Inhibition by ATP and activation by ADP in the regulation of flagellar movement in sea urchin sperm

cAMP and the Fibrous Sheath Protein CABYR (Ca2+-Binding Tyrosine-Phosphorylation-Regulated Protein) Is Required for 4D Sperm Movement

A new life starts with successful fertilization whereby one sperm from a pool of millions fertilizes the oocyte. Sperm motility is one key factor for this selection process, which depends on a coordinated flagellar movement. The flagellar beat cycle is regulated by Ca2+ entry via CatSper, cAMP, Mg2+, ADP and ATP. This study characterizes the effects of these parameters for 4D sperm motility, especially for flagellar movement and the conserved clockwise (CW) path chirality of murine sperm. Therefore, we use detergent-extracted mouse sperm and digital holographic microscopy (DHM) to show that a balanced ratio of ATP to Mg2+ in addition with 18 µM cAMP and 1 mM ADP is necessary for controlled flagellar movement, induction of rolling along the long axis and CW path chirality. Rolling along the sperm’s long axis, a proposed mechanism for sperm selection, is absent in sea urchin sperm, lacking flagellar fibrous sheath (FS) and outer-dense fibers (ODFs). In sperm lacking CABYR, a Ca2+-binding tyrosine-phosphorylation regulated protein located in the FS, the swim path chirality is preserved. We conclude that specific concentrations of ATP, ADP, cAMP and Mg2+ as well as a functional CABYR play an important role for sperm motility especially for path chirality.

Light-Powered Reactivation of Flagella and Contraction of Microtubule Networks: Toward Building an Artificial Cell

Artificial systems capable of self-sustained movement with self-sufficient energy are of high interest with respect to the development of many challenging applications, including medical treatments, but also technical applications. The bottom-up assembly of such systems in the context of synthetic biology is still a challenging task. In this work, we demonstrate the biocompatibility and efficiency of an artificial light-driven energy module and a motility functional unit by integrating light-switchable photosynthetic vesicles with demembranated flagella. The flagellar propulsion is coupled to the beating frequency, and dynamic ATP synthesis in response to illumination allows us to control beating frequency of flagella in a light-dependent manner. In addition, we verified the functionality of light-powered synthetic vesicles in in vitro motility assays by encapsulating microtubules assembled with force-generating kinesin-1 motors and the energy module to investigate the dynamics of a contractile filamentous network in cell-like compartments by optical stimulation. Integration of this photosynthetic system with various biological building blocks such as cytoskeletal filaments and molecular motors may contribute to the bottom-up synthesis of artificial cells that are able to undergo motor-driven morphological deformations and exhibit directional motion in a light-controllable fashion.

Enzyme activity in energy supply of spermatozoon motility in two taxonomically distant fish species (sterlet Acipenser ruthenus, Acipenseriformes and common carp Cyprinus carpio, Cypriniformes)

As spermatozoon motility duration differs significantly among fish species, the mechanism of ATP generation-regeneration and its distribution along the flagellum may be species-dependent. The present study compared the role of creatine kinase (CK) with that of adenylate kinase (AK) in ATP regeneration during motility of demembranated spermatozoa of taxonomically distant fish species, sterlet, and common carp, allowing investigation for the presence of the creatine-phosphocreatine (PCr) shuttle in sterlet spermatozoa. The flagellar beat frequency of demembranated spermatozoa was measured in reactivating media in the presence or absence of ATP, ADP, PCr, and CK and AK inhibitors. After demembranation, AK, CK, and total ATPase activity was measured in spermatozoon extracts. Beat frequency of demembranated spermatozoa was found to be positively correlated with ATP levels in reactivating medium and to reach a plateau at 0.8 mM and 0.6 mM ATP for carp and sterlet, respectively. It was shown for the first time that sterlet axonemal dynein ATPases have a higher affinity for ATP than do those of carp. Supplementation of reactivating medium with ADP and PCr without ATP resulted in beat frequencies comparable to that measured with 0.3 to 0.5-mM ATP for both studied species. The presence of the PCr-CK phosphagen system and its essential role in ATP regeneration were first confirmed for sturgeon spermatozoa. The inhibition of CK exerted a high impact on spermatozoon energy supply in both species, whereas the inhibition of AK was more pronounced in sterlet than in carp. This was confirmed by the quantification of enzyme activity in spermatozoon extracts. We concluded that spermatozoa of these taxonomically distant species use similar systems to supply energy for flagella motility, but with different efficacy.

