Characterization of plasminogen as an adhesive ligand for integrins alphaMbeta2 (Mac-1) and alpha5beta1 (VLA-5)

αMI-domain of integrin Mac-1 binds the cytokine pleiotrophin using multiple mechanisms

The integrin Mac-1 (αMβ2, CD11b/CD18, CR3) is an adhesion receptor expressed on macrophages and neutrophils. Mac-1 is also a promiscuous integrin that binds a diverse set of ligands through its αMI-domain. However, the binding mechanism of most ligands remains unclear. We have characterized the interaction of αMI-domain with the cytokine pleiotrophin (PTN), a protein known to bind αMI-domain and induce Mac-1-mediated cell adhesion and migration. Our data show that PTN's N-terminal domain binds a unique site near the N- and C-termini of the αMI-domain using a metal-independent mechanism. However, a stronger interaction is achieved when an acidic amino acid in a zwitterionic motif in PTN's C-terminal domain chelates the divalent cation in the metal ion-dependent adhesion site of active αMI-domain. These results indicate that αMI-domain can bind ligands using multiple mechanisms and that the active αMI-domain has a preference for motifs containing both positively and negatively charged amino acids.

αMI-domain of Integrin Mac-1 Binds the Cytokine Pleiotrophin Using Multiple Mechanisms

The integrin Mac-1 (αMβ2, CD11b/CD18, CR3) is an important adhesion receptor expressed on macrophages and neutrophils. Mac-1 is also the most promiscuous member of the integrin family that binds a diverse set of ligands through its αMI-domain. However, the binding mechanism of most ligands is not clear. We have determined the interaction of αMI-domain with the cytokine pleiotrophin (PTN), a cationic protein known to bind αMI-domain and induce Mac-1-mediated cell adhesion and migration. Our data show that PTN's N-terminal domain binds a unique site near the N- and C-termini of the αMI-domain using a metal-independent mechanism. However, stronger interaction is achieved when an acidic amino acid in a zwitterionic motif in PTN's C-terminal domain chelates the divalent cation in the metal ion-dependent adhesion site of the active αMI-domain. These results indicate that αMI-domain can bind ligands using multiple mechanisms, and suggest that active αMI-domain prefers acidic amino acids in zwitterionic motifs.

Wound Healing Complications With Tranexamic Acid: Not the Silver Bullet After All

Tranexamic acid (TXA) has been popularized as an adjunct to decrease the risk of bleeding and subsequent bruising and edema in aesthetic surgery. The most notable risks of TXA are thrombus and seizures, which are associated with higher plasma concentrations of the acid. In an effort to mitigate these risks, surgeons have begun using TXA locally, either as a topical irrigation or mixed into the local anesthetic. Although local use is thought to be safer from a side-effect standpoint, because there is decreased systemic absorption, its use is not without risk. We present 4 patients who developed wound healing complications thought to be related to locally administered TXA. One patient had TXA delivered topically, and 3 patients had TXA mixed into their local anesthetic. These adverse events have not been published in the literature previously. This case report serves as a warning to other surgeons about using locally administered TXA.

LEVEL OF EVIDENCE: 4

Hemostatic Alginate/Nano-Hydroxyapatite Composite Aerogel Loaded with Tranexamic Acid for the Potential Protection against Alveolar Osteitis

Wound control in patients on anticoagulants is challenging and often leads to poor hemostasis. They have a higher tendency to develop alveolar osteitis after tooth extraction. The application of a hemostatic dressing that has a high absorbing capacity and is loaded with an antifibrinolytic drug could help in controlling the bleeding. Alginate/nano-hydroxyapatite (SA/Nano-HA) composite aerogels loaded with tranexamic acid (TXA) were prepared. Nano-HA served as a reinforcing material for the alginate matrix and a source of calcium ions that helps in blood clotting. It influenced the porosity and the water uptake capacity. TXA release from SA/Nano-HA aerogels showed a biphasic profile for up to 4 h. Blood coagulation studies were performed on human whole blood. The TXA-loaded aerogel significantly reduced the clotting time by 69% compared to the control (p < 0.0001). Recalcification time was significantly reduced by 80% (p < 0.0001). Scanning electron microscopy analysis revealed the porous nature of the aerogels and the ability of the optimum aerogel to activate and adhere platelets to its porous surface. The cell migration assay showed that there was a delay in wound healing caused by the TXA aerogel compared to the control sample after treating human fibroblasts. Results suggest that the developed aerogel is a promising dressing that will help in hemostasis after tooth extraction.

