Interactions of the streptococcal C5a peptidase with human fibronectin

Molecular-level studies of extracellular matrix proteins conducted using atomic force microscopy

Extracellular matrix (ECM) proteins provide anchorage and structural strength to cells and tissues in the body and, thus, are fundamental molecular components for processes of cell proliferation, growth, and function. Atomic force microscopy (AFM) has increasingly become a valuable approach for studying biological molecules such as ECM proteins at the level of individual molecules. Operational modes of AFM can be used to acquire the measurements of the physical, electronic, and mechanical properties of samples, as well as for viewing the intricate details of the surface chemistry of samples. Investigations of the morphology and properties of biomolecules at the nanoscale can be useful for understanding the interactions between ECM proteins and biological molecules such as cells, DNA, and other proteins. Methods for preparing protein samples for AFM studies require only basic steps, such as the immersion of a substrate in a dilute solution or protein, or the deposition of liquid droplets of protein suspensions on a flat, clean surface. Protocols of nanolithography have been used to define the arrangement of proteins for AFM studies. Using AFM, mechanical and force measurements with tips that are coated with ECM proteins can be captured in ambient or aqueous environments. In this review, representative examples of AFM studies are described for molecular-level investigations of the structure, surface assembly, protein-cell interactions, and mechanical properties of ECM proteins (collagen, elastin, fibronectin, and laminin). Methods used for sample preparation as well as characterization with modes of AFM will be discussed.

Fibronectin and Its Role in Human Infective Diseases

Fibronectin is a multidomain glycoprotein ubiquitously detected in extracellular fluids and matrices of a variety of animal and human tissues where it functions as a key link between matrices and cells. Fibronectin has also emerged as the target for a large number of microorganisms, particularly bacteria. There are clear indications that the binding of microorganism’ receptors to fibronectin promotes attachment to and infection of host cells. Each bacterium may use different receptors which recognize specific fibronectin domains, mostly the N-terminal domain and the central cell-binding domain. In many cases, fibronectin receptors have actions over and above that of simple adhesion: In fact, adhesion is often the prerequisite for invasion and internalization of microorganisms in the cells of colonized tissues. This review updates the current understanding of fibronectin receptors of several microorganisms with emphasis on their biochemical and structural properties and the role they can play in the onset and progression of host infection diseases. Furthermore, we describe the antigenic profile and discuss the possibility of designing adhesion inhibitors based on the structure of the fibronectin-binding site in the receptor or the receptor-binding site in fibronectin.

Pilus-1 Backbone Protein RrgB of Streptococcus pneumoniae Binds Collagen I in a Force-Dependent Way

Attachment to host tissue is a prerequisite for successful host colonization and invasion of pathogens. Many pathogenic bacteria use surface appendices, called pili, to bind and firmly attach to host tissue surfaces. Although it has been speculated that the laterally positioned D3 domain of the pilus-1 backbone protein RrgB of Streptococcus pneumoniae may promote bacterial-host interaction, via adhesion to extracellular matrix molecules, such as collagen, earlier studies showed no affinity of RrgB to collagen I. Using atomic force microscopy-based single molecule force spectroscopy combined with lateral force microscopy, we show that under mechanical load, RrgB in fact binds to human collagen I in a force-dependent manner. We observe exceptionally strong interactions, with interaction forces reaching as much as 1500 pN, and we show that high force loading and shearing rates enhance and further strengthen the interaction. In addition, the affinity of RrgB to collagen I under mechanical load not only depends on the orientation of the D3 domain but also on the orientation of the collagen fibrils, relative to the pulling direction. Both exceptionally high binding forces and force-induced bond strengthening resemble the behavior of so-called catch bonds, which have recently been observed in bacterial adhesins, but have not been reported for multimeric backbone subunits of virulence related pili.

The Double Life of Group B Streptococcus: Asymptomatic Colonizer and Potent Pathogen

Group B streptococcus (GBS) is a β-hemolytic gram-positive bacterium that colonizes the lower genital tract of approximately 18% of women globally as an asymptomatic member of the gastrointestinal and/or vaginal flora. If established in other host niches, however, GBS is highly pathogenic. During pregnancy, ascending GBS infection from the vagina to the intrauterine space is associated with preterm birth, stillbirth, and fetal injury. In addition, vertical transmission of GBS during or after birth results in life-threatening neonatal infections, including pneumonia, sepsis, and meningitis. Although the mechanisms by which GBS traffics from the lower genital tract to vulnerable host niches are not well understood, recent advances have revealed that many of the same bacterial factors that promote asymptomatic vaginal carriage also facilitate dissemination and virulence. Furthermore, highly pathogenic GBS strains have acquired unique factors that enhance survival in invasive niches. Several host factors also exist that either subdue GBS upon vaginal colonization or alternatively permit invasive infection. This review summarizes the GBS and host factors involved in GBS's state as both an asymptomatic colonizer and an invasive pathogen. Gaining a better understanding of these mechanisms is key to overcoming the challenges associated with vaccine development and identification of novel strategies to mitigate GBS virulence.

