The FbaB-type fibronectin-binding protein of Streptococcus pyogenes promotes specific invasion into endothelial cells

A Versatile Magnetic Nanoplatform for Plug-and-Play Functionalization: Genetically Programmable Cargo Loading to Bacterial Magnetosomes by SpyCatcher "Click Biology"

Bacterial magnetosomes ("MAGs") represent a promising class of magnetic iron oxide nanoparticles with exceptional material characteristics and high application potential in the biomedical and biotechnological field. For the surface functionalization of MAGs with different protein cargos, their enveloping membrane can be addressed by genetic means. However, the expression of foreign polypeptides as translational fusion to magnetosome membrane proteins is still laborious and lacks versatility as the generated particles are monospecific and thus restricted to predetermined functions. Utilizing the SpyTag-SpyCatcher (ST-SC) bioconjugate system, we here establish a flexible platform for the targeted nanoassembly of multifunctional MAGs that combines the rapidity of chemical coupling (e.g., by cross-linking reactions) and the unmatched selectivity and controllability of in vivo functionalization. MAGs genetically engineered to display either SC- or ST-connectors are shown to efficiently bind a variety of complementary tagged (protein) cargo. Specifically, we cover a broad spectrum of representative functional moieties and foreign cargo (such as enzymes, antibodies, fluorophores, and silica beads) with relevance in biotechnology and biomedicine and demonstrate the interchangeability of the MAGs-adapted ST-SC system. For the controlled generation of artificial shells surrounding the particles, SC-MAGs are effectively coated by protein-corona proteins. The potential of the here-provided toolkit is even more enhanced by using SC-MAGs as an affinity tool for selective protein pulldown in vitro and in vivo. Overall, this innovative technology turns bacterial MAGs into a flexible magnetic nanoscaffold for the targeted plug-and-play display of virtually unlimited additional functionalities, thereby generating a multitude of magnetic hybrid materials that can be used in many applications.

Modular Bioorthogonal Lipid Nanoparticle Modification Platforms for Cardiac Homing

Lipid nanoparticles (LNPs) are becoming widely adopted as vectors for the delivery of therapeutic payloads but generally lack intrinsic tissue-homing properties. These extracellular vesicle (EV) mimetics can be targeted toward the liver, lung, or spleen via charge modification of their lipid headgroups. Homing to other tissues has only been achieved via covalent surface modification strategies using small-molecule ligands, peptides, or monoclonal antibodies─methods that are challenging to couple with large-scale manufacturing. Herein, we design a novel modular artificial membrane-binding protein (AMBP) platform for the modification of LNPs postformation. The system is composed of two protein modules that can be readily coupled using bioorthogonal chemistry to yield the AMBP. The first is a membrane anchor module comprising a supercharged green fluorescent protein (scGFP) electrostatically conjugated to a dynamic polymer surfactant corona. The second is a functional module containing a cardiac tissue fibronectin homing sequence from the bacterial adhesin CshA. We demonstrate that LNPs modified using the AMBP exhibit a 20-fold increase in uptake by fibronectin-rich C2C12 cells under static conditions and a 10-fold increase under physiologically relevant shear stresses, with no loss of cell viability. Moreover, we show targeted localization of the AMBP-modified LNPs in zebrafish hearts, highlighting their therapeutic potential as a vector for the treatment of cardiac disease and, more generally, as a smart vector.

Biological Applications of Synthetic Binders Isolated from a Conceptually New Adhiron Library

BACKGROUND

Adhirons are small (10 kDa) synthetic ligands that might represent an alternative to antibody fragments and to alternative scaffolds such as DARPins or affibodies.

METHODS

We prepared a conceptionally new adhiron phage display library that allows the presence of cysteines in the hypervariable loops and successfully panned it against antigens possessing different characteristics.

RESULTS

We recovered binders specific for membrane epitopes of plant cells by panning the library directly against pea protoplasts and against soluble C-Reactive Protein and SpyCatcher, a small protein domain for which we failed to isolate binders using pre-immune nanobody libraries. The best binders had a binding constant in the low nM range, were produced easily in bacteria (average yields of 15 mg/L of culture) in combination with different tags, were stable, and had minimal aggregation propensity, independent of the presence or absence of cysteine residues in their loops.

DISCUSSION

The isolated adhirons were significantly stronger than those isolated previously from other libraries and as good as nanobodies recovered from a naïve library of comparable theoretical diversity. Moreover, they proved to be suitable reagents for ELISA, flow cytometry, the western blot, and also as capture elements in electrochemical biosensors.

Functionalization of OMVs for Biocatalytic Applications

Outer membrane vesicles (OMVs) are miniature versions of gram-negative bacteria that contain almost the same content as their parent cells, particularly in terms of membrane composition. Using OMVs as biocatalysts is a promising approach due to their potential benefits, including their ability to be handled similarly to bacteria while lacking potentially pathogenic organisms. To employ OMVs as biocatalysts, they must be functionalized with immobilized enzymes to the OMV platform. Various enzyme immobilization techniques are available, including surface display and encapsulation, each with advantages and disadvantages depending on the objectives. This review provides a concise yet comprehensive overview of these immobilization techniques and their applications in utilizing OMVs as biocatalysts. Specifically, we discuss the use of OMVs in catalyzing the conversion of chemical compounds, their role in polymer degradation, and their performance in bioremediation.

