Membrane lipids of rat liver lysosomes prepared by freeflow electrophoresis

Comparative Molecular Dynamics Simulation Studies of Realistic Eukaryotic, Prokaryotic, and Archaeal Membranes

We present a comparative all-atom molecular dynamics simulation study of 18 biomembrane systems with lipid compositions corresponding to eukaryotic, bacterial, and archaebacterial membranes together with three single-component lipid bilayers. A total of 105 lipid types used in this study include diverse sterols and glycerol-based lipids with acyl chains of various lengths, unsaturation degrees, and branched or cyclic moieties. Our comparative analysis provides deeper insight into the influences of sterols and lipid unsaturation on the structural and mechanical properties of these biomembranes, including water permeation into the membrane hydrocarbon core. For sterol-containing membranes, sterol fraction is correlated with the membrane thickness, the area compressibility modulus, and lipid order but anticorrelated with the area per lipid and sterol tilt angles. Similarly, for all 18 biomembranes, lipid order is correlated with the membrane thickness and area compressibility modulus. Sterols and lipid unsaturation produce opposite effects on membrane thickness, but only sterols influence water permeation into the membrane. All membrane systems are accessible for public use in CHARMM-GUI Archive. They can be used as templates to expedite future modeling of realistic cell membranes with transmembrane and peripheral membrane proteins to study their structure, dynamics, molecular interactions, and function in a nativelike membrane environment.

A proteomic view on lysosomes

Lysosomes are the main degradative organelles of almost all eukaryotic cells. They fulfil a crucial function in cellular homeostasis, and impairments in lysosomal function are connected to a continuously increasing number of pathological conditions. In recent years, lysosomes are furthermore emerging as control centers of cellular metabolism, and major regulators of cellular signaling were shown to be activated at the lysosomal surface. To date, >300 proteins were demonstrated to be located in/at the lysosome, and the lysosomal proteome and interactome is constantly growing. For the identification of these proteins, and their involvement in cellular mechanisms or disease progression, mass spectrometry (MS)-based proteomics has proven its worth in a large number of studies. In this review, we are recapitulating the application of MS-based approaches for the investigation of the lysosomal proteome, and their application to a diverse set of research questions. Numerous strategies were applied for the enrichment of lysosomes or lysosomal proteins and their identification by MS-based methods. This allowed for the characterization of the lysosomal proteome, the investigation of lysosome-related disorders, the utilization of lysosomal proteins as biomarkers for diseases, and the characterization of lysosome-related cellular mechanisms. While these >60 studies provide a comprehensive picture of the lysosomal proteome across several model organisms and pathological conditions, various proteomics approaches have not been applied to lysosomes yet, and a large number of questions are still left unanswered.

CE analysis of the acidic organelles of a single cell

The properties of organelles within a cell have been shown to be highly heterogeneous. Until now, it has been unclear just how much of this heterogeneity is endemic to the organelle subpopulations themselves and how much is actually due to stochastic cellular noise. An attractive approach for investigating the origins of heterogeneity among the organelles of a single cell is CE with LIF detection (CE-LIF). As a proof of principle, in this report we optimize and use a single cell CE-LIF method to investigate the properties of endocytic (acidic) organelles. Our results show that the properties of individual acidic organelles containing Alexa Fluor 488 Dextran suggest that there are two groups of CCRF-CEM cells: a group with a high dextran content per cell, and a group with a low dextran content per cell. Furthermore, the individual organelle measurements of the single cells allow us to compare in each group the distributions of doxorubicin content per acidic organelle and electrophoretic mobilities of these organelles.

Analysis of mitochondria isolated from single cells

Bulk studies are not suitable to describe and study cell-to-cell variation, which is of high importance in biological processes such as embryogenesis, tissue differentiation, and disease. Previously, capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) was used to measure the properties of organelles isolated from millions of cells. As such, these bulk measurements reported average properties for the organelles of cell populations. Similar measurements for organelles released from single cells would be highly relevant to describe the subcellular variations among cells. Toward this goal, here we introduce an approach to analyze the mitochondria released from single mammalian cells. Osteosarcoma 143B cells are labeled with either the fluorescent mitochondrion-specific 10-N-nonyl acridine orange (NAO) or via expression of the fluorescent protein DsRed2. Subsequently, a single cell is introduced into the CE-LIF capillary where the organelles are released by a combined treatment of digitonin and trypsin. After this treatment, an electric field is applied and the released organelles electromigrate toward the LIF detector. From an electropherogram, the number of detected events per cell, their individual electrophoretic mobilities, and their individual fluorescence intensities are calculated. The results obtained from DsRed2 labeling, which is retained in intact mitochondria, and NAO labeling, which labels all mitochondria, are the basis for discussion of the strengths and limitations of this single-cell approach.

