Bardet-biedl syndrome proteins control the cilia length through regulation of actin polymerization

Abstract

Primary cilia are cellular appendages important for signal transduction and sensing the environment. Bardet-Biedl syndrome proteins form a complex that is important for several cytoskeleton-related processes such as ciliogenesis, cell migration and division. However, the mechanisms by which BBS proteins may regulate the cytoskeleton remain unclear. Wediscovered that Bbs4-and Bbs6-deficient renal medullary cells display a characteristic behaviour comprising poor migration, adhesionanddivisionwithaninability to form lamellipodialand filopodial extensions.Moreover, fewer mutant cells were ciliated [48%±6 for wild-type (WT) cells versus23%± 7 for Bbs4 null cells; P < 0.0001] and their cilia were shorter (2.55 μm±0.41 for WT cells versus 2.16 μm±0.23 for Bbs4 null cells; P < 0.0001). While the microtubular cytoskeleton and cortical actin were intact, actin stress fibre formation was severely disrupted, forming abnormal apical stress fibre aggregates. Furthermore, we observed over-abundant focal adhesions (FAs) in Bbs4-, Bbs6-and Bbs8-deficient cells. In view of these findings and the role of RhoA in regulation of actin filament polymerization, we showed that RhoA-GTP levels were highly upregulated in the absence of Bbs proteins. Upon treatment of Bbs4-deficient cells with chemical inhibitors of RhoA, we were able to restore the cilia length and number as well as the integrity of the actin cytoskeleton. Together these findings indicate that Bbs proteins play a central role in the regulation of the actin cytoskeleton and control the cilia length through alteration of RhoA levels. © The Author 2013.