Copper(II) complexes of rat amylin fragments

Role of mass spectrometry in the study of interactions between amylin and metal ions

Amylin (islet amyloid polypeptide [IAPP]) is a neuroendocrine hormone synthesized with insulin in the beta cells of pancreatic islets. The two hormones act in different ways: in fact insulin triggers glucose uptake in muscle and liver cells, removing glucose from the bloodstream and making it available for energy use and storage, while amylin regulates glucose homeostasis. Aside these positive physiological aspects, human amyloid polypeptide (hIAPP) readily forms amyloid in vitro. Amyloids are aggregates of proteins and in the human body amyloids are considered responsible of the development of various diseases. These aspects have been widely described and discussed in literature and to give a view of the highly complexity of this biochemical behavior the different physical, chemical, biological and medical aspects are shortly described in this review. It is strongly affected by the presence on metal ions, responsible for or inhibiting the formation of fibrils. Mass spectrometry resulted (and still results) to be a particularly powerful tool to obtain valid and effective experimental data to describe the hIAPP behavior. Aside classical approaches devoted to investigation on metal ion-hIAPP structures, which reflects on the identification of metal-protein interaction site(s) and of possible metal-induced conformational changes of the protein, interesting results have been obtained by ion mobility mass spectrometry, giving, on the basis of collisional cross-section data, information on both the oligomerization processes and the conformation changes. Laser ablation electrospray ionization-ion mobility spectrometry-mass spectrometry (LAESI-IMS-MS), allowed to obtain information on the binding stoichiometry, complex dissociation constant, and the oxidation state of the copper for the amylin-copper interaction. Alternatively to inorganic ions, small organic molecules have been tested by ESI-IMS-MS as inhibitor of amyloid assembly. Also in this case the obtained data demonstrate the validity of the ESI-IMS-MS approach as a high-throughput screen for inhibitors of amyloid assembly, providing valid information concerning the identity of the interacting species, the nature of binding and the effect of the ligand on protein aggregation. Effects of Cu²⁺ and Zn²⁺ ions in the degradation of human and murine IAPP by insulin-degrading enzyme were studied by liquid chromatography/mass spectrometry (LC/MS). The literature data show that mass spectrometry is a highly valid and effective tool in the study of the amylin behavior, so to individuate medical strategies to avoid the undesired formation of amyloids in in vivo conditions.

Thermodynamic surprises of Cu(II)-amylin analogue complexes in membrane mimicking solutions

Membrane environment often has an important effect on the structure, and therefore also on the coordination mode of biologically relevant metal ions. This is also true in the case of Cu(II) coordination to amylin analogues—rat amylin, amylin_1–19, pramlintide and Ac-pramlintide, which offer N-terminal amine groups and/or histidine imidazoles as copper(II) anchoring sites. Complex stabilities are comparable, with the exception of the very stable Cu(II)–amylin_1–19, which proves that the presence of the amylin C-terminus lowers its affinity for copper(II); although not directly involved, its appropriate arrangement sterically prevents early metal binding. Most interestingly, in membrane-mimicking solution, the Cu(II) affinities of amylin analogues are lower than the ones in water, probably due to the crowding effect of the membrane solution and the fact that amide coordination occurs at higher pH, which happens most likely because the α-helical structure, imposed by the membrane-mimicking solvent, prevents the amides from binding at lower pH, requiring a local unwinding of the α-helix.

The human adenovirus type 5 E1B 55kDa protein interacts with RNA promoting timely DNA replication and viral late mRNA metabolism

The E1B 55kDa produced by human adenovirus type 5 is a multifunctional protein that participates in the regulation of several steps during the viral replication cycle. Previous studies suggest this protein plays an important role in postranscriptional regulation of viral and cellular gene expression, as it is required for the selective accumulation of maximal levels of viral late mRNA in the cytoplasm of the infected cell; however the molecular mechanisms that are altered or regulated by this protein have not been elucidated. A ribonucleoprotein motif that could implicate the direct interaction of the protein with RNA was initially predicted and tested in vitro, but the interaction with RNA could not be detected in infected cells, suggesting the interaction may be weak or transient. Here it was determined that the E1B 55kDa interacts with RNA in the context of the viral infection in non-transformed human cells, and its contribution to the adenovirus replication cycle was evaluated. Using recombinant adenoviruses with amino acid substitutions or a deletion in the ribonucleoprotein motif the interaction of E1B 55kDa with RNA was found to correlate with timely and efficient viral DNA replication and viral late mRNA accumulation and splicing.

