Abstract
Glycogen synthase kinase 3 (GSK3) was identified as an enzyme regulating sperm protein phosphatase. The GSK3 alpha paralog, but not GSK3 beta, is essential for sperm function. Sperm lacking GSK3 alpha display altered motility and are unable to undergo hyperactivation, which is essential for fertilization. Male mice lacking sperm-specific calcineurin (PP2B), a calcium regulated phosphatase, in testis and sperm, are also infertile. Loss of PP2B results in impaired epididymal sperm maturation and motility. The phenotypes of GSK3 alpha and PP2B knockout mice are similar, prompting us to examine the interrelationship between these two enzymes in sperm. High calcium levels must exist to permit catalytically active calcineurin to function during epididymal sperm maturation. Total and free calcium levels are high in immotile compared to motile epididymal sperm. Inhibition of calcineurin by FK506 results in an increase in the net phosphorylation and a consequent decrease in catalytic activity of sperm GSK3. The inhibitor FK506 and an isoform-selective inhibitor of GSK3 alpha, BRD0705, also inhibited fertilization of eggs in vitro. Interrelated functions of GSK3 alpha and sperm PP2B are essential during epididymal sperm maturation and during fertilization. Our results should enable the development of male contraceptives targeting one or both enzymes.