The annexins and exocytosis

Fc epsilon RI-mediated recruitment of p53/56lyn to detergent-resistant membrane domains accompanies cellular signaling

Detergent-resistant plasma membrane structures, such as caveolae, have been implicated in signalling, transport, and vesicle trafficking functions. Using sucrose gradient ultracentrifugation, we have isolated low-density, Triton X-100-insoluble membrane domains from RBL-2H3 mucosal mast cells that contain several markers common to caveolae, including a src-family tyrosine kinase, p53/56lyn. Aggregation of Fc epsilon RI, the high-affinity IgE receptor, causes a significant increase in the amount of p53/56lyn associated with these low-density membrane domains. Under our standard conditions for lysis, IgE-Fc epsilon RI fractionates with the majority of the solubilized proteins, whereas aggregated receptor complexes are found at a higher density in the gradient. Stimulated translocation of p53/56lyn is accompanied by increased tyrosine phosphorylation of several proteins in the low-density membrane domains as well as enhanced in vitro tyrosine kinase activity toward these proteins and an exogenous substrate. With a lower detergent-to-cell ratio during lysis, significant Fc epsilon RI remains associated with these membrane domains, consistent with the ability to coimmunoprecipitate tyrosine kinase activity with Fc epsilon RI under similar lysis conditions [Pribluda, V. S., Pribluda, C. & Metzger, H. (1994) Proc. Natl. Acad. Sci. USA 91, 11246-11250]. These results indicate that specialized membrane domains may be directly involved in the coupling of receptor aggregation to the activation of signaling events.

Fus2 localizes near the site of cell fusion and is required for both cell fusion and nuclear alignment during zygote formation

Zygote formation occurs through tightly coordinated cell and nuclear fusion events. Genetic evidence suggests that the FUS2 gene product promotes cell fusion during zygote formation in Saccharomyces cerevisiae, functioning with the Fus1 plasma membrane protein at or before cell wall and plasma membrane fusion. Here we report the sequence of the FUS2 gene, localization of Fus2 protein, and show that fus1 and fus2 mutants have distinct defects in cell fusion. FUS2 encodes a unique open reading frame of 617 residues that only is expressed in haploid cells in response to mating pheromone. Consistent with a role in cell fusion, Fus2 protein localizes with discrete structures that could be of cytoskeletal or vesicular origin that accumulate at the tip of pheromone-induced shmoos and at the junction of paired cells in zygotes. Fus2 is predicted to be a coiled-coil protein and fractionates with a 100,000 g pellet, suggesting that it is associated with cytoskeleton, membranes, or other macromolecular structures. Fus2 may interact with structures involved in the alignment of the nuclei during cell fusion, because fus2 mutants have strong defects in karyogamy and fail to orient microtubules between parental nuclei in zygotes. In contrast, fus1 mutants show no karyogamy defects. These, and other results suggest that Fus2 defines a novel cell fusion function and subcellular structure that is also required for the alignment of parental nuclei before nuclear fusion.

In vivo and in vitro phosphorylation of annexin II in T cells: potential regulation by annexin V

In order to understand how signal transduction occurs during T cell activation, it is necessary to identify the key regulatory molecules whose function is influenced by phosphorylation. Annexins II (A-II) and V (A-V) belong to a large family of Ca(2+)-dependent phospholipid-binding proteins. Among many putative functions, annexins may be involved in signal transduction during cellular proliferation and differentiation. In the present study we show that A-II is phosphorylated in vivo in the Jurkat human T cell line. Indeed, A-II is phosphorylated after stimulation by phorbol myristate acetate and on serine residues after T cell antigen receptor (TcR) stimulation. In cytosol from Jurkat cells, A-II is phosphorylated only by Ca2+/phospholipid-stimulated kinases such as Ca(2+)-dependent protein kinases C (cPKCs). A-V inhibits the phosphorylation of A-II and other substrates of cPKCs and has no effect on kinases activated only by phospholipids. In conclusion, A-II is phosphorylated both in vitro and in vivo in Jurkat cells, and may play a role as a substrate during signal transduction in lymphocytes via the TcR through the PKC pathway. On the other hand, A-V could act as a potent modulator of cPKCs in Jurkat cells.

Localization of intracellular calcium during the acrosome reaction in ram spermatozoa

Calcium was identified by a pyroantimonate-osmium fixation technique in ram spermatozoa undergoing a spontaneous acrosome reaction induced by incubation of diluted semen at 39 degrees C. Intracellular calcium was only detected in diluted spermatozoa and increased in amount and distribution over 4 hr At 4 hr, the majority of the spermatozoa displayed ultrastructural evidence of an acrosome reaction. Calcium was initially evident on the outer acrosomal membrane in multiparticulate clusters, which were seen to be located on scalloped crests of acrosomal membrane as fusion developed; it was also located in the region of the acrosomal ridge beneath the outer acrosomal membrane. Vesiculation commenced just anterior to the equatorial segment and proceeded anteriorly. As vesiculation advanced, calcium particles became associated with the periphery of the vesicles attached in the region of the fusion between the two membranes, but were never seen inside the vesicles. The equatorial segment was not labelled until much later in the reaction, at which time calcium particles were also evident on the nuclear membrane; vesiculation of the equatorial segment was also noted at this time. Dense labelling of the postacrosomal dense lamina was seen in all incubated spermatozoa. At the anterior margin of this structure the labelling was seen to be in a "sawtooth" arrangement. The disposition of the calcium both temporally and spatially is discussed in relation to its possible mechanisms in bringing about membrane fusion.

