PAM, a novel plasminogen-binding protein from Streptococcus pyogenes

The Recruitment and Activation of Plasminogen by Bacteria-The Involvement in Chronic Infection Development

The development of infections caused by pathogenic bacteria is largely related to the specific properties of the bacterial cell surface and extracellular hydrolytic activity. Furthermore, a significant role of hijacking of host proteolytic cascades by pathogens during invasion should not be disregarded during consideration of the mechanisms of bacterial virulence. This is the key factor for the pathogen evasion of the host immune response, tissue damage, and pathogen invasiveness at secondary infection sites after initial penetration through tissue barriers. In this review, the mechanisms of bacterial impact on host plasminogen-the precursor of the important plasma serine proteinase, plasmin-are characterized, principally focusing on cell surface exposition of various proteins, responsible for binding of this host (pro)enzyme and its activators or inhibitors, as well as the fibrinolytic system activation tactics exploited by different bacterial species, not only pathogenic, but also selected harmless residents of the human microbiome. Additionally, the involvement of bacterial factors that modulate the process of plasminogen activation and fibrinolysis during periodontitis is also described, providing a remarkable example of a dual use of this host system in the development of chronic diseases.

C-Terminal Lysine Residue of Pneumococcal Triosephosphate Isomerase Contributes to Its Binding to Host Plasminogen

The main causative agent of pneumonia, Streptococcus pneumoniae, is also responsible for invasive diseases. S. pneumoniae recruits human plasminogen for the invasion and colonization of host tissues. We previously discovered that S. pneumoniae triosephosphate isomerase (TpiA), an enzyme involved in intracellular metabolism that is essential for survival, is released extracellularly to bind human plasminogen and facilitate its activation. Epsilon-aminocaproic acid, a lysine analogue, inhibits this binding, suggesting that the lysine residues in TpiA are involved in plasminogen binding. In this study, we generated site-directed mutant recombinants in which the lysine residue in TpiA was replaced with alanine and analyzed their binding activities to human plasminogen. Results from blot analysis, enzyme-linked immunosorbent assay, and surface plasmon resonance assay revealed that the lysine residue at the C-terminus of TpiA is primarily involved in binding to human plasminogen. Furthermore, we found that TpiA binding to plasminogen through its C-terminal lysine residue was required for the promotion of plasmin activation by activating factors.

Inactivation of the lysine binding sites of human plasminogen (hPg) reveals novel structural requirements for the tight hPg conformation, M-protein binding, and rapid activation

Accelerated activation of the human plasminogen zymogen (hPg) to two-chain active plasmin (hPm) is achieved following conformational changes induced by ligand-binding at the lysine-binding sites (LBSs) in four of the five hPg kringle domains. In this manner, pattern D skin-trophic strains of Group A streptococci (GAS), through the expression of surface plasminogen-binding M-protein (PAM), immobilize surface hPg, thereby enabling rapid hPg activation by GAS-secreted streptokinase (SK). Consequently, GAS enhances virulence by digesting extracellular and tight cellular junctional barriers using hPm activity. Many studies have demonstrated the singular importance of the kringle-2 domain of hPg (K2hPg) to PAM-binding using hPg fragments. Recently, we showed, using full-length hPg, that K2hPg is critical for PAM binding. However, these studies did not eliminate any modulatory effects of the non-K2hPg LBS on this interaction. Moreover, we sought to establish the significance of the intramolecular interaction between Asp219 of the LBS of K2hPg and its serine protease domain binding partner, Lys708, to conformational changes in hPg. In the current study, selective inactivation of the LBS of K1hPg, K4hPg, and K5hPg revealed that the LBS of these kringle domains are dispensable for hPg binding to PAM. However, the attendant conformational change upon inactivation of K4hPg LBS increased the affinity of hPg for PAM by an order of magnitude. This finding suggests that the native hPg conformation encloses PAM-binding exosites or sterically hinders access to K2hPg. While simultaneous inactivation of the LBS of K1hPg, K4hPg, and K5hPg inhibited hPg/SK association alongside hPg activation, the replacement of Lys708 generated a slight conformational change that optimally accelerated hPg activation. Thus, we accentuate disparate functions of hPg LBS and conclude, using intact proteins, that K2hPg plays a central role in regulating hPg activation.

High-Resolution Single-Particle Cryo-EM Hydrated Structure of Streptococcus pyogenes Enolase Offers Insights into Its Function as a Plasminogen Receptor

Cellular plasminogen (Pg) receptors (PgRs) are utilized to recruit Pg; stimulate its activation to the serine protease, plasmin (Pm); and sterically protect the surface Pm from inactivation by host inhibitors. One such PgR is the moonlighting enzyme, enolase, some of which leaves the cytoplasm and resides at the cell surface to potentially function as a PgR. Since microbes employ conscription of host Pg by PgRs as one virulence mechanism, we explored the structural basis of the ability of Streptococcus pyogenes enolase (Sen) to function in this manner. Employing single-particle cryo-electron microscopy (cryo-EM), recombinant Sen from S. pyogenes was modeled at 2.6 Å as a stable symmetrical doughnut-shaped homooctamer with point group 422 (D4) symmetry, with a monomeric subunit molecular weight of ∼49 kDa. Binding sites for hPg were reported in other studies to include an internal K^252,255 and the COOH-terminal K^434,435 residues of Sen. However, in native Sen, the latter are buried within the minor interfaces of the octamer and do not function as a Pg-binding epitope. Whereas Sen and hPg do not interact in solution, when Sen is bound to a surface, hPg interacts with Sen independently of K^{252,255,434,435}. PgRs devoid of COOH-terminal lysine utilize lysine isosteres comprising a basic residue, "i", and an anionic residue at "i + 3" around one turn of an α-helix. We highlight a number of surface-exposed potential hPg-binding lysine isosteres and further conclude that while the octameric structure of Sen is critical for hPg binding, disruption of this octamer without dissociation exposes hPg-binding epitopes.

Relationships Between Plasminogen-Binding M-Protein and Surface Enolase for Human Plasminogen Acquisition and Activation in Streptococcus pyogenes

The proteolytic activity of human plasmin (hPm) is utilized by various cells to provide a surface protease that increases the potential of cells to migrate and disseminate. Skin-trophic Pattern D strains of Streptococcus pyogenes (GAS), e.g., GAS isolate AP53, contain a surface M-protein (PAM) that directly and strongly interacts (K_d  ~ 1 nM) with human host plasminogen (hPg), after which it is activated to hPm by a specific coinherited bacterial activator, streptokinase (SK2b), or by host activators. Another ubiquitous class of hPg binding proteins on GAS cells includes "moonlighting" proteins, such as the glycolytic enzyme, enolase (Sen). However, the importance of Sen in hPg acquisition, especially when PAM is present, has not been fully developed. Sen forms a complex with hPg on different surfaces, but not in solution. Isogenic AP53 cells with a targeted deletion of PAM do not bind hPg, but the surface expression of Sen is also greatly diminished upon deletion of the PAM gene, thus confounding this approach for defining the role of Sen. However, cells with point deletions in PAM that negate hPg binding, but fully express PAM and Sen, show that hPg binds weakly to Sen on GAS cells. Despite this, Sen does not stimulate hPg activation by SK2b, but does stimulate tissue-type plasminogen activator-catalyzed activation of hPg. These data demonstrate that PAM plays the dominant role as a functional hPg receptor in GAS cells that also contain surface enolase.

