Mechanism of action of the tuberculosis and Crohn disease risk factor IRGM in autophagy.

Polymorphisms in the IRGM gene, associated with Crohn disease (CD) and tuberculosis, are among the earliest identified examples documenting the role of autophagy in human disease. Functional studies have shown that IRGM protects against these diseases by modulating autophagy, yet the exact molecular mechanism of IRGM's activity has remained unknown. We have recently elucidated IRGM's mechanism of action. IRGM functions as a platform for assembling, stabilizing, and activating the core autophagic machinery, while at the same time physically coupling it to conventional innate immunity receptors. Exposure to microbial products or bacterial invasion increases IRGM expression, which leads to stabilization of AMPK. Specific protein-protein interactions and post-translational modifications such as ubiquitination of IRGM, lead to a co-assembly with IRGM of the key autophagy regulators ULK1 and BECN1 in their activated forms. IRGM physically interacts with two other CD risk factors, ATG16L1 and NOD2, placing these three principal players in CD within the same molecular complex. This explains how polymorphisms altering expression or function of any of the three factors individually can affect the same process - autophagy. Furthermore, IRGM's interaction with NOD2, and additional pattern recognition receptors such as NOD1, RIG-I and select TLRs, transduces microbial signals to the core autophagy apparatus. This work solves the long-standing enigma of how IRGM controls autophagy.

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