Thermal stability and domain-domain interactions in natural and recombinant protein C

Biochemical characterization of LR769, a new recombinant factor VIIa bypassing agent produced in the milk of transgenic rabbits

BACKGROUND

The bypassing agent factor VII (FVIIa) is a first-line therapy for the treatment of acute bleeding episodes in patients with haemophilia and high-titre inhibitors. FVIIa is a highly post-translationally modified protein that requires eukaryotic expression systems to produce a fully active molecule. A recombinant FVIIa was produced in the milk of transgenic rabbits to increase expression and provide an efficient, safe and affordable product after purification to homogeneity (LR769).

AIM

To present the biochemical and functional in vitro characteristics of LR769.

RESULTS

Mass spectrometric analyses of the intact protein and of heavy and light chains revealed a fully activated, mature and properly post-translationally modified protein notably regarding N/O-glycosylations and γ-carboxylation. Primary structure analysis, performed by peptide mapping, confirmed 100% of the sequence and the low level or absence of product-derived impurities such as oxidized, deamidated and glycated forms. Low levels of aggregates and fragments were observed by different chromatographic methods. Higher order structure investigated by circular dichroism showed appropriate secondary/tertiary structures and conformational change in the presence of Ca²⁺ ions. Finally, activated partial thromboplastin time and thrombin generation assays showed the ability of LR769 to decrease coagulation time and to generate thrombin in haemophiliac-A-plasmas, even in the presence of inhibitors.

CONCLUSION

The innovative expression system used to produce LR769 yields a new safe and effective rhFVIIa for the treatment of haemophilia A or B patients with inhibitors.

Identification of an ordered compact structure within the recombinant bovine fibrinogen alphaC-domain fragment by NMR

The NMR solution structure of the bovine fibrinogen alphaC-domain fragment, including residues Aalpha374-538, reveals a type-I' beta-hairpin, restricted at the base by a C423-C453 disulfide linkage and a short turn preceding C423. Although both faces of the hairpin are formed mainly by hydrophilic residues, one of them is uncharged while the other has a characteristic pattern of charged residues which are highly conserved among vertebrate species. Chemical shift indexing and relaxation data indicate the presence of a collapsed hydrophobic region next to the hairpin that includes approximately 30 residues with slower concerted motion and higher content of nonpolar residues and, according to a previous study (Tsurupa, G., Tsonev, L., and Medved, L. (2002) Biochemistry 41, 6449-6459), may cooperate with the hairpin to form a compact cooperative unit (domain). Structure and relaxation data show that the region between C423 and C453 is populated by both random coil and beta-structure, suggesting that the cooperative structure in the isolated alphaC-domain is intrinsically unstable. This observation is in agreement with a very low energy of stabilization of the Aalpha374-538 fragment determined in unfolding experiments. The low stability of the alphaC-domain suggests a possible explanation for the previously observed intra- and intermolecular interactions of these domains in fibrinogen and fibrin.

Endothelial protein C receptor-dependent inhibition of migration of human lymphocytes by protein C involves epidermal growth factor receptor

The protein C pathway is an important regulator of the blood coagulation system. Protein C may also play a role in inflammatory and immunomodulatory processes. Whether protein C or activated protein C affects lymphocyte migration and possible mechanisms involved was tested. Lymphocyte migration was studied by micropore filter assays. Lymphocytes that were pretreated with protein C (Ceprotin) or activated protein C (Xigris) significantly reduced their migration toward IL-8, RANTES, MCP-1, and substance P, but not toward sphingosine-1-phosphate. The inhibitory effects of protein C or activated protein C were reversed by Abs against endothelial protein C receptor and epidermal growth factor receptor. Evidence for the synthesis of endothelial protein C receptor by lymphocytes is shown by demonstration of receptor mRNA expression and detection of endothelial protein C receptor immunoreactivity on the cells' surface. Data suggest that an endothelial protein C receptor is expressed by lymphocytes whose activation with protein C or activated protein C arrests directed migration. Exposure of lymphocytes to protein C or activated protein C stimulates phosphorylation of Tyr845 of epidermal growth factor receptor, which may be relevant for cytoprotective effects of the protein C pathway.

