Bending patterns of chlamydomonas flagella: III. A radial spoke head deficient mutant and a central pair deficient mutant

Loss of an extensive ciliary connectome induces proteostasis and cell fate switching in a severe motile ciliopathy

Motile cilia have essential cellular functions in development, reproduction, and homeostasis. Genetic causes for motile ciliopathies have been identified, but the consequences on cellular functions beyond impaired motility remain unknown. Variants in CCDC39 and CCDC40 cause severe disease not explained by loss of motility. Using human cells with pathological variants in these genes, Chlamydomonas genetics, cryo-electron microscopy, single cell RNA transcriptomics, and proteomics, we identified perturbations in multiple cilia-independent pathways. Absence of the axonemal CCDC39/CCDC40 heterodimer results in loss of a connectome of over 90 proteins. The undocked connectome activates cell quality control pathways, switches multiciliated cell fate, impairs microtubule architecture, and creates a defective periciliary barrier. Both cilia-dependent and independent defects are likely responsible for the disease severity. Our findings provide a foundation for reconsidering the broad cellular impact of pathologic variants in ciliopathies and suggest new directions for therapies.

Unraveling the Kinematics of Sperm Motion by Reconstructing the Flagellar Wave Motion in 3D

Sperm swim through the female reproductive tract by propagating a 3D flagellar wave that is self-regulatory in nature and driven by dynein motors. Traditional microscopy methods fail to capture the full dynamics of sperm flagellar activity as they only image and analyze sperm motility in 2D. Here, an automated platform to analyze sperm swimming behavior in 3D by using thin-lens approximation and high-speed dark field microscopy to reconstruct the flagellar waveform in 3D is presented. It is found that head-tethered mouse sperm exhibit a rolling beating behavior in 3D with the beating frequency of 6.2 Hz using spectral analysis. The flagellar waveform bends in 3D, particularly in the distal regions, but is only weakly nonplanar and ambidextrous in nature, with the local helicity along the flagellum fluctuating between clockwise and counterclockwise handedness. These findings suggest a nonpersistent flagellar helicity. This method provides new opportunities for the accurate measurement of the full motion of eukaryotic flagella and cilia which is essential for a biophysical understanding of their activation by dynein motors.

The Transport Behavior of a Biflagellated Microswimmer before and after Cargo Loading

The changes in the transport behavior of a microswimmer before and after cargo loading are crucial to understanding and control of the motion of a biohybrid microbot. In this work, we show the change in swimming behavior of biflagellated microalgae Chlamydomonas reinhardtii picking up a 4.5 μm polystyrene microbead upon collision. The microswimmer changed from linear forward motion into helical motion upon the attachment of the cargo and swam with a decreased swimming velocity. We revealed the helical motion of the microswimmer upon cargo loading due to suppression of flagella by image analysis of magnified time-lapse images of C. reinhardtii with one microbead attached at the anterior end (between the flagella). Furthered suppression on the flagellum imposed by the loading of the second cargo has led to increased oscillation per displacement traveled and decreased swimming velocity. Moreover, the microswimmer with a microbead attached at the posterior end swam with swimming velocity close to free swimming microalgae and did not exhibit helical swimming behavior. The experimental results and analysis showed that the loading location of the cargo has a great influence over the swimming behavior of the microswimmer. Furthermore, the work balance calculation and mathematical analysis based on Lighthill's model are well consistent with our experimental findings.

How Does Cilium Length Affect Beating?

The effects of cilium length on the dynamics of cilia motion were investigated by high-speed video microscopy of uniciliated mutants of the swimming alga, Chlamydomonas reinhardtii. Cells with short cilia were obtained by deciliating cells via pH shock and allowing cilia to reassemble for limited times. The frequency of cilia beating was estimated from the motion of the cell body and of the cilium. Key features of the ciliary waveform were quantified from polynomial curves fitted to the cilium in each image frame. Most notably, periodic beating did not emerge until the cilium reached a critical length between 2 and 4 μm. Surprisingly, in cells that exhibited periodic beating, the frequency of beating was similar for all lengths with only a slight decrease in frequency as length increased from 4 μm to the normal length of 10-12 μm. The waveform average curvature (rad/μm) was also conserved as the cilium grew. The mechanical metrics of ciliary propulsion (force, torque, and power) all increased in proportion to length. The mechanical efficiency of beating appeared to be maximal at the normal wild-type length of 10-12 μm. These quantitative features of ciliary behavior illuminate the biophysics of cilia motion and, in future studies, may help distinguish competing hypotheses of the underlying mechanism of oscillation.

Analysis of unstable modes distinguishes mathematical models of flagellar motion

The mechanisms underlying the coordinated beating of cilia and flagella remain incompletely understood despite the fundamental importance of these organelles. The axoneme (the cytoskeletal structure of cilia and flagella) consists of microtubule doublets connected by passive and active elements. The motor protein dynein is known to drive active bending, but dynein activity must be regulated to generate oscillatory, propulsive waveforms. Mathematical models of flagellar motion generate quantitative predictions that can be analysed to test hypotheses concerning dynein regulation. One approach has been to seek periodic solutions to the linearized equations of motion. However, models may simultaneously exhibit both periodic and unstable modes. Here, we investigate the emergence and coexistence of unstable and periodic modes in three mathematical models of flagellar motion, each based on a different dynein regulation hypothesis: (i) sliding control; (ii) curvature control and (iii) control by interdoublet separation (the 'geometric clutch' (GC)). The unstable modes predicted by each model are used to critically evaluate the underlying hypothesis. In particular, models of flagella with 'sliding-controlled' dynein activity admit unstable modes with non-propulsive, retrograde (tip-to-base) propagation, sometimes at the same parameter values that lead to periodic, propulsive modes. In the presence of these retrograde unstable modes, stable or periodic modes have little influence. In contrast, unstable modes of the GC model exhibit switching at the base and propulsive base-to-tip propagation.

