A Streptococcal Collagen-like Protein Interacts with the α2β1 Integrin and Induces Intracellular Signaling

The streptococcal collagen-like protein-1 (Scl1) is a significant determinant for biofilm formation by group A Streptococcus

Background

Group A Streptococcus (GAS) is a human-specific pathogen responsible for a number of diseases characterized by a wide range of clinical manifestations. During host colonization GAS-cell aggregates or microcolonies are observed in tissues. GAS biofilm, which is an in vitro equivalent of tissue microcolony, has only recently been studied and little is known about the specific surface determinants that aid biofilm formation. In this study, we demonstrate that surface-associated streptococcal collagen-like protein-1 (Scl1) plays an important role in GAS biofilm formation.

Results

Biofilm formation by M1-, M3-, M28-, and M41-type GAS strains, representing an intraspecies breadth, were analyzed spectrophotometrically following crystal violet staining, and characterized using confocal and field emission scanning electron microscopy. The M41-type strain formed the most robust biofilm under static conditions, followed by M28- and M1-type strains, while the M3-type strains analyzed here did not form biofilm under the same experimental conditions. Differences in architecture and cell-surface morphology were observed in biofilms formed by the M1- and M41-wild-type strains, accompanied by varying amounts of deposited extracellular matrix and differences in cell-to-cell junctions within each biofilm. Importantly, all Scl1-negative mutants examined showed significantly decreased ability to form biofilm in vitro. Furthermore, the Scl1 protein expressed on the surface of a heterologous host, Lactococcus lactis, was sufficient to induce biofilm formation by this organism.

Conclusions

Overall, this work (i) identifies variations in biofilm formation capacity among pathogenically different GAS strains, (ii) identifies GAS surface properties that may aid in biofilm stability and, (iii) establishes that the Scl1 surface protein is an important determinant of GAS biofilm, which is sufficient to enable biofilm formation in the heterologous host Lactococcus. In summary, the GAS surface adhesin Scl1 may have an important role in biofilm-associated pathogenicity.

Entry of Bacillus anthracis spores into epithelial cells is mediated by the spore surface protein BclA, integrin α2β1 and complement component C1q

Inhalational anthrax is initiated by pulmonary exposure to Bacillus anthracis spores. Spore entry into lung epithelial cells is observed both in vitro and in vivo and evidence suggests it is important for bacterial dissemination and virulence. However the specific host receptor and spore factor that mediate the entry process were unknown. Here, we report that integrin α2β1 is a major receptor for spore entry. This is supported by results from blocking antibodies, siRNA knock-down, colocalization, and comparison of spore entry into cells that do or do not express α2. BclA, a major spore surface protein, is found to be essential for entry and α2β1-mediated entry is dependent on BclA. However, BclA does not appear to bind directly to α2. Furthermore, spore entry into α2-expressing cells is dramatically reduced in the absence of serum, suggesting that additional factors are involved. Finally, complement component C1q, also an α2β1 ligand, appears to act as a bridging molecule or a cofactor for BclA/α2β1-mediated spore entry and BclA binds to C1q in a dose-dependent and saturable manner. These findings suggest a novel mechanism for pathogen entry into host cells as well as a new function for C1q-integrin interactions. The implications of these findings are discussed.

Streptococcal collagen-like surface protein 1 promotes adhesion to the respiratory epithelial cell

Background

Collagen-like surface proteins Scl1 and Scl2 on Streptococcus pyogenes contain contiguous Gly-X-X triplet amino acid motifs, the characteristic structure of human collagen. Although the potential role of Scl1 in adhesion has been studied, the conclusions may be affected by the use of different S. pyogenes strains and their carriages of various adhesins. To explore the bona fide nature of Scl1 in adherence to human epithelial cells without the potential interference of other streptococcal surface factors, we constructed a scl1 isogenic mutant from the Scl2-defective S. pyogenes strain and a Scl1-expressed Escherichia coli.

Results

Loss of Scl1 in a Scl2-defective S. pyogenes strain dramatically decreased the adhesion of bacteria to HEp-2 human epithelial cells. Expression of Scl1 on the surface of the heterologous bacteria E. coli significantly increased adhesion to HEp-2. The increase in adhesion was nullified when Scl1-expressed E. coli was pre-incubated with proteases or antibodies against recombinant Scl1 (rScl1) protein. Treatment of HEp-2 cells with rScl protein or pronase drastically reduced the binding capability of Scl1-expressed E. coli. These findings suggest that the adhesion is mediated through Scl1 on bacterial surface and protein receptor(s) on epithelial cells. Further blocking of potential integrins revealed significant contributions of α2 and β1 integrins in Scl1-mediated binding to epithelial cells.

Conclusions

Together, these results underscore the importance of Scl1 in the virulence of S. pyogenes and implicate Scl1 as an adhesin during pathogenesis of streptococcal infection.