A case of a young boy with hyper-fibrinolysis associated with natural fibrin precipitates suspected to have occurred through a novel coagulation and fibrinolysis mechanism

An 8-year-old Japanese boy with no underlying disease presented with severe intramuscular hematoma of the hip, and was admitted for a disseminated intravascular coagulation-like state with fibrinolytic dominance. Laboratory examinations revealed severe hyper-fibrinolysis with elevated markers, markedly shortened euglobulin clot lysis time, mildly decreased prothrombin, and severely decreased fibrinogen and factor XIII. Natural fibrin precipitates rapidly appeared in citrate-treated, ethylene-diamine-tetra-acetic-treated, and heparin-treated samples, but not in argatroban-treated samples, indicating that the mechanism of thrombin and fibrin formation was Ca²⁺-independent. The precipitates were physically similar to thrombin-triggered plasma fibrin. A global coagulation assay revealed that thrombin generation potentials were normal throughout the clinical course, whereas plasmin generation was already detected before initiation of fibrin formation in the acute phase. This phenomenon disappeared with time. Changes in coagulation abnormalities and nature of fibrinolysis paralleled those seen in specific markers for streptococcal infections. Streptokinase was possibly involved in this disease, as SDS-polyacrylamide gel electrophoresis revealed that plasmin derived from streptokinase-plasminogen complex proteolyzed the prothrombin to approximately 35-kDa α-thrombin consisting of the A-B single chain, which was identified by NH₂-terminal sequence analysis. The involvement of streptokinase-plasminogen-prothrombin caused by streptococcal infection may be one mechanism that produces marked hyper-fibrinolysis associated with natural fibrin precipitates.

Binding of the kringle-2 domain of human plasminogen to streptococcal PAM-type M-protein causes dissociation of PAM dimers

The direct binding of human plasminogen (hPg), via its kringle-2 domain (K2hPg ), to streptococcal M-protein (PAM), largely contributes to the pathogenesis of Pattern D Group A Streptococcus pyogenes (GAS). However, the mechanism of complex formation is unknown. In a system consisting of a Class II PAM from Pattern D GAS isolate NS88.2 (PAMNS88.2 ), with one K2hPg binding a-repeat in its A-domain, we employed biophysical techniques to analyze the mechanism of the K2hPg /PAMNS88.2 interaction. We show that apo-PAMNS88.2 is a coiled-coil homodimer (M.Wt. ~80 kDa) at 4°C-25°C, and is monomeric (M.Wt. ~40 kDa) at 37°C, demonstrating a temperature-dependent dissociation of PAMNS88.2 over a narrow temperature range. PAMNS88.2 displayed a single tight binding site for K2hPg at 4°C, which progressively increased at 25°C through 37°C. We isolated the K2hPg /PAMNS88.2 complexes at 4°C, 25°C, and 37°C and found molecular weights of ~50 kDa at each temperature, corresponding to a 1:1 (m:m) K2hPg /PAMNS88.2  monomer complex. hPg activation experiments by streptokinase demonstrated that the hPg/PAMNS88.2  monomer complexes are fully functional. The data show that PAM dimers dissociate into functional monomers at physiological temperatures or when presented with the active hPg module (K2hPg ) showing that PAM is a functional monomer at 37°C.

Lysine Residues in the MK-Rich Region Are Not Required for Binding of the PbsP Protein From Group B Streptococci to Plasminogen

Binding to plasminogen (Plg) enables bacteria to associate with and invade host tissues. The cell wall protein PbsP significantly contributes to the ability of group B streptococci, a frequent cause of invasive infection, to bind Plg. Here we sought to identify the molecular regions involved in the interactions between Plg and PbsP. The K4 Kringle domain of the Plg molecule was required for binding of Plg to whole PbsP and to a PbsP fragment encompassing a region rich in methionine and lysine (MK-rich domain). These interactions were inhibited by free L-lysine, indicating the involvement of lysine binding sites in the Plg molecule. However, mutation to alanine of all lysine residues in the MK-rich domain did not decrease its ability to bind Plg. Collectively, our data identify a novel bacterial sequence that can interact with lysine binding sites in the Plg molecule. Notably, such binding did not require the presence of lysine or other positively charged amino acids in the bacterial receptor. These data may be useful for developing alternative therapeutic strategies aimed at blocking interactions between group B streptococci and Plg.

The Role of Fibrin(ogen) in Wound Healing and Infection Control

Fibrinogen, one of the most abundant plasma proteins playing a key role in hemostasis, is an important modulator of wound healing and host defense against microbes. In the current review, we address the role of fibrin(ogen) throughout the process of wound healing and subsequent tissue repair. Initially fibrin(ogen) acts as a provisional matrix supporting incoming leukocytes and acting as reservoir for growth factors. It later goes on to support re-epithelialization, angiogenesis, and fibroplasia. Importantly, removal of fibrin(ogen) from the wound is essential for wound healing to progress. We also discuss how fibrin(ogen) functions through several mechanisms to protect the host against bacterial infection by providing a physical barrier, entrapment of bacteria in fibrin(ogen) networks, and by directing immune cell function. The central role of fibrin(ogen) in defense against bacterial infection has made it a target of bacterial proteins, evolved to interact with fibrin(ogen) to manipulate clot formation and degradation for the purpose of promoting microbial virulence and survival. Further understanding of the dual roles of fibrin(ogen) in wound healing and infection could provide novel means of therapy to improve recovery from surgical or chronic wounds and help to prevent infection from highly virulent bacterial strains, including those resistant to antibiotics.

Klebsiella pneumoniae enolase-like membrane protein interacts with human plasminogen

Many models assessing the risk of sepsis utilize the knowledge of the constituents of the plasminogen system, as it is proven that some species of bacteria can activate plasminogen, as a result of interactions with bacterial outer membrane proteins. However, much is yet to be discovered about this interaction since there is little information regarding some bacterial species. This study is aimed to check if Klebsiella pneumoniae, one of the major factors of nosocomial pneumonia and a factor for severe sepsis, has the ability to bind to human plasminogen. The strain used in this study, PCM 2713, acted as a typical representative of the species. With use of various methods, including: electron microscopy, 2-dimensional electrophoresis, immunoblotting and peptide fragmentation fingerprinting, it is shown that Klebsiella pneumoniae binds to human plasminogen, among others, due to plasminogen-bacterial enolase-like protein interaction, occurring on the outer membrane of the bacterium. Moreover, the study reveals, that other proteins, such as: phosphoglucomutase, and phosphoenolpyruvate carboxykinase act as putative plasminogen-binding factors. These information may virtually act as a foundation for future studies investigating: the: pathogenicity of Klebsiella pneumoniae and means for prevention from the outcomes of Klebsiella-derived sepsis.

Inhibition of Fibrinolysis by Streptococcal Phage LysinSM1

Expression of bacteriophage lysin_SM1 by Streptococcus oralis strain SF100 is thought to be important for the pathogenesis of infective endocarditis, due to its ability to mediate bacterial binding to fibrinogen. To better define the lysin_SM1 binding site on fibrinogen Aα, and to investigate the impact of binding on fibrinolysis, we examined the interaction of lysin_SM1 with a series of recombinant fibrinogen Aα variants. These studies revealed that lysin_SM1 binds the C-terminal region of fibrinogen Aα spanned by amino acid residues 534 to 610, with an affinity of equilibrium dissociation constant (K_D) of 3.23 × 10^-5 M. This binding site overlaps the known binding site for plasminogen, an inactive precursor of plasmin, which is a key protease responsible for degrading fibrin polymers. When tested in vitro, lysin_SM1 competitively inhibited plasminogen binding to the αC region of fibrinogen Aα. It also inhibited plasminogen-mediated fibrinolysis, as measured by thromboelastography (TEG). These results indicate that lysin_SM1 is a bi-functional virulence factor for streptococci, serving as both an adhesin and a plasminogen inhibitor. Thus, lysin_SM1 may facilitate the attachment of bacteria to fibrinogen on the surface of damaged cardiac valves and may also inhibit plasminogen-mediated lysis of infected thrombi (vegetations) on valve surfaces. IMPORTANCE The interaction of streptococci with human fibrinogen and platelets on damaged endocardium is a central event in the pathogenesis of infective endocarditis. Streptococcus oralis can bind platelets via the interaction of bacteriophage lysin_SM1 with fibrinogen on the platelet surface, and this process has been associated with increased virulence in an animal model of endocarditis. We now report that lysin_SM1 binds to the αC region of the human fibrinogen Aα chain. This interaction blocks plasminogen binding to fibrinogen and inhibits fibrinolysis. In vivo, this inhibition could prevent the lysis of infected vegetations, thereby promoting bacterial persistence and virulence.

