The Akt/PKB pathway is constitutively activated in Theileria-transformed leucocytes, but does not directly control constitutive NF-kappaB activation

Hijacking of GPCRs and RTKs by pathogens

Pathogens exploit multiple cellular and molecular pathways in the host organisms for their entry, survival and dissemination. The cell surface receptors such as G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) constitute the targets of many pathogens. This is due to the ubiquitous expression of these two receptor families in the organism and their pivotal role in various cellular and physiological processes. At the molecular level, receptor hijacking implies either direct or indirect interactions between pathogens' effectors or toxins with GPCRs and RTKs at the cell surface thereby interfering with their activation and their downstream signaling pathways inside the host cells. As a result, the pathogens manipulate and redirect GPCR/RTK-mediated signaling pathways and different aspects of cell function for their benefit. The review presents a compilation of the major examples of pathogen infections where GPCRs and RTKs and their related intracellular signaling pathways are targeted. This provides a molecular basis for pathogens hijacking cell signaling and their virulence. Our understanding of such complex host-pathogen interactions at the molecular level will open new opportunities to develop new prophylactic and therapeutic approaches against infections. In this context, the pharmacological targeting of GPCRs and RTKs may be a promising approach.

A parasite DNA binding protein with potential to influence disease susceptibility acts as an analogue of mammalian HMGA transcription factors

Intracellular pathogens construct their environmental niche, and influence disease susceptibility, by deploying factors that manipulate infected host cell gene expression. Theileria annulata is an important tick-borne parasite of cattle that causes tropical theileriosis. Excellent candidates for modulating host cell gene expression are DNA binding proteins bearing AT-hook motifs encoded within the TashAT gene cluster of the parasite genome. In this study, TashAT2 was transfected into bovine BoMac cells to generate three expressing and three non-expressing (opposite orientation) cell lines. RNA-Seq was conducted and differentially expressed (DE) genes identified. The resulting dataset was compared with genes differentially expressed between infected cells and non-infected cells, and DE genes between infected cell lines from susceptible Holstein vs tolerant Sahiwal cattle. Over 800 bovine genes displayed differential expression associated with TashAT2, 209 of which were also modulated by parasite infection. Network analysis showed enrichment of DE genes in pathways associated with cellular adhesion, oncogenesis and developmental regulation by mammalian AT-hook bearing high mobility group A (HMGA) proteins. Overlap of TashAT2 DE genes with Sahiwal vs Holstein DE genes revealed that a significant number of shared genes were associated with disease susceptibility. Altered protein levels encoded by one of these genes (GULP1) was strongly linked to expression of TashAT2 in BoMac cells and was demonstrated to be higher in infected Holstein leucocytes compared to Sahiwal. We conclude that TashAT2 operates as an HMGA analogue to differentially mould the epigenome of the infected cell and influence disease susceptibility.

Activation of telomerase activity and telomere elongation of host cells by Theileria annulata infection

Theileria annulata-transformed cells share many phenotypes with cancer cells, including uncontrolled proliferation, immortalization, and dissemination. Telomeres are DNA-protein complex at the end of eukaryotic chromosomes that function to maintain genome stability and cell replicative capacity. Telomere length maintenance is primarily dependent on telomerase activity. In up to 90% of human cancer cells, telomerase is reactivated through expression of its catalytic subunit TERT. However, the effect of T. annulata infection on telomere and telomerase activity in bovine cells has not yet been described. In the present study, we confirmed that telomere length and telomerase activity are upregulated after T. annulata infection in three types of cell lines. This change depends on the presence of parasites. After eliminating Theileria from cells with antitheilerial drug buparvaquone, telomerase activity and the expression level of bTERT were decreased. In addition, inhibition of bHSP90 by novobiocin led to decreased AKT phosphorylation levels and telomerase activity, indicating that the bHSP90-AKT complex is a potent factor modulates telomerase activity in T. annulata-infected cells.

Longitudinal transcriptome analysis of cattle infected with Theileria parva

The apicomplexan cattle parasite Theileria parva is a major barrier to improving the livelihoods of smallholder farmers in Africa, killing over one million cattle on the continent each year. Although exotic breeds not native to Africa are highly susceptible to the disease, previous studies have illustrated that such breeds often show innate tolerance to infection by the parasite. The mechanisms underlying this tolerance remain largely unclear. To better understand the host response to T. parva infection we characterised the transcriptional response over 15 days in tolerant and susceptible cattle (n = 29) naturally exposed to the parasite. We identify key genes and pathways activated in response to infection as well as, importantly, several genes differentially expressed between the animals that ultimately survived or succumbed to infection. These include genes linked to key cell proliferation and infection pathways. Furthermore, we identify response expression quantitative trait loci containing genetic variants whose impact on the expression level of nearby genes changes in response to the infection. These therefore provide an indication of the genetic basis of differential host responses. Together these results provide a comprehensive analysis of the host transcriptional response to this under-studied pathogen, providing clues as to the mechanisms underlying natural tolerance to the disease.

Changes in TFG gene expression in bovine leucocytes transformed by Theileria annulata

Theileria annulata schizont-infected host cells in culture in vitro show unlimited proliferation similar to tumor cells; thus far, T. annulata and T. parva are the only eukaryotes that have been found to transform mammalian cells (immortalized). The transformation of these cells is reversible; when the parasite is eliminated in transformed cells by buparvaquone (BW720c), the host cells show normal growth and apoptosis. TFG is a tropomyosin-receptor kinase fused gene that is conserved among many species and is an important proto-oncogene. In this study, the bovine TFG gene was amplified by PCR from the cDNA of T. annulata schizont-transformed cells, cloned into the pGEX-4T-1 vector and expressed in Escherichia coli BL21 (DE3). After purification, the fusion protein was injected into rabbits to produce polyclonal antibodies. Using T. annulata-transformed cells together with BW720c treatment to kill the parasite, we aimed to identify changes in TFG gene expression by real-time PCR and Western blotting. The results showed that the bovine TFG gene was ~582 bp in size; SDS-PAGE analysis showed that the fusion protein was expressed in BL21 (DE3) cells with a molecular mass of 48 kD, and Western blotting indicated that the polyclonal antibodies could react with bovine TFG proteins from T. annulata-transformed cells and showed high specificity. Compared with that in the control group, the transcription level of the host TFG gene decreased significantly in the BW720c test group, and the expression of host tumor-related TFG protein decreased sharply after 72 h of drug treatment, suggesting that the TFG protein expression in transformed cells was directly related to T. annulata. This finding laid a foundation for further study on the interaction between T. annulata and host cells.

Susceptibility to disease (tropical theileriosis) is associated with differential expression of host genes that possess motifs recognised by a pathogen DNA binding protein

Background

Knowledge of factors that influence the outcome of infection are crucial for determining the risk of severe disease and requires the characterisation of pathogen-host interactions that have evolved to confer variable susceptibility to infection. Cattle infected by Theileria annulata show a wide range in disease severity. Native (Bos indicus) Sahiwal cattle are tolerant to infection, whereas exotic (Bos taurus) Holstein cattle are susceptible to acute disease.

