Defining the overlap of molecular mechanisms of degeneration in genetic ALS

Amyotrophic Lateral Sclerosis (ALS) is a progressive and fatal neurodegenerative disease that is characterized by the selective death of upper and lower motor neurons (MNs). Understanding the mechanisms that cause ALS and the discovery of treatments have been hampered by the inaccessibility of human motor neurons. While the majority of ALS cases are sporadic, mutations in a number of genes that code for proteins with diverse functions account for ~15% of all ALS. We propose to use induced pluripotent stem cells (iPSCs) and reprogramming technologies to address two important outstanding questions in the field: Are there common molecular pathways that unify genetically distinct familial ALS cases? And do disturbances in electrical excitability patterns of motor neurons contribute to ALS pathophysiology? Defining the overlap (or lack thereof) in the molecular mechanisms that lead to the degeneration of human MNs in different genetic ALS cases will address whether the disease represents a syndrome or a single nosological entity. I have previously shown that MNs derived from iPSCs carrying an SOD1 A4V mutation exhibit elevated levels of apoptosis, reduced dendritic densities and soma sizes, which are associated with an irregular transcriptome, increased levels of ER stress, mitochondrial abnormalities and alterations in electrophysiological excitability. Importantly, fixing the SOD1 mutation using genetic engineering reversed these phenotypes. I propose to employ this strategy to determine whether similar phenotypes are induced in MNs harboring disease-causing mutations in the majority of familial ALS genes including C9orf72, TDP43, FUS, UBQLN2 and CHCHD10. This approach will stratify ALS patients based on the molecular pathways that are altered in their MNs. I have previously shown that mutant SOD1 MNs generated from ALS patients exhibit electrical excitability alterations that are directly dependent on the disease-causing mutations. Using patch clamp I showed that the defective firing patterns are likely due to reduced K+ currents. When mutant MNs are treated with a K+ opener, normal excitability patterns are restored and survival of MNs is extended. I further demonstrated that these alterations are connected to an inherent elevated level of ER stress that human MNs exhibit relative to other neural subtypes. I hypothesize that this relationship may contribute to the selective vulnerability of MNs in ALS and that it is central to ALS pathophysiological mechanisms. To further define the connection between these pathways and how they interact with ALS-causing genetic mutations I propose to manipulate electric activity via optogenetic stimulation and characterize the cellular response of MNs at the functional and transcriptional levels in ALS and isogenic healthy controls.