Frontotemporal dementia (FTD) is the leading dementia before the age of 65 and the second most common form of dementia after Alzheimer’s disease (AD). There is no cure. FTD is caused by focal but progressive atrophy of frontal and/or anterior temporal cortices that leads to changes in personality, apathy, loss of empathy, disinhibition, and language disability at early-mid stages, and general memory and cognitive deteriorations requiring a full-time care at later stages. Thus, synaptic and circuit dysfunctions underlying behavior impairments may precede massive neuronal cell loss and disability. However, molecular mechanisms underlying disease initiation and early symptoms as well as disease progression are not well understood. FTD is linked clinically, pathologically, genetically, and mechanistically to amyotrophic lateral sclerosis (ALS). Up to 50% of FTD are familial and associated with mutations of at least 15 genes of diverse functions. Remarkably, at least 10 of these genes are involved in autophagy, which has emerged as a central mechanism in FTD/ALS. However, it remains enigmatic how autophagy is dysregulated in FTD/ALS and how exactly autophagy dysfunctions cause the diseases, presenting a major hurdle and knowledge gap in development of autophagy-based therapeutic strategies. Recently, a gain of function mutation in the CYLD gene is identified in FTD/ALS patients, placing CYLD as the newest member of the FTD/ALS-causing gene family. CYLD encodes a Lys63-specific deubiquiting enzyme and interacts with several FTD gene products, including p62/SQSTM1, Optineurin, and TBK1, suggesting a potential role for CYLD in autophagy related to FTD. CYLD is best known as a tumor suppressor linked to familial cylindromatosis and immune signaling, but its roles in neurons and synapses are largely unknown. Our published and unpublished studies indicate that CYLD is an abundant Lys63-specific synaptic deubiquitinase that has a major role in synapse maintenance, function, and plasticity through regulation of neuronal autophagy. The goals of this R01 application are to define the molecular details and functional consequences of CYLD-dependent autophagy (Aim 1), to generate an inducible transgenic mouse model and delineate the role of FTD-causing mutation CYLDM719V in FTD pathogenesis (Aim 2), and to validate the pathogenic role of CYLDM719V in FTD and explore therapeutic strategies in human induced pluripotent stem cell (iPSC)-derived cortical neurons (Aim 3). Our study represents the first attempt to investigate the role of a new disease gene in FTD pathogenesis. Our proposed studies are fundamentally important and highly significant because they have the potential to uncover novel pathogenic mechanisms and treatment strategies for FTD and related neurodegenerative diseases.