PMC:3432770 / 37070-44168 JSONTXT

{"target":"https://pubannotation.org/docs/sourcedb/PMC/sourceid/3432770","sourcedb":"PMC","sourceid":"3432770","source_url":"http://www.ncbi.nlm.nih.gov/pmc/3432770","text":"Discussion\nIn this paper we investigated the function of the ciliopathy protein Rpgrip1l in planar polarity. We show that, in the mammalian cochlear sensory epithelium and in the zebrafish floor plate, Rpgrip1l is required for correct positioning of the basal body along the PCP axis. We demonstrate that Rpgrip1l is essential for stabilizing dishevelled in different experimental systems and that in the zebrafish floor plate this stabilization of dishevelled by Rpgrip1l mediates basal body positioning. Finally, our experiments in cell culture suggest that Rpgrip1l acts in a complex with nephrocystin-4 and inversin to finely regulate dishevelled stability.\nOur data uncover a function for Rpgrip1l in several PCP processes characterized by a stereotyped position of the centrioles along the PCP axis (Jones and Chen, 2008; Borovina et al., 2010; Sepich et al., 2011). Rpgrip1l could act at several levels in these processes. It could be a general modulator of planar polarity, controlling the polarized localization of core PCP pathway proteins. Alternatively, it could be involved in interpreting a preexisting polarity information that would result in the asymmetric positioning of the basal body. Finally, it could act in parallel to and independently of the PCP pathway. Our results are not in favor of the first possibility. First, in cochleae, Rpgrip1l mutation leads to a mild disruption of the orientation of the stereociliary bundles, confirming that distinct planar polarity events in these cells can be uncoupled in ciliary mutants (Jones et al., 2008; Sipe and Lu, 2011). Moreover, the rescue of CE defects and of cilium positioning in the zebrafish floor plate by dishevelled shows that Rpgrip1l function is mediated at least in part by dishevelled stabilization. This places Rpgrip1l either downstream of, or in parallel to, cortical asymmetric cues, and upstream of intracellular basal body positioning. In the Drosophila wing (Goodrich and Strutt, 2011) as in the Xenopus skin (Mitchell et al., 2009), dishevelled acts cell-autonomously to establish polarity within cells, while Strabismus/Vangl and Frizzled transmembrane proteins also act in the coordination of planar polarity between adjacent cells. Thus, we propose that Rpgrip1l, by maintaining dishevelled levels, is specifically involved in one planar polarity process, i.e., the asymmetric localization of the basal body.\nThe function of dishevelled in basal body positioning in the zebrafish floor plate is consistent with what is described in the mouse node (Hashimoto et al., 2010). In the cochlea, dishevelled loss of function perturbs stereocilia orientation (Wang et al., 2006), whereas Rpgrip1l loss of function does not. In this system, dishevelled has been described in a cortical crescent located at the distal membrane of the hair cells, where the centrioles are anchored (Wang et al., 2006; Etheridge et al., 2008; Sipe and Lu, 2011). It would be very interesting to know whether Rpgrip1l is required for dishevelled stability in this system, and we are currently investigating this question. Also, dishevelled effectors in asymmetric basal body localization in the zebrafish floor plate remain to be identified. A candidate is Rac1, which is a downstream effector of dishevelled in other cellular contexts (Gao and Chen, 2010; Ishida-Takagishi et al., 2012), and whose inhibition prevents both posterior positioning of the basal body in node cells (Hashimoto et al., 2010) and positioning of the kinocilium on the distal side of cochlear hair cells (Sipe and Lu, 2011).