Novel interactions between urokinase and its receptor

Crystal structure and cellular functions of uPAR dimer

Receptor dimerization of urokinase-type plasminogen activator receptor (uPAR) was previously identified at protein level and on cell surface. Recently, a dimeric form of mouse uPAR isoform 2 was proposed to induce kidney disease. Here, we report the crystal structure of human uPAR dimer at 2.96 Å. The structure reveals enormous conformational changes of the dimer compared to the monomeric structure: D1 of uPAR opens up into a large expanded ring that captures a β-hairpin loop of a neighboring uPAR to form an expanded β-sheet, leading to an elongated, highly intertwined dimeric uPAR. Based on the structure, we identify E49P as a mutation promoting dimer formation. The mutation increases receptor binding to the amino terminal fragment of its primary ligand uPA, induces the receptor to distribute to the basal membrane, promotes cell proliferation, and alters cell morphology via β1 integrin signaling. These results reveal the structural basis for uPAR dimerization, its effect on cellular functions, and provide a basis to further study this multifunctional receptor.

Endogenous plasminogen activators mediate progressive intracerebral hemorrhage after traumatic brain injury in mice

Persistent intracerebral hemorrhage (ICH) is a major cause of death and disability after traumatic brain injury (TBI) for which no medical treatment is available. Delayed bleeding is often ascribed to consumptive coagulopathy initiated by exposed brain tissue factor. We examined an alternative hypothesis, namely, that marked release of tissue-type plasminogen activator (tPA) followed by delayed synthesis and release of urokinase plasminogen activator (uPA) from injured brain leads to posttraumatic bleeding by causing premature clot lysis. Using a murine model of severe TBI, we found that ICH is reduced in tPA(-/-) and uPA(-/-) mice but increased in PAI-1(-/-) mice compared with wild-type (WT) mice. tPA(-/-), but not uPA(-/-), mice developed a systemic coagulopathy post-TBI. Tranexamic acid inhibited ICH expansion in uPA(-/-)mice but not in tPA(-/-) mice. Catalytically inactive tPA-S(481)A inhibited plasminogen activation by tPA and uPA, attenuated ICH, lowered plasma d-dimers, lessened thrombocytopenia, and improved neurologic outcome in WT, tPA(-/-), and uPA(-/-) mice. ICH expansion was also inhibited by tPA-S(481)A in WT mice anticoagulated with warfarin. These data demonstrate that protracted endogenous fibrinolysis induced by TBI is primarily responsible for persistent ICH and post-TBI coagulopathy in this model and offer a novel approach to interrupt bleeding.

Ligand binding alters dimerization and sequestering of urokinase receptors in raft-mimicking lipid mixtures

Lipid heterogeneities, such as lipid rafts, are widely considered to be important for the sequestering of membrane proteins in plasma membranes, thereby influencing membrane protein functionality. However, the underlying mechanisms of such sequestration processes remain elusive, in part, due to the small size and often transient nature of these functional membrane heterogeneities in cellular membranes. To overcome these challenges, here we report the sequestration behavior of urokinase receptor (uPAR), a glycosylphosphatidylinositol-anchored protein, in a planar model membrane platform with raft-mimicking lipid mixtures of well-defined compositions using a powerful optical imaging platform consisting of confocal spectroscopy XY-scans, photon counting histogram, and fluorescence correlation spectroscopy analyses. This methodology provides parallel information about receptor sequestration, oligomerization state, and lateral mobility with single molecule sensitivity. Most notably, our experiments demonstrate that moderate changes in uPAR sequestration are not only associated with modifications in uPAR dimerization levels, but may also be linked to ligand-mediated allosteric changes of these membrane receptors. Our data show that these modifications in uPAR sequestration can be induced by exposure to specific ligands (urokinase plasminogen activator, vitronectin), but not via adjustment of the cholesterol level in the planar model membrane system. Good agreement of our key findings with published results on cell membranes confirms the validity of our model membrane approach. We hypothesize that the observed mechanism of receptor translocation in the presence of raft-mimicking lipid mixtures is also applicable to other glycosylphosphatidylinositol-anchored proteins.

