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

Modulatory mechanisms of sliding of nine outer doublet microtubules for generating planar and half-helical flagellar waves

It has been shown that sperm flagellar motility is generated and modulated by metachronal sliding and two types of synchronous sliding of the outer doublet microtubules. Metachronal sliding propagates around the axoneme circumferentially from one doublet to another along the flagellum, whereas the two types of synchronous sliding occur synchronously throughout an extended region along the doublet microtubules. Oscillatory synchronous sliding occurs between most pairs of the nine doublet microtubules, whereas non-oscillatory synchronous sliding occurs between a specific pair of the nine doublet microtubules. These types of sliding coexist in the flagellum and create beat cycles of flagellar movement. The circumferential propagation of active sliding around the nine doublet microtubules in the metachronal sliding suggests that it is easier for a flagellum to produce helical waves than planar waves. Most sperm flagellar movements are planar to a certain extent. Therefore, mechanisms that modulate the helical waves into planar waves may be present. Structures such as the central pair microtubules in 9 + 2 sperm flagella and the fusion of fibrous-sheath and 3-,8-doublet microtubules in mammalian sperm flagella partition the nine outer doublet microtubules into two groups. Accordingly, the sliding between these two groups generates planar flagellar waves. A similar effect is caused by the sliding between a specific pair of the nine doublet microtubules of the non-oscillatory synchronous sliding, occurring in a Ca2+ concentration-dependent manner. These hard- and soft-wired systems produce the nearly planar flagellar waves required for the efficient propulsion of spermatozoa.

Four-dimensional analysis by high-speed holographic imaging reveals a chiral memory of sperm flagella

Here high-speed Digital Holographic Microscopy (DHM) records sperm flagellar waveforms and swimming paths in 4 dimensions (X, Z, and t). We find flagellar excursions into the Z-plane nearly as large as the envelope of the flagellar waveform projected onto the XY-plane. These Z-plane excursions travel as waves down the flagellum each beat cycle. DHM also tracks the heads of free-swimming sperm and the dynamics and chirality of rolling of sperm around their long axis. We find that mouse sperm roll CW at the maximum positive Z-plane excursion of the head, then roll CCW at the subsequent maximum negative Z-plane excursion. This alternating chirality of rolling indicates sperm have a chiral memory. Procrustes alignments of path trajectories for sequences of roll-counterroll cycles show that path chirality is always CW for the cells analyzed in this study. Human and bull sperm lack distinguishable left and right surfaces, but DHM still indicates coordination of Z-plane excursions and rolling events. We propose that sperm have a chiral memory that resides in a hypothetical elastic linkage within the flagellar machinery, which stores some of the torque required for a CW or CCW roll to reuse in the following counter-roll. Separate mechanisms control path chirality.

Regulatory mechanisms of sperm flagellar motility by metachronal and synchronous sliding of doublet microtubules

STUDY QUESTION

What is the role of metachronal and synchronous sliding in sperm flagellar motility?

SUMMARY ANSWER

Both metachronal and oscillatory synchronous sliding are essential for sperm flagellar motility, while the change in mode of synchronous sliding between the non-oscillatory synchronous sliding of a specific pair of the doublet microtubules and the oscillatory synchronous sliding between most pairs of doublet microtubules modulates the sperm flagellar motility.

WHAT IS KNOWN ALREADY

Metachronal and synchronous sliding of doublet microtubules are involved in sperm flagellar motility and regulation of these sliding movements controls flagellar bend formation.

STUDY DESIGN, SIZE, DURATION

To study the regulatory mechanisms of metachronal and synchronous sliding in flagellar movement of golden hamster spermatozoa, changes in these sliding movements during hyperactivation were examined by measuring the angle of the tangent to the flagellar shaft with reference to the central axis of the sperm head (the shear angle) along the flagellum. Golden hamster spermatozoa were obtained from the caudal epididymis of five sexually mature golden hamsters. Results from three experiments were averaged. The number of spermatozoa analyzed is 15 activated sperm, 22 hyperactivated sperm and 20 acrosome-reacted sperm.

