The functional significance of the autolysis loop in protein C and activated protein C

Selective modulation of activated protein C activities by a nonactive site-targeting nanobody library

Activated protein C (APC) is a pleiotropic coagulation protease with anticoagulant, anti-inflammatory, and cytoprotective activities. Selective modulation of these APC activities contributes to our understanding of the regulation of these physiological mechanisms and permits the development of therapeutics for the pathologies associated with these pathways. An antibody library targeting the nonactive site of APC was generated using llama antibodies (nanobodies). Twenty-one nanobodies were identified that selectively recognize APC compared with the protein C zymogen. Overall, 3 clusters of nanobodies were identified based on the competition for APC in biolayer interferometry studies. APC functional assays for anticoagulant activity, histone H3 cleavage, and protease-activated receptor 1 (PAR1) cleavage were used to understand their diversity. These functional assays revealed 13 novel nanobody-induced APC activity profiles via the selective modulation of APC pleiotropic activities, with the potential to regulate specific mechanisms for therapeutic purposes. Within these, 3 nanobodies (LP2, LP8, and LP17) inhibited all 3 APC functions. Four nanobodies (LP1, LP5, LP16, and LP20) inhibited only 2 of the 3 functions. Monofunction inhibition specific to APC anticoagulation activity was observed only by 2 nanobodies (LP9 and LP11). LP11 was also found to shift the ratio of APC cleavage of PAR1 at R46 relative to R41, which results in APC-mediated biased PAR1 signaling and APC cytoprotective effects. Thus, LP11 has an activity profile that could potentially promote hemostasis and cytoprotection in bleedings associated with hemophilia or coagulopathy by selectively modulating APC anticoagulation and PAR1 cleavage profile.

Protein C deficiency (a novel mutation: ala291Thr) with systemic lupus erythematosus leads to the deep vein thrombosis

: The current study aims to explore the phenotype and genotype of a mutation Ala291Thr, which responsible for type I protein C (PC) deficiency in a Chinese woman. The PROC antigen was tested with chromogenic substrate method. PROC gene were amplified by PCR with direct sequencing. Bioinformatics and model analysis were used to study the harm of the mutation. PC activity (PC: A) levels of three members were reduced to 39, 57 and 56%, respectively, PC: antigen was decreased parallelly same as PC: A. Sequencing analysis showed proband with a novel heterozygous c.997G>A point mutation in exon 9 of PROC gene resulting in Ala291Thr. The Ala291Thr mutation is responsible for the decrease of PC: A, which is cross-reacting material negative deficiency and the first reported in the world. This mutation alone may not have significant clinical symptoms, whereas it will cause deep vein thrombosis when combined with systemic lupus erythematosus.

The structure-function relationship of activated protein C

Protein C is the central enzyme of the natural anticoagulant pathway and its activated form APC (activated protein C) is able to proteolyse non-active as well as active coagulation factors V and VIII. Proteolysis renders these cofactors inactive, resulting in an attenuation of thrombin formation and overall down-regulation of coagulation. Presences of the APC cofactor, protein S, thrombomodulin, endothelial protein C receptor and a phospholipid surface are important for the expression of anticoagulant APC activity. Notably, APC also has direct cytoprotective effects on cells: APC is able to protect the endothelial barrier function and expresses anti-inflammatory and anti-apoptotic activities. Exact molecular mechanisms have thus far not been completely described but it has been shown that both the protease activated receptor 1 and EPCR are essential for the cytoprotective activity of APC. Recently it was shown that also other receptors like sphingosine 1 phosphate receptor 1, Cd11b/CD18 and tyrosine kinase with immunoglobulin-like and EGFlike domains 2 are likewise important for APC signalling. Mutagenesis studies are being performed to map the various APC functions and interactions onto its 3D structure and to dissect anticoagulant and cytoprotective properties. The results of these studies have provided a wealth of structure-function information. With this review we describe the state-of-the-art of the intricate structure-function relationships of APC, a protein that harbours several important functions for the maintenance of both humoral and tissue homeostasis.

