Disruption of Folliculin interacting protein-1 results in loss of sIg+ B cells and inhibits autophagy

Abstract Folliculin interacting protein-1 (Fnip1) was discovered through its interaction with Folliculin (Flcn), a tumor suppressor mutated in the rare disorder Birt-Hogg Dubé syndrome. The Fnip1/Flcn complex interacts with the catabolic regulator AMP kinase. Previous studies have shown that Fnip1 is required for the development of B-cells past the late pre-B stage (Park et al Immunity 2012) and for the development of invariant natural killer T (iNKT) cells (Park et al PNAS 2014). We previously found Fnip1-deficient immune cells exhibited concurrently increased activation of mTORC1 and AMPK. In this study, we used a floxed Fnip1 allele combined with poly inosine:cytosine (poly I:C)-inducible Mx1-Cre to investigate the effects of conditional Fnip1 depletion on immature and mature B cells, and utilized genetic experiments to investigate the mechanism(s) of the B cell developmental block. We show loss of sIg+ B cells in the bone marrow and spleen after conditional Fnip1 depletion. Fnip1-deficient B cells exhibited significantly increased metabolism (oxidative phosphorylation and glycolysis) upon stimulation, possibly as a result of increased mitochondrial number and mTOR activation. Bcl-xL and c-Myc overexpression rescued a small fraction of sIg+ B cells in Fnip1-deficient mice, while p53 deficiency (Trp53−/−) and mTORC1 inactivation (Raptorfl/flmb1-cre) did not. Finally, we show that Fnip1-deficient B cells expressing LC3-GFP have lower mean GFP fluorescence and do not respond as robustly as wild type B cells to induction of autophagy. These results suggest that Fnip1 mediates autophagy in B cells, and that tight regulation of autophagic flux is necessary for development and survival of immature and mature B cells.