Phosphorylation of multiple sites in a 15,000 dalton proteolipid from rat skeletal muscle sarcolemma, catalyzed by adenosine 3',5'-monophosphate-dependent and calcium/phospholipid-dependent protein kinases

A century of exercise physiology: effects of muscle contraction and exercise on skeletal muscle Na+,K+-ATPase, Na+ and K+ ions, and on plasma K+ concentration-historical developments

This historical review traces key discoveries regarding K+ and Na+ ions in skeletal muscle at rest and with exercise, including contents and concentrations, Na+,K+-ATPase (NKA) and exercise effects on plasma [K+] in humans. Following initial measures in 1896 of muscle contents in various species, including humans, electrical stimulation of animal muscle showed K+ loss and gains in Na+, Cl- and H20, then subsequently bidirectional muscle K+ and Na+ fluxes. After NKA discovery in 1957, methods were developed to quantify muscle NKA activity via rates of ATP hydrolysis, Na+/K+ radioisotope fluxes, [3H]-ouabain binding and phosphatase activity. Since then, it became clear that NKA plays a central role in Na+/K+ homeostasis and that NKA content and activity are regulated by muscle contractions and numerous hormones. During intense exercise in humans, muscle intracellular [K+] falls by 21 mM (range - 13 to - 39 mM), interstitial [K+] increases to 12-13 mM, and plasma [K+] rises to 6-8 mM, whilst post-exercise plasma [K+] falls rapidly, reflecting increased muscle NKA activity. Contractions were shown to increase NKA activity in proportion to activation frequency in animal intact muscle preparations. In human muscle, [3H]-ouabain-binding content fully quantifies NKA content, whilst the method mainly detects α2 isoforms in rats. Acute or chronic exercise affects human muscle K+, NKA content, activity, isoforms and phospholemman (FXYD1). Numerous hormones, pharmacological and dietary interventions, altered acid-base or redox states, exercise training and physical inactivity modulate plasma [K+] during exercise. Finally, historical research approaches largely excluded female participants and typically used very small sample sizes.

Epigenetic remodelling of Fxyd1 promoters in developing heart and brain tissues

FXYD1 is a key protein controlling ion channel transport. FXYD1 exerts its function by regulating Na⁺/K⁺-ATPase activity, mainly in brain and cardiac tissues. Alterations of the expression level of the FXYD1 protein cause diastolic dysfunction and arrhythmias in heart and decreased neuronal dendritic tree and spine formation in brain. Moreover, FXYD1, a target of MeCP2, plays a crucial role in the pathogenesis of the Rett syndrome, a neurodevelopmental disorder. Thus, the amount of FXYD1 must be strictly controlled in a tissue specific manner and, likely, during development. Epigenetic modifications, particularly DNA methylation, represent the major candidate mechanism that may regulate Fxyd1 expression. In the present study, we performed a comprehensive DNA methylation analysis and mRNA expression level measurement of the two Fxyd1 transcripts, Fxyd1a and Fxyd1b, in brain and heart tissues during mouse development. We found that DNA methylation at Fxyd1a increased during brain development and decreased during heart development along with coherent changes in mRNA expression levels. We also applied ultra-deep methylation analysis to detect cell to cell methylation differences and to identify possible distinct methylation profile (epialleles) distribution between heart and brain and in different developmental stages. Our data indicate that the expression of Fxyd1 transcript isoforms inversely correlates with DNA methylation in developing brain and cardiac tissues suggesting the existence of a temporal-specific epigenetic program. Moreover, we identified a clear remodeling of epiallele profiles which were distinctive for single developmental stage both in brain and heart tissues.

