Structure and mechanism of the essential two-component signal-transduction system WalKR in Staphylococcus aureus.

Most low GC Gram-positive bacteria possess an essential walKR two-component system (TCS) for signal transduction involved in regulating cell wall homoeostasis. Despite the well-established intracellular regulatory mechanism, the role of this TCS in extracellular signal recognition and factors that modulate the activity of this TCS remain largely unknown. Here we identify the extracellular receptor of the kinase 'WalK' (erWalK) as a key hub for bridging extracellular signal input and intracellular kinase activity modulation in Staphylococcus aureus. Characterization of the crystal structure of erWalK revealed a canonical Per-Arnt-Sim (PAS) domain for signal sensing. Single amino-acid mutation of potential signal-transduction residues resulted in severely impaired function of WalKR. A small molecule derived from structure-based virtual screening against erWalK is capable of selectively activating the walKR TCS. The molecular level characterization of erWalK will not only facilitate exploration of natural signal(s) but also provide a template for rational design of erWalK inhibitors.

Calprotectin Increases the Activity of the SaeRS Two Component System and Murine Mortality during Staphylococcus aureus Infections.

Calprotectin, the most abundant cytoplasmic protein in neutrophils, suppresses the growth of Staphylococcus aureus by sequestering the nutrient metal ions Zn and Mn. Here we show that calprotectin can also enhance the activity of the SaeRS two component system (TCS), a signaling system essential for production of over 20 virulence factors in S. aureus. The activity of the SaeRS TCS is repressed by certain divalent ions found in blood or neutrophil granules; however, the Zn bound-form of calprotectin relieves this repression. During staphylococcal encounter with murine neutrophils or staphylococcal infection of the murine peritoneal cavity, calprotectin increases the activity of the SaeRS TCS as well as the production of proinflammatory cytokines such as IL-1ß and TNF-α, resulting in higher murine mortality. These results suggest that, under certain conditions, calprotectin can be exploited by S. aureus to increase bacterial virulence and host mortality.

The extracytoplasmic linker peptide of the sensor protein SaeS tunes the kinase activity required for staphylococcal virulence in response to host signals.

Bacterial pathogens often employ two-component systems (TCSs), typically consisting of a sensor kinase and a response regulator, to control expression of a set of virulence genes in response to changing host environments. In Staphylococcus aureus, the SaeRS TCS is essential for in vivo survival of the bacterium. The intramembrane-sensing histidine kinase SaeS contains, along with a C-terminal kinase domain, a simple N-terminal domain composed of two transmembrane helices and a nine amino acid-long extracytoplasmic linker peptide. As a molecular switch, SaeS maintains low but significant basal kinase activity and increases its kinase activity in response to inducing signals such as human neutrophil peptide 1 (HNP1). Here we show that the linker peptide of SaeS controls SaeS's basal kinase activity and that the amino acid sequence of the linker peptide is highly optimized for its function. Without the linker peptide, SaeS displays aberrantly elevated kinase activity even in the absence of the inducing signal, and does not respond to HNP1. Moreover, SaeS variants with alanine substitution of the linker peptide amino acids exhibit altered basal kinase activity and/or irresponsiveness to HNP1. Biochemical assays reveal that those SaeS variants have altered autokinase and phosphotransferase activities. Finally, animal experiments demonstrate that the linker peptide-mediated fine tuning of SaeS kinase activity is critical for survival of the pathogen. Our results indicate that the function of the linker peptide in SaeS is a highly evolved feature with very optimized amino acid sequences, and we propose that, in other SaeS-like intramembrane sensing histidine kinases, the extracytoplasmic linker peptides actively fine-control their kinases.

Molecular mechanism of quinone signaling mediated through S-quinonization of a YodB family repressor QsrR.

