Title: Identification and Biophysical Characterization of Small Molecules Modulating Protein Disulfide Isomerase in Neurodegenerative Diseases
Description: Neurodegenerative disorders constitute a class of diseases that express characteristic misfolded proteins that aggregate and induce neuronal toxicity and death. Huntington’s disease is one such fatal protein misfolding disease. Currently no therapeutic avenue can delay or stop the progression of the disease. In this context, there is a need to identify therapeutic pathways and drug targets that can prevent or delay pathogenesis in neurodegenerative diseases involving protein misfolding. This dissertation describes how our search for new drug targets have led us to identify protein disulfide isomerase and three unique small molecules that modulate its activity as a means to protect neuronal cells from neurodegenerative protein misfolding diseases, such as Huntington’s disease. Protein disulfide isomerase is a thiol-oxidoreductase in the endoplasmic reticulum that has garnered increased attention because of its implicated role in numerous human diseases, including cancer, human immunodeficiency virus pathogenesis, and thrombosis.

Validating protein disulfide isomerase as target for neurodegenerative disorders may open up new therapeutic strategies to understand and treat these diseases. First, I describe the identification and validation of protein disulfide isomerase as a target of the neuroprotective small molecule, 16F16. I show that 16F16 is an irreversible inhibitor of protein disulfide isomerase that binds covalently to both cysteines in the active site. This inhibition is protective in cell and brain-slice models of Huntington’s disease, as well as in the brain-slice model of Alzheimer’s disease. Next, I describe the neuroprotective small molecule IBS141 that was originally incorrectly annotated with a chemical structure. I elucidate the correct structure of the active compound using analytical chemistry, revealing it to be the natural product securinine. Furthermore, I identify the binding site of securinine to protein disulfide isomerase and show that the inhibition of the protein is protective in cell and brain-slice models of neurodegenerative diseases.

In addition to finding this unexpected activity of securinine, I provide a systematic roadmap to those who encounter compounds with incorrect structural annotation in the course of screening campaigns. Last, I describe the discovery of LOC14, a nanomolar, reversible, modulator of protein disulfide isomerase that protects cells and medium spiny neurons from the toxic mutant huntingtin protein. I find that this protection results from LOC14 binding adjacent to the active site and inducing protein disulfide isomerase to adopt an oxidized conformation. LOC14, has dramatically improved potency for protein disulfide isomerase over previously identified inhibitors and displays favorable pharmaceutical properties, making it an idea compound to evaluate the therapeutic potential of modulating protein disulfide isomerase in in vivo models of neurodegenerative diseases.