Lessons from natural and engineered mutations

Function of the activated protein C (APC) autolysis loop in activated FVIII inactivation

Activated protein C (APC) binds to its substrates activated factor V (FVa) and activated factor VIII (FVIIIa) with a basic exosite that consists of loops 37, 60, 70 and the autolysis loop. These loops have a high density of basic residues, resulting in a positive charge on the surface of APC. Many of these residues are important in the interaction of APC with FVa and FVIIIa. The current study focused on the function of the autolysis loop in the interaction with FVIIIa. This loop was previously shown to interact with FVa, and it inhibits APC inactivation by plasma serpins. Charged residues of the autolysis loop were individually mutated to alanine and the activity of these mutants was assessed in functional FVIIIa inactivation assays. The autolysis loop was functionally important for FVIIIa inactivation. Mutation of R306, K311 and R314 each resulted in significantly reduced FVIIIa inactivation. The inactivating cleavages of FVIIIa at R336 and R562 were affected equally by the mutations. Protein S and FV stimulated cleavage at R562 more than cleavage at R336, independent of mutations in the autolysis loop. Together, these results confirmed that the autolysis loop plays a significant role as part of the basic exosite on APC in the interaction with FVIIIa.

Regulation of the protein C anticoagulant and antiinflammatory pathways

Protein C is a vitamin K-dependent anticoagulant serine protease zymogen in plasma which upon activation by the thrombin-thrombomodulin complex down-regulates the coagulation cascade by degrading cofactors Va and VIIIa by limited proteolysis. In addition to its anticoagulant function, activated protein C (APC) also binds to endothelial protein C receptor (EPCR) in lipid-rafts/caveolar compartments to activate protease- activated receptor 1 (PAR-1) thereby eliciting antiinflammatory and cytoprotective signaling responses in endothelial cells. These properties have led to FDA approval of recombinant APC as a therapeutic drug for severe sepsis. The mechanism by which APC selects its substrates in the anticoagulant and antiinflammatory pathways is not well understood. Recent structural and mutagenesis data have indicated that basic residues of three exposed surface loops known as 39-loop (Lys-37, Lys-38, and Lys-39), 60-loop (Lys-62, Lys- 63, and Arg-67), and 70-80-loop (Arg-74, Arg-75, and Lys-78) (chymotrypsin numbering) constitute an anion binding exosite in APC that interacts with the procoagulant cofactors Va and VIIIa in the anticoagulant pathway. Furthermore, two negatively charged residues on the opposite side of the active-site of APC on a helical structure have been demonstrated to determine the specificity of the PAR-1 recognition in the cytoprotective pathway. This article will review the mechanism by which APC exerts its proteolytic function in two physiologically inter-related pathways and how the structure- function insights into determinants of the specificity of APC interaction with its substrates in two pathways can be utilized to tinker with the structure of the molecule to obtain APC derivatives with potentially improved therapeutic profiles.

Activated protein C action in inflammation

Activated protein C (APC) is a natural anticoagulant that plays an important role in coagulation homeostasis by inactivating the procoagulation factor Va and VIIIa. In addition to its anticoagulation functions, APC also has cytoprotective effects such as anti-inflammatory, anti-apoptotic, and endothelial barrier protection. Recently, a recombinant form of human APC (rhAPC or drotrecogin alfa activated; known commercially as 'Xigris') was approved by the US Federal Drug Administration for treatment of severe sepsis associated with a high risk of mortality. Sepsis, also known as systemic inflammatory response syndrome (SIRS) resulting from infection, is a serious medical condition in critical care patients. In sepsis, hyperactive and dysregulated inflammatory responses lead to secretion of pro- and anti-inflammatory cytokines, activation and migration of leucocytes, activation of coagulation, inhibition of fibrinolysis, and increased apoptosis. Although initial hypotheses focused on antithrombotic and profibrinolytic functions of APC in sepsis, other agents with more potent anticoagulation functions were not effective in treating severe sepsis. Furthermore, APC therapy is also associated with the risk of severe bleeding in treated patients. Therefore, the cytoprotective effects, rather than the anticoagulant effect of APC are postulated to be responsible for the therapeutic benefit of APC in the treatment of severe sepsis.

The role of autolysis loop in determining the specificity of coagulation proteases