Quinone molecules are intracellular electron-transport carriers, as well as critical intra- and extracellular signals. However, transcriptional regulation of quinone signaling and its molecular basis are poorly understood. Here, we identify a thiol-stress-sensing regulator YodB family transcriptional regulator as a central component of quinone stress response of Staphylococcus aureus, which we have termed the quinone-sensing and response repressor (QsrR). We also identify and confirm an unprecedented quinone-sensing mechanism based on the S-quinonization of the essential residue Cys-5. Structural characterizations of the QsrR-DNA and QsrR-menadione complexes further reveal that the covalent association of menadione directly leads to the release of QsrR from operator DNA following a 10° rigid-body rotation as well as a 9-Å elongation between the dimeric subunits. The molecular level characterization of this quinone-sensing transcriptional regulator provides critical insights into quinone-mediated gene regulation in human pathogens.

SAP is required for the development of innate phenotype in H2-M3--restricted Cd8(+) T cells.

H2-M3--restricted T cells have a preactivated surface phenotype, rapidly expand, and produce cytokines upon stimulation, and, as such, are classified as innate T cells. Unlike most innate T cells, M3-restricted T cells also express CD8αß coreceptors and a diverse TCR repertoire: hallmarks of conventional MHC Ia-restricted CD8(+) T cells. Although invariant NKT cells are also innate T cells, they are selected exclusively on hematopoietic cells (HC), whereas M3-restricted T cells can be selected on either hematopoietic or thymic epithelial cells. Moreover, their phenotypes differ depending on what cells mediate their selection. Although there is a clear correlation between selection on HC and development of innate phenotype, the underlying mechanism remains unclear. Signaling lymphocyte activation molecule-associated protein (SAP) is required for the development of invariant NKT cells and mediates signals from signaling lymphocyte activation molecule receptors that are exclusively expressed on HC. Based on their dual selection pathway, M3-restricted T cells present a unique model for studying the development of innate T cell phenotype. Using both polyclonal and transgenic mouse models, we demonstrate that although M3-restricted T cells are capable of developing in the absence of SAP, SAP is required for HC-mediated selection, development of preactivated phenotype, and heightened effector functions of M3-restricted T cells. These findings are significant because they directly demonstrate the need for SAP in HC-mediated acquisition of innate T cell phenotype and suggest that, due to their SAP-dependent HC-mediated selection, M3-restricted T cells develop a preactivated phenotype and an intrinsic ability to proliferate faster upon stimulation, allowing for an important role in the early response to infection.

The auxiliary protein complex SaePQ activates the phosphatase activity of sensor kinase SaeS in the SaeRS two-component system of Staphylococcus aureus.

In bacterial two-component regulatory systems (TCSs), dephosphorylation of phosphorylated response regulators is essential for resetting the activated systems to the pre-activation state. However, in the SaeRS TCS, a major virulence TCS of Staphylococcus aureus, the mechanism for dephosphorylation of the response regulator SaeR has not been identified. Here we report that two auxiliary proteins from the sae operon, SaeP and SaeQ, form a protein complex with the sensor kinase SaeS and activate the sensor kinase's phosphatase activity. Efficient activation of the phosphatase activity required the presence of both SaeP and SaeQ. When SaeP and SaeQ were ectopically expressed, the expression of coagulase, a sae target with low affinity for phosphorylated SaeR, was greatly reduced, while the expression of alpha-haemolysin, a sae target with high affinity for phosphorylated SaeR, was not, demonstrating a differential effect of SaePQ on sae target gene expression. When expression of SaePQ was abolished, most sae target genes were induced at an elevated level. Since the expression of SaeP and SaeQ is induced by the SaeRS TCS, these results suggest that the SaeRS TCS returns to the pre-activation state by a negative feedback mechanism.

Construction, and in vitro and in vivo analyses of tetravalent immunoadhesins.