Number and brightness image analysis reveals ATF-induced dimerization kinetics of uPAR in the cell membrane

We studied the molecular forms of the GPI-anchored urokinase plasminogen activator receptor (uPAR-mEGFP) in the human embryo kidney (HEK293) cell membrane and demonstrated that the binding of the amino-terminal fragment (ATF) of urokinase plasminogen activator is sufficient to induce the dimerization of the receptor. We followed the association kinetics and determined precisely the dimeric stoichiometry of uPAR-mEGFP complexes by applying number and brightness (N&B) image analysis. N&B is a novel fluctuation-based approach for measuring the molecular brightness of fluorophores in an image time sequence in live cells. Because N&B is very sensitive to long-term temporal fluctuations and photobleaching, we have introduced a filtering protocol that corrects for these important sources of error. Critical experimental parameters in N&B analysis are illustrated and analyzed by simulation studies. Control experiments are based on mEGFP-GPI, mEGFP-mEGFP-GPI, and mCherry-GPI, expressed in HEK293. This work provides a first direct demonstration of the dimerization of uPAR in live cells. We also provide the first methodological guide on N&B to discern minor changes in molecular composition such as those due to dimerization events, which are involved in fundamental cell signaling mechanisms.

Screening small-molecule compound microarrays for protein ligands without fluorescence labeling with a high-throughput scanning microscope

We describe a high-throughput scanning optical microscope for detecting small-molecule compound microarrays on functionalized glass slides. It is based on measurements of oblique-incidence reflectivity difference and employs a combination of a y-scan galvometer mirror and an x-scan translation stage with an effective field of view of 2 cm x 4 cm. Such a field of view can accommodate a printed small-molecule compound microarray with as many as 10,000 to 20,000 targets. The scanning microscope is capable of measuring kinetics as well as endpoints of protein-ligand reactions simultaneously. We present the experimental results on solution-phase protein reactions with small-molecule compound microarrays synthesized from one-bead, one-compound combinatorial chemistry and immobilized on a streptavidin-functionalized glass slide.

A novel high-throughput scanning microscope for label-free detection of protein and small-molecule chemical microarrays

We describe a novel scanning optical microscope based on a polarization-modulated nulling ellipsometry. The new microscope employs a combination of scanning mirror and sample translation and thus enables high-throughput label-free detection of biomolecular microarrays with more than 10 000 protein or small-molecule targets. For illustration, we show the image of a 2760-spot protein microarray on a functionalized glass slide obtained with such a microscope. The new scanning microscope is also capable of determining, in parallel, the real-time binding kinetics of multiple molecular species under aqueous conditions.

One-step affinity purification of recombinant urokinase-type plasminogen activator receptor using a synthetic peptide developed by combinatorial chemistry

Several lines of evidence have pointed to a role of urokinase-type plasminogen activator receptor (uPAR) as a modulator of certain biochemical processes that are active during tumor invasion and metastasis. Consequently, the structure and function of this receptor have been studied extensively, using recombinantly produced uPAR that has been purified by either affinity chromatography using its cognate ligand, the urokinase-type plasminogen activator (uPA), or a monoclonal anti-uPAR antibody (R2), or by hydroxyapatite. Here, we present a new method for the efficient one-step affinity purification of recombinant uPAR exploiting a high-affinity synthetic peptide antagonist (AE152). The corresponding parent peptide was originally identified in a random phage-display library and subsequently subjected to affinity maturation by combinatorial chemistry. This study compares the affinity purification of a soluble, recombinant uPAR using the monoclonal antibody R2 or the peptide AE152 immobilized on Sepharose. The two affinity ligands perform equally well in purifying uPAR from Drosophila melanogaster Schneider 2 cell culture medium and yield products of comparable purity, activity, and stability as judged by SDS-PAGE, size exclusion chromatography and surface plasmon resonance analysis. The general availability of peptide synthesis renders the present AE152-based affinity purification of uPAR more accessible than the traditional protein-based affinity purification strategies. In this way, large amounts of recombinant uPAR can conveniently be purified for further structural and functional studies.

Looking towards label-free biomolecular interaction analysis in a high-throughput format: a review of new surface plasmon resonance technologies

Surface plasmon resonance (SPR) biosensors have enabled a wide range of applications in which researchers can monitor biomolecular interactions in real time. Owing to the fact that SPR can provide affinity and kinetic data, unique features in applications ranging from protein-peptide interaction analysis to cellular ligation experiments have been demonstrated. Although SPR has historically been limited by its throughput, new methods are emerging that allow for the simultaneous analysis of many thousands of interactions. When coupled with new protein array technologies, high-throughput SPR methods give users new and improved methods to analyze pathways, screen drug candidates and monitor protein-protein interactions.

