Cholesterol-dependent interaction of polyunsaturated phospholipids with Na,K-ATPase

Differential expression of two ATPases revealed by lipid raft isolation from gills of euryhaline teleosts with different salinity preferences

In euryhaline teleosts, Na⁺, K⁺-ATPase (NKA) and V-type H + -ATPase A (VHA A) are important ion-transporters located in cell membrane. Lipid rafts (LR) are plasma membrane microdomains enriched in cholesterol, sphingolipids, and proteins (e.g., flotillin). Flotillin is a LR-associated protein, commonly used as the LR marker. Previous mammalian studies showed that LR may play a crucial role in ion exchanges. Meanwhile, studies on mammals and rainbow trout showed that NKA were found to be present mainly in LR. However, little is known about LR in fish. Therefore, the present study aimed to investigate the involvement of branchial LR in osmoregulation of tilapia and milkfish, two euryhaline teleosts with different salinity preferences, by (i) extracting LR from the gills of euryhaline teleosts; (ii) detecting the abundance of LR marker protein (flotillin-2) and ion-transporters (NKA and VHA A) in branchial LR and non-LR of fresh water- and seawater-acclimated milkfish and tilapia. The results indicated that the protein abundance of LR marker, flotillin-2, changed with environmental salinities in branchial LR of tilapia. In addition, flotillin-2 and NKA were only found in LR in both tilapia and milkfish gills, while VHA A were mainly present in non-LR. Relative protein abundance of NKA was found to be significantly higher in gills of freshwater milkfish and seawater tilapia, while VHA A was significantly higher in gills of freshwater tilapia and milkfish. This study illustrated differential distribution and salinity-dependent expression of NKA and VHA A in cell membrane of gill tissues of euryhaline teleosts with different salinity preferences.

Overview on solubilization and lipid reconstitution of Na,K-ATPase: enzyme kinetic and biophysical characterization

Na,K-ATPase is a membrane protein which plays a vital role. It pumps Na⁺ and K⁺ ions across the cellular membranes using energy from ATP hydrolysis, and is responsible for maintaining the osmotic equilibrium and generating the membrane potential. Moreover, Na,K-ATPase has also been involved in cell signaling, interacting with partner proteins. Cardiotonic steroids bind specifically to Na,K-ATPase triggering a number of signaling pathways. Because of its importance, many efforts have been employed to study the structure and function of this protein. Difficulties associated with its removal from natural membranes and the concomitant search for appropriate replacement conditions to keep the protein in solution have presented a challenge that had to be overcome prior to carrying out biophysical and biochemical studies in vitro. In this review, we summarized all of the methods and techniques applied by our group in order to obtain information about Na,K-ATPase in respect to solubilization, reconstitution into mimetic system, influence of lipid composition, stability, oligomerization, and aggregation.

Hypoxia-induced remodelling of goldfish membranes

Hypoxia-tolerant animals use metabolic suppression as an essential strategy to survive low oxygen. Ectotherms can alter membrane lipid composition in response to changes in environmental temperature, but it is currently unknown whether chronic hypoxia can also elicit membrane restructuring. The goal of this study was to investigate a possible physiological link between membrane remodelling and metabolic suppression in goldfish exposed to prolonged hypoxia (4 weeks at 10% air saturation). We have tested the hypothesis that chronic hypoxia would modulate membrane lipid composition in ways that are consistent with known mechanisms of ion pump inhibition. Because homeoviscous membrane restructuring could interfere with the response to hypoxia, measurements were made at 2 temperatures. Results show that hypoxic goldfish suppress metabolic rate by 74% (at 13 °C) and 63% (at 20 °C). This study is the first to reveal that cold-acclimated animals undergo extensive, tissue-specific restructuring of membrane lipids as they reach minimal metabolic rates. However, hypoxia does not affect membrane composition in fish acclimated to 20 °C. The strong membrane response of cold-acclimated fish involves increases in cholesterol abundance (in white muscle and gills) and in fatty acid saturation, mainly caused by a reduction in %22:6 (docosahexaenoic acid in gills and liver). Major ion pumps like Na⁺/K⁺-ATPase are known to be inhibited by cholesterol and activated by 22:6. Because ion pumping by membrane-bound ATPases accounts for a large fraction of basal cellular energy use, we propose that the membrane responses reported here could be a novel mechanism to promote metabolic suppression in cold-acclimated animals.

