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

Impact of vascular Ehlers-Danlos Syndrome-associated Gly substitutions on structure, function, and mechanics using bacterial collagen

Vascular Ehlers-Danlos syndrome (vEDS) arises from mutations in collagen-III, a major structural component of the extracellular matrix (ECM) in vascularized tissues, including blood vessels. Fibrillar collagens form a triple-helix that is characterized by a canonical (Gly-X-Y)n sequence. The substitution of another amino acid for Gly within this conserved repeating sequence is associated with several hereditary connective tissue disorders, including vEDS. The clinical severity of vEDS depends on the identity of the substituted amino acid and its location. In this study, we engineered recombinant bacterial collagen-like proteins (CLPs) with previously reported Gly→X (X=Ser or Arg) vEDS substitutions within the integrin-binding site. Employing a combination of biophysical techniques, enzymatic digestion assays, integrin binding affinity assays, and computational modeling, we assessed the impact of Gly→X substitutions on structure, stability, function, and mechanical properties. While constructs with Ser or Arg substitutions maintained a triple-helix structure, Arg substitution significantly reduced global thermal stability, heightened susceptibility to trypsin digestion, and altered integrin α2-inserted (α2I) domain binding. Molecular dynamics (MD) simulations also demonstrated distinct effects of different Gly substitutions on the triple-helix structure - Arg substitutions induced notable bulging at the substitution site and disrupted interchain hydrogen bonds compared to Ser substitutions. Additionally, steered MD simulations revealed that Arg substitution led to a significant decrease in the Young's modulus of the triple-helix. Bacterial CLPs have proved to be a powerful model for studying the underlying mechanisms of vEDS-causing mutations in collagen-III. Serine and arginine substitutions differentially perturb cell-matrix interactions and ECM in a manner consistent with clinical vEDS severity.

The Legionella collagen-like protein employs a distinct binding mechanism for the recognition of host glycosaminoglycans

Bacterial adhesion is a fundamental process which enables colonisation of niche environments and is key for infection. However, in Legionella pneumophila, the causative agent of Legionnaires' disease, these processes are not well understood. The Legionella collagen-like protein (Lcl) is an extracellular peripheral membrane protein that recognises sulphated glycosaminoglycans on the surface of eukaryotic cells, but also stimulates bacterial aggregation in response to divalent cations. Here we report the crystal structure of the Lcl C-terminal domain (Lcl-CTD) and present a model for intact Lcl. Our data reveal that Lcl-CTD forms an unusual trimer arrangement with a positively charged external surface and negatively charged solvent exposed internal cavity. Through molecular dynamics simulations, we show how the glycosaminoglycan chondroitin-4-sulphate associates with the Lcl-CTD surface via distinct binding modes. Our findings show that Lcl homologs are present across both the Pseudomonadota and Fibrobacterota-Chlorobiota-Bacteroidota phyla and suggest that Lcl may represent a versatile carbohydrate-binding mechanism.

Self-Assembling Triple-Helix Recombinant Collagen Hydrogel Enriched with Tyrosine

The self-assembly of collagen within the human body creates a complex 3D fibrous network, providing structural integrity and mechanical strength to connective tissues. Recombinant collagen plays a pivotal role in the realm of biomimetic natural collagen. However, almost all of the reported recombinant collagens lack the capability of self-assembly, severely hindering their application in tissue engineering and regenerative medicine. Herein, we have for the first time constructed a series of self-assembling tyrosine-rich triple helix recombinant collagens, mimicking the structure and functionality of natural collagen. The recombinant collagen consists of a central triple-helical domain characterized by the (Gly-Xaa-Yaa)n sequence, along with N-terminal and C-terminal domains featuring the GYY sequence. The introduction of GYY has a negligible impact on the stability of the triple-helical structure of recombinant collagen while simultaneously promoting its self-assembly into fibers. In the presence of [Ru(bpy)3]Cl2 and APS as catalysts, tyrosine residues in the recombinant collagen undergo covalent cross-linking, resulting in a hydrogel with exceptional mechanical properties. The recombinant collagen hydrogel exhibits outstanding biocompatibility and bioactivity, significantly enhancing the proliferation, adhesion, migration, and differentiation of HFF-1 cells. This innovative self-assembled triple-helix recombinant collagen demonstrates significant potential in the fields of tissue engineering and medical materials.

Designing collagens to shed light on the multi-scale structure-function mapping of matrix disorders

Collagens are the most abundant structural proteins in the extracellular matrix of animals and play crucial roles in maintaining the structural integrity and mechanical properties of tissues and organs while mediating important biological processes. Fibrillar collagens have a unique triple helix structure with a characteristic repeating sequence of (Gly-X-Y)n. Variations within the repetitive sequence can cause misfolding of the triple helix, resulting in heritable connective tissue disorders. The most common variations are single-point missense mutations that lead to the substitution of a glycine residue with a bulkier amino acid (Gly → X). In this review, we will first discuss the importance of collagen's triple helix structure and how single Gly substitutions can impact its folding, structure, secretion, assembly into higher-order structures, and biological functions. We will review the role of "designer collagens," i.e., synthetic collagen-mimetic peptides and recombinant bacterial collagen as model systems to include Gly → X substitutions observed in collagen disorders and investigate their impact on structure and function utilizing in vitro studies. Lastly, we will explore how computational modeling of collagen peptides, especially molecular and steered molecular dynamics, has been instrumental in probing the effects of Gly substitutions on structure, receptor binding, and mechanical stability across multiple length scales.

The Legionella collagen-like protein employs a unique binding mechanism for the recognition of host glycosaminoglycans

Bacterial adhesion is a fundamental process which enables colonisation of niche environments and is key for infection. However, in Legionella pneumophila, the causative agent of Legionnaires' disease, these processes are not well understood. The Legionella collagen-like protein (Lcl) is an extracellular peripheral membrane protein that recognises sulphated glycosaminoglycans (GAGs) on the surface of eukaryotic cells, but also stimulates bacterial aggregation in response to divalent cations. Here we report the crystal structure of the Lcl C-terminal domain (Lcl-CTD) and present a model for intact Lcl. Our data reveal that Lcl-CTD forms an unusual dynamic trimer arrangement with a positively charged external surface and a negatively charged solvent exposed internal cavity. Through Molecular Dynamics (MD) simulations, we show how the GAG chondroitin-4-sulphate associates with the Lcl-CTD surface via unique binding modes. Our findings show that Lcl homologs are present across both the Pseudomonadota and Fibrobacterota-Chlorobiota-Bacteroidota phyla and suggest that Lcl may represent a versatile carbohydrate binding mechanism.

Purification of recombinant bacterial collagens containing structural perturbations

Streptococcus pyogenes-derived recombinant bacterial collagen-like proteins (CLPs) are emerging as a potential biomaterial for biomedical research and applications. Bacterial CLPs form stable triple helices and lack specific interactions with human cell surface receptors, thus enabling the design of novel biomaterials with specific functional attributes. Bacterial collagens have been instrumental in understanding collagen structure and function in normal and pathological conditions. These proteins can be readily produced in E. coli, purified using affinity chromatography, and subsequently isolated after cleavage of the affinity tag. Trypsin is a widely used protease during this purification step since the triple helix structure is resistant to trypsin digestion. However, the introduction of Gly→X mutations or natural interruptions within CLPs can perturb the triple helix structure, making them susceptible to trypsin digestion. Consequently, removing the affinity tag and isolating collagen-like (CL) domains containing mutations is impossible without degradation of the product. We present an alternative method to isolate CL domains containing Gly→X mutations utilizing a TEV protease cleavage site. Protein expression and purification conditions were optimized for designed protein constructs to achieve high yield and purity. Enzymatic digestion assays demonstrated that CL domains from wild-type CLPs could be isolated by digestion with either trypsin or TEV protease. In contrast, CLPs containing Gly→Arg mutations are readily digested by trypsin while digestion with TEV protease cleaved the His6-tag, enabling the isolation of mutant CL domains. The developed method can be adapted to CLPs containing various new biological sequences to develop multifunctional biomaterials for tissue engineering applications.

