Interrelated lipid alterations and their influence on the proliferation and fusion of cultured myogenic cells

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.

Azide-Modified Membrane Lipids: Synthesis, Properties, and Reactivity

In the present work, we describe the synthesis and the temperature-dependent behavior of photoreactive membrane lipids as well as their capability to study peptide/lipid interactions. The modified phospholipids contain an azide group either in the middle part or at the end of an alkyl chain and also differ in the linkage (ester vs ether) of the second alkyl chain. The temperature-dependent aggregation behavior of the azidolipids was studied using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and small-angle X-ray scattering (SAXS). Aggregate structures were visualized by stain and cryo transmission electron microscopy (TEM) and were further characterized by dynamic light scattering (DLS). We show that the position of the azide group and the type of linkage of the alkyl chain at the sn-2 position of the glycerol influences the type of aggregates formed as well as their long-term stability: P10AzSPC and r12AzSHPC show the formation of extrudable liposomes, which are stable in size during storage. In contrast, azidolipids that carry a terminal azido moiety either form extrudable liposomes, which show time-dependent vesicle fusion (P15AzPdPC), or self-assemble in large sheet-like, nonextrudable aggregates (r15AzPdHPC) where the lipid molecules are arranged in an interdigitated orientation at temperatures below T_m (L_βI phase). Finally, a P10AzSPC:DMPC mixture was used for photochemically induced cross-linking experiments with a transmembrane peptide (WAL-peptide) to demonstrate the applicability of the azidolipids for the analysis of peptide/lipid interactions. The efficiency of photo-cross-linking was monitored by attenuated total reflection infrared (ATR-IR) spectroscopy and mass spectrometry (MS).

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.

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.

Heterogeneous distribution of acetylcholine receptors in chick myocytes induced by cholesterol enrichment

The cholesterol concentration at the cell surface of cultured chick myocytes was increased in order to determine the effects of high levels of cholesterol on the ion channel properties of the nicotinic acetylcholine receptor. Single channel recordings and fluorescence polarization studies using 1,6-diphenyl-1,3,5-hexatriene (DPH) were performed under equivalent conditions for normal and cholesterol enriched myocytes. In cell attached patches from myocytes with a cholesterol to phospholipid molar ratio (c/p) of 0.24 and a microviscosity of 1.35 poise a single conductance of 51 pS was detected. The cholesterol enriched myocytes with a c/p of 0.52 and a microviscosity of 2.05 poise showed two conductances, a 54 pS and a 39 pS channel: both were blocked by alpha-bungarotoxin. The 39 pS channel was detected with the simultaneous appearance of a slow component of tau m (modulation time) for DPH fluorescence measured by phase demodulation. The 80% reduction in the open time constant (tau 2) of the 39 pS channel suggest an inhibition of the normal conformational state. The combined results suggest that cholesterol enrichment may induced a more heterogeneous lipid environment and that the two types of channel properties could result from the distribution of the receptors in different domains.

Effect of soman and sarin on phosphatidylcholine asymmetry in the electroplax from the electric eel

Some effects of organophosphorus anticholinesterase compounds that are unrelated to cholinesterase inhibition and that are sometimes long lasting may be due to alterations at the cellular membrane level. Phosphatidylcholine exchange protein was used to assess the effects of sarin and soman on phosphatidylcholine asymmetry in the inner and outer leaflets of the plasma membrane bilayer of the electroplax. Exposure of electroplax (30 min in vitro) to soman (10(-4), 10(-6) M) or sarin (10(-4), 10(-6), 5 x 10(-9) M) increased the percentage of phosphatidylcholine in the outer monolayer of the innervated plasma membrane bilayer and decreased the percentage in the inner monolayer. These changes by sarin were observed at concentrations that produced 100% cholinesterase inhibition (10(-4), 10(-6) M) and at a concentration (5 x 10(-9) M) where no inhibition occurred, suggesting that these effects are not directly due to cholinesterase inhibition. A 1-week exposure of live eels to soman (10(-8) M) in vivo caused an increase in phosphatidylcholine labeling in the outer monolayer of the innervated and noninnervated surfaces of the electroplax. Two weeks after stopping exposure to soman, increased labeling was still observed, suggesting that this may be a long-term effect. Because the organophosphates did not increase the permeability of the electroplax, all of these changes in labeling appear to be due to a redistribution of phosphatidylcholine from the inner to the outer monolayer of the plasma membrane bilayer.

Cation-induced aggregation of acidic phospholipid vesicles: the role of fatty acid unsaturation and cholesterol

Cation-induced aggregation of acidic phospholipid vesicles consisting of dimyristoylphosphatidylglycerol (DMPG), dipalmitoylphosphatidylserine (DPPS), phosphatidylserine from bovine brain (brPS), and phosphatidylglycerol from egg yolk (eggPG) was studied. Significant differences were evident in the NaCl-induced aggregation of fully saturated and unsaturated acidic phospholipid vesicles. The threshold NaCl concentration of vesicle aggregation ([NaCl]Thr) for DPPS vesicles was 320 mM compared to 610 mM observed for brPS vesicles. For DMPG vesicles the [NaCl]Thr was 430 mM and no aggregation of eggPG vesicles could be observed upon addition of NaCl. The threshold CaCl2 concentrations of aggregation of DMPG and eggPG vesicles were 2.3 and 4.9 mM, respectively. The corresponding threshold CaCl2 concentrations for DPPS and brPS vesicles were 0.85 mM and 1.3 mM, respectively. The inclusion of cholesterol into vesicles attenuated NaCl- and CaCl2-induced aggregation of DMPG and DPPS vesicles. However, enhancement of aggregation by inclusion of cholesterol was observed in the case of NaCl-induced aggregation of brPS vesicles. It is concluded that cation mediated membrane-membrane interactions depend, in addition to polar headgroup structure, on the fatty acid composition of the phospholipids also.

