title: A function for the Joubert syndrome protein Arl13b in ciliary membrane extension and ciliary length regulation.
abstract: 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.
authors.affiliation: Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore.
authors.forename: Hao
authors.initials: H
authors.lastname: Lu
authors.affiliation: Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore.
authors.forename: Meng Tiak
authors.initials: MT
authors.lastname: Toh
authors.affiliation: Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore.
authors.forename: Vijayashankaranarayanan
authors.initials: V
authors.lastname: Narasimhan
authors.affiliation: Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore.
authors.forename: Surin Kumar
authors.initials: SK
authors.lastname: Thamilselvam
authors.affiliation: Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore.
authors.forename: Semil P
authors.initials: SP
authors.lastname: Choksi
authors.affiliation: Institute of Molecular and Cell Biology, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore; Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543, Singapore; Department of Pediatrics, National University of Singapore, 1E Kent Ridge Road, Singapore 119228, Singapore. Electronic address: sudipto@imcb.a-star.edu.sg.
authors.forename: Sudipto
authors.initials: S
authors.lastname: Roy
mesh_terms: D020727:ADP-Ribosylation Factors / Q000235:genetics / Q000378:metabolism*; D000015:Abnormalities, Multiple; D000818:Animals; D054468:Axoneme / Q000378:metabolism / Q000502:physiology*; D002526:Cerebellar Diseases / Q000235:genetics*; D002531:Cerebellum / Q000002:abnormalities; D002923:Cilia / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D003001:Cloning, Molecular; D017931:DNA Primers / Q000235:genetics; D005124:Eye Abnormalities / Q000235:genetics*; D051858:Forkhead Transcription Factors; D006801:Humans; D017403:In Situ Hybridization; D052177:Kidney Diseases, Cystic / Q000235:genetics*; D051379:Mice; D046529:Microscopy, Electron, Transmission; D008856:Microscopy, Fluorescence; D041681:NIH 3T3 Cells; D012160:Retina / Q000002:abnormalities*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015027:Zebrafish / Q000196:embryology* / Q000235:genetics; D029961:Zebrafish Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D051858:Forkhead Transcription Factors; C533681:Foxj1a protein, zebrafish; D029961:Zebrafish Proteins; D020727:ADP-Ribosylation Factors; C545718:Arl13b protein, zebrafish
keywords: Arl13b; Cilia; Ciliary length; Ciliary membrane; GTPase; Joubert syndrome