New Insights into the Molecular Events of Mammalian Fertilization

Abstract

Currently, infertility affects ∼16% of couples worldwide. The causes are reported to involve both male and female factors, including fertilization failure between mature spermatozoa and eggs. However, the molecular mechanisms involved in each step of mammalian fertilization are yet to be fully elucidated. Although some of these steps can be rescued with assisted reproductive technologies, it is important to clarify the molecular mechanisms involved for the treatment and diagnosis of infertile couples. This review illustrates recent findings in mammalian fertilization, discovered by combining gene modification techniques with other new approaches, and aims to show how these findings will guide future research in mammalian fertilization.

Keywords: CRISPR/Cas9 system; Genetically modified animals; egg activation; mammalian fertilization; sperm–egg fusion.

Figure 1.. (Key Figure) Mammalian sperm–egg fusion and egg activation processes.

Gene deletion analysis revealed that both IZUMO1 on the spermatozoa and JUNO on the egg are required for sperm–egg fusion. PLCζ1 is a sperm-borne oocyte activation factor that induces Ca²⁺ oscillations, subsequent cell cycle resumption, and other postfusion processes such as the blocks to polyspermy.

Figure 2.. Interaction between IZUMO1 and JUNO.

(A) complex are shown as a ribbon diagram (Protein Date Bank code 5F4E; [15]). Alpha helixes or β-sheets in the HB domain, hinge region, and Ig-like domain of IZUMO1 are colored in orange, green, and blue, respectively. Residues shown in (C) are colored in red. (B) Conformational changes in IZUMO1 upon interaction with JUNO. In Ohto et al. reported no conformational change in IZUMO1 [11]; on the other hand, Aydin et al. reported that IZUMO1 changes its structure from a boomerang-like shape to an upright or right-angled conformation [15]. (C) Significant residues in the interactive surface between IZUMO1 and JUNO suggested from in vitro affinity assays using recombinant proteins.

Figure 3.. Egg activation mechanisms with or without PLCζ1

Without sperm-borne PLCζ1 (upper), eggs exhibit atypical Ca²⁺ oscillations (delayed onset and low Ca²⁺ spike frequency) leading to higher incidences of polyspermy and activation failure compared with PLCζ1dependent and robust egg activation (lower). The PLCζ1-independent egg activation is observed only following natural fertilization (in vivo or in vitro) during sperm–egg interactions, but not after ICSI.