Nephronophthisis-related ciliopathies (NPHP-RC) are recessive disorders featuring dysplasia or degeneration preferentially in kidney, retina, and cerebellum. Here we combine homozygosity mapping with candidate gene analysis by performing “ciliopathy candidate exome capture” followed by massively-parallel sequencing. We detect 12 different truncating mutations of SDCCAG8 in 10 NPHP-RC families. We demonstrate that SDCCAG8 is localized at both centrioles and directly interacts with NPHP-RC-associated OFD1. Depletion of sdccag8 causes kidney cysts and a body axis defect in zebrafish and induces cell polarity defects in 3D renal cell cultures. This work identifies SDCCAG8 loss of function as a novel cause of a retinal-renal ciliopathy and validates exome capture analysis for broadly heterogeneous single-gene disorders.

Cilia intraflagellar transport and ciliogenesis are regulated by two small GTPases that maintain binding between IFT subcomplexes.

The bidirectional movement of intraflagellar transport (IFT) particles, which are composed of motors, IFT-A and IFT-B subcomplexes, and cargos, is required for cilia biogenesis and signaling 1, 2. A successful IFT cycle depends on the massive IFT particle to be properly assembled at the ciliary base and turned around from anterograde to retrograde transport at the ciliary tip. However, how IFT assembly and turnaround are regulated in vivo remains elusive. From a whole-genome mutagenesis screen in C. elegans, we identified two hypomorphic mutations in dyf-2 and bbs-1 as the only mutants showing normal anterograde IFT transport but defective IFT turnaround at the ciliary tip. Further analyses revealed that the BBSome 3, 4, a group of conserved proteins affected in human Bardet-Biedl syndrome (BBS) 5, assembles IFT complexes at the ciliary base, then binds to anterograde IFT particle in a DYF-2- (an ortholog of human WDR19) and BBS-1-dependent manner, and lastly reaches the ciliary tip to regulate proper IFT recycling. Our results unravel the BBSome as the key player regulating IFT assembly and turnaround in cilia.

Sensory organelle cilia play critical roles in mammalian embryonic development and tissue homeostasis. Intraflagellar transport (IFT) machinery is required for the assembly and maintenance of cilia. Yet how this large complex passes through the size-dependent barrier at the ciliary base remains enigmatic. Here we report that FBF1, a highly conserved transition fibre protein, is required for the ciliary import of assembled IFT particles at the cilia base. We cloned dyf-19, the C. elegans homolog of human FBF1, in a whole-genome screen for ciliogenesis mutants. DYF-19 localizes specifically to transition fibres and interacts directly with the IFT-B component DYF-11/IFT54. Although not a structural component of transition fibres, DYF-19 is essential for the transit of assembled IFT particles through the ciliary base. Furthermore, we found that human FBF1 shares conserved localization and function with its worm counterpart. We conclude that FBF1 is a key functional transition fibre component that facilitates the ciliary entry of assembled IFT machinery.

Defective primary cilia are causative to a wide spectrum of human genetic disorders, termed ciliopathies. Although the regulation of ciliogenesis is intensively studied, how it is initiated remains unclear. Here we show that type Iγ phosphatidylinositol 4-phosphate (PtdIns(4)P) 5-kinase (PIPKIγ) and inositol polyphosphate-5-phosphatase E (INPP5E), a Joubert syndrome protein, localize to the centrosome and coordinate the initiation of ciliogenesis. PIPKIγ counteracts INPP5E in regulating tau-tubulin kinase-2 (TTBK2) recruitment to the basal body, which promotes the removal of microtubule capping protein CP110 and the subsequent axoneme elongation. Interestingly, INPP5E and its product—PtdIns(4)P—accumulate at the centrosome/basal body in non-ciliated, but not ciliated, cells. PtdIns(4)P binding to TTBK2 and the distal appendage protein CEP164 compromises the TTBK2-CEP164 interaction and inhibits the recruitment of TTBK2. Our results reveal that PtdIns(4)P homoeostasis, coordinated by PIPKIγ and INPP5E at the centrosome/ciliary base, is vital for ciliogenesis by regulating the CEP164-dependent recruitment of TTBK2.

Tubulin polyglutamylation is a predominant axonemal post-translational modification. However, if and how axoneme polyglutamylation is essential for primary cilia and contribute to ciliopathies are unknown. Here, we report that Joubert syndrome protein ARL13B controls axoneme polyglutamylation, which is marginally required for cilia stability but essential for cilia signaling. ARL13B interacts with RAB11 effector FIP5 to promote cilia import of glutamylase TTLL5 and TTLL6. Hypoglutamylation caused by a deficient ARL13B-RAB11-FIP5 trafficking pathway shows no effect on ciliogenesis, but promotes cilia disassembly and, importantly, impairs cilia signaling by disrupting the proper anchoring of sensory receptors and trafficking of signaling molecules. Remarkably, depletion of deglutamylase CCP5, the predominant cilia deglutamylase, effectively restores hypoglutamylation-induced cilia defects. Our study reveals a paradigm that tubulin polyglutamylation is a major contributor for cilia signaling and suggests a potential therapeutic strategy by targeting polyglutamylation machinery to promote ciliary targeting of signaling machineries and correct signaling defects in ciliopathies.

Type Iγ phosphatidylinositol-4-phosphate 5-kinase and Exo70 cooperate in the directed targeting of E-cadherin on the plasma membrane to newly formed adherens junctions. This promotes the regional accumulation of E-cadherin, expansion and maturation of adherens junctions, and differentiation of the lateral membrane domain.

Cilia harbor sensory receptors for various signaling cascades critical for vertebrate development. However, the mechanisms underlying the ciliary homeostasis of sensory receptors remain elusive. Here, we demonstrate that BBS-4 and BBS-5, two distinct BBSome components, show unexpected functional redundancy in the context of cilia in C. elegans. BBS-4 directly interacts with BBS-5 and the interaction can be disrupted by a conserved mutation identified in human BBS4. Surprisingly, we found that BBS-4 and BBS-5 act redundantly in the BBSome to regulate the ciliary removal, rather than the ciliary entry or retrograde IFT transport, of various sensory receptors. Further analyses indicate that co-depletion of BBS-4 and BBS-5 disrupts the lysosome-targeted degradative sorting of ciliary sensory receptors. Moreover, mammalian BBS4 and BBS5 also interact directly and coordinate the ciliary removal of polycystin 2. Hence, we reveal a novel and highly conserved role for the BBSome in fine-tuning ciliary signaling by regulating the ciliary removal of sensory receptors for lysosomal degradation.