Dynein-deficient flagella respond to increased viscosity with contrasting changes in power and recovery strokes

Changes in the flagellar waveform in response to increased viscosity were investigated in uniflagellate mutants of Chlamydomonas reinhardtii. We hypothesized that the waveforms of mutants lacking different dynein arms would change in different ways as viscosity was increased, and that these variations would illuminate the feedback pathways from force to dynein activity. Previous studies have investigated the effects of viscosity on cell body motion, propulsive force, and power in different mutants, but the effect on waveform has not yet been fully characterized. Beat frequency decreases with viscosity in wild-type uniflagellate (uni1) cells, and outer dynein arm deficient (oda2) mutants. In contrast, the inner dynein arm mutant ida1 (lacking I1/f) maintains beat frequency at high viscosity but alters its flagellar waveform more than either wild-type or oda2. The ida1 waveform is narrower than wild-type, primarily due to an abbreviated recovery stroke; this difference is amplified at high viscosity. The oda2 mutant in contrast, maintains a consistent waveform at high and low viscosity with a slightly longer power stroke than wild-type. Analysis of the delays and shear displacements between bends suggest that direct force feedback in the outer dynein arm system may initiate switching of dynein activity. In contrast, I1/f dynein appears to delay switching, most markedly at the initiation of the power stroke, possibly by controlling inter-doublet separation.

DRC3 connects the N-DRC to dynein g to regulate flagellar waveform

The nexin-dynein regulatory complex (N-DRC), which is a major hub for the control of flagellar motility, contains at least 11 different subunits. A major challenge is to determine the location and function of each of these subunits within the N-DRC. We characterized a Chlamydomonas mutant defective in the N-DRC subunit DRC3. Of the known N-DRC subunits, the drc3 mutant is missing only DRC3. Like other N-DRC mutants, the drc3 mutant has a defect in flagellar motility. However, in contrast to other mutations affecting the N-DRC, drc3 does not suppress flagellar paralysis caused by loss of radial spokes. Cryo-electron tomography revealed that the drc3 mutant lacks a portion of the N-DRC linker domain, including the L1 protrusion, part of the distal lobe, and the connection between these two structures, thus localizing DRC3 to this part of the N-DRC. This and additional considerations enable us to assign DRC3 to the L1 protrusion. Because the L1 protrusion is the only non-dynein structure in contact with the dynein g motor domain in wild-type axonemes and this is the only N-DRC-dynein connection missing in the drc3 mutant, we conclude that DRC3 interacts with dynein g to regulate flagellar waveform.

The awesome power of dikaryons for studying flagella and basal bodies in Chlamydomonas reinhardtii

Cilia/flagella and basal bodies/centrioles play key roles in human health and homeostasis. Among the organisms used to study these microtubule-based organelles, the green alga Chlamydomonas reinhardtii has several advantages. One is the existence of a temporary phase of the life cycle, termed the dikaryon. These cells are formed during mating when the cells fuse and the behavior of flagella from two genetically distinguishable parents can be observed. During this stage, the cytoplasms mix allowing for a defect in the flagella of one parent to be rescued by proteins from the other parent. This offers the unique advantage of adding back wild-type gene product or labeled protein at endogenous levels that can used to monitor various flagellar and basal body phenotypes. Mutants that show rescue and ones that fail to show rescue are both informative about the nature of the flagella and basal body defects. When rescue occurs, it can be used to determine the mutant gene product and to follow the temporal and spatial patterns of flagellar assembly. This review describes many examples of insights into basal body and flagellar proteins' function and assembly that have been discovered using dikaryons and discusses the potential for further analyses.

The N-DRC forms a conserved biochemical complex that maintains outer doublet alignment and limits microtubule sliding in motile axonemes

The nexin–dynein regulatory complex (N-DRC) is implicated in the control of dynein activity as a structural component of the nexin link. This study identifies several new subunits of the N-DRC and demonstrates for the first time that it forms a discrete biochemical complex that maintains outer doublet integrity and regulates microtubule sliding.

The nexin–dynein regulatory complex (N-DRC) is proposed to coordinate dynein arm activity and interconnect doublet microtubules. Here we identify a conserved region in DRC4 critical for assembly of the N-DRC into the axoneme. At least 10 subunits associate with DRC4 to form a discrete complex distinct from other axonemal substructures. Transformation of drc4 mutants with epitope-tagged DRC4 rescues the motility defects and restores assembly of missing DRC subunits and associated inner-arm dyneins. Four new DRC subunits contain calcium-signaling motifs and/or AAA domains and are nearly ubiquitous in species with motile cilia. However, drc mutants are motile and maintain the 9 + 2 organization of the axoneme. To evaluate the function of the N-DRC, we analyzed ATP-induced reactivation of isolated axonemes. Rather than the reactivated bending observed with wild-type axonemes, ATP addition to drc-mutant axonemes resulted in splaying of doublets in the distal region, followed by oscillatory bending between pairs of doublets. Thus the N-DRC provides some but not all of the resistance to microtubule sliding and helps to maintain optimal alignment of doublets for productive flagellar motility. These findings provide new insights into the mechanisms that regulate motility and further highlight the importance of the proximal region of the axoneme in generating flagellar bending.

Methods for analysis of calcium/calmodulin signaling in cilia and flagella

The axonemal microtubules of cilia/flagella act as a scaffold for assembly of the protein complexes that ultimately regulate dynein activity to control the size and shape of ciliary bends. Despite our general understanding of the contribution of microtubule sliding to ciliary and flagellar motility, many questions regarding the regulation of dynein remain unanswered. For example, we know that the second messenger calcium plays an important role in modulating dynein activity in response to extracellular cues, but it remains unclear how calcium-binding proteins anchored to the axoneme contribute to this regulation. Recent work has focused on determining the identity and specific functions of these axonemal calcium-binding proteins. Here, we review our current knowledge of calcium-mediated motility and highlight key experiments that have substantially aided our understanding of calcium signaling within the axoneme.

The Pcdp1 complex coordinates the activity of dynein isoforms to produce wild-type ciliary motility

Generating the complex waveforms characteristic of beating cilia requires the coordinated activity of multiple dynein isoforms anchored to the axoneme. We previously identified a complex associated with the C1d projection of the central apparatus that includes primary ciliary dyskinesia protein 1 (Pcdp1). Reduced expression of complex members results in severe motility defects, indicating that C1d is essential for wild-type ciliary beating. To define a mechanism for Pcdp1/C1d regulation of motility, we took a functional and structural approach combined with mutants lacking C1d and distinct subsets of dynein arms. Unlike mutants completely lacking the central apparatus, dynein-driven microtubule sliding velocities are wild type in C1d- defective mutants. However, coordination of dynein activity among microtubule doublets is severely disrupted. Remarkably, mutations in either outer or inner dynein arm restore motility to mutants lacking C1d, although waveforms and beat frequency differ depending on which isoform is mutated. These results define a unique role for C1d in coordinating the activity of specific dynein isoforms to control ciliary motility.

