Increased membrane fluidity precedes fusion of muscle cells

Molecular regulation of myocyte fusion

Myocyte fusion is a pivotal process in the development and regeneration of skeletal muscle. Failure during fusion can lead to a range of developmental as well as pathological consequences. This review aims to comprehensively explore the intricate processes underlying myocyte fusion, from the molecular to tissue scale. We shed light on key players, such as the muscle-specific fusogens - Myomaker and Myomixer, in addition to some lesser studied molecules contributing to myocyte fusion. Conserved across vertebrates, Myomaker and Myomixer play a crucial role in driving the merger of plasma membranes of fusing myocytes, ensuring the formation of functional muscle syncytia. Our multiscale approach also delves into broader cell and tissue dynamics that orchestrate the timing and positioning of fusion events. In addition, we explore the relevance of muscle fusogens to human health and disease. Mutations in fusogen genes have been linked to congenital myopathies, providing unique insights into the molecular basis of muscle diseases. We conclude with a discussion on potential therapeutic avenues that may emerge from manipulating the myocyte fusion process to remediate skeletal muscle disorders.

Generating Heterokaryotic Cells via Bacterial Cell-Cell Fusion

Fusion of cells is an important and common biological process that leads to the mixing of cellular contents and the formation of multinuclear cells. Cell fusion occurs when distinct membranes are brought into proximity of one another and merge to become one. Fusion holds promise for biotechnological innovations, for instance, for the discovery of urgently needed new antibiotics. Here, we used antibiotic-producing bacteria that can proliferate without their cell wall as a model to investigate cell-cell fusion. We found that fusion between genetically distinct cells yields heterokaryons that are viable, contain multiple selection markers, and show increased antimicrobial activity. The rate of fusion induced using physical and chemical methods was dependent on membrane fluidity, which is related to lipid composition as a function of cellular age. Finally, by using an innovative system of synthetic membrane-associated lipopeptides, we achieved targeted fusion between distinctly marked cells to further enhance fusion efficiency. These results provide a molecular handle to understand and control cell-cell fusion, which can be used in the future for the discovery of new drugs. IMPORTANCE Cell-cell fusion is instrumental in introducing different sets of genes in the same environment, which subsequently leads to diversity. There is need for new protocols to fuse cells of different types together for biotechnological applications like drug discovery. We present here wall-deficient cells as a platform for the same. We identify the fluidity of the membrane as an important characteristic for the process of fusion. We demonstrate a cell-specific approach for fusion using synthetically designed peptides yielding cells with modified antibiotic production profiles. Overall, wall-deficient cells can be a chassis for innovative metabolite production by providing an alternative method for cell-cell fusion.

Doubly Stimulated Corrole for Organelle-Selective Antitumor Cytotoxicity

Balancing between safety and efficacy of cancer chemotherapeutics is achievable by relying on internal and/or external stimuli for selective and on-demand antitumor cytotoxicity. We now introduce the difluorophosphorus(V) corrole PC-Im, a theranostic agent with a pH-sensitive N-methylimidazole moiety. Structure/activity relationships, via comparison with the permanently charged PC-ImM+ and the lipophilic PC, uncovered the exceptional features of PC-Im: nanoparticular and monomeric at neutral and low pH, respectively, 10-fold increased light-induced singlet oxygen production at acidic pH, internalization into malignant cells within minutes, and selective accumulation within lysosomes. Submillimolar PC-Im concentrations are tolerable in the dark, while illumination induces nanomolar cytotoxic effects due to a multiplicity of cellular deleterious events: endoplasmic reticulum fragmentation, lysosome fusion and exocytosis, calcium leakage, mitochondrial fission, and swelling. PC-Im emerges as an antitumor agent, whose potency is triggered by endogenous and exogenous stimuli, assuring its cytotoxicity will occur selectively upon lysosomal accumulation and solely upon light activation.

Promoting Cell Fusion by Polyvalent DNA Ligands

Artificially induced in vitro cell fusion is one essential technique that has been extensively used for biological studies. Nevertheless, there is a lack of robust and efficient method to produce fused cells efficiently. Herein, we proposed to use cell-membrane-anchored polyvalent DNA ligands (PDL) to bring cells into close proximity by forming clusters to enhance PEG-induced cell fusion. PDL of complementary sequences are separately anchored onto different population of cells through cholesterol-induced hydrophobic insertion into lipid membrane. Cells are clustered via mixing cells of complementary PDL prior to cell fusion. PDL exhibited strong stability on cell membrane, induced efficient cell clustering, and eventually achieved cell fusion efficiently in combination with PEG induction. We demonstrated homogeneous and heterogeneous cell fusion of high yield on various cell types. This report presented a programmable yet robust technique for achieving efficient cell fusion that hold great application potentials.

Regulation of the myoblast fusion reaction for muscle development, regeneration, and adaptations

Fusion of plasma membranes is essential for skeletal muscle development, regeneration, exercise-induced adaptations, and results in a cell that contains hundreds to thousands of nuclei within a shared cytoplasm. The differentiation process in myocytes culminates in their fusion to form a new myofiber or fusion to an existing myofiber thereby contributing more synthetic material to the syncytium. The choice for two cells to fuse and become one could be a dangerous event if the two cells are not committed to an allied function. Thus, fusion events are highly regulated with positive and negative factors to fine-tune the process, and requires muscle-specific fusogens (Myomaker and Myomerger) as well as general cellular machinery to achieve the union of membranes. While a unified vertebrate myoblast fusion pathway is not yet established, recent discoveries should make this pursuit attainable. Not only does myocyte fusion impact the normal biology of skeletal muscle, but new evidence indicates dysregulation of the process impacts pathologies of skeletal muscle. Here, I will highlight the molecular players and biochemical mechanisms that drive fusion events in muscle, and discuss how this key myogenic process impacts skeletal muscle diseases.

