Myogenic differentiation of the muscle clonal cell line BC3H-1 is accompanied by changes in its lipid composition

The Mef2c/AdipoR1 axis is responsible for myogenic differentiation and is regulated by resistin in skeletal muscles

Previous studies have shown that accumulated lipid and insulin resistance emerges in skeletal muscle after the onset of obesity and diabetes. We have previously shown that resistin significantly increases lipid contents in C2C12 cells. However, studies evaluating the effects of resistin on skeletal muscle cells and tissues are limited; despite that, an understanding of resistin action and function on lipid alteration in skeletal muscle tissues is critical for understanding obesity-related diseases. In this study, we document that resistin increases lipid deposition both in vitro and in vivo. Further, resistin promotes fiber type transformation, decreases enzyme activities, inhibits myogenic differentiation, and decreases muscle grip and excise endurance. In addition, adiponectin signaling is activated during myocyte differentiation, but it is inhibited at elevated resistin concentrations. Mechanistic investigation revealed that mef2c is responsible for adiponectin signaling pathway inhibition by inhibiting adipoR1 expression at the transcriptional level. In conclusion, this is the first study to document that resistin increases ectopic lipid deposition in skeletal muscles via a mef2c-adipoR1 signaling pathway, which reveals for the first time the presence of crosstalk between resistin and adiponectin in skeletal muscles.

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.

Cholesterol modulates the rate and mechanism of acetylcholine receptor internalization

Stability of the nicotinic acetylcholine receptor (AChR) at the cell surface is key to the correct functioning of the cholinergic synapse. Cholesterol (Chol) is necessary for homeostasis of AChR levels at the plasmalemma and for ion translocation. Here we characterize the endocytic pathway followed by muscle-type AChR in Chol-depleted cells (Chol(-)). Under such conditions, the AChR is internalized by a ligand-, clathrin-, and dynamin-independent mechanism. Expression of a dominant negative form of the small GTPase Rac1, Rac1N17, abolishes receptor endocytosis. Unlike the endocytic pathway in control CHO cells (1), accelerated AChR internalization proceeds even upon disruption of the actin cytoskeleton. Under Chol(-) conditions, AChR internalization is furthermore found to require the activity of Arf6 and its effectors Rac1 and phospholipase D. The Arf6-dependent mechanism may constitute the default endocytic pathway followed by the AChR in the absence of external ligands, membrane Chol levels acting as a key homeostatic regulator of cell surface receptor levels.

Sphingomyelin Levels in the Plasma Membrane Correlate with the Activation State of Muscle Satellite Cells

Satellite cells are responsible for postnatal growth, hypertrophy, and regeneration of skeletal muscle. They are normally quiescent, and must be activated to fulfill these functions, yet little is known of how this is regulated. As a first step in determining the role of lipids in this process, we examined the dynamics of sphingomyelin in the plasma membrane. Sphingomyelin contributes to caveolae/lipid rafts, which act to concentrate signaling molecules, and is also a precursor of several bioactive lipids. Proliferating or differentiated C2C12 muscle cells did not bind lysenin, a sphingomyelin-specific binding protein, but noncycling reserve cells did. Quiescent satellite cells also bound lysenin, revealing high levels of sphingomyelin in their plasma membranes. On activation, however, the levels of sphingomyelin drop, so that lysenin did not label proliferating satellite cells. Although most satellite cell progeny differentiate, others stop cycling, maintain Pax7, downregulate MyoD, and escape immediate differentiation. Importantly, many of these Pax7-positive/MyoD-negative cells also regained lysenin binding on their surface, showing that the levels of sphingomyelin had again increased. Our observations show that quiescent satellite cells are characterized by high levels of sphingomyelin in their plasma membranes and that lysenin provides a novel marker of myogenic quiescence.

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.

Cells defective in sphingolipids biosynthesis express low amounts of muscle nicotinic acetylcholine receptor

