Zinc-induced Alzheimer's Aβ1–40 Aggregation Is Mediated by Conformational Factors

Abstract

The heterogeneous precipitates of A␤ that accumulate in the brain cortex in Alzheimer's disease possess varying degrees of resistance to resolubilization. We previously found that A␤1-40 is rapidly precipitated in vitro by physiological concentrations of zinc, a neurochemical that is highly abundant in brain compartments where A␤ is most likely to precipitate. We now present evidence that the zinc-induced precipitation of A␤ is mediated by a peptide dimer and favored by conditions that promote ␣-helical and diminish ␤-sheet conformations. The manner in which the synthetic peptide is solubilized was critical to its behavior in vitro. Zincinduced A␤ aggregation was dependent upon the presence of NaCl, was enhanced by ␣-helical-promoting solvents, but was abolished when the peptide stock solution was stored frozen. The A␤ aggregates induced by zinc were reversible by chelation, but could then be reprecipitated by zinc for several cycles, indicating that the peptide's conformation is probably preserved in the zinc-mediated assembly. In contrast, A␤ aggregates induced by low pH (5.5) were not resolubilized by returning the pH milieu to 7.4. The zinc-A␤ interaction exhibits features resembling the gelation process of zinc-mediated fibrin assembly, suggesting that, in events such as clot formation or injury, reversible A␤ assembly could be physiologically purposive. Such a mechanism is contemplated in the early evolution of diffuse plaques in Alzheimer's disease and suggests a possible therapeutic strategy for the resolubilization of some forms of A␤ deposit in the disease.The pathological hallmark of Alzheimer's disease is the abundant accumulation in the brain of A␤, a 39 -43-amino acid peptide, as morphologically heterogeneous deposits in the neuropil (senile plaques) and cerebral blood vessels (congophilic angiopathy) (1, 2). A␤ is a soluble component of cerebrospinal fluid where it is found in concentrations in the low nanomolar range (3-5). Many studies now indicate that synthetic A␤ becomes toxic to cultured neuronal cells when in a specific ␤-sheet conformation that involves incubating the synthetic peptide in a aqueous solution over time periods of days to weeks (6 -14). Whereas the ␤-sheet conformation of the peptide is found in highly insoluble fibrillar amyloid deposits in Alzheimer's disease, a substantial proportion of A␤ precipitates into nonfibrillar deposits that can be resolubilized by extraction into aqueous solvents (15).We have recently reported that A␤ itself specifically and saturably binds zinc, manifesting high affinity binding (K D ϭ 107 nM) with a 1:1 (zinc:A␤) stoichiometry and low affinity binding (K D ϭ 5.2 M) with a 2:1 stoichiometry (16). This binding is probably histidine-mediated since it is abolished by acidic pH (no binding at pH 6). The zinc-binding site was mapped to a stretch of contiguous residues between positions 6 -28 of the A␤ sequence. Occupation of the zinc-binding site, which straddles the lysine 16 position of ␣-secretase cleavage (17, 18), inhibits ␣-secretase type (tryp...

Contribution by synaptic zinc to the gender-disparate plaque formation in human Swedish mutant APP transgenic mice

“…Evidences obtained in vitro suggest that zinc may contribute to A␤ aggregation and thus plaque formation (5)(6)(7)(8). The recent demonstration that metal chelation ameliorates A␤ plaque formation in hAPP ϩ mice strongly supports this hypothesis (13).…”

“…First, zinc or copper induces the rapid aggregation of synthetic A␤ in an aqueous environment (5,6), likely by binding to histidine residues within A␤ (7,8). Second, concentrations of the transition metals including zinc and copper are elevated in AD brains, more so around plaques (9)(10)(11).…”

“…Second, thus far, synaptic zinc is the only zinc that has been demonstrated to be released with neuronal activity into the extracellular space (17,24), where A␤ mainly accumulates. Although extracellular zinc concentrations are normally low (Ͻ1 M), at the peak of neuronal activity, they may exceed 100 M (24), a concentration sufficient to promote aggregation of A␤ (7,12).…”

Zinc and copper modulate Alzheimer Aβ levels in human cerebrospinal fluid

“…The toxicity of Aβ in cell culture depends upon the peptide binding to copper [2] or iron [39] through generating radical chemistry (Aβ42>Aβ40) [20,31]. Zinc, copper and iron rapidly precipitate Aβ [2,6,13,19], which then becomes resistant to proteolysis [9]. The brain and CSF are protected from fluctuations in plasma metal concentrations by the bloodbrain barrier.…”

In vivo reduction of amyloid-β by a mutant copper transporter

“…In addition to the ectodomain Cu-binding site of APP, A␤ peptide also contains binding sites for Zn and Cu (13)(14)(15). A␤-metal interaction may drive both fibril formation and free radical production (13,(16)(17)(18), findings potentially relevant to AD pathogenesis in vivo, given that metal chelators can resolubilize A␤ aggregates from postmortem AD brain (19). On the other hand, studies of APP processing in cultured cells have revealed stimulation of the ␣-secretase pathway for APP processing by extracellular Cu ions (20).…”