No abstract

Galectin-8 has two different carbohydrate recognition domains (CRDs), the N-terminal Gal-8N and the C-terminal Gal-8C linked by a peptide, and has various effects on cell adhesion and signaling. To understand the mechanism for these effects further, we compared the binding activities of galectin-8 in solution with its binding and activation of cells. We used glycan array analysis to broaden the specificity profile of the two galectin-8 CRDs, as well as intact galectin-8s (short and long linker), confirming the unique preference for sulfated and sialylated glycans of Gal-8N. Using a fluorescence anisotropy assay, we examined the solution affinities for a subset of these glycans, the highest being 50 nM for NeuAcalpha2,3Lac by Gal-8N. Thus, carbohydrate-protein interactions can be of high affinity without requiring multivalency. More importantly, using fluorescence polarization, we also gained information on how the affinity is built by multiple weak interactions between different fragments of the glycan and its carrier molecule and the galectin CRD subsites (A-E). In intact galectin-8 proteins, the two domains act independently of each other in solution, whereas at a surface they act together. Ligands with moderate or weak affinity for the isolated CRDs on the array are bound strongly by intact galectin-8s. Also galectin-8 binding and signaling at cell surfaces can be explained by combined binding of the two CRDs to low or medium affinity ligands, and their highest affinity ligands, such as sialylated galactosides, are not required.

The extravasation of leukocytes from the peripheral blood stream to inflammatory sites is a key feature in the innate immune response to infection (1). Different chemoattractants (e.g. N-formylated peptides, C5a, interleukin-8, leukotriene B 4 , and platelet-activating factor) and chemokines induce leukocyte infiltration and activation through binding to G proteincoupled seven-transmembrane cell-surface receptors (2, 3). The chemoattractant-mediated dissociation of G␣ i2 from the G␤␥ subunit complex results in the activation of several downstream signaling effector enzymes that promote intracellular calcium mobilization, modifications in the metabolism of phosphoinositides, and activation of mitogen-activated protein kinases (4). The integration by the cell of the different chemoattractant-activated signaling pathways results in directed cell migration, recruitment of new receptors from the granules to the cell surface, release of proteolytic enzymes, production of large amounts of superoxide by the neutrophil NADPH oxidase, and increased gene transcription (5-8). The extent of the cellular response is dependent on the identity of the agonist and on the level of expression and desensitization of the receptors involved in the activation process (9).Two synthetic hexapeptides, Trp-Lys-Tyr-Met-Val-Met-NH 2 (WKYMVM)andTrp-Lys-Tyr-Met-Val-D-Met-NH 2 (WKYMVm), that stimulate phosphoinositide hydrolysis in myeloid cells were identified by screening a peptide library (10, 11). The D-methionine-containing hexapeptide (WKYMVm) was found to be a very potent activator of several leukocyte effector functions such as chemotaxis, mobilization of complement receptor-3, and activation of the NADPH oxidase (11). The peptide WKYMVm activates neutrophils through both the N-formyl peptide receptor (FPR) 1 and FPRL1 (N-formyl peptide receptor-like-1) (12, 13). The latter was originally cloned from human phagocytes by low-stringency hybridization of a cDNA library with the FPR cDNA sequence, and it was initially defined as an orphan receptor (14 -16). FPRL1 was later referred to as the LXA 4 receptor since it was shown to bind lipoxin A 4 with high affinity (17). In addition, several different peptides/proteins have been reported to stimulate this receptor. These include a leucine zipper-like domain of the HIV-1

Over the last decade a vast amount of reports have shown that galectin-1 and galectin-3 are important mediators of inflammation. In this review we describe how the galectins may be involved in several parts of the inflammatory process, including the recruitment of neutrophils into an infected tissue and the recognition and killing of bacteria by activation of the tissue destructive phagocytic respiratory burst. During bacterial infection or aseptic inflammatory processes, galectins are produced and released by e.g. infected epithelium, activated tissue-resident macrophages and endothelial cells. These extracellular galectins may facilitate binding of neutrophils to the endothelium by cross-linking carbohydrates on the respective cells. Further the galectins improve binding of the neutrophil to the extracellular matrix proteins laminin and fibronectin, and are potential chemotactic factors, inducing migration through the extracellular matrix towards the inflammatory focus. When the cells encounter bacteria, galectin-3 could function as an opsonin, cross-linking bacterial lipopolysaccharide or other carbohydrate-containing surface structures to phagocyte surface glycoconjugates. Both galectin-1 and galectin-3 have the capacity to induce a respiratory burst in neutrophils, provided that the cells have been primed by degranulation and receptor upregulation. The reactive oxygen species produced may be destructive to the invading micro-organisms as well as to the surrounding host tissue, pointing out the possible role of galectins, not only in defence toward infection, but also in inflammatory-induced tissue destruction.

Human neutrophils have an important role in host defense against microbial infection. At different stages of an infectious process, neutrophils progressively up-regulate receptors and release various effector molecules. These are stored in several distinct types of granules with varying propensity to be secreted. Heparin-binding protein (HBP), also known as CAP37 or azurocidin, is a multifunctional, inactive serine-protease homologue. The present work shows that HBP is released from neutrophils on stimulation with secretagogues that do not trigger the secretion of azurophilic granule content. Therefore, the subcellular localization of HBP was investigated in more detail. Immunofluorescence microscopy revealed that HBP was localized close to the plasma membrane. Further analysis by fractionation of postnuclear supernatants from cavitated neutrophils showed that HBP is stored in azurophilic granules and secretory vesicles but that it is also detected to a minor extent in the plasma membrane. IntroductionPolymorphonuclear leukocytes (PMNs) have an important role in early host defense against invading microorganisms (for reviews, see references 1 and 2). Recruitment of these cells from the bloodstream to a site of infection involves their recognition of inflammatory mediators, their binding to adhesion molecules of the vascular endothelium, and their migration across the endothelial barrier. 3 How efficiently neutrophils perform these tasks depends on a sophisticated mobilization mechanism that triggers the release of granule contents and the concomitant up-regulation of various receptors to the plasma membrane. 4 Secretory processes are also important for the extravascular migration of neutrophils through tissues. Once the cells have reached the focus of infection, they are fully activated and are able to fight the infection by secreting reactive oxygen intermediates, antimicrobial peptides, and degradative enzymes. 2 These substances can be preferentially targeted to phagosome compartments to achieve efficient killing and degradation of internalized microorganisms.Lately, much interest has been focused on the various granule types of neutrophils and their sequential mobilization during the inflammatory process (for review, see reference 5). Analysis of these granules by electron microscopy and subcellular fractionation has demonstrated that neutrophils have at least 4 different granule or vesicle types. [6][7][8][9] These are the primary or azurophilic granules that contain myeloperoxidase (MPO), bactericidal proteins, and proteinases; the secondary or specific granules that store lactoferrin and enzymes such as collagenase and gelatinase; the tertiary or gelatinase granules that, like specific granules, contain tissuedegrading enzymes; and the secretory vesicles, an easily mobilizable compartment, that contain alkaline phosphatase and plasma proteins such as human serum albumin. The 4 granule types are mobilized at different stages of the inflammatory process; secretory vesicles are more readily secreted than th...