Annexin V perturbs or stabilises phospholipid membranes in a calcium-dependent manner

Annexin V-containing cubosomes for targeted early detection of apoptosis in degenerative retinal tissue

ANX–PS–Phy cubosomes could be applied as a safe and robust drug delivery vehicle for targeting damaged, apoptotic cells in ocular diseases.

Inhibition of Rho-ROCK signaling induces apoptotic and non-apoptotic PS exposure in cardiomyocytes via inhibition of flippase

Subsequent to myocardial infarction, cardiomyocytes within the infarcted areas and border zones expose phosphatidylserine (PS) in the outer plasma membrane leaflet (flip-flop). We showed earlier that in addition to apoptosis, this flip-flop can be reversible in cardiomyocytes. We now investigated a possible role for Rho and downstream effector Rho-associated kinase (ROCK) in the process of (reversible) PS exposure and apoptosis in cardiomyocytes. In rat cardiomyoblasts (H9c2 cells) and isolated adult ventricular rat cardiomyocytes Clostridium difficile Toxin B (TcdB), a Rho GTPase family inhibitor, C3 transferase (C3), a Rho(A,B,C) inhibitor and the ROCK inhibitors Y27632 and H1152 were used to inhibit Rho-ROCK signaling. PS exposure was assessed via flow cytometry and fluorescent digital imaging microscopy using annexin V. Akt expression and phosphorylation were analyzed via Western blot, and Akt activity was inhibited by wortmannin. The cellular concentration activated caspase 3 was determined as a measure of apoptosis, and flippase activity was assessed via flow cytometry using NBD-labeled PS. TcdB, C3, Y27632 and H1152 all significantly increased PS exposure. TcdB, Y27632 and H1152 all significantly inhibited phosphorylation of the anti-apoptotic protein Akt and Akt inhibition by wortmannin lead to increased PS exposure. However, only TcdB and C3, but not ROCK- or Akt inhibition led to caspase 3 activation and thus apoptosis. Notably, pancaspase inhibitor zVAD only partially inhibited TcdB-induced PS exposure indicating the existence of apoptotic and non-apoptotic PS exposure. The induced PS exposure coincided with decreased flippase activity as measured with NBD-labeled PS flip-flop. In this study, we show a regulatory role for a novel signaling route, Rho-ROCK-flippase signaling, in maintaining asymmetrical membrane phospholipid distribution in cardiomyocytes.

Study of two markers of apoptosis and meiotic segregation in ejaculated sperm of chromosomal translocation carrier patients

BACKGROUND

To try to explain the infertility of chromosomal translocation carrier patients, we compared the expression of two markers of apoptosis in the sperm of patients and of fertile donors, and we studied the meiotic segregation in the ejaculated sperm of these translocation carriers.

METHODS

Twenty semen samples of translocation carriers, [reciprocal (n=14) and Robertsonian translocations (n=6)], were compared with the semen samples of donors (n=20). Different tests were applied: annexin V binding assay; terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL); and fluorescence in situ hybridization (FISH).

RESULTS

The annexin V binding assay in sperm of patients with chromosomal translocation (n=17) showed a significantly increased proportion of sperm with externalized phosphatidylserine (PS) than in the control group (n=20, P<or=0.05). The rates of DNA fragmentation investigated by TUNEL reaction were higher in samples of translocation carriers (n=14) than in donors (n=20, P<0.0001). The measures by FISH technique showed that the proportions of balanced or normal gametes were predominant in the reciprocal translocation group (alternate: n=7; from 33.0 to 58.8%; adjacent I: n=7; from 4.6 to 43.8%) and in the Robertsonian translocation group (normal: n=5; from 76.0 to 88.5%).

CONCLUSIONS

Our data show a predominant proportion of balanced gametes in sperm of chromosomal translocation carrier patients. Moreover, PS externalization and DNA fragmentation rates are significantly higher in ejaculated sperm of these patients than in donor sperm. These tests could be used to predict the outcome of ICSI for these patients.

Annexins: from structure to function

Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.

