Ca(2+)-dependent phospholipid and arachidonic acid binding by the placental annexins VI and IV

ANXA6: a key molecular player in cancer progression and drug resistance

Annexin-A6 (ANXA6), a Ca²⁺-dependent membrane binding protein, is the largest of all conserved annexin families and highly expressed in the plasma membrane and endosomal compartments. As a multifunctional scaffold protein, ANXA6 can interact with phospholipid membranes and various signaling proteins. These properties enable ANXA6 to participate in signal transduction, cholesterol homeostasis, intracellular/extracellular membrane transport, and repair of membrane domains, etc. Many studies have demonstrated that the expression of ANXA6 is consistently altered during tumor formation and progression. ANXA6 is currently known to mediate different patterns of tumor progression in different cancer types through multiple cancer-type specific mechanisms. ANXA6 is a potentially valuable marker in the diagnosis, progression, and treatment strategy of various cancers. This review mainly summarizes recent findings on the mechanism of tumor formation, development, and drug resistance of ANXA6. The contents reviewed herein may expand researchers' understanding of ANXA6 and contribute to developing ANXA6-based diagnostic and therapeutic strategies.

Role of annexin A3 in breast cancer (Review)

Annexins are a large group of proteins occurring in numerous cell types. Annexins have roles in events such as coagulation inhibition, endocytosis, exocytosis, signal transduction, proliferation and programmed cell death. The association of annexins with numerous diseases has been reported. There are 12 annexin proteins in total and the association of annexin A3 (ANXA3) with numerous malignant tumor types, such as breast cancer, prostate cancer, lung cancer, stomach cancer and colon cancer, has been reported. Studies investigating the relationship between ANXA3 and breast cancer were analyzed in the present review and it was observed that ANXA3 is expressed at higher levels in breast cancer cells. Furthermore, high ANXA3 levels are a poor prognostic factor, increase the invasion ability of breast cancer cells and may be a novel therapeutic target.

Reduced Annexin A3 in schizophrenia

The cellular and molecular mechanisms underlying onset and development of schizophrenia have not yet been completely elucidated, but the association of disturbed neuroplasticity and inflammation has gained particular relevance recently. These mechanisms are linked to annexins functions. ANXA3, particularly, is associated to inflammation and membrane metabolism cascades. The aim was to determine the ANXA3 levels in first-onset drug-naïve psychotic patients. We investigated by western blot the protein expression of annexin A3 in platelets of first-onset, drug-naïve psychotic patients (diagnoses according to DSM-IV: 28 schizophrenia, 27 bipolar disorder) as compared to 30 age- and gender-matched healthy controls. Annexin A3 level was lower in schizophrenia patients as compared to healthy controls (p < 0.001) and to bipolar patients (p < 0.001). Twenty out of 28 schizophrenic patients had undetectable annexin A3 levels, as compared to none from the bipolar and none from the control subjects. ANXA3 was reduced in drug-naïve patients with schizophrenia. ANXA3 affects neuroplasticity, inflammation and apoptosis, as well as it modulates membrane phospholipid metabolism. All these processes have been discussed in regard to the biology of schizophrenia. In face of these data, we feel that further studies with larger samples are warranted to investigate the possible role of reduced ANXA3 as a possible risk marker for schizophrenia.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

Role of annexin A6 in cancer

Annexin A6 (AnxA6) is a member of a conserved superfamily of Ca²⁺-dependent membrane-binding annexin proteins. It participates in membrane and cytoskeleton organization, cholesterol homeostasis, membrane trafficking, cell adhesion and signal transduction. The expression levels of AnxA6 are closely associated with melanoma, cervical cancer, epithelial carcinoma, breast cancer, gastric cancer, prostate cancer, acute lymphoblastic leukemia, chronic myeloid leukemia, large-cell lymphoma and myeloma. AnxA6 exhibits dual functions in cancer, acting either as a tumor suppressor or promoter, depending on the type of cancer and the degree of malignancy. In several types of cancer, AnxA6 acts via Ras, Ras/MAPK and/or FAK/PI3K signaling pathways by mainly mediating PKCα, p120GAP, Bcr-Abl and YY1. In the present review, the roles of AnxA6 in different types of cancer are summarized.

Dances with Membranes: Breakthroughs from Super-resolution Imaging

Biological membrane organization mediates numerous cellular functions and has also been connected with an immense number of human diseases. However, until recently, experimental methodologies have been unable to directly visualize the nanoscale details of biological membranes, particularly in intact living cells. Numerous models explaining membrane organization have been proposed, but testing those models has required indirect methods; the desire to directly image proteins and lipids in living cell membranes is a strong motivation for the advancement of technology. The development of super-resolution microscopy has provided powerful tools for quantification of membrane organization at the level of individual proteins and lipids, and many of these tools are compatible with living cells. Previously inaccessible questions are now being addressed, and the field of membrane biology is developing rapidly. This chapter discusses how the development of super-resolution microscopy has led to fundamental advances in the field of biological membrane organization. We summarize the history and some models explaining how proteins are organized in cell membranes, and give an overview of various super-resolution techniques and methods of quantifying super-resolution data. We discuss the application of super-resolution techniques to membrane biology in general, and also with specific reference to the fields of actin and actin-binding proteins, virus infection, mitochondria, immune cell biology, and phosphoinositide signaling. Finally, we present our hopes and expectations for the future of super-resolution microscopy in the field of membrane biology.

