Three types of muscle-specific gene expression in fusion-blocked rat skeletal muscle cells: translational control in EGTA-treated cells

Postnatal skeletal muscle myogenesis governed by signal transduction networks: MAPKs and PI3K-Akt control multiple steps

Skeletal muscle myogenesis represents one of the most intensively and extensively examined systems of cell differentiation, tissue formation, and regeneration. Muscle regeneration provides an in vivo model system of postnatal myogenesis. It comprises multiple steps including muscle stem cell (or satellite cell) quiescence, activation, migration, myogenic determination, myoblast proliferation, myocyte differentiation, myofiber maturation, and hypertrophy. A variety of extracellular signaling and subsequent intracellular signal transduction pathways or networks govern the individual steps of postnatal myogenesis. Among them, MAPK pathways (the ERK, JNK, p38 MAPK, and ERK5 pathways) and PI3K-Akt signaling regulate multiple steps of myogenesis. Ca2+, cytokine, and Wnt signaling also participate in several myogenesis steps. These signaling pathways often control cell cycle regulatory proteins or the muscle-specific MyoD family and the MEF2 family of transcription factors. This article comprehensively reviews molecular mechanisms of the individual steps of postnatal skeletal muscle myogenesis by focusing on signal transduction pathways or networks. Nevertheless, no or only a partial signaling molecules or pathways have been identified in some responses during myogenesis. The elucidation of these unidentified signaling molecules and pathways leads to an extensive understanding of the molecular mechanisms of myogenesis.

Weak Electromagnetic Fields Accelerate Fusion of Myoblasts

Weak electromagnetic fields (WEF) alter Ca²⁺ handling in skeletal muscle myotubes. Owing to the involvement of Ca²⁺ in muscle development, we investigated whether WEF affects fusion of myoblasts in culture. Rat primary myoblast cultures were exposed to WEF (1.75 µT, 16 Hz) for up to six days. Under control conditions, cell fusion and creatine kinase (CK) activity increased in parallel and peaked at 4–6 days. WEF enhanced the extent of fusion after one and two days (by ~40%) vs. control, but not thereafter. Exposure to WEF also enhanced CK activity after two days (almost four-fold), but not afterwards. Incorporation of ³H-thymidine into DNA was enhanced by one-day exposure to WEF (~40%), indicating increased cell replication. Using the potentiometric fluorescent dye di-8-ANEPPS, we found that exposure of cells to 150 mM KCl resulted in depolarization of the cell membrane. However, prior exposure of cells to WEF for one day followed by addition of KCl resulted in hyperpolarization of the cell membrane. Acute exposure of cells to WEF also resulted in hyperpolarization of the cell membrane. Twenty-four hour incubation of myoblasts with gambogic acid, an inhibitor of the inward rectifying K⁺ channel 2.1 (K_ir2.1), did not affect cell fusion, WEF-mediated acceleration of fusion or hyperpolarization. These data demonstrate that WEF accelerates fusion of myoblasts, resulting in myotube formation. The WEF effect is associated with hyperpolarization but WEF does not appear to mediate its effects on fusion by activating K_ir2.1 channels.

M-Ras is activated by bone morphogenetic protein-2 and participates in osteoblastic determination, differentiation, and transdifferentiation

The small GTPase M-Ras is highly expressed in the central nervous system and plays essential roles in neuronal differentiation. However, its other cellular and physiological functions remain to be elucidated. Here, we clarify the novel functions of M-Ras in osteogenesis. M-Ras was prominently expressed in developing mouse bones particularly in osteoblasts and hypertrophic chondrocytes. Its expression was elevated in C3H/10T1/2 (10T1/2) mesenchymal cells and in MC3T3-E1 preosteoblasts during differentiation into osteoblasts. Treatment of C2C12 skeletal muscle myoblasts with bone morphogenetic protein-2 (BMP-2) to bring about transdifferentiation into osteoblasts also induced M-Ras mRNA and protein expression. Moreover, the BMP-2 treatment activated the M-Ras protein. Stable expression of the constitutively active M-Ras(G22V) in 10T1/2 cells facilitated osteoblast differentiation. M-Ras(G22V) also induced transdifferentiation of C2C12 cells into osteoblasts. In contrast, knockdown of endogenous M-Ras by RNAi interfered with osteoblast differentiation in 10T1/2 and MC3T3-E1 cells. Osteoblast differentiation in M-Ras(G22V)-expressing C2C12 cells was inhibited by treatment with inhibitors of p38 MAP kinase (MAPK) and c-Jun N-terminal kinase (JNK) but not by inhibitors of MAPK and ERK kinase (MEK) or phosphatidylinositol 3-kinase. These results imply that M-Ras, induced and activated by BMP-2 signaling, participates in the osteoblastic determination, differentiation, and transdifferentiation under p38 MAPK and JNK regulation.

Physiological factors influencing the growth of skeletal muscle

The growth of muscle can be regulated by developmental changes or by alterations in hormone levels or in the rate or amount of work demanded. The mechanisms and structures involved in growth processes can be studied by controlling these factors. The models used are chicken anterior latissimus dorsi (ALD) muscle under the influence of overloading and rabbit tibialis anterior (TA) muscle under the influence of chronic nerve stimulation. Both models involve changes in the isoform of myosin that is expressed. Methods of study include quantitative ultrastructural analysis, immunofluorescence and in situ mRNA hybridization. In overloaded chick ALD fibres polysomes are nonuniformly distributed between the myofibrils and in a peripheral annulus even though subcellular concentrations of the new isoform are not found. In normal rabbit muscle the highest concentration of myosin mRNA detected by in situ hybridization is found in the subsarcolemmal zone. In stimulated TA polysomes are found between myofibrils. It appears that the myosin mRNA accumulates at specific cell locations before translation; then diffusion of isomyosin and rapid exchange into myofibrils follows. Therefore, regulation of growth may be possible at the transcriptional, translational and assembly stages.

DA-Raf1, a competent intrinsic dominant-negative antagonist of the Ras-ERK pathway, is required for myogenic differentiation

Ras activates Raf, leading to the extracellular-regulated kinase (ERK)–mitogen-activated protein kinase pathway, which is involved in a variety of cellular, physiological, and pathological responses. Thus, regulators of this Ras–Raf interaction play crucial roles in these responses. In this study, we report a novel regulator of the Ras–Raf interaction named DA-Raf1. DA-Raf1 is a splicing isoform of A-Raf with a wider tissue distribution than A-Raf. It contains the Ras-binding domain but lacks the kinase domain, which is responsible for activation of the ERK pathway. As inferred from its structure, DA-Raf1 bound to activated Ras as well as M-Ras and interfered with the ERK pathway. The Ras–ERK pathway is essential for the negative regulation of myogenic differentiation induced by growth factors. DA-Raf1 served as a positive regulator of myogenic differentiation by inducing cell cycle arrest, the expression of myogenin and other muscle-specific proteins, and myotube formation. These results imply that DA-Raf1 is the first identified competent, intrinsic, dominant-negative antagonist of the Ras–ERK pathway.

Sustained activation of M-Ras induced by nerve growth factor is essential for neuronal differentiation of PC12 cells

Neuronal differentiation in PC12 cells induced by nerve growth factor (NGF) requires sustained activation of ERK/MAP kinase pathway (Raf-MEK-ERK cascade). Although classical Ras (H-Ras, K-Ras, and N-Ras) activated by NGF signaling induces activation of ERK pathway, the activation is transient and not sufficient for PC12 cell differentiation. Instead, it has been widely accepted that NGF signaling-mediated Rap1 activation causes sustained activation of ERK pathway. There has been no direct evidence, however, that Rap1 participates in neuronal differentiation. Here we show that NGF signaling induces sustained activation of M-Ras and subsequent sustained activation of ERK pathway and the transcription factor CREB leading to PC12 cell differentiation. Exogenously expressed constitutively active mutant of M-Ras caused neurite outgrowth in PC12 cells and activating phosphorylation of ERK, whereas activated Rap1 did not. Knockdown of endogenous M-Ras by small interfering RNAs as well as the expression of a dominant-negative mutant of M-Ras interfered with NGF-induced neuritogenesis. Since MEK inhibitors prevented M-Ras-induced neurite outgrowth, ERK pathway participates in this differentiation pathway. Furthermore, M-Ras brought about ERK pathway-mediated activating phosphorylation of CREB and the CREB-mediated transcription. In addition, a dominant-negative mutant of CREB inhibited M-Ras-induced neuritogenesis. Taken together, NGF-induced PC12 cell differentiation requires M-Ras-ERK pathway-mediated activation of CREB. M-Ras was predominantly expressed in the hippocampus and cerebellum of mouse brain and in the gray matter of the spinal cord. All these properties of M-Ras were apparently indistinguishable from those of H-Ras. However, NGF stimulation caused transient activation of classical Ras proteins but sustained activation of M-Ras as well as sustained activating phosphorylation of ERK and CREB. Therefore, M-Ras is essential for neuronal differentiation in PC12 cells by inducing sustained activation of ERK pathway.