Previous observations demonstrated that various immunosuppressive agents and their combination therapies can increase allograft survival rates. However, these treatments may have serious side effects and cannot substantially improve or prolong graft survival in acute graft-versus-host disease (GVHD). To improve the therapeutic potency of divalent immunoadhesins, we have constructed and produced several tetravalent forms of immunoadhesins comprising each of cytotoxic T-lymphocyte-associated antigen-4 (CTLA4), CD2, and lymphocyte activation gene-3 (LAG3). Flow cytometric and T cell proliferation analyses displayed that tetravalent immunoadhesins have a higher binding affinity and more potent efficacy than divalent immunoadhesins. Although all tetravalent immunoadhesins possess better efficacies, tetravalent forms of CTLA4-Ig and LAG3-Ig revealed higher inhibitory effects on T cell proliferation than tetravalent forms of TNFR2-Ig and CD2-Ig. In vitro mixed lymphocytes reaction (MLR) showed that combined treatment with tetravalent CTLA4-Ig and tetravalent LAG3-Ig was highly effective for inhibiting T cell proliferation in both human and murine allogeneic stimulation. In addition, both single tetravalent-form and combination treatments can prevent the lethality of murine acute GVHD. The results of this study demonstrated that co-blockade of the major histocompatibility complex class (MHC)II:T cell receptor (TCR) and CD28:B7 pathways by using tetravalent human LAG3-Ig and CTLA4-Ig synergistically prevented murine acute GVHD.

Quorum-sensing agr mediates bacterial oxidation response via an intramolecular disulfide redox switch in the response regulator AgrA.

Oxidation sensing and quorum sensing significantly affect bacterial physiology and host-pathogen interactions. However, little attention has been paid to the cross-talk between these two seemingly orthogonal signaling pathways. Here we show that the quorum-sensing agr system has a built-in oxidation-sensing mechanism through an intramolecular disulfide switch possessed by the DNA-binding domain of the response regulator AgrA. Biochemical and mass spectrometric analysis revealed that oxidation induces the intracellular disulfide bond formation between Cys-199 and Cys-228, thus leading to dissociation of AgrA from DNA. Molecular dynamics (MD) simulations suggest that the disulfide bond formation generates a steric clash responsible for the abolished DNA binding of the oxidized AgrA. Mutagenesis studies further established that Cys-199 is crucial for oxidation sensing. The oxidation-sensing role of Cys-199 is further supported by the observation that the mutant Staphylococcus aureus strain expressing AgrAC199S is more susceptible to H(2)O(2) owing to repression of the antioxidant bsaA gene under oxidative stress. Together, our results show that oxidation sensing is a component of the quorum-sensing agr signaling system, which serves as an intrinsic checkpoint to ameliorate the oxidation burden caused by intense metabolic activity and potential host immune response.

Organizational requirements of the SaeR binding sites for a functional P1 promoter of the sae operon in Staphylococcus aureus.

In Staphylococcus aureus, the SaeRS two-component system controls the expression of multiple virulence factors. Of the two promoters in the sae operon, P1 is autoinduced and has two binding sites for the response regulator SaeR. In this study, we examined the organizational requirements of the SaeR binding sites in P1 for transcription activation. Mutational studies showed that both binding sites are essential for binding to phosphorylated SaeR (P-SaeR) and transcription activation. When the 21-bp distance between the centers of the two SaeR binding sites was altered to 26 bp, 31 bp, 36 bp, or 41 bp, only the 31-bp mutant retained approximately 40% of the original promoter activity. When the -1-bp spacing (i.e.,1-bp overlap) between the primary SaeR binding site and the -35 promoter region was altered, all mutant P1 promoters failed to initiate transcription; however, when the first nucleotide of the -35 region was changed from A to T, the mutants with 0-bp or 22-bp spacing showed detectable promoter activity. Although P-SaeR was essential for the binding of RNA polymerase to P1, it was not essential for the binding of the enzyme to the alpha-hemolysin promoter. When the nonoptimal spacing between promoter elements in P1 or the coagulase promoter was altered to the optimal spacing of 17 bp, both promoters failed to initiate transcription. These results suggest that SaeR binding sites are under rather strict organizational restrictions and provide clues for understanding the molecular mechanism of sae-mediated transcription activation.

Targeting MgrA-mediated virulence regulation in Staphylococcus aureus.

