Molecular mechanisms of plasminogen activation: bacterial cofactors provide clues

C-terminal lysine residues enhance plasminogen activation by inducing conformational flexibility and stabilization of activator complex of staphylokinase with plasmin

Staphylokinase (SAK), a potent fibrin-specific plasminogen activator secreted by Staphylococcus aureus, carries a pair of lysine at the carboxy-terminus that play a key role in plasminogen activation. The underlaying mechanism by which C-terminal lysins of SAK modulate its function remains unknown. This study has been undertaken to unravel role of C-terminal lysins of SAK in plasminogen activation. While deletion of C-terminal lysins (Lys135, Lys136) drastically impaired plasminogen activation by SAK, addition of lysins enhanced its catalytic activity 2-2.5-fold. Circular dichroism analysis revealed that C-terminally modified mutants of SAK carry significant changes in their beta sheets and secondary structure. Structure models and RING (residue interaction network generation) studies indicated that the deletion of lysins has conferred extensive topological alterations in SAK, disrupting vital interactions at the interface of SAK.plasmin complex, thereby leading significant impairment in its functional activity. In contrast, addition of lysins at the C-terminus enhanced its conformational flexibility, creating a stronger coupling at the interface of SAK.plasmin complex and making it more efficient for plasminogen activation. Taken together, these studies provided new insights on the role of C-terminal lysins in establishment of precise intermolecular interactions of SAK with the plasmin for the optimal function of activator complex.

Chemical profiles and pharmacological insights of Anisomeles indica Kuntze: An experimental chemico-biological interaction

Anisomeles indica (L.) Kuntze is an ethnomedicinally important plant that has long been used in traditional medicine to treat a variety of ailments, including dyspepsia, abdominal pain, colic, allergies, inflammation, and rheumatic arthritis. However, the scientific framework underlying these medicinal properties is not well known. This study aimed to investigate the antidepressive, antidiarrheal, thrombolytic, and anti-inflammatory potential of a methanol extract of A. indica (MeOH-AI). The potential bioactive compounds in the MeOH-AI were identified using gas chromatography-mass spectrometry (GC-MS), and antidepressant activities were evaluated using the tail suspension test (TST) and forced swim test (FST). Antidiarrheal effects were also assayed in castor oil-induced diarrhea and gastrointestinal motility studies. The anti-inflammatory activities were explored by examining the effects on protein inhibition and denaturation in heat- and hypotonic solution-induced hemolysis assays. The thrombolytic activity was evaluated using the clot lysis test in human blood. BIOVIA and Schrödinger Maestro (v11.1) were applied for docking analysis to determine binding interactions, and the absorption, distribution, metabolisms, excretion/toxicity (ADME/T) properties of bioactive compounds were explored using a web-based method. The GC-MS analysis of MeOH-AI revealed the presence of several bioactive compounds. MeOH-AI administration resulted in significant (p < 0.01) reductions in the immobility times for both the FST and TST compared with those in the control group. MeOH-AI also induced significant (p < 0.01) reductions in castor oil-induced diarrhea severity and gastrointestinal motility in a mouse model. In addition, the in vitro anti-inflammatory and thrombolytic activity studies produced remarkable responses. The binding assay showed that 4-dehydroxy-N-(4,5-methylenedioxy-2-nitrobenzylidene) tyramine interacts favorably with monoamine oxidase and serotonin and M3 muscarinic acetylcholine receptors, displaying good pharmacokinetic properties, which may mediate the effects of MeOH-AI on depression and diarrhea. Overall, the research findings indicated that MeOH-AI has significant antidepressant, antidiarrheal, and anti-inflammatory effects and may represent an alternative source of novel therapeutic factors.

Fagonia arabica L.: A review of its phytochemistry, pharmacology and traditional uses

BACKGROUND

Fagonia arabica, belongs to family Zygophyllaceae, is a medicinal plant, widely distributed in the desert areas of the world including Saudi Arabia, Pakistan, India and South Africa. The present review aims to explore the published information on the traditional uses, ethnobotanical knowledge, phytochemistry and various biological activities like antioxidant, antimicrobial, thrombolytic activities and anticoagulant effects of Fagonia arabica with critical analysis on the gaps and future perspectives.

METHOD

A literature survey was performed by searching the digital libraries and the scientific databases including Scopus, Google Scholar, SciFinder, ACS, Web of Science and published books.

RESULTS

Fagonia arabica plant has been reported to have a wide range of traditional uses in sore mouth, smallpox, hematological, neurological, endocrinological, inflammatory, cooling agent in stomatitis, vertigo and endothermic reaction in the body. Several bioactive constituents including glycosides, flavonoids, terpenoids, saponins, alkaloids and trace elements were recorded from Fagonia arabica plant. The isolation and identification of two flavonoid glycosides (kaempferol-7-O-rhamnoside and acacetin-7-O-rhamnoside) were also reported. Fagonia arabica has been studied for its wide range of biological activities, which include antioxidant, antimicrobial, cardioprotective and anticoagulant.

CONCLUSION

It is apparent from the literature that Fagonia arabica plant possesses a wide range of medicinal and pharmacological uses and has been studied for its various pharmacological activities and medicinal applications. Critical analysis reveals that the plant has the huge potential for pharmaceutical and pharmacological applications.

Fibrinolysis: A Primordial System Linked to the Immune Response

The fibrinolytic system provides an essential means to remove fibrin deposits and blood clots. The actual protease responsible for this is plasmin, formed from its precursor, plasminogen. Fibrin is heralded as it most renowned substrate but for many years plasmin has been known to cleave many other substrates, and to also activate other proteolytic systems. Recent clinical studies have shown that the promotion of plasmin can lead to an immunosuppressed phenotype, in part via its ability to modulate cytokine expression. Almost all immune cells harbor at least one of a dozen plasminogen receptors that allows plasmin formation on the cell surface that in turn modulates immune cell behavior. Similarly, a multitude of pathogens can also express their own plasminogen activators, or contain surface proteins that provide binding sites host plasminogen. Plasmin formed under these circumstances also empowers these pathogens to modulate host immune defense mechanisms. Phylogenetic studies have revealed that the plasminogen activating system predates the appearance of fibrin, indicating that plasmin did not evolve as a fibrinolytic protease but perhaps has its roots as an immune modifying protease. While its fibrin removing capacity became apparent in lower vertebrates these primitive under-appreciated immune modifying functions still remain and are now becoming more recognised.

Integration of VEK-30 peptide enhances fibrinolytic properties of staphylokinase

Staphylokinase (SAK), a 136 amino acid bacterial protein with profibrinolytic properties, has emerged as an important thrombolytic agent because of its fibrin specificity and reduced inhibition by α-2 antiplasmin. In an attempt to enhance the clot dissolution ability of SAK, a 30 amino acid peptide (VEK-30) derived from a plasminogen (Pg) binding protein (PAM), was fused at the C-terminal end of SAK with a RGD (Arg-Gly-Asp) linker. The chimeric protein, SAKVEK, was expressed in E. coli and purified as a soluble protein. Pg activation by equimolar complexes of SAKVEK and SAK with plasmin revealed that the fusion of VEK-30 peptide has significantly enhanced the catalytic activity of SAK. The kinetic constant, k_cat /K_m , of SAKVEK for the substrate Pg appeared 2.7 times higher than that of SAK and the time required for the fibrin and platelet rich clot lysis was shortened by 30% and 50%, respectively. The binary activator complex of SAKVEK with plasmin gets inhibited by α2- antiplasmin but remains protected in the presence of fibrin, very similar to SAK. Thus, the present study suggests that SAKVEK is more potent and effective as a thrombolytic agent due to its higher catalytic activity for Pg activation in a fibrin-specific manner and its ability to clear platelet-rich plasma clot faster than SAK.