Interaction between endogenous circulating sulfated-glycosaminoglycans and plasma proteins

Interaction between endogenous 35S-labelled plasma glycosaminoglycans and proteins in murine plasma was demonstrated by coelution from gel chromatography of circulating 35S-labelled glycosaminoglycans with a wide range of plasma proteins. Autoradiography of electrophoretic tracing of proteins from 35S-sulfate labelled plasma showed that labelled glycosaminoglycans were associated with alpha 1, alpha 2, beta globulins and albumin, but not with gamma globulins. Analysis by gel chromatography on Sepharose CL-6B of delipidated 35S-labelled plasma after either proteolysis or beta-elimination, suggested that 35S-labelled glycosaminoglycan chains were covalently bound to proteins. Lipids were probably involved in the supramolecular assembly of GAGs with plasma proteins, as shown by hydrophobic interaction chromatography. In addition, strong, non-covalent interaction between glycosaminoglycan chains and proteins was responsible for the difficulty in extracting 'free' glycosaminoglycans from plasma. Consistently, ion-exchange chromatography of 35S-sulfate labelled delipidated plasma after alkali treatment, revealed that the anionic properties of glycosaminoglycans were hampered when plasma proteins were present.

Vinblastine sensitivity of leukaemic lymphoblasts modulated by serum lipid

The high-resolution proton magnetic resonance spectrum of leukaemic lymphoblasts is characteristic of neutral lipid in an isotropic environment. When such lymphoblasts are selected for resistance to the anticancer drug vinblastine, the intensity of this spectrum increases with increasing drug resistance. A reversal of this trend can be achieved by growing cells in delipidated serum, whereby lipid spectrum and drug resistance are diminished. However, both can be restored by subsequent regrowth in normal medium. Thus, although detectable genetic changes accompany the development of vinblastine resistance, the expression of these changes can be modulated by environmental lipid.

Acetylated lipoproteins impair erythroid growth factor release from endothelial cells

Endothelial cells are a known source of hematopoietic growth-enhancing factors, including platelet-derived growth factor (PDGF). In addition, endothelium interacts directly with plasma lipoproteins which have been shown to modulate hematopoiesis. To determine the relationship of these properties, we measured the release of an erythroid growth-enhancing factor from bovine endothelial cells under lipid-loaded and control conditions. Human bone marrow cells cultured under serum-free conditions form more erythroid, granulocyte/macrophage, and mixed hematopoietic colonies when supplemented with endothelial cell-conditioned medium (ECCM) than do controls (P less than 0.05). The activity is expressed over a wide range of erythropoietin, lymphocyte-conditioned medium (LCM), recombinant human interleukin-3, and colony-stimulating factor (CSF) concentrations, and is related to ECCM dose. In contrast, enhancing activity in ECCM prepared with 0-400 micrograms/ml acetylated low density lipoproteins (AcLDL) or native LDL is diminished to 0% in a dose-dependent fashion (relative to ECCM from unexposed cells or from cells incubated with very low density lipoproteins, P less than 0.05). Upon dilution, medium prepared from cells incubated with LDL shows a rightward shift in the dose-response curve for erythroid colony formation, while that prepared from AcLDL loaded cells demonstrates a downward shift, indicating that the inhibitory activities are kinetically distinct. Delipidation of ECCM prior to addition to marrow culture removes the inhibitory action of native LDL (P less than 0.05) but not that of AcLDL (P greater than 0.10). Immunochemical analysis suggests that the erythropoietic activity in ECCM is unrelated to that of PDGF, recombinant human CSF, and erythroid burst-promoting activity (BPA) present in LCM. This conclusion is supported by Northern blot analysis of endothelial cells using a cDNA probe for the v-sis homologue of the PDGF beta chain and by immunoprecipitation of metabolically labeled PDGF. The relative amounts of c-sis transcripts and of secreted PDGF were similar in endothelial cells incubated with or without AcLDL. We conclude that AcLDL impair the synthesis or release of an erythropoietic growth-enhancing factor(s) which is biologically distinct from PDGF and BPA present in LCM.

Ca2+-induced fusion of large unilamellar phosphatidylserine/cholesterol vesicles

The effect of cholesterol on the Ca2+-induced aggregation and fusion of large unilamellar phosphatidylserine (PS) vesicles has been investigated. Mixing of aqueous vesicle contents was followed continuously with the Tb/dipicolinate assay, while the dissociation of pre-encapsulated Tb/dipicolinate complex was taken as a measure of the release of vesicle contents. Vesicles consisting of pure PS or PS/cholesterol mixtures at molar ratios of 4:1, 2:1 and 1:1 were employed at three different lipid concentrations, each at four different Ca2+ concentrations. The results could be well simulated in terms of a mass-action kinetic model, providing separately the rate constants of vesicle aggregation, c11, and of the fusion reaction itself, f11. In the analyses the possibility of deaggregation of aggregated vesicles was considered explicitly. Values of both c11 and f11 increase steeply with the Ca2+ concentration increasing from 2 to 5 mM. With increasing cholesterol content of the vesicles the value of c11 decreases, while the rate of the actual fusion reaction, f11, increases. Remarkably, the effect of cholesterol on both aggregation and fusion is quite moderate. The presence of cholesterol in the vesicle bilayer does not affect the leakage of vesicle contents during fusion.

