Engineering the substrate and inhibitor specificities of human coagulation Factor VIIa

Factor XI/XIa Inhibition: The Arsenal in Development for a New Therapeutic Target in Cardio- and Cerebrovascular Disease

Despite major advancements in the development of safer and more effective anticoagulant agents, bleeding complications remain a significant concern in the treatment of thromboembolic diseases. Improvements in our understanding of the coagulation pathways highlights the notion that the contact pathway-specifically factor XI (FXI)-has a greater role in the etiopathogenesis of thrombosis than in physiological hemostasis. As a result, a number of drugs targeting FXI are currently in different stages of testing and development. This article aims to review the different strategies directed towards FXI-inhibition with a brief summation of the agents in clinical development, and to comment on the therapeutic areas that could be explored for potential indications. Therapeutics targeting FXI/FXIa inhibition have the potential to usher in a new era of anticoagulation therapy.

Iripin-3, a New Salivary Protein Isolated From Ixodes ricinus Ticks, Displays Immunomodulatory and Anti-Hemostatic Properties In Vitro

Tick saliva is a rich source of pharmacologically and immunologically active molecules. These salivary components are indispensable for successful blood feeding on vertebrate hosts and are believed to facilitate the transmission of tick-borne pathogens. Here we present the functional and structural characterization of Iripin-3, a protein expressed in the salivary glands of the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Belonging to the serpin superfamily of protease inhibitors, Iripin-3 strongly inhibited the proteolytic activity of serine proteases kallikrein and matriptase. In an in vitro setup, Iripin-3 was capable of modulating the adaptive immune response as evidenced by reduced survival of mouse splenocytes, impaired proliferation of CD4+ T lymphocytes, suppression of the T helper type 1 immune response, and induction of regulatory T cell differentiation. Apart from altering acquired immunity, Iripin-3 also inhibited the extrinsic blood coagulation pathway and reduced the production of pro-inflammatory cytokine interleukin-6 by lipopolysaccharide-stimulated bone marrow-derived macrophages. In addition to its functional characterization, we present the crystal structure of cleaved Iripin-3 at 1.95 Å resolution. Iripin-3 proved to be a pluripotent salivary serpin with immunomodulatory and anti-hemostatic properties that could facilitate tick feeding via the suppression of host anti-tick defenses. Physiological relevance of Iripin-3 activities observed in vitro needs to be supported by appropriate in vivo experiments.

Recent advances in the discovery and development of factor XI/XIa inhibitors

Factor XIa (FXIa) is a serine protease homodimer that belongs to the intrinsic coagulation pathway. FXIa primarily catalyzes factor IX activation to factor IXa, which subsequently activates factor X to factor Xa in the common coagulation pathway. Growing evidence suggests that FXIa plays an important role in thrombosis with a relatively limited contribution to hemostasis. Therefore, inhibitors targeting factor XI (FXI)/FXIa system have emerged as a paradigm-shifting strategy so as to develop a new generation of anticoagulants to effectively prevent and/or treat thromboembolic diseases without the life-threatening risk of internal bleeding. Several inhibitors of FXI/FXIa proteins have been discovered or designed over the last decade including polypeptides, active site peptidomimetic inhibitors, allosteric inhibitors, antibodies, and aptamers. Antisense oligonucleotides (ASOs), which ultimately reduce the hepatic biosynthesis of FXI, have also been introduced. A phase II study, which included patients undergoing elective primary unilateral total knee arthroplasty, revealed that a specific FXI ASO effectively protects patients against venous thrombosis with a relatively limited risk of bleeding. Initial findings have also demonstrated the potential of FXI/FXIa inhibitors in sepsis, listeriosis, and arterial hypertension. This review highlights various chemical, biochemical, and pharmacological aspects of FXI/FXIa inhibitors with the goal of advancing their development toward clinical use.