Recombinant protein polymer-antibody conjugates for applications in nanotechnology and biomedicine

Currently, there are over 100 antibody-based therapeutics on the market for the treatment of various diseases. The increasing importance of antibody treatment is further highlighted by the recent FDA emergency use authorization of certain antibody therapies for COVID-19 treatment. Protein-based materials have gained momentum for antibody delivery due to their biocompatibility, tunable chemistry, monodispersity, and straightforward synthesis and purification. In this review, we discuss progress in engineering the molecular features of protein-based biomaterials, in particular recombinant protein polymers, for introducing novel functionalities and enhancing the delivery properties of antibodies and related binding protein domains.

Antibody responses to collagen peptides and streptococcal collagen-like 1 proteins in acute rheumatic fever patients

Acute rheumatic fever (ARF) is a serious post-infectious immune sequelae of Group A streptococcus (GAS). Pathogenesis remains poorly understood, including the events associated with collagen autoantibody generation. GAS express streptococcal collagen-like proteins (Scl) that contain a collagenous domain resembling human collagen. Here, the relationship between antibody reactivity to GAS Scl proteins and human collagen in ARF was investigated. Serum IgG specific for a representative Scl protein (Scl1.1) together with collagen-I and collagen-IV mimetic peptides were quantified in ARF patients (n = 36) and healthy matched controls (n = 36). Reactivity to Scl1.1 was significantly elevated in ARF compared to controls (P < 0.0001) and this was mapped to the collagen-like region of the protein, rather than the N-terminal non-collagenous region. Reactivity to collagen-1 and collagen-IV peptides was also significantly elevated in ARF cases (P < 0.001). However, there was no correlation between Scl1.1 and collagen peptide antibody binding, and hierarchical clustering of ARF cases by IgG reactivity showed two distinct clusters, with Scl1.1 antigens in one and collagen peptides in the other, demonstrating that collagen autoantibodies are not immunologically related to those targeting Scl1.1. Thus, anti-collagen antibodies in ARF appear to be generated as part of the autoreactivity process, independent of any mimicry with GAS collagen-like proteins.

Current Insights on the Diverse Structures and Functions in Bacterial Collagen-like Proteins

The dearth of knowledge on the diverse structures and functions in bacterial collagen-like proteins is in stark contrast to the deep grasp of structures and functions in mammalian collagen, the ubiquitous triple-helical scleroprotein that plays a central role in tissue architecture, extracellular matrix organization, and signal transduction. To fill and highlight existing gaps due to the general paucity of data on bacterial CLPs, we comprehensively reviewed the latest insight into their functional and structural diversity from multiple perspectives of biology, computational simulations, and materials engineering. The origins and discovery of bacterial CLPs were explored. Their genetic distribution and molecular architecture were analyzed, and their structural and functional diversity in various bacterial genera was examined. The principal roles of computational techniques in understanding bacterial CLPs' structural stability, mechanical properties, and biological functions were also considered. This review serves to drive further interest and development of bacterial CLPs, not only for addressing fundamental biological problems in collagen but also for engineering novel biomaterials. Hence, both biology and materials communities will greatly benefit from intensified research into the diverse structures and functions in bacterial collagen-like proteins.

Thinking Outside the Bug: Targeting Outer Membrane Proteins for Burkholderia Vaccines

Increasing antimicrobial resistance due to misuse and overuse of antimicrobials, as well as a lack of new and innovative antibiotics in development has become an alarming global threat. Preventative therapeutics, like vaccines, are combative measures that aim to stop infections at the source, thereby decreasing the overall use of antibiotics. Infections due to Gram-negative pathogens pose a significant treatment challenge because of substantial multidrug resistance that is acquired and spread throughout the bacterial population. Burkholderia spp. are Gram-negative intrinsically resistant bacteria that are responsible for environmental and nosocomial infections. The Burkholderia cepacia complex are respiratory pathogens that primarily infect immunocompromised and cystic fibrosis patients, and are acquired through contaminated products and equipment, or via patient-to-patient transmission. The Burkholderia pseudomallei complex causes percutaneous wound, cardiovascular, and respiratory infections. Transmission occurs through direct exposure to contaminated water, water-vapors, or soil, leading to the human disease melioidosis, or the equine disease glanders. Currently there is no licensed vaccine against any Burkholderia pathogen. This review will discuss Burkholderia vaccine candidates derived from outer membrane proteins, OmpA, OmpW, Omp85, and Bucl8, encompassing their structures, conservation, and vaccine formulation.

Growth factor-mimicking 3,4-dihydroxyphenylalanine-encoded bioartificial extracellular matrix like protein promotes wound closure and angiogenesis

The present work reports a new route to prepare a "smart biomaterial" by mimicking long-acting cellular growth factor showing enhanced cell-material interactions by promoting cell proliferation and angiogenesis. For that, reactive non-proteogenic amino acid 3,4-dihydroxyphenylalanine (DOPA) was genetically introduced into an intrinsic triple-helical hierarchical structure forming protein to initiate hierarchical self-assembly to form a macromolecular structure. The self-assembled scaffold displayed vascular endothelial growth factor mimicking the pro-angiogenic reactive group for repairing and remodeling of damaged tissue cells. We customized the recombinant collagen-like protein (CLP) with DOPA to promote rapid wound healing and cell migrations. Selective incorporation of catechol in variable and C-terminal region of CLP enhanced interaction between inter- and intra-triple-helical collagen molecules that resulted in a structure resembling higher-order native collagen fibril. Turbidity analysis indicated that the triple-helical CLP self-assembled at neutral pH via a catechol intra-crosslinking mechanism. After self-assembly, only DOPA-encoded CLP formed branched filamentous structures suggesting that catechol mediated network coordination. The catechol-encoded CLP also acted as a "smart material" by mimicking long-acting cellular growth factor showing enhanced cell-material interactions by promoting cell proliferation and angiogenesis. It eliminates release rate, stability, and shelf-life of hybrid growth factor conjugated biomaterials. The newly synthesized CLP has the potential to promote accelerated cell migration, pro-angiogenesis, and biocompatibility and could be used in the field of implantable medical devices and tissue engineering.

Burkholderia collagen-like protein 8, Bucl8, is a unique outer membrane component of a putative tetrapartite efflux pump in Burkholderia pseudomallei and Burkholderia mallei

Bacterial efflux pumps are an important pathogenicity trait because they extrude a variety of xenobiotics. Our laboratory previously identified in silico Burkholderia collagen-like protein 8 (Bucl8) in the hazardous pathogens Burkholderia pseudomallei and Burkholderia mallei. We hypothesize that Bucl8, which contains two predicted tandem outer membrane efflux pump domains, is a component of a putative efflux pump. Unique to Bucl8, as compared to other outer membrane proteins, is the presence of an extended extracellular region containing a collagen-like (CL) domain and a non-collagenous C-terminus (Ct). Molecular modeling and circular dichroism spectroscopy with a recombinant protein, corresponding to this extracellular CL-Ct portion of Bucl8, demonstrated that it adopts a collagen triple helix, whereas functional assays screening for Bucl8 ligands identified binding to fibrinogen. Bioinformatic analysis of the bucl8 gene locus revealed it resembles a classical efflux-pump operon. The bucl8 gene is co-localized with downstream fusCDE genes encoding fusaric acid (FA) resistance, and with an upstream gene, designated as fusR, encoding a LysR-type transcriptional regulator. Using reverse transcriptase (RT)-qPCR, we defined the boundaries and transcriptional organization of the fusR-bucl8-fusCDE operon. We found exogenous FA induced bucl8 transcription over 80-fold in B. pseudomallei, while deletion of the entire bucl8 locus decreased the minimum inhibitory concentration of FA 4-fold in its isogenic mutant. We furthermore showed that the putative Bucl8-associated pump expressed in the heterologous Escherichia coli host confers FA resistance. On the contrary, the Bucl8-associated pump did not confer resistance to a panel of clinically-relevant antimicrobials in Burkholderia and E. coli. We finally demonstrated that deletion of the bucl8-locus drastically affects the growth of the mutant in L-broth. We determined that Bucl8 is a component of a novel tetrapartite efflux pump, which confers FA resistance, fibrinogen binding, and optimal growth.