Methodology/Principal findings

We used RNA-seq to assess whether Theileria infected cell lines from Sahiwal cattle display a different transcriptome profile compared to Holstein and screened for altered expression of parasite factors that could generate differences in host cell gene expression.

Significant differences (<0.1 FDR) in the expression level of a large number (2211) of bovine genes were identified, with enrichment of genes associated with Type I IFN, cholesterol biosynthesis, oncogenesis and parasite infection. A screen for parasite factors found limited evidence for differential expression. However, the number and location of DNA motifs bound by the TashAT2 factor (TA20095) were found to differ between the genomes of B. indicus vs. B. taurus, and divergent motif patterns were identified in infection-associated genes differentially expressed between Sahiwal and Holstein infected cells.

Conclusions/Significance

We conclude that divergent pathogen-host molecular interactions that influence chromatin architecture of the infected cell are a major determinant in the generation of gene expression differences linked to disease susceptibility.

Inherited Tolerance in Cattle to the Apicomplexan Protozoan Theileria parva is Associated with Decreased Proliferation of Parasite-Infected Lymphocytes

Theileria parva is the causative agent of East Coast fever and Corridor disease, which are fatal, economically important diseases of cattle in eastern, central and southern Africa. Improved methods of control of the diseases are urgently required. The parasite transforms host lymphocytes, resulting in a rapid, clonal expansion of infected cells. Resistance to the disease has long been reported in cattle from T. parva-endemic areas. We reveal here that first- and second-generation descendants of a single Bos indicus bull survived severe challenge with T. parva, (overall survival rate 57.3% compared to 8.7% for unrelated animals) in a series of five field studies. Tolerant cattle displayed a delayed and less severe parasitosis and febrile response than unrelated animals. The in vitro proliferation of cells from surviving cattle was much reduced compared to those from animals that succumbed to infection. Additionally, some pro-inflammatory cytokines such as IL1β, IL6, TNFα or TGFβ which are usually strongly expressed in susceptible animals and are known to regulate cell growth or motility, remain low in tolerant animals. This correlates with the reduced proliferation and less severe clinical reactions observed in tolerant cattle. The results show for the first time that the inherited tolerance to T. parva is associated with decreased proliferation of infected lymphocytes. The results are discussed in terms of whether the reduced proliferation is the result of a perturbation of the transformation mechanism induced in infected cells or is due to an innate immune response present in the tolerant cattle.

Curcumin induced oxidative stress causes autophagy and apoptosis in bovine leucocytes transformed by Theileria annulata

Bovine tropical theileriosis is a tick-borne disease, caused by Theileria annulata which is a protozoan parasite that resides within the B-cells and macrophages. T. annulata is a unique parasite that can transform bovine leucocytes which leads to the cancer hallmarks in the infected cells. Previously, curcumin has been shown to possess multiple pharmacological activities such as anti-inflammatory and anti-cancer activities. In this study, we demonstrated that curcumin inhibits the proliferation of Theileria-transformed bovine leucocytes by promoting apoptosis and autophagy. The transcriptome analysis of curcumin treated cells showed that the genes involved in cell death and autophagy are also differentially regulated. We further elucidated the mechanism of action of curcumin on Theileria infected bovine cells. We found that curcumin induced the generation of reactive oxygen species (ROS) which activated caspase 8 and destabilized the mitochondrial membrane potential leading to the release of cytochrome c from mitochondria. This subsequently led to the activation of caspase 3 and PARP cleavage, finally leading to apoptosis in the infected cells. Furthermore, curcumin induced the process of autophagy which was characterized by the formation of acidic vesicular organelles, LC3B accumulation with lysosome inhibitor, E64d, and the presence of autophagosomes as visualized by transmission electron microscopy (TEM). Curcumin treatment suppressed the mTOR and increased the expression of autophagy-related proteins. We also found that N- acetylcysteine, an inhibitor of ROS, could rescue the infected cells from curcumin induced apoptosis and autophagy mediated cell death. Intriguingly, curcumin had no effect on uninfected bovine PBMCs. Altogether, these data suggest the therapeutic potential of curcumin against bovine tropical theileriosis.

Identification and characterisation of a Theileria annulata proline-rich microtubule and SH3 domain-interacting protein (TaMISHIP) that forms a complex with CLASP1, EB1, and CD2AP at the schizont surface

Theileria annulata is an apicomplexan parasite that modifies the phenotype of its host cell completely, inducing uncontrolled proliferation, resistance to apoptosis, and increased invasiveness. The infected cell thus resembles a cancer cell, and changes to various host cell signalling pathways accompany transformation. Most of the molecular mechanisms leading to Theileria-induced immortalization of leukocytes remain unknown. The parasite dissolves the surrounding host cell membrane soon after invasion and starts interacting with host proteins, ensuring its propagation by stably associating with the host cell microtubule network. By using BioID technology together with fluorescence microscopy and co-immunoprecipitation, we identified a CLASP1/CD2AP/EB1-containing protein complex that surrounds the schizont throughout the host cell cycle and integrates bovine adaptor proteins (CIN85, 14-3-3 epsilon, and ASAP1). This complex also includes the schizont membrane protein Ta-p104 together with a novel secreted T. annulata protein (encoded by TA20980), which we term microtubule and SH3 domain-interacting protein (TaMISHIP). TaMISHIP localises to the schizont surface and contains a functional EB1-binding SxIP motif, as well as functional SH3 domain-binding Px(P/A)xPR motifs that mediate its interaction with CD2AP. Upon overexpression in non-infected bovine macrophages, TaMISHIP causes binucleation, potentially indicative of a role in cytokinesis.

Cell penetrating peptides to dissect host-pathogen protein-protein interactions in Theileria-transformed leukocytes

One powerful application of cell penetrating peptides is the delivery into cells of molecules that function as specific competitors or inhibitors of protein-protein interactions. Ablating defined protein-protein interactions is a refined way to explore their contribution to a particular cellular phenotype in a given disease context. Cell-penetrating peptides can be synthetically constrained through various chemical modifications that stabilize a given structural fold with the potential to improve competitive binding to specific targets. Theileria-transformed leukocytes display high PKA activity, but PKA is an enzyme that plays key roles in multiple cellular processes; consequently genetic ablation of kinase activity gives rise to a myriad of confounding phenotypes. By contrast, ablation of a specific kinase-substrate interaction has the potential to give more refined information and we illustrate this here by describing how surgically ablating PKA interactions with BAD gives precise information on the type of glycolysis performed by Theileria-transformed leukocytes. In addition, we provide two other examples of how ablating specific protein-protein interactions in Theileria-infected leukocytes leads to precise phenotypes and argue that constrained penetrating peptides have great therapeutic potential to combat infectious diseases in general.