Correcting deregulated Fxyd1 expression rescues deficits in neuronal arborization and potassium homeostasis in MeCP2 deficient male mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the MECP2 gene. In the absence of MeCP2, expression of FXYD domain-containing transport regulator 1 (FXYD1) is deregulated in the frontal cortex (FC) of mice and humans. Because Fxyd1 is a membrane protein that controls cell excitability by modulating Na⁺, K⁺-ATPase activity (NKA), an excess of Fxyd1 may reduce NKA activity and contribute to the neuronal phenotype of Mecp2 deficient (KO) mice. To determine if Fxyd1 can rescue these RTT deficits, we studied the male progeny of Fxyd1 null males bred to heterozygous Mecp2 female mice. Maximal NKA enzymatic activity was not altered by the loss of MeCP2, but it increased in mice lacking one Fxyd1 allele, suggesting that NKA activity is under Fxyd1 inhibitory control. Deletion of one Fxyd1 allele also prevented the increased extracellular potassium (K⁺) accumulation observed in cerebro-cortical neurons from Mecp2 KO animals in response to the NKA inhibitor ouabain, and rescued the loss of dendritic arborization observed in FC neurons of Mecp2 KO mice. These effects were gene-dose dependent, because the absence of Fxyd1 failed to rescue the MeCP2-dependent deficits, and mimicked the effect of MeCP2 deficiency in wild-type animals. These results indicate that excess of Fxyd1 in the absence of MeCP2 results in deregulation of endogenous K⁺ conductances functionally associated with NKA and leads to stunted neuronal growth.

Molecular Mechanisms and Kinetic Effects of FXYD1 and Phosphomimetic Mutants on Purified Human Na,K-ATPase

Phospholemman (FXYD1) is a single-transmembrane protein regulator of Na,K-ATPase, expressed strongly in heart, skeletal muscle, and brain and phosphorylated by protein kinases A and C at Ser-68 and Ser-63, respectively. Binding of FXYD1 reduces Na,K-ATPase activity, and phosphorylation at Ser-68 or Ser-63 relieves the inhibition. Despite the accumulated information on physiological effects, whole cell studies provide only limited information on molecular mechanisms. As a complementary approach, we utilized purified human Na,K-ATPase (α1β1 and α2β1) reconstituted with FXYD1 or mutants S63E, S68E, and S63E,S68E that mimic phosphorylation at Ser-63 and Ser-68. Compared with control α1β1, FXYD1 reduces Vmax and turnover rate and raises K0.5Na. The phosphomimetic mutants reverse these effects and reduce K0.5Na below control K0.5Na. Effects on α2β1 are similar but smaller. Experiments in proteoliposomes reconstituted with α1β1 show analogous effects of FXYD1 on K0.5Na, which are abolished by phosphomimetic mutants and also by increasing mole fractions of DOPS in the proteoliposomes. Stopped-flow experiments using the dye RH421 show that FXYD1 slows the conformational transition E2(2K)ATP → E1(3Na)ATP but does not affect 3NaE1P → E2P3Na. This regulatory effect is explained simply by molecular modeling, which indicates that a cytoplasmic helix (residues 60-70) docks between the αN and αP domains in the E2 conformation, but docking is weaker in E1 (also for phosphomimetic mutants). Taken together with previous work showing that FXYD1 also raises binding affinity for the Na(+)-selective site III, these results provide a rather comprehensive picture of the regulatory mechanism of FXYD1 that complements the physiological studies.

CaMKII regulation of cardiac ryanodine receptors and inositol triphosphate receptors

Ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3Rs) are structurally related intracellular calcium release channels that participate in multiple primary or secondary amplified Ca(2+) signals, triggering muscle contraction and oscillatory Ca(2+) waves, or activating transcription factors. In the heart, RyRs play an indisputable role in the process of excitation-contraction coupling as the main pathway for Ca(2+) release from sarcoplasmic reticulum (SR), and a less prominent role in the process of excitation-transcription coupling. Conversely, InsP3Rs are believed to contribute in subtle ways, only, to contraction of the heart, and in more important ways to regulation of transcription factors. Because uncontrolled activity of either RyRs or InsP3Rs may elicit life-threatening arrhythmogenic and/or remodeling Ca(2+) signals, regulation of their activity is of paramount importance for normal cardiac function. Due to their structural similarity, many regulatory factors, accessory proteins, and post-translational processes are equivalent for RyRs and InsP3Rs. Here we discuss regulation of RyRs and InsP3Rs by CaMKII phosphorylation, but touch on other kinases whenever appropriate. CaMKII is emerging as a powerful modulator of RyR and InsP3R activity but interestingly, some of the complexities and controversies surrounding phosphorylation of RyRs also apply to InsP3Rs, and a clear-cut effect of CaMKII on either channel eludes investigators for now. Nevertheless, some effects of CaMKII on global cellular activity, such as SR Ca(2+) leak or force-frequency potentiation, appear clear now, and this constrains the limits of the controversies and permits a more tractable approach to elucidate the effects of phosphorylation at the single channel level.

