Smoothened and ciliary GPCRs regulate ciliary protein kinase A activity involved in Hedgehog signal transduction

Hedgehog (HH) signaling in vertebrates is dependent on the primary cilium, an organelle that scaffolds signal transduction. HH signals induce ciliary enrichment of Smoothened (SMO) and ciliary departure of the G protein-coupled receptor (GPCR) GPR161 to trigger GLI activation of the HH transcriptional program. Recently, SMO has been shown to inhibit protein kinase A (PKA). To test the hypothesis that SMO inhibits PKA at cilia to activate the HH signal transduction pathway, we developed a ciliary PKA reporter. Ciliary PKA activity was graded during zebrafish development. Activation of the HH signal transduction pathway by either Sonic hedgehog (SHH) or SMO agonist (SAG) inhibited ciliary PKA activity. Blocking SMO phosphorylation by GRK2/3 prevented ciliary SMO from inhibiting ciliary PKA activity. Converting the SMO C-terminal PKA pseudosubstrate site into a consensus PKA substrate blocked SMO-mediated inhibition of ciliary PKA activity. A ciliary GPCR, SSTR3, activated ciliary PKA and induced HH transcriptional responses in NIH/3T3 cells via a different mechanism: activation of Gαi/o. A different ciliary GPCR, GPR161, possesses an A-Kinase Anchoring Protein (AKAP), which we found was critical for the ciliary localization of the catalytic subunit of PKA (PKA-C) to promote ciliary PKA activity. We propose that HH signal transduction is inhibited by GPR161-mediated ciliary enrichment of PKA-C, and activated by GRK2/3-phosphorylated SMO inhibition of ciliary PKA activity.