Evidence for annexin A6-dependent plasma membrane remodelling of lipid domains

BACKGROUND AND PURPOSE

Annexin A6 (AnxA6) is a calcium-dependent phospholipid-binding protein that can be recruited to the plasma membrane to function as a scaffolding protein to regulate signal complex formation, endo- and exocytic pathways as well as distribution of cellular cholesterol. Here, we have investigated how AnxA6 influences the membrane order.

EXPERIMENTAL APPROACH

We used Laurdan and di-4-ANEPPDHQ staining in (i) artificial membranes; (ii) live cells to investigate membrane packing and ordered lipid phases; and (iii) a super-resolution imaging (photoactivated localization microscopy, PALM) and Ripley's K second-order point pattern analysis approach to assess how AnxA6 regulates plasma membrane order domains and protein clustering.

KEY RESULTS

In artificial membranes, purified AnxA6 induced a global increase in membrane order. However, confocal microscopy using di-4-ANEPPDHQ in live cells showed that cells expressing AnxA6, which reduces plasma membrane cholesterol levels and modifies the actin cytoskeleton meshwork, displayed a decrease in membrane order (∼15 and 30% in A431 and MEF cells respectively). PALM data from Lck10 and Src15 membrane raft/non-raft markers revealed that AnxA6 expression induced clustering of both raft and non-raft markers. Altered clustering of Lck10 and Src15 in cells expressing AnxA6 was also observed after cholesterol extraction with methyl-β-cyclodextrin or actin cytoskeleton disruption with latrunculin B.

CONCLUSIONS AND IMPLICATIONS

AnxA6-induced plasma membrane remodelling indicated that elevated AnxA6 expression decreased membrane order through the regulation of cellular cholesterol homeostasis and the actin cytoskeleton. This study provides the first evidence from live cells that support current models of annexins as membrane organizers.

ISG15 functions as an interferon-mediated antiviral effector early in the murine norovirus life cycle

Human noroviruses (HuNoV) are the leading cause of nonbacterial gastroenteritis worldwide. Similar to HuNoV, murine noroviruses (MNV) are enteric pathogens spread via the fecal-oral route and have been isolated from numerous mouse facilities worldwide. Type I and type II interferons (IFN) restrict MNV-1 replication; however, the antiviral effectors impacting MNV-1 downstream of IFN signaling are largely unknown. Studies using dendritic cells, macrophages, and mice deficient in free and conjugated forms of interferon-stimulated gene 15 (ISG15) revealed that ISG15 conjugation contributes to protection against MNV-1 both in vitro and in vivo. ISG15 inhibited a step early in the viral life cycle upstream of viral genome transcription. Directly transfecting MNV-1 RNA into IFN-stimulated mouse embryonic fibroblasts (MEFs) and bone marrow-derived dendritic cells (BMDC) lacking ISG15 conjugates bypassed the antiviral activity of ISG15, further suggesting that ISG15 conjugates restrict the MNV-1 life cycle at the viral entry/uncoating step. These results identify ISG15 as the first type I IFN effector regulating MNV-1 infection both in vitro and in vivo and for the first time implicate the ISG15 pathway in the regulation of early stages of MNV-1 replication.

IMPORTANCE

Type I IFNs are important in controlling murine norovirus 1 (MNV-1) infections; however, the proteins induced by IFNs that restrict viral growth are largely unknown. This report reveals that interferon-stimulated gene 15 (ISG15) mitigates MNV-1 replication both in vitro and in vivo. In addition, it shows that ISG15 inhibits MNV-1 replication by targeting an early step in the viral life cycle, MNV-1 entry and/or uncoating. These results identify ISG15 as the first type I IFN effector regulating MNV-1 infection both in vitro and in vivo and for the first time implicate the ISG15 pathway in the regulation of viral entry/uncoating.

Enzymatic and transcriptional regulation of the cytoplasmic acetyl-CoA hydrolase ACOT12

Acyl-CoA thioesterase 12 (ACOT12) is the major enzyme known to hydrolyze the thioester bond of acetyl-CoA in the cytosol in the liver. ACOT12 contains a catalytic thioesterase domain at the N terminus and a steroidogenic acute regulatory protein-related lipid transfer (START) domain at the C terminus. We investigated the effects of lipids (phospholipids, sphingolipids, fatty acids, and sterols) on ACOT12 thioesterase activity and found that the activity was inhibited by phosphatidic acid (PA) in a noncompetitive manner. In contrast, the enzymatic activity of a mutant form of ACOT12 lacking the START domain was not inhibited by the lipids. These results suggest that the START domain is important for regulation of ACOT12 activity by PA. We also found that PA could bind to thioesterase domain, but not to the START domain, and had no effect on ACOT12 dissociation. ACOT12 is detectable in the liver but not in hepatic cell lines such as HepG2, Hepa-1, and Fa2N-4. ACOT12 mRNA and protein levels in rat primary hepatocytes decreased following treatment with insulin. These results suggest that cytosolic acetyl-CoA levels in the liver are controlled by lipid metabolites and hormones, which result in allosteric enzymatic and transcriptional regulation of ACOT12.

