Project Summary Amyotrophic lateral sclerosis (ALS) is marked by progressive degeneration of motor neurons resulting in near total paralysis. While in familial forms of ALS all cells in the body carry associated mutations, a subset of cranial motor neurons (CMN) resist degeneration, allowing patients to retain eye movements until late stages of the disease. Therefore, spinal motor neurons (SMN), which control most voluntary movements, degenerate at earlier stages of the disease than CMNs. The nature of the intrinsic resistance of this subpopulation of motor neurons to degeneration is not understood and represents an under-exploited area of ALS research that will lead to a better understanding of the disease mechanism and the development of new therapeutic strategies. This project aims to discover how CMNs resist ALS and devise strategies to transfer ALS resistance to SMNs. While it is known that a subpopulation of CMNs in humans resist degeneration in ALS, direct comparison of sensitive and resistant cell types is hindered by challenges in extracting and culturing sufficient numbers of CMNs from tissue. We overcome this hurdle by employing a stem cell-based platform wherein we can express ALS-relevant mutations in populations of CMNs and SMNs obtained by differentiating embryonic stem cells. By taking advantage of our ability to both generate SMNs and CMNs from mouse embryonic stem cells at high efficiency, we model ALS in vitro. Aim 1 tests the hypothesis that SMNs and CMNs utilize different degradation machineries to degrade ALS-causing misfolded SOD1. We will compare between induced iCMNs and iSMNs the aggregation of overexpressed SOD1 mutants and assess the roles of the ubiquitin/proteasome system and autophagy. Aim 2 tests the hypothesis that CMNs and SMNs are differentially sensitive to generalized proteostatic stress and that the same mechanisms which control hSOD1 aggregation control how CMNs and SMNs respond to proteostatic stress. Thus, we will assess survival differences between iCMNs and iSMNs to proteostatic stress and assess the roles of the ubiquitin/proteasome system and autophagy. Aim 3 will investigate if Phox2a expression (the master regulator of CMN fate) will be sufficient to protect SMNs from ALS stress. Thus, we will induce expression of Phox2a in differentiated iSMNs and assess if Phox2a can increase iSMN resistance to hSOD1 aggregation and proteostasis stress. Furthermore, we will derive predictive regulatory networks to identify additional candidate transcription factors that might confer iCMN resistance to ALS stress. We will then experimentally test the ability of these transcription factors to rescue iSMN sensitivity to ALS inducing mutations. This research program seeks to rationally design ALS therapies based on the intrinsic resistance of particular motor neuron populations. Since neuronal-specific sensitivity is not unique to ALS, we believe it will also set a paradigm for other neurodegenerative diseases.