Hao Lu1, Meng Tiak Toh1, Vijayashankaranarayanan Narasimhan1, Surin Kumar Thamilselvam1, Semil P. Choksi1, Sudipto Roy1,2,3,4
1 Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore
2 Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543, Singapore
3 Department of Pediatrics, National University of Singapore, 1E Kent Ridge Road, Singapore 119228, Singapore
Author for correspondence: email@example.com
Published in Developmental Biology 397: 225-236
Cilia perform a variety of functions in a number of developmental and physiological contexts, and are implicated in the pathogenesis of a wide spectrum of human disorders. While the ciliary axoneme is assembled by intraflagellar transport, how ciliary membrane length is regulated is not completely understood. Here, we show that zebrafish embryos as well as mammalian cells overexpressing the ciliary membrane protein Arl13b, an ARF family small GTPase that is essential for ciliary differentiation, showed pronounced increase in ciliary length. Intriguingly, this increase in cilia length occurred as a function of the amounts of overexpressed Arl13b. While the motility of Arl13b overexpressing excessively long motile cilia was obviously disrupted, surprisingly, the abnormally long immotile primary cilia seemed to retain their signaling capacity. arl13b is induced by FoxJ1 and Rfx, and these ciliogenic transcription factors are unable to promote ciliary length increase when Arl13b activity is inhibited. Conversely, overexpression of Arl13b was sufficient to restore ciliary length in zebrafish embryos deficient in FoxJ1 function. We show that Arl13b increases cilia length by inducing protrusion of the ciliary membrane, which is then followed by the extension of the axonemal microtubules. Using mutant versions of Arl13b, one of which has been shown to be causative of the ciliopathy Joubert syndrome, we establish that the GTPase activity of the protein is essential for ciliary membrane extension. Taken together, our findings identify Arl13b as an important effector of ciliary membrane biogenesis and ciliary length regulation, and provide insights into possible mechanisms of dysfunction of the protein in Joubert syndrome.
Cover image of Journal:
Primary cilia and basal bodies in the somites of the zebrafish embryo labeled with antibodies to ciliary membrane protein Arl13b (green) and gamma-tubulin (red), respectively. The cell membranes of the somitic cells are highlighted with antibodies to beta-catenin (blue) and the nuclei with DAPI (grey). Arl13b is a critical regulator of ciliary length. See article by Lu et al.
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