Utilization of membranous lipid substrates by membranous enzymes. Hydrolysis of sphingomyelin in erythrocyte 'ghosts' and liposomes by the membranous sphingomyelinase of chicken erythrocyte 'ghosts'

Role of sphingolipids in the biogenesis and biological activity of extracellular vesicles

Extracellular vesicles (EVs) are membrane vesicles released by both eukaryotic and prokaryotic cells; they not only serve physiological functions, such as disposal of cellular components, but also play pathophysiologic roles in inflammatory and degenerative diseases. Common molecular mechanisms for EV biogenesis are evident in different cell biological contexts across eukaryotic phyla, and inhibition of this biogenesis may provide an avenue for therapeutic research. The involvement of sphingolipids (SLs) and their enzymes on EV biogenesis and release has not received much attention in current research. Here, we review how SLs participate in EV biogenesis by shaping membrane curvature and how they contribute to EV action in target cells. First, we describe how acid and neutral SMases, by generating the constitutive SL, ceramide, facilitate biogenesis of EVs at the plasma membrane and inside the endocytic compartment. We then discuss the involvement of other SLs, such as sphingosine-1-phosphate and galactosyl-sphingosine, in EV formation and cargo sorting. Last, we look ahead at some biological effects of EVs mediated by changes in SL levels in recipient cells.

A sphingomyelinase-resistant pool of sphingomyelin in the nuclear membrane of hen erythrocytes

Experiments in which hen erythrocytes were exposed to the action of exogenous sphingomyelinase (Staphylococcus aureus) or to their endogenous plasma membrane sphingomyelinase showed that about 15% of the total sphingomyelin was resistant to breakdown either in intact or lysed cells. This resistant pool of sphingomyelin seems likely to reside in the nuclear membranes of the cells, so that essentially all the plasma membrane sphingomyelin can be broken down by exogenous sphingomyelinase acting on intact cells, suggesting that plasma membrane sphingomyelin is exclusively localised in the outer lipid leaflet. Paradoxically, introduction of Ca2+ into the intact cells using A23187 causes the breakdown of up to 30% of total cell sphingomyelin inside the cells but without apparently affecting the putative nuclear pool of sphingomyelin and this suggests that Ca2+ may alter the original disposition of sphingomyelin in the membrane so that originally outer leaflet sphingomyelin becomes accessible to the endogenous sphingomyelinase inside the cells. No differences were seen in the fatty acid compositions of sphingomyelin degradable by exogenous sphingomyelinase, sphingomyelin degradable in the presence of A23187/Ca2+ or the enzyme-resistant pool of sphingomyelin.

Hydrolysis of membrane-associated phosphoglycerides by mitochondrial phospholipase A2

Conversion of membrane-bound substrates by membrane-associated enzymes can proceed in principle via intramembrane and intermembrane action. By using rat-liver mitochondria containing labeled phosphatidylethanolamine and inactivated phospholipase A2 as substrate source, and mitochondria containing unlabeled substrate and active enzyme, it is shown that hydrolysis of phosphatidylethanolamine by mitochondrial phospholipase A2 proceeds nearly entirely via intramembrane enzyme action. A study of the characteristics of this mode of enzyme action showed that all mitochondrial phosphoglycerides were hydrolyzed. Plots of approximate initial velocities of hydrolysis against the remaining amounts of each individual phospholipid, indicated that phosphatidylethanolamine was hydrolyzed fastest, with a rate about twice that for phosphatidylcholine and about 10-fold that for cardiolipin. The initial rates remained nearly constant in the initial phase of the hydrolysis, suggesting that the enzyme is surrounded by excess substrate.

Utilization of membranous lipid substrates by membranous enzymes: activation of the latent sphingomyelinase of hen erythrocyte membrane

Previous studies demonstrated that hen erythrocytes have an inoperative, latent sphingomyelinase which is activated when the cells are hemolyzed in a hypotonic medium. Within minutes after hemolysis about 60-80% of the sphingomyelin (SPM) of the RBC "ghost" membrane was hydrolyzed. In this paper, expression of sphingomyelinase activity was further investigated. The percentage of total SPM hydrolyzed depended on the volume of the hypotonic hemolyzing buffer. Thus, suspending the erythrocytes in 4 vol of the buffer resulted in clumping of the hemolyzed "ghosts" and no hydrolysis of SPM. In comparison, suspension in 19 vol of the hypotonic buffer showed no clumping and sphingomyelinase activity was fully expressed. But centrifugation of the latter or, alternatively, addition of concanavalin A induced clumping and elimination of sphingomyelinase activity. Hen RBC could also be hemolyzed in an isotonic medium in the presence of Triton X-100, mellitin, halothane, and phospholipase C. Activation of the latent sphingomyelinase occurred at concentrations of these reagents which caused cell lysis. Hen RBC were dispersed in an isotonic medium containing glutaraldehyde (0.1%) or formaldehyde (10%). This rendered the cells resistant to hemolysis, even when subsequently dispersed in a hypotonic medium or water. But incubation of the "fixed" cells in a hypotonic or isotonic medium activated the enzyme, resulting in hydrolysis of 60% of the cellular SPM. In contrast, when glutaraldehyde was included in the hypotonic buffer, hemolysis occurred but sphingomyelinase activity was eliminated.