ASFV proteins presented at the surface of T7 phages induce strong antibody responses in mice

African swine fever virus (ASFV) infection causes substantial economic losses to the swine industry worldwide, and there are still no safe and effective vaccines or therapeutics available. The granulated virus antigen improves the antigen present process and elicits high antibody reaction than the subunit antigen. In this study, the SpyTag peptide-p10 fusion protein was altered and displayed on the surface of the T7 phage to construct an engineered phage (T7-ST). At the same time, ASFV antigen-Spycatcher C-terminal-fused protein (antigen-SC) was expressed and purified by an E. coli prokaryotic expression system. Five virus-like particles (VLPs) displaying the main ASFV antigenic proteins P30, P54, P72, CD2v, and K145R were reconstructed by the isopeptide bond between SpyTag and antigen-SC proteins. The stability of five ASFV VLPs in high temperature and extreme pH conditions was evaluated by transmission electron microscopy (TEM) and plaque analysis. All ASFV VLPs induced a high titer antigen-specific antibody response in mice. Our results showed that the granulated antigen displaying ASFV protein on the surface of the T7 phage provides a robust potential vaccine and diagnostic tool to address the challenge of the ASFV pandemic.

Sequentially co-immobilized PET and MHET hydrolases via Spy chemistry in calcium phosphate nanocrystals present high-performance PET degradation

Accumulation of polyethylene terephthalate (PET) has brought an enormous threat to the ecosystem. The recently reported PET hydrolase (DuraPETase) and MHET hydrolase (MHETase) can synergistically catalyze the complete PET degradation. Hence, this work was designed to develop a bienzymatic cascade catalysis by co-immobilizing the two enzymes for PET biodegradation. DuraPETase and MHETase were sequentially co-immobilized in calcium phosphate nanocrystals (CaP) through SpyTag/SpyCatcher system. MHETase-SpyCatcher was first embedded inside the nanocrystals via biomimetic mineralization, and DuraPETase-SpyTag was then conjugated on the outlayer (~1.5 µm). The bienzyme compartmentalization facilitated DuraPETase interaction with the solid substrate, and the layered structures of the nanocrystals protected the enzymes, thus enhancing their stability. The high specific surface area of the nanocrystals and the proximity effects from the bienzymatic cascade were beneficial to the improved enzyme activity. Experimental data and molecular dynamics simulations revealed the activation effect of Ca²⁺ on DuraPETase. Taken together, the final results indicate that the PET degradation efficiency of DuraPETase-MHETase@CaP increased by 6.1 and 1.5 times over the free bienzyme system within 10 d at 40 °C and 50 °C, with weight losses at 32.2% and 50.3%, respectively. The bienzymatic cascade with DuraPETase-MHETase@CaP can completely degrade PET, contributing to the recycling of PET.

A Magnetosome-Based Platform for Flow Biocatalysis

Biocatalysis in flow reactor systems is of increasing importance for the transformation of the chemical industry. However, the necessary immobilization of biocatalysts remains a challenge. We here demonstrate that biogenic magnetic nanoparticles, so-called magnetosomes, represent an attractive alternative for the development of nanoscale particle formulations to enable high and stable conversion rates in biocatalytic flow processes. In addition to their intriguing material characteristics, such as high crystallinity, stable magnetic moments, and narrow particle size distribution, magnetosomes offer the unbeatable advantage over chemically synthesized nanoparticles that foreign protein "cargo" can be immobilized on the enveloping membrane via genetic engineering and thus, stably presented on the particle surface. To exploit these advantages, we develop a modular connector system in which abundant magnetosome membrane anchors are genetically fused with SpyCatcher coupling groups, allowing efficient covalent coupling with complementary SpyTag-functionalized proteins. The versatility of this approach is demonstrated by immobilizing a dimeric phenolic acid decarboxylase to SpyCatcher magnetosomes. The functionalized magnetosomes outperform similarly functionalized commercial particles by exhibiting stable substrate conversion during a 60 h period, with an average space-time yield of 49.2 mmol L^-1 h^-1. Overall, our results demonstrate that SpyCatcher magnetosomes significantly expand the genetic toolbox for particle surface functionalization and increase their application potential as nano-biocatalysts.

An antigen display system of GEM nanoparticles based on affinity peptide ligands

Gram-positive enhancer matrix (GEM) nanoparticles are often used in mucosal immunity, preparation of subunit vaccines or as an immune adjuvant due to its good immunological activities in recent years. Here, we designed and screened out a high affinity peptide ligand PL23, which could specifically target the non-epitope region of Classic Swine Fever Virus (CSFV) E2 protein, by virtual screening technology, enzyme linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR) test. The OD value of PL23 at 450 nm was reached 1.982, and the K_D value of it was 90.12 nM. Its binding capacity to protein was verified by SDS-PAGE as well. PL23 was subsequently conjugated to GEM nanoparticles by dehydration synthesis generating GEM-PL23 particles, and the GEM-PL-E2 particles were assembled after incubated with CSFV E2 protein. The cytotoxic test indicated that PL23, CSFV E2 protein, GEM nanoparticles, GEM-PL23 particles and GEM-PL-E2 particles were not toxic to cells and GEM nanoparticles could significantly promote the growth of APCs at high concentration for 1 h, p<0.001. In addition, GEM nanoparticles could promote the uptake of antigen by APCs. The cytokines tests suggested that GEM-PL-E2 particles could promote innate immune responses, regulate adaptive immune responses generated by T cells and APCs, and promote the differentiation and maturation of dendritic cells without producing inflammasomes. The results of immunological activity identification showed GEM-PL-E2 particles induced higher levels of both neutralizing antibodies and anti-CSFV antibodies than CSFV E2 protein in mice. This strategy provided a new, simpler, faster and cheaper method for assembling GEM nanoparticles, using an affinity peptide ligand replaced the protein anchor (PA), and provided a better application prospect for the application of GEM particles.

