Cilia microtubules

Figure 1. Cross-sectional view of an idealized cilium. For clarity only three outer doublet pairs of microtubules are shown. (This diagram is constructed from data previously published for example, see Alberts et al., 1988.)...

Dynein sidearms interact with the walls of B-microtubules of adjacent doublets by means of a sliding-filament mechanism to produce ciliary movement. The process is energized by ATP hydrolysis. Movement of the cilium occurs in two stages, termed the power stroke and the recovery stroke. 

An isolated flagellum will continue to bend actively, indicating that this function is linked to its intrinsic structure. Treatment of cilia from the protozoan Tetra-hymena with the proteolytic enzyme trypsin selectively dissolves the nexin links and radial spokes but leaves unaffected the microtubules and dynein arms. If such a preparation is treated with a small amount of ATP, the loosened microtubule doublets slide against each other and through longitudinal overlap, extend for a distance that is up to nine times the original length of the cilium (Warner and Mitchell, 1981). 

Figure 19-23 (A) Diagram of a cross-sectional view of the outer portion of a lamellibranch gill cilium. This has the 9+2 axoneme structure as shown in Fig. 1-8 and in (B). The viewing direction is from base to tip. From M. A. Sleigh.329 (B, C) Thin-section electron micrographs of transverse (B) and longitudinal (C) sections of wild-type Chlamydomonas axonemes. In transverse section labels A and B mark A and B subtubules of microtubule doublets oa, ia, outer and inner dynein arms, respectively sp, spokes cpp, central pair projections bk, beaks. From Smith and Sale.329a...

A typical cross section of a cilium shows a ring formed by nine pairs of microtubules and two central tubules, i.e., the so-called nine + two pattern. Each doublet contains an A and a B subfibril with an inner and an outer dynein arm (a complex protein with ATPase activity) located on the A subfibril with radial spokes extending toward the central doublet. The ciliary membrane, which is an extension from the cell membrane of the epithelial cell, encloses the microtubules. The motion of the cilia is dependent on the sliding of the outer doublets past one another with the energy provided by adenosine triphosphate (ATP) through dynein ATPase activity. 

In photographs of cilia taken by an electron microscope, several different types of connectors can be seen tying together the individual microtubules (see Figure 3-2). There is a protein that bridges the two central single microtubules in the middle of the cilium. Also, from each of the double microtubules, a radial spoke projects toward the center of the cilium. The structure ends in a knobby mass called the spoke head. Finally, a protein called nexin connects each outer double microtubule to the one beside it. 

Now, let us sit back, review the workings of the cilium, and consider what they imply. What components are needed for a cilium to work Ciliary motion certainly requires microtubules otherwise, there would be no strands to slide. Additionally it requires a motor, or else the mi-... 

The function of the cilium is to be a motorized paddle. In order to achieve this function microtubules, nexin linkers, and motor proteins all have to be ordered in a precise fashion. They have to recognize each other intimately, and interact exactly. The function is not present if any of the components is missing. Furthermore, many more factors besides those listed are required to make the system useful for a living cell the cilium has to be positioned in the right place, oriented correctly, and turned on or off according to the needs of the cell. 

In a more technical vein, Miller excitedly announced that some components of IC biochemical systems I discuss have other roles in the cell, such as the ciliary proteins tubulin and dynein.11 But I myself pointed that out when I first wrote Darwin s Black Box ten years ago For example, in Chapter 3 I wrote that microtubules occur in many cells and are usually used as mere structural supports, like girders, to prop up cell shape.12 Furthermore, motor proteins also are involved in other cell functions, such as transporting cargo from one end of the cell to another. Nonetheless, I emphasized that such other roles don t help with the irreducible complexity of the cilium an evolutionary story for the cilium must envision a circuitous route, perhaps adapting parts that were originally used for other purposes to build a cilium. And I went on to show why indirect routes are quite implausible.13 Toothpicks do not... 

There are also other connectors in this system. For example, the contacts the dynein arm makes with the microtubule also serve as a connector. As mentioned previously, a system can be more complex than the simplest system imaginable, and the cilium is an example of such a system. 

