Folliculin-interacting protein 1 (FNIP1) was identified in 2006 by Baba et al. as a protein which interacts with the C-terminus of FLCN. Although the function of FNIP1 is unknown, the protein was also found to interact with and be phosphorylated by AMPK, an enzyme involved in cellular energy homeostasis. A recent paper by Park et al. (2012) has now identified a role for FNIP1 in B lymphocyte development.
The authors used an ethylnitrosourea (ENU) mutagenesis screen to identify genes involved in immune cell development. One pedigree of mice, which had an absence of B lymphocytes, was found to have a deletion in the Fnip1 gene. The Fnip1-/- mice had several additional phenotypes, including alterations in skeletal muscle, increased liver glycogen content and hypertrophic cardiomyopathy. In wild-type mice, Fnip1 was found to be highly expressed in many tissues and at an equal level throughout B-cell development.
Loss of Fnip1 appeared to halt B-cell development at the pre-B cell stage. This was investigated further and the authors found the block to occur at the transition from large pre-B cell to small pre-B cell. Fnip1-/- pre-B cells showed no defects in cell division; therefore only maturation of the B cell was inhibited. B cell development was rescued by retroviral expression of Fnip1, confirming Fnip1 is directly responsible for the observed phenotype.
To identify the cause of this inhibition of B cell development, gene expression levels were compared between Fnip1-/- and wild-type mice using cDNA microarray technology. Compared to the wild-type, over 500 genes were differentially expressed in Fnip1-/- mice. The majority of these genes were involved in cell metabolism and mitochondrial biogenesis, and in particular, there was an increase in the expression of Pgc1α and Pgc1β (which are regulators of fatty acid oxidation). These mice also had increased numbers of mitochondria and an increase in phospho-S6 ribosomal protein (S6R), a downstream target of mTORC1, suggesting an increase in mTOR-mediated metabolism.
In addition, the effect of Fnip1 loss on AMPK signalling in pre-B cells was investigated and it was seen that the phosphorylation of the mTORC1 component Raptor by AMPK was unaffected. However, where AMPK activation in wild-type pre-B cells inhibited the phosphorylation of S6R, suggesting a decrease in mTOR function, in Fnip1-/- pre-B cells, activation of AMPK failed to inhibit this phosphorylation. These results suggest that Fnip1 is not required for the activation of AMPK, but it is important for AMPK to inhibit mTOR.
The authors suggest that Fnip1 loss results in a nutrient and energy deficit and that the role of Fnip1 in B cells could be to help maintain metabolic homeostasis. When Fnip1 is lost, homeostasis is dysregulated and B-cell development is arrested in response to nutrient stress. It would be interesting to investigate the role of FNIP1 in BHD Syndrome and the function of its interaction with FLCN. Does FNIP1 also have a role to play in the regulation of metabolism in BHD Syndrome? It is known that metabolism and mitochondrial biogenesis are dysregulated in BHD (Preston et al., 2011; Klomp et al., 2010). Perhaps dysregulation of FNIP1, through loss of FLCN, could contribute to these cellular effects and the observed symptoms of BHD.
- Park H, Staehling K, Tsang M, Appleby MW, Brunkow ME, Margineantu D, Hockenbery DM, Habib T, Liggitt HD, Carlson G, & Iritani BM (2012). Disruption of fnip1 reveals a metabolic checkpoint controlling B lymphocyte development. Immunity, 36 (5), 769-81 PMID: 22608497
- Baba M, Hong SB, Sharma N, Warren MB, Nickerson ML, Iwamatsu A, Esposito D, Gillette WK, Hopkins RF 3rd, Hartley JL, Furihata M, Oishi S, Zhen W, Burke TR Jr, Linehan WM, Schmidt LS, & Zbar B (2006). Folliculin encoded by the BHD gene interacts with a binding protein, FNIP1, and AMPK, and is involved in AMPK and mTOR signaling. Proceedings of the National Academy of Sciences of the United States of America, 103 (42), 15552-7 PMID: 17028174
- Klomp JA, Petillo D, Niemi NM, Dykema KJ, Chen J, Yang XJ, Sääf A, Zickert P, Aly M, Bergerheim U, Nordenskjöld M, Gad S, Giraud S, Denoux Y, Yonneau L, Méjean A, Vasiliu V, Richard S, MacKeigan JP, Teh BT, & Furge KA (2010). Birt-Hogg-Dubé renal tumors are genetically distinct from other renal neoplasias and are associated with up-regulation of mitochondrial gene expression. BMC medical genomics, 3 PMID: 21162720
- Preston RS, Philp A, Claessens T, Gijezen L, Dydensborg AB, Dunlop EA, Harper KT, Brinkhuizen T, Menko FH, Davies DM, Land SC, Pause A, Baar K, van Steensel MA, & Tee AR (2011). Absence of the Birt-Hogg-Dubé gene product is associated with increased hypoxia-inducible factor transcriptional activity and a loss of metabolic flexibility. Oncogene, 30 (10), 1159-73 PMID: 21057536