Functional differences between Streptococcus pyogenes cluster 1 and cluster 2b streptokinases are determined by their β-domains

Group a Streptococcus remains viable inside fibrin clots and gains access to human plasminogen for subsequent fibrinolysis and dissemination

Group A Streptococcus (GAS) is a major human pathogen that causes several invasive diseases including necrotizing fasciitis. The host coagulation cascade initiates fibrin clots to sequester bacteria to prevent dissemination into deeper tissues. GAS, especially skin-tropic bacterial strains, utilize specific virulence factors, plasminogen binding M-protein (PAM) and streptokinase (SK), to manipulate hemostasis and activate plasminogen to cause fibrinolysis and fibrin clot escape. A major unresolved question regards the temporal dynamics of how GAS enmeshed in a fibrin clot can access plasminogen for clot dissolution and eventual dissemination. Here, we reveal through live imaging studies that GAS trapped inside a fibrin clot can remain viable in a latent state, until access to plasminogen activates fibrinolysis and dissemination. RNA-sequencing (RNA-seq) analysis shows marked changes in the wild-type (WT)-GAS transcriptome from the time bacteria were enmeshed inside the clot (4 h) to when dissemination was initiated (8 h). To gain a more fully realized model of how GAS trapped in fibrin clots can disseminate in the blood system, we utilized a novel 3D endothelial microfluidic device to demonstrate that GAS is fully capable of fibrinolysis in an endothelial environment, revealing a major underappreciated route by which GAS may cause more invasive outcomes. Our findings reveal for the first time that GAS can engage a latent, growth-suspended phase whereby physical structures such as fibrin clots that immobilize an invading pathogen allow bacteria to remain viable until sufficient access to plasminogen allows it to initiate fibrinolysis and escape into surrounding blood system and tissues.

IMPORTANCE

Group A Streptococcus (GAS) is a human-specific bacterial pathogen that causes infections ranging in severity from mild to severe infections that can often be fatal. To protect the host, the innate immune system creates fibrin clots to trap bacteria and prevent deeper spread. GAS produces several factors that can initiate the dissolution of these fibrin clots to spread to deeper tissues, but we lack specific understanding of the timing of these events. Our studies demonstrate for the first time that GAS can delay their escape from fibrin clots to gain access to deeper tissues during infection, suggesting a key strategy that GAS utilize to cause more invasive disease.

Evolution of Streptococcus pyogenes has maximized the efficiency of the Sortase A cleavage motif for cell wall transpeptidation

Trafficking of M-protein (Mprt) from the cytosol of Group A Streptococcus pyogenes (GAS) occurs via Sec translocase membrane channels that associate with Sortase A (SrtA), an enzyme that catalyzes cleavage of Mprt at the proximal C-terminal [-LPST355∗GEAA-] motif and subsequent transpeptidation of the Mprt-containing product to the cell wall (CW). These steps facilitate stable exposure of the N-terminus of Mprt to the extracellular milieu where it interacts with ligands. Previously, we found that inactivation of SrtA in GAS cells eliminated Mprt CW transpeptidation but effected little reduction in its cell surface exposure, indicating that the C-terminus of Mprt retained in the cytoplasmic membrane (CM) extends its N-terminus to the cell surface. Herein, we assessed the effects of mutating the Thr355 residue in the WT SrtA consensus sequence (LPST355∗GEAA-) in a specific Mprt, PAM. In vitro, we found that synthetic peptides with mutations (LPSX355GEAA) in the SrtA cleavage site displayed slower cleavage activities with rSrtA than the WT peptide. Aromatic residues at X had the lowest activities. Nonetheless, PAM/[Y355G] still transpeptidated the CW in vivo. However, when using isolated CMs from srtA-inactivated GAS cells, rapid cleavage of PAM/[LPSY355GEAA] occurred at E357∗ but transpeptidation did not take place. These results show that another CM-resident enzyme nonproductively cleaved PAM/[LPSYGE357∗AA]. However, SrtA associated with the translocon channel in vivo cleaved and transpeptidated PAM/[LPSX355∗GEAA] variants. These CM features allow diverse cleavage site variants to covalently attach to the CW despite the presence of other potent nonproductive CM proteases.

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.

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.

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.

Contribution of Streptokinase-Domains from Groups G and A (SK2a) Streptococci in Amidolytic/Proteolytic Activities and Fibrin-Dependent Plasminogen activation: A Domain-Exchange Study

BACKGROUND

Streptokinase (SK), a heterogeneous plasminogen activator (PA) protein from groups A, C, and G streptococci (GAS, GCS, GGS, respectively) contains three structural domains (SKα, SKβ, and SK). Based on the variable region of SKβ, GAS-SK (ska) are clustered as SK1 and SK2 (including cluster2-streptokinase (SK2a)/SK2b), which show low and high fibrinogen (FG)-dependent plasminogen (Plg) activation properties, respectively. Despite being co-clustered as SK2a, GCS/GGS-SK (skcg) variants display properties similar to SK1. Herein, by SKβ exchange between GGS (G88) and GAS-SK2a (STAB902) variants, the potential roles of SK domains in amidolytic/proteolytic activity and FG-bound-Plg activation are represented.

METHODS

Two parental SKG88 and SKSTAB902 genes were cloned into the NdeI/XhoI site of pET26b expression vector. The two chimeric SKβ-exchanged constructs (SKC1: αG88-βSTAB-γG88 and SKC2; αSTAB-βG88-γSTAB) were constructed by BstEII/BsiWI digestion/cross-ligation in parental plasmids. SK were expressed in E. coli and purified by nickel-nitriloacetic acid chromatography. PA potencies of SKs were measured by colorimetric assay.

