Influence of phospholipid fatty acyl composition on sarcolemmal and sarcoplasmic reticular cation transporters

Lysophosphatidylinositol-acyltransferase-1 is involved in cytosolic Ca2+ oscillations in macrophages

Lysophosphatidylinositol-acyltransferase-1 (LPIAT1) specifically catalyzes the transfer of arachidonoyl-CoA to lysophosphoinositides. LPIAT^-/- mice have been shown to have severe defects in the brain and liver; however, the exact molecular mechanisms behind these conditions are not well understood. As immune cells have been implicated in liver inflammation based on disfunction of LPIAT1, we generated Raw264.7 macrophages deficient in LPIAT1, using shRNA and CRISPR/Cas9. The amount of C38:4 species in phosphoinositides, especially in PtdInsP₂ , was remarkably decreased in these cells. Unlike in wild-type cells, LPIAT1-deficient cells showed prolonged oscillations of intracellular Ca²⁺ upon UDP stimulation, which is known to activate phospholipase Cβ through the Gq-coupled P2Y6 receptor, even in the absence of extracellular Ca²⁺ . It is speculated that the prolonged Ca²⁺ response may be relevant to the increased risk of liver inflammation induced by LPIAT1 disfunction.

The complex and intriguing lives of PIP2 with ion channels and transporters

Phosphatidylinositol-4,5-bisphosphate (PIP(2)), the precursor of several signaling molecules in eukayotic cells, is itself also used by cells to signal to membrane-associated proteins. PIP(2) anchors numerous signaling molecules and cytoskeleton at the cell membrane, and the metabolism of PIP(2) is closely connected to membrane trafficking. Recently, ion transporters and channels have been discovered to be regulated by PIP(2). Systems reported to be activated by PIP(2) include (i) plasmalemmal calcium pumps (PMCA), (ii) cardiac sodium-calcium exchangers (NCX1), (iii) sodium-proton exchangers (NHE1-4), (iv) a sodium-magnesium exchanger of unknown identity, (v) all inward rectifier potassium channels (KATP, IRK, GIRK, and ROMK channels), (vi) epithelial sodium channels (ENaC), and (vii) ryanodine-sensitive calcium release channels (RyR). Systems reported to be inhibited by PIP(2) include (i) cyclic nucleotide-gated channels of the rod (CNG), (ii) transient receptor potential-like (TRPL) Drosophila phototransduction channels, (iii) capsaicin-activated transient receptor potential (TRP) channels (VR1), and (iv) IP(3)-gated calcium release channels (IP3R). Systems that appear to be completely insensitive to PIP(2) include (i) voltage-gated sodium channels, (ii) most voltage-gated potassium channels, (iii) sodium-potassium pumps, (iv) several neurotransmitter transporters, and (v) cystic fibrosis transmembrane receptor (CFTR)-type chloride channels. Presumably, local changes of the concentration of PIP(2) in the plasma membrane represent cell signals to those mechanisms sensitive to PIP(2) changes. Unfortunately, our understanding of how local PIP(2) concentrations are regulated remains very limited. One important complexity is the probable existence of phospholipid microdomains, or lipid rafts. Such domains may serve to localize PIP(2) and thereby PIP(2) signaling, as well as to organize PIP(2) binding partners into signaling complexes. A related biological role of PIP(2) may be to control the activity of ion transporters and channels during biosynthesis or vesicle trafficking. Low PIP(2) concentrations in the secretory pathway would inactivate all of the systems that are stimulated by PIP(2). How, in detail, is PIP(2) used by cells to control ion channel and transporter activities? Further progress requires an improved understanding of lipid kinases and phosphatases, how they are regulated, where they are localized in cells, and with which ion channels and transporters they might localize.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Renal Na+, K+-ATPase in SHR: studies of activity and gene expression

The mechanism by which increased dietary intake of calcium reduces blood pressure in the spontaneously hypertensive rat is unknown. The present studies were designed to determine if there were alterations in the activity of the major membrane ion translocating pump, sodium, potassium-ATPase (NKA), in the kidneys of hypertensive rats and whether increased dietary calcium intake affected the activity of this enzyme. Fifteen-week old SHR's were found to have lower total ATPase activity in microsomal preparations from the kidney than age matched Wistar-Kyoto animals. Both the ouabain-sensitive component (NKA) and the ouabain-insensitive component were lower in SHR. Increasing dietary calcium intake from 1% to 3% elevated both components of the ATPase activity in SHR, but was without effect in WKY. Measurement of membrane phospholipid composition suggested that altered phospholipid composition did not account for the reduced ATPase activity observed, but indicated a reduced density of ATPase in SHR. A technique has been devised for qualitative and quantitative analysis of Na, K-ATPase alpha isoforms using RT-PCR. This technique reveals that the alpha 1 isoform is the sole catalytic isoform present in the nephron. Accurate and precise quantification of the amount of gene expression in individual nephron segments is reported and will be applied to determine whether dietary calcium influences blood pressure by a mechanism which alters nephron NKA gene expression.

