The streptococcal collagen-binding protein CNE specifically interferes with alphaVbeta3-mediated cellular interactions with triple helical collagen

Collagen Binding Proteins of Gram-Positive Pathogens

Collagens are the primary structural components of mammalian extracellular matrices. In addition, collagens regulate tissue development, regeneration and host defense through interaction with specific cellular receptors. Their unique triple helix structure, which requires a glycine residue every third amino acid, is the defining structural feature of collagens. There are 28 genetically distinct collagens in humans. In addition, several other unrelated human proteins contain a collagen domain. Gram-positive bacteria of the genera Staphylococcus, Streptococcus, Enterococcus, and Bacillus express cell surface proteins that bind to collagen. These proteins of Gram-positive pathogens are modular proteins that can be classified into different structural families. This review will focus on the different structural families of collagen binding proteins of Gram-positive pathogen. We will describe how these proteins interact with the triple helix in collagens and other host proteins containing a collagenous domain and discuss how these interactions can contribute to the pathogenic processes.

Collagen-binding proteins: insights from the Collagen Toolkits

The Collagen Toolkits are libraries of 56 and 57 triple-helical synthetic peptides spanning the length of the collagen II and collagen III helices. These have been used in solid-phase binding assays to locate sites where collagen receptors and extracellular matrix components bind to collagens. Truncation and substitution allowed exact binding sites to be identified, and corresponding minimal peptides to be synthesised for use in structural and functional studies. 170 sites where over 30 proteins bind to collagen II have been mapped, providing firm conclusions about the amino acid distribution within such binding sites. Protein binding to collagen II is not random, but displays a periodicity of approximately 28 nm, with several prominent nodes where multiple proteins bind. Notably, the vicinity of the collagenase-cleavage site in Toolkit peptide II-44 is highly promiscuous, binding over 20 different proteins. This may reflect either the diverse chemistry of that locus or its diverse function, together with the interplay between regulatory binding partners. Peptides derived from Toolkit studies have been used to determine atomic level resolution of interactions between collagen and several of its binding partners and are finding practical application in tissue engineering.

Integrin αV β3 can substitute for collagen-binding β1 -integrins in vivo to maintain a homeostatic interstitial fluid pressure

New Findings

What is the central question of this study?

Collagen‐binding β₁‐integrins function physiologically in cellular control of dermal interstitial fluid pressure (P_IF) in vivo and thereby participate in control of extravascular fluid volume. During anaphylaxis, simulated by injection of compound 48/80, integrin α_Vβ₃ takes over this physiological function. Here we addressed the question whether integrin α_Vβ₃ can replace collagen‐binding β₁‐integrin to maintain a long‐term homeostatic P_IF.

What is the main finding and its importance?

Mice lacking the collagen‐binding integrin α₁₁β₁ show a complex dermal phenotype with regard to the interstitial physiology apparent in the control of P_IF. Notably dermal P_IF is not lowered with compound 48/80 in these animals. Our present data imply that integrin α_Vβ₃ is the likely candidate that has taken over the role of collagen‐binding β₁‐integrins for maintaining a steady‐state homeostatic P_IF. A better understanding of molecular processes involved in control of P_IF is instrumental for establishing novel treatment regimens for control of oedema formation in anaphylaxis and septic shock.

Abstract

Accumulated data indicate that cell‐mediated contraction of reconstituted collagenous gels in vitro can serve as a model for cell‐mediated control of interstitial fluid pressure (P_IF) in vivo. A central role for collagen‐binding β₁‐integrins in both processes has been established. Furthermore, integrin α_Vβ₃ takes over the role of collagen‐binding β₁‐integrins in mediating contraction after perturbations of collagen‐binding β₁‐integrins in vitro. Integrin α_Vβ₃ is also instrumental for normalization of dermal P_IF that has been lowered due to mast cell degranulation with compound 48/80 (C48/80) in vivo. Here we demonstrate a role of integrin α_Vβ₃ in maintaining a long term homeostatic dermal P_IF in mice lacking the collagen‐binding integrin  α₁₁β₁ (α11^−/− mice). Measurements of P_IF were performed after circulatory arrest. Furthermore, cell‐mediated integrin α_Vβ₃‐directed contraction of collagenous gels in vitro depends on free access to a collagen site known to bind several extracellular matrix (ECM) proteins that form substrates for α_Vβ₃‐directed cell attachment, such as fibronectin and fibrin. A streptococcal collagen‐binding protein, CNE, specifically binds to and blocks this site on the collagen triple helix. Here we show that whereas CNE perturbed α_Vβ₃‐directed and platelet‐derived growth factor BB‐induced normalization of dermal P_IF after C48/80, it did not affect α_Vβ₃‐dependent maintenance of a homeostatic dermal P_IF. These data imply that dynamic modification of the ECM structure is needed during acute patho‐physiological modulations of P_IF but not for long‐term maintenance of a homeostatic P_IF. Our data thus show that collagen‐binding β₁‐integrins, integrin α_Vβ₃ and ECM structure are potential targets for novel therapy aimed at modulating oedema formation and hypovolemic shock during anaphylaxis.