\nOur data point to a functional interaction between dishevelled and Rpgrip1l, which leads to dishevelled stabilization. What are the mechanisms of this functional interaction? Our experiments in HEK293T cells suggest that a physical interaction between Rpgrip1l, nephrocystin-4, inversin, and dishevelled can occur. The Rpgrip1l–dishevelled interaction is likely to be indirect, through a macromolecular complex comprising nephrocystin-4 and inversin (Simons et al., 2005; Delous et al., 2007; Burcklé et al., 2011). Rpgrip1l-dependent integrity of the cilium does not seem to be required because Rpgrip1l stabilizes dishevelled in HEK293T cells that do not form a cilium. We favor an alternative possibility, in which Rpgrip1l would be required for the stability and/or function of a pericentriolar platform involved in dishevelled stabilization. This is suggested by the reduction of the pericentriolar material and associated proteins in MDCK cells upon RPGRIP1L knock-down. Although Rpgrip1l is mainly found at the transition zone in many cell types, it is known to interact with pericentriolar proteins (Coene et al., 2011).\nSeveral ciliopathy proteins, in particular the Rpgrip1l interactors inversin and nephrocystin-4, have been shown to target cytoplasmic dishevelled for proteasomal degradation (Simons et al., 2005; Burcklé et al., 2011; Wallingford and Mitchell, 2011). Here we show that, in contrast, Rpgrip1l protects dishevelled from proteasomal degradation. How is the balance in dishevelled stability achieved, and is the subcellular localization of the complex important for this balance? Our data show that Rpgrip1l does not compete with binding of nephrocystin-4 or inversin to dishevelled. Rpgrip1l could act by anchoring an inversin–nephrocystin-4–dishevelled complex to the basal body. Indeed, in C. elegans neurons, nephrocystin-4 localization at the ciliary transition zone depends on Rpgrip1l (Williams et al., 2011). Rpgrip1l could also modify the activity of nephrocystin-4 and inversin on dishevelled, allowing a precise tuning of dishevelled stability. The stoichiometry or the conformation of the different ciliary proteins in the complex, depending on the cellular context, could modulate the recruitment and/or the activity of proteins involved in proteasomal degradation. For instance, it could modulate the recruitment or activity of the ubiquitin ligase APC-C, which interacts with both dishevelled and inversin and regulates dishevelled stability (Morgan et al., 2002; Ganner et al., 2009). These two different mechanisms, localization of the complex to the pericentriolar region, and modulation of the activity of the complex, are not mutually exclusive and could both participate in fine-tuning dishevelled stability.\nIn conclusion, we describe here a novel, central function of an Rpgrip1l-dependent nephrocystin complex in stabilizing dishevelled, and we provide compelling evidence that this function is required for planar localization of the basal body. In mice, altered Wnt-PCP signaling results in kidney cysts appearing before birth, associated with CE defects in the elongating renal tubules (Karner et al., 2009). Thus, the function of Rpgrip1l in planar polarity may help interpret defects such as kidney dysfunctions found in mouse Ftm mutants and in humans presenting RPGRIP1L mutations. In this respect, our observation that RPGRIP1L mutations found in Joubert syndrome type B fail to stabilize dishevelled highlights possible physiopathological mechanisms occurring in