Resolution of Michaelis complex, acylation, and conformational change steps in the reactions of the serpin, plasminogen activator inhibitor-1, with tissue plasminogen activator and trypsin

tPA supplementation preserves neurovascular and cognitive function in Tg2576 mice

INTRODUCTION

Amyloid beta (Aβ) impairs the cerebral blood flow (CBF) increase induced by neural activity (functional hyperemia). Tissue plasminogen activator (tPA) is required for functional hyperemia, and in mouse models of Aβ accumulation tPA deficiency contributes to neurovascular and cognitive impairment. However, it remains unknown if tPA supplementation can rescue Aβ-induced neurovascular and cognitive dysfunction.

METHODS

Tg2576 mice and wild-type littermates received intranasal tPA (0.8 mg/kg/day) or vehicle 5 days a week starting at 11 to 12 months of age and were assessed 3 months later.

RESULTS

Treatment of Tg2576 mice with tPA restored resting CBF, prevented the attenuation in functional hyperemia, and improved nesting behavior. These effects were associated with reduced cerebral atrophy and cerebral amyloid angiopathy, but not parenchymal amyloid.

DISCUSSION

These findings highlight the key role of tPA deficiency in the neurovascular and cognitive dysfunction associated with amyloid pathology, and suggest potential therapeutic strategies involving tPA reconstitution.

HIGHLIGHTS

Amyloid beta (Aβ) induces neurovascular dysfunction and impairs the increase of cerebral blood flow induced by neural activity (functional hyperemia). Tissue plasminogen activator (tPA) deficiency contributes to the neurovascular and cognitive dysfunction caused by Aβ. In mice with florid amyloid pathology intranasal administration of tPA rescues the neurovascular and cognitive dysfunction and reduces brain atrophy and cerebral amyloid angiopathy. tPA deficiency plays a crucial role in neurovascular and cognitive dysfunction induced by Aβ and tPA reconstitution may be of therapeutic value.

Cobalamin decyanation by the membrane transporter BtuM

BtuM is a bacterial cobalamin transporter that binds the transported substrate in the base-off state, with a cysteine residue providing the α-axial coordination of the central cobalt ion via a sulfur-cobalt bond. Binding leads to decyanation of cobalamin variants with a cyano group as the β-axial ligand. Here, we report the crystal structures of untagged BtuM bound to two variants of cobalamin, hydroxycobalamin and cyanocobalamin, and unveil the native residue responsible for the β-axial coordination, His28. This coordination had previously been obscured by non-native histidines of His-tagged BtuM. A model in which BtuM initially binds cobinamide reversibly with low affinity (KD = 4.0 μM), followed by the formation of a covalent bond (rate constant of 0.163 s-1), fits the kinetics data of substrate binding and decyanation of the cobalamin precursor cobinamide by BtuM. The covalent binding mode suggests a mechanism not used by any other transport protein.

Macrophage Function Modulated by tPA Signaling in Mouse Experimental Kidney Disease Models

Macrophage infiltration and accumulation is a hallmark of chronic kidney disease. Tissue plasminogen activator (tPA) is a serine protease regulating the homeostasis of blood coagulation, fibrinolysis, and matrix degradation, and has been shown to act as a cytokine to trigger various receptor-mediated intracellular signal pathways, modulating macrophage function in response to kidney injury. In this review, we discuss the current understanding of tPA-modulated macrophage function and underlying signaling mechanisms during kidney fibrosis and inflammation.

Molecular Mechanism and Pathways of Normal Human Parturition in Different Gestational Tissues: A Systematic Review of Transcriptome Studies

Objective: Genome-wide transcriptomic studies on gestational tissues in labor provide molecular insights in mechanism of normal parturition. This systematic review aimed to summarize the important genes in various gestational tissues around labor onset, and to dissect the underlying molecular regulations and pathways that trigger the labor in term pregnancies.

Data sources: PubMed and Web of Science were searched from inception to January 2021.

Study Eligibility Criteria: Untargeted genome-wide transcriptomic studies comparing the gene expression of various gestational tissues in normal term pregnant women with and without labor were included.

Methods: Every differentially expressed gene was retrieved. Consistently expressed genes with same direction in different studies were identified, then gene ontology and KEGG analysis were conducted to understand molecular pathways and functions. Gene-gene association analysis was performed to determine the key regulatory gene(s) in labor onset.

Results: A total of 15 studies, including 266 subjects, were included. 136, 26, 15, 7, and 3 genes were significantly changed during labor in the myometrium (seven studies, n = 108), uterine cervix (four studies, n = 64), decidua (two studies, n = 42), amnion (two studies, n = 44) and placenta (two studies, n = 41), respectively. These genes were overrepresented in annotation terms related to inflammatory and immune responses. TNF and NOD-like receptor signaling pathways were overrepresented in all mentioned tissues, except the placenta. IL6 was the only gene included in both pathways, the most common reported gene in all included studies, and also the gene in the central hub of molecular regulatory network.

Conclusions: This systematic review identified that genes involved in immunological and inflammatory regulations are expressed in specific gestational tissues in labor. We put forward the hypothesis that IL6 might be the key gene triggering specific mechanism in different gestational tissues, eventually leading to labor onset through inducing uterine contraction, wakening fetal membranes and stimulating cervical ripening.

Systematic Review Registration: Identifier [CRD42020187975].

Dissecting molecular details and functional effects of the high-affinity copper binding site in plasminogen activator Inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the primary inhibitor for plasminogen activators, tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). As a unique member in the serine protease inhibitor (serpin) family, PAI-1 is metastable and converts to an inactive, latent structure with a half-life of 1-2 hr under physiological conditions. Unusual effects of metals on the rate of the latency conversion are incompletely understood. Previous work has identified two residues near the N-terminus, H2 and H3, which reside in a high-affinity copper-binding site in PAI-1 [Bucci JC, McClintock CS, Chu Y, Ware GL, McConnell KD, Emerson JP, Peterson CB (2017) J Biol Inorg Chem 22:1123-1,135]. In this study, neighboring residues, H10, E81, and H364, were tested as possible sites that participate in Cu(II) coordination at the high-affinity site. Kinetic methods, gel sensitivity assays, and isothermal titration calorimetry (ITC) revealed that E81 and H364 have different roles in coordinating metal and mediating the stability of PAI-1. H364 provides a third histidine in the metal-coordination sphere with H2 and H3. In contrast, E81 does not appear to be required for metal ligation along with histidines; contacts made by the side-chain carboxylate upon metal binding are perturbed and, in turn, influence dynamic fluctuations within the region encompassing helices D, E, and F and the W86 loop that are important in the pathway for the PAI-1 latency conversion. This investigation underscores a prominent role of protein dynamics, noncovalent bonding networks and ligand binding in controlling the stability of the active form of PAI-1.

Molecular mechanism of two nanobodies that inhibit PAI-1 activity reveals a modulation at distinct stages of the PAI-1/plasminogen activator interaction

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1), a key inhibitor of plasminogen activators (PAs) tissue-type PA (tPA) and urokinase-type PA (uPA) plays a crucial role in many (patho)physiological processes (e.g., cardiovascular disease, tissue fibrosis) as well as in many age-related pathologies. Therefore, much effort has been put into the development of small molecule or antibody-based PAI-1 inhibitors.

OBJECTIVE

To elucidate the molecular mechanism of nanobody-induced PAI-1 inhibition.

METHODS AND RESULTS

Here we present the first crystal structures of PAI-1 in complex with two neutralizing nanobodies (Nbs). These structures, together with biochemical and biophysical characterization, reveal that Nb VHH-2g-42 (Nb42) interferes with the initial PAI-1/PA complex formation, whereas VHH-2w-64 (Nb64) redirects the PAI-1/PA interaction to PAI-1 deactivation and regeneration of active PA. Furthermore, whereas vitronectin does not have an impact on the inhibitory effect of Nb42, it strongly potentiates the inhibitory effect of Nb64, which may contribute to a strong inhibitory potential of Nb64 in vivo.

CONCLUSIONS

These findings illuminate the molecular mechanisms of PAI-1 inhibition. Nb42 and Nb64 can be used as starting points to engineer further improved antibody-based PAI-1 inhibitors or guide the rational design of small molecule inhibitors to treat a wide range of PAI-1-related pathophysiological conditions.