How molecular motors shape the flagellar beat

Cilia and eukaryotic flagella are slender cellular appendages whose regular beating propels cells and microorganisms through aqueous media. The beat is an oscillating pattern of propagating bends generated by dynein motor proteins. A key open question is how the activity of the motors is coordinated in space and time. To elucidate the nature of this coordination we inferred the mechanical properties of the motors by analyzing the shape of beating sperm: Steadily beating bull sperm were imaged and their shapes were measured with high precision using a Fourier averaging technique. Comparing our experimental data with wave forms calculated for different scenarios of motor coordination we found that only the scenario of interdoublet sliding regulating motor activity gives rise to satisfactory fits. We propose that the microscopic origin of such "sliding control" is the load dependent detachment rate of motors. Agreement between observed and calculated wave forms was obtained only if significant sliding between microtubules occurred at the base. This suggests a novel mechanism by which changes in basal compliance could reverse the direction of beat propagation. We conclude that the flagellar beat patterns are determined by an interplay of the basal properties of the axoneme and the mechanical feedback of dynein motors.

Building blocks of the nexin-dynein regulatory complex in Chlamydomonas flagella

The directional flow generated by motile cilia and flagella is critical for many processes, including human development and organ function. Normal beating requires the control and coordination of thousands of dynein motors, and the nexin-dynein regulatory complex (N-DRC) has been identified as an important regulatory node for orchestrating dynein activity. The nexin link appears to be critical for the transformation of dynein-driven, linear microtubule sliding to flagellar bending, yet the molecular composition and mechanism of the N-DRC remain largely unknown. Here, we used proteomics with special attention to protein phosphorylation to analyze the composition of the N-DRC and to determine which subunits may be important for signal transduction. Two-dimensional electrophoresis and MALDI-TOF mass spectrometry of WT and mutant flagellar axonemes from Chlamydomonas identified 12 N-DRC-associated proteins, including all seven previously observed N-DRC components. Sequence and PCR analyses identified the mutation responsible for the phenotype of the sup-pf-4 strain, and biochemical comparison with a radial spoke mutant revealed two components that may link the N-DRC and the radial spokes. Phosphoproteomics revealed eight proteins with phosphorylated isoforms for which the isoform patterns changed with the genotype as well as two components that may play pivotal roles in N-DRC function through their phosphorylation status. These data were assembled into a model of the N-DRC that explains aspects of its regulatory function.

The dynein regulatory complex is the nexin link and a major regulatory node in cilia and flagella

Elegant cryoelectron tomography reveals that the nexin link between microtubule doublets in 9 + 2 axonemal structures, critical for their ability to bend, is the dynein regulatory complex.

Cilia and flagella are highly conserved microtubule (MT)-based organelles with motile and sensory functions, and ciliary defects have been linked to several human diseases. The 9 + 2 structure of motile axonemes contains nine MT doublets interconnected by nexin links, which surround a central pair of singlet MTs. Motility is generated by the orchestrated activity of thousands of dynein motors, which drive interdoublet sliding. A key regulator of motor activity is the dynein regulatory complex (DRC), but detailed structural information is lacking. Using cryoelectron tomography of wild-type and mutant axonemes from Chlamydomonas reinhardtii, we visualized the DRC in situ at molecular resolution. We present the three-dimensional structure of the DRC, including a model for its subunit organization and intermolecular connections that establish the DRC as a major regulatory node. We further demonstrate that the DRC is the nexin link, which is thought to be critical for the generation of axonemal bending.

Efficient spatiotemporal analysis of the flagellar waveform of Chlamydomonas reinhardtii

The 9 + 2 axoneme is a microtubule-based machine that powers the oscillatory beating of cilia and flagella. Its highly regulated movement is essential for the normal function of many organs; ciliopathies cause congenital defects, chronic respiratory tract infections and infertility. We present an efficient method to obtain a quantitative description of flagellar motion, with high spatial and temporal resolution, from high speed video recording of bright field images. This highly automated technique provides the shape, shear angle, curvature, and bend propagation speeds along the length of the flagellum, with approximately 200 temporal samples per beat. We compared the waveforms of uniflagellated wild-type and ida3 mutant cells, which lack the I1 inner dynein complex. Video images were captured at 350 fps. Rigid-body motion was eliminated by fast Fourier transform (FFT)-based registration, and the Cartesian (x-y) coordinates of points on the flagellum were identified. These x-y "point clouds" were embedded in two data dimensions using Isomap, a nonlinear dimension reduction method, and sorted by phase in the flagellar cycle. A smooth surface was fitted to the sorted point clouds, which provides high-resolution estimates of shear angle and curvature. Wild-type and ida3 cells exhibit large differences in shear amplitude, but similar maximum and minimum curvature values. In ida3 cells, the reverse bend begins earlier and travels more slowly relative to the principal bend, than in wild-type cells. The regulation of flagellar movement must involve I1 dynein in a manner consistent with these results.

Calcium regulation of ciliary motility analysis of axonemal calcium-binding proteins

Substantial data have contributed to a model in which the axonemal microtubules act as a scaffold for the assembly of molecules that form a signal transduction pathway that ultimately regulates dynein. We have also known for some time that for virtually all motile cilia and flagella, the second messenger, calcium, impacts upon these signaling pathways to modulate beating in response to extracellular cues. Yet we are only beginning to identify the axonemal proteins that bind this second messenger and determine their role in regulating dynein-driven microtubule sliding to alter the size and shape of ciliary bends. Here, we review our current understanding of calcium regulation of motility, emphasizing recent advances in the detection and characterization of calcium-binding proteins anchored to the axoneme.

An axonemal dynein particularly important for flagellar movement at high viscosity. Implications from a new Chlamydomonas mutant deficient in the dynein heavy chain gene DHC9

Ciliary and flagellar axonemes contain multiple inner arm dyneins of which the functional difference is largely unknown. In this study, a Chlamydomonas mutant, ida9, lacking inner arm dynein c was isolated and shown to carry a mutation in the DHC9 dynein heavy chain gene. The cDNA sequence of DHC9 was determined, and its information was used to show that >80% of it is lost in the mutant. Electron microscopy and image analysis showed that the ida9 axoneme lacked electron density near the base of the S2 radial spoke, indicating that dynein c localizes to this site. The mutant ida9 swam only slightly slower than the wild type in normal media. However, swimming velocity was greatly reduced when medium viscosity was modestly increased. Thus, dynein c in wild type axonemes must produce a significant force when flagella are beating in viscous media. Because motility analyses in vitro have shown that dynein c is the fastest among all the inner arm dyneins, we can regard this dynein as a fast yet powerful motor.

