Expanding horizons: ciliary proteins reach beyond cilia

Abstract

Once obscure, the cilium has come into the spotlight during the past decade. It is now clear that aside from generating locomotion by motile cilia, both motile and immotile cilia serve as signaling platforms for the cell. Through both motility and sensory functions, cilia play critical roles in development, homeostasis, and disease. To date, the cilium proteome contains more than 1,000 different proteins, and human genetics is identifying new ciliopathy genes at an increasing pace. Although assigning a function to immotile cilia was a challenge not so long ago, the myriad of signaling pathways, proteins, and biological processes associated with the cilium have now created a new obstacle: how to distill all these interactions into specific themes and mechanisms that may explain how the organelle serves to maintain organism homeostasis. Here, we review the basics of cilia biology, novel functions associated with cilia, and recent advances in cilia genetics, and on the basis of this framework, we further discuss the meaning and significance of ciliary connections.

Figure 1

Morphology and structure of the cilium. (a) Illustration depicting the cell surface location and three distinct structural segments of the cilium: axoneme (blue), transition zone (TZ) (orange), and basal body (BB) (green). The axoneme is composed of nine outer microtubule doublets surrounded by the ciliary membrane, which is contiguous with, but distinct from, the plasma membrane (PM) (gray). Transition fibers (TF) (magenta) found at the base of the cilium interconnect the microtubules and the plasma membrane. (b) Detailed examination of a microtubule doublet, which consists of A and B subfibers (violet). In motile cilia, the A subfiber displays outer dynein arms (ODAs) (cyan), inner dynein arms (IDAs) (magenta), and radial spokes (orange). (c) Transverse section of the basal body, detailing nine microtubule triplets (violet) and cartwheel (orange). (d) Transverse section of the axoneme, which is found in two configurations: with a central pair (9+2) and without the central pair (9+0). In the 9+2 configuration, nexins interconnect the nine microtubule doublets, and a sheath surrounds the central pair. (e) Longitudinal transmission electron microscopy (TEM) section of a 9+2 flagellum from a wild-type Chlamydomonas cell, with the axoneme, transition zone, and basal body highlighted. (f) Transverse TEM section of a 9+2 flagellar axoneme from a wild-type Chlamydomonas cell, with the microtubule doublets and central pair apparent. (g) Scanning electron microscopy (SEM) micrograph of an immotile cilium on the endoderm of a stage 17 Xenopus tropicalis embryo. (h) SEM micrograph of a motile multiciliated cell on the epidermis of a Xenopus tropicalis embryo. EM images courtesy of Mustafa Khokha, Dennis Diener, and Joel Rosenbaum (Yale University).

Figure 2

Zebrafish as a vertebrate model for cilia analysis. (a) Brightfield image of a wild-type and lrrc6^hi3308 zebrafish embryo at five days postfertilization (dpf). lrrc6^hi3308 mutants display bilateral renal cysts (arrowhead and inset image) and a ventral body curvature (asterisk) due to ciliary defects. Body curvature is a hallmark of ciliary zebrafish mutants and the severity of the defect can be quantified by measuring the angle between the eye, yolk extension, and tail tip (cyan). (b–g) Fluorescent immunostaining of numerous ciliated tissues in wild-type zebrafish embryos, with cilia labeled by an antibody against acetylated-α-tubulin (green). (b) Kupffer’s vesicle (KV) at the 8-somite stage. The apical membrane is labeled with an antibody against atypical protein kinase C (PKC) (violet). (c) Pronephric duct (PND) at 2 dpf. The basolateral membrane is labeled with an antibody against Cdh17 (violet). (d) Otic vesicle (OV) at 20 h postfertilization (hpf). (e) Lateral line (LL) at 36 hpf. (f) Olfactory placode (OP) at 36 hpf. (g) Magnification of a peripheral edge of the OP. Nuclei are labeled with TOTO3 (blue).