A plasma proteolysis pathway comprising blood coagulation proteases

Coagulation factors are essential for hemostasis. Here, we show that these factors also team up to degrade plasma proteins that are unrelated to hemostasis. Prolidase, SRC and amyloid β1-42 (Aβ1-42) are used as probes. Each probe, upon entering the blood circulation, binds and activates factor XII (FXII), triggering the intrinsic and common coagulation cascades, which in turn activate factor VII, a component of the extrinsic coagulation cascade. Activated factor VII (FVIIa) rapidly degrades the circulating probes. Therefore, FXII and FVIIa serve as the sensor/initiator and executioner, respectively, for the proteolysis pathway. Moreover, activation of this pathway by one probe leads to the degradation of all three probes. Significant activation of this pathway follows tissue injury and may also occur in other disorders, e.g., Alzheimer's disease, of which Aβ1-42 is a key driver. However, enoxaparin, a clinically used anticoagulant, inhibits the proteolysis pathway and elevates plasma levels of the probes. Enoxaparin may also mitigate potential impact of activators of the proteolysis pathway on coagulation. Our results suggest that the proteolysis pathway is important for maintaining low levels of various plasma proteins. Our finding that enoxaparin inhibits this pathway provides a means to control it. Inhibition of this pathway may facilitate the development of disease biomarkers and protein therapeutics, e.g., plasma Aβ1-42 as a biomarker of Alzheimer's disease or recombinant human prolidase as an antitumor agent.

Anticoagulants inhibit proteolytic clearance of plasma amyloid beta

We recently discovered a plasma proteolysis pathway, termed the FXII-FVII pathway which is composed of coagulation proteases, and found it to be mainly responsible for the clearance of Aβ42 in the plasma in mice. Aβ42 and Aβ40 are the main Aβ forms in Alzheimer’s disease (AD). In the present study, in vitro assays, wild type (WT) mice and J20 mice (a transgenic AD model) are used to assess the degradation of Aβ40 and Aβ42 by the FXII-FVII pathway and the impact of anticoagulants on such degradation. Four clinically available and mechanistically distinct anticoagulants are evaluated, including dabigatran, enoxaparin (EP), rivaroxaban and warfarin. Each anticoagulant significantly elevates plasma level of synthetic Aβ42 in WT mice, among which EP is the most effective. The differential efficacies of the anticoagulants in elevating plasma Aβ42 level match closely with their inhibitory mechanisms towards the FXII-FVII pathway. Plasma Aβ40 is also degraded by the FXII-FVII pathway and is protected by EP. Moreover, the FXII-FVII pathway is significantly activated in J20 mice, but EP inhibits the activation and significantly elevates plasma levels of both Aβ40 and Aβ42. Taken together, our results shed new light on Aβ metabolism, reveal a novel function of anticoagulants, and suggest a novel approach to potentially developing plasma Aβ as an AD biomarker.

Coagulation factor VII variants resistant to inhibitory antibodies

Replacement therapy is currently used to prevent and treat bleeding episodes in coagulation factor deficiencies. However, structural differences between the endogenous and therapeutic proteins might increase the risk for immune complications. This study was aimed at identifying factor (F)VII variants resistant to inhibitory antibodies developed after treatment with recombinant activated factor VII (rFVIIa) in a FVII-deficient patient homozygous for the p.A354V-p.P464Hfs mutation, which predicts trace levels of an elongated FVII variant in plasma. We performed fluorescent bead-based binding, ELISA-based competition as well as fluorogenic functional (activated FX and thrombin generation) assays in plasma and with recombinant proteins. We found that antibodies displayed higher affinity for the active than for the zymogen FVII (half-maximal binding at 0.54 ± 0.04 and 0.78 ± 0.07 BU/ml, respectively), and inhibited the coagulation initiation phase with a second-order kinetics. Isotypic analysis showed a polyclonal response with a large predominance of IgG1. We hypothesised that structural differences in the carboxyl-terminus between the inherited FVII and the therapeutic molecules contributed to the immune response. Intriguingly, a naturally-occurring, poorly secreted and 5-residue truncated FVII (FVII-462X) escaped inhibition. Among a series of truncated rFVII molecules, we identified a well-secreted and catalytically competent variant (rFVII-464X) with reduced binding to antibodies (half-maximal binding at 0.198 ± 0.003 BU/ml) as compared to the rFVII-wt (0.032 ± 0.002 BU/ml), which led to a 40-time reduced inhibition in activated FX generation assays. Taken together our results provide a paradigmatic example of mutation-related inhibitory antibodies, strongly support the FVII carboxyl-terminus as their main target and identify inhibitor-resistant FVII variants.