Towards the molecular basis for the regulation of mitochondrial dehydrogenases by calcium ions

In mammalian cells, increases in calcium concentration cause increases in oxidative phosphorylation. This effect is mediated by the activation of four mitochondrial dehydrogenases by calcium ions; FAD-glycerol 3-phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol 3-phosphate dehydrogenase, being located on the outer surface of the inner mitochondrial membrane, is exposed to fluctuations in cytoplasmic calcium concentration. The other three enzymes are located within the mitochondrial matrix. While the kinetic properties of all of these enzymes are well characterised, the molecular basis for their regulation by calcium is not. This review uses information derived from calcium binding studies, analysis of conserved calcium binding motifs and comparison of amino acid sequences from calcium sensitive and non-sensitive enzymes to discuss how the recent cloning of several subunits from the four dehydrogenases enhances our understanding of the ways in which these enzymes bind calcium. FAD-glycerol 3-phosphate dehydrogenase binds calcium ions through a domain which is part of the polypeptide chain of the enzyme. In contrast, it is possible that the calcium sensitivity of the other three dehydrogenases may involve separate calcium binding subunits.

Annexin II tetramer: structure and function

The annexins are a family of proteins that bind acidic phospholipids in the presence of Ca2+. The interaction of these proteins with biological membranes has led to the suggestion that these proteins may play a role in membrane trafficking events such as exocytosis, endocytosis and cell-cell adhesion. One member of the annexin family, annexin II, has been shown to exist as a monomer, heterodimer or heterotetramer. The ability of annexin II tetramer to bridge secretory granules to plasma membrane has suggested that this protein may play a role in Ca(2+)-dependent exocytosis. Annexin II tetramer has also been demonstrated on the extracellular face of some metastatic cells where it mediates the binding of certain metastatic cells to normal cells. Annexin II tetramer is a major cellular substrate of protein kinase C and pp60src. Phosphorylation of annexin II tetramer is a negative modulator of protein function.

Host cellular annexin II is associated with cytomegalovirus particles isolated from cultured human fibroblasts

A significant amount of host cellular annexin II was found to be associated with human cytomegalovirus isolated from cultured human fibroblasts (approximately 1,160 molecules per virion). This composition was established by four different analytical approaches that included (i) Western blot (immunoblot) analysis of gradient-purified virions with a monoclonal antibody specific for annexin II, (ii) peptide mapping and sequence analysis of virus-associated proteins and proteins dissociated from virus following EDTA treatment, (iii) electron microscopic immunocytochemistry of gradient-purified virions, and (iv) labeling of virus-associated proteins by lactoperoxidase-catalyzed radioiodination. These results indicated that annexin II was primarily localized to the viral surface, where it bound in a divalent cation-dependent manner. In functional experiments, a rabbit antiserum raised against annexin II inhibited cytomegalovirus plaque formation in human foreskin fibroblast monolayers in a concentration-dependent manner. Cumulatively, these studies demonstrate an association of host annexin II with cytomegalovirus particles and provide evidence for the involvement of this cellular protein in virus infectivity.

Antitumor effects of liposomal IL1 alpha and TNF alpha against the pulmonary metastases of the B16F10 murine melanoma in syngeneic mice

Interleukin 1 alpha (IL1 alpha) and tumor necrosis factor alpha (TNF alpha) have been successfully incorporated into specific phosphatidylcholine (PC) and phosphatidylserine (PS) multilamellar vesicle (MLV) liposomes by modifying the concentration of calcium ion and pH of the encapsulation buffer. Under these conditions, some of the cytokines may attach to the exterior surface of the MLV and therefore be readily accessible to target cells for receptor binding and signal transduction. These cytokine-associated liposomes are stable for up to 2 weeks in serum-free buffer, and leakage of cytokines into medium containing 10% fetal bovine serum was about 50% at the end of a 3-day incubation period at 37 degrees C. The biological activities mediated by liposomal IL1 alpha and TNF alpha were specific: the stimulation of thymidine uptake in T-helper D10 lymphocytes and the cytolysis of TNF alpha-sensitive L929 target cells could be blocked by specific neutralizing antibodies in a dose-dependent fashion. When administered intravenously into C57BL/6 mice bearing the syngeneic B16F10 murine melanoma cells, dual entrapment of liposomal IL1 alpha and TNF alpha significantly reduced the number of metastatic tumor nodules in the lungs and prolonged the life span of the animals. Thus, liposomal IL1 alpha and TNF alpha displayed significant in vivo antitumor activity against the IL1 alpha- and TNF alpha-resistant B16F10 metastatic murine melanoma.

Endothelial caveolae have the molecular transport machinery for vesicle budding, docking, and fusion including VAMP, NSF, SNAP, annexins, and GTPases

Transport by discrete vesicular carriers is well established at least in part because of recent discoveries identifying key protein mediators of vesicle formation, docking, and fusion. A general mechanism sensitive to N-ethylmaleimide (NEM) is required for the transport of a divergent group of vesicular carriers in all eukaryotes. Many endothelia have an abundant population of non-coated plasmalemmal vesicles or caveolae, which have been reported with considerable controversy to function in transport. We recently have shown that like other vesicular transport systems, caveolae-mediated endocytosis and transcytosis are inhibited by NEM (Schnitzer, J. E., Allard, J., and Oh, P. (1995) Am. J. Physiol. 268, H48-H55). Here, we continue this work by utilizing our recently developed method for purifying endothelial caveolae from rat lung tissue (Schnitzer, J. E., Oh, P., Jacobson, B. S., and Dvorak, A. M. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 1759-1763) to show that these caveolae contain key proteins known to mediate different aspects of vesicle formation, docking, and/or fusion including the vSNARE VAMP-2, monomeric and trimeric GTPases, annexins II and VI, and the NEM-sensitive fusion factor NSF along with its attachment protein SNAP. Like neuronal VAMPs, this endothelial VAMP is sensitive to cleavage by botulinum B and tetanus neurotoxins. Caveolae in endothelium are indeed like other carrier vesicles and contain similar NEM-sensitive molecular machinery for transport.

