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J. Biol. Chem., Vol. 282, Issue 52, 37730-37737, December 28, 2007

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Fertilization and Nicotinic Acid Adenine Dinucleotide Phosphate Induce pH Changes in Acidic Ca²⁺ Stores in Sea Urchin Eggs^*

From the Department of Pharmacology, University of Oxford, Oxford OX1 3QT, United Kingdom

The second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) releases Ca²⁺ from the acidic Ca²⁺ stores of many organisms, including those of the sea urchin egg. We investigated whether the pH within the lumen of these acidic organelles changes in response to stimuli. Fertilization activates the egg by Ca²⁺ release dependent upon NAADP, and accordingly, we report that fertilization also alters organellar pH in a spatio-temporally complex manner. Upon sperm fusion, vesicles deep in the egg center slowly acidify, whereas cortical vesicles undergo a rapid alkalinization. The cortical vesicle alkalinization is independent of exocytosis and cytosolic pH but coincides with the NAADP-dependent fertilization Ca²⁺ wave. Microinjection of NAADP mimicked the fertilization cortical response, suggesting that it occurred within NAADP-sensitive acidic Ca²⁺ stores. Our data show that NAADP and physiological stimuli alter the pH within intracellular organelles and suggest that NAADP signals through pH as well as Ca²⁺.

Received for publication, June 5, 2007 , and in revised form, October 17, 2007.

^* This work was supported by a senior fellowship from the Wellcome Trust (to A. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

¹ To whom correspondence should be addressed: Dept. of Pharmacology, University of Oxford, Mansfield Rd., Oxford OX1 3QT, UK. Tel.: 44-1865-271606; Fax: 44-1865-271853; E-mail: Anthony.morgan{at}pharm.ox.ac.uk.

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