Changes in myoblast membrane order during differentiation as measured by EPR

Cancer stem cell markers: what is their diagnostic value?

IMPORTANCE OF THE FIELD

Cancer resistance to conventional therapies has been attributed to cancer stem cells (CSCs). Although a variety of markers have been reported, a universal marker has not yet been found to identify CSCs. Better identification of these CSCs may lead to new therapies that selectively target these cells and thereby result in more effective treatment. This article categorizes the types of marker that have been identified and explores their potential diagnostic and therapeutic value.

AREAS COVERED IN THIS REVIEW

A focused literature review of studies relating to CSCs and their identification was conducted. Databases evaluated include MEDLINE and Web of Science through 2009.

WHAT THE READER WILL GAIN

The ideal identification method needs to be effective and practical in terms of application. The measurement of aldehyde dehydrogenase activity is simple to accomplish compared with other reported identification methods; however, cell surface antigens have been studied most frequently in the therapeutic targeting of CSCs.

TAKE HOME MESSAGE

Although specific targeting methods have been reported for various cancers, there does not appear to be a proven universal marker for CSCs that would apply to all cancers. Each particular identification method appears to have advantages and disadvantages. From a therapeutic standpoint, targeting of these CSCs should improve prognosis.

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.

Resistance of multicellular aggregates to pharmorubicin observed in human hepatocarcinoma cells

The objective of the present study was to investigate the multicellular resistance of human hepatocarcinoma cells BEL-7402 to pharmorubicin. Cells (1 x 10(4)) and 200 microcarrier Cytodex-3 beads were seeded onto a 24-well plate and cultured in RPMI 1640 medium. After the formation of multicellular aggregates, morphology and cell viability were analyzed by scanning electron microscopy, transmission electron microscopy and flow cytometry, respectively. The IC50 was determined by flow cytometry and MTT assay after the cells cultured in aggregates and monolayers were treated with pharmorubicin. The culture products exhibited structural characteristics somewhat similar to those of trabecular hepatocarcinoma in vivo. Among the microcarriers, cells were organized into several layers. Intercellular spaces were 0.5-2.0 microm wide and filled with many microvilli. The percent of viable cells was 87%. The cells cultured as multicellular aggregates were resistant to pharmorubicin with IC50 4.5-fold and 7.7-fold that of monolayer culture as determined by flow cytometry and MTT assay, respectively. This three-dimensional culture model may be used to investigate the mechanisms of multicellular drug resistance of hepatocarcinoma and to screen new anticancer drugs.

Apoptosis, cell adhesion and the extracellular matrix in the three-dimensional growth of multicellular tumor spheroids

In the last few years, it has become increasingly apparent that cell survival and death, especially apoptosis, strongly depend on cell adhesion and the extracellular matrix. In addition, it has also become clear that the use of three-dimensional multicellular tumor spheroids, which mimick more closely solid tumors in vivo, are a realistic experimental model to investigate many aspects of tumor biology. In the present review, after a general overview of the current knowledge regarding apoptosis, cell adhesion and the extracellular matrix, the results obtained utilizing multicellular tumor spheroids in these types of studies are discussed. The main conclusion that may be drawn from a synthesis of the literature on these topics is that investigations with multicellular tumor spheroids yield much useful information that is sometimes in contradiction to that obtained with monolayer cultures, but is closer to that derived from in vivo studies. Consequently, the authors encourage that these three-dimensional systems be used in many studies in which cell death and adhesion are being examined.

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.

Probucol reduces hepatic cholesterol secretion in hyperlipidemic Yoshida rats

In this study, perfused livers from Yoshida rats, either on a normal diet or on a diet with 0.3% probucol, were examined. The analysis of liver lipid content and of bile and lipoprotein secretion changes showed that probucol had a relevant effect on liver lipid biosynthesis. In particular, it reduced the production of triacylglycerols and, to a much greater extent that of cholesterol. In addition, probucol reduced plasma cholesterol concentration by decreasing esterified cholesterol in HDL1 and HDL2 fractions. Furthermore, HDL1 composition of both hepatic neosynthetized and circulating particles was strongly modified by probucol. Finally, probucol did not appear to induce significant differences in lipid bile secretion while phospholipid secretion from perfused livers was increased. These facts suggest that the hypolipidemic action of probucol is not mediated by an increase in bile steroid secretion, but rather by a direct reduction in hepatic lipoprotein cholesterol secretion. This secretion induces a modified plasma profile of HDL particles such that these variations are advantageous in terms of reverse cholesterol transport.

