Role of phospholipase A₂pathway in regulating activation ofBufo arenarumoocytes

Abstract

Transient increases in the concentration of cytosolic Ca(2+) are essential for triggering egg activation events. Increased Ca(2+) results from its rapid release from intracellular stores, mainly mediated by one or both intracellular calcium channels: the inositol trisphosphate receptor (IP3R) and the ryanodine receptor (RyR). Several regulatory pathways that tailor the response of these channels to the specific cell type have been proposed. Among its many modulatory actions, calcium can serve as an activator of a cytosolic phospholipase A(2) (cPLA2), which releases arachidonic acid from phospholipids of the endoplasmic reticulum as well as from the nuclear envelope. Previous studies have suggested that arachidonic acid and/or its metabolites were able to modulate the activity of several ion channels. Based on these findings, we have studied the participation of the phospholipase A(2) (PLA(2)) pathway in the process of Bufo arenarum oocyte activation and the interrelation between any of its metabolites and the ion channels involved in the calcium release from the intracellular reservoirs at fertilization. We found that addition of both melittin, a potent PLA(2) activator, and arachidonic acid, the main PLA(2) reaction metabolite, was able to induce activation events in a bell-shaped manner. Differential regulation of IP3Rs and RyRs by arachidonic acid and its products could explain melittin and arachidonic acid behaviour in Bufo arenarum egg activation. The concerted action of arachidonic acid and/or its metabolites could provide controlled mobilization of calcium from intracellular reservoirs and useful tools for understanding calcium homeostasis in eggs that express both types of receptors.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

“…PLA2 catalyzes the breakdown of certain species of PC to LPC and a free fatty acid such as arachidonic acid (Petcoff et al, 2008). Supporting a role in fertilization, PLA2 inhibitors did not block the calcium increase in sea urchin fertilization, but did inhibit fertilization envelope formation (an event dependent upon the membrane fusion event of cortical granule exocytosis)(Ajmat et al, 2013). LPC can induce membrane fusion of the sperm plasma membrane and acrosomal membrane (Liguori et al, 2004) as well as human sperm and hamster egg fusion (Riffo and PArraga, 1997).…”

“…As Alzheimer’s disease may involve abnormal exocytosis, it is interesting that blood LPC (18:2) decreases during development of the disease (levels of PC and PI also decrease)(Mapstone et al, 2014). The other typical product of PLA2 action, arachidonic acid, inhibits IP 3 receptors in Bufo arenarum (Ajmat et al, 2013). Surprisingly, during fertilization, there was no detectable decrease in 20:4n6 arachidonate species of cardiolipin, LPC, PC, combined PS/PI, or SM (arachidonate PE actually increased)(Petcoff et al, 2008).…”

Identification of phospholipase activity inRhinella arenarumsperm extract capable of inducing oocyte activation

“…In the mature oocytes of Rhinella arenarum the presence of RyRs has been described, their participation in the activation process having also been demonstrated (Ajmat et al ., 2013). Our results show that if the RyRs is blocked, F 1 is unable to induce activation, thus confirming the importance of this receptor in the activation of the oocytes of this species.…”

“…The participation of inositol trisphosphate and RyRs in Rhinella arenarum oocyte activation was demonstrated previously by Ajmat et al . (2011, 2013).…”

Participation of PLA2 and PLC in DhL-induced activation ofRhinella arenarumoocytes

“…Studies in Rhinella arenarum oocyte suggest that PLA2 plays a fundamental role in activation signalling and that this effect is mediated by an increase in PLA2 activity (Ajmat et al , 2013). It was found that the addition of melittin, a potent PLA2 activator, and arachidonic acid, the main PLA2 reaction metabolite, was able to induce activation events such as cortical granule exocytosis and subsequent fertilization envelope formation probably through calcium-triggered sequential reactions.…”