General and specific interactions of the phospholipid bilayer with P-type ATPases

Protein structure and function are modulated via interactions with their environment, representing both the surrounding aqueous media and lipid membranes that have an active role in shaping the structural topology of membrane proteins. Compared to a decade ago, there is now an abundance of crystal structural data on membrane proteins, which together with their functional studies have enhanced our understanding of the salient features of lipid-protein interactions. It is now important to recognize that membrane proteins are regulated by both (1) general lipid-protein interactions, where the general physicochemical properties of the lipid environment affect the conformational flexibility of a membrane protein, and (2) by specific lipid-protein interactions, where lipid molecules directly interact via chemical interactions with specific lipid-binding sites located on the protein. However, due to local differences in membrane composition, thickness, and lipid packing, local membrane physical properties and hence the associated lipid-protein interactions also differ due to membrane location, even for the same protein. Such a phenomenon has been shown to be true for one family of integral membrane ion pumps, the P2-type adenosine triphosphatases (ATPases). Despite being highly homologous, individual members of this family have distinct structural and functional activity and are an excellent candidate to highlight how the local membrane physical properties and specific lipid-protein interactions play a vital role in facilitating the structural rearrangements of these proteins necessary for their activity. Hence in this review, we focus on both the general and specific lipid-protein interactions and will mostly discuss the structure-function relationships of the following P2-type ATPases, Na⁺,K⁺-ATPase (NKA), gastric H⁺,K⁺-ATPase (HKA), and sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), in concurrence with their lipid environment.

Cholesterol depletion inhibits Na+,K+-ATPase activity in a near-native membrane environment

Cholesterol's effects on Na+,K+-ATPase reconstituted in phospholipid vesicles have been extensively studied. However, previous studies have reported both cholesterol-mediated stimulation and inhibition of Na+,K+-ATPase activity. Here, using partial reaction kinetics determined via stopped-flow experiments, we studied cholesterol's effect on Na+,K+-ATPase in a near-native environment in which purified membrane fragments were depleted of cholesterol with methyl-β-cyclodextrin (mβCD). The mβCD-treated Na+,K+-ATPase had significantly reduced overall activity and exhibited decreased observed rate constants for ATP phosphorylation (ENa3+ → E2P, i.e. phosphorylation by ATP and Na+ occlusion from the cytoplasm) and K+ deocclusion with subsequent intracellular Na+ binding (E2K2+ → E1Na3+). However, cholesterol depletion did not affect the observed rate constant for K+ occlusion by phosphorylated Na+,K+-ATPase on the extracellular face and subsequent dephosphorylation (E2P → E2K2+). Thus, partial reactions involving cation binding and release at the protein's intracellular side were most dependent on cholesterol. Fluorescence measurements with the probe eosin indicated that cholesterol depletion stabilizes the unphosphorylated E2 state relative to E1, and the cholesterol depletion-induced slowing of ATP phosphorylation kinetics was consistent with partial conversion of Na+,K+-ATPase into the E2 state, requiring a slow E2 → E1 transition before the phosphorylation. Molecular dynamics simulations of Na+,K+-ATPase in membranes with 40 mol % cholesterol revealed cholesterol interaction sites that differ markedly among protein conformations. They further indicated state-dependent effects on membrane shape, with the E2 state being likely disfavored in cholesterol-rich bilayers relative to the E1P state because of a greater hydrophobic mismatch. In summary, cholesterol extraction from membranes significantly decreases Na+,K+-ATPase steady-state activity.

Topographical and mechanical properties of liposome surfaces harboring Na,K-ATPase by means of atomic force microscopy