Lipid products of phosphoinositide 3-kinase abrogate genistein-induced fusion inhibition in myoblasts

Genistein (4',5,7-trihydroxyisoflavone) is a tyrosine kinase inhibitor. Although the agent has shown to inhibit myoblast differentiation, neither intracellular target(s) as a tyrosine kinase inhibitor nor action mechanism of the agent is well known. Here we studied the effect of genistein on the differentiation of myoblasts. Genistein strongly but reversibly blocked both myoblast fusion and synthesis of the muscle-specific proteins. The agent also reversibly reduced the phosphorylation level of focal adhesion kinase (FAK), a cytoplasmic tyrosine kinase, and its interaction with p85, the regulatory subunit of phosphoinositide 3-kinase (PI3-kinase). In addition, genistein indirectly inhibited PI3-kinase activity and blocked calcium influx which is required for myoblast fusion. However, both genistein-induced inhibition of cell fusion and calcium influx were abrogated by the lipid products of PI3-kinase. These results demonstrate that genistein can exert their effect on the signaling pathway from FAK to calcium influx via PI3-kinase in the differentiation of myoblasts.

Overexpression of calpastatin inhibits L8 myoblast fusion

The formation of skeletal muscle fibers involves cessation of myoblast division, myoblast alignment, and fusion to multinucleated myofibers. Calpain is one of the factors shown to be involved in myoblast fusion. Using L8 rat myoblasts, we found that calpain levels did not change significantly during myoblast differentiation, whereas calpastatin diminished prior to myoblast fusion and reappeared after fusion. The transient diminution in calpastatin allows the Ca2+-promoted activation of calpain and calpain-induced membrane proteolysis, which is required for myoblast fusion. Here we show that calpastatin overexpression in L8 myoblasts does not inhibit cell proliferation and alignment, but prevents myoblast fusion and fusion-associated protein degradation. In addition, calpastatin appears to modulate myogenic gene expression, as indicated by the lack of myogenin (a transcription factor expressed in differentiating myoblasts) in myoblasts overexpressing calpastatin. These results suggest that, in addition to the role in membrane disorganization in the fusing myoblasts, the calpain-calpastatin system may also modulate the levels of factors required for myoblast differentiation.

Vascular endothelial growth factor stimulates skeletal muscle regeneration in vivo

Vascular endothelial growth factor (VEGF) is a major regulator of blood vessel formation during development and in the adult organism. Recent evidence indicates that this factor also plays an important role in sustaining the proliferation and differentiation of different cell types, including progenitor cells of different tissues, including bone marrow, bone, and the central nervous system. Here we show that the delivery of the 165-aa isoform of VEGF-A cDNA using an adeno-associated virus (AAV) vector exerts a powerful effect on skeletal muscle regeneration in vivo. Following ischemia-, glycerol-, or cardiotoxin-induced damage in mouse skeletal muscle, the delivery of AAV-VEGF markedly improved muscle fiber reconstitution with a dose-dependent effect. The expression of both VEGF receptor-1 (VEGFR-1) and VEGFR-2 was upregulated both in the satellite cells of the damaged muscles and during myotube formation in vitro; the VEGF effect was mediated by the VEGFR-2, since the transfer of PlGF, a VEGF family member interacting with the VEGFR-1, was ineffective. These results are consistent with the observation that VEGF promotes the growth of myogenic fibers and protects the myogenic cells from apoptosis in vitro and prompt a therapeutic use for VEGF gene transfer in a variety of muscular disorders.

Myocyte differentiation generates nuclear invaginations traversed by myofibrils associating with sarcomeric protein mRNAs

Certain types of cell both in vivo and in vitro contain invaginated or convoluted nuclei. However, the mechanisms and functional significance of the deformation of the nuclear shape remain enigmatic. Recent studies have suggested that three types of cytoskeleton, microfilaments, microtubules and intermediate filaments, are involved in the formation of nuclear invaginations, depending upon cell type or conditions. Here, we show that undifferentiated mouse C2C12 skeletal muscle myoblasts had smoothsurfaced spherical or ellipsoidal nuclei, whereas prominent nuclear grooves and invaginations were formed in multinucleated myotubes during terminal differentiation. Conversion of mouse fibroblasts to myocytes by the transfection of MyoD also resulted in the formation of nuclear invaginations after differentiation. C2C12 cells prevented from differentiation did not have nuclear invaginations, but biochemically differentiated cells without cell fusion exhibited nuclear invaginations. Thus, biochemical differentiation is sufficient for the nuclear deformation. Although vimentin markedly decreased both in the biochemically and in the terminally differentiated cells, exogenous expression of vimentin in myotubes did not rescue nuclei from the deformation. On the other hand, non-striated premyofibrils consisting of sarcomeric actinmyosin filament bundles and cross-striated myofibrils traversed the grooves and invaginations. Time-lapse microscopy showed that the preformed myofibrillar structures cut horizontally into the nuclei. Prevention of myofibril formation retarded the generation of nuclear invaginations. These results indicate that the myofibrillar structures are, at least in part, responsible for the formation of nuclear grooves and invaginations in these myocytes. mRNA of sarcomeric proteins including myosin heavy chain and alpha-actin were frequently associated with the myofibrillar structures running along the nuclear grooves and invaginations. Consequently, the grooves and invaginations might function in efficient sarcomeric protein mRNA transport from the nucleus along the traversing myofibrillar structures for active myofibril formation.

Small GTPase Tc10 and its homologue RhoT induce N-WASP-mediated long process formation and neurite outgrowth

Rho family small GTPases regulate multiple cellular functions through reorganization of the actin cytoskeleton. Among them, Cdc42 and Tc10 induce filopodia or peripheral processes in cultured cells. We have identified a member of the family, designated as RhoT, which is closely related to Tc10. Tc10 was highly expressed in muscular tissues and brain and remarkably induced during differentiation of C2 skeletal muscle cells and neuronal differentiation of PC12 and N1E-115 cells. On the other hand, RhoT was predominantly expressed in heart and uterus and induced during neuronal differentiation of N1E-115 cells. Tc10 exogenously expressed in fibroblasts generated actin-filament-containing peripheral processes longer than the Cdc42-formed filopodia, whereas RhoT produced much longer and thicker processes containing actin filaments. Furthermore, both Tc10 and RhoT induced neurite outgrowth in PC12 and N1E-115 cells, but Cdc42 did not do this by itself. Tc10 and RhoT as well as Cdc42 bound to the N-terminal CRIB-motif-containing portion of N-WASP and activated N-WASP to induce Arp2/3-complex-mediated actin polymerization. The formation of peripheral processes and neurites by Tc10 and RhoT was prevented by the coexpression of dominant-negative mutants of N-WASP. Thus, N-WASP is essential for the process formation and neurite outgrowth induced by Tc10 and RhoT. Neuronal differentiation of PC12 and N1E-115 cells induced by dibutyryl cyclic AMP and by serum starvation, respectively, was prevented by dominant-negative Cdc42, Tc10 and RhoT. Taken together, all these Rho family proteins are required for neuronal differentiation, but they exert their functions differentially in process formation and neurite extension. Consequently, N-WASP activated by these small GTPases mediates neuronal differentiation in addition to its recently identified role in glucose uptake.

