The Ciliary Rootlet Maintains Long-Term Stability of Sensory Cilia

Abstract

The striated ciliary rootlet is a prominent cytoskeleton originating from basal bodies of ciliated cells. Although a familiar structure in cell biology, its function has remained unresolved. In this study, we carried out targeted disruption in mice of the gene for rootletin, a component of the rootlet. In the mutant, ciliated cells are devoid of rootlets. Phototransduction and ciliary beating in sensory and motile cilia initially exhibit no apparent functional deficits. However, photoreceptors degenerate over time, and mutant lungs appear prone to pathological changes consistent with insufficient mucociliary clearance. Further analyses revealed a striking fragility at the ciliary base in photoreceptors lacking rootlets. In vitro assays suggest that the rootlet is among the least dynamic of all cytoskeletons and interacts with actin filaments. Thus, a primary function of the rootlet is to provide structural support for the cilium. Inasmuch as photoreceptors elaborate an exceptionally enlarged sensory cilium, they are especially dependent on the rootlet for structural integrity and long-term survival.The ciliary rootlet originates from the proximal ends of basal bodies, the centriole-related structure that anchors the cilia, and extends proximally toward the cell nuclei (5, 25). Prominent rootlets are associated with sensory cilia of photoreceptor cells and motile cilia, such as those lining the respiratory tract, oviduct, and brain ventricles. The retinal photoreceptor elaborates a single enlarged distal cilium known as the outer segment. The outer segment is packed with photosensitive membranous disks and specializes in phototransduction. It is among the largest of all mammalian cilia, spanning approximately 25 m in length and more than 1 m in diameter. The outer segment is linked to the cell body, the inner segment, through a thin bridge called the connecting cilium (3). In a photoreceptor, the rootlet appears as a very thick striated filament that traverses the entire cell body all the way to the synaptic terminal (17,19,26). In epithelial cells bearing motile cilia, rootlets appear as robust subapical filamentous networks.

Reproductive morphology of Brittanichthys axelrodi (Teleostei: Characidae), a miniature inseminating fish from South America

“…Striated rootlets are also present in the spermatozoa of many invertebrates (e.g., Platyhelminthes; Justine, 1995), as well as in the spermatids of Postorchigenes (Platyhelminthes: Trematoda: Lecithodendriidae; Gracenea et al, 1997). Work by Yang et al (2002Yang et al ( , 2005 on striated rootlets of ciliated cells demonstrated that rootlets are critical structures for support of cilia and are composed of a large protein called rootletin. Although rootlets were not required for ciliary motility, their presence increased survival and enabled significantly more normal ciliated cells to withstand mechanical stress compared to mutant ciliated cells that lacked rootlets (Yang et al, 2005).…”

“…Work by Yang et al (2002Yang et al ( , 2005 on striated rootlets of ciliated cells demonstrated that rootlets are critical structures for support of cilia and are composed of a large protein called rootletin. Although rootlets were not required for ciliary motility, their presence increased survival and enabled significantly more normal ciliated cells to withstand mechanical stress compared to mutant ciliated cells that lacked rootlets (Yang et al, 2005). In a spermatozoon, the rootlet may therefore act to stabilize the cell during periods of flagellar swimming.…”

Structure and duplication of the centrosome

“…C-Nap1 interacts with rootletin, a conserved component of the ciliary rootlet. The ciliary rootlet is a cytoskeletal structure found in many ciliated cells that originates from the basal body and extends proximally toward the nucleus, providing structural support for the cilium (Yang et al, 2005). Rootletin is, however, also found in cells devoid of a ciliary rootlet, forming fibers that emanate from the proximal ends of centrioles.…”

Mechanotransduction and auditory transduction in Drosophila

“…Striated rootlets are a general feature of ciliated cells and some are capable of calcium-dependent contraction [78]. Rootlet components include isoforms of centrin/caltractin, a calmodulin-related protein that regulates rootlet contractility [105,106], and rootletin, a large coiledcoil protein required to maintain mammalian sensory cilia [109,110]. Drosophila has genes encoding two centrin isoforms and a rootletin homolog.…”