Insights into molecular pathways and fatty acid membrane composition during the temperature stress response in the murine C2C12 cell model

Daily and seasonal temperature fluctuations are inevitable due to climate change, which highlights the importance of studying the detrimental effects of temperature fluctuations on the health, productivity, and product quality of farm animals. Muscle membrane composition and the molecular signals are vital for muscle cell differentiation and muscle growth, but their response to temperature stress is not well characterized. Temperature changes can lead to modification of membrane components of the cell, which may affect its surroundings and intracellular signaling pathways. Using C2C12 myoblast cells as a model of skeletal muscle development, this study was designed to investigate the effects of high temperature (39 °C and 41 °C) and low temperature (35 °C) on molecular pathways in the cells as well as the cell membrane fatty acid composition. Our results show that several genes were differentially expressed in C2C12 cells cultured under heat or cold stress, and these genes were enriched important KEGG pathways including PI3K-Akt signaling pathway, lysosome and HIF- signaling pathway, Wnt signaling pathway and AMPK signaling pathway. Our analysis further reveals that several membrane transporters and genes involved in lipid metabolism and fatty acid elongation were also differentially expressed in C2C12 cells cultured under high or low temperature. Additionally, temperature stress shifts the fatty acid composition in the cell membranes, including the proportion of saturated, monounsaturated and polyunsaturated fatty acids. This study revealed an interference between fatty acid composition in the membranes and changing molecular pathways including lipid metabolism and fatty acids elongation mediated under thermal stress. These findings will reinforce a better understanding of the adaptive mechanisms in skeletal muscle under temperature stress.

Subcompartmentalization and Pseudo-Division of Model Protocells

Membrane enclosed intracellular compartments have been exclusively associated with the eukaryotes, represented by the highly compartmentalized last eukaryotic common ancestor. Recent evidence showing the presence of membranous compartments with specific functions in archaea and bacteria makes it conceivable that the last universal common ancestor and its hypothetical precursor, the protocell, may have exhibited compartmentalization. To the authors' knowledge, there are no experimental studies yet that have tested this hypothesis. They report on an autonomous subcompartmentalization mechanism for protocells which results in the transformation of initial subcompartments to daughter protocells. The process is solely determined by the fundamental materials properties and interfacial events, and does not require biological machinery or chemical energy supply. In the light of the authors' findings, it is proposed that similar events may have taken place under early Earth conditions, leading to the development of compartmentalized cells and potentially, primitive division.

Rapid Growth and Fusion of Protocells in Surface-Adhered Membrane Networks

Elevated temperatures might have promoted the nucleation, growth, and replication of protocells on the early Earth. Recent reports have shown evidence that moderately high temperatures not only permit protocell assembly at the origin of life, but can have actively supported it. Here, the fast nucleation and growth of vesicular compartments from autonomously formed lipid networks on solid surfaces, induced by a moderate increase in temperature, are shown. Branches of the networks, initially consisting of self-assembled interconnected nanotubes, rapidly swell into microcompartments which can spontaneously encapsulate RNA fragments. The increase in temperature further causes fusion of adjacent network-connected compartments, resulting in the redistribution of the RNA. The experimental observations and the mathematical model indicate that the presence of nanotubular interconnections between protocells facilitates the fusion process.

Photoinduced Reversible Solid-to-Liquid Transitions for Photoswitchable Materials

Heating and cooling can induce reversible solid-to-liquid transitions of matter. In contrast, athermal photochemical processes can induce reversible solid-to-liquid transitions of some newly developed azobenzene compounds. Azobenzene is photoswitchable. UV light induces trans-to-cis isomerization; visible light or heat induces cis-to-trans isomerization. Trans and cis isomers usually have different melting points (T_m ) or glass transition temperatures (T_g ). If T_m or T_g of an azobenzene compound in trans and cis forms are above and below room temperature, respectively, light may induce reversible solid-to-liquid transitions. In this Review, we introduce azobenzene compounds that exhibit photoinduced reversible solid-to-liquid transitions, discuss the mechanisms and design principles, and show their potential applications in healable coatings, adhesives, transfer printing, lithography, actuators, fuels, and gas separation. Finally, we discuss remaining challenges in this field.

Regulation of muscle plasticity and trophism by fatty acids: A short review

The skeletal muscle tissue has a remarkable ability to alter its plastic structural and functional properties after a harmful stimulus, regulating the expression of proteins in complex events such as muscle regeneration. In this context, considering that potential therapeutic agents have been widely studied, nutritional strategies have been investigated in order to improve the regenerative capacity of skeletal muscle. There is evidence of the modulatory action of fatty acids, such that oleic and linoleic acids, that are abundant in Western diets, on muscle function and trophism. Thus, fatty acids appear to be potential candidates to promote or impair the recovery of muscle mass and function during regeneration, since they modulate intracellular pathways that regulate myogenesis. This study is the first to describe and discuss the effect of fatty acids on muscle plasticity and trophism, with emphasis on skeletal muscle regeneration and in vitro differentiation of muscle cells.

Targeted Lipidomic Analysis of Myoblasts by GC-MS and LC-MS/MS

Lipids represent ∼10% of the cell dry mass and play essential roles in membrane composition and physical properties, energy storage, and signaling pathways. In the developing or the regenerating skeletal muscle, modifications in the content or the flipping between leaflets of membrane lipid components can modulate the fusion capacity of myoblasts, thus constituting one of the regulatory mechanisms underlying myofiber growth. Recently, few genes controlling these qualitative and quantitative modifications have started to be unraveled. The precise functional characterization of these genes requires both qualitative and quantitative evaluations of a global lipid profile. Here, we describe a lipidomic protocol using mass spectrometry, allowing assessing the content of fatty acids, glycerophospholipids, and cholesterol in the routinely used C2C12 mouse myoblast cell line, or in primary cultures of mouse myoblasts.

LPS-Induced Macrophage Activation and Plasma Membrane Fluidity Changes are Inhibited Under Oxidative Stress

Macrophage activation is essential for a correct and efficient response of innate immunity. During oxidative stress membrane receptors and/or membrane lipid dynamics can be altered, leading to dysfunctional cell responses. Our aim is to analyze membrane fluidity modifications and cell function under oxidative stress in LPS-activated macrophages. Membrane fluidity of individual living THP-1 macrophages was evaluated by the technique two-photon microscopy. LPS-activated macrophage function was determined by TNFα secretion. It was shown that LPS activation causes fluidification of macrophage plasma membrane and production of TNFα. However, oxidative stress induces rigidification of macrophage plasma membrane and inhibition of cell activation, which is evidenced by a decrease of TNFα secretion. Thus, under oxidative conditions macrophage proinflammatory response might develop in an inefficient manner.