Effect of hormones on phospholipid metabolism in human cultured fibroblasts

The effect of hormones on phospholipid metabolism, pool size, 32P labeling and changes in fatty acid of human adult fibroblasts was determined. Simultaneously the change in membrane fluidity of single cells was recorded via fluorescence recovery after photobleaching under the influence of hormones. From all substances tested (isoproterenol, phenylephrine, adrenalin, histamine, angiotensin II, dansylcadaverine, propranolol) only isoproterenol and adrenalin slightly decreased total amount of phosphatidylcholine (PC). The amount of the other phospholipids analyzed remained unchanged. The 32P incorporation rate into phospholipids (PC, phosphatidylinositol (PI), phosphatidylethanolamine (PE)) was affected basicly different analyzing either PC, PI or PE. Histamine and propranolol provoked the highest incorporation of 32P (240% increase in PI labeling). Isoproterenol and adrenalin decreased PC labeling (45% and 18%) whereas isoproterenol decreased 32P incorporation into PI (18%), and adrenalin led to an increase (37%). PE labeling showed no or a slight increase in 32P incorporation applying the other agonists or antagonists. The fatty acid pattern of the respective phospholipids changed only to a minor extend. A decrease in hexadecanoic acid content of PI was found after administration of either isoproterenol, adrenalin or histamine. Parallel determination of membrane fluidity of single cells by fluorescence recovery after photobleaching showed an increase in the diffusion coefficient of a fluorescent lipid probe sticking in the membrane, following administration of isoproterenol and adrenalin, other substances tested exerted no effect. A relationship to changes in phospholipid metabolism became obvious. These results are discussed considering known mechanisms of receptor coupling and change in phospholipid metabolism and fluidity.

The influence of saturated fatty acid modulation of bilayer physical state on cellular and membrane structure and function

Cultured chick fibroblasts supplemented with stearic acid in the absence of serum at 37 degrees C degenerate and die in contrast to cells grown at 41 degrees C which appear normal in comparison with controls. These degenerative effects at 37 degrees C are alleviated by addition to stearate-containing media of fatty acids known to fluidize bilayers. These observations suggest that cell degeneration at 37 degrees C may involve alterations in the physical state of the membrane. Fatty acid analysis of plasma membrane obtained from stearate-supplemented cells clearly demonstrates the enrichment of this fatty acid species into bilayer phospholipids. Moreover, the extent of enrichment is similar in cells grown at both 37 and 41 degrees C. Stearate enrichment at either temperature does not appear to alter significantly membrane cholesterol or polar lipid content. Fluorescence anisotropy measurements for perylene and diphenylhexatriene incorporated into stearate-enriched membranes reveals changes suggestive of decreased bilayer fluidity. Moreover, analysis of temperature dependence of probe anisotropy indicates that a similarity in bilayer fluidity exists between stearate-enriched membranes at 41 degrees C and control membranes at 37 degrees C. Calorimetric data from liposomes prepared from polar lipids isolated from these membranes show similar melting profiles, consistent with the above lipid and fluorescence analyses. Arrhenius plot of stearate-enriched membrane glucose transporter function reveals breaks which coincide with the main endotherm of the pure phospholipid phase transition, indicating the sensitivity of the transporter to this transition which is undetectable in these native bilayers. These data suggest the existence of regions of bilayer lipid microheterogeneity which affect integral enzyme function, cell homeostasis and viability.

Metabolic incorporation of 9-(2-anthryl)-nonanoic acid, a new fluorescent and photoactivable probe, into the membrane lipids of Chinese hamster ovary cells

9-(2-Anthryl)-nonanoic acid is a new fluorescent and photoactivable probe, which has been designed for studying the lateral diffusion rate and the lateral distribution of lipids in biological membranes by means of the anthracene photodimerization reaction. It is shown that this anthracene fatty acid is metabolically incorporated into the glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol) of the eukaryotic Chinese hamster ovary cells in culture. Under our culture conditions (Eagle's minimal essential medium plus delipidized fetal calf serum) this incorporation proceeded with a very good rate (up to 45 mol/100 mol, after two days culture) and could be easily modulated depending on the way the cells were fed with the anthracene fatty acid. It occurred to a similar extent at the sn-1 (55 +/- 5%) or at the sn-2 (45 +/- 5%) position on the phospholipid glycerol backbone, without any degradation or elongation. No double labelling at the sn-1 and sn-2 positions was detected. Although incorporation of the anthracene fatty acid affected the cell growth rate (generation time of 48 h compared to a generation time of 21 h for control cells) it did not bring about cell mortality. This incorporation took place not only into the phospholipids but also into the triglycerides with, as a consequence, the appearance of strongly fluorescent lipid vesicles inside the cells. It affected the whole cell fatty acid composition by slightly increasing the amount of palmitic acid and markedly decreasing the amount of stearic and oleic acids.

