The solution structure of rat Aβ-(1–28) and its interaction with zinc ion: insights into the scarcity of amyloid deposition in aged rat brain

Molecular details of aluminium-amyloid β peptide interaction by nuclear magnetic resonance

Alzheimer's disease (AD) is a devastating neurodegenerative condition that poses major challenges to human health. Both amyloid β (Aβ) and metal ions such as aluminium are implicated in the development of AD. By the means of NMR, the interactions of Al³⁺ with Aβ_1-28 peptide as well as the Aβ_1-28 analogues were studied, and the key binding sites of Al³⁺ in Aβ determined. NMR data showed Al³⁺ interacts with Aβ_1-28 at the NH and αH of numerous residues by exhibiting upfield shifts. Using Aβ analogues where His6, His13 and His14 were individually replaced by alanine residue(s), including Aβ H6A, Aβ H13A, Aβ H14A, and Aβ H6,13,14A, the results demonstrated that the histidine residues (His6, His13 and His14) and N-terminal Asp1 were involved in the Al³⁺ coordination. These findings provide, for the first time, the details of the molecular interaction between Al³⁺ and Aβ, which points to the potential role of Al³⁺ in Aβ aggregation, hence in AD development.

Distinct impact of glycation towards the aggregation and toxicity of murine and human amyloid-β

Glycation of human Aβ (hAβ) is implicated to induce the deposition of amyloid aggregates found in the Alzheimer's disease (AD)-affected brain. Murine Aβ (mAβ) differs from hAβ in three different amino acid residues (Gly5, Phe10, and Arg13) and is less likely to form amyloid aggregates. Herein, we report that the advanced glycated end products of mAβ40 over hAβ40 are distinctly generated. The different glycation between the two peptides can govern their aggregation kinetics, structural transition, and cytotoxicity.

Tailoring the nuclear Overhauser effect for the study of small and medium-sized molecules by solvent viscosity manipulation

The nuclear Overhauser effect (NOE) is a consequence of cross-relaxation between nuclear spins mediated by dipolar coupling. Its sensitivity to internuclear distances has made it an increasingly important tool for the determination of through-space atom proximity relationships within molecules of sizes ranging from the smallest systems to large biopolymers. With the support of sophisticated FT-NMR techniques, the NOE plays an essential role in structure elucidation, conformational and dynamic investigations in liquid-state NMR. The efficiency of magnetization transfer by the NOE depends on the molecular rotational correlation time, whose value depends on solution viscosity. The magnitude of the NOE between ¹H nuclei varies from +50% when molecular tumbling is fast to -100% when it is slow, the latter case corresponding to the spin diffusion limit. In an intermediate tumbling regime, the NOE may be vanishingly small. Increasing the viscosity of the solution increases the motional correlation time, and as a result, otherwise unobservable NOEs may be revealed and brought close to the spin diffusion limit. The goal of this review is to report the resolution of structural problems that benefited from the manipulation of the negative NOE by means of viscous solvents, including examples of molecular structure determination, conformation elucidation and mixture analysis (the ViscY method).

A benchmark of optimally folded protein structures using integer programming and the 3D-HP-SC model

The Protein Structure Prediction (PSP) problem comprises, among other issues, forecasting the three-dimensional native structure of proteins using only their primary structure information. Most computational studies in this area use synthetic data instead of real biological data. However, the closer to the real-world, the more the impact of results and their applicability. This work presents 17 real protein sequences extracted from the Protein Data Bank for a benchmark to the PSP problem using the tri-dimensional Hydrophobic-Polar with Side-Chains model (3D-HP-SC). The native structure of these proteins was found by maximizing the number of hydrophobic contacts between the side-chains of amino acids. The problem was treated as an optimization problem and solved by means of an Integer Programming approach. Although the method optimally solves the problem, the processing time has an exponential trend. Therefore, due to computational limitations, the method is a proof-of-concept and it is not applicable to large sequences. For unknown sequences, an upper bound of the number of hydrophobic contacts (using this model) can be found, due to a linear relationship with the number of hydrophobic residues. The comparison between the predicted and the biological structures showed that the highest similarity between them was found with distance thresholds around 5.2-8.2 Å. Both the dataset and the programs developed will be freely available to foster further research in the area.

