BBSome-deficient cells activate intraciliary CDC42 to trigger actin-dependent ciliary ectocytosis

Abstract

Bardet-Biedl syndrome (BBS) is a pleiotropic ciliopathy caused by dysfunction of the BBSome, a cargo adaptor essential for export of transmembrane receptors from cilia. Although actin-dependent ectocytosis has been proposed as compensatory cellular process for defective cargo retrieval, the underlying molecular mechanism is poorly understood, particularly in the context of BBS pathology. In this study, we investigated how actin polymerization and ectocytosis are regulated within the cilium. Our findings reveal that ciliary CDC42, a RHO-family GTPase triggers in situ actin polymerization, ciliary ectocytosis, and cilia shortening in BBSome-deficient cells. Furthermore, activation of the Sonic Hedgehog pathway further enhances CDC42 activity in BBSome-deficient cilia, but not in healthy cells. Inhibition of CDC42 decreased the frequency and duration of ciliary actin polymerization events and lead to the accumulation of the GPR161 receptor in bulges along the axoneme in BBSome-deficient cells during Sonic Hedgehog signaling. Overall, our study identifies CDC42 as a key trigger of ciliary ectocytosis. Moreover, the hyperactivated ciliary CDC42-actin-ectocytosis axis in BBSome-deficient cells results in cilia shortening and loss of ciliary material, potentially impacting disease severity.