Endycytosis and exocytosis: role of microfilaments and involvement of phospholipids in membrane fusion

Artificial Organelles with Digesting Characteristics: Imitating Simplified Lysosome- and Macrophage-Like Functions by Trypsin-Loaded Polymersomes

Defects in cellular protein/enzyme encoding or even in organelles are responsible for many diseases. For instance, dysfunctional lysosome or macrophage activity results in the unwanted accumulation of biomolecules and pathogens implicated in autoimmune, neurodegenerative, and metabolic disorders. Enzyme replacement therapy (ERT) is a medical treatment that replaces an enzyme that is deficient or absent in the body but suffers from short lifetime of the enzymes. Here, this work proposes the fabrication of two different pH-responsive and crosslinked trypsin-loaded polymersomes as protecting enzyme carriers mimicking artificial organelles (AOs). They allow the enzymatic degradation of biomolecules to mimic simplified lysosomal function at acidic pH and macrophage functions at physiological pH. For optimal working of digesting AOs in different environments, pH and salt composition are considered the key parameters, since they define the permeability of the membrane of the polymersomes and the access of model pathogens to the loaded trypsin. Thus, this work demonstrates environmentally controlled biomolecule digestion by trypsin-loaded polymersomes also under simulated physiological fluids, allowing a prolonged therapeutic window due to protection of the enzyme in the AOs. This enables the application of AOs in the fields of biomimetic therapeutics, specifically in ERT for dysfunctional lysosomal diseases.

Influence of compounds affecting synthesis, modification and transport of proteins on the expression and release of interleukin-2 receptor

One of the most central events during lymphocyte activation is the synthesis and release of IL-2. IL-2 induces the synthesis and expression of the IL-2 receptor alpha-chain (IL-2R) on the lymphocyte as well as the release of a truncated form of IL-2R (sIL-2R). The two proteins are identical except for the absence of the transmembrane and intracellular part on sIL-2R. We have in an in vitro model investigated the influence of certain compounds, affecting different parts of the proliferative response, on the release and expression of IL-2R. We found the generation of the two molecules to have different sensitivity to blocking of protein synthesis, glycosylation, microtubular assembly and proteolytic activity. However, blocking of intracellular transport from Golgi to the endoplasmic reticulum, disassembly of actin filaments and disturbances in the intracellular sodium/calcium balance had identical effects on expression and release of the respective IL-2R. These findings indicate a more complex and specific mechanism behind the generation of sIL-2R than simply by shedding through the action of proteases on the membrane-bound IL-2R.

Latrunculin A is a potent inhibitor of phagocytosis by macrophages

We have found that latrunculin A, a Red Sea sponge toxin, is a potent inhibitor of immunological phagocytosis by mouse peritoneal macrophages, but does not block the binding (recognition) of the immune complexes (erythrocytes sensitized with IgG antibodies) to the cells. The inhibition begins to be appreciable around 12 nM latrunculin A, and is complete with a toxin concentration of 60 nM. This inhibitory effect does not interfere with the cell viability, and can be reversed when the macrophages are incubated in fresh medium. Since latrunculin A is a disrupting agent of microfilament organization, these results strengthen the evidence for the active participation of microfilaments in the mechanism of phagocytosis and at the same time provide a new tool for the investigation of phagocytosis at the molecular level.

Mechanism of phagocytosis by Schwann cells

33B rat Schwannoma cell line is known to exhibit phagocytic properties analogous to those of normal Schwann cells. The mechanism of phagocytosis by this cell line was investigated by studying the effect of known modulators of phagocytosis on the uptake of latex particles by these cells. Treatments which block energy production of the host cell, such as incubation at 4 degrees C and treatment with sodium azide, markedly inhibited the phagocytosis of latex particles by these cells. Phagocytosis was dose-dependently, and completely, inhibited by cytochalasin B, demonstrating an important role of microfilaments. Colchicine produced a minor inhibition of phagocytosis only at the highest concentration (10(-3) M) tested, suggesting that intact microtubules are not crucial for latex phagocytosis. Dibutyryl cyclic AMP was without any effect on the phagocytosis. Thus, latex phagocytosis by rat Schwannoma cells is an active, energy-dependent process requiring intact microfilaments with only a minor dependence on microtubules.

