Functional and genetic analysis of annexin VI

Annexin Animal Models-From Fundamental Principles to Translational Research

Routine manipulation of the mouse genome has become a landmark in biomedical research. Traits that are only associated with advanced developmental stages can now be investigated within a living organism, and the in vivo analysis of corresponding phenotypes and functions advances the translation into the clinical setting. The annexins, a family of closely related calcium (Ca²⁺)- and lipid-binding proteins, are found at various intra- and extracellular locations, and interact with a broad range of membrane lipids and proteins. Their impacts on cellular functions has been extensively assessed in vitro, yet annexin-deficient mouse models generally develop normally and do not display obvious phenotypes. Only in recent years, studies examining genetically modified annexin mouse models which were exposed to stress conditions mimicking human disease often revealed striking phenotypes. This review is the first comprehensive overview of annexin-related research using animal models and their exciting future use for relevant issues in biology and experimental medicine.

Two translation initiation codons direct the expression of annexin VI 64kDa and 68kDa isoforms

Annexin A6 is a multicompetent, multifunctional protein involved in several biological processes within and outside of the cell. Whereas HeLa cells express annexin A6 only as a 68/67-kDa doublet, indicating alternative splicing (Smith PD et al. (1994) Proc Natl Acad Sci USA 91, 2713-2717), the GMO2784 human fibroblast cell line expresses two additional isoforms at 64 and 58kDa. In both cell lines, annexin A6 is located intracellularly and on the plasma membrane. In vitro eukaryotic protein synthesis of pIRESneoAnxA6 cDNA and pIRESneoAnxA6/Met^1- or Met^33- using a reticulocyte lysate coupled transcription/translation system revealed that this gene contains two translation start codons, Met¹ and Met³³. Immunoprecipitation of the products obtained from the transcription/translation system using various anti-annexin A6 antibodies confirmed the presence of several isoforms and suggested that this protein might be present in different configurations.

Annexin A6 is an organizer of membrane microdomains to regulate receptor localization and signalling

Annexin A6 (AnxA6) belongs to the conserved annexin protein family--a group of Ca(2+) -dependent membrane binding proteins. It is the largest of all annexin proteins and upon activation, binds to negatively charged phospholipids in the plasma membrane and endosomes. In addition, AnxA6 associates with cholesterol-rich membrane microdomains termed lipid rafts. Membrane cholesterol triggers Ca(2+) -independent translocation of AnxA6 to membranes and AnxA6 levels determine the number of caveolae, a form of specialized rafts at the cell surface. AnxA6 also has an F-actin binding domain and interacts with cytoskeleton components. Taken together, this suggests that AnxA6 has a scaffold function to link membrane microdomains with the organization of the cytoskeleton. Such a link facilitates AnxA6 to participate in plasma membrane repair and it would also impact on receptor signalling at the cell surface, growth factor, and lipoprotein receptor trafficking, Ca(2+) -channel activity and T cell activation. Hence, the regulation of cell surface receptors by AnxA6 may be facilitated by its unique structure that allows recruitment of interaction partners and simultaneously bridging specialized membrane domains with cortical actin surrounding activated receptors.

Annexin VI is a mannose-6-phosphate-independent endocytic receptor for bovine β-glucuronidase

Endocytosis and transport of bovine liver β-glucuronidase to lysosomes in human fibroblasts are mediated by two receptors: the well-characterized cation-independent mannose 6-phosphate receptor (IGF-II/Man6PR) and an IGF-II/Man6PR-independent receptor, which recognizes a Ser-Trp*-Ser sequence present on the ligand. The latter receptor was detergent extracted from bovine liver membranes and purified. LC/ESI-MS/MS analysis revealed that this endocytic receptor was annexin VI (AnxA6). Several approaches were used to confirm this finding. First, the binding of bovine β-glucuronidase to the purified receptor from bovine liver membranes and His-tagged recombinant human AnxA6 protein was confirmed using ligand-blotting assays. Second, western blot analysis using antibodies raised against IGF-II/Man6PR-independent receptor as well as commercial antibodies against AnxA6 confirmed that the receptor and AnxA6 were indeed the same protein. Third, double immunofluorescence experiments in human fibroblasts confirmed a complete colocalization of the bovine β-glucuronidase and the AnxA6 receptor on the plasma membrane. Lastly, two cell lines were stably transfected with a plasmid containing the cDNA for human AnxA6. In both transfected cell lines, an increase in cell surface AnxA6 and in mannose 6-phosphate-independent endocytosis of bovine β-glucuronidase was detected. These results indicate that AnxA6 is a novel receptor that mediates the endocytosis of the bovine β-glucuronidase.