Ermelin, an endoplasmic reticulum transmembrane protein, contains the novel HELP domain conserved in eukaryotes

We have cloned a cDNA encoding a novel protein referred to as ermelin from mouse C2 skeletal muscle cells. This protein contained six hydrophobic amino acid stretches corresponding to transmembrane domains, two histidine-rich sequences, and a sequence homologous to the fusion peptides of certain fusion proteins. Ermelin also contained a novel modular sequence, designated as HELP domain, which was highly conserved among eukaryotes, from yeast to higher plants and animals. All these HELP domain-containing proteins, including mouse KE4, Drosophila Catsup, and Arabidopsis IAR1, possessed multipass transmembrane domains and histidine-rich sequences. Ermelin was predominantly expressed in brain and testis, and induced during neuronal differentiation of N1E-115 neuroblastoma cells but downregulated during myogenic differentiation of C2 cells. The mRNA was accumulated in hippocampus and cerebellum of brain and central areas of seminiferous tubules in testis. Epitope-tagging experiments located ermelin and KE4 to a network structure throughout the cytoplasm. Staining with the fluorescent dye DiOC(6)(3) identified this structure as the endoplasmic reticulum. These results suggest that at least some, if not all, of the HELP domain-containing proteins are multipass endoplasmic reticulum membrane proteins with functions conserved among eukaryotes.

Invertebrate connectin spans as much as 3.5 microm in the giant sarcomeres of crayfish claw muscle

In crayfish claw closer muscle, the giant sarcomeres are 8.3 microm long at rest, four times longer than vertebrate striated muscle sarcomeres, and they are extensible up to 13 microm upon stretch. Invertebrate connectin (I-connectin) is an elastic protein which holds the A band at the center of the sarcomere. The entire sequence of crayfish I-connectin was predicted from cDNA sequences of 53 424 bp (17 352 residues; 1960 kDa). Crayfish I-connectin contains two novel 68- and 71-residue repeats, and also two PEVK domains and one kettin region. Kettin is a small isoform of I-connectin. Immunoblot tests using antibody to the 68-residue repeats revealed the presence of I-connectin also in long sarcomeres of insect leg muscle and barnacle ventral muscle. Immunofluorescence microscopy demonstrated that the two repeats, the long spacer and the two PEVK domains contribute to sarcomere extension. These regions rich in charged amino acids, occupying 63% of the crayfish I-connectin molecule, may allow a span of a 3.5 microm distance as a new class of composite spring.

Meltrin alpha cytoplasmic domain interacts with SH3 domains of Src and Grb2 and is phosphorylated by v-Src

Meltrin alpha/ADAM12 is a member of the ADAM/MDC family proteins characterized by the presence of metalloprotease and disintegrin domains. This protein also contains a single transmembrane domain and a relatively long cytoplasmic domain containing several proline-rich sequences. These sequences are compatible with the consensus sequences for binding the Src homology 3 (SH3) domains. To determine whether the proline-rich sequences interact with SH3 domains in several proteins, binding of recombinant SH3 domains to the meltrin alpha cytoplasmic domain was analysed by pull-down assays. The SH3 domains of Src and Yes bound strongly, but that of Abl or phosphatidylinositol 3-kinase p85 subunit did not. Full-length Grb2/Ash bound strongly, whereas its N-terminal SH3 domain alone did less strongly. Src and Grb2 in bovine brain extracts also bound to meltrin alpha cytoplasmic domain on affinity resin. Furthermore, immunoprecipitation with a monoclonal antibody to meltrin alpha resulted in coprecipitation of Src and Grb2 with meltrin alpha in cell extracts, suggesting that Src and Grb2 are associated in vivo with meltrin alpha cytoplasmic domain. This notion was also supported by the findings that exogenously expressed meltrin cytoplasmic domain coexisted with Src and Grb2 on the membrane ruffles. The C-terminal Tyr901 of meltrin alpha was phosphorylated both in vitro and in cultured cells by v-Src. These results may imply that meltrin alpha cytoplasmic domain is involved in a signal transduction for some biological function through the interaction with SH3-containing proteins.

Tenotomy decreases reporter protein synthesis via the 3'-untranslated region of the beta-myosin heavy chain mRNA

We tested the hypothesis that the beta-myosin heavy chain (beta-MHC) 3'-untranslated region (UTR) mediates decreased protein expression after tenotomy of the rat soleus. We also tested the hypothesis that decreased protein expression is the result of RNA-protein interactions within the 3'-UTR. beta-MHC was chosen for study because of its critical role in the function of postural muscles such as soleus. Adult rat soleus muscles were directly injected with luciferase (LUC) reporter constructs containing either the beta-MHC or SV40 3'-UTR. After 48 h of tenotomy, there was no significant effect on LUC expression in the SV40 3'-UTR group. In the beta-MHC 3'-UTR group, LUC expression was 37.3 +/- 4% (n = 5, P = 0.03) of that in sham controls. Gel mobility shift assays showed that a protein factor specifically interacts with the beta-MHC 3'-UTR and that tenotomy significantly increases the level of this interaction (25 +/- 7%, n = 5, P = 0.02). Thus the beta-MHC 3'-UTR is directly involved in decreased protein expression that is probably due to increased RNA-protein binding within the UTR.

Effects of thiol protease inhibitors on myoblast fusion and myofibril assembly in vitro

To investigate the roles of thiol proteases such as cathepsins and calpains in muscle differentiation, we have treated primary cultures of pectoralis muscle with a variety of protease inhibitors and examined the effects these agents have on myoblast fusion and myofibrillogenesis. We have found that a membrane-permeable inhibitor, E64D, has dramatic effects on both events of muscle differentiation. Cells treated with this inhibitor display gross morphological changes, severe delays in myofibril assembly, and reduced ability to fuse in culture. These morphological changes are correlated with a build up of beta1-integrin throughout the cytoplasm. These effects could also be produced using NH4Cl, a lysosomotrophic agent. In addition, we show that two nonpermeable inhibitors (leupeptin and E64) slightly decrease myoblast fusion, but have no effects on the ability of the cells to form mature myofibrils. These results are discussed in terms of their relevance to the inheritable disease of muscular dystrophy and I-cell disease (mucolipodosis II).

Involvement of SPARC in in vitro differentiation of skeletal myoblasts

SPARC (secreted protein acidic and rich in cysteine) is an extracellular Ca(2+)-binding glycoprotein associated with the morphogenesis and remodeling of various tissues. Here, involvement of SPARC in the myogenesis of skeletal myoblasts was investigated in vitro. First, the differential expression of SPARC mRNA during the myogenesis was initially identified by a differential display reverse transcription (DDRT)-PCR method. The expression of the SPARC gene was significantly up-regulated during the differentiation of C2C12 mouse myoblasts. Second, the treatment with anti-SPARC antibody almost completely prevented the differentiation of myoblasts. Third, the treatment with EGTA, a Ca(2+) chelator that is known to inhibit the fusion of C2C12 myoblasts, reversibly inhibited the up-regulation of SPARC gene expression. On the other hand, the treatment with A23187, a Ca(2+) ionophore, rapidly and dramatically increased the level of SPARC transcript. Taken together, these results suggest that SPARC may play a critical role(s) in the morphological change of myoblasts, and that the expression of SPARC gene may be controlled by Ca(2+)-dependent pathway in myogenesis.

Distinct effects of Rac1 on differentiation of primary avian myoblasts

Rho family GTPases have been implicated in the regulation of the actin cytoskeleton in response to extracellular cues and in the transduction of signals from the membrane to the nucleus. Their role in development and cell differentiation, however, is little understood. Here we show that the transient expression of constitutively active Rac1 and Cdc42 in unestablished avian myoblasts is sufficient to cause inhibition of myogenin expression and block of the transition to the myocyte compartment, whereas activated RhoA affects myogenic differentiation only marginally. Activation of c-Jun N-terminal kinase (JNK) appears not to be essential for block of differentiation because, although Rac1 and Cdc42 GTPases modestly activate JNK in quail myoblasts, a Rac1 mutant defective for JNK activation can still inhibit myogenic differentiation. Stable expression of active Rac1, attained by infection with a recombinant retrovirus, is permissive for terminal differentiation, but the resulting myotubes accumulate severely reduced levels of muscle-specific proteins. This inhibition is the consequence of posttranscriptional events and suggests the presence of a novel level of regulation of myogenesis. We also show that myotubes expressing constitutively active Rac1 fail to assemble ordered sarcomeres. Conversely, a dominant-negative Rac1 variant accelerates sarcomere maturation and inhibits v-Src-induced selective disassembly of I-Z-I complexes. Collectively, our findings provide a role for Rac1 during skeletal muscle differentiation and strongly suggest that Rac1 is required downstream of v-Src in the signaling pathways responsible for the dismantling of tissue-specific supramolecular structures.

