Nicotine Induces Polyspermy in Sea Urchin Eggs through a Non-Cholinergic Pathway Modulating Actin Dynamics

Abstract

While alkaloids often exert unique pharmacological effects on animal cells, exposure of sea urchin eggs to nicotine causes polyspermy at fertilization in a dose-dependent manner. Here, we studied molecular mechanisms underlying the phenomenon. Although nicotine is an agonist of ionotropic acetylcholine receptors, we found that nicotine-induced polyspermy was neither mimicked by acetylcholine and carbachol nor inhibited by specific antagonists of nicotinic acetylcholine receptors. Unlike acetylcholine and carbachol, nicotine uniquely induced drastic rearrangement of egg cortical microfilaments in a dose-dependent way. Such cytoskeletal changes appeared to render the eggs more receptive to sperm, as judged by the significant alleviation of polyspermy by latrunculin-A and mycalolide-B. In addition, our fluorimetric assay provided the first evidence that nicotine directly accelerates polymerization kinetics of G-actin and attenuates depolymerization of preassembled F-actin. Furthermore, nicotine inhibited cofilin-induced disassembly of F-actin. Unexpectedly, our results suggest that effects of nicotine can also be mediated in some non-cholinergic pathways.

Cellular and molecular aspects of oocyte maturation and fertilization: a perspective from the actin cytoskeleton

“…b' Egg treated with 1-MA for 70 min experience a normal Ca 2+ response with a CF (arrow) which precedes the single Ca 2+ wave. c′ Two Ca 2+ waves (arrowheads) as a result of polyspermic fertilization in overripe eggs inseminated 6 h after hormonal treatment pathways but is mediated by changes in the egg cortical Factin meshwork and its altered polymerization dynamics [187].…”

Effects of Dithiothreitol on Fertilization and Early Development in Sea Urchin

“…In a series of previous studies on starfish and sea urchin eggs, we demonstrated that polyspermy is closely linked to an anomaly in the actin cytoskeleton of the egg cortex. Indeed, when the actin cytoskeleton was artificially altered through the use of actin-targeting toxins and chemicals, an actin-binding protein cofilin, anti-depactin antibody, nicotine, or even by “ageing”, the eggs showed a strong tendency of polyspermic fertilization [ 27 , 29 , 39 , 55 , 56 , 57 , 58 ]. At variance with these experimental conditions that caused cytoskeletal alterations, the pretreatment of P. lividus eggs with DTT or TCEP in our experimental condition did not induce discernable changes in the structure of the cortical actin cytoskeleton (data not shown).…”

“…This may be one of the reasons why the DTT- and TCEP-pretreated eggs were not prominently polyspermic ( Table 1 and Table 2 ). Nonetheless, it is possible that the concerted rearrangement of the actin cytoskeleton taking place in the fertilized eggs during egg activation [ 25 , 28 , 29 ] may not be influenced by the DTT pretreatment. In other words, the reducing agent may not have a profound effect on the actin filaments inside a quiescent egg, but it may still affect the mobilization kinetics in post-fertilization eggs.…”

“…In sea urchin, a fertilized egg exhibits two modes of Ca 2+ increase: immediate Ca 2+ influx and slow propagation of a Ca 2+ wave (see below) [ 20 , 24 ]. In parallel to this, some cortical actin filaments translocate to the inner cytoplasm, while others elongate the microvilli in the perivitelline space [ 25 , 26 , 27 , 28 , 29 ]. These progressive changes of the Ca 2+ signals and the reorganization of the cortical actin cytoskeleton are believed to play diverse roles in egg activation and in early development [ 30 , 31 , 32 , 33 ].…”

“…The mobilization of intracellular Ca 2+ and actin filaments at fertilization should be under tight control because their deregulation is often accompanied by developmental problems [ 18 , 27 , 28 , 30 , 34 ]. The present study is on the extension of a series of our previous studies on egg quality and the roles of the cortical actin cytoskeleton in fertilization [ 18 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. This time, we focused on the effect of reducing agents.…”