PROJECT SUMMARY Neurodegenerative disorders remain a public health burden and lack effective treatments. Rare genetic disorders can cause neurodegeneration in children, which is particularly devastating. Regardless of whether rare or common, the mechanisms underlying neurodegeneration remain incompletely understood. For instance, why neurons may be more susceptible to cell death when a genetic mutation is present throughout the body? Such is the case in TBCK encephalopathy (TBCKE), a rare autosomal recessive disorder that causes developmental delay and neurodegeneration in children. We previously characterized the neurologic phenotype of Puerto Rican children with a homozygous null mutation (p.R126X) in TBCK, and later found mitochondrial respiratory defects and evidence for abnormal mitochondrial quality control (i.e. mitophagy) in patient-derived fibroblasts. Nevertheless, the (1) physiologic role of TBCK and (2) how TBCK-deficiency leads to mitochondrial dysfunction and neurodegeneration, remain unclear. Our data support that Tbck protein may be part of a novel mRNA transport complex (Five-subunit Endosomal Rab5 and RNA/ribosome intermediary- FERRY). This complex may attach to early endosomes to deliver RNA transcripts, and therefore contribute to newly synthesized proteins, to cellular compartments distant from the nucleus, such as axons. Deficits in RNA transport and local protein translation, particularly to distant axonal mitochondria, have been recently proposed as a novel mechanism underlying a common neurodegenerative disorder (ALS, amyotrophic lateral sclerosis). Hence, our central hypothesis is that neuronal vulnerability in TBCK-deficiency stems from impaired transport of mRNA and/or local protein translation disrupting mitochondrial function. We predict this leads to compartment-specific mitochondrial deficits, with distal axonal mitochondria being more susceptible to TBCK- deficiency than those in the neuronal soma. In Aim 1 we will test how TBCK-deficiency impacts mitochondrial function and mitophagy, using human iPSC-derived neurons (iNeu). Then in Aim 2, we will test how TBCK- deficiency may affect the function of the FERRY complex, by examining effects in the RNA transcriptome in a compartment-specific fashion (neuronal soma vs axons). We will also directly assay local protein synthesis to test if TBCK is leading to mitochondrial dysfunction because of impaired protein translation in axonal compartments. These experiments will address a gap in knowledge regarding the role of neuronal RNA transport defects in mediating neurodegeneration, how these defects may particularly impact mitochondria and the role that Tbck protein may play in the novel FERRY complex. Support from this R01 award will be instrumental in growing my independent research program as a physician-scientist in a superb institutional environment. It will also contribute to my long-term goal of untangling disease mechanisms of pediatric neurodegenerative disorders ...