Brain region-specific expression of Fxyd1, an Mecp2 target gene, is regulated by epigenetic mechanisms

Fxyd1 encodes a trans-membrane protein that modulates Na(+) ,K(+) -ATPase activity and is a substrate for multiple protein kinases. Fxyd1 expression is repressed by methyl CpG-binding protein 2 (Mecp2) in the frontal cortex (FC) but not in the cerebellum (CB) of the mouse brain. Consistently with these observations, FXYD1 mRNA abundance is increased in the FC of Rett syndrome (RTT) patients with MECP2 mutations. Because Fxyd1 is implicated in the regulation of neuronal excitability, understanding how Fxyd1 expression is controlled is important. Here we report that basal expression of Fxyd1a and Fxyd1b, the two main alternatively spliced forms of Fxyd1 mRNA, is lower in the FC than in the CB. This difference is accompanied by increased Mecp2 recruitment to the promoter region of these two Fxyd1 mRNA forms. DNA methylation of both promoters is more frequent in the FC than in the CB, and in both cases the most frequently methylated CpG dinucleotides are adjacent to [A/T](4) sequences required for high-affinity Mecp2 binding. Consistently with these features of epigenetic silencing, histone 3 acetylated at lysines 9 and 14 (H3K9/14ac) and histone 3 methylated at lysine 4 (H3K4me3), both activating histone marks, were associated with the Fxyd1 promoter to a lesser degree in the FC than in the CB. These results indicate that differential Fxyd1 expression in these two brain regions is, at least in part, regulated by an epigenetic mechanism involving increased DNA methylation of the two alternative Fxyd1 promoters, enhanced Mecp2 recruitment, and reduced association of activating histones.

Functional interactions of phospholemman (PLM) (FXYD1) with Na+,K+-ATPase. Purification of alpha1/beta1/PLM complexes expressed in Pichia pastoris

Human FXYD1 (phospholemman, PLM) has been expressed in Pichia pastoris with porcine alpha1/His10-beta1 subunits of Na+,K+-ATPase or alone. Dodecyl-beta-maltoside-soluble complexes of alpha1/beta1/PLM have been purified by metal chelate chromatography, either from membranes co-expressing alpha1,His10-beta1, and PLM or by in vitro reconstitution of PLM with alpha1/His10-beta1 subunits. Comparison of functional properties of purified alpha1/His10-beta1 and alpha1/His10-beta1/PLM complexes show that PLM lowered K0.5 for Na+ ions moderately (approximately 30%) but did not affect the turnover rate or Km of ATP for activating Na+,K+-ATPase activity. PLM also stabilized the alpha1/His10-beta1 complex. In addition, PLM markedly (>3-fold) reduced the K0.5 of Na+ ions for activating Na+-ATPase activity. In membranes co-expressing alpha1/His10-beta1 with PLM the K0.5 of Na+ ions was also reduced, compared with the control, excluding the possibility that detergent or lipid in purified complexes compromise functional interactions. When expressed in HeLa cells with rat alpha1, rat PLM significantly raised the K0.5 of Na+ ions, whereas for a chimeric molecule consisting of transmembranes segments of PLM and extramembrane segments of FXYD4, the K0.5 of Na+ ions was significantly reduced, compared with the control. The opposite functional effects in P. pastoris and HeLa cells are correlated with endogenous phosphorylation of PLM at Ser68 or unphosphorylated PLM, respectively, as detected with antibodies, which recognize PLM phosphorylated at Ser68 (protein kinase A site) or unphosphorylated PLM. We hypothesize that PLM interacts with alpha1/His10-beta1 subunits at multiple locations, the different functional effects depending on the degree of phosphorylation at Ser68. We discuss the role of PLM in regulation of Na+,K+-ATPase in cardiac or skeletal muscle cells.