In this study, we obtained unprecedented AFM images of the Na,K-ATPase (NKA) pump after being reconstituted into DPPC and DPPC:DPPE liposomes. The mechanical properties observed in the phase images were associated with protrusions correlated to NKA microdomains, which are the darker areas seen in the AFM phase images. Protrusions in the DPPC-NKA proteoliposomes ranged from 38 to 115 nm, with 74 ± 21 nm diameter and 2.1 ± 1.4 nm height. DPPC:DPPE-NKA proteoliposomes showed protrusions from 21 to 78 nm, with 38 ± 16 nm diameter and 0.7 ± 0.5 nm height. We have estimated the presence of annular lipids in the microdomains considering that the areas of the protrusions should contain αβ oligomers and annular phospholipids. For DPPC-NKA proteoliposomes, we hypothesize that 40 phospholipids surround an (αβ)₂ dimer and 46 phospholipids are present for the DPPC:DPPE-NKA proteoliposomes in an αβ monomer. Catalytic activity measurements of both lipid compositions of proteoliposomes harboring NKA provide strong evidence regarding the protein orientation in the biomembrane. AFM data suggest that DPPC-NKA proteoliposomes are also rightside-out protein orientated, where the protrusions have an average height of 2.1 nm, while for DPPC:DPPE-NKA proteoliposomes, the majority of the protein reconstituted should be inside-out orientated, where the protrusions' average height is 0.5 nm. This result corroborates with the enzymatic analysis, where 61% and 91% of the enzymatic activity was recovered, respectively. Thus, a new application of AFM as a tool for the determination of topological features of protrusions in proteoliposomes has been brought to the scientific community, in addition to revealing the distinct catalytic orientation of enzymes present in the biomembranes model.

Are Polyunsaturated Fatty Acids Implicated in Histaminergic Dysregulation in Bipolar Disorder?: AN HYPOTHESIS

Bipolar disorder (BD) is an extremely disabling psychiatric disease, characterized by alternate states of mania (or hypomania) and depression with euthymic states in between. Currently, patients receive pharmacological treatment with mood stabilizers, antipsychotics, and antidepressants. Unfortunately, not all patients respond well to this type of treatment. Bipolar patients are also more prone to heart and metabolic diseases as well as a higher risk of suicide compared to the healthy population. For a correct brain function is indispensable a right protein and lipids (e.g., fatty acids) balance. In particular, the amount of fatty acids in the brain corresponds to a 50–70% of the dry weight. It has been reported that in specific brain regions of BD patients there is a reduction in the content of unsaturated n-3 fatty acids. Accordingly, a diet rich in n-3 fatty acids has beneficial effects in BD patients, while their absence or high levels of saturated fatty acids in the diet are correlated to the risk of developing the disease. On the other hand, the histamine system is likely to be involved in the pathophysiology of several psychiatric diseases such as BD. Histamine is a neuromodulator involved in arousal, motivation, and energy balance; drugs acting on the histamine receptor H3 have shown potential as antidepressants and antipsychotics. The histaminergic system as other neurotransmission systems can be altered by fatty acid membrane composition. The purpose of this review is to explore how polyunsaturated fatty acids content alterations are related to the histaminergic system modulation and their impact in BD pathophysiology.

Membrane lipid rafts are disturbed in the response of rat skeletal muscle to short-term disuse

Marked loss of skeletal muscle mass occurs under various conditions of disuse, but the molecular and cellular mechanisms leading to atrophy are not completely understood. We investigate early molecular events that might play a role in skeletal muscle remodeling during mechanical unloading (disuse). The effects of acute (6-12 h) hindlimb suspension on the soleus muscles from adult rats were examined. The integrity of plasma membrane lipid rafts was tested utilizing cholera toxin B subunit or fluorescent sterols. In addition, resting intracellular Ca²⁺ level was analyzed. Acute disuse disturbed the plasma membrane lipid-ordered phase throughout the sarcolemma and was more pronounced in junctional membrane regions. Ouabain (1 µM), which specifically inhibits the Na-K-ATPase α2 isozyme in rodent skeletal muscles, produced similar lipid raft changes in control muscles but was ineffective in suspended muscles, which showed an initial loss of α2 Na-K-ATPase activity. Lipid rafts were able to recover with cholesterol supplementation, suggesting that disturbance results from cholesterol loss. Repetitive nerve stimulation also restores lipid rafts, specifically in the junctional sarcolemma region. Disuse locally lowered the resting intracellular Ca²⁺ concentration only near the neuromuscular junction of muscle fibers. Our results provide evidence to suggest that the ordering of lipid rafts strongly depends on motor nerve input and may involve interactions with the α2 Na-K-ATPase. Lipid raft disturbance, accompanied by intracellular Ca²⁺ dysregulation, is among the earliest remodeling events induced by skeletal muscle disuse.

