Membrane fusion

Spontaneous formation of multinucleated giant cells with bone resorbing activity by long culture of human peripheral blood CD14-positive monocytes in vitro

Several reports indicate that multinucleated giant cells that derived from human peripheral blood CD14-positive monocytes have osteoclastic properties, and although the mechanism is not completely understood, the authors have previously demonstrated that spontaneous osteoclastogenesis from monocytes can occur. Here, the authors investigated the effect of detachment and long-term cultures in this process. When monocytes were incubated for 2 weeks, spontaneous formation of polykaryocytes was rarely observed. In addition, when monocytes precultured for 2 weeks were detached by a cell scraper and further subcultured, almost all cells died. Surprisingly, when monocytes were incubated for 8 weeks without any pro-osteoclastogenic factors and without detachment, the authors observed the spontaneous formation of tartrate-resistant acid phosphatase (TRAP)-positive polykaryocytes that were able of lacunae resorption. These findings indicate that cell adhesion is a prerequisite for differentiation and survival of CD14-positive monocytes, and that a long incubation period spontaneously induces multinucleation and bone-resorbing activity of monocytes, even in the absence of osteoclastogenesis-stimulating factors.

Reticulocyte-secreted exosomes bind natural IgM antibodies: involvement of a ROS-activatable endosomal phospholipase iPLA2

Reticulocytes release small membrane vesicles termed exosomes during their maturation into erythrocytes. It has been suggested that reticulocytes remodel the plasma membrane of the immature red cell during erythropoiesis by specifically eliminating various proteins. We report here that exosome release is associated with a physiologic cascade induced by the expression of a 15-lipoxygenase at the reticulocyte stage. We found that the phospholipase iPLA2 specifically associated with the endosomal and exosomal membranes could be activated by reactive oxygen species (ROSs) produced during mitochondria degeneration induced by 15-lipoxygenase. Since iPLA2 has recently been demonstrated to participate in the clearance of apoptotic cells, we investigated its role in vesicle removal. We found that exosomes isolated directly from the blood of an anemic rat or released during in vitro maturation of rat reticulocytes bind IgM antibodies on their surface, in contrast to immature and mature red cells. These natural IgM antibodies recognize lysophosphatidylcholine and are able to specifically bind to apoptotic cells. Finally, evidence of C3 deposition on the exosome surface leads us to hypothesize that this cascade may favor the clearance of exosomes by cells once released into the bloodstream, via a mechanism similar to that involved in the elimination of apoptotic cells.

Reconstituted Proteolipid Vesicles Prepared from Mycoplasma fermentans Membranes Are Able to Bind and Fuse with Molt-3 Cells

We describe and characterize reconstituted proteolipid vesicles (rPLV) prepared from solubilized Mycoplasma fermentans membranes and studied their binding to and fusion with host Molt-3 cells. The rPLV were prepared following membrane solubilization by Triton X-100 and detergent removal by SM-2 resin beads. The vesicles thus obtained had a rather uniform diameter of about 1 microm and were sealed as monitored by measuring in an assay that measures the quenching by sodium dithionite of a hydrophobic fluorescent probe incorporated into the rPLV membrane. The rPLV adhered to Molt-3 cells and, based on measurements of lipid mixing, fused with the host cells at a similar rate and to about the same extent as intact M. fermentans. Preliminary experiments showed that a chimeric protein, GnRH-PE66, could be encapsulated within these rPLV, opening the way to develop a system for the transfer of high-molecular weight soluble molecules, encapsulated in the rPLV, to target eukaryotic cells.

Cell signaling through the protein kinases cAMP-dependent protein kinase, protein kinase Cepsilon, and RAF-1 regulates amphotropic murine leukemia virus envelope protein-induced syncytium formation

Amphotropic murine leukemia virus (A-MuLV) utilizes the PiT2 sodium-dependent phosphate transporter as its cell surface receptor to infect mammalian cells. The process of A-MuLV infection requires cleavage of the R peptide from the envelope protein. This occurs within virions thereby rendering them competent to fuse with target cells. Envelope proteins lacking the inhibitory R peptide (e.g. envelope (R-) proteins) induce viral envelope-mediated cell-cell fusion (syncytium). Here we have performed studies to determine if cell signaling through protein kinases is involved in the regulation of PiT2-mediated A-MuLV envelope (R-)-induced syncytium formation. Truncated A-MuLV retroviral envelope protein lacking the inhibitory R peptide (R-) was used to induce viral envelope-mediated cell-cell fusion. Signaling through cyclic AMP to activate PKA was found to inhibit envelope-induced cell-cell fusion, whereas treatment of cells with PKA inhibitors H89, KT5720, and PKA Catalpha siRNA all enhanced this cell fusion process. It was noted that activation of PKC, as well as overexpression of PKCepsilon, up-regulated A-MuLV envelope protein-induced cell-cell fusion, whereas exposure to PKC inhibitors and expression of a kinase-inactive dominant-negative mutant of PKCepsilon (K437R) inhibited syncytium formation. v-ras transformed NIH3T3 cells were highly susceptible to A-MuLV envelope-induced cell-cell fusion, whereas expression of a dominant-negative mutant of Ras (N17Ras) inhibited this cell fusion process. Importantly, activation of Raf-1 protein kinase also is required for A-MuLV envelope-induced syncytium formation. Expression of constitutively active BXB Raf supported, whereas expression of a dominant-negative mutant of Raf-1 (Raf301) blocked, A-MuLV-induced cell-cell fusion. These results indicate that specific cell signaling components are involved in regulating PiT2-mediated A-MuLV-induced cell-cell fusion. Selective pharmacological modulation of these signaling components may be an effective means of altering cell susceptibility to viral-mediated cytopathic effects.

The functional interaction between CD98 and CD147 in regulation of virus-induced cell fusion and osteoclast formation

Membrane fusion is an important event in the functioning of a living organism. Life starts as a sperm fuses with the membrane of an egg, leading to its fertilization. Membrane fusion is also required for myogenesis, osteogenesis and placenta formation. Multinucleated giant cells originating from monocytes-macrophages are associated with granulomatous lesions formed in response to foreign bodies, viruses, and bacteria. The CD4 molecule acts as a receptor for HIV. The major virus envelope glycoprotein, gp120, attaches to CD4 molecules expressed on the host cell surface. After binding to CD4 on the target cells, HIV is internalized via direct, pH-independent fusion of the viral and cell membranes. However, attachment of HIV to CD4 on the target cells is not sufficient for fusion. Interaction of gp160-expressing cells with neighboring cells bearing surface CD4 molecules is also required for syncytium formation. Syncytium formation and subsequent generalized cell fusion have been reported as potentially important mechanisms of virus-induced cytotoxic effects. Some antibodies against CD98/FRP-1 suppressed virus-induced cell fusion and CD98-mediated cell fusion of monocytes, indicating that CD98/FRP-1 molecules are able to regulate cell fusion. In this study, the functional interaction between CD98 and CD147 was investigated. Three kinds (Ab1, 2, and 3) of anti-CD147 and three kinds of anti-CD98 antibodies were used. Ab1 suppressed CD98-mediated cell fusion, but showed no effect on cell aggregation of Cd(+)U2ME-7 cells, U937-2 cells expressing HIV gp160. On the other hand, Ab2 enhanced the CD98-mediated cell fusion. Ab1 showed suppressive effect at early stage and Ab2 showed enhancing effect at later stage. Ab2 and 3 suppressed the spontaneous cell agglutination and cell fusion of Cd(+)JME-2 cells, Jurkat cells expressing HIV gp160. Ab2 suppressed CD98-mediated cell fusion, but showed no effect on cell aggregation of Cd(+)JME-2 cells. Ab2 cancelled suppression of cell fusion induced by suppressive antibody against CD98. Ab2 and 3 also suppressed CD98-mediated cell fusion of monocytes. This study indicates the functional interaction between CD98 and CD147 in the regulation of cell fusion.

