Human neutrophil annexin I promotes granule aggregation and modulates Ca(2+)-dependent membrane fusion

Role of the IgG4-related cholangitis autoantigen annexin A11 in cholangiocyte protection

BACKGROUND & AIMS

Annexin A11 was identified as autoantigen in IgG4-related cholangitis (IRC), a B-cell driven disease. Annexin A11 modulates calcium-dependent exocytosis, a crucial mechanism for insertion of proteins into their target membranes. Human cholangiocytes form an apical 'biliary bicarbonate umbrella' regarded as defense against harmful hydrophobic bile acid influx. The bicarbonate secretory machinery comprises the chloride/bicarbonate exchanger AE2 and the chloride channel ANO1. We aimed to investigate the expression and function of annexin A11 in human cholangiocytes and a potential role of IgG1/IgG4-mediated autoreactivity against annexin A11 in the pathogenesis of IRC.

METHODS

Expression of annexin A11 in human liver was studied by immunohistochemistry and immunofluorescence. In human control and ANXA11 knockdown H69 cholangiocytes, intracellular pH, AE2 and ANO1 surface expression, and bile acid influx were examined using ratio microspectrofluorometry, cell surface biotinylation, and 22,23-3H-glycochenodeoxycholic acid permeation, respectively. The localization of annexin A11-mEmerald and ANO1-mCherry was investigated by live-cell microscopy in H69 cholangiocytes after incubation with IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies or disease control serum.

RESULTS

Annexin A11 was strongly expressed in human cholangiocytes, but not hepatocytes. Knockdown of ANXA11 led to reduced plasma membrane expression of ANO1, but not AE2, alkalization of intracellular pH and uncontrolled bile acid influx. High intracellular calcium conditions led to annexin A11 membrane shift and colocalization with ANO1. Incubation with IRC patient serum inhibited annexin A11 membrane shift and reduced ANO1 surface expression.

CONCLUSION

Cholangiocellular annexin A11 mediates apical membrane abundance of the chloride channel ANO1, thereby supporting biliary bicarbonate secretion. Insertion is inhibited by IRC patient serum containing anti-annexin A11 IgG1/IgG4-autoantibodies. Anti-annexin A11 autoantibodies may contribute to the pathogenesis of IRC by weakening the 'biliary bicarbonate umbrella'.

LAY SUMMARY

We previously identified annexin A11 as a specific autoantigen in immunoglobulin G4-related cholangitis (IRC), a B-cell driven disease affecting the bile ducts. Human cholangiocytes are protected against harmful hydrophobic bile acid influx by a defense mechanism referred to as the 'biliary bicarbonate umbrella'. We found that annexin A11 is required for the formation of a robust bicarbonate umbrella. Binding of patient-derived annexin A11 autoantibodies inhibits annexin A11 function, possibly contributing to bile duct damage by weakening the biliary bicarbonate umbrella in patients with IRC.

Role of Annexin A1 in NLRP3 Inflammasome Activation in Murine Neutrophils

This study evaluated the role of endogenous and exogenous annexin A1 (AnxA1) in the activation of the NLRP3 inflammasome in isolated peritoneal neutrophils. C57BL/6 wild-type (WT) and AnxA1 knockout mice (AnxA1-/-) received 0.3% carrageenan intraperitoneally and, after 3 h, the peritoneal exudate was collected. WT and AnxA1-/- neutrophils were then stimulated with lipopolysaccharide, followed by the NLRP3 agonists nigericin or ATP. To determine the exogenous effect of AnxA1, the neutrophils were pretreated with the AnxA1-derived peptide Ac2-26 followed by the NLRP3 agonists. Ac2-26 administration reduced NLRP3-derived IL-1β production by WT neutrophils after nigericin and ATP stimulation. However, IL-1β release was impaired in AnxA1-/- neutrophils stimulated by both agonists, and there was no further impairment in IL-1β release with Ac2-26 treatment before stimulation. Despite this, ATP- and nigericin-stimulated AnxA1-/- neutrophils had increased levels of cleaved caspase-1. The lipidomics of supernatants from nigericin-stimulated WT and AnxA1-/- neutrophils showed potential lipid biomarkers of cell stress and activation, including specific sphingolipids and glycerophospholipids. AnxA1 peptidomimetic treatment also increased the concentration of phosphatidylserines and oxidized phosphocholines, which are lipid biomarkers related to the inflammatory resolution pathway. Together, our results indicate that exogenous AnxA1 negatively regulates NLRP3-derived IL-1β production by neutrophils, while endogenous AnxA1 is required for the activation of the NLRP3 machinery.

The role of annexin A1 in Candida albicans and Candida auris infections in murine neutrophils

Annexin A1 (AnxA1) is an anti-inflammatory protein expressed in various cell types, especially macrophages and neutrophils. Because neutrophils play important roles in infections and inflammatory processes and the relationship between AnxA1 and Candida spp. infections is not well-understood, our study examined whether AnxA1 can serve as a target protein for the regulation of the immune response during fungal infections. C57BL/6 wild-type (WT) and AnxA1 knockout (AnxA1^-/-) peritoneal neutrophils were coinfected with Candida albicans or Candida auris for 4 h. AnxA1^-/- neutrophils exhibited a marked increase in cyclooxygenase 2 (COX-2), phosphorylated extracellular signal-related kinase (ERK), p-38, and c-Jun N-terminal kinase (JNK) levels after coinfection with both Candida spp. A lipidomics approach showed that AnxA1 deficiency produced marked differences in the supernatant lipid profiles of both control neutrophils and neutrophils coinfected with Candida spp. compared with WT cells, especially the levels of glycerophospholipids and glycerolipids. Our results showed that endogenous AnxA1 regulates the neutrophil response under fungal infection conditions, altering lipid membrane organization and metabolism.

Annexin A1 drives macrophage skewing to accelerate muscle regeneration through AMPK activation

Understanding the circuits that promote an efficient resolution of inflammation is crucial to deciphering the molecular and cellular processes required to promote tissue repair. Macrophages play a central role in the regulation of inflammation, resolution, and repair/regeneration. Using a model of skeletal muscle injury and repair, herein we identified annexin A1 (AnxA1) as the extracellular trigger of macrophage skewing toward a pro-reparative phenotype. Brought into the injured tissue initially by migrated neutrophils, and then overexpressed in infiltrating macrophages, AnxA1 activated FPR2/ALX receptors and the downstream AMPK signaling cascade, leading to macrophage skewing, dampening of inflammation, and regeneration of muscle fibers. Mice lacking AnxA1 in all cells or only in myeloid cells displayed a defect in this reparative process. In vitro experiments recapitulated these properties, with AMPK-null macrophages lacking AnxA1-mediated polarization. Collectively, these data identified the AnxA1/FPR2/AMPK axis as an important pathway in skeletal muscle injury regeneration.

