Plasma Clearance of Coagulation Factor VIII and Extension of Its Half-Life for the Therapy of Hemophilia A: A Critical Review of the Current State of Research and Practice

Factor VIII (FVIII) is an important component of blood coagulation as its congenital deficiency results in life-threatening bleeding. Current prophylactic therapy of the disease (hemophilia A) is based on 3-4 intravenous infusions of therapeutic FVIII per week. This poses a burden on patients, demanding reduction of infusion frequency by using FVIII with extended plasma half-life (EHL). Development of these products requires understanding FVIII plasma clearance mechanisms. This paper overviews (i) an up-to-date state of the research in this field and (ii) current EHL FVIII products, including recently approved efanesoctocog alfa, for which the plasma half-life exceeds a biochemical barrier posed by von Willebrand factor, complexed with FVIII in plasma, which results in ~1 per week infusion frequency. We focus on the EHL FVIII products' structure and function, in particular related to the known discrepancy in results of one-stage clotting (OC) and chromogenic substrate (CS) assays used to assign the products' potency, dosing, and for clinical monitoring in plasma. We suggest a possible root cause of these assays' discrepancy that is also pertinent to EHL factor IX variants used to treat hemophilia B. Finally, we discuss approaches in designing future EHL FVIII variants, including those to be used for hemophilia A gene therapy.

Characterization of interaction between blood coagulation factor VIII and LRP1 suggests dynamic binding by alternating complex contacts

BACKGROUND

Deficiency in blood coagulation factor VIII (FVIII) results in life-threating bleeding (hemophilia A) treated by infusions of FVIII concentrates. To improve disease treatment, FVIII has been modified to increase its plasma half-life, which requires understanding mechanisms of FVIII catabolism. An important catabolic actor is hepatic low density lipoprotein receptor-related protein 1 (LRP1), which also regulates many other clinically significant processes. Previous studies showed complexity of FVIII site for binding LRP1.

OBJECTIVES

To characterize binding sites between FVIII and LRP1 and suggest a model of the interaction.

METHODS

A series of recombinant ligand-binding complement-type repeat (CR) fragments of LRP1 including mutated variants was generated in a baculovirus system and tested for FVIII interaction using surface plasmon resonance, tissue culture model, hydrogen-deuterium exchange mass spectrometry, and in silico.

RESULTS

Multiple CR doublets within LRP1 clusters II and IV were identified as alternative FVIII-binding sites. These interactions follow the canonical binding mode providing major binding energy, and additional weak interactions are contributed by adjacent CR domains. A representative CR doublet was shown to have multiple contact sites on FVIII.

CONCLUSIONS

FVIII and LRP1 interact via formation of multiple complex contacts involving both canonical and non-canonical binding combinations. We propose that FVIII-LRP1 interaction occurs via switching such alternative binding combinations in a dynamic mode, and that this mechanism is relevant to other ligand interactions of the low-density lipoprotein receptor family members including LRP1.

Isolated Variable Domains of an Antibody Can Assemble on Blood Coagulation Factor VIII into a Functional Fv-like Complex

Single-chain variable fragments (scFv) are antigen-recognizing variable fragments of antibodies (FV) where both subunits (V_L and V_H) are connected via an artificial linker. One particular scFv, iKM33, directed against blood coagulation factor VIII (FVIII) was shown to inhibit major FVIII functions and is useful in FVIII research. We aimed to investigate the properties of iKM33 enabled with protease-dependent disintegration. Three variants of iKM33 bearing thrombin cleavage sites within the linker were expressed using a baculovirus system and purified by two-step chromatography. All proteins retained strong binding to FVIII by surface plasmon resonance, and upon thrombin cleavage, dissociated into V_L and V_H as shown by size-exclusion chromatography. However, in FVIII activity and low-density lipoprotein receptor-related protein 1 binding assays, the thrombin-cleaved iKM33 variants were still inhibitory. In a pull-down assay using an FVIII-affinity sorbent, the isolated V_H, a mixture of V_L and V_H, and intact iKM33 were carried over via FVIII analyzed by electrophoresis. We concluded that the isolated V_L and V_H assembled into scFv-like heterodimer on FVIII, and the isolated V_H alone also bound FVIII. We discuss the potential use of both protease-cleavable scFvs and isolated Fv subunits retaining high affinity to the antigens in various practical applications such as therapeutics, diagnostics, and research.

