Modulation of apoptosis during infection with Chlamydia

Keeping the host alive - lessons from obligate intracellular bacterial pathogens

Mammals have evolved sophisticated host cell death signaling pathways as an important immune mechanism to recognize and eliminate cell intruders before they establish their replicative niche. However, intracellular bacterial pathogens that have co-evolved with their host have developed a multitude of tactics to counteract this defense strategy to facilitate their survival and replication. This requires manipulation of pro-death and pro-survival host signaling pathways during infection. Obligate intracellular bacterial pathogens are organisms that absolutely require an eukaryotic host to survive and replicate, and therefore they have developed virulence factors to prevent diverse forms of host cell death and conserve their replicative niche. This review encapsulates our current understanding of these host-pathogen interactions by exploring the most relevant findings of Anaplasma spp., Chlamydia spp., Rickettsia spp. and Coxiella burnetii modulating host cell death pathways. A detailed comprehension of the molecular mechanisms through which these obligate intracellular pathogens manipulate regulated host cell death will not only increase the current understanding of these difficult-to-study pathogens but also provide insights into new tools to study regulated cell death and the development of new therapeutic approaches to control infection.

The STING pathway in response to chlamydial infection

The past decades have witnessed significant progress in discovery and characterize cytosolic DNA sensing and signaling, especially the understanding of the stimulator of interferon genes (STING). This pathway to foreign nucleic acids enables the initiation of robust anti-pathogenic responses to protect the host, and provides a new understanding for therapeutic intervention in a growing infectious disease, including chlamydial infection. Chlamydiae are obligate intracellular pathogenic bacterium causing widespread human diseases such as sexually transmitted infections and respiratory tract infections. Previous studies have shown that IFN production and autophagy are well recognized as being two critical processes induced by STING, and these two processes were also activated during chlamydial infection. In this review, we summarize the important characteristics of the STING activation pathway and recent snapshots about the role of STING in chlamydial infection. Studying the role of STING in chlamydial infection could provide valuable information to further understand the pathogenesis and treatment of chlamydial infection.

Effect of Sulfated Polysaccharides from Brown Algae on Apoptosis of Human Peripheral Blood Lymphocytes

We studied the influence of fucoidans from brown algae Fucus evanescens, Laminaria cichorioides and Laminaria japonica on apoptosis of human peripheral blood lymphocytes. It was demonstrated that fucoidans induced lymphocyte apoptosis, increased the proportion of cells with low mitochondrial transmembrane potential, and inhibit expression of Bcl-xL gene in blood lymphocytes. These findings suggest that lymphocyte apoptosis induced by the analyzed sulfated polysaccharides is mediated by mitochondrial pathway.

Tarp regulates early Chlamydia-induced host cell survival through interactions with the human adaptor protein SHC1

Many bacterial pathogens translocate effector proteins into host cells to manipulate host cell functions. Here, we used a protein microarray comprising virtually all human SRC homology 2 (SH2) and phosphotyrosine binding domains to comprehensively and quantitatively assess interactions between host cell proteins and the early phase Chlamydia trachomatis effector protein translocated actin-recruiting phosphoprotein (Tarp), which is rapidly tyrosine phosphorylated upon host cell entry. We discovered numerous novel interactions between human SH2 domains and phosphopeptides derived from Tarp. The adaptor protein SHC1 was among Tarp's strongest interaction partners. Transcriptome analysis of SHC1-dependent gene regulation during infection indicated that SHC1 regulates apoptosis- and growth-related genes. SHC1 knockdown sensitized infected host cells to tumor necrosis factor-induced apoptosis. Collectively, our findings reveal a critical role for SHC1 in early C. trachomatis-induced cell survival and suggest that Tarp functions as a multivalent phosphorylation-dependent signaling hub that is important during the early phase of chlamydial infection.

Inactivation of the fliY gene encoding a flagellar motor switch protein attenuates mobility and virulence of Leptospira interrogans strain Lai

Background

Pathogenic Leptospira species cause leptospirosis, a zoonotic disease of global importance. The spirochete displays active rotative mobility which may contribute to invasion and diffusion of the pathogen in hosts. FliY is a flagellar motor switch protein that controls flagellar motor direction in other microbes, but its role in Leptospira, and paricularly in pathogenicity remains unknown.

