RO1 (Renewal Application) Abstract Understanding biomarkers of cognitive decline in Lewy body diseases While patients with Lewy body diseases (LBD) share the core feature of deposition of misfolded alpha-synuclein (aSyn) into neuropathological inclusions, they exhibit pronounced heterogeneity in both initial clinical phenomenology, as well as in trajectory of outcomes. The underlying reasons for these differences in phenomenology among LBD patients – patients with Parkinson’s disease (PD), with or without dementia, as well as patients with dementia with Lewy bodies (DLB) – are poorly understood. However, these differences matter greatly for quality of life for patients and their families, as well as costs to the healthcare system, making cognitive decline in the LBD one of the most important Alzheimer’s disease related dementias (ADRD) facing our aging population. This RO1 renewal application seeks to uncover, using biomarker screening approaches, the molecular correlates of cognitive heterogeneity in the LBD. However, we do not plan to stop there. Instead, based on data generated in the first period of funding, we posit that biomarker-derived leads represent an untapped opportunity to identify key players in the pathogenesis of cognitive decline in the LBD. We thus propose to also investigate our more mature leads in model systems, with the goal of developing targets for therapeutic development. We have two specific aims, focused on the underlying premise that host characteristics (reflected by biomarker signatures) and “proteinopathy” characteristics (reflected by differences in aSyn) together determine cognitive trajectory in the LBD. SPECIFIC AIM 1: Investigate the contribution of host characteristics to cognitive heterogeneity in the LBD. We will screen CSF samples from 150 PD patients to identify proteins associating with cognitive outcome, and we will cross-reference “hits” against plasma biomarkers or genomic variants previously found to associate with the same phenotype. We will elucidate the role of our previously-discovered, biomarker-derived leads Apolipoprotein A1 (ApoA1) and Melanoma inhibitory activity protein (MIA) in the pathogenesis of LBD, using induced pluripotent stem-cell derived neurons (iPSC-N) and microglia (iMicroglia), as well as in vivo models. SPECIFIC AIM 2: Investigate the contribution of aSyn species to cognitive heterogeneity in the LBD. We have discovered that plasma species of aSyn recognized by two conformation-selective monoclonal antibodies (termed Strain A and Strain B aSyn) discriminate PD from DLB. We will biochemically and cell biologically characterize these two strains of aSyn using proteinase K digestion, mass-spectrometry-based methods, real- time quaking-induced conversion (RT-QuIC) assays, and iPSC-N seeding models.