Cytochemical localization of calcium in prefusion myoblasts from the chick embryo myotome

Myoblast fusion is a Ca2+-dependent process. The aim of this report was to study the localization of Ca2+ in prefusion myoblasts from the brachial somites of chick embryos (51-108 h of incubation), using the potassium pyroantimonate cytochemical method. When observed under a transmission electron microscope, electron-dense precipitates of Ca2+-antimonate were found in the basement membrane of the myotome, which separates the myotome from the adjacent mesenchyma. Within myoblasts, triads and sarcoplasmic reticulum associated with the first newly formed sarcomeres were observed, but a T-tubule network was not found. Moreover, Ca2+-antimonate precipitates were not observed in structures resembling T-tubules or sarcoplasmic reticulum. The results suggest that sarcomerogenesis and sarcoplasmic reticulum development occur simultaneously and that prefusion myoblasts have neither a T-tubule network nor Ca2+ deposits on sarcoplasmic reticulum. Small Ca2+ pools were found in the myoblast nuclei, cytoplasmic vesicles and mitochondrias. Ca2+-antimonate precipitates periodically distributed at the cell periphery, close to the cell membrane, were observed. These precipitates could represent internal Ca2+ stores located in the peripheral couplings and it is proposed that these pools of Ca2+ could be mobilized before fusion, leading to the increase in free intracellular Ca2+ that precedes myoblast fusion.

Alteration of myogenic regulatory components in a rat myoblast GLUT 3(-) mutant

Myogenesis is a complex process characterized by both biochemical and morphological differentiation. Recent transfection studies suggested a close relationship between the GLUT 3 transporter and the myogenic ability of rat skeletal L6 myoblast. In this study, the myogenic properties of GLUT 3- mutants were examined. Studies using three different GLUT 3- mutants (D2, D9 and D23) revealed that these mutants were defective not only in the GLUT 3 transporter, but also in the expression of myogenesis-associated genes. The properties of mutant D23 were further characterized. Overexpression of an exogenous functional GLUT 3 transporter was unable to restore the myogenic defects of this mutant. This mutant was subsequently found to be altered in components acting on at least two different sites of the L6 myogenic pathway. Transfection studies revealed that mutant D23 was deficient in component(s) essential for the myogenin promoter activity. The second component was required for the transcription of muscle-specific protein genes, as overexpression of myogenin was unable to rescue the inability of this mutant to express muscle-specific genes and to form myotubes. Mutant D23 was therefore thought to be deficient in a regulatory component which controlled the expression of GLUT 3, myogenin and muscle-specific genes.

Inhibitors of the proteasome block the myogenic differentiation of rat L6 myoblasts

Myogenesis is characterized by membrane fusion and accumulation of muscle specific proteins. We have previously shown that nitric oxide acts as a messenger for membrane fusion. Here we show that inhibitors of the proteasome, such as lactacystin, reversibly block both the fusion of L6 myoblasts and the accumulation of muscle specific proteins, such as myosin heavy chain (MHC). The inhibitors also reversibly prevented the induction of the NF-kappaB activity, which is required for the expression of nitric oxide synthase (NOS). Moreover, the inhibition of the NF-kappaB activity occurred in parallel with that of the NOS activity upon treatment with increasing concentrations of lactacystin. While pyrrolidine dithiocarbamate, an inhibitor of NF-kappaB, blocked both membrane fusion and accumulation of MHC, N(G)-monomethyl-L-arginine, a specific inhibitor of NOS, inhibited only the fusion. These results suggest that the proteasome plays an essential role in the regulation of myogenic differentiation through the activation of NF-kappaB and that the target of NF-kappaB for the expression of muscle specific proteins is distinct from that for myoblast fusion.

Calpastatin-modulation of m-calpain activity is required for myoblast fusion

Previous studies have demonstrated a role for m-calpain in myoblast fusion. Moreover, the presence, in differentiated cells, of a highly specific endogenous inhibitor of calpain, calpastatin, has led to the hypothesis that a regulation of or a protection against m-calpain activity by calpastatin could also occur during the earlier stages of muscle cell differentiation. In order to verify this hypothesis, we have investigated, in myoblast culture, the appearance of calpastatin-mRNA and its corresponding protein. Our results provide evidence that calpastatin is already present at the earlier stages of myoblast differentiation and that a significant decrease of the levels of calpastatin mRNA and its protein precedes myoblast fusion. In addition, the induction of an artificial decrease in calpastatin level, via an appropriate antisense oligodeoxyribonucleotide methodology, leads to earlier and faster myoblast fusion. Together with previous studies, these results indicate that m-calpain and calpastatin are functionally involved in myoblast fusion. Our findings also demonstrate that an acute "hyperactivity" of m-calpain resulting from the decrease of calpastatin synthesis is necessary during the early stages of this step of differentiation.

Drosophila myogenesis and insights into the role of nautilus

Several aspects of muscle development appear to be conserved between Drosophila and vertebrate organisms. Among these is the conservation of genes that are critical to the myogenic process, including transcription factors such as nautilus. From a simplistic point of view, Drosophila therefore seems to be a useful organism for the identification of molecules that are essential for myogenesis in both Drosophila and in other species. nautilus, the focal point of this review, appears to be involved in the specification and/or differentiation of a specific subset of muscle founder cells. As with several of its vertebrate and invertebrate counterparts, it is capable of inducing a myogenic program of differentiation reminiscent of that of somatic muscle precursors when expressed in other cell types. We therefore favor the model that nautilus implements the specific differentiation program of these founder cells, rather than their specification. Further analyses are necessary to establish the validity of this working hypothesis. Studies have revealed a critical role for Pax-3 in specifying a particular subset of myogenic cells, the progenitors of the limb muscles. These myogenic cells migrate from the somite into the periphery of the organism, where they differentiate. These myoblasts do not express MyoD or myf5 until they have arrived at their destination and begin the morphologic process of myogenesis (Bober et al., 1994; Goulding et al., 1994; Williams and Ordahl, 1994). They then begin to express these genes, possibly to put the myogenic plan into action. Thus, as with nautilus, MyoD and myf5 may be necessary for the manifestation of a muscle-specific commitment that has already occurred. By comparison with vertebrates, it was anticipated that the single Drosophila gene would serve the purpose of all four vertebrate genes. However, its restricted pattern of expression and apparent loss-of-function phenotype are inconsistent with this expectation. It remains to be determined whether nautilus functions in a manner similar to just one of the vertebrate genes. Since the myf5- and MyoD-expressing myoblasts are proliferative, the loss of one cell type appears to be compensated by proliferation of the remaining cell type. This apparent plasticity may obscure differences in mutant phenotype resulting from the loss of particular cells that express each of these genes. In Drosophila, by comparison, nautilus-expressing cells committed to the myogenic program undergo few, if any, additional cell divisions, and thus no other cells are available to compensate for the loss of nautilus. Therefore, the apparent differences between the Drosophila nautilus gene and its vertebrate counterparts may reflect, at least in part, differences in the developmental systems rather than differences in the function of the genes themselves.