Monoclonal anti-annexin V antibody inhibits trophoblast gonadotropin secretion and induces syncytiotrophoblast apoptosis

The pathogenic role of anti-annexin V antibodies remains unclear. Anti-annexin V antibodies are frequently associated with higher incidences of intrauterine fetal loss, preeclampsia, and arterial and venous thrombosis. The present study investigated the in vitro ability of anti-annexin V antibody to bind human trophoblast cells, to affect trophoblast gonadotropin secretion and invasiveness, and to induce placental apoptosis. Cytotrophoblast cells were dispersed in Ringer bicarbonate buffer containing trypsin and DNase I, filtered, and layered over a Percoll gradient in Hanks balanced salt solution. In the case of monoclonal anti-annexin V antibody, the highest binding was found when the cells displayed the greatest amount of syncytium formation. Anti-annexin V antibody, but not its negative control, induced trophoblast apoptosis and significantly reduced trophoblast gonadotropin secretion. These findings suggest that recognition by anti-annexin V antibody of adhered annexin V on trophoblast cell structures might represent a potential pathogenic mechanism by which these antibodies can cause defective placentation.

S100A1 and S100B interactions with annexins

Members of the annexin protein family interact with members of the S100 protein family thereby forming heterotetramers in which an S100 homodimer crossbridges two copies of the pertinent annexin. Previous work has shown that S100A1 and S100B bind annexin VI in a Ca(2+)-dependent manner and that annexin VI, but not annexin V, blocks the inhibitory effect of S100A1 and S100B on intermediate filament assembly. We show here that both halves of annexin VI (i.e., the N-terminal half or annexin VI-a and the C-terminal half or annexin VI-b) bind individual S100s on unique sites and that annexin VI-b, but not annexin VI-a, blocks the ability of S100A1 and S100B to inhibit intermediate filament assembly. We also show that the C-terminal extension of S100A1 (and, by analogy, S100B), that was previously demonstrated to be critical for S100A1 and S100B binding to several target proteins including intermediate filament subunits, is not part of the S100 surface implicated in the recognition of annexin VI, annexin VI-a, or annexin VI-b. Evaluation of functional properties with a liposome stability and a calcium influx assay reveals the ability of both S100 proteins to permeabilize the membrane bilayer in a similar fashion like annexins. When tested in combinations with different annexin proteins both S100 proteins mostly lead to a decrease in the calcium influx activity although not all annexin/S100 combinations behave in the same manner. Latter observation supports the hypothesis that the S100-annexin interactions differ mechanistically depending on the particular protein partners.

Annexin 24 from Capsicum annuum. X-ray structure and biochemical characterization

This work provides the first three-dimensional structure of a member of the plant annexin family and correlates these findings with biochemical properties of this protein. Annexin 24(Ca32) from Capsicum annuum was purified as a native protein from bell pepper and was also prepared by recombinant techniques. To overcome the problem of precipitation of the recombinant wild-type protein in crystallization trials, two mutants were designed. Whereas an N-terminal truncation mutant turned out to be an unstable protein, the N-terminal His-tagged annexin 24(Ca32) was crystallized, and the three-dimensional structure was determined by x-ray diffraction at 2. 8 A resolution. The structure refined to an R-factor of 0.216 adopts the typical annexin fold; the detailed structure, however, is different from non-plant annexins, especially in domains I and III and in the membrane binding loops on the convex side. Within the unit cell there are two molecules per asymmetric unit, which differ in conformation of the IAB-loop. Both conformers show Trp-35 on the surface. The loop-out conformation is stabilized by tight interactions of this tryptophan with residue side chains of a symmetry-related molecule and enforced by a bound sulfate. Characterization of this plant annexin using biophysical methods revealed calcium-dependent binding to phospholipid vesicles with preference for phosphatidylcholine over phosphatidylserine and magnesium-dependent phosphodiesterase activity in vitro as shown with adenosine triphosphate as the substrate. A comparative unfolding study of recombinant annexin 24(Ca32) wild type and of the His-tag fusion protein indicates higher stability of the latter. The effect of this N-terminal modification is also visible from CD spectra. Both proteins were subjected to a FURA-2-based calcium influx assay, which gave high influx rates for the wild-type but greatly reduced influx rates for the fusion protein. We therefore conclude that the N-terminal domain is indeed a major regulatory element modulating different annexin properties by allosteric mechanisms.