The calcium-dependent myoblast adhesion that precedes cell fusion is mediated by glycoproteins

Presumptive myoblasts from explants of chick embryo pectoral muscle proliferate, differentiate, and fuse to form multinucleate myotubes. One event critical to multinucleate cell formation is the specific adhesion of myoblasts before union of their membranes. In the studies reported here five known inhibitors of myotube formation--trifluoperazine, sodium butyrate, chloroquine, 1,10 phenanthroline, and tunicamycin--were tested for their effect on the Ca++-dependent myoblast adhesion step. The first four inhibitors of myotube formation do not perturb myoblast adhesion but rather block fusion of aggregated cells, which suggests that these agents perturb molecular events required for the union of the lipid bilayers. By contrast, tunicamycin exerts its effect by inhibiting the myoblast adhesion step, thereby blocking myotube formation. The effect of tunicamycin can be blocked by a protease inhibitor, however, which implies that the carbohydrate residues protect the glycoproteins from proteolytic degradation rather than participate directly in cell-cell adhesion. Whereas trypsin treatment of myoblasts in the absence of Ca++ destroys the cells' ability to exhibit Ca++-dependent adhesion, the presence of Ca++ during trypsin treatment inhibits the enzyme's effect, which suggests that myoblast adhesion is mediated by a glycoprotein(s) that has a conformation affected by Ca++. Finally, myoblast adhesion is inhibited by an antiserum raised against fusion-competent myoblasts. The effect of the antiserum is blocked by a fraction from the detergent extract of pectoral muscle that binds to immobilized wheat germ agglutinin, which again suggests that glycoproteins mediate Ca++-dependent myoblast adhesion.

Remodeling of the myoblast membrane accompanies development

We have prepared a library of cloned hybridomas that produce monoclonal antibodies reactive with the surface of E63 rat myoblasts. Using immunofluorescence analysis of antigens on single cells we have studied the expression of determinants at distinct stages of development. Conditions were established for quantitative photometry and were used to confirm the diversity in stage-specific expression that accompanies development. The remodeling of the myoblast membrane also involves stage-specific and transient changes in topography and aggregation of many antigens, and the period surrounding fusion is one of particular activity. The localization of antigens on the upper and attached surfaces of myogenic cells was often distinct, and quantitative and spatial stage-specific reorganizations of antigens differed with respect to these two surfaces. This polarity represents an additional level of complexity in the continuous remodeling of the muscle cell membrane. Comparisons of quantitative and topographic analyses of antigens on E63 cells with Rat-1 fibroblasts and developmentally defective (fu-) myoblasts indicate that the outer membranes of these nonmyogenic cells are distinct from differentiating myoblasts. One determinant, H36, is absent on Rat-1 cells and on all fu- lines tested, and undergoes interesting stage-specific changes in expression and topography.

Freeze-fracture study of filipin binding in photoreceptor outer segments and pigment epithelium of dystrophic and normal retinas

We have studied sterol distribution in the retinal pigment epithelial (RPE) microvillous and outer segment disc membranes of rats with inherited retinal degeneration (RCS; RCS-p/+) and of normal genetic controls (RCS-rdy+, RCS-rdy+-p/+) by using the polyene antibiotic filipin, which binds specifically to 3-B-hydroxy-sterols, and freeze-fracture techniques. Retinas were perfusion-fixed, incubated with filipin in the same fixative, and prepared routinely for freeze-fracture electron microscopy. In the normal retina, the distribution of filipin binding sites on both RPE microvillous and outer segment disc membranes changes during development. Prior to outer segment elongation and the onset of phagocytosis (10 days postnatal), filipin sterol complexes are homogeneously distributed in both microvillous and outer segment membranes. With the onset of phagocytosis (2 weeks postnatal and later) filipin binding in both tissues forms a proximal-to-distal gradient, and binding sites decrease as distance from the cell body increases. In the normal RPE microvillous membranes, binding sites are numerous proximally and sparse on the distal tips. In the normal outer segment disc membranes, binding sites are often present on the basal discs, but are sparse on the intact apical discs prior to shedding. As the discs are cast off and engulfed by the RPE, however, filipin binding increases on both disc and phagosome membranes. In the dystrophic retina, the distribution of filipin binding sites differs from the normal. First, in the microvillous membranes, the proximal-to-distal gradient in filipin binding is rarely present at 2 weeks postnatal and becomes prominent only after the buildup of membranous debris has begun (3-5 weeks postnatal). Second, as the photoreceptors degenerate and the membrane debris disappears (4 months postnatal), filipin binding on the microvillous membranes becomes relatively sparse and homogeneous. Third, filipin binding on the intact disc membranes does not change with outer segment elongation, and numerous filipin binding sites are present on both apical and basal outer segment disc membranes. Fourth, large aggregates of filipin binding sites occupy the vast expanses of particle-free areas of debris membranes which accumulate between the photoreceptors and the RPE. These changes in the amount and distribution of filipin binding sites in the dystrophic retina add to the evidence that the disease process involves outer segment as well as RPE membranes and suggest that alterations in cholesterol distribution could contribute to the phagocytic defect.

The fusion of myoblasts

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Effects of ethanol on the lipid composition of bovine vascular smooth muscle cells in culture

Ethanol (50 mM) had no effect on the growth rate or viability of arterial smooth muscle cells over 3.5 days. The cholesterol:phospholipid ratio of the cells was unchanged after 7 days exposure. The major phospholipid components phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol were unchanged by ethanol exposure. Sphingomyelin content fell significantly within 12 hr. There were major changes in the fatty acid composition of the phospholipids with a reduction in saturated fatty acids and an increase in unsaturated fatty acids.