Separation of endosomes by aqueous two-phase partition and free-flow electrophoresis

We have developed two endosome models to evaluate the separation of endosome populations by aqueous two-phase partition. In the first model, bovine kidney endosomes were used. In the second model. HeLa endosomes were identified in homogenates by means of a latent drug-(capsaicin-)inhibited NADH oxidase (NOX). Endosomes were first isolated by aqueous two-phase partition. To separate early and late endosomes, the endosomes were incubated with ATP to acidify the endosome interiors by activating a proton-translocating ATPase. Thus far, we have been able to resolve the early and late endosomes from any source only by preparative free-flow electrophoresis and not by phase-partition. Previous studies have shown that gravitational forces may be important for separation of endosomes by phase partition. Low-speed centrifugation (< or =12.5 g) during phase resolution altered the activity of the latent NADH oxidase used as a marker for HeLa cell endosomes.

Ovine ceroid-lipofuscinosis. I: Lipopigment composition is indicative of a lysosomal proteinosis

The ceroid-lipofuscinoses are inherited lysosomal storage diseases of children and animals characterised by a fluorescent lipopigment stored in a variety of tissues. Defects in lipid metabolism or the control of lipid peroxidation have been postulated to explain their pathogenesis but the underlying biochemical defect is still unknown. In the present study lipopigment was isolated from liver, kidney, pancreas and brain of sheep affected with ceroid-lipofuscinosis. Approximately two-thirds of the lipopigment mass was protein. Sodium dodecyl sulphate polyacrylamide gel electrophoresis showed a major polypeptide band of Mr 14,800, heterogeneous polypeptides between 5,000-9,000 Mr and a major band of Mr 3,500. These were not normal lysosomal proteins. I125 radiolabeling studies indicated that they were 47% of the pancreatic lipopigment mass, the 3,500 Mr polypeptides alone accounting for 26%. Lipopigment polypeptides were not subunits of a larger protein held together by disulphide bonds. The presence of the 3,500 Mr proteins in whole affected tissue homogenates distinguished them from homogenates of normal tissues. Lipopigment phospholipids were the same species as normal lysosomal phospholipids, including bis (monoacylglycero) phosphate, a lysosomal marker. Similarly the neutral lipids, notably dolichol, ubiquinone and dolichyl esters were typical of those in lysosomal membranes. Lipopigments contained 1-1.7% metals. Analyses of them indicated a functional lysosomal origin for the lipopigment. It was concluded that low Mr proteins are specifically stored in ovine ceroid-lipofuscinosis and that this disease is a lysosomal proteinosis.

Comparison of the ability of phospholipids from rat liver lysosomes to reconstitute glucocerebrosidase

The in situ lipid activator of rat liver glucocerebrosidase was investigated. Rat liver lysosomes were purified (42.9-fold relative to the crude homogenate) by sequential isopycnic centrifugation in sucrose and metrizamide gradients. Lipids were extracted with chloroform:methanol (2:1) and phospholipids were separated by one-dimensional thin-layer chromatography. The phospholipid content of the lysosome preparation was 0.28 mumol lipid phosphorus/mg protein. Phosphatidylcholine was present as the major nonacidic phospholipid (39.3%). Of the acidic phospholipids, phosphatidylinositol and phosphatidylserine were present in the greatest amounts (12.0 and 19.7%, respectively). The resolved phospholipids were tested separately and in the presence of a heat-stable factor from Gaucher spleen for their ability to reconstitute butanol-delipidated rat liver glucocerebrosidase activity. Alone or in the presence of the heat-stable factor, phosphatidylserine and phosphatidylinositol were the most effective activators of glucocerebrosidase. Bis(monoacylglyceryl) phosphate derived from rat liver tritosomes or rabbit lung macrophages was also effective in reconstituting beta-glucosidase activity.

Enhanced lysosomal phospholipid degradation and lysophospholipid production due to free radicals

To pursue the hypothesis that peroxidized lipids may become preferred substrates for endogenous phospholipases, we injured hepatic lysosomes by adding an exogenous free radical generating system [dihydroxyfumurate + Fe3+-ADP]; this system rapidly lysed hepatic lysosomes at pH 6.0, with maximal changes at 30 min. The production of malondialdehyde [MDA] plateaued rapidly. At 20 min the degradation of phosphatidylethanolamine [PE] was greater than phosphatidylcholine [PC]: 52% and 17%, respectively. Sphingomyelin and neutral lipids did not decrease. Most interesting was the significant increase of lysoPC [329%; p less than 0.05] at 10 min and [381%; p less than 0.01] after 20 min of incubation; lysoPE production became significant [766%; p less than 0.05] at 20 min. This enhanced production of lysoPC and lysoPE suggests a new mechanism to increase the production of amphiphilic lipids during ischemia, that is active at moderately acid pH without added calcium.