Characterization of platelet aminophospholipid externalization reveals fatty acids as molecular determinants that regulate coagulation

Aminophospholipid (APL) trafficking across the plasma membrane is a key event in cell activation, apoptosis, and aging and is required for clearance of dying cells and coagulation. Currently the phospholipid molecular species externalized are unknown. Using a lipidomic method, we show that thrombin, collagen, or ionophore-activated human platelets externalize two phosphatidylserines (PSs) and five phosphatidylethanolamines (PEs). Four percent of the total cellular PE/PS pool (∼300 ng/2 × 10(8) cells, thrombin), is externalized via calcium mobilization and protease-activated receptors-1 and -4, and 48% is contained in microparticles. Apoptosis and energy depletion (aging) externalized the same APLs in a calcium-dependent manner, and all stimuli externalized oxidized phospholipids, termed hydroxyeicosatetraenoic acid-PEs. Transmembrane protein-16F (TMEM-16F), the protein mutated in Scott syndrome, was required for PE/PS externalization during thrombin activation and energy depletion, but not apoptosis. Platelet-specific APLs optimally supported tissue factor-dependent coagulation in human plasma, vs. APL with longer or shorter fatty acyl chains. This finding demonstrates fatty acids as molecular determinants of APL that regulate hemostasis. Thus, the molecular species of externalized APL during platelet activation, apoptosis, and energy depletion were characterized, and their ability to support coagulation revealed. The findings have therapeutic implications for bleeding disorders and transfusion therapy. The assay could be applied to other cell events characterized by APL externalization, including cell division and vesiculation.

Overexpression of annexin A4 is associated with chemoresistance in papillary serous adenocarcinoma of the ovary

Annexin A4 study in ovarian cancer has been primarily focused on clear cell carcinoma, which exhibits strong resistance to chemotherapy. The aim of this study was to examine the expression and cellular localization of annexin A4 in serous ovarian carcinomas. We evaluated the expression of annexin A4 with real-time polymerase chain reaction in 40 ovarian serous carcinoma tissues. Furthermore, the distribution of the protein within the tumor was studied by immunohistochemistry in 70 epithelial ovarian carcinoma tissues. The levels of annexin A4 transcripts were higher in 14 chemoresistant tumors than those in 26 chemosensitive tumors (P = .013). Immunohistochemical expressions showed that nuclear expression was detected in 14 (20.0%) of 70 samples, and cytoplasmic expression was detected in 17 (24.3%) of 70 samples. The results showed that 35.7% of women with nuclear expression were resistant to platinum-based chemotherapy, whereas only 14.3% of women without expression were chemoresistant (P = .065). In addition, patients with nuclear staining had significantly shorter disease-free survival than did patients who showed negative staining. Multivariate proportional hazards model revealed that the stage and nuclear annexin A4 expression were independent prognostic factors (hazard ratios, 6.34 [P = .001] and 2.85 [P = .011], respectively). This study showed that overexpression and nuclear localization of annexin A4 are related to chemoresistance and poor survival in patients with serous papillary ovarian carcinomas. Future studies are required to develop new therapies targeting annexin A4 in patients with ovarian epithelial adenocarcinoma.

Proteomics analyses of human optic nerve head astrocytes following biomechanical strain

We investigate the role of glial cell activation in the human optic nerve caused by raised intraocular pressure, and their potential role in the development of glaucomatous optic neuropathy. To do this we present a proteomics study of the response of cultured, optic nerve head astrocytes to biomechanical strain, the magnitude and mode of strain based on previously published quantitative models. In this case, astrocytes were subjected to 3 and 12% stretches for either 2 h or 24 h. Proteomic methods included nano-liquid chromatography, tandem mass spectrometry, and iTRAQ labeling. Using controls for both stretch and time, a six-plex iTRAQ liquid chromatography- tandem MS (LC/MS/MS) experiment yielded 573 proteins discovered at a 95% confidence limit. The pathways included transforming growth factor β1, tumor necrosis factor, caspase 3, and tumor protein p53, which have all been implicated in the activation of astrocytes and are believed to play a role in the development of glaucomatous optic neuropathy. Confirmation of the iTRAQ analysis was performed by Western blotting of various proteins of interest including ANXA 4, GOLGA2, and αB-Crystallin.

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.