In vivo neuroprotective, antinociceptive, anti-inflammatory potential in Swiss albino mice and in vitro antioxidant and clot lysis activities of fractionated Holigarna longifolia Roxb. bark extract

Background The study investigated the in vivo neuroprotective, antinociceptive, anti-inflammatory potential and in vitro antioxidant and clot lysis activities of crude methanol extract with its different solvent-soluble fractions like petroleum ether (PESF), carbon tetrachloride (CTSF), chloroform (CSF) and aqueous (AQSF) of Holigarna longifolia Roxb. Methods Phenobarbitone-induced sleeping time method was used for the neuroprotective activity, writhing response experimental model introduced by acetic acid was designed for antinociceptive efficacy, carrageenan-induced paw edema model was carried out for anti-inflammatory activity, DPPH free radical scavenging activity was assessed for antioxidant activity and clot lysis model was investigated for the thrombolytic potential of the plant. Results On investigation it was found that methanol extract and CS fraction revealed statistically meaningful (p<0.05) neuroprotective activity by increasing phenobarbitone-induced sleeping time of mice, produced substantial (p<0.05) inflammation inhibitory efficacy compared to standard diclofenac sodium and also exhibited statistically significant (p<0.01) oxidative stress inhibitory efficacy by inhibiting free radical formation compared to ascorbic acid as standard. Only methanol extract produced significant (p<0.05) antinociceptive activity by inhibiting abdominal writhes produced by acetic acid compared to standard analgesic drug diclofenac sodium. And only aqueous soluble fraction exhibited moderate clot lysis activity compared to streptokinase as standard. Conclusion The findings demonstrate that H. longifolia could be potential neuroprotective due to its justified antioxidative capacity as well as clot lysis properties.

Toxins and Superantigens of Group A Streptococci

Streptococcus pyogenes (i.e., the group A Streptococcus ) is a human-restricted and versatile bacterial pathogen that produces an impressive arsenal of both surface-expressed and secreted virulence factors. Although surface-expressed virulence factors are clearly vital for colonization, establishing infection, and the development of disease, the secreted virulence factors are likely the major mediators of tissue damage and toxicity seen during active infection. The collective exotoxin arsenal of S. pyogenes is rivaled by few bacterial pathogens and includes extracellular enzymes, membrane active proteins, and a variety of toxins that specifically target both the innate and adaptive arms of the immune system, including the superantigens; however, despite their role in S. pyogenes disease, each of these virulence factors has likely evolved with humans in the context of asymptomatic colonization and transmission. In this article, we focus on the biology of the true secreted exotoxins of the group A Streptococcus , as well as their roles in the pathogenesis of human disease.

Proteases: Pivot Points in Functional Proteomics

Proteases drive the life cycle of all proteins, ensuring the transportation and activation of newly minted, would-be proteins into their functional form while recycling spent or unneeded proteins. Far from their image as engines of protein digestion, proteases play fundamental roles in basic physiology and regulation at multiple levels of systems biology. Proteases are intimately associated with disease and modulation of proteolytic activity is the presumed target for successful therapeutics. "Proteases: Pivot Points in Functional Proteomics" examines the crucial roles of proteolysis across a wide range of physiological processes and diseases. The existing and potential impacts of proteolysis-related activity on drug and biomarker development are presented in detail. All told the decisive roles of proteases in four major categories comprising 23 separate subcategories are addressed. Within this construct, 15 sets of subject-specific, tabulated data are presented that include identification of proteases, protease inhibitors, substrates, and their actions. Said data are derived from and confirmed by over 300 references. Cross comparison of datasets indicates that proteases, their inhibitors/promoters and substrates intersect over a range of physiological processes and diseases, both chronic and pathogenic. Indeed, "Proteases: Pivot Points …" closes by dramatizing this very point through association of (pro)Thrombin and Fibrin(ogen) with: hemostasis, innate immunity, cardiovascular and metabolic disease, cancer, neurodegeneration, and bacterial self-defense.

Streptococcal inhibitor of complement (SIC) modulates fibrinolysis and enhances bacterial survival within fibrin clots

Some strains of the bacterial pathogen Streptococcus pyogenes secrete protein SIC (streptococcal inhibitor of complement), including strains of the clinically relevant M1 serotype. SIC neutralizes the effect of a number of antimicrobial proteins/peptides and interferes with the function of the host complement system. Previous studies have shown that some S. pyogenes proteins bind and modulate coagulation and fibrinolysis factors, raising the possibility that SIC also may interfere with the activity of these factors. Here we show that SIC interacts with both human thrombin and plasminogen, key components of coagulation and fibrinolysis. We found that during clot formation, SIC binds fibrin through its central region and that SIC inhibits fibrinolysis by interacting with plasminogen. Flow cytometry results indicated that SIC and plasminogen bind simultaneously to S. pyogenes bacteria, and fluorescence microscopy revealed co-localization of the two proteins at the bacterial surface. As a consequence, SIC-expressing bacteria entrapped in clots inhibit fibrinolysis, leading to delayed bacterial escape from the clots as compared with mutant bacteria lacking SIC. Moreover, within the clots SIC-expressing bacteria were protected against killing. In an animal model of subcutaneous infection, SIC-expressing bacteria exhibited a delayed systemic spread. These results demonstrate that the bacterial protein SIC interferes with coagulation and fibrinolysis and thereby enhances bacterial survival, a finding that has significant implications for S. pyogenes virulence.

Impact of Bradykinin Generation During Thrombolysis in Ischemic Stroke

Ischemic stroke is one of the leading causes of death and disability worldwide. Current medical management in the acute phase is based on the activation of the fibrinolytic cascade by intravenous injection of a plasminogen activator (such as tissue-type plasminogen activator, tPA) that promotes restauration of the cerebral blood flow and improves stroke outcome. Unfortunately, the use of tPA is associated with deleterious effects such as hemorrhagic transformation, symptomatic brain edema, and angioedema, which limit the efficacy of this therapeutic strategy. Preclinical and clinical evidence suggests that intravenous thrombolysis generates large amounts of bradykinin, a peptide with potent pro-inflammatory, and pro-edematous effects. This tPA-triggered generation of bradykinin could participate in the deleterious effects of thrombolysis and is a potential target to improve neurological outcome in tPA-treated patients. The present review aims at summarizing current evidence linking thrombolysis, bradykinin generation, and neurovascular damage.

