Open Sesame: How Transition Fibers and the Transition Zone Control Ciliary Composition

Abstract

Cilia are plasma membrane protrusions that act as cellular propellers or antennae. To perform these functions, cilia must maintain a composition distinct from those of the contiguous cytosol and plasma membrane. The specialized composition of the cilium depends on the ciliary gate, the region at the ciliary base separating the cilium from the rest of the cell. The ciliary gate's main structural features are electron dense struts connecting microtubules to the adjacent membrane. These structures include the transition fibers, which connect the distal basal body to the base of the ciliary membrane, and the Y-links, which connect the proximal axoneme and ciliary membrane within the transition zone. Both transition fibers and Y-links form early during ciliogenesis and play key roles in ciliary assembly and trafficking. Accordingly, many human ciliopathies are caused by mutations that perturb ciliary gate function.

Figure 1.

Ultrastructure of the ciliary gate. (A) Schematic showing the different parts of a cilium, with emphasis on the ciliary gate comprised by the transition fibers and the transition zone. (From Reiter et al. 2012; adapted, with permission, from the authors.) (B) Electron microscopic images of the transition fibers and transition zone. Left panels show transition fibers in transverse. (Top, from O’Toole et al. 2003; adapted, with permission, from the American Society for Cell Biology © 2003; and longitudinal [bottom] views from Tateishi et al. 2013; reprinted under a Creative Commons License [Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at creativecommons.org/licenses/by-nc-sa/3.0].) Arrows point to transition fibers (tf). Right panels show transverse and longitudinal views of transition zones from rat photoreceptor connecting cilia (From data in Horst et al. 1987; adapted, with permission, from the authors.) Arrows point to Y-links. Asterisk marks the distal end of the transition zone (TZ). (C) Freeze-fracture etching of the ciliary necklace of a rat tracheal cilium. (Image adapted from data in Gilula and Satir 1972.) Arrowheads point to beads in the ciliary necklace.

Figure 2.

Superresolution microscopy of the ciliary gate. (A) Lateral view of Cep89 localizing to centriolar distal appendages, as observed using photoactivated localization microscopy (PALM). A comparable electron microscopic image is shown at the bottom. Black arrows indicate distal appendages. Red lines indicate the angle at which distal appendages emerge from the centriole. (From Sillibourne et al. 2011; adapted, with permission, from John Wiley and Sons © 2011.) (B) Longitudinal views of Caenorhabditis elegans sensory cilia transition zones with the indicated proteins localized using stimulated emission depletion (STED) microscopy. Arrowheads indicate axial and radial periodicity of TMEM-231::GFP and NPHP-1::GFP, respectively. Scale bars, 200 nm. (Panel B is from Lambacher et al. 2016; adapted, with permission, from Nature Publishing Group © 2015.) (C) Transverse STED images of cilia from human renal proximal tubule epithelial cells (RPTECs) stained for the indicated proteins. Arrowheads indicate radial periodicity. Ring diameters indicated in each image. Scale bars, 100 nm. (Panel C is from Lambacher et al. 2016; adapted, with permission, from Nature Publishing Group © 2015.) (D) Schematic of transition zone (TZ) protein localization in nematode and mammalian cilia. (Panel D is from Lambacher et al. 2016; adapted, with permission, from Nature Publishing Group © 2015.) (E) Summary of STED data from human retinal pigment epithelium (RPE) cell cilia. Left panel shows overlayed STED signals, colored as indicated. The center panel shows an electron microscopic image for reference in which ciliary gate structures are indicated. The right panel shows merge. Scale bar, 200 nm. (F) Schematic of the data shown in E. (Images E and F are from Yang et al. 2015; reprinted courtesy of Nature Publishing Group under Creative Commons CC-BY Licensing.) MT, Microtubule, CN, ciliary necklace; TF, transition fiber; CP, ciliary pocket; CM, ciliary membrane; BB, basal body.

Figure 3.

The transition zone interactome. (A) Domain structure of human MKS module proteins. Orange rectangles, transmembrane helices; purple rectangles, signal peptides; TCTN. Tectonic domains (also known as DUF1619); CYS, cysteine-rich domain; CC, coiled-coil; SH3, Src homology-3 motif; β-prop, β-propeller domain. With the exception of CEP290, the schematic lengths scale with amino acid number. (Panel A from Garcia-Gonzalo and Reiter 2012; adapted, with permission, from the authors.) (B) Domain structure of human NPHP module proteins, including those of the inversin subcomplex. MSP, major sperm protein domain; CC, coiled coil; SH3, Src-homology-3 motif; IQ, IQ calmodulin-binding motif; TPR, tetratricopeptide repeats; Armadillo, Armadillo-like domain; Ankyrin, ankyrin repeats; Ser/Thr, serine/threonine kinase domain; RCC1, RCC1-like domain; SAM, sterile α motif. (Panel B from Garcia-Gonzalo and Reiter 2012; adapted, with permission, from the authors.) (C) Model of the MKS and NPHP protein interaction network. TMEM proteins are only indicated by their number. The TCTN and B9 subcomplexes are shown as compact structures with the individual protein names indicated inside.