Zn2+ Aggravates Tau Aggregation and Neurotoxicity

Alzheimer's disease (AD) is a neurodegenerative disease with high morbidity that has received extensive attention. However, its pathogenesis has not yet been completely elucidated. It is mainly related to β-amyloid protein deposition, the hyperphosphorylation of tau protein, and the loss of neurons. The main function of tau is to assemble tubulin into stable microtubules. Under pathological conditions, tau is hyperphosphorylated, which is the major component of neurofibrillary tangles (NFT) in AD. There is considerable evidence showing that the dyshomeostasis of Zn2+ is closely related to the development of AD. Herein, by using the third repeat unit of the microtubule-binding domain of tau (tau-R3), we investigated the effect of Zn2+ on the aggregation and neurotoxicity of tau. Experimental results showed that tau-R3 probably bound Zn2+ via its Cys residue with moderate affinity (association constant (Ka) = 6.82 ± 0.29 × 10⁴ M-1). Zn2+ accelerated tau-R3 aggregation and promoted tau-R3 to form short fibrils and oligomers. Compared with tau-R3, Zn2+-tau-R3 aggregates were more toxic to Neuro-2A (N2A) cells and induced N2A cells to produce higher levels of reactive oxygen species (ROS). The dendrites and axons of Zn2+-tau-R3-treated neurons became fewer and shorter, resulting in a large number of neuronal deaths. In addition, both tau-R3 and Zn2+-tau-R3 aggregates were found to be taken up by N2A cells, and more Zn2+-tau-R3 entered the cells compared with tau-R3. Our data demonstrated that Zn2+ can aggravate tau-R3 aggregation and neurotoxicity, providing clues to understand the relationship between Zn2+ dyshomeostasis and the etiology of Alzheimer's disease.

Alzheimer's disease and metabolic syndrome: A link from oxidative stress and inflammation to neurodegeneration

Alzheimer's disease (AD) is the most common cause of dementia and one of the most important causes of morbidity and mortality among the aging population. AD diagnosis is made post-mortem, and the two pathologic hallmarks, particularly evident in the end stages of the illness, are amyloid plaques and neurofibrillary tangles. Currently, there is no curative treatment for AD. Additionally, there is a strong relation between oxidative stress, metabolic syndrome, and AD. The high levels of circulating lipids and glucose imbalances amplify lipid peroxidation that gradually diminishes the antioxidant systems, causing high levels of oxidative metabolism that affects cell structure, leading to neuronal damage. Accumulating evidence suggests that AD is closely related to a dysfunction of both insulin signaling and glucose metabolism in the brain, leading to an insulin-resistant brain state. Four drugs are currently used for this pathology: Three FDA-approved cholinesterase inhibitors and one NMDA receptor antagonist. However, wide varieties of antioxidants are promissory to delay or prevent the symptoms of AD and may help in treating the disease. Therefore, therapeutic efforts to achieve attenuation of oxidative stress could be beneficial in AD treatment, attenuating Aβ-induced neurotoxicity and improve neurological outcomes in AD. The term inflammaging characterizes a widely accepted paradigm that aging is accompanied by a low-grade chronic up-regulation of certain pro-inflammatory responses in the absence of overt infection, and is a highly significant risk factor for both morbidity and mortality in the elderly.

Revisiting rodent models: Octodon degus as Alzheimer's disease model?