Lipid metabolism in photoreceptor membranes: regulation and mechanisms

Lipid metabolism in photoreceptor rod outer segments has attracted considerable attention because of its importance in providing the appropriate environment for supporting an efficient phototransduction mechanism. Recent studies suggest that lipid metabolism in these membranes is involved in the generation of second messengers and in signal transduction mechanisms. Phospholipid turnover is tightly regulated by phosphorylation-dephosphorylation reactions and light, and provides, in turn, with molecules capable of activating protein kinases and cellular processes such as membrane fusion or light-adaptation. These findings suggest that photoreceptor membrane lipids are more than just important structural components of the visual cell rod outer segment.

Acyl-CoA:lysophosphatidylcholine acyltransferase activity in bovine retina rod outer segments

In the present paper the properties of acyl-CoA:lysophosphatidylcholine acyltransferase activity associated with rod outer segments (ROS) have been studied. Under adequate experimental conditions, ROS acyl-CoA:lysophosphatidylcholine acyltransferase activity presented a maximum at pH 7.0. The enzyme was able to incorporate as much as 60% of the label offered as [1-14C]oleoyl-CoA into phosphatidylcholine after 5 min of incubation. The use of varying concentrations of oleoyl-CoA and 46 microM lysophosphatidylcholine gave an apparent K(m) value for oleoyl-CoA of 100 microM and a Vmax value of 153 nmol x h-1 x (mg protein)-1. The use of varying concentrations of lysophosphatidylcholine and 100 microM oleoyl-CoA gave an apparent K(m) value for lysophosphatidylcholine of 27 microM and a Vmax value of 155 nmol x h-1 x (mg protein)-1. The enzyme was inhibited by 25% when ROS membranes were incubated in the presence of 10 mM MgCl2. The acyltransferase was able to incorporate other acyl-CoAs (palmitoyl-CoA and arachidonoyl-CoA) into ROS phospholipids and to acylate other lysophospholipids but less efficiently than lysophosphatidylcholine. Lysophoshatidylcholine was preferentially acylated with arachidonic acid followed by oleic acid and, less efficiently, with palmitic acid. The high specific activity of acyl-CoA lysophosphatidylcholine acyltransferase found in purified ROS compared to the activity found in other subcellular fractions of the bovine retina suggests that this enzymatic activity is native to the ROS.

Oleic acid supplementation reduces oxidant-mediated dysfunction of cultured porcine pulmonary artery endothelial cells

We have previously shown that supplementing cultured porcine pulmonary artery endothelial cells (PAEC) with exogenous oleic acid (18:1 omega 9) alters the fatty acid composition of the cells and reduces oxidant-mediated cytotoxicity. Because the mechanisms by which lipid alterations modulate oxidant susceptibility have not been defined, the ability of 18:1 to reduce hydrogen peroxide (H2O2)-mediated PAEC dysfunction was evaluated. PAEC monolayers on polycarbonate filters were incubated for 3 h in maintenance medium supplemented with either 0.1 mM 18.1 in ethanol vehicle (ETOH) or with an equivalent volume of vehicle alone. Twenty-four hours later monolayers were treated for 30 min with 50 or 100 microM H2O2 in Hanks' balanced salt solution (HBSS) or with HBSS alone (nonoxidant control). As a functional index of PAEC monolayer integrity, the permeability of monolayers to albumin was then measured for 3 h. Treatment with 100 microM H2O2 caused cytotoxicity and progressive increases in PAEC monolayer permeability that were attenuated by 18:1 supplementation, whereas 50 microM H2O2 caused only a transient increase in permeability without cytotoxicity. Supplementation with 18:1 also attenuated H2O2-induced reductions in PAEC adenosine triphosphate (ATP) content and disruption of PAEC microfilament architecture. The ATP content of PAEC monolayers was reversibly reduced in the absence of oxidant stress by incubation with glucose-depleted medium containing deoxyglucose and antimycin A. Metabolic inhibitor-induced ATP depletion increased monolayer permeability and altered cytoskeletal architecture, alterations that resolved during recovery of PAEC ATP content. These results demonstrate that ATP depletion plays a critical role in barrier dysfunction and suggests that the ability of 18:1 to reduce oxidant-mediated PAEC dysfunction and injury may relate directly to its ability to preserve PAEC ATP content.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.