Changes in sperm glycogen synthase kinase-3 serine phosphorylation and activity accompany motility initiation and stimulation. 
Sperm motility is regulated by protein phosphorylation.  We have shown that the signaling kinase, glycogen synthase kinase-3 alpha (GSK-3 alpha), is present in spermatozoa.  In somatic cells, GSK-3 is regulated by serine and tyrosine phosphorylation.  In this report, we document that both GSK-3 alpha and GSK-beta isoforms are present in spermatozoa, with GSK-3 alpha being the predominant isoform.  The relationship between GSK-3 serine phosphorylation and motility was investigated.  Serine phosphorylation of GSK-3 increases significantly in spermatozoa during their passage through the epididymis.  Initiation and stimulation of motility in vitro by isobutyl-methyl-xanthine, 2-chloro-2'-deoxy-adenosine, and calyculin A lead to a dramatic increase in GSK-3 serine phosphorylation.  The concentration-dependent induction of motility by calyculin A is closely associated with GSK-3 serine phosphorylation.  Immunoprecipitation of GSK-3 alpha and GSK-3 beta shows that both of the GSK-3 isoforms are more active in caput than in caudal spermatozoa.  Calyculin A treatment decreased the activity of both isoforms.  Column chromatography was used to purify inactive GSK-3 alpha from the caudal sperm extracts.  This GSK-3 alpha species was phosphorylated at amino acid residues serine 21 and tyrosine 214.  Inactive GSK-3 alpha is present in caudal but not in caput epididymal spermatozoa.  The enzymes protein kinase B (PKB; also known as cAkt) and phosphoinositide 3-kinase (PI3-kinase), the upstream signaling proteins involved in GSK-3 phosphorylation, are both present in spermatozoa.  Fluorescence immunocytochemistry showed that GSK-3 is present in the head and tail regions of sperm.  Our work suggests a novel role for the signaling system involving GSK-3 in the regulation of sperm motility.