The plasminogen binding protein PbsP is required for brain invasion by hypervirulent CC17 Group B streptococci

Streptococcus agalactiae (Group B Streptococcus or GBS) is a frequent cause of serious disease in newborns and adults. Epidemiological evidence indicates a strong association between GBS strains belonging to the hypervirulent CC17 clonal complex and the occurrence of meningitis in neonates. We investigate here the role of PbsP, a cell wall plasminogen binding protein, in colonization of the central nervous system by CC17 GBS. Deletion of pbsP selectively impaired the ability of the CC17 strain BM110 to colonize the mouse brain after intravenous challenge, despite its unchanged capacity to persist at high levels in the blood and to invade the kidneys. Moreover, immunization with a recombinant form of PbsP considerably reduced brain infection and lethality. In vitro, pbsP deletion markedly decreased plasmin-dependent transmigration of BM110 through brain microvascular endothelial cells. Although PbsP was modestly expressed in bacteria grown under standard laboratory conditions, pbsP expression was markedly upregulated during in vivo infection or upon contact with cultured brain endothelial cells. Collectively, our studies indicate that PbsP is a highly conserved Plg binding adhesin, which is functionally important for invasion of the central nervous system by the hypervirulent CC17 GBS. Moreover, this antigen is a promising candidate for inclusion in a universal GBS vaccine.

Stable Expression of Modified Green Fluorescent Protein in Group B Streptococci To Enable Visualization in Experimental Systems

Group B streptococcus (GBS) is a Gram-positive bacterium associated with various diseases in humans and animals. Many studies have examined GBS physiology, virulence, and microbe-host interactions using diverse imaging approaches, including fluorescence microscopy. Strategies to label and visualize GBS using fluorescence biomarkers have been limited to antibody-based methods or nonspecific stains that bind DNA or protein; an effective plasmid-based system to label GBS with a fluorescence biomarker would represent a useful visualization tool. In this study, we developed and validated a green fluorescent protein (GFP)-variant-expressing plasmid, pGU2664, which can be applied as a marker to visualize GBS in experimental studies. The synthetic constitutively active CP25 promoter drives strong and stable expression of the GFPmut3 biomarker in GBS strains carrying pGU2664. GBS maintains GFPmut3 activity at different phases of growth. The application of fluorescence polarization enables easy discrimination of GBS GFPmut3 activity from the autofluorescence of culture media commonly used to grow GBS. Differential interference contrast microscopy, in combination with epifluorescence microscopy to detect GFPmut3 in GBS, enabled visualization of bacterial attachment to live human epithelial cells in real time. Plasmid pGU2664 was also used to visualize phenotypic differences in the adherence of wild-type GBS and an isogenic gene-deficient mutant strain lacking CovR (the control of virulence regulator) in adhesion assays. The system for GFPmut3 expression in GBS described in this study provides a new tool for the visualization of this organism in diverse research applications. We discuss the advantages and consider the limitations of this fluorescent biomarker system developed for GBS.IMPORTANCE Group B streptococcus (GBS) is a bacterium associated with various diseases in humans and animals. This study describes the development of a strategy to label and visualize GBS using a fluorescence biomarker, termed GFPmut3. We show that this biomarker can be successfully applied to track the growth of bacteria in liquid medium, and it enables the detailed visualization of GBS in the context of live human cells in real-time microscopic analysis. The system for GFPmut3 expression in GBS described in this study provides a new tool for the visualization of this organism in diverse research applications.