Fusion of lamellar body with plasma membrane is driven by the dual action of annexin II tetramer and arachidonic acid

Annexin II has been implicated in membrane fusion during the exocytosis of lamellar bodies from alveolar epithelial type II cells. Most previous studies were based on the fusion assays by using model membranes. In the present study, we investigated annexin II-mediated membrane fusion by using isolated lamellar bodies and plasma membrane as determined by the relief of octadecyl rhodamine B (R18) self-quenching. Immunodepletion of annexin II from type II cell cytosol reduced its fusion activity. Purified annexin II tetramer (AIIt) induced the fusion of lamellar bodies with the plasma membrane in a dose-dependent manner. This fusion is Ca2+-dependent and is highly specific to AIIt because other annexins (I and II monomer, III, IV, V, and VI) were unable to induce the fusion. Modification of the different functional residues of AIIt by N-ethylmaleimide, nitric oxide, or peroxynitrite abolished AIIt-mediated fusion. Arachidonic acid enhanced AIIt-mediated fusion and reduced its Ca2+ requirement to an intracellularly achievable level. This effect is due to membrane-bound arachidonic acid, not free arachidonic acid. Other fatty acids including linolenic acid, palmitoleic acid, myristoleic acid, stearic acid, palmitic acid, and myristic acid had little effect. AIIt-mediated fusion was suppressed by the removal of arachidonic acid from lamellar body and plasma membrane using bovine serum albumin. The addition of arachidonic acid back to the arachidonic acid-depleted membranes restored its fusion activity. Our results suggest that the fusion between lamellar bodies with the plasma membrane is driven by the synergistic action of AIIt and arachidonic acid.

Association of cPLA2-alpha and COX-1 with the Golgi apparatus of A549 human lung epithelial cells

Cytosolic phospholipase A2-alpha (cPLA2-alpha) is an 85 kDa, Ca2+-sensitive enzyme involved in receptor-mediated prostaglandin synthesis. In airway epithelial cells, the release of prostaglandins is crucial in regulating the inflammatory response. Although prostaglandin release has been studied in various epithelial cell models, the subcellular location of cPLA2-alpha in these cells is unknown. Using high-resolution confocal microscopy of the human A549 lung epithelial cell line, we show that cPLA2-alpha relocates from the cytosol and nuclei to a juxtanuclear region following stimulation with the Ca2+ ionophore A23187. Double staining with rhodamine-conjugated wheat germ agglutinin confirmed this region to be the Golgi apparatus. Markers specific for Golgi subcompartments revealed that cPLA2-alpha is predominantly located at the trans-Golgi stack and the trans-Golgi network following elevation of cytosolic Ca2+. Furthermore, treatment of cells with the Golgi-disrupting agent brefeldin A caused a redistribution of cPLA2-alpha, confirming that cPLA2-alpha associates with Golgi-derived membranes. Finally, a specific co-localization of cPLA2-alpha with cyclooxygenase-1 but not cyclooxygenase-2 was evident at the Golgi apparatus. These results, combined with recent data on the role of PLA2 activity in maintaining Golgi structure and function, suggest that Golgi localization of cPLA2-alpha may be involved in membrane trafficking in epithelial cells.

Alternative to fluorescence assays to monitor fusion between Acholeplasma laidlawii cells and liposomes

AIMS

To develop a new technique as an alternative to the fluorescence assays and electron microscopy for the purpose of monitoring the cell-liposome fusion.

METHODS AND RESULTS

Acholeplasma laidlawii whole cells did not oxidize Glucose-6-phosphate (G6P) or Fructose-1,6 diphosphate (F1,6DP) as free (unentrapped) substrates, at concentrations 47 and >270 mM, respectively. Lysed A. laidlawii cells oxidized G6P and F1,6DP at lower concentration of 0.8 and 15 mM, respectively. When these substrates were entrapped inside liposomes, at a final concentration of 1.5 mM, and interacted with A. laidlawii whole cells, in an oxygen electrode chamber, an increase in oxygen uptake was evident. This interaction does not have any effect on cell viability.

SIGNIFICANCE AND IMPACT OF THE STUDY

The experimental system described here is advantageous over classical fluorescence assays in determining the fate of liposome-entrapped material and raises the possibility of studying the kinetics of metabolic substrates, which are normally excluded from the cell by the cell membrane.

Essential role of Ca2+ -dependent phospholipase A2 in estradiol-induced lysosome activation

The mechanism of lysosome activation by 17beta-estradiol has been studied in mussel blood cells. Cell treatment with estradiol induced a sustained increase of cytosolic free Ca2+ that was completely prevented by preincubating the cells with the Ca2+ chelator BAPTA-AM. Estradiol treatment was also followed by destabilization of the lysosomal membranes, as detected in terms of the lysosomes' increased permeability to neutral red. The effect of estradiol on lysosomes was almost completely prevented by preincubation with the inhibitor of cytosolic Ca2+ -dependent PLA2 (cPLA2), arachidonyl trifluoromethyl ketone (AACOCF3), and was significantly reduced by preincubation with BAPTA-AM. In contrast, it was virtually unaffected by preincubation with the inhibitor of Ca2+ -independent PLA2, (E)-6-(bromomethylene)tetrahydro-3-(1-naphtalenyl)-2H-pyran-2-one (BEL). The Ca2+ ionophore A-23187 yielded similar effects on [Ca2+](i) and lysosomes. Exposure to estradiol also resulted in cPLA2 translocation from cytosol to membranes, lysosome enlargement, and increased protein degradation. These results suggest that the destabilization of lysosomal membranes following cell exposure to estradiol occurs mainly through a Ca2+ -dependent mechanism involving activation of Ca2+ -dependent PLA2. This mechanism promotes lysosome fusion and catabolic activities and may mediate short-term estradiol effects.

Vesicles in the subacrosomal space and partial diaphragms in the subacrosomal nuclear envelope of round spermatids of a rat injected intravenously with gold labeled-testosterone-bovine serum albumin conjugate: vesicular trafficking from acrosome to nucleus

Colloidal gold labeled-testosterone-bovine serum albumin conjugate (testosterone-BSA-gold) injected into the vascular system of rats is taken up by endocytosis into round spermatids. Based on observation of silver deposits indicating testosterone-BSA-gold with silver enhancement, we have suggested that testosterone-BSA-gold enters the nuclei through not only the postacrosomal nuclear envelope but also the subacrosomal nuclear envelope (SNE) via the acrosome (Nishimura and Nakano, 1997). However, it was unclear how testosterone-BSA-gold in the acrosome entered the nucleoplasm. Spermatids showing silver deposits on the subacrosomal space were observed under electron microscope without silver enhancement, to clarify the courses of translocation. In the spermatids, vesicles with the gold particles were seen in the subacrosomal space. Some of the vesicles were in contact with the SNE. A part of the outer nuclear membrane projected into the space. Furthermore, local single-bilayer nuclear membranes, which seemed to partially lack nuclear lamina, were present in the SNE. These results indicate the possibility that the vesicles mediate the transport of testosterone-BSA-gold from acrosome to nucleus, and that the vesicle membrane fuses with not only the outer nuclear membrane but also a shared bilayer in the SNE.