The ANXA2/S100A10 Complex—Regulation of the Oncogenic Plasminogen Receptor

The generation of the serine protease plasmin is initiated by the binding of its zymogenic precursor, plasminogen, to cell surface receptors. The proteolytic activity of plasmin, generated at the cell surface, plays a crucial role in several physiological processes, including fibrinolysis, angiogenesis, wound healing, and the invasion of cells through both the basement membrane and extracellular matrix. The seminal observation by Albert Fischer that cancer cells, but not normal cells in culture, produce large amounts of plasmin formed the basis of current-day observations that plasmin generation can be hijacked by cancer cells to allow tumor development, progression, and metastasis. Thus, the cell surface plasminogen-binding receptor proteins are critical to generating plasmin proteolytic activity at the cell surface. This review focuses on one of the twelve well-described plasminogen receptors, S100A10, which, when in complex with its regulatory partner, annexin A2 (ANXA2), forms the ANXA2/S100A10 heterotetrameric complex referred to as AIIt. We present the theme that AIIt is the quintessential cellular plasminogen receptor since it regulates the formation and the destruction of plasmin. We also introduce the term oncogenic plasminogen receptor to define those plasminogen receptors directly activated during cancer progression. We then discuss the research establishing AIIt as an oncogenic plasminogen receptor-regulated during EMT and activated by oncogenes such as SRC, RAS, HIF1α, and PML-RAR and epigenetically by DNA methylation. We further discuss the evidence derived from animal models supporting the role of S100A10 in tumor progression and oncogenesis. Lastly, we describe the potential of S100A10 as a biomarker for cancer diagnosis and prognosis.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

Assessing Plasmin Generation in Health and Disease

Fibrinolysis is an important process in hemostasis responsible for dissolving the clot during wound healing. Plasmin is a central enzyme in this process via its capacity to cleave fibrin. The kinetics of plasmin generation (PG) and inhibition during fibrinolysis have been poorly understood until the recent development of assays to quantify these metrics. The assessment of plasmin kinetics allows for the identification of fibrinolytic dysfunction and better understanding of the relationships between abnormal fibrin dissolution and disease pathogenesis. Additionally, direct measurement of the inhibition of PG by antifibrinolytic medications, such as tranexamic acid, can be a useful tool to assess the risks and effectiveness of antifibrinolytic therapy in hemorrhagic diseases. This review provides an overview of available PG assays to directly measure the kinetics of plasmin formation and inhibition in human and mouse plasmas and focuses on their applications in defining the role of plasmin in diseases, including angioedema, hemophilia, rare bleeding disorders, COVID-19, or diet-induced obesity. Moreover, this review introduces the PG assay as a promising clinical and research method to monitor antifibrinolytic medications and screen for genetic or acquired fibrinolytic disorders.

Plasminogen Receptors and Fibrinolysis

The ability of cells to promote plasminogen activation on their surfaces is now well recognized, and several distinct cell surface proteins have been demonstrated to function as plasminogen receptors. Here, we review studies demonstrating that plasminogen bound to cells, in addition to plasminogen directly bound to fibrin, plays a major role in regulating fibrin surveillance. We focus on the ability of specific plasminogen receptors on eukaryotic cells to promote fibrinolysis in the in vivo setting by reviewing data obtained predominantly in murine models. Roles for distinct plasminogen receptors in fibrin surveillance in intravascular fibrinolysis, immune cell recruitment in the inflammatory response, wound healing, and lactational development are discussed.

Structural Characterization of the Interaction between the αMI-Domain of the Integrin Mac-1 (αMβ2) and the Cytokine Pleiotrophin

Integrin Mac-1 (α_Mβ₂) is an adhesion receptor vital to many functions of myeloid leukocytes. It is also the most promiscuous member of the integrin family capable of recognizing a broad range of ligands. In particular, its ligand-binding α_MI-domain is known to bind cationic proteins/peptides depleted in acidic residues. This contradicts the canonical ligand-binding mechanism of αI-domains, which requires an acidic amino acid in the ligand to coordinate the divalent cation within the metal ion-dependent adhesion site (MIDAS) of αI-domains. The lack of acidic amino acids in the α_MI-domain-binding sequences suggests the existence of an as-yet uncharacterized interaction mechanism. In the present study, we analyzed interactions of the α_MI-domain with a representative Mac-1 ligand, the cationic cytokine pleiotrophin (PTN). Through NMR chemical shift perturbation analysis, cross saturation, NOESY, and mutagenesis studies, we found the interaction between the α_MI-domain and PTN is divalent cation-independent and mediated mostly by hydrophobic contacts between the N-terminal domain of PTN and residues in the α5-β5 loop of α_MI-domain. The observation that increased ionic strength weakens the interaction between the proteins indicates electrostatic forces may also play a significant role in the binding. On the basis of the results from these experiments, we formulated a model of the interaction between the α_MI-domain and PTN.

Functions of the plasminogen receptor Plg-RKT

Plg-RKT is a structurally unique transmembrane plasminogen receptor with both N- and C-terminal domains exposed on the extracellular face of the cell. Its C-terminal lysine functions to tether plasminogen to cell surfaces. Overexpression of Plg-RKT increases cell surface plasminogen binding capacity while genetic deletion of Plg-RKT decreases plasminogen binding. Plasminogen binding to Plg-RKT results in promotion of plasminogen activation to the broad spectrum serine protease plasmin. This function is promoted by the physical association of Plg-RKT with the urokinase receptor (uPAR). Plg-RKT is broadly expressed in cells and tissues throughout the organism and its sequence is remarkably conserved phylogenetically. Plg-RKT also is required for lactation and, thus, is necessary for survival of the species. This review provides an overview of established and emerging functions of Plg-RKT and highlights major roles for Plg-RKT in both the initiation and resolution of inflammation. While the roles for Plg-RKT in the inflammatory response are predominantly plasmin(ogen)-dependent, its role in lactation requires both plasminogen-dependent and plasminogen-independent mechanisms. Furthermore, the functions of Plg-RKT are dependent on sex. In view of the broad tissue distribution of Plg-RKT , its role in a broad array of physiological and pathological processes should provide a fruitful area for future investigation.

