Recombinant Factor IX Fc Fusion Protein Maintains Full Procoagulant Properties and Exhibits Prolonged Efficacy in Hemophilia B Mice

Introduction

Hemophilia B is an inherited X chromosome–linked disorder characterized by impaired blood clotting owing to the absence of functional coagulation factor IX. Due to the relatively short half-life of factor IX, patients with hemophilia B require frequent factor IX infusions to maintain prophylaxis. We have developed a recombinant factor IX (rFIX) fused to the Fc region of IgG (rFIXFc) with an extended half-life in animals and humans.

Materials and Methods

Procoagulant properties of rFIXFc and rFIX (BENEFIX^®) were compared to determine the effect of the Fc region on rFIXFc hemostatic function. Specifically, we assessed rFIXFc activation, intermolecular interactions within the Xase complex, inactivation by antithrombin III (AT) and thrombin generation potential compared with rFIX. We also assessed the acute and prophylactic efficacy profiles of rFIXFc and rFIX in vivo in hemophilia B mouse bleeding models.

Results and Conclusions

The activation by factor XIa or factor VIIa/tissue factor, inhibition by AT, interaction profiles with phospholipids, affinities for factor VIIIa within the context of the Xase complex, and thrombin generation profiles were similar for rFIXFc and rFIX. Xase complexes formed with either molecule exhibited similar kinetic profiles for factor Xa generation. In acute efficacy models, mice infused with rFIXFc or rFIX were equally protected from bleeding. However, in prophylactic efficacy models, protection from bleeding was maintained approximately three times longer in rFIXFc-dosed mice than in those given rFIX; this prolonged efficacy correlates with the previously observed half-life extension. We conclude that rFIXFc retains critical FIX procoagulant attributes and that the extension in rFIXFc half-life translates into prolonged efficacy in hemophilia B mice.     

Expression of Bax in yeast affects not only the mitochondria but also vacuolar integrity and intracellular protein traffic

Bax-induced lethality in yeast is accompanied by morphological changes in mitochondria, giving rise to a reduced number of swollen tubules. Although these changes are completely abolished upon coexpression of the Bax inhibitor, Bcl-2, coexpression of Bax with Bax inhibiting-glutathione S-transferase (BI-GST) leads to aggregation, but not fusion of the mitochondria. In addition, Bax affects the integrity of yeast vacuoles, resulting in the disintegration and eventual loss of the organelles, and the disruption of intracellular protein traffic. While Bcl-2 coexpression only partially corrects this phenotype, coexpression of BI-GST fully restores the organelles, indicating a different mode of protection exerted by Bcl-2 and BI-GST.     

The caspase-1 digestome identifies the glycolysis pathway as a target during infection and septic shock

Caspase-1 is an essential effector of inflammation, pyroptosis, and septic shock. Few caspase-1 substrates have been identified to date, and these substrates do not account for its wide range of actions. To understand the function of caspase-1, we initiated the systematic identification of its cellular substrates. Using the diagonal gel proteomic approach, we identified 41 proteins that are directly cleaved by caspase-1. Among these were chaperones, cytoskeletal and translation machinery proteins, and proteins involved in immunity. A series of unexpected proteins along the glycolysis pathway were also identified, including aldolase, triose-phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, alpha-enolase, and pyruvate kinase. With the exception of the latter, the identified glycolysis enzymes were specifically cleaved in vitro by recombinant caspase-1, but not caspase-3. The enzymatic activity of wild-type glyceraldehyde-3-phosphate dehydrogenase, but not a non-cleavable mutant, was dampened by caspase-1 processing. In vivo, stimuli that fully activated caspase-1, including Salmonella typhimurium infection and septic shock, caused a pronounced processing of these proteins in the macrophage and diaphragm muscle, respectively. Notably, these stimuli inhibited glycolysis in wild-type cells compared with caspase-1-deficient cells. The systematic characterization of caspase-1 substrates identifies the glycolysis pathway as a caspase-1 target and provides new insights into its function during pyroptosis and septic shock.     

Inflammasomes in infection and inflammation

Two of the main challenges that multicellular organisms faced during evolution were to cope with invading microorganisms and eliminate and replace dying cells. Our innate immune system evolved to handle both tasks. Key aspects of innate immunity are the detection of invaders or tissue injury and the activation of inflammation that alarms the system through the action of cytokine and chemokine cascades. While inflammation is essential for host resistance to infections, it is detrimental when produced chronically or in excess and is linked to various diseases, most notably auto-immune diseases, auto-inflammatory disorders, cancer and septic shock. Essential regulators of inflammation are enzymes termed “the inflammatory caspases”. They are activated by cellular sensors of danger signals, the inflammasomes, and subsequently convert pro-inflammatory cytokines into their mature active forms. In addition, they regulate non-conventional protein secretion of alarmins and cytokines, glycolysis and lipid biogenesis, and the execution of an inflammatory form of cell death termed “pyroptosis”. By acting as key regulators of inflammation, energy metabolism and cell death, inflammatory caspases and inflammasomes exert profound influences on innate immunity and infectious and non-infectious inflammatory diseases. Christian R. McIntire and Garabet Yeretssian have contributed equally to this review.     