Specific phospholipid binding to Na,K-ATPase at two distinct sites

Membrane protein function can be affected by the physical state of the lipid bilayer and specific lipid-protein interactions. For Na,K-ATPase, bilayer properties can modulate pump activity, and, as observed in crystal structures, several lipids are bound within the transmembrane domain. Furthermore, Na,K-ATPase activity depends on phosphatidylserine (PS) and cholesterol, which stabilize the protein, and polyunsaturated phosphatidylcholine (PC) or phosphatidylethanolamine (PE), known to stimulate Na,K-ATPase activity. Based on lipid structural specificity and kinetic mechanisms, specific interactions of both PS and PC/PE have been inferred. Nevertheless, specific binding sites have not been identified definitively. We address this question with native mass spectrometry (MS) and site-directed mutagenesis. Native MS shows directly that one molecule each of 18:0/18:1 PS and 18:0/20:4 PC can bind specifically to purified human Na,K-ATPase (α₁β₁). By replacing lysine residues at proposed phospholipid-binding sites with glutamines, the two sites have been identified. Mutations in the cytoplasmic αL8-9 loop destabilize the protein but do not affect Na,K-ATPase activity, whereas mutations in transmembrane helices (TM), αTM2 and αTM4, abolish the stimulation of activity by 18:0/20:4 PC but do not affect stability. When these data are linked to crystal structures, the underlying mechanism of PS and PC/PE effects emerges. PS (and cholesterol) bind between αTM 8, 9, 10, near the FXYD subunit, and maintain topological integrity of the labile C terminus of the α subunit (site A). PC/PE binds between αTM2, 4, 6, and 9 and accelerates the rate-limiting E₁P-E₂P conformational transition (site B). We discuss the potential physiological implications.

Dipole-Potential-Mediated Effects on Ion Pump Kinetics

The kinetics of conformational changes of P-type ATPases necessary for the occlusion or deocclusion of transported ions are known to be sensitive to the composition of the surrounding membrane, e.g., phospholipid content, mole percentage of cholesterol, and the presence of lipid-bound anions. Research has shown that many membrane components modify the dipole potential of the lipid head-group region. Based on the observation that occlusion/deocclusion reactions of ion pumps perturb the membrane surrounding the protein, a mechanism is suggested whereby dipole potential modifiers induce preferential stabilization or destabilization of occluded or nonoccluded states of the protein, leading to changes in the forward and backward rate constants for the transition. The mechanism relies on the assumption that conformational changes of the protein are associated with changes in its hydrophobic thickness that requires a change in local lipid packing density to allow hydrophobic matching with the membrane. The changes in lipid packing density cause changes in local lipid dipole potential that are responsible for the dependence of conformational kinetics on dipole potential modifiers. The proposed mechanism has the potential to explain effects of lipid composition on the kinetics of any membrane protein undergoing significant changes in its membrane cross-sectional area during its activity.

Specialized Functional Diversity and Interactions of the Na,K-ATPase

Na,K-ATPase is a protein ubiquitously expressed in the plasma membrane of all animal cells and vitally essential for their functions. A specialized functional diversity of the Na,K-ATPase isozymes is provided by molecular heterogeneity, distinct subcellular localizations, and functional interactions with molecular environment. Studies over the last decades clearly demonstrated complex and isoform-specific reciprocal functional interactions between the Na,K-ATPase and neighboring proteins and lipids. These interactions are enabled by a spatially restricted ion homeostasis, direct protein-protein/lipid interactions, and protein kinase signaling pathways. In addition to its "classical" function in ion translocation, the Na,K-ATPase is now considered as one of the most important signaling molecules in neuronal, epithelial, skeletal, cardiac and vascular tissues. Accordingly, the Na,K-ATPase forms specialized sub-cellular multimolecular microdomains which act as receptors to circulating endogenous cardiotonic steroids (CTS) triggering a number of signaling pathways. Changes in these endogenous cardiotonic steroid levels and initiated signaling responses have significant adaptive values for tissues and whole organisms under numerous physiological and pathophysiological conditions. This review discusses recent progress in the studies of functional interactions between the Na,K-ATPase and molecular microenvironment, the Na,K-ATPase-dependent signaling pathways and their significance for diversity of cell function.