Engineering of a membrane-triggered activity switch in coagulation factor VIIa

Coagulation factor VIIa (FVIIa) is an intrinsically poor serine protease that requires assistance from its cofactor tissue factor (TF) to trigger the extrinsic pathway of blood coagulation. TF stimulates FVIIa through allosteric maturation of its active site and by facilitating substrate recognition. The surface dependence of the latter property allowed us to design a potent membrane-triggered activity switch in FVIIa by engineering a disulfide cross-link between an allosterically silent FVIIa variant and soluble TF. These results show that optimization of substrate recognition remote from the active site represents a promising new route to simultaneously enhance and localize the procoagulant activity of FVIIa for therapeutic purposes.

Recombinant factor VIIa (FVIIa) variants with increased activity offer the promise to improve the treatment of bleeding episodes in patients with inhibitor-complicated hemophilia. Here, an approach was adopted to enhance the activity of FVIIa by selectively optimizing substrate turnover at the membrane surface. Under physiological conditions, endogenous FVIIa engages its cell-localized cofactor tissue factor (TF), which stimulates activity through membrane-dependent substrate recognition and allosteric effects. To exploit these properties of TF, a covalent complex between FVIIa and the soluble ectodomain of TF (sTF) was engineered by introduction of a nonperturbing cystine bridge (FVIIa Q64C-sTF G109C) in the interface. Upon coexpression, FVIIa Q64C and sTF G109C spontaneously assembled into a covalent complex with functional properties similar to the noncovalent wild-type complex. Additional introduction of a FVIIa-M306D mutation to uncouple the sTF-mediated allosteric stimulation of FVIIa provided a final complex with FVIIa-like activity in solution, while exhibiting a two to three orders-of-magnitude increase in activity relative to FVIIa upon exposure to a procoagulant membrane. In a mouse model of hemophilia A, the complex normalized hemostasis upon vascular injury at a dose of 0.3 nmol/kg compared with 300 nmol/kg for FVIIa.

Interdependence of Inhibitor Recognition in HIV-1 Protease

Molecular recognition is a highly interdependent process. Subsite couplings within the active site of proteases are most often revealed through conditional amino acid preferences in substrate recognition. However, the potential effect of these couplings on inhibition and thus inhibitor design is largely unexplored. The present study examines the interdependency of subsites in HIV-1 protease using a focused library of protease inhibitors, to aid in future inhibitor design. Previously a series of darunavir (DRV) analogs was designed to systematically probe the S1′ and S2′ subsites. Co-crystal structures of these analogs with HIV-1 protease provide the ideal opportunity to probe subsite interdependency. All-atom molecular dynamics simulations starting from these structures were performed and systematically analyzed in terms of atomic fluctuations, intermolecular interactions, and water structure. These analyses reveal that the S1′ subsite highly influences other subsites: the extension of the hydrophobic P1′ moiety results in 1) reduced van der Waals contacts in the P2′ subsite, 2) more variability in the hydrogen bond frequencies with catalytic residues and the flap water, and 3) changes in the occupancy of conserved water sites both proximal and distal to the active site. In addition, one of the monomers in this homodimeric enzyme has atomic fluctuations more highly correlated with DRV than the other monomer. These relationships intricately link the HIV-1 protease subsites and are critical to understanding molecular recognition and inhibitor binding. More broadly, the interdependency of subsite recognition within an active site requires consideration in the selection of chemical moieties in drug design; this strategy is in contrast to what is traditionally done with independent optimization of chemical moieties of an inhibitor.