Metal-dependent hormone function: the emerging interdisciplinary field of metalloendocrinology

For over 100 years, there has been an incredible amount of knowledge amassed concerning hormones in the endocrine system and their central role in human health. Hormones represent a diverse group of biomolecules that are released by glands, communicate signals to their target tissue, and are regulated by feedback loops to maintain organism health. Many disease states, such as diabetes and reproductive disorders, stem from misregulation or dysfunction of hormones. Increasing research is illuminating the intricate roles of metal ions in the endocrine system where they may act advantageously in concert with hormones or deleteriously catalyze hormone-associated disease states. As the critical role of metal ions in the endocrine system becomes more apparent, it is increasingly important to untangle the complex mechanisms underlying the connections between inorganic biochemistry and hormone function to understand and control endocrinological phenomena. This tutorial review harmonizes the interdisciplinary fields of endocrinology and inorganic chemistry in the newly-termed field of "metalloendocrinology". We describe examples linking metals to both normal and aberrant hormone function with a focus on highlighting insight to molecular mechanisms. Hormone activities related to both essential metal micronutrients, such as copper, iron, zinc, and calcium, and disruptive nonessential metals, such as lead and cadmium are discussed.

DFT Protocol for EPR Prediction of Paramagnetic Cu(II) Complexes and Application to Protein Binding Sites

With the aim to provide a general protocol to interpret electron paramagnetic resonance (EPR) spectra of paramagnetic copper(II) coordination compounds, density functional theory (DFT) calculations of spin Hamiltonian parameters g and A for fourteen Cu(II) complexes with different charges, donor sets, and geometry were carried out using ORCA software. The performance of eleven functionals was tested, and on the basis of the mean absolute percent deviation (MAPD) and standard deviation (SD), the ranking of the functionals for Az is: B3LYP > B3PW91 ~ B3P86 > PBE0 > CAM-B3LYP > TPSSh > BH and HLYP > B2PLYP > MPW1PW91 > ω-B97x-D >> M06; and for gz is: PBE0 > BH and HLYP > B2PLYP > ω-B97x-D > B3PW91~B3LYP~B3P86 > CAM-B3LYP > TPSSh~MPW1PW91 >> M06. With B3LYP the MAPD with respect to A z exp t l is 8.6% with a SD of 4.2%, while with PBE0 the MAPD with respect to g z exp t l is 2.9% with a SD of 1.1%. The results of the validation confirm the fundamental role of the second order spin-orbit contribution to Az. The computational procedure was applied to predict the values of gz and Az of the adducts formed by Cu(II) with albumin and two fragments of prion protein, 106–126 and 180–193.

Cu and Zn coordination to amyloid peptides: From fascinating chemistry to debated pathological relevance

Several diseases share misfolding of different peptides and proteins as a key feature for their development. This is the case of important neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and type II diabetes mellitus. Even more, metal ions such as copper and zinc might play an important role upon interaction with amyloidogenic peptides and proteins, which could impact their aggregation and toxicity abilities. In this review, the different coordination modes proposed for copper and zinc with amyloid-β, α-synuclein and IAPP will be reviewed as well as their impact on the aggregation, and ROS production in the case of copper. In addition, a special focus will be given to the mutations that affect metal binding and lead to familial cases of the diseases. Different modifications of the peptides that have been observed in vivo and could be relevant for the coordination of metal ions are also described.

