This project examines the novel hypothesis that patent foramen ovale (PFO) induces neurovascular sequelae that may lead to vascular contribution to cognitive impairment and dementia (VCID). PFO, a connection between the right and left cardiac atria present in one out of four normal individuals, is known to cause stroke by allowing venous clots to travel directly to the brain. We have found that PFO is not just a door to allow clots through, but that PFO right-to-left shunting can also alter blood biochemistry in ways that are injurious to the brain vasculature, potentially increasing the risk of white-matter disease (WMD) and VCID. We previously focused on the clinical endpoint of recurrent stroke. We now propose to study the new long-term outcome of vascular cognitive impairment, and to conduct novel complementary in vitro and clinical CSF/blood studies to advance the mechanism of shunting-related brain injury as a foundation for future intervention. We are grateful for prior critiques that found the study “innovative,” “extremely productive,” and viewed our translational/clinical approach to probe long term vascular cognitive impairment as a strength. In response, we have extensively revised and strengthened this resubmission with the addition of new aims and preliminary data, revised approach, and provided in-depth analysis and new publications as advised. To investigate whether PFO is indeed a novel risk factor for VCID, building on the foundation of a well characterized and prospectively enrolled cohort complete with biomarkers/omics and cognitive data over more than a decade, we will use a combined clinical translational approach to: 1) understand how our newly- discovered PFO shunting state contributes to VCID over the long term, via subclinical ischemia as noted by WMD and clinical measures of cognitive impairment, as well as recurrent stroke. 2) Investigate mechanisms of novel shunt markers in vitro to better understand their contribution to white-matter dysfunction in the oligovascular niche, where crosstalk between oligodendrocyte precursor cells (OPCs) and brain endothelium is vital to the BBB and the integrity of white matter health, whose disruption contributes to AD/ADRD; 3) Investigate clinical blood and cerebrospinal fluid (CSF) levels and functions of PFO shunt markers with respect to mechanisms of BBB permeability to understand the impact of these markers on cognitive impairment. Ultimately, the project aims to advance the scientific understanding VCID by investigating the mechanisms of PFO physiology, not only for high risk patients excluded from PFO trials, but also as a novel risk factor for long term development of cognitive impairment, to individualize future treatment and preventive strategies.