Na,K-ATPase expression in C2C12 cells during myogenesis: minimal contribution of alpha 2 isoform to Na,K transport

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

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

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Thyroid hormone up-regulates Na+/K+ pump alpha2 mRNA but not alpha2 protein isoform in cultured skeletal muscle

Thyroid hormone (T(3)) is known to up-regulate the physiological expression of the Na(+)/K(+) pump in cultured skeletal muscle. We recently reported that primary cultured rat skeletal muscle expresses only the alpha(1), beta(1) and beta(2) protein isoforms of Na(+)/K(+) pump. Interestingly, alpha(2) mRNA is detectable while the alpha(2) protein isoform is not. We therefore examined whether T(3) might up-regulate the expression of Na(+)/K(+) pump alpha(2) isoform at the protein and mRNA level. We also examined the regulation by this hormone of the other isoforms of the pump. Primary cultures were treated with T3 for 48 h from day 4 to day 6 of differentiation. Protein and mRNA isoforms of Na(+)/K(+) pump were identified by Western blotting and Northern blotting, respectively. T(3) induced a marked increase in the beta(1) protein and a slight increase in the alpha(1) protein. T(3) did not affect expression of the beta(2) protein. The alpha(2) protein was not detected in either untreated or T(3)-treated cells. In contrast, alpha(2) mRNA was highly up-regulated by T(3) treatment compared to the other isoforms. The lack of expression of the alpha(2) protein isoform following T(3) treatment suggests that posttranscriptional events related to this isoform may be dependent on other growth factors or hormones.

Na(+)/K(+) pump expression in the L8 rat myogenic cell line: effects of heterologous alpha subunit transfection

We have characterized the physiological and biochemical properties of the Na(+)/K(+) pump and its molecular expression in L8 rat muscle cells. Pump properties were measured by [(3)H]ouabain binding and (86)Rb uptake. Scatchard plot analysis of specific ouabain binding indicated the presence of a single family of binding sites with a B(max) of approximately 135 fmol/ mg P and a K(D) of 3.3 x 10(-8). (86)Rb uptake due to specific pump activity was found to be 20% of the total in L8 cells. The results indicated lower affinity of L8 cells for ouabain and lower activity of the pump than that reported for chick or rat skeletal muscle in primary culture. Both the alpha(1) and beta(1) protein and mRNA isoforms were expressed in myoblasts and in myotubes, while the alpha(2), alpha(3), and beta(2) isoforms were not detectable. We attempted to overcome low physiological expression of the Na(+)/K(+) pump by employing a vector expressing an avian high affinity alpha subunit. This allowed identification of the transfected subunit separate from that endogenously expressed in L8 cells. Successful transfection into L8 myoblasts and myotubes was recognized by anti-avian alpha subunit monoclonal antibodies. Fusion index, Na(+)/K(+) pump activity, and the level of the transmembrane resting potential were all significantly greater in transfected L8 (tL8) cells than in non-tL8. The total amount of alpha subunit (avian and rat) in tL8 cells was greater than that (only rat) in non-tL8 cells. This relatively high abundance of the Na(+)/K(+) pump in transfected cells may indicate that avian and rat alpha subunits hybridize to form functional pump complexes.

Na+ -transport modulation induces isoform-specific expression of Na+,K+ -Atpase alpha-subunit isoforms in C2C12 skeletal muscle cell

Changes in demands for Na+ transport alter expression of the Na+,K+ -ATPase subunit isoforms. In skeletal muscle, the effects of these changes on expression the alpha2 isoform, the major isoform expressed in differentiated muscle cell, is not known. Therefore, this study examines regulation of the alpha-subunit isoforms by Na+ in the C2C12 skeletal muscle cell that expresses the alpha1 and alpha2 isoforms. Western blot analysis showed that in differentiating C2C12 muscle cell, but not in undifferentiated myoblast, veratridine, a Na+ channel activator, greatly increased expression of the alpha2 isoform; expression of alpha1 was unaltered. Because the level of alpha-actinin was unaltered, the data suggest that veratridine treatment did not significantly alter the progression of cell differentiation. Furthermore, a reduction in Na+ transport by tetrodotoxin again failed to alter expression of alpha1. Thus, in C2C12 skeletal muscle cell, changes in Na+ transport alters expression of the alpha2, but not the alpha1 isoform. These results differ from those observed previously in muscle cells that express only the alpha1 isoform. Because mammalian skeletal muscle expresses both the alpha1- and alpha2-subunit isoforms, the differential regulation that was observed may be physiologically relevant in these muscle cells in vivo.

