Structure and mass of mammalian respiratory ciliary outer arm 19S dynein

Generation of a three-dimensional ultrastructural model of human respiratory cilia

The ultrastructures of cilia and flagella are highly similar and well conserved through evolution. Consequently, Chlamydomonas is commonly used as a model organism for the study of human respiratory cilia. Since detailed models of Chlamydomonas axonemes were generated using cryoelectron tomography, disparities among some of the ultrastructural features have become apparent when compared with human cilia. Extrapolating information on human disease from the Chlamydomonas model may lead to discrepancies in translational research. This study aimed to establish the first three-dimensional ultrastructural model of human cilia. Tomograms of transverse sections (n = 6) and longitudinal sections (n = 9) of human nasal respiratory cilia were generated from three healthy volunteers. Key features of the cilium were resolved using subatomic averaging, and were measured. For validation of the method, a model of the well characterized structure of Chlamydomonas reinhardtii was simultaneously generated. Data were combined to create a fully quantified three-dimensional reconstruction of human nasal respiratory cilia. We highlight key differences in the axonemal sheath, microtubular doublets, radial spokes, and dynein arms between the two structures. We show a decreased axial periodicity of the radial spokes, inner dynein arms, and central pair protrusions in the human model. We propose that this first human model will provide a basis for research into the function and structure of human respiratory cilia in health and in disease.

Computer-assisted image analysis of human cilia and Chlamydomonas flagella reveals both similarities and differences in axoneme structure

In the past decade, investigations from several different fields have revealed the critical role of cilia in human health and disease. Because of the highly conserved nature of the basic axonemal structure, many different model systems have proven useful for the study of ciliopathies, especially the unicellular, biflagellate green alga Chlamydomonas reinhardtii. Although the basic axonemal structure of cilia and flagella is highly conserved, these organelles often perform specialized functions unique to the cell or tissue in which they are found. These differences in function are likely reflected in differences in structural organization. In this work, we directly compare the structure of isolated axonemes from human cilia and Chlamydomonas flagella to identify similarities and differences that potentially play key roles in determining their functionality. Using transmission electron microscopy and 2D image averaging techniques, our analysis has confirmed the overall structural similarity between these two species, but also revealed clear differences in the structure of the outer dynein arms, the central pair projections, and the radial spokes. We also show how the application of 2D image averaging can clarify the underlying structural defects associated with primary ciliary dyskinesia (PCD). Overall, our results document the remarkable similarity between these two structures separated evolutionarily by over a billion years, while highlighting several significant differences, and demonstrate the potential of 2D image averaging to improve the diagnosis and understanding of PCD.

Force generation and dynamics of individual cilia under external loading

Motile cilia are unique multimotor systems that display coordination and periodicity while imparting forces to biological fluids. They play important roles in normal physiology, and ciliopathies are implicated in a growing number of human diseases. In this work we measure the response of individual human airway cilia to calibrated forces transmitted via spot-labeled magnetic microbeads. Cilia respond to applied forces by 1), a reduction in beat amplitude (up to an 85% reduction by 160-170 pN of force); 2), a decreased tip velocity proportionate to applied force; and 3), no significant change in beat frequency. Tip velocity reduction occurred in each beat direction, independently of the direction of applied force, indicating that the cilium is "driven" in both directions at all times. By applying a quasistatic force model, we deduce that axoneme stiffness is dominated by the rigidity of the microtubules, and that cilia can exert 62 +/- 18 pN of force at the tip via the generation of 5.6 +/- 1.6 pN/dynein head.

Regulation of mammalian ciliary beating

Recent advances in our understanding of the structure-function relationship of motile cilia with the 9 + 2 microtubular arrangement have helped explain some of the mechanisms of ciliary beat regulation by intracellular second messengers. These second messengers include cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) as well as calcium and pH. cAMP activates protein kinase A (PKA), which is localized to the axoneme. The cAMP-dependent phosphorylation of PKA's main target, originally described as p29 in Paramecium, seems to increase ciliary beat frequency (CBF) directly. The mechanism by which cGMP increases CBF is less well defined but involves protein kinase G and possibly PKA. Protein kinase C inhibits ciliary beating. The regulation mechanisms of CBF by calcium remain somewhat controversial, favoring an immediate, direct action of calcium on ciliary beating and a second cyclic nucleotide-dependent phase. Finally, intracellular pH likely affects CBF through direct influences on dynein arms.

