The basic mechanism of ciliary motility is now reasonably well understood, although many details of how waveform is generated and propagated remain obscure. While much of the information about this mechanism was derived from protistan and invertebrate organisms (Satir 1985
), the basic principles clearly apply to mammalian cilia of the respiratory and reproductive tract, and to brain ependymal cilia. An electron micrograph of cilia of the mouse oviduct is shown in Fig. .
Fig. 1 Classic transmission electron micrograph of mouse oviduct cilia. Cross-sections show the 9 + 2 axoneme of motile cilia (asterisk). The axoneme grows from a basal body, with a basal foot (arrowhead) pointing in the direction of the effective (more ...)
Ciliary motility is caused by the relative sliding of the nine outer axonemal doublets, operating as two opposing sets, one (doublets 1–4) to produce the effective stroke (or principal bend in flagella), one (doublets 6–9) to produce the recovery stroke (or reverse bend), powered by ATP and a set of molecular motors, the axonemal dyneins. The axonemal dyneins are arranged in two sets of arms: the outer dynein arms (ODAs) consisting of two heavy-chain dynein isoforms in mammalian cilia, packaged with intermediate and light chains, with four identical ODAs, each aligned along 96-nm long doublet microtubule (Nicastro et al. 2006
), and the more complex inner dynein arms (IDAs). The IDAs, more centrally located, consist of at least seven heterodimeric and monomeric heavy-chain dynein isoforms per 96 nm. Generally, the ODAs and IDAs function together, but the ODAs principally regulate beat frequency, in part by cAMP-dependent phosphorylation of an ODA regulatory light chain, while the IDAs control beat form. Both frequency and form change by changing the rate of sliding and the switching of activity between the doublet sets. Sliding is converted into propagated bending, most efficiently by control of IDA activity via phosphorylation and mechanical interaction involving the radial spoke, central pair complex.
In effect, the 9 + 2 axoneme is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines. The enzymes that control axonemal response, for example protein kinase A, are structurally part of the complexes. Strong support for this mechanism of ciliary motility comes from studies of paralyzed and other mutants of protistan cilia, where a structural defect and a corresponding genetic mutation can directly be correlated with a change in ciliary beat. Similarly, strong support for the applicability of the mechanism to human cilia comes from studies, mainly of ciliary structural defects and genetic mutations associated with impaired mucociliary function in the respiratory system, which produces sinusitis and bronchiectasis. The genetic diseases are now called primary ciliary dyskinesia (PCD). Clinical features of PCD, which include chronic rhinitis/sinusitis, otitis media, male infertility and an increased incidence of hydrocephalus, point to physiological processes where ciliary motility is essential (Afzelius 2004
). As might be anticipated, prominent causes of PCD are mutations that affect the assembly or function of the dynein arms—most commonly a mutation in DNAH5
, a gene encoding a dynein heavy chain of the ODAs (Olbrich et al. 2002
Any mutation affecting the ciliary beat mechanism will potentially produce PCD, depending on the severity of beat impairment. An instructive example is hydin, a component of the central pair projection complex that interacts with the radial spokes in the switching mechanism between doublet sets. Hydin-deficient flagella of Chlamydomonas
become intermittently paralyzed, stopping for long times at the end of the effective or recovery strokes, where the direction of beat is reversed (Lechtreck and Witman 2007
). Mice defective in Hydin
develop hydrocephalus and die shortly after birth. Lechtreck et al. (2008
) analyzed ciliary structure and motility in these mice. The central pair projection assigned to the hydin complex was missing. Mutant cilia were unable to bend properly and frequently stalled, implying that because central pair structure was defective, interactions between the central pair and the radial spokes that affected dynein arm switching were abnormal, and consequently cilia-generated flow was impaired. Motility of tracheal cilia was also impaired. One would predict that human mutations in HYDIN
are also likely to result in hydrocephalus or a form of PCD.
Many people diagnosed with PCD have situs inversus totalis that is reversal of left–right asymmetry of body organs such as the heart. This is now known to be produced, because fluid flow produced by motile cilia at the embryonic node is defective (Hirokawa et al. 2006
). The usual flow pattern produced by the cilia is unidirectional right-to-left, and this activates a left-side-only gene cascade, probably by impinging on primary cilia at the left side of the node (McGrath et al. 2003
). The nodal cilia are mainly missing the central pair of axonemal microtubules, hence, although motile, are often 9 + 0, but with some unique ODA dynein isoforms (Supp et al. 1999
). It is not known whether the response of the primary cilia at the node is to mechanical displacement (flow sensing) or if it involves morphogens produced in membrane parcels in the node (Tanaka et al. 2005
). Morphogen-containing multimembrane vesicles are a feature of nodal response that could also apply respiratory and other ciliated epithelia, where flow occurs.
While mouse mutants or knockouts that are missing cilia develop situs inversus, hydin mutants do not show this phenotype, presumably because their motility is sufficient to generate appropriate nodal flow. In a similar way, by examining the PCD phenotype of mutations in various ciliary proteins, identified by proteomics, it should be possible to dissect details of the function of the protein in the mechanism of beat generation (Avidor-Reiss et al. 2004
; Badano et al. 2006
; Blacque et al. 2005