Expression of biologically active human antithrombin III by recombinant baculovirus in Spodoptera frugiperda cells

Efficient single step chromatographic purification of recombinant human antithrombin (rhAT) from Saccharomyces cerevisiae

Antithrombin (AT) is a glycoprotein that inactivates the several physiological target enzymes of coagulation system. The effect of purification strategies plays a crucial role in getting maximum recovery of yield, purity and biological activity of recombinant human antithrombin (rhAT). In the present work, the task of purifying rhAT from Saccharomyces cerevisiae BY4741 has been carried out using two different approaches such as cross flow filtration (CFF) system and chromatography methods. In the first approach, the protein was concentrated and partially purified through CFF to achieve maximum recovery yield and purity of 87 and 94 %, respectively. In the second approach, purification involved a single step chromatography with various types of ion exchange and size exclusion resins to analyze the maximum rhAT recovery yield and purity. From the experimental results, it has been observed that the size exclusion chromatography (SEC) technique with Superose 12 matrix was suitable for the purification of rhAT and achieved the maximum recovery yield and purity of 51 and 97 %, respectively. Further, to acquire a high recovery yield and purity of rhAT, the effect of various chromatographic conditions such as mobile phase, mobile phase pH, flow rate, sample volume and sample concentration were also investigated. Under the optimal chromatographic conditions, rhAT was significantly recovered and purified in a single step with maximum recovery yield, purity and biological activity of 67, 99 % and 410 IU/L, respectively. Based on these investigations, it was concluded that SEC with Superose 12 matrix was a more suitable and a potential method for the purification of rhAT.

Comparison of biological activities of human antithrombins with high-mannose or complex-type nonfucosylated N-linked oligosaccharides

The structure of the N-linked oligosaccharides attached to antithrombin (AT) has been shown to affect its anticoagulant activity and pharmacokinetics. Human AT has biantennary complex-type oligosaccharides with the unique feature of lacking a core fucose, which affects its biological activities by changing its heparin-binding affinity. In human plasma, AT circulates as a mixture of the α-form bearing four oligosaccharides and the β-form lacking an oligosaccharide at Asn135. However, it remains unclear how the immature high-mannose-type oligosaccharides produced by mammalian cells affect biological activities of AT. Here, we succeeded in directly comparing the activities between the high-mannose and complex types. Interestingly, although there were no substantial differences in thrombin inhibitory activity, the high-mannose type showed higher heparin-binding affinity. The anticoagulant activities were increased by heparin and correlated with the heparin-binding affinity, resulting in the strongest anticoagulant activity being displayed in the β-form with the high-mannose type. In pharmacokinetic profiling, the high-mannose type showed a much shorter plasma half-life than the complex type. The β-form was found to have a prolonged plasma half-life compared with the α-form for the high-mannose type; conversely, the α-form showed a longer half-life than the β-form for the complex-type. The present study highlights that AT physiological activities are strictly controlled not only by a core fucose at the reducing end but also by the high-mannose-type structures at the nonreducing end. The β-form with the immature high-mannose type appears to function as a more potent anticoagulant than the AT typically found in human plasma, once it emerges in the blood.

High level expression of recombinant human antithrombin in the mammary gland of rabbits by adenoviral vectors infection

Expression of recombinant pharmaceutical proteins in the mammalian mammary gland is of great interest for the medical industry. This study was designed to express recombinant human antithrombin (rhAT) in the mammary gland of rabbits by adenovirus vectors infection. Replication-defective adenovirus encoding human antithrombin complementary DNA (cDNA) was constructed and directly infused into the mammary gland of rabbits via the teat canal. The milk serum was collected from the infected mammary gland 48 h post-infection and subjected to Western blot analysis, Enzyme-linked immunosorbent assay (ELISA), and antithrombotic activity assay. In this way, the target protein was verified, and a high expression level of rhAT up to 4.8 g/L was obtained, and antithrombotic activity of the rhAT was not different than that of a standard human antithrombin protein (p > 0.05). Compared to previous attempts to produce human antithrombin in the mammary gland of transgenic animals or fractionation the plasma of blood donors, the method for rhAT expression we established would reduce production cost and further increase production efficacy.

Mutation of the H-helix in antithrombin decreases heparin stimulation of protease inhibition

Blood clotting proceeds through the sequential proteolytic activation of a series of serine proteases, culminating in thrombin cleaving fibrinogen into fibrin. The serine protease inhibitors (serpins) antithrombin (AT) and protein C inhibitor (PCI) both inhibit thrombin in a heparin-accelerated reaction. Heparin binds to the positively charged D-helix of AT and H-helix of PCI. The H-helix of AT is negatively charged, and it was mutated to contain neutral or positively charged residues to see if they contributed to heparin stimulation or protease specificity in AT. To assess the impact of the H-helix mutations on heparin stimulation in the absence of the known heparin-binding site, negative charges were also introduced in the D-helix of AT. AT with both positively charged H- and D-helices showed decreases in heparin stimulation of thrombin and factor Xa inhibition by 10- and 5-fold respectively, a decrease in affinity for heparin sepharose, and a shift in the heparin template curve. In the absence of a positively charged D-helix, changing the H-helix from neutral to positively charged increased heparin stimulation of thrombin inhibition 21-fold, increased heparin affinity and restored a normal maximal heparin concentration for inhibition.