An engineered alpha1 integrin-binding collagenous sequence

Collagen is an extracellular matrix structural component that can regulate cellular processes through its interaction with the integrins, α1β1, α2β1, α10β1, and α11β1. Collagen-like proteins have been identified in a number of bacterial species. Here, we used Scl2 from Streptococcus pyogenes serotype M28 strain MGAS6274 as a backbone for the introduction of discrete integrin-binding sequences. The introduced sequences GLPGER, GFPGER, or GFPGEN did not affect triple helix stability of the Scl (Streptococcal collagen-like) protein. Using ELISA and surface plasmon resonance, we determined that Scl2(GLPGER) and Scl2(GFPGER) bound to recombinant human α1 and α2 I-domains in a metal ion-dependent manner and without a requirement for hydroxyproline. We predicted a novel and selective integrin-binding sequence, GFPGEN, through the use of computer modeling and demonstrated that Scl2(GFPGEN) shows specificity toward the α1 I-domain and does not bind the α2 I-domain. Using C2C12 cells, we determined that intact integrins interact with the modified Scl2 proteins with the same selectivity as recombinant I-domains. These modified Scl2 proteins also acted as cell attachment substrates for fibroblast, endothelial, and smooth muscle cells. However, the modified Scl2 proteins were unable to aggregate platelets. These results indicate that Scl2 is a suitable backbone for the introduction of mammalian integrin-binding sequences, and these sequences may be manipulated to individually target α1β1 and α2β1.

Bioactive hydrogels based on Designer Collagens

Designer Collagens are based on streptococcal collagen-like (Scl) proteins that form a triple helix similar to mammalian collagens but that are non-platelet aggregating. In contrast to the numerous cell-binding sites on collagen, Scl2 from Streptococcus pyogenes serotype M28 does not contain any known cell-binding sites and thus provides a blank slate in terms of cellular interactions. In the current study, Scl2 protein was modified to include receptor binding motifs that interact with alpha1 and/or alpha2 integrin subunits. The modfied Scl2 proteins have been demonstrated to mediate differential endothelial cell (EC) and smooth muscle cell (SMC) adhesion via these integrins and to retain the non-platelet aggregating properties of the "parent" Scl2. Thromboresistant scaffolds which selectively bind ECs vs. SMCs would be desirable for vascular repair or replacement. Despite the potential of these Scl proteins in vascular applications, the utility of this recombinant protein family is currently limited to coatings due to the inability of Scl proteins to assemble into stable three-dimensional networks. To address this limitation, the Scl2 proteins were functionalized with photocrosslinking sites to enable incorporation into a hydrogel matrix. Characterization studies confirmed that the functionalization of the Scl2 proteins did not disrupt triple helix conformation, integrin binding or cell adhesion. Bioactive hydrogels were fabricated by combining the functionalized Scl2 proteins with poly(ethylene glycol) diacrylate (PEGDA) and photocrosslinking. EC and SMC adhesion studies confirmed cell-specific adhesion due to selective integrin binding to the two receptor binding motifs investigated. These results serve to highlight the potential of this novel biomaterial platform in the development of improved tissue engineered vascular grafts.

Binding of the human complement regulators CFHR1 and factor H by streptococcal collagen-like protein 1 (Scl1) via their conserved C termini allows control of the complement cascade at multiple levels

Group A streptococci (GAS) utilize soluble human complement regulators to evade host complement attack. Here, we characterized the binding of the terminal complement complex inhibitor complement Factor H-related protein 1 (CFHR1) and of the C3 convertase regulator Factor H to the streptococcal collagen-like proteins (Scl). CFHR1 and Factor H, but no other member of the Factor H protein family (CFHR2, CFHR3, or CFHR4A), bound to the two streptococcal proteins Scl1.6 and Scl1.55, which are expressed by GAS serotypes M6 and M55. The two human regulators bound to the Scl1 proteins via their conserved C-terminal attachment region, i.e. CFHR1 short consensus repeats 3-5 (SCR3-5) and Factor H SCR18-20. Binding was affected by ionic strength and by heparin. CFHR1 and the C-terminal attachment region of Factor H did not bind to Scl1.1 and Scl2.28 proteins but did bind to intact M1-type and M28-type GAS, which express Scl1.1 and Scl2.28, respectively, thus arguing for the presence of an additional binding mechanism to CFHR1 and Factor H. Furthermore mutations within the C-terminal heparin-binding region and Factor H mutations that are associated with the acute renal disease atypical hemolytic uremic syndrome blocked the interaction with the two streptococcal proteins. Binding of CFHR1 affected the complement regulatory functions of Factor H on the level of the C3 convertase. Apparently, streptococci utilize two types of complement regulator-acquiring surface proteins; type A proteins, as represented by Scl1.6 and Scl1.55, bind to CFHR1 and Factor H via their conserved C-terminal region and do not bind the Factor H-like protein 1 (FHL-1). On the contrary, type B proteins, represented by M-, M-like, and the fibronectin-binding protein Fba proteins, bind Factor H and FHL-1 via domain SCR7 and do not bind CFHR1. In conclusion, binding of CFHR1 is at the expense of Factor H-mediated regulatory function at the level of C3 convertase and at the gain of a regulator that controls complement at the level of the C5 convertase and formation of the terminal complement complex.