Inhibition of myogenesis by ouabain: effect on protein synthesis

Ouabain, a specific inhibitor of the sodium- and potassium-activated adenosine triphosphatase, causes reversible inhibition of the fusion of myoblasts to form myotubes. We further examined this observation to investigate whether control of Na/K-ATPase activity may normally contribute to the regulation of myogenesis. In control cultures, fusion was preceded by a small decrease in intracellular sodium concentration, but intracellular sodium and potassium increased significantly during fusion. Levels of ouabain that produce prolonged inhibition of fusion (400 microM) virtually eliminated sodium and potassium gradients. However, lower ouabain levels (10-100 microM) also produced significant changes in intracellular potassium and/or sodium along with little apparent decrease in the eventual extent of fusion. The effect of ouabain on protein synthesis was also examined. Low levels of ouabain (<50 microM) that did not affect myogenesis also did not affect incorporation of radiolabeled amino acids, while higher concentrations produced a decline in protein synthesis that paralleled decreases in the rate of myoblast fusion. Levels of metabolic labeling were reduced 90% in cultures treated with 400 microM ouabain. Inhibition of protein synthesis would prevent membrane remodeling required for fusion and other events in myogenesis. Thus, our results do not support any specific role for the sodium- and potassium-activated adenosine triphosphatase in regulating myogenesis.

Rat skeletal muscle in culture expresses the alpha1 but not the alpha2 protein subunit isoform of the Na+/K+ pump

Studies from this laboratory have shown that the physiological expression of the Na+/K+ pump in primary cultures of rat skeletal muscle increases with development. The molecular mechanisms underlying these changes are not known. Therefore, we have examined the expression of alpha and beta subunits of the Na+/K+ pump at both the protein and mRNA levels during myogenesis of primary skeletal muscle cell cultures obtained from newborn rats. Protein isoforms were identified by Western blotting techniques with specific monoclonal and polyclonal antibodies and subunit mRNA was studied with specific cDNA probes. Freshly isolated skeletal muscle from newborn rats expressed both alpha1 and alpha2 protein subunits. From day 1 after plating, primary cultures expressed only the alpha1 protein isoform. In contrast, both beta1 and beta2 isoforms were expressed in freshly isolated muscle and in primary cultures, with beta1 expression being stronger in both preparations. Studies on RNA expression showed that mRNA for alpha1, alpha2, beta1, and beta2 isoforms was identified both in freshly isolated muscle and after plating of cells in culture. These findings indicate that the lack of alpha2 protein expression in primary muscle cell cultures reflects a form of posttranscriptional regulation. There did not appear to be a quantitative difference in isoform expression as a function of age or of fusion in spite of developmental increases in Na+/K+ pump activity and its dependence on cell fusion. The lack of expression of the alpha2 subunit isoform suggests that the developmental changes in physiological expression of the Na+/K+ pump in primary cultures of skeletal muscle may be attributable either to the changes in activity of the alpha1 subunit or to differential activities of alphabeta complexes involving either of the beta subunits.

Alpha 2 mRNA of Na+K+ ATPase is increased in astroctyes of rat hippocampus after treatment with kainic acid

The Na+K+ ATPase (Na+ pump) plays a central role in regulating cation homeostasis and is thought to have an important role in cell proliferation. The multitude of subunit isoforms comprising the functional Na+K+ ATPase has raised the possibility that specific subunit isoform combinations may be involved in different cellular processes. We have investigated the involvement of the specific isoforms in neurons and glia at the site of a CNS lesion. Intracerebroventricular injection of kainic acid was used to induce neuronal cell loss and reactive gliosis in rat hippocampus and levels of Na+K+ ATPase subunit isoform mRNA levels were determined in cells of rat hippocampus using in situ hybridization. alpha 2 mRNA levels increased 35-40% in CA1 and CA3 astrocytes between 1-3 weeks after KA injection with no significant change in other subunit isoform mRNA levels. In addition alpha 3 mRNA levels in CA1 pyramidal neurons were decreased by approx. 35%. Small neurons in the CA1 and CA3 region showed no changes in mRNA levels for any of the Na+K+ ATPase subunit isoforms. These results may indicate a possible role for alpha 2 subunit isoform in the conversion of glial cells from a normal phenotype to the reactive phenotype characteristic in this model of CNS injury.