Protease-independent action of tissue plasminogen activator in brain plasticity and neurological recovery after ischemic stroke

Emerging evidence suggests that tissue plasminogen activator (tPA), currently the only FDA-approved medication for ischemic stroke, exerts important biological actions on the CNS besides its well-known thrombolytic effect. In this study, we investigated the role of tPA on primary neurons in culture and on brain recovery and plasticity after ischemic stroke in mice. Treatment with recombinant tPA stimulated axonal growth in culture, an effect independent of its protease activity and achieved through epidermal growth factor receptor (EGFR) signaling. After permanent focal cerebral ischemia, tPA knockout mice developed more severe sensorimotor and cognitive deficits and greater axonal and myelin injury than wild-type mice, suggesting that endogenously expressed tPA promotes long-term neurological recovery after stroke. In tPA knockout mice, intranasal administration of recombinant tPA protein 6 hours poststroke and 7 more times at 2 d intervals mitigated white matter injury, improved axonal conduction, and enhanced neurological recovery. Consistent with the proaxonal growth effects observed in vitro, exogenous tPA delivery increased poststroke axonal sprouting of corticobulbar and corticospinal tracts, which might have contributed to restoration of neurological functions. Notably, recombinant mutant tPA-S478A lacking protease activity (but retaining the EGF-like domain) was as effective as wild-type tPA in rescuing neurological functions in tPA knockout stroke mice. These findings demonstrate that tPA improves long-term functional outcomes in a clinically relevant stroke model, likely by promoting brain plasticity through EGFR signaling. Therefore, treatment with the protease-dead recombinant tPA-S478A holds particular promise as a neurorestorative therapy, as the risk for triggering intracranial hemorrhage is eliminated and tPA-S478A can be delivered intranasally hours after stroke.

Protease-activated receptor 2 signaling modulates susceptibility of colonic epithelium to injury through stabilization of YAP in vivo

Hippo signaling plays critical roles in intestinal regeneration. However, the mechanisms which regulate its activity in vivo are largely unknown. We hypothesize that protease-activated receptor 2 (PAR2) signaling, which could be activated by trypsin, might affect YAP activity in the setting of tissue damage and regeneration. It is found that knockout of PAR2 severely aggravates the mucosal damage induced by dextran sodium sulfate (DSS) in mouse, which correlated with notable repression of YAP protein in colonic epithelial cells. Although the cytokine expression is reduced, the damage of colonic crypt is more severe after DSS-induced colitis in PAR2-/- mouse. In vitro, PAR2 activation causes the accumulation of YAP, while knockdown of PAR2 with shRNA dramatically represses the expression of YAP protein in different intestinal epithelial cell lines. Moreover, forced expression of YAP significantly reduces the production of reactive oxygen species (ROS) and the sensitivity to nitric oxide-induced apoptosis in PAR2-deficient condition. Further studies show that PAR2 signaling stabilizes YAP protein but independent of Lats. Nevertheless PAR2 activation increased the binding of YAP with protein phosphatase PP1. Inhibition of PP1 with specific siRNA blocked PAR2-induced dephosphorylation of YAP. Taken together, PAR2 signaling might modulate susceptibility of colonic epithelium to injury through stabilization of YAP.

Resolving distinct molecular origins for copper effects on PAI-1

Components of the fibrinolytic system are subjected to stringent control to maintain proper hemostasis. Central to this regulation is the serpin plasminogen activator inhibitor-1 (PAI-1), which is responsible for specific and rapid inhibition of fibrinolytic proteases. Active PAI-1 is inherently unstable and readily converts to a latent, inactive form. The binding of vitronectin and other ligands influences stability of active PAI-1. Our laboratory recently observed reciprocal effects on the stability of active PAI-1 in the presence of transition metals, such as copper, depending on the whether vitronectin was also present (Thompson et al. Protein Sci 20:353–365, 2011). To better understand the molecular basis for these copper effects on PAI-1, we have developed a gel-based copper sensitivity assay that can be used to assess the copper concentrations that accelerate the conversion of active PAI-1 to a latent form. The copper sensitivity of wild-type PAI-1 was compared with variants lacking N-terminal histidine residues hypothesized to be involved in copper binding. In these PAI-1 variants, we observed significant differences in copper sensitivity, and these data were corroborated by latency conversion kinetics and thermodynamics of copper binding by isothermal titration calorimetry. These studies identified a copper-binding site involving histidines at positions 2 and 3 that confers a remarkable stabilization of PAI-1 beyond what is observed with vitronectin alone. A second site, independent from the two histidines, binds metal and increases the rate of the latency conversion.

Slow fusion pore expansion creates a unique reaction chamber for co-packaged cargo

A lumenal secretory granule protein can slow fusion pore dilation and thus its own discharge. Bohannon et al. demonstrate another outcome: the creation of a nanoscale chemical reaction chamber for granule contents in which the pH is suddenly neutralized upon fusion.

A lumenal secretory granule protein, tissue plasminogen activator (tPA), greatly slows fusion pore dilation and thereby slows its own discharge. We investigated another outcome of the long-lived narrow fusion pore: the creation of a nanoscale chemical reaction chamber for granule contents in which the pH is suddenly neutralized upon fusion. Bovine adrenal chromaffin cells endogenously express both tPA and its primary protein inhibitor, plasminogen activator inhibitor 1 (PAI). We found by immunocytochemistry that tPA and PAI are co-packaged in the same secretory granule. It is known that PAI irreversibly and covalently inactivates tPA at neutral pH. We demonstrate with zymography that the acidic granule lumen protects tPA from inactivation by PAI. Immunocytochemistry, total internal reflection fluorescence (TIRF) microscopy, and polarized TIRF microscopy demonstrated that co-packaged PAI and tPA remain together in granules for many seconds in the nanoscale reaction chamber, more than enough time to inhibit tPA and create a new secreted protein species.

Inhibitory serpins. New insights into their folding, polymerization, regulation and clearance

Serpins are a widely distributed family of high molecular mass protein proteinase inhibitors that can inhibit both serine and cysteine proteinases by a remarkable mechanism-based kinetic trapping of an acyl or thioacyl enzyme intermediate that involves massive conformational transformation. The trapping is based on distortion of the proteinase in the complex, with energy derived from the unique metastability of the active serpin. Serpins are the favoured inhibitors for regulation of proteinases in complex proteolytic cascades, such as are involved in blood coagulation, fibrinolysis and complement activation, by virtue of the ability to modulate their specificity and reactivity. Given their prominence as inhibitors, much work has been carried out to understand not only the mechanism of inhibition, but how it is fine-tuned, both spatially and temporally. The metastability of the active state raises the question of how serpins fold, whereas the misfolding of some serpin variants that leads to polymerization and pathologies of liver disease, emphysema and dementia makes it clinically important to understand how such polymerization might occur. Finally, since binding of serpins and their proteinase complexes, particularly plasminogen activator inhibitor-1 (PAI-1), to the clearance and signalling receptor LRP1 (low density lipoprotein receptor-related protein 1), may affect pathways linked to cell migration, angiogenesis, and tumour progression, it is important to understand the nature and specificity of binding. The current state of understanding of these areas is addressed here.

Structural basis of specific inhibition of tissue-type plasminogen activator by plasminogen activators inhibitor-1

Thrombosis is a leading cause of death worldwide [1]. Recombinant tissue-type plasminogen activator (tPA) is the FDA-approved thrombolytic drug for ischemic strokes, myocardial infarction and pulmonary embolism. tPA is a multi-domain serine protease of the trypsin-family [2] and catalyses the critical step in fibrinolysis [3], converting the zymogen plasminogen to the active serine protease plasmin, which degrades the fibrin network of thrombi and blood clots. tPA is rapidly inactivated by endogenous plasminogen activators inhibitor-1 (PAI-1) [4] (Fig. 1). Engineering on tPA to reduce its inhibition by PAI-1 without compromising its thrombolytic effect is a continuous effort [5]. Tenecteplase (TNK-tPA) is a newer generation of tPA variant showing slower inhibition by PAI-1 [6]. Extensive studies to understand the molecular interactions between tPA and PAI-1 have been carried out [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], however, the precise details at atomic resolution remain unknown. We report the crystal structure of tPA·PAI-1 complex here. The methods required to achieve these data include: (1) recombinant expression and purification of a PAI-1 variant (14-1B) containing four mutations (N150H, K154T, Q319L, and M354I), and a tPA serine protease domain (tPA-SPD) variant with three mutations (C122A, N173Q, and S195A, in the chymotrypsin numbering) [19]; (2) formation of a tPA-SPD·PAI-1 Michaëlis complex in vitro [19]; and (3) solving the three-dimensional structure for this complex by X-ray crystallography [deposited in the PDB database as 5BRR]. The data explain the specificity of PAI-1 for tPA and uPA [19], [20], and provide structural basis to design newer generation of PAI-1-resistant tPA variants as thrombolytic agents [19].

