Factor X bound to the surface of activated human platelets is preferentially activated by platelet-bound factor IXa

Platelets: Physiology and Biochemistry

This article represents a republication of an article originally published in STH in 2005. This republication is to help celebrate 50 years of publishing for STH. The original abstract follows.Platelets are specialized blood cells that play central roles in physiologic and pathologic processes of hemostasis, inflammation, tumor metastasis, wound healing, and host defense. Activation of platelets is crucial for platelet function that includes a complex interplay of adhesion and signaling molecules. This article gives an overview of the activation processes involved in primary and secondary hemostasis, for example, platelet adhesion, platelet secretion, platelet aggregation, microvesicle formation, and clot retraction/stabilization. In addition, activated platelets are predominantly involved in cross-talk to other blood and vascular cells. Stimulated "sticky" platelets enable recruitment of leukocytes at sites of vascular injury under high shear conditions. Platelet-derived microparticles as well as soluble adhesion molecules, sP-selectin and sCD40L, shed from the surface of activated platelets, are capable of activating, in turn, leukocytes and endothelial cells. This article focuses further on the new view of receptor-mediated thrombin generation of human platelets, necessary for the formation of a stable platelet-fibrin clot during secondary hemostasis. Finally, special emphasis is placed on important stimulatory and inhibitory signaling pathways that modulate platelet function.

Thrombin activity propagates in space during blood coagulation as an excitation wave

Injury-induced bleeding is stopped by a hemostatic plug formation that is controlled by a complex nonlinear and spatially heterogeneous biochemical network of proteolytic enzymes called blood coagulation. We studied spatial dynamics of thrombin, the central enzyme of this network, by developing a fluorogenic substrate-based method for time- and space-resolved imaging of thrombin enzymatic activity. Clotting stimulation by immobilized tissue factor induced localized thrombin activity impulse that propagated in space and possessed all characteristic traits of a traveling excitation wave: constant spatial velocity, constant amplitude, and insensitivity to the initial stimulation once it exceeded activation threshold. The parameters of this traveling wave were controlled by the availability of phospholipids or platelets, and the wave did not form in plasmas from hemophilia A or C patients who lack factors VIII and XI, which are mediators of the two principal positive feedbacks of coagulation. Stimulation of the negative feedback of the protein C pathway with thrombomodulin produced nonstationary patterns of wave formation followed by deceleration and annihilation. This indicates that blood can function as an excitable medium that conducts traveling waves of coagulation.

Systems biology of coagulation initiation: kinetics of thrombin generation in resting and activated human blood

Blood function defines bleeding and clotting risks and dictates approaches for clinical intervention. Independent of adding exogenous tissue factor (TF), human blood treated in vitro with corn trypsin inhibitor (CTI, to block Factor XIIa) will generate thrombin after an initiation time (T_i) of 1 to 2 hours (depending on donor), while activation of platelets with the GPVI-activator convulxin reduces T_i to ∼20 minutes. Since current kinetic models fail to generate thrombin in the absence of added TF, we implemented a Platelet-Plasma ODE model accounting for: the Hockin-Mann protease reaction network, thrombin-dependent display of platelet phosphatidylserine, VIIa function on activated platelets, XIIa and XIa generation and function, competitive thrombin substrates (fluorogenic detector and fibrinogen), and thrombin consumption during fibrin polymerization. The kinetic model consisting of 76 ordinary differential equations (76 species, 57 reactions, 105 kinetic parameters) predicted the clotting of resting and convulxin-activated human blood as well as predicted T_i of human blood under 50 different initial conditions that titrated increasing levels of TF, Xa, Va, XIa, IXa, and VIIa. Experiments with combined anti-XI and anti-XII antibodies prevented thrombin production, demonstrating that a leak of XIIa past saturating amounts of CTI (and not “blood-borne TF” alone) was responsible for in vitro initiation without added TF. Clotting was not blocked by antibodies used individually against TF, VII/VIIa, P-selectin, GPIb, protein disulfide isomerase, cathepsin G, nor blocked by the ribosome inhibitor puromycin, the Clk1 kinase inhibitor Tg003, or inhibited VIIa (VIIai). This is the first model to predict the observed behavior of CTI-treated human blood, either resting or stimulated with platelet activators. CTI-treated human blood will clot in vitro due to the combined activity of XIIa and XIa, a process enhanced by platelet activators and which proceeds in the absence of any evidence for kinetically significant blood borne tissue factor.