Molecular chaperone RAP interacts with LRP1 in a dynamic bivalent mode and enhances folding of ligand-binding regions of other LDLR family receptors

The low-density lipoprotein receptor (LDLR) family of receptors are cell-surface receptors that internalize numerous ligands and play crucial role in various processes, such as lipoprotein metabolism, hemostasis, fetal development, etc. Previously, receptor-associated protein (RAP) was described as a molecular chaperone for LDLR-related protein 1 (LRP1), a prominent member of the LDLR family. We aimed to verify this role of RAP for LRP1 and two other LDLR family receptors, LDLR and vLDLR, and to investigate the mechanisms of respective interactions using a cell culture model system, purified system, and in silico modelling. Upon coexpression of RAP with clusters of the ligand-binding complement repeats (CRs) of the receptors in secreted form in insect cells culture, the isolated proteins had increased yield, enhanced folding, and improved binding properties compared with proteins expressed without RAP, as determined by circular dichroism and surface plasmon resonance. Within LRP1 CR-clusters II and IV, we identified multiple sites comprised of adjacent CR doublets, which provide alternative bivalent binding combinations with specific pairs of lysines on RAP. Mutational analysis of these lysines within each of isolated RAP D1/D2 and D3 domains having high affinity to LRP1 and of conserved tryptophans on selected CR-doublets of LRP1, as well as in silico docking of a model LRP1 CR-triplet with RAP, indicated a universal role for these residues in interaction of RAP and LRP1. Consequently, we propose a new model of RAP interaction with LDLR family receptors based on switching of the bivalent contacts between molecules over time in a dynamic mode.

Characterization of protein unable to bind von Willebrand factor in recombinant factor VIII products

BACKGROUND

Therapeutic products with coagulation factor VIII (FVIII) have a wide range of specific activities, implying presence of protein with altered structure. Previous studies showed that recombinant FVIII products (rFVIII) contain a fraction (FVIIIFT ) unable to bind von Willebrand factor (VWF) and reported to lack activity. Because of loss of function(s), FVIIIFT can be defined as a product-related impurity, whose properties and levels in rFVIII products should be investigated.

OBJECTIVE

To isolate and characterize the FVIIIFT fraction in rFVIII products.

METHODS

Protein fractions unable (FVIIIFT ) and able (FVIIIEL ) to bind VWF were isolated from rFVIII products using immobilized VWF affinity chromatography (IVAC) and characterized by gel electrophoresis, immunoblotting, FVIII activity test, surface plasmon resonance, mass spectrometry, and for plasma clearance in mice.

RESULTS AND CONCLUSIONS

A robust IVAC methodology was developed and applied for analysis of 10 rFVIII products marketed in the United States. FVIIIFT was found at various contents (0.4%-21.5%) in all products. Compared with FVIIIEL , FVIIIFT had similar patterns of polypeptide bands by gel electrophoresis, but lower functional activity. In several representative products, FVIIIFT was found to have reduced sulfation at Tyr1680, important for VWF binding, decreased interaction with a low-density lipoprotein receptor-related protein 1 fragment, and faster plasma clearance in mice. These findings provide basic characterization of FVIIIFT and demonstrate a potential for IVAC to control this impurity in rFVIII products to improve their efficacy in therapy of hemophilia A.