Actin filament alterations in glomerular epithelial cells of adriamycin-induced nephrotic rats

The alterations of actin filaments in the glomerular epithelial cells of adriamycin-induced nephrotic rats were studied by electron microscopy and immunohistochemistry using both of the peroxidase-antiperoxidase and avidin-biotin methods. After the administration of adriamycin, the glomerular epithelial cells showed cell swelling, deformation of cell shape, vacuole formations, retraction or fusion of foot processes, detachment of the plasma membrane from the glomerular besement membrane (GBM), and an increase of cellular organelles. In parallel with these alterations, abnormal distribution of intracytoplasmic microfilaments of 5-6 nm in diameter was observed, which formed clusters, particularly gathering densely along the plasma membrane of the fused foot processes that abutted the GBM. Immnohistochemically, actin was demonstrated on the site of the above described filaments. The above results suggest that actin filaments may closely relate to the morphological maintenance of the glomerular epithelial cells and to their function as well.

Transfer of amphotericin B from gel state vesicles to mycoplasma cells: biphasic action on potassium transport and permeability

The action of amphotericin B on the K+ permeability of Mycoplasma mycoides var. capri cells, containing either cholesterol or ergosterol in their membranes, was studied. When the drug, solubilized in dimethyl sulfoxide, was added directly to the cell suspension, a slightly greater sensitivity to permeabilization was observed for ergosterol-containing cells, confirming the data reported in the literature. When amphotericin B bound to gel state phospholipid vesicles was added to the cell suspension, two effects on cholesterol-containing cells were observed. First, the K+ active transport rates increased; membrane permeabilization and K+ leakage were subsequently detected. For ergosterol-containing cells these sequential events were observed only at amphotericin B concentrations below 10(-6) M. At higher concentrations only K+ leakage was observed. The second permeabilization effect varied with the amphotericin B concentration in different ways in the two types of cells. The permeabilization of ergosterol-containing membranes depended on the amphotericin B/phospholipid molar ratio, whereas the permeabilization of cholesterol-containing membranes did not. In general, the latter remained fairly constant when the total amphotericin B concentration in the medium varied.

Distribution of actin in mesangial cell of rat glomerulus

The localization of actin in the renal glomerulus of normal and streptozotocin (SZ) or aminonucleoside (AN) administered rats was studied by immunofluorescent and electron microscopy. The immunofluorescence (IF) for actin in normal rats showed a fine granular mesangial pattern, but the intensity was weak. An increase in positivity and an enlargement of the distribution of IF were observed in both SZ and AN administered rats, and the pattern of staining was still mesangial. Under an electron microscope, the mesangial cells showed an increase in number of microfilaments of 5 nm in diameter as well as in other cellular organelles, such as rERs and free ribosomes. These results suggest that the IF-positive substance for actin in the mesangium and the microfilaments observed in the mesangial cells by electron microscopy are identical with each other, i.e. actin filaments. Further, the mesangial cells may react by stimulation from injurious agents, such as SZ or AN, to increase the amount of actin as well as the number of other cellular organelles.

Plasma membrane-mediated leakage of liposomes induced by interaction with murine thymocytic leukemia cells

The interaction of liposomes with BW 5147 murine thymocytic leukemia cells was studied using fluorescent probes (entrapped carboxyfluorescein and fluorescent phosphatidylethanolamine) in conjunction with a Ficoll-Paque discontinous gradient system for rapid separation of liposomes from cells. Reversible liposomal binding to discrete sites on the BW cell surface was found to represent the major form of interaction; uptake of intact liposomal contents by a process such as liposome-BW cell membrane fusion was found to apparently represent a minor pathway of interaction (2%). Liposomal lysis was found to be associated with the process of liposomal binding (perhaps as a result of the binding itself). Lysis was followed by release of the entrapped carboxyfluorescein into the media and its subsequent uptake by the cells. This lysis was shown to be dependent upon discrete membrane-associated sites that have some of the properties of proteins. The results of these studies suggest that liposomal binding to the cells, subsequent lysis of the liposomes and cellular uptake of their contents should be seriously considered in all studies of liposome-cell interactions as an alternate mode of interaction to the four modes (fusion, endocytosis, adsorption and lipid exchange) previously emphasized in the literature.