Unique charge-dependent constraint on collagen recognition by integrin α10β1

The collagen-binding integrins recognise collagen through their inserted (I) domain, where co-ordination of a Mg^2 + ion in the metal ion-dependent site is reorganised by ligation by a collagen glutamate residue found in specific collagen hexapeptide motifs. Here we show that GROGER, found in the N-terminal domain of collagens I and III, is only weakly recognised by α10β1, an important collagen receptor on chondrocytes, contrasting with the other collagen-binding integrins. Alignment of I domain sequence and molecular modelling revealed a clash between a unique arginine residue (R215) in α10β1 and the positively-charged GROGER. Replacement of R215 with glutamine restored binding. Substituting arginine at the equivalent locus (Q214) in integrins α1 and α2 I domains impaired their binding to GROGER. Collagen II, abundant in cartilage, lacks GROGER. GRSGET is uniquely expressed in the C-terminus of collagen II, but this motif is similarly not recognised by α10β1. These data suggest an evolutionary imperative to maintain accessibility of the terminal domains of collagen II in tissues such as cartilage, perhaps during endochondral ossification, where α10β1 is the main collagen-binding integrin.

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Integrin α10β1 binding to collagen is mapped onto Collagen Toolkits.

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Charged residue in α10 I domain clashes with some binding sites that are unique to collagen II.

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Mutant constructs of other integrin I domains mimic this charge effect.

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Implications for evolution of collagens and cartilage with reference to bone formation

Collagen-binding proteins of Streptococcus mutans and related streptococci

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

Preparation and Characterization of an <i>In Situ</i> Hydrogel of Self-Assembly Type I Collagen from Shark Skin/Methylcellulose for Central Nerve System Regeneration

Central nerve system degeneration is a crucial problem for many patients. To use an in situ hydrogel formation is an attractive method to treat that problem. An in situ hydrogel was developed for central nerve system regeneration. An acid soluble collagen (ASC) and pepsin soluble collagen (PSC) from the shark skin of the brownbanded bamboo shark (Chiloscyllium punctatum) were used to produce hybridized hydrogels by the biomimetic approach. Collagen was mixed with methylcellulose and used 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) as a crosslinker. The hydrogels had various ratios of collagen:methylcellulose: 100:0, 70:30, 50:50, 30:70, and 0:100. Structural, molecular, and morphological organization were characterized and observed by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). The DSC results showed that the peak of denatured collagen fibril shifted higher in a 30:70 ratio of collagen:methylcellulose in both ASC and PSC. The FT-IR results indicated that the structure of hydrogels from both ASC and PSC were organized into complex structures. The SEM results demonstrated that the collagen fibril networks were formed in both ASC and PSC hydrogels. The results indicated that the samples containing collagen promise to be an in situ hydrogel for central nerve regeneration.

Identification and characterization of enolase as a collagen-binding protein in Lactobacillus plantarum

Collagen is a target of pathogens for adhesion, colonization, and invasion of host tissue. Probiotic bacteria can mimic the same mechanism as used by the pathogens in the colonization process, expressing cell surface proteins that specifically interact with extracellular matrix component proteins. The capability to bind collagen is expressed by several Lactobacillus isolates, including some Lactobacillus plantarum strains. In this study we report the involvement of the L. plantarum EnoA1 alfa-enolase in type I collagen (CnI) binding. By adhesion assays, we show that the mutant strain LM3-CC1, carrying a null mutation in the enoA1 gene, binds to immobilized collagen less efficiently than wild type strain. CnI overlay assay and Elisa tests, performed on the purified EnoA1, show that this protein can bind collagen both under denaturing and native conditions. By using truncated recombinant enolase proteins, we also show that the region spanning from 73rd to the 140th amino acid residues is involved in CnI binding.

Structural and functional analysis of the fibronectin-binding protein FNE from Streptococcus equi spp. equi

Streptococcus equi is a horse pathogen belonging to Lancefield group C. Infection by S. equi ssp. equi causes strangles, a serious and highly contagious disease of the upper respiratory tract. S. equi ssp. equi secretes a fibronectin (Fn)-binding protein, FNE, that does not contain cell wall-anchoring motifs. FNE binds to the gelatin-binding domain (GBD) of Fn, composed of the motifs (6) FI (12) FII (789) FI . FNE lacks the canonical Fn-binding peptide repeats observed in many microbial surface components recognizing adhesive matrix molecules. We found that the interaction between FNE and the human GBD is mediated by the binding of the disordered C-terminal region (residues 208-262) of FNE to the (789) FI GBD subfragment. The crystal structure of FNE showed that it is similar to the minor pilus protein Spy0125 of Streptococcus pyogenes, found at the end of pilus polymers and responsible for adhesion. FNE and Spy0125 both have a superimposable internal thioester bond between highly conserved Cys and Gln residues. Small-angle X-ray scattering of the FNE-(789) FI complex provided a model that aligns the C-terminal peptide of FNE with the E-strands of the FI domains, adopting the β-zipper extension model observed in previous structures of microbial surface components recognizing adhesive matrix molecule adhesion peptides bound to FI domains.

