α-Synuclein (αS) is an abundant brain protein that regulates neurotransmission. αS is also implicated in Parkin- son’s disease (PD), PD with dementia (PDD), dementia with Lewy bodies (DLB), and certain forms of Alzheimer’s disease (AD). PDD and DLB are regarded as Lewy Body (LB) Dementia (LBD). Remarkably, about 90% of αS in the LB and Lewy neurites is in its serine 129 phosphorylated form (pS129). Therefore, pS129 is widely used as a surrogate marker for pathology. However, we recently demonstrated that physiological pS129 is triggered by neuronal activity and positively regulates synaptic transmission. These new results raise the critical question of whether normal and abnormal pS129 can be distinguished. Distinguishing normal and abnormal pS129 will help us better understand the processes leading to the pathology and refine pS129 as a marker for PD and LBD. Eventually, these will contribute to developing meaningful therapeutic interventions. A lack of disease-modifying treatments for PD and LBD is partly due to a lack of insight into how native αS dynamics become aberrant and its consequences. The sequence of molecular events that turns the states of normal αS into pathological forms over time is a complex topic, but environmental factors, excess αS, missense mutations in αS, and other genetic risk factors influence the conversion of “good” αS into “bad”. Our long-term goal is to understand the normal αS biology and test how their functions are compromised under pathological states by exploiting factors that accel- erate αS dyshomeostasis. We then seek to convert “bad αS” back to “good αS”. Our objectives are to (i) compare the biochemical properties of physiological and pathological pS129 and (ii) describe whether or to what extent the normal synaptic transmission mediated by pS129 is compromised in PD and LBD models. The central hy- pothesis flows from our recent data on pS129 being part of normal αS homeostasis: we hypothesize that dynamic activity-dependent pS129 reversibility is impaired when normal αS homeostasis is pathologically perturbed. The rationale for this project is that understanding normal and abnormal pS129 in neurons is likely to help us quantify signatures of αS pathology. To achieve the overall objectives, two specific aims will be pursued: 1. Determine whether or to what extent dynamic activity-dependent pS129 is impaired in familial and sporadic models of PD and LBD, and 2. Determine whether normal functions of pS129 are affected in αS-based PD and LBD models. In many ways, the concept of physiological pS129 is new, particularly the dynamic reversibility of pS129. In that regard, the research proposed in this application is innovative because it uses the concept of dynamic reversi- bility of pS129 to distinguish “good” and “bad” pS129. Our contributions are expected to be significant because a) our work will elucidate the differences in the biological properties of normal and abnormal pS129 that occur in physiological an...