Kinetics of iron import into developing mouse organs determined by a pup-swapping method

A Large Proportion of the Neonatal Iron Pool Is Acquired from the Gestational Diet in a Murine Model

BACKGROUND

Iron is crucial for growth and development, but excess iron is harmful. Neonatal mice have elevated concentrations of circulating iron, but the source of this iron is unclear. This lack of understanding makes it difficult to optimize early life iron balance.

OBJECTIVES

Identify the origins of neonatal tissue-specific iron pools using dietary manipulation and cross-fostering murine models.

METHODS

To determine whether tissue-specific neonatal iron was primarily acquired during gestation or after birth, pups born to iron-sufficient or iron-deficient dams were cross-fostered, and tissues were harvested at postnatal days 3-5 to measure iron content. A separate set of female mice were fed a diet enriched with the stable iron isotope 57 (57Fe) for 4 generations to replace naturally abundant liver iron isotope 56 (56Fe) stores with 57Fe. To quantify the proportions of neonatal iron acquired during gestation, pups born to dams with 56Fe or 57Fe stores were cross-fostered, and tissues were harvested at postnatal day 3-5 to determine 56Fe:57Fe ratios by inductively coupled plasma mass spectrometry. Finally, to quantify the proportion of neonatal iron acquired from the maternal diet, female mice with 56Fe or 57Fe stores switched diets upon mating, and pup tissues were harvested on P0 to determine 56Fe:57Fe ratios by inductively coupled plasma mass spectrometry.

RESULTS

Perinatal iron deficiency resulted in smaller pups, and gestational iron deficiency resulted in lower neonatal serum and liver iron. Cross-fostering between dams with 56Fe and 57Fe stores demonstrated that ≤70% of neonatal serum, liver, and brain iron were acquired during gestation. Dietary manipulation experiments using dams with 56Fe and 57Fe stores showed that over half of neonatal serum, liver, and brain iron were from the dam's gestational diet rather than preconception iron stores.

CONCLUSIONS

This study provides quantitative values for the sources of neonatal iron, which may inform approaches to optimize neonatal iron status.

Iron-tracking strategies: Chaperones capture iron in the cytosolic labile iron pool

Cells express hundreds of iron-dependent enzymes that rely on the iron cofactors heme, iron-sulfur clusters, and mono-or di-nuclear iron centers for activity. Cells require systems for both the assembly and the distribution of iron cofactors to their cognate enzymes. Proteins involved in the binding and trafficking of iron ions in the cytosol, called cytosolic iron chaperones, have been identified and characterized in mammalian cells. The first identified iron chaperone, poly C-binding protein 1 (PCBP1), has also been studied in mice using genetic models of conditional deletion in tissues specialized for iron handling. Studies of iron trafficking in mouse tissues have necessitated the development of new approaches, which have revealed new roles for PCBP1 in the management of cytosolic iron. These approaches can be applied to investigate use of other nutrient metals in mammals.

Bioanalytical applications of Mössbauer spectroscopy

Data on the applications of Mössbauer spectroscopy in the transmission (mainly on 57Fe nuclei) and emission (on 57Co nuclei) variants for analytical studies at the molecular level of metal-containing components in a wide range of biological objects (from biocomplexes and biomacromolecules to supramolecular structures, cells, tissues and organisms) and of objects that are participants or products of biological processes, published in the last 15 years are discussed and systematized. The prospects of the technique in its biological applications, including the developing fields (emission variant, use of synchrotron radiation), are formulated.

The bibliography includes 248 references.

Iron and manganese transport in mammalian systems

Studies in recent years have significantly expanded, refined, and redefined the repertoire of transporters and other proteins involved in iron and manganese (Mn) transport and homeostasis. In this review, we discuss highlights of the recent literature on iron and Mn transport, focusing on the roles of membrane transporters and related proteins. Studies are considered from the vantage point of main organs, tissues, and cell types that actively control whole-body iron or Mn homeostasis, with emphasis on studies in which in vivo metal transport was measured directly or implicated by using knockout mouse models. Overviews of whole-body and cellular iron and Mn homeostasis are also provided to give physiological context for key transporters and to highlight how they participate in the uptake, intracellular trafficking, and efflux of each metal. Important similarities and differences in iron and Mn transport are noted, and future research opportunities and challenges are identified.