Identification of dynein heavy chain 7 as an inner arm component of human cilia that is synthesized but not assembled in a case of primary ciliary dyskinesia

Although the basic structure of the axoneme has been highly conserved throughout evolution, the varied functions of specialized axonemes require differences in structure and regulation. Cilia lining the respiratory tract propel mucus along airway surfaces, providing a critical function to the defense mechanisms of the pulmonary system, yet little is known of their molecular structure. We have identified and cloned a dynein heavy chain that is a component of the inner dynein arm. Bronchial epithelial cells were obtained from normal donors and from a patient with primary ciliary dyskinesia (PCD) whose cilia demonstrated an absence of inner dynein arms by electron microscopy. Cilia from normal and PCD cells were compared by gel electrophoresis, and mass spectrometry was used to identify DNAH7 as a protein absent in PCD cilia. The full-length DNAH7 cDNA was cloned and shares 68% similarity with an inner arm dynein heavy chain from Drosophila. DNAH7 was induced during ciliated cell differentiation, and immunohistochemistry demonstrated the presence of DNAH7 in normal cilia. In cilia from PCD cells, DNAH7 was undetectable, whereas intracellular DNAH7 was clearly present. These studies identify DNAH7 as an inner arm component of human cilia that is synthesized but not assembled in a case of PCD.

Characterization of an axonemal dynein heavy chain expressed early in airway epithelial ciliogenesis

The most conspicuous evidence of airway epithelial maturation and vitality is the presence of motile cilia. In an effort to generate genetic and antigenic markers of airway maturation, injury, and repair, we characterized airway epithelial expression of a gene identified by two human expressed sequence tags that encoded peptides with sequence similarity to an invertebrate ciliary dynein heavy chain (DHC). Molecular analyses showed that the gene has a very large RNA transcript that encodes a very high molecular weight polypeptide with biochemical properties that are characteristic of a dynein heavy chain. Expression of the gene transcript correlated with the presence of ciliated cells in tissues, and immunohistochemical localization of the gene product confirmed its presence in the cilia of mature airway epithelium. In epithelium undergoing ciliogenesis ex vivo, expression of the gene transcript preceded ciliation of the epithelium and the gene product was present in the cytoplasm and at the apical border of nonciliated cells. These data suggested that the gene encodes an axonemal DHC that is expressed early during ciliogenesis, before the appearance of cilia.

Outer arm dynein from Newt lung respiratory cilia: purification and polypeptide composition

Dyneins are multimeric ATPases that comprise the inner and outer arms of cilia and flagella. It previously has been shown that salt extraction of newt lung axonemes selectively removes > 95% of the outer arm dynein (OAD), and that the beat frequency of OAD-depleted axonemes cannot be activated as compared to controls [Hard et al., 1992: Cell Motil. Cytoskeleton 21:199-209]. Therefore, expression of the activated state appears to require the presence of outer dynein arms. The present study was undertaken to ascertain basic information on the structure and molecular composition of newt OAD. Populations of demembranated axonemes were extracted with 0.375 M salt. Each lung released approximately 1.4 x 10(7) axonemes during isolation, yielding approximately 120 ng of salt extractable OAD. Electron microscopy of negatively stained samples revealed that newt OAD consisted of two globular heads joined together by a Y-shaped stem, similar to sea urchin and trout sperm OAD. Each head appeared to be roughly spherical in shape, measuring approximately 17 nm in diameter. Electrophoretic analysis of whole axonemes revealed more than six dynein heavy chains when resolved in silver stained 0-8 M urea, 3-5% acrylamide gradients. Extracted OAD, either crude in high salt or purified by alloaffinity, was composed of two heavy chains. UV-induced (366 nm) photolytic cleavage at the V1 site, performed in the presence of Mg2+, vanadate, and ATP, produced four new polypeptides (M(r) 234, 232, 197, and 189 kD). Photolysis was supported by Mg2+ and Ca2+, but did not occur in the presence of Mn2+. The apparent M(r) of the dynein heavy chains was determined to lie between 430-420 kD. Eight discrete polypeptides (putative intermediate chains, IC1-IC8, M(r), 175-56 kD) copurified with the alpha- and beta-heavy chains by microtubule-alloaffinity. Based on its extraction characteristics, polypeptide composition in purified and crude samples, and structure, we conclude that this two-headed particle represents the entire newt respiratory outer arm dynein.

Axonemes paralyzed by the presence of dyneins unable to use ribose-modified ATP

Substrate analogs are useful for studying the structures of active sites and for distinguishing between similar enzyme activities. Fluorescent ribose-modified ATP analogs were used to investigate the functional differences between dynein ATPases. These analogs reactivate (support the movement of) sea urchin sperm axonemes, yet they do not reactivate wild-type Chlamydomonas axonemes. Surprisingly, the analogs reactivate the axonemes of mutants completely missing the outer arm dyneins. Competition experiments using ATP and these analogs provide strong evidence that the analogs bind to all dynein active sites but fail to release a subset of dyneins from rigor. We suggest that this subset of Chlamydomonas outer arm dyneins unable to use the analogs remains in rigor in the presence of the analogs and paralyzes the axoneme.

