Abstract
Glycogen synthase kinase 3 (GSK3) is a highly conserved protein kinase regulating key cellular functions. Its two isoforms, GSK3α and GSK3β, are encoded by distinct genes. In most tissues the two isoforms are functionally interchangeable, except in the developing embryo where GSK3β is essential. One functional allele of either of the two isoforms is sufficient to maintain normal tissue functions. Both GSK3 isoforms, present in sperm from several species including human, are suggested to play a role in epididymal initiation of sperm motility. Using genetic approaches, we have tested requirement for each of the two GSK3 isoforms in testis and sperm. Both GSK3 isoforms are expressed at high levels during the onset of spermatogenesis. Conditional knockout of GSK3α, but not GSK3β, in developing testicular germ cells in mice results in male infertility. Mice lacking one allele each of GSK3α and GSK3β are fertile. Despite overlapping expression and localization in differentiating spermatids, GSK3β does not substitute for GSK3α. Loss of GSK3α impairs sperm hexokinase activity resulting in low ATP levels. Net adenine nucleotide levels in caudal sperm lacking GSK3α resemble immature caput epididymal sperm. Changes in the association of the protein phosphatase PP1γ2 with its protein interactors occurring during epididymal sperm maturation is impaired in sperm lacking GSK3α. The isoform-specific requirement for GSK3α is likely due to its specific binding partners in the sperm principal piece. Testis and sperm are unique in their specific requirement of GSK3α for normal function and male fertility.
Highlights
The enzyme Glycogen synthase kinase 3 (GSK3), a serine/threonine protein kinase, was termed GSK-3 and was discovered after PKA and phosphorylase kinase (GSK-1 and GSK-2) along with two other GS kinases (GSK-4 and -5) based on relative elution from phosphocellulose chromatography of muscle extracts
GSK3α in sperm is essential for male fertility
Expression and localization of GSK3α and GSK3β in testis We have previously shown that both isoforms of GSK3 are present in sperm [11]
Summary
The enzyme GSK3, a serine/threonine protein kinase, was termed GSK-3 and was discovered after PKA and phosphorylase kinase (GSK-1 and GSK-2) along with two other GS kinases (GSK-4 and -5) based on relative elution from phosphocellulose chromatography of muscle extracts. An array of functions attributed to GSK3 includes insulin action, regulation of cell survival, apoptosis, embryonic development, Wnt/β-catenin and hedgehog signaling, and growth factor action. It is a target for drug development due to involvement in several clinical disorders including cancer [3]. GSK3 is ubiquitous and is expressed as two isoforms, GSK3α and GSK3β, encoded by different genes. The catalytic domains of the two isoforms are 98% identical, while their N- and C- termini are distinctive [4]. While there are reports ascribing distinct roles for each of the isoforms [5] under most circumstances, the two isoforms appear redundant and functionally interchangeable
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