Formation of free fatty acid and ceramide during brain handling: lability of sphingomyelin

Intact brain and brain homogenates readily form free fatty acids and ceramides, even in the cold during subcellular isolation procedures. The fatty acid formation is slightly stimulated by chelators and might be due to phospholipid hydrolysis by lysosomal phospholipases. The ceramide formation is accompanied by loss of sphingomyelin and is apparently due to the action of neutral, metal ion-activated sphingomyelinase. The latter reaction is inhibited by EDTA whereas both degradative processes are inhibited by mercuriphenylsulfonate, the thiol-reacting inhibitor. Cerebroside does not seem to be a source of accumulated ceramide.

Equilibrium-driven mechanism for preferential adhesion between chick embryo cells

The experiments presented here confirm the hypothesis according to which, in our experimental system of differential cell adhesion (where we studied the kinetics of the earliest period of adhesion of a suspension of chick embryo neuroblasts to layers of astroblasts or fibroblasts), the mechanism of adhesion appears to consist of two steps, the first of which is a short-term reversible phase corresponding to a binding equilibrium. In fact, adhesion of neuroblasts to each of the two cell layers occurs according to kinetic constants and attains levels which are characteristic for each of the two adhesion systems. In both systems, neuroblasts that have not adhered at equilibrium are able to adhere if inoculated over a fresh cell layer of the same type, as they do during the first inoculation; conversely, neuroblasts that have adhered to a cell layer can be made to de-adhere by substituting cell-free fresh medium to the inoculation medium containing non-adhering neuroblasts. This shows that, as predicted for a reversible equilibrium system, removal of adhering neuroblasts from the system at equilibrium provokes adhesion, and removal of non-adhered neuroblasts provokes de-adhesion. Furthermore the level of adhesion at equilibrium is, in all cases, the same. The reversibility of adhesion, which is almost quantitative during the onset of the equilibrium, gradually decreases with time, indicating the presence of a process of irreversible attachment between cells after the first reversible step. The developmental implications of the complete sequential mechanisms are briefly discussed.

Utilization of membranous lipid substrates by membrane-bound enzymes. Intramembrane and intermembrane hydrolysis of diacylglycerol by lipase of rat liver microsomes

The membranous lipase of rat liver microsomes was used to hydrolyze diacylglycerol (DG), generated within the microsomal membrane by treatment with phospholipase C, in two separate interactions. For an intramembrane enzyme-substrate interaction, the enzyme and DG were present in the same microsomes. For intermembrane interactions, native microsomes of rat liver were used as carriers of the enzyme, while heated and phospholipase C-treated microsomes of rat liver or brain were employed as carriers of the substrate. The v vs S curves of the intermembrane interaction were hyperbolic while those of the intramembrane utilization were parabolic.

Microvesiculation and sphingomyelinase activation in chicken erythrocytes treated with ionophore A23187 and Ca2+

Treatment of chicken erythrocytes with ionophore A23187 and Ca2+ leads to the disappearance of the marginal band of microtubules and to a release of the constraints which normally maintain the nucleus in a central position in the cells. The consequent close apposition of the nucleus to the plasma membrane may allow nuclear-plasma membrane fusion to occur and subsequently results in the release of microvesicles from the hybrid surface membrane. The remnant cells are spherical, and have nuclei which appear to be partly exocytosed. Concomitant with these morphological changes, there is a breakdown of 20-30% of the total cell sphingomyelin by an endogenous sphingomyelinase which does not require Ca2+ and which releases phosphorylcholine only into the cell interior. It is suggested that the pool of sphingomyelin which is broken down as a consequence of Ca2+ entry into the cells is present in the nuclear membrane and that it becomes available to the plasma membrane sphingomyelinase as a result of the close apposition of nucleus and plasma membrane induced by Ca2+.

Lysophospholipase-catalyzed hydrolysis of lysophospholipids in Mycoplasma gallisepticum membranes

Mycoplasma gallisepticum strains have a membrane-bound lysophospholipase which hydrolyzes lysophospholipid generated in these membranes by treatment with an external phospholipase. This paper studies the hydrolysis of the membranous lysophospholipids by an enzyme residing in the same membrane (intramembrane utilization) or in adjacent membranes (intermembrane utilization). To study intermembrane hydrolysis, the phospholipids of M. gallisepticum were labeled with [3H]oleic acid. Membranes were prepared, heated at 65 degrees C, and subsequently treated with pancreatic phospholipase A2. This resulted in membranes whose enzyme was heat inactivated, but which contained lysophospholipid. When these membranes were mixed with M. gallisepticum cells or membranes, the lysophospholipid was hydrolyzed by the membranous lysophospholipase. To study intramembrane hydrolysis, [3H]oleyl-labeled membranes of M. gallisepticum were treated with pancreatic phospholipase A2 at pH 5.0. At this pH, lysophospholipid was generated but not hydrolyzed. Adjustment of the pH to 7.4 resulted in hydrolysis of the lysophospholipid by the membranous lysophospholipase. These procedures permitted measuring the initial rates of intramembrane and intermembrane hydrolysis of the lysophospholipid, showing that the time course and dependence on endogenous substrate concentration were different in the intramembrane and intermembrane modes of utilization. They also permitted calculation of the molar concentration of the lysophospholipid in the membrane and its rate of hydrolysis, expressed as moles per minute per cell or per square centimeter of cell surface.

A micromethod for the determination of ceramide

An enzymatic micromethod for the determination of ceramide is presented. The enzyme, E. coli diglyceride kinase was used to phosphorylate ceramide, as well as diglyceride with high specific activity gamma-[32P]ATP, and the two products are differentiated by their alkali stability. This method was applied to the detection of endogenous phospholipases and sphingolipases in several membrane systems and could have wide application.