Failure of bacterial screening to detect Staphylococcus aureus: the English experience of donor follow-up

BACKGROUND AND OBJECTIVES

Between February 2011 and December 2016, over 1·6 million platelet units, 36% pooled platelets, underwent bacterial screening prior to issue. Contamination rates for apheresis and pooled platelets were 0·02% and 0·07%, respectively. Staphylococcus aureus accounted for 21 contaminations, including four pooled platelets, one confirmed transfusion-transmitted infection (TTI) and three 'near-miss' incidents detected on visual inspection which were negative on screening. We describe follow-up investigations of 16 donors for skin carriage of S. aureus and molecular characterisation of donor and pack isolates.

MATERIALS AND METHODS

Units were screened by the BacT/ALERT 3D detection system. Contributing donors were interviewed and consent requested for skin and nasal swabbing. S. aureus isolates were referred for spa gene type and DNA macrorestriction profile to determine identity between carriage strains and packs.

RESULTS

Donors of 10 apheresis and two pooled packs screen positive for S. aureus were confirmed as the source of contamination; seven had a history of skin conditions, predominantly eczema; 11 were nasal carriers. The 'near-miss' incidents were associated with apheresis donors, two donors harboured strains indistinguishable from the pack strain. The TTI was due to a screen-negative pooled unit, and a nasal isolate of one donor was indistinguishable from that in the unit.

CONCLUSION

Staphylococcus aureus contamination is rare but potentially harmful in platelet units. Donor isolates showed almost universal correspondence in molecular type with pack isolates, thus confirming the source of contamination. The importance of visual inspection of packs prior to transfusion is underlined by the 'near-miss' incidents.

Assessment of the antioxidant, thrombolytic, analgesic, anti-inflammatory, antidepressant and anxiolytic activities of leaf extracts and fractions of Tetracera sarmentosa (L.) Vahl

BACKGROUND

The plant under investigation (Tetracera sarmentosa) is a dicotyledonous flowering plant and belongs to the family Dilleniaceae. The goal of our investigation was to determine whether the leaf extracts of this plant held any significant medicinal properties.

METHODS

Leaves of T. sarmentosa were extracted with pure ethanol (EETS) and methanol (METS), and then methanol extract fractioned with n-hexane (NHFMETS) and chloroform (CHFMETS). The extracts and fractions were tested for antioxidant activity, which was measured by using qualitative and quantitative procedures. Thrombolytic activity was evaluated by the clot lysis test. Analgesic activity was evaluated employing the acidic acid-induced writhing test, the formalin-induced paw licking test and tail immersion on Swiss albino mice. The anti-inflammatory activity test was studied using the paw edema test. Forced swimming, tail suspension, elevated plus maze and hole board model tests were used to evaluate neuropharmacological activity.

RESULTS

All the extracts and fractions possessed antioxidant effects. All the extracts, fractions and streptokinase exhibited significant (p<0.0001) clot lysis. The extracts and fractions produced significant analgesic effects as evaluated by the acetic acid writhing test, the formalin-induced paw licking test and the tail immersion method. Similarly, carrageenan-induced inflammation was significantly antagonized by the treatments. The extracts and fractions also significantly showed neuropharmacological (antidepressant and anxiolytic) effects.

CONCLUSIONS

The overall results suggested that this plant deserves further investigation to isolate the active compounds which are responsible for these activities and to establish the mechanism of action.

Epic Immune Battles of History: Neutrophils vs. Staphylococcus aureus

Neutrophils are the most abundant leukocytes in human blood and the first line of defense after bacteria have breached the epithelial barriers. After migration to a site of infection, neutrophils engage and expose invading microorganisms to antimicrobial peptides and proteins, as well as reactive oxygen species, as part of their bactericidal arsenal. Ideally, neutrophils ingest bacteria to prevent damage to surrounding cells and tissues, kill invading microorganisms with antimicrobial mechanisms, undergo programmed cell death to minimize inflammation, and are cleared away by macrophages. Staphylococcus aureus (S. aureus) is a prevalent Gram-positive bacterium that is a common commensal and causes a wide range of diseases from skin infections to endocarditis. Since its discovery, S. aureus has been a formidable neutrophil foe that has challenged the efficacy of this professional assassin. Indeed, proper clearance of S. aureus by neutrophils is essential to positive infection outcome, and S. aureus has developed mechanisms to evade neutrophil killing. Herein, we will review mechanisms used by S. aureus to modulate and evade neutrophil bactericidal mechanisms including priming, activation, chemotaxis, production of reactive oxygen species, and resolution of infection. We will also highlight how S. aureus uses sensory/regulatory systems to tailor production of virulence factors specifically to the triggering signal, e.g., neutrophils and defensins. To conclude, we will provide an overview of therapeutic approaches that may potentially enhance neutrophil antimicrobial functions.

Bacterial screening of platelet components by National Health Service Blood and Transplant, an effective risk reduction measure

BACKGROUND

Bacterial contamination of blood components remains a major cause of sepsis in transfusion medicine. Between 2006 and 2010 in the 5 years before the introduction of bacterial screening of platelet (PLT) components by National Health Service Blood and Transplant (NHSBT), seven cases of PLT component-associated transmission of bacterial infection were recorded for 10 patients, three of which were fatal.

STUDY DESIGN AND METHODS

Sampling of individual PLT components was undertaken at 36 to 48 hours after donation and tested in the BacT/ALERT system with 8 mL inoculated into each of aerobic and anaerobic culture bottles. Bottles were incubated until the end of the 7-day shelf life and initial reactive bottles were examined for contamination. Bacterial screened time-expired PLTs were tested as in the screen method.

RESULTS

From February 2011 to September 2015, a total of 1,239,029 PLT components were screened. Initial-reactive, confirmed-positive, and false-positive rates were 0.37, 0.03, and 0.19%, respectively. False-negative cultures, all with Staphylococcus aureus, occurred on four occasions; three were visually detected before transfusion and one confirmed transmission resulted in patient morbidity. The NHSBT screening protocol effectively reduced the number of clinically adverse transfusion transmissions by 90% in this reporting period, compared to a similar time period before implementation. Delayed testing of 4515 time-expired PLT units after screening revealed no positives.

CONCLUSION

The implementation of bacterial screening of PLT components with the NHSBT BacT/ALERT protocol was an effective risk reduction measure and increased the safety of the blood supply.

Antithrombotic and cytotoxic activities of four Bangladeshi plants and PASS prediction of their isolated compounds

BACKGROUND

This study aims to investigate whether tested organic extracts possess antithrombotic properties with minimal or no toxicity and to predict the activity of some of their isolated compounds.

METHODS

An in vitro thrombolytic model was used to check the clot lysis effect of four Bangladeshi herbal extracts viz., roots of Curculigo recurvata W.T. Aiton (Satipata), leaf of Amorphophallus bulbifer Roxb. (Olkachu), leaf of Phyllanthus sikkimensis Muell. Arg., and whole plant of Thunbergia grandiflora Roxb. (Nillata) using streptokinase as a positive control and water as a negative control. Cytotoxicity was screened by brine shrimp lethality bioassay using vincristine sulfate as positive control. In silico prediction of activity spectra for substances (PASS) prediction was applied for phytoconstituents, namely, nyasicoside, glucomannan, grandifloric acid, serine, and alanine.