TGF-β2 induces Grb2 to recruit PI3-K to TGF-RII that activates JNK/AP-1-signaling and augments invasiveness of Theileria-transformed macrophages

Theileria-infected macrophages display many features of cancer cells such as heightened invasive capacity; however, the tumor-like phenotype is reversible by killing the parasite. Moreover, virulent macrophages can be attenuated by multiple in vitro passages and so provide a powerful model to elucidate mechanisms related to transformed macrophage virulence. Here, we demonstrate that in two independent Theileria-transformed macrophage cell lines Grb2 expression is down-regulated concomitant with loss of tumor virulence. Using peptidimer-c to ablate SH2 and SH3 interactions of Grb2 we identify TGF-receptor II and the p85 subunit of PI3-K, as Grb2 partners in virulent macrophages. Ablation of Grb2 interactions reduces PI3-K recruitment to TGF-RII and decreases PIP3 production, and dampens JNK phosphorylation and AP-1-driven transcriptional activity down to levels characteristic of attenuated macrophages. Loss of TGF-R>PI3-K>JNK>AP-1 signaling negatively impacts on virulence traits such as reduced JAM-L/ITG4A and Fos-B/MMP9 expression that contribute to virulent macrophage adhesion and invasiveness.

Cell cycle-dependent phosphorylation of Theileria annulata schizont surface proteins

The invasion of Theileria sporozoites into bovine leukocytes is rapidly followed by the destruction of the surrounding host cell membrane, allowing the parasite to establish its niche within the host cell cytoplasm. Theileria infection induces host cell transformation, characterised by increased host cell proliferation and invasiveness, and the activation of anti-apoptotic genes. This process is strictly dependent on the presence of a viable parasite. Several host cell kinases, including PI3-K, JNK, CK2 and Src-family kinases, are constitutively activated in Theileria-infected cells and contribute to the transformed phenotype. Although a number of host cell molecules, including IkB kinase and polo-like kinase 1 (Plk1), are recruited to the schizont surface, very little is known about the schizont molecules involved in host-parasite interactions. In this study we used immunofluorescence to detect phosphorylated threonine (p-Thr), serine (p-Ser) and threonine-proline (p-Thr-Pro) epitopes on the schizont during host cell cycle progression, revealing extensive schizont phosphorylation during host cell interphase. Furthermore, we established a quick protocol to isolate schizonts from infected macrophages following synchronisation in S-phase or mitosis, and used mass spectrometry to detect phosphorylated schizont proteins. In total, 65 phosphorylated Theileria proteins were detected, 15 of which are potentially secreted or expressed on the surface of the schizont and thus may be targets for host cell kinases. In particular, we describe the cell cycle-dependent phosphorylation of two T. annulata surface proteins, TaSP and p104, both of which are highly phosphorylated during host cell S-phase. TaSP and p104 are involved in mediating interactions between the parasite and the host cell cytoskeleton, which is crucial for the persistence of the parasite within the dividing host cell and the maintenance of the transformed state.

MDM2 regulates a novel form of incomplete neoplastic transformation of Theileria parva infected lymphocytes

Our efforts are concerned with identifying features of incomplete malignant transformation caused by non viral pathogens. Theileria parva (T. parva) is a tick-transmitted protozoan parasite that can cause a fatal lymphoproliferative disease in cattle. The T. parva-infected lymphocytes display a transformed phenotype and proliferate in culture media like the other tumor cells, however those cells will return to normal after antiprotozoal treatment reflecting the incomplete nature of transformation. To identify signaling pathways involved in this form of transformation of T. parva-infected cells, we screened a library of anticancer compounds. Among these, TIBC, a specific inhibitor of MDM2, markedly inhibited proliferation of T. parva-infected lymphocytes and promoted apoptosis. Therefore we analyzed MDM2 function in T. parva-infected cells. Several T. parva-infected cell lines showed increased expression level of MDM2 with alternatively spliced isoforms compared to the lymphoma cells or ConA blasts. In addition, buparvaquone affected MDM2 expression in T. parva transformed cells. Moreover, p53 protein accumulation and function were impaired in T. parva-infected cells after cisplatin induced DNA damage despite the increased p53 transcription level. Finally, the treatment of T. parva-infected cells with boronic-chalcone derivatives TIBC restored p53 protein accumulation and induced Bax expression. These results suggest that the overexpression of MDM2 is closely linked to the inhibition of p53-dependent apoptosis of T. parva-infected lymphocytes. Aberrant expression of host lymphocyte MDM2 induced by cytoplasmic existence of T. parva, directly and/or indirectly, is associated with aspects of this type of transformation of T. parva-infected lymphocytes. This form of transformation shares features of oncogene induced malignant phenotype acquisition.

Modulation of activation-associated host cell gene expression by the apicomplexan parasite Theileria annulata

Infection of bovine leucocytes by Theileria annulata results in establishment of transformed, infected cells. Infection of the host cell is known to promote constitutive activation of pro-inflammatory transcription factors that have the potential to be beneficial or detrimental. In this study we have compared the effect of LPS activation on uninfected bovine leucocytes (BL20 cells) and their Theileria-infected counterpart (TBL20). Gene expression profiles representing activated uninfected BL20 relative to TBL20 cells were also compared. The results show that while prolonged stimulation with LPS induces cell death and activation of NF-κB in BL20 cells, the viability of Theileria-infected cells was unaffected. Analysis of gene expression networks provided evidence that the parasite establishes tight control over pathways associated with cellular activation by modulating reception of extrinsic stimuli and by significantly altering the expression outcome of genes targeted by infection-activated transcription factors. Pathway analysis of the data set identified novel candidate genes involved in manipulation of cellular functions associated with the infected transformed cell. The data indicate that the T. annulata parasite can irreversibly reconfigure host cell gene expression networks associated with development of inflammatory disease and cancer to generate an outcome that is beneficial to survival and propagation of the infected leucocyte.

Cytoplasmic sequestration of p53 promotes survival in leukocytes transformed by Theileria

The function of the p53 protein as the central effector molecule of the p53 apoptotic pathway was investigated in a reversible model of epigenetic transformation. The infection of bovine leukocytes by the intracellular protozoan parasite Theileria annulata results in parasite-dependent transformation and proliferation of the host cells. We found p53 to be largely localized in the host cell cytoplasm and associated with the parasite membrane of isolated schizonts. Curing infected cells of the parasite with the theilericidal drug buparvaquone resulted in a time-dependent translocation of p53 into the host cell nucleus and the upregulation of the proapoptotic Bax and Apaf-1 and the downregulation of the anti-apoptotic Bcl-2 proteins. Although buparvaquone treatment led to apoptosis of the host cell, inhibition of either p53 or Bax significantly reduced buparvaquone-induced apoptosis of the transformed cells. Thus, the p53 apoptotic pathway of host cells is not induced by infection and transformation with Theileria by a mechanism involving cytoplasmic sequestration of p53. The close association of host cell p53 with the parasite membrane implies that the parasite either interacts directly with p53 or mediates cytoplasmic sequestration of p53 by interacting with other host cell proteins regulating p53 localization.

