On metachronism in ciliary systems: a model describing the dependence of the metachronal wave properties on the intrinsic ciliary parameters

A mathematical model is proposed to explain the dependence of the direction and the length of the metachronal wave on parameters that characterize the ciliary beat, the dimensions of the cilia, and the geometry of their arrangement on the ciliated surface. The metachronal wave is decomposed into two mutually perpendicular components, which are chosen in such a way that the direction of one of them is in the direction of the effective stroke. The magnitudes of the two components are determined by using the concept of the time of delay between adjacent cilia. The properties of the metachronal wave are then calculated as a function of the ciliary parameters. The results obtained with the present model predict that the direction of the wave propagation is strongly dependent on the type of metachronism in the direction of the effective stoke and the polarization in time and in space of the ciliary beat. The metachronal wavelength is found to depend on four parameters: the ciliary length, the angle of the arc projected on the cell surface by the ciliary tip during the recovery stroke, the degree of asymmetry of ciliary beat, and the portion of the cycle occupied by the pause. The metachronal wavelength is also found to be only weakly dependent on the ciliary frequency. At this stage there exists relatively little experimental information with which to characterize fully the metachronal properties of ciliary systems. Even when only partial information exists, the model allows prediction, to within a certain range, of the direction of the wave propagation. It also suggests a possible mechanism for the influence of changes in environmental conditions on wave direction and wavelength. In several cases in which full information does exist, good agreement between the experimental findings and the predictions of the model is found. According to this model it will be worthwhile to invest more effort in measuring the time and space polarization of ciliary beating and times of delay between cilia.     

Ciliary activity under normal conditions and under viscous load

Ciliary metachronism and motility were examined optically in muco-ciliary tissue cultures from three different systems: a) frog's palate epithelium, b) frog's oesophagus, and c) human nasal polyps. In addition, lateral cilia of Mytilus edulis (water transporting cilia) were examined. It was revealed that the degree of synchronization between muco-ciliary systems is lower than that of water transporting cilia. There are no significant differences between different muco-ciliary systems, within the accuracy of our measurement although relatively large statistical ensembles were used. In addition the wavelength and wave direction of the metachronal wave was examined. All four systems exhibit similar wavelength. The metachronal parameters of muco-ciliary systems exhibit fluctuations (as was demonstrated by the degree of synchronization), however, the magnitude and repetitivity of these fluctuations, is dependent on the loading of the ciliary system. We have loaded the system by increasing the viscosity of the medium. Under viscous load the frequency of the beating decreased. The metachronal wavelength became longer and the metachronal coordination type more orthoplectic.     

Synchronization between beating cilia.

A novel quantitative parameter is proposed to define and measure the degree of synchronization between two small ciliary areas. These areas can be close to or far from one another. The Pearson correlation factor is used to define the degree of synchronization by a single number. This approach is based on a computerized, dual photoelectric method which simulataneously measures the scattered light from two small areas on the ciliary epithelium or its tissue culture. The measurements were performed on tissue culture from frog's palate epithelium. It was found that: (a) the degree of synchronization decreases, as a function of distance; (b) the correlation is fairly high even at relatively large separations, when measured on the same patch; (c) on a given patch the synchronization factor is independent of the direction of the metachronal wave; (d) close disconnected ciliary cells exhibit fairly high correlation; (e) disconnected randomly choosen ciliary cells at relatively large separation distances exhibit relatively low correlation, smaller by a factor of 2 than the correlation factor at the same distances when measured along the metachronal wave; (f) the average frequencies' ratio and the metachronal wavelength can be used as first-order indicators of ciliary synchronization; (g) there is a spread of metachronal wavelengths even over a single well-organized patch.     

Extracellular ATP induces hyperpolarization and motility stimulation of ciliary cells.