Annexin I in female rabbit reproductive organs: varying levels in relation to maturity and pregnancy

The level of annexin I, a 36 kDa calcium-dependent phospholipid-binding protein (36 kDa PLBP) in the reproductive organs of young, mature, and pregnant rabbits was determined immunologically with antibodies raised against purified rabbit lung annexin I. In the cytosolic fractions of the ovary, fallopian tube, uterus, and placenta, annexin I was the only major immunoreactive protein. The reproductive organs appeared to have higher annexin I levels than most nonreproductive organ tissues, except the lung and the spleen which were also rich in annexin I. A small amount of annexin I and a nearly equal amount of its hydrolytic product, a 33 kDa polypeptide, were detected in the amniotic fluid between 21 and 27 days gestation. Structural similarity of annexin I in the reproductive organs and in the lung was suggested by their identical isoelectric point values. Annexin I in the ovary of adult rabbits was 70% higher than that in the respective organ of immature rabbits. The uterus of pregnant rabbits had about 84% higher annexin I contents than that of the nonpregnant rabbits. The placenta had more annexin I per mg cytosolic protein than either the ovary or the uterus during pregnancy. The high concentration of annexin I in the reproductive organs may reflect specific functions of these organs in the reproductive years and during the reproductive cycle.

Collagen binding activity of recombinant and N-terminally modified annexin V (anchorin CII)

We have cloned the full coding cDNA sequence of chicken annexin V and of a mutant lacking 8 amino acid residues of the N-terminal tail for prokaryotic expression. Both proteins were synthesized in Escherichia coli upon induction with isopropyl thio-beta-D-galactoside, and were purified following two different protocols: one based on the ability of these proteins to interact reversibly with liposomes in the presence of calcium, and the other based on two sequential ion-exchange chromatographic steps. Spectroscopical analysis of recombinant annexin V revealed that binding of calcium did not change the circular dichroism spectra indicating no significant changes on the secondary structure; however, a conformational change affecting the exposition to the solvent of the tryptophan residue 187 was detected by analysis of fluorescence emission spectra. Recombinant annexin V binds with high affinity to collagen types II and X, and with lower affinity to collagen type I in a calcium-independent manner. Heat denaturing of collagen decreases this interaction while pepsin-treatment of collagen almost completely abolishes annexin V binding. Mutated annexin V interacts with collagen in a similar way as the nonmutated recombinant protein, indicating that the N-terminal tail of annexin V is not essential for collagen binding.

cDNA cloning and tissue-specific regulation of expression of rat calcium-binding protein 65/67. Identification as a homologue of annexin VI

We isolated a cDNA encoding the rat membrane-associated 65/67-kDa calcium-binding protein, CBP 65/67, from a lambda ZAP II cDNA-expression library of rat liver by immunoscreening using monospecific polyclonal anti-(CBP 65/67) antibodies and monoclonal anti-(CBP 65/67) IgG. The product of this cDNA expressed in Escherichia coli was confirmed as CBP 65/67 both by immunostaining and by comparison of the molecular mass with the CBP 65/67 isolated from rat liver by SDS/PAGE. The cDNA sequence and the deduced amino acid sequence of CBP 65/67 both show a high degree of identity to human p68 and human calelectrin, which belong to a family of calcium-dependent, membrane-associated, phospholipid-binding proteins, called annexins. This means that CBP 65/67 is a homolog of the two human proteins just mentioned above. We are not aware that a rat annexin VI has previously been isolated and sequenced. The mRNA expression of CBP 65/67 in different rat organs during development was investigated by Northern blot analysis. In adult tissues, high mRNA levels of CBP 65/67 were found in lung, heart, muscle, spleen and especially in thymus and pancreas, whereas in liver, kidney, intestine, stomach and brain only low levels of CBP 65/67 mRNA could be detected. The amount of mRNA during tissue development in kidney, stomach and muscle showed only slight changes. In contrast, a significant increase of CBP 65/67 expression was observed in liver, lung, heart and brain. In most of the organs investigated, the level of mRNA correlated closely with the level of protein expression, indicating that the expression of CBP 65/67 in most organs is controlled primarily at the transcriptional level.

The heterotrimeric G-protein Gi is localized to the insulin secretory granules of beta-cells and is involved in insulin exocytosis

Mastoparan, a tetradecapeptide found in wasp venom that stimulates G-proteins, increases insulin secretion from beta-cells. In this study, we have examined the role of heterotrimeric G-proteins in mastoparan-induced insulin secretion from the insulin-secreting beta-cell line beta-TC3. Mastoparan stimulated insulin secretion in a dose-dependent manner from digitonin-permeabilized beta-TC3 cells. Active mastoparan analogues mastoparan 7, mastoparan 8, and mastoparan X also stimulated secretion. Mastoparan 17, an inactive analogue of mastoparan, did not increase insulin secretion from permeabilized beta-TC3 cells. Mastoparan-induced insulin secretion from permeabilized beta-TC3 cells was inhibited by pretreatment of the cells with pertussis toxin, suggesting that mastoparan-induced insulin secretion is mediated through a pertussis toxin-sensitive G-protein present distally in exocytosis. Enriched insulin secretory granules (ISG) were prepared by sucrose/nycodenz ultracentrifugation. Western immunoblotting performed on beta-TC3 homogenate and ISG demonstrated that G alpha i was dramatically enriched in ISG. Levels of G alpha o and G alpha q were comparable in homogenate and ISG. Mastoparan stimulated ISG GTPase activity in a pertussis toxin-sensitive manner. Mastoparan 7 and mastoparan 8 also stimulated GTPase activity in the ISG, while the inactive analogue mastoparan 17 had no effect. Selective localization of G alpha i to ISG was confirmed with electron microscopic immunocytochemistry in beta-TC3 cells and beta-cells from rat pancreas. In contrast to G alpha o and G alpha q, G alpha was clearly localized to the ISG. Together, these data suggest that mastoparan may act through the heterotrimeric G-protein G alpha i located in the ISG of beta-cells to stimulate insulin secretion.

