Limits...
Identification of the microsporidian Encephalitozoon cuniculi as a new target of the IFNγ-inducible IRG resistance system.

Ferreira-da-Silva Mda F, da Fonseca Ferreira-da-Silva M, Springer-Frauenhoff HM, Bohne W, Howard JC - PLoS Pathog. (2014)

Bottom Line: We hypothesized that unusual features of the entry mechanisms and intracellular replicative niches of these two organisms, neither of which resembles a phagosome, might hint at a common principle.The suppression of E. cuniculi growth by IFNγ is completely reversed in cells lacking regulatory (GMS subfamily) IRG proteins, cells that effectively lack all IRG function.The phylogenetic divergence of the three organisms whose vacuoles are now known to be involved in IRG-mediated immunity and the non-phagosomal character of the vacuoles themselves strongly suggests that the IRG system is triggered not by the presence of specific parasite components but rather by absence of specific host components on the vacuolar membrane.

View Article: PubMed Central - PubMed

Affiliation: Institute for Genetics, University of Cologne, Cologne, Germany.

ABSTRACT
The IRG system of IFNγ-inducible GTPases constitutes a powerful resistance mechanism in mice against Toxoplasma gondii and two Chlamydia strains but not against many other bacteria and protozoa. Why only T. gondii and Chlamydia? We hypothesized that unusual features of the entry mechanisms and intracellular replicative niches of these two organisms, neither of which resembles a phagosome, might hint at a common principle. We examined another unicellular parasitic organism of mammals, member of an early-diverging group of Fungi, that bypasses the phagocytic mechanism when it enters the host cell: the microsporidian Encephalitozoon cuniculi. Consistent with the known susceptibility of IFNγ-deficient mice to E. cuniculi infection, we found that IFNγ treatment suppresses meront development and spore formation in mouse fibroblasts in vitro, and that this effect is mediated by IRG proteins. The process resembles that previously described in T. gondii and Chlamydia resistance. Effector (GKS subfamily) IRG proteins accumulate at the parasitophorous vacuole of E. cuniculi and the meronts are eliminated. The suppression of E. cuniculi growth by IFNγ is completely reversed in cells lacking regulatory (GMS subfamily) IRG proteins, cells that effectively lack all IRG function. In addition IFNγ-induced cells infected with E. cuniculi die by necrosis as previously shown for IFNγ-induced cells resisting T. gondii infection. Thus the IRG resistance system provides cell-autonomous immunity to specific parasites from three kingdoms of life: protozoa, bacteria and fungi. The phylogenetic divergence of the three organisms whose vacuoles are now known to be involved in IRG-mediated immunity and the non-phagosomal character of the vacuoles themselves strongly suggests that the IRG system is triggered not by the presence of specific parasite components but rather by absence of specific host components on the vacuolar membrane.

No MeSH data available.


Related in: MedlinePlus

Scheme of the IRG resistance system and its target organisms.In IFNγ-stimulated mouse cells, GMS proteins localise mainly to endomembranes such as the ER and keep membrane-bound or cytosolic GKS proteins in a GDP-bound inactive state. Our current view is that the plasma membrane is protected by a hypothetical unknown factor that inhibits GKS protein-mediated damage. During host cell infection by T. gondii or E. cuniculi, invagination of the plasma membrane creates a parasitophorous vacuole that excludes the hypothetical factor and also does not carry GMS proteins. This “missing-self” allows GKS proteins to activate and accumulate on the PVM leading to the PV disruption, pathogen elimination and ultimately host cell death. However, bacteria entering via phagocytic mechanisms do not actively exclude the hypothetical factor and are therefore targeted for endolysosomal degradation.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4214799&req=5

ppat-1004449-g008: Scheme of the IRG resistance system and its target organisms.In IFNγ-stimulated mouse cells, GMS proteins localise mainly to endomembranes such as the ER and keep membrane-bound or cytosolic GKS proteins in a GDP-bound inactive state. Our current view is that the plasma membrane is protected by a hypothetical unknown factor that inhibits GKS protein-mediated damage. During host cell infection by T. gondii or E. cuniculi, invagination of the plasma membrane creates a parasitophorous vacuole that excludes the hypothetical factor and also does not carry GMS proteins. This “missing-self” allows GKS proteins to activate and accumulate on the PVM leading to the PV disruption, pathogen elimination and ultimately host cell death. However, bacteria entering via phagocytic mechanisms do not actively exclude the hypothetical factor and are therefore targeted for endolysosomal degradation.