General and specific lipid-protein interactions in Na,K-ATPase

The molecular activity of Na,K-ATPase and other P2 ATPases like Ca(2+)-ATPase is influenced by the lipid environment via both general (physical) and specific (chemical) interactions. Whereas the general effects of bilayer structure on membrane protein function are fairly well described and understood, the importance of the specific interactions has only been realized within the last decade due particularly to the growing field of membrane protein crystallization, which has shed new light on the molecular details of specific lipid-protein interactions. It is a remarkable observation that specific lipid-protein interactions seem to be evolutionarily conserved, and conformations of specifically bound lipids at the lipid-protein surface within the membrane are similar in crystal structures determined with different techniques and sources of the protein, despite the rather weak lipid-protein interaction energy. Studies of purified detergent-soluble recombinant αβ or αβFXYD Na,K-ATPase complexes reveal three separate functional effects of phospholipids and cholesterol with characteristic structural selectivity. The observations suggest that these three effects are exerted at separate binding sites for phophatidylserine/cholesterol (stabilizing), polyunsaturated phosphatidylethanolamine (stimulatory), and saturated PC or sphingomyelin/cholesterol (inhibitory), which may be located within three lipid-binding pockets identified in recent crystal structures of Na,K-ATPase. The findings point to a central role of direct and specific interactions of different phospholipids and cholesterol in determining both stability and molecular activity of Na,K-ATPase and possible implications for physiological regulation by membrane lipid composition. This article is part of a special issue titled "Lipid-Protein Interactions."

Stimulation, inhibition, or stabilization of Na,K-ATPase caused by specific lipid interactions at distinct sites

The activity of membrane proteins such as Na,K-ATPase depends strongly on the surrounding lipid environment. Interactions can be annular, depending on the physical properties of the membrane, or specific with lipids bound in pockets between transmembrane domains. This paper describes three specific lipid-protein interactions using purified recombinant Na,K-ATPase. (a) Thermal stability of the Na,K-ATPase depends crucially on a specific interaction with 18:0/18:1 phosphatidylserine (1-stearoyl-2-oleoyl-sn-glycero-3-phospho-L-serine; SOPS) and cholesterol, which strongly amplifies stabilization. We show here that cholesterol associates with SOPS, FXYD1, and the α subunit between trans-membrane segments αTM8 and -10 to stabilize the protein. (b) Polyunsaturated neutral lipids stimulate Na,K-ATPase turnover by >60%. A screen of the lipid specificity showed that 18:0/20:4 and 18:0/22:6 phosphatidylethanolamine (PE) are the optimal phospholipids for this effect. (c) Saturated phosphatidylcholine and sphingomyelin, but not saturated phosphatidylserine or PE, inhibit Na,K-ATPase activity by 70-80%. This effect depends strongly on the presence of cholesterol. Analysis of the Na,K-ATPase activity and E1-E2 conformational transitions reveals the kinetic mechanisms of these effects. Both stimulatory and inhibitory lipids poise the conformational equilibrium toward E2, but their detailed mechanisms of action are different. PE accelerates the rate of E1 → E2P but does not affect E2(2K)ATP → E13NaATP, whereas sphingomyelin inhibits the rate of E2(2K)ATP → E13NaATP, with very little effect on E1 → E2P. We discuss these lipid effects in relation to recent crystal structures of Na,K-ATPase and propose that there are three separate sites for the specific lipid interactions, with potential physiological roles to regulate activity and stability of the pump.

Na,K-ATPase reconstituted in ternary liposome: the presence of cholesterol affects protein activity and thermal stability

Differential scanning calorimetry (DSC) was applied to investigate the effect of cholesterol on the thermotropic properties of the lipid membrane (DPPC and DPPE). The thermostability and unfolding of solubilized and reconstituted Na,K-ATPase in DPPC:DPPE:cholesterol-liposomes was also studied to gain insight into the role of cholesterol in the Na,K-ATPase modulation of enzyme function and activity. The tertiary system (DPPC:DPPE:cholesterol) (molar ratio DPPC:DPPE equal 1:1) when cholesterol content was increased from 0% up to 40% results in a slight decrease in the temperature of transition and enthalpy, and an increase in width. We observed that, without heating treatment, at 37°C, the activity was higher for 20mol% cholesterol. However, thermal inactivation experiments showed that the enzyme activity loss time depends on the cholesterol membrane content. The unfolding of the enzyme incorporated to liposomes of DPPC:DPPE (1:1mol) with different cholesterol contents, ranging from 0% to 40% mol was also studied by DSC. Some differences between the thermograms indicate that the presence of lipids promotes a conformational change in protein structure and this change is enough to change the way Na,K-ATPase thermally unfolds.

