Involvement of cyclic GMP in the fusion of chick embryonic myoblasts in culture

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

FoxO1a-cyclic GMP-dependent kinase I interactions orchestrate myoblast fusion

The regulatory circuits that orchestrate mammalian myoblast cell fusion during myogenesis are poorly understood. The transcriptional activity of FoxO1a directly regulates this process, yet the molecular mechanisms governing FoxO1a activity during muscle cell differentiation remain unknown. Here we show an autoregulatory loop in which FoxO1a directly activates transcription of the cyclic GMP-dependent protein kinase I (cGKI) gene and where the ensuing cGKI activity phosphorylates FoxO1a and abolishes its DNA binding activity. These findings establish the FoxO1a-to-cGKI pathway as a novel feedback loop that allows the precise tuning of myoblast fusion. Interestingly, this pathway appears to operate independently of muscle cell differentiation programs directed by myogenic transcription factors.

NMDA receptor-mediated calcium influx plays an essential role in myoblast fusion

Ca2+ influx is known to be prerequisite for myoblast fusion during skeletal muscle differentiation. Here, we show that the N-methyl-D-aspartate (NMDA) receptor is involved in the Ca2+ influx of C2C12 myoblasts. NMDA receptor (NR) 1 and NR2D were expressed in the myoblasts during muscle differentiation. Using Ca2+ imaging analysis, Ca2+ influx through NRs was directly measured at a single-cell level. l-Glutamate increased myoblast fusion as well as intracellular Ca2+ levels, and both effects were completely blocked by MK801, a selective antagonist of NRs. Furthermore, treatment with the Ca2+ ionophore A23187 recovered MK801-mediated inhibition of myoblast fusion. These results suggest that the NRs may play an important role in myoblast fusion by mediating Ca2+ influx.

Hyperpolarization, but not depolarization, increases intracellular Ca(2+) level in cultured chick myoblasts

Ca(2+) influx appears to be important for triggering myoblast fusion. It remains, however, unclear how Ca(2+) influx rises prior to myoblast fusion. The present study examines a possible involvement of the voltage-dependent Ca(2+) influx pathways. Treatment with the L-type Ca(2+) channel blockers, diltiazem, and nifedipine did not alter cytosolic Ca(2+) levels. Depolarization with high K(+) solution and activation of Ca(2+) channel with Bay K 8644, and agonist of voltage dependent Ca(2+) channels, failed to elicit increases intracellular Ca(2+) level, indicating the absence of depolarization-operated mechanisms. In contrast, phloretin, an agonist of Ca(2+)-activated potassium (K(Ca)) channels, was able to hyperpolarize membrane potential and promoted Ca(2+) influx. These effects were completely abolished by treatment of charybdotoxin, a specific inhibitor of K(Ca) channels. In addition, gadolinium, a potent stretch-activated channel (SAC) blocker, prevented the phloretin-mediated Ca(2+) increase, indicating the involvement of SACs in Ca(2+) influx. Furthermore, phloretin stimulated precocious myoblast fusion and this effect was blocked with gadolinium or charybdotoxin. Taken together, these results suggest that induced hyperpolarization, but not depolarization increases Ca(2+) influx through stretch-activated channels, and in turn triggers myoblast fusion.

Transplacental exposure to methylene blue initiates teratogenesis in the mouse: preliminary evidence for a mechanistic implication of cyclic GMP pathway disruption

BACKGROUND

The vital dye methylene blue (MB) has been shown to be teratogenic when injected into the amnion in the second trimester. On the other hand, the teratogenic potential of transplacental exposure to MB has not been determined.

METHODS

MB was administered subcutaneously to ICR (CD-1) mice at 0, 35, 50, 60, or 70 mg/kg on gestation day 8 (plug day = day 0). Teratological assessments were carried out at term gestation, on gestation day 18. Since MB inhibits soluble guanylate cyclase enzyme activity, zaprinast (ZPN), a selective cGMP-phosphodiesterase type V inhibitor, was administered to prevent developmental disorders initiated by MB at 50 mg/kg.

RESULTS

There was a dose-dependent increment of embryolethality. MB treatment also produced axial skeleton and neural tube defects. Coadministration of ZPN (20 mg/kg per three times) abolished completely MB-induced neural tube defects and reduced by one-half the incidence of fetuses exhibiting axial skeletal defects. ZPN did not provide protection against the embryocidal effects of MB.

