Assessment of CE-ICP/MS hyphenation for the study of uranyl/protein interactions

Recent Advances in Metalloproteomics

Interactions between proteins and metal ions and their complexes are important in many areas of the life sciences, including physiology, medicine, and toxicology. Despite the involvement of essential elements in all major processes necessary for sustaining life, metalloproteomes remain ill-defined. This is not only owing to the complexity of metalloproteomes, but also to the non-covalent character of the complexes that most essential metals form, which complicates analysis. Similar issues may also be encountered for some toxic metals. The review discusses recently developed approaches and current challenges for the study of interactions involving entire (sub-)proteomes with such labile metal ions. In the second part, transition metals from the fourth and fifth periods are examined, most of which are xenobiotic and also tend to form more stable and/or inert complexes. A large research area in this respect concerns metallodrug-protein interactions. Particular attention is paid to separation approaches, as these need to be adapted to the reactivity of the metal under consideration.

The plasma membrane-associated cation-binding protein PCaP1 of Arabidopsis thaliana is a uranyl-binding protein

Uranium (U) is a naturally-occurring radionuclide that is toxic to living organisms. Given that proteins are primary targets of U(VI), their identification is an essential step towards understanding the mechanisms of radionuclide toxicity, and possibly detoxification. Here, we implemented a chromatographic strategy including immobilized metal affinity chromatography to trap protein targets of uranyl in Arabidopsis thaliana. This procedure allowed the identification of 38 uranyl-binding proteins (UraBPs) from root and shoot extracts. Among them, UraBP25, previously identified as plasma membrane-associated cation-binding protein 1 (PCaP1), was further characterized as a protein interacting in vitro with U(VI) and other metals using spectroscopic and structural approaches, and in planta through analyses of the fate of U(VI) in Arabidopsis lines with altered PCaP1 gene expression. Our results showed that recombinant PCaP1 binds U(VI) in vitro with affinity in the nM range, as well as Cu(II) and Fe(III) in high proportions, and that Ca(II) competes with U(VI) for binding. U(VI) induces PCaP1 oligomerization through binding at the monomer interface, at both the N-terminal structured domain and the C-terminal flexible region. Finally, U(VI) translocation in Arabidopsis shoots was affected in pcap1 null-mutant, suggesting a role for this protein in ion trafficking in planta.

A Simple Fluorescence Affinity Assay to Decipher Uranyl-Binding to Native Proteins

Determining the affinity of proteins for uranyl is key to understand the toxicity of this cation and to further develop decorporation strategies. However, usual techniques to achieve that goal often require specific equipment and expertise. Here, we propose a simple, efficient, fluorescence‐based method to assess the affinity of proteins and peptides for uranyl, at equilibrium and in buffered solution. We first designed and characterized an original uranyl‐binding fluorescent probe. We then built a reference scale for uranyl affinity in solution, relying on signal quenching of our fluorescent probe in presence of high‐affinity uranyl‐binding peptides. We finally validated our approach by re‐evaluating the uranyl‐binding affinity of four native proteins. We envision that this tool will facilitate the reliable and reproducible assessment of affinities of peptides and proteins for uranyl.

Uranyl Binding to Proteins and Structural-Functional Impacts

The widespread use of uranium for civilian purposes causes a worldwide concern of its threat to human health due to the long-lived radioactivity of uranium and the high toxicity of uranyl ion (UO₂²⁺). Although uranyl–protein/DNA interactions have been known for decades, fewer advances are made in understanding their structural-functional impacts. Instead of focusing only on the structural information, this article aims to review the recent advances in understanding the binding of uranyl to proteins in either potential, native, or artificial metal-binding sites, and the structural-functional impacts of uranyl–protein interactions, such as inducing conformational changes and disrupting protein-protein/DNA/ligand interactions. Photo-induced protein/DNA cleavages, as well as other impacts, are also highlighted. These advances shed light on the structure-function relationship of proteins, especially for metalloproteins, as impacted by uranyl–protein interactions. It is desired to seek approaches for biological remediation of uranyl ions, and ultimately make a full use of the double-edged sword of uranium.

Fetuin exhibits a strong affinity for plutonium and may facilitate its accumulation in the skeleton

After entering the blood, plutonium accumulates mainly in the liver and the bones. The mechanisms leading to its accumulation in bone are, however, completely unknown. We already know that another uptake pathway not involving the transferrin-mediated pathways is suspected to intervene in the case of the liver. Fetuin, a protein playing an important role in bone metabolism, is proposed as a potential transporter of Pu from serum to bone. For the first time, the binding constants of these two proteins (transferrin and fetuin) with tetravalent plutonium at physiological pH (pH 7.0) were determined by using capillary electrophoresis (CE) coupled with inductively coupled plasma mass spectrometry (ICP-MS). Their very close values (log₁₀ K_PuTf = 26.44 ± 0.28 and log₁₀ K_PuFet = 26.20 ± 0.24, respectively) suggest that transferrin and fetuin could compete to chelate plutonium, either in the blood or directly at bone surfaces in the case of Pu deposits. We performed competition reaction studies demonstrating that the relative distribution of Pu-protein complexes is fully explained by thermodynamics. Furthermore, considering the average concentrations of transferrin and fetuin in the blood, our calculation is consistent with the bio-distribution of Pu observed in humans.