Alzheimer's disease primarily occurs as sporadic disease and is accompanied with vast socio-economic problems. The mandatory basic research relies on robust and reliable disease models to overcome increasing incidence and emerging social challenges. Rodent models are most efficient, versatile, and predominantly used in research. However, only highly artificial and mostly genetically modified models are available. As these 'engineered' models reproduce only isolated features, researchers demand more suitable models of sporadic neurodegenerative diseases. One very promising animal model was the South American rodent Octodon degus, which was repeatedly described as natural 'sporadic Alzheimer's disease model' with 'Alzheimer's disease-like neuropathology'. To unveil advantages over the 'artificial' mouse models, we re-evaluated the age-dependent, neurohistological changes in young and aged Octodon degus (1 to 5-years-old) bred in a wild-type colony in Germany. In our hands, extensive neuropathological analyses of young and aged animals revealed normal age-related cortical changes without obvious signs for extensive degeneration as seen in patients with dementia. Neither significant neuronal loss nor enhanced microglial activation were observed in aged animals. Silver impregnation methods, conventional, and immunohistological stains as well as biochemical fractionations revealed neither amyloid accumulation nor tangle formation. Phosphoepitope-specific antibodies against tau species displayed similar intraneuronal reactivity in both, young and aged Octodon degus.In contrast to previous results, our study suggests that Octodon degus born and bred in captivity do not inevitably develop cortical amyloidosis, tangle formation or neuronal loss as seen in Alzheimer's disease patients or transgenic disease models.

A possible significant role of zinc and GPR39 zinc sensing receptor in Alzheimer disease and epilepsy

Zinc the essential trace element, plays a significant role in the brain development and in the proper brain functions at every stage of life. Misbalance of zinc (Zn(2+)) ions in the central nervous system is involved in the pathogenesis of numerous neurodegenerative disorders such as Alzheimer's disease, Depression, and Epilepsy. In brain, Zn(2+) has been identified as a ligand, capable of activating and inhibiting the receptors including the NMDA-type glutamate receptors (NMDARs), GABAA receptors, nicotinic acetylcholine receptors (nAChRs), glycine receptors (glyR) and serotonin receptors (5-HT3). Recently GPR39 has been identified as a zinc-specific receptor, widely expressed in brain tissues including the frontal cortex, amygdala, and hippocampus. GPR39, when binding with Zn(2+) has shown promising therapeutic potentials. This review presents current knowledge regarding the role of GPR39 zinc sensing receptor in brain, with a focus on Alzheimer's disease and Epilepsy. Although the results are encouraging, further research is needed to clarify zinc and GPR39 role in the treatment of Alzheimer's disease and Epilepsy.

[Cytokines neuroinflammatory reaction to the action of homoaggregatic and liposomal forms of b-amyloid 1-40 in rats]

An injection model of preclinical stages of Alzheimer's disease has been reproduced in rats. It was accompanied by the decrease in the latent period of conditioned reflex avoidance, increasing levels of endogenous b-amyloid peptide 1-40 and activation of inflammatory cytokines (IL-1b, TNF-a, IL-6, IL-10) in the cerebral cortex, hippocampus and blood serum of experimental animals. We belive that changes identified at the biochemical level are prerequisite to modulate neuronal functions in rats induced by Ab40_Human administration. The toxic effect of exogenous b-amyloid 1-40 homoaggregates caused intense response of the cytokine system, while its liposomal form caused the soft information signal to the activation of innate immunity.

Nanopore investigation of the stereoselective interactions between Cu(2+) and D,L-histidine amino acids engineered into an amyloidic fragment analogue