Expression and characterization of a codon-optimized blood coagulation factor VIII

Essentials Recombinant factor VIII (FVIII) is known to be expressed at a low level in cell culture. To increase expression, we used codon-optimization of a B-domain deleted FVIII (BDD-FVIII). This resulted in 7-fold increase of the expression level in cell culture. The biochemical properties of codon-optimized BDD-FVIII were similar to the wild-type protein.

SUMMARY

Background Production of recombinant factor VIII (FVIII) is challenging because of its low expression. It was previously shown that codon-optimization of a B-domain-deleted FVIII (BDD-FVIII) cDNA resulted in increased protein expression. However, it is well recognized that synonymous mutations may affect the protein structure and function. Objectives To compare biochemical properties of a BDD-FVIII variants expressed from codon-optimized and wild-type cDNAs (CO and WT, respectively). Methods Each variant of the BDD-FVIII was expressed in several independent Chinese hamster ovary (CHO) cell lines, generated using a lentiviral platform. The proteins were purified by two-step affinity chromatography and analyzed in parallel by PAGE-western blot, mass spectrometry, circular dichroism, surface plasmon resonance, and chromogenic, clotting and thrombin generation assays. Results and conclusion The average yield of the CO was 7-fold higher than WT, whereas both proteins were identical in the amino acid sequences (99% coverage) and very similar in patterns of the molecular fragments (before and after thrombin cleavage), glycosylation and tyrosine sulfation, secondary structures and binding to von Willebrand factor and to a fragment of the low-density lipoprotein receptor-related protein 1. The CO preparations had on average 1.5-fold higher FVIII specific activity (activity normalized to protein mass) than WT preparations, which was attributed to better preservation of the CO structure as a result of considerably higher protein concentrations during the production. We concluded that the codon-optimization of the BDD-FVIII resulted in significant increase of its expression and did not affect the structure-function properties.

Cluster III of low-density lipoprotein receptor-related protein 1 binds activated blood coagulation factor VIII

Low-density lipoprotein receptor-related protein 1 (LRP) mediates clearance of blood coagulation factor VIII (FVIII). In LRP, FVIII binds the complement-type repeats (CRs) of clusters II and IV, which also bind a majority of other LRP ligands. No ligand is known for LRP cluster I, and only three ligands, including the LRP chaperone alpha-2 macroglobulin receptor-associated protein (RAP), bind cluster III. Using surface plasmon resonance, we found that in addition to clusters II and IV, activated FVIII (FVIIIa) binds cluster III. The specificity of this interaction was confirmed using an anti-FVIII antibody fragment, which inhibited the binding. Recombinant fragments of cluster III and its site-directed mutagenesis were used to localize the cluster's site for binding FVIIIa to CR.14-19. The interactive site of FVIIIa was localized within its A1/A3'-C1-C2 heterodimer (HDa), which is a major physiological remnant of FVIIIa. In mice, the clearance of HDa was faster than that of FVIII and prolonged in the presence of RAP, which is known to inhibit interactions of LRP with its ligands. In accordance with this, the cluster III site for RAP (CR.15-19) was found to overlap that for FVIIIa. Altogether, our findings support the involvement of LRP in FVIIIa catabolism and suggest a greater significance of the biological role of cluster III compared to that previously known.

Mapping the binding region on the low density lipoprotein receptor for blood coagulation factor VIII

Low density lipoprotein receptor (LDLR) was shown to mediate clearance of blood coagulation factor VIII (FVIII) from the circulation. To elucidate the mechanism of interaction of LDLR and FVIII, our objective was to identify the region of the receptor necessary for binding FVIII. Using surface plasmon resonance, we found that LDLR exodomain and its cluster of complement-type repeats (CRs) bind FVIII in the same mode. This indicated that the LDLR site for FVIII is located within the LDLR cluster. Similar results were obtained for another ligand of LDLR, α-2-macroglobulin receptor-associated protein (RAP), a common ligand of receptors from the LDLR family. We further generated a set of recombinant fragments of the LDLR cluster and assessed their structural integrity by binding to RAP and by circular dichroism. A number of fragments overlapping CR.2-5 of the cluster were positive for binding RAP and FVIII. The specificity of these interactions was tested by site-directed mutagenesis of conserved tryptophans within the LDLR fragments. For FVIII, the specificity was also tested using a single-chain variable antibody fragment directed against the FVIII light chain as a competitor. Both cases resulted in decreased binding, thus confirming its specificity. The mutagenic study also showed an importance of the conserved tryptophans in LDLR for both ligands, and the competitive binding results showed an involvement of the light chain of FVIII in its interaction with LDLR. In conclusion, the region of CR.2-5 of LDLR was defined as the binding site for FVIII and RAP.

