Studies on the eel sperm flagellum. 3. Vibratile motility and rotatory bending

The mechanics of cilia and flagella: What we know and what we need to know

In this review, we provide a condensed overview of what is currently known about the mechanical functioning of the flagellar/ciliary axoneme. We also present a list of 10 specific areas where our current knowledge is incomplete and explain the benefits of further experimental investigation. Many of the physical parameters of the axoneme and its component parts have not been determined. This limits our ability to understand how the axoneme structure contributes to its functioning in several regards. It restricts our ability to understand how the mechanics of the structure contribute to the regulation of motor function. It also confines our ability to understand the three-dimensional workings of the axoneme and how various beating modes are accomplished. Lastly, it prevents accurate computational modeling of the axoneme in three-dimensions.

Melatonin regulates ATP content and fertilising capacity of Onychostoma macrolepis spermatozoa by inhibiting ROS accumulation during semen storage in vitro

Melatonin (MLT) is an efficient antioxidant that protects spermatozoa against damages caused by oxidative stress. In this study, to maintain good function of Onychostoma macrolepis spermatozoa during semen preservation invitro at 4°C, different concentrations of MLT (0.5, 1 and 2μM) were added to the semen. After storage (0, 24, 48 and 72h), 1μM MLT in semen markedly improved sperm quality, as reflected by better plasma membrane integrity, the relative steady level of reactive oxygen species (ROS) and slower rate of decrease in mitochondrial membrane potential. Activated spermatozoa in semen with 1μM MLT had higher kinematic performance (i.e. percentage of motile and progressive spermatozoa and the beat cross frequency; P<0.05) and longer duration of sperm motility (P<0.05) compared with spermatozoa in semen withother MLT concentrations. Furthermore, 1μM MLT maintained higher ATP concentrations in spermatozoa during semen storage and significantly improved the fertilising capacity of spermatozoa after 72h semen storage compared with the other MLT concentrations. To expand wild resources of O. macrolepis, 1μM MLT can be used as a semen additive to maintain better sperm function and enhance sperm fertilising capacity in artificial insemination (AI).

Rsph4a is essential for the triplet radial spoke head assembly of the mouse motile cilia

Motile cilia/flagella are essential for swimming and generating extracellular fluid flow in eukaryotes. Motile cilia harbor a 9+2 arrangement consisting of nine doublet microtubules with dynein arms at the periphery and a pair of singlet microtubules at the center (central pair). In the central system, the radial spoke has a T-shaped architecture and regulates the motility and motion pattern of cilia. Recent cryoelectron tomography data reveal three types of radial spokes (RS1, RS2, and RS3) in the 96 nm axoneme repeat unit; however, the molecular composition of the third radial spoke, RS3 is unknown. In human pathology, it is well known mutation of the radial spoke head-related genes causes primary ciliary dyskinesia (PCD) including respiratory defect and infertility. Here, we describe the role of the primary ciliary dyskinesia protein Rsph4a in the mouse motile cilia. Cryoelectron tomography reveals that the mouse trachea cilia harbor three types of radial spoke as with the other vertebrates and that all triplet spoke heads are lacking in the trachea cilia of Rsph4a-deficient mice. Furthermore, observation of ciliary movement and immunofluorescence analysis indicates that Rsph4a contributes to the generation of the planar beating of motile cilia by building the distal architecture of radial spokes in the trachea, the ependymal tissues, and the oviduct. Although detailed mechanism of RSs assembly remains unknown, our results suggest Rsph4a is a generic component of radial spoke heads, and could explain the severe phenotype of human PCD patients with RSPH4A mutation.