Mentions: In summary, every property of the IRG-dependent resistance mechanism that has been analysed for T. gondii is probably also valid against E. cuniculi, and, to the extent that it is known, also against Chlamydia. Since effective resistance dependent on IRG proteins seems to be perfectly correlated with the accumulation of IRG proteins on the parasitophorous vacuole, the challenge is to determine the common factor that enables IRG proteins to accumulate on the vacuoles of these three organisms but not on the vacuoles of other organisms. These three organisms cover three kingdoms of life: protozoa, bacteria, and fungi. The broad phylogenetic distribution makes it unlikely a priori that the IRG proteins target a common ligand expressed by all restricted pathogens on their vacuolar membranes. Our preferred view builds on a hypothesis first formulated by Martens [53] to account for the targeting of IRG proteins to the T. gondii vacuole rather than to other cellular organelles. Martens proposed the existence of a self-derived factor X expressed on the membranes of cellular organelles that inhibits the accumulation and activation of IRG proteins on these sites, thereby protecting these organelles from IRG protein mediated damage. Parasitophorous vacuoles, lacking factor X, would be exposed to IRG accumulation and activation. This elegant “missing self” model was confirmed some time later and “factor X” was revealed to be the three GMS proteins, Irgm1, Irgm2 and Irgm3, which are bound to distinct subsets of organellar membranes and act as guanine nucleotide dissociation inhibitors of the effector GKS proteins at these sites [16]. In the absence of one or more GMS proteins, GKS proteins form activated, GTP-bound assemblies in the cytoplasm, probably associated with “unprotected” organellar membranes [23], [41]. The GMS IRG proteins seem to fulfil exactly the role of Martens' Factor X for the distinction between intracellular organelles and a parasitophorous vacuole. However, GKS effector IRG proteins do not accumulate or activate on the plasma membrane, which to the best of our knowledge is not protected by any GMS protein. We are therefore forced to introduce a new hypothetical inhibitor associated with the plasma membrane that inhibits GKS activation at that location (Figure 8). Parasitophorous vacuoles are formed by invagination of the plasma membrane, and as we know with some precision from experiments with T. gondii, the vacuoles are receptive to IRG loading and activation immediately after parasite entry [18]. Thus entry of the parasite and formation of the parasitophorous vacuole must entail loss of the hypothetical plasma membrane-bound inhibitor. We propose that this is the essential distinction between those organisms that do, and those that do not, engage the IRG system, and that loss of the plasma membrane inhibitor is due to the unusual, non-phagocytic entry mechanisms of all three parasites.


Identification of the microsporidian Encephalitozoon cuniculi as a new target of the IFNγ-inducible IRG resistance system.

Ferreira-da-Silva Mda F, da Fonseca Ferreira-da-Silva M, Springer-Frauenhoff HM, Bohne W, Howard JC - PLoS Pathog. (2014)

Scheme of the IRG resistance system and its target organisms.In IFNγ-stimulated mouse cells, GMS proteins localise mainly to endomembranes such as the ER and keep membrane-bound or cytosolic GKS proteins in a GDP-bound inactive state. Our current view is that the plasma membrane is protected by a hypothetical unknown factor that inhibits GKS protein-mediated damage. During host cell infection by T. gondii or E. cuniculi, invagination of the plasma membrane creates a parasitophorous vacuole that excludes the hypothetical factor and also does not carry GMS proteins. This “missing-self” allows GKS proteins to activate and accumulate on the PVM leading to the PV disruption, pathogen elimination and ultimately host cell death. However, bacteria entering via phagocytic mechanisms do not actively exclude the hypothetical factor and are therefore targeted for endolysosomal degradation.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4214799&req=5