Cis-elements involved in differential expression of Na(+)-K(+)-ATPase alpha 2 subunit gene in muscle differentiation

The mRNA of the Na(+)-K(+)-ATPase alpha 2 subunit gene (Atpla2) increases during myogenesis in rat following birth. To investigate the mechanism of the transcriptional regulation underlying this process, we examined the promoter activity of Atpla2 by transient transfection assays in myocyte differentiation, using C2C12 murine skeletal myoblast cell line. The promoter activity was increased about 5-fold in the differentiated cells, comparing with that in the growing cells. By analyses of 5'-deletion mutations of the gene, the responsible elements were localized in the region between -108 and +60. We identified a binding factor that exists only in the nuclear extract from the growing cells but not in the extract from the differentiated cells with a probe DNA fragment from +20 to +50 by gel retardation assays. The element to which the factor binds can mediate the inhibition of alpha 2 gene expression in the growing C2C12 cells but not in the differentiated cells. Finally we revealed protein occupancy in this region in the growing cells by in vivo footprinting. We propose a model that the increased expression of Atpla2 during muscle differentiation is mediated by the disappearance of a negative factor.

Influence of amphotericin B on the sodium pump of porcine lens epithelium

Active transport by Na(+)-K(+)-ATPase in the monolayer of lens epithelium is vital for the regulation of sodium and potassium levels within the mass of fiber cells that make up the bulk of the lens. In this study, experiments were conducted using porcine lenses to test whether Na(+)-K(+)-ATPase activity in the epithelium is altered when the permeability of lens cell plasma membranes is increased by the ionophore amphotericin B. After 24 h, sodium was significantly (P < 0.01) elevated in lenses exposed to 5 or 10 microM amphotericin B. Amphotericin B stimulated 86Rb uptake, probably through an increase of cytoplasmic sodium concentration due to increased inward sodium leak; the rate of ouabain-sensitive potassium (86Rb) uptake by intact lenses was significantly increased by amphotericin B at 5 microM (P < 0.05) and 10 microM (P < 0.01). After 24 h, the epithelium from lenses exposed to amphotericin B had an Na(+)-K(+)-ATPase activity that was more than twofold higher (P < 0.01) than the Na(+)-K(+)-ATPase activity in control lenses. By immunoblot, there was an increase in Na(+)-K(+)-ATPase catalytic (alpha) subunit immunoreactive polypeptide in the epithelium of lenses exposed to amphotericin B. The increase stemmed from a marked increase of Na(+)-K(+)-ATPase alpha 2-immunoreactive polypeptide but little change in the amount of alpha 1-immunoreactive protein. As judged by immunoblot experiments, the amount of Na(+)-K(+)-ATPase beta 1-immunoreactive polypeptide also appeared to be higher in the epithelium of amphotericin B-treated lenses compared with control lenses. In summary, these results suggest that in response to a permeability challenge with amphotericin B, the porcine lens epithelium is able to increase the activity of Na(+)-K(+)-ATPase. The same permeability challenge also appears to stimulate the biosynthesis of Na(+)-K(+)-ATPase catalytic subunit as well as glycoprotein subunit polypeptides.

Expression of phospholamban in C2C12 cells and regulation of endogenous SERCA1 activity

Phospholamban (PLB) is a regulator of the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2) expressed in cardiac, slow-twitch skeletal, and smooth muscles. Phospholamban is not expressed in the sarcoplasmic reticulum of fast-twitch skeletal muscle, but it can regulate the sarcoplasmic reticulum Ca(2+)-ATPase activity (SERCA1) expressed in this muscle, in vitro. To determine whether phospholamban can regulate SERCA1 activity in its native membrane environment, phospholamban was stably transfected into a cell line (C2C12) derived from murine fast-twitch skeletal muscle. Differentiation of C2C12 myoblasts to myotubes was associated with induction of SERCA1 expression, assessed by Western blotting analysis using Ca(2+)-ATPase isoform specific antibodies. The expressed phospholamban protein was localized in the microsomal fraction isolated from C2C12 myotubes. To determine the effect of phospholamban expression on SERCA1 activity, microsomes were isolated from transfected and nontransfected C2C12 cell myotubes, and the initial rates of 45Ca(2+)-uptake were determined over a wide range of Ca2+ concentrations (0.1-10 microM). Expression of phospholamban was associated with inhibition of the initial rates of Ca(2+)-uptake at low [Ca2+] and this resulted in a decrease in the affinity of SERCA1 for Ca2+ (0.27 +/- 0.02 microM in nontransfected vs. 0.41 +/- 0.03 microM in PLB transfected C2C12 cells). These findings indicate that phospholamban expression in C2C12 cells is associated with inhibition of the endogenous SERCA1 activity and provide evidence that phospholamban is capable of regulating this Ca(2+)-ATPase isoform in its native membrane environment.