RESULTS

Using an in vitro thrombolytic model, C. recurvata, A. bulbifer, P. sikkimensis, and T. grandiflora showed 28.10±1.64%, 42.47±1.96%, 32.86±1.92%, and 25.51±1.67% of clot lysis, respectively. Reference drug streptokinase exhibited 75.00±3.04% clot lysis. Examined herbs showed significant (p<0.001) percentage (%) of clot lysis compared to negative control. In brine shrimp cytotoxic assay, C. recurvata, A. bulbifer, P. sikkimensis, and T. grandiflora showed LC50 values 210.64±3.44, 98.51±1.47, 187.29±2.01, and 386.43±3.02 μg/mL, respectively, with reference to vincristine sulfate (LC50 0.76±0.04). PASS predicted that examined phytoconstituents have a wide range of biological activity.

CONCLUSIONS

Through our study it was found that A. bulbifer and P. sikkimensis could be considered as very promising and beneficial thrombolytic agents.

Functional Regulation of the Plasma Protein Histidine-Rich Glycoprotein by Zn2+ in Settings of Tissue Injury

Divalent metal ions are essential nutrients for all living organisms and are commonly protein-bound where they perform important roles in protein structure and function. This regulatory control from metals is observed in the relatively abundant plasma protein histidine-rich glycoprotein (HRG), which displays preferential binding to the second most abundant transition element in human systems, Zinc (Zn²⁺). HRG has been proposed to interact with a large number of protein ligands and has been implicated in the regulation of various physiological and pathological processes including the formation of immune complexes, apoptotic/necrotic and pathogen clearance, cell adhesion, antimicrobial activity, angiogenesis, coagulation and fibrinolysis. Interestingly, these processes are often associated with sites of tissue injury or tumour growth, where the concentration and distribution of Zn²⁺ is known to vary. Changes in Zn²⁺ levels have been shown to modify HRG function by altering its affinity for certain ligands and/or providing protection against proteolytic disassembly by serine proteases. This review focuses on the molecular interplay between HRG and Zn²⁺, and how Zn²⁺ binding modifies HRG-ligand interactions to regulate function in different settings of tissue injury.

Activity Regulation by Fibrinogen and Fibrin of Streptokinase from Streptococcus Pyogenes

Streptokinase is a virulence factor of streptococci and acts as a plasminogen activator to generate the serine protease plasmin which promotes bacterial metastasis. Streptokinase isolated from group C streptococci has been used therapeutically as a thrombolytic agent for many years and its mechanism of action has been extensively studied. However, group A streptococci are associated with invasive and potentially fatal infections, but less detail is available on the mechanism of action of streptokinase from these bacteria. We have expressed recombinant streptokinase from a group C strain to investigate the therapeutic molecule (here termed rSK-H46A) and a molecule isolated from a cluster 2a strain from group A (rSK-M1GAS) which is known to produce the fibrinogen binding, M1 protein, and is associated with life-threatening disease. Detailed enzyme kinetic models have been prepared which show how fibrinogen-streptokinase-plasminogen complexes regulate plasmin generation, and also the effect of fibrin interactions. As is the case with rSK-H46A our data with rSK-M1GAS support a "trigger and bullet" mechanism requiring the initial formation of SK•plasminogen complexes which are replaced by more active SK•plasmin as plasmin becomes available. This model includes the important fibrinogen interactions that stimulate plasmin generation. In a fibrin matrix rSK-M1GAS has a 24 fold higher specific activity than the fibrin-specific thrombolytic agent, tissue plasminogen activator, and 15 fold higher specific activity than rSK-H46A. However, in vivo fibrin specificity would be undermined by fibrinogen stimulation. Given the observed importance of M1 surface receptors or released M1 protein to virulence of cluster 2a strain streptococci, studies on streptokinase activity regulation by fibrin and fibrinogen may provide additional routes to addressing bacterial invasion and infectious diseases.

Cross-inhibition of pathogenic agents and the host proteins they exploit

The major limitations of pathogen-directed therapies are the emergence of drug-resistance and their narrow spectrum of coverage. A recently applied approach directs therapies against host proteins exploited by pathogens in order to circumvent these limitations. However, host-oriented drugs leave the pathogens unaffected and may result in continued pathogen dissemination. In this study we aimed to discover drugs that could simultaneously cross-inhibit pathogenic agents, as well as the host proteins that mediate their lethality. We observed that many pathogenic and host-assisting proteins belong to the same functional class. In doing so we targeted a protease component of anthrax toxin as well as host proteases exploited by this toxin. We identified two approved drugs, ascorbic acid 6-palmitate and salmon sperm protamine, that effectively inhibited anthrax cytotoxic protease and demonstrated that they also block proteolytic activities of host furin, cathepsin B, and caspases that mediate toxin's lethality in cells. We demonstrated that these drugs are broad-spectrum and reduce cellular sensitivity to other bacterial toxins that require the same host proteases. This approach should be generally applicable to the discovery of simultaneous pathogen and host-targeting inhibitors of many additional pathogenic agents.

Molecular Interactions of Human Plasminogen with Fibronectin-binding Protein B (FnBPB), a Fibrinogen/Fibronectin-binding Protein from Staphylococcus aureus

Staphylococcus aureus is a commensal bacterium that has the ability to cause superficial and deep-seated infections. Like several other invasive pathogens, S. aureus can capture plasminogen from the human host where it can be converted to plasmin by host plasminogen activators or by endogenously expressed staphylokinase. This study demonstrates that sortase-anchored cell wall-associated proteins are responsible for capturing the bulk of bound plasminogen. Two cell wall-associated proteins, the fibrinogen- and fibronectin-binding proteins A and B, were found to bind plasminogen, and one of them, FnBPB, was studied in detail. Plasminogen captured on the surface of S. aureus- or Lactococcus lactis-expressing FnBPB could be activated to the potent serine protease plasmin by staphylokinase and tissue plasminogen activator. Plasminogen bound to recombinant FnBPB with a KD of 0.532 μm as determined by surface plasmon resonance. Plasminogen binding did not to occur by the same mechanism through which FnBPB binds to fibrinogen. Indeed, FnBPB could bind both ligands simultaneously indicating that their binding sites do not overlap. The N3 subdomain of FnBPB contains the full plasminogen-binding site, and this includes, at least in part, two conserved patches of surface-located lysine residues that were recognized by kringle 4 of the host protein.

Streptococcus pyogenes triggers activation of the human contact system by streptokinase

Severe invasive infectious diseases remain a major and life-threatening health problem. In serious cases, a systemic activation of the coagulation cascade is a critical complication that is associated with high mortality rates. We report here that streptokinase, a group A streptococcal plasminogen activator, triggers the activation of the human contact system. Activation of contact system factors at the surface of the Streptococcus pyogenes serotype M49 is dependent on streptokinase and plasminogen. Our results also show that secreted streptokinase is an efficient contact system activator, independent from a contact surface. This results in the processing of high-molecular-weight kininogen and the release of bradykinin, a potent vascular mediator. We further investigated whether the ability of 50 different clinical S. pyogenes isolates to activate the contact system is associated with an invasive phenotype. The data reveal that isolates from invasive infections trigger an activation of the contact system more potently than strains isolated from noninvasive infections. The present study gives new insights into the mechanisms by which S. pyogenes triggers the human contact system and stresses the function of soluble and surface located plasmin exploited as a group A streptococcal virulence factor through the action of streptokinase.