Clotting of blood involves a series of reactions wherein at each step an inactive zymogen is converted to an active enzyme by the product of the previous step, sometimes in plasma and usually on efficient catalytic surfaces provided by the activating platelet. The protein Tissue Factor (TF) initiates this cascade when blood vessels are disrupted, but how this cascade is triggered in the absence of exogenous TF remains the subject of much debate. First, we validated a high throughput experimental system that allowed the noninvasive quantification of thrombin generation dynamics. Next, we showed that “contact activation,” despite use of the best available inhibitor (CTI) to prevent it, builds up enough autocatalytic strength to trigger coagulation without exogenous TF, particularly upon activated platelets. Further, we build an ODE based model to predict the stability of blood resulting from multiple perturbations with active enzymes at various physiologically realizable concentrations. Unlike existing models, we consider the dynamics of platelet activation on reaction rates due to phosphatiylserine exposure. The “Platelet-Plasma” model lays the groundwork for integration of coagulation reaction kinetics and donor specific descriptions of platelet function.

Antidote-controlled antithrombotic therapy targeting factor IXa and von Willebrand factor

Thrombotic disorders and their common clinical phenotypes of acute myocardial infarction, ischemic stroke, and venous thromboembolism are the proximate cause of substantial morbidity, mortality, and health care expenditures worldwide. Accordingly, therapies designed to attenuate thrombus initiation and propagation, reflecting integrated platelet-mediated and coagulation protease-mediated events, respectively, represent a standard of care. Unfortunately, there are numerous inherent limitations of existing therapies that include target nonselectivity, variable onset and offset of pharmacodynamic effects, a narrow efficacy-safety profile, and the absence of a safe and reliable platform for either accurate titration, based on existing patient-specific, disease-specific, and clinical conditions, or active reversibility. Herein, we summarize our experience with oligonucleotide antithrombotic agents and their complementary antidotes, targeting the platelet adhesive protein von Willebrand factor and the pivotal coagulation protease factor IXa.

PAR-1-stimulated factor IXa binding to a small platelet subpopulation requires a pronounced and sustained increase of cytoplasmic calcium

We previously reported that only a subpopulation of PAR-1-stimulated platelets binds coagulation factor IXa, since confirmed by other laboratories. Since calcium changes have been implicated in exposure of procoagulant aminophospholipids, we have now examined calcium fluxes in this subpopulation by measuring fluorescence changes in Fura Red/AM-loaded platelets following PAR-1 stimulation. While fluorescence changes in all platelets indicated calcium release from internal stores and influx of external calcium, a subpopulation of platelets displayed a pronounced increase in calcium transients by 15 s and positive factor IXa binding by 2 min, with calcium transients sustained for 45 min. Pretreatment of platelets with Xestospongin C to inhibit IP3-mediated dense tubule calcium release, and the presence of impermeable calcium channel blockers nifedipine, SKF96365, or LaCl3, inhibited PAR-1-induced development of a subpopulation with pronounced calcium transients, factor IXa binding, and platelet support of FXa generation, suggesting the importance of both release of calcium from internal stores and influx of extracellular calcium. When platelets were stimulated in EDTA for 5-20 min before addition of calcium, factor IXa binding sites developed on a smaller subpopulation but with unchanged rate, indicating sustained opening of calcium channels and continued availability of signaling elements required for binding site exposure. While pretreatment of platelets with 100 microM BAPTA/AM (Kd 160 nM) had minimal effects, 100 microM 5,5'-dimethylBAPTA/AM (Kd 40 nM) completely inhibited the appearance and function of the platelet subpopulation, indicating the importance of minor increases of cytoplasmic calcium. We conclude that PAR-1-stimulated development of factor IXa binding sites in a subpopulation of platelets is dependent upon release of calcium from internal stores leading to sustained and pronounced calcium transients.