Insect cell-based expression and characterization of a single-chain variable antibody fragment directed against blood coagulation factor VIII

A recombinant single-chain variable antibody fragment (scFv) KM33 was previously described as a ligand that can inhibit the function of blood coagulation factor VIII (FVIII). This scFv was previously derived from an individual with anti-FVIII antibodies manifested in FVIII functional deficiency (Hemophilia A) and expressed in bacteria. In the present work, we describe an alternative approach for fast and easy production of KM33 in insect cells (Spodoptera frugiperda). The KM33 gene was codon-optimized and expressed in secreted form using a baculovirus system. The protein was isolated using metal-affinity and size-exclusion chromatography to purity of about 96% and yield of 0.4-1.2 mg per 120 mL of culture, based on several independent expression experiments. In a binding assay using surface plasmon resonance, the insect cell-derived KM33 (iKM33) was qualified as a high-affinity ligand for FVIII. Epitope specificity of iKM33 on FVIII (C1 domain) was confirmed by testing the binding with a relevant mutant of FVIII. In several FVIII functional tests (factor Xa generation, APTT clotting, thrombin generation and video microscopy clot growth assays), iKM33 strongly inhibited FVIII activity in accordance with the clinical effect of the parental antibody. Therefore, the expressed protein was concluded to be fully functional and applicable in various assays with FVIII.

Prostate cancer outcome and tissue levels of metal ions

BACKGROUND

There are several studies examining prostate cancer and exposure to cadmium, iron, selenium, and zinc. Less data are available on the possible influence of these metal ions on prostate cancer outcome. This study measured levels of these ions in prostatectomy samples in order to examine possible associations between metal concentrations and disease outcome.

METHODS

We obtained formalin fixed paraffin embedded tissue blocks of prostatectomy samples of 40 patients with PSA recurrence, matched 1:1 (for year of surgery, race, age, Gleason grading, and pathology TNM classification) with tissue blocks from 40 patients without recurrence (n = 80). Case-control pairs were compared for the levels of metals in areas adjacent to tumors. Inductively coupled plasma-mass spectrometry (ICP-MS) was used for quantification of Cd, Fe, Zn, and Se.

RESULTS

Patients with biochemical (PSA) recurrence of disease had 12% lower median iron (95 µg/g vs. 111 µg/g; P = 0.04) and 21% lower zinc (279 µg/g vs. 346 µg/g; P = 0.04) concentrations in the normal-appearing tissue immediately adjacent to cancer areas. Differences in cadmium (0.489 µg/g vs. 0.439 µg/g; 4% higher) and selenium (1.68 µg/g vs. 1.58 µg/g; 5% higher) levels were not statistically significant in recurrence cases, when compared to non-recurrences (P = 0.40 and 0.21, respectively).

CONCLUSIONS

There is an association between low zinc and low iron prostate tissue levels and biochemical recurrence in prostate cancer. Whether these novel findings are a cause or effect of more aggressive tumors, or whether low zinc and iron prostatic levels raise implications for therapy, remains to be investigated.