Structure-activity relationship studies for multitarget antithrombotic drugs

Thrombosis is a complex process involving multiple pathways. Currently, therapy relies on the combination of two or more antithrombotic drugs, showing that inhibiting more than one target provides benefits in the prevention and treatment of thrombosis. This review focuses on structure-activity relationship studies of molecules possessing multiple actions against thrombosis, namely, dual inhibitors of coagulation, dual inhibitors of coagulation and platelet aggregation, and also dual inhibitors of platelet aggregation. EP217609 has just entered clinical trials, which raise the expectations on the multitarget strategy to prevent or treat thrombosis.

Differential functional readthrough over homozygous nonsense mutations contributes to the bleeding phenotype in coagulation factor VII deficiency

Essentials Potentially null homozygous Factor(F)7 nonsense mutations are associated to variable bleeding symptoms. Readthrough of p.Ser112X (life-threatening) and p.Cys132X (moderate) stop codons was investigated. Readthrough-mediated insertion of wild-type or tolerated residues produce functional proteins. Functional readthrough over homozygous F7 nonsense mutations contributes to the bleeding phenotype.

SUMMARY

Background Whereas the rare homozygous nonsense mutations causing factor (F)VII deficiency may predict null conditions that are almost completely incompatible with life, they are associated with appreciable differences in hemorrhagic symptoms. The misrecognition of premature stop codons (readthrough) may account for variable levels of functional full-length proteins. Objectives To experimentally evaluate the basal and drug-induced levels of FVII resulting from the homozygous p.Cys132X and p.Ser112X nonsense mutations that are associated with moderate (132X) or life-threatening (112X) symptoms, and that are predicted to undergo readthrough with (132X) or without (112X) production of wild-type FVII. Methods We transiently expressed recombinant FVII (rFVII) nonsense and missense variants in human embryonic kidney 293 cells, and evaluated secreted FVII protein and functional levels by ELISA, activated FX generation, and coagulation assays. Results The levels of functional FVII produced by p.Cys132X and p.Ser112X mutants (rFVII-132X, 1.1% ± 0.2% of wild-type rFVII; rFVII-112X, 0.5% ± 0.1% of wild-type rFVII) were compatible with the occurrence of spontaneous readthrough, which was magnified by the addition of G418 - up to 12% of the wild-type value for the rFVII-132X nonsense variant. The predicted missense variants arising from readthrough abolished (rFVII-132Trp/Arg) or reduced (rFVII-112Trp/Cys/Arg, 22-45% of wild-type levels) secretion and function. These data suggest that the appreciable rescue of p.Cys132X function was driven by reinsertion of the wild-type residue, whereas the minimal p.Ser112X function was explained by missense changes permitting FVII secretion and function. Conclusions The extent of functional readthrough might explain differences in the bleeding phenotype of patients homozygous for F7 nonsense mutations, and prevent null conditions even for the most readthrough-unfavorable mutations.

Crystal structure of human interferon-γ receptor 2 reveals the structural basis for receptor specificity

Interferon-γ receptor 2 is a cell-surface receptor that is required for interferon-γ signalling and therefore plays a critical immunoregulatory role in innate and adaptive immunity against viral and also bacterial and protozoal infections. A crystal structure of the extracellular part of human interferon-γ receptor 2 (IFNγR2) was solved by molecular replacement at 1.8 Å resolution. Similar to other class 2 receptors, IFNγR2 has two fibronectin type III domains. The characteristic structural features of IFNγR2 are concentrated in its N-terminal domain: an extensive π-cation motif of stacked residues KWRWRH, a NAG-W-NAG sandwich (where NAG stands for N-acetyl-D-glucosamine) and finally a helix formed by residues 78-85, which is unique among class 2 receptors. Mass spectrometry and mutational analyses showed the importance of N-linked glycosylation to the stability of the protein and confirmed the presence of two disulfide bonds. Structure-based bioinformatic analysis revealed independent evolutionary behaviour of both receptor domains and, together with multiple sequence alignment, identified putative binding sites for interferon-γ and receptor 1, the ligands of IFNγR2.