HACD1, a regulator of membrane composition and fluidity, promotes myoblast fusion and skeletal muscle growth

The reduced diameter of skeletal myofibres is a hallmark of several congenital myopathies, yet the underlying cellular and molecular mechanisms remain elusive. In this study, we investigate the role of HACD1/PTPLA, which is involved in the elongation of the very long chain fatty acids, in muscle fibre formation. In humans and dogs, HACD1 deficiency leads to a congenital myopathy with fibre size disproportion associated with a generalized muscle weakness. Through analysis of HACD1-deficient Labradors, Hacd1-knockout mice, and Hacd1-deficient myoblasts, we provide evidence that HACD1 promotes myoblast fusion during muscle development and regeneration. We further demonstrate that in normal differentiating myoblasts, expression of the catalytically active HACD1 isoform, which is encoded by a muscle-enriched splice variant, yields decreased lysophosphatidylcholine content, a potent inhibitor of myoblast fusion, and increased concentrations of ≥C18 and monounsaturated fatty acids of phospholipids. These lipid modifications correlate with a reduction in plasma membrane rigidity. In conclusion, we propose that fusion impairment constitutes a novel, non-exclusive pathological mechanism operating in congenital myopathies and reveal that HACD1 is a key regulator of a lipid-dependent muscle fibre growth mechanism.

The role of Na+/K+-ATPase during chick skeletal myogenesis

The formation of a vertebrate skeletal muscle fiber involves a series of sequential and interdependent events that occurs during embryogenesis. One of these events is myoblast fusion which has been widely studied, yet not completely understood. It was previously shown that during myoblast fusion there is an increase in the expression of Na⁺/K⁺-ATPase. This fact prompted us to search for a role of the enzyme during chick in vitro skeletal myogenesis. Chick myogenic cells were treated with the Na⁺/K⁺-ATPase inhibitor ouabain in four different concentrations (0.01-10 μM) and analyzed. Our results show that 0.01, 0.1 and 1 μM ouabain did not induce changes in cell viability, whereas 10 μM induced a 45% decrease. We also observed a reduction in the number and thickness of multinucleated myotubes and a decrease in the number of myoblasts after 10 μM ouabain treatment. We tested the involvement of MEK-ERK and p38 signaling pathways in the ouabain-induced effects during myogenesis, since both pathways have been associated with Na⁺/K⁺-ATPase. The MEK-ERK inhibitor U0126 alone did not alter cell viability and did not change ouabain effect. The p38 inhibitor SB202190 alone or together with 10 μM ouabain did not alter cell viability. Our results show that the 10 μM ouabain effects in myofiber formation do not involve the MEK-ERK or the p38 signaling pathways, and therefore are probably related to the pump activity function of the Na⁺/K⁺-ATPase.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

Dynamic clustering and dispersion of lipid rafts contribute to fusion competence of myogenic cells

Recent research indicates that the leading edge of lamellipodia of myogenic cells (myoblasts and myotubes) contains presumptive fusion sites, yet the mechanisms that render the plasma membrane fusion-competent remain largely unknown. Here we show that dynamic clustering and dispersion of lipid rafts contribute to both cell adhesion and plasma membrane union during myogenic cell fusion. Adhesion-complex proteins including M-cadherin, beta-catenin, and p120-catenin accumulated at the leading edge of lamellipodia, which contains the presumptive fusion sites of the plasma membrane, in a lipid raft-dependent fashion prior to cell contact. In addition, disruption of lipid rafts by cholesterol depletion directly prevented the membrane union of myogenic cell fusion. Time-lapse recording showed that lipid rafts were laterally dispersed from the center of the lamellipodia prior to membrane fusion. Adhesion proteins that had accumulated at lipid rafts were also removed from the presumptive fusion sites when lipid rafts were laterally dispersed. The resultant lipid raft- and adhesion complex-free area at the leading edge fused with the opposing plasma membrane. These results demonstrate a key role for dynamic clustering/dispersion of lipid rafts in establishing fusion-competent sites of the myogenic cell membrane, providing a novel mechanistic insight into the regulation of myogenic cell fusion.

Morphological changes and spatial regulation of diacylglycerol kinase-zeta, syntrophins, and Rac1 during myoblast fusion

The fusion of mononuclear myoblasts into multinucleated myofibers is essential for the formation and growth of skeletal muscle. Myoblast fusion follows a well-defined sequence of cellular events, from initial recognition and adhesion, to alignment, and finally plasma membrane fusion. These processes depend upon coordinated remodeling of the actin cytoskeleton. Our recent studies suggest diacylglycerol kinase-zeta (DGK-zeta), an enzyme that metabolizes diacylglycerol to yield phosphatidic acid, plays an important role in actin reorganization. Here, we investigated whether DGK-zeta has a role in the fusion of cultured C2C12 myoblasts. We show that DGK-zeta and syntrophins, scaffold proteins of the dystrophin glycoprotein complex that bind directly to DGK-zeta, are spatially regulated during fusion. Both proteins accumulated with the GTPase Rac1 at sites where fine filopodia mediate the initial contact between myoblasts. In addition, DGK-zeta codistributed with the Ca(2+)-dependent cell adhesion molecule N-cadherin at nascent, but not previously established cell contacts. We provide evidence that C2 cells are pulled together at cell-cell junctions by N-cadherin-containing filopodia reminiscent of epithelial adhesion zippers, which guide the advance of lamellipodia from apposing cells. At later times, vesicles with properties of macropinosomes formed close to cell-cell junctions. Reconstruction of confocal optical sections showed these form dome-like protrusions from the dorsal surface of contacting cells. Collectively, these results suggest DGK-zeta and syntrophins play a role at multiple stages of the fusion process. Moreover, our findings provide a potential link between changes in the lipid content of the membrane bilayer and reorganization of the actin cytoskeleton during myoblast fusion.

Study of muscle regeneration using in vitro 2D 1H spectroscopy

The in vivo spectrum of regenerating muscles shows a specific cross-correlation signal assigned to the (n-3) fatty acyl chain, which peaks during the myoblast fusion phase. In order to identify the origin of this signal and to take all the lipid metabolites into account, we investigated the degeneration-regeneration process by 1H 2D NMR of lipid muscle extracts. We observed an increase in the total amount of lipids during the regeneration process, although the lipid profile did not show any drastic change during this process. The changes in the NMR signal observed in vivo and, in particular, the appearance of the specific (n-3) fatty acyl chain signal appears to arise from mobile lipid compartments located in fusing cells.