Increasing antibiotic resistance in human pathogens necessitates the development of new approaches against infections. Targeting virulence regulation at the transcriptional level represents a promising strategy yet to be explored. A global transcriptional regulator, MgrA in Staphylococcus aureus, was identified previously as a key virulence determinant. We have performed a fluorescence anisotropy (FA)-based high-throughput screen that identified 5, 5-methylenedisalicylic acid (MDSA), which blocks the DNA binding of MgrA. MDSA represses the expression of α-toxin that is up-regulated by MgrA and activates the transcription of protein A, a gene down-regulated by MgrA. MDSA alters bacterial antibiotic susceptibilities via an MgrA-dependent pathway. A mouse model of infection indicated that MDSA could attenuate S. aureus virulence. This work is a rare demonstration of utilizing small molecules to block protein-DNA interaction, thus tuning important biological regulation at the transcriptional level.

Identification of the P3 promoter and distinct roles of the two promoters of the SaeRS two-component system in Staphylococcus aureus.

In Staphylococcus aureus, the SaeRS two-component system (TCS) encoded by the saePQRS operon controls expression of major virulence factors, such as coagulase and alpha-hemolysin. The saePQRS operon has two promoters: P1 and P3. The P1 promoter, a strong promoter, is autoinduced and can transcribe all four genes. Compared with P1, P3 shows fairly low but constitutive promoter activity, and it transcribes only saeR and saeS, the two genes encoding response regulator SaeR and sensor kinase SaeS. However, the role of each promoter in sae signaling has not been rigorously defined. In this study, we found that the genuine transcription start site (TSS) of P3 is located 78 nucleotides downstream of the previously reported TSS. Subsequently, the P3 promoter sequence was identified and validated by mutagenesis analyses. Deletion of the saePQ region including the P1 promoter did not significantly alter the expression patterns of coagulase and alpha-hemolysin, two well-known sae target genes. Due to its L18P substitution in a transmembrane domain, SaeS in strain Newman has a constitutive kinase activity. Interestingly, the mutation also rendered the protein unstable, but the protein stability was restored by SaeQ, suggesting a possible SaeQ-SaeS interaction. Ironically, the same mutation seems to increase mRNA stability. SaeR appears to be stabilized by SaeS, possibly by a protein-protein interaction. Chromosomal mutation of P1 did not affect the expression pattern of coagulase and alpha-hemolysin. Based on these results, we conclude that transcription of saeRS from P3 is sufficient for target gene activation and that P1 is not involved in the activation.

Positive selecting cell type determines the phenotype of MHC class Ib-restricted CD8+ T cells.

Several studies have demonstrated an apparent link between positive selection on hematopoietic cells (HCs) and an "innate" T-cell phenotype. Whereas conventional CD8(+) T cells are primarily selected on thymic epithelial cells (TECs) and certain innate T cells are exclusively selected on HCs, MHC class Ib-restricted CD8(+) T cells appear to be selected on both TECs and HCs. However, whether TEC- and HC-selected T cells represent distinct lineages or whether the same T-cell precursors have the capacity to be selected on either cell type is unknown. Using an M3-restricted T-cell receptor transgenic mouse model, we demonstrate that not only are MHC class Ib-restricted CD8(+) T cells capable of being selected on either cell type but that selecting cell type directly affects the phenotype of the resulting CD8(+) T cells. M3-restricted CD8(+) T cells selected on HCs acquire a more activated phenotype and possess more potent effector functions than those selected on TECs. Additionally, these two developmental pathways are active in the generation of the natural pool of M3-restricted CD8(+) T cells. Our results suggest that these two distinct populations may allow MHC class Ib-restricted CD8(+) T cells to occupy different immunological niches playing unique roles in immune responses to infection.

Nonconventional CD8+ T cell responses to Listeria infection in mice lacking MHC class Ia and H2-M3.