Different requirements for NSF, SNAP, and Rab proteins in apical and basolateral transport in MDCK cells

We used an in vitro system based on streptolysin O-permeabilized MDCK cells to study the involvement of NSF, SNAP, SNAREs, and Rab proteins in polarized membrane transport of epithelial cells. In MDCK cells, transport from the trans-Golgi network (TGN) to the basolateral plasma membrane is inhibited by anti-NSF antibodies and stimulated by alpha-SNAP. In contrast, transport from the TGN to the apical cell surface is not affected by anti-NSF antibodies or alpha-SNAP. Furthermore, apical transport is insensitive to Rab-GDI and tetanus and botulinum neurotoxins, which inhibit basolateral transport. These results provide evidence that the Rab-NSF-SNAP-SNARE mechanism operates in basolateral transport, while other molecules constitute the machinery for vesicular delivery in the apical pathway.

The in vivo role of annexin VII (synexin): characterization of an annexin VII-deficient Dictyostelium mutant indicates an involvement in Ca(2+)-regulated processes

Dictyostelium discoideum cells harbor two annexin VII isoforms of 47 and 51 kDa which are present throughout development. In immunofluorescence and cell fractionation studies annexin VII was found in the cytoplasm and on the plasma membrane. In gene disruption mutants lacking both annexin VII isoforms growth, pinocytosis, phagocytosis, chemotaxis and motility were not significantly impaired under routine laboratory conditions, and the cells were able to complete the developmental cycle on bacterial plates. On non-nutrient agar plates development was delayed by three to four hours and a significant number of aggregates was no longer able to form fruiting bodies. Exocytosis as determined by measuring extracellular cAMP phosphodiesterase, alpha-fucosidase and alpha-mannosidase activity was unaltered, the total amounts of these enzymes were however lower in the mutant than in the wild type. The mutant cells were markedly impaired when they were exposed to low Ca2+ concentrations by adding EGTA to the nutrient medium. Under these conditions growth, motility and chemotaxis were severely affected. The Ca2+ concentrations were similar in mutant and wild-type cells both under normal and Ca2+ limiting conditions; however, the distribution was altered under low Ca2+ conditions in SYN-cells. The data suggest that annexin VII is not required for membrane fusion events but rather contributes to proper Ca2+ homeostasis in the cell.

Structure of the Drosophila melanogaster annexin X gene

The annexin X gene was cloned in the P1 recombinant phage carrying a genomic sequence of approximately 70 kb long. This DNA fragment encompasses at least two annexin X copies and several 7.8-kb tandem units represented by an anonymous sequence fused to the 3' truncated part of the annexin X gene. The proteins of annexin family contain a variable amino-terminal domain and a core domain; the latter includes four structurally conserved repeats that presumably arose as a result of duplications. The annexin X gene of Drosophila is about 2 kb long and contains four exons. Exon 1 encodes four amino-terminal amino acids, exon 2 encodes the remaining part of the amino-terminal domain and the three conserved repeats, and exon 3 and exon 4 encode the fourth repeat. The positions of introns 2 and 3 are strictly conserved with respect to both the amino acid position and codon phase as compared to introns 10 and 12 of the fourth repeat in vertebrate annexin genes. We propose the existence of a primordial annexin coding structure comprising at least two introns whose duplications during evolution have been followed by the loss of ancient introns in the first three repeats of Drosophila and vertebrates. Acquisition of new introns in vertebrates is supposed taking into account that exon borders are not found at homologous locations in four repeats of a given vertebrate annexin. Transcription of the annexin gene was detected in embryonic cell cultures. No profound effects of ecdysterone on the annexin X message content in cell cultures were observed.

Annexins: a novel family of calcium- and membrane-binding proteins in search of a function

Although the annexins have been extensively studied and much detailed structural information is available, their in vivo function has yet to be established.

Annexin XIIIb: a novel epithelial specific annexin is implicated in vesicular traffic to the apical plasma membrane

The sorting of apical and basolateral proteins into vesicular carriers takes place in the trans-Golgi network (TGN) in MDCK cells. We have previously analyzed the protein composition of immunoisolated apical and basolateral transport vesicles and have now identified a component that is highly enriched in apical vesicles. Isolation of the encoding cDNA revealed that this protein, annexin XIIIb, is a new isoform of the epithelial specific annexin XIII sub-family which includes the previously described intestine-specific annexin (annexin XIIIa; Wice, B. M., and J. I. Gordon. 1992. J. Cell Biol. 116:405-422). Annexin XIIIb differs from annexin XIIIa in that it contains a unique insert of 41 amino acids in the NH2 terminus and is exclusively expressed in dog intestine and kidney. Immunofluorescence microscopy demonstrated that annexin XIIIb was localized to the apical plasma membrane and underlying punctate structures. Since annexins have been suggested to play a role in membrane-membrane interactions in exocytosis and endocytosis, we investigated whether annexin XIIIb is involved in delivery to the apical cell surface. To this aim we used permeabilized MDCK cells and a cytosol-dependent in vitro transport assay. Antibodies specific for annexin XIIIb significantly inhibited the transport of influenza virus hemagglutinin from the TGN to the apical plasma membrane while the transport of vesicular stomatitis virus glycoprotein to the basolateral cell surface was unaffected. We propose that annexin XIIIb plays a role in vesicular transport to the apical plasma membrane in MDCK cells.