Changes in serum influence the fatty acid composition of established cell lines

The fatty acid composition of different kinds of commercially available serum used to supplement cell culture media differs widely. As compared with fetal bovine serum, horse and bovine calf serum have a very high content of linoleic acid (18:2) and are low in arachidonic acid (20:4). (Fatty acids are abbreviated as number of carbon atoms:number of double bonds). Swine serum contains substantial amounts of both 18:2 and 20:4. Only fetal bovine serum contains more than 1% docosahexaenoic acid (22:6). Considerable differences in fatty acid composition occur when cells are grown in media containing any of these different serum supplements. The 18:2 and 20:4 content of 3T3 mouse fibroblast phospholipids is highest when the medium contains horse serum, intermediate with bovine calf serum, and lowest with swine or fetal bovine serum. Likewise, the highest phospholipid 18:2 content in Madin-Darby canine kidney cells (MDCK) occurs when the medium contains horse serum. With MDCK cells, however, growth in swine serum produces the highest 20:4 content. The 3T3 cell phospholipids accumulate more than 1% 22:6 only when the medium contains fetal bovine serum, whereas in no case do the MDCK cell phospholipids accumulate appreciable amounts of 22:6. The fact that the cellular fatty acid composition is likely to change should be taken into account when changes are contemplated in the serum used to grow established cell lines.

Regulation of skeletal muscle satellite cell proliferation by bovine pituitary fibroblast growth factor

Satellite cells in skeletal muscle have been implicated in muscle growth processes and regeneration. However, very little is known about the regulation of their proliferation and differentiation. The effect of fibroblast growth factor (FGF) on the proliferation of myogenic cells from adult rat skeletal muscle, presumably satellite cells, has been examined, and FGF has been found to be a potent mitogen for these cells. The mitogenic properties of serum were also documented and studied in conjunction with FGF. Even under conditions of maximal stimulation by serum, the addition of FGF caused a substantial increase in proliferation of satellite cells. The additive nature of the FGF and serum-stimulatory activity suggests that FGF-like molecules are not the active agents in serum and that more than one pathway may be involved in stimulating satellite cell proliferation.

Rings of membrane sterols surround the openings of vesicles and fenestrae, in capillary endothelium

We investigated the distribution of sterols in the cell membrane of microvascular endothelium (mouse pancreas, diaphragm, brain, heart, lung, kidney, thyroid, adrenal, and liver) with the polyene antibiotic filipin, which reportedly has binding specificity for free 3-beta-hydroxysterols. In some experiments, concomitantly, cell-surface anionic sites were detected with cationized ferritin. Vessels were perfused in situ with PBS, followed by light fixation and filipin administration for 10 to 60 min. Tissues were further processed for thin-section and freeze-fracture electron microscopy. Short exposure (10 min) to filipin-glutaraldehyde solution resulted in the initial appearance, on many areas, of rings of characteristic filipin-sterol complexes within the rim surrounding stomata of most plasmalemmal vesicles, transendothelial channels, and fenestrae. Such rings were absent from the rims of the large openings of the sinusoid endothelium (liver, adrenal), coated pits and phagocytic vacuoles. After longer exposure (30-60 min), filipin-sterol complexes labeled randomly the rest of plasma membrane (except for coated pits, and partially the interstrand areas of junctions), and also marked most plasmalemmal vesicles. These peristomal rings of sterols were displayed mostly on the P face, and, at their full development, consisted of 6-8 units around a vesicle stoma, and 10-12 units around a fenestra. At their level, the intramembranous particles and the cell surface anionic sites were virtually excluded. Peristomal rings of sterols were also detected on the plasma membrane of pericytes and smooth muscle cells of the microvascular wall, which otherwise were poorly labeled with filipin-sterol complexes as compared to endothelial plasmalemma. It is presumed that the peristomal rings of cholesterol may represent important contributors to the local transient stabilization of plasma membrane and to the phase separation between cell membrane and vesicle membrane at a certain stage of their fusion/fission process.

Trifluoperazine, a calmodulin antagonist, inhibits muscle cell fusion

We investigated the effect of trifluoperazine (TFP), a calmodulin antagonist, on the fusion of chick skeletal myoblasts in culture. TFP was found to inhibit myoblast fusion. This effect occurs at concentrations that have been reported to inhibit Ca2+-calmodulin in vitro, and is reversed upon removal of TFP. In addition, other calmodulin antagonists, including chlorpromazine, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W7), and N-(6-aminohexyl)-1-naphthalene-sulfonamide (W5), inhibit fusion at doses that correspond closely to the antagonistic effects of these drugs on calmodulin. The expression of surface acetylcholine receptor, a characteristic aspect of muscle differentiation, is not impaired in TFP-arrested myoblasts. Myoblasts inhibited from fusion by 10 microM TFP display impaired alignment. In the presence of the Ca2+ ionophore A23187, the fusion block by 10 microM TFP is partially reversed and myoblast alignment is restored. The presence and distribution of calmodulin in both prefusional myoblasts and fused muscle cells was established by immunofluorescence. We observed an apparent redistribution of calmodulin staining that is temporally correlated with the onset of myoblast fusion. Our findings suggest a possible role for calmodulin in the regulation of myoblast fusion.

