GRK5 dysfunction accelerates tau hyperphosphorylation in APP (swe) mice through impaired cholinergic activity

Recent studies have suggested that G-protein-coupled receptor kinase 5 (GRK5) deficiency plays a significant role in the pathogenesis of early Alzheimer's disease. Mild soluble beta-amyloid accumulation can result in reduced membrane (functional) and elevated cytosolic levels of GRK5. Dysfunction of GRK5 impairs the desensitization of presynaptic muscarinic 2 (M2) autoreceptors, which results in presynaptic M2 hyperactivity and inhibits acetylcholine (ACh) release. GRK dysfunction also promotes a deleterious cycle that further increases beta-amyloid accumulation and exaggerates tau hyperphosphorylation in the hippocampus. However, the pathogenic effect of GRK5 dysfunction through targeting tau hyperphosphorylation remains unclear. Here we examined not only the reduced membrane (functional) and elevated cytosolic levels of GRK5 but also the increased levels of hyperphosphorylated tau in the hippocampi of aged APP(swe) mice (11 months of age). Moreover, western blotting analyses revealed the changes in the location of activity of both protein kinase C (PKC) and glycogen synthase kinase3 beta (GSK3 beta) in the hippocampus of aged APP(swe) mice in which GRK5 translocation occurred. Moreover, treatment with methoctramine, a selective M2 antagonist, partially corrected the difference between wild-type control mice and GRK5-dysfunctional APP (swe) mice in hippocampal ACh release, PKC and GSK3 beta activities, as well as tau hyperphosphorylation. In contrast, the GSK3 beta inhibitor lithium chloride significantly reduced tau hyperphosphorylation in GRK5-defective APP (swe) mice, but failed to enhance PKC activity and ACh release in the hippocampi of GRK5-defective APP (swe) mice. Taken together, these findings indicate that GRK5 dysfunction accelerated tau hyperphosphorylation in APP(swe) mice by activating GSK3 beta through impaired cholinergic activity.