Functional modulation of IFT kinesins extends the sensory repertoire of ciliated neurons in Caenorhabditis elegans

Cilia: tuning in to the cell's antenna

Cilia are microtubule-based organelles that project like antennae from the surface of most cells in the body. Motile cilia move fluid past cells, for example mucus in the airway. Non-motile primary cilia, however, transduce a multitude of sensory stimuli, including chemical concentrations of growth factors, hormones, odorants, and developmental morphogens, as well as osmolarity, light intensity, and fluid flow. Cilia have evolved a complex ultrastructure to accommodate these diverse functions, and an extensive molecular machinery has developed to support the assembly of these organelles. Defects in the cilia themselves, or the machinery required to assemble them, lead to a broad spectrum of human disease symptoms, including polycystic kidney disease, nephronophthisis, hydrocephalus, polydactyly, situs inversus, retinal degeneration, and obesity. While these diseases highlight the pivotal roles of cilia in physiology and development, the mechanistic link between cilia, physiology, and disease remains unclear.

The roles of cilia in developmental disorders and disease

Cilia are highly conserved organelles that have diverse motility and sensory functions. Recent discoveries have revealed that cilia also have crucial roles in cell signaling pathways and in maintaining cellular homeostasis. As such, defects in cilia formation or function have profound effects on the development of body pattern and the physiology of multiple organ systems. By categorizing syndromes that are due to cilia dysfunction in humans and from studies in vertebrate model organisms, molecular pathways that intersect with cilia formation and function have come to light. Here, we summarize an emerging view that in order to understand some complex developmental pathways and disease etiologies, one must consider the molecular functions performed by cilia.

Caenorhabditis elegans DYF-2, an orthologue of human WDR19, is a component of the intraflagellar transport machinery in sensory cilia

The intraflagellar transport (IFT) machinery required to build functional cilia consists of a multisubunit complex whose molecular composition, organization, and function are poorly understood. Here, we describe a novel tryptophan-aspartic acid (WD) repeat (WDR) containing IFT protein from Caenorhabditis elegans, DYF-2, that plays a critical role in maintaining the structural and functional integrity of the IFT machinery. We determined the identity of the dyf-2 gene by transgenic rescue of mutant phenotypes and by sequencing of mutant alleles. Loss of DYF-2 function selectively affects the assembly and motility of different IFT components and leads to defects in cilia structure and chemosensation in the nematode. Based on these observations, and the analysis of DYF-2 movement in a Bardet-Biedl syndrome mutant with partially disrupted IFT particles, we conclude that DYF-2 can associate with IFT particle complex B. At the same time, mutations in dyf-2 can interfere with the function of complex A components, suggesting an important role of this protein in the assembly of the IFT particle as a whole. Importantly, the mouse orthologue of DYF-2, WDR19, also localizes to cilia, pointing to an important evolutionarily conserved role for this WDR protein in cilia development and function.

General and cell-type specific mechanisms target TRPP2/PKD-2 to cilia

Ciliary localization of the transient receptor potential polycystin 2 channel (TRPP2/PKD-2) is evolutionarily conserved, but how TRPP2 is targeted to cilia is not known. In this study, we characterize the motility and localization of PKD-2, a TRPP2 homolog, in C. elegans sensory neurons. We demonstrate that GFP-tagged PKD-2 moves bidirectionally in the dendritic compartment. Furthermore, we show a requirement for different molecules in regulating the ciliary localization of PKD-2. PKD-2 is directed to moving dendritic particles by the UNC-101/adaptor protein 1 (AP-1) complex. When expressed in non-native neurons, PKD-2 remains in cell bodies and is not observed in dendrites or cilia, indicating that cell-type specific factors are required for directing PKD-2 to the dendrite. PKD-2 stabilization in cilia and cell bodies requires LOV-1, a functional partner and a TRPP1 homolog. In lov-1 mutants, PKD-2 is greatly reduced in cilia and forms abnormal aggregates in neuronal cell bodies. Intraflagellar transport (IFT) is not essential for PKD-2 dendritic motility or access to the cilium, but may regulate PKD-2 ciliary abundance. We propose that both general and cell-type-specific factors govern TRPP2/PKD-2 subcellular distribution by forming at least two steps involving somatodendritic and ciliary sorting decisions.

The emerging complexity of the vertebrate cilium: new functional roles for an ancient organelle

Cilia and flagella are found on the surface of a strikingly diverse range of cell types. These intriguing organelles, with their unique and highly adapted protein transport machinery, have been studied extensively in the context of cellular locomotion, sexual reproduction, or fluid propulsion. However, recent studies are beginning to show that in vertebrates particularly, cilia have been recruited to perform additional developmental and homeostatic roles. Here, we review advances in deciphering the functional components of cilia, and we explore emerging trends that implicate ciliary proteins in signal transduction and morphogenetic pathways.

Intraflagellar transport and cilium-based signaling

Cilia are specialized structures that not only play diverse roles in cell motility but also transmit signals to the cytoplasm and nucleus to control gene expression, cell function, animal development, and behavior. Cilia are assembled and maintained by the intraflagellar transport (IFT) machinery, which coordinates rapid, bidirectional transport between the cell body and the distal tip of the cilium. A new study (Wang et al., 2006) illuminates the role of IFT in cilium-based signaling during mating in the alga Chlamydomonas.

Ciliary targeting of olfactory CNG channels requires the CNGB1b subunit and the kinesin-2 motor protein, KIF17

Nonmotile cilia on olfactory sensory neurons (OSNs) compartmentalize signaling molecules, including odorant receptors and cyclic nucleotide-gated (CNG) channels, allowing for efficient, spatially confined responses to sensory stimuli . Little is known about the mechanisms of the ciliary targeting of olfactory CNG channels, composed of three subunits: CNGA2, CNGA4, and CNGB1b . Recent reports suggest that subunit composition of the retinal CNG channel influences localization, leading to disease . However, the mechanistic role of subunits in properly targeting native olfactory CNG channels remains unclear. Here, we show that heteromeric assembly with CNGB1b, containing a critical carboxy-terminal motif (RVxP), is required for ciliary trafficking of olfactory CNG channels. Movement of proteins within the cilia is governed by intraflagellar transport (IFT), a process that facilitates bidirectional movement of cargo along microtubules. Work in C. elegans has established that heterotrimeric and homodimeric kinesin-2 family members play a critical role in anterograde transport . In mammalian systems, the heterotrimeric KIF3a/KIF3b/KAP-3 complex plays a clear role in IFT; however, no role has been established for KIF17, the mammalian homolog of OSM-3 . Here, we demonstrate that KIF17 is required for olfactory CNG channel targeting, providing novel insights into mechanisms of mammalian ciliary transport.