SUMMARY Alzheimer’s disease (AD) afflicts 5.8 million in the USA, but there remains no disease-modifying therapy. Critical for progression from pre-symptomatic to mild cognitive impairment (MCI) to dementia is the loss of synapses in relevant brain regions. We seek to define the molecular basis of this synapse loss. In the previous cycle, we demonstrated an essential role for aberrant metabotropic glutamate receptor 5 (mGluR5) activation in synapse dysfunction, synaptic loss and memory deficits using cellular assays, brain slices and transgenic mice. Our Preliminary studies here demonstrate that synapse loss is mGluR5-dependent in gene knock-in as well as transgenic models, and that it can be tracked by longitudinal PET imaging. We have discovered a sub-nanomolar potent and orally available mGluR5 silent allosteric modulator (SAM, BMS-984923) that blocks this AD process, while preserving physiological Glu signaling. We now aim to address two key issues regarding this clinically targeted AD synapse loss pathway. Single cell transcriptomic profiling will be utilized to understand how synaptic mGluR5 modulation interacts with diverse molecular pathways and glial cells in AD. Single particle cryo-electron microscope studies will be employed to understand the basis for differential mGluR5 regulation by Aßo/PrPC vs Glu, and for SAM action. Hypotheses generated from the transcriptomic and protein studies will be tested in human iPSC-derived neurons and in mouse gene knock-in models. To maximize relevance, signaling will be triggered by Aßo purified from human AD brain.