Results

A suicide plasmid for the fliY gene of Leptospira interrogans serogroup Icterohaemorrhagiae serovar Lai strain Lai that was disrupted by inserting the ampicillin resistance gene (bla) was constructed, and the inactivation of fliY gene in a mutant (fliY^-) was confirmed by PCR and Western Blot analysis. The inactivation resulted in the mRNA absence of fliP and fliQ genes which are located downstream of the fliY gene in the same operon. The mutant displayed visibly weakened rotative motion in liquid medium and its migration on semisolid medium was also markedly attenuated compared to the wild-type strain. Compared to the wild-type strain, the mutant showed much lower levels of adhesion to murine macrophages and apoptosis-inducing ability, and its lethality to guinea pigs was also significantly decreased.

Conclusion

Inactivation of fliY, by the method used in this paper, clearly had polar effects on downstream genes. The phentotypes observed, including lower pathogenicity, could be a consequence of fliY inactivation, but also a consequence of the polar effects.

Expression of purinergic receptors and modulation of P2X7 function by the inflammatory cytokine IFNgamma in human epithelial cells

The cervical epithelial cell line, HeLa, is one of the oldest and most commonly used cell lines in cell biology laboratories. Although a truncated P2X(7) receptor has recently been identified in HeLa cells, the expression of other purinergic receptors or the function of the P2X(7) protein has not been characterized. We here show that HeLa cells express transcripts for most P2X and P2Y purinergic receptors. Treatment of cells with ATP or other P2X(7) agonists does not stimulate cell death, but can induce atypical calcium fluxes and ion currents. Cervical epithelial cells represent an important target for sexually-transmitted pathogens and are commonly exposed to pro-inflammatory cytokines such as IFNgamma. Stimulation of HeLa cells with IFNgamma upregulates expression of P2X(7) mRNA and full-length protein, modifies ATP-dependent calcium fluxes, and renders the cells sensitive to ATP-induced apoptosis, which can be blocked by a P2X(7) antagonist. IFNgamma treatment also increased dramatically the sensitivity of the intestinal epithelial cell line, HCT8, to ATP-induced apoptosis. Significantly, IFNgamma also stimulated P2X(7) expression on human intestinal tissues. Responses to other purinergic receptor ligands suggest that HeLa cells may also express functional P2Y(1), P2Y(2) and P2Y(6) receptors, which could be relevant for modulating ion homeostasis in the cells.

Comparison of invasion of fibroblasts and macrophages by high- and low-virulence Leptospira strains: colonization of the host-cell nucleus and induction of necrosis by the virulent strain

The infection cycle of low- and high-virulence strains of Leptospira interrogans was compared in fibroblasts and macrophages. L. interrogans serovar Lai strain Lai was used as a representative high-virulence strain, while L. interrogans serovars Pomona strain Luo was used as a low-virulence strain. L. biflexa serovar Patoc strain Patoc I, a nonparasitic strain of Leptospira, was used as a control. Both the high- and low-virulence strains could adhere to fibroblasts and macrophages using one or both ends of the spirochete, which was followed by phagocytosis of both strains. Both strains adhered more strongly to macrophages than fibroblasts. However, the high-virulence strain could invade the host-cell nucleus, while the low-virulence strain remained in phagosomes. The L. biflexa strain neither adhered to nor invaded either cell type. Both of the L. interrogans strains also induced cell death (mostly necrosis) of macrophages, whether or not the spirochetes were viable, suggesting that leptospiral virulence is unrelated to macrophage death. However, the high-virulence strain induced mainly necrosis in fibroblasts, while the low-virulence strain induced more apoptosis. Thus, the main feature distinguishing the two L. interrogans strains is the ability of the high-virulence strain to invade the host-cell nucleus and induce pro-inflammatory necrosis in fibroblasts.

Recruitment of BAD by the Chlamydia trachomatis vacuole correlates with host-cell survival