Streptococcus agalactiae Non-Pilus, Cell Wall-Anchored Proteins: Involvement in Colonization and Pathogenesis and Potential as Vaccine Candidates

Group B Streptococcus (GBS) remains an important etiological agent of several infectious diseases including neonatal septicemia, pneumonia, meningitis, and orthopedic device infections. This pathogenicity is due to a variety of virulence factors expressed by Streptococcus agalactiae. Single virulence factors are not sufficient to provoke a streptococcal infection, which is instead promoted by the coordinated activity of several pathogenicity factors. Such determinants, mostly cell wall-associated and secreted proteins, include adhesins that mediate binding of the pathogen to host extracellular matrix/plasma ligands and cell surfaces, proteins that cooperate in the invasion of and survival within host cells and factors that neutralize phagocytosis and/or modulate the immune response. The genome-based approaches and bioinformatics tools and the extensive use of biophysical and biochemical methods and animal model studies have provided a great wealth of information on the molecular structure and function of these virulence factors. In fact, a number of new GBS surface-exposed or secreted proteins have been identified (GBS immunogenic bacterial adhesion protein, leucine-rich repeat of GBS, serine-rich repeat proteins), the three-dimensional structures of known streptococcal proteins (αC protein, C5a peptidase) have been solved and an understanding of the pathogenetic role of "old" and new determinants has been better defined in recent years. Herein, we provide an update of our current understanding of the major surface cell wall-anchored proteins from GBS, with emphasis on their biochemical and structural properties and the pathogenetic roles they may have in the onset and progression of host infection. We also focus on the antigenic profile of these compounds and discuss them as targets for therapeutic intervention.

Single Molecule Force Spectroscopy Reveals Two-Domain Binding Mode of Pilus-1 Tip Protein RrgA of Streptococcus pneumoniae to Fibronectin

For host cell adhesion and invasion, surface piliation procures benefits for bacteria. A detailed investigation of how pili adhere to host cells is therefore a key aspect in understanding their role during infection. Streptococcus pneumoniae TIGR 4, a clinical relevant serotype 4 strain, is capable of expressing pilus-1 with terminal RrgA, an adhesin interacting with host extracellular matrix (ECM) proteins. We used single molecule force spectroscopy to investigate the binding of full-length RrgA and single RrgA domains to fibronectin. Our results show that full-length RrgA and its terminal domains D3 and D4 bind to fibronectin with forces of 51.6 (full length), 52.8 (D3), and 46.2 pN (D4) at force-loading rates of around 1500 pN/s. Selective saturation of D3 and D4 binding sites on fibronectin showed that both domains can interact simultaneously with fibronectin, revealing a two-domain binding mechanism for the pilus-1 tip protein. The high off rates and the corresponding short lifetime of the RrgA Fn bond (τ = 0.26 s) may enable piliated pneumococci to form and maintain a transient contact to fibronectin-containing host surfaces and thus to efficiently scan the surface for specific receptors promoting host cell adhesion and invasion. These molecular properties could be essential for S. pneumoniae pili to mediate initial contact to the host cells and-shared with other piliated Gram-positive bacteria-favor host invasion.

Group B Streptococcal Maternal Colonization and Neonatal Disease: Molecular Mechanisms and Preventative Approaches

Group B Streptococcus (GBS) colonizes the gastrointestinal and vaginal epithelium of a significant percentage of healthy women, with potential for ascending intrauterine infection or transmission during parturition, creating a risk of serious disease in the vulnerable newborn. This review highlights new insights on the bacterial virulence determinants, host immune responses, and microbiome interactions that underpin GBS vaginal colonization, the proximal step in newborn infectious disease pathogenesis. From the pathogen perspective, the function GBS adhesins and biofilms, β-hemolysin/cytolysin toxin, immune resistance factors, sialic acid mimicry, and two-component transcriptional regulatory systems are reviewed. From the host standpoint, pathogen recognition, cytokine responses, and the vaginal mucosal and placental immunity to the pathogen are detailed. Finally, the rationale, efficacy, and potential unintended consequences of current universal recommended intrapartum antibiotic prophylaxis are considered, with updates on new developments toward a GBS vaccine or alternative approaches to reducing vaginal colonization.

Characterization of Afb, a novel bifunctional protein in Streptococcus agalactiae

BACKGROUND AND OBJECTIVES

Streptococcus agalactiae is the leading cause of bacterial sepsis and meningitis in newborns and results in pneumonia and bacteremia in adults. A number of S. agalactiae components are involved in colonization of target cells. Destruction of peptidoglycan and division of covalently linked daughter cells is mediated by autolysins. In this study, autolytic activity and plasma binding ability of AFb novel recombinant protein of S. agalactiae was investigated.

MATERIALS AND METHODS

The gbs1805 gene was cloned and expressed. E. coli strains DH5α and BL21 were used as cloning and expression hosts, respectively. After purification, antigenicity and binding ability to plasma proteins of the recombinant protein was evaluated.