Temporal transcription program of recombinant Autographa californica multiple nucleopolyhedrosis virus

Baculoviruses, a family of large, rod-shaped viruses that mainly infect lepidopteran insects, have been widely used to transduce various cells for exogenous gene expression. Nonetheless, how a virus controls its transcription program in cells is poorly understood. With a custom-made baculovirus DNA microarray, we investigated the recombinant Autographa californica multiple nucleopolyhedrosis virus (AcMNPV) gene expression program in lepidopteran Sf21 cells over the time course of infection. Our analysis of transcription kinetics in the cells uncovered sequential viral gene expression patterns possibly regulated by different mechanisms during different phases of infection. To gain further insight into the regulatory network, we investigated the transcription program of a mutant virus deficient in an early transactivator (pe38) and uncovered several pe38-dependent and pe38-independent genes. This study of baculovirus dynamic transcription programs in different virus genetic backgrounds provides new molecular insights into how gene expression in viruses is regulated.

Recombinant functional human lactoferrin expressed in baculovirus system

Human lactoferrin (hLf) is a multifunctional iron-binding glycoprotein. In this study, we amplified hLf cDNA by reverse transcription-polymerase chain reaction from normal human mammary gland. The nucleotide sequence of the hLf was identical to the known hLf. We constructed a recombinant virus, vBm-hLf, harboring the hLf gene and exploited the BmN cells as host to produce recombinant human lactoferrin (rhLf). It was found that a recombinant protein with a molecular mass of approximately 78 kDa was expressed. Approximately 13.5 microg rhLf was purified from 1-2x10(5) BmN cells infected by vBm-hLf and the rhLf proved to be biologically active. This method established in our study will pave the way for efficient production of rhLf for further application of this protein in the future.

Purification and characterization of recombinant human antithrombin containing the prelatent form in Chinese hamster ovary cells

Antithrombin (AT) is a serine proteinase inhibitor and a major regulator of the blood coagulation cascade. AT in human plasma has two isoforms, a predominant alpha-isoform and a minor beta-isoform; the latter lacks N-glycosylation at Asn 135 and has a higher heparin affinity. From the difference in its folding states, the AT molecule can be separated into three forms: a native form, a denatured and inactive form known as the latent form, and a partially denatured form called the prelatent form. In this study, we purified and characterized recombinant human AT (rAT) containing the prelatent form produced by Chinese hamster ovary (CHO) cells. When rAT was purified at physiological pH, its specific activity was lower than that of plasma-derived human AT (pAT). The latent and prelatent forms were detected in rAT by using hydrophobic interaction chromatography analysis. However, when rAT was purified at alkaline pH, the prelatent form was reversibly folded to the native form and the inhibitory activity of rAT increased to a value similar to that of pAT. Highly purified rAT was analyzed and compared with pAT by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, circular dichroism spectroscopy, amino acid composition, N-terminal sequence, monosaccharide composition, peptide mapping, and heparin-binding affinity. From these analyses, rAT was found to be structurally identical to pAT, except for carbohydrate side-chains. rAT in CHO cells had a high beta-isoform content and it caused a higher heparin affinity than by pAT and also pH-dependent reversible inhibitory activity.

Characterization of N-linked oligosaccharides attached to recombinant human antithrombin expressed in the yeast Pichia pastoris

We studied the structures of four N-linked oligosaccharide chains of the recombinant human antithrombin (rAT) expressed in the yeast Pichia pastoris. rAT was fully glycosylated at Asn 96 and Asn 155, whereas the glycosylation on Asn 135 and Asn 192 was partial. The glycosylation level on Asn 135 was only 12% and this reduction is assumed to be one of the reasons for a higher heparin-binding affinity of rAT than plasma-derived human antithrombin (pAT). In order to determine the sizes and electrostatic charges of the N-linked oligosaccharides, rAT was treated with PNGase F, and the reduced ends were labelled by pyridylamination followed by analysis using anion exchange and amide adsorption columns. The N-linked oligosaccharides were 78% neutral and 22% phosphomannosylated. The neutral oligosaccharides were thought to be Man(9-12)GlcNAc(2) as their major components. The phosphomannosylated oligosaccharides were then subjected to mild acid hydrolysis and/or digestion with alkaline phosphatase, and their charge shifts were analysed by the affinity to an anion exchange column. Among phosphomannosylated oligosaccharides, monophosphate diester type was predominant, whereas negatively charged diphosphate diester and monophosphate monoester types were minor components. The mannose residues at the non-reducing end(s) of Man(9-12)GlcNAc(2) were phosphomannosylated or phosphorylated and these are the major components. Because rAT is less negatively charged than pAT, which has disialyl biantennary N-glycans, it might be less repulsive to pentasaccharide-bearing anticoagulantly active heparan sulphate proteoglycan molecules exposed on the surface of the damaged vascular vessels.

