Figure 3.

The functional relationship between primary cilia and autophagy. (a) In the absence of Sonic Hedgehog (Shh), the Patched (Ptc) receptor restricts Smoothened (Smo) entry into the primary cilium, which results in processing of the transcriptional effector GLI into its repressor form or targeting for degradation. Autophagy is not induced, OFD1 at centriolar satellites is not degraded and ciliogenesis is impaired. Basal autophagy degrades IFT20 when nutrient levels are normal. This reduces trafficking of IFT components to the pericentriolar material, thus impairing the ciliary transport required for growth of primary cilia. (b) Primary ciliogenesis and autophagy are induced during starvation or periods of low-nutrient availability. Autophagy induction is dependent on Shh signaling through the primary cilium. Shh signal transduction is mediated by Smo, which undergoes localization to the primary cilium following Shh binding to Ptc. Smo accumulation within primary cilia promotes GLI protein processing into its transcriptional activator form (GLI-A), enabling downstream signal transduction. Shh-mediated signal transduction promotes the autophagic degradation of OFD1 at centriolar satellites, which is required for ciliogenesis