Definition of a Novel Pathway Centered on Lysophosphatidic Acid To Recruit Monocytes during the Resolution Phase of Tissue Inflammation

Blood-derived monocytes remove apoptotic cells and terminate inflammation in settings as diverse as atherosclerosis and Alzheimer’s disease. They express high levels of the proresolving receptor ALX/FPR2, which is activated by the protein annexin A1 (ANXA1), found in high abundance in inflammatory exudates. Using primary human blood monocytes from healthy donors, we identified ANXA1 as a potent CD14⁺CD16⁻ monocyte chemoattractant, acting via ALX/FPR2. Downstream signaling pathway analysis revealed the p38 MAPK-mediated activation of a calcium independent phospholipase A₂ with resultant synthesis of lysophosphatidic acid (LPA) driving chemotaxis through LPA receptor 2 and actin cytoskeletal mobilization. In vivo experiments confirmed ANXA1 as an independent phospholipase A₂–dependent monocyte recruiter; congruently, monocyte recruitment was significantly impaired during ongoing zymosan-induced inflammation in AnxA1^−/− or alx/fpr2/3^−/− mice. Using a dorsal air-pouch model, passive transfer of apoptotic neutrophils between AnxA1^−/− and wild-type mice identified effete neutrophils as the primary source of soluble ANXA1 in inflammatory resolution. Together, these data elucidate a novel proresolving network centered on ANXA1 and LPA generation and identify previously unappreciated determinants of ANXA1 and ALX/FPR2 signaling in monocytes.

Membrane tubulovesicular extensions (cytonemes): secretory and adhesive cellular organelles

In this review, we summarized data on the formation and structure of the long and highly adhesive membrane tubulovesicular extensions (TVEs, membrane tethers or cytonemes) observed in human neutrophils and other mammalian cells, protozoan parasites and bacteria. We determined that TVEs are membrane protrusions characterized by a uniform diameter (130-250 nm for eukaryotic cells and 60-90 nm for bacteria) along the entire length, an outstanding length and high rate of development and a high degree of flexibility and capacity for shedding from the cells. This review represents TVEs as protrusions of the cellular secretory process, serving as intercellular adhesive organelles in eukaryotic cells and bacteria. An analysis of the physical and chemical approaches to induce TVEs formation revealed that disrupting the actin cytoskeleton and inhibiting glucose metabolism or vacuolar-type ATPase induces TVE formation in eukaryotic cells. Nitric oxide is represented as a physiological regulator of TVE formation.

Extracellular annexins and dynamin are important for sequential steps in myoblast fusion

Annexins A1 and A5 are important for initial lipid mixing, whereas subsequent stages of myoblast fusion depend on dynamin, phosphatidylinositol(4,5)bisphosphate, and cellular metabolism.

Myoblast fusion into multinucleated myotubes is a crucial step in skeletal muscle development and regeneration. Here, we accumulated murine myoblasts at the ready-to-fuse stage by blocking formation of early fusion intermediates with lysophosphatidylcholine. Lifting the block allowed us to explore a largely synchronized fusion. We found that initial merger of two cell membranes detected as lipid mixing involved extracellular annexins A1 and A5 acting in a functionally redundant manner. Subsequent stages of myoblast fusion depended on dynamin activity, phosphatidylinositol(4,5)bisphosphate content, and cell metabolism. Uncoupling fusion from preceding stages of myogenesis will help in the analysis of the interplay between protein machines that initiate and complete cell unification and in the identification of additional protein players controlling different fusion stages.

Enhanced neutrophil expression of annexin-1 in coronary artery disease

The systemic inflammatory activity in patients with stable coronary artery disease (CAD) is associated with a dysregulated cortisol response. Moreover, an aberrant activation status of neutrophils in CAD has been discussed; and the question of glucocorticoid resistance has been raised. The anti-inflammatory actions of glucocorticoids are mediated by annexin-1 (ANXA1). We investigated the expression of glucocorticoid receptors (GR) and ANXA1, as well as the exogenous effects of ANXA1 on neutrophils in CAD patients and related the data to diurnal salivary cortisol. Salivary cortisol levels were measured in the morning and evening during 3 consecutive days in 30 CAD patients and 30 healthy individuals. The neutrophil expression of GR and ANXA1 was determined by flow cytometry. The effect of exogenous ANXA1 was determined in a neutrophil stimulation assay. The patients showed a flattened diurnal cortisol pattern compared with healthy subjects, involving higher levels in the evening. The neutrophil expression of GR-total and GR-alpha was decreased, whereas the GR-beta expression did not differ compared with controls. The neutrophil expression of ANXA1 was significantly increased in patients. Ex vivo, ANXA1 impaired the leukotriene B(4)-induced neutrophil production of reactive oxygen species in patients but not in controls. Our findings indicate a persistent overactivation of the hypothalamic-pituitary-adrenal axis in CAD patients but do not give any evidence for glucocorticoid resistance, as assessed by the neutrophil expression of GR and ANXA1. The altered neutrophil phenotype in CAD may thus represent a long-term response to disease-related activation.

The Influence of Different Cultivating Conditions on Polymorphonuclear Leukocyte Apoptotic Process In Vitro, II: Ultrastructural Characteristics of PMN Populations Incubated with Proteinase 3 Anti-neutrophil Autoantibodies

The present study shows the effects of proteinase 3 anti-neutrophil cytoplasmic autoantibodies (PR3 ANCA) on polymorphonuclear leukocytes (PMN) apoptotic processes in vitro. The results are part of a generalized morphological analysis of 3 identical experiments on the influence of different cultivating conditions on the apoptotic processes. As controls, the authors use the results on spontaneous PMN apoptosis (Guejes L, Zurgil N, Deutsch M, Gilburd B, Shoenfeld Y. Ultrastruct Pathol. 2003;27: 23-32) and PMN populations incubated with normal human IgG. Interaction of PR3 ANCA with the target antigen proteinase 3 (PR3) is one of the crucial pathogenic factors in Wegener granulomatosis (systemic autoimmune vasculitis). Following 40min and 12h incubation, PMN populations were evaluated by light microscopy, transmission electron microscopy, and immunogold electron microscopy. Twelve-hour cultures, either control or incubated with PR3 ANCA, contained different cell forms ranging from normal cells to cells at the final stages of apoptosis. Neutrophils at the state of complete manifestation of apoptotic phenotype were analyzed and compared. Three morphologically distinct apoptotic cell lines were characteristic for all PMN populations studied, regardless of cultivating conditions. As in spontaneous apoptosis, these cell lines are code-named "first," "second," and "third." The present study has shown, firstly, that in the presence of PR3 ANCA, all 3 apoptotic lines were modified or altered. Secondly, the modifications or alterations of apoptotic cell lines effected by PR3 ANCA are specific for each cell line: the "first" line is characterized by intensification and modification of activation; the "second" by vacuolized cell forms; and the "third" by pronounced lytic alterations of the nuclei, while the cytoplasm is fully identical to that of control cell lines.

Endogenous lipid mediators in the resolution of airway inflammation

Acute inflammation in the lung is fundamentally important to host defence, but chronic or excessive inflammation leads to several common respiratory diseases, including asthma and acute respiratory distress syndrome. The resolution of inflammation is an active process. In health, events at the onset of acute inflammation establish biosynthetic circuits for specific chemical mediators that later serve as agonists to orchestrate a return to tissue homeostasis. In addition to an overabundance of pro-inflammatory stimuli, pathological inflammation can also result from defects in resolution signalling. The understanding of anti-inflammatory, pro-resolution molecules and their counter-regulatory signalling pathways is providing new insights into the molecular pathophysiology of lung disease and opportunities for the design of therapeutic strategies. In the present review, the growing family of lipid mediators of resolution is examined, including lipoxins, resolvins, protectins, cyclopentenones and presqualene diphosphate. Roles are uncovered for these compounds, or their structural analogues, in regulating airway inflammation.

