Progesterone-induced Acrosome Exocytosis Requires Sequential Involvement of Calcium-independent Phospholipase A2β (iPLA2β) and Group X Secreted Phospholipase A2 (sPLA2)

Abstract

Phospholipase A 2 (PLA 2 ) activity has been shown to be involved in the sperm acrosome reaction (AR), but the molecular identity of PLA 2 isoforms has remained elusive. Here, we have tested the role of two intracellular (iPLA 2 ␤ and cytosolic PLA 2 ␣) and one secreted (group X) PLA 2 s in spontaneous and progesterone (P4)-induced AR by using a set of specific inhibitors and knock-out mice. iPLA 2 ␤ is critical for spontaneous AR, whereas both iPLA 2 ␤ and group X secreted PLA 2 are involved in P4-induced AR. Cytosolic PLA 2 ␣ is dispensable in both types of AR. P4-induced AR spreads over 30 min in the mouse, and kinetic analyses suggest the presence of different sperm subpopulations, using distinct PLA 2 pathways to achieve AR. At low P4 concentration (2 M), sperm undergoing early AR (0 -5 min post-P4) rely on iPLA 2 ␤, whereas sperm undergoing late AR (20 -30 min post-P4) rely on group X secreted PLA 2 . Moreover, the role of PLA 2 s in AR depends on P4 concentration, with the PLA 2 s being key actors at low physiological P4 concentrations (<2 M) but not at higher P4 concentrations (ϳ10 M).

Mouse sperm begin to undergo acrosomal exocytosis in the upper isthmus of the oviduct

“…Progesterone is actively produced by cumulus cells after ovulation and its concentration within the cumulus matrix has not been assessed accurately. In mouse, it was recently reported that its concentration within the cumulus cells is in the micromolar range (Abi Nahed et al, 2015). In the oviductal fluid of rabbit and hamster, progesterone was found to be in the range of 2 to 100 nM depending on the phase of the estrous cycle (Libersky and Boatman, 1995; Richardson and Oliphant, 1981).…”

Lipid Regulation of Acrosome Exocytosis

“…In an elegant series of experiments with these inhibitors and different concentrations of progesterone, it was found that iPLA 2 β had more effect on early stages of exocytosis at lower, more physiological concentrations of 2 μM (Abi Nahed et al 2015). Later stages of exocytosis relied more on X sPLA 2 (Abi Nahed et al 2015); these results support the notion that lipid modulation is important at different stages of the regulated exocytotic event described by the acrosome exocytosis model.…”

“…Of great interest, different isoforms seem to be responsible, suggesting possible dysfunction in specific compartments of cells undergoing spontaneous exocytosis. For example, intracellular, calcium-independent iPLA 2 β is critical for spontaneous exocytosis, whereas both it and secreted group X sPLA 2 are involved in progesteroneinduced acrosome exocytosis, to extents that differ depending on the concentration of progesterone and the kinetics of the sperm's response (Abi Nahed et al 2015).…”

“…These results do not contradict the abovementioned findings with PLB. Indeed, some sperm were still able to undergo acrosome exocytosis in the presence of inhibitors of both iPLA 2 β and group X sPLA 2 , suggesting that all three phospholipases likely participate (Abi Nahed et al 2015). In much the same way that membrane fluidity induced by bicarbonate and sterol efflux are mutually reinforcing and can occur independently, it makes sense that sperm would have several reinforcing and potentially redundant mechanisms in place to promote membrane fusion.…”

2-Oxoamides based on dipeptides as selective calcium-independent phospholipase A 2 inhibitors

“…These inhibitors have been used to study the role of the enzyme ex vivo and in vivo and in particular in autoimmune diseases. 19–22 FKGK11 showed strong reduction in the clinical severity and progression of experimental autoimmune encephalomyelitis. 19 Administration of FKGK18 to non-obese diabetic (NOD) mice significantly reduced diabetes incidence in association with reduced insulitis, improved glucose homeostasis, and β-cell preservation.…”