PROJECT SUMMARY ALS, also known as Lou Gehrig’s disease, is an incurable neurodegenerative disease caused by the loss of motor neurons leading to paralysis and eventually death. To understand the disease mechanism and develop therapeutics, mammalian models that phenocopy human disease are indispensable. For more than two decades, transgenic animals expressing mutant SOD1 gene were the only model available that faithfully represented the human disease and played an essential role in advancing our understandings of ALS and enabling therapeutic development. However, due to the lack of animal models from other ALS-causal genes, it has been difficult to verify and generalize the mechanistic and therapeutic findings from the mutant SOD1 models. Consequently, we do not know whether the mechanistic findings from the SOD1 models are common to different mutant genes or it is specific for SOD1 mutations alone. This has hampered our understanding of ALS and therapeutic development. Thus, construction of additional mammalian models that mimic ALS disease process in human is crucial in our fight against ALS. However, efforts in developing new mammalian models with progressive ALS phenotypes leading to motor neuron loss, clinical paralysis and death has proven difficult. To solve this problem, we have constructed a transgenic mouse model by expressing mutant profilin1 gene (PFN1C71G), a recently discovered ALS gene. We show that expression of the mutant, but not the wild type (PFN1WT) gene, caused a late onset motor dysfunction phenotype that subsequently progressed to paralysis and death. Furthermore, the mutant mice developed a relentless progression of motor neuron degeneration and lose a majority of their motor neurons at the end stage. These results demonstrate that mutant PFN1 causes ALS by a gain of toxicity and establish a progressive ALS disease model that closely phenocopy the human disease. These mice provide a new in vivo system for study of disease mechanisms and therapeutics for ALS. This proposal take advantage of this new model and seeks mechanistic insights on motor neuron degeneration. Aim 1 will differentiate between two mechanisms of mutant PFN1 toxicity, a gain of novel toxic property by the mutant protein vs. a dominant-negative inhibition of the normal PFN1 allele. Aim 2 will explore the underlying cellular determinants for the disease onset and the rate of disease progression. Aim 3 will investigate the role of ER damage in mutant PFN1-induced motor neuron degeneration. We will compare the findings from these aims with the findings from mutant SOD1 models to determine common or gene-specific mechanisms. By these understanding, better therapeutic approaches may be achieved.