Frontotemporal degeneration (FTD) is an understudied clinical neurodegenerative condition that is the most common dementia after Alzheimer disease (AD) in people younger than 65. The most common pathology associated with FTD is frontotemporal lobar degeneration due to transactive DNA/RNA binding protein of ~43 kD (TDP-43 (FTLD-TDP), and this is also the underlying pathology in the vast majority of patients who have co-occurring amyotrophic lateral sclerosis (FTD-ALS) spectrum disorders as well as a critical force in age- related disorders such as limbic-predominant age-related TDP-43 encephalopathy (LATE). Since discovering a role for TDP-43 pathology in human disease, important progress has been made in experimental cellular and animal models of disease. However, the human brain has many unique properties associated with distinctly human clinical disorders that are not easily replicated in these experimental models. Major gaps in knowledge thus constrain the development of disease-modifying treatment trials. Among these is our limited knowledge of the pathophysiologic consequences of the accumulation and progression of abnormal TDP-43 at a molecular level. At a microscopic level, a major limitation is that most patients with accumulating TDP-43 have sporadic disease that can be identified reliably only at autopsy, although ~20% of cases have familial FTLD (fFTLD) with known pathology during life. We are limited at translating this knowledge to a macroscale level where FTLD- TDP pathology is manifested in humans with heterogeneous clinical features as diverse as emotional dysregulation and impaired language both with and without a motor disorder. Moreover, there is limited knowledge of the factors contributing to the highly varying rates of disease progression. In five novel, independent but synergistic Projects and five Cores that support each of the Projects, this unique, multidisciplinary, Program Project Grant (PPG) adopts the innovative perspective of investigating the TDP-43-associated breakdown of neural networks at molecular, microscopic and macroscale levels in humans. We hypothesize that our novel, well-integrated, network perspective will fill major gaps in knowledge by elucidating mechanistic insights into the pathophysiology of abnormal TDP-43 and the associated pattern of disease progression, and offer a fresh perspective on the identification of accumulating TDP-43 pathology during life and its longitudinal course. This proposal is consistent with the highest priorities for FTD at the 2019 Alzheimer’s Disease and Related Disorders (ADRD) summit. By focusing on disruption of neural networks at molecular, microscopic and macroscale levels of brain functioning, our multidisciplinary network approach will elucidate the pathophysiology and spread of abnormal TDP-43 in humans, and examine the consequences of TDP-43 pathology for clinical disease during life in dementia and aging using fresh approaches to improve our mechanistic understan...