SummaryCilia 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 actinbased 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.

The female steroid hormone progesterone regulates ovulation and supports pregnancy, but also controls human sperm function within the female reproductive tract. Progesterone causes elevation of sperm intracellular Ca 2+ leading to sperm hyperactivation, acrosome reaction, and perhaps chemotaxis toward the egg. Although it has been suggested that progesterone-dependent Ca 2+ influx into human spermatozoa is primarily mediated by cationic channel of sperm (CatSper), the principal flagellar Ca 2+ channel of sperm, conclusive loss-of-function genetic evidence for activation of CatSper by progesterone has yet to be provided. Moreover, it is not clear whether the responsiveness of CatSper to progesterone is an innate property of human spermatozoa or is acquired as the result of exposure to the seminal plasma. Here, by recording ionic currents from spermatozoa of an infertile CatSper-deficient patient, we demonstrate that CatSper is indeed the principal Ca 2+ channel of human spermatozoa, and that it is strongly potentiated by progesterone. In addition, by recording CatSper currents from human epididymal and testicular spermatozoa, we show that CatSper sensitivity to progesterone arises early in sperm development and increases gradually to a peak when spermatozoa are ejaculated. These results unambiguously establish an important role of CatSper channel in human sperm nongenomic progesterone signaling and demonstrate that the molecular mechanism responsible for activation of CatSper by progesterone arises early in sperm development concurrently with the CatSper channel itself.CatSper ion channel | male fertility | nongenomic steroid action | sperm physiology