Chlamydiae replicate intracellularly in a vacuole called an inclusion. Chlamydial-infected host cells are protected from mitochondrion-dependent apoptosis, partly due to degradation of BH3-only proteins. The host-cell adapter protein 14-3-3β can interact with host-cell apoptotic signaling pathways in a phosphorylation-dependent manner. In Chlamydia trachomatis-infected cells, 14-3-3β co-localizes to the inclusion via direct interaction with a C. trachomatis-encoded inclusion membrane protein. We therefore explored the possibility that the phosphatidylinositol-3 kinase (PI3K) pathway may contribute to resistance of infected cells to apoptosis. We found that inhibition of PI3K renders C. trachomatis-infected cells sensitive to staurosporine-induced apoptosis, which is accompanied by mitochondrial cytochrome c release. 14-3-3β does not associate with the Chlamydia pneumoniae inclusion, and inhibition of PI3K does not affect protection against apoptosis of C. pneumoniae-infected cells. In C. trachomatis-infected cells, the PI3K pathway activates AKT/protein kinase B, which leads to maintenance of the pro-apoptotic protein BAD in a phosphorylated state. Phosphorylated BAD is sequestered via 14-3-3β to the inclusion, but it is released when PI3K is inhibited. Depletion of AKT through short-interfering RNA reverses the resistance to apoptosis of C. trachomatis-infected cells. BAD phosphorylation is not maintained and it is not recruited to the inclusion of Chlamydia muridarum, which protects poorly against apoptosis. Thus, sequestration of BAD away from mitochondria provides C. trachomatis with a mechanism to protect the host cell from apoptosis via the interaction of a C. trachomatis-encoded inclusion protein with a host-cell phosphoserine-binding protein.

Chlamydia trachomatis is the most common cause of sexually transmitted bacterial infections in humans. These bacteria survive and replicate within a vacuole in the infected cell called an inclusion, producing up to a thousand bacteria per inclusion within a day of infection. Despite the large size of the inclusion, the infected cell survives long enough for the pathogens to complete their infection cycle and then infect new host cells. The researchers describe a novel mechanism for protection of the host cell by Chlamydia, namely activation of enzymes involved in host-cell survival. These enzymes, called kinases, cause the phosphorylation and inactivation of a protein, BAD, which can promote “cell suicide” (apoptosis) of uninfected cells. BAD phosphorylation is accompanied by recruitment of BAD to the chlamydial inclusion, where BAD binds to a cellular adapter protein, 14-3-3β. The adapter protein, in turn, is attracted to the inclusion by a membrane protein produced by Chlamydia. Thus, the chlamydial inclusion sequesters BAD away from mitochondria, where BAD could induce host-cell apoptosis. The cross talk between chlamydiae and host-cell signaling pathways demonstrates a novel mechanism of pathogen modulation of host-cell biology, and represents a potential therapeutic target for blocking infection by this prevalent pathogen.

Toxoplasma gondii inhibits ultraviolet light-induced apoptosis through multiple interactions with the mitochondrion-dependent programmed cell death pathway

Cells infected with the protozoan parasite Toxoplasma gondii are resistant to diverse apoptotic stimuli. In this study, we perform a detailed analysis of the manipulation of the mitochondrial arm of the apoptotic cascade by the parasite. Apoptosis was induced using irradiation with ultraviolet light (UV), and the kinetics of caspase activation, cytochrome c release and activation of the upstream signalling pathways were examined. The evidence clearly points to T. gondii targeting multiple steps in the transmission [inhibition of c-Jun N-terminal kinase (JNK) activation in response to UV], triggering (inhibition of cytochrome c release by affecting the balance of pro- and anti-apoptotic BCL-2 family members) and execution (inhibition of caspase 9 and caspase 3) phases of the apoptotic cascade. Interestingly, the multilevel pattern of inhibition that emerges suggests that the global inhibition of the mitochondrial arm of apoptosis is not likely to be contributed to by the small subset of mitochondria recruited to the T. gondii parasitophorous vacuole membrane.

Chlamydia and programmed cell death

Discordant views regarding host cell death induction by Chlamydia are likely owing to the different methods used for evaluation of apoptosis. Apoptotic and non-apoptotic death owing to both caspase-dependent and -independent activation of the Bax protein occur late in the productive growth cycle. Evidence also suggests that Chlamydia inhibits apoptosis during productive growth as part of its intracellular survival strategy. This is in part owing to proteolytic degradation of the BH3-only family of pro-apoptotic proteins in the mitochondrial pathway. Chlamydia also inhibits apoptosis during persistent growth or in phagocytes, but induces apoptosis in T cells, which suggests that apoptosis has an immunomodulatory role in chlamydial infections. The contribution of apoptosis in disease pathogenesis remains a focus for future research.