Insights into the serine protease mechanism based on structural observations of the conversion of a peptidyl serine protease inhibitor to a substrate

BACKGROUND

Serine proteases are one of the most studied group of enzymes. Despite the extensive mechanistic studies, some crucial details remain controversial, for example, how the cleaved product is released in the catalysis reaction. A cyclic peptidyl inhibitor (CSWRGLENHRMC, upain-1) of a serine protease, urokinase-type plasminogen activator (uPA), was found to become a slow substrate and cleaved slowly upon the replacement of single residue (W3A).

METHODS

By taking advantage of the unique property of this peptide, we report the high-resolution structures of uPA in complex with upain-1-W3A peptide at four different pH values by X-ray crystallography.

RESULTS

In the structures obtained at low pH (pH4.6 and 5.5), the cyclic peptide upain-1-W3A was found to be intact and remained in the active site of uPA. At 7.4, the scissile bond of the peptide was found cleaved, showing that the peptide became a uPA substrate. At pH9.0, the C-terminal part of the substrate was no longer visible, and only the P1 residue occupying the S1 pocket was identified.

CONCLUSIONS

The analysis of these structures provides explanations why the upain-1-W3A is a slow substrate. In addition, we clearly identified the cleaved fragments of the peptide at both sides of the scissile bond in the active site of the enzyme, showing a slow release of the cleaved peptide.

GENERAL SIGNIFICANCE

This work indicates that the quick release of the cleaved P' fragment after the first step of hydrolysis may not always be needed for the second hydrolysis.

Distinct encounter complexes of PAI-1 with plasminogen activators and vitronectin revealed by changes in the conformation and dynamics of the reactive center loop

Plasminogen activator inhibitor-1 (PAI-1) is a biologically important serine protease inhibitor (serpin) that, when overexpressed, is associated with a high risk for cardiovascular disease and cancer metastasis. Several of its ligands, including vitronectin, tissue-type and urokinase-type plasminogen activator (tPA, uPA), affect the fate of PAI-1. Here, we measured changes in the solvent accessibility and dynamics of an important unresolved functional region, the reactive center loop (RCL), upon binding of these ligands. Binding of the catalytically inactive S195A variant of tPA to the RCL causes an increase in fluorescence, indicating greater solvent protection, at its C-terminus, while mobility along the loop remains relatively unchanged. In contrast, a fluorescence increase and large decrease in mobility at the N-terminal RCL is observed upon binding of S195A-uPA to PAI-1. At a site distant from the RCL, binding of vitronectin results in a modest decrease in fluorescence at its proximal end without restricting overall loop dynamics. These results provide the new evidence for ligand effects on RCL conformation and dynamics and differences in the Michaelis complex with plasminogen activators that can be used for the development of more specific inhibitors to PAI-1. This study is also the first to use electron paramagnetic resonance (EPR) spectroscopy to investigate PAI-1 dynamics.

SIGNIFICANCE

Balanced blood homeostasis and controlled cell migration requires coordination between serine proteases, serpins, and cofactors. These ligands form noncovalent complexes, which influence the outcome of protease inhibition and associated physiological processes. This study reveals differences in binding via changes in solvent accessibility and dynamics within these complexes that can be exploited to develop more specific drugs in the treatment of diseases associated with unbalanced serpin activity.

Single fluorescence probes along the reactive center loop reveal site-specific changes during the latency transition of PAI-1

The serine protease inhibitor (serpin), plasminogen activator inhibitor-1 (PAI-1), is an important biomarker for cardiovascular disease and many cancers. It is therefore a desirable target for pharmaceutical intervention. However, to date, no PAI-1 inhibitor has successfully reached clinical trial, indicating the necessity to learn more about the mechanics of the serpin. Although its kinetics of inhibition have been extensively studied, less is known about the latency transition of PAI-1, in which the solvent-exposed reactive center loop (RCL) inserts into its central β-sheet, rendering the inhibitor inactive. This spontaneous transition is concomitant with a large translocation of the RCL, but no change in covalent structure. Here, we conjugated the fluorescent probe, NBD, to single positions along the RCL (P13-P5') to detect changes in solvent exposure that occur during the latency transition. The results support a mousetrap-like RCL-insertion that occurs with a half-life of 1-2 h in accordance with previous reports. Importantly, this study exposes unique transitions during latency that occur with a half-life of ∼5 and 25 min at the P5' and P8 RCL positions, respectively. We hypothesize that the process detected at P5' represents s1C detachment, while that at P8 results from a steric barrier to RCL insertion. Together, these findings provide new insights by characterizing multiple steps in the latency transition.

Crystal Structure of the Michaelis Complex between Tissue-type Plasminogen Activator and Plasminogen Activators Inhibitor-1

Thrombosis is a leading cause of death worldwide. Recombinant tissue-type plasminogen activator (tPA) is the Food and Drug Administration-approved thrombolytic drug. tPA is rapidly inactivated by endogenous plasminogen activator inhibitor-1 (PAI-1). Engineering on tPA to reduce its inhibition by PAI-1 without compromising its thrombolytic effect is a continuous effort. Precise details, with atomic resolution, of the molecular interactions between tPA and PAI-1 remain unknown despite previous extensive studies. Here, we report the crystal structure of the tPA·PAI-1 Michaelis complex, which shows significant differences from the structure of its urokinase-type plasminogen activator analogue, the uPA·PAI-1 Michaelis complex. The PAI-1 reactive center loop adopts a unique kinked conformation. The structure provides detailed interactions between tPA 37- and 60-loops with PAI-1. On the tPA side, the S2 and S1β pockets open up to accommodate PAI-1. This study provides structural basis to understand the specificity of PAI-1 and to design newer generation of thrombolytic agents with reduced PAI-1 inactivation.

Repression of liver colorectal metastasis by the serpin Spn4A a naturally occurring inhibitor of the constitutive secretory proprotein convertases

Liver is the most common site of metastasis from colorectal cancers, and liver of patients with liver colorectal metastasis have abnormal levels of the proprotein convertases (PCs). These proteases are involved in the activation and/or expression of various colon cancer-related mediators, making them promising targets in colorectal liver metastasis therapy. Here, we revealed that the serpin Spn4 from Drosophila melanogaster inhibits the activity of all the PCs found in the constitutive secretory pathway and represses the metastatic potential of the colon cancer cells HT-29 and CT-26. In these cells, Spn4A inhibited the processing of the PCs substrates IGF-1R and PDGF-A that associated their reduced anchorage-independent growth, invasiveness and survival in response to apoptotic agents. In vivo, Spn4A-expressing tumor cells showed repressed subcutaneous tumor development and liver metastases formation in response to their intrasplenic inoculation. In these cells Spn4A induced the expression of molecules with anti-metastatic functions and inhibited expression of pro-tumorigenic molecules. Taken together, our findings identify Spn4A as the only endogenous inhibitor of all the constitutive secretory pathway PCs, which is able to repress the metastatic potential of colon cancer cells. These results suggest the potential use of Spn4A and/or derivates as a useful adduct colorectal liver metastasis prevention.