Complex formation of nickel(ii) and zinc(ii) ions with peptide fragments of rat amylin

For nickel(ii)-SSNX-NH₂ an equilibrium between the common (NH₂,3N⁻(peptide)) and (NH₂,2N⁻(peptide),N⁻(asparagine)) coordination modes was observed in a basic solution resulting in an increased stability of the complexes.

Regulation of the assembly and amyloid aggregation of murine amylin by zinc

The secretory granule of the pancreatic β-cells is a zinc-rich environment copopulated with the hormones amylin and insulin. The human amylin is shown to interact with zinc ions with major contribution from the single histidine residue, which is absent in amylin from other species such as cat, rhesus and rodents. We report here the interaction of murine amylin with zinc ions in vitro. The self-assembly of murine amylin is tightly regulated by zinc and pH. Ion mobility mass spectrometry revealed zinc interaction with monomers and oligomers. Nuclear magnetic resonance confirms the binding of zinc to murine amylin. The aggregation process of murine amylin into amyloid fibrils is accelerated by zinc. Collectively these data suggest a general role of zinc in the modulation of amylin variants oligomerization and amyloid fibril formation.

Rapid assessment of human amylin aggregation and its inhibition by copper(II) ions by laser ablation electrospray ionization mass spectrometry with ion mobility separation

Native electrospray ionization (ESI) mass spectrometry (MS) is often used to monitor noncovalent complex formation between peptides and ligands. The relatively low throughput of this technique, however, is not compatible with extensive screening. Laser ablation electrospray ionization (LAESI) MS combined with ion mobility separation (IMS) can analyze complex formation and provide conformation information within a matter of seconds. Islet amyloid polypeptide (IAPP) or amylin, a 37-amino acid residue peptide, is produced in pancreatic beta-cells through proteolytic cleavage of its prohormone. Both amylin and its precursor can aggregate and produce toxic oligomers and fibrils leading to cell death in the pancreas that can eventually contribute to the development of type 2 diabetes mellitus. The inhibitory effect of the copper(II) ion on amylin aggregation has been recently discovered, but details of the interaction remain unknown. Finding other more physiologically tolerated approaches requires large scale screening of potential inhibitors. Here, we demonstrate that LAESI-IMS-MS can reveal the binding stoichiometry, copper oxidation state, and the dissociation constant of human amylin-copper(II) complex. The conformations of hIAPP in the presence of copper(II) ions were also analyzed by IMS, and preferential association between the β-hairpin amylin monomer and the metal ion was found. The copper(II) ion exhibited strong association with the -HSSNN- residues of the amylin. In the absence of copper(II), amylin dimers were detected with collision cross sections consistent with monomers of β-hairpin conformation. When copper(II) was present in the solution, no dimers were detected. Thus, the copper(II) ions disrupt the association pathway to the formation of β-sheet rich amylin fibrils. Using LAESI-IMS-MS for the assessment of amylin-copper(II) interactions demonstrates the utility of this technique for the high-throughput screening of potential inhibitors of amylin oligomerization and fibril formation. More generally, this rapid technique opens the door for high-throughput screening of potential inhibitors of amyloid protein aggregation.

Potentiometric and spectroscopic studies on the copper(II) complexes of rat amylin fragments. The anchoring ability of specific non-coordinating side chains

Copper(ii) complexes of peptides modelling the sequence of the 17-22 residues of rat amylin have been studied by potentiometric, UV-Vis, CD and ESR spectroscopic methods. The peptides were synthesized in N-terminally free forms, NH2-VRSSNN-NH2, NH2-VRSSAA-NH2, NH2-VRAANN-NH2, NH2-VRSS-NH2, NH2-SSNN-NH2, NH2-SSNA-NH2 and NH2-AANN-NH2, providing a possibility for the comparison of the metal binding abilities of the amino terminus and the -SSNN- domain. The amino terminus was the primary ligating site in all cases and the formation of only mononuclear complexes was obtained for the tetrapeptides. The thermodynamic stability of the (NH2, N(-), N(-)) coordinated complexes was, however, enhanced by the asparaginyl moiety in the case of NH2-SSNN-NH2, NH2-SSNA-NH2 and NH2-AANN-NH2. Among the hexapeptides the formation of dinuclear complexes was characteristic for NH2-VRSSNN-NH2 demonstrating the anchoring ability of the -SSNN- (SerSerAsnAsn) domain. The complexes of the heptapeptide NH2-GGHSSNN-NH2 were also studied and the data supported the above mentioned anchoring ability of the -SSNN- site.