Expression and characterization of recombinant human antithrombin III in Pichia pastoris

Antithrombin III (ATIII) is a member of the serpin superfamily and a major regulator of the blood coagulation cascade. To express recombinant human ATIII (rATIII) in the methylotrophic yeast Pichia pastoris, we constructed an rATIII expression plasmid which contained the ATIII cDNA encoding mature protein region connected with the truncated mAOX2 promoter and the SUC2 secretion signal, introduced it into the P. pastoris genome, and screened for a single copy transformant. The secretion of rATIII from the transformant reached a level of 320 IU/L in the culture broth at 169 h. From the culture-supernatant, rATIII was purified to over 99% by heparin-affinity chromatography and other column chromatography methods. We characterized rATIII and compared it with human plasma-derived ATIII (pATIII). The purified rATIII possessed correct N-terminal amino acid sequence, and its molecular weight by SDS-PAGE of 56,000 Da was slightly different from the 58,000 Da of pATIII. Sequence and mass spectrometry analysis of BrCN fragments revealed that posttranslational modifications had occurred in rATIII. O-linked mannosylation was found at Ser 3 and Thr 9, and in some rATIII molecules, modification with O-linked mannosyl-mannose had probably occurred at Thr 386, close to the reactive center. Although the heparin-binding affinity of rATIII was 10-fold higher than that of pATIII, its inhibitory activity against thrombin was only half. As the conformation of rATIII and pATIII by circular dichroism spectroscopy was similar, O-glycosylation in the reactive center loop was assumed to be mainly responsible for the decreased inhibitory activity. pATIII can inactivate thrombin through formation of a stable thrombin-ATIII complex, but rATIII modified with O-glycosylation in the reactive center loop may act as a substrate rather than an inhibitor of thrombin.

The efficient catabolism of thrombin-protease nexin 1 complexes is a synergistic mechanism that requires both the LDL receptor-related protein and cell surface heparins

Protease nexin 1 (PN1) is a serine protease inhibitor (SERPIN) that acts as a suicide substrate for thrombin (Th) and urokinase-type plasminogen activator (uPA). PN1 forms 1:1 stoichiometric complexes with these proteases, which are then rapidly bound, internalized, and degraded. The low density lipoprotein receptor-related protein (LRP) is the receptor responsible for the internalization of protease-PN1 complexes. However, we found that the LRP is not significantly involved in the initial cell surface binding of thrombin-PN1, leading us to investigate what cellular component was responsible for this initial interaction. Since Th-PN1 complexes retain a high-affinity for heparin after complex formation, unlike several of the other SERPINs, we tested the possibility that cell surface heparins were involved in initial complex binding. Soluble heparin was found to be a potent inhibitor of the binding of Th-PN1 to the cell surface and greatly facilitated the dissociation of Th-PN1 complexes pre-bound in the absence of soluble heparin. To ascertain the role of cell surface heparins, further studies were done using complexes of thrombin and PN1(K7E), a variant of PN1 in which the heparin binding site was rendered non-functional. When added at equal initial concentrations of complexes, Th-PN1(K7E) was catabolized 5- to 10-fold less efficiently than Th-PN1, a direct result of the greatly diminished initial binding of the Th-PN1(K7E) complexes. These data demonstrate the sizable contribution of cell surface heparins to Thrombin-PN1 complex binding and support a model in which these heparins act to concentrate the complexes at the cell surface facilitating their subsequent LRP-dependent endocytosis.

Characterization of human lactoferrin produced in the baculovirus expression system

Lactoferrin, an iron-binding 80-kDa glycoprotein, is a major component of human milk whose structure is now well defined. The binding site of lactoferrin to the membrane receptor of lymphocyte has been located in the region 4-52, but the amino acids directly involved in the interaction have not been identified yet. To gain further insights into the structure-function relationships of the lactoferrin binding site, we first expressed the cDNA encoding human lactoferrin in the lepidoptera Spodoptera frugiperda cells (Sf9) using a recombinant baculovirus. The selected transformant secreted and N-glycosylated protein of 78 kDa which was immunoprecipitated by specific anti-lactoferrin antibodies. To confirm the structure and the function of the recombinant lactoferrin, the protein was purified by ion-exchange chromatography and its physical, biochemical, and biological properties were compared with those of the native protein. In particular, the N-terminal amino acid sequence and the iron-binding stability as a function of pH, of both proteins, were identical. The main difference concerns the glycosylation which leads to glycans of lower molecular masses as detected by the electrophoretic mobility of lactoferrin after N-glycosidase F treatment and matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. Despite the different glycosylation features, the recombinant lactoferrin retained the binding property to the Jurkat human lymphoblastic T-cell line of the native lactoferrin. On the basis of these analyses, production of protein mutants generated by site-directed mutagenesis is now in process.