Role of annexin A6 isoforms in catecholamine secretion by PC12 cells: distinct influence on calcium response

Noradrenaline and adrenaline are secreted by adrenal medulla chromaffin cells via exocytosis. Exocytosis of catecholamines occurs after cell stimulation with various endogenous activators such as nicotine or after depolarization of the plasma membrane and is regulated by calcium ions. Cytosolic [Ca(2+)] increases in response to cell excitation and triggers a signal-initiated secretion. Annexins are known to participate in the regulation of membrane dynamics and are also considered to be involved in vesicular trafficking. Some experimental evidence suggests that annexins may participate in Ca(2+)-regulated catecholamine secretion. In this report the effect of annexin A6 (AnxA6) isoforms 1 and 2 on catecholamine secretion has been described. Overexpression of AnxA6 isoforms and AnxA6 knock-down in PC12 cells were accompanied by almost complete inhibition or a 20% enhancement of dopamine secretion, respectively. AnxA6-1 and AnxA6-2 overexpression reduced Delta[Ca(2+)](c) upon depolarization by 32% and 58%, respectively, while AnxA6 knock-down increased Delta[Ca(2+)](c) by 44%. The mechanism of AnxA6 action on Ca(2+) signalling is not well understood. Experimental evidence suggests that two AnxA6 isoforms interact with different targets engaged in regulation of calcium homeostasis in PC12 cells.

Annexin A6-Linking Ca(2+) signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca(2+)-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA(2)) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca(2+)-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca(2+)-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

In situ proteomics with imaging mass spectrometry and principal component analysis in the Scrapper-knockout mouse brain

Imaging MS is emerging as a useful tool for proteomic analysis. We utilized this technique to analyze gene knockout (KO) mice in addition to traditional 2-DE analysis. The Scrapper-knockout (SCR-KO) mouse brain showed two types of neurodegenerative pathologies, the spongiform neurodegeneration and shrinkage of neuronal cells. 2-DE analysis of the whole brain lysates of SCR-KO mice indicated slight changes in annexin A6, Rap1 GTPase, and glyoxalase domain containing four spots while most of the main components did not show significant changes. By imaging MS analysis based on principal component analysis (PCA), we could find numerous alterations in the KO mouse brain. Furthermore, we could also know the information on the position of altered substances all together. PCA provides information about which molecules in tissue microdomains have altered and is helpful in analyzing large dataset of imaging MS, while exact identification of each molecule from peaks in MALDI imaging MS may require additional analyses such as MS/MS. Direct imaging with PCA is a powerful tool to perform in situ proteomics and will lead to novel findings. Our study shows that imaging MS yields information complementary to conventional 2-DE analysis.

Annexin A6 inhibits Ras signalling in breast cancer cells

Overexpression of epidermal growth factor receptor (EGFR) is associated with enhanced activation of wild-type (hyperactive) Ras in breast cancer. Little is known about the regulation of Ras inactivation and GTPase-activating proteins (GAPs), such as p120GAP, in cells with hyperactive Ras. Recently, we showed that in EGFR-overexpressing A431 cells, which lack endogenous Annexin A6 (AnxA6), ectopic expression of AnxA6 stimulates membrane recruitment of p120GAP to modulate Ras signalling. We now demonstrate that, AnxA6 is downregulated in a number of EGFR-overexpressing and estrogen receptor (ER)-negative breast cancer cells. In these cells, AnxA6 overexpression promotes Ca(2+)- and EGF-inducible membrane targeting of p120GAP. In ER-negative MDA-MB-436 cells, overexpression of p120GAP, but not CAPRI or a p120GAP mutant lacking the AnxA6-binding domain inhibits Ras/MAPK activity. AnxA6 knockdown in MDA-MB-436 increases Ras activity and cell proliferation in anchorage-independent growth assays. Furthermore, AnxA6 co-immunoprecipitates with H-Ras in a Ca(2+)- and EGF-inducible manner and fluorescence resonance energy transfer (FRET) microscopy confirmed that AnxA6 is in close proximity of active (G12V), but not inactive (S17N) H-Ras. Thus, association of AnxA6 with H-Ras-containing protein complexes may contribute to regulate p120GAP/Ras assembly in EGFR-overexpressing and ER-negative breast cancer cells.

