[Show abstract][Hide abstract] ABSTRACT: Programmed cell death is an essential mechanism of the host to combat infectious agents and to regulate immunity during infection. Consequently, activation and deactivation of the hosts' cell death pathways by protozoan parasites play critical roles in parasite control, pathogenesis, immune evasion and parasite dissemination within the host. Here, we discuss advances in the understanding of these fascinating host-parasite interactions with special emphasis on how protozoa can modulate the cell death apparatus of its host.
No preview · Article · Oct 2009 · Microbes and Infection
[Show abstract][Hide abstract] ABSTRACT: In order to accomplish their life style, intracellular pathogens, including the apicomplexan Toxoplasma gondii, subvert the innate apoptotic response of infected host cells. However, the precise mechanisms of parasite interference with the mitochondrial apoptotic pathway remain unknown. Here, we used the conditional expression of the BH3-only protein Bim(S) to pinpoint the interaction of T. gondii with the intrinsic pathway of apoptosis. Infection of epithelial cells with T. gondii dose-dependently abrogated Bim(S)-triggered release of cytochrome c from host-cell mitochondria into the cytosol, induction of activity of caspases 3, 7 and 9, and chromatin condensation. Furthermore, inhibition of apoptosis in parasite-infected lymphocytes counteracted death of Toxoplasma-infected host cells. Although total cellular levels and mitochondrial targeting of Bim(S) was not altered by the infection, the activation of pro-apoptotic effector proteins Bax and Bak was strongly impaired. Inhibition of Bax and Bak activation by T. gondii was seen with regard to their conformational changes, the cytosol-to-mitochondria targeting and the oligomerization of Bax but not their cellular protein levels. Blockade of Bax and Bak activation was not mediated by the upregulation of anti-apoptotic Bcl-2-like proteins following infection. Further, the BH3-mimetic ABT-737 failed to overcome the Toxoplasma-imposed inhibition of Bim(S)-triggered apoptosis. These results indicate that T. gondii targets activation of pro-apoptotic Bax and Bak to inhibit the apoptogenic function of mitochondria and to increase host-cell viability.
Preview · Article · Oct 2009 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: Apoptosis plays crucial roles for the outcome of infection with various infectious agents. The host's apoptotic program may be modulated after infection in order to combat the pathogen or to restrict the immune response. In addition, distinct microorganisms alter the apoptotic program of the host in order to meet the requirements for their further distribution. The activation of caspases (i.e., cysteine proteases with specificity for aspartic acid residues) preludes the disassembly of the cell in response to apoptosis-inducing stimuli. This depends on the proteolytic cleavage of inactive proforms into catalytically active subunits. Analyses of the proteolysis and the enzymatic activity of caspases therefore represent valuable tools to study apoptotic programs during infection. The apicomplexan parasite Toxoplasma gondii interferes with the caspase cascade of its host cell in order to facilitate intracellular survival. The modulation of caspase activation by T. gondii is determined by SDS-PAGE and immunoblotting with caspase-specific antibodies. Furthermore, the impact of the parasite on caspase activity is fluorimetrically determined by measuring the cleavage of caspase-specific substrate analogues.
No preview · Article · Feb 2009 · Methods in Molecular Biology
[Show abstract][Hide abstract] ABSTRACT: The intracellular protozoan Toxoplasma gondii induces persistent infections in various hosts and is an important opportunistic pathogen of humans with immature or deficient
immune responses. The ability to survive intracellularly largely depends on the blocking of different proapoptotic signaling
cascades of its host cell. Fas/CD95 triggers an apoptotic cascade that is crucial for immunity and the outcome of infectious
diseases. We have determined the mechanism by which T. gondii counteracts death receptor-mediated cell death in type II cells that transduce Fas/CD95 ligation via caspase 8-mediated activation
of the mitochondrial amplification loop. The results showed that infection with T. gondii significantly reduced Fas/CD95-triggered apoptosis in HeLa cells by inhibiting the activities of initiator caspases 8 and
9 and effector caspase 3/7. Parasitic infection dose dependently diminished cleavage of caspase 8, the BH3-only protein Bid,
and the downstream caspases 9 and 3. Importantly, interference with Fas/CD95-triggered caspase 8 and caspase 3/7 activities
after parasitic infection was largely dependent on the presence of caspase 9. Within the mitochondrial amplification loop,
T. gondii significantly inhibited the Fas/CD95-triggered release of cytochrome c into the host cell cytosol. These results indicate that T. gondii inhibits Fas/CD95-mediated apoptosis in type II cells primarily by decreasing the apoptogenic function of mitochondria.
Full-text · Article · Aug 2008 · Infection and immunity
[Show abstract][Hide abstract] ABSTRACT: Ligation of the death receptor Fas/CD95 activates an apoptotic cascade and plays critical roles during infectious diseases. Previous work has established that infection with the intracellular parasite Toxoplasma gondii renders cells resistant to multiple inducers of apoptosis. However, the effect of T. gondii on the death receptor pathway is poorly characterized. Here we have determined the impact of the parasite on apoptosis in type I cells that transduce Fas/CD95 engagement via the death receptor pathway without the need of a mitochondrial amplification loop. The results have shown that T. gondii significantly reduced Fas/CD95-triggered apoptosis by impairing activation of the initiator caspase 8. Parasitic infection diminished the cellular amount of procaspase 8, resulting in its decreased recruitment to the death-inducing signalling complex and the impaired activation of effector caspases. Remarkably, downregulation of caspase 8 protein in T. gondii-infected cells also occurred in the absence of Fas/CD95 engagement and was associated with the appearance of non-canonical caspase 8 cleavage fragments. Distinct parasite proteins were associated with caspase 8 and its proteolytic fragments. These findings indicate that T. gondii aberrantly processes and finally degrades the initiator caspase 8, thereby, blocking Fas/CD95-mediated apoptosis which signals independently of the apoptogenic function of host cell mitochondria.
[Show abstract][Hide abstract] ABSTRACT: Toxoplasma gondii is an intracellular parasite able to both promote and inhibit apoptosis. T. gondii renders infected cells resistant to programmed cell death induced by multiple apoptotic triggers. On the other hand, increased apoptosis of immune cells after in vivo infection with T. gondii may suppress the immune response to the parasite. Glycosylphosphatidylinositol (GPI)-anchored proteins dominate the surface of T. gondii tachyzoites and GPIs are involved in the pathogenicity of protozoan parasites. In this report, we determine if GPIs are responsible for inhibition or induction of host cell apoptosis. We show here that T. gondii GPIs fail to block apoptosis that was triggered in human-derived cells via extrinsic or intrinsic apoptotic pathways. Furthermore, characteristics of apoptosis, e.g. caspase-3/7 activity, phosphatidylserine exposition at the cell surface or DNA strand breaks, were not observed in the presence of T. gondii GPIs. These results indicate that T. gondii GPIs are not involved in survival or in apoptosis of host cells. This absence of effect on apoptosis could be a feature common to GPIs of other parasites.
[Show abstract][Hide abstract] ABSTRACT: 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.