PAI-1 stability: the role of histidine residues

Targeting PAI-1 in Cardiovascular Disease: Structural Insights Into PAI-1 Functionality and Inhibition

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor (serpin) superfamily with antiprotease activity, is the main physiological inhibitor of tissue-type (tPA) and urokinase-type (uPA) plasminogen activators (PAs). Apart from being crucially involved in fibrinolysis and wound healing, PAI-1 plays a pivotal role in various acute and chronic pathophysiological processes, including cardiovascular disease, tissue fibrosis, cancer, and age-related diseases. In the prospect of treating the broad range of PAI-1-related pathologies, many efforts have been devoted to developing PAI-1 inhibitors. The use of these inhibitors, including low molecular weight molecules, peptides, antibodies, and antibody fragments, in various animal disease models has provided ample evidence of their beneficial effect in vivo and moved forward some of these inhibitors in clinical trials. However, none of these inhibitors is currently approved for therapeutic use in humans, mainly due to selectivity and toxicity issues. Furthermore, the conformational plasticity of PAI-1, which is unique among serpins, poses a real challenge in the identification and development of PAI-1 inhibitors. This review will provide an overview of the structural insights into PAI-1 functionality and modulation thereof and will highlight diverse approaches to inhibit PAI-1 activity.

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.

Positive and negative ions of the amino acid histidine formed in low-energy electron collisions

Histidine is an aromatic amino acid crucial for the biological functioning of proteins and enzymes. When biological matter is exposed to ionising radiation, highly energetic particles interact with the surrounding tissue which leads to efficient formation of low-energy electrons. In the present study, the interaction of low-energy electrons with gas-phase histidine is studied at a molecular level in order to extend the knowledge of electron-induced reactions with amino acids. We report both on the formation of positive ions formed by electron ionisation and negative ions induced by electron attachment. The experimental data were complemented by quantum chemical calculations. Specifically, the free energies for possible fragmentation reactions were derived for the τ and the π tautomer of histidine to get insight into the structures of the formed ions and the corresponding neutrals. We report the experimental ionisation energy of (8.48 ± 0.03) eV for histidine which is in good agreement with the calculated vertical ionisation energy. In the case of negative ions, the dehydrogenated parent anion is the anion with the highest mass observed upon dissociative electron attachment. The comparison of experimental and computational results was also performed in view of a possible thermal decomposition of histidine during the experiments, since the sample was sublimated in the experiment by resistive heating of an oven. Overall, the present study demonstrates the effects of electrons as secondary particles in the chemical degradation of histidine. The reactions induced by those electrons differ when comparing positive and negative ion formation. While for negative ions, simple bond cleav ages prevail, the observed fragment cations exhibit partly restructuring of the molecule during the dissociation process.

High-energy collision-induced dissociation of histidine ions [His + H]+ and [His - H]- and histidine dimer [His2 + H]. RAPID COMMUNICATIONS IN MASS SPECTROMETRY : RCM 2018;32:113-120. [PMID: 29108138 DOI: 10.1002/rcm.8027]

RATIONALE

Histidine (His) is an essential amino acid, whose side group consists of an aromatic imidazole moiety that can bind a proton or metal cation and act as a donor in intermolecular interactions in many biological processes. While the dissociation of His monomer ions is well known, information on the kinetic energy released in the dissociation is missing.

METHODS

Using a new home-built electrospray ionization (ESI) source adapted to a double-focusing mass spectrometer of BE geometry, we investigated the fragmentation reactions of protonated and deprotonated His, [His + H]⁺ and [His - H]^- , and the protonated His dimer [His₂  + H]⁺ , accelerated to 6 keV in a high-energy collision with helium gas. We evaluated the kinetic energy release (KER) for the observed dissociation channels.

RESULTS

ESI of His solution in positive mode led to the formation of His clusters [His_n + H]⁺ , n = 1-6, with notably enhanced stability of the tetramer. [His + H]⁺ dissociates predominantly by loss of (H₂ O + CO) with a KER of 278 meV, while the dominant dissociation channel of [His - H]^- involves loss of NH₃ with a high KER of 769 meV. Dissociation of [His₂ + H]⁺ is dominated by loss of the monomer but smaller losses are also observed.