RESULTS

AFb, the 18KDa protein was purified successfully. The insoluble mature protein revealed the ability to bind to fibrinogen and fibronectin. This insoluble mature protein revealed that it has the ability to bind to fibrinogen and fibronectin plasma proteins. Furthermore, in silico analysis demonstrated the AFb has an autolytic activity.

CONCLUSIONS

AFb is a novel protein capable of binding to fibrinogen and fibronectin. This findings lay a ground work for further investigation of the role of the bacteria in adhesion and colonization to the host.

Soluble CD163 promotes recognition, phagocytosis and killing of Staphylococcus aureus via binding of specific fibronectin peptides

CD163 is a multi-ligand scavenger receptor exclusively expressed by monocytes and macrophages, which is released after their activation during sepsis (sCD163). The biological relevance of sCD163, however, is not yet clear. We now demonstrate that sCD163 exhibits direct antimicrobial effects by recognizing a specific subfragment ((6) F1(1) F2(2) F2(7) F1) of fibronectin (FN) bound to staphylococcal surface molecules. Moreover, contact with staphylococci promotes sCD163-shedding from monocyte surface via induction of metalloproteinases ADAM10 and ADAM17. sCD163 subsequently binds to Staphylococcus aureus via FN peptides and strongly amplifies phagocytosis as well as killing by monocytes and to a lesser extend by neutrophils. This mechanism exhibits additional paracrine effects because staphylococci additionally opsonized by sCD163 induce higher activation and more efficient killing activity of non-professional phagocytes like endothelial cells. Targeting pathogen-bound FN by sCD163 would be a very sophisticated strategy to attack S. aureus as any attempt of the pathogen to avoid this defence mechanism will automatically bring about loss of adherence to the host protein FN, which is a pivotal patho-mechanism of highly invasive staphylococcal strains. Thus, we report a novel function for sCD163 that is of particular importance for immune defence of the host against S. aureus infections.

The OmpL37 surface-exposed protein is expressed by pathogenic Leptospira during infection and binds skin and vascular elastin

Pathogenic Leptospira spp. shed in the urine of reservoir hosts into freshwater can be transmitted to a susceptible host through skin abrasions or mucous membranes causing leptospirosis. The infection process involves the ability of leptospires to adhere to cell surface and extracellular matrix components, a crucial step for dissemination and colonization of host tissues. Therefore, the elucidation of novel mediators of host-pathogen interaction is important in the discovery of virulence factors involved in the pathogenesis of leptospirosis. In this study, we assess the functional roles of transmembrane outer membrane proteins OmpL36 (LIC13166), OmpL37 (LIC12263), and OmpL47 (LIC13050), which we recently identified on the leptospiral surface. We determine the capacity of these proteins to bind to host tissue components by enzyme-linked immunosorbent assay. OmpL37 binds elastin preferentially, exhibiting dose-dependent, saturating binding to human skin (K(d), 104±19 nM) and aortic elastin (K(d), 152±27 nM). It also binds fibrinogen (K(d), 244±15 nM), fibrinogen fragment D (K(d), 132±30 nM), plasma fibronectin (K(d), 359±68 nM), and murine laminin (K(d), 410±81 nM). The binding to human skin elastin by both recombinant OmpL37 and live Leptospira interrogans is specifically enhanced by rabbit antiserum for OmpL37, suggesting the involvement of OmpL37 in leptospiral binding to elastin and also the possibility that host-generated antibodies may promote rather than inhibit the adherence of leptospires to elastin-rich tissues. Further, we demonstrate that OmpL37 is recognized by acute and convalescent leptospirosis patient sera and also by Leptospira-infected hamster sera. Finally, OmpL37 protein is detected in pathogenic Leptospira serovars and not in saprophytic Leptospira. Thus, OmpL37 is a novel elastin-binding protein of pathogenic Leptospira that may be promoting attachment of Leptospira to host tissues.

Understanding the regulation of Group B Streptococcal virulence factors

Bacterial infections remain a significant threat to the health of newborns and adults. Group B Streptococci (GBS) are Gram-positive bacteria that are common asymptomatic colonizers of healthy adults. However, this opportunistic organism can also subvert suboptimal host defenses to cause severe invasive disease and tissue damage. The increasing emergence of antibiotic-resistant GBS raises more concerns for sustained measures in treatment of the disease. A number of factors that are important for virulence of GBS have been identified. This review summarizes the functions of some well-characterized virulence factors, with an emphasis on how GBS regulates their expression. Regulatory and signaling molecules are attractive drug targets in the treatment of bacterial infections. Consequently, understanding signaling responses of GBS is essential for elucidation of pathogenesis of GBS infection and for the identification of novel therapeutic agents.