Increased intracellular pH ([pH]i) activates dynein in sea urchin and mammalian sperm and induces activation of flagellar motility. It is thought that cAMP-dependent protein phosphorylation is associated with motility

Eukaryotic cilia and flagella are found on various types of cells, from single-cell protozoa to epithelial cells of multicellular organs. Most have a well-conserved structure called the axoneme, which generates ciliary and flagellar movements. The axoneme is a microtubule-based, highly organized structure composed of more than 250 kinds of proteins. Molecular motors, called the dyneins, exert force to drive the active sliding of microtubules, which is converted into the bending wave of cilia and flagella. It is known that the phosphorylation of axonemal proteins plays a crucial role in the regulation of motility (Inaba, 2003). An increase in intracellular pH ([pH]i) is reported to be involved in the activation of sperm flagellar motility in sea urchin (Christen et al., 1982; Lee et al., 1983), marine teleosts (Oda and Morisawa, 1993) and mammals (Wong et al., 1981; Babcock et al., 1983). It is thought that cAMP-dependent protein phosphorylation is associated with motility activation through increased [pH]i (Brokaw, 1987; Goltz et al., 1988; Carr and Acott, 1989), but little attention has been given to the cAMP-independent phosphorylation induced by the increased [pH]i. According to Carr and Acott (1989), two proteins are phosphorylated in intact bull sperm during motility activation induced by an increase in [pH]i, and the phosphorylation is probably not due to changes in cAMP levels. However, it is not clear whether these phosphoproteins are associated with the signaling pathway induced by an increase in [pH]i nor whether the phosphorylation occurs independently of cAMP. In addition, these phosphoproteins are located in the membrane (Carr and Acott, 1989). In the present study, we demonstrate in starfish sperm that an increase in [pH]i induces phosphorylation of axonemal proteins and activation of flagellar motility independently of cAMP. It is reported that starfish testicular sperm are generally immotile in seawater, but motility is activated by histidine, which has a strong zinc-binding capacity (Fujii et al., 1955). EDDA and TPEN, high-affinity chelators of Zn2+, are also effective in activating motility (Mohri et al., 1990; Morisawa, et al., 2004). Therefore the liberation of Zn2+ from the sperm is thought to be a key factor for motility activation in starfish (Fujii et al., 1955; Mohri et al., 1990; Morisawa et al., 2004), but downstream events in the signaling pathway for motility activation remain unknown. As the first step, we show that an increase in [pH]i is also involved in the activation of intact starfish sperm motility. We then demonstrate pH-dependent but The Journal of Experimental Biology 208, 4411-4418 Published by The Company of Biologists 2005 doi:10.1242/jeb.01906