Annexin 1 cleavage in activated neutrophils: a pivotal role for proteinase 3

Annexin 1 is an anti-inflammatory protein that plays a key role in innate immunity by modulating the activation of several types of cells, including neutrophils. Here we have developed a cleavage assay using tagged annexin 1 and observed marked activity in the membrane fraction of activated neutrophils. A combination of inhibitors, transfected cells, and proteomic analyses allowed us to identify proteinase 3 as the main enzyme responsible for this cleavage in the N terminus region of the protein, at least in the context of neutrophil activation. Because annexin 1 is an important endogenous anti-inflammatory mediator, blocking its cleavage by proteinase 3 would augment its homeostatic pro-resolving actions and could represent an opportunity for innovative anti-inflammatory drug discovery.

Requirement for annexin A1 in plasma membrane repair

Ca2+ entering a cell through a torn or disrupted plasma membrane rapidly triggers a combination of homotypic and exocytotic membrane fusion events. These events serve to erect a reparative membrane patch and then anneal it to the defect site. Annexin A1 is a cytosolic protein that, when activated by micromolar Ca2+, binds to membrane phospholipids, promoting membrane aggregation and fusion. We demonstrate here that an annexin A1 function-blocking antibody, a small peptide competitor, and a dominant-negative annexin A1 mutant protein incapable of Ca2+ binding all inhibit resealing. Moreover, we show that, coincident with a resealing event, annexin A1 becomes concentrated at disruption sites. We propose that Ca2+ entering through a disruption locally induces annexin A1 binding to membranes, initiating emergency fusion events whenever and wherever required.

Maturation of Human Neutrophil Phagosomes Includes Incorporation of Molecular Chaperones and Endoplasmic Reticulum Quality Control Machinery

A Human neutrophils are an essential component of the innate immune response. Although significant progress has been made toward understanding mechanisms of phagocytosis and microbicidal activity, a comprehensive analysis of proteins comprising neutrophil phagosomes has not been conducted. To that end, we used subcellular proteomics to identify proteins associated with human neutrophil phagosomes following receptor-mediated phagocytosis. Proteins (n = 411 spots) resolved from neutrophil phagosome fractions were identified by MALDI-TOF MS and/or LC-MS/MS analysis. Those associated with phagocytic vacuoles originated from multiple subcellular compartments, including the cytosol, plasma membrane, specific and azurophilic granules, and cytoskeleton. Unexpectedly several enzymes typically associated with mitochondria were identified in phagosome fractions. Furthermore proteins characteristic of the endoplasmic reticulum, including 11 molecular chaperones, were resolved from phagosome preparations. Confocal microscopy confirmed that proteins representing these major subcellular compartments were enriched on phagosomes of intact neutrophils. Notably calnexin and glucose-regulated protein 78 co-localized with gp91(phox) in human neutrophils and were thus likely delivered to phagosomes by fusion of specific granules. We conclude that neutrophil phagosomes have heretofore unrecognized complexity and function, which includes potential for antigen processing events.

Annexin 1 and its bioactive peptide inhibit neutrophil-endothelium interactions under flow: indication of distinct receptor involvement

We have tested the effects of annexin 1 (ANXA1) and its N-terminal peptide Ac2-26 on polymorphonuclear leukocyte (PMN) recruitment under flow. Differential effects of the full-length protein and its peptide were observed; ANXA1 inhibited firm adhesion of human PMNs, while Ac2-26 significantly attenuated capture and rolling without effect on firm adhesion. Analysis of the effects of ANXA1 and Ac2-26 on PMN adhesion molecule expression supported the flow chamber results, with Ac2-26 but not ANXA1 causing l-selectin and PSGL-1 shedding. ANXA1 and its peptide act via the FPR family of receptors. This was corroborated using HEK-293 cells transfected with FPR or FPRL-1/ALX (the 2 members of this family expressed by human PMNs). While Ac2-26 bound both FPR and FPRL-1/ALX, ANXA1 bound FPRL-1/ALX only. ANXA1 and Ac2-26 acted as genuine agonists; Ac2-26 binding led to ERK activation in both FPR- and FPRL-1/ALX-transfected cells, while ANXA1 caused ERK activation only in cells transfected with FPRL-1/ALX. Finally, blockade of FPRL-1/ALX with a neutralizing monoclonal antibody was found to abrogate the effects of ANXA1 in the flow chamber but was without effect on Ac2-26-mediated inhibition of rolling. These findings demonstrate for the first time distinct mechanisms of action for ANXA1 and its N-terminal peptide Ac2-26.

A twist in anti-inflammation: annexin 1 acts via the lipoxin A4 receptor

The inflammatory response is a life-saving protective process mounted by the body to overcome pathogen infection and injury; however, in chronic inflammatory pathologies this response can become deregulated. The existence of specialized anti-inflammatory pathways/mediators that operate in the body to down-regulate inflammation have now emerged. Thus, persistence of inflammation leading to pathology could be due to malfunctioning of one or more of these counter-regulatory pathways. Here we focus on one of them, the anti-inflammatory mediator annexin 1, and provide an update on its inhibitory effects upon the leukocyte trafficking process. In particular, recent evidence that receptors of the formyl-peptide family, which includes also the lipoxin A4 receptor, could be the annexin 1 receptor(s) in the context of anti-inflammation might provide new avenues for exploiting this pathway for drug discovery.

Annexin 1-deficient neutrophils exhibit enhanced transmigration in vivo and increased responsiveness in vitro

The role of the endogenous anti-inflammatory mediator annexin 1 (AnxA1) in controlling polymorphonuclear leukocyte (PMN) trafficking and activation was addressed using the recently generated AnxA1 null mouse. In the zymosan peritonitis model, AnxA1 null mice displayed a higher degree (50-70%) of PMN recruitment compared with wild-type littermate mice, and this was associated with reduced numbers of F4/80+ cells. Intravital microscopy analysis of the cremaster microcirculation inflamed by zymosan (6 h time-point) indicated a greater extent of leukocyte emigration, but not rolling or adhesion, in AnxA1 null mice. Real-time analysis of the cremaster microcirculation did not show spontaneous activation in the absence of AnxA1; however, superfusion with a direct-acting PMN activator (1 nM platelet-activating factor) revealed a subtle yet significant increase in leukocyte emigration, but not rolling or adhesion, in this genotype. Changes in the microcirculation were not secondary to alterations in hemodynamic parameters. The phenotype of the AnxA1 null PMN was investigated in two in vitro assays of cell activation (CD11b membrane expression and chemotaxis): the data obtained indicated a higher degree of cellular responses irrespective of the stimulus used. In conclusion, we have used a combination of inflammatory protocols and in vitro assays to address the specific counter-regulatory role of endogenous AnxA1, demonstrating its inhibitory control on PMN activation and the consequent impact on the inflamed microcirculation.

