Murine heparin cofactor II: purification, cDNA sequence, expression, and gene structure

Identification and characterization of a novel isoform of heparin cofactor II in human liver

Heparin cofactor II (HCII) is predominantly expressed in the liver and inhibits thrombin in blood plasma to influence the blood coagulation cascade. Its deficiency is associated with arterial thrombosis. Its cleavage by neutrophil elastase produces fragment that helps in neutrophil chemotaxis in the acute inflammatory response in human. In the present study, we have identified a novel alternatively spliced transcript of the HCII gene in human liver. This novel transcript includes an additional novel region in continuation with exon 3 called exon 3b. Exon 3b acts like an alternate last exon, and hence its inclusion in the transcript due to alternative splicing removes exon 4 and encodes for a different C-terminal region to give a novel protein, HCII-N. MD simulations of HCII-N and three-dimensional structure showed a unique 51 amino acid sequence at the C-terminal having unique RCL-like structure. The HCII-N protein purified from bacterial culture showed a protein migrating at lower molecular weight (MW 55 kDa) as compared to native HCII (MW 66 kDa). A fluorescence-based analysis revealed a more compact structure of HCII-N that was in a more hydrophilic environment. The HCII-N protein, however, showed no inhibitory activity against thrombin. Due to large conformational variation observed in comparison with native HCII, HCII-N may have alternate protease specificity or a non-inhibitory role. Western blot of HCII purified from large plasma volume showed the presence of a low MW 59 kDa band with no thrombin activity. This study provides the first evidence of alternatively spliced novel isoform of the HCII gene.

Vascular dermatan sulfate and heparin cofactor II

Heparin cofactor II (HCII) is a plasma protease inhibitor of the serpin family that inactivates thrombin by forming a covalent 1:1 complex. The rate of complex formation increases more than 1000-fold in the presence of dermatan sulfate (DS). Endothelial injury allows circulating HCII to enter the vessel wall, where it binds to DS and presumably becomes activated. Mice that lack HCII develop carotid artery thrombosis more rapidly than wild-type mice after oxidative damage to the endothelium. These mice also have increased arterial neointima formation following mechanical injury and develop more extensive atherosclerotic lesions when made hypercholesterolemic. Similarly, low plasma HCII levels appear to be a risk factor for atherosclerosis and in-stent restenosis in human subjects. These observations suggest that a major function of the HCII-DS system is to regulate the physiologic response to arterial injury.

Effect of microgravity on gene expression in mouse brain

Changes in gravitational force such as that experienced by astronauts during space flight induce a redistribution of fluids from the caudad to the cephalad portion of the body together with an elimination of normal head-to-foot hydrostatic pressure gradients. To assess brain gene profile changes associated with microgravity and fluid shift, a large-scale analysis of mRNA expression levels was performed in the brains of 2-week control and hindlimb-unloaded (HU) mice using cDNA microarrays. Although to different extents, all functional categories displayed significantly regulated genes indicating that considerable transcriptomic alterations are induced by HU. Interestingly, the TIC class (transport of small molecules and ions into the cells) had the highest percentage of up-regulated genes, while the most down-regulated genes were those of the JAE class (cell junction, adhesion, extracellular matrix). TIC genes comprised 16% of those whose expression was altered, including sodium channel, nonvoltage-gated 1 beta (Scnn1b), glutamate receptor (Grin1), voltage-dependent anion channel 1 (Vdac1), calcium channel beta 3 subunit (Cacnb3) and others. The analysis performed by GeneMAPP revealed several altered protein classes and functional pathways such as blood coagulation and immune response, learning and memory, ion channels and cell junction. In particular, data indicate that HU causes an alteration in hemostasis which resolves in a shift toward a more hyper-coagulative state with an increased risk of venous thrombosis. Furthermore, HU treatment seems to impact on key steps of synaptic plasticity and learning processes.

Vascular dermatan sulfate regulates the antithrombotic activity of heparin cofactor II

Heparin cofactor II (HCII)-deficient mice form occlusive thrombi more rapidly than do wild-type mice following injury to the carotid arterial endothelium. Dermatan sulfate (DS) and heparan sulfate (HS) increase the rate of inhibition of thrombin by HCII in vitro, but it is unknown whether vascular glycosaminoglycans play a role in the antithrombotic effect of HCII in vivo. In this study, we found that intravenous injection of either wild-type recombinant HCII or a variant with low affinity for HS (K173H) corrected the abnormally short thrombosis time of HCII-deficient mice, while a variant with low affinity for DS (R189H) had no effect. When HCII was incubated with frozen sections of the mouse carotid artery, it bound specifically to DS in the adventitia. HCII was undetectable in the wall of the uninjured carotid artery, but it became concentrated in the adventitia following endothelial injury. These results support the hypothesis that HCII interacts with DS in the vessel wall after disruption of the endothelium and that this interaction regulates thrombus formation in vivo.