Gill tissue lipids of salmon (Salmo salar L.) presmolts and smolts from anadromous and landlocked populations

Composition of membrane lipids from the gills of juvenile Atlantic salmon (Salmo salar) in presmolt and smolt phases of development was compared among anadromous and non-anadromous populations. Three stocks migrating from spawning rivers to either lake (landlocked stock), brackish water or full strength sea water were grown under common garden conditions, and gill lipids and their acyl and alkenyl chains were examined in February (presmolts) and at the end of May (smolts) by mass spectrometry and gas-liquid chromatography. The most remarkable changes upon transition from the presmolt phase to the smolt phase were: (i) increase in the cholesterol/phospholipid ratio, (ii) decrease in the abundance of phosphatidylinositol (PI) content, (iii) increase in the amount of sulfatides, (iv) increase in phosphatidylcholine (PC) and phosphatidylethanolamine (PE) species with two highly unsaturated acyl chains, and finally (v) convergence of interstock differences in PC and PE species composition towards a similar lipid composition. Increases in the gill membrane content of cholesterol and sulfatides are discussed as pre-adaptation of salmon gills for salt-secretion, which may occur by increases in membrane microdomains (rafts) harboring ion channels and pumps. The decreases of PI were likely related to adjusting the gill membrane permeability to ions by diminishing prostanoid production. The similarity of those changes among three salmon stocks and the convergence of initially (presmolt phase) different PC and PE species profiles between the stocks towards similar lipid composition suggests that smoltification process of the gill epithelium is largely similar in anadromous and landlocked populations.

Neutral phospholipids stimulate Na,K-ATPase activity: a specific lipid-protein interaction

Membrane proteins interact with phospholipids either via an annular layer surrounding the transmembrane segments or by specific lipid-protein interactions. Although specifically bound phospholipids are observed in many crystal structures of membrane proteins, their roles are not well understood. Na,K-ATPase is highly dependent on acid phospholipids, especially phosphatidylserine, and previous work on purified detergent-soluble recombinant Na,K-ATPase showed that phosphatidylserine stabilizes and specifically interacts with the protein. Most recently the phosphatidylserine binding site has been located between transmembrane segments of αTM8-10 and the FXYD protein. This paper describes stimulation of Na,K-ATPase activity of the purified human α1β1 or α1β1FXYD1 complexes by neutral phospholipids, phosphatidylcholine, or phosphatidylethanolamine. In the presence of phosphatidylserine, soy phosphatidylcholine increases the Na,K-ATPase turnover rate from 5483 ± 144 to 7552 ± 105 (p < 0.0001). Analysis of α1β1FXYD1 complexes prepared with native or synthetic phospholipids shows that the stimulatory effect is structurally selective for neutral phospholipids with polyunsaturated fatty acyl chains, especially dilinoleoyl phosphatidylcholine or phosphatidylethanolamine. By contrast to phosphatidylserine, phosphatidylcholine or phosphatidylethanolamine destabilizes the Na,K-ATPase. Structural selectivity for stimulation of Na,K-ATPase activity and destabilization by neutral phospholipids distinguish these effects from the stabilizing effects of phosphatidylserine and imply that the phospholipids bind at distinct sites. A re-examination of electron densities of shark Na,K-ATPase is consistent with two bound phospholipids located between transmembrane segments αTM8-10 and TMFXYD (site A) and between TM2, -4, -6, -and 9 (site B). Comparison of the phospholipid binding pockets in E2 and E1 conformations suggests a possible mechanism of stimulation of Na,K-ATPase activity by the neutral phospholipid.

Physiological adaptation of an Antarctic Na+/K+-ATPase to the cold

Because enzymatic activity is strongly suppressed by the cold, polar poikilotherms face significant adaptive challenges. For example, at 0°C the catalytic activity of a typical enzyme from a temperate organism is reduced by more than 90%. Enzymes embedded in the plasma membrane, such as the Na(+)/K(+)-ATPase, may be even more susceptible to the cold because of thermal effects on the lipid bilayer. Accordingly, adaptive changes in response to the cold may include adjustments to the enzyme or the surrounding lipid environment, or synergistic changes to both. To assess the contribution of the enzyme itself, we cloned orthologous Na(+)/K(+)-ATPase α-subunits from an Antarctic (Pareledone sp.; -1.8°C) and a temperate octopus (Octopus bimaculatus; ∼18°C), and compared their turnover rates and temperature sensitivities in a heterologous expression system. The primary sequences of the two pumps were found to be highly similar (97% identity), with most differences being conservative changes involving hydrophobic residues. The physiology of the pumps was studied using an electrophysiological approach in intact Xenopus oocytes. The voltage dependence of the pumps was equivalent. However, at room temperature the maximum turnover rate of the Antarctic pump was found to be 25% higher than that of the temperate pump. In addition, the Antarctic pump exhibited a lower temperature sensitivity, leading to significantly higher relative activity at lower temperatures. Orthologous Na(+)/K(+) pumps were then isolated from two tropical and two Arctic octopus. The temperature sensitivities of these pumps closely matched those of the temperate and Antarctic pumps, respectively. Thus, reduced thermal sensitivity appears to be a common mechanism driving cold adaptation in the Na(+)/K(+)-ATPase.

