Douglas M. Tollefsen

Highly Sulfated Dermatan Sulfates from Ascidians STRUCTURE VERSUS ANTICOAGULANT ACTIVITY OF THESE GLYCOSAMINOGLYCANS

Dermatan sulfates with the same backbone structure [4-α-l-IdceA-1→3-β-d-GalNAc-1] n but with different patterns of sulfation substitutions have been isolated from the ascidian body. All the ascidian dermatan sulfates have a high content of 2-O-sulfated α-l-iduronic acid residues but differ in the pattern of sulfation of the N-acetyl-β-d-galactosamine units. Styela plicata and Halocynthia pyriformis have 4-O-sulfated units, but inAscidian nigra they are 6-O-sulfated. This collection of ascidian dermatan sulfates (together with native and oversulfated mammalian dermatan sulfate), where the extent and position of sulfate substitution have been fully characterized, were tested in anticoagulant assays. Dermatan sulfate from A. nigra has no discernible anticoagulant activity, which indicates that 4-O-sulfation of theN-acetyl-β-d-galactosamine is essential for the anticoagulant activity of this glycosaminoglycan. In contrast dermatan sulfates from S. plicata and H. pyriformis are potent anticoagulants due to potentiation of thrombin inhibition by heparin cofactor II. These ascidian dermatan sulfates have ∼10-fold and ∼6-fold higher activity with heparin cofactor II than native and an oversulfated mammalian dermatan sulfate, respectively. They have no effect on thrombin or factor Xa inhibition by antithrombin. These naturally oversulfated ascidian dermatan sulfates are sulfated at selected sites required for interaction with heparin cofactor II and thus have specific and potent anticoagulant activity.

Detection of a new heparin-dependent inhibitor of thrombin in human plasma.

We have demonstrated that human plasma contains a heparin-dependent inhibitor of thrombin that is distinguishable from antithrombin III (AT III). When a 1:50 dilution of plasma was incubated with greater than or equal to 0.01 U/ml heparin and 1 U/ml 125I-thrombin, the labeled thrombin B-chains became incorporated into two complexes of Mr-96,000 and Mr-85,000 that were separated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and beta-mercaptoethanol. Neither complex was detectable at heparin concentrations less than 0.01 U/ml. When a limiting amount of 125I-thrombin was present, the proportion of radioactivity incorporated into each of the two complexes varied with the heparin concentration. Thus, the Mr-85,000 complex predominated at 0.01-5 U/ml heparin, whereas the Mr-96,000 complex predominated at 5-100 U/ml heparin. The Mr-85,000 complex reacted with antibodies to human AT III and comigrated with the purified thrombin-AT III complex. The Mr-96,000 complex did not react with antibodies to AT III or to alpha 1-antitrypsin, and it was detected in normal quantities after incubating 125I-thrombin with plasma immunodepleted of AT III, alpha 2-antiplasmin, alpha 2-macroglobulin, C1 inactivator, alpha 1-antichymotrypsin, or inter-alpha-trypsin inhibitor. The protein that combines with thrombin to form the Mr-96,000 complex was estimated to be present at a minimum concentration of 90 +/- 26 micrograms/ml (mean +/- SD) in identical to any of the known plasma protease inhibitors and that at relatively high heparin concentrations in vitro it reacts with thrombin more rapidly than does AT III.

Heparin cofactor II inhibits arterial thrombosis after endothelial injury

Heparin cofactor II (HCII) is a plasma protein that inhibits thrombin rapidly in the presence of dermatan sulfate, heparan sulfate, or heparin. HCII has been proposed to regulate coagulation or to participate in processes such as inflammation, atherosclerosis, and wound repair. To investigate the physiologic function of HCII, about 2 kb of the mouse HCII gene, encoding the N-terminal half of the protein, was deleted by homologous recombination in embryonic stem cells. Crosses of F1 HCII(+/-) animals produced HCII(-/-) offspring at the expected mendelian frequency. Biochemical assays confirmed the absence of dermatan sulfate-dependent thrombin inhibition in the plasma of HCII(-/-) animals. Crosses of HCII(-/-) animals produced litters similar in size to those obtained from heterozygous matings. At 1 year of age, HCII-deficient animals were grossly indistinguishable from their wild-type littermates in weight and survival, and they did not appear to have spontaneous thrombosis or other morphologic abnormalities. In comparison with wild-type animals, however, they demonstrated a significantly shorter time to thrombotic occlusion of the carotid artery after photochemically induced endothelial cell injury. This abnormality was corrected by infusion of purified HCII but not ovalbumin. These observations suggest that HCII might inhibit thrombosis in the arterial circulation.

