaptic genes upon denervation. Inactivation of HDAC4 prevents denervation-induced synaptic acetyl-choline receptor (nAChR) and MUSK transcription whereas forced expression of HDAC4 mimics denervation and activates ectopic nAChR transcription throughout myofibers. We determined that HDAC4 executes activity-dependent transcription by regulating the Dach2-myogenin transcriptional cascade where inhibition of the repressor Dach2 by HDAC4 permits the induction of the transcription factor myogenin, which in turn activates synaptic gene expression. Our findings establish HDAC4 as a neural activity-regulated deacetylase and a key signaling component that relays neural activity to the muscle transcriptional machinery.

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.

Unique signaling properties of B cell antigen receptor in mature and immature B cells: implications for tolerance and activation

Immature B cells display increased sensitivity to tolerance induction compared with their mature counterparts. The molecular mechanisms underlying these differences are poorly defined. In this study, we demonstrate unique maturation stage-dependent differences in B cell Ag receptor (BCR) signaling, including BCR-mediated calcium mobilization responses. Immature B cells display greater increases in intracellular calcium concentrations following Ag stimulation. This has consequences for the induction of biologically relevant responses: immature B cells require lower Ag concentrations for activation than mature B cells, as measured by induction of receptor editing and CD86 expression, respectively. BCR-induced tyrosine phosphorylation of CD79a, Lyn, B cell linker protein, and phospholipase Cgamma2 is enhanced in immature B cells and they exhibit greater capacitative calcium entry in response to Ag. Moreover, B cell linker protein, Bruton's tyrosine kinase, and phospholipase Cgamma2, which are crucial for the induction of calcium mobilization responses, are present at approximately 3-fold higher levels in immature B cells, potentially contributing to increased mobilization of calcium. Consistent with this possibility, we found that the previously reported lack of inositol-1,4,5-triphosphate production in immature B cells may be explained by enhanced inositol-1,4,5-triphosphate breakdown. These data demonstrate that multiple mechanisms guarantee increased Ag-induced mobilization of calcium in immature B cells and presumably ensure elimination of autoreactive B cells from the repertoire.

Role of the sarcoplasmic reticulum in regulating the activity-dependent expression of the glycogen phosphorylase gene in contractile skeletal muscle cells

Nerve-evoked contractile activity in skeletal muscle regulates transcript and protein levels of many metabolic genes in a coordinate fashion, including the muscle isozyme of glycogen phosphorylase (MGP). Cellular signaling mechanisms mediating the activity-dependent modulation of MGP transcript levels were investigated in a spontaneously contractile rat skeletal muscle cell line (Rmo). Mechanisms regulating MGP mRNA levels in Rmo myotubes were compared with those previously shown to modulate the gene encoding the alpha subunit of the acetylcholine receptor (alphaAChR). Reducing the resting membrane potential from -78 to -30 mV, either electrochemically (KCl) or by increasing Na(+) permeability (veratridine): (1) prevented activation of transverse tubules, (2) impeded calcium release by the sarcoplasmic reticulum (SR), and (3) blocked Rmo contractility. MGP mRNA levels decreased to 30% of control levels and alphaAChR levels increased to 350% following 24 h of depolarization. Differing mechanisms appear to mediate this voltage-dependent regulation of MGP and alphaAChR. Inhibition of SR calcium efflux selectively decreased MGP mRNA levels by 30-50% when using dantrolene, thapsigargin, or a dose of ryanodine shown to inactivate Ca(2+)-induced SR Ca(2+) release (CICR). By contrast, blockade of voltage sensors in transverse tubules with nifedipine, a dihydroaminopyridine (DHAP) antagonist, selectively increased alphaAChR mRNA levels by twofold. These data indicate that the voltage-dependent regulation of AChR gene expression differs from that modulating the MGP gene. KCl-induced depolarization and dantrolene both inhibit pulsatile SR Ca(2+) efflux in Rmo myotubes, but by differing mechanisms. Depolarization and dantrolene comparably reduced MGP mRNA levels and decreased MGP transcript stability from a t(1/2) of 24 h to 14.5 and 16 h, respectively. Reduced transcript stability can account for the observed reduction in mRNA levels of MGP in noncontractile Rmo myotubes and could be a significant regulatory mechanism in skeletal muscle that coordinates the activity-dependent expression of MGP with other glycogenolytic genes.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Intracellular signaling molecules involved in an inhibitory factor-induced decrease in fetal-type AChR expression

The innervation-induced down-regulation of fetal-type acetylcholine receptor (AChR) expression in developing muscle fibers has largely been attributed to nerve-evoked muscle activity; however, there is increasing evidence that a neural trophic factor also contributes to this receptor down-regulation. Previous studies from this laboratory have shown that neural extracts contain a factor which decreases fetal-type AChR expression in skeletal muscle cell lines and therefore may account for the proposed inhibitory neurotrophic influence. The current study investigated possible intracellular signaling molecules involved in this receptor down-regulation and demonstrated that activation of protein kinase C and p70(S6k) appeared to be important in receptor down-regulation. Decreases in AChR density were independent of myogenin. In addition, the receptor down-regulation was independent of neuregulin, which also induces p70(S6k) activity. These studies demonstrate that neural extracts contain an inhibitory factor which can down-regulate fetal-type AChR expression independently of nerve-evoked muscle activity through intracellular signaling molecules which are known to regulate AChR expression.