Cellular membrane potential and ciliary motility were examined in tissues cultures prepared from frog palate and esophagus epithelia. Addition of micromolar concentrations of extracellular ATP caused membrane hyperpolarization and enhanced the beat frequency. These two effects of ATP were 1) dose dependent, reaching a maximum at 10 microM ATP; 2) dependent on the presence of extracellular Ca2+ or Mg2+; 3) insensitive to inhibitors of voltage-gated calcium channels; 4) abolished after depleting the intracellular Ca2+ stores with thapsigargin; 5) attenuated by quinidine (1 mM), Cs+ (5-20 mM), and replacement of extracellular Na+ by K+; 6) insensitive to charybdotoxin (5-20 nM), TEA (1-20 microM), and apamin (0.1-1 microM); 7) independent of initial membrane potential; and 8) unaffected by amiloride. In addition, extracellular ATP induced an appreciable rise in intracellular Ca2+. Addition of thapsigargin caused an initial enhancement of the ciliary beat frequency and membrane hyperpolarization. These results strongly suggest the involvement of calcium-dependent potassium channels in the response to ATP. The results show that moderate hyperpolarization is closely associated with a sustained enhancement of ciliary beating by extracellular ATP.     

Extracellular ATP mobilizes intracellular Ca2+ in T51B rat liver epithelial cells: a study involving single cell measurements

T51B rat liver epithelial cells were stimulated with extracellular ATP. Changes in cytoplasmic free Ca2+ concentration [( Ca2+]i) were measured by fura-2 both in a large population of cells on coverslips in a cuvette and in single cells in a microscopic system. Extracellular ATP evoked a prompt increase in [Ca2+]i in both the presence and absence of extracellular Ca2+, although the effect was less pronounced in the latter case. These findings indicate that at least part of the [Ca2+]i increase is due to mobilization of intracellularly bound calcium. Stimulation with ATP did not mobilize the total pool of intracellular releasable Ca2+, as evidenced from experiments where subsequent addition of ionomycin evoked a pronounced increase in [Ca2+]i in the absence of extracellular Ca2+. The effect of ATP was maintained at room temperature but was markedly impaired in the absence of continuous stirring of the buffer solution. In the absence of stirring, ATP had to be increased to the millimolar range in order to evoke a pronounced effect. Single cell measurements revealed a heterogenous Ca2+ response to ATP, with some cells failing to respond with a detectable increase in [Ca2+]i. The actual increase in [Ca2+]i was not uniform throughout the cytoplasm, but seemed to start in one part of the cell. Even if part of the [Ca2+]i increase might be accounted for by ATP promoting the hydrolysis of phosphatidylinositol 4,5-bisphosphate and thereby a generation of InsP3 and diacylglycerol, there was no initiation of DNA synthesis under the present conditions. Hence, extracellular growth factors exert either a quantitative difference in second messenger production or additional stimulatory effects by activating intracellular signal pathways beyond these represented by [Ca2+]i and protein kinase C.     

Possible mechanism of ciliary stimulation by extracellular ATP: Involvement of calcium-dependent potassium channels and exogenous Ca2+

Summary Ciliary motility was examined optically in tissue cultures from frog palate epithelium and frog's esophagus as a function of extracellular concentration of adenosine 5-triphosphate (ATP) and related compounds. The addition of micromolar concentration of ATP caused a strong enhancement of frequency and wave velocity in the direction of the effective stroke. Since adenosine 5-[, imido]-triphosphate (AMP-PNP), a nonhydrolyzable analog of ATP, produces the same effects, ATP hydrolysis is not required. The overall potency is ATP AMP-PNP>ADP adenosine>AMP. It is suggested that both the phosphate and the base moieties are involved in ATP binding.The enhancement of ciliary activity by extracellular ATP is dependent on the presence of extracellular Ca²⁺, which can be replaced by extracellular Mg²⁺. The effect of a number of potent inhibitors of the voltage-gated calcium channels on the stimulation of ciliary activity by ATP were examined. No effect was detected in the concentration range within which these agents are specific. On the other hand, quinidine, a potent inhibitor of K⁺ (calcium-dependent) channels, inhibits the effect of ATP.The following model is suggested: exogenous ATP interacts with a membrane receptor in the presence of Ca²⁺, a cascade of events occurs which mobilizes intracellular calcium, thereby increasing the cytosolic free Ca²⁺ concentration which consequently opens the calcium-activated K⁺ channels, which then leads to a change in membrane potential. The ciliary response to these changes is the enhancement of ciliary activity.This work was supported by a grant from the Fund for Basic Research administered by the Israel Academy of Science and Humanities.     