Role of cholesterol in the capping of surface immunoglobulin receptors on murine lymphocytes

Previously, we have shown that the capping of surface immunoglobulins on murine lymphocytes can be affected by modulating the lipid environment of the surface membrane with free fatty acids. In the present study, murine lymphocytes were depleted of cholesterol by incubation with phospholipid vesicles. As the cellular cholesterol:phospholipid ratio decreased, the capping of the surface immunoglobulin was seen to decrease. This inhibition of capping could not be reversed by calcium and is not accompanied by changes in either the cytoskeletal element alpha-actinin or cellular ATP levels. Incubation of the cholesterol-depleted cells with cholesterol-containing phospholipid vesicles raised both the cholesterol:phospholipid ratio and capping levels to values close to those of untreated control cells. Remarkably, stearic acid, a saturated fatty acid, could also restore the capping levels in the cholesterol-depleted cells. On the basis of the present data and measurements of the fluorescence polarization of the probe diphenyl hexatriene, we propose a model in which the protein(s) involved in capping is located in a gel-like lipid domain, and that removal of cholesterol makes this domain less gel-like and inhibits capping. Restoration of the gel-like nature of this domain by the addition of either cholesterol or stearic acid enables the protein(s) to function normally.

The coordination of sterol and phospholipid synthesis in cultured myogenic cells. Effect of cholesterol synthesis inhibition on the synthesis of phosphatidylcholine

The coordination of biosynthesis of cholesterol and phosphatidylcholine has been investigated in a myoblast cell line L6, grown in lipid-depleted medium. The addition of 25-hydroxycholesterol or compactin to this medium inhibits cholesterol synthesis by over 95%. The rate of [3H]choline incorporation into phosphatidylcholine begins to decline after 6 h and eventually falls to 45% of control. Measurements of choline flux through the CDPcholine pathway and of the pool sizes of choline-containing intermediates indicate that the formation of CDPcholine is the rate-limiting step in phosphatidylcholine synthesis in L6. The rate of CDPcholine synthesis was measured in vivo by pulse-chase experiments. Culturing cells with 25-hydroxycholesterol or compactin results in an inhibition of this step, which parallels the inhibition of incorporation of [3H]choline into phosphatidylcholine. The specific activities of the enzymes of phosphatidylcholine synthesis were assayed under optimal substrate conditions. Growth in the presence of sterol-synthesis inhibitors for 24 h has a significant, but variable, effect on the activity of microsomal and cytosolic cholinephosphate cytidylytransferase. Inhibition is seen in approximately one-half of the preparations and ranges up to 60%. The degree of inhibition of the enzyme in vitro correlates with an elevation of cytosolic triacylglycerol and phospholipid levels, and is not eliminated by the inclusion of excess stimulatory phospholipids in the assay. The pool sizes of the substrates, cholinephosphate and CTP, are unaffected by cholesterol synthesis inhibition. In contrast to the effects on cholinephosphate cytidylytransferase, the microsomal enzymes glycerol-3-phosphate acyltransferase and choline phosphotransferase are stimulated 2-fold or more. Choline kinase specific activity was inhibited 2-fold after 24 h of treatment with 25-hydroxycholesterol; however, no effect on this step was observed in vivo. These results indicate that the coordination of cholesterol and phosphatidylcholine synthesis involves regulation at the cytidylytransferase-catalyzed step.

Differentiation-related differences in the plasma membrane phospholipid asymmetry of myogenic and fibrogenic cells

We have determined the asymmetric distribution of two aminophospholipids phosphatidylethanolamine and phosphatidylserine in the plasma membrane of chick embryo fibroblast and myoblasts. Right-side-out membrane preparations were incubated with two different amidating reagents, trinitrobenzenesulfonate and isethionylacetimidate, under nonpenetrating conditions. Inside-out membranes were incubated with trinitrobenzenesulfonate. In fibroblasts, the similar plateau values suggested that 35% of the phosphatidylethanolamine and 20% of the phosphatidylserine is externally disposed. These values agree with previous measurements on fibroblast plasma membranes. In myoblasts, however, labelling plateaux were achieved which suggested that 65% of the phosphatidylethanolamine and 45% of the phosphatidylserine is externally disposed. This represents a 2-3-fold increase in potentially fusogenic lipids on the external leaflet of the plasma membrane. This unique distribution of aminophospholipids in myoblasts extends through the stage of development during which myoblasts become competent to fuse and form myotubes in culture. Two inferences may be drawn from these results. First, the external concentration of aminophospholipids in myoblasts is enriched significantly over that of fibroblasts or erythrocytes. This orientation may contribute to its fusion competence. Second, although large amounts of externally disposed aminophospholipid may be necessary for myoblast fusion, they do not confer fusion competence.

Morphological studies of synapses and varicosities in dissociated cell cultures of sympathetic neurons

Neurons dissociated from the superior cervical ganglia of newborn rats can be grown under conditions which support either adrenergic or cholinergic differentiation. In both cases, the neurons form numerous morphologically specialized synaptic terminals or synapses as well as relatively unspecialized varicosities. The ultrastructure of both types of terminal was compared in mature neuronal cultures and the effects of growth conditions on terminal morphology examined. After aldehyde-osmium fixation, synapses in cultures grown under adrenergic or cholinergic conditions were characterized by asymmetrical membrane specializations comparable to type I or asymmetric synapses; bismuth iodide and ethanolic phosphotungstic acid impregnation of neuronal cultures revealed the presence of characteristic synaptic membrane specializations: a presynaptic grid of dense projections and a wide postsynaptic dense band of uniform thickness. No membrane specializations were apparent in varicosities after aldehyde-osmium fixations or with these stains. Intramembranous particle distributions were examined in freeze-fracture replicas of neurons. Aggregates of large, 10-12 nm particles were found on P-face membrane leaflets of cell bodies and large diameter processes; this distribution is the same as that of synapses in thin-sectioned preparations. These particle aggregates may represent postsynaptic membrane specializations or acetylcholine receptors. The cytoplasmic leaflet of boutons contained large, 12-14 nm particles, which appeared to be concentrated at the region of synaptic contact at putative synapses, but were diffusely distributed in varicosity membranes. Similar large particles were also seen at a much lower density in the membrane E-face. None of these ultrastructural characteristics appeared to vary with transmitter identity or growth conditions. Synaptic vesicle shape, however, did vary in glutaraldehyde-fixed cultures. At all ages examined, neurons grown on monolayers of heart cells contained predominantly round vesicles, whereas neurons grown in the virtual absence of non-neuronal cells possessed pleiomorphic synaptic vesicles. This difference in vesicle shape appeared to be correlated more closely with growth in the presence of non-neuronal cells than with the transmitter present at the time of fixation.

