|Pablo E. Visconti
Professor of Veterinary and Animal Sciences, University of Massachusetts
Ph.D.: University of Buenos Aires
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, Ca 2+ and NaHCO 3 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 HCO 3- and/or Ca 2+ 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
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:
1) Which are the protein substrates for tyrosine phosphorylation and how are they involved in the capacitation process?
2) Which are the tyrosine kinase/s and/or phosphotyrosine phosphatases responsible for the increase in protein tyrosine phosphorylation observed during capacitation?
3) How capacitation and the capacitation-associated hyperpolarization are regulated by components of the capacitation medium?
4) How are changes in cAMP, protein tyrosine phosphorylation and hyperpolarization of the sperm plasma membrane integrated to regulate capacitation?
5) Where in the sperm do the changes in cAMP, protein tyrosine phosphorylation and hyperpolarization occur during capacitation?
6) How cholesterol removal affects signaling pathways during sperm capacitation?
Maier, B, Moreno, S., Sleight S.B., Visconti, P.E. and Scrable H. (2003) Developmental association of the synaptic activity-regulated protein arc with the mouse acrosomal organelle and the sperm tail. Biology of Reproduction In press.
Scott Ficarro2, Olga Chertihin1, Anne Westbrook1, Forest White2, Friederike Jayes1, Petr Kalab, Jeffrey Shabanowitz2, John C. Herr1, Donald Hunt2,3 and Pablo E. Visconti (2003) Phosphoproteome of capacitated human sperm reveals tyrosine phosphorylation and translocation of p97. J. Biol. Chem. In press.
Ignacio A. Demarco, Felipe Espinosa, Jennifer Edwards, Julian Sosnik, Jose Luis de la Vega-Beltrán, Joel W. Hockensmith, Gregory S. Kopf, Alberto Darszon and Pablo E. Visconti. Involvement of a Na+/HCO3- cotransporter in mouse sperm capacitation. J. Biol. Chem. In press.
Visconti, P.E., Westbrook, V.A., Chertihin, O., Demarco, I. Sleight, S. and Diekman, A.B. (2002) Novel signaling pathways involved in sperm acquisition of fertilizing capacity. J. of Reprod. Immunol. 53:133-150
Tomes, CN., Michaut, M, De Blas, G., Visconti, PE, Matti, U. and Mayorga, L. (2002) SNARE complex assembly is required for human sperm acrosome reaction. Developmental Biology 243, 326-338
Westbrook, V.A., Diekman, A.B. and Visconti, P.E. (2002) Novel signaling pathways involved in sperm acquisition of fertilizing capacity. In: Introduction to Mammalian Reproduction. (Ed. D.R. Tulsiani). Kluwer Academic/Plenum Publishers, New York, NY.
Coonrod, SA, Bolling, LC, Wright, PW, Visconti, PE and Herr, JC. (2001) A Morpholino Phenocopy of the Mouse mos Mutation. Dev. Gen. Genesis 30: 198-200
Visconti PE, Hao, Z., Purdon, M., Stein, P, Balsara, BR, Testa, JR, Herr, JC, Moss, S and Kopf, GS (2001) Cloning and chromosomal localization of a novel serine/threonine kinase belonging to the subfamily of testis-specific kinases tssk. Genomics. 77: 163-170.
Espinosa, F., López-González, I., Muñoz-Garay, C., Felix, R., De la Vega Beltrán, Kopf, G. S., Visconti, P. E. and Darszon, A. Dual regulation of the T-type Ca2+ current by serum albumin and b-estradiol in mammalian spermatogenic cells. FEBS Lett. 475: 251-256, 2000
Visconti, PE, Galantino-Homer, H, Ning, XP, Moore, GD, Valenzuela, JP, Alvarez, J.A. and Kopf, GS. (1999) Cholesterol efflux-mediated signal transduction in mammalian sperm: b-cyclodextrins initiate transmembrane signaling leading to an increase in protein tyrosine phosphorylation and capacitation. J. Biol. Chem.. 274, 3235-3242.