Loss of the interferon-γ-inducible regulatory immunity-related GTPase (IRG), Irgm1, causes activation of effector IRG proteins on lysosomes, damaging lysosomal function and predicting the dramatic susceptibility of Irgm1-deficient mice to infection.
BACKGROUND
The interferon-γ (IFN-γ)-inducible immunity-related GTPase (IRG), Irgm1, plays an essential role in restraining activation of the IRG pathogen resistance system. However, the loss of Irgm1 in mice also causes a dramatic but unexplained susceptibility phenotype upon infection with a variety of pathogens, including many not normally controlled by the IRG system. This phenotype is associated with lymphopenia, hemopoietic collapse, and death of the mouse.


RESULTS
We show that the three regulatory IRG proteins (GMS sub-family), including Irgm1, each of which localizes to distinct sets of endocellular membranes, play an important role during the cellular response to IFN-γ, each protecting specific membranes from off-target activation of effector IRG proteins (GKS sub-family). In the absence of Irgm1, which is localized mainly at lysosomal and Golgi membranes, activated GKS proteins load onto lysosomes, and are associated with reduced lysosomal acidity and failure to process autophagosomes. Another GMS protein, Irgm3, is localized to endoplasmic reticulum (ER) membranes; in the Irgm3-deficient mouse, activated GKS proteins are found at the ER. The Irgm3-deficient mouse does not show the drastic phenotype of the Irgm1 mouse. In the Irgm1/Irgm3 double knock-out mouse, activated GKS proteins associate with lipid droplets, but not with lysosomes, and the Irgm1/Irgm3(-/-) does not have the generalized immunodeficiency phenotype expected from its Irgm1 deficiency.


CONCLUSIONS
The membrane targeting properties of the three GMS proteins to specific endocellular membranes prevent accumulation of activated GKS protein effectors on the corresponding membranes and thus enable GKS proteins to distinguish organellar cellular membranes from the membranes of pathogen vacuoles. Our data suggest that the generalized lymphomyeloid collapse that occurs in Irgm1(-/-) mice upon infection with a variety of pathogens may be due to lysosomal damage caused by off-target activation of GKS proteins on lysosomal membranes and consequent failure of autophagosomal processing.
Institute for Genetics, University of Cologne, Cologne, Germany.
Jelena
J
Maric-Biresev
Department for Biochemistry and Cell Biology, Faculty of Veterinary Medicine and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands.
Julia P
JP
Hunn
Institute for Medical Microbiology, Immunology and Hygiene, University Hospital Cologne, Cologne, Germany.
Oleg
O
Krut
Department for Biochemistry and Cell Biology, Faculty of Veterinary Medicine and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands.
J Bernd
JB
Helms
Max F. Perutz Laboratories, University of Vienna, Vienna, Austria.
Sascha
S
Martens
Institute for Genetics, University of Cologne, Cologne, Germany. jhoward@igc.gulbenkian.pt.|Instituto Gulbenkian de Ciência, Rua da Quinta Grande 6, Oeiras, Portugal. jhoward@igc.gulbenkian.pt.|Max-Planck Institute for Plant Breeding Research, Cologne, Germany. jhoward@igc.gulbenkian.pt.
Jonathan C
JC
Howard
D000818:Animals; D001343:Autophagy; D002460:Cell Line; D020558:GTP Phosphohydrolases / Q000032:analysis / Q000235:genetics / Q000276:immunology*; D019204:GTP-Binding Proteins / Q000032:analysis / Q000235:genetics / Q000276:immunology*; D017353:Gene Deletion; D007113:Immunity, Innate; D007239:Infections / Q000235:genetics / Q000276:immunology; D008247:Lysosomes / Q000276:immunology* / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL
C096000:Ifi1 protein, mouse; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; C101131:Igtp protein, mouse; C491070:Iigp1 protein, mouse
Autophagy; IRGs; Immunity-related GTPases; Irgm1; Irgm3; Lysosomes; Pathogen recognition