Motility initiation of sterlet sturgeon (Acipenser ruthenus) spermatozoa: Describing the propagation of the first flagellar waves

In the present study, for the first time in fish spermatozoa, we describe the precise chronology of motility initiation of sterlet (sturgeon) sperm from completely immotile flagella to regular full wave propagation. The successive activation steps were investigated by high-speed video microscopy, using specific experimental situation, where sperm motility initiation was delayed in time up to several seconds (10 ± 2.68 seconds). Starting from fully immotile, the flagellum shows some trembling for a brief period, soon followed by appearance of the first real bend (so-called "principal bend") with a large wave amplitude 4.28 ± 0.65 μm, then by the "reverse bend," the latter presenting a lower (P < 0.05) wave amplitude (1.14 ± 0.32 μm). This couple of first bends formed at the basal region begins to propagate toward the flagellar tip but gradually fades when reaching the midflagellum, wherein consequently the sperm cell remains nonprogressive. This behavior repeats several times until a stage where the amplitude of the reverse bend gradually reaches a value similar that of the principal bend: The larger amplitude of this couple of bends finally leads to sustain a real "takeoff" of the sperm cell characterized by a full flagellar wave propagation generating an active forward displacement similar to that occurring during regular steady state motility (several seconds after activation). Starting from the earliest stages of motility initiation, the wave propagation along the flagellum and formation of new waves proceeded in a helical manner leading to a 3-dimensional rotation of the whole spermatozoon. Eventually, we estimated that the time period needed from the activation signal (contact with fresh water) to full wave propagation ranges from 0.4 to 1.2 seconds.

Motor domain-based motility system and motile properties of alpha heavy chain in Tetrahymena outer arm dynein

Axonemal dynein plays an essential role in ciliary motility, and impaired ciliary motility causes human diseases such as primary ciliary dyskinesia (PCD). The motor domain of axonemal dynein powers ciliary motility and its function is regulated by several accessary proteins bound to the tail region. Therefore, to understand the essential properties of dynein motility, examining the motile properties of the motor domain without the tail is necessary. In this study, the functional motor domain of the alpha heavy chain in Tetrahymena outer arm dynein was purified, and its motile properties were examined using an in vitro motility system. The purified protein caused microtubules to glide at a velocity of 5.0μm/s with their minus-end trailing, and motility was inhibited in an ATP concentration-dependent manner, which is in contrast with kinesin1. This method could be applicable to other axonemal dyneins and will enable further molecular studies on diverse axonemal dyneins and ciliary motility.

Biology of purinergic signalling: its ancient evolutionary roots, its omnipresence and its multiple functional significance

The purinergic signalling system, which utilises ATP, related nucleotides and adenosine as transmitter molecules, appeared very early in evolution: release mechanisms and ATP-degrading enzymes are operative in bacteria, and the first specific receptors are present in single cell eukaryotic protozoa and algae. Further evolution of the purinergic signalling system resulted in the development of multiple classes of purinoceptors, several pathways for release of nucleotides and adenosine, and a system of ectonucleotidases controlling extracellular levels of purinergic transmitters. The purinergic signalling system is expressed in virtually all types of tissues and cells, where it mediates numerous physiological reactions and contributes to pathological responses in a variety of diseases.

Measuring the regulation of dynein activity during flagellar motility

Flagellar and ciliary motility are driven by the activity of dynein, which produces microtubule sliding within the axonemes. Our goal is to understand how dynein motile activity is regulated to produce the characteristic oscillatory movement of flagella. Analysis of various parameters, such as frequency and shear angle in beating flagella, is important for understanding the time-dependent changes of microtubule sliding amounts along the flagellum. Demembranated flagella can be reactivated in a wide range of ATP concentrations (from 2 μM to several mM) and the beat frequency increases with an increase in ATP. By imposed vibration of a micropipette that caught a sperm head by suction, however, the oscillatory motion can be modulated so as to synchronize to the vibration frequency over a range of 20-70Hz at 2mM ATP. The time-averaged sliding velocity calculated as a product of shear angle and vibration frequency decreases when the imposed frequency is below the undriven flagellar beat frequency, but at higher imposed frequencies, it remains constant. In addition to the role of ATP, the mechanical force of bending is involved in the activation of dynein. In elastase-treated axonemes, bending-dependent regulation of microtubule sliding is achieved. This chapter provides an overview of several approaches, using sea urchin sperm flagella, to studying the measurements in the regulation of dynein activity with or without mechanical force.