Liposome-mediated DNA vaccination: the effect of vesicle composition

Liposome-entrapped DNA has been shown to enhance the potency of DNA vaccines, possibly by facilitating uptake of the plasmid by antigen-presenting cells (APC). In this paper, we have investigated the influence of the liposomal composition and surface charge on such potency. Plasmid DNA pRc/CMV HBS encoding the S (small) region of hepatitis B surface antigen was entrapped within cationic liposomes of various compositions and surface charges with high efficiency (88-97% of the amount used) by the dehydration-rehydration method that generates dehydration-rehydration vesicles (DRV). Cryo-electron microscopy revealed that DNA-containing DRV (DRV(DNA)) were multilamellar. In immunisation studies, female Balb/c mice were given two to four intramuscular injections of 10 microg naked or liposome-entrapped pRc/CMV HBS and bled at time intervals. Results indicate that the lipid composition of the DRV(DNA) influences the strength of the humoural response (immunoglobulin (Ig)G subclasses) with inclusion of dioleoyl phosphatidylethanolamine (DOPE) or phosphatidylethanolamine (PE) in the liposomal structure contributing to greater responses. DRV(DNA) in which the DOPE or PE were omitted or substituted with cholesterol led to significant reduction of humoural responses against the encoded antigen. Replacing phosphatidylcholine (PC) in the DRV(DNA) with the high-melting distearoyl phosphatidylcholine also contributed to lower responses. In other experiments, IgG responses were monitored in mice immunised with pRc/CMV HBS entrapped in DRV composed of PC and DOPE as before but incorporating increasing amounts of DOTAP (1-16 micromol). Maximal IgG responses were observed at 10 weeks after the first of four injections and suggested a trend of higher responses when 4 or 8 micromol DOTAP was present in the DRV(DNA) formulation. Cell-mediated immunity (measured in terms of endogenous antigen-specific splenic interferon-gamma) in mice immunised with pRc/CMV HBS entrapped in liposomes composed of PC, DOPE and DOTAP (16:8:4 molar ratio) was much greater than in animals treated with naked plasmid. These results indicate that liposome-mediated DNA immunisation is more effective than the use of naked DNA, and also suggest that the presence of fusogenic phosphatidylethanolamine in DRV in conjunction with a low-melting phosphatidylcholine and an appropriate content of cationic lipid might contribute to more effective liposomal DNA vaccines. The notion that liposomes improve immune responses to the plasmid-encoded vaccine by facilitating the latter's uptake by APC was supported by the observation that in Balb/c mice injected intramuscularly with liposome-entrapped pCMV. Enhanced green fluorescent protein, expression of the gene in terms of fluorescence intensity in the draining lymph nodes, was much greater than in animals treated with the naked plasmid.

Cross-talk between RANKL and FRP-1/CD98 Systems: RANKL-mediated osteoclastogenesis is suppressed by an inhibitory anti-CD98 heavy chain mAb and CD98-mediated osteoclastogenesis is suppressed by osteoclastogenesis inhibitory factor

The two pathways to osteoclastogenesis, RANKL-mediated and CD98-mediated osteoclastogenesis, have recently been reported. RANKL, OCIF, and TIMP-3 mRNAs are not found in monocytes freshly isolated or incubated with anti-FRP-1/CD98hc antibody. RANK, TACE, and M-CSF mRNAs can be detected in these cells. Interestingly, the expressed amount of RANK mRNA increases by cultivation of monocytes with anti-CD98hc antibody and maximal expression is observed in osteoclast-like cells. CD98-mediated cell aggregation and multinucleated giant cell formation are blocked by OCIF. OCIF also suppressed the CD98-mediated induction of Sp1 and c-src mRNAs in monocytes. Soluble RANK shows no effect on CD98-mediated cell aggregation and multinucleated giant cell formation. When blood monocytes were incubated with RANKL and M-CSF, c-src and Sp1 mRNAs were first found in blood monocytes incubated with these cytokines for 7 days. On the contrary, c-src mRNA could be detected 3 h after treatment of blood monocytes with anti-CD98hc mAb. LAT-1 mRNA was not found, and the expression levels of Y(+)LAT-1 and Y(+)LAT-2 mRNAs were not changed in monocytes stimulated without or with anti-CD98hc mAb or RANKL and M-CSF. An inhibitory mAb directed against CD98hc, HBJ 127, shows a suppressive effect on RANKL-mediated cell aggregation and cell fusion. Thus, there is cross-talk between these two pathways.

Aggregation and fusion of vesicles composed of N-palmitoyl derivatives of membrane phospholipids

N-Acylphosphatidylethanolamines and N-acylphosphatidylserines have been isolated from mammalian cells and have been associated with some tissue degenerative changes, although the relationship between their synthesis and the uncontrolled sequence of events that ends in irreversible tissue damage is not completely established. Our results show that monovalent and divalent cations induce aggregation and fusion of liposomes constituted by N-palmitoylphosphatidylethanolamine (NPPE) and N-palmitoylphosphatidylserine (NPPS). The effectiveness of cations to induce the aggregation of NPPE and NPPS liposomes is Ca2+ > Mg2+ >> Na+. NPPS liposomes aggregate at lower concentrations of divalent cations than NPPE liposomes, but with sodium NPPE liposomes aggregate to a higher extent than NPPS liposomes. The reaction order for the aggregation processes depends on the lipid and the cation nature and range from 1.04 to 1.64. Dynamic light scattering shows an irreversible increase of the size of the aggregates in the presence of all cations tested. The irreversibility of the aggregation process and the intermixing of bilayer lipids, as studied by resonance energy transfer assay, suggest that fusion, rather than aggregation, occurs. The existence of a real fusion was demonstrated by the coalescence of the aqueous contents of both NPPS and NPPE liposomes in the presence of either monovalent or divalent cations. The different binding sensitivity of Ca2+ to NPPS and NPPE liposomes, determined by zeta potential measurements, agrees with the results obtained in the aggregation and fusion assays. Our results suggest that the synthesis in vivo of N-acylated phospholipids can introduce important changes in membrane-mediated processes.

Ability of osteoclast formation from peripheral monocytes using anti-fusion regulatory protein-1/CD98/4F2 monoclonal antibodies in patients with osteoporosis

We investigated the difference in osteoclast formation between patients with osteoporosis and two healthy control groups by inducing it from peripheral blood monocytes with use of anti-fusion regulatory protein- monoclonal antibody. The group of patients with osteoporosis consisted of 35 women and excluded secondary osteoporosis, and the control groups consisted of 12 young healthy volunteers (control I) or 10 individuals age-matched to the patients with osteoporosis (control II). Osteoclast formation declined with age between the two control groups, but this decline was not significant. Fusion rate and the mean number of nuclei in osteoclasts were significantly less in the patients with osteoporosis than in the young or age-matched controls. It was clearly demonstrated that the ability of monocytes to fuse declines significantly in patients with osteoporosis.