Cytoplasmic targeting signals mediate delivery of phospholemman to the plasma membrane

The FXYD protein family consists of several small, single-span membrane proteins that exhibit a high degree of homology. The best-known members of the family include the gamma-subunit of the Na(+)-K(+)-ATPase and phospholemman (PLM), a phosphoprotein of cardiac sarcolemma. Other members of the family include corticosteroid hormone-induced factor (CHIF), mammary tumor protein of 8 kDa (Mat-8), and related to ion channels (RIC). The exact physiological roles of the FXYD proteins remain unknown. To better characterize the function of the members of the FXYD protein family, we expressed several members of the family in Madin-Darby canine kidney (MDCK) cells. All of the FXYD proteins, with the exception of PLM, were primarily found in the basolateral plasma membrane. Surprisingly, PLM, a previously characterized plasma membrane protein, was found to colocalize with the endoplasmic reticulum marker protein disulfide isomerase. Treatment of MDCK cells expressing PLM with an agonist of PKC caused some of the PLM to be redistributed to the plasma membrane. Site-directed mutagenesis of residues within the cytoplasmic domain of PLM indicated that a negative charge at Ser69 is necessary to shift the localization of PLM to the plasma membrane. In addition, other regions of PLM necessary for either its endoplasmic reticulum or plasma membrane localization have been elucidated. In contrast to PLM, the plasma membrane localization of CHIF and RIC was not altered by mutation of potential cytoplasmic phosphorylation sites. Overall, these results suggest that phosphorylation of specific residues of PLM may direct PLM from an intracellular compartment to the plasma membrane.

Expression of phospholemman and its association with Na+-K+-ATPase in skeletal muscle: effects of aging and exercise training

Phospholemman (PLM) is a recently identified accessory protein of the Na+-K+-ATPase (NKA), with a high level of expression in skeletal muscle. The objectives of this study are to characterize the PLM in skeletal muscle and to test the hypothesis that, as an accessory protein of NKA, expression of PLM and its association with the α-subunits of NKA is regulated during aging and with exercise training. PLM was characterized in skeletal muscle of 6- and 16-mo-old sedentary middle-aged rats (Ms), and the effects of aging and exercise training were studied in Ms, 29-mo-old sedentary senescent, and 29-mo-old treadmill-exercised senescent rats. Expression of PLM was muscle-type dependent, and immunofluorescence study showed that PLM distributed predominantly on the sarcolemmal membrane of the muscle fibers. Anti-PLM antibody reduced activity of NKA, and thus PLM appears to be required for NKA to express its full activity in skeletal muscle. Expression of PLM was not altered with aging but increased after exercise training. Coimmunoprecipitation studies demonstrated that PLM associates with both the α1- and α2-subunit isoforms of NKA. Compared with Ms rats, levels of PLM-associated α1-subunit increased in 29-mo-old sedentary senescent rats, and treadmill exercise has a tendency to partially reverse it. There was no significant change in PLM-associated α2-subunit with age, and exercise training has a tendency to increase that level. It is concluded that, in skeletal muscle, PLM appears to be a protein integral to the NKA complex and that PLM has the potential to modulate NKA in an isoform-specific and muscle type-dependent manner in aging and after exercise training.

Identification of a cytoskeleton-bound form of phospholemman with unique C-terminal immunoreactivity

Phospholemman (PLM) is a 72-amino acid transmembrane protein thought to function in Na,K-ATPase regulation or assembly, similar to other members of the FXYD family of proteins. Unique to PLM among these regulatory proteins are sites for C-terminal phosphorylation by PKA and PKC, although a role for phosphorylation in PLM function remains unclear. To study PLM phosphorylation, we used PLM phosphopeptides to generate antibodies to specifically detect phosphorylated PLM. Peptide affinity chromatography isolated two populations of antibodies: one reacting with standard PLM, a collection of closely-spaced 15-kDa protein bands by SDS-PAGE. About 20% of PLM antibodies reacted specifically with a single distinct form of PLM. Levels of this second immunological form (PLM-b) were increased with overexpression of PLM cDNA, and also reacted with a monoclonal antibody against the PLM N-terminus. In complete contrast to standard PLM, however, PLM-b was quantitatively insoluble in nonionic detergents and was released from tight binding by colchicine. Antibodies to PLM-b were present in two different antisera raised to the phosphorylated C-terminal peptide (residues 57-70), but not in antiserum raised to the non-phosphorylated C-terminal peptide. Despite an apparent relationship between PLM-b and phosphorylated PLM, PLM-b levels were not affected by treatment of heart cells with isoproterenol. PLM-b appears to represent a cytoskeleton-attached detergent-insoluble form of PLM with distinctive C-terminal immunoreactivity that might have implications for PLM structure and function.

