The challenges involved in elucidating the molecular basis of sperm-egg recognition in mammals and approaches to overcome them

Abstract

Sexual reproduction is used by many different organisms to create a new generation of genetically distinct progeny. Cells originating from separate sexes or mating types segregate their genetic material into haploid gametes which must then recognize and fuse with each other in a process known as fertilization to form a diploid zygote. Despite the central importance of fertilization, we know remarkably little about the molecular mechanisms that are involved in how gametes recognize each other, particularly in mammals, although the proteins that are displayed on their surfaces are almost certainly involved. This paucity of knowledge is largely due to both the unique biological properties of mammalian gametes (sperm and egg) which make them experimentally difficult to manipulate, and the technical challenges of identifying interactions between membrane-embedded cell surface receptor proteins. In this review, we will discuss our current knowledge of animal gamete recognition, highlighting where important contributions to our understanding were made, why particular model systems were helpful, and why progress in mammals has been particularly challenging. We discuss how the development of mammalian in vitro fertilization and targeted gene disruption in mice were important technological advances that triggered progress. We argue that approaches employed to discover novel interactions between cell surface gamete recognition proteins should account for the unusual biochemical properties of membrane proteins and the typically highly transient nature of their interactions. Finally, we describe how these principles were applied to identify Juno as the egg receptor for sperm Izumo1, an interaction that is essential for mammalian fertilization.

Keywords: Egg; Fertilization; Izumo1; Juno; Membrane protein.

Fig. 1

Biological, technical and ethical challenges that have limited progress in the molecular basis of gamete recognition in mammals. Sperm and eggs are terminally differentiated cells whose unique biological properties make investigating the molecular basis of their function difficult. Here, we list some of these properties for both gametes

Fig. 2

The use of a highly avid ectodomain probe and an expression cloning approach to identify the Izumo1-Juno interaction. a Pools of plasmid clones from a mouse oocyte cDNA expression library were transiently transfected into adherent HEK293T cells. A highly avid (pentameric) tagged Izumo1 ectodomain (brown ovals) binding probe was used to identify plasmid pools that contained an Izumo1 binding protein by positively staining the transfected cells. b Individual plasmids within the positive pools were tested for their ability to confer gain of Izumo1 binding. Identified plasmids were then sequenced to reveal the identity of the Izumo1 binding protein which, in all cases, was Juno