Group A Streptococcus-Induced Activation of Human Plasminogen Is Required for Keratinocyte Wound Retraction and Rapid Clot Dissolution

Invasive outcomes of Group A Streptococcus (GAS) infections that involve damage to skin and other tissues are initiated when these bacteria colonize and disseminate via an open wound to gain access to blood and deeper tissues. Two critical GAS virulence factors, Plasminogen-Associated M-Protein (PAM) and streptokinase (SK), work in concert to bind and activate host human plasminogen (hPg) in order to create a localized proteolytic environment that alters wound-site architecture. Using a wound scratch assay with immortalized epithelial cells, real-time live imaging (RTLI) was used to examine dynamic effects of hPg activation by a PAM-containing skin-trophic GAS isolate (AP53R⁺S^-) during the course of infection. RTLI of these wound models revealed that retraction of the epithelial wound required both GAS and hPg. Isogenic AP53R⁺S^- mutants lacking SK or PAM highly attenuated the time course of retraction of the keratinocyte wound. We also found that relocalization of integrin β1 from the membrane to the cytoplasm occurred during the wound retraction event. We devised a combined in situ-based cellular model of fibrin clot-in epithelial wound to visualize the progress of GAS pathogenesis by RTLI. Our findings showed GAS AP53R⁺S^- hierarchically dissolved the fibrin clot prior to the retraction of keratinocyte monolayers at the leading edge of the wound. Overall, our studies reveal that localized activation of hPg by AP53R⁺S^- via SK and PAM during infection plays a critical role in dissemination of bacteria at the wound site through both rapid dissolution of the fibrin clot and retraction of the keratinocyte wound layer.

Mycobacterium tuberculosis glyceraldehyde-3-phosphate dehydrogenase plays a dual role-As an adhesin and as a receptor for plasmin(ogen)

The spread of infection is directly determined by the ability of a pathogen to invade and infect host tissues. The process involves adherence due to host-pathogen interactions and traversal into deeper tissues. Mycobacterium tuberculosis (Mtb) primarily infects the lung but is unique in its ability to infect almost any other organ of the human host including immune privileged sites such as the central nervous system (CNS). The extreme invasiveness of this bacterium is not fully understood. In the current study, we report that cell surface Mtb glyceraldehyde-3-phosphate dehydrogenase (GAPDH) functions as a virulence factor by multiple mechanisms. Firstly, it serves as a dual receptor for both plasminogen (Plg) and plasmin (Plm). CRISPRi-mediated silencing of this essential enzyme confirmed its role in the recruitment of Plg/Plm. Our studies further demonstrate that soluble GAPDH can re-associate on Mtb bacilli to promote plasmin(ogen) recruitment. The direct association of plasmin(ogen) via cell surface GAPDH or by the re-association of soluble GAPDH enhanced bacterial adherence to and traversal across lung epithelial cells. Furthermore, the association of GAPDH with host extracellular matrix (ECM) proteins coupled with its ability to recruit plasmin(ogen) may endow cells with the ability of directed proteolytic activity vital for tissue invasion.

Nonimmune antibody interactions of Group A Streptococcus M and M-like proteins

M and M-like proteins are major virulence factors of the widespread and potentially deadly bacterial pathogen Streptococcus pyogenes. These proteins confer resistance against innate and adaptive immune responses by recruiting specific human proteins to the streptococcal surface. Nonimmune recruitment of immunoglobulins G (IgG) and A (IgA) through their fragment crystallizable (Fc) domains by M and M-like proteins was described almost 40 years ago, but its impact on virulence remains unresolved. These interactions have been suggested to be consequential under immune conditions at mucosal surfaces and in secretions but not in plasma, while other evidence suggests importance in evading phagocytic killing in nonimmune blood. Recently, an indirect effect of Fc-binding through ligand-induced stabilization of an M-like protein was shown to increase virulence. Nonimmune recruitment has also been seen to contribute to tissue damage in animal models of autoimmune diseases triggered by S. pyogenes infection. The damage was treatable by targeting Fc-binding. This and other potential therapeutic applications warrant renewed attention to Fc-binding by M and M-like proteins.

Streptococcus co-opts a conformational lock in human plasminogen to facilitate streptokinase cleavage and bacterial virulence

Virulent strains of Streptococcus pyogenes (gram-positive group A Streptococcus pyogenes [GAS]) recruit host single-chain human plasminogen (hPg) to the cell surface-where in the case of Pattern D strains of GAS, hPg binds directly to the cells through a surface receptor, plasminogen-binding group A streptococcal M-protein (PAM). The coinherited Pattern D GAS-secreted streptokinase (SK2b) then accelerates cleavage of hPg at the R561-V562 peptide bond, resulting in the disulfide-linked two-chain protease, human plasmin (hPm). hPm localizes on the bacterial surface, assisting bacterial dissemination via proteolysis of host defense proteins. Studies using isolated domains from PAM and hPg revealed that the A-domain of PAM binds to the hPg kringle-2 module (K2hPg), but how this relates to the function of the full-length proteins is unclear. Herein, we use intact proteins to show that the lysine-binding site of K2hPg is a major determinant of the activation-resistant T-conformation of hPg. The binding of PAM to the lysine-binding site of K2hPg relaxes the conformation of hPg, leading to a greatly enhanced activation rate of hPg by SK2b. Domain swapping between hPg and mouse Pg emphasizes the importance of the Pg latent heavy chain (residues 1-561) in PAM binding and shows that while SK2b binds to both hPg and mouse Pg, the activation properties of streptokinase are strictly attributed to the serine protease domain (residues 562-791) of hPg. Overall, these data show that native hPg is locked in an activation-resistant conformation that is relaxed upon its direct binding to PAM, allowing hPm to form and provide GAS cells with a proteolytic surface.

A local α-helix drives structural evolution of streptococcal M-protein affinity for host human plasminogen

Plasminogen-binding group A streptococcal M-protein (PAM) is a signature surface virulence factor of specific strains of Group A Streptococcus pyogenes (GAS) and is an important tight binding protein for human plasminogen (hPg). After activation of PAM-bound hPg to the protease, plasmin (hPm), GAS cells develop invasive surfaces that are critical for their pathogenicity. PAMs are helical dimers in solution, which are sensitive to temperature changes over a physiological temperature range. We previously categorized PAMs into three classes (I-III) based on the number and nature of short tandem α-helical repeats (a1 and a2) in their NH2-terminal A-domains that dictate interactions with hPg/hPm. Class II PAMs are special cases since they only contain the a2-repeat, while Class I and Class III PAMs encompass complete a1a2-repeats. All dimeric PAMs tightly associate with hPg, regardless of their categories, but monomeric Class II PAMs bind to hPg much weaker than their Class I and Class III monomeric counterparts. Additionally, since the A-domains of Class II PAMs comprise different residues from other PAMs, the issue emerges as to whether Class II PAMs utilize different amino acid side chains for interactions with hPg. Herein, through NMR-refined structural analyses, we elucidate the atomic-level hPg-binding mechanisms adopted by two representative Class II PAMs. Furthermore, we develop an evolutionary model that explains from unique structural perspectives why PAMs develop variable A-domains with regard to hPg-binding affinity.