PARTICIPANTS/MATERIALS, SETTING, METHODS

For detailed field-by-field analysis, an individual flagellar image was tracked automatically using the Autotrace module of image analysis software. The coordinate values of the flagellar shaft were used to calculate the shear angle, which is proportional to the amount of microtubule sliding at any given position along the flagellum. The maximum shear angles of metachronal and synchronous sliding were obtained from the mean shear angles between the maximum shear angles of pro-hook bends and the absolute values of the minimum shear angles of anti-hook bends, which represent the amplitude of a set of successive shear angle curves, with 3-12 shear curves covering one beat cycle of sperm flagellar movement. Asymmetry of flagellar waves was expressed by the mean shear angle between the maximum shear angle of pro-hook bends and the minimum shear angle of anti-hook bends at 100 μm from the head-midpiece junction.

MAIN RESULTS AND THE ROLE OF CHANCE

The asymmetrical flagellar movements observed in the activated (non-hyperactivated) and hyperactivated spermatozoa were characterized by the non-oscillatory synchronous sliding of a specific pair of the doublets; the large asymmetrical flagellar movement in the hyperactivated spermatozoa was generated by the large non-oscillatory synchronous sliding. Both the metachronal and synchronous sliding increased during the hyperactivation; however, the large symmetrical flagellar movement of the acrosome-reacted spermatozoa was characterized by the oscillatory synchronous sliding between most pairs of doublets. These results demonstrated that the metachronal and synchronous sliding are involved in generation and modulation of sperm flagellar motility; however, two types of synchronous sliding, non-oscillatory and oscillatory sliding, modulate the sperm flagellar motility by enhancing the sliding of a specific pair of the doublets or the sliding between most pairs of the doublets.

LARGE SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

This is an indirect study of the metachronal and synchronous sliding of doublet microtubules. Studies based on the direct observation of behavior of dynein are needed to clarify the sliding microtubule theory of flagellar movement of spermatozoa.

WIDER IMPLICATIONS OF THE FINDINGS

Both the metachronal and oscillatory synchronous sliding of doublet microtubule generate and modulate sperm flagellar motility, while the change in mode of synchronous sliding between the non-oscillatory synchronous sliding and oscillatory synchronous sliding modulates the sperm flagellar motility. The coordination between these sliding leads to various types of flagellar and ciliary motility, including the asymmetrical beating in flagellar and ciliary movement and planar or helical beating in sea urchin spermatozoa. Moreover, the finding that the metachronal sliding and two types of synchronous sliding generate and modulate the flagellar motility will open a new avenue for quantitative analysis of flagellar and ciliary motility.

STUDY FUNDING AND COMPETING INTEREST(S)

The authors have no conflict of interest and no funding to declare.

Functional anatomy of the mammalian sperm flagellum

The eukaryotic flagellum is the organelle responsible for the propulsion of the male gamete in most animals. Without exception, sperm of all mammalian species use a flagellum for swimming. The mammalian sperm has a centrally located 9 + 2 arrangement of microtubule doublets and hundreds of accessory proteins that together constitute an axoneme. However, they also possess several characteristic peri-axonemal structures that make the mammalian sperm tail function differently. These modifications include nine outer dense fibers (ODFs) that are paired with the nine outer microtubule doublets of the axoneme, and are anchored in a structure called the connecting piece located at the base. The presence of the ODFs and connecting piece, and the absence of a basal body, dictate that physical forces generated by the dynein motors are transmitted to the base of the flagellum through the ODFs. Mammalian sperm flagella also possess a mitochondrial and a fibrous sheath that encircle most of the axoneme. These sheaths and the ODFs add mechanical rigidity to the flagellum creating the functional effect of increasing bend wavelength, which requires the entrainment of more dynein motors in the production of a single wave. The sheaths also act as a retinaculum and maintain the integrity of the central axoneme when large bending torques are generated by dynein. Large torque production is crucial to the process of hyperactivation and the unique motility transitions associated with effective fertilizing capacity. Consequently, these specialized anatomical features are essential for the effective interaction of sperm with the female reproductive tract and ovum. © 2016 Wiley Periodicals, Inc.