Virulence Associated Gene 8 of Bordetella pertussis Enhances Contact System Activity by Inhibiting the Regulatory Function of Complement Regulator C1 Inhibitor

Current understanding of Bordetella-induced cough

Typical pathogenic bacteria of the genus Bordetella cause respiratory diseases, many of which are characterized by severe coughing in host animals. In human infections with these bacteria, such as whooping cough, coughing imposes a heavy burden on patients. The pathophysiology of this severe coughing had long been uncharacterized because convenient animal models that reproduce Bordetella-induced cough have not been available. However, rat and mouse models were recently shown as useful for understanding, at least partially, the causative factors and the mechanism of Bordetella-induced cough. Many types of coughs are induced under various physiological conditions, and the neurophysiological pathways of coughing are considered to vary among animal species, including humans. However, the neurophysiological mechanisms of the coughs in different animal species have not been entirely understood, and, accordingly, the current understanding of Bordetella-induced cough is still incomplete. Nevertheless, recent research findings may open the way for the development of prophylaxis and therapeutic measures against Bordetella-induced cough.

Phylogenetic Classification and Functional Review of Autotransporters

Autotransporters are the core component of a molecular nano-machine that delivers cargo proteins across the outer membrane of Gram-negative bacteria. Part of the type V secretion system, this large family of proteins play a central role in controlling bacterial interactions with their environment by promoting adhesion to surfaces, biofilm formation, host colonization and invasion as well as cytotoxicity and immunomodulation. As such, autotransporters are key facilitators of fitness and pathogenesis and enable co-operation or competition with other bacteria. Recent years have witnessed a dramatic increase in the number of autotransporter sequences reported and a steady rise in functional studies, which further link these proteins to multiple virulence phenotypes. In this review we provide an overview of our current knowledge on classical autotransporter proteins, the archetype of this protein superfamily. We also carry out a phylogenetic analysis of their functional domains and present a new classification system for this exquisitely diverse group of bacterial proteins. The sixteen phylogenetic divisions identified establish sensible relationships between well characterized autotransporters and inform structural and functional predictions of uncharacterized proteins, which may guide future research aimed at addressing multiple unanswered aspects in this group of therapeutically important bacterial factors.

What Causes the Cough in Whooping Cough?

What causes the cough in whooping cough (pertussis) has been a longstanding question in the field but has been difficult to answer because of the perceived lack of convenient small animal models. Y. Hiramatsu, K. Suzuki, T. Nishida, N. Onoda, et al. (mBio 13:e01397-21, 2022, https://doi.org/10.1128/mbio.03197-21) used a mouse model and cellular studies to investigate bacterial and host factors that contribute to cough production during Bordetella pertussis infection. In elegant studies, they found that the bacterial factors pertussis toxin, lipooligosaccharide, and Vag8 function cooperatively to produce cough. These factors induce production of host bradykinin, a known cough inducer that sensitizes the ion channel TRPV1 on neurons, and they investigated host signaling pathways altered by the bacterial factors that exacerbate cough responses. This is a highly significant and important finding that not only elucidates mechanisms underlying the pathophysiology of the severe cough, but also may reveal potential novel therapeutic approaches to treat individuals suffering from the debilitating effects of cough in pertussis.

Serum proteases prevent bacterial biofilm formation: role of kallikrein and plasmin

Biofilm formation is a general strategy for bacterial pathogens to withstand host defense mechanisms. In this study, we found that serum proteases inhibit biofilm formation by Neisseria meningitidis, Neisseria gonorrhoeae, Haemophilus influenzae, and Bordetella pertussis. Confocal laser-scanning microscopy analysis revealed that these proteins reduce the biomass and alter the architecture of meningococcal biofilms. To understand the underlying mechanism, the serum was fractionated through size-exclusion chromatography and anion-exchange chromatography, and the composition of the fractions that retained anti-biofilm activity against N. meningitidis was analyzed by intensity-based absolute quantification mass spectrometry. Among the identified serum proteins, plasma kallikrein (PKLK), FXIIa, and plasmin were found to cleave neisserial heparin-binding antigen and the α-peptide of IgA protease on the meningococcal cell surface, resulting in the release of positively charged polypeptides implicated in biofilm formation by binding extracellular DNA. Further experiments also revealed that plasmin and PKLK inhibited biofilm formation of B. pertussis by cleaving filamentous hemagglutinin. We conclude that the proteolytic activity of serum proteases toward bacterial adhesins involved in biofilm formation could constitute a defense mechanism for the clearance of pathogens.

