Coordination of outer arm dynein activity along axonemal doublet microtubules

Torque generating properties of Tetrahymena ciliary three-headed outer-arm dynein

Eukaryotic cilia/flagella are cellular bio-machines that drive the movement of microorganisms. Molecular motor axonemal dyneins in the axoneme, which consist of an 9 + 2 arrangement of microtubules, play an essential role in ciliary beating. Some axonemal dyneins have been shown to generate torque coupled with the longitudinal motility of microtubules across an array of dyneins fixed to the coverglass surface, resulting in a corkscrew-like translocation of microtubules. In this study, we performed three-dimensional tracking of a microbead coated with axonemal outer-arm dyneins on a freely suspended microtubule. We found that microbeads coated with multiple outer-arm dyneins exhibited continuous right-handed helical trajectories around the microtubule. This unidirectional helical motion differs from that of other types of cytoplasmic dyneins, which exhibit bidirectional helical motility. We also found that, in an in vitro microtubule gliding assay, gliding microtubules driven by outer-arm dyneins tend to turn to the left, causing a curved path, suggesting that the outer-arm dynein itself is able to rotate on its own axis. Two types of torque generated by the axonemal dyneins, corresponding to the forces used to rotate the microtubule unidirectionally with respect to the long and short axes, may regulate ciliary beating with complex waveforms.

Structures of outer-arm dynein array on microtubule doublet reveal a motor coordination mechanism

Thousands of outer-arm dyneins (OADs) are arrayed in the axoneme to drive a rhythmic ciliary beat. Coordination among multiple OADs is essential for generating mechanical forces to bend microtubule doublets (MTDs). Using electron microscopy, we determined high-resolution structures of Tetrahymena thermophila OAD arrays bound to MTDs in two different states. OAD preferentially binds to MTD protofilaments with a pattern resembling the native tracks for its distinct microtubule-binding domains. Upon MTD binding, free OADs are induced to adopt a stable parallel conformation, primed for array formation. Extensive tail-to-head (TTH) interactions between OADs are observed, which need to be broken for ATP turnover by the dynein motor. We propose that OADs in an array sequentially hydrolyze ATP to slide the MTDs. ATP hydrolysis in turn relaxes the TTH interfaces to effect free nucleotide cycles of downstream OADs. These findings lead to a model explaining how conformational changes in the axoneme produce coordinated action of dyneins.

A computational model of dynein activation patterns that can explain nodal cilia rotation

Normal left-right patterning in vertebrates depends on the rotational movement of nodal cilia. In order to produce this ciliary motion, the activity of axonemal dyneins must be tightly regulated in a temporal and spatial manner; the specific activation pattern of the dynein motors in the nodal cilia has not been reported. Contemporary imaging techniques cannot directly assess dynein activity in a living cilium. In this study, we establish a three-dimensional model to mimic the ciliary ultrastructure and assume that the activation of dynein proteins is related to the interdoublet distance. By employing finite-element analysis and grid deformation techniques, we simulate the mechanical function of dyneins by pairs of point loads, investigate the time-variant interdoublet distance, and simulate the dynein-triggered ciliary motion. The computational results indicate that, to produce the rotational movement of nodal cilia, the dynein activity is transferred clockwise (looking from the tip) between the nine doublet microtubules, and along each microtubule, the dynein activation should occur faster at the basal region and slower when it is close to the ciliary tip. Moreover, the time cost by all the dyneins along one microtubule to be activated can be used to deduce the dynein activation pattern; it implies that, as an alternative method, measuring this time can indirectly reveal the dynein activity. The proposed protein-structure model can simulate the ciliary motion triggered by various dynein activation patterns explicitly and may contribute to furthering the studies on axonemal dynein activity.

Additive-free digital microfluidics

Digital microfluidics, a technique for manipulation of droplets, is becoming increasingly important for the development of miniaturized platforms for laboratory processes. Despite the enthusiasm, droplet motion is frequently hindered by the desorption of proteins or other analytes to surfaces. Current approaches to minimize this unwanted surface fouling involve the addition of extra species to the droplet or its surroundings, which might be problematic depending on the droplet content. Here, a new strategy is introduced to move droplets containing cells and other analytes on solid substrates, without extra moieties; in particular, droplets with bovine serum albumin could be moved at a concentration 2000 times higher than previously reported (without additives). This capability is achieved by using a soot-based superamphiphobic surface combined with a new device geometry, which favors droplet rolling. Contrasting with electrowetting, wetting forces are not required for droplet motion.

Ciliary behaviour and mechano-transduction in the embryonic node: computational testing of hypotheses

Left-right symmetry breaking in the mammalian embryo is believed to occur in a transient embryonic structure, the node: rotational motion of cilia within this structure creates a leftward flow of liquid that is the first asymmetric event observed. A hypothesis, often referred to as the "two-cilia" hypothesis, proposes that the node contains two kinds of primary cilia: motile cilia, driven by motor proteins, that rotate clockwise generating the leftward flow and passive cilia that act as mechano-sensors, reacting mechanically to the emerging flow. The exact mechanism that underlies the initial breaking of symmetry remains unclear, in spite of several studies that have attempted to elucidate the processes involved. In this paper, we present two computational models to (i) simulate the unidirectional flow induced by the active ciliary motion as well as their propulsion on the passive cilia and to (ii) investigate the protein activity that produces the active ciliary rotation-like movement. The models presented incorporate methodologies from computational fluid dynamics, deformable mesh computational techniques and fluid-structure interaction analysis. By solving the three-dimensional unsteady transport equations, with suitable boundary conditions, we confirm that the whirling motion of active cilia is capable of inducing the unidirectional flow and that the passive cilia are pushed by this flow towards the left with a visible deformation of 41.7% of the ciliary length on the tip, supporting the plausibility of the two-cilia hypothesis. Further, by applying finite element analysis and grid deformation techniques, we investigate the ciliary motion triggered by the activation of protein motors and propose a possible dynein activation pattern that is able to produce the clockwise rotation of embryonic cilia.

Nucleotide-induced global conformational changes of flagellar dynein arms revealed by in situ analysis

Outer and inner dynein arms generate force for the flagellar/ciliary bending motion. Although nucleotide-induced structural change of dynein heavy chains (the ATP-driven motor) was proven in vitro, our lack of knowledge in situ has precluded an understanding of the bending mechanism. Here we reveal nucleotide-induced global structural changes of the outer and inner dynein arms of Chlamydomonas reinhardtii flagella in situ using electron cryotomography. The ATPase domains of the dynein heavy chains move toward the distal end, and the N-terminal tail bends sharply during product release. This motion could drive the adjacent microtubule to cause a sliding motion. In contrast to in vitro results, in the presence of nucleotides, outer dynein arms coexist as clusters of apo or nucleotide-bound forms in situ. This implies a cooperative switching, which may be related to the mechanism of bending.

Flagellar and ciliary beating: the proven and the possible

The working mechanism of the eukaryotic flagellar axoneme remains one of nature's most enduring puzzles. The basic mechanical operation of the axoneme is now a story that is fairly complete; however, the mechanism for coordinating the action of the dynein motor proteins to produce beating is still controversial. Although a full grasp of the dynein switching mechanism remains elusive, recent experimental reports provide new insights that might finally disclose the secrets of the beating mechanism: the special role of the inner dynein arms, especially dynein I1 and the dynein regulatory complex, the importance of the dynein microtubule-binding affinity at the stalk, and the role of bending in the selection of the active dynein group have all been implicated by major new evidence. This Commentary considers this new evidence in the context of various hypotheses of how axonemal dynein coordination might work.