Glycogen synthase kinase-3 levels and phosphorylation undergo large fluctuations in mouse brain during development

In a landmark study, Kessler and colleagues (1) found that 50% of all lifetime cases of mental disorders begin by the age of 14 years, and 75% by the age of 24 years. This prevalent onset in young people is reflected in the recent large increase in diagnosis and prescriptions of psychotropic medications given to juveniles and adolescents (2, 3). These medications particularly include antidepressants that increase serotonergic signaling, and stimulants that increase dopaminergic signaling. The National Institute of Mental Health (NIMH) has concluded that interventions targeting young, high-risk populations may have remarkable benefits in reducing the risk for psychopathology over the lifespan, which currently afflicts approximately 25% of the population of the United States. However, it is unclear what predisposes young people during development to many major psychiatric diseases, such as mood disorders, schizophrenia, and attention-deficit hyperactivity disorder, and if administered psychotropic medications during development have equivalent effects as in adults (4). This study was designed to examine glycogen synthase kinase-3 (GSK3) in juvenile, adolescent, and adult mice as a possible risk factor for increased vulnerability to psychiatric diseases during development and as a possible contributor to the potential for differential medication responses in young people and adults. GSK3, comprising GSK3α and GSK3β isoforms, is an important modulator of many cellular functions, such as regulating gene expression, neural plasticity, cellular architecture, and cell survival (5-8). Unlike most protein kinases, GSK3 is constitutively partially active, and most signaling pathways converging on GSK3 decrease its activity. Phosphorylation is a major mechanism regulating GSK3 activity, whereas the expression levels of GSK3 are generally stable. The activity of GSK3 is optimal when phosphorylated on the regulatory tyrosine-279-GSK3α and tyrosine-216-GSK3β, but mechanisms regulating GSK3 tyrosine-phosphorylation remain unclear. In contrast, GSK3 is inhibited by serine-phosphorylation on serine-9 of GSK3β and serine-21 of GSK3α. Many signaling pathways regulate GSK3 activity by this modification, which can be catalyzed by several kinases, particularly Akt/protein kinase B (9, 10). GSK3 was first linked to mood disorders by the discovery that the mood stabilizer lithium selectively inhibits GSK3 by directly binding the enzyme (11, 12). This direct inhibition by lithium appears to be amplified in vivo at therapeutic lithium levels by an indirect mechanism that increases the inhibitory serine-phosphorylation of GSK3 (13, 14). Antidepressants and antipsychotics also increase inhibitory serine-phosphorylation of GSK3 in vivo (15-20). There is no evidence addressing whether these regulatory effects of therapeutic drugs on GSK3 differ during development from adults. In mice, inhibitors of GSK3 modify several behaviors that may model mood, such as reducing immobility in the forced swim test (indicative of antidepressant-like effects) and reducing amphetamine-induced hyperactivity (a model of manic-like behavior) (21-25). These and many additional findings in animals and humans support the hypothesis that dysregulated GSK3 contributes to susceptibility to mood disorders (26-28). Little is known about GSK3 during development. Leroy and Brion (29) reported that in whole rat brain the level of GSK3β from birth to 10 days of age was 50–60% higher than in adults. Since GSK3 may contribute to the onset of mood disorders and other psychiatric diseases that commonly occur in young people, we considered that if GSK3 is elevated at susceptible stages of development it may contribute to heightened developmental susceptibility to psychiatric diseases. Therefore, we analyzed developmental changes in the levels of both isoforms of GSK3 in two brain regions of mice, its regulatory phosphorylation, and the regulation of GSK3 by lithium and fluoxetine administered in vivo during development.