Calpain and calpastatin in myoblast differentiation and fusion: effects of inhibitors

Myoblast differentiation and fusion to multinucleated muscle cells can be studied in myoblasts grown in culture. Calpain (Ca(2+)-activated thiol protease) induced proteolysis has been suggested to play a role in myoblast fusion. We previously showed that calpastatin (the endogenous inhibitor of calpain) plays a role in cell membrane fusion. Using the red cell as a model, we found that red cell fusion required calpain activation and that fusibility depended on the ratio of cell calpain to calpastatin. We found recently that calpastatin diminishes markedly in myoblasts during myoblast differentiation just prior to the start of fusion, allowing calpain activation at that stage; calpastatin reappears at a later stage (myotube formation). In the present study, the myoblast fusion inhibitors TGF-beta, EGTA and calpeptin (an inhibitor of cysteine proteases) were used to probe the relation of calpastatin to myoblast fusion. Rat L8 myoblasts were induced to differentiate and fuse in serum-poor medium containing insulin. TGF-beta and EGTA prevented the diminution of calpastatin. Calpeptin inhibited fusion without preventing diminution of calpastatin, by inhibiting calpain activity directly. Protein levels of mu-calpain and m-calpain did not change significantly in fusing myoblasts, nor in the inhibited, non-fusing myoblasts. The results indicate that calpastatin level is modulated by certain growth and differentiation factors and that its continuous presence results in the inhibition of myoblast fusion.

Drosophila myoblast city encodes a conserved protein that is essential for myoblast fusion, dorsal closure, and cytoskeletal organization

The Drosophila myoblast city (mbc) locus was previously identified on the basis of a defect in myoblast fusion (Rushton et al., 1995. Development [Camb.]. 121:1979-1988). We describe herein the isolation and characterization of the mbc gene. The mbc transcript and its encoded protein are expressed in a broad range of tissues, including somatic myoblasts, cardial cells, and visceral mesoderm. It is also expressed in the pole cells and in ectodermally derived tissues, including the epidermis. Consistent with this latter expression, mbc mutant embryos exhibit defects in dorsal closure and cytoskeletal organization in the migrating epidermis. Both the mesodermal and ectodermal defects are reminiscent of those induced by altered forms of Drac1 and suggest that mbc may function in the same pathway. MBC bears striking homology to human DOCK180, which interacts with the SH2-SH3 adapter protein Crk and may play a role in signal transduction from focal adhesions. Taken together, these results suggest the possibility that MBC is an intermediate in a signal transduction pathway from the rho/rac family of GTPases to events in the cytoskeleton and that this pathway may be used during myoblast fusion and dorsal closure.

Effects of divalent cations on M-cadherin expression and distribution during primary rat myogenesis in vitro

In the process of myogenesis, cadherins are thought to be involved in the initial cell-cell recognition and possible initiation of myoblast fusion to form multinucleated myotubes. Of the cadherins, M-cadherin, but not N-cadherin, is down-regulated upon inhibition of myogenesis, suggesting that M-cadherin may be a key receptor involved in myogenesis. M-cadherin binds in a calcium-dependent manner, and depletion of divalent cations inhibits myoblast fusion. We analyzed the regulation of M-cadherin protein and mRNA levels in primary rat myogenic cultures in the presence and absence of divalent cations. In untreated cultures M-cadherin was localized to various myogenic cell-cell contacts. M-cadherin protein and mRNA levels showed a peak at day 2 after the initiation of growth. When divalent cations were removed from the cell culture medium, myoblast fusion was inhibited and immunocytochemical analysis revealed a failure of M-cadherin to localize to cell-cell contacts. Analysis of M-cadherin protein and mRNA in fusion-inhibited cultures still revealed a peak at day 2. However, by day 3, M-cadherin protein levels in the fusion-inhibited cultures were reduced in both the detergent-soluble and -insoluble fractions in comparison with the untreated cultures. Interestingly, beta-catenin, a protein associated with cadherins, was frequently observed at intercellular contacts in the fusion-inhibited cultures. We could also show that the intracellular levels of beta-catenin protein remained constant regardless of the presence or absence of divalent cations. In summary, the dynamic regulation of M-cadherin in muscle-fusion-related events is an indication of the importance of M-cadherin for myoblast fusion and myogenic differentiation.

Two-step delivery of retroviruses to postmitotic, terminally differentiated cells

Recombinant replication-defective retroviral vectors are currently the most commonly used vectors for introducing foreign genes into human cells in gene therapy protocols. Their genomes stably incorporate in the host chromosomes of mitotic cells, thus ensuring stable expression. However, the applications of retroviruses to gene therapy are limited by their inability to infect postmitotic cells such as muscle fibers. In an attempt to overcome such limitations, we have developed a novel two-step transduction protocol that allows integration and expression of retroviral genes in differentiated cells. We induced DNA synthesis in terminally differentiated cultured mouse myotubes derived from both established myogenic cell lines and from primary myoblasts. We infected the postmitotic cells with a recombinant replication-defective adenoviral vector encoding the SV40 large T antigen as a mitogen. Subsequently we transduced the adenovirus-infected cells with a Moloney retroviral vector bearing the LacZ gene. Histochemical analysis revealed the coincident expression of LacZ gene in those myotubes that had been induced to synthesize DNA.

Expression of the plasma membrane Ca2+-ATPase in myogenic cells

To study the physiological function of the plasma membrane calmodulin-dependent calcium ATPase (PMCA) in intact cells, L6 myogenic cell lines stably overexpressing the human PMCA isoform 4CI (= human PMCA isoform 4b) were generated. Several independent L6 clones and controls stably transfected with the empty expression vector were analyzed in detail. The resting cytosolic calcium level in hPMCA4CI-overexpressing muscle cells (measured by the Fura-2 method) was significantly reduced by 20-30% compared with controls. This was shown in a cytosolic window of 1322 single cells (p < 0.01). Furthermore, the differentiation process of these cells was remarkably accelerated compared with control myoblasts and parental nontransfected L6 cells as assessed by multinucleated myotube formation and creatine phosphokinase activity elevation. After 4 and 6 days of differentiation, PMCA-overexpressing L6 cells from four independent clones displayed a 3- and 4-fold higher creatine phosphokinase activity compared with controls (n = 5, p < 0.02). These results may extend the concept of the function of the PMCA from simple prevention of calcium overload to an active involvement in intracellular calcium regulation with potentially important consequences for cellular functions.

Distinct troponin T genes are expressed in embryonic/larval tail striated muscle and adult body wall smooth muscle of ascidian

During development of the ascidian Halocynthia roretzi, the tadpole larva hatched from the tailbud embryo metamorphoses to the sessile adult with a body wall muscle. Although the adult body wall muscle is morphologically nonsarcomeric smooth muscle, it contains troponin complex consisting of three subunits (T, I, and C) as do vertebrate striated muscles. Different from vertebrate troponins, however, the smooth muscle troponin promotes actomyosin Mg2+-ATPase activity in the presence of high concentration of Ca2+, and this promoting property is attributable to troponin T. To address whether the embryonic/larval tail striated muscle and the adult smooth muscle utilize identical or different regulatory machinery, we cloned troponin T cDNAs from each cDNA library. The embryonic and the adult troponin Ts were encoded by distinct genes and shared only <60% identity with each other. Northern blotting and whole mount in situ hybridization revealed that these isoforms were specifically expressed in the embryonic/larval tail striated muscle and the adult smooth muscle, respectively. These results may imply that these isoforms regulate actin-myosin interaction in different manners. The adult troponin T under forced expression in mouse fibroblasts was unexpectedly located in the nuclei. However, a truncated protein with a deletion including a cluster of basic amino acids colocalized with tropomyosin on actin filaments. Thus, complex formation with troponin I and C immediately after the synthesis is likely to be essential for the protein to properly localize on the thin filaments.

Molecular cloning of a partial cDNA clone encoding the C terminal region of chicken breast muscle connectin

The cDNA sequence encoding the C terminal region of chicken skeletal muscle connectin was described. Its predicted amino acid sequence had 1,021 amino acids comprising six motif Ils (Immunoglobulin C2 domain) and five interdomains. The sequence showed 70-75% homology with that of human cardiac connectin, but 168 amino acids including one motif II were missing in chicken skeletal muscle connectin. The C terminal sequence of chicken skeletal muscle connectin reported by the previous work (Maruyama et al., 1994) was erroneous due to the accidental ligation of the cDNA clone encoding a N terminal region of connectin with a partial porin cDNA clone.