CD8(+) T cells restricted to MHC class Ib molecules other than H2-M3 have been shown to recognize bacterial Ags. However, the contribution of these T cells to immune responses against bacterial infection is not well defined. To investigate the immune potential of MHC class Ib-restricted CD8(+) T cells, we have generated mice that lack both MHC class Ia and H2-M3 molecules (K(b-/-)D (b-/-)M3(-/-)). The CD8(+) T cells present in K(b-/-)D (b-/-)M3(-/-) mice display an activated surface phenotype and are able to secrete IFN-γ rapidly upon anti-CD3 and anti-CD28 stimulation. Although the CD8(+) T cell population is reduced in K(b-/-)D (b-/-)M3(-/-) mice compared with that in K(b-/-)D (b-/-) mice, this population retains the capacity to expand significantly in response to primary infection with the bacteria Listeria monocytogenes. However, K(b-/-)D (b-/-)M3(-/-) CD8(+) T cells do not expand upon secondary infection, similar to what has been observed for H2-M3-restricted T cells. CD8(+) T cells isolated from Listeria-infected K(b-/-)D (b-/-)M3(-/-) mice exhibit cytotoxicity and secrete proinflammatory cytokines in response to Listeria-infected APCs. These T cells are protective against primary Listeria infection, as Listeria-infected K(b-/-)D (b-/-)M3(-/-) mice exhibit reduced bacterial burden compared with that of infected ß(2)-microglobulin-deficient mice that lack MHC class Ib-restricted CD8(+) T cells altogether. In addition, adoptive transfer of Listeria-experienced K(b-/-)D (b-/-)M3(-/-) splenocytes protects recipient mice against subsequent Listeria infection in a CD8(+) T cell-dependent manner. These data demonstrate that other MHC class Ib-restricted CD8(+) T cells, in addition to H2-M3-restricted T cells, contribute to antilisterial immunity and may contribute to immune responses against other intracellular bacteria.

Differential requirements for the Ets transcription factor Elf-1 in the development of NKT cells and NK cells.

E26 Transformation specific (Ets) family transcription factors control the expression of a large number of genes regulating hematopoietic cell development and function. Two such transcription factors, Ets-1 and myeloid Elf-1-like factor (MEF), have been shown to play critical roles in both natural killer (NK)- and NKT-cell development, but not in the development of conventional T cells. In this study, we address the role of E74-like factor 1 (Elf-1), another Ets family transcription factor that is closely related to MEF but divergent from Ets-1, in NK- and NKT-cell development using Elf-1-deficient (Elf-1(-/-)) mice. Whereas the proportion of NK cells in Elf-1(-/-) mice was normal, the proportion of NKT cells was significantly reduced in the thymus and periphery of Elf-1(-/-) mice compared with wild-type (WT) mice. Although Ets-1-deficient mice lack NKT cells altogether, Elf-1(-/-) mice exhibited only a partial block in NKT-cell development caused by a cell-intrinsic defect in the selection, survival, and maturation of NKT cells. In addition, residual NKT cells found in Elf-1(-/-) mice produced less cytokine upon antigen stimulation compared with WT NKT cells. Our data demonstrate that Elf-1 plays an important and nonredundant role in the development and function of NKT cells, but is not involved in NK-cell development.

CcpA mediates proline auxotrophy and is required for Staphylococcus aureus pathogenesis.

Human clinical isolates of Staphylococcus aureus, for example, strains Newman and N315, cannot grow in the absence of proline, albeit their sequenced genomes harbor genes for two redundant proline synthesis pathways. We show here that under selective pressure, S. aureus Newman generates proline-prototrophic variants at a frequency of 3 x 10(-6), introducing frameshift and missense mutations in ccpA or IS1811 insertions in ptsH, two regulatory genes that carry out carbon catabolite repression (CCR) in staphylococci and other Gram-positive bacteria. S. aureus Newman variants with mutations in rocF (arginase), rocD (ornithine aminotransferase), and proC (Delta(1)-pyrroline 5-carboxylate [P5C] reductase) are unable to generate proline-prototrophic variants, whereas a variant with a mutation in ocd (ornithine cyclodeaminase) is unaffected. Transposon insertion in ccpA also restored proline prototrophy. CcpA was shown to repress transcription of rocF and rocD, encoding the first two enzymes, but not of proC, encoding the third and final enzyme in the P5C reductase pathway. CcpA bound to the upstream regions of rocF and rocD but not to that of proC. CcpA's binding to the upstream regions was greatly enhanced by phosphorylated HPr. The CCR-mediated proline auxotrophy was lifted when nonpreferred carbohydrates were used as the sole carbon source. The ccpA mutant displayed reduced staphylococcal load and replication in a murine model of staphylococcal abscess formation, indicating that carbon catabolite repression presents an important pathogenesis strategy of S. aureus infections.