Topical moistening of mastectomy wounds with diluted tranexamic acid to reduce bleeding: randomized clinical trial

BACKGROUND

Topical administration of tranexamic acid (TXA) may be an alternative to intravenous administration to reduce bleeding with a lower risk of systemic adverse events. The aim of this study was to investigate whether moistening a surgical wound with TXA before closure, leaving a thin film of drug only, would reduce postoperative bleeding.

METHODS

This was a two-centre, stratified, parallel-group, placebo-controlled, double-blind RCT. Patients undergoing mastectomy with or without axillary lymph node clearance were randomized 1 : 1 to moistening of wound surface before closure with either 25 mg/ml TXA or 0·9 per cent sodium chloride (placebo). The primary endpoint was postoperative bleeding as measured by drain production in the first 24 h. Secondary endpoints were early haematoma, total drain production, postoperative complications and late aspirations of seroma within 3 months.

RESULTS

Between 1 January 2016 and 31 August 2018, 208 patients were randomized. Two patients were converted to a different surgical procedure at surgery, and four did not receive the intervention owing to technical error. Thus, 202 patients were included in the study (101 in the TXA and 101 in the placebo group). TXA reduced mean drain production at 24 h (110 versus 144 ml; mean difference 34 (95 per cent c.i. 8 to 60) ml, P = 0·011). One patient in the TXA group had early haematoma compared with seven in the placebo group (odds ratio (OR) 0·13 (95 per cent c.i. 0·02 to 1·07); P = 0·057). There was no significant difference in postoperative complications between TXA and placebo (13 versus 10; OR 1·11 (0·45 to 2·73), P = 0·824) or need for late seroma aspirations (79 versus 67 per cent; OR 1·83 (0·91 to 3·68), P = 0·089).

CONCLUSION

Moistening the wound with TXA 25 mg/ml before closure reduces postoperative bleeding within the first 24 h in patients undergoing mastectomy. Registration number: NCT02627560 (https://clinicaltrials.gov).

Cytotoxicity and effect on wound re-epithelialization after topical administration of tranexamic acid

Background

Topical administration of tranexamic acid (TXA) reduces bleeding from surgical wounds similarly to intravenous use, but with negligible risk of adverse systemic events. Topical use is expanding, but is off‐label. Surgeons lack guidelines regarding safe topical dosages and modes of administration. The effects of topical TXA on skin cells and wound healing are unknown. This study investigated whether topical TXA might be cytotoxic or affect wound re‐epithelialization.

Methods

Human keratinocytes and fibroblast cell cultures and an ex vivo human skin wound model were subjected to both short (limited) and long (chronic) exposure to various clinically relevant concentrations of TXA to mimic different modalities of topical administration. Cytotoxicity and effects on wound re‐epithelialization were evaluated.

Results

In cell culture, toxicity from chronic exposure was associated with increasing concentration and exposure time. Limited exposure to TXA did not cause significant cytotoxicity even at high concentrations. Re‐epithelialization was completely absent in wounds chronically exposed to TXA concentrations of 25 mg/ml or above, and 50–100 mg/ml induced epidermolysis of normal epithelium, possibly by a non‐toxic mechanism. Wound re‐epithelialization was slightly delayed, but not impaired, by limited exposure to 100 mg/ml or chronic exposure to 6·25 mg/ml.

Conclusion

Although short exposure to even high concentrations of topical TXA seems well tolerated in vitro, prolonged exposure can be cytotoxic and may affect wound re‐epithelialization. Surgeons should adjust the TXA concentration to the planned mode of topical administration in clinical practice.

Leukocyte arrest: Biomechanics and molecular mechanisms of β2 integrin activation

Integrins are a group of heterodimeric transmembrane receptors that play essential roles in cell-cell and cell-matrix interaction. Integrins are important in many physiological processes and diseases. Integrins acquire affinity to their ligand by undergoing molecular conformational changes called activation. Here we review the molecular biomechanics during conformational changes of integrins, integrin functions in leukocyte biorheology (adhesive functions during rolling and arrest) and molecules involved in integrin activation.

Selective photonic disinfection of cell culture using a visible ultrashort pulsed laser

Microbial contamination of cell culture is a major problem encountered both in academic labs and in the biotechnology/pharmaceutical industries. A broad spectrum of microbes including mycoplasma, bacteria, fungi, and viruses are the causative agents of cell culture contamination. Unfortunately, the existing disinfection techniques lack selectivity and/or lead to the development of drug-resistance, and more importantly there is no universal method to address all microbes. Here, we report a novel, chemical-free visible ultrashort pulsed laser method for cell culture disinfection. The ultrashort pulsed laser technology inactivates pathogens with mechanical means, a paradigm shift from the traditional pharmaceutical and chemical approaches. We demonstrate that ultrashort pulsed laser treatment can efficiently inactivate mycoplasma, bacteria, yeast, and viruses with good preservation of mammalian cell viability. Our results indicate that this ultrashort pulsed laser technology has the potential to serve as a universal method for the disinfection of cell culture.