Factor VIIIa regulates substrate delivery to the intrinsic factor X-activating complex

Activation of coagulation factor X (fX) by activated factors IX (fIXa) and VIII (fVIIIa) requires the assembly of the enzyme-cofactor-substrate fIXa-fVIIIa-fX complex on negatively charged phospholipid membranes. Using flow cytometry, we explored formation of the intermediate membrane-bound binary complexes of fIXa, fVIIIa, and fX. Studies of the coordinate binding of coagulation factors to 0.8-microm phospholipid vesicles (25/75 phosphatidylserine/phosphatidylcholine) showed that fVIII (fVIIIa), fIXa, and fX bind to 32 700 +/- 5000 (33 200 +/- 14 100), 20 000 +/- 4500, and 30 500 +/- 1300 binding sites per vesicle with apparent K(d) values of 76 +/- 23 (71 +/- 5), 1510 +/- 430, and 223 +/- 79 nm, respectively. FVIII at 10 nm induced the appearance of additional high-affinity sites for fIXa (1810 +/- 370, 20 +/- 5 nm) and fX (12 630 +/- 690, 14 +/- 4 nm), whereas fX at 100 nm induced high-affinity sites for fIXa (541 +/- 67, 23 +/- 5 nm). The effects of fVIII and fVIIIa on the binding of fIXa or fX were similar. The apparent Michaelis constant of the fX activation by fIXa was a linear function of the fVIIIa concentration with a slope of 1.00 +/- 0.12 and an intrinsic K(m) value of 8.0 +/- 1.5 nm, in agreement with the hypothesis that the reaction rate is limited by the fVIIIa-fX complex formation. In addition, direct correlation was observed between the fX activation rate and formation of the fVIIIa-fX complex. Titration of fX, fVIIIa, phospholipid concentration and phosphatidylserine content suggested that at high fVIIIa concentration the reaction rate is regulated by the concentration of free fX rather than of membrane-bound fX. The obtained results reveal formation of high-affinity fVIIIa-fX complexes on phospholipid membranes and suggest their role in regulating fX activation by anchoring and delivering fX to the enzymatic complex.

Mathematical Modeling and Computer Simulation in Blood Coagulation

Over the last two decades, mathematical modeling has become a popular tool in study of blood coagulation. The in silico methods were able to yield interesting and significant results in the understanding of both individual reaction mechanisms and regulation of large sections of the coagulation cascade. The objective of this paper is to review the development of theoretical research in hemostasis and thrombosis, to summarize the main findings, and outline problems and possible prospects in the use of mathematical modeling and computer simulation approaches. This review is primarily focused on the studies dealing with: (1) the membrane-dependent reactions of coagulation; (2) regulation of the coagulation cascade, including effects of positive and negative feedback loops, diffusion of coagulation factors, and blood flow.

An ordered sequential mechanism for Factor IX and Factor IXa binding to platelet receptors in the assembly of the Factor X-activating complex

