Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of large motor neurons in the brain and spinal cord, resulting in progressive wasting and paralysis of voluntary muscles, respiratory failure and ultimately death. Familial ALS (FALS) accounts for around 10% of all ALS cases, with the majority (90%) of the disease presenting as sporadic ALS (SALS). Discovery of genetic etiologies of ALS has been the driving force in ALS research for the last 20 years. These discoveries have provided fresh insight into pathogenesis and animal models. However, in spite of this effort, global treatments have not been possible for ALS because a common molecular mechanism downstream from the distinct etiologies has not become apparent. It is clear that discovery of additional etiologies and common downstream mechanisms will greatly advance the goals for a molecular understanding of ALS and in finding appropriate treatments. In this application we propose an advance in direction of those goals. We have discovered mutations in UBQLN2 as cause of X-linked dominant ALS and ALS/dementia of the FTD type. The pathology is novel, distinct and unique with ubiquilin2 positive inclusions in the spinal cord and cortex in ALS patients and additional inclusions in the hippocampus of patients with ALS/FTD. To our surprise we found similar ubiquilin2 positive inclusions in all cases of SALS and ALS/FTD and FALS cases studied thus far. None of those cases had mutations in UBQLN2, arguing for a posttranslational role for ubiquilin2 in ALS as a whole. Our initial in vitro and in vivo data suggest a dysfunction of the ubiquilin proteasome syndrome (UPS) and autophagy in the presence of mutant ubiquilin2. In addition, cognitive and behavioral deficits and decreased long term potentiation in far field recordings were observed in transgenic mice expressing a pathogenic UBQLN2 mutation (P497H). In this project, we propose four specific aims to understand the disease causing mechanism(s) of mutant ubiquilin2. We will establish pathological, cognitive and other behavioral phenotypes and their electrophysiological correlates in our current and to be developed mouse models of mutant ubiquiln2. We will also use our already established ubqln2 knockout mouse model and develop ubqln1 knockout and double knockouts of ubqln1/ubqln2 to test if mutant ubiquilin2 defects are manifestation of a loss of function of ubiquilin2 in relationto the UPS and autophagy systems or whether mutant ubiquilin2 exhibits a novel toxic property. Successful completion of this project will not only provide insight into understanding the pathogenic mechanism of X-linked ALS, but also rapidly provide the wider research community with useful reagents for future studies and for screening and testing potential therapies. The outcome of this project will also have important implications in the understanding of the pathogenesis and treatment of other neurodegenerative diseases.