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.

Zinc ions promote the interaction between heparin and heparin cofactor II

The effects of bivalent cations on heparin binding, structure, and thrombin inhibition rates of heparin cofactor II were examined. Zn(2+) - and to a lesser extent Cu(2+) and Ni(2+) - enhanced the interaction between heparin cofactor II and heparin as demonstrated by heparin affinity chromatography and surface plasmon resonance experiments. Metal chelate chromatography and increased intrinsic protein fluorescence in the presence of Zn(2+) indicated that heparin cofactor II has metal ion-binding properties. The results are compatible with the hypothesis that Zn(2+) induces a conformational change in heparin cofactor II that favors its interaction with heparin.

Tyrosine sulfation and N-glycosylation of human heparin cofactor II from plasma and recombinant Chinese hamster ovary cells and their effects on heparin binding

The structure of post-translational modifications of human heparin cofactor II isolated from human serum and from recombinant Chinese hamster ovary cells and their effects on heparin binding have been characterized. Oligosaccharide chains were found attached to all three potential N-glycosylation sites in both protein preparations. The carbohydrate structures of heparin cofactor II circulating in blood are complex-type diantennary and triantennary chains in a ratio of 6 : 1 with the galactose being > 90% sialylated with alpha 2-->6 linked N-acetylneuraminic acid. About 50% of the triantennary structures contain one sLe(x) motif. Proximal alpha 1-->6 fucosylation of oligosacharides from Chinese hamster ovary cell-derived HCII was detected in > 90% of the diantennary and triantennary glycans, the latter being slightly less sialylated with exclusively alpha 2-->3-linked N-acetylneuraminic acid units. Applying the ESI-MS/ MS-MS technique, we demonstrate that the tryptic peptides comprising tyrosine residues in positions 60 and 73 were almost completely sulfated irrespective of the protein's origin. Treatment of transfected Chinese hamster ovary cells with chlorate or tunicamycin resulted in the production of heparin cofactor II molecules that eluted with higher ionic strength from heparin-Sepharose, indicating that tyrosine sulfation and N-linked glycans may affect the inhibitor's interaction with glycosaminoglycans.

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, antithrombin-beta and their complexes with thrombin in human tissues

In the presence of glycosaminoglycans, thrombin is rapidly inactivated by two natural inhibitors secreted from liver: antithrombin (AT) is presumed to be the principal thrombin inhibitor in circulating blood, while for heparin cofactor II (HCII), a role outside circulation has been proposed. In this study, we show that HCII and AT differ with respect to their association with human tissues. Aside from brain, each of these inhibitors was found in sodium dodecyl sulphate (SDS) soluble extracts of various human organs, with a preponderance of HCII in placenta. AT levels, however, predominated in liver. Compared to plasma, the beta-variant of AT was found to be strongly enriched in human organs, while tissue-resident HCII did not differ in its electrophoretic mobility from the circulating form. In placenta, comparable amounts of HCII/thrombin and AT/thrombin complexes were detected, indicating that HCII may exert a thrombin regulating role in that organ under conditions of tissue or blood vessel damage. Transcripts coding for HCII and AT were detected in all tissues examined. The low levels of their mRNAs suggest that most of the tissue-associated thrombin inhibitor molecules originate from circulation and are retained in organs, possibly by specific receptors. The differential presence of HCII and AT in organs is in accordance with individual physiological roles of these inhibitors.

Amino acid residues of heparin cofactor II required for stimulation of thrombin inhibition by sulphated polyanions

A variety of sulphated polyanions in addition to heparin and dermatan sulphate stimulate the inhibition of thrombin by heparin cofactor II (HCII). Previous investigations indicated that the binding sites on HCII for heparin and dermatan sulphate overlap but are not identical. In this study we determined the concentrations (IC50) of various polyanions required to stimulate thrombin inhibition by native recombinant HCII in comparison with three recombinant HCII variants having decreased affinity for heparin (Lys-173-->Gln), dermatan sulphate (Arg-189-->His), or both heparin and dermatan sulphate (Lys-185-->Asn). Pentosan polysulphate, sulphated bis-lactobionic acid amide, and sulphated bis-maltobionic acid amide resembled dermatan sulphate, since their IC50 values were increased to a much greater degree (>/=8-fold) by the mutations Arg-189-->His and Lys-185-->Asn than by Lys-173-->Gln (</=1.5-fold). By contrast, the IC50 values for fucosylated chondroitin sulphate, chondroitin sulphate E, dextran sulphate, and fucoidan were minimally affected. Only in the case of heparin was the IC50 increased to a greater degree by both Lys-173-->Gln and Lys-185-->Asn (>/=6-fold) than by Arg-189-->His (</=1.5-fold). None of the polyanions significantly stimulated inhibition of thrombin by an N-terminal deletion mutant of HCII (Delta1-74). These results suggest that, like dermatan sulphate and heparin, other polyanions stimulate HCII primarily by an allosteric mechanism requiring the N-terminal acidic domain.