Analysis of iron, zinc, selenium and cadmium in paraffin-embedded prostate tissue specimens using inductively coupled plasma mass-spectrometry

Formalin-fixed paraffin-embedded (FFPE) tissue specimens represent a valuable and abundant resource of pathologic material for various biomedical studies. In the present study, we report the application of high-resolution inductively coupled mass-spectrometry (ICP-MS) for quantification of Fe, Zn, Se and Cd in FFPE prostate tissue. These elements have a possible role in the development of prostate diseases: while Zn and Se are needed for a healthy prostate, Cd shows multiple toxic and carcinogenic effects. Excessive accumulation of Fe induces the production of highly reactive hydroxyl radical species, which may play a role in cancer etiopathogenesis. To assess whether the levels of these metals in the FFPE prostate tissue represent their original content, we compared their levels with those in the fresh tissue (on dry weight basis) in samples obtained from 15 patients. We found that in FFPE tissue, the recoveries of Se, Fe, Cd and Zn were progressively decreased, 97+/-11% (r=0.88), 82+/-22% (r=0.86), 59+/-23% (r=0.69) and 24+/-11% (r=0.38), respectively. Thus, the use of correction factors, determined as k=0.16 for Se, k=0.20 for Fe, k=0.27 for Cd and k=0.67 for Zn, is required to estimate the retrospective levels of these elements in the parental non-processed fresh (wet) prostate tissue. The technique used in this study enables the analysis of archival FFPE prostate tissue for the concentrations of Fe, Zn, Se and Cd to study association between the levels of these metals and prostate disease.

Localization of the low-density lipoprotein receptor-related protein regions involved in binding to the A2 domain of coagulation factor VIII

Catabolism of coagulation factor VIII (FVIII) is mediated by low-density lipoprotein receptor-related protein (LRP). The ligand-binding sites of LRP are formed by complement-type repeats (CR), and CR clusters II and IV bind most of LRP ligands. FVIII contains two major LRP-binding sites located in the A2 and A3 domains. This study was aimed to identify specific complement-type repeats of LRP involved in interaction with the A2 site and to probe their functional importance in A2 catabolism. We generated individual LRP clusters II, III and IV, along with nine overlapping CR triplets encompassing clusters II and IV in a baculovirus expression system and studied their interaction with isolated A2. In surface plasmon resonance (SPR) assay, A2 bound to clusters II and IV with KDs 22 and 39 nM, respectively, and to the majority of CR triplets with affinities in the range of KDs 25-90 nM. Similar affinities were determined for A2 interaction with a panel of CR doublets overlapping cluster II (CR 3-4, 4-5, 5-6, 6-7 and 7-8). These LRP fragments inhibited the binding of 125I-A2 to LRP in solid-phase assay, LRP-mediated internalization of 125I-A2 in cell culture and 125I-A2 clearance from the mouse circulation. Point mutations of critical A2 residues of the LRPbinding site resulted in differential reduction or abolishment of its binding to LRP fragments. We conclude that A2 interacts with LRP via multiple binding sites spanning CR 3-8 in cluster II and CR 23-29 in cluster IV, and the minimal A2-binding unit of LRP is formed by two adjacent CR.

Identification of Coagulation Factor VIII A2 Domain Residues Forming the Binding Epitope for Low-Density Lipoprotein Receptor-Related Protein,

Regulation of the coagulation factor VIII (fVIII) level in circulation involves a hepatic receptor low-density lipoprotein receptor-related protein (LRP). One of two major LRP binding sites in fVIII is located within the A2 domain (A2), likely exposed within the fVIII complex with von Willebrand factor and contributing to regulation of fVIII via LRP. This work aimed to identify A2 residues forming its LRP-binding site, previously shown to involve residues 484-509. Isolated A2 was subjected to alanine-scanning mutagenesis followed by expression of a set of mutants in a baculovirus system. In competition and surface plasmon resonance assays, affinities of A2 mutants K466A, R471A, R484A, S488A, R489A, R490A, H497A, and K499A for LRP were found to be decreased by 2-4-fold. This correlated with 1.3-1.5-fold decreases in the degree of LRP-mediated internalization of the mutants in cell culture. Combining these mutations into pairs led to cumulative effects, i.e., 7-13-fold decrease in affinity for LRP and 1.6-2.2-fold decrease in the degree of LRP-mediated internalization in cell culture. We conclude that the residues mentioned above play a key role in formation of the A2 binding epitope for LRP. Experiments in mice revealed an approximately 4.5 times shorter half-life for A2 in the circulation in comparison with that of fVIII. The half-lives of A2 mutant R471A/R484A or A2 co-injected with receptor-associated protein, a classical ligand of LRP, were prolonged by approximately 1.9 and approximately 3.5 times, respectively, compared to that of A2. This further confirms the importance of the mutated residues for interaction of A2 with LRP and suggests the existence of an LRP-dependent mechanism for removing A2 as a product of dissociation of activated fVIII from the circulation.

