A growing body of evidence suggests that the degenerative process of Alzheimer's disease begins as much as 20-30 years before clinical symptoms appear. The earliest and most consistent neurochemical change is loss of cortical cholinergic markers. In addition, the M1 muscarinic receptor is uncoupled from its G-protein, and other components of the signal transduction system become dysfunctional. Defects in signal transduction could limit the effectiveness of cholinergic replacement therapy. Furthermore, recent work suggests that impaired signal transduction may lead to increases in beta-amyloid, a potentially neurotoxic peptide. The hypothesis of this proposal is that degeneration of cholinergic neurons early in Alzheimer's disease leads to uncoupling of M1 muscarinic receptors and other defects in signal transduction, with subsequent increases in production of beta-amyloid. We further propose that this process might be prevented by early intervention with cholinergic replacement therapy.
Specific Aim 1 : Are alterations in M1 receptor-mediated signal transduction observed in pre-clinical stages of Alzheimer's disease, and how do these correlate with cholinergic indiced, senile plaques and levels of beta-amyloid? These experiments will be performed in samples of human brain from non- demented elderly individuals with senile plaques. Age-matched plaque-free patients will be used as controls. Many if not all of the patients with plaques are likely to be in the pre-clinical stages of Alzheimer's disease. Loss of cholinergic input will be assessed histochemically and with enzyme assays. Levels of beta-amyloid will be measured with an immunoassay. Defects in signal transduction will be assessed by examining the ability of each component in the system to stimulate hydrolysis of phosphatidylinositol. Direct coupling of the M1 muscarinic receptor to G-proteins will be estimated by measuring the shift from high to low affinity states of the receptor when muscarinic antagonist binding is displaced by agonists.
Specific Aim 2 : Can defects in M1 receptor-mediated signal transduction and beta-amyloid increases be prevented by cholinergic replacement therapy? Cholinergic inputs to the cerebral cortex and hippocampus will be lesioned in rats with the selective immunotoxin 192 IgG saporin, which has been shown to uncouple the M1 receptor. The selective M1 muscarinic agonist xanomeline will be administered to lesioned rats to determine whether pharmacologic therapy can prevent lesion-induced alterations in M1 receptor/G-protein coupling and signal transduction. If so, this suggests that uncoupling of the M1 receptor in Alzheimer brains can be reversed or prevented by cholinergic replacement therapy, and may prevent the development of uncoupling and defects in signal transduction in Alzheimer's disease.