Regulation of Brain Iron Acquisition and Misappropriation in Alzheimer's Disease
Open Access
- Author:
- Baringer, Stephanie
- Graduate Program:
- Biomedical Sciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 17, 2023
- Committee Members:
- Xuemei Huang, Major Field Member
David Degraff, Outside Field Member
Elizabeth Proctor, Major Field Member
Yongsoo Kim, Outside Unit Member
James Connor, Chair & Dissertation Advisor
Ralph Keil, Program Head/Chair - Keywords:
- blood-brain barrier
iron
transferrin
Alzheimer's disease - Abstract:
- Brain iron homeostasis is essential to proper neurological functioning, with high levels of brain iron being implicated as causative factors in neurodegenerative diseases and low levels leading to cognitive impairment and Restless Legs Syndrome. As such, uptake of iron in the brain is tightly regulated at the site of endothelial cells (ECs) of the blood-brain barrier (BBB). The working model is that as cells in the brain can signal their iron needs, based on their iron consumption, and control iron release from ECs in the form of apo (iron free)- and holo (iron bound)- transferrin (Tf). Our group has previously shown using in vitro models that apo-Tf indicates an iron deficient environment and stimulates iron release whereas holo-Tf indicates an iron saturated environment and suppresses iron release. In Chapter 2, I determined if the delivery protein of iron or the sex of the organism could impact their regulatory mechanism by performing steady-state infusions of apo- and holo-Tf into the brain of male and female mice and then intraperitoneal injecting the mice with radioactive iron bound to Tf or H-ferritin, another iron delivery protein. I found that only iron delivered via Tf to the brain is influenced by brain side ratios of apo- and holo-Tf, while iron found to H-ferritin was not regulated by this mechanism. Additionally, I discovered a sex specific response to modulating the ratio of apo- and holo-Tf in the brain. In Chapter 3, I investigated the molecular mechanism of apo- and holo-Tf’s respective influence of iron release, and I found that holo-Tf incubation causes ubiquitination and subsequent degradation of ferroportin (Fpn), the only known iron exporter protein. Using orthogonal methods, I discovered that apo-Tf directly interacts with hephaestin, a ferroxidase that aims Fpn, and holo-Tf directly interacts with Fpn. Hepcidin is an inflammatory hormone peptide and long thought to be the primary iron release regulator. Thus, to understand how physiological and pathophysiological levels of hepcidin influence these protein-protein interactions, I uncovered that only hepcidin levels consistent with disease interrupt the interaction between holo-Tf and Fpn, while no amount interrupts the interaction between apo-Tf and hephaestin. Furthermore, I found that the interaction disruption is due to hepcidin internalizing Fpn faster than holo-Tf. These data suggest that hepcidin may be deployed to abruptly stop iron release in systemic stress while holo-Tf is likely the primary regulator of iron release in homeostasis. While numerous diseases display altered brain iron regulation, Alzheimer’s disease (AD) exhibits excessive brain iron accumulation that can be used to predict cognitive decline. What’s more, brain iron accumulation occurs early in the disease and prior to widespread amyloid-β (Aβ) deposition, suggesting an element of malfunction in the regulation of iron uptake. To explore iron release dysfunction is response to Aβ, in Chapter 4, I used induced pluripotent stem cell derived- ECs and astrocytes and I found that media collected from astrocytes exposed to low levels of Aβ stimulated iron release from ECs without damaging the cells. In response to low levels of Aβ, astrocytes increased their own iron uptake and mitochondrial activity, resulting in elevated levels of apo-Tf and iron deficient media. These data are the first to demonstrate how disease pathology can misappropriate iron release regulatory mechanisms and further disease dysfunction. Taken together, the findings presented in this body of work aid in deciphering the important regulatory process of iron release from ECs of the BBB and present a novel interpretation of how the process can be misconstrued in disease. Furthermore, the findings shift the paradigm of conventional iron release regulation and suggest a novel mechanism in cells throughout the body.