Effect of different phospholipid-cholesterol membrane compositions on liposome-mediated formation of calcium phosphates

Effect of cholesterol content on affinity and stability of factor VIII and annexin V binding to a liposomal bilayer membrane

To investigate the effect of cholesterol composition on the binding of factor VIII (FVIII) and annexin V (AV) to membranes, liposomal membranes with phospholipid bilayers of various compositions of phosphatidylcholine (PC), phosphatidylserine (PS), and cholesterol were constructed. A surface plasmon resonance (SPR) biosensor system was employed to measure the equilibrium and rate constants of the bindings. As expected, PS was found to play a dominant role in the binding of AV; its binding level was directly proportional to the PS composition in a liposome. The binding levels of FVIII and AV to liposome increased with an increase in cholesterol composition in liposome. It seemed to suggest that cholesterol in liposome acts as a 'phospholipid arrangement' factor by inducing the formation of PS-rich microdomains. However, in the absence of PS (20% on a mole basis), cholesterol could not exert the binding enhancement effect, which again confirmed the critical role of PS in the bindings. Stability of the AV binding was significantly improved by the increase in cholesterol content; for AV, the dissociation rate constant was decreased approximately fivefold, from 1.7 x 10(-3)s(-1) in the absence of cholesterol to 3.3 x 10(-4)s(-1) in the presence of only 10% cholesterol. But, for FVIII the binding stability was not so much influenced by the cholesterol addition (up to 50% on a mole basis). In summary, by using liposomes on an SPR system, we were able to demonstrate quantitatively the apparent effects of cholesterol on the binding affinity and stability of the membrane-binding proteins.

Matrix vesicles in the fibrous cap of atherosclerotic plaque: possible contribution to plaque rupture

Plaque rupture is the most common type of plaque complication and leads to acute ischaemic events such as myocardial infarction and stroke. Calcification has been suggested as a possible indicator of plaque instability. Although the role of matrix vesicles in the initial stages of arterial calcification has been recognized, no studies have yet been carried out to examine a possible role of matrix vesicles in plaque destabilization. Tissue specimens selected for the present study represented carotid specimens obtained from patients undergoing carotid endarterectomy. Serial frozen cross-sections of the tissue specimens were cut and mounted on glass slides. The thickness of the fibrous cap (FCT) in each advanced atherosclerotic lesion, containing a well developed lipid/necrotic core, was measured at its narrowest sites in sets of serial sections. According to established criteria, atherosclerotic plaque specimens were histologically subdivided into two groups: vulnerable plaques with thin fibrous caps (FCT <100 μm) and presumably stable plaques, in which fibrous caps were thicker than 100 μm. Twenty-four carotid plaques (12 vulnerable and 12 presumably stable plaques) were collected for the present analysis of matrix vesicles in fibrous caps. In order to provide a sufficient number of representative areas from each plaque, laser capture microdissection (LCM) was carried out. The quantification of matrix vesicles in ultrathin sections of vulnerable and stable plaques revealed that the numbers of matrix vesicles were significantly higher in fibrous caps of vulnerable plaques than those in stable plaques (8.908±0.544 versus 6.208±0.467 matrix vesicles per 1.92 μm² standard area; P= 0.0002). Electron microscopy combined with X-ray elemental microanalysis showed that some matrix vesicles in atherosclerotic plaques were undergoing calcification and were characterized by a high content of calcium and phosphorus. The percentage of calcified matrix vesicles/microcalcifications was significantly higher in fibrous caps in vulnerable plaques compared with that in stable plaques (6.705±0.436 versus 5.322±0A94; P= 0.0474). The findings reinforce a view that the texture of the extracellular matrix in the thinning fibrous cap of atherosclerotic plaque is altered and this might contribute to plaque destabilization.

Are calcifying matrix vesicles in atherosclerotic lesions of cellular origin?