Annexin-1 is an endogenous gastroprotective factor against indomethacin-induced damage

Adherence of neutrophils to the vascular endothelium is an early and critical event in the pathogenesis of gastric injury induced by NSAIDs. Pretreatment with glucocorticoids has been shown to prevent NSAID-induced neutrophil adherence and, in turn, to protect the stomach from injury. Some of the anti-inflammatory effects of glucocorticoids, including inhibition of neutrophil adherence, are mediated via the release of annexin-1. In this study, we assessed the contribution of annexin-1 to the protective actions of a glucocorticoid (dexamethasone) against indomethacin-induced gastric damage. Dexamethasone pretreatment markedly reduced the extent of indomethacin-induced gastric damage in rats. Immunoneutralization of annexin-1 resulted in a reversal of the gastroprotective actions of dexamethasone. Similarly, pretreatment with either of two antagonists of the formyl peptide receptor family, to which annexin-1 binds, reversed the gastroprotective effects of dexamethasone. The inhibitory effects of dexamethasone on indomethacin-induced leukocyte adherence in the mesenteric microcirculation were abolished by pretreatment with an antibody directed against annexin-1 or with an antagonist of the formyl peptide receptors. These results demonstrate that annexin-1 mediates the gastroprotective effects of a glucocorticoid against NSAID-induced damage. We propose that in some circumstances, annexin-1 plays an important role as an endogenous mediator of mucosal defense.

Annexin 1 and the biology of the neutrophil

This overview will focus on one aspect of neutrophil biology, which is the selective activation of the annexin 1 system in relation to the process of cell extravasation. Besides the current view about the biochemistry of annexin 1 and annexin 1 receptor(s) up-regulation within the microenvironment of the adherent neutrophils, we will also comment on the final result achieved by activation of the system, which is inhibition of neutrophil recruitment. In view of the historical link between annexin 1 and glucocorticoids, the potential for the annexin 1 system in mediating at least some of the anti-inflammatory actions of these powerful drugs is also discussed.

Ceramide inhibition of phospholipase D and its relationship to RhoA and ARF1 translocation in GTP gamma S-stimulated polymorphonuclear leukocytes

Phospholipase D (PLD) regulates the polymorphonuclear leukocyte (PMN) functions of phagocytosis, degranulation, and oxidant production. Ceramide inhibition of PLD suppresses PMN function. In streptolysin O-permeabilized PMNs, PLD was directly activated by guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) stimulation of adenosine diphosphate (ADP)-ribosylation factor (ARF) and Rho, stimulating release of lactoferrin from specific granules of permeabilized PMNs; PLD activation and degranulation were inhibited by C2-ceramide but not dihydro-C2-ceramide. To investigate the mechanism of ceramide's inhibitory effect on PLD, we used a cell-free system to examine PLD activity and translocation from cytosol to plasma membrane of ARF, protein kinase C (PKC)alpha and beta, and RhoA, all of which can activate PLD. GTP gamma S-activated cytosol stimulated PLD activity and translocation of ARF, PKC alpha and beta, and RhoA when recombined with cell membranes. Prior incubation of PMNs with 10 microM C2-ceramide inhibited PLD activity and RhoA translocation, but not ARF1, ARF6, PKC alpha, or PKC beta translocation. However, in intact PMNs stimulated with N-formyl-1-methionyl-1-leucyl-1-phenylalamine (FMLP) or permeabilized PMNs stimulated with GTP gamma S, C2-ceramide did not inhibit RhoA translocation. Exogenous RhoA did not restore ceramide-inhibited PLD activity but bound to membranes despite ceramide treatment. These observations suggest that, although ceramide may affect RhoA in some systems, ceramide inhibits PLD through another mechanism, perhaps related to the ability of ceramide to inhibit phosphatidylinositol-bisphosphate (PIP2) interaction with PLD.

Neutrophil granules and secretory vesicles in inflammation

The neutrophil is a major effector cell of innate immunity. Exocytosis of granules and secretory vesicles plays a pivotal role in most neutrophil functions from early activation to the destruction of phagocytosed microorganisms. Neutrophil granules contain a multitude of antimicrobial and potentially cytotoxic substances that are delivered to the phagosome or to the exterior of the cell following degranulation. This review summarises current knowledge of granule biology and highlights the effects of neutrophil degranulation in the acute inflammatory response.

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.

Kinase-dependent regulation of the secretion of thyrotrophin and luteinizing hormone by glucocorticoids and annexin 1 peptides

Our previous studies have identified a role for annexin 1 (ANXA1), a protein produced by the pituitary folliculostellate cells, as a paracrine/juxtacrine mediator of the acute regulatory effects of glucocorticoids on the release of adrenocorticotropic hormone and other pituitary hormones. In the present study, we focused on the secretion of thyroid stimulating hormone (TSH) and luteinizing hormone (LH) and used a battery of ANXA1-derived peptides to identify the key domains in the ANXA1 molecule that are critical to the inhibition of peptide release. In addition, as ANXA1 is a substrate for protein kinase C (PKC) and tyrosine kinase, we examined the roles of these kinases in the manifestation of the ANXA1-dependent inhibitory actions of dexamethasone on TSH and LH release. Dexamethasone suppressed the forskolin-induced release of TSH and LH from rat anterior pituitary tissue in vitro. Its effects were mimicked by human recombinant ANXA1 (hrANXA1) and a truncated protein, ANXA1(1-188). ANXA1(Ac2-26), also suppressed stimulated peptide release but it lacked both the potency and the efficacy of the parent protein. Shorter N-terminal ANXA1 sequences were without effect. The PKC inhibitor PKC(19-36) abolished the inhibitory actions of dexamethasone on the forskolin-evoked release of TSH and LH; it also attenuated the inhibitory actions of ANXA1(Ac2-26). Similar effects were produced by annexin 5 (ANXA5) which sequesters PKC in other systems. By contrast, the tyrosine kinase inhibitors, p60v-src (137-157) and genistein, had no effect on the secretion of TSH or LH alone or in the presence of forskolin and/or dexamethasone. Dexamethasone caused the translocation of a tyrosine-phosphorylated species of ANXA1 to the surface of pituitary cells. The total amount of ANXA1 exported from the cells in response to the steroid was unaffected by tyrosine kinase blockade. However, the degree of tyrosine-phosphorylation of the exported protein was markedly reduced by genistein. These results suggest that (i) the ANXA1-dependent inhibitory actions of dexamethasone on the release of TSH and LH require PKC and sequences in the N-terminal domain of ANXA1, but are independent of tyrosine kinase, and (ii) while dexamethasone induces the cellular exportation of a tyrosine-phosphorylated species of ANXA1, tyrosine phosphorylation per se is not critical to the steroid-induced passage of ANXA1 across the membrane.