Reconsideration of the evaluation criteria for bull ejaculated sperm motility in the context of rotation

Progressive movement of spermatozoa has conventionally been regarded as a good indicator of motility. However, bull spermatozoa exhibit two types of progressive movement: progressive/planar movement without rotation and progressive/helical movement with rotation. The aim of this study was to reconsider the evaluation criteria of bull ejaculated sperm motility in the context of rotation. Here, we compared the movement patterns of ejaculated spermatozoa with relatively high and low protein kinase A (PKA)-mediated signaling activities, because sperm motility is positively regulated by PKA-mediated signaling activities. We prepared sperm samples with high and low PKA-mediated signaling activities by suspending spermatozoa in media containing either the stimulator (NaHCO₃) or inhibitor (KH-7) of adenylyl cyclase 10, and we then investigated movement patterns and relative velocities using a microscopic high-speed camera and recording system. In the control medium without NaHCO₃ and KH-7, most spermatozoa exhibited round/planar movement without rotation and asymmetrical bends in the principal pieces. NaHCO₃ significantly promoted changes in movement patterns from round/planar movement to progressive/planar movement (without rotation) as well as symmetrization of flagellar bends and increased relative velocities. KH-7 significantly increased spermatozoa exhibiting progressive/helical movement (with rotation), decreased relative velocities, and symmetrized flagellar bends with a reduction in their size. These indicate that progressive/planar movement (without rotation) and fast movement characterize the movement patterns of bull ejaculated spermatozoa with high PKA-mediated signaling activities. A sign of reduced PKA-mediated signaling activity is not only slow movement but also helical movement (with rotation). Thus, it is beneficial to add a new parameter of “rotation” to the evaluation criteria of bull ejaculated sperm motility.

Absence of Radial Spokes in Mouse Node Cilia Is Required for Rotational Movement but Confers Ultrastructural Instability as a Trade-Off

Determination of left-right asymmetry in mouse embryos is established by a leftward fluid flow that is generated by clockwise rotation of node cilia. How node cilia achieve stable unidirectional rotation has remained unknown, however. Here we show that brief exposure to the microtubule-stabilizing drug paclitaxel (Taxol) induces randomly directed rotation and changes the ultrastructure of node cilia. In vivo observations and a computer simulation revealed that a regular 9+0 arrangement of doublet microtubules is essential for stable unidirectional rotation of node cilia. The 9+2 motile cilia of the airway, which manifest planar beating, are resistant to Taxol treatment. However, the airway cilia of mice lacking the radial spoke head protein Rsph4a undergo rotational movement instead of planar beating, are prone to microtubule rearrangement, and are sensitive to Taxol. Our results suggest that the absence of radial spokes allows node cilia to rotate unidirectionally but, as a trade-off, renders them ultrastructurally fragile.

Subpopulation pattern of eel spermatozoa is affected by post-activation time, hormonal treatment and the thermal regimen

There has been a marked reduction in natural stocks of eels (genus Anguilla) over the past 60 years, and the culture of eels is still based on the capture of very large quantities of juveniles. It is necessary to close the life cycle in captivity in order to ease the pressure on wild populations. The aims of the present study were to evaluate sperm subpopulations (through cluster analysis of computer-aided sperm analysis data) in the European eel (Anguilla anguilla) and to assess the effects of motility acquisition time after activation (i.e. at 30, 60 and 90 s), the thermal regimen (i.e. 10°C (T10) or 15°C (T15) and up to 20°C, or constant at 20°C (T20)) and hormonal treatments (i.e. human chorionic gonadotropin (hCG), recombinant (r) hCG or pregnant mare serum gonadotropin (PMSG)) on these subpopulations. In all cases, we obtained three subpopulations of spermatozoa: low velocity and linear (S1); high velocity with low linearity (S2); and high velocity and linear (S3; considered high quality). Total motility and S1 were affected by acquisition time; thus, 30 s is recommended as the standard time for motility acquisition. When eels were kept at 20°C (T20), motility data fitted quadratic models, with the highest motility and proportion of S3 between Weeks 8 and 12 after the first injection. Lower temperatures (T10, T15) delayed spermiation and the obtaining of high-quality spermatozoa (S3), but did not seem to alter the spermiation process (similar subpopulation pattern). Conversely, the hormonal treatments altered both the dynamics of the subpopulation pattern and the onset of spermiation (with PMSG delaying it). Total motility and the yield of S3 with the widely used hCG treatment varied throughout the spermiation period. However, using rhCG allowed us to obtain high-quality and constant motility for most of the study (Weeks 7–20), and the S3 yield was also higher overall (61.8 ± 1.3%; mean ± s.e.m.) and more stable over time than the other hormonal treatments (averaging 53.0 ± 1.4%). Using T20 and rhCG would be more economical and practical, allowing us to obtain a higher number of S3 spermatozoa over an extended time.