Lipid vesicles and membrane fusion

Membrane fusion is essential for cell survival and has attracted a great deal of both theoretical and experimental interest. Fluorescence (de)quenching measurements were designed to distinguish between bilayermerging and vesicle-mixing. Theoretical studies and various microscopic and diffraction methods have elucidated the mechanism of membrane fusion. These have revealed that membrane proximity and high defect density in the adjacent bilayers are the only prerequisites for fusion. Intermediates, such as stalk or inverse micellar structures can, but need not, be involved in vesicle fusion. Nonlamellar phase creation is accompanied by massive membrane fusion although it is not a requirement for bilayer merging. Propensity for membrane fusion is increased by increasing the local membrane disorder as well by performing manipulations that bring bilayers closer together. Membrane rigidification and enlarged bilayer separation opposes this trend. Membrane fusion is promoted by defects created in the bilayer due to the vicinity of lipid phase transition, lateral phase separation or domain generation, high local membrane curvature, osmotic or electric stress in or on the membrane; the addition of amphiphats or macromolecules which insert themselves into the membrane, freezing or other mechanical membrane perturbation have similar effects. Lowering the water activity by the addition of water soluble polymers or by partial system dehydration invokes membrane aggregation and hence facilitates fusion; as does the membrane charge neutralization after proton or other ion binding to the lipids and intermembrane scaffolding by proteins or other macromolecules. The alignment of defect rich domains and polypeptides or protein binding is pluripotent: not only does it increase the number of proximal defects in the bilayers, it triggers the vesicle aggregation and is fusogenic. Exceptions are the bound molecules that create steric or electrical barriers between the membranes which prevent fusion. Membrane fusion can be non-leaky but it is very common to lose material from the vesicle interior during the later stages of membrane unification, that is, after a few hundred microseconds following the induction of fusion.

Gene expression during osteoclast-like cell formation induced by antifusion regulatory protein-1/CD98/4F2 monoclonal antibodies (MAbs): c-src is selectively induced by anti-FRP-1 MAb

Human blood monocytes can differentiate into osteoclast-like cells when they are cultured in the presence of anti-FRP-1. Messenger (mRNA) expression of markers related to osteoclasts was analyzed during differentiation of osteoclasts from monocytes. As markers related to osteoclasts, we selected cathepsin-K, carbonic anhydrase (CA) II, vacuolar H(+)-ATPase (v-ATPase), vitronectin receptor (VNR), tartrate-resistant acid phosphatase (TRAP), osteopontin (OPN), galectin-3, c-src, c-fos, and c-fms. The mRNAs other than c-src mRNA were expressed in freshly isolated monocytes or monocytes incubated with control antibody or anti-FRP-1 monoclonal antibody (MAb) for 14 days. Of these mRNAs, cathepsin-K, CA II, v-ATPase, VNR, TRAP, OPN, and c-fms mRNAs were expressed at higher levels in the osteoclast-like cells than those in monocytes cultured with control antibody. On the other hand, galectin-3 mRNA was expressed at lower levels in the osteoclast-like cells, and there was no significant difference in c-fos mRNA expression between the monocytes cultured with control antibody and anti-FRP-1 MAb. c-src mRNA could not be detected in monocytes freshly isolated or incubated with control antibody. Surprisingly, expression of c-src mRNA was induced in monocytes by anti-FRP-1 MAb and was detectable as early as 3 h after anti-FRP-1 MAb treatment, indicating that c-src is selectively induced by anti-FRP-1 MAb treatment. Furthermore, the osteoclast-like cells expressed calcitonin receptor. Receptor activator of NF-kappaB (RANK) mRNA was detectable in freshly isolated monocytes or monocytes cultured with control antibody or anti-FRP-1 MAbs. Maximal expression of RANK was observed in osteoclast-like cells. On the other hand, no receptor activator of NF-KB ligand (RANKL) mRNA was detectable in any of the samples, suggesting that anti-FRP-1 mAb can induce osteoclast-like cells from blood monocytes without RANKL.

Suppression of FRP-1/CD98-mediated multinucleated giant cell and osteoclast formation by an anti-FRP-1/CD98 mAb, HBJ 127, that inhibits c-src expression

When anti-CD98 mAb 6-1-13, 4-5-1, or 38-2-2 was added to the culture fluids of monocytes, extensive cell aggregation and polykaryocyte formation were induced. These multinucleated giant cells were tartrate-resistant acid phosphatase (TRAP) positive. On the other hand, when monocytes were incubated with another anti-CD98 mAb, HBJ 127, polykaryocyte formation was not detected, although extensive cell aggregation was induced. When HBJ 127 and 6-1-13 were simultaneously added to the culture fluids, anti-CD98 mAb-induced cell fusion was inhibited almost completely. HBJ 127 suppressed formation of 6-1-13-induced cell fusion in a dose-dependent manner. If, however, HBJ 127 was added after incubation of monocytes with mAb 6-1-13 for 6 h, an appreciable degree of TRAP-positive polykaryocyte formation was found. The bindings of 6-1-13 and HBJ 127 were not mutually competed. When monocytes were incubated with 6-1-13 or HBJ 127, 6-1-13 induced c-src mRNA, while HBJ 127 did not. Furthermore, when monocytes were incubated with both 6-1-13 and HBJ 127, c-src mRNA could not be detected, showing that HBJ 127 suppresses c-src expression. Therefore, CD98-mediated osteoclast formation can be regulated by modification of CD98 system.

N-stearoyl-phosphatidylserine: synthesis and role in divalent-cation-induced aggregation and fusion

N-Acylphosphatidylserines have been isolated from intact and injured tissues, but the participation of such acidic phospholipids in membrane aggregation and fusion has not been demonstrated. We have synthesized N-stearoylphosphatidylserine (NSPS) and examined divalent-cation-induced aggregation of NSPS-liposomes, which leads to membrane destabilization and fusion. The purified lipid was characterized by its chromatographic and spectroscopic (infrared and 1H nuclear magnetic resonance) properties and by its chemical degradation pattern. Aggregation of unilamellar NSPS-liposomes was studied as a function of calcium and magnesium concentration. The ability of calcium and magnesium to induce vesicle aggregation is higher for phosphatidylserine (PS)-liposomes (threshold concentration 1.5 mM for calcium and 4.6 mM for magnesium) than for NSPS-liposomes (threshold concentration 2.8 mM for calcium and 6.6 mM for magnesium). The irreversibility of the aggregation reactions after adding EDTA suggests that vesicle fusion might occur in the presence of calcium and magnesium. Preliminary studies, based on mixing of both lipid and internal aqueous contents, show that fusion rather than aggregation of NSPS-liposomes occurs in the presence of calcium ions. The tendency of NSPS-liposomes to aggregate at higher cation concentrations than PS-liposomes suggests that N-acylation of phosphatidylserine protects the membrane against degenerative damage caused by aggregation and fusion.

Cationic lipids, phosphatidylethanolamine and the intracellular delivery of polymeric, nucleic acid-based drugs (review)

Polymeric, nucleic acid drugs must be protected from endogenous nucleases and delivered to target cell nuclei in order to maximize their activity. Constructs expressing therapeutic genes, antisense oligonucleotides and ribozymes can be delivered into cells by viral vectors, but concerns over safety and clinical utility have led to research into the development of alternative, non-viral delivery systems. Antisense and ribozyme drug development has focused upon modifications to the natural oligonucleotide chemistry which make the molecules resistant to nuclease degradation. These novel oligonucleotides cannot be generated by transgenes and must be administered in similar fashion to conventional drugs. However, oligonucleotides cannot cross membranes by passive diffusion and intracellular delivery for these drugs is very inefficient. Here we review the recent advances in forming lipid-DNA particles designed to mimic viral delivery of DNA. Most evidence now supports the hypothesis that lipid-DNA drugs enter target cells by endocytosis and disrupt the endosomal membrane, releasing nucleic acid into the cytoplasm. The mechanisms of particle formation and endosome disruption are not well understood. Cationic lipids are employed to provide an electrostatic interaction between the lipid carrier and polyanionic nucleic acids, and they are critical for efficient packaging of the drugs into a form suitable for systemic administration. However, their role in endosome disruption and other aspects of successful delivery leading to gene expression or inhibition of mRNA translation are less clear. We discuss the propensity of lipid-nucleic acid particles to undergo lipid mixing and fusion with adjacent membranes, and how phosphatidylethanolamine and other lipids may act as factors capable of disrupting bilayer structure and the endosomal pathway. Finally, we consider the challenges that remain in bringing nucleic acid based drugs into the realm of clinical reality.