Timing of incorporation of docosahexaenoic acid into brain and muscle phospholipids during precocial and altricial modes of avian development

We investigated the possibilities that the proportion of docosahexaenoic acid (DHA) in phospholipids of brain and skeletal muscle at hatch, and the ontogenetic timing of the DHA accretion spurt in these tissues, might serve as indices of neonatal functional maturity that discriminate between precocial and altricial avian developmental modes. Comparison of the fatty acid profiles of the initial and residual yolks of two free-living altricial species, the swallow (Hirundo rustica) and the sparrow (Passer domesticus), reveals that, in contrast to precocial birds, there is no preferential uptake of DHA from the yolk during embryonic development. At hatch, the proportions of DHA in brain phospholipid (wt.% of fatty acids) of the swallow and sparrow, at 8.1% and 5.0%, respectively, are far lower than the values (16.9-19.6%) reported for non-altricial species. This reflects a marked difference in the timing of the brain DHA accretion spurt, which occurs during the first half of the embryonic period of precocial birds, but is largely delayed until after hatching in the altricial species. By the time of fledging, the proportion of DHA in the swallow brain phospholipid has increased to 14.3%. For non-altricial birds, the brain DHA concentration at hatch shows little interspecies variation, despite major differences in yolk DHA content. The proportions of DHA in leg muscle phospholipid of the newly hatched swallow and sparrow, at 2.9% and 2.5%, respectively, are far lower than the value (6.7%) for the precocial chicken. Again, this relates to differences in developmental timing, with muscle DHA accretion occurring in the first half of the chicken's embryonic period, whereas, in the swallow, this increase is delayed until after hatching. By the time of fledging in the swallow, DHA forms 9.3% of muscle phospholipid fatty acids, equivalent to the level attained in chicken muscle at the mid-embryo stage. The results indicate a clear distinction between altricial and non-altricial avian species in the timing of tissue DHA accretion during development, presumably reflecting differences in neonatal functional maturity.

Formation and characterization of spontaneously formed heterokaryons between quail myoblasts and 3T3-L1 preadipocytes: correlation between differential plasticity and degree of differentiation

Skeletal muscle cells and adipose cells have a close relationship in developmental lineage. Our previous study has shown that the heterokaryons between quail myoblasts and undifferentiated 3T3-L1 cells (preadipocytes) normally differentiated into myotubes, whereas the heterokaryons between myoblasts and differentiated 3T3-L1 cells (adipocytes) failed myogenic differentiation. These results suggest differences between preadipocytes and adipocytes. The purpose of this study was to clarify whether preadipocytes have flexibility in differentiation before terminal adipose differentiation. Presumptive quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) and mouse 3T3-L1 cells (either preadipocytes or adipocytes) were co-cultured for 48 h under conditions allowing myogenic differentiation. On co-culture between myoblasts and undifferentiated 3T3-L1 cells, heterokaryotic myotubes formed spontaneously, but not on co-culture with differentiated 3T3-L1 cells. In addition, the heterokaryotic myotubes expressed mouse myogenin derived from the 3T3-L1 cell gene. Our previous study indicated that the fusion sensitivity of differentiating myoblasts change with decreasing cholesterol of the cell membrane during myoblast fusion. Thus we compared the level of membrane cholesterol between undifferentiated and differentiated 3T3-L1 cells. The result showed that the level of membrane cholesterol in 3T3-L1 cells increases during adipose differentiation. Corresponding to the increase in membrane cholesterol content, differentiated 3T3-L1 cells had lower sensitivity to HVJ (Sendai virus)-mediated cell fusion than undifferentiated 3T3-L1 cells. This study demonstrated that 3T3-L1 cells at an undifferentiated state have a capacity for spontaneous fusion with differentiating myoblasts following myogenic differentiation, and that the capacity is lost after terminal adipose differentiation.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Phosphocholine and phosphoethanolamine during chick embryo myogenesis: a (31)P-NMR study

Elevated contents of phosphoethanolamine (Etn-P) and/or phosphocholine (Cho-P), a common feature of most tumours with respect to normal counterparts, may also occur in non-cancerous proliferating tissues. The significance of these alterations in relation to cell proliferation, differentiation and maturation is scarcely understood. In this work, the Cho-P and Etn-P pools were measured by (31)P-NMR in extracts of chick embryo pectoral muscle at different days of development. The average concentration of these metabolites exhibited the highest values (respectively, 1.5 and 3.0 micromol/mg DNA) on days 9-11 and decreased at later stages of myogenesis. While, however, Cho-P maintained substantial levels (above 1.0 micromol/mg DNA) also during myotube formation (days 11-18) and stepwise decreased (to about 0.5 micromol/mg DNA) upon fibres' maturation, Etn-P gradually decreased between day 11 and hatching time (down to about 0.2 micromol/mg DNA). These results demonstrate that significant changes may occur in the steady-state pools of these metabolites during normal in vivo cellular development and differentiation, and are consistent with: (a) high rates of phospholipid biosynthesis reported in the literature for proliferating myoblasts; (b) sustained phosphatidylcholine synthesis maintained also during myoblast fusion; and (c) decreased requirement of phospholipid synthesis in the last phase of in ovo myofibre maturation.

Role of hyperpolarization attained by linoleic acid in chick myoblast fusion

Our previous report has suggested that hyperpolarization generated by reciprocal activation of calcium-activated potassium (K(Ca)) channels and stretch-activated channels induces calcium influx that triggers myoblast fusion. Here we show that linoleic acid is involved in the process of generating hyperpolarization in cultured chick myoblasts and hence in promotion of the cell fusion. Linoleic acid dramatically hyperpolarized the membrane potential from -14 +/- 3 to -58 +/- 5 mV within 10 min. This effect was partially blocked by 1 mM tetraethylammonium (TEA) or 30 nM charybdotoxin, a selective K(Ca) channel inhibitor, and completely abolished by 10 mM TEA. Single-channel recordings revealed that linoleic acid activates TEA-resistant potassium channels as well as K(Ca) channels. Furthermore, linoleic acid induced calcium influx from extracellular solution, and this effect was partially blocked by 1 mM TEA and completely prevented at 10 mM, similar to the effect of TEA on linoleic acid-mediated hyperpolarization. Since the valinomycin-mediated hyperpolarization promoted calcium influx, hyperpolarization itself appears capable of inducing calcium influx. In addition, gadolinium prevented the valinomycin-mediated increase in intracellular calcium level under hypotonic conditions, revealing the involvement of stretch-activated channels in calcium influx. Furthermore, linoleic acid stimulated myoblast fusion, and this stimulatory effect could completely be prevented by 10 mM TEA. These results suggest that linoleic acid induces hyperpolarization of membrane potential by activation of potassium channels, which induces calcium influx through stretch-activated channels, and thereby triggers myoblast fusion.