Stereochemistry is an essential theme for a number of industries and applications, constructed around discriminating various chiral enantiomers, including amino acids, chiral metal complexes, and drugs. In this work, we designed a set of peptide mutants of the human amyloidic Aβ1-16 sequence, known to display an effective Cu(2+) coordinating pocket provided mainly by the intramolecular His-6, His-13, and His-14 residues, that were engineered to contain L- and D-His enantiomers in positions 6 and 13 and provide a local coordination environment with distinct Cu(2+) binding geometries and affinities. We examined the mechanism of selective chiral recognition of Cu(2+) by such mutant peptides, by quantifying their stochastic sensing in real time with a single α-hemolysin (α-HL) protein immobilized in a planar lipid membrane, while incubated in various concentrations of Cu(2+). Our data reveal that the Cu(2+)-binding affinity lies within the micromolar range, and decreases by orders of magnitude as L-His is replaced with its Denantiomer, with the effect being prevalent when such changes were inflicted on the His-6 residue. The presented results demonstrate the feasibility of tuning the metal selectivity in a relatively simple peptide substrate by enantiomeric replacement of key metal binding residues and illustrates the potential of the protein nanopores as a promising approach to quantify the chiral recognition of l/d amino acids by metals.

Alzheimer's disease--a panorama glimpse

The single-mutation of genes associated with Alzheimer's disease (AD) increases the production of Aβ peptides. An elevated concentration of Aβ peptides is prone to aggregation into oligomers and further deposition as plaque. Aβ plaques and neurofibrillary tangles are two hallmarks of AD. In this review, we provide a broad overview of the diverses sources that could lead to AD, which include genetic origins, Aβ peptides and tau protein. We shall discuss on tau protein and tau accumulation, which result in neurofibrillary tangles. We detail the mechanisms of Aβ aggregation, fibril formation and its polymorphism. We then show the possible links between Aβ and tau pathology. Furthermore, we summarize the structural data of Aβ and its precursor protein obtained via Nuclear Magnetic Resonance (NMR) or X-ray crystallography. At the end, we go through the C-terminal and N-terminal truncated Aβ variants. We wish to draw reader's attention to two predominant and toxic Aβ species, namely Aβ4-42 and pyroglutamate amyloid-beta peptides, which have been neglected for more than a decade and may be crucial in Aβ pathogenesis due to their dominant presence in the AD brain.

The role of intracellular zinc release in aging, oxidative stress, and Alzheimer's disease

Brain aging is marked by structural, chemical, and genetic changes leading to cognitive decline and impaired neural functioning. Further, aging itself is also a risk factor for a number of neurodegenerative disorders, most notably Alzheimer’s disease (AD). Many of the pathological changes associated with aging and aging-related disorders have been attributed in part to increased and unregulated production of reactive oxygen species (ROS) in the brain. ROS are produced as a physiological byproduct of various cellular processes, and are normally detoxified by enzymes and antioxidants to help maintain neuronal homeostasis. However, cellular injury can cause excessive ROS production, triggering a state of oxidative stress that can lead to neuronal cell death. ROS and intracellular zinc are intimately related, as ROS production can lead to oxidation of proteins that normally bind the metal, thereby causing the liberation of zinc in cytoplasmic compartments. Similarly, not only can zinc impair mitochondrial function, leading to excess ROS production, but it can also activate a variety of extra-mitochondrial ROS-generating signaling cascades. As such, numerous accounts of oxidative neuronal injury by ROS-producing sources appear to also require zinc. We suggest that zinc deregulation is a common, perhaps ubiquitous component of injurious oxidative processes in neurons. This review summarizes current findings on zinc dyshomeostasis-driven signaling cascades in oxidative stress and age-related neurodegeneration, with a focus on AD, in order to highlight the critical role of the intracellular liberation of the metal during oxidative neuronal injury.

Effects of Zn2+ binding on the structural and dynamic properties of amyloid β peptide associated with Alzheimer's disease: Asp1 or Glu11?

