The H67D HFE Gene Variant is Associated with Hormetic Neuroprotection in Brain Regions Vulnerable to Neurodegeneration

Open Access
- Author:
- Marshall Moscon, Savannah
- Graduate Program:
- Biomedical Sciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 28, 2024
- Committee Members:
- Xuemei Huang, Outside Unit Member
Zachary Simmons, Outside Field Member
Anirban Paul, Major Field Member
Elizabeth Proctor, Chair & Dissertation Advisor
Lisa Shantz, Program Head/Chair - Keywords:
- Hormesis
Adaptation
Neurodegeneration
Parkinson's
ALS - Abstract:
- Genes involved in brain iron homeostasis play a pivotal role in modification of age-related disorders, such as neurodegenerative disease, due to iron’s ability to catalyze reactive oxygen species production. The common H63D variant in the Iron Regulatory Gene, HFE, is implicated in modification of neurodegenerative disease progression due to its role in increasing brain iron loading.1 In mice, the H67D HFE variant (the mouse homolog of H63D) has repeatedly been shown to be associated with increased brain iron, neuroinflammation, and oxidative damage.1 However, these mice have also demonstrated robust neuroprotection against toxins which increase the risk of neurodegenerative disease as well as improved recovery from intracerebral hemorrhage.2–5 Our lab’s findings support the hypothesis that increased free iron in H67D mice falls within a hormetic dose of stress (eustress) and leads to a concerted adaptive mechanism which leads to heightened neuroprotection, at least partially driven by upregulation of Nrf2-driven antioxidant mechanisms. This stress response has been particularly evident in the lumbar spinal cord (LSC) and ventral midbrain (VM), both relevant to neurodegeneration.2,6 In this thesis, C57BL6/129 mice with homozygous H67D HFE were compared to mice with WT HFE to determine differences which may elucidate mechanisms of neuroprotection development. I used Immunohistochemistry to analyze dopaminergic (in the VM) and motor (in the LSC) neuron population maturation during the first 3 months. I used immunoblotting to measure protein carbonyl content as well as expression of oxidative phosphorylation complexes within LSC and VM tissue. I then used a Seahorse assay to analyze metabolic states of mitochondria isolated from the LSC and VM of 3 month old mice. I then performed a Nanostring transcriptomic analysis to measure expression of genes relevant to neurodegeneration within the LSC and VM of 3 month old mice. Finally, I performed DTI MRI to analyze connectivity and structure of the nigrostriatal pathway. Compared to WT mice, I found no difference in the viability of motor neurons in the LSC, but dopaminergic neurons in H67D mice experienced significant decline between 2 and 3 months of age. Both regions in H67D mice had alterations in oxidatively modified proteins as well as differences in oxidative phosphorylation complex expression indicative of stress adaptation. Mitochondria from the LSC of female H67D mice demonstrated alterations indicative of adaptation to heightened ROS. However, mitochondria from the VM of male H67D mice appeared to be in a stressed state. Transcriptional differences in both the LSC and VM of H67D mice were mostly related to cell structure and connectivity as well as cell signaling. Despite significant dopaminergic neuron death, MRI analysis demonstrated H67D mice have no decreased number of fiber tracts in the nigrostriatal pathway (from the substantia nigra to dorsal striatum). However, evidence of heightened microstructural organization and thinner fibers within the dorsal striatum suggests that axons have been sufficiently replaced within the substantia nigra but have not yet fully innervated the DS. Overall, our data sheds light on the resilience of the LSC and VM in the presence of low dose iron-related stress and informs hormetic mechanisms in response to eustress.