The kidneys play an important role in the clearance of rFVIIa in rats

INTRODUCTION

Previous distribution and histological studies have indicated that the kidneys and renal proximal tubular cells play a role in clearance of rFVIIa. However, the relative importance of the kidneys in clearance of rFVIIa has not previously been addressed. The objective of the present study was to evaluate the importance of the kidneys in the clearance process of rFVIIa after iv administration to rats using a nephrectomy model.

MATERIALS AND METHODS

A nephrectomized rat model was established and validated using inulin, a compound primarily cleared by the kidneys, as a test substance and several physiological parameters were monitored to ensure viability and robustness of the model. The model was then used for pharmacokinetic evaluation of renal clearance of rFVIIa. The pharmacokinetic parameters for rFVIIa were evaluated both by use of standard non-compartmental methods and by use of mixed effects methods, where a pharmacokinetic model was used to simultaneously model all data from healthy, sham operated, and nephrectomized rats.

RESULTS

Nephrectomized animals showed stable rectal temperature, SpO2 and pulse and as expected, clearance of inulin was essentially abolished compared to control animals (p<0.001). For rFVIIa, nephrectomy resulted in a clearance and terminal half-life of 34mL/h/kg and 2.8h compared to 68mL/h/kg and1.9h in rats exposed to sham surgery (p<0.0001 for both parameters).

CONCLUSION

The present data show that about 50% of the total clearance of rFVIIa from circulation in rats under isoflurane anaesthesia is due to renal clearance.

Antibody-induced enhancement of factor VIIa activity through distinct allosteric pathways

In the absence of its cofactor tissue factor (TF), coagulation factor VIIa (FVIIa) predominantly exists in a zymogen-like, catalytically incompetent state. Here we demonstrate that conformation-specific monoclonal antibodies (mAbs) can be used to characterize structural features determining the activity of FVIIa. We isolated two classes of mAbs, which both increased the catalytic efficiency of FVIIa more than 150-fold. The effects of the antibodies were retained with a FVIIa variant, which has been shown to be inert to allosteric activation by the natural activator TF, suggesting that the antibodies and TF employ distinct mechanisms of activation. The antibodies could be classified into two groups based on their patterns of affinities for different conformations of FVIIa. Whereas one class of antibodies affected both the K(m) and k(cat), the other class mainly affected the K(m). The antibody-induced activity enhancement could be traced to maturation of the S1 substrate binding pocket and the oxyanion hole, evident by an increased affinity for p-aminobenzamidine, an increased rate of antithrombin inhibition, an increased rate of incorporation of diisopropylfluorophosphate, and an enhanced fraction of molecules with a buried N terminus of the catalytic domain in the presence of antibodies. As demonstrated by site-directed mutagenesis, the two groups of antibodies appear to have overlapping, although clearly different, epitopes in the 170-loop. Our findings suggest that binding of ligands to specific residues in the 170-loop or its spatial vicinity may stabilize the S1 pocket and the oxyanion hole, and they may have general implications for the molecular understanding of FVIIa regulatory mechanisms.