Effect of inhibitors of cholesterol synthesis on muscle differentiation

Aggregation, the first step in the fusion of cultured chick myoblasts to form myotubes, is inhibited by 90% after only 2 h exposure to 25-hydroxycholesterol, an inhibitor of cholesterol synthesis. In this study, the effect of this agent on the concentration of an integral membrane protein in the plasma membrane, the synthesis of two differentiation-controlled proteins and total protein synthesis were examined. While it is possible that any of these mechanisms could be responsible for the inhibition of fusion, none of them appear to account for the attenuation of fusion induced by the inhibition of cholesterol synthesis.

Dynamics and topographical distribution of surface glycoproteins during myoblast fusion: a resonance energy transfer study

We have investigated changes in topography and lateral translational mobility of concanavalin A (Con A) receptors on the surface of cultured chick muscle cells during the period of myoblast fusion. A temporal correlation between these phenomena and the alteration in membrane fluidity known to occur during this time period is established. Receptor topography and mobility are studied by means of a resonance energy transfer technique employing pyrene- and FITC-Con A conjugates. All measurements are performed through a microscope on single cells. Our results reveal that during the period of myoblast fusion Con A receptors undergo a dramatic redistribution on the cell surface. Furthermore, our data suggest that the changes in membrane fluidity observed during muscle differentiation serve to modulate the lateral mobility of these receptors.

Selective isoactin release from cultured embryonic skeletal muscle cells

The culture medium of embryonic quail myoblasts, labeled for 24 h with [35S]L-methionine, was analyzed by two-dimensional gel autoradiography. The major polypeptide observed had a 43,000 molecular weight and an isoelectric point of 5.4. This polypeptide could be specifically adsorbed to DNAse-I Sepharose. A tryptic peptide map of the [35S]methionine-labeled peptides of intracellular actin and the extracellular major polypeptide were virtually identical. These findings identify the released polypeptide as actin. A comparison of two-dimensional gel patterns of intracellular and extracellular labeled polypeptides showed a large number of differences indicating the actin release did not result from general cellular breakdown. The released actin was not filamentous as judged by its behavior during Bio-Gel A-5m chromatography (Bio-Rad Laboratories, Richmond, Calif.) The released actin did not originate solely from contaminating fibroblasts in the culture because actin was also observed in the medium in clonal myoblast cultures and in purified myotube preparations. Finally, the nonmuscle isoactins, as opposed to muscle alpha-isoactin, were released preferentially. These results indicate that within the developing muscle cell where both muscle and nonmuscle specific isoactins are simultaneously present, the different isoactins may be physically or functionally compartmentalized with the nonmuscle isoactins existing primarily at or near the cell surface.

Accumulation of N-3 polyunsaturated fatty acids cultured human Y79 retinoblastoma cells

The metabolism of the n-3 class of polyunsaturated fatty acids, which occur in relatively high quantities in neural tissues, was studied in human Y79 retinoblastoma cells. These cells contained low levels of n-3 polyunsaturates when grown in culture media supplemented with fetal bovine serum. The cells readily incorporated performed docosahexaenoic acid (22.6 n-3) into phospholipids, but human skin fibroblasts did this to a similar extent. When 10 to 30 mumol/ml linolenic acid (18:3 n-3) was added, the cell also accumulated 22:6 in phospholipids. The capacity to convert appreciable amounts of 18:3 to 22:6 appears to be a unique property of the retinoblastoma cells as compared with other continuously cultured cell lines. More 18:3 than linoleic acid (18:2 n-6) was incorporated into phospholipids by the retinoblastoma cultures, and 18:3 was channeled to a larger extent into the ethanolamine glycerophospholipid fraction. These findings indicate that retinoblastoma cells handle n-3 polyunsaturated fatty acids in a manner very similar to neural tissue in vivo. Based on the results obtained with this model system, it appears that three processes may contribute to the accumulation of 22:6 in retina and neural tissue: increased ability to incorporate 18:3, the capacity to convert 18:3 to 22:6, and channeling of 18:3 and its metabolites into ethanolamine glycerophospholipids.

Effects of retinol, fatty acids and glycerol monooleate on the fusion of chick embryo myoblasts in vitro

Cell fusion of embryonic chick myoblasts has been studied in the presence of fat-soluble agents that induce erythrocytes to fuse. Retinol inhibited myoblast fusion but the cells recovered their ability to fuse within 48 h of removal of the retinol from the medium. Myristic acid, oleic acid, glycerol monooleate, linolenic acid and arachidonic acid similarly prevented fusion in myogenic cultures. By contrast, linoleic acid moderately enhanced the fusion of chick skeletal myoblasts. In addition, stearic acid, which does not fuse erythrocytes, inhibited myoblast fusion whereas the saturated, non-fusogenic fatty acid, arachidic acid, was without effect.

