Metaphase I arrest of starfish oocytes induced via the MAP kinase pathway is released by an increase of intracellular pH

Abstract

Reinitiation of meiosis in oocytes usually occurs as a two-step process during which release from the prophase block is followed by an arrest in metaphase of the first or second meiotic division [metaphase I (MI) or metaphase II (MII)]. The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization does not occur. Here we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning into seawater,activation of Na+/H+ antiporters via a heterotrimeric G protein coupling to a 1-methyladenine receptor in the oocyte leads to an intracellular pH increase that can overcome the MI arrest even in the presence of active MAP kinase.

Transitioning from egg to embryo: Triggers and mechanisms of egg activation

“…Top: Shrimp eggs preloaded with a Ca 2ϩ -sensitive fluorescent dye, Fluo-3, exhibit a wave of fluorescence corresponding to an increase in internal Ca 2ϩ , upon application of external MgCl 2 to the egg [reprinted from Lindsay et al (1992), with permission from Elsevier]. Middle: Starfish oocytes [reproduced from Harada et al (2003) with permission of the Company of Biologists] spawned into seawater (SW) complete the first and second meiotic divisions within 90 and 120 min, respectively, as measured by extrusion of the first and second polar bodies (arrowheads). However, in seawater lacking Na ϩ (0 NaSW), meiosis does not complete.…”

“…The completion of meiosis is triggered instead by exposure to Na ϩ in seawater upon spawning, because eggs spawned into artificial seawater lacking Na ϩ do not complete meiosis ( Fig. 3C; Harada et al, 2003).…”

“…Upon stimulation by the hormone 1-methyladenine (1-MA), oocytes undergo a process of maturation and progress to metaphase of meiosis I, where they arrest again (Kanatani et al, 1969;Harada et al, 2003). This can be visualized by collecting 1-MA stimulated oocytes immediately after spawning and staining for DNA and tubulin ( Fig.…”

“…This can be visualized by collecting 1-MA stimulated oocytes immediately after spawning and staining for DNA and tubulin ( Fig. 3C; Harada et al, 2003). The eggs then activate, upon exposure to seawater, which has a higher Na ϩ concentration than ovarian fluid.…”

“…Meiosis resumes approximately 30 min after exposure to seawater Na ϩ , and completes within approximately 90 min ( Fig. 3C; Harada et al, 2003). Exposure to seawater Na ϩ has been hypothesized to activate Na ϩ /H ϩ antiporters on the oocyte plasma membrane to trigger an increase in pH within the egg.…”