Substrate specificity of Staphylococcus aureus cysteine proteases--Staphopains A, B and C

Human strains of Staphylococcus aureus secrete two papain-like proteases, staphopain A and B. Avian strains produce another homologous enzyme, staphopain C. Animal studies suggest that staphopains B and C contribute to bacterial virulence, in contrast to staphopain A, which seems to have a virulence unrelated function. Here we present a detailed study of substrate preferences of all three proteases. The specificity of staphopain A, B and C substrate-binding subsites was mapped using different synthetic substrate libraries, inhibitor libraries and a protein substrate combinatorial library. The analysis demonstrated that the most efficiently hydrolyzed sites, using Schechter and Berger nomenclature, comprise a P2-Gly↓Ala(Ser) sequence motif, where P2 distinguishes the specificity of staphopain A (Leu) from that of both staphopains B and C (Phe/Tyr). However, we show that at the same time the overall specificity of staphopains is relaxed, insofar as multiple substrates that diverge from the sequences described above are also efficiently hydrolyzed.

Engineering of substrate selectivity for tissue factor.factor VIIa complex signaling through protease-activated receptor 2

The complex of factor VIIa (FVIIa) with tissue factor (TF) triggers coagulation by recognizing its macromolecular substrate factors IX (FIX) and X (FX) predominantly through extended exosite interactions. In addition, TF mediates unique cell-signaling properties in cancer, angiogenesis, and inflammation that involve proteolytic cleavage of protease-activated receptor 2 (PAR2). PAR2 is cleaved by FVIIa in the binary TF.FVIIa complex and by FXa in the ternary TF.FVIIa.FXa complex, but physiological roles of these signaling complexes are incompletely understood. In a screen of FVIIa protease domain mutants, three variants (Q40A, Q143N, and T151S) activated macromolecular coagulation substrates and supported signaling of the ternary TF.FVIIa-Xa complex normally but were severely impaired in binary TF.FVIIa.PAR2 signaling. The residues identified were located in the model-predicted S2' pocket of FVIIa, and complementary PAR2 P2' Leu-38 replacements demonstrated that the P2' side chain was indeed crucial for PAR2 cleavage by TF.FVIIa. In addition, PAR2 was activated more efficiently by FVIIa T99Y, consistent with further contributions from the S2 subsite. The P2 residue preference of FVIIa and FXa predicted additional PAR2 mutants that were efficiently activated by TF.FVIIa but resistant to cleavage by the alternative PAR2 activator FXa. Thus, contrary to the paradigm of exosite-assisted cleavage of PAR1 by thrombin, the cofactor-associated protease FVIIa recognizes PAR2 predominantly by catalytic cleft interactions. Furthermore, the delineated molecular details of this substrate interaction enabled protein engineering of protease-selective PAR2 receptors that will aid further studies to dissect the roles of TF signaling complexes in vivo.

Using specificity to strategically target proteases

Proteases are a family of naturally occurring enzymes in the body whose dysregulation has been implicated in numerous diseases and cancers. Their ability to selectively and catalytically turnover substrate adds both signal amplification and functionality as parameters for the detection of disease. This review will focus on the development of activity-based methodologies to characterize proteases, and in particular, the use of positional scanning, synthetic combinatorial libraries (PS-SCL's), and substrate activity screening (SAS) assays. The use of these approaches to better understand a protease's natural substrate will be discussed as well as the technologies that emerged.

Activity and regulation of glycoPEGylated factor VIIa analogs

BACKGROUND

Recombinant coagulation factor VIIa (rFVIIa) has proven to be a safe and effective drug for treatment of bleeding episodes in hemophilic patients with inhibitors. However, rFVIIa is cleared from the circulation relatively quickly. Protein modification with poly(ethylene glycol) (PEG) can prolong the circulatory lifetime of proteins but it could also impair protein function by molecular shielding of the protein surface.

OBJECTIVES

To characterize the interaction of glycoPEGylated rFVIIa - rFVIIa-10K PEG and rFVIIa-40K PEG - with tissue factor (TF), factor X (FX) and plasma inhibitors, tissue factor pathway inhibitor (TFPI) and antithrombin (AT).

METHODS

The amidolytic and FX activation assays were employed to investigate the interaction of glycoPEGylated rFVIIa with its macromolecular substrate and inhibitors.