Heparin Cofactor II

Heparin cofactor II (HCII) is a serpin that inhibits thrombin rapidly in the presence of dermatan sulfate or heparin. Both of these glycosaminoglycans bind to HCII and increase the rate of inhibition of thrombin >1000-fold. This review will focus on the biochemistry of HCII and the mechanism by which glycosaminoglycans stimulate its activity.

Chemistry and biology of serpins

Introduction: Serpins: From the Way It Was to the Way It Is J. Travis. Serpins: A Mechanistic Class of Their Own S.R. Stone et al.. Coagulation: Antithrombin--A Bloody Important Serpin I. Bjork, S.T. Olson. Heparin Cofactor II D.M. Tollefsen. Neurobiology and Cancer: Regulation of Neurons and Astrocytes by Thrombin and Protease Nexin--l: Relationship to Brain Injury D.D. Cunningham, F.M. Donovan. Maspin: A Tumor Suppressing Serpin R. Sager et al.. Fibrinolysis: The Role of Reactive--Center Loop Mobility in the Serpin Inhibitory Mechanism D.A. Lawrence. Substrate Specificity of Tissue Type Plasminogen Activator E.L. Madison. Development and Reproduction: Biology of Progesterone-Induced Uterine Serpins (P.J. Hansen, W.--J. Liu). Serpins from an Insect, Manduca Sexta M.R. Kanost, H. Jiang. Inflammation: Serpins and Programmed Cell Death G.S. Salvesen. Noninhibitor Serpins: Structure--Function Studies on PEDF: A Noninhibitory Serpin with Neurotropic Activity S.P Becerra. Abstracts: Coagulation, Neurobiology and Cancer. Fibrinolysis, Development and Reproduction. Inflammation and Noninhibitor Serpins. 10 Additional Articles. Index.

Inhibition of human platelet aggregation by monovalent antifibrinogen antibody fragments.

Monovalent goat antibody fragments (Fab) that were monospecific for human fibrinogen were isolated by affinity chromatography on fibrinogen-Sepharose and used as a direct probe for the involvement of fibrinogen in platelet aggregation and the release reaction. The antifibrinogen Fab inhibited aggregation of washed human platelets induced by thrombin (0.1-10 U/ml) by 50-95%, but had no effect on (14-C)-serotinin release and only a slight inhibitory effect on 125-I-thrombin binding to platelets. Inhibition of aggregation was not observed with nonimmune goat Fab or rabbit antihuman albumie bound tightly at saturation to surface fibrinogen molecules. After washing the platelets once to remove unbound Fab, aggregation by subsequently added thrombin was no longer inhibited. The antifibrinogen Fab inhibited the clotting of fibrinogen by thrombin but did not effect the rate of fibrinopeptide A release, indicating that the Fab inhibits clotting by interfering with the polymerization of fibrin monomers. Our experiments suggest that fibrinogen released from platelets is directly involved in thrombin-induced aggregation of washed platelets, perhaps through polymerization of fibrin monomers generated by proteolytic cleavage of released fibrinogen.

Thrombin‐inhibiting perfluorocarbon nanoparticles provide a novel strategy for the treatment and magnetic resonance imaging of acute thrombosis