Two Anti-radial Spoke Monoclonal Antibodies Inhibit Chlamydomonas Axonemal Motility by Different Mechanisms

In the 9 + 2 axoneme, radial spokes are structural components attached to the A-tubules of the nine outer doublet microtubules. They protrude toward the central pair microtubule complex with which they have transient but regular interactions for the normal flagellar motility to occur. Flagella of Chlamydomonas mutants deficient in entire radial spokes or spoke heads are paralyzed. In this study the importance of two radial spoke proteins in the flagellar movement is exemplified by the potent inhibitory action of two monoclonal antibodies on the axonemal motility of demembranated-reactivated Chlamydomonas models. We show that one of these proteins is localized on the stalk of the radial spokes, whereas the other is a component of the head of the same structure and most likely correspond to radial spoke protein 2 and 1, respectively. Fine motility analysis by videomicrography further indicates that these two anti-radial spoke protein antibodies at low concentration affect motility of demembranated-reactivated Chlamydomonas by changing the flagellar waveform without modifying axonemal beat frequency. They also modify wave amplitude differently during motility inhibition. This brings more direct evidence for the involvement of both radial spoke stalk and head in the fine tuning of the waveform during flagellar motility.

Flagellar quiescence in Chlamydomonas: Characterization and defective quiescence in cells carrying sup-pf-1 and sup-pf-2 outer dynein arm mutations

Chlamydomonas reinhardtii can use their flagella for two distinct types of movement: swimming through liquid or gliding on a solid substrate. Cells switching from swimming to gliding motility undergo a reversible flagellar quiescence. This phenomenon appears to involve the outer dynein arms, since mutants having altered outer arm beta and gamma dyneins (sup-pf-1 and sup-pf-2) show a diminished ability to quiesce. Sup-pf-1 and sup-pf-2 were originally isolated as gain-of-function mutations that suppress the flagellar paralysis resulting from radial spoke or central pair defects. Defective quiescence is also a gain-of-function phenomenon, as cells completely lacking outer arm heavy chains show a normal quiescence phenotype. These data suggest that regulation of outer arm dynein activity is essential for flagellar quiescence and furthermore that regulation of quiescence involves a signal transduction pathway that shares elements with the radial spoke/central pair system.

Orientation of the central pair complex during flagellar bend formation inChlamydomonas

Thin section electron micrographs of rapidly fixed Chlamydomonas cells were used to establish a relationship between flagellar bends and orientation of the central pair microtubule complex. Using conditions that preserve flagellar waveforms during both forward swimming (asymmetric bends) and backward swimming (symmetric bends), we found that central pair orientation differs in bent regions and straight regions. During forward swimming, a plane through the two central pair microtubules is parallel to the bend plane throughout principal bends, in both effective stroke and recovery stroke phases of the beat cycle. In these curved segments, the C1 microtubule always faces the outer edge of the curve. This parallel orientation twists in straight regions both proximal and distal to bends. During backward swimming episodes induced by photoshock, when Chlamydomonas flagella beat with principal and reverse bends of similar magnitude, the central pair twists by 180 degrees between successive bends. These observations support a model in which central pair orientation in Chlamydomonas is linked to doublet-specific dynein activation, and bend propagation is linked to rotation of the central pair complex.

Structural-functional relationships of the dynein, spokes, and central-pair projections predicted from an analysis of the forces acting within a flagellum

In the axoneme of eukaryotic flagella the dynein motor proteins form crossbridges between the outer doublet microtubules. These motor proteins generate force that accumulates as linear tension, or compression, on the doublets. When tension or compression is present on a curved microtubule, a force per unit length develops in the plane of bending and is transverse to the long axis of the microtubule. This transverse force (t-force) is evaluated here using available experimental evidence from sea urchin sperm and bull sperm. At or near the switch point for beat reversal, the t-force is in the range of 0.25-1.0 nN/ micro m, with 0.5 nN/ micro m the most likely value. This is the case in both beating and arrested bull sperm and in beating sea urchin sperm. The total force that can be generated (or resisted) by all the dyneins on one micron of outer doublet is also approximately 0.5 nN. The equivalence of the maximum dynein force/ micro m and t-force/ micro m at the switch point may have important consequences. Firstly, the t-force acting on the doublets near the switch point of the flagellar beat is sufficiently strong that it could terminate the action of the dyneins directly by strongly favoring the detached state and precipitating a cascade of detachment from the adjacent doublet. Secondly, after dynein release occurs, the radial spokes and central-pair apparatus are the structures that must carry the t-force. The spokes attached to the central-pair projections will bear most of the load. The central-pair projections are well-positioned for this role, and they are suitably configured to regulate the amount of axoneme distortion that occurs during switching. However, to fulfill this role without preventing flagellar bend formation, moveable attachments that behave like processive motor proteins must mediate the attachment between the spoke heads and the central-pair structure.

Asymmetry of the central apparatus defines the location of active microtubule sliding in Chlamydomonas flagella

Regulation of ciliary and flagellar motility requires spatial control of dynein-driven microtubule sliding. However, the mechanism for regulating the location and symmetry of dynein activity is not understood. One hypothesis is that the asymmetrically organized central apparatus, through interactions with the radial spokes, transmits a signal to regulate dynein-driven microtubule sliding between subsets of doublet microtubules. Based on this model, we hypothesized that the orientation of the central apparatus defines positions of active microtubule sliding required to control bending in the axoneme. To test this, we induced microtubule sliding in axonemes isolated from wild-type and mutant Chlamydomonas cells, and then used electron microscopy to determine the orientation of the central apparatus. Transverse sections of wild-type axonemes revealed that the C1 microtubule is predominantly oriented toward the position of active microtubule sliding. In contrast, the central apparatus is randomly oriented in axonemes isolated from radial spoke deficient mutants. For outer arm dynein mutants, the C1 microtubule is oriented toward the position of active microtubule sliding in low calcium buffer, but is randomly oriented in high calcium buffer. These results provide evidence that the central apparatus defines the position of active microtubule sliding, and may regulate the size and shape of axonemal bends through interactions with the radial spokes. In addition, our results indicate that in high calcium conditions required to generate symmetric waveforms, the outer dynein arms are potential targets of the central pair-radial spoke control system.

