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Salmonella-induced caspase-2 activation in macrophages: a novel mechanism in pathogen-mediated apoptosis.

Jesenberger V, Procyk KJ, Yuan J, Reipert S, Baccarini M - J. Exp. Med. (2000)

Bottom Line: By using caspase-2 knockout macrophages and chemical inhibition, we establish a role for caspase-2 in both caspase-1-dependent and -independent apoptosis.Particularly, activation of caspase-1 during fast Salmonella-induced apoptosis partially relies on caspase-2.The ability of Salmonella to induce caspase-1-independent macrophage apoptosis may play a role in situations in which activation of this protease is either prevented or uncoupled from the induction of apoptosis.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell- and Microbiology, Institute of Microbiology and Genetics, Vienna Biocenter, Vienna, Austria.

ABSTRACT
The enterobacterial pathogen Salmonella induces phagocyte apoptosis in vitro and in vivo. These bacteria use a specialized type III secretion system to export a virulence factor, SipB, which directly activates the host's apoptotic machinery by targeting caspase-1. Caspase-1 is not involved in most apoptotic processes but plays a major role in cytokine maturation. We show that caspase-1-deficient macrophages undergo apoptosis within 4-6 h of infection with invasive bacteria. This process requires SipB, implying that this protein can initiate the apoptotic machinery by regulating components distinct from caspase-1. Invasive Salmonella typhimurium targets caspase-2 simultaneously with, but independently of, caspase-1. Besides caspase-2, the caspase-1-independent pathway involves the activation of caspase-3, -6, and -8 and the release of cytochrome c from mitochondria, none of which occurs during caspase-1-dependent apoptosis. By using caspase-2 knockout macrophages and chemical inhibition, we establish a role for caspase-2 in both caspase-1-dependent and -independent apoptosis. Particularly, activation of caspase-1 during fast Salmonella-induced apoptosis partially relies on caspase-2. The ability of Salmonella to induce caspase-1-independent macrophage apoptosis may play a role in situations in which activation of this protease is either prevented or uncoupled from the induction of apoptosis.

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Electron microscopy of wt and caspase-1–deficient macrophages infected with invasive or invasion-deficient Salmonella. (A) Caspase-1–deficient macrophage after a 4-h infection with the sipB− strain; the size ratio between nucleus and cytoplasm shows no significant differences in comparison with uninfected macrophages. Moreover, both the nucleus and the cytoplasm do not show any morphological differences in comparison to control cells (A), and mitochondria do not display any signs of morphological alterations. A number of Salmonella (marked by asterisks) within the section indicate the infection of the cell. (B) Caspase-1–deficient macrophage after a 4-h infection with invasive Salmonella; the nucleus is slightly reduced in size and displays electron-dense material as a result of strong nucleolar segregation. The ratio in size between nucleus and cytoplasm is significantly reduced. The cytoplasm contains giant vacuoles. Absence of vacuoles and a dramatic reduction of cytoplasmic content in a late stage of cell death suggest a release of the vacuolar content into the culture medium (data not shown). Similar to control cells (A), mitochondria appear to be elongated and structurally intact.
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Figure 6: Electron microscopy of wt and caspase-1–deficient macrophages infected with invasive or invasion-deficient Salmonella. (A) Caspase-1–deficient macrophage after a 4-h infection with the sipB− strain; the size ratio between nucleus and cytoplasm shows no significant differences in comparison with uninfected macrophages. Moreover, both the nucleus and the cytoplasm do not show any morphological differences in comparison to control cells (A), and mitochondria do not display any signs of morphological alterations. A number of Salmonella (marked by asterisks) within the section indicate the infection of the cell. (B) Caspase-1–deficient macrophage after a 4-h infection with invasive Salmonella; the nucleus is slightly reduced in size and displays electron-dense material as a result of strong nucleolar segregation. The ratio in size between nucleus and cytoplasm is significantly reduced. The cytoplasm contains giant vacuoles. Absence of vacuoles and a dramatic reduction of cytoplasmic content in a late stage of cell death suggest a release of the vacuolar content into the culture medium (data not shown). Similar to control cells (A), mitochondria appear to be elongated and structurally intact.

Mentions: To characterize the Salmonella-induced death of caspase-1–deficient macrophages ultrastructurally, we performed transmission electron microscopy of cells after a 4-h infection either with wt bacteria or with a sipB− mutant (Fig. 6). The cell infected with wt strain showed a morphology compatible with apoptosis, with cell and nuclear shrinkage and chromatin condensation (Fig. 6 B). Most of the cell was occupied by large vacuoles, and the mitochondria were reduced in number. In contrast to the situation observed in wt macrophages infected with invasive Salmonella (Fig. 2 B), however, these organelles did not show any gross anomalies. Caspase-1–deficient macrophages infected with a sipB− mutant strain were viable, contained fewer vacuoles and some bacteria, and showed no signs of cell damage (Fig. 6 A). Thus, Salmonella is capable to induce caspase-1–independent apoptosis in macrophages, and this requires a functional sipB gene.