RESULTS

SDS-PAGE and Western-blot analyses confirmed the proper expression of 47-kDa SK. Analyses indicated that the catalytic efficiency (Kcat/Km) for amidolytic and proteolytic activity were less and moderately dependent on SKβ, respectively. The increase of FG-bound-Plg activation for SKSTAB902/SKC1 containing SK2aβ was around six times, whereas for SKG88/SKC2 containing skcgβ, it was four times.

CONCLUSION

Although SKβ has noticeable contribution in FG-bound-Plg activation activity, it had minor contribution in fibrin-independent, amidolytic activity. These data might be of interest for engineering fibrin-specific versions of SK.

The β-domain of streptokinase affects several functionalities, including specific/proteolytic activity kinetics

Streptokinase (SK) is a plasminogen activator which converts inactive plasminogen (Pg) to active plasmin (Pm), which cleaves fibrin clots. SK secreted by groups A, C, and G Streptococcus (SKA/SKC/SKG) is composed of three domains: SKα, SKβ and SKγ. Previous domain-swapping studies between SK1/SK2b-cluster variants revealed that SKβ plays a major role in the activation of human Pg. Here, we carried out domain-swapping between skcg-SK/SK2-cluster variants to determine the involvement of SKβ in several SK functionalities, including specific/proteolytic activity kinetics, fibrinogen-bound Pg activation and α2 -antiplasmin resistance. Our results indicate that SKβ has a minor to determining role in these diverse functionalities for skcg-SK and SK2b variants, which might potentially be accompanied by few critical residues acting as hot spots. Our findings enhance our understanding of the roles of SKβ and hot spots in different functional characteristics of SK clusters and may aid in the engineering of fibrin-specific variants of SK for breaking down blood clots with potentially higher efficacy and safety.

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.

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.

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.

Direct Host Plasminogen Binding to Bacterial Surface M-protein in Pattern D Strains of Streptococcus pyogenes Is Required for Activation by Its Natural Coinherited SK2b Protein

Streptokinase (SK), secreted by Group A Streptococcus (GAS), is a single-chain ∼47-kDa protein containing three consecutive primary sequence regions that comprise its α, β, and γ modules. Phylogenetic analyses of the variable β-domain sequences from different GAS strains suggest that SKs can be arranged into two clusters, SK1 and SK2, with a subdivision of SK2 into SK2a and SK2b. SK2b is secreted by skin-tropic Pattern D M-protein strains that also express plasminogen (human Pg (hPg)) binding Group A streptococcal M-protein (PAM) as its major cell surface M-protein. SK2a-expressing strains are associated with nasopharynx tropicity, and many of these strains express human fibrinogen (hFg) binding Pattern A-C M-proteins, e.g. M1. PAM interacts with hPg directly, whereas M1 binds to hPg indirectly via M1-bound hFg. Subsequently, SK is secreted by GAS and activates hPg to plasmin (hPm), thus generating a proteolytic surface on GAS that enhances its dissemination. Due to these different modes of hPg/hPm recognition by GAS, full characterizations of the mechanisms of activation of hPg by SK2a and SK2b and their roles in GAS virulence are important topics. To more fully examine these subjects, isogenic chimeric SK- and M-protein-containing GAS strains were generated, and the virulence of these chimeric strains were analyzed in mice. We show that SK and M-protein alterations influenced the virulence of GAS and were associated with the different natures of hPg activation and hPm binding. These studies demonstrate that GAS virulence can be explained by disparate hPg activation by SK2a and SK2b coupled with the coinherited M-proteins of these strains.

Unique genomic arrangements in an invasive serotype M23 strain of Streptococcus pyogenes identify genes that induce hypervirulence

The first genome sequence of a group A Streptococcus pyogenes serotype M23 (emm23) strain (M23ND), isolated from an invasive human infection, has been completed. The genome of this opacity factor-negative (SOF(-)) strain is composed of a circular chromosome of 1,846,477 bp. Gene profiling showed that this strain contained six phage-encoded and 24 chromosomally inherited well-known virulence factors, as well as 11 pseudogenes. The bacterium has acquired four large prophage elements, ΦM23ND.1 to ΦM23ND.4, harboring genes encoding streptococcal superantigen (ssa), streptococcal pyrogenic exotoxins (speC, speH, and speI), and DNases (spd1 and spd3), with phage integrase genes being present at one flank of each phage insertion, suggesting that the phages were integrated by horizontal gene transfer. Comparative analyses revealed unique large-scale genomic rearrangements that result in genomic rearrangements that differ from those of previously sequenced GAS strains. These rearrangements resulted in an imbalanced genomic architecture and translocations of chromosomal virulence genes. The covS sensor in M23ND was identified as a pseudogene, resulting in the attenuation of speB function and increased expression of the genes for the chromosomal virulence factors multiple-gene activator (mga), M protein (emm23), C5a peptidase (scpA), fibronectin-binding proteins (sfbI and fbp54), streptolysin O (slo), hyaluronic acid capsule (hasA), streptokinase (ska), and DNases (spd and spd3), which were verified by PCR. These genes are responsible for facilitating host epithelial cell binding and and/or immune evasion, thus further contributing to the virulence of M23ND. In conclusion, strain M23ND has become highly pathogenic as the result of a combination of multiple genetic factors, particularly gene composition and mutations, prophage integrations, unique genomic rearrangements, and regulated expression of critical virulence factors.

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.

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.