Phosphorylation of CD19 Y484 and Y515, and Linked Activation of Phosphatidylinositol 3-Kinase, Are Required for B Cell Antigen Receptor-Mediated Activation of Bruton’s Tyrosine Kinase

Bruton’s tyrosine kinase (Btk) plays a critical role in B cell Ag receptor (BCR) signaling, as indicated by the X-linked immunodeficiency and X-linked agammaglobulinemia phenotypes of mice and men that express mutant forms of the kinase. Although Btk activity can be regulated by Src-family and Syk tyrosine kinases, and perhaps by phosphatidylinositol 3,4,5-trisphosphate, BCR-coupled signaling pathways leading to Btk activation are poorly understood. In view of previous findings that CD19 is involved in BCR-mediated phosphatidylinositol 3-kinase (PI3-K) activation, we assessed its role in Btk activation. Using a CD19 reconstituted myeloma model and CD19 gene-ablated animals we found that BCR-mediated Btk activation and phosphorylation are dependent on the expression of CD19, while BCR-mediated activation of Lyn and Syk is not. Wortmannin preincubation inhibited the BCR-mediated activation and phosphorylation of Btk. Btk activation was not rescued in the myeloma by expression of a CD19 mutant in which tyrosine residues previously shown to mediate CD19 interaction with PI3-K, Y484 and Y515, were changed to phenylalanine. Taken together, the data presented indicate that BCR aggregation-driven CD19 phosphorylation functions to promote Btk activation via recruitment and activation of PI3-K. Resultant phosphatidylinositol 3,4,5-trisphosphate probably functions to localize Btk for subsequent phosphorylation and activation by Src and Syk family kinases.

Role for calcium from the sarcoplasmic reticulum in coupling muscle activity to nicotinic acetylcholine receptor gene expression in rat

Neurally evoked muscle electrical activity suppresses nicotinic acetylcholine receptor (nAChR) gene expression in extrajunctional domains of adult muscle fibers. It has been proposed that this regulation is mediated by calcium influx through voltage-dependent L-type calcium channels but bypasses the sarcoplasmic reticulum in chick and mouse C2C12 cells. Here we report that in rat muscle calcium influx through L-type calcium channels preferentially reduced nAChR epsilon-subunit RNA via a post-transcriptional mechanism. In contrast, calcium release from the sarcoplasmic reticulum (SR) suppressed nAChR subunit RNA levels as a result of decreasing nAChR subunit promoter activity. Finally, we show that this decreased promoter activity is mediated through the same DNA sequences that control activity-dependent gene expression. Therefore, we propose that in rat muscle, calcium release from the SR participates in coupling muscle depolarization to nAChR gene expression.

Nuclear clustering in myotubes: a proposed role in acetylcholine receptor mRNA expression

We investigated the functional relationship between nuclear topology, as expressed by degree and type of nuclear aggregation, and appearance of acetylcholine receptor (AChR) subunit mRNAs. Embryonic chick muscle cell cultures treated with the muscle activity blocking agents decamethonium (DCM), d-tubocurare (TBC), and tetrodotoxin (TTX) or co-cultured with cholinergic neurons were examined for the influence of muscle activity on nuclear aggregation and its effects on AChR alpha-, gamma-, and delta-subunit message expression. mRNA was measured by in situ hybridization and nuclei were visualized by bis-benzimide DNA staining. DCM and TBC treatments, as well as neuronal co-culture, resulted in increased nuclear clustering within myotubes and a per nucleus upregulation in mRNA expression relative to control for each subunit. The pattern of nuclear aggregation was treatment dependent, with more and larger aggregates found when myotubes were co-cultured with neurons. Moreover, as nuclear aggregates became larger: (1) nearly all nuclei within active aggregates expressed mRNA and (2) local accumulation (mRNA per unit area) was elevated relative to single nuclei, while per nucleus mRNA production decreased. To determine whether mRNA expression was transient and did not result in steady-state upregulation of AChR receptor protein, we performed a double labeling of surface AChRs with 125I-alpha-bungarotoxin (125I-alpha-BTX) concomitant to the in situ hybridization for mRNA quantification on TTX treated muscle cells. Surface receptor expression tracked mRNA expression forall types of nuclear topology observed, indicating that message levels are in fact reliable indicators of receptor population on the plasma membrane surface in myotubes. We propose that nuclear clustering is an organelle-level, accessory mechanism whereby cells concentrate relatively large amounts of AChR mRNA/protein in specific myotube regions.