Polymeric immunoglobulin receptor-mediated invasion of Streptococcus pneumoniae into host cells requires a coordinate signaling of SRC family of protein-tyrosine kinases, ERK, and c-Jun N-terminal kinase

Streptococcus pneumoniae are commensals of the human nasopharynx with the capacity to invade mucosal respiratory cells. PspC, a pneumococcal surface protein, interacts with the human polymeric immunoglobulin receptor (pIgR) to promote bacterial adherence to and invasion into epithelial cells. Internalization of pneumococci requires the coordinated action of actin cytoskeleton rearrangements and the retrograde machinery of pIgR. Here, we demonstrate the involvement of Src protein-tyrosine kinases (PTKs), focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) but not p38 mitogen-activated protein kinases (MAPK) in pneumococcal invasion via pIgR. Pharmacological inhibitors of PTKs and MAPKs and genetic interference with Src PTK and FAK functions caused a significant reduction of pIgR-mediated pneumococcal invasion but did not influence bacterial adhesion to host cells. Furthermore, pneumococcal ingestion by host cells induces activation of ERK1/2 and JNK. In agreement with activated JNK, its target molecule and DNA-binding protein c-Jun was phosphorylated. We also show that functionally active Src PTK is essential for activation of ERK1/2 upon pneumococcal infections. In conclusion, these data illustrate the importance of a coordinated signaling between Src PTKs, ERK1/2, and JNK during PspC-pIgR-mediated uptake of pneumococci by host epithelial cells.

Noncollagenous region of the streptococcal collagen-like protein is a trimerization domain that supports refolding of adjacent homologous and heterologous collagenous domains

Proper folding of the (Gly-Xaa-Yaa)(n) sequence of animal collagens requires adjacent N- or C-terminal noncollagenous trimerization domains which often contain coiled-coil or beta sheet structure. Collagen-like proteins have been found recently in a number of bacteria, but little is known about their folding mechanism. The Scl2 collagen-like protein from Streptococcus pyogenes has an N-terminal globular domain, designated V(sp), adjacent to its triple-helix domain. The V(sp) domain is required for proper refolding of the Scl2 protein in vitro. Here, recombinant V(sp) domain alone is shown to form trimers with a significant alpha-helix content and to have a thermal stability of T(m) = 45 degrees C. Examination of a new construct shows that the V(sp) domain facilitates efficient in vitro refolding only when it is located N-terminal to the triple-helix domain but not when C-terminal to the triple-helix domain. Fusion of the V(sp) domain N-terminal to a heterologous (Gly-Xaa-Yaa)(n) sequence from Clostridium perfringens led to correct folding and refolding of this triple-helix, which was unable to fold into a triple-helical, soluble protein on its own. These results suggest that placement of a functional trimerization module adjacent to a heterologous Gly-Xaa-Yaa repeating sequence can lead to proper folding in some cases but also shows specificity in the relative location of the trimerization and triple-helix domains. This information about their modular nature can be used in the production of novel types of bacterial collagen for biomaterial applications.

The Scl1 of M41-type group A Streptococcus binds the high-density lipoprotein

Streptococcal collagen-like protein 1 (Scl1) is a virulence factor on the surface of group A Streptococcus (GAS). We have reported previously that several Scl1 proteins derived from various M-type GAS strains, including M41, can bind to low-density lipoprotein, but the Scl1 protein derived from the M6-type GAS strain cannot. Here, we demonstrated that recombinant protein, designated C176, derived from Scl1.41 of the GAS M41-type strain also binds both plasma and purified high-density lipoprotein (HDL). Next, we determined that the intact noncollagenous region of C176 was necessary and sufficient for HDL binding. C176-HDL interaction could be eliminated by the presence of low concentrations of the nonionic detergent, Tween 20, indicating the hydrophobic nature of this interaction. We finally showed that whole GAS cells expressing native Scl1.41 protein absorbed HDL from human plasma in the absence of Tween 20, but M6-type GAS cells did not. Altogether, our results add further evidence to the importance of GAS-lipoprotein binding.

Requirement of alpha(4)beta(1) and alpha(5)beta(1) integrin expression in bone-marrow-derived progenitor cells in preventing endotoxin-induced lung vascular injury and edema in mice

The goal of this study was to determine the role of integrin-mediated adhesion of bone-marrow-derived progenitor cells (BMPCs) as a requirement for the endothelial barrier protection in a lung injury model. C57BL mice were used as the source for BMPCs, which were characterized as CD34(+) and fetal liver kinase-1 (Flk1)(+) and also an expression of a repertoire of integrins. We used a mouse model of bacterial lipopolysaccharide (LPS)-induced lung vascular injury and edema formation to test the effects of BMPC integrin expression in preventing endothelial barrier injury. Adhesion of BMPCs to purified extracellular matrix proteins induced focal adhesion kinase (Fak) phosphorylation and formation of branching point structures in a alpha(4) and alpha(5) integrin-dependent manner. BMPCs expressing red fluorescent protein (RFP) were administered via the retro-orbital venous route in mice treated intraperitonially with LPS (7.5 mg/kg body weight). We observed increased retention of RFP-labeled Flk1(+) and CD34(+) BMPCs for up to 8 weeks in mice injured with LPS. BMPC transplantation increased survival by 50% (at 72-96 hours after LPS) and reduced lung vascular injury and extravascular water content induced by LPS. However, blocking with anti-alpha(4) or anti-alpha(5) integrin antibody or shRNA-mediated silencing of alpha(4) or alpha(5) integrins in donor BMPCs failed to prevent the vascular injury or edema formation and mortality. Thus, alpha(4) and alpha(5) integrin-dependent adhesion of BMPCs in lung tissue plays a critical role in preventing lung vascular injury and increasing survival in a mouse model of LPS-induced acute lung injury.

