PROJECT SUMMARY Dementia has significant social and economic implications in terms of direct medical and social care costs. Lewy bodies dementia (LBD) is one of the most common causes of dementia, including Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB). Approximately 30% of Alzheimer' disease (AD) patients also suffer from LBD resulting in a more rapid and severe cognition decline than AD alone. LBD is associated with abnormal deposits of a protein called α-synuclein (α-syn) in the brain. Substantial postmortem studies by Braak et al. show that α-syn pathology spreads in a stereotypical fashion in PDD, and the onset of motor symptoms occurs with loss of dopaminergic neurons in the substantia nigra (SN), and ~80% of patients finally progress to PDD with α-syn pathology in the cortex. In brief, the spread of pathogenic α-syn acts as a major driver of cognitive impairment (CI) in LBD. Recent studies support the notion that pathogenic α-syn may behave in a manner similar to strain-specific prions exhibiting distinct biochemical and pathologic phenotypes. Even recombinant α-syn aggregates (one strain) can convert to another strain after cyclic aggregation, and these two strains exhibited different neurotoxicity and immunoblot patterns after digestion with proteinase K. When injected intracerebrally, MSA (multiple system atrophy) brain homogenates remarkably promote α-syn pathology spreading compared to PD homogenates, suggesting that MSA and PD have different strains of α-syn. Strain- specific difference were observed in the signs of neurological illness, time to disease onset, morphology of cerebral α-syn deposits and the conformation properties of the induced aggregates. Moreover, different strains targeted distinct cellular populations and cell types within the brain, recapitulating the selective targeting observed among α-synucleinopathies. To investigate the role of α-syn strains in PDD progression, we collected cerebrospinal fluid (CSF) samples from the clinically well-characterized patients followed longitudinally. In a double-blinded manner, we amplified α-syn aggregates templated by CSF (containing α-syn seeds) of patients, using a well-established strain amplification technique--PMCA (protein misfolding cyclic amplification). We characterized these α-syn aggregates from patients with well-established neurotoxicity, biochemical, and biophysical assays. However, given that misfolded α-syn aggregates exhibit heterogeneous strain properties and dynamic conversion, particularly in the complicated interplay with environmental, genetic, aging factors, it is necessary to build a fingerprinting method to define these α-syn strains from LBD patients. Fortunately, small molecule microarray (SMM) screening has provided a platform to combine the affinity profile of a diverse panel of tens of thousands of small molecules to certain protein targets. Analysis of this “big data” by comparing and contracting these affinity probes...