Cholesterol attenuates and prevents bilayer damage and breakdown in lipoperoxidized model membranes. A spin labeling EPR study

The stabilizing effect of cholesterol on oxidized membranes has been studied in planar phospholipid bilayers and multilamellar 1-palmitoyl-2-linoleoyl-phosphatidylcholine vesicles also containing either 1-palmitoyl-2-glutaroyl-phosphatidylcholine or 1-palmitoyl-2-(13-hydroxy-9,11-octadecanedienoyl)-phosphatidylcholine oxidized phosphatidylcholine in variable ratio. Lipid peroxidation-dependent membrane alterations in the absence and in the presence of cholesterol were analyzed using Electron Paramagnetic Resonance spectroscopy of the model membranes spin labelled with either cholestane spin label (3-DC) or phosphatidylcholine spin label (5-DSPC). Cholesterol, added to lipid mixtures up to 40% final molar ratio, decreased the inner bilayer disorder as compared to cholesterol-free membranes and strongly reduced bilayer alterations brought about by the two oxidized phosphatidylcholine species. Furthermore, Sepharose 4B gel-chromatography and cryo electron microscopy of aqueous suspensions of the lipid mixtures clearly showed that cholesterol is able to counteract the micelle forming tendency of pure 1-palmitoyl-2-glutaroyl-phosphatidylcholine and to sustain multilamellar vesicles formation. It is concluded that membrane cholesterol may exert a beneficial and protective role against bilayer damage caused by oxidized phospholipids formation following reactive oxygen species attack to biomembranes.

Activity of BK(Ca) channel is modulated by membrane cholesterol content and association with Na+/K+-ATPase in human melanoma IGR39 cells

Interaction of large conductance Ca(2+)- and voltage-activated K(+) (BK(Ca)) channels with Na(+)/K(+)-ATPase, caveolin-1, and cholesterol was studied in human melanoma IGR39 cells. Functional BK(Ca) channels were enriched in caveolin-rich and detergent-resistant membranes, i.e. rafts, and blocking of the channels by a specific BK(Ca) blocker paxilline reduced proliferation of the cells. Disruption of rafts by selective depletion of cholesterol released BK(Ca) channels from these domains with a consequent increase in their activity. Consistently, cholesterol enrichment of the cells increased the proportion of BK(Ca) channels in rafts and decreased their activity. Immunocytochemical analysis showed that BK(Ca) channels co-localize with Na(+)/K(+)-ATPase in a cholesterol-dependent manner, thus suggesting their co-presence in rafts. Supporting this, ouabain, a specific blocker of Na(+)/K(+)-ATPase, inhibited BK(Ca) whole-cell current markedly in control cells but not in cholesterol-depleted ones. This inhibition required the presence of external Na(+). Collectively, these data indicate that the presence of Na(+)/K(+)-ATPase in rafts is essential for efficient functioning of BK(Ca) channels, presumably because the pump maintains a low intracellular Na(+) proximal to the BK(Ca) channel. In conclusion, cholesterol could play an important role in cellular ion homeostasis and thus modulate many cellular functions and cell proliferation.

Close encounters of the oily kind: regulation of transporters by lipids

Neurotransmitter transporters are membrane proteins that serve as key regulators of extracellular neurotransmitter concentrations and have been long viewed as important targets for drug development by the pharmaceutical industry. Although many cellular signaling systems are known to modulate transport activity, much less is known about how transporters communicate with and are regulated by the various components of the lipid sea in which they reside. Variations in lipid content clearly affect the activity of a variety of transport systems, and with advances in techniques for lipid analysis and a clearer vision of carrier structure, this area of research appears poised for major advances.