PROJECT SUMMARY Aging is the greatest risk factor to α-synucleinopathy, a group of neurodegenerative diseases with severe cognitive impairmentand progressive motor dysfunction and dementia, such as Parkinson's disease (PD), dementia with Lewy bodies (DLB) and Parkinson's disease with dementia (PDD) and half of Alzheimer's disease patients (AD). Dementia is a common symptom in α-synucleinopathies: DLB is the 2nd most common dementia after Alzheimer's disease (AD) accounting for 30% of dementia cases; Around 30% of AD cases also suffer from α-synucleinopathy resulting in a more rapid and severe cognition decline than AD alone. PD is the 2nd most common neurodegenerative disease, and greater than 50% of PD cases develop PDD. In addition to cognitive and memory dysfunctions, patients with dementia also suffer from anxiety, depression and mood swings. Although α-syn pathology is highly associated with dementia, the underlying aging-related mechanism driving the pathogenesis and contributing to their progression is not known and there is no available disease modifying therapy yet. Based on substantial postmortem analysis, Braak et al. demonstrated that α-syn pathology spreads in a stereotyped fashion from the vagus to the brain, which may initiate in the gastrointestinal tract. Particularly, nearly all the DLB and PDD cases present with α-syn pathology in the gut. Both clinical and experimental observations support that pathogenic α-syn spreading is a master trigger that drives α-synucleinopathy. In our gut-brain α-synucleinopathy (GBAS) mouse model, gut-injection of pathogenic α-synuclein (α-syn) can recapitulate α-syn pathology gut-brain spreading and cognitive impairment. In our preliminary studies, heterochronic blood exchange (HBE) from young mice inhibited pathogenic α-syn transmission and neuroinflammation in aged mice, suggesting an HBE-transferred phenotype that may effectively inhibit α-synucleinopathy. We identified lymphocyte-activation gene 3 (LAG3)1, a major receptor of pathologic α-syn transmission. To identify the mechanism underlying rejuvenation and accelerated aging event, we further identified two novel LAG3-related and aging-regulating proteins that can mediate pathogenic α-syn transmission. Our studies support the feasibility to modulate plasma levels of FGL1 and sLAG3 in aged mice by the HBE approach. To determine the underlying mechanism if FGL1 and sLAG3 in the plasma are molecular mediators essential for the inhibitory effects of HBE on α-synucleinopathy an d related cognitive impairment, we have established a rigorous and robust experimental system combining the HBE approach, the GBAS model, genetically engineered mice without these factors, and recombinant FGL1 and sLAG3 proteins, for comprehensive gain- and loss-of-function analysis. Our Central Hypothesis is to identify the underlying mechanism that HBE inhibits α-synucleinopathy and related cognitive impairment through FGL1 and sLAG3. FGL1 functions as a rejuvenation fact...