Phospholipase C-dependent Ca2+ release by worm and mammal sperm factors

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

Characterization of phospholipases C beta and gamma and their possible roles in Chaetopterus egg activation

Intracellular calcium release from the endoplasmic reticulum is a hallmark at egg activation of both vertebrates and invertebrates. This fertilization-associated calcium release results from generation of the second messenger inositol 1,4,5-trisphosphate (IP(3)) by one or more phospholipases C (PLC). We characterized Chaetopterus PLCbeta and gamma by reverse transcription/degenerate oligonucleotide primed PCR and rapid amplification of cDNA end PCR. Phylogenetic analyses suggested that the deduced PLCbeta protein shared the greatest homology with mammalian PLCbeta4; the deduced PLCgamma protein shared the greatest homology with starfish PLCgamma and diverged from mammalian PLCgamma before mammalian the PLCgamma1 and gamma2 isoforms diverged. Western blot analyses with specific anti-PLCbeta and gamma antibodies, respectively, revealed that 135 and 150 kDa proteins were expressed in eggs. The general PLC antagonist U-73122 blocked fertilization-induced egg activation; however, the inactive analog, U-73343, had no effect on egg activation. We further tested whether egg activation was G protein-PLCbeta and/or protein tyrosine kinase-PLCgamma dependent. Cholera and pertussis toxins, well-known effectors of G proteins, had no effect on egg activation; while two antagonists of PTK, genistein and tyrphostin B42, inhibited both fertilization-induced and artificial egg activation. Taken together, our studies suggested that PLC activity from eggs contributes to Chaetopterus egg activation and PLCgamma might play an important role during this biological process.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

Simulation of calcium waves in ascidian eggs: insights into the origin of the pacemaker sites and the possible nature of the sperm factor

Fertilization triggers repetitive waves of cytosolic Ca2+ in the egg of many species. The mechanism involved in the generation of Ca2+ waves has been studied in much detail in mature ascidian eggs, by raising artificially the level of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] or of its poorly metabolizable analogue, glycero-myo-phosphatidylinositol 4,5-bisphosphate [gPtdIns(4,5)P2]. Here, we use this strategy and the experimental results it provides to develop a realistic theoretical model for repetitive Ca2+ wave generation and propagation in mature eggs. The model takes into account the heterogeneous spatial distribution of the endoplasmic reticulum. Our results corroborate the hypothesis that Ca2+ wave pacemakers are associated with cortical accumulations of endoplasmic reticulum. The model is first tested and validated by the adequate match between its theoretical predictions and the observed effects of localized injections of massive amounts of Ins(1,4,5)P3 analogues. In a second step, we use the model to make some propositions about the possible characteristics of the sperm factor. We find that to account for the spatial characteristics of the first series of Ca2+ waves seen at fertilization in ascidian eggs, it has to be assumed that, if the sperm factor is a phospholipase C, it is Ca2+-sensitive and highly diffusible. Although the actual state of knowledge does not allow us to explain the observed relocalization of the Ca2+ wave pacemaker site, the model corroborates the assumption that PtdIns(4,5)P2, the substrate for phospholipase C is distributed over the entire egg. We also predict that the dose of sperm factor injected into the egg should modulate the temporal characteristics of the first, long-lasting fertilization wave.