The opioid peptide dynorphin A induces leukocyte responses via integrin Mac-1 (αMβ2, CD11b/CD18)

Background

Opioid peptides, including dynorphin A, besides their analgesic action in the nervous system, exert a broad spectrum of effects on cells of the immune system, including leukocyte migration, degranulation and cytokine production. The mechanisms whereby opioid peptides induce leukocyte responses are poorly understood. The integrin Mac-1 (α_Mβ₂, CD11b/CD18) is a multiligand receptor which mediates numerous reactions of neutrophils and monocyte/macrophages during the immune-inflammatory response. Our recent elucidation of the ligand recognition specificity of Mac-1 suggested that dynorphin A and dynorphin B contain Mac-1 recognition motifs and can potentially interact with this receptor.

Results

In this study, we have synthesized the peptide library spanning the sequence of dynorphin AB, containing dynorphin A and B, and showed that the peptides bound recombinant α_MI-domain, the ligand binding region of Mac-1. In addition, immobilized dynorphins A and B supported adhesion of the Mac-1-expressing cells. In binding to dynorphins A and B, Mac-1 cooperated with cell surface proteoglycans since both anti-Mac-1 function-blocking reagents and heparin were required to block adhesion. Further focusing on dynorphin A, we showed that its interaction with the α_MI-domain was activation independent as both the α7 helix-truncated (active conformation) and helix-extended (nonactive conformation) α_MI-domains efficiently bound dynorphin A. Dynorphin A induced a potent migratory response of Mac-1-expressing, but not Mac-1-deficient leukocytes, and enhanced Mac-1-mediated phagocytosis of latex beads by murine IC-21 macrophages.

Conclusions

Together, the results identify dynorphins A and B as novel ligands for Mac-1 and suggest a role for the Dynorphin A-Mac-1 interactions in the induction of nonopiod receptor-dependent effects in leukocytes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0027-0) contains supplementary material, which is available to authorized users.

Exploitation of plasmin(ogen) by bacterial pathogens of veterinary significance

The plasminogen (Plg) system plays an important homeostatic role in the degradation of fibrin clots, extracellular matrices and tissue barriers important for cellular migration, as well as the promotion of neurotransmitter release. Plg circulates in plasma at physiologically high concentrations (150-200μg ml(-1)) as an inactive proenzyme. Proteins enriched in lysine and other positively charged residues (histidine and arginine) as well as glycosaminoglycans and gangliosides bind Plg. The binding interaction initiates a structural adjustment to the bound Plg that facilitates cleavage by proteases (plasminogen activators tPA and uPA) that activate Plg to the active serine protease plasmin. Both pathogenic and commensal bacteria capture Plg onto their cell surface and promote its conversion to plasmin. Many microbial Plg-binding proteins have been described underpinning the importance this process plays in how bacteria interact with their hosts. Bacteria exploit the proteolytic capabilities of plasmin by (i) targeting the mammalian fibrinolytic system and degrading fibrin clots, (ii) remodeling the extracellular matrix and generating bioactive cleavage fragments of the ECM that influence signaling pathways, (iii) activating matrix metalloproteinases that assist in the destruction of tissue barriers and promote microbial metastasis and (iv) destroying immune effector molecules. There has been little focus on the exploitation of the fibrinolytic system by veterinary pathogens. Here we describe several pathogens of veterinary significance that possess adhesins that bind plasmin(ogen) onto their cell surface and promote its activation to plasmin. Cumulative data suggests that these attributes provide pathogenic and commensal bacteria with a means to colonize and persist within the host environment.

Ligand recognition specificity of leukocyte integrin αMβ2 (Mac-1, CD11b/CD18) and its functional consequences

The broad recognition specificity exhibited by integrin α(M)β2 (Mac-1, CD11b/CD18) has allowed this adhesion receptor to play innumerable roles in leukocyte biology, yet we know little about how and why α(M)β2 binds its multiple ligands. Within α(M)β2, the α(M)I-domain is responsible for integrin's multiligand binding properties. To identify its recognition motif, we screened peptide libraries spanning sequences of many known protein ligands for α(M)I-domain binding and also selected the α(M)I-domain recognition sequences by phage display. Analyses of >1400 binding and nonbinding peptides derived from peptide libraries showed that a key feature of the α(M)I-domain recognition motif is a small core consisting of basic amino acids flanked by hydrophobic residues. Furthermore, the peptides selected by phage display conformed to a similar pattern. Identification of the recognition motif allowed the construction of an algorithm that reliably predicts the α(M)I-domain binding sites in the α(M)β2 ligands. The recognition specificity of the α(M)I-domain resembles that of some chaperones, which allows it to bind segments exposed in unfolded proteins. The disclosure of the α(M)β2 binding preferences allowed the prediction that cationic host defense peptides, which are strikingly enriched in the α(M)I-domain recognition motifs, represent a new class of α(M)β2 ligands. This prediction has been tested by examining the interaction of α(M)β2 with the human cathelicidin peptide LL-37. LL-37 induced a potent α(M)β2-dependent cell migratory response and caused activation of α(M)β2 on neutrophils. The newly revealed recognition specificity of α(M)β2 toward unfolded protein segments and cationic proteins and peptides suggests that α(M)β2 may serve as a previously proposed "alarmin" receptor with important roles in innate host defense.

