Interferon-inducible transmembrane proteins (Ifitm Proteins) act as membrane organizers by influencing the pathways involved in endocytosis and inflammation

The innate immune response Interferon-inducible transmembrane (Ifitm) proteins have been characterized as influencing immune cellular proliferation, aiding signaling complexes and restricting virus infections following interferon (IFN) activation. A number of recent observations further support the evidence that Ifitm proteins play a key role in antiviral activity by influencing the intracellular trafficking pathway. However, the underlying mechanisms have remained largely unknown until our recent work. We have analyzed the function(s) of the Ifitm proteins in the mouse by comparing the localization of known membrane endocytic constituents between cells obtained from wild type (WT) mice and mice lacking the five Ifitm genes (IfitmDel). Cells obtained from IfitmDel mice and treated with IFNβ demonstrated a loss of clathrin from the surface of cells, compared to WT, and a loss of clathrin-mediated endocytosis. The pH of endosomes of cells obtained from IfitmDel, with or without IFN activation, was altered (more basic), suggesting the function of the vacuolar ATPase (v-ATPase) proton pumps in such cells was altered by the loss of the Ifitm proteins. Co-immunoprecipation of Ifitm proteins with the proton-transporter complex constituent Atp6v0b demonstrated a direct interaction between the Ifitm proteins and the vacuolar ATPases. These data suggest that the Ifitm proteins serve as protein complex organizers, maintaining the appropriate cellular localization and function of membrane proteins following type I IFN activation. The IfitmDel mouse lacks all five of the Ifitm genes via LoxP deletion. This iv animal breeds normally with no obvious defect in development but exhibits an obese phenotype. Further, the IfitmDel animals also develop insulin resistance and hyperleptinemia. Analysis of the gene expression profiles in the hypothalamus of IfitmDel animals, compared to WT, demonstrated an altered ratio of Pomc and Npy neuropeptide expression, which likely impairs the satiation response of the IfitmDel animal, leading to an increased eating behavior. Anatomical analysis of the hypothalamus using immunohistochemistry revealed that microglia exhibit an abnormal morphology in IfitmDel animals and respond abnormally to Poly:IC challenge. This novel phenotype for the IfitmDel mouse could be related to abnormal neuropeptide production, inflammatory status and microglia status in the hypothalamus.