Surface-replica topography of retinal pigment epithelium during phagocytosis

The initial step in retinal phagocytosis may be mediated by complementary recognition molecules on the pigment epithelial microvilli and outer segment membrane surfaces. These molecules may display a surface morphology which is related topographically to phagocytic events. In order to explore this, we have developed a method for replicating the membrane surfaces of rat pigment epithelial explants during phagocytosis of latex beads. The explants are fixed in a mixed aldehyde and osmium fixative and the basal surface glued to a coverslip. After dehydration and critical point drying, the tissue is replicated with platinum and carbon in a freeze-fracture apparatus using either rotary or unidirectional coating and the replicas are examined by transmission electron microscopy. The membrane surfaces of the pigment epithelial cells and their microvilli are studded with numerous surface particles varying in size from 20 to 50 nm that are closely packed and give a cobblestone appearance to the membrane topography. During the early stages of phagocytosis, microvillous processes appear to spread over the beads and form branched processes as they contact bead surfaces. During bead engulfment the microvilli have shortened into flattened sheets with interdigitating processes overlapping the beads. These uptake sites resemble Venus' fly-traps, which as they close over the beads create craters in the membrane surfaces of the pigment epithelium. The crater-like uptake sites appear to flatten out in the later stages and form flattened membrane domains which are surrounded by crenulated membrane that displays an irregular particulate morphology.

Effect of actin-binding protein on the sedimentation properties of actin

Actin and actin-binding protein (ABP) have recently been purified from human platelet cytoskeletons (S. Rosenberg, A. Stracher, and R.C. Lucas, 1981, J. Cell Biol. 91:201-211). Here, the effect of ABP on the sedimentation of actin was studied. When ABP was added to preformed F-actin filaments, it bound until a maximum ratio of 1:9 (ABP:actin, mol:mol) was reached. however, when actin was polymerized in the presence of ABP, two and a half times more ABP was able to bind to the actin- that is, every 3.4 actin monomers were now bound by an ABP dimer. ABP was not able to induce the sedimentation of actin under nonpolymerizing conditions but was able to reduce the time and concentration of actin required for sedimentation under slow polymerizing conditions. ABP, therefore, exerts its effect of G-actin by either nucleating polymerization or by cross-linking newly formed oligomers into a more sedimentable form.

Effect of various extraction solutions and thrombin activation on the composition of the platelet cytoskeleton

When human blood platelets were immersed in an ice-cold solution containing 1% Triton X-1200, 40 mM KCl, 10 mM EGTA, 10 mM imidazole-HCl, and 2 mM NaN3 pH 7.0, a flocculent precipitate appeared immediately in the tube. This precipitate was collected at 3,000g and SDS-polyacrylamide gel analysis showed it to consist mainly of actin, alpha-actinin, actin-binding protein (ABP), and varying amounts of myosin. Any modifications of this solution used to isolate the platelets' Triton-insoluble cytoskeleton caused profound changes in the nature of the cytoskeleton isolated. Increasing the KCl concentration resulted in a lower yield of cytoskeletal actin and ABP. Inclusion of EDTA in the solution resulted in an increased amount of myosin associated with the cytoskeleton, whereas including MgATP decreased the myosin yield. Experiments with the purified proteins showed that ABP and myosin can each protect the actin from depolymerizing when dialyzed into the Triton solubilization solution. In addition, it was found that when platelets were stimulated with thrombin for 2 min prior to the addition of the Triton solution, 3-4 times more myosin was associated with the cytoskeletal precipitate. The results suggest, therefore, that any variations in solution conditions used for isolating the cytoskeleton from resting platelets, which results in alterations in the amount of ABP, may have profound effects on the state of actin polymerization. Likewise, in thrombin-activated platelets, it is suggested that the increased association of myosin with the cytoskeleton results in a greater stabilization of the F-actin associated with the cytoskeleton. These factors must be considered when interpreting the results regarding the nature of actin transformations in the resting and activated platelet.

Modulation of phagosome-lysosome fusion in mouse macrophages

A previously described fluorescence assay has been used to characterize factors that modulate phagosome-lysosome (P-L) fusion in mouse macrophages. Fusion was not affected by enzymatic modification or by concanavalin A cross-linking of the plasma membrane or by coating the phagocytic particle with concanavalin A or immune serum. Pretreatment of cells with 10-5-10-4 M colchicine, or treatment immediately after ingestion with 1-10 microgram/ml cytochalasin did not alter P-L fusion; implying that the cytoskeleton does not control fusion in a rate-limiting way. Fusion was strikingly elevated in 5-h cultures of activated macrophages from immune-boosted mice. A lower enhancement was seen in cells activated by proteose-peptone, a nonspecific inflammatory agent.