To define the contributions of the Omega-loop of the Gla (gamma-carboxyglutamic acid) domain and the EGF2 (second epidermal growth factor) domain of FIXa (Factor IXa) in the assembly of the FX-activating complex on activated platelets and phospholipid membranes, three recombinant FIXa chimeras were prepared with corresponding residues from the homologous coagulation protein, FVII: (i) Gly4-Gln11 (FIXa7Omegaloop), (ii) Cys88-Cys124 (FIXa7EGF2), and (iii) both Gly4-Gln11 and Cys88-Cys124 (FIXa7Omegaloop7EGF2). All three chimeras were similar to wild-type FIXa, as assessed by SDS/PAGE, active-site titration, content of Gla residues, activation rates by FXIa and rates of FXa generation in solution. Titrations of FX or FVIIIa on SFLLRN peptide-activated platelets and on phospholipid vesicles in the presence of FVIIIa revealed normal substrate and cofactor binding to all chimeras. In kinetic assays in the presence of phospholipid vesicles and FVIIIa, compared with wild-type FIXa K(d, app) approximately 4 nM, the FIX7Omegaloop chimera showed a 1.6-fold increase in K(d, app), the FIX7EGF2 chimera had a 7.4-fold increase in K(d, app), and the FIX7Omegaloop7EGF2 chimera showed a 21-fold increase in K(d, app). In kinetic assays and equilibrium platelet-binding assays with activated platelets and FVIIIa, compared with wild-type FIXa (V(max) approximately 5 nM min(-1); K(d, app) approximately 0.5 nM; B(max) approximately 550 sites/platelet; K(d) approximately 0.5 nM), the FIX7Omegaloop chimera displayed 2-fold decreases in V(max) and B(max) and 2-fold increases in K(d, app) and K(d). The FIX7EGF2 chimera displayed 2-fold decreases in V(max) and B(max) and 10-fold increases in K(d, app) and K(d). The FIX7Omegaloop7EGF2 chimera showed non-saturable curves and severely impaired rates of FXa generation, and non-saturable, non-specific, low-level binding to activated platelets. Thus both the Gla domain Omega-loop (Gly4-Gln11) and the EGF2 domain (Cys88-Cys124) are required to mediate the normal assembly of the FX-activating complex on activated platelets and on phospholipid membranes.

Kinetics of Factor X activation by the membrane-bound complex of Factor IXa and Factor VIIIa

Intrinsic tenase consists of activated Factors IX (IXa) and VIII (VIIIa) assembled on a negatively charged phospholipid surface. In vivo, this surface is mainly provided by activated platelets. In vitro, phosphatidylcholine/phosphatidylserine vesicles are often used to mimic natural pro-coagulant membranes. In the present study, we developed a quantitative mathematical model of Factor X activation by intrinsic tenase. We considered two situations, when complex assembly occurs on either the membrane of phospholipid vesicles or the surface of activated platelets. On the basis of existing experimental evidence, the following mechanism for the complex assembly on activated platelets was suggested: (i) Factors IXa, VIIIa and X bind to their specific platelet receptors; (ii) bound factors form complexes on the membrane: platelet-bound Factor VIIIa provides a high-affinity site for Factor X and platelet-bound Factor IXa provides a high-affinity site for Factor VIIIa; (iii) the enzyme-cofactor-substrate complex is assembled. This mechanism allowed the explanation of co-operative effects in the binding of Factors IXa, VIIIa and X to platelets. The model was reduced to obtain a single equation for the Factor X activation rate as a function of concentrations of Factors IXa, VIIIa, X and phospholipids (or platelets). The equation had a Michaelis-Menten form, where apparent V(max) and K(m) were functions of the factors' concentrations and the internal kinetic constants of the system. The equation obtained can be used in both experimental studies of intrinsic tenase and mathematical modelling of the coagulation cascade. The approach of the present study can be applied to research of other membrane-dependent enzymic reactions.

