Mutations in the S1 subunit of pertussis toxin that affect secretion

Contribution of a Novel Pertussis Toxin-Like Factor in Mediating Persistent Otitis Media

Chronic otitis media (COM) is the long-term infection and inflammation of the middle ears typically caused by upper respiratory tract pathogens that are able to ascend the Eustachian tube. Our understanding of contributing factors is limited because human otopathogens cannot naturally colonize or persist in the middle ears of mice. We recently described a natural COM in mice caused by Bordetella pseudohinzii and proposed this as an experimental system to study bacterial mechanisms of immune evasion that allow persistent infection of the middle ear. Here we describe a novel pertussis toxin (PTx)-like factor unique to B. pseudohinzii, apparently acquired horizontally, that is associated with its particularly efficient persistence and pathogenesis. The catalytic subunit of this toxin, PsxA, has conserved catalytic sites and substantial predicted structural homology to pertussis toxin catalytic subunit PtxA. Deletion of the gene predicted to encode the catalytic subunit, psxA, resulted in a significant decrease in persistence in the middle ears. The defect was not observed in mice lacking T cells, indicating that PsxA is necessary for persistence only when T cells are present. These results demonstrate the role of a novel putative toxin in the persistence of B. pseudohinzii and its generation of COM. This PsxA-mediated immune evasion strategy may similarly be utilized by human otopathogens, via other PTx-like toxins or alternative mechanisms to disrupt critical T cell functions necessary to clear bacteria from the middle ear. This work demonstrates that this experimental system can allow for the detailed study of general strategies and specific mechanisms that otopathogens use to evade host immune responses to persist in the middle ear to cause COM.

Investigating pertussis toxin and its impact on vaccination

Whooping cough, caused by Bordetella pertussis, remains a major global health problem. Each year around 40 million of pertussis cases resulting in 200,000-400,000 annual deaths occur worldwide. Pertussis toxin is a major virulence factor of B. pertussis. Murine studies have shown its importance in bacterial colonization and in immunomodulation to evade innate or adaptive immunity. The toxin is composed of an A protomer expressing ADP-ribosyltransferase activity and a B oligomer, responsible for toxin binding to target cells. The toxin is also a major protective antigen in all currently available vaccines. However, vaccine escape mutants with altered toxin expression have recently been isolated in countries with high vaccination coverage illustrating the need for improved pertussis vaccines.

The ins and outs of pertussis toxin

Pertussis toxin, produced and secreted by the whooping cough agent Bordetella pertussis, is one of the most complex soluble bacterial proteins. It is actively secreted through the B. pertussis cell envelope by the Ptl secretion system, a member of the widespread type IV secretion systems. The toxin is composed of five subunits (named S1 to S5 according to their decreasing molecular weights) arranged in an A-B structure. The A protomer is composed of the enzymatically active S1 subunit, which catalyzes ADP-ribosylation of the α subunit of trimeric G proteins, thereby disturbing the metabolic functions of the target cells, leading to a variety of biological activities. The B oligomer is composed of 1S2:1S3:2S4:1S5 and is responsible for binding of the toxin to the target cell receptors and for intracellular trafficking via receptor-mediated endocytosis and retrograde transport. The toxin is one of the most important virulence factors of B. pertussis and is a component of all current vaccines against whooping cough.

Bordetella pertussis risA, but not risS, is required for maximal expression of Bvg-repressed genes

Expression of virulence determinants by Bordetella pertussis, the primary etiological agent of whooping cough, is regulated by the BvgAS two-component regulatory system. The role of a second two-component regulatory system, encoded by risAS, in this process is not defined. Here, we show that mutation of B. pertussis risA does not affect Bvg-activated genes or proteins. However, mutation of risA resulted in greatly diminished expression of Bvg-repressed antigens and decreased transcription of Bvg-repressed genes. In contrast, mutation of risS had no effect on the expression of Bvg-regulated molecules. Mutation of risA also resulted in decreased bacterial invasion in a HeLa cell model. However, decreased invasion could not be attributed to the decreased expression of Bvg-repressed products, suggesting that mutation of risA may affect the expression of a variety of genes. Unlike the risAS operons in B. parapertussis and B. bronchiseptica, B. pertussis risS is a pseudogene that encodes a truncated RisS sensor. Deletion of the intact part of the B. pertussis risS gene does not affect the expression of risA-dependent, Bvg-repressed genes. These observations suggest that RisA activation occurs through cross-regulation by a heterologous system.

