Selective loss of basal forebrain cholinergic neurons by 192 IgG-saporin is associated with decreased phosphorylation of Ser⁹ glycogen synthase kinase-3β

Abstract

Glycogen synthase kinase-3β (GSK-3β) is a multifunctional enzyme involved in a variety of biological events including development, glucose metabolism and cell death. Its activity is inhibited by phosphorylation of the Ser⁹ residue and up-regulated by Tyr²¹⁶ phosphorylation. Activated GSK-3β increases phosphorylation of tau protein and induces cell death in a variety of cultured neurons, whereas phosphorylation of phosphatidylinositol-3 (PI-3) kinase-dependent protein kinase B (Akt), which inhibits GSK-3β activity, is one of the best characterized cell survival signaling pathways. In the present study, the cholinergic immunotoxin 192 IgG-saporin was used to address the potential role of GSK-3β in the degeneration of basal forebrain cholinergic neurons, which are preferentially vulnerable in Alzheimer's disease (AD) brain. GSK-3β co-localized with a subset of forebrain cholinergic neurons and loss of these neurons was accompanied by a transient decrease in PI-3 kinase, phospho-Ser⁴⁷³Akt and phospho-Ser⁹GSK-3β levels, as well as an increase in phospho-tau levels, in the basal forebrain and hippocampus. Total Akt, GSK-3β, tau and phospho-Tyr²¹⁶GSK-3β levels were not significantly altered in these brain regions in animals treated with 192 IgG-saporin. Systemic administration of the GSK-3β inhibitor LiCl did not significantly affect cholinergic marker or phospho-Ser⁹GSK-3β levels in control rats but did preclude 192-IgG saporin-induced alterations in PI-3 kinase/phospho-Akt, phospho-Ser⁹GSK-3β and phospho-tau levels, and also partly protected cholinergic neurons against the immunotoxin. These results provide the first evidence that increased GSK-3β activity, via decreased Ser⁹ phosphorylation, can mediate, at least in part, 192-IgG saporin-induced in vivo degeneration of forebrain cholinergic neurons by enhancing tau phosphorylation. The partial protection of these neurons following inhibition of GSK-3β kinase activity suggests a possible therapeutic role for GSK-3β inhibitors in attenuating the loss of basal forebrain cholinergic neurons observed in AD.