Region-specific dysregulation of glycogen synthase kinase-3β and β-catenin in the postmortem brains of subjects with bipolar disorder and schizophrenia

Several lines of evidence suggest that disturbances in GSK-3β and β-catenin, two important components of the Wnt signaling pathway, may be involved in the pathophysiology of bipolar illness and schizophrenia (SZ). The Wnt signaling pathway is activated by the interaction of a Wnt protein with a frizzled receptor, which leads to the phosphorylation of disheveled (DVL) and the inactivation of GSK-3β by DVL. The inactivation of GSK-3β results in the cytoplasmic accumulation of β-catenin and its subsequent translocation to the nucleus where it binds to TCF/LEF (T-cell factor and Lymphoid Enhancer Factor), thus forming a transcription factor complex that activates the transcription of several genes (1-4). The main evidence suggesting a disturbance in Wnt signaling in bipolar disorder (BP) is derived from the observation that mood stabilizing drugs, such as lithium, used in the treatment of BP affect the Wnt signaling pathway, presumably by inhibiting GSK-3β activity and thus increasing β-catenin levels. It was first shown by Klein and Menton (5) that lithium causes inhibition of GSK-3β activity. Subsequently, it was shown that lithium inhibits GSK-3 both in intact cells and in the mammalian brain in vivo. Jope and Roh (6) and De Sarno et al. (7) found that chronic in vivo treatment with lithium increased the phosphorylation of Ser-9 in GSK-3β in the brain of mice. It was later shown by other investigators (8) that valproic acid treatment, an anticonvulsant and a mood-stabilizing drug, also directly inhibited GSK-3β activity. However, not all groups agree that valproate is a GSK-3β inhibitor (9). It was shown by Gould et al. (10) that lithium treatment inhibited GSK-3β in brain of rats. The observation that both mood-stabilizing drugs, lithium and valproic acid, cause inhibition of GSK-3β suggests that abnormalities of the Wnt signaling pathway in general, and GSK-3β in particular, may be associated with the pathophysiology of BP. The involvement of GSK-3β in BP has also been supported by the observation that monozygotic twins discordant for BP have differential expression of genes in Wnt signaling (11). It has been observed that a single nucleotide polymorphism in the GSK-3β promoter gene influences the onset of illness in patients with BP (12). GSK-3β has been studied in the platelets, lymphocytes, and postmortem brain of subjects with BP and SZ. Pandey et al. (13) found decreased GSK-3β protein expression in platelets of drug-free patients with BP. Nadri et al. (14) found no difference in GSK-3α and GSK-3β mRNA levels, GSK-3β protein levels, or total GSK-3 activity between patients with SZ and normal control (NC) subjects. Some studies used postmortem brain samples from subjects with SZ and BP. Lesort et al. (15) determined Wnt signaling in the postmortem brains of subjects with BP and observed that there are no significant differences in the protein expression levels of GSK-3β or β-catenin in subjects with BP compared with matched NC subjects. Similar results were reported by Kozlvosky et al. (16) who also did not find differences in the levels of GSK-3β between subjects with BP and NC subjects. Since antipsychotics also increase β-catenin levels in the rat brain and inhibit GSK-3β activity (17), GSK-3β and β-catenin have been studied in the postmortem brain of patients with SZ. Cotter et al. (18) and Lesort et al. (15) found decreased β-catenin in the postmortem brains of the patients with SZ. Emamian et al. (19) found decreased GSK-3β phosphorylation (i.e., increased activity) in the brains of subjects with SZ and a non-significant decrease in total GSK-3β. Beasley et al. (20) found significantly decreased GSK-3β protein levels in subjects with SZ. Kozlovsky et al. (16, 21) found lower GSK-3β levels in the frontal cortex of patients with SZ compared with NC subjects. However, they did not find significant differences in GSK-3β between subjects with BP and NC subjects (21). Studies of GSK-3β in the postmortem brain of subjects with SZ by Nadri et al. (22) were inconclusive. The suggestion that abnormalities of GSK-3β and/or β-catenin may be involved in the pathophysiology of bipolar illness and SZ is, thus, generally indirect, since the studies of GSK-3β in the postmortem brain were unclear. One of the reasons that the studies in the postmortem brain were inconclusive or unclear was because they studied only the protein expression of GSK-3β in the brain samples. However, determination of GSK-3β protein expression does not indicate or does not provide any index of its activity. GSK-3β is a multi-functional serine/threonine kinase, and its activity is regulated negatively by serine 9 (pGSK-3β-ser-9) and positively by the tyrosine 216 (pGSK-3β-tyr-216) phosphorylation. GSK-3β phosphorylation at Ser-9 appears to be an index of GSK-3β activity (23), and it is therefore important to determine both the protein expression of GSK-3β and pGSK-3β-ser-9, as well as its mRNA in order to obtain a comprehensive evaluation of the role of GSK-3β in the postmortem brain of subjects with BP and SZ. To further clarify the role of the Wnt signaling pathway in general, and GSK-3β and β-catenin in particular, we determined the protein expression of GSK-3β, pGSK-3β-ser-9, and β-catenin (in both cytosol and nuclear fractions) in the dorsolateral prefrontal cortex (DLPFC), cingulate gyrus (CG), and temporal cortex (TEMP) from subjects with BP and SZ and in NC subjects.