The properties of the nicotinic acetylcholine receptor (AChR) are modulated by its lipid microenvironment. Studies of such modulation are hampered by the cell's homeostatic mechanisms that impede sustained modification of membrane lipid composition. We have devised a novel strategy to circumvent this problem and study the effect of changes in plasma membrane lipid composition on the functional properties of AChR. This approach is based on the stable transfection of AChR subunit cDNAs into cells defective in a specific lipid metabolic pathway. In the present work we illustrate this new strategy with the successful transfection of a temperature-sensitive Chinese hamster ovary (CHO) cell line, SPB-1, with the genes corresponding to the four adult mouse AChR subunits. The new clone, SPB-1/SPH, carries a mutation of the gene coding for serine palmitoyl transferase, the enzyme that catalyses the first step in sphingomyelin (Sph) biosynthesis. This defect causes a decrease of Sph de novo synthesis at non-permissive temperatures. The IC50 for inhibition of alpha-BTX binding with the agonist carbamoylcholine exhibited values of 3.6 and 2.7 microm in the wild-type and Sph-deficient cell lines, respectively. The corresponding IC50 values for the competitive antagonist D-tubocurarine (D-TC) were 2.8 and 3.4 microm, respectively. No differences in single-channel properties were observed between wild-type and mutant cell lines grown at the non-permissive, lipid defect-expressing temperature using the patch-clamp technique. Both cells exhibited two open times with mean values of 0.35 +/- 0.05 and 1.78 +/- 0.2 ms at 12 degrees C. Taken together, these results suggest that the AChR is expressed as the complete heteroligomer. However, only 10-20% of the total AChR synthesized reached the surface membrane in the mutant cell line and exhibited a higher metabolic turnover, with a half-life about 50% shorter than the wild-type cells. When control CHO-K1/A5 cells were treated with fumonisin B1, an inhibitor of sphingosine (sphinganine) N-acetyltransferase (ceramide synthase), a 45.5% decrease in cell surface AChR expression was observed. The results suggest that sphingomyelin deficiency conditions AChR targeting to the plasma membrane.

Heterologous retinal cultured neurons and cell adhesion molecules induce clustering of acetylcholine receptors and polynucleation in mouse muscle BC3H-1 clonal cell line

Several features of the clonal cell line BC3H-1 resemble those of embryonic muscle cells at their early stage of development (Patrick et al.: J Biol Chem 252:2143-2153, 1977). Under normal culture conditions, fully differentiated BC3H-1 cells possess a spindle-shaped, muscle-like morphology with a single nucleus. Like embryonic muscle cell counterparts, they neither express the epsilon subunit nor exhibit the clustered organization of the nicotinic acetylcholine receptor (AChR) characteristic of mature myocytes. Instead, AChRs are evenly distributed upon the sarcolemma upon differentiation. Here we report that BC3H-1 cells can be induced to express AChR clusters by co-culturing the cell line with heterologous retinal neurons, which establish contacts with former cells. Clustering was also triggered by pretreating the culture substratum with several cell adhesion molecules. Polynucleation, a phenomenon observed in mature myotubes in vivo, was also observed in BC3H-1 cells under the two sets of experimental conditions. AChR clustering and polynucleation, both characteristic of the mature postsynaptic region, can thus be induced in BC3H-1 cells by at least two quite distinct pathways: neuronal cell contact and/or cell adhesion molecules. It is suggested that heterologous retinal neurons can elicit only an incomplete differentiation of BC3H-1 cells, failing to concentrate the clusters beneath the neuronal endings, and failing to induce expression of the epsilon subunit, characteristic of the mature AChR.

N-3 and n-6 fatty acid metabolism in undifferentiated and differentiated human intestine cell line (Caco-2)

Metabolism of n-6 and n-3 fatty acids in the undifferentiated and differentiated human adenocarcinoma colon cell line (Caco-2) was studied. In cells incubated with either 18:2n-6 or 18:3n-3, no significant amounts of long chain n-6 and n-3 metabolites were found. Incubation with either 18:3n-6 or 18:4n-3 raised significantly the levels of 20:3n-6 and 20:4n-3, respectively. In the undifferentiated cells, significant proportions of 20:3n-6 and 20:4n-3 were further delta 5-desaturated to form 20:4n-6 and 20:5n-3, respectively. Incubation with either 20:4n-6 or 20:5n-3 raised the levels of their direct elongation products, 22:4n-6 and 22:5n-3, respectively. Incubation with 22:4n-6 or 22:5n-3 increased the levels of 20:4n-6 and 20:5n-6. These results suggest that delta 6-desaturation in the Caco-2 cells is less active in comparison with elongation, delta 5-desaturation and retro-conversion. These enzymes were modulated by the state of differentiation, and appeared to be non-specific to n-3 and n-6 fatty acids. When cells were incubated with 18:3n-6 and 18:4n-3 concomitantly, the levels of incorporation of total n-6 fatty acids into cellular lipids were greater than those of the n-3 fatty acids, whereas the ratios of 20+22 carbon metabolites to 18-carbon precursor favored n-3 over n-6 fatty acids. These results suggest that n-3 and n-6 fatty acids were not metabolized identically in Caco-2 cells.