Overcoming Symmetry Mismatch in Vaccine Nanoassembly through Spontaneous Amidation

Matching of symmetry at interfaces is a fundamental obstacle in molecular assembly. Virus-like particles (VLPs) are important vaccine platforms against pathogenic threats, including Covid-19. However, symmetry mismatch can prohibit vaccine nanoassembly. We established an approach for coupling VLPs to diverse antigen symmetries. SpyCatcher003 enabled efficient VLP conjugation and extreme thermal resilience. Many people had pre-existing antibodies to SpyTag:SpyCatcher but less to the 003 variants. We coupled the computer-designed VLP not only to monomers (SARS-CoV-2) but also to cyclic dimers (Newcastle disease, Lyme disease), trimers (influenza hemagglutinins), and tetramers (influenza neuraminidases). Even an antigen with dihedral symmetry could be displayed. For the global challenge of influenza, SpyTag-mediated display of trimer and tetramer antigens strongly induced neutralizing antibodies. SpyCatcher003 conjugation enables nanodisplay of diverse symmetries towards generation of potent vaccines.

Overcoming Symmetry Mismatch in Vaccine Nanoassembly through Spontaneous Amidation

Matching of symmetry at interfaces is a fundamental obstacle in molecular assembly. Virus-like particles (VLPs) are important vaccine platforms against pathogenic threats, including Covid-19. However, symmetry mismatch can prohibit vaccine nanoassembly. We established an approach for coupling VLPs to diverse antigen symmetries. SpyCatcher003 enabled efficient VLP conjugation and extreme thermal resilience. Many people had pre-existing antibodies to SpyTag:SpyCatcher but less to the 003 variants. We coupled the computer-designed VLP not only to monomers (SARS-CoV-2) but also to cyclic dimers (Newcastle disease, Lyme disease), trimers (influenza hemagglutinins), and tetramers (influenza neuraminidases). Even an antigen with dihedral symmetry could be displayed. For the global challenge of influenza, SpyTag-mediated display of trimer and tetramer antigens strongly induced neutralizing antibodies. SpyCatcher003 conjugation enables nanodisplay of diverse symmetries towards generation of potent vaccines.

Live long and active: Polypeptide-mediated assembly of antibody variable fragments

Antibodies possess multiple biologically relevant features that have been engineered into new therapeutic formats. Two examples include the adaptable specificity of their variable (Fv) region and the extension of plasma circulation times through their crystallizable (Fc) region. Since the invention of the single chain variable fragment (scFv) in 1988, antibody variable regions have been re-engineered into a wide variety of multifunctional nanostructures. Among these strategies, peptide-mediated self-assembly of variable regions through heterologous expression has become a powerful method to produce homogenous, functional biomaterials. This manuscript reviews recent reports of antibody fragments assembled through fusion with peptides and proteins, including elastin-like polypeptides (ELPs), collagen-like polypeptides (CLPs), albumin, transmembrane proteins, leucine zippers, silk protein, and viruses. This review further discusses the current clinical status of engineered antibody fragments and challenges to overcome.

Covalent protein display on Hepatitis B core-like particles in plants through the in vivo use of the SpyTag/SpyCatcher system

Virus-like particles (VLPs) can be used as nano-carriers and antigen-display systems in vaccine development and therapeutic applications. Conjugation of peptides or whole proteins to VLPs can be achieved using different methods such as the SpyTag/SpyCatcher system. Here we investigate the conjugation of tandem Hepatitis B core (tHBcAg) VLPs and the model antigen GFP in vivo in Nicotiana benthamiana. We show that tHBcAg VLPs could be successfully conjugated with GFP in the cytosol and ER without altering VLP formation or GFP fluorescence. Conjugation in the cytosol was more efficient when SpyCatcher was displayed on tHBcAg VLPs instead of being fused to GFP. This effect was even more obvious in the ER, showing that it is optimal to display SpyCatcher on the tHBcAg VLPs and SpyTag on the binding partner. To test transferability of the GFP results to other antigens, we successfully conjugated tHBcAg VLPs to the HIV capsid protein P24 in the cytosol. This work presents an efficient strategy which can lead to time and cost saving post-translational, covalent conjugation of recombinant proteins in plants.

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.

A 192-heme electron transfer network in the hydrazine dehydrogenase complex

Anaerobic ammonium oxidation (anammox) is a major process in the biogeochemical nitrogen cycle in which nitrite and ammonium are converted to dinitrogen gas and water through the highly reactive intermediate hydrazine. So far, it is unknown how anammox organisms convert the toxic hydrazine into nitrogen and harvest the extremely low potential electrons (-750 mV) released in this process. We report the crystal structure and cryo electron microscopy structures of the responsible enzyme, hydrazine dehydrogenase, which is a 1.7 MDa multiprotein complex containing an extended electron transfer network of 192 heme groups spanning the entire complex. This unique molecular arrangement suggests a way in which the protein stores and releases the electrons obtained from hydrazine conversion, the final step in the globally important anammox process.