Over recent years, the role of matrix vesicles in the initial stages of arterial calcification has been recognized. Matrix calcifying vesicles have been isolated from atherosclerotic arteries and the biochemical composition of calcified vesicles has been studied. No studies have yet been carried out to examine the fine structure of matrix vesicles in order to visualize the features of the consequent stages of their calcification in arteries. In the present work, a high resolution ultrastructural analysis has been employed and the study revealed that matrix vesicles in human atherosclerotic lesions are heterogeneous with two main types which we classified. Type I calcified vesicles were presented by vesicles surrounded by two electron-dense layers and these vesicles were found to be resistant to the calcification process in atherosclerotic lesions in situ. Type II matrix vesicles were presented by vesicles surrounded by several electron-dense layers and these vesicles were found to represent calcifying vesicles in atherosclerotic lesions. To test the hypothesis that calcification of matrix vesicles surrounded by multilayer sheets may occur simply as a physicochemical process, independently from the cell regulation, we produced multilamellar liposomes and induced their calcification in vitro in a manner similar to that occurring in matrix vesicles in atherosclerotic lesions in situ.

Chick embryo anchored alkaline phosphatase and mineralization process in vitro

Bone alkaline phosphatase with glycolipid anchor (GPI-bALP) from chick embryo femurs in a medium without exogenous inorganic phosphate, but containing calcium and GPI-bALP substrates, served as in vitro model of mineral formation. The mineralization process was initiated by the formation of inorganic phosphate, arising from the hydrolysis of a substrate by GPI-bALP. Several mineralization media containing different substrates were analysed after an incubation time ranging from 1.5 h to 144 h. The measurements of Ca/Pi ratio and infrared spectra permitted us to follow the presence of amorphous and noncrystalline structures, while the analysis of X-ray diffraction data allowed us to obtain the stoichiometry of crystals. The hydrolysis of phosphocreatine, glucose 1-phosphate, glucose 6-phosphate, glucose 1,6-bisphosphate by GPI-bALP produced hydroxyapatite in a manner similar to that of beta-glycerophosphate. Several distinct steps in the mineral formation were observed. Amorphous calcium phosphate was present at the onset of the mineral formation, then poorly formed hydroxyapatite crystalline structures were observed, followed by the presence of hydroxyapatite crystals after 6-12 h incubation time. However, the hydrolysis of either ATP or ADP, catalysed by GPI-bALP in calcium-containing medium, did not lead to the formation of any hydroxyapatite crystals, even after 144 h incubation time, when hydrolysis of both nucleotides was completed. In contrast, the hydrolysis of AMP by GPI-bALP led to the appearance of hydroxyapatite crystals after 12 h incubation time. The hydroxyapatite formation depends not only on the ability of GPI-bALP to hydrolyze the organic phosphate but also on the nature of substrates affecting the nucleation process or producing inhibitors of the mineralization.

Changes in phospholipid extractability and composition accompany mineralization of chicken growth plate cartilage matrix vesicles

Matrix vesicles are lipid bilayer-enclosed structures that initiate extracellular mineral formation. Little attention has been given to how newly formed mineral interacts with the lipid constituents and then emerges from the lumen. To explore whether specific lipids bind to the incipient mineral and if breakdown of the membrane is involved, we analyzed changes in lipid composition and extractability during vesicle-induced calcification. Isolated matrix vesicles were incubated in synthetic cartilage lymph to induce mineral formation. At various times, samples of the lipids were taken for analysis, extracted both before and after demineralization to remove deposited mineral. Phosphatidylserine and phosphatidylinositol both rapidly disappeared from extracts made before decalcification, indicating rapid degradation. However, extracts made after demineralization revealed that phosphatidylserine had become complexed with newly forming mineral. Concomitantly, its levels actually increased, apparently by base-exchange with phosphatidylethanolamine. Though partially complexed with the mineral, phosphatidylinositol was nevertheless rapidly broken down. Sphingomyelin and phosphatidylethanolamine also underwent rapid breakdown, but phosphatidylcholine was degraded more slowly, all accompanied by a buildup of free fatty acids. The data indicate that phosphatidylserine forms complexes that accompany mineral formation, while degradation of other membrane phospholipids apparently enables egress of crystalline mineral from the vesicle lumen.