Expression and Purification of Collagen-Like Proteins of Group A Streptococcus

Prokaryotic proteins with extended collagen domain are found in many bacterial species that are pathogenic to humans and animals. The collagen domain is often fused to additional ligand-binding domains and plays both structural and functional roles in modular "bacterial collagens." Here, we describe the step-by-step expression and purification of the recombinant streptococcal collagen-like proteins, rScl, using the Strep-tag II system. The integrity and structural characterization of recombinant collagen-like proteins is very important for defining their function.

Collagen-like sequences encoded by extremophilic and extremotolerant bacteria

Collagens and collagen-like proteins are found in a wide range of organisms. The common feature of these proteins is a triple helix fold, requiring a characteristic pattern of amino acid sequences, composed of Gly-X-Y tripeptide repeats. Collagen-like proteins from bacteria are heterogeneous in terms of length and amino acid composition of their collagenous sequences. However, different bacteria live in different environments, some at extreme temperatures and conditions. This study explores the occurrence of collagen-like sequences in the genomes of different extreme condition-adapted bacteria, and investigates features that could be linked to conditions where they thrive. Our results show that proteins containing collagen-like sequences are encoded by genomes of various extremophiles. Some of these proteins contain conservative domains, characteristic of cell or endospore surface proteins, while most other proteins are unknown. The characteristics of collagenous sequences may depend on both, the phylogenetic relationship and the living conditions of the bacteria.

A self-assembly and higher order structure forming triple helical protein as a novel biomaterial for cell proliferation

Collagen plays a critical role in the structural design of the extracellular matrix (ECM) and cell signaling in mammals, which makes it one of the most promising biomaterials with versatile applications. However, there is considerable concern regarding the purity and predictability of the product performance. At present, it is mainly derived as a mixture of collagen (different types) from animal tissues, where the selective enrichment of a particular type of collagen is generally difficult and expensive. Collagen derived from bovine sources poses the risk of transmitting diseases and can cause adverse immunologic and inflammatory responses. Hence, recombinant collagen can be a good alternative. Nevertheless, the necessity of post-translational hydroxyproline (Hyp) modification limits large-scale recombinant collagen production. Here, we recombinantly expressed the collagen-like protein (CLTP) and genetically introduced the Hyp in the CLTP to form a higher order self-assembled fibril structure, similar to human collagen. During the current study, it was observed that the Hyp incorporated CLTP protein (CLTHP) formed a stable triple helical polyproline-II like structure and self-assembled to form fibrils at neutral pH, which had an initial lag phase followed by a growth phase similar to animal collagen. In contrast, the higher order fibrillar assembly was missing in the nonhydroxylated CLTP. This study demonstrated that CLTHP self-association is based on the common underlying lateral interactions between triple helical structured proteins, where the hydroxyproline forms the significantly stable hydration network. Hence, this work will be the first fundamental empirical research for flexible modifications of recombinant collagen for structural analysis and biomedical applications.

Streptococcal Collagen-like Protein 1 Binds Wound Fibronectin: Implications in Pathogen Targeting

Group A Streptococcus (GAS) infections are responsible for significant morbidity and mortality worldwide. The outlook for an effective global vaccine is reduced because of significant antigenic variation among GAS strains worldwide. Other challenges in GAS therapy include the lack of common access to antibiotics in developing countries, as well as allergy to and treatment failures with penicillin and increasing erythromycin resistance in the industrialized world. At the portal of entry, GAS binds to newly deposited extracellular matrix, which is rich in cellular fibronectin isoforms with extra domain A (EDA, also termed EIIIA) via the surface adhesin, the streptococcal collagen-like protein 1 (Scl1). Recombinant Scl1 constructs, derived from diverse GAS strains, bind the EDA loop segment situated between the C and C' β-strands. Despite the sequence diversity in Scl1 proteins, multiple sequence alignments and secondary structure predictions of Scl1 variants, as well as crystallography and homology modeling studies, point to a conserved mechanism of Scl1-EDA binding. We propose that targeting this interaction may prevent the progression of infection. A synthetic cyclic peptide, derived from the EDA C-C' loop, binds to recombinant Scl1 with a micromolar dissociation constant. This review highlights the current concept of EDA binding to Scl1 and provides incentives to exploit this binding to treat GAS infections and wound colonization.

Adaptation of the group A Streptococcus adhesin Scl1 to bind fibronectin type III repeats within wound-associated extracellular matrix: implications for cancer therapy

The human‐adapted pathogen group A Streptococcus (GAS) utilizes wounds as portals of entry into host tissue, wherein surface adhesins interact with the extracellular matrix, enabling bacterial colonization. The streptococcal collagen‐like protein 1 (Scl1) is a major adhesin of GAS that selectively binds to two fibronectin type III (FnIII) repeats within cellular fibronectin, specifically the alternatively spliced extra domains A and B, and the FnIII repeats within tenascin‐C. Binding to FnIII repeats was mediated through conserved structural determinants present within the Scl1 globular domain and facilitated GAS adherence and biofilm formation. Isoforms of cellular fibronectin that contain extra domains A and B, as well as tenascin‐C, are present for several days in the wound extracellular matrix. Scl1‐FnIII binding is therefore an example of GAS adaptation to the host's wound environment. Similarly, cellular fibronectin isoforms and tenascin‐C are present in the tumor microenvironment. Consistent with this, FnIII repeats mediate GAS attachment to and enhancement of biofilm formation on matrices deposited by cancer‐associated fibroblasts and osteosarcoma cells. These data collectively support the premise for utilization of the Scl1‐FnIII interaction as a novel method of anti‐neoplastic targeting in the tumor microenvironment.

Polymorphisms of a Collagen-Like Adhesin Contributes to Legionella pneumophila Adhesion, Biofilm Formation Capacity and Clinical Prevalence

Legionellosis is a severe respiratory illness caused by the inhalation of aerosolized water droplets contaminated with the opportunistic pathogen Legionella pneumophila. The ability of L. pneumophila to produce biofilms has been associated with its capacity to colonize and persist in human-made water reservoirs and distribution systems, which are the source of legionellosis outbreaks. Nevertheless, the factors that mediate L. pneumophila biofilm formation are largely unknown. In previous studies we reported that the adhesin Legionella collagen-like protein (Lcl), is required for auto-aggregation, attachment to multiple surfaces and the formation of biofilms. Lcl structure contains three distinguishable regions: An N-terminal region with a predicted signal sequence, a central region containing tandem collagen-like repeats (R-domain) and a C-terminal region (C-domain) with no significant homology to other known proteins. Lcl R-domain encodes tandem repeats of the collagenous tripeptide Gly-Xaa-Yaa (GXY), a motif that is key for the molecular organization of mammalian collagen and mediates the binding of collagenous proteins to different cellular and environmental ligands. Interestingly, Lcl is polymorphic in the number of GXY tandem repeats. In this study, we combined diverse biochemical, genetic, and cellular approaches to determine the role of Lcl domains and GXY repeats polymorphisms on the structural and functional properties of Lcl, as well as on bacterial attachment, aggregation and biofilm formation. Our results indicate that the R-domain is key for assembling Lcl collagenous triple-helices and has a more preponderate role over the C-domain in Lcl adhesin binding properties. We show that Lcl molecules oligomerize to form large supramolecular complexes to which both, R and C-domains are required. Furthermore, we found that the number of GXY tandem repeats encoded in Lcl R-domain correlates positively with the binding capabilities of Lcl and with the attachment and biofilm production capacity of L. pneumophila strains. Accordingly, the number of GXY tandem repeats in Lcl influences the clinical prevalence of L. pneumophila strains. Therefore, the number of Lcl tandem repeats could be considered as a potential predictor for virulence in L. pneumophila isolates.

