Iron and zinc sensing in cells and the body

The Iron Metabolism Group at King's College London was founded in 1998 and has a long tradition of organizing focused, one day symposia on various topics related to human metal metabolism in health and disease. In December 2013, it convened the 45th such conference on “Iron and zinc sensing in cells and the body.” This meeting explored how biological cells sense whether or not their metal concentrations are adequate, and how chemical biology is employed to investigate metabolically active metal ion concentrations using fluorescent probes and sensors. The presenters at this conference have agreed to summarize their work, which we deemed to be sufficiently novel and original to deserve wider attention. The articles collected in this issue of Metallomics address the different levels of control of cellular iron, the molecular mechanisms of iron sites in proteins in regulation, and the integration of control into metabolism. The meeting also explored how free zinc(II) and iron(II) ion concentrations are measured fluorimetrically with low molecular weight probes and genetically encoded protein sensors.

Among the three articles that focus on iron, the first deals with measuring iron(II) using chemical probes, while the other two deal with how cells sense and control their iron concentrations and how this control of iron integrates with metabolism.

Robert Hider (Ma, Abate, and Hider) provides a so far unique summary of the chemistry and biology of fluorescent chelating agents (iron-sensitive probes) to measure the cellular labile iron pools (LIP). The concentration of the cytosolic pool is between 0.2 and 5 μM, with Fe(II)/glutathione species as its major component. The quantitation of lysosomal and mitochondrial pools with probes targeted to these compartments underlies considerable uncertainty, and probes for other subcellular compartments have yet to be developed.

Lukas Kühn discusses the central and differential functions of the two iron-regulatory proteins, IRP1 and IRP2, in the post-transcriptional control of cellular iron homeostasis. These proteins bind to the iron-regulatory elements (IREs) in ferritin and transferrin mRNA and control their translation, therefore affecting the uptake and storage of iron, but also to the mRNA of a splice variant of DMT-1 (divalent metal transporter 1) and ferroportin. This control extends to systemic iron homeostasis in tissues and integrates with oxygen metabolism and other basic cellular control mechanisms. Iron coordination chemistry is involved in the mechanism of iron-sensing proteins (and thus presumably in controlling LIP levels) through the assembly of a 4Fe–4S cluster to convert IRP1 into an aconitase inactivating mRNA binding and sensing oxygen and iron through the assembly of a hemerythrin dinculear iron center in FBXL5, which induces the degradation of IRP2, a process that also abrogates mRNA binding.

Andrew McKie (Simpson and McKie) adds to the theme of iron regulation and integration into metabolism by including transcriptional mechanisms, and expands on the theme of how sensing of the two elements, oxygen and iron, is interrelated. Iron and oxygen dependent prolyl hydroxylases (PHDs) have a central role in regulating the hypoxia-inducible factors HIF1α and HIF2α, which serve an additional role in iron metabolism e.g. as transcriptional regulators of the iron transporters ferroportin and Dcytb, and together with the IRPs they form an iron-regulatory network.

There are also three articles focusing on zinc. The first two deal with the fluorescence techniques employed to measure free zinc ion concentrations, which are orders of magnitude lower than free iron (LIP) concentrations. The last article deals with the factors that determine the affinity of zinc in the coordination environments of proteins, and how these affinities relate to the free zinc ion concentrations and their fluctuations, which are important for zinc(II) ions serving as signalling ions in intra- and intercellular regulation.

Maarten Merkx (Hessels and Merkx) reviews the development and applications of genetically encoded FRET (fluorescence resonance energy transfer) sensors for zinc(II) ions. These sensors can be engineered for zinc affinity, fluorescence properties and targeting in the cell. As they are based on the metal specificity of native proteins, they have distinct advantages in affording control of cellular concentrations and localization.

Wolfgang Maret, after a short introduction into how the cell controls cellular zinc homeostasis with numerous proteins, compares low molecular weight zinc-chelating agents (probes) and genetically encoded proteins (sensors) for zinc(II) ions and their limitations in the quantitative determination of cellular zinc ion concentrations. As pointed out in the Merkx article, cytosolic concentrations are picomolar but vast differences pertain in measurements of zinc ion concentrations in subcellular compartments.

Artur Krezel (Kochańczyk, Drodz and Krężel) provides an overview of the zinc coordination environments in proteins, discusses the affinities of proteins for zinc, and identifies the structural elements that affect this affinity. He then relates zinc binding to function, in particular, in sites where zinc has to associate and dissociate as part of its function in regulating the proteins and their interactions.

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