Localization of a metal-dependent epitope to the amino terminal residues 33-40 of human factor IX

Characterization of missense mutations in the signal peptide and propeptide of FIX in hemophilia B by a cell-based assay

Many mutations in the signal peptide and propeptide of factor IX (FIX) cause hemophilia B. A FIX variants database reports 28 unique missense mutations in these regions that lead to FIX deficiency, but the underlying mechanism is known only for the mutations on R43 that interfere with propeptide cleavage. It remains unclear how other mutations result in FIX deficiency and why patients carrying the same mutation have different bleeding tendencies. Here, we modify a cell-based reporter assay to characterize the missense mutations in the signal peptide and propeptide of FIX. The results show that the level of secreted conformation-specific reporter (SCSR), which has a functional γ-carboxyglutamate (Gla) domain of FIX, decreases significantly in most mutations. The decreased SCSR level is consistent with FIX deficiency in hemophilia B patients. Moreover, we find that the decrease in the SCSR level is caused by several distinct mechanisms, including interfering with cotranslational translocation into the endoplasmic reticulum, protein secretion, γ-carboxylation of the Gla domain, and cleavage of the signal peptide or propeptide. Importantly, our results also show that the SCSR levels of most signal peptide and propeptide mutations increase with vitamin K concentration, suggesting that the heterogeneity of bleeding tendencies may be related to vitamin K levels in the body. Thus, oral administration of vitamin K may alleviate the severity of bleeding tendencies in patients with missense mutations in the FIX signal peptide and propeptide regions.

Engineering of a membrane-triggered activity switch in coagulation factor VIIa

Coagulation factor VIIa (FVIIa) is an intrinsically poor serine protease that requires assistance from its cofactor tissue factor (TF) to trigger the extrinsic pathway of blood coagulation. TF stimulates FVIIa through allosteric maturation of its active site and by facilitating substrate recognition. The surface dependence of the latter property allowed us to design a potent membrane-triggered activity switch in FVIIa by engineering a disulfide cross-link between an allosterically silent FVIIa variant and soluble TF. These results show that optimization of substrate recognition remote from the active site represents a promising new route to simultaneously enhance and localize the procoagulant activity of FVIIa for therapeutic purposes.

Recombinant factor VIIa (FVIIa) variants with increased activity offer the promise to improve the treatment of bleeding episodes in patients with inhibitor-complicated hemophilia. Here, an approach was adopted to enhance the activity of FVIIa by selectively optimizing substrate turnover at the membrane surface. Under physiological conditions, endogenous FVIIa engages its cell-localized cofactor tissue factor (TF), which stimulates activity through membrane-dependent substrate recognition and allosteric effects. To exploit these properties of TF, a covalent complex between FVIIa and the soluble ectodomain of TF (sTF) was engineered by introduction of a nonperturbing cystine bridge (FVIIa Q64C-sTF G109C) in the interface. Upon coexpression, FVIIa Q64C and sTF G109C spontaneously assembled into a covalent complex with functional properties similar to the noncovalent wild-type complex. Additional introduction of a FVIIa-M306D mutation to uncouple the sTF-mediated allosteric stimulation of FVIIa provided a final complex with FVIIa-like activity in solution, while exhibiting a two to three orders-of-magnitude increase in activity relative to FVIIa upon exposure to a procoagulant membrane. In a mouse model of hemophilia A, the complex normalized hemostasis upon vascular injury at a dose of 0.3 nmol/kg compared with 300 nmol/kg for FVIIa.

Intraarticular factor IX protein or gene replacement protects against development of hemophilic synovitis in the absence of circulating factor IX

Hemophilic bleeding into joints causes synovial and microvascular proliferation and inflammation (hemophilic synovitis) that contribute to end-stage joint degeneration (hemophilic arthropathy), the major morbidity of hemophilia. New therapies are needed for joint deterioration that progresses despite standard intravenous (IV) clotting factor replacement. To test whether factor IX within the joint space can protect joints from hemophilic synovitis, we established a hemophilia B mouse model of synovitis. Factor IX knockout (FIX(-/-)) mice received a puncture of the knee joint capsule with a needle to induce hemarthrosis; human factor IX (hFIX) was either injected through the needle into the joint space (intraarticularly) or immediately delivered IV. FIX(-/-) mice receiving intraarticular FIX protein were protected from synovitis compared with mice receiving same or greater doses of hFIX IV. Next, adeno-associated virus (AAV) gene transfer vectors expressing hFIX were injected into knee joints of FIX(-/-) mice. Joints treated with 10(10) vector genomes (vg)/joint AAV2-, AAV5-, or AAV8-hFIX or 2.5 x 10(9) vg/joint AAV5-hFIX developed significantly fewer pathologic changes 2 weeks after injury compared with the pathology of control injured contralateral hind limbs. Extravascular factor activity and joint-directed gene transfer may ameliorate hemophilic joint destruction, even in the absence of circulating FIX.