Functional architecture of the outer arm dynein conformational switch

Dynein light chain 1 (LC1/DNAL1) is one of the most highly conserved components of ciliary axonemal outer arm dyneins, and it associates with both a heavy chain motor unit and tubulin located within the A-tubule of the axonemal outer doublet microtubules. In a variety of model systems, lack of LC1 or expression of mutant forms leads to profound defects in ciliary motility, including the failure of the hydrodynamic coupling needed for ciliary metachronal synchrony, random stalling during the power/recovery stroke transition, an aberrant response to imposed viscous load, and in some cases partial failure of motor assembly. These phenotypes have led to the proposal that LC1 acts as part of a mechanical switch to control motor function in response to alterations in axonemal curvature. Here we have used NMR chemical shift mapping to define the regions perturbed by a series of mutations in the C-terminal domain that yield a range of phenotypic effects on motility. In addition, we have identified the subdomain of LC1 involved in binding microtubules and characterized the consequences of an Asn → Ser alteration within the terminal leucine-rich repeat that in humans causes primary ciliary dyskinesia. Together, these data define a series of functional subdomains within LC1 and allow us to propose a structural model for the organization of the dynein heavy chain-LC1-microtubule ternary complex that is required for the coordinated activity of dynein motors in cilia.     

Triggering and regulatory mechanisms of ciliary motion in the ctenophore Bolinopsis. III. Electric excitability and inherent contractility of ciliated cells]

Rhythmical electrostimulation is able to produce in a tissue strip of Bolinopsis series of beats of combs-plates. However, the beat frequency, within one series, is only poorly controlled by electrical stimuli. Isolated comb-plate cells with a cilium cut off maintain rhythmical contractions stimulated both mechanically and electrically. A lot of microtubules has been found in the apical region of these cells. It is supposed that it is these microtubules, probably together with the associated root filaments, that are responsible for cell motility. Mutual mechanical stimulation of ciliary cells in these motions is, presumably, a signal providing intratissue propagation of metachronal wave.     

Rotation and twist of the central-pair microtubules in the cilia of Paramecium

The orientation and configuration of the central-pair microtubules in cilia were studied by serial thin-section analysis of "instantaneously fixed" paramecia. Cilia were frozen in various positions in metachronal waves by such a fixation. The spatial sequence of these positions across the wave represents the temporal sequence of the positions during the active beat cycle of a cilium. Systematic shifts of central- pair orientation across the wave indicate that the central pair rotates 360 degrees counterclockwise (viewed from outside) with each ciliary beat cycle (C. K. Omoto, 1979, Thesis, University of Wisconsin, Madison; C. K. Omoto and C. Kung, 1979, Nature [Lond.] 279:532-534). This is true even for paramecia with different directions of effective stroke as in forward- or backward-swimming cells. The systematic shifts of central-pair orientation cannot be seen in Ni++-paralyzed cells or sluggish mutants which do not have metachronal waves. Both serial thin- section and thick-section high-voltage electron microscopy show that whenever a twist in the central pair is seen, it is always left-handed. This twist is consistent with the hypothesis that the central pair continuously rotates counterclockwise with the rotation originating at the base of the cilium. That the rotation of the central pair is most likely with respect to the peripheral tubules as well as the cell surface is discussed. These results are incorporated into a model in which the central-pair complex is a component in the regulation of the mechanism needed for three-dimensional ciliary movement.     

Self-Sustained Oscillatory Sliding Movement of Doublet Microtubules and Flagellar Bend Formation

It is well established that the basis for flagellar and ciliary movements is ATP-dependent sliding between adjacent doublet microtubules. However, the mechanism for converting microtubule sliding into flagellar and ciliary movements has long remained unresolved. The author has developed new sperm models that use bull spermatozoa divested of their plasma membrane and midpiece mitochondrial sheath by Triton X-100 and dithiothreitol. These models enable the observation of both the oscillatory sliding movement of activated doublet microtubules and flagellar bend formation in the presence of ATP. A long fiber of doublet microtubules extruded by synchronous sliding of the sperm flagella and a short fiber of doublet microtubules extruded by metachronal sliding exhibited spontaneous oscillatory movements and constructed a one beat cycle of flagellar bending by alternately actuating. The small sliding displacement generated by metachronal sliding formed helical bends, whereas the large displacement by synchronous sliding formed planar bends. Therefore, the resultant waveform is a half-funnel shape, which is similar to ciliary movements.     

