Glycogen Synthase Kinase-3 Isoform Variants and Their Inhibitory Phosphorylation in Human Testes and Spermatozoa

Abstract

Purpose: To clarify (phospho-) glycogen synthase kinase-3 (GSK3) isoform variants in the gemiline and soma of human testes and spermatozoa. Materials and Methods: GSK3 isoform variants in normospermatogenic and Sertoli cell-only (SCO) testicular biopsies and spermatozoa were examined. Results: In normospermatogenic testes, GSK3 alpha, and GSK3 beta variants 1 and 2 different in low complexity region (LCR) were expressed and their levels were decreased in SCO testes. GSK3 beta variant 3 was only expressed in SCO testes. GSK3 beta as well as GSK3 alpha, the dominant isoforms in testes were decreased in SCO testes. In normospermatogenic testes, GSK3 beta were found in spermatogonia and markedly decreased in meiotic germ cells in which GSK3 alpha was dominant. p-GSK3 alpha/beta were marginal in spermatogonia and early spermatocytes. In SCO testes, GSK3 alpha/beta immunoreactivity in seminiferous epithelia was weaker than those of normospermatogenic testes whereas p-GSK3 alpha/beta(Ser) immunoreactivity was visibly increased in Sertoli cells. GSK3 alpha was dominant in ejaculated spermatozoa in which GSK3 alpha and p-GSK3 alpha(Ser) were found in the head, midpiece, and tail. In acrosome-reacted spermatozoa, GSK3 alpha was found in the equatorial region of head, midpiece, and tail, and p-GSK3 alpha(Ser) was only found in midpiece. During sperm capacitation, p-GSK3 alpha(Ser) was significantly increased together with phosphotyrosine proteins and motility. Conclusions: In human male germ cells, GSK3 isoforms different in LCRs switch from GSK3 beta to GSK3 alpha during meiotic entry, suggesting the isoform-specific roles of GSK3 alpha and GSK3 beta in meiosis and sternness or proliferation of spermatogonia, respectively. In dormant Sertoli cells of SCO testes kinase activity of GSK3 might be downregulated via inhibitory phosphorylation. In spermatozoa, inhibitory phosphorylation of GSK3 alpha might be coupled with activation of motility during capacitation.