A Thrombin-Activatable Factor X Variant Corrects Hemostasis in a Mouse Model for Hemophilia A.

Engineered recombinant factor X (FX) variants represent a promising strategy to bypass the tenase complex and restore hemostasis in hemophilia patients. Previously, a thrombin-activatable FX variant with fibrinopeptide-A replacing the activation peptide (FX-delAP/FpA) has been described in this regard. Here we show that FX-delAP/FpA is characterized by a sixfold shorter circulatory half-life compared with wild-type FX, limiting its therapeutical applicability. We therefore designed a variant in which the FpA sequence is inserted C-terminal to the FX activation peptide (FX/FpA). FX/FpA displayed a similar survival to wt-FX in clearance experiments and could be converted into FX by thrombin and other activating agents. In in vitro assays, FX/FpA efficiently restored thrombin generation in hemophilia A and hemophilia B plasmas, even in the presence of inhibitory antibodies. Expression following hydrodynamic gene transfer of FX/FpA restored thrombus formation in FVIII-deficient mice in a laser-induced injury model as well as hemostasis in a tail-clip bleeding model. Hemostasis after tail transection in FVIII-deficient mice was also corrected at 5 and 90 minutes after injection of purified FX/FpA. Our data indicate that FX/FpA represents a potential tenase-bypassing agent for the treatment of hemophilia patients with or without inhibitors.

Macrophage receptor SR-AI is crucial to maintain normal plasma levels of coagulation factor X.

Beside its classical role in the coagulation cascade, coagulation factor X (FX) is involved in several major biological processes including inflammation and enhancement of virus-induced immune responses. We recently reported that the long circulatory half-life of FX is linked to its interaction with liver-resident macrophages. Importantly, we now observed that macrophages, but not undifferentiated monocytes, support this interaction. Using cell biology approaches with primary and THP1-derived macrophages as well as transfected cells, we further identified the scavenger receptor type A member I (SR-AI) to be a macrophage-specific receptor for FX. This result was confirmed using SR-AI-deficient mice, which exhibit reduced circulating levels of FX in vivo and loss of FX-macrophage interactions in vitro. Binding studies using purified proteins revealed that FX binds specifically (half-maximal binding, 3 µg/mL) to the extracellular domain of SR-AI. Altogether, we demonstrate that macrophages regulate FX plasma levels in an SR-AI-dependent manner.

Liver-specific transcriptional modules identified by genome-wide in silico analysis enable efficient gene therapy in mice and non-human primates.

The robustness and safety of liver-directed gene therapy can be substantially improved by enhancing expression of the therapeutic transgene in the liver. To achieve this, we developed a new approach of rational in silico vector design. This approach relies on a genome-wide bio-informatics strategy to identify cis-acting regulatory modules (CRMs) containing evolutionary conserved clusters of transcription factor binding site motifs that determine high tissue-specific gene expression. Incorporation of these CRMs into adeno-associated viral (AAV) and non-viral vectors enhanced gene expression in mice liver 10 to 100-fold, depending on the promoter used. Furthermore, these CRMs resulted in robust and sustained liver-specific expression of coagulation factor IX (FIX), validating their immediate therapeutic and translational relevance. Subsequent translational studies indicated that therapeutic FIX expression levels could be attained reaching 20-35% of normal levels after AAV-based liver-directed gene therapy in cynomolgus macaques. This study underscores the potential of rational vector design using computational approaches to improve their robustness and therefore allows for the use of lower and thus safer vector doses for gene therapy, while maximizing therapeutic efficacy.

Coagulation factor X interaction with macrophages through its N-glycans protects it from a rapid clearance.

Factor X (FX), a plasma glycoprotein playing a central role in coagulation has a long circulatory half-life compared to closely related coagulation factors. The activation peptide of FX has been shown to influence its clearance with two N-glycans as key determinants of FX's relatively long survival. To decipher FX clearance mechanism, organ biodistribution and cellular interactions of human plasma FX (pd-FX), recombinant FX (rFX), N-deglycosylated FX (N-degly-FX) and recombinant FX mutated at both N-glycosylation sites (rFX(N181A-N191A)) were evaluated. Biodistribution analysis of (125)I-labelled FX proteins after administration to mice revealed liver as major target organ for all FX variants. Liver tissue sections analysis showed an interaction of pd-FX and N-degly-FX to different cell types. These findings were confirmed in cell binding studies revealing that FX and FX without N-glycans interact with macrophages and hepatocytes, respectively. N-degly-FX appeared to be degraded in hepatocytes while interestingly pd-FX was not by macrophages. Furthermore, the chemical inactivation of macrophages by gadolinium chloride resulted in a significant decrease of circulating pd-FX into mice and not of N-degly-FX. Altogether our data lead to the conclusion that FX interaction with macrophages through its N-glycans protects it from a rapid clearance explaining its relatively long circulatory half-life.

