Formation of the transition zone by Mks5/Rpgrip1L establishes a ciliary zone of exclusion (CIZE) that compartmentalises ciliary signalling proteins and controls PIP₂ ciliary abundance

Abstract

Cilia are thought to harbour a membrane diffusion barrier within their transition zone (TZ) that compartmentalises signalling proteins. How this "ciliary gate" assembles and functions remains largely unknown. Contrary to current models, we present evidence that Caenorhabditis elegans MKS-5 (orthologue of mammalian Mks5/Rpgrip1L/Nphp8 and Rpgrip1) may not be a simple structural scaffold for anchoring > 10 different proteins at the TZ, but instead, functions as an assembly factor. This activity is needed to form TZ ultrastructure, which comprises Y-shaped axoneme-tomembrane connectors. Coiled-coil and C2 domains within MKS-5 enable TZ localisation and functional interactions with two TZ modules, consisting of Meckel syndrome (MKS) and nephronophthisis (NPHP) proteins. Discrete roles for these modules at basal body-associated transition fibres and TZ explain their redundant functions in making essential membrane connections and thus sealing the ciliary compartment. Furthermore, MKS-5 establishes a ciliary zone of exclusion (CIZE) at the TZ that confines signalling proteins, including GPCRs and NPHP-2/inversin, to distal ciliary subdomains. The TZ/CIZE, potentially acting as a lipid gate, limits the abundance of the phosphoinositide PIP 2 within cilia and is required for cell signalling. Together, our findings suggest a new model for Mks5/Rpgrip1L in TZ assembly and function that is essential for establishing the ciliary signalling compartment.

Endocytic Control of Cellular Signaling at the Primary Cilium

“…The ciliary membrane is continuous with the cell body plasma membrane, but the transition zone at the ciliary base functions as a diffusion barrier and selective gate for the passage of soluble and membrane-bound components into and out of the ciliary compartment, thereby endowing cilia with a unique protein and lipid composition ( Figure 1A) [15][16][17][18]. In many cell types and organisms, such as Trypanosoma and mammalian fibroblasts, the proximal region of the cilium is surrounded by a cytoplasmic invagination of the periciliary membrane, known as the ciliary pocket (CiPo; Figures 1A and 2), which serves as a hotspot for exo-and endocytosis of vesicles for the delivery and retrieval of ciliary membrane components [19].…”

“…As we discuss in a later section of this intraflagellar transport (IFT), and trafficking processes that control ciliary assembly and targeting of receptors to the ciliary membrane. The ciliary membrane (red) is enriched in PI(4)P whereas PI(4,5)P 2 is abundant in the periciliary membrane [18,106,107]. (B) Images of primary cilia (arrows) analyzed by scanning electron microscopy of renal proximal tubule epithelial cells (left image reproduced, with permission, from [108]) and by immunofluorescence microscopy of mouse embryonic fibroblasts using anti-ARL13B (cyan) to mark the cilium, which lies in close proximity to the Golgi (anti-giantin, green) and the nucleus (DAPI, blue) (right image).…”

Hypomorphism of Fto and Rpgrip1l causes obesity in mice

“…Retinitis pigmentosa GTPase regulator-interacting protein-1 like (RPGRIP1L), located <100 bp upstream and in opposite transcriptional orientation to FTO ( Figure 1A), is a transition zone structural component of the primary cilium (16), an organelle implicated in human energy homeostasis (17). We have shown that the obesity-protective allele at FTO intronic SNP rs8050136 promotes the DNA-binding of the cut-like homeobox 1 (CUX1) isoform P110 that acts as an activator of FTO and RPGRIP1L expression in vitro (18,19).…”

Mutations in DZIP1L, which encodes a ciliary transition zone protein, cause autosomal recessive polycystic kidney disease

“…The ciliary axoneme is anchored by the centriolederived basal body, and is ensheathed by the ciliary membrane. Located between the basal body and axoneme, the transition zone forms a diffusion barrier that modulates the selective passage of proteins to and from the cilium [4][5][6] .…”