Intracellular Streptococcal Uptake and Persistence: A Potential Cause of Erysipelas Recurrence

Erysipelas is a severe streptococcal infection of the skin primarily spreading through the lymphatic vessels. Penicillin is the treatment of choice. The most common complication consists in relapses which occur in up to 40% or more of patients despite appropriate antibiotic treatment. They cause lymphatic damage resulting in irreversible lymphedema and ultimately elephantiasis nostras and lead to major health restrictions and high socio-medical costs. Prevention of relapses is an unmet need, because even long-term prophylactic penicillin application does eventually not reduce the risk of recurrence. In this article we assess risk factors and causes of erysipelas recurrence. A systematic literature search for clinical studies addressing potential causes and measures for prevention of erysipelas recurrence was combined with a review of experimental and clinical data assessing the ability and clinical relevance of streptococci for intracellular uptake and persistence. The literature review found that venous insufficiency, lymphedema, and intertrigo from fungal infections are considered to be major risk factors for recurrence of erysipelas but cannot adequately explain the high recurrence rate. As hitherto unrecognized likely cause of erysipelas relapses we identify the ability of streptococci for intracellular uptake into and persistence within epithelial and endothelial cells and macrophages. This creates intracellular streptococcal reservoirs out of reach of penicillins which do not reach sufficient bactericidal intracellular concentrations. Incomplete streptococcal elimination due to intracellular streptococcal persistence has been observed in various deep tissue infections and is considered as cause of relapsing streptococcal pharyngitis despite proper antibiotic treatment. It may also serves as endogenous infectious source of erysipelas relapses. We conclude that the current antibiotic treatment strategies and elimination of conventional risk factors employed in erysipelas management are insufficient to prevent erysipelas recurrence. The reactivation of streptococcal infection from intracellular reservoirs represents a plausible explanation for the frequent occurrence erysipelas relapses. Prevention of erysipelas relapses therefore demands for novel antibiotic strategies capable of eradicating intracellular streptococcal persistence.

NAD-Glycohydrolase Depletes Intracellular NAD+ and Inhibits Acidification of Autophagosomes to Enhance Multiplication of Group A Streptococcus in Endothelial Cells

Group A Streptococcus (GAS) is a human pathogen causing a wide spectrum of diseases, from mild pharyngitis to life-threatening necrotizing fasciitis. GAS has been shown to evade host immune killing by invading host cells. However, how GAS resists intracellular killing by endothelial cells is still unclear. In this study, we found that strains NZ131 and A20 have higher activities of NADase and intracellular multiplication than strain SF370 in human endothelial cells (HMEC-1). Moreover, nga mutants of NZ131 (SW957 and SW976) were generated to demonstrate that NADase activity is required for the intracellular growth of GAS in endothelial cells. We also found that intracellular levels of NAD+ and the NAD+/NADH ratio of NZ131-infected HMEC-1 cells were both lower than in cells infected by the nga mutant. Although both NZ131 and its nga mutant were trapped by LC3-positive vacuoles, only nga mutant vacuoles were highly co-localized with acidified lysosomes. On the other hand, intracellular multiplication of the nga mutant was increased by bafilomycin A1 treatment. These results indicate that NADase causes intracellular NAD+ imbalance and impairs acidification of autophagosomes to escape autophagocytic killing and enhance multiplication of GAS in endothelial cells.

Caveolin 1 restricts Group A Streptococcus invasion of nonphagocytic host cells

Caveolae are composed of 2 major proteins, caveolin 1 (CAV1) and cavin 1 or polymerase transcript release factor I (CAVIN1). Here, we demonstrate that CAV1 levels modulate invasion of Group A Streptococcus (GAS) into nonphagocytic mammalian cells. GAS showed enhanced internalisation into CAV1-knockout mouse embryonic fibroblasts and CAV1 knockdown human epithelial HEp-2 cells, whereas overexpression of CAV1 in HEp-2 cells reduced GAS invasion. This effect was not dependent on the expression of the GAS fibronectin binding protein SfbI, which had previously been implicated in caveolae-mediated uptake. Nor was this effect dependent on CAVIN1, as knockout of CAVIN1 in mouse embryonic fibroblasts resulted in reduced GAS internalisation. Although CAV1 restricted GAS invasion into host cells, we observed only minimal association of invading GAS (strain M1T1⁵⁴⁴⁸ ) with CAV1 by immunofluorescence and very low association of invading M1T1⁵⁴⁴⁸ with caveolae by transmission electron microscopy. These observations suggest that physical interaction with caveolae is not needed for CAV1 restriction of invading GAS. An indirect mechanism of action is also consistent with the finding that changing membrane fluidity reverses the increased invasion observed in CAV1-null cells. Together, these results suggest that CAV1 protects host cells against GAS invasion by a caveola-independent mechanism.

Clinical and molecular characteristics of infective β-hemolytic streptococcal endocarditis

Streptococcus pyogenes (S. pyogenes) and Streptococcus dysgalactiae subspecies equisimilis (SDSE) cause considerable morbidity and mortality, and show similarities in disease manifestations and pathogenic mechanisms. Their involvement in infective endocarditis, however, has not been well described. Invasive S. pyogenes and SDSE infections in Health Region Bergen, Norway, in the period 1999-2013 were reviewed, and sixteen cases of endocarditis were identified. The median duration of symptoms was 2.5days, the frequency of embolic events 50%, 38% received valve replacement and the 30-day mortality was 25%. In S. pyogenes, a significant correlation was observed between the repertoire of fibronectin-binding genes, phenotypic binding ability to fibronectin and disease manifestations. Conversely, no associations between phenotypic and genotypic characteristics were detected in SDSE. S. pyogenes and SDSE endocarditis is characterized by rapid and severe clinical manifestations. The pathogenesis is multifactorial, but our results infer a potential role of fibronectin binding in the development of S. pyogenes endocarditis.