Increased annexin-V and decreased TNF-α serum levels in chronic-medicated patients with schizophrenia

Schizophrenia (SZ) is a chronic severe mental disorder. Increased inflammatory processes have been shown in acute and chronic SZ. Apoptotic processes may alter the neuronal network and are involved in the pathogenesis of several neurodegenerative diseases, such as SZ. Annexin-V seems to have a role on inhibition of pro-inflammatory activities during apoptosis. Tumor necrosis factor (TNF-alpha) is a cytokine involved in systemic inflammation and is a member of a group of cytokines which stimulate acute phase reactions. A chronic immune activation in SZ has been shown. The aim of this study was to compare annexin-V and TNF-alpha serum levels in chronic medicated patients with SZ and healthy controls. Thirty-eight outpatients from the HCPA Schizophrenia Program and 38 healthy controls were enrolled to this study protocol. Annexin-V and TNF-alpha serum levels were measured with ELISA. Serum annexin-V levels were significantly higher in patients with SZ than in controls (p<0.001) and TNF-alpha significantly lower (p<0.001). The present result of increased annexin-V and decreased serum levels of TNF-alpha compared to controls may be a result of the stabilization phase of psychosis and a reduction in metabolic brain aggression. In this complex picture, increased levels of annexin-V and decreased levels of TNF-alpha in our sample would be explained by illness stability and chronic treatment. Our findings support the hypothesis of a state dependant process of inflammation in SZ. Further prospective studies to clarify the findings described in this paper are needed.

Targeting annexin A4 to counteract chemoresistance in clear cell carcinoma of the ovary

IMPORTANCE OF THE FIELD

Epithelial ovarian cancer (EOC) is the leading cause of death from gynecological malignancies in Western countries. Among the four major histological subtypes of EOC, clear cell carcinoma (CCC) of the ovary is highly resistant to platinum-based chemotherapy and is consequently associated with poor patient prognosis in advanced stages.

AREAS COVERED IN THIS REVIEW

An overview of the clinical characteristics of ovarian CCC; the role of annexin family proteins in tumor development and progression; the role of annexin A4 in enhancing cellular drug resistance; recent studies linking annexin A4 overexpression to chemoresistance in tumors of ovarian CCC.

WHAT THE READER WILL GAIN

Insight into the emerging role for annexin A4 in enhancing chemoresistance in ovarian CCC.

TAKE HOME MESSAGE

Annexin A4 enhances cancer cell chemoresistance and is overexpressed in tumors of patients with ovarian CCC. Targeting of annexin A4 may represent a future strategy to counteract resistance to chemotherapy in ovarian CCC.

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.

Annexin A4 interacts with the NF-kappaB p50 subunit and modulates NF-kappaB transcriptional activity in a Ca2+-dependent manner

Previously, we identified annexin A4 (ANXA4) as a candidate substrate of caspase-3. Proteomic studies were performed to identify interacting proteins with a view to determining the roles of ANXA4. ANXA4 was found to interact with the p105. Subsequent studies revealed that ANXA4 interacts with NF-kappaB through the Rel homology domain of p50. Furthermore, the interaction is markedly increased by elevated Ca(2+) levels. NF-kappaB transcriptional activity assays demonstrated that ANXA4 suppresses NF-kappaB transcriptional activity in the resting state. Following treatment with TNF-alpha or PMA, ANXA4 also suppressed NF-kappaB transcriptional activity, which was upregulated significantly early after etoposide treatment. This difference may be due to the intracellular Ca(2+) level. Additionally, ANXA4 translocates to the nucleus together with p50, and imparts greater resistance to apoptotic stimulation by etoposide. Our results collectively indicate that ANXA4 differentially modulates the NF-kappaB signaling pathway, depending on its interactions with p50 and the intracellular Ca(2+) ion level.

alpha14-Giardin (annexin E1) is associated with tubulin in trophozoites of Giardia lamblia and forms local slubs in the flagella

In a previous study, we reported that the novel annexin XX1 (annexin E1), identical to alpha14-giardin, is specifically localized to the flagella and to the median body of the trophozoites. However, the mode of interaction and the direct partners involved remained unclear. In the present study, we show that alpha4-giardin obviously does not evenly distribute over the full length of the axonemes, but rather, resides at local slubs near the proximal part and the ends of the flagella. In immunocytochemical co-localization studies, the anti-giardin primary antibody exclusively reacted with distinct regions of the flagella in permeabilized cells, whereas the anti-tubulin antibody bound to all areas of the axonemes in the cells and to isolated cytoskeletons. Isolated cytoskeletons did not react with anti-giardin antibodies. alpha14-Giardin itself is able to assemble to multimeric structures. Taken together, our findings suggest that alpha14-giardin adheres to microtubules of the flagella via self-assembly that may regulated by Ser/Thr-phosphorylation.