We recently demonstrated that the substitution of the autolysis loop (residues 143 to 154 in the chymotrypsin numbering system) of activated protein C (APC) with the corresponding loop of factor Xa (fXa) renders the APC mutant (APC/fX143-154) susceptible to inhibition by antithrombin (AT) in the presence of pentasaccharide. Our recent results further indicated, that in addition to an improvement in the reactivity of APC/fX143-154 with AT, both the amidolytic and anti-factor Va activities of the mutant APC have also been significantly increased. Since the autolysis loop of APC is five residues longer than the autolysis loop of fXa, it could not be ascertained whether this loop in the mutant APC specifically interacts with the activated conformation of AT or if a shorter autolysis loop is responsible for a global improvement in the catalytic activity of the mutant protease. To answer this question, we prepared another APC mutant in which the autolysis loop of the protease was replaced with the corresponding loop of trypsin (APC/Tryp143-154). Unlike an approximately 500-fold improvement in the reactivity of APC/fX143-154 with AT in the presence of pentasaccharide, the reactivity of APC/Tryp143-154 with the serpin was improved approximately 10-fold. These results suggest that both the length and structure of residues of the autolysis loop are critical for the specificity of the coagulation protease interaction with AT. Further factor Va inactivation studies with the APC mutants revealed a similar role for the autolysis loop of APC in the interaction with its natural substrate.

Autolysis loop restricts the specificity of activated protein C: analysis by FRET and functional assays

We previously demonstrated that the substitution of the autolysis loop (residues 143-154 in chymotrypsin numbering) of APC with the corresponding loop of trypsin (APC-Tryp 143-154) has no influence on the proteolytic activity of the protease toward fVa, however, this substitution increases the reactivity of APC with plasma inhibitors so that the mutant exhibits no anticoagulant activity in plasma. To further investigate the role of the autolysis loop in APC and determine whether this loop is a target for modulation by protein S, we evaluated the activity of APC-Tryp 143-154 toward fVa and several plasma inhibitors both in the absence and presence of protein S. Furthermore, we evaluated the active-site topography of APC-Tryp 143-154 by determining the average distance of the closest approach (L) between a fluorescein dye tethered to a tripeptide inhibitor, attached to the active-site of APC-Tryp 143-154, and octadecylrhodamine dyes incorporated into PCPS vesicles both in the absence and presence of protein S. The activity of APC-Tryp 143-154 toward fVa was identical to that of wild-type APC both in the presence and absence of protein S. However, the reactivity of APC-Tryp 143-154 with plasma inhibitors was preferentially improved independent of protein S. The FRET analysis revealed a dramatic change in the active-site topography of APC both in the absence and presence of protein S. Anisotropy measurements revealed that the fluorescein dye has a remarkable degree of rotational freedom in the active-site of APC-Tryp 143-154. These results suggest that the autolysis loop of APC may not be a target for modulation by protein S. This loop, however, plays a critical role in restricting both the specificity and spatial environment of the active-site groove of APC.

Autolysis of a novel multidomain subtilase-cold-adapted deseasin MCP-01 is pH-dependent and the surface loops in its catalytic domain, the linker, and the P_proprotein domain are susceptible to proteolytic attack

Cold-adapted deseasin MCP-01 is a novel type subtilase with a multidomain structure containing a catalytic domain, a linker, a P_proprotein domain, and a PKD domain. Its autolysis was pH-dependent due to its flexible structure. N-terminal sequence analysis of the autolytic peptides revealed four autolytic sites in the catalytic domain. Three of these are in the same loops as mesophilic subtilases and one is unlike anything previously reported. Two autolytic sites were deduced in its linker and three in its P_proprotein domain, indicating the linker and the P_proprotein domain are flexible and susceptible to proteolytic attacks. Therefore, during MCP-01 autolysis, the linker and the P_proprotein domain of MCP-01 were easily attacked by proteolysis, resulting in cleavage of the C-terminal region. At the same time, some autolytic sites in the surface loops of the catalytic domain were cleaved. This is the first report describing the autolytic mechanism of a multidomain subtilase.

Identification and computationally-based structural interpretation of naturally occurring variants of human protein C

Protein C (PC) is a key regulator of blood clotting and inflammation. Its inherited deficiency is associated with venous thromboembolism, and recombinant activated PC is currently used to increase survival in severe sepsis. The molecular basis of inherited PC deficiency is heterogeneous. Due to its multiple physiologic interactions and functions, and its modular structure, natural variants aid in the understanding of the relationship between critical residues and discrete functions. This knowledge has important therapeutic implications in the planning of a recombinant activated PC with a specific therapeutic target and devoid of major collateral effects. A way of predicting important functional consequences of residue variation is the use of molecular modeling and structural interpretation of amino acidic substitutions. A study of 21 out of 32 identified PC gene (PROC) variants is presented. For three of them, localized in the active site, electrostatic potential variation was calculated. For more than half of the studied variants, an explanation for the functional impairment could be derived from computational analysis, allowing a focused choice of which variants it is worthwhile pursuing.