The broad, long-term objective of this grant application is to understand the mechanisms involved in neuroprotective action of sigma 1 receptor (S1R) agonists Alzheimer’s disease (AD). Recent clinical and preclinical data suggest that activation of S1R may promote neuronal resilience and result in synaptoprotective and neuroprotective effects within an AD context. Mutations in S1R are linked with degeneration of motor neurons and polymorphisms in SIGMA1R gene have been linked with the risk of developing late onset AD. These links have prompted on-going and planned phase 2 and phase 3 clinical trials of S1R agonists in AD patients. Despite its importance for neuronal physiology and as a drug target, the biological function of S1R is poorly understood and mechanism of action of S1R agonists in preclinical and clinical studies is not well established. The main aim of this grant proposal is to evaluate the hypothesis that synaptoprotective and neuroprotective effects of S1R agonists in AD are mediated in part by their remodeling of mitochondria-associated membranes (MAMs) and other endoplasmic reticulum (ER) cholesterol-rich microdomains. This hypothesis is based on our recent discovery of biological importance of S1R association with cholesterol and its role in forming MAMs and ER microdomains. To test this hypothesis, specifically we will (1) investigate an importance of cholesterol-binding site in S1R sequence for in vivo synaptic rescue in AD mice; (2) analyze effects of S1R activation on changes of lipid composition of MAMs in AD; (3) determine a role of enhanced ApoE release from astrocytes for neuroprotective effects of S1R agonists; (4) establish an importance of neuronal calcium signaling changes resulting from S1R activation. Results obtained in the proposed studies will provide essential information regarding mechanism of action of S1R agonists in models of AD, facilitate interpretation of on-going AD clinical trials of S1R agonists, and potentially lead to development of the next generation of S1R-targeting therapeutic agents for AD treatment.