Low-molecular-mass iron in healthy blood plasma is not predominately ferric citrate

Blood contains a poorly characterized pool of labile iron called non-transferrin-bound iron (NTBI). In patients with iron-overload diseases such as hemochromatosis, NTBI accumulates in the liver, heart, and other organs. This material is probably nonproteinaceous and low molecular mass (LMM). However, the number, concentration, mass, and chemical composition of NTBI species remain unknown despite decades of effort. Here, solutions of plasma from humans, pigs, horses, and mice were passed through a 10 kDa cutoff membrane, affording flow-through solutions (FTSs) containing ∼1 μM iron. The FTSs were subjected to size-exclusion liquid chromatography at pH 8.5, 6.5, and 4.5. Iron was detected by an online inductively-coupled-plasma mass spectrometer. LC-ICP-MS chromatograms of the FTSs exhibited 2-6 iron-containing species with apparent masses between 400 and 2500 Da. Their approximate concentrations in plasma were 10-8-10-7 M. Not every FTS sample contained every LMM iron species, indicating individual variations. The most reproducible iron species had apparent masses of 400 and 500 Da. Chromatograms of the FTSs from established hemochromatosis patients exhibited no significant differences relative to controls. The peak positions and intensities depended on column pH. Some FTS iron adsorbed onto the column, especially at higher pH. Column-adsorbing-iron coordinated apo-transferrin whereas the more tightly coordinated iron species did not. Ferric citrate standards exhibited LMM iron peaks that were similar to but not the same as those obtained in FTSs. The results indicate that the LMM iron species in healthy blood plasma is not primarily ferric citrate; however, this may be one of many contributing complexes.

Transport of Non-Transferrin Bound Iron to the Brain: Implications for Alzheimer's Disease

A direct correlation between brain iron and Alzheimer's disease (AD) raises questions regarding the transport of non-transferrin-bound iron (NTBI), a toxic but less researched pool of circulating iron that is likely to increase due to pathological and/or iatrogenic systemic iron overload. Here, we compared the distribution of radiolabeled-NTBI (59Fe-NTBI) and transferrin-bound iron (59Fe-Tf) in mouse models of iron overload in the absence or presence of inflammation. Following a short pulse, most of the 59Fe-NTBI was taken up by the liver, followed by the kidney, pancreas, and heart. Notably, a strong signal of 59Fe-NTBI was detected in the brain ventricular system after 2 h, and the brain parenchyma after 24 h. 59Fe-Tf accumulated mainly in the femur and spleen, and was transported to the brain at a much slower rate than 59Fe-NTBI. In the kidney, 59Fe-NTBI was detected in the cortex after 2 h, and outer medulla after 24 hours. Most of the 59Fe-NTBI and 59Fe-Tf from the kidney was reabsorbed; negligible amount was excreted in the urine. Acute inflammation increased the uptake of 59Fe-NTBI by the kidney and brain from 2-24 hours. Chronic inflammation, on the other hand, resulted in sequestration of iron in the liver and kidney, reducing its transport to the brain. These observations provide direct evidence for the transport of NTBI to the brain, and reveal a complex interplay between inflammation and brain iron homeostasis. Further studies are necessary to determine whether transient increase in NTBI due to systemic iron overload is a risk factor for AD.

Sample treatment in Mössbauer spectroscopy for protein-related analyses: Nondestructive possibilities to look inside metal-containing biosystems

In this review, the unique possibilities are considered of the ⁵⁷Fe transmission (TMS) and ⁵⁷Co emission (EMS) variants of Mössbauer (nuclear γ-resonance) spectroscopy as nondestructive techniques with minimal sample preparation/treatment and a significant analytical potential, with a focus on the analysis of cation-binding sites in metalloproteins. The techniques are shown to provide unique structural and quantitative information on the coordination microenvironment, the chemical state and transformations of the Mössbauer nuclides in sophisticated metal-containing proteins, including those within complicated supramolecular structures, and in microbial cells or tissues. Recent representative examples of analyses of Fe-containing proteins by ⁵⁷Fe TMS are briefly discussed, along with the newly emerging data on using ⁵⁷Co EMS for probing the structural organisation of ⁵⁷Co-doped cation-binding sites in sophisticated biocomplexes including metalloenzymes. Finally, some rare or exotic applications of Mössbauer spectroscopy (including the synchrotron-based methodology) in protein-related studies are outlined.

