Cilia and polycystic kidney disease.

Polycystic kidney disease (PKD), comprising autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD), is characterized by incessant cyst formation in the kidney and liver. ADPKD and ARPKD represent the leading genetic causes of renal disease in adults and children, respectively. ADPKD is caused by mutations in PKD1 encoding polycystin1 (PC1) and PKD2 encoding polycystin 2 (PC2). PC1/2 are multi-pass transmembrane proteins that form a complex localized in the primary cilium. Predominant ARPKD cases are caused by mutations in polycystic kidney and hepatic disease 1 (PKHD1) gene that encodes the Fibrocystin/Polyductin (FPC) protein, whereas a small subset of cases are caused by mutations in DAZ interacting zinc finger protein 1 like (DZIP1L) gene. FPC is a type I transmembrane protein, localizing to the cilium and basal body, in addition to other compartments, and DZIP1L encodes a transition zone/basal body protein. Apparently, PC1/2 and FPC are signaling molecules, while the mechanism that cilia employ to govern renal tubule morphology and prevent cyst formation is unclear. Nonetheless, recent genetic and biochemical studies offer a glimpse of putative physiological malfunctions and the pathomechanisms underlying both disease entities. In this review, I summarize the results of genetic studies that deduced the function of PC1/2 on cilia and of cilia themselves in cyst formation in ADPKD, and I discuss studies regarding regulation of polycystin biogenesis and cilia trafficking. I also summarize the synergistic genetic interactions between Pkd1 and Pkhd1, and the unique tissue patterning event controlled by FPC, but not PC1. Interestingly, while DZIP1L mutations generate compromised PC1/2 cilia expression, FPC deficiency does not affect PC1/2 biogenesis and ciliary localization, indicating that divergent mechanisms could lead to cyst formation in ARPKD. I conclude by outlining promising areas for future PKD research and highlight rationales for potential therapeutic interventions for PKD treatment.

Super-Resolution Microscopy and FIB-SEM Imaging Reveal Parental Centriole-Derived, Hybrid Cilium in Mammalian Multiciliated Cells.

Motile cilia are cellular beating machines that play a critical role in mucociliary clearance, cerebrospinal fluid movement, and fertility. In the airways, hundreds of motile cilia present on the surface of a multiciliated epithelia cell beat coordinately to protect the epithelium from bacteria, viruses, and harmful particulates. During multiciliated cell differentiation, motile cilia are templated from basal bodies, each extending a basal foot-an appendage linking motile cilia together to ensure coordinated beating. Here, we demonstrate that among the many motile cilia of a multiciliated cell, a hybrid cilium with structural features of both primary and motile cilia is harbored. The hybrid cilium is conserved in mammalian multiciliated cells, originates from parental centrioles, and its cellular position is biased and dependent on ciliary beating. Furthermore, we show that the hybrid cilium emerges independently of other motile cilia and functions in regulating basal body alignment.

The centrosomal/basal body protein OFD1 is required for microtubule organization and cell cycle progression.

Oral-Facial-Digital type I (OFD1) is a rare inherited form of renal cystic disease associated with ciliary dysfunction. This disorder is due to mutations in the OFD1 gene that encodes a protein localized to centrosomes and basal bodies in different cell types. Immunofluorescence analysis demonstrated that OFD1 displays a dynamic distribution during cell cycle. High-content microscopy analysis of Ofd1-depleted fibroblasts revealed impaired cell cycle progression. Immunofluorescence analysis and cell proliferation assays also indicated the presence of a variety of defects such as centrosome accumulation, nuclear abnormalities and aneuploidy. In addition, Ofd1-depleted cells displayed an abnormal microtubule network that may underlie these defects. All together our results suggest that OFD1 contributes to the function of the microtubule organizing center (MTOC) in the cell, controlling cell cycle progression both in vitro and in vivo.

De Novo Mutations in FOXJ1 Result in a Motile Ciliopathy with Hydrocephalus and Randomization of Left/Right Body Asymmetry.