Effects of iodide on the coupling between ATP hydrolysis and motile activity in axonemal dynein

Dynein transduces the chemical energy of ATP hydrolysis into mechanical work through conformational changes. To identify the factors governing the coupling between the ATPase activity and the motile activity of the dynein molecule, we examined the effects of potassium iodide, which can unfold protein tertiary structures, on dynein activity in reactivated sea urchin sperm flagella. The presence of low concentrations of KI (0.05-0.1 M) in the reactivating solution did not influence the stable beating of demembranated flagella at 0.02-1 mM ATP, when the total concentration of potassium was kept at 0.15 M by adding K-acetate. However, double-reciprocal plots of ATP concentration and beat frequency showed a mixed type of inhibition by KI, indicating the possibility that KI inhibits the ATP hydrolysis and decreases the maximum sliding velocity. The ATPase activity of 21S dynein with or without microtubules did not decrease with the KI concentration. In the elastase-treated axonemes, KI decreased the velocity of sliding disintegration, while it increased the frequency of occurrence of axonemes showing no sliding. This may be related to some defect in the coordination of dynein activities. On 21S dynein adsorbed on a glass surface, however, the velocity of microtubule sliding was increased by KI, while KI lowered the dynein-microtubule affinity. The velocity further increased under lower salt conditions enhancing the dynein-microtubule interactions. The results suggest the importance of organized regulation of the dynamic states of dynein-microtubule interactions through the stalk for the coupling between the ATPase activity and the motile activity of dynein in beating flagella.

Analysis of the role of nucleotides in axonemal dynein function

Axonemal dynein in flagella and cilia is a motor molecule that produces microtubule sliding, powered by the energy of ATP hydrolysis. Our goal is to understand how dynein motile activity is controlled to produce the characteristic oscillatory movement of flagella. ATP, the energy source for dynein, is also important as a regulator of dynein activity. Among the four nucleotide-binding sites of a dynein heavy chain, one is the primary ATP hydrolyzing site while the others are noncatalytic sites and thought to perform regulatory functions. Stable binding of both ATP and ADP to these regulatory sites is probably essential for the chemomechanical energy transduction in dynein. Although the ATP concentration in beating flagella is physiologically high and constant, at any moment in the oscillatory cycle some dynein molecules are active while others are not, and the motile activity of dynein oscillates temporally and spatially in the axoneme. It is likely that the basic mechanism underlying the highly dynamic control of dynein activity involves the ATP-dependent inhibition and ADP-dependent activation (or release of inhibition) of dynein. How the inhibition and activation can be induced in beating flagella is still unknown. It seems, however, that the mechanical force of bending is involved in the activation of dynein, probably through the control of noncatalytic nucleotide binding to dynein. This chapter provides an overview of several approaches, using sea urchin sperm flagella, to studying the roles of ATP and ADP in the regulation of dynein activity with or without the mechanical signal of bending.

Asymmetry of inner dynein arms and inter-doublet links in Chlamydomonas flagella

Although the widely shared “9 + 2” structure of axonemes is thought to be highly symmetrical, axonemes show asymmetrical bending during planar and conical motion. In this study, using electron cryotomography and single particle averaging, we demonstrate an asymmetrical molecular arrangement of proteins binding to the nine microtubule doublets in Chlamydomonasreinhardtii flagella. The eight inner arm dynein heavy chains regulate and determine flagellar waveform. Among these, one heavy chain (dynein c) is missing on one microtubule doublet (this doublet also lacks the outer dynein arm), and another dynein heavy chain (dynein b or g) is missing on the adjacent doublet. Some dynein heavy chains either show an abnormal conformation or were replaced by other proteins, possibly minor dyneins. In addition to nexin, there are two additional linkages between specific pairs of doublets. Interestingly, all these exceptional arrangements take place on doublets on opposite sides of the axoneme, suggesting that the transverse functional asymmetry of the axoneme causes an in-plane bending motion.