Aureusimines in Staphylococcus aureus are not involved in virulence.

BACKGROUND: Recently, dipeptide aureusimines were reported to activate expression of staphylococcal virulence genes, such as alpha-hemolysin, and increase S. aureus virulence. Surprisingly, most of the virulence genes affected by aureusimines form part of the regulon of the SaeRS two component system (TCS), raising the possibility that SaeRS might be directly or indirectly involved in the aureusimine-dependent signaling process. METHODOLOGY/PRINCIPAL FINDINGS: Using HPLC analyses, we confirmed that a transposon mutant of ausA, the gene encoding the aureusimine dipeptide synthesis enzyme, does not produce dipeptides. However, the transposon mutant showed normal hemolysis activity and alpha-hemolysin/SaeP production. Furthermore, the P1 promoter of the sae operon, one of the targets of the SaeRS TCS, showed normal transcription activity. Moreover, in contrast to the original report, the ausA transposon mutant did not exhibit attenuated virulence in an animal infection model. DNA sequencing revealed that the ausA deletion mutant used in the original study has an 83 nt-duplication in saeS. Hemolysis activity of the original mutant was restored by a plasmid carrying the sae operon. A mutant of the sae operon showed elevated resistance to chloramphenicol and erythromycin, two antibiotics widely used during staphylococcal mutagenesis. At 43°C in the presence of erythromycin and aeration, the conditions typically employed for staphylococcal mutagenesis, an saeR transposon mutant grew much faster than a control mutant and the saeR mutant was highly enriched in a mixed culture experiment. CONCLUSIONS/SIGNIFICANCE: Our results show that the previously reported roles of aureusimines in staphylococcal gene regulation and virulence were due to an unintended mutation in saeS, which was likely selected due to elevated resistance of the mutant to environmental stresses. Thus, there is no evidence indicating that the dipeptide aureusimines play a role in sae-mediated virulence factor production or contribute to staphylococcal virulence.

Bacterial infection alters the kinetics and function of iNKT cell responses.

CD1d-restricted Valpha14 invariant NKT cells (iNKT) are innate-like, immunoregulatory lymphocytes that play critical roles in autoimmunity, tumor surveillance, and infectious disease. Although iNKT cells are activated during microbial infection, the impacts of infection on the function of iNKT cells have not been fully characterized. Using a Listeria monocytogenes (LM) infection model, we found that iNKT cells failed to expand after infection, resulting in prolonged loss in the spleen, in contrast to the typical expansion and contraction of conventional T cells. iNKT cells from LM-infected mice responded more rapidly to secondary LM infection; however, they became functionally hyporesponsive to antigenic challenge for at least 1 month. This infection-induced hyporesponsiveness was also induced by Mycobacteria infection and was more profound in LM-infected, thymectomized mice, suggesting that infection-primed iNKT cells might have altered functionality. Interestingly, activation with alpha-galactosylceramide-loaded dendritic cells was able to overcome infection-induced hyporesponsiveness of iNKT cells, suggesting a role for extrinsic factors in this functional deficit. Taken together, these findings suggest that infection affects iNKT cell responses quantitatively and qualitatively. As humans are under constant microbial insult, predictions of iNKT cell function based on naïve animal models may not accurately reflect iNKT cell behavior in a clinical setting.