Incorporation of hydroxyproline in bacterial collagen from Streptococcus pyogenes

Bacterial collagen-like proteins differ from vertebrate collagens in that they do not contain hydroxyproline, which is seen as a characteristic of the vertebrate collagens, and which provides a significant contribution to the stability of the collagen triple-helix at body temperature. Despite this difference, the bacterial collagens are stable at around body temperature through inclusion of other stabilising sequence elements. Another difference is the lack of aggregation, and certain vertebrate collagen binding domains that can be introduced into the bacterial sequence lack full function when hydroxyproline is absent. In the present study we have demonstrated that a simple method utilising co-translational incorporation during fermentation can be used to incorporate hydroxyproline into the recombinant bacterial collagen. The presence and amount of hydroxyproline incorporation was shown by amino acid analysis and by mass spectrometry. A small increase in thermal stability was observed using circular dichroism spectroscopy. STATEMENT OF SIGNIFICANCE: Recombinant bacterial collagens provide a new opportunity for biomedical materials as they are readily produced in large quantity in E. coli. Unlike animal collagens, they are stable without the need for inclusion of a secondary modification system for hydroxyproline incorporation. In animal collagens, however, introduction of hydroxyproline is essential for stability and is also important for functional molecular interactions within the mammalian extracellular matrix. The present study has shown that hydroxyproline can be readily introduced into recombinant S. pyogenes bacterial collagen through direct co-translational incorporation of this modified imino acid during expression using the codons for proline in the introduced gene construct. This hydroxylation further improves the stability of the collagen and is available to enhance any introduced molecular functions.

Collagen degradation in tuberculosis pathogenesis: the biochemical consequences of hosting an undesired guest

The scenario of chemical reactions prompted by the infection by Mycobacterium tuberculosis is huge. The infection generates a localized inflammatory response, with the recruitment of neutrophils, monocytes, and T-lymphocytes. Consequences of this immune reaction can be the eradication or containment of the infection, but these events can be deleterious to the host inasmuch as lung tissue can be destroyed. Indeed, a hallmark of tuberculosis (TB) is the formation of lung cavities, which increase disease development and transmission, as they are sites of high mycobacterial burden. Pulmonary cavitation is associated with antibiotic failure and the emergence of antibiotic resistance. For cavities to form, M. tuberculosis induces the overexpression of host proteases, like matrix metalloproteinases and cathepsin, which are secreted from monocyte-derived cells, neutrophils, and stromal cells. These proteases destroy the lung parenchyma, in particular the collagen constituent of the extracellular matrix (ECM). Namely, in an attempt to destroy infected cells, the immune reactions prompted by mycobacterial infections induce the destruction of vital regions of the lung, in a process that can become fatal. Here, we review structure and function of the main molecular actors of ECM degradation due to M. tuberculosis infection and the proposed mechanisms of tissue destruction, mainly attacking fibrillar collagen. Importantly, enzymes responsible for collagen destruction are emerging as key targets for adjunctive therapies to limit immunopathology in TB.

Influence of collagen-based integrin α1 and α2 mediated signaling on human mesenchymal stem cell osteogenesis in three dimensional contexts

Collagen I interactions with integrins α1 and α2 are known to support human mesenchymal stem cell (hMSC) osteogenesis. Nonetheless, elucidating the relative impact of specific integrin interactions has proven challenging, in part due to the complexity of native collagen. In the present work, we employed two collagen-mimetic proteins-Scl2-2 and Scl2-3- to compare the osteogenic effects of integrin α1 versus α2 signaling. Scl2-2 and Scl2-3 were both derived from Scl2-1, a triple helical protein lacking known cell adhesion, cytokine binding, and matrix metalloproteinase sites. However, Scl2-2 and Scl2-3 were each engineered to display distinct collagen-based cell adhesion motifs: GFPGER (binding integrins α1 and α2 ) or GFPGEN (binding only integrin α1 ), respectively. hMSCs were cultured within poly(ethylene glycol) (PEG) hydrogels containing either Scl2-2 or Scl2-3 for 2 weeks. PEG-Scl2-2 gels were associated with increased hMSC osterix expression, osteopontin production, and calcium deposition relative to PEG-Scl2-3 gels. These data indicate that integrin α2 signaling may have an increased osteogenic effect relative to integrin α1 . Since p38 is activated by integrin α2 but not by integrin α1 , hMSCs were further cultured in PEG-Scl2-2 hydrogels in the presence of a p38 inhibitor. Results suggest that p38 activity may play a key role in collagen-supported hMSC osteogenesis. This knowledge can be used toward the rational design of scaffolds which intrinsically promote hMSC osteogenesis. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2594-2604, 2018.

Surface-exposed loops and an acidic patch in the Scl1 protein of group A Streptococcus enable Scl1 binding to wound-associated fibronectin

Keratinized epidermis constitutes a powerful barrier of the mucosa and skin, effectively preventing bacterial invasion, unless it is wounded and no longer protective. Wound healing involves deposition of distinct extracellular matrix (ECM) proteins enriched in cellular fibronectin (cFn) isoforms containing extra domain A (EDA). The streptococcal collagen-like protein 1 (Scl1) is a surface adhesin of group A Streptococcus (GAS), which contains an N-terminal variable (V) domain and a C-terminally located collagen-like domain. During wound infection, Scl1 selectively binds EDA/cFn isoforms and laminin, as well as low-density lipoprotein (LDL), through its V domain. The trimeric V domain has a six-helical bundle fold composed of three pairs of anti-parallel α-helices interconnected by hypervariable loops, but the roles of these structures in EDA/cFn binding are unclear. Here, using recombinant Scl (rScl) constructs to investigate structure-function determinants of the Scl1-EDA/cFn interaction, we found that full-length rScl1, containing both the globular V and the collagen domains, is necessary for EDA/cFn binding. We established that the surface-exposed loops, interconnecting conserved α-helices, guide recognition and binding of Scl1-V to EDA and binding to laminin and LDL. Moreover, electrostatic surface potential models of the Scl1-V domains pointed to a conserved, negatively charged pocket, surrounded by positively charged and neutral regions, as a determining factor for the binding. In light of these findings, we propose an updated model of EDA/cFn recognition by the Scl1 adhesin from GAS, representing a significant step in understanding the Scl1-ECM interactions within the wound microenvironment that underlie GAS pathogenesis.