Domain structure and molecular conformation in annexin V/1,2-dimyristoyl-sn-glycero-3-phosphate/Ca2+ aqueous monolayers: a Brewster angle microscopy/infrared reflection-absorption spectroscopy study

Annexins comprise a family of proteins that exhibit a Ca2+-dependent binding to phospholipid membranes that is possibly relevant to their in vivo function. Although substantial structural information about the ternary (protein/lipid/Ca2+) interaction in bulk phases has been derived from a variety of techniques, little is known about the temporal and spatial organization of ternary monolayer films. The effect of Ca2+ on the interactions between annexin V (AxV) and anionic DMPA monolayers was therefore investigated using three complementary approaches: surface pressure measurements, infrared reflection-absorption spectroscopy (IRRAS), and Brewster angle microscopy (BAM). In the absence of Ca2+, the injection of AxV into an aqueous subphase beneath a DMPA monolayer initially in a liquid expanded phase produced BAM images revealing domains of protein presumably surrounded by liquid-expanded lipid. The protein-rich areas expanded with time, resulting in reduction of the area available to the DMPA and, eventually, in the formation of condensed lipid domains in spatial regions separate from the protein film. There was thus no evidence for a specific binary AxV/lipid interaction. In contrast, injection of AxV/Ca2+ at a total Ca2+ concentration of 10 microM beneath a DMPA monolayer revealed no pure protein domains, but rather the slow formation of pinhead structures. This was followed by slow (>2 h) rigidification of the whole film accompanied by an increase in surface pressure, and connection of solid domains to form a structure resembling strings of pearls. These changes were characteristic of this specific ternary interaction. Acyl chain conformational order of the DMPA, as measured by nu(sym)CH2 near 2850 cm(-1), was increased in both the AxV/DMPA and AxV/DMPA/Ca2+ monolayers compared to either DMPA monolayers alone or in the presence of Ca2+. The utility of the combined structural and temporal information derived from these three complementary techniques for the study of monolayers in situ at the air/water interface is evident from this work.

Localization of the apoptosis-inducing activity of lupus anticoagulant in an annexin V-binding antibody subset

Lupus anticoagulant (LAC) is associated with arterial and venous thrombosis, thrombocytopenia, and recurrent fetal loss. We have reported previously that plasma with LAC activity induces apoptosis in endothelial cells and binds annexin V (Nakamura, N., Y. Shidara, N. Kawaguchi, C. Azuma, N. Mitsuda, S. Onishi, K. Yamaji, and Y. Wada. 1994. Biochem. Biophys. Res. Commun. 205:1488-1493). In this study, we separated two IgG antibody fractions, one with and one without affinity for annexin V, from 10 patients with LAC. LAC and apoptotic activities were localized in the annexin V-binding fraction in all 10 patients. DNA fragmentation was dose-dependent, paralleling the amount of IgG added to the human umbilical vein endothelial cell culture medium, and was inhibited by preincubation with annexin V. Removal of the antiphospholipid antibodies from patient IgG with phospholipid liposomes did not abolish the apoptosis-inducing activities or binding to annexin V. These results imply that patients with LAC often have antibodies that do not bind phospholipids and are responsible for the induction of apoptosis in endothelial cells.

Inhibition of secreted phospholipases A2 by annexin V. Competition for anionic phospholipid interfaces allows an assessment of the relative interfacial affinities of secreted phospholipases A2

