Project Summary/Abstract Alzheimer's Disease and Related Dementias (AD/ADRD) are the most common forms of dementia. No treatments exist to stop or prevent them. AD/ADRD are caused in part by the aggregation of one, or more often a combination of, specific proteins. TDP-43 is one of a handful of such proteins. It characteristically transitions from a dynamic liquid phase of assembly to a rigid and pathogenic phase in a large fraction of dementia cases. This transition occurs against a backdrop of progressive changes in the interactions between different cell types in the brain. An interdependence of these molecular and cellular changes likely determines the clinical courses of AD/ADRD, but to understand that interdependence, we need new tools with which to compare protein phase transitions and their corresponding phenotype effects in the context of interacting brain cell types. We propose here to develop such a tool by modifying a cell-based biophysical method, Distributed Amphifluoric FRET (DAmFRET) for use in human induced pluripotent stem cell (hiPSC)-derived brain tissue models (Aim 1). We will simultaneously use DAmFRET to develop TDP-43 as a reporter of cell type-specific differences in the homeostasis of protein phase transitions (Aim 2). By completing these aims, we will have created and validated a uniquely powerful tool for better understanding the complex molecular and cellular causes of AD/ADRD.