Purification by Strep-Tactin affinity chromatography of a delete envelope gp51 protein of Bovine Leukaemia virus expressed in Sf21 insect cells

Bovine leukaemia virus (BLV) causes disease in cattle and it is related to human T lymphotrofic viruses HTLV-1 and HTLV-2. The objective of this study was to express and purify deleted and stable forms of the gp51 envelope glycoprotein of BLV using a baculovirus system. Two forms of the gp51 were synthesised: one comprised the gp51 N-terminal 174 amino acids and a single 6xHis tag (Delta(175-268)gp51-His) and the second form contained the same amino acid sequence and a C-terminal Strep-tag II in addition to the 6xHis tag (Delta(175-268)gp51-STH). The two proteins were expressed and purified by immobilized metal-affinity chromatography (IMAC) or by Strep-Tactin column. The Strep-Tactin technology was more efficient than IMAC method and achieved a high pure recombinant deleted gp51. In addition the Delta(175-268)gp51-STH protein was further concentrated by IMAC. This purified antigen could be used for the isolation of immunoreactive molecules and to develop a competitive ELISA test.

A Streptococcus pyogenes derived collagen-like protein as a non-cytotoxic and non-immunogenic cross-linkable biomaterial

A range of bacteria have been shown to contain collagen-like sequences that form triple-helical structures. Some of these proteins have been shown to form triple-helical motifs that are stable around body temperature without the inclusion of hydroxyproline or other secondary modifications to the protein sequence. This makes these collagen-like proteins particularly suitable for recombinant production as only a single gene product and no additional enzyme needs to be expressed. In the present study, we have examined the cytotoxicity and immunogenicity of the collagen-like domain from Streptococcus pyogenes Scl2 protein. These data show that the purified, recombinant collagen-like protein is not cytotoxic to fibroblasts and does not elicit an immune response in SJL/J and Arc mice. The freeze dried protein can be stabilised by glutaraldehyde cross-linking giving a material that is stable at >37 degrees C and which supports cell attachment while not causing loss of viability. These data suggest that bacterial collagen-like proteins, which can be modified to include specific functional domains, could be a useful material for medical applications and as a scaffold for tissue engineering.

Scl1, the multifunctional adhesin of group A Streptococcus, selectively binds cellular fibronectin and laminin, and mediates pathogen internalization by human cells

The streptococcal collagen-like protein-1, Scl1, is widely expressed by the well-recognized human pathogen group A Streptococcus (GAS). Screening of human ligands for binding to recombinant Scl1 identified cellular fibronectin and laminin as binding partners. Both ligands interacted with the globular domain of Scl1, which is also able to bind the low-density lipoprotein. Native Scl1 mediated GAS adherence to ligand-coated glass cover slips and promoted GAS internalization into HEp-2 cells. This work identifies new ligands of the Scl1 protein that are known to be important in GAS pathogenesis and suggests a novel ligand-switching mechanism between blood and tissue environments, thereby facilitating host colonization and GAS dissemination.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Self-association of streptococcus pyogenes collagen-like constructs into higher order structures

A number of bacterial collagen-like proteins with Gly as every third residue and a high Pro content have been observed to form stable triple-helical structures despite the absence of hydroxyproline (Hyp). Here, the high yield cold-shock expression system is used to obtain purified recombinant collagen-like protein (V-CL) from Streptococcus pyogenes containing an N-terminal globular domain V followed by the collagen triple-helix domain CL and the modified construct with two tandem collagen domains V-CL-CL. Both constructs and their isolated collagenous domains form stable triple-helices characterized by very sharp thermal transitions at 35-37 degrees C and by high values of calorimetric enthalpy. Procedures for the formation of collagen SLS crystallites lead to parallel arrays of in register V-CL-CL molecules, as well as centrosymmetric arrays of dimers joined at their globular domains. At neutral pH and high concentrations, the bacterial constructs all show a tendency towards aggregation. The isolated collagen domains, CL and CL-CL, form units of diameter 4-5 nm which bundle together and twist to make larger fibrillar structures. Thus, although this S. pyogenes collagen-like protein is a cell surface protein with no indication of participation in higher order structure, the triple-helix domain has the potential of forming fibrillar structures even in the absence of hydroxyproline. The formation of fibrils suggests bacterial collagen proteins may be useful for biomaterials and tissue engineering applications.

Identification of active variants of PARF in human pathogenic group C and group G streptococci leads to an amended description of its consensus motif

Certain streptococcal M proteins bind collagen via an octapeptide motif that is located in their hypervariable N-terminal region. The interaction with this extracellular matrix protein enhances adhesion to the host tissue and thereby facilitates infection. Moreover, it has the side effect of eliciting collagen autoimmune responses, a phenomenon which is also observed in patients with acute rheumatic fever. Therefore, the octapeptide motif was named peptide associated with rheumatic fever (PARF). Only a comprehensive characterization of the collagen-binding M proteins and their collagen-binding motifs will allow the investigation of their associations with certain streptococcal infections and their sequelae. Therefore, a collection of Streptococcus dysgalactiae equisimilis strains that were isolated from infected humans was examined, in order to identify collagen-binding proteins and motifs. Strains that bound collagen independent of a hyaluronic acid capsule belonged to 7 distinct types of the emm gene, which codes for the M protein (emm types). Only one of these emm types was previously described as collagen-binding. Five possessed a PARF sequence. The other 2 emm types stC2sk.0 and stG2574 had PARF-like motifs that diverged from the previously described consensus sequence AXYLZZLN but were able to induce collagen autoimmunity when injected into mice. The results led to the amended PARF consensus (A/E/T)XYLXXLN. Moreover, they demonstrate a predictive power regarding collagen-binding and elicitation of collagen autoimmunity, indicating that PARF may be one of the markers for strains that cause collagen-dependent acute rheumatic fever.