Contraction accelerates myosin heavy chain synthesis rates in adult cardiocytes by an increase in the rate of translational initiation

The purpose of this study was to determine the mechanism by which contraction acutely accelerates the synthesis rate of the contractile protein myosin heavy chain (MHC). Laminin-adherent adult feline cardiocytes were maintained in a serum-free medium and induced to contract at 1 Hz via electrical field stimulation. Electrical stimulation of contraction accelerated rates of MHC synthesis 28%, p < 0.05 by 4 h as determined by incorporation of [3H]phenylalanine into MHC. MHC mRNA expression as measured by RNase protection was unchanged after 4 h of electrical stimulation. MHC mRNA levels in messenger ribonucleoprotein complexes and translating polysomes were examined by sucrose gradient fractionation. The relative percentage of polysomebound MHC mRNA was equal at 47% in both electrically stimulated and control cardiocytes. However, electrical stimulation of contraction resulted in a reproducible shift of MHC mRNA from smaller polysomes into larger polysomes, indicating an increased rate of initiation. This shift resulted in significant increases in MHC mRNA levels in the fractions containing the larger polysomes of electrically stimulated cardiocytes as compared with nonstimulated controls. These data indicate that the rate of MHC synthesis is accelerated in contracting cardiocytes via an increase in translational efficiency.

Involvement of M-cadherin in terminal differentiation of skeletal muscle cells

Cadherins are a gene family encoding calcium-dependent cell adhesion proteins which are thought to act in the establishment and maintenance of tissue organization. M-cadherin, one member of the family, has been found in myogenic cells of somitic origin during embryogenesis and in the adult. These findings have suggested that M-cadherin is involved in the regulation of morphogenesis of skeletal muscle cells. Therefore, we investigated the function of M-cadherin in the fusion of myoblasts into myotubes (terminal differentiation) in cell culture. Furthermore, we tested whether M-cadherin might influence (a) the expression of troponin T, a typical marker of biochemical differentiation of skeletal muscle cells, and (b) withdrawal of myoblasts from the cell cycle (called terminal commitment). The studies were performed by using antagonistic peptides which correspond to sequences of the putative M-cadherin binding domain. Analogous peptides of N-cadherin have previously been shown to interfere functionally with the N-cadherin-mediated cell adhesion. In the presence of antagonistic M-cadherin peptides, the fusion of myoblasts into myotubes was inhibited. Analysis of troponin T revealed that it was downregulated at the protein level although its mRNA was still detectable. In addition, withdrawal from the cell cycle typical for terminal commitment of muscle cells was not complete in fusion-blocked myogenic cells. Finally, expression of M-cadherin antisense RNA reducing the expression of the endogenous M-cadherin protein interfered with the fusion process of myoblasts. Our data imply that M-cadherin-mediated myoblast interaction plays an important role in terminal differentiation of skeletal muscle cells.

Inhibition of myogenic differentiation in myoblasts expressing a truncated type II TGF-beta receptor

Transforming growth factor-beta (TGF-beta) is thought to play a role in mesenchymal cell development and, specifically, in muscle differentiation, yet its precise role in the latter process remains unclear. TGF-beta has been shown to both inhibit and induce myoblast maturation in vitro, depending on the culture conditions. Whether the type I or type II TGF-beta receptor mediates the various TGF-beta effects on myogenesis is not known. In the present study, C2C12 myoblasts were transfected with an expression vector for a truncated type II TGF-beta receptor, which has been shown to act as a dominant negative inhibitor of type II receptor signaling. In contrast to the parental cells, the transfected clones did not efficiently form myotubes or induce expression of MyoD, myogenin and several other differentiation markers following incubation in low serum media. However, some muscle differentiation markers continued to be expressed in the transfected cells suggesting that at least two pathways are involved in muscle cell differentiation. These cells could still growth arrest in low serum media, showing that decreased proliferation can be dissociated from differentiation. Unlike several oncogenes known to block myogenic differentiation, expression of the truncated TGF-beta receptor did not result in myoblast transformation. Injection of the parental or the transfected C2C12 cells into the limb muscle of nude mice revealed quantitative and qualitative differences in their behavior, and suggested that myoblasts expressing the truncated TGF-beta receptor cannot fuse in vivo. Finally, retrovirus-mediated expression of MyoD in the transfected cells restored their ability to form myotubes in vitro, indicating that inhibition of myoblast differentiation by the truncated TGF-beta receptor may depend on decreased MyoD expression. We propose that TGF-beta signaling through the type II receptor is required for several distinct aspects of myogenic differentiation and that TGF-beta acts as a competence factor in this multistep process.

Morulae at compaction and the pattern of protein synthesis in mouse embryos

Compaction of mouse embryos at the 8-cell stage causes a drastic change in cell form and in cell-to-cell contacts in 3-4 h. We have studied the effect of inhibitors of transcription (alpha-amanitin), DNA replication (aphidicolin) and compaction (cytochalasin D, EGTA, alpha-lactalbumin and Con A) on the pattern of protein synthesis using gel electrophoresis. Our results show that the pattern of protein synthesis is regulated principally by passage through S phase during each early cell cycle rather than by de novo transcription, while changes induced in cell form or contacts do not alter the pattern significantly.

Sequential appearance of muscle-specific proteins in myoblasts as a function of time after cell division: evidence for a conserved myoblast differentiation program in skeletal muscle

Based on the assumption that a conserved differentiation program governs the assembly of sarcomeres in skeletal muscle in a manner analogous to programs for viral capsid assembly, we have defined the temporal and spatial distribution of 10 muscle-specific proteins in mononucleated myoblasts as a function of the time after terminal cell division. Single cells in mitosis were identified in monolayer cultures of embryonic chicken pectoralis, followed for selected time points (0-24 h postmitosis) by video time-lapse microscopy, and then fixed for immunofluorescence staining. For convenience, the myoblasts were termed x-h-old to define their age relative to their mitotic "birthdate." All 6 h myoblasts that emerged in a mitogen-rich medium were desmin+ but only 50% were positive for a alpha-actin, troponin-I, alpha-actinin, MyHC, zeugmatin, titin, or nebulin. By 15 h postmitosis, approximately 80% were positive for all of the above proteins. The up-regulation of these 7 myofibrillar proteins appears to be stochastic, in that many myoblasts were alpha-actinin+ or zeugmatin+ but MyHC- or titin- whereas others were troponin-I+ or MyHC+ but alpha-actinin- or alpha-actin-. In 15-h-old myoblasts, these contractile proteins were organized into nonstriated myofibrils (NSMFs). In contrast to striated myofibrils (SMFs), the NSMFs exhibited variable stoichiometries of the sarcomeric proteins and these were not organized into any consistent pattern. In this phase of maturation, two other changes occurred: (1) the microtubule network was reorganized into parallel bundles, driving the myoblasts into polarized, needle-shaped cells; and (2) the sarcolemma became fusion-competent. A transition from NSMFs to SMFs took place between 15 and 24 h (or later) postmitosis and was correlated with the late appearance of myomesin, and particularly, MyBP-C (C protein). The emergence of one, or a string of approximately 2 mu long sarcomeres, was invariably characterized by the localization of myomesin and MyBP-C to their mature positions in the developing A-bands. The latter group of A-band proteins may be rate-limiting in the assembly program. The great majority of myoblasts stained positively for desmin and myofibrillar proteins prior to, rather than after, fusing to form myotubes. This sequential appearance of muscle-specific proteins in vitro fully recapitulates myofibrillar assembly steps in myoblasts of the myotome and limb bud in vivo, as well as in nonmuscle cells converted to myoblasts by MyoD. We suggest that this cell-autonomous myoblast differentiation program may be blocked at different control points in immortalized myogenic cell lines.