Dual role of the leukocyte integrin αMβ2 in angiogenesis

Polymorphonuclear neutrophils (PMNs) and macrophages are crucial contributors to neovascularization, serving as a source of chemokines, growth factors, and proteases. α(M)β(2)(CD11b/CD18) and α(L)β(2)(CD11a/CD18) are expressed prominently and have been implicated in various responses of these cell types. Thus, we investigated the role of these β2 integrins in angiogenesis. Angiogenesis was analyzed in wild-type (WT), α(M)-knockout (α(M)(-/-)), and α(L)-deficient (α(L)(-/-)) mice using B16F10 melanoma, RM1 prostate cancer, and Matrigel implants. In all models, vascular area was decreased by 50-70% in α(M)(-/-) mice, resulting in stunted tumor growth as compared with WT mice. In contrast, α(L) deficiency did not impair angiogenesis and tumor growth. The neovessels in α(M)(-/-) mice were leaky and immature because they lacked smooth muscle cell and pericytes. Defective angiogenesis in the α(M)(-/-) mice was associated with attenuated PMN and macrophage recruitment into tumors. In contrast to WT or the α(L)(-/-) leukocytes, the α(M)(-/-) myeloid cells showed impaired plasmin (Plm)-dependent extracellular matrix invasion, resulting from 50-75% decrease in plasminogen (Plg) binding and pericellular Plm activity. Surface plasmon resonance verified direct interaction of the α(M)I-domain, the major ligand binding site in the β(2) integrins, with Plg. However, the α(L)I-domain failed to bind Plg. In addition, endothelial cells failed to form tubes in the presence of conditioned medium collected from TNF-α-stimulated PMNs derived from the α(M)(-/-) mice because of severely impaired degranulation and secretion of VEGF. Thus, α(M)β(2) plays a dual role in angiogenesis, supporting not only Plm-dependent recruitment of myeloid cells to angiogenic niches, but also secretion of VEGF by these cells.

New insights into the role of Plg-RKT in macrophage recruitment

Plasminogen (PLG) is the zymogen of plasmin, the major enzyme that degrades fibrin clots. In addition to its binding and activation on fibrin clots, PLG also specifically interacts with cell surfaces where it is more efficiently activated by PLG activators, compared with the reaction in solution. This results in association of the broad-spectrum proteolytic activity of plasmin with cell surfaces that functions to promote cell migration. Here, we review emerging data establishing a role for PLG, plasminogen receptors and the newly discovered plasminogen receptor, Plg-RKT, in macrophage recruitment in the inflammatory response, and we address mechanisms by which the interplay between PLG and its receptors regulates inflammation.

Thrombomodulin functions as a plasminogen receptor to modulate angiogenesis

Urokinase-type plasminogen activator (uPA) activates plasminogen (Plg) through a major pericellular proteolytic system involved in cell migration and angiogenesis; however, the Plg receptor that participates in uPA-mediated Plg activation has not yet been identified. In this study, we demonstrated that thrombomodulin (TM), a type I transmembrane glycoprotein, is a novel Plg receptor that plays a role in pericellular proteolysis and cell migration. Plg activation at the cell surface and the extent of its cell migration- and invasion-promoting effect are cellular TM expression dependent. Direct binding of Plg and the recombinant TM extracellular domain, with a KD of 0.1-0.3 μM, was determined through surface plasmon resonance analysis. Colocalization of TM, Plg, and the uPA receptor within plasma membrane lipid rafts, at the leading edge of migrating endothelial cells, was demonstrated and was also shown to overlap with areas of major pericellular proteolysis. Moreover, the roles of TM and Plg in neoangiogenesis were demonstrated in vivo through the skin wound-healing model. In conclusion, we propose that TM is a novel Plg receptor that regulates uPA/uPA receptor-mediated Plg activation and pericellular proteolysis within lipid rafts at the leading edge of migrating cells during angiogenesis.

Plasminogen receptors: the first quarter century

The interaction of plasminogen with cell surfaces results in promotion of plasmin formation and retention on the cell surface. This results in arming cell surfaces with the broad-spectrum proteolytic activity of plasmin. Over the past quarter century, key functional consequences of the association of plasmin with the cell surface have been elucidated. Physiologic and pathophysiologic processes with plasmin-dependent cell migration as a central feature include inflammation, wound healing, oncogenesis, metastasis, myogenesis, and muscle regeneration. Cell surface plasmin also participates in neurite outgrowth and prohormone processing. Furthermore, plasmin-induced cell signaling also affects the functions of inflammatory cells, via production of cytokines, reactive oxygen species, and other mediators. Finally, plasminogen receptors regulate fibrinolysis. In this review, we highlight emerging data that shed light on longstanding controversies and raise new issues in the field. We focus on (1) the impact of the recent X-ray crystal structures of plasminogen and the development of antibodies that recognize cell-induced conformational changes in plasminogen on our understanding of the interaction of plasminogen with cells; (2) the relationship between apoptosis and plasminogen binding to cells; (3) the current status of our understanding of the molecular identity of plasminogen receptors and the discovery of a structurally unique novel plasminogen receptor, Plg-RKT; (4) the determinants of the interplay between distinct plasminogen receptors and cellular functions; and (5) new insights into the role of colocalization of plasminogen and plasminogen activator receptors on the cell surface.

