Construction and Characterization of Thrombin-resistant Variants of Recombinant Human Protein S

The anticoagulant function of coagulation factor V

Almost two decades ago an anticoagulant function of factor V (FV) was discovered, as an anticoagulant cofactor for activated protein C (APC). A natural mutant of FV in which the R506 inactivation site was mutated to Gln (FVLeiden) was inactivated slower by APC, but also could not function as anticoagulant cofactor for APC in the inactivation of activated factor VIII (FVIIIa). This mutation is prevalent in populations of Caucasian descent, and increases the chance of thrombotic events in carriers. Characterisation of the FV anticoagulant effect has elucidated multiple properties of the anticoagulant function of FV: 1) Cleavage of FV at position 506 by APC is required for anticoagulant function. 2) The C-terminal part of the FV B domain is required and the B domain must have an intact connection with the A3 domain of FV. 3) FV must be bound to a negatively charged phospholipid membrane. 4) Protein S also needs to be present. 5) FV acts as a cofactor for inactivation of both FVa and FVIIIa. 6) The prothrombotic function of FVLeiden is a function of both reduced APC cofactor activity and resistance of FVa to APC inactivation. However, detailed structural and mechanistic properties remain to be further explored.

Protein C anticoagulant and cytoprotective pathways

Plasma protein C is a serine protease zymogen that is transformed into the active, trypsin-like protease, activated protein C (APC), which can exert multiple activities. For its anticoagulant action, APC causes inactivation of the procoagulant cofactors, factors Va and VIIIa, by limited proteolysis, and APC's anticoagulant activity is promoted by protein S, various lipids, high-density lipoprotein, and factor V. Hereditary heterozygous deficiency of protein C or protein S is linked to moderately increased risk for venous thrombosis, while a severe or total deficiency of either protein is linked to neonatal purpura fulminans. In recent years, the beneficial direct effects of APC on cells which are mediated by several specific receptors have become the focus of much attention. APC-induced signaling can promote multiple cytoprotective actions which can minimize injuries in various preclinical animal injury models. Remarkably, pharmacologic therapy using APC demonstrates substantial neuroprotective effects in various murine injury models, including ischemic stroke. This review summarizes the molecules that are central to the protein C pathways, the relationship of pathway deficiencies to venous thrombosis risk, and mechanisms for the beneficial effects of APC.

The extended cleavage specificity of human thrombin

Thrombin is one of the most extensively studied of all proteases. Its central role in the coagulation cascade as well as several other areas has been thoroughly documented. Despite this, its consensus cleavage site has never been determined in detail. Here we have determined its extended substrate recognition profile using phage-display technology. The consensus recognition sequence was identified as, P2-Pro, P1-Arg, P1'-Ser/Ala/Gly/Thr, P2'-not acidic and P3'-Arg. Our analysis also identifies an important role for a P3'-arginine in thrombin substrates lacking a P2-proline. In order to study kinetics of this cooperative or additive effect we developed a system for insertion of various pre-selected cleavable sequences in a linker region between two thioredoxin molecules. Using this system we show that mutations of P2-Pro and P3'-Arg lead to an approximate 20-fold and 14-fold reduction, respectively in the rate of cleavage. Mutating both Pro and Arg results in a drop in cleavage of 200-400 times, which highlights the importance of these two positions for maximal substrate cleavage. Interestingly, no natural substrates display the obtained consensus sequence but represent sequences that show only 1-30% of the optimal cleavage rate for thrombin. This clearly indicates that maximal cleavage, excluding the help of exosite interactions, is not always desired, which may instead cause problems with dysregulated coagulation. It is likely exosite cooperativity has a central role in determining the specificity and rate of cleavage of many of these in vivo substrates. Major effects on cleavage efficiency were also observed for residues as far away as 4 amino acids from the cleavage site. Insertion of an aspartic acid in position P4 resulted in a drop in cleavage by a factor of almost 20 times.

Protein S inherited qualitative deficiency: novel mutations and phenotypic influence

BACKGROUND

Only a few mutations associated with qualitative protein S deficiency have already been described. Sensitivity and specificity for type II PROS1 mutations of commercially available reagents for measuring Protein S (PS) activity are not well established. Whether these mutations are significant risk factors for thrombosis remains an unresolved question.

METHODS

In order to address the first point, we present and discuss the results of PROS1 analysis performed in the 30 probands with type II PS-inherited deficiency suspicion and 35 relatives, studied in our laboratory between 2000 and 2008. In order to investigate the influence of type II mutations on the coagulability level, thrombin generation tests were performed on plasma from 102 PROS1 type II, type I/III or PS Herleen mutation heterozygous carriers and controls.

RESULTS

Mutations (12 novel, six already described) which probably explain the qualitative phenotype, were found in 27 (90%) out of the 30 probands studied. In relatives, 78% of heterozygotes presented with a type II phenotype. An APC resistance phenotype was documented in type II and type I/III defects heterozygous carriers; however, the effect of type II was milder than the effect of type I/III PS mutations.