Junctional sites of erythrocyte skeletal proteins are specific targets of tert-butylhydroperoxide oxidative damage

The oxidative denaturation of the erythrocyte membrane, which is considered a major cause of the haemolytic process, was evaluated upon 'in vitro' oxidative stress with tertbutylhydroperoxide. Biochemical and ultrastructural analyses were performed to point out the effect of this substance on the skeletal network, which is mainly responsible for red cell shape and viability. Moreover, cell morphology was observed by scanning electron microscopy and membrane rigidity assessed by EPR measurements. The most relevant features of the membrane denaturation were, (i) lipid peroxidation, as assessed by malonidialdehyde production, (ii) spectrin and ankyrin degradation with simultaneous globin binding to the membrane, as evidenced by electrophoretic pattern of red cell ghosts. These phenomena were related to the drug concentration in the incubation medium, and accompanied by depletion of intracellular reduced glutathione. The denaturation of protein components hindered the release of spectrin in a hypotonic extraction medium and could be only partially reversed by dithiothreitol. The extensive membrane protein and lipid degradation, at high drug concentration, was coherent with a marked increase of membrane order (membrane 'rigidity'). No clustering of intramembrane proteins was shown by the transmission electron microscopy images. At the same time scanning electron microscopy demonstrated shrinking and disco-stomatocytic deformation of erythrocytes. Ultrastructural analysis of the membrane skeleton by fluorescence-labelling of spectrin and actin, allowed to point out that exposure to t-BHP caused the marginalization of spectrin and the rearrangement of actin molecules with formation of micro aggregates, so that a detachment of actin from the spectrin network was suggested. In addition to the generalized damage of red cell membrane, tertbutylhydroperoxide was found to induce a specific alteration of the skeletal network at the horizontal junction sites involving spectrin, actin, and protein 4.1 and thus to modify the cytoskeletal assembly. This effect on the membrane skeletal components was consistent with the hypothesis that oxidative stress plays a key role in the haemolytic process.

Different susceptibilities to cell death induced by t-butylhydroperoxide could depend upon cell histotype-associated growth features

The effects of the oxidizing agent t-butylhydroperoxide (t-BHP) were investigated on three human cell lines of different origin and growth features (A431 epithelial cells, ADF astrocytoma cells and U937 leukemic cells) using electron microscopy and electron paramagnetic resonance spectroscopy. The results indicate that important biophysical and ultrastructural modifications are induced in the plasma and mitochondrial membranes of these cells and that these changes can ultimately lead to cell death. In addition, the cell cytoskeleton also appears to be a target of hydroperoxide-mediated stress. In particular, all three cell types undergo cytoskeletal alterations leading to surface blebbing, a typical characteristic of cell damage. However, the timing and extent of this damage as well as that occurring at the mitochondrial and plasma membrane levels seems to be different: cells with weak (ADF) or absent (U937) cell-to-cell and cell-substrate contacts and a poorly developed cytoskeleton appear to be more susceptible than other cell types (e.g., A431) to t-BHP-mediated injury. These diverse cell susceptibilites to hydroperoxide-mediated oxidative stress could thus depend upon cell histotype-associated growth features.

A static magnetic field does not affect the dielectric properties of chick embryo myoblast membranes

We have recently demonstrated using dielectic relaxation studies in the radiowave frequency range that sinusoidal 50 Hz magnetic fields (with intensities ranging from 1 to 10 mT) induce a nonlinear change in both membrane conductivity and permittivity of primary chick embryo myoblasts in vitro. It was the aim of the present study to determine if a DC-induced static magnetic field is capable of generating similar variations in the membrane conductivity and/or the membrane permittivity of chick embryo myoblasts. The results indicate that when the myogenic cells are exposed to a static magnetic field of either 1, 3 or 5 mT (values comparable with the previous extremely low frequency study), no changes in the membrane electrical parameters can be observed with respect to controls. Differences in the characteristics of static and extremely low frequency fields as well as the possible mechanisms underlying the contrasting results with these two types of magnetic fields are discussed.