Summary.  Background: As a regulator of the penultimate step in the coagulation cascade, thrombin represents a principal target of direct and specific anticoagulants. Objective: A potent thrombin inhibitor complexed with a colloidal nanoparticle was devised as a first‐in‐class anticoagulant with prolonged and highly localized therapeutic impact conferred by its multivalent thrombin‐absorbing particle surface. Methods: PPACK (Phe[D]‐Pro‐Arg‐Chloromethylketone) was secured covalently to the surface of perfluorocarbon‐core nanoparticle structures. PPACK and PPACK nanoparticle inhibition of thrombin were assessed in vitro via thrombin activity against a chromogenic substrate. In vivo antithrombotic activity of PPACK, heparin, non‐functionalized nanoparticles and PPACK nanoparticles was assessed through intravenous (i.v.) administration prior to acute photochemical injury of the common carotid artery. Perfluorocarbon particle retention in extracted carotid arteries from injured mice was assessed via 19F magnetic resonance spectroscopy (MRS) and imaging (MRI) at 11.7 T. Activated partial thromboplastin time (APTT) measurements determined the systemic effects of the PPACK nanoparticles at various times after injection. Results: An optical assay verified that PPACK nanoparticles exceeded PPACK’s intrinsic activity against thrombin. Application of an in vivo acute arterial thrombosis model demonstrated that PPACK nanoparticles outperformed both heparin (P = 0.001) and uncomplexed PPACK (P = 0.0006) in inhibiting thrombosis. 19F MRS confirmed that PPACK nanoparticles specifically bound to sites of acute thrombotic injury. APTT normalized within 20 min of PPACK nanoparticles injection. Conclusions: PPACK nanoparticles present thrombin‐inhibiting surfaces at sites of acutely forming thrombi that continue to manifest local clot inhibition even as systemic effects rapidly diminish and thus represent a new platform for localized control of acute thrombosis.

Heparin Cofactor II Deficiency

OBJECTIVES To review of the state of the art relating to congenital heparin cofactor II deficiency as a potential risk factor for thrombosis, as reflected by the medical literature and the consensus opinion of recognized experts in the field, and to make recommendations for the use of laboratory assays for assessing this thrombotic risk in individual patients. DATA SOURCES Review of the medical literature, primarily from the last 10 years. DATA EXTRACTION AND SYNTHESIS After an initial assessment of the literature, including review of clinical study design and laboratory methods, a draft manuscript was prepared and circulated to participants in the College of American Pathologists Conference XXXVI: Diagnostic Issues in Thrombophilia. Recommendations were accepted if a consensus of experts attending the conference was reached. The results of the discussion were used to revise the manuscript into its final form. CONCLUSIONS Consensus was reached that there is insufficient evidence to recommend testing for heparin cofactor II deficiency in patients with thromboembolic disease.

Heparin cofactor II modulates the response to vascular injury.

Heparin cofactor II (HCII) has several biochemical properties that distinguish it from other serpins: (1) it specifically inhibits thrombin; (2) the mechanism of inhibition involves binding of an acidic domain in HCII to thrombin exosite I; and (3) the rate of inhibition increases dramatically in the presence of dermatan sulfate molecules having specific structures. Human studies suggest that high plasma HCII levels are protective against in-stent restenosis and atherosclerosis. Studies with HCII knockout mice directly support the hypothesis that HCII interacts with dermatan sulfate in the arterial wall after endothelial injury and thereby exerts an antithrombotic effect. In addition, HCII deficiency appears to promote neointima formation and atherogenesis in mice. These results suggest that HCII plays a unique and important role in vascular homeostasis.

Identification of a monoclonal thrombin inhibitor associated with multiple myeloma and a severe bleeding disorder.

We investigated a patient with a long‐standing IgGκ monoclonal gammopathy who developed severe haemorrhagic complications. At IgG concentrations of ∼50 g/l the patient had severe bleeding associated with prolongation of the thrombin time, activated partial thromboplastin time, and reptilase time. Plasmapheresis resulted in improvement in the thrombin time and resolution of bleeding. Depletion of the IgG by absorption of plasma with protein G–Sepharose in vitro resulted in normalization of the thrombin time and reptilase time. The purified IgG bound to immobilized thrombin and immunoprecipitated human α‐, β‐ and γ‐thrombin but not prothrombin, other vitamin K‐dependent coagulation factors, or fibrinogen. Purified IgG at concentrations >1×10−2 g/l decreased (∼50%) the rate of hydrolysis of a chromogenic substrate by thrombin. Addition of purified IgG to normal pooled plasma at concentrations >1×10−2 g/l prolonged the thrombin time and activated partial thromboplastin time, but the reptilase time was prolonged only at IgG concentrations >1 g/l. This finding suggests that at low concentrations the IgG produces a specific antithrombin effect, but at higher concentrations it also affects fibrin polymerization; the combination of these effects probably produced clinical bleeding. This is the first report of a monoclonal antithrombin antibody associated with bleeding in a patient with multiple myeloma.