Stress Induced Nuclear Granules form within the Nucleus in Response to Environmental Stress in C. elegans

Abstract

Protein misfolding caused by environmental stress can be damaging to a cell and it has been linked to many human diseases. Cells contend with misfolded proteins by employing an arsenal of protein quality control mechanisms (PQC). One PQC mechanism that aids in destroying misfolded and damaged proteins is the ubiquitin proteasome system (UPS). PQC mechanisms have been observed in the cytosol, endoplasmic reticulum, and mitochondria. However, little information is known about the PQC mechanisms in the nucleus. The nucleus has been suggested to be a site of protein degradation based on ubiquitin and proteasome localization in the nucleus under normal conditions. The research in this dissertation aims to understand how nuclear UPS is involved in environmental stress in C. elegans. Results from this study have shown that ubiquitin, proteasome, and TIA-1/TIAL RNA binding protein homolog (TIAR-2) localize into distinct structures termed stress induced nuclear granules (SINGs) in response to osmotic stress, oxidative stress, and starvation. SINGs were found to be enriched in K48 polyubiquitin chains and their formation was inhibited by proteasome inhibitors, which indicates SINGs being sites of protein degradation. Knockdown of ubiquitin conjugating enzymes ubc-18, ubc-20, and ubc-22 decreased the appearance of SINGs during stress, which indicates they are part of the pathway leading to SING formation. Similar results were seen with knockdown of the ubiquitin ligase chn-1 and nuclear import factors ima-1, ran-1, and smo-1. The formation of SINGs can be inhibited by a brief exposure to heat shock, which was found to be HSF-1 dependent. This finding suggests that increased chaperone expression is able to prevent SINGs. In addition, this experiment indicates that the accumulation of misfolded proteins is required to induce SING formation. Embryos containing SINGs are unable to complete cell division suggesting that SINGs interfere with cell cycle progression. These results suggest a model in which the UPS is triggered in response to misfolded proteins in the nucleus.