Cross-talk between transcriptional regulation by thyroid hormone and myogenin: new aspects of the Ca2+-dependent expression of the fast-type sarcoplasmic reticulum Ca2+-ATPase

We have previously demonstrated an interaction between the major determinants of skeletal muscle phenotype by showing that continuous contractile activity represses the thyroid hormone (3,3', 5-tri-iodothyronine; T3)-dependent transcriptional activity of fast-type sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase (SERCA1), a characteristic of the fast phenotype. Both the free cytosolic Ca2+ concentration ([Ca2+]i) and the myogenic determination factors MyoD and myogenin have been implicated as mediators of the effect of contractile activity on skeletal muscle phenotype. Using L6 cells we have shown that an increase in the steady-state [Ca2+]i above the resting level of 120 nM indeed can mimic the effect of contractile activity on T3-dependent SERCA1 expression. We now show that the repressing effect of increased [Ca2+]i on T3-dependent SERCA1 expression in L6 cells is exerted at a pre-translational level and is accompanied by increased myogenin mRNA expression. Myogenin overexpression in these cells revealed that increased expression of myogenin alone strongly decreases the T3-dependent stimulation of SERCA1 promoter activity. These results suggest a pathway for the regulation of skeletal muscle phenotype in which [Ca2+]i mediates the effect of contractile activity by regulating the expression of myogenin, which in turn interferes with transcriptional regulation by T3.

Opposite effect of intracellular Ca2+ and protein kinase C on the expression of inwardly rectifying K+ channel 1 in mouse skeletal muscle

The level of inwardly rectifying K+ channel 1 (IRK1) mRNA decreased upon denervation and increased during muscle differentiation in mouse skeletal muscle. To identify the mechanism(s) underlying the regulation of IRK1 mRNA expression, we examined its expression using the well differentiated C2C12 mouse skeletal muscle cell line as a model system. Since nerve-induced muscle activity results in contraction, it was questioned whether the changes in IRK1 expression might be relevant to the increased intracellular calcium that functions as a cytoplasmic messenger in excitation-contraction coupling. Indeed, activation of either L-type calcium channels or ryanodine receptors increased the level of IRK1 mRNA. More directly, ionomycin activated the IRK1 expression in time- and dose-dependent manners, which was abolished by treatment with EGTA. Genistein, a tyrosine kinase inhibitor, also abolished the stimulating effect of ionomycin. Meanwhile, activation of protein kinase C by 12-O-tetradecanoylphorbol acetate (TPA) markedly decreased the level of IRK1 mRNA, which required ongoing protein synthesis. Actinomycin D experiments revealed that ionomycin increased the half-life of IRK1 mRNA from 0.86 to 1.97 h, but TPA decreased it to 0.38 h. However, neither ionomycin nor TPA appreciably altered the rate of IRK1 gene transcription. Based on these observations, we conclude that intracellular calcium and protein kinase C are oppositely involved in the muscle activity-dependent regulation of IRK1 gene expression and that both act at the level of mRNA stability.

Expression of nerve-regulated genes in muscles of mouse mutants affected by spinal muscular atrophies and muscular dystrophies

The expression of the genes for the alpha-subunit of AChR (AChR alpha), for the myogenic factors myogenin and MyoD, for the calcium-binding protein parvalbumin (PV), and for the muscular chloride channel CIC-1 was studied in the three mouse spinal muscular atrophies (SMAs). These were the mutants "wobbler" (WR), "muscle deficient" (MDF) and "progressive motor neuronopathy" (PMN). Murine myopathies "muscular dystrophy with myositis" (MDM) and "X-linked muscular dystrophy" (MDX) were used as controls. AChR alpha and myogenin mRNA levels were strongly elevated in muscles affected by SMAs (reflecting denervation), whereas only myogenin mRNA was moderately elevated in MDX and MDM muscles, probably due to fiber regeneration. As in denervated muscle, CIC-1 and PV mRNA levels were lowered in SMAs. No changes were seen in muscles of up to 222-day-old symptomless ciliary neurotrophic factor (CNTF) knockout mice. The patterns of gene expression were characteristic for the type of muscle disease, indicating their possible usefulness for clinical diagnosis.

Depolarization-transcription signals in skeletal muscle use calcium flux through L channels, but bypass the sarcoplasmic reticulum

Membrane depolarization inactivates acetylcholine receptor (AChR) genes in skeletal muscle. We have studied this process in C2C12 cells, focusing on the role of calcium. Cytoplasmic calcium was monitored with fluo-3, and the activity of receptor genes was measured with a sensitive transcript elongation assay. Removal of extracellular calcium or blockage of L-type calcium channels disrupts signaling, even when release of calcium from the sarcoplasmic reticulum (SR) is not impeded, whereas L channel agonists induce signaling without membrane depolarization or release of calcium from intracellular stores. Activators of calcium release from the SR do not inhibit AChR genes, either in C2C12 or in chicken skeletal muscle in vivo. It appears that calcium ions do not act as messengers between sarcolemma and nucleus but target a sensor near their port of entry where they initiate a signal that bypasses the SR.