Mouse C2 myoblast cells resist HVJ (Sendai virus)-mediated cell fusion in the proliferating stage but become capable of fusion after differentiation

To investigate the mechanism of myoblast fusion, we attempted to prepare artificial myotubes of mouse C2 myoblast cells using the hemagglutinating virus of Japan (HVJ, Sendai virus). Proliferating C2 cells showed strong resistance to HVJ-mediated cell fusion and remained morphologically unchanged even though massive numbers of virions adsorbed onto their surface. They showed no membrane disruption, which occurs in the early stage of cell fusion induced by HVJ. These observations suggest that proliferating C2 cells are resistant to HVJ-mediated cell fusion. However, upon induction of differentiation, C2 cells gradually became capable of fusion induced by HVJ and then even generated heterokaryons with Ehrlich ascites tumor cells. When differentiated C2 cells that had become fusion-sensitive were treated with HVJ in the presence of EDTA, they did not fuse but degenerated, suggesting that their cell membranes were transiently disrupted by interaction with HVJ. These results suggest that the cell membranes of myoblasts change to a fusion-capable state during the process of differentiation.

Potential m-calpain substrates during myoblast fusion

Many studies have demonstrated that m-calpain was implicated in cell membrane reorganization-related phenomena during fusion via a regulation by calpastatin, the specific Ca2+-dependent proteolytic inhibitor. However, the real biological role of this protease is unclear because many targeted proteins are still unknown. Using different digestion experiments we have demonstrated that desmin, vimentin, talin, and fibronectin represent very good substrates for this proteinase capable of cleaving them in fragments which are immediately degraded by other enzymatic systems. Concerning intermediate filaments, we showed that during the phenomenon of fusion, the amount of desmin was significantly reduced while the concentration of vimentin presented a steady level. On the other hand, we have conducted biological assays on cultured myoblasts supplemented by exogenous factors such as calpain inhibitors or antisense oligonucleotides capable of stimulating or inhibiting m-calpain activity. The effect of such factors on fusion and concomitantly on the targeted substrates was analyzed and quantified. When m-calpain activity and myoblast fusion were prevented by addition of calpain inhibitors entering the cells, the amounts of desmin, talin, and fibronectin were increased, whereas the amount of vimentin was unchanged. Using antisense strategy, similar results were obtained. In addition, when the phenomenon of fusion was enhanced by preventing calpastatin synthesis, the amounts of desmin, talin, and fibronectin were significantly reduced. Taken together, these results support the hypothesis that m-calpain is involved in myoblast fusion by cleaving certain proteins identified here. This cleavage could modify membrane and cytoskeleton organization for the myoblasts to fuse.

Alterations in membrane lipid dynamics of leukemic cells undergoing growth arrest and differentiation: dependency on the inducing agent

The effect of various differentiation inducers on membrane cell dynamics was studied using HL-60 and K562 leukemic cell lines. Membrane lipid dynamics was measured by the steady-state fluorescence polarization (P) method utilizing either 1,6-diphenyl-1,3,5-hexatriene (DPH) or the trimethyl ammonium derivative of DPH (TMA-DPH), which ascertains anchorage of the label to the membrane-water-lipid interface. Decrease in membrane microfluidity was observed in HL-60 cells undergoing differentiation into macrophages by 1,25-dihydroxyvitamin D3 and by K562 cells induced to differentiate by DMSO. Sodium butyrate caused an increase in membrane fluidity in K562 cells undergoing differentiation into erythroid-like cells while in HL-60 cells a dual effect was observed. At 0.4 mM concentration, in which the cells were induced to differentiate along the monocyte pathway, a decrease in membrane fluidity was observed, while at 1 mM concentration an increase in membrane fluidity occurred. Interferon-gamma (IFN-gamma) induced an increase in membrane fluidity in both cell lines. Using HL-60 cells fluorescently labeled by TMA-DPH, similar results indicating fluidization of the membrane following IFN-gamma treatment were obtained. Advanced fluorescence lifetime measurements, evaluated either by phase modulation spectrofluorometry or by single photon correlation fluorometry confirmed that the decrease in fluorescence polarization by IFN-gamma resulted from membrane fluidization and not from elongation of the probe's excited state lifetime. It is suggested that the inducer mode of action, and not the differentiation route, determine the outcome of changes in membrane microviscosity.

Is intercellular communication via gap junctions required for myoblast fusion?

Fusion of myoblasts to form syncitial muscle cells results from a complex series of sequential events including cell alignment, cell adhesion and cell communication. The aim of the present investigation was to assess whether intercellular communication through gap junctions would be required for subsequent membrane fusion. The presence of the gap junction protein connexin 43 at areas of contact between prefusing rat L6 myoblasts was established by immunofluorescent staining. These myoblasts were dye-coupled, as demonstrated by the use of the scrape-loading/dye transfer technique. L6 myoblast dye coupling was reversibly blocked by heptanol in short term experiments as well as after chronic treatment. After a single addition of 3.5 mM heptanol, gap junctions remained blocked for up to 8 hours, then this inhibitory effect decreased gradually, likely because the alcohol was evaporated. Changing heptanol solutions every 8 hours during the time course of L6 differentiation resulted in a lasting drastic inhibition of myoblast fusion. We further investigated the effect of heptanol and of other uncoupling agents on the differentiation of primary cultures of embryonic chicken myoblasts. These cells are transiently coupled by gap junctions before myoblast fusion and prolonged application of heptanol, octanol and 18-beta-glycyrrhetinic acid also inhibited their fusion. The effect of heptanol and octanol was neither due to a cytotoxic effect nor to a modification of cell proliferation. Moreover, heptanol treatment did not alter myoblast alignment and adhesion. Taken together these observations suggest that intercellular communication might be a necessary step for myoblast fusion.

Muscle fiber formation in vitro is delayed by low power laser irradiation

The myogenic cell culture provides a good in vitro model for studying the differentiation process of the muscle tissue. Although the growth of the mononucleated myoblasts is predetermined, in that they will fuse to form multinucleated muscle fibers, some control on the process of fusion can be achieved in vitro. The low power laser irradiation (LPLI) has been shown to enhance in cultured mammalian cells DNA synthesis and motility of cells. In our rat myogenic cell line (L8) system the LPLI induced a delay of 5 to 6 hours in the onset of fusion of the myoblasts compared to the nonirradiated cells. The creatine kinase activity and the incorporation of labelled thymidine of the irradiated cultures were similar to the pattern of behaviour of these parameters in the control cultures. Thus, we have extended the longevity of the myoblasts population. We assume that the delay in fusion was induced by the increase of the motility of the myoblasts in culture, so that rearrangements of physical contacts or of membrane components were needed to resume fusion.

