A second messenger role for monoacylglycerols is suggested by their activating effects on the sodium pump

Greasing the wheels or a spanner in the works? Regulation of the cardiac sodium pump by palmitoylation

The ubiquitous sodium/potassium ATPase (Na pump) is the most abundant primary active transporter at the cell surface of multiple cell types, including ventricular myocytes in the heart. The activity of the Na pump establishes transmembrane ion gradients that control numerous events at the cell surface, positioning it as a key regulator of the contractile and metabolic state of the myocardium. Defects in Na pump activity and regulation elevate intracellular Na in cardiac muscle, playing a causal role in the development of cardiac hypertrophy, diastolic dysfunction, arrhythmias and heart failure. Palmitoylation is the reversible conjugation of the fatty acid palmitate to specific protein cysteine residues; all subunits of the cardiac Na pump are palmitoylated. Palmitoylation of the pump's accessory subunit phospholemman (PLM) by the cell surface palmitoyl acyl transferase DHHC5 leads to pump inhibition, possibly by altering the relationship between the pump catalytic α subunit and specifically bound membrane lipids. In this review, we discuss the functional impact of PLM palmitoylation on the cardiac Na pump and the molecular basis of recognition of PLM by its palmitoylating enzyme DHHC5, as well as effects of palmitoylation on Na pump cell surface abundance in the cardiac muscle. We also highlight the numerous unanswered questions regarding the cellular control of this fundamentally important regulatory process.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Interactions of K+ATP channel blockers with Na+/K+-ATPase

Two K(+) (ATP) channel blockers, 5-hydroxydecanoate (5-HD) and glyburide, are often used to study cross-talk between Na(+)/K(+)-ATPase and these channels. The aim of this work was to characterize the effects of these blockers on purified Na(+)/K(+)-ATPase as an aid to appropriate use of these drugs in studies on this cross-talk. In contrast to known dual effects (activating and inhibitory) of other fatty acids on Na(+)/K(+)-ATPase, 5-HD only inhibited the enzyme at concentrations exceeding those that block mitochondrial K(+) (ATP) channels. 5-HD did not affect the ouabain sensitivity of Na(+)/K(+)-ATPase. Glyburide had both activating and inhibitory effects on Na(+)/K(+)-ATPase at concentrations used to block plasma membrane K(+) (ATP) channels. The findings justify the use of 5-HD as specific mitochondrial channel blocker in studies on the relation of this channel to Na(+)/K(+)-ATPase, but question the use of glyburide as a specific blocker of plasma membrane K(+) (ATP) channels, when the relation of this channel to Na(+)/K(+)-ATPase is being studied.

Receptor-independent actions of cannabinoids on cell membranes: Focus on endocannabinoids

Cannabinoids are a structurally diverse group of mostly lipophilic molecules that bind to cannabinoid receptors. In fact, endogenous cannabinoids (endocannabinoids) are a class of signaling lipids consisting of amides and esters of long-chain polyunsaturated fatty acids. They are synthesized from lipid precursors in plasma membranes via Ca(2+) or G-protein-dependent processes and exhibit cannabinoid-like actions by binding to cannabinoid receptors. However, endocannabinoids can produce effects that are not mediated by these receptors. In pharmacologically relevant concentrations, endocannabinoids modulate the functional properties of voltage-gated ion channels including Ca(2+) channels, Na(+) channels, various types of K(+) channels, and ligand-gated ion channels such as serotonin type 3, nicotinic acetylcholine, and glycine receptors. In addition, modulatory effects of endocannabinoids on other ion-transporting membrane proteins such as transient potential receptor-class channels, gap junctions and transporters for neurotransmitters have also been demonstrated. Furthermore, functional properties of G-protein-coupled receptors for different types of neurotransmitters and neuropeptides are altered by direct actions of endocannabinoids. Although the mechanisms of these effects are currently not clear, it is likely that these direct actions of endocannabinoids are due to their lipophilic structures. These findings indicate that additional molecular targets for endocannabinoids exist and that these targets may represent novel sites for cannabinoids to alter either the excitability of the neurons or the response of the neuronal systems. This review focuses on the results of recent studies indicating that beyond their receptor-mediated effects, endocannabinoids alter the functions of ion channels and other integral membrane proteins directly.