Hypertrophy, increased ejection fraction, and reduced Na-K-ATPase activity in phospholemman-deficient mice

Phospholemman (FXYD1), a 72-amino acid transmembrane protein abundantly expressed in the heart and skeletal muscle, is a major substrate for phosphorylation in the cardiomyocyte sarcolemma. Biochemical, cellular, and electrophysiological studies have suggested a number of possible roles for this protein, including ion channel modulator, taurine-release channel, Na(+)/Ca(2+) exchanger modulator, and Na-K-ATPase-associated subunit. We have generated a phospholemman-deficient mouse. The adult null mice exhibited increased cardiac mass, larger cardiomyocytes, and ejection fractions that were 9% higher by magnetic resonance imaging compared with wild-type animals. Notably, this occurred in the absence of hypertension. Total Na-K-ATPase activity was 50% lower in the phospholemman-deficient hearts. Expression (per unit of membrane protein) of total Na-K-ATPase was only slightly diminished, but expression of the minor alpha(2)-isoform, which has been specifically implicated in the control of contractility, was reduced by 60%. The absence of phospholemman thus results in a complex response, including a surprisingly large reduction in intrinsic Na-K-ATPase activity, changes in Na-K-ATPase isoform expression, increase in ejection fraction, and increase in cardiac mass. We hypothesize that a primary effect of phospholemman is to modulate the Na-K-ATPase and that its reduced activity initiates compensatory responses.

Phospholemman, a single-span membrane protein, is an accessory protein of Na,K-ATPase in cerebellum and choroid plexus

Phospholemman (FXYD1) is a homolog of the Na,K-ATPase gamma subunit (FXYD2), a small accessory protein that modulates ATPase activity. Here we show that phospholemman is highly expressed in selected structures in the CNS. It is most abundant in cerebellum, where it was detected in the molecular layer, in Purkinje neurons, and in axons traversing the granule cell layer. Phospholemman was particularly enriched in choroid plexus, the organ that secretes CSF in the ventricles, where it colocalized with Na,K-ATPase in the apical membrane. It was also enriched, with Na,K-ATPase, in certain tanycytes or ependymal cells of the ventricle wall. Two different experimental approaches demonstrated that phospholemman physically associated with the Na,K-ATPase in cerebellum and choroid plexus: the proteins copurified after detergent treatment and co-immunoprecipitated from solubilized crude membranes using either anti-phospholemman or anti-Na,K-ATPase antibodies. Phospholemman antibodies precipitated all three Na,K-ATPase alpha subunit isoforms (alpha1-alpha3) from cerebellum, indicating that the interaction is not specific to a particular alpha isoform and consistent with the presence of phospholemman in both neurons and glia. Antibodies against the C-terminal domain of phospholemman reduced Na,K-ATPase activity in vitro without effect on Na+ affinity. At least two other FXYD family members have been detected in the CNS, suggesting that additional complexity of sodium pump regulation will be found.

Identification of a phospholemman-like protein from shark rectal glands. Evidence for indirect regulation of Na,K-ATPase by protein kinase c via a novel member of the FXYDY family

The Na,K-ATPase provides the driving force for many ion transport processes through control of Na(+) and K(+) concentration gradients across the plasma membranes of animal cells. It is composed of two subunits, alpha and beta. In many tissues, predominantly in kidney, it is associated with a small ancillary component, the gamma-subunit that plays a modulatory role. A novel 15-kDa protein, sharing considerable homology to the gamma-subunit and to phospholemman (PLM) was identified in purified Na,K-ATPase preparations from rectal glands of the shark Squalus acanthias, but was absent in pig kidney preparations. This PLM-like protein from shark (PLMS) was found to be a substrate for both PKA and PKC. Antibodies to the Na, K-ATPase alpha-subunit coimmunoprecipitated PLMS. Purified PLMS also coimmunoprecipitated with the alpha-subunit of pig kidney Na, K-ATPase, indicating specific association with different alpha-isoforms. Finally, PLMS and the alpha-subunit were expressed in stoichiometric amounts in rectal gland membrane preparations. Incubation of membrane bound Na,K-ATPase with non-solubilizing concentrations of C(12)E(8) resulted in functional dissociation of PLMS from Na,K-ATPase and increased the hydrolytic activity. The same effects were observed after PKC phosphorylation of Na,K-ATPase membrane preparations. Thus, PLMS may function as a modulator of shark Na,K-ATPase in a way resembling the phospholamban regulation of the Ca-ATPase.