Preparation of vitamin K-dependent proteins, such as clotting factors II, VII, IX and X and clotting inhibitor protein C

A review is given of preparative methods for the isolation of the vitamin K-dependent clotting factors II, VII, IX, X and clotting inhibitor protein C, all derived from human plasma. Factor II, activated factor VII and activated protein C are also obtained from recombinant animal cells. The methods for their purification are described. The problem of difference in posttranslational modifications between plasma derived and recombinant protein is discussed with regard to therapeutic proteins.

Secretion, gamma-carboxylation, and endoplasmic reticulum-associated degradation of chimeras with mutually exchanged Gla domain between human protein C and prothrombin

Warfarin, an antagonist of vitamin K, causes diminution of vitamin K-dependent coagulation factors in the circulation. Although all vitamin K-dependent factors have Gla domains, the warfarin-induced decrease in their plasma concentration differs among factors. In warfarin-treated HepG2 cells, we found modest and severe intracellular degradation of prothrombin and protein C, respectively. To investigate the structural features of these proteins that contribute to their warfarin sensitivity, chimeric prothrombin containing the prepropeptide and Gla domain of protein C was expressed in baby hamster kidney (BHK) cells. This chimera showed similar secretion kinetics and warfarin sensitivity to those of wild-type prothrombin, demonstrating that the Gla domain cannot solely explain the warfarin sensitivity of protein C. In contrast, two chimeric protein Cs containing either the Gla domain alone or the prepropeptide and Gla domain of prothrombin showed impaired secretion. Even though gamma-carboxylation proceeded normally, both chimeras were degraded intracellularly by the proteasome. From these results, we conclude that not only the folding of the Gla domain, but the entire structure and conformation of protein C and prothrombin, contribute to their quality control and susceptibility to warfarin-induced ER (endoplasmic reticulum)-associated degradation.

Transgenic animal bioreactors in biotechnology and production of blood proteins

The regulatory elements of genes used to target the tissue-specific expression of heterologous human proteins have been studied in vitro and in transgenic mice. Hybrid genes exhibiting the desired performance have been introduced into large animals. Complex proteins like protein C, factor IX, factor VIII, fibrinogen and hemoglobin, in addition to simpler proteins like alpha 1-antitrypsin, antithrombin III, albumin and tissue plasminogen activator have been produced in transgenic livestock. The amount of functional protein secreted when the transgene is expressed at high levels may be limited by the required posttranslational modifications in host tissues. This can be overcome by engineering the transgenic bioreactor to express the appropriate modifying enzymes. Genetically engineered livestock are thus rapidly becoming a choice for the production of recombinant human blood proteins.

Domain organization of the 39-kDa receptor-associated protein

The 39-kDa receptor-associated protein (RAP) is an endoplasmic reticulum resident protein that binds to the low density lipoprotein receptor-related protein (LRP) as well as certain members of the low density lipoprotein receptor superfamily and antagonizes ligand binding. In order to identify important functional regions of RAP, studies were performed to define the domain organization and domain boundaries of this molecule. Differential scanning calorimetry (DSC) experiments revealed that the process of thermal denaturation of RAP is highly reversible and occurs in a broad temperature range with two well resolved heat absorption peaks. A good fit of the endotherm was obtained with four two-state transitions suggesting these many cooperative domains in the molecule. A number of recombinant fragments of RAP were expressed in bacteria, and their domain composition and stability were characterized by DSC, circular dichroism, and fluorescence spectroscopy. The results confirmed that RAP is composed of four independently folded domains, D1, D2, D3, and D4, that encompass residues 1-92, 93-163, 164-216, and 217-323, respectively. The first and the fourth domains preserved their structure and stability when isolated, whereas the compact structure of the fragment corresponding to D2 seems to be altered when isolated from the parent molecule. Isolated D3 was partially degraded during isolation from bacterial lysates. The isolated D4 was capable of binding with high affinity to LRP whereas neither D1 nor D2 bound. At the same time a fragment containing both D1 and D2 exhibited high affinity binding to LRP. These facts combined with the thermodynamic analysis of the melting process of the fragments containing D1 and D2 indicate that these two domains interact with each other and that the proper folding of the second domain into a native-like active conformation requires presence of the first domain.