The ability of annexins, particularly annexin 1 (lipocortin 1), to inhibit phospholipase A2 (PLA2) is well known and a substrate depletion mechanism is now widely accepted as the explanation for most inhibitory studies. In this investigation we have examined the substrate depletion mechanism of annexin V using a variety of phospholipid substrates and secreted PLA2's (sPLA2). The results suggest that the term interfacial competition best describes the inhibitory effect of annexin V although the overall inhibitory process remains one of substrate sequestration by the annexin. We have utilised the competitive nature of the interaction of enzyme and annexin V for a phospholipid interface as a means of quantifying the relative affinity of sPLA2's for anionic phospholipid vesicles. The results highlight the very high affinity of the human non-pancreatic sPLA2 for such vesicles (Kd<<10-(10) M) while the Naja naja venom PLA2 and porcine pancreatic sPLA2 showed lower affinities. Hydrolysis of mixed vesicles containing phosphatidylserine and phosphatidylcholine by the venom and pancreatic enzymes were differentially inhibited by annexin V. This difference must reflect the preference of both annexin V and the pancreatic enzyme for an anionic phospholipid interface. In contrast, the venom enzyme is able to readily hydrolyse phosphatidylcholine domains that would be minimally affected by annexin V. Annexin V was an effective inhibitor of cardiolipin hydrolysis by the pancreatic PLA2, however the inhibition was of a more complex nature than seen with other phospholipids tested. Overall the results highlight the ability of annexin V to inhibit phospholipid hydrolysis by sPLA2's by an interfacial competition (substrate depletion) mechanism. The effectiveness of annexin V as an apparent inhibitor depends on the nature of the enzyme and the phospholipid substrate.

Inhibition of human cytosolic phospholipase A2 by human annexin V

The ability of annexins, particularly annexin 1 (lipocortin 1), to inhibit phospholipase A2 (PLA2) is well known and a substrate depletion mechanism is now widely accepted as the explanation for most inhibitory studies. However, there are only a very limited number of reported studies involving annexins and the high-molecular-mass cytosolic PLA2 (cPLA2). In this study we have examined the effect of human recombinant annexin V, a potentially abundant cytosolic protein, on the ability of human recombinant cPLA2 to hydrolyse a variety of phospholipid substrates. The results show clearly that, under the conditions of our study, annexin V can inhibit cPLA2 activity by a mechanism of substrate depletion and that this inhibition is dependent on the nature of the phospholipids and the concentration of Ca2+ ions in the assay. The hydrolysis of 1-stearoyl 2-arachidonyl phosphatidylcholine by cPLA2 was not significantly affected by annexin V over a range of Ca2+ concentrations (1 microM-2.5 mM), a result that presumably reflects the zwitterionic nature of the phospholipid and the known inability of annexins to bind to such interfaces. In contrast, the hydrolysis of dioleoyl phosphatidylglycerol, which is an effective anionic phospholipid substrate for this enzyme, and more significantly that of 1-stearoyl 2-arachidonyl phosphatidic acid, were readily inhibited by annexin V, although these effects were Ca2+-dependent. The Ca2+ concentrations required for inhibition in the assay system in vitro are greater than those associated with Ca2+-stimulated events within the cell, suggesting that a role for annexin V in regulating cPLA2 activity might not involve a substrate depletion mechanism in vivo unless factors in addition to Ca2+ and phospholipids contribute to the binding of annexin V to cell membranes.

Voltage dependent binding of annexin V, annexin VI and annexin VII-core to acidic phospholipid membranes

Annexin V, VI and VII-core (delta1-107) are members of the annexin protein family and bind to acidic phospholipid membranes in a calcium dependent manner. They also show ion channel activity under certain conditions. As annexins bind peripherally to lipid membranes, ion channel formation must consist of at least two steps: An adsorption reaction regulating the binding of annexin to the membrane surface and the opening and closing of the active species controlling the channel activity. By using the baseline current through the patch clamp seal as a probe for unoccupied binding sites at the membrane, we show that the adsorption of annexins to membranes is not only calcium dependent but also strongly voltage dependent. Whereas the free transfer energies at low calcium concentrations are similar for all three annexins, the binding of annexin V becomes much tighter with higher calcium levels, compared to annexin VI and VII-core. This correlates with the finding that annexin VI and VII-core display channel activity much more often than annexin V if one assumes that a high coverage of the membrane surface with annexins stabilizes the bilayer. At higher protein concentrations weaker binding is observed in agreement with the previously reported anti-cooperativity of membrane binding.