Identification of the first prokaryotic collagen sequence motif that mediates binding to human collagen receptors, integrins alpha2beta1 and alpha11beta1

Many pathogenic bacteria interact with human integrins to enter host cells and to augment host colonization. Group A Streptococcus (GAS) employs molecular mimicry by direct interactions between the cell surface streptococcal collagen-like protein-1 (Scl1) and the human collagen receptor, integrin alpha2beta1. The collagen-like (CL) region of the Scl1 protein mediates integrin-binding, although, the integrin binding motif was not defined. Here, we used molecular cloning and site-directed mutagenesis to identify the GLPGER sequence as the alpha2beta1 and the alpha11beta1 binding motif. Electron microscopy experiments mapped binding sites of the recombinant alpha2-integrin-inserted domain to the GLPGER motif of the recombinant Scl (rScl) protein. rScl proteins and a synthetic peptide harboring the GLPGER motif mediated the attachment of C2C12-alpha2+myoblasts expressing the alpha2beta1 integrin as the sole collagen receptor. The C2C12-alpha11+myoblasts expressing the alpha11beta1 integrin also attached to GLPGER-harboring rScl proteins. Furthermore, the C2C12-alpha11+cells attached to rScl1 more efficiently than C2C12-alpha2+cells, suggesting that the alpha11beta1 integrin may have a higher binding affinity for the GLPGER sequence. Human endothelial cells and dermal fibroblasts adhered to rScl proteins, indicating that multiple cell types may recognize and bind the Scl proteins via their collagen receptors. This work is a stepping stone toward defining the utilization of collagen receptors by microbial collagen-like proteins that are expressed by pathogenic bacteria.

Trimeric autotransporters of Haemophilus parasuis: generation of an extensive passenger domain repertoire specific for pathogenic strains

Haemophilus parasuis is the agent responsible for causing Glässer's disease, but little is known about the pathogenic determinants of this major pig disease. Here we describe, for the pathogenic strain Nagasaki, the molecular characterization of 13 trimeric autotransporters as assessed by the presence of YadA C-terminal translocator domains which were classified into three groups. All passenger domains possess motifs and repeats characteristic of adhesins, hemagglutinins, and invasins with various centrally located copies of collagen-like repeats. This domain architecture is shared with two trimeric autotransporter proteins of H. somnus 129Pt. Genomic comparison by microarray hybridization demonstrated homologies among H. parasuis virulent strains and high divergence with respect to nonvirulent strains. Therefore, these genes were named vtaA (virulence-associated trimeric autotransporters). The sequencing of 17 homologous vtaA genes of different invasive strains highlighted an extensive mosaic structure. Based also on the presence of DNA uptake signal sequences within the vtaA genes, we propose a mechanism of evolution by which gene duplication and the accumulation of mutations and recombinations, plus the lateral gene transfer of the passenger domain, led to the diversity of this multigene family. This study provides insights to help understand the tissue colonization and invasiveness characteristic of H. parasuis pathogenic strains.

Genome sequence of a Lancefield group C Streptococcus zooepidemicus strain causing epidemic nephritis: new information about an old disease

Outbreaks of disease attributable to human error or natural causes can provide unique opportunities to gain new information about host-pathogen interactions and new leads for pathogenesis research. Poststreptococcal glomerulonephritis (PSGN), a sequela of infection with pathogenic streptococci, is a common cause of preventable kidney disease worldwide. Although PSGN usually occurs after infection with group A streptococci, organisms of Lancefield group C and G also can be responsible. Despite decades of study, the molecular pathogenesis of PSGN is poorly understood. As a first step toward gaining new information about PSGN pathogenesis, we sequenced the genome of Streptococcus equi subsp. zooepidemicus strain MGCS10565, a group C organism that caused a very large and unusually severe epidemic of nephritis in Brazil. The genome is a circular chromosome of 2,024,171 bp. The genome shares extensive gene content, including many virulence factors, with genetically related group A streptococci, but unexpectedly lacks prophages. The genome contains many apparently foreign genes interspersed around the chromosome, consistent with the presence of a full array of genes required for natural competence. An inordinately large family of genes encodes secreted extracellular collagen-like proteins with multiple integrin-binding motifs. The absence of a gene related to speB rules out the long-held belief that streptococcal pyrogenic exotoxin B or antibodies reacting with it singularly cause PSGN. Many proteins previously implicated in GAS PSGN, such as streptokinase, are either highly divergent in strain MGCS10565 or are not more closely related between these species than to orthologs present in other streptococci that do not commonly cause PSGN. Our analysis provides a comparative genomics framework for renewed appraisal of molecular events underlying APSGN pathogenesis.