A serpin shapes the extracellular environment to prevent influenza A virus maturation

Interferon-stimulated genes (ISGs) act in concert to provide a tight barrier against viruses. Recent studies have shed light on the contribution of individual ISG effectors to the antiviral state, but most have examined those acting on early, intracellular stages of the viral life cycle. Here, we applied an image-based screen to identify ISGs inhibiting late stages of influenza A virus (IAV) infection. We unraveled a directly antiviral function for the gene SERPINE1, encoding plasminogen activator inhibitor 1 (PAI-1). By targeting extracellular airway proteases, PAI-1 inhibits IAV glycoprotein cleavage, thereby reducing infectivity of progeny viruses. This was biologically relevant for IAV restriction in vivo. Further, partial PAI-1 deficiency, attributable to a polymorphism in human SERPINE1, conferred increased susceptibility to IAV in vitro. Together, our findings reveal that manipulating the extracellular environment to inhibit the last step in a virus life cycle is an important mechanism of the antiviral response.

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SERPINE1/PAI-1 was identified as an unconventional ISG that acts extracellularly

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PAI-1 inhibits influenza A virus (IAV) spread by inhibiting glycoprotein cleavage

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Endogenous PAI-1 blocks IAV spread in human and murine cells, ex vivo and in vivo

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PAI-1 potentially inhibits other viruses requiring extracellular maturation

A t-PA/nanoparticle conjugate with fully retained enzymatic activity and prolonged circulation time

A major issue in the therapeutic use of tissue plasminogen activator (t-PA) for the treatment of thrombotic diseases is its very short half-life in the circulation due to the effects of inhibitors. The present study aims to resolve the issue using a t-PA/gold nanoparticle (t-PA/AuNP) conjugate prepared via bio-affinity ligation under physiological conditions. The ligation is based on the specific interactions between t-PA and ε-lysine (a ligand that has affinity to a specific domain in t-PA) immobilized on the AuNP surface through polyvinyl pyrrolidone (PVP) as a spacer. The conjugate can not only retain almost full enzymatic activity and clot dissolving efficiency, but also protect t-PA from inhibition by PAI-1 to some extent as compared with free t-PA in vitro. Moreover, the conjugate showed prolonged circulation time in vivo.

Surface immobilization of a protease through an inhibitor-derived affinity ligand: a bioactive surface with defensive properties against an inhibitor

Surface immobilization of a protease through its inhibitor-derived peptide was shown to be advantageous in retaining the enzymatic activity of the protease and protecting the protease from being inhibited by its inhibitor.

Structure-function relationship of SW-AT-1, a serpin-type protease inhibitor in silkworm

Although SW-AT-1, a serpin-type trypsin inhibitor from silkworm (Bombyx mori), was identified in previous study, its structure-function relationship has not been studied. In this study, SW-AT-1 was cloned from the body wall of silkworm and expressed in E. coli. rSW-AT-1 inhibited both trypsin and chymotrypsin in a concentration-dependent manner. The association rate constant for rSW-AT-1 and trypsin is 1.31×10-5 M-1s-1, for rSW-AT-1 and chymotrpsin is 2.85×10-6 M-1s-1. Circular dichroism (CD) assay showed 33% α-helices, 16% β-sheets, 17% turns, and 31% random coils in the secondary structure of the protein. Enzymatic and CD analysis indicated that rSW-AT-1 was stable at wide pH range between 4-10, and exhibited the highest activity at weakly acidic or alkaline condition. The predicted three-dimensional structure of SW-AT-1 by PyMOL (v1.4) revealed a deductive reactive centre loop (RCL) near the C-terminus, which was extended from the body of the molecule. In addition to trypsin cleavage site in RCL, matrix-assisted laser desorption ionization time of flight mass spectrometry indicated that the chymotrypsin cleavage site of SW-AT-1 was between F336 and T337 in RCL. Directed mutagenesis indicated that both the N- and C-terminal sides of RCL have effects on the activity, and G327 and E329 played an important role in the proper folding of RCL. The physiological role of SW-AT-1 in the defense responses of silkworm were also discussed.

t-PA acts as a cytokine to regulate lymphocyte-endothelium adhesion in experimental autoimmune encephalomyelitis

In this study, the capacity for t-PA to affect T cell-brain microvascular endothelial cell adhesion by acting as a cytokine was investigated. Following the treatment of a brain-derived endothelial cell line, bEnd.3, with various concentrations of t-PA, adhesion and transwell migration assays were performed. In the presence of t-PA, enhanced adhesion of T cells to bEnd.3 cells was observed. Using western blot analysis, an increase in ICAM-1 expression was detected for both t-PA-treated bEnd.3 cells and bEnd.3 cells treated with a non-enzymatic form of t-PA. In contrast, when LRP1 was blocked using a specific antibody, upregulation of ICAM-1 was inhibited and cAMP-PKA signaling was affected. Furthermore, using an EAE mouse model, administration of t-PA was associated with an increase in ICAM-1 expression by brain endothelial cells. Taken together, these findings suggest that t-PA can induce ICAM-1 expression in brain microvascular endothelial cells, and this may promote the development of EAE.

Kinetic intermediates en route to the final serpin-protease complex: studies of complexes of α1-protease inhibitor with trypsin

Serpin protein protease inhibitors inactivate their target proteases through a unique mechanism in which a major serpin conformational change, resulting in a 70-Å translocation of the protease from its initial reactive center loop docking site to the opposite pole of the serpin, kinetically traps the acyl-intermediate complex. Although the initial Michaelis and final trapped acyl-intermediate complexes have been well characterized structurally, the intermediate stages involved in this remarkable transformation are not well understood. To better characterize such intermediate steps, we undertook rapid kinetic studies of the FRET and fluorescence perturbation changes of site-specific fluorophore-labeled derivatives of the serpin, α1-protease inhibitor (α1PI), which report the serpin and protease conformational changes involved in transforming the Michaelis complex to the trapped acyl-intermediate complex in reactions with trypsin. Two kinetically resolvable conformational changes were observed in the reactions, ascribable to (i) serpin reactive center loop insertion into sheet A with full protease translocation but incomplete protease distortion followed by, (ii) full conformational distortion and movement of the protease and coupled serpin conformational changes involving the F helix-sheet A interface. Kinetic studies of calcium effects on the labeled α1PI-trypsin reactions demonstrated both inactive and low activity states of the distorted protease in the final complex that were distinct from the intermediate distorted state. These studies provide new insights into the nature of the serpin and protease conformational changes involved in trapping the acyl-intermediate complex in serpin-protease reactions and support a previously proposed role for helix F in the trapping mechanism.

Remarkable stabilization of plasminogen activator inhibitor 1 in a "molecular sandwich" complex

Plasminogen activator inhibitor 1 (PAI-1) levels are elevated in a number of life-threatening conditions and often correlate with unfavorable outcomes. Spontaneous inactivation due to active to latent transition limits PAI-1 activity in vivo. While endogenous vitronectin (Vn) stabilizes PAI-1 by 1.5-2.0-fold, further stabilization occurs in a "molecular sandwich" complex (MSC) in which a ligand that restricts the exposed reactive center loop is bound to PAI-1/Vn. The effects of S195A two-chain urokinase (tcuPA) and Vn on inactivation of wild-type (wt) glycosylated (Gl-PAI-1), nonglycosylated (rPAI-1), and nonglycosylated Q123K PAI-1 (lacks Vn binding) forms were studied. S195A tcuPA decreased the rate constant (kL) for spontaneous inactivation at 37 °C for rPAI-1, Q123K, and Gl-PAI-1 by 6.7-, 3.4-, and 7.8-fold, respectively, and both S195A tcuPA and Vn by 66.7-, 5.5-, and 103.3-fold, respectively. Analysis of the temperature dependences of kL revealed a synergistic increase in the Gibbs free activation energy for spontaneous inactivation of wt Gl-PAI-1 and rPAI-1 in MSC from 99.8 and 96.1 to 111.3 and 107.0 kJ/mol, respectively, due to an increase in the activation enthalpy and a decrease in the activation entropy. Anti-PAI-1 monoclonal antibodies (mAbs) competing with proteinase also stabilize PAI-1/Vn. The rate of inhibition of target proteinases by MSCs, with a stoichiometry close to unity, was limited by the dissociation (k = 10(-4) to 10(-3) s(-1)) of S195A tcuPA or mAb. The stabilization of PAI-1 in MSCs in vivo may potentiate uncontrolled thrombosis or extravascular fibrin deposition, suggesting a new paradigm for using PAI-1 inhibitors and novel potential targets for therapy.