Inhibition of gliding movement by calcium in doublet microtubules on Tetrahymena ciliary dyneins in vitro.

We examined the effects of Ca ions on the gliding movement of Tetrahymena ciliary doublet microtubules induced by 14S or 22S dyneins in an in vitro motility assay system. The doublet microtubule appeared as circular-arc in solution, about 5 to 6 microns in length [1]. The doublet microtubules glided distal-end first on a 14S or 22S dynein-coated glass surface either clockwise or counterclockwise following the addition of ATP. The diameter of the circular path changed according to Ca concentration in the solution. Gliding velocity was from 1 to 5 microns/s. The addition of 0.1% Nonidet P-40 was necessary to induce the gliding movement on 22S dynein. This movement on 22S dynein was strongly inhibited above 0.5 mM ATP in the presence of 10(-9) M Ca, and at 0.05 to 1 mM ATP in the presence of 10(-3) M Ca. Many studies have indicated that Ca ions regulate ciliary movement [2-8] in which dyneins and doublet microtubule in the axoneme may play an essential role. The inhibition of the gliding movement of doublet microtubule on dyneins at appropriate concentrations of Ca and ATP as observed in this study may be the key for understanding Ca regulation of ciliary motility.     

Calcium regulation of ciliary activity in rabbit tracheal epithelial explants and outgrowth

Ciliated epithelial cells from rabbit trachea were employed to examine the role of Ca2+ in the regulation of ciliary motility. Tracheal explants and outgrowths were maintained in culture, and ciliary frequency was determined using a photomultiplier interfaced with a spectrum analyzer capable of Fast Fourier transform analysis. Relative cellular Ca2+ levels were determined by measuring 45Ca2+ uptake and efflux. Elevated cellular Ca2+, from exposure to 10(-5) M calcium ionophore A23187, led to an increase in ciliary frequency followed by inhibition of motility after prolonged treatment. A decrease in ciliary frequency was observed upon lowering intracellular Ca2+ by exposing the epithelium to 1 mM EGTA. Exposure of ciliated cells to 10(-4) M trifluoperazine resulted in inhibition of ciliary motility, a result suggesting a possible role for calmodulin- or phospholipid-sensitive Ca2+-dependent protein kinases in ciliary function. These results support the hypothesis that intracellular Ca2+ is actively involved in modulating the frequency of ciliary beat.     

An Outer Arm Dynein Conformational Switch Is Required for Metachronal Synchrony of Motile Cilia in Planaria

Motile cilia mediate the flow of mucus and other fluids across the surface of specialized epithelia in metazoans. Efficient clearance of peri-ciliary fluids depends on the precise coordination of ciliary beating to produce metachronal waves. The role of individual dynein motors and the mechanical feedback mechanisms required for this process are not well understood. Here we used the ciliated epithelium of the planarian Schmidtea mediterranea to dissect the role of outer arm dynein motors in the metachronal synchrony of motile cilia. We demonstrate that animals that completely lack outer dynein arms display a significant decline in beat frequency and an inability of cilia to coordinate their oscillations and form metachronal waves. Furthermore, lack of a key mechanosensitive regulatory component (LC1) yields a similar phenotype even though outer arms still assemble in the axoneme. The lack of metachrony was not due simply to a decrease in ciliary beat frequency, as reducing this parameter by altering medium viscosity did not affect ciliary coordination. In addition, we did not observe a significant temporal variability in the beat cycle of impaired cilia. We propose that this conformational switch provides a mechanical feedback system within outer arm dynein that is necessary to entrain metachronal synchrony.     