Bending-induced switching of dynein activity in elastase-treated axonemes of sea urchin sperm--roles of Ca2+ and ADP

Flagellar beating is caused by microtubule sliding, driven by the activity of dynein, between adjacent two of the nine doublet microtubules. An essential process in the regulation of dynein is to alternate its activity (switching) between the two sides of the central pair microtubules. The switching of dynein activity can be detected, in an in vitro system using elastase-treated axonemes of sea urchin sperm flagella, as a reversal of the relative direction of ATP-induced sliding between the two bundles of doublets at high Ca(2+) (10(-4) M) at pH 7.8-8.0. The reversal is triggered by externally applied bending of the doublet bundle. However, the mechanism of this bending-induced reversal (or backward sliding) remains unclear. To understand how the switching of dynein activity in flagella can be induced by bending, we studied the roles of ADP, which is an important factor for the dynein motile activity, and of Ca(2+) in the bending-induced reversal of microtubule sliding between two bundles of doublets at pH 7.5 and 7.2. We found that the reversal of sliding direction was induced regardless of the concentrations of Ca(2+) at low pH, but occurred more frequently at low Ca(2+) (<10(-9) M) than at high Ca(2+). At pH 7.5, an application of ADP increased the frequency of occurrence of backward sliding at high as well as low concentrations of Ca(2+). The results indicate that ADP-dependent activation of dynein, probably resulting from ADP-binding to dynein, is involved in the regulation of the bending-induced switching of dynein activity in flagella.

Central pair apparatus enhances outer-arm dynein activities through regulation of inner-arm dyneins

The beating of eukaryotic cilia and flagella is controlled by multiple species of inner-arm and outer-arm dyneins. To clarify the regulation on axonemal beating by nucleotide conditions and central-pair microtubules, microtubule sliding in disintegrating Chlamydomonas axonemes of various mutants and in vitro microtubule gliding by isolated axonemal dyneins were examined. In the in vitro motility assays with outer-arm dyneins (alphabeta and gamma), microtubule translocation velocity decreased at high concentrations of ATP, while this inhibition was canceled by the simultaneous presence of ADP or ribose-modified analogues, mantATP/ADP. In contrast, motility of inner-arm dyneins was rather insensitive to these nucleotides. The velocity of sliding disintegration in axonemes lacking the central pair was less than that in wild-type axonemes at high ATP concentrations, but was overcome by the presence of ADP or mantATP/ADP. While these nucleotides did not activate the sliding velocity in other mutant axonemes, they increased the extent of sliding, except for axonemes lacking outer-arm dynein. Experiments with axonemes lacking inner-arm dynein f using casein kinase 1 inhibitor suggest that the regulation of outer-arm dynein by the central pair is effected through the activation of inner-arm dynein f, and possibly by other interactions. These results indicate that the central pair activates outer-arm dyneins on specific outer-doublet, resulting in amplification of the axonemal bending force.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Dynein pulls microtubules without rotating its stalk

Dynein is a microtubule motor that powers motility of cilia and flagella. There is evidence that the relative sliding of the doublet microtubules is due to a conformational change in the motor domain that moves a microtubule bound to the end of an extension known as the stalk. A predominant model for the movement involves a rotation of the head domain, with its stalk, toward the microtubule plus end. However, stalks bound to microtubules have been difficult to observe. Here, we present the clearest views so far of stalks in action, by observing sea urchin, outer arm dynein molecules bound to microtubules, with a new method, "cryo-positive stain" electron microscopy. The dynein molecules in the complex were shown to be active in in vitro motility assays. Analysis of the electron micrographs shows that the stalk angles relative to microtubules do not change significantly between the ADP.vanadate and no-nucleotide states, but the heads, together with their stalks, shift with respect to their A-tubule attachments. Our results disagree with models in which the stalk acts as a lever arm to amplify structural changes. The observed movement of the head and stalk relative to the tail indicates a new plausible mechanism, in which dynein uses its stalk as a grappling hook, catching a tubulin subunit 8 nm ahead and pulling on it by retracting a part of the tail (linker).

Mechanism of flagellar oscillation–bending-induced switching of dynein activity in elastase-treated axonemes of sea urchin sperm

Oscillatory movement of eukaryotic flagella is caused by dynein-driven microtubule sliding in the axoneme. The mechanical feedback from the bending itself is involved in the regulation of dynein activity, the main mechanism of which is thought to be switching of the activity of dynein between the two sides of the central pair microtubules. To test this, we developed an experimental system using elastase-treated axonemes of sperm flagella, which have a large Ca2+-induced principal bend (P-bend) at the base. On photoreleasing ATP from caged ATP, they slid apart into two bundles of doublets. When the distal overlap region of the slid bundles was bent in the direction opposite to the basal P-bend, backward sliding of the thinner bundle was induced along the flagellum including the bent region. The velocity of the backward sliding was significantly lower than that of the forward sliding, supporting the idea that the dynein activity alternated between the two sides of the central pair on bending. Our results show that the combination of the direction of bending and the conformational state of dynein-microtubule interaction induce the switching of the dynein activity in flagella, thus providing the basis for flagellar oscillation.