Immunization with a streptococcal multiple-epitope recombinant protein protects mice against invasive group A streptococcal infection

Streptococcus pyogenes (group A Streptococcus; GAS) causes clinical diseases, including pharyngitis, scarlet fever, impetigo, necrotizing fasciitis and streptococcal toxic shock syndrome. A number of group A streptococcus vaccine candidates have been developed, but only one 26-valent recombinant M protein vaccine has entered clinical trials. Differing from the design of a 26-valent recombinant M protein vaccine, we provide here a vaccination using the polyvalence epitope recombinant FSBM protein (rFSBM), which contains four different epitopes, including the fibronectin-binding repeats domain of streptococcal fibronectin binding protein Sfb1, the C-terminal immunogenic segment of streptolysin S, the C3-binding motif of streptococcal pyrogenic exotoxin B, and the C-terminal conserved segment of M protein. Vaccination with the rFSBM protein successfully prevented mortality and skin lesions caused by several emm strains of GAS infection. Anti-FSBM antibodies collected from the rFSBM-immunized mice were able to opsonize at least six emm strains and can neutralize the hemolytic activity of streptolysin S. Furthermore, the internalization of GAS into nonphagocytic cells is also reduced by anti-FSBM serum. These findings suggest that rFSBM can be applied as a vaccine candidate to prevent different emm strains of GAS infection.

Bcl-xL Affects Group A Streptococcus-Induced Autophagy Directly, by Inhibiting Fusion between Autophagosomes and Lysosomes, and Indirectly, by Inhibiting Bacterial Internalization via Interaction with Beclin 1-UVRAG

Anti-apoptotic Bcl-2 and Bcl-xL are proposed to regulate starvation-induced autophagy by directly interacting with Beclin 1. Beclin 1 is also thought to be involved in multiple vesicle trafficking pathways such as endocytosis by binding to Atg14L and UVRAG. However, how the interaction of Bcl-2 family proteins and Beclin 1 regulates anti-bacterial autophagy (xenophagy) is still unclear. In this study, we analyzed these interactions using Group A Streptococcus (GAS; Streptococcus pyogenes) infection as a model. GAS is internalized into epithelial cells through endocytosis, while the intracellular fate of GAS is degradation by autophagy. Here, we found that Bcl-xL but not Bcl-2 regulates GAS-induced autophagy. Autophagosome-lysosome fusion and the internalization process during GAS infection were promoted in Bcl-xL knockout cells. In addition, knockout of Beclin 1 phenocopied the internalization defect of GAS. Furthermore, UVRAG interacts not only with Beclin 1 but also with Bcl-xL, and overexpression of UVRAG partially rescued the internalization defect of Beclin 1 knockout cells during GAS infection. Thus, our results indicate that Bcl-xL inhibits GAS-induced autophagy directly by suppressing autophagosome-lysosome fusion and indirectly by suppressing GAS internalization via interaction with Beclin 1-UVRAG.

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.

Streptococcus pyogenes adhesion and colonization

Streptococcus pyogenes (group A Streptococcus, GAS) is a human-adapted pathogen responsible for a wide spectrum of disease. GAS can cause relatively mild illnesses, such as strep throat or impetigo, and less frequent but severe life-threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. GAS is an important public health problem causing significant morbidity and mortality worldwide. The main route of GAS transmission between humans is through close or direct physical contact, and particularly via respiratory droplets. The upper respiratory tract and skin are major reservoirs for GAS infections. The ability of GAS to establish an infection in the new host at these anatomical sites primarily results from two distinct physiological processes, namely bacterial adhesion and colonization. These fundamental aspects of pathogenesis rely upon a variety of GAS virulence factors, which are usually under strict transcriptional regulation. Considerable progress has been made in better understanding these initial infection steps. This review summarizes our current knowledge of the molecular mechanisms of GAS adhesion and colonization.

Tissue tropisms in group A Streptococcus: what virulence factors distinguish pharyngitis from impetigo strains?

PURPOSE OF REVIEW

Group A streptococci (GAS) are a common cause of pharyngitis and impetigo, and distinct throat strains and skin strains have been long recognized. This review aims to describe recent advances in molecular differences between throat and skin strains, and the pathogenic mechanisms used by virulence factors that may distinguish between these two groups.

RECENT FINDINGS

Recent findings include a new typing scheme for GAS strains based on sequence clusters of genes encoding the entire surface-exposed portion of M protein; correlations between emm-based typing schemes, clinical disease and surface adhesins; covalent bond formation mediated by GAS pili and other adhesins in binding to host ligands; a key role for superantigens in oropharyngeal infection via binding major histocompatibility complex class II antigen; and migration of GAS-specific Th17 cells from the upper respiratory tract to the brain, which may be relevant to autoimmune sequelae.

SUMMARY

The gap between molecular markers of disease (correlation) and virulence mechanisms (causation) in the establishment of tissue tropisms for GAS infection currently remains wide, but the gap also continues to narrow. Whole genome sequencing combined with mutant construction and improvements in animal models for oropharyngeal infection by GAS may help pave the way for new discoveries.

Insufficient Acidification of Autophagosomes Facilitates Group A Streptococcus Survival and Growth in Endothelial Cells

Group A streptococcus (GAS) is an important human pathogen, and its invasion via blood vessels is critically important in serious events such as bacteremia or multiorgan failure. Although GAS was identified as an extracellular bacterium, the internalization of GAS into nonphagocytic cells may provide a strategy to escape from immune surveillance and antibiotic killing. However, GAS has also been reported to induce autophagy and is efficiently killed within lysosome-fused autophagosomes in epithelial cells. In this study, we show that GAS can replicate in endothelial cells and that streptolysin O is required for GAS growth. Bacterial replication can be suppressed by altering GAS gene expression in an acidic medium before internalization into endothelial cells. The inhibitory effect on GAS replication can be reversed by treatment with bafilomycin A1, a specific inhibitor of vacuolar-type H⁺-ATPase. Compared with epithelial cells in which acidification causes autophagy-mediated clearance of GAS, there was a defect in acidification of GAS-containing vesicles in endothelial cells. Consequently, endothelial cells fail to maintain low pH in GAS-containing autophagosomes, thereby permitting GAS replication inside LAMP-1- and LC3-positive vesicles. Furthermore, treatment of epithelial cells with bafilomycin A1 resulted in defective GAS clearance by autophagy, with subsequent bacterial growth intracellularly. Therefore, low pH is a key factor for autophagy-mediated suppression of GAS growth inside epithelial cells, while defective acidification of GAS-containing vesicles results in bacterial growth in endothelial cells.