CONCLUSIONS

A PS functional assay (Staclot PS, Stago) was efficient in screening for PROS1 type II defects, particularly in probands. A significant positive influence of type II mutations on ex vivo thrombin generation was demonstrated. However, whether these mutations increase the risk of venous thromboembolism requires further investigation.

Activated protein C cofactor function of protein S: a critical role for Asp95 in the EGF1-like domain

Protein S has an established role in the protein C anticoagulant pathway, where it enhances the factor Va (FVa) and factor VIIIa (FVIIIa) inactivating property of activated protein C (APC). Despite its physiological role and clinical importance, the molecular basis of its action is not fully understood. To clarify the mechanism of the protein S interaction with APC, we have constructed and expressed a library of composite or point variants of human protein S, with residue substitutions introduced into the Gla, thrombin-sensitive region (TSR), epidermal growth factor 1 (EGF1), and EGF2 domains. Cofactor activity for APC was evaluated by calibrated automated thrombography (CAT) using protein S-deficient plasma. Of 27 variants tested initially, only one, protein S D95A (within the EGF1 domain), was largely devoid of functional APC cofactor activity. Protein S D95A was, however, gamma-carboxylated and bound phospholipids with an apparent dissociation constant (Kd(app)) similar to that of wild-type (WT) protein S. In a purified assay using FVa R506Q/R679Q, purified protein S D95A was shown to have greatly reduced ability to enhance APC-induced cleavage of FVa Arg306. It is concluded that residue Asp95 within EGF1 is critical for APC cofactor function of protein S and could define a principal functional interaction site for APC.

Proteolytic cleavage of protein S during the hemostatic response

BACKGROUND

Protein S is a vitamin K-dependent protein with anticoagulant properties. It contains a so-called thrombin-sensitive region (TSR), which is susceptible to cleavage by coagulation factor Xa (FXa) and thrombin. Upon cleavage, the anticoagulant activity of protein S is abolished.

OBJECTIVE

The aim of the present study was to determine whether protein S is cleaved within the TSR during activation of the coagulation system under near physiological conditions.

RESULTS

In a reconstituted coagulation system containing apart from protein S only procoagulant constituents and synthetic phospholipid vesicles, protein S was cleaved at Arg60 by the FXa generated (3 mol min(-1) mol(-1) enzyme). FXa-catalyzed cleavage of protein S, however, was inhibited by factor Va and prothrombin by more than 70%. During clotting of recalcified citrated plasma in the presence of a synthetic lipid membrane, no FXa-catalyzed proteolysis of protein S was observed. Substituting platelets for phospholipid vesicles resulted both in the reconstituted system and in plasma in cleavage of the TSR. Cleavage was at Arg60 and was observed upon platelet activation, irrespective of the presence of FXa (13 pmol min(-1) 10(-8) platelets). No cleavage by thrombin was observed in either the reconstituted coagulation system or clotting plasma.

CONCLUSION

These findings suggest that in vivo the anticoagulant activity of protein S is not down-regulated by FXa or thrombin during activation of coagulation. Our results rather suggest a role for a platelet protease in down-regulating the anticoagulant activity of protein S during the hemostatic response.

Activated protein C-dependent and -independent anticoagulant activities of protein S have different structural requirements

Plasma protein S exhibits multiple anticoagulant activities. About 20% of protein S normally circulates in a form that is cleaved in its thrombin-sensitive region (TSR, residues 47-72) and this cleaved protein S is inactive as a cofactor for activated protein C (APC). To clarify whether the same cleavage(s) in the TSR neutralizes both APC-cofactor and APC-independent direct anticoagulant activities, protein S was treated with several proteases, and activities and cleavages were monitored. Thrombin cleaved protein S first at Arg49, which abolished protein S APC-cofactor activity, but not APC-independent activity. A slower second thrombin cleavage at Arg70 abolished the direct prothrombinase inhibitory activity of protein S and its ability to bind phospholipids. Factor Xa cleaved protein S only at Arg60 and abolished APC-cofactor activity but not APC-independent anticoagulant activity. The snake venom enzyme Protac C efficiently cleaved protein S at two sites in the TSR, which impaired both types of protein S anticoagulant activity in the presence of phospholipids. Protac C-cleaved protein S did not compete with Factor Xa for limiting phospholipid surfaces but could still inhibit prothrombinase activity in the absence of phospholipids. Thus, the APC-cofactor activity protein S is significantly more sensitive to structural changes in the TSR than is the APC-independent activity of protein S.

