05/09/2022
By Luc Francois

The Zuckerberg College of Health Sciences, Department of Pharmaceutical Sciences, invites you to attend a doctoral dissertation defense by Luc Francois on “Loss of Huntingtin Disrupts Iron-Dependent Processes that Regulate Growth and Development in the Model Organism Dictyostelium Discoideum.”

Candidate Name: Luc E. Francois
Defense Date: Friday, May 20, 2022
Time: 8 to 10 a.m.
Location: ETIC 445, North Campus
Dissertation Title: Loss of Huntingtin Disrupts Iron-Dependent Processes that Regulate Growth and Development in the Model Organism Dictyostelium Discoideum

Committee Chair (Advisor): Jeffrey R. Moore, Professor, Graduate Coordinator

Committee Members:

  • Frederic Chain, Assistant Professor, Biological Sciences, University of Massachusetts Lowell
  • Brenda Geiger, Assistant Dean, Associate Teaching Professor, Biomedical & Nutritional Sciences, University of Massachusetts Lowell
  • Matthew Nugent, Associate Dean for Research, Innovation and Partnerships, Professor, Biological Sciences, University of Massachusetts Lowell

Brief Abstract:
Huntington disease is a devastating neurodegenerative disease that is manifested by progressive motor dysfunction, cognitive decline, and psychiatric disturbance. The disease is characterized by a CAG trinucleotide repeat expansion mutation in exon 1 of the huntingtin gene that leads to a polyglutamine stretch of more than 35 repeats in the N-terminal region of the huntingtin protein. The polyglutamine mutation confers a toxic gain of function in the protein that leads to selective neuronal death. Huntingtin is a pleiotropic protein that has function in many cellular processes. N-terminal huntingtin fragments and iron accumulate in regions of the brain where neurons die, while elimination of huntingtin is embryonically lethal in mice and leads to iron homeostasis dysregulation. To investigate whether iron accumulation is a result of the disease, we expressed full-length human huntingtin with expanded polyglutamine repeats in huntingtin deficient Dictyostelium discoideum, an organism that is easily amenable and comprises both cellular and multicellular developmental stages. We assessed growth and development in response to iron dosage, followed by examination of intracellular iron trafficking and differential gene expression. Huntingtin-deficiency resulted in modest to no growth and abnormal mound formation in development. Furthermore, there was abnormal morphology of the contractile vacuole, which is the organelle involved in iron storage and efflux in Dictyostelium. Insertion of human huntingtin rescued growth and chemotaxis in development, suggesting restoration of key functions of the wild-type protein. Our RNA sequencing analysis showed various genes differentially expressed between wild type and huntingtin-deficient cells in nutrient-rich media and media deprived of, and overloaded with iron, including genes involved in iron homeostasis, mitochondrial function, cytoskeletal and membrane formation. Among the differentially expressed genes, we found the expression of glutathione transferase, a key protector in cellular response to oxidative stress, was significantly down-regulated in HTT-null cells compared to our full-length human huntingtin expressing cells and wild-type cells.

Our study has shed light on the effects of huntingtin on growth, development, and iron homeostasis in Dictyostelium discoideum through interference of cellular processes including gene expression and protein-protein interactions. Low iron condition, ferrous iron supplementation and insertion of full-length human huntingtin are conditions that improved growth or development in huntingtin-deficient cells. Consequently, we observe changes in cellular survival response through the expression of glutathione transferase and response to oxidative stress when huntingtin is deficient indicating cellular stress as a cause of cell death. Future work targeting glutathione activities and preventing iron accumulation can offer more insights not only on early pathological events of HD but potentially help uncover conserved processes responsible for related neurodegenerative disorders.

All interested students and faculty members are invited to attend.