RESULTS

Both the glycoPEGylated rFVIIa analogs exhibited similar amidolytic activity as that of rFVIIa in the absence or the presence of relipidated TF. The analogs were as effective as rFVIIa in activating FX in the absence of TF. In the presence of TF, the glycoPEGylated rFVIIa variants, relative to rFVIIa, were slightly less effective at lower concentrations, but no significant differences were found among them in activating FX at saturating concentrations. Both AT/heparin and TFPI effectively inhibited the glycoPEGylated rFVIIa bound to relipidated TF or TF on stimulated endothelial cells. In contrast to their normal interaction with TF, the glycoPEGylated rFVIIa variants appeared to interact poorly with phospholipids.

CONCLUSIONS

The glycoPEGylated rFVIIa variants retained their catalytic activity and interacted efficiently with TF, FX and the plasma inhibitors. Further work with appropriate in vitro and in vivo model systems is needed to determine the feasibility of using glycoPEGylated rFVIIa to improve therapeutic options for bleeding disorders.

A cyclic peptidylic inhibitor of murine urokinase-type plasminogen activator: changing species specificity by substitution of a single residue

uPA (urokinase-type plasminogen activator) is a potential therapeutic target in a variety of pathological conditions, including cancer. In order to find new principles for inhibiting uPA in murine cancer models, we screened a phage-displayed peptide library with murine uPA as bait. We thereby isolated several murine uPA-binding peptide sequences, the predominant of which was the disulfide-bridged constrained sequence CPAYSRYLDC, which we will refer to as mupain-1. A chemically synthesized peptide corresponding to this sequence was found to be a competitive inhibitor of murine uPA, inhibiting its activity towards a low-molecular-mass chromogenic substrate as well as towards its natural substrate plasminogen. The K(i) value for inhibition as well as the K(D) value for binding were approx. 400 nM. Among a variety of other murine and human serine proteases, including trypsin, mupain-1 was found to be highly selective for murine uPA and did not even measurably inhibit human uPA. The cyclic structure of mupain-1 was indispensable for binding. Alanine scanning mutagenesis identified Arg(6) of mupain-1 as the P1 residue and indicated an extended binding interaction including the P5, P3, P2, P1 and P1' residues of mupain-1 and the specificity pocket, the catalytic triad and amino acids 41, 99 and 192 located in and around the active site of murine uPA. Exchanging His(99) of human uPA by a tyrosine residue, the corresponding residue in murine uPA, conferred mupain-1 susceptibility on to the latter. Peptide-derived inhibitors, such as mupain-1, may provide novel mechanistic information about enzyme-inhibitor interactions, provide alternative methodologies for designing effective protease inhibitors, and be used for target validation in murine model systems.

Activation mechanism and substrate specificity of the Drosophila initiator caspase DRONC

Drosophila Nedd2-like caspase (DRONC), an initiator caspase in Drosophila melanogaster and ortholog of human caspase-9, is cleaved during its activation in vitro and in vivo. We show that, in contrast to conclusions from previous studies, cleavage is neither necessary nor sufficient for DRONC activation. Instead, our data suggest that DRONC is activated by dimerization, a mechanism used by its counterparts in humans. Subsequent cleavage at Glu352 stabilizes the active dimer. Since cleavage is at a Glu residue, it has been proposed that DRONC is a dual Asp- and Glu-specific caspase. We used positional-scanning peptide libraries to define the P1-P4 peptide sequence preferences of DRONC, and show that it is indeed equally active on optimized tetrapeptides containing either Asp or Glu in P1. Furthermore, mutagenesis reveals that Asp and Glu residues are equally tolerated at the primary autoprocessing site of DRONC itself. However, when its specificity is tested on a natural substrate, the Drosophila executioner caspase DRICE, a clear preference for Asp emerges. The formerly proposed Glu preference is thus incorrect. DRONC does not differentiate between Asp and Glu in poor substrates, but prefers Asp when tested on a good substrate.