Evaluation of late outgrowth endothelial progenitor cell and umbilical vein endothelial cell responses to thromboresistant collagen-mimetic hydrogels

Bioactive coatings which support the adhesion of late-outgrowth peripheral blood endothelial progenitor cells (EOCs) are actively being investigated as a means to promote rapid endothelialization of "off-the-shelf," small-caliber arterial graft prostheses following implantation. In the present work, we evaluated the behavior of EOCs on thromboresistant graft coatings based on the collagen-mimetic protein Scl2-2 and poly(ethylene glycol) (PEG) diacrylate. Specifically, the attachment, proliferation, migration, and phenotype of EOCs on PEG-Scl2-2 hydrogels were evaluated as a function of Scl2-2 concentration (4, 8, and 12 mg/mL) relative to human umbilical vein endothelial cells (HUVECs). Results demonstrate the ability of each PEG-Scl2-2 hydrogel formulation to support EOC and HUVEC adhesion, proliferation, and spreading. However, only the 8 and 12 mg/mL PEG-Scl2-2 hydrogels were able to support stable EOC and HUVEC confluence. These PEG-Scl2-2 formulations were, therefore, selected for evaluation of their impact on EOC and HUVEC phenotype relative to PEG-collagen hydrogels. Cumulatively, both gene and protein level data indicated that 8 mg/mL PEG-Scl2-2 hydrogels supported similar or improved levels of EOC maturation relative to PEG-collagen controls based on evaluation of CD34, VEGFR2, PECAM-1, and VE-Cadherin. The 8 mg/mL PEG-Scl2-2 hydrogels also appeared to support similar or improved levels of EOC homeostatic marker expression relative to PEG-collagen hydrogels based on von Willebrand factor, collagen IV, NOS3, thrombomodulin, and E-selectin assessment. Combined, the present results indicate that PEG-Scl2-2 hydrogels warrant further investigation as "off-the-shelf" graft coatings. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1712-1724, 2017.

Collagen-like proteins of pathogenic streptococci

The collagen domain, which is defined by the presence of the Gly-X-Y triplet repeats, is amongst the most versatile and widespread known structures found in proteins from organisms representing all three domains of life. The streptococcal collagen-like (Scl) proteins are widely present in pathogenic streptococci, including Streptococcus pyogenes, S. agalactiae, S. pneumoniae, and S. equi. Experiments and bioinformatic analyses support the hypothesis that all Scl proteins are homotrimeric and cell wall-anchored. These proteins contain the rod-shaped collagenous domain proximal to cell surface, as well as a variety of outermost non-collagenous domains that generally lack predicted functions but can be grouped into one of six clusters based on sequence similarity. The well-characterized Scl1 proteins of S. pyogenes show a dichotomous switch in ligand binding between human tissue and blood environments. In tissue, Scl1 adhesin specifically recognizes the wound microenvironment, promotes adhesion and biofilm formation, decreases bacterial killing by neutrophil extracellular traps, and modulates S. pyogenes virulence. In blood, ligands include components of complement and coagulation-fibrinolytic systems, as well as plasma lipoproteins. In all, the Scl proteins signify a large family of structurally related surface proteins, which contribute to the ability of streptococci to colonize and cause diseases in humans and animals.

Bioengineered Collagens

There is a great deal of interest in obtaining recombinant collagen as an alternative source of material for biomedical applications and as an approach for obtaining basic structural and biological information. However, application of recombinant technology to collagen presents challenges, most notably the need for post-translational hydroxylation of prolines for triple-helix stability. Full length recombinant human collagens have been successfully expressed in cell lines, yeast, and several plant systems, while collagen fragments have been expressed in E. coli. In addition, bacterial collagen-like proteins can be expressed in high yields in E. coli and easily manipulated to incorporate biologically active sequences from human collagens. These expression systems allow manipulation of biologically active sequences within collagen, which has furthered our understanding of the relationships between collagen sequences, structure and function. Here, recombinant studies on collagen interactions with cell receptors, extracellular matrix proteins, and matrix metalloproteinases are reviewed, and discussed in terms of their potential biomaterial and biomedical applications.

Engineering specific chemical modification sites into a collagen-like protein from Streptococcus pyogenes

Recombinant bacterial collagens provide a new opportunity for safe biomedical materials. They are readily expressed in Escherichia coli in good yield and can be readily purified by simple approaches. However, recombinant proteins are limited in that direct secondary modification during expression is generally not easily achieved. Thus, inclusion of unusual amino acids, cyclic peptides, sugars, lipids, and other complex functions generally needs to be achieved chemically after synthesis and extraction. In the present study, we have illustrated that bacterial collagens that have had their sequences modified to include cysteine residue(s), which are not normally present in bacterial collagen-like sequences, enable a range of specific chemical modification reactions to be produced. Various model reactions were shown to be effective for modifying the collagens. The ability to include alkyne (or azide) functions allows the extensive range of substitutions that are available via "click" chemistry to be accessed. When bifunctional reagents were used, some crosslinking occurred to give higher molecular weight polymeric proteins, but gels were not formed. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 806-813, 2017.

Unique Footprint in the scl1.3 Locus Affects Adhesion and Biofilm Formation of the Invasive M3-Type Group A Streptococcus

The streptococcal collagen-like proteins 1 and 2 (Scl1 and Scl2) are major surface adhesins that are ubiquitous among group A Streptococcus (GAS). Invasive M3-type strains, however, have evolved two unique conserved features in the scl1 locus: (i) an IS1548 element insertion in the scl1 promoter region and (ii) a nonsense mutation within the scl1 coding sequence. The scl1 transcript is drastically reduced in M3-type GAS, contrasting with a high transcription level of scl1 allele in invasive M1-type GAS. This leads to a lack of Scl1 expression in M3 strains. In contrast, while scl2 transcription and Scl2 production are elevated in M3 strains, M1 GAS lack Scl2 surface expression. M3-type strains were shown to have reduced biofilm formation on inanimate surfaces coated with cellular fibronectin and laminin, and in human skin equivalents. Repair of the nonsense mutation and restoration of Scl1 expression on M3-GAS cells, restores biofilm formation on cellular fibronectin and laminin coatings. Inactivation of scl1 in biofilm-capable M28 and M41 strains results in larger skin lesions in a mouse model, indicating that lack of Scl1 adhesin promotes bacterial spread over localized infection. These studies suggest the uniquely evolved scl1 locus in the M3-type strains, which prevents surface expression of the major Scl1 adhesin, contributed to the emergence of the invasive M3-type strains. Furthermore these studies provide insight into the molecular mechanisms mediating colonization, biofilm formation, and pathogenesis of group A streptococci.

Collagen structure: new tricks from a very old dog

The main features of the triple helical structure of collagen were deduced in the mid-1950s from fibre X-ray diffraction of tendons. Yet, the resulting models only could offer an average description of the molecular conformation. A critical advance came about 20 years later with the chemical synthesis of sufficiently long and homogeneous peptides with collagen-like sequences. The availability of these collagen model peptides resulted in a large number of biochemical, crystallographic and NMR studies that have revolutionized our understanding of collagen structure. High-resolution crystal structures from collagen model peptides have provided a wealth of data on collagen conformational variability, interaction with water, collagen stability or the effects of interruptions. Furthermore, a large increase in the number of structures of collagen model peptides in complex with domains from receptors or collagen-binding proteins has shed light on the mechanisms of collagen recognition. In recent years, collagen biochemistry has escaped the boundaries of natural collagen sequences. Detailed knowledge of collagen structure has opened the field for protein engineers who have used chemical biology approaches to produce hyperstable collagens with unnatural residues, rationally designed collagen heterotrimers, self-assembling collagen peptides, etc. This review summarizes our current understanding of the structure of the collagen triple helical domain (COL×3) and gives an overview of some of the new developments in collagen molecular engineering aiming to produce novel collagen-based materials with superior properties.