Calcium- and pH-dependent localization of annexin A6 isoforms in Balb/3T3 fibroblasts reflecting their potential participation in vesicular transport

Annexin A6 (AnxA6), calcium- and membrane-binding protein, is involved in membrane dynamics. It exists in the cell in two isoforms, AnxA6-1 and AnxA6-2, varying only by the VAAEIL sequence. In most cells, AnxA6-1 predominates. A limited number of observations suggests that both isoforms differ from each other functionally. The EGF-dependent Ca(2+) influx in A431 cells is inhibited only by AnxA6-1. Moreover, AnxA6-2 was found to exhibit higher affinity for Ca(2+). In this report we addressed the potential significance of the VAAEIL deletion in AnxA6-2. For this purpose, we expressed AnxA6 isoform cDNAs in bacteria or mouse Balb/3T3 fibroblasts. The recombinant AnxA6-2 was characterized by a less extended molecular shape than that of AnxA6-1 and required a narrower [Ca(2+)] range to bind liposomes. Upon lowering pH in the presence of EGTA recombinant AnxA6-2 became less hydrophobic than AnxA6-1 as revealed by the Triton X-114 partition. Furthermore, AnxA6-2 revealed stronger F-actin binding than that of AnxA6-1. Immunofluorescence microscopy showed that the EGFP-tagged AnxA6 isoforms expressed in Balb/3T3 fibroblasts relocate in a Ca(2+)- and H(+)-sensitive manner to the vesicular structures in a perinuclear region or in cytosol. Cell fractionation showed that in resting conditions AnxA6-1 is associated with early endosomes and AnxA6-2 with late endosomes, and an increase in [Ca(2+)] and/or [H(+)] induced their opposite distribution. These findings suggest a potentially independent regulation, localization, and function of AnxA6 isoforms in Balb/3T3 fibroblasts. More generally, our findings suggest distinct functions of AnxA6 isoforms in membrane dynamics.

Identification of a novel interaction between the Ca2+-binding protein S100A11 and the Ca2+- and phospholipid-binding protein annexin A6

S100A11 is a member of the S100 family of EF-hand Ca2+-binding proteins, which is expressed in smooth muscle and other tissues. Ca2+binding to S100A11 induces a conformational change that exposes a hydrophobic surface for interaction with target proteins. Affinity chromatography with immobilized S100A11 was used to isolate a 70-kDa protein from smooth muscle that bound to S100A11 in a Ca2+-dependent manner and was identified by mass spectrometry as annexin A6. Direct Ca2+-dependent interaction between S100A11 and annexin A6 was confirmed by affinity chromatography of the purified bacterially expressed proteins, by gel overlay of annexin A6 with purified S100A11, by chemical cross-linking, and by coprecipitation of S100A11 with annexin A6 bound to liposomes. The expression of S100A11 and annexin A6 in the same cell type was verified by RT-PCR and immunocytochemistry of isolated vascular smooth muscle cells. The site of binding of S100A11 on annexin A6 was investigated by partial tryptic digestion and deletion mutagenesis. The unique NH2terminal head region of annexin A6 was not required for S100A11 binding, but binding sites were identified in both NH2- and COOH-terminal halves of the molecule. We hypothesize that an agonist-induced increase in cytosolic free [Ca2+] leads to formation of a complex of S100A11 and annexin A6, which forms a physical connection between the plasma membrane and the cytoskeleton, or plays a role in the formation of signaling complexes at the level of the sarcolemma.