Two different pathways for necrotic cell death induced by free radicals

Plasma membrane modifications have been widely recognized as crucial factors in cell injury and death. One of these modifications, surface blebbing, has been considered as an injury-marker associated with a series of biochemical and physiological modifications. Our study focused on the different effects of free radical-induced cell damage by quinone menadione (2-methyl-1,4-naphthoquinone) and by hyperthermic shock (45 degrees C) on the erythroleukemic cell line K562. Different techniques including immunofluorescence, freeze-fracturing, and electron paramagnetic resonance spectroscopy were employed. Menadione induced the formation of surface blebs, accompanied by a rearrangement of the microfilament system and changes in the distribution of plasma membrane proteins. In contrast, heat-shocked cells showed neither blebbing nor important cytoskeletal changes. Finally, the electron paramagnetic resonance results showed an increase in membrane order not specifically related to the type of free radical-induced stress. These cell death features appear to suggest the existence of two different types of pathways for necrotic cell death: both treatments induce cell injury and eventual death by modifying plasma membrane integrity and function. However, one involves cytoskeleton-dependent surface blebbing, whereas the other does not.

A 50-Hz magnetic field induces structural and biophysical changes in membranes

The effects of a 50-Hz extremely low frequency magnetic field on cultured K562 cells growing in suspension were studied by means of scanning electron microscopy and electron paramagnetic resonance spectroscopy. Exposure of K562 cells at 2.5 mT for periods to 96 hours induced significant changes in cell-surface structure and physiology without modification of proliferative capability as indicated by quantitative analysis. Thus extremely low frequency fields seem able to induce injurious, sublethal cell alterations, and the plasma membrane seems to play an important role in this effect.

The fusion radiosensitivity of differentiating chick embryo myoblasts in vitro is not determined by the plasma membrane

We have recently demonstrated by dielectric relaxation studies in the radiofrequency range that the sharp drop in the conductivity and permittivity of the membranes of chick embryo myoblasts in vitro, representative of fusion, is either delayed or completely blocked by sublethal doses of ionizing radiation (Santini et al. 1990a). The lowest of the doses investigated (3.25 Gy) caused a 10 h delay in myoblast membrane fusion when the cells were exposed at 24 h of culture, indicating that radiation-induced membrane injury had occurred. The purpose of this study was to determine if the myoblast system under investigation shows the same radiosensitive characteristics if irradiated with 3.25 Gy at various stages of differentiation. Consequently, the myoblasts were exposed to this dose at two different stages of differentiation (12 h or 48 h of culture). We show here that the time at which the myogenic cells are irradiated (state of differentiation) does not seem to affect the magnitude of the fusion delay (which was used as a measure of radiosensitivity of the myoblasts). In fact, the sharp drop in both membrane conductivity and membrane permittivity occurs with the same 10 h delay independent of the time of exposure. The role played by the plasma membrane in determining myoblast response to radiation damage is discussed.

Changes in membrane fluidity and Na+/K(+)-ATPase activity during human trophoblast cell culture

The human placenta plays an essential role in embryo development, in particular regulating the transport of ions, nutrients and immunoglobulins from the maternal to the fetal circulation. Trophoblast organization into a syncytial layer involves structural and functional steps that may be monitored and elucidated by in vitro studies. The structural stages by which the syncytial trophoblast is formed are not yet understood. In order to clarify the mechanism of trophoblast development, we studied the morphological characteristics of the syncytial trophoblast formation in culture and the functional changes (transport properties and membrane microviscosity) accompanying the structural modifications. By using both 5-nitroxystearate and 16-nitroxystearate as spin labels, we observed an initial increase in membrane order over 0-24 h of culture, which can be associated with two events: recovery of cell membranes from trypsin and initial aggregation of cytotrophoblasts. The similar behaviour of the order parameters determined with both probes indicates that membrane order changes both inside and in the outer part of the lipid bilayer. The subsequent decrease in membrane order observed at 36-48 h might be related to the process of cellular fusion. The increase in sodium/potassium pump activity in the first 24 h of culture might be an expression of cell recovery following trypsin treatment. The subsequent decrease might represent an adaptive mechanism by which metabolic energy is mainly used for morphogenetic changes.

