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J Gen Physiol 2018 Sep 03;1509:1239-1248. doi: 10.1085/jgp.201812069.
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PLC and IP3-evoked Ca2+ release initiate the fast block to polyspermy in Xenopus laevis eggs.

Wozniak KL , Tembo M , Phelps WA , Lee MT , Carlson AE .

The prevention of polyspermy is essential for the successful progression of normal embryonic development in most sexually reproducing species. In external fertilizers, the process of fertilization induces a depolarization of the egg's membrane within seconds, which inhibits supernumerary sperm from entering an already-fertilized egg. This fast block requires an increase of intracellular Ca2+ in the African clawed frog, Xenopus laevis, which in turn activates an efflux of Cl- that depolarizes the cell. Here we seek to identify the source of this intracellular Ca2+ Using electrophysiology, pharmacology, bioinformatics, and developmental biology, we explore the requirement for both Ca2+ entry into the egg from the extracellular milieu and Ca2+ release from an internal store, to mediate fertilization-induced depolarization. We report that although eggs express Ca2+-permeant ion channels, blockade of these channels does not alter the fast block. In contrast, insemination of eggs in the presence of Xestospongin C-a potent inhibitor of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release from the endoplasmic reticulum (ER)-completely inhibits fertilization-evoked depolarization and increases the incidence of polyspermy. Inhibition of the IP3-generating enzyme phospholipase C (PLC) with U73122 similarly prevents fertilization-induced depolarization and increases polyspermy. Together, these results demonstrate that fast polyspermy block after fertilization in X. laevis eggs is mediated by activation of PLC, which increases IP3 and evokes Ca2+ release from the ER. This ER-derived Ca2+ then activates a Cl- channel to induce the fast polyspermy block. The PLC-induced cascade of events represents one of the earliest known signaling pathways initiated by fertilization.

PubMed ID: 30012841
PMC ID: PMC6122927
Article link: J Gen Physiol
Grant support: [+]

Species referenced: Xenopus laevis
Genes referenced: ano1 cacna1d cacng6 dennd5a dennd5b itpr1 mcu nadsyn1 ncs1 orai2 pkd2 plcb1 slc24a3 slc24a5 tmco1 tmem37 tpcn2 trpc3 trpv2 trpv4
GO keywords: prevention of polyspermy

Article Images: [+] show captions
References [+] :
Berridge, Inositol trisphosphate and calcium signalling mechanisms. 2009, Pubmed