Previous reports showed that GAS can induce autophagy and is efficiently killed within lysosome-fused autophagosomes in epithelial cells. In endothelial cells, in contrast, induction of autophagy is not sufficient for GAS killing. In this study, we provide the first evidence that low pH is required to prevent intracellular growth of GAS in epithelial cells and that this mechanism is defective in endothelial cells. Treatment of GAS with low pH altered GAS growth rate and gene expression of virulence factors and resulted in enhanced susceptibility of GAS to intracellular lysosomal killing. Our findings reveal the existence of different mechanisms of host defense against GAS invasion between epithelial and endothelial cells.

From Single Cells to Engineered and Explanted Tissues: New Perspectives in Bacterial Infection Biology

Cell culture techniques are essential for studying host-pathogen interactions. In addition to the broad range of single cell type-based two-dimensional cell culture models, an enormous amount of coculture systems, combining two or more different cell types, has been developed. These systems enable microscopic visualization and molecular analyses of bacterial adherence and internalization mechanisms and also provide a suitable setup for various biochemical, immunological, and pharmacological applications. The implementation of natural or synthetical scaffolds elevated the model complexity to the level of three-dimensional cell culture. Additionally, several transwell-based cell culture techniques are applied to study bacterial interaction with physiological tissue barriers. For keeping highly differentiated phenotype of eukaryotic cells in ex vivo culture conditions, different kinds of microgravity-simulating rotary-wall vessel systems are employed. Furthermore, the implementation of microfluidic pumps enables constant nutrient and gas exchange during cell cultivation and allows the investigation of long-term infection processes. The highest level of cell culture complexity is reached by engineered and explanted tissues which currently pave the way for a more comprehensive view on microbial pathogenicity mechanisms.

Superglue from bacteria: unbreakable bridges for protein nanotechnology

Biotechnology is often limited by weak interactions. We suggest that an ideal interaction between proteins would be covalent, specific, require addition of only a peptide tag to the protein of interest, and form under a wide range of conditions. Here we summarize peptide tags that are able to form spontaneous amide bonds, based on harnessing reactions of adhesion proteins from the bacterium Streptococcus pyogenes. These include the irreversible peptide-protein interaction of SpyTag with SpyCatcher, as well as irreversible peptide-peptide interactions via SpyLigase. We describe existing applications, including polymerization to enhance cancer cell capture, assembly of living biomaterial, access to diverse protein shapes, and improved enzyme resilience. We also indicate future opportunities for resisting biological force and extending the scope of protein nanotechnology.

SpyLigase peptide-peptide ligation polymerizes affibodies to enhance magnetic cancer cell capture

Individual proteins can now often be modified with atomic precision, but there are still major obstacles to connecting proteins into larger assemblies. To direct protein assembly, ideally, peptide tags would be used, providing the minimal perturbation to protein function. However, binding to peptides is generally weak, so assemblies are unstable over time and disassemble with force or harsh conditions. We have recently developed an irreversible protein-peptide interaction (SpyTag/SpyCatcher), based on a protein domain from Streptococcus pyogenes, that locks itself together via spontaneous isopeptide bond formation. Here we develop irreversible peptide-peptide interaction, through redesign of this domain and genetic dissection into three parts: a protein domain termed SpyLigase, which now ligates two peptide tags to each other. All components expressed efficiently in Escherichia coli and peptide tags were reactive at the N terminus, at the C terminus, or at internal sites. Peptide-peptide ligation enabled covalent and site-specific polymerization of affibodies or antibodies against the tumor markers epidermal growth factor receptor (EGFR) and HER2. Magnetic capture of circulating tumor cells (CTCs) is one of the most promising approaches to improve cancer prognosis and management, but CTC capture is limited by inefficient recovery of cells expressing low levels of tumor antigen. SpyLigase-assembled protein polymers made possible the isolation of cancerous cells expressing lower levels of tumor antigen and should have general application in enhancing molecular capture.

The M1 protein of Streptococcus pyogenes triggers an innate uptake mechanism into polarized human endothelial cells

Serotype M1 Streptococcus pyogenes is a major human pathogen associated with severe invasive diseases causing high morbidity and mortality. In a substantial number of cases, invasive disease develops in previously healthy individuals with no obvious port of entry. This has led to the hypothesis that the source of streptococci in these cases is a transient bacteraemia. This study focuses on the analysis of interaction of tissue-invasive serotype M1 S. pyogenes with human endothelial cells (EC) of the vascular system. We identify the M1 surface protein of S. pyogenes as the EC invasin which is recognised by polarized human blood EC, thereby triggering rapid, phagocytosis-like uptake of streptococci into polarized EC layers. Upon internalization, the M1 S. pyogenes serotype is incorporated into phagosomes which traffic via the endosomal/lysosomal pathway. However, some of the streptococci successfully evade this innate killing process and hereby mediate their escape into the cytoplasm of the host cell. The results of this study demonstrate that blood EC possess an efficient uptake mechanism for serotype M1 S. pyogenes. Despite efficient phagocytosis, streptococcal survival within EC constitutes one potential mechanism which favours intracellular persistence and thus facilitates continuous infection and dissemination from the primary side of infection into deep tissue.

