Cell type‐specific regulation of ciliary transition zone assembly in vertebrates

Abstract

Ciliopathies are life‐threatening human diseases caused by defective cilia. They can often be traced back to mutations of genes encoding transition zone (TZ) proteins demonstrating that the understanding of TZ organisation is of paramount importance. The TZ consists of multimeric protein modules that are subject to a stringent assembly hierarchy. Previous reports place Rpgrip1l at the top of the TZ assembly hierarchy in Caenorhabditis elegans. By performing quantitative immunofluorescence studies in RPGRIP1L−/− mouse embryos and human embryonic cells, we recognise a different situation in vertebrates in which Rpgrip1l deficiency affects TZ assembly in a cell type‐specific manner. In cell types in which the loss of Rpgrip1l alone does not affect all modules, additional truncation or removal of vertebrate‐specific Rpgrip1 results in an impairment of all modules. Consequently, Rpgrip1l and Rpgrip1 synergistically ensure the TZ composition in several vertebrate cell types, revealing a higher complexity of TZ assembly in vertebrates than in invertebrates.

Rpgrip1l controls ciliary gating by ensuring the proper amount of Cep290 at the vertebrate transition zone

“…In this study, we shed light on the role of Rpgrip1l in regulating the ciliary gating function of the TZ. Our previous investigations revealed that mutations in RPGRIP1L cause deadly ciliopathies [25], that Rpgrip1l localises to the vertebrate TZ [26] and that it is a decisive factor in vertebrate TZ assembly [27]. Our current work demonstrates that Rpgrip1l deficiency results in an altered ciliary protein composition and that Rpgrip1l governs ciliary gating by ensuring the proper amount of Cep290 at the vertebrate TZ.…”

“…Among the proteins allowed to cross the TZ are receptors and mediators of signalling pathways [13,14,17,19,[28][29][30][31]. In a former study, we showed that Rpgrip1l deficiency leads to a reduction of the ciliary Arl13b amount in all analysed mouse cells in vitro and in vivo [in mouse embryonic fibroblasts (MEFs), in mouse embryonic kidneys and in mouse limb buds] [27]. However, we could not detect an alteration of the ciliary Smo amount in Rpgrip1l -/-MEFs [26] raising the question whether the effect of Rpgrip1l is Arl13bspecific or whether it functions as a more general ciliary gatekeeper at the vertebrate TZ.…”

“…In this context, Centrosome And Spindle Pole-Associated Protein 1 (Cspp1) was an interesting object of investigation since its truncation in humans results in a reduced ciliary amount of Arl13b and Ac3 [34] and since it interacts with Rpgrip1l [35]. Previously, we and others reported that Rpgrip1l ensures the proper amount of many proteins at the base of vertebrate cilia [19,27] suggesting that it serves as a vital scaffold protein. Thus, we quantified 5 the amount of Cspp1 at the ciliary base of Rpgrip1l -/-MEFs but we could not detect any alteration ( Figure S1A) indicating that Rpgrip1l exerts its ciliary gatekeeper function independently of Cspp1.…”

“…Neither Sept2 nor Sept7 was altered by the loss of Rpgrip1l ( Figure S1B and C). We then turned to the TZ proteins Nephrocystin 4 (Nphp4), Nephrocystin 1 (Nphp1), Centrosomal Protein 290 (Cep290) and Inversin (Invs) whose amount at the vertebrate TZ is decreased in the absence of Rpgrip1l [27].…”

CEP290 myosin-tail homology domain is essential for protein confinement between inner and outer segments in photoreceptors

“…Our study provides insights into the pathomechanisms of retinal degenerations associated with compromised ciliary gates.the absence of CEP290 in these organisms, ciliary protein compositions are altered [4,6]. In mammalian primary cilia, CEP290 is localized to the transition zone [7][8][9], and loss of CEP290 reduces ARL13B and ADCY3 levels within cilia while increasing the rate of ciliary entry of SMO in fibroblasts [10]. These studies establish the current model for CEP290 function: a ciliary gatekeeper that regulates protein trafficking in and out of the ciliary compartment at the transition zone [4][5][6][7][10][11][12][13].…”