An initial proteomic analysis of human preterm labor: placental membranes

Human preterm labor (PL) is the single most significant problem in modern Obstetrics and Gynecology, affecting approximately 10% of pregnancies worldwide, constituting the leading cause of perinatal mortality and morbidity, and contributing significantly to chronic childhood disease. Currently, our molecular understanding of PL remains staggeringly inadequate to reliably diagnose or rationally intervene in PL events; several molecular alterations have been implicated in PL, but these have proven of limited value as diagnostic/prognostic markers. The majority of PL events remain spontaneous and unpredictable: critical care emergencies. Here, we apply functional proteomics to dissect molecular mechanisms of human PL. Human placental tissue was collected in clearly differentiated cases of preterm and term labor. Highly refined two-dimensional gel electrophoresis (2DE) was used for protein separation, coupled with automated differential gel image analysis to compare the resulting proteomic maps. For this initial study, only the most important protein differences were selected for further analysis, that is, proteins that were unique to one sample, and absent from the other, with 100% reproducibility across the sample population. In total, 11 such proteins were identified by tandem mass spectrometry, falling into three distinct functional classes: structural/cytoskeletal components, ER lumenal proteins with enzymatic or chaperone functions, and proteins with anticoagulant properties. These expression changes form the groundwork for further molecular investigation of this devastating medical condition. This approach therefore holds the potential not only to define the underlying molecular components, but also to identify novel diagnostic tools and targets for rational drug intervention.

Annexin A4 self-association modulates general membrane protein mobility in living cells

Annexins are Ca2+-regulated phospholipid-binding proteins whose function is only partially understood. Annexin A4 is a member of this family that is believed to be involved in exocytosis and regulation of epithelial Cl- secretion. In this work, fluorescent protein fusions of annexin A4 were used to investigate Ca2+-induced annexin A4 translocation and self-association on membrane surfaces in living cells. We designed a novel, genetically encoded, FRET sensor (CYNEX4) that allowed for easy quantification of translocation and self-association. Mobility of annexin A4 on membrane surfaces was investigated by FRAP. The experiments revealed the immobile nature of annexin A4 aggregates on membrane surfaces, which in turn strongly reduced the mobility of transmembrane and plasma membrane associated proteins. Our work provides mechanistic insight into how annexin A4 may regulate plasma membrane protein function.

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

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

Biophysical and molecular properties of annexin-formed channels

The annexins are water soluble proteins possessing a hydrophilic surface, which belong to a family of proteins which (a) bind ('annex') both calcium and phospholipids, and (b) form voltage-dependent calcium channels within planar lipid bilayers. Annexins types are diverse (94 annexins in 45 species) and they belong to an enormous multigene family that ranges throughout all eukaryotic kingdoms. Although the structure of these proteins is now well known their functional and physiological roles remain largely unknown and circumstantial. Various experimental approaches provided evidence that annexins function as Ca(2+) channels that could act as regulators of membrane fusion. The identity of annexins is derived from the conserved 34 kDa C-terminal domain which comprises four repeats - except for annexin VI, with eight repeats - of a sequence of approximately seventy amino acids, which holds the area known as the 'endonexin fold', with its identifying GXGTDE. Annexins have been placed into three subgroups of (1) tetrad core and short amino terminal, (2) tetrad core and long amino terminal, and (3) octad core and short amino terminal. The repeats are highly conserved, each forming a compact alpha-helical domain comprising five alpha-helices wound in a right-handed superhelix. Four domains are formed, arranged in a nearly flat and cyclical array, with domains I and IV, and II and III respectively forming two tightly organised modules with almost twofold symmetry. A hydrophilic pore lies at the centre of the molecule, forming a prominent ion channel coated with charged and highly conserved residues. The annexin molecule is slightly curved, with both a convex and a concave face. The cation/anion permeability ratios and the selectivity sequence of the ion channels formed by several annexins confirm the selectivity of the annexins for Ca(2+) over other divalent cations, and reveals the importance of structural sites, e.g. amino acid positions 17, 78, 95 and 112 for the identification of the ion channel's position, function and regulation. Some are sensitive to low doses of the phenothiazine drugs, trifluoperazine (an anti-schizophrenia drug) and promethazine (anti nausea drug) La(3+) and Cd(2+), (blockers of voltage-gated Ca(2+) channels) nifedipine (an inhibitor of non-activating Ca(2+) channels). There are two main competing models used to explain in vitro ion channel activity of annexins: one involves changes in the conductance of ion via electrostatic disturbance of the membrane surface; the other involves a much more extensive alteration in protein structure and a correspondingly deeper penetration into the membrane.

Investigating the role played by protein-lipid and protein-protein interactions in the membrane association of the p120GAP CaLB domain

The GTPase activating protein, p120GAP, contains an amino acid sequence motif called the Ca2+-dependent lipid binding domain (CaLB) which mediates a protein-protein interaction between p120GAP and annexin VI and also binds to negatively charged phospholipids. Because membrane association of p120GAP is important for the regulation of p21 Ras activity, we have studied the roles played by Ca2+, phospholipids and annexin VI in the membrane association of p120GAP. Here we demonstrate that a truncated CaLB domain GST fusion protein (GSTGAP618-632), lacking the ability to bind to phospholipids, is able to bind to rat fibroblast membranes in a Ca2+- and concentration-dependent manner. In addition, this fusion protein also binds to annexin VI in an amino acid sequence specific but Ca2+ independent manner. Also, when bound to annexin VI in the presence of Ca2+, this fusion protein has the ability to co-bind to phosphatidylserine vesicles. Thus, annexin VI may simultaneously mediate an interaction with p120GAP and also an interaction with membrane phospholipids. This may in part explain the mechanism by which p120GAP associates with membranes in response to Ca2+ elevation and suggests the potential importance of annexin VI in the regulation of p21 Ras and the role CaLB domains may play in the specific recognition of cellular membranes.