Thrombin inhibition by the serpins

Thrombin is the central protease in the blood coagulation network. It has multiple substrates and cofactors, and it appears that four serpins are responsible for inhibiting the thrombin produced in haemostasis and thrombosis. Structural studies conducted over the last 10 years have resolved how thrombin recognises these serpins with the aid of cofactors. Although antithrombin (AT), protein C inhibitor (PCI), heparin cofactor II (HCII) and protease nexin-1 (PN1) all share a common fold and mechanism of protease inhibition, they have evolved radically different mechanisms for cofactor-assisted thrombin recognition. This is likely to be due to the varied environments in which thrombin is found. In this review, I discuss the unusual structural features of thrombin that are involved in substrate and cofactor recognition, the serpin mechanism of protease inhibition and the fate of thrombin in the complex, and how the four thrombin-specific serpins exploit the special features of thrombin to accelerate complex formation.

The plasminogen activator system in the ovine placentome during late gestation and stage-two of parturition

The process of placental separation is not completely understood. In domestic animals, especially cattle, it is important that expulsion of the fetal membranes takes place in a timely manner in order to achieve maximal reproductive efficiency. The activity of the matrix-metalloprotease (MMP) family of proteases is known to be reduced in placentomes from cases of retained placenta. Members of the MMP family are known to be activated by the plasminogen activator (PA) family of proteases. We hypothesized that the expression and activity of the PA family increase in the cotyledon and/or caruncle as parturition approaches, with maximal expression and activity at parturition. To test this hypothesis, we performed reverse-transcriptase quantitative PCR and plasminogen-casein zymography to detect the presence and activity of PA family members in the placentome leading up to and during parturition in spontaneous and dexamethasone-induced parturient ewes. The results from our experiments indicated that serine proteases inhibitor E1 (SERPINE1) mRNA abundance in the cotyledon was different between treatment groups (P = 0.0002). In the caruncle, gene expression for plasminogen activator urokinase-type (PLAU) was different (P = 0.0154), and there was a strong trend for differences in SERPINE1 expression (P = 0.0565). These results demonstrate that expression of the PA system in the placentome changes from late pregnancy to parturition, and the presence or activity of these enzymes may occur after fetal expulsion.

Tissue plasminogen activator activates NF-κB through a pathway involving annexin A2/CD11b and integrin-linked kinase

NF-κB activation is central to the initiation and progression of inflammation, which contributes to the pathogenesis of CKD. Tissue plasminogen activator (tPA) modulates the NF-κB pathway, but the underlying mechanism remains unknown. We investigated the role of tPA signaling in macrophage NF-κB activation and found that tPA activated NF-κB in a time- and dose-dependent manner. tPA also induced the expression of the NF-κB-dependent chemokines IP-10 and MIP-1α. The protease-independent action of tPA required its membrane receptor, annexin A2. tPA induced the aggregation and interaction of annexin A2 with integrin CD11b, and ablation of CD11b or administration of anti-CD11b neutralizing antibody abolished the effect of tPA. Knockdown of the downstream effector of CD11b, integrin-linked kinase, or disruption of its engagement with CD11b also blocked tPA-induced NF-κB signaling. In vivo, tPA-knockout mice had reduced NF-κB signaling, fewer renal macrophages, and less collagen deposition than their counterparts. Taken together, these data suggest that tPA activates the NF-κB pathway in macrophages through a signaling pathway involving annexin A2/CD11b-mediated integrin-linked kinase.

Metals affect the structure and activity of human plasminogen activator inhibitor-1. II. Binding affinity and conformational changes

Human plasminogen activator inhibitor type 1 (PAI-1) is a serine protease inhibitor with a metastable active conformation. The lifespan of the active form of PAI-1 is modulated via interaction with the plasma protein, vitronectin, and various metal ions. These metal ions fall into two categories: Type I metals, including calcium, magnesium, and manganese, stabilize PAI-1 in the absence of vitronectin, whereas Type II metals, including cobalt, copper, and nickel, destabilize PAI-1 in the absence of vitronectin, but stabilize PAI-1 in its presence. To provide a mechanistic basis for understanding the unusual modulation of PAI-1 structure and activity, the binding characteristics and conformational effects of these two types of metals were further evaluated. Steady-state binding measurements using surface plasmon resonance indicated that both active and latent PAI-1 exhibit a dissociation constant in the low micromolar range for binding to immobilized nickel. Stopped-flow measurements of approach-to-equilibrium changes in intrinsic protein fluorescence indicated that the Type I and Type II metals bind in different modes that induce distinct conformational effects on PAI-1. Changes in the observed rate constants with varying concentrations of metal allowed accurate determination of binding affinities for cobalt, nickel, and copper, yielding dissociation constants of ∼40, 30, and 0.09 μM, respectively. Competition experiments that tested effects on PAI-1 stability were consistent with these measurements of affinity and indicate that copper binds tightly to PAI-1.

Metals affect the structure and activity of human plasminogen activator inhibitor-1. I. Modulation of stability and protease inhibition

Human plasminogen activator inhibitor type 1 (PAI-1) is a serine protease inhibitor with a metastable active conformation. Under physiological conditions, half of the inhibitor transitions to a latent state within 1-2 h. The interaction between PAI-1 and the plasma protein vitronectin prolongs this active lifespan by ∼50%. Previously, our group demonstrated that PAI-1 binds to resins using immobilized metal affinity chromatography (Day, U.S. Pat. 7,015,021 B2, March 21, 2006). In this study, the effect of these metals on function and stability was investigated by measuring the rate of the transition from the active to latent conformation. All metals tested showed effects on stability, with the majority falling into one of two types depending on their effects. The first type of metal, which includes magnesium, calcium and manganese, invoked a slight stabilization of the active conformation of PAI-1. A second category of metals, including cobalt, nickel and copper, showed the opposite effects and a unique vitronectin-dependent modulation of PAI-1 stability. This second group of metals significantly destabilized PAI-1, although the addition of vitronectin in conjunction with these metals resulted in a marked stabilization and slower conversion to the latent conformation. In the presence of copper and vitronectin, the half-life of active PAI-1 was extended to 3 h, compared to a half-life of only ∼30 min with copper alone. Nickel had the largest effect, reducing the half-life to ∼5 min. Together, these data demonstrate a heretofore-unknown role for metals in modulating PAI-1 stability.

Plasminogen activator inhibitor 1 is an intracellular inhibitor of furin proprotein convertase

Proprotein convertases (PCs) are a family of serine proteases that are involved in the post-translational processing and activation of a wide range of regulatory proteins. The upstream role of PCs in the control of many physiological and pathological processes generates a growing interest in understanding their regulation. Here, we demonstrate that the serine protease inhibitor plasminogen activator inhibitor 1 (PAI-1) forms an SDS-stable complex with the PC furin, which leads to the inhibition of the intra-Golgi activity of furin. It is known that elevated PAI-1 plasma levels are correlated with the occurrence of the metabolic syndrome and type 2 diabetes, and we show that PAI-1 reduces the furin-dependent maturation and activity of the insulin receptor and ADAM17: two proteins involved in the onset of these metabolic disorders. In addition to demonstrating that PAI-1 is an intracellular inhibitor of furin, this study also provides arguments in favor of an active role for PAI-1 in the development of metabolic disorders.

Structural basis for recognition of urokinase-type plasminogen activator by plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1), together with its physiological target urokinase-type plasminogen activator (uPA), plays a pivotal role in fibrinolysis, cell migration, and tissue remodeling and is currently recognized as being among the most extensively validated biological prognostic factors in several cancer types. PAI-1 specifically and rapidly inhibits uPA and tissue-type PA (tPA). Despite extensive structural/functional studies on these two reactions, the underlying structural mechanism has remained unknown due to the technical difficulties of obtaining the relevant structures. Here, we report a strategy to generate a PAI-1·uPA(S195A) Michaelis complex and present its crystal structure at 2.3-Å resolution. In this structure, the PAI-1 reactive center loop serves as a bait to attract uPA onto the top of the PAI-1 molecule. The P4-P3' residues of the reactive center loop interact extensively with the uPA catalytic site, accounting for about two-thirds of the total contact area. Besides the active site, almost all uPA exosite loops, including the 37-, 60-, 97-, 147-, and 217-loops, are involved in the interaction with PAI-1. The uPA 37-loop makes an extensive interaction with PAI-1 β-sheet B, and the 147-loop directly contacts PAI-1 β-sheet C. Both loops are important for initial Michaelis complex formation. This study lays down a foundation for understanding the specificity of PAI-1 for uPA and tPA and provides a structural basis for further functional studies.

