The primary cilium is critical to vertebrate development and the prevention of disease. Severe defects in this organelle lead to prenatal lethality in extreme cases and a variety of structural birth defects and degenerative diseases in less extreme cases. It is thought that the primary cilium serves as a cellular antenna to monitor the extracellular environment and feed information back to the cell to coordinate its action with that of the surrounding cells. A large number of signaling pathways are directly or indirectly regulated by cilia but of these, hedgehog is particularly important in vertebrate development. Understanding how the cilium is assembled and how the signaling environment is created and maintained is critical to understanding how this organelle functions in the etiology of human diseases. The cilium is assembled by the process of intraflagellar transport (IFT) where large protein complexes called IFT particles are carried along the ciliary microtubules by kinesin and dynein motors. These large complexes transport proteins made in the cell body into the cilium to build and maintain the structure. In addition to building cilia it is now clear that IFT is an integral part of hedgehog and other ciliary signal transduction cascades. The propagation of the hedgehog signal involves a complex set of dynamic movements of receptors and effectors into and out of cilia. We found that Ift25 and Ift27, two components of the IFT system, are not required for ciliary assembly but are required for hedgehog signaling. In their absence, hedgehog receptors fail to be removed from cilia at appropriate times during signaling and transcription factors are mislocalized. Under normal conditions, activation of the pathway is initiated by hedgehog ligand binding to a ciliary-localized receptor called Ptch1. This causes Ptch1 to exit the cilium and relieves a negative inhibition on a second receptor called Smo. Activation of Smo causes it to accumulate in cilia and this drives a third receptor, Gpr161, out of cilia. The Gli transcription factors then accumulate at the ciliary tip and become activated before moving into the nucleus to regulate gene expression. When Ift25/Ift27 are missing, Ptch1 and Gpr161 fail to be removed from cilia upon pathway activation, Smo is inappropriately localized to cilia when the pathway is inactive and the Gli transcription factors are not concentrated at the ciliary tip upon pathway activation. Our current proposal seeks to understand how Ift25/Ift27 in conjunction with the rest of the IFT system regulates the removal of the hedgehog receptors at the appropriate times during signaling. Emerging evidence suggests that hedgehog receptors are ubiquitinated in pathway dependent manner. Our preliminary evidence indicates that Ift25/Ift27 remove ubiquitinated Smo from cilia. This suggests a model where hedgehog signaling regulates ubiquitination of the hedgehog receptors and this in turn controls their interaction with the IFT system to determine whether the receptors localize to the cilium or cell body.
Severe defects in the primary cilium cause prenatal lethality while less severe defects result in a variety of diseases including polycystic kidney disease and blindness along with a group of syndromes called the ciliopathies. These syndromes, which include Bardet-Biedl, Meckel Gruber and Jeune Syndromes, often present with cystic kidney disease and blindness along with a variety of symptoms including obesity, mental retardation, and structural birth defects. This proposal seeks to understand the mechanism by which cilia are assembled and function to ultimately drive the identification of a treatment or cure for these diseases.
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