Pathogen activators of plasminogen

Group A streptococci (GAS) express soluble and surface-bound virulence factors. Secreted streptokinase (SK) allelic variants exhibit varying abilities to activate host plasminogen (Pg), and GAS pathogenicity is associated with Pg activation and localization of the resulting plasmin (Pm) on the bacterial surface to promote dissemination. The various mechanisms by which GAS usurp the host proteolytic system are discussed, including the molecular sexuality mechanism of conformational activation of the Pg zymogen (Pg*) and subsequent proteolytic activation of substrate Pg by the S•KPg* and SK•Pm catalytic complexes. Substantial progress has been made to delineate both processes in a unified mechanism. Pm coats the bacteria by direct and indirect binding pathways involving plasminogen-binding group A streptococcal M-like (PAM) protein and host fibrin(ogen). Transgenic mouse models using human Pg are being optimized to mimic infections by SK variants in humans and to define in vivo combined mechanisms of these variants and PAM.

Antithrombotic Effects of Five Organic Extracts of Bangladeshi Plants In Vitro and Mechanisms in In Silico Models

This research was carried out to investigate the thrombolytic effects of the methanolic extracts of five Bangladeshi plants. Phytochemical metabolites of those plants have been identified to elucidate whether the plant-derived metabolites are linked with the thrombolytic effects. Potential computer aided models were adopted in this study to find out a structure-function correlation between the phytochemical constituents and thrombolytic effects using the secondary metabolites as ligands and tissue plasminogen activator (t-PA) as receptor for the best fit ligand-receptor interaction.

Bilobed shape of PadA reveals the connectivity from single to multi-domain bacterial plasminogen activators

The bacterial plasminogen activator, PadA activates bovine, ovine and caprine plasminogen but remains inert toward human plasminogen. It shows high sequence homology with human plasminogen activator, staphylokinase (SAK) but generates active-site in bovine plasminogen non-proteolytically, similar to streptokinase (SK). To examine the structural requirements for the function of this unique cofactor, attempts were made to visualize solution structure of the PadA using small-angle X-ray scattering (SAXS) data and compare its shape profile with structural models based on crystal structures of staphylokinase and streptokinase domains. The bilobal shape solved for the PadA matched closely with the structural model of α-domain of SK rather than its sequence homolog, SAK. The SAXS based solution structure of the PadA exhibited an extra volume and high mobility around Y(90)DKAEK(95) and P(104)ITES(108) loop regions that were found to play a crucial role in its cofactor function. Structure and sequence analysis of bacterial cofactors and mammalian plasminogens displayed evolutionary conservation of crucial complimentary amino acids required for making a functional binary activator complex between bacterial plasminogen activators and their cognate partner plasminogen. These studies highlighted the importance of structure-function related evolutionary strategies adopted by bacteria for exploiting mammalian plasminogen activation system and its understanding may help in designing and the development of new thrombolytic agents for clinical interventions.

Effects of organic extracts and their different fractions of five Bangladeshi plants on in vitro thrombolysis

Background

The increasingly high incidence of ischemic stroke caused by thrombosis of the arterial vessels is one of the major factors that threaten people’s health and lives in the world. The present treatments for thrombosis are still unsatisfactory. Herbal preparations have been used since ancient times for the treatment of several diseases. The aim of this study was to investigate whether herbal preparations possess thrombolytic activity or not.

Methods

An in vitro thrombolytic model was used to check the clot lysis effect of the crude extracts and fractions of five Bangladeshi plant viz., Trema orientalis L., Bacopa monnieri L., Capsicum frutescens L., Brassica oleracea L. and Urena sinuata L. using streptokinase as a positive control and water as a negative control. Briefly, venous blood drawn from twenty healthy volunteers was allowed to form clots which were weighed and treated with the test plant materials to disrupt the clots. Weight of clot after and before treatment provided a percentage of clot lysis.

Results

Using an in vitro thrombolytic model, different fractions of five Bangladeshi medicinal plants namely T. orientalis, B. monnieri, C. frutescens, B. oleracea and U. sinuata showed various range of clot lysis activity. Chloroform fractions of T. orientalis, B. monnieri, C. frutescens, B. oleracea and U. sinuata showed highest significant (P < 0.05 and P < 0.001) clot lysis activity viz., 46.44 ± 2.44%, 48.39 ± 10.12%, 36.87 ± 1.27%, 30.24 ± 0.95% and 47.89 ± 6.83% respectively compared with positive control standard streptokinase (80.77 ± 1.12%) and negative control sterile distilled water (5.69 ± 3.09%). Other fractions showed moderate to low clot lysis activity. Order of clot lysis activity was found to be: Streptokinase > Chloroform fractions > Methanol (crude) extract > Hydro-methanol fractions > Ethyl acetate fractions > n-hexane fractions > Water.

Conclusions

Our study suggests that thrombolytic activity of T. orientalis, B. monnieri and U. sinuata could be considered as very promising and beneficial for the Bangladeshi traditional medicine. Lower effects of other extracts might suggest the lack of bio-active components and/or insufficient quantities in the extract. In vivo clot dissolving property and active component(s) of T. orientalis and B. monnieri for clot lysis could lead the plants for their therapeutic uses. However, further work will establish whether or not, chloroform soluble phytochemicals from these plants could be incorporated as a thrombolytic agent for the improvement of the patients suffering from atherothrombotic diseases.

[Role of angiostatins in diabetic complications]

Angiogenesis is a process through which new blood vessels form from pre-existing vessels. Angiogenesis is regulated by a number of factors of peptide nature. Disbalance of angiogenic system appears to be the major causative factor contributing vascular abnormalities in diabetes mellitus, resulting in various complications. Angiostatins, which are kringle-containing fragments of plasminogen/plasmin, are known to be powerful physiological inhibitors of neovascularization. In the present review, current literature data on peculiarities of production of angiostatins and their functioning at diabetes mellitus are summarized and analyzed for the first time. Also, role of angiostatins in the pathogenesis of typical diabetic complications, including retinopathies, nephropathies and cardiovascular diseases, is discussed. Data presented in this review may be useful for elaboration of novel effective approaches for diagnostics and therapy of vascular abnormalities in diabetes mellitus.

Transcriptional adaptations during long-term persistence of Staphylococcus aureus in the airways of a cystic fibrosis patient

The lungs of Cystic fibrosis (CF) patients are often colonized and/or infected by Staphylococcus aureus for years, mostly by one predominant clone. For long-term survival in this environment, S. aureus needs to adapt during its interactions with host factors, antibiotics, and other pathogens. Here, we study long-term transcriptional as well as genomic adaptations of an isogenic pair of S. aureus isolates from a single patient using RNA sequencing (RNA-Seq) and whole genome sequencing (WGS). Mimicking in vivo conditions, we cultivated the S. aureus isolates using artificial sputum medium before harvesting RNA for subsequent analysis. We confirmed our RNA-Seq data using quantitative real-time (qRT)-PCR and additionally investigated intermediate isolates from the same patient representing in total 13.2 years of persistence in the CF airways. Comparative RNA-Seq analysis of the first and the last ("late") isolate revealed significant differences in the late isolate after 13.2 years of persistence. Of the 2545 genes expressed in both isolates that were cultivated aerobically, 256 genes were up- and 161 were down-regulated with a minimum 2-fold change (2f). Focusing on 25 highly (≥8f) up- (n=9) or down- (n=16) regulated genes, we identified several genes encoding for virulence factors involved in immune evasion, bacterial spread or secretion (e.g. spa, sak, and esxA). Moreover, these genes displayed similar expression trends under aerobic, microaerophilic and anaerobic conditions. Further qRT-PCR-experiments of highly up- or down-regulated genes within intermediate S. aureus isolates resulted in different gene expression patterns over the years. Using sequencing analysis of the differently expressed genes and their upstream regions in the late S. aureus isolate resulted in only few genomic alterations. Comparative transcriptomic analysis revealed adaptive changes affecting mainly genes involved in host-pathogen interaction. Although the underlying mechanisms were not known, our results suggest adaptive processes beyond genomic mutations triggered by local factors rather than by activation of global regulators.

