Mapping of the catalytic site of CHO-t-PA and the t-PA variant BM 06.022 by synthetic inhibitors and substrates

Recombinant Expression of Thrombolytic Agent Reteplase in Marine Microalga Tetraselmis subcordiformis (Chlorodendrales, Chlorophyta)

Tetraselmis subcordiformis, a unicellular marine green alga, is used widely in aquaculture as an initial feeding for fish, bivalve mollusks, penaeid shrimp larvae, and rotifers because of its rich content of amino acids and fatty acids. A stable nuclear transformation system using the herbicide phosphinothricin (PPT) as a selective reagent was established previously. In this research, the recombinant expression in T. subcordiformis was investigated by particle bombardment with the rt-PA gene that encodes the recombinant human tissue-type plasminogen activator (Reteplase), which is a thrombolytic agent for acute myocardial infarction treatment. Transgenic algal strains were selected by their resistance to PPT, and expression of rt-PA was validated by PCR, Southern blotting, and Western blotting, and bioactivity of rt-PA was confirmed by the fibrin agarose plate assay for bioactivity. The results showed that rt-PA was integrated into the genome of T. subcordiformis, and the expression product was bioactive, indicating proper post-transcriptional modification of rt-PA in T. subcordiformis. This report contributes to efforts that take advantage of marine microalgae as cell factories to prepare recombinant drugs and in establishing a characteristic pathway of oral administration in aquaculture.

The evolution of recombinant thrombolytics: Current status and future directions

Cardiovascular disorders are on the rise worldwide due to alcohol abuse, obesity, hypertension, raised blood lipids, diabetes and age-related risks. The use of classical antiplatelet and anticoagulant therapies combined with surgical intervention helped to clear blood clots during the inceptive years. However, the discovery of streptokinase and urokinase ushered the way of using these enzymes as thrombolytic agents to degrade the fibrin network with an issue of systemic hemorrhage. The development of second generation plasminogen activators like anistreplase and tissue plasminogen activator partially controlled this problem. The third generation molecules, majorly t-PA variants, showed desirable properties of improved stability, safety and efficacy with enhanced fibrin specificity. Plasmin variants are produced as direct fibrinolytic agents as a futuristic approach with targeted delivery of these drugs using liposome technlogy. The novel molecules from microbial, plant and animal origin present the future of direct thrombolytics due to their safety and ease of administration.

Serine-proteases as plasminogen activators in terms of fibrinolysis

OBJECTIVES

This review should give an overview about the natural human plasminogen activators and their various modified variants as well as similar substances isolated from animals, microorganisms and plants. When a blood clot is formed in a blood vessel, it avoids the oxygen supply of the surrounding tissue. A fast fibrinolytic therapy should redissolve the blood vessel and reduce the degradation of the tissue. All proteases that are part of the human blood coagulation and fibrinolytic system belong to the serine protease family. t-PA (tissue plasminogen activator) and u-PA (urokinase plasminogen activator) are the naturally occurring fibrinolytic agents that are also used in therapy.

KEY FINDINGS

Despite many years of research, t-PA is still the gold standard in fibrinolytic therapy. But it has to be given as an infusion, which needs time. Modified fibrinolytic substances are, were, or perhaps will be in the market. They have different advantages over t-PA, but often the disadvantages predominate.

CONCLUSION

Many substances have been developed but an optimal fibrinolytic agent combined with a simple administration is not in therapeutic use to date.

Recombinant expression of rt-PA gene (encoding Reteplase) in gametophytes of the seaweed Laminaria japonica (Laminariales, Phaeophyta)

The life cycle of seaweed Laminaria japonica involves a generation alternation between diploid sporophyte and haploid gametophte. The expression of foreign genes in sporophte has been proved. In this research, the recombinant expression in gametophyte was investigated by particle bombardment with the rt-PA gene encoding the recombinant human tissue-type plasminogen activator (Reteplase), which is a thrombolytic agent for acute myocardial infarction (AMI). Transgenic gametophytes were selected by their resistance to herbicide phosphiothricin (PPT), and proliferated in an established bubble column photo-bioreactor. According to the results from quantitative ELISA, Southern blotting, and fibrin agarose plate assay (FAPA) for bioactivity, it was showed that the rt-PA gene had been integrated into the genome of gametophytes of L. japonica, and the expression product showed the expected bioactivity, implying the proper post-transcript modification in haploid gametophyte.