In vitro inhibition of membrane-mediated calcification by novel phosphonates

The effects of a series of novel phosphonates on the kinetics of mineral development in an ionophore-primed 7:2:1 phosphatidylcholine (PC): dicetylphosphate (DCP): cholesterol (Chol) liposomal model system are reported. When present at 2.5 micromol/liter or 25 micromol/liter concentrations in the solution surrounding the liposomes, the investigated phosphonates did not significantly delay the initial formation of hydroxyapatite-like calcium phosphate salts (HAP) within the liposomes or the penetration of HAP crystals through the enclosing membranes. However, the phosphonates variably retarded the subsequent growth and proliferation of the HAP crystals once they became directly exposed to the phosphonate-containing solution. The effectiveness of phosphonates in inhibiting extraliposomal precipitation strongly depended on their structure. The inhibitory action on active surface growth sites of released intraliposomal crystals was found to be the most effective if the phosphonate molecule contained two phosphonic groups linked to the same C atom. At a phosphonate concentration of 25 micromol/liter, the following general order of effectiveness was established: geminal bisphosphonate >/= geminal tetrakisphosphonate > bisacylphosphonates > monoacylphosphonate > bisalkylphosphonate. Within the bisacylphosphonate family, the highest inhibitory action was observed when four or five -CH2- groups separated the ketophosphonic groups.

Persistence of complexed acidic phospholipids in rapidly mineralizing tissues is due to affinity for mineral and resistance to hydrolytic attack: in vitro data

Acidic phospholipids, complexed with calcium and inorganic phosphate, are components of extracellular matrix vesicles. Both the complexed acidic phospholipids and matrix vesicles have previously been shown to serve as hydroxyapatite (HA) nucleators in solution and when implanted in a muscle pouch. The present study supplies evidence that complexed acidic phospholipids can persist in mineralizing tissues both because of their affinity for HA and because of their resistance to hydrolysis by phospholipase A2. Calcium-phosphatidylserine-phosphate complex (CPLX-PS) synthesized with 14C-labeled phosphatidylserine (PS) was used to measure CPLX-PS affinity for HA using a Langmuir adsorption isotherm model. The affinity was shown to be higher and more specific than that of PS itself (K = 8.66 ml/mumol; N, the number of binding sites = 20.4 mumol/m2 as compared with previously reported values for PS of K = 3.33 ml/mumol, and N = 4.87 mumol/m2). Incorporated into synthetic liposomes and incubated in a calcium phosphate solution in which mineralization is induced by an ionophore, CPLX-PS showed behavior distinct from free PS. As previously reported, PS in these liposomes totally blocked HA formation. On the other hand, CPLX-PS in similar concentrations had a varied response, having no effect, slightly inhibiting, or actually promoting HA formation. CPLX-PS was also shown to be a poorer substrate for phospholipase A2 than PS, with Km = 4.63 mM for CPLX-PS and Km = 0.27 mM for PS; and Vmax = 0.029 ml/minute for CPLX-PS and Vmax = 0.066 ml/minute for PS. These data explain how complexed acidic phospholipids may persist in the growth plate and facilitate initial mineral deposition.

Mixed phospholipid liposome calcification

Synthetic lipid vesicle (liposome) suspensions have been used to experimentally model many of the calcium phosphate precipitation steps observed in matrix vesicle (MV) calcification. In particular, precipitate development in liposomes can be made to preferentially follow the progression seen in MV, i.e. to occur initially in intraliposomal spaces and then to expand into the surrounding suspending medium. This paper reviews results from studies by us which show that certain phospholipid (PL) constituents of the liposomal membrane can modulate this progression. Of greatest relevance to MV calcification is the observation that phosphatidylserine and sphingomyelin, two lipids selectively enriched in MV, slow the expansion of the precipitation from inside to outside the liposome.