Hydrocephalus is one of the most prevalent form of developmental central nervous system (CNS) malformations. Cerebrospinal fluid (CSF) flow depends on both heartbeat and body movement. Furthermore, it has been shown that CSF flow within and across brain ventricles depends on cilia motility of the ependymal cells lining the brain ventricles, which play a crucial role to maintain patency of the narrow sites of CSF passage during brain formation in mice. Using whole-exome and whole-genome sequencing, we identified an autosomal-dominant cause of a distinct motile ciliopathy related to defective ciliogenesis of the ependymal cilia in six individuals. Heterozygous de novo mutations in FOXJ1, which encodes a well-known member of the forkhead transcription factors important for ciliogenesis of motile cilia, cause a motile ciliopathy that is characterized by hydrocephalus internus, chronic destructive airway disease, and randomization of left/right body asymmetry. Mutant respiratory epithelial cells are unable to generate a fluid flow and exhibit a reduced number of cilia per cell, as documented by high-speed video microscopy (HVMA), transmission electron microscopy (TEM), and immunofluorescence analysis (IF). TEM and IF demonstrate mislocalized basal bodies. In line with this finding, the focal adhesion protein PTK2 displays aberrant localization in the cytoplasm of the mutant respiratory epithelial cells.

BBS4 is required for intraflagellar transport coordination and basal body number in mammalian olfactory cilia.

Bardet-Beidl syndrome (BBS) manifests from genetic mutations encoding for one or more BBS proteins. BBS4 loss impacts olfactory ciliation and odor detection, yet the cellular mechanisms remain unclear. Here, we report that Bbs4-/- mice exhibit shorter and fewer olfactory sensory neuron (OSN) cilia despite retaining odorant receptor localization. Within Bbs4-/- OSN cilia, we observed asynchronous rates of IFT-A/B particle movements, indicating miscoordination in IFT complex trafficking. Within the OSN dendritic knob, the basal bodies are dynamic, with incorporation of ectopically expressed centrin-2 and γ-tubulin occurring after nascent ciliogenesis. Importantly, BBS4 loss results in the reduction of basal body numbers separate from cilia loss. Adenoviral expression of BBS4 restored OSN cilia lengths and was sufficient to re-establish odor detection, but failed to rescue ciliary and basal body numbers. Our results yield a model for the plurality of BBS4 functions in OSNs that includes intraciliary and periciliary roles that can explain the loss of cilia and penetrance of ciliopathy phenotypes in olfactory neurons.

KIAA0556 is a novel ciliary basal body component mutated in Joubert syndrome.

BACKGROUND: Joubert syndrome (JBTS) and related disorders are defined by cerebellar malformation (molar tooth sign), together with neurological symptoms of variable expressivity. The ciliary basis of Joubert syndrome related disorders frequently extends the phenotype to tissues such as the eye, kidney, skeleton and craniofacial structures. RESULTS: Using autozygome and exome analyses, we identified a null mutation in KIAA0556 in a multiplex consanguineous family with hallmark features of mild Joubert syndrome. Patient-derived fibroblasts displayed reduced ciliogenesis potential and abnormally elongated cilia. Investigation of disease pathophysiology revealed that Kiaa0556 (-/-) null mice possess a Joubert syndrome-associated brain-restricted phenotype. Functional studies in Caenorhabditis elegans nematodes and cultured human cells support a conserved ciliary role for KIAA0556 linked to microtubule regulation. First, nematode KIAA0556 is expressed almost exclusively in ciliated cells, and the worm and human KIAA0556 proteins are enriched at the ciliary base. Second, C. elegans KIAA0056 regulates ciliary A-tubule number and genetically interacts with an ARL13B (JBTS8) orthologue to control cilium integrity. Third, human KIAA0556 binds to microtubules in vitro and appears to stabilise microtubule networks when overexpressed. Finally, human KIAA0556 biochemically interacts with ciliary proteins and p60/p80 katanins. The latter form a microtubule-severing enzyme complex that regulates microtubule dynamics as well as ciliary functions. CONCLUSIONS: We have identified KIAA0556 as a novel microtubule-associated ciliary base protein mutated in Joubert syndrome. Consistent with the mild patient phenotype, our nematode, mice and human cell data support the notion that KIAA0556 has a relatively subtle and variable cilia-related function, which we propose is related to microtubule regulation.