Inhibition of annexin V-dependent Ca2+ movement in large unilamellar vesicles by K201, a new 1,4-benzothiazepine derivative

Examination was made of the effect of annexin V on Ca2+ movement into large unilamellar vesicles (LUV) using fura-2, a calcium-sensitive fluorescent dye. To avoid the possible difficulties relating to the addition of annexin V and/or Ca2+ in fura-2-loaded LUV, the burst method was used. LUV, preincubated with rat annexin V in the presence of Ca2+, were collected by centrifugation and resuspended, and then burst with Triton X-100 in the presence of fura-2. Inward Ca2+ movement across the artificial lipid membrane was measured by determination of fura-2 fluorescence due to the leaked Ca2+ from ruptured LUV. The observed Ca2+ signal increased dependent on annexin V and Ca2+ concentrations, whereas bovine serum albumin did not affect this signal up to 1 microM. Thus, annexin V shows Ca2+ channel activity in LUV. K201, a novel 1,4-benzothiazepine, inhibited inward Ca2+ movement into LUV caused by annexin V in a dose-dependent manner. In the presence of 50 nM annexin V and 400 microM Ca2+, 3 microM K201 showed significant inhibition of Ca2+ movement due to annexin V, and 50% inhibition was achieved at 25 microM K201. On the other hand, diltiazem had no such effect even at 30 microM. K201 is thus shown to have inhibitory activity on inward Ca2+ movement due to annexin V in artificial vesicles and may prove useful as a probe for elucidating the functions of annexin V in vivo.

Presence and comparison of Ca2+ transport activity of annexins I, II, V, and VI in large unilamellar vesicles

This study evaluated whether annexins I, II and VI possess Ca2+ transport activity in phospholipid membranes by the burst method, and the activity of each was compared with that of annexin V. Briefly, in the presence of 400 microM Ca2+, each annexin at 50 nM was added to large unilamellar vesicles (LUV) which were then burst in fura-2 solution with 0.2% Triton X-100, followed by examination of Ca2+ signals. Annexins I, II, V and VI were all shown to express, each to a different degree, Ca2+ activity toward phosphatidylserine/phosphatidyl- ethanolamine-LUV. Ca2+ signal intensity increased dependent on annexin concentration, and the Ca2+ transport activity of annexin V and VI was higher than that of annexin I and II. However, none of annexin I, II, V and VI expressed Ca2+ transport activity in LUV produced using phosphatidylcholine. Ca(2+)-incorporated LUV with no annexin showed signals whose intensity was proportional to Ca2+ concentration. The Ca2+ transport activity of the annexins could be effectively measured by the burst method. Ca2+ signal intensity would thus appear to be unique for each of the annexins and to be determined by the particular function and specificity of each of those considered in this study.

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Intrapituitary distribution and effects of annexin 5 on prolactin release

Annexin 5 is expressed by rat anterior pituitary cells and a depolarizing stimulus results in increased extracellular display and, depending on local calcium concentrations, potential release into the extracellular environment. In order to further investigate the role of annexin 5 in anterior pituitary function, we have examined the intracellular distribution by immunocytochemistry and the effects of annexin 5 on the release of a major secretory product, prolactin. Prolactin was chosen because we could easily monitor effects on basal release and effects on the immediate and sustained phases of thyroid stimulating hormone releasing hormone (TRH)-stimulated release. Immunocytochemical localization of annexin 5 showed staining of the majority of anterior pituitary cells. Labeling was predominantly on the nuclear envelope and plasma membrane. For the chosen secretory product, prolactin, annexin 5 was found in most, but not all prolactin positive cells. When recombinant annexin 5 (50 ng/mL) was added to a 3 h static culture incubation of rat anterior pituitary cells, prolactin release was inhibited by about 30% (p<0.05). A lower dose had a reduced effect and higher doses had no further inhibitory effect, indicating that the effect was specific to annexin 5 and not a nonspecific toxic effect of some contaminant in the preparation. This interpretation was further strengthened in a time-course experiment demonstrating that when TRH and annexin 5 were added together, there was no effect of annexin 5 on the amount of prolactin released. After a 3 h preincubation in annexin 5, however, prolactin release, in response to TRH, was suppressed by about 30% in both the acute and sustained phases. These data suggest that annexin 5 may be a local regulator of release in the anterior pituitary, but a slow onset effect on both phases of TRH-stimulated release suggests that this is not an effect at the plasma membrane such as local extracellular calcium depletion by plasma membrane-bound annexin 5.