The kringle domain of urokinase-type plasminogen activator potentiates LPS-induced neutrophil activation through interaction with {alpha}V{beta}3 integrins

Urokinase plasminogen activator (uPA) is a serine protease that catalyzes the conversion of plasminogen to plasmin. In addition, uPA has been shown to have proinflammatory properties, particularly in potentiating lipopolysaccharide (LPS)-induced neutrophil responses. To explore the mechanisms by which uPA exerts these effects, we examined the ability of specific uPA domains to increase cytokine expression in murine and human neutrophils stimulated with LPS. Whereas the addition of intact uPA to neutrophils cultured with LPS increased mRNA and protein levels of interleukin-1beta, macrophage-inflammatory protein-2, and tumor necrosis factor alpha, deletion of the kringle domain (KD) from uPA resulted in loss of these potentiating effects. Addition of purified uPA KD to LPS-stimulated neutrophils increased cytokine expression to a degree comparable with that produced by single-chain uPA. Inclusion of the arginine-glycine-aspartic but not the arginine-glycine-glutamic peptide to neutrophil cultures blocked uPA kringle-induced potentiation of proinflammatory responses, demonstrating that interactions between the KD and integrins were involved. Antibodies to alpha(V) or beta(3) integrins or to the combination of alpha(V)beta(3) prevented uPA kringle-induced enhancement of expression of proinflammatory cytokines and also of adhesion of neutrophils to the uPA KD. These results demonstrate that the KD of uPA, through interaction with alpha(V)beta(3) integrins, potentiates neutrophil activation.

Ly6 family member C4.4A binds laminins 1 and 5, associates with galectin-3 and supports cell migration

C4.4A is a member of the Ly6 family, with low homology to uPAR. It has been detected mainly on metastasizing carcinoma cells and proposed to be involved in wound healing. So far, C4.4A has been observed as an orphan receptor, and its functional activity has not been explored. Using recombinant rat C4.4A (rrC4.4A) made in a eukaryotic expression system, we demonstrate by immunohistology that C4.4A ligands are strongly expressed in tissues adjacent to squamous epithelia of, e.g., tongue and esophagus, the expression pattern partly overlapping with laminin (LN) and complementing the C4.4A expression that is found predominantly on the basal layers of squamous epithelium. ELISA screening of several components of the extracellular matrix revealed selective binding of rrC4.4A to LN1 and LN5 and that transfection of the BSp73AS tumor line with C4.4A cDNA (BSp73AS-1B1) promoted LN1 and LN5 binding. Binding of BSp73AS-1B1 to LN5 and, less markedly, LN1 induced spreading, lamellipodia formation and migration. C4.4A also associates with galectin-3 in nontransformed tissues and tumor lines. There is evidence that the association of C4.4A with galectin-3 influences LN adhesion. C4.4A was described originally as a metastasis-associated molecule. Our findings that LN1 and LN5 are C4.4A ligands, that galectin-3 associates with C4.4A and that C4.4A ligand binding confers a migratory phenotype are well in line with the supposed metastasis association.

Regulation of urokinase receptor proteolytic function by the tetraspanin CD82

The high affinity interaction between the urokinase-type plasminogen activator (uPA) and its glycolipid-anchored cellular receptor (uPAR) promotes plasminogen activation and the efficient generation of pericellular proteolytic activity. We demonstrate here that expression of the tetraspanin CD82/KAI1 (a tumor metastasis suppressor) leads to a profound effect on uPAR function. Pericellular plasminogen activation was reduced by approximately 50-fold in the presence of CD82, although levels of components of the plasminogen activation system were unchanged. uPAR was present on the cell surface and molecularly intact, but radioligand binding analysis with uPA and anti-uPAR antibodies revealed that it was in a previously undetected cryptic form unable to bind uPA. This was not due to direct interactions between uPAR and CD82, as they neither co-localized on the cell surface nor could be co-immunoprecipitated. However, expression of CD82 led to a redistribution of uPAR to focal adhesions, where it was shown by double immunofluorescence labeling to co-localize with the integrin alpha(5)beta(1), which was also redistributed in the presence of CD82. Co-immunoprecipitation experiments showed that, in the presence of CD82, uPAR preferentially formed stable associations with alpha(5)beta(1), but not with a variety of other integrins, including alpha(3)beta(1). These data suggest that CD82 inhibits the proteolytic function of uPAR indirectly, directing uPAR and alpha(5)beta(1) to focal adhesions and promoting their association with a resultant loss of uPA binding. This represents a novel mechanism whereby tetraspanins, integrins, and uPAR, systems involved in cell adhesion and migration, cooperate to regulate pericellular proteolytic activity and may suggest a mechanism for the tumor-suppressive effects of CD82/KAI1.