LDL acts as an opsonin enhancing the phagocytosis of group A Streptococcus by monocyte and whole human blood

Low-density lipoprotein (LDL) binds to group A Streptococcus (GAS) through Sc11 protein, and scavenger receptor CD36 of monocyte mediates the endocytosis of native or modified LDL. Therefore, we hypothesized that LDL might be an opsonin enhancing the phagocytosis of LDL-bound GAS by monocyte. The results showed that LDL could significantly promote U937 cell to phagocytose M28 (ATCC BAA1064) and M41 (ATCC 12373, AM41)-type GAS, and the phagocytosis rates were significantly increased, compared with LDL-free group. LDL, however, did not enhance the phagocytosis of M41 (CMCC 32198, CM41) or M6 (ATCC BAA946)-type GAS since these two strains did not bind to LDL. CD36 was the major scavenger receptor mediating the uptake of LDL-bound GAS by monocyte U937 cells since anti-CD36 antibody abolished the phagocytosis of LDL-opsonized GAS but anti-CD4 antibody did not. Most of AM41-type GAS cells were killed in human blood, whereas only a few CM41-type cells were phagocytosed. Moreover, recombinant Scl1 (rScl1) derived from M41-type GAS could significantly decrease the opsonophagocytosis of AM41 but not CM41-type GAS because the rScl1 competitively blocked the binding of AM41-type GAS to LDL. Therefore, our findings suggest that LDL may be an opsonin to enhance CD36-dependent opsonic phagocytosis of GAS by monocyte.

Endothelial cell response to chemical, biological, and physical cues in bioactive hydrogels

The highly tunable biological, chemical, and physical properties of bioactive hydrogels enable their use in an array of tissue engineering and drug delivery applications. Systematic modulation of these properties can be used to elucidate key cell-material interactions to improve therapeutic effects. For example, the rate and extent of endothelialization are critical to the long-term success of many blood-contacting devices. To this end, we have developed a bioactive hydrogel that could be used as coating on cardiovascular devices to enhance endothelial cell (EC) adhesion and migration. The current work investigates the relative impact of hydrogel variables on key endothelialization processes. The bioactive hydrogel is based on poly(ethylene glycol) (PEG) and a streptococcal collagen-like (Scl2-2) protein that has been modified with integrin α1β1 and α2β1 binding sites. The use of PEG hydrogels allows for incorporation of specific bioactive cues and independent manipulation of scaffold properties. The selective integrin binding of Scl2-2 was compared to more traditional collagen-modified PEG hydrogels to determine the effect of integrin binding on cell behavior. Protein functionalization density, protein concentration, and substrate modulus were independently tuned with both Scl2-2 and collagen to determine the effect of each variable on EC adhesion, spreading, and migration. The findings here demonstrate that increasing substrate modulus, decreasing functionalization density, and increasing protein concentration can be utilized to increase EC adhesion and migration. Additionally, PEG-Scl2-2 hydrogels had higher migration speeds and proliferation over 1 week compared with PEG-collagen gels, demonstrating that selective integrin binding can be used to enhance cell-material interactions. Overall, these studies contribute to the understanding of the effects of matrix cues on EC interactions and demonstrate the strong potential of PEG-Scl2-2 hydrogels to promote endothelialization of blood-contacting devices.

A Unique Set of the Burkholderia Collagen-Like Proteins Provides Insight into Pathogenesis, Genome Evolution and Niche Adaptation, and Infection Detection

Burkholderia pseudomallei and Burkholderia mallei, classified as category B priority pathogens, are significant human and animal pathogens that are highly infectious and broad-spectrum antibiotic resistant. Currently, the pathogenicity mechanisms utilized by Burkholderia are not fully understood, and correct diagnosis of B. pseudomallei and B. mallei infection remains a challenge due to limited detection methods. Here, we provide a comprehensive analysis of a set of 13 novel Burkholderia collagen-like proteins (Bucl) that were identified among B. pseudomallei and B. mallei select agents. We infer that several Bucl proteins participate in pathogenesis based on their noncollagenous domains that are associated with the components of a type III secretion apparatus and membrane transport systems. Homology modeling of the outer membrane efflux domain of Bucl8 points to a role in multi-drug resistance. We determined that bucl genes are widespread in B. pseudomallei and B. mallei; Fischer’s exact test and Cramer’s V² values indicate that the majority of bucl genes are highly associated with these pathogenic species versus nonpathogenic B. thailandensis. We designed a bucl-based quantitative PCR assay which was able to detect B. pseudomallei infection in a mouse with a detection limit of 50 CFU. Finally, chromosomal mapping and phylogenetic analysis of bucl loci revealed considerable genomic plasticity and adaptation of Burkholderia spp. to host and environmental niches. In this study, we identified a large set of phylogenetically unrelated bucl genes commonly found in Burkholderia select agents, encoding predicted pathogenicity factors, detection targets, and vaccine candidates.

Chronic Wound Dressings Based on Collagen-Mimetic Proteins

Objective: Chronic wounds are projected to reach epidemic proportions due to the aging population and the increasing incidence of diabetes. There is a strong clinical need for an improved wound dressing that can balance wound moisture, promote cell migration and proliferation, and degrade at an appropriate rate to minimize the need for dressing changes. Approach: To this end, we have developed a bioactive, hydrogel microsphere wound dressing that incorporates a collagen-mimetic protein, Scl2_GFPGER, to promote active wound healing. A redesigned Scl2_GFPGER, engineered collagen (eCol_GFPGER), was created to reduce steric hindrance of integrin-binding motifs and increase overall stability of the triple helical backbone, thereby resulting in increased cell adhesion to substrates. Results: This study demonstrates the successful modification of the Scl2_GFPGER protein to eCol_GFPGER, which displayed enhanced stability and integrin interactions. Fabrication of hydrogel microspheres provided a matrix with adaptive moisture technology, and degradation rates have potential for use in human wounds. Innovation: This collagen-mimetic wound dressing was designed to permit controlled modulation of cellular interactions and degradation rate without impact on other physical properties. Its fabrication into uniform hydrogel microspheres provides a bioactive dressing that can readily conform to irregular wounds. Conclusion: Overall, this new eCol_GFPGER shows strong promise in the generation of bioactive hydrogels for wound healing as well as a variety of tissue scaffolds.

Collagen-mimetic hydrogels promote human endothelial cell adhesion, migration and phenotypic maturation

This work evaluates the response of human aortic endothelial cells (HAECs) to thromboresistant collagen-mimetic hydrogel coatings toward improving the biocompatibility of existing "off-the-shelf" small-caliber vascular grafts. Specifically, bioactive hydrogels - previously shown to support α₁/α₂ integrin-mediated cell adhesion but to resist platelet activation - were fabricated by combining poly(ethylene glycol) (PEG) with a 120 kDa, triple-helical collagen-mimetic protein(Scl2-2) containing the GFPGER adhesion sequence. Analysis of HAECs seeded onto the resulting PEG-Scl2-2 hydrogels demonstrated that HAEC adhesion increased with increasing Scl2-2 concentration, while HAEC migration rate decreased over this same concentration range. In addition, evaluation of HAEC phenotype at confluence indicated significant differences in the gene expression of NOS3, thrombomodulin, and E-selectin on the PEG-Scl2-2 hydrogels relative to PEG-collagen controls. At the protein level, however, only NOS3 was significantly different between the PEG-Scl2-2 and PEG-collagen surfaces. Furthermore, PECAM-1 and VE-cadherin expression on PEG-Scl2-2 hydrogels versus PEG-collagen controls could not be distinguished at either the gene or protein level. Cumulatively, these data indicate the PEG-Scl2-2 hydrogels warrant further investigation as "off-the-shelf" graft coatings. In future studies, the Scl2-2 protein can potentially be modified to include additional extracellular matrix or cytokine binding sites to further improve endothelial cell responses.