Dissecting the cellular functions of annexin XI using recombinant human annexin XI-specific autoantibodies cloned by phage display

Functional studies of cellular proteins are often complicated by the lack of well-defined monoclonal antibodies, the production of which is hampered by the highly conserved nature of these cellular proteins across species. Annexin XI, a member of the Ca2+-dependent, phospholipid-binding protein family, is an example of such a protein and was used in studies to devise a strategy using human autoimmune phage display libraries to generate reagents for biological studies of conserved cellular proteins. An IgG phage display library was generated from bone marrow of an autoimmune patient with high serum antibody titer against annexin XI, which was identified recently as an autoantigen targeted by autoantibodies in several systemic autoimmune diseases. From this phage library, a panel of human monoclonal annexin XI-specific Fabs were isolated and applied to studies of the cellular functions of annexin XI. Confocal microscopy showed a cell cycle-specific redistribution of annexin XI from the cytoplasm to the mitotic spindle. In metaphase, annexin XI was up-regulated and costained with alpha-tubulin. The subcellular distribution of annexin XI in COS-7 cells was shown to be Ca2+-dependent, and exhibited a predominantly nuclear pattern at low concentrations and a cytoplasmic pattern at high Ca2+ concentrations. Calcyclin, found previously to bind annexin XI in vitro, in vivo coated the nuclear membrane of cultured cell lines and did not colocalize with annexin XI. Ultrastructural analysis by immunoelectron microscopy revealed that annexin XI associated with specific granules in both neutrophils and eosinophils, suggesting a role in the exocytotic pathway. Our results illuminate the multifunctional nature of human annexin XI, provide the first evidence that annexin XI associates with the mitotic spindles and might play a role in cell division, and clearly illustrate the potential of phage display-derived human autoantibodies in broader analyses of the function of highly conserved cellular proteins.

A novel calcium-dependent proapoptotic effect of annexin 1 on human neutrophils

The glucocorticoid-inducible protein annexin (ANXA) 1 is an anti-inflammatory mediator that down-regulates the host response. Endogenously, ANXA1 is released in large amounts from adherent polymorphonuclear neutrophils (PMN) and binds to their cell surface to inhibit their extravasation into inflamed tissues. The present study determined the effects of exogenous ANXA1 on several functions of human PMN in vitro. Addition of 0.1-1 microM human recombinant ANXA1 to the PMN provoked rapid and transient changes in intracellular Ca2+ concentrations that were blocked by the Ca2+ channel inhibitor SKF-96365. Although ANXA1 did not affect oxidant production and only minimally affected PMN chemotactic properties, the ANXA1-promoted Ca2+ influx was associated with two important functional effects: shedding of L-selectin and acceleration of PMN apoptosis. The latter effect was confirmed using three distinct technical procedures, namely, cell cycle, Hoechst staining, and ANXA5 binding assay. ANXA1-induced PMN apoptosis was insensitive to inhibitors of L-selectin shedding, whereas it appeared to be associated with dephosphorylation of the proapoptotic intracellular mediator BAD. In conclusion, exogenous ANXA1 displayed selective actions on human PMN. We propose that the new proapoptotic effect reported here may be part of the spectrum of ANXA1-mediated events involved in the resolution of acute inflammation.

Modulation of annexin I and cyclooxygenase-2 in smokeless tobacco-induced inflammation and oral cancer

Smokeless tobacco usage is a growing public health concern in the United States. Epidemiological evidence shows a correlation between use of chewing tobacco, lesions of the oral cavity and the incidence of oral and other cancers. However, the molecular mechanism(s) underlying the oral cancer causation are yet unknown. The major constituents of tobacco are known to cause inflammation, DNA damage and cell death. We propose modulation of inflammatory mediators by smokeless tobacco as a novel mechanism for the development of oral cancer. Exposure of hamster cheek pouches to smokeless tobacco extract (STE) results in cleavage of the anti-inflammatory peptide from the anti-inflammatory protein annexin I. Annexin I is produced from cultured oral epithelial cells and its expression is modulated by STE. We further show that STE exposure of oral epithelial cells results in upregulation of the pro-inflammatory protein COX-2. COX-2 is also upregulated in immortalized human oral epithelial cells, human squamous cell carcinoma cells and in primary tumor tissues from head and neck cancer. In summary, we find that exposure to smokeless tobacco results in loss of the anti-inflammatory activity of annexin I and upregulation of the pro-inflammatory COX-2 in oral cells. The dual effect of these regulatory events leads the cells down the carcinogenic pathway.

Leukocyte antiadhesive actions of annexin 1: ALXR- and FPR-related anti-inflammatory mechanisms

Recent investigations conducted with human neutrophils have indicated an involvement for the receptor for formylated peptides, termed FPR, and its analog FPRL1 (or ALXR because it is the receptor for the endogenous ligand lipoxin A(4)) in the in vitro inhibitory actions of the glucocorticoid-regulated protein annexin 1 and its peptidomimetics. To translate these findings in in vivo settings, we have used an ischemia/reperfusion (I/R) procedure to promote leukocyte-endothelium interactions in the mouse mesenteric microcirculation. In naive mice, the annexin 1 mimetic peptide Ac2-26 (20 to 100 microg administered intravenously prior to reperfusion) abolished I/R-induced cell adhesion and emigration, but not cell rolling. In FPR-deficient mice, peptide Ac2-26 retained significant inhibitory actions (about 50% of the effects in naive mice), and these were blocked by an FPR antagonist, termed butyloxycarbonyl-Phe-Leu-Phe-Leu-Phe, or Boc2. In vitro, neutrophils taken from these animals could be activated at high concentrations of formyl-Met-Leu-Phe (30 microM; fMLP), and this effect was blocked by cell incubation with peptide Ac2-26 (66 microM) or Boc2 (100 microM). FPR-deficient neutrophils expressed ALXR mRNA and protein. Both ALXR agonists, lipoxin A(4) and peptide Ac2-26, provoked detachment of adherent leukocytes in naive as well as in FPR-deficient mice, whereas the CXC chemokine KC or fMLP were inactive. The present findings demonstrate that endogenous regulatory autocoids such as lipoxin A(4) and annexin 1-derived peptides function to disengage adherent cells during cell-cell interactions.

Unaltered expression of Fas (CD95/APO-1), caspase-3, Bcl-2 and annexins in brains of fetal Down syndrome: evidence against increased apoptosis

Apoptosis is the mechanism by which cells are programmed to die under a wide range of physiological and developmental stimuli. Accumulating evidence indicates that enhanced apoptosis (programmed cell death) in Down syndrome (DS) may play a role in mental retardation and precocious neurodegeneration of the Alzheimer-type. In this regard, alteration of several apoptosis related proteins have been reported in adult DS brain. Fetal DS neurons exhibited increased reactive oxygen species leading to early apoptosis, however, expression of apoptosis related proteins in fetal DS, has never been considered. To address this issue, we investigated the expression of proteins involved in apoptosis including Fas (CD95, APO-1), caspase-3, Bcl-2 and annexins in the cerebral cortex of control and DS fetal brain by western blot and two dimensional electrophoresis. Here, we report that no detectable changes were obtained in fetal DS brain in the expression of Fas, caspase-3, Bcl-2 and Annexins (I, II, V, and VI) compared to controls. In parallel experiment, we also examined the expression of neuron specific enolase (NSE), a neuronal marker found to be decreased in adult DS brain, to see if there is any neuronal loss and no difference was observed between the two groups. Protein expression did not correlate with age. The unchanged levels of Fas, Bcl-2 and annexins together with unaltered caspase-3 expression, a predominant caspase that executes apoptosis in the developing nervous system, suggest that enhanced apoptosis may not be apparent in fetal DS brain as demonstrated for adult DS brain.

