A role for glycogen synthase kinase 3, its activity modulators, and the serine/threonine phosphatase pp172 in the regulation of sperm function

We have reported that the phosphatase PP172, an I2-like inhibitor, and glycogen synthase kinase-3 (GSK-3) are present in mammalian sperm. Our data suggest that PP172 and GSK-3 play key roles in motility initiation. The objective of this report was to determine how changes in activities of PP172 and GSK-3 regulate sperm motility. PP172 and GSK-3 activities were measured following immunoprecipitation in extracts from immotile caput and motile caudat bovine epidîdymal sperm. The activities of these enzyme were higher in immotile than motile sperm. In the cytosol, PP172 was bound to a 36 kDa, GSK 3 phosphorylatible protein distinct from somatic cell inhibitor 12. In the insoluble sperm flagellar fraction, the enzyme was bound to an unidentified 72 kDa protein. Western blot showed that PI3-kinase and protein kinase B (PKB), the signal transduction enzymes postulated to play a role in GSK-3 regulation in somatic cells, are present in high levels in sperm. Whether these enzymes change in parallel with GSK-3 inactivation and sperm motility initiation is under investigation. Immunocytochemistry showed PP172, GSK-3, and Pl3-kinase in the flagellum, consistent with their roles in regulating sperm motility. We suggest that PP172 activity is modulated by phosphorylation of its targeting proteins by GSK-3, and that GSK-3, in turn, is regulated by PI3-kinase and PKB. Our work documents, for the first time, a physiological role for GSK-3 and PP172 in the regulation of sperm function.