On the Environmental Factors Affecting the Structural and Cytotoxic Properties of IAPP Peptides

Pancreatic islets in type 2 diabetes mellitus (T2DM) patients are characterized by reduced β-cells mass and diffuse extracellular amyloidosis. Amyloid deposition involves the islet amyloid polypeptide (IAPP), a neuropancreatic hormone cosecreted with insulin by β-cells. IAPP is physiologically involved in glucose homeostasis, but it may turn toxic to β-cells owing to its tendency to misfold giving rise to oligomers and fibrils. The process by which the unfolded IAPP starts to self-assemble and the overall factors promoting this conversion are poorly understood. Other open questions are related to the nature of the IAPP toxic species and how exactly β-cells die. Over the last decades, there has been growing consensus about the notion that early molecular assemblies, notably small hIAPP oligomers, are the culprit of β-cells decline. Numerous environmental factors might affect the conformational, aggregation, and cytotoxic properties of IAPP. Herein we review recent progress in the field, focusing on the influences that membranes, pH, and metal ions may have on the conformational conversion and cytotoxicity of full-length IAPP as well as peptide fragments thereof. Current theories proposed for the mechanisms of toxicity will be also summarized together with an outline of the underlying molecular links between IAPP and amyloid beta (Aβ) misfolding.

The role of copper(II) in the aggregation of human amylin

Amylin is a 37-residue peptide hormone produced by the islet β-cells of pancreas and the formation of amylin aggregates is strongly associated with β-cell degeneration in type 2 diabetes, as demonstrated by more than 95% of patients exhibiting amylin amyloid upon autopsy. It is widely recognized that metal ions such as copper(II) have been implicated in the aggregation process of amyloidogenic peptides such as Aβ and α-synuclein and there is evidence that amylin self-assembly is also largely affected by copper(II). For this reason, in this work, the role of copper(II) in the aggregation of amylin has been investigated by several different experimental approaches. Mass spectrometric investigations show that copper(II) induces significant changes in the amylin structure, which decrease the protein fibrillogenesis as observed by ThT measurements. Accordingly, solid-state NMR experiments together with computational analysis carried out on a model amylin fragment confirmed the non-fibrillogenic nature of the copper(II) induced aggregated structure. Finally, the presence of copper(II) is also shown to have a major influence on amylin proneness to be degraded by proteases and cytotoxicity studies on different cell cultures are reported.

The role of copper(II) and zinc(II) in the degradation of human and murine IAPP by insulin-degrading enzyme

Amylin or islet amyloid polypeptide (IAPP) is a 37-residue peptide hormone secreted from the pancreatic islets into the blood circulation and is cleared by peptidases in the kidney. IAPP aggregates are strongly associated with β-cell degeneration in type 2 diabetes, as demonstrated by the fact that more than 95% of patients exhibit IAPP amyloid upon autopsy. Recently, it has been reported that metal ions such as copper(II) and zinc(II) are implicated in the aggregation of IAPP as well as able to modulate the proteolytic activity of IAPP degrading enzymes. For this reason, in this work, the role of the latter metal ions in the degradation of IAPP by insulin-degrading enzyme (IDE) has been investigated by a chromatographic and mass spectrometric combined method. The latter experimental approach allowed not only to assess the overall metal ion inhibition of the human and murine IAPP degradation by IDE but also to have information on copper- and zinc-induced changes in IAPP aggregation. In addition, IDE cleavage site preferences in the presence of metal ions are rationalized as metal ion-induced changes in substrate accessibility.