Regulation of proteases by protein inhibitors of the serpin superfamily

The serpins comprise an ancient superfamily of proteins, found abundantly in eukaryotes and even in some bacteria and archea, that have evolved to regulate proteases of both serine and cysteine mechanistic classes. Unlike the thermodynamically determined lock-and-key type inhibitors, such as those of the Kunitz and Kazal families, serpins use conformational change and consequent kinetic trapping of an enzyme intermediate to effect inhibition. By combining interactions of both an exposed reactive center loop and exosites outside this loop with the active site and complementary exosites on the target protease, serpins can achieve remarkable specificity. Together with the frequent use of regulatory cofactors, this permits a sophisticated time- and location-dependent mode of protease regulation. An understanding of the structure and function of serpins has suggested that they may provide novel scaffolds for engineering protease inhibitors of desired specificity for therapeutic use.

Development of inhibitors of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serine protease inhibitor super family (serpin) and is the primary inhibitor of both the tissue-type (tPA) and urokinase-type (uPA) plasminogen activators. PAI-1 has been implicated in a wide range of pathological processes where it may play a direct role in a variety of diseases. These observations have made PAI-1 an attractive target for small molecule drug development. However, PAI-1's structural plasticity and its capacity to interact with multiple ligands have made the identification and development of such small molecule PAI-1 inactivating agents challenging. In the following pages, we discuss the difficulties associated with screening for small molecule inactivators of PAI-1, in particular, and of serpins, in general. We discuss strategies for high-throughput screening (HTS) of chemical and natural product libraries, and validation steps necessary to confirm identified hits. Finally, we describe steps essential to confirm specificity of active compounds, and strategies to examine potential mechanisms of compound action.

tPA activates LDL receptor-related protein 1-mediated mitogenic signaling involving the p90RSK and GSK3beta pathway

In renal fibrosis, interstitial fibroblasts have an increased proliferative phenotype, and the numbers of interstitial fibroblasts closely correlate with the extent of kidney damage. The mechanisms underlying proliferation and resulting expansion of the interstitium remain largely unknown. Here we define the intracellular signaling events by which tissue plasminogen activator (tPA) promotes renal interstitial fibroblast proliferation. tPA promoted the proliferation of renal interstitial fibroblasts independent of its protease activity. The mitogenic effect of tPA required Tyr(4507) phosphorylation of the cytoplasmic tail of its receptor LDL receptor-related protein 1. tPA triggered sequential proliferative signaling events involving Erk1/2, p90RSK, GSK3β phosphorylation, and cyclin D1 induction. Blockade of Erk1/2 activation or knockdown of p90RSK suppressed tPA-induced GSK3β phosphorylation, cyclin D1 expression, and fibroblast proliferation. In contrast, expression of constitutively active Mek1 mimicked tPA in inducing GSK3β phosphorylation and cyclin D1 expression. Ectopic overexpression of an uninhibitable GSK3β mutant eliminated tPA-induced cyclin D1 expression. In the murine obstruction model, tPA deficiency reduced renal GSK3β phosphorylation and induction of PCNA and FSP-1. These findings show that tPA induces Tyr(4507) phosphorylation of LDL receptor-related protein 1, which in turn leads to the downstream phosphorylation of Erk1/2, p90RSK, and GSK3β, followed by the induction of cyclin D1 in murine interstitial fibroblasts. This study implicates tPA as a mitogen that promotes interstitial fibroblast proliferation, leading to expansion of these cells.

Interactions of plasminogen activator inhibitor-1 with vitronectin involve an extensive binding surface and induce mutual conformational rearrangements

In order to explore early events during the association of plasminogen activator inhibitor-1 (PAI-1) with its cofactor vitronectin, we have applied a robust strategy that combines protein engineering, fluorescence spectroscopy, and rapid reaction kinetics. Fluorescence stopped-flow experiments designed to monitor the rapid association of PAI-1 with vitronectin indicate a fast, concentration-dependent, biphasic binding of PAI-1 to native vitronectin but only a monophasic association with the somatomedin B (SMB) domain, suggesting that multiple phases of the binding interaction occur only when full-length vitronectin is present. Nonetheless, in all cases, the initial fast interaction is followed by slower fluorescence changes attributed to a conformational change in PAI-1. Complementary experiments using an engineered, fluorescently silent PAI-1 with non-natural amino acids showed that concomitant structural changes occur as well in native vitronectin. Furthermore, we have measured the effect of vitronectin on the rate of insertion of the reactive center loop into beta-sheet A of PAI-1 during reaction with target proteases. With a variety of PAI-1 variants, we observe that both full-length vitronectin and the SMB domain have protease-specific effects on the rate of loop insertion but that the two exhibit clearly different effects. These results support a model for PAI-1 binding to vitronectin in which the interaction surface extends beyond the region of PAI-1 occupied by the SMB domain. In support of this model are recent results that define a PAI-1-binding site on vitronectin that lies outside the somatomedin B domain (Schar, C. R., Blouse, G. E., Minor, K. H., and Peterson, C. B. (2008) J. Biol. Chem. 283, 10297-10309) and the complementary site on PAI-1 (Schar, C. R., Jensen, J. K., Christensen, A., Blouse, G. E., Andreasen, P. A., and Peterson, C. B. (2008) J. Biol. Chem. 283, 28487-28496).

Engineering functional antithrombin exosites in alpha1-proteinase inhibitor that specifically promote the inhibition of factor Xa and factor IXa

We have previously shown that residues Tyr-253 and Glu-255 in the serpin antithrombin function as exosites to promote the inhibition of factor Xa and factor IXa when the serpin is conformationally activated by heparin. Here we show that functional exosites can be engineered at homologous positions in a P1 Arg variant of the serpin alpha1-proteinase inhibitor (alpha1PI) that does not require heparin for activation. The combined effect of the two exosites increased the association rate constant for the reactions of alpha1PI with factors Xa and IXa 11-14-fold, comparable with their rate-enhancing effects on the reactions of heparin-activated antithrombin with these proteases. The effects of the engineered exosites were specific, alpha1PI inhibitor reactions with trypsin and thrombin being unaffected. Mutation of Arg-150 in factor Xa, which interacts with the exosite residues in heparin-activated antithrombin, abrogated the ability of the engineered exosites in alpha1PI to promote factor Xa inhibition. Binding studies showed that the exosites enhance the Michaelis complex interaction of alpha1PI with S195A factor Xa as they do with the heparin-activated antithrombin interaction. Replacement of the P4-P2 AIP reactive loop residues in the alpha1PI exosite variant with a preferred IEG substrate sequence for factor Xa modestly enhanced the reactivity of the exosite mutant inhibitor with factor Xa by approximately 2-fold but greatly increased the selectivity of alpha1PI for inhibiting factor Xa over thrombin by approximately 1000-fold. Together, these results show that a specific and selective inhibitor of factor Xa can be engineered by incorporating factor Xa exosite and reactive site recognition determinants in a serpin.

Kinetic characterization of the protein Z-dependent protease inhibitor reaction with blood coagulation factor Xa

Protein Z-dependent protease inhibitor (ZPI) is a recently identified member of the serpin superfamily that functions as a cofactor-dependent regulator of blood coagulation factors Xa (FXa) and XIa. Here we show that ZPI and its cofactor, protein Z (PZ), inhibit procoagulant membrane-bound factor Xa by the branched pathway acyl-intermediate trapping mechanism used by other serpins, but with significant variations of this mechanism that are unique to ZPI. Rapid kinetic analyses showed that the reaction proceeded by the initial assembly of a membrane-associated PZ-ZPI-FXa Michaelis complex (K(M) 53+/-5 nM) followed by conversion to a stable ZPI-FXa complex (k(lim) 1.2+/-0.1 s(-1)). Cofactor premixing experiments together with independent kinetic analyses of ZPI-PZ and factor Xa-PZ-membrane complex formation suggested that assembly of the Michaelis complex through either ZPI-PZ-lipid or factor Xa-PZ-lipid intermediates was rate-limiting. Reaction stoichiometry analyses and native PAGE showed that for every factor Xa molecule inhibited by ZPI, two serpin molecules were cleaved. Native PAGE and immunoblotting showed that PZ dissociated from ZPI once ZPI forms a stable complex with FXa, and kinetic analyses confirmed that PZ acted catalytically to accelerate the membrane-dependent ZPI-factor Xa reaction. The ZPI-FXa complex was only transiently stable and dissociated with a rate constant that showed a bell-shaped pH dependence indicative of participation of factor Xa active-site residues. The complex was detectable by SDS-PAGE when denatured at low pH, consistent with it being a kinetically trapped covalent acyl-intermediate. Together our findings show that ZPI functions like other serpins to regulate the activity of FXa but in a manner uniquely dependent on protein Z, procoagulant membranes, and pH.