The Streptococcal Protease SpeB Antagonizes the Biofilms of the Human Pathogen Staphylococcus aureus USA300 through Cleavage of the Staphylococcal SdrC Protein

Streptococcus pyogenes, or group A Streptococcus (GAS), is both a pathogen and an asymptomatic colonizer of human hosts and produces a large number of surface-expressed and secreted factors that contribute to a variety of infection outcomes. The GAS-secreted cysteine protease SpeB has been well studied for its effects on the human host; however, despite its broad proteolytic activity, studies on how this factor is utilized in polymicrobial environments are lacking. Here, we utilized various forms of SpeB protease to evaluate its antimicrobial and antibiofilm properties against the clinically important human colonizer Staphylococcus aureus, which occupies niches similar to those of GAS. For our investigation, we used a skin-tropic GAS strain, AP53CovS+, and its isogenic ΔspeB mutant to compare the production and activity of native SpeB protease. We also generated active and inactive forms of recombinant purified SpeB for functional studies. We demonstrate that SpeB exhibits potent biofilm disruption activity at multiple stages of S. aureus biofilm formation. We hypothesized that the surface-expressed adhesin SdrC in S. aureus was cleaved by SpeB, which contributed to the observed biofilm disruption. Indeed, we found that SpeB cleaved recombinant SdrC in vitro and in the context of the full S. aureus biofilm. Our results suggest an understudied role for the broadly proteolytic SpeB as an important factor for GAS colonization and competition with other microorganisms in its niche.IMPORTANCEStreptococcus pyogenes (GAS) causes a range of diseases in humans, ranging from mild to severe, and produces many virulence factors in order to be a successful pathogen. One factor produced by many GAS strains is the protease SpeB, which has been studied for its ability to cleave and degrade human proteins, an important factor in GAS pathogenesis. An understudied aspect of SpeB is the manner in which its broad proteolytic activity affects other microorganisms that co-occupy niches similar to that of GAS. The significance of the research reported herein is the demonstration that SpeB can degrade the biofilms of the human pathogen Staphylococcus aureus, which has important implications for how SpeB may be utilized by GAS to successfully compete in a polymicrobial environment.

The M Protein of Streptococcus pyogenes Strain AP53 Retains Cell Surface Functional Plasminogen Binding after Inactivation of the Sortase A Gene

Streptococcus pyogenes (Lancefield group A Streptococcus [GAS]) is a β-hemolytic human-selective pathogen that is responsible for a large number of morbid and mortal infections in humans. For efficient infection, GAS requires different types of surface proteins that provide various mechanisms for evading human innate immune responses, thus enhancing pathogenicity of the bacteria. Many such virulence-promoting proteins, including the major surface signature M protein, are translocated after biosynthesis through the cytoplasmic membrane and temporarily tethered to this membrane via a type 1 transmembrane domain (TMD) positioned near the COOH terminus. In these proteins, a sorting signal, LPXTG, is positioned immediately upstream of the TMD, which is cleaved by the membrane-associated transpeptidase, sortase A (SrtA), leading to the covalent anchoring of these proteins to newly emerging l-Ala-l-Ala cross-bridges of the growing peptidoglycan cell wall. Herein, we show that inactivation of the srtA gene in a skin-tropic pattern D GAS strain (AP53) results in retention of the M protein in the cell membrane. However, while the isogenic AP53 ΔsrtA strain is attenuated in overall pathogenic properties due to effects on the integrity of the cell membrane, our data show that the M protein nonetheless can extend from the cytoplasmic membrane through the cell wall and then to the surface of the bacteria and thereby retain its important properties of productively binding and activating fluid-phase host plasminogen (hPg). The studies presented herein demonstrate an underappreciated additional mechanism of cell surface display of bacterial virulence proteins via their retention in the cell membrane and extension to the GAS surface.IMPORTANCE Group A Streptococcus pyogenes (GAS) is a human-specific pathogen that produces many surface factors, including its signature M protein, that contribute to its pathogenicity. M proteins undergo specific membrane localization and anchoring to the cell wall via the transpeptidase sortase A. Herein, we explored the role of sortase A function on M protein localization, architecture, and function, employing, a skin-tropic GAS isolate, AP53, which expresses a human plasminogen (hPg)-binding M (PAM) Protein. We showed that PAM anchored in the cell membrane, due to the targeted inactivation of sortase A, was nonetheless exposed on the cell surface and functionally interacted with host hPg. We demonstrate that M proteins, and possibly other sortase A-processed proteins that are retained in the cell membrane, can still function to initiate pathogenic processes by this underappreciated mechanism.

Solution structural model of the complex of the binding regions of human plasminogen with its M-protein receptor from Streptococcus pyogenes

VEK50 is a truncated peptide from a Streptococcal pyogenes surface human plasminogen (hPg) binding M-protein (PAM). VEK50 contains the full A-domain of PAM, which is responsible for its low nanomolar binding to hPg. The interaction of VEK50 with kringle 2, the PAM-binding domain in hPg (K2hPg), has been studied by high-resolution NMR spectroscopy. The data show that each VEK50 monomer in solution contains two tight binding sites for K2hPg, one each in the a1- (RH1; R17H18) and a2- (RH2; R30H31) repeats within the A-domain of VEK50. Two mutant forms of VEK50, viz., VEK50[RH1/AA] (VEK50ΔRH1) and VEK50[RH2/AA] (VEK50ΔRH2), were designed by replacing each RH with AA, thus eliminating one of the K2hPg binding sites within VEK50, and allowing separate study of each binding site. Using 13C- and 15N-labeled peptides, NMR-derived solution structures of VEK50 in its complex with K2hPg were solved. We conclude that the A-domain of PAM can accommodate two molecules of K2hPg docked within a short distance of each other, and the strength of the binding is slightly different for each site. The solution structure of the VEK50/K2hPg, complex, which is a reductionist model of the PAM/hPg complex, provides insights for the binding mechanism of PAM to a host protein, a process that is critical to S. pyogenes virulence.

Surface Proteins on Gram-Positive Bacteria

Surface proteins are critical for the survival of gram-positive bacteria both in the environment and to establish an infection. Depending on the organism, their surface proteins are evolutionarily tailored to interact with specific ligands on their target surface, be it inanimate or animate. Most surface molecules on these organisms are covalently anchored to the peptidoglycan through an LPxTG motif found at the C-terminus. These surface molecules are generally modular with multiple binding or enzymatic domains designed for a specific survival function. For example, some molecules will bind serum proteins like fibronectin or fibrinogen in one domain and have a separate function in another domain. In addition, enzymes such as those responsible for the production of ATP may be generally found on some bacterial surfaces, but when or how they are used in the life of these bacteria is currently unknown. While surface proteins are required for pathogenicity but not viability, targeting the expression of these molecules on the bacterial surface would prevent infection but not death of the organism. Given that the number of different surface proteins could be in the range of two to three dozen, each with two or three separate functional domains (with hundreds to thousands of each protein on a given organism), exemplifies the complexity that exists on the bacterial surface. Because of their number, we could not adequately describe the characteristics of all surface proteins in this chapter. However, since the streptococcal M protein was one of the first gram-positive surface protein to be completely sequenced, and perhaps one of the best studied, we will use M protein as a model for surface proteins in general, pointing out differences with other surface molecules when necessary.

Variations in the secondary structures of PAM proteins influence their binding affinities to human plasminogen

M-proteins (M-Prts) are major virulence determinants of Group A Streptococcus pyogenes (GAS) that are covalently anchored to the cell wall at their conserved COOH-termini while the NH₂-terminal regions extend through the capsule into extracellular space. Functional M-Prts are also secreted and/or released from GAS cells where they exist as helical coiled-coil dimers in solution. Certain GAS strains (Pattern D) uniquely express an M-protein (plasminogen-binding group A streptococcal M-protein; PAM) that directly interacts with human plasminogen (hPg), a process strongly implicated in the virulence of these strains. M-Prt expressed by the emm gene is employed to serotype over 250 known strains of GAS, ∼20 of which are hitherto found to express PAMs. We have developed a modular structural model of the PAM dimer that describes the roles of different domains of this protein in various functions. While the helical COOH-terminal domains of PAM are essential for dimerization in solution, regions of its NH₂-terminal domains also exhibit a weak potential to dimerize. We find that temperature controls the open (unwound) or closed (wound) states of the functional NH₂-terminal domains of PAM. As temperature increases, α-helices are dramatically reduced, which concomitantly destabilizes the helical coiled-coil PAM dimers. PAMs with two a-repeats within the variable NH₂-terminal A-domain (class I/III) bind to hPg tightly, but natural PAM isolates with a single a-repeat in this domain (class II) display dramatic changes in hPg binding with temperature. We conclude that coexistence of two a-repeats in PAM is critical to achieve optimal binding to hPg, especially in its monomeric form, at the biologically relevant temperature.

