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Caspase-Independent Exposure of Aminophospholipids and Tyrosine Phosphorylation in Bicarbonate Responsive Human Sperm Cells¹

Institute for Biomembranes,³ Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands Graduate School of Animal Health,⁴ Department of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands Department of Molecular and Experimental Medicine,⁵ The Scripps Research Institute, La Jolla, California 92037

Only capacitated sperm cells are able to fertilize egg cells, and this process is triggered by high levels of bicarbonate. Bicarbonate renders the plasma membrane more fluid, which is caused by protein kinase A (PKA)-mediated alterations in the phospholipid (PL) bilayer. We studied exposure of phosphatidylserine (PS) and phosphatidylethanolamine (PE) in human sperm cells. Surface exposure of PS and PE on sperm cell activation in vitro was found to be bicarbonate dependent and restricted to the apical area of the head plasma membrane. The PL scrambling in bicarbonate-triggered human sperm was not related to apoptosis, because the incubated cells did not show any signs of caspases or degeneration of mitochondria or DNA. The PL scramblase (PLSCR) gene family has been implicated in this nonspecific, bidirectional PL movement. A 25-kDa isoform of PLSCR was identified that was homogeneously distributed in human sperm cells. We propose that compartment-dependent activation of PKA is required for the surface exposure of aminophospholipids at the apical plasma membrane of sperm cells. Bicarbonate-induced PL scrambling appears to be an important event in the capacitation process, because the entire intact scrambling sperm subpopulation showed extensive tyrosine phosphorylation, which was absent in the nonscrambling subpopulation. The proportion of live cells with PL scrambling corresponded with that showing capacitation-specific chlortetracyclin staining.

¹ Supported in part by grant HL36946 from the National Institutes of Health of the United States (to P.J.S. and T.W.). K.J.V. is a postdoctoral fellow financed by the Graduate School of Animal Health.

² Correspondence: B.M. Gadella, Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 2, 3584 CM Utrecht, The Netherlands. FAX: 31 30 2535492; b.gadella{at}vet.uu.nl

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