Molecular Basis for Bordetella pertussis Interference with Complement, Coagulation, Fibrinolytic, and Contact Activation Systems: the Cryo-EM Structure of the Vag8-C1 Inhibitor Complex

Complement, contact activation, coagulation, and fibrinolysis are serum protein cascades that need strict regulation to maintain human health. Serum glycoprotein, a C1 inhibitor (C1-INH), is a key regulator (inhibitor) of serine proteases of all the above-mentioned pathways. Recently, an autotransporter protein, virulence-associated gene 8 (Vag8), produced by the whooping cough pathogen, Bordetella pertussis, was shown to bind to C1-INH and interfere with its function. Here, we present the structure of the Vag8-C1-INH complex determined using cryo-electron microscopy at a 3.6-Å resolution. The structure shows a unique mechanism of C1-INH inhibition not employed by other pathogens, where Vag8 sequesters the reactive center loop of C1-INH, preventing its interaction with the target proteases.IMPORTANCE The structure of a 10-kDa protein complex is one of the smallest to be determined using cryo-electron microscopy at high resolution. The structure reveals that C1-INH is sequestered in an inactivated state by burial of the reactive center loop in Vag8. By so doing, the bacterium is able to simultaneously perturb the many pathways regulated by C1-INH. Virulence mechanisms such as the one described here assume more importance given the emerging evidence about dysregulation of contact activation, coagulation, and fibrinolysis leading to COVID-19 pneumonia.

Identification of the minimum region of Bordetella pertussis Vag8 required for interaction with C1 inhibitor

An autotransporter of Bordetella pertussis, virulence‐associated gene 8 (Vag8), binds and inactivates the complement regulator, C1 inhibitor (C1‐Inh), and plays a role in evasion of the complement system. However, the molecular interaction between Vag8 and C1‐Inh remains unclear. Here, we localized the minimum region of Vag8 required for interaction with C1‐Inh by examining the differently truncated Vag8 derivatives for the ability to bind and inactivate C1‐Inh. The truncated Vag8 containing amino‐acid residues 102–548, but not 102–479 and 202–648, showed the full activity of intact Vag8, suggesting that the separate 102–202 and 548–648 amino‐acid regions of Vag8 mediate the interaction with C1‐Inh.

Design and characterization of α1-antitrypsin variants for treatment of contact system-driven thromboinflammation

The contact system produces the inflammatory peptide bradykinin and contributes to experimental thrombosis. C1 esterase-inhibitor (C1INH) deficiency or gain-of-function mutations in factor XII (FXII) cause hereditary angioedema, a life-threatening tissue swelling disease. C1INH is a relatively weak contact system enzyme inhibitor. Although α1-antitrypsin (α1AT) does not naturally inhibit contact system enzymes, a human mutation (M358R; α1AT-Pittsburgh) changes it into a powerful broad-spectrum enzyme inhibitor. It blocks the contact system, but also thrombin and activated protein C (APC), making it an unattractive candidate for therapeutic contact system blockade. We adapted the reactive center loop of α1AT-Pittsburgh (AIPR/S) to overcome these obstacles. Two α1AT variants (SMTR/S and SLLR/S) strongly inhibit plasma kallikrein, activated FXII, and plasmin. α1AT-SMTR/S no longer inhibits thrombin, but residually inhibits APC. In contrast, α1AT-SLLR/S residually inhibits thrombin, but no longer APC. Additional modification at the P1' position (S→V) eliminates residual inhibition of thrombin and APC for both variants, while retaining their properties as contact system inhibitors. Both α1AT-SMTR/V and -SLLR/V are superior to C1INH in reducing bradykinin production in plasma. Owing to their capacity to selectively block contact system-driven coagulation, both variants block vascular occlusion in an in vivo model for arterial thrombosis. Furthermore, both variants block acute carrageenan-induced tissue edema in mice. Finally, α1AT-SLLR/V, our most powerful candidate, suppresses epithelial leakage of the gut in a mouse model of colitis. Our findings confirm that redesign of α1AT strongly alters its inhibitory behavior and can be used for the treatment of contact system-mediated thrombosis and inflammation.

Is there a link between genetic defects in the complement cascade and Porphyromonas gingivalis in Alzheimer's disease?

Defects, as determined by Genome-Wide Association Studies (GWAS), in the complement cascade of innate immunity have been suggested to play a key role in Alzheimer's disease (AD). These defective genes encode sub-component 1s (C1s), complement receptor 1, complement component 9, and clusterin, a fluid-phase regulatory protein. A dysregulated complement cascade has been shown to relate to cell activation, defective complement mediated clearance and possible cognitive decline in AD patients. Porphyromonas gingivalis, a putative keystone pathogen of periodontal disease, has been reported to be associated with human AD. The inflammatory burden following experimental oral infection in mice and putative entry of this bacterium into the brain appears to drive the formation of amyloid-beta plaques and neurofibrillary tangles with loss of cognition. P. gingivalis is a master of immune subversion in this inflammatory cascade and may establish microbial dysbiosis where it is located. Here we discuss if P. gingivalis may enhance the detrimental effects of the defective GWAS complement cascade protein genes.