Contributions of different modules of the plasminogen-binding Streptococcus pyogenes M-protein that mediate its functional dimerization

Group A Streptococcus pyogenes (GAS) is a causative agent of pharyngeal and dermal infections in humans. A major virulence determinant of GAS is its dimeric signature fibrillar M-protein (M-Prt), which is evolutionarily designed in modules, ranging from a hypervariable extracellular N-terminal region to a progressively more highly conserved C-terminus that is covalently anchored to the cell wall. Of the >250 GAS isolates classified, only the subset of skin-trophic Pattern D strains expresses a specific serotype of M-Prt, PAM, that directly binds to host human plasminogen (hPg) via its extracellular NH2-terminal variable A-domain region. This interaction allows these GAS strains to accumulate components of the host fibrinolytic system on their surfaces to serve extracellular functions. While structure-function studies have been accomplished on M-Prts from Pattern A-C GAS isolates with different direct ligand binding properties compared to PAM, much less is known regarding the structure-function relationships of PAM-type M-Prts, particularly their dimerization determinants. To examine these questions, PAMs from seven GAS strains with sequence variations in the NH2-terminal ligand binding domains, as well as truncated versions of PAM, were designed and studied. The results from bioinformatic and biophysical analyses show that the different domains of PAM are disparately engaged in dimerization. From these data, we propose an experimentally-based model for PAM secondary and quaternary structures that is highly dependent on the conserved helical C-terminal C-D-domains. In addition, while the N-terminal regions of PAMs are variable in sequence, the binding properties of hPg and its activated product, plasmin, to the A-domain, remain intact.

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.

Crossing the Blood-Brain Barrier by Neuroinvasive Pathogens

The penetration of the blood-brain barrier (BBB) and invasion of the central nervous system (CNS) are important steps for all neuroinvasive pathogens. All of the ways of pathogens passing through the BBB are still unclear. Among known pathways, pathogen traversal can occur paracellularly, transcellularly or using a “Trojan horse” mechanism. The first step of translocation across the BBB is the interactions of the pathogen’s ligands with the receptors of the host brain cells. Lyme disease, the most common vector-borne disease in the temperate zones of Europe and North America, are caused by Borreliella species (former Borrelia burgdorferi sensu lato) that affects the peripheral and the CNS. In this review, we have presented various pathogen interactions with endothelial cells, which allow the disruption of the BBB so that the pathogens can pass across the BBB.

Virulence Role of the GlcNAc Side Chain of the Lancefield Cell Wall Carbohydrate Antigen in Non-M1-Serotype Group A Streptococcus

Classification of streptococci is based upon expression of unique cell wall carbohydrate antigens. All serotypes of group A Streptococcus (GAS; Streptococcus pyogenes), a leading cause of infection-related mortality worldwide, express the group A carbohydrate (GAC). GAC, the classical Lancefield antigen, is comprised of a polyrhamnose backbone with N-acetylglucosamine (GlcNAc) side chains. The immunodominant GlcNAc epitope of GAC is the basis of all rapid diagnostic testing for GAS infection. We previously identified the 12-gene GAC biosynthesis gene cluster and determined that the glycosyltransferase GacI was required for addition of the GlcNAc side chain to the polyrhamnose core. Loss of the GAC GlcNAc epitope in serotype M1 GAS resulted in attenuated virulence in two animal infection models and increased GAS sensitivity to killing by whole human blood, serum, neutrophils, and antimicrobial peptides. Here, we report that the GAC biosynthesis gene cluster is ubiquitous among 520 GAS isolates from global sources, representing 105 GAS emm serotypes. Isogenic ΔgacI mutants were constructed in M2, M3, M4, M28, and M89 backgrounds and displayed an array of phenotypes in susceptibility to killing by whole human blood, baby rabbit serum, human platelet releasate, human neutrophils, and antimicrobial peptide LL-37. The contribution of the GlcNAc side chain to GAS survival in vivo also varied by strain, demonstrating that it is not a prerequisite for virulence in the murine infection model. Thus, the relative contribution of GAC to virulence in non-M1 serotypes appears to depend on the quorum of other virulence factors that each strain possesses.IMPORTANCE The Lancefield group A carbohydrate (GAC) is the species-defining antigen for group A Streptococcus (GAS), comprising ~50% of the cell wall of this major human pathogen. We previously showed that the GlcNAc side chain of GAC contributes to the innate immune resistance and animal virulence phenotypes of the globally disseminated strain of serotype M1 GAS. Here, we use isogenic mutagenesis to examine the role of GAC GlcNAc in five additional medically relevant GAS serotypes. Overall, the GlcNAc side chain of GAC contributes to the innate immune resistance of GAS, but the relative contribution varies among individual strains. Moreover, the GAC GlcNAc side chain is not a universal prerequisite for GAS virulence in the animal model.

Conformationally organized lysine isosteres in Streptococcus pyogenes M protein mediate direct high-affinity binding to human plasminogen

The binding of human plasminogen (hPg) to the surface of the human pathogen group A Streptococcus pyogenes (GAS) and subsequent hPg activation to the protease plasmin generate a proteolytic surface that GAS employs to circumvent host innate immunity. Direct high-affinity binding of hPg/plasmin to pattern D GAS is fully recapitulated by the hPg kringle 2 domain (K2hPg) and a short internal peptide region (a1a2) of a specific subtype of bacterial surface M protein, present in all GAS pattern D strains. To better understand the nature of this binding, critical to the virulence of many GAS skin-tropic strains, we used high-resolution NMR to define the interaction of recombinant K2hPg with recombinant a1a2 (VKK38) of the M protein from GAS isolate NS455. We found a 2:1 (m/m) binding stoichiometry of K2hPg/VKK38, with the lysine-binding sites of two K2hPg domains anchored to two regions of monomeric VKK38. The K2hPg/VKK38 binding altered the VKK38 secondary structure from a helical apo-peptide with a flexible center to an end-to-end K2hPg-bound α-helix. The K2hPg residues occupied opposite faces of this helix, an arrangement that minimized steric clashing of K2hPg We conclude that VKK38 provides two conformational lysine isosteres that each interact with the lysine-binding sites in K2hPg Further, the adoption of an α-helix by VKK38 upon binding to K2hPg sterically optimizes the side chains of VKK38 for maximal binding to K2hPg and minimizes steric overlap between the K2hPg domains. The mechanism for hPg/M protein binding uncovered here may facilitate targeting of GAS virulence factors for disease management.

Variable region in streptococcal M-proteins provides stable binding with host fibrinogen for plasminogen-mediated bacterial invasion

Dimeric M-proteins (M-Prt) in group A Streptococcus pyogenes (GAS) are surface-expressed virulence factors implicated in processes that contribute to the pathogenicity of infection. Sequence analyses of various GAS M-Prts have shown that they contain a highly conserved sortase A-dependent cell wall-anchored C terminus, whereas the surface-exposed N terminus is highly variable, a feature used for identification and serotyping of various GAS strains. This variability also allows for strain-specific responses that suppress host defenses. Previous studies have indeed identified the N-terminal M-Prt B-domain as the site interacting with antiphagocytotic human-host fibrinogen (hFg). Herein, we show that hFg strongly interacts with M-Prts containing highly variable B-domains. We further demonstrate that specific GAS clinical isolates display high affinity for the D-domain of hFg, and this interaction allowed for subsequent surface binding of human-host plasminogen (hPg) to the E-domain of hFg. This GAS surface-bound hPg is then activated by GAS-secreted streptokinase, leading to the generation of an invasive proteolytic bacterial surface. Our results underscore the importance of the human fibrinolytic system in host-pathogen interactions in invasive GAS infections.