Mechanism of Stabilization of a Bacterial Collagen Triple Helix in the Absence of Hydroxyproline

The Streptococcus pyogenes cell-surface protein Scl2 contains a globular N-terminal domain and a collagen-like domain, (Gly-Xaa-X'aa)(79), which forms a triple helix with a thermal stability close to that seen for mammalian collagens. Hyp is a major contributor to triple-helix stability in animal collagens, but is not present in bacteria, which lack prolyl hydroxylase. To explore the basis of bacterial collagen triple-helix stability in the absence of Hyp, biophysical studies were carried out on recombinant Scl2 protein, the isolated collagen-like domain from Scl2, and a set of peptides modeling the Scl2 highly charged repetitive (Gly-Xaa-X'aa)(n) sequences. At pH 7, CD spectroscopy, dynamic light scattering, and differential scanning calorimetry of the Scl2 protein all showed a very sharp thermal transition near 36 degrees C, indicating a highly cooperative unfolding of both the globular and triple-helix domains. The collagen-like domain isolated by trypsin digestion showed a sharp transition at the same temperature, with an enthalpy of 12.5 kJ/mol of tripeptide. At low pH, Scl2 and its isolated collagen-like domain showed substantial destabilization from the neutral pH value, with two thermal transitions at 24 and 27 degrees C. A similar destabilization at low pH was seen for Scl2 charged model peptides, and the degree of destabilization was consistent with the strong pH dependence arising from the GKD tripeptide unit. The Scl2 protein contained twice as much charge as human fibril-forming collagens, and the degree of electrostatic stabilization observed for Scl2 was similar to the contribution Hyp makes to the stability of mammalian collagens. The high enthalpic contribution to the stability of the Scl2 collagenous domain supports the presence of a hydration network in the absence of Hyp.

Thrombin-activatable Fibrinolysis Inhibitor Binds to Streptococcus pyogenes by Interacting with Collagen-like Proteins A and B

Regulation of proteolysis is a critical element of the host immune system and plays an important role in the induction of pro- and anti-inflammatory reactions in response to infection. Some bacterial species take advantage of these processes and recruit host proteinases to their surface in order to counteract the host attack. Here we show that Thrombin-activatable Fibrinolysis Inhibitor (TAFI), a zinc-dependent procarboxypeptidase, binds to the surface of group A streptococci of an M41 serotype. The interaction is mediated by the streptococcal collagen-like surface proteins A and B (SclA and SclB), and the streptococcal-associated TAFI is then processed at the bacterial surface via plasmin and thrombin-thrombomodulin. These findings suggest an important role for TAFI in the modulation of host responses by streptococci.

Scl1-dependent internalization of group A Streptococcus via direct interactions with the alpha2beta(1) integrin enhances pathogen survival and re-emergence

The molecular pathogenesis of infections caused by group A Streptococcus (GAS) is not fully understood. We recently reported that a recombinant protein derived from the collagen-like surface protein, Scl1, bound to the human collagen receptor, integrin alpha(2)beta(1). Here, we investigate whether the same Scl1 variant expressed by GAS cells interacts with the integrin alpha2beta(1) and affects the biological outcome of host-pathogen interactions. We demonstrate that GAS adherence and internalization involve direct interactions between surface expressed Scl1 and the alpha2beta(1) integrin, because (i) both adherence and internalization of the scl1-inactivated mutant were significantly decreased, and were restored by in-trans complementation of Scl1 expression, (ii) GAS internalization was reduced by pre-treatment of HEp-2 cells with anti-alpha2 integrin-subunit antibody and type I collagen, (iii) recombinant alpha2-I domain bound the wild-type GAS cells and (iv) internalization of wild-type cells was significantly increased in C2C12 cells expressing the alpha2beta(1) integrin as the only collagen-binding integrin. Next, we determined that internalized GAS re-emerges from epithelial cells into the extracellular environment. Taken together, our data describe a new molecular mechanism used by GAS involving the direct interaction between Scl1 and integrins, which increases the overall capability of the pathogen to survive and re-emerge.

Proteomic analysis and identification of Streptococcus pyogenes surface-associated proteins

Streptococcus pyogenes is a gram-positive human pathogen that causes a wide spectrum of disease, placing a significant burden on public health. Bacterial surface-associated proteins play crucial roles in host-pathogen interactions and pathogenesis and are important targets for the immune system. The identification of these proteins for vaccine development is an important goal of bacterial proteomics. Here we describe a method of proteolytic digestion of surface-exposed proteins to identify surface antigens of S. pyogenes. Peptides generated by trypsin digestion were analyzed by multidimensional tandem mass spectrometry. This approach allowed the identification of 79 proteins on the bacterial surface, including 14 proteins containing cell wall-anchoring motifs, 12 lipoproteins, 9 secreted proteins, 22 membrane-associated proteins, 1 bacteriophage-associated protein, and 21 proteins commonly identified as cytoplasmic. Thirty-three of these proteins have not been previously identified as cell surface associated in S. pyogenes. Several proteins were expressed in Escherichia coli, and the purified proteins were used to generate specific mouse antisera for use in a whole-cell enzyme-linked immunosorbent assay. The immunoreactivity of specific antisera to some of these antigens confirmed their surface localization. The data reported here will provide guidance in the development of a novel vaccine to prevent infections caused by S. pyogenes.

Phylogenetic analysis of the tenascin gene family: evidence of origin early in the chordate lineage

BACKGROUND

Tenascins are a family of glycoproteins found primarily in the extracellular matrix of embryos where they help to regulate cell proliferation, adhesion and migration. In order to learn more about their origins and relationships to each other, as well as to clarify the nomenclature used to describe them, the tenascin genes of the urochordate Ciona intestinalis, the pufferfish Tetraodon nigroviridis and Takifugu rubripes and the frog Xenopus tropicalis were identified and their gene organization and predicted protein products compared with the previously characterized tenascins of amniotes.