The regulation of the Na+,K+ pump in contracting skeletal muscle

Increased passive Na+,K+ fluxes necessitate an efficient activation of the Na+,K+ pump in working muscles to limit the rundown of the Na+,K+ chemical gradients and ensuing loss of excitability. Several studies have demonstrated an increase in Na+,K+-pump rate in working muscles, and in electrically stimulated muscles up to a 22-fold increase in active Na+,K+ transport has been observed. Excitation-induced increase in intracellular Na+ is believed to be the primary stimulus for Na+,K+ pumping in a contracting muscle. In muscles recovering from electrical stimulation, however, the activity of the pump may stay elevated even after intracellular Na+ has been reduced to below the resting level. Moreover, in rat soleus muscles 10-s stimulation at 60 Hz induced a 5-fold increase in the activity of the Na+,K+ pump although mean intracellular [Na+] was unchanged. These findings strongly suggest that a substantial part of the excitation-induced increase in Na+,K+-pump activity is caused by mechanisms other than increased intracellular [Na+]. The mechanism behind this activation is not clear, but may involve a change in the affinity of the Na+,K+ pump for intracellular Na+. In addition to intracellular [Na+], the Na+,K+ pump may be stimulated in contracting muscles by other factors such as catecholamines, calcitonin gene-related peptide (CGRP), free fatty acids and cytoskeletal links. Together, this activation may form a feed forward mechanism protecting muscles from loss of excitability during periods of contraction by increasing Na+,K+-pump activity prior to erosion of the Na+,K+ chemical gradients. During exercise of high intensity, however, intracellular [Na+] increases substantially constituting an additional stimulus for the pump.

Kinetics of Na+, K+-ATPase inhibition by a rat brain endogenous factor (II-E)

Previous work from this laboratory led to the isolation by gel filtration and anionic exchange HPLC of a rat brain fraction named II-E, which highly inhibits synaptosomal membrane Na+, K+-ATPase activity. In this study we evaluated the kinetics of such inhibition and found that inhibitory potency was independent of Na+ (1.56-200 mM), K+ (1.25-40 mM), or ATP (1-8 mM) concentration. Hanes-Woolf plots indicated that II-E decreases Vmax but does not alter KM value, and suggested uncompetitive inhibition for Na+, K+ or ATP. However, II-E became a stimulator at 0.5 mM ATP concentration. It is postulated that this brain factor may modulate ionic transport at synapses, thus participating in central neurotransmission.

Inhibition by 2-arachidonoylglycerol, a novel type of possible neuromodulator, of the depolarization-induced increase in intracellular free calcium in neuroblastoma x glioma hybrid NG108-15 cells

2-Arachidonoylglycerol was found to inhibit the depolarization-induced increase in [Ca2+]i in NG108-15 cells differentiated with prostaglandin E1 and theophylline in a dose-dependent manner. Such an effect appears to be rather specific to polyunsaturated fatty acid-containing monoacylglycerols such as 2-arachidonoylglycerol. Neither 2-palmitoylglycerol nor free arachidonic acid exhibited appreciable inhibitory activity. These observations raise the possibility that 2-arachidonoylglycerol attenuates the increase in [Ca2+]i, thereby modulating several neural functions in this type of cell.

Biosynthesis, release and degradation of the novel endogenous cannabimimetic metabolite 2-arachidonoylglycerol in mouse neuroblastoma cells