Salmonella-induced caspase-2 activation in macrophages: a novel mechanism in pathogen-mediated apoptosis.

Jesenberger V, Procyk KJ, Yuan J, Reipert S, Baccarini M - J. Exp. Med. (2000)

Electron microscopy of wt and caspase-1–deficient macrophages infected with invasive or invasion-deficient Salmonella. (A) Caspase-1–deficient macrophage after a 4-h infection with the sipB− strain; the size ratio between nucleus and cytoplasm shows no significant differences in comparison with uninfected macrophages. Moreover, both the nucleus and the cytoplasm do not show any morphological differences in comparison to control cells (A), and mitochondria do not display any signs of morphological alterations. A number of Salmonella (marked by asterisks) within the section indicate the infection of the cell. (B) Caspase-1–deficient macrophage after a 4-h infection with invasive Salmonella; the nucleus is slightly reduced in size and displays electron-dense material as a result of strong nucleolar segregation. The ratio in size between nucleus and cytoplasm is significantly reduced. The cytoplasm contains giant vacuoles. Absence of vacuoles and a dramatic reduction of cytoplasmic content in a late stage of cell death suggest a release of the vacuolar content into the culture medium (data not shown). Similar to control cells (A), mitochondria appear to be elongated and structurally intact.
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Related In: Results  -  Collection

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Figure 6: Electron microscopy of wt and caspase-1–deficient macrophages infected with invasive or invasion-deficient Salmonella. (A) Caspase-1–deficient macrophage after a 4-h infection with the sipB− strain; the size ratio between nucleus and cytoplasm shows no significant differences in comparison with uninfected macrophages. Moreover, both the nucleus and the cytoplasm do not show any morphological differences in comparison to control cells (A), and mitochondria do not display any signs of morphological alterations. A number of Salmonella (marked by asterisks) within the section indicate the infection of the cell. (B) Caspase-1–deficient macrophage after a 4-h infection with invasive Salmonella; the nucleus is slightly reduced in size and displays electron-dense material as a result of strong nucleolar segregation. The ratio in size between nucleus and cytoplasm is significantly reduced. The cytoplasm contains giant vacuoles. Absence of vacuoles and a dramatic reduction of cytoplasmic content in a late stage of cell death suggest a release of the vacuolar content into the culture medium (data not shown). Similar to control cells (A), mitochondria appear to be elongated and structurally intact.
Mentions: To characterize the Salmonella-induced death of caspase-1–deficient macrophages ultrastructurally, we performed transmission electron microscopy of cells after a 4-h infection either with wt bacteria or with a sipB− mutant (Fig. 6). The cell infected with wt strain showed a morphology compatible with apoptosis, with cell and nuclear shrinkage and chromatin condensation (Fig. 6 B). Most of the cell was occupied by large vacuoles, and the mitochondria were reduced in number. In contrast to the situation observed in wt macrophages infected with invasive Salmonella (Fig. 2 B), however, these organelles did not show any gross anomalies. Caspase-1–deficient macrophages infected with a sipB− mutant strain were viable, contained fewer vacuoles and some bacteria, and showed no signs of cell damage (Fig. 6 A). Thus, Salmonella is capable to induce caspase-1–independent apoptosis in macrophages, and this requires a functional sipB gene.

Bottom Line: By using caspase-2 knockout macrophages and chemical inhibition, we establish a role for caspase-2 in both caspase-1-dependent and -independent apoptosis.Particularly, activation of caspase-1 during fast Salmonella-induced apoptosis partially relies on caspase-2.The ability of Salmonella to induce caspase-1-independent macrophage apoptosis may play a role in situations in which activation of this protease is either prevented or uncoupled from the induction of apoptosis.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell- and Microbiology, Institute of Microbiology and Genetics, Vienna Biocenter, Vienna, Austria.

ABSTRACT
The enterobacterial pathogen Salmonella induces phagocyte apoptosis in vitro and in vivo. These bacteria use a specialized type III secretion system to export a virulence factor, SipB, which directly activates the host's apoptotic machinery by targeting caspase-1. Caspase-1 is not involved in most apoptotic processes but plays a major role in cytokine maturation. We show that caspase-1-deficient macrophages undergo apoptosis within 4-6 h of infection with invasive bacteria. This process requires SipB, implying that this protein can initiate the apoptotic machinery by regulating components distinct from caspase-1. Invasive Salmonella typhimurium targets caspase-2 simultaneously with, but independently of, caspase-1. Besides caspase-2, the caspase-1-independent pathway involves the activation of caspase-3, -6, and -8 and the release of cytochrome c from mitochondria, none of which occurs during caspase-1-dependent apoptosis. By using caspase-2 knockout macrophages and chemical inhibition, we establish a role for caspase-2 in both caspase-1-dependent and -independent apoptosis. Particularly, activation of caspase-1 during fast Salmonella-induced apoptosis partially relies on caspase-2. The ability of Salmonella to induce caspase-1-independent macrophage apoptosis may play a role in situations in which activation of this protease is either prevented or uncoupled from the induction of apoptosis.

Show MeSH
Related in: MedlinePlus