The ability of the microtubules to slide past each other, propelled by the ATP-dependent mechano-chemical cycling of the dynein arms, brings about ciliary motility. Since the microtubules are constrained at the ciliary tip, it is possible to imagine how the sliding of microtubules on one side of the cilium might cause the cilium to bend. How such sliding is translated into a full beat cycle is still the subject of extensive research. 

Figure 9.6 Diagram showing the infrastructure of an airway cilium. CM, central microtubules NX, nexin link RS, radial spoke SH, spoke head CP, projections of the central microtubules, (redrawn from Sanderson, M.J. (1997) Mechanisms controlling airway ciliary activity. In Rogers, D.F. andLethem, M.I. (eds.) Airway Mucus Basic Mechanisms and Clinical Perspectives. Birkhauser Verlag, Basel).

Fig. S-36 Representations of ccntrioles. basal bodies and axonemes within a cilium. la) Ccntriole/basal body showing the nine sets of triplet microtubules, (h) Cross-section of a cilium/flagellum showing the 9 + 2" array of microtubules and accessory proteins of the axoneme. (c) View of a cell indicating the organizing role of ccntrioles and basal bodies.

Figure I. Structure of a ciliated photoreceptor cell in vertebrates. (A) Scheme of a rod photoreceptor cell. (B) Transmission electronmicroscopy image of a part of a mouse rod photoreceptor cell. The apical extensions of cells of the retinal pigment epithelium (RPE) evolve the tips of photoreceptors light-sensitive outer segments (OS). The OS is linked via a connecting cilium (CC) to an inner segment (IS) which bears the basal body complex (BB) in its apical region. Synaptic terminals (S) link the photoreceptor cell and the 2nd-order neurons, bipolar and horizontal cells. N = nucleus in B, arrow point to axonemal microtubules projecting into the OS. Bar in B = 0.2 p,m...

Figure 2. Schematic representations of a prototypic cilium and the photoreceptor cilium in comparison. (A) Scheme of a prototypic cilium, in longitudinal extension and cross sections through subciliary compartments axoneme (9x2 + 2 microtubule arrangement), transition zone (9x2 + 0 microtubule arrangement) and centriole (9x3 + 0 microtubule arrangement) of the basal body. (B) Scheme of the ciliary part of a rod photoreceptor cell. Axonemal microtubules (MX) project into the outer segment (OS). The OS is linked via the connecting cilium (CC) to the inner segment (IS). The CC corresponds to the transition zone of a prototypic cilium. The basal body complex (BB) is localized in the apical region of the IS. The calycal process (CP) of the IS is linked by extracellular fibers with the membrane of the CC.

Figure 3. Model for centrin-G-protein complex assembly in die connecting cilium of photoreceptor cell. Schematic representations of a part of the inner lumen of the photoreceptor connecting cilium. Centrins ( centrin isoforms 1 and 2) (Cen) are physically linked to the inner surface of the microtubule of the connecting cilium(CC). (A) Scenario at high free Ca " concentrations in CC Cen are specifically dephosphorylated by protein phosphatase PP2Cf. Ca "-binding to Cen induces Cen oligomerization and increases affinity of the Gtf -subunit of the visual heterotrimeric G-protein transducin (Gta-GtPy). This may result in trapping G-protein molecules in the connecting cilium and G-protein diffiision is inhibited (Barrier hypothesis, Wolfrum et al. 2002). (B) Scenario at low free Ca " concentrations in CC Cen are specifically phosphorylated by protein kinase CK2. Cen-P decreases affinity of G-protein to Cen. Arrow indicates that free difihision of G-protein is possible, (for references, please see text)...

The outer segment of vertebrate photoreceptor cells can be considered as the highly modified distal part of an immotile cilium (Rohlich 1975 Besharse and Horst 1990) and contains only a small number of axonemal microtubules (see Introduction). The RPl protein is specifically associated with these microtubules and is required for... 

Horst, C.J., Forestner, D.M. and Besharse, J.C. (1987) Cytoskeletal-membrane interactions Between cell surface glycoconjugates and doublet microtubules of the photoreceptor connecting cilium. J. Cell Biof 705,2973-2987. 

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