The urokinase receptor (uPAR) and the uPAR-associated protein (uPARAP/Endo180): membrane proteins engaged in matrix turnover during tissue remodeling

The breakdown of the barriers formed by extracellular matrix proteins is a pre-requisite for all processes of tissue remodeling. Matrix degradation reactions take part in specific physiological events in the healthy organism but also represent a crucial step in cancer invasion. These degradation processes involve a highly organized interplay between proteases and their cellular binding sites as well as specific substrates and internalization receptors. This review article is focused on two components, the urokinase plasminogen activator receptor (uPAR) and the uPAR-associated protein (uPARAP, also designated Endo180), that are considered crucially engaged in matrix degradation. uPAR and uPARAP have highly diverse functions, but on certain cell types they interact with each other in a process that is still incompletely understood. uPAR is a glycosyl-phosphatidylinositol-anchored glycoprotein on the surface of various cell types that serves to bind the urokinase plasminogen activator and localize the activation reactions in the proteolytic cascade system of plasminogen activation. uPARAP is an integral membrane protein with a pronounced role in the internalization of collagen for intracellular degradation. Both receptors have additional functions that are currently being unraveled. The present discussion of uPAR and uPARAP is centered on their protein structure and molecular and cellular function.

The kringle stabilizes urokinase binding to the urokinase receptor

The structural basis of the interaction between single-chain urokinase-type plasminogen activator (scuPA) and its receptor (uPAR) is incompletely defined. Several observations indicated the kringle facilitates the binding of uPA to uPAR. A scuPA variant lacking the kringle (Delta K-scuPA) bound to soluble uPAR (suPAR) with the similar "on-rate" but with a faster "off-rate" than wild-type (WT)-scuPA. Binding of Delta K-scuPA, but not WT-scuPA, to suPAR was comparably inhibited by its growth factor domain (GFD) and amino-terminal fragment (ATF). ATF and WT-scuPA, but not GFD, scuPA lacking the GFD (Delta GFD-scuPA), or Delta K-scuPA reconstituted the isolated domains of uPAR. ATF completely inhibited the enzymatic activity of WT-scuPA-suPAR unlike comparable concentrations of GFD. Variants containing mutations that alter the charge, length, or flexibility of linker sequence (residues 43-49) between the GFD and the kringle displayed a lower affinity for uPAR, were unable to reconstitute uPAR domains, and their binding to uPAR was inhibited by GFD in the same manner as Delta K-scuPA. A scuPA variant in which the charged amino acids in the heparin binding site (HBS) in the kringle domain were mutated to alanines behaved like Delta K-scuPA, indicating that that the structure of the kringle as well as its interaction with the GFD govern receptor binding. These data demonstrate an important role for the kringle in stabilizing the binding of scuPA to uPAR.

Critical role of integrin alpha 5 beta 1 in urokinase (uPA)/urokinase receptor (uPAR, CD87) signaling

