Loss of the ciliary protein Chibby1 in mice leads to exocrine pancreatic degeneration and pancreatitis.

Primary cilia protrude from the apical surface of many cell types and act as a sensory organelle that regulates diverse biological processes ranging from chemo- and mechanosensation to signaling. Ciliary dysfunction is associated with a wide array of genetic disorders, known as ciliopathies. Polycystic lesions are commonly found in the kidney, liver, and pancreas of ciliopathy patients and mouse models. However, the pathogenesis of the pancreatic phenotype remains poorly understood. Chibby1 (Cby1), a small conserved coiled-coil protein, localizes to the ciliary base and plays a crucial role in ciliogenesis. Here, we report that Cby1-knockout (KO) mice develop severe exocrine pancreatic atrophy with dilated ducts during early postnatal development. A significant reduction in the number and length of cilia was observed in Cby1-KO pancreta. In the adult Cby1-KO pancreas, inflammatory cell infiltration and fibrosis were noticeable. Intriguingly, Cby1-KO acinar cells showed an accumulation of zymogen granules (ZGs) with altered polarity. Moreover, isolated acini from Cby1-KO pancreas exhibited defective ZG secretion in vitro. Collectively, our results suggest that, upon loss of Cby1, concomitant with ciliary defects, acinar cells accumulate ZGs due to defective exocytosis, leading to cell death and progressive exocrine pancreatic degeneration after birth.

Chibby promotes ciliary vesicle formation and basal body docking during airway cell differentiation.

Airway multiciliated epithelial cells play crucial roles in the mucosal defense system, but their differentiation process remains poorly understood. Mice lacking the basal body component Chibby (Cby) exhibit impaired mucociliary transport caused by defective ciliogenesis, resulting in chronic airway infection. In this paper, using primary cultures of mouse tracheal epithelial cells, we show that Cby facilitates basal body docking to the apical cell membrane through proper formation of ciliary vesicles at the distal appendage during the early stages of ciliogenesis. Cby is recruited to the distal appendages of centrioles via physical interaction with the distal appendage protein CEP164. Cby then associates with the membrane trafficking machinery component Rabin8, a guanine nucleotide exchange factor for the small guanosine triphosphatase Rab8, to promote recruitment of Rab8 and efficient assembly of ciliary vesicles. Thus, our study identifies Cby as a key regulator of ciliary vesicle formation and basal body docking during the differentiation of airway ciliated cells.

Cby1 promotes Ahi1 recruitment to a ring-shaped domain at the centriole-cilium interface and facilitates proper cilium formation and function.

Defects in centrosome and cilium function are associated with phenotypically related syndromes called ciliopathies. Cby1, the mammalian orthologue of the Drosophila Chibby protein, localizes to mature centrioles, is important for ciliogenesis in multiciliated airway epithelia in mice, and antagonizes canonical Wnt signaling via direct regulation of ß-catenin. We report that deletion of the mouse Cby1 gene results in cystic kidneys, a phenotype common to ciliopathies, and that Cby1 facilitates the formation of primary cilia and ciliary recruitment of the Joubert syndrome protein Arl13b. Localization of Cby1 to the distal end of mature centrioles depends on the centriole protein Ofd1. Superresolution microscopy using both three-dimensional SIM and STED reveals that Cby1 localizes to an ∼250-nm ring at the distal end of the mature centriole, in close proximity to Ofd1 and Ahi1, a component of the transition zone between centriole and cilium. The amount of centriole-localized Ahi1, but not Ofd1, is reduced in Cby1(-/-) cells. This suggests that Cby1 is required for efficient recruitment of Ahi1, providing a possible molecular mechanism for the ciliogenesis defect in Cby1(-/-) cells.

Generation and characterization of monoclonal antibodies against human Chibby protein.

Chibby (Cby) was originally identified as an antagonist of the Wnt/ß-catenin signaling pathway. It physically interacts with the key co-activator ß-catenin and inhibits ß-catenin-mediated transcriptional activation. More recently, we demonstrated that Cby protein localizes to the base of motile cilia and is required for ciliogenesis in the respiratory epithelium of mice. To gain further insight into the physiological function of Cby, we developed mouse monoclonal antibodies (MAbs) against human Cby protein and characterized two Cby MAbs, designated 8-2 and 27-11, in depth. Western blot analysis revealed that 8-2 reacts with both human and mouse Cby proteins, whereas 27-11 is specific to human Cby. The epitopes of 8-2 and 27-11 were narrowed down to the middle portion (aa 49-63) and N-terminal region (aa 1-31) of the protein, respectively. We also determined their isotypes and found that 8-2 and 27-11 belong to IgG2a and IgG1 with κ light chains, respectively. Both MAbs can be employed for immunoprecipitation assays. Moreover, 8-2 detects endogenous Cby protein on Western blots, and marks the ciliary base of motile cilia in the murine lung and trachea as shown by immunofluorescence staining. These Cby MAbs therefore hold promise as useful tools for the investigation of Wnt signaling and ciliogenesis.

Altered lung morphogenesis, epithelial cell differentiation and mechanics in mice deficient in the Wnt/ß-catenin antagonist Chibby.

The canonical Wnt/ß-catenin pathway plays crucial roles in various aspects of lung morphogenesis and regeneration/repair. Here, we examined the lung phenotype and function in mice lacking the Wnt/ß-catenin antagonist Chibby (Cby). In support of its inhibitory role in canonical Wnt signaling, expression of ß-catenin target genes is elevated in the Cby(-/-) lung. Notably, Cby protein is prominently associated with the centrosome/basal body microtubule structures in embryonic lung epithelial progenitor cells, and later enriches as discrete foci at the base of motile cilia in airway ciliated cells. At birth, Cby(-/-) lungs are grossly normal but spontaneously develop alveolar airspace enlargement with reduced proliferation and abnormal differentiation of lung epithelial cells, resulting in altered pulmonary function. Consistent with the Cby expression pattern, airway ciliated cells exhibit a marked paucity of motile cilia with apparent failure of basal body docking. Moreover, we demonstrate that Cby is a direct downstream target for the master ciliogenesis transcription factor Foxj1. Collectively, our results demonstrate that Cby facilitates proper postnatal lung development and function.