Modulation of Xenopus oocyte-expressed phospholemman-induced ion currents by co-expression of protein kinases

Phospholemman (PLM), the major sarcolemmal substrate for phosphorylation by cAMP-dependent kinase (PKA) protein kinase C (PKC) and NIMA kinase in muscle, induces hyperpolarization-activated anion currents in Xenopus oocytes, most probably by enhancing endogenous oocyte currents. PLM peptides from the cytoplasmic tail are phosphorylated by PKA at S68, by NIMA kinase at S63, and by PKC at both S63 and S68. We have confirmed the phosphorylation sites in the intact protein, and we have investigated the role of phosphorylation in the regulatory activity of PLM using oocyte expression experiments. We found: (1) the cytoplasmic domain is not essential for inducing currents in oocytes; (2) co-expression of PKA increased the amplitude of oocyte currents and the amount of PLM in the oocyte membrane largely, but not exclusively, through phosphorylation of S68; (3) co-expression of PKA had no effect on a PLM mutant in which all putative phosphorylation sites had been inactivated by serine to alanine mutation (SSST 62, 63, 68, 69 AAAA); (4) co-expression of PKC had no effect in this system; (5) co-expression of NIMA kinase increased current amplitude and membrane protein level, but did not require PLM phosphorylation. These findings point to a role for phosphorylation in the function of PLM.

Mechanism of hexosamine-induced insulin resistance in transgenic mice overexpressing glutamine:fructose-6-phosphate amidotransferase: decreased glucose transporter GLUT4 translocation and reversal by treatment with thiazolidinedione

Hexosamines have been hypothesized to mediate aspects of glucose sensing and toxic effects of hyperglycemia. For example, insulin resistance results when the rate-limiting enzyme for hexosamine synthesis, glutamine:fructose-6-phosphate amidotransferase (GFA), is overexpressed in muscle and adipose tissue of transgenic mice. The glucose infusion rates required to maintain euglycemia at insulin infusion rates of 0.5, 2, 15, and 20 mU/kg x min were 39-90% lower in such transgenic mice, compared with their control littermates (P < or = 0.01). No differences were observed in hepatic glucose output, serum insulin levels, or muscle ATP levels. Uptake of 2-deoxyglucose, measured under conditions of hyperinsulinemia, was significantly lower in transgenic hindlimb muscle, compared with controls (85.9 +/- 17.8 vs. 166.8 +/- 15.1 pmol deoxyglucose/g x min). The decrease in glucose uptake by transgenic muscle was associated with a disruption in the translocation of the insulin-stimulated glucose transporter GLUT4. Fractionation of muscle membranes on a discontinuous sucrose gradient revealed that insulin stimulation of control muscle led to a 28.8% increase in GLUT4 content in the 25% fraction and a 61.2% decrease in the 35% fraction. In transgenic muscle, the insulin-stimulated shifts in GLUT4 distribution were inhibited by over 70%. Treatment of the transgenic animals with the thiazolidinedione troglitazone completely reversed the defect in glucose disposal without changing GFA activity or the levels of uridine 5'-diphosphate-N-acetylglucosamine. Overexpression of GFA in skeletal muscle thus leads to defects in glucose transport similar to those seen in type 2 diabetes. These data support the hypothesis that excess glucose metabolism through the hexosamine pathway may be responsible for the diminished insulin sensitivity and defective glucose uptake that are seen with hyperglycemia.

Structural domains in phospholemman: a possible role for the carboxyl terminus in channel inactivation