Zn(2+)-selective purification of recombinant proteins from the milk of transgenic animals

The milk of transgenic livestock is becoming a viable, large-scale source of post-translationally complex, recombinant therapeutic proteins. Recombinant vitamin K-dependent proteins such as human protein C (rhPC) and Factor IX can be produced in milk. However, rate limitations in post-translational modification such as intrachain proteolytic cleavage and gamma-carboxylation occur in the mammary gland. Thus, most desirable recombinant products often exist as sub-populations in milk because the mammary gland tends to secrete incompletely processed polypeptides. In general, a nonaffinity purification strategy by which to purify mature recombinant proteins from milk is desirable. Zn2+ is used to selectively modify ion-exchange adsorption behavior of endogenous and recombinant milk proteins through conformational changes which cause aggregation and or precipitation. Zn(2+)-selective precipitation of milk and recombinant proteins results in the purification of active rhPC at high yield from the milk of transgenic pigs using expanded bed chromatography. This method selects for rhPC which is both heterodimeric and properly gamma-carboxylated. Due to the homology of milk proteins among different species, this same Zn(2+)-selective precipitation strategy is useful for developing purification methods for other recombinant proteins from the milk of transgenic livestock.

The shape of thrombomodulin and interactions with thrombin as determined by electron microscopy

Studies have been carried out to investigate aspects of the structure of thrombomodulin, an endothelial cell glycoprotein that binds thrombin and accelerates both the thrombin-dependent activation of protein C and the inhibition of antithrombin III. We have determined the shape of SolulinTM, a soluble recombinant form of human thrombomodulin missing the transmembrane and cytoplasmic domains, by electron microscopy of preparations rotary-shadowed with tungsten. Solulin appears to be an elongated molecule about 20 nm long that has a large nodule at one end and a smaller nodule near the other end from which extends a thin strand. About half of the molecules form bipolar dimers apparently via interactions between these thin strands. Electron microscopy of complexes formed between Solulin and human alpha-thrombin revealed that a single thrombin molecule appears to bind to the smaller nodule of Solulin, suggesting that this region contains the epidermal growth factor-like domains 5 and 6. Epidermal growth factor-like domains 1-4 comprise the connector between the small and large nodule, which is the lectin-like domain; the thin strand at the other end of the molecule is the carbohydrate-rich region. With chondroitin sulfate-containing soluble thrombomodulin produced from either human melanoma cells Bowes or Chinese hamster ovary cells, a higher percentage of molecules bound thrombin and, in some cases, two thrombin molecules were attached to one soluble thrombomodulin in approximately the same region. These structural studies provide insight into the structure of thrombomodulin and its interactions with thrombin as well as aspects of the mechanisms of its actions.

Recombinant protein production in transgenic animals

The engineering of animals for recombinant protein production has gone beyond the stage of identifying proper regulatory sequences. Efforts are now spent on the generation of transgenic animals that process heterologous proteins more efficiently. Another line of research is the development of strategies aimed at bypassing pronuclear microinjection.

Structural changes in factor VIIa induced by Ca2+ and tissue factor studied using circular dichroism spectroscopy

Factor VIIa (fVIIa) is composed of four discrete domains, a gamma-carboxyglutamic acid (Gla)-containing domain, two epidermal growth factor (EGF)-like domains, and a serine protease domain, all of which appear to be involved, to different extents, in an optimal interaction with tissue factor (TF). All except the second EGF-like domain contain at least one Ca2+ binding site and many properties of fVIIa, e.g., TF and phospholipid binding and amidolytic activity, are Ca(2+)-dependent. A CD study was performed to characterize and locate the conformational changes in fVIIa induced by Ca2+ and TF binding. In addition to intact fVIIa, derivatives lacking the Gla domain or the protease domain were used. Assignment of the Ca(2+)-induced changes in the far-UV region of the fVIIa spectrum to the Gla domain could be made by comparing the CD spectra obtained with these fVIIa derivatives. The changes primarily appeared to reflect a Ca(2+)-induced ordering of alpha-helices existing in the apo state of fVIIa. This was corroborated by models of the apo and Ca2+ forms of fVIIa, obtained as difference spectra between fVIIa derivatives, were very similar to those of isolated Gla peptides from other vitamin K-dependent plasma proteins. The near-UV CD spectrum of fVIIa was dominated by aromatic residues residing in the protease domain and specific bands affected by Ca2+ were indicative of tertiary structural alterations. The formation of a fVIIa:TF complex led to secondary structural changes that appeared to be restricted to the catalytic domain, possibly shedding light on the mechanism by which TF induces an enhancement of fVIIa catalytic activity.

