Vesicular transport to the primary cilium
The membrane of the primary cilium is continuous with the plasma membrane. Despite this continuity, ciliary and plasma membranes contain distinct complements of lipids and proteins, thus making the cilium a bona fide compartment exposed to the extracellular milieu. Consequently, the cilium has been compared to a "cellular antenna" that captures and transduces developmental signals to the inside of the cell. But how do the relevant signaling receptors reach the ciliary membrane instead of the plasma membrane? This question of vesicular targeting to the cilium has recently been illuminated by our discovery of two key ciliogenic factors, the BBSome and Rab8. Consistent with the general paradigm of the small GTPases of the Rab family, Rab8 likely mediates the docking and fusion of transport vesicles with the base of the primary cilium. But where and when does Rab8 precisely function in this transport process? What effectors does Rab8 utilize to specifically target vesicles to the primary cilium? How is the subsequent step of entry into the cilium proper linked to this fusion step?
Conversely to Rab8, the molecular activity of the BBSome is still very poorly understood. The BBSome is a complex of 7 proteins encoded by genes defective in Bardet-Biedl Syndrome (BBS), an obesity/retinopathy ciliopathy. We plan to develop in vitro and in vivo assays to pinpoint the role of the BBSome in transport to the cilium and to define what type of molecular machine the BBSome represents. To this end, a structural analysis of the BBSome has been initiated in collaboration with the lab of Chris Garcia.
Regulated traffic to and from the primary cilium
While constitutive transport to and from the cilium is required for ciliogenesis, the utilization of the cilium as a signaling organelle posits the existence of regulated transport to and from the cilium. Indeed, others have shown that distinct proteins in the Hedgehog signaling cascade move in and out of the cilium upon pathway stimulation. However, very little is known about the molecular processes that regulate protein targeting to the cilium. Since the proteomic analysis of the BBSome interactome uncovered known regulators of signaling, we will be pursuing the analysis of these factors in the context of a novel signaling pathway.
Discovery of novel ciliary signaling pathways
Given the variety of symptoms found in Bardet-Biedl syndrome, it is expected that organ-specific developmental pathways are defective in BBS patients. Following our hypothesis that the BBSome acts in vesicular transport to the primary cilium, one would predict that the BBSome targets tissue-specific signaling receptors to the ciliary membrane (depicted in green on the figure) and that a defective BBSome will render these pathways inactive. In collaboration with the lab of Val Sheffield, we have engineered a mouse strain that will allow us to biochemically isolate the BBSome and its associated factors. We will subject specific tissues to in-depth proteomics analysis of the BBSome with the aim of identifying specific signaling factors. Our goal is to uncover novel signaling pathways that utilize the cilium for signal capture or processing and to shed light on the molecular bais of symptoms such as obesity and retinal degeneration.