RESULTS

A single tenascin gene was identified in the genome of C. intestinalis that encodes a polypeptide with domain features common to all vertebrate tenascins. Both pufferfish genomes encode five tenascin genes: two tenascin-C paralogs, a tenascin-R with domain organization identical to mammalian and avian tenascin-R, a small tenascin-X with previously undescribed GK repeats, and a tenascin-W. Four tenascin genes corresponding to tenascin-C, tenascin-R, tenascin-X and tenascin-W were also identified in the X. tropicalis genome. Multiple sequence alignment reveals that differences in the size of tenascin-W from various vertebrate classes can be explained by duplications of specific fibronectin type III domains. The duplicated domains are encoded on single exons and contain putative integrin-binding motifs. A phylogenetic tree based on the predicted amino acid sequences of the fibrinogen-related domains demonstrates that tenascin-C and tenascin-R are the most closely related vertebrate tenascins, with the most conserved repeat and domain organization. Taking all lines of evidence together, the data show that the tenascins referred to as tenascin-Y and tenascin-N are actually members of the tenascin-X and tenascin-W gene families, respectively.

CONCLUSION

The presence of a tenascin gene in urochordates but not other invertebrate phyla suggests that tenascins may be specific to chordates. Later genomic duplication events led to the appearance of four family members in vertebrates: tenascin-C, tenascin-R, tenascin-W and tenascin-X.

Binding of the low-density lipoprotein by streptococcal collagen-like protein Scl1 of Streptococcus pyogenes

Several bacterial genera express proteins that contain collagen-like regions, which are associated with variable (V) non-collagenous regions. The streptococcal collagen-like proteins, Scl1 and Scl2, of group A Streptococcus (GAS) are members of this 'prokaryotic collagen' family, and they too contain an amino-terminal non-collagenous V region of unknown function. Here, we use recombinant rScl constructs, derived from several Scl1 and Scl2 variants, and affinity chromatography to identify Scl ligands present in human plasma. First, we show that Scl1, but not Scl2, proteins from different GAS serotypes bind the same ligand identified as apolipoprotein B (ApoB100), which is a major component of the low-density lipoprotein (LDL). Scl1 binding to purified ApoB100 and LDL is specific and concentration-dependent. Furthermore, the non-collagenous V region of the Scl1 protein is responsible for LDL/ApoB100 binding because only those rScls, constructed by domain swapping, which contain the V region from Scl1 proteins, were able to bind to ApoB100 and LDL ligands, and this binding was inhibited by antibodies directed against the Scl1-V region. Electron microscopy images of Scl1-LDL complexes showed that the globular V domain of Scl1 interacted with spherical particles of LDL. Importantly, live M28-type GAS cells absorbed plasma LDL on the cell surface and this binding depended on the surface expression of the Scl1.28, but not Scl2.28, protein. Phylogenetic analysis showed that the non-collagenous globular domains of Scl1 and Scl2 evolved independently to form separate lineages, which differ in amino acid sequence, and these differences may account for the variations in binding patterns of Scl1 and Scl2 proteins. Present studies provide insight into the structure-function relationship of the Scl proteins and also underline the importance of lipoprotein binding by GAS.

Assessment of prokaryotic collagen-like sequences derived from streptococcal Scl1 and Scl2 proteins as a source of recombinant GXY polymers

Collagen triple helix, composed of the repeating Gly-Xaa-Yaa (GXY) sequence, is a structural element found in all multicellular animals and also in some prokaryotes. Long GXY polymers are highly regarded components used in food, cosmetic, biomedical, and pharmaceutical industries. In this study, we explore a new concept for the production of recombinant GXY polymers which are based on the sequence of "prokaryotic collagens", the streptococcal collagen-like proteins Scl1 and Scl2. Analysis of 50 Scl variants identified the amino acid distribution and GXY-repeat usage that are involved in the stabilization of the triple helix in Scls. Using circular dichroism spectroscopy and electron microscopy, we show that significantly different recombinant rScl polypeptides form stable, unhydroxylated homotrimeric triple helices that can be produced both intra- and extracellularly in the Escherichia coli. These rScl constructs containing 20 to 129 GXY repeats had mid-point melting temperatures between 32 and 39 degrees C. Altogether, Scl-derived collagens, which are different from the mammalian collagens, can form stable triple helices under physiological conditions and can be used for the production of recombinant GXY polymers with a wide variety of potential applications.

A fibronectin-binding protein from Streptococcus equi binds collagen and modulates cell-mediated collagen gel contraction

The N-terminal fragment (FNZN) of the fibronectin-binding protein FNZ from Streptococcus equi subspecies zooepidemicus was investigated as to effects on murine cell interactions with extracellular matrix proteins. FNZN bound to immobilized fibronectin (FN) and native, but not denatured, collagen type I. FNZN had no effect on primary adhesion of cells from the murine myoblastic C2C12 cell line to immobilized fibronectin. C2C12 cells adhered to immobilized FNZN, a process that was not inhibited by anti-human FN IgG or by an inhibitor of integrin alphaVbeta3. C2C12 cells lack collagen-binding beta1 integrins and neither adhere to native collagen nor mediate contraction of three-dimensional collagen gels. FNZN stimulated collagen gel contraction by C2C12 cells but not adhesion of C2C12 cells to collagen. Experiments with an alphaVbeta3-inhibitor suggested that FNZN promoted contraction by a process requiring alphaVbeta3. Our data suggest that FNZN by binding to cells, collagen, and FN modulate complex adhesive processes mediated by the alphaVbeta3 integrin. Since alphaVbeta3-mediated contractile events function to counteract edema formation during inflammation, it is possible that FNZN and its secreted homologue FNE modulate edema responses in infected tissues.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.