Domain structure, stability and interactions in streptokinase

The structural organization of streptokinase was established through detailed study of its denaturation by differential scanning calorimetry. Streptokinase exhibited a complex endotherm whose shape was sensitive to changing pH. In all cases the endotherms were easily described by four two-state transitions indicating unambiguously the presence of four independently folded domains in the molecule. Two of them were slightly destabilized by lowering pH from 7.0 to 3.8 while the other two were stabilized in this pH range. Two proteolytic fragments of streptokinase were examined, a 37-kDa fragment beginning at Ile1 with a cleavage following Phe62, and a 17-kDa fragment beginning at Lys 147. At pH 8.5, three two-state transitions were observed in the former and two in the latter indicating this many domains in each and suggesting that the fragments are formed by a step-wise removal of individual domains from the parent molecule. Comparison of the melting of these fragments with that of streptokinase allowed the first two transitions in the parent protein to be assigned to the melting of two NH2-terminal domains and the two higher-temperature transitions to the melting of the two COOH-terminal domains. The latter two domains strongly interact with each other since the absence of the most stable extreme COOH-terminal domain in both fragments resulted in a strong destabilization of its neighbor whose melting occurred with a midpoint near room temperature. The two NH2-terminal domains seem to be more independent. One of them melts similarly in the parent protein and both fragments while the other, formed by the 1-146 region, is less stable in the 37-kDa fragment. This destabilization is most probably due to the cleavage after Phe62 which, based on the sequence similarity of streptokinase with serine proteases, may be part of a surface-oriented loop.

Activation of recombinant human protein C

We have produced recombinant human Protein C (rHPC) in the milk of transgenic swine. After purification, we have analyzed the interaction of teh zymogen with Protac, thrombin/thrombomodulin and thrombin alone. The amidolytic and anticoagulant activities of rAPC after Protac activation were approximately 80% those of its human plasma counterpart. Upon the excision of the activation peptide by thrombin/thrombomodulin complex, both the natural and recombinant activation products had similar enzymatic and biological activities. This observation can be attributed to the difference in the mechanism of action between the two activators and structural differences between HPC and rHPC.

Thermal stability of individual domains in platelet glycoprotein IIbIIIa

Thermal denaturation of platelet glycoprotein IIbIIIa (integrin alpha IIb beta 3) was investigated by spectrofluorimetry and differential scanning calorimetry (DSC). Two forms of the protein were compared: active IIbIIIa, i.e., that fraction that binds to RGD-Sepharose, and inactive IIbIIIa, the non-binding fraction. At pH 8.5 in the presence of octyl glucoside and Ca2+ both forms exhibited a broad complex endotherm consisting of a well expressed low-temperature heat-absorption peak in the range of 40-65 degrees C followed by a broad peak stretching over 65-110 degrees C. Each endotherm could be deconvoluted into at least eight transitions reflecting the melting of at least this many independently folded domains. The first two transitions in the region of the low-temperature peak had similar positions in both forms while at least some of the other transitions occurred at higher temperature in the active protein suggesting that some of the domains are more stable in the latter. When both fractions of IIbIIIa were heated in the fluorometer a sigmoidal transition was observed in the region of the first endothermic peak where the two thermolabile domains melt. This transition was destabilized by 15 degrees C in the presence of EDTA, suggesting that these domains are formed by the 243-468 region of the IIb subunit which contains four Ca(2+)-binding motifs. It was further stabilized by 3 degrees C upon addition of the GRGDSPK peptide in the presence of Ca2+ while in EDTA the peptide had no effect. This is consistent with the involvement of Ca(2+)-binding region in the formation of the ligand-binding site. A 66-kDa chymotryptic fragment, containing the 17-kDa NH2-terminal portion of the IIIa subunit disulfide-linked to its 50-kDa COOH-terminal portion including the cysteine-rich core, exhibited a fluorescence-detected Ca(2+)-independent transition in the region where the higher temperature DSC-detected transitions occur suggesting that some of the latter may be connected with the melting of the corresponding portions of IIbIIIa.