Cell surface heparan sulfate proteoglycans participate in factor VIII catabolism mediated by low density lipoprotein receptor-related protein

We have demonstrated previously that catabolism of a coagulation factor VIII (fVIII) from its complex with von Willebrand factor (vWf) is mediated by low density lipoprotein receptor-related protein (LRP) (Saenko, E. L., Yakhyaev, A. V., Mikhailenko, I., Strickland, D. K., and Sarafanov, A. G. (1999) J. Biol. Chem. 274, 37685-37692). In the present study, we found that this process is facilitated by cell surface heparan sulfate proteoglycans (HSPGs). This was demonstrated by simultaneous blocking of LRP and HSPGs in model cells, which completely prevented fVIII internalization and degradation from its complex with vWf. In contrast, the selective blocking of either receptor had a lesser effect. In vivo studies of clearance of (125)I-fVIII-vWf complex in mice also demonstrated that the simultaneous blocking of HSPGs and LRP led to a more significant prolongation of fVIII half-life (5.5-fold) than blocking of LRP alone (3.5-fold). The cell culture and in vivo experiments revealed that HSPGs are also involved in another, LRP-independent pathway of fVIII catabolism. In both pathways, HSPGs act as receptors providing the initial binding of fVIII-vWf complex to cells. We demonstrated that this binding occurs via the A2 domain of fVIII, since A2, but not other portions of fVIII or isolated vWf, strongly inhibited cell surface binding of fVIII-vWf complex, and the affinities of A2 and fVIII-vWf complex for the cells were similar. The A2 site involved in binding to heparin was localized to the region 558-565, based on the ability of the corresponding synthetic peptide to inhibit A2 binding to heparin, used as a model for HSPGs.

Role of the low density lipoprotein-related protein receptor in mediation of factor VIII catabolism

In the present study, we found that catabolism of coagulation factor VIII (fVIII) is mediated by the low density lipoprotein receptor-related protein (LPR), a liver multiligand endocytic receptor. In a solid phase assay, fVIII was shown to bind to LRP (K(d) 116 nM). The specificity was confirmed by a complete inhibition of fVIII/LRP binding by 39-kDa receptor-associated protein (RAP), an antagonist of all LRP ligands. The region of fVIII involved in its binding to LRP was localized within the A2 domain residues 484-509, based on the ability of the isolated A2 domain and the synthetic A2 domain peptide 484-509 to prevent fVIII interaction with LRP. Since vWf did not inhibit fVIII binding to LRP, we proposed that LRP receptor may internalize fVIII from its complex with vWf. Consistent with this hypothesis, mouse embryonic fibroblasts that express LRP, but not fibroblasts genetically deficient in LRP, were able to catabolize (125)I-fVIII complexed with vWf, which was not internalized by the cells. These processes could be inhibited by RAP and A2 subunit of fVIII, indicating that cellular internalization and degradation were mediated by interaction of the A2 domain of fVIII with LRP. In vivo studies of (125)I-fVIII.vWf complex clearance in mice demonstrated that RAP completely inhibited the fast phase of the biphasic (125)I-fVIII clearance that is responsible for removal of 60% of fVIII from circulation. Inhibition of the RAP-sensitive phase prolonged the half-life of (125)I-fVIII in circulation by 3.3-fold, indicating that LRP receptor plays an important role in fVIII clearance.

