The Arf GAP ASAP1 provides a platform to regulate Arf4- and Rab11-Rab8-mediated ciliary receptor targeting

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

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. We discovered that Bbs4- and Bbs6-deficient renal medullary cells display a characteristic behaviour comprising poor migration, adhesion and division with an inability to form lamellipodial and filopodial extensions. Moreover, fewer mutant cells were ciliated [48% ± 6 for wild-type (WT) cells versus 23% ± 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.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Crosstalk of Arf and Rab GTPases en route to cilia

Small GTPases are versatile temporal and spatial regulators of virtually all cellular processes including signal transduction, cytoskeleton dynamics and membrane trafficking. They function as molecular switches, aided by a multitude of regulatory and effector proteins that link them into functional networks. A picture is beginning to emerge whereupon scaffold proteins with many functional domains perform the regulatory and effector functions, thus allowing the ordered recruitment and activation of small GTPases. This leads to the formation of scaffolding patches that coordinate cargo concentration and capture, with the recruitment and activation of the membrane tethering complexes and fusion regulators. This review will focus on the crosstalk of Arf and Rab GTPases at the Golgi complex and the scaffolds that facilitate their activation during trafficking of sensory receptors to primary cilia. The evolutionary conservation of the GTPase cascades in ciliogenesis and yeast budding will be discussed.