Schizonts of Theileria annulata interact with the microtubuli network of their host cell via the membrane protein TaSP

Intracellular leukoproliferative Theileria are unique as eukaryotic organisms that transform the immune cells of their ruminant host. Theileria utilize the uncontrolled proliferation for rapid multiplication and distribution into host daughter cells. The parasite distribution into the daughter cells is accompanied by a tight association with the host cell mitotic apparatus. Since the molecular basis for this interaction is largely unknown, we investigated the possible involvement of the immunodominant Theileria annulata surface protein, TaSP, in the attachment of the parasite to host cell microtubule network. Confocal microscopic analyses showed co-localization of the TaSP protein with alpha-tubulin and reciprocal immuno-co-precipitation experiments demonstrated an association of TaSP with alpha-tubulin in vivo. In addition, the partially expressed predicted extracellular domain of TaSP co-localized with the mitotic spindle of dividing cells and was co-immunoprecipitated with alpha-tubulin in transiently transfected Cos-7 cells devoid of other T. annulata expressed proteins. Pull-down studies showed that there is a direct interaction between TaSP and polymerized microtubules. Analysis of the interaction of TaSP and host microtubulin during host cell mitosis indicated that TaSP co-localizes and interacts with the spindle poles, the mitotic spindle apparatus and the mid-body. Moreover, TaSP was demonstrated to be localized to the microtubule organizing center and to physically interact with gamma-tubulin. These data support the notion that the TaSP-microtubule interaction may be playing a potential role in parasite distribution into daughter host cells and give rise to the speculation that TaSP may be involved in regulation of microtubule assembly in the host cell.

Modulation of NF-kappaB activation in Theileria annulata-infected cloned cell lines is associated with detection of parasite-dependent IKK signalosomes and disruption of the actin cytoskeleton

Apicomplexan parasites within the genus Theileria have the ability to induce continuous proliferation and prevent apoptosis of the infected bovine leukocyte. Protection against apoptosis involves constitutive activation of the bovine transcription factor NF-kappaB in a parasite-dependent manner. Activation of NF-kappaB is thought to involve recruitment of IKK signalosomes at the surface of the macroschizont stage of the parasite, and it has been postulated that additional host proteins with adaptor or scaffolding function may be involved in signalosome formation. In this study two clonal cell lines were identified that show marked differences in the level of activated NF-kappaB. Further characterization of these lines demonstrated that elevated levels of activated NF-kappaB correlated with increased resistance to cell death and detection of parasite-associated IKK signalosomes, supporting results of our previous studies. Evidence was also provided for the existence of host- and parasite-dependent NF-kappaB activation pathways that are influenced by the architecture of the actin cytoskeleton. Despite this influence, it appears that the primary event required for formation of the parasite-dependent IKK signalosome is likely to be an interaction between a signalosome component and a parasite-encoded surface ligand.

Intracellular survival of apicomplexan parasites and host cell modification

The intracellular stages of apicomplexan parasites are known to extensively modify their host cells to ensure their own survival. Recently, considerable progress has been made in understanding the molecular details of these parasite-dependent effects for Plasmodium-, Toxoplasma- and Theileria-infected cells. We have begun to understand how Plasmodium liver stage parasites protect their host hepatocytes from apoptosis during parasite development and how they induce an ordered cell death at the end of the liver stage. Toxoplasma parasites are also known to regulate host cell survival pathways and it has been convincingly demonstrated that they block host cell major histocompatibility complex (MHC)-dependent antigen presentation of parasite epitopes to avoid cell-mediated immune responses. Theileria parasites are the masters of host cell modulation because their presence immortalises the infected cell. It is now accepted that multiple pathways are activated to induce Theileria-dependent host cell transformation. Although it is now known that similar host cell pathways are affected by the different parasites, the outcome for the infected cell varies considerably. Improved imaging techniques and new methods to control expression of parasite and host cell proteins will help us to analyse the molecular details of parasite-dependent host cell modifications.

Promotion of cell death or neurite outgrowth in PC-12 and N2a cells by the fungal alkaloid militarinone A depends on basal expression of p53

The fungal alkaloid militarinone A (MiliA) was recently found to stimulate neuronal outgrowth in PC-12 cells by persistant activation of pathways that are also involved in NGF-mediated differentiation, namely the PI3-K/PKB and the MEK/ERK pathways. Application of equal concentrations of MiliA to other cells such as the murine neuroblastoma cell line N2a resulted in immediate onset of apoptosis by nuclear translocation of apoptosis inducing factor (AIF), activation of caspases and c-Jun/AP-1 transcription factor without an intermediate differentiated phenotype, although minor transient phosphorylation of PKB and MAPK as well as activation of NF-kappaB were also observed. Translocation of AIF was preceded by p53 phosphorylation at Ser15 and blocked by pifithrin alpha, a known inhibitor of p53-transcriptional activity. We here show that both cell types activate the same pathways albeit in different time scales. This is mainly due to contrasting basal expression levels of p53, which in turn regulates expression of AIF. In PC-12 cells, continuous activation of these pathways after prolonged treatment with 40 muM MiliA first led to up-regulation of p53, phosphorylation of p53, release of AIF from mitochondria and its translocation into the nucleus. Additionally, also activation of the c-Jun/AP-1 transcription factor was observed, and PC-12 cells subsequently underwent apoptosis 48-72 h post-treatment. We report that similar pathways working on different levels are able to initially shape very divergent cellular responses.

Multiphoton imaging of ultrashort pulse laser ablation in the intracellular parasite Theileria

Theileria annulata is an intracellular parasite that infects and transforms bovine leukocytes, inducing continuous proliferation of its host cell both in vivo and in vitro. Theileria-infected cells can easily be propagated in the laboratory and serve as a good model for laser ablation studies. Using single pulses from an amplified ultrashort pulse laser system, we developed a technique to introduce submicrometer holes in the plasma membrane of the intracellular schizont stage of Theileria annulata. This was achieved without compromising either the viability of the organisms or that of the host cell that harbors the parasite in its cytoplasm. Multiphoton microscopy was used to generate image stacks of the parasite before and after ablation. The high axial resolution allowed precise selection of the region of the membrane that was ablated. It also allowed observation of the size of the holes generated (in fixed, stained cells) and determination of the structural changes in the parasite resulting from the laser pulses (in living cells in vitro). This technique opens a new possibility for the transfection of Theileria or delivery of molecules to the schizont that may prove useful in the study of this special host-parasite relationship.

Infection with Toxoplasma gondii results in dysregulation of the host cell cycle

Mammalian cells infected with Toxoplasma gondii are characterized by a profound reprogramming of gene expression. We examined whether such transcriptional responses were linked to changes in the cell cycle of the host. Human foreskin fibroblasts (HFFs) in the G(0)/G(1) phase of the cell cycle were infected with T. gondii and FACS analysis of DNA content was performed. Cell cycle profiles revealed a promotion into the S phase followed by an arrest towards the G(2)/M boundary with infection. This response was markedly different from that of growth factor stimulation which caused cell cycle entry and completion. Transcriptional profiles of T. gondii-infected HFF showed sustained increases in transcripts associated with a G(1)/S transition and DNA synthesis coupled to an abrogation of cell cycle regulators critical in G(2)/M transition relative to growth factor stimulation. These divergent responses correlated with a distinct temporal modulation of the critical cell cycle regulator kinase ERK by infection. While the kinetics of ERK phosphorylation by EGF showed rapid and sustained activation, infected cells displayed an oscillatory pattern of activation. Our results suggest that T. gondii infection induces and maintains a 'proliferation response' in the infected cell which may fulfill critical growth requirements of the parasite during intracellular residence.