On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines

Immunology often conveys the image of large molecules, either in the soluble state or in the membrane of leukocytes, forming multiple contacts with a target for actions of the immune system. Avidity names the ability of a polyvalent molecule to form multiple connections of the same kind with ligands tethered to the same surface. Polyvalent interactions are vastly stronger than their monovalent equivalent. In the present review, the functional consequences of polyvalent interactions are explored in a perspective of recent theoretical advances in understanding the thermodynamics of such binding. From insights on the structural biology of soluble pattern recognition molecules as well as adhesion molecules in the cell membranes or in their proteolytically shed form, this review documents the prominent role of polyvalent interactions in making the immune system a formidable barrier to microbial infection as well as constituting a significant challenge to the application of nanomedicines.

Potential role of kringle-integrin interaction in plasmin and uPA actions (a hypothesis)

We previously showed that the kringle domains of plasmin and angiostatin, the N-terminal four kringles (K1–4) of plasminogen, directly bind to integrins. Angiostatin blocks tumor-mediated angiogenesis and has great therapeutic potential. Angiostatin binding to integrins may be related to the antiinflammatory action of angiostatin. We reported that plasmin induces signals through protease-activated receptor (PAR-1), and plasmin-integrin interaction may be required for enhancing plasmin concentration on the cell surface, and enhances its signaling function. Angiostatin binding to integrin does not seem to induce proliferative signals. One possible mechanism of angiostatin's inhibitory action is that angiostatin suppresses plasmin-induced PAR-1 activation by competing with plasmin for binding to integrins. Interestingly, plasminogen did not interact with αvβ3, suggesting that the αvβ3-binding sites in the kringle domains of plasminogen are cryptic. The kringle domain of urokinase-type plasminogen activator (uPA) also binds to integrins. The uPA-integrin interaction enhances uPA concentrations on the cell surface and enhances plasminogen activation on the cell surface. It is likely that integrins bind to the kringle domain, and uPAR binds to the growth factor-like domain (GFD) of uPA simultaneously, making the uPAR-uPA-integrin ternary complex. We present a docking model of the ternary complex.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Fibrinogen counteracts the antiadhesive effect of fibrin-bound plasminogen by preventing its activation by adherent U937 monocytic cells

BACKGROUND

Fibrinogen and plasminogen strongly reduce adhesion of leukocytes and platelets to fibrin clots, highlighting a possible role for these plasma proteins in surface-mediated control of thrombus growth and stability. In particular, adsorption of fibrinogen on fibrin clots renders their surfaces non-adhesive, while the conversion of surface-bound plasminogen to plasmin by transiently adherent blood cells results in degradation of a superficial fibrin layer, leading to cell detachment. Although the mechanisms whereby these proteins exert their antiadhesive effects are different, the outcome is the same: the formation of a mechanically unstable surface that does not allow firm cell attachment.

OBJECTIVES

Since fibrin clots in circulation are exposed to both fibrinogen and plasminogen, their combined effect on adhesion of monocytic cells was examined.

METHODS

Fibrin gels were coated with plasminogen and its activation by adherent U937 monocytic cells in the presence of increasing concentrations of fibrinogen was examined by either measuring (125) I-labeled fibrin degradation products or plasmin amidolytic activity.

RESULTS

Unexpectedly, the antiadhesive effects of two fibrin binding proteins were not additive; in fact, in the presence of fibrinogen, the effect of plasminogen was strongly reduced. An investigation of the underlying mechanism revealed that fibrinogen prevented activation of fibrin-bound plasminogen by cells. Confocal microscopy showed that fibrinogen accumulates in a thin superficial layer of a clot, where it exerts its blocking effect on activation of plasminogen.

CONCLUSION

 The results point to a complex interplay between the fibrinogen- and plasminogen-dependent antiadhesive systems, which may contribute to the mechanisms that control the adhesiveness of a fibrin shell on the surface of hemostatic thrombi.

Fertilization outcome could be regulated by binding of oviductal plasminogen to oocytes and by releasing of plasminogen activators during interplay between gametes

OBJECTIVE

To detect plasminogen and plasminogen activators (PA) in oviduct and oocytes and to clarify the role of the plasminogen/plasmin system on mammalian fertilization.

DESIGN

Experimental prospective study.

SETTING

Mammalian reproduction research laboratory.

ANIMAL(S)

Oviducts and ovaries from porcine and bovine females were collected at slaughterhouse. A total of 52 oviducts and 2,292 oocytes were used. Boar and bull ejaculated spermatozoa were also used.

