The sperm cytoplasm has recently been found to contain enzymes that activate egg metabolism, as well as RNA fragments that may alter gene expression (Sharma et al. 2016). As in every other animal studied, a transient rise in cytoplasmic Ca2+ is necessary for egg activation in mammals (Yeste et al. 2017; Kashir et al. 2018). The sperm induces a series of Ca2+ waves that can last for hours, terminating in egg activation (i.e., resumption of meiosis, cortical granule exocytosis, and release of the inhibition on maternal mRNAs) and the formation of the male and female pronuclei. And, again as in sea urchins, fertilization triggers intracellular Ca2+ release through the production of IP3 by the enzyme phospholipase C (PLC) (Swann et al. 2006; Igarashi et al. 2007).

However, the mammalian PLC responsible for egg activation and pronucleus formation may in fact come from the sperm rather than from the egg. Some of the first observations for a sperm-derived PLC came from studies of intracytoplasmic sperm injection (ICSI), an experimental treatment for curing infertility. Here, sperm are directly injected into oocyte cytoplasm, bypassing any interaction with the egg cell membrane. To the surprise of many biologists (who had assumed that sperm binding to an egg receptor protein was critical for egg activation), this treatment worked. The human egg was activated, and pronuclei formed. Injecting mouse sperm into mouse eggs will also induce fertilization-like Ca2+ oscillations in the egg and lead to complete development (Kimura and Yanagimachi 1995).

It appeared that an activator of Ca2+ release was stored in the sperm head. This activator turned out to be a soluble sperm PLC enzyme, PLCζ (zeta), which is delivered to the egg by gamete fusion. In mice, expression of PLCζ mRNA in the egg produces Ca2+ oscillations, and removing PLCζ from mouse sperm (by antibodies or RNAi) abolishes the sperm’s calcium-inducing activity, as shown in Figure 1. (Saunders et al. 2002; Yoda et al. 2004; Knott et al. 2005). PLCζ is also responsible for initiating the membrane block to polyspermy by removing Juno from the egg cell membrane (Nozawa et al. 2018). Human sperm that are unsuccessful in ICSI have been shown to have little or no functional PLCζ. In fact, normal human sperm can activate Ca2+ oscillations when injected into mouse eggs, but sperm lacking PLCζ do not (Yoon et al. 2008).

Whereas sea urchin eggs usually are activated as a single wave of Ca2+ crosses from the point of sperm entry, the mammalian egg is traversed by numerous waves of Ca2+ (Miyazaki et al. 1992; Ajduk et al. 2008; Ducibella and Fissore 2008). The extent (amplitude, duration, and number) of these Ca2+ oscillations appears to regulate the timing of mammalian egg activation events (Ducibella et al. 2002; Ozil et al. 2005; Toth et al. 2006). In this way, cortical granule exocytosis occurs just before the nucleus resumes meiosis and much before the translation of maternal mRNAs.

In mammals, the Ca2+ released by IP3 binds to a series of proteins, including calmodulin-activated protein kinase (which will be important in eliminating the inhibitors of mRNA translation), MAP kinase (which allows the resumption of meiosis), and synaptotagmin (which helps initiate cortical granule fusion). Unused Ca2+ is pumped back into the endoplasmic reticulum, and additional Ca2+ is acquired from outside the cell. This recruitment of extracellular Ca2+ appears to be necessary for the egg to complete meiosis. If Ca2+ influx is blocked, the second polar body does not form; instead, the result is two nonviable (triploid) egg pronuclei (Maio et al. 2012; Wakai et al. 2013).

After C. M. Saunders et al. 2002. Development 129: 3533–3544.

 

FIGURE 1 

Importance of PLCζ-induced Ca2+ oscillations. Ca2+ fluxes across the mouse oocyte were monitored after in vitro fertilization with mouse sperm (A) or after microinjection with cRNA encoding wild-type PLCζ at 0.02 mg/ml (B), PLCδ1 at 2 mg/ml (C), or a mutated form of PLCζ at 2 mg/ml (D). The Ca2+ fluxes were produced by the PLCζ but not by the mutated PLCζ or the relatively similar enzyme PLCδ1.