Natural variant of collagen-like protein a in serotype M3 group a Streptococcus increases adherence and decreases invasive potential

Group A Streptococcus (GAS) predominantly exists as a colonizer of the human oropharynx that occasionally breaches epithelial barriers to cause invasive diseases. Despite the frequency of GAS carriage, few investigations into the contributory molecular mechanisms exist. To this end, we identified a naturally occurring polymorphism in the gene encoding the streptococcal collagen-like protein A (SclA) in GAS carrier strains. All previously sequenced invasive serotype M3 GAS possess a premature stop codon in the sclA gene truncating the protein. The carrier polymorphism is predicted to restore SclA function and was infrequently identified by targeted DNA sequencing in invasive strains of the same serotype. We demonstrate that a strain with the carrier sclA allele expressed a full-length SclA protein, while the strain with the invasive sclA allele expressed a truncated variant. An isoallelic mutant invasive strain with the carrier sclA allele exhibited decreased virulence in a mouse model of invasive disease and decreased multiplication in human blood. Further, the isoallelic invasive strain with the carrier sclA allele persisted in the mouse nasopharynx and had increased adherence to cultured epithelial cells. Repair of the premature stop codon in the invasive sclA allele restored the ability to bind the extracellular matrix proteins laminin and cellular fibronectin. These data demonstrate that a mutation in GAS carrier strains increases adherence and decreases virulence and suggest selection against increased adherence in GAS invasive isolates.

Preparation and characterization of monomers to tetramers of a collagen-like domain from Streptococcus pyogenes

The collagen like domain Scl2 from Streptococcus pyogenes has been proposed as a potential biomedical material. It is non-cytotoxic and non-immunogenic and can be prepared in good yield in fermentation. The Scl2 collagen domain is about a quarter of the length, 234 residues, of the main collagen type, mammalian type I collagen (1014 residues) that is currently used in biomedical devices. In the present study we have made constructs comprising 1 to 4 copies of the Scl2 collagen domain, plus these same constructs with a CysCys sequence at the C-terminal, analogous to that found in mammalian type III collagens. The yields of these constructs were examined from 2 L fermentation studies. The yields of both series declined with increasing size. Circular dichroism showed that the addition of further collagen domains did not lead to a change in the melting temperature compared to the monomer domain. Addition of the CysCys sequence led to a small additional stabilization of about 2-3°C for the monomer construct when the folding (V) domain was present.

Bioengineered collagens: emerging directions for biomedical materials

Mammalian collagen has been widely used as a biomedical material. Nevertheless, there are still concerns about the variability between preparations, particularly with the possibility that the products may transmit animal-based diseases. Many groups have examined the possible application of bioengineered mammalian collagens. However, translating laboratory studies into large-scale manufacturing has often proved difficult, although certain yeast and plant systems seem effective. Production of full-length mammalian collagens, with the required secondary modification to give proline hydroxylation, has proved difficult in E. coli. However, recently, a new group of collagens, which have the characteristic triple helical structure of collagen, has been identified in bacteria. These proteins are stable without the need for hydroxyproline and are able to be produced and purified from E. coli in high yield. Initial studies indicate that they would be suitable for biomedical applications.

Polyproline and triple helix motifs in host-pathogen recognition

Secondary structure elements often mediate protein-protein interactions. Despite their low abundance in folded proteins, polyproline II (PPII) and its variant, the triple helix, are frequently involved in protein-protein interactions, likely due to their peculiar propensity to be solvent-exposed. We here review the role of PPII and triple helix in mediating host-pathogen interactions, with a particular emphasis to the structural aspects of these processes. After a brief description of the basic structural features of these elements, examples of host-pathogen interactions involving these motifs are illustrated. Literature data suggest that the role played by PPII motif in these processes is twofold. Indeed, PPII regions may directly mediate interactions between proteins of the host and the pathogen. Alternatively, PPII may act as structural spacers needed for the correct positioning of the elements needed for adhesion and infectivity.

Recent investigations have highlighted that collagen triple helix is also a common target for bacterial adhesins. Although structural data on complexes between adhesins and collagen models are rather limited, experimental and theoretical studies have unveiled some interesting clues of the recognition process. Interestingly, very recent data show that not only is the triple helix used by pathogens as a target in the host-pathogen interaction but it may also act as a bait in these processes since bacterial proteins containing triple helix regions have been shown to interact with host proteins.

As both PPII and triple helix expose several main chain non-satisfied hydrogen bond acceptors and donors, both elements are highly solvated. The preservation of the solvation state of both PPII and triple helix upon protein-protein interaction is an emerging aspect that will be here thoroughly discussed.

Crystallization and preliminary X-ray crystallographic analysis of the variable domain of Scl2.3, a streptococcal collagen-like protein from invasive M3-type Streptococcus pyogenes

Streptococcal collagen-like proteins (Scls) are widely expressed by the well recognized human pathogen Streptococcus pyogenes. These surface proteins contain a signature central collagen-like region and an amino-terminal globular domain, termed the variable domain, which is protruded away from the cell surface by the collagen-like domain. Despite their recognized importance in bacterial pathogenicity, no structural information is presently available on proteins of the Scl class. The variable domain of Scl2 from invasive M3-type S. pyogenes has successfully been crystallized using vapour-diffusion methods. The crystals diffracted to 1.5 Å resolution and belonged to space group H32, with unit-cell parameters a = 44.23, b = 44.23, c = 227.83 Å. The crystal structure was solved by single-wavelength anomalous dispersion using anomalous signal from a europium chloride derivative.|

Bioactive hydrogels with enhanced initial and sustained cell interactions

The highly tunable properties of poly(ethylene glycol) (PEG)-based hydrogel systems permit their use in a wide array of regenerative medicine and drug delivery applications. One of the most valuable properties of PEG hydrogels is their intrinsic resistance to protein adsorption and cell adhesion, as it allows for a controlled introduction of desired bioactive factors including proteins, peptides, and drugs. Acrylate-PEG-N-hydroxysuccinimide (Acr-PEG-NHS) is widely utilized as a PEG linker to functionalize bioactive factors with photo-cross-linkable groups. This enables their facile incorporation into PEG hydrogel networks or the use of PEGylation strategies for drug delivery. However, PEG linkers can sterically block integrin binding sites on functionalized proteins and reduce cell-material interactions. In this study we demonstrate that reducing the density of PEG linkers on protein backbones during functionalization results in significantly improved cell adhesion and spreading to bioactive hydrogels. However, this reduction in functionalization density also increases protein loss from the matrix over time due to ester hydrolysis of the Acr-PEG-NHS linkers. To address this, a novel PEG linker, acrylamide-PEG-isocyanate (Aam-PEG-I), with enhanced hydrolytic stability was synthesized. It was found that decreasing functionalization density with Aam-PEG-I resulted in comparable increases in cell adhesion and spreading to Acr-PEG-NHS systems while maintaining protein and bioactivity levels within the hydrogel network over a significantly longer time frame. Thus, Aam-PEG-I provides a new option for protein functionalization for use in a wide range of applications that improves initial and sustained cell-material interactions to enhance control of bioactivity.

Construction and characterization of molecular nonwoven fabrics consisting of cross-linked poly(γ-methyl-L-glutamate)

Molecular nonwoven fabrics in the form of ultrathin layer-by-layer (LbL) helical polymer films with covalent cross-linking were assembled on substrates by an alternate ester-amide exchange reaction between poly(γ-methyl L-glutamate) (PMLG) and cross-linking agent ethylene diamine or 4,4'-diamino azobenzene. The regular growth of helical monolayers without excessive adsorption and the formation of amide bonds were confirmed by ultraviolet-visible (UV-vis) spectrophotometry, quartz crystal microbalance (QCM), ellipsometry, and infrared reflection-absorption spectroscopy (IR-RAS) measurements. Nanostructures with high uniformity and ultrathin films with few defects formed by helical rod segments of PMLG were characterized by atomic force microscopy (AFM) and Kelvin probe force microscopy (KFM).