The reaction center profile structure derived from neutron diffraction

Both reaction center protein from the photosynthetic bacteria Rhodopseudomonas sphaeroides and egg phosphatidylcholine can be deuterium labelled; the reaction center protein can be incorporated into the phosphatidylcholine bilayers forming a homogeneous population of unilamellar vesicles. The lipid profile and the reaction center profile within these reconstituted membrane profiles were directly determined to 32 A resolution using lamellar neutron diffraction from oriented membrane multilayers containing either deuterated or protonated reaction centers, and either deuterated or protonated phosphatidylcholine. The 32 A resolution reaction center profile shows that the protein spans the membranes, and has an asymmetric mass distribution along the perpendicular to the membrane plane. These results were combined with previously described X-ray diffraction results in order to extend the resolution of the derived reaction center profile to 9 A.

Retention of human skin fibroblast fatty acid modifications during maintenance culture

The fatty acid composition of cultured human skin fibroblasts was modified by adding either oleic or linoleic acid to the growth medium. After the cultures became confluent, they were washed and transferred to different maintenance media in order to determine the stability of the various fatty acyl modifications. Some changes in fatty acid composition occurred under all conditions. When the maintenance medium was supplemented with fatty acid, the cellular neutral lipid and phospholipid fatty acyl composition were altered markedly within 16 to 24 hr. If no supplemental fatty acid was available during the maintenance period, however, the modified fatty acyl compositions were sufficiently retained so that appreciable differences between the cells enriched with oleate and linoleate persisted for at least 48 to 72 hr. This considerable degree of stability occurred when either 10% delipidized fetal bovine serum or 10% fetal bovine serum containing its inherent lipids were present in the maintenance medium. Although the triglyceride content of the fatty acid-modified cells was quite labile, neither the cholesterol nor phospholipid content changed appreciably during culture in any of the maintenance media. Since the fatty acid compositional differences persisted during several days of maintenance under certain conditions, these modified cultures appear to be a useful experimental system for assessing the effect of lipid structure on fairly long-term cellular functions.

The fluidity of plasma membranes of Dictyostelium discoideum. The effects of polyunsaturated fatty acid incorporation assessed by fluorescence depolarization and electron paramagnetic resonance

Two probe techniques, fluorescence depolarization (using diphenylhexatriene) and electron paramagnetic resonance (using 5-doxyl stearic acid), have been used to assess the fluidity of the purified plasma membranes of Dictyostelium discoideum. Both techniques indicate that a large incorporation of polyunsaturated fatty acids into the plasma membranes does not significantly change membrane fluidity In addition, phosphatidylcholines isolated from cells grown on both polyunsaturated fatty acid-supplemented and unsupplemented media exhibit similar mobilities of an incorporated spin probe when dispersed in aqueous solution. This result suggests that the enrichment of a membrane already high in content of fatty acyl chains containing two double bonds with those containing three or more double bonds does not markedly change fluidity.

Cholesterol availability modulates myoblast fusion

The requirement of cholesterol for myoblast fusion has been linked to the primary step in the fusion process, calcium-dependent aggregation (recognition). Inhibition of cholesterol synthesis with 25-hydroxycholesterol or compactin in the absence of exogenous lipid dramatically inhibits calcium-mediated aggregation and concomitant fusion within several hours. Restimulating cholesterol synthesis or supplying exogenous cholesterol rapidly restores aggregation activity. Over this time period, however, the sterol:phospholipid ratio is unaltered, suggesting a local rather than a general membrane cholesterol requirement for the expression of aggregation activity. The aggregation response to a change in sterol availability occurs on a shorter time scale than that required to inhibit the synthesis of the protein(s) with aggregation activity; thus, the cholesterol-requiring step is posttranslational. We suggest that the assembly or maintenance of the aggregation activity depends on a continued local supply of cholesterol.

Apparent coordination of the biosynthesis of lipids in cultured cells: its relationship to the regulation of the membrane sterol:phospholipid ratio and cell cycling

The coordination of the syntheses of the several cellular lipid classes with one another and with cell cycle control were investigated in proliferating L6 myoblasts and fibroblasts (WI-38 and CEF). Cells cultured in lipid-depleted medium containing one of two inhibitors of hydroxymethylglutaryl-CoA reductase, 25-hydroxycholesterol or compactin, display a rapid, dose-dependent inhibition of cholesterol synthesis. Inhibition of the syntheses of each of the other lipid classes is first apparent after the rate of sterol synthesis is depressed severalfold. 24 h after the addition of the inhibitor, the syntheses of DNA, RNA, and protein also decline. The inhibition of sterol synthesis leads to a threefold reduction in the sterol:phospholipid ratio that parallels the development of proliferative and G1 cell cycle arrests and alterations in cellular morphology. All of these responses are reversed upon reinitiation of cholesterol synthesis or addition of exogenous cholesterol. A comparison of the timing of these responses with respect to the development of the G1 arrest indicates that the primary factor limiting cell cycling is the availability of cholesterol provided either from an exogenous source or by de novo synthesis. The G1 arrest appears to be responsible for the general inhibition of macromolecular synthesis in proliferating cells treated with 25-hydroxycholesterol. In contrast, the apparent coordinated inhibition of lipid synthesis is not a consequence of the G1 arrest but may in fact give rise to it. Sequential inhibition of lipid syntheses is also observed in cycling cells when the synthesis of choline-containing lipids is blocked by choline deprivation and is observed in association with G1 arrests caused by confluence or differentiation. In the nonproliferating cells, the syntheses of lipid and protein do not appear coupled.