Induction of human osteoclast-like cells by treatment of blood monocytes with anti-fusion regulatory protein-1/CD98 monoclonal antibodies

We have developed a new and simple system of human osteoclast formation by fusing peripheral blood monocytes with anti-Fusion Regulatory Protein-1 (anti-FRP-1) monoclonal antibody (mAb). When human blood monocytes were cultured in the presence of anti-FRP-1/CD98 mAbs, polykaryocytes began to appear at approximately 15 h and increased in size with time until 3-4 days of incubation with anti-FRP-1 mAb. These fused cells showed positive staining in tartrate-resistant acid phosphatase, possessed numerous calcitonin receptors, and were capable of bone resorption. These results strongly suggest that anti-FRP-1 antibody-induced multinucleated cells are osteoclasts. Furthermore, FRP-1 antigens were detected in osteoclasts isolated from human bone and in the osteoclast-like cells obtained from human giant cell tumors of bone.

Characterization of phospholipase A2 activity in reticulocyte endocytic vesicles

Electron spin resonance spectroscopy was used to investigate the presence of phospholipase A2 activity in endocytic vesicles prepared from reticulocytes and to define some of its characteristics. Using spin-labeled phospholipid analogues, we measured the hydrolysis rate of the ester bond at position 2 during incubation with reticulocyte endocytic vesicles. We have shown that this phospholipase A2 activity was membrane-associated, enriched in endocytic vesicles as compared to cytosol and plasma membrane. Enzymic activity was also observed in exosomes, vesicles coming from the endocytic compartment and released by reticulocytes during their maturation in erythrocytes. Neither the hydrolytic activity nor the membrane association was found to be Ca(2+)-dependent. Spin-labeled phospholipids with choline and serine polar heads were better substrates than glycerophosphoethanolamine analogues. Optimal pH was found to be close to neutral. 5,5'-Dithiobis(2-nitrobenzoic acid) and diisopropyl fluorophosphate very efficiently inhibited spin-labeled phospholipid hydrolysis. This phospholipase A2 activity was confirmed using a radioactive assay, although with much lower sensitivity. (E)-6-(Bromomethylene)-tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one, a specific mechanism-based inhibitor of calcium-independent phospholipases A2, was found to abolish the enzymic activity present in endocytic vesicles.

Monoclonal antibodies which recognize equatorial segment epitopes presented de novo following the A23187-induced acrosome reaction of guinea pig sperm

Acrosome-intact mammalian sperm can adhere to zona pellucida-free oocytes but are only capable of fusing if they have previously undergone the acrosome reaction. This suggests that the acrosome reaction results in presentation of at least one novel epitope which plays a role in sperm-oocyte fusion. Monoclonal antibodies were raised against unfixed acrosome-reacted guinea pig sperm and screened by indirect immunofluorescence for binding to the equatorial segment. They were back-screened against unfixed acrosome-intact sperm for absence of binding. Using this approach, two antibodies, G11 and M13, were identified which detect equatorial segment epitopes presented de novo by sperm following an A23187-induced acrosome reaction. The localization of these epitopes to the equatorial segment was confirmed at the ultrastructural level by indirect immunogold-labelling. Fluorescein isothiocyanate-labelled Fab fragments of these two antibodies also localized to the equatorial segment. Affinity chromatography and western blotting established that the two mAbs recognize the same proteins, which have M(r)s of 34, 46, 48 and 51 × 10(3). When sperm were induced to undergo the acrosome reaction with A23187 and incubated with their discharged acrosomal contents, a further band was produced with an M(r) of 30 × 10(3). Production of this band was inhibited in the combined presence of 100 microM phenylmethylsulphonyl fluoride and 100 microM p-aminobenzamidine even though these compounds do not inhibit acrosomal exocytosis. Neuraminidase and O-glycosidase were without effect on the proteins detected by antibodies G11 and M13. Endoglycosidase F, however, eliminated the bands of M(r) 46, 48 and 51 × 10(3) and replaced them with a strong band of M(r) 44 × 10(3) and two minor bands of M(r) 43 and 45 × 10(3). Formaldehyde fixation of acrosome-intact sperm caused partial rupture of the acrosome with loss of the characteristic rouleaux (stacks) of guinea pig sperm. Indirect labelling of these formaldehyde-fixed sperm with fluorescein isothiocyanate- or gold-labelled second antibody, with or without permeabilization with 0.05% Triton X-100, showed dense labelling on the cytoplasmic face of the plasma membrane overlying the convex surface of the acrosome but little labelling elsewhere. Cryosections of acrosome-intact sperm labelled indirectly with immuno-gold showed labelling consistent with the same location, as well as sporadic labelling at other intracellular sites overlying the acrosome. Since there is no evidence that sperm can translocate intact membrane protein from the cytoplasmic face to the extracellular face of the plasma membrane during the acrosome reaction, the evidence suggests that there are two isolated antigen pools.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced surface area in apoptotic rounding of human Chang liver cells from serum deprivation

BACKGROUND

The early stages of apoptosis (programmed cell death) are said to be characterized by internucleosomal DNA fragmentation and "condensation of the cytoplasm" in which cells round up, detach, and increase in density. We studied the causation of apoptotic rounding.

METHODS

Human Chang liver cells in normal monolayer culture were compared with apoptotic counterparts derived from serum growth factor deprivation. Cell-by-cell analysis using the Coulter EPICS PROFILE II flow cytometer studied 1) the cell cycle from propidium iodide-DNA bindings, 2) uptake of neutral red (NR) dye, a viable cell marker, and 3) cytosolic pH (pHi) modulations from 2',7'-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF) fluorescence ratios with NH4Cl prepulsing and forward scatter bitmapping of cell surface area. Morphometric studies were done in the Quantimet 570 image analyser. Uptake of trypan blue, neutral red, and 2 million mol.wt fluoresceinated dextrans was studied by light microscopy. Cytological profiles were examined in light microscopy and transmission and scanning electron microscopy.

RESULTS

Three days of serum growth factor deprivation caused confluent flat substrate-attached cells to retract and round up, tethering tenuously to the substrate via thin microvillus attachments only. Ninety percent of cell surface area was lost with this flat-to-round change. There was high trypan blue staining with total loss of proliferative potential, and the entire genome was just fragmented DNA making up the solitary Ao (apoptotic) peak in cell cycle profiles. However, these rounded apoptotic cells also internalized huge 2 million mol.wt dextran particles and impermeant neutral red which is an established viable cell marker. The rounded apoptotic cells had an intensely acidic (pH 5.6) cytosol and therefore a steep [H+]i/[H+]o gradient promoting proton extrusion. The pHi upshifted dynamically upon acidification, recovering and even exceeding resting level by a whole pH unit. Surface area reduction occurred concomitantly in real time with pHi upshifts in these apoptotic cells. Acidification and recovery in apoptotic cells also produced enhanced uptake of neutral red. Cytological profiles showed abundant large endocytic channels and endosomes in the rounded apoptotic cells.

