In situ architecture of the ciliary base reveals the stepwise assembly of intraflagellar transport trains.

The cilium is an antenna-like organelle that performs numerous cellular functions, including motility, sensing, and signaling. The base of the cilium contains a selective barrier that regulates the entry of large intraflagellar transport (IFT) trains, which carry cargo proteins required for ciliary assembly and maintenance. However, the native architecture of the ciliary base and the process of IFT train assembly remain unresolved. In this work, we used in situ cryo-electron tomography to reveal native structures of the transition zone region and assembling IFT trains at the ciliary base in Chlamydomonas. We combined this direct cellular visualization with ultrastructure expansion microscopy to describe the front-to-back stepwise assembly of IFT trains: IFT-B forms the backbone, onto which bind IFT-A, dynein-1b, and finally kinesin-2 before entry into the cilium.

Kinesin-1 activity recorded in living cells with a precipitating dye.

Kinesin-1 is a processive motor protein that uses ATP-derived energy to transport a variety of intracellular cargoes toward the cell periphery. The ability to visualize and monitor kinesin transport in live cells is critical to study the myriad of functions associated with cargo trafficking. Herein we report the discovery of a fluorogenic small molecule substrate (QPD-OTf) for kinesin-1 that yields a precipitating dye along its walking path on microtubules (MTs). QPD-OTf enables to monitor native kinesin-1 transport activity in cellulo without external modifications. In vitro assays show that kinesin-1 and MTs are sufficient to yield fluorescent crystals; in cells, kinesin-1 specific transport of cargo from the Golgi appears as trails of fluorescence over time. These findings are further supported by docking studies, which suggest the binding of the activity-based substrate in the nucleotide binding site of kinesin-1.

Biliary-Atresia-Associated Mannosidase-1-Alpha-2 Gene Regulates Biliary and Ciliary Morphogenesis and Laterality.

BACKGROUND/AIMS: Infectious and genetic factors are invoked, respectively in isolated biliary atresia (BA), or syndromic BA, with major extrahepatic anomalies. However, isolated BA is also associated with minor extrahepatic gut and cardiovascular anomalies and multiple susceptibility genes, suggesting common origins. METHODS: We investigated novel susceptibility genes with genome-wide association, targeted sequencing and tissue staining in BA requiring liver transplantation, independent of BA subtype. Candidate gene effects on morphogenesis, developmental pathways, and ciliogenesis, which regulates left-right patterning were investigated with zebrafish knockdown and mouse knockout models, mouse airway cell cultures, and liver transcriptome analysis. RESULTS: Single nucleotide polymorphisms in Mannosidase-1-α-2 (MAN1A2) were significantly associated with BA and with other polymorphisms known to affect MAN1A2 expression but were not differentially enriched in either BA subtype. In zebrafish embryos, man1a2 knockdown caused poor biliary network formation, ciliary dysgenesis in Kupffer's vesicle, cardiac and liver heterotaxy, and dysregulated egfra and other developmental genes. Suboptimal man1a2 knockdown synergized with suboptimal EGFR signaling or suboptimal knockdown of the EGFR pathway gene, adenosine-ribosylation-factor-6, which had minimal effects individually, to reproduce biliary defects but not heterotaxy. In cultured mouse airway epithelium, Man1a2 knockdown arrested ciliary development and motility. Man1a2 -/- mice, which experience respiratory failure, also demonstrated portal and bile ductular inflammation. Human BA liver and Man1a2 -/- liver exhibited reduced Man1a2 expression and dysregulated ciliary genes, known to cause multisystem human laterality defects. CONCLUSION: BA requiring transplantation associates with sequence variants in MAN1A2. man1a2 regulates laterality, in addition to hepatobiliary morphogenesis, by regulating ciliogenesis in zebrafish and mice, providing a novel developmental basis for multisystem defects in BA.

Genetic link between renal birth defects and congenital heart disease.

Structural birth defects in the kidney and urinary tract are observed in 0.5% of live births and are a major cause of end-stage renal disease, but their genetic aetiology is not well understood. Here we analyse 135 lines of mice identified in large-scale mouse mutagenesis screen and show that 29% of mutations causing congenital heart disease (CHD) also cause renal anomalies. The renal anomalies included duplex and multiplex kidneys, renal agenesis, hydronephrosis and cystic kidney disease. To assess the clinical relevance of these findings, we examined patients with CHD and observed a 30% co-occurrence of renal anomalies of a similar spectrum. Together, these findings demonstrate a common shared genetic aetiology for CHD and renal anomalies, indicating that CHD patients are at increased risk for complications from renal anomalies. This collection of mutant mouse models provides a resource for further studies to elucidate the developmental link between renal anomalies and CHD.

Respiratory motile cilia dysfunction in a patient with cranioectodermal dysplasia.

Ciliopathies such as cranioectodermal dysplasia, Sensenbrenner syndrome, short-rib polydactyly, and Jeune syndrome are associated with respiratory complications arising from rib cage dysplasia. While such ciliopathies have been demonstrated to involve primary cilia defects, we show motile cilia dysfunction in the airway of a patient diagnosed with cranioectodermal dysplasia. While this patient had mild thoracic dystrophy not requiring surgical treatment, there was nevertheless newborn respiratory distress, restrictive airway disease with possible obstructive airway involvement, repeated respiratory infections, and atelectasis. High-resolution videomicroscopy of nasal epithelial biopsy showed immotile/dyskinetic cilia and nasal nitric oxide was reduced, both of which are characteristics of primary ciliary dyskinesia, a sinopulmonary disease associated with mucociliary clearance defects due to motile cilia dysfunction in the airway. Exome sequencing analysis of this patient identified compound heterozygous mutations in WDR35, but no mutations in any of the 30 known primary ciliary dyskinesia genes or other cilia-related genes. Given that WDR35 is only known to be required for primary cilia function, we carried out WDR35 siRNA knockdown in human respiratory epithelia to assess the role of WDR35 in motile cilia function. This showed WDR35 deficiency disrupted ciliogenesis in the airway, indicating WDR35 is also required for formation of motile cilia. Together, these findings suggest patients with WDR35 mutations have an airway mucociliary clearance defect masked by their restrictive airway disease.

ANKS6 is the critical activator of NEK8 kinase in embryonic situs determination and organ patterning.

The ciliary kinase NEK8 plays a critical role in situs determination and cystic kidney disease, yet its exact function remains unknown. In this study, we identify ANKS6 as a target and activator of NEK8. ANKS6 requires NEK8 for localizing to the ciliary inversin compartment (IC) and activates NEK8 by binding to its kinase domain. Here we demonstrate the functional importance of this interaction through the analysis of two novel mouse mutations, Anks6(Streaker) and Nek8(Roc). Both display heterotaxy, cardiopulmonary malformations and cystic kidneys, a syndrome also characteristic of mutations in Invs and Nphp3, the other known components of the IC. The Anks6(Strkr) mutation decreases ANKS6 interaction with NEK8, precluding NEK8 activation. The Nek8(Roc) mutation inactivates NEK8 kinase function while preserving ANKS6 localization to the IC. Together, these data reveal the crucial role of NEK8 kinase activation within the IC, promoting proper left-right patterning, cardiopulmonary development and renal morphogenesis.