Actin, more than just a housekeeping protein at the scene of fertilization

Abstract

Since the first demonstration of sperm entry into the fertilized eggs of Mediterranean sea urchin Paracentrotus lividus by Hertwig (1876), enormous progress and insights have been made on this topic. However, the precise molecular mechanisms underlying fertilization are largely unknown. The two most dramatic changes taking place in the zygote immediately after fertilization are: (i) a sharp increase of intracellular Ca 2+ that initiates at the sperm interaction site and traverses the egg cytoplasm as a wave, and (ii) the concomitant dynamic rearrangement of the actin cytoskeleton. Traditionally, this has been studied most extensively in the sea urchin eggs, but another echinoderm, starfish, whose eggs are much bigger and transparent, has facilitated experimental approaches using microinjection and fluorescent imaging methodologies. Thus in starfish, it has been shown that the sperm-induced Ca 2+ increase in the fertilized egg can be recapitulated by several Ca 2+ -evoking second messengers, namely inositol 1,4,5-trisphosphate (InsP 3 ), cyclic ADP-ribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP), which may play distinct roles in the generation and propagation of the Ca 2+ waves. Interestingly, it has also been found that the dynamic rearrangement of the actin cytoskeleton in the fertilized eggs plays pivotal roles in guiding monospermic sperm entry and in the fine modulation of the intracellular Ca 2+ signaling. As it is well known that Ca 2+ regulates the structure of the actin cytoskeleton, our finding that Ca 2+ signaling can be reciprocally affected by the state of the actin cytoskeleton raises an intriguing possibility that actin and Ca 2+ signaling may form a 'positive feedback loop' that accelerates the downstream events of fertilization. Perturbation of the cortical actin networks also inhibits cortical granules exocytosis. Polymerizing actin bundles also compose the 'acrosome process,' a tubular structure protruding from the head of fertilizing sperm. Hence, actin, which is one of the most strictly conserved proteins in eukaryotes, modulates almost all major aspects of fertilization. starfish, calcium, egg maturation, fertilization, PIP2, actin cytoskeleton, cofilin Citation:Santella L, Chun J T. Actin, more than just a housekeeping protein at the scene of fertilization.

Early events of fertilization in sea urchin eggs are sensitive to actin-binding organic molecules

“…Alternatively, the elevated plateau at the decaying phase of Ca 2+ response might have happened because the monomeric actin pools in the cytoplasm were depleted due to the hyperpolymerization. Binding of Ca 2+ by G-actin and its incorporation into actin filaments could serve as a supplemental mechanism to absorb Ca 2+ and alleviate its local surge [19,[36][37][38][39]. Thus, overstabilization of the actin filaments by PHAL and JAS might have repressed the Ca 2+ -removal process by suppressing treadmilling.…”

Assessment of the relationship between follicular fluid calcium ion concentration and reproductive outcomes

“…The detailed signal transduction pathways that lead to the intracellular Ca 2+ liberation during hormonal and sperm stimulation have been extensively studied. 15,16 Recently a higher intracellular Ca 2+ concentrations in granulosa cells have been reported to increase the fertilization ratio and oocyte quality in patients undergoing IVF. 17 Hence we postulated that a higher intracellular concentration of Ca 2+ could be represented as a higher FF level of Ca 2+, which might have help us speculate about reproductive capability of the individual.…”

Progress in protein structure and function studies in China during 2010–2011

“…Ryanodien receptors are large ion channels that facilitate the release of calcium cations from the endoplasmic or sarcoplasmic reticulum. Santella and Chun [58] reviewed the roles played by actin and its interactions with Ca 2+ signaling in fertilization. Maki et al [59] reviewed structural and functional studies on ALG-2, a penta-EF-hand calcium-dependent adaptor protein.…”