A potential endogenous ligand of annexin IV in the exocrine pancreas. Carbohydrate structure of GP-2, a glycosylphosphatidylinositol-anchored glycoprotein of zymogen granule membranes

We demonstrated previously that annexins IV, V, and VI, proteins of the calcium/phospholipid-binding annexin family, have glycosaminoglycan binding properties (Ishitsuka, R., Kojima, K., Utsumi, H., Ogawa, H., and Matsumoto, I. (1998) J. Biol. Chem. 273, 9935-9941). In this study, we investigated the endogenous ligands of annexin IV in the exocrine pancreas. Immunohistochemical study of bovine pancreas showed that annexin IV localized in the apical cytoplasmic region of pancreatic acinar cells where zymogen granules are concentrated. Because it is the major component of the zymogen granule membrane, the glycosylphosphatidylinositol-anchored glycoprotein GP-2 was suggested to play a role in apical sorting and secretion of zymogens. We isolated GP-2 from porcine pancreas extract and determined the structure of its N-linked oligosaccharides by two-dimensional mapping. The major carbohydrate structures of porcine GP-2 were trisialo-triantennary and tetrasialo-tetra-antennary complex-type oligosaccharides. Dot-blot assay showed that annexin IV interacts with GP-2 in the presence of calcium and that it recognizes the terminal sialic acid residues linked through alpha2-3 linkages to the carbohydrate of GP-2. Lectin blot assay showed that Maackia amurensis mitogen, a plant lectin specific for the trisaccharide sequence Sia(alpha)2-3Galbeta1-4GlcNAc of N-linked oligosaccharides, has strong affinity for GP-2. Thus, M. amurensis mitogen was used as a specific probe for GP-2 in the histochemical staining of the bovine pancreas. GP-2 was found to localize exclusively in the same apical cytoplasmic region of pancreatic acinar cells as annexin IV does. These results suggest that GP-2 is an endogenous ligand of annexin IV in the exocrine pancreas.

Annexin 6 is a putative cell surface receptor for chondroitin sulfate chains

Chondroitin sulfate proteoglycans, including PG-M/versican, inhibit cell-substratum adhesion. They achieve this through their chondroitin sulfate chains. In order to define the molecular mechanism for this inhibition, we investigated the influence of these chains on cell attachment to substratum,the first step in cell adhesion. Chondroitin sulfate chains did not prevent cell attachment. In fact, a variety of cells attached to chondroitin sulfate,implying the existence of putative receptors and/or binding proteins for this extracellular matrix glycosaminoglycan. Detergent-extracted human fibroblast membrane protein extracts were examined by affinity chromatography in the presence of Ca2+ on chondroitin sulfate immobilized on agarose CL-6B. A 68 kDa and a 35 kDa protein were isolated, sequenced and demonstrated to be annexin 6 and annexin 4, respectively. Next we used A431 cells devoid of annexin 6 expression to verify that annexin 6 is the receptor for this glycosaminoglycan. We confirmed that A431 cells were unable to attach to the chondroitin sulfate substratum and that the stable transfectants expressing annexin 6 conferred the ability to attach to chondroitin sulfate chains. Further, the presence of annexin 6 on the cell surface was confirmed by fluorescence-activated cell sorting analysis using the annexin 6 antibody;annexin 4 is not present on the cell surface. In summary, annexin 6 is a candidate receptor for chondroitin sulfate chains.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Late endocytic compartments are major sites of annexin VI localization in NRK fibroblasts and polarized WIF-B hepatoma cells

Annexin VI is an abundant calcium- and phospholipid-binding protein whose intracellular distribution and function are still controversial. Using a highly specific antibody, we have studied the distribution of annexin VI in NRK fibroblasts and the polarized hepatic cell line WIF-B by confocal microscopy. In NRK cells, annexin VI was almost exclusively found associated with endocytic compartments, which were defined by their ability to receive fluid-phase marker internalized from the cell surface. However, extensive colocalization of annexin VI and the endocytic marker was only observed after about 45 min, indicating that annexin VI was primarily in late endocytic compartments or (pre)lysosomes. Consistent with this, annexin VI was predominantly seen on structures that contained the lysosomal protein lgp120, although not on dense core lysosomes by electron microscopy. Two major populations of annexin VI-containing structures were present in polarized WIF-B hepatocytes. One correlated to lgp120-positive (pre)lysosomes and was still observed after treatment with brefeldin A (BFA), while the other appeared to be partially associated with Golgi membranes and was BFA-sensitive. The striking association with prelysosomal compartments in NRK and WIF-B cells suggests that annexin VI could play a role in fusion events in the late endocytic pathway, possibly by acting as a tether between membranes.

