Molecular Determinants of Zinc Regulation of Mammalian Sperm

Zinc is the most abundant trace metal in the body, and the highest zinc concentrations are found in semen. Human semen is enriched with 1-3 mM zinc, a 100-fold higher concentration than the zinc contents of other parts of the body. Moreover, low seminal zinc has been correlated with male infertility. High concentrations of zinc are also found surrounding newly fertilized eggs. Despite several observations documenting that sperm encounter high concentrations of zinc multiple times between mating and fertilization, how zinc regulates sperm is unknown. The overall objectives of this application are to uncover how zinc alters sperm swimming and establish the mechanism by which zinc homeostasis changes between when sperm are first mixed with the seminal fluids and when sperm arrive at the site and time of fertilization. The central hypothesis is that zinc derived from the seminal fluid and released by the egg at fertilization regulates sperm swimming and their ability to fertilize. The rationale for this project is that uncovering a role for zinc in the processes that sperm undergo between mating and fertilization will provide the conceptual foundation needed to develop strategies for fertility treatments, and the development of non-hormonal contraception can be developed. The central hypothesis will be tested by pursuing three specific aims: 1) Establish the mechanisms by which zinc regulates sperm at mating; 2) Delineate the mechanisms of zinc depletion during capacitation; and 3) Define how zinc alters hyperactivation of mouse and human sperm. Under the first aim, we will use proteomics, computer-assisted semen analysis, and biochemistry approaches to define how zinc enters and regulates sperm at mating. For the second aim, we will use fluorescence-based experiments to understand how intracellular zinc is depleted during capacitation. For the third aim, we will use electrophysiology and computer-assisted semen analysis to understand how zinc regulates the Ca2+ channel CatSper and the swimming pattern known as hyperactivation. The research proposed in this application is innovative because it focuses on zinc as a central regulator of sperm physiology, combines experimentation on both mouse and human sperm, and uses mass spectrometry to identify plasma membrane-localized proteins in sperm. The proposed research is significant because it is expected to change our understanding of the essential events of fertilization. Ultimately, this knowledge has the potential to offer new opportunities for the development of innovative fertility interventions.