Structure and function of vertebrate cilia, towards a new taxonomy

In this review, we propose a new classification of vertebrate cilia/flagella and discuss the evolution and prototype of cilia. Cilia/flagella are evolutionarily well-conserved membranous organelles in eukaryotes and serve a variety of functions, including motility and sensation. Vertebrate cilia have been traditionally classified into conventional motile cilia and sensory primary cilia. However, an avalanche of emerging evidence on the variations of cilia has made it almost impossible to classify them in a simple dichotomic manner. For example, conventional motile cilia are also involved in the sensation of bitter taste to facilitate the beating of cilia as a defense system of the respiratory system. On the other hand, the primary cilium, often regarded as a non-motile sensory organelle, has been revealed to be motile in vertebrate embryonic nodes, where they play a crucial role in the determination of left-right asymmetry of the body. Moreover, choroid plexus epithelial cells in the cerebral ventricular system exhibit multiple primary cilia on a single cell. Considering these lines of evidence on the diversity of cilia, we believe the classification of cilia should be based on their structure and function, and include more detailed criteria. Another intriguing issue is how in the evolution of cilia, their function and morphology are combined. For example, has motility been acquired from originally sensory cilia, or vice versa? Alternatively, were they originally hybrid in nature? These questions are inseparable from the classification of cilia per se. We would like to address these conundrums in this review article, principally from the standpoint of differentiation of the animal cell.

Molecular and physiological study of the artificial maturation process in European eel males: from brain to testis

European eel males can be artificially matured (1.5IU hCG/g fish), but the regulatory mechanisms of their reproductive development are practically unknown. Spermatogenic stages (S1-S6), biometric characters [eye index (EI), gonadosomatic index (GSI), hepatosomatic index (HSI)] and sperm quality parameters (motility, viability and head spermatozoa morphometry) were analysed. Moreover, the present study evaluated the expression of GnRHs (mammal and chicken II Gonadotropin Release Hormone I) and gonadotrophins (FSHbeta and LHbeta) during hormonal treatment, as well as 11-ketotestosterone (11-KT) and 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) plasma levels. One week was enough to observe the S2 of gonad development, but it was necessary to reach the 7th week of treatment to obtain animals that presented the most advanced stage of development (S6). Differential regulation of the two GnRH expressions was found, supporting the main role of mGnRH in the control of gonadotrophin release. One hCG injection was enough to dramatically decrease the FSHbeta expression, being close to zero during the rest of the treatment. LHbeta expression and 17,20beta-P registered a significant increase in the same stage of development, S3/4, confirming the role of this gonadotrophin in the last steps of maturation and 17,20beta-P in the spermatozoa maturation. The 11-KT increased with GSI, and the highest 11-KT values coincided with the advanced steps of spermatogenesis prior to spermiation. Being consistent with the known role of the steroid in these processes. Furthermore, this study supports a role for 11-KT in stimulating eye growth, presenting high values when EI increased. Sperm production was obtained from the 4th week of treatment, but it was in the 8th week when a significant increase was observed in sperm quality [viability, high motility (>75%)].

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.