Inactivation of human coagulation factor X by a protease of the pathogen Capnocytophaga canimorsus

Essentials Capnocytophaga canimorsus causes severe dog bite related blood stream infections. We investigated if C. canimorsus contributes to bleeding abnormalities during infection. The C. canimorsus protease CcDPP7 causes factor X dysfunction by N-terminal cleavage. CcDPP7 inhibits coagulation in vivo, which could promote immune evasion and trigger hemorrhage.

SUMMARY

Background Capnocytophaga canimorsus is a Gram-negative bacterium that is present in the oral flora of dogs and causes fulminant sepsis in humans who have been bitten, licked, or scratched. In patients, bleeding abnormalities, such as petechiae, purpura fulminans, or disseminated intravascular coagulation (DIC), occur frequently. Objective To investigate whether C. canimorsus could actively contribute to these bleeding abnormalities. Methods Calibrated automated thrombogram and clotting time assays were performed to assess the anticoagulant activity of C. canimorsus 5 (Cc5), a strain isolated from a fatal human infection. Clotting factor activities were measured with factor-deficient plasma. Factor X cleavage was monitored with the radiolabeled zymogen and western blotting. Mutagenesis of Cc5 genes encoding putative serine proteases was performed to identify the protease that cleaves FX. Protein purification was performed with affinity chromatography. Edman degradation allowed the detection of N-terminal cleavage of FX. Tail bleeding times were measured in mice. Results We found that Cc5 inhibited thrombin generation and increased the prothrombin time and the activated partial thromboplastin time of human plasma via FX cleavage. A mutant that was unable to synthesize a type 7 dipeptidyl peptidase (DPP7) of the S46 serine protease family failed to proteolyse FX. The purified protease (CcDPP7) cleaved FX heavy and light chains from the N-terminus, and was active in vivo after intravenous injection. Conclusions This is, to our knowledge, the first study demonstrating a detailed mechanism for FX inactivation by a bacterial protease, and it is the first functional study associating DPP7 proteases with a potentially pathogenic outcome.

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.

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 Employs Strain-dependent Mechanisms of C3b Inactivation to Inhibit Phagocytosis and Killing of Bacteria

Evasion of complement-mediated opsonophagocytosis enables group A Streptococcus pyogenes (GAS) to establish infection. Different strain-dependent mechanisms are employed by the host to accomplish this goal. In general, GAS inhibits the amplification of the complement cascade on its cell surface by facilitating the degradation of C3b, an opsonin, to an inactive product, inactivated C3b (iC3b), in a step catalyzed by factor I (FI) and its cofactor, factor H (FH), with or without the participation of human host plasmin (hPm). GAS recruits FH to its cell surface via FH receptors, which are transcriptionally controlled by the two-component cluster of virulence responder-sensor system. The manner in which FI-FH and hPm function together on GAS cells is unknown. Using GAS strain AP53, which strongly binds host human plasminogen/plasmin (hPg/hPm) directly via an hPg/hPm surface receptor (PAM), we show that both FI-FH and hPm sequentially cleave C3b. Whereas FI-FH proteolytically cleaves C3b into iC3b, PAM-bound hPm catalyzes cleavage of iC3b into multiple smaller peptides. Unlike AP53, GAS strain M23ND weakly binds FH and recruits hPg/hPm to its cell surface indirectly via fibrinogen bound to M-protein, M23. In this case, FH-FI cleaves C3b into iC3b, with negligible degradation of iC3b by hPm that is bound to fibrinogen on the cells. AP53 and M23ND display similar resistance to human neutrophil-mediated phagocytosis, which results in a corresponding high lethality in mice after injection of these cells. These results suggest that GAS utilizes diverse mechanisms to degrade C3b and thus to protect bacterial cells from the complement response of the host.

Progress in the biological function of alpha-enolase

Alpha-enolase (ENO1), also known as 2-phospho-D-glycerate hydrolase, is a metalloenzyme that catalyzes the conversion of 2-phosphoglyceric acid to phosphoenolpyruvic acid in the glycolytic pathway. It is a multifunctional glycolytic enzyme involved in cellular stress, bacterial and fungal infections, autoantigen activities, the occurrence and metastasis of cancer, parasitic infections, and the growth, development and reproduction of organisms. This article mainly reviews the basic characteristics and biological functions of ENO1.

Group A Streptococcus exploits human plasminogen for bacterial translocation across epithelial barrier via tricellular tight junctions

Group A Streptococcus (GAS) is a human-specific pathogen responsible for local suppurative and life-threatening invasive systemic diseases. Interaction of GAS with human plasminogen (PLG) is a salient characteristic for promoting their systemic dissemination. In the present study, a serotype M28 strain was found predominantly localized in tricellular tight junctions of epithelial cells cultured in the presence of PLG. Several lines of evidence indicated that interaction of PLG with tricellulin, a major component of tricellular tight junctions, is crucial for bacterial localization. A site-directed mutagenesis approach revealed that lysine residues at positions 217 and 252 within the extracellular loop of tricellulin play important roles in PLG-binding activity. Additionally, we demonstrated that PLG functions as a molecular bridge between tricellulin and streptococcal surface enolase (SEN). The wild type strain efficiently translocated across the epithelial monolayer, accompanied by cleavage of transmembrane junctional proteins. In contrast, amino acid substitutions in the PLG-binding motif of SEN markedly compromised those activities. Notably, the interaction of PLG with SEN was dependent on PLG species specificity, which influenced the efficiency of bacterial penetration. Our findings provide insight into the mechanism by which GAS exploits host PLG for acceleration of bacterial invasion into deeper tissues via tricellular tight junctions.

Bacterial pathogens activate plasminogen to breach tissue barriers and escape from innate immunity

Both coagulation and fibrinolysis are tightly connected with the innate immune system. Infection and inflammation cause profound alterations in the otherwise well-controlled balance between coagulation and fibrinolysis. Many pathogenic bacteria directly exploit the host's hemostatic system to increase their virulence. Here, we review the capacity of bacteria to activate plasminogen. The resulting proteolytic activity allows them to breach tissue barriers and evade innate immune defense, thus promoting bacterial spreading. Yersinia pestis, streptococci of group A, C and G and Staphylococcus aureus produce a specific bacterial plasminogen activator. Moreover, surface plasminogen receptors play an established role in pneumococcal, borrelial and group B streptococcal infections. This review summarizes the mechanisms of bacterial activation of host plasminogen and the role of the fibrinolytic system in infections caused by these pathogens.

Cloning and Characterization of Surface-Localized α-Enolase of Streptococcus iniae, an Effective Protective Antigen in Mice

Streptococcus iniae is a major fish pathogen that can also cause human bacteremia, cellulitis and meningitis. Screening for and identification of protective antigens plays an important role in developing therapies against S. iniae infections. In this study, we indicated that the α-enolase of S. iniae was not only distributed in the cytoplasm and associated to cell walls, but was also secreted to the bacterial cell surface. The functional identity of the purified recombinant α-enolase protein was verified by its ability to catalyze the conversion of 2-phosphoglycerate (2-PGE) to phosphoenolpyruvate (PEP), and both the recombinant and native proteins interacted with human plasminogen. The rabbit anti-rENO serum blockade assay shows that α-enolase participates in S. iniae adhesion to and invasion of BHK-21 cells. In addition, the recombinant α-enolase can confer effective protection against S. iniae infection in mice, which suggests that α-enolase has potential as a vaccine candidate in mammals. We conclude that S. iniae α-enolase is a moonlighting protein that also associates with the bacterial outer surface and functions as a protective antigen in mice.