Site-restricted plasminogen activation mediated by group A streptococcal streptokinase variants

By examining the roles of bacterial and host-derived cofactors in streptokinase-mediated plasminogen activation, we find that phenotypic streptokinase variation functionally underpins a pathogenic mechanism whereby streptokinase variants differentially focus plasminogen activation, leading to specific niche adaption within the host.

Effects of organic extracts of six Bangladeshi plants on in vitro thrombolysis and cytotoxicity

Background

Thrombus formed in blood vessels lead to atherothrombotic diseases such as myocardial or cerebral infarction. Thrombolytic agents are used to dissolve the already formed clots in the blood vessels; however, these drugs sometimes cause serious and fatal consequences. Herbal preparations have been used since ancient times for the treatment of several diseases although they show little toxicity in some cases. Aqueous extracts of herbs used in thrombolysis have been reported before with cytotoxic data, however, the organic extracts of herbs have not been documented. This study aims to investigate whether organic extracts possess thrombolytic properties with minimal or no toxicity.

Methods

An in vitro thrombolytic model was used to check the clot lysis effect of six Bangladeshi herbal extracts viz., Ageratum conyzoides L., Clausena suffruticosa, Leea indica (Burm.f.) Merr., Leucas aspera Willd., Senna sophera L. Roxb., and Solanum torvum Swartz. using streptokinase as a positive control and water as a negative control. Briefly, venous blood drawn from twenty healthy volunteers was allowed to form clots which were weighed and treated with the test plant materials to disrupt the clots. Weight of clot after and before treatment provided a percentage of clot lysis. Cytotoxicity was screened by brine shrimp lethality bioassay using vincristine sulfate as positive control.

Results

Using an in vitro thrombolytic model, Ageratum conyzoides, Clausena suffruticosa, Leea indica, Leucas aspera, Senna sophera and Solanum torvum showed 18.12 ± 2.34%, 48.9 ± 2.44%, 39.30 ± 0.96%, 37.32 ± 2.00%, 31.61 ± 2.97% and 31.51 ± 0.57% and clot lysis respectively. Among the herbs studied Clausena suffruticosa, Leea indica and Leucas aspera showed very significant (p < 0.0001) percentage (%) of clot lysis compared to reference drug streptokinase (75.00 ± 3.04%). In brine shrimp cytotoxic assay, the extracts Ageratum conyzoides, Clausena suffruticosa, Leea indica, Leucas aspera, Senna sophera and Solanum torvum showed LC₅₀ values 508.86 ± 6.62,41.16 ± 1.26, 2.65 ± 0.16, 181.67 ± 1.65, 233.37 ± 7.74 and 478.40 ± 3.23 μg/ml, respectively, with reference to vincristine sulfate (LC₅₀ 0.76 ± 0.04).

Conclusion

Through our study it was found that Clausena suffruticosa, Leea indica and Leucas aspera possessed effective thrombolytic properties whereas Senna sophera and Solanum torvum showed moderate to mild thrombolytic effects while Ageratum conyzoides showed no significant effect. No extract was found cytoxic compared to positive control. Clausena suffruticosa, Leea indica and Leucas aspera could be incorporated as a thrombolytic agent with in vivo effects to improve the atherothrombotic patients. However, Clausena suffruticosa could be the best one to use in this purpose.

A bacterial pathogen co-opts host plasmin to resist killing by cathelicidin antimicrobial peptides

The bacterial pathogen Group A Streptococcus (GAS) colonizes epithelial and mucosal surfaces and can cause a broad spectrum of human disease. Through the secreted plasminogen activator streptokinase (Ska), GAS activates human plasminogen into plasmin and binds it to the bacterial surface. The resulting surface plasmin protease activity has been proposed to play a role in disrupting tissue barriers, promoting invasive spread of the bacterium. We investigated whether this surface protease activity could aid the immune evasion role through degradation of the key innate antimicrobial peptide LL-37, the human cathelicidin. Cleavage products of plasmin-degraded LL-37 were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Ska-deficient GAS strains were generated by targeted allelic exchange mutagenesis and confirmed to lack surface plasmin activity after growth in human plasma or media supplemented with plasminogen and fibrinogen. Loss of surface plasmin activity left GAS unable to efficiently degrade LL-37 and increased bacterial susceptibility to killing by the antimicrobial peptide. When mice infected with GAS were simultaneously treated with the plasmin inhibitor aprotinin, a significant reduction in the size of necrotic skin lesions was observed. Together these data reveal a novel immune evasion strategy of the human pathogen: co-opting the activity of a host protease to evade peptide-based innate host defenses.

Bacterial plasminogen receptors utilize host plasminogen system for effective invasion and dissemination

In order for invasive pathogens to migrate beyond the site of infection, host physiological barriers such as the extracellular matrix, the basement membrane, and encapsulating fibrin network must be degraded. To circumvent these impediments, proteolytic enzymes facilitate the dissemination of the microorganism. Recruitment of host proteases to the bacterial surface represents a particularly effective mechanism for enhancing invasiveness. Plasmin is a broad spectrum serine protease that degrades fibrin, extracellular matrices, and connective tissue. A large number of pathogens express plasminogen receptors which immobilize plasmin(ogen) on the bacterial surface. Surface-bound plasminogen is then activated by plasminogen activators to plasmin through limited proteolysis thus triggering the development of a proteolytic surface on the bacteria and eventually assisting the spread of bacteria. The host hemostatic system plays an important role in systemic infection. The interplay between hemostatic processes such as coagulation and fibrinolysis and the inflammatory response constitutes essential components of host defense and bacterial invasion. The goal of this paper is to highlight mechanisms whereby pathogenic bacteria, by engaging surface receptors, utilize and exploit the host plasminogen and fibrinolytic system for the successful dissemination within the host.

Crystal structure of the native plasminogen reveals an activation-resistant compact conformation

BACKGROUND

Plasminogen is the zymogen form of plasmin and the precursor of angiostatin. It has been implicated in a variety of disease states, including thrombosis, bleeding and cancers. The native plasminogen, known as Glu-plasminogen, contains seven domains comprising the N-terminal peptide domain (NTP), five kringle domains (K1-K5) and the C-terminal serine protease domain (SP). Previous studies have established that the lysine binding site (LBS) of the conserved kringle domains plays a crucial role in mediating the regulation of plasminogen function. However, details of the related conformational mechanism are unknown.