P2X and P2Y purinergic receptors on human intestinal epithelial carcinoma cells: effects of extracellular nucleotides on apoptosis and cell proliferation

Extracellular nucleotides interact with purinergic receptors, which regulate ion transport in a variety of epithelia. With the use of two different human epithelial carcinoma cell lines (HCT8 and Caco-2), we have shown by RT-PCR that the cells express mRNA for P2X1, P2X3, P2X4, P2X5, P2X6, P2X7, P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, and P2Y12 receptors. Protein expression for P2Y1 and P2Y2 receptors was also demonstrated immunohistochemically, and P2X receptor subtype protein was present in the following decreasing order: P2X4 > P2X7 > P2X1 > P2X3 > P2X6 > P2X5 >> P2X2. The functional presence of P2X7, P2Y1, P2Y2, and P2Y4 receptors was shown based on the effect of extracellular nucleotides on apoptosis or cell proliferation, and measurement of nucleotide-dependent calcium fluxes using a fluorometric imaging plate reader in the presence of different selective agonists and antagonists. ATP, at high concentrations, induced apoptosis through ligation of P2X7 and P2Y1 receptors; conversely, ATP, at lower concentrations, and UTP stimulated proliferation, probably acting via P2Y2 receptors. We therefore propose that stimulation or dysfunction of purinergic receptors may contribute at least partially to modulation of epithelial carcinoma cell proliferation and apoptosis.

Comparison of wild type with recombinant Clostridium difficile toxin A

Toxins A and B from Clostridium difficile are single-chain proteins of 308,000 and 270,000 Da, respectively. They possess transferase activity to monoglucosylate proteins of the Rho GTPase family whereby Rho, Rac, and Cdc42 are the canonical substrates. For application of these toxins as specific Rho GTPase inhibitors the highest possible purity is of crucial interest. We, therefore, expressed recombinant His-tagged toxin A using the Bacillus megaterium expression system. Specific antisera raised against the native toxin A from C. difficile and the recombinant toxin, respectively, showed identical sensitivity and specificity in Western blot and ELISA analyses towards both toxins. By comparison of both toxins in functional studies we showed that the recombinant toxin was about two times more cytotoxic than the native toxin, and the glucosyltransferase-activity of the recombinant toxin was even 10-fold increased. However, recombinant toxin A showed one essential difference to the classically purified one. The reported transferase-independent effect of toxin A to release cytochrome c from isolated mitochondria was not exhibited by the recombinant toxin A. This putative mitochondrial effect decreased with increased purity of toxin A, and was absent with recombinant toxin, strongly suggesting an clostridial contamination responsible. In summary, we tested the recombinant toxin A to be at least an adequate substitute for the native toxin, bearing the advantage of a rapid single-step purification and the absence of biological active contaminations.

Isolation and characterization of Psalmopeotoxin I and II: two novel antimalarial peptides from the venom of the tarantula Psalmopoeus cambridgei

Two novel peptides that inhibit the intra-erythrocyte stage of Plasmodium falciparum in vitro were identified in the venom of the Trinidad chevron tarantula, Psalmopoeus cambridgei. Psalmopeotoxin I (PcFK1) is a 33-residue peptide and Psalmopeotoxin II (PcFK2) has 28-amino acid residues; both have three disulfide bridges and belong to the Inhibitor Cystine Knot superfamily. The cDNAs encoding both peptides were cloned, and nucleotide sequence analysis showed that the peptides are synthesized with typical signal peptides and pro-sequences that are cleaved at a basic doublet before secretion of the mature peptides. The IC(5O) of PcFK1 for inhibiting P. falciparum growth was 1.59+/-1.15 microM and that of PcFK2 was 1.15+/-0.95 microM. PcFK1 was adsorbed strongly to uninfected erythrocytes, but PcFK2 was not. Neither peptide has significant hemolytic activity at 10 microM. Electrophysiological recordings in isolated frog and mouse neuromuscular preparations revealed that the peptides (at up to 9.3 microM) do not affect neuromuscular transmission or quantal transmitter release. PcFK1 and PcFK2 do not affect the growth or viability of human epithelial cells, nor do they have any antifungal or antibacterial activity at 20 microM. Thus, PcFK1 and PcFK2 seem to interact specifically with infected erythrocytes. They could therefore be promising tools for antimalaria research and be the basis for the rational development of antimalarial drugs.

Activation of the phosphatidylinositol 3-kinase/Akt pathway contributes to survival of primary epithelial cells infected with the periodontal pathogen Porphyromonas gingivalis

Porphyromonas gingivalis, an important periodontal pathogen, infects primary gingival epithelial cells (GECs). Despite the large number of bacteria that replicate inside the GECs, the host cell remains viable. We demonstrate that P. gingivalis triggers rapid and reversible surface phosphatidylserine exposure through a mechanism requiring caspase activation. However, after 1 day of infection, the bacteria no longer induce phosphatidylserine externalization and instead protect infected cells against apoptosis. Infection exerts its effect at the level of mitochondria, as P. gingivalis also blocks depolarization of the mitochondrial transmembrane potential and cytochrome c release. Interestingly, protein kinase B/Akt is phosphorylated during infection, which can be blocked with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002. Suppression of the PI3K/Akt pathway following staurosporine treatment results in mitochondrial-membrane depolarization, cytochrome c release, DNA fragmentation, and increased apoptosis of infected GECs. Thus, P. gingivalis stimulates early surface exposure of phosphatidylserine, which could downmodulate the inflammatory response, while also promoting host cell survival through the PI3K/Akt pathway.

