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Cyclic 3',5'-AMP Causes ADAM1/ADAM2 to Rapidly Diffuse Within the Plasma Membrane of Guinea Pig Sperm¹

Population Council,³ Center for Biomedical Research, Rockefeller University, New York, New York 10065 Center for Cell Analysis and Modeling⁴ and Department of Molecular, Microbial and Structural Biology,⁵ University of Connecticut Health Center, Farmington, Connecticut 06030

ABSTRACT

Because sperm cannot synthesize new proteins as they journey to the egg, they use multiple mechanisms to modify the activity of existing proteins, including changes in the diffusion coefficient of some membrane proteins. Previously, we showed that during capacitation the guinea pig heterodimeric membrane protein ADAM1/ADAM2 (fertilin) transforms from a stationary state to one of rapid diffusion within the lipid bilayer. The cause for this biophysical change, however, was unknown. In this study we examined whether an increase in cAMP, such as occurs during capacitation, could trigger this change. We incubated guinea pig cauda sperm with the membrane-permeable cAMP analog dibutyryl cAMP (db-cAMP) and the phosphodiesterase inhibitor papaverine and first tested for indications of capacitation. We observed hypermotility and acrosome-reaction competence. We then used fluorescence redistribution after photobleaching (FRAP) to measure the lateral mobility of ADAM1/ADAM2 after the db-cAMP treatment. We observed that db-cAMP caused roughly a 12-fold increase in lateral mobility of ADAM1/ADAM2, yielding diffusion similar to that observed for sperm capacitated in vitro. When we repeated the FRAP on testicular sperm incubated in db-cAMP, we found only a modest increase in lateral mobility of ADAM1/ADAM2, which underwent little redistribution. Interestingly, testicular sperm also cannot be induced to undergo capacitation. Together, the data suggest that the release of ADAM1/ADAM2 from its diffusion constraints results from a cAMP-induced signaling pathway that, like others of capacitation, is established during epididymal sperm maturation..

ADAM1, ADAM2, cAMP, capacitation, fertilin, fertilization, fluorescent redistribution after photobleaching, FRAP, gamete biology, membrane diffusion, sperm, sperm capacitation, sperm maturation

¹Supported by the National Institute of Health research grant HD038807.

Correspondence: ²Gary Hunnicutt, Population Council, 1230 York Ave, New York, NY 10065. FAX: 212 327 7678; e-mail: ghunnicutt{at}popcbr.rockefeller.edu