Effect of von Willebrand Factor and its proteolytic fragments on kinetics of interaction between the light and heavy chains of human factor VIII

It was previously shown that vWF increases the rate of divalent cation-mediated fVIII reconstitution from isolated light chain (LCh) and heavy chain (HCh) subunits. We examined the effect of vWF on kinetic parameters for interaction between LCh and HCh in the presence of Ca2+ and Mn2+ ions, the most effective mediators of fVIII reconstitution from isolated subunits, and determined the minimal structural portion of vWF able to enhance fVIII formation. We found that affinity (Kd) for LCh/HCh binding mediated by Ca2+ and Mn2+ was 91 and 34.9 nM in the absence of vWF and 15.5 and 5.6 nM in its presence. This decrease of Kd resulted from a sixfold increase of the association rate constant (k(on)) for this interaction. The value of the dissociation rate constant (k(off)) for LCh/HCh complex was lower in the presence of Mn2+ (k(off) 4.6x 10(-6) s(-1)) than Ca2+ (k(off) 8.4 x 10(-6) s(-1)) but in both cases vWF had no effect on k(off). This indicates that at physiological concentration of 1 nM the rate of fVIII inactivation via dissociation to subunits would be entirely determined by the k(off) value, and it should not depend on the presence of vWF. Indeed, our experiments demonstrated that vWF did not have any effect on the rate of fVIII inactivation resulting from its dissociation to subunits at the physiological concentrations of the fVIII and vWF proteins. We identified the minimal portion of the vWF molecule, able to enhance reconstitution of fVIII from isolated subunits. Only vWF large proteolytic N-terminal homodimeric fragment SPIII (vWF residues 1-1365), but not small monomeric N-terminal fragment SPIII-T4 (1-272), both of which are known to contain a major fVIII binding site, was able to support reconstitution of fVIII activity from isolated LCh and HCh subunits in the presence of Mn2+ or Ca2+. The effect of SPIII on the LCh/HCh association was similar to that of vWF, because both proteins identically increased of the value of k(on) and did not alter the k(off) value.

Role of activation of the coagulation factor VIII in interaction with vWf, phospholipid, and functioning within the factor Xase complex

Blood coagulation factor VIII (fVIII) in its nonactivated form circulates in plasma in a complex with von Willebrand factor (vWf). Upon activation by thrombin- or factor Xa-mediated site-specific proteolysis, activated fVIII (fVIIIa) serves as a cofactor for factor IXa. This protein complex assembled on a phospholipid surface (factor Xase) activates factor X. This complex plays the key role in the intrinsic pathway of blood coagulation. We reviewed the molecular events triggered by fVIII activation, which are required for the assembly and functioning of the Xase complex, including fVIIIa dissociation from vWf and a significant increase of fVIII affinity for binding to the phospholipid surface. Both events are mediated by activation-related cleavage within fVIII light chain (LCh), releasing the 40 amino-acid N-terminal LCh peptide, which is followed by a conformational change within the C2 domain. The conformational change within LCh is also required for the optimal fVIII cofactor functioning within the factor Xase complex, exerted via fVIIIa interactions with phospholipid, factor IXa, and factor X. Since factor IXa not only stabilizes but also proteolytically inactivates fVIIIa within the factor Xase complex, the stability of the membrane-bound fVIIIa in the presence and absence of factor IXa is discussed. In conclusion, we outline some new possible directions of the research. One of them arises from the recently demonstrated ability of plasma lipoproteins to provide a phospholipid surface for the assembly of the factor Xase complex in vitro. This finding raises a possibility that lipoproteins participate in factor Xase functioning in vivo and suggests a direct link between elevated levels of lipoproteins associated with atherosclerosis and increased thrombogenicity associated with this disease.

