As discussed in a previous blog post, the Rab family of small GTPases are known to be involved in membrane trafficking. Two members of this family have now been associated with FLCN: Rab27B, using microarray studies (Hong et al., 2010; Klomp et al., 2010; Reiman et al., 2012), reviewed here, and Rab35, in an in vitro guanine nucleotide exchange assay (Nookala et al., 2012), reviewed here. However, the exact function of these Rabs is yet to be defined and further research is now shedding light on this issue.
Work by Charrasse et al. (2012) showed that Rab35 co-localises and interacts with N-cadherin and M-cadherin at cell-cell contacts, using both immunofluorescence and immunoprecipitation experiments in C2C12 and HeLa cells. shRNA studies in C2C12 cells then went on to show that Rab35 is necessary for N- and M-cadherin localisation at cell-cell contacts, and that it is involved in adherens junction formation. This is of particular interest as studies have suggested a role for FLCN in cadherin signalling (Hong et al., 2010; Reiman et al., 2012) and cell-cell adhesion (Nahorski et al., 2012; Medvetz et al., 2012). Consequently, could FLCN-loss impact on these cellular processes through dysregulated Rab35?
Additionally, a clinical study by McGrath et al. (2012) suggested that a mutation in the Rab27B effector protein EXPH5 plays a role in the development of an inherited skin disorder. Closer analysis of the skin lesions by electron microscopy showed a disruption of keratinocyte adhesion in the lower epidermis, as well as aggregated keratin filaments and a perinuclear accumulation of vesicles within the cells. A similar alteration in the keratin filament network and keratinocyte adhesion was noted after shRNA knockdown of EXPH5 in normal human keratinocytes. Notably, Medvetz et al. observed that keratin levels were regulated by FLCN and PKP4. Thus, could Rab27B and EXPH5 also be involved in this process?
It is encouraging that studies have identified both Rab27B and Rab35 as potential targets of FLCN, and that similar themes appear to be emerging from additional studies. However, Rabs are known to be involved with a number of different organelles and processes (Galvez et al., 2012), so it must be noted that current knowledge does not exclude the possibility that FLCN may interact with other Rabs or even an alternative family of small GTPases.
- Charrasse S, Comunale F, De Rossi S, Echard A, & Gauthier-Rouvière C (2012). Rab35 regulates cadherin-mediated adherens junction formation and myoblast fusion. Molecular biology of the cell PMID: 23197472
- Galvez T, Gilleron J, Zerial M, & O’Sullivan GA (2012). SnapShot: Mammalian Rab proteins in endocytic trafficking. Cell, 151 (1), 234-23400 PMID: 23021225
- Hong SB, Oh H, Valera VA, Stull J, Ngo DT, Baba M, Merino MJ, Linehan WM, & Schmidt LS (2010). Tumor suppressor FLCN inhibits tumorigenesis of a FLCN-null renal cancer cell line and regulates expression of key molecules in TGF-beta signaling. Molecular cancer, 9 PMID: 20573232
- 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
- McGrath JA, Stone KL, Begum R, Simpson MA, Dopping-Hepenstal PJ, Liu L, McMillan JR, South AP, Pourreyron C, McLean WH, Martinez AE, Mellerio JE, & Parsons M (2012). Germline Mutation in EXPH5 Implicates the Rab27B Effector Protein Slac2-b in Inherited Skin Fragility. American journal of human genetics, 91 (6), 1115-21 PMID: 23176819
- Medvetz DA, Khabibullin D, Hariharan V, Ongusaha PP, Goncharova EA, Schlechter T, Darling TN, Hofmann I, Krymskaya VP, Liao JK, Huang H, & Henske EP (2012). Folliculin, the Product of the Birt-Hogg-Dube Tumor Suppressor Gene, Interacts with the Adherens Junction Protein p0071 to Regulate Cell-Cell Adhesion. PloS one, 7 (11) PMID: 23139756
- Nahorski MS, Seabra L, Straatman-Iwanowska A, Wingenfeld A, Reiman A, Lu X, Klomp JA, Teh BT, Hatzfeld M, Gissen P, & Maher ER (2012). Folliculin interacts with p0071 (plakophilin-4) and deficiency is associated with disordered RhoA signalling, epithelial polarization and cytokinesis. Human molecular genetics, 21 (24), 5268-79 PMID: 22965878
- Nookala RK, Langemeyer L, Pacitto A, Ochoa-Montaño B, Donaldson JC, Blaszczyk BK, Chirgadze DY, Barr FA, Bazan JF, & Blundell TL (2012). Crystal structure of folliculin reveals a hidDENN function in genetically inherited renal cancer. Open biology, 2 (8) PMID: 22977732
- Reiman A, Lu X, Seabra L, Boora U, Nahorski MS, Wei W, & Maher ER (2012). Gene Expression and Protein Array Studies of Folliculin-regulated Pathways. Anticancer research, 32 (11), 4663-70 PMID: 23155228