Urokinase-type plasminogen activator (uPA) induces cell adhesion and chemotactic movement. uPA signaling requires its binding to uPA receptor (uPAR/CD87), but how glycosylphosphatidylinositol-anchored uPAR mediates signaling is unclear. uPAR is a ligand for several integrins (e.g. alpha 5 beta 1) and supports cell-cell interaction by binding to integrins on apposing cells (in trans). We studied whether binding of uPAR to alpha 5 beta 1 in cis is involved in adhesion and migration of Chinese hamster ovary cells in response to immobilized uPA. This process was temperature-sensitive and required mitogen-activated protein kinase activation. Anti-uPAR antibody or depletion of uPAR blocked, whereas overexpression of uPAR enhanced, cell adhesion to uPA. Adhesion to uPA was also blocked by deletion of the growth factor domain (GFD) of uPA and by anti-GFD antibody, whereas neither the isolated uPA kringle nor serine protease domain supported adhesion directly. Interestingly, anti-alpha 5 antibody, RGD peptide, and function-blocking mutations in alpha 5 beta 1 blocked adhesion to uPA. uPA-induced cell migration also required GFD, uPAR, and alpha 5 beta 1, but alpha 5 beta 1 alone did not support uPA-induced adhesion and migration. Thus, binding of uPA causes uPAR to act as a ligand for alpha 5 beta 1 to induce cell adhesion, intracellular signaling, and cell migration. We demonstrated that uPA induced RGD-dependent binding of uPAR to alpha 5 beta 1 in solution. These results suggest that uPA-induced adhesion and migration of Chinese hamster ovary cells occurs as a consequence of (a) uPA binding to uPAR through GFD, (b) the subsequent binding of a uPA.uPAR complex to alpha 5 beta 1 via uPAR, and (c) signal transduction through alpha 5 beta 1.

Noncatalytic domain of uPA stimulates human extravillous trophoblast migration by using phospholipase C, phosphatidylinositol 3-kinase and mitogen-activated protein kinase

The serine protease urokinase-type plasminogen activator (uPA) promotes matrix degradation by many cell types, including the invasive extravillous trophoblast (EVT) of the human placenta. The noncatalytic amino-terminal end of uPA binds to uPA receptors (uPARs) expressed by these cells. A highly polarized expression of uPAR-bound uPA at the migration front of EVT cells in situ suggests a functional role of uPA:uPAR interaction in EVT cell migration. The present study examined whether uPA stimulates EVT cell migration, independent of proteolytic function, and investigated some of the signaling pathways involved. Using in vitro-propagated EVT cells in Transwell migration assays, both uPA and its noncatalytic amino-terminal fragment (ATF) were shown to stimulate migration through multiporous polycarbonate (pore size 8 microm) membranes. A uPAR-blocking antibody inhibited basal and ATF-stimulated migration. Migration was found to be stimulated by hypoxic conditions, which upregulates uPAR expression; this stimulation was abrogated with the uPAR-blocking antibody, indicating the role of endogenous uPA in EVT cell migration. Spectrofluorometric measurement of cytosolic calcium in cells treated with uPA and ATF demonstrated a rapid rise in [Ca2+](i), which was prevented by pretreatment of cells with thapsigargin, indicating a release from intracellular stores. Both basal and ATF-mediated migratory responses were suppressed in the presence of selective pharmacological inhibitors LY294002, U73122, and U0126, implicating the respective roles of phosphatidinylinositol 3-kinase (PI 3-K), phospholipase C (PLC), and MEK1/2 in basal and ATF-stimulated migratory capacity. Taken together, these results demonstrate that uPA:uPAR interaction stimulates EVT cell migration, independent of uPA enzymatic activity, using the mitogen-activated protein kinase pathway and calcium signaling events including the participation of PI 3-K and PLC. These findings are relevant to clinical conditions of aberrant trophoblast migration, including spontaneous abortion, preeclampsia, and choriocarcinoma.

Anti-angiogenic activity of the recombinant kringle domain of urokinase and its specific entry into endothelial cells

Urokinase plasminogen activator (uPA) belongs to a family of proteins that contains kringle domain and plays an important role in inflammation, tissue remodeling, angiogenesis, and tumor metastasis by pericellular plasminogen activation. Kringle domains of plasminogen have been shown to demonstrate anti-angiogenic and anti-tumor activities. Here, we report our investigation of the kringle domain of uPA for anti-angiogenic activity and a possible cellular mechanism of action. The recombinant kringle domain of uPA (Asp(45)-Lys(135)) (UK1) inhibited endothelial cell proliferation stimulated by basic fibroblast growth factor, vascular endothelial growth factor (VEGF), or epidermal growth factor. It also inhibited migration of endothelial cells induced by VEGF or uPA, and in vivo angiogenesis on the chick chorioallantoic membrane. It did not block plasminogen activation by activated uPA in clot lysis and chromogenic substrate assays. Neither binding of UK1 to immobilized uPA receptor nor competitive inhibition of uPA binding were confirmed by real-time interaction analysis. However, internalization of UK1 followed by translocation from cytosol to nucleus was determined to be specific to endothelial cells. It also elicited a transient increase of Ca(2+) flux of more than 2-fold within 2 min of exposure in an endothelial cell-specific manner. These results suggest that the kringle domain of uPA exhibits anti-angiogenic activity and that its anti-angiogenic activity may occur through a different mechanism from inhibition of uPA-uPA receptor interaction or uPA proteolytic activity and may be associated with endothelial-cell specific internalization not mediated by the uPA receptor.

