PROJECT SUMMARY This proposal will address the mechanisms underlying neuromuscular degeneration in Charcot-Marie-Tooth disease (CMT). CMT is a genetically and phenotypically heterogeneous neuromuscular disorder with causative mutations found in over 100 genes. While considered a rare disease, CMT is the most common inherited disorder of the peripheral nervous system, affecting ~1 in 3,500 people worldwide. Dominant mutations in 6 different tRNA synthetases (aaRSs) cause forms of CMT (aaRS-CMT), making them the largest family of CMT-associated genes. Each of these genes is involved in protein synthesis suggesting a common mechanism that leads to defects in protein production and ultimately CMT pathologies. Our recently published work uncovered a potential mechanism underlying aaRS-CMT. We found that mutant aaRSs inappropriately sequester tRNAs from the ribosome, which stalls ribosome function and activates an integrated stress response (ISR) via a sensor protein, GCN2. ISR activation causes two major cellular events: 1) shutdown of a major form of protein synthesis and, 2) upregulation of the transcription factor, ATF4, and its target genes. The relative contributions of each of these events is currently unknown. One goal of this project is to determine the role of ATF4 and target genes in the pathophysiology observed in aaRS-CMT. Preliminary results show that ATF4 overexpression is toxic to motor neurons and produces a CMT- like phenotype in mice, evidence that ATF4 could be a viable therapeutic target for aaRS-CMT. In Aim 1 we will manipulate ATF4 expression levels in validated mouse models of aaRS-CMT to determine whether the disease pathology is driven by decreased protein translation or by increased expression of the ATF4 gene. To advance toward therapeutic applications we need to establish that human motor neurons also activate the ISR in response to aaRS-CMT mutations. Therefore, in Aim 2 we will establish and validate human induced pluripotent stem cell (hiPSC)-derived motor neuron cultures which have been genetically engineered to model aaRS-CMT. We will also test therapeutic strategies in these human cell-based models. Interestingly, ATF4 expression is common in many different types of neurodegeneration. Therefore, in Aim 3, we will integrate data from ATF4 mice in Aim 1, and hiPSC-derived motor neurons in Aim 2 to identify common genes and cellular pathways involved in ATF4-mediated neurodegeneration. These hiPSC-based models will be a powerful tool to help identify and develop new targets or pathways for potential therapeutic interventions. This MOSAIC (Maximizing Opportunities for Scientific and Academic Independent Careers) Postdoctoral Career Transition Award to Promote Diversity will be supported by excellent career development resources and a mentoring team of globally recognized experts in CMT (R.W. Burgess) and human stem cells (M.F. Pera) at The Jackson Laboratory for Mammalian Genetics.