A Critical Role for System A Amino Acid Transport in the Regulation of Dendritic Development by Brain-derived Neurotrophic Factor (BDNF)

Dendritic development is essential for the establishment of a functional nervous system. Among factors that control dendritic development, brain-derived neurotrophic factor (BDNF) has been shown to regulate dendritic length and complexity of cortical neurons. However, the cellular and molecular mechanisms that underlie these effects remain poorly understood. In this study, we examined the role of amino acid transport in mediating the effects of BDNF on dendritic development. We show that BDNF increases System A amino acid transport in cortical neurons by selective up-regulation of the sodium-coupled neutral amino acid transporter (SNAT)1. Up-regulation of SNAT1 expression and System A activity is required for the effects of BDNF on dendritic growth and branching of cortical neurons. Further analysis revealed that induction of SNAT1 expression and System A activity by BDNF is necessary in particular to enhance synthesis of tissue-type plasminogen activator, a protein that we demonstrate to be essential for the effects of BDNF on cortical dendritic morphology. Together, these data reveal that stimulation of neuronal differentiation by BDNF requires the up-regulation of SNAT1 expression and System A amino acid transport to meet the increased metabolic demand associated with the enhancement of dendritic growth and branching.

Third-generation thrombolytic drugs

Several third-generation thrombolytic agents have been developed. They are either conjugates of plasminogen activators with monoclonal antibodies against fibrin, platelets, or thrombomodulin; mutants, variants, and hybrids of alteplase and prourokinase (amediplase); or new molecules of animal (vampire bat) or bacterial (Staphylococcus aureus) origin. These variations may lengthen the drug's half-life, increase resistance to plasma protease inhibitors, or cause more selective binding to fibrin. Compared with the second-generation agent (alteplase), third-generation thrombolytic agents such as monteplase, tenecteplase, reteplase, lanoteplase, pamiteplase, and staphylokinase result in a greater angiographic patency rate in patients with acute myocardial infarction, although, thus far, mortality rates have been similar for those few drugs that have been studied in large-scale trials. Bleeding risk, however, may be greater.

Design of benzamidine-type inhibitors of factor Xa

A series of derivatives of rac-benzenesulfonyl-glycyl-phenylalanine or its ethyl ester with a combination of thioamido/amidino or amidino/amidino substituents in the benzene rings was synthesized as potential inhibitors of factor Xa (fXa). Among these, the racemic 4'-amidinobenzenesulfonyl-glycyl-4-amidinophenylalanine ethyl ester was found to exhibit the highest affinity for fXa despite the unfavored location of the amidino substituent in the para position. X-ray structural analysis of the trypsin complex with this bis-benzamidine compound revealed a retro-binding mode if compared to those of similar compounds, so far analyzed in complexes with trypsin or fXa. This noncanonical binding mode as well as its slow plasma clearance rates in rats, if compared to those of other benzamidine derivatives, suggests this compound as an interesting new lead structure for the design of fXa inhibitors.

Tissue plasminogen activator mediates reverse occlusion plasticity in visual cortex

Preventing visual input to one eye (monocular deprivation) in early postnatal development reduces cortical responses to stimulation of the deprived eye, with a significant loss of thalamocortical connections. These effects are reversible by opening the deprived eye and closing the previously open eye (reverse occlusion). We show that intracortical blockade of tissue plasminogen activator or plasmin selectively prevents recovery of cortical function and thalamic neuron size during reverse occlusion, without affecting the monocular deprivation response. Therefore, a proteolytic cascade consisting of plasmin generated by tissue plasminogen activator may selectively mediate reverse-occlusion-induced cortical plasticity, perhaps via structural remodeling of axons.

Tissue-type plasminogen activator: variants and crystal/solution structures demarcate structural determinants of function

NMR and crystal structure of many components of tissue-type plasminogen activator (t-PA) are now available: the finger-EGF pair and the kringle-2 domain structures have been solved, as have the proteolytic domains of vampire bat PA and human t-PA in two- and single-chain forms. These structures confirm the trypsin-like arrangement of the proteolytic domain of t-PA and show how surface loops near the catalytic centre contribute to the narrow specificity of t-PA. Together with mutational experiments, they identify the Lys156 sidechain as a cause of the amidolytic activity of single-chain t-PA, as it can provide a substitute salt bridge partner for Asp194 in the absence of the Ile16 N terminus of the two-chain form. These new findings provide new ideas for the design of PA variants with improved therapeutic properties.

