Which cells have cilia and flagella

The first is the PM. Some ciliary membrane proteins — including Agglutinin Hunnicutt et al. For instance, the binding of the ligand Sonic Hedgehog Shh to its receptor Patched 1 Ptc1 triggers Smo accumulation within the ciliary membrane and activation of Shh signaling.

Smo is transported laterally from the PM into the ciliary membrane through the diffusion barrier at the base of the cilium Rohatgi et al. After ciliary membrane proteins reach the PM, some undergo endocytosis followed by polarized exocytosis at the periciliary membrane.

For example, the ciliary localization of the membrane protein Kim1 involves Rab5-dependent endocytosis from the PM via the recycling pathway, followed by Rab8-dependent polarized exocytosis to the base of the cilium Boehlke et al. In many cell types or organisms, such as Trypanosoma and mammalian fibroblasts, the proximal region of the cilium is surrounded by a cytoplasmic invagination of the periciliary membrane, known as the ciliary pocket CiPo.

The CiPo is enriched in clathrin-coated pits, which make it a hotspot for exocytosis and endocytosis of vesicles for the delivery and retrieval of ciliary membrane component Pedersen et al.

BLOC1 is also required for selective membrane protein trafficking from endosomes to primary cilia Monis et al. The second destination for vesicles carrying ciliary membrane proteins is the periciliary membrane, a specialized membrane region distinct from the PM and ciliary membrane. Some ciliary membrane proteins are transported directly from vesicles to the periciliary membrane. In polarized epithelial cells, the periciliary membrane is characterized by enrichment of galectin-3, exclusion of cortical actin cytoskeleton, and high endocytosis and exocytosis Vieira et al.

In , Dennis et al. These results suggest that the proteins located in the TZ are highly conserved between species, but their roles in the formation of diffusion barriers, and how they coordinate with each other to regulate the function of diffusion barriers, remain unclear. A crucial function of the diffusion barrier is distinguishing ciliary membrane from PM and maintaining the specific compositions of proteins and lipids in ciliary membrane.

Studies in mammalian cells indicate that membrane proteins can shuttle between PM and ciliary membrane. The IFT-A complex regulates the transport of ciliary membrane proteins into cilia, whereas the BBSome complex mediates the transport of ciliary membrane proteins out of cilia. When a cilium-related signal is activated, as in Hedgehog signaling, the responding ciliary membrane receptors bind to the IFT-A complex and are transported into the cilia with the help of the membrane protein TULP3.

When a subunit of the BBSome complex is mutated, GPCRs cannot be smoothly transported back to the cell body and are secreted to the outside of the cell along with the ciliary ectosomes.

This is another way in which ciliary membrane proteins can exit cilia Nachury, The function of the diffusion barrier is regulated by other specific proteins. For example, the GTPase Septin 2, a component of the diffusion barrier localized to the junction between ciliary membrane and PM in both mammalian cells and Chlamydomonas , helps regulate the transport of ciliary membrane receptors.

When Septin 2 expression is knocked down, most cells have no cilia, a few have short cilia, and the accumulation of the ciliary membrane protein Smo in cilia is reduced; subsequently, the transduction of the Sonic hedgehog signal is disturbed Hu et al. When the mouse Septin 4 gene is mutated, the diffusion barrier of sperm tail does not form and the localization of flagellar membrane proteins is altered Kwitny et al.

The abundances of certain membrane proteins are greatly reduced in nphp4 mutant cilia, and cellular housekeeping proteins larger than 50 kDa are not excluded from these cilia.

Thus, NPHP4 functions as an essential part of a barrier that regulates both the membrane and soluble protein compositions of cilia Awata et al. The Kharon protein of Trypanosoma localizes at the ciliary base and interacts with calcium channels to regulate their transport to cilia Sanchez et al. Two Rab-effector-related proteins, Rab-interacting lysosomal protein-like 1 Rilpl1 and Rilpl2, regulate protein localization in ciliary membrane.

Depletion of Rilpl1 and Rilpl2 results in the accumulation of signaling proteins such as fibrocystin in the ciliary membrane and prevents proper epithelial cell organization in three-dimensional culture Schaub and Stearns, In summary, when the function of the diffusion barrier is abnormal, several characteristics are observed at the cellular level: 1 cilium growth is inhibited; 2 the ability of cells to sense external signals is reduced or lost; 3 the transport of membrane proteins is abnormal; and 4 the lipid composition of the ciliary membrane changes Hunnicutt et al.

These characteristics lay the foundation for screening mutants that are defective in diffusion barrier formation and regulation through forward-genetic methods. Apart from transport into or out of cilia, the homeostasis of ciliary membrane proteins is also regulated by the secretion of ciliary vesicles Figure 1. These small 50— nm vesicles bud from the ciliary membrane and are secreted to the extracellular environment Wood et al. The secretion function of cilia was first discovered in the model organism Chlamydomonas , in which the ciliary ectosomes carry bioactive enzymes that digest the mother cell wall and release the daughter cells during reproduction Wood et al.

