A Study of the Mechanism of Motor Neuron Death in Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a fatal adult-onset paralytic disorder for which there is currently no cure. Underlying the disease mechanism of ALS is the spontaneous pathologic degeneration of motor neurons (MNs). Understanding the molecular mechanisms underlying spontaneous and selective MN demise is critical to the development of rational therapeutic strategies. In the current work, utilizing established in vitro models of ALS, I demonstrate that necroptosis, a form of caspase-independent programmed cell death (PCD), drives MN death. Pharmacologic inhibition and/or genetic silencing of receptor interacting protein kinase-1 (RIPK1), receptor interacting protein kinase-3 (RIPK3), and mixed lineage kinase domain-like-protein (MLKL) rescued MN death in vitro. While this core machinery was conserved, the requirement of nuclear factor kappa-B (NF-κB) and Bcl-2-associated X protein (Bax) deviated from known models of necroptosis. This divergence led me to consider that there may be a MN-specific program of necroptosis. Thus, I then used unbiased approaches, by meta-analyzing a gene expression signature captured from MNs undergoing cell death in vitro, to explore MN cell death drivers that may be engaged upstream or downstream to RIPK1/RIPK3/MLKL. I also explored the relevance of necroptosis to MN disease in vivo, in part by deleting RIPK3 from a genetic mouse model of familial ALS. Overall this approach did not rescue motor neuron loss, and there was no improvement in motor function, disease onset, or survival in these animals. I conclude that while necroptosis machinery drives motor neuron death in in vitro models of ALS, more work needs to be done to (1) assess the motor neuron-specific cell death program, and (2) evaluate the relationship, if any, of necroptosis to motor neuron disease in vivo.