Extensive experimental and computational studies have suggested that multiple Zn(2+) binding modes in amyloid β (Aβ) peptides could exist simultaneously. However, consistent results have not been obtained for the effects of Zn(2+) binding on Aβ structure, dynamics, and kinetics in particular. Some key questions such as why it is so difficult to distinguish the polymorphic states of metal ions binding to Aβ and what the underlying rationale is, necessitate elucidation. In this work, two 3N1O Zn(2+) binding modes were constructed with three histidines (His(6), His(13), and His(14)), and Asp(1)/Glu(11) of Aβ40 coordinated to Zn(2+). Results from molecular dynamics simulations reveal that the conformational ensembles of different Zn(2+)-Aβ40 complexes are nonoverlapping. The formation of turn structure and, especially, the salt bridge between Glu(22)/Asp(23) and Lys(28) is dependent on specific Zn(2+) binding mode. Agreement with available NMR observations of secondary and tertiary structures could be better achieved if the two simulation results are considered together. The free energy landscape constructed by combining both conformations of Aβ40 indicates that transitions between distinct Aβ40 conformations thar are ready for Zn(2+) binding could be possible in aqueous solution. Markov state model analyses reveal the complex network of conformational space of Aβ40 modeulated by Zn(2+) binding, suggesting various misfolding pathways. The binding free energies evaluated using a combination of quantum mechanics calculations and the MM/3D-RISM method suggest that Glu(11) is the preferred oxygen ligand of Zn(2+). However, such preference is dependent on the relative populations of different conformations with specific Zn(2+) binding modes, and therefore could be shifted when experimental or simulation conditions are altered.

NMR solution structure of rat aβ(1-16): toward understanding the mechanism of rats' resistance to Alzheimer's disease

In an attempt to reveal the mechanism of rats' resistance to Alzheimer's disease, we determined the structure of the metal-binding domain 1-16 of rat β-amyloid (rat Aβ(1-16)) in solution in the absence and presence of zinc ions. A zinc-induced dimerization of the domain was detected. The zinc coordination site was found to involve residues His-6 and His-14 of both peptide chains. We used experimental restraints obtained from analyses of NMR and isothermal titration calorimetry data to perform structure calculations. The calculations employed an explicit water environment and a simulated annealing molecular-dynamics protocol followed by quantum-mechanical/molecular-mechanical optimization. We found that the C-tails of the two polypeptide chains of the rat Aβ(1-16) dimer are oriented in opposite directions to each other, which hinders the assembly of rat Aβ dimers into oligomeric aggregates. Thus, the differences in the structure of zinc-binding sites of human and rat Aβ(1-16), their ability to form regular cross-monomer bonds, and the orientation of their hydrophobic C-tails could be responsible for the resistance of rats to Alzheimer's disease.

In vivo monitoring of the transfer kinetics of trace elements in animal brains with hyphenated inductively coupled plasma mass spectrometry techniques

The roles of metal ions to sustain normal function and to cause dysfunction of neurological systems have been confirmed by various studies. However, because of the lack of adequate analytical method to monitor the transfer kinetics of metal ions in the brain of a living animal, research on the physiopathological roles of metal ions in the CNS remains in its early stages and more analytical efforts are still needed. To explicitly model the possible links between metal ions and physiopathological alterations, it is essential to develop in vivo monitoring techniques that can bridge the gap between metalloneurochemistry and neurophysiopathology. Although inductively coupled plasma mass spectrometry (ICP-MS) is a very powerful technique for multiple trace element analyses, when dealing with chemically complex microdialysis samples, the detection capability is largely limited by instrumental sensitivity, selectivity, and contamination that arise from the experimental procedure. As a result, in recent years several high efficient and clean on-line sample pretreatment systems have been developed and combined with microdialysis and ICP-MS for the continuous and in vivo determination of the concentration-time profiles of metal ions in the extracellular space of rat brain. This article reviews the research relevant to the development of analytical techniques for the in vivo determination of dynamic variation in the concentration levels of metal ions in a living animal.