Convergence of the adhesive and fibrinolytic systems: recognition of urokinase by integrin alpha Mbeta 2 as well as by the urokinase receptor regulates cell adhesion and migration

Previous studies demonstrated that integrin alpha(M)beta(2) (CD11b/18, Mac-1) forms a physical complex with the urokinase-type plasminogen activator receptor (uPAR/CD87) on leukocytes. In this study, we used human peripheral blood neutrophils and transfected cells expressing alpha(M)beta(2), uPAR, or both receptors to show that the integrin can directly interact with urokinase (uPA). We demonstrate that alpha(M)beta(2) supported adhesion and migration of these cells to uPA, and, in each case, blockade of alpha(M)beta(2) suppressed the response. Within uPA, both the kringle and proteolytic domains are recognized by alpha(M)beta(2), which are distinct from the growth factor domain that binds to uPAR. Within the alpha(M) subunit of the integrin, the I domain interacts with uPA, which is distinct from the region that interacts with uPAR. On cells expressing uPAR and alpha(M)beta(2), both receptors mediated adhesion and migration. This cooperation was particularly apparent in the responses of neutrophils to uPA, where blockade of alpha(M)beta(2) reduced uPAR-mediated responses and engagement of uPAR enhanced recognition of uPA by alpha(M)beta(2). Thus, recognition of uPA by alpha(M)beta(2) allows for formation of a multicontact trimolecular complex, in which a single uPA ligand may bind simultaneously to both uPAR and alpha(M)beta(2). This complex may play an important role in the control of inflammatory cell migration and vascular homeostasis.

Biospecific interaction analysis: a tool for drug discovery and development

The recent development of surface plasmon resonance (SPR)-based biosensor technologies for biospecific interaction analysis (BIA) enables the monitoring of a variety of molecular reactions in real-time. The biomolecular interactions occur at the surface of a flow cell of a sensor chip between a ligand immobilized on the surface and an injected analyte. SPR-based BIA offers many advantages over most of the other methodologies available for the study of biomolecular interactions, including full automation, no requirement for labeling, and the availability of a large variety of activated sensor chips that allow immobilization of DNA, RNA, proteins, peptides and cells. The assay is rapid and requires only small quantitities of both ligand and analyte in order to obtain informative results. In addition, the sensor chip can be re-used many times, leading to low running costs. Aside from the analysis of all possible combinations of peptide, protein, DNA and RNA interactions, this technology can also be used for screening of monoclonal antibodies and epitope mapping, analysis of interactions between low molecular weight compounds and proteins or nucleic acids, interactions between cells and ligands, and real-time monitoring of gene expression. Applications of SPR-based BIA in medicine include the molecular diagnosis of viral infections and genetic diseases caused by point mutations. Future perspectives include the combinations of SPR-based BIA with mass spectrometry, the use of biosensors in proteomics, and the application of this technology to design and develop efficient drug delivery systems.

Urokinase regulates vitronectin binding by controlling urokinase receptor oligomerization