Synthesis and structure-activity relationships of potent thrombin inhibitors: piperazides of 3-amidinophenylalanine

Thrombin is the key enzyme in the blood coagulation system, and inhibitors of its proteolytic activity are of therapeutic interest since they are potential anticoagulants. The most potent inhibitor of the benzamidine type is N alpha-[(2-naphthylsulfonyl)glycyl]-4-amidinophenylalanylpiperid ide (NAPAP). However, NAPAP and other benzamidine derivatives do not show favorable pharmacological properties; above all, they have very low systemic bioavailability after oral administration. The goal of designing new compounds was to obtain potent inhibitors with improved pharmacokinetic properties. Piperazide derivatives of 3-amidinophenylalanine as the key building block were synthesized. The piperazine moiety opened the possibility to introduce quite different substituents on the second nitrogen using common synthetic procedures. Some of the newly synthesized compounds are potent inhibitors of thrombin and offer an approach to study structure-function relationships for inhibition of thrombin and related enzymes and for the improvement of their pharmacokinetic properties.

Structural mapping of the active site specificity determinants of human tissue-type plasminogen activator. Implications for the design of low molecular weight substrates and inhibitors

The recent structure determination of the catalytic domain of tissue-type plasminogen activator (tPA) suggested residue Arg174 could play a role in P3/P4 substrate specificity. Six synthetic chromogenic tPA substrates of the type R-Xaa-Gly-Arg-p-nitroanilide, in which R is an N-terminal protection group, were synthesized to test this property. Although changing the residue Xaa (in its L or D form) at position P3 from the hydrophobic Phe to an acidic residue, Asp or Glu, gave no improvement in catalytic efficiency, comparative analysis of the substrates indicated a preference for an acidic substituent occupying the S3 site when the S4 site contains a hydrophobic or basic moiety. The 2.9 A structure determination of the catalytic domain of human tPA in complex with the bis-benzamidine inhibitor 2, 7-bis-(4-amidinobenzylidene)-cycloheptan-1-one reveals a three-site interaction, salt bridge formation of the proximal amidino group of the inhibitor with Asp189 in the primary specificity pocket, extensive hydrophobic surface burial, and a weak electrostatic interaction between the distal amidino group of the inhibitor and two carbonyl oxygens of the protein. The latter position was previously occupied by the guanidino group of Arg174, which swings out to form the western edge of the S3 pocket. These data suggest that the side chain of Arg174 is flexible, and does not play a major role in the S4 specificity of tPA. On the other hand, this residue would modulate S3 specificity, and may be exploited to fine tune the specificity and selectivity of tPA substrates and inhibitors.

Reteplase. A review of its pharmacological properties and clinical efficacy in the management of acute myocardial infarction

Reteplase (BM 06.022; r-PA) is a recombinant peptide which consists of the kringle 2 and protease domains of human tissue-type plasminogen activator. It has been developed as a thrombolytic treatment for acute myocardial infarction (AMI). The half-life of reteplase allows administration as a double-bolus injection (second injection given 30 minutes after the first) rather than by the prolonged and, in some cases, more complex intravenous infusion regimens that are required for most other thrombolytic agents. Reteplase produced rapid and effective coronary artery thrombolysis in a number of dose-finding and comparative studies. Double-bolus administration of reteplase 10U + 10U produced significantly higher coronary artery patency rates than accelerated alteplase (100mg as a 1.5-hour infusion) in patients with AMI in the RAPID-II study. The 10U + 10U reteplase regimen produced a 35-day survival rate at least equivalent to that seen with a 1-hour infusion of streptokinase 1.5 million units in 5986 patients in the INJECT study, which was designed to demonstrate equivalence between treatments. As with other thrombolytics, bleeding was the most common adverse event seen in reteplase recipients. No significant differences in the overall risk of haemorrhage were observed between reteplase and either accelerated alteplase or standard streptokinase treatment in clinical trials. The risk of stroke in reteplase recipients appears to be similar to that for other thrombolytic agents [1.2% incidence in 3288 patients treated with reteplase 10U + 10U in clinical trials (0.76% for haemorrhagic stroke)], although accurate statistical assessment of the relative risk is not possible for the data available to date. Thus, reteplase is an effective thrombolytic agent which can be administered as a double-bolus injection regimen rather than as a prolonged infusion. Together with acquisition cost and general pharmacoeconomic data (which are not yet available), the results of GUSTO-III (a trial comparing double-bolus reteplase with accelerated alteplase in 15 000 patients) will have a major influence on the pattern of use of reteplase. In the meantime, data from the available clinical trials suggest that reteplase is a fast-acting and effective thrombolytic treatment for patients with AMI.