Resistance and susceptibility to a protozoan parasite of cattle—Gene expression differences in macrophages from different breeds of cattle

Cattle infected with the tick-borne protozoan, Theileria annulata, usually undergo severe morbidity, and mortality ensues in a high proportion of animals. However, we have shown that a Bos indicus breed, the Sahiwal, which originates in a T. annulata endemic area, is more resistant to the parasite. Although Sahiwals become infected, the breed exhibits fewer clinical signs and recovers from a dose of parasite which is fatal in the Holstein B. taurus breed. The Sahiwals have a significantly lower fever response, and lower levels of parasite than the Holsteins. One unusual feature of this disease is the production of acute phase proteins (APP), indicating that the parasite induces high systemic levels of pro-inflammatory cytokines. In the Holsteins there is prolonged production of the APP, alpha1-glycoprotein, which, in contrast, is only slightly elevated in the Sahiwals. As the parasite infects macrophages (mphi), our hypothesis is that the Sahiwals can control the excessive production of pro-inflammatory cytokines in response to infection, and that this control is expressed at the level of the mphi. We thus reasoned that the genes underlying the observed difference in resistance to tropical theileriosis, might be identified by investigating gene expression differences in mphi from both breeds. It is possible that relevant polymorphisms might in themselves result in gene expression differences, so initially we targeted likely candidates. However, we detected no differences in expression of the pro-inflammatory cytokines, tumour necrosis factor-alpha (TNFalpha), interleukin-1beta (IL-1beta) or IL-6, in infected mphi. As it is more likely that polymorphisms in candidate genes influence the expression of other genes involved in interrelated pathways, we undertook a more global approach. We designed a bovine mphi specific cDNA microarray, which contains representatives of 5000 different genes expressed in mphi, and investigated the transcriptional responses of mphi from both breeds in response to a variety of stimuli, including infection with T. annulata. Our results indicate that there are fundamental differences in gene expression in mphi from both breeds in the way they respond to infection, and even in their pre-infection resting state.

Suicide prevention: disruption of apoptotic pathways by protozoan parasites

The modulation of apoptosis has emerged as an important weapon in the pathogenic arsenal of multiple intracellular protozoan parasites. Cryptosporidium parvum, Leishmania spp., Trypanosoma cruzi, Theileria spp., Toxoplasma gondii and Plasmodium spp. have all been shown to inhibit the apoptotic response of their host cell. While the pathogen mediators responsible for this modulation are unknown, the parasites are interacting with multiple apoptotic regulatory systems to render their host cell refractory to apoptosis during critical phases of intracellular infection, including parasite invasion, establishment and replication. Additionally, emerging evidence suggests that the parasite life cycle stage impacts the modulation of apoptosis and possibly parasite differentiation. Dissection of the host-pathogen interactions involved in modulating apoptosis reveals a dynamic and complex interaction that recent studies are beginning to unravel.

Leishmania promastigotes activate PI3K/Akt signalling to confer host cell resistance to apoptosis

Previous reports have shown that cells infected with promastigotes of some Leishmania species are resistant to the induction of apoptosis. This would suggest that either parasites elaborate factors that block signalling from apoptosis inducers or that parasites engage endogenous host signalling pathways that block apoptosis. To investigate the latter scenario, we determined whether Leishmania infection results in the activation of signalling pathways that have been shown to mediate resistance to apoptosis in other infection models. First, we showed that infection with the promastigote form of Leishmania major, Leishmania pifanoi and Leishmania amazonensis activates signalling through p38 mitogen-activated protein kinase (MAPK), NFkappaB and PI3K/Akt. Then we found that inhibition of signalling through the PI3K/Akt pathway with LY294002 and Akt IV inhibitor reversed resistance of infected bone marrow-derived macrophages and RAW 264.7 macrophages to potent inducers of apoptosis. Moreover, reduction of Akt levels with small interfering RNAs to Akt resulted in the inability of infected macrophages to resist apoptosis. Further evidence of the role of PI3K/Akt signalling in the promotion of cell survival by infected cells was obtained with the finding that Bad, which is a substrate of Akt, becomes phosphorylated during the course of infection. In contrast to the observations with PI3K/Akt signalling, inhibition of p38 MAPK signalling with SB202190 or NFkappaB signalling with wedelolactone had limited effect on parasite-induced resistance to apoptosis. We conclude that Leishmania promastigotes engage PI3K/Akt signalling, which confers to the infected cell, the capacity to resist death from activators of apoptosis.

Pro- and anti-apoptotic activities of protozoan parasites

During infection, programmed cell death, i.e. apoptosis, is an important effector mechanism of innate and adaptive host responses to parasites. In addition, it fulfils essential functions in regulating host immunity and tissue homeostasis. Not surprisingly, however, adaptation of parasitic protozoa to their hosts also involves modulation or even exploitation of cell death in order to facilitate parasite survival in a hostile environment. During recent years, considerable progress has been made in our understanding of apoptosis during parasitic infections and there is now convincing evidence that apoptosis and its modulation by protozoan parasites has a major impact on the parasite-host interaction and on the pathogenesis of disease. This review updates our current knowledge on the diverse functions apoptosis may fulfil during infections with diverse protozoan parasites including apicomplexans, kinetoplastids and amoebae. Furthermore, we also summarize common mechanistic themes of the pro- and anti-apoptotic activities of protozoan parasites. The diverse and complex effects which parasitic protozoa exert on apoptotic cell death within the host highlight fascinating interactions of parasites and their hosts. Importantly, they also stress the importance of further investigations before the modulation of host cell apoptosis can be exploited to combat parasitic infections.

Theileria-induced constitutive IKK activation is independent of functional Hsp90

The intracellular parasite Theileria induces uncontrolled proliferation and host cell transformation. Parasite-induced transformation is accompanied by constitutive activation of IkappaB kinase (IKK), resulting in permanently high levels of activated nuclear factor (NF)-kappaB. IKK activation pathways normally require heat shock protein 90 (Hsp90), a chaperone that regulates the stability and activity of signalling molecules and can be blocked by the benzoquinone ansamycin compound geldanamycin (GA). In Theileria-transformed cells, IkappaBalpha and p65 phosphorylation, NF-kappaB nuclear translocation and DNA binding activity are largely resistant to GA and also NF-kappaB-dependent reporter gene expression is only partly affected. Our findings indicate that parasite-induced IKK activity does not require functional Hsp90.

Post-translational signal peptide cleavage controls differential epitope recognition in the QP-rich domain of recombinant Theileria parva PIM

The presence of the schizont stage of the obligate intracellular parasites Theileria parva or T. annulata in the cytoplasm of an infected leukocyte results in host cell transformation via a mechanism that has not yet been elucidated. Proteins, secreted by the schizont, or expressed on its surface, are of interest as they can interact with host cell molecules that regulate host cell proliferation and/or survival. The major schizont surface protein is the polymorphic immunodominant molecule, PIM, which contains a large glutamine- and proline-rich domain (QP-rd) that protrudes into the host cell cytoplasm. Analyzing QP-rd generated by in vitro transcription/translation, we found that the signal peptide was efficiently cleaved post-translationally upon addition of T cell lysate or canine pancreatic microsomes, whereas signal peptide cleavage of a control protein only occurred cotranslationally and in the presence of microsomal membranes. The QP-rd of PIM migrated anomalously in SDS-PAGE and removal of the 19 amino acids corresponding to the predicted signal peptide caused a decrease in apparent molecular mass of 24kDa. The molecule was analyzed using monoclonal antibodies that recognize a set of previously defined PIM epitopes. Depending on the presence or the absence of the signal peptide, two conformational states could be demonstrated that are differentially recognized, with N-terminal epitopes becoming readily accessible upon signal peptide removal, and C-terminal epitopes becoming masked. Similar observations were made when the QP-rd of PIM was expressed in bacteria. Our observations could also be of relevance to other schizont proteins. A recent analysis of the proteomes of T. parva and T. annulata revealed the presence of a large family of potentially secreted proteins, characterized by the presence of large stretches of amino acids that are also particularly rich in QP-residues.