CONCLUSIONS

The KER for HCOOH loss from both [His + H]⁺ and [His - H]^- is similar at 278 and 249 meV, respectively, which suggests that the collision-induced dissociation takes place via a similar mechanism. The loss of COOH and C₂ H₅ NO₂ from the dimer suggests that the dimer of His binds through a shared proton between the imidazole moieties.

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.

Functional stability of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is the main inhibitor of plasminogen activators, such as tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), and a major regulator of the fibrinolytic system. PAI-1 plays a pivotal role in acute thrombotic events such as deep vein thrombosis (DVT) and myocardial infarction (MI). The biological effects of PAI-1 extend far beyond thrombosis including its critical role in fibrotic disorders, atherosclerosis, renal and pulmonary fibrosis, type-2 diabetes, and cancer. The conversion of PAI-1 from the active to the latent conformation appears to be unique among serpins in that it occurs spontaneously at a relatively rapid rate. Latency transition is believed to represent a regulatory mechanism, reducing the risk of thrombosis from a prolonged antifibrinolytic action of PAI-1. Thus, relying solely on plasma concentrations of PAI-1 without assessing its function may be misleading in interpreting the role of PAI-1 in many complex diseases. Environmental conditions, interaction with other proteins, mutations, and glycosylation are the main factors that have a significant impact on the stability of the PAI-1 structure. This review provides an overview on the current knowledge on PAI-1 especially importance of PAI-1 level and stability and highlights the potential use of PAI-1 inhibitors for treating cardiovascular disease.

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.

Structures of importance for the stability of antiplasmin as studied by site-directed mutagenesis

Human antiplasmin, a fast-acting inhibitor of plasmin in plasma, belongs to the serpin super-family of proteins. Like other members of this family, antiplasmin has a scissile peptide bond exposed within a reactive centre loop, typically present at the surface of the molecule. Antiplasmin is stable at neutral pH, but at acidic pH or at elevated temperatures it rapidly becomes inactivated. Data regarding "native" antiplasmin have demonstrated that both polymerization processes and formation of latent molecules are important in this respect. In this work we used site-directed mutagenesis to produce 11 single-site mutants (mainly within Abeta-sheet, Bbeta-sheet and reactive centre loop), which were expressed in Drosophila S2 cells, purified and characterized. Five of the 11 mutants were found to have a deviating stability at decreased pH. Glu346Thr was the only mutant with a lesser stability as compared to wt-antiplasmin, but the other 4 were more stable. The most stable mutant, His341Thr, was 7-fold more stable at pH 4.9 as compared to wt-antiplasmin. The wt-antiplasmin had a much more pronounced tendency to polymerize at decreased pH, as compared to "native" antiplasmin. However, many of the mutants clearly rather formed latent molecules, as judged both from PAGE-analysis at non-denaturing condition and reactivation experiments.

Inactivation of antiplasmin at low pH: evidence for the formation of latent molecules

Several serine proteinase inhibitors (serpins) are metastable proteins which under certain conditions may undergo conformational changes resulting in the insertion of the reactive centre loop into the so-called Abeta-sheet and hence forming latent molecules. Here we have studied the inactivation of antiplasmin as a function of pH and temperature with time. At decreased pH (4.9-5.8) and at room temperature, antiplasmin activity decreased following first-order kinetics. Analysis by polyacrylamide gel electrophoresis under non-denaturing conditions demonstrated that only minor amounts of polymerized material formed after extensive incubation (4 days) at room temperature. However, on incubation at elevated temperatures (45 or 55 degrees C), a rapid formation of polymerized material was observed. We also demonstrated that antiplasmin inactivated by treatment at pH approximately 5 at room temperature spontaneously slowly regained some activity if incubated in a buffer of neutral pH. Furthermore, by treatment with 4 M guanidinium chloride for about 30 min, followed by dialysis against a neutral phosphate buffer, considerable activity (almost 40%) was regained. Thus, we conclude that antiplasmin, at least partially, at lower temperatures is transformed into a latent form, which could be reactivated, in a similar manner as PAI-1. At increased temperature, however, polymerization seems to be the predominant reason for inactivation.