Membrane-bound 3D structures reveal the intrinsic flexibility of annexin VI

Several quasi-ordered arrays and three two-dimensional crystal forms of annexin VI were obtained on artificial lipid monolayers. Three-dimensional reconstructions of the crystal forms exhibit marked differences in the orientations of the two lobes, revealing flexibility of the linker between the two lobes of annexin VI. Evidence is presented that the lobes may bind the monolayer in a parallel orientation, or an antiparallel orientation, in which the second lobe is turned away from the monolayer. It is hypothesized that annexin VI may also adopt several conformations in vivo, underlying different functional roles.

Mapping the site of interaction between annexin VI and the p120GAP C2 domain

Annexin VI is a Ca(2+)-dependent membrane and phospholipid binding protein. It mediates a protein-protein interaction with the Ras p21 regulatory protein p120GAP. In this study we have mapped the binding site of GAP within the annexin VI protein. Using Far Western overlay binding assays and cell lysate competition studies we have mapped the site of interaction to the inter-lobe linker region; amino acids 325-363. Finally, using a GST fusion protein corresponding to this linker region we have demonstrated that cellular loading of the fusion protein into Rat-1 fibroblasts by electroporation blocks the interaction and co-immunoprecipitation of annexin VI and GAP.

Crystal structure of bovine annexin VI in a calcium-bound state

The crystal structure of a calcium-bound form of bovine annexin VI has been determined with X-ray diffraction data to 2.9 A by molecular replacement. Six Ca2+ ions were found, five in AB loops, one in a DE loop. Two loops (II-AB, which binds calcium, and V-AB, which does not) have conformations that differ significantly from those in calcium-free, human recombinant annexin VI. There are only small differences between the calci- and the apo-annexin VI in the rest of the molecule. Calcium by itself does not promote a major conformational change.

Modulation of annexin VI--driven aggregation of phosphatidylserine liposomes by ATP

Annexin (Anx) VI has been implicated in mediating the endosome aggregation and vesicle fusion in secreting epithelia during exocytosis. In addition, AnxVI of porcine liver is an ATP-binding protein, and ATP in vitro modulates its interaction with membranes and cytoskeletal elements (Bandorowicz-Pikuła and Awasthi, FEBS Lett. 409 (1997) 300-306). In this study, we examined the effect of ATP on phosphatidylserine (PtdSer) aggregation in the presence of annexin and on calcium-dependent binding of protein to liposomes, and found that ATP stimulates the former process, although it increases the calcium concentration necessary for half-maximal binding of AnxVI to membranes. These results were corroborated by the experiments with fluorescent analog of ATP, in which binding of ATP to AnxVI was affected by binding of Ca2+ and/or phospholipids to the protein. Taken together they favour an idea of ATP being a functional ligand for AnxVI, which even in the relative absence of Ca2+ may modulate interaction of AnxVI with PtdSer-enriched membranes.

Glycosaminoglycan binding properties of annexin IV, V, and VI

We have previously demonstrated that annexin IV, one of the calcium/phospholipid-binding annexin family proteins, binds to glycosaminoglycans (GAGs) in a calcium-dependent manner (Kojima, K., Yamamoto, K., Irimura, T., Osawa, T., Ogawa, H., and Matsumoto, I. (1996) J. Biol. Chem. 271, 7679-7685). In this study, we investigated the GAG binding specificities of annexins IV, V, and VI by affinity chromatography and solid phase assays. Annexin IV was found to bind in a calcium-dependent manner to all the GAG columns tested. Annexin V bound to heparin and heparan sulfate columns but not to chondroitin sulfate columns. Annexin VI was adsorbed to heparin and heparan sulfate columns in a calcium-independent manner, and to chondroitin sulfate columns in a calcium-dependent manner. An N-terminal half fragment (A6NH) and a C-terminal half fragment (A6CH) of annexin VI, each containing four units, were prepared by digestion with V8 protease and examined for GAG binding activities. A6NH bound to heparin in the presence of calcium but not to chondroitin sulfate C, whereas A6CH bound to heparin calcium-independently and to chondroitin sulfate C calcium-dependently. The results showed that annexin IV, V, and VI have different GAG binding properties. Some annexins have been reported to be detected not only in the cytoplasm but also on the cell surface or in extracellular components. The findings suggest that the some annexins function as recognition elements for GAGs in extracellular space.