N-cadherin and N-CAM in myoblast fusion: compared localisation and effect of blockade by peptides and antibodies

The expression and distribution of two cell adhesion molecules, N-cadherin and N-CAM, at the surface of cultured leg muscle cells from 11-day-old chicken embryos were studied and compared. N-cadherin, which was expressed by fusing myoblasts, was down-regulated on old myotubes while N-CAM was still present. Both molecules, as viewed by confocal microscopy, appeared to have coaccumulated at the areas of contact between fusing myoblasts. However, immunogold electron microscopy did not reveal significant colocalization of N-cadherin and N-CAM, and their segregation after antibody-induced patching suggested the absence of direct interactions between N-cadherin and N-CAM. The role of the Ca2+ dependent cell adhesion molecule N-cadherin in myogenesis was investigated. Myoblast fusion was inhibited (1) with a synthetic peptide containing the H-A-V sequence and (2) with a monoclonal anti-N-cadherin antibody, demonstrating that N-cadherin-mediated cell adhesion is required for myoblast fusion. Under the same conditions no effect of anti-N-CAM antibodies was observed. Taken together these observations suggest that N-cadherin, acting independently from N-CAM, is a major cell adhesion molecule involved in embryonic myoblast fusion in vitro.

Role of hydrophobic forces in bilayer adhesion and fusion

With the aim of gaining more insight into the forces and molecular mechanisms associated with bilayer adhesion and fusion, the surface forces apparatus (SFA) was used for measuring the forces and deformations of interacting supported lipid bilayers. Concerning adhesion, we find that the adhesion between two bilayers can be progressively increased by up to two orders of magnitude if they are stressed to expose more hydrophobic groups. Concerning fusion, we find that the most important force leading to direct fusion is the hydrophobic attraction acting between the (exposed) hydrophobic interiors of bilayers; however, the occurrence of fusion is not simply related to the strength of the attractive interbilayer forces but also to the internal bilayer stresses (intrabilayer forces). For all the bilayer systems studied, a single basic fusion mechanism was found in which the bilayers do not "overcome" their short-range repulsive steric-hydration forces. Instead, local bilayer deformations allow these repulsive forces to be "bypassed" via a mechanism that is like a first-order phase transition, with a sudden instability occurring at some critical surface separation. Some very slow relaxation processes were observed for fluid bilayers in adhesive contact, suggestive of constrained lipid diffusion within the contact zone.

Regulation of glycolipid synthesis during differentiation of clonal murine muscle cells

The two clonal murine muscle cell lines G7 and G8, originally derived from the M114 line, represent unique models for comparative studies of myogenesis. Glycolipid synthesis was examined during differentiation using [3H]-galactose and [3H]-glucosamine as precursors. Upon G7 contact glucosylceramide labeling increased and nLcOse5Cer labeling stopped. During membrane fusion, glucosylceramide labeling stopped and lactosylceramide became the major synthetic product. G8 cells presented a different pattern, with increased labeling of GbOse3Cer during myogenesis. The major ganglioside synthesized by both myoblasts was GM3, and more complex structures were observed following completion of myotube formation. Total glycopeptide labeling increased when G8 myoblasts fused and remained elevated in myotubes, whereas no differences during fusion of G7 cells were noted. Upon comparison of the two clonal lines, the only consistent observation was a significant increase in the synthesis of total gangliosides and neutral glycolipid during cell contact and membrane fusion (p less than 0.02). The results suggest that changes in the synthesis of specific glycolipid structures during myogenesis are unique to each muscle cell line examined. However, transient increases in synthesis of total myoblast gangliosides and neutral glycolipids may be a more general phenomenon, possibly by curbing proliferation or by altering myoblast membrane fluidity characteristics during differentiation.

The cesium-induced delay in myoblast membrane fusion is accompanied by changes in isolated membrane lipids

We have recently demonstrated that cesium ions delay the sharp decrease in both membrane conductivity and membrane permittivity of chick embryo myoblasts seen at fusion (Santini, M.T., Bonincontro, A., Cametti, C. and Indovina, P.L. (1988) Biochim. Biophys. Acta 945, 56-64). Analysis of the conductivity dispersion data (obtained in the radiowave frequency range) indicated that cesium delays fusion by about 30 h. We suggested that cesium is affecting both active ionic transport by blocking potassium channels as well as interfering with membrane lipid and/or protein charges. In the present study, we have investigated both the possible role of membrane lipids in myoblast fusion and the possible effects of cesium on these lipids. Our data indicate that lipid changes do occur in the isolated myoblast plasma membrane of controls during myogenic differentiation especially prior to fusion and that in cesium cultures these variations do not occur. These variations are in accordance with current membrane fusion theory. Specifically, there is a decrease in bilayer-stabilizing lipids (phosphatidylcholine) and an increase in bilayer-destabilizing ones (phosphatidylethanolamine and phosphatidic acid) and cholesterol during the fusion process. In addition, although slight, during fusion there appears to be a decrease in phosphatidylinositol which is believed to be involved in the inositol phosphate second messenger system. In cesium cultures, in which fusion is greatly delayed, the same lipid changes do not take place and those that are observed seem to reflect the fusion delay.

Effects of cesium on in vitro myoblast differentiation: an electron microscopic study

This paper describes the microscopic evidence supporting a cesium-induced delay in the fusion of chick embryo myoblast membranes during in vitro myogenic differentiation. We have recently demonstrated that the sharp decrease in the conductivity and permittivity of the membranes of these myogenic cells at the time of fusion is delayed 30 h by the addition of cesium to the culture medium (Santini et al., Biochim. Biophys. Acta 945:56-64; 1988). We report here that this delay in fusion is substantiated by direct microscopic observation and that cesium also induces ultrastructural changes in the myoblast cells themselves. Possible mechanisms by which cesium may cause both the delay in fusion as well as the ultrastructural changes observed are discussed.