Toxoplasma gondiitriggers Gi-dependent PI 3-kinase signaling required for inhibition of host cell apoptosis

Infection with the intracellular parasite Toxoplasma gondii renders cells resistant to multiple pro-apoptotic signals, but underlying mechanisms have not been delineated. The phosphoinositide 3-kinase (PI 3-kinase) pathway and the immediate downstream effector protein kinase B (PKB/Akt) play important roles in cell survival and apoptosis inhibition. Here, we show that Toxoplasma infection of mouse macrophages activates PKB/Akt in vivo and in vitro. In a mixed population of infected and non-infected macrophages, activation is only observed in parasite-infected cells. The PI 3-kinase inhibitors wortmannin and LY294002 block parasite-induced PKB phosphorylation. PKB activation occurs independently of Toll-like receptor adaptor protein MyD88 but uncoupling of Gi-protein-mediated signaling with pertussis toxin prevents PKB phosphorylation. Moreover, in the presence of PI 3-kinase inhibitors or pertussis toxin, not only PKB activation but also ERK1/2 activation during T. gondii infection is defective. Most importantly, the parasite's ability to induce macrophage resistance to pro-apoptotic signaling is prevented by incubation with PI 3-kinase inhibitors. This study demonstrates that T. gondii exploits host Gi-protein-dependent PI 3-kinase signaling to prevent induction of apoptosis in infected macrophages.

Regulation of host cell survival by intracellular Plasmodium and Theileria parasites

Plasmodium and Theileria parasites are obligate intracellular protozoa of the phylum Apicomplexa. Theileria infection of bovine leukocytes induces transformation of host cells and infected leukocytes can be kept indefinitely in culture. Theileria-dependent host cell transformation has been the subject of interest for many years and the molecular basis of this unique phenomenon is quite well understood. The equivalent life cycle stage of Plasmodium is the infection of mammalian hepatocytes, where parasites reside for 2-7 days depending on the species. Some of the molecular details of parasite-host interactions in P. berghei-infected hepatocytes have emerged only very recently. Similar to what has been shown for Theileria-infected leukocytes these data suggest that malaria parasites within hepatocytes also protect their host cell from programmed cell death. However, the strategies employed to inhibit host cell apoptotic pathways appear to be different to those used by Theileria. This review discusses similarities and differences at the molecular level of Plasmodium- and Theileria-induced regulation of the host cell survival machinery.

Constitutively activated CK2 potentially plays a pivotal role in Theileria-induced lymphocyte transformation

Activation of casein kinase II (CK2) was one of the first observations made on how Theileria parasites manipulate host cell signal transduction pathways and we argue that CK2 induction may in fact contribute to many of the different activation events that have been described since 1993 for Theileria-infected lymphocytes such as sustained activation of transcription factors c-Myc and NF-κB. CK2 also contributes to infected lymphocyte survival by inhibiting caspase activation and is probably behind constitutive PI3-K activation by phosphorylating PTEN. Finally, we also discuss how CK2A may act not only as a kinase, but also as a stimulatory subunit for the protein phosphatase PP2A, so dampening down the MEK/ERK and Akt/PKB pathways and for all these reasons we propose CK2 as a central player in Theileria-induced lymphocyte transformation.

Alteration of host cell phenotype by Theileria annulata and Theileria parva: mining for manipulators in the parasite genomes

The apicomplexan parasites Theileria annulata and Theileria parva cause severe lymphoproliferative disorders in cattle. Disease pathogenesis is linked to the ability of the parasite to transform the infected host cell (leukocyte) and induce uncontrolled proliferation. It is known that transformation involves parasite dependent perturbation of leukocyte signal transduction pathways that regulate apoptosis, division and gene expression, and there is evidence for the translocation of Theileria DNA binding proteins to the host cell nucleus. However, the parasite factors responsible for the inhibition of host cell apoptosis, or induction of host cell proliferation are unknown. The recent derivation of the complete genome sequence for both T. annulata and T. parva has provided a wealth of information that can be searched to identify molecules with the potential to subvert host cell regulatory pathways. This review summarizes current knowledge of the mechanisms used by Theileria parasites to transform the host cell, and highlights recent work that has mined the Theileria genomes to identify candidate manipulators of host cell phenotype.

The strategies of the Theileria parasite: a new twist in host-pathogen interactions

Theileria parasites infect and transform cells of the ruminant immune system. Continuous proliferation and survival of Theileria-transformed cells involves the well-orchestrated activation of several host-cell signalling pathways. Constitutive NF-kappa B (nuclear factor kappa B) activation is accomplished by recruiting the IKK (I kappa B kinase) complex, a central regulator of NF-kappa B pathways, to the surface of the transforming schizont, where it becomes permanently activated. Constitutive activation of the PI-3K-PKB [phosphoinositide 3-kinase-(Akt) protein kinase B] pathway is likely to be indirect and is essential for continuous proliferation. Theileria-transformed T cells express a range of anti-apoptotic proteins that can be expected to provide protection against apoptosis induced by death receptors, as well as cellular control mechanisms that are mobilised to eliminate cells that entered a cycle of uncontrolled proliferation.

Accelerated recycling of transferrin receptor in Theileria-transformed B cells

We found that phoshatidylinositol-3 kinase (PI3-K) markedly contributes to the increased surface expression of bovine transferrin receptor (TfR) on Theileria-infected lymphocytes. We observed that all aspects of TfR turnover are upregulated in parasitized B cells and we were able to detect TfR colocalizing with EEA1 (early endosome antigen 1) and Rab11 at the ultrastructure level in Theileria-infected B cells. We demonstrated recycling of TfR through Rab5- and Rab11-positive compartments by transfection of dominant negative guanosine diphosphate (GDP)-on mutants of the GTPases. Therefore, in Theileria-transformed B cells constitutive PI3-K activity leads to accelerated TfR recycling through Rab5- and Rab11-positive compartments.