Changes in erythrocyte membrane lipid composition affect the transient decrease in membrane order which accompanies insulin receptor down-regulation

We have recently demonstrated, using electron paramagnetic resonance (EPR) spectroscopy, that insulin receptor internalization in response to insulin incubation (down-regulation) in human erythrocytes is accompanied by a transient decrease in membrane order, as measured by the 2T' parallel order parameter. Since membrane lipids play such an important role in receptor internalization, we investigated the possible effects that an alteration of the normally-occurring lipid profile might have on down-regulation and the concomitant transient decrease in membrane order. Consequently, human erythrocytes enriched with cholesterol and erythrocytes from cirrhotic patients were examined, because both of these groups of cells have a higher cholesterol/phospholipid molar ratio (CH/PL) than controls. The 5-nitroxystearate spin label, which inserts into the lipid bilayer of cell membranes, was used to monitor changes in 2T' parallel for a 3-h period at 37 degrees C. We report here that both cholesterol-enriched and cirrhotic erythrocytes do not down-regulate, as demonstrated by binding assays, and that they do not show the typical transient decrease in membrane order observed in controls. The results seem to indicate that a more ordered membrane inhibits internalization of the insulin receptor in erythrocytes, and that an increase in membrane disorder is necessary for insulin receptor down-regulation.

The cesium-induced delay in myoblast membrane fusion is accompanied by changes in cellular subfraction lipid composition

We have recently demonstrated that the delay in myoblast membrane fusion induced by cesium is accompanied by changes in isolated membrane lipids (Santini, M.T., Indovina, P.L., Cantafora, A. and Blotta, I. (1990) Biochim. Biophys. Acta 1023, 298-304). In the present study, we have investigated changes in the lipid profile of total cell homogenates and microsomal membrane fractions during myoblast membrane fusion as well as the effects that addition of cesium may have on these lipid variations in order to try to understand the production and translocation of lipids during this myogenic process. The data presented here indicate that the lipid composition of cell homogenates and microsomes varies in a different manner from isolated plasma membranes during myogenic fusion. In addition, cesium affects, in a different manner, the normally-occurring lipid production and distribution which takes place in each subcellular fraction.

Prostaglandin-dependent phosphatidylinositol signaling during embryonic chick myogenesis

Previous investigations suggested that binding of prostaglandin to a myoblast membrane receptor initiates a second messenger cascade which is essential for subsequent myogenesis. Initial evidence of the sensitivity of myogenesis to lithium suggested the involvement of inositol phosphate metabolism. That possibility is investigated here. The accumulation of inositol monophosphate in response to prostaglandin binding was studied in aggregate cultures of chick embryo myoblasts in vitro. At 22 or 28 h in culture mononucleated myoblasts were labeled with [3H]inositol, which was then incorporated into phosphoinositides. After experimental manipulations of prostaglandin metabolism and the addition of Li+ prior to prostaglandin binding at 33 h, [3H]inositol monophosphate accumulation was measured by anion-exchange chromatography between 33 and 37 h. Inositol monophosphate was found to accumulate rapidly following 33 h. However, after 36 h of myogenesis, no inositol monophosphate accumulation was observed. The accumulation was dependent on prostaglandin as indomethacin, which also blocks subsequent membrane events in myogenesis, blocked inositol phosphate accumulation. Like subsequent myogenesis, inositol phosphate accumulation was restored by the addition of exogenous prostaglandin. Finally, the accumulation of inositol phosphate began only after the binding of prostaglandin. The results demonstrate that an inositol phosphate signal transduction mechanism connects prostaglandin binding to membrane events in embryonic chick myogenesis.

