Exploration of the Potential Role for Aβ in Delivery of Extracellular Copper to Ctr1

Abstract

Amyloid beta (Aβ) peptides are notorious for their involvement in Alzheimer's disease (AD), by virtue of their propensity to aggregate to form oligomers, fibrils, and eventually plaques in the brain. Nevertheless, they appear to be essential for correct neurophysiology on the synaptic level and may have additional functions including antimicrobial activity, sealing the blood−brain barrier, promotion of recovery from brain injury, and even tumor suppression. Aβ peptides are also avid copper chelators, and coincidentally copper is significantly dysregulated in the AD brain. Copper (Cu) is released in significant amounts during calcium signaling at the synaptic membrane. Aβ peptides may have a role in maintaining synaptic Cu homeostasis, including as a scavenger for redoxactive Cu and as a chaperone for clearing Cu from the synaptic cleft. Here, we employed the Aβ 1−16 and Aβ 4−16 peptides as well-established non-aggregating models of major Aβ species in healthy and AD brains, and the Ctr 1−14 peptide as a model for the extracellular domain of the human cellular copper transporter protein (Ctr1). With these model peptides and a number of spectroscopic techniques, we investigated whether the Cu complexes of Aβ peptides could provide Ctr1 with either Cu(II) or Cu(I). We found that Aβ 1−16 fully and rapidly delivered Cu(II) to Ctr 1−14 along the affinity gradient. Such delivery was only partial for the Aβ 4−16 /Ctr 1−14 pair, in agreement with the higher complex stability for the former peptide. Moreover, the reaction was very slow and took ca. 40 h to reach equilibrium under the given experimental conditions. In either case of Cu(II) exchange, no intermediate (ternary) species were present in detectable amounts. In contrast, both Aβ species released Cu(I) to Ctr 1−14 rapidly and in a quantitative fashion, but ternary intermediate species were detected in the analysis of XAS data. The results presented here are the first direct evidence of a Cu(I) and Cu(II) transfer between the human Ctr1 and Aβ model peptides. These results are discussed in terms of the fundamental difference between the peptides' Cu(II) complexes (pleiotropic ensemble of open structures of Aβ 1−16 vs the rigid closed-ring system of amino-terminal Cu/Ni binding Aβ 4−16 ) and the similarity of their Cu(I) complexes (both anchored at the tandem His13/His14, bis-His motif). These results indicate that Cu(I) may be more feasible than Cu(II) as the cargo for copper clearance from the synaptic cleft by Aβ peptides and its delivery to Ctr1. The arguments in favor of Cu(I) include the fact that cellular Cu export and uptake proteins (ATPase7A/B and Ctr1, respectively) specifically transport Cu(I), the abundance of extracellular ascorbate reducing agent in the brain, and evidence of a potential associative (hand-off) mechanism of Cu(I) transfer that may mirror the mechanisms of intracellular Cu chaperone proteins.

Intermediate Cu(II)-Thiolate Species in the Reduction of Cu(II)GHK by Glutathione: A Handy Chelate for Biological Cu(II) Reduction

“…The above results allowed us to propose the scheme of Cu(II)GHK interaction with GSH. Reaction 2 provides the overall stoichiometry of Cu(II) reduction, where [CuGHK]* is a postulated reduction intermediate detected by electrochemistry but not visible to spectroscopic methods due to its very short lifetime, with the follow-up rapid formation of Cu(I) 4 GSH 6 driven by a huge difference of Cu(I) affinities to nitrogen ligands (nano- to picomolar) 66 and GSH (attomolar). 47 …”

Exploration of the Potential Role of Serum Albumin in the Delivery of Cu(I) to Ctr1

“…A similar ternary complex has been proposed in the exchange of Cu(I) between an Aβ peptide and Ctr 1−14 , and such ternary complexes are hypothesized to be intermediates in the Cu(I)-transfer reaction pathway. 60 Alternatively, or in addition, there may be one or more ternary complexes that effectively "trap" Cu(I) from transferring and as such would not contribute toward the final Cu(I) transfer to Ctr 1−14 .…”

Nerve Growth Factor Peptides Bind Copper(II) with High Affinity: A Thermodynamic Approach to Unveil Overlooked Neurotrophin Roles

“…The affinity values for copper ions of a peptide encompassing the sequence 1-16 of Aβ, functionalized with a polyethylene glycol moiety (Aβ 1-16 -PEG) to increase peptide solubility, were calculated from reported stability constant values [ 54 ]. Analogously, the values for copper complexes formed by the N-terminal fragment of hCtr-1 were calculated from potentiometric data reported in literature [ 40 ].…”

“…It has been hypothesized that Aβ can chelate copper ions in the synaptic cleft and act as an extracellular metal chaperone for its delivery to hCtr1 protein. The release of copper from Aβ should be driven by the higher affinity of hCtr1 [ 40 , 41 ]. In competition with Aβ, NGF could perform a similar activity transporting copper to hCtr1.…”

“…Aβ may play the role of handling copper cellular influx/efflux via binding metal and transfer it to human copper transporter 1 (hCtr1), the main protein responsible for Cu import into cells [ 40 ]. Studies carried out with the peptide model showed that a peptide encompassing the first 14 residues of hCtr1 binds copper ions with higher affinity than N-terminal Aβ peptides [ 41 ].…”