In Vitro Activity Differences between Proteins of the ADF/Cofilin Family Define Two Distinct Subgroups†

The actin depolymerizing factor (ADF)/cofilins are an essential group of proteins that are important regulators of actin filament turnover in vivo. Although protists and yeasts express only a single member of this family, metazoans express two or more members in many cell types. In cells expressing both ADF and cofilin, differences have been reported in the regulation of their expression, their pH sensitivity, and their intracellular distribution. Each member has qualitatively similar interactions with actin, but quantitative differences have been noted. Here we compared quantitative differences between chick ADF and chick cofilin using several assays that measure G-actin binding, actin filament length distribution, and assembly/disassembly dynamics. Quantitative differences were measured in the critical concentrations of the complexes required for assembly, in the effects of nucleotide and divalent metal on actin monomer binding, in pH-dependent severing, in enhancement of filament minus end off-rates, and in steady-state filament length distributions generated in similar mixtures. Some of these assays were used to compare the activities of several ADF/cofilins from across phylogeny, most of which fall into one of two groups based upon their behavior. The ADF-like group has higher affinities for Mg(2+)-ATP-G-actin than the cofilin-like group and a greater pH-dependent depolymerizing activity.

Mutational analysis of plasminogen activator inhibitor-1

The serpin plasminogen activator inhibitor-1 (PAI-1) is a fast and specific inhibitor of the plasminogen activating serine proteases tissue-type and urokinase-type plasminogen activator and, as such, an important regulator in turnover of extracellular matrix and in fibrinolysis. PAI-1 spontaneously loses its antiproteolytic activity by inserting its reactive centre loop (RCL) as strand 4 in beta-sheet A, thereby converting to the so-called latent state. We have investigated the importance of the amino acid sequence of alpha-helix F (hF) and the connecting loop to s3A (hF/s3A-loop) for the rate of latency transition. We grafted regions of the hF/s3A-loop from antithrombin III and alpha1-protease inhibitor onto PAI-1, creating eight variants, and found that one of these reversions towards the serpin consensus decreased the rate of latency transition. We prepared 28 PAI-1 variants with individual residues in hF and beta-sheet A replaced by an alanine. We found that mutating serpin consensus residues always had functional consequences whereas mutating nonconserved residues only had so in one case. Two variants had low but stable inhibitory activity and a pronounced tendency towards substrate behaviour, suggesting that insertion of the RCL is held back during latency transition as well as during complex formation with target proteases. The data presented identify new determinants of PAI-1 latency transition and provide general insight into the characteristic loop-sheet interactions in serpins.

The spontaneous polymerization of plasminogen activator inhibitor type-2 and Z-antitrypsin are due to different molecular aberrations

The wild-type form of plasminogen activator inhibitor type-2 (PAI-2) and the pathogenic Z-mutant of alpha(1)-antitrypsin (alpha(1)AT) are serpins that spontaneously polymerize by the loop-sheet mechanism. Compared to the consensus serpin sequence, both PAI-2 and Z-alpha(1)AT have deviations in the so-called breach region located at the top of the A beta-sheet. In the case of Z-alpha(1)AT, conformational perturbations caused by a single amino acid substitution result in polymerization in vivo and predisposes to disease. To test whether the polymerization of PAI-2 is due to aberrations in the breach region, we constructed substitution mutants of PAI-2 with conserved residues in this region. Analysis of the mutants revealed that deviations in the breach region modulate but are not the major cause of PAI-2 polymerization. Rather, PAI-2 exists in a highly polymerogenic conformation and does not require conformational rearrangements before polymerization can take place.

The molecular basis for anti-proteolytic and non-proteolytic functions of plasminogen activator inhibitor type-1: roles of the reactive centre loop, the shutter region, the flexible joint region and the small serpin fragment

The serine proteinase inhibitor plasminogen activator inhibitor type-1 (PAI-1) is the primary physiological inhibitor of the tissue-type and the urokinase-type plasminogen activator (tPA and uPA, respectively) and as such an important regulator of proteolytic events taking place in the circulation and in the extracellular matrix. Moreover, a few non-proteolytic functions have been ascribed to PAI-1, mediated by its interaction with vitronectin or the interaction between the uPA-PAI-1 complex bound to the uPA receptor and members of the low density lipoprotein receptor family. PAI-1 belongs to the serpin family, characterised by an unusual conformational flexibility, which governs its molecular interactions. In this review we describe the anti-proteolytic and non-proteolytic functions of PAI-1 from both a biological and a biochemical point of view. We will relate the various biological roles of PAI-1 to its biochemistry in general and to the different conformations of PAI-1 in particular. We put emphasis on the intramolecular rearrangements of PAI-1 that are required for its antiproteolytic as well as its non-proteolytic functions.