Zn(II)- and Cu(II)-induced non-fibrillar aggregates of amyloid-beta (1-42) peptide are transformed to amyloid fibrils, both spontaneously and under the influence of metal chelators

Aggregation of amyloid-beta (Abeta) peptides is a central phenomenon in Alzheimer's disease. Zn(II) and Cu(II) have profound effects on Abeta aggregation; however, their impact on amyloidogenesis is unclear. Here we show that Zn(II) and Cu(II) inhibit Abeta(42) fibrillization and initiate formation of non-fibrillar Abeta(42) aggregates, and that the inhibitory effect of Zn(II) (IC(50) = 1.8 micromol/L) is three times stronger than that of Cu(II). Medium and high-affinity metal chelators including metallothioneins prevented metal-induced Abeta(42) aggregation. Moreover, their addition to preformed aggregates initiated fast Abeta(42) fibrillization. Upon prolonged incubation the metal-induced aggregates also transformed spontaneously into fibrils, that appear to represent the most stable state of Abeta(42). H13A and H14A mutations in Abeta(42) reduced the inhibitory effect of metal ions, whereas an H6A mutation had no significant impact. We suggest that metal binding by H13 and H14 prevents the formation of a cross-beta core structure within region 10-23 of the amyloid fibril. Cu(II)-Abeta(42) aggregates were neurotoxic to neurons in vitro only in the presence of ascorbate, whereas monomers and Zn(II)-Abeta(42) aggregates were non-toxic. Disturbed metal homeostasis in the vicinity of zinc-enriched neurons might pre-dispose formation of metal-induced Abeta aggregates, subsequent fibrillization of which can lead to amyloid formation. The molecular background underlying metal-chelating therapies for Alzheimer's disease is discussed in this light.

COL25A1 triggers and promotes Alzheimer's disease-like pathology in vivo

Collagen XXV alpha 1 (COL25A1) is a collagenous type II transmembrane protein purified from senile plaques of Alzheimer's disease (AD) brains. COL25A1 alleles have been associated with increased risk for AD in a Swedish population. COL25A1 is specifically expressed in neurons and binds to aggregated Abeta in vitro. However, its contribution to the pathogenesis of AD and in vivo function are unknown. Here, we report that over-expression of COL25A1 in transgenic mice increases p35/p25 and beta-site APP-cleaving enzyme 1 (BACE1) levels, facilitates intracellular aggregation and extracellular matrix deposits of Abeta, and causes synaptophysin loss and astrocyte activation. COL25A1 mice displayed reduced anxiety-like behavior in elevated plus maze and open field tests and significantly slower swimming speed in Morris water maze. In stable cell lines, motifs in noncollagenous domains of COL25A1 were important for the induction of BACE1 expression. These findings demonstrate that COL25A1 leads to AD-like pathology in vivo. Modulation of COL25A1 function may represent an alternative therapeutic intervention for AD.

Energy landscapes of the monomer and dimer of the Alzheimer's peptide Abeta(1-28)

The cytotoxicity of Alzheimer's disease has been linked to the self-assembly of the 4042 amino acid of the amyloid-beta (Abeta) peptide into oligomers. To understand the assembly process, it is important to characterize the very first steps of aggregation at an atomic level of detail. Here, we focus on the N-terminal fragment 1-28, known to form fibrils in vitro. Circular dichroism and NMR experiments indicate that the monomer of Abeta(1-28) is alpha-helical in a membranelike environment and random coil in aqueous solution. Using the activation-relaxation technique coupled with the OPEP coarse grained force field, we determine the structures of the monomer and of the dimer of Abeta(1-28). In agreement with experiments, we find that the monomer is predominantly random coil in character, but displays a non-negligible beta-strand probability in the N-terminal region. Dimerization impacts the structure of each chain and leads to an ensemble of intertwined conformations with little beta-strand content in the region Leu17-Ala21. All these structural characteristics are inconsistent with the amyloid fibril structure and indicate that the dimer has to undergo significant rearrangement en route to fibril formation.