CONCLUSIONS

This study showed that transplacental exposure to MB is teratogenic in the mouse. Coadministration of ZPN prevented partly MB-induced teratogenesis, which supports the hypothesis that imbalance of cGMP pathway accounts, in part, for the teratogenicity of MB.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Molecular cloning and expression of nitric oxide synthase gene in chick embryonic muscle cells

The chick skeletal muscle nitric oxide synthase (NOS) gene was cloned in order to further define the involvement of NOS in the differentiation of skeletal muscle cells. The respective cDNA had an open reading frame of 1136 amino acid residues, predicting a protein of 129,709.85 Da, and recognition sites for FAD, FMN, NADPH, and a calmodulin-binding site like those of other mammalian NOS's. Alignment of the deduced amino acid sequence revealed high homology with mammalian inducible NOS (iNOS), but not other NOS isoforms, suggesting chick skeletal muscle NOS may be an iNOS isoform. Immunoblots showed that NOS expression was highly restricted in embryonic muscle, but not in adult skeletal muscle: NOS expression markedly increased from embryonic day 9, reached a maximum by embryonic day 13, and then gradually declined until it was no longer detectable on embryonic day 19. When muscle cells obtained on embryonic day 12 were cultured, NOS expression increased transiently prior to the onset of differentiation and decreased thereafter. Inhibition of NOS expression by PDTC completely prevented muscle cell differentiation, as indicated by the inhibition of expression of myosin heavy chain and creatine kinase. The inhibitory effect of PDTC was completely reversed by addition of sodium nitroprusside, a compound that produces NO. These results clearly indicate that NOS is significantly involved in the differentiation of chick skeletal muscle cells.

NF-kappaB-dependent expression of nitric oxide synthase is required for membrane fusion of chick embryonic myoblasts

The activity of nitric oxide synthase (NOS) has recently been shown to increase transiently but dramatically in chick embryonic myoblasts that are competent for fusion and that NO acts as a messenger for the cell fusion. Here we show that the rise in NOS activity is tightly correlated with an increase in NOS protein level, and its synthesis is under transcriptional control. In addition, a partial cDNA sequence of NOS obtained by reverse transcription PCR on total RNA from chick myoblasts was found to be identical with that of the inducible type of NOS (iNOS) from chick macrophage. Thus chick myoblast NOS must belong to the family of iNOS. Consistently, pyrrolidine dithiocarbamate, a potent inhibitor of nuclear factor kappaB (NF-kappaB), prevented the expression of myoblast NOS. Furthermore the antioxidant also strongly inhibited cell fusion, and its inhibitory effect was reversed by treatment with sodium nitroprusside, an NO-generating agent. In addition, nuclear extracts obtained from myoblasts that were competent for fusion, but not those from proliferating cells or from fully differentiated myotubes, were capable of binding to the consensus NF-kappaB site in the promoter region of the gene encoding iNOS. These results suggest that NF-kappaB-dependent expression of NOS is an important step in membrane fusion of chick embryonic myoblasts.

Myoblast fusion requires cytosolic calcium elevation but not activation of voltage-dependent calcium channels

Many studies of in vitro skeletal myogenesis have demonstrated that fusion of myoblasts into multinucleated myotubes is regulated by calcium-dependent processes. Calcium ions appear to be necessary at the outer face of the membrane, and an additional internal calcium increase seems required to promote fusion of aligned myoblasts. It has been proposed that a calcium influx could take place prior to fusion and that this may be mediated by voltage-dependent calcium channels. Previously, we showed that two types of voltage-dependent calcium currents were expressed in multinucleated myotubes but not in rat myoblasts growing in primary culture before the withdrawal of the growth medium. We also showed that the previous formation of multinucleated synticia was not a prerequisite of developmental appearance of calcium currents, suggesting that the two events were time-correlated but not sequentially dependent. These features led us to investigate changes in internal calcium activity and the possible appearance of voltage-dependent calcium influx pathways just after the promotion of fusion by the change of culture medium. The results confirm that a rise in cytosolic calcium activity occurs slightly before fusion in confluent myoblasts and remained in newly formed myotubes. Reducing this elevation by internal calcium buffering lowered myoblast fusion and, reciprocally, blocking cell fusion prevented calcium increase. Treatment with the organic calcium channel blockers nifedipine (5 microM) and PN 200-110 (1 microM) did not alter cytosolic calcium changes nor cell fusion, and voltage-dependent calcium currents were never observed by the perforated patch-clamp technique in aligned fusion-competent myoblasts. Other voltage-operated mechanisms of calcium rise were not detected since depolarization with hyperpotassium solutions failed to elicit increases in intracellular calcium. On the contrary, acetylcholine was able to promote extracellular calcium-dependent calcium transients. Our results confirm the requirement of an increase in resting calcium during fusion, but do not support the hypothesis of an influx through voltage-dependent channels or other voltage-operated pathways. The elevation of internal calcium activity may result from other mechanisms, such as a cholinergic action for example.