OBJECTIVES

We aim to understand in more detail the conformational mechanism of plasminogen activation involving the kringles.

METHODS

We crystallized the native plasminogen under physiologically relevant conditions and determined the structure at 3.5 Å resolution. We performed structural analyses and related these to the literature data to gain critical understanding of the plasminogen activation.

RESULTS AND CONCLUSIONS

The structure reveals the precise architecture of the quaternary complex. It shows that the Glu-plasminogen renders its compact form as an activation-resistant conformation for the proteolytic activation. The LBSs of all kringles, except K1, are engaged in intra-molecular interactions while only K1-LBS is readily available for ligand binding or receptor anchorage. The structure also provides insights into the interactions between plasminogen and α2-antiplasmin, the primary physiological inhibitor of plasmin. Furthermore, the data presented explain why a conformational transition to the open form is necessary for plasminogen activation as well as angiostatin generation, and provide a rationale for the functional hierarchy of the different kringles.

Human pathogens utilize host extracellular matrix proteins laminin and collagen for adhesion and invasion of the host

Laminin (Ln) and collagen are multifunctional glycoproteins that play an important role in cellular morphogenesis, cell signalling, tissue repair and cell migration. These proteins are ubiquitously present in tissues as a part of the basement membrane (BM), constitute a protective layer around blood capillaries and are included in the extracellular matrix (ECM). As a component of BMs, both Lns and collagen(s), thus function as major mechanical containment molecules that protect tissues from pathogens. Invasive pathogens breach the basal lamina and degrade ECM proteins of interstitial spaces and connective tissues using various ECM-degrading proteases or surface-bound plasminogen and matrix metalloproteinases recruited from the host. Most pathogens associated with the respiratory, gastrointestinal, or urogenital tracts, as well as with the central nervous system or the skin, have the capacity to bind and degrade Lns and collagen(s) in order to adhere to and invade host tissues. In this review, we focus on the adaptability of various pathogens to utilize these ECM proteins as enhancers for adhesion to host tissues or as a targets for degradation in order to breach the cellular barriers. The major pathogens discussed are Streptococcus, Staphylococcus, Pseudomonas, Salmonella, Yersinia, Treponema, Mycobacterium, Clostridium, Listeria, Porphyromonas and Haemophilus; Candida, Aspergillus, Pneumocystis, Cryptococcus and Coccidioides; Acanthamoeba, Trypanosoma and Trichomonas; retrovirus and papilloma virus.

Antibiotic modulation of the plasminogen binding ability of viridans group streptococci

The ability of viridans group streptococci to bind human plasminogen and its subsequent activation into plasmin may contribute to the pathogenesis of infective endocarditis (IE) by leading to a decreased stability of the streptococcal vegetation and facilitating dehiscence of emboli. At levels greater than or equal to their MICs, penicillin, vancomycin, and linezolid are efficacious in the treatment of streptococcal endocarditis. However, at sub-MICs, antibiotics can modulate the expression of bacterial genes, including virulence-associated genes, which can have counterproductive effects on the treatment of endocarditis. The effects of 1/8× and 1/4× MICs of penicillin, vancomycin, and linezolid on the plasminogen binding ability of IE isolates Streptococcus mitis 881/956, Streptococcus oralis 12601, and Streptococcus sanguinis 12403 were assessed phenotypically and the expression of plasminogen receptors α-enolase and glyceraldehyde 3-phosphate dehydrogenase of S. oralis 12601 when exposed to 1/4× MIC of penicillin, was analyzed through quantitative reverse transcription (qRT)-PCR. The plasminogen binding ability of S. mitis 881/956 and S. sanguinis 12403 remained unaffected by exposure to sub-MICs of all of the antibiotics tested, while that of S. oralis 12601 was significantly enhanced by all of the antibiotics tested at sub-MICs. qRT-PCR analysis of S. oralis 12601 demonstrated an upregulation of the eno and gapdh genes, indicating an overexpression of plasminogen receptors. These findings suggest that for some endocarditis isolates, the effect of antibiotic sub-MICs, in addition to a reduced antibacterial effect, may influence the clinical response to nonsurgical therapy. It remains difficult to accurately predict isolate responses to sub-MIC antimicrobials since there appears to be interspecies variation.

Identification through combinatorial random and rational mutagenesis of a substrate-interacting exosite in the gamma domain of streptokinase

To identify new structure-function correlations in the γ domain of streptokinase, mutants were generated by error-prone random mutagenesis of the γ domain and its adjoining region in the β domain followed by functional screening specifically for substrate plasminogen activation. Single-site mutants derived from various multipoint mutation clusters identified the importance of discrete residues in the γ domain that are important for substrate processing. Among the various residues, aspartate at position 328 was identified as critical for substrate human plasminogen activation through extensive mutagenesis of its side chain, namely D328R, D328H, D328N, and D328A. Other mutants found to be important in substrate plasminogen activation were, namely, R319H, N339S, K334A, K334E, and L335Q. When examined for their 1:1 interaction with human plasmin, these mutants were found to retain the native-like high affinity for plasmin and also to generate amidolytic activity with partner plasminogen in a manner similar to wild type streptokinase. Moreover, cofactor activities of the mutants precomplexed with plasmin against microplasminogen as the substrate as well as in silico modeling studies suggested that the region 315-340 of the γ domain interacts with the serine protease domain of the macromolecular substrate. Overall, our results identify the presence of a substrate specific exosite in the γ domain of streptokinase.

The role of streptococcal plasmin(ogen) binding in infective endocarditis

The ability of viridans group streptococci (VGS) to bind human plasminogen and its subsequent activation into plasmin may contribute to the pathogenesis of streptococcal endocarditis. The increased proteolytic activity acquired through cell-bound plasmin may lead to a decreased stability of the streptococcal vegetation and possible embolisation. Twenty-two infective endocarditis isolates and 16 non-infective endocarditis isolates were screened for their ability to bind plasminogen through the quantification of its active form plasmin, using the colorimetric substrate D-Val-Leu-Lys p-nitroanilide. The species of the VGS assessed expressed a universal capability to bind human plasminogen, although they did so with differing affinities and independently of the site of isolation.

Probing the primary structural determinants of streptokinase inter-domain linkers by site-specific substitution and deletion mutagenesis

The bacterial protein streptokinase (SK) contains three independently folded domains (alpha, beta and gamma), interconnected by two flexible linkers with noticeable sequence homology. To investigate their primary structure requirements, the linkers were swapped amongst themselves i.e. linker 1 (between alpha and beta domains) was swapped with linker 2 (between beta and gamma domains) and vice versa. The resultant construct exhibited very low activity essentially due to an enhanced proteolytic susceptibility. However, a SK mutant with two linker 1 sequences, which was proteolytically as stable as WT-rSK retained about 10% of the plasminogen activator activity of rSK When the native sequence of each linker was substituted with 9 consecutive glycine sequences, in case of the linker 1 substitution mutant substantial activity was seen to survive, whereas the linker 2 mutant lost nearly all its activity. The optimal length of linkers was then studied through deletion mutagenesis experiments, which showed that deletion beyond three residues in either of the linkers resulted in virtually complete loss of activator activity. The effect of length of the linkers was then also examined by insertion of extraneous pentapeptide sequences having a propensity for adopting either an extended conformation or a relatively rigid conformation. The insertion of poly-Pro sequences into native linker 2 sequence caused up to 10-fold reduction in activity, whereas its effect in linker 1 was relatively minor. Interestingly, most of the linker mutants could form stable 1:1 complexes with human plasminogen. Taken together, these observations suggest that (i) the functioning of the inter-domain linkers of SK requires a critical minimal length, (ii) linker 1 is relatively more tolerant to insertions and sequence alterations, and appears to function primarily as a covalent connector between the alpha and beta domains, and (iii) the native linker 2 sequence is virtually indispensable for the activity of SK probably because of structural and/or flexibility requirements in SK action during catalysis.