A Subpopulation of Platelets Responds to Thrombin- or SFLLRN-stimulation with Binding Sites for Factor IXa

Strong agonists cause platelets to expose a procoagulant surface supporting the assembly of two important coagulation enzyme complexes. Equilibrium binding has determined the density of high affinity saturable factor IXa binding sites to be 500-600 sites/platelet. We have now used flow cytometry to visualize the binding of factor IX and IXa to thrombin- or SFLLRN-activated platelets. Concentrations of these agonists that are half-maximal or maximal in kinetic studies resulted in only a small subpopulation (4-20%) of platelets binding factor IX or IXa with the density of binding sites for factor IX being about half of that for factor IXa, consistent with previous equilibrium binding studies. A small subpopulation (5 +/- 1.5%) of platelets stimulated with either agonist also exposed annexin V binding sites, and this subpopulation of platelets also bound factor IXa. Annexin V decreased factor IXa binding in the presence or absence of factor VIIIa, and factor IXa could also decrease annexin V binding on some platelets indicating a common binding site in agreement with previous studies. All platelets binding factor IXa were positive for glycoprotein IX, at the same glycoprotein IX surface density as seen in platelets negative for factor IXa binding. These studies refine the results from equilibrium binding studies and suggest that, on average, only a small subpopulation (approximately 10%) of PAR 1-stimulated platelets expose approximately 6000 factor IXa binding sites/platelet.

The function of factor XI in tissue factor-initiated thrombin generation

The influence of plasma and platelet factor (F)XI on thrombin generation initiated with 10 pm tissue factor (TF) in a synthetic coagulation model was evaluated in the presence of either 2 x 108 mL-1 platelets or the equivalent (2 microm) phospholipids. In either system, with all proteins present at physiological concentrations, FXI (30 nm) had no effect on thrombin generation. With phospholipids in the absence of FXI, an increase in vitamin K-dependent proteins (VKDP) (up to 500%) significantly prolonged the initiation phase of thrombin generation and decreased maximum thrombin levels. The inhibition was principally caused by the elevated prothrombin and FIX concentrations. When 30 nm FXI was added with elevated VKDP and phospholipids, the initiation phase was decreased and the maximum thrombin levels generated substantially increased. In experiments with platelets (with and without plasma FXI), an increase in VKDP had little effect on the initiation phase of thrombin generation. These data indicate that (i) FXI has no effect on thrombin generation at 10 pm TF and physiological concentrations of VKDP; (ii) platelets and plasma FXI are able to compensate for the inhibitory effects of elevated VKDP.

The assembly of the factor X-activating complex on activated human platelets

Platelet membranes provide procoagulant surfaces for the assembly and expression of the factor X-activating complex and promote the proteolytic activation and assembly of the prothrombinase complex resulting in normal hemostasis. Recent studies from our laboratory and others indicate that platelets possess specific, high-affinity, saturable, receptors for factors XI, XIa, IX, IXa, X, VIII, VIIIa, V, Va and Xa, prothrombin, and thrombin. Studies described in this review support the hypothesis that the factor X-activating complex on the platelet surface consists of three receptors (for the enzyme, factor IXa; the substrate, factor X; and the cofactor, factor VIIIa), the colocalization of which results in a 24 million-fold acceleration of the rate of factor X activation. Whether the procoagulant surface of platelets is defined exclusively by procoagulant phospholipids, or whether specific protein receptors exist for the coagulant factors and proteases, is currently unresolved. The interaction between coagulation proteins and platelets is critical to the maintenance of normal hemostasis and is pathogenetically important in human disease.

Platelets and thrombin generation

This review examines the evidence that platelets play a major role in localizing and controlling the burst of thrombin generation leading to fibrin clot formation. From the first functional description of platelets, it has been recognized that platelets supply factors that support the activation of prothrombin. Studies have demonstrated that on activation, the amount of one specific lipid, phosphatidylserine, is significantly increased on the outer leaflet of platelet membranes. When it was found that phosphatidylserine containing lipid extracts could be substituted for platelets in clotting assays, this suggested the possibility that changes in platelet lipid composition were necessary and sufficient to account for platelet surface thrombin generation. Because a growing body of data suggest that platelet-binding proteins provide much of the specificity for platelet thrombin generation, we review in this report data suggesting that changes in lipid composition are necessary but not sufficient to account for platelet surface regulation of thrombin generation. Also, we review data suggesting that platelets from different individuals differ in their capacity to generate thrombin, whereas platelets from a single subject support thrombin generation in a reproducible manner. Individual differences in platelet thrombin generation might be accounted for by differences in platelet-binding proteins.

