Evolution of Pathologic RNA-Protein Aggregates in Motor Neuron Disease Amyotrophic Lateral Sclerosis (ALS) is a progressive, idiopathic neurodegenerative disease of the human neuromuscular system. A prominent, yet enigmatic feature in over 90% of ALS patients is the accumulation of large, neuronal RNA and protein aggregates. The role of these aggregates in disease pathology is unknown; however, these aggregates share many molecular similarities to RNA and protein aggregates, called stress granules, which form naturally in all cells. Stress granules are highly conserved, transient cytoplasmic aggregates of RNA and protein that assemble under cellular stress, and disassemble when stress is removed. Stress granules are known to be important for cellular survival during stress by regulating stress-responsive signaling and apoptosis. Strikingly, many ALS mutations are in proteins that regulate the assembly or clearance of stress granules. A key barrier in understanding the role of stress granules during a normal stress response and their relation to neurodegenerative disease has been a lack of methods to systematically characterize stress granule RNA and protein composition. We have overcome this limitation by developing a broadly applicable, new purification technique that allows for the characterization of stress granule RNA and protein in both normal and pathological states. In preliminary work, we have shown that stress granules are normally enriched in proteins that regulate stress-responsive gene expression. The objective of this proposal is to provide the first comprehensive functional analysis of stress granule RNA-protein composition in both normal (Aim 1) and cells expressing ALS pathogenic mutations (Aim 2). We hypothesize that persistent stress granules abnormally sequester RNA processing factors and select RNAs resulting in altered post-transcriptional control of subsets of mRNAs. Successful completion will likely provide fundamental knowledge into normal RNA biology and how this may become dysregulated in neurodegenerative diseases such as ALS. It will determine the biogenic pathway for pathological RNA-protein aggregate formation when driven by mutations that affect neurodegenerative disease and alter stress granule dynamics. Finally, the changes in stress granule composition and pathological formation will be related to effects on mRNA levels and protein expression, thereby defining how normal and aberrant stress granules affect gene expression. Taken together, these experiments will provide a new paradigm for understanding the normal function of stress granules and how perturbation of stress granule metabolism can alter gene expression and lead to ALS. Presently, no therapeutics are available that either cure or prolong ALS disease progression more than a few months. As protein and RNA aggregates are a pathological hallmark of ALS, understanding the basic biology of these aggregates is essential towards future therapeutic design.