Actin-dependent activation of ion conductances in bronchial epithelial cells

Activation of Cl- and K+ channels is necessary to drive ion secretion in epithelia. There is substantial evidence from previous reports that vesicular transport and exocytosis are involved in the regulation of ion channels. In the present study we examined the role of cytoskeletal elements and components of intracellular vesicle transport on ion channel activation in bronchial epithelial cells. To this end, cells were incubated with a number of different compounds which interact with either microtubules or actin microfilaments, or which interfere with vesicle transport in the Golgi apparatus. The effectiveness of these agents was verified by fluorescence staining of cellular microtubules and actin. The function was examined in 36Cl- efflux studies as well as in whole-cell (WC) patch-clamp and cell-attached studies. The cells were studied under control conditions and after exposure to (in mmol/l) ATP (0.1), forskolin (0.01), histamine (0.01) and hypotonic bath solution (HBS, NaCl 72.5). In untreated control cells, ATP primarily activated a K+ conductance whilst histamine and forskolin induced mainly a Cl- conductance. HBS activated both K+ and Cl- conductances. Incubation of the cells with brefeldin A (up to 100 mumol/l) did not inhibit WC current activation and 36Cl- efflux. Nocodazole (up to 170 mumol/l) reduced the ATP-induced WC current, and mevastatin (up to 100 mumol/l) the cell-swelling-induced WC current. Neither had any effect on the WC current induced by forskolin and histamine. Also 36Cl- efflux induced by HBS, ATP, forskolin and histamine was unaltered by these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and ultrastructural localization of annexin VI from mitochondria

Annexin VI, a member of a family of related intracellular proteins that associate reversibly with membrane phospholipids in a Ca(2+)-dependent manner, has been purified from bovine liver mitochondria and characterized. Moreover, biochemical and immunocytochemical lines of evidence are presented which strongly suggest that annexin VI is closely associated with the cristae in the inner membrane of mitochondria. These findings are consistent with a calcium channel activity of annexin VI in mitochondria.

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.

The annexins: specific markers of midline structures and sensory neurons in the developing murine central nervous system

The annexins are a family of cytoplasmic proteins that have been shown to have numerous actions within a cell. Recent evidence suggests that at least one of these proteins plays a role in the development of the central nervous system (CNS). The present study examines the temporal expression and spatial distribution of annexins I, II, IV, V, and VI during development and at maturity in the murine CNS by immunocytochemical analysis. The results demonstrate that annexins I, II and IV exhibit clear immunolabeling in the murine CNS with distinct patterns of temporal and spatial expression. Annexin IV is the first annexin to be expressed on embryonic day (E) 9.5 while annexin I is the last to be expressed (E11.5). Annexins I, II and IV are found in the floor plate region, but to differing rostrocaudal extents. Annexin I has a very restricted distribution, only present in the midline raphe of the brainstem. Annexin II is present in the spinal cord, brainstem and mesencephalon. Annexin IV has the widest midline distribution, being observed in the floor and roof plates of the developing CNS. Additionally, antibodies against annexin II and IV immunolabel most dorsal root and sensory ganglion cells and their axons. During early postnatal development, immunolabeling with each antibody gradually disappears in many structures, and only first order sensory neurons and their fibers are immunopositive for annexins II and IV at weaning. Three functions of the annexins are suggested by the present findings: (1) to help establish the midline structures of the floor and roof plates, (2) to help direct the decussation of sensory fibers, and (3) to regulate some aspect of sensory neuron processing, such as signal transduction.

Stilbene disulfonic acids inhibit synexin-mediated membrane aggregation and fusion

Stilbene disulfonic acids inhibit surfactant secretion from lung epithelial type II cells by an undefined mechanism, and inhibit CD4 mediated cell-cell fusion. We have previously shown that lung synexin promotes in vitro fusion of lamellar bodies and plasma membranes, an obligatory process for surfactant secretion. This study investigates the effect of stilbene disulfonic acids, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), and 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (AMDS), on synexin-mediated liposome aggregation and fusion. Structurally, these three stilbene compounds differ in the number of isothiocyano groups present (DIDS = 2, SITS = 1, and AMDS = 0). At 10 micrograms synexin/ml, DIDS and SITS inhibited synexin-mediated liposome aggregation with an EC50 of 3.5 microM and 148 microM, respectively. In comparison, AMDS was least inhibitory (EC50 > 1 mM). Thus, the inhibitory potency (DIDS > SITS > AMDS) was partly dependent upon the number of isothiocyano groups. The EC50 was also dependent on synexin concentration. Stilbene disulfonic acids were also inhibitory for arachidonic acid-enhanced synexin-mediated liposome fusion. The EC50 for DIDS and SITS for fusion were similar to that for liposome aggregation. Ca(2+)-induced synexin polymerization, measured by 90 degrees light scattering, was increased by DIDS, suggesting binding of stilbene disulfonic acids to synexin. The binding of DIDS to synexin was dependent on the molar ratio of synexin to DIDS. These results indicate that stilbene disulfonic acids interact directly with synexin to inhibit membrane aggregation and fusion. Our results suggest that such inhibition of synexin activity may contribute towards inhibition of surfactant secretion by DIDS, and support a physiological role for synexin in lung surfactant secretion.