ciliopathies.","divisions":[{"label":"Title","span":{"begin":0,"end":10}}],"tracks":[{"project":"2_test","denotations":[{"id":"22927466-19147007-56740307","span":{"begin":822,"end":826},"obj":"19147007"},{"id":"22927466-20305649-56740308","span":{"begin":845,"end":849},"obj":"20305649"},{"id":"22927466-21205798-56740309","span":{"begin":866,"end":870},"obj":"21205798"},{"id":"22927466-18066062-56740310","span":{"begin":1563,"end":1567},"obj":"18066062"},{"id":"22927466-21752934-56740311","span":{"begin":1582,"end":1586},"obj":"21752934"},{"id":"22927466-21521735-56740312","span":{"begin":1969,"end":1973},"obj":"21521735"},{"id":"22927466-19427216-56740313","span":{"begin":2016,"end":2020},"obj":"19427216"},{"id":"22927466-20098415-56740314","span":{"begin":2559,"end":2563},"obj":"20098415"},{"id":"22927466-16571627-56740315","span":{"begin":2658,"end":2662},"obj":"16571627"},{"id":"22927466-16571627-56740316","span":{"begin":2877,"end":2881},"obj":"16571627"},{"id":"22927466-19008950-56740317","span":{"begin":2901,"end":2905},"obj":"19008950"},{"id":"22927466-21752934-56740318","span":{"begin":2920,"end":2924},"obj":"21752934"},{"id":"22927466-20006983-56740319","span":{"begin":3314,"end":3318},"obj":"20006983"},{"id":"22927466-22643886-56740320","span":{"begin":3345,"end":3349},"obj":"22643886"},{"id":"22927466-20098415-56740321","span":{"begin":3460,"end":3464},"obj":"20098415"},{"id":"22927466-21752934-56740322","span":{"begin":3556,"end":3560},"obj":"21752934"},{"id":"22927466-15852005-56740323","span":{"begin":4029,"end":4033},"obj":"15852005"},{"id":"22927466-17558409-56740324","span":{"begin":4050,"end":4054},"obj":"17558409"},{"id":"22927466-21498478-56740325","span":{"begin":4072,"end":4076},"obj":"21498478"},{"id":"22927466-21685204-56740326","span":{"begin":4685,"end":4689},"obj":"21685204"},{"id":"22927466-15852005-56740327","span":{"begin":4883,"end":4887},"obj":"15852005"},{"id":"22927466-21498478-56740328","span":{"begin":4905,"end":4909},"obj":"21498478"},{"id":"22927466-21289065-56740329","span":{"begin":4937,"end":4941},"obj":"21289065"},{"id":"22927466-21422230-56740330","span":{"begin":5499,"end":5503},"obj":"21422230"},{"id":"22927466-12471060-56740331","span":{"begin":6062,"end":6066},"obj":"12471060"},{"id":"22927466-19805045-56740332","span":{"begin":6083,"end":6087},"obj":"19805045"},{"id":"22927466-19543268-56740333","span":{"begin":6718,"end":6722},"obj":"19543268"}],"attributes":[{"subj":"22927466-19147007-56740307","pred":"source","obj":"2_test"},{"subj":"22927466-20305649-56740308","pred":"source","obj":"2_test"},{"subj":"22927466-21205798-56740309","pred":"source","obj":"2_test"},{"subj":"22927466-18066062-56740310","pred":"source","obj":"2_test"},{"subj":"22927466-21752934-56740311","pred":"source","obj":"2_test"},{"subj":"22927466-21521735-56740312","pred":"source","obj":"2_test"},{"subj":"22927466-19427216-56740313","pred":"source","obj":"2_test"},{"subj":"22927466-20098415-56740314","pred":"source","obj":"2_test"},{"subj":"22927466-16571627-56740315","pred":"source","obj":"2_test"},{"subj":"22927466-16571627-56740316","pred":"source","obj":"2_test"},{"subj":"22927466-19008950-56740317","pred":"source","obj":"2_test"},{"subj":"22927466-21752934-56740318","pred":"source","obj":"2_test"},{"subj":"22927466-20006983-56740319","pred":"source","obj":"2_test"},{"subj":"22927466-22643886-56740320","pred":"source","obj":"2_test"},{"subj":"22927466-20098415-56740321","pred":"source","obj":"2_test"},{"subj":"22927466-21752934-56740322","pred":"source","obj":"2_test"},{"subj":"22927466-15852005-56740323","pred":"source","obj":"2_test"},{"subj":"22927466-17558409-56740324","pred":"source","obj":"2_test"},{"subj":"22927466-21498478-56740325","pred":"source","obj":"2_test"},{"subj":"22927466-21685204-56740326","pred":"source","obj":"2_test"},{"subj":"22927466-15852005-56740327","pred":"source","obj":"2_test"},{"subj":"22927466-21498478-56740328","pred":"source","obj":"2_test"},{"subj":"22927466-21289065-56740329","pred":"source","obj":"2_test"},{"subj":"22927466-21422230-56740330","pred":"source","obj":"2_test"},{"subj":"22927466-12471060-56740331","pred":"source","obj":"2_test"},{"subj":"22927466-19805045-56740332","pred":"source","obj":"2_test"},{"subj":"22927466-19543268-56740333","pred":"source","obj":"2_test"}]}],"config":{"attribute types":[{"pred":"source","value type":"selection","values":[{"id":"2_test","color":"#93ecad","default":true}]}]}}