Use of micro-rotation imaging to study JNK-mediated cell survival inTheileria parva-infected B-lymphocytes

Lymphocytes infected with the protozoan parasiteTheileria parvaare transformed to permanently proliferating cells, an event underlying the pathology of the disease. However, the molecular signalling mediating this process is complex and poorly understood. Here, we show that down-regulation of JNK signalling by transient over expression of a dominant-negative mutant of JNK (JNK-APF) significantly increases Annexin-V-phycoerythrin (V-PE) labelling on infected B cell populations observed using flow cytometry. To establish whether this increase was specifically due to apoptosis, we used a novel single-cell imaging method: micro-rotation (MR)-imaging, designed to allow high-resolution 3-dimensional imaging of single cells in suspension. With this method we visualized subcellular patterns of V-PE uptake and chromatin organization in lymphocytes co-transfected with JNK-APF and GFP-tagged histone-H2B. This single-cell approach allowed us to clearly reveal characteristic apoptotic phenotypes, whose patterns reflected progressive states of programmed cell death due to JNK down-regulation. Our results strongly suggest a role for JNK in the survival ofTheileria-infected B cells, and demonstrate the powerful utility of a new and unique 3-dimensional imaging method for living cells in suspension.

TashHN, a Theileria annulata encoded protein transported to the host nucleus displays an association with attenuation of parasite differentiation

The intracellular apicomplexan parasite, Theileria annulata, manipulates its bovine host cell by over-riding the cells natural apoptotic response and inducing proliferation of the infected leukocyte. We have recently identified a T. annulata encoded family of polypeptides (TashATs) with characteristics that indicate that they are involved in control of host cell gene expression. Here we present data on another member of this family, TashHN, showing that it is located to the parasite and host cell nucleus. Immunoblot analysis demonstrated that, unlike TashAT2 and 3, TashHN displays three forms, the largest of which is enriched in the host nuclear fraction and appears to be phosphorylated. Northern and 5 prime race analyses identified multiple TashHN RNA species in infected cells that have retained the ability to differentiate. These transcripts showed subtly different kinetics, but all decreased during differentiation to the merozite, and two showed reduced levels prior to down-regulation of the other TashATs. In addition, analyses of multiple cell lines that have become severely attenuated in their potential to differentiate, indicated a substantial increase in TashHN expression, with host nuclear reactivity particularly enhanced.

Activation of NF-kappaB by Toxoplasma gondii correlates with increased expression of antiapoptotic genes and localization of phosphorylated IkappaB to the parasitophorous vacuole membrane

Mammalian cells infected with Toxoplasma gondii are resistant to apoptosis induced by a variety of stimuli. We have demonstrated that the host transcription factor NF-kappaB plays a pivotal role in the T.-gondii-mediated blockade of apoptosis because inhibition is lost in cells lacking the p65 (RelA) subunit of NF-kappaB (p65-/-). In the present study, we examined the effects of T. gondii infection on NF-kappaB activation and the expression of genes involved in the apoptotic cascade. Infection of wild-type mouse embryonic fibroblasts (MEFs) with T.-gondii-induced nuclear translocation of the p50 and p65 subunits of NF-kappaB as examined by immunoblotting of nuclear extracts, immunofluorescence and electrophoretic mobility shift assays. A comparison of apoptotic gene expression profiles from wild-type and p65-/- MEFs revealed distinct patterns of induction in response to T. gondii infection. In particular, the differences seen in the Bcl-2 and IAP families are consistent with the antiapoptotic responses observed in the resistant wild-type cells compared with the sensitive p65-/- fibroblasts. Consistent with NF-kappaB activation, T. gondii infection promoted phosphorylation of the inhibitor IkappaB. Interestingly, phosphorylated IkappaB was concentrated on the parasitophorous vacuole membrane (PVM), suggesting a parasite-directed event. Results from this study suggest that activation of NF-kappaB plays an important role in stimulation of antiapoptotic gene expression by T. gondii. Furthermore, recruitment of phosphorylated IkappaB to the PVM implies the presence of intrinsic factor(s) in T. gondii that might be used to manipulate the NF-kappaB signaling pathway in the host to elicit a survival response during infection.

A tumour necrosis factor alpha autocrine loop contributes to proliferation and nuclear factor-kappaB activation of Theileria parva-transformed B cells

Theileria infection of bovine leucocytes induces uncontrolled proliferation and a transformed phenotype comparable to tumour cells. Infected cells have many characteristics of activated leucocytes and use autocrine loops to augment proliferation. We have shown previously that, in infected B cells, PI3-K controls a granulocyte-macrophage colony-stimulating factor (GM-CSF) autocrine loop to increase both proliferation and activation of the activator protein 1 (AP-1) transcription factor. We show here that the same infected B cells also use a tumour necrosis factor (TNF) alpha autocrine loop that again contributes to proliferation and augments nuclear factor (NF)-kappaB activation. Interestingly, both pharmacological inhibition of TNF synthesis and neutralizing anti-TNF antibodies lead to a reduction in proliferation and a 50% drop in NF-kappaB activation, without inducing apoptosis.

Initiation of translation and cellular localization of Theileria annulata casein kinase IIalpha: implication for its role in host cell transformation

Theileria annulata and T. parva are protozoa that infect bovine leukocytes which leads to subsequent transformation and uncontrolled proliferation of these cells. It has been proposed that the CKIIalpha subunit of T. parva induces mitogenic pathways of host leukocytes by being exported into the host cell. The evidence for this is the existence of a predicted N-terminal secretion signal-like peptide. We tested this hypothesis by analyzing gene structure, translation, and protein localization of the T. annulata CKIIalpha (TaCKIIalpha). The determined TaCKIIalpha-ORF potentially codes for a 50 kDa protein with an N-terminal extension including a possible signal sequence not present in CKIIalpha proteins of non-Theileria species. However, antisera raised against TaCKIIalpha recognized a protein of a molecular weight of about 40 kDa and, therefore, inconsistent with this predicted molecular weight. We demonstrate by in vitro transcription/translation that this discrepancy is due to translation from a downstream initiation site omitting the putative N-terminal signal sequence and thus excluding the notion that the protein product is secreted via the classical secretory pathway. In corroboration immunofluorescence investigations suggest that the TaCKIIalpha subunit is confined to the parasite schizonts within the host cell. On the basis of the above findings it seems highly unlikely that export via the classical pathway of the parasite CKIIalpha is the way in which this protein possibly contributes to host cell transformation.

Theileria parva-transformed T cells show enhanced resistance to Fas/Fas ligand-induced apoptosis

Lymphocyte homeostasis is regulated by mechanisms that control lymphocyte proliferation and apoptosis. Activation-induced cell death is mediated by the expression of death ligands and receptors, which, when triggered, activate an apoptotic cascade. Bovine T cells transformed by the intracellular parasite Theileria parva proliferate in an uncontrolled manner and undergo clonal expansion. They constitutively express the death receptor Fas and its ligand, FasL but do not undergo apoptosis. Upon elimination of the parasite from the host cell by treatment with a theilericidal drug, cells become increasingly sensitive to Fas/FasL-induced apoptosis. In normal T cells, the sensitivity to death receptor killing is regulated by specific inhibitor proteins. We found that anti-apoptotic proteins such as cellular (c)-FLIP, which functions as a catalytically inactive form of caspase-8, and X-chromosome-linked inhibitor of apoptosis protein (IAP) as well as c-IAP, which can block downstream executioner caspases, are constitutively expressed in T. parva-transformed T cells. Expression of these proteins is rapidly down-regulated upon parasite elimination. Antiapoptotic proteins of the Bcl-2 family such as Bcl-2 and Bcl-x(L) are also expressed but, in contrast to c-FLIP, c-IAP, and X-chromosome-linked IAP, do not appear to be tightly regulated by the presence of the parasite. Finally, we show that, in contrast to the situation in tumor cells, the phosphoinositide 3-kinase/Akt pathway is not essential for c-FLIP expression. Our findings indicate that by inducing the expression of antiapoptotic proteins, T. parva allows the host cell to escape destruction by homeostatic mechanisms that would normally be activated to limit the continuous expansion of a T cell population.