BsaB, a novel adherence factor of group B Streptococcus

Streptococcus agalactiae (group B Streptococcus [GBS]) is a leading cause of neonatal sepsis and meningitis, peripartum infections in women, and invasive infections in chronically ill or elderly individuals. GBS can be isolated from the gastrointestinal or genital tracts of up to 30% of healthy adults, and infection is thought to arise from invasion from a colonized mucosal site. Accordingly, bacterial surface components that mediate attachment of GBS to host cells or the extracellular matrix represent key factors in the colonization and infection of the human host. We identified a conserved GBS gene of unknown function that was predicted to encode a cell wall-anchored surface protein. Deletion of the gene and a cotranscribed upstream open reading frame (ORF) in GBS strain 515 reduced bacterial adherence to VK2 vaginal epithelial cells in vitro and reduced GBS binding to fibronectin-coated microtiter wells. Expression of the gene product in Lactococcus lactis conferred the ability to adhere to VK2 cells, to fibronectin and laminin, and to fibronectin-coated ME-180 cervical epithelial cells. Expression of the recombinant protein in L. lactis also markedly increased biofilm formation. The adherence function of the protein, named bacterial surface adhesin of GBS (BsaB), depended both on a central BID1 domain found in bacterial intimin-like proteins and on the C-terminal portion of the BsaB protein. Expression of BsaB in GBS, like that of several other adhesins, was regulated by the CsrRS two-component system. We conclude that BsaB represents a newly identified adhesin that participates in GBS attachment to epithelial cells and the extracellular matrix.

The fibronectin-binding motif within FlpA facilitates Campylobacter jejuni adherence to host cell and activation of host cell signaling

Campylobacter jejuni is a gram-negative, curved and rod-shaped bacterium that causes human gastroenteritis. Acute disease is associated with C. jejuni invasion of the intestinal epithelium. Epithelial cells infected with C. jejuni strains containing mutations in the FlpA and CadF fibronectin (Fn)-binding proteins exhibit reduced invasion of host cells and a C. jejuni CadF FlpA double mutant is impaired in the activation of epidermal growth factor receptor (EGFR) and Rho GTPase Rac1. Although these observations establish a role for Fn-binding proteins during C. jejuni invasion, their mechanistic contributions to invasion-associated signaling are unclear. We examined FlpA, a C. jejuni Fn-binding protein composed of three FNIII-like repeats D1, D2 and D3, to identify the interactions required for cellular adherence on pathogen-induced host cell signaling. We report that FlpA binds the Fn gelatin-binding domain via a motif within the D2 repeat. Epithelial cells infected with a flpA mutant exhibited decreased Rac1 activation and reduced membrane ruffling that coincided with impaired delivery of the secreted Cia proteins and reduced cell association. Phosphorylation of the Erk1/2 kinase, a downstream effector of EGFR signaling, was specifically associated with FlpA-mediated activation of β₁-integrin and EGFR signaling. In vivo experiments revealed that FlpA is necessary for C. jejuni disease based on bacterial dissemination to the spleen of IL-10^−/− germ-free mice. Thus, a novel Fn-binding motif within FlpA potentiates activation of Erk1/2 signaling via β₁-integrin during C. jejuni infection.

Contribution of fibronectin-binding protein to pathogenesis of Streptococcus equi ssp. zooepidemicus

Streptococcus equi ssp. zooepidemicus (S. zooepidemicus) is responsible for a wide variety of infections in many species. Fibronectin-binding protein is a bacterial cell surface protein, which specifically binds fibronectin (FN). Considering the specific role of FN-binding protein in host-pathogen interactions, we investigated the function of a novel FN-binding domain in the FN-binding protein (FNZ) of S. zooepidemicus. Five recombinant FNZ gene fragments [N1 (amino acids, 38-197), N2 (amino acids, 38-603), N3 (amino acids, 41-315), N4 (amino acids, 192-370), and N5 (amino acids, 38-225)] were expressed in Escherichia coli, and their FN-binding activities were tested. The results showed that amino acids 192-225 in the NH2 -terminal region of FNZ could be responsible for binding fibronectin. The FNZ knockout mutant was constructed in S. zooepidemicus, which results in the reduced capacity to adhere to HEp-2 cell, defective virulence in vivo, decreased biofilm formation, and decreased colonization capacity in blood, liver, lung, and spleen tissues of mice as compared to the wild type. These results suggest that FNZ participates in biofilm formation, FN binding, cell adhesion, and pathogenesis of S. zooepidemicus. Furthermore, this work offers a novel FN-binding domain within FNZ, which will help in further characterization of S. zooepidemicus FN-binding properties.

Coiled-coil irregularities of the M1 protein structure promote M1-fibrinogen interaction and influence group A Streptococcus host cell interactions and virulence

Group A Streptococcus (GAS) is a human pathogen causing a wide range of mild to severe and life-threatening diseases. The GAS M1 protein is a major virulence factor promoting GAS invasiveness and resistance to host innate immune clearance. M1 displays an irregular coiled-coil structure, including the B-repeats that bind fibrinogen. Previously, we found that B-repeat stabilisation generates an idealised version of M1 (M1) characterised by decreased fibrinogen binding in vitro. To extend these findings based on a soluble truncated version of M1, we now studied the importance of the B-repeat coiled-coil irregularities in full length M1 and M1 expressed in live GAS and tested whether the modulation of M1-fibrinogen interactions would open up novel therapeutic approaches. We found that altering either the M1 structure on the GAS cell surface or removing its target host protein fibrinogen blunted GAS virulence. GAS expressing M1 showed an impaired ability to adhere to and to invade human endothelial cells, was more readily killed by whole blood or neutrophils and most importantly was less virulent in a murine necrotising fasciitis model. M1-mediated virulence of wild-type GAS was strictly dependent on the presence and concentration of fibrinogen complementing our finding that M1-fibrinogen interactions are crucial for GAS virulence. Consistently blocking M1-fibrinogen interactions by fragment D reduced GAS virulence in vitro and in vivo. This supports our conclusion that M1-fibrinogen interactions are crucial for GAS virulence and that interference may open up novel complementary treatment options for GAS infections caused by the leading invasive GAS strain M1.