Activated platelets but not endothelial cells participate in the initiation of the consolidation phase of blood coagulation

To address the question of whether initiation of the consolidation phase of coagulation occurs on platelets or on endothelium, we have examined the interaction of coagulation factor XI with human umbilical vein endothelial cells (HUVEC) and with platelets. In microtiter wells factor XI binds to more sites in the absence of HUVEC (1.8 x 10(10) sites/well, K(D) = 2.6 nm) than in their presence (1.3 x 10(10) sites/well, K(D) = 12 nm) when high molecular weight kininogen (HK) and zinc are present. Binding was volume-dependent and abrogated by HUVEC or Chinese hamster ovary cells and was a function of nonspecific binding of HK to the artificial plastic surface. Factor XI did not bind to HUVEC or to HEK293 cell monolayers anchored to microcarrier beads. Activation of HUVEC resulted in von Willebrand's factor secretion, but factor XI binding was not observed. Only activated platelets supported factor XI binding in the presence of HK and zinc (K(D) = 8 nm, B(max) = 1319 sites/cell). Activation of factor XI was observed in plasma in the presence of platelets activated by the thrombin receptor activation peptide but not with activated HUVEC. These results support the concept that activated platelets, but not endothelial cells, expose a procoagulant surface for binding and activating factor XI, thereby initiating the consolidation phase of coagulation.

The factor IXa second epidermal growth factor (EGF2) domain mediates platelet binding and assembly of the factor X activating complex

Previously we have determined that residues 88-109 (but not Arg(94)) in the second epidermal growth factor (EGF2)-like domain of factor IXa (FIXa) are important for assembly of the factor X (FX) activating complex on phospholipid vesicles (Wilkinson, F. H., London, F. S., and Walsh, P. N. (2002) J. Biol. Chem. 277, 5725-5733). Here we report that these residues are important for platelet binding affinity, stoichiometry, and assembly of the FX activating complex. We prepared several chimeric FIXa proteins using homologous sequences from factor VII (FVII): FIXa(FVIIEGF2) (FIX Delta 88-124,inverted Delta FVII91-127), FIXa(loop1) (FIX Delta 88-99,inverted Delta FVII91-102), FIXa(loop2) (FIX Delta 95-109,inverted Delta FVII98-112), and FIXa(loop3) (FIX Delta 111-124,inverted Delta FVII114-127) and tested their ability to bind to thrombin-activated platelets. Binding affinities (K(d) values in 10(-9) m) for the proteins were as follows in the presence and absence of FVIIIa, respectively: FIXa(N) (0.55 +/- 0.06, 2.9 +/- 0.45), FIXa(WT) (0.80 +/- 0.08, 3.5 +/- 0.5), FIXa(loop1) (19 +/- 4.0, 27 +/- 5.0), FIXa(loop2) (35 +/- 9.0, 65 +/- 12.0), and FIXa(loop3) (1.1 +/- 0.09, 5.0 +/- 0.90). These K(d) values are in good agreement with K((d)(app)) values (in 10(-9) m) determined from the activation of FX (in the presence and absence of FVIIIa, respectively): FIXa(N) (0.46 +/- 0.05, 1.40 +/- 0.14), FIXa(WT) (0.72 +/- 0.08, 3.8 +/- 0.08), FIXa(loop1) (3.2 +/- 0.72, 14.0 +/- 1.60), FIXa(loop2) (18.4 +/- 1.60, 26.3 +/- 3.40), and FIXa(loop3) (0.7 +/- 0.05, 3.0 +/- 0.15). Moreover, the stoichiometry of binding (sites/platelet) showed an agreement with V(max) of FX activation and was reduced in those proteins that also showed a decreased platelet binding affinity. A peptide corresponding to the FIX EGF2 domain (Leu(84)-Val(128)) was an effective inhibitor of FIXa binding to platelets in both the presence (K(i) = 0.7 x 10(-6) m) and the absence (K(i) = 1.5 x 10(-6) m) of FVIIIa and FX. We conclude that residues 88-109 of the FIXa EGF2 domain mediate binding to platelets and assembly of the FX activating complex.ut not Ar