The molecular genetics of familial venous thrombosis

In the past few years, important advances have been made in the identification of factors predisposing to familial thrombophilia. Particular attention has been paid to the characterization of known inherited defects and their genotype-phenotype relationship, and to studying the interaction between single or multiple inherited conditions and acquired risk factors for venous thrombosis. The recent discovery of 'new' and very common genetic lesions predisposing to thrombosis has greatly expanded the interest in this field. Hereditary predisposition to venous thrombosis may be related to lesions in one or more of 10-15 genes encoding antithrombin, Protein C, Protein S, Factor V, prothrombin, enzymes of the homocysteine metabolic pathway, fibrinogen, heparin cofactor II, plasminogen and thrombomodulin. About 500 different gene lesions (substitutions, deletions, insertions) have so far been reported to affect these genes in patients with thrombotic disease. Because there are potentially multiple interactions between genetic and environmental factors, familial thrombophilia is now considered to be a multifactorial disease. The aim of this chapter is to review aspects of the molecular genetics of familial thrombophilia. In particular, those gene/protein defects for which there is convincing evidence of an association with familial thrombosis will be examined in detail.

Chemical synthesis of human protein S thrombin-sensitive module and first epidermal growth factor module

Human plasma protein S is a nonenzymatic cofactor for activated protein C (APC) in the inactivation of coagulation factors Va and VIIIa, and helps to provide an essential negative feedback on blood coagulation. Previous indirect evidence suggested that the thrombin-sensitive region (TSR: residues 47-75, 1 disulfide) and the first epidermal growth factorlike region (EGF1: residues 76-116, 3 disulfides) of protein S may be functionally important for expression of its APC cofactor activity. To study the functional importance of these modules directly, access to the isolated TSR and EGF1 modules would be preferred. Recombinant expression of protein S intact TSR and correctly folded EGF1 has not been possible. Here we describe the synthesis of both TSR and EGF1 modules by stepwise solid phase peptide synthesis using the in situ neutralization/2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluron ium hexafluorophosphate activation procedure for tert-butoxycarbonyl chemistry. For the TSR, correct intramodular disulfide bonding was confirmed. To overcome folding difficulties with the EGF1, a two-step oxidation procedure was used in which the cysteines involved in the middle, crossing, disulfide bond (Cys85-Cys102) remained protected with acetamidomethyl (Acm) groups after hydrogen fluoride treatment of the peptide resin. Selective formation of the first two disulfide bonds (Cys80-Cys93 and Cys104-Cys113) was followed by release of the Acm groups and subsequent formation of the third disulfide bond (Cys85-Cys102). CD studies revealed 54% of beta-sheet/turn in the EGF1 that is characteristic for EGF modules. Deuterium exchange studies suggested a very tightly packed core in EGF1 that is not accessible to the bulk solvent, likely a result from the compact structure caused by its three disulfide bonds. The 30% beta-sheet structure observed in the TSR involved amide protons that could be readily exchanged by deuterons, likely reflecting a more flexible structure of the TSR loop in contrast to the rigid structure of EGF1. The establishment of synthetic access to the TSR and EGF1 of protein S provides a versatile tool to study interactions of these modules with the blood coagulation components of the anticoagulant plasma protein C pathway.

Involvement of amino acid residues 423-429 of human protein S in binding to C4b-binding protein

Human protein S binds to C4b-binding protein (C4BP) both in plasma and in a system using purified proteins. Amino acid residues 420-434 of the first disulfide loop of the sex hormone binding globulinlike domain of protein S are involved in the interaction of protein S with C4BP. To define the involvement of specific polar amino acids within residues 420-434, we studied in parallel synthetic protein S peptides and recombinant protein S variants containing the same amino acid replacements, K423E, E424K, Q427E and K429E. Synthetic peptide analogs of peptide PSP-420 (residues 420-434) were assayed for binding C4BP and as inhibitors of complex formation. The PSP-420 peptide and the analogous peptide with the substitution E424K, but not the peptides containing the substitutions K423E and K429E, were able to bind C4BP. Recombinant proteins with mutations of K423E, Q427E and K429E showed reduced affinity for C4BP compared to plasma protein S, recombinant wild type protein S, or E424K-protein S. These results suggest that Lys-423, Gln-427 and Lys-429 of protein S are important for normal binding to C4BP. The anti-protein S monoclonal antibody LJ-56, raised against peptide PSP-420, recognizes only free protein S and inhibits complex formation with C4BP. Antibody LJ-56 recognized the E424K and Q427E peptides but not the K423E or K429E peptides. Similarly, the E424K and Q427E protein S mutants were recognized by LJ-56, whereas the K423E and K429E protein S mutants were not recognized. This suggests that both in the peptide PSP-420 and in protein S, Lys-423 and Lys-429 significantly contribute to binding to antibody LJ-56. These results demonstrate that protein S residues 423, 427 and 429, but not residue 424, are involved in binding to both the antibody LJ-56 and to C4BP. When peptides PSP 420 and SL-6 (residues 447-460) with carboxyterminal amide or carboxylate moieties were compared to their ability to inhibit C4BP-protein S complexation, PSP-420-amide was the most potent. This finding together with the other results described here supports the hypothesis that the residues 420 and 434 in protein S provides a major binding site for C4BP.