Differential interactions of Streptococcus gordonii and Staphylococcus aureus with cultured osteoblasts

The impedance of normal osteoblast function by microorganisms is at least in part responsible for the failure of dental or orthopedic implants. Staphylococcus aureus is a major pathogen of bone, and exhibits high levels of adhesion and invasion of osteoblasts. In this article we show that the commensal oral bacterium Streptococcus gordonii also adheres to and is internalized by osteoblasts. Entry of S. gordonii cells had typical features of phagocytosis, similar to S. aureus, with membrane protrusions characterizing initial uptake, and closure of the osteoblast membrane leading to engulfment. The sensitivities of S. gordonii internalization to inhibitors cytochalasin D, colchicine and monensin indicated uptake through endocytosis, with requirement for actin accumulation. Internalization levels of S. gordonii were enhanced by expression of S. aureus fibronectin-binding protein A (FnBPA) on the S. gordonii cell surface. Lysosomal-associated membrane protein-1 phagosomal membrane marker accumulated with intracellular S. aureus and S. gordonii FnBPA, indicating trafficking of bacteria into the late endosomal/lysosomal compartment. Streptococcus gordonii cells did not survive intracellularly for more than 12 h, unless expressing FnBPA, whereas S. aureus showed extended survival times (>48 h). Both S. aureus and S. gordonii DL-1 elicited a rapid interleukin-8 response by osteoblasts, whereas S. gordonii FnBPA was slower. Only S. aureus elicited an interleukin-6 response. Hence, S. gordonii invades osteoblasts by a mechanism similar to that exhibited by S. aureus, and elicits a proinflammatory response that may promote bone resorption.

Integrins and small GTPases as modulators of phagocytosis

Phagocytosis is the mechanism whereby cells engulf large particles. This process has long been recognized as a critical component of the innate immune response, which constitutes the organism's defense against microorganisms. In addition, phagocytic internalization of apoptotic cells or cell fragments plays important roles in tissue homeostasis and remodeling. Phagocytosis requires target interactions with receptors on the plasma membrane of the phagocytic cell. Integrins have been identified as important mediators of particle clearance, in addition to their well-established roles in cell adhesion, migration and mechanotransduction. Indeed, these ubiquitously expressed proteins impart phagocytic capacity to epithelial, endothelial and mesenchymal cell types. The importance of integrins in particle internalization is emphasized by the ability of microbial and viral pathogens to exploit their signaling pathways to invade host cells, and by the wide variety of disorders that arise from abnormalities in integrin-dependent phagocytic uptake.

The PavA-like fibronectin-binding protein of Enterococcus faecalis, EfbA, is important for virulence in a mouse model of ascending urinary tract infection

Enterococcus faecalis is an established nosocomial pathogen, yet the pathogenesis of enterococcal infections, particularly of urinary tract infections (UTIs), remains to be fully elucidated. Fibronectin-binding proteins have been identified as potent adhesins in pathogenic Gram-positive cocci. Here, we characterized EfbA, which is encoded by the enterococcal orthologue of Streptococcus pneumoniae pavA. Similar to PavA, the anchorless EfbA protein was localized to the enterococcal cell outer surface and bound to immobilized human fibronectin. In addition to abrogated EfbA expression, deletion of the efbA gene eliminated EfbA from the cell surface and drastically reduced the enterococcal cell binding to immobilized fibronectin. The ΔefbA deletion mutant was highly attenuated vs wild-type in a murine ascending UTI model, consistent with an increased tropism for the kidney relative to the bladder. These results provide the first evidence that EfbA of E. faecalis plays a role in UTIs, probably contributing to the pathogenesis in this site.

Peptide tag forming a rapid covalent bond to a protein, through engineering a bacterial adhesin

Protein interactions with peptides generally have low thermodynamic and mechanical stability. Streptococcus pyogenes fibronectin-binding protein FbaB contains a domain with a spontaneous isopeptide bond between Lys and Asp. By splitting this domain and rational engineering of the fragments, we obtained a peptide (SpyTag) which formed an amide bond to its protein partner (SpyCatcher) in minutes. Reaction occurred in high yield simply upon mixing and amidst diverse conditions of pH, temperature, and buffer. SpyTag could be fused at either terminus or internally and reacted specifically at the mammalian cell surface. Peptide binding was not reversed by boiling or competing peptide. Single-molecule dynamic force spectroscopy showed that SpyTag did not separate from SpyCatcher until the force exceeded 1 nN, where covalent bonds snap. The robust reaction conditions and irreversible linkage of SpyTag shed light on spontaneous isopeptide bond formation and should provide a targetable lock in cells and a stable module for new protein architectures.

Adherence and invasion of streptococci to eukaryotic cells and their role in disease pathogenesis

Streptococcal adhesion, invasion, intracellular trafficking, dissemination, and persistence in eukaryotic cells have a variety of implications in the infection pathogenesis. While cell adhesion establishes the initial host contact, adhering bacteria exploit the host cell for their own benefit. Internalization into the host cell is an essential step for bacterial survival and subsequent dissemination and persistence, thus playing a key role in the course of infection. This chapter summarizes the current knowledge about the diverse mechanisms of streptococcal adhesion to and invasion into different eukaryotic cells and the impact on dissemination and persistence which is reflected by consequences for the pathogenesis of streptococcal infections.