PROJECT SUMMARY The goal of this proposal is to understand the mechanisms by which alterations in central and peripheral neuroimmune signaling crosstalk contribute to neurodegeneration in frontotemporal dementia with progranulin mutations (FTD-GRN), an Alzheimer’s Disease (AD)-related dementia (ADRD), and the most common dementia in people younger than 60. GRN mutations are among the leading causes of dominantly inherited FTD. These GRN mutations typically cause PGRN haploinsufficiency, with plasma progranulin levels reduced by >50% relative to controls. PGRN has been implicated in a variety of functions, including inflammation and lysosomal homeostasis. It is unclear whether neurodegenerative processes are driven solely by dysfunctional neurons and brain-resident microglia or whether hyperactivation of the peripheral immune system contributes to neuroinflammatory and neurodegenerative processes. As blood-brain barrier leakage has been reported in patients with AD and FTD, it is possible that infiltrating monocytes play a key role in synaptic pruning and neurodegeneration in FTD that has so far been over-looked. Thus, our studies will focus on investigating the extent of dysfunctional crosstalk between peripheral immune cells and microglia and its effects on neuronal integrity and function in various mouse and human iPSC-derived models of FTD-GRN. In Aim 1, we propose to use human models of FTD based on iPSC-derived neurons (i-N), microglia (i-MG) and monocytes/macrophages (i-MO/i-MAC) in 2D or 3D culture, together with an innovative longitudinal imaging technology, robotic microscopy (RM), to investigate mechanisms of human central and peripheral myeloid cell dysfunction and its associated contribution to neurodegeneration in the context of FTD. In addition, we will test several drugs currently in clinical development for other medical conditions that increase PGRN expression and reverse neurodegeneration of mouse FTD-GRN neurons for their ability to rescue the disease phenotype of human i- MG, i-MO/i-MAC, and i-N cultures, which will support their potential development as treatments for FTD patients. In Aim 2, we will conduct complementary in vivo studies in mouse models of FTD-GRN with the GrnR493X/+ mutation and PGRN haploinsufficiency in which we previously reported neurodegeneration and lysosomal dysfunction in vitro. We will test the hypothesis that infiltration of peripheral immune cells with lysosomal dysfunction contributes to neuroinflammation and neurodegeneration in this animal model of FTD-GRN. In Aim 3, we will define and validate a robust, conserved, and functionally defined disease-associated signature of FTD- associated microglia/monocytes for systems-level comparative analysis, bioinformatic driver analysis, and future chemical genomics screens.