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Subcellular localization

cilia, Vesicles, Cytosol

Functional category

• Metabolism
• Signaling (Hedgehog
• GPCRs
• ion channels)

Function

PALD1 accumulates within cilia of selected cell types upon Hh stimulation. While CYC promoted ciliary accumulation of SMO in IMCD3 cells, it did not increase the ciliary levels of PALD1 (Fig. 6, E and F), indicating that ciliary accumulation of SMO is not sufficient to drive PALD1 into cilia and that SMO must be activated for PALD1 to accumulate in cilia. To test whether PALD1 entry into cilia is sensitive to the reduction in [cAMP]cilia downstream of SMO, we resorted to the ciliary Gαi-coupled GPCR SSTR3. Strikingly, SSTR3 activation is sufficient to recruit PALD1 to primary cilia (Fig. 6, G and H), suggesting that, similarly to GPR161 exit from cilia, ciliary PALD1 accumulation responds to decreases in ciliary cAMP levels. While PALD1 was absent from unstimulated C2C12 cilia, PALD1 became enriched in primary cilia in response to Hh signal (Fig. 7, C and D). These results indicate that the association of PALD1 with Hh signaling can be detected in multiple cell lines but is not a universal element of the Hh response. PALD1 is a cell type–specific attenuator of Hh signaling. To further investigate why pathway activity is increased in Pald1−/− IMCD3 cells, we assessed the cilia localization of SMO and GPR161 in response to Hh pathway stimulation. The Hh-dependent ciliary accumulation of SMO was unperturbed by deletion of Pald1 (Fig. 8, D and E). Meanwhile, GPR161 levels were reduced in cilia of unstimulated Pald1−/− IMCD3 cells compared with unstimulated WT cells (Fig. 8, D and F), consistent with a mild derepression of the Hh pathway. Nonetheless, GPR161 still exited cilia upon Hh pathway activation in the absence of PALD1. In conclusion, Pald1−/− IMCD3 cells respond to Hh stimulation, however, they display elevated basal pathway activity. Hence, unlike other negative regulators such as SUFU and PKA, PALD1 is not strictly required for Hh signaling but rather attenuates Hh signals in certain cell types to fine-tune cellular responses(33856408).