CONCLUSIONS

Gross surface area reduction with evidence of distinctive endocytic activity including uptake of huge 2 million mol.wt dextran particles suggested large channel endocytic internalization as a causal factor in apoptotic rounding, in common with rounding in M-phase and interphase cells with pHi upshifting where concomitant surface area reduction and uptake of impermeant particles were similarly demonstrable. The reduction in size of the cell envelope, together with consequential concentration pressures, could account for the observed rise in cell density and shrinkage in cell size. As a symptom of continual pHi upshifting, apoptotic rounding appears to be a recovery-associated response rather than a direct consequence of the disruptive forces causing its death.

In vitro and in vivo 13C and 31P NMR analyses of phosphocholine metabolism in rat glioma cells

In vivo magnetic resonance spectroscopy (MRS) has revealed that phosphomonoesters (PME) such as phosphocholine (PCho) and phosphoethanolamine (PEth) are elevated in tumors and rapidly proliferating tissues. The regulation of PME levels and their relationship to proliferation are not well known. In the present study, we investigated the regulation of PCho and PEth levels in rat glioma cells grown in vivo and in vitro using 31P and 13C MRS. However, the ability of cells to produce choline endogenously is variable. To fully understand regulation of PCho levels, it is necessary to characterize the activity of the endogenous pathway, if it exists. This was first investigated by following the metabolic fate of 13C-labeled methionine of 9L glioma tumors in vivo. Our results indicate that there is a significant amount of de novo choline synthesis in vivo. However, similar experiments performed in vitro using cells cultured in bioreactors indicated that glioma cells themselves are unable to synthesize choline de novo, suggesting that the in vivo results were due to the involvement of extra-tumoral organs, e.g., liver. Further in vitro experiments demonstrated that the uptake and phosphorylation of physiologically relevant concentrations of exogenous choline is very active in these systems. Thus, it appears that the exogenous pathway for PCho biosynthesis predominates and regulates PCho levels in glioma cells. Our results also demonstrate that PCho levels are lowest, and PEth levels are highest, in non-proliferating cells. These observations indicate that there is a decrease in the biosynthesis of PCho concomitant with a reduction in culture growth. The source of the increased PEth is, as yet, undefined.

Structure and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine bilayer membranes

The structural and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine (C(18):C(2)-PC) were studied as a function of hydration. A combination of differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the phase behavior of C(18):C(2)-PC. At low hydration (e.g., 20% H2O), the differential scanning calorimetry heating curve shows a single reversible endothermic transition at 44.6 degrees C with transition enthalpy delta H = 6.4 kcal/mol. The x-ray diffraction pattern at -8 degrees C shows a lamellar structure with a small bilayer periodicity d = 46.3 A and two wide angle reflections at 4.3 and 3.95 A, characteristic of a tilted chain, L beta' bilayer gel structure. Above the main transition temperature, a liquid crystalline L alpha phase is observed with d = 53.3 A. Electron density profiles at 20% hydration suggest that C(18):C(2)-PC forms a fully interdigitated bilayer at -8 degrees C and a noninterdigitated, liquid crystalline phase above its transition temperature (T > Tm). Between 30 and 50% hydration, on heating C(18):C(2)-PC converts from a highly ordered, fully interdigitated gel phase (L beta') to a less ordered, interdigitated gel phase (L beta), which on further heating converts to a noninterdigitated liquid crystalline L alpha phase. However, the fully hydrated (> 60% H2O) C(18):C(2)-PC, after incubation at 0 degrees C, displays three endothermic transitions at 8.9 degrees C (transition I, delta H = 1.6 kcal/mol), 18.0 degrees C (transition II), and 20.1 degrees C (transition III, delta HII+III = 4.8 kcal/mol). X-ray diffraction at -8 degrees C again showed a lamellar gel phase (L beta') with a small periodicity d = 52.3 A. At 14 degrees C a less ordered, lamellar gel phase (L beta) is observed with d = 60.5 A. However, above the transition III, a broad, diffuse reflection is observed at approximately 39 A, consistent with the presence of a micellar phase. The following scheme is proposed for structural changes of fully hydrated C(18):C(2)-PC, occurring with temperature: L beta' (interdigitated)-->L beta (interdigitated)-->L alpha(noninterdigitated)-->Micelles. Thus, at low temperature C(18):C(2)-PC forms a bilayer gel phase (L beta') at all hydrations, whereas above the main transition temperature it forms a bilayer liquid crystalline phase L alpha at low hydrations and a micellar phase at high hydrations (> 60 wt% water).

Intermembrane lipid transfer during Trypanosoma cruzi-induced erythrocyte membrane destabilization

The ability of Trypanosoma cruzi to induce erythrocyte membrane destabilization in vitro was studied. Epimastigote forms adhered to human erythrocytes and caused fusion or lysis of the red cells, depending on the conditions of the interaction. Red cells were fused in the presence of calcium, while haemolysis was induced in the absence of the cation. Dextran 60 C facilitated fusion but delayed lysis. Optimum pH and temperature for fusion were 7.4 and 37 degrees C, respectively. Lipid alterations were produced in the plasma membrane of the red cell during the interaction with the parasite. A Ca(2+)-independent increase of lysophospholipids and free fatty acids was common to both the lysis and fusion processes. A relative increase of 1,2-diacylglycerides was unique to the fusion process and these changes were dependent on Ca2+. The transfer of free fatty acids and lysophospholipids from T. cruzi to erythrocyte membranes was demonstrated using parasites pre-labelled with radioactive phospholipids. Pre-treatment of parasites with exogenous phospholipase A2 abolished the fusogenicity, while lysis was increased. Neither fusion nor haemolysis occurred when the parasites were pre-treated with fatty acid free albumin, phospholipase A2 inhibitors or when these compounds were present in the medium during the parasite-erythrocyte interaction. Our results suggest that T. cruzi induces erythrocyte membrane destabilization in vitro by transfer of lipid material in a calcium independent manner and that this ion is necessary for other membrane alterations that lead to erythrocyte fusion.

Direct analysis and significance of cardiolipin transverse distribution in mitochondrial inner membranes

The distribution of cardiolipin across the inner mitochondrial membrane was directly determined by using the ability of the fluorescent dye 10-N-nonyl-3,6-bis(dimethylamino)acridine (10-N-nonyl acridine orange) to form dimers when it interacts with the diacidic phospholipid. Two independent methods were employed: (a) a spectrophotometric measurement of 10-N-nonyl acridine orange binding to isolated rat liver mitochondria, mitoplasts and inside-out submitochondrial particles, and (b) a flow-cytometric analysis of specific red fluorescence, emitted when two dye molecules are bound to one membrane cardiolipin; the stoichiometry of 10-N-nonyl acridine orange binding to phosphatidylserine and phosphatidylinositol, 1 mol dye/mol phospholipid, prevented dye dimerisation and subsequent red-fluorescence appearance. 57% total cardiolipin was present in the outer leaflets of inner membranes of isolated organelles, a distribution confirmed by saturation measurements for mitoplasts and inside-out submitochondrial particles. The same asymmetry was directly observed in situ with mitochondrial membranes of quiescent L1210 cells, and with mitochondrial membranes of respiring yeasts. Nevertheless, alterations in ATP synthesis and inhibition of mitochondrial protein synthesis revealed that cardiolipin distribution was apparently tightly correlated with mitochondrial membrane assembly and activity.