Inhibition and recovery of mammalian respiratory ciliary function after formaldehyde exposure

Formaldehyde (HCHO) has been reported to impair mucociliary clearance. The present investigation using rabbit and porcine tracheal explants in vitro examined (1) the impairment of ciliary activity, an essential component of mucociliary clearance; (2) the reversibility of ciliary dysfunction after HCHO exposure; and (3) the mechanism by which ciliary activity is reduced by HCHO. HCHO treatment of rabbit tracheal rings significantly decreased zones of active ciliated epithelium in direct proportion to concentration and exposure duration. There was also a significant concentration-dependent reduction of ciliary beat frequency. Removal of HCHO permitted recovery of zones of ciliary activity to normal beat frequencies; greater inhibitory concentrations of HCHO required greater time for return of function. Treatment of porcine tracheae with increasing concentrations of HCHO for time periods inhibitory to rabbit ciliary activity correspondingly reduced the yield of cilia extractable from treated epithelium. Furthermore, the specific activity of ATPase of extracted ciliary axonemes was diminished with increasing HCHO concentration, indicating loss of function. A recovery period following identical exposures of the porcine tracheae to the lower HCHO concentrations resulted in normal yields of functionally intact ciliary axonemes. Similarly, a recovery period after the highest HCHO concentration produced more functional axonemes than obtained from exposed tracheae without a recovery period, although less than normal yields. Therefore, ciliary dysfunction elicited by a defined range of HCHO concentrations is reversible. The yield and functional integrity of ciliary axonemes from epithelium exposed to HCHO with a recovery period are significantly greater than those without such a recovery period, suggesting an alteration and subsequent repair of epithelial surface components following HCHO exposure.

Outer-arm dynein from trout spermatozoa: substructural organization

Outer-arm dynein purified from trout spermatozoa was disrupted by low-ionic-strength dialysis, and the resulting subunits were separated by sucrose density-gradient centrifugation. The intact 19 S dynein, containing the alpha- an beta-heavy chains, intermediate chains (ICs) 1-5 and light chains (LCs) 1-6, yielded several discrete particles: a 17.5 S adenosine triphosphatase (ATPase) composed of the alpha- and beta-chains ICs 3-5 and LC 1; a 9.5 S complex containing ICs 1 and 2 together with LCs 2, 3, 4, and 6; and a single light chain (LC 5), which sedimented at approximately 4 S. In some experiments, ICs 3-5 also separated from the heavy chain complex and were obtained as a distinct subunit. Further dissociation of the 17.5 S particle yielded a 13.1 S ATPase that contained the beta-heavy chain and ICs 3-5. The polypeptide compositions of the complexes provide new information on the intermolecular associations that occur within dynein. Substructural features of the trout dynein polypeptides also were examined. The heavy chains were subjected to vanadate-mediated photolysis at the V1 sites by irradiation at 365 nm in the presence of Mg2+, ATP, and vanadate. Fragment pairs of relative molecular mass (Mr) 245,000/185,000 and 245,000/170,000 were obtained from the alpha- and beta-heavy chains, respectively. Photolysis of these molecules at their V2 sites, by irradiation in the presence of vanadate and Mn2+, yielded fragments of Mr 160,000/270,000 and 165,000/250,000, respectively. These values confirm that the alpha- and beta-heavy chains have masses of 430,000 and 415,000 daltons, respectively. Immunological analysis using monoclonal antibodies revealed that one intermediate chain from trout dynein (IC 2) contains epitopes present in two different intermediate chains from Chlamydomonas dynein. This indicates that specific sequences within the dynein intermediate chains have been highly conserved throughout evolution.

A squid dynein isoform promotes axoplasmic vesicle translocation

Axoplasmic vesicles that translocate on isolated microtubules in an ATP-dependent manner have an associated ATP-binding polypeptide with a previously estimated relative molecular mass of 292 kD (Gilbert, S. P., and R. D. Sloboda. 1986. J. Cell Biol. 103:947-956). Here, data are presented showing that this polypeptide (designated H1) and another high molecular mass polypeptide (H2) can be isolated in association with axoplasmic vesicles or optic lobe microtubules. The H1 and H2 polypeptides dissociate from microtubules in the presence of MgATP and can be further purified by gel filtration chromatography. The peak fraction thus obtained demonstrates MgATPase activity and promotes the translocation of salt-extracted vesicles (mean = 0.87 microns/s) and latex beads (mean = 0.92 microns/s) along isolated microtubules. The H1 polypeptide binds [alpha 32P]8-azidoATP and is thermosoluble, but the H2 polypeptide does not share these characteristics. In immunofluorescence experiments with dissociated squid axoplasm, affinity-purified H1 antibodies yield a punctate pattern that corresponds to vesicle-like particles, and these antibodies inhibit the bidirectional movement of axoplasmic vesicles. H2 is cleaved by UV irradiation in the presence of MgATP and vanadate to yield vanadate-induced peptides of 240 and 195 kD, yet H1 does not cleave under identical conditions. These experiments also demonstrate that the actual relative molecular mass of the H1 and H2 polypeptides is approximately 435 kD. On sucrose density gradients, H1 and H2 sediment at 19-20 S, and negatively stained samples reveal particles comprised of two globular heads with stems that contact each other and extend to a common base. The results demonstrate that the complex purified is a vesicle-associated ATPase whose characteristics indicate that it is a squid isoform of dynein. Furthermore, the data suggest that this vesicle-associated dynein promotes membranous organelle motility during fast axoplasmic transport.