Studies on the mechanism of membrane fusion. Role of head-group composition in calcium- and magnesium-induced fusion of mixed phospholipid vesicles

We have investigated the contribution of various phospholipids to membrane fusion induced by divalent cations. Fusion was followed by means of a new fluorescence assay monitoring the mixing of internal aqueous contents of large (0.1 micrometer diameter) unilamellar liposomes. The rate and extent of fusion induced by Ca2+ in mixed phosphatidylserine/phosphatidylcholine vesicles were lower compared to those in pure phosphatidylserine vesicles. The presence of 50% phosphatidylcholine completely inhibited fusion, although the vesicles aggregated upon Ca2+ addition. When phosphatidylserine was mixed with phosphatidylethanolamine, however, rapid fusion could be induced by Ca2+ even in mixtures that contained only 25% phosphatidylserine. Phosphatidylethanolamine also facilitated fusion by Mg2+ which could not fuse pure phosphatidylserine vesicles. In phosphatidylserine/phosphatidylethanolamine/phosphatidylcholine mixtures, in which the phosphatidylcholine content was kept at 25%, phosphatidylethanolamine could not substitute for phosphatidylserine, and the fusogenic capacity of Mg2+ was abolished by the presence of merely 10% phosphatidylcholine. The initial rate of release of vesicle contents was slower than the rate of fusion in all the mixtures used. The presence of phosphate effected a considerable decrease in the threshold concentration of Ca2+ and also enhanced the rate and extent of fusion. Mg2+ had a synergistic effect on Ca2+-induced fusion of phosphatidylserine/phosphatidylethanolamine vesicles. We suggest that the role of phospholipids in membrane fusion is related to their ability to form dehydrated intermembrane complexes with divalent cations.

A morphological study of plasma and phagosome membranes during endocytosis in Acanthamoeba

Particle ingestion by Acanthamoeba is rapid. Within 40 s bound particles can be surrounded by pseudopods, brought into the cytoplasm, and released as phagosomes into the cytoplasmic stream. In electron micrographs the phagosome appears as a flasklike invagination of the surface. Separation from the surface occurs by fragmentation of the attenuated "neck+ of the invagination. The separated phagosome membrane has a three- to fourfold greater density of intramembrane particles than the plasma membrane from which it derives. This change is evident within 15 min of ingestion and is detectable while the membrane is still tightly apposed to the particle. There is no direct evidence for the mechanism of this increase; no increase in particle density was seen in the membrane at an early stage in the forming phagosomes still connected to the surface. These morphological observations are consistent with chemical analyses, to be reported in a separate communication, that show that the phagosome membrane has a higher protein to phospholipid ratio and a higher glycosphingolipid content than the plasma membrane. Enlarged phagosomes (presumptive phagolysosomes) show multiple small vesiculations of characteristic morphology. The small vesicles are postulated to be the major route of membrane return to the cell surface.

Protein and glycoprotein components of phagosome membranes derived from mouse L cells

1. Sodium dodecylsulfate--polyacrylamide gel electrophoresis has been employed to analyze the protein and glycorprotein components of phagosome membranes prepared from mouse L cells by the polystyrene latex bead method. 2. These experiments showed that phagosome membranes contain at least 20 major membrane protein species having apparent molecular weights between 27,000 and 250,000; the most abundant proteins have molecular weights of 112,000, 103,000, 76,000, 68,000, 62,000, 42,000 and 36,000. 3. Phagosome membrane glycoproteins, which were detected on the gels by staining with periodic acid-Schiff's reagent, were found to migrate in two broad zones centered at positions on the gel corresponding to proteins of mol. wt 140,000 and 85,000. 4. Comparison of the phagosome membrane results with the results of similar experiments carried out with cell surface membranes revealed a high degree of similarity between the two. All major protein and glycoprotein components present in phagosome membranes could be identified in both of two types of cell surface membrane preparations analyzed. 5. These results strongly support the view that phagosome membranes contain a representative, not a highly selected, sample of surface membrane proteins and glycoproteins.