INTERVENTION(S)

Plasminogen concentration in oviductal fluid (OF) through the cycle was measured. Immunolocalization of plasminogen and PAs in oocytes was carried out before and after fertilization. Porcine and bovine oocytes were in vitro fertilized, with plasminogen and plasmin added to the culture medium at different concentrations.

MAIN OUTCOME MEASURE(S)

Plasminogen concentration in OF. Plasminogen and PAs immunolocalization in oocytes. Penetration and monospermy rates, number of spermatozoa in the ooplasma and on the zona pellucida (ZP) after IVF.

RESULT(S)

Oviductal fluid contains about 92 μg/mL of plasminogen. The mature oocyte shows immunoreactivity toward plasminogen and toward PAs on its oolemma and ZP. After fertilization, plasminogen and PAs immunolabeling decreases in the oocyte, suggesting its conversion into plasmin. When exogenous plasminogen is added to the IVF medium, sperm entry into the oocyte is hampered, suggesting that the role of plasminogen activation during fertilization is to reduce the number of (or to select) penetrating spermatozoa.

CONCLUSION(S)

The plasminogen/plasmin system is activated during gamete interaction and regulates the sperm entry into the oocyte.

Streptococcus pyogenes M49 plasminogen/plasmin binding facilitates keratinocyte invasion via integrin-integrin-linked kinase (ILK) pathways and protects from macrophage killing

The entry into epithelial cells and the prevention of primary immune responses are a prerequisite for a successful colonization and subsequent infection of the human host by Streptococcus pyogenes (group A streptococci, GAS). Here, we demonstrate that interaction of GAS with plasminogen promotes an integrin-mediated internalization of the bacteria into keratinocytes, which is independent from the serine protease activity of potentially generated plasmin. α(1)β(1)- and α(5)β(1)-integrins were identified as the major keratinocyte receptors involved in this process. Inhibition of integrin-linked kinase (ILK) expression by siRNA silencing or blocking of PI3K and Akt with specific inhibitors, reduced the GAS M49-plasminogen/plasmin-mediated invasion of keratinocytes. In addition, blocking of actin polymerization significantly reduced GAS internalization into keratinocytes. Altogether, these results provide a first model of plasminogen-mediated GAS invasion into keratinocytes. Furthermore, we demonstrate that plasminogen binding protects the bacteria against macrophage killing.

Bioengineered scaffolds for spinal cord repair

Spinal cord injury can lead to devastating and permanent loss of neurological function, affecting all levels below the site of trauma. Unfortunately, the injured adult mammalian spinal cord displays little regenerative capacity and little functional recovery in large part due to a tissue environment that is nonpermissive for regenerative axon growth. Artificial tissue repair scaffolds may provide a physical guide to allow regenerative axon growth that bridges the lesion cavity and restores functional neural connectivity. By integrating different strategies, including the use of various biomaterials and microstructures as well as incorporation of bioactive molecules and living cells, combined or synergistic effects for spinal cord repair through regenerative axon growth may be achieved. This article briefly reviews the development of bioengineered scaffolds for spinal cord repair, focusing on spinal cord injury and the subsequent cellular response, scaffold materials, fabrication techniques, and current therapeutic strategies. Key issues and challenges are also identified and discussed along with recommendations for future research.

New coumarin-based anti-inflammatory drug: putative antagonist of the integrins alphaLbeta2 and alphaMbeta2

This study was conducted to investigate putative antagonism of integrin receptors alphaMbeta2 and alphaLbeta2 by a novel coumarin derivative (BOL-303225-A), its efficacy in-vivo after retinal ischaemia-reperfusion injury, and its bioavailability in rat plasma. A cellular adhesion assay in Jurkat and U937 cells, and a flow cytometry assay with an antibody against the beta2 subunit were conducted. BOL-303225-A bioavailability in rat plasma and the retinal levels of myeloperoxidase (MPO) after ischaemia-reperfusion injury were evaluated after oral administration (10 mg kg(-1)). In-vitro cell viability assays revealed no cytotoxicity for BOL-303225-A over a wide dose range, and IC50 values of 32.3 +/- 1.5 muM and 84.95 +/- 2.3 muM were found for Jurkat and U937 cells, respectively. The drug showed specific binding to the alphaMbeta2 and alphaLbeta2 integrin receptors expressed by U937 and Jurkat cells, respectively, producing a fluorescence shift towards lower values in a concentration-dependent manner. The pharmacokinetic profile of BOL-303225-A exhibited rapid absorption following oral administration in the rat. A significant reduction of retinal MPO levels was observed in drug-treated rats. This study demonstrated that BOL-303225-A acts as an antagonist of the integrin alphaLbeta2 and alphaMbeta2 receptors, suggesting that this drug could be used for ocular diseases such as diabetic retinopathy.