Computer simulation of flagellar movement VIII: coordination of dynein by local curvature control can generate helical bending waves

Computer simulations have been carried out with a model flagellum that can bend in three dimensions. A pattern of dynein activation in which regions of dynein activity propagate along each doublet, with a phase shift of approximately 1/9 wavelength between adjacent doublets, will produce a helical bending wave. This pattern can be termed "doublet metachronism." The simulations show that doublet metachronism can arise spontaneously in a model axoneme in which activation of dyneins is controlled locally by the curvature of each outer doublet microtubule. In this model, dyneins operate both as sensors of curvature and as motors. Doublet metachronism and the chirality of the resulting helical bending pattern are regulated by the angular difference between the direction of the moment and sliding produced by dyneins on a doublet and the direction of the controlling curvature for that doublet. A flagellum that is generating a helical bending wave experiences twisting moments when it moves against external viscous resistance. At high viscosities, helical bending will be significantly modified by twist unless the twist resistance is greater than previously estimated. Spontaneous doublet metachronism must be modified or overridden in order for a flagellum to generate the planar bending waves that are required for efficient propulsion of spermatozoa. Planar bending can be achieved with the three-dimensional flagellar model by appropriate specification of the direction of the controlling curvature for each doublet. However, experimental observations indicate that this "hard-wired" solution is not appropriate for real flagella.

Regulation of flagellar dynein by calcium and a role for an axonemal calmodulin and calmodulin-dependent kinase

Ciliary and flagellar motility is regulated by changes in intraflagellar calcium. However, the molecular mechanism by which calcium controls motility is unknown. We tested the hypothesis that calcium regulates motility by controlling dynein-driven microtubule sliding and that the central pair and radial spokes are involved in this regulation. We isolated axonemes from Chlamydomonas mutants and measured microtubule sliding velocity in buffers containing 1 mM ATP and various concentrations of calcium. In buffers with pCa > 8, microtubule sliding velocity in axonemes lacking the central apparatus (pf18 and pf15) was reduced compared with that of wild-type axonemes. In contrast, at pCa4, dynein activity in pf18 and pf15 axonemes was restored to wild-type level. The calcium-induced increase in dynein activity in pf18 axonemes was inhibited by antagonists of calmodulin and calmodulin-dependent kinase II. Axonemes lacking the C1 central tubule (pf16) or lacking radial spoke components (pf14 and pf17) do not exhibit calcium-induced increase in dynein activity in pCa4 buffer. We conclude that calcium regulation of flagellar motility involves regulation of dynein-driven microtubule sliding, that calmodulin and calmodulin-dependent kinase II may mediate the calcium signal, and that the central apparatus and radial spokes are key components of the calcium signaling pathway.

Axonemal activity relative to the 2D/3D-waveform conversion of the flagellum

The waveform of the flagellum of the sea urchin spermatozoon is mainly planar, but its 3D-properties were evoked for dynamic reasons and described as helical. In 1975, the apparent twisting pattern of the sea urchin axoneme was described [Gibbons I. 1975. The molecular basis of flagellar motility in sea urchin spermatozoa. In: Inoué S, Stephens R, editors. Molecular and cellular movement. New York: Raven Press, p. 207-232.] and was considered to be one of the main elements involved in axonemal behaviour. Recently, planar, quasi-planar, and helical waveforms were observed when the flagellum of sea urchin sperm cells was submitted to an increase in viscosity. The quasi-planar conformation seemed to be due to the alternating torsion of the inter-bend segments [Woolley D, Vernon G. 2001. A study of helical and planar waves on sea urchin sperm flagella, with a theory of how they are generated. J. Exp. Biol. 204:1333-1345]. These three waveforms, which are due to a change in axonemal activity, are possibly used by the sperm cells to adapt their movement to variations in the physico-chemical characteristics of the medium (seawater) in which the cells normally swim. We constructed a simple model to describe qualitatively the central shear (between the axonemal doublets and the central pair) and the tangential shear (between the doublets themselves). In this model, the 3D-bending is resolved into components in two perpendicular planes and each of the nine planes of inter-doublet interaction defines a potential bending plane that is independently regulated. These shears were calculated for the three waveforms and their inter-conversion. This allowed us to propose that axoneme is resolved in successive modules delineated by abscissas where the sliding is always nil. We discuss these data concerning the axonemal machinery, and especially the alternating activity of opposite sides of (two) neutral surface(s) that seem(s) to be responsible for this inter-conversion, and for the possible twist of the axoneme during the beating.

Casein kinase I is anchored on axonemal doublet microtubules and regulates flagellar dynein phosphorylation and activity

Flagellar dynein activity is regulated by phosphorylation. One critical phosphoprotein substrate in Chlamydomonas is the 138-kDa intermediate chain (IC138) of the inner arm dyneins (Habermacher, G., and Sale, W. S. (1997) J. Cell Biol. 136, 167-176). In this study, several approaches were used to determine that casein kinase I (CKI) is physically anchored in the flagellar axoneme and regulates IC138 phosphorylation and dynein activity. First, using a videomicroscopic motility assay, selective CKI inhibitors rescued dynein-driven microtubule sliding in axonemes isolated from paralyzed flagellar mutants lacking radial spokes. Rescue of dynein activity failed in axonemes isolated from these mutant cells lacking IC138. Second, CKI was unequivocally identified in salt extracts from isolated axonemes, whereas casein kinase II was excluded from the flagellar compartment. Third, Western blots indicate that within flagella, CKI is anchored exclusively to the axoneme. Analysis of multiple Chlamydomonas motility mutants suggests that the axonemal CKI is located on the outer doublet microtubules. Finally, CKI inhibitors that rescued dynein activity blocked phosphorylation of IC138. We propose that CKI is anchored on the outer doublet microtubules in position to regulate flagellar dynein.

Computer simulation of flagellar movement: VII. Conventional but functionally different cross-bridge models for inner and outer arm dyneins can explain the effects of outer arm dynein removal

Outer arm dynein removal from flagella by genetic or chemical methods causes decreased frequency and power, but little change in bending pattern. These results suggest that outer arm dynein operates within bends to increase the speed of bend propagation, but does not produce forces that alter the bending pattern established by inner arm dyneins. A flagellar model incorporating different cross-bridge models for inner and outer arm dyneins has been examined. The inner arm dynein model has a hyperbolic force-velocity curve, with a maximum average force at 0 sliding velocity of about 14 pN for each 96 nm group of inner arm dyneins. The outer arm dynein model has a very different force-velocity curve, with a maximum force at about 10-15% of V(max). The outer arm dynein model is adjusted so that the unloaded sliding velocity for a realistic mixture of inner and outer arm dyneins is twice the unloaded sliding velocity for the inner arm dynein model alone. With these cross-bridge models, a flagellar model can be obtained that reduces its sliding velocity and frequency by approximately 50% when outer arm dyneins are removed, with little change in bending pattern. The addition of outer arm dyneins, therefore, gives an approximately 4-fold increase in power output against viscous resistances, and outer arm dyneins may generate 90% or more of the power output. Cell Motil.

