The Daily Bulletin Archive

Please join the Biology Department and the Great Lakes Center for the seminar "Signaling Mechanisms of the Fast Block to Polyspermy in Xenopus laevis," presented by Anne Carlson, assistant professor in the University of Pittsburgh's Department of Biological Sciences, today, December 5, at 4:30 p.m. in Bulger Communication Center West 2.

All faculty, staff, and students are welcome.

Abstract
Fertilization of an egg by more than one sperm, a condition known as polyspermy, presents one of the earliest and most prevalent barriers to sexual reproduction. Polyspermy induces chromosomal abnormalities and is embryonic lethal. Accordingly, most sexually reproducing species have evolved mechanisms to inhibit the entry of more than one sperm. The two most ubiquitous blocks to polyspermy, the slow and fast block, create a physical barrier to prevent additional sperm entry minutes after fertilization and induce an electrical barrier seconds after fertilization, respectively. Currently, the signaling pathways underlying the fast block have not yet been elucidated in any species. In Xenopus laevis, the fast block is known to require increase in intracellular Ca²⁺, and an efflux of Cl^- to depolarize the egg membrane. Using pharmacological approaches, we sought to identify the source of increased intracellular Ca²⁺ and the molecular identity of the Cl^- channel mediating the fast block in X. laevis. We found that blocking the de novo synthesis of inositol-trisphosphate (IP₃), as well as inhibiting IP₃ from binding to activation of its receptors, evoked Ca2+ release from the endoplasmic reticulum (ER) that dramatically slowed or completely inhibited the fast block. By contrast, altering extracellular Ca²⁺ concentrations or blocking Ca^2+-permeant channels on the membrane did not alter the fast block. We also found that blocking the Ca²⁺-activated Cl^- channel, TMEM16a, similarly slowed or prevented the fast block. Together, these results indicate that fertilization signals an increase in the IP₃ content of the egg to release Ca²⁺ from the ER, which then binds to and opens the TMEM16a channel. Together these findings provide the most complete characterization of the fast block in any species.