Recent advances in uranyl binding in proteins thanks to biomimetic peptides

Uranium is an element belonging to the actinide series. It is ubiquitous in rock, soil, and water. Uranium is found in the ecosystem due to mining and milling industrial activities and processing to nuclear fuel, but also to the extensive use of phosphate fertilizers. Understanding uranium binding in vivo is critical, first to deepen our knowledge of molecular events leading to chemical toxicity, but also to provide new mechanistic information useful for the development of efficient decorporation treatments to be applied in case of intoxication. The most stable form in physiological conditions is the uranyl cation (UO₂²⁺), in which uranium oxidation state is +VI. This short review presents uranyl coordination properties and chelation, and what is currently known about uranium binding to proteins. Although several target proteins have been identified, the UO₂²⁺ binding sites have barely been identified. Biomimetic approaches using model peptides are good options to shed light on high affinity uranyl binding sites in proteins. A strategy based on constrained cyclodecapeptides allowed recently to propose a tetraphosphate binding site for uranyl that provides an affinity similar to the one measured with the phosphoprotein osteopontin.

What do we know about actinides-proteins interactions?

Since the early 40s when the first research related to the development of the atomic bomb began for the Manhattan Project, actinides (An) and their association with the use of nuclear energy for civil applications, such as in the generation of electricity, have been a constant source of interest and fear. In 1962, the first Society of Toxicology (SOT), led by H. Hodge, was established at the University of Rochester (USA). It was commissioned as part of the Manhattan Project to assess the impact of nuclear weapons production on workers’ health. As a result of this initiative, the retention and excretion rates of radioactive heavy metals, their physiological impact in the event of acute exposure and their main biological targets were assessed. In this context, the scientific community began to focus on the role of proteins in the transportation and in vivo accumulation of An. The first studies focused on the identification of these proteins. Thereafter, the continuous development of physico-chemical characterization techniques has made it possible to go further and specify the modes of interaction with proteins from both a thermodynamic and structural point of view, as well as from the point of view of their biological activity. This article reviews the work performed in this area since the Manhattan Project. It is divided into three parts: first, the identification of the most affine proteins; second, the study of the affinity and structure of protein-An complexes; and third, the impact of actinide ligation on protein conformation and function.

Phosphate-Rich Biomimetic Peptides Shed Light on High-Affinity Hyperphosphorylated Uranyl Binding Sites in Phosphoproteins

Some phosphoproteins such as osteopontin (OPN) have been identified as high-affinity uranyl targets. However, the binding sites required for interaction with uranyl and therefore involved in its toxicity have not been identified in the whole protein. The biomimetic approach proposed here aimed to decipher the nature of these sites and should help to understand the role of the multiple phosphorylations in UO₂ ²⁺ binding. Two hyperphosphorylated cyclic peptides, pS₁₆₈ and pS₁₃₆₈ containing up to four phosphoserine (pSer) residues over the ten amino acids present in the sequences, were synthesized with all reactions performed in the solid phase, including post-phosphorylation. These β-sheet-structured peptides present four coordinating residues from four amino acid side chains pointing to the metal ion, either three pSer and one glutamate in pS₁₆₈ or four pSer in pS₁₃₆₈ . Significantly, increasing the number of pSer residues up to four in the cyclodecapeptide scaffolds produced molecules with an affinity constant for UO₂ ²⁺ that is as large as that reported for osteopontin at physiological pH. The phosphate-rich pS₁₃₆₈ can thus be considered a relevant model of UO₂ ²⁺ coordination in this intrinsically disordered protein, which wraps around the metal ion to gather four phosphate groups in the UO₂ ²⁺ coordination sphere. These model hyperphosphorylated peptides are highly selective for UO₂ ²⁺ with respect to endogenous Ca²⁺ , which makes them good starting structures for selective UO₂ ²⁺ complexation.

Is hydroxypyridonate 3,4,3-LI(1,2-HOPO) a good competitor of fetuin for uranyl metabolism?

Identification of stable HOPO–UO₂²⁺–fetuin ternary complexes after a chromatographic separation process.

Intracellular uranium distribution: Comparison of cryogenic fixation versus chemical fixation methods for SIMS analysis

Localization of uranium within cells is mandatory for the comprehension of its cellular mechanism of toxicity. Secondary Ion Mass Spectrometry (SIMS) has recently shown its interest to detect and localize uranium at very low levels within the cells. This technique requires a specific sample preparation similar to the one used for Transmission Electronic Microscopy, achieved by implementing different chemical treatments to preserve as much as possible the living configuration uranium distribution into the observed sample. This study aims to compare the bioaccumulation sites of uranium within liver or kidney cells after chemical fixation and cryomethods preparations of the samples: SIMS analysis of theses samples show the localization of uranium soluble forms in the cell cytoplasm and nucleus with a more homogenous distribution when using cryopreparation probably due to the diffusible portion of uranium inside the cytoplasm.