Ca2+-dependent waveform conversion in the flagellar axoneme of Chlamydomonas mutants lacking the central-pair/radial spoke system

Chlamydomonas flagella undergo a striking waveform conversion from an asymmetrical ciliary type to a symmetrical flagellar type when the cell is stimulated by intense light and the Ca2+ concentration within the flagellum is increased above approximately 10(-6) M. To see whether the central-pair/radial spoke system is needed for this conversion as suggested by previous studies, we examined the effect of Ca2+ on the reactivated axonemes of the mutants lacking the central pair (pf18) or the radial spokes (pf14). Although the flagella of these mutants are paralyzed in vivo, demembranated axonemes can be reactivated to beat under certain nucleotide conditions such as in the presence of low concentrations (< 100 microM) of ATP. We examined the waveform of the axonemes reactivated at 20 microM ATP in the presence of 10(-8)-10(-4) M Ca2+ and found that these axonemes, as well as the wild-type axonemes, undergo a waveform conversion over a Ca2+ concentration range of 10(-7)-10(-5) M: a highly asymmetrical waveform at <10(-6) M Ca2+ and a symmetrical waveform at >=10(-5) M Ca2+. Although the waveform is different between the mutants and the wild type, the Ca2+ concentration at which the waveform conversion occurred was similar. These results indicate that the central pair/radial spoke system is not essential for the waveform conversion.

Transient disruptions of axonemal structure and microtubule sliding during bend propagation by Ciona sperm flagella

Demembranated sperm flagella of Ciona were reactivated at increased salt concentrations (0.45 to 0.5 M K acetate). In addition to a decrease in amplitude of propagated bends, some flagella switch between "stable" and "transient" bending cycles. In the transient bending cycles, there is increased intermicrotubule sliding, in the direction that forms a new principal bend at the base of the flagellum, during the first half of a bending cycle. The magnitude of this increased sliding may be as much as 1 radian, or 0.06 micron between adjacent doublet microtubules. Most transient bending patterns also show a characteristic disruption of axonemal structure, involving separation between strands of microtubule doublets over a distance of up to 5 microns, occurring within a principal bend, typically about 16 microns from the base of the flagellum. The disruptions usually disappear after the principal bend propagates beyond the region of the disruption. Formation of these disruptions requires additional sliding, in the direction that would form a principal bend at the base of the flagellum, of up to about 0.3 micron. Formation of these disruptions may be explained by weakening of structural interactions by increased salt concentration and transverse forces, proportional to curvature and transmitted force, that will tend to separate doublets in a bend. These observations indicate that an actively beating flagellum possesses active sliding capability that is activated but not expressed during normal bend initiation and propagation. The initiation and propagation of flagellar bends may not be explicable solely in terms of local activation and inactivation of dynein-driven sliding.

Bending patterns of ATP-reactivated sea urchin sperm flagella following high salt extraction for removal of outer dynein arms

Two procedures were used for extraction of demembranated sea urchin sperm flagella with increased KCl concentrations, to remove outer dynein arms. Extraction with 0.55 M KCl in the Triton-demembranation solution produced a rapid fall in average sliding velocity to 50% of its unextracted value, with extensive changes in bending behavior of the distal half of the flagellum. Extraction with 0.42 M KCl following demembranation and activation by incubation with cAMP produced a more gradual fall in sliding velocity, reaching 50% of the unextracted value after 180 sec extraction. This procedure produced somewhat more normal bending patterns. In both cases, the bending pattern of the basal region of the flagellum is altered very little by extraction, in agreement with data from Chlamydomonas mutant flagella deficient in outer arm dyneins.

Calcium regulation of microtubule sliding in reactivated sea urchin sperm flagella

The changes in the bending pattern of flagella induced by an increased intracellular Ca(2+) concentration are caused by changes in the pattern and velocity of microtubule sliding. However, the mechanism by which Ca(2+) regulates microtubule sliding in flagella has been unclear. To elucidate it, we studied the effects of Ca(2+) on microtubule sliding in reactivated sea urchin sperm flagella that were beating under imposed head vibration. We found that the maximum microtubule sliding velocity obtainable by imposed vibration, which was about 170–180 rad/second in the presence of 250 microM MgATP and &lt;10(−9) M Ca(2+), was decreased by 10(−6)-10(−5) M Ca(2+) by about 15–20%. Similar decrease of the sliding velocity was observed at 54 and 27 microM MgATP. The Ca(2+)-induced decrease of the sliding velocity was due mainly to a decrease in the reverse bend angle. When the plane of beat was artificially rotated by rotating the plane of vibration of the pipette that held the sperm head, the asymmetric bending pattern also rotated at 10(−5) M Ca(2+) as well as at &lt;10(−9) M Ca(2+). The rotation of the bending pattern was observed at MgATP higher than 54 microM (approximately 100 microM ATP). These results indicate that the Ca(2+)-induced decrease of the sliding velocity is mediated by a rotatable component or components (probably the central pair) at high MgATP, but is not due to specific dynein arms on particular doublets. We further investigated the effects of a mild trypsin treatment and of trifluoperazine on the Ca(2+)-induced decrease in sliding velocity. Axonemes treated for 3 minutes with a low concentration (0.1 microgram/ml) of trypsin beat with a more symmetrical waveform than before the treatment. Also, their microtubule sliding velocity and reverse bend angle were not affected by high Ca(2+) concentrations. Trifluoperazine (25-50 microM) had no effect on the decrease of the sliding velocity in beating flagella at 10(−5) M Ca(2+). However, the flagella that had been ‘quiescent’ at 10(−4) M Ca(2+) resumed asymmetrical beating following an application of 10–50 microM trifluoperazine. In such beating flagella, both the sliding velocity and the reverse bend angle were close to their respective values at 10(−5) M Ca(2+). Trypsin treatment induced a similar recovery of beating in quiescent flagella at 10(-)(4) M Ca(2+), albeit with a more symmetrical waveform. These results provide first evidence that, at least at ATP concentrations higher than approximately 100 microM, 10(−6)-10(−5) M Ca(2+) decreases the maximum sliding velocity of microtubules in beating flagella through a trypsin-sensitive regulatory mechanism which possibly involves the central pair apparatus. They also suggest that calmodulin may be associated with the mechanism underlying flagellar quiescence induced by 10(−4) M Ca(2+).

