Amyotrophic lateral sclerosis (ALS) is an incurable disease marked by a relentless dying-off of motor neurons that causes paralysis then death. Ninety percent of cases occur sporadically and the cause of these cases is not known. Strong evidence indicates that abnormal aggregation of some cell protein or proteins causes sporadic ALS, but these proteins have not been conclusively identified, nor has the cause of the aggregation been determined. The hypothesis behind our proposal is that protein aggregation in ALS is driven by a self-propagating, or "autocatalytic" process of protein aggregation that is analogous to the process of aggregation that causes prion diseases, but involves a protein different from the prion protein. We aim to exploit recently developed methods, known as protein misfolding cyclic amplification (PMCA), that efficiently propagate aggregated prion protein in vitro to identify this putative protein that aggregates to cause ALS. In outline, the procedure we propose is as follows: A small amount ("seed") of homogenized spinal cord tissue from persons who died of sporadic ALS will be mixed with a larger amount of normal spinal cord tissue. Simultaneously, a homogenate of normal cord seeded with normal cord will be prepared. The two homogenate mixes will be subjected to a modified PMCA procedure. Putative autocatalytically aggregating proteins from the ALS spinal cord will cause the same protein to specifically aggregate in the normal cord. We will compare the aggregated proteins in the two samples. Those proteins that aggregate only when "seeded" by ALS cord are likely to be autocatalytic aggregates and potentially the cause of sporadic ALS. PMCA procedures were not designed for the proteome-wide studies we proposed, so we will systematically modify the procedures to minimize non-autocatalytic aggregation while maintaining autocatalytic aggregation, using the prion protein as a surrogate for the putative ALS-causing protein. We will also use PMCA to look for autocatalytic aggregation of TDP-43, a protein recently identified to aggregate in spinal cord neurons of persons with sporadic ALS. These studies have the potential to determine the cause of sporadic ALS, a disease that kills almost 6000 Americans annually, and for which there is no effective treatment. A specific target protein and a pathogenic process for curative therapies may be identified. Further, if successful, the approach can be readily modified to use as an tool for high-throughput screening of potentially therapeutic compounds. [unreadable] [unreadable] [unreadable]