The role of high molecular weight kininogen and prothrombin as cofactors in the binding of factor XI A3 domain to the platelet surface

We have reported that prothrombin (1 microm) is able to replace high molecular weight kininogen (45 nm) as a cofactor for the specific binding of factor XI to the platelet (Baglia, F. A., and Walsh, P. N. (1998) Biochemistry 37, 2271-2281). We have also determined that prothrombin fragment 2 binds to the Apple 1 domain of factor XI at or near the site where high molecular weight kininogen binds. A region of 31 amino acids derived from high molecular weight kininogen (HK31-mer) can also bind to factor XI (Tait, J. F., and Fujikawa, K. (1987) J. Biol. Chem. 262, 11651-11656). We therefore investigated the role of prothrombin fragment 2 and HK31-mer as cofactors in the binding of factor XI to activated platelets. Our experiments demonstrated that prothrombin fragment 2 (1 microm) or the HK31-mer (8 microm) are able to replace high molecular weight kininogen (45 nm) or prothrombin (1 microm) as cofactors for the binding of factor XI to the platelet. To localize the platelet binding site on factor XI, we used mutant full-length recombinant factor XI molecules in which the platelet binding site in the Apple 3 domain was altered by alanine scanning mutagenesis. The recombinant factor XI with alanine substitutions at positions Ser(248), Arg(250), Lys(255), Leu(257), Phe(260), or Gln(263) were defective in their ability to bind to activated platelets. Thus, the interaction of factor XI with platelets is mediated by the amino acid residues Ser(248), Arg(250), Lys(255), Leu(257), Phe(260), and Gln(263) within the Apple 3 domain.

Regulation of tissue plasminogen activator activity by cells. Domains responsible for binding and mechanism of stimulation

A number of cell types have previously been shown to bind tissue plasminogen activator (tPA), which in some cases can remain active on the cell surface resulting in enhanced plasminogen activation kinetics. We have investigated several cultured cell lines, U937, THP1, K562, Molt4, and Nalm6 and shown that they bind both tPA and plasminogen and are able to act as promoters of plasminogen activation in kinetic assays. To understand what structural features of tPA are involved in cell surface interactions, we performed kinetic assays with a range of tPA domain deletion mutants consisting of full-length glycosylated and nonglycosylated tPA (F-G-K1-K2-P), DeltaFtPA (G-K1-K2-P), K2-P tPA (BM 06.022 or Reteplase), and protease domain (P). Deletion variants were made in Escherichia coli and were nonglycosylated. Plasminogen activation rates were compared with and without cells, over a range of cell densities at physiological tPA concentrations, and produced maximum levels of stimulation up to 80-fold with full-length, glycosylated tPA. Stimulation for nonglycosylated full-length tPA dropped to 45-60% of this value. Loss of N-terminal domains as in DeltaFtPA and K2P resulted in a further loss of stimulation to 15-30% of the full-length glycosylated value. The protease domain alone was stimulated at very low levels of up to 2-fold. Thus, a number of different sites are involved in cell interactions especially within finger and kringle domains, which is similar to the regulation of tPA activity by fibrin. A model was developed to explain the mechanism of stimulation and compared with actual data collected with varying cell, plasminogen, or tPA concentrations and different tPA variants. Experimental data and model predictions were generally in good agreement and suggest that stimulation is well explained by the concentration of reactants by cells.