Effects of the central pair apparatus on microtubule sliding velocity in sea urchin sperm flagella

To produce oscillatory bending movement in cilia and flagella, the activity of dynein arms must be regulated. The central-pair microtubules, located at the centre of the axoneme, are often thought to be involved in the regulation, but this has not been demonstrated definitively. In order to determine whether the central-pair apparatus are directly involved in the regulation of the dynein arm activity, we analyzed the movement of singlet microtubules that were brought into contact with dynein arms on bundles of doublets obtained by sliding disintegration of elastase-treated flagellar axonemes. An advantage of this new assay system was that we could distinguish the bundles that contained the central pair apparatus from those that did not, the former being clearly thicker than the latter. We found that microtubule sliding occurred along both the thinner and the thicker bundles, but its velocity differed between the two kinds of bundles in an ATP concentration dependent manner. At high ATP concentrations, such as 0.1 and 1 mM, the sliding velocity on the thinner bundles was significantly higher than that on the thicker bundles, while at lower ATP concentrations the sliding velocity did not change between the thinner and the thicker bundles. We observed similar bundle width-related differences in sliding velocity after removal of the outer arms. These results provide first evidence suggesting that the central pair and its associated structures may directly regulate the activity of the inner (and probably also the outer) arm dynein.

Characterization of a Chlamydomonas insertional mutant that disrupts flagellar central pair microtubule-associated structures

Two alleles at a new locus, central pair-associated complex 1 (CPC1), were selected in a screen for Chlamydomonas flagellar motility mutations. These mutations disrupt structures associated with central pair microtubules and reduce flagellar beat frequency, but do not prevent changes in flagellar activity associated with either photophobic responses or phototactic accumulation of live cells. Comparison of cpc1 and pf6 axonemes shows that cpc1 affects a row of projections along C1 microtubules distinct from those missing in pf6, and a row of thin fibers that form an arc between the two central pair microtubules. Electron microscopic images of the central pair in axonemes from radial spoke-defective strains reveal previously undescribed central pair structures, including projections extending laterally toward radial spoke heads, and a diagonal link between the C2 microtubule and the cpc1 projection. By SDS-PAGE, cpc1 axonemes show reductions of 350-, 265-, and 79-kD proteins. When extracted from wild-type axonemes, these three proteins cosediment on sucrose gradients with three other central pair proteins (135, 125, and 56 kD) in a 16S complex. Characterization of cpc1 provides new insights into the structure and biochemistry of the central pair apparatus, and into its function as a regulator of dynein-based motility.

Computer simulation of flagellar movement: VII. Conventional but functionally different cross-bridge models for inner and outer arm dyneins can explain the effects of outer arm dynein removal

Outer arm dynein removal from flagella by genetic or chemical methods causes decreased frequency and power, but little change in bending pattern. These results suggest that outer arm dynein operates within bends to increase the speed of bend propagation, but does not produce forces that alter the bending pattern established by inner arm dyneins. A flagellar model incorporating different cross-bridge models for inner and outer arm dyneins has been examined. The inner arm dynein model has a hyperbolic force-velocity curve, with a maximum average force at 0 sliding velocity of about 14 pN for each 96 nm group of inner arm dyneins. The outer arm dynein model has a very different force-velocity curve, with a maximum force at about 10-15% of V(max). The outer arm dynein model is adjusted so that the unloaded sliding velocity for a realistic mixture of inner and outer arm dyneins is twice the unloaded sliding velocity for the inner arm dynein model alone. With these cross-bridge models, a flagellar model can be obtained that reduces its sliding velocity and frequency by approximately 50% when outer arm dyneins are removed, with little change in bending pattern. The addition of outer arm dyneins, therefore, gives an approximately 4-fold increase in power output against viscous resistances, and outer arm dyneins may generate 90% or more of the power output. Cell Motil.

The sup-pf-2 mutations of Chlamydomonas alter the activity of the outer dynein arms by modification of the gamma-dynein heavy chain

The sup-pf-2 mutation is a member of a group of dynein regulatory mutations that are capable of restoring motility to paralyzed central pair or radial spoke defective strains. Previous work has shown that the flagellar beat frequency is reduced in sup-pf-2, but little else was known about the sup-pf-2 phenotype (Huang, B., Z. Ramanis, and D.J.L. Luck. 1982. Cell. 28:115-125; Brokaw, C.J., and D.J.L. Luck. 1985. Cell Motil. 5:195-208). We have reexamined sup-pf-2 using improved biochemical and structural techniques and by the analysis of additional sup-pf-2 alleles. We have found that the sup-pf-2 mutations are associated with defects in the outer dynein arms. Biochemical analysis of sup-pf-2-1 axonemes indicates that both axonemal ATPase activity and outer arm polypeptides are reduced by 40-50% when compared with wild type. By thin-section EM, these defects correlate with an approximately 45% loss of outer dynein arm structures. Interestingly, this loss is biased toward a subset of outer doublets, resulting in a radial asymmetry that may reflect some aspect of outer arm assembly. The defects in outer arm assembly do not appear to result from defects in either the outer doublet microtubules or the outer arm docking structures, but rather appear to result from defects in outer dynein arm components. Analysis of new sup-pf-2 mutations indicates that the severity of the outer arm assembly defects varies with different alleles. Complementation tests and linkage analysis reveal that the sup-pf-2 mutations are alleles of the PF28/ODA2 locus, which is thought to encode the gamma-dynein heavy chain subunit of the outer arm. The sup-pf-2 mutations therefore appear to alter the activity of the outer dynein arms by modification of the gamma-dynein heavy chain.

PF16 encodes a protein with armadillo repeats and localizes to a single microtubule of the central apparatus in Chlamydomonas flagella

Several studies have indicated that the central pair of microtubules and their associated structures play a significant role in regulating flagellar motility. To begin a molecular analysis of these components we have generated central apparatus-defective mutants in Chlamydomonas reinhardtii using insertional mutagenesis. One paralyzed mutant recovered in our screen, D2, is an allele of a previously identified mutant, pf16. Mutant cells have paralyzed flagella, and the C1 microtubule of the central apparatus is missing in isolated axonemes. We have cloned the wild-type PF16 gene and confirmed its identity by rescuing pf16 mutants upon transformation. The rescued pf16 cells were wild-type in motility and in axonemal ultrastructure. A full-length cDNA clone for PF16 was obtained and sequenced. Database searches using the predicted 566 amino acid sequence of PF16 indicate that the protein contains eight contiguous armadillo repeats. A number of proteins with diverse cellular functions also contain armadillo repeats including pendulin, Rch1, importin, SRP-1, and armadillo. An antibody was raised against a fusion protein expressed from the cloned cDNA. Immunofluorescence labeling of wild-type flagella indicates that the PF16 protein is localized along the length of the flagella while immunogold labeling further localizes the PF16 protein to a single microtubule of the central pair. Based on the localization results and the presence of the armadillo repeats in this protein, we suggest that the PF16 gene product is involved in protein-protein interactions important for C1 central microtubule stability and flagellar motility.