Functional identification of alpha 1-giardin as an annexin of Giardia lamblia

A protein with a relative molecular mass of 31 kDa was specifically extracted by EGTA from a detergent-insoluble fraction of Giardia lamblia. N-terminal sequencing showed this protein to be identical to alpha 1-giardin, a component of the ventral disc which, based on its predicted amino acid sequence, has been classified as annexin XIX. Purified alpha 1-giardin associated with multilamellar phosphatidyl serine-containing vesicles in a Ca(2+)-dependent manner, confirming that it is a functional annexin. Molecular modelling of the amino acid sequence of the giardial annexin into the X-ray structure of annexin V suggests that the Ca(2+)-binding sites, which, as in other annexins, are all located on the convex surface of the molecule, are of the low-affinity type III.

Annexin XIIIb associates with lipid microdomains to function in apical delivery

A member of the annexin XIII sub-family, annexin XIIIb, has been implicated in the apical exocytosis of epithelial kidney cells. Annexins are phospholipid-binding proteins that have been suggested to be involved in membrane trafficking events although their actual physiological function remains open. Unlike the other annexins, annexin XIIIs are myristoylated. Here, we show by immunoelectron microscopy that annexin XIIIb is localized to the trans-Golgi network (TGN), vesicular carriers and the apical cell surface. Polarized apical sorting involves clustering of apical proteins into dynamic sphingolipid-cholesterol rafts. We now provide evidence for the raft association of annexin XIIIb. Using in vitro assays and either myristoylated or unmyristoylated recombinant annexin XIIIb, we demonstrate that annexin XIIIb in its native myristoylated form stimulates specifically apical transport whereas the unmyristoylated form inhibits this route. Moreover, we show that formation of apical carriers from the TGN is inhibited by an anti-annexin XIIIb antibody whereas it is stimulated by myristoylated recombinant annexin XIIIb. These results suggest that annexin XIIIb directly participates in apical delivery.

The relationship between the binding of ATP and calcium to annexin IV. Effect of nucleotide on the calcium-dependent interaction of annexin with phosphatidylserine

With the use of ATP analogues, we have found that porcine liver annexin (Anx) IV can be covalently labelled with 8-azido[gamma-32P]-ATP in the presence of Ca2+ (Kd 4.2 microM) and that the labelling is prevented by asolectin/cholesterol liposomes or chelation of calcium ions. On the other hand, non-covalent binding of 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP) to AnxIV occurs optimally in the presence of liposomes and Ca2+ (Kd 7 microM). These observations were further confirmed by the results of intrinsic fluorescence quenching of AnxIV with various nucleotides, suggesting the existence of a relationship between Ca(2+)-, phospholipid- and ATP-binding sites within the annexin molecule. The interaction of AnxIV with nucleotides does not significantly affect its in vitro properties concerning the binding to phosphatidylserine (PS) monolayers.

Functional and genetic analysis of annexin VI

This study is concerned with the determination of the function of the 68kDa calcium-binding protein, annexin VI. Studies on the structure and regulation of the gene include a detailed analysis of annexin VI expressed heterologously in human A431 carcinoma cells. We have recently discovered that annexin VI is subject to a novel growth dependent post-translational modification. Interestingly, the protein exerts a negative effect on A431 cells. This effect was manifested as a partial reversal of the transformed phenotype. We are currently exploring the hypothesis that the post-translational modification of annexin VI is required for sub-cellular targeting, and that correct localisation within the cell is essential for function.

EPR study of annexin V-cardiolipin Ca-mediated interaction in phospholipid vesicles and isolated mitochondria

The properties of the binding of annexin V to variously composed phospholipid vesicles have been studied by applying a recently developed EPR method, using an annexin V spin label. By this approach, this protein is seen to bind to acidic phospholipid-containing vesicles, as reported, thus confirming the reliability of the method. In addition, binding of this annexin to cardiolipin-containing vesicles has been studied in more depth, and the protein has been shown to have a distinct affinity for this phospholipid. As a cardiolipin-rich natural membrane system, mitochondrial membranes and mitoplasts from rat liver were considered, and a strong binding of AV to these membranes was observed. Having compared this binding with that to phospholipid vesicles, cardiolipin-rich microdomains in the mitochondrial membranes are proposed as the putative mitochondrial binding sites for annexin V.

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.

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.

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.