Expression and localization of annexin VII (synexin) in muscle cells

Annexin VII (synexin) is a member of the annexin family of proteins, which are characterized by Ca(2+)-dependent binding to phospholipids. We used PCR to isolate from a lambda gt11-mouse fibroblast library annexin VII cDNA fragments corresponding to the two isoforms found in both humans and Dictyostelium discoideum. The two isoforms of 47 kDa and 51 kDa differed by 22 amino acids inserted into the proximal third of the hydrophobic N terminus. Annexin VII-specific polypeptides expressed in Escherichia coli were used to generate isoform-specific monoclonal antibodies. Expression of the two isoforms during myogenesis was followed in the myogenic cell lines BC3H1 and L6. Only the 47 kDa isoform was present in undifferentiated L6 or BC3H1 myoblasts. The 51 kDa isoform appeared after myogenesis had been induced and in striated muscle only the 51 kDa isoform was observed. Immunofluorescence showed that annexin VII was located in the cytosol of mononucleated and fused polynucleated cultured cells, whereas in striated muscle, annexin VII was located preferentially at the plasma membrane and the transverse tubules. However, there was also some residual cytosolic staining, which was more abundant in type II (fast twitch) than in type I (slow twitch) fibers. Permeabilization of L6 cells with digitonin in the presence of 5 mM EGTA led to a release of annexin VII from the cells, which paralleled the loss of cytosolic lactate dehydrogenase (LDH) at low detergent concentrations (50 microM). In the presence of 100 microM extracellular Ca2+, annexin VII remained bound to the plasma membrane even in the presence of high digitonin concentrations. Incubation with the Ca(2+)-specific ionophore A23187 and 100 microM extracellular Ca2+ led to a redistribution of annexin VII from the cytosol to the plasma membrane after 30 minutes of incubation. The results obtained indicate a developmentally and Ca(2+)-regulated localization and expression of annexin VII and raise the possibility that annexin VII may play a role in excitation-contraction coupling in skeletal muscle.

Synaptic vesicle proteins and regulated exocytosis

The recent identification of novel proteins associated with the membranes of synaptic vesicles has ignited the field of molecular neurobiology to probe the function of these molecules. Evidence is mounting that the vesicle proteins vamp (synaptobrevin), rab3A, synaptophysin, synaptotagmin (p65) and SV2 play an important role in regulated exocytosis, by regulating neurotransmitter uptake, vesicle targeting and fusion with the presynaptic plasma membrane.

Levels and localization of group II phospholipase A2 and annexin I in interleukin- and dexamethasone-treated rat mesangial cells: evidence against annexin mediation of the dexamethasone-induced inhibition of group II phospholipases A2

The mechanism by which glucocorticosteroids inhibit the synthesis and secretion of pro-inflammatory arachidonate metabolites is still controversial. Initially it was postulated that glucocorticoids can induce the formation of PLA2 inhibitory proteins termed annexins. We have previously shown that the cytokine-induced 14 kDa PLA2 activity and the synthesis of prostaglandin E2 in rat mesangial cells is dose-dependently blocked by pretreatment of the cells with dexamethasone (Schalkwijk et al. (1991) Biochem. Biophys. Res. Commun. 180, 46-52). Concurrently, the synthesis of 14 kDa group II PLA2 is suppressed. The regulation of PLA2 activity is complex and may well involve superimposable mechanisms. Thus, although the decrease in PLA2 protein levels could in itself explain the dexamethasone-induced decrease in PLA2 activity, a contribution of the glucocorticoid-induced anti-phospholipase A2 protein annexin cannot be ruled out a priori. To investigate this possibility we analyzed the level of annexin I by Western blotting and immunostaining in mesangial cells treated with interleukin-1 beta and/or dexamethasone. Under conditions where 14 kDa group II PLA2 activity and protein levels were dramatically affected by interleukin-1 and dexamethasone, the level of annexin I in the cells remained constant. Dexamethasone also did not induce the secretion of annexin I. In addition, no evidence for dexamethasone-induced translocation of annexin I from the cytosol to membranes, thereby possibly sequestering the substrates for PLA2, was obtained. Immunofluorescence studies localized the cytokine-induced PLA2 to the Golgi area and punctate structures in the cytoplasm. We have also studied the subcellular localization of annexin I in rat mesangial cells using confocal microscopy. These studies located annexin I mainly in the cytoplasma and the nucleus. We conclude from these experiments that the dexamethasone-induced inhibition of 14 kDa group II PLA2 in rat mesangial cells is not mediated by annexin I and is solely due to the suppression of PLA2 gene expression.

Identification and partial sequence analysis of novel annexins in Lytechinus pictus oocytes

The annexins are a major class of calcium-binding proteins with unknown functions. In an attempt to define novel model systems in which to study members of the annexin family, we have investigated the expression of annexins in eggs from the sea urchin Lytechinus pictus. Western blot analysis of L. pictus eggs using antisera raised against human annexins I, V and VI revealed the presence of immunoreactive proteins of approximately 34 kDa, 35 kDa and 68 kDa respectively. The sea urchin annexins behaved similarly to their mammalian counterparts, both during purification and in their ability to bind calcium-dependently to anionic phospholipids. Of the three sea urchin annexins, the 34 kDa form was most abundant, yielding sufficient quantities for peptide microsequencing. The amino acid sequences derived in this way showed the L. pictus annexin to be closely related both to mammalian annexin I and to annexins IX, X and XII from Drosophila and Hydra. However, N-terminal sequence from the L. pictus annexin showed it to be a novel member of the annexin super-gene family. The results are interesting in view of the complex evolution of the annexin gene family, and also point to the potential usefulness of echinoderm eggs as a model system in which to study annexin function.

Evidence for the existence of lipid-diffusion barriers in the equatorial segment of human spermatozoa

Liposomes consisting of negatively charged phospholipids interact almost exclusively with the equatorial segment (ES) of human spermatozoa provided the cells have undergone the acrosome reaction (AR) [Arts, Kuiken, Jager and Hoekstra (1993) Eur. J. Biochem. 217, 1001-1009]. Using fluorescently tagged liposomes, this interaction can be observed by fluorescence microscopy, showing either a diffuse fluorescence in the ES region (pattern ESd, presumably reflecting membrane-incorporated lipids as a result of fusion) or a punctate fluorescence (pattern ESp, representing adhering liposomes). These distribution patterns remain unchanged during prolonged incubation, up to 40 min. Not only do these observations suggest the existence of fairly specific liposomal binding sites, associated with the ES region, but also that a barrier to lipid lateral diffusion seems to exist in the ES membrane. Using liposomes that contain fluorescent lipid analogues in either both leaflets or in the inner leaflet only, we demonstrate that this putative barrier entails both membrane leaflets. Treatment with EDTA caused fluorescence to spread from the ES towards other membrane domains. Since only spermatozoa displaying pattern ESd were affected by the chelator, the randomization was not caused by EDTA-induced fusion activity. Therefore, this observation provides further evidence that in spermatozoa displaying pattern ESd the fluorescent lipid analogues were incorporated in the ES membrane as a result of fusion. Furthermore, these experiments support the view of the existence of a transmembranous block to lipid lateral diffusion in the ES, the stability of which may be governed by bivalent cations.