Patch clamp studies of single intact secretory granules

The membrane of secretory granules is involved in the molecular events that cause exocytotic fusion. Several of the proteins that have been purified from the membrane of secretory granules form ion channels when they are reconstituted in lipid bilayers and, therefore, have been thought to form part of the molecular structure of the exocytotic fusion pore. We have used the patch clamp technique to study ion conductances in single isolated secretory granules from beige mouse mast cells. We found that the membrane of the intact granule had a conductance of < 50 pS. No abrupt changes in current corresponding to the opening and closing of ion channels were observed, even under conditions where exocytotic fusion occurred. However, mechanical tension or a large voltage pulse caused the breakdown of the granule membrane resulting in the abrupt opening of a pore with an ion conductance of about 1 nS that fluctuated rapidly and could expand to an immeasurably large conductance or close completely. Surprisingly, the behavior of these pores resembled the pattern of conductance changes of exocytotic fusion pores observed in degranulating beige mast cells. This similarity supports the view that the earliest fusion pore is formed upon the breakdown of a bilayer such as that formed during hemifusion.

Inhibition of endosome fusion by phospholipase A2 (PLA2) inhibitors points to a role for PLA2 in endocytosis

Fusion of intracellular membrane-bound compartments is a common step in the transport of macromolecules along the endocytic and secretory pathways. A large number of factors active in the fusion process or its regulation have been identified; however, the actual sequence of events leading to membrane fusion is still unknown. In this study, we have assessed a possible role for PLA2 in endosome fusion by using an in vitro reconstitution assay and by examining endocytosis in intact cells. Several PLA2 inhibitors blocked endosome fusion in a broken-cell preparation. Inhibition was reversed by addition of arachidonic acid. At the electron microscope level, endosome clusters were observed even in the presence of inhibitors; however, actual fusion between endosomes was largely reduced. Fusion frequency increased upon the addition of arachidonic acid. A membrane-permeable PLA2 inhibitor blocked mixing of ligands internalized sequentially but did not affect internalization. The results indicate that vesicle fusion along the endocytic pathway requires a PLA2 activity. The effect of this activity would be, at least in part, mediated by arachidonic acid release.

Fusion of Spiroplasma floricola cells with small unilamellar vesicles is dependent on the age of the culture

Small unilamellar vesicles were labeled with the fluorescent probe octadecylrhodamine B chloride and mixed with intact Spiroplasma floricola cells. The increase in fluorescence observed was interpreted as a result of the dilution of the probe in the unlabeled S. floricola membranes because of lipid mixing upon fusion. The progression of S. floricola cultures to the stationary phase of growth was accompanied by a sharp decrease in the ability of the cells to fuse with small unilamellar vesicles. Low fusogenic activity was also detected in cells from cultures that were aged in a growth medium maintained at pH 7.5 throughout the growth cycle. Chemical analysis of the cell membrane preparations isolated from cells harvested at the various phases of growth revealed that the phospholipid content and composition and the cholesterol/phospholipid molar ratio were changed very little upon aging of the cultures. Likewise, no changes in the fatty acid composition of membrane lipids were detected, with palmitic and oleic acids predominating throughout the cycle. Nonetheless, upon aging of S. floricola cultures, a pronounced increase in the levels of both cholesteryl esters, incorporated from the growth medium, and organic peroxides was observed. A decrease in both fluorescence anisotropy of diphenylhexatriene and merocyanine 540 binding to membranes of aged cells was also detected. The possible influence of these changes on the fusogenic activity of the cells is discussed.

Phospholipids in animal eukaryotic membranes: transverse asymmetry and movement

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Fusion activity of an amphiphilic polypeptide having acidic amino acid residues: generation of fusion activity by alpha-helix formation and charge neutralization

A sequential polypeptide, poly(Glu-Aib-Leu-Aib) (Aib represents 2-aminoisobutyric acid), was synthesized and the pH-dependence of fusogenic activity of the polypeptide was studied. The polypeptide was designed to take amphiphilic structure upon the formation of alpha-helix. Circular dichroism spectra of the polypeptide showed a negative Cotton effect with double minima, indicative of an alpha-helical conformation. The alpha-helix content was increased with lowering pH and/or increasing the ionic strength. It was found that the polypeptide induces remarkable leakage of calcein from egg-yolk phosphatidylcholine (EYPC) vesicles loaded in the inner aqueous phase with lowering pH and/or increasing ionic strength. The polypeptide caused fusion of EYPC liposomes and dipalmitoylphosphatidylcholine liposomes more strongly with decreasing pH. Moreover, two distinct increases of fusogenic activity of the polypeptide were observed near pH 6.0 and below pH 4.5. The former corresponds to the midpoint of pH-dependent change in helical content of the polypeptide and the latter the pKa of the gamma-carboxyl group of glutamic acid. These results indicate that elevation of the fusogenic activity of the polypeptide is related to the increase in two factors, alpha-helix content and hydrophobicity.

Cholesterol is required for the fusion of single unilamellar vesicles with Mycoplasma capricolum

Small unilamellar vesicles (SUV) were prepared from the total lipid extract of Mycoplasma capricolum. The SUV were labeled with the fluorescent probe octadecylrhodamine B chloride (R18) to a level at which the R18 fluorescence was self-quenched. At pH 7.4 and 37 degrees C, and in the presence of 5% polyethylene glycol, an increase in the R18 fluorescence with time was observed when the R18-labeled SUV were introduced to a native M. capricolum cell suspension. The fluorescence dequenching resulting from dilution of the R18 into the unlabeled membranes of M. capricolum, was interpreted as a result of lipid mixing during fusion between the SUV and the mycoplasma cells. The presence of cholesterol in the SUV was found to be obligatory to allow SUV-mycoplasma fusion to occur. Adaptation of M. capricolum cells to grow in a medium containing low cholesterol concentration provided cells in which the unesterified cholesterol content was as low as 17 micrograms/mg cell protein. The fusion activity of the adapted cells was very low or nonexistent. Nonetheless, when an early exponential phase culture of the adapted cells was transferred to a cholesterol-rich medium, the cells accumulated cholesterol and regained their fusogenic activity. The cholesterol requirement for fusion in the target mycoplasma membrane was met by a variety of planar sterols having a free beta-hydroxyl group, but differing in the aliphatic side chain, e.g., beta-sitosterol or ergosterol, even though these sterols, having a bulky side chain, are preferentially localized in the outer leaflet of the lipid bilayer. It is suggested that the role of cholesterol in mycoplasma-SUV fusion is not at the level of bulk bilayer viscosity but rather, affecting local lipid-lipid or lipid-protein interactions that are relevant to the fusion event.

Reduced surface area in mitotic rounding of human Chang liver cells

The rounding up of mitotic human Chang liver cells in monolayer culture was studied quantitatively. It was surprising to find significant reduction in cell surface area considering that endocytosis has been demonstrated to be at a complete standstill in M phase. Uptake studies using impermeant BCECF (2',7'-bis(2-carboxyethyl)-4(5)-carboxyfluorescein free acid) pH indicator and particulate neutral red dye in aqueous buffer showed preferential internalization into mitotic cells in direct contrast to expectation since interphase cells do not have arrested endocytosis. However, infolded plasma membrane ruffles and internalized extracellular material were demonstrated in prophase cells, much like those seen in interphase rounding via the induction of intracellular alkalinizations. Raised intracellular pH (pHi) is a universal and consistent finding in M phase cells. Despite cessation of small pit endocytosis, it remains possible for plasma membrane internalization to be a causal factor in the observed surface area reduction in mitotic rounding.

Membrane fusion

Common themes are emerging from the study of viral, cell-cell, intracellular, and liposome fusion. Viral and cellular membrane fusion events are mediated by fusion proteins or fusion machines. Viral fusion proteins share important characteristics, notably a fusion peptide within a transmembrane-anchored polypeptide chain. At least one protein involved in a cell-cell fusion reaction resembles viral fusion proteins. Components of intracellular fusion machines are utilized in multiple membrane trafficking events and are conserved through evolution. Fusion pores develop during and intracellular fusion events suggesting similar mechanisms for many, if not all, fusion events.

