1 - Introduction

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Iron is a chemical element present in key biochemical processes of virtually every living cell. It exists in two interconvertible ionic forms, Fe2+ (ferrous ion, reduced) and Fe3+ (ferric ion, oxidized). This is the basis for numerous oxido-reductive electron transfers which are a vital element of living cells. The high reactivity of ionic iron, however, is also a danger, since it may lead to chemical radicals which are detrimental to biological macromolecules. An example for this is a reaction of the so-called Fenton type, where ferrous iron becomes oxidized by hydrogen peroxide. The reaction produces a hydroxyl anion and a hydroxyl radical, which can damage cell membranes and essential components like DNA or proteins.

Fe2+ + H2O2 → Fe3+ + OH· + OH

It is therefore important for the organism to exert a critical control over its free iron, otherwise its beneficial reactive characteristics can turn into a threat for the cell.

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In mammals, a very important role of iron is related to its presence in hemoglobin, a protein directly involved in oxygen transport through the body. The synthesis of hemoglobin is an essential step in the production of red blood cells (RBC). In the human body this step requires approximately 20 mg iron per day. In adult humans as well as in many other mammals the RBC production takes place predominantly in the bone marrow and represents the greatest demand of iron in the body.

In the muscles, iron is present in myoglobin. This protein provides a reservoir of readily accessible oxygen. This is intracellular buffer for the case of intermittent anoxia [1 ]. Other cell types of the body contain iron as a reserve store, bound to ferritin. This protein is able to bind free iron in considerable amounts and keeps it non-toxic, releasing it only until required by other metabolic functions.

Every cell of the body incorporates a certain amount of iron into a host of iron-containing proteins (heme proteins, Fe-S-cluster proteins) which fulfill essential functions of cellular life.

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The only natural source of iron for mammals is the diet. A tightly controlled mechanism exists to determine the exact amount allowed to enter the body. Disorders in this absorptive process, in either direction – too much or too little iron – have serious public health implications. Iron deficiency anemia is the most prevalent nutrition disease worldwide and affects every society, irrespective of race, cultural and social-background [2 ]. On the other hand is hemochromatosis a hereditary disease that provokes an excessive intake of iron from the diet. Since the human body is incapable of excreting iron in a well-regulated manner, an accumulation of this metal can take place, which damages the liver and other parenchymous organs and leads to liver cirrhosis and finally to liver cancer. The liver absorbs the excess of iron and so protects other organs, but unfortunately ends up damaging itself.

In the brain, iron varies according to three factors: the anatomic region, the developmental stage of the organism and the species being studied [3 ]. In this organ iron plays a not-well characterized role. A strong correlation was observed between accumulation of this metabolite and neurodegenerative diseases like Parkinson, Alzheimer and Huntington [4 , 5 ].

Uptake and distribution of iron in the body have been investigated in detail, but we have no complete picture of the molecular mechanisms that regulate these processes. There are still missing components that are at present being revealed through the use of modern molecular techniques. The use of transgenic mice technology opened a range of possibilities and helps to elucidate the regulatory pathways of iron metabolism.

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From a systemic point of view the iron metabolism displays two different hierarchical levels. One level concerns the well regulated iron metabolism within the multifarious types of cells and tissues of the body. The other level is the regulated exchange of iron between cells and tissues and the control of its uptake. The cellular and the organismal aspect are intimately connected and cannot be satisfactorily understood in isolation of each other. The study of isolated cells has led to a deeper understanding of the regulation within certain cells. However, the interpretation has been limited by the fact that the cell lines so studied were usually not fully functional and could not communicate with the extracellular environment and with other organs. On the other hand, the study of iron flux between tissues has led to important quantitative data, but was limited to a phenomenological level that described aptly what happened in the body, but not why it happened as it did. These two limitations have now been overcome by the modern gene construction techniques. They allow the study of animal iron metabolism applying certain well-designed genetic constructions which reveal, by knock-out or by enforced gene expression, the fine-tuning of iron-related reactions in the healthy as well as in the diseased organism. The so-called Cre-Lox-technology makes it even possible to change a certain gene in a selected target cell, by causing the attempted effect (knock-out, knock-in or enforced expression) only under the control of cell-specific promoters. So it became possible to address certain cell types and tissues with experimental changes, thereby avoiding the often deleterious effect of whole-body genetic mutations.

A holistic understanding of iron metabolism in its various physiological and pathological states requires a deeper systemic understanding. This can be advanced by the method of mathematical modeling. Many ingenious mathematical studies of iron metabolism of the whole body have been published. Most of the earlier work concentrated on the interpretation of the tracer elimination curve in human blood plasma after an intravenous injection [6 , 7]. Marsaglia, in cooperation with Finch and Hosain [8 ] devised a method to estimate the passage time through bone marrow and the return time of tracer into blood. Pollycove and Mortimer [9 ] published a study that tried to estimate the organ distribution of iron fluxes on the basis of scintillation measurements of tracer projected to the body surface. Nathanson and coworkers [10 ] studied the absorption and distribution kinetics of iron in dogs. Berzuini et al. [11 ] and later on Stefanelli et al. [12 ] developed whole-body models on data from human subjects, after tracer injection into blood or as colloidal tracer absorbed by the reticulo-endothelial system. A whole-body iron distribution study by Vácha et al. [13 ] attempted at a quantitative description of a mouse strain (C57BL/10ScSnPh) which is related to the strain to be modelled in this dissertation. This paper contained a series of ad-hoc assumptions on fluxes for which a precise biochemical characterization was not possible, but the resulting mathematical model fitted the measured ferrokinetic data quite satisfactorily. A first attempt to model iron metabolism as compartment system with inclusion of the recently discovered hormonal signals (especially the hepcidin loop) was published by Lao and Kamei [14 ]. The intracellular aspect has in all these papers been studied only in a black-box manner because iron motion within the cell occurs in a complex membranous environment. This precludes the classical biochemical kinetics which has been so successfully applied to the analysis of cytosolic and mitochondrial biochemistry.

The dissertation presented here proposes a comprehensive description of iron metabolism in the form of an in silico simulation of the iron exchange and its regulation for the mouse strain C57BL6. We chose this special model animal for two reasons: It is the preferred strain for the afore-mentioned genetic constructs, and it is possible to obtain most of the experimental data that are required for a quantitative description of iron metabolism. A generic cell model will be presented which comprises the main features of iron metabolism that are common to all cell types, due to the fact that every cell expresses the same set of iron-related mRNA and proteins. Specific cellular flavor is obtained by adapting the values of certain crucial parameters of the cell model accordingly. The different cell and tissue types the iron profile of which is being specified in this way are then integrated in accordance with their relative abundance into a whole-body balance sheet to which the most important regulatory signals (iron-related hormone hepcidin, erythron-related hormone erythropoietin, intracellular regulators of the IRP/IRE system) are added. We show that certain of the salient features of iron metabolism in the normal state as well as under physiological or pathological challenge are being satisfactorily simulated with quantitative approximation to experimental data. 


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