Annexins in rat enterocyte and hepatocyte: an immunogold electron-microscope study

In the present study, immunogold labeling of ultrathin sections of rat small intestine and liver has been used to obtain insights into the ultrastructural localization and possible functions of annexins. In enterocytes, annexins II, IV, and VI are found at the periphery of the core of each microvillus and of the rootlets, but are absent from the interrootlet space. Annexins II, IV, and VI are also observed close to the interdigitated plasma membrane. In hepatocytes, only annexin VI is found to be concentrated within the microvilli in the bile canaliculi, on the inner face of the sinusoidal cell surface, particularly in the space of Disse, and all along the plasma membrane. Annexin VI is also detected in mitochondria of enterocytes and hepatocytes. These localizations are in agreement with the concept of a close calcium-dependent association of annexins with membranes and cytoskeletal proteins, particularly with actin. Moreover, they support the hypothesis of an involvement of annexins in exocytotic and endocytotic processes, which take place in epithelial cells.

Identification and characterization of ATPase activity associated with maize (Zea mays) annexins

An ATPase activity is associated with maize (Zea mays) annexins. It has a pH optimum of 6.0, shows Michaelis-Menten kinetics and is not stimulated by Ca2+, Mg2+, EDTA or KCl; it is not inhibited by vanadate, molybdate, nitrate or azide, but N-ethylmaleimide inhibits by approximately 30% at 1-2 mM. These properties indicate that the activity is unlike other ATPases, although it has many features in common with the myosin ATPase. Gel filtration shows that the ATPase activity is mainly associated with a 68 kDa protein that is extracted with the p33/p35 annexins and cross-reacts with antibodies to these proteins.

Expression and characterization of a Dictyostelium discoideum annexin

The annexins are calcium-dependent phospholipid-binding proteins. Recently the gene encoding the homologue of a mammalian annexin has been identified in Dictyostelium discoideum. Analysis of cDNA and genomic clones showed that the transcript for Dictyostelium annexin is alternatively spliced (Greenwood, M. and Tsang, A. (1991) Biochim. Biophys. Acta 1088, 429-432; Döring, V., Schleicher, M and Noegel, A. (1991) J. Biol. Chem. 266, 17509-17515). Here, we showed that the Dictyostelium annexin DNA hybridized to two populations of transcripts. We used a recombinant annexin polypeptide to raise polyclonal antibody. Immunoblot analysis revealed that the antibody recognized two polypeptides of 48 kDa and 54 kDa in developing D. discoideum cells. The molecular sizes of these polypeptides correspond well with the expected sizes of the alternatively spliced products. The 48-kDa and 54-kDa polypeptides were purified by isoelectric focusing to more than 70% homogeneity. The partially purified proteins were found to associate with phosphatidylserine vesicles in a calcium-dependent manner. These results suggest that the 48- and 54-kDa polypeptides are the products of alternative splicing of the annexin transcripts. During development the two polypeptides accumulate at different rates to about 60 times the level detected in vegetative cells. On the other hand, RNA blot analysis showed that the level of the annexin transcripts in multicellular aggregates was about 5 times that of vegetative cells.

Expression of annexin VI in A431 carcinoma cells suppresses proliferation: a possible role for annexin VI in cell growth regulation

Human A431 cells exhibit many characteristics typical of transformed cells, such as lack of contact inhibition and reduced growth factor requirement. We have used these cells as a model for the study of annexin VI function, since they do not normally express this protein. In this study we isolated two stably transfected clones, both of which were found to express annexin VI at physiological levels, and examined various growth parameters associated with the transformed phenotype. In low serum, normal A431 cells had doubling times similar to those observed in high serum. However, although the annexin VI transfectants grew only slightly more slowly than controls in high serum, their doubling time was significantly increased in low serum. Moreover, in low serum the annexin VI transfectants stopped proliferating after reaching confluence, indicating contact inhibition. Fluorescence activated cell sorting analysis revealed that the annexin VI+ cells were growth arrested in the G1 phase of the cell cycle when cultured in low serum, whereas annexin VI- clones exhibited the same proportion of mitotic cells in both low and high serum. Thus, expression of annexin VI in a heterologous cell line has a moderating influence on cell proliferation suggesting a possible role for annexin VI in cell growth regulation.

Annexin VI isoforms are differentially expressed in mammalian tissues

Purified annexin VI migrates as a closely spaced doublet when separated by SDS-PAGE. Immunolocalization of annexin VI in heart demonstrates staining at different defined subcellular compartments. Moss et al. identified two cDNAs, one having an insert of 18 bases encoding VAAEIL at the beginning of repeat domain seven. We have identified the splicing site of the murine annexin VI gene. It contains a single small exon of 18 bases. PCR amplification of reverse transcribed (RT) mRNA demonstrates that, in all tissues tested, the mRNA isoform containing the insert is predominant. Site-directed antibody was produced and affinity purified against peptides reflecting the insert and deletion sequences. The steady-state isoform ratio of the annexin VI protein is consistent with the RT-PCR data. Chromatographic experiments demonstrate that the annexin VI protein isoforms have biochemical differences. These differences may target the individual isoforms to unique cellular compartments or alter functional properties.

