Role of P62 in Protein Aggregation and Neurodegeneration in ALS

Protein aggregates containing mutant copper-zinc superoxide dismutase (SOD1) are a hallmark of familial amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease), an age-related neurodegenerative disease. Conformational differences between wild-type (WT) and mutant SOD1 proteins have been demonstrated by many in vitro studies using a variety of biophysical techniques, but in vivo details of the aggregation process remain to be defined. It is also unclear whether protein aggregates are toxic or beneficial. P62/Sequestome 1 (referred as p62 in this proposal) is a multifunctional protein involved in both of the two major protein degradation mechanisms, ubiquitin-proteasome system (UPS) and autophagy-lysosome pathway. We recently reported that the endogenous mouse p62 levels significantly increase:d prior to the disease onset in G93A SOD1 transgenic mice. In addition, p62 was co-localized with mutant SOD1 and ubiquitin in the protein aggregates in spinal motor neurons in G93A SOD1 transgenic mice. We have established collaboration with Dr. Marie Wooten at Auburn University who is the leading scientist studying p62 in signaling cascades and who will provide the p62 knockout mice for the study. The long term goal of this collaborative project is to understand the role of p62 in protein aggregation caused by ALS-Iinked SOD1 mutants and motor neuron degeneration in ALS. The innovative aspect of this project is that the role of p62 in the ALS etiology is largely undefined. The central hypothesis to be tested in the project is that p62 can recognize misfolded mutant SOD1 and that p62 can ameliorate the mutant SOD1 induced toxicity by shuttling such misfolded proteins to the ubiquitin-proteasome system (UPS) and/or autophagy. Our preliminary studies demonstrate that p62 specifically recognizes mutant SOD1 and the ubiquitin-association I[UBA) domain of p62 is not essential to such interaction. Our results also show that p62 enhances the formation of aggresome-like inclusion of mutant SOD1 but had little effect on WT SOD1. Recent results from other groups showed that p62 can directly bind to autophagy effector protein Atg8/LC3 and that the formation of autophagy was increased in the G93A SOD1 transgenic mice. These data strongly support the central hypothesis. Three specific aims are designed to determine the mechanisms by which p62 recognizes mutant SOD1, the downstream pathways activated by such interaction, and the functional role of p62 in neuronal degeneration. Aim 1 is to map the domains of p62 essential for recognizing and interacting with mutant SOD1. Aim 2 is to determine whether and how p62 mediates the autophagy activation induced by mutant SOD1. Aim 3 is to study how p62 influences protein aggregation and ALS disease progression in vivo using p62 KO mice. The results from this study will provide invaluable insights into the role of p62 in protein aggregation and neurodegeneration in ALS, which will result in better understanding of ALS etiology and potential discovery of new therapeutic taq~et for ALS treatment.