Preferential Acquisition and Activation of Plasminogen Glycoform II by PAM Positive Group A Streptococcal Isolates

Plasminogen (Plg) circulates in the host as two predominant glycoforms. Glycoform I Plg (GI-Plg) contains glycosylation sites at Asn289 and Thr346, whereas glycoform II Plg (GII-Plg) is exclusively glycosylated at Thr346. Surface plasmon resonance experiments demonstrated that Plg binding group A streptococcal M protein (PAM) exhibits comparative equal affinity for GI- and GII-Plg in the "closed" conformation (for GII-Plg, KD = 27.4 nM; for GI-Plg, KD = 37.0 nM). When Plg was in the "open" conformation, PAM exhibited an 11-fold increase in affinity for GII-Plg (KD = 2.8 nM) compared with that for GI-Plg (KD = 33.2 nM). The interaction of PAM with Plg is believed to be mediated by lysine binding sites within kringle (KR) 2 of Plg. PAM-GI-Plg interactions were fully inhibited with 100 mM lysine analogue ε-aminocaproic acid (εACA), whereas PAM-GII-Plg interactions were shown to be weakened but not inhibited in the presence of 400 mM εACA. In contrast, binding to the KR1-3 domains of GII-Plg (angiostatin) by PAM was completely inhibited in the presence 5 mM εACA. Along with PAM, emm pattern D GAS isolates express a phenotypically distinct SK variant (type 2b SK) that requires Plg ligands such as PAM to activate Plg. Type 2b SK was able to generate an active site and activate GII-Plg at a rate significantly higher than that of GI-Plg when bound to PAM. Taken together, these data suggest that GAS selectively recruits and activates GII-Plg. Furthermore, we propose that the interaction between PAM and Plg may be partially mediated by a secondary binding site outside of KR2, affected by glycosylation at Asn289.

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.

Post-infectious group A streptococcal autoimmune syndromes and the heart

There is a pressing need to reduce the high global disease burden of rheumatic heart disease (RHD) and its harbinger, acute rheumatic fever (ARF). ARF is a classical example of an autoimmune syndrome and is of particular immunological interest because it follows a known antecedent infection with group A streptococcus (GAS). However, the poorly understood immunopathology of these post-infectious diseases means that, compared to much progress in other immune-mediated diseases, we still lack useful biomarkers, new therapies or an effective vaccine in ARF and RHD. Here, we summarise recent literature on the complex interaction between GAS and the human host that culminates in ARF and the subsequent development of RHD. We contrast ARF with other post-infectious streptococcal immune syndromes - post-streptococcal glomerulonephritis (PSGN) and the still controversial paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS), in order to highlight the potential significance of variations in the host immune response to GAS. We discuss a model for the pathogenesis of ARF and RHD in terms of current immunological concepts and the potential for application of in depth "omics" technologies to these ancient scourges.

Stability of the octameric structure affects plasminogen-binding capacity of streptococcal enolase

Group A Streptococcus (GAS) is a human pathogen that has the potential to cause invasive disease by binding and activating human plasmin(ogen). Streptococcal surface enolase (SEN) is an octameric α-enolase that is localized at the GAS cell surface. In addition to its glycolytic role inside the cell, SEN functions as a receptor for plasmin(ogen) on the bacterial surface, but the understanding of the molecular basis of plasmin(ogen) binding is limited. In this study, we determined the crystal and solution structures of GAS SEN and characterized the increased plasminogen binding by two SEN mutants. The plasminogen binding ability of SEN^K312A and SEN^K362A is ~2- and ~3.4-fold greater than for the wild-type protein. A combination of thermal stability assays, native mass spectrometry and X-ray crystallography approaches shows that increased plasminogen binding ability correlates with decreased stability of the octamer. We propose that decreased stability of the octameric structure facilitates the access of plasmin(ogen) to its binding sites, leading to more efficient plasmin(ogen) binding and activation.

Photonic activation of plasminogen induced by low dose UVB

Activation of plasminogen to its active form plasmin is essential for several key mechanisms, including the dissolution of blood clots. Activation occurs naturally via enzymatic proteolysis. We report that activation can be achieved with 280 nm light. A 2.6 fold increase in proteolytic activity was observed after 10 min illumination of human plasminogen. Irradiance levels used are in the same order of magnitude of the UVB solar irradiance. Activation is correlated with light induced disruption of disulphide bridges upon UVB excitation of the aromatic residues and with the formation of photochemical products, e.g. dityrosine and N-formylkynurenine. Most of the protein fold is maintained after 10 min illumination since no major changes are observed in the near-UV CD spectrum. Far-UV CD shows loss of secondary structure after illumination (33.4% signal loss at 206 nm). Thermal unfolding CD studies show that plasminogen retains a native like cooperative transition at ~70 ºC after UV-illumination. We propose that UVB activation of plasminogen occurs upon photo-cleavage of a functional allosteric disulphide bond, Cys737-Cys765, located in the catalytic domain and in van der Waals contact with Trp761 (4.3 Å). Such proximity makes its disruption very likely, which may occur upon electron transfer from excited Trp761. Reduction of Cys737-Cys765 will result in likely conformational changes in the catalytic site. Molecular dynamics simulations reveal that reduction of Cys737-Cys765 in plasminogen leads to an increase of the fluctuations of loop 760–765, the S1-entrance frame located close to the active site. These fluctuations affect the range of solvent exposure of the catalytic triad, particularly of Asp646 and Ser74, which acquire an exposure profile similar to the values in plasmin. The presented photonic mechanism of plasminogen activation has the potential to be used in clinical applications, possibly together with other enzymatic treatments for the elimination of blood clots.

Current views of haemolytic streptococcal pathogenesis

PURPOSE OF REVIEW

Increasing disease caused by beta-haemolytic streptococci indicates the need for improved understanding of pathogenesis.

RECENT FINDINGS

Streptococcus pyogenes, or group A Streptococcus (GAS), causes significant disease worldwide. The closely related Streptococcus dysgalactiae subspecies equisimilis (SDSE) is increasingly recognized as causing a similar disease spectrum. Whole-genome sequencing applied to the study of outbreaks may reveal factors that contribute to pathogenesis and changes in epidemiology. The role of quorum sensing in biofilm formation, and interspecies communication with other streptococci, is discussed. GAS has evolved multiple mechanisms to evade the humoral arm of innate immunity, including complement, which is well known in protecting the host from bacteria, and the coagulation-fibrinolytic system, which is increasingly recognized as an innate immune effector.

SUMMARY

Molecular biology has enhanced our understanding of the intricate balance of host-pathogen interactions that result in clearance or establishment of invasive streptococcal infection. Although the skin and oropharynx remain the usual ecological niche of GAS and SDSE, occasionally the bacteria find themselves within deeper tissues and blood. Recent research has armed us with better knowledge of bacterial adaptations to this alternative environment. However, the challenge is to translate this knowledge into clinical practice, through the development of novel therapeutic options and ultimately a vaccine against GAS.

Mutual exclusivity of hyaluronan and hyaluronidase in invasive group A Streptococcus

A recent analysis of group A Streptococcus (GAS) invasive infections in Australia has shown a predominance of M4 GAS, a serotype recently reported to lack the antiphagocytic hyaluronic acid (HA) capsule. Here, we use molecular genetics and bioinformatics techniques to characterize 17 clinical M4 isolates associated with invasive disease in children during this recent epidemiology. All M4 isolates lacked HA capsule, and whole genome sequence analysis of two isolates revealed the complete absence of the hasABC capsule biosynthesis operon. Conversely, M4 isolates possess a functional HA-degrading hyaluronate lyase (HylA) enzyme that is rendered nonfunctional in other GAS through a point mutation. Transformation with a plasmid expressing hasABC restored partial encapsulation in wild-type (WT) M4 GAS, and full encapsulation in an isogenic M4 mutant lacking HylA. However, partial encapsulation reduced binding to human complement regulatory protein C4BP, did not enhance survival in whole human blood, and did not increase virulence of WT M4 GAS in a mouse model of systemic infection. Bioinformatics analysis found no hasABC homologs in closely related species, suggesting that this operon was a recent acquisition. These data showcase a mutually exclusive interaction of HA capsule and active HylA among strains of this leading human pathogen.