Phospholemman (PLM) is a small (72-amino acid) transmembrane protein found in cardiac sarcolemma that is a major substrate for several protein kinases in vivo. Detailed structural data for PLM is lacking, but several studies have described an ion conductance that results from PLM expression in oocytes. Moreover, addition of purified PLM to lipid bilayers generates similar ion currents, suggesting that the PLM molecule itself might be sufficient for channel formation. To provide a framework for understanding the function of PLM, we investigated PLM topology and structure in sarcolemmal membrane vesicles and analyzed purified recombinant PLM. Immunoblot analyses with site-specific antibodies revealed that the extracellular segment (residues 1 to 17) exists in a protected configuration highly resistant to proteases, even in detergent solutions. The intracellular portion of the molecule (residues 38 to 72), in contrast, was highly susceptible to proteases. Trypsin treatment produced a limit peptide (residues 1 to 43), which showed little change in electrophoretic mobility in SDS gels and retained the ion-channel activity in lipid bilayers that is characteristic of the full-length protein. In addition, we found that conductance through PLM channels exhibited rapid inactivation during depolarizing ramps at voltages greater than +/- 50 mV, Channels formed by trypsinized PLM or recombinant PLM 1-43 exhibited dramatic reductions in voltage-dependent inactivations. Our data point to distinct domains within the PLM molecule that may correlate with functional properties of channel activity observed in oocytes and lipid bilayers.

Characterization of the human and rat phospholemman (PLM) cDNAs and localization of the human PLM gene to chromosome 19q13.1

Previous reports have demonstrated that the phospholemman (PLM), a 72-residue plasma-membrane protein enriched in skeletal muscle and heart, is a major substrate phosphorylated in response to insulin and adrenergic stimulation. Here we describe the isolation and characterization of human and rat PLM cDNA from the heart. Both PLM proteins share significant nucleotide and amino acid sequence and structural similarities with the previously published canine PLM and, to a lesser degree, with Na+/K(+)-ATPase gamma subunit, Mat-8 protein, and CHIF protein. Despite the functional diversity, all these proteins are quite small and possess a single transmembrane domain. Human PLM appears to be a unique gene localized on chromosome 19q13.1. The PLM mRNA is widely distributed in human tissues, with the highest expression in skeletal muscle and heart, suggesting a functional role in muscle contraction. Like canine PLM, both human and rat PLM induce a hyperpolarization-activated chloride current when expressed in Xenopus oocytes. The high degree of sequence and functional conservation among the mammalian PLM proteins indicates that this gene is conserved throughout evolution.

Glucosamine induces insulin resistance in vivo by affecting GLUT 4 translocation in skeletal muscle. Implications for glucose toxicity

Glucosamine (Glmn), a product of glucose metabolism via the hexosamine pathway, causes insulin resistance in isolated adipocytes by impairing insulin-induced GLUT 4 glucose transporter translocation to the plasma membrane. We hypothesized that Glmn causes insulin resistance in vivo by a similar mechanism in skeletal muscle. We performed euglycemic hyperinsulinemic clamps (12 mU/kg/min + 3H-3-glucose) in awake male Sprague-Dawley rats with and without Glmn infusion at rates ranging from 0.1 to 6.5 mg/kg/min. After 4h of euglycemic clamping, hindquarter muscles were quick-frozen and homogenized, and membranes were subfractionated by differential centrifugation and separated on a discontinuous sucrose gradient (25, 30, and 35% sucrose). Membrane proteins were solubilized and immunoblotted for GLUT 4. With Glmn, glucose uptake (GU) was maximally reduced by 33 +/- 1%, P < 0.001. The apparent Glmn dose to reduce maximal GU by 50% was 0.1 mg/kg/min or 1/70th the rate of GU on a molar basis. Control galactosamine and mannosamine infusions had no effect on GU. Relative to baseline, insulin caused a 2.6-fold increase in GLUT 4 in the 25% membrane fraction (f), P < 0.01, and a 40% reduction in the 35%f, P < 0.05, but had no effect on GLUT 4 in the 30% f, P= NS. Addition of Glmn to insulin caused a 41% reduction of GLUT 4 in the 25%f, P < 0.05, a 29% fall in the 30%f, and prevented the reduction of GLUT 4 in the 35% f. The 30%f membranes were subjected to a second separation with a 27 and 30% sucrose gradient. Insulin mobilized GLUT 4 away from the 30%f, P < 0.05, but not the 27% f. In contrast, Glmn reduced GLUT 4 in the 27%f, P < 0.05, but not the 30%f. Thus Glmn appears to alter translocation of an insulin-insensitive GLUT 4 pool. Coinfusion of Glmn did not alter enrichment of the sarcolemmal markers 5'-nucleotidase, Na+/K+ATPase, and phospholemman in either 25, 30, or 35% f. Thus Glmn completely blocked movement of Glut 4 induced by insulin. Glmn is a potent inducer of insulin resistance in vivo by causing (at least in part) a defect intrinsic to GLUT 4 translocation and/or trafficking. These data support a potential role for Glmn to cause glucose-induced insulin resistance (glucose toxicity).