Mössbauer Spectra of Mouse Hearts Reveal Age-dependent Changes in Mitochondrial and Ferritin Iron Levels

Cardiac function requires continuous high levels of energy, and so iron, a critical player in mitochondrial respiration, is an important component of the heart. Hearts from ⁵⁷Fe-enriched mice were evaluated by Mössbauer spectroscopy. Spectra consisted of a sextet and two quadrupole doublets. One doublet was due to residual blood, whereas the other was due to [Fe₄S₄]²⁺ clusters and low-spin Fe^II hemes, most of which were associated with mitochondrial respiration. The sextet was due to ferritin; there was no evidence of hemosiderin, a ferritin decomposition product. Iron from ferritin was nearly absent in young hearts, but increased steadily with age. EPR spectra exhibited signals similar to those of brain, liver, and human cells. No age-dependent EPR trends were apparent. Hearts from HFE^-/- mice with hemochromatosis contained slightly more iron overall than controls, including more ferritin and less mitochondrial iron; these differences typify slightly older hearts, perhaps reflecting the burden due to this disease. HFE^-/- livers were overloaded with ferritin but had low mitochondrial iron levels. IRP2^-/- hearts contained less ferritin than controls but normal levels of mitochondrial iron. Hearts of young mice born to an iron-deficient mother contained normal levels of mitochondrial iron and no ferritin; the heart from the mother contained low ferritin and normal levels of mitochondrial iron. High-spin Fe^II ions were nearly undetectable in heart samples; these were evident in brains, livers, and human cells. Previous Mössbauer spectra of unenriched diseased human hearts lacked mitochondrial and blood doublets and included hemosiderin features. This suggests degradation of iron-containing species during sample preparation.

Modelling Systemic Iron Regulation during Dietary Iron Overload and Acute Inflammation: Role of Hepcidin-Independent Mechanisms

Systemic iron levels must be maintained in physiological concentrations to prevent diseases associated with iron deficiency or iron overload. A key role in this process plays ferroportin, the only known mammalian transmembrane iron exporter, which releases iron from duodenal enterocytes, hepatocytes, or iron-recycling macrophages into the blood stream. Ferroportin expression is tightly controlled by transcriptional and post-transcriptional mechanisms in response to hypoxia, iron deficiency, heme iron and inflammatory cues by cell-autonomous and systemic mechanisms. At the systemic level, the iron-regulatory hormone hepcidin is released from the liver in response to these cues, binds to ferroportin and triggers its degradation. The relative importance of individual ferroportin control mechanisms and their interplay at the systemic level is incompletely understood. Here, we built a mathematical model of systemic iron regulation. It incorporates the dynamics of organ iron pools as well as regulation by the hepcidin/ferroportin system. We calibrated and validated the model with time-resolved measurements of iron responses in mice challenged with dietary iron overload and/or inflammation. The model demonstrates that inflammation mainly reduces the amount of iron in the blood stream by reducing intracellular ferroportin transcription, and not by hepcidin-dependent ferroportin protein destabilization. In contrast, ferroportin regulation by hepcidin is the predominant mechanism of iron homeostasis in response to changing iron diets for a big range of dietary iron contents. The model further reveals that additional homeostasis mechanisms must be taken into account at very high dietary iron levels, including the saturation of intestinal uptake of nutritional iron and the uptake of circulating, non-transferrin-bound iron, into liver. Taken together, our model quantitatively describes systemic iron metabolism and generated experimentally testable predictions for additional ferroportin-independent homeostasis mechanisms.