The exocytotic fusion pore modeled as a lipidic pore

Freeze-fracture electron micrographs from degranulating cells show that the lumen of the secretory granule is connected to the extracellular compartment via large (20 to 150 nm diameter) aqueous pores. These exocytotic fusion pores appear to be made up of a highly curved bilayer that spans the plasma and granule membranes. Conductance measurements, using the patch-clamp technique, have been used to study the fusion pore from the instant it conducts ions. These measurements reveal the presence of early fusion pores that are much smaller than those observed in electron micrographs. Early fusion pores open abruptly, fluctuate, and then either expand irreversibly or close. The molecular structure of these early fusion pores is unknown. In the simplest extremes, these early fusion pores could be either ion channel like protein pores or lipidic pores. Here, we explored the latter possibility, namely that of the early exocytotic fusion pore modeled as a lipid-lined pore whose free energy was composed of curvature elastic energy and work done by tension. Like early exocytotic fusion pores, we found that these lipidic pores could open abruptly, fluctuate, and expand irreversibly. Closure of these lipidic pores could be caused by slight changes in lipid composition. Conductance distributions for stable lipidic pores matched those of exocytotic fusion pores. These findings demonstrate that lipidic pores can exhibit the properties of exocytotic fusion pores, thus providing an alternate framework with which to understand and interpret exocytotic fusion pore data.

Human Chang liver cells show large surface openings and endocytic channels that resemble those in the amoeba

Cytolysis of host cells by pathogenic Entamoeba histolytica can be blocked by specific lysozyme inhibitions and is recently reported to be enhanced by phosphoinositide (PI) signal transduction activation. However the mechanistic relationship between PI second messenger targets and massive lysosomal secretion needed to achieve rapid host cell lysis is unclear. We have previously shown that intracellular alkalinization associated with activated PI hydrolysis produces a massive endocytosis of huge proportions which would force a corresponding exocytosis for the maintenance of overall cell dimensions. These endosomes are processed by primary lysosomes. Apparently then, the massive exocytosis secretory pathway could provide the means for the ejection of lysozymes over target cells. We show here using human Chang liver cells that intracellular alkalinization produced large surface pittings similar to those seen in pathogenic E. histolytica in a rounded state. The SEM profile is correlated with the TEM profile of large endosomes containing extracellular debris and endosomes associated with primary lysosomal vesicles, which could support the notion that some of the pittings seen in the rounded Chang cells and the pathogenic amoebae are exit portals for endosome-lysosomes.

Freeze-fracture analysis of the effects of intermediates of the phosphatidylinositol cycle on fusion of rough endoplasmic reticulum membranes

While searching for the identity of the effector of the putative GTP-binding protein involved in fusion of rough endoplasmic reticulum (RER) cell-free incubation conditions were found permitting fusion in a GTP-independent manner. Membrane fusion was obtained using medium required to study synthesis of phosphatidylinositol (PI). We now report on the effects of various co-factors and intermediates of the PI cycle on the interaction of rough microsomes. By freeze-fracture, fusion of rough microsomes was defined as the appearance of fracture-planes of membrane larger than those of unincubated membrane. Cytosine triphosphate (CTP, 3 mM) in the presence of 2 mM MnCl2 was most effective in stimulating fusion. Guanosine triphosphate (GTP) at the same concentration, could substitute for CTP to stimulate fusion, ATP, ITP, UTP and guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) could not. When combined together in the same medium CTP potentiated the effect of GTP. Arachidonic acid (20 micrograms/ml) also stimulated fusion in the presence of MnCl2. This led to the appearance of large fracture-planes of membrane with a heterogeneous distribution of intramembranous particles. Other saturated fatty acids at the same concentration did not stimulate fusion. Phosphatidylinositol (PI, 50 micrograms) and 2 mM MnCl2 had a similar effect as arachidonic acid and MnCl2 in stimulating fusion. The PI effect was largely augmented in the presence of CTP. Our results are consistent with the concept that metabolism of phospholipids may modulate GTP-dependent fusion of RER membranes.

Differential susceptibility of phosphatidylcholine small unilamellar vesicles to phospholipases A2, C and D in the presence of membrane active peptides

Activities of phospholipases C and D along with A2 were followed on egg phosphatidylcholine small unilamellar vesicles in the presence of membrane active peptides melittin, gramicidin S and alamethicin. Decrease in the activity of phospholipase C and D and enhancement of phospholipases A2 activity suggest that these enzymes are sensitive to alterations in the lipid packing in the membranes in the presence of these peptides. Phospholipase C and D, which have not been used to study peptide--membrane interactions, have potential use in studying membrane perturbations, since their activities are very sensitive to lipid packing.

Phospholipase C activity-induced fusion of pure lipid model membranes. A freeze fracture study

The structural effects of in situ production of diacylglycerol by phospholipase C in pure lipid model membranes have been examined by freeze fracture electron microscopy. Phospholipase C-activity induces massive aggregation and fusion of large unilamellar lipid vesicles and leads to the formation of a 'sealed' lipid aggregate; the outer membrane of this aggregate appears to be continuous. In some areas lipid arranges into a honeycomb structure; this structure is probably a precursor of a discontinuous inverted (type II) cubic phase. Similarly, enzyme treatment of multilamellar vesicles leads to extensive membrane fusion and vesiculation. Thus morphological evidence is obtained showing the ability of phospholipase C to induce bilayer destabilization and fusion. It is speculated that phospholipase C-induced membrane fusion involves a type II fusion intermediate induced by diacylglycerol produced locally.

The interaction of synthetic analogs of the N-terminal fusion sequence of influenza virus with a lipid monolayer. Comparison of fusion-active and fusion-defective analogs

The amino terminus of subunit-2 of influenza virus hemagglutinin (NHA2) plays a crucial role in the induction of fusion between viral and endosomal membranes leading to the infection of a cell. Three synthetic analogs with an amino acid sequence corresponding to NHA2 of variant hemagglutinins were studied in a monolayer set up. Comparison of the interaction of a fusion-active and two fusion-defective analogs with a lipid monolayer revealed a greater surface activity of the fusion-active analog. Pronounced differences were found if the pure peptides were spread at the air/water interface; the fusion-active analog showed a higher collapse pressure and a greater limiting molecular area. Circular dichroism measurements on collected lipid monolayers indicated a high content of alpha-helical structure for the fusion-active and one of the fusion-defective analogs. A simple relation between alpha-helical content and fusogenicity does not seem to exist. Instead, the extent of penetration, a defined tertiary structure or orientation of the alpha-helical peptide may be essential for its membrane perturbing activity.

Replacement of vertebrate serum with lipids and other factors in the culture of invertebrate cells, tissues, parasites, and pathogens

Culture medium supplementation with vertebrate serum results in the selection of fibroblastoid insect cell lines and a general decline during continuous subculturing of both morphologic and functional differentiation of the surviving cells. Essential lipid mixtures can substitute for vertebrate serum in the culture of insect and some vertebrate cells, tissues, parasites, and pathogens. The provision of sterols and essential (with nonessential) polyunsaturated fatty acids as phospholipids in oxidation-protected peptoliposomes or proteoliposomes allows cells in culture to duplicate in vivo specific membranes more accurately. Such lipid-corrected membranes allow cultured cells to communicate with neighboring cells through the extracellular matrix, effectively transmit hormonal signals directly and via receptor control, and respond with various tissue-specific functions and differentiation states as directed.