Human erythrocyte insulin receptor down-regulation is accompanied by a transient decrease in membrane order

Insulin receptor internalization in response to insulin incubation (down-regulation) has been shown to occur in human erythrocytes as well as in human erythrocyte ghosts. It is also known that changes in cell membrane events can be detected with electron paramagnetic resonance (EPR) spectroscopy using spin labels. In the present study, changes in erythrocyte membrane order during down regulation as measured by the 2T'II parameter were investigated using EPR. The spin label, 5-nitroxystearate, which inserts into the lipid bilayer of cell membranes, was used. Changes in 2T'II at 37 degrees C were followed over a 3 h time period. A transient decrease in erythrocyte membrane order began within 30 min of the start of insulin incubation and reached a minimum level of 52.5 Gauss (G) within 90 min. This represented nearly a 2 G decrease from the zero incubation time value. Membrane order returned to the initial value by 2.5 h. These time-related changes in membrane order corresponded well with the insulin receptor internalization process as followed by surface binding assays. Surface insulin binding began to decrease within 30 min of the start of insulin incubation, and was reduced to 30% of control values within 2 h. Similar correlations between membrane order and receptor internalization were observed at 23 degrees C. Erythrocytes incubated with denatured insulin, and ATP-depleted erythrocytes incubated with native insulin, did not down-regulate their insulin receptors. Under these conditions, these erythrocytes also did not exhibit the transient decreases in membrane order. These findings are consistent with the hypothesis that an increase in membrane disorder is part of the mechanism of insulin receptor down-regulation.

Dose-dependent effects of ionizing radiation on in vitro myoblast fusion

We have recently demonstrated by dielectric relaxation studies in the radio-frequency range that there is a sharp decrease in the conductivity and permittivity of the membranes of chick embryo myoblasts in vitro at the time of fusion (60 h) (Bonincontro et al. 1987). This sharp fall in membrane electrical parameters was subsequently shown to be due to changes in ionic flux, particularly of the Na+/K+ equilibrium (Santini et al. 1988). Ionizing radiation induces a wide variety of effects on biological membranes, including variations in membrane ionic transport. We wished to investigate if sublethal doses of gamma-irradiation could affect membrane electrical parameters and thus myoblast membrane fusion. Consequently, chick embryo myoblast aggregate cultures were irradiated with 3.25, 5.15 or 6.35 Gy at 24 h of culture. We found that the lower dose delays membrane fusion by about 10 h while the two higher doses block fusion up to 120 h of culture. Aggregates showed a very high cell viability. The possible mechanisms by which ionizing radiation causes these variations in myoblast membrane electrical properties and fusion are discussed.

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.

Requirement for G protein activity at a specific time during embryonic chick myogenesis

Signaling between embryonic myoblasts involves prostaglandin metabolism, the activation of a membrane receptor and changes in polyphosphatidyl inositol metabolism. Many of these membrane-localized events occur between 33 to 35 h of differentiation, concomitant with a dramatic change in membrane organization, in myoblast aggregates in culture. Since many receptors affect inositol phosphate metabolism by activating a GTP-binding protein (G protein), we asked if there was evidence for such a protein in myogenic signaling. We show that during the period of differentiation in culture when prostaglandin is needed to bind to a transient receptor, a pertussis toxin-sensitive but cholera toxin-insensitive G protein must act. If this activation is blocked, the characteristic change in myoblast cell adhesion and subsequent membrane fusion do not occur. We suggest that a G protein couples the activated prostaglandin receptor and the change in polyphosphatidyl inositol metabolism and that this membrane transduction step is necessary for subsequent membrane differentiation events during myogenesis.

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.

Protein kinase C activity and phorbol ester binding to rat myogenic cells during growth and differentiation

Phorbol esters have been reported to induce opposite responses in fetal myoblasts and in satellite cells isolated from adult skeletal muscles. We examined the possibility that different levels of protein kinase C (PKC) activity and different phorbol ester binding characteristics account for these responses. For this purpose, the subcellular distributions of PKC were compared in primary cultures of myogenic cells from fetal and adult rat muscles and in the L6 cell line. Cells were used at the proliferative stage or after differentiation into myotubes. Binding of phorbol dibutyrate (PDBu) was assayed. In all three cell types, the levels of PKC specific activity were comparable at the proliferating and the differentiated stages, and partial translocation of PKC activity from the membrane to the cytosolic compartment was observed after differentiation into myotubes. PDBu binding, which had a Kd of 6 to 13 nM in proliferative cells, rose to between 30 and 52 nM in myotubes. Simultaneously, a small increase was observed in the total number of PDBu binding sites. These results suggest that the role of PKC might change with the stage of differentiation. They also imply that the difference described by others between the sensitivity to phorbol esters of fetal myoblasts and satellite cells is not connected with the phorbol ester receptor (i.e., PKC), but might be caused by events subsequent to PKC activation.