Regulation of polymorphonuclear leukocyte degranulation and oxidant production by ceramide through inhibition of phospholipase D

Exogenous C(2)-ceramide has been shown to inhibit polymorphonuclear leukocyte (PMN) phagocytosis through inhibition of phospholipase D (PLD) and downstream events, including activation of extracellular signal-regulated kinases 1 and 2, leading to the hyphothesis that the sphingomyelinase pathway is involved in termination of phagocytosis. Here it is postulated that increased PLD activity generating phosphatidic acid and diacylglycerol (DAG) is essential for superoxide release and degranulation and that ceramide, previously shown to be generated during PMN activation, inhibits PLD activation, thereby leading to inhibition of PMN function. When PMNs were primed with granulocyte colony-stimulating factor (G-CSF) and then activated with N-formyl-methionyl-leucyl-phenylalanine (FMLP), C(2)-ceramide (10 microM) completely inhibited release of superoxide, lactoferrin, and gelatinase; the DAG analog sn-1,2-didecanoylglycerol (DiC10) (10 microM) restored oxidase activation and degranulation in the ceramide-treated cells. Similarly, C(2)-ceramide inhibited oxidase activity and degranulation of PMNs treated with cytochalasin B followed by FMLP, and DiC10 restored function. In contrast, C(2)-ceramide did not inhibit phosphorylation of p47phox or p38 mitogen-activated protein kinase, or translocation of p47phox, PLD-containing organelles, adenosine diphosphate-ribosylation factor 1, RhoA, protein kinase C (PKC)-beta or PKC-alpha to the plasma membrane in G-CSF or cytochalasin B-treated, FMLP-activated PMNs. PLD activity increased by 3-fold in G-CSF-primed PMNs stimulated by FMLP and by 30-fold in cytochalasin B-treated PMNs stimulated by FMLP. Both PLD activities were completely inhibited by 10 microM C(2)-ceramide. In conclusion, superoxide, gelatinase, and lactoferrin release require activation of the PLD pathway in primed PMNs and cytochalasin B-treated PMNs. Ceramide may affect protein interactions with PLD in the plasma membrane, thereby attenuating PMN activation.

Identification and cloning of the SNARE proteins VAMP-2 and syntaxin-4 from HL-60 cells and human neutrophils

Degranulation and membrane fusion by neutrophils are essential to host defense. We sought homologues of neuron-specific fusion proteins in human neutrophils and in their precursors, the promyelocytic cell line HL-60. We screened a differentiated HL-60 library and obtained an 848 bp sequence with a 351 bp open reading frame, identical to that published for human VAMP-2 and including 5' and 3' untranslated regions. RNA from HL-60 cells during differentiation into the neutrophil lineage was subjected to Northern blot analysis. which revealed a transcript of approximately 1050 bp at all stages of differentiation. The amount of these transcripts increased approximately threefold during differentiation, a finding confirmed by quantitative RT-PCR. We also detected mRNA for VAMP-2 in human neutrophils and monocytes using RT-PCR. In like fashion, transcripts of syntaxin-4, another fusion protein, were recovered from a neutrophil cDNA library. As with VAMP-2, expression of syntaxin-4 (determined by Northern blots) also increased, but by only 50%, during differentiation of HL-60 cells. These studies demonstrate that neutrophils and their progenitors possess mRNA for the fusion proteins VAMP-2 and syntaxin-4, and that their transcription increases during differentiation, concurrent with the functional maturation of myeloid cells.

Neutrophil interaction with inflamed postcapillary venule endothelium alters annexin 1 expression

Annexin 1 (ANX-A1) exerts antimigratory actions in several models of acute and chronic inflammation. This is related to its ability to mimic the effect of endogenous ANX-A1 that is externalized on neutrophil adhesion to the postcapillary endothelium. In the present study we monitored ANX-A1 expression and localization in intravascular and emigrated neutrophils, using a classical model of rat peritonitis. For this purpose, a pair of antibodies raised against the ANX-A1 N-terminus (ie, able to recognize intact ANX-A1) or the whole protein (ie, able to interact with all ANX-A1 isoforms) was used by immunofluorescence and immunocytochemistry analyses. The majority ( approximately 50%) of ANX-A1 on the plasma membrane of intravascular neutrophils was intact. Extravasation into the subendothelial matrix caused loss of this pool of intact protein (to approximately 6%), concomitant with an increase in total amount of the protein; only approximately 25% of the total protein was now recognized by the antibody raised against the N-terminus (ie, it was intact). In the cytoplasm of these cells, ANX-A1 was predominantly associated with large vacuoles, possibly endosomes. In situ hybridization confirmed de novo synthesis of ANX-A1 in the extravasated cells. In conclusion, biochemical pathways leading to the externalization, proteolysis, and synthesis of ANX-A1 are activated during the process of neutrophil extravasation.

Intact Ca(2+)-binding sites are required for targeting of annexin 1 to endosomal membranes in living HeLa cells

Annexin 1 is a Ca(2+)-regulated membrane binding protein and a major substrate of the epidermal growth factor receptor kinase. Because of its properties and intracellular distribution, the protein has been implicated in endocytic trafficking of the receptor, in particular in receptor sorting occurring in multivesicular endosomes. Up to now, however, the localization of annexin 1 to cellular membranes has been limited to subcellular fractionation and immunocytochemical analyses of fixed cells. To establish its localization in live cells, we followed the intracellular fate of annexin 1 molecules fused to the Green Fluorescent Protein (GFP). We show that annexin 1-GFP associates with distinct, transferrin receptor-positive membrane structures in living HeLa cells. A GFP chimera containing the Ca(2+)/phospholipid-binding protein core of annexin 1 also shows a punctate intracellular distribution, although the structures labeled here do not resemble early but, at least in part, late endosomes. In contrast, the cores of annexins 2 and 4 fused to GFP exhibit a cytoplasmic or a different punctate distribution, respectively, indicating that the highly homologous annexin core domains carry distinct membrane specificities within live cells. By inactivating the three high-affinity Ca(2+) binding sites in annexin 1 we also show that endosomal membrane binding of the protein in live HeLa cells depends on the integrity of these Ca(2+) binding sites. More detailed analysis identifies a single Ca(2+) site in the second annexin repeat that is crucially involved in establishing the membrane association. These results reveal for the first time that intracellular membrane binding of an annexin in living cells requires Ca(2+) and is mediated in part through an annexin core domain that is capable of establishing specific interactions.

Annexin XI may be involved in Ca2+ - or GTP-gammaS-induced insulin secretion in the pancreatic beta-cell

The aim of this study was to investigate possible involvement of annexin XI in the insulin secretory machinery. In fluorescence immunocytochemistry, annexin XI was found in the cytoplasm of pancreatic endocrine cells and a pancreatic beta-cell line, MIN6, in a granular pattern. MIN6 cells also possessed weak and diffused annexin XI immunoreactivity in the cytoplasm. Immunoelectron microscopy revealed annexin XI in the insulin granules. Insulin secretion from streptolysin-O-permeabilized MIN6 cells was inhibited by anti-annexin XI antibody, when the release was stimulated by either Ca2+ or GTP-gammaS, but not by a protein kinase C-activating phorbol ester. Inhibition of insulin release by anti-annexin XI antibody was reproduced in permeabilized rat islets. These findings suggest that annexin XI may be involved in the regulation of insulin secretion from the pancreatic beta-cells.

