Cellular responses to the accumulation of neurodegeneration associated protein fragments

Cellular protein quality control consists of an integrated network of molecular pathways engaged in protein folding and protein degradation to ensure protein homeostasis. The loss of protein homeostasis, due to excess production of aberrant proteins or defects in protein degradation, contributes to many diseases including neurodegeneration. As part of the etiology of neurodegeneration, specific neuronal proteins are proteolytically digested into fragments that misfold and form aggregates within cells. In this study, we investigated the cellular response to aggregation-prone protein fragments. To examine the influence of N-terminus on fragment behavior, our studies employed two C-terminal fragments of human TDP43 that are associated with amyotrophic lateral sclerosis and frontal temporal dementia. These fragments are largely identical but differ in their N-termini. Using genetic models in cultured cells, yeast, and mice, as well as fluorescence microscopy, biochemistry, and molecular cloning techniques including the CRISPR/Cas9 system, we found that the N-termini of otherwise similar protein fragments have profound effects on fragment behavior and may influence clinical outcomes in neurodegeneration associated with aggregation. In a follow up study, we discovered that the molecular chaperone, BAG6, can recognize hydrophobic regions of neurodegeneration-associated protein fragments and prevent their intracellular aggregation. Our studies also implicate RNF126, an E3‐ubiquitin ligase, in the degradation of these fragments. We also examined how cells respond to the aggregation-prone fragments in the absence of their degradation and we synthesized a cell-based fluorescent reporter to use in screening applications to discover modulators of protein degradation. These studies contribute to our understanding of how cells cope with toxic misfolded proteins and will aid in the search for novel therapeutics for neurodegenerative disease.