Requirement of lysine residues outside of the proposed pentasaccharide binding region for high affinity heparin binding and activation of human antithrombin III

Variant forms of human antithrombin III with glutamine or threonine substitutions at Lys114, Lys125, Lys133, Lys136, and Lys139 were expressed in insect cells to evaluate their roles in heparin binding and activation. Recombinant native ATIII and all of the variants had very similar second order rate constants for thrombin inhibition in the absence of heparin, ranging from 1.13 x 10(5) M-1min-1 to 1.66 x 10(5) M-1min-1. Direct binding studies using 125I-flouresceinamine-heparin yielded a Kd of 6 nM for the recombinant native ATIII and K136T, whereas K114Q and K139Q bound heparin so poorly that a Kd could not be determined. K125Q had a moderately reduced affinity. Heparin binding affinity correlated directly with heparin cofactor activity. Recombinant native ATIII was nearly identical to plasma-purified ATIII, whereas K114Q and K139Q were severely impaired in heparin cofactor activity. K125Q and K136T were only slightly impaired. Based on these data, Lys114 and Lys139, which are outside of the putative pentasaccharide binding site, play pivotal roles in the high affinity binding of heparin to ATIII and the activation of thrombin inhibitory activity.

Antithrombin and its inherited deficiencies

Human antithrombin is the major inhibitor of the coagulation serine proteases accounting for approximately 80% of the thrombin inhibitory activity of plasma. It is a member of the serpin family of serine protease inhibitors and in common with some other members of this family it undergoes a dramatic increase in its inhibitory activity in the presence of heparin and other sulphated glycosaminoglycans. Two functional domains in antithrombin are recognised, the reactive site domain which interacts with the active site serine residue of the protease and the heparin binding domain. The gene for antithrombin has been cloned and its entire nucleotide sequence determined. A deficiency or functional abnormality of antithrombin may result in an increased risk of thromboembolic disease. Such deficiencies are estimated to affect as many as 1:300 of the general population and 3 to 5% of patients with thrombotic disease. On the basis of functional and immunological antithrombin assays, antithrombin deficiency may be subdivided into Types I and II. Type I disease is due to a wide variety of heterogeneous DNA mutations whilst in Type II disease missense mutations leading to single amino acid substitutions have been identified in all cases. Clinically, Type I antithrombin deficiency is associated with recurrent thromboembolic disease whereas in Type II deficiency the risk of thrombosis is closely related to the position of the mutation within the protein. Thus, heterozygotes with mutations within the heparin binding domain of antithrombin have a relatively low risk of thrombosis compared to those with mutations at or close to the reactive site of the molecule.

Post-translational processing of barley beta-glucan endohydrolases in the baculovirus-insect cell expression system

Two cDNAs encoding barley (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzymes EI and EII have been expressed in Spodoptera frugiperda (Sf9) cell cultures using the baculovirus AcNPV vector. Modifications to both the 5' and 3' ends of the cDNAs were required before satisfactory levels of expression were obtained. The modified cDNAs directed high levels of (1-->3,1-->4)-beta-glucanase expression in the Sf9 insect cell cultures, with yields of approximately 10 mg/liter of isoenzyme EI (expEI) and 15 mg/liter of isoenzyme EII (expEII). Amino acid sequence analyses showed that the expressed enzymes were processed correctly at their amino termini. However, affinity chromatography of the isoenzyme expEII on concanavalin-A (conA)-Sepharose indicated that, although the enzyme is glycosylated, the structures of the carbohydrate chains differ from those of the native enzyme. When a cDNA encoding the homologous barley (1-->3)-beta-glucanase (EC 3.2.1.39) isoenzyme GII was expressed in insect cells, aberrant amino-terminal processing of the nascent polypeptide was sometimes observed. The forms with incompletely removed signal peptides retained their substrate specificity, but exhibited slightly reduced catalytic efficiency, altered chromatographic behavior, and reduced stability at elevated temperatures. The results show that high levels of expression of recombinant plant proteins can be obtained in insect cells, but they emphasize the need to characterize thoroughly the products that are expressed in the heterologous insect cell system before comparisons are made with the native enzyme or with engineered enzyme mutants.