Selective inhibition of proprotein convertases represses the metastatic potential of human colorectal tumor cells.

The proprotein convertases (PCs) are implicated in the activation of various precursor proteins that play an important role in tumor cell metastasis. Here, we report their involvement in the regulation of the metastatic potential of colorectal tumor cells. PC function in the human and murine colon carcinoma cell lines HT-29 and CT-26, respectively, was inhibited using siRNA targeting the PCs furin, PACE4, PC5, and PC7 or by overexpression of the general PC inhibitor alpha1-antitrypsin Portland (alpha1-PDX). We found that overexpression of alpha1-PDX and knockdown of furin expression inhibited processing of IGF-1 receptor and its subsequent activation by IGF-1 to induce IRS-1 and Akt phosphorylation, all important in colon carcinoma metastasis. These data suggest that the PC furin is a major IGF-1 receptor convertase. Expression of alpha1-PDX reduced the production of TNF-alpha and IL-1alpha by human colon carcinoma cells, and incubation of murine liver endothelial cells with conditioned media derived from these cells failed to induce tumor cell adhesion to activated murine endothelial cells, a critical step in metastatic invasion. Furthermore, colon carcinoma cells in which PC activity was inhibited by overexpression of alpha1-PDX when injected into the portal vein of mice showed a significantly reduced ability to form liver metastases. This suggests that inhibition of PCs is a potentially promising strategy for the prevention of colorectal liver metastasis.

Role of the alpha-helix 163-170 in factor Xa catalytic activity.

Factor Xa (FXa) is a key protease of the coagulation pathway whose activity is known to be in part modulated by binding to factor Va (FVa) and sodium ions. Previous investigations have established that solvent-exposed, charged residues of the FXa alpha-helix 163-170 (h163-170), Arg(165) and Lys(169), participate in its binding to FVa. In the present study we aimed to investigate the role of the other residues of h163-170 in the catalytic functions of the enzyme. FX derivatives were constructed in which point mutations were made or parts of h163-170 were substituted with the corresponding region of either FVIIa or FIXa. Purified FXa derivatives were compared with wild-type FXa. Kinetic studies in the absence of FVa revealed that, compared with wild-type FXa, key functional parameters (catalytic activity toward prothrombin and tripeptidyl substrates and non-enzymatic interaction of a probe with the S1 site) were diminished by mutations in the NH(2)-terminal portion of h163-170. The defective amidolytic activity of these FXa derivatives appears to result from their impaired interaction with Na(+) because using a higher Na(+) concentration partially restored normal catalytic parameters. Furthermore, kinetic measurements with tripeptidyl substrates or prothrombin indicated that assembly of these FXa derivatives with an excess of FVa in the prothrombinase complex improves their low catalytic efficiency. These data indicate that residues in the NH(2)-terminal portion of the FVa-binding h163-170 are energetically linked to the S1 site and Na(+)-binding site of the protease and that residues Val(163) and Ser(167) play a key role in this interaction.

Role of the Gla and first epidermal growth factor-like domains of factor X in the prothrombinase and tissue factor-factor VIIa complexes.

Factor X (FX) has high structure homology with other proteins of blood coagulation such as factor IX (FIX) and factor VII (FVII). These proteins present at their amino-terminal extremity a gamma-carboxyglutamic acid containing domain (Gla domain), followed by two epidermal growth factor-like (EGF1 and EGF2) domains, an activation peptide, and a serine protease domain. After vascular damage, the tissue factor-FVIIa (TF-FVIIa) complex activates both FX and FIX. FXa interacts stoichiometrically with tissue pathway inhibitor (TFPI), regulating TF-FVIIa activity by forming the TF-FVIIa-TFPI-FXa quaternary complex. Conversely, FXa boosts coagulation by its association with its cofactor, factor Va (FVa). To investigate the contribution of the Gla and EGF1 domains of FX in these complexes, FX chimeras were produced in which FIX Gla and EGF1 domains substituted the corresponding domains of FX. The affinity of the two chimeras, FX/FIX(Gla) and FX/FIX(EGF1), for the TF-FVIIa complex was markedly reduced compared with that of wild-type-FX (wt-FX) independently of the presence of phospholipids. Furthermore, the association rate constants of preformed FX/FIX(Gla)-TFPI and FX/FIX(EGF1)-TFPI complexes with TF-FVIIa were, respectively, 10- and 5-fold slower than that of wt-FXa-TFPI complex. Finally, the apparent affinity of FVa was 2-fold higher for the chimeras than for wt-FX in the presence of phospholipids and equal in their absence. These data demonstrate that FX Gla and EGF1 domains contain residues, which interact with TF-FVIIa exosites contributing to the formation of the TF-FVIIa-FX and TF-FVIIa-TFPI-FXa complexes. On the opposite, FXa Gla and EGF1 domains are not directly involved in FVa binding.