Structural differences between active forms of plasminogen activator inhibitor type 1 revealed by conformationally sensitive ligands

Plasminogen activator inhibitor type 1 (PAI-1) is a serine protease inhibitor (serpin) in which the reactive center loop (RCL) spontaneously inserts into a central beta-sheet, beta-sheet A, resulting in inactive inhibitor. Available x-ray crystallographic studies of PAI-1 in an active conformation relied on the use of stabilizing mutations. Recently it has become evident that these structural models do not adequately explain the behavior of wild-type PAI-1 (wtPAI-1) in solution. To probe the structure of native wtPAI-1, we used three conformationally sensitive ligands: the physiologic cofactor, vitronectin; a monoclonal antibody, 33B8, that binds preferentially to RCL-inserted forms of PAI-1; and RCL-mimicking peptides that insert into beta-sheet A. From patterns of interaction with wtPAI-1 and the stable mutant, 14-1B, we propose a model of the native conformation of wtPAI-1 in which the bottom of the central sheet is closed, whereas the top of the beta-sheet A is open to allow partial insertion of the RCL. Because the incorporation of RCL-mimicking peptides into wtPAI-1 is accelerated by vitronectin, we further propose that vitronectin alters the conformation of the RCL to allow increased accessibility to beta-sheet A, yielding a structural hypothesis that is contradictory to the current structural model of PAI-1 in solution and its interaction with vitronectin.

Tissue-type plasminogen activator promotes murine myofibroblast activation through LDL receptor-related protein 1-mediated integrin signaling

The activation of interstitial fibroblasts to become alpha-SMA-positive myofibroblasts is an essential step in the evolution of chronic kidney fibrosis, as myofibroblasts are responsible for the production and deposition of the ECM components that are a hallmark of the disease. Here we describe a signaling pathway that leads to this activation. Tissue-type plasminogen activator (tPA) promoted TGF-beta1-mediated alpha-SMA and type I collagen expression in rat kidney interstitial fibroblasts. This fibrogenic effect was independent of its protease activity but required its membrane receptor, the LDL receptor-related protein 1 (LRP-1). In rat kidney fibroblasts, tPA induced rapid LRP-1 tyrosine phosphorylation and enhanced beta1 integrin recruitment by facilitating the LRP-1/beta1 integrin complex formation. Blockade or knockdown of beta1 integrin abolished type I collagen and alpha-SMA expression. Furthermore, inhibition of the integrin-linked kinase (ILK), a downstream effector of beta1 integrin, or disruption of beta1 integrin/ILK engagement, abrogated the tPA action, whereas ectopic expression of ILK mimicked tPA in promoting myofibroblast activation. In murine renal interstitium after obstructive injury, tPA and alpha-SMA colocalized with LRP-1, and tPA deficiency reduced LRP-1/beta1 integrin interaction and myofibroblast activation. These findings show that tPA induces LRP-1 tyrosine phosphorylation, which in turn facilitates the LRP-1-mediated recruitment of beta1 integrin and downstream ILK signaling, thereby leading to myofibroblast activation. This study implicates tPA as a fibrogenic cytokine that promotes the progression of kidney fibrosis.

tPA protects renal interstitial fibroblasts and myofibroblasts from apoptosis

Activation and expansion of interstitial fibroblasts and myofibroblasts play an essential role in the evolution of renal fibrosis. After obstructive injury, mice lacking tissue-type plasminogen activator (tPA) have fewer myofibroblasts and less interstitial fibrosis than wild-type controls. This suggests that tPA controls the size of the fibroblast/myofibroblast population in vivo, and this study sought to determine the underlying mechanism. In vitro, tPA inhibited staurosporine or H(2)O(2)-induced caspase-3 activation, prevented cellular DNA fragmentation, and suppressed the release of cytochrome C from mitochondria into the cytosol in a rat interstitial fibroblast cell line (NRK-49F). tPA also protected TGF-beta1-activated myofibroblasts from apoptosis. This antiapoptotic effect of tPA was independent of its protease activity but required its membrane receptor, the LDL receptor-related protein 1 (LRP-1). Deletion or knockdown of LRP-1 abolished tPA-mediated cell survival, whereas re-introduction of an LRP-1 minigene in a mouse LRP-1-deficient fibroblast cell line (PEA-13) restored the cytoprotective ability of tPA. tPA triggered a cascade of survival signaling involving extracellular signal-regulated kinase 1/2 (Erk1/2), p90RSK, and phosphorylation of Bad. Blockade of Erk1/2 activation abrogated the antiapoptotic effect of tPA, whereas expression of constitutively active MEK1 promoted cell survival similar to tPA. In vivo, compared with wild-type controls, apoptosis of interstitial myofibroblasts was increased in tPA(-/-) mice after obstructive injury, and myofibroblasts were completely depleted 4 wk after relief of the obstruction. Together, these findings illustrate that tPA is a survival factor that prevents apoptosis of renal interstitial fibroblasts and myofibroblasts through an LRP-1-, Erk1/2-, p90RSK-, and Bad-dependent mechanism.

Short-lived protease serpin complexes: partial disruption of the rat trypsin active site

Serpins inhibit serine proteases by mechanically disrupting the protease active site. The protease first reacts with the serpin's reactive center loop (RCL) to form an acylenzyme. Then the RCL inserts into a beta-sheet in the body of the serpin, translocating the attached protease approximately 70 A and deforming the protease active site, thereby trapping the acylenzyme. Loop insertion (approximately 1 s(-1)) is an order of magnitude slower than hydrolysis of a typical substrate acylenzyme (approximately 50 s(-1)), indicating that the protease is inhibited during translocation. We have previously trapped a partially translocated covalent complex of rat trypsin and alpha1-proteinase inhibitor (EpartI*) resulting from attractive interactions between cationic dyes and anionic rat trypsin. Here, using single pair Förster resonance energy transfer, we demonstrate that EpartI* is a metastable complex that can dissociate to free protease and cleaved serpin (I*) as well as convert to the canonical fully translocated complex EfullI*. The partitioning between these two pathways is pH dependent, with conversion favored at low pH and dissociation favored at high pH. The short lifetime of EpartI* (approximately 3 h at pH 7.4) and the pH dependence of EpartI* dissociation suggest that, unlike in EfullI*, the catalytic triad is intact in EpartI*. These results also demonstrate that interactions between target proteases and the body of the serpin can hinder protease translocation leading to short-lived covalent complexes.

A deletion mutant of vitronectin lacking the somatomedin B domain exhibits residual plasminogen activator inhibitor-1-binding activity

Vitronectin and plasminogen activator inhibitor-1 (PAI-1) are important physiological binding partners that work in concert to regulate cellular adhesion, migration, and fibrinolysis. The high affinity binding site for PAI-1 is located within the N-terminal somatomedin B domain of vitronectin; however, several studies have suggested a second PAI-1-binding site within vitronectin. To investigate this secondary site, a vitronectin mutant lacking the somatomedin B domain (rDeltasBVN) was engineered. The short deletion had no effect on heparin-binding, integrin-binding, or cellular adhesion. Binding to the urokinase receptor was completely abolished while PAI-1 binding was still observed, albeit with a lower affinity. Analytical ultracentrifugation on the PAI-1-vitronectin complex demonstrated that increasing NaCl concentration favors 1:1 versus 2:1 PAI-1-vitronectin complexes and hampers formation of higher order complexes, pointing to the contribution of charge-charge interactions for PAI-1 binding to the second site. Furthermore, fluorescence resonance energy transfer between differentially labeled PAI-1 molecules confirmed that two independent molecules of PAI-1 are capable of binding to vitronectin. These results support a model for the assembly of higher order PAI-1-vitronectin complexes via two distinct binding sites in both proteins.