Rescue of the Li+-induced delay of embryonic myogenesis in vitro by added inositol

Signaling between embryonic myoblasts to coordinate gene expression is part of normal skeletal muscle development in the embryo. An unanswered question is the nature of the second messengers carrying the information to the nucleus. We have investigated the cell membrane events associated with the binding of prostaglandin to a transient receptor on the embryonic chick myoblast membrane in vitro. The membrane events include a transient change in membrane order seen by electron paramagnetic resonance (EPR), a change in cell-cell adhesion, a rapid decrease in membrane permeability and fusion of the membrane bilayers. The addition of 20 mM Li+, an inhibitor of inositol phosphate phosphatase, perturbed the transient change in membrane order and delayed the change in cell-cell adhesion and conductivity for 2-6 h. Other alkali metal ions had no such effects. The addition of inositol to the culture medium in the continued presence of Li+ restored the normal timing of the two latter events. We interpret this as evidence for an inositol phosphate second messenger system which might connect the activation of the prostaglandin receptor with the change in cell-cell adhesion, the changes in membrane conductivity and perhaps bilayer fusion. We suggest that Li+, by blocking the regeneration of polyphosphatidylinositol from inositol phosphate, reduced the efficiency of the second messenger system such that further differentiation of the myoblast membrane was delayed. The exogenous inositol provided an alternative source and membrane differentiation was unaffected.

Opposing effects of calcium entry and phorbol esters on fusion of chick muscle cells

Studies utilizing cultured muscle cells have shown that myoblast fusion requires extracellular Ca2+ and involves transient coordinated changes in cell membrane topography and cytoskeletal organization. However, neither the mechanisms by which Ca2+ influences these changes nor its cellular sites of action are known. We have investigated the effects of Ca2+ channel modulators and phorbol esters on fusion of embryonic chick myoblasts in culture. Myoblast fusion was inhibited by the Ca2+ channel blockers D600 and nitrendipine and stimulated by the Ca2+ channel activator Bay K 8644. We have obtained evidence that the tumor promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits fusion through activation of protein kinase C. Myoblasts prevented from fusing by Ca2+ channel blockers or TPA display a distinctive elongated morphology that is characteristic of cells prevented from fusion by Ca2+ deprivation. The inhibition of fusion by D600 and TPA is significantly diminished in the presence of the Ca2+ ionophore A23187. TPA arrest of myoblast fusion was found to be accompanied by an increase in phosphorylation of the 20-kDa light chain of cytoplasmic myosin in a dose- and time-dependent manner. The effects of TPA on myoblast fusion and phosphorylation of myosin light chain were mimicked by the cell permeant diacylglycerol sn-1,2-dioctanoylglycerol, a potent activator of protein kinase C. The present results suggest that activators of protein kinase C block fusion by interfering with a Ca2+ signal transduction pathway and that this interference may be associated with a protein kinase C catalyzed inhibitory phosphorylation of myosin light chain.

Purothionin from wheat endosperm reversibly blocks myogenic differentiation of chick embryonic muscle cells in culture

Purothionin from wheat endosperm is a cysteine-rich, basic polypeptide of about 5000 Da, which modifies membrane permeability of cultured mammalian cells. This peptide was found to block fusion of chick embryonic muscle cells in culture but allows proliferation and alignment. A purothionin concentration of 6 micrograms/ml (1.2 microM) was necessary for the complete prevention of myotube formation. Under similar conditions, incorporation of [35S]methionine occurred normally but the synthesis of muscle-specific proteins including creatine kinase and acetylcholine receptor was strongly inhibited. In addition, purothionin blocked the uptake of 86Rb+, immediately after its addition to the cultured myoblasts. No such effects were found with the purothionin chemically modified with acetic or succinic anhydride. Thus, the basic residues in purothionin appear to be associated with the inhibition of myogenic differentiation. These results suggest that purothionin exerts its regulatory effect on the transition from proliferative to differentiative myoblasts by interfering with membrane permeability or intercellular contact and recognition, which are necessary for the initiation of muscle differentiation.

Membrane-cytoskeleton associations during myogenesis deviate from traditional definitions

Plasma membrane-cytoskeleton associations involving four membrane proteins (A5, H58, H36, and I20) were studied in developing L8E63 rat skeletal muscle cells using immunofluorescence microscopy and photometry on the basis of three criteria: Triton-insolubility, colocalization with cytoskeletal components, and sensitivity to cytoskeleton-directed drugs. The results presented demonstrate that there are developmental stage-specific associations between membrane proteins and the cytoskeleton during skeletal myogenesis. Several inconsistencies were found with traditional expectations of membrane-cytoskeleton associations. For example, although A5 is Triton-insoluble and sensitive to cytochalasin, its distribution generally does not correspond with any known cytoskeletal structure. Furthermore, the topography of A5 is dependent on the integrity of the plasma membrane. H36 and I20 are completely soluble in Triton and therefore by accepted definitions would not be expected to be associated with any cytoskeletal component. Yet H36 and actin codisrupt in the presence of cytochalasin, while I20, whose distribution does not correspond with microtubules, is uniquely sensitive to their disruption. These results demonstrate that (i) neither Triton-solubility nor colocalization alone predicts all membrane-cytoskeleton associations; some associations between the membrane and cytoskeleton are unstable in nonionic detergent; (ii) the native distribution of proteins in the membrane may not reflect their cytoskeletal associations; and (iii) the topography of some membrane proteins with no apparent association with the cytoskeleton may be greatly influenced by the cell cytoskeleton.

Lateral motion of fluorescent molecules embedded into cell membranes of clonal myogenic cells, L6, changes upon cell maturation

The lateral motion of fluorescent molecules embedded into cell membranes of myogenic cell line, L6, was measured. The motion of S-F-ConA became faster at cell fusion stage, and became slower after fusion. On the other hand, the motion of lipid analog, F18, was not changed at cell fusion stage. However, after fusion when myotubes were formed, the motion of F18 became slower. At cell fusion stage, there was a large variation in the motion of S-F-ConA. This means that at this stage the properties of myoblasts change drastically and rapidly.

Changes in myoblast membrane electrical properties during cell-cell adhesion and fusion in vitro

Characteristic of the process of myogenesis are the changes in the composition and organization of the cell membrane. While poorly understood, these changes have biochemical and biophysical relevance. Recently, changes in molecular order of the myoblast membrane which accompany differentiation in vitro have been observed (Santini, M.T., Indovina, P.L. and Hausman, R.E. (1987) Biochim. Biophys. Acta 896, 19-25). To further investigate these cell fusion processes we have examined additional physical parameters: conductivity and permittivity of the myoblast membrane during differentiation which reflect the molecular arrangement of the membrane. The determination of these parameters is possible because in the radio frequency range suspensions of cells in an electrolyte buffer show a characteristic conductivity dispersion due to the interfacial polarization. An analysis of our experimental data based on a 'single-shell' model showed that conductivity and permittivity of the membrane of pre- and post-fusion myoblasts varied significantly and abruptly. The conductivity of the cell interior (cytosol) remained constant. We discuss the significance of the observed changes in these membrane parameters for myogenesis.