A nucleotide-binding domain of porcine liver annexin VI. Proteolysis of annexin VI labelled with 8-azido-ATP, purification by affinity chromatography on ATP-agarose, and fluorescence studies

Porcine liver annexin VI (AnxVI) of Mr 68.000 is an ATP-binding protein as evidenced by specific and saturable UV-dependent labelling with 8-azido-[gamma-32P]ATP or the fluorescent analog of ATP, 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine triphosphate and by binding of AnxVI to ATP-agarose. These characteristics of purified AnxVI were used to identify and characterize preliminary nucleotide-binding domain of the protein. AnxVI labelled with 8-azido-ATP was subjected to limited proteolysis and the proteolytic fragments of AnxVI that retained the covalently-bound nucleotide were separated by means of gel electrophoresis and visualized by exposure of the gel to a phosphor storage screen. It was found that the AnxVI proteolytic fragments of Mr 34-36.000 and smaller retained the nucleotide. In a reciprocal experiment, AnxVI was digested with proteolytic enzymes and in an ATP eluate from an ATP-agarose column protein fragments of similar Mr to these labelled with 8-azido-ATP were identified. The extent of AnxVI labelling with 8-azido-ATP and the distribution of proteolytic fragments varied upon calcium concentration. These results lead to the conclusion that there is a nucleotide-binding domain within the AnxVI molecule that is functionally similar to the nucleotide-binding domains of other nucleotide-binding proteins. The nucleotide-binding domain is located close to the tryptophan residue 343 of AnxVI and in close vicinity to the Ca2+- and phospholipid-binding sites of the protein. This is confirmed by the observation that the tryptophan fluorescence intensity of AnxVI decreases in the presence of a fluorescence analog of ATP in a calcium-dependent manner, due to the quenching properties of the nucleotide and/or fluorescence energy transfer from AnxVI tryptophan to fluorophore. Both processes were modulated by the presence of phospholipid molecules.

The alveolar macrophage as a model of calcium signaling in oxidative stress

Regulation of the free intracellular calcium concentration, [Ca2+]i, plays a major role in physiological signal transduction. Many of the essential enzymes in signaling cascades are Ca(2+)-dependent, as are numerous proteins that participate in the regulated function. Oxidative stress, which for many years was considered synonymous with cell and tissue injury, has more recently been demonstrated to alter signal transduction in both positive and negative directions. The realization that hydrogen peroxide and lipid hydroperoxides are produced as part of normal metabolism has led to the proposal that these oxidants function as second messengers. Exposure to environmental and other agents that produce hydroperoxides or the addition of exogenous hydroperoxides also causes elevation of [Ca2+]i in some cells. At sublethal exposure to hydroperoxides, the elevation in [Ca2+]i can either alter or mimic physiological stimulation. In addition to endoplasmic reticulum, mitochondria, and the extracellular space, the phospholipid- and Ca(2+)-binding proteins known as annexins constitute a Ca2+ pool from which this ion may be released under situations of oxidative stress. In this article, the source and consequences of Ca2+ elevation are reviewed with an emphasis on studies done with alveolar macrophages. These phagocytes, which modulate much of the physiological and immunological function of the lung, are susceptible targets for environmental oxidants.

Genes induced in programmed cell death of neuronal PC12 cells and developing sympathetic neurons in vivo

To identify primary response genes induced during early stages of neuronal programmed cell death (PCD), we screened by differential hybridization a subtracted cDNA library prepared from neuronal PC12 cells deprived of NGF for 6 hr in the presence of cycloheximide. Eight induced cDNA sequences were identified and designated message up-regulated during death (mud)-1-8. To determine which cloned sequences might be involved in neuronal PCD in vivo, expression of mud genes was analyzed in developing rat superior cervical ganglia (SCG) undergoing programmed cell death, using a combination of reverse Southern, reverse transcription polymerase chain reaction (RT-PCR), and in situ hybridization. Five sequences (mud-1, -3, -5/8, -6, and -7) are induced in SCG undergoing cell death in vivo, and induction of at least three of these (mud-3, -6, and -7) occurs in neurons. Partial sequence analysis reveals that mud-1 corresponds to annexin VI; mud-3 corresponds to rat PC3, mouse TIS21; mud-4 appears to be the rat homolog of human TAFII70; mud-5 and -8 are >85% identical members of the rodent gene family of B2-transcribed repeats; and mud-6 appears to be the rat homolog of human Ring 3 and Drosophila female sterile homeotic (fsh). Mud-2 and mud-7 encode novel sequences. These new candidate genes provide markers for early stages of neuronal PCD, are potentially involved in the cell death process, and serve to expand our view of cell death control in the developing nervous system.