NMR studies of the Zn2+ interactions with rat and human beta-amyloid (1-28) peptides in water-micelle environment

Alzheimer's disease is a fatal neurodegenerative disorder involving the abnormal accumulation and deposition of peptides (amyloid-beta, Abeta) derived from the amyloid precursor protein. Here, we present the structure and the Zn2+ binding sites of human and rat Abeta(1-28) fragments in water/sodium dodecyl sulfate (SDS) micelles by using 1H NMR spectroscopy. The chemical shift variations measured after Zn2+ addition at T>310 K allowed us to assign the binding donor atoms in both rat and human zinc complexes. The Asp-1 amine, His-6 Ndelta, Glu-11 COO-, and His-13 Nepsilon of rat Abeta28 all enter the metal coordination sphere, while His-6 Ndelta, His-13, His-14 Nepsilon, Asp-1 amine, and/or Glu-11 COO- are all bound to Zn2+ in the case of human Abeta28. Finally, a comparison between the rat and human binding abilities was discussed.

Solution 1H NMR investigation of Zn2+ and Cd2+ binding to amyloid-beta peptide (Aβ) of Alzheimer's disease

Elevated levels of zinc2+ and copper2+ are found chelated to the amyloid-beta-peptide (Abeta) in isolated senile plaque cores of Alzheimer's disease (AD) patients. However, the precise residues involved in Zn2+ ligation are yet to be established. We have used 1H NMR and CD to probe the binding of Zn2+ to Abeta(1-28). Zinc binding to Abeta causes a number of 1H NMR resonances to exhibit intermediate exchange broadening upon Zn2+ addition, signals in slow and fast exchange are also observed. In addition, there is a general loss of signal for all resonances with Zn2+ addition, suggestive of the formation of high molecular weight polymeric species. Perturbations in specific 1H NMR resonances between residues 6 and 14, and analysis of various Abeta analogues in which each of the three His residues have been replaced by alanine, indicates that His6, His13 and His14 residues are implicated in Zn-Abeta binding. Complementary studies with Cd2+ ions cause perturbations to 1H NMR spectra that are strikingly similar to that observed for Zn2+. Binding monitored at Val12 indicates a 1:1 stoichiometry with Abeta for both Zn2+ and Cd2+ ions. Circular Dichroism (CD) studies in the far-UV indicate quite minimal ordering of the main-chain with Zn2+ or Cd2+ addition. Changes in the far-UV are quite different from that obtained with Cu2+ additions indicating that Zn2+ coordination is distinct from that of Cu2+ ions. Taken together, these observations seem to suggest that Zn2+ coordination is dominated by inter-molecular coordination and the formation of polymeric species.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Characterization of the ZnII Binding to the Peptide Amyloid-β1-16 linked to Alzheimer's Disease

Aggregation of the human peptide amyloid-beta (Abeta) is a key event in Alzheimer's disease (AD). Zinc ions play an important role in AD and in Abeta aggregation. In vitro, Zn(II) binds to Abeta and accelerates its aggregation. In this work we have investigated Zn(II) binding to the synthetic peptide Abeta1-16, which contains the metal-binding domain of Abeta. Cd(II) was used to probe the Zn(II) site. Abeta1-16 bound one equivalent of Zn(II) with an apparent dissociation constant (Kd) of 10(-4) M. This Kd value is in the same range as the Zn concentration needed to precipitate Abeta. Circular dichroism and NMR indicated predominantly random-coil secondary structures of apo-Abeta1-16, Zn(II)-Abeta1-16 and Cd(II)-Abeta1-16, which were all highly dynamic and flexible. The three histidines at positions 6, 13 and 14 were suggested to be ligands to Zn(II) and Cd(II). Evidence that the aspartate at position 1 served as a fourth ligand to Zn(II) and Cd(II) was found at pH 8.7. 111Cd(II) NMR showed a resonance at 84 ppm, in line with a mixed oxygen-/nitrogen-ligand environment. The tyrosine at position 10 could be excluded as a ligand.