Sperm chemotaxis is mediated by calcium extrusion via plasma membrane Ca2+-ATPase and Na+/Ca2+ exchanger (#212)
Chemotactic behavior of sperm is an important event to enhance the probability of fertilization. In the ascidian, Ciona intestinalis, the sperm-activating and -attracting factor (SAAF) was already identified from eggs1 . Transient [Ca2+]i increases in the spermatozoon are elicited in a SAAF gradient, and that they trigger a series of stereotypic responses of flagellar waveforms that comprises turning and straight-swimming2 . Ca2+ regulates several cellular processes and its concentration is finely regulated by various channels, pumps and exchangers, however the coordination of all these Ca2+ modulators concerning sperm chemotaxis are unsolved. The Ca2+ extrusion via the Na+/Ca2+ exchanger (NCX) was important for Ciona sperm chemotaxis3 . Concerning the other molecules for Ca2+ extrusion, the plasma membrane Ca2+-ATPase (PMCA) has not been investigated in Ciona. Indeed, we obtained SAAF binding proteins by pull-down assay, and their proteomic analysis by peptide mass fingerprinting revealed a PMCA as a candidate for SAAF binding protein. We confirmed the interaction between recombinant Ciona PMCA and SAAF using QCM. Thus, PMCA is a potent candidate for the SAAF receptor and might mediate the chemotactic responses. Inhibition of NCX or PMCA caused decrease in chemotactic behavior. The inhibition of NCX prolonged duration of the Ca2+ bursts and increased maximum concentrations of the Ca2+ bursts. Furthermore, modification of [Ca2+]ex disrupted the Ca2+ bursts and sperm chemotactic behavior. In the low [Ca2+]ex conditions, the Ca2+ bursts were almost inhibited. In contrast, the sperm in a high [Ca2+]ex condition showed a chemotaxis-like behavior, i.e., turning and straight swimming, but its duration and timing were erroneous, resulting in the failure of the sperm to approach to the attractant source. Thus, the formation of a swimming trajectory in chemotactic behavior requires the extrusion of Ca2+ via PMCA and NCX to occur with accurate timing in a SAAF gradient.
- Yoshida, M. Proc. Natl. Acad. Sci. U S A 99, 14831-6 (2002).
- Shiba, K. Proc. Natl. Acad. Sci. U S A 105, 19312-7 (2008).
- Shiba, K. Cell. Motil. Cytoskeleton. 63, 623-32 (2006).