Active site labeling with dansyl-glutamyl-glycyl-arginyl chloromethyl ketone demonstrates the full activity of the refolded and purified tissue-type plasminogen activator variant BM 06.022

BM 06.022 is a tissue-type plasminogen activator deletion variant that is comprised of the kringle 2 and the protease domain of the native molecule. BM 06.022 is expressed as inactive inclusion bodies in E. coli and transferred into the active enzyme by an in vitro folding process. Active site labeling with dansyl-glutamyl-glycyl-arginyl chloromethyl ketone provides evidence that the purified BM 06.022 is fully active and that misfolded species are completely removed by affinity chromatography on ETI-Sepharose. The comparison of the kinetics of the inhibition of BM 06.022 with that of CHO-t-PA indicates that the active centers of both enzymes are rather similar. The further evaluation of the site of interaction of BM 06.022 and DnsEGRck by mass spectroscopy and amino acid sequence analysis revealed that the inhibitor is bound selectively to His322, which is part of the catalytic triad of this serine protease.

Thrombolytic agents in development

The quest continues for thrombolytic agents with a higher thrombolytic potency, specific thrombolytic activity and/or a better fibrin selectivity. Several lines of research towards improvement of thrombolytic agents are being explored, including the construction of mutants and variants of plasminogen activators (PAs), chimaeric PAs, conjugates of PAs with monoclonal antibodies, and PAs from animal or bacterial origin. Some of these new thrombolytic agents have shown promise in animal models of venous or arterial thrombosis and in pilot clinical studies. Such molecules include numerous mutants of tissue-type PA (t-PA) with prolonged in vivo half-life and/or resistance to protease inhibitors, and chimaeric PAs consisting of different regions of t-PA and of urokinase-type PA (u-PA). Several molecular forms of the thrombolytic substance in the saliva of the vampire bat have been characterised and cloned. Vampire bat PA exhibits 85% homology to human t-PA but lacks kringle 2 and the plasmin-sensitive cleavage site. A thrombolytic enzyme of 203 amino acids is present in the venom of a southern copperhead snake. This polypeptide, termed fibrolase, is now produced by recombinant technology. Fibrolase does not activate plasminogen or protein C, but directly degrades the alpha and beta chains of fibrin and fibrinogen. Recombinant staphylokinase is not an enzyme, but it forms a 1:1 stoichiometric complex with plasminogen, which becomes active after conversion of plasminogen to plasmin. It is a potent and highly fibrin specific thrombolytic agent in animals and patients.

Tissue-type plasminogen activator domain-deletion mutant BM 06.022: modular stability, inhibitor binding, and activation cleavage

Recombinant BM 06.022 (M(r) 39,589) is a domain-deletion mutant of the human tissue-type plasminogen activator (tPA) structured by the kringle 2 and protease modules. Unfolding under various conditions was investigated via 1H-NMR spectroscopy by monitoring the well-resolved high-field methyl resonances at approximately -0.97 ppm (kringle 2) and approximately -0.29 and -0.54 ppm (protease). Reversible acid/base unfolding is manifest under low pH (< 4.8) conditions. It is observed that, relative to the protease, the kringle exhibits higher overall stability at low pH. At pH 4.6, BM 06.022 undergoes two distinct thermal melting transitions, at approximately 334 and approximately 352 K, assigned to an irreversible denaturation of the protease and a reversible unfolding of the kringle 2, respectively. Under the same conditions, the protease reacted with the active site inhibitor 1,5 dansyl-L-glutamylglycyl-L-arginine chloromethyl ketone (EGRck) exhibits a higher (approximately 10 K) thermal stability than the inhibitor-free protease. Upon acidification, the EGRck-modified protease unfolds irreversibly around pH 3.4. As exemplified by BM 06.022, a single-chain protein, as defined by continuity of the polypeptide backbone, can exhibit simultaneous folding reversibility and irreversibility for autonomous segments of the sequence. Conversion of the isolated (single-chain) protease or intact BM 06.022 to their catalytically active two-chain forms via plasminolytic cleavage of the Arg275-Ile276 peptide bond leaves the kringle 2 spectrum unaffected while perturbing the resolved high-field methyl resonances stemming from the protease. The latter also shift when the protease is reacted with EGRck, indicating that these signals are sensitive to events at the binding pocket.(ABSTRACT TRUNCATED AT 250 WORDS)