Fluorescence spectroscopic studies on interactions between liver annexin VI and nucleotides--a possible role for a tryptophan residue

Annexin VI is a 68-kDa calcium-, phospholipid-, and cytoskeletal-element-binding protein, which has been implicated in various processes, including calcium release and sequestration in calcifying cartilage, in a receptor-mediated endocytosis in human fibroblasts, and in secretion from chromaffin granules. In these processes it was found that, in addition to Ca2+ and annexin, the presence of ATP is also a prerequisite. In the present report we show that annexin VI binds ATP and the binding of nucleotide to protein is accompanied by quenching of an intrinsic fluorescence of annexin VI, which was found to be specific for 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, GTP and ATP, and dependent on the annexin conformation. The nucleotide-binding site within an annexin VI molecule is likely to be close to the tryptophan-containing domain of annexin VI. We propose that ATP plays the role of a physiological ligand for annexin VI, and its binding to annexin VI may represent an alternative cellular mechanism for the regulation of annexin-membrane interactions coupled to overall energy transitions in the cell.

Hydroperoxide-induced increases in intracellular calcium due to annexin VI translocation and inactivation of plasma membrane Ca2+-ATPase

Oxidative stress can cause changes in intracellular free calcium concentration ([Ca2+]i) that resemble those occurring under normal cell signaling. In the alveolar macrophage, hydroperoxide-induced elevation of [Ca2+]i modulates the respiratory burst and other important physiologic functions. The source of Ca2+ released by hydroperoxide is intracellular but separate from the endoplasmic reticulum pool released by receptor-mediated stimuli (Hoyal, C. R., Gozal, E., Zhou, H., Foldenauer, K., and Forman, H. J. (1996) Arch. Biochem. Biophys. 326, 166-171). Previous studies in other cells have suggested that mitochondria are a potential source of oxidant-induced [Ca2+]i elevation. In this study we have identified another potential source of hydroperoxide-releasable intracellular calcium, that bound to annexin VI on the inner surface of the plasma membrane. Translocation of annexin VI from the membrane during exposure to t-butyl hydroperoxide matched elevation of [Ca2+]i as a function of time and t-butyl hydroperoxide concentration. The translocation was possibly due to a combination of ATP depletion and oxidative modification of membrane lipids and proteins. A sustained increase in [Ca2+]i occurring > 50 pmol/10(6) cells (50 microM under these conditions) appeared to be a consequence of membrane Ca2+-ATPase dysfunction. These results suggest that exposure to oxidative stress results in early alterations to the plasma membrane and concomitant release of Ca2+ into the cytosol. In addition it suggests a mechanism for participation of annexin VI translocation that may underlie the alterations in macrophage function by oxidative stress.

The Ca2+-dependent lipid binding domain of P120GAP mediates protein-protein interactions with Ca2+-dependent membrane-binding proteins. Evidence for a direct interaction between annexin VI and P120GAP

The CaLB domain is a 43-amino acid sequence motif found in a number of functionally diverse signaling proteins including three Ras-specific GTPase activating proteins (GAPs). In the Ras GTPase activating protein, P120(GAP), this domain has the ability to confer membrane association in response to intracellular Ca2+ elevation. Here we have isolated three proteins, p55, p70, and p120, which interact with the P120(GAP) CaLB domain in vitro. We identify p70 as the Ca2+-dependent phospholipid-binding protein annexin VI. Using co-immunoprecipitation studies, we have shown that the interaction between P120(GAP) and annexin VI is also detectable in rat fibroblasts, suggesting that this interaction may have a physiological role in vivo. Thus, the CaLB domain in P120(GAP) appears to have the ability to direct specific protein-protein interactions with Ca2+-dependent membrane-associated proteins. In addition, annexin VI is known to have tumor suppressor activity. Therefore, it is possible that the interaction of annexin VI with P120(GAP) may be important in the subsequent modulation of p21(ras) activity.