Fibrin binds to collagen and provides a bridge for αVβ3 integrin-dependent contraction of collagen gels

The functional significance of fibrin deposits typically seen in inflammatory lesions, carcinomas and in healing wounds is not fully understood. In the present study, we demonstrate that fibrinogen/fibrin specifically bound to native Col I (collagen type I) and used the Col I fibre network as a base to provide a functional interface matrix that connects cells to the Col I fibres through αVβ3 integrins. This allowed murine myoblast C2C12 cells to contract the collagenous composite gel via αVβ3 integrin. We show that fibrinogen specifically bound to immobilized native Col I at the site known to bind matrix metalloproteinase-1, discoidin domain receptor-2 and fibronectin, and that binding had no effect on Col I fibrillation. A specific competitive inhibitor blocking the Col-I-binding site for fibrinogen abolished the organization of fibrin into discernable fibrils, as well as the C2C12-mediated contraction of Col I gels. Our data show that fibrin can function as a linkage protein between Col I fibres and cells, and suggest that fibrin at inflammatory sites indirectly connects αVβ3 integrins to Col I fibres and thereby promotes cell-mediated contraction of collagenous tissue structures.

The putative functions of extravascular fibrin in pathologies are poorly characterized. We show that fibrinogen binds specifically to a defined protein-binding site of the native collagen. Through this binding, fibrin provides an interface matrix allowing αVβ3 integrin-mediated collagen gel contraction.

Bacterial adhesion to animal tissues: protein determinants for recognition of extracellular matrix components

The extracellular matrix (ECM) is present within all animal tissues and organs. Actually, it surrounds the eukaryotic cells composing the four basic tissue types, i.e. epithelial, muscle, nerve and connective. ECM does not solely refer to connective tissue but composes all tissues where its composition, structure and organization vary from one tissue to another. Constituted of the four main fibrous proteins, i.e. collagen, fibronectin, laminin and elastin, ECM components form a highly structured and functional network via specific interactions. From the basement membrane to interstitial matrix, further heterogeneity exists in the organization of the ECM in various tissues and organs also depending on their physiological state. Back to a molecular level, bacterial proteins represent the most significant part of the microbial surface components recognizing adhesive matrix molecules (MSCRAMM). These cell surface proteins are secreted and localized differently in monoderm and diderm-LPS bacteria. While one collagen-binding domain (CBD) and different fibronectin-binding domains (FBD1 to 8) have been registered in databases, much remains to be learned on specific binding to other ECM proteins via single or supramolecular protein structures. Besides theinteraction of bacterial proteins with individual ECM components, this review aims at stressing the importance of fully considering the ECM at supramolecular, cellular, tissue and organ levels. This conceptual view should not be overlooked to rigorously comprehend the physiology of bacterial interaction from commensal to pathogenic species.

Blocking αvβ3 integrin by a recombinant RGD disintegrin impairs VEGF signaling in endothelial cells

Vascular endothelial growth factor (VEGF) and αvβ3 integrin are key molecules that actively participate in tumor angiogenesis and metastasis. Some integrin-blocking molecules are currently under clinical trials for cancer and metastasis treatment. However, the mechanism of action of such inhibitors is not completely understood. We have previously demonstrated the anti-angiogenic and anti-metastatic properties of DisBa-01, a recombinant His-tag RGD-disintegrin from Bothrops alternatus snake venom in some experimental models. DisBa-01 blocks αvβ3 integrin binding to vitronectin and inhibits integrin-mediated downstream signaling cascades and cell migration. Here we add some new information on the mechanism of action of DisBa-01 in the tumor microenvironment. DisBa-01 supports the adhesion of fibroblasts and MDA-MB-231 breast cancer cells but it inhibits the adhesion of these cells to type I collagen under flow in high shear conditions, as a simulation of the blood stream. DisBa-01 does not affect the release of VEGF by fibroblasts or breast cancer cells but it strongly decreases the expression of VEGF mRNA and of its receptors, vascular endothelial growth factor receptors 1 and 2 (VEGFR1 and VEGFR2) in endothelial cells. DisBa-01 at nanomolar concentrations also modulates metalloprotease 2 (MMP-2) and 9 (MMP-9) activity, the latter being decreased in fibroblasts and increased in MDA-MB-231 cells. In conclusion, these results demonstrate that αvβ3 integrin inhibitors may induce distinct effects in the cells of the tumor microenvironment, resulting in blockade of angiogenesis by impairing of VEGF signaling and in inhibition of tumor cell motility.