Modulators of calcium signalling at fertilization

Abstract

Calcium (Ca²⁺) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca²⁺ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca²⁺ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca²⁺. Following the Ca²⁺ rise, the reuptake of Ca²⁺ into intracellular stores or efflux of Ca²⁺ out of the egg drive the return of cytoplasmic Ca²⁺ back to baseline levels. The molecular mediators of these Ca²⁺ fluxes in different organisms include Ca²⁺ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca²⁺ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

Keywords: calcium channels; calcium signalling; egg activation; fertilization; oocyte.

Figure 1.

Elements of Ca²⁺ toolbox in endoplasmic reticulum, mitochondria, lysosomes and plasma membrane of somatic cells. Orange dots indicate Ca²⁺; grey arrows show direction of Ca²⁺ flow. cADPR, cyclic ADP ribose; CAX, Ca²⁺/proton exchanger; IP3, inositol trisphosphate; IP3R, IP3 receptor; MCU, mitochondrial uniporter; NCLX, mitochondrial sodium–Ca²⁺ exchanger; NAADP, nicotinic acid adenine dinucleotide phosphate; NCX, sodium/Ca²⁺ exchanger; ORAI, Ca²⁺ release-activated Ca²⁺ channel protein; PIP2, phosphatidylinositol 4,5-bisphosphate; PLC, phospholipase C; PMCA, plasma membrane Ca²⁺ ATPase; RyR, ryanodine receptor; SERCA, sarco/endoplasmic reticulum Ca²⁺ ATPase pump; STIM, stromal interaction molecule; TPC, two-pore channel.

Figure 2.

Diagrams of typical cytoplasmic Ca²⁺ changes at fertilization across various species. From top to bottom: nematode (Caenorhabditis elegans) [62], fruit fly (Drosophila melanogaster) [63], purple mussel (Septifer virgatus) [7], sea urchin (Paracentrotus lividus) [64], zebrafish (Danio rerio) [65], frog (Xenopus laevis) [66], newt (Cynops pyrrhogaster) [67] and mouse (Mus musculus) [68]. Time indicated on scale bar for each Ca²⁺ trace. The three lines on the newt trace represent spatially distinct signals induced by different sperm during physiological polyspermy. Traces adapted from indicated references.

Figure 3.

Generation of a Ca²⁺ wave. Localized production of IP3 (teal circles) by PLC (teal semicircles) leads to IP3-mediated Ca²⁺ release from clusters of IP3 receptors in endoplasmic reticulum (ER) membranes. The released Ca²⁺ promotes Ca²⁺-induced Ca²⁺ release from nearby clusters, increasing the cytoplasmic Ca²⁺ gradient. In addition, the released Ca²⁺ stimulates PLC to generate additional IP3 in a positive feedback loop. Continued IP3 production and Ca²⁺ diffusion lead eventually to Ca²⁺ release from distal IP3 receptor clusters in distinct ER regions and a wave of Ca²⁺ release across the cell. Orange dots indicate Ca²⁺; grey arrows show the direction of IP3 or Ca²⁺ flow.

Figure 4.

Cycle of Ca²⁺ transient generation in mammalian eggs at fertilization. Starting at the top left, the large grey arrows show temporal order. For each panel, the cytoplasmic Ca²⁺ trace is coloured orange at the portions of the trace that are generated mainly due to the steps illustrated in that panel. Top left: Sperm PLCζ acts on PIP2 in intracellular vesicles to generate IP3, which stimulates IP3R-mediated Ca²⁺ release and subsequent Ca²⁺-induced Ca²⁺ release. Top right: Ca²⁺ stimulates mitochondrial ATP production; ATP is required for SERCA pump activity. Bottom right: Ca²⁺ is pumped back into the ER through SERCA pumps and out of the egg through PMCA pumps and NCX. Bottom left: Ca²⁺ flows into the cytoplasm through TRMP7, Ca_V3.2 and TRPV3 channels and is then available for SERCA pumps to replenish ER Ca²⁺ stores in preparation for the next Ca²⁺ release event. Orange dots indicate Ca²⁺ at its destination; small grey arrows show the direction of flow. Ca_V3.2, T-type voltage-dependent Ca²⁺ channel; IP3, inositol trisphosphate; IP3R, IP3 receptor; MCU, mitochondrial uniporter; NCX, sodium/Ca²⁺ exchanger; PIP2, phosphatidylinositol 4,5-bisphosphate; PLCζ, phospholipase C zeta; PMCA, plasma membrane Ca²⁺ ATPase; SERCA, sarco/endoplasmic reticulum Ca²⁺ ATPase pump; TRPM7, transient receptor potential cation channel subfamily M member 7; TRPV3, transient receptor potential cation channel subfamily V member 3.

Figure 5.

Representative Ca²⁺ traces resulting from egg activation by various methods. Top row, chemical inducers of artificial egg activation (AEA) in bovine eggs [234]. Bottom row, in vitro fertilization (IVF) [68], Plcz1 cRNA injection [123] and intracytoplasmic sperm injection (ICSI) [235] in mouse eggs; ICSI using sperm lacking PLCζ activity, in medium containing CaCl₂, followed by two treatments with ionomycin in human egg [235]. Scale bar applies to all traces. Traces adapted from indicated references.