Structural insights into microtubule doublet interactions in axonemes

Coordinated sliding of microtubule doublets, driven by dynein motors, produces periodic beating of eukaryotic cilia and flagella. Recent structural studies of the axoneme, which forms the core of cilia and flagella, have used cryo-electron tomography to reveal new details of the interactions between some of the multitude of proteins that form the axoneme and regulate its movement. Connections between the several types of dyneins, in particular, suggest ways in which their action might be coordinated. Study of the molecular architecture of isolated doublets has provided a structural basis for understanding mechanical properties related to the bending of the axoneme, and has also offered insight into the potential role of doublets in the mechanism of dynein activity regulation.

Changes in Mesopotamian spiny eel, Mastacembelus mastacembelus (Bank & Solender in Russell, 1794) (Mastacembelidae) milt quality during a spawning period

This study was carried out to determine the milt quality in male Mesopotamian spiny eel (Mastacembelus mastacembelus; Bank & Solender in Russell, 1794) during spawning season. Review was performed using 58 M. mastecembulus males captured from Atatürk Dam Lake in Turkey. Milt samples were collected and evaluated daily during sampling period. In collected milt, volume of milt, sperm motility percentage, the duration of forward motility, sperm concentration, milt production and milt pH were evaluated. On June 10, 2005, 72% of the males were spermiating, which increased to 100% in the middle of June and dropped to 72% by late June. Sperm motility percentage showed a significant improvement after beginning of spermiation (P<0.001). Sperm concentration changed significantly (P<0.001) in milt collected during sampling period. Duration of sperm motility increased significantly (P<0.001) from beginning (8.22 min) through end (37.33 min) of spermiation. Milt production was maximal on June 15th, decreasing from a mean value of 3.12-0.46 x 10(9)/ml. The high pH in milt was observed during the week of spermiation (P<0.001). In conclusion, the results show that season has a significant influence on semen quality in male M. mastecembulus corresponding to spawning season in hot arid. Milt of good quality was especially collected in the middle of June. Spawning season in this fish species was found to be short.

An integrated approach to the characterization of cell movement

BACKGROUND

Most phenomena in developmental biology involve or depend upon cell migration. This article describes a comprehensive framework for the characterization and analysis of trajectories defined by cell movement. The following two perspectives are considered: (a) the behavior of each individual cell and (b) interactions between neighboring pairs of cells.

METHODS

The measurements considered for individual trajectories include the velocity magnitude and orientation, maximum spatial dispersion, displacement effectiveness, and displacement entropies. Interactions between two trajectories are characterized by comparing the respective velocities.

RESULTS

The potential of the overall framework is illustrated using data of moving cells in different biological environments. The work shows that it is possible to use the new algorithm presented here to characterize cell motility.

CONCLUSIONS

The features of the algorithm were successful in determining the motility changes under different experimental conditions.

Cyclical interactions between two outer doublet microtubules in split flagellar axonemes

The beating of cilia and flagella is based on the localized sliding between adjacent outer doublet microtubules; however, the mechanism that produces oscillatory bending is unclear. To elucidate this mechanism, we examined the behavior of frayed axonemes of Chlamydomonas by using high-speed video recording. A pair of doublet microtubules frequently displayed association and dissociation cycles in the presence of ATP. In many instances, the dissociation of two microtubules was not accompanied by noticeable bending, suggesting that the dynein-microtubule interaction is not necessarily regulated by the microtubule curvature. On rare occasions, association and dissociation occurred simultaneously in the same interacting pair, resulting in a tip-directed movement of a stretch of gap between the pair. Based on these observations, we propose a model for cyclical bend propagation in the axoneme.

Nodal cilia dynamics and the specification of the left/right axis in early vertebrate embryo development

Nodal cilia dynamics is a key factor for left/right axis determination in mouse embryos through the induction of a leftward fluid flow. So far it has not been clearly established how such dynamics is able to induce the asymmetric leftward flow within the node. Herein we propose that an asymmetric two-phase nonplanar beating cilia dynamics that involves the bending of the ciliar axoneme is responsible for the leftward fluid flow. We support our proposal with a host of hydrodynamic arguments, in silico experiments and in vivo video microscopy data in wild-type embryos and inv mutants. Our phenomenological modeling approach underscores how the asymmetry and speed of the flow depends on different relevant parameters. In addition, we discuss how the combination of internal and external mechanisms might cause the two-phase beating cilia dynamics.

