Abstract
Cilia and flagella are highly conserved and important microtubule-based organelles that project from the surface of eukaryotic cells and act as antennae to sense extracellular signals. Moreover, cilia have emerged as key players in numerous physiological, developmental, and sensory processes such as hearing, olfaction, and photoreception. Genetic defects in ciliary proteins responsible for cilia formation, maintenance, or function underlie a wide array of human diseases like deafness, anosmia, and retinal degeneration in sensory systems. Impairment of more than one sensory organ results in numerous syndromic ciliary disorders like the autosomal recessive genetic diseases Bardet-Biedl and Usher syndrome. Here we describe the structure and distinct functional roles of cilia in sensory organs like the inner ear, the olfactory epithelium, and the retina of the mouse. The spectrum of ciliary function in fundamental cellular processes highlights the importance of elucidating ciliopathy-related proteins in order to find novel potential therapies.
Highlights
Cilia and flagella are highly conserved microtubule-based organelles consisting of nine peripheral doublet microtubules (9 × 2) that project from the surface of eukaryotic cells
Cilia have received increasing attention as they are involved in multiple cellular functions and adopt crucial roles in vertebrate development
Ciliary malfunctions can result in human diseases called ciliopathies [3,10] (Table 1)
Summary
Cilia and flagella are highly conserved microtubule-based organelles consisting of nine peripheral doublet microtubules (9 × 2) that project from the surface of eukaryotic cells. Anterograde IFT from the base to the tip of the cilium depends on IFT B proteins and the microtubule plus-end-directed kinesin II motor protein family. Retrograde IFT from the tip to the base of the cilium depends on IFT A proteins and the minus-end-directed cytoplasmic dynein 2 motor protein. In most non-dividing cells the centrioles of the centrosome migrate to the cell surface, where the mother centriole forms a basal body which anchors the nine peripheral doublet microtubules and organizes formation of the axoneme (Figure 1F). Anterograde trafficking from the base to the tip of the cilium (minus to plus end) depends on microtubule plus-end-directed kinesin II motor proteins associated with IFT B protein complexes. The corresponding organs can be affected by mutations in ciliary proteins These ciliopathies are caused by deficient formation and dysfunction of cilia leading to sensory impairments. Impairment of more than one sensory organ results, for instance, in Bardet-Biedl [28,29] or Usher syndrome [30,31], two autosomal recessive genetic diseases (Table 1)
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