Analysis of subassemblies of pertussis toxin subunits in vivo and their interaction with the ptl transport apparatus

Pertussis toxin (PT) has an AB(5) structure that is typical of many bacterial protein toxins; however, this toxin is more complex than many toxins since it is composed of five different subunit types, subunits S1 to S5. Little is known about how PT assembles in vivo and how and when it interacts with its secretion apparatus, known as the Ptl transporter. In order to better understand these events, we expressed subsets of the genes encoding the S1, S2, and/or S4 subunits of PT in strains of Bordetella pertussis that either did or did not produce the Ptl proteins. We found evidence to suggest that certain subassemblies of the toxin, including subassemblies consisting of the S1 subunit and incomplete forms of the B oligomer, can form in vivo, at least transiently. These results suggest that the B oligomer of the toxin does not need to completely form before interactions between the S1 subunit and B-oligomer subunits can occur in vivo. All subassemblies localized primarily to the membrane fraction of the cell. Moreover, we found that Ptl-mediated secretion occurs in a strain that produces S1 and an incomplete complement of B-oligomer subunits. These results indicate that subassemblies of the toxin consisting of the S1 subunit and a partial B oligomer can interact with the Ptl system.

Temporal expression of pertussis toxin and Ptl secretion proteins by Bordetella pertussis

Pertussis toxin is an AB(5) toxin comprised of protein subunits S1 through S5. The individual subunits are secreted by a Sec-dependent mechanism into the periplasm, where the toxin is assembled. The Ptl type IV secretion system mediates secretion of assembled toxin past the outer membrane. In this study, we examined the time course of protein expression, toxin assembly, and secretion as a function of the bacterial growth cycle. Logarithmic growth was observed after a 1-h lag phase. Secreted toxin was first observed at 3 h. Secretion continued throughout the logarithmic growth phase and decreased as the culture entered the stationary phase after about 24 h. On a per cell basis, toxin secretion occurred at a constant rate of 3 molecules/min/cell from 2 to 18 h. More of toxin subunits S1, S2, and S3 were produced than were secreted, resulting in periplasmic accumulation. Periplasmic S1, S2, and S3 were found to be soluble in the periplasm, as well as membrane associated. About one-half of the periplasmic S1, S2 and S3 subunits were incorporated into holotoxin. Secretion component PtlF was present at a low level at time zero, and the level increased between 2 and 24 h from 30 to 1,000 molecules per cell; however, the initial level of PtlF, 30 molecules per cell, supported maximal secretion. The accumulation of both periplasmic toxin and secretion components suggests that translation rates exceed the rate of secretion and that secretion, not toxin and Ptl complex assembly, is rate limiting.

Reduced glutathione is required for pertussis toxin secretion by Bordetella pertussis

The abilities of cysteine-containing compounds to support growth of Bordetella pertussis and influence pertussis toxin transcription, assembly, and secretion were examined. Cysteine is an essential amino acid for B. pertussis and must be present for protein synthesis and bacterial growth. However, cysteine can be metabolized to sulfate, and high concentrations of sulfate can selectively inhibit transcription of the virulence factors, including pertussis toxin, via the BvgAS two-component regulatory system in a process called modulation. In addition, pertussis toxin possesses several disulfide bonds, and the cysteine-containing compound glutathione can influence oxidation-reduction reactions and perhaps disulfide bond formation. Bacterial growth was not observed in the absence of a source of cysteine. Oxidized glutathione, as a sole source of cysteine, also did not support bacterial growth. Cysteine, cystine, and reduced glutathione did support bacterial growth, and none of these compounds caused modulation at the concentrations tested. Similar amounts of periplasmic pertussis toxin were detected regardless of the source of cysteine; however, in the absence of reduced glutathione, pertussis toxin was not efficiently secreted. Addition of the reducing agent dithiothreitol was unable to compensate for the lack of reduced glutathione and did not promote secretion of pertussis toxin. These results suggest that reduced glutathione does not affect the accumulation of assembled active pertussis toxin in the periplasm but plays a role in efficient pertussis toxin secretion by the bacterium.

