A Role for Ceramide in Meiosis

Abstract

Meiotic maturation of Xenopus laevis oocytes is a widely used model for the study of cell cycle regulation. Progesterone, the physiological regulator of maturation, triggers cell cycle progression of prophase arrested oocytes through meiosis I and into meiosis II where the mature oocyte remains arrested in metaphase II until fertilization. Although extensively studied, the mechanism of reinitiation remains poorly understood. Progesterone addition to oocytes caused within minutes a transient decrease in cAMP levels which was necessary for maturation; agents which elevate cAMP inhibit progesterone-induced maturation.1 The molecular mechanism(s) by which progesterone elicits these changes is unclear. However, inhibition of adenylate cyclase2 in a G-protein dependent manner3 appears to be involved. Subsequent microinjection experiments demonstrated that the catalytic subunit of cAMPdependent protein kinase (PKA) inhibited maturation,4 while the regulatory subunit induced maturation,5 providing evidence that PKA is a negative regulator of oocyte maturation. While the role of PKA in oocyte maturation is now well established, the initial event(s) following the addition of progesterone is undefined. A transmembrane signaling mechanism is required since, unlike the classical steroid hormone receptor for transcriptional activation, the progesterone receptor is localized to the plasma membrane.6,7 Numerous studies have proposed a phospholipase-mediated transmembrane signaling mechanism.8–11 While progesterone has been observed to stimulate the turnover of the major glycerophospholipids with the generation of potential second messengers, these are not sufficient to induce maturation in the absence of hormone. In contrast, treatment of oocytes with exogenous sphingomyelinase (Staphylococcus aureus) is a potent inducer of maturation.12,13 These investigators additionally showed that microinjection of sphingosine was capable of inducing maturation in the absence of hormone. These observations are consistent with the emerging role of sphingolipids as regulators of critical cellular processes.14 In particular, Okazaki et al15 originally described a sphingomyelin cycle where ceramide, generated by sphingomyelinase hydrolysis of membrane sphingomyelin, acts as a lipid second messenger functioning as a modulator of monocytic differentiation,15 cell cycle regulation,16,17 and apoptosis.18,19 At the time of the original report by Varnold and Smith, sphingosine had recently been reported as a potent inhibitor of protein kinase C (PKC)20 and ceramide was not known to act as a lipid second messenger. Thus, these investigators assumed that sphingosine levels were increased by sphingomyelinase treatment of oocytes leading to inhibition of PKC and subsequently, reinitiation of oocyte maturation. Since ceramide is the more proximal metabolite of sphingomyelinase hydrolysis and sphingosine could be acylated to form ceramide, we further investigated the role of sphingolipids in the regulation of oocyte maturation. From these studies, we have defined a role for ceramide in the maturation pathway which suggests it may be functionally important in mediating the reinitiation of the meiotic cell cycle triggered by progesterone.21 The involvement of sphingolipid turnover in progesterone-induced meiosis has been strengthened by recently reported observations showing that agonists which increase the level of ceramide in cells trigger a normal process of maturation and that at suboptimal concentrations, these agonists can potentiate progesterone-induced maturation.22,23 This chapter details the observations which support a role for ceramide in meiosis and presents the current hypothesis on the function of sphingolipid turnover in reinitiation of oocyte maturation.