Hydroxyproline-free single composition ABC collagen heterotrimer

Hydroxyproline plays a major role in stabilizing collagenous domains in eukaryotic organisms. Lack of this modification is associated with significant lowering in the thermal stability of the collagen triple helix and may also affect fibrillogenesis and folding of the peptide chains. In contrast, even though bacterial collagens lack hydroxyproline, their thermal stability is comparable to that of fibrillar collagen. This has been attributed to the high frequency of charged amino acids found in bacterial collagen. Here we report a thermally stable hydroxyproline-free ABC heterotrimeric collagen mimetic system composed of decapositive and decanegative peptides and a zwitterionic peptide. None of the peptides contain hydroxyproline, and furthermore the zwitterionic peptide does not even contain proline. The heterotrimer is electrostatically stabilized via multiple interpeptide lysine-aspartate and lysine-glutamate salt bridges and maintains good thermal stability with a melting temperature of 37 °C. The ternary peptide mixture also populates a single composition ABC heterotrimer as confirmed by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy. This system illustrates the power of axial salt bridges to direct and stabilize the self-assembly of a triple helix and may be useful in analogous designs in expression systems where the incorporation of hydroxyproline is challenging.

Collagen--emerging collagen based therapies hit the patient

The choice of biomaterials available for regenerative medicine continues to grow rapidly, with new materials often claiming advantages over the short-comings of those already in existence. Going back to nature, collagen is one of the most abundant proteins in mammals and its role is essential to our way of life. It can therefore be obtained from many sources including porcine, bovine, equine or human and offer a great promise as a biomimetic scaffold for regenerative medicine. Using naturally derived collagen, extracellular matrices (ECMs), as surgical materials have become established practice for a number of years. For clinical use the goal has been to preserve as much of the composition and structure of the ECM as possible without adverse effects to the recipient. This review will therefore cover in-depth both naturally and synthetically produced collagen matrices. Furthermore the production of more sophisticated three dimensional collagen scaffolds that provide cues at nano-, micro- and meso-scale for molecules, cells, proteins and bulk fluids by inducing fibrils alignments, embossing and layered configuration through the application of plastic compression technology will be discussed in details. This review will also shed light on both naturally and synthetically derived collagen products that have been available in the market for several purposes including neural repair, as cosmetic for the treatment of dermatologic defects, haemostatic agents, mucosal wound dressing and guided bone regeneration membrane. There are other several potential applications of collagen still under investigations and they are also covered in this review.

The group A streptococcal collagen-like protein-1, Scl1, mediates biofilm formation by targeting the extra domain A-containing variant of cellular fibronectin expressed in wounded tissue

Wounds are known to serve as portals of entry for group A Streptococcus (GAS). Subsequent tissue colonization is mediated by interactions between GAS surface proteins and host extracellular matrix components. We recently reported that the streptococcal collagen-like protein-1, Scl1, selectively binds the cellular form of fibronectin (cFn) and also contributes to GAS biofilm formation on abiotic surfaces. One structural feature of cFn, which is predominantly expressed in response to tissue injury, is the presence of a spliced variant containing extra domain A (EDA/EIIIA). We now report that GAS biofilm formation is mediated by the Scl1 interaction with EDA-containing cFn. Recombinant Scl1 proteins that bound cFn also bound recombinant EDA within the C-C' loop region recognized by the α(9)β(1) integrin. The extracellular 2-D matrix derived from human dermal fibroblasts supports GAS adherence and biofilm formation. Altogether, this work identifies and characterizes a novel molecular mechanism by which GAS utilizes Scl1 to specifically target an extracellular matrix component that is predominantly expressed at the site of injury in order to secure host tissue colonization.

The influence of specific binding of collagen-silk chimeras to silk biomaterials on hMSC behavior

Collagen-like proteins in the bacteria Streptococcus pyogenes adopt a triple-helix structure with a thermal stability similar to that of animal collagens, can be expressed in high yield in Escherichia coli and can be easily modified through molecular biology techniques. However, potential applications for such recombinant collagens are limited by their lack of higher order structure to achieve the physical properties needed for most biomaterials. To overcome this problem, the S. pyogenes collagen domain was fused to a repetitive Bombyx mori silk consensus sequence, as a strategy to direct specific non-covalent binding onto solid silk materials whose superior stability, mechanical and material properties have been previously established. This approach resulted in the successful binding of these new collagen-silk chimeric proteins to silk films and porous scaffolds, and the binding affinity could be controlled by varying the number of repeats in the silk sequence. To explore the potential of collagen-silk chimera for regulating biological activity, integrin (Int) and fibronectin (Fn) binding sequences from mammalian collagens were introduced into the bacterial collagen domain. The attachment of bioactive collagen-silk chimeras to solid silk biomaterials promoted hMSC spreading and proliferation substantially in comparison to the controls. The ability to combine the biomaterial features of silk with the biological activities of collagen allowed more rapid cell interactions with silk-based biomaterials, improved regulation of stem cell growth and differentiation, as well as the formation of artificial extracellular matrices useful for tissue engineering applications.

Multilayer vascular grafts based on collagen-mimetic proteins

A major roadblock in the development of an off-the-shelf, small-caliber vascular graft is achieving rapid endothelialization of the conduit while minimizing the risk of thrombosis, intimal hyperplasia, and mechanical failure. To address this need, a collagen-mimetic protein derived from group A Streptococcus, Scl2.28 (Scl2), was conjugated into a poly(ethylene glycol) (PEG) hydrogel to generate bioactive hydrogels that bind to endothelial cells (ECs) and resist platelet adhesion. The PEG-Scl2 hydrogel was then reinforced with an electrospun polyurethane mesh to achieve suitable biomechanical properties. In the current study, initial evaluation of this multilayer design as a potential off-the-shelf graft was conducted. First, electrospinning parameters were varied to achieve composite burst pressure, compliance, and suture retention strength that matched reported values of saphenous vein autografts. Composite stability following drying, sterilization, and physiological conditioning under pulsatile flow was then demonstrated. Scl2 bioactivity was also maintained after drying and sterilization as indicated by EC adhesion and spreading. Evaluation of platelet adhesion, aggregation, and activation indicated that PEG-Scl2 hydrogels had minimal platelet interactions and thus appear to provide a thromboresistant blood contacting layer. Finally, evaluation of EC migration speed demonstrated that PEG-Scl2 hydrogels promoted higher migration speeds than PEG-collagen analogs and that migration speed was readily tuned by altering protein concentration. Collectively, these results indicate that this multilayer design warrants further investigation and may have the potential to improve on current synthetic options.

Collagen Mimetic Peptides: Progress Towards Functional Applications

Traditionally, collagen mimetic peptides (CMPs) have been used for elucidating the structure of the collagen triple helix and the factors responsible for its stabilization. The wealth of fundamental knowledge on collagen structure and cell-extracellular matrix (ECM) interactions accumulated over the past decades has led to a recent burst of research exploring the potential of CMPs to recreate the higher order assembly and biological function of natural collagens for biomedical applications. Although a large portion of such research is still at an early stage, the collagen triple helix has become a promising structural motif for engineering self-assembled, hierarchical constructs similar to natural tissue scaffolds which are expected to exhibit unique or enhanced biological activities. This paper reviews recent progress in the field of collagen mimetic peptides that bears both direct and indirect implications to engineering collagen-like materials for potential biomedical use. Various CMPs and collagen-like proteins that mimic either structural or functional characteristics of natural collagens are discussed with particular emphasis on providing helpful information to bioengineers and biomaterials scientists interested in collagen engineering.