Transfection of Na,K-ATPase alpha-subunit: regulation of enzyme abundance

DNA-mediated gene transfer into mammalian cells was used as a model for investigating the regulation of Na,K-ATPase abundance. Complementary DNA encoding the catalytic alpha 1-submit from rat was introduced into ouabain-sensitive monkey kidney cells, and transfectants were selected by their ability to survive in normally cytotoxic concentrations of ouabain. The overall specific activity of Na,K-ATPase in the membranes of transfectants was not significantly different from that in control cells, suggesting that there was a partial replacement, rather than an addition, of introduced alpha 1 for the endogenous subunit in the functional enzyme. Immunoblotting with specific antibodies confirmed the similarities in overall alpha abundance between control and transfected cells. Hybridization analysis of total RNA, however, revealed a higher abundance of the mRNA encoding total alpha 1 in transfected cells. The results suggest that endogenous and introduced alpha-submit compete for a limited amount of beta, with rapid degradation of unassembled subunits.

Stimulation of Na(+)-K(+)-ATPase by thyrotropin in cultured thyroid follicular cells

Thyroid-stimulating hormone (TSH; thyrotropin) produces a pleiotropic response in the thyroid gland, accelerating nearly every aspect of metabolic turnover within the follicular epithelia. We examined the effects of TSH on expression of Na(+)-K(+)-ATPase in FRTL-5 cells, a cell line derived from rat thyroid. TSH (10 mU/ml) produced a nearly twofold increase in abundance of the mRNA encoding the catalytic alpha 1-subunit within 6 h of treatment. With the four mRNAs encoding the beta 1-subunit, TSH produced a striking increase in abundance, but this regulation was discoordinate, and some species increased more than others. Similar increases in mRNA abundance were elicited by activators of the adenosine 3',5'-cyclic monophosphate second messenger system. In contrast to the alpha 1- and beta 1-mRNAs, the abundance of the mRNA encoding the beta 2-subunit was unchanged with TSH after 6 h, indicating that the effects of thyrotropin were not universal or indiscriminate. Thyrotropin also caused a 76% increase in Na(+)-K(+)-ATPase activity and a 46% increase in pump-mediated transport after 48 h. These studies suggest that the changes in metabolic turnover initiated by TSH during hormone synthesis include upregulation of the N(+)-K+ pump.

Increased Na(+)-K+ pump number and decreased pump activity in soleus muscles in SHR

We have previously demonstrated electrophysiological and contractile abnormalities in soleus muscles of the spontaneously hypertensive rat (SHR). The age-related decrease in force and fatigue resistance observed in SHR muscles may be produced by alterations in sarcolemmal ion conductance and/or Na+ pump function. The experiments reported in the present paper were designed to assess the functional capacity of the Na+ pump in 6- to 8- and 24- to 28-wk-old SHR and Wistar-Kyoto (WKY) soleus muscles and to correlate pump activity with Na+ pump number and binding affinity ([3H]ouabain binding). Functional capacity was determined by measuring the change in resting membrane potential (RMP) of soleus muscle fibers in response to agents that stimulate (epinephrine and insulin) or inhibit (ouabain) the pump and by measuring maximum ouabain-suppressible 86Rb+ uptake in Na(+)-loaded muscles. Na+ pump number and affinity were quantified by determining the specific binding of [3H]ouabain in soleus muscle slices. SHR soleus muscles contain a greater number of Na+ pump sites (ouabain binding sites) than are present in age-matched WKY muscles but also experience a significant decrease in pump activity with age. SHR may upregulate pump number in response to the significantly higher intracellular Na+ concentration found in soleus muscles at all the ages examined. The apparent reduction in pump capacity with age may play a major role in the observed age-related decrease in SHR soleus force and fatigue resistance.