Increased apoptosis in U937 cells over-expressing lipocortin 1 (annexin I)

The potential involvement of endogenous lipocortin 1 in the process of cellular apoptosis, particularly in cells of the myelo-monocytic lineage, has been investigated. U937 cells were transfected either with an antisense or a sense DNA for lipocortin 1 and the stable clones 36.4AS clone (20-40% lower lipocortin 1 levels) and 15S (30% higher lipocortin 1 levels) were obtained. Cell apoptosis was induced by incubation with tumor necrosis factor-alpha: optimal responses were observed within a 24 h incubation period at a 5 ng/ml concentration. Apoptosis was assessed both morphologically, by annexin V binding and cell cycle analysis with propidium iodide. Whilst no consistent difference was seen between wild type cells and clone 36.4AS, a higher incidence of apoptosis (ranging from +30% to + 60%) was observed in the 15S clone. Release of arachidonic acid from loaded cells was promoted by 24 h incubation with the cytokine, and a higher degree of release was measured in the 15S clone. These data indicate that endogenous intracellular lipocortin 1 is involved in the promotion of apoptosis in cells of the myelo-monocytic derivation.

Annexin V binds to the actin-based cytoskeleton at the plasma membrane of activated platelets

Immunocytochemical studies demonstrate that annexin V relocates to the plasma membranes of intact stimulated blood platelets. Anti-annexin V antibodies label the cytoplasmic side of the substrate-adherent plasma membrane of mechanically unroofed, glass-activated platelets and colocalize with actin. In addition, crosslinking experiments using detergent-solubilized membranes of activated platelets have identified an 85-kDa complex containing annexin V. The 85-kDa complex is also recognized by antibodies against actin, suggesting that annexin V interacts with actin. In addition, annexin V was found to associate with filamentous actin in the presence of millimolar Ca(2+). Annexin V was also shown by immunofluorescence microscopy to be associated with platelet cytoskeletons, colocalizing with actin in the presence of micromolar Ca(2+). These findings provide the first evidence for annexin V binding to the plasma membrane and to the actin-based cytoskeleton in activated platelets and indicate that annexin V may function in both cytoskeletal and membrane domains.

Characterization of the calcyclin (S100A6) binding site of annexin XI-A by site-directed mutagenesis

Residues in annexin XI-A essential for binding of calcyclin (S100A6) were examined by site-directed mutagenesis. GST fusion proteins with the calcyclin binding site of annexin XI-A, GST-AXI 34-62 and GST-AXI 49-77 bound to calcyclin-Sepharose Ca2+-dependently. The mutants GST-AXI L52E, M55E, A56E and M59E lost the binding ability, whereas GST-AXI A57E retained the ability. These results demonstrate that the hydrophobic residues L52, M55, A56 and M59 on one side surface of the alpha-helix are critical for the binding. Assays with GST fusion proteins and synthesized peptides corresponding to the calcyclin binding site indicated that other regions around the calcyclin binding site are important to stabilize the conformation.

The formation of ceramide-1-phosphate during neutrophil phagocytosis and its role in liposome fusion

Ceramide, a product of agonist-stimulated sphingomyelinase activation, is known to be generated during the phagocytosis of antibody-coated erythrocytes by polymorphonuclear leukocytes. Agonist-stimulated formation of ceramide-1-phosphate is now shown to occur in 32PO4-labeled neutrophils. Ceramide-1-phosphate is formed by a calcium-dependent ceramide kinase, found predominately in the neutrophil plasma membrane. The neutrophil kinase is specific for ceramide because, in contrast to the bacterial diglyceride kinase, ceramide is not phosphorylated under conditions specific for diglyceride phosphorylation. Conversely, 1,2-diacylglycerol does not serve as substrate for the neutrophil ceramide kinase. Ceramide kinase activation occurs in a time-dependent fashion, reaching peak activity 10 min after formyl peptide stimulation and challenge with antibody-coated erythrocytes. The lipid kinase activity is optimal at pH 6.8. Because the formation of the phagolysosome is a critical event in phagocytosis, the effect of ceramide-1-phosphate in promoting the fusion of liposomes was determined. Both the addition of increasing concentrations of sphingomyelinase D and ceramide-1-phosphate promoted liposomal fusion. In summary, ceramide-1-phosphate is formed during phagocytosis through activation of ceramide kinase. Ceramide-1-phosphate may promote phagolysosome formation.

Lipocortin 1 and chemokine modulation of granulocyte and monocyte accumulation in experimental inflammation

1. Migration of blood-derived leukocytes to tissue sites of inflammation is a hallmark of the response that the host organizes to counteract an insult or a trauma or an infection. A cascade of events is then activated to allow interaction between the leukocyte and the endothelium of postcapillary venule, and this cascade is finely regulated such that mechanisms of negative control are operating side by side with pathways that promote and sustain the extravasation process. Examples of both these positive and negative regulatory systems are discussed here. 2. In vivo accumulation of specific subtypes of leukocytes in response to application of selective chemokines operates through an indirect mechanism that includes the perivenular mast cell and, in particular, the mast cell-derived amines, such as histamine and serotonin. In fact, treatments of animals with (1) histamine H1 or serotonin antagonists or with (2) the mast cell stabilizer cromolyn or with (3) prior depletion of intact mast cells are maneuvers that successfully reduce eosinophil, neutrophil and monocyte extravasation in response to eotaxin, interleukin-8 or monocyte chemoattractant protein-1, respectively. A model in which histamine provides a P-selectin-dependent rolling phenomenon is then postulated. 3. The discovery that neutrophil-derived lipocortin 1 acts as an autocrine mediator with an inhibitory action on the emigration (diapedesis) process confirms the growing body of experimental data that showed that exogenously administered lipocortin 1 and lipocortin 1 mimetics (peptide Ac2-26) potently inhibit neutrophil extravasation in response to different stimuli. Externalization of lipocortin 1 on the plasma membrane of adherent neutrophils reduces their rate of passage through the endothelial gaps. Because cell-associated lipocortin 1 levels are under the partial control of corticosterone (endogenous circulating glucocorticoid hormone in rodents) and dexamethasone (a synthetic glucocorticoid hormone with a potent anti-inflammatory profile), a model is proposed in which a balance between anti-inflammatory (lipocortin 1, etc.) and pro-inflammatory (adhesion molecules, cytokines and chemokines) mediators explains the difference in the rate of leukocyte accumulation during the different stages of the host inflammatory response. 4. In conclusion, this review emphasizes the importance of in vivo experimental systems as a valid way of obtaining pertinent observations and reiterates the importance of negative regulatory mechanisms on the leukocyte extravasation process operating within the host.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Regulation of calcyclin (S100A6) binding by alternative splicing in the N-terminal regulatory domain of annexin XI isoforms