Dimerization is not a determining factor for functional high affinity human plasminogen binding by the group A streptococcal virulence factor PAM and is mediated by specific residues within the PAM a1a2 domain

A emm53 subclass of Group A Streptococcus pyogenes (GAS) interacts tightly with human plasma plasminogen (hPg) and plasmin (hPm) via the kringle 2 (K2hPg) domain of hPg/hPm and the N-terminal a1a2 regions of a GAS coiled-coil M-like protein (PAM). Previous studies have shown that a monomeric PAM fragment, VEK30 (residues 97-125 + Tyr), interacted specifically with isolated K2hPg. However, the binding strength of VEK30 (KD = 56 nm) was ∼60-fold weaker than that of full-length dimeric PAM (KD = 1 nm). To assess whether this attenuated binding was due to the inability of VEK30 to dimerize, we defined the minimal length of PAM required to dimerize using a series of peptides with additional PAM residues placed at the NH2 and COOH termini of VEK30. VEK64 (PAM residues 83-145 + Tyr) was found to be the smallest peptide that adopted an α-helical dimer, and was bound to K2hPg with nearly the same affinity as PAM (KD = 1-2 nm). However, addition of two PAM residues (Arg(126)-His(127)) to the COOH terminus of VEK30 (VEK32) maintained a monomeric peptidic structure, but exhibited similar K2hPg binding affinity as full-length dimeric PAM. We identified five residues in a1a2 (Arg(113), His(114), Glu(116), Arg(126), His(127)), mutation of which reduced PAM binding affinity for K2hPg by ∼ 1000-fold. Replacement of these critical residues by Ala in the GAS genome resulted in reduced virulence, similar to the effects of inactivating the PAM gene entirely. We conclude that rather than dimerization of PAM, the five key residues in the binding domain of PAM are essential to mediate the high affinity interaction with hPg, leading to increased GAS virulence.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

The β-domain of cluster 2b streptokinase is a major determinant for the regulation of its plasminogen activation activity by cellular plasminogen receptors

Cluster 2b streptokinase (SK2b), secreted by invasive skin-trophic strains of Streptococcus pyogenes (GAS), is a human plasminogen (hPg) activator that optimally functions when human plasma hPg is bound, via its kringle-2 domain, to cognizant bacterial cells through the a1a2 domain of the major cellular hPg receptor, Plasminogen-binding group A streptococcal M-like protein (PAM). Another class of streptokinases (SK1), secreted primarily by GAS strains that possess affinity for pharyngeal infections, does not require PAM-bound hPg for optimal activity. We find herein that replacement of the central β-domain of SK2b with the same module from SK1 reduces the dependency of SK2b on PAM, and the converse is true when the β-domain of SK1 is replaced with this same region of SK2b. These data suggest that simple evolutionary shuttling of protein domains in GAS can be employed by GAS to rapidly generate strains that differ in tissue tropism and invasive capability and allow the bacteria to survive different challenges by the host.

Full time course kinetics of the streptokinase-plasminogen activation pathway

Our previously hypothesized mechanism for the pathway of plasminogen (Pg) activation by streptokinase (SK) was tested by the use of full time course kinetics. Three discontinuous chromogenic substrate initial rate assays were developed with different quenching conditions that enabled quantitation of the time courses of Pg depletion, plasmin (Pm) formation, transient formation of the conformationally activated SK·Pg* catalytic complex intermediate, formation of the SK·Pm catalytic complex, and the free concentrations of Pg, Pm, and SK. Analysis of full time courses of Pg activation by five concentrations of SK along with activity-based titrations of SK·Pg* and SK·Pm formation yielded rate and dissociation constants within 2-fold of those determined previously by continuous measurement of parabolic chromogenic substrate hydrolysis and fluorescence-based equilibrium binding. The results obtained with orthogonal assays provide independent support for a mechanism in which the conformationally activated SK·Pg* complex catalyzes an initial cycle of Pg proteolytic conversion to Pm that acts as a trigger. Higher affinity binding of the formed Pm to SK outcompetes Pg binding, terminating the trigger cycle and initiating the bullet catalytic cycle by the SK·Pm complex that converts the residual Pg into Pm. The new assays can be adapted to quantitate SK-Pg activation in the context of SK- or Pg-directed inhibitors, effectors, and SK allelic variants. To support this, we show for the first time with an assay specific for SK·Pg* that fibrinogen forms a ternary SK·Pg*·fibrinogen complex, which assembles with 200-fold enhanced SK·Pg* affinity, signaled by a perturbation of the SK·Pg* active site.

Complement-mediated opsonization of invasive group A Streptococcus pyogenes strain AP53 is regulated by the bacterial two-component cluster of virulence responder/sensor (CovRS) system

Group A Streptococcus pyogenes (GAS) strain AP53 is a primary isolate from a patient with necrotizing fasciitis. These AP53 cells contain an inactivating mutation in the sensor component of the cluster of virulence (cov) responder (R)/sensor (S) two-component gene regulatory system (covRS), which enhances the virulence of the primary strain, AP53/covR(+)S(-). However, specific mechanisms by which the covRS system regulates the survival of GAS in humans are incomplete. Here, we show a key role for covRS in the regulation of opsonophagocytosis of AP53 by human neutrophils. AP53/covR(+)S(-) cells displayed potent binding of host complement inhibitors of C3 convertase, viz. Factor H (FH) and C4-binding protein (C4BP), which concomitantly led to minimal C3b deposition on AP53 cells, further showing that these plasma protein inhibitors are active on GAS cells. This resulted in weak killing of the bacteria by human neutrophils and a corresponding high death rate of mice after injection of these cells. After targeted allelic alteration of covS(-) to wild-type covS (covS(+)), a dramatic loss of FH and C4BP binding to the AP53/covR(+)S(+) cells was observed. This resulted in elevated C3b deposition on AP53/covR(+)S(+) cells, a high level of opsonophagocytosis by human neutrophils, and a very low death rate of mice infected with AP53/covR(+)S(+). We show that covRS is a critical transcriptional regulator of genes directing AP53 killing by neutrophils and regulates the levels of the receptors for FH and C4BP, which we identify as the products of the fba and enn genes, respectively.

Cooperative plasminogen recruitment to the surface of Streptococcus canis via M protein and enolase enhances bacterial survival

Streptococcus canis is a zoonotic pathogen capable of causing serious invasive diseases in domestic animals and humans. Surface-exposed M proteins and metabolic enzymes have been characterized as major virulence determinants in various streptococcal species. Recently, we have identified SCM, the M-like protein of S. canis, as the major receptor for miniplasminogen localized on the bacterial surface. The present study now characterizes the glycolytic enzyme enolase as an additional surface-exposed plasminogen-binding protein. According to its zoonotic properties, purified S. canis enolase binds to both human and canine plasminogen and facilitates degradation of aggregated fibrin matrices after activation with host-derived urokinase-type plasminogen activator (uPA). Unlike SCM, which binds to the C terminus of human plasminogen, the S. canis enolase interacts N terminally with the first four kringle domains of plasminogen, representing angiostatin. Radioactive binding analyses confirmed cooperative plasminogen recruitment to both surface-exposed enolase and SCM. Furthermore, despite the lack of surface protease activity via SpeB in S. canis, SCM is released and reassociated homophilically to surface-anchored SCM and heterophilically to surface-bound plasminogen. In addition to plasminogen-mediated antiphagocytic activity, reassociation of SCM to the bacterial surface significantly enhanced bacterial survival in phagocytosis analyses using human neutrophils.

IMPORTANCE

Streptococcal infections are a major issue in medical microbiology due to the increasing spread of antibiotic resistances and the limited availability of efficient vaccines. Surface-exposed glycolytic enzymes and M proteins have been characterized as major virulence factors mediating pathogen-host interaction. Since streptococcal infection mechanisms exert a subset of multicombinatorial processes, the investigation of synergistic activities mediated via different virulence factors has become a high priority. Our data clearly demonstrate that plasminogen recruitment to the Streptococcus canis surface via SCM and enolase in combination with SCM reassociation enhances bacterial survival by protecting against phagocytic killing. These data propose a new cooperative mechanism for prevention of phagocytic killing based on the synergistic activity of homophilic and heterophilic SCM binding in the presence of human plasminogen.