Calaxin is essential for the transmission of Ca2+-dependent asymmetric waves in sperm flagella

Regulation of waveform asymmetry in sperm flagella is critical for changes in sperm swimming trajectory as seen during sperm chemotaxis towards eggs. Ca2+ is known as an important regulator of asymmetry in flagellar waveforms. A calcium sensor protein, calaxin, which is associated with the outer arm dynein, plays a key role in the sperm waveform regulation in a Ca2+-dependent manner. However, the molecular mechanism underlying the regulation of asymmetric waves by Ca2+ and calaxin remains unclear. We performed experiments using caged ATP to elucidate the formation and propagation of asymmetric flagellar waves in the sperm of the ascidian Ciona intestinalis. Demembranated sperm cells were suspended in a solution containing caged ATP and reactivated using UV flash photolysis. Initial bends were formed at the base and propagated towards the tip of flagella; however, the bend direction was different between asymmetric and symmetric waves. A calaxin inhibitor, repaglinide, had no effect on initial bend formation, but significantly inhibited the generation of the second flagellar bend in the reverse direction, resulting in the failure of asymmetric wave formation and propagation. These results suggest that calaxin plays a critical role in Ca2+-dependent transmission of flagellar asymmetric waveforms.