The annexin II2p11(2) complex is the major protein component of the triton X-100-insoluble low-density fraction prepared from MDCK cells in the presence of Ca2+

Annexin II2p11(2) is present in the submembranous region of cells expressing both subunits of the complex. Most probably, this subcellular distribution is maintained through the interaction of annexin II2p11(2) with membrane phospholipids and/or elements of the cortical cytoskeleton known to occur in vitro in a Ca(2+)-dependent manner. To determine whether membrane or cytoskeleton interactions are primarily responsible for anchoring annexin II2p11(2) in the cell cortex, we subjected Madin-Darby canine kidney (MDCK) cells to serial extractions using different detergents and identified annexin II and p11 in the different fractions employing specific antibodies. The complex but not monomeric annexin II remains insoluble when the cells are extracted with Triton X-100 in the presence of Ca2+. However, treatment of the Triton X-100-insoluble cell remnants with a series of other detergents known to solubilize GPI-anchored proteins leads to a partial extraction of annexin II2p11(2) even in the presence of Ca2+. Using sucrose density gradient analysis in the presence of Ca2+ as a different means of fractionating the Triton X-100-insoluble cell remnants we show that the majority of annexin II2p11(2) copurifies with a low-density fraction which has been reported to contain GPI-anchored proteins, certain glycolipids, and VIP21/caveolin. Annexin II2p11(2) is by far the most abundant protein component in this fraction indicating that its association with the low-density material occurs via lipid binding and is not due to the interaction with a certain protein.

The expression of different annexins in the fish embryo is developmentally regulated

The expression of annexins, a family of Ca2+/phospholipid-binding proteins, was analyzed by biochemical and immunological criteria in the fish Misgurnus fossilis (loach), which is a good model system to study embryonic events. Five different annexins (loach annexins A to E) are present as a maternal pool in the unfertilized eggs. Only annexin E is newly synthesized in the early embryo. Its synthesis, already apparent at mid-blastula, decreases in later stages when two additional annexins (F and G) appear. They are present among the newly synthesized polypeptides of mid-gastrula and later embryonic stages and are also found in loach larvae. The developmentally controlled expression of several annexins indicates a specific role of these proteins at certain embryonic stages.

Annexin V binding to the outer leaflet of small unilamellar vesicles leads to altered inner-leaflet properties: 31P- and 1H-NMR studies

Calcium-dependent binding to phospholipid membranes is closely associated with annexin functional properties. In these studies, 31P- and 1H-nuclear magnetic resonance (NMR) experiments have been performed to study the effects of binding of recombinant rat annexin V to sonicated small unilamellar vesicles (SUVs). High-resolution 31P-NMR spectra of SUVs containing mixtures of synthetic phosphatidic acid (PA) and phosphatidylcholine (PC) show resolvable resonances corresponding to the inner-leaflet PA, outer-leaflet PA, and PC phosphoryl groups. When annexin binding occurs, the outer-leaflet PA 31P resonance shifts while that of PC is unaffected, consistent with selective binding of the protein to the phosphoryl moiety of the PA component. Further, annexin V binding to membrane outer-leaflet phospholipids has a measurable effect on inner-leaflet phospholipids of intact vesicles. 1H-NMR T1 relaxation measurements of SUVs containing acyl-chain-perdeuterated PC show no effects on the PA hydrocarbon-chain segmental motions upon annexin binding. Circular dichroism measurements indicate that the protein does not undergo a significant conformational change upon binding to the vesicles. The observed NMR changes do not correspond to proton or calcium gradients, nor to lateral segregation of extended patches of homogeneous phospholipids. The combined evidence suggests that selective, peripheral annexin-membrane interactions influence the environment of the inner vesicular surface. The mechanism proposed is a protein-induced change in vesicle morphology that corresponds to reduced curvature.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Evaluation of annexin V as a platelet-directed thrombus targeting agent

Annexin V is a human phospholipid binding protein (M(r) 36,000) that binds with high affinity to activated platelets in vitro. We studied the biodistribution and thrombus binding of annexin V in rabbit and swine models of fully occlusive arterial thrombi formed 1-2 h prior to injection of annexin V. Iodinated annexin V was cleared from blood in a rapid early phase (t1/2 = 6.4 min, 76% of radioactivity) and a slower late phase (t1/2 = 71 min, 24% of radioactivity). Organ uptake was highest in the kidney and spleen and lowest in heart and skeletal muscle. Thrombus/blood uptake ratios were (mean +/- SEM): 6.39 +/- 1.80 for rabbit iliac artery, 6.97 +/- 1.45 for swine carotid artery, and 7.68 +/- 1.70 for swine femoral artery (all p values < 0.01 versus control artery); a control protein, ovalbumin, showed an uptake ratio of 0.59 +/- 0.08 in swine femoral artery thrombi. These results indicate that annexin V is useful as an agent for selective targeting of platelet-containing thrombi.

Molecular cloning of a novel N-terminal variant of annexin II from rat basophilic leukaemia cells

Rat annexin II cDNA clones were isolated from a rat basophilic leukaemia cell plasmid library by cross-species hybridization with a mouse probe, and fully sequenced using the dideoxy-chain-termination method. Alignment of the derived amino-acid sequence with those of other mammalian annexin II species revealed a high level of conservation, characteristic of the annexin family of proteins. One of the cDNAs isolated contained an additional six nucleotides close to the N-terminus, lying in-frame and at a point corresponding to an intron/exon boundary in the human annexin II gene. As the two rat cDNAs were identical apart from the six nucleotide insert, it is likely that these represent alternatively spliced transcripts of a single gene, rather than the products of two separate genes. The six nucleotides encode serine-glutamine and therefore introduce an additional potential phosphorylation site into a region already containing one tyrosine and two serine phosphorylation sites. The discovery of this novel annexin II variant may have important implications both for p11 binding and for regulation of annexin II function by phosphorylation.