Investigation of uranium interactions with calcium phosphate-binding proteins using ICP/MS and CE-ICP/MS

During long-term exposure, uranium accumulates in bone. Since uranium in U(vi) complexes shares similar coordination properties to calcium, this toxicant is assumed to be exchanged with calcium ions at the surfaces of bone mineral crystals. Recently, two proteins involved in bone turnover, fetuin A and osteopontin, were shown to exhibit a high affinity for U(vi). A common biochemical feature of both fetuin A and osteopontin is their inhibiting role in calcium phosphate precipitation. Therefore it is conceivable that complexation of U(vi) with these proteins may alter their interaction with calcium and/or calcium phosphate. Quantitative analyses of calcium, phosphorus and uranium performed using inductively coupled plasma/mass spectrometry (ICP/MS) demonstrated the inhibition of the precipitation of calcium phosphate by fetuin A and osteopontin for 2 h. In addition, the presence of U(vi) did not seem to alter the duration of this process. However, dynamic light scattering studies revealed that the size of the colloidal particles formed with osteopontin was altered by the presence of U(vi) in a concentration-dependent manner. Finally, using hyphenated capillary electrophoresis-ICP/MS (CE-ICP/MS), we showed that in these systems, at a low concentration of U(vi) (protein : U(vi) 8 : 1), U(vi) might remain in solution by forming a complex with proteins and not by sequestration of a precipitate of either autunite or uranyl orthophosphate.

Cyclic Phosphopeptides to Rationalize the Role of Phosphoamino Acids in Uranyl Binding to Biological Targets

The specific molecular interactions responsible for uranium toxicity are not yet understood. The uranyl binding sites in high-affinity target proteins have not been identified yet and the involvement of phosphoamino acids is still an important question. Short cyclic peptide sequences, with three glutamic acids and one phosphoamino acid, are used as simple models to mimic metal binding sites in phosphoproteins and to help understand the mechanisms involved in uranium toxicity. A combination of peptide design and synthesis, analytical chemistry, extended X-ray absorption fine structure (EXAFS) spectroscopy, and DFT calculations demonstrates the involvement of the phosphate group in the uranyl coordination sphere together with the three carboxylates of the glutamate moieties. The affinity constants measured with a reliable analytical competitive approach at physiological pH are significantly enhanced owing to the presence of the phosphorous moiety. These findings corroborate the importance of phosphoamino acids in uranyl binding in proteins and the relevance of considering phosphoproteins as potential uranyl targets in vivo.

Reactivity of U-associated osteopontin with lactoferrin: a one-to-many complex

A highly-simplified scenario of LF/U-fOPN interaction. The U content of the U-fOPN complexes refers to the CE-ICP/MS experiments.

Evidence of isotopic fractionation of natural uranium in cultured human cells

The study of the isotopic fractionation of endogen elements and toxic heavy metals in living organisms for biomedical applications, and for metabolic and toxicological studies, is a cutting-edge research topic. This paper shows that human neuroblastoma cells incorporated small amounts of uranium (U) after exposure to 10 µM natural U, with preferential uptake of the ²³⁵U isotope with regard to ²³⁸U. Efforts were made to develop and then validate a procedure for highly accurate n(²³⁸U)/n(²³⁵U) determinations in microsamples of cells. We found that intracellular U is enriched in ²³⁵U by 0.38 ± 0.13‰ (2σ, n = 7) relative to the exposure solutions. These in vitro experiments provide clues for the identification of biological processes responsible for uranium isotopic fractionation and link them to potential U incorporation pathways into neuronal cells. Suggested incorporation processes are a kinetically controlled process, such as facilitated transmembrane diffusion, and the uptake through a high-affinity uranium transport protein involving the modification of the uranyl (UO₂²⁺) coordination sphere. These findings open perspectives on the use of isotopic fractionation of metals in cellular models, offering a probe to track uptake/transport pathways and to help decipher associated cellular metabolic processes.

In vitro induction and proteomics characterisation of a uranyl-protein interaction network in bovine serum

Uranyl ions (UO2(2+)) were shown to interact with a number of foetal serum proteins, leading to the formation of a complex that could be isolated by ultracentrifugation. The molecular weight of the complex was estimated based on size-exclusion chromatography as 650 000 Da. Online ICP AES detection indicated that UO2(2+) in the complex co-eluted with minor amounts of calcium and phosphorous, but not with magnesium. A 1D gel electrophoresis of the U-complex produced more than 10 bands of similar intensity compared with only 2-3 intense bands corresponding to the main serum proteins in the control serum, indicative of the specific interaction of UO2(2+) with minor proteins. A proteomics approach allowed for the identification of 74 proteins in the complex. Analysis of the protein-protein interaction network in the UO2(2+) complex identified 32 proteins responsible for protein-protein complex formation and 34 with demonstrated ion-binding function, suggesting that UO2(2+) stimulates the formation of protein functional networks rather than using a particular molecule as its target.