Modulation of Neuronal Signal Transduction Systems by Extracellular ATP

The secretion of ATP by stimulated nerves is well documented. Following repetitive stimulation, extracellular ATP at the synapse can accumulate to levels estimated to be well over 100 microM. The present study examined the effects of extracellular ATP in the concentration range of 0.1-1.0 mM on second-messenger-generating systems in cultured neural cells of the clones NG108-15 and N1E-115. Cells in a medium mimicking the physiological extracellular environment were used to measure 45Ca2+ uptake, changes in free intracellular Ca2+ levels by the probes aequorin and Quin-2, de novo generation of cyclic GMP and cyclic AMP from intracellular GTP and ATP pools prelabeled with [3H]guanosine and [3H]adenine, respectively, and phosphoinositide metabolism in cells preloaded with [3H]inositol and assayed in the presence of LiCl. Extracellular ATP induced a concentration-dependent increase of 45Ca2+ uptake by intact cells, which was additive with the uptake induced by K+ depolarization. The increased uptake involved elevation of intracellular free Ca2+ ions, evidenced by measuring aequorin and Quin-2 signals. At the same concentration range (0.1-1.0 mM), extracellular ATP induced an increase in [3H]cyclic GMP formation, and a decrease in prostaglandin E1-stimulated [3H]cyclic AMP generation. In addition, extracellular ATP (1 mM) caused a large (15-fold) increase in [3H]inositol phosphates accumulation, and this effect was blocked by including La3+ ions in the assay medium. In parallel experiments, we found in NG108-15 cells surface protein phosphorylation activity that had an apparent Km for extracellular ATP at the same concentration required to produce half-maximal effects on Ca2+ uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

The mechanisms of reversible immobilization of fowl spermatozoa at body temperature

Intact fowl spermatozoa became almost immotile at 40 degrees C, but motility increased significantly at 30 degrees C. The oxygen consumption at both temperatures was 8-11 microliters O2/10(10) spermatozoa.min-1. The ATP concentration at 40 degrees C was higher than that at 30 degrees C but ADP concentration at 30 degrees C was higher than that at 40 degrees C. Consequently, the ATP/ADP ratio at 30 degrees C (1.9-2.2) increased to 3.5-3.7 at 40 degrees C. The motility of intact spermatozoa at 40 degrees C was effectively restored by 2 mM-Ca2+, 10% seminal plasma and 10% peritoneal fluid taken at the time of ovulation. In contrast, these effectors did not restore the motility of demembranated spermatozoa at 40 degrees C. Motility of demembranated spermatozoa was restored at 30 degrees C. These results suggest that the immobilization of fowl spermatozoa at 40 degrees C occurs due to a decrease in flagellar dynein ATPase activity. Furthermore, the action of effectors for motility such as Ca2+ may not be directly on the axoneme, but mediated by solubilized substances which have been removed by demembranation of the spermatozoa.     

Regulation of airway ciliary activity by Ca2+: simultaneous measurement of beat frequency and intracellular Ca2+.

Airway ciliary activity is influenced by [Ca2+]i, but this mechanism is not fully understood. To investigate this relationship, ciliary activity and [Ca2+]i were measured simultaneously from airway epithelial ciliated cells. Ciliary beat frequency was determined, for each beat cycle, with phase-contrast optics and high-speed video imaging (at 240 images s-1) and correlated with [Ca2+]i determined, at the ciliary base, by fast imaging (30 images s-1) of fura-2 fluorescence. As a mechanically induced intercellular Ca2+ wave propagated through adjacent cells, [Ca2+]i was elevated from a baseline concentration of 45 to 100 nM, to a peak level of up to 650 nM. When the Ca2+ wave reached the ciliary base, the beat frequency rapidly increased, within a few beat cycles, from a basal rate of 6.4 to 11.6 Hz at 20-23 degrees C, and from 17.2 to 26.7 Hz at 37 degrees C. Changes in [Ca2+]i, above 350 nM, had no effect on the maximum beat frequency. We suggest that airway ciliary beat frequency is 1) controlled by a low range of [Ca2+]i acting directly at an axonemal site at the ciliary base and 2) that a maximum frequency is induced by a change in [Ca2+]i of approximately 250-300 nM.     

