Signal Transduction Pathways During Sperm Capacitation
Mammalian sperm are not able to fertilize eggs immediately after ejaculation. They acquire fertilization capacity after residing in the female tract for a finite period of time. The physiological changes occurring in the female reproductive tract that render the sperm able to fertilize constitute the phenomenon of sperm capacitation.
Using the mouse as an experimental paradigm, we have demonstrated that conditions conducive to capacitation of cauda epididymal sperm promote the tyrosine phosphorylation of a subset of proteins of Mr 40,000 - 120,000. The increase in protein tyrosine phosphorylation is dependent on the presence of BSA, Ca2+ and NaHCO3 in the medium, and the concentrations of these compounds needed for protein tyrosine phosphorylation to occur are correlated with those required for capacitation.
Serum albumin, usually bovine serum albumin (BSA), is believed to function during capacitation in vitro as a sink for the removal of cholesterol from the sperm plasma membrane. We have demonstrated that cholesterol removal is also essential in the regulation of intracellular signaling that occurs during sperm capacitation.
The transmembrane movements of HCO3- and/or Ca2+ could be responsible for the regulation of sperm cAMP metabolism, since the mammalian sperm adenylyl cyclase is markedly stimulated by these ions. We have also demonstrated that the increase in protein tyrosine phosphorylation as well as capacitation were regulated by a cAMP-dependent pathway involving protein kinase A (PKA).
Simultaneously with our findings, Zeng et al. (Dev. Biol. 1995; 171:554-563) reported that capacitation is accompanied by hyperpolarization of the sperm plasma membrane. Although the molecular basis of capacitation is not well understood, recent work from many laboratories is beginning to lead to a unified hypothesis of how capacitation is controlled and this is summarized in the following model:
Transmembrane and intracellular signaling pathways hypothesized to play a role in regulating sperm capacitation. This model is based on the work from a number of different laboratories.
(-) indicates negative regulation;
(+) indicates positive regulation.
BSA: bovine serum albumin
5'AMP, 5' adenosine monophosphate; PTK, protein tyrosine kinase
PTyr-Ptase, phosphotyrosine phosphatase
PDE, cyclic nucleotide phosphodiesterase.
My long term goal is to understand how the sperm acquire fertilizing capacity at the molecular level. In particular, I am focusing in several basic questions that arise from the capacitation model in the figure such as:
- Which are the protein substrates for tyrosine phosphorylation and how are they involved in the capacitation process?
- Which are the tyrosine kinase/s and/or phosphotyrosine phosphatases responsible for the increase in protein tyrosine phosphorylation observed during capacitation?
- How capacitation and the capacitation-associated hyperpolarization are regulated by components of the capacitation medium?
- How are changes in cAMP, protein tyrosine phosphorylation and hyperpolarization of the sperm plasma membrane integrated to regulate capacitation?
- Where in the sperm do the changes in cAMP, protein tyrosine phosphorylation and hyperpolarization occur during capacitation?
- How cholesterol removal affects signaling pathways during sperm capacitation?
Postdotoral Research Associate
|gmohanty [at] umass.edu
Graduate Student - MCB
|snayyab [at] umass.edu
ABBS Graduate Student
|lokeshsaini [at] umass.edu
Graduate Student - ABBS
|mtaghavi [at] umass.edu
Weber, W. David
Postdoctoral Research Associate
|wweber [at] umass.edu