The long-term goal of our lab is to understand cholinergic muscarinic receptor-mediated signal transduction as it relates to the etiology of Alzheimer's disease (AD). This knowledge will aid in the development of pharmacotherapies for the treatment of this devastating disease. Given the well documented cholinergic deficits observed in AD, in addition to the implicated roles of cholinergic neurotransmission in the neural processes of learning and memory, pharmacological treatments for AD have primarily focused on cholinomimetic compounds which either directly stimulate cholinergic receptors (e.g. bethanechol) or enhance the synaptic availability of acetylcholine (e.g. tacrine, arecept). However, an underlying assumption of this therapeutic strategy is that post-synaptic cholinergic signal transduction mechanisms remain intact in AD. Several independent groups, including our lab, have demonstrated a decrement of M1 cholinergic muscarinic receptor (M1AchR)-G protein coupling in post-mortem AD brains using a variety of experimental endpoints. The attenuation of signal transduction at the M1AchR in AD may potentially contribute to the relative lack of success of cholinomimetic-based pharmacotherapy, and suggest alternative targets for the development of novel medicinal intervention. Muscarinic receptors are functionally regulated by phosphorylation, resulting in an attenuation of receptor-mediated signal transduction. Biochemical studies of AD brain tissue indicate a disease-related increase in the phosphorylation state of many proteins, including the microtubule-associated protein tau. The phosphorylation state of proteins is determined by the balance of kinase and phosphatase activities. We have demonstrated a significant reduction in calcineurin activity in AD. Calcineurin (protein phosphatase 2B) regulates the phosphorylation state and function of both cytoskeletal proteins and neurotransmitter receptors. Direct phosphorylation of brain muscarinic receptors impairs receptor-G protein interactions in vitro. In preliminary studies from our lab, the uncoupling of the M1AchR from its G protein was reversed by in vitro activation of calcineurin. Therefore, we hypothesize that the impairment of M1AchR-G protein coupling in AD is due to an increase in the phosphorylation state of the M1AchR itself, which may be secondary to a reduction in the activity of cholinergic receptor phosphatases particularly calcineurin.
| Cook, Casey N; Hejna, Matthew J; Magnuson, Debra J et al. (2005) Expression of calcipressin1, an inhibitor of the phosphatase calcineurin, is altered with aging and Alzheimer's disease. J Alzheimers Dis 8:63-73 |
| Muma, Nancy A; Mariyappa, Ramesh; Williams, Kyle et al. (2003) Differences in regional and subcellular localization of G(q/11) and RGS4 protein levels in Alzheimer's disease: correlation with muscarinic M1 receptor binding parameters. Synapse 47:58-65 |
| Lian, Q; Ladner, C J; Magnuson, D et al. (2001) Selective changes of calcineurin (protein phosphatase 2B) activity in Alzheimer's disease cerebral cortex. Exp Neurol 167:158-65 |
