The LIM protein Ajuba is required for ciliogenesis and left-right axis determination in medaka

Cilia are microtubule-based organelles that are present on the surfaces of almost all vertebrate cells. Most cilia function as sensory or molecular transport structures. Malfunctions of cilia have been implicated in several diseases of human development. The assembly of cilia is initiated by the centriole (or basal body), and several centrosomal proteins are involved in this process. The mammalian LIM protein Ajuba is a well-studied centrosomal protein that regulates cell division but its role in ciliogenesis is unknown. In this study, we isolated the medaka homolog of Ajuba and showed that Ajuba localizes to basal bodies of cilia in growth-arrested cells. Knockdown of Ajuba resulted in randomized left-right organ asymmetries and altered expression of early genes responsible for left-right body axis determination. At the cellular level, we found that Ajuba function was essential for ciliogenesis in the cells lining Kupffer’s vesicle; it is these cells that induce the asymmetric fluid flow required for left-right axis determination. Taken together, our findings identify a novel role for Ajuba in the regulation of vertebrate ciliogenesis and left-right axis determination.     

Decoding cilia function: defining specialized genes required for compartmentalized cilia biogenesis

The evolution of the ancestral eukaryotic flagellum is an example of a cellular organelle that became dispensable in some modern eukaryotes while remaining an essential motile and sensory apparatus in others. To help define the repertoire of specialized proteins needed for the formation and function of cilia, we used comparative genomics to analyze the genomes of organisms with prototypical cilia, modified cilia, or no cilia and identified approximately 200 genes that are absent in the genomes of nonciliated eukaryotes but are conserved in ciliated organisms. Importantly, over 80% of the known ancestral proteins involved in cilia function are included in this small collection. Using Drosophila as a model system, we then characterized a novel family of proteins (OSEGs: outer segment) essential for ciliogenesis. We show that osegs encode components of a specialized transport pathway unique to the cilia compartment and are related to prototypical intracellular transport proteins.     

Protein transport in growing and steady‐state cilia

Cilia and eukaryotic flagella are threadlike cell extensions with motile and sensory functions. Their assembly requires intraflagellar transport (IFT), a bidirectional motor‐driven transport of protein carriers along the axonemal microtubules. IFT moves ample amounts of structural proteins including tubulin into growing cilia likely explaining its critical role for assembly. IFT continues in non‐growing cilia contributing to a variety of processes ranging from axonemal maintenance and the export of non‐ciliary proteins to cell locomotion and ciliary signaling. Here, we discuss recent data on cues regulating the type, amount and timing of cargo transported by IFT. A regulation of IFT‐cargo interactions is critical to establish, maintain and adjust ciliary length, protein composition and function.     

Motile ciliogenesis and the mitotic prism

Motile cilia of epithelial multiciliated cells transport vital fluids along organ lumens to promote essential respiratory, reproductive and brain functions. Progenitors of multiciliated cells undergo massive and coordinated organelle remodelling during their differentiation for subsequent motile ciliogenesis. Defects in multiciliated cell differentiation lead to severe cilia‐related diseases by perturbing cilia‐based flows. Recent work designated the machinery of mitosis as the orchestrator of the orderly progression of differentiation associated with multiple motile cilia formation. By examining the events leading to motile ciliogenesis with a methodological prism of mitosis, we contextualise and discuss the recent findings to broaden the spectrum of questions related to the differentiation of mammalian multiciliated cells.     

Cilia and polycystic kidney disease,kith and kin

In the past decade, cilia have been found to play important roles in renal cystogenesis. Many genes, such as PKD1 and PKD2 which, when mutated, cause autosomal dominant polycystic kidney disease (ADPKD), have been found to localize to primary cilia. The cilium functions as a sensor to transmit extracellular signals into the cell. Abnormal cilia structure and function are associated with the development of polyscystic kidney disease (PKD). Cilia assembly includes centriole migration to the apical surface of the cell, ciliary vesicle docking and fusion with the cell membrane at the intended site of cilium outgrowth, and microtubule growth from the basal body. This review summarizes the most recent advances in cilia and PKD research, with special emphasis on the mechanisms of cytoplasmic and intraciliary protein transport during ciliogenesis. Birth Defects Research (Part C) 102:174–185, 2014 . © 2014 Wiley Periodicals, Inc .     

Dynein-2 Affects the Regulation of Ciliary Length but Is Not Required for Ciliogenesis in Tetrahymena thermophila

Eukaryotic cilia and flagella are assembled and maintained by the bidirectional intraflagellar transport (IFT). Studies in alga, nematode, and mouse have shown that the heavy chain (Dyh2) and the light intermediate chain (D2LIC) of the cytoplasmic dynein-2 complex are essential for retrograde intraflagellar transport. In these organisms, disruption of either dynein-2 component results in short cilia/flagella with bulbous tips in which excess IFT particles have accumulated. In Tetrahymena, the expression of the DYH2 and D2LIC genes increases during reciliation, consistent with their roles in IFT. However, the targeted elimination of either DYH2 or D2LIC gene resulted in only a mild phenotype. Both knockout cell lines assembled motile cilia, but the cilia were of more variable lengths and less numerous than wild-type controls. Electron microscopy revealed normally shaped cilia with no swelling and no obvious accumulations of material in the distal ciliary tip. These results demonstrate that dynein-2 contributes to the regulation of ciliary length but is not required for ciliogenesis in Tetrahymena.     

IFT25 links the signal-dependent movement of Hedgehog components to intraflagellar transport

The intraflagellar transport (IFT) system is required for building primary cilia, sensory organelles that cells use to respond to their environment. IFT particles are composed of about 20 proteins, and these proteins are highly conserved across ciliated species. IFT25, however, is absent from some ciliated organisms, suggesting that it may have a unique role distinct from ciliogenesis. Here, we generate an Ift25 null mouse and show that IFT25 is not required for ciliary assembly but is required for proper Hedgehog signaling, which in mammals occurs within cilia. Mutant mice die at birth with multiple phenotypes, indicative of Hedgehog signaling dysfunction. Cilia lacking IFT25 have defects in the signal-dependent transport of multiple Hedgehog components including Patched-1, Smoothened, and Gli2, and fail to activate the pathway upon stimulation. Thus, IFT function is not restricted to building cilia where signaling occurs, but also plays a separable role in signal transduction events.