Exchange and shuttling of electrons by nitroxide spin labels

The ability of nitroxide spin labels to act as oxidizers of reduced nitroxides (hydroxylamines) in biological and model systems was demonstrated. All of the nitroxides tested were able to act as oxidizing agents with respect to hydroxylamine derivatives of nitroxides. The rates of these reactions were first order with respect to nitroxide concentration and with respect to hydroxylamine concentration, making the reaction second order overall. The second-order rate constants are reported for a number of these reactions. These reactions proceeded to an equilibrium state and the equilibrium constants for several combinations of reactants are presented. Both the rate constants and the equilibrium constants were found to be dependent on the ring structure of the nitroxide and hydroxylamine, with piperidines being reduced more easily and pyrrolidines and oxazolidines being oxidized more easily. All of the hydroxylamine derivatives were oxidized by air to their respective nitroxides, with the rate of this oxidation greater for pyrrolidines than for piperidines. Furthermore, hydroxylamines that are permeable to lipid bilayers were able to act as shuttles of reducing equivalents to liposome-encapsulated nitroxides that were otherwise inaccessible to reducing agents. This mechanism of shuttling of electrons was able to explain the relatively rapid reduction by cells of a nonpermeable nitroxide in the presence of a permeable nitroxide.

Cesium ions delay membrane fusion of chick embryo myoblasts in vitro: a conductivity study

Cesium has a wide range of effects on biological systems. However, the effects of this ion on muscle differentiation are not known. We have recently demonstrated that there is a sharp decrease in the conductivity and permittivity of the membranes of chick embryo myoblasts at the time of fusion (Bonincontro, A., Cametti, C., Hausman, R.E., Indovina, P.L. and Santini, M.T. (1987) Biochim. Biophys. Acta 903, 89-95). Analysis of the conductivity dispersion data in the radiowave frequency range using a 'single-shell' model showed that individual myoblasts and unfused myoballs have significantly higher membrane conductivity and membrane permittivity than fused myoballs. We show here that the sharp fall in these membrane electrical parameters occurs at 60 h of culture and is indeed very abrupt, taking place within one hour. In addition, we also demonstrate that cesium ions delay the sharp decrease in both the conductivity and permittivity of myoblast membranes by about 30 h. We discuss the possible mechanisms by which cesium perturbs potassium transport across these membranes and how this perturbation may affect fusion itself.

Prostaglandin dependence of membrane order changes during myogenesis in vitro

Myogenic differentiation in vitro involves at least three events at the cell surface: binding of prostaglandin to cells, cell-cell adhesion, and fusion of the myoblast membranes into syncytia. Previous work has suggested that binding of prostaglandin is causal to the change in cell-cell adhesion and that both are accompanied by a characteristic reorganization of the myoblast membrane detected as a transient increase in membrane order by electron paramagnetic resonance. We show here that this membrane order change, which reaches a maximum at 38 h of development in vitro, was the last membrane order change before bilayer fusion which begins several hours later. This membrane order change, which accompanies the change in cell-cell adhesion, was dependent on the availability of prostaglandin. In myoblasts maintained in indomethacin, where further differentiation is known to be blocked at the prostaglandin binding step, the membrane order change did not occur. However, if myoblasts are provided with exogenous prostaglandin, the membrane order change occurred and differentiation proceeded. The results indicate that the basis of the membrane order change was the reorganization of myoblast membranes to allow increased adhesion and prepare the membrane for bilayer fusion. They also demonstrate that, like the increase in myoblast adhesion, the membrane order change was dependent on prostaglandin being available to bind to its receptor.

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.