Neutrophil apoptosis: selective regulation by different ligands of integrin alphaMbeta2

Neutrophils undergo spontaneous apoptosis, but their survival can be extended during inflammatory responses. alpha(M)beta(2) is reported either to delay or accelerate neutrophil apoptosis, but the mechanisms by which this integrin can support such diametrically opposed responses are poorly understood. The abilities of closely related alpha(M)beta(2) ligands, plasminogen and angiostatin, derived from plasminogen, as well as fibrinogen and its two derivative alpha(M)beta(2) recognition peptides, P1 and P2-C, differed markedly in their effects on neutrophil apoptosis. Plasminogen, fibrinogen, and P2-C suppressed apoptosis via activation of Akt and ERK1/2 kinases, while angiostatin and P1 failed to activate these prosurvival pathways and did not prevent neutrophil apoptosis. Using cells transfected with alpha(M)beta(2) or its individual alpha(M) or beta(2) subunits, and purified receptors and its constituent chains, we show that engagement of both subunits with prosurvival ligands is essential for induction of the prosurvival response. Hence, engagement of a single integrin by closely related ligands can induce distinct signaling pathways, which can elicit distinct cellular responses.

Antiadhesive effect of fibrinogen: a safeguard for thrombus stability

The recruitment of phagocytic leukocytes to sites of vessel wall injury plays an important role in thrombus dissolution by proteases elaborated on their adhesion. However, leukocyte adhesion to the fibrin clot can be detrimental at the early stages of wound healing when hemostatic plug integrity is critical for preventing blood loss. Adhesion of circulating leukocytes to the insoluble fibrin(ogen) matrix is mediated by integrins and occurs in the presence of a high concentration of plasma fibrinogen. In this study, the possibility that soluble fibrinogen could protect fibrin from excessive adhesion of leukocytes was examined. Fibrinogen was a potent inhibitor of adhesion of U937 monocytoid cells and neutrophils to fibrin gel and immobilized fibrin(ogen). An investigation of the mechanism by which soluble fibrinogen exerts its influence on leukocyte adhesion indicated that it did not block integrins but rather associated with the fibrin(ogen) substrate. Consequently, leukocytes that engage fibrinogen molecules loosely bound to the surface of fibrin(ogen) matrix are not able to consolidate their grip on the substrate; subsequently, cells detach. This conclusion is based on the evidence obtained in adhesion studies using various cells and performed under static and flow conditions. These findings reveal a new role of fibrinogen in integrin-mediated leukocyte adhesion and suggest that this mechanism may protect the thrombus from premature dissolution.

Lipoprotein(a) in atherosclerotic plaques recruits inflammatory cells through interaction with Mac‐1 integrin

Lipoprotein(a) [Lp(a)], consisting of LDL and the unique constituent apolipoprotein(a) [apo(a)], which contains multiple repeats resembling plasminogen kringle 4, is considered a risk factor for the development of atherosclerotic disorders. However, the underlying mechanisms for the atherogenicity of Lp(a) are not completely understood. Here, we define a novel function of Lp(a) in promoting inflammatory cell recruitment that may contribute to its atherogenicity. Through its apo(a) moiety Lp(a) specifically interacts with the beta2-integrin Mac-1, thereby promoting the adhesion of monocytes and their transendothelial migration in a Mac-1-dependent manner. Interestingly, the interaction between Mac-1 and Lp(a) was strengthened in the presence of proatherogenic homocysteine and was blocked by plasminogen/angiostatin kringle 4. Through its interaction with Mac-1, Lp(a) induced activation of the proinflammatory transcription factor NFkappaB, as well as the NFkappaB-related expression of prothrombotic tissue factor. In atherosclerotic coronary arteries Lp(a) was found to be localized in close proximity to Mac-1 on infiltrating mononuclear cells. Taken together, our data demonstrate that Lp(a), via its apo(a) moiety, is a ligand for the beta2-integrin Mac-1, thereby facilitating inflammatory cell recruitment to atherosclerotic plaques. These observations suggest a novel mechanism for the atherogenic properties of Lp(a).

Integrin alphaDbeta2, an adhesion receptor up-regulated on macrophage foam cells, exhibits multiligand-binding properties

Integrin alphaDbeta2, the most recently discovered member of the beta2 subfamily of integrin adhesion receptors, is up-regulated on macrophage foam cells. Although other members of the subfamily have been subjects of extensive research, the recognition specificity and the molecular basis for alphaDbeta2 ligand binding remain unknown. Based on the high extent of structural homology between alphaDbeta2 and the major myeloid-cell-specific integrin alphaMbeta2 (Mac-1), noted for its capacity to bind multiple ligands, we considered that the 2 integrins have similar recognition specificity. In this study, using recombinant and natural alphaDbeta2-expressing cells, we demonstrate that alphaDbeta2 supports adhesion and migration to many extracellular matrix proteins in a fashion similar to alphaMbeta2. Consistent with these data, the recombinant alphaDI-domain of the receptor bound selected ligands. The binding was activation-dependent because the alphaDI-domain with its C-terminal alpha7 helix truncated, but not the form with the C-terminal part extended, bound ligands. When the alphaDI-domain segment Lys244-Lys260 (highly homologous to its alphaMI-domain counterpart Lys245-Arg261 responsible for alphaMbeta2 multiligand-binding properties) was inserted into the mono-specific alphaLI-domain, the chimeric protein bound many ligands with affinities similar to those of wild-type alphaDI-domain. These results establish integrin alphaDbeta2 as a multiligand receptor and indicate that the mechanism whereby alphaDbeta2 exhibits broad ligand specificity resembles that used by alphaMbeta2, the most promiscuous member of the integrin family.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.