Characterization of a gene encoding two isoforms of a mitochondrial protein up-regulated by cyclosporin A in activated T cells

Cyclosporin A (CSA) is an immunosuppressor used in organ transplantation. A recent proteomic analysis has revealed that activation of T cells in the presence of CSA induces the synthesis of hundreds of new proteins. Here we used representational difference analysis to characterize some of the corresponding induced genes. After cDNA bank screening we focused on one of these genes, which we named CSA-conditional, T cell activation-dependent (CSTAD) gene. This gene produces two mRNAs resulting from alternative splicing events. They encode two proteins of 104 and 141 amino acids, CSTADp-S and CSTADp-L, for the short and long forms, respectively. FK506 had the same effect as CSA, whereas rapamycin did not affect the level of CSTAD gene expression, demonstrating that inhibition of the calcineurin activation pathway is involved in CSTAD gene up-regulation. CSA also led to overexpression of CSTAD in mice immunized in the presence of CSA, confirming the in vitro analysis. Microscopic and cytofluorimetric analysis of cells expressing green fluorescent protein-tagged CSTADp-L and CSTADp-S showed that both proteins colocalize with mitochondrial markers and depolarize the mitochondrial transmembrane potential without causing release of cytochrome c, apoptosis, or necrosis. Both CSTADp isoforms are sensitive to proteinase K, implying that they are located in the mitochondrial outer membrane. These data reveal a new mechanism of action for CSA, which involves up-regulation of a gene whose products are sorted to mitochondria and depolarize the mitochondrial membrane.

Inhibition of chlamydial infectious activity due to P2X7R-dependent phospholipase D activation

Chlamydia trachomatis survives within host cells by inhibiting fusion between Chlamydia vacuoles and lysosomes. We show here that treatment of infected macrophages with ATP leads to killing of chlamydiae through ligation of the purinergic receptor, P2X(7)R. Chlamydial killing required phospholipase D (PLD) activation, as PLD inhibition led to rescue of chlamydiae in ATP-treated macrophages. However, there was no PLD activation nor chlamydial killing in ATP-treated P2X(7)R-deficient macrophages. P2X(7)R ligation exerts its effects by promoting fusion between Chlamydia vacuoles and lysosomes. P2X(7)R stimulation also resulted in macrophage death, but fusion with lysosomes preceded macrophage death and PLD inhibition did not prevent macrophage death. These results suggest that P2X(7)R ligation leads to PLD activation, which is directly responsible for inhibition of infection.

Role of proapoptotic BAX in propagation of Chlamydia muridarum (the mouse pneumonitis strain of Chlamydia trachomatis) and the host inflammatory response

The BCL-2 family member BAX plays a critical role in regulating apoptosis. Surprisingly, bax-deficient mice display limited phenotypic abnormalities. Here we investigate the effect of BAX on infection by the sexually transmitted pathogen, Chlamydia muridarum (the mouse pneumonitis strain of Chlamydia trachomatis). Bax(-/-) cells are relatively resistant to Chlamydia-induced apoptosis, and fewer bacteria are recovered after two infection cycles from Bax(-/-) cells than from wild-type cells. These results suggest that BAX-dependent apoptosis may be used to initiate a new round of infection, most likely by releasing Chlamydia-containing apoptotic bodies from infected cells that could be internalized by neighboring uninfected cells. Nonetheless, infected Bax(-/-) cells die through necrosis, which is normally associated with inflammation, more often than infected wild-type cells. These studies were confirmed in mice infected intravaginally with C. muridarum; since the infection disappears more quickly from Bax(-/-) mice than from wild-type mice, secretion of proinflammatory cytokines is increased in Bax(-/-) mice, and large granulomas are present in the genital tract of Bax(-/-) mice. Taken together, these data suggest that chlamydia-induced apoptosis via BAX contributes to bacterial propagation and decreases inflammation. Bax deficiency results in lower infection and an increased inflammatory cytokine response associated with more severe pathology.