Haemosiderin and tissue damage

High levels of haemosiderin occur in iron overload syndromes such as idiopathic haemochromatosis or secondary iron overload in thalassaemic patients; haemosiderin is the predominant iron-storage compound in such cases. It consists of a large aggregate of FeOOH cores, many of which have an incomplete shell of protein, and is probably derived from ferritin by lysosomal proteolysis. In addition, some chemical degradation of the ferritin cores appears to occur on conversion to haemosiderin. Other biochemical components are phosphate and magnesium, which may be adsorbed to the core surface, and perhaps certain lipids. Haemosiderin may have a central role, either directly or indirectly, in iron cytotoxicity and therefore the chemistry and biochemistry of this material warrants further study.

Lead-induced regional lipid peroxidation in brain

Rats exposed to lead (2% lead acetate in drinking water for a period of 10 days) showed altered levels of phospholipids, lipid peroxide and lead ions. Lead ion concentrations were significantly increased in all brain regions with a maximum retention in the spinal cord and cerebellum. The cerebral cortex showed a higher percent elevation of lead ions as compared to the other areas of the brain. The phospholipid contents were significantly depressed in the cerebral cortex, cerebellum and spinal cord. Subsequently, the rate of lipid peroxidation has been enhanced in all brain regions following toxicosis. Increased rate of lipid peroxidation and decreased levels of phospholipids followed the similar rank order as that of increasing lead concentration was spinal cord, cerebellum, cerebral cortex and brain stem after lead intoxication.

Involvement of lysosomes in the uptake of macromolecular material by bloodstream forms of Trypanosoma brucei

To investigate whether the lysosomes of Trypanosoma brucei are capable of uptake of macromolecules after internalization by the cell, we used Triton WR-1339, a non-digestible macromolecular compound, which is known to cause a marked decrease in the density of hepatic lysosomes due to massive intralysosomal storage. Intraperitoneal administration of 0.4 g/kg Triton WR-1339 to rats infected with T. brucei led to the development of a large vacuole in the trypanosomes between nucleus and kinetoplast within 22 h. Higher doses (2 g/kg) led to the disappearance of the trypanosomes from the blood and resulted in permanent cures (greater than 100 days). Lysosomes isolated from the trypanosomes of animals treated with a sub-curative dose showed a decrease in equilibrium density of 0.03 g/cm3 in sucrose gradients. These lysosomes were partly damaged as evidenced by a reduction in latency and an increase in the non-sedimentable part of lysosomal enzymes. We conclude that acid proteinase and alpha-mannosidase-containing organelles of T. brucei take up exogenous macromolecules and must therefore be considered as true lysosomes and that Triton WR-1339 acts in T. brucei as a true lysosomotropic drug. Its trypanocidal action probably results from an interference with lysosomal function.

Involvement of periodic deacylation-acylation cycles of mitochondrial phospholipids during Sr2+-induced oscillatory ion transport in rat liver mitochondria

Lysophosphatidylcholine and lysophosphatidylethanolamine levels were determined during Sr2+-induced oscillating ion fluxes in mitochondria prelabelled in vivo with 32Pi. Periodic fluctuations of both lyso compounds were established with an increase at the stage of simultaneously monitored K+ influx and a decrease at K+ efflux. The periodic activations and inactivations of phospholipase were found to be associated with periodic changes in the incorporation rates of labelled polyunsaturated fatty acids with an apparent phase difference of 180 degrees. Periodic deacylation-acylation cycles of phospholipids accompanying the periodic cycles of reversible ion accumulation and release are suggested to be involved in the trigger mechanism generating the permeability changes during oscillatory ion transport.