Role of prolyl cis/trans isomers in cyclophilin-assisted Pseudomonas syringae AvrRpt2 protease activation

In a process contributing to the innate immunity of higher plants, Arabidopsis thaliana cyclophilin ROC1 induces the self-cleavage of Pseudomonas syringae putative cysteine protease AvrRpt2, triggering limited cleavage of A. thaliana RIN4, a negative regulator of plant immunity. We report an increase in AvRpt2 activity in hydrolysis of decapeptide substrates at -GG- sites of more than 5 orders of magnitude, in the presence of cyclophilin-like peptidyl prolyl cis/trans isomerases including ROC1 or hCyp18. Both full-length AvrRpt2 and its 21 kDa self-cleavage product (AvrRpt2(72-255)) were found to be equally active under these conditions. In contrast to classical isomer-specific proteolysis, inertness toward cleavage of a cis/trans prolyl bond isomer at the substrate P4 subsite is not the cause of cyclophilin-mediated activation of the proteolytic reaction. Monitoring single- and double-jump kinetics of proteolytic reactions in the presence of the PPIase inhibitor cyclosporin A revealed that the cis/trans ratio of potentially relevant prolyl bonds of AvrRpt2(72-255) remained the same in the functionally inactive state of AvrRpt2(72-255) and the productive AvrRpt2(72-255)-cyclophilin-substrate complex.

Defining the structural basis of human plasminogen binding by streptococcal surface enolase

The flesh-eating bacterium group A Streptococcus (GAS) binds and activates human plasminogen, promoting invasive disease. Streptococcal surface enolase (SEN), a glycolytic pathway enzyme, is an identified plasminogen receptor of GAS. Here we used mass spectrometry (MS) to confirm that GAS SEN is octameric, thereby validating in silico modeling based on the crystal structure of Streptococcus pneumoniae alpha-enolase. Site-directed mutagenesis of surface-located lysine residues (SEN(K252 + 255A), SEN(K304A), SEN(K334A), SEN(K344E), SEN(K435L), and SEN(Delta434-435)) was used to examine their roles in maintaining structural integrity, enzymatic function, and plasminogen binding. Structural integrity of the GAS SEN octamer was retained for all mutants except SEN(K344E), as determined by circular dichroism spectroscopy and MS. However, ion mobility MS revealed distinct differences in the stability of several mutant octamers in comparison with wild type. Enzymatic analysis indicated that SEN(K344E) had lost alpha-enolase activity, which was also reduced in SEN(K334A) and SEN(Delta434-435). Surface plasmon resonance demonstrated that the capacity to bind human plasminogen was abolished in SEN(K252 + 255A), SEN(K435L), and SEN(Delta434-435). The lysine residues at positions 252, 255, 434, and 435 therefore play a concerted role in plasminogen acquisition. This study demonstrates the ability of combining in silico structural modeling with ion mobility-MS validation for undertaking functional studies on complex protein structures.

Molecular mechanisms underlying group A streptococcal pathogenesis

Group A Streptococcus (GAS) is a versatile human pathogen causing diseases ranging from uncomplicated mucosal infections to life-threatening invasive disease. The development of human-relevant animal models of GAS infection and introduction of new technologies have markedly accelerated the pace of discoveries related to GAS host-pathogen interactions. For example, recently investigators have identified pili on the GAS cell surface and learned that they are key components for adherence to eukaryotic cell surfaces. Similarly, the recent development of a transgenic mouse expressing human plasminogen has resulted in new understanding of the molecular processes contributing to invasive infection. Improved understanding of the molecular mechanisms underlying the pathogenesis of GAS pharyngeal, invasive and other infections holds the promise of assisting with the development of novel preventive or therapeutic agents for this prevalent human pathogen.

Structural consideration of the formation of the activation complex between the staphylokinase-like streptococcal plasminogen activator PadA and bovine plasminogen

The characteristics of a streptococcal plasminogen activator (PA) displaying specificity for ruminant plasminogen (Plg) were defined using molecular approaches. The 16-kDa secreted protein PadA was found to be prevalent in Streptococcus dysgalactiae subspecies dysgalactiae isolated from cases of bovine mastitis and septic arthritis in lambs. PadA was able to activate bovine, ovine and caprine Plg, but not human Plg. Amino acid sequence analysis identified a limited level of homology to other streptococcal PAs, including streptokinase; however, PadA was found to align well with and match in size the staphylococcal PA, staphylokinase. Recombinant PadA was used to investigate interaction with bovine Plg, leading to formation of an activator complex that was capable of recruiting and converting further substrate Plg into plasmin. Individual non-overlapping peptides of PadA or bovine microplasminogen were found to block the interaction between PadA and bovine Plg, preventing the formation of the activation complex. Homology modelling based upon structures of staphylokinase complexed with human microplasminogen supported these findings by placing critical residues in close proximity to the plasmin component of the activation complex.

Role of the 88-97 loop in plasminogen activation by streptokinase probed through site-specific mutagenesis

The role of a prominent surface-exposed loop (residues 88-97) in the alpha domain of streptokinase (SK), in human plasminogen (HPG) activation was explored through its selective mutagenesis and deletion studies. We first made a conformationally constrained derivative of the loop by the substitution of sequences known to possess a strong propensity for beta-turn formation. The mutant so formed (termed SK88-97-Beta Turn), when tested for co-factor activity against substrate HPG, after first forming a 1:1 molar complex with human plasmin (HPN), showed a nearly 6-fold decreased co-factor activity compared to the wild-type, native SK. The major catalytic change was observed to be at the k(cat) level, with relatively minor changes in Km values against HPG. Real-time binary interaction (i.e. the 1:1 complexation between SK, or its mutant/s, with HPG), and ternary complexation studies (i.e. the docking of a substrate HPG molecule into the preformed SK-HPG complex) using Surface Plasmon Resonance were done. These studies revealed minor alterations in binary complex formation but the ternary interactions of the substitution and/or deletion mutants were found to be decreased for full-length HPG compared to that for native SK.HPG. In contrast, their ternary interactions with the isolated five-kringle domain unit of plasminogen (K1-5) showed Kd values comparable to that seen with the native SK.HPG complex. Taking into consideration the overall alterations observed in catalytic levels after site-specific mutagenesis and complete loop deletion of the 88-97 loop, on the one hand, and its known position at the SK-HPG interface in the binary complex, suggests the importance of this loop. The present results suggest that the 88-97 loop of the alpha domain of SK contributes towards catalytic turn-over, even though its individual contribution towards enzyme-substrate affinity per se is minimal.