Two-dimensional display and whole genome comparison of bacterial pathogen genomes of high G+C DNA content

High-resolution comparison of bacterial genomes facilitates the identification of the genetic changes responsible for clinically relevant phenotypes. For this purpose we have established a method for the display and comparison of high G+C bacterial genomes in two dimensions. Here we describe the application of two-dimensional bacterial genomic display to resolve the genomes of Bordetella pertussis, Mycobacterium avium and Mycobacterium tuberculosis, and its utility in strain comparison and detection of insertion and substitution mutations.

DsbA and DsbC are required for secretion of pertussis toxin by Bordetella pertussis

The Dsb family of enzymes catalyzes disulfide bond formation in the gram-negative periplasm, which is required for folding and assembly of many secreted proteins. Pertussis toxin is arguably the most complex toxin known: it is assembled from six subunits encoded by five genes (for subunits S1 to S5), with 11 intramolecular disulfide bonds. To examine the role of the Dsb enzymes in assembly and secretion of pertussis toxin, we identified and mutated the Bordetella pertussis dsbA, dsbB, and dsbC homologues. Mutations in dsbA or dsbB resulted in decreased levels of S1 (the A subunit) and S2 (a B-subunit protein), demonstrating that DsbA and DsbB are required for toxin assembly. Mutations in dsbC did not impair assembly of periplasmic toxin but resulted in decreased toxin secretion, suggesting a defect in the formation of the Ptl secretion complex.

Growth phase influences complement resistance of Bordetella pertussis

The BrkA protein of Bordetella pertussis inhibits killing by the antibody-dependent classical pathway of complement; however, susceptibility to complement can be highly variable. Log-phase bacteria grown in Stainer-Scholte (SS) broth plated on Bordet-Gengou (BG) agar were about 500 times more sensitive to killing by complement than stationary-phase SS-BG cultures. While always more susceptible to complement than the wild-type strain, a BrkA mutant displayed a similar growth phase variation in susceptibility to complement. Growth phase susceptibility to complement was also observed for a mutant constitutive for Bvg activation of BrkA, suggesting that modulation of virulence factor expression was not responsible for sensitivity to complement. Susceptibility was not due to differential antigenic expression, since serum adsorbed with complement-resistant, stationary-phase SS-BG cultures lacked bactericidal activity against B. pertussis harvested at all times during the growth cycle. These results suggest that log-phase susceptibility to complement is not due to variable expression of BrkA or antigenic differences and may be an inherent property of rapidly growing cultures. Implications for vaccine development are discussed.

Bordetella pertussis autoregulates pertussis toxin production through the metabolism of cysteine

Pertussis toxin (Ptx) expression and secretion in Bordetella pertussis are regulated by a two-component signal transduction system encoded by the bvg regulatory locus. However, it is not known whether the metabolic pathways and growth state of the bacterium influence synthesis and secretion of Ptx and other virulence factors. We have observed a reduction in the concentration of Ptx per optical density unit midway in fermentation. Studies were conducted to identify possible factors causing this reduction and to develop culture conditions that optimize Ptx expression. Medium reconstitution experiments demonstrated that spent medium and a fraction of this medium containing components with a molecular weight of <3,000 inhibited the production of Ptx. A complete flux analysis of the intermediate metabolism of B. pertussis revealed that the sulfur-containing amino acids methionine and cysteine and the organic acid pyruvate accumulated in the media. In fermentation, a large amount of internal sulfate (SO4(2-)) was observed in early stage growth, followed by a rapid decrease as the cells entered into logarithmic growth. This loss was later followed by the accumulation of large quantities of SO4(2-) into the media in late-stage fermentation. Release of SO4(2-) into the media by the cells signaled the decoupling of cell growth and Ptx production. Under conditions that limited cysteine, a fivefold increase in Ptx production was observed. Addition of barium chloride (BaCl2) to the culture further increased Ptx yield. Our results suggest that B. pertussis is capable of autoregulating the activity of the bvg regulon through its metabolism of cysteine. Reduction of the amount of cysteine in the media results in prolonged vir expression due to the absence of the negative inhibitor SO4(2-). Therefore, the combined presence and metabolism of cysteine may be an important mechanism in the pathogenesis of B. pertussis.