The ciliary pocket: an endocytic membrane domain at the base of primary and motile cilia

TLDR

Cilia and flagella are eukaryotic organelles involved in multiple cellular functions, with the primary cilium being non‑motile and present in many vertebrate cell types to control key signalling pathways; although they share a common architecture, ultrastructural data suggest differences in organization, and the ciliary pocket resembles the flagellar pocket of Trypanosomatids, a trafficking‑specialized membrane domain at the base of the flagellum. Here, we report the first detailed characterisation of the ciliary pocket, a depression of the plasma membrane in which the primary cilium is rooted. Transmission and scanning electron microscopy, immunofluorescence analysis and videomicroscopy revealed that the ciliary pocket establishes closed links with the actin‑based cytoskeleton and is enriched in active and dynamic clathrin‑coated pits. The ciliary pocket is present at low frequency in kidney epithelial cells but associates with virtually all primary cilia in retinal pigment epithelial cells, is confirmed in mouse tissues bearing primary cilia, motile cilia, and flagella, and our data highlight a conserved role of membrane trafficking near cilia.

Abstract

Cilia and flagella are eukaryotic organelles involved in multiple cellular functions. The primary cilium is generally non motile and found in numerous vertebrate cell types where it controls key signalling pathways. Despite a common architecture, ultrastructural data suggest some differences in their organisation. Here, we report the first detailed characterisation of the ciliary pocket, a depression of the plasma membrane in which the primary cilium is rooted. This structure is found at low frequency in kidney epithelial cells (IMCD3) but is associated with virtually all primary cilia in retinal pigment epithelial cells (RPE1). Transmission and scanning electron microscopy, immunofluorescence analysis and videomicroscopy revealed that the ciliary pocket establishes closed links with the actin-based cytoskeleton and that it is enriched in active and dynamic clathrin-coated pits. The existence of the ciliary pocket was confirmed in mouse tissues bearing primary cilia (cumulus), as well as motile cilia and flagella (ependymal cells and spermatids). The ciliary pocket shares striking morphological and functional similarities with the flagellar pocket of Trypanosomatids, a trafficking-specialised membrane domain at the base of the flagellum. Our data therefore highlight the conserved role of membrane trafficking in the vicinity of cilia.