Identification and purification of vitamin K-dependent proteins and peptides with monoclonal antibodies specific for gamma -carboxyglutamyl (Gla) residues

Novel monoclonal antibodies that specifically recognize gamma-carboxyglutamyl (Gla) residues in proteins and peptides have been produced. As demonstrated by Western blot and time-resolved immunofluorescence assays the antibodies are pan-specific for most or all of the Gla-containing proteins tested (factors VII, IX, and X, prothrombin, protein C, protein S, growth arrest-specific protein 6, bone Gla protein, conantokin G from a cone snail, and factor Xa-like proteins from snake venom). Only the Gla-containing light chain of the two-chain proteins was bound. Decarboxylation destroyed the epitope(s) on prothrombin fragment 1, and Ca(2+) strongly inhibited binding to prothrombin. In Western blot, immunofluorescence, and surface plasmon resonance assays the antibodies bound peptides conjugated to bovine serum albumin that contained either a single Gla or a tandem pair of Gla residues. Binding was maintained when the sequence surrounding the Gla residue(s) was altered. Replacement of Gla with glutamic acid resulted in a complete loss of the epitope. The utility of the antibodies was demonstrated in immunochemical methods for detecting Gla-containing proteins and in the immunopurification of a factor Xa-like protein from tiger snake venom. The amino acid sequences of the Gla domain and portions of the heavy chain of the snake protein were determined.

Identification of residues in the Gla-domain of human factor IX involved in the binding to conformation specific antibodies

The binding of Ca2+ induces a conformational change in factor IX which can be monitored with conformation specific antibodies. Anti-FIX:Mg(II) antibodies recognize a conformational epitope (FIX') that can be induced by several metal ions such as Ca2+, Mg2+, Mn2+ and Ba2+, while anti-FIX:Ca(II) antibodies recognize a conformational epitope (FIX*) that can be only induced by Ca2+ and Sr2+ ions (Liebman et al., J. Biol. Chem., vol. 262 (1987) pp. 7605-7612). The latter conformation is essential for the function of factor IX. In this study we tried to identify residues in the Gla-domain of factor IX which are involved in binding to anti-factor IX:Mg(II) and anti-factor IX:Ca(II) antibodies. For this we substituted residues in recombinant human factor IX for those of factor X or factor VII. The substitution of residues 1-40 of factor IX by those of factor VII eliminated binding to both types of antibodies. Re-introduction of factor IX specific residues increased the binding to conformation specific anti-factor IX antibodies, but reduced the binding to conformation specific anti-factor VII antibodies, indicating that the structural integrity of the Gla-domain was not seriously affected by the mutations. We provide evidence that residues 33, 39 and 40 of human factor IX are important for binding to anti-factor IX:Mg(II) antibodies, while residues 1-11 are important for binding to anti-factor IX:Ca(II) antibodies.

gamma-Carboxyglutamic acids 36 and 40 do not contribute to human factor IX function

The gamma-carboxyglutamic acid (Gla) domains of the vitamin K-dependent blood coagulation proteins contain 10 highly conserved Gla residues within the first 33 residues, but factor IX is unique in possessing 2 additional Gla residues at positions 36 and 40. To determine their importance, factor IX species lacking these Gla residues were isolated from heterologously expressed human factor IX. Using ion-exchange chromatography, peptide mapping, mass spectrometry, and N-terminal sequencing, we have purified and identified two partially carboxylated recombinant factor IX species; factor IX/gamma 40E is uncarboxylated at residue 40 and factor IX/gamma 36,40E is uncarboxylated at both residues 36 and 40. These species were compared with the fully gamma-carboxylated recombinant factor IX, unfractionated recombinant factor IX, and plasma-derived factor IX. As monitored by anti-factor IX:Ca (II)-specific antibodies and by the quenching of intrinsic fluorescence, all these factor IX species underwent the Ca(II)-induced conformational transition required for phospholipid membrane binding and bound equivalently to phospholipid vesicles composed of phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine. Endothelial cell binding was also similar in all species, with half-maximal inhibition of the binding of 125I-labeled plasma-derived factor IX at concentrations of 2-6 nM. Functionally, factor IX/gamma 36,40E and factor IX/gamma 40E were similar to fully gamma-carboxylated recombinant factor IX and plasma-derived factor IX in their coagulant activity and in their ability to participate in the activation of factor X in the tenase complex both with synthetic phospholipid vesicles and activated platelets. However, Gla 36 and Gla 40 represent part of the epitope targeted by anti-factor IX:Mg(II)-specific antibodies because these antibodies bound factor IX preferentially to factor IX/gamma 36,40E and factor IX/gamma 40E. These results demonstrate that the gamma-carboxylation of glutamic acid residues 36 and 40 in human factor IX is not required for any function of factor IX examined.

Localization of a calcium-dependent epitope to the amino terminal region of the Gla domain of human factor IX

We have used site-directed mutagenesis to define the epitope of calcium-dependent monoclonal antibodies to human factor IX. We demonstrate that the calcium-specific epitope includes residues 1-11 of factor IX, with apparent contributions from other regions of the protein. Antibodies JK.IX-1, -3, and -4 had critical portions of their epitopes in the first eleven amino acids of the Gla domain. These three antibodies could bind to a chimera containing the factor VII Gla domain, in which surface residues 3, 4, 5, 9, 10, and 11 were modified to those of factor IX. In contrast, the epitope of JK.IX-2 was unaffected by mutations in residues 3-11 of factor IX, but was dependent on the amino terminal tyrosine residue. Furthermore, the calcium-dependent monoclonal antibodies, JK.IX-1, -3, and -4, whose epitope include residues 3 through 11. inhibit factor IX's binding to endothelial cells, for which the binding site on factor IX has been localized to this region. Our results, together with previous studies, confirm the existence of discrete calcium and metal-dependent epitopes within the Gla domain of factor IX and show that the calcium-specific epitope lies near the amino terminus region of this domain.