Human carcinoembryonic antigen, an intercellular adhesion molecule, blocks fusion and differentiation of rat myoblasts

Human carcinoembryonic antigen (CEA), a widely used tumor marker, is a member of a family of cell surface glycoproteins that are overexpressed in many carcinomas. CEA has been shown to function in vitro as a homotypic intercellular adhesion molecule. This correlation of overproduction of an adhesion molecule with neoplastic transformation provoked a test of the effect of CEA on cell differentiation. Using stable CEA transfectants of the rat L6 myoblast cell line as a model system of differentiation, we show that fusion into myotubes and, in fact, the entire molecular program of differentiation, including creatine phosphokinase upregulation, myogenin upregulation, and beta-actin downregulation are completely abrogated by the ectopic expression of CEA. The blocking of the upregulation of myogenin, a transcriptional regulator responsible for the execution of the entire myogenic differentiation program, indicates that CEA expression intercepts the process at a very early stage. The adhesion function of CEA is essential for this effect since an adhesion-defective N domain deletion mutant of CEA was ineffective in blocking fusion and CEA transfectants treated with adhesion-blocking peptides fused normally. Furthermore, CEA transfectants maintain their high division potential, whereas control transfectants lose division potential with differentiation similarly to the parental cell line. Thus the expression of functional CEA on the surface of cells can block terminal differentiation and maintain proliferative potential.

Use of p112-deficient myoblasts to determine the temporal order of the in vitro expression of myogenic components

The present investigation examines the function and site(s) of involvement of an ecto-protein kinase and its substrate protein (a cell surface 112 kDa protein) in the in vitro myogenic pathway. The phosphorylated 112 kDa protein (p112) has recently been shown to be involved in myogenesis. Not much information is currently available on the role of the ecto-protein kinase and the 112 kDa protein in modulating the expression of the myogenic factors and various muscle-specific proteins. Five different p112-deficient rat myoblasts were used to examine the temporal order of the in vitro expression of the myogenic components; namely, L6 myoblasts treated with BrdUrd or phloretin, a conditional p112-defective mutant (clone D1), an ecto-protein kinase-deficient mutant (clone F72), and a mutant defective in the 112 kDa protein (clone D1/S4). All these p112-deficient myoblasts were also impaired in myogenesis. The absence of p112, ecto-protein kinase, and/or the 112 kDa protein was found to have no effect on the Myf-5 mRNA level. However, the expected increase in NCAM and Myf-4 mRNAs was not observed in any of the p112-deficient myoblasts examined. This suggests that the p112 site of action is probably located upstream of the Myf-4 and NCAM sites in the myogenic pathway. While 7-28 fold increases in the MLC, MHC, and TnT transcripts were observed during myogenesis, such increases were not observed in the p112-deficient myoblasts. However, when mutant D1/S4 was transfected with the myf-4 cDNA, expression of Myf-4 in the transfectant resulted in increased level of the MLC, MHC, and TnT mRNAs, and in myotube formation, even though the Myf-5 and NCAM mRNA levels and p112 were not altered. This suggests that p112 may function by activating transcription of Myf-4, which will subsequently promote the expression of muscle-specific proteins and myotube formation. In the absence of p112, Myf-5 cannot activate the expression of Myf-4, NCAM, MLC, MHC, TnT, and myotube formation. If all these components are involved in the same myogenic pathway, then p112 may be acting downstream from Myf-5, and upstream from NCAM and Myf-4.

Use of a conditional MyoD transcription factor in studies of MyoD trans-activation and muscle determination

DNA sequences encoding the hormone-binding domains of several steroid hormone receptors were fused in frame to the MyoD gene. When the gene for this chimeric protein was expressed in NIH 3T3 or 10T1/2 fibroblasts, these cells displayed hormone-dependent induction of myogenesis. Our experiments focused on cell lines expressing estrogen receptor-MyoD chimeras. Induction of these lines in the presence of estradiol and an inhibitor of protein synthesis, cycloheximide, resulted in the activation of the endogenous myogenin gene but did not activate the muscle-specific creatine kinase or cardiac alpha-actin gene. This result suggests that MyoD is not a "direct" activator of these downstream myogenic genes but must first activate myogenin as an intermediary. Once muscle is induced by estrogen receptor-MyoD the muscle phenotype is very stable and does not need the continued presence of estradiol for its maintenance.

Translational repression of EF-1 alpha mRNA in vitro

In this report we show that when 10,000 x g supernatant extracts of growth arrested murine erythroleukemia (MEL) cells are incubated there is a rapid conversion of essentially all mRNAs to non-translating messenger ribonucleoprotein (RNP) particles. Most of these RNPs are readily translated in an initiation-dependent manner when added to a nuclease-treated rabbit reticulocyte lysate. A notable exception is the RNP containing eucaryotic elongation factor 1 alpha (EF-1 alpha) mRNA. The mRNA for poly(A)-binding protein behaved similarly to EF-1 alpha. Previous work has demonstrated that the translation of both these mRNAs are repressed in vivo when the growth of a number of different mammalian cells is arrested [Slobin L. I. and Jordan, P. (1984) Eur J. Biochem. 145, 1984; Thomas, G. and Thomas, G. (1986) J. Cell Biol. 103, 1986]. Translational activity of EF-1 alpha mRNA could be restored by treating RNP particles with 0.5 M KCl, provided that the RNPs were separated from salt wash by chromatography on oligo(dT)-cellulose. Addition of the salt wash to total MEL cell mRNA significantly and selectively inhibited EF-1 alpha mRNA translation, suggesting that a component of the salt wash acts as a trans-acting translational repressor of EF-1 alpha mRNA.

Ascidian entactin/nidogen. Implication of evolution by shuffling two kinds of cysteine-rich motifs

Entactin/nidogen, a major component of the basement membrane, has a domain structure comprising three globular domains, and thread-like and rod-like domains connecting them. It contains six epidermal-growth-factor-(EGF)-like motifs and one thyroglobulin-like motif. In the present study, ascidian entactin/nidogen has been identified by a monoclonal antibody technique. We prepared anti-(ascidian entactin/nidogen)IgG, named anti-AsEnt1, then cloned the cDNA of ascidian entactin/nidogen using anti-AsEnt1 as a probe, and determined its entire sequence. Mainly because the deduced amino acid sequence exhibited high similarity to mouse entactin and human nidogen, and because the antigen localized in basement membrane of ascidian body-wall muscle, we have concluded that the antigen anti-AsEnt1 corresponds to the ascidian entactin/nidogen homologue. The deduced amino acid sequence of ascidian entactin/nidogen clearly showed that the ascidian homologue also has a domain structure. However, the ascidian homologue lacked the thread-like domain, and the rod-like domain differed from that of mouse entactin in composition, consisting of two kinds of cysteine-rich motifs, that is, the EGF-like motif and the thyroglobulin-like motif. These results suggest that entactin/nidogen have evolved by modifying the domains, especially by shuffling the two kinds of cysteine-rich motifs.

Increase in the level of m-calpain correlates with the elevated cleavage of filamin during myogenic differentiation of embryonic muscle cells

The activity of Ca(2+)-activated proteinase requiring millimolar Ca2+ (m-calpain) was found to increase dramatically in cultured chick embryonic myoblasts during the early period of myogenic differentiation. Furthermore, the protein level of m-calpain also markedly increased in parallel with the rise in its activity, and both remained elevated thereafter. On the other hand, the activity level of calpastatin, an endogenous inhibitor of the proteinase, remained similar during the entire period of the culture. In addition, the activity of Ca(2+)-activated proteinase requiring micromolar Ca2+ (mu-calpain) was not detected in either proliferating or differentiated myoblasts. Thus, the overall capacity of Ca(2+)-dependent proteolysis is likely to increase in differentiating myoblasts and should be contributed by m-calpain. Filamin (250 kDa), that is known to facilitate actin microfilament assembly and interfere with actin-myosin filament formation, was found to be cleaved in cultured myoblasts to 240 kDa products. This filamin-cleavage occurred in a manner similar to the in vitro cleavage of the cytoskeletal protein by the purified m-calpain. Moreover, the filamin-cleavage was most evident at the period of the cell fusion. Thus, it seems likely that the in vivo cleavage of filamin is mediated by m-calpain. These results suggest that m-calpain may play an important role in cytoskeletal reorganization that is requisite for myoblast fusion.

Sodium butyrate inhibits myogenesis by interfering with the transcriptional activation function of MyoD and myogenin

Sodium butyrate reversibly inhibits muscle differentiation and blocks the expression of many muscle-specific genes in both proliferating myoblasts and differentiated myotubes. We investigated the role of the basic helix-loop-helix (bHLH) myogenic determinator proteins MyoD and myogenin in this inhibition. Our data suggest that both MyoD and myogenin are not able to function as transcriptional activators in the presence of butyrate, although both apparently retain the ability to bind DNA. Transcription of MyoD itself is extinguished in butyrate-treated myoblasts and myotubes, an effect that may be due to the inability of MyoD to autoactivate its own transcription. We present evidence that the HLH region of MyoD is essential for butyrate inhibition of MyoD. In contrast to MyoD and myogenin, butyrate does not inhibit the ubiquitous basic HLH protein E2-5 from functioning as a transcriptional activator.

Alteration of translation and stability of mRNA for the poly(A)-binding protein during myogenesis

The regulation of synthesis of various factors involved in mRNA translation during differentiation of muscle cells was examined. The steady-state levels of mRNAs coding for eukaryotic initiation factor (eIF) 2 alpha, 2 beta and elongation factor (eEF)-1 alpha were measured in both proliferating rat L6 myoblast and differentiated myotubes. The steady-state levels of these mRNAs were not altered during myogenesis. Furthermore, the distribution of these mRNAs between repressed and translated populations remained unchanged. Recent studies suggest a role for poly(A)-binding protein (PABP) in translation initiation. Therefore, we also examined the expression of PABP mRNA during myogenesis. The PABP mRNA was less abundant in myotubes compared to myoblasts. However, the synthesis of PABP remained unchanged. In myoblasts, approximately 50-60% of the total mRNA was associated with polyribosomes, whereas in myotubes more than 80% of the mRNA was associated with polyribosomes. These results, therefore, suggest that the PABP mRNA was more efficiently translated in differentiated myotubes than in the proliferating myoblasts. Measurement of the stability and transcription of PABP mRNA showed that, while transcription was not affected during myogenesis, the stability of the mRNA was reduced in differentiated cells. The t1/2 of PABP mRNA in myoblasts was 13 h compared to 7.5 h in myotubes. This observation suggests that the reduced steady-state level of PABP mRNA in myotube were largely due to the change in stability of this mRNA during myogenesis.

Sodium butyrate inhibits myogenesis by interfering with the transcriptional activation function of MyoD and myogenin

Sodium butyrate reversibly inhibits muscle differentiation and blocks the expression of many muscle-specific genes in both proliferating myoblasts and differentiated myotubes. We investigated the role of the basic helix-loop-helix (bHLH) myogenic determinator proteins MyoD and myogenin in this inhibition. Our data suggest that both MyoD and myogenin are not able to function as transcriptional activators in the presence of butyrate, although both apparently retain the ability to bind DNA. Transcription of MyoD itself is extinguished in butyrate-treated myoblasts and myotubes, an effect that may be due to the inability of MyoD to autoactivate its own transcription. We present evidence that the HLH region of MyoD is essential for butyrate inhibition of MyoD. In contrast to MyoD and myogenin, butyrate does not inhibit the ubiquitous basic HLH protein E2-5 from functioning as a transcriptional activator.

Density-dependent modulation of vascular smooth muscle alpha-actin biosynthetic processing in differentiated BC3H1 myogenic cells

The expression of vascular smooth muscle (VSM) alpha-actin mRNA during BC3H1 myogenic cell differentiation is specifically stimulated by conditions of high cell density. Non-proteolytic dissociation of cell-cell and cell-matrix contacts in post-confluent cultures of BC3H1 myocytes using EDTA promotes loss of the differentiated morphological phenotype. EDTA-dispersed myocytes exhibit an undifferentiated fibroblastoid appearance and contained reduced levels of both VSM and skeletal alpha-actin mRNA. Muscle alpha-actin mRNA levels in EDTA-dispersed myocytes were not restored to that observed in confluent myocyte preparations by experimental manipulation of cell density conditions. Pulse-labeling techniques using L-[35S]cysteine to identify muscle actin biosynthetic intermediates revealed that EDTA-dispersed myocytes expressed nascent forms of both the VSM and skeletal muscle alpha-actin polypeptide chains. However EDTA-dispersed myocytes were less efficient in the post-translational processing of immature VSM alpha-actin compared to non-dispersed myocytes. Simple cell-to-cell contact may mediate VSM alpha-actin processing efficiency since high-density preparations of EDTA-dispersed myocytes processed more VSM alpha-actin intermediate than myocytes plated at low density. The actin isoform selectivity of the response to modulation of intercellular contacts suggests that actin biosynthesis in BC3H1 myogenic cells involves mechanisms capable of discriminating between different isoform classes of nascent actin polypeptide chains.

Translational regulation of ribosomal protein synthesis in Xenopus cultured cells: mRNA relocation between polysomes and RNP during nutritional shifts

Translational control of ribosomal protein mRNA was analyzed in a Xenopus cell line during growth-rate changes induced by serum deprivation and readdition. After being transferred into serum-free medium, the cells rapidly decrease their DNA, RNA and protein synthesis, while addition of serum to the culture after a few hours of deprivation causes a rapid recovery. During these growth-rate changes, we observed a shift in ribosomal protein mRNA distribution between polysomes and RNP. The proportion of mRNA on polysomes for the four ribosomal proteins analyzed changed from 70-80% during rapid growth to 25-35% during the downshift and back to 70-80% after the upshift. Northern blot analysis showed that ribosomal protein mRNA level was constant during the shifts even in the presence of the transcriptional inhibitor actinomycin D. This indicates that the distribution changes were due to a reversible transfer of ribosomal protein mRNA between polysomes and RNP without altering mRNA stability. We have also compared the kinetics of ribosomal protein mRNA distribution changes with the kinetics of the changes in the partition of ribosomes between free monomers and polysomes. The results obtained show that the change in ribosomal protein mRNA localization is very fast, allowing short-term adjustments of ribosome synthesis rate. Moreover, our observations are consistent with the hypothesis that the amount of free ribosomes present in the cell could affect ribosomal protein mRNA utilization.

Ca2+/calmodulin-dependent phosphorylation of the 100-kDa protein in chick embryonic muscle cells in culture

The pattern of protein phosphorylation was found to change in differentiating chick embryonic myoblasts in culture. The extent of phosphorylation of 42-, 50-, and 100-kDa proteins increased while that of a 63-kDa protein declined in extracts of myoblasts that had been cultured for increasing periods. Of these, the increase in phosphorylation of the 100-kDa protein occurred most dramatically in extracts of myoblasts in an early stage of differentiation and was specifically inhibited by trifluoperazine (TFP) and other calmodulin (CaM) antagonists including chlorpromazine and N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7). Treatment of increasing concentrations of TFP to culture medium also decreased the phosphorylation state of the 100-kDa protein and the degree of myoblast fusion in parallel. In addition, levels of both the kinase activity and the 100-kDa protein but not of CaM appeared to rise in the cells cultured for longer periods. These results suggest that (1) a Ca2+/CaM-dependent protein kinase is responsible for phosphorylation of the 100-kDa protein, (2) the TFP-mediated myoblast fusion block may be associated with the inhibitory effect of the drug against the kinase activity, and (3) the increase in phosphorylation state of the 100-kDa protein during myogenic differentiation is due to the rise in levels of the kinase and its substrate.

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

We found that a transient rise in cGMP levels, which was closely associated with the Ca2+ influx, occurred concomitant with the onset of myoblast fusion. The Ca2+ channel blocker D600 decreased both the cell fusion and the normal rise in cGMP levels. In contrast, the Ca2+ ionophore A23187 transiently increased cGMP levels and induced precocious fusion. In addition, the cGMP analog 8-Br-cGMP induced precocious fusion as A23187 did. The guanylate cyclase inhibitor, methylene blue delayed the fusion in a dose-dependent manner without significantly affecting cell alignment, proliferation, or muscle-specific protein expression. Furthermore, methylene blue delayed the normal rise in cGMP levels, and the fusion block imposed by methylene blue was significantly recovered by 8-Br-cGMP. On the basis of our present findings, we suggest that a Ca2+ influx-dependent rise in cGMP levels is an important step in myoblast fusion.