Analogous ultrastructure and surface properties during capping and phagocytosis in leukocytes

Ultrastructural analyses have revealed striking similarities between Concanavalin A capping and phagocytosis in leukocytes. Both processes involve extensive membrane movement to form a protuberance or pseudopods; a dense network of microfilaments is recruited into both the protuberance and the pseudopods; microtubules are disassembled either generally (capping) or in the local region of the pseudopods (phagocytosis); and cells generally depleted of microtubules by colchicine show polarized phagocytosis via the microfilament-rich protuberance rather than uniform peripheral ingestion of particles via individual pseudopods. Cap formation can thus be viewed as occurring as an exaggeration of the same ultrastructural events that mediate phagocytosis. Similar changes in cell surface topography also accompany capping and phagocytosis. Thus, in nonfixed cells, Concanavalin A-receptor complexes aggregate into the region of the protuberance in colchicine-treated leukocytes (conventional capping) or into the region of pseudopod formation in phagocytizing leukocytes. In the latter case, the movement of lectin-receptor complexes occurs from membrane overlying peripheral microtubules into filament-rich pseudopods that exclude microtubules. These data provide evidence against a role for microtubules as "anchors" for lectin receptors. Rather, they indicate a preferential movement of cell surface Concanavalin A-receptor complexes towards areas of extensive (the protuberance) or localized (pseudopods) microfilament concentration. In conventional capping, Concanavalin A must be added to the colchicine-treated cells before fixation in order to demonstrate movement of receptors from a diffuse distribution into the protuberance. However, Convanavalin A receptors are enriched in the membrane associated with phagocytic particles as compared to the remaining membrane. This particle-induced redistribution of receptors is particularly prominent in colchicine-treated cells that phagocytize and are then fixed and Concanavalin A labeled; both lectin receptors and beads are concentrated over the protuberance. Thus, the final analogy between conventionally capped and phagocytic cells is that in both cases the properties of the plasma membrane in regions of microfilament concentration are modified by Concanavalin A itself (capping) or by the phagocytized particle, to limit locally the diffusion of Concanavalin A receptors.

The effect of serum protein fractions on liposome-cell interactions in cultured cells and the perfused rat liver

We have studied the interactions of liposomes with human skin fibroblasts and mouse P815Y mastocytoma cells in culture, and the perfused rat liver, with the following findings. 1. In all the systems studied serum was found to cause an increase in the uptake of a [14C]cholesterol label into cells from anionic and neutral liposomes and a decrease in the uptake of the label from cationic liposomes. 2. Evidence suggests that albumin enhances the exchange/transfer of [14C]-cholesterol between liposomes and cultured cells. 3. With [14C]cholesterol in the liposome bilayer and [3H]methotrexate entrapped in the aqueous spaces of the liposome, the alpha- and beta-globulin fractions of serum decreased the transfer of both labels from cationic liposomes into cultured cells and the perfused rat liver. The beta-globulin fraction caused increased leakage of methotrexate from fluid liposomes of all charges. 4. The alpha- and beta-globulin fractions of serum appear to enhance the uptake of anionic liposomes into the perfused rat liver.

Cytochalasin B inhibits actin-related gelation of HeLa cell extracts

When the 100,000 g supernatant fraction (extract) of HeLa cells lysed in a buffer containing sucrose, ATP, DTE, EGTA, imidazole, and Triton X-100 is incubated at 25 degrees C, it gels, and actin and a HMWP are progressively enriched in the extract and in gel isolated from extract. CB (greater than or equal to 0.25 muM) inhibits gelation and specifically lowers the concentrations of actin and the HMWP in the fraction which sediments at 100,000 g after incubation. These results indicate that actin and HMWP are partly disaggregated by cytochalasin treatment, and thus that their aggregation is related gelation. Inasmuch as previous results showed that actin is present and HMWP is enriched in the plasma membrane fraction of HeLa cells, the results also point to a possible relation between plasma membrane-associated gel and in vivo effects of CB.