Analytical characterization and purification of plasma membrane from cultured hepatoma cells (HTC cells)

The plasma membrane of the hepatoma cell line, HTC cells, has been characterized and purified by cell fractionation techniques. In the absence of true 5'-nucleotidase in HTC cells, alkaline phosphodiesterase I has been used as a marker enzyme, following conclusions gained from differential and isopycnic centrifugation studies (Lopez-Saura, P., Trouet, A. and Tulkens, P. (1978) Biochim. Biophys. Acta 543, 430-449). To confirm this localization, HTC cells were exposed to anti-plasma membrane IgG at 4 degrees C and fractionated. Alkaline phosphodiesterase I and IgG showed superimposable distribution patterns in linear sucrose gradients. Alkaline phosphodiesterase I is, however, only poorly resolved from enzyme markers of other organelles, especially NADPH-cytochrome c reductase (endoplasmic reticulum) and galactosyltransferase (Golgi complex). Maximal purification from the homogenate is only 13-fold, on a protein basis, even when using a microsomal fraction (67 and 13% of alkaline phosphodiesterase I and protein, respectively) as the starting material. Improved resolution can be obtained after the addition of small quantities of digitonin (equimolar with respect to the cholesterol content). Digitonin increases the buoyant density of alkaline phosphodiesterase I by approx. 0.05 g/cm3, whereas the buoyant densities of galactosyltransferase and NADPH-cytochrome c reductase are increased only by 0.03 and 0.015 g/cm3, respectively. Accordingly, a procedure has been designed which yields a fraction containing 22.8% of alkaline phosphodiesterase I with a purification of 21-fold on a protein basis. The content of NADPH-cytochrome c reductase and galactosyltransferase is 1.2 and 2.1%, respectively. Electron microscopy shows smooth surface membrane elements and vesicles, with only occasional other recognizable elements.

Kininogenase activity in plasma membranes and cell organelles from rabbit kidney cortex: subcellular localization of renal kallikrein by free-flow electrophoresis and density-gradient fractionation

Subcellular fractions were prepared from a rabbit kidney cortex homogenate by density gradient and free-flow electrophoresis techniques. After enzymatic and morphologic characterization, we determined the kininogenase activity in the different fractions. This activity was present in those plasma membranes that also contained high specific activities Na-K-ATPase and in lysosomal-like particles. No activity was found in the lumen, that is, the microvillous part of the proximal tubule cell. The kallikrein-like nature of this kininogenase activity was established by several methods.

The permeability of the lysosomal membrane to small ions

The permeability of the lysosomal membrane to small anions and cations was studied at 37 degrees C and pH 7.0 in a lysosomal-mitochondrial fraction isolated from the liver of untreated rats. The extent of osmotic lysis following ion influx was used as a measure of ion permeancy. In order to preserve electroneutrality, anion influx was coupled to an influx of K+ in the presence of valinomycin, and cation influx was coupled to an efflux of H+ using the protonophore 3-tert-butyl-5,2'-dichloro-4'-nitrosalicilylanilide. Lysosomal lysis was monitored by observing the loss of latency of two lysosomal hydrolases. The order of permeability of the lysosomal membrane to anions was found to be SCN- greater than I- greater than CH3COO- greater than Cl- approximately Pi greater than SO24- and that to cations Cs+ greater than K+ greater than Na+ greater than H+. These orders are largely in agreement with the lyotropic series of anions and cations. The implications of these findings for the mechanism by means of which a low intralysosomal pH is produced and maintained are discussed.

Effect of cholesterol content on some physical and functional properties of mitochondria isolated from adult rat liver, fetal liver, cholesterol-enriched liver and hepatomas AH-130, 3924A and 5123

The cholesterol to phospholipid ratio in mitochondria from hepatomas AH-130, 3924A and 5123 is higher than in the particles isolated from adult or fetal rat livers. Nearly all the cholesterol of hepatoma mitochondria is located in membranes. As in liver mitochondria, in the particles isolated from hepatoma AH-130 there is more cholesterol in the outer than in the inner membrane. In mitochondria from cholesterol-enriched liver and hepatomas, there occurs a decrease in extent of hypoosmotic and phosphate-induced swelling and a decrease of conformational changes linked to energy states. The phenomenon is more marked in particles which exhibit higher cholesterol to phospholipid ratios. A statistically significant negative correlation exists between the cholesterol to phospholipid ratio and extent of volume or conformational changes. No significant modifications of these parameters were found in fetal liver mitochondria. Cholesterol content does not influence K+ uptake by cholesterol-enriched or hepatoma mitochondria. Nor does cholesterol content affect the respiratory increment related to this uptake. As a consequence of K+ uptake, total mitochondrial water exchangeable with tritiated water rises 20% while sucrose-impermeable water rises 42-48% in both adult rat liver and hepatoma AH-130 mitochondria. Absorbance changes linked to ion uptake do not correspond merely to variations in mitochondrial water content. Water content is apparently not influenced by the cholesterol to phospholipid ratio. However, the ratio is significantly correlated to both extent and initial rate of absorbance decrease of mitochondrial suspensions during K+ uptake. The higher the ratio, the lower the extent and initial rate of absorbance decrease.