Regulation of brain iron uptake by apo- and holo-transferrin is dependent on sex and delivery protein

Abstract

Background The brain requires iron for a number of processes, including energy production. Inadequate or excessive amounts of iron can be detrimental and lead to a number of neurological disorders. As such, regulation of brain iron uptake is required for proper functioning. Understanding both the movement of iron into the brain and how this process is regulated is crucial to both address dysfunctions with brain iron uptake in disease and successfully use the transferrin receptor uptake system for drug delivery. Methods Using in vivo steady state infusions of apo- and holo-transferrin into the lateral ventricle, we demonstrate the regulatory effects of brain apo- and holo-transferrin ratios on the delivery of radioactive 55Fe bound to transferrin or H-ferritin in male and female mice. In discovering sex differences in the response to apo- and holo-transferrin infusions, ovariectomies were performed on female mice to interrogate the influence of circulating estrogen on regulation of iron uptake. Results Our model reveals that apo- and holo-transferrin significantly regulate iron uptake into the microvasculature and subsequent release into the brain parenchyma and their ability to regulate iron uptake is significantly influenced by both sex and type of iron delivery protein. Furthermore, we show that cells of the microvasculature act as reservoirs of iron and release the iron in response to cues from the interstitial fluid of the brain. Conclusions These findings extend our previous work to demonstrate that the regulation of brain iron uptake is influenced by both the mode in which iron is delivered and sex. These findings further emphasize the role of the microvasculature in regulating brain iron uptake and the importance of cues regarding iron status in the extracellular fluid.

Brain iron acquisition: An overview of homeostatic regulation and disease dysregulation

“…The constant utilization of iron for metabolic processes likely leads to an increased requirement of iron uptake into the brain. Our in vivo study supports this, where we found that, within 24 hours, brain iron uptake increased with apo‐Tf infusions in males but not in females (Baringer et al, 2022), potentially because the iron uptake signaling (apo‐Tf) in females was already maximally elevated.…”

“…Indeed, Chiou et al reported that FTH1 is released as an iron delivery protein for ECs (Chiou et al, 2019) and showed that basal apo‐ and holo‐Tf affected the amount of FTH1‐bound iron that was transported across the ECs in a similar manner as Tf‐delivered iron (Chiou et al, 2019). However, our in vivo study found no change in FTH1 uptake when the levels of apo‐ and holo‐Tf were manipulated via intraventricular infusion (Baringer et al, 2022). The differences between the results of these two studies are likely due to the complex dynamics of iron uptake in vivo including differences in signaling that regulates uptake of Tf and FTH1.…”

“…As the evidence suggesting that ECs are reservoirs for iron grows, it is important to specifically investigate the process of iron release. Our in vivo studies have shown that the rates of iron uptake and release are correlated, but they differ in value, with iron uptake into the microvessels being significantly more than what is initially released into the brain parenchyma (Baringer et al, 2022). If TfR is the rate‐limiting step for iron uptake, then Fpn, the only known iron exporter in all barrier cells, is the rate‐limiting step for free iron export.…”

“…However, this theory did not consider the iron needs of the ECs nor did it acknowledge the clear need for the brain to exert regulation. More recently, it has been demonstrated that apo (iron free)‐Tf and holo (iron loaded)‐Tf have influence over the release of iron from ECs (Baringer et al, 2022; Burdo et al, 2003; Chiou et al, 2019; McCarthy & Kosman, 2015a; Simpson et al, 2015), and thus may be the source of brain iron uptake regulation the field has long ignored. Understanding the regulation of brain iron uptake has significant implications in the optimized harnessing of the brain iron uptake mechanisms for drug delivery and the treatment of iron uptake dysregulation in a variety of diseases.…”

Exosomes are involved in iron transport from human blood–brain barrier endothelial cells and are modified by endothelial cell iron status

“…Tf-bound iron is taken up via transferrin receptor (TfR) expressed on the luminal side of the blood–brain barrier endothelial cells (BBBECs), which leads to receptor-mediated endocytosis ( 8 , 9 ), and studies have shown receptor-mediated transcytosis of Tf-bound iron directly from the luminal side of the BBBECs to the brain ( 10 , 11 ). We have established that H-ferritin (FTH1) can also transport iron to the brain both in vitro and in vivo ( 12 , 13 ). Ferritin is an iron storage protein capable of binding 4500 iron atoms, that is made up of two subunits: FTH1 and L-ferritin ( 14 , 15 ).…”