Lipid vesicle-cell interactions. I. Hemagglutination and hemolysis

The interaction of lipid vesicles (liposomes) of several different compositions with erythrocytes has been investigated. Lecithin liposomes, rendered positively charged with stearylamine, exhibit potent hemagglutination activity in media containing low concentrations of electrolytes. The hemagglutination titer is found to be a linear function of the zeta potential of the lipid vesicles. Hemagglutination is reduced when the surface potential of the cells is made more positive by pH adjustment or enzyme treatment. Similarly, hemagglutination is reduced by increasing concentrations of electrolytes. Hemagglutination is examined theoretically and is shown to be consistent with vesicle-cell interactions that are due to only electrostatic forces. Vesicles containing lysolecithin in addition to lecithin and stearylamine cause lysis of erythrocytes, provided the lipids of the vesicles are above the crystal-liquid crystal phase transition temperature. In addition, hemolysis requires close juxtaposition of the vesicle to the cell membrane; vesicles precoated with antibodies exhibit severely diminished hemolytic activities, only a small fraction of which can be attributed to a reduction in hemagglutination titer. Evidence is presented indicating that a single vesicle is sufficient to lyse one cell. With regard to hemagglutination and hemolysis, lipid vesicles of simple composition mimic paramyxoviruses such as Sendai virus.

Lipid vesicles as carriers for introducing materials into cultured cells: influence of vesicle lipid composition on mechanism(s) of vesicle incorporation into cells

The mechanisms involved in the uptake of uni- and multi-lamellar lipid vesicles by BALB/c mouse 3T3 cells have been investigated. Vesicles are incorporated into cells both by endocytosis and by a nonendocytotic mechanism which we propose involves fusion of vesicles with the plasma membrane. The nonendocytotic pathway predominates in the uptake of negatively charged vesicles composed of phospholipids that are "fluid" (phosphatidylserine/phosphatidylcholine) at 37 degrees. Neutral fluid vesicles (phosphatidylcholine) and negatively charged vesicles prepared from "solid" phospholipids (phosphatidylserine/distearylphosphatidylcholine/dipalmitoylphosphatidylcholine) are instead incorporated largely by endocytosis. Uptake of the latter classes of vesicle is reduced (80-90% inhibition) by inhibitors of cellular energy metabolism and by cytochalasin B.

The fluid double polar-nonpolar-polar leaflet model for biological membranes

A model for biological membranes is proposed according to which the plasma membrane consists of two functionally different polar-nonpolar-polar leaflets separated by a polar space. The binding of water-soluble proteins, integral lipoproteins and spanning proteins to a biological membrane as well as possible conformations of interphase peptides partitioned between polar and nonpolar layers are discussed. A model for the diffusion of water soluble proteins across nonpolar layers of a membrane is described. Three complete biological membranes containing two leaflets and an inter-leaflet space are defined. These are: 1: The inner nuclear membrane + the perinuclear space and the endoplasmatic cisternae + the outer nuclear membrane and the endoplasmatic reticulum, 2: the innner mitochondrial membrane + the mitochondrial intermembraneous space + the outer mitochondrial membrane and 3: the cytoplasmic leaflet of the plasma membrane + an intramembraneous space in the plasma membrane + the outer leaflet of the plasma membrane.

Interaction of phospholipid vesicles with cells. Endocytosis and fusion as alternate mechanisms for the uptake of lipid-soluble and water-soluble molecules

Depending on their phospholipid composition, liposomes are endocytosed by, or fuse with, the plasma membrane, of Acanthamoeba castellanii. Unilamellar egg lecithin vesicles are endocytosed by amoeba at 28 degrees C with equal uptake of the phospholipid bilayer and the contents of the internal aqueous space of the vesicles. Uptake is inhibited almost completely by incubation at 4 degrees C or in the presence of dinitrophenol. After uptake at 28 degrees C, the vesicle phospholipid can be visualized by electron microscope autoradiography within cytoplasmic vacuoles. In contrast, uptake of unilamellar dipalmitoyl lecithin vesicles and multilamellar dipalmitoyl lecithin liposomes is only partially inhibited at 4 degrees C, by dinitrophenol and by prior fixation of the amoebae with glutaraldehyde, each of which inhibits pinocytosis. Vesicle contents are taken up only about 40% as well as the phospholipid bilayer. Electron micrographs are compatible with the interpretation that dipalmitoyl lecithin vesicles fuse with the amoeba plasma membrane, adding their phospholipid to the cell surface, while their contents enter the cell cytoplasm. Dimyristoyl lecithin vesicles behave like egg lecithin vesicles while distearoyl lecithin vesicles behave like dipalmitoyl lecithin vesicles.