Serpins in plants and green algae

Control of proteolysis is important for plant growth, development, responses to stress, and defence against insects and pathogens. Members of the serpin protein family are likely to play a critical role in this control through irreversible inhibition of endogenous and exogenous target proteinases. Serpins have been found in diverse species of the plant kingdom and represent a distinct clade among serpins in multicellular organisms. Serpins are also found in green algae, but the evolutionary relationship between these serpins and those of plants remains unknown. Plant serpins are potent inhibitors of mammalian serine proteinases of the chymotrypsin family in vitro but, intriguingly, plants and green algae lack endogenous members of this proteinase family, the most common targets for animal serpins. An Arabidopsis serpin with a conserved reactive centre is now known to be capable of inhibiting an endogenous cysteine proteinase. Here, knowledge of plant serpins in terms of sequence diversity, inhibitory specificity, gene expression and function is reviewed. This was advanced through a phylogenetic analysis of amino acid sequences of expressed plant serpins, delineation of plant serpin gene structures and prediction of inhibitory specificities based on identification of reactive centres. The review is intended to encourage elucidation of plant serpin functions.

Mechanism by Which Exosites Promote the Inhibition of Blood Coagulation Proteases by Heparin-activated Antithrombin

Heparin activates the serpin, antithrombin, to inhibit its target blood-clotting proteases by generating new protease interaction exosites. To resolve the effects of these exosites on the initial Michaelis docking step and the subsequent acylation and conformational change steps of antithrombin-protease reactions, we compared the reactions of catalytically inactive S195A and active proteases with site-specific fluorophore-labeled antithrombins that allow monitoring of these reaction steps. Heparin bound to N,N'-dimethyl-N-(acetyl)-N'-(7-nitrobenz-3-oxa-1,3-diazol-4-yl)ethylenediamine (NBD)-fluorophore-labeled antithrombins and accelerated the reactions of the labeled inhibitor with thrombin and factor Xa similar to wild type. Equilibrium binding of NBD-labeled antithrombins to S195A proteases showed that exosites generated by conformationally activating antithrombin with a heparin pentasaccharide enhanced the affinity of the serpin for S195A factor Xa minimally 100-fold. Moreover, additional bridging exosites provided by a hexadecasaccharide heparin activator enhanced antithrombin affinity for both S195A factor Xa and thrombin at least 1000-fold. Rapid kinetic studies showed that these exosite-mediated enhancements in Michaelis complex affinity resulted from increases in k(on) and decreases in k(off) and caused antithrombin-protease reactions to become diffusion-controlled. Competitive binding and kinetic studies with exosite mutant antithrombins showed that Tyr-253 was a critical mediator of exosite interactions with S195A factor Xa; that Glu-255, Glu-237, and Arg-399 made more modest contributions to these interactions; and that exosite interactions reduced k(off) for the Michaelis complex interaction. Together these results show that exosites generated by heparin activation of antithrombin function both to promote the formation of an initial antithrombin-protease Michaelis complex and to favor the subsequent acylation of this complex.

Neutralization of Plasminogen Activator Inhibitor I (PAI-1) by the Synthetic Antagonist PAI-749 via a Dual Mechanism of Action

PAI-749 is a potent and selective synthetic antagonist of plasminogen activator inhibitor 1 (PAI-1) that preserved tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA) activities in the presence of PAI-1 (IC(50) values, 157 and 87 nM, respectively). The fluorescence (Fl) of fluorophore-tagged PAI-1 (PAI-NBD119) was quenched by PAI-749; the apparent K(d) (254 nM) was similar to the IC(50) (140 nM) for PAI-NBD119 inactivation. PAI-749 analogs displayed the same potency rank order for neutralizing PAI-1 activity and perturbing PAI-NBD119 Fl; hence, binding of PAI-749 to PAI-1 and inactivation of PAI-1 activity are tightly linked. Exposure of PAI-1 to PAI-749 for 5 min (sufficient for full inactivation) followed by PAI-749 sequestration with Tween 80 micelles yielded active PAI-1; thus, PAI-749 did not irreversibly inactivate PAI-1, a known metastable protein. Treatment of PAI-1 with a PAI-749 homolog (producing less assay interference) blocked the ability of PAI-1 to displace p-aminobenzamidine from the uPA active site. Consistent with this observation, PAI-749 abolished formation of the SDS-stable tPA/PAI-1 complex. PAI-749-mediated neutralization of PAI-1 was associated with induction of PAI-1 polymerization as assessed by native gel electrophoresis. PAI-749 did not turn PAI-1 into a substrate for tPA; however, PAI-749 promoted plasmin-mediated degradation of PAI-1. In conclusion, PAI-1 inactivation by PAI-749 using purified components can result from a dual mechanism of action. First, PAI-749 binds directly to PAI-1, blocks PAI-1 from accessing the active site of tPA, and abrogates formation of the SDS-stable tPA/PAI-1 complex. Second, binding of PAI-749 to PAI-1 renders PAI-1 vulnerable to plasmin-mediated proteolytic degradation.

Modulation of serpin reaction through stabilization of transient intermediate by ligands bound to alpha-helix F

Mechanism-based inhibition of proteinases by serpins involves enzyme acylation and fast insertion of the reactive center loop (RCL) into the central beta-sheet of the serpin, resulting in mechanical inactivation of the proteinase. We examined the effects of ligands specific to alpha-helix F (alphaHF) of plasminogen activator inhibitor-1 (PAI-1) on the stoichiometry of inhibition (SI) and limiting rate constant (k(lim)) of RCL insertion for reactions with beta-trypsin, tissue-type plasminogen activator (tPA), and urokinase. The somatomedin B domain of vitronectin (SMBD) did not affect SI for any proteinase or k(lim) for tPA but decreased the k(lim) for beta-trypsin. In contrast to SMBD, monoclonal antibodies MA-55F4C12 and MA-33H1F7, the epitopes of which are located at the opposite side of alphaHF, decreased k(lim) and increased SI for every enzyme. These effects were enhanced in the presence of SMBD. RCL insertion for beta-trypsin and tPA is limited by different subsequent steps of PAI-1 mechanism as follows: enzyme acylation and formation of a loop-displaced acyl complex (LDA), respectively. Stabilization of LDA through the disruption of the exosite interactions between PAI-1 and tPA induced an increase in the k(lim) but did not affect the SI. Thus it is unlikely that LDA contributes significantly to the outcome of the serpin reaction. These results demonstrate that the rate of RCL insertion is not necessarily correlated with SI and indicate that an intermediate, different from LDA, which forms during the late steps of PAI-1 mechanism, and could be stabilized by ligands specific to alphaHF, controls bifurcation between the inhibitory and the substrate pathways.

Maspin Binds to Urokinase-type and Tissue-type Plasminogen Activator through Exosite-Exosite Interactions

Maspin is a member of the serpin family with a reactive center loop that is incompatible with proteinase inhibition by the serpin conformational change mechanism. Despite this there are reports that maspin might regulate uPA-dependent processes in vivo. Using exogenous and endogenous fluorescence, we demonstrate here that maspin can bind uPA and tPA in both single-chain and double-chain forms, with K(d) values between 300 and 600 nM. Binding is at an exosite on maspin close to, but outside of, the reactive center loop and is therefore insensitive to mutation of Arg(340) within the reactive center loop. The binding site on tPA does not involve the proteinase active site, with the result that maspin can bind to S195A tPA that is already complexed to plasminogen activator inhibitor-1. The ability of maspin to bind these proteinases without involvement of the reactive center loop leaves the latter free to engage in additional, as yet unidentified, maspin-protein interactions that may serve to regulate the properties of the exosite-bound proteinase. This may help to reconcile apparently conflicting studies that demonstrate the importance of the reactive center loop in certain maspin functions, despite the inability of maspin to directly inhibit tPA or uPA catalytic activity in in vitro assays through engagement between its reactive center loop and the active site of the proteinase.