Electrodynamics of microtubular motors: the building blocks of a new model

Microtubules are ubiquitous components of the cytoskeleton. They participate in many motility processes ranging from intracellular transport or chromosome movement during mitosis to ciliary and flagellar beating. The biophysical mechanism inherent in the generation and control of movement in all these motility phenomena has not yet been entirely elucidated. The authors propose a new model based on a charge transfer mechanism capable of shedding a new light on the molecular foundations of all motility processes. Electron transfer along the microtubular lattice is responsible for activation and control of all microtubule-associated ATPases (i.e. force generating enzymes). Microtubules are thus shown to be the basic motors of cell dynamics. The model is first applied to intracellular transport and ciliary and flagellar beating. Through two additional examples, the authors show the heuristic capabilities of the suggested hypothesis. The application of charge transfer control to the Protozoan Euglena gracilis leads to a plausible model capable of accounting for its phototactic response mechanism. Furthermore, the model allows a new interpretation of the electrophysiological response in vertebrate photoreceptors.

Regulation of Chlamydomonas flagellar dynein by an axonemal protein kinase

Genetic, biochemical, and structural data support a model in which axonemal radial spokes regulate dynein-driven microtubule sliding in Chlamydomonas flagella. However, the molecular mechanism by which dynein activity is regulated is unknown. We describe results from three different in vitro approaches to test the hypothesis that an axonemal protein kinase inhibits dynein in spoke-deficient axonemes from Chlamydomonas flagella. First, the velocity of dynein-driven microtubule sliding in spoke-deficient mutants (pf14, pf17) was increased to wild-type level after treatment with the kinase inhibitors HA-1004 or H-7 or by the specific peptide inhibitors of cAMP-dependent protein kinase (cAPK) PKI(6-22)amide or N alpha-acetyl-PKI(6-22)amide. In particular, the peptide inhibitors of cAPK were very potent, stimulating half-maximal velocity at 12-15 nM. In contrast, kinase inhibitors did not affect microtubule sliding in axonemes from wild-type cells. PKI treatment of axonemes from a double mutant missing both the radial spokes and the outer row of dynein arms (pf14pf28) also increased microtubule sliding to control (pf28) velocity. Second, addition of the type-II regulatory subunit of cAPK (RII) to spoke-deficient axonemes increased microtubule sliding to wild-type velocity. Addition of 10 microM cAMP to spokeless axonemes, reconstituted with RII, reversed the effect of RII. Third, our previous studies revealed that inner dynein arms from the Chlamydomonas mutants pf28 or pf14pf28 could be extracted in high salt buffer and subsequently reconstituted onto extracted axonemes restoring original microtubule sliding activity. Inner arm dyneins isolated from PKI-treated axonemes (mutant strain pf14pf28) generated fast microtubule sliding velocities when reconstituted onto both PKI-treated or control axonemes. In contrast, dynein from control axonemes generated slow microtubule sliding velocities on either PKI-treated or control axonemes. Together, the data indicate that an endogenous axonemal cAPK-type protein kinase inhibits dynein-driven microtubule sliding in spoke-deficient axonemes. The kinase is likely to reside in close association with its substrate(s), and the substrate targets are not exclusively localized to the central pair, radial spokes, dynein regulatory complex, or outer dynein arms. The results are consistent with a model in which the radial spokes regulate dynein activity through suppression of a cAMP-mediated mechanism.

Components of a "dynein regulatory complex" are located at the junction between the radial spokes and the dynein arms in Chlamydomonas flagella

Previous studies of flagellar mutants have identified six axonemal polypeptides as components of a "dynein regulatory complex" (DRC). The DRC is though to coordinate the activity of the multiple flagellar dyneins, but its location within the axoneme has been unknown (Huang et al., 1982; Piperno et al., 1992). We have used improved chromatographic procedures (Kagami and Kamiya, 1992) and computer averaging of EM images (Mastronarde et al., 1992) to analyze the relationship between the DRC and the dynein arms. Our results suggest that some of the DRC components are located at the base of the second radial spoke in close association with the inner dynein arms. (a) Averages of axoneme cross-sections indicate that inner arm structures are significantly reduced in three DRC mutants (pf3 < pf2 < sup-pf-3 < wt). (b) These defects are more pronounced in distal/medial regions of the axoneme than in proximal regions. (c) Analysis of flagellar extracts by fast protein liquid chromatography and SDS-PAGE indicates that a specific dynein I2 isoform is missing in pf3 and reduced in pf2 and sup-pf-3. Comparison with ida4 and pf3ida4 extracts reveals that this isoform differs from those missing in ida4. (d) When viewed in longitudinal section, all three DRC mutants lack a crescent-shaped density above the second radial spoke, and pf3 axonemes lack additional structures adjacent to the crescent. We propose that the crescent corresponds in part to the location of the DRC, and that this structure is also directly associated with a subset of the inner dynein arms. This position is appropriate for a complex that is thought to mediate signals between the radial spokes and the dynein arms.

Movement of Myzostomum spermatozoa: calcium ion regulation of swimming direction

Spermatozoa of the small myzostomid worm Myzostomum cirriferum usually swim with the flagellum foremost but occasionally stop and then swim with the head foremost. The spermatozoa have axoneme of the 9 + 0 type; thus each lacks the central pair microtubules. The flagellum emerges in the anterior end of the cell body and attaches to it with junctions. To understand the mechanism regulating the swimming direction of the spermatozoa, we recorded the sperm and their flagellar movements using a video camera with a high-speed shutter. The effects of calcium and viscosity on these movements were also examined. The cell body with the flagellum attached to it formed a curved plate during beating, while the free portion of the flagellum beats with small helical bends. Motive force to propel a spermatozoon was mainly due to the bends in the cell body. The spermatozoa reversed the direction of their swimming as a result of a change in the direction of bend propagation. The direction of bend propagation was regulated by calcium; the bends in the cell body propagated from the end of the head toward the free portion of the flagellum at low concentrations of Ca2+, whereas the direction of bend propagation was reversed at high concentrations of this ion. High viscosity of the medium stimulated a change in the direction of bend propagation.