Theileria-induced leukocyte transformation

The intracellular protozoan parasites Theileria parva and T. annulata transform the cells they infect, inducing uncontrolled proliferation. This is not a trivial event as, in addition to permanently switching on the complex pathways that govern all steps of the cell cycle, the built-in apoptotic safety mechanisms that prevent 'illegitimate' cell replication also need to be inactivated. Recent experiments show that the NF-kappa B and phosphoinositide 3-kinase (PtdIns-3K) pathways are important participants in the transformation process. I kappa B kinase (IKK), a pivotal kinase complex in the NF-kappa B pathway, is recruited to the parasite surface where it becomes activated. The PtdIns-3K/Akt/PKB pathway is also constitutively activated in a parasite-dependent manner, but contrary to IKK, activation is probably not triggered by direct association with the parasite.

Hijacking of host cell IKK signalosomes by the transforming parasite Theileria

Parasites have evolved a plethora of mechanisms to ensure their propagation and evade antagonistic host responses. The intracellular protozoan parasite Theileria is the only eukaryote known to induce uncontrolled host cell proliferation. Survival of Theileria-transformed leukocytes depends strictly on constitutive nuclear factor kappa B (NF-kappaB) activity. We found that this was mediated by recruitment of the multisubunit IkappaB kinase (IKK) into large, activated foci on the parasite surface. IKK signalosome assembly was specific for the transforming schizont stage of the parasite and was down-regulated upon differentiation into the nontransforming merozoite stage. Our findings provide insights into IKK activation and how pathogens subvert host-cell signaling pathways.

Characterization of the bovine IkappaB kinases (IKK)alpha and IKKbeta, the regulatory subunit NEMO and their substrate IkappaBalpha

The Nuclear factor (NF)-kappaB signalling pathway plays a critical role in the regulation and coordination of a wide range of cellular events such as cell growth, apoptosis and cell differentiation. Activation of the IKK (inhibitor of NF-kappaB kinase) complex is a crucial step and a point of convergence of all known NF-kappaB signalling pathways. To analyse bovine IKKalpha (IKK1), IKKbeta (IKK2) and IKKgamma (or NF-kappaB Essential MOdulator, NEMO) and their substrate IkappaBalpha (Inhibitor of NF-kappaB), the corresponding cDNAs of these molecules were isolated, sequenced and characterized. A comparison of the amino acid sequences with those of their orthologues in other species showed a very high degree of identity, suggesting that the IKK complex and its substrate IkappaBalpha are evolutionarily highly conserved components of the NF-kappaB pathway. Bovine IKKalpha and IKKbeta are related protein kinases showing 50% identity which is especially prominent in the kinase and leucine zipper domains. Co-immunoprecipitation assays and GST-pull-down experiments were carried out to determine the composition of bovine IKK complexes compared to that in human Jurkat T cells. Using these approaches, the presence of bovine IKK complexes harbouring IKKalpha, IKKbeta, NEMO and the interaction of IKK with its substrate IkappaBalpha could be demonstrated. Parallel experiments using human Jurkat T cells confirmed the high degree of conservation also at the level of protein-protein interactions. Finally, a yeast two-hybrid analysis showed that bovine NEMO molecules, in addition to the binding to IKKalpha and IKKbeta, also strongly interact with each other.

Involvement of inhibitory nuclear factor-kappaB (NFkappaB)-independent NFkappaB activation in the gonadotropic regulation of X-linked inhibitor of apoptosis expression during ovarian follicular development in vitro

Increased X-linked inhibitor of apoptosis (XIAP) expression and suppressed follicular apoptosis are important determinants in the regulation of follicular development by FSH. The objective of the present study was to examine the role and regulation of nuclear factor-kappaB (NFkappaB) in the gonadotropic control of granulosa cell XIAP expression and follicular growth in vitro. FSH (100 ng/ml) increased rat granulosa cell XIAP mRNA abundance and protein content. The gonadotropin also induced granulosa cell p65 subunit-containing NFkappaB translocation from cytoplasm to nucleus and increased NFkappaB-DNA binding activity. Supershift EMSA indicated the FSH-activated NFkappaB contained p65 and p50 subunits. Unlike TNFalpha, FSH failed to elicit a significant change in granulosa cell phospho- and total-inhibitory NFkappaB (IkappaB) IkappaB contents in vitro and dominant-negative IkappaB expression was ineffective in blocking the increase in NFkappaB-DNA-binding activity and XIAP protein content induced by the gonadotropin. In contrast, SN50 (a cell permeable inhibitory peptide of NFkappaB translocation, 50-200 ng/ml) suppressed FSH-stimulated NFkappaB-DNA binding, XIAP expression, and follicular growth. FSH also increased granulosa cell phospho-Akt contents, a response sensitive to the PI-3K inhibitor LY294002 (10 microM). In conclusion, the present studies demonstrate that the FSH-induced XIAP expression is mediated through the NFkappaB pathway through activation of phosphatidylinositol 3-kinase rather than the classical IkappaB kinase.

Inhibition of apoptosis by intracellular protozoan parasites

Protozoan parasites which reside inside a host cell avoid direct destruction by the immune system of the host. The infected cell, however, still has the capacity to counteract the invasive pathogen by initiating its own death, a process which is called programmed cell death or apoptosis. Apoptotic cells are recognised and phagocytosed by macrophages and the parasite is potentially eliminated together with the infected cell. This potent defence mechanism of the host cell puts strong selective pressure on the parasites which have, in turn, evolved strategies to modulate the apoptotic program of the host cell to their favour. Within the last decade, the existence of cellular signalling pathways which inhibit the apoptotic machinery has been demonstrated. It is not surprising that intracellular pathogens subvert these pathways to ensure their own survival in the infected cell. Molecular mechanisms which interfere with apoptotic pathways have been studied extensively for viruses and parasitic bacteria, but protozoan parasites have come into focus only recently. Intracellular protozoan parasites which have been reported to inhibit the apoptotic program of the host cell, are Toxoplasma gondii, Trypanosoma cruzi, Leishmania sp., Theileria sp., Cryptosporidium parvum, and the microsporidian Nosema algerae. Although these parasites differ in their mechanism of host cell entry and in their final intracellular localisation, they might activate similar pathways in their host cells to inhibit apoptosis. In this respect, two families of molecules, which are known for their capacity to interrupt the apoptotic program, are currently discussed in the literature. First, the expression of heat shock proteins is often induced upon parasite infection and can directly interfere with molecules of the cellular death machinery. Secondly, a more indirect effect is attributed to the parasite-dependent activation of NF-kappaB, a transcription factor that regulates the transcription of anti-apoptotic molecules.