A novel RING finger protein,human enhancer of invasion 10, alters mitotic progression through regulation of cyclin B levels

The process of cellular morphogenesis is highly conserved in eukaryotes and is dependent upon the function of proteins that are centrally involved in specification of the cell cycle. The human enhancer of invasion clone 10 (HEI10) protein was identified from a HeLa cell library based on its ability to promote yeast agar invasion and filamentation. Through two-hybrid screening, the mitotic cyclin B1 and an E2 ubiquitin-conjugating enzyme were isolated as HEI10-interacting proteins. Mutation of the HEI10 divergent RING finger motif (characteristic of E3 ubiquitin ligases) and Cdc2/cyclin binding and phosphorylation sites alter HEI10-dependent yeast phenotypes, including delay in G(2)/M transition. In vertebrates, the addition of HEI10 inhibits nuclear envelope breakdown and mitotic entry in Xenopus egg extracts. Mechanistically, HEI10 expression reduces cyclin B levels in cycling Xenopus eggs and reduces levels of the cyclin B ortholog Clb2p in yeast. HEI10 is itself a specific in vitro substrate of purified cyclin B/cdc2, with a TPVR motif as primary phosphorylation site. Finally, HEI10 is itself ubiquitinated in egg extracts and is also autoubiquitinated in vitro. These and other points lead to a model in which HEI10 defines a divergent class of E3 ubiquitin ligase, functioning in progression through G(2)/M.     

A Single Chain Variant of Factor VIII Fc Fusion Protein Retains Normal In Vivo Efficacy but Exhibits Altered In Vitro Activity

Recombinant factor VIII Fc (rFVIIIFc) is a fusion protein consisting of a single B-domain-deleted (BDD) FVIII linked recombinantly to the Fc domain of human IgG1 to extend half-life. To determine if rFVIIIFc could be further improved by maintaining the heavy and light chains within a contiguous single chain (SC), we evaluated the activity and function of SC rFVIIIFc, an isoform that is not processed at residue R1648. SC rFVIIIFc showed equivalent activity in a chromogenic assay compared to rFVIIIFc, but approximately 40% activity by the one-stage clotting assay in the presence of von Willebrand Factor (VWF), with full activity in the absence of VWF. Moreover, SC rFVIIIFc demonstrated markedly delayed thrombin-mediated release from VWF, but an activity similar to that of rFVIIIFc upon activation in FXa generation assays. Therefore, the apparent reduction in specific activity in the aPTT assay appears to be primarily due to delayed release of FVIII from VWF. To assess whether stability and activity of SC rFVIIIFc were affected in vivo, a tail vein transection model in Hemophilia A mice was utilized. The results demonstrated similar pharmacokinetic profiles and comparable efficacy for SC rFVIIIFc and rFVIIIFc. Thus, while the single chain configuration did not promote enhanced half-life, it reduced the rate of release of FVIII from VWF required for activation. This impaired release may underlie the observed reduction in the one-stage clotting assay, but does not appear to affect the physiological activity of SC rFVIIIFc.     

Molecular regulation of inflammation and cell death

Cell death and innate immunity are ancient evolutionary conserved processes that utilize a dazzling number of related molecular effectors and parallel signal transduction mechanisms. The investigation of the molecular mechanisms linking the sensing of a danger signal (pathogens or tissue damage) to the induction of an inflammatory response has witnessed a renaissance in the last few years. This was initiated by the identification of pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and more recently cytosolic Nod-like receptors (NLRs), that brought innate immunity to center stage and opened the field to the study of signal transduction pathways, adaptors and central effectors linked to PRRs. This led to the characterization of the inflammasome, a macromolecular complex, scaffolded by NLRs, that recruits and activates inflammatory caspases, which are essential effectors in inflammation and cell death responses. In this review, we describe the molecular pathways of cell death and innate immunity with a focus on recent advancements in both fields and an emphasis on the striking analogies between NLR innate immunity and mitochondrial apoptosis pathways.     

Caspase-12 modulates NOD signaling and regulates antimicrobial peptide production and mucosal immunity

Bacterial sensing by intracellular Nod proteins and other Nod-like receptors (NLRs) activates signaling pathways that mediate inflammation and pathogen clearance. Nod1 and Nod2 associate with the kinase Rip2 to stimulate NF-kappaB signaling. Other cytosolic NLRs assemble caspase-1-activating multiprotein complexes termed inflammasomes. Caspase-12 modulates the caspase-1 inflammasome, but unlike other NLRs, Nod1 and Nod2 have not been linked to caspases, and mechanisms regulating the Nod-Rip2 complex are less clear. We report that caspase-12 dampens mucosal immunity to bacterial infection independent of its effects on caspase-1. Caspase-12 deficiency enhances production of antimicrobial peptides, cytokines, and chemokines to entric pathogens, an effect dependent on bacterial type III secretion and the Nod pathway. Mechanistically, caspase-12 binds to Rip2, displacing Traf6 from the signaling complex, inhibiting its ubiquitin ligase activity, and blunting NF-kappaB activation. Nod activation and resulting antimicrobial peptide production constitute an early innate defense mechanism, and caspase-12 inhibits this mucosal antimicrobial response.