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

A study of helical and planar waves on sea urchin sperm flagella, with a theory of how they are generated

When the spermatozoon of Echinus esculentus swims in sea water containing methyl cellulose (viscosity 1.5-4 Pa s), its flagellum may generate either a helical or a planar waveform, each type being stable. The helical wave, which is dextral, is complicated by the concurrent passage of miniature waves along it. These miniature waves have a pulsatile origin in the neck region of the spermatozoon. Our videotape analysis indicates that there are two pulses of mechanical activity for each true cycle of the helical wave. (The true helical frequency was obtained from the apparent wave frequency and the roll frequency of the sperm head, the latter being detectable in some sperm when lit stroboscopically.) The planar wave has a meander shape. During the propagation of planar waves, the sliding displacements are adjustable in either direction; moribund flagella can undergo unrestricted sliding. The planar waves are, in fact, exactly planar only at interfaces. Otherwise, there tend to be torsions in the interbend segments between planar bends. Mechanical stimulation of the flagellum can cause a sudden transition from the helical to the planar waveform. To account for the two modes of beating, we advance the hypothesis that circumferential linkages yield beyond a threshold strain. Whether this yield point is exceeded, we suggest, depends upon the balance between the active shear force and the external viscosity (among other factors). We propose that a subthreshold force originates in one array and then triggers the other dynein arrays circumferentially, but unidirectionally, around the base of the flagellum; whereas a suprathreshold force provokes bi-directional circumferential triggering. These may be the two patterns of activation that result in helical and planar waveforms, respectively. The transition from helical to planar bending may result from an increment in the force produced by the dynein motors. The pulsatile origin of the helical wave resembles behaviour described previously for spermatozoa of Ciona intestinalis and of the quail Coturnix coturnix.

Three-dimensional motion of avian spermatozoa

Observations have been made on spermatozoa from the domestic fowl, quail and pigeon (non-passerine birds) and also from the starling and zebra finch (passerine birds). In free motion, all these spermatozoa roll (spin) continuously about the progression axis, whether or not they are close to a plane surface. Furthermore, the direction of roll is consistently clockwise (as seen from ahead). The flagellar wave has been shown to be helical and dextral (as predicted) for domestic fowl sperm when they swim rapidly in low viscosity salines. Calculations have shown that their forward velocity is consistent with their induced angular velocity but that the size of the sperm head is suboptimal for progression speed under these conditions. Dextrally helical waves also occur on the distal flagellum of fowl, quail and pigeon sperm in high viscosity solutions. But in other cases, the mechanism of torque-generation is more problematical. The problem is most profound for passerine sperm, in that typically these cells spin rapidly while seeming to remain virtually straight. Because there is no evidence for a helical wave on these flagella, we have considered other possible means whereby rotation about the local flagellar axis (self-spin) might be achieved. Sometimes, passerine sperm, while maintaining their spinning motion, adopt a fixed curvature; this must be an instance of bend-transfer circumferentially around the axonemal cylinder-though the mechanism is obscure. It is suggested that the self-spin phenomenon may be occurring in non-passerine sperm that in some circumstances spin persistently, yet without expressing regular helical waves. More complex waves are apparent in non-passerine sperm swimming in high viscosity solutions: added to the small scale bends is a large scale, sinistrally helical curvature of the flagellum. It is argued that the flagellum follows this sinistrally helical path (i.e. "screws" though the fluid) because of the shape of the sperm head and the angle at which the flagellum is inserted into it. These conclusions concerning avian sperm motility are thought to have relevance to other animal groups. Also reported are relevant aspects of flagellar ultrastructure for pigeon and starling sperm.