Annexin XI is a Ca2+/phospholipid-binding protein that interacts with a member of S100 protein family, calcyclin (S100A6), in a Ca2+-dependent manner. There are two isoforms of annexin XI, annexin XI-A and -B, generated by alternative splicing in the N-terminal regulatory domain. To determine the role of the alternative splicing region in the calcyclin-binding, we identified and characterized its calcyclin binding site. Experiments with glutathione S-transferase fusion proteins with N-terminal sites of annexin XI-A showed the calcyclin binding site to be in residues Gln49-Thr62 of rabbit annexin XI-A, which contains part of the splicing region. A synthesized peptide corresponding to Tyr43-Thr62 of annexin XI-A inhibited the interaction of annexin XI with calcyclin in liposome co-pelleting assay. The calcyclin binding site possesses a hydrophobic residue cluster conserved among S100 binding sites of annexin I and II. Recombinant annexin XI isoforms were expressed in Sf9 cells using a baculovirus expression system. In contrast to annexin XI-A, it was found that annexin XI-B protein could not bind to calcyclin by the liposome co-pelleting assay. In Sf9 cells coexpressing calcyclin with annexin XI isoforms, the calcyclin binding was observed only for annexin XI-A isoform. These results indicate that the calcyclin binding ability of annexin XI is an annexin XI-A isoform-specific character, suggesting that annexin XI isoforms might play distinct roles in cells through each alternative splicing regions.

The interaction of lung annexin I with phospholipid monolayers at the air/water interface

Lung annexin I (LAI), a calcium-ion-dependent phospholipid-binding protein, has been shown earlier to cause aggregation and fusion of bilayered vesicles containing phospholipids found in lung surfactant, and to be a very likely factor in the assembly of lung surfactant into the lamellar bodies stored in the Type II cell. In this study, we have measured the accumulation of LAI into spread monolayers of some major lipid components of lung surfactant, dipalmitoyl-phosphatidylcholine (DPPC), dipalmitoyl-phosphatidylglycerol (DPPG), palmitoyl-oleyoyl-phosphatidylglycerol (POPG), and selected mixtures, as a function of calcium-ion concentration and surface concentration (degree of packing) of the phospholipid monolayer. The ability of LAI to significantly penetrate such monolayers was calcium-ion-dependent and only occurred in the presence of DPPG or POPG. The relative extent of penetration into DPPG and POPG was directly related to the available free area in the monolayer, penetration being greater with POPG. Fluorescence microscopy measurements revealed that DPPC mixed with either DPPG or POPG caused a change in surface phase behavior in a manner believed to be related to certain types of bilayer fusion. A chemical breakdown product of LAI, LAI-bp, previously found not to cause aggregation and fusion of bilayers, did not exhibit comparable monolayer penetration or surface phase separation to LAI.

Binding to phosphatidyl serine membranes causes a conformational change in the concave face of annexin I

Recent studies have revealed that binding of annexin I to phospholipids induces the formation of a second phospholipid binding site. It is shown that the N terminus on the concave side of membrane-bound annexin I is cleaved much faster by trypsin or cathepsin than the N terminus of the free protein. The reactivity of the unique disulfide bond located near the concave face was similarly increased by membrane binding. These results demonstrate that Ca(2+)-dependent membrane binding induces a conformational change on the concave side of the annexin I molecule and support the notion that this face of the molecule may contribute to the formation of the secondary membrane-binding site.

Mobilizing lipocortin 1 in adherent human leukocytes downregulates their transmigration

Polymorphonuclear leukocyte (PMN) migration into sites of inflammation is fundamental to the host defense response. Activation of endothelial cells and PMNs increases the expression or activation of adhesion molecules, culminating in rolling and subsequent adherence of these cells to the vascular wall. Further activation of adherent PMNs, possibly by endothelial cell ligands, leads, within a few minutes, to extravasation itself. This process is not clearly understood, but adhesion molecules or related proteins, as well as endogenous chemokines, may play an important role. The anti-inflammatory glucocorticoids delay extravasation, which implies that an inhibitory regulatory system exists. Resting PMNs contain abundant cytoplasmic lipocortin 1 (LC1, also called annexin I)', and the activity profile of this protein suggests that it could reduce PMN responsiveness. To investigate this we have assessed neutrophil transmigration both in vivo and in vitro and examined the content and subcellular distribution of LC1 in PMNs by fluorescence-activated cell-sorting (FACS) analysis, western blotting and confocal microscopy. We report that LC1 is mobilized and externalized following PMN adhesion to endothelial monolayers in vitro or to venular endothelium in vivo and that the end point of this process is a negative regulation of PMN transendothelial passage.

Diverse effects of different neutrophil organelles on truncation and membrane-binding characteristics of annexin I

A neutrophil annexin I-related protein, detected after translocation of cytosolic proteins to specific granules and secretory vesicles/plasma membrane (Sjölin et al. (1994) Biochem. J. 300, 325-330), has been characterized with respect to origin and organelle-binding properties. The annexin I-related protein is formed as a result of annexin I cleavage, and this occurs during translocation of annexin I to the specific granules and secretory vesicles/plasma membrane, but not when annexin I is translocated to azurophil granules. The cleavage required calcium and it was facilitated in the presence of specific granules or secretory vesicles/plasma membrane, but not in the presence of azurophil granules. We conclude that the membranes of specific granules and secretory vesicles/plasma membrane contain a protease which is able to cleave annexin I into a truncated 38 kDa fragment, which retains the ability to bind to these organelles. The azurophil granules lack the capacity to cleave annexin I as well as the ability to bind the 38 kDa fragment. These findings may implicate a role for annexin I in the divergent regulation of exocytosis of the different neutrophil granules.

Translocation of annexin I to plasma membranes and phagosomes in human neutrophils upon stimulation with opsonized zymosan: possible role in phagosome function

Annexin I in the cytosol of resting neutrophils was translocated to the plasma membranes upon addition of opsonized zymosan (OZ). Maximum translocation could be detected 1 min after stimulation with OZ, and decreased thereafter. Subcellular fractionation studies demonstrated that annexin I could not be detected in the granule fractions in either resting or activated cells, but was found in association with the phagosome fraction. The marked translocation of annexin I was unique to OZ, since formyl-Met-Leu-Phe induced only slight translocation of annexin I to the plasma membranes, and phorbol 12-myristate 13-acetate had no effect at all. The mechanism regulating the translocation of annexin I is not clear. Annexin I is not phosphorylated in resting or stimulated cells. The correlation between the elevation in the intracellular calcium ion concentration ([Ca2+]i) and the degree of translocation of annexin I to the plasma membranes induced by the different stimuli, together with the inhibition of these processes by the addition of EGTA, indicate that the translocation of annexin I can probably be attributed to the rise in [Ca2+]i. However, this cannot be the sole mechanism since ionomycin, which caused an increase in [CA2+]i similar to that induced by OZ, was less efficient than OZ in inducing translocation of annexin I. The induction of annexin I translocation to the plasma membrane by OZ, which was the only agent that induced phagosome formation, and the detection of annexin I in the phagosome fraction, suggest that annexin I participates in phagosome function.