Adhesion of monocytes to the extracellular matrix is mediated by a direct high affinity interaction between cell-surface urokinase-type plasminogen activator (uPA) receptor (uPAR) and the extracellular matrix protein vitronectin. We demonstrate a tight connection between uPA-regulated uPAR oligomerization and high affinity binding to immobilized vitronectin. We find that binding of soluble uPAR (suPAR) to immobilized vitronectin is strictly ligand-dependent with a linear relationship between the observed binding and the concentration of ligand added. Nevertheless, a comparison of experimentally obtained binding curves to those generated using a simple equilibrium model suggests that the high affinity vitronectin-binding pro-uPA.suPAR complex contains two molecules of suPAR. In co-immunoprecipitation experiments, using different epitope-tagged suPAR molecules, suPAR/suPAR co-immunoprecipitation displayed a similar uPA dose dependence as that observed for vitronectin binding, demonstrating that the high affinity vitronectin-binding complex indeed contains oligomeric suPAR. Structurally, the kringle domain of uPA was found to be critical for the formation of the vitronectin-binding competent complex because the amino-terminal fragment, but not the growth factor-like domain, behaved as a full-length uPA. Our data represent the first demonstration of functional, ligand-induced uPAR oligomerization having extensive implications for glycosylphosphatidylinositol-anchored receptors in general, and for the biology of the uPA/uPAR system in particular.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

Ligand binding regions in the receptor for urokinase-type plasminogen activator

The interaction between urokinase plasminogen activator (uPA) and its cellular receptor (uPAR) is a key event in cell surface-associated plasminogen activation, relevant for cell migration and invasion. In order to define receptor recognition sites for uPA, we have expressed uPAR fragments as fusion products with the minor coat protein on the surface of M13 bacteriophages. Sequence analysis of cDNA fragments encoding uPA-binding peptides indicated the existence of a composite uPA-binding structure including all three uPAR domains. This finding was confirmed by experiments using an overlapping 15-mer peptide array covering the entire uPAR molecule. Four regions within the uPAR sequence were found to directly bind to uPA: two distinct regions containing amino acids 13--20 and amino acids 74--84 of the uPAR domain I, and regions in the putative loop 3 of the domains II and III. All the uPA-binding fragments from the three domains were shown to have an agonistic effect on uPA binding to immobilized uPAR. Furthermore, uPAR-(154--176) increased uPAR-transfected BAF3-cell adhesion on vitronectin in the presence of uPA, whereas uPAR-(247--276) stimulated the cell adhesion both in the absence or presence of uPA. The latter fragment was also able to augment the binding of vitronectin to uPAR in a purified system, thereby mimicking the effect of uPA on this interaction. These results indicate that uPA binding can take place to particular part(s) on several uPAR molecules and that direct uPAR-uPAR contacts may contribute to receptor activation and ligand binding.

Cutting edge: evidence for a signaling partnership between urokinase receptors (CD87) and L-selectin (CD62L) in human polymorphonuclear neutrophils

Leukocyte urokinase plasminogen activator receptors (uPARs) cluster at adhesion interfaces and at migratory fronts where they participate in adhesion, chemotaxis, and proteolysis. uPAR aggregation triggers activation signaling even though this glycolipid-anchored protein must associate with membrane-spanning proteins to access the cell interior. This study demonstrates a novel partnership between uPAR and L-selectin in human polymorphonuclear neutrophils. Fluorescence resonance energy transfer demonstrated a direct physical association between uPAR and L-selectin. To examine the role of L-selectin in uPAR-mediated signaling, uPAR was cross-linked and intracellular Ca(2+) concentrations were measured by spectrofluorometry. A mAb reactive against the carbohydrate binding domain (CBD) of L-selectin substantially inhibited uPAR-mediated Ca(2+) mobilization, whereas mAbs against the beta(2) integrin complement receptor 3 (CR3), another uPAR-binding adhesion protein, had no effect. Similarly, fucoidan, a sulfated polysaccharide that binds to L-selectin CBD, inhibited the Ca(2+) signal. We conclude that uPAR associates with the CBD region of L-selectin to form a functional signaling complex.

Urokinase receptor: a molecular organizer in cellular communication

In a variety of cell types, the glycolipid-anchored urokinase receptor (uPAR) is colocalized pericellularly with components of the plasminogen activation system and endocytosis receptors. uPAR is also coexpressed with caveolin and members of the integrin adhesion receptor superfamily. The formation of functional units with these various proteins allows the uPAR to mediate the focused proteolysis required for cell migration and invasion and to contribute both directly and indirectly to cell adhesive processes in a non-proteolytic fashion. This dual activity, together with the initiation of signal transduction pathways by uPAR, is believed to influence cellular behaviour in angiogenesis, inflammation, wound repair and tumor progression/metastasis and open up the way for uPAR-based therapeutic approaches.