Phospholipid determinants for annexin V binding sites and the role of tryptophan 187

Annexin V is part of a family of Ca(2+)-dependent phospholipid-binding proteins, whose purported functions are related to their interactions with biological membranes. While Ca(2+)-dependent binding to phospholipids is well-established, the specific structural interactions within the phospholipid-binding sites have only been inferred to resemble those of phospholipase A2, with no direct structural evidence. In this study, the binding avidity of various phospholipid analogs, with variations at the headgroup or sn-2 acyl chain, was monitored in a C12E8 detergent micelle system using the increase in fluorescence of tryptophan 187. Micelles also contained excess negative surface charge to saturate a nonspecific component of the binding. The Ca2+ and phospholipid concentrations required for the binding of annexin V to various phospholipid headgroups were very similar, except for the relatively weak binding to phosphatidylinositol (PI). The unique close proximity of the PI sugar ring to the phosphate group may lead to steric hindrance in this case. Binding was also strongly dependent on the presence of an sn-3 phosphate group and an sn-2 acyl chain, as previously observed. The relatively shallow nature of the annexin V phospholipid-binding sites was reflected by the nearly equivalent binding of D and L versions of phospholipids, i.e., a large shift in the position of the sn-1 acyl chain is accommodated in this case. Binding of annexin V does not specifically require an ester carbonyl oxygen, as it occurs with ether-linked, amide-linked, and phosphonate-linked sn-2 hydrocarbon chains, under these conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of cardiac annexin V from the beagle dog heart and changes in its localization in the ischemic rat heart

We isolated and purified 35 kDa protein from the myocardium of the beagle dog and identified it to be annexin V from partial amino acid sequence determination. It was confirmed that anticanine cardiac annexin V rabbit polyclonal antibody, which was produced using the 35 kDa protein, cross-reacts with annexin V of the myocardium, lung, liver, kidney, and brain of the rat. The localization of cardiac annexin V and the effect of ischemia for 30-180 min in the rat were immunohistochemically studied with the use of the Langendorff perfusion heart. In the normal myocardium, annexin V, accompanied by cross-striation, was observed throughout the cell. In ischemia of 30 min, extracellular leakage of annexin V was observed with uneven staining in the cytoplasm. When the ischemic time exceeded 60 min, annexin V was observed in the cell membrane with a decrease of annexin V in the cytoplasm. Also, extracellular leakage of annexin V was observed prominently. In ischemia for 180 min, almost all the annexin V in the cytoplasm disappeared. These results suggest that the level of ischemia can be estimated from the changes in localization of annexin V.

Interdependence of phospholipid specificity and calcium binding in annexin I as shown by site-directed mutagenesis

We have mutated the lysine 128 of domain II of annexin I, which flanks a putative calcium-binding loop, into a glutamic acid residue. The properties of the mutated recombinant protein were compared to those of the wild-type recombinant protein. A change in the isotherm of calcium binding in the presence of lipids was observed. A slight decrease in the affinity for lipids was evident. When tested for the vesicle aggregation property, the mutation induced a change in lipid specificity; unlike the wild-type protein, the mutant protein aggregates vesicles containing phosphatidylserine plus phosphatidylethanolamine better than vesicles containing only phosphatidylserine. These experiments are in agreement with a model which suggests that a lipid molecule is inserted into the calcium-binding loop of annexin I and that the conserved lysine residue is involved in the specificity of annexins for anionic phospholipids.

Endocytosis occurs independently of annexin VI in human A431 cells

Annexin VI is one of a family of calcium-dependent phospholipid-binding proteins. Although the function of this protein is not known, various physiological roles have been proposed, including a role in the budding of clathrin-coated pits (Lin et al., 1992. Cell. 70:283-291.). In this study we have investigated a possible endocytotic role for annexin VI in intact cells, using the human squamous carcinoma cell line A431, and report that these cells do not express endogenous annexin VI, as judged by Western and Northern blotting and PCR/Southern blotting. To examine whether endocytosis might in some way be either facilitated or inhibited by the presence of annexin VI, a series of A431 clones were isolated in which annexin VI expression was achieved by stable transfection. These cells expressed annexin VI at similar levels to other human cell types. Using assays for endocytosis and recycling of the transferrin receptor, we report that each of these cellular processes occurs with identical kinetics in both transfected and wild-type A431 cells. In addition, purified annexin VI failed to support the scission of coated pits in permeabilized A431 cells. We conclude that annexin VI is not an essential component of the endocytic pathway, and that in A431 cells, annexin VI fails to exert any influence on internalization and recycling of the transferrin receptor.

Calcium-dependent annexin V binding to phospholipids: stoichiometry, specificity, and the role of negative charge