The monoacylglycerol 2-arachidonoylglycerol (2-AG) has been recently suggested as a possible endogenous agonist at cannabinoid receptors both in brain and peripheral tissues. Here we report that a widely used model for neuronal cells, mouse N18TG2 neuroblastoma cells, which contain the CB1 cannabinoid receptor, also biosynthesize, release and degrade 2-AG. Stimulation with ionomycin (1-5 microM) of intact cells prelabelled with [3H]arachidonic acid ([3H]AA) led to the formation of high levels of a radioactive component with the same chromatographic behaviour as synthetic standards of 2-AG in TLC and HPLC analyses. The amounts of this metabolite were negligible in unstimulated cells, and greatly decreased in cells stimulated in the presence of the Ca2+-chelating agent EGTA. The purified component was further characterized as 2-AG by: (1) digestion with Rhizopus arrhizus lipase, which yielded radiolabelled AA; (2) gas chromatographic-MS analyses; and (3) TLC analyses on borate-impregnated plates. Approx. 20% of the 2-AG produced by stimulated cells was found to be released into the incubation medium when this contained 0.1% BSA. Subcellular fractions of N18TG2 cells were shown to contain enzymic activity or activities catalysing the hydrolysis of synthetic [3H]2-AG to [3H]AA. Cell homogenates were also found to convert synthetic [3H]sn-1-acyl-2-arachidonoylglycerols (AcAGs) into [3H]2-AG, suggesting that 2-AG might be derived from AcAG hydrolysis. When compared with ionomycin stimulation, treatment of cells with exogenous phospholipase C, but not with phospholipase D or A2, led to a much higher formation of 2-AG and AcAGs. However, treatment of cells with phospholipase A2 10 min before ionomycin stimulation caused a 2.5-3-fold potentiation of 2-AG and AcAG levels with respect to ionomycin alone, whereas preincubation with the phospholipase C inhibitor neomycin sulphate did not inhibit the effect of ionomycin on 2-AG and AcAG levels. These results suggest that the Ca2+-induced formation of 2-AG proceeds through the intermediacy of AcAGs but not necessarily through phospholipase C activation. By showing for the first time the existence of molecular mechanisms for the inactivation and the Ca2+-dependent biosynthesis and release of 2-AG in neuronal cells, the present paper supports the hypothesis that this cannabimimetic monoacylglycerol might be a physiological neuromodulator.

Activation of Na+/K(+)-ATPase by fatty acids, acylglycerols, and related amphiphiles: structure-activity relationship

A number of fatty acids and derivatives have been shown to activate Na+/K(+)-ATPase when ATP is suboptimal. To explore the relation of the structures of these amphiphiles to enzyme activation, the effects of varying amphiphile concentrations on the activity of the highly purified kidney Na+/K(+)-ATPase at 50 microM ATP were determined. Among fatty acids, efficacy (maximal level of activation) and potency were found to be dependent, in different ways, on chain length and unsaturation. Compared to fatty acids, the corresponding alcohols had lower efficacies. Methyl esters of fatty acids inhibited, but CoA esters and monoacyl esters of glycerol activated the enzyme. Relation between chain length and potency among CoA esters and monoacylglycerols was the same as that observed with acids. Diacylglycerols did not activate, but they antagonized the effects of the activator amphiphiles. The substantial specificities of the amphiphile effects support the hypothesis that these ligands bind to a distinct amphipathic peptide segment of the intracellular central loop of the alpha-subunit to regulate ATP binding to the enzyme. The findings also suggest that direct effects of the changing intracellular levels of fatty acids and derivatives on Na+/K(+)-ATPase should be considered as a possible mechanism for the regulation of its function in the intact cell.

A possible pathway of phosphoinositide metabolism through EDTA-insensitive phospholipase A1 followed by lysophosphoinositide-specific phospholipase C in rat brain

Incubation of [2-3H]glycerol-labeled phosphatidylinositol with a crude cytosol fraction of rat brain in the presence of EDTA yielded [3H]lysophosphatidylinositol predominantly without accumulation of labeled monoacylglycerol and diacylglycerol. The pH optimum of this phospholipase A activity was 8.0. The activity for phosphatidylinositol was twofold higher than for phosphatidylethanolamine, whereas phosphatidylcholine, phosphatidylserine, and phosphatidic acid were not hydrolyzed significantly under the conditions used. The phospholipase A activity for phosphatidylethanolamine was resolved in part from that for phosphatidylinositol by ammonium sulfate fractionation of the cytosol, indicating the existence of at least two forms of EDTA-insensitive phospholipase A. The positional specificity of the phosphatidylinositol-hydrolyzing activity was found to be that of a phospholipase A1, as radioactive lysophosphatidylinositol was produced from 1-stearoyl-2-[1-14C]arachidonyl-sn-glycero-3-phosphoinositol without release of free arachidonate. A phospholipase C activity specific for lysophosphoinositides was found in a membrane fraction from rat brain, which was similar to that characterized in porcine platelets. The phospholipase C was demonstrated to hydrolyze the 2-acyl isomer as well as the 1-acyl isomer of lysophosphatidylinositol. Taken together, our results suggest a possible pathway through which phosphatidylinositol is selectively degraded to the 2-acyl isomer of lysophosphatidylinositol in a Ca(2+)-independent manner, and subsequently converted to a 2-monoacylglycerol in rat brain.