The importance of iron in many physiological processes relies on its ability to participate in reduction-oxidation reactions. This property also leads to potential toxicity if concentrations of free iron are not properly managed by cells and tissues. Multicellular organisms therefore evolved intricate regulatory mechanisms to control systemic iron levels. A central regulatory mechanism is the binding of the hormone hepcidin to the iron exporter ferroportin, which controls the major fluxes of iron into blood plasma. Here, we present a mathematical model that is fitted and validated against experimental data to simulate the iron content in different organs following dietary changes and/or inflammatory states, or genetic perturbation of the hepcidin/ferroportin regulatory system. We find that hepcidin mediated ferroportin control is essential, but not sufficient to quantitatively explain several of our experimental findings. Thus, further regulatory mechanisms had to be included in the model to reproduce reduced serum iron levels in response to inflammation, the preferential accumulation of iron in the liver in the case of iron overload, or the maintenance of physiological serum iron concentrations if dietary iron levels are very high. We conclude that hepcidin-independent mechanisms play an important role in maintaining systemic iron homeostasis.

Maternal intestinal HIF-2α is necessary for sensing iron demands of lactation in mice

The mechanisms that are essential for the maintenance of nutrient status in breast milk are unclear. Our data demonstrate that the intestine via hypoxia-inducible factor (HIF)-2α is an essential regulatory mechanism for maintaining the quality of breast milk. During lactation, intestinal HIF-2α is highly increased, leading to an adaptive induction of apical and basolateral iron transport genes. Disruption of intestinal HIF-2α (but not HIF-1α) or the downstream target gene divalent metal transporter (DMT)-1 in lactating mothers did not alter systemic iron homeostasis in the mothers, but led to anemia, decreased growth, and truncal alopecia in pups which was restored following weaning. Moreover, pups born from mothers with a disruption of intestinal HIF-2α led to long-term cognitive defects. Cross-fostering experiments and micronutrient profiling of breast milk demonstrated that the defects observed were due to decreased maternal iron delivery via milk. Increasing intestinal iron absorption by activation of HIF-2α or parenteral administration of iron-dextran in HIF-2α knockout mothers ameliorated anemia and restored neonatal development and adult cognitive functions. The present work details the importance of breast milk iron in neonatal development and uncovers an unexpected molecular mechanism for the regulation of nutritional status of breast milk through intestinal HIF-2α.

Speciation of iron in mouse liver during development, iron deficiency, IRP2 deletion and inflammatory hepatitis

The iron content of livers from (57)Fe-enriched C57BL/6 mice of different ages were investigated using Mössbauer spectroscopy, electron paramagnetic resonance (EPR), electronic absorption spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS). About 80% of the Fe in an adult liver was due to blood; thus removal of blood by flushing with buffer was essential to observe endogenous liver Fe. Even after exhaustive flushing, ca. 20% of the Fe in anaerobically dissected livers was typical of deoxy-hemoglobin. The concentration of Fe in newborn livers was the highest of any developmental stage (∼1.2 mM). Most was stored as ferritin, with little mitochondrial Fe (consisting primarily of Fe-S clusters and haems) evident. Within the first few weeks of life, about half of ferritin Fe was mobilized and exported, illustrating the importance of Fe release as well as Fe storage in liver function. Additional ferritin Fe was used to generate mitochondrial Fe centres. From ca. 4 weeks of age to the end of the mouse's natural lifespan, the concentration of mitochondrial Fe in liver was essentially invariant. A minor contribution from nonhaem high-spin Fe(II) was observed in most liver samples and was also invariant with age. Some portion of these species may constitute the labile iron pool. Livers from mice raised on an Fe-deficient diet were highly Fe depleted; they were devoid of ferritin and contained 1/3 as much mitochondrial Fe as found in Fe-sufficient livers. In contrast, brains of the same Fe-deficient mice retained normal levels of mitochondrial Fe. Livers from mice with inflammatory hepatitis and from IRP2(-/-) mice hyper-accumulated Fe. These livers had high ferritin levels but low levels of mitochondrial Fe.