During mating, both ciliary touching and signaling depend on a protein called SAG1. Afterward, SAG1 disappear from cilia without returning to the PM; instead, it is packaged within ciliary ectosomes and then shed from cilia into the external environment. Isolated ciliary ectosomes are capable of activating cilium-generated signaling Cao et al.

Cilia of Chlamydomonas vegetative cells and gametes both undergo constitutive membrane shedding of ciliary ectosomes. Work by Cao et al. Thus, receptor-signaling-induced shedding of ciliary ectosomes is much more rapid in vegetative cells than in normal growing cells. In future studies, it will be interesting to explore the distinct regulatory mechanism controlling these effects. ESCRT proteins mediate ectosome release and thereby influence flagellar shortening and mating Long et al.

Although the ciliary ectosomes are derived from ciliary membrane, their protein composition is different. Ciliary ectosomes are enriched in a subset of ciliary membrane proteins, proteases, proteins from the endosomal sorting complex required for transport, small GTPases, and ubiquitinated proteins Long et al. These results suggest that the secretion of ciliary ectosomes regulates the homeostasis of ciliary membrane proteins and is in turn strictly regulated.

The turnover of ciliary membrane proteins through ciliary ectosome secretion is conserved in many species. The biogenesis of ECVs occurs via budding from the periciliary membrane and not via fusion of multivesicular bodies. ECVs isolated from the cultured plate of wild-type worms function in cellular communication and induced mating-related male tail-chasing behavior Wang et al. Bloodstream African Trypanosomes produce membranous nanotubes that originate from the flagellar membrane and disassociate into free ECVs.

Trypanosome ECVs contain several flagellar membrane proteins, mediate non-hereditary virulence factor transfer, and cause host erythrocyte remodeling that induces anemia Szempruch et al. Actin and the actin regulators Drebrin and Myosin 6 mediate this process. Therefore, signal-dependent ciliary ectosome release is a selective and effective process that removes activated signaling receptors from cilia Nager et al. INPP5E loss in cilia triggers actin polymerization in primary cilia, which excises cilia tips in a process known as decapitation.

Decapitation induces mitogenic signaling and constitutes a molecular link between the cilium life cycle and the cell-division cycle Phua et al. In future studies, it will be useful to expand the investigation of ciliary secretion function. These studies can further elucidate the molecular mechanism and regulation of ciliary ectosome release, the cellular processes and corresponding mechanisms involved in ciliary ectosome secretion, and identify more of the genes involved in regulating the ciliary secretion function.

The sensory function of cilia is mainly dependent on ciliary membrane proteins. In addition to investigating the processes controlling the activity of these proteins in situ , it is important to assess how cells regulate the amounts of membrane proteins localized in cilia. Thus, it is critical to identify factors involved in the trafficking of ciliary membrane proteins to their destination.

Based on current research into the processes that build and maintain the homeostasis of ciliary membrane proteins in cilia, several areas are worthy of further investigation: 1 the coordination of different organelles, such as the PM, ER, and Golgi, during ciliary membrane protein transport; 2 the roles of lipids in sorting, localization, and transport of ciliary membrane proteins; 3 the regulation and complete composition of the diffusion barrier; 4 the molecular mechanism of IFT involved in moving the ciliary membrane proteins in cilia; and 5 the molecular mechanism of ciliary ectosome secretion in regulating the turnover of ciliary membrane proteins.

Studies in these areas will deepen our understanding of the pathology of ciliopathies resulting from defects in ciliary membrane traffic. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Adams, N. The retinal ciliopathies. Ophthalmic Genet. Awata, J.

NPHP4 controls ciliary trafficking of membrane proteins and large soluble proteins at the transition zone. Cell Sci. Axelrod, J. Basal bodies, kinocilia and planar cell polarity. Badgandi, H. Tubby family proteins are adapters for ciliary trafficking of integral membrane proteins.

Cell Biol. Baker, S. The outer segment serves as a default destination for the trafficking of membrane proteins in photoreceptors. Berbari, N. Identification of ciliary localization sequences within the third intracellular loop of G protein-coupled receptors.

Featured Video. Cite this Article Format. Bailey, Regina. Cilia and Flagella. Epithelial Tissue: Function and Cell Types. The Structure and Function of a Cell Wall. Ribosomes - The Protein Builders of a Cell. What Are Prokaryotic Cells? Structure, Function, and Definition. Microtubules, the Structural Foundation of Your Cells.

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Aa Aa Aa. The Primary Cilium. Chlamydomonas and Intraflagellar Transport. Figure 1: Diagram of ciliary structure. The organelle is membrane-bound and contains multiple microtubules running along its length. Cilia as Sensory Receptors. Figure 2: The effect of fluid flow on the uptake of calcium in normal and PKD kidney cells. The primary cilia in normal cells respond to the mechanical signal produced by the flowing liquid to initiate rapid uptake of calcium.

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