The Ciliary Rootlet Provides Structural Support for Photoreceptor Outer Segments

Abstract

Abstract: : Purpose: The striated ciliary rootlet is a prominent cytoskeleton originating from basal bodies of ciliated cells. Although it is a familiar structure in cell biology, its function remains unresolved. Ciliary rootlets are composed of polymerized and bundled fibers of rootletin, a large protein with extended coiled–coil domains. To uncover the in vivo role of the rootlet, we carried out targeted disruption of the rootletin gene and performed detailed phenotype analyses in mutant mice. Methods: Gene targeting was carried out by homologous recombination in embryonic stem cells followed by blastocyst injection of the targeted clones. Mutant mice were analyzed by light and electron microscopy, immunofluorescence, western blotting, electroretinography (ERG), and single cell recording. Results: Disruption of rootletin expression resulted in the loss of the ciliary rootlet in all cells that normally develop this structure. Retinal photoreceptors exhibited no apparent functional deficits in phototransduction initially. However, rods and cones degenerated over time, so that by 18 months of age, significant cell loss and reduction in ERG amplitudes were evident. Further analyses revealed a striking fragility at the ciliary base in photoreceptors lacking rootlets. Conclusions: While other in vivo roles of the rootlet are possible and under investigation, a primary function of the rootlet is to provide structural support for cilia. Inasmuch as disruptions of the exceptionally enlarged sensory cilium (the light sensing outer segment) appear to be lethal to retinal photoreceptors, they are especially dependent on the rootlet for their long–term survival.

Keywords: cytoskeleton • retinal degenerations: cell biology • retina: distal (photoreceptors, horizontal cells, bipolar cells)