Murine lipid phosphate phosphohydrolase-3 acts as a cell-associated integrin ligand

Lipid phosphate phosphohydrolase-3 (LPP3) is a cell surface protein that exhibits ectoenzyme activity. Previously, we identified human LPP3 in a functional assay of angiogenesis and showed that the Arg-Gly-Asp (RGD) motif in the proposed second extracellular domain interacts with a subset of integrins to mediate cell-cell adhesion. In contrast to the RGD domain of human LPP3, murine Lpp3 contains a variant sequence, Arg-Gly-Glu (RGE). Whether the RGE motif of murine Lpp3 mediates cell-cell interaction has not been studied. In this report, we test the hypothesis that the cell adhesion function of the LPP3 protein is conserved across mouse and human. A glutathione S-transferase (GST) fusion protein of the proposed second extracellular loop of the murine Lpp3 sequence (GST-mLpp3-RGE) promoted attachment of cells in a long-term cell adhesion assay. GST-mLpp3-RGE interacted with alpha(5)beta(1) and alpha(v)beta(3) integrins in a solid-phase ELISA, while a mutant control, GST-hLPP3-RAD, did not. Long-term adhesion of endothelial cells to GST-mLpp3-RGE induced phosphorylation of FAK, SHC, and CAS, whereas adhesion to GST-hLPP3-RAD failed to do so. Upon long-term adhesion both the GST-hLPP3-RGD and GST-mLpp3-RGE substrates bound to the alpha(5)beta(1) integrin of FRT-alpha(5)(+) cells, an interaction that was inhibited by an anti-alpha(5) integrin antibody. In addition, a cell aggregation assay showed that the intact mLpp3-RGE protein interacts with alpha(5)beta(1) and alpha(v)beta(3) integrins expressed by adjacent cells, an interaction that can be blocked by GRGDSP peptides and anti-LPP3-RGD antibodies. These data, together with the known importance of integrins in angiogenesis, provide a mechanism for the function of LPP3 in cell-cell interactions in both human and mouse.

Anti-lipid phosphate phosphohydrolase-3 (LPP3) antibody inhibits bFGF- and VEGF-induced capillary morphogenesis of endothelial cells

Background

Angiogenesis, or the remodeling of existing vasculature serves as a lifeline to nourish developing embryos and starved tissues, and to accelerate wound healing, diabetic retinopathy, and tumor progression. Recent studies indicate that angiogenesis requires growth factor activity as well as cell adhesion events mediated by α₅β₁ and α_vβ₃ integrins. We previously demonstrated that human lipid phosphate phosphohydrolase-3 (LPP3) acts as a cell-associated ligand for α₅β₁ and α_vβ₃ integrins. Here, we test the hypothesis that an anti-LPP3 antibody can inhibit basic fibroblast growth factor (bFGF)-and vascular endothelial growth factor (VEGF)-induced capillary morphogenesis of endothelial cells (ECs).

Results

We report that bFGF and VEGF up-regulate LPP3 protein expression in ECs. Immunoprecipitation analyses show that LPP3 is a cell surface protein and undergoes N-glycosylation. Fluorescent activated cell sorting (FACS) data suggest that anti-LPP3-RGD detects native neoepitope on the surface of activated ECs. Moreover, we demonstrate LPP3 protein expression in tumor endothelium alongside VEGF. The embedding of ECs into three-dimensional type I collagen in the presence of bFGF and VEGF induce capillary formation. Importantly, we show that the addition of an anti-LPP3 antibody specifically and significantly blocks bFGF- and VEGF-induced capillary morphogenesis of ECs.

Conclusion

These data suggest that activated ECs as well as tumor endothelium express LPP3 protein. In an in vitro assay, the anti-LPP3-RGD specifically blocks bFGF and VEGF induced capillary morphogenesis of ECs. Our results, therefore, suggest a role for LPP3 in angiogenesis.

A novel binding site in collagen type III for integrins alpha1beta1 and alpha2beta1

Previously identified high affinity integrin-binding motifs in collagens, GFOGER and GLOGER, are not present in type III collagen. Here, we first characterized the binding of recombinant I domains from integrins alpha(1) and alpha(2) (alpha(1)I and alpha(2)I) to fibrillar collagen types I-III and showed that each I domain bound to the three types of collagens with similar affinities. Using rotary shadowing followed by electron microscopy, we identified a high affinity binding region in human type III collagen recognized by alpha(1)I and alpha(2)I. Examination of the region revealed the presence of two sequences that contain the critical GER motif, GROGER and GAOGER. Collagen-like peptides containing these two motifs were synthesized, and their triple helical nature was confirmed by circular dichroism spectroscopy. Experiments show that the GROGER-containing peptide was able to bind both alpha(1)I and alpha(2)I with high affinity and effectively inhibit the binding of alpha(1)I and alpha(2)I to type III and I collagens, whereas the GAOGER-containing peptide was considerably less effective. Furthermore, the GROGER-containing peptide supported adhesion of human lung fibroblast cells when coated on a culture dish. Thus, we have identified a novel high affinity binding sequence for the collagen-binding integrin I domains.