Insights into mammalian sperm-egg adhesion and fusion from studies of knockout mouse models of male infertility

According to the CDC's 2006 Assisted Reproductive Technology (ART) report, ∼18% of U.S. couples that underwent an ART procedure did so because of male factor infertility. Some of the causes of male infertility include disruptions in spermatogenesis, the loss or mutation of key molecules that mediate an essential sperm function, or indirect effects that lead to abnormalities in sperm or reproductive tract function. Knockout mouse models are useful tools to investigate the causes of male infertility. This thesis research focused on two specific mouse models, the knockout of the inositol polyphosphate phosphatase Inpp5b and the knockout of the tyrosylprotein sulfotransferase Tspt2. Both of these genes are expressed widely throughout the body, but the knockouts specifically affect male fertility. These animals produce sperm with normal morphology, but were speculated to have abnormalities in sperm-egg membrane interactions. To gain insights into the molecular basis of gamete membrane interactions as well as into male reproductive function in general, these knockouts were analyzed on multiple levels: physiological (performance of males in mating trials), cellular (performance of sperm in IVF assays), and molecular (analysis of sperm proteins of interest). IZUMO1 and the A Disintegrin and A Metalloprotease (ADAM) family of proteins were sperm proteins examined in this molecular analysis because they have roles in sperm-egg interactions. The analysis of Inpp5b-/- male reproductive function revealed variability in the different phenotypes. Multivariate analysis was used to assess the relationships between the variability in the molecular characteristics and the variability of the in vitro and in vivo physiological functions. Tpst2-/- mice were used to determine how tyrosine O-sulfation plays a role in sperm-egg interactions. Tpst2-null sperm have an increased ability to fuse with the egg, despite containing no detectable ADAM6 or ADAM3. Interestingly, both of the knockout models studied here did not to have any obvious abnormities in IZUMO1, the only sperm protein known to be essential for sperm-egg fusion. This study of male reproductive function on in Inpp5b and Tpst2-deficient mice has lead to new insights into sperm-egg interactions and intriguing future directions for infertility analysis and the study of regulation of gamete interactions.