[Determination and analysis of the primary structure of a genomic sequence adjacent to the 3'-end of the human tissue plasminogen activator gene]

Primary structure was determined for the recently cloned f1/BglII-fragment [19] containing 2102 b.p. of the human tissue plasminogen activator (tPA) gene 3' end and adjacent DNA region. Computer analysis has revealed an Alu-repeat 820 b.p. downstream the tPA gene; the sequence proved to have a considerable homology (86-88%) with the Alus from the 3'-untranslated regions (3'UTRs) of cytochrome P-450, lysozyme and p53 protein human mRNAs. The same homology was estimated for this Alu in reversed orientation and Alus from the 3'UTRs of some other human mRNAs. In contrast, the homology between this 3' end tPA gene flanking Alu-repeat and other Alus dispersed throughout the gene introns either direct or reversed, was less than 70%. The polyadenylation signal AATAAA downstream the Alu and two nearby signals CACAG and GTGTT resembling consensus sequences CACAG and YGTGTTYY, respectively, were also detected. The two latter motifs located close to the 3' ends in most mammalian genes are likely to regulate mature mRNA formation. The comparison of the sequenced spaser flank adjacent to the tPA gene with short homologous sequence from the same genomic region primary structure reported previously has revealed discrepancies (substitutions, deletions or insertions) in 21 nucleotide positions. The nucleotide sequence of E. coli uvrB gene fragment (980 b.p.) is also reported. This E. coli gene fragment was cloned accidentally within the f1/BglII-fragment being an artifact of the host-vector system used.

5S rRNA gene hybridizes to human chromosome 1 at two sites (1q42.11-->q42.13 and 1q43)

Hybridization of a loach (Misgurnus fossilis) oocyte 5S rRNA gene repeat with human chromosomes was carried out to refine the localization of the 5S RNA gene in the human genome. Preliminary in situ hybridization analysis showed that this repeat hybridized with human chromosome region 1q42-->q43, the same location previously reported for the human 5S rRNA gene. High-resolution banding revealed the presence of two sites of hybridization, with a main peak in chromosome region 1q42.1 and a second peak in 1q43. These results are consistent with our data on the existence in the human genome of different 5S rRNA gene clusters specified by BamHI and HindIII restriction sites in spacer DNA (Timofeeva et al., 1993) and suggest a multicluster organization of the human 5S rRNA genes.

[Isolation and characteristics of DNA fragments for the region of the tissue plasminogen activator genes and areas adjacent to it in the human genome]

Fragments overlapping the tPA gene and its 5'- and 3'-flanking regions were isolated from human liver DNA library cloned in lambda Charon4A vector. A BglII fragment comprising the 3' end and the adjacent genomic region (total length 3.7 kb) was subcloned in plasmid pUC19 and its restriction map was determined. The nucleotide sequence of the 5' region of this fragment was compared with the 3' end region of the tPA gene and the corresponding regions of five published variants of tPA mRNA cDNA from different tissues; discrepancies in seven positions were revealed, which might be caused by intragenomic polymorphism.

[Organization of a 5S ribosomal RNA gene cluster in the human genome]

Method in situ hybridization was used to localize 5S rRNA genes on human chromosomes. It was shown that 5S genes are situated on the 1-st chromosome, positions q42.11-42.13 and q 43. The genome organization of 5S rRNA gene cluster was tested by blot-hybridization of fibroblast DNA restriction fragments separated by electrophoresis. It was shown that cluster comprises of repeats with different length and restriction site specificity. The size of HindIII individual repeats-2.6 kb, BamHI-repeats-2.3 and 1.8 kb. The most part of 5S repeats were organized in oligomers. Long 5S rDNA fragments (100 kb) in which BamHI, HindIII, and AccI restriction sites are absent were demonstrated. The results obtained allow to suggest the multicluster organization of human 5S rRNA genes, these clusters are localized in a short distance from each other on the long arm of 1-st chromosome.