ppat-1004449-g008: Scheme of the IRG resistance system and its target organisms.In IFNγ-stimulated mouse cells, GMS proteins localise mainly to endomembranes such as the ER and keep membrane-bound or cytosolic GKS proteins in a GDP-bound inactive state. Our current view is that the plasma membrane is protected by a hypothetical unknown factor that inhibits GKS protein-mediated damage. During host cell infection by T. gondii or E. cuniculi, invagination of the plasma membrane creates a parasitophorous vacuole that excludes the hypothetical factor and also does not carry GMS proteins. This “missing-self” allows GKS proteins to activate and accumulate on the PVM leading to the PV disruption, pathogen elimination and ultimately host cell death. However, bacteria entering via phagocytic mechanisms do not actively exclude the hypothetical factor and are therefore targeted for endolysosomal degradation.
Mentions: In summary, every property of the IRG-dependent resistance mechanism that has been analysed for T. gondii is probably also valid against E. cuniculi, and, to the extent that it is known, also against Chlamydia. Since effective resistance dependent on IRG proteins seems to be perfectly correlated with the accumulation of IRG proteins on the parasitophorous vacuole, the challenge is to determine the common factor that enables IRG proteins to accumulate on the vacuoles of these three organisms but not on the vacuoles of other organisms. These three organisms cover three kingdoms of life: protozoa, bacteria, and fungi. The broad phylogenetic distribution makes it unlikely a priori that the IRG proteins target a common ligand expressed by all restricted pathogens on their vacuolar membranes. Our preferred view builds on a hypothesis first formulated by Martens [53] to account for the targeting of IRG proteins to the T. gondii vacuole rather than to other cellular organelles. Martens proposed the existence of a self-derived factor X expressed on the membranes of cellular organelles that inhibits the accumulation and activation of IRG proteins on these sites, thereby protecting these organelles from IRG protein mediated damage. Parasitophorous vacuoles, lacking factor X, would be exposed to IRG accumulation and activation. This elegant “missing self” model was confirmed some time later and “factor X” was revealed to be the three GMS proteins, Irgm1, Irgm2 and Irgm3, which are bound to distinct subsets of organellar membranes and act as guanine nucleotide dissociation inhibitors of the effector GKS proteins at these sites [16]. In the absence of one or more GMS proteins, GKS proteins form activated, GTP-bound assemblies in the cytoplasm, probably associated with “unprotected” organellar membranes [23], [41]. The GMS IRG proteins seem to fulfil exactly the role of Martens' Factor X for the distinction between intracellular organelles and a parasitophorous vacuole. However, GKS effector IRG proteins do not accumulate or activate on the plasma membrane, which to the best of our knowledge is not protected by any GMS protein. We are therefore forced to introduce a new hypothetical inhibitor associated with the plasma membrane that inhibits GKS activation at that location (Figure 8). Parasitophorous vacuoles are formed by invagination of the plasma membrane, and as we know with some precision from experiments with T. gondii, the vacuoles are receptive to IRG loading and activation immediately after parasite entry [18]. Thus entry of the parasite and formation of the parasitophorous vacuole must entail loss of the hypothetical plasma membrane-bound inhibitor. We propose that this is the essential distinction between those organisms that do, and those that do not, engage the IRG system, and that loss of the plasma membrane inhibitor is due to the unusual, non-phagocytic entry mechanisms of all three parasites.

Bottom Line: We hypothesized that unusual features of the entry mechanisms and intracellular replicative niches of these two organisms, neither of which resembles a phagosome, might hint at a common principle.The suppression of E. cuniculi growth by IFNγ is completely reversed in cells lacking regulatory (GMS subfamily) IRG proteins, cells that effectively lack all IRG function.The phylogenetic divergence of the three organisms whose vacuoles are now known to be involved in IRG-mediated immunity and the non-phagosomal character of the vacuoles themselves strongly suggests that the IRG system is triggered not by the presence of specific parasite components but rather by absence of specific host components on the vacuolar membrane.

View Article: PubMed Central - PubMed

Affiliation: Institute for Genetics, University of Cologne, Cologne, Germany.

ABSTRACT
The IRG system of IFNγ-inducible GTPases constitutes a powerful resistance mechanism in mice against Toxoplasma gondii and two Chlamydia strains but not against many other bacteria and protozoa. Why only T. gondii and Chlamydia? We hypothesized that unusual features of the entry mechanisms and intracellular replicative niches of these two organisms, neither of which resembles a phagosome, might hint at a common principle. We examined another unicellular parasitic organism of mammals, member of an early-diverging group of Fungi, that bypasses the phagocytic mechanism when it enters the host cell: the microsporidian Encephalitozoon cuniculi. Consistent with the known susceptibility of IFNγ-deficient mice to E. cuniculi infection, we found that IFNγ treatment suppresses meront development and spore formation in mouse fibroblasts in vitro, and that this effect is mediated by IRG proteins. The process resembles that previously described in T. gondii and Chlamydia resistance. Effector (GKS subfamily) IRG proteins accumulate at the parasitophorous vacuole of E. cuniculi and the meronts are eliminated. The suppression of E. cuniculi growth by IFNγ is completely reversed in cells lacking regulatory (GMS subfamily) IRG proteins, cells that effectively lack all IRG function. In addition IFNγ-induced cells infected with E. cuniculi die by necrosis as previously shown for IFNγ-induced cells resisting T. gondii infection. Thus the IRG resistance system provides cell-autonomous immunity to specific parasites from three kingdoms of life: protozoa, bacteria and fungi. The phylogenetic divergence of the three organisms whose vacuoles are now known to be involved in IRG-mediated immunity and the non-phagosomal character of the vacuoles themselves strongly suggests that the IRG system is triggered not by the presence of specific parasite components but rather by absence of specific host components on the vacuolar membrane.

No MeSH data available.


Related in: MedlinePlus