Rapid changes in the exon/intron structure of a mammalian thrombin inhibitor gene

The genomic organization of the heparin cofactor II (HCII) gene from rat and mouse was investigated and compared with their human counterpart. The genes share a common core structure consisting of five exons interrupted by four introns, but the mouse and rat gene reveal individual additional features. A unique differentially spliced exon is present in the 5'-untranslated region of the rat gene, which most probably has arisen de novo by point mutations in intronic sequences of the ancestor gene. In the mouse HCII gene, a novel intron/exon boundary has been created due to the presence of an additional DNA segment, which simultaneously provides a 3'-splice site and a polypyrimidine stretch leading to an alternatively used exon of increased size. Our data suggest that, in contrast to most other mammalian genes, the exon/intron pattern of the gene coding for HCII is in dynamic evolution.

Comparative metabolism and distribution of rabbit heparin cofactor II and rabbit antithrombin in rabbits

The metabolic characteristics of two rabbit plasma thrombin inhibitors, heparin cofactor II (HCII) and antithrombin (AT), have been compared in healthy young rabbits. Purified HCII and AT-alpha were differentially radiolabeled (125I, 131I) and injected intravenously; blood samples were taken at prescribed intervals over 7 days. From the plasma clearance curves of protein-bound radioactivities, fractional catabolic rates and compartmental distributions were calculated using a three-compartment model. The whole body fractional catabolic rate for HCII (jt, 0.43/day, equivalent to t1/2 = 1.61 days) was significantly faster than for AT (jt, 0.37/day; t1/2 = 1.89 days; P < 0.005). The fractional distribution of HCII in the intravascular compartment (Ap, 0.20) and in the extravascular compartment (Ac, 0.63) differed significantly from AT (Ap, 0.30; Ac, 0.56). From the catabolic data and blood concentrations, absolute quantities of HCII and AT catabolized by a 3-kg rabbit amounted to 12.8 and 19.9 mg/day, respectively, equivalent to a molar ratio, AT/HCII, of 1.7. The catabolic molar ratio was compared with the relative release rates of HCII and AT from perfused rabbit livers. Both proteins were released from the liver, the molar ratio in the perfusate rising to approximately 1.4 at 2.5 h. This report increases our understanding of the in vivo dynamics of these two proteins.

Role of the proposed serpin-enzyme complex receptor recognition site in binding and internalization of thrombin-heparin cofactor II complexes by hepatocytes

Several serpin-enzyme complexes bind to a receptor on hepatocytes that mediates their endocytosis and lysosomal degradation. Joslin et al. (Joslin, G., Fallon, R. J., Bullock, J., Adams, S. P., and Perlmutter, D. H.(1991) J. Biol. Chem. 266, 11282-11288) proposed that a sequence near the C-terminal end of the serpin (e.g. FVFLM in alpha1-antitrypsin) binds to the serpin-enzyme complex receptor (SEC receptor). In experiments with synthetic peptides, they found that substitution of alanine at the fourth or fifth position in this sequence reduced the affinity of peptide binding to Hep G2 cells. To test the hypothesis that the corresponding sequence in heparin cofactor II (HCII), FLFLI (residues 456-460), mediates binding and uptake of the thrombin-HCII complex by Hep G2 cells, we constructed five recombinant HCII variants, F456A, L457A, F458A, L459A, and I460A. At 4 degrees C, the 125I-thrombin-HCII(native) complex bound reversibly to 0.6-2.6 x 10(5) sites per Hep G2 cell with a Kd of 19-32 nM. Binding was inhibited by excess unlabeled thrombin-HCII(native), thrombin-antithrombin, or elastase-alpha1-antitrypsin, but not by free HCII or thrombin, which is consistent with the reported specificity of the SEC receptor. However, complexes of thrombin with each of the HCII variants inhibited binding as effectively as the complex with native HCII. Competitive binding experiments with various concentrations of unlabeled thrombin-HCII(native) or thrombin-HCII(I460A) indicated that these complexes bind to Hep G2 cells with equal affinity. At 37 degrees C, complexes of 125I-thrombin with each of the five HCII variants were internalized and degraded at the same rate as the complex with native HCII. Our data suggest that the pentapeptide FLFLI in HCII is not involved in binding, internalization, and degradation of thrombin-HCII complexes by Hep G2 cells.