Abstract
When a spermatozoon shows chemotactic behavior, transient [Ca2+]i increases in the spermatozoon are induced by an attractant gradient. The [Ca2+]i increase triggers a series of stereotypic responses of flagellar waveforms that comprise turning and straight-swimming. However, the molecular mechanism of [Ca2+]i modulation controlled by the attractants is not well defined. Here, we examined receptive mechanisms for the sperm attractant, SAAF, in the ascidian, Ciona intestinalis, and identified a plasma membrane Ca2+-ATPase (PMCA) as a SAAF-binding protein. PMCA is localized in sperm flagella membranes and seems to interact with SAAF through basic amino acids located in the second and third extracellular loops. ATPase activity of PMCA was enhanced by SAAF, and PMCA inhibitors, 5(6)-Carboxyeosin diacetate and Caloxin 2A1, inhibited chemotactic behavior of the sperm. Furthermore, Caloxin 2A1 seemed to inhibit efflux of [Ca2+]i in the sperm, and SAAF seemed to competitively reduce the effect of Caloxin 2A1. On the other hand, chemotactic behavior of the sperm was disordered not only at low-Ca2+, but also at high-Ca2+ conditions. Thus, PMCA is a potent candidate for the SAAF receptor, and direct control of Ca2+ efflux via PMCA is a fundamental mechanism to mediate chemotactic behavior in the ascidian spermatozoa.
Highlights
Ca2+ is a well-known second messenger that plays important roles relating to numerous events in virtually all types of cells[1]
The most abundant sperm-activating and attracting factor (SAAF)-binding protein, the 370-kDa protein (Fig. 1A), was a product of the predicted gene model KH.C8.156, which is similar to the human PMCA3 (Supplemental Fig. S1C)
We concluded that plasma membrane Ca2+-ATPase (PMCA) is a potent candidate for the SAAF receptor
Summary
Ca2+ is a well-known second messenger that plays important roles relating to numerous events in virtually all types of cells[1]. In the case of the sea urchin and starfish, cGMP produced by the receptor guanylyl cyclase opens the K+-selective cGMP-gated channel, resulting in hyperpolarization[29] This seems to activate the Na+/H+ exchanger and leads to alkalization, resulting in Ca2+ increase via the alkalization-gated channel CatSper[30]. Identification of the SAAF receptor is required to understand Ca2+ signaling and the molecular mechanisms of ascidian sperm chemotaxis. In the case of any other species, Ca2+ influx and [Ca2+]i increases in the sperm cell are focused in chemotactic behavior, and Ca2+ efflux and [Ca2+]i decreases are scarcely examined; despite the need for prompt [Ca2+]i decrease, it has been observed only in sperm activation of the sea urchin[33]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
