Flagellar quiescence response in sea urchin sperm induced by electric stimulation

A coordinated sequence of distinct flagellar waveforms enables a sharp flagellar turn mediated by squid sperm pH-taxis

Animal spermatozoa navigate by sensing ambient chemicals to reach the site of fertilization. Generally, such chemicals derive from the female reproductive organs or cells. Exceptionally, squid spermatozoa mutually release and perceive carbon dioxide to form clusters after ejaculation. We previously identified the pH-taxis by which each spermatozoon can execute a sharp turn, but how flagellar dynamics enable this movement remains unknown. Here, we show that initiation of the turn motion requires a swim down a steep proton gradient (a theoretical estimation of ≥0.025 pH/s), crossing a threshold pH value of ~5.5. Time-resolved kinematic analysis revealed that the turn sequence results from the rhythmic exercise of two flagellar motions: a stereotypical flagellar ‘bent-cane’ shape followed by asymmetric wave propagation, which enables a sharp turn in the realm of low Reynolds numbers. This turning episode is terminated by an ‘overshoot’ trajectory that differs from either straight-line motility or turning. As with bidirectional pH-taxes in some bacteria, squid spermatozoa also showed repulsion from strong acid conditions with similar flagellar kinematics as in positive pH-taxis. These findings indicate that squid spermatozoa might have a unique reorientation mechanism, which could be dissimilar to that of classical egg-guided sperm chemotaxis in other marine invertebrates.

Mechanism regulating Ca2+-dependent mechanosensory behaviour in sea urchin spermatozoa

Flagellar movement of the sea urchin sperm is regulated by intracellular Ca(2+). Flagellasialin, a polysialic acid-containing glycoprotein, as well as other membrane proteins seems responsible for the Ca(2+) control. To elucidate the mechanism of Ca(2+) dynamics underlying flagellar movement, we analysed the sperm's mechanosensory behavioural responses by using microtechniques. In sea water containing 10 mM Ca(2+), the sperm swim in circular paths. When a mechanical stimulus was applied to the sperm head with a glass microstylus, the sperm showed a series of flagellar responses, consisting of a stoppage of beating (quiescence) and a recovery of swimming in a straight path, followed by swimming in a circular path again; as the result the sperm avoided the obstacle. Ca(2+)-imaging with Fluo-4 showed that the intracellular Ca(2+) was high in the quiescence and gradually decreased after that. The effects of blockers and antibodies against candidate components revealed that the Ca(2+) influx was induced by Ca(2+) channels and the Ca(2+) efflux was induced by a flagellasialin-related Ca(2+)-efflux system, plasma membrane Ca(2+)-ATPases and the K(+)-dependent Na(+)/Ca(2+) exchanger. The results show that the Ca(2+)-dependent mechanosensory behaviour of the sea urchin sperm is regulated by organized functioning of the membrane environment including the plasma membrane proteins and flagellasialin.

Progression of Flagellar Stages during Artificially Delayed Motility Initiation in Sea Urchin Sperm

Transition from immotile to motile flagella may involve a series of states, in which some of regulatory mechanisms underlying normal flagellar movement are working with others being still suppressed. To address ourselves to the study of starting transients of flagella, we analyzed flagellar movement of sea urchin sperm whose motility initiation had been retarded in an experimental solution, so that we could capture the instance at which individual spermatozoa began their flagellar beating. Initially straight and immotile flagella began to shiver at low amplitude, then propagated exclusively the principal bend (P bend), and finally started stable flagellar beating. The site of generation of the P bend in the P-bend propagating stage varied in position in the basal region up to 10 microm from the base, indicating that the ability of autonomous bend generation is not exclusively possessed by the very basal region but can be unmasked throughout a wider region when the reverse bend (R bend) is suppressed. The rate of change in the shear angle, the curvature of the R bend and the frequency and regularity of beating substantially increased upon transition from P-bend propagating to full-beating, while the propagation velocity of bends remained unchanged. These findings indicate that artificially delayed motility initiation may accompany sequential modification of the motile system and that mechanisms underlying flagellar motility can be analyzed separately under experimentally retarded conditions.

Ca(v)2.3 (alpha1E) Ca2+ channel participates in the control of sperm function

To know the function of the Ca2+ channel containing alpha(1)2.3 (alpha1E) subunit (Ca(v)2.3 channel) in spermatozoa, we analyzed Ca2+ transients and sperm motility using a mouse strain lacking Ca(v)2.3 channel. The averaged rising rates of Ca2+ transients induced by alpha-D-mannose-bovine serum albumin in the head region of Ca(v)2.3-/- sperm were significantly lower than those of Ca(v)2.3+/+ sperm. A computer-assisted sperm motility assay revealed that straight-line velocity and linearity were greater in Ca(v)2.3-/- sperm than those in Ca(v)2.3+/+ sperm. These results suggest that the Ca(v)2.3 channel plays some roles in Ca2+ transients and the control of flagellar movement.

Conversion of beating mode in Chlamydomonas flagella induced by electric stimulation

Electric stimulation of a single Chlamydomonas cell by means of a suction electrode induced a temporary conversion of flagellar waveform from an asymmetric forward mode to a symmetric reverse mode. The reverse mode continued for about 0.5 seconds, after which the forward mode was resumed. Anodic stimulation (current passing outward through the membrane outside the suction pipette) was more effective in inducing the flagellar response than cathodic stimulation. No flagellar response was induced in the absence of free Ca2+ or in the presence of calcium channel inhibitors, pimozide (5 microM) and diltyazem (0.3 mM). These findings indicate that the flagellar response by membrane depolarization followed by a Ca2+ influx through voltage-dependent calcium channels. This experimental system allowed us to quantitatively analyze the behavior of flagella during the waveform conversion. The flagellar bending pattern quickly changed from the forward mode to the reverse mode and, thereafter, gradually resumed the forward mode through two discrete phases: changes during reverse mode beating (phase I) and a distinct transitional phase (phase II). Recovery in curvature and sliding velocity of principal bends occurred mostly in phase I. Almost all of the recovery of reverse bends, returning the curvature to the low values characteristic of asymmetric forward mode beating, occurred in phase II. Beat frequency recovered through both phases. Phase II was often interrupted by a temporary stoppage of beating. These findings indicate that the bending pattern is converted through multiple steps that are controlled by Ca2+.

A model for flagellar motility

Experimental investigation has provided a wealth of structural, biochemical, and physiological information regarding the motile mechanism of eukaryotic flagella/cilia. This chapter surveys the available literature, selectively focusing on three major objectives. First, it attempts to identify those conserved structural components essential to providing motile function in eukaryotic axonemes. Second, it examines the relationship between these structural elements to determine the interactions that are vital to the mechanism of flagellar/ciliary beating. Third, the vital principles of these interactions are incorporated into a tractable theoretical model, referred to as the Geometric Clutch, and this hypothetical scheme is examined to assess its compatibility with experimental observations.