STUDIES ON CILIA : The Fixation of the Metachronal Wave

Upon excision into spring water, the lateral cilia of the gill of the freshwater mussel Elliptio complanatus (Solander) stop beating, but 0.04 M potassium ion can activate the gill so that these cilia again beat with metachronal rhythm. One per cent osmium tetroxide quickly pipetted onto a fully activated gill fixes the lateral cilia in a pattern that preserves the form and arrangement of the metachronal wave, and permits the cilia to be studied with the electron microscope in all stages of their beat cycle. Changes are seen in the fixed active preparation that are not present in the inactive control, i.e., in the packing of the cilia, the position of the axis of the ciliary cross-section, and the diameter of the ring of peripheral filaments. Analysis of these parameters may lead to new correlations between ciliary fine structure and function.     

Fibroblast growth factor-2 and remodeled type I collagen control membrane protrusion in human vascular smooth muscle cells: biphasic activation of Rac1

Plasma membrane protrusion is fundamental to cell motility, but its regulation by the extracellular environment is not well elucidated. We have quantified lamellipodial protrusion dynamics in human vascular smooth muscle cells exposed to fibroblast growth factor 2 (FGF-2) and type I collagen, two distinct ligands presented to vascular cells during arterial remodeling. Video microscopy revealed that FGF-2 stimulated a modest increase in lamellipodial protrusion rate that peaked within 15 min. This response was associated with immediate but transient activation of Rac1 and was inhibited in cells infected with retrovirus containing cDNA encoding dominant-negative Rac1. A 1-h exposure to FGF-2 also set up a second phase of more striking lamellipodial protrusion evident at 24-36 h. This delayed response was most pronounced when cells were on type 1 collagen and was associated with FGF-2-induced expression of collagenase-1 that localized to the edge of protruding lamellipodia. Moreover, late membrane protrusion was inhibited when cells were on collagenase-resistant type I collagen, implicating degraded collagen as a mediator. For cells on collagen, the immediate activation of Rac1 by FGF-2 was followed by a sustained wave of Rac1 activation that was inhibited when cleavage of the collagen triple helix was prevented and also by blockade of alpha(v)beta(3) integrin. We conclude that lamellipodial protrusion in smooth muscle cells can be regulated by waves of Rac1 activation, corresponding to the sequential presentation of FGF-2 and remodeled collagen. The findings thus reveal a previously unrecognized level of coordination among extracellular input that enables cells to maintain protrusive activity over prolonged periods.     

Effect of intracellular ATP on La(3+)-induced aggregation and fusion of human erythrocytes

The influence of intracellular ATP level on the aggregation and fusion of human erythrocytes, induced by La3+ in the concentration range 20-330 microM was studied. The aggregation of intact red blood cells differs from that of cells with increased and decreased contents of ATP. Incubation of erythrocyte aggregates at 37 degrees C did not lead to cell fusion. At the same time, incubation of erythrocyte aggregates with decreased and increased ATP contents in the presence of La3+ induced a pronounced cell fusion.     

Stimulus-response coupling in mammalian ciliated cells. Demonstration of two mechanisms of control for cytosolic [Ca2+]

Changes of cytosolic [Ca2+] have been proposed to couple stimulation of ciliary movement, however, quantitative measurements of fluctuations of intracellular free [Ca2+] associated with stimulation of ciliated cells have not been investigated. In primary cultures of rabbit oviductal ciliated cells, the stimulation of ciliary activity produced by micromolar concentrations of adenosine triphosphate (ATP) and prostaglandin F2 alpha (PGF2 alpha) was associated with a transient increase of intracellular [Ca2+]. Whereas the increase of cytosolic [Ca2+] and beat frequency produced by ATP were inhibited by the Ca-channel blocker LaCl3, the rise of cytosolic [Ca2+] and frequency of ciliary beat produced by PGF2 alpha was not affected by LaCl3. These results are the first direct demonstration that fluctuations of cytosolic [Ca2+] are associated with increased ciliary beat frequency in mammalian epithelial cells. The present findings suggest two different calcium-dependent mechanisms for stimulus-coupling in ciliary epithelium: ATP acting via purinergic receptor coupled to transmembrane influx of Ca2+, and PGF2 alpha acting via receptor-mediated release of intracellular sequestered Ca.