Annexin V is a Ca(2+)-dependent, phospholipid-binding protein that may have one or more membrane-related functions. The binding of annexin V to phospholipids in a detergent micelle matrix was studied to attempt to determine directly the stoichiometry of specific phospholipid-binding sites and the importance of negative charge. When annexin V binds to phospholipids, a large increase (severalfold) of the emission intensity of tryptophan 187 is observed. This intensity change was used to monitor the binding to phosphatidylcholine (PC) or phosphatidylserine (PS) at varying ratios with the detergent, octaethylene glycol monododecyl ether (C12E8). No binding to PC alone in these micelles could be observed, while approximately 10 PS molecules per micelle were required to observe binding. However, inclusion of negatively charged amphiphiles in the micelles, such as oleic acid or dodecyl sulfate, allowed the observation of binding to PC and decreased the number of phospholipids per micelle necessary for binding to both PS and PC. By including increasing proportions of dodecyl sulfate in the C12E8 micelles, a minimum average number of PS or PC per micelle of approximately 3-4 was required for complete binding. Labeling with photoreactive phospholipids under similar conditions led to an average of approximately 4-5 phospholipids covalently bound per annexin V monomer. Since annexin V has four similar domains, it is reasonable to suggest that one phospholipid binding site is associated with each domain, although as few as three functional domains may be sufficient for binding. Efficient binding required certain structural features of the phospholipid, including a phosphate group, an sn-2 acyl chain, and at least a few carbons on the sn-2 chain. Phospholipid headgroups were almost irrelevant, except for important surface charge effects on the interfacial ionic double layer. A negative surface charge on the micellar aggregate nonspecifically increases the Ca2+ concentration near the micelle surface and may also directly enhance the affinity of annexin V for phospholipids, as shown by the decreased two-dimensional phospholipid concentration necessary for binding. The ability to bind to zwitterionic phospholipids in the presence of a nonspecific negative surface charge may be relevant to the extracellular functions of this protein. Relatively weak individual phospholipid-binding sites that easily exchange were observed, suggesting rapid exchange of phospholipids between the sites on membrane-bound annexin V. These data suggest a working hypothesis that includes approximately four binding sites specific for phospholipid phosphate groups and sn-2 acyl chains.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase C and annexins: unusual calcium response elements in the cell

Protein kinase C and the annexins appear to share some unusual and potentially important membrane- and calcium-binding properties. While these proteins are calcium response elements, they are not calcium-binding proteins in the formal sense; at intracellular calcium concentrations, they only bind significant amounts of calcium when membranes or other suitable surfaces are present. The number of calcium ions bound per protein is large (> 8) and this stoichiometry, at the protein-membrane interface, may provide the large number of contact points needed for the very high-affinity interaction that is observed. The further ability of annexins and PKC to form structures with properties of integral membrane proteins may be important to provide a type of long-term cell signalling that produces a constitutively active kinase or ion channel activity. Selectivity for phospholipids in bilayer form is modest with respect to the acidic phospholipids but there is a surprising preference for phosphatidylethanolamine as the neutral phospholipid matrix. Along with other unusual properties, these proteins offer the potential for unique types of cell regulation events.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.

Arachidonic acid and cell signalling in the ovary and placenta

Arachidonic acid (AA) and its metabolites make up a diverse group of signalling molecules important to mediation of metabolic and endocrine function of ovarian and placental cell membranes. This paper reviews recent literature examining AA and eicosanoid involvement in the functional dynamics of follicular development, ovulation and corpus luteum function. The putative roles of AA metabolites in establishment and maintenance of pregnancy are reviewed with reference to decidualization, trophoblast invasion and implantation, maintenance of perfusion of the feto-placental unit and lipid transfer. Finally, recent evidence implicating AA metabolism in mediation of enzyme activity following hormone-receptor coupling within various cells types comprising the placental membranes is reviewed.

Importance of phosphatidylethanolamine for association of protein kinase C and other cytoplasmic proteins with membranes

Biological membranes exhibit an asymmetric distribution of phospholipids. Phosphatidylserine (PS) is an acidic phospholipid that is found almost entirely on the interior of the cell where it is important for interaction with many cellular components. A less well understood phenomenon is the asymmetry of the neutral phospholipids, where phosphatidylcholine (PC) is located primarily on exterior membranes while phosphatidylethanolamine (PE) is located primarily on interior membranes. The effect of these neutral phospholipids on protein-phospholipid associations was examined using four cytoplasmic proteins that bind to membranes in a calcium-dependent manner. With membranes containing PS at a charge density characteristic of cytosolic membranes, protein kinase C and three other proteins with molecular masses of 64, 32, and 22 kDa all showed great selectively for membranes containing PE rather than PC as the neutral phospholipid; the calcium requirements for membrane-protein association of the 64- and 32-kDa proteins were about 10-fold lower with membranes containing PE; binding of the 22-kDa protein to membranes required the presence of PE and could not even be detected with membranes containing PC. Variation of the PS/PE ratio showed that membranes containing about 20% PS/60% PE provided optimum conditions for binding and were as effective as membranes composed of 100% PS. Thus, PE, as a phospholipid matrix, eliminated the need for membranes with high charge density and/or reduced the calcium concentrations needed for protein-membrane association. A surprising result was that PKC and the 64- and 32-kDa proteins were capable of binding to neutral membranes composed entirely of PE/PC or PC only. The different phospholipid headgroups altered only the calcium required for membrane-protein association. For example, calcium concentrations at the midpoint for association of the 64-kDa protein with membranes containing PS, PE/PC, or PC occurred at 6, 100, and 20,000 microM, respectively. Thus, biological probes detected major differences in the surface properties of membranes containing PE versus PC, despite the fact that both of these neutral phospholipids are often thought to provide "inert" matrices for the acidic phospholipids. The selectivity for membranes containing PE could be a general phenomenon that is applicable to many cytoplasmic proteins. The present study suggested that the strategic location of PE on the interior of the membranes may be necessary to allow some membrane-protein associations to occur at physiological levels of calcium and PS.