Protein kinase C and cyclic AMP-dependent protein kinase phosphorylate phospholemman, an insulin and adrenaline-regulated membrane phosphoprotein, at specific sites in the carboxy terminal domain

Phospholemman, a transmembrane, 72 residue protein enriched in striated muscle and heart [Palmer, Scott and Jones (1991) J. Biol. Chem. 266, 11126-11130], is phosphorylated in response to insulin [Walaas, Horn and Walaas (1991) Biochim. Biophys. Acta 1094, 92-102]. The present study is aimed at identifying the phosphorylation sites of this protein. A synthetic peptide, GTFRSS63IRRLS68TRRR (in the single letter code) and consisting of phospholemman residues 58-72, is a substrate for both protein kinase C and cyclic AMP (cAMP)-dependent protein kinase, with Km values of 6-7 microM for both enzymes. Amino acid sequencing of the phosphopeptide shows that protein kinase C phosphorylates both Ser-63 and Ser-68, while cAMP-dependent protein kinase phosphorylates Ser-68. Thermolytic phosphopeptide mapping of 32P-labelled phospholemman from rat diaphragms shows that treatment with insulin results in labelling of phosphopeptides containing both Ser-63 and Ser-68, whereas treatment with adrenaline results in labelling of the phosphopeptide containing Ser-68. Hence, insulin and adrenaline regulate the phosphorylation of phospholemman, presumably through protein kinase C and cAMP-dependent protein kinase, respectively, on partly overlapping phosphorylation sites.

Insulin and phorbol ester stimulate phosphorylation of a 15,000 dalton membrane protein in rat diaphragm in a similar manner

The effects of insulin on the phosphorylation of a 15 kilodalton (kDa) membrane protein in rat diaphragm in situ have been investigated. Incubation of the diaphragm with insulin or tumor-promoting phorbol ester increased the 32P-labelling of the 15 kDa protein at serine residues by 50 +/- 8% and 64 +/- 11%, (mean +/- S.E.), respectively. Thermolytic peptide mapping of the 15 kDa protein after insulin treatment of the diaphragm yielded two major phosphopeptides, one of which was absent from digests from control diaphragms. The same two phosphopeptides were identified after incubation of the diaphragm with phorbol ester and after phosphorylation of sarcolemma in vitro with [gamma-32P]ATP and protein kinase C. Additional experiments indicated that pretreatment of diaphragms with insulin or phorbol ester both increased the state of phosphorylation of the 15 kDa sarcolemma protein on phosphorylation sites regulated by protein kinase C. The stimulatory effect of insulin was decreased by staurosporine or by preincubation of the diaphragms with phorbol esters. These results indicate that the insulin-induced increases in protein kinase C activity previously found in rat diaphragm (Walaas et al. (1987) FEBS Lett. 220, 311-318) may be involved in insulin-mediated regulation of phosphorylation of the 15 kDa protein in situ.

Skeletal muscle sarcolemma proteins as targets for adenosine 3':5'-monophosphate-dependent and calcium-dependent protein kinases

The present study documents the existence in rat skeletal muscle plasma membrane (sarcolemma) of a distinct set of proteins, most of which represent unknown protein species, which can be phosphorylated in vitro by addition of cAMP-dependent or calcium-dependent protein kinases. Under the experimental conditions used, cAMP-regulated protein phosphorylation appeared to be the most important phosphorylation system in these membranes, followed by the calcium/phospholipid-regulated, and, with only a few substrates detected, the calcium/calmodulin-regulated systems. No specific substrate for cGMP-dependent protein kinase was found. In contrast, calcium/calmodulin-regulated protein phosphorylation was the most important in the sarcoplasmic reticulum fraction. Most of the cAMP-regulated and calcium/phospholipid-regulated sarcolemma phosphoproteins appeared to be intrinsic membrane proteins, at least three of which appeared to be phosphorylated by both these protein kinases. These phosphoproteins may represent membrane targets for multiple hormone or transmitter actions in skeletal muscle cells. Our results, therefore, suggest that protein phosphorylation systems, particularly those regulated by cAMP or calcium/phospholipid, may be more important in the regulation of sarcolemma function than hitherto believed.