Changes in myoblast membrane order during differentiation as measured by EPR

The events which make possible the characteristic fusion of the cell membranes of embryonic myoblasts are known to involve modification of the cell membrane (Hausman, R.E., Dobi, E.T., Woodford, E.J., Petrides, S., Ernst, M. and Nichols E.B. (1986) Dev. Biol. 113, 40-48). Myoblasts from chick embryos were allowed to differentiate in gyrotory aggregate culture and the order of their membranes was measured by EPR. Two spin-labels which insert at different depths into the lipid bilayer were used. Measurement with the 5-nitroxystearate label showed an increase in myoblast membrane order (2T' parallel) from 0-15 h of culture and again from 26-38 h of culture. Measurement with the 12-nitroxystearate label showed the 0-15 h increase in order but the second increase was greatly reduced and shifted in time. While the specific sources of these changes in membrane order cannot yet be identified, the changes observed correlated well with known events of myogenic differentiation in vitro. The initial increase in membrane order occurred while the myoblasts were recovering from the effects of trypsin dissociation and undergoing gyrotory aggregation. The second increase in membrane order occurred during the known period of prostaglandin receptor activity and increased cell-cell adhesion.

Myoblast fusion is regulated by a prostanoid of the one series independently of a rise in cyclic AMP

The role of prostanoids in the regulation of chick myoblast differentiation has been investigated. At 3 X 10(-6) M, indomethacin and chloroquine specifically inhibit cell fusion. They do not affect cell proliferation, alignment, or the expression of two muscle-specific proteins, namely, the acetylcholine receptor and the muscle-specific form of creatine phosphokinase. The results demonstrate that it is indomethacin's activity as an inhibitor of prostaglandin synthesis at the cyclooxygenase step that causes the block of cell fusion, whereas chloroquine probably acts at the earlier step of phospholipase A. Prostaglandin E1 (PGE1), but not prostaglandin E2 (PGE2), rapidly reverses the inhibition of fusion imposed by indomethacin or chloroquine. The dose response of the myoblasts to PGE1 is a bell-shaped curve with a 100% reversal of fusion at approximately 10(-9) M. Eicosatrienoate and linoleate reverse the inhibition of fusion with similar kinetics, whereas arachidonate is completely ineffective. The ability of PGE1 and eicosatrienoate but not PGE2 and arachidonate to restore fusion to control levels implies that fusion is specifically regulated by a prostanoid of the one series. The reversal of the fusion-block by linoleate further suggests that this fatty acid provides the necessary source of eicosatrienoate in the myoblast plasma membrane. At 10(-8) M and above, PGE1 and PGE2 stimulate adenylate cyclase and depress control fusion as does 10(-5) M isoproterenol. The beta-adrenergic blocker propranolol abolishes both isoproterenol's inhibition of myoblast fusion and its activation of adenylate cyclase. The similar depressions imposed on cell fusion by 10(-8)-10(-6) M prostanoid and 10(-5) M isoproterenol suggest that in both cases the depressive effects are mediated by cyclic AMP. It is concluded that a prostanoid of the one series regulates fusion by a cyclic AMP-independent mechanisms.

Organization of filaments underneath the plasma membrane of developing chicken skeletal muscle cells in vitro revealed by the freeze-dry and rotary replica method

Cytoskeletal organization and its association with plasma membranes in embryonic chick skeletal muscle cells in vitro was studied by the freeze-drying and rotary-shadowing method of physically ruptured cells. The cytoskeletal filaments underlying the plasma membranes were sparse in myogenic cells at the stage when cells exhibited great lipid fluidity in plasma membranes (fusion competent mononucleated myoblasts and recently fused young myotubes). Myotubes at more advanced stages of development possessed a highly interconnected dense filamentous network just underneath the cell membrane. This subsarcolemmal network was composed predominantly of 8-10 nm filaments; they were identified as actin filaments because of their decoration with myosin subfragment-1. Fine fibrils having a diameter of 3-5 nm were found on the protoplasmic surface of the plasmalemma at both the early and advanced stages of development. They were associated with the subsarcolemmal cytoskeletal filaments. Short 2-5 nm cross-linking filaments were occasionally seen between filaments in the subsarcolemmal network. We conclude that, although the subsarcolemmal cytoskeletal network contains many actin filaments, this domain appears to play some role in preserving the cell shape in the form of the membrane skeleton rather than membrane mobility.

Modifications of the adenylate cyclase complex during differentiation of cultured myoblasts

Alterations in receptor-independent activation of adenylate cyclase during proliferation and differentiation of L6E9 myoblasts were studied using Mn2+, forskolin, and Gpp(NH)p. Analyses were performed 3, 6, and 10 days following subculture, corresponding to onset of proliferation, end of proliferation with start of differentiation, and completion of differentiation, respectively. The apparent activation constant for Mn2+ decreases with the age of the culture; the apparent activation constant for Mg2+ does not. Bimodal activation by Mn2+, i.e., at concentrations greater than 10 mM, results in total adenylate cyclase activity less than the Vmax and occurs exclusively in differentiated cultures. Independent of the presence of Mg2+, forskolin activation occurs with low-and high-affinity constants in differentiated cultures and with a low affinity constant in youngest cultures; intermediate cultures (day 6) demonstrate low- and high-affinity activation only in the presence of high Mg2+. In contrast, the Vmax for forskolin increases with increasing Mg2+ in all culture ages. Although Gpp(NH)p-dependent adenylate cyclase activation occurs with an apparent activation constant independent of culture age and Mg2+, low Mg2+ fosters bimodal activation by Gpp(NH)p, i.e., above 100 microM nucleotide, total adenylate cyclase activity is less than the Vmax. The loss of stimulatory capacity by high Gpp(NH)p is greatest in differentiated cultures. Additional experiments are presented to substantiate that bimodal activation by Gpp(NH)p is specific. Cholera- and pertussis toxin-